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Title: Physiology - The Science of the Body
Author: Martin, Ernest G.
Language: English
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[Illustration: _Muscular Coordination_
_Weissmuller’s horizontal plunge into a swimming pool._]
POPULAR SCIENCE LIBRARY
EDITOR-IN-CHIEF
GARRETT P. SERVISS
AUTHORS
WILLIAM J. MILLER · HIPPOLYTE GRUENER · A. RUSSELL BOND
D. W. HERING LOOMIS HAVEMEYER · ERNEST G. MARTIN
ARTHUR SELWYN-BROWN · ROBERT CHENAULT GIVLER
ERNEST INGERSOLL · WILFRED MASON BARTON
WILLIAM B. SCOTT · ERNEST J. STREUBEL
NORMAN TAYLOR · DAVID TODD
CHARLES FITZHUGH TALMAN
ROBIN BEACH
ARRANGED IN SIXTEEN VOLUMES
WITH A HISTORY OF SCIENCE, GLOSSARIES
AND A GENERAL INDEX
_ILLUSTRATED_
[Illustration]
VOLUME NINE
P. F. COLLIER & SON COMPANY
NEW YORK
Copyright 1922
BY P. F. COLLIER & SON COMPANY
MANUFACTURED IN U. S. A.
PHYSIOLOGY
THE SCIENCE OF THE BODY
BY
ERNEST G. MARTIN
PROFESSOR OF PHYSIOLOGY, STANFORD UNIVERSITY
[Illustration]
P. F. COLLIER & SON COMPANY
NEW YORK
PREFACE
When Alexander Pope wrote “The proper study of mankind is man,” he was
thinking rather of man as a social being than as the possessor of an
amazingly complex and interesting body. It is nevertheless true that to
one who finds enjoyment in the study of intricate mechanisms or to one
for whom that amazing sequence of events which we call life has appeal
there is no more fascinating study than the study of the living body.
That part of the study of the body which concerns itself primarily with
activity and only secondarily with form and structure, makes up the
science of Physiology. The way the body works is the central theme.
The practical value of Physiology to the general reader lies in the fact
that it forms the basis of all sound rules of hygiene. Life is made up
of bodily activities which may be carried on correctly or incorrectly.
Carried on correctly they mean health, carried on incorrectly, unhealth.
The world is flooded with health-preserving or health-restoring systems,
urged upon the public, for the most part, by promoters in search of
gain. Such of these as have merit are based on definite physiological
principles, and anyone who has a common-sense working knowledge of his
own body can order his life in accordance with them, at little or no
expense. Moreover, a sound appreciation of Physiology drives home the
truth that when the body is really out of order its restoration can be
safely intrusted only to the properly trained physician: the man or
woman who through years of painstaking study has won insight into the
intricacies of the human mechanism and whose honest appreciation of the
difficulties of his profession, and courageous sincerity in grappling
with them, justify to the full the confidence in which he is held by his
community.
ERNEST G. MARTIN.
CONTENTS
CHAPTER PAGE
I. THE SIGNS OF LIFE 9
II. THE MAINTAINING OF LIFE 18
III. THE SOURCES OF FOOD 24
IV. THE USES OF FOOD 31
V. BODY CELLS 41
VI. THE SUPPORTING FRAMEWORK 53
VII. MOTION 73
VIII. SENSATION--INTERNAL AND CONTACT SENSES 86
IX. SENSATION--DISTANCE SENSES 96
X. THE NERVOUS SYSTEM AND SIMPLE NERVOUS
ACTIONS 122
XI. THE BRAIN AND COMPLEX NERVOUS ACTIONS 139
XII. SOME SPECIAL NERVOUS ACTIONS; SMOOTH
MUSCLE AND GLAND CONTROL 155
XIII. THE BODY FLUIDS 173
XIV. THE CONVEYER SYSTEM OF THE BODY 191
XV. THE SERVICE OF SUPPLY OF FOOD 226
XVI. THE SERVICE OF SUPPLY OF OXYGEN 253
XVII. THE SERVICE OF REMOVAL OF WASTE 271
XVIII. MORE ABOUT THE USE OF FOOD BY THE BODY 277
XIX. KEEPING WARM IN WINTER AND COOL IN SUMMER 305
XX. THE PERPETUATION OF THE RACE 324
XXI. CHILD AND MAN 345
LIST OF ILLUSTRATIONS
MUSCULAR COORDINATION--WEISSMULLER’S HORIZONTAL
PLUNGE INTO A SWIMMING POOL _Frontispiece_
FACING PAGE
FACTORY’S COOPERATION IN AIDING PHYSICAL FITNESS,
UNDERWEIGHT EMPLOYES ARE GIVEN A MILK LUNCH 32
WEIGHT TEST IN A CHILD’S PHYSICAL EXAMINATION TO
DETERMINE HOW NEARLY THE CHILD APPROACHES
CORRECT STANDARDS OF PHYSICAL DEVELOPMENT 33
FRENCH METHOD OF MEASURING VERTICAL CONFORMATION,
CONSIDERED VALUABLE IN ADVISING AND
TRAINING ATHLETES 64
CURVE OF THE SPINE MEASURED AND DRAWN FOR STUDY
OF AN ATHLETE’S POSSIBILITIES 65
MUSCULAR DEVELOPMENT OF AN ATHLETE--THE DISCUS
THROWER OF MYRON 80
A MODERN “VICTORY”--MISS SABIE AT PRACTICE 81
HOSPITAL DESIGNED TO SUPPLY THE MAXIMUM OF LIGHT
AND AIR--CHIEF REQUISITES FOR HEALTH--TO EACH
ROOM 96
ARCHITECT’S PLAN MADE WITH THE NEED FOR LIGHT
AND AIR IN MIND 97
SOFT, RESTFUL COLORS OF A HOSPITAL ROOM 112
MAGNET PUT TO THE SERVICE OF REMOVING IRON FILINGS
FROM THE EYE 113
TEST FOR BLOOD PRESSURE 192
INDICAN TEST IN URINALYSIS 193
MICROSCOPIC STUDY OF THE BLOOD 208
MICROPHOTOGRAPH OF BRAIN TISSUE 209
X-RAYING THE TEETH AND JAW--HEALTH OFTEN DEPENDS
ON THEIR GOOD CONDITION 224
THROAT EXAMINATION--IMPORTANT IN DETERMINING
PHYSICAL CONDITION 225
LUNCH HOUR IN FRENCH SCHOOL WHERE EXERCISE,
OUT-OF-DOOR LIFE, SUNSHINE AND FRESH AIR ARE
CONSIDERED ESSENTIALS 288
DIET KITCHEN ON A U. S. HOSPITAL SHIP 289
CARING FOR THE EARS--IMPORTANT FOR ADULTS AND
BABIES ALIKE 336
BABY’S FOOTPRINT--A MEANS OF POSITIVE IDENTIFICATION 336
BABIES, PHYSICALLY AND MENTALLY ACTIVE 337
CHAPTER I
THE SIGNS OF LIFE
Physiology is the study of living things, so the first thing to be asked
when we begin to think about physiology is how we are to know whether
anything is alive or not. It is usually pretty easy to tell whether a
dog or cat is alive or dead, although sometimes when a dog is stretched
out on the road we have to look closely to tell whether he has already
met his end or is merely courting it by sleeping in the public highway.
There are in the world hosts of animals with which we are not familiar,
and to tell whether these are alive or dead is often a puzzle. More than
one picnicker has been thoroughly surprised by seeing what looked like a
bit of dead twig begin to walk away, and recognized the walking stick.
On the whole we will agree that the sign of life which we find most
reliable is motion of some sort on the part of the living animal. If the
stretched-out dog makes breathing movements, we pronounce him alive; if
not, we decide that he is dead. It is because the walking stick moves
off when disturbed that we know it is not a twig. But while motion is
the thing we look for in living animals we know perfectly well that it
would be foolish to assert that anything that moves is alive. When the
wind is blowing the air may be full of dead leaves and butterflies, all
moving, but only part, the butterflies, alive. Unless the motion is
produced by the animal itself it is not good as a sign of life. So
widespread among animals is the making of movements, either on their own
account, or when disturbed, that we shall not often find ourselves
mistaken if we decide that an object which remains quiet indefinitely is
not a living animal. Now how about the other side of the question? Is
anything that moves on its own account or when disturbed to be judged
alive?
Suppose that the inhabitants of Mars have finally succeeded in
perfecting a flying boat that can be hermetically sealed and shot across
space from that planet to our earth. Suppose further, that the first
exploring party has set forth on a voyage of discovery, and has reached
a point high in our sky from which objects on the earth’s surface begin
to be distinguishable. Of course the huge landmarks, cities, lakes, and
rivers, have been in view for a long while, and now the explorers are on
the lookout for signs of living things. They are watching, just as we
would be, for moving objects. The first moving thing that they see will
probably be a train, and we can imagine their speculations as to whether
they are actually looking or not at an inhabitant of the earth. As their
craft sinks toward the surface the make-up of the train becomes
perceptible as also the fact that it runs on rails, showing that it is a
mechanical contrivance and not a living being. As smaller objects come
into view black, shiny specks are seen moving about. These show every
appearance of life; they start and stop; pass each other without
interference; in fact conduct themselves about as animals usually do. If
their apparent great power has the effect of discouraging the exploring
party, so that they give up further investigation and fly away to Mars,
the inhabitants of that planet will always suppose the earth to be
populated by automobiles. We know that automobiles are not alive, yet,
as this little allegory shows, they behave enough like living beings to
deceive distant observers. There must be some sign of life which will
apply to an animal and not to an automobile; what is it? Evidently what
the Martian explorers missed was the fact that the automobile does not
really start or stop itself, or guide itself past obstructions. If it
had been alive, it would have done these things of itself. It is not so
much the power of motion, then, that proves that the thing is alive as
the power of making motions that are under the control of the animal
itself.
The sight of an automobile which is not alive behaving as though it were
because it is under control of a driver who is alive may lead us to ask
whether the animal that we know to be living is actually alive in all
its parts, or is a dead mechanism of some sort which has somewhere
within it a living controller, corresponding to the living driver of the
car. The animals with which we are most familiar are ourselves; how is
it with our own bodies? Are they alive in all their parts, or is the
brain the only part of us which is living? When a patient goes under
ether on the operating table, or even when he is sound asleep, the signs
of life are not conspicuously present; the heart goes on beating, to be
sure, but so does the engine of an automobile go on running when the
driver is away, provided he has not shut it off. A favorite belief among
the Hindus is that when they go into a trance the body actually becomes
lifeless while the living spirit soars among the heights. How are we to
decide whether the Hindus are right or not? Evidently we shall have to
look deeper than we have thus far, and learn something of what is
actually going on in the different parts of our bodies when we are
asleep and when we are awake.
Nearly everyone learns in school the main facts about the construction
of the body; that there is a bony skeleton which supports the softer
parts; that motions are made by muscles; that sense organs inform us as
to what is going on in the world around; that the brain is the seat of
the mind; that heart, lungs, stomach, kidneys, and other organs
contribute in various ways to our well-being. Not so many go into detail
as to the make-up of these organs, or into the way in which they do
their work. This is not a simple matter, for several reasons. The first
is that the construction units are so tiny that they cannot be seen by
the unaided eye, but must be studied under the high magnification of a
first-class microscope. It is much harder to make out the manner of the
working of tiny pieces of machinery than of those that are of convenient
size. When the parts are as small as those that make up our bodies, the
task of finding out how they operate is so difficult that even now,
after years of study, there are many details about which we know very
little.
The construction units have been named cells. In some tiny animals the
whole body consists of but one cell; all higher animals, including
ourselves, have millions of cells making up the body. Undoubtedly some
cells are alive; our question is as to whether all of them are, or
whether there are some that are alive and some that are not. There are
parts of our bodies, and of the bodies of nearly all other kinds of
animals, as well, that are certainly not alive. Examples are the hair,
the nails, the enamel of the teeth, and the hard parts of the bones.
Actual living stuff is very soft and liquid. It is too fragile to hold
its own structure except in the very tiniest animals; those that are
larger need some additional supporting framework. In a body the size of
a man’s the supporting framework amounts to a very considerable
percentage of the entire weight (25 per cent). Not only is there the
large bony skeleton, but between and among the individual cells is a
framework made up of fine fibers and sheets which hold the cells in
place. This latter framework is called connective tissue; we run across
it in the gristly parts of meat. It makes up the stringy mass that
clings to the cutter of the meat grinder when beef is being ground for
Hamburg steak. We shall consider later how all this supporting material
is made and put in place. Just now we are interested in the cells, and
in determining whether all of them are alive or not.
There are many different kinds of cells in the body; some are muscle
cells, others nerve cells, still others gland cells, and so on. Careful
study shows, however, that at bottom all cells are alike. All are
composed of one kind of substance to which has been given the name of
protoplasm, meaning first or primary flesh. It is because some, at
least, of this protoplasm is alive that our bodies are alive, and our
physical life consists of nothing more than the combined life of all the
living protoplasm which our bodies contain. Is there any way by which to
tell whether any particular mass of protoplasm is alive or not? In other
words, what are the signs of life of protoplasm as contrasted with the
signs of life of whole animals?
We shall scarcely expect it to be as simple a matter to tell whether the
tiny mass of protoplasm that we call a cell is alive or not as to decide
whether a dog is dead or alive. For one thing, our most useful test of
life, namely motion, cannot always be applied to single cells. We have
in our bodies a great many cells, those in the brain, that we know are
alive if any part of us is, but aside from the exceedingly gradual
shifts in position that take place during growth the brain cells never
make any motions at all, so far as anyone has ever been able to find
out. Of course in the body of any ordinary animal most of the cells are
hidden from view beneath the skin, but there are enough small
transparent animals whose internal parts can be watched through the
microscope to let us say with certainty that some of the cells which we
know to be alive do not move. Tests of life that can be applied to all
kinds of cells will necessarily be difficult to use, and we shall have
to take the word of experts as to whether they have found particular
cells alive or not, but the principle on which the tests are based is
simple enough so that we can examine it. To do this, it will be
necessary to turn our attention for a little while to some of the very
tiniest of all living animals, those whose whole bodies consist of but
one cell.
When these tiny one-celled animals are watched through the microscope as
they swim about it can be seen that in one important feature they behave
just as we do ourselves; that is in their care not to neglect mealtime.
To be sure, mealtime comes for them whenever they happen to hit against
any tinier particle than themselves, which they can take in and digest.
But for them, as for us, the taking of food from time to time is a
necessity of life. Only a small part of the food thus taken in is added
permanently to the bulk of the animal. In other words, the growth does
not go on as fast as does the taking of food. Of course in ourselves,
after we have reached full size, there is little or no increase in
permanent bulk even though we do keep right on eating. Evidently in the
tiny one-celled animal, and in us as well, food is constantly being used
for something besides growth. It can be proved that this food is used
for precisely the same purpose that gasoline is used in the automobile,
namely to run the machine. In a very real sense every living thing is a
machine, and will no more run without a supply of power than will any
other machine. From the engineering standpoint animals can be classified
along with automobiles and locomotives as “prime movers,” namely, as
machines which develop their power within themselves. There are not many
kinds of power which can be developed by prime movers. By far the
commonest is that seen in locomotives and automobiles, namely the
burning of some kind of fuel. We have always known that the locomotive
operates by the burning of coal or oil in the fire box. A moment’s
thought will show us, if we have not realized it before, that the
explosion of the air-gas mixture in the automobile cylinders is also a
burning. Every steam-driven power plant depends on burning fuel for its
power. Evidently fuel materials contain abundant power, if it can be
extracted, and burning is a good method for doing the extracting. The
word “burning” is the common name for a chemical process known
technically as “oxidation,” meaning the union of oxygen with the fuel.
The air is one-fifth oxygen, so there is plenty available, and fuel will
usually oxidize readily after it is properly started.
Not only do animals correspond with other machines in using fuel as
their source of power; they correspond also in that the power is
extracted through the process of oxidation. To be sure, the oxidation in
animals is not accompanied by flame and smoke as it usually is in power
plants, nor do any parts of the animal get as hot as does the furnace
where fuel is ordinarily burned; but in spite of these differences the
fundamental fact is the same, namely that the extraction of power is by
means of oxidation. What this shows is that great heat, flame, and smoke
are not necessary in oxidation, but only in the kinds of oxidation with
which we are most familiar.
As soon as we have described one more feature of animal power
development, we shall be ready to apply what has been said to the topic
in hand, namely the signs of life in single cells. The point that
remains to be made is that in living cells power development has to go
on all the time whether the cell is active or not. This means that fuel
is constantly being burned, and oxygen is constantly being taken in to
do the burning. There has been, and still is, a great deal of debate as
to how much the oxidation can be reduced in living cells without
destroying life. It is evident that it can be cut down to a very low
level indeed, for seeds remain alive for years without using up, or even
noticeably depleting, the store of fuel material which they contain.
Most botanists of the present time doubt the truth of the tale that
grains of wheat have sprouted after being taken from the wrappings of
mummies, where they had lain for thousands of years. Careful efforts
have been made to preserve wheat under as favorable conditions as
existed in the mummy wrappings, but in every case the power to sprout
was lost within a comparatively few years. So far as experiment enables
us to judge, the complete cessation of power development in cells,
either of plants or animals, means their death.
Here we have our sign of life that is applicable to all kinds of cells
wherever they are located, whether making up the whole of a microscopic
animal or deeply imbedded in the body of a large animal which consists
of millions of them. If power development is going on, the cell is
alive; if no power is being developed, the cell is not alive. When this
test is applied it is found that all the protoplasmic cell masses which
are present in the body of a plant or animal are alive, and since such
masses are everywhere throughout the body, life is present in all parts
of it, and not confined to the brain or to any other single region. We
might admit that the Hindus are correct in assuming that the spirit can
sometimes soar away and leave the lifeless body behind, but we cannot
accept the possibility that it can return and establish life within it
again. When life is resumed after a trance, that fact is proof positive
that life continued throughout the trance itself.
CHAPTER II
THE MAINTAINING OF LIFE
Equal in importance to being alive is the power to go on living;
therefore, having described the signs of life, our next task is to
consider how that life is maintained. When the primary fact of life was
given as continuous power development, the foundation was laid for this
topic, for life cannot fail to go on if continuous power development is
maintained.
Power development in living animals as in locomotives depends on fuel
and oxygen; evidently continuous supplies of these must be provided if
life is to go on. The living animal differs from the locomotive in this:
that while some one attends to supplying the locomotive with fuel, most
living animals, except the very young, have to attend to providing
themselves. There are exceptions to this rule. The tapeworms that
inhabit the intestines of animals, and sometimes of men, live in a
stream of food; they are put to no trouble to obtain it. The same is
true of many kinds of parasites. Except for these, however, it holds
true that animals must attend to their own wants. We shall now begin to
see the utility of the most conspicuous sign of life spoken of in the
first chapter, namely, motion, for food must be gotten where it is; only
tapeworms and similar animals swim in it. All the rest, including
ourselves, must go to where the food is. Even animals like oysters,
that are anchored to the rocks, have to use motion in getting food. In
their case the motion consists in setting up a current in the sea water
into and through their bodies, from which current they sift out the tiny
food particles which abound in the ocean.
If an animal happens to live in the ocean, where every drop furnishes
its particle or two of foodstuff, and especially if the animal is small,
or sluggish, like the oyster, almost any kind of motion will serve to
bring the animal all the food it needs. The simplest of the one-celled
animals, that must be watched through the microscope to see how they
behave, blunder about aimlessly, and in the course of their blundering
bump up against food particles often enough to keep themselves fed. If
an animal happens to live where food is scarcer, or if it is big and
active, and so must have large quantities of food, aimless blundering
about will never get it enough to keep it alive. It must have some means
of finding out where the food is. Since we ourselves come under the head
of animals whose food needs are so large that we must locate food
supplies, and not depend on happening onto them, we can identify in
ourselves the means which are used for doing this. We all know that our
sense organs, the eyes, ears, nose, and finger tips are what we depend
on for telling us where food is to be found. The same is true of all
animals that are able to hunt for food; they have some sort of sense
organs to help in guiding them to where the food is.
The story of the machinery for finding food is not yet quite complete,
for the muscles which actually make the movements by which the animal
gets about are in one part of the body, while the sense organs which
are to furnish the information by which the movements are guided are in
quite a different part, and in animals as large as ourselves, some
distance away. From our eyes to our leg muscles is quite a space, and it
is evident that this space must be bridged somehow if our legs are to
move in obedience to information which our eyes bring in. In ourselves
and in almost all other animals this space is bridged by means of
special machinery for the purpose. We are familiar with it under the
name of the nervous system. We may not have been in the habit of
thinking of the nervous system in just this way, but at bottom this is
exactly what the nervous system does for us, namely, guides our muscles
according to the information brought in by our sense organs. There is
more to nervous activity than just this, but this is the starting point
and groundwork for all the rest, as we shall try to show presently.
Continuous food supplies are the main necessity for continued life, but
there are some other things that have to be looked out for in addition.
The favorite food for large numbers of animals, and, indeed, in many
cases the only food, consists of the bodies of other animals. All the
flesh-eating sorts prey on other animals for their food. This places the
other animals on the defensive, so that a large part of their activity
consists in escaping the attacks of the beasts that wish to eat them.
For most kinds of animals the greater part of their waking life is taken
up with movements which serve either to get them food or to prevent them
from becoming food for others. If we add to these the movements that are
necessary to preserve the animals against other kinds of danger than the
danger of being eaten, and those connected with the propagation and
care of the young, we shall have about covered the list of what we may
call the serious activities of animals, and of men as well. Many kinds
are active besides in play. This is particularly true of young animals,
although grown-ups, both among animals and men, find play both agreeable
and beneficial when not overindulged.
Protective motions need to be even more accurately made than those whose
purpose is the getting of food, for if the food is missed at one effort
another trial may be more successful, but if an attempt to escape fails
there will probably be no more chances to try. The sense organs and the
nervous system are just as deeply concerned, therefore, in avoiding harm
as in finding food materials, and it is as important for them to do
their work well in the one case as in the other. When we think of the
activities of animals, for whatever purpose they are carried on, we must
think of them as made up of the combined actions of the muscles, the
nerves, and the sense organs, and not of any of them working by
themselves.
These parts of us that are so closely concerned in the maintaining of
our life by getting us food and keeping us safe from harm make up, also,
the only parts of us which really share in what we may call conscious
living. When we come right down to it we could spare our other
organs--heart, lungs, stomach, and the rest--and never miss them so far
as adding anything to our happiness is concerned. In fact, the less
these organs intrude themselves into our attention the better off we
are; only when we are ailing do we begin to think about them. Of course,
they are absolutely necessary to us, and we should die instantly if one
of the more important of them were to stop working, but the part they
play is not one which enters actively into our consciousness, as do the
muscles, nerves, and sense organs.
Naturally, we will ask what all these other organs are for if they do
not share in our conscious life. Why can we not get along with just
those that we use for getting food, for avoiding harm, for play, and for
the other activities which they carry on? The answer to this question is
found in the fact emphasized above that continuous power development is
necessary to continued life. By themselves the muscles, nerves, and
sense organs cannot carry on power development; they require the aid of
a great many other organs to do this. Just how these other organs work
will be described later; at present it will be enough to recall that
every muscle, every nerve, and every sense organ is actually made up of
a great many of the tiny construction units--the cells about which we
were talking a few pages back--and that every one of these cells must be
developing power all the time if it is to go on living. In order to be
able to do this they must, every one, be able to oxidize fuel
continuously, and this means that they must receive constant supplies,
both of the fuel itself and of the oxygen with which it combines. Some
system of delivering these materials must be in operation, and in case
the materials have to be prepared for use beforehand this must be
provided for also. The heart, the lungs, the stomach, and the various
other organs that are useful but not conspicuous, are concerned in these
necessary jobs. In an automobile factory we have a similar situation;
the men that stand at the machines actually make the parts that go into
the finished automobiles, but unless other men are hard at work
preparing the castings, and bringing them to where the machine operators
can get them, not many automobiles will be turned out. So in the body,
unless the various organs are carrying on their work of preparing and
delivering materials to the muscles, the nerves, and the sense organs,
these latter cannot perform their tasks of getting the food for the
whole body and of securing the body against harm, nor can they carry on
the pleasant, but not absolutely necessary, activities of play and
recreation.
CHAPTER III
THE SOURCES OF FOOD
We have talked about the necessity of power development in all living
things, and have seen that power development depends on the oxidation of
fuel. Of course, our fuel is the food that we eat. No substance is
suitable for fuel unless it contains power which can be gotten out by
oxidation, and unless, in addition, it is suited to the particular kind
of oxidation that goes on in the body, and can be handled by the body.
Wood is excellent fuel for some purposes, but as a food for man it has
no value, even when ground fine and mixed with flour as was done in some
European countries during the Great War, because wood cannot be handled
by the body in the way in which a usable fuel must be. Although wood is
not good food, closely related materials are, and in fact make up the
bulk of it. All fuel food is either vegetable or animal. All animal food
traces back finally to the vegetable world, and it is an interesting
fact that we do not usually care to eat flesh that is more than one
remove from the vegetable kingdom. Animals that are flesh eaters are not
considered fit for food, except in the case of fish and birds, and the
flesh that these eat is not commonly thought of as being such, since it
consists mostly of the flesh of insects, frogs, and fish themselves.
The real sources of food, then, are in the vegetable world. Of the
countless thousands of kinds of plants that exist a few dozen have
proven to be of enough use for human food, or for food for the animals
on which human beings feed, to justify us in taking the trouble to raise
them on our farms and in our gardens. There must be something about
these particular plants to make us prefer them. If we look into the
reason for the preference we shall understand something of the qualities
which make plants good for food in the first place. At the beginning of
the chapter were set down the things which make a substance fit for
food. These are: the ability to yield power by oxidation, and a
composition suitable to be used by the body. The ability to yield power
involves the possession of a store of it. Power, or energy, which means
the same thing in our present use of the words, is never present
anywhere except as the result of an earlier exhibition of power. It is
not made out of nothing. The sun is a reservoir of energy on which the
earth draws, and we do not know with any certainty from whence the sun
got its power. The heated center of the earth itself is a reservoir of
power on which we may draw at some time in the future, when cheaper
sources are used up. Except for energy from these sources and for
trifling amounts that may be brought in by meteorites, there is none on
the earth’s surface that has not always been here. On the other hand,
the earth is constantly losing energy into space. The amount that
reaches us from the sun balances our losses into space, so that the
total energy present holds fairly steady. The energy that comes to us
from the sun is chiefly in the two forms of heat and light. In actual
horsepower the heat far outweighs the light, but in importance to
mankind one stands about on a par with the other, for while without the
sun’s heat the earth would become so cold that we would all die, without
its light there would be no food and we would all starve. This is
another way of saying that the energy that plants store up, and that we
get when we eat them, comes originally from the light of the sun.
Plants, like animals, are made up of cells. Those with which we are
familiar consist of a great many cells, of a good many different kinds.
Some are in the roots, others in the stems, still others in the leaves;
the blossoms, fruits, and seeds are made up, likewise, of cells. The
cells near the surface of the leaves, and, in many kinds of plants, near
the surface of the stems as well, contain a green substance known as
chlorophyll. This substance enables the cells in which it is present,
although we do not know just how, to manufacture sugar, utilizing the
energy of the sunlight for the purpose.
Sugar is composed of three very common chemical elements, carbon,
hydrogen, and oxygen. As we all know, hydrogen and oxygen in combination
of two atoms of hydrogen to one of oxygen form water; the most familiar
of chemical symbols is that expressing this combination, namely H{2}O.
Carbon, which we know in an almost pure state in anthracite coal, and in
even purer form in diamonds, forms a combination with oxygen known as
carbon dioxide. This is a gas; it makes up a small fraction of the air.
The amount in the air is increased whenever coal or any other
carbon-containing material is burned, since carbon dioxide is the
product of the oxidation of carbon. Except in the arid regions of the
earth there is always some water in the soil a greater or less distance
below the surface of the ground. Water and carbon dioxide between them
contain all the elements of which sugar is composed. The
chlorophyll-containing cells are the factory; the sunlight is the power;
and the carbon dioxide and water are the raw materials. Sugar is the
finished product, and wherever sunlight is falling on green plants,
whether directly or through a layer of cloud, its manufacture is going
on. Sugar will oxidize readily, and in so doing will yield abundant
power. The energy which it contains was derived by transformation from
the energy of the sunlight. With the exception of a few kinds of
bacteria every living thing on the earth depends for its food, and so
for its energy, either directly or indirectly on the sugar which green
plants manufacture. Since sugar dissolves in water it cannot easily be
held in storage, so by a simple chemical process the plant changes it to
starch, and it is in this latter form that we get it, except in the case
of a few plants, like sugar cane.
The green parts of plants are the only places where sugar is made. We
eat a certain amount of green food in lettuce and asparagus and similar
vegetables, but for the most part the sugar or starch we eat comes from
parts of plants that are not green. There is evidently a transportation
from the point of manufacture to points of storage. The means of
transport is in the sap; since starch is not soluble in water, it must
be changed back into sugar. This is done, and then, by the movement of
sap the dissolved sugar is carried to the points of storage, roots in
such vegetables as beets, underground stems in potatoes, above ground
stems in sugar cane, fruits or seeds in orchard and grain crops. In such
of these as are sweet, the sugar itself is held in storage; in most
kinds it is changed back into starch. Where the storage is in the form
of starch the vegetable ordinarily keeps better than when sugar is the
substance on deposit.
A few kinds of plants--olives, peanuts, and cocoanuts, for
example--convert the sugar into oil and store their surplus material in
that form. The chemical elements in oils and fats are the same as in
starch and sugar, although the proportions are not the same. Weight for
weight oil has more than twice the energy value of sugar; in making a
given amount of peanut oil the peanut vine used up more than twice the
amount of starch or sugar; but since energy value is what counts rather
than bulk the plant is just as well off, and perhaps better on account
of the smaller bulk occupied by the stored material. One of the very
interesting examples of oil storage is found in the very tiny plants,
called diatoms, which abound in the water of the ocean. Each tiny diatom
stores within itself an even more tiny drop of oil. Although by
themselves single oil drops would make no impression, if enough of them
could be brought into one place a respectable accumulation of oil would
result. This is precisely what the geologists tell us has happened in
past ages; the bodies of diatoms have accumulated through thousands of
years, and finally the oil accumulations have been covered over with
sediment of one kind or another. When we tap through the sediment we
strike into the “oil sand,” which contains this residue of the diatoms,
and an oil well results.
Since we depend for our food, and so for our life, on the sugar-making
activities of green plants, it will be worth our while to think for a
moment of the slowness with which the process goes on. The slice of
bread which we may eat in a dozen bites represents the result of a
season’s growth of several wheat plants, every one of which was
absorbing the sun’s energy and laying up starch grains during every
daylight minute throughout the growing season. From the standpoint of
the plant which does the storing the material which serves us as food is
the excess over the plant’s own daily needs. In most cases it would be
utilized at the beginning of the next season’s growth before the plant
had put out a leaf system, if the course of events were not disturbed to
satisfy the needs of man.
In addition to starch, sugar, and fat there is another kind of food
material manufactured by plants, known as protein. This substance is
much more complex chemically than any of the others; it contains, in
addition to the three chemical elements--carbon, oxygen, and
hydrogen--that are found in them, the element nitrogen, and usually some
phosphorus and sulphur. These materials are dissolved in the soil water
in the form of simple chemical substances, and are taken up by the plant
along with the water which enters the roots and flows as sap up to the
leaves. The same cells of the plant that make sugar have the power to
make protein, using as raw materials some of the sugar along with the
substances brought in with the soil water. The energy for the
manufacture of protein comes from the oxidation of some of the sugar or
starch in the leaf. The finished protein has about the same energy
value, weight for weight, as has the starch from which it was mainly
derived.
When an animal eats a plant or part of one, he is eating for the sake of
the sugar or sugar products which the plant has made. There is one sugar
product that is useful as food for many animals, but not for man,
except possibly to a very slight extent. That is the woody substance,
cellulose, which is formed in plants mainly as a support to the delicate
living protoplasm. Cotton fiber is nearly pure cellulose. Cellulose is
very similar to starch chemically, and is an excellent fuel wherever it
can be burned. The human digestive tract is unable to handle it in a
manner to make it usable, although grazing animals do so quite
efficiently. A good many plants make products that are either
disagreeable in flavor or actually poisonous. Of course, in such cases
the plants become useless as food unless a treatment can be devised that
will remove the objectionable material or convert it into something
harmless. The few dozen kinds of plants that we raise for food are those
that are free from harmful substances and that yield large quantities of
stored food materials, or in some cases that taste especially good, even
though they may not have much food value. Tomatoes, lettuce, and the
like, come in this latter class. The world has been pretty well
ransacked for food grains, fruits, and herbs, but probably there are
others yet to be found besides those we now have.
CHAPTER IV
THE USES OF FOOD
We have had a good deal to say thus far about power development in
living animals, and have talked about food in connection with its use as
fuel for the purpose. While we are on the topic it may be as well to say
something about other uses to which food is put in animals besides that
of serving as fuel, and also something about what is done with the power
that is developed by the burning of such food as is used for fuel. To
begin with, it is evident that one use that is made of food is to build
the body itself. The new-born infant usually weighs somewhere between 5
and 12 pounds. From birth until the body gets its growth there is an
almost continuous gain in weight until a total which may range anywhere
between 90 and 250 pounds is reached. Of course, every bit of this
additional material came into the body in the form of food. The whole
mass of the body divides itself, as has been said before, into living
protoplasm and nonliving substance. We do not know accurately what
proportion of the whole weight is made up by protoplasm; it has been
estimated at about 60 per cent, but any estimate can be only very rough
because about half of the nonliving substance consists of fat deposits
which vary greatly in different people.
In any case, that part of the food which goes to make gain in weight is
passed over to the living
[Illustration: DIAGRAM SHOWING THE RATE OF GROWTH IN MAN]
cells. If we accept the rough estimate given above, about 60 per cent is
then used for the actual manufacture of new protoplasm; the remainder is
worked over by cells specially devoted for the purpose and put into
place to serve as supporting structure, or to be held in reserve as fat.
Living protoplasm is chemically a very complex mixture. In consistency
it resembles a rather thin, transparent jelly; the thickness of the
jelly depends on how much water it contains and this varies greatly in
different kinds of protoplasm. The watery part of the protoplasm has
dissolved in it several substances; among them may be mentioned ordinary
table salt; also salts of potash and lime. Only tiny amounts of these
are present, but it is a curious fact that without these tiny amounts of
salts protoplasm cannot live. The chief solid substance in protoplasm is
protein; this
[Illustration: A FACTORY’S COOPERATION IN AIDING PHYSICAL FITNESS--THE
UNDERWEIGHT EMPLOYEES ARE GIVEN A MILK LUNCH]
[Illustration: Photo, Paul Thompson
THE WEIGHT TEST IN A CHILD’S PHYSICAL EXAMINATION TO DETERMINE HOW
NEARLY IT APPROACHES CORRECT STANDARDS OF PHYSICAL DEVELOPMENT]
material, which is one of the most complex substances known to
chemistry, has certain peculiarities which seem to fit it specially to
serve as the chemical basis of life. Evidently of all the foodstuffs
protein is the most important for the manufacture of new protoplasm, in
other words for growth. In the case of a tiny one-celled animal, whose
body is made up of protoplasm, not much else would be needed, but any
animal that has a bony skeleton has to build this up to keep pace with
the growth of the soft parts of the body. For this purpose mineral
substances are needed, chiefly lime salts.
In addition to these foods which are actually used for making new body
substance it has recently been discovered that proper growth in the
higher animals, including man, depends on the presence in the diet of
certain dietary accessories, whose use is not at all understood,
although there is no doubt of their importance. These materials, to
which has been given the rather cumbersome name of “growth-promoting
vitamines,” are found dissolved in certain food fats. Apparently they
are insoluble in water and soluble in oil. Most animal fats appear to
contain them in small amounts, while most vegetable fats do not. Milk
and eggs, which are growth foods in an especial sense, are richer in
these accessories than any other articles of the diet. The discovery of
these facts has emphasized the importance of including animal fats in
the diet of growing children, milk and eggs particularly. Since milk is
also rich in the lime salts which are necessary for bone formation it
forms the best single foodstuff for children that there is. When very
young children have to be fed on cow’s milk, which differs somewhat in
proportion from mother’s milk, it is often found necessary to feed the
top milk diluted with water, instead of the whole milk. When this is
done, lime water is usually used in part for diluting the milk, instead
of all ordinary water. In this way the proportion of lime is brought up
enough to insure that the child will get plenty of it.
In addition to the use of protein as a growth food it has another use
which no other kind of foodstuff can share. This is also because protein
is the foundation material of living protoplasm. We do not know a great
deal about what goes on in living protoplasm to make up what we call the
life processes, but we do know that these processes are of a chemical
nature, and that in connection with them there is a steady wastage of
protein. The protein that thus goes to waste is broken down into simpler
chemical compounds which are expelled from the cells. Why this occurs we
do not know, but since it does it is evident that unless the wastage is
made good the time will presently come when so much protein will have
been lost from the protoplasm that it can no longer exist as such and
must die. As a matter of fact, one might go on a diet excessively rich
in starchy foods and fats and still starve to death if there were no
protein present. This use of protein is called cell maintenance to
distinguish it from the other special use of protein in cell growth.
Evidently, whatever may be missing from the diet, protein must not be
left out. Fortunately most of our common foods contain protein. It is
especially abundant in lean meat, in dried beans and peas, and in grain.
Potatoes and most garden vegetables are deficient in protein, as are
almost all common fruits. Bread and meat are our chief stand-bys as
furnishers of protein.
Just as there are vitamines that are important for growth, so are there
vitamines that are necessary for cell maintenance. Many years ago Dr.
Sylvester Graham made himself prominent by arguing that the outer coats
of wheat grains contain something that is needed in the diet, which is
removed in the process of manufacturing white flour. He accordingly
invented a form of flour, familiar to us all under his name, which
includes some of the bran from the outer layers of the wheat. This idea,
which originated with Dr. Graham, has since been substantiated, although
not precisely as Graham had it. We know that there are necessary
accessories to the diet, but we know, also, that they are much more
widespread than Graham thought. They occur in so many kinds of
foodstuffs that anyone who eats a mixed diet usually gets enough of them
for his needs. The ill effects of their lack are most evident when the
diet is restricted to a few kinds of food which happen not to contain
them. A striking example of bodily injury directly due to the absence of
these vitamines from the food is seen among Orientals whose diet is apt
to be made up of rice plus small amounts of other substances. Of recent
years the natives of Japan and China and the Philippines have suffered
much from a disease of the nerves known as beriberi. Investigation has
shown that this disease is due to the absence from the diet of needed
vitamines, and dates from the time when rice-milling machinery was
introduced. The old hand methods of milling rice were so imperfect that
much of the hull was left clinging to the grains, but machinery
polishes the rice clean of every trace of hull. The hulls of rice
contain the accessory that is wanting from the polished grains. Wherever
it has been possible to bring about the use of unpolished (brown) rice
instead of the usual polished kind, beriberi has disappeared. Or the
same result can be secured by adding small amounts of beans to the diet.
It is probable, also, that the hulls of most grains, including wheat,
contain some of the same, or a similar accessory, so to that extent Dr.
Graham was right in emphasizing the importance of adding hulls to the
flour. Quite recently it has been shown that raw foods are richer in
these accessories than cooked, and that ordinary compressed yeast
contains more of them than any other easily obtainable material. Many
people are being benefited by taking part or all of a yeast cake daily
in a glass of milk.
For growth, or the making of new protoplasm, and for maintenance, or the
repair of protoplasmic wastage, then, we must eat protein-containing
foods, also foods containing various kinds of salts, and foods
containing the necessary vitamines. All these are to provide required
materials; the actual substances built into the protoplasm. There
remains the requirement of power, for both growth and maintenance
represent chemical activity on the part of the cell, and this activity
depends on power just as does any other activity. In saying this we are
merely saying over again in different words what was set down at the
very beginning of the book as the chief sign of life, namely, the
necessity on the part of living cells of continuous power development.
The use of food as a source of energy or power has been talked about
already, but it is necessary to say something about the different sorts
of power development that may go on in cells, and since we shall have
to talk about this a good deal, right here is a good place to bring in
for the first time a word that has come to be used whenever the matter
of the chemical activities of living cells is being mentioned. The word
is _metabolism_; when we speak of cell metabolism we mean the chemical
processes that are going on in the cells. Hereafter, instead of saying
power development, the word metabolism will be used as meaning
practically the same thing.
First of all, in describing the various kinds of metabolism that cells
may show, we have the metabolism of rest. By this we mean the power
development that is going on when the cell is doing nothing more than
keeping alive; neither growing nor showing any special activity. This is
evidently the minimum amount that any cell can show, so it is often
referred to as the _basic metabolism_. We know of at least two things
that may change the amount of basic metabolism; the first of these is a
change in temperature; when a cell is cold, its basic metabolism is less
than when it is warm. There is a very simple chemical reason for this,
namely, that chemical processes as a rule go on more slowly the lower
the temperature. Since all metabolism consists of chemical processes,
this rule applies not only to basic metabolism, but to all other kinds
as well, and, as we shall see, explains why the lower animals show such
marked differences in behavior in cold and warm weather. The second
thing that influences the amount of basic metabolism is the percentage
of water in the protoplasm of the cell. Highly organized animals, like
ourselves, are destroyed if the cells lose more than a small fraction
of their water, but there are many of the lower animals that can be
dried until their bodies contain only a very little water and still
live. This applies to microscopic forms that live in puddles and similar
places; when the puddle dries up the animal dries up too, until all that
is left of it is a tiny particle of highly concentrated protoplasm. But
this tiny particle preserves all the original cells, or at least enough
of them to make a fresh start, and a very sluggish metabolism goes on in
each cell. Of course, the advantage of this is that the stored food
materials will not be used up as rapidly as they would if metabolism
went on at the usual rate, and so there is a better chance that the
animal may survive until more water falls or drains into the puddle, or
until the particle of dust which the animal has become may be blown by
the wind where it will fall into another one. Whenever either of these
things happens the protoplasm takes up water again and the former rate
of metabolism is resumed. It is only by means of this reduction in rate
of metabolism that many kinds of animals are able to persist, for in
large parts of the globe there is a period of each year when conditions
become so unfavorable that the usual rate of metabolism could not
possibly be maintained.
Next in order to basic metabolism comes the metabolism of growth, by
which we mean the energy necessary for the making of new protoplasm. Not
a great deal is known about growth metabolism; in fact, about the only
reason for believing that it requires any energy at all is that the
metabolism of young animals, whenever it has been studied, has been
found to be greater in proportion than that of animals that are fully
grown. It is hard to account for this, unless the growth process
itself, namely, the making of new protoplasm, requires energy. When we
think of the extreme complexity of living protoplasm, we can easily
believe that its formation involves the expenditure of energy, perhaps
in considerable amounts.
The last kind of power development to be considered is the metabolism of
special activity. Most kinds of cells, particularly in highly organized
animals, have some special kind of work to do. For example, the muscle
cells have the task of making the motions; the gland cells of
manufacturing the secretions, and so on. These we speak of as the
particular functions of the cells, and the metabolism by which they are
performed as _functional_ metabolism. In some of the lower animals one
can scarcely tell where basic metabolism leaves off and functional
begins. There is a small shrimp, about a half inch long, that is found
quite commonly in small ponds. This little animal has several pairs of
legs by which he swims about, and the strokes of these legs go on
continuously, day and night, with almost no interruptions, at the rate
of a hundred or more a minute, for days or even weeks. It looks as
though this, and other animals, that are continuously on the move, were
organized without any sharp line between basic and functional
metabolism; their protoplasm liberates energy by the oxidation of food,
and various things happen as the result; among them are the maintenance
of the protoplasm and the making of motions. In the higher animals the
distinction between basic and functional metabolism is sharp, and,
necessarily so, for the well-being of any of the higher animals requires
that he shall have pretty complete control over the activities of his
protoplasm, and this he could not have if the functional metabolism were
blended in with the basic. In other words, it is as important for bodily
well-being that the cells be able to become inactive as that they be
capable of activity.
CHAPTER V
BODY CELLS
A good deal has been said thus far about living cells without anything
at all having been said to tell what they look like, or how they are
made up, beyond the statement that they consist of living protoplasm,
which is of a jellylike consistency. To look at living cells through a
microscope would almost surely be a disappointment at first, for
protoplasm is so transparent that not much of its form can be seen on
direct inspection. Fortunately for our knowledge of how cells are made
up, protoplasm that has been properly killed and preserved takes stain
very well, and different chemical substances in the protoplasm stain
differently. Thus features that could not be made out at all in the
living cells become clearly visible after killing and staining. The
first thing that attracts the attention when cells thus prepared are
studied is that every cell has somewhere within it, and usually near its
middle, a spot which is more deeply stained than any other part of the
cell. This indicates the presence of a substance or substances that take
stain more readily than the mass of the protoplasm. This peculiarity led
to the naming of the deeply staining portion of the protoplasm
_chromatin_, referring to the ease of staining. The part of the cell
which contains chromatin is called the _nucleus_. In many kinds of cells
the nucleus can be made out by an expert observer without resorting to
stains, although the details of structure cannot be seen in that way.
[Illustration: NUCLEUS OF A CELL, SHOWING CHROMATIN NETWORK
(After Martin’s “Human Body”)]
We now know that the nucleus, or rather the chromatin that it contains,
plays a remarkable and interesting rôle in the life of the cell. To this
we shall return presently. The remainder of the protoplasm, outside of
the nucleus, shows the greatest possible variety of form, according to
the kind of cell at which we happen to be looking. In some of the
simpler types this part of the protoplasm seems to be merely a nearly
uniform mass, perhaps with tiny particles scattered through it. In other
types the protoplasm is drawn out into long slender threads, and these
threads may have many branches; or the protoplasm may be distorted into
a thin shell inclosing a mass of fat; or it may be subdivided into dense
and thin portions with sharp lines of division between them. These
various forms are related to the special functions which the cells have,
and we shall learn more about them as we take up the different functions
in order. On the whole, study of cell structure shows clearly that the
protoplasm outside the nucleus carries on the greater part of the
metabolism or power development, and is correspondingly important as the
seat of the special functions shown by the cell. If it is a muscle
cell, this is the part that does the moving; if a gland cell, this is
the part that secretes. Nevertheless, the nucleus is a vital part of the
cell. It has been definitely proven that a cell from which the nucleus
is lost cannot survive more than a brief time. To gain some idea of the
actual part played by the nucleus, we shall have to return to it in some
detail.
[Illustration: DIAGRAM SHOWING CELL SUBDIVISION
_A_, a cell; _B_ to _F_, successive stages in its subdivision; _a_,
cell-sac; _b_, cell contents; _c_, nucleus. (From Martin’s “Human
Body”)]
Before undertaking a further description of the nucleus itself, we shall
be helped to an understanding of its function if we trace briefly the
history of the cells which make up our body. At the beginning, as we
probably all know, we start life as a single cell. This cell, after a
series of events which will be described in a later chapter, begins the
process known as development. Development consists of a series of
subdivisions of cell material. At first the single cell divides into
two; each of these then divides, giving four. At the next stage eight
are formed, then sixteen and so on, until finally the millions of cells
that make up the body are produced, all derived from the original single
cell. We know that in the adult body there are very many different kinds
of cells. Since they are all derived from a single cell, these
differences must have put in their appearance during the course of the
various cell divisions. In fact, this happens all along; at definite
points in the process the two cells that come from the subdivision of
some particular one will not be alike. The special kinds of cells that
are thus produced become the starting points for whole masses of similar
cells in the fully developed body. In human beings, and probably in most
other kinds of animals, the very first subdivision does not result in
any difference between the cells. The proof of this is that sometimes,
in fact fairly often, the two cells become separated. When this happens
twinning results, and the twins are exactly alike, being known as
“identical twins.” Not only are they alike in all other respects, but
they are always of the same sex, a fact that has escaped the attention
of some writers of fiction, who have made twins, identical in all other
features, brother and sister, instead of both boys or both girls. Twins
that are not identical come from different original cells that happened
to start developing together. Such twins need have no more resemblance
than any members of the same family, and may or may not be of the same
sex.
In every cell division the first step consists in a division of the
chromatin of the nucleus, which is followed by a division of the rest of
the protoplasm. The process by which the chromatin is subdivided is so
curious as to be worth a brief description. The
[Illustration: DIAGRAM ILLUSTRATING DIVISION CHANGES IN A CELL NUCLEUS
WITH FOUR CHROMOSOMES
(From Martin’s “Human Body”)]
chromatin material is not a simple lump in the nucleus. It looks rather
like a tiny string of beads thrown down carelessly, so as to become all
mixed together. Each bead is a single bit of chromatin, and these bits
are strung on a tiny thread. In an ordinary cell the beads are so mixed
together that no order can be distinguished among them, but if a cell
that is about to begin dividing is looked at it is found that the string
has straightened itself out, and also that it has broken into pieces.
The individual pieces are called _chromosomes_ and their number is
always the same for any one kind of animal or plant. There is a
parasitic worm whose cells have only four chromosomes, and the number
ranges from this up to as many as forty-eight in human beings. It may be
that other species have even more, but they become so hard to count when
there are as many as forty-eight that the number cannot be stated with
certainty. So far as can be judged, the number of chromosomes has little
to do with the complexity of the animal or plant, for some complex forms
have few chromosomes, and some simple forms many.
At the same time as the chromatin is breaking up into chromosomes two
tiny spots put in their appearance in the protoplasm of the cell on
opposite sides of the nucleus, and tiny threads extend from one spot to
the other through the nucleus. There are as many threads as there are
chromosomes, the whole group making up a spindle-shaped figure. The
chromosomes now become arranged at the middle of the spindle, and
apparently each chromosome becomes fastened to a thread. Next each
chromosome splits lengthwise through the middle and by what looks like a
shortening of the threads the split halves are pulled apart and drawn
to opposite tips of the spindle. The purpose of this elaborate scheme
seems to be to insure an exactly equal division of the chromosomes
between the cells, and the necessity of such an equal division will
become clear when we learn something of what the chromatin is for.
Meanwhile the description of cell division can be finished by saying
that after the halves of the chromosomes are pulled apart the whole mass
of protoplasm divides through the middle. As we stated above, sometimes
the cells thus produced are alike and sometimes they are different,
according to whether they are destined to become parts of similar or of
different structures. In either case the chromatin material that goes
into the two cells is exactly alike, so that if the cells themselves
become different there must have developed a difference in the
protoplasm at the two ends of the cell from which they came. Our bodies
are made up of millions of cells, of a great many different kinds, but
however different they may be the chromatin of each exactly duplicates
that of every other one, or did when the cells were first formed; there
is reason to believe that the chromatin may become changed during the
lifetime of the cells, at least in some cases.
We may be interested in inquiring how long this process of cell division
keeps up. Many children do not get through growing until they are twenty
years old or more. Does cell division keep on during all this time? More
than that; are there any cases of cell division that continue after full
growth is reached? The answer to both these questions can be given in a
brief paragraph. There are some tissues, particularly the outer layer of
the skin, the connective tissues, the blood-corpuscle-forming tissues,
and the reproductive tissues, in which cell division continues during
all or most of life. The others finish at birth or shortly thereafter.
We are born with the precise number of muscle cells with which we shall
die, unless accident deprives us of some meanwhile; and if this happens
no new ones will be formed to replace those that are lost. The same is
true of gland cells. The last cell divisions among nerve cells are
believed to occur within a few months after birth. As most of us have
observed in our own cases, bodily injuries, if at all severe, are
followed by the formation of scars. This means that connective tissue
has grown in to fill the place of the cells destroyed by the injury,
which cannot be replaced by cells of their own sort, since they have
lost the power of cell division.
We have tried, in the above paragraphs, to get some idea of what living
cells are like, and how they are derived, but have not attempted any
detailed picture of particular kinds of cells. That will have to wait
till we reach the story of the different kinds of bodily activity, when
the cells that carry on each kind will have to be described more
exactly. Something has also been told of the chromosomes, but the full
account of them and their meaning is to be taken up in a later chapter,
devoted to the matter of heredity and reproduction. In what remains of
the present chapter we wish to talk about the conditions in which cells
live so that we shall easily picture how they carry on their metabolism.
As an introduction to this topic a word may be said about the wide
differences of complexity that are found in animals. They range from the
simplest imaginable, a single cell with its nucleus and with protoplasm
that appears almost uniform throughout, to a highly organized body like
that of man, composed of millions of cells of many different kinds.
