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Title: A Treatise on Physiology and Hygiene - For Educational Institutions and General Readers
Author: Hutchison, Joseph Chrisman
Language: English
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Copyright Status: Not copyrighted in the United States. If you live elsewhere check the laws of your country before downloading this ebook. See comments about copyright issues at end of book.

*** Start of this Doctrine Publishing Corporation Digital Book "A Treatise on Physiology and Hygiene - For Educational Institutions and General Readers" ***

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A TREATISE ON PHYSIOLOGY AND HYGIENE ***



Transcriber's note: A few typographical errors have been corrected: they
are listed at the end of the text.

       *       *       *       *       *


[Illustration: THE VISCERA IN POSITION.]

       *       *       *       *       *


A

TREATISE

ON

PHYSIOLOGY AND HYGIENE

FOR

EDUCATIONAL INSTITUTIONS AND GENERAL READERS.

_FULLY ILLUSTRATED._

BY

JOSEPH C. HUTCHISON, M. D.,

_President of the New York Pathological Society, Vice-President of the New
York
Academy of Medicine, Surgeon to the Brooklyn City Hospital, late
President
of the Medical Society of the State of New York, etc._

       *       *       *       *       *

NEW YORK:

CLARK & MAYNARD, PUBLISHERS,

5 BARCLAY STREET.

1872.

       *       *       *       *       *


Entered according to Act of Congress, in the year 1870,
By CLARK & MAYNARD.
In the Office of the Librarian of Congress, at Washington.

  Stereotyped by LITTLE, RENNIE & CO.
      645 and 647 Broadway.

       *       *       *       *       *


TO MY WIFE,

WHOSE SYMPATHY HAS, FOR MORE THAN TWENTY YEARS, LIGHTENED THE
CARES INCIDENT TO

_AN ACTIVE PROFESSIONAL LIFE_,

THIS HUMBLE VOLUME

IS AFFECTIONATELY INSCRIBED.

       *       *       *       *       *


{3}

PREFACE.

------o------

This work is designed to present the leading facts and principles of human
Physiology and Hygiene in clear and concise language, so that pupils in
schools and colleges, and readers not familiar with the subjects, may
readily comprehend them. Anatomy, or a description of the structure of an
organ, is of course necessary to the understanding of its Physiology, or
its uses. Enough of the former study has, therefore, been introduced, to
enable the pupil to enter intelligently upon the latter.

Familiar language, as far as practicable, has been employed, rather than
that of a technical character. With a view, however, to supply what might
seem to some a deficiency in this regard, a Pronouncing Glossary has been
added, which will enable the inquirer to understand the meaning of many
scientific terms not in common use.

In the preparation of the work the writer has carefully examined all the
best material at his command, and freely used it; the special object being
to have it abreast of the present knowledge on the subjects treated, as far
as such is possible in a work so elementary as this. The discussion of
disputed points has been avoided, it being manifestly inappropriate in a
work of this kind.

Instruction in the rudiments of Physiology in schools does not necessitate
the general practice of dissections, or of experiments upon animals. The
most important subjects may be illustrated by {4} drawings, such as are
contained in this work. Models, especially those constructed by AUZOUX of
Paris, dried preparations of the human body, and the organs of the lower
animals, may also be used with advantage.

The writer desires to acknowledge his indebtedness to R. M. WYCKOFF, M.D.,
for valuable aid in the preparation of the manuscript for the press; and to
R. CRESSON STILES, M.D., a skilful microscopist and physician, for the
chapter "On the Use of the Microscope in the Study of Physiology." Mr. AVON
C. BURNHAM, the well-known teacher of gymnastics, furnished the drawing of
the parlor gymnasium and the directions for its use.

_Brooklyn, N. Y., 1870._

       *       *       *       *       *


{5}

CONTENTS.

  CHAPTER I.

                                                                       PAGE

  THE FRAMEWORK OF THE BODY                                              15

  _The Bones--Their form and composition--The Properties of Bone--The
  Skeleton--The Joints--The Spinal Column--The Growth of Bone--The
  Repair of Bone._

  CHAPTER II.

  THE MUSCLES                                                            25

  _The Muscles--Flexion and Extension--The Tendons--Contraction--Physical
  Strength--Necessity for Exercise--Its Effects--Forms of
  Exercise--Walking--Riding--Gymnastics--Open-air Exercise--Sleep--
  Recreation._

  CHAPTER III.

  THE INTEGUMENT, OR SKIN                                                41

  _The Integument--Its Structure--The Nails and Hair--The Complexion--The
  Sebaceous Glands--The Perspiratory Glands--Perspiration
  and its uses--Importance of Bathing--Different kinds of Baths--Manner
  of Bathing--The Benefits of the Sun--Importance of
  Warm Clothing--Poisonous Cosmetics._

  CHAPTER IV.

  THE CHEMISTRY OF FOOD                                                  53

  _The Source of Food--Inorganic Substances--Water--Salt--Lime--Iron--
  Organic Substances--Albumen, Fibrin, and Casein--The Fats or
  Oils--The Sugars, Starch, and Gum--Stimulating Substances--Necessity
  of a Regulated Diet._

  {6}
  CHAPTER V.

  FOOD AND DRINK                                                         64

  _Necessity for Food--Waste and Repair--Hunger and Thirst--Amount
  of Food--Renovation of the Body--Mixed Diet--Milk--Eggs--Meat--Cooking
  --Vegetable Food--Bread--The Potato--Fruits--Purity
  of Water--Action of Water upon Lead--Coffee, Tea, and Chocolate--Effects
  of Alcohol._

  CHAPTER VI.

  DIGESTION                                                              80

  _The Principal Processes of Nutrition--The General Plan of Digestion--
  Mastication--The Teeth--Preservation of the Teeth--Insalivation--The
  Stomach and the Gastric Juice--The Movements of the
  Stomach--Gastric Digestion--The Intestines--The Bile and Pancreatic
  Juice--Intestinal Digestion--Absorption by means of Blood-vessels
  and Lacteals--The Lymphatic or Absorbent System--The
  Lymph--Conditions which affect Digestion--The Quality, Quantity,
  and Temperature of the Food--The Influence of Exercise and
  Sleep._

  CHAPTER VII.

  THE CIRCULATION                                                       101

  _The Blood--Its Plasma and Corpuscles--Coagulation of the Blood--The
  Uses of the Blood--Transfusion--Change of Color--The Organs of
  the Circulation--The Heart, Arteries, and Veins--The Cavities
  and Valves of the Heart--Its Vital Energy--Passage of the Blood
  through the Heart--The Frequency and Activity of its Movements--The
  Pulse--The Sphygmograph--The Capillary Blood-vessels--The
  Rate of the Circulation--Assimilation--Injuries to the Blood-vessels._

  CHAPTER VIII.

  RESPIRATION                                                           123

  _The Objects of Respiration--The Lungs--The Air-Passages--The Movements
  of Respiration--Expiration and Inspiration--The Frequency
  of Respiration--Capacity of the Lungs--The Air we Breathe--Changes
  in the Air from Respiration--Changes in the Blood--Interchange
  of Gases in the Lungs--Comparison between Arterial and
  {7}
  Venous Blood--Respiratory Labor--Impurities of the Air--Dust--Carbonic
  Acid--Effects of Impure Air--Nature's Provision for
  Purifying the Air--Ventilation--Animal Heat_.

  CHAPTER IX.

  THE NERVOUS SYSTEM                                                    148

  _Animal and Vegetative Functions--Sensation, Motion, and Volition--The
  Structure of the Nervous System--The White and Gray Substances--The
  Brain--Its Convolutions--Cerebellum--The Spinal
  Cord and its System of Nerves--The Anterior and Posterior
  Roots--The Sympathetic System of Nerves--The Properties of Nervous
  Tissue--Excitability of Nervous Tissues--The Functions of the
  Spinal Nerves and Cord--The Direction of the Fibres of the Cord--Reflex
  Activity and its Uses--The Functions of the Medulla Oblongata
  and the Cranial Ganglia--The Reflex Action of the Brain._

  CHAPTER X.

  THE SPECIAL SENSES                                                    177

  _The Production of Sensations--Variety of Sensations--General
      Sensibility--Pain
  and its Function--Special Sensation, Touch, Taste,
  Smell, Sight, and Hearing--The Hand, the Organ of Touch--The
  Sense of Touch--Delicacy of Touch--Sensation of Temperature and
  Weight--The Tongue the Organ of Taste--The Nerves of Taste--The
  Sense of Taste, and its Relations with the other Senses--The
  Influence of Education on the Taste--The Nasal Cavities, or the
  Organs of Smell--The Olfactory Nerve--The Uses of the Sense of
  Smell--The Sense of Sight--Light--The Optic Nerve--The
  Eyeball and its Coverings--The Function of the Iris--The
  Sclerotic, Choroid, and Retina--The Tears and their Function--The
  Movements of the Eyeball--The Function of Accommodation--The
  Sense of Hearing and Sound--The Ear, or
  the organ of Hearing--The External, Middle, and Internal Ear._

  CHAPTER XI.

  THE VOICE                                                             227

  _Voice and Speech--The Larynx, or the Organ of the Voice--The Vocal
  Cords--The Laryngoscope--The Production of the Voice--The Use
  of the Tongue--The different Varieties of Voice--The Change of
  Voice--Its Compass--Purity of Tone--Ventriloquy._

  {8}
  CHAPTER XII.

  THE USE OF THE MICROSCOPE IN THE STUDY OF
  PHYSIOLOGY                                                            236

  _The Law of Tissues--Necessity of the Microscope--Different kinds of
  Microscopes--Additional Apparatus--Preliminary Studies--The
  Study of Human Tissues--Tissues of the Inferior Animals--Incentives
  to Study._

  APPENDIX.

  POISONS AND THEIR ANTIDOTES                                           247

  GLOSSARY                                                              252

{9}

LIST OF ILLUSTRATIONS

  FIG.                                               PAGE
  FRONTISPIECE,       }
  VISCERA IN POSITION,}
  1. Section of bone,                                  17
  2. Structure of bone, magnified,                     17
  3. The skeleton,                                     18
  4. Cells of cartilage,                               20
  5. Elbow-joint,                                      21
  6. Spinal column,                                    22
  7. The muscles,                                      24
  8. Muscular tissue, magnified,                       25
  9. Biceps muscle of the arm,                         26
  10. View of knee-joint,                              27
  11. Appliance for strengthening the muscles,         35
  12. Appliance for strengthening the muscles,         35
  13. Parlor gymnasium,                                36
  14. Root and transverse section of hair, magnified,  43
  15. Granules of potato starch,                       61
  16. Section of the trunk,                            81
  17. Section of a tooth,                              82
  18. Section of the jaws,                             82
  19. Section of the jaws--right side,                 84
  20. Structure of a salivary gland,                   87
  21. Head of a horse, showing salivary gland, etc.    87
  22. Section of chest and abdomen,                    90
  23. Organs of digestion,                             91
  24. The lacteals,                                    97
  25. Blood corpuscles,                               102
  26. Blood corpuscles of man and lower animals,      103
  27. Circulation of the blood,                       108
  {10}
  28. Heart and large vessels,                        109
  29. Section of the heart,                           110
  30. Form of the pulse,                              116
  31. Valves of the veins,                            117
  32. Web of frog's foot, magnified,                  119
  33. Circulation in a frog's foot,                   119
  34. Organs of the chest,                            124
  35. Larynx, trachea, and bronchial tubes,           125
  36. Diagram of the structure of the air-cells,      125
  37. Section of the lungs,                           126
  38. Section of mouth and throat,                    127
  39. Ciliated cells,                                 128
  40. Cerebro-spinal system,                          151
  41. Upper surface of the cerebrum,                  153
  42. Vertical section of the brain,                  154
  43. Base of the brain,                              155
  44. Brain and spinal cord,                          156
  45. Sense of touch,                                 185
  46. Section of nasal cavity,                        193
  47. Front view of the eye,                          200
  48. Vertical section of eye,                        202
  49. Diagram for blind point of eye,                 207
  50. Retinal image,                                  210
  51. Different shapes of the globe of the eye,       212
  52. Function of accommodation,                      214
  53. Diagram of the ear,                             218
  54. Section of the ear,                             221
  55. Section of larynx and trachea,                  229
  56. View of vocal cords by the laryngoscope,        232
  57. Different positions of vocal cords,             232
  58. Simple microscope,                              238
  59. Compound microscope,                            239
  60. Household microscope,                           240
  61. Popular microscope,                             241

{11}

INTRODUCTION.

------o------

The Human Body is the abode of an immortal spirit, and is the most complete
and perfect specimen of the Creator's handiwork. To examine its structure,
to ascertain the uses and modes of action of its various parts, how to
protect it from injury, and maintain it in a healthy condition, is the
design of this work.

The departments of knowledge which are concerned in these investigations,
are the science of Human Physiology and the art of Hygiene.

PHYSIOLOGY treats of the vital actions and uses of the various parts of
living bodies, whether vegetable or animal. Every living thing, therefore,
has a Physiology. We have a _Vegetable_ Physiology, which relates to
plants; and an _Animal_ Physiology, relating to the animal kingdom. The
latter is also divided into _Comparative_ Physiology, which treats of the
inferior races of animals, and _Human_ Physiology, which teaches the uses
of the various parts of the human body.

HYGIENE, or the art of preserving health, is the practical use of
Physiology. It teaches us how to cultivate our bodily and mental powers, so
as to increase our strength and to fit us for a higher enjoyment of life.
It also shows us how to prevent some of the accidents which may befall the
body, and to avoid disease. It is proper that we should {12} understand the
construction and powers of our bodies; but it is our duty, as rational
beings, to know the laws by which health and strength may be maintained and
disease warded off.

There are various means by which we gain important information respecting
the Physiology of man. Plants aid us in understanding the minute structure
of the human body, its circulation, and absorption. From inferior animals
we learn much in respect to the workings of the different _organs_, as we
call those parts of the system which have a particular duty to perform. In
one of them, as in the foot of the frog, we can study the circulation of
the blood; in another, we can study the action of the brain.

By _vivisection_, or the laying bare of some organ of a living animal, we
are able to investigate certain vital processes which are too deeply hidden
in the human body to be studied directly. This is not necessarily a cruel
procedure, as we can, by the use of anæsthetics, so blunt the sensibility
of the animal under operation, that he need not suffer while the experiment
is being performed. There are other means by which we gather our
information. There are occasionally men, who, from some accident, present
certain parts, naturally out of view, in exposed positions. In these cases,
our knowledge is of much greater value than when obtained from creatures
lower in the scale of being than man.

We are greatly aided, also, by the use of various instruments of modern
invention. Chief among these is the microscope, which is, as we shall learn
hereafter, an arrangement and combination of lenses in such a way as
greatly to magnify the objects we wish to examine. {13}

We have much to say of Life, or vital activity, in the course of our study
of Physiology; but the most that we know of it is seen in its results. What
Life is, or where its precise position is, we are not able to determine. We
discover one thing, however, that all the parts of the body are united
together with wonderful sympathy, so that one part cannot be injured and
other parts not suffer damage. It is further evident that all organs are
not equally important in carrying on the work of Life; for some may
temporarily suspend their action, without serious results to the system,
while others must never cease from acting. Yet there is nothing superfluous
or without aim in our frames, and no part or organ can suffer harm without
actual loss to the general bodily health. On this point Science and Holy
Writ strictly agree.

       *       *       *       *       *


{14}

{15}

PHYSIOLOGY,

AND

HYGIENE.

CHAPTER I.

THE FRAMEWORK OF THE BODY.

_The Bones--Their Form and Composition--The Properties of Bone--The
Skeleton--The Joints--The Spinal Column--The Growth of Bone--The Repair of
Bone._

[Sidenote: 1. The framework of the body? The superstructure? Softness and
delicacy of the organs? How protected?]

1. THE BONES.--The framework which sustains the human body is composed of
the _Bones_. The superstructure consists of the various organs on which the
processes of life depend. These organs are soft and delicately formed, and,
if unprotected, would, in most cases, rapidly be destroyed when subjected
to violence, however slight. The bones, having great strength and power of
resistance, afford the protection required.

[Sidenote: 2. The more delicate the organ? Example in relation to the
brain? The eye? The lungs? The services performed by the bones?]

2. The more delicate the organ, the more completely does Nature shield it.
For example: the brain, which is soft in structure, is enclosed on all
sides by a complete box of bone; the eye, though it must be near the
surface of the body to command an extensive view, is sheltered from injury
within a deep recess of bone; the lungs, requiring freedom of motion as
well as protection, are surrounded by a large case of bone and muscle. The
bones serve other useful purposes. They give permanence of form to the
body, by {16} holding the softer parts in their proper places. They assist
in movement, by affording points of attachment to those organs which have
power of motion--the muscles.

[Sidenote: 3. Their shape and size? Of what composed? Possibility of being
separated? Effect of fire? Of dilute acid?]

3. THE FORM AND COMPOSITION OF THE BONES.--Their shape and size vary
greatly in different parts of the body, but generally they are arranged in
pairs, one bone for each side of the body. They are composed of both
mineral and animal substances, united in the proportion of two parts of the
former to one of the latter; and we may separate each of these substances
from the other for examination. First, if we expose a bone to the action of
fire, the animal substance is driven off, or "burned out." We now find
that, though the shape of the bone is perfectly retained, what is left is
no longer tough, and does not sustain weight as before. Again, we may
remove the mineral portion, which is a form of lime, by placing a bone into
a dilute acid. The lime will be dissolved out, and the shape of the bone
remain as before; but now its firmness has disappeared, and it may be bent
without breaking.

[Sidenote: 4. Effect of deficiency of ingredient? Usefulness of the lime?
Of the animal substance? Effect of their union? Condition, in youth? Old
age?]

4. If, for any reason, either of these ingredients is disproportionate in
the bone during life, the body is in danger. The lime is useful in giving
rigidity of form, while the animal substance insures toughness and
elasticity. By their union, we are able to withstand greater shocks and
heavier falls than would be possible with either alone. In youth, the
period of greatest activity, the animal constituent is in excess: a bone
then does not break so readily, but, when broken, unites with great
rapidity and strength. On the other hand, the bones of old persons are more
easily broken, and in some cases fail to unite. The mineral matter being
then in excess, indicates that the period of active exertion is drawing to
a close.

{17}

[Illustration: FIG. 1.--SECTION OF BONE.]

[Illustration: FIG. 2.

Structure of bone enlarged.]

[Sidenote: 5. In what respect admirably fashioned? Its formation?
Microscopic examination? The inference? "Line of beauty?"]

5. THE STRUCTURE OF THE BONES.--If we examine one of the long bones, which
has been sawn through lengthwise, we observe that it is admirably fashioned
for affording lightness as well as strength (Fig. 1). Its exterior is hard
and resisting, but it is porous at the broad extremities, while through the
central portion there is a cavity or canal which contains an oily
substance, called _marrow_. Let us now take a thin section of bone, and
examine it under the microscope; we discover that it is pierced by numerous
fine tubes (Fig. 2), about which layers of bone-substance are arranged.
Accordingly, though a bone be as hard as stone externally, it is by no
means as heavy as stone, by reason of its light interior texture. Another
element of power is found in the curved outline of the bones. The curved
line is said to be "the line of beauty," as it certainly is the line of
strength, and is uniformly employed in the bones whose position exposes
them to accident.

{18}

[Illustration: FIG. 3.--THE SKELETON]

{19}

[Sidenote: 6. Number of bones? Skeleton? The skull? Chest? The trunk? The
trunk and skull, how maintained? What of the arms? Legs?]

6. THE SKELETON.--The number of bones in the human body exceeds two
hundred. When these are joined together in the proper places, they form
what is termed the _Skeleton_ (Fig. 3). It embraces three important
cavities. The first, surmounting the frame, is a box of bone, called the
_skull;_ below this, is a bony case, or "chest;" and lower down is a bony
basin, called the _pelvis_. The two latter compose the trunk. The trunk and
skull are maintained in their proper relations by the "spinal column."
Branching from the trunk are two sets of limbs: the arms, which are
attached to the chest by means of the "collar-bone" and "shoulder-blade;"
and the legs, directly joined to the lower part of the trunk.

[Sidenote: 7. Design of the cavities? Give the examples.]

7. The cavities of which we have spoken, are designed for the lodgment and
protection of the more delicate and perishable parts of the system. Thus,
the skull, together with the bones of the face, shelters the brain and the
organs of four senses--sight, hearing, smell, and taste. The chest contains
the heart, lungs, and great blood-vessels, while the lower part of the
trunk sustains the liver, stomach, and other organs.

[Sidenote: 8. Joint or articulation? Movable joints, how compacted? The
ligaments of the movable joints? What is a sprain? Consequence of a serious
sprain?]

8. THE JOINTS.--The point of union of two or more bones forms a joint or
_articulation_, the connection being made in various ways according to the
kind and amount of motion desired. The movable joints are compacted
together by certain strong fibrous bands, called ligaments. These ligaments
are of a shining, silvery whiteness, and very unyielding; so much so, that
when sudden violence is brought to bear in the vicinity of a joint, the
bone to which a ligament is attached may be broken, while the ligament
itself remains uninjured. When this connecting material of the joints is
strained or lacerated by an {20} accident, a "sprain" is the consequence.
An injury of this sort may be, and frequently is, quite as serious as the
breaking of a bone.

[Illustration: FIG. 4.--CELLS OF CARTILAGE.]

[Sidenote: 9. Office of the ligament? What must it have? How accomplished?
Describe it. Synovia?]

9. The ligament, then, secures firmness to the joint; it must also have
flexibility and smoothness of motion. This is accomplished by a beautiful
mechanism the perfection of which is only feebly imitated by the most
ingenious contrivance of man. The ends of the bones are covered by a thin
layer of _cartilage_, which being smooth and elastic, renders all the
movements of the joint very easy. In addition to this, there is an
arrangement introduced for "lubricating" the joint, by means of a delicate
sac containing fluid. This fluid is constantly supplied in small
quantities, but only so fast as it is used up in exercise. In appearance,
it is not unlike the white of an egg, and hence its name _synovia_, or
egg-like.

[Sidenote: 10. What do we observe as regards the composition of a joint?
The ligament and cartilage? What varies? Example of the skull? Other
examples? The ball-and-socket joint?]

10. Thus, we observe, that two very different substances enter into the
composition of a joint. The ligament, very unyielding, affords strength,
while the cartilage, elastic and moist, gives ease and smoothness of
motion. The amount of motion provided for varies greatly in different
joints. In some there is none at all, as in the skull, where one bone is
dove-tailed into another by what are termed _sutures_. Others have a
hinge-like motion, such as those of the elbow, wrist, ankle, and knee; the
most complete of these being the elbow-joint (Fig. 5). Belonging to another
class, the {21} ball-and-socket joint, is that at the shoulder, possessing
a freedom of motion greater than any other in the body.

[Illustration: FIG. 5.--ELBOW JOINT. A, Bone of the arm; B, C, Bones of the
fore-arm.]

[Sidenote: 11. What is the spinal column? What does it connect and form?
Joints of the vertebræ? Amount of motion? Result?]

11. THE SPINAL COLUMN.--The spinal column is often spoken of as the
"back-bone," as if it were a single bone, while, in reality, it is composed
of a chain of twenty-six small bones, called _vertebræ_. The spinal column
is a wonderful piece of mechanism. It not only connects the important
cavities of the body, as has already been shown, but, also, itself forms a
canal, which contains the spinal cord. The joints of the vertebræ are
remarkable for the thick layers of cartilage which separate the adjacent
surfaces of bone. The amount of motion between any two of these bones is
not great; but these little movements, taken together, admit of very
considerable flexibility, in several directions, without endangering the
supporting power of the column.

{22}

[Sidenote: 12. Elasticity of the frame? Protection of the brain from
shocks? Tallness of persons? Effects of reclining?]

[Illustration: FIG. 6--THE SPINAL COLUMN.]

12. The abundant supply of intervertebral cartilage has another important
use, namely, it adds greatly to the elasticity of the frame. It is due, in
part, to this elastic material, and in part to the frequent curves of the
spine, that the brain and other delicate organs are protected from the
shock of sudden falls or jars. During the day, the constant pressure upon
these joints, while the body is erect, diminishes the thickness of the
cartilages; so that a person is not so tall in the evening as in the
morning. The effects of this compression pass away when the body reclines
in a horizontal position.

[Sidenote: 13. Change in bone? Example--animal and madder. Rapidity of
change in color? Waste and repair?]

13. THE GROWTH OF BONE.--Bone, like all the other tissues of the body, is
constantly undergoing change, old material being withdrawn, and new
particles taking their place. This has been shown conclusively by
experiments. If an animal be fed with madder--a red coloring matter--for a
day or two, the bones soon become tinged; then, if the madder be
discontinued for a few days, the original color returns. If, however, this
material be alternately given and withheld, at short intervals, the bone
will be marked by a succession of red and white rings. In very young
animals, all the bones become colored in a single day; in older ones, a
longer time is required. The process of waste and repair, therefore, is
constantly taking place in this hard substance, and with astonishing
rapidity. {23}

14. THE REPAIR OF BONE.--Nature's provision for uniting broken bones is
very complete. At first, blood is poured out around the ends of the bone,
as a result of the injury. This is gradually absorbed, and gives place to a
watery fluid, which, thickening from day to day, acquires, at the end of
two weeks, the consistency of jelly. This begins to harden, by a deposit of
new bone-substance, until, at the expiration of five or six weeks, the
broken bone may be said to be united. It is, however, still fragile, and
must be used carefully a few weeks longer. The process of hardening
continues, and months must pass before the union can be said to be
complete.

QUESTIONS FOR TOPICAL REVIEW.

                                                                    PAGE
  1. What useful purposes do the bones serve?                     15, 16
  2. State what you can of the composition of the bones.              16
  3. Of the usefulness of lime in the bones.                          16
  4. Of the usefulness of animal substance in the bones.              16
  5. State what you can of the structure of the bones.                17
  6. Of the strength belonging to the bones.                  15, 16, 17
  7. What is meant by the human skeleton?                             19
  8. Give a description of its construction.                          19
  9. What is meant by a joint in the human frame?                     19
  10. State what you can of the movable joints.                   19, 20
  11. What office is performed by the ligaments of the joints?    19, 20
  12. What by the cartilage at the joints?                            20
  13. What movable joints are there?                              20, 21
  14. Describe the construction of the spinal column.                 21
  15. What properties and powers does the spinal column possess?  21, 22
  16. When is a person taller than at other times?                    22
  17. Give the reason for this.                                       22
  18. What can you state of the growth of bone?                       22
  19. Describe the process by which a broken bone is repaired.        23

       *       *       *       *       * {24}


[Illustration: FIG. 7.--THE MUSCLES.]

{25}

CHAPTER II.

THE MUSCLES.

_The Muscles--Flexion and Extension--The Tendons--Contraction--Physical
Strength--Necessity for Exercise--Its Effects--Forms of
Exercise--Walking--Riding--Gymnastics--Open-air
Exercise--Sleep--Recreation._

[Sidenote: 1. What are the muscles? Their number? The design of most of
them? Of a few?]

1. THE MUSCLES.-- The great mass of the body external to the skeleton, is
composed of the flesh, or _Muscles_, which largely determines its outline
and weight. The muscles are the organs of motion. Their number is about
four hundred, and to each of them is assigned a separate and distinct
office. They have all been studied, one by one, and a name given to each,
by the anatomist. Each is attached to bones which it is designed to move. A
few are circular in form, and enclose cavities, the size of which they
diminish by contraction.

[Illustration: FIG. 8.--MUSCULAR TISSUE.

_a_, _b_, Striped muscular fibres: _c_, The same more highly magnified.]

[Sidenote: 2. The structure of flesh? Its color, etc.? The composition of
the fibres? How marked?]

2. If we examine a piece of flesh, we observe that it is soft, and of a
deep red color. Its structure appears to be composed of layers and bundles
of small fibres. Let us further examine these fibres under the microscope.
We now discover that they are, in turn, made up of still finer fibres, of
_fibrillæ_: these are seen in Fig. 8. The fibres are beautifully {26}
marked by parallel wavy lines, about ten thousand to an inch, which give
the fibre its name of the _striped_ muscular fibre. All of the voluntary
muscles present this appearance.

[Sidenote: 3. Arrangement of the muscles? Their action? Flexion and
extension? Action of the muscles when we stand erect?]

3. FLEXION AND EXTENSION.--The muscles are, for the most part, so arranged
in pairs, or corresponding sets, that when motion is produced in one
direction by one set, there is, opposite to it, another muscle, or group of
muscles, which brings the limb back to its place. When they act
alternately, a to-and-fro movement results. When a joint is bent, the
motion is called _flexion_; and when it is made straight again, it is
called _extension_. When both sets act equally, and at the same moment, no
motion is produced, but the body or limb is maintained in a fixed position:
this occurs when we stand erect. The muscles which produce extension are
more powerful than those opposite to them.

[Illustration: FIG. 9.--A, Biceps muscle of the arm: B, C, Its tendons.]

[Sidenote: 4. Kinds of muscles? The voluntary? Involuntary? The heart? Give
the example. The hand? Arm?]

4. The muscles are also distinguished, on the other hand, as the voluntary
and involuntary muscles, according as they are, or are not, under the
control of the will. The heart is an example of the involuntary variety. We
cannot change its action in the least by an effort of the will. When we
sleep, and the will ceases to act, the heart continues to beat without
cessation. The voluntary muscles, on the other hand, are such as are used
only when we wish or _will_ to use them--as the muscles of the hand or arm
(Fig. 9).

{27}

[Sidenote: 5. What are the tendons or sinews? Their strength? Color?
Location? Tendon of Achilles? The fable? Muscles of the leg?]

5. THE TENDONS.--Tendons, or sinews, are the extremities of muscles, and
are compactly fastened upon bone. They are very strong, and of a silvery
whiteness. They may be felt just beneath the skin, in certain parts of the
body, when the muscles are being used, as at the bend of the elbow or knee.
The largest tendon of the body is that which is inserted into the heel,
called the tendon of Achilles, after the hero of the Grecian poet, the
fable relating that it was at this point that he received his death-wound,
no other part of his body being vulnerable. The muscles which extend into
the leg unite to form a single and very powerful tendon, and enclose a
small bone called the knee-pan, which, acting like a pulley, greatly
increases their power, and at the same time protects the front of the
knee-joint (Fig. 10).

[Illustration: FIG. 10.--VIEW OF KNEE-JOINT. A, Thigh bone: B, Knee-pan: C,
D, Leg bones.]