Between these extremes almost every possible form is seen. The
one-celled animals themselves show a wide range of complexity, and as
soon as animals begin to be formed of numbers of cells grouped together
the possibilities of complexity increase in proportion. One important
difference between one-celled and many-celled animals needs to be
emphasized; that is the matter of size. There are definite limits to the
size that a single cell may attain; these limits are just over the
boundary of naked eye vision. If animals are to attain larger sizes,
they must necessarily be composed of many cells. The life of a
single-celled animal presents no special problem, since it has only to
take in through its outer layer from the surrounding water the various
food materials and the oxygen which its metabolism requires, and to
discharge into the same water any chemical products that may result from
that same metabolism, and the question of whether it will live or die
depends only on whether the water in which it happens to be contains
sufficient materials and is otherwise suitable as a place to live. A
many-celled animal, whose cells are arranged in not more than two
layers, is in practically the same situation, for every cell has a
frontage on the water and so can carry on interchanges of material
directly; but the moment complexity reaches a stage where any cells are
buried beneath other cells some special arrangement must be provided so
that the buried cells can obtain the needed substances for their
metabolism. The arrangement consists, in general, of furnishing what
may be called an internal water frontage for the buried cells. In other
words, complex animals have spaces all through their bodies, and these
spaces are filled with fluid. There are no living tissues so dense that
the cells of which they are composed are completely cut off from contact
with body fluid. In thinking of our own bodies we should realize that
this same arrangement applies; every one of our millions of living cells
has contact with the fluid with which all the spaces of our bodies are
filled, and it is from this fluid that the cells obtain the materials
for their metabolism, and into this same fluid they discharge whatever
substances their metabolism may produce.
The total amount of body fluid is not large, for the spaces among the
cells are in most cases extremely tiny; it follows that with all the
millions of cells absorbing food materials and oxygen from this fluid
and discharging waste materials into it the time will soon come when no
more food or oxygen will be left to be absorbed and there will be no
more capacity for holding waste substances. If this state of affairs
were actually to happen, metabolism would come to an end and death would
be the result; evidently there must be some means of keeping the body
fluids constantly renewed in respect to the things which the cells need
for their metabolism, and constantly drained of the waste substances
which the cells pour out. The way in which this renewal is accomplished
is simple; part of the body fluid is separated off from the rest in a
system of pipes, known to us as the blood vessels, and this part is kept
in motion; at intervals along the system are stations at which the
moving fluid can exchange substances with the fluid which actually comes
in
[Illustration: DIAGRAM SHOWING HOW THE MOVING BODY FLUID RENEWS THE
STATIONARY, AND IS ITSELF RENEWED IN LUNGS, DIGESTIVE TRACT, AND
KIDNEYS]
contact with the cells; thus the stationary fluid can obtain from the
moving fluid the materials which the cells, in turn, are constantly
withdrawing from it, and can pass on to the moving fluid the products
with which the cells are continuously charging it. All that is necessary
to complete the successful operation of the system is to have additional
stations at which the moving fluid can obtain supplies of food materials
and of oxygen, and stations where it can get rid of the wastes which it
accumulates from the stationary fluid, and there must be a pump by which
the moving fluid is kept in motion. We are familiar with the moving
fluid under the name of blood; the system of pipes in which it moves are
the blood vessels; the pump which keeps it in motion is the heart; the
various supply stations include the digestive organs, the lungs, and the
kidneys. In later chapters the operation of all these stations will be
described in detail. The present outline has been given to show in a
general way how the problem of metabolism is handled in highly organized
bodies in which the individual cells have no direct access to food or
oxygen supplies.
CHAPTER VI
THE SUPPORTING FRAMEWORK
Since protoplasm is so very soft and fragile it must be supported in all
animals and plants except the very tiniest. The nature of the supporting
framework has a great deal to do with both the form and the working of
the body, so it is desirable that we become familiar with it before
trying to go further in the examination of the living protoplasm itself.
A large heavy body like that of man requires an arrangement for support
that shall meet several conditions. In the first place there must be
strength and stiffness, combined with flexibility, so that the body as a
whole shall be firm, yet not rigid. The weight, also, must be kept as
small as possible. Then every single cell, and every grouping of cells
that we call an organ, must be supported in its place securely but
without hindering the free performance of its function. Not only must
the protoplasm be held in place, but on account of its fragility it has
also to be protected against injury; the vital parts require more
careful protection than those that are less immediately essential to
life. Finally, bodily motions of all sorts depend on the framework to
give purchase to the muscles, which are the actual organs of motion, and
so to make their movements effective. For support, for protection, and
for motion, then, the framework is important.
The material that does the real supporting is not, of course, alive, for
living protoplasm lacks the necessary qualities needed here. It is
manufactured and put in place, however, by living cells. They do this by
withdrawing the special materials needed from the body fluid which
surrounds them; in large part what they get from the fluid is not the
finished substance but material from which the living cells make the
finished substance. It is then passed outside their bodies and deposited
in the surrounding space. Of course this is a gradual process. Bit by
bit the structure, bone, cartilage, or connective tissue, as the case
may be, is built up by the combined activities of many cells.
Of the three kinds of supporting material mentioned above, bone is the
most familiar. No description of its appearance is necessary, for
everyone has seen it as it appears in meat animals and in poultry, and
it looks precisely the same in man. There are several things about bone,
however, that are worth describing. One is the arrangement by which the
very hard, compact material is deposited in large masses without cutting
off the cells which are doing the depositing from their contact with the
body fluid, and so destroying them and bringing their work to an end.
The way this is managed can be made out by examination of the figure,
showing the structure of bone. At the beginning the bone cells are lying
near one of the tiny blood vessels known as _capillaries_, which are the
exchange stations for material between blood and the stationary part of
the body fluid. Thus these cells are favorably located for obtaining
materials from which bone can be constructed. As they proceed with the
formation of bone they always leave tiny passages open between
themselves and the blood capillary. Finally the capillary may become
completely surrounded by bone, but all along it will be left the
passages through which fluid can make its way from the blood to where
the cells are imprisoned within the bony walls of their own
construction. The metabolism of bone cells is not on a very active
scale; the amount of bone substance that a single bone cell has to
produce in a day is only a fraction of the amount of saliva, for
instance, that a single cell of the salivary gland turns out in the same
time; so the bone cell can manage even though its supply of material has
to come to it through a few very tiny passages in the bone.
[Illustration: CROSS SECTION OF COMPACT BONE FROM THE SHAFT OF THE
HUMERUS
_A_, bone cells; _B_, blood capillaries. (From Martin’s “Human Body”)]
Another interesting feature of bone is the ease with which it can be
remodeled. We are apt to think of bone as permanent, after it has once
been formed, but as a matter of fact bone is about as subject to change
as any of the softer tissues. This is because there are in and around
the bones, in addition to the bone-forming cells, a great many cells of
different appearance which may be named bone-destroying cells. These
latter have the ability to dissolve out the hard material which the
bone-forming cells have deposited. Good examples of their work are seen
in the hollows of the long bones. We know, of course, that the bones in
a child’s leg are so much smaller than those in the leg of an adult that
they could almost be fitted into the hollows of the latter. Evidently
the bone substance has been moved bodily outward during the course of
growth. As the bone-forming cells add material to the outer surface of
the bone, the bone-destroying cells dissolve it away from the inner
surface. The same thing happens all over the body. A child’s face grows
by an increase in size of the bones. Again the inner surfaces are
dissolved away. Apparently one condition which makes the bone-destroying
cells active is constant pressure. A good example of this is seen in
what is known as a gumboil. If a tooth becomes ulcerated, gas and pus
are formed at its root, and cannot escape since this is completely
surrounded by bone. They accordingly press upon the surrounding bone,
and also upon the sensitive tissues, resulting in extreme pain. The
pressure upon the bone starts the bone-destroying cells into great
activity and in the course of a few days they will dissolve a hole right
through the bone, allowing the gas and pus to escape to the outside, and
relieving the pain.
Of recent years school authorities have had much to say about the
importance of adjusting school seats and desks so that they shall be at
the proper height for the particular children that are to occupy them.
This is because if the feet hang clear of the ground for hours at a
time, as they will if the seat is too high, or if the body must be
screwed around to enable the child to work at his desk, as happens when
the desk is too low, there is real danger that some of the bones may
become misshapen. Most of the stoop shoulders and many of the crooked
backs that we see are the result of the habitual taking of wrong
postures. Children, and adults as well, should form habits of standing
and sitting so straight that none of the bones are put under a pressure
that may tend to distort them.
After the teeth are lost the bony sockets in which they lie are
dissolved away, making the jaws much shallower than formerly, a fact
that accounts for the shortening of the distance between chin and nose
in aged people. An important result of this dissolving away of bone by
the bone-destroying cells is that the bones are kept as light as
possible, without undue sacrifice of strength.
A second kind of supporting material is cartilage. This is both softer
and more flexible than bone. It is found in places where flexibility is
more important than great strength, as in the ears, the parts of the
nose just below the bridge, the Adam’s apple and wind pipe. The chief
difference in make-up between bone and cartilage is that while in bone
about three-fourths of the nonliving substance consists of lime salts,
in cartilage there is almost none of this material, organic substances
making up the entire mass. There are no living cells in the body that
are more poorly located with respect to obtaining supplies from the body
fluids than the cartilage cells, for as these deposit the cartilage
around themselves they leave no definite passages through which fluid
may pass; the material incloses the cells completely. Although cartilage
looks as though it were altogether nonporous, there must be some degree
of sponginess present, since the cells do succeed in getting the
materials on which their life depends. Cartilage seems to be a more
primitive kind of supporting substance than bone. This is shown by the
fact that it makes up the entire skeleton in the lowest fishes, and also
by the fact that in the higher animals, including man, the bony skeleton
starts, in large part, as cartilage. In the parts in which this happens
a mass of cartilage is deposited in the place which is later to be
occupied by bone. Then at certain points the cartilage begins to be
dissolved away by cartilage-destroying cells, which are precisely like
bone-destroying cells, and the bone-forming cells come in and build up
the real bone as fast as the cartilage is removed. This process of
replacing cartilage by bone is practically completed at birth, except in
the long bones of legs and arms. These bones, which will about double in
length during the growth of the body to adult size, as well as the other
bones, which grow to some extent, retain plates of cartilage near each
end during all the growth period, and the increase in length is obtained
by a continuous formation of new cartilage, which is continuously
replaced by bone.
The third kind of supporting material is connective tissue. This is
composed of tiny threads or fibers, some of which are inelastic, others
are very elastic. The inelastic fibers are found in places where a
flexible, but unyielding support is required; the elastic fibers are
located where elasticity is particularly important. Either kind of fiber
may be grouped into sheets, or into loose networks, or into stout
cords. A good example of inelastic connective tissue in sheet form is in
the _mesentery_ which holds the organs of the abdominal cavity in place.
Just under the skin, anchoring it loosely to the underlying muscles, is
connective tissue in network form. The tendons by which most of the
muscles are attached to the bones upon which they pull are made up of
inelastic connective tissue in the form of cords. The best example of
elastic connective tissue is in the large arteries, which are just as
elastic as best quality rubber tubing. Another good example is the large
and strong elastic cord which passes along the back of the neck in
cattle and sheep, and helps to support the weight of the head.
Connective tissue fibers are deposited by living connective tissue
cells. Since connective tissue is of open and relatively loose
construction, there is no problem presented in supplying the cells with
material. The meshes among the fibers are filled with fluid, and this
fluid has ready connection, in turn, with the blood. Use is made of the
abundance of body fluid in the connective tissue spaces whenever a
subcutaneous injection is given, for what is done is to inject the
desired material into the fluid which fills the spaces in the connective
tissue just beneath the skin, trusting that it will pass from there to
the blood, which it does rather gradually, and so is distributed about
the whole body.
While we are on the topic of the supporting framework, something must be
said about the grouping of the bones into what we know as the skeleton.
Of course it is evident that the effectiveness of the bony part of the
framework depends almost altogether on the way in which the individual
bones are grouped together. If the whole skeleton were
[Illustration: THE BONY AND CARTILAGINOUS SKELETON
(From Martin’s “Human Body”)]
composed of one great bone, or of different bones anchored solidly
together, the body would be perfectly rigid; since motion is necessary
to life, flexible connections between some of the bones are absolutely
essential. Our movements are actually made by means of muscles, but
nearly all of them become effective through the motions of bones to
which the muscles are fastened. The bones are often very irregular in
shape; careful study shows that the irregularities are due either to
provision for the contact of one bone with another in the joints, a
contact that must allow in most cases for motion of one on the other, or
to provision of places to which muscles can be fastened in such a way as
to make their pull effective. It is, of course, out of the question for
us to examine the skeleton bone by bone. Figures are given of a number
of typical bones: all that we can do in addition is to mention some of
the interesting features of the skeleton.
The skeleton of the head is called the skull; its chief features are the
brain case, the eye sockets, and the parts about the nose and mouth. The
brain case is made up of eight bones firmly joined together by saw-tooth
margins to make up a roughly spherical box which holds the brain, and
protects this delicate and vitally important organ from all injury
except the most severe. There are a number of small openings out from
the brain case through which nerves pass, and one large opening below
and at the back through which the spinal cord merges into the brain. The
bones which make up the sides of the brain case are much thickened just
behind the ears. A hollow extends from each ear into the bone, and
within this hollow, securely protected from harm, is the actual organ of
hearing. There are extensions of the hollow backward which are not
occupied by any organs, and which communicate with the cavity of the
ear. These sometimes become infected from the ear, causing the condition
known as mastoiditis. Not only is this condition excruciatingly painful,
but on account of the thin layer of bone which separates it from the
brain itself it is highly dangerous. For this reason any ear trouble
should be carefully watched lest it develop into mastoid trouble.
[Illustration: A SIDE VIEW OF THE SKULL]
Of the bones that make up the eye sockets not much need be said, except
that they have a great deal to do with determining the shape of the
upper part of the face and so the appearance. There are bones within the
nostrils that are very irregular in outline. Their effect is to increase
greatly the surface over which the air that is breathed must pass,
enabling it to become both warm and moist before entering the lungs. The
jaw bones serve as receptacles for the teeth; the lower jaw, which is
the only movable bone of the head, except for the tiny bones within the
ears, has also the duty of operating as a mill in reducing the food to
suitable form for swallowing. To aid in this function the lower jaw is
hinged to the rest of the skull in such a way that it not only opens and
closes but can slide forward and back or from side to side. All these
motions are used in chewing. There are twenty-two bones altogether in
the skull, not counting the three tiny ones in each ear which will be
described later.
[Illustration: FRONT VIEW OF TRUNK AND LIMB ARCHES
_c_, collar bone; _s_, shoulder blade; _Oc_, innominate bone (From
Martin’s “Human Body”)]
[Illustration: SIDE VIEW OF THE SPINAL COLUMN
_C_ 1-7, cervical; _D_ 1-12, dorsal; _L_ 1-5, lumbar; _S_ 1, sacrum;
_Co_ 1-4, coccygeal. (From Martin’s “Human Body”)]
The body consists of trunk and limbs, and each part has its skeleton.
The skeleton of the trunk consists of the spinal column, the rib cage,
the shoulder girdle, and the hip girdle. The skeletons
[Illustration: Photo, A. N. Mirzaoff
A FRENCH METHOD OF MEASURING VERTICAL CONFORMATION, CONSIDERED IMPORTANT
FOR ATHLETES]
[Illustration: Photo, A. N. Mirzaoff
THE CURVE OF THE SPINE MEASURED AND DRAWN FOR STUDY OF AN ATHLETE’S
POSSIBILITIES]
of the limbs are all according to a single plan to be described in a
moment. The spinal column is a remarkable example of strength combined
with flexibility and elasticity. It is made up of thirty-three bones or
_vertebræ_; each of these has a disk-shaped part known as the _body_,
and these disks are placed in line as shown in the figure. Between each
disk and its neighbor is an elastic pad composed of a mixture of
cartilage and elastic connective tissue. There is a small amount of give
in each pad and this, taken along the whole length of the spinal column,
is enough to give it the great flexibility which it enjoys. During the
day the weight of the body packs these pads down hard, so that it is
said that a man may be as much as an inch shorter at night than in the
morning. Behind the disk each vertebra has an arch of bone, and beyond
and beside this arch most of them have projections. All the arches
together make up a bony canal which contains and protects the spinal
cord. The projections serve for the attachment of the ribs and the back
muscles by which the bending motions of the body are made.
The rib cage includes the breastbone and twelve pairs of ribs. It serves
two purposes: to protect the heart and lungs from injury; and to take
part in the movements of breathing. The latter function involves some
degree of motion of the rib cage. All the ribs are attached behind to
the vertebral column in a fashion that permits of a little motion up and
down. All except the last two are fastened in front, seven pairs to the
breastbone, three pairs each to the rib above it. In breathing the
breastbone and ribs are moved up and down by muscles attached to them.
The shoulder girdle is made up of the collar bones and shoulder blades.
Each collar bone is fastened at its inner end to the upper edge of the
breastbone; this is the only direct contact the shoulder girdle has with
any other part of the skeleton of the trunk; at the point where collar
bone and shoulder blade meet, there is a shallow cup into which the head
of the skeleton of the arm fits. The arrangement is favorable to great
freedom of movement of the arm. Not only is the shoulder joint very
flexible owing to the shallowness of the cup into which the arm bone
fits, but the shoulder blade itself is capable of a considerable range
of movement. This is because it is imbedded in and held in place by
muscles. If one watches a person with bare shoulders while he raises his
arms, it will be seen that the shoulder blades do not move much while
the arms are being lifted to the horizontal position, but as that point
is passed they begin to swing outward rapidly, so that when the arms are
high above the head the shoulder blades are in a quite different
position from that which they have when the arms are down.
The hip girdle consists of five bones of the vertebral column welded
firmly together to make up what is called the sacrum, and two other
large bones known as the innominate bones, each of which, in turn, is
made up of three bones tightly fused together. The innominate bones are
firmly joined to the sacrum at the back and they meet in front, also in
a firm joint. The hip girdle or _pelvis_ is rigid, suiting it to bear
the strains that come upon it on account of its position at the junction
of the legs with the trunk. At the outer side of each innominate bone is
a cup, much deeper than the corresponding cup of the shoulder girdle,
and into this fits the head of the skeleton of the leg. The arrangement
is a typical ball and socket, and has been much copied in machinery
where a flexible joint is required. In a good many people the union of
the innominate bones to the sacrum is not so firm but that it yields
somewhat when strains are put on it. Ordinary strains in these cases
produce severe backache. Heavy strains may cause an excessively painful
as well as disabling dislocation. In either case medical attention is
needed.
Each arm can be subdivided into upper arm, forearm, wrist, and hand. The
skeleton of the upper arm is a single long bone. The forearm has two
bones, one of which is hinged at the elbow to the bone of the upper arm
in a way to limit the movement to the single back and forth swing of
which the elbow is capable. The other bone of the forearm can be rolled
over the one which is fast at the elbow; this is what happens whenever
the hand is changed from the palm up to the palm down position. There
are eight bones in the wrist; these are irregular in shape, and are so
grouped as to permit of a very wide range of movement. The bones of the
hand and fingers make up five rows numbering four bones each for the
fingers and three for the thumb. The joints are all practically simple
hinges except for the one where the thumb joins the wrist, which is a
much more flexible joint; flexible enough, in fact, to allow the thumb
to be brought opposite any of the fingers. No animal except man enjoys
this degree of flexibility in the thumb, so no animal equals man in the
nicety of the grasp, particularly of small tools. When we recall how
constantly we take advantage of this property of our hands we can
realize how greatly our superiority over the lower animals has been
aided by this rather slight structural difference between our hands and
theirs.
[Illustration: THE BONES OF THE ARM
_a_, upper arm; _b_, forearm; _c_, wrist; _d_, hand
(From Martin’s “Human Body”)]
The leg subdivides along the same lines as the arm into upper leg, or
thigh, lower leg, or shin, and foot. The order of bones is, on the
whole, the same; one in the thigh; two in the lower leg. Instead of a
flexible wrist the corresponding bones of the foot are grouped into a
less flexible, but much stronger, heel and upper instep. Two of the
bones of this group are fused together into one, reducing the total
number from eight to seven. The bones of the lower instep and toes
correspond in number and arrangement to those of the hand and fingers,
but the great toe does not have superior flexibility as does the thumb.
There is one bone in the leg, the knee cap or _patella_, that does not
correspond to any bone in the arm, although it does correspond to a part
of a bone, namely, the projection, at the elbow, of the long bone of the
forearm. A feature of the skeleton of the foot that is worth a word is
the arching of the instep. This undoubtedly adds greatly to the ease of
walking. The natural position for the foot is, of course, with both the
heel and the ball of the foot on the ground. For some reason it has
become the universal custom among civilized people to raise the heel off
the ground by adding a heel to the shoe. This does not seem to make much
difference as long as the heel is not too high. In fact soldiers wearing
properly fitted heel shoes can march as far and fast as can be expected.
Excessively high heels throw the weight too much on the ball of the
foot, thus doing away with the benefits that come from the arching of
the instep. The effect on the gait is very apparent in any one who walks
in high-heeled shoes. The foot itself does not appear to be greatly
harmed by the wearing of high heels provided the shoes are otherwise
well fitting. Whether the heels are high or low, the fit of the shoe is
of utmost importance to the preservation of the feet. Crowding the feet
into shoes that are too small in any direction is a fruitful means of
bringing on foot trouble. Wearing shoes that are loose enough to allow
the foot to turn over inside the shoe is nearly as bad. If the shoes
are properly fitted in the beginning and then the heels are kept squared
up, so that the feet will always stand straight on the ground, there
will usually be little trouble with fallen arches or other foot
disturbances.
[Illustration: BONES OF THE LEG
_a_, thigh; _b_, shin; _c_, foot; _d_, knee cap (From Martin’s “Human
Body”)]
The bones are fastened together at the movable joints by stout sheets or
bands of connective tissue known as ligaments. These hold them in place
very securely and as additional support the muscles which surround
every joint help to prevent the bones from slipping out of place. At
nearly all the joints of the body the combined action of ligaments and
muscles is sufficient to guarantee the joint against dislocation; the
shoulder joint, and to a less extent the hip joint, is more likely to
suffer this accident. The reason is that in obtaining flexibility of
movement security of attachment is somewhat lessened. If the ligaments
at the shoulder were tight enough to prevent the joint from ever
becoming dislocated they would bind it to a serious degree. Most of the
ligaments are of inelastic connective tissue, but those that fasten the
separate vertebræ of the spinal column together are elastic, allowing of
the bending in every direction which makes our backs as flexible as they
are. The only movable joints which are bound by other means than
ligaments are the connections of the ribs with the breastbone. These are
of cartilage, but the movement here is so slight that the cartilage
yields enough to permit it.
This completes our account of the bony skeleton. We shall finish the
description of the supporting framework by a word about what may be
called the connective tissue skeleton. The bony skeleton serves to
support the body as a whole and to permit the muscles to do their work;
the individual organs and the cells which make them up are held in place
by sheets and bands of connective tissue. These are coarse and strong
when their purpose is to support a large and heavy organ like the
stomach; they become finer and finer as the parts to be supported become
smaller, and when the individual cells are reached the connective tissue
which surrounds them is almost inconceivably delicate. So completely
does connective tissue permeate the whole body that it has been said
that if everything else could be dissolved away, leaving only this
tissue in place, there would still remain a model of the body, complete
to the last detail.
CHAPTER VII
MOTION
Our account of the body has now reached the point where we can take up
in detail the special activities of the different kinds of cells. The
first to be considered is motion, both because it is the familiar sign
of life, as pointed out in the first chapter, and because it has so much
to do with everything that enters into life. There are probably no
animals that live out their entire lives without making any active
motions, although some parasites, like the tapeworm, are stationary most
of the time. There are a number of different ways in which movements are
brought about. The very simplest animals, which consist of nothing but a
bit of protoplasm, move by causing the protoplasm to flow bodily in one
direction or another, a projection of part of the protoplasm being
balanced by withdrawal of an equal part on the opposite side, and the
whole mass progresses in the direction of the first projection. Next
beyond this simplest means comes motion by tiny threads of protoplasm
which project beyond the surface of the cell and whip back and forth.
The stroke of these threads or _cilia_, as they are called, is stronger
in one direction than in the other, so the effect of their beating is to
propel the cell of which they are part in one direction through the
water; or if they are on a surface which is stationary they set up a
current in the water itself. This latter is the means by which oysters
and similar animals which are anchored to the rocks get their food
supplies. In some one-celled animals there are only one or two large
cilia at one end; these beat back and forth, propelling the animal much
as a fish swims.
The commonest, as well as the most effective, means of making motions is
by cells specially developed for that purpose. These are called muscle
cells, and every highly organized animal depends on them for most if not
all of the motions which take place in its body. In muscle cells the
functional metabolism takes the form of forcible changes in shape of the
cells by which bodily motions are brought about. A muscle cell might be
described as a mechanical device for transforming the chemical energy of
burning fuel into the energy of motion. We have something comparable in
the automobile cylinder, where the energy obtained from the explosive
burning of the air-gas mixture drives the piston and so propels the car.
Of course the two devices are not even remotely alike in the actual way
in which they operate; their resemblance is purely the general one of
converting one type of energy (chemical) into another type (motion).
There are three kinds of muscle cells in our bodies. The simplest are
those that are found in the wall of the stomach and intestines and other
internal organs that are capable of movements; the next kind is found
only in the heart; the third, and most complex, makes up the bulk of our
muscular tissue; it is the muscle that moves the bones. The first kind,
because it shows no particular markings when examined through the
microscope, is usually called smooth muscle; the second kind is known as
heart muscle; the third kind, because it moves the skeleton, is named
skeletal muscle. We shall devote most of our attention to this third
kind of muscle, because it is a much more efficient machine than the
others, and also because it has to do with our general bodily movements
instead of with motions of internal organs.
A single skeletal muscle cell is an exceedingly slender fiber, much
smaller than the finest thread; it may also be very short, not more than
a twenty-fifth of an inch long, or it may be as much as an inch long. A
muscle is made up of many of these fibers grouped side by side in
bundles, and also, if the muscle is long, placed end to end. The fibers
are held in place and fastened together by connective tissue. Lean meat
consists of thousands of these muscle cells with their connective tissue
fastenings. In coarse meat there is relatively more connective tissue
and less actual muscle tissue than in the finer grades. In every muscle
the connective tissue is loose enough to allow body fluid to penetrate
among the muscle cells. Blood vessels are also distributed through the
mass of the muscles between and among the cells; thus their nutrition is
provided for.
Although not all muscle cells are exactly equal in power, on the whole
the _force_ that muscle can show is the force of one cell multiplied by
the number of cells that can join in the pull. A strong muscle must have
many cells side by side; in other words it must be thick. Also, the
_distance_ through which muscles can make movements depends on their
length, so a muscle that has to pull for a considerable way must be
long, and since single muscle cells are short there will have to be a
good many cells end to end to make the whole muscle long enough for its
task. The actual make-up and arrangement of muscles in the body depends
in part, therefore, on the thickness and length needed for the
particular work to be done, and in part on the architecture of the part
of the body where the muscle is located. For example, the strongest
muscle in the body is that by which one rises on the toes. This muscle
operates by pulling upward at the back of the heel. If it were located
right at the ankle, where it would have to be if attached directly to
the place where its force is exerted, the resulting clumsiness can
easily be imagined. By shifting it up to the middle of the lower leg
room is found for the large mass of muscle needed for the work. The
connection with the heel is made by means of a long and very strong
tendon, known as the tendon of Achilles, because that was the part
Achilles’ mother failed to immerse when she was dipping the infant in
the river Styx to make him invulnerable. Other equally good examples are
the muscles for operating the fingers. If placed in the hands, the
latter would be too bulky and clumsy for any kind of efficient use. By
placing them up in the forearms out of the way, and connecting them with
the fingers by long tendons, delicacy is secured for the hands.
The muscles of the arms and legs are arranged in groups about the
joints, and these groups always include opposing sets. Thus if the joint
is a simple hinge, as at the elbow, where the only motion possible is
back and forth, there will be one muscle to bend the joint and another
opposing muscle to straighten it out again. The first is known as a
_flexor_; the second as an _extensor_. In the arm the biceps, on the
upper surface, is the flexor and the triceps, on the under side, the
extensor. Joints that permit of motion in several directions have
correspondingly more opposing sets of muscles acting upon them. The same
scheme applies to the trunk, but since in the trunk instead of a few
very movable joints we have the whole row of slightly movable vertebræ,
the grouping of muscles is more complicated. Not all the skeletal
muscles work about joints. The tongue, the muscles of the lips and about
the eyes, those along the front of the abdomen, and some others are
attached to bones only at one end or not at all, and do their work by
pulling upon one another.
[Illustration: THE BICEPS MUSCLE AND THE ARM BONES
(From Martin’s “Human Body”)]
In earlier paragraphs we have seen that the movements made by muscles
represent their functional metabolism, and also that the actions of
whole muscles are merely the sum of the actions of the individual cells.
Our present task is to see how muscles act; in other words to examine
their functional metabolism. One feature that must be in mind from the
very beginning is that the functional metabolism of muscle cells is
under control; they do not go off at random, but only when started. This
is more or less true of the functional metabolism of all the cells in
highly organized animals. The agency that starts them off is named a
stimulus. To picture how stimuli act we shall have to think for a moment
of the state of affairs in cells at rest. As we have tried to make
clear, cells at rest are not stagnating; a more or less active basic
metabolism goes on within them all the time. This metabolism is of such
a sort that it does not disturb the balance existing within the cell.
The various chemical processes go on, using up material and producing
wastes, but without arousing the additional chemical processes of
functional metabolism. Meanwhile the substances that are required for
this latter are present in the cell, so that when the disturbance that
we call a stimulus comes along there is an increase in the total amount
of metabolism, the extra chemical processes being those which perform
the special function of the cell. In the case of muscle cells the
stimulus ordinarily reaches them by way of the nervous system, although
electric shocks, sharp blows, some irritating chemicals, and perhaps one
or two other kinds of disturbance can act as stimuli. The effect of the
stimulus is to start certain chemical processes; these in turn bring
about the forcible shortening which is the thing that happens in active
muscle. In skeletal muscle the shortening may be very rapid; the muscle
can contract and relax again more quickly than the eye can follow. This
is true at the temperature of our bodies. In cold-blooded animals, like
fish or frogs, muscles become sluggish when they are cold. We see here
one of the advantages we enjoy in having bodies that stay at the same
temperature the year around; if our bodies cooled off in cold weather as
do those of frogs, we should have to do as they do, become inactive
whenever the weather becomes cold. As each muscle cell shortens it pulls
upon the connective tissue that surrounds it; this communicates with the
connective tissue of other cells, and all the connective tissue within
the mass of the muscle fastens to the very stout sheets or cords of the
same at the ends which are called tendons, by which the muscles are
attached to the bones. Thus, although the pull of any single cell is so
feeble as to be scarcely measurable, when hundreds or thousands of them
pull all at once the effect may be very powerful.
We are familiar with the very wide range of effort that our muscles can
show. They may contract with utmost delicacy, as when we hold a humming
bird’s egg in our fingers, or they may pull with a force, in our largest
muscles, of several hundred pounds. Of course this possibility of
variation is of great advantage in our use of our muscles. It depends
upon the very large number of individual fibers of which even our
smallest muscles are made up. Whenever any single fiber contracts, it
pulls to its full extent; if only a few become active, the pull of the
whole muscle will be slight; as more come into action, more force will
be exerted; the muscle will show its utmost power when all the fibers
are contracting at once. We are conscious of greater mental effort when
we make a powerful muscular contraction. This can be explained as due to
the greater nervous discharge required to excite all the muscle fibers
at once.
One feature of muscular action calls for an additional word. This is the
temporary loss of power, resulting from too long-continued use, which
is called _fatigue_. We know that a well-constructed machine can operate
day in and day out without having to stop to rest; why cannot our
muscles do the same? Evidently the necessity of resting cuts down the
possibilities of life more than any other one thing; our real life is
only two-thirds as long as it counts up in years because we have to
spend one-third of the time in sleep. Of course muscular fatigue is not
the only kind; there is nervous fatigue, as well, about which something
will be said later. The activity of our muscles is based on functional
metabolism; it follows, therefore, that fatigue is also due to
metabolism. We can think of two ways in which metabolism might cause
fatigue; the first of these is by using up the materials which furnish
energy; clearly no cells can go on working after they have exhausted
their supplies of fuel. The second results from the fact that metabolism
produces waste products. It is a familiar fact of chemistry that when
the substances formed in chemical processes are not removed they
interfere with the processes themselves. In active muscles very rapid
metabolism is going on and large quantities of waste substance are being
formed; these have to be discharged from the cells into the surrounding
fluid, and removed from there in turn by the blood. We can easily
imagine that this might not take place as fast as necessary to keep the
cells from becoming more or less clogged; in fact this clogging is
exactly what happens, so that muscles begin to show fatigue some time
before their supplies of fuel material are used up.
One familiar fact of muscular fatigue is that soreness, which indicates
that fatigue has really
[Illustration: Photo, Metropolitan Museum
MUSCULAR DEVELOPMENT OF AN ATHLETE--THE DISCUS THROWER OF MYRON]
[Illustration: A MODERN “VICTORY”--MISS SABIE AT PRACTICE]
been present in large amount, occurs much more often when we use our
muscles in ways to which we are not accustomed than when they are
exercised according to habit. It is the experience of every one who does
manual labor that when he gets a new job, one that calls for different
use of the muscles than he has been in the habit of, his muscles are
very sore until he is “broken in.” After that, although he does as much
or even more work than at first, he no longer becomes sore. This is
explained as being due to two things. First, whenever we make an
unaccustomed movement we overstimulate our muscles; that is, we call
more fibers into action than are necessary to do the job; as the motion
becomes familiar we cut down the action to that which just meets the
demand. Thus there is a great deal more metabolism than necessary when
unfamiliar motions are being made. Then, secondly, there is a spot in
every muscle cell, just at the point where the nerve makes its
connection with the muscle, that is more easily fatigued than any other
part of the muscle cell. This spot, by becoming fatigued first, tends to
cause metabolism to stop in time to prevent the rest of the cell from
being seriously fatigued. Only when we are so much interested in what we
are doing that we pay no attention to the fatigue of this safety spot,
or when necessity keeps us at work after we would quit if we had our own
way, do we push the metabolism so far that muscular soreness results.
Other types of fatigue, including feelings of exhaustion, are due to
effects on the nervous system, and will be described when we have that
system before us.
Before we leave the subject of the skeletal muscles it will be
interesting to say a word about the different kinds of motions that
they bring about. We have already seen that they work by pulling at the
joints, and we have no intention of enlarging on that topic. What we
want to do here is to group the bodily motions into a few classes,
regardless of what joints are actually moved. First, and most important,
comes _locomotion_; by that we mean any motions that move the body from
one place to another. Under that head we have walking, running,
swimming, jumping; in birds, flying. Next in order comes _grasping_;
this includes all motions by which we take hold of anything. We can
realize the importance of this group of movements when we think that our
fore limbs are specifically grasping organs, while in the great majority
of animals they are organs of locomotion along with the hind limbs.
Originally grasping had to do, undoubtedly, with the taking of food and
not much else. In civilized man we have in addition the use of all kinds
of tools from the coarsest to the finest. In most four-legged animals
the chief organ of grasping is the mouth. We still use our mouths to
some extent as grasping organs, and could probably learn to make even
more use of them in that direction if forced to it. _Chewing_ and
_swallowing_ make up a group of movements concerned primarily with the
handling of food after it has been grasped. Not much need be said about
them. Of small extent but great importance are the motions connected
with _sense perception_; these include chiefly the motions of the body,
neck, and eyes in _vision_; we are constantly turning to look at
something; in such animals as horses movements of the ears help greatly
in _hearing_; and both man and animals make sniffing motions to increase
the keenness of _smelling_. There is a group of motions devoted to
_voice production_ (including breathing). In man the vocal cords,
tongue, and muscles of the cheeks are the chief muscles that have to do
with the voice, not including the muscles of breathing, which, of
course, are essential. The interesting things about the vocal cords are
the excessive fineness of their operation, enabling expert singers to
produce tones that vary by only a few vibrations a second, and the
amazing exactitude of the control that the nerves have over them, so
that good singers can set them at the tension needed for producing a
particular tone with absolute certainty. The tongue is not a single
muscle, but a mass of several muscles working one upon the other. It
plays a part both in voice production and in the chewing of food. As an
organ of voice production it helps by changing the shape of the mouth
cavity. Speech depends very largely on this, since not the tension of
the vocal cords but the shape of the mouth and throat determines the
making of letters and syllables.
In addition to these familiar uses of the muscles there is a use which
is just as important but about which we are apt to think less. This is
their use in connection with posture, the taking and holding of
particular bodily positions. Posture is unlike other muscular activities
in several things. In the first place there is a steady, but rather
feeble, tension which can be held without marked fatigue for long
periods; all other forms of muscular contraction become severely
fatiguing rather quickly if held steadily. In the second place the
nervous control of posture seems to be different in some respects from
our ordinary control of our muscles. Finally there is some doubt as to
whether the contractions of the muscles themselves are the same.
Measurements of the functional metabolism of posture show that it is
much less than would be expected if the muscular action were of the
ordinary type. This, of course, explains why posture is less fatiguing
than other forms of activity.
The other two kinds of muscle, heart muscle and smooth muscle, must have
a word of description. Heart muscle contracts quickly and powerfully, as
does skeletal. It differs from skeletal in not depending on nervous
stimulation to make it contract; the heart can be cut clean out of the
body and will go on beating for a short time; in cold-blooded animals,
like frogs or turtles, for a long time. This could not be true if the
heart muscle had to be aroused to activity by nerves. Besides being
automatic, heart muscle shows the peculiarity that whenever it contracts
all the fibers join. We do not have a varying strength of pull shown by
heart muscle as we do in skeletal. As we shall see, it would be a
serious disadvantage rather than an advantage if heart muscle were to be
like skeletal in this respect.
Smooth muscle has the duty of operating the internal organs. For this
function no great strength is required; the motions do not have to be
powerful. Nor is rapid motion important. Smooth muscle does not have to
be so highly developed, then, as is skeletal. It is sluggish and rather
feeble in its actions. There are, however, two points of superiority
about smooth muscle, which fit in well with its special task. The first
of these is its freedom from fatigue. There are in the body numerous
smooth muscle masses that are in contraction practically all the time.
This would be impossible if fatigue were to develop. These masses make
up what are called the _sphincters_, rings of muscle surrounding
openings like that from the esophagus to the stomach or from the stomach
to the small intestine. It is the duty of these sphincters to hold the
openings closed all the time except occasionally when they open for just
an instant to let material through. The second point about smooth muscle
which fits it for its work is that it is capable of stretching out
greatly or contracting sharply without much difference in the force with
which it is pulling. For example, at the beginning of a meal the walls
of the stomach are drawn up, so that the food that is swallowed enters a
small space. With the progress of the meal the stomach enlarges, so that
at the end it has a much greater bulk than at the beginning. But the
actual pressure of the stomach upon its contents is about the same as at
the beginning. If the stomach were an ordinary elastic bag this could
not happen; the walls would have to stretch as the stomach filled, and
the stretching would mean greater pressure. Since the stomach walls are
of smooth muscle they adjust themselves to the progress of the meal. It
is important to note that there is a limit to this possibility of
adjustment. If one is so greedy as to keep on stuffing after the stomach
has reached its full size, stretching does occur, and if this is
repeated it may lead to a diseased condition known as “dilated stomach,”
which will cause much digestive trouble.
CHAPTER VIII
SENSATION--INTERNAL AND CONTACT SENSES
We have talked a good deal about muscles and the different sorts of
activities they can perform. We have also mentioned the fact that the
skeletal muscles are under accurate nervous control. Our next task is to
investigate the control of this nervous control; in other words to find
out just what it is that causes the nerves to stimulate the muscles so
that they shall perform as skillfully and usefully as they do. In
Chapter II we saw that our bodily movements are adjusted to our needs
through the sense organs. These bring information of the situation and
we act accordingly. We may group the kinds of information which the
sense organs furnish under three heads; first, what is going on inside
our bodies; second, what is happening at the surface of the body, and
third, what is happening at a distance from us. The senses which bring
the first kind of information are called the _internal_ senses; the
second group are the _contact_ senses; and the third are the _distance_
senses. We need to remember that the primary purpose of the senses is to
guide our muscles, and that our muscles are to find food for us, to keep
us from bodily harm, and to assist in the perpetuation of life by
propagating and caring for the young. By keeping these facts in mind we
shall have no difficulty in understanding the way in which the various
senses do their work.
Pain, hunger, and thirst are the internal senses with which we are most
familiar. Pain is evidently a protective sense. It is never aroused
unless something is amiss; for that reason pain should never be
neglected. Of course, in the majority of cases the pain is due to some
simple disturbance which can be located, and if no permanent harm is to
follow, or if no relief is possible, the heroic bearing of the pain is
meritorious; but thousands of women, thinking mistakenly that to
complain of suffering is a sign of weakness, or hoping to spare loved
ones distress, bear in secret or make light of pains that are the signs
of insidious disease, curable if taken in hand early enough, but sure to
cause acutest suffering and untimely death if allowed to go on
unchecked. Unfortunately our most dangerous internal enemies, the
organisms of infectious disease, do not give warning of their attack by
causing pain until the disease itself is so far advanced that there is
no escaping it. In this respect pain falls short of being efficient as a
means of warning us against impending injury.
Hunger and thirst are the stimuli which drive us to the taking of food
and water. It is interesting to think that of all the living things that
roam the earth only men have discovered the connection between the
taking of food and the avoidance of starvation; all other animals are
impelled to nourish themselves wholly through the operation of these
senses. There are two distinct phases to hunger. The first is appetite,
and this by itself seems not to be a sense in the strict meaning of the
word, but rather a memory of agreeable experiences associated with the
taking of food. In man appetite is often sufficient by itself to lead to
eating, as is proved by the frequency with which food is eaten between
meals when there cannot possibly be any genuine hunger, but probably in
animals it acts to arouse genuine hunger, rather than to cause eating by
itself. Genuine hunger is a sense as definite as any other. It is
aroused by spasmodic contractions of the stomach. These contractions
cannot occur except when the wall of the stomach is in a certain state
of tension. Various things can influence the coming on of this degree of
tension in the stomach, and so the possibility of hunger. Appetite
itself probably does this very effectively. Habit seems also to have
something to do with it. Hunger is usually felt just as mealtime draws
near, and it is often much keener at noon or night than before
breakfast, although the stomach has been longer empty at breakfast than
at any other meal. A curious fact about hunger is that it may disappear
completely after a few days of complete starvation. Contrary to the
popular idea that hunger becomes more and more acute as starvation
continues, the testimony of practically all persons who have starved for
more than a few days is that all sensations of hunger, as well as all
strong longings for food, subside and do not return. This is especially
true if the body is kept quiet and if the mind is diverted, so that
recollection of meals particularly enjoyed shall not come up.
Thirst is due to actual drying of the throat. When the cells lining that
region become deficient in moisture the sense is aroused. The drying may
occur from without or from within. When it occurs from without, as in
sleeping with the mouth open, relief can usually be obtained by merely
swallowing saliva copiously. The same treatment helps for the moment
when the lack of moisture is due to deficiency in the amount in the
body, but in this latter case no permanent relief can be had except by
the taking of water. When the amount in the body falls below the proper
level no comfort can be had until the loss has been made good. An
interesting thing about thirst is that it is the only sense which is
said never to be lost or seriously impaired by disease.
In addition to these familiar internal senses we have some that are less
well known. They are for the purpose of what may be described as the
routine guidance of the muscles. The act of walking, as we well know, is
made up of a series of muscular movements which are both accurately
timed and accurately graded. We obtain startling realization of this
when we come to the bottom step on our way down stairs without noticing
that we have arrived there. This timing and grading are done for us by
our bodies without our having to attend to it. The amount of labor that
is saved is shown by walking upon railroad ties. These are irregularly
spaced, and on that account it is necessary for us to pay attention to
every step. There is no comparison between the fatigue of this kind of
walking and ordinary progress along a smooth path. The senses that keep
track of the position of the body and of individual muscles are known
respectively as the equilibrium sense and the muscle-and-joint sense.
The equilibrium sense has as its organ a part of the internal ear.
Deeply imbedded in the bone is a series of chambers and canals lined
with a delicate membrane and filled with liquid. The canals, which are
three in number in each ear, are semicircular in shape, and accordingly
have been named the _semicircular canals_. One of them is horizontal;
the other two are vertical, and the two vertical canals lie at right
angles to one another. This arrangement makes it inevitable that any
movement of the head, in any direction whatsoever, will register
differently on the canal system than any other movement, which is
exactly what is required to make the apparatus efficient as an organ by
which motions of the body are kept track of and guided. Along with the
semicircular canals is a structure known as the _vestibule_ which
registers the position of the head, and so indirectly of the body, when
no movements are being made. We are not ordinarily conscious of the
working of these senses; they carry on their guidance of muscular
movement without our attention. We can, however, pay attention to what
they show if we wish. For example, one who is swimming under water is
never in doubt as to whether his head is turned up or down, even though
his eyes may be shut. His knowledge of position in such a place is
obtained from his equilibrium organ, even though he may not be aware of
the fact. Sometimes the organ becomes diseased. The results, so far as
the victim is concerned, are highly distressing. He usually has to stay
in bed because he cannot balance himself well enough to get about.
The organs for muscle-and-joint sense consist of tiny spindles
distributed around the joints and embedded within the mass of the
muscles. They are arranged so as to be affected by every motion of a
joint or every contraction of a muscle. They register not only the fact
of motion but also the extent. There is a disease, commonly known as
locomotor ataxia, in which the muscle-and-joint sense is impaired or
lost, particularly in the legs. The result is that walking becomes
difficult and unsteady, and usually impossible when the eyes are shut or
the room is dark. This is because the victim learns to make his sight
serve instead of his muscle-and-joint sense for guiding his muscular
movements, and when this also is withdrawn all knowledge of where his
legs are or what they are doing fails, and the only course is to fall
down or lie down as quickly as possible.
We have some additional bodily sensations, such as nausea, repletion,
fatigue, ill feeling or _malaise_, which guide our conduct more or less,
and are not very different in consciousness from hunger or thirst. So
far as is known there are no sense organs by which these sensations are
aroused. They are not strictly senses, therefore. We do not know enough
about how they originate to say anything more about them.
The _contact senses_ are touch, warmth, cold, and taste. Pain that comes
as the result of bodily injury might also be classified as a contact
sense, since its cause is something that comes in direct contact with
the body from outside, but it differs from internal pain only in its
source and not at all in the sensations it arouses, so there is no need
of describing it over again. The sense of touch is the fundamental
sense; the very lowest animals, even those that have no specially
developed sense organs, and few organs of any kind, react to the contact
of objects with their bodies just as the highest animals react to the
sense of touch. When no other information is available, that of simple
contact guides the animal in its securing of food and its avoidance of
harm. In accordance with this primitive character of the touch sense,
the psychologists tell us that we interpret the information from our
more highly developed sense organs, sight particularly, in terms of the
feel of objects. When we look at anything our judgment of it actually
consists in an idea of how it would feel if we were to take hold of it.
Our touch organs consist of tiny spots scattered all over the surface of
the body. They are much closer together on some parts than on others.
The total number is estimated at a half million or more. A good way to
test their sensitiveness is by pressing down on different parts of the
skin with fine hairs. When this is done it is found that the most
sensitive regions--the tip of the tongue, for instance--are fifty or
sixty times as sensitive as the dullest regions, like the small of the
back. To obtain sensations of touch it is necessary that there be
unaffected points alongside those that are affected. If all are acted on
alike, there will be no more sensation than if none is acted upon. This
can be shown by dipping the hand into quicksilver. The very heavy liquid
presses on all the touch points hard enough to affect them, but since it
presses on all alike nothing at all can be felt except along the line
where the hand enters the quicksilver where the pressure is strongly
marked. It is this feature of the touch sense that makes the wearing of
clothing bearable. If we had to feel the contact of the clothes
constantly we should presently find them so trying that we could no
longer endure them. We do feel rough places and are often seriously
annoyed by them, so we can judge what would be the effect if the whole
surface were felt as plainly.
Closely related to touch is the sensation of tickling or itching.
Curious facts about this sensation are the violence of the feeling that
may be aroused by very delicate irritation, drawing a thread along the
corner of the nose, for example; the persistence of the feeling beyond
the actual irritation; and the effectiveness of scratching as a means of
alleviating the condition. Almost nothing is known in explanation of any
of these peculiarities.
In addition to organs of touch the skin contains two kinds of organs for
perceiving differences of temperature. The first of these detects
warmth; the second cold. It is by means of these senses that we judge
whether the place where we are is of a suitable temperature in which to
remain; whether we should be quiet or active; whether special
provisions, like changes in the clothing, are necessary. In the case of
both senses the temperature of the skin is the comparison point. We
judge that an object is warm or cold according as its temperature is
above or below that of the skin which touches it. The ears are usually a
few degrees cooler than the hands; thus it is possible for one and the
same object to feel cold to the hands and warm to the ears. The two
kinds of temperature organs are side by side in the skin, although there
are many more “cold” spots than “warmth” spots. Very warm objects affect
both kinds, and then we get the sensation that we call “hot,” as
distinguished from merely warm. The cold spots are a little nearer the
surface of the skin than are the warmth spots; for this reason a hot
bath may feel cold at the very instant of stepping into it, although the
sensation changes to hot almost at once. We need to remember that our
sensations of warmth or cold depend altogether on the state of the skin,
and tell us nothing at all about whether our bodies as a whole are warm
or cold. Because the blood is always warm a flushed skin always feels
warm, and to produce flushing by means of alcohol has long been used as
a means of making the body feel warm and comfortable. This may be a
serious mistake in cold weather, for to drive the blood to the surface
then may mean that the body as a whole will cool off to the point of
actual injury. It is better to feel cold and conserve the body’s heat
than to feel warm and waste it.
[Illustration: TASTE BUDS
(From Martin’s “Human Body”)]
The last of the contact senses is that of taste. This is found only on
the tongue. Scattered about on that organ are many tiny sense organs
known as taste buds. These are usually in little hollows, so they cannot
be affected unless liquids which can enter the hollows are on the
surface of the tongue. If the tongue is wiped dry and then dry sugar is
sprinkled on it, no sweet taste will develop so long as the dryness
continues. The purpose of taste is evidently to give final information
about the food after it has passed the inspection of the other senses
and has been inserted into the mouth, but before it is swallowed. In the
higher animals there has been a subdivision of this sense into two. The
other is the sense of smell. In large part smell is a distance sense,
and will be treated when we are talking about the distance senses. Smell
has monopolized most of the properties of food-judging, so there is left
for taste proper only four kinds of perception. These are sweet, sour,
salty and bitter. We have, apparently, four kinds of taste buds, one for
each of these kinds of taste. All the other sensations that we call
taste are flavors, and are really smells. Of the four tastes sour and
bitter would probably be called warning and sweet and salty
recommending. Only by practice do we come to care for bitter foods, and
children usually object just as strongly to those that are sour.
Tropical savages, for whom salt is a rarity, esteem it much more highly
than sugar, which they can usually get in abundance.
In concluding this chapter we need to remember that the contact senses
make up the court of last resort; by the time anything comes close
enough to the body to act upon any of them it is so close that the
effect in guiding the muscles must be immediate; there is no time for
deliberation; whatever the muscles are going to do in response to
information thus obtained must be done at once. Later we shall see how
this affects our whole bodily make-up.
CHAPTER IX
SENSATION--DISTANCE SENSES
The three senses that give us information of what is happening beyond
the surface of our bodies are smell, hearing, and sight. Since smell is
closely related to taste, which was talked about in the last chapter, we
shall take it up first. Smell is like taste in that it is aroused by
chemical substances, but to be smelled these must be in gaseous form,
not dissolved in water, as for taste. The organ for smell is in the
upper part of the nasal chamber. There are really two of them, one in
each nostril. They are made up simply of little patches of mucous
membrane, as the membrane that lines the nose is called, in which are
many of the particular kind of cells that are affected by odors. An
interesting thing about these patches is that they are not in the part
of the nostrils through which the main current of air sweeps in
breathing, but in a little pocket off this main channel. If air
containing an odorous substance is breathed in or out, a little of it
works its way into the side pocket and is smelled. If we wish to get
more of the odor we do it by sniffing, which is changing the shape of
the nostrils to throw the air current more directly against the smell
organs.
These organs are amazingly sensitive. It is hard to appreciate the
minuteness of the amounts of material that can be smelled. Especially is
this true of those animals that have a really keen sense of
[Illustration: Photo, Fifth Avenue Hospital
A NEW HOSPITAL DESIGNED TO SUPPLY THE MAXIMUM OF LIGHT AND AIR--CHIEF
REQUISITES FOR HEALTH--TO EACH ROOM]
[Illustration: Copyright, Fifth Avenue Hospital
THE ARCHITECT’S PLAN WAS MADE WITH THE NEED FOR LIGHT AND AIR IN MIND]
smell. The amount of substance that rubs off from a rabbit’s feet onto
the ground at each step cannot be much to begin with; yet this continues
for hours to give off gas into the air, and a dog coming along at any
time meanwhile will get enough of the gas into his nostrils to smell it.