[Sidenote: 6. Contraction of the muscles? Bending of the arm or finger?
Other agencies? Automatic movements? In cold-blooded animals?]

6. MUSCULAR CONTRACTION.--The muscles, when acted upon by the appropriate
stimulus, contract, or so change {28} their shape, that their extremities
are brought nearer together. The bending of the arm, or of a finger, is
effected in this manner, by the will; but the will is not the only means of
producing this effect. Electricity, a sharp blow over a muscle, and other
stimuli, also cause it. Contraction does not always cease with life. In
man, after death from cholera, automatic movements of hands and feet have
been observed, lasting not less than an hour. In certain cold-blooded
animals, as the turtle, contraction has been known to take place for
several days after the head has been cut off.

[Sidenote: 7. Contractility? Give the illustration. What was supposed? What
is the case?]

7. The property which, in muscle, enables these movements to take place is
called _contractility_. If we grasp a muscle while in exercise (for
example, the large muscle in the front of the arm), we notice the alternate
swelling and decrease of the muscle, as we move the forearm to and fro. It
was at one time supposed that the muscle actually increased in volume
during contraction. This, however, is not the case; for the muscle, while
gaining in thickness, loses in length in the same proportion; and thus, the
volume remains the same in action and at rest.

[Sidenote: 8. What further in relation to contraction? Weariness of a
muscle? Beating of the heart? Standing and walking?]

8. Contraction is not the permanent, or normal, state of a muscle. It
cannot long remain contracted, but after a shorter or longer time, it
wearies and is obliged to relax. After a short rest, it can then again
contract. It is for this reason that the heart can beat all through life,
night and day, by having, as we shall hereafter see, a brief interval of
rest between successive pulsations. For the same reason, it is more
fatiguing to stand for any great length of time in one position, than to be
walking the same period.

[Sidenote: 9. Muscular power of animals? How tested? Man's power? Horse's?
The comparison?]

9. RELATIVE STRENGTH OF ANIMALS.--The amount of muscular power which
different animals can exert, has {29} been tested by experiment. By
determining the number of pounds which an animal can drag upon a level
surface, and afterward comparing that with its own weight, we can judge of
its muscular force. It is found that man is able to drag a little less than
his own weight. A draught-horse can exert a force equal to about two-thirds
of his weight. The horse, therefore, though vastly heavier than man, is
relatively not so powerful.

[Sidenote: 10. Power of insects? Beetles? Give the conclusion.]

10. Insects are remarkable for their power of carrying objects larger and
heavier than themselves. Many of them can drag ten, and even twenty times
their weight. Some of the beetles have been known to move bodies more than
forty times their own weight. So far, therefore, from it being a fact that
animals have strength in proportion to their weight and bulk, the reverse
of that statement seems to be the law.

[Sidenote: 11. Difference in strength of individuals? How caused?]

11. PHYSICAL STRENGTH.--The difference in strength, as seen in different
individuals, is not due to any original difference in their muscles. Nature
gives essentially the same kind and amount of muscles to each person, and
the power of one, or the weakness of another, arises, in great part, from
the manner in which these organs are used or disused.

[Sidenote: 12. Complaint in relation to degeneracy? How true? How
determined by armor? The fair supposition?]

12. Many authors complain of the physical degeneracy of men at the present
day, as compared with past generations. There is room for doubt as to the
correctness of this statement. Certain experiments have recently been made
with the metallic armor worn seven hundred years ago, by which it is found
that any man, of ordinary height and muscular development, can carry the
armor and wield the weapons of an age supposed to be greatly our superior
in strength. When we consider that in those days, only very strong men
could endure the hardships of soldier-life, {30} it is fair to suppose that
our age has not so greatly degenerated in respect to physical strength.

[Sidenote: 13 Action? Use of organs? Training of the mind? The child's
brain? Education of the body?]

13. IMPORTANCE OF EXERCISE.--Action is the law of the living body. Every
organ demands use to preserve it in full vigor, and to obtain from it its
best services. The value of that training of the mind, which we call
education, is everywhere recognized. The child is early put to school, and
for many years continues to study, in order that his brain, which is the
great centre of mental power, may act healthfully and with force. It is
important that the body, also, should receive its education by exercise.
This is especially true of persons belonging to certain classes of society,
whose occupation confines them within doors, and requires chiefly
brain-work.

[Sidenote: 14 Work in the open air? A perfect business? The consequence of
universal perfect business? Occupation of children?]

14. Persons who are engaged in manual labor in the open air obtain all the
exercise necessary for bodily health in their regular business: their need
is more likely to be a discipline or exercise of the mind. A perfect
business of life, therefore, would be one which would combine both physical
and mental labor in their proper proportions. If such a business were
possible for all the human race, life would thereby be vastly prolonged.
Such is, in fact, to a large extent, the occupation pertaining to one
period of life--childhood. A part of the time is spent by the child in
improving his mind by study, and another part of the time he has physical
exercise in his games and sports.

[Sidenote: 15 In what does exercise consist? Effects of it?]

15. THE EFFECTS OF EXERCISE.--Exercise consists in a well-regulated use of
the voluntary muscular system. The effects, however, are not limited to the
parts used. Other organs, which are not under the control of the will, are
indirectly influenced by it. For instance, the heart beats more rapidly,
the skin acts more freely, and {31} becomes hotter, as well as the parts
beneath it, and the appetite and power of digestion are increased. An
increased exhalation from the lungs and skin purifies the current of the
circulation, and the body as a whole thrives under its influence.

[Sidenote: 16. General effect upon the muscles? Special effect? Effects of
inaction? Of excessive exercise?]

16. The immediate effect of exercise, however, is upon the muscles
themselves; for by use they become firm and large, and increase in power.
If we examine a muscle thus improved by exercise, we find that its fibres
have become larger and more closely blended together, that its color is of
a darker red, and that the supply of blood-vessels has increased. Without
exercise the muscle appears thin, flabby, and pale. On the other hand,
excessive exercise, without sufficient relaxation, produces in the muscle a
condition not very different from that which follows disuse. The muscle is
worn out faster than nature builds it up, and it becomes flabby, pale, and
weak.

[Sidenote: 17. Of violent and spasmodic efforts? Strength, how attained?
Give the particulars.]

17. Violent exercise is not beneficial; and spasmodic efforts to increase
the muscular strength are not calculated to secure such a result. Strength
is the result of a gradual growth, and is most surely acquired if the
exercise be carried to a point short of fatigue, and after an adequate
interval of rest. To gain the most beneficial results, the exercise should
be at regular hours, and during a regular period. The activity of the
exercise, and the time devoted to it must vary, of course, with the
strength of the individual, and should be carefully measured by it.

[Sidenote: 18. What may walking be called? What further is said of
walking?]

18. DIFFERENT MODES OF EXERCISE.--There are very few who have not the power
to walk. There is required for it no expensive apparatus, nor does it
demand a period of preliminary training. _Walking may be called the
universal exercise._ With certain foreign nations, the English {32}
especially, it is a very popular exercise, and is practised habitually by
almost every class of society; by the wealthy as well as by those who have
no carriages; by women as well as by men.

[Sidenote: 19. What is said of running, and other like movements? What, as
related to childhood? What instances are alluded to? Example?]

19. Running, leaping, and certain other more rapid and violent movements,
are the forms of exercise that are most enjoyed in childhood. For the
child, they are not too severe, but they may be so prolonged as to become
injurious. Instances have been recorded where sudden death has resulted
after violent playing, from overtaxing the heart: for example, we have the
case of a young girl who, while skipping the rope, and endeavoring to excel
her playmates by jumping the greatest number of times, fell dead from
rupture of the heart.

[Sidenote: 20. Carriage-riding? Horseback-riding?]

20. Carriage-riding, as a means of passive exercise, is particularly well
suited to invalids, and persons advanced in life. Horseback exercise brings
into use a greater number of muscles than any other one exercise, and with
it there is an exhilaration of feeling which refreshes the mind at the same
time. It is one of the manliest of exercises, but not less suitable for
women than for men. To be skilful in riding, it is best to begin its
practice in youth; but there are very few kinds of exercise of which the
same is not equally true.

[Sidenote: 21. Boating, swimming, and skating?]

21. For those who live near streams or bodies of water, there are the
delightful recreations of boating, swimming, and skating. Certain of these
exercises have a practical importance aside from and above their use in
increasing the physical vigor. This is especially true of boating and
swimming, since they are often the means of saving life. Practice in these
exercises also teaches self-reliance, courage, and presence of mind.
Persons who have become proficient in these vigorous exercises are
generally the ones, {33} who, in times of danger, are the quickest to act
and the most certain to do so with judgment.

[Sidenote: 22. What kind of exercise yields the best results? What advice
is given?]

22. PHYSICAL CULTURE.--That form of exercise which interests and excites
the mind, will yield the best results; but to some persons no kind of
exertion whatever is, at first, agreeable. They should, nevertheless, make
a trial of some exercise, in the expectation that, as they become
proficient in it, it will become more pleasant. In exercise, as many sets
of muscles should be employed as possible, open-air exercise being the
best. Parlor gymnastics, and the discipline of the gymnasium are desirable,
but they should not be the sole reliance for physical culture. No in-door
exercise, however excellent in itself, can fill the place of hearty and
vigorous activity in the open air.

[Sidenote: 23. Physical culture among the ancients? In Greece? In schools
and colleges at the present time? Result to the body and mind?]

23. GYMNASTIC EXERCISES FOR SCHOOLS AND COLLEGES.--In the system of
education among the ancients, physical culture predominated. In ancient
Greece, physical exercises in schools were prescribed and regulated by law,
and hence these schools were called _gymnasia_. At the present time, on the
contrary, this culture is almost wholly unknown, as a part of the course of
education, in our schools and colleges. In a few of our institutions of
learning, however, physical exercises have been introduced, with manifest
advantage to the students, and they form a part of the regular curriculum
of exercises,--as much so as the recitations in geography, grammar, or
Greek. The good effect of the experiments, as shown in improved scholarship
as well as increased bodily vigor, in the institutions where the plan has
been tried, will, it is hoped, lead to its universal adoption. We should
then hear less frequently of parents being obliged to withdraw their
children from school, because they become exhausted {34} or, perchance,
have lost their health from intense and protracted mental application.

[Sidenote: 24. The result of gymnastics in our colleges and other
institutions of learning?]

24. Were gymnastics more common in our educational institutions we should
not so often witness the sad spectacle of young men and women leaving our
colleges and seminaries, with finished educations it may be, but with
constitutions so impaired, that the life which should be devoted to the
accomplishment of noble purposes must be spent in search of health. Spinal
curvatures, which, according to the experience of physicians, are now
extremely frequent, especially among ladies, would give place to the steady
gait and erect carriage which God designed his human creatures should
maintain.

25. All the exercises necessary for the proper development of the body may
be obtained from the use of a few simple contrivances that every one can
have at home, at little cost--less by far than is spent for useless toys.
Many of these may be made available in the parlor or chamber, though all
exercises are far more useful in the open air. A small portion of the day
thus spent will afford agreeable recreation as well as useful exercise. The
Indian club, the wand, the ring, and the dumb-bells answer ordinary
purposes very well. Illustrations are here introduced of a few simple
contrivances that may be useful for general exercises, and are specially
suitable for persons with _weak spines_, or with spines that are the
subject of lateral curvature.

26. One of the simplest appliances for strengthening the muscles of the
spine, designed chiefly to exercise the muscles on either side of the
spine, consists of two wooden handles attached to india-rubber cords, one
of which is attached to a hook made fast in the ceiling, or in the top of
the door-case; and the other to another hook fastened in the wall,
door-post, or window-casing, about the height {35} of the shoulder. When
traction is made with the left hand, it exercises the muscles on the left
side of the spine, while those on the opposite side are left almost at
rest, owing to the oblique direction given to the shoulders when the right
hand grasps the horizontal cord. (This appliance will be understood by
referring to Fig. 13.)

[Illustration: FIG. 11.]

[Illustration: FIG. 12.]

{36}

27. Fig. 11 shows an appliance consisting of two strong elastic cords, with
handles, secured to a hook in the floor, so arranged that the patient has
to stoop forward to reach them. On raising the body the spinal muscles are
powerfully exercised. Fig. 12 shows other modes of using the elastic cords
for strengthening the spine and chest.

[Illustration: FIG. 13.]

28. These various appliances have been combined so as to form a system of
gymnastics suitable for parlor use; other appliances have been added by
which the muscles of {37} the legs may be called into action as well as
those of the spine and upper part of the body (Fig. 13). Combinations of
cords suitable for particular cases may also be made, and by using one or
several cords on the same hook, their power may be adapted to the strength
of the most robust as well as to that of the invalid, or of the most
delicate child. The entire apparatus is quite simple in its construction
and inexpensive, requiring but little space, and at the same time affording
a great variety of exercises.

EXERCISES THAT MAY BE PRACTISED ON THIS APPARATUS.

    EXERCISE I. (Fig. 13).--Stand erect under the cords and place the heels
    together. Grasp the handles firmly, keeping the knees and elbows stiff,
    and pull downward and forward until the fingers nearly touch the toes.
    Return slowly to the erect position. Repeat.

    EXERCISE II. (Fig. 13).--Stand erect, and having grasped the handles
    overhead firmly, separate them and bring them down slowly until they
    touch the sides: then return them slowly to the original position.
    Repeat.

    EXERCISE III. (Fig. 13).--Stand erect, heels together, grasp the
    handles overhead, and charge forward with the right foot. Return to
    first position, and then charge with the left. Repeat, using the right
    and left foot alternately.

    EXERCISE IV. (Fig. 13).--Stand erect, heels together. Grasp the handle
    overhead, and charge forward with the right foot, knee bent. Remain in
    this position and bring the arms down to the sides so that the arm and
    fore-arm may form a right angle. Still holding the handles, thrust
    forward, first the right hand and then the left, until the arm is
    straight. Repeat. Return to first position, then charge forward with
    the left foot, performing the same movements as before.

    EXERCISE V. (Fig. 13).--In this exercise we change to the pulleys
    leading from the side posts, which can be used in several different
    ways. 1st. Stand erect, heels together, facing one of the posts, grasp
    the handle with the right hand, the arm being extended, then flex the
    fore-arm on the arm. Repeat. Perform the same movements with the left
    hand. 2d. Stand with back to the post; grasp the pulley behind with the
    right hand, then gradually bring the hand forward until it is extended
    in a straight line in front. Repeat. Perform the same exercise with the
    left hand.

    {38} EXERCISE VI. (Fig. 13).--This exercise is especially adapted to
    the legs. Stirrups are so arranged that they can be attached to the
    pulleys overhead, and can hang down to within three or four feet of the
    floor. Place the foot in the stirrup, and then press down until it
    touches the floor. Repeat. Exercise the left foot in the same way.

    EXERCISE VII. (Fig. 13.)--This exercise requires a little attention in
    the adjustment of the apparatus. Under the pulleys in the floor are
    passed ropes which can be attached to the snap-hooks that hold the
    handles overhead. Stoop forward with the knees stiff, and take hold of
    the handles, and then raise the body to the erect position. Repeat.

    EXERCISE VIII. (Fig. 13).--Sit on the floor or on a seat three or four
    inches high; bend forward, take hold of the handles, and perform the
    same movements that you would in rowing a boat.

    EXERCISE IX. (Fig. 13).--The trapeze can now be let down; take hold of
    it with both hands, sustaining the weight of the body with the arms,
    then rotate the body first from right to left, then from left to right
    alternately. This exercise is especially suitable for females.