Fortunately for our comfort the sense of smell fatigues very rapidly. An
odor that is excessively disagreeable at first presently no longer
troubles us. If it were not for this, it would be almost, if not quite,
impossible to obtain laborers in those industries where the odor is
necessarily bad. There is, however, a source of danger in this quick
subsidence of smell perception. About our only method of judging offhand
as to the ventilation of a room is by the smell, and this fails as a
guide when we have been in the room for a time. Persons coming in from
outside are often struck by the bad state of the air in rooms whose
occupants are not conscious that there is anything amiss. Because of
this the ventilation of schoolrooms usually is not, and never should be,
left merely to the judgment of the teacher, but definite rules are laid
down as to opening of ventilators or windows.
When the gas that is smelled is part of an inward air current we
recognize it as coming from the outside and call it an odor; when it is
part of an outward current we call it a flavor. On account of the rapid
fatigue of the sense of smell we are unconscious of the smell of our own
breath, but can get fresh smells from within, and these come,
practically always, from materials that have just been taken into the
mouth. In comparison with taste flavor furnishes great variety of
perception. As persons become connoisseurs in food their enjoyment
depends more and more on flavor and less and less on taste. The
sensation from spices is a combination of flavor with irritation of the
tongue that is partly pain and partly touch. The sense of smell has
evidently a two-fold use; it makes us aware that there is food in the
vicinity, or sometimes that disagreeable things are near at hand; and it
shares with taste the duty of enabling us to judge of the food as it is
being eaten. Agreeable tastes, flavors, and odors add much to the
enjoyment of life. Within reasonable limits it is well to cultivate this
kind of enjoyment, for while there is no doubt that it can be overdone,
as in the excessive lengths to which the decadent Romans went to gratify
their taste and smell, neither is there any doubt that bodily health in
general, and the bodily function of digestion in particular, benefit
definitely from the kind of enjoyment that savory food and delightful
odors bring. It is the duty of those charged with the responsibility of
preparing and serving food to take pains that full advantage is taken of
the possibilities present in what food is to be prepared. This does not
mean expensive food; what it does call for is skillful handling of all
food, whether cheap or costly.
In introducing the subject of hearing we shall have to say a few words
about that which is heard, namely sound. Any object that has any degree
of elasticity at all is apt, if struck or rubbed or otherwise set
suddenly in motion, to start vibrating back and forth; the vibrations
will nearly always be regular, and will occur at a rate that is the same
for that particular object whenever it vibrates. The rate depends on the
size, the character, and the degree of stretch of the object. Air is to
all intents and purposes perfectly elastic; it is set vibrating by any
object that is vibrating in it, but since it has no particular size nor
degree of stretch it takes the vibration rate of the object that started
it going in the first place. The vibrations once started in air spread
in all directions, just as waves spread from a stone thrown into a pond,
and when these air waves strike upon another object that is free to
vibrate they will set it going at the same rate. The human hearing
apparatus is a device which is set in vibration by air waves, and the
result is called sound. The ear is limited in its ability to respond to
vibrations; they must be neither too fast nor too slow; if slower than
16 a second, most people will fail to hear them, and the same is true if
they are more rapid than about 40,000 a second. Between these limits
vibrations that strike upon the ear are heard as sounds.
Differences in vibration rate between one sound and another can be
recognized by the ear; the difference is a matter of pitch. By the pitch
of a tone we mean the vibration rate which it has. More rapid vibrations
give tones of higher pitch; a slow rate means a low pitch. Middle C on
the piano has a rate of either 256 or 261 a second according to the
system used by the tuner. The human voice has an extreme range starting
with the lowest note that the bass voice can compass with a rate of
about 80 vibrations a second, to the highest note that famous sopranos
can attain at about 1,400 a second. There is a record of a singer who
could achieve a tone with a rate of 2,100 a second, but this has not
been duplicated so far as is known. Of course no single voice can cover
more than a fraction of this range. Most men produce all their tones at
rates of between 90 and 500 a second, and women between 200 and 800 a
second. Not every different vibration rate is heard as a tone of
different pitch; the ear is not sensitive enough for that. The interval
between one note and the next includes several vibrations, more the
higher one goes in the scale. A perfectly true tone has exactly the rate
called for; a departure of one or two vibrations a second may not be
noticed, but if the error is greater the singer is sharping or flatting
his tone, according as he is above or below the true rate. A note that
has just double the rate of another one is said to be its octave. For
convenience the interval of the octave has been split up into twelve
tones, and all our music is constructed on that basis.
It is evident that the ear must be a very complicated organ; not only
must it perceive differences in pitch, as just indicated, but
differences in loudness must also register differently. More than that,
the ear has to be able to deal with sounds made up of a great many tones
coming into it all at once. When we listen to an orchestra or band, the
waves that strike our ears represent the commotion set up in the air by
all the instruments together. It is a remarkable fact that in this case,
instead of getting a meaningless jumble, we actually get a blend of
tones from which, if we are sufficiently musical, we can pick out the
individual elements.
The ear consists, in the first place, of a vibrator that will respond
accurately to any vibration rate or combination of vibration rates
within its range, and secondly of a sensitive apparatus that is acted
upon by the vibrator. The vibrator must respond freely to feeble
impulses, and, what is of prime importance, to any vibration rate as
readily as to any other. Almost all elastic bodies have a preferred
vibration rate; that is, they will respond better to some rates than to
others. About the only exception to this rule
[Illustration: DIAGRAM OF THE EAR
_A_, auditory canal, leading to the eardrum _B_; _C_, cavity of the
middle ear, communicating by the Eustachian tube with the throat _D_ and
containing the ear bones; _E_, semicircular canals; _F_, true hearing
organ; _G_, auditory nerve. (“The Human Mechanism,” by Hough and
Sedgwick.)]
is in the case of membranes that are not tightly stretched. A stretched
membrane, like a drumhead, has its own vibration rate, but one that is
not on the stretch is able to vibrate at almost any rate. This fact is
taken advantage of in the telephone and the phonograph, both of which
depend on being able to vibrate at various rates almost equally well. In
the ear, also, an unstretched membrane is the vibrator. We are familiar
with it as the eardrum. It is located at the bottom of the ear canal,
but cannot be seen by looking therein, because of a slight curve at
about the middle. When the ear specialist wants to examine the eardrum
he thrusts a small metal tube into the canal. This straightens it out
enough to bring the drum into view.
The eardrum does not act directly upon the sensitive hearing apparatus,
but its vibrations are transmitted across a space known as the middle
ear. The necessity for this space is found in the fact that atmospheric
pressure is not constant, but changes frequently from day to day,
besides falling off as one ascends higher above sea level. The free
action of the eardrum depends on its not being stretched; if there were
no means of readjustment it might be properly set for one air pressure,
but greater pressures would bulge it inward, putting it on the stretch,
and so cutting down its ability to respond to a wide range of tones. The
middle ear, which is the space behind the drum, connects with the
outside air by a tube leading from it to the back of the throat, which
latter communicates freely with the air through the nose, as well as
through the mouth whenever it is open. The tube is known as the
Eustachian tube. Its walls are ordinarily collapsed, so it is not an
open passage, but every time one swallows the tube is pulled open, thus
allowing differences in air pressure on the two sides of the eardrum to
equalize. Whenever one ascends a high hill quickly, as by train or
automobile, or even in going to the top of a high building by elevator,
the difference in air pressure behind and in front of the eardrum can be
felt. The sensation is disagreeable, and there is definite impairment of
hearing. Repeated swallowing gives relief.
The vibrations of the eardrum are transmitted across the space of the
middle ear by a chain of three tiny bones; these are very irregular in
shape, and are attached to one another in such a way that every
movement of the drum is followed exactly as to time and direction, but
with reduced size and increased power. The hearing apparatus, which is
part of the internal ear, but not the same part as makes up the
equilibrium organ, contains liquid which is set moving by the last of
the chain of bones, and this liquid acts upon the actual sensitive cells
which make up the sense organ proper. These latter are arranged in an
exceedingly complicated fashion. There are various theories as to the
precise manner in which the vibrations of the liquid in the inner ear
arouse the sensitive cells. It is thought that different cells respond
to tones of different pitch, but exactly how this is accomplished is not
known.
Deafness may result from failure of the sensitive inner ear to respond,
or from poor transmission of vibrations across the middle ear by the
chain of bones, or by interference with the freedom of action of the
eardrum, or by preventing the air waves from striking upon the drum.
Injury to the inner ear is rare, because of its secure position within
the bone. A common form of deafness is the result of hardening of the
connections between the ear bones, so that the chain no longer follows
well the vibrations of the drum. This usually begins to come on during
the twenties or thirties, and causes almost complete deafness by the age
of fifty. In most if not all cases it is hereditary. Interference with
the action of the eardrum may be due to the partial destruction of the
drum itself. Scarlet fever and measles are particularly likely to leave
the drum in a delicate condition, and any strain upon it then may
rupture it beyond repair. Continuous closing of the Eustachian tube, by
preventing equalization of air pressure on the two sides of the drum,
causes partial deafness. Inflammation accompanying a cold may cause
this, or the growth of adenoids in the back of the throat. Adenoids will
be described later; here they are mentioned only because they may press
upon and close the Eustachian tubes. There is danger from the closure of
the tubes by inflammation, because the inflammation may creep up to the
middle ear and cause serious trouble, both there and in the mastoid
region adjoining. Earache in children should be carefully watched, since
it usually means that the middle ear has been invaded by the same
inflammatory condition that is present in the throat in colds, and may
do serious damage to the delicate structures there. Often in children,
and sometimes in adults, the hearing is impaired by the accumulation of
wax in the ear canal. This wax is a sticky secretion that serves to
catch particles entering the ear canal and to prevent them from striking
against the drum. Unless the ear canals are washed out frequently with
_hot_ water the wax accumulates and hardens into a plug which closes the
ear canal, shutting off faint sounds. The wax dissolves in hot water,
but not in cold, so its accumulation is to be prevented by taking pains
that hot water actually gets to the bottom of the ear canal once in a
while. Digging the wax out with hard instruments should be done only by
an expert with greatest caution, lest the drum be injured.
The last of the senses to be described is sight; this is the one of
which we make the most use ordinarily, and curiously enough is the only
one that we can turn on and off. Loud noises or penetrating smells must
be endured, but by shutting our eyes tightly we can escape sight
whenever we want to. Altogether there are three kinds of information
which the sense of sight brings to us; the first is the knowledge
simply of light and darkness; the second is the knowledge of the shape
and size of objects; the third is the knowledge of color. Nearly all
animals seem to have some power of distinguishing between light and
shade. Even the one-celled kinds, that have no eyes or anything
corresponding to eyes, behave in a way that proves them to have this
power. A lot of them can be put in a dish of water that is well lighted
on one side and in shade on the other and in a few minutes all of them
will be found to have traveled to one side or the other according as
they happen to be a light-seeking kind or a dark-seeking species. As we
go higher up the animal scale we find that parts of the body show this
same power of distinguishing light from darkness. In the case of the
common angleworm, or earthworm as it is more properly called, the front
end has the power but the rear end has not. In very highly organized
animals only the special organs known as eyes possess the power; all the
rest of the body has lost it. The next feature of sight, the ability to
perceive the shape and size of objects, requires a special apparatus,
the eye, so animals that lack eyes cannot perceive objects, although
they may be able to tell light from darkness.
In order to see an object it is necessary that a pattern or image of it
be thrown on a sensitive surface; this surface registers the details of
the pattern, and so the object is seen. What we have to do here is to
find out how these patterns or images are formed in our eyes. In the
first place we must realize that every visible object has rays of light
going off from every part of its surface in every direction. So-called
_self-luminous_ objects, like lamps or the sun, produce the light
within themselves; all others merely reflect light that falls on them
from some source. Whenever light falls on any object, unless it is a
perfect mirror, part is absorbed by the object and the rest is
reflected; that which is reflected presently strikes against another
object and is again in part absorbed and in part reflected; as this
process is repeated over and over again the light becomes so broken up
that rays of it are traveling in every direction from every point on
every object. Any spot so protected that no rays can strike it evidently
cannot reflect any out again, and such a spot will be absolutely dark.
For the formation of an image of any object all that is necessary is
that some of the rays of light from every point on the object be caused
to fall in exactly corresponding positions on the image. The simplest
possible means of doing this takes advantage of the fact that rays of
light travel in straight lines. If we inclose an incandescent bulb in a
tight box with a round hole in one side of it, every spot on the
incandescent filament will be giving off light in every direction, but
all the light will be cut off by the box except that which happens to
have the direction which takes it out through the hole. From every
incandescent spot, then, there will be a beam of light in the form of a
cone escaping from the box through the hole. The tip of the cone will be
the incandescent spot; the slope of its sides will depend upon the size
of the hole. If a screen is placed in front of the hole, all these cones
of light will strike on it, and it will be illuminated in a pattern
which is made up of all the cones from all the incandescent spots which
make up the filament, but these will overlap so much that one cannot be
told from another. Now if the hole in
[Illustration: DIAGRAM OF A PINHOLE CAMERA
Showing how clear images can be formed by the use of a hole so small
that only pencils of light can pass through it to strike on the screen
at the back.]
the box is made small enough, a pinhole, in fact, and if the screen is
placed close to the hole, the cones of light from the different
incandescent spots become so narrow that when they strike the screen
they overlap scarcely at all, and what we get is a tiny spot of light on
the screen corresponding to every incandescent spot on the filament and
straight in line with it through the hole. Here we have exactly what we
have been talking about, namely a pattern or image of an object. The
image will be upside down, because those rays from the top of the bulb
that strike the tiny hole will be below on the outside, and those from
the bottom will be above. The same thing can be worked exactly in
reverse; we can place a box with a pinhole in it in front of any object
and get an image inside the box on the back; by placing a photographic
plate or film there an excellent picture can be taken. There is just one
reason why this scheme is not used in all cameras; that is that unless
the object is very brightly illuminated indeed the amount of light that
passes through the pinhole is not enough to affect the plate or film
except on long exposure. Perfect pictures can be taken with a pinhole
camera wherever long exposures are possible, or wherever the object
shines brightly enough. This difficulty is gotten around in the ordinary
camera by gathering up all the light in a wide cone from each spot on
the object and condensing it again on the plate or film. The image is
formed just as before, but now each spot on the image includes, not only
the beam of light that comes in a straight line from the corresponding
spot on the object, but in addition that in a wide cone surrounding the
straight beam. It is naturally brighter the wider the cone; which
explains why in poor light we open the diaphragm of the camera wider
than when the light is good; a brightly illuminated object will pass
enough light through a narrow opening, but a dimmer object must have as
wide an opening as possible in order that enough may get through.
The method of condensing the light in a spreading cone so that it shall
come back to a point again is by means of a lens; not only is this true
of cameras, but also of the eye; in fact everything that has been said
thus far about cameras applies perfectly to the eye. There is one thing
about the way in which light is brought to a point by a lens that makes
the formation of images by this method troublesome in comparison with
their formation in pinhole cameras. That is that the cone of light which
strikes the lens is condensed as an opposite cone on the other side, and
since the formation of an image requires that every point of the object
shall be reproduced as a _point_ in the image, there is only one place
where the image can come, which is where the tips of the cones of light
are. This place is spoken of as the _focus_. Unless the screen or film
is exactly there the image will be made up of overlapping circles of
light instead of points, and so will be blurred. In a pinhole camera the
cones are so small that they cannot overlap, so there is no one place
where the image is better than elsewhere; in other words, there is no
necessity of focusing. The chief reason that focusing gives trouble is
that the farther away from the lens the object is the closer to the lens
will be its image; hence if the field of view consists of several
objects at different distances they will not all focus at the same
level; the distant objects will focus near the lens; the near objects
farther back. In practice this trouble is met by using a thick lens,
which has a very short focus to begin with, so that a considerable range
of distances will be covered without serious blurring, and for finer
work by adjusting the distance between the lens and the back of the
camera, where the film or plate is, so that there shall be a good focus
of the particular object that is desired.
In the eye the clear part that projects between the lids, and is called
the _cornea_, is the important lens. Just behind is the arrangement that
corresponds to the diaphragm of the camera; this is the colored part
with a round hole in the center; it is called the _iris_ and the round
hole is the _pupil_. Behind the iris, and resting right against it, is
the secondary lens of the eye, known as the _crystalline lens_. The eye
as a whole is a globe just under an inch in diameter; at the back of it,
straight behind the lenses and pupil, is the sensitive surface upon
which images are formed. This is the _retina_; it extends pretty well
around to the sides, but the part we use most in seeing is the small
portion straight in line with the pupil. The cornea by itself is a lens
whose focus is
[Illustration: VERTICAL SECTION OF THE RIGHT EYE AND ITS LIDS
_c_, cornea; _l_, crystalline lens, its margin shielded by _i_, iris;
_p_, pupil; _r_, retina; _m_, muscles that move the eyeball; _o. n._,
optic nerve. (From “Human Physiology,” Stiles.)]
longer than the length of the eyeball, and the crystalline lens by
itself also has a long focus, but the two in combination give a focus
that just corresponds with the length of the eyeball, so that the images
of all objects at a distance of eighteen feet or more fall sharply on
the retina. Since near objects focus farther away from the lens than far
objects, the effect of this is to make objects nearer than eighteen feet
out of focus. For them a longer eyeball would be needed, and it would
have to become longer and longer the nearer the object was brought to
the eye. We all know that when we look at near objects we make an
adjustment in the eyes. This is known as accommodation; for a long time
it was supposed that _accommodation_ was actually secured by lengthening
or shortening the eyeball to bring the focus right, but we now know
that the eyeball does not change in shape when we accommodate. The same
result is secured by another means, namely, by letting the crystalline
lens bulge out and become thicker. It was stated incidentally a few
pages back that thick lenses have shorter focuses than thin. So when the
crystalline lens thickens it shortens the focus of the eye, and throws
the image forward. This will locate the image of near objects on the
retina, instead of behind it, as in the unaccommodated eye. The
crystalline lens is not stiff like glass, but rather like a thick jelly;
it is in a transparent bag, or capsule, which is fastened to the inside
of the eyeball all around, and the pull on the capsule stretches it out
pretty flat, making the lens thin. There are tiny muscles inside the
eyeball, known as the _ciliary_ muscles. These are so fastened that when
they contract they pull the eyeball forward and inward, loosening the
tension on the capsule of the lens. This then bulges out, taking up all
the slack. When we learn, as babies, to accommodate for near objects we
find out just how much the capsule must be loosened to give the proper
adjustment for any distance. As people get along in years the
crystalline lens often loses its elasticity so that it does not bulge
when the capsule is loosened. After this happens vision for near objects
is no longer clear. Relief is obtained by wearing reading glasses. These
are merely glass lenses worn in front of the eyes selected so that their
focus fits in with that of the lenses of the eye itself to bring the
image of objects at reading distance sharply on the retina. It is
usually necessary to replace these glasses from time to time as the
crystalline lens becomes stiffer and stiffer and ordinary accommodation
fails more and more.
There is a disease known as _cataract_ in which the crystalline lenses
become cloudy and finally completely opaque. Of course this means
blindness, since the passage of light to the retina is interfered with.
Relief is obtained by the simple expedient of removing the opaque lenses
bodily. This is possible merely because the crystalline lens is not the
chief lens of the eye. To be sure the cornea by itself will not focus on
the retina, but a glass lens can be placed in front of it which will add
itself to the cornea and the combined lenses will. There is no
possibility of accommodation in a case like this, so the patient has to
be furnished with bifocal lenses; the main part gives clear distance
vision; the lower section gives clear vision at the reading distance.
The patient has to get along with blurred vision in the regions between.
Not all eyes are exactly the right size so that distant objects shall
focus sharply on the retina. In fact a large proportion of them are
either too long or too short. It is clear that in an eyeball that is too
long the image of distant objects will fall in front of the retina, but
near objects that happen to be at just the right distance will focus
exactly on the retina. The distance at which this happens depends, of
course, on how much too long the eyeball is. Persons that have unduly
long eyeballs are, therefore, nearsighted. The condition is called
_myopia_. The correction for it consists in the use of lenses in front
of the eyes that instead of shortening the focus shall lengthen it.
Concave lenses, namely, those that are thick at the edge and thin in the
middle, will do this, and these are the kind that are worn by
near-sighted people.
When the eyeball is too short the image of distant objects falls behind
the retina, and of course that of near objects tends to fall farther
back yet. Since by
[Illustration: Photo, Fifth Avenue Hospital
SOFT, RESTFUL COLORS AND ATTRACTIVE SURROUNDINGS ARE CONDUCIVE TO GOOD
SPIRITS AND THEREFORE TO HEALTH]
[Illustration: Copyright, Paul Thompson
THE MAGNET IS HERE PUT TO THE UNUSUAL SERVICE OF REMOVING IRON FILINGS
FROM THE EYE]
accommodation the focus can be thrown forward, most persons with short
eyeballs can see distant objects clearly by accommodating for them, and
near objects that are not too near by extreme accommodation. For this
reason _hyperopia_, as this condition is called, is usually not
discovered until the person begins to feel the strain of the constant
accommodation that is necessary whenever the eyes are open. Eyestrain
usually shows itself in headaches; in fact, so large a proportion of
headaches come from this cause that anyone who suffers from them at all
frequently should have his eyes examined by a competent oculist. Relief
for hyperopia is by means of glasses that shorten the focus and thus
bring the image of distant objects forward to where the retina is in the
short eyeball.
[Illustration: EYE LENS--NORMAL AND DEFECTIVE
_A_, normal eye; _B_, myopic eye; _C_, hyperopic eye (From Martin’s
“Human Body”)]
There is one other defect of vision that is so common as to call for a
word; this is _astigmatism_. It is the condition in which the cornea is
not curved equally in all directions; the vertical curvature may be
greater or less than the horizontal. The effect is that points on
objects do not focus sharply as points in the image, but as little
elliptical spots. If one adjusts the accommodation so that the top and
bottom edges of objects are sharp the sides will be blurred and vice
versa. Usually the blurring is not great enough to be noticeable, but
only enough to make the person unconsciously dissatisfied with the
accommodation. He, therefore, constantly tries to improve it by changing
the tension of his ciliary muscles, and so brings on eyestrain. The
correction for this condition is glasses that are not equally curved in
all directions, but so selected that their less curvatures shall fit in
with the greater curvatures of the cornea. In fact, glasses that are
curved only in one direction are usually used, this curvature being just
enough to bring up the total curvature in that direction to equal the
other curvatures of the cornea. It ought not be necessary to give
warning that only competent persons should be allowed to examine and
prescribe for the eyes. Great skill is needed to determine accurately
just how far from correct the eyeball is, and unless this is known there
is no means except guesswork by which to decide on a prescription.
The third feature of vision is the perception of color. Color is to
light what pitch is to sound; that is, it depends on the vibration rate
of the light waves. Light, as already explained, is one of the forms in
which energy reaches the earth from the sun. Heat is another form. Both
are portions of a great energy stream to which we give the name of
_radiant energy_. This, as it comes from the sun, is made up of a
mixture of vibrations having almost every imaginable rate, except that
the slowest are many times faster than the highest pitched sound. At a
certain rate, and one that for these vibrations ranks as slow, the
energy is what we know as heat, and over a considerable range it
continues to be called heat; more rapid vibrations, and they are so
rapid that they have to be expressed in trillions per second, cause the
effect on our retinas that we call light. The slowest that we can see
give the sensation of red; the most rapid the sensation of violet; the
other colors of the rainbow, which in order after red are orange,
yellow, green and blue, are vibration rates between those that give red
and those that give violet. It will be noticed that only six colors are
given for the rainbow instead of seven. This is because there is not
enough real difference between blue and indigo to justify making them
separate colors. The distinction was made at the time when it was
supposed that there was something specially wonderful about the number
seven, which made it necessary that every important feature in nature
should show that number. We now know of no reason why seven should have
virtue over any other number. When all the vibration rates are mixed
together, as they are in the sunlight, the sensation is white. There are
other mixtures of colors that give white also, but they are not exactly
equivalent to the white of sunlight, as is proven when one tries to
match colors under artificial light. A great deal of labor has been
devoted to the attempt to get an artificial light that shall be
practically equivalent to sunlight, and only lately have good results
been obtained. When no light enters the eye the sensation is of black,
and it is worth while to note that so far as our sensations are
concerned black is as much a color as white or any other,
notwithstanding the fact that no light falls on the retina when the
black sensation is being felt. Contrary to ordinary belief, blind
persons who are blind because their eyes have been destroyed do not see
black all the time; they simply have no sensations at all from the eyes.
On the other hand, persons blind because of cataract do see black,
because in them the retinas are still present, but no light falls on
them.
The perception of color is very complicated, and not at all well
understood. Persons who do not have the same color perception as most of
us are called color-blind, and by learning some things about
color-blindness we shall best get an idea of color perception itself.
About four men in one hundred have defective color sense; the proportion
in women is only about a tenth as great. By far the commonest type is
one in which neither red nor green is seen correctly, but both are seen
as neutral tints, and in many cases look so much alike that the person
cannot tell one from the other. The practical importance of knowing
whether or not this defect is present is seen when we think that red and
green lights are used more than any other colors in signaling, so that
railroad men and others who work by signals must have normal color
sense. It has been discovered that even people that have normal color
vision are color-blind in the margins of the retina. This can easily be
demonstrated by bringing a red or green disk slowly around in front of
the eye of a person who, meanwhile, keeps looking straight ahead. He
will see the disk out of the corner of his eye some time before he can
tell what color it is. In fact, if a red or green disk is used, he
usually will not be certain as to the color until it is almost straight
in front. Blue or yellow disks can be told with certainty much farther
out, but even these colors are not perceived clear to the edge of the
field of vision. These experiments show that the retina becomes more and
more highly developed as an organ for perceiving color as we get closer
and closer to the center; at the extreme edges there is no color sense
at all, but only the primitive ability to tell light from darkness;
closer in blue and yellow are distinguished and not red and green; only
in the central part are all colors clearly seen.
In addition to the kinds of information which have been described thus
far, that the distance senses bring in, there is another kind, fully as
important as any in our actual use of our senses; that is information as
to the “direction from us” of the object or objects which are arousing
the sense. We can get this through all the distance senses, but much
more perfectly in the case of sight than in the others. We locate the
direction of objects that we smell by turning the head this way and
that, sniffing meanwhile, and noting the position in which the odor is
caught most clearly. Animals with a keen sense of smell, like dogs, can
locate directions very accurately by this means. In the case of hearing
the method is to turn the head until the sound is equally loud in both
ears. We would expect that a person who was hard of hearing in one ear
would never be able to locate sounds by this method; but, as a matter of
fact, such persons unconsciously allow for the difference in hearing in
the two ears, and so can judge the direction of sounds about as well as
any of us. Animals, like horses or rabbits, that have very movable outer
ears, undoubtedly can locate sounds much more accurately than we can.
Our outer ears are of almost no use in hearing; persons who have had the
misfortune to lose them hear practically as well as anyone.
[Illustration: DIAGRAM SHOWING HOW DISTANCE AFFECTS THE SIZE OF THE
IMAGE]
We locate directions with the sense of sight with perfect accuracy,
because unless the image of the object we are looking at falls on the
center of the retina it is not seen clearly. The only way to make the
image fall just there is to look directly at the object. The muscle
sense in the eye muscles is extremely delicate, so that if the eyes are
rolled at all in looking at anything we know it and can judge, also, how
much they are turned from the straight position. In this way we are able
to tell exactly the direction from us of any object we can see.
We can judge the _distance_ of a near object very accurately by noting
the degree to which the two eyes have to be turned in in order to see it
clearly with both. We are quite unconscious of this means of making the
judgment; all we know is that we can tell. It is easy to prove that it
depends on the two eyes by closing one and trying to make movements that
depend on accurate knowledge of distance. A good example is threading a
needle sideways. With both eyes open this can be done fairly easily, but
with one shut it cannot be done at all, except by chance. Objects so far
away that the eyes are not turned in perceptibly in looking at them are
judged as to distance wholly on the basis of their size. It is clear
that the actual size of any image on the retina will depend in part on
how large the object is, and in part on how far away it is. If an object
that we know to be large casts a small image on the retina, we conclude
that it is far away. It follows that unless there are some familiar
objects in view, judgments of distance are not at all trustworthy. A
good illustration is in looking up a bare hillside, and trying to
estimate the distance to the top. If, while this estimation is being
made, a man or horse suddenly comes into view at the top, the man or
horse will nearly always appear unexpectedly large, showing that the top
of the hill is not actually as far away as it was judged to be.
The possession of two eyes instead of one is an advantage to us in
another way, in addition to helping in the estimation of near distances.
This is in making objects appear solid, or in other words, in helping
the estimation of depth. When we look about us we have no difficulty in
realizing that some objects are near and others far, and that the
objects themselves have some parts that are nearer to us than other
parts. A great many things assist us in this realization. First and
foremost comes that which is known in art as perspective, namely the
tendency of distant objects or distant parts of objects to appear
smaller than those that are near. This can best be illustrated in the
case of parallel lines extending away from the eye, as when one stands
on a straight railroad track and looks along it. Although we know
perfectly well that the rails are the same distance apart all along, if
we were to believe our eyesight implicitly we should think that they
came gradually together. It is on account of this matter of perspective
that drawings of solid objects must show supposedly parallel sides
nearer together at the far end than at the near. Besides perspective
there are the shadows to be taken into account. Only solid objects cast
shadows, so if we see a shadow apparently cast by any object we
naturally conclude that it is solid. Both perspective and shadows can be
and are used by artists in making drawings and paintings look real. In
fact, they have almost no other means of doing this. As we all know,
even the cleverest paintings do not give an impression of depth equal to
that which comes from actually looking at solid objects. This is because
of help we get from the two eyes in the latter case. The reason for the
difference is that when we look at a solid object with both eyes the
view we get with one eye is not exactly the same as with the other; we
see a little farther around on the left side with the left eye, and on
the right side with the right eye. The combined view with the two eyes
gives us an impression of solidity that cannot possibly be had when the
view with the two eyes is exactly the same, as when we look at a
picture. The only way in which the impression of depth can really be
gotten in a picture is by using the familiar method of the stereoscope,
where two pictures are taken simultaneously by two cameras, placed a
little farther apart than the two eyes; and then the two are looked at
together through a special pair of prisms.
By means of the three distance senses, smell, hearing, and sight, we are
informed pretty completely as to what is around us. All three give an
idea as to the direction from us of objects; although sight does this
better than either of the others. Sight, also, lets us know accurately
as to the distance away of objects, provided they are fairly near. Smell
and hearing, as well as sight, may give us some idea as to the distance
of far objects, but only when we are dealing with familiar sensations. A
very faint smell, or faint sound, means a distant object provided we
know enough about the source to know that if the object were near the
sensations would be keener. Our judgment of distant objects by sight is
better than this, but not by any means perfect. When we contrast the
distance senses with the contact senses we see at once that the great
advantage coming from the possession of distance senses is in the time
that is permitted for action. In the chapter on contact senses we
emphasized the fact that the response to them must be immediate; there
is no time to pick and choose. When we can learn of the presence of
objects before they reach us, and something of their direction and
distance as well, we can usually take time to select the most fitting
course of action. We do not have to jump into a mud puddle to escape an
automobile if we see it soon enough. We shall learn in a later chapter
how this opportunity of choice is wrapped up with our development into
highly intelligent animals.
CHAPTER X
THE NERVOUS SYSTEM AND SIMPLE NERVOUS ACTIONS
In the second chapter we saw that to make our muscles act in accordance
with the information brought in by the sense organs some means of
communication between them is necessary; we saw, also, that this means
consists of the nervous system. Now that we have learned something about
both muscles and sense organs we are ready to look into the way in which
communication between them is carried on. First of all, we must realize
that living protoplasm does this. The nerve cells are alive and have
their basic metabolism just as do all other living cells. They also have
their functional metabolism; but in them, instead of taking the form of
forcible motion, as in muscles, or of the manufacture of special
materials, as in gland cells, it takes the form of the transmission of a
disturbance from one part of the cell to another. An interesting and
important fact about this transmission of disturbances is that the
actual amount of functional metabolism required by it is very small.
Only by the most careful measurements has it been shown that nerve cells
that are functioning have a greater metabolism than those that are at
rest. For a long time it was thought that a nerve cell acted very much
like a telephone or a telegraph wire, transmitting some kind of a
disturbance which was set up in it, but not having any active part
itself in the process. We now know that the special activity of nerve
cells is a form of functional metabolism, just as is the special
activity of muscle cells or gland cells.
The nerve cells have to make communication between sense organs and
muscles, and, as we have already seen, these are often quite a distance
apart. It is necessary, therefore, that the nerve cells be long enough
to reach over these distances. As a matter of fact, it is not necessary
for single cells to have this great length, because it is possible for
them to be arranged end to end, making a path of living protoplasm
consisting not of one cell but of a chain of them. If we look at a nerve
cell under the microscope we see that it is made up of a little central
mass of protoplasm to which has been given the name of “cell body.” From
this cell body extends a tiny thread of living protoplasm. This thread
is called the _axon_. It is so very slender that it cannot be seen
except under a powerful microscope, and yet in our own bodies and the
bodies of all large animals many of these are three feet or more in
length without a break. This tiny protoplasmic thread, the axon, was
formed originally by growing out from the cell body. As it grew it
became surrounded by a sheath, which probably gives it strength and
decreases the danger of its being broken. Another thing which helps to
keep the axons from being injured is that they are always in bundles.
Instead of one of these very slender axons lying all by itself, it will
be bound up with several hundred others; the arrangement is similar to
that in a telephone cable, where a great many single wires are bound
together in the large and very strong cable. The living protoplasm of
the nerve cell has a gray color, so that wherever this shows we have
what is commonly called gray matter. We saw a moment ago that every axon
is inclosed in a sheath. Some of these sheaths are transparent, so that
the gray color can be seen underneath, but most of them have a layer of
white material, which makes them look white instead of gray. The bundles
of axons corresponding to the telephone cables make up what we call the
nerves. Nearly all nerves are white in color because of the white
material in the sheaths.
The sense organs, as we have seen in Chapters VIII and IX, are some of
them inside the body, others spread over the surface of the skin, and
the rest in the special sense organs, like the eyes or the ears. A very
complex organ, like the eye or the ear, has thousands of axons leading
from it. In the case of the eye these are grouped into a large nerve
leading away from it at the back, which is called the optic nerve. There
is a similar large nerve leading from the ear. When any sense organ is
acted upon, as when light falls in the eye or sound on the ear, it
starts a disturbance in some or all of the axons leading away from it.
As we have said over and over, the purpose of the nervous system is to
arouse the muscles to activity, and to guide them in that activity. We
must ask next, then, how the nerves are distributed to the muscles. If
we dissect the body of any animal or bird, we can find nerves passing to
various parts. For example, a large nerve goes down each leg; this nerve
subdivides here and there. Since we know that the nerve consists of a
great many axons bundled together, we will realize that this
subdivision is not a real branching, but simply a passing of some of
the axons away from the main trunk along the smaller stem. Some of these
smaller stems can be traced to endings in the skin; these contain the
axons connecting with sense organs. Others lead directly into muscles.
Some of these axons may also connect with sense organs, since, as we
have already seen, every muscle has embedded in it the organs of muscle
sense, but in addition any nerve that leads to a muscle contains a great
many axons which pass directly to the muscle fibers. These are the axons
by which the muscles are aroused to activity. It is a general rule of
the nervous system that no nerve cell extends without a break from any
sense organ to any muscle fiber. The axon which communicates with the
sense organ belongs to one nerve cell; the axon which connects with the
muscle fiber belongs to a different nerve cell. The first is called a
_sensory_ nerve cell, the second a _motor_ nerve cell. Some idea of the
appearance of these cells can be gotten from the figures on page 126.
It will be seen that the cell body of the sensory cell appears to be off
on a little side branch. As a matter of fact, the branch is double, so
that when a nervous disturbance passing along from the sense organ comes
to the beginning of this branch it can pass up to the cell body and then
out from the cell body along the second part of the branch, and so along
the other part of the axon. This part of the axon is seen in the figure
to have several branches; these are really branches of protoplasm and
not separate axons coming off, as in the case of the nerve trunk. The
use of these branches we shall see in a moment. Also at the tips of
each branch there is a tiny feathering. We shall explain this
presently. Let us look first at the figure of the motor nerve cell. This
has a cell body and long axon, and, besides these, has a great many
short protoplasmic branches sticking out in all directions from the cell
body. Since a nervous disturbance to get from a sense organ to a muscle
has to pass over a sensory nerve cell, and also over a motor nerve cell,
evidently there will have to be some point at which it leaves the
sensory cell and gets into the motor. This is accomplished by having the
tiny feathering at the tip of the sensory cell interwoven with the fine
processes projecting from the body of the motor cell. This arrangement
we may call a nerve junction. In the whole body there are, of course,
millions of these nerve junctions.
[Illustration: MOTOR NERVE CELL
SENSORY NERVE CELL
(From Martin’s “Human Body”)]
We have just described the simplest arrangement of a nerve path from a
sense organ to a muscle; it consists of the sensory nerve cell, a nerve
junction, and a motor nerve cell. This arrangement will answer where the
sense organ and the muscle are in the same part of the body, but it may
happen that the sense organ is in one part of the body and the muscle is
in a distant part, as, for example, the eye and the muscles of the hand.
To make these distant connections there must be additional nerve cells,
and these we find in the body in the form of a kind of nerve cell that
serves as a connecting link between sensory cells and motor cells. It
may be called simply a connecting nerve cell. In appearance it is like
the motor nerve cell, except that it has many branches which do not
terminate in muscle fibers, but in fine feathering like that at the tips
of the sensory nerve cell. When these connecting cells are present in
the chain, the arrangement is as follows: from the sense organ the
sensory nerve cell will pass just as previously described, the
feathering at the tip will form a nerve junction with a connecting cell
instead of with a motor cell. This connecting cell also has feathering
at the tips of its branches, and these featherings will form a nerve
junction with another nerve cell. This may be a motor cell, in which
case the pathway is completed, or it may be another connecting cell,
which in turn may lead into motor cells or connecting cells.
The nervous system is made up, then, of chains of nerve cells. Now why
is this arrangement present? In very many cases sense organs and muscles
are side by side or within a short distance of each other; why does not
a single nerve cell reach directly from the sense organ to the muscle?
The
[Illustration: CONNECTING NERVE CELL
(From Martin’s “Human Body”)]
answer is simple, if we think for a moment of how the body works. The
information that comes in by way of particular sense organs cannot
always be used to arouse particular muscles to activity. Things that we
see will sometimes cause us to move one hand, sometimes another hand,
sometimes the legs, sometimes muscles of the head, and so on. This means
that the eye is able to make connection with a very large number of
different muscles. The same thing is true of the other sense organs. The
body could not possibly work as it does, if certain sense organs
connected with certain muscles and no others. As a matter of fact, it is
not an exaggeration to say that the proper working of the body requires
any sense organ to be able to make connection with any muscle. It might
be possible to do this by giving every sense organ as many sensory nerve
cells as there are muscles, but this would be as bunglesome as to
attempt to provide every business house in a large city with a separate
telephone wire to every other business house.
The arrangement of the nervous system is very much like that of a city
telephone system. The sensory nerve cells all lead into a part of the
nervous system to which is given the name of the _Central Nervous
System_, just as the telephone wires all lead into a “central exchange.”
From this central nervous system or “exchange” all the motor nerve cells
extend to the muscles. There is one important difference between the
arrangement of the nervous system and that of the telephone exchange;
namely, that the nervous system is a “one-way” system. As we all know, a
telephone instrument can be used either for sending or receiving and the
same wires conduct the messages in both directions. This is not true of
the nervous system. The messages from the sense organs pass into the
center by way of the sensory nerves and out from the center by way of
the motor nerves. The central nervous system operates as an “exchange,”
connections can be made from any sensory cell to any of the motor cells.
Since this ability of the central nervous system to make connections
here and there within itself is about the most important of all our
nervous activities, as we shall see shortly, we must try to form an idea
of how it is done. If we look again at the figures of the sensory and
connecting nerve cells, we shall note that both kinds are branched; the
tip of every branch makes a nerve junction. This means that every
sensory cell, for example, has as many outlets as it has branches. If
every one of these outlets were to communicate directly with a motor
nerve cell, the number of connections that
[Illustration: DIAGRAM OF NERVE CELL CONNECTIONS
_S C_, spinal cord; _S_, sense organ; _M_, muscle; _a_, sensory nerve
cell; _b_, connecting nerve cell; _c_, motor nerve cell]
this sensory cell could make would depend on the number of its branches;
but, as a matter of fact, most of the branches from the sensory cells
make nerve junctions with connecting cells, and not with motor cells
directly. These connecting cells in turn are branched and many of these
branches lead again to connecting cells, so we see that the number of
connections that a single sensory cell can make quickly becomes very
large. The arrangement is shown in the accompanying diagram. Not only
does one sensory cell have in this way the possibility of nerve
connection with all the muscles, but the reverse is also true; namely,
that every muscle has the possibility of being connected with every
sense organ. This means that there must be a number of nerve junctions
connecting with the cell body of each of the motor nerve cells. If we
look back at the diagram of the motor nerve cell, we shall see that it
has a great many tiny branches leading off it. These are numerous enough
to enable the feathery tips of a great many sensory or connecting nerve
cells to interweave with them, and so enable any motor cell to be acted
upon from a great many different directions.
The central nervous system, which is the place where all these
much-branched pathways are and where all the nerve junctions are
located, is made up of two chief parts, the brain and the spinal cord.
The brain is inside the skull and the spinal cord is an extension down
the back. It lies in the tunnel made up of the arches of the bones of
the vertebral column as described in Chapter VI. The cables which
contain the axons of both sensory and motor nerve cells extend from the
brain or from the spinal cord out to the different parts of the body
where the sense organs and the muscles are located. They start as large
nerve trunks which divide and subdivide as they get farther and farther
away from the central nervous system. The large nerve trunks are
arranged in pairs--those that spring from the brain are called _cranial_
nerves; those that spring from the spinal cord are called _spinal_
nerves. The cranial nerves, with the exception of one, lead only to
points in the head or neck, and so are short; the spinal nerves on the
other hand reach to the various parts of the trunk or down the arms and
legs, so that some of these may be three feet long or more. An idea of
the arrangement of the nervous system is given in the accompanying
figure.
A good way to realize the actual working of the nervous system is to
take a particular action and follow it through. Suppose a barefooted boy
steps on a sharp thorn. The thorn arouses some of the sense organs in
the sole of the foot. These in turn start a disturbance in the sensory
nerve cells which pass up the leg to the lower end of the spinal cord in
the small of the back. Within the spinal cord the sensory cells branch
and the disturbance set up by the prick of the thorn spreads all over
these branches to their tips. Some of the nerve junctions thus affected
lead to muscles which will cause the foot to be jerked up; others
communicate with connecting nerve cells which extend all the way up the
spinal cord and into the brain and make the boy aware of the fact that
he has stepped on the thorn; still others may make connection with
muscles which would cause him to sit down and look at the bottom of his
foot; still others may lead to the vocal muscles and the tear glands,
causing the boy to cry. Of course these are not the only possible nerve
connections; every muscle in the body might theoretically be aroused to
action as the result of the stepping on the thorn. As a matter of fact,
there is a condition in which this will happen. The drug strychnine has
such an effect upon the nervous system that the stimulation of any sense
organ actually does arouse all
[Illustration: DIAGRAM ILLUSTRATING THE GENERAL ARRANGEMENT OF THE
NERVOUS SYSTEM
(From Martin’s “Human Body”)]
the muscles in the body, giving what is called a convulsion. There are
some other poisons which may act similarly. Convulsions are not at all
uncommon in young children. The way in which a convulsion is produced is
by the spreading of the disturbance all over from a single sense organ.
What we have, then, in the nervous system is an arrangement whereby
under special conditions a nervous disturbance can pass from any sense
organ to any or all of the muscles, but under ordinary conditions the
disturbance spreads to a particular muscle or group of muscles only. It
is evident that the nervous system would be of no use at all, if this
latter arrangement did not exist. In order for our muscles to serve us,
they must act in obedience to information brought in by the sense
organs, and this can happen only when certain groups of muscles work in
accordance with the information brought in by certain sense organs or
groups of sense organs. We explain the behavior of the central nervous
system by saying that there are preferred pathways through it, or, to
put it in a slightly different way, when a nervous disturbance spreads
over any nerve cell it extends equally over all parts of it, but does
not pass with equal ease over all the nerve junctions. Some of the nerve
junctions allow the disturbance to pass more readily than others, and it
is this difference in the ease of passing the nerve junctions that
determines which pathway the disturbance shall follow. We know no other
means by which this picking out of particular paths from the huge number
of possible paths could be accomplished.
What we are describing now is the simple foundation on which all our
nervous activities rest. For that reason we are not taking up at this
point the working of the brain, but only the direct connections between
sense organs and muscles. In some very low animals the whole nervous
system is made up of such simple connections.
[Illustration: DIAGRAM ILLUSTRATING A REFLEX
An object _O_ suddenly appears in front of the eye. Its image, formed on
the retina _R_, a sense organ, starts impulses along the fibers of the
sensory nerve cell _s_, which in turn stimulate the motor nerve cells
_m_. These in turn stimulate the appropriate muscles of the eyelid,
compelling a wink. (After Hough and Sedgwick, “The Human Mechanism.”)]
A nervous activity which consists of the passage of a disturbance from a
sense organ to a muscle is called a _reflex_. We have many examples in
ourselves; if we inadvertently touch something hot, the hand is jerked
away. Tickling the soles of the feet in one who is asleep will cause
them to be drawn up; irritation in the throat causes us to cough, or in
the nose to sneeze; the flashing of a bright light into the eye compels
us to wink; all these are examples of the direct passage of nervous
disturbances from sense organs to muscles. In every case the path is
from the sense organ over the sensory nerve cell to some point in the
central nervous system, then either by a direct nerve junction to a
motor nerve cell or over one or more connecting nerve cells to a motor
nerve cell and so to the appropriate muscles. These reflex actions
follow the arousing of the sense organ with no more delay than is
required for the passage of the disturbance over the nervous pathway.
There is a delay of a small fraction of a second at every nerve
junction, which makes some reflexes slower in their action than others.
For example, there is a reflex known as the knee jerk; this is an
outward kick which results from a sharp blow on the front of the leg
just below the knee. The kick follows so closely after the blow that
there cannot be more than one or at the most two nerve junctions in the
pathway. The reflex of winking, on the other hand, takes several times
longer, although the eye is much nearer the central nervous system than
is the place on the leg which is struck in arousing the knee jerk. Since
the actual length of nerve to be passed by the disturbance is much
shorter in the winking than in the knee jerk, while the time for the
reflex is a good deal longer, we conclude that the nerve pathway which
is used in arousing winking contains a great many more nerve junctions,
and therefore includes a great many more connecting cells than does the
path for the knee jerk. In this particular case the reason why the
pathway of winking contains so many connecting cells is that it is a
brain pathway, while that for the knee jerk includes only the lower end
of the spinal cord, where the arrangement of nerve cells is very much
simpler.
It is important for us to get clearly in mind the working of the
reflexes in order to be able to understand the more complex nervous
actions which will be described in the next chapter. We need to remember
that the ordinary way of starting nervous disturbances is from the sense
organs. With a few exceptions, to be described later, whenever a
nervous action occurs anywhere in our bodies it can be traced back,
although often very indirectly, to the sense organ from which the
disturbance originally came. The part played in this by the brain and by
what we call our mental processes will be described in the next chapter.
Before going on to that topic we have a word to say about nervous
fatigue. We mentioned the fact in Chapter VII that much of our actual
feeling of fatigue is nervous rather than muscular. Not as much is known
about nervous fatigue as about the fatigue that comes on when the
muscles are overworked. One thing that seems pretty evident is that the
place where the fatigue actually is located is in the very delicate
nerve junctions. These junctions offer some resistance to the passing of
nervous disturbances over them, and if they are compelled to submit too
often to this passing they appear to offer still more resistance; in
other words, to become fatigued. We must remember that the nerve
junctions are exceedingly delicate things, consisting, as they do, of
the interweaving of the almost inconceivably tiny featherings at the
tips of sensory or connecting nerve cells with the equally tiny
featherlike branches from the cell bodies of connecting or motor nerve
cells. It is likely, also, that on account of their delicacy they are
easily affected by the waste products that may be circulating about in
the blood stream from the active muscles. In either case, the way to
recover from nervous fatigue is simply by resting. It is not hard for
the delicate nerve junctions to throw off fatigue if given a chance. The
way to give them this chance is not to use them. As we shall see in the
next chapter, mental processes are made up of nervous disturbances
passing here and there in the brain. If we allow ourselves to be
occupied too continuously with the same lines of thought, we are
evidently sending nervous disturbances over the same nerve junctions
over and over again. In order to give those nerve junctions a chance to
rest, what we have to do is to think about something entirely different.
The word that best expresses what we have in mind is “diversion.” In the
strict sense diversion means a turning aside from what we have been
doing to something different, and that is the best way to allow the
brain to rest. The man who takes his business home with him, and dwells
on it during the hours that are supposed to be set aside for rest, may
be able to achieve more for the moment than if he were really to rest,
although even that is doubtful; but in the long run there is no doubt
that continuous efficiency depends on allowing the fatigued nerve
junctions ample opportunity to recover, which means that the thoughts
must be directed into entirely different channels.
CHAPTER XI
THE BRAIN AND COMPLEX NERVOUS ACTIONS
We have just seen that the underlying arrangement of the nervous system
is one which makes communication possible between any sense organ in the
body and any muscle; also that in the working of the nervous system
there are certain paths from given sense organs to particular muscles
which we may call the selected paths, and over which nervous
disturbances easily pass. These paths work out in such a way that the
muscles that are thrown into activity as the result of the arousing of
any sense organ are those most likely to serve the needs of the animal.
There are a number of respects in which a nervous system, having no more
than we have thus far described, would fall short of meeting the
requirements of any animal except perhaps the very simplest. The thing
that is missing that is of most importance, perhaps, is the ability to
perform actions in response to information received at some past time;
in other words, the ability to profit by experience. As the nervous
system has been described up to this point, only immediate effects upon
the sense organs are transmitted to the muscles to throw them into
activity. As a matter of fact, we know that we ourselves, as well as
many of the higher animals, are able to profit by past experience. That
is to say, we can make our muscles move in accordance with information
received at some time in the past. There must be in the nervous system,
then, some arrangement for doing this, and our present task is to
describe it. In the chapter on the distance sense organs considerable
emphasis was placed on the fact that the advantage of having distance
sense organs is to allow time for a choice among the various responses
that the animal might make to the information received by the sense
organs. It is this opportunity for choice that lies at the foundation of
the higher nervous activities that we are now beginning to describe. As
soon as the element of choice enters, the nervous disturbance cannot
pass from the sense organ to the muscle in the least possible time. In
order for there to be a choice there must be a delay while the selection
of the muscle to be aroused to activity is being made. We have, then, a
new feature in connection with the operation of distance sense organs in
that the nervous disturbance can come to a stop at some point in the
course of its progress from sense organs to muscles. Every animal that
has distance sense organs shows this feature of possible delay in the
movement of the nervous disturbance over the nervous pathway, and in
every one the particular part of the nervous system in which this occurs
is known as the brain. In fact no other nerve cells except some of the
nerve cells of the brain have this ability of stopping the nervous
disturbance and holding it for a time before sending it on.
One thing about the information that comes in through the distance sense
organs is that much of it does not require an immediate activity. For
example, a hungry fox, seeing a fowl roosting in an accessible place,
might pounce upon it at once, but if he happened at the moment to be
either fully satisfied or to be carrying a fowl which he had captured a
moment ago, there would be no occasion for him to seize this one; rather
would it be desirable for him to set it aside in his mind to be captured
in the future. In the case used in the illustration the nervous action
goes on in the beginning just as we have been picturing it; that is, the
sense organ is aroused and this in turn starts a disturbance over the
nerve path; when it comes to the brain, however, it does not go on from
there to cause immediate muscular activity, but instead is held in the
brain and can cause activity at some future time, as for example if the
fox, after taking the fowl which he is carrying home to his young, comes
back in search of the one that he saw roosting. We are perfectly
familiar with this ability to stop nervous disturbances; we know it
under the name of _memory_, although we may not have thought of memory
in exactly this sense. If we consider what memory really is, we shall
see that it is just what we have been describing, namely the stopping of
nervous disturbances that come in from the sense organs and holding
them, so that they may in turn set up at some future time a nervous
disturbance that shall produce activity. If we try to picture what
actually goes on in the nerve cells where this memory is located, we can
say only that when the nervous disturbance strikes upon these cells, it
does not pass on through at once leaving them very much as they were
before, but stops in them and brings about in them some kind of
permanent change. An interesting thing about memory is its persistence.
We may act upon memories that we have held for years. More than one
person moving away from a particular town early in life, and going back
to it in later years has remembered things he saw in childhood well
enough to find his way to them again.
Since memory is registered in the brain cells as a permanent or fairly
permanent change, it can become the source of nervous disturbance over
and over again. This in fact is one of its greatest advantages, because
when we have once learned a thing, we can make use of it a great many
times, and do not have to have the sense organs freshly aroused every
time it is desirable to use this particular bit of information.
In order that the brain cells may receive and store up memories, it is
evidently necessary that nerve paths from the sense organs should lead
into the brain; so, if we go back to the sensory nerve cells, we shall
find that in nearly every one of them, if not in every one, one of the
branches makes a nerve junction with a connecting cell, which either
extends directly into the brain, or links with other connecting cells
which do. This is true not only of the distance sense organs, but also
of the contact sense organs and to some extent of those inside the body.
The path by which the brain is reached from the sense organs that are
located in the body is over the sensory nerve cells to the spinal cord,
and along the spinal cord by way of connecting nerve cells to the brain.
All the distance sense organs are located in the head, so their nerve
paths lead into the brain directly over cranial nerves; for the eye, the
optic nerve; for the ear, the auditory nerve; and for the organ of
smell, the olfactory nerve. One very interesting fact about the
connection of the sense organs with the brain is that the nerve paths in
every case cross from the left-hand
[Illustration: DIAGRAM OF NERVE PATHWAYS
_B_, brain; _S. C._, spinal cord]
side to the right-hand side, and vice versa, so that all the sense
organs in the left half of the body have their connection with the right
half of the brain, and those in the right half of the body with the left
half of the brain. We know of no reason why this should be so. It is
merely an interesting fact. A diagram showing the path from a sense
organ in the body and from one of the head senses to the brain is given
in the accompanying figure.
[Illustration: DIAGRAM ILLUSTRATING THE GENERAL RELATIONSHIPS OF THE
PARTS OF THE BRAIN
_A_, cerebrum; _B_, cerebellum; _b_, _C_, _D_, brain stem. (From
Martin’s “Human Body”)]
The brain in man and in all the higher animals is a very complicated
organ made up of a number of different parts. To simplify the
description as much as possible we shall omit all account of smaller
subdivisions and speak only of the most important parts. These are three
in number, named the _cerebrum_, the _cerebellum_, and the _brain stem_.
Their location with reference to one another is shown herewith.
As the figure shows, the _cerebrum_ is the main part of the brain. It in
fact is the part we ordinarily think of, when we have the brain in mind.
The brain stem is really the upper extension of the spinal cord within
the head. It is a very important part of the brain, because all the
nerve pathways in from the sense organs, and from the brain out to the
muscles, pass through it. Besides that it has some activities of its
own, which will be described a little later. The _cerebellum_ is a
subdivision of the brain which plays a very important part, but is not
concerned in those complicated nervous actions which make up our mental
processes. All these are conducted wholly in the cerebrum. In fact, this
section of the brain is the only part which has the ability to stop the
progress of nervous disturbances; the property of memory, which was
described a moment ago, is found only in the cerebrum. Since the
cerebrum is the seat of memory and of our mental life in general, it is
both the most interesting and the most important part of the whole body,
and a very large amount of study has been given to it. Many years ago
attempts were made to show that the brain is subdivided into a number of
parts, each of which has control over certain mental characteristics.
Nothing very important came of these attempts, although they gave rise
to the false science of phrenology, which has been widely exploited by
fakers. Notwithstanding the failure of these early attempts at locating
particular mental activities in particular parts of the brain, recent
studies have shown that there is something of the sort, although it is
not at all what the phrenology charts would indicate. It has been shown,
however, that the various sense organs do make connections with
particular parts of the cerebrum. Comparing the pathways over which the
nervous disturbances pass to railroad tracks these places in the brain
are often spoken of as the “arrival platforms” of the various senses.
Thus the arrival platform for the sense of
[Illustration: THE BRAIN FROM THE LEFT SIDE
_Cb_, cerebrum; _Cbl_, cerebellum; _BS_, brain stem. (From Martin’s
“Human Body”)]
sight is at the back of the cerebrum down at its lower margin; the
arrival platform for the senses of touch, pain, and the like are just
about at the top of the brain. The arrival platform for the sense of
hearing is down at the side. The location of these arrival platforms has
been worked out by studying the effects of brain diseases. It has been
found, for example, that persons who are blind, although there is
nothing the matter with their eyes, are so because of a disease of the
lower back part of the cerebrum, and persons who are deaf, even though
their ears are perfect, are so because of diseases in the parts of the
cerebrum at the sides. The only way to explain these findings is by
supposing that the arrival platforms of these senses are located in the
parts that are found diseased. The cells which stop the nervous
disturbances coming in from the sense organs and so serve as the seat of
memories of sensations are in the arrival platforms, so that when we
recall how something looked, for example, the nerve cells which are
active in this recollection are those at the lower back part of the
cerebrum. All these arrival platforms taken together occupy only a very
small fraction of the whole of the cerebrum, so that evidently it has
more to do than simply to register these memories.
One additional thing we shall look for in the cerebrum is a pathway from
the brain to the muscles whereby the memories that are stored in the
brain can make themselves effective in arousing the muscles to activity.
There is such a pathway, and it is one of the best marked of all the
nerve pathways of the body. It is made up of nerve cells which start
just about at the top of the cerebrum, a little in front of the middle
and pass down into the brain stem; these cross over from the side in
which they started to the opposite side; and go on down the spinal cord
to make connection by nerve junctions directly with the motor nerve
cells. Thus from the brain to the muscles the nerve pathway is made up
of just two nerve cells. Since this pathway crosses over from one side
to the other in the brain stem, the whole right half of the body, both
sensory and motor, connects with the left half of the brain, and vice
versa. The part of the cerebrum in which this nerve pathway starts is
called the _motor area_ of the brain.
We have now seen how nervous disturbances from the various sense organs
can come into the brain and be registered there as memories. We have
also seen that there is a nerve pathway by which nervous disturbances
starting in the motor area can pass out to the muscles, and arouse them
to activity. We have left to see how the connections are made between
the incoming and the outgoing disturbances, or in
[Illustration: DIAGRAM OF MOTOR PATHWAY
_B_, brain; _S. C._, spinal cord; _M_, muscle]
other words to see how the memories that are registered in the brain act
upon the outgoing pathways. We shall expect to find connecting nerve
cells reaching across from the various arrival platforms to the motor
area, and such connecting cells exist, but they are not simple and
direct connections for a reason which we shall now try to make clear.
We all know from our own experience that our memories are never the pure
registering of a single sense. What we mean is that the sight of
something by itself or the sound of something by itself never remains as
a separate memory, but is always worked in with some other memories from
some of the other sense organs. This putting together of the simple
sensory memories began in earliest childhood, long before we were old
enough to think about our mental processes, and see how they are carried
on, so that unless our attention has been called to it, we have probably
failed to realize how complicated our simplest memories are. A good
example of this is in the experience of a baby with its mother’s voice.
So far as the baby is concerned, the voice is an influence affecting the
organ of hearing, arousing nervous disturbances which pass to the
arrival platform for hearing in the brain and are registered there as
memories, but it does not take the infant long to learn that some other
memories that have come to it by way of other sense organs belong with
this particular memory and always go with it. For example there is the
sight of the mother, or the feel of the mother’s face and hands, all
these are influences affecting different sense organs and registering in
the child’s brain in different arrival platforms. Yet within his brain
they become fused into a single composite memory of the mother, and
after this fusion has once occurred, the arousing of any part of the
memory brings up the whole of it, so that the child may hear the
mother’s voice from the next room, but the memory that will be aroused
as the result will be not simply of the voice but of the mother as a
whole. This is an illustration of how our pure sense memories are fused
into complex memories. After one of these memory complexes is once
started, we add to it any time any sense organ is acted upon by anything
that has relationship to the complex. For example, the child’s idea of
the mother at first is a very simple one made up of a few sensory
impressions, but as time goes on and more and more sensory impressions
of the mother are received, the child’s idea of her becomes more and
more complex. This process of memory fusion is called association or
sometimes association of ideas. Strictly it is an association of
memories and this is the method by which all our mental activities are
carried on. The thought process consists of putting together various
memories in various ways and so building up associations of different
kinds. Of course adults of wide experience can form associations which
are made of literally hundreds or perhaps thousands of separate memories
of sense impressions. We receive sense impressions at the rate of
hundreds every day, and very many of these, perhaps all of them, are
registered as memories and are fitted into their proper associations.
The nervous machinery for carrying on different processes of association
consists of nerve cells of the brain. These are all of the kind known as
connecting cells. It will be remembered that the connecting cells may
have many nerve junctions leading into them and they in turn are much
branched so that many nerve junctions can lead out from them. The
connecting cells of the brain are richer in these respects than those in
any other part of the body, and the cells of the human brain than the
brain of any of the lower animals. We suppose that the associations are
formed by the passage of nervous disturbances from the arrival
platforms where the sense memories are registered over various
connecting nerve cells to a common meeting point in some cell or in some
group of cells, where the associated memories are all brought together
into a single memory complex. It is supposed that the large areas of the
brain which are not taken up either by the arrival platforms or by the
motor areas are the regions in which these associations take place.
There is one particular group of associations that are so interesting as
to call for special mention. These are the associations concerned with
language. We are so in the habit of using language that we are likely
never to have thought of it in its real meaning as a part of our mental
activity. We know that we have two kinds of language, spoken and
written, and that spoken language consists of certain sounds, and
written language of certain visual symbols. Two of the distance senses
then are concerned, sight for written language, hearing for spoken, and
language itself consists simply of sense impressions coming in through
one or the other of these sensory channels. The important feature of
language is that mankind has selected arbitrarily certain sounds or
certain written symbols to stand for particular things. When a child is
learning to understand what is said, what he is really doing is fitting
a particular set of arbitrary sense impressions into their proper places
in his associations. We can illustrate this by the same example that was
used a moment ago. The child becomes thoroughly familiar with its
mother, so far as sense impressions are concerned that come directly
from her. In course of time it adds to the associations thus formed an
additional one made up of the sound of the word “mother.” Of course
there is no particular reason why this sound should have that meaning
rather than any other. The proof is that different languages have
different sounds which stand for the same thing. After this sound has
once been selected and learned, it becomes as much a part of the idea of
mother as any other of the associations concerned, and thereafter,
whenever that particular sound strikes upon the child’s ear, the
association of mother is aroused. Precisely the same sort of thing is
true of written language. Arbitrarily selected symbols act through the
sense of sight to arouse nervous disturbances, which are built into
particular associations, and here again it makes no particular
difference what the symbol is; one will do as well as another, provided
a number of people have agreed to use that symbol to stand for the same
thing. One very useful feature of language is that we can make a single
word stand for a complex group of sensory impressions or even
associations. To illustrate, the word physiology, whether spoken or
written, is in itself simply a sound or a visible symbol, but it stands
for a highly complex group of associations in the human mind. In this
respect language is a kind of shorthand.
It is hard to overestimate the importance of language to the human race.
This is because of the enormous extension it gives to the ability to
profit by experience. The lower animals are able to profit by
experience, but the only experience that they can get is that which
comes to them individually or to another animal which they may observe.
Human beings, on the other hand, may profit by the experience of their
contemporaries through learning of them by word of mouth, or may profit
by the experience of present or past generations as recorded in
writing. It is not too much to say that the progress of civilization
depends on our ability to profit by the experience of past generations
as made known to us in writing. We are further advanced than our remote
ancestors, not because we are actually superior to them in mental power,
but because we have a much larger background of experience than they
had, and this background depends altogether on our ability to use
language.
The complex nervous activities that have been described up to this point
are purely a matter of the association of memories, or what we speak of
as intellectual processes. Besides these our mental life includes what
we call the feelings or emotions, and these make up so large a part of
our mental life, that we cannot leave the consideration of the brain
without saying a word about them. For convenience we may subdivide the
feelings into the two classes of agreeable and disagreeable. If we try
to get at the meaning of these, we can perceive that the agreeable
feelings are fundamentally associated with one of two things: either a
condition in which bodily well-being is assured, or conditions
associated with the perpetuation of the race. An example of this latter
is the pleasure that parents find in the care of children. The
disagreeable feelings are fundamentally concerned with the immediate
preservation of the body from injury. Thus fear and anger are two of our
most disagreeable emotions, and both of these are aroused ordinarily
only when danger threatens or when bodily well-being is otherwise
interfered with. One other disagreeable emotion, worry or anxiety, is
interesting in that it applies to ill-being that is expected in the
future, and while it is well to be on guard against trouble, this
emotion is really for the most part futile, because the expected
ill-being commonly fails to materialize. Not only do the feelings share
with the intellectual processes in making up our mental life as a whole,
but they also show themselves in certain bodily changes, which are so
important that we shall return to them at greater length in the next
chapter.
The feelings as well as the intellectual processes are made up of
nervous disturbances passing over the brain cells. We do not know
whether those which are concerned with the emotions are in different
parts of the brain from those that are concerned with the simple
associations or not. There are some differences between the two kinds of
nervous activity; particularly is this true of the vividness of the
memories concerned. Every intellectual process, presumably, results in
the formation of some associations that were not previously present, and
the memory of the new associations should be and usually is reasonably
sharp. Emotions on the other hand, although they may be remembered, do
not register themselves in such a way that a new emotion is aroused
every time the old one is remembered. It is probably true that for a
while after any strong emotion the recollection of it will tend to bring
up the emotional state again, but in course of time there is a dulling,
so that although we may remember clearly the occasion of the emotion,
the feelings are no longer actively aroused.
CHAPTER XII
SOME SPECIAL NERVOUS ACTIONS; SMOOTH MUSCLE AND GLAND CONTROL
In the last two chapters we have talked about the arrangement of the
nervous system and its working as seen in simple reflex actions, and the
special activities of the brain which make up the intellectual processes
and the emotions. Before we can leave the subject of nervous activity,
there are some special kinds of reflexes to be spoken of, and there are
also some additional things to say about the working of the emotions.
When we speak of special reflex actions we have in mind some of the most
useful of our own reflexes. They are not different from the simple
reflexes that are described in Chapter X, except that they involve a
larger number of sense organs and many muscles, and are correspondingly
more complicated. Furthermore, on account of this complication a special
part of the nervous system is devoted to carrying them on. Back in
Chapter VIII, under the heading of motion, the different types of bodily
movement were listed. Chief of these was the act of locomotion; that is,
the means by which we get from one place to another, an act which is, of
course, of prime importance in maintaining life, since it is concerned
both with the securing of food and the escaping of harm. Locomotion is
performed in a number of different ways; in ourselves, walking, running,
leaping, swimming, riding a bicycle are all forms of locomotion. Any of
these acts, as stated in Chapter VIII, requires the accurate timing and
accurate grading of the contractions of a great many different muscles.
As we saw in that same chapter there are special sense organs which
furnish the information upon which this accurate control of the muscles
of locomotion is based. We have now to consider the part played by the
nervous system in carrying it on. Whenever one starts to walk or to
perform any of the other acts of locomotion, nervous disturbances will
be set up in great quantity from the organs of muscle and joint sense in
the parts of the body that are active, and also from the organs of
equilibrium in the ear. It is necessary that the whole volume of nervous
disturbance that is pouring into the central nervous system from these
sources be translated into a series of streams of disturbances passing
out over the motor nerves to cause the muscles to work just as they
should in performing the act. All this is reflex in the strict sense.
That is to say, the muscular action follows the arousing of the sense
organs with no more delay than is necessary for the passage of the
nervous disturbance. The work of transforming the incoming stream into a
suitable outgoing stream is performed by a part of the brain about which
little has been said thus far, namely the _cerebellum_. This is
connected with other parts of the central nervous system in such a way
that pathways from organs of equilibrium and from the organs of muscle
and joint sense lead into it. From it in turn come a great many
connecting nerve cells whose axons pass along the spinal cord and form
nerve junctions with the motor nerve cells; and so nervous disturbances
coming from the cerebellum can arouse the muscles to activity. We all
know that walking or any other locomotor act, after it has once been
learned, is carried on successfully without our performing any
intellectual work in connection with it. What we do have to do is to
start the act, and this is done through a discharge from the cerebrum;
either some sense organ is aroused and sends a disturbance into the
cerebrum, which arouses in us an association which indicates the
necessity for going somewhere, or else a pure memory not stimulated from
outside has the same effect. In either case the nervous disturbance
passes over from the cell where the association resides to the motor
area, and from there down to start the act of walking; but as soon as it
is started, the reflex takes hold and carries it on without any further
thought on our part except as some additional associations enter to
cause us to change or perhaps to stop the action. It is possible that
the complex muscular acts which we group under the head of grasping are
operated in similar fashion. For example, the act of writing is
essentially a modified grasping movement. When we are writing we think
not about the muscular movements but about the sense of what is being
written, and it may be that the actual control of the muscles in this
case is something like that which we know exists in locomotion, although
this is by no means certain. It is of course a very great advantage to
us to have these complicated activities, which we are performing so
constantly, carried on as reflexes without conscious effort on our part.
If we had to think about the movement of every muscle, whenever we
started to walk, our minds would be so taken up with the act itself that
there would be no freedom for anything else; but since the reflex
machinery takes care of these muscular movements for us, our minds are
left free to deal with other matters at the same time as the muscular
actions are going on.
Not only is the simple act of walking carried on for us without mental
effort on our part, but to a very large extent the guidance through the
sense of sight becomes reflex also. Thus, as we go along the street, we
step down from or up on to the curb at the crossings without noticing
particularly what we are doing. The approach of the curb is registered
through the eyes, and the sensory disturbances thus set up are carried
into the brain and intermingled with those from the organs of muscle
sense and equilibrium, so that the motions are adjusted to carry us
safely down or up. This reflex guidance through the eye becomes so
perfect that one who habitually walks over a certain course, as a man in
going from his home to his place of business, ordinarily goes the entire
distance with scarcely any conscious attention to what he is doing; in
fact, persons who move from one locality to another frequently, when
they start to walk home, unless they pay attention, find themselves
going to the old home instead of the new. The reflex machinery which
guided them to their old home works so well that only by conscious
interference with it are they able to direct themselves to the new one.
Whenever an act of locomotion is learned, it is carried on at first by
nervous disturbances from the _cerebrum_, just as are muscular actions
of all sorts; but, as time goes on, there seems to be a transference of
the control from the _cerebrum_ to the _cerebellum_, and as this happens
the act is more accurately performed. We have an amusing illustration of
this in persons who are learning to ride a bicycle. So long as they are
making movements of the handle bars and at the same time pedaling by
pure acts of the will, they proceed with the very greatest difficulty;
after these actions become habitual, they are carried on easily and
skillfully. The great effort of athletic trainers is to develop what is
called “form.” What this really means is that they try to teach the
athlete the best way of making the desired motions, and then drill him
in that particular way, so that it becomes completely reflex with him.
The more fully this is done, the better will the athlete perform.
All that has been said about the nervous system thus far has applied to
its duty of controlling the activities of skeletal muscles; namely, the
muscles by which the body carries on its principal motions. These, as
already stated in Chapter VII, are not the only kind of muscles in the
body. There are also the smooth muscles and heart muscle. There is also
another kind of functional metabolism besides the metabolism of muscular
motion over which the nerves have control. This is the metabolism of
secretion, or the manufacture of special chemical substances by gland
cells. In later sections of the book we shall hear more about what the
glands do, but here we have to say something about the way they are
controlled through the nervous system. We speak of them along with the
smooth muscles, because it happens that the same nervous arrangements
are found in both cases; the smooth muscles and the glands have a
precisely similar kind of nervous control. In appearance the nerves
which lead from the central nervous system to the smooth muscles and the
glands do not differ much from other nerves. They are composed of nerve
cells, just as are all the other nerve pathways. It happens to be true
that the sheaths which inclose the axons of these particular nerves
usually have little or no white material in them, so that nerve trunks
which are made up of these fibers look gray instead of white, as do the
other nerves. Just why there should be this difference, we do not know.
Of more importance than this difference in appearance is the fact that
the connection from the central nervous system to the smooth muscle cell
and the gland cell in no case is made up of a single nerve cell, but
always of a chain of two. In the case of nerve paths to skeletal muscles
the cell body of the motor nerve cell lies either in the spinal cord or
the brain and its axon extends all the way from there directly to the
muscle fibers. In the case of smooth muscles and glands a connecting
nerve cell extends out from the central nervous system some distance and
makes its nerve junction with the motor nerve cell part way between the
central nervous system and the muscle or gland. These connecting cells
are much branched, as are all connecting cells; also the motor cells
which connect with smooth muscles or glands are very much branched; the
result is that a single nerve cell starting from the spinal cord finally
connects with a large number of smooth muscle cells or gland cells. We
do not have, then, in these organs the fine subdivision of control that
there is in the skeletal muscles, for a single skeletal motor cell in
the spinal cord connects with one or, at the most, a very few muscle
fibers. If we consider the uses to which smooth muscles are put, we
shall see that finely subdivided control is not important, as it is in
the case of the skeletal muscles, where the movements have to be graded
very accurately. Smooth muscles are present in the stomach and
intestines and other abdominal organs which make movements. They are
found also in the walls of the small blood vessels where their
contractions or relaxations can change the caliber of the vessels, and
so affect the amount of blood passing through them. A good example of
this action is seen in the color of the face; when one is pale, it is
because the small blood vessels just under the skin of the face are much
contracted, and very little blood can pass through; when flushed, on the
other hand, the blood vessels are relaxed and there is a large amount of
blood passing. These contractions and relaxations of the blood vessels
are performed by movements of the smooth muscles. Another interesting
place where there are smooth muscles is at the roots of the hairs.
Everybody knows that the hair is supposed to stand on end when one is
frightened. Most of us have thought of this as a picturesque figure of
speech, but as a matter of fact it actually does happen. A human being
has such a small equipment of hair that its rising on end does not make
much of a showing, but in an angry dog or frightened cat we see it in
all its glory. If one looks at the forearm when it is so chilly that
“goose-fleshing” occurs, he sees his hair then standing clearly on end.
In fact, goose-fleshing is caused by contractions of these smooth
muscles just under the skin. Every hair has a few smooth muscle cells at
its base and they are attached in such a way that, when they contract,
the hair is pulled from the slanting to the erect position. The surface
of the skin acts as the fulcrum on which the motion is made. Besides
these places where smooth muscle is found there are some in the eye. The
muscles of accommodation and also those by which the size of the pupil
is changed are smooth muscles. There are some also in the small
bronchial passages. In persons who suffer from asthma these smooth
muscles become contracted, and the difficulty in breathing experienced
is the result of the partial closing of the air passages caused by this
contraction. Glands which are under the control of the same kind of
nerves as control the smooth muscles are found in many places in the
body; in the head we have the tear glands and the salivary glands; in
the trunk there are the gastric glands, the pancreatic, and various
others about which we shall hear later. With the single exception of the
muscles of accommodation there is no case either among the smooth
muscles or the glands where there is any occasion for a finely
subdivided control. When the muscles of the stomach or intestines become
active, a great many fibers work together. When the hair stands on end,
a great many hairs become affected at once. A flushing or paling of the
skin is also a matter of a large surface. The same is true of gland
secretion. In an active gland there is no picking out of a few cells to
do the secretion, but a large part of the gland, if not the whole of it,
becomes active, whenever any of it is. The advantage of the construction
of the nerve paths from the spinal cord to the smooth muscles or to the
glands is that a single nervous disturbance from the central nervous
system can affect many muscle or gland cells all at once. In the case of
the skeletal muscles, on the other hand, the only way a great many
fibers can be made active at once, is by sending out nervous
disturbances through a great many nerve cells. There is undoubted
economy in having the control of the smooth muscles and glands carried
on by means of comparatively few discharges from the central nervous
system.
A peculiarity of the structures that we are talking about now is that we
have very little voluntary control over them. This contrasts sharply
with the completeness of our control over the activities of our skeletal
muscles. With very few exceptions we can start, stop, or regulate
skeletal muscle activities at pleasure, but the activities of our smooth
muscles and of our glands are not under our control in the same way. We
cannot cause ourselves to turn pale by an act of the will, nor can we
set the stomach to working when we wish. This latter inability is a real
misfortune, for there are many cases of acute indigestion that are due
to stagnation of the stomach, and if we could set the stomach into
operation at will, these would be avoided. Since we have not this
voluntary control, it is of the utmost importance for us to learn how
the control is really carried on, so that the motions necessary to
health can be induced in a normal manner.
To a very large extent the smooth muscles and glands are subject to
reflex control; that is, nervous disturbances coming in from the sense
organs pass over by way of connecting nerve cells to the nerve pathways
leading out to the smooth muscles or to the glands. Examples of this
kind of control are seen in the paling of the skin, when one is chilly,
or its flushing, when one is warm; in the copious secretion of saliva
into the mouth that one can get by chewing a rubber band; or in the
secretion of tears that comes when the front of the eye is hit.
A second kind of control of the smooth muscles and glands is their
control through the emotions. We have already given illustrations of
this in the paling of the skin when one is frightened, or the erection
of the hairs in the tail of an angry cat. For a long time it has been
known that most of our emotions, if not all of them, are accompanied by
some sort of bodily change; laughter and crying are familiar
illustrations. To a certain extent, as in these instances, skeletal
muscles share in the bodily effects of emotions, but their most
important examples are found in the smooth muscles and glands. Every
serious display of emotion is accompanied by a discharge of nervous
disturbances into some of the smooth muscles or some of the glands. This
fact has long been known, but its meaning has become clear only very
recently. Before attempting to explain this, an additional word will be
necessary about the nervous control of smooth muscles. These muscles
differ from skeletal muscles in one important particular, which is this:
when a skeletal muscle is aroused to contraction, it will relax as soon
as the stream of nervous disturbances stops. To keep it in contraction a
continuous stream must come into it through the nervous system. This is
not true of smooth muscle. When a smooth muscle has been caused to
contract, it is very likely to stay contracted without further action on
the part of the nervous system. In fact, in very many cases another kind
of nervous disturbance must be sent into it in order to make it relax.
This means that the smooth muscles, or at least most of them, have two
sets of nerves coming to them; one set which causes them to contract,
the other set which causes them to relax. Both these sets of nerves are
of the type described above as supplying the smooth muscles and glands.
A good illustration of the double action is found in the stomach. There
is one set of nerves whose activity causes the walls of the stomach to
perform the churning motions which go on ordinarily during digestion.
There is another set of nerves whose effect upon the stomach is to cause
all motions in it to stop. This same double arrangement of nerves is
found in many of our most important smooth muscle organs. We said a
moment ago that every emotional display is accompanied by an outflow of
nervous disturbances to the smooth muscle or gland structures. We now
see that what happens in the body is determined by which of the possible
pathways is followed by the nervous disturbance. One turns pale, when
frightened, because in the emotion of fright the nervous discharge is
over the nerves that cause the blood vessels of the face to contract.
The mouth waters at the sight of delicious food, because the pleasurable
emotions aroused thereby cause the nerves to the salivary glands to
become active. As was said a moment ago, for a long time there seemed to
be no rhyme or reason about these emotional reactions. Why, for example,
should worry cause the muscles of the stomach to stop completely, thus
bringing on thousands of attacks of acute indigestion? We now have a
clue to the meaning of these facts, which is about as follows: The
emotions are to be looked upon as part of the machinery by which bodily
well-being is promoted. Emotions of pleasure signify that matters are
progressing favorably, and the nervous discharges accompanying these
emotions are those in accord with well-being. For example, the
activities of the alimentary tract in digestion are of a kind that
prosper when the body as a whole is well off. Disagreeable emotions, on
the other hand, are associated with the necessity for self-preservation.
Thus anger or fear arise in the presence of an emergency calling either
for combat or flight. The nervous discharges that accompany these
emotions are found on analysis to bring about bodily changes whose
effect is to prepare the body successfully for one or the other. This is
best shown by actually naming over some of the bodily changes that
occur. We may begin with the top of the head and work down. The hair
stands on end; this is of no importance in human beings, but in many
animals contributes materially to the ferocity of the appearance. The
pupils are dilated, aiding vision; the face is pale, and the mouth is
dry; not because these in themselves are of importance but because they
represent a diversion of energy from unimportant to important parts of
the body. The blood that ordinarily flows through the face is shunted
out of it into the brain and muscles, where it will really help in the
emergency. The activity of the salivary glands is stopped to free the
energy that they would consume for use in the brain or muscles. The
heart beat is quickened; this again, by making the circulation more
efficient, helps the animal to fight effectively. The activity of the
stomach is suspended for the same reason as is that of the salivary
glands. What we have is a marshaling of the bodily forces into the
organs which are of immediate importance in meeting the emergency;
namely, the nervous system and the muscles. The other parts of the body,
which are necessary in the long run, are allowed to stop functioning for
the moment. This behavior of the body in an emergency illustrates better
than anything else could the point that our muscles, nerves, and sense
organs make up the really important parts of us and the rest of the body
has simply the duty of keeping these in good order. In time of
emergency all the energies are devoted to the task of keeping these
parts of us at highest efficiency, however bad may be the effect on the
other less necessary organs; so it is that one who worries himself into
an attack of indigestion is really preparing for an emergency by
sacrificing the less essential for the more essential activities. The
practical difficulty in this is that the worry almost always fails to be
of use, either because the expected emergency does not arise at all or
else arises so far in the future that the immediate bodily changes
caused by the worry do no good. Evidently worry is an emotion to be
combated. The old adage about not crossing the bridge until it is
reached has really a great deal more to do with the preservation of
health than most of us realize.
We have described the control of smooth muscles and glands through
reflexes and in connection with the emotions. We have still to mention
briefly a third way in which nervous disturbances leading to these
organs may be originated. In the last chapter the brain was described as
consisting of three chief parts: the cerebrum, which is the seat of
memory and association, and so of all thought processes; the cerebellum,
which governs the reflex of locomotion; and the brain stem about which
nothing was said except that it is a region through which all nervous
disturbances have to pass on their way into or out from the brain. The
brain stem has a special function of its own in connection with the
control of what are often called the “vital processes.” Since most of
these vital processes are concerned with smooth muscle or gland action
they are of interest to us here. In the brain stem are located groups
of nerve cells to which are given the name of “centers.” From these
extend nervous pathways to various bodily structures. For example, there
are two centers having to do with the activity of the heart, and both of
these have nerve pathways leading to that organ. Over one of them pass
the discharges which cause the heart to beat more rapidly, over the
other the discharges which slow it down. There are also centers for
controlling the caliber of the various small blood vessels and so the
amount of blood that flows through the various organs of the body. Still
another center has control of the secretion of sweat. These are a few of
the centers which are present in the brain stem. It is evident that
these centers must be played upon directly from the sense organs in the
reflex control of the smooth muscles and glands and also from the
cerebrum in their emotional control. The third way in which these
centers can be affected is by a direct action of the blood which
circulates through them. Of course, every center is made up of nerve
cells which are alive and which share with all other living cells the
necessity of being sufficiently nourished. This means that there must be
small blood vessels here and there among the nerve cells, so that
interchanges of material can take place between the blood and the fluid
immediately surrounding the cells. Among the materials which may come
from the blood are some special chemical substances which have the
ability to arouse these cells to activity. For example, the very rapid
beating of the heart following vigorous muscular exercise is due, in
part at least, to the presence of chemical substances that are poured
out into the blood from the laboring muscles and which have the effect
of arousing the center whose action speeds up the heart. In at least
one case, that of the center controlling the sweat glands, the nerve
cells are aroused by an increase in temperature; on a very hot day or
when one exercises briskly the temperature of the blood begins to go up;
this arouses the center which controls the secretion of sweat; nervous
disturbances are poured out to the sweat glands and they become active.
The effect of their activity is to remove heat from the body, and so
help to lower its temperature. Here we have an example of an automatic
regulating device in which nerve cells are controlled through the blood.
Except for this one instance of the effect of blood temperature the
actions of this class are thought to be wholly by means of chemical
substances, and for that reason we speak commonly of the third method of
control of the smooth muscles and glands as chemical control. We have
then altogether three methods of control; through reflexes, through the
emotions, and through chemical substances in the blood.
Since we have spoken of chemical control it will be appropriate to close
this chapter with just a further word about it. In addition to the
regulation of smooth muscle and gland activity there are a number of
other bodily processes which are affected more or less through chemical
agents brought to them by the blood. Many of these processes are of a
kind that cannot well be controlled through the nervous system. For
example, growth is something which goes on very slowly and yet which
distinctly requires some kind of control. It has not perhaps occurred to
us that it is more than chance that our ears are substantially the same
size, our arms about the same length, our legs the same length, our
feet about the same size, and so on. Yet if we pause a moment we realize
that as the ears grow there would seem to be no particular reason why
they should both stop when they have reached the same size, unless there
is some definite regulation. This regulation is present in the form of
chemical substances. Of late years a very large amount of study has been
given to the materials which have the power of acting as chemical
regulators, and a good deal has been learned about them, although there
is every reason to believe that a great deal more remains to be found
out. It has been necessary to invent a name for substances which act as
chemical regulators, because, without such a name, talking about them
becomes too cumbersome. The name that has been adopted is _hormone_,
from a Greek word meaning to arouse or to stimulate, referring to the
ability these substances have of acting upon living cells. Thus far we
have spoken of hormones only in connection with their ability to excite
those nerve cells by which smooth muscles and glands are controlled. As
a matter of fact, their action is much wider than that. They have the
power of affecting the metabolism of very many body cells. Some of them
appear to have the special function of regulating the rate of
metabolism. These we shall talk about in detail, when we return to the
study of metabolism in a later chapter. One of the hormones is
interesting in connection with the relation of emotions to smooth muscle
and gland activity. This is a secretion manufactured by a pair of small
glands located in the abdomen near the kidneys. They are known as the
_adrenal bodies_. These glands secrete a substance to which has been
given the name of adrenalin, which has been shown to be a very
efficient chemical excitant to a number of bodily processes. We are
particularly interested here in a property it has of arousing smooth
muscles and glands in precisely the same way as they are aroused in
connection with the disagreeable emotions, such as fright or anger. For
example the injection of a little of this adrenalin into the veins of a
person or animal will cause his heart beat to be quickened, the blood
flow into the brain and the muscles to be increased at the expense of
the flow through the skin and abdominal organs, and the other effects to
occur that were described above as accompanying these emotions. These
facts about the effects of adrenalin had been known for some time before
it was realized that they fit in with the reaction by which the body
prepares itself to operate efficiently in time of stress. Now we have
learned to look upon adrenalin as the “emergency hormone,” meaning that
it is a chemical substance which does the same things to the body that
are done through the emotions in time of emergency. We look upon this as
a reenforcement of the nervous action, indicating the efficiency with
which the fitting of the body for time of stress works out. The adrenal
bodies are themselves glands and are acted upon like other glands
through the part of the nervous system that we have just been
describing. In time of emergency a stream of nervous discharges pours
into them; they secrete an abundant supply of adrenalin which passes out
into the blood and is carried by the blood to all parts of the body,
exerting its special functions wherever it comes. There is no doubt that
the ability of the body to care for itself in time of need is helped by
the outpouring of adrenalin. In one respect, however, this arrangement
does not work out perfectly. This is because chemical substances that
are poured out into the blood cannot be gotten rid of instantly, and are
likely to linger for a shorter or longer time after the actual emergency
subsides, so the bodily effects persist for a time after the occasion
for them has disappeared. We are all familiar with the fact, for
example, that the heart goes on thumping for a long time after a sudden
shock. It may not have occurred to us to wonder what kept it going at
that rate after the immediate disturbance was passed, but we now see
that it must do so until the adrenalin which was poured out in
connection with the shock has been gotten rid of, which is a more or
less gradual process.
CHAPTER XIII
THE BODY FLUIDS
We have learned to think of the cells which make up the body as
dependent on the fluid which surrounds them for the various materials
they require, and as a place into which they discharge the products of
their metabolism. We have seen furthermore that the fluids which bathe
the cells directly must be constantly renewed. The renewal is
accomplished by interchanges between this fluid and the blood, which
constantly flows through the tiny blood vessels that are everywhere
present in the body. In its course, in turn, it passes through the blood
vessels of the organs in which it is to be itself renewed; the digestive
organs for food supplies, the lungs for oxygen, the kidneys for the
discharge of waste material. We must now look further into the nature
and action of the various body fluids. Of course the foundation of all
of them is water. In this water must be dissolved everything that is
used by any of the cells for food or anything that any of these cells
produces. Under this latter head we have the waste products of ordinary
metabolism, or in the case of some cells special products of functional
metabolism. The presence of all these various materials would be bound
to make the body fluid an extremely complex mixture. In addition to
these various materials there are certain substances present besides
water to make up what we may call the structure of the body fluids as
distinguished from the materials which they are carrying from one place
to another. These structural materials include a number of salts of
which ordinary table salt (sodium chloride) is the most abundant as well
as the most familiar. In addition there are salts of lime and potash and
magnesium; all these latter in very small proportions. Just why the body
fluids should contain these salts is not very clear. We know that if
they were not present the cells of our bodies could not live, yet it is
true that there are a great many kinds of living cells that get along
perfectly well in fresh water, which may have no salts dissolved in it
at all. On the other hand there are more kinds living in the ocean and
exposed to the rather strong concentration of salts which make up ocean
water, and there are even some kinds of animals that live in strong
brine, so that evidently living protoplasm can adjust itself to
surroundings in which the strength of the salt in solution is widely
different. It is true that very few kinds of animals can endure being
changed suddenly from ocean water to fresh water or the reverse. One of
the best ways to clean the bottom of a ship that has become foul through
long sailing about the sea is to transfer it into a fresh water lake or
stream, where the accumulation of living animals and plants will be
killed and will drop off. Most kinds of ocean animals die rather
promptly if changed to fresh water, or fresh water animals if put into
the ocean. There are a few kinds of fish, like the salmon and shad,
which live in the ocean but lay their eggs in rivers, and these are able
to endure the change from the one kind of water to the other without
being destroyed. They, to be sure, make the transition gradually,
swimming up from the ocean into water that is less and less salty, until
they finally reach the fresh water stream itself. There are other kinds
of living things which can endure a much more abrupt change from salt to
fresh water and back again. In various parts of the world are large
rivers emptying into the ocean and so situated relative to towns and
cities that steamers make regular round trips from the ocean up to the
fresh water of the river and back again, and it will be found that on
the bottoms of these steamers are various plants and water animals which
endure the frequent shift from salt water to fresh and back again
without harm.
The percentage of salt in the body fluids of all the higher animals
including ourselves is only about one-fifth that in the ocean.
Furthermore the fluids of most kinds of land animals have about the same
percentage of salts dissolved in them. Naturally there has been much
speculation as to why there should be this percentage of salt in
preference to any other. One ingenious theory supposes that back in the
beginning of things, when the earth cooled down below the boiling point
of water, so that it was possible for water to collect on the earth, the
water in all the oceans was fresh. This would have to be true, since
water must have fallen in the form of rain; but in course of time some
of the salts in the earth’s crust would be dissolved, making the water
salty, and as time went on the ocean would become saltier and saltier.
This is still happening, for every river that discharges into the ocean
carries with it materials that it has dissolved from the underlying soil
during its passage from its source, and such material, when it once
enters the ocean, must stay there until the ocean water becomes
saturated with that particular substance. According to this theory there
was a time, then, when the ocean water was just about as salty as our
body fluids at the present time, and it supposes also that that was the
time when the ancestors of present land animals crawled out of the ocean
and took up their abode on land. Of course there is no way to prove that
this is so, but it does account for the particular percentage of salts
in our bodies as well as any other explanation we know anything about.
In addition to these various salts our body fluids contain in solution
moderate amounts of very complex chemical substances belonging to the
class of proteins. A fact about proteins which has not yet been
emphasized is that they make liquids in which they are dissolved sticky
or gelatinous. An excellent example of this is ordinary raw white of
egg, which is a solution of protein in water, and which shows the
gelatinous character very strikingly. Because of the protein that is in
solution in the body fluids, they have also this gelatinous character,
although to a much less extent than in the white of egg, because the
amount of protein in solution is so much less. As we shall show later,
this sticky quality of the fluid is of a good deal of importance in its
actual use in the body. It may be that the protein in the body fluids
serves other purposes as well. One interesting fact that needs to be
emphasized is that it is not used for fuel or building material for the
cells. The protein that comes to them as part of their food supply is
entirely distinct from that about which we are now talking, which is
part of the permanent structure of the body fluids. We shall speak of
the protein that serves to nourish the cells in a moment, when we are
talking about the relations of the fluids to the transportation of food
material.
In addition to the salts and proteins we have also dissolved a great
many very complex materials which may be looked upon as permanent or
relatively permanent constituents of the fluid, but about which we know
practically nothing chemically. We are sure that they are present,
because of certain effects which they produce, but the substances
themselves have never been made out by chemical analysis. These are
materials which are concerned with the resistance of the body to
infectious disease, and it will be necessary to say just a word about
infection to make clear the part played by them.
What we call an infection is the invasion of the body from the outside
by minute living organisms, either plant or animal, and the establishing
of them within the body, so that they grow and multiply. They carry on
their metabolism just as do all other living cells and produce various
chemical products as a result. There are many organisms living within
our bodies whose metabolic products apparently do us no harm, and so we
serve as hosts for these unbidden guests year in and year out without
even knowing of their existence. The products from other kinds of
organisms are poisonous to us, and when some of these organisms multiply
within us we discover it, because we are poisoned and become ill. Only
organisms whose products of metabolism are poisonous are counted
ordinarily as causing infection. In the strict sense we might be said to
be infected by the harmless organisms of which so many thrive within us,
but in the usual use of the word we speak of one as having an infection
only when his body has been invaded by organisms that produce poison. In
the case of most kinds of injurious organisms their multiplication
within the body unchecked would lead finally to its destruction. It is
necessary, therefore, that the body have some means either of checking
the development of the organisms or of neutralizing the poisons which
they produce. The body has this power, and the machinery for it consists
very largely of substances or structures in the body fluids. The
detailed story of these is too complex to be told here. We shall content
ourselves by saying that when an infection becomes established, as of
scarlet fever, for example, the poisons that are produced are poured out
by the organisms into the body fluids of the part where they happen to
be located, and are taken up from there by the blood and distributed all
over the body. The fever, headache, and other disagreeable symptoms are
due to these poisons. The interesting thing about it is that the poisons
themselves act toward the body as chemical regulators or hormones,
exciting some or perhaps all of the cells of the body to a special kind
of functional metabolism, which results in the manufacture of materials
which neutralize the poisons. Thus, one of the very important properties
of living protoplasm is to respond to the poisons from the metabolism of
other cells by producing neutralizing material. Whether one dies from an
infection or recovers from it depends on whether the cells are able to
produce enough neutralizing material to prevent themselves from being
killed or whether the poison is so abundant or so malignant that the
cells are destroyed in spite of their activity in pouring out the
neutralizing material.
Still another interesting thing about this whole matter is that every
kind of infecting organism has its own kind of poison, which differs
from that of the other kinds, and so the chemical effect of the poison
upon the cells is not the same for one infection as for another. The
functional metabolism of the cells in turn is adjusted to the kind of
poison, so that the material they pour out is suitable to neutralize the
particular poison which aroused them to activity in the first place, and
in most cases no other. If one gets well from any infection, there is a
surplus of the neutralizing material left in his body fluids, and, as
long as it remains, he is secure from another infection of the same
kind. This condition is defined as immunity. Since the neutralizing
materials are different for different infections, immunity against one
is in most cases of no avail against another. One may be immune against
scarlet fever, but be just as likely to catch pneumonia as a person who
has never suffered from any infection at all. It follows that an
individual who has had and recovered from a great many infections has a
correspondingly large assortment of neutralizing materials in his body
fluids. Some of these appear to persist throughout life, others
disappear fairly soon.
The next group of permanent constituents to be described consists of
some materials which seem to have nothing at all to do as long as
everything is going well: the body fluids bathing the cells, or, in the
case of the blood, circulating about from part to part through the blood
vessels. These come into play only when, as the result of injury, the
fluids begin to escape; namely, in the case of bleeding, or, as it is
technically called, hemorrhage. It is clear that if any injury is
started sufficient to allow the fluid to escape, there would be no more
reason why it should stop running out, unless prevented, than there is
why water should stop running out of an open faucet of itself. Since, as
we know, we do not bleed to death every time we get a slight cut, but
after a long or shorter time the bleeding stops of itself, there must be
some automatic arrangement by which the opening is plugged. All of us
are familiar with the way in which this is done. We know that in the
course of a few minutes after the bleeding begins, the blood tends to
set into a firm jelly, which is called the _clot_. This clotting is the
result of a chemical transformation which goes on in the blood as the
result of its escape from the blood vessels and its exposure to the
outside. The details of the chemical processes are too complicated to be
described here; all we need to remember is that the blood within the
body contains certain special materials which are soluble and therefore
float in solution along with the other soluble materials. When any blood
vessel is injured, so that the blood begins to escape, a series of
chemical changes is started automatically by which this soluble material
is changed and becomes insoluble, so that instead of remaining dissolved
in the blood it is precipitated out. In this form it is called _fibrin_.
Fibrin is a very sticky, stringy mass which forms a spongy network,
spreads itself over the injury, and clings firmly to the edges, thus
plugging the opening, unless the rush of blood is so strong that it
keeps washing the fibrin away as fast as it is formed. Bleeding from
small wounds will presently stop of itself, but if the hemorrhage is too
great for this, it is necessary that the blood flow be slowed down or
stopped artificially. This is done by locating and pressing upon the
large blood vessel through which the blood is escaping. In order to do
this successfully, one has to know something about the course of the
blood vessels, and this will be described in a later chapter.
There are a few people whose blood lacks some of the necessary chemical
substances to enable it to clot; such persons are known technically as
“bleeders.” Even a slight injury in one of them will cause serious, or
even fatal, hemorrhage, unless the escape of blood is stopped
artificially, since it will not stop of itself. An interesting fact
about this condition is that it runs in families; in other words it is
hereditary.
In addition to all these constituents of the body fluids which are
_dissolved_ in them, there are in that part of the fluids confined to
the blood vessels, which we call the blood, three kinds of structures
floating; these we have next to describe briefly. The first of these are
the _red corpuscles_. They give the blood its red color, although if
looked at singly they appear yellowish rather than red. Red corpuscles
are almost inconceivably tiny. They are red flexible disks, a little bit
thinner in the middle than at the edges, about one three-thousandth of
an inch in diameter. Some idea of the enormous numbers in the body can
be gathered, when we say that a drop of blood the size of the head of a
pin would contain four or five million of them. The red corpuscles are
made up of a sort of framework of protein within which is inclosed a red
coloring matter or pigment, known as _hemoglobin_. It is this pigment
that gives the blood its color, and in some respects it is one of the
most important of the nonliving substances in
[Illustration: STRUCTURE OF A DROP OF BLOOD AS SEEN UNDER THE MICROSCOPE
Two white (colorless) corpuscles _a_ appear. The remainder are red
corpuscles sticking together, forming _rouleaux_. (From Martin’s “Human
Body.”)]
the body. This is because it is the means by which the cells obtain
sufficient supplies of oxygen. As we have already seen, every cell is
constantly drawing from the body fluids about it the oxygen which is
required for carrying on its metabolism. The fluids in turn get oxygen
from the blood. It is necessary, therefore, for the blood to convey
abundant supplies. Oxygen will dissolve in water, as is proven by the
fact that fish and other aquatic animals are able to get enough oxygen
from the water in which they live to serve their needs; but it is not
sufficiently soluble to supply the needs of an active body like that of
man. It is necessary, therefore, to have a special additional means of
conveying oxygen besides its simply dissolving in the blood. This
additional means is furnished by the hemoglobin, which is an
“oxygen-carrying” pigment. What this means is that the hemoglobin has
the property of taking up oxygen chemically, whenever it is exposed to a
region where there is oxygen in abundance, and of giving it up again
whenever it passes through a region where oxygen is scarce. It is thus
that oxygen is conveyed from the lungs to the active tissues of the
body. We shall have more to say about this in the chapter devoted
especially to the matter of the oxygen supply.
We said a moment ago that the red corpuscles consist of a protein
framework inclosing hemoglobin. They are not living. They must,
therefore, have been made by living cells and poured out into the blood
stream. We might suppose that this was done once for all and that the
same red corpuscles are floating in our blood now that started floating
there when the blood was first formed; but, as a matter of fact, this is
not the case. There is a continuous breaking down of red corpuscles
which must be made good by a continuous manufacture of new ones. Most of
the larger bones in our bodies have a sort of spongy framework by which
the ends, where the joints are, are made stronger. Within the space of
these frameworks is a kind of marrow, known as _red marrow_, because it
has such a very abundant blood supply. It is in this red marrow that the
manufacture of red corpuscles goes on. There are throughout the red
marrow living cells which are constantly dividing and subdividing,
forming more and more so-called _daughter cells_. Within these daughter
cells hemoglobin is presently deposited; a little later they lose the
nucleus and probably the remainder of the living protoplasm as well,
leaving just the framework of nonliving protein with its contained store
of hemoglobin. This is the finished red corpuscle, and it breaks loose
from the red marrow and floats out into the blood stream. The rate of
manufacture of red corpuscles is very rapid; undoubtedly millions of
them are formed daily in the various red marrow regions of the body.
The total number of red corpuscles does not increase correspondingly,
because they are broken down at the same rate as they are formed. It is
now believed that in the _spleen_, which is a large organ of the abdomen
whose function has always been obscure, those red corpuscles which are
destined for destruction are picked out of the blood and broken down. We
commonly suppose that this fate overtakes corpuscles that are worn out
and are no longer efficient oxygen carriers, but we do not know, as a
matter of fact, that the corpuscles do lose their effectiveness in
course of time, nor have we any idea how the spleen could select out of
the millions in the blood those particular ones which are no longer
useful.
What the spleen does to the corpuscle is to break it up so that the
protein and the hemoglobin in it are set free in the blood stream. We do
not know what becomes of the protein; probably it is taken up and
utilized. We do know that the hemoglobin is decomposed in the liver. One
constituent of hemoglobin, in fact the constituent which gives it its
ability to carry oxygen, is the element iron. Iron is not particularly
abundant in living things, and we find that the body is thrifty with
regard to it. When the liver decomposes the hemoglobin, the iron is
saved in some way which enables the blood to carry it back to the red
marrow, where it can be used over again. There are also some portions of
the hemoglobin which are valuable as food material; the remainder, which
is of no further use, is discharged from the body as a part of the bile.
Besides the red corpuscles the blood contains what are known as the
white or, better, the colorless corpuscles. These, instead of being dead
structures, as are the red corpuscles, are actual living cells, which
float along in the blood stream or have the power of clinging to the
walls of the blood vessels and crawling along them; in places where the
blood vessels are very thin, they work their way right through, and so
get out into the spaces among the tissues. There are not nearly so many
colorless as red corpuscles, in fact, the latter are eight or nine
hundred times as numerous as the former. A great deal of study has been
devoted to colorless corpuscles to find out just what they do. We are by
no means certain that we understand fully all their activities, but we
do know that one very important thing that they do is to absorb into
their own bodies tiny foreign particles that may be present in the blood
stream or in the spaces among the tissues. There are not many kinds of
foreign particles that can get into these places. In fact, about the
only sort that can are the tiny plant or animal cells which are
responsible for disease. When these foreign organisms invade the body,
the colorless corpuscles may engulf them into their own bodies and
destroy them. In this way a great deal of infection is prevented. Of
course it may happen that the invading organisms are so numerous that
the colorless corpuscles can not get rid of all of them, or the
corpuscles themselves may show a diminished activity. It is an
interesting fact that both the number of colorless corpuscles in the
body and the vigor with which each colorless corpuscle attacks foreign
organisms vary from time to time, so that we are much more secure
against infection at some times than at others. In general we may say
that when the body is in a condition of vigorous health the colorless
corpuscles will be efficient. When we are run down, on the other hand,
these cells share with all other body cells this state of low vitality.
This explains why people who allow themselves to become run down are
more likely to fall victims to infection than those who are in vigorous
health, and emphasizes, of course, the importance of a habit of life
that shall keep the body vigorously healthy at all times. It should be
understood that the colorless corpuscles do not show the same
effectiveness against all kinds of foreign organisms; some kinds of
disease germs are able to bring about infection in the body quite
regardless of the activity of the colorless corpuscles. From other
kinds, on the other hand, they give the body very complete protection.
The most familiar example of the action of the colorless corpuscles is
in the formation of what we all know as pus. There are a few kinds of
organisms that, instead of getting into the body and becoming scattered
through its fluids, establish themselves at certain points and by growth
and multiplication accumulate at those places in large numbers. Examples
are pimples and boils. In these cases the pus-forming organisms have
located just under the skin and are multiplying there at a great rate.