    EXERCISE X. (Fig. 13).--Grasp the trapeze as before, bearing all the
    weight with the arms: then draw the body up slowly until you can place
    the chin over the bars. This requires strength of muscle, and might
    strain if done too violently; if slowly performed there is no danger.

    These are but a few of the exercises that can be practised with this
    apparatus. As these become familiar they can easily be modified, and
    new ones can be arranged to meet the requirements of particular cases.
    Most of the exercises described can be practised with one hand so as to
    strengthen the muscles on one side.

[Sidenote: 29. Need of repose? How do we obtain rest? Alfred the Great? The
eight-hour division of time?]

29. REST.--We cannot always be active: repose must succeed labor. We obtain
this rest partly by suspending all exertion, as in sleep, and partly by a
change of employment. It is said that Alfred the Great recommended that
each day should be divided in the following manner: "Eight hours for work,
eight hours for recreation, and eight hours for sleep." This division of
time is as good as any that could now be made, if it be borne in mind that,
when the work is physical, the time of recreation should {39} be devoted to
the improvement of the mind; and when mental, we should then recreate by
means of physical exercise.

[Sidenote: 30. Cessation of voluntary activity? Temperature of the body?
Consequence? Body and mind during sleep? Nutrition? Describe it.
Consequences of insufficient sleep?]

30. During sleep, all voluntary activity ceases, the rapidity of the
circulation and breathing diminishes, and the temperature of the body falls
one or two degrees. In consequence, the body needs warmer coverings than
during the hours of wakefulness. During sleep, the body seems wholly at
rest, and the mind is also inactive, if we except those involuntary mental
wanderings which we call dreams. Nevertheless a very active and important
physical process is going on. Nutrition, or the nourishing of the tissues,
now takes place. While the body is in action, the process of pulling down
predominates, but in sleep, that of building up takes place more actively.
In this way we are refreshed each night, and prepared for the work and
pleasures of another day. If sleep is insufficient, the effects are seen in
the lassitude and weakness which follow. Wakefulness is very frequently the
forerunner of insanity, especially among those who perform excessive mental
labor.

[Sidenote: 31. Amount of sleep in different persons? Cases? Frederick the
Great? Bonaparte? Instances of long deprivation of sleep?]

31. All persons do not require the same amount of sleep, but the average of
men need from seven to nine hours. There are well-authenticated cases where
individuals have remained without sleep for many days without apparent
injury. Frederick the Great required only five hours of sleep daily.
Bonaparte could pass days with only a few hours of rest. But this long
continued absence of sleep is attended with danger. After loss of sleep for
a long period, in some instances, stupor has come on so profoundly, that
there has been no awaking.

[Sidenote: 32. Instances of sailors? French soldiers? During torture?]

32. There are instances related of sailors falling asleep {40} on the
gun-deck of their ships while in action. On the retreat from Moscow, the
French soldiers would fall asleep on the march, and could only be aroused
by the cry, "The Cossacks are coming!" Tortured persons are said to have
slept upon the rack in the intervals of their torture. In early life, while
engaged in a laborious country practice, the writer not unfrequently slept
soundly on horseback. These instances, and others, show the imperative
demand which nature makes for rest in sleep.

QUESTIONS FOR TOPICAL REVIEW.

                                                                       PAGE
  1. What can you state of the number and division of the muscles?   25, 26
  2. Describe the structure of the muscles.                          25, 26
  3. Their arrangement in pairs and consequent action.                   26
  4. What is the difference between the motion called flexion and that
        called extension?                                                26
  5. Describe their action, and state which are the more powerful.       26
  6. What is the difference between voluntary and involuntary muscles?   26
  7. Illustrate the difference between the two.                          26
  8. State all you can of the tendons or sinews.                         27
  9. What is meant by contraction of the muscles?                    27, 28
  10. In how many and what ways may contraction be effected?             28
  11. What is stated of after-death contraction?                         28
  12. Why cannot a muscle in life continue contracted a long time?       28
  13. How then can the constant beating of the heart be explained?       28
  14. How does the strength of a man compare with that of a horse?       29
  15. What can you state in relation to the relative strength
        of animals?                                                  28, 29
  16. What, in relation to physical strength?                            29
  17. What, in relation to physical degeneracy?                      29, 30
  18. What, in relation to the importance of exercise?                   30
  19. What is the effect of exercise upon the heart, skin,
        and appetite?                                                30, 31
  20. How does exercise affect the current of the body's circulation?    31
  21. How does judicious exercise affect the muscles?                    31
  22. What is stated of violent and spasmodic exercise?                  31
  23. Of the exercise of walking?                                31, 32, 33
  24. Of running, leaping, and other modes of exercise?                  32
  25. Of physical culture, in connection with out-door exercises?        33
  26. Of the importance of gymnastics in our schools and colleges?   33, 34
  27. Of the importance of rest from labor or exercise?              38, 39
  28. What processes take place during sleep?                            39
  29. What effects follow insufficient sleep?                            39

       *       *       *       *       *


{41}

CHAPTER III.

THE INTEGUMENT, OR SKIN.

_The Integument--Its Structure--The Nails and Hair--The Complexion--The
Sebaceous Glands--The Perspiratory Glands--Perspiration and its
Uses--Importance of Bathing--Different kinds of Baths--Manner of
Bathing--The Benefits of the Sun--Importance of Warm Clothing--Poisonous
Cosmetics._

[Sidenote: 1. What is the skin? Parts directly beneath? What is shown?]

1. THE INTEGUMENT.--The skin is the outer covering of the body. The parts
directly beneath it are very sensitive, and require protection. This is
shown whenever by accident the skin is broken, pierced, or torn off, the
bared surface being very tender, and painful to the touch. Nature has
provided the body with a garment that is soft, pliable, close-fitting, and
very thin, and yet sufficiently strong to enable us to come in contact with
the objects that surround us, without inconvenience or suffering.

[Sidenote: 2. Microscopic examination? What is the cutis? The cuticle?
Their union? How separated? What further is said of the cuticle?]

2. THE STRUCTURE OF THE SKIN.--When examined with the aid of the
microscope, the skin is found to be made up of two layers--the outer and
the inner. The inner one is called the _cutis_, or true skin; the outer one
is the _epidermis_, or scarf-skin. The latter is also known as the
_cuticle_. These two layers are closely united, but they may be separated
from each other. This separation takes place whenever, from a burn, or
other cause, a blister is formed; a watery fluid is poured out between the
two layers, and lifts the epidermis from the true skin.

Of the two layers, the cuticle is the thinner in most parts of the body,
and has the appearance of a whitish membrane. It is tough and elastic, is
without feeling, and does {42} not bleed, when cut. Examine it more
closely, and we observe that it is composed of minute flat cells, closely
compacted, and arranged layer upon layer.

[Sidenote: 3. Wearing out of the cuticle? What then? Variety in thickness
of cuticle? How accounted for?]

3. The outer layer is constantly being worn out, and falls from the body in
the form of very fine scales. It is, also, continually forming anew on the
surface of the inner layer. Its thickness varies in different parts of the
body. Where exposed to use, it is thick, hard, and horn-like, as may be
seen on the soles of the feet, or on the palms of the hands, especially of
those who are accustomed to perform much manual labor. This is an admirable
provision for the increased protection of the sensitive parts below the
skin against all extraordinary exposure. Even the _liabilities_ of these
parts to injury, are thus kindly provided for by "the Hand that made us."

[Sidenote: 4. Location and office of the cutis? What further is said of it?
Papillæ? Touch?]

4. The cutis, or true skin, lies beneath the epidermis, and is its origin
and support. It is firm, dense, elastic, very sensitive, and is freely
supplied with blood-vessels. It is closely connected with the tissues below
it, but may be separated by means of a sharp instrument. The surface of the
cutis is not smooth, but is covered here and there with minute elevations,
called _papillæ_. These are arranged in rows, along fine lines, or ridges,
such as those which mark the palm and fingers; their number is about 80 to
the square line (a line being one-twelfth of an inch). These _papillæ_
contain the blood-vessels which carry the supply of blood needed by the
ever-wasting skin. They contain nerves also, and are largely concerned in
the sense of touch; hence they are particularly abundant where the touch is
most delicate, as at the ends of the fingers.

[Sidenote: 5. What are the nails and hair? The growth of the nail? The
rapidity of its growth? Accident to the nail?]

{43} [Illustration: Fig. 14.

_a, b._ THE ROOT OF A HAIR.

1, 2, 3. The skin forming the hair sac. 4. Sebaceous glands. 5. The hair
sac.

_c._ TRANSVERSE SECTION OF A HAIR.]

5. THE NAILS AND HAIR.--These are appendages of the skin, and although very
unlike the cuticle as it appears on the surface of the body, they are, in
reality, modified forms of that layer of the skin. The nail grows from a
fold of the cuticle at the root, and from the under surface. As fast as it
is formed, it is constantly being pushed outward. The rapidity of its
growth can be ascertained by filing a slight groove on its surface, and
noticing how the space between it and the root of the nail increases, in
the course of a few weeks. When the nail is removed by any accident, it
will be replaced by a new one, if the root be not injured.

[Sidenote: 6. How are the hairs produced? Difference in their length?]

6. The hairs are produced in a similar manner; the skin forming
depressions, or hair sacs, from the bottom of which they grow and are
nourished (Fig. 14). They are found, of greater or less length, on almost
all parts of the surface, except the palms of the hands and soles of the
feet. On certain parts of the body, they grow to great length; on other
parts they are so short, that they do not rise beyond the hair sac in which
they originate.

[Sidenote: 7. Root of the hair? Shaft? Firmness and softness of the hair?]

7. The bulb, or root, from which the hair arises, is lodged in a small
pouch, or depression in the skin. The shaft is the part which grows out
beyond the level of the skin. Its growth is altogether in one direction, in
length alone. The outer part of the hair is quite firm, while its {44}
interior is softer, and probably conveys the fluids by which it is
nourished. The hair is more glossy in health than at other times.

[Sidenote: 8. Office of the nail? Of the hair? Give the illustrations.]

8. The nail serves as a protection to the end of the finger, and also
enables us to grasp more firmly, and to pick up small objects. The hair,
too, is a protection to the parts it covers. On the head, it shields the
brain from extremes of heat and cold, and moderates the force of blows upon
the scalp. On the body, it is useful in affording a more extensive surface
for carrying off the perspiration.

[Sidenote: 9. On what does the complexion depend? Light and dark races?
Freckles?]

9. COMPLEXION.--In the deeper cells of the cuticle lies a pigment, or
coloring matter, consisting of minute colored grains. On this pigment
_complexion_ depends; and, according as it is present in less or greater
amount, occasions the difference of hue, that exists between the light and
dark races of men, and between the blonde and brunette of the white races.
Freckles are due to an irregular increase of coloring matter.

[Sidenote: 10. Influence of the sun? How illustrated? Jews?]

10. The sun has a powerful influence over the development of this pigment,
as is shown by the swarthy hue of those of the white race who have
colonized in tropical climates. It is also well illustrated by the fact,
that among the Jews who have settled in northern Europe, there are many who
are fair complexioned, while those residing in India, are as dark as the
Hindoos around them.

[Sidenote: 11. What is an Albino? Where are Albinos found?]

11. An Albino is a person who may be said to have no complexion; that is,
there is an entire absence of coloring matter from the skin, hair, and
_iris_ of the eye. This condition more frequently occurs among the dark
races, and in hot climates, although it has been observed in almost every
race and clime.

[Sidenote: 12. What are sebaceous glands? How do they act? Sebaceous glands
of the face? How do they act?]

12. SEBACEOUS GLANDS.--There are in the skin certain {45} small glands,
which produce an oily substance, called _sebaceous_ matter. These glands
are little rounded sacs, usually connected with the hair-bulbs; and upon
these bulbs, they empty their product of oil, which acts as a natural and
adequate dressing for the hair (4, Fig. 14). A portion of the sebaceous
matter passes out upon the surface, and prevents the cuticle from becoming
dry and hard. The glands situated upon the face and forehead, open directly
upon the skin. In these, the sebaceous matter is liable to collect, and
become too hard to flow off naturally.

[Sidenote: 13. Black points, called worms? Animalcules? Service performed
by sebaceous matter?]

13. These glands on the face and forehead frequently appear, on the faces
of the young, as small black points, which are incorrectly called "worms."
It is true, that occasionally living animalcules are found in this
thickened sebaceous matter, but they can only be detected by the aid of the
microscope. This sebaceous matter acts not only to keep the skin flexible,
and furnish for the hair an oily dressing, but it especially serves to
protect the skin and hair, from the acridity arising from the perspiration.

[Sidenote: 14. Perspiration? Sweat glands? Of what do they consist?
Dimension of the tubes?]

14. THE PERSPIRATORY GLANDS.--The chief product of the skin's action is the
perspiration. For the formation of this, there are furnished countless
numbers of little sweat-glands in the true skin. They consist of fine
tubes, with globe-like coils at their deeper extremity. Their mouths or
openings may be seen with an ordinary magnifying glass, upon the fine
ridges which mark the fingers. These tubes, if uncoiled, measure about
one-tenth of an inch in length. In diameter, they are about one
three-hundredth of an inch, and upon certain parts of the body there are
not far from three thousand of these glands to the square inch. Their whole
number in the body is, therefore, very great; and, in fact, it is computed
if they were all united, end to end, their combined measurement would
exceed three miles. {46}

[Sidenote: 15. What is sensible perspiration? Insensible perspiration?]

15. THE SENSIBLE AND INSENSIBLE PERSPIRATION.--The pores of the skin are
constantly exhaling a watery fluid; but, under ordinary circumstances,
there is no moisture apparent upon the surface, for it evaporates as
rapidly as it is formed. This is called insensible perspiration. Under the
influence of heat or exercise, however, this fluid is excreted more
abundantly, and appears on the surface in the form of minute, colorless
drops. It is then termed sensible perspiration.

[Sidenote: 16. Components of perspiration? Upon what does perspiration
depend? Amount of perspiration daily?]

16. Water is the chief component of this fluid, there being about
ninety-eight parts of water to two parts of solid matter. The quantity
escaping from the body varies greatly, according to the temperature of the
air, the occupation of the individual, and other circumstances. The average
daily amount of this excretion, in the adult, is not far from thirty
ounces, nearly two pints, or more than nine grains each minute.

[Sidenote: 17. What does perspiration set free from the blood? What other
service does perspiration perform? Explain the process.]

17. THE USES OF THE PERSPIRATION.--Besides liberating from the blood this
large amount of water, with the effete matter it contains, the perspiration
serves to regulate the temperature of the body. That is to say, as
evaporation always diminishes temperature, so the perspiration, as it
passes off in the form of fine vapor, cools the surface. Accordingly, in
hot weather this function is much more active, and the cooling influence
increases in proportion. When the air is already charged with moisture, and
does not readily receive this vapor of the body, the heat of the atmosphere
apparently increases, and the discomfort therefrom is relatively greater.

[Sidenote: 18. Effect of interruption of excretion? What experiments are
mentioned?]

18. The importance of this excretion is shown by the effects that often
follow its temporary interruption, namely, headache, fever, and the other
symptoms that accompany {47} "taking cold." When the perspiration is
completely checked, the consequences are very serious. Experiments have
been performed upon certain smaller animals, as rabbits, to ascertain the
results of closing the perspiratory tubes. When they are covered by a
coating of varnish impervious to water and gases, death ensues in from six
to twelve hours; the attendant symptoms resembling those of suffocation.