They produce poison which is absorbed from the place where they are and
distributed through the body. This poison appears to have some sort of
chemical attraction for the colorless corpuscles; at any rate the
corpuscles gather from all around to the place where these organisms are
located and engulf as many of them as they are able, but in so doing
they themselves are destroyed, and pus, as we know it, is simply a mass
made up of the dead bodies of the colorless corpuscles along with the
organisms which they have destroyed and which in turn have destroyed
them. In the ordinary pimple or boil the colorless corpuscles ultimately
win the combat and the infection is completely overcome. It happens
occasionally, however, that a pus-forming infection becomes established
in some place where it manages to maintain itself in spite of the
attacks of the colorless corpuscles. This happens very frequently in the
tonsils, so that persons who have infected tonsils may have pus
formation going on month in and month out. This may also take place at
the roots of the teeth. In fact it is now commonly believed that
whenever the nerve to the tooth dies, infection is certain sooner or
later to become established at the tip of the root. These places where
pus formation is going on continually are known as “pus pockets.” For a
long time very little attention was paid to them. Persons occasionally
suffered acute distress from gumboils or had attacks of sore throat
owing to the infected condition of the tonsils, but beyond these no
particular attention was paid to the pus-forming organisms, unless, as
occasionally happened, an especially virulent type became established
which brought on the condition commonly known as blood poisoning. Within
the last few years the discovery has been made that there is a steady
discharge of poison from every pus pocket into the body fluids and so to
the blood stream. The amount is ordinarily so small that the body as a
whole is not seriously affected, but now and then, either because of a
larger production of poison or because of a lower resistance on the part
of the body, serious ill effects are produced. Among them may be
mentioned acute (inflammatory) rheumatism. This is not only an extremely
painful condition, but it is very likely to leave serious after-effects,
as, for example, injured heart valves, to give trouble for the rest of
life. The discovery of these facts has given us great respect for pus
pockets, so we no longer treat them carelessly. Infected tonsils are
looked after, not simply because they bring about sore throat now and
then, but even more because of the poison which they are likely to send
through the body. It is probably not too much to say that the practice
of dentistry has been revolutionized since the significance of pus
pockets has been discovered. Formerly the killing of nerves to relieve
aching teeth or to insure them against future aching was a common
practice. The modern dentist, on the other hand, kills nerves to teeth
with the very greatest reluctance and only as a last resort, because he
knows full well that in so doing he is opening the way for the
establishment of pus pockets with the train of ills that is likely to
follow. At the present time methods of curing pus pockets at the base of
the teeth are not very adequate, so that the extraction of the tooth has
to be resorted to, but there is every reason to believe that the dental
profession will shortly find methods by which pus pockets can be
controlled without having to sacrifice the teeth.
The third kind of structures which are present in the blood stream are
much smaller than the red corpuscles, but are nowhere near so numerous.
They are called the _platelets_ and are disk-shaped bodies composed
chiefly of protein material, and probably, although not certainly,
living cells. Their presence in the blood remained unsuspected up to
about the end of the last century, not so much because of their very
small size, as because they go to pieces very quickly after the blood is
shed. By the time a drop of blood could be gotten under the microscope
they would be all gone. They were discovered only as the result of the
adoption of a method of treating the blood which preserved them long
enough so that they would still be present when the blood was looked at
under the microscope. They seem to have something to do with the changes
that take place in the blood by which it is caused to clot. Whether this
is their only function, we do not know.
We have now described the substances which are present in the blood and
in the other tissue fluids as a fairly permanent part of their make-up.
In addition to these there are present all the materials that are in
transit to the cells or from them. These include all kinds of foodstuffs
on their way from the digestive organs to the cells. As we shall see in
detail later, the digestive organs work over the food that we eat before
passing it on to the blood, so that the actual food materials that are
being transported by the blood are the digested products of the food
rather than the food itself. For example, there are to be found in the
blood, in addition to the regular blood proteins which were described a
moment ago, the digestion products of the food proteins on their way to
serve the needs of the various cells of the body. There will be found
also the digestion products of the other classes of foodstuffs. The body
fluids contain also the waste products of cell metabolism on the way to
be discharged from the body and the special products, such as the
hormones, which are manufactured by certain cells and carried through
the blood stream to act upon other cells. Of course we realize that not
all of the materials that are manufactured by cells are poured out into
the blood stream. Such materials as saliva, gastric juice and the like
are passed directly from the cells in which they are manufactured into
tubes by which they are conveyed to the digestive canal, where they
carry on their special work of digesting the food.
Among the things which are present in the body fluids should be
mentioned the two gases oxygen and carbon dioxide. We should expect
oxygen to be present in the body fluids, because it is necessary for the
metabolism of cells and can get to them only by being carried in the
blood stream. We have seen in the red corpuscles the special method by
which an abundance of oxygen is transported. Carbon dioxide is the
gaseous product of the oxidation of carbon and is found in large amounts
wherever there is burning, since carbon is the chief constituent of all
fuel and whenever carbon is burned, carbon dioxide is formed. Since the
fuel materials that are burned in the living cells consist largely of
carbon, carbon dioxide is produced in them as well. They have to get rid
of it, and the only way they can do so is by passing it out into the
fluids that surround them, which in turn pass it on to the blood. The
way in which the body handles these two gases makes up a special
subdivision of the subject of physiology and will be so treated in the
chapter on respiration.
CHAPTER XIV
THE CONVEYER SYSTEM OF THE BODY
In the last chapter we talked about the body fluids and saw that they
can be subdivided into the tissue fluids, which surround the cells, and
the blood, which is inclosed in a system of pipes and which carries the
materials to and from the tissue fluids. We now have to take up this
matter of transporting the material in more detail. The first step will
be to see how materials that are in the blood get from it to the tissue
fluids, and how materials that are in the tissue fluids get from them
into the blood. Unless these interchanges can take place freely, there
is no way in which the blood can serve as a conveyer system. In order to
see how the interchanges are carried on, we shall have to look first at
some features of the construction of the system of pipes through which
the blood flows. As we all know, the large blood vessels are either
arteries or veins, the arteries being blood vessels which are carrying
blood away from the heart, and the veins vessels that are carrying blood
toward the heart. If we start with an artery and trace it through the
body, we find that it is continually giving off branches which in turn
give off smaller branches, until finally the subdivisions are so small
that we cannot trace them any further with the naked eye. In the days
before the microscope was discovered, there was a great deal of question
as to how these finest branches ended. At first no one suspected that
there was connection between the fine subdivisions of the arteries and
the fine subdivisions of the veins through which the blood could pass.
About the beginning of the seventeenth century William Harvey became
convinced that there must be fine vessels leading across from the
smallest arteries to the smallest veins and that the blood must pass
through these. He came to this conclusion without ever having seen these
small vessels, since at that time there was no microscope by which such
tiny structures could be seen. Before Harvey’s time it was supposed that
the blood ebbed and flowed in the arteries and in the veins. He showed
that the blood flows in one direction constantly, leaving the heart by
way of the arteries and coming back into it by the veins. Harvey was,
therefore, the discoverer of the circulation, one of the most important
discoveries in physiology. After the microscope was perfected the tiny
tubes connecting the finest arteries with the finest veins were made
out. They were found to be very small in diameter and to have very thin
and delicate walls. They were also found to be extremely numerous. The
finest subdivisions of the arteries that can be seen with the naked eye
are scattered very thickly through all the tissues of the body which
have a blood supply, and they go on subdividing microscopically, so that
the finest vessels are scattered more and more thickly through the mass
of living substance. The very finest of all, which are the tubes
connecting the smallest arteries with the smallest veins, are called
_capillaries_, from a Latin word meaning a hair, to indicate their very
small size. They are so close together in most parts of the body that it
would be difficult to thrust a
[Illustration: Photo, Paul Thompson
THE TEST FOR BLOOD PRESSURE]
[Illustration: Photo, Paul Thompson
PART OF PROCESS OF URINALYSIS INDICAN TEST]
[Illustration: A NETWORK OF CAPILLARIES
The artery _a_ and vein _v_ (highly magnified). (From “The Human
Mechanism,” Hough and Sedgwick)]
pin in anywhere for any distance without striking against one or more of
them. The capillaries are spread so thickly that there are not many
places in the body where living cells are more than a very small
fraction of an inch from one of them. The cells do not, however, lie
right against the capillaries, but are separated from them by tiny
spaces filled with tissue fluid. In order for material to get from the
blood to any living cell, then, it must pass through the wall of the
capillary into the fluid which fills the tissue space and from that in
turn to the cell itself. The wall of the capillaries is so delicate that
if the blood flowing through any capillary contains more of any
substance than is present in the tissue fluid surrounding that
capillary, some of it will pass out through the wall and into the
tissue fluid, just about as freely as though there were no wall there at
all. The arrangement can be illustrated by a familiar example; if a drop
of ink is allowed to fall into a glass of water, it will color only a
small part of the water at first, but quickly spreads out until each
part of the water is as deeply colored as any other part. If the glass
of water were to be divided in half by a very delicate membrane, and the
ink dropped in on one side, it would spread out in the same way, passing
through the membrane in so doing, until again all the water in the glass
was equally colored. Of course, the quickness with which the ink could
pass through the membrane would depend on how delicate the membrane was.
We could imagine membranes which would not let any ink at all through,
and every degree from that up to membranes so delicate as to offer no
obstruction at all to the passage of the ink. The walls of the
capillaries rank as membranes of such delicacy as apparently to offer
almost no obstruction to the passage of materials through them. They
hold back the red corpuscles and the platelets fairly well, so that they
do not pass out of the blood and into the tissue spaces, unless the
capillaries are actually injured. The colorless corpuscles are able to
make their way through the capillary walls and so also do nearly all the
substances that are dissolved in the blood. It is an interesting fact
that the blood proteins do not pass freely through the capillary walls,
although the digestion products of food proteins, do. It will be
remembered that in the chapter on Body Fluids the importance of the
sticky quality of the blood proteins was spoken of. It is now believed
that it is because of this gelatinous nature that the blood proteins
are not able to pass out through the capillary walls, and this is
supposed to be important in the proper working of the circulation. In
fact there is a condition of greatly lowered vitality to which the name
“shock” is applied, in which the blood proteins escape through the
capillary walls to so great an extent as to interfere with the proper
working of the body. It has been found possible to prevent this in a
very large measure by the simple expedient of injecting some substance
like mucilage directly into the blood stream. We are to think of the
capillary walls, then, as allowing materials to pass freely through them
in either direction, from the blood into the tissue spaces or from the
tissue spaces into the blood, with the exception of the red corpuscles,
platelets, and the blood proteins, and as thus keeping the tissue fluids
supplied with whatever materials the blood contains or taking from the
tissue fluids the waste products of cell metabolism, which the cells are
pouring out. With this arrangement clearly in mind, all that remains for
the understanding of the conveyer system is to see how the blood is kept
in motion and distributed among the various organs of the body, and then
to consider where the blood in turn gets its supplies of materials which
it can pass on to the tissue fluids, or how it gets rid of the
substances which the tissue fluids have passed on to it from the cells.
At the beginning of the chapter we said something about the arteries and
veins and their branching into smaller and smaller subdivisions with the
final connecting link between the smallest arteries and the smallest
veins in the form of capillaries. We are now to consider in detail the
movement of the blood through these tubes, and to do that it will be
necessary to speak again of this arrangement. In describing the
circulation we usually begin with the heart. The heart itself will be
taken up presently. First let us trace the blood vessels from the heart
through the body and back to it again. The large main artery leading
from the heart is known as the _aorta_. This springs from the upper side
of the heart, bends over in an arch, and passes down through the chest
into the abdomen.
[Illustration: THE HEART AND BLOOD VESSELS DIAGRAMMATICALLY REPRESENTED
_L_, lung; _M_, intestine; _P_, liver; dotted lines lymphatics.
(Martin’s “Human Body”)]
Branches are given off from the aorta all along its length. The very
first of these come off before the aorta gets away from the heart, and
are the arteries by which the tissues of the heart itself are supplied
with blood. A little farther on are large arteries, one for the left arm
with a large branch running up the left side of the neck, another for
the right arm with a large branch running up the right side of the
neck. Each of these in turn gives off branches all along to provide for
the tissues of the arms, neck, and head. It is worth while noting that
the arteries running up the neck to the head are large in proportion to
the size of the head itself; this is because the brain, as the most
important organ in the body, requires and receives a disproportionately
large blood supply. Besides the brain the head contains numerous muscles
and also the salivary and tear glands, all of which carry on active
metabolism and therefore require abundant blood supply. The main
branches of the aorta to the head and arms are given off from the arch;
as the aorta passes down through the chest it gives off small branches
to the muscles of the chest wall, and then passes into the abdomen. Here
are located two of the three important arrangements for renewing the
blood; namely, the digestive organs and the organs of excretion
(kidneys). Large branches from the aorta pass to the digestive organs
and others to the kidneys; smaller branches lead to the muscles of the
abdominal wall, and also to the various secreting glands that are
located in the abdomen. At the lower end of the abdomen the aorta
divides, giving one large branch for each leg. As we have already seen,
if we follow any of these subdivisions through its finer and finer
branchings, we shall finally be able, with the aid of a microscope, to
trace it to capillaries, where the interchanges between blood and tissue
fluid occur, and beyond the capillaries to tiny veins which unite with
other tiny veins from other capillaries into larger veins. These again
continue to come together into main veins corresponding in every part of
the body with the main arteries. All these veins finally unite into two,
one for the lower part of the body, called the _inferior vena cava_,
and one for the upper part of the body, called the _superior vena cava_.
These two come together just at the entrance to the heart. One special
feature of the blood supply to the digestive organs may as well be
mentioned here; it is that the blood which flows through the capillaries
of the stomach and intestines is all reassembled into a vein known as
the _portal vein_, which instead of passing directly into the inferior
vena cava goes first to the liver, where the vein breaks up into another
set of capillaries, the liver capillaries, beyond which is another vein
which leads into the inferior vena cava. The result of this arrangement
is that all blood passing into the capillaries of the stomach or
intestines is obliged to pass again through the capillaries of the liver
before going on into the main stream of the circulation. This is an
important feature of the renewal of the food supplies of the blood.
We have now traced the blood from the heart through the body back to the
heart again, and have seen how in its course some of it will pass
through such active tissues as muscles or brain or glands, so that the
interchanges can go on by which the fluids in these active tissues can
take up needed materials and give off wastes. Also a part of it flows
through the digestive organs, where food materials can be taken up, and
another part flows into the kidneys, where wastes can be gotten rid of.
This leaves us to consider only the passage of the blood through the
lungs, where the supply of oxygen is to be taken up and the gaseous
waste product, carbon dioxide, is to be disposed of.
The most urgent requirement of the body is the requirement for oxygen.
There is under ordinary circumstances at all times some surplus of food
materials stored in the cells, so that even though the renewal of their
surrounding fluids from the blood should stop, they could keep on going
for a time on the material that is stored within them; but there is no
such storage of oxygen. The cells in the body lead almost a
hand-to-mouth existence so far as their oxygen supply is concerned. They
are constantly withdrawing oxygen from the tissue fluids surrounding
them, and these fluids are just as constantly withdrawing it from the
blood; therefore any failure of the blood to be properly supplied with
oxygen results very promptly in a condition of oxygen hunger in the
cells. This means prompt cutting off of metabolism, since metabolism is
a matter of oxidizing fuel, and oxidation cannot go on unless the oxygen
is provided. This urgent need for oxygen is met in the body by having
the arrangement for supplying it to the blood much more perfect than the
other renewal arrangements. We saw a moment ago that only part of the
whole blood stream passes through the digestive organs at any given time
and only part of the stream passes through the kidneys. The whole
stream, on the other hand, passes through the capillaries of the lungs.
This is brought about by having an arrangement whereby the combined venæ
cavæ after entering the heart communicate with an outlet in the form of
an artery leading to the lungs. This artery, which is called the
_pulmonary artery_, breaks up into capillaries in the lungs, which
reunite into the _pulmonary vein_ which comes back to the heart again.
It is from the pulmonary vein that connection is made with the aorta,
starting the blood on its course through the body again. We see then
that the blood passes through the heart twice in each complete round,
once as it comes in from the body at large on its way to the lungs, and
again as it comes in from the lungs on its way to the body at large.
We have spoken of the heart thus far as a single organ; it is actually
two hearts side by side, and these would work just as well if they were
at a distance instead of being built into one organ. There has probably
been more misunderstanding of the heart by people in general than of any
of the other parts of the body. This is because from the earliest times
the heart has been looked upon as the seat of the affections, and so
powers and properties have been attributed to it to which it is not at
all entitled. As we tried to make perfectly clear in a former chapter
all intelligence and all feelings are located in the brain; the heart
cannot possibly take any more active part in these than can the stomach,
liver, kidneys or any of the other parts of the body which are concerned
with the maintaining of the tissues in good working order. Probably no
one really knows how it came about originally that the heart was endowed
with these peculiar gifts. It is true that in time of strong emotion
there are changes in the activity of the heart which we can perceive.
These occur because the heart is under the same kind of nervous control
as are the smooth muscles and glands, and shares with them in the
disturbances which accompany emotion; but the real seat of these
emotions is, of course, the brain. As a matter of fact the heart is
nothing but a muscular pump whose sole function is to keep the blood in
motion. From what has already been said it is clear that the heart
cannot relax its activity for more than an instant without disastrous
results; the pressing need of the tissues for oxygen requires that the
blood be kept moving. If there were any other way in which the needs of
the cells could be supplied except through the movement of the blood,
the heart could be dispensed with perfectly well. We have emphasized
this about the heart because it is much easier to understand its working
if one thinks of it simply as a pumping organ than if one is attributing
to it mystic functions connected with our higher emotions.
[Illustration: DIAGRAM SHOWING THE RELATION OF THE TWO HALVES OF THE
HEART
_ra_ and _rv_, right auricle and ventricle; _la_ and _lv_, left auricle
and ventricle; _ao_, aorta; _vc_, venæ cavæ; _pa_, pulmonary artery;
_pc_, pulmonary capillaries; _pv_, pulmonary vein. (Martin’s “Human
Body.”)]
We showed a moment ago that the heart is really a double pump. The
relation of the two halves is shown in the diagram. One of the two
pumps, that on the right side of the heart, receives the blood from the
body at large and pumps it out into the pulmonary artery and through
the capillaries of the lungs; the pump on the left side of the heart
receives the blood from the lungs through the pulmonary vein and pumps
it out into the aorta and so through all the other capillaries of the
body. Since the circuit of the body is much more extensive than the
circuit of the lungs, the work of pumping is correspondingly greater,
and we find the left part of the heart a much more powerful pump than
the right. The heart operates as a _reciprocating pump_, by which we
mean that it alternately fills and empties. In this respect it is like
ordinary pumps except those of the rotary variety. Any reciprocating
pump must have a chamber which will fill and which can then be emptied
forcibly. In order that it shall not empty itself back through the pipe
from which it filled there must be a valve in the intake pipe which
shall close as the pump is being emptied. If, as is the case in the
heart, it is emptying itself into a system which permits backflow, there
must be another valve in the outlet pipe to prevent the fluid that has
been expelled from running back in. Each of the heart pumps consists,
then, of a chamber, which alternately fills and empties itself, and two
valves, one on the intake and one on the outflow side. In ordinary pumps
the forcible emptying is performed by a piston which moves through the
pump chamber expelling the liquid ahead of it and then has to draw back,
making room for the chamber to fill again. In the heart the forcible
emptying is accomplished by muscular action. The wall of the heart
consists of a great many muscle fibers so arranged that when they
contract they pull the walls of the heart together, making the cavity
smaller, or even obliterating it completely. The contraction of these
fibers makes up what we are familiar with as the beat of the heart. The
frequency with which they contract varies a good deal in different
individuals. The average is about seventy-two a minute; but it may be as
slow as forty-eight or fifty, or may run up to one hundred and forty or
one hundred and fifty a minute. Whatever the rate, in every case there
is an alternation of contraction and relaxation; during the relaxation
the cavity is filling with blood through the intake valve, the outflow
valve being closed, so that no blood that has once been pumped out can
rush back in again. By the contraction of the muscles the heart is
emptied, the outflow valve being open, and the intake valve being closed
to prevent an escape of blood backward into the veins through which it
flowed in. The part of the heart that carries on this active pumping
work is known as the _ventricle_; that on the right side, which receives
the blood from the body and pumps it to the lungs, is the right
ventricle; and that on the left side, which receives the blood from the
lungs and pumps it to the body, is the left ventricle.
In addition to the ventricles, which are the active pumps, each side of
the heart has an additional chamber known as the _auricle_, whose
purpose is to serve as a reservoir into which blood can flow during the
time that the ventricles are emptying themselves. If it were not for the
auricles, the movement of blood into the heart would have to stop with
every beat, because while the ventricles are contracting the intake
valves are closed and there would be no place to which blood could flow,
but since each side has its auricle, the flow of blood goes on during
the beat of the ventricles, the auricles filling up. The intake valve,
in order to operate properly, should be located between the auricle and
ventricle, and this is where it is. The vein opens directly into the
auricle without any valve between; the auricle opens into the ventricle
with the intake valve at the point of junction. The intake valves are
given rather formidable names; they are sometimes spoken of as the
_auriculo-ventricular valves_; that on the right side of the heart
between the right auricle and the right ventricle is composed of three
flaps of membrane, and has therefore been named the _tricuspid valve_.
The intake valve on the left side of the heart, which is composed of but
two flaps, is known as the _mitral valve_. As soon as the beat of the
ventricles is over and the ventricular muscle relaxes, the blood which
has accumulated in the auricles presses the intake valve open and blood
begins to flow through it directly into the ventricle. Both the intake
and outflow valves are composed of stout but thin sheets of membrane, so
that very little pressure is required to operate them. The weight of the
blood that is accumulated in the auricles during the beat of the
ventricle is more than sufficient to force the valve open and allow the
blood to flow on through into the ventricle. In a heart that is beating
seventy-two times a minute, there cannot be much time occupied either in
filling or emptying. As a matter of fact both these intervals are
measured in tenths of seconds. If we take a heart that is beating at the
average rate of seventy-two times a minute the whole of a single beat
amounts to eight-tenths of a second. The beat of the ventricle takes
about three-tenths of a second or three-eighths of the whole time; the
period of relaxation of the ventricle, during which it is filling with
blood through the open intake valve, is about five-tenths of a second or
five-eighths of the whole time. The movement of blood is rapid enough
so that this five-tenths of a second allows the ventricle to fill; in
fact much less time than this is required, for in a heart that is
beating at twice the average rate, the ventricle still fills with blood
between beats.
A word remains to be said about the beat of the auricle. During most of
the period when the ventricle is relaxed the auricle is also quiet and
blood is pouring directly through it from the veins into the ventricle;
but just an instant before the ventricular beat begins, one-tenth of a
second to be exact, the auricle contracts, emptying what blood it
contains into the nearly filled ventricle; thus, when the ventricle
beats, which it does immediately, closing the intake valve at the same
time, the auricle is empty and so is able to accommodate the inflow of
blood from the vein during the three-tenths of a second that the intake
valve is shut. Both sides of the heart work exactly together, the two
auricles beating simultaneously, and the two ventricles. Of course it is
necessary that this be so, for if they did not keep pace exactly, one
with the other, there would be a piling up of the blood either in the
lungs or in the veins leading from the body to the heart, and the
efficiency of the circulation would be seriously impaired.
We can get a good deal of information about the way our hearts are
behaving simply by holding the hand against the chest directly over the
heart or by pressing the ear against the chest of some one else and
listening to the heart’s action. The physician makes use of a
stethoscope, which is merely an apparatus for conducting clearly the
sounds which the heart makes, so that it is not necessary to apply the
ear to the chest. When one listens thus to the heart he finds that with
every beat there are two distinct sounds: the first is a rather dull
sound which comes just at the beginning of the beat of the ventricle,
the second is a sharp sound occurring just at the end of the ventricular
beat. As we saw in Chapter IX, sound is always the result of vibrations,
and a great deal of study has been devoted to an attempt to find out
where the vibrations come from that cause the heart sounds. It is now
generally believed that the first sound is partly the result of
vibrations set up in the contracting heart muscle and partly due to
vibrations from the sharp closing of the intake valves. The second sound
is known to be wholly due to the sudden closing of the outflow valves.
The sounds are chiefly of importance in that they enable the physician
to determine whether the valves are holding tight or whether there is a
leakage of blood through them. In case the intake valve leaks, there
will be a backward jet of blood from the ventricle into the auricle with
every beat of the heart. This will cause a sort of hissing or murmur
which can be heard with the stethoscope in connection with the first
sound. If the outflow valve is the one that leaks, blood will squirt
back into the ventricle from the aorta, while the ventricle is relaxing.
The murmur in this case will come just after the second sound. The
skillful physician by comparing the loudness of the murmur when the
stethoscope is pressed at different points on the chest and back can
determine whether the leaky valves are on the right side or the left
side. Thus an accurate diagnosis of imperfect valves can be obtained. Of
course the heart will not work well if its valves are not tight any more
than will an ordinary pump, so that persons suffering from this trouble
cannot have as good a circulation as those whose valves have nothing
the matter with them. It is true that in most cases of imperfect valve
action there is a compensation in the form of an increase in the size
and strength of the heart muscle, so that the circulation is maintained
in spite of poor valve action by harder work on the part of the heart.
It is evident that in a case of this kind exceptional strains on the
heart are more dangerous than if the heart is normal to begin with, so
that persons with faulty valve action must avoid physical strains, such
as sharp running after street cars or trains, which would be borne with
impunity by ordinary individuals. Since faulty valves are a frequent
result of acute rheumatism, which in turn comes from pus pockets, and
since no way is known to cure a defective valve, once the trouble has
developed, it is evident that prevention is of the utmost importance.
Physical efficiency is very seriously hampered by poor heart action.
One feature of the heart action with which we are all perfectly familiar
is that both the rate and the vigor of the heartbeat vary greatly from
time to time. When one is lying quietly, the heartbeat is at its lowest
point. It becomes more rapid as one sits up, still more rapid upon
standing, increasing still more with the taking of any form of muscular
exercise, and in vigorous muscular exercise attains its greatest
rapidity and force. The rate in this latter case may be fully double
that of the quiet standing position, and, as the vigorous thumping tells
us, the force is also very much increased. As we saw in Chapter VII the
heart muscle works automatically, contracting and relaxing without being
stimulated through the nervous system. The _variations_ in rate and
force, however, are the result of nervous action. The heart muscle, as
we have already seen, is under the same sort of nervous control as the
smooth muscles and glands. It has passing to it two sets of nerves, one
to slow it down, the other to speed it up. Both these sets of nerves
arise from centers in the brain stem, and both these centers appear to
be discharging continuously. So it works out that the heart muscle is
under the constant influence of two opposing sets of nerves, and its
actual rate and vigor depend upon the balance between them. This has the
effect of making the heart extremely responsive to nervous influences.
The slightest relaxation on the part of the nerves whose function is to
cause slowing will lead to a prompt increase of rate, since the nerves
that tend to cause increase are active all the time. Various things may
bring about changes in the nervous balance governing the heart; chief of
these are muscular activity and emotional disturbance. Practically all
the changes in the heart action that we observe from moment to moment
can be explained as being due to one or the other of these causes. There
are, however, two additional points to be noted briefly; the first is
that after muscular exercise the heart slows down very gradually, not
returning to its ordinary resting rate for a half hour to an hour after
the exercise is over, depending on how long the exercise was kept up.
The explanation of this long-continued rapid beat is found in the great
outpouring of waste products as the result of the exercise. We have
already learned that the functional metabolism of muscular work involves
the oxidation of a large amount of energy-yielding material and
therefore brings about the production of large amounts of oxidation
products. Their presence in the blood serves as a stimulus to
[Illustration: THE MICROSCOPE IS NECESSARY TO UNDERSTAND THE STRUCTURE
OF THE BODY
In this laboratory the workers are examining blood smears]
[Illustration: Photo, Cornell University Medical School
MICROPHOTOGRAPH OF BRAIN TISSUE]
the nerve center in the brain stem, which acts to quicken the heart, and
this keeps the rapid beat going until these products are gradually
gotten rid of from the body. Another somewhat similar case is the
prolonged rapid heartbeat following a violent emotion. The explanation
of this we saw a couple of chapters ago in the outpouring of adrenalin
that accompanies emotion. One property of adrenalin, as already noted,
is to quicken the heart; so, as long as any adrenalin remains in the
blood stream, the heartbeat will be faster than normal.
In the above paragraphs we have tried to make clear that the blood is
kept in motion through the body by the work of the heart, and that the
heart’s activity varies in accordance with the needs of the body; in
muscular exercise there is a great increase in metabolism, which means a
greatly increased demand both for food supplies and for oxygen. To meet
this increased demand it is necessary that the blood circulate more
abundantly, and in the automatic speeding up of the heart through the
nervous system we have the means by which this is done. In the case of
strong emotion, as already emphasized, the bodily reactions are such as
put the body into the best possible condition for meeting the emergency.
Evidently a quickened heartbeat, by insuring abundant supplies of oxygen
and of foodstuffs, contributes to this end. The slowing of the
heartbeat, when one lies down, is evidently helpful in enabling the
heart itself to recover from any strains that may have been put upon it.
The heart is a muscle, and like any other muscle carries on its
functional metabolism, which means that it is oxidizing fuel materials
and producing waste products. Since it is absolutely necessary that the
heart go on beating regularly year in and year out for perhaps eighty
or a hundred years, any relief from activity that it can get by slowing
down during sleep is evidently an advantage. It has been calculated that
the heart muscle really enjoys an “eight-hour day,” by which is meant
that on the average the functional metabolism of contraction is going on
in heart muscle only about one-third of the time, eight hours out of
each twenty-four. During the active waking time the metabolism takes a
larger percentage than that, but during sleep enough less to even up.
The heart empties itself into the large arteries; the left heart into
the aorta, the right heart into the pulmonary artery. Both these
arteries, as well as their subdivisions, are highly elastic. The very
best quality of rubber tubing is not superior to our arteries as samples
of elastic tubes. The blood, as we have already seen, is quite sticky,
and the capillaries through which it must pass in its course around the
body are microscopically tiny. The heart pumps the blood out of itself
at the rate of four or five quarts a minute or more, according to
whether it is working moderately or at high speed. To force this amount
of the sticky blood through the tiny capillaries evidently requires very
considerable force. As a matter of fact, the force is sufficient so that
if it were applied to working a fountain it would force a jet to the
height of nearly eight feet. Evidently pumping blood into elastic
arteries with this force and against the resistance offered by the tiny
capillaries causes the arteries themselves to be not only filled but
overfilled, so that their walls are greatly stretched. This fact, that
our arteries are elastic and are kept on the stretch by the pressure of
the blood within them, is of the very greatest importance to the proper
flow of blood and this in turn is so important to our well-being that
some of our most serious chronic diseases are traceable to the loss of
elasticity on the part of the arteries.
We must remember that once every second, or oftener, the heart is
shooting a jet of blood into the large artery which is already stretched
with blood and which can empty itself only through the tiny capillaries
at the tips of its finest subdivisions. On account of the inertia of the
blood stream, room is made for this additional jet of blood by
stretching the arteries near the heart more than they were stretched
before. The result is that there is an inequality in the amount to which
the arteries are stretched, those near the heart being stretched more
than those farther along. As quickly as possible this inequality of
stretch will be equalized by a spreading of the additional tension out
over all the arteries in the form of a wave. This wave makes up what we
know as the pulse. It can be felt in any artery that is near enough to
the surface so that the finger tips can press upon it. The radial artery
at the wrist is the one commonly used by physicians for feeling it.
There is a large artery in the neck in which the pulse can also be felt
readily, and if one takes the pulse of another person in the neck with
one hand and in the wrist with the other he can easily satisfy himself
that the pulse in the neck always comes an instant earlier than that in
the wrist. This is simply because the pulse spreads from the heart as a
wave, and the distance to the neck is not so great as that to the wrist.
By the time the finest subdivisions have been reached, the tension is
equalized throughout the arterial system, and there is no more pulse.
The advantage of this is that the blood flows through the capillaries
in a steady stream and not in a series of jerks. This, in fact, is the
chief, but not the only, benefit we derive from having elastic arteries.
Since the heart operates as an intermittent pump, it is evident that
unless the arteries were elastic and so could take up the shock, the
blood would have to pass through the capillaries in a series of jerks,
exactly corresponding with the beats of the heart. There is abundant
proof, which we shall return to in a moment, that to have the blood move
through the capillaries in this jerky fashion would be disastrous.
Before taking that up, however, we wish to show that by having elastic
arteries the actual work of the heart is less than it would be if the
arteries were stiff. The reason is really very simple. As was stated a
few pages back, the heart is actually emptying itself only during
three-eighths of every beat. If the arteries were stiff tubes, and
therefore not able to take up any of the blood within themselves,
exactly as much would have to pass out through the capillaries during
this three-eighths of the beat as was pumped in by the heart. In other
words, if the heart were pumping five quarts a minute, five quarts would
have to pass through the capillaries in three-eighths of the minute
instead of having the whole minute in which to do it. Since the arteries
are actually elastic, they are able, by stretching a little more, to
make room for part of the blood and so spread the time of its passage
through the capillaries out over the whole time instead of confining it
just to the period when the ventricle is contracting. Evidently it would
take more work to pump five quarts of blood through the capillaries in
three-eighths of a minute than in a whole minute.
We measure the work of the heart by what we call blood pressure, about
which we are hearing so much these days, so that it will be well to
explain as clearly as possible just what is meant by it. The blood
pressure really means the pressure of the blood within the large
arteries. It could be measured with any ordinary pressure gauge, if it
were not for the fact that we cannot very well cut into our bodies to
apply gauges to the arteries. For this reason it has been necessary to
invent means of finding out what the blood pressure is from the outside.
The way it is done is to put a band around the arm, press this band down
upon the arm until it squeezes the large arm artery shut, and then, by
means of a suitable gauge, find out how much pressure was required. Of
course, it is necessary to be able to tell when the artery has actually
been squeezed shut, so that the determination of blood pressure in human
beings is the work of an expert. Furthermore, blood pressure, as should
be clear from what has already been said, varies with every heartbeat.
It is at its maximum the instant the heart finishes emptying itself into
the artery, and falls off steadily, reaching a minimum just before the
next beat comes. The more elastic the arteries, the less difference
there will be between the maximum and the minimum blood pressure. The
reason for this will be clear when we think that if the arteries were
entirely rigid there would have to be a very high pressure during the
time the heart was actually beating to force the blood out through the
capillaries, but that between beats the pressure would fall off to zero.
The more elastic the arteries are, the more nearly do they exert a
steady pressure on the blood within them, and so the less will be the
difference between the maximum and the minimum pressure. When the
physician determines blood pressure, he really determines both the
maximum and the minimum pressure, in order that he may be able to judge
whether or not the arteries are as elastic as they should be. High blood
pressure just by itself might not mean much more than that the heart was
beating more rapidly than it should, but a high maximum pressure and a
low minimum pressure means nonelastic arteries. This in turn means that
the blood is forced through the capillaries in jets rather than in a
steady stream, and we may judge of the importance of having a steady
flow through the capillaries when we recall the well-known medical
proverb that “a man is as old as his arteries.” It is an actual fact
that the chronological age of an individual need not have much to do
with his physical age. If his arteries continue elastic over a long
period of years, he will be physiologically youthful, while if his
arteries become rigid he will be physiologically aged, no matter how few
his actual years upon earth may have been. Unfortunately we do not know
as much as we would like to about the causes of loss of elasticity in
the arteries. It does appear, however, that self-indulgence of various
kinds is apt to lead to loss of elasticity. For example, even the
moderate use of alcohol is now generally recognized by the medical
profession as a cause of impairment of elasticity in the arteries. It is
probable that intemperance in the use of various foods and drugs leads
also to this same condition.
We have just seen that the heart is obliged to maintain high blood
pressure in order to force the blood through the tiny capillaries. It
will be clear that the actual amount of pressure will depend in part
upon how much blood is forced through in a minute and in part upon the
extent to which the capillaries offer resistance. It is a familiar law
of friction that the smaller the tube the greater will be the resistance
it will offer to the passage of liquid through it, so that if the
capillaries change in size their resistance to the flow of blood through
them is bound to vary. The walls of the capillaries are very sparsely
provided with muscle fibers, but the very finest subdivisions of the
arteries, which are really no larger in diameter than the capillaries,
have much more smooth muscle in their walls. These muscles, as we have
already seen, can by their contraction or relaxation make the tiny
vessels smaller or larger. We have examples of this in the flushing and
pallor of the skin. What we wish to do now is to show how the flow of
blood through different parts of the body is controlled by changes in
the caliber of these tiny tubes. The muscles in the vessel walls have
the double nervous control commonly found in smooth muscles, and both
sets of nerves trace back to centers in the brain stem. One of these
centers causes the muscles to contract and the vessels to become
smaller; the effect of this is, of course, to increase the resistance to
the passage of blood through them. The nervous center which brings this
about is called _vasomotor_, or, more properly, the _vasoconstrictor_
center. Vasoconstriction means literally causing contraction of the
vessels, which is exactly what this center does. Not all the blood
vessels in the body are acted upon through the vasoconstrictor center.
Those of the skin and of the abdominal organs are under its control, but
those of the skeletal muscles are not. The result of activity of this
center, then, is to make it more difficult for blood to flow through the
skin and through the abdominal organs, but the ease of flow of blood
through the muscles is not affected. Of course, it will follow
automatically that the blood stream will be diverted in a large part
from the former regions into the latter. The skin and abdomen together
make up so large a part of the whole body that marked constriction of
the blood vessels in these two regions is bound to cause a considerable
increase in blood pressure.
A fairly high blood pressure is necessary for bodily well-being, because
only thereby is the brain assured of sufficient nourishment. To see why
this is so, we have only to remember that the brain is unfavorably
situated for receiving ample supplies of blood. It is at the top of the
body, so that the influence of gravity has to be overcome in forcing
blood up to it. Also it is completely inclosed in the bony skull, which
it in turn fills so completely that there is almost no room for the
accommodation of extra blood in it. In all other parts of the body a
rise in blood pressure stretches the arteries and so leads to there
being actually more blood within them, but there is no room for the
arteries in the brain to stretch, so that the total quantity of blood in
the brain cannot vary greatly from time to time. The only way in which
an increased blood supply can be obtained is by causing the blood to
flow more rapidly. This is precisely what happens every time the blood
pressure in the body rises. Whenever the vasoconstrictor center sends
nervous discharges into the blood vessels of the skin and abdominal
organs, causing them to contract, there is a diversion of blood from
them directly into the skeletal muscles and also a rise in blood
pressure due to the increase in resistance to the circulation, which
causes blood to flow more rapidly through the brain. The net result,
then, is improvement in the blood supply to the skeletal muscles and to
the brain. So important is the blood supply to the brain that the
vasoconstrictor center discharges actively throughout the waking part of
the day. If for any cause the activity of this center diminishes, there
will be an increased circulation in the skin and in the abdominal
organs, the blood pressure will fall, the circulation through the brain
will decrease, and along with it there will be a decrease in brain
function. After this passes a certain point unconsciousness will result.
This is what happens when one faints. For some reason or other the
vasoconstrictor center becomes inactive, and the series of events just
described is set in motion. Fainting ordinarily cures itself
automatically, because when consciousness is lost, the individual falls
over; this brings his head down on the level with the rest of his body,
makes it easier for the blood to flow through it, and so in a moment or
two consciousness will be regained. It is a mistake to try to scramble
to one’s feet immediately, because until the vasoconstrictor center
recovers its ordinary activity, raising of the head above the level of
the rest of the body is bound to result in its failure to receive
sufficient blood, and so faintness will come on again. It has long been
a practice to dash cold water in the face of a fainting person. The
physiological value of this is in the sudden stimulation of the sensory
nerves in the face by which is set up a stream of nervous discharges
which will play upon the vasomotor center, and arouse it again to
activity. Almost any vigorous sensory stimulation may have the same
effect.
During sleep the vasoconstrictor center is usually not very active; the
cause of sleep is not completely understood, but one of the most
satisfactory theories regarding it is that during the period of waking
the vasoconstrictor center becomes gradually more and more fatigued, and
so requires more and more stimulation to keep it active. This
stimulation may come in part through the ordinary channels of the sense
organs and in part from the higher brain centers, as when one keeps
awake by an effort of the will. Upon going to bed sensory stimulation is
cut off to a very large extent, also the will to remain awake is no
longer present. Under these circumstances the fatigued vasoconstrictor
center is under a minimum of stimulation and tends, therefore, to lessen
its activity. The result is that the blood pressure falls and presently
the circulation through the brain drops below the level of consciousness
and the individual is asleep. During the period of sleep the fatigued
center recuperates, so that it becomes more susceptible to sensory
stimulations and, in course of time, is aroused by such stimuli as
accompany the returning day to sufficient activity to restore the brain
to consciousness. Of course it will be perceived that there are many
things about sleep which are not satisfactorily explained on this
theory; in fact no one at the present time pretends that we can account
for it completely on this basis of changes of circulation through the
brain. It is believed, however, that they have a good deal to do with it
and it is certainly true that in healthy individuals the course of sleep
follows very closely the activity of the vasoconstrictor center. Undue
wakefulness in persons not suffering from disease can nearly always be
explained on the basis of excessive activity on the part of this
center. The activity may be the result of chemical stimulation, as when
persons are kept awake by drinking coffee or strong tea in the evening,
or by the persistence of adrenalin in the blood following a period of
great excitement. Mental activity, itself, tends to keep the
vasoconstrictor center whipped up, so that one who allows his mind to
work actively during the time when he should be asleep is very apt to
find sleep refusing to come when it is desired. The center is often
stimulated from the digestive tract; gastric irritation, even though not
acute enough to be recognized as indigestion, may cause wakefulness by
arousing nervous disturbances which play upon the vasoconstrictor
center. Flatulence, namely the presence of large volumes of gas in the
digestive tract, frequently acts as a mechanical source of irritation by
which wakefulness is induced. Evidently the factors favoring healthy
sleep are the inducing of fatigue, preferably by physical exercise, the
avoidance of chemical irritation or of excitement in the hours just
before going to bed, and finally the adoption of dietary habits which
shall insure good digestion. If an individual in whom all these
precautions are combined still continues chronically wakeful, the
trouble is sufficiently deep-seated to call for competent medical
attention.
Besides the vasoconstrictor center about which we have just been talking
there is in the brain stem a center which relaxes the tension of the
muscles in the walls of the blood vessels. This is known as the
_vasodilator_ center, and it acts in opposition to the vasoconstrictor.
In most parts of the body it does not appear to have any very great
importance for the simple reason that the blood is under such high
pressure that any relaxation of effort on the part of the
vasoconstrictor center leads at once to a forcing open of the blood
vessels. There are a few regions, however, in which the action of the
vasodilator center is of real importance; one of these is in the
skeletal muscles. These, as should be perfectly clear by this time, are
the seats of our most active functional metabolism. When the muscles are
active, great amounts of oxygen and food are being withdrawn from the
blood and large amounts of waste material, including carbon dioxide,
poured out into it. Only a very rapid circulation will take care of this
situation. At the same time the skeletal muscles are the most compact of
our living tissues. The cells are crowded together in making up the very
strong muscular machine by which our movements are performed. As the
muscles contract, they squeeze hard on the blood vessels passing through
them. In view of this situation it is very important that the blood
vessels be opened as widely as possible during muscular activity, and so
we find that the vasodilator center acts to improve the circulation
through the muscles, while they are active. In time of special
emergency, as we saw a moment ago, the vasoconstrictor center is at the
same time engaged in cutting down the blood supply to the skin and to
the abdominal organs, thus insuring to the muscles the maximum possible
nourishment through the blood stream.
Besides the skeletal muscles there is active functional metabolism in
the various secreting glands; a feature of their activity is that it
must be very great at certain times, but falls to little or nothing at
others; thus during the actual taking and digesting of food the various
glands which secrete digestive juices are exceedingly active, but in
the intervals they may be doing little or no work. They require a very
copious blood supply when they are functioning, but need very little
between times. The blood vessels flowing through all these glands are
under the influence of the vasodilator center, so that they are opened
as widely as possible while the glands are active. They, therefore,
receive much more blood in proportion to their size than they would if
it were not for the action of this center. Between times their blood
supply falls off to that which suffices for inactive tissues generally.
In time of emergency the action of the vasoconstrictor center upon the
vessels through these glands is such as to cut off the blood supply to
them almost completely. This is well illustrated in the dry mouth of the
frightened man, showing that the salivary glands have suspended
activity, a suspension due largely, if not wholly, to the cutting down
of the blood supply through them.
In the section just completed we have tried to give some idea of the way
in which the circulating blood provides the various tissues with the
materials they require and is adjusted automatically to meet variations
in demand from different tissues. Just one more point needs to be noted
in completing the account of the conveyer system. The tissue fluids
which serve as the connecting links between the circulating blood and
the individual cells are necessarily full of waste products, because
these come directly from the cells to the tissue fluids and afterward
pass on from them to the blood. The result is that the cells are
constantly bathed in a solution of their own waste products. There is
only one way in which relief from this condition can be obtained and
that is by moving the used tissue fluid away bodily and letting it be
replaced by fresh fluid. As a matter of fact, this happens; there is an
oozing of fluid through the walls of the capillaries from the blood into
the tissue spaces; this of course pushes the fluid already in those
spaces on ahead of it. If there were no place to which the fluid could
go, the result would be a swelling, as the tissue became more and more
filled with fluid. This is avoided ordinarily by a drainage system
whereby the tissue fluids are carried off as fast as more fluid comes in
from the blood, but when liquid is poured out faster than it will drain
off, as from a bruise or wrench, we do get a swelling.
The drainage system consists of very delicate vessels known as the
_lymphatics_, which come together into larger and larger vessels, just
as the veins do, and finally empty into the vena cava just at the point
where it enters the heart. There is no back pressure of blood here, so
that the movement of fluid through the lymphatics is not hindered. There
is no pump for forcing the lymph to move along; the very gradual motion,
which is all that is necessary to keep the fluid from accumulating in
the tissue spaces, is brought about by pressure upon the lymphatics
resulting from the bodily movements. The lymphatic vessels are like the
veins in having valves here and there along them. Whenever by any bodily
movement either a lymphatic vessel or a vein is squeezed, the valves
insure that the liquid shall be passed along in the direction toward the
heart and never in the reverse direction. In the case of the veins this
action is not absolutely necessary, since the heart itself is able to
maintain the circulation, although it does help, particularly in
bringing back the blood from the extremities. In the lymphatics this is
the only way by which movement of fluid is brought about. The result is
that when the body is perfectly quiet, there is very little movement of
fluid through the lymphatics; muscular activity, on the other hand,
leads to rather active movement through these vessels. The fact is well
illustrated in ourselves. One who sits for a long time humped over a
desk finds himself feeling very dull; to obtain relief he stirs about,
stretches, and yawns. The dullness was merely the result of the
stagnation of fluid in his tissues, causing the cells to be more or less
poisoned by their own waste products. By making active movements, these
stagnating fluids were forced along to be replaced by fresh liquid
direct from the blood and the beneficial effect is felt immediately.
_Massage_ properly applied has very much the same effect, although it is
doubtful whether as good results can ever be obtained thus as by actual
vigorous exercise.
At various places along the lymphatics are little spongelike lumps of
tissue known as _lymph nodes_; the particular spongy substance of which
they are composed is called _adenoid tissue_. This adenoid tissue acts
as a filter for the fluid passing through it. Any foreign particles,
living or nonliving, that get into the stream are caught in the lymph
nodes and held there more or less permanently. Most of the nonliving
particles that get into our tissue fluids are from the dust that we
inhale, which works its way through the mucous membrane of the
respiratory passages and so into the body fluids. The lymphatics that
drain the lungs carry along these dust particles and they lodge in lymph
nodes at the base of the neck. Persons who have lived in dusty regions
or have pursued a dusty livelihood, such as coal heaving, will have by
the end of their lives lymph nodes which are literally black with dirt.
The tonsils are lymph nodes at the base of the tongue. Unfortunately
they are so near the surface of the throat that they frequently become
infected from the throat itself, and so become the seats of pus pockets,
as already noted. Closely related to the tonsils are the masses of
adenoid tissue at the back of the throat which frequently grow to an
undue size in children, and are then known as adenoids. The harm done by
adenoids is chiefly mechanical; they may block the Eustachian tubes, and
so cause deafness, as already mentioned in Chapter IX, or they may
interfere with the free movement of the air through the nasal passages.
Children in whom this condition exists are mouth breathers. Adenoids,
like tonsils, are subject to infection, and so may give trouble by
becoming the sites of poison formation. Adenoids represent always an
overgrowth and for that reason may be removed without any possibility of
hampering the proper working of the body. Experience has shown that
children whose development appears to be hindered by the presence of
adenoids are almost invariably benefited by having them removed. The
tonsils are normal parts of the bodily structure and as such undoubtedly
have a regular work to do, but here again experience has shown that harm
from persistent pus pockets is so much greater than harm from loss of
function following their removal as to justify taking them out, whenever
pus pockets develop in them. There are enough lymph nodes in the region
about the throat, so that if tonsils or adenoids are removed any foreign
matters that get into the body fluids will still be filtered out.
[Illustration: Copyright, Keystone View Co.
X-RAYING THE TEETH AND JAW. HEALTH OFTEN DEPENDS ON THEIR GOOD
CONDITION]
[Illustration: Photo, Paul Thompson
A THROAT EXAMINATION--IMPORTANT IN DETERMINING PHYSICAL FITNESS]
One more function of the lymph nodes must be mentioned; this is their
property in preventing the spread of cancer cells. We now know that
so-called secondary cancers are the result of the spread of cancer cells
from the original seat of the cancer to other parts of the body, and
that this spread is much hindered by the ability of the lymph nodes to
catch the cancer cells and hold them. Unfortunately sooner or later some
of the cells will escape beyond the lymph nodes and so spread the
malignant growth throughout the body, but until this happens the cancer
is confined to the region where it started, and it is during this period
that complete cure by surgical means is possible. It is because of the
imminent danger of the escape of cancer cells beyond the restraining
lymph nodes that relief by surgery should be sought at the very earliest
possible moment. Delay, whether due to carelessness or any other cause,
is as certainly fatal in the case of cancer as in any other disease for
which a cure is known.
CHAPTER XV
THE SERVICE OF SUPPLY OF FOOD
In order for the blood that circulates through the body to pass on to
the body cells the materials which they need, it is evident that the
blood itself must have some source from which to obtain the materials.
Our present task is to examine this source. In Chapter IV we talked
about food and its uses; here we are concerned with the way in which the
foodstuffs are taken into the body and prepared for use. We know that
the material which we take in is part solid and part liquid, giving rise
to the familiar distinction between food and drink. For our present
purposes this distinction has no importance and will not be made. What
we do have to note is that of the complex mixture of materials which
makes up any ordinary meal, some of the substances are ready to be taken
up at once by the blood and to be distributed around the body; others
have to go through a preliminary course of preparation. All of the
dietary accessories, so far as we know, pass from the digestive organs
into the blood without change. All or nearly all of the energy-yielding
foods, on the other hand, must have a preliminary preparation to which
we give the name of digestion; the operation of digestion being to break
down the complex foodstuffs that are eaten into simpler materials of
which the body can make use. Digestion is made up of a series of
chemical changes by which the large molecules of the original foodstuffs
are broken into smaller and simpler molecules. This breaking down of
large molecules into smaller is a very common kind of chemical process.
A feature of it is that under ordinary circumstances the breaking down
of the large molecule goes on very slowly, but if the right conditions
are provided the breaking down proceeds rapidly. The particular
condition which is necessary is the presence in the solution where the
large molecules are breaking down of something which will hurry up the
process. There are a good many kinds of substances which have this
ability to hasten molecular decomposition; those that do it in carrying
on digestion are given the name of _enzymes_. We do not know just how
the enzymes act; we merely know that when they are put along with the
substance which is to be decomposed, it goes to pieces very much more
rapidly than if none of them was present. The enzymes themselves are not
used up in the process, so if time enough is allowed, a very small
amount of enzyme can bring about the decomposition of a very large
amount of material. In our bodies the whole digestive process consists
of a succession of decompositions of complex materials into simpler ones
under the speeding-up influence of enzymes. We have a number of
different kinds of foodstuffs to be decomposed and a corresponding
number of enzymes.
The process begins in the mouth; here the food is chewed and moistened
with saliva before being swallowed; both the chewing and the moistening
are important to insure good digestion later on. The enzymes have no
particular ability to penetrate a mass of food material; what they do is
to attack it from the outside and work in as it decomposes. Since
enzyme action is thus a surface action, it is evident that the larger
the surface the more efficient will be the action. Chewing is nothing in
the world but a mechanical breaking up of the food to get the largest
possible surface. We have sufficient proof of its importance in the
digestive disturbances that arise as the direct consequence of improper
chewing. Of recent years it has been realized that undernutrition,
particularly in children, is often a result of the failure to chew the
food properly; it has been found, furthermore, that bad teeth or
improperly shaped mouths are very frequently responsible. For this
reason in most of our large cities dental clinics are being established
for the purpose of inspecting and, if necessary, caring for the mouths
of school children. The result of this work is to improve the general
average of health among children simply by increasing the extent to
which the food is chewed. This fact, together with that described in
Chapter XIII as to pus pockets, should impress upon both parents and
teachers the fundamental importance of proper care of the teeth. This
includes not only the prevention of decay by daily thorough cleaning and
the securing at intervals of not more than six months of dental
inspection with treatment where necessary, but also in the case of
children with deformed jaws the special treatment necessary to bring the
teeth into position for effective chewing. Too much stress can scarcely
be laid on the importance of these precautions.
The moistening of the food with saliva is likewise important to good
digestion. As a matter of fact, without it even the act of swallowing
would be impossible. One can easily prove that insufficient moistening
prevents swallowing by eating a rather large quantity of a very dry
food, like crackers. A common habit of people is to moisten the food
with water which they drink instead of waiting for it to be properly
moistened by saliva. Contrary to popular belief there is absolutely no
objection to drinking water at mealtime. In fact, the presence of the
additional liquid in the alimentary tract is probably beneficial rather
than the reverse; there is, however, a very serious objection to using
water for washing down half-chewed food. The best possible way to judge
whether the food is sufficiently chewed is by observing whether it has
been sufficiently moistened so that it will be swallowed easily. If so,
the chances are that the chewing has been sufficient. Between mouthfuls
as much water may be taken as one desires, although if ice water is
drunk, it should be held in the mouth until the worst chill is taken off
before being swallowed, so that it will not chill the stomach. Since the
comfort of drinking ice water is in the cooling of the mouth and throat
and not at all in the cooling of the stomach, this increases rather than
diminishes the enjoyment one gets from a glass of it on a hot day. So
much satisfaction can be obtained by proper drinking habits, that it is
a pity to allow the health to be injured by improper habits to such an
extent as to necessitate, as frequently happens, the complete
abandonment of water drinking with meals.
The saliva is primarily for the purpose of moistening the food, but
besides this it is also a definite digestive juice, because it contains
one of the digestive enzymes. This enzyme, to which is given the name
_ptyalin_, acts upon starch, changing it to sugar, but not to the
particular sugar of which the body makes use. Perhaps we should digress
for a moment to say that there are chemically a number of sugars. These
are in many respects alike, although they also vary a good deal among
themselves. For example, some are much sweeter than others; glucose,
which is a sugar that can be made from cornstalks and other plant
products, is not nearly so sweet as cane or beet sugar, although one is
fully equal to the other so far as nutritive value is concerned. Of all
the various kinds of sugar that exist, only those in the glucose class
can actually be used by the body, so that all other sugars have to be
changed by digestion into a glucoselike sugar before passing on into the
blood. The enzyme of saliva converts starch not into glucose but into a
more complex sugar to which is given the name of maltose. It, therefore,
starts digestion, but does not carry it through to completion. Starch
digestion begins during the course of chewing and mixing the food with
saliva; it goes on while the food is being swallowed, and for a short
time after it enters the stomach, being stopped there sooner or later by
means which will be described in a moment. No other real digestion
occurs in the mouth, although the chewing and moistening are of very
great importance in preparing the food for the digestion that is to come
further along.
The food that is swallowed passes down the esophagus and enters the
stomach. Before speaking of this it will be necessary to recall what was
said in Chapter VII about the behavior of the stomach walls. Between
meals these are in a relaxed and flabby state, with the opposite walls
lying more or less in contact. There is usually a little liquid and some
swallowed air in the stomach, so that it is not actually empty, even
when we speak of it as being so. Just before mealtime the walls of the
stomach draw up so that instead of a flabby bag we have a fairly tense
tubular organ. It is at this time that the contractions of the stomach
wall, which we recognize as hunger, begin to come on. There is a
sphincter muscle between the esophagus and stomach which closes the
opening tightly and so prevents the pressure within the stomach from
forcing gas or liquid back up into the esophagus. This sphincter opens
automatically only in connection with the act of swallowing. Every time
we make a swallowing movement a sort of wave passes down the esophagus,
and when this wave arrives at the stomach the sphincter relaxes,
allowing whatever was moving down the esophagus to enter. If one watches
a horse or cow in the act of swallowing, the rather deliberate progress
of this wave down the neck can be followed. What happens as we eat a
meal is that every mouthful, as it is swallowed, is passed through the
sphincter into the stomach and room is made for it by a gradual
relaxation of the stomach wall, so that, as we saw in Chapter VII, the
pressure of the stomach against its contents stays fairly steady, in
spite of the fact that more and more material is being taken into the
stomach from the esophagus. One result of this behavior of the stomach
is that the first food that is swallowed is nearest to the stomach
walls, and that which is swallowed afterward is nearer the middle, being
inclosed on all sides by the previously swallowed food. This is fairly
important because the gastric juice is secreted by glands in the wall
of the stomach and so will get in contact first with the food that is
swallowed first, and only afterward will reach the food that was
swallowed later. Occasionally the sphincter between the stomach and the
esophagus relaxes unexpectedly; this is said to happen more often in
smokers than in nonsmokers. The result is that some of the sharply acid
stomach contents are forced up into the esophagus and vigorous
swallowing is necessary to crowd them back down into the stomach. The
burning sensation which accompanies this is known as heartburn,
although, as we have just seen, it is really entirely a matter of the
esophagus and has nothing whatever to do with the heart.
Although the stomach carries on a certain part of the work of digestion,
its primary purpose, as we shall see presently, is to serve as a storage
place into which a considerable amount of food can be placed in a few
moments and so enable us to do our eating at three or four definite
meals instead of little by little throughout the day. When we rise from
table after any meal, we have in our stomachs roughly one-third of our
total food supply for the day, the exact proportion depending, of
course, on our individual habits as regards our distribution of
food-taking among the three meals. During the two to four hours
following the meal this accumulated material in the stomach will be
passed along little by little to the small intestine, which is the real
digestive organ of the body. In the course of this time there will be
some additional digestion within the stomach, but not enough to prepare
any foodstuffs for actual use by the tissues. The outlet from the
stomach to the small intestine is guarded by
[Illustration: THE ALIMENTARY CANAL
_A_, stomach; _J_, _I_, small intestine; near _CC_, vermiform appendix;
_AC_, _TC_, _DC_, large intestine; _R_, rectum. (Martin’s “Human Body”)]
another sphincter similar to that which closes the opening between the
esophagus and stomach. This sphincter is ordinarily closed tightly,
relaxing occasionally to let a little food through into the small
intestine. It is interesting to note, however, that the sphincter does
not hold against some materials; for example, water that is drunk
between meals passes rapidly from the stomach on into the small
intestine. It is said that raw oysters, raw white of egg, and other
materials of a similar consistency also pass the sphincter immediately.
Most other kinds of food material cannot pass unless the sphincter
actually relaxes.
Shortly after food begins to enter the stomach churning movements are
set up. These consist of regular contractions of the smooth muscle
beginning at about the middle of the stomach in the form of a ring and
so causing a deep depression to form right around the stomach, which
then travels along toward the sphincter at the outlet. This, of course,
crowds the food up toward that end; but if the sphincter does not relax,
the food instead of escaping simply squirts back through the space left
in the middle of the ringlike construction and so is actively churned. A
regular procession of these waves travels over this part of the stomach
during the whole time that gastric digestion is going on. The outer half
of the stomach, the side toward the esophagus, does not join in this
churning motion. The walls here remain quiet, merely pressing upon the
food to crowd it up into the part where the churning is going on as fast
as room is made for it there. At intervals during this process the
sphincter relaxes and a small mass of food is crowded through into the
small intestine; then the sphincter closes again, preventing more from
passing until the former mass has had time to be acted upon by the
digestive juices in the intestine. The way in which this sphincter is
controlled is one of the very interesting facts of physiology, but
before describing it we shall have to say something about actual gastric
digestion.
The juice which the glands in the stomach wall secrete is called gastric
juice. This contains three important constituents, first of which is
hydrochloric acid. From the standpoint of chemistry it is a very
interesting thing that the gastric glands should he able to manufacture
a mineral acid like hydrochloric acid. That they do so, however, is
proven by the facts of digestion in everybody. In addition to
hydrochloric acid gastric juice contains two enzymes; the first is
pepsin, which begins the digestion of protein foods, although it does
not carry it through to completion. The other constituent is the enzyme
rennin, which clots milk; this property appears to be useful in that
clotted milk will not pass the sphincter into the small intestine as
quickly as it would if it were liquid, and so the digestive processes
can go on in milk after clotting just as it goes on in any other of our
foods. Since milk is the chief, and in many cases the only, food of the
young it is of course very important that its digestion should be very
efficiently carried on. As the food is churned by the stomach muscles,
it becomes mixed with gastric juice. Any milk that is present will be
clotted and any protein will begin to be digested. The food will also be
mixed with hydrochloric acid. It happens that the salivary enzyme,
ptyalin, will not act in the presence of an acid, so that as soon as any
of the food in the stomach comes in contact with hydrochloric acid of
the gastric juice, the digestion of starch by ptyalin stops. Since
gastric juice is mixed only with that part of the food which is being
churned, the other portion, that in the part of the stomach toward the
esophagus, goes right on undergoing salivary digestion. So, contrary to
the old idea that salivary digestion is unimportant because there is not
time enough during the chewing and swallowing of the food for it to go
on effectively, we now know that the latter parts of the meal
particularly may be very thoroughly acted upon by ptyalin before the
action is stopped through the contact of this part of the food with the
acid of the gastric juice.
The sphincter between the stomach and small intestine is operated by
hydrochloric acid, making up a remarkably ingenious arrangement for
securing the emptying of the stomach as fast as the food is ready to be
passed on into the small intestine. In order that gastric digestion may
have full opportunity, it is necessary that the food should be
thoroughly mixed with the gastric juice. As soon as this has happened,
however, it is proper that the food should be passed along to be acted
on by the rest of the digestive juices. Until that part of the food
which is next to the outlet sphincter is thoroughly mixed with gastric
juice there will be no surplus of hydrochloric acid by which to operate
the sphincter, but as soon as the mixing is complete there will be a
surplus. We have here a simple case of stimulation by a chemical
substance; the mere contact of the acid with the part of the sphincter
which fronts on the stomach is sufficient to stimulate the smooth
muscles in it to relax, and, of course, as soon as relaxation occurs,
the pressure of the churning movements upon the food will crowd that
part of it which is nearest to the sphincter through into the small
intestine.
Now we encounter the second part of the action which is just as
interesting as the first part. The small intestine is a rather narrow
tube; its cavity is not much more than three-quarters of an inch or so
in diameter. If the outlet sphincter from the stomach were to stay
relaxed after it once let go, the rapid pressing of the food from the
stomach would fill the small intestine quickly to a distance of several
feet. It happens, however, that the next of the digestive juices to act
upon the food are the pancreatic juice and bile, which are secreted
respectively by the pancreas and liver, and are poured into the small
intestine by ducts from these organs just a few inches beyond the outlet
sphincter from the stomach. In order for these juices to mix well with
the food it is important that only a little food come into the small
intestine at a time. Otherwise a large part of it would pass the opening
of these ducts so quickly that there would be no chance for it to become
mixed with the juice from them. What actually happens is that the
sphincter relaxes under the stimulus of surplus hydrochloric acid and
then as soon as a little food has passed through closes again. The
closing as well as the opening is operated by hydrochloric acid, but
there is this difference: the opening is the result of the stimulation
of acid upon the stomach side of the sphincter; when the same acid comes
in contact with the intestinal side of the sphincter, its presence there
causes the sphincter to contract. Of course, this means that whenever
any food passes out from the stomach into the small intestine the
sphincter is stimulated by acid simultaneously from both sides, but
under that condition the stimulus which causes the sphincter to contract
is more potent than that which causes it to relax, so that whenever
there is acid on both sides the sphincter will be shut. The only way in
which it can be opened to allow more food to pass out is to get rid of
the acid on the intestinal side. This is fairly quickly accomplished,
because both the pancreatic juice and bile are strongly alkaline
liquids, so that as fast as they mix with the food they tend to
neutralize the acid; the result is, of course, that by the time these
digestive juices have impregnated the food mass thoroughly and so have
accomplished their purpose, the stimulus which keeps the sphincter
closed has disappeared and there is left only the stimulus to open it,
due to the presence of acid in the stomach. As soon as this condition is
reached, the sphincter will open again, another mass of food will pass
through bringing its acid with it, which, of course, closes the
sphincter promptly, and so the whole story goes on again. This action
has been described in detail because it is one of the very best examples
we have of the remarkably ingenious arrangements by which the complex
bodily functions are carried on automatically.
From what we have just said it is evident that the proper emptying of
the stomach and, therefore, the proper carrying on of the whole process
of digestion, depends upon the formation and outpouring of ample
supplies of acid-containing gastric juice. A failure of gastric
secretion is bound to be followed by a failure of the stomach to empty
itself. Instead of going on to the small intestine where digestion is
completed and the digestion products passed on to the blood stream to be
used by the body, the food mass lies inert in the stomach. After a few
hours, if not immediately, the acute symptoms of indigestion are certain
to develop. We are describing not a theoretical possibility but an
actual happening in the life of thousands of people, sometimes almost
daily. Indigestion is one of the great causes of misery and of impaired
efficiency throughout the civilized world. It is said that Napoleon lost
the Battle of Waterloo on account of an acute indigestion which befell
him on the morning of the crucial day.
There are several causes of indigestion. Among them should be mentioned
food poisoning; that is, the inadvertent taking of some material which
instead of being a food proves to be actively poisonous. A second type
about which we shall have something to say presently is the result of
poisoning by substances which are products of the putrefaction of the
intestinal contents. A third type, and the one in which we are
particularly interested at this moment, is the one which results from
the failure of gastric juice to be properly secreted. A fourth
occasional cause of indigestion, which should be mentioned in passing,
is that which results from eyestrain, and is avoided by properly fitted
glasses. There is comparatively little food poisoning; most persons who
are susceptible to poisoning from particular kinds of foods find it out
promptly and learn to avoid the article which poisons them. A good deal
of indigestion is the direct result of intestinal sluggishness; this
usually comes on gradually enough to give warning of its approach. Sharp
attacks for which there seems to be no justification are nearly always
the result of the failure of gastric secretion. Because of the practical
importance of this topic a very large amount of study has been devoted
to it, and its story makes one of the very fascinating chapters of
physiology.
Our space permits us to touch on it only very lightly. It begins with
the time back in the middle of the eighteenth century when a couple of
Italian investigators fed sponges inside perforated metal capsules to
birds and animals and after recovering and squeezing the sponges
demonstrated that the juice which had been soaked up would dissolve
meat. The scene then shifts from Italy in the middle of the eighteenth
century to our own country at the close of the first quarter of the
nineteenth, when a French-Canadian lad, by the name of Alexis St.
Martin, was brought in to the frontier army post on Mackinac Island with
a gaping gunshot wound in his side. The victim of this accident
recovered, but meanwhile a permanent opening into his stomach was left.
The army doctor on duty at that post, William Beaumont by name,
perceiving the unique opportunity for the study of digestion that was in
his hands, employed the young man in his family and for many years
studied the digestive process as it went on in his stomach. Next we turn
to Russia, where an eminent physiologist, who was still living at last
reports, demonstrated by making artificially an opening into the stomach
of animals, similar to the one which was made by accident in the case of
St. Martin, that the secretion of gastric juice is under the control of
the nervous system and, furthermore, that it is under the control of
that part of the nervous system which we have already learned regulates
the functioning of smooth muscles and glands. Our present knowledge of
the nervous system would lead us to expect nothing else, but at the time
when these discoveries were made much less was known than now about how
the glands, as well as the smooth muscles, are controlled. This Russian
investigator showed that it is not necessary for food to enter the
stomach in order for gastric juice to be secreted. The enjoyment that
accompanies the taking of food into the mouth and so getting its taste
and flavor is sufficient to arouse nervous disturbances which excite the
gastric glands to activity. On the other hand, unfavorable emotions such
as anger, fright, or worry, prevent the discharge of these nervous
disturbances and so the glands remain inactive. Hand in hand with this,
as we saw in Chapter XII, there will be a complete absence of muscular
activity in the stomach; not only does the food fail to be churned, but
gastric juice fails to be poured out. No digestion goes on in the
stomach nor is the food passed on into the small intestine to be
digested there. We can expect nothing else than that indigestion will
come on. It appears that in human beings, with their high nervous
organization, simple weariness may suffice to hinder the outpouring of
the nervous disturbances upon which gastric secretion depends, so that
one may not be worried at mealtime, but may be simply overtired and
still fail of digesting the food. Fortunately we have means of avoiding
this; it has been shown that meat extracts have a definite chemical
effect upon the gastric glands, arousing them directly to activity. A
hot, well-flavored soup at the beginning of a meal is a great aid to
digestion, both because it tends to arouse us to active enjoyment of the
meal and also because of the chemical influence it exerts directly upon
our gastric glands. Cheerful conversation at table is an aid to
digestion, because it contributes to our enjoyment of our food. Heated
or angry discussion is to be avoided because of the danger that
unfavorable emotional disturbances may be aroused, hampering or even
putting a stop to the digestive activities.
We have here a powerful argument in favor of good cookery; some may be
inclined to think that food is food and that one kind is about as
nourishing as another. Not only theoretically, but in practice, food
that is attractive both in appearance and in flavor benefits the body
more than unattractive and badly cooked food. Of course, we must
recognize that no statement of this sort has absolutely general
application. The old proverb which says that “hunger is the best sauce”
continues true. Hard-working manual laborers and actively playing
children enjoy and digest food that is indifferently attractive. Brain
workers and people of moderate health are better for an attractively
prepared dietary.
We have now traced the food into the small intestine and will remember
that up to this point the actual act of digestion has not gone very far.
The enzyme of saliva has decomposed part of the starch, or perhaps all
of it, into a complex sugar; the enzyme of gastric juice has split some
of the very complex proteins into simpler ones. Of the final products of
digestion none has yet been formed; their formation is the work of the
small intestine. Into the upper end of it, as we have already seen,
pancreatic juice and bile are poured from the pancreas and liver
respectively. The pancreatic juice contains three enzymes; one of them
is identical with that of saliva and completes the decomposition of any
starch that the saliva has failed to act upon; the second is an enzyme
which acts upon proteins, differing from pepsin of gastric juice in that
it carries the splitting of proteins to completion; the third is the
enzyme which acts upon fats. Fats are the most troublesome of all the
foodstuffs from the digestive standpoint, because they are insoluble in
water. Not only do they themselves offer difficulty, but they smear over
the other food masses and make it hard for the digestive juices to get
at them. Although the enzyme for digesting fat is secreted by the
pancreas, its successful working depends on the presence of bile from
the liver. If there is an obstruction of the bile passages, so that bile
cannot be poured out into the small intestine, the digestion of fats
stops and, along with it to a considerable extent, all the other
digestive processes of the intestine. There is a familiar common
condition known as jaundice, in which the bile passages become stopped,
usually as the result of inflammation. When one is suffering from this
condition, it is very important that not much fat be included in the
diet; otherwise there is likely to be trouble with indigestion.
In addition to the pancreatic juice and bile there is a secretion known
as intestinal juice which is poured out by small glands scattered along
the wall of the small intestine. These secrete enzymes which convert the
various kinds of sugar into the particular sugar, glucose, which is the
one the body can use. The glands also secrete an enzyme which can carry
on to completion protein digestion. Thus there is ample insurance that
all the foodstuffs shall be made ready for the use of the body.
The next thing in order is for the digested foods to pass from the
digestive cavity into the blood. This process is known as absorption.
The first part of the alimentary tract from which absorption might occur
is the stomach, but, as we have just seen, the process of digestion is
not carried far enough in the stomach to fit the foods for going on into
the blood stream. Absorption from the stomach is, therefore, undesirable
rather than desirable. This is interesting because many condiments,
particularly the hot spices and mustard, irritate the stomach lining in
such a way as to promote absorption through it. Formerly this was looked
upon as an argument in favor of the use of these condiments, but we now
realize that it is rather an argument against them. The same applies to
alcohol. Alcohol irritates the lining of the stomach and produces
absorption through it. Unfortunately for those who desire a
physiological argument in favor of the use of alcohol at meals this is a
disadvantage rather than an advantage. The great region of absorption is
the small intestine, where the digestive process is completed and where,
therefore, absorption can properly go on. As to the method of absorption
not a great deal needs to be said; there is a very delicate membrane
lining the intestine, and just beneath this are the very numerous
capillaries through which the blood is flowing; also in the spaces
between the capillaries is the tissue fluid which drains into the
lymphatic system. The products of protein digestion and glucose pass
through the lining membrane of the intestine and through the capillary
walls directly into the blood stream and are carried along in it. The
digested fats, on the other hand, after passing through the intestinal
lining enter the tissue fluids instead of the blood. Incidentally it
should be noted that they are rebuilt into tiny fat droplets during the
process of absorption, so that what we find in the tissue fluids of the
intestine are true fats and not digestion products of fat. These tiny
fat droplets pass away from the intestine by way of the lymphatic
system. This, as we saw in the last chapter, drains finally through
large lymphatic vessels into the great veins just at the entrance of the
heart, and so the fat finally reaches the blood stream, but not so
directly as do glucose and the digestion products of protein.
There is a reason for this difference in the handling of fats and the
other foodstuffs which will be clear when we recall the point made in
the last chapter that all the blood which flows through the walls of the
intestine is collected in the portal vein and must pass again through
the capillaries of the liver before entering the circulation at large.
What happens is that all the digestion products, except the fats, pass
through the capillaries of the liver, but the fat gets into the blood
stream by another way and does not go through the liver en route; for
some reason or other it seems to be to the advantage of the body that
the fat should not be allowed to circulate through the liver
capillaries. It is, as we shall see in a later chapter, a distinct
advantage to have the glucose pass through the liver, and there is
probably also a good reason for having the digestion products of protein
take that course.
The digesting food is moved along the small intestine by contractions of
its muscular wall, which travel along slowly in the form of a wave, very
much as the wave of swallowing passes down the neck of a cow or horse.
These waves do not occur regularly but only now and then; what happens
is that several inches or two or three feet of the small intestine will
be filled with a food mass which stays for a while at that place,
digestion going on in it, and the digested food being absorbed through
the wall. Both the digestion and the absorption are aided by a kind of
churning motion made up of a series of contractions spaced about an inch
apart along the part of the intestine where the food mass lies. These
contractions form and disappear quite regularly at the rate of several a
minute. They have the effect of squeezing the food mass rhythmically,
but do not move it away from the place where it is lying. After this
churning has been going on for a while it subsides, leaving that part of
the intestine entirely quiet; then a contraction wave sets in at the end
of the food mass toward the stomach and pushes the mass bodily along the
intestine to a new section where the wave dies out and the churning
motion begins again. This process is repeated with every food mass as it
comes through from the stomach into the intestine until, finally,
usually three or four hours after the first food has commenced to come
into the intestine from the stomach, the whole mass has been propelled
the length of the small intestine and what is left has been passed on
through into the large intestine. The small intestine is about
twenty-five feet long in man; flesh-eating animals of about the same
size have intestines only about one-half as long; grazing animals the
same size, on the other hand, have intestines two or three times longer
than those of man; this intermediate length of the human intestine is
ordinarily looked upon as indicating that man is adapted to a mixed
diet, being neither strictly a flesh-eating nor strictly a
vegetable-eating animal.
During the slow passage of the food mass through the small intestine the
processes of digestion and absorption are completed, so that very little
material enters the large intestine except the indigestible parts of the
original food. These are made up mostly of cellulose and similar
indigestible vegetable materials, but they include also indigestible
fragments of gristle and other meat remnants. It is also worthy of note
that the amount of water does not decrease much during the passage of
material through the small intestine; in other words the material that
enters the large intestine is about as liquid as that which passes from
the stomach in to the small intestine. This does not mean that there is
no absorption of water from the small intestine; in fact, water that we
drink is probably absorbed quite rapidly; what it does mean is that
enough water is secreted into the intestine from the blood in the lower
parts to keep the whole mass liquid; this is of course important as an
aid both to digestion and absorption.
The large intestine, as its name implies, is a tube of much larger
diameter than the small; it begins low down on the right-hand side,
passes up to about the level of the stomach, then across the body and
down on the left-hand side, making thus a sort of inverted “U” in the
abdomen; the space within the “U” is filled up by the loops and coils of
the small intestine. The first part of the large intestine is devoted to
the absorption of water; the result of this absorption is to leave the
contents of the large intestine in a semidry condition; absorption of
water goes on continuously, so that the longer the material remains in
the large bowel, the drier will it become. In the ordinary course of
events the material, as it is dried out, is passed along through the
large intestine and at intervals, which should not exceed twenty-four
hours, the accumulated material is discharged from the body.
Various facts about the functioning of the intestine which have seemed
to many people mysterious are easily explained on the basis of what we
have next to talk about, which is the presence both in the small and in
the large intestine of flourishing colonies of bacteria. It has perhaps
not occurred to most of us that we are in the position of involuntary
hosts to enormous numbers of these microscopic plants, but that is
exactly what we are. There is no possibility of preventing this, since
the combination of warmth, moisture, and abundant food, which is
afforded within the intestine, is the most favorable possible situation
for many kinds of bacteria, and no effort on our part would enable us to
be free from them. Fortunately, in the ordinary course of events, we are
not affected one way or the other by their presence. In the small
intestine their chief diet consists of the sugars which are produced as
the result of starch digestion or are eaten directly. In connection with
nourishing themselves from these sugars, they bring about what is known
as alcoholic fermentation; that is, some of the sugar is converted into
alcohol with a by-product of carbon dioxide. Exactly the same thing
happens when yeast develops in a mass of dough; in the latter case the
bubbles of carbon dioxide are what we are after, since they are what
makes the bread light, the alcohol being driven off by the heat of
cooking. In the alcoholic fermentation that goes on in our small
intestines the carbon dioxide is a waste product and is passed on out of
the body, but the alcohol is absorbed and used as fuel. Little has been
said thus far about the physiological effects of alcohol. At this point
all we care to say is that alcohol can be oxidized by the body tissues
with the liberation of energy and so is a perfectly good fuel food. In
the minute amounts in which it is absorbed from the small intestine it
is utilized completely and there is not the slightest reason to suppose
that it has any ill effects whatever. The objections to the use of
alcohol, which have led to its abolition as a beverage in this country,
depend on certain definite pernicious drug effects which it shows when
consumed in any but the smallest amounts, and which destroy the
usefulness of what would otherwise be a valuable article of human diet.
Fermentation within the small intestine ordinarily goes on entirely
unperceived by us. Certain foods, or the invasion of the intestinal
tract by certain species of bacteria, may change the fermentation in
such a way that irritating substances are produced which cause the
movements of the small intestine to be very greatly increased; its
contents are swept on through in very much less than the usual time and
we have the condition known as diarrhea.
Within the large intestine bacterial action goes on fully as vigorously
as in the small, and because of the smaller relative amounts of
digestible food and of water a much larger proportion of the intestinal
contents consists of the bodies of the bacteria and of the products of
their metabolism. Since the sugars have been completely absorbed out by
the time the food reaches the large intestine and the remaining
materials upon which the bacteria can feed are more largely of the
protein class, the bacterial action changes from fermentation to that
which we commonly describe as putrefaction. This type of action, instead
of giving rise to alcohol and carbon dioxide, produces a number of
highly offensive compounds; many of these can pass through the lining
membrane of the large intestine into the blood and so can circulate
around the body. Of late years we have realized that these products of
intestinal putrefaction are highly poisonous, especially if present in
the blood stream in any considerable amount. The term “auto-intoxication,”
which we run across frequently in health literature, means strictly the
poisoning of the body by the products of intestinal putrefaction. Of
course, the condition is much aggravated if putrefaction is allowed to
go on for too long a time. The ill feelings which result from
constipation and which in the life of very many people constitute a
really serious impairment of health are the direct result of poisoning
by these putrefaction products. The obvious remedy is the avoidance of
intestinal sluggishness; unfortunately this is easier in theory than it
often is in practice. Probably two conditions of modern life are chiefly
responsible: the first is our tendency to make the diet more and more
highly concentrated; that is, to leave out of it more and more the
indigestible parts. The result is that not enough indigestible material
enters the large intestine to make a sufficient bulk upon which the
intestinal muscle can work in moving the mass forward. While bulk is
accumulating, both absorption of water and putrefaction are going on,
until by the time a sufficient mass is present, it is so dry that the
muscles are not able to move it along and it has produced undue
quantities of poisons. Obviously the way to treat this condition is by
eating more indigestible material. For this purpose there is nothing
better than apples in their season. The old proverb “An apple a day
keeps the doctor away” has very sound common sense back of it. Apples
are not ideal for this purpose for all people, since they frequently
cause a distressing evolution of gas or even headache. Nearly everybody,
though, by experimenting, can find a time of day at which an apple can
be eaten without any digestive trouble whatever, and frequently with
considerable benefit. Apples are valuable simply for their bulk of
indigestible substance. The process of cooking converts most of this
into digestible material, so that for this particular purpose they must
be eaten raw. There are various other fruits such as figs, prunes, and
raisins which function similarly and in addition have a direct
stimulating effect upon the intestinal movements and so favor the
discharge of material. Some people can eat popcorn to good advantage,
although others suffer from gas distress if they do so.
The second condition of modern life which favors intestinal sluggishness
is the sedentary habit which so many of us, both men and women, have. A
vigorous outdoor existence is practically never complicated by
auto-intoxication. Anyone who can maintain habits of active exercise
will usually find himself troubled little by this condition. One further
point should be made, and this can scarcely be over-emphasized, since it
probably has as much to do as any other single factor in the avoidance
of auto-intoxication; this is the development of regular habits in the
matter of evacuation of the bowel. It is a general fact of the operation
of smooth muscle that it readily develops certain habits. We have
already seen an example of this in the behavior of the stomach in
connection with mealtime. As we have noted, the stomach lies flabby
between meals and enters into a state of tension just about the time
that we are in the habit of eating. This adjustment is made no matter
what particular habits individuals may have. Those who habitually eat
only two meals a day will have this tightening of the stomach twice a
day; others whose habit it is to eat five times a day will have a
similar tightening five times in the twenty-four hours. Similarly the
large intestine can establish a regular habit with regard to
evacuation. This is best done in childhood, so that parents by insisting
upon regularity in their children in this respect can usually assure
them of a lifetime in which there will be little trouble from
auto-intoxication. On the other hand, parents who are neglectful of
their children’s welfare in this respect are laying up for them a
lifetime of trouble and very much discomfort.
In concluding this part of our discussion we should note that artificial
stimulation to intestinal activity should be regarded as a measure of
last resort and under no circumstances as a habitual means of inducing
evacuation. Persons who allow themselves to become dependent upon
laxatives are laying up for themselves trouble, since these invariably
become less and less effective, making it necessary to increase the dose
and finally establishing a condition in which only the vigorous efforts
of a physician will restore the body to normal. A safe general rule is
that anyone who finds himself becoming dependent upon laxatives should
immediately put himself under competent medical care for the purpose of
restoring his system to normal functioning in which he will not be
dependent upon drugs.
CHAPTER XVI
THE SERVICE OF SUPPLY OF OXYGEN
Every cell must have oxygen for its metabolism. This it must get from
the tissue fluid upon which it fronts, and tissue fluid in turn must get
it from the circulating blood. The blood in turn has to get it from
somewhere and the place from which the blood gets it is called a lung or
a gill, according as the animal breathes air directly or gets its oxygen
out of the water. The purpose of the present chapter is to trace oxygen
from the air through the blood to the tissue fluids and so to the living
cells. We saw in the chapter on “Blood” that we have a special
substance, the hemoglobin, which helps in the transportation of oxygen.
We shall have occasion here to see how it does this. It is well to bear
in mind that for practical purposes we include the methods by which
carbon dioxide is gotten rid of along with the study of the supply of
oxygen, so that although this chapter carries the heading “The Service
of Supply of Oxygen,” we shall also study in it how the carbon dioxide
is carried away. This is a convenient way of dealing with the subject,
because the two gases are handled in very much the same manner; it is
also made necessary by the fact that the control of the apparatus by
which these gases are handled is so interwoven that the transportation
of one could not well be studied without giving attention also to the
transportation of the other.
The problem of the oxygen supply and of the removal of the carbon
dioxide is in theory very simple; the air contains a large percentage of
oxygen and a very small percentage (three parts in ten thousand) of
carbon dioxide. If the blood is exposed to air with nothing between but
a very delicate membrane, oxygen will diffuse from the air into the
blood until the blood has taken up all that it is capable of holding. As
the blood circulates around the body and comes to the tissue spaces
where there is little or no oxygen, because the living cells are
constantly taking it up and using it, the oxygen which previously
diffused into the blood will diffuse out into the tissue spaces. The
only special arrangements that have to be provided are a sufficiently
great surface of exposure to the air, so that no matter how rapidly the
blood may be flowing it shall be able to take up all the oxygen it can
hold, and an arrangement for making sure that the blood can hold and
carry as much oxygen as the tissues need. The first of these
requirements is met by the special construction of the lung or gill; the
second by having present in the blood stream a chemical substance
(hemoglobin) which automatically takes up large quantities of oxygen and
so insures that sufficient shall be transported.
In principle the structure of a gill corresponds with that of a lung;
since we are particularly interested in the working of our own bodies we
shall content ourselves with describing only a lung. We saw in the
chapter on the Circulation that the pulmonary artery which leads away
from the right side of the heart breaks up into a system of capillaries.
These capillaries are thousands in number and they are spread out over
the whole lung surface. The lung itself consists of a hollow bag with
very thin and very elastic walls connecting with the throat by means of
the windpipe. In reality the bag is double, for the windpipe splits at
its lower end into two tubes, known as the chief bronchial tubes, and
these subdivide repeatedly until their fine terminals end in the elastic
lung sacs themselves. We spoke of the lung as a bag; in reality it is a
system of thousands upon thousands of separate tiny bags. The structure
is comparable to that of a bunch of grapes, the stem representing the
chief bronchial tube, the smaller stems the subdivisions and the
individual grapes the lung sacs proper. The advantage of this
arrangement is, of course, in the very large surface which it gives;
every one of the individual lung sacs has its wall filled with
capillaries and there are so many of the tiny individual sacs that the
total surface over which the blood is spread is measured in hundreds of
square feet. We cannot imagine any other arrangement by which so large a
surface of exposure could be packed away into a cavity the size of the
human chest.
Of course, we see immediately one serious defect of this arrangement of
the lung surface; every one of the individual sacs is full of air and so
the blood vessels which line its wall have exposure to air, but between
the individual lung sac and the outside atmosphere is, first, the very
tiny bronchial tube with which the sac connects, then the somewhat
larger one into which that opens, then a still larger one, and so on
until we come by way of the chief bronchial tube and windpipe up to the
throat, and so through the mouth or nose to the outside. It is quite
evident that this system of passages does not permit of a very free
movement of air. We must realize also that the blood which flows
through the walls of the lung sacs must constantly take up oxygen from
the air within the sacs, if it is to meet the needs of the body tissues.
Simple diffusion through the narrow bronchial tubes could not possibly
bring oxygen into the lung sacs fast enough to supply the requirements
of the blood flowing through their walls. The situation is met by active
lung ventilation; that is, by forcibly changing the air in the lungs at
frequent intervals. The way in which this is done is, as we all know, by
breathing. Breathing is nothing but a bellows movement of the chest by
which air is alternately expelled and allowed to enter. It does not
require a very active lung ventilation to keep the air in the lung sacs
sufficiently supplied with oxygen under conditions of bodily quiet. Our
ordinary breathing movements are gentle, less than a quart of air is
breathed in and out again in every breath, and we breathe only fifteen
or sixteen times a minute. Of course, when there is vigorous functional
metabolism, as in brisk muscular exercise, the oxidation processes in
the tissues go on at a very much more rapid rate, and correspondingly
larger amounts of oxygen must be carried by the blood to meet the
demand. Under these circumstances there is an improvement in lung
ventilation, the movements of the chest are greater and also happen more
times in a minute.
The act of breathing is carried on by ordinary skeletal muscles. This is
the only act connected with bodily maintenance of which this is true.
Our other “vital” organs are operated by means of smooth muscles. On
account of this difference we have a certain degree of control over our
movements of breathing. As we all know, we can hold the breath for a
short time without difficulty, or can breathe more quickly or more
deeply any time we choose. In this respect breathing differs strikingly
from either the heart action or the movements of the digestive organs,
over which we have no direct control at all. Our control of the muscles
of breathing is, however, rather limited; we cannot hold the breath
indefinitely. This means that the nervous mechanism which causes the
muscles to contract will work in spite of the efforts of our will to
prevent it. The actual machinery is very much like that which has
already been talked about in connection with other “vital processes.” We
have a “center” in the brain stem from which the nervous discharges
come. This center is located immediately adjoining the vasoconstrictor
center about which we learned in Chapter XIV. Because of the location of
these two important centers in a single very small space, the spot where
they are was named by a French physiologist more than a hundred years
ago “the vital knot”; the point of this is that death can be induced
more quickly and with less actual tissue destruction here than anywhere
else in the body.
The center which controls breathing has been named the “respiratory
center.” It discharges automatically about fifteen or sixteen times a
minute, causing the muscles of breathing to contract and so the bellows
action of the chest to be carried on. Like the other centers in the
brain stem this one can be acted upon by nervous disturbances passing
into the brain stem from the sense organs. Perhaps the best example of
this is the modification of breathing that comes as the result of a dash
of cold water on the skin. Most of us have noticed that we give a sort
of gasp upon stepping suddenly under a cold shower or plunging into
cold water. It may not have occurred to us that this gasp is entirely
involuntary, but we can easily prove that it is by trying to breathe
with perfect regularity at the moment of stepping under a cold shower.
We shall easily convince ourselves that this modification of the
breathing is something over which we have no control. It is, as a matter
of fact, an excellent example of a simple reflex. Coughing and sneezing
are other reflexes in which sensory irritation of some sort acts upon
the respiratory center modifying its discharges. In addition to these
reflex changes in breathing we have also the familiar effects of
muscular exercise. We know that after even moderate exercise the
breathing is quickened somewhat, and after vigorous exercise it becomes
very rapid and deep, and that after very severe exertion, particularly
in an untrained person, the puffing and blowing is not only pronounced
but even distressful. We shall see presently how muscular exercise
brings these changes about, but before doing so it will be necessary for
us to take up the movement of the gases between the lungs and the
tissues, by way of the blood; oxygen from lungs to tissues, carbon
dioxide from tissues to lungs.
By lung ventilation the air in the tiny individual lung sacs is kept
supplied with oxygen and also measurably free from carbon dioxide. From
this air there is a continuous diffusion of oxygen into the blood. The
first oxygen that diffuses in may dissolve in the blood liquid just as
oxygen will dissolve in any water to which it is exposed, but as this
goes on the hemoglobin of the red corpuscles begins to take up oxygen,
forming a chemical compound to which is given the name of
_oxyhemoglobin_. If there is enough of the gas present, every molecule
of hemoglobin will take up oxygen to its full capacity. The amount that
will dissolve directly in blood is so slight that to all intents and
purposes the ability of the blood to carry oxygen depends on the extent
to which hemoglobin can combine with it. It is important to emphasize
this, because it means that there is a definite limit to the amount of
oxygen that the blood can carry, a limit which is reached as soon as the
hemoglobin is saturated. Hemoglobin has so great a power of combining
with oxygen that the moderate lung ventilation which ordinary quiet
breathing gives suffices usually to saturate it. Now and then we
encounter statements which give the impression that there is a virtue in
deep breathing in improving the amount of oxygen which becomes available
for our tissues. As a matter of fact, this is not the case; ordinary
quiet breathing when the body is at rest saturates the blood with
oxygen, which means that it is carrying its full cargo, and evidently no
amount of deep breathing can make it do more than that. We should not be
understood as intimating that deep breathing is not a valuable exercise;
the point which we wish to emphasize is that its value does not lie in
affording an increased supply of oxygen.
Oxyhemoglobin is of a bright scarlet color; hemoglobin itself, not
combined with oxygen, is a very dark purplish color; partial
combinations are brighter and brighter as they contain more oxygen, so
that an expert can judge of the degree to which any specimen of
hemoglobin is combined with oxygen by noting its color in comparison
with fully saturated hemoglobin. The combination of hemoglobin with
oxygen takes place as the blood is passing through the capillaries of
the lungs; therefore the blood which leaves the lungs has the bright
scarlet color characteristic of oxyhemoglobin. This blood is called
arterial blood, the reason is that it is the kind that is found in the
arteries of the body in general. It happens that it makes its first
appearance in the pulmonary vein, by which it is conveyed from the lungs
to the left side of the heart, so that the expression arterial blood
does not mean anything in particular except to describe blood in which
the hemoglobin is saturated with oxygen. This blood is pumped out by the
left side of the heart to all the parts of the body; in its passage
through the capillaries it is in a region where there is active
utilization of oxygen by the living cells. These are steadily taking up
oxygen from the tissue fluids about them, so the blood in the
capillaries, which is carrying an abundant supply of oxygen, is brought
in contact with tissue fluids containing little or none, with only the
delicate wall of the capillaries between. Under these circumstances
rapid diffusion of oxygen from the blood into the tissue fluids takes
place and accompanying this there is a breakdown of oxyhemoglobin, so
that not only most of the oxygen which was dissolved in the blood passes
out, but also a considerable part of that which was formerly in
combination with hemoglobin. Under ordinary circumstances only from
one-fourth to one-third of the oxyhemoglobin decomposes during the rapid
passage of the blood through the capillaries; thus the blood that goes
on into the veins will still be carrying two-thirds or more of the total
oxygen cargo. The color of venous blood will be darker than that of
arterial blood, because it contains a good deal less oxyhemoglobin, but
it is nowhere near so dark as is blood in which all the oxyhemoglobin
has been decomposed. This is well recognized in melodramatic fiction
where the wounds of persons who have met death by strangulation are
described as oozing black blood. It is an actual fact that blood from
which all the oxygen has been withdrawn is so much darker than ordinary
venous blood that it gives the impression of being black, although it is
really a dark purple.
If we should be moved to inquire why so small a fraction of its whole
store of oxygen is given up by the blood to the tissues ordinarily, we
shall find the answer in remembering that the demand of the body for
oxygen is extremely variable; every increase in functional metabolism
means an increase in the amount of fuel that is oxidized and therefore
an increase in the amount of oxygen that is required. Actual
measurements have shown that in very vigorous muscular exercise the
oxygen consumption may be approximately ten times as great as in
complete rest. In order that this very greatly increased metabolism may
be carried on it must be possible for the blood to deliver approximately
ten times as much oxygen to the active tissues as it delivers to them
when quiet. There are just two ways in which this can be done; one is by
a more complete decomposition of the oxyhemoglobin by which all its
oxygen is set free; the other is by a more rapid movement of the blood.
It is by a combination of these two that the oxygen requirements of the
body in times of vigorous metabolism are taken care of. As has already
been said, the heart rate is just about doubled in vigorous exercise.
There has also been shown to be some increase in the amount of blood
that it pumps with every beat. The result is that more than twice as
much blood leaves the heart in a minute under these circumstances as in
time of rest. The oxyhemoglobin is also completely decomposed when the
tissues are active, and these two facts together are sufficient to
account for the great increase in the oxygen supply.
Hand in hand with the increased consumption of oxygen, there is of
course an increased production of carbon dioxide and of water, since the
oxidation of fuel substances produces these waste products. The cells
are always pouring both out into the tissue fluids, but to a very much
greater extent when they are actively functioning. We need make no
effort to keep track of the water, since it merely adds itself to the
water already present, and we shall consider later how the water
supplies of the body are handled. The carbon dioxide, however, must be
gotten rid of, and the mechanism for getting rid of it must work
efficiently, otherwise metabolism itself will be hampered, since it is a
familiar law of chemical action that if the products of an action are
allowed to accumulate they interfere with its further progress. The
method of getting rid of carbon dioxide is by simple diffusion from the
cells into the tissue fluids and from the tissue fluids into the blood.
Carbon dioxide is many times as soluble as oxygen, so that a great deal
more of it can be handled by merely dissolving. This is not sufficient,
however, to take care of all the carbon dioxide; the remainder must go
into chemical combination with some substances that are in the blood.
There is no single conspicuous material for carrying carbon dioxide like
the hemoglobin which transports the oxygen. There are, however, a number
of compounds in the blood which are able to combine with carbon dioxide,
among them the blood proteins of which much was made in Chapter XIII.
The carbon dioxide distributes itself among these various substances and
so is transported. It should be noted that the blood does not become
saturated with carbon dioxide as it does with oxygen. Arterial blood
ordinarily carries practically all the oxygen it is able to take up;
venous blood on the other hand probably never comes anywhere near being
as fully charged with carbon dioxide as it is able to be.
During the passage of the blood through the capillaries of the lungs an
outward diffusion of carbon dioxide into the air in the lung sacs is
going on simultaneously with the inward movement of oxygen from this air
into the blood. The diffusion is never so complete as to deprive the
blood of all its carbon dioxide; there is in fact only a little less of
it in arterial blood than in venous, although the diffusion is
sufficiently rapid so that as much carbon dioxide as is produced in the
whole body in a minute is passed out into the air of the lung sacs in
the same time. The effect of this outward diffusion is naturally to
increase the amount of carbon dioxide in the air of the lung sacs, and
if this increase is allowed to go on unhampered, there will presently be
so much carbon dioxide there as to stop further outward diffusion, and
so to put an end to the escape of carbon dioxide from the blood. This is
avoided by lung ventilation. Every time a breath is drawn some air that
is almost free from carbon dioxide enters the lung spaces to replace the
carbon dioxide-laden air that was expelled at the previous exhalation.
The description of gas transportation that we have just given opens the
way for an account of the control of breathing. From what has just been
said it should be clear that the amount of carbon dioxide in the blood
corresponds closely with the amount that is in the air of the lung sacs.
As the percentage of carbon dioxide in this air goes up, outward
diffusion becomes less free, and so the amount of carbon dioxide in the
blood will have to increase. The tissues are all the time producing and
pouring out carbon dioxide, and so there will be a steady increase in
the amount of carbon dioxide in the blood. This applies to the arterial
blood as well as to the venous, since, as we saw a moment ago, there is
nearly as much carbon dioxide in the former as in the latter. This is
the fact which is utilized in the body for operating the breathing
machinery. The respiratory center in the brain stem is susceptible to
carbon dioxide; the more of this gas there is in the blood, the more
tendency there will be for the center to discharge. There is a certain
level of carbon dioxide below which it is entirely inactive, but when
this level is passed nervous discharges begin and become more and more
powerful as the amount of carbon dioxide in the blood goes up. Now we
can see what makes us breathe. Let us imagine that there is not very
much carbon dioxide in our blood, but that the tissues are constantly
producing it and giving it off. Since we are supposing the amount is not
enough to excite the respiratory center, there will be no movements of
breathing. There will be a steady increase of the amount of carbon
dioxide in the blood and at the same time a corresponding increase of
the amount of carbon dioxide in the air sacs of the lungs; presently
there will be enough in the blood to arouse the respiratory center to
discharge. This will cause a breath to be drawn; the effect of this will
be to sweep out much of the accumulated carbon dioxide from the lung
sacs; this in turn enables more rapid diffusion of carbon dioxide from
the blood to occur, and so the amount of it in the blood may fall below
the level at which the respiratory center is made active. In a moment,
of course, the continued outpouring of carbon dioxide from the tissues
will raise the level again to the point of exciting the respiratory
center, and so we will have a rhythmically recurring discharge of that
center causing a rhythmic drawing of breath.
According to the account just given the activity of the respiratory
center is determined exclusively by the carbon dioxide in the blood; it
could be so regulated, but, as a matter of fact, in all higher animals,
including man, the carbon dioxide control of the respiratory center is
interwoven with a complicated nervous control whose effect is to make us
breathe more often in a minute, but to make the individual breaths
shallower than they would be if the control of breathing were
exclusively by means of carbon dioxide. The net result in lung
ventilation is exactly the same, but the rapid shallow breaths are
advantageous in that they avoid large fluctuations in the amount of
carbon dioxide in the blood, while they do serve fully to provide
sufficient oxygen.
The rate and vigor of breathing are ordinarily adjusted automatically to
the amount of carbon dioxide in the blood stream, but, as we know, we
can, of our own will, breathe quite differently. Let us see what will
happen if while we are sitting quietly we begin to breathe deeply and
rapidly, overventilating the lungs. So far as oxygen is concerned, this
will make no difference at all, since, as we have already seen, the
ordinary automatic breathing keeps the blood charged with all the oxygen
it can hold. What overventilation does is to sweep out the carbon
dioxide from the lung sacs more rapidly than usual and this permits of a
correspondingly more rapid outward diffusion of carbon dioxide from the
blood. The result will be that carbon dioxide will leave the blood
faster than it is being poured into it from the tissues, and so the
total amount of the gas in the body will be cut down. The first effect
of this we would expect to be the removal of the automatic stimulation
of the respiratory center, so that, after a period of excessive
breathing, one would not at once resume breathing spontaneously. This,
as a matter of fact, is the case; anyone can easily prove on himself, by
breathing deeply and rapidly for a minute or two, that the automatic
control of breathing is temporarily suspended immediately after. It
follows naturally that one can hold the breath a good deal longer if the
lungs are overventilated for a short time just before the attempt is
made. This also can be easily proved. Prolonged overventilation of the
lungs has, likewise, a number of other effects, all of which are due to
cutting down the total amount of carbon dioxide in the body. The most
conspicuous is a feeling of dizziness or light-headedness that comes on.
If pushed to excess, there is a very definite feeling as though one were
about to soar away into space, and this is followed by unconsciousness.
Certain religious cults in India have interpreted this sensation
resulting from deep breathing as an actual severance of soul from body,
and maintain that during the time of unconsciousness the spirit really
floats freely in space. Without venturing any statement as to the
relation between the soul and the body during either consciousness or
unconsciousness, we would point out that these bodily sensations are
definitely due to the very simple fact that there is less carbon dioxide
in the blood than is normal on account of the overventilation of the
lungs, and just as soon as the metabolism that goes on all of the time
in the tissues pours out enough carbon dioxide to bring the amount up to
normal, consciousness will return and the ordinary condition of affairs
will be resumed.
Although this finishes what we have to say about the movements of gases
into and out of the body, the general subject cannot be completed
without a word concerning the conditions that should be maintained in
the air immediately surrounding us. This makes up the topic of
_ventilation_. We all know that some air is much more fit to breathe
than other; until very recently, however, our ideas as to the conditions
which make air fit or unfit to breathe have been hazy or entirely
erroneous. Fortunately, of late years, the subject of ventilation has
been actively investigated and we now have a satisfactory knowledge of
its laws.
There are, of course, two things that must be true of any air that is to
be breathed; these are that it must contain enough oxygen and must not
contain too much carbon dioxide. So far as the oxygen supply is
concerned we may state that only with the greatest difficulty are
conditions reached in which there is not enough oxygen in the air. As we
all know, the air becomes thinner the higher we go above the surface of
the earth; both mountain climbers and aviators have attained heights at
which the amount of oxygen in the air is only about one-third that of
ordinary air and have been able to obtain enough oxygen for their
bodily needs even under those extreme conditions. It is quite evident
that a room could scarcely be so poorly ventilated as to bring the
oxygen supply down below this figure, so that no attention need be paid
to the oxygen supply in working out practical methods of ventilation.
Air which contains carbon dioxide to the extent of four per cent could
not be breathed because the carbon dioxide being produced in the body
would not diffuse out fast enough into an atmosphere containing that
amount of carbon dioxide to keep the body alive. This again is a
percentage of carbon dioxide that is practically never reached. Probably
the most famous case in history of death from poor ventilation is the
“Black Hole of Calcutta,” a dungeon room about twenty feet square with
only two small windows, in which one hundred and fifty British soldiers
were imprisoned over one night; all but twenty-three of these died, but
it is doubtful whether their death was actually due either to deficiency
of oxygen or to excess of carbon dioxide. This is because there were
enough other factors which would make the air unbreathable to bring on
death before either of these could come into play. The modern science of
ventilation concerns itself with these other factors; chief among them
is the factor of moisture. As we shall see in the next chapter our
bodies are constantly giving off from the lungs and by evaporation from
the sweat glands water vapor into the air. This causes the humidity to
go up rapidly in rooms where people are congregated, and particularly so
where there are many people present. Also everyone gives off a great
deal of heat. We now know that the feeling of closeness which we ascribe
to a poorly ventilated room is due to the combination of warmth and
moisture. We also know that the discomfort which comes from being in
such rooms is due to the same causes. Actual vitiation of the air is
much less disagreeable than is the accumulation of heat and moisture. In
theory, of course, the best ventilation is secured by keeping rooms
flooded with outdoor air. In practice, however, this does not always
work out; for example, in many cities the air is so laden with dust and
smoke as to be bad for everybody and even dangerous for sick people.