[Sidenote: 19. Give the story in relation to the boy covered with gold
foil.]

19. It is related that, at the coronation of one of the Popes about three
hundred years ago, a little boy was chosen to act the part of an angel; and
in order that his appearance might be as gorgeous as possible, he was
covered from head to foot with a coating of gold foil. He was soon taken
sick, and although every known means were employed for his recovery, except
the removal of his fatal golden covering, he died in a few hours.

[Sidenote: 20. Give the quotation. Perspiration?]

20. THE IMPORTANCE OF BATHING.--From these considerations, it is evident
that health must greatly depend upon the free action of the skin. "He who
keeps the skin ruddy and soft, shuts many gates against disease." When the
watery portion of the perspiration evaporates, the solid matter is left
behind on the surface. There, also, remain the scales of the worn-out
cuticle, and the excess of sebaceous matter. In order to secure the natural
action of the skin, these impurities require to be removed by the frequent
application of water.

[Sidenote: 21. Ablution in warm climates? What advice is given?]

21. In warm climates, and during hot weather, ablution should be more
frequently practised. For a person in good health, a daily cold bath is
advisable. To this should be added occasionally a tepid bath, with soap,
water alone not being sufficient to remove impurities of a greasy nature.
Soap facilitates this, by forming with such substances a chemical mixture,
which is readily soluble in water, and is by it removed from the body. {48}

[Sidenote: 22. Liebig's maxim? What further is added?]

22. There is a maxim by the chemist Liebig, to the effect, that the
civilization of a nation is high, in proportion to the amount of soap that
it consumes; and that it is low, in proportion to its use of perfumes. In
some degree, we may apply the same test to the refinement of an individual.
The soap removes impurity; the perfume covers, while retaining it.

[Sidenote: 23. What is said about cold bathing?]

23. THE DIFFERENT KINDS OF BATHS.--All persons are not alike able to use
the cold bath. When the health is vigorous, and the system does not feel a
shock after such a bath, a prompt reaction and glow upon the surface will
show that it is beneficial. Where this pleasurable feeling is not
experienced, but rather a chill and sense of depression ensues, we are
warned that the system will not, with impunity, endure cold bathing.

[Sidenote: 24. What is said about warm bathing?]

24. It should also be borne in mind, that the warm or hot bath cannot be
continued so long, or repeated so frequently as the cold, on account of the
enervating effect of unusual heat so applied to the body. For persons who
are not in robust health, one warm bath each week is sufficient; this class
should be careful to avoid every extreme in reference to bathing, clothing,
and whatever greatly affects the action of the skin.

[Sidenote: 25. What is said about sea-bathing?]

25. Sea-bathing is even more invigorating than fresh-water bathing. Those
who cannot endure the fresh water, are often benefited by the salt-water
baths. This may be accounted for, in part, by the stimulant action upon the
surface, of the saline particles of the sea-water; but the exciting scenes
and circumstances of sea-bathing also exert an important influence. The
open-air exercise, the rolling surf, the genial weather, and usually the
cheerful company, add to its intrinsic benefits. {49}

[Sidenote: 26. What is said as to the time and manner of bathing?]

26. TIME AND MANNER OF BATHING.--A person in sound health may take a bath
at almost any time, except directly after a full meal. The most appropriate
time is about three hours after a meal, the noon-hour being probably the
best. For the cold bath, taken rapidly, no time is better than immediately
after rising. Those beginning the use of cold baths should first try them
at 70° Fahr., and gradually use those of a lower temperature. From five to
twenty minutes may be considered the proper limit of time to remain in a
bath; but a sensation of chilliness is a signal to withdraw instantly,
whether at home, or at the sea-side. Two sea-baths may be taken daily; one
of any other kind is sufficient.

[Sidenote: 27. Condition of the body when bathing? Direction, after
bathing?]

27. The body should be warm, rather than cold, when stepping into the bath;
and after it, the skin should be thoroughly dried with a coarse towel. It
is best to continue friction until there is a sensation of warmth or "glow"
throughout the entire surface. This reaction is the test of the good
effects of the bath. If reaction is still incomplete, a short walk may be
taken, especially in the sunshine. It is very congenial, however, both to
health and comfort, to rest for a short time directly after bathing, or to
take some light refreshment. This is better than severe exercise or a full
meal.

[Sidenote: 28. Bathing among the ancients? Baths of Rome?]

28. BATHING AMONG THE ANCIENTS.--The Romans and other nations of antiquity
made great use of the vapor-bath as a means of preserving the health, but
more particularly as a luxury. Their method was not unlike that employed in
northern Europe at the present day. The public baths of Rome and other
cities are among the grandest and most interesting monuments of ancient
luxury and splendor; and from their ruins have been recovered some of the
most beautiful works of art. {50}

[Sidenote: 29. After the bath? Swimming among the ancients?]

29. The Thermæ, as the baths of Rome were called, were of great extent,
built very substantially, and ornamented at vast expense. They were
practically free to all, the cost of a bath having been less than a cent.
It is related that some persons bathed seven times a day. After the bath
their bodies were anointed with perfumed oil. If the weather was fine, they
passed directly from the Thermæ into the gymnasium, and engaged in some
gentle exercise previous to taking the midday meal. Between two and three
in the afternoon was the favorite hour for this ancient luxury. Swimming
was a favorite exercise, and a knowledge of it was regarded as necessary to
every educated man. Their common expression, when speaking of an ignorant
person, was, "He can neither read nor swim."

[Sidenote: 30. The Sun-bath? The story of Pliny?]

30. THE SUN-BATH.--Some also were accustomed daily to anoint themselves,
and lie or walk in apartments arranged for the purpose, with naked bodies
exposed to the direct rays of the sun. There is an interesting allusion to
this practice, in a letter of the younger Pliny to the historian Tacitus,
describing the destruction of Pompeii by an eruption of Vesuvius. "My
uncle," (Pliny the elder,) "was at that time in command of the fleet at
Misenum. On the 24th of August, about one in the afternoon, my mother
desired him to notice a cloud which seemed of unusual shape and dimensions.
He had just returned from _taking the benefit of the sun_, and after a cold
bath, and a slight repast, had retired to his study." Then follows a
description of the destruction of Pompeii, and the death of the elder
Pliny.

[Sidenote: 31. Benefit of the sun? Effect upon plants? Skin?]

31. We may judge somewhat of "the benefits of the sun," by observing the
unnatural and undeveloped condition of plants and animals which are
deprived of light. Plants become blanched and tender; the fish of {51}
subterranean lakes, where no light enters, are undersized, and have no
eyes; tadpoles kept in the dark do not develop into frogs; men growing up
in mines are sallow, pale, and deformed. Besides the well-known effect of
solar light in tanning the skin, it also makes it thicker and better able
to resist exposure; though the complexion may be thereby injured, the
health gains more than compensates for the loss of beauty. "To make good
the loss of the lily, where the sun has cast his ray, he seldom fails to
plant the rose."

[Sidenote: 32. Direction about clothing? Exposing limbs of children?
Clothing, night and day?]

32. CLOTHING.--In reference to clothing, we are far more apt, in our
changeful climate, to use too little than too much. An aphorism of
Boerhaave, worth remembering, if not of adopting, is, "We should put off
our winter clothing on midsummer's day, and put it on again the day after."
He also says, "Only fools and beggars suffer from the cold; the latter not
being able to get sufficient clothes, the others not having the sense to
wear them." The practice of exposing the limbs and necks of young children,
for the alleged purpose of "hardening" them, is quite hazardous. It is not
to be denied that some seem to be made tough by the process; but it is so
only with the rugged children, the delicate ones will invariably suffer
under this fanciful treatment. As has been stated before, the skin is
constantly acting, by night as well as by day. It is therefore conducive
both to cleanliness and comfort to change entirely the clothing on retiring
for the night. The day-clothing should be aired during the night, and the
bedding should be aired in the morning, for the same reason.

[Sidenote: 33. Cosmetics? Painters' colic?]

33. POISONOUS COSMETICS.--The extensive use of _cosmetics_ for the
complexion is a fertile source of disease. The majority of these
preparations contain certain poisonous mineral substances, chiefly lead.
Now, the skin rapidly absorbs the fine particles of lead, and the system
{52} experiences the same evil effects that are observed among the
operatives in lead works and painters, namely, "painters' colic," and
paralysis of the hands, called "wrist-drop."

34. Certain hair-dyes also contain lead, together with other noxious and
filthy ingredients. These do not work as great harm as the cosmetics, since
they are purposely kept away from the skin, but they rob the hair of its
vitality. Eye-washes, too, are made from solutions of lead, and many an eye
has been ruined by their use. They deposit a white metallic scale on the
surface of the eye, which becomes a permanent obstruction to the vision.

QUESTIONS FOR TOPICAL REVIEW.

                                                                      PAGE

  1. What are the characteristics of the skin, and what office
         does it perform?                                               41
  2. What can you state of the structure of the skin?                   41
  3. Describe the cuticle and tell its use.                         41, 42
  4. Describe the cutis or true skin and tell its use.                  42
  5. What can you state of the nature and growth of the nail?       42, 43
  6. Of the nature and growth of the hair?                      42, 43, 44
  7. Of the offices performed by the nails and hair?                    44
  8. How is the difference in complexion in different persons
        accounted for?                                                  44
  9. How is the presence of freckles accounted for?                     44
  10. How does Nature provide a dressing for the hair?              44, 45
  11. What other service do the sebaceous glands perform?               45
  12. State what you can of the perspiratory glands.                    45
  13. What is the difference between sensible and insensible
         perspiration?                                                  46
  14. State the uses and importance of perspiration.                46, 47
  15. What impurities gather naturally on the skin?                     47
  16. Repeat what is said of the importance of bathing.             47, 48
  17. When should we indulge in cold, warm, and sea bathing?        48, 49
  18. What is the effect in each case?                                  48
  19. What directions are given as to the time and manner for
         bathing?                                                       49
  20. What is related of bathing among the ancients?                49, 50
  21. What is related to show the antiquity of sun-bathing?             50
  22. What are the effects of sun-bathing?                          50, 51
  23. What directions are given in relation to clothing the body?       51
  24. What can you state of poisonous cosmetics?                    51, 52
  25. Of hair-dyes and eye-washes?                                      52

       *       *       *       *       *


{53}

CHAPTER IV.

THE CHEMISTRY OF FOOD.

_The Source of Food--Inorganic Substances--Water--Salt--Lime--Iron--Organic
Substances--Albumen, Fibrin, and Casein--The Fats or Oils--The Sugars,
Starch, and Gum--Stimulating Substances--Necessity of a Regulated Diet._

[Sidenote: 1. The term food? Source of food? Need of preparing food?]

1. THE SOURCE OF FOOD.--The term _food_ includes all those substances,
whether liquid or solid, which are necessary for the nourishment of the
body. The original source of all food is the earth, which the poet has
fitly styled the "Mother of all living." In her bosom, and in the
atmosphere about her, are contained all the elements on which life depends.
But man is unable to obtain nourishment directly from such crude chemical
forms as he finds in the inorganic world. They must, with a few exceptions,
be prepared for his use, by being transformed into new and higher
combinations, more closely resembling the tissues of his own body.

[Sidenote: 2. Usefulness and hurtfulness of plants? What then must man do?
Parts of the same plant or tree?]

2. This transformation is effected, first, by the vegetable world. But all
plants are not alike useful to man; while some are absolutely hurtful.
Accordingly, he must learn to discriminate between that which is poisonous
and that which is life-supporting. Again, all parts of the same plant or
tree are not alike beneficial: in some, the fruit, in others, the leaves,
and in others, the seeds only are sufficiently refined for his use. These
he must learn to select; he must also learn the proper modes of preparing
each kind for his table, whether by cooking or other processes. {54}

[Sidenote: 3. Certain forms of vegetable creation? Example of the bee?
Cattle? The inference?]

3. Again, certain forms of the vegetable creation which are unfit, in their
crude state, for man's food, and which he rejects, are chosen as food by
some of the lower animals, and are, by them, made ready for his use. Thus
the bee takes the clover, that man cannot eat, and from it collects honey.
The cattle eat the husks of corn and the dried grass, that are by far too
coarse for man, and in their own flesh convert them into tissues closely
resembling his muscular tissue. In this way, by the aid of the transforming
processes of the vegetable and animal creations, the simple chemical
elements of the mineral kingdom are elaborated into our choice articles of
food.

[Sidenote: 4. What classification? Define organic substances. Inorganic.
Organic, how spoken of? The inorganic? Water and salt?]

4. INORGANIC SUBSTANCES.--The substances we use as food are classified as
_organic_ and _inorganic_. By organic substances are meant those derived
from living forms, such as vegetables and animals. Inorganic substances are
those simpler inanimate forms which belong to the mineral kingdom. The
former alone are commonly spoken of as food, but the latter enter very
largely into the constitution of the body, and must therefore be present in
our food. With the exception of two articles, water and common salt, these
substances only enter the system when blended with organic substances.

[Sidenote: 5. Water in physiology? Where found? Computation? Water in the
teeth? Muscle, tendons, and ligaments? How ascertained? Water in the fluids
of the body? What is the advantage?]

5. WATER.--Water, from a physiological point of view, is the most important
of all the articles of food. It is everywhere found in the body, even in
the bones and the teeth. It has been computed that as large a proportion as
two-thirds of the body is water. The teeth, the densest of the solids in
the human system, contain ten per cent. of water. The muscles, tendons, and
ligaments are more than half water; for it is found that they lose more
than half their {55} weight when dried with moderate heat. But it is in the
_fluids_ of the body that water is found most abundantly. It gives to them
the power of holding a great variety of substances in solution, and is the
great highway by which new supplies are conveyed to the point where they
are required, and by which old particles of matter, that have served their
uses, are brought to the outlets of the body to be thus removed from the
system.

[Sidenote: 6. Length of time man can do without food or water? Give the
comparison? Bulk of drinks? Constituent of meats, etc.? Fruits?]

6. Man can remain a longer time without solid food than without water. He
may be deprived of the former for ten to twelve hours without great
suffering, but deprivation of water for the same length of time will
produce both severe pain and great weakness. The food should contain not
less than two parts of water to one of solid nutriment. Water constitutes
the great bulk of all our drinks, and is also a large constituent of the
meats, vegetables, and fruits which come upon the table. Fruits,
especially, contain it in great abundance, and, in their proper season,
furnish most agreeable and refreshing supplies of the needed fluid.

[Sidenote: 7. Salt, how obtained? Where found? In the human body?
Importance of salt? What else can you state of the value of salt?]

7. COMMON SALT.--Salt, or sodium chloride, as an article of food, is
obtained chiefly from the mineral kingdom; although plants contain it in
small quantities, and it is also found in the tissues of nearly all animals
used as food. In the human body, it is an ingredient of all the solids and
fluids. The importance of salt to animal life in general, is shown by the
great appetite for it manifested by domestic animals, and also by the
habitual resort of herds of wild beasts to the "salt-licks" or springs. In
those parts of the world where salt is obtained with difficulty, man places
a very high price upon it.

[Sidenote: 8. Experiments upon animals?]

8. Experiments upon domestic animals show that the withdrawal of salt from
their food, not only makes their {56} hides rough and causes the hair to
fall out, but also interferes with the proper digestion of food. If it be
withheld persistently, they become entirely unable to appropriate
nourishment, and die of starvation.