Before such air is breathed the smoke and dirt should be gotten out of
it. This is done sometimes by forcing it through fine mesh cloth bags,
or the most modern scheme is by passing it through a thin screen of
water and so washing the dirt and smoke out of it. The second practical
difficulty with flooding rooms with outside air is the expense in cold
weather of warming the large volumes that would be required. For this
reason it has been found feasible in churches and public halls that are
occupied only occasionally to use the same air over and over by keeping
down the temperature and moisture. Of course, this cuts down very
greatly the expense of heating.
There is one other source of harmful effect from bad air besides the
high humidity and undue warmth; this is the presence in it of ammonia
and other poisonous compounds that are given off from the bodies of
people. It used to be believed that organic poisons were exhaled from
the lungs with every breath, but we now know that the amount of these,
if any are present, is too small to be important in comparison with the
very much larger amounts that come off from the evaporating sweat, from
decaying teeth, and from the digestive tract; there is no doubt that in
any assemblage of people the air will be vitiated by organic poisons
from these sources. The more cleanly the individuals are, the less will
be the contamination. It is generally believed, although perhaps not
absolutely proven, that the bad health found in sweatshops and crowded
slums generally is due largely to chronic poisoning from the constant
breathing of effluvia from the unwashed bodies and clothing of the
inhabitants. The obvious remedy is insistence upon personal cleanliness,
although this does not lessen the desirability of breathing as pure air
as can be gotten. The point to be emphasized is that where personal
cleanliness prevails, the closeness of rooms is chiefly due to excessive
moisture ordinarily accompanied by too high a temperature. Ventilation
measures should be carried out with this in mind.
CHAPTER XVII
THE SERVICE OF REMOVAL OF WASTE
The metabolism that goes on in all our body cells results in the
formation of waste products. The chief of these, carbon dioxide, is
taken care of by means which were described in the last chapter. Besides
this gaseous substance there are produced a number of compounds which
are soluble in water and which are poured out from the cells into the
tissue spaces surrounding them and which pass thence into the blood
stream. The special places in the body wherein these substances leave
the blood stream make up the apparatus for the removal of waste. This
apparatus must be able to take out of the blood stream all the waste
products of cell metabolism except carbon dioxide, also any materials
which may have been absorbed into the blood stream from the digestive
tract, but not used by the tissues, and finally all the accumulations of
water. The discharge of water makes up a topic by itself which will be
considered after a word has been said about the apparatus by which the
removal of wastes is carried on. Sharing in this function we have first
the kidneys, second the liver, and third the sweat glands.
The kidneys are the most important of the organs for removing waste.
They are a pair of bean-shaped bodies lying in the small of the back.
Each
[Illustration: DIAGRAM OF KIDNEY
(From Martin’s “Human Body”)]
one is made up of thousands of tiny tubes; each of these tubes starts as
a little sac in which is a knot of blood vessels which is really a tiny
filter, filtering out from the blood water and the inorganic salts that
are dissolved in it.
All the other materials that are in the blood are held back by the
filter, so that nothing escapes from it in these places except water and
salts. Further along each of the tiny tubes is a section which is
surrounded by a network of fine blood vessels where two things take
place; the first of these is the taking out from the blood of the
various kinds of waste products; the second is the absorbing back into
the blood of part of the water which filtered out through the knot at
the tip of the tube. Beyond the point where these things happen the tube
does nothing except to convey the finished kidney secretion, or urine,
down to the outlet where opens a very large tube, the _ureter_, by which
the kidney secretion is carried down to the bladder. The formation of
urine is made up then of three stages; first the filtration of water and
salts out of the blood, second the escape from the blood into the kidney
tube of the various waste products of cell activity, and third, the
reabsorption into the blood of part of the water. It is estimated that
about four times as much water filters out from the thousands of tiny
filters at the tips of the kidney tubes as comes out at their ends to be
carried down to the bladder. We do not understand exactly why this
should happen; one result of it is to make the urine contain about four
times as much salt as the blood; the water that filters out at the
beginning of the kidney tube carries with it just the amount of salt
that is dissolved; after the water is reabsorbed the salt is left behind
in the kidney tube, so that all the salt that filters out stays in the
urine, but only about a quarter of the water. The discharge of waste
products is easier to understand than the way in which the water is
handled; all organic substances which happen to be in the blood and
which do not belong there pass out from the blood during its passage
through the fine network surrounding the kidney tubes. Of course we do
not know just how this is carried on; there must be some special
features about the protoplasm in this part of the kidney to cause these
substances to pass out from the blood here and nowhere else. Not only
waste products of cellular activity, but also all or nearly all the
organic accessories of the diet and all drugs taken as medicine leave
the blood in this region.
An interesting thing about the removal of water from the blood by the
kidneys is the relation it shows to the activity of the sweat glands.
The total amount of water gotten rid of in any day is the amount taken
in in drinking during the same time. The amount we drink is controlled
chiefly by the sense of thirst, assuming that we are in a place where
water can be had. The amount of water that passes out in the form of
sweat is not under our control; in a later chapter we shall see how the
formation of sweat is regulated. There is also a constant and fairly
steady loss of water from the lungs, since every breath that we exhale
carries with it all the water vapor which it can hold. What the kidneys
have to do is to get rid of the water that is not passed out either from
the sweat glands or the lungs. The amount of urine that is formed in a
day is, therefore, extremely variable; on days that are warm and when a
great deal of exercise is taken there will be so very much sweat formed
that in spite of copious drinking comparatively little water will be
discharged from the kidneys. On the other hand on cold days in which not
much exercise is taken there will be a large amount of urine formed. It
is interesting to note that some substances like asparagus, and to a
less extent coffee, cause a large increase in the amount of urine. We do
not know certainly whether these act by increasing filtration from the
tips of the kidney tubes or by diminishing the reabsorption in the part
farther along, but so far as our information goes either or both may
occur.
The kidney is not the only organ by which wastes are removed; the liver
has a part in this as well. The action of the liver is not, however,
simply to assist the kidney; there are certain special substances which
are taken care of by the liver and which are never gotten rid of through
the kidneys. The first of these are the broken-down blood pigments. We
have already seen that after the red corpuscles become worn out they are
decomposed in the spleen and the hemoglobin carried to the liver where
the iron and other usable parts are saved and the useless remainder is
passed on into the bile to be conveyed out of the body. Bile is green in
vegetable-eating animals and golden brown in flesh-eating; the
difference depends on the so-called bile pigments which are the waste
remains of hemoglobin. The difference in color is the result of a slight
chemical difference in the pigments which in turn depends on the diet,
so that in man the bile is either green or golden brown according to
whether the diet is vegetarian or chiefly of flesh. The liver also takes
out from the blood a waste substance which has the formidable name of
_cholesterin_. The interesting thing about this substance is that unless
the bile is altogether normal chemically it will not be dissolved and so
will be thrown down in the form of little grains or lumps. So long as
these grains are small they can pass down the bile duct to the small
intestine and be carried out of the body, but after they become bunched
together into lumps it is difficult or even impossible for them to be
carried down the narrow bile duct. These lumps make up what are commonly
called gallstones. The acute pain known as gallstone colic is due to the
stretching of the bile duct by the passage of one or more of these
stones down it. At the present time no way is known to prevent the
formation of gallstones in persons in whom it starts, nor of softening
them after they have once been formed; the only relief is through
surgery, which, fortunately, is entirely adequate, so that thousands of
persons are living in great comfort to-day who would be dead or living
in acute suffering if the surgeon had not been able to remove the
accumulation of gallstones from the passages in their livers.
The sweat glands, as we have already seen, make up part of the service
of the removal of waste in that they help to carry away water from the
body. To a very slight extent, but much slighter than is commonly
believed, they get rid also of waste products of cell activity; for
example ammonia, which is one such product, can frequently be detected
in the sweat by its odor; practically, however, the importance of sweat
glands is not in their discharge of waste organic substances, but only
in their discharge of water. In this they are playing a very important
part in the regulation of body temperature, so that the further account
of the sweat glands as well as of the skin of which they form a part is
left for the chapter in which the regulation of temperature is
described.
CHAPTER XVIII
MORE ABOUT THE USE OF FOOD BY THE BODY
We have now brought our description of the various things that happen in
our bodies up to the point where we may begin to make some kind of
summary of them; particularly in respect to what goes on within our
individual cells. We have seen how some of our cells are muscle cells by
which we make motions; others are the cells of sense organs by which we
get the necessary information for guiding our activities; still others
are in the nervous system and carry on the adjustments by which we act
in accordance with the indications of our surroundings and also of our
past experience; finally we have the cells which manufacture materials,
as in the various digestive glands or in the glands which make hormones.
We have also looked into the kind of materials which these cells
require, where they come from, and how they are prepared in the
digestive tract, and we have seen how these prepared materials and
oxygen are conveyed to the different parts of the body where the cells
can get them and finally how the waste products which all these cells
give off are gotten rid of. We are now to turn to the process of
metabolism itself as it takes place within the cells. In Chapters III
and IV we looked into the use of food for power development and for
repairing the wastage of protoplasm, as well as for the making of new
protoplasm in growth. We have seen also that the body contains much
nonliving material, as in the bones and teeth, which must come from the
food and which must be put in place as the result of metabolism on the
part of living cells.
The first thing which we wish to take up here is the use of protein in
the repair of protoplasmic wastage and in growth. We saw in Chapter III
that protein is manufactured originally by the living cells of green
plants. We have also seen that protein is the only material that can
repair protoplasmic wastage or that can make new protoplasm. We have
omitted to say thus far that the most important place from which we can
obtain protein is from living protoplasm. It is true that most seeds
store up within themselves nonliving protein to be used by the young
sprout as it forms, and seeds make up a large part of our diet; but
except in grain and other seed foods we obtain our supplies of protein
by eating protoplasm. This protein is to be used by us for repairing our
protoplasmic wastage or, in parts of us that are growing, for making new
protoplasm. We have already seen that the protein which we eat must go
through a process of digestion before we can use it for these purposes,
and our present task is to explain just why this is necessary and to
show how the protein is actually used in our bodies.
A thing about protein which fits it specially to be the chief material
of living protoplasm is that it is very much the same sort of substance
wherever we find it and yet can differ enough to account for the
differences that exist among animals and plants. In spite of the fact
that protoplasm analyzes about the same, no matter where it comes from,
we are bound to believe that the difference between a dog and an oak
tree is at bottom a chemical difference; they are unlike because the
protoplasm of one is not the same substance chemically as the protoplasm
of the other, and the difference is a difference in the proteins. We can
come even nearer home than that and say that the differences between the
races of mankind are probably chemical differences between their
proteins. Human protein is undoubtedly different from the protein of
beef or pork or mutton. What we have then is a substance which can be at
the same time similar and different; also since we can make human
protein out of beef protein or any other kind which we happen to eat it
must be fairly easy to change one into the other.
Protein is about the most complex substance that we know anything about;
it is made up of a number of organic acids combined chemically. These
organic acids all contain nitrogen, which puts them into a class to
which is given the name of _amino acids_. To the chemist the name amino
acid shows a certain kind of chemical formation; to us it need mean no
more than an organic acid which contains nitrogen. The proteins which
are in our bodies are as complex as any that exist and some of them are
made up of as many as eighteen different amino acids. The same eighteen
acids are present in the proteins of all the higher animals. When we eat
lean beef or pork we get exactly the same eighteen amino acids that are
in our own proteins, but not put together in precisely the same way.
What we have to do with these proteins is to break them up into the
amino acids of which they are composed and then put these together again
in the combination which makes up human protein. The breaking up of the
proteins is carried on in the digestive organs; we have said a good deal
about it in Chapter XV; what were called in that chapter digestion
products of proteins we now see are amino acids. These are taken up into
the blood stream, carried around the body to the tissue fluids and by
the living cells taken up to be built into human protein. Exactly the
same thing happens to any proteins that we eat.
One of the great differences between animal protein and plant protein is
that the percentages of the different amino acids are very different.
Some of the amino acids that make up a large proportion of animal
protein are very scantily represented in plant protein. To make human
protein we must not only have all the amino acid ingredients, but we
must also have enough of every one. On a purely vegetarian diet to get
enough of those amino acids which are scantily present in plants we have
to eat a large surplus of those amino acids which are specially
abundant. In this respect plants as providers of amino acids are less
economical than animals, because animal proteins have more nearly the
same proportions of the different amino acids as do our own human
proteins. Of course, as we will see at once, in theory cannibalism is
the most economical way of getting protein; if we were to eat human
protein we would have exactly the correct proportion of the different
amino acids and so could get along with a minimum amount. This is not to
be interpreted as an argument for the practice of cannibalism among
human beings, although we may as well face the fact that there is no
physiological or dietary reason for avoiding the practice. In some of
the lower animals, particularly in rats, cannibalism is a regular part
of the life habit. Rats do not have the instinct of storing up food
supplies as do squirrels and some other kinds of animals. When food is
abundant they multiply very rapidly and then when food becomes scarce
the stronger feed upon the weaker. It is largely for this reason that
the unsanitary and extremely expensive rat nuisance is so hard to abate.
The amino acids that are circulating in the blood stream after every
meal are primarily to be used for repairing protoplasmic wastage; also
they serve for the manufacture of new protoplasm, provided growth is
going on. In theory an adult who is through with all his growth except
in the skin and one or two other minor tissues should be able to get
along with just the amount of protein which will make good his
protoplasmic wastage. Since protein is an expensive food and likely to
be hard to get in times of scarcity, the question of how much protein
should be eaten is of great practical importance. There are several ways
of studying the problem; one is by the actual study of diets to find out
how much protein people do habitually eat; another is by finding out how
much the daily protoplasmic wastage amounts to. If no more protein is
being eaten than is necessary for the protoplasmic wastage, these two
figures should be about the same. The way of finding out how much
protoplasmic wastage there is is to go on a diet which contains abundant
starch and fat for energy supplies, but no protein or amino acids. When
one is on that kind of diet he knows that he will not have to burn up
any of his own tissues to supply him with the energy for his metabolism;
whatever breakdown of protoplasm occurs on such a diet is the natural
wastage of the body and not the result of using tissues for fuel, as is
the case in complete starvation. It is fairly easy to keep track of the
decomposition of protein in the body, because protein contains nitrogen
and the nitrogen is given off almost wholly in the waste products that
are passed out from the kidneys. By collecting the urine and analyzing
it for nitrogen the amount of protoplasmic breakdown in the body can be
determined, provided no nitrogen-containing compounds were taken in with
the food. Otherwise, of course, one could not be sure that nitrogen
appearing in the urine had actually come from the wastage of protoplasm.
The fact is that when an average-sized human being goes on a diet which
contains no protein, but is ample in other respects, he loses daily from
his body about an ounce of protein. This is proven by the occurrence in
the urine of an amount of nitrogen which stands for that much protein.
The person may be gaining or losing weight meanwhile; if his consumption
of fats, starches, and sugars is excessive, he may deposit some fat, in
which case he might gain weight in spite of the loss of some of his
actual living protoplasm. Usually though, in experiments of this kind,
there is a steady loss of weight made up of the ounce of protein and of
three or four ounces of water. We have to remember that living
protoplasm is three-fourths or more water, so that whenever any
protoplasm wastes away, some water will be lost as well as protein. It
is a very interesting fact that this protoplasmic wastage goes on
steadily at the rate of about an ounce a day whether the body is active
or inactive; this means that the wastage is a matter of the basic
metabolism and not of the functional metabolism. The former goes on all
the time day and night, in sleep and in waking, and in connection with
it the living protoplasm shows this small amount of wastage. Functional
metabolism does not, at least under ordinary conditions, increase the
amount. Speaking of the body as though it were a machine, we would say
that it rusts out just as fast as it wears out. This is one of the
features in which the living machine differs from most mechanical
devices of human manufacture.
Although the loss of protein due to wastage is only about an ounce a
day, nobody can get along on a diet which contains no more than that
amount. Between three and four ounces of protein is the average daily
consumption of adults in this country. We should not forget that our
diet consists of meat, bread, vegetables, fruits, etc., which are
mixtures of proteins with the other food materials and with a large
percentage of water, so that in order to get three or four ounces of
protein we have to eat four or five times that weight of ordinary
foodstuffs. There has been much debate as to whether it is necessary or
even desirable for adults to eat three or four times as much protein as
the body requires for making up its wastage. The decision will have to
rest in part on what use the body makes of the surplus. Since from time
immemorial human beings have habitually eaten every day this large
surplus, it is evident that they have been wasting enormous amounts of
good food or else that some use is made of it even though it does not
serve its purpose of repairing the body waste. The surplus materials are
present in the body in the form of amino acids, since what the cells do
in repairing their wastage is to take up from the whole quantity of
amino acids in the tissue fluids as much as they require for making good
their loss. The mixture of amino acids that is left over will make
perfectly good fuel provided the nitrogen that is in it is gotten rid
of, and this is what happens in the body. All the amino acids in excess
of the amount needed for restoring the tissues are decomposed in such a
way that the nitrogen is abstracted in the form of ammonia and the
substance that is left, which is a starchlike compound, joins with the
other starch products and fats to be burned in the body as a source of
energy. We do not know certainly which tissues have the power of
decomposing the surplus amino acids. At the present time it is believed
that all or nearly all of them can do it, so that as they take up from
the tissue fluids the particular amino acids which they need for making
good their own wastage they take up also the surplus which they
decompose, utilizing the starchlike part for fuel and turning the
ammonia back into the body fluid as a waste product.
Ammonia is a very poisonous substance and it quickly poisons the body if
allowed to accumulate in the tissue fluids. This is prevented by the
action of the liver in changing the poisonous ammonia into a harmless
substance known as urea. This urea is carried by the blood stream from
the liver to the kidneys where it is passed out to become the chief
organic substance in urine. The more protein one eats the more surplus
amino acids will there be, and so the more urea will be formed and
passed out of the body. Flesh-eating animals and men (Eskimos for an
example of the latter) eat a very large surplus of proteins, the fuel
for their metabolism being furnished almost altogether either from the
usable remains of the decomposed amino acids or the fats that were in
the flesh they ate. Some people have been inclined to believe the
production of so much ammonia and its subsequent conversion into
correspondingly large amounts of urea to be injurious. As a matter of
fact, there is no particular reason for thinking this to be the case; it
is part of the duty of the liver to change all the ammonia that comes to
it to urea and of the kidneys to pass out all the urea that comes to it;
so long as these organs are healthy they are able to fulfill these
duties effectively, so this does not seem to be a good reason for
cutting down the percentage of meat in the diet. It is generally
believed that meat has special effects upon the nervous system, such as
to incite to cruelty and bloodthirstiness. There is no real scientific
proof as to whether this is true or not. The scientific fact is that man
is fitted for a mixed diet, neither exclusively of flesh nor exclusively
vegetarian. He has lived for thousands of years on that kind of diet and
can apparently go on for thousands of years more. We need to remember
that the various dietary fads which come into great prominence from time
to time are rarely based on a well-established scientific foundation nor
have any of them any long experience back of them. On the other hand,
the common mixed diet which all of us eat in accordance with custom and
the dictates of our appetites has the sanction of thousands of years of
successful maintenance of the human race. It is quite true that one can
get along on almost any kind of a diet provided it contains enough
protein to make good the daily body wastage and enough fuel material to
provide for the demands of metabolism. Anyone who is disposed to adopt
for himself a dietary fad will rarely suffer seriously from it; on the
other hand, those who prefer to eat as our fore-fathers have eaten need
not feel conscience-stricken because there is agitation against the
commonly accepted diet.
While on the topic of diet, a word should be said about cooking. In a
previous chapter the advantage of good cookery as an aid to digestion
was emphasized. We would add here merely the comment that in defending
the ancestral diet we do not intend to imply that their cookery was
always what it should have been. Over most of America there has
prevailed from pioneer days a habit of frying food in preference to
other means of cooking it. Our hardy pioneer ancestors throve on fried
meats; an outdoor life of muscular toil makes almost any kind of cookery
both acceptable and digestible. As labor-saving machines tend more and
more to diminish the amount of muscular labor that most of us do, we
find it harder and harder to maintain good digestion on fried foods. The
objection to frying is simple; fats are the hardest of all foods to
digest and fried foods are smeared all over with fat. It is only logical
to expect fried foods to be harder to digest than other kinds; it is
undeniable that the flavor of many fried foods is so agreeable that we
would be unwilling to omit them from the diet altogether. What is realty
objectionable is the practice of smearing all the food with fat in the
process of cooking it.
We have finished what we have to say about the use of food for the
repair of bodily wastage. While we are on the topic, a few words about
the use of food in growth will come in well, since the growth process is
closely related to the process of tissue repair. The chief difference
between them is that the process of growth comes to an end in all but a
few of our body tissues as soon as we become adults. The tissues which
go on growing are the layers of the skin just under the surface, the
reproductive tissues, and the blood-corpuscle-forming tissues.
Connective tissue has the power to grow at any time during life,
although it does not actually keep on growing as does the skin. Whenever
an injury is suffered which actually destroys muscular tissue or the
deep layers of the skin, there is no growth of new tissue to take the
place of either. Repair is made by a growth of connective tissue to fill
up the space. The result is the formation of a scar. If the edges of the
injury can be brought together skillfully enough, the outer layers of
the skin which do have the power of growth may bridge across the space
so that no scar will result.
A second thing to be mentioned about growth is a discovery which has
attracted a great deal of attention of late years; we will realize of
course that the chief thing in the making of new protoplasm is the
building together of protein out of amino acids. It is evident that for
the manufacture of new protein all the eighteen amino acids must be
present in sufficient proportion to give enough of each. There are some
plant proteins which lack one or two of the amino acids that are present
in human proteins and when a growing animal goes on a diet in which
these are the only proteins present it at once stops growing. Most of
the experiments proving this have been done on white rats, and it has
been found possible to keep a rat for more than a year at the size it
was when only a few weeks old simply by feeding it proteins in which one
or two amino acids were lacking. The fact that the animal lived during
this time proves that his protoplasmic wastage was made good and
therefore that there are proteins which can replace the body wastage but
cannot manufacture new protein. There is only one conclusion to be drawn
from this, namely that the daily wastage which the protein suffers does
not include all of it; some amino acids when once built in are there for
good; others, on the other hand, are constantly being lost in the
process of wastage, and these are the ones which must be replaced. There
is a common protein, gelatin, which is used a great deal for food, but
which by itself will not serve either for repair or growth because it
not only lacks some of the amino acids that are in living protoplasm,
but also some that are lost in the process of wastage. Gelatin is useful
as a food, therefore, only in combination with other proteins that
contain the necessary amino acids; or after the nitrogen has been taken
out, it becomes a good fuel.
One interesting question that was settled by the experiments described
above was whether the ability to grow is exclusively a matter of youth;
as we know, under ordinary conditions only young animals grow. When they
reach a certain age, they become mature and thereafter no increase in
size takes place. In the case of these white rats which were kept for
more than a year at the size of partially grown rats it was found that
as soon as their diet was changed to one in which all the necessary
amino acids were present they would begin at once to grow and grow to
full size, when they would stop growing just as they would have when
young. This proves that growth is not a matter of age, but is a matter
of achieving a certain size, and is controlled by factors which we do
not at present understand. An animal that has not been able to attain
[Illustration: Photo, Keystone View Co.
THE LUNCH HOUR IN A FRENCH SCHOOL WHERE EXERCISE, OUT-OF-DOOR LIFE,
SUNSHINE AND FRESH AIR ARE CONSIDERED ESSENTIAL]
[Illustration: Copyright, Paul Thompson
THE DIET KITCHEN ON A U. S. HOSPITAL SHIP]
full growth because it has been denied the amino acids necessary for
making new protein retains the power of growth, so that even though it
may be long past the ordinary age of maturity it can go on growing as
soon as the necessary materials are provided. The dependence of growth
on certain dietary accessories was spoken of in Chapter IV and need not
be repeated here.
The final use of food is as a source of energy for carrying on
metabolism. A good deal was said about that in an earlier chapter, but
there are a few additional points to be brought out here. The energy for
metabolism can come from any of the foodstuffs; these are present in the
blood stream in the form of sugar or fat or amino acids. A moment ago we
saw that in the case of the amino acids the nitrogen is removed before
they are ready for use as fuel. After this has happened the part that
remains is so similar to sugar that it can be thought of as the same
material and will be so considered by us. We have then to follow only
the two food substances, sugar and fat, through their use by the body
cells. Fat will be dealt with first since we have less to say about it.
It passes from the digestive tract into the blood stream in the form
known as an _emulsion_; all this means is that it is broken up into very
tiny particles which are kept from running together by some sort of a
film. In the case of the fats in the blood it is likely that this film
is composed of ordinary soap. In this state the fat circulates in the
blood stream until it is taken out by the tissues and burned. In the
process of this burning some very poisonous substances are likely to be
produced, but this happens only when large amounts of fat are being
burned by themselves. If sugar is present and is being burned at the
same time, there is no danger. Ordinarily in the body sugar is always
present, but in a certain disease, diabetes, about which more will be
said later, the body is not able to burn sugar as well as it ordinarily
does, and under these circumstances poisoning from the products of the
fat burning is apt to happen. This makes up, in fact, the serious danger
in diabetes. As we all know, fat is chiefly important as the form in
which food is stored in the body against a time of future need. We shall
return to the way in which the body does this after we have spoken of
the use of sugar as fuel.
Sugar is the great fuel substance of the body. About two-thirds or our
food ordinarily consists of starchy materials which are digested into
sugar. When we add to this the sugarlike remains of all the protein food
which is not actually used for the repair of tissues we see that this
substance makes up the great bulk of the material which is carried by
the blood to the tissues. This material is handled in the body in an
interesting way which depends on the curious fact that although sugar is
the most important fuel for living cells they cannot endure its presence
in them or in the fluids surrounding them except in very small amounts.
Sugar, as we all know, is very soluble in water, and it would be
perfectly possible for the blood to dissolve all the sugar that enters
it from the digestive tract and simply carry it in solution until the
tissues withdrew it for their needs; but this would mean that
immediately after every meal the percentage of sugar in the blood would
mount up to a high point from which it would gradually sink as the sugar
was taken out, to mount again after the next meal when absorption began
again. This does not actually happen because it is prevented by the
liver, which has as the most important of its many functions that of
storing the sugar that is taken up by the blood from the digestive tract
and dealing it out little by little as the cells of the body need it.
Back in the chapter on digestion we saw that all the blood that passes
through the intestinal tract is gathered up by the portal vein and
passes through the capillaries of the liver. It is during this passage
through the liver that the sugar is taken out of the blood and stored in
the form of a less easily dissolved material known as animal starch or
_glycogen_. The liver cells have the ability to convert sugar into
glycogen and they do this whenever the amount of sugar in the blood
passing through them is greater than the very small amount which is
suitable for the body cells. The blood that leaves the liver carries in
it only this small percentage. The liver cells have the ability to
change glycogen back into sugar, and this they do whenever the blood
that enters them is deficient in it, so that the blood leaving the liver
tends always to have the same amount of sugar in it. Whenever it enters
with more, there is a conversion of sugar into glycogen; whenever it
enters with less, there is a conversion of glycogen back into sugar.
So important is the protection of the body cells against having too much
sugar in the fluids surrounding them that the kidney acts to prevent
undue accumulation; this it does by withdrawing from the blood and
passing out into the urine any sugar that may be in the blood in excess
of the small amount which the tissues are able to endure. Thus we see
that if the liver did not have its function of converting the sugar into
glycogen we would have to change our eating habits completely, taking
only a little food at a time instead of eating it in three meals, since
otherwise most of our food would be wasted by being passed out from the
kidneys as fast as it was poured into the blood from the digestive
organs. There is a limit to the ability of the liver to change sugar
into glycogen. If the amount in the portal vein at any one time goes
above a certain figure, not all will be saved: a part will escape into
the blood stream, and since this will raise the percentage, the kidney
comes into action and passes it out. In order for this to happen, there
must be a very large amount of digested sugar in the small intestine
leading to rapid absorption. Since some starchy foods are easier to
digest than others some diets are more likely to result in the
appearance of sugar in the urine than others. Some of our common foods,
notably honey and corn sirup, consist largely of the kind of sugar the
body uses. These require no digestion at all, but are ready for
absorption as soon as they enter the small intestine. Naturally, if they
are eaten in any quantity, they are likely to flood the liver with sugar
beyond the amount which it can change to glycogen. Common table sugar
and the sugar of milk have to be digested before they are absorbed and
so are less likely to flood the liver. It is true, however, that either
of them if taken in very large amounts may do this. The digestion of
starch goes on much more slowly and so is absorbed more gradually and it
is doubtful whether the liver is ever flooded on a starch diet. Since
the presence of sugar in the urine is a common indication of diabetes,
it is necessary to know that other conditions may bring it about.
Obviously, as in the case of an examination for life insurance, a
perfectly healthy person might be rejected on account of the presence
of sugar in his urine, if it were not that the examining physician knows
of this other possibility and is on his guard against it.
The liver ordinarily turns glycogen back into sugar at just the rate
necessary to keep the amount in the blood constant. This means that when
functional metabolism is going on glycogen is being turned into sugar
more rapidly than when the body is quiet. One of the very interesting
discoveries of recent years is that in times of strong excitement
leading to the outpouring of adrenalin into the blood the rate of change
of glycogen into sugar is much increased, so that instead of the usual
small amount there is present in the blood a large concentration of
sugar. This is evidently advantageous in insuring ample fuel supplies to
the muscles at the time of an emergency. It is wasteful in that a large
part of the surplus sugar is passed out by the kidneys. The fact is
illustrated that in marshaling the bodily functions for meeting an
emergency economy is lost sight of.
In addition to these healthy conditions in which sugar may appear in the
urine there is the disease diabetes, in which the presence of sugar in
the urine is a conspicuous symptom. In diabetes there is a serious
disturbance of the whole fuel-supplying mechanism; the liver does not
carry on its function of changing sugar to glycogen and glycogen back to
sugar as perfectly as it should, and what is of much more importance the
muscles which are the chief users of sugar as fuel cannot use sugar in
anything like their usual manner. In fact in severe cases they appear to
be almost wholly unable to use sugar as fuel. Since the protein from
which the nitrogen has been removed classes itself with sugar in this
regard, the muscles are thrown back upon fat as their only source of
fuel, and this confronts the body with the danger already mentioned that
in the burning of fat when little or no sugar is being burned along with
it very poisonous products may be formed. Medical investigators have
devoted a vast amount of labor to the attempts to find a diet that can
be successfully eaten by diabetics. Starches and, almost equally,
proteins are not serviceable because they simply flood the tissues with
sugar, making an environment which is not good for the cells, and
keeping the kidneys busy getting rid of the surplus. Fats are dangerous
for the reason just stated. Quite recently real progress has been made
by means of the discovery that when the body is living on its own tissue
there will be no accumulation of sugar in the body fluids nor outpouring
of it from the kidneys. One who is being starved is living on his own
tissues and so by simply starving a diabetic his symptoms can be
relieved. This is not in itself a very promising expedient, since
evidently without food one cannot go on living very long. The point of
the treatment is that after starvation has proceeded until the body is
actually living on its own tissue it is possible to begin feeding
proteins cautiously until little by little a protein diet can be
established in which there is little or no indication of surplus sugar.
In other words it seems that when the body is compelled to live upon its
own tissue it uses proteins efficiently and will then go on using them
efficiently when they are supplied to it in the diet.
In an earlier section of the chapter we talked about the amount of
protein that the body requires; now we have to take up the matter of the
amounts of energy-yielding foods. It is evident that the amounts of
these depend upon the amount of metabolism; the total metabolism is made
up of the basic metabolism which is steady, shifting little day in and
day out, plus the functional metabolism which depends upon how actively
the body works. The main functional metabolism is that of the muscles
and it is that which varies from day to day. In the case of children
there is the additional metabolism of growth for which energy is
required and for which food must be eaten. In order to talk
intelligently about the use of foods for metabolism we must have a word
by which to express a definite amount of it. The word that we use for
this is “calory.” Primarily this word stands for a certain amount of
energy in the form of heat; since one kind of energy can be transformed
into another kind without changing the actual energy value we can use
this word as a measure of all kinds of energy and this has become the
custom. The calory as a unit of energy means really the amount of energy
in the form of heat required to raise the temperature of 1,000 grams of
water by 1 degree centigrade. We can translate it into more familiar
terms by stating that it equals almost exactly the amount of energy
required to raise a weight of 300 pounds to a height of 10 feet, or 30
pounds to a height of 100 feet, or any other combination of weight in
pounds multiplied by distance in feet which figures up to 3,000.
The total metabolism, as we said a moment ago, varies greatly day by day
because the extent to which we use our muscles is so different. The
basic metabolism is very steady and the functional metabolism of the
vital processes like breathing, the heartbeat, etc., is also pretty
steady, so that the metabolism for one who makes no use of his muscles
is fairly uniform. Of course it differs in large people as compared with
small, although curiously enough the difference is not proportional to
the weight but to the body surface. Just why this is so we do not know.
The average figure for the total metabolism of a resting man is about
1,900 calories a day; this includes the motions that are necessary for
eating and swallowing food, since life cannot go on indefinitely without
making those motions, but supposes that all other activities are done
away with. This figure, as we said a moment ago, holds pretty steady day
in and day out. To find the total metabolism on any day we have simply
to add to it the figure for the energy expenditure from the use of the
muscles, which will vary with the amount of work that is done. In
figuring this we have to reckon with the fact that the muscles are like
other machines in working at what is called in engineering a low
efficiency; by that we mean that the amount of energy that can be gotten
out in the form of useful work is less than the energy that is actually
consumed; there is a waste of energy which takes the form of heat. Our
muscles are under ordinary circumstances about twenty per cent
efficient, which means that for every calory-worth of actual muscular
work we do we use up five calories-worth of fuel; the energy of four
wasted calories takes the form of heat and we all know from experience
that the amount of heat thus produced inside our bodies warms us up very
quickly, when we are using our muscles actively.
The actual muscular work done in a day by individuals varies from almost
nothing in the case of invalids confined to bed through about forty
calories for a person of decidedly sedentary habits and about 120
calories for the average clerical or professional man up to from 300 to
400 in the case of manual laborers. These figures are for the actual
muscular work done; to obtain the energy expenditure we have to multiply
each of them by five on account of the inefficiency of the muscles. If
we do this and then add to each product the constant figure of 1,900 for
the metabolism of rest we obtain for the total metabolism of a decidedly
sedentary person an average of about 2,100 calories; for an average
clerical or professional man 2,500 calories, for manual laborers from
3,200 to 4,000 calories. Of course, individuals may exceed even these
latter figures. It is believed that athletes in extreme competitions
such as for example a six-day bicycle race may liberate energy at the
rate of 10,000 calories a day, although they probably cannot keep this
up long enough actually to do that amount in a single day.
In order to satisfy the requirements of metabolism the food that is
eaten must yield corresponding amounts of energy. If it does not do so
enough of the tissues will be consumed to make up the deficiency. Of
these the first to be drawn upon will be the stored glycogen in the
liver and secondly the body fat. Only when the deficiency is great
enough so that all these are used up, does the protoplasm itself begin
to be drawn upon as a source of fuel. This happens in cases of prolonged
starvation and it is interesting to note that in this case the tissues
drawn upon are the muscles. When one wastes away as the result of
starvation, the only tissues that suffer serious loss at first are the
muscles; the rest of the body is fed at their expense. In this process
no muscle cells are actually destroyed--apparently each can sacrifice a
little of its protoplasm without being itself injured; the material thus
obtained is converted into amino acids and from most of these the
nitrogen is removed, leaving a fuel material which can be burned in the
cells all over the body to keep them going. It is only after extreme
starvation, when the muscles can no longer yield of their substance
without being themselves destroyed, that the other tissues begin to show
serious wastage. This explains why the brain of a starving man remains
clear almost up to the end.
If a surplus of food is eaten over the energy requirements, the liver
will store it in the form of glycogen so far as it is able; but if this
will not suffice, the excess will be changed into fat and stored in the
body in what are called adipose tissues. These are located in various
regions, one of the most important being directly under the skin. It is
the loading of this with fat that causes the bodily enlargement of fat
people. We do not know exactly how the fat is made; we do know that it
is not ordinarily food fat that has been simply transported to these
tissues and deposited there. There is abundant proof that the body can
manufacture fat even if there is none in the diet. Grazing cows that get
no fat of any kind produce milk with its regular percentage of fat in
the form of cream and do this day in and day out, showing that the fat
that the body makes does not have to come from fat in the food. Since
body fat represents ordinarily a storage of fuel against a future need,
we ordinarily think of it as made whenever the temporary storage in the
form of glycogen becomes inadequate to take care of the surplus of food
over the amount consumed in carrying on the metabolism. Since the vast
majority of people are neither gaining nor losing weight, the amount of
food that they take in each day must balance the average metabolism.
This is interesting because the amount of food that is eaten is
regulated chiefly by the sense of hunger, or by the hunger and appetite
together, and it is remarkable that these should cause us to eat so
accurately just the amount of food our metabolism requires. In order
that we may get some idea of how much energy is furnished in our common
foods a table is given below. The figures are for the numbers of
calories in a pound of the food material as purchased in the market. In
most vegetables and meats there is a loss of about ten per cent in
preparing them for the table, or, in the case of meat, in the bones. The
figures were prepared by officials of the United States Government in
arranging dietaries for the Army.
CALORIES PER POUND AS PURCHASED
Apples, fresh 219
Bacon 2,979
Bananas 298
Beans, dried 1,603
Beef, fresh 1,009
Bread 1,300
Butter 3,478
Cabbage 124
Celery 70
Cheese, American 1,948
Chocolate 2,858
Eggs 614
Fish, salmon, canned 679
Fish, fresh 368
Flour 1,651
Lard and substitutes 4,218
Milk 302
Pork, salt 2,948
Potatoes, white 311
Rice 1,631
Sugar 1,860
Tomatoes 106
Very few of our foodstuffs are exclusively of one kind of material; that
is they are not exclusively protein or exclusively fat or exclusively of
sugar or starch compounds. Most vegetables are mixtures of starch with
protein, fruits are mixtures of starch and sugar with a little protein,
both fruits and vegetables contain so much water that their actual fuel
value per pound amounts to little. Meats are mixtures of proteins with
fats. Milk is a mixture of proteins, sugar, and fat. Table sugar and
butter are as near pure unmixed foodstuffs as any of the things we
commonly eat. In an earlier part of the chapter we saw that about
two-thirds of our ordinary diet is of starch or starchlike materials.
This figure is given, not in weight of material, but in the energy
value. What it means is that about two-thirds of the energy for our
metabolism comes from the starch and sugar that we eat; the other
one-third is divided between fat and protein in the proportion of about
two to one. The energy we get from fat being about double that which we
get from protein--since a given weight of fat has about twice the energy
value of the same weight of protein--we actually eat about the same
amount of protein as of fat. The combined weight of the starch and sugar
is between four and five times that of the protein. These, of course,
are average figures representing not what we eat at any one meal, but
the way in which the foodstuffs are found to be divided in our diet
taken as a whole. Where meals have to be planned on a large scale, as in
armies or in institutions where the persons to eat the food have not
much choice in selecting them, it is necessary that the diets be
arranged both to give the proper amounts of material and also to furnish
them in about the correct proportions. The dietary experts who have
charge of these matters do this with tables similar to the one given
above. In domestic feeding arrangements, although usually the choice of
foods is determined by the state of the markets, the preferences of the
various members of the family, and the abilities of the cook, it is
astonishing how closely the result will correspond in the long run to
the figures here given. Most of us, without any effort on our part to do
so, eat a diet which is made up week in and week out in just about the
proportions here given. The experience of ourselves and of all our
ancestors indicates that these are the correct proportions for the human
race.
In talking about metabolism thus far we have spoken as though the
metabolism of the body at rest were about the same day in and day out
regardless of conditions. This is true in the main for healthy persons;
there is one condition which may bring about a change in the resting
metabolism of health which must be mentioned, and one or two coming
under the category of disease about which also something must be said.
The change in resting metabolism that comes about in health is one that
is seen when the diet contains an especially large percentage of
protein. For some reason which we do not understand the digestion,
absorption, and utilization of protein stimulate the resting metabolism
so that during the time that this protein is being used the metabolism
is higher than at other times. The curious thing about it is that the
increase of metabolism is not just enough to take care of the protein
itself, but goes so far to cause sugars or even fats to be burned at a
more rapid rate than usual. This fact is taken advantage of in treatment
for reducing flesh; where one lays on flesh it is evident that more food
has been taken than was required for metabolism so that the surplus has
been stored in the form of fat. The only way to reduce the weight is to
compel the body to burn up that stored fat. In theory the simplest way
to do this is simply to starve. If starvation is combined with very
vigorous exercise, the reduction of weight is bound to be rapid, since
metabolism cannot be carried on without fuel, and if not enough fuel is
supplied in the form of food, the body will have to furnish it and the
stored fat is the place from which it will be taken. Unfortunately this
is much simpler in theory than it is in practice. A good deal of
discomfort and sometimes even disturbance of health results from too
drastic efforts to reduce the weight by starvation. It is perfectly
feasible to do it by adopting and sticking to a practice of never eating
quite enough. This is a perfectly successful method, but requires great
strength of will to carry it out. Probably the easiest way to reduce
weight is to combine self-denial with a diet which consists chiefly of
protein. Thus the stimulating effect upon metabolism is obtained and the
result will be a gradual burning away of the body fat. The selection of
a diet to fit any particular individual can best be made under competent
medical advice, since personal peculiarities have to be taken into
account in selecting among the various foods those best adapted for the
purpose.
The variations in resting metabolism that fall under the head of disease
are, first, the increase of metabolism in fever, about which we shall
speak in detail in the next chapter, and, secondly, variations in
metabolism that result from variations in the activity of the thyroid
gland. The thyroid gland is an organ at the front of the neck; when it
is enlarged, as it is in some people, we have the condition known as
goiter. This gland is now known to manufacture and pour out into the
blood a hormone which is a regulator of metabolism. When it is produced
in normal amounts, the metabolism goes on at the rate that we find in
healthy people. If the gland is inactive and does not secrete enough of
the hormone, there is a reduction in the metabolism. Since this implies
a lowering of the vigor of the life processes, we might expect it to
have important effects. The most marked of these are in the nervous
system. Persons whose thyroid glands are relatively inactive are
mentally sluggish; the less active the gland is, the more marked is this
sluggishness, and in case of practically complete absence of the hormone
the condition amounts to idiocy. Occasionally a child is born without an
active thyroid gland; this child is doomed for life, unless artificial
aid can be procured, not only to complete idiocy but to all the other
results of lowered metabolism; these show themselves in dwarfishness and
a misshapen body. One of the conspicuous and beneficent discoveries of
medicine in comparatively recent years has been that extracts of the
thyroid glands of meat animals, when eaten by persons whose own thyroids
are not sufficiently active, supply the lack, and so they may be
restored to the normal condition. There are many people alive to-day who
are in all respects normal, but who, if they were to discontinue taking
thyroid extract, would relapse rapidly into a condition of idiocy.
The thyroid gland may sometimes become overactive as well as
underactive; when the former happens we have an increase in the resting
metabolism and a group of symptoms that indicate, so far as the nervous
system is concerned, a condition of overexcitability. Unfortunately
this does not mean exceptional mental power but rather mental
instability. The victims of this condition are exceptionally quick
nervously, but they are quick to take offense and quick to be disturbed
by all sorts of conditions. If the overactivity of the gland becomes too
pronounced, mental instability or even marked insanity results. Another
fact of the heightened metabolism is that large amounts of food must be
eaten to carry it on. Sufferers from overactivity of the thyroid gland
eat voraciously, but, in spite of doing so, are thin or even emaciated.
They have rapid heartbeat, high blood pressure, and other symptoms
indicative of too great activity of the gland. The successful treatment
of this condition depends on the removal by surgery of enough of the
thyroid gland to reduce the outpouring of the hormone to the normal
amount. This feat is now accomplished successfully by our most skillful
surgeons, and the result has been the restoring to health and happiness
of large numbers of people whose lives were rendered miserable through
no fault of their own, but because their thyroid glands had become
unduly active. We do not know how the thyroid gland itself is
controlled; there is evidently something which causes it in the vast
majority of us to produce its hormone at the rate which keeps the
metabolism steady at what we look upon as the level of health. Deficient
thyroid is, at least in some cases, hereditary. Excessive thyroid
activity seems to be rather the secondary result of some preceding
disturbance of the nervous system, but as to that we cannot say with
confidence.
CHAPTER XIX
KEEPING WARM IN WINTER AND COOL IN SUMMER
A good deal of what has been said thus far in the book applies to nearly
all kinds of animals about as well as it does to man. We have now to
take up a feature found only in two great groups of the animal
kingdom--birds, and the four-legged animals, to which are given the name
of mammals. This peculiarity is commonly called warm-bloodedness. What
we really mean when we say that an animal is warm-blooded is that the
temperature of its body runs about the same summer and winter, day in
and day out. An animal that we call cold-blooded, on the other hand,
cools down when in a cold place, but is warm when in a warm place. It
therefore has a very variable temperature as compared with the almost
constant temperature of warm-blooded animals. The maintaining of the
warm-blooded condition, which means really the maintaining of a constant
temperature, involves only one thing, namely that the amount of heat
lost from the body shall exactly balance the amount of heat produced in
it. If more heat is produced than is lost, the temperature must rise; if
more heat is lost than is produced, the temperature must fall. These are
simple facts of physics which apply as well to the body of an animal as
to any other source of heat.
All animals produce heat, because in all of them metabolism is going on,
which means that in all of them energy is being liberated, and it is one
of the fundamental laws of physics that whenever any energy is liberated
all of it which does not take some other definite form is certain to
appear as heat. The other possible forms that the energy liberation of
the body may take are chemical energy in the manufacture of various
materials and the energy of motion. Whenever either of these processes
takes place a large amount of heat is produced in connection; this
means, from the standpoint of mechanics, as we have already seen, that
the body is an inefficient machine; if it were perfectly efficient, the
energy that it liberates might all go into the form either of motion or
of the manufacture of materials, with none left over to appear as heat;
but since it is only 20 per cent efficient, four-fifths of all the
energy that is displayed actually takes the form of heat. While this is
a marked inefficiency from the mechanical standpoint, from the
standpoint of bodily well-being it is by no means a bad thing, since it
is this constant production of heat which makes possible in birds and
mammals the maintenance of a constant and rather high temperature. That
it is a great advantage to the body to be warm all of the time is clear
when we compare the possibilities of our lives with those of
cold-blooded animals, like insects or frogs. Whenever the weather cools
down they necessarily become inactive since, as we saw in the third
chapter, when protoplasm is cooled, its metabolism necessarily slows
down, and when the cooling reaches a certain point the protoplasm
becomes completely inactive. The temperature of 98½ degrees F., which
our bodies maintain, is a temperature which is very well suited for an
active metabolism; by keeping this temperature all the year around they
are able to show this metabolism all the time instead of only in summer,
as in the case of the cold-blooded animals.
As we said a moment ago, the maintaining of a constant temperature is
altogether a matter of making the loss of heat balance the production of
heat; the production of heat is altogether a matter of the metabolism;
practically the whole of the resting metabolism takes the form of heat,
since apparently the expenditure of energy in keeping the protoplasm
alive is extremely wasteful; nearly all the energy that is actually
liberated takes the form of heat, almost none of it being used in actual
manufacture of material. This is, as has already been said, a very
constant metabolism; the functional metabolism on the other hand is
extremely variable depending on how hard we use our muscles; the total
amount of heat produced from hour to hour is scarcely ever the same
except when we are asleep; this means that the loss of heat, which has
to balance the production of heat, must vary from hour to hour exactly
parallel with the latter. In birds and in all mammals, except man, the
adjustment of heat loss to heat production is almost wholly automatic;
the animal or bird does very little to control it. We do see, however, a
few examples of effort on the part of animals either to prevent heat
from being lost too rapidly or to favor its more rapid loss. Thus, on a
cold day, a cat or dog is apt to lie curled up or with its legs bunched
under it; on a very warm day, on the other hand, it will lie stretched
out as much as possible, and in the case of the dog, with the tongue
hanging out full length. Birds sometimes are seen with feathers ruffled
up apparently in an effort to keep warm. Both birds and mammals seek
sheltered places in which to sleep, where they will be as little exposed
as possible to cold winds or rains. In man the automatic adjustment of
heat loss to heat production is very imperfect, in fact, it would not
enable men to live naked except in the tropics. It is true that outside
the arctic regions men might go naked and still make the heat production
equal the heat loss during the waking hours by very vigorous exercise,
but during the hours of sleep the loss of heat would be certain to be
much more rapid than its production and so the body temperature would
fall in severe weather enough to cause death. Man maintains himself
outside the tropics, then, by using artificial aids for maintaining the
balance between heat production and heat loss; these are of three sorts;
first the use of clothing, second the use of shelter, and third the use
of artificial heat. It is really a very curious fact when one stops to
think of it that, although many animals enjoy artificial heat and gladly
bask in it when opportunity offers, no animal has ever discovered the
simple fact that throwing sticks on a fire will keep it going, and so no
animals, except man, have ever made any real use of artificial heat.
When we think that the progress of human civilization has been
accomplished wholly in parts of the earth where clothing, shelter, and
fire are necessary to human existence, we realize that these, instead of
being mere incidents to our life, lie really at the very basis of
advancement. Hairless man is evidently a tropical animal; if he had not
devised means of maintaining himself outside the tropics, there is no
reason to suppose that he would have behaved any differently than have
the savages that inhabit those regions at the present time.
Clothing, shelter, and fire all operate to prevent us from losing heat
too rapidly and so have their great value when the weather is cool; we
also can and do bring about an increased heat production in cold weather
partly by exercising more actively and partly by increasing the amount
of protein in the diet, and so bringing into play the stimulating effect
of that substance on metabolism. This latter is seen perhaps most
strikingly in the experience of the Eskimos; when one of them comes in
from a hunting trip in the depth of winter very much chilled, he
scarcely stops to warm himself at all, but with the greatest haste gulps
huge quantities of meat that are barely thawed out, not really cooked in
our sense of the word; in fact, frequently the meat that is eaten is so
cold that it makes the Eskimo chillier than before; as soon, however, as
digestion and absorption have commenced, so that the stimulation of
metabolism can begin, the rapid production of heat in the body warms it
up to the point where complete comfort is obtained.
We have no other means of increasing heat production in our bodies
except by muscular exercise or eating protein. It is interesting to
note, however, that there is a form of involuntary muscular exercise
which comes into play when we need to produce more heat in order to
balance too rapid loss; this involuntary muscular action is the familiar
shivering which one does when chilly. It is reflex in the strict sense,
that is to say, the stimulation of cold on the skin arouses nervous
disturbances which pass to the muscles in various parts of the body and
set them into the violent movements which we call shivering. The
functional metabolism of shivering is much greater than we might at
first suppose, and the production of heat is correspondingly rapid. In
fact, if one who feels inclined to shiver encourages it instead of
attempting to keep from doing it, he will very quickly produce enough
heat within his body so that he will no longer have a disposition to
shiver.
We have seen above the various ways in which heat is produced in the
body, and the artificial means we employ to prevent heat from being lost
too rapidly. It is clear that all of these make up what we may call
coarse adjustments. They tend on the whole to cause heat to be produced
more rapidly when more is needed, or to prevent too rapid loss when
conditions are of a sort to bring it about. Nothing that has yet been
said accounts for the fine adjustment, that is, for the actual
maintaining from moment to moment of a balance of the heat loss with the
heat production. This fine adjustment is purely automatic and is carried
on in us by two distinct means. The first of these is by an automatic
regulation of the amount of blood flowing through the skin and so,
within certain limits, of the temperature of the skin. It is evident
that if the skin is warm heat will be given off from it more rapidly
than if it is cool; a condition in which the balance is being upset by
the failure of the body to lose heat rapidly enough can be corrected, at
least in part, by causing the skin to become warmer and so more heat to
be given off. The machinery for regulating the amount of blood flowing
through the skin is the vasomotor system, about which a good deal has
already been said. This system operates reflexly, stimulation of cold on
the skin tends to diminish the flow of blood through it by contracting
the blood vessels, while warmth on the skin has just the opposite
effect, causing the blood vessels to become flushed and the blood flow
to be more rapid. It is true that there is a reddening of exposed parts
of the skin in extreme cold, as we all see frequently on our nose or
ears, but this is a purely local effect and does not mean that the blood
is flowing through the region rapidly enough to keep up its temperature
and so favor the loss of heat. These changes in the amount of blood in
the skin are very effective in the fine regulation of heat loss. The
body can get rid of heat very much more effectively when the skin is
flushed than when it is pale. There are, however, limits to the
usefulness of this arrangement. Even when the skin is as pale as it can
become it still is warm, and on a cold day it continues to lose heat
more rapidly than is desirable. It is to prevent this that clothing is
worn. Clothing operates for people as fur does for animals. It
establishes a nonconducting layer between the skin and the outside. This
nonconducting layer warms up to the temperature of the body and so
hinders the escape of heat. The actual nonconducting material is the air
which is caught in the meshes of the fabric, or in the case of
fur-bearing animals among the different strands of the fur. The
effectiveness of clothing for conserving heat depends altogether on the
degree to which it imprisons air in its meshes. In this respect wool is
the most effective of all fabrics, cotton next, silk and linen having
very little effectiveness. This explains why wool is preferred for
winter clothing, linen and silk for summer.
The temperature of the body as a whole is, as we have seen, about 98½
degrees F.; this figure represents the maximum temperature that the skin
can reach, even when it is flushed to its utmost. In the warm weather of
the summertime the temperature of the air frequently mounts to or even
above that point and it is evident that under such circumstances the
body cannot give heat directly from itself to the surrounding air, and
no amount of flushing of the skin will enable it to do so. It is under
these circumstances that the second method of getting rid of heat comes
into play, namely, the evaporation of sweat. We have spoken of the sweat
glands in a previous chapter, but have left their action and the
description of the skin in which they lie for this point.
The skin serves a number of purposes; it is the great protective layer
for all the parts beneath; in order that it may serve for this it must
itself be reasonably free from injury. This is secured by having the
outer layers of the skin composed of a horny dead material. Just beneath
this dead layer is a layer of living cells tightly packed together and
having exposure to the body fluids only at their under side. Cell
multiplication goes on in these cells continuously and rapidly; as the
cells divide, they grow, the result being that those that are underneath
are constantly crowding those above them farther and farther out toward
the surface; this cuts them off from their food supplies and so they
die. This layer of dead cells becomes very closely packed and makes up
what is commonly called the horny layer of the skin or the “cuticle.”
The outer part of this cuticle is constantly rubbing off and, of
course, as this happens any dirt that may be clinging is rubbed off as
well. A large factor of cleanliness is the constant rubbing off from the
surface of the body of its outermost layer. The extent to which this
happens can be appreciated by anyone who has ever had a broken bone and
has had to have an arm or leg put up in a splint or cast for several
weeks. At the end of that time there is a great accumulation of dead
skin waiting to be rubbed off.
Bathing is a very efficient aid to cleanliness; when baths are for this
purpose, warm or hot water, with an abundance of soap, should be used;
the choice of soap is an individual matter determined largely by the
sensitiveness of the skin. Chemically there is no very great difference
between one kind of soap and another. Some of them contain materials
which are more irritating than those found in others and are to be
avoided by persons whose skins are easily irritated. The common belief
that a bath should not be taken immediately after eating rests on the
feeling that the flushing of the skin that results from the contact with
the warm water may divert blood unduly from the alimentary tract and so
interfere with the process of digestion. This may very well be true if a
very hot bath is taken; where the bath is only a degree or two above
body temperature little trouble is to be feared from this cause. A cold
bath shortly after eating sometimes gives rise to cramps either in the
stomach or in the muscles. Not much is known about these cramps or about
the causes which bring them about. In connection with its use as the
protective surface of the body the skin has scattered over it numerous
glands known as the _sebaceous glands_, which secrete an oily substance
that is spread more or less completely over the skin. This oil probably
helps to keep the skin soft and also to some degree to make the skin
waterproof. One of the reasons why soap has to be used in bathing is to
dissolve off this thin film of oil which is ordinarily over the surface
of the body.
Another use of the skin is as a great sense organ; as we have already
seen, the structures for touch, for temperature, and for pain are
present in the deeper layers of the skin. These consist of tiny cell
masses into which the tips of the sensory nerves pass. In the case of
the nerves of pain there are believed to be no special end organs, but
the nerves end nakedly among the cells of the skin.
The third use of the skin, and that with which we are particularly
concerned here, is as the fine regulator of body temperature. We have
already seen in part how the skin works in this connection, through
being warmer when there is more blood flowing through it and cooler when
there is less. Its other heat-regulating mechanism is made up of the
_sweat glands_. These, as the name implies, are tiny glands in the skin;
they consist of tubes opening on the surface and at their inner end
coiled up into a sort of knot. In this knot are a great many fine blood
vessels, so that each sweat gland has an abundant blood supply. So far
as we can tell, the sweat glands act as filters, corresponding in that
respect to the filtering tips of the kidney tubes. One interesting thing
about them is that the filtering action is controlled by nerves. These
nerves belong to the system which controls the smooth muscles and
glands; when the nerves become active the sweat glands are also active,
and there is rapid filtration of water and inorganic salts from the
blood into the sweat glands and out to the outside of the body. Small
amounts of organic materials come out also in the sweat, and these are
responsible for its characteristic odor. They evidently differ in
different people, since all dogs and some persons are able to recognize
individuals by their odor, and body odor is largely the odor of sweat.
The sweat glands are carrying on their filtering activity most if not
all of the time; we are not usually aware of it because the sweat comes
out in exceedingly fine drops which evaporate as fast as they are
formed; it is only when the sweat is poured out faster than it
evaporates that we become aware of its presence. If the air is very full
of moisture, sweat will stand on our bodies even though it is not being
formed more abundantly than usual. This is because of its inability to
evaporate into air already laden with moisture. Usually, however, when
sweat appears on the surface of the body it is because the sweat glands
have become more active. A couple of paragraphs ago we saw that the
sweat glands are under the control of nerves; these arise like the other
nerves that control smooth muscles and glands from a center in the brain
stem; this particular one is called the sweat center. It may become
active through various influences. The one most commonly arousing it is
a rise in the temperature of the blood. Whenever we begin to exercise,
heat is produced very rapidly, warming up the tissues in which the
activity is going on and, since the blood is flowing through them
rapidly, warming the blood up as well. This warm blood circulates all
through the body, tending to raise all parts of it to the temperature
of the active regions, or, as it passes through the skin, to cool by
loss of heat from the surface. When this warm blood enters the brain
stem it arouses the sweat center and an increased secretion of sweat
results. The importance of this is that the evaporation of water, no
matter at what temperature, requires a large amount of heat. This heat
is abstracted from the nearest place, which in this case would be the
body itself, so that the evaporation of sweat acts powerfully to cool
off the body. In hot weather this is really our only effective means of
getting rid of heat, for if the body is no warmer than its surroundings
it cannot lose heat directly, but sweat can evaporate, taking heat with
it, no matter how warm the surroundings may be. We said a moment ago
that a high percentage of water in the air would hamper the evaporation
of sweat. The practical workings of this fact are seen in connection
with heat prostrations in the summertime. Careful scrutiny of the
weather reports will show that these are much more numerous as well as
more severe on days of high relative humidity than on days that are
simply hot, but without much moisture in the air. As the sweat
evaporates from the body, the resulting water vapor has a tendency to
linger in the neighborhood and so to interfere with further evaporation.
Under these circumstances it is desirable that this moisture-laden air
be moved away and fresh air brought into its place. This is the benefit
we obtain from fanning. The air that is brought upon us by a fan feels
cooler than it really is; this we realize when we notice how comfortable
the draft of a fan is even on the very hottest days. When one fans
oneself in the old-fashioned way, the heat produced in working the fan
is often almost enough to balance the loss through the improved
evaporation of sweat. An electric fan, on the other hand, is a very
powerful aid to comfort in hot weather. In fact it is hard to think of
any mechanical device that has contributed more in recent years. An
amusing illustration of the importance of moisture in the air in
relation to bodily comfort is in the case of the man who thought he
would obtain a cool-weather office in hot weather by installing a block
of ice and causing his electric fan to play upon it. Very much to his
surprise and quite contrary to his expectations the room quickly became
unbearably sultry. The rapid evaporation of the ice filled the air so
full of water vapor that evaporation of sweat could not take place and
the persons in the room were much worse off than if they had contented
themselves with merely keeping the warm and comparatively dry air of the
room in rapid motion. Although the evaporation of sweat is a very
effective means of getting rid of heat from the body it does not work
perfectly in times of great muscular exertion in very hot weather.
Careful studies of men in steel industries and other places where hard
work is done in intense heat have shown that the production of heat
during the course of the working day slightly out-strips its loss, so
that by the end of the day it is the regular thing for the body
temperature to be up two or three degrees, to 100 or 101 degrees
Fahrenheit. Upon finishing the work the temperature quickly falls to
normal, so that there is no reason at present for supposing this daily
rise to be seriously harmful.
In fever the production of heat becomes greater than its loss for a time
and so the temperature goes up. It must not be forgotten that presently
the loss again balances the production; if it did not, the temperature
would keep on rising until death resulted; usually the greater loss of
heat from the body at the fever temperature is sufficient to restore the
equilibrium, but it will be clear that the temperature cannot fall again
until the heat loss exceeds the heat production. There has been a good
deal of discussion as to whether fevers are caused by an increase of
heat production or by an impairment of the machinery for getting rid of
heat. There is no doubt that in fevers the resting metabolism is much
greater than in health; it is not by any means always the case, however,
that the total metabolism of an individual lying quietly in bed with a
fever is greater than that of a healthy individual doing heavy manual
labor. We must therefore look to an interference with the process of
losing heat to account for the rise in temperature. The distribution of
blood through the skin in fever does not differ from that at other times
when the body is warm; it is the usual thing in fevers for the skin to
be much flushed. There is, on the other hand, a definite impairment of
the sweat-secreting mechanism. It is one of the most familiar facts
about fever that the skin is not only hot but very dry. It is also well
known that by inducing sweating the temperature can often be brought
down. It is evident that the poisons which are responsible for the fever
have at least two effects; to increase general resting metabolism to a
high level and at the same time to interfere with the ordinary
regulation of the sweat glands, so that, even though the temperature of
the blood is several degrees above normal, the sweat glands are not
stimulated to active production of sweat.
A fever is very uncomfortable to the person suffering from it, but is
not in itself particularly dangerous, unless it mounts to a very high
degree. The body wastes away rapidly, of course, because of the
increased metabolism which is usually not balanced by any taking of
food. Theoretically fever patients should eat as abundantly as laborers;
practically this is impossible of accomplishment in most cases because
of the depressed condition of mind and body of the patient. In some
particularly wasting diseases, like typhoid, feeding has been carried on
with some degree of success. The old practice with regard to fevers was
to restore the patient to comfort as quickly as possible by getting rid
of the fever, that is, by lowering his temperature to normal; at the
present time the tendency in medicine is not so much to strive with the
result of the poisoning as to seek to rid the body as quickly as
possible of the poison and of its source. For that reason modern
physicians are much less concerned with the degree of fever shown by the
patient than with the extent to which the poisons in his body are being
controlled or are becoming masters.
In all our discussion of the regulation of body temperature it is
important to remember that our feelings of cold or of warmth depend
altogether on the temperature of the outermost one-quarter inch of our
body surface. No matter how cold or how warm we may feel, as soon as we
get to that depth below the surface, we find the body temperature the
same, namely 98½ degrees. We feel cold when the skin is cool; the skin
is cool because not much blood is flowing through it; the stimulation of
the surrounding cold on the skin has aroused reflex vasoconstriction and
so diverted the blood out of the skin into other parts of the body.
This means that the mechanism for preventing loss of heat is working as
effectively as it can and, therefore, that the heat which the body
produces is being conserved. Experience shows that in reality the
feeling of cold is very largely a matter of the condition of the
extremities. If the hands and feet are warm, one rarely finds the cold
uncomfortable; on the other hand, cold extremities produce a very
general feeling of discomfort, even though the rest of the body may be
warm. The lesson to be drawn from this is to pay particular attention to
keeping the extremities warm. Where workmen are standing on a floor it
is much more important that the floor be warm than that the air of the
room as a whole be so. Of course, it must not be cold enough to allow
the fingers to be chilled and stiff where manual work is being done, but
it has been found that a considerably lower factory temperature is
endured when the floors are warm than when the floors are cold.
We feel cold in cold weather because the body is conserving heat; in
warm weather, on the other hand, we feel warm because the skin is
flushed and the body is losing heat as rapidly as it can. The practical
bearing of this is that it might be disastrous to feel particularly warm
in cold weather. Not, of course, if the feeling of warmth be due to a
very rapid production of heat, as in vigorous exercise, but if it is due
simply to a flushing of the skin when not much heat is being produced,
the body is being deprived of what heat it has and a condition which
cannot long be endured in cold weather will result. It happens that
exactly this situation is met with in connection with the use of
alcohol. One of the important effects of alcohol is to produce a
flushing of the skin; this is a direct drug effect and has nothing
whatever to do with the amount of heat that is being produced in the
body. The result is that one who, by a few drinks, fortifies himself,
when starting out for a long cold ride, feels very warm and comfortable
as long as the alcoholic effect persists; but as soon as this passes off
he is very much colder than he otherwise would be, since during all this
time he has been losing heat rapidly instead of conserving it. It used
formerly to be not at all uncommon in the cold parts of the country for
persons to perish from exposure on long drives because they had
attempted to keep themselves warm by taking alcohol, when if they had
conserved their bodily heat instead of wasting it for the sake of
comfort, they would probably have reached their destinations in safety.
While we are on the topic of temperature regulation and the part played
by the skin generally in bodily activities a word should be said about
bathing for other purposes than cleanliness. There is a good deal of
discussion about whether the morning cold bath is really a health
measure or is simply a fad; there can be very little doubt that persons
who react well to the cold bath find that it contributes definitely to
the enjoyment of life. It is particularly true that after a night of
disturbed rest the morning cold bath makes one feel much more ready for
the duties of the day, whether he actually is so or not. The value seems
really to be in what is known as the _reaction_ in which the first chill
is followed by a warm glow. The bath should under no circumstances be
prolonged beyond the time when a good reaction is obtained. Brisk
rubbing with a harsh towel helps the oncoming of the reaction very much.
For those who would like to take a cold bath but have found it too much
of a hardship a hint may be helpful, namely, that if one enters the bath
with the feet warm there is practically no discomfort connected with
taking it. To step into a cold bath when the feet are already cold
causes them to begin aching almost instantly and usually nullifies all
the benefits the bath affords. One should either enter the bath
immediately after rising, before there has been any time for the body to
become chilly, or if one prefers to shave or carry on other activities
first, it is a good plan to give the feet a momentary preliminary bath
in hot water. When this condition is fulfilled, the cold bath will be
both enjoyable and beneficial to the majority of people. Very hot baths
have a distinct relaxing effect; they favor the onset of sleep and can
be taken to best advantage just before retiring. There is nothing in the
common idea that a hot bath opens the pores of the skin; the only pores
the skin has are the openings of the sweat glands, and these do not vary
in size. What a hot bath does do, is to cause a marked flushing of the
skin as the result of which the body loses heat rapidly. It is
necessary, therefore, after a hot bath to wrap oneself warmly so that
excessive loss of heat shall not occur.
In finishing this topic, the subject of catching cold must be brought
up, together with its relation to the regulation of body temperature.
There is no doubt at the present time that common colds result from
infection; in that sense they are acute diseases. It seems to be the
fact, however, that the organisms which cause them are pretty constantly
present in the air passages of our throats and lungs ready to invade the
body whenever opportunity offers. Very recently it has been shown that
rapid chilling of the skin is accompanied by contraction of the blood
vessels in the linings of the throat and air passages. The effect of
thus cutting down the blood supply to these linings seems to be to make
them more ready of access by the organisms of colds. At any rate the
fact that when the skin is suddenly chilled a cold frequently develops
is best explained on this basis. Since colds are infectious, the
organisms causing them may sometimes succeed in penetrating the linings
of the air passages even when no change in the latter has occurred. It
may therefore happen, and frequently does, that one develops a cold
without being able to refer it to any time when there was a sudden
chilling of the skin. Also it can easily happen that the chilling may
not lead to a cold, since if the organisms do not happen to be abundant
or if the general bodily resistance is high, the infection will not get
a foothold, even though the linings of the air passages would permit the
organisms to enter. One’s best precaution against colds would naturally
be to avoid having the organisms which produce them present in the
throat and lungs. Since this usually cannot be done, the remaining
practical measures are to keep the general bodily health as good as
possible and to avoid conditions which lead to a sudden rapid chilling,
such as sitting in a cold draft when heated.
CHAPTER XX
THE PERPETUATION OF THE RACE
We have tried to take up one by one the chief things that happen in the
body, but thus far have emphasized their importance entirely in
connection with bodily well-being; that is, we have seen how the body
maintains itself against the competition of other living things and in a
world full of hazards. Before leaving the subject entirely, a short
account must be given of the way in which the _race_ is maintained upon
the earth as distinct from the maintenance of individuals. Early in the
book we showed that every one of us starts life as a single cell which,
by dividing and subdividing, along with continuous growth, finally
develops into our large and complicated body. This cell is really the
union of two cells; one furnished by each parent; in the body of the
parent it formed part of what is known as the _germinal tissue_, this
name being applied because the cell from which we start may be spoken of
as the germ of life. As the cell from which the body is to come begins
to develop, the cells formed from it quickly become different one from
the other, so that very early it can be distinguished that certain cells
are to form the future nervous system, others the future muscles, and so
on. One of the earliest of these groups of cells to become
distinguishable is that which is to become the germinal tissue in the
newly formed individual. A great deal of importance has been attached
of recent years to this fact that the germinal tissue is set aside
almost from the beginning of development, and when we remember that the
next generation will be derived from this germinal tissue and from no
other tissue, we see that there is a very close relationship of the
germinal tissue from generation to generation. The importance of this is
in connection with heredity, namely, with the question of the
resemblance of the child to the parent. We all know that sometimes
children resemble their parents very closely, at other times there is
almost no resemblance at all. The main fact of heredity is, as we have
just seen, that the child comes from germinal tissue of the parent and
not from the parent as a whole. We can think of an individual as a
complex body carrying within itself and nourishing an independent group
of cells which are to serve as the starting point for the next
generation, but with which the individual himself has nothing to do
except to provide nourishment. A little later we shall try to show just
why we think of the germinal tissue as so little dependent on the rest
of the body. What we want to do first is to point out how this notion
must affect our views of heredity. If the germinal tissue is simply
harbored and nourished by the body, but otherwise quite independent of
it, it follows as a matter of course that things the parent does,
provided that they do not disturb the nourishment of the germinal
tissue, can have very little effect upon it. For example, anything that
the parent may achieve during his lifetime cannot be passed on by
heredity to his offspring. This is entirely contrary to the idea of
heredity which is held by nearly everybody. Just a little over one
hundred years ago a Frenchman by the name of Lamarck published writings
in which he argued very strongly in favor of the inheritance of
“acquired characters,” meaning by this that the achievements of parents
could be and were transmitted to the offspring. It is evident that this
would be a very great advantage; the progress of the race would be
secure if the children could inherit directly the achievements of their
parents. Of course, it might work the other way around and children
might inherit unfavorable acquisitions of their parents as well as
favorable. Since the time of Lamarck there has been not only hot
discussion of his ideas but also a vast amount of experimentation to
find out whether acquired characters actually can be inherited. One of
the most famous experiments is that of a man who cut off the tails of
mice at birth and did this generation after generation in the hope of
being able to produce a race of tailless mice; after a great many
generations he abandoned the attempt with the conclusion that the
artificial removal of tails from parents would not cause the offspring
to be born in the tailless condition. More recently similar experiments
have been carried out on kinds of animals that have much shorter lives
and in which the generations are correspondingly more frequent. In some
cases thousands of generations have been studied without any indication
that changes brought about during the lifetime of the parents can be
transmitted to the offspring.
One of the first questions that is sure to be asked as soon as this
inheritance of acquired characters is denied, is how changes can then be
brought about; the answer to that question is found in the experience of
animal and plant breeders. One fact of nature which should be
emphasized, is that individuals are never exactly alike except perhaps
in the case of so-called identical twins which have a special heredity
spoken of in Chapter V. Even brothers and sisters always have pretty
marked differences. These differences among individuals are some of them
probably more or less accidental results of the way in which the germ
develops into the complete body; others are the result of differences in
the germ cells themselves. The distinction between these is that where
the germ cells are different, the difference will be transmitted by
heredity to future generations; if it is a modification that comes on in
connection with development and is not due to a difference in the germ
cell, it will not show itself in future generations. What the animal and
plant breeders have to do is to watch for changes in their stock, and
when the kinds that they are looking for appear, they breed them in the
hope that the desired features will prove to be hereditary, so that a
race can be established showing them. A very striking example of this
kind of breeding is in the development of hornless cattle. Of late years
it has been the practice to dehorn cattle regularly and this dehorning
has never caused the offspring to be born without horns, but on the
other hand it has happened occasionally that calves have been born which
were naturally hornless and since this absence of horns was due to a
difference in the germ tissue, the character was found to be hereditary
and it has been possible to establish breeds of hornless cattle. Thus by
selecting cases where a change occurred naturally in the germ tissue, a
result has been obtained which could not possibly be gotten by any
amount of work directly upon the parents.
What we have tried to make clear thus far is that heredity is absolutely
a matter of the condition of the germinal tissue. What we have now to do
is to see what the germinal tissue is like and how the process of
heredity actually goes on. In Chapter V something was said about the
nucleus; it will be remembered that the nucleus contains a substance
known as _chromatin_, which, as seen under the microscope, looks like a
tangled skein or network of fine threads, but is in reality a number of
tiny structures known as _chromosomes_. We now know that these
chromosomes are the actual controllers of heredity. Of course we do not
know at all how they exert their control; what we do know is that when
certain factors are present in the chromosomes of the germinal tissue of
the parents, the offspring derived from that germinal tissue will have
traits that would not be present if the chromosomes had been different.
Space does not permit us to tell at length how the facts that are now to
be described were discovered. They date from the gardening experiments
of an Austrian monk by the name of Mendel, who for many years grew
ordinary garden peas and studied from season to season the varieties
that appeared. The facts of heredity that he found to be true of peas,
have since been shown to apply just as well to ourselves, and since this
book deals with human beings we shall try to make the description apply
directly to human heredity. The first thing to get clearly in mind is
that every single thing about any one of us which can or does differ
from the corresponding feature in another person, may be a _hereditary
trait_, and, if it is, will have a factor controlling it somewhere among
our chromosomes. Since the number of hereditary traits is legion,
including not only the size and shape of all parts of our bodies inside
and out, but our mental peculiarities and moral tendencies as well,
there must be a huge number of controlling factors, or _determiners_, as
they are often called. As a matter of fact, there are probably not quite
so many determiners as traits, because a single determiner may govern
more than one trait. But even so, the number of determiners is too large
for comfort in trying to describe their working. The best way to go
about it is by pretending that we are not so complicated as we really
are. We shall do this by setting the number of different determiners
human beings may possess at fifty-two, not because that is anywhere near
their real number, but because it is the number of combined large and
small letters in the alphabet, and we propose to use letters to stand
for determiners, as an easy way to keep them separate in the
description.
Every cell in our bodies has its nucleus with its equipment of
chromosomes. An interesting fact already spoken of in Chapter V is that
according to our best knowledge the chromosomes in any cell of any one
of us are exactly like those in all the other cells of the same person.
The chromosomes in the muscle cells are exactly like those in the nerve
cells, and both correspond exactly with the chromosomes of the ordinary
cells in the germinal tissue. In every one of these cells the
chromosomes are arranged in pairs. It happens that in human beings the
number of pairs is twenty-four, with one pair incomplete in some of us
for a reason that will be explained presently; this is an awkwardly
large number for our present purpose, since we have allotted only
fifty-two determiners altogether, so we shall do some more pretending
and set the number of pairs at nine. One more change will have to be
made from the real state of affairs before we can go on with the
description; this is to suppose that our various bodily features can
show only one difference, instead of the many of which they are really
capable. For example, we shall suppose that the hair can be either black
or light, but never red; the nose can be Roman or Greek, but never
Irish. By making this supposition, we can let the large letters stand
for one set of hereditary determiners and the small for the same
features but with exactly contrary traits. According to this
arrangement, if we had two persons side by side one of whom had only
large-letter determiners in his chromosomes and the other only small,
they would have the same general human make-up, but in every possible
detail one would be the exact opposite of the other.
It has been proven by complicated studies which we cannot take time to
describe that the determiners are grouped in the different chromosomes
in a definite plan. We saw a moment ago that the chromosomes are in
pairs. This pairing is an essential part of the arrangement, for every
hereditary bodily feature actually has two determiners governing it,
which lie in corresponding positions in the two chromosomes of the pair.
To illustrate how this works out, let us suppose that the chromosomes of
the first pair contain determiners A, B, and C. Each of these three
determiners will be present in both members of pair number one, and if
this is true of any cell it will be true of all the cells, and in any
other human being either they or the corresponding small-letter
determiners, a, b, and c, will occupy pair one of the chromosomes.
Furthermore, they will lie in a row within the chromosomes, always in
the same order; thus if A and C are at the ends with B in the middle in
one chromosome, every other chromosome that contains these three
determiners will have them in the same order.
[Illustration: DIAGRAM ILLUSTRATING VARIOUS ARRANGEMENTS OF DETERMINERS
IN A SINGLE CHROMOSOME PAIR
1 and 2, Pure; 3, Hybrid.]
Thus far we have planned our diagrams as though only large letter
determiners were concerned; but we saw a moment ago that there is a
complete set also of small-letter determiners, which control a set of
contrary hereditary traits, and we intimated that these will sometimes
be found in the chromosomes in positions corresponding with those
occupied by the equivalent large-letter determiners. It might happen,
for example, that pair one of the chromosomes would contain large-letter
determiners A, B, and C, while pair two would contain small-letter
determiners a, b, and c. Evidently the person in whom this combination
was present would differ from one all of whose chromosomes contained
large-letter determiners, since part of his traits would be established
by small-letter determiners.
We are now ready to go back to the germinal tissue and trace the process
of heredity as it actually works out in the developing offspring. In the
ordinary cells of the germinal tissue, as we have already seen, the
chromosomes are exactly like those in the other cells of the body. Cell
multiplication goes on actively in the germinal tissue of both parents;
in the mother this leads to the production of germ cells which are
called eggs, and in the father to the production of cells that are
called sperm. During the process certain changes occur, so that neither
eggs nor sperm are exactly like the original cells of the germinal
tissue. If we look back at the description of cell division in Chapter
V, we shall recall that the chromosomes split lengthwise and are pulled
apart. Now that we have learned about determiners, we will realize that
every determiner splits in half, because otherwise there would not be an
equal distribution of determiners between the two cells. Much of the
cell division in the germinal tissue is precisely like this, but at a
certain stage in the production of both eggs and sperm there is one cell
division in which the pairs of chromosomes are simply pulled apart,
without there having been any previous lengthwise splitting. The result
of this is to leave the resulting cells with only half as many
chromosomes as the other cells of the body have. Some further changes
take place in these cells before they become ripe eggs or ripe sperm,
but there is no further disturbance of the chromosomes, so that eggs and
sperm contain only one member of each pair, instead of both members, as
do all other body cells.
The first step in development is the coming together of the egg cell
with one sperm in the process that we call fertilization. The sperm
penetrates the egg and its chromosomes line themselves up with the
egg’s, restoring the pairing that is present regularly. Immediately
afterward the cell divisions begin that make up development, and in all
of these the usual lengthwise splitting of determiners takes place.
Every cell in our body contains its pairs of chromosomes, one member of
each pair tracing back directly to the egg cell while the other traces
directly to the sperm. Thus half of our determiners came from the
maternal germ tissue and half from the paternal.
[Illustration: CHROMOSOMES IN CELL DIVISION
Figure 1, chromosome splitting in ordinary cell division, in which each
determiner splits in half, contrasts with Figure 2, reduction division,
in which the chromosomes of the pair are simply pulled apart.]
We shall now begin to see how heredity works out. Suppose chromosome one
in the egg has large-letter determiners A, B, and C, while the
corresponding chromosome in the sperm has small-letter determiners a, b,
and c. When these line up after fertilization, restoring pair one, we
have A opposite a, B opposite b, and C opposite c. Since we have
supposed the large and small letters to stand for contrary hereditary
traits we introduce here a conflict and must ask at once how it is
settled. One of the things the monk Mendel worked out in his studies of
heredity in peas was this particular problem. He found that where there
is conflicting heredity one of the determiners usually dominates over
the other, and when this happens the trait in the offspring will be like
that of the parent which contributed the dominant determiner. To
illustrate: suppose A is dominant over a, then in the case in question
the offspring will be like the mother in feature A. In some kinds of
animals and plants conflicting characters blend in the offspring,
producing an intermediate appearance. A good example of this latter case
is in the common flower, the four-o’clock. In this plant white blossoms
and red blossoms are due to determiners that occupy the same positions
in the chromosomes; therefore, if white and red flowered plants are
crossed, these conflicting determiners come together when the sperm and
egg chromosomes pair. Since neither determiner dominates over the other,
the color of the flowers in the offspring is neither white nor red, but
pink.
Any individual whose chromosome pairs contain conflicting determiners is
called a “hybrid”; there may be every degree of hybridism, from the
simplest, in which all the chromosome pairs are alike except one, up to
the most complete, in which all the pairs are unlike. It is easy to tell
a hybrid by its appearance in the cases in which there is blending
inheritance, but not so easy when one trait dominates over the other,
for then the hybrid will look like the parent that furnished the
dominant determiner. The sure way to detect hybrids is by the study of
their offspring. Suppose we have two parents, both of whom are hybrids
in respect to chromosome pair one. This pair in both will contain
determiners A, B, and C lined up opposite a, b, and c. Since we have
supposed the large letters to be dominant over the small, both will have
the same appearance, dependent on the presence in their chromosomes of
A, B, and C. Since in the formation of eggs and sperm the chromosome
pairs are pulled apart, half the eggs produced by the mother will
contain determiners A, B, and C, and the other half a, b, and c, and
half the sperm of the father will similarly contain one set and half the
other. Since it is an absolute matter of chance which sperm encounters
which egg in fertilization we can safely conclude that in the long run
all the chances will be realized equally. Calling, for convenience, the
large-letter eggs E and the small-letter e, and, similarly, the
large-letter sperm S and the small-letter s, this means that E can be
fertilized either by S or s, and e also by either S or s. The possible
combinations are ES, Es, eS, and es. In terms of actual determiners
these combinations are ABCABC, ABCabc, abcABC, and abcabc.
The combinations just given represent the possible offspring from a pair
of hybrid parents. If we look them over, we see at once that only half
of them are hybrid, namely, combinations ABCabc and abcABC; the other
half are pure breed, the chromosome pairs being exactly alike; but these
pure breeds are of two kinds, one having only large-letter determiners,
the other only small-letter. If it is a case where the large letters
dominate over the small, the large letter pure breeds and the hybrids
will look alike, but the small-letter pure breeds will look different,
since in them the traits governed by the small-letter determiners have a
chance to show themselves. A very good illustration of this is seen in
eye color in human beings. Brown eyes are dominant over blue; in other
words, the determiner that causes eyes to be brown dominates over that
responsible for blueness in cases where both come together in hybrids. A
person who has brown eyes may be either a pure breed in that respect or
may be a hybrid; there is no way to tell the difference from the
appearance; but if the brown-eyed person has offspring, and any of them
turn up with blue eyes, it is proof positive that the parent is a hybrid
so far as eye color is concerned. Moreover, the blue-eyed child is not a
hybrid in this respect; his brown-eyed brethren may or may not be; in
the long run two-thirds of them will prove so; the other third will be
pure breed, having in their chromosome pairs only brown-eye determiners.
Where the hybrids differ from the pure breeds, as in the case of the
four-o’clocks, given earlier, it is easy, of course, to tell which are
pure and which are hybrid. When pink four-o’clocks are interbred, the
chromosomes will combine just as described above for hybrids, since the
plants that have pink flowers are hybrid. One-half the offspring will
have pink flowers, showing that they are hybrid; one-fourth will have
white flowers, proving that in them the white-flower determiners have
separated out, and the other fourth will have red flowers, because in
them the red-flower determiners are the only kind present. This and
similar experiments have been tried hundreds of times, and whenever the
numbers of offspring have been great enough to allow the chances to
equalize, the proportion of different kinds of offspring has always
agreed almost exactly with expectation. Of course in human beings the
families are not large enough for this always to work out accurately,
but even so the agreement is often striking.
[Illustration: CARING FOR THE BABY’S EARS
Important for adults and babies alike]
[Illustration: THE BABY’S FOOTPRINT
A means of positive identification]
[Illustration: BABIES PHYSICALLY AND MENTALLY ACTIVE]
In practical animal breeding the blended inheritance just described is
not very useful, for even though a blended character might appear which
is just what the breeder has been looking for, it will not occur in more
than half the offspring and can not ever be depended on to show itself
in any particular individual. This explains largely why pure-bred stock
is always more desirable than hybrid, and why breeders strive so eagerly
to obtain desired traits in pure-bred animals. In plants, blended
characters are much more valuable, for two reasons; first, because the
offspring are so numerous that even though half of them come out pure,
and so lack the desired blend, there are enough left that have it to
make the crop worth while; and second, because propagation by cuttings
is possible in very many kinds of plants, which means that the same
plant is kept going in hundreds of places, and for tens or even hundreds
of years. A trait that is desirable can be perpetuated indefinitely by
this means, even though it may be a blending of several hereditary
traits, which would separate out in a few generations by ordinary means
of propagation.
There are several more things in heredity that must be taken up while we
are on the subject, so we shall have to return to the chromosomes for a
while. We have seen that there are several determiners to each
chromosome; for convenience, we assigned three apiece to our
chromosomes, except the ninth, which has to get along with two; but in
reality the number to each chromosome is often much greater. This
grouping of the determiners, several to a chromosome, carries an
interesting consequence with it, in that all the hereditary traits
controlled by one chromosome have to go together in reproduction. In
the example we have already used A, B, and C are together; therefore any
individual that shows character A must show B and C as well. The most
striking instances of this are certain traits that are bound up with
sex, but we cannot describe these further until we have looked into the
heredity of sex, which we shall do in a minute. First a word must be
said about occasional exceptions that turn up to the rule that we have
just stated. In the study of thousands of specimens now and then one has
been found in which there has evidently been a swapping about of
determiners. We can illustrate the situation by supposing chromosome one
is found to contain determiners A, b, and C, instead of A, B, and C; one
small-letter determiner has traded places with a large. Of course, the
effect of this is to permit different combinations of hereditary traits
than ordinarily occur, and at the present time students of heredity are
actively engaged in following this up to see how it happens, and what
advantage can be taken of it. This crossing over of determiners from one
chromosome to another takes place only among such as are actually in
contact at times within the nucleus as seen under the microscope, which
confines it to the members of corresponding pairs.
In man, and in many of the lower animals, sex is a hereditary character.
That means that there is a determiner for it which is grouped with other
determiners in one of the chromosomes. In man the determiner is for
femaleness; there is no special determiner for producing the male sex;
it is produced whenever the female determiner is missing from one
chromosome of the pair, and this is brought about by having the whole
chromosome that should make up this pair absent. At the beginning of
this chapter the fact was mentioned incidentally that a good many of us
have only 47 chromosomes, instead of the 48 that are characteristic of
human beings. The distribution is really almost exactly half and half,
for all males have 47 and all females 48. This means that the cells of
the germinal tissue of females have 24 complete pairs, while the
corresponding cells in males have only 23 complete pairs and one
chromosome over. This extra chromosome is the one that contains the
determiner for femaleness; each of the chromosomes of pair 24 in females
contains this determiner also. These are often spoken of as sex
chromosomes.
Now when in the course of the production of egg cells within the
mother’s germinal tissue the pairs of chromosomes are pulled apart, each
separate cell, and so each egg, will contain the full number of
chromosomes, 24, including the sex chromosome. But when the same thing
happens in the course of the production of sperm only every other one
will have the full number; the remaining half having only 23, and all of
this half lacking the chromosome that contains the determiner for
femaleness. There are, then, always equal numbers of two kinds of sperm,
one with 24 and the other with only 23 chromosomes. If the egg is
fertilized by a sperm containing 24, including the sex chromosome, the
pairing of chromosomes is complete in the egg, and the offspring will be
a female; if, on the other hand, the fertilizing sperm is one that
contains only 23 chromosomes the pairing in the egg will be incomplete;
the single sex chromosome of the egg will not be paired with a
corresponding one from the sperm and the egg will develop into a male.
Since it is a pure chance whether fertilization will be accomplished by
a sperm of 24 or one of 23 chromosomes, we should expect the sexes to
appear in exactly equal numbers, taking the world as a whole. As a
matter of fact, whenever extensive birth data have been accumulated they
have shown a very slight excess of male births over female. We are not
able to explain this at the present time. It is possible that the 23
chromosome sperms are a little more vigorous for some reason than those
that have 24, and so are able to fertilize slightly more than their
share of eggs.
We spoke a moment ago of hereditary traits whose determiners are bound
up in the sex chromosomes. All such behave interestingly in heredity for
the simple reason that they can never be transmitted from father to son,
but only from father to daughter. This is because, as we have just seen,
the sex chromosome in the sperm always causes the egg which that sperm
fertilizes to develop into a female. The single-sex chromosome which
males possess invariably comes from the mother. An interesting example
is the common type of color blindness known as Daltonism. Normal color
vision is hereditary and the determiners which establish it are in the
sex chromosomes. Occasionally a person is found in whom these
determiners are defective. If this person is a male, he will be color
blind, but if a female not, unless both sex chromosomes are defective in
this regard, since normal color vision is dominant over color blindness;
so if one sex chromosome is normal the vision will be also. The woman,
in this case, will be a hybrid with respect to color vision; one of her
sex chromosomes containing a normal determiner, the other a defective.
This works out in heredity as follows: If a color-blind man is married
to a woman who has no color blindness in her heredity, none of his
children will be color-blind because he cannot transmit the sex
chromosomes which carry the determiners for color blindness to his sons,
but only to his daughters; all the latter will be hybrid with respect to
the character, since all of them come from fertilized eggs which
received sex chromosomes from the sperm. If these daughters, in turn,
marry men who are free from color blindness, some of their sons may be
color-blind, but none of their daughters can be. The only way in which
women can be color blind through inheritance is by descent from
color-blind fathers and from mothers who are either themselves
color-blind or are hybrid with respect to the trait. The result of this
difference in the heredity of the two sexes is to make color blindness
many times as frequent among men as among women. In round numbers four
men out of every hundred have this type of color blindness, while only
six or seven women in ten thousand show it.
We have left for discussion only one topic dealing with heredity, but
this is the most baffling of all, since it deals with the problem of how
the various kinds of determiners came into existence. It is evident that
if given one parent with all large-letter determiners and the other with
all small-letter, we might, in the course of many generations, get a
great variety of combinations and so a great many different-appearing
individuals. But unless we have various kinds of determiners to start
with, there is no way in which this can be done. We do not pretend to
know very much about how the innumerable determiners that are in
existence came about, but we have one clue that is thought to point the
way. In some animals, and many plants, descendants put in their
appearance from time to time that are so different from their ancestors
as not to be accountable according to ordinary laws of heredity. These
have long been known, and the name of “sport” has been applied to them
by breeders. Since the facts about determiners have been learned, it has
been clear that these “sports” cannot have all their determiners like
those present in their parents, and it has come to be believed that
occasionally spontaneous changes take place within individual
determiners. Since the determiners are undoubtedly complex chemical
structures, we know of no reason why this might not happen. Probably it
is much more common an occurrence in some kinds of plants and animals
than in others. The name of “mutant” has been applied to the plant or
animal in which this has happened, and the process is called “mutation.”
It is important since it is the most likely way in which the innumerable
kinds of determiners that are now in existence came into being.
We suppose that since life first put in its appearance on the earth
there have been uncounted mutations, a vastly greater number than are
now represented by determiners. Many of the mutants could not compete
with their brothers and sisters of ordinary descent and so promptly
died, but occasionally it might happen that a mutant would be as well
fitted for life as its relatives, in which case it would establish
itself, and in course of time become ancestor to a whole line like
itself. If this happened often enough, and time were allowed for it to
work out, all the kinds of plants and animals that are now in existence
might have come by descent from a very few ancestors. The geological
history of the earth shows that there has been plenty of time, even
though valuable mutations did not occur oftener than once in a thousand
years.
Our description of racial perpetuation should be finished by an account
of the development of the fertilized egg. Snugly ensconced within the
body of the mother, in an organ devoted solely to the purpose, the egg
passes rapidly through the early stages, living at first on fats and
proteins stored within itself. After these are exhausted it draws
supplies from the body fluids of the mother. In the course of a few
weeks it has developed its own conveyer system, with its own beating
heart and its own stock of blood. There is never any actual mingling of
the blood of the developing child with that of the mother; capillaries
of the maternal circulation come into intimate contact with capillaries
of the circulation of the child. Here interchange of all sorts of
material goes on; food and oxygen pass from the mother’s blood to the
child’s and waste materials from the child’s blood to the mother’s.
During all this time the mother is eating, breathing, and excreting
wastes for two. She cannot bring any nervous influences to bear on the
developing child, since there is no connection between her nervous
system and the child’s; she can, however, influence it chemically
through the blood. Poisons that get into the blood of the mother can
pass from it into the blood of the child. These may be the poisons of
auto-intoxication, or drugs that the mother has taken. In either case
they may do the child harm. We do not know very much about this, but it
may be that a considerable percentage of children that are born with
abnormalities that are not hereditary come by them through chemical
influences received from the mother’s blood.
When the development of the child has gone far enough so that it can do
its own breathing, feeding, and discharging of waste materials, it is
expelled from the body of the mother in the process that we know as
birth. This does not imply by any means that parental care and
responsibility are at an end. Food, protection, and warmth must be
provided. Education must be attended to, for the nervous system of the
new-born infant is absolutely undeveloped. It has, through heredity,
certain possibilities of achievement; their realization hinges upon the
bringing to bear of worth-while influences. Upon the attainment of
maturity the child will be expected to assume his place in society, and
society has a right to the best that he is able to offer. In preparation
for this it is the duty, both of the parents and of society itself, to
provide throughout the formative years as nearly as possible the
environment best suited to the development of those traits which make
for usefulness. Environment cannot overcome the limitations of heredity,
but environment can bring out the best that is in us.
CHAPTER XXI
CHILD AND MAN
Everyone is familiar with the beguilingly helpless picture the tiny baby
presents. The disproportionately large head, with aimlessly rolling eyes
and toothless mouth, the frail and delicate limbs, waving in the air or
clutching spasmodically at anything within reach, the expressionless
face, on which, for the first few days, only sensations of discomfort
are registered, all mark a creature whose survival hinges absolutely on
unremitting care; a far cry from the competent self-sufficiency of the
average person of mature years. These surface marks of helplessness are
by no means the most significant. Buried from view beneath the soft and
velvety skin are characteristics of even greater meaning to those on
whom falls the responsibility for the rearing of the new life.
At the time of birth the bony skeleton is very incomplete; there is a
spot just above the forehead where the skull bones have not yet grown
completely together, leaving a space where the brain is protected from
injury only by the overlying skin. This spot can be detected easily in
very young infants by its pulsations in time with the heartbeats. In
most of the other bones the deposit of lime salts to which they owe
their stiffness has gone only a little way, so that it would be
impossible for the child to stand or walk even though it knew how. In
this respect the contrast between the human infant and the new-born of
such animals as horses or cattle is very striking, for the latter walk
stumblingly from almost the moment of birth, and efficiently within a
few hours thereafter.
Not only is the infant devoid of teeth, but in various other regards his
digestive apparatus is undeveloped. Not only is he unable to chew solid
food, but he could not digest most of it if it were served already
chewed. During the early months of life the child is emphatically a
one-diet being. His alimentary tract deals successfully with the mixture
of proteins, fats, and sugars of which milk consists, provided the
proportions are substantially those of mother’s milk and the quantity at
a feeding is not too great. He can do this because the enzymes needed
for digesting these particular substances are manufactured by his
digestive glands from the very beginning, and because the muscles of his
stomach and intestines can churn and propel the soft curd into which the
milk is converted as soon as it enters the stomach. The fact that cow’s
milk sets into a tough curd accounts for much of the difficulty some
babies have in thriving upon cow’s milk.
There is no starch in milk, and neither the saliva nor the pancreatic
juice of the infant contains the enzyme by which starch is digested. It
is wholly useless to begin feeding starch-containing foods until this
enzyme begins to be manufactured, which usually takes place when the
child is about eight months old. Even then the introduction of starch
into the diet should be gradual and cautious.
Modern science has discovered no better food for infants than mother’s
milk, and no substitute more satisfactory in general than suitably
modified cow’s or goat’s milk. Under modern conditions of life,
particularly in cities where milk has to be transported over long
distances, and where much time necessarily elapses before the milk can
be placed in the hands of the consumer, there is serious danger of
contamination of the milk with disease germs. In all enlightened
communities this danger is fully recognized, and the entire milk supply,
so far as possible, but that part of it destined for the feeding of
children in particular, is safeguarded by all available means. A fairly
reliable index to the degree of enlightenment of any community is the
quality of the milk which its children receive. One recent discovery of
considerable importance, and incidentally an interesting illustration of
the way in which correct procedure may be hit upon in advance of the
scientific knowledge which justifies it, is the finding that orange
juice contains one of the dietary accessories, known as vitamines,
essential to health, and present in raw milk, but destroyed when milk is
heated. For years, physicians had recommended orange juice for babies.
It is of vital importance when the milk must be pasteurized.
Both the heartbeat and the breathing in the young child are much more
rapid than in the grown person. It is believed that this quickness of
heart action and of breathing rate are related to the smaller size of
the infant as compared with the adult and are of no very marked
significance. At any rate it is true in general that the smaller the
animal the more rapidly does its heart beat and the more quickly does it
breathe. A very noticeable fact about young children is the
susceptibility to all sorts of influences of the mechanism by which the
breathing is controlled. Every passing interest reflects itself in
heightened breathing. Violent emotion often leads to such extreme
overbreathing as to drain the child’s blood of considerable of its store
of carbon dioxide, whereupon the rapid breathing gives place to
prolonged holding of the breath. Many a young mother has been seriously
alarmed by the length of time her offspring in a tantrum is able to
refrain from drawing its breath. Contrary to the appearance of things,
which would indicate that the child is holding its breath out of spite,
and in the hope of getting even with its parent, the cessation of
breathing is largely or wholly automatic, indicating the way in which
nervous and chemical influences have interacted to suspend the
respiration.
The child is born with all its muscles in place, and all fully formed in
that every muscle fiber the child will ever have has been produced
previous to birth. In fact, as soon as the full equipment of muscle
fibers has been laid down the body loses the power to form more, so that
if, through injury, one is so unfortunate as to have some of his muscle
fibers destroyed he will have to get along for the rest of his life with
those that are left. The gaps in the muscle tissue that are produced by
injuries are filled up by a kind of connective tissue known as scar
tissue. The muscle fibers are all present, but smaller and weaker than
they will be later.
The connections between muscles and nerves are also pretty well
established at the time of birth, so that the body and limbs can be
moved freely, even if not at all efficiently.
Not only are the motor nerves formed and in connection with the muscles
at and even before birth, but the sensory nerves and most of the central
nervous system are ready to begin functioning as well. Some reflex
actions, including a few that require quite elaborate muscular and
nervous coordination, are performed very shortly after birth. One famous
example, that has been much cited as tending to reinforce the belief in
man’s descent from tree-dwelling ancestors, is the curling of the
fingers about a slender rod that is pressed against the palms. In most
very tiny babies the grasp thus secured is strong enough so that the
child can be raised and held in the air, supported wholly by its own
grip. Sneezing, which is really a very complex act, requiring accurate
cooperation on the part of many muscles, is done successfully by very
young infants. The reflex of sucking, which is of paramount importance,
in that without it the child would almost inevitably starve, is present
practically from birth. Another important early reflex is that of
crying. It is a curious thought that this reflex, by which bodily
discomfort is made known, and through which relief may be summoned, is
revealed at the very instant of birth, in connection with the drawing of
the first breath, while the contrary reflex of laughter, by which bodily
well-being is expressed, puts in an appearance only after some weeks or
even months.
It is difficult to determine just how far the sense organs have arrived
in their development at the time when the infant begins its independent
existence. That touch and those senses related to bodily discomfort, of
which pain is most important, are operative from the first is shown by
the occurrence of the reflexes described above, which are brought into
action by those particular senses. There is good reason to believe that
the sense of hunger comes into play within two or three days at the
latest. The sense of thirst does not have much chance to reveal itself
early in life, for with a diet exclusively liquid, and feedings
separated by only a few hours, there is no reason why the child should
become thirsty. As the feedings become less frequent, and particularly
as solid food begins to be added, there is real danger that the child
may be insufficiently supplied with water.
Muscle sense and the equilibrium sense, if present at all, must be in a
very imperfect condition at first. They seem to differ from the senses
described thus far in that they depend on practice for their
development. At any rate the bodily movements are largely aimless in the
beginning, and it will be observed that the baby has the appearance of
experimenting with its extremities, placing them repeatedly in
particular positions and seeming to gain precision thereby. The eye
movements, and especially those by which both eyes are focused on a
single object, depend for their accuracy on the working of muscle sense.
In the estimation of the parents a distinct mark of progress is
registered the first time the baby follows a movement with its eyes. As
soon as it does this accurately, and also brings both eyes to bear on
any object, its muscle sense is known to be in efficient operation, so
far, at least, as the eye muscles are concerned. Equilibrium sense first
shows itself when real balancing motions of the body are made.
The senses of taste and smell may be operative to some degree in very
young children, but it is doubtful whether they have either the breadth
or the acuteness that will be shown later in life. Recognition of
disagreeable tastes or smells seems to appear earlier than appreciation
of agreeable. This is in line with the general fact that the
self-protective reactions are developed very early.
Hearing and sight are probably in working order practically from birth.
It is customary to test the sight of the new-born by passing a light
directly in front of the eyes. If sight is present there will be some
appreciable eye movement, suggesting that the eyes are attempting to
follow the moving light. There is no reason to believe that there is any
such thing as definite looking at objects thus early. So long as the
eyes continue to roll aimlessly about, and before they begin to focus
accurately, they are more likely concerned with distinctions between
light and shadow, than with perceptions of form or size. In general we
may say of the senses that those concerned immediately with bodily
discomfort are about as fully developed at birth, or shortly thereafter,
as they ever will be, while those that have to do with the general
adjustments of the body to its environment reach full efficiency more
gradually.
The higher parts of the brain, especially those concerned on the one
hand with the mental life (the cerebrum), and on the other with the
complicated reflex acts involved in locomotion (the cerebellum), are not
ready to begin active functioning when the child is born. Indeed some
parts of the cerebellum do not take on final form for from eight months
to two years afterward. It is thought by some that the question of
whether a child will learn to walk early or late depends, in part at
least, on how soon his cerebellum reaches complete development.
Most parents are fully alive to the importance of abundant warm covering
when their children are to be taken out into the cold, but there is much
less appreciation of the harm that may be done by too much clothing in
extremely hot weather. Of special importance is the avoidance of
exposure to sharp drops in temperature. The adult adjusts himself more
or less automatically to these, whereas the infant does so to only a
limited extent.
Finally, the hold of the infant upon life, that quality that we know as
ruggedness or vitality, is much slighter than it will be after a few
years. Not only is the susceptibility to many kinds of infectious
diseases very much greater, and the power of resisting them very much
less, but the ill effects of poisons, whether taken in with the food or
breathed in with the air, are more pronounced. Thus the vitiated air of
slum dwellings, saturated with the effluvium from unwashed bodies and
unclean clothing, while trying enough for the average grown person, is
deadly for all but the toughest babies. Even in the ordinarily well-kept
home, especially in the winter time when ventilation is apt to be
neglected, the air within the house tends to become unsatisfactory from
the standpoint of the infant’s welfare.
The wise parent, and wise he must be at this time, relaxes his care in
just proportion as the child achieves ability to do things for himself.
Since bodily development is more rapid than mental the close supervision
of food, clothing, and physical occupation is necessary only during the
early years, but the task of building up, through the slow processes of
education, the sort of mind which will be able to do its proper share in
dealing with the difficulties which confront the coming generations is
one to which may well be devoted the best thought and effort not only of
the parents, but of organized society as a whole.
Typographical errors corrected by the etext transcriber:
that their less curvaturos=> that their less curvatures {pg 114}
as fast as it it formed.=> as fast as it is formed. {pg 180}
Russian invesigator showed=> Russian investigator showed {pg 240}
quantities of axygen=> quantities of oxygen {pg 254}
unless artificial air can be procured=> unless artificial aid can be
procured {pg 303}
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