[Sidenote: 9. Salt, how taken into the system? Its use in cooking?
Consumption?]

9. Salt is usually taken into the system in sufficient quantities in our
food. Even the water we drink often has traces of it. The habitual use of
much salt in cooking, or as a seasoning at the table, is not wise; and
while it may not lead to consumption, as some writers declare, it is a bad
habit in itself, and leads to the desire for other and more injurious
condiments.

[Sidenote: 10. Lime in the bones? What does it impart? Chief ingredient of
the bones and teeth? Where else found?]

10. LIME.--This is the mineral substance which we have spoken of before as
entering very largely into the composition of the bones. It is the
important element which gives solidity and permanence to the framework upon
which the body is built. Calcium tri-phosphate, or "bone-earth," is the
chief ingredient of the bones and teeth, but is found in the cartilages and
other parts of the body in smaller quantities.

[Sidenote: 11. How does lime find its way into the body? Early life? Effect
of its derivation?]

11. How does this substance find its way into the body? Meat, milk, and
other articles obtained from the animal kingdom contain it, and it is
abundantly stored away also in the grains from which our bread is made, in
wheat, rye, and Indian corn. In early life, while the body is growing, the
supplies of this substance should be carefully provided. The evil effects
of the deprivation of it are too often and painfully evident in the
softening of the bones, and in the predisposition to curvature of the
spine--deformities which are most deplorable and which continue through
life.

[Sidenote: 12. Iron, its abundance and diffusion? Where found? What part of
the blood is it? How supplied to the system? In case of loss of blood or
wasting disease?]

12. IRON.--This substance is probably the most abundant and widely diffused
of the metals. It is found in {57} most of the vegetables, and is a very
important component of animal tissues. It enters into the composition of
human blood in about one part per thousand. Ordinarily, the food conveys to
the system enough iron for its use, but it must sometimes be introduced
separately as a remedy, especially after great loss of blood, or after some
wasting disease. Under its influence the blood seems to be rapidly
restored, and a natural color of the lips and skin replaces the pallor
caused by disease.

[Sidenote: 13. Soda, potash, and magnesia? How do they occur?]

13. OTHER INORGANIC SUBSTANCES.--In addition to the substances mentioned,
the mineral kingdom supplies compounds of soda, potash, and magnesia, which
are essential for the use of the body. They occur in small quantities in
the body, and enter it in combination with the various articles of diet.

[Sidenote: 14. Organic substances, whence derived? What do they comprise?
Groups?]

14. ORGANIC SUBSTANCES.--These substances are derived from the vegetable
and animal creations. They comprise all those articles which are commonly
spoken of as "food," and which are essential to sustain the body in life
and strength. They are divided into three groups, namely: the Albuminoid
substances, the Fats, and Sugars.

[Sidenote: 15. The Albuminoid class, includes what? These compounds
constitute what? The food? Their importance? Their properties?]

15. THE ALBUMINOIDS.--This class includes three important nutritive
substances--(1) _Albumen_, which gives it its name; (2) _Fibrin_, including
_gluten_; and (3) _Casein_. These compounds constitute a large part of the
human body, and the food contains them in proportionally large quantities.
Their importance is so great, and the system so promptly suffers from their
absence, that they have been styled the "_nutritious_ substances." The
properties which they hold in common are, that they do not crystallize, and
have a jelly-like form, except when heat is applied to them, when they
harden, or _coagulate_. {58}

[Sidenote: 16. Decomposition? Effect of cold? Illustrations? Elephants?]

16. They likewise decompose, or _putrefy_, under the influence of warmth
and moisture. Hence the decay of all dead animal tissues. Cold arrests this
process. It is well known that milk, eggs, and the like, "keep" much longer
in winter than at other seasons. The bodies of elephants, caught in the ice
many hundred years ago, are occasionally borne by the icebergs to the coast
of Siberia, completely frozen, but preserved almost perfectly in form and
limb.

[Sidenote: 17. In what substances does albumen exist? What further is said
of the egg?]

17. ALBUMEN exists in milk, meat, the grains, and the juices of many
plants; but the purest form is obtained from the white of egg. When we
consider that an egg is composed chiefly of albumen and water--namely, six
parts in seven; and when we also consider the numerous, diverse, and
complex tissues--the muscles, bones, internal organs, bill, claws, and
feathers--with which the chick is equipped on leaving his shell, we are
impressed with the importance of these apparently simple constituents of
the food and body.

[Sidenote: 18. Fibrin, gluten, clotting of the blood?]

18. FIBRIN is derived from meats, and exists in the blood both of man and
the lower animals. _Gluten_, or vegetable fibrin, resembles closely true
fibrin, and is abundantly furnished in wheat and other grains from which
flour is commonly made. Animal fibrin coagulates spontaneously when it is
removed from the body, and thus causes the "clotting" of the blood.

[Sidenote: 19. Casein? Its coagulation? Effect of rennet? Making of
cheese?]

19. CASEIN is the curdy ingredient of milk, and a highly important
food-substance. Its coagulation in milk takes place not from heat, but by
the addition of an acid, and also when milk becomes sour from exposure to
the air. It is commonly effected, however, by introducing a piece of
_rennet_, a preparation made from a calf's stomach. The _curds_, or casein,
may then be separated from the _whey_, {59} and made into cheese, by
pressing it sufficiently to drive off the water.

[Sidenote: 20. What are the fats? The oils? How supplied? How alike?
Emulsifying? Example? How do we know it?]

20. THE FATS OR OILS.--This is the second group of organic foods. Those
which are more solid are called _fats_: the more fluid ones are the _oils_.
Oleaginous substances are supplied in both animal and vegetable food; but,
from whatever source derived, they are chemically much alike. They are
insoluble in water, and yet they unite readily with the watery fluids of
the body, and are by them conveyed to its various parts for their
nourishment. This is due to their property of "emulsifying;" that is, they
are held in suspension, in a finely divided state, in water. Ordinary milk
is an example of an _emulsion_. We know that it contains fat; for butter is
obtained from it, and, under the microscope, the minute oil-globules may be
distinctly seen.

[Sidenote: 21. Whence are fatty articles of food derived?]

21. In our country and climate, and also in colder climates, fatty articles
of food are principally derived from the animal creation, such as meat or
flesh, milk and butter. But most of the bread-stuffs contain more or less
fat or oil; Indian meal as much as nine parts in a hundred.

[Sidenote: 22. Appetite of persons in cold climates? What do they require?
Upon what must they rely? Why? The Esquimaux? Laplander? Olive and palm?]

22. Among persons living in cold climates, the appetite for oleaginous food
is especially eager; and they require large quantities of it to enable them
to resist the depressing influences of cold. Since vegetation is scanty and
innutritious, and the waters of the frozen regions abound in animal life,
they must rely wholly upon a diet derived from the latter source. The
Esquimaux consumes daily from ten to fifteen pounds of meat or blubber, a
large proportion of which is fat. The Laplander will drink train-oil, and
regards tallow-candles as a great delicacy. In hot climates, on the
contrary, where flourish the olive {60} and the palm, this kind of food may
be obtained from vegetable sources in abundant quantities.

[Sidenote: 23. Which are the third of the organic groups? What do they
embrace? Points of resemblance?]

23. THE SUGARS, OR THE SACCHARINE SUBSTANCES.--These constitute the third,
and last, group of the organic substances, which are employed as food. This
group embraces, in addition to the different kinds of _Sugar_, the
varieties of starch and gum, from whatever source derived. The two
substances last named do not, at first sight, present many points of
similarity to sugar; but they closely resemble it in respect to their
ultimate chemical composition, being made up of the same elements, in
nearly the same proportions. And their office in the system is the same,
since they are all changed into sugar by the processes of digestion.

[Sidenote: 24. Origin of the sugars? Ordinary sugar? Beetroot? Maple-sugar?
Grape-sugar? Cane-sugar?]

24. SUGAR is chiefly of vegetable origin; the animal varieties being
obtained from honey and milk. The most noticeable characteristic of this
substance is its agreeable, sweet taste, which makes it everywhere a
favorite article of food. But this quality of sweetness is not possessed by
all the varieties of sugar in the same degree; that obtained from milk, for
instance, has a comparatively feeble taste, but rather imparts a gritty
feeling to the tongue. The other important properties of sugar are, its
power to crystallize when evaporated from watery solutions, such as the
juices of many plants; a tendency to ferment, by which process alcohol is
produced; and a ready solubility in water. This latter quality renders it
very easy of digestion, and more so than any other of the saccharine group.
It is computed that the annual production of sugar, in all parts of the
world, is more than one million of tons. The kind of sugar that is in
ordinary use, in this country, is prepared from the juice of the
sugar-cane, which contains eighteen per cent. of sugar. In France it is
manufactured from the {61} beet root, which holds about nine per cent.; the
maple-tree of our climate yields a similar sugar. The sweet taste of fruits
is due to the presence of grape-sugar: the white grains seen on raisins
belong to this variety. Cane-sugar is more soluble than the latter, and has
twice the sweetening power.

[Illustration: FIG. 15.--GRANULES OF POTATO STARCH.]

[Sidenote: 25. Starch, how widely distributed? Its qualities? Its
constituents? Its solubility?]

25. STARCH.--This is the most widely distributed of the vegetable
principles. It is tasteless, inodorous, and does not crystallize. It
consists of minute rounded granules, which, under the microscope, reveal a
somewhat uniform structure (Fig. 15). Starch will not dissolve in cold
water, but in boiling water the small grains burst open, and may then be
dissolved and digested.

[Sidenote: 26. How much starch in bread-stuffs? In rice? Unripe fruits?
Ripe fruits?]

26. The bread stuffs, wheat, corn, and rye flours, are more than one-half
starch. Rice, which is the "staff of life" to one-third of the human
family, contains eighty per cent. Unripe fruits have much starch in them,
which renders them indigestible when eaten uncooked; for the grains of raw
starch are but slightly acted upon within the body. But, under the potent
chemistry of the sun's ray, this crude material is converted into sugar.
Thus are the fruits prepared by the careful hand of Nature, so that when
ripe they may be freely used without further preparation.

[Sidenote: 27. Gum, where found? Its composition? Gum Arabic?]

27. GUM is commonly found in those articles which {62} also contain starch;
and has the same chemical composition as the latter, but is much less
nutritious. In the East, gum-arabic and similar substances are largely
employed as food. Persons who travel by caravan across vast, sandy deserts,
find such substances well adapted to their wants, since they are not
perishable, and are easily packed and carried.

[Sidenote: 28. The three classes of food principles? What besides? What is
said of them? Name the articles not nutritious.]

28. STIMULATING SUBSTANCES.--The three classes of food-principles already
considered--the Albuminoids, the Fats, and the Sugars--comprise all the
more important organic ingredients of our food. There are, besides, a great
variety of coloring and flavoring matters that stimulate or increase the
appetite for food by appealing to the eye and taste; but they are not
nutritious, and are quickly separated from the truly useful substances, and
do not long remain in the body. Among these may be classed spices, flavors
of fruits, tea, coffee, and vegetable acids.

[Sidenote: 29. What is said of experiments that have been tried?]

29. NECESSITY OF A REGULATED DIET.--A great variety of experiments have
been tried in order to test the relative value of the different nutritive
principles. They have been practised to some extent upon man, but chiefly
upon those inferior animals which require a similar diet to man.

[Sidenote: 30. What has been demonstrated in the first place? Example?
Second demonstration? Example? Give the illustration in relation to
convertibility.]

30. By this means it has been demonstrated that--first, when any one of
these substances is eaten exclusively, the body is imperfectly nourished,
and life is shortened. Dogs fed exclusively upon either albumen, fat, or
sugar, soon die of starvation. Second, a diet long deprived of either of
these principles, is a fertile cause of disease; for example, on
ship-board, where fresh vegetables are not dealt out for a long period,
_scurvy_ becomes prevalent among the sailors. They are, however, to a
certain extent mutually convertible, and thus the missing article is
indirectly supplied. For {63} instance, sugar changes to fat in the body;
and hence, as is well known, the "hands" on a sugar plantation grow fat
during the sugar season, by partaking freely of the ripened juices of the
cane.

31. That is the best diet therefore which contains some of each of these
principles, in due proportion; and that is the worst which excludes the
most of them. The cravings and experience of man had unerringly guided him
to a correct regulation of his diet, long before the chemistry of food was
understood; so that his ordinary meals long ago combined these various
principles, the necessity and value of which are now explained.

QUESTIONS FOR TOPICAL REVIEW.

                                                                       PAGE
  1. What is understood by the term food?                                53
  2. What can you state in relation to the source of food?               53
  3. What discriminations and selections are necessary?              53, 54
  4. How can you tell the organic from the inorganic substances?         54
  5. What relative position does water hold as an article of food?       54
  6. In what parts of the body is water found?                       54, 55
  7. In what articles that we eat is it found?                           55
  8. If you were required to go without water or solid food for a
         number of days, which would you prefer to have, and why?        55
  9. What can you state of the importance of salt as an article
  of food?                                                           55, 56
  10. How abundant is salt, and how does it find its way into the
        human system?                                                55, 56
  11. What can you state of the importance of lime in the body?      56
  12. What, of the importance of iron?                               56, 57
  13. What further is stated of other inorganic substances?              57
  14. What in relation to organic substances?                            57
  15. What can you state in relation to the albuminoids?             57, 58
  16. What, in relation to albumen?                                      58
  17. What, in relation to casein?                                   58, 59
  18. In relation to the fats or oils, and how generally consumed?   59, 60
  19. What do we understand by the sugars or saccharine substances?      60
  20. State what you can of sugar--its origin and various qualities. 60, 61
  21. Of starch--its varieties and qualities.                            61
  22. Of the abundance of starch, and its importance as a food
        principle.                                                       61
  23. What is stated in relation to stimulating substances?              62
  24. Of the necessity for regulation in diet?                       62, 63

       *       *       *       *       *


{64}

CHAPTER V.

FOOD AND DRINK.

_Necessity for Food--Waste and Repair--Hunger and Thirst--Amount of
Food--Renovation of the Body--Mixed
Diet--Milk--Eggs--Meat--Cooking--Vegetable Food--Bread--The
Potato--Fruits--Purity of Water--Action of Water upon Lead--Coffee, Tea,
and Chocolate--Effects of Alcohol._

[Sidenote: 1. What follow activity? Examples? Necessity for food?]

1. NECESSITY FOR FOOD.--Activity is everywhere followed by waste. The
engine uses up coal and water to produce motion, the stream wears away its
bank, the growing corn-blade draws tribute from the soil. When the human
body acts, and it is always in action during life, some of its particles
are worn out and thrown off. This waste must constantly be repaired, or the
body suffers. In this fact is seen the necessity for food. The particles,
thus worn out, being henceforth useless, are removed from the body. Our
_food_ and _drink_ are rapidly transformed into a new supply of living,
useful material, to be in turn used up and replaced by a fresher supply.

[Sidenote: 2. Give the theory in relation to waste and repair.]

2. WASTE AND REPAIR.--In this way the healthful body, though always
wasting, is always building up, and does not greatly change in size, form,
or weight. At two periods of life the processes of waste and repair are not
exactly balanced. In early life the process of building up is more active,
and in consequence the form is plump, and the stature increases. Repair now
exceeds waste. On the other hand, when old age comes on, the wasting
process is more active, the flesh and weight diminish, the skin falls in
wrinkles, and the senses become dull. Only during the prime of life--from
about twenty to sixty years of age--is the balance exact between loss and
gain. {65}

[Sidenote: 3. System deprived of food? Warning? What is the pain? How
proved?]

3. HUNGER AND THIRST.--When the system is deprived of its supply of solid
food during a longer time than usual, nature gives warning by the sensation
of hunger, to repair the losses that have taken place. This sensation or
pain appears to be located in the stomach, but it is really a distress of
the system at large. Let a sufficient quantity of nourishment be introduced
into the system in any other way than by the mouth, and it will appease
hunger just as certainly as when taken in the usual manner.

[Sidenote: 4. Feeling of thirst? Seat of the pain? How proved? Time a
person can exist without food?]

4. The feeling of thirst, in like manner, is evidence that the system is
suffering from the want of water. The apparent seat of the distress of
thirst is in the throat; but the injection of water into the blood-vessels
is found to quench thirst, and by the immersion of the body in water, the
skin will absorb sufficient to satisfy the demands of the system. The
length of time that man can exist without food or drink is estimated to be
about seven days. If water alone be supplied, life will last much longer;
there being cases recorded where men have lived twenty days and over,
without taking any solid food.

[Sidenote: 5. Amount of food required? The young and others? Those living
in hot and cold climates? Habits?]

5. QUANTITY OF FOOD.--The quantity of food required varies greatly,
according to the individual and his mode of life. The young, and others who
lead active lives, or who live in the open air, require more food than the
old, the inactive, or the sedentary. Those who live in cold regions require
more than the inhabitants of hot climates. Habit, also, has much to do with
the quantity of food required. Some habitually eat and drink more than they
actually need, while a few eat less than they should.

[Sidenote: 6. Quantity of food daily? How divided? Compare with the weight
of the body?]

6. The average daily quantity of food and drink for a healthy man of active
habits is estimated at six pounds. This amount may be divided in about the
following {66} proportions: the mineral kingdom furnishes three and
one-half pounds, including water and salt; the vegetable kingdom, one and
one-half pounds, including bread, vegetables, and fruits; the animal
kingdom, one pound, comprising meat, eggs, butter, and the like. This
quantity is about one twenty-fourth the weight of the body, as it is
generally computed; the average weight of an adult man being placed at 140
pounds. A man, therefore, consumes an amount of solid and liquid nutriment
every twenty-four days equal in weight to that of his body, a corresponding
amount being _excreted_, or removed from the system in the same time.

[Sidenote: 7. How often then might the body be renewed? Why is it not?
Opinion? How correct? What further is stated?]

7. RENOVATION OF THE BODY.--By this process, so far as weight is concerned,
the body might be renewed every twenty-four days; but these pounds of food
are not all real nutriment. A considerable portion of that which we eat is
innutritious, and though useful in various ways, is not destined to repair
the losses of the system. An opinion has prevailed that the body is renewed
throughout once in seven years; how correct this may be it is not easy to
decide, but probably the renovation of the body takes place in a much
shorter period. Some parts are very frequently renewed, the nutritive
fluids changing more or less completely, several times during the day. The
muscles, and other parts in frequent exercise, change often during a year;
the bones not so often, and the enamel of the teeth probably never changes
after being once fully formed.

[Sidenote: 8. Habits of nations? Give the different cases.]

8. MIXED DIET.--The habits of different nations in respect to diet exhibit
the widest and strangest diversity. The civilized, cook their food, while
savages often eat it in a raw state. Some prefer it when fresh, others
allow it to remain until it has become tainted with decay. Those dwelling
in the far north subsist almost wholly on {67} animal food, while those
living in hot climates have bountiful supplies of delicious fruits with
which to satisfy all their bodily wants. One race subsists upon the banana,
another upon the blubber of seals. In temperate climates, a diet composed
partly of vegetable and partly of animal food is preferred.

[Sidenote: 9. The point to consider? Vegetable diet? Louis Cornaro? John
the Baptist?]

9. The important point to consider is, however, not one of origin, but
whether the chemical principles (mentioned in the last chapter) enter into
the composition of the diet. A purely vegetable diet may be selected which
would contain all the principles necessary to sustain life. It is recorded
of Louis Cornaro, a Venetian noble, that he supported himself comfortably
for fifty-eight years on a daily allowance of twelve ounces of vegetable
food, and about a pint of light wine. On the other hand, the food of John
the Baptist, consisting of "locusts and wild honey," is an example of the
sustaining power of a diet chiefly animal in its origin.

[Sidenote: 10. What has been found in our climate? Exclusive vegetable
diet?]

10. In our climate, those who lead active lives crave an allowance of
animal food; and it has been found by experience that with it they can
accomplish more work and are less subject to fatigue, than without it.
Among nations where an exclusively vegetable diet is employed, indigestion
is a disorder especially prevalent.

[Sidenote: 11. Necessity for change in diet? Continuous use of the same
diet? Exception? Why? Too rich diet? Horses?]

11. The necessity for occasionally changing or varying the diet, is seen in
the fact that no single article comprises all the necessary principles of
food, and that the continuous use of any one diet, whether salt or fresh,
is followed by defective nutrition and disease. There is one exception to
this rule: in infancy, milk alone is best calculated to support life; for
then the digestive powers are incompletely developed, and the food must be
presented in the simplest form possible. It should also be remembered {68}
that too rich diet is injurious, just as truly as one that is inadequate.
When the food of horses is too nutritious, instinct leads them to gnaw the
wood-work of their mangers.

[Sidenote: 12. Milk as a model food? Cow's milk? The constituents when
separated?]

12. DIFFERENT ARTICLES OF DIET--MILK.--Milk is the earliest nutriment of
the human race, and in the selection and arrangement of its constituents,
may be regarded as a model food, no other single article being capable of
sustaining life so long. Cow's milk holds casein, one of the albuminoids,
about five parts in one hundred; a fatty principle, when separated, known
as butter, about four parts; sugar of milk four parts; water and salts
eighty-seven parts. The casein and fatty substance are far more digestible
in milk, than after they have been separated from it in the form of cheese
and butter.

[Sidenote: 13. Milk as a beverage? Milk sold in cities? How to detect the
cheat?]

13. Since milk, in itself, is so rich an article of food, the use of it as
a beverage is unwise, unless the quantity of the other articles consumed be
reduced at the same time. The milk sold in cities is apt to be diluted with
water. The way to detect the cheat is by testing the specific gravity of
the article. Good milk is about 1030; skimmed milk 1035; but milk diluted
one-fifth is 1024. An instrument called the lactometer is also used, by
which the amount of cream present is ascertained.

[Sidenote: 14. Composition of eggs? Yolk? How should eggs be eaten? Why?
How boiled? Why?]

14. EGGS.--The egg is about two-thirds water, the balance is pure albumen
and fat in nearly equal proportions. The fat is in the yolk, and gives it
its yellow color. Eggs contain none of the sugar-principles, and should be
eaten with bread or vegetables that contain them. Soft-boiled eggs are more
wholesome than those which are hard-boiled or fried, as the latter require
longer time to digest.

[Sidenote: 15. Meats, whence derived? Why important? Flesh of young
animals?]

15. MEATS.--The meats, so called, are derived from the muscular parts of
various animals. They are most {69} important articles of food for adults,
inasmuch as they are richly stored with albuminoid substances, and contain
more or less fat. Such food is very nourishing and easily digested if eaten
when fresh,--veal and pork being exceptions. The flesh of young animals is
more tender and, in general, more digestible than that of older ones. All
meat is more tough immediately after the killing of the animal, but
improves by being kept a certain length of time.

[Sidenote: 16. Preference of persons? Venison? Mutton? Cheese? Uncooked
flesh?]

16. Some persons prefer flesh that has begun to show signs of
decomposition, or is unmistakably putrid. By some, venison is not
considered to have its proper flavor until it is tainted. In England,
people prefer mutton that is in a similar condition, just as on the
continent of Europe many delight in cheese that is in a state of
decomposition. In certain less civilized countries flesh is not only eaten
uncooked, but in a mouldy, rotten condition. The use of such food is not
always immediately injurious, but it predisposes to certain diseases, as
indigestion and fevers.

[Sidenote: 17. Cold as a preserver? Meat in Russia? Beef and pork, how
preserved? Salted meat as food? Scurvy?]

17. Cold is one means of preserving meat from decay. In the markets of
northern Russia, the frozen carcases of animals stand exposed for sale in
the winter air for a considerable time, and are sawn in pieces, like sticks
of wood, as the purchases are made; such meat, when thawed, being entirely
fit for food. Beef and pork are preserved by salting down in brine, and in
this condition may be carried on long voyages or kept for future use.
Salted meat is not as nutritious as fresh, since the brine absorbs its rich
juices and hardens its fibres. Long continued use of salt meats, without
fresh vegetables, gives rise to the disease called scurvy, formerly very
prevalent on shipboard and in prisons; but now scarcely known.

[Sidenote: 18. The antiquity of the custom of cooking food? Object of
cooking? The oyster? Raw meat as an occasional food?]

18. COOKING.--The preparation of food by the agency {70} of fire is of
almost universal practice, even among the rudest nations. The object of
cooking is to render food more easy of digestion by softening it, to
develop its flavor, and to raise its temperature more nearly to that of the
body. A few articles of flesh-food are eaten uncooked in civilized lands,
the oyster being an instance. Raw meat is occasionally eaten by invalids
with weak digestive powers, and by men training for athletic contests.

[Sidenote: 19. Effect of boiling meat? How may the cooking be done? The
proper method? Effect? Making of soup?]

19. In boiling meat, the water in which it is placed tends to dissolve its
nutrient juices. In fact, the cooking may be so conducted as to rob the
meat of its nourishment, its tenderness, and even of its flavor. The proper
method, in order to preserve or promote these qualities, is to place the
meat in boiling water, which, after a few minutes, should be reduced in
temperature. In this way the intense heat, at first, coagulates the
exterior layers of albumen, and imprisons the delicate juices; after that,
moderate heat best softens it throughout. When soup is to be made, an
opposite course should be pursued; for then the object is to extract the
juices and reject the fibre. Meat, for such purpose, should be cut in small
pieces and put into cold water, which should then be gradually raised to
boiling heat.

[Sidenote: 20. Roasting? How should it be done? Give the philosophy of the
process. Frying?]

20. Roasting is probably the best method of cooking meat, especially
"joints" or large pieces, as by this process the meat is cooked in its own
juices. Roasting should begin with intense heat, and be continued at a
moderate temperature, in order to prevent the drying out of the nutritious
juices, as by this process an outer coating or crust of coagulated albumen
is formed. During this process the meat loses one-fourth of its weight, but
the loss is almost wholly water, evaporated by the heat. Too {71} intense
or prolonged heat will dry the meat, or burn it. Frying is the worst
possible method, as the heated fat, by penetrating the meat, or other
article placed in it, dries and hardens it, and thus renders it
indigestible.

[Sidenote: 21. What is "Trichina?" How guarded against?]

21. TRICHINA.--It should be remembered that ham, sausages, and other forms
of pork, should never be eaten in a raw or imperfectly cooked condition.
The muscle of the pig is often infested by a minute animal parasite, or
worm, called _trichina spiralis_. This worm may be introduced alive into
the human body in pork food, where it multiplies with great rapidity, and
gives rise to a painful and serious disease. This disease has been
prevalent in Germany, and cases of it occur from time to time in this
country.

[Sidenote: 22. What part of fish is eaten? What does it resemble? Fish as
food for digestion? Fish as a diet?]

22. FISH.--The part of fish that is eaten is the muscle, just as in the
case of the meats and poultry. It closely resembles flesh in its
composition, but is more watery. Some varieties are very easy of digestion,
such as salmon, trout, and cod; others are quite indigestible, especially
lobsters, clams, and shell-fish generally. A diet in which fish enters as
the chief article, is ill adapted to strengthen mind or body, while its
continued use is said to be the fertile source of nearly every form of
disease of the skin. Some persons are so constituted that they can eat no
kind of fish without experiencing unpleasant results.

[Sidenote: 23. List of vegetable articles? Usefulness of the different
vegetables? Strychnia? What further is said in relation to the nourishing
and other qualities of vegetables?]

23. VEGETABLE FOOD.--The list of vegetable articles of diet is a very long
one, including the grains from which our bread-stuffs are made, the
vegetables from the garden, and the fruits. All the products of the
vegetable kingdom are not alike useful. Some are positively hurtful;
indeed, the most virulent poisons, as strychnia and prussic acid, are
obtained from certain vegetables. Again, of such {72} articles as have been
found good for food, some are more nourishing than others: some require
very little preparation for use, while others are hard and indigestible,
and can only be used after undergoing many preparatory processes. Great
care must therefore be exercised, and many experiments made, before we can
arrive at a complete knowledge in reference to these articles of diet. Tea,
coffee, and other substances from which drinks are made, are of vegetable
origin.

[Sidenote: 24. Wheat? "Staff of life?" White flour? Hard-grain wheats?
Bolting? Graham bread?]

24. BREAD.--Wheat is the principal and most valuable kind of grain for the
service of man. Bread made from wheat-flour has been in use for many
hundreds of years, and on this account, as well as because of its highly
nourishing properties, has been aptly called "the staff of life." We never
become tired of good bread as an article of daily food.

The white kinds of flour contain more starch and less gluten than the
darker, and are therefore less nutritious. The hard-grain wheat yields the
best flour. In grinding wheat, the chaff or bran is separated by a process
called "bolting." Unbolted flour is used for making brown or Graham bread.

[Sidenote: 25. Leavened bread? Unleavened? Hot bread?]

25. The form of bread most easily digested is that which has been
"leavened," or rendered porous by the use of yeast, or by some similar
method. Unleavened bread requires much more mastication. Hot bread is
unwholesome, because it is not firm enough to be thoroughly masticated, but
is converted into a pasty, heavy mass that is not easily digested.

[Sidenote: 26. Wheaten bread? Bread and butter? Experiment on the dog?]

26. Wheaten bread contains nearly every principle requisite for sustaining
life, except fat. This is commonly added in other articles of diet,
especially in butter,--"bread and butter," consequently, forming an almost
perfect article of {73} food. The following experiment is recorded: "A dog
eating _ad libitum_ of white bread, made of pure wheat, and freely supplied
with water, did not live beyond fifty days. He died at the end of that time
with all the signs of gradual exhaustion." Death took place, not because
there was anything hurtful in the bread, but because of the absence of one
or more of the food-principles.

[Sidenote: 27. State what is said of the Irish potato?]

27. THE POTATO.--The common or Irish potato is the vegetable most
extensively used in this country and Great Britain. Among the poorer
classes in Ireland it is the main article of food. While it is not so rich
in nutritious substances as many others, it has some very useful qualities.
It keeps well from season to season, and men do not weary of its continuous
use. It is more than two-thirds water, the balance being chiefly starch,
with a little albumen.

[Sidenote: 28. Sweet potato? Nightshades? Potatoes when germinating?]

28. The sweet potato differs from the white or common, in containing more
water and a small proportion of sugar. The common potato and the tomato
belong to the same botanical order as the "nightshades," but do not possess
their poisonous qualities, unless we except potatoes that are in the
process of germination or sprouting, when they are found injurious as food.

[Sidenote: 29. Fruits? Use of ripe fruit? Nutriment they contain? Starch in
unripe fruits? Cooking of unripe fruits?]

29. FRUITS.--These are produced, in this country, in great abundance, and
are remarkable alike for their variety and delicious flavor; consequently
they are consumed in large quantities, especially during the warmer months.
The moderate use of ripe fruits, in their season, is beneficial, because
they offer a pleasant substitute for the more concentrated diet that is
used in cold weather. The amount of solid nutriment they contain is,
however, small. The percentage of water in cherries is seventy-five, in
grapes eighty-one, in apples eighty-two. Unripe fruits contain starch,
which, during the process of ripening, {74} is converted into sugar. Such
fruits are indigestible and should be avoided: cooking, however, in part
removes the objections to them.

[Sidenote: 30. How should drinking-water be as regards color and smell?
Chemically pure water? How obtained? Agreeableness of perfectly pure
water?]

30. PURE WATER.--It is important that the water we drink and use in the
preparation of food should be pure. It should be clear and colorless, with
little or no taste or smell, and free from any great amount of foreign
ingredients. Chemically pure water does not occur in nature: it is only
obtained by the condensation of steam, carefully conducted, and is not as
agreeable for drinking purposes as the water furnished by springs and
streams. Rain-water is the purest occurring in nature; but even this
contains certain impurities, especially the portion which falls in the
early part of a shower; for in its descent from the clouds, the particles
floating in the air are caught by the falling drops.

[Sidenote: 31. Spring and well water? Whence the sparkle, or life? The
water supply of cities? Croton water? Ridgewood?]

31. Water from springs and wells always contains more or less foreign
matter of mineral origin. This imparts to the drink its pleasant taste--the
sparkle, or "life," coming from the gases absorbed by the water during its
passage under ground. The ordinary supply of cities is from some pure
stream or pond conveyed from a distance through pipes, the limpid fluid
containing generally only a small amount of impurity. Croton water, the
supply of New York City, is very pure, and contains only four and a half
grains to a gallon: the Ridgewood water of Brooklyn holds even less foreign
matter.

[Sidenote: 32. Impurities in drinking-water? Mineral springs?]

32. Drinking-water may contain as large a proportion as sixty to seventy
grains per gallon of impurity, but a much larger quantity renders it
unwholesome. The mineral spring waters, used popularly as medicines, are
highly charged with mineral substances. Some of them, such as {75} the
waters at Saratoga, contain three hundred grains and more to the gallon.

[Sidenote: 33. What is stated of the action of water upon lead?]

33. ACTION OF WATER UPON LEAD.--The danger of using water that has been in
contact with certain metals is well known. Lead is one of the most readily
soluble, and probably the most poisonous of these substances in common use.
When pure water and an untarnished surface of lead come in contact, the
water gradually corrodes the metal, and soon holds an appreciable quantity
of it in solution. When this takes place the water becomes highly
injurious: the purer the water, and the more recent the use of the metal,
the greater will be the danger.

[Sidenote: 34. Lead in pipes and other things? Advice? What takes place
after the articles of lead have been used much? What is wise?]

34. In cities, lead pipes are commonly used to convey water through the
houses; lead being also used in the construction of roofs, cisterns, and
vessels for keeping water and other liquids. After the articles of lead
have been in use several months, the danger of lead-poisoning diminishes.
An insoluble coating of the sulphate of lead forms upon the exposed
surface, thus protecting it from further corrosion. It is, however, a wise
precaution, at all times to reject the water or other fluid that has been
in contact with leaden vessels over night, or for a number of hours. Allow
the water in pipes to run freely before using.

[Sidenote: 35. Coffee as an article of diet? Of what does it consist? How
does the water affect the coffee? The peculiar stimulant? How does it
affect most persons?]

35. COFFEE.--This is an important addition to diet, and if moderately used
is beneficial to persons of adult age. As commonly employed, it consists of
an infusion in boiling water of the roasted and ground berry. The water
extracts certain flavoring and coloring matters, but that which gives it
its peculiar stimulant qualities is the alkaloid _caffein_. With most
persons its action is that of a gentle stimulant, without any injurious
reaction. It produces a restful feeling after exhausting efforts of mind or
{76} body; it tranquilizes, but does not disqualify for labor; and hence it
is highly esteemed by persons of literary pursuits.

[Sidenote: 36. Another property of coffee? Miners of Belgium? The Caravans?
Among armies? Taken with meals?]

36. Another property of coffee is, that it diminishes the waste of the
tissues, and consequently permits the performance of excessive labor upon
an economical and inadequate diet. This has been tested among the miners of
Belgium. Their allowance of solid food was below that found necessary in
prisons and elsewhere; but, with the addition of about four pints of coffee
daily, they were enabled to undergo severe labor without reducing their
muscular strength. The caravans which traverse the deserts are supported by
coffee during long journeys and lengthened privation of food. Among armies
it is indispensable in supplementing their imperfect rations, and in
relieving the sense of fatigue after great exposure and long marches. When
taken with meals, coffee is also thought to promote digestion.

[Sidenote: 37. Effects of tea-drinking? Peculiar principle? The tea
beverage, how made? Black and green tea? Excessive use of tea or coffee?]

37. TEA.--The effects of tea-drinking are very similar to those of coffee,
and are due to a peculiar principle called _thein_. This principle is
probably the same as that found in coffee, _caffein_, since the chemical
composition of both is precisely alike. Tea, as a beverage, is made from
the dried leaves of the plant by the addition of hot water; if the tea is
boiled, the oil which gives it its agreeable flavor is driven off with the
steam. There are two kinds of tea--the black and the green: the latter is
sometimes injurious, producing wakefulness and other nervous symptoms. The
excessive use of either coffee or tea will cause wakefulness.

[Sidenote: 38. Experiments made during Kane's expedition?]

38. During Dr. Kane's expedition in the Arctic regions, the effects of
these articles were compared. "After {77} repeated trials, the men took
most kindly to coffee in the morning and tea in the evening. The coffee
seemed to continue its influence throughout the day, and they seemed to
grow hungry less rapidly than after drinking tea, while tea soothed them
after a day's hard labor, and the better enabled them to sleep. They both
operated upon fatigued men like a charm, and their superiority over
alcoholic stimulants was very decided."

[Sidenote: 39. State what is said of chocolate.]

39. Chocolate is made from the seeds of the cocoa-tree, a native of
tropical America. Its effects resemble somewhat those of tea and coffee,
but it is very rich in nutriment. Linnæus, the botanist, was so fond of
this beverage, that he gave to the cocoa-tree the name, _Theobroma_--"the
Food of the Gods." Its active principle is _theobromin_.

[Sidenote: 40. Use of alcoholic drinks, how general? The rule given?]

40. ALCOHOL.--The list of beverages that are consumed for the sake of the
alcohol they contain is a very long one. Their use is almost universally
prevalent, every civilized nation, and nearly every barbarous one, having
its favorite alcoholic drink; and, as a general rule, the nations which
stand the highest in civilization have the greatest varieties of these
beverages,--at the same time using them the most intelligently and wisely.

[Sidenote: 41. The beverages produced by fermentation? The ardent spirits?
Grains and fruits employed? Long use of wine? Of distilled liquors?]

41. The wines and malt liquors that contain a small amount of alcohol are
produced by fermentation. The beverages that hold a large proportion of
alcohol, the "ardent spirits," are made by distillation. Enormous
quantities of grains and fruits are thus yearly diverted from their proper
uses as food; some of these being corn, wheat, rye, barley, potatoes, and
rice; also the grape, apple, pear, peach, sugar-cane, cherry, fig, and
orange. Wine, the fermented juice of the grape, has been in use from time
immemorial, while the introduction of distilled liquors dates from a
comparatively recent period. {78}

[Sidenote: 42. Describe the action of alcohol upon the human system?
Experience of Dr. Hayes and others?]

42. What is the physiological action of alcohol? Its first and most evident
action is stimulation: this effect is transient, and is followed by a
variable degree of depression. At first it sharpens the appetite and
quickens digestion, but its habitual use impairs both. This stimulation is
efficient in giving the system an artificial strength during some temporary
derangement, and in enabling the endurance of unusual fatigue or exposure.
The experience of Dr. Hayes, and other explorers of the polar regions, is
that alcohol does not enable the body to resist the influence of cold, but,
on the contrary, is always injurious.

[Sidenote: 43. Another property of alcohol? How do we explain the
restorative influence of wines and liquors?]

43. Another property it has in common with tea and coffee. It supports the
powers of life, economizes food, and retards the waste of tissues; in other
words, it "banks the fires," and prevents their burning wastefully. On this
principle we explain the restorative influence of wines or liquors during
exhausting diseases, in convalescence, and after excessive labors of mind
or body.

[Sidenote: 44. Alcohol, a poison? Moderate stimulants? Prevailing opinion?
Hence?]

44. Pure alcohol, or an excessive quantity of ardent spirits, is an
undoubted poison, and has been frequently known to produce fatal results.
Stimulants in moderate quantities have been thought to increase strength,
and in this view they have been called "alcoholic foods." This is not now
conceded by scientific men. The prevailing opinion is, that they serve no
useful purpose as an article of diet, and that their beneficial influence
is limited to cases where the system is enfeebled, where some unnatural
demand is made upon the vital powers, or where the supply of food is
insufficient. Hence, while alcohol has not the power to build up, it may
obstruct the process of pulling down. {79}

QUESTIONS FOR TOPICAL REVIEW.

                                                                       PAGE
  1. How is the necessity for food shown?                                64
  2. To what process of waste and repair is the body constantly
        subjected?                                                       64
  3. How do you account for the sensations of hunger and thirst?         65
  4. What further can you state having relation to the subject?          65
  5. What can you state in regard to the quantity of food required
        for the support of life?                                     65, 66
  6. What circumstances change the needs of persons, old and young,
        as regards food and drink?                                   65, 67
  7. What becomes of all the food and drink we consume?                  66
  8. What further can you state in relation to the process of
         renovation through which the body passes?                       66
  9. What can you state of the habits of nations in respect to
         diet?                                                   66, 67, 69
  10. What in relation to the selection of articles for food?            67
  11. What as respects the necessity for changing or varying the diet?   67
  12. What has been proved as regards animal food?                       67
  13. Of what importance is milk as an article of food?              67, 68
  14. What are the constituents of milk?                                 68
  15. What can you state of eggs as an article of food?                  68
  16. Of the meats, so called, as an article of food?                68, 69
  17. What effect does cold have upon meats?                             69
  18. In what other way may beef and pork be preserved?                  69
  19. What can you state of salted meat as food, and of its
        continued use?                                                   69
  20. What change does meat undergo in the cooking?                  70, 71
  21. What directions are given for boiling meat?                        70
  22. What for roasting, and with what results?                      70, 71
  23. What is said about the frying of meats?                            71
  24. Give the statement in relation to trichina.                        71
  25. State what is said in relation to fish.                            71
  26. What is stated of the usefulness and other properties of the
         products of the vegetable kingdom?                          71, 72
  27. What further is said of vegetable food?                        71, 72
  28. Why is bread made of wheat flour so important as an article
        of food?                                                         72
  29. State whatever else you can in relation to bread.              72, 73
  30. Give the statement respecting the potato.                          73
  31. What is stated of fruits, the use of them, their nutritious
        qualities, etc.?                                             73, 74
  32. How general is the existence of perfectly pure water?              74
  33. What is stated in relation to drinking water?                  74, 75
  34. How does the action of water upon lead affect lead?                75
  35. What further can you state on the subject?                         75
  36. What properties has coffee as an article of diet?              75, 76
  37. In what circumstances has coffee been found peculiarly
        beneficial?                                                      76
  38. What comparison is made between coffee, tea, and chocolate?        76
  39. How are the wines, and malt and other alcoholic beverages
        produced?                                                        77
  40. What articles are employed in their production?                    77
  41. Describe the physiological action of alcohol.                      78
  42. What comparison is made between tea, coffee, and alcohol?          78
  43. What can you state of alcohol, as a poison, a stimulant, and
        article of diet?                                                 78
  44. What, then, can be said of alcohol as a recommendation?            78

       *       *       *       *       *


{80}

CHAPTER VI.

DIGESTION.

_The Principal Processes of Nutrition--The General Plan of
Digestion--Mastication--The Teeth--Preservation of the
Teeth--Insalivation--The Stomach and the Gastric Juice--The Movements of
the Stomach--Gastric Digestion--The Intestines--The Bile and Pancreatic
Juice--Intestinal Digestion--Absorption by means of Blood-vessels and
Lacteals--The Lymphatic or Absorbent System--The Lymph--Conditions which
affect Digestion--The Quality, Quantity, and Temperature of the Food--The
Influence of Exercise and Sleep._

[Sidenote: 1. Design of food? How accomplished?]

1. NUTRITION.--The great design of food is to give _nutriment_ or
nourishment to the body. But this is not accomplished directly, as the food
must first pass through certain preparatory changes, as follows: (1),
_Digestion_, by which the food is reduced to a soluble condition; (2),
_Absorption_, by which, when digested, it is imbibed into the blood; (3),
_Circulation_, which carries the enriched blood to the various parts of the
system; and (4), _Assimilation_, by which each tissue derives from the
blood the materials necessary for its support.

[Sidenote: 2. Sustaining power of food? Simile of the engine? Operation in
the human body?]

2. By these means the sustaining power of food is gradually developed and
employed, and the vital machinery kept in working order, somewhat after the
manner of the steam-engine. To operate the latter, the force imprisoned
within the coal and water is set free and converted into motion by the
combustion of the fuel and the vaporization of the water. It will be seen,
however, when we come to study these operations in the human body, that
they are conducted silently and harmoniously, with marvellous delicacy and
completeness, and without that friction, and {81} consequent loss of power,
which attend the working of the most perfect machinery of man's invention.

[Sidenote: 3. Change of food in digestion? Process of digestion? Describe
the alimentary canal.]

3. GENERAL PLAN OF DIGESTION.--The great change which food undergoes in
digestion is essentially a reforming process, reducing articles of diet,
which are at first more or less solid, crude, and coarse, to a liquid and
finely comminuted condition, suitable for absorption into the blood. The
entire process of digestion takes place in what is called the alimentary
canal, a narrow, tortuous tube, about thirty feet in its entire length.
This canal begins in the mouth, extends thence downward through the gullet
to the stomach (a receptacle in which the principal work of digestion is
performed), and thence onward through the small and large intestines.

[Illustration: FIG. 16.--SECTION OF THE TRUNK SHOWING THE CAVITIES OF THE
CHEST AND ABDOMEN.

A, Cavity of Chest; B, Diaphragm; C, Abdomen; D, E, Spinal Column.]

[Sidenote: 4. Situation of the stomach and intestines? Action of the food?
Mechanical action? Chemical?]

4. The stomach and intestines are situated in the cavity of the abdomen
(Fig. 16, C, and Fig. 22), and occupy about two-thirds of its space. The
action to which the food is subjected in these organs is of two
kinds--mechanical and chemical. By the former it is crushed, agitated, and
carried onward from one point to another; by the latter it is changed in
form through the solvent power of the various digestive juices. {82}

[Sidenote: 5. Describe the process of mastication? How many and what
movements?]

5. MASTICATION.--As soon as solid food is taken into the mouth, it
undergoes mastication, or chewing. It is caught between the opposite
surfaces of the teeth, and by them is cut and crushed into very small
fragments. In the movements of chewing, the lower jaw plays the chief part;
the upper jaw, having almost no motion, acts simply as a point of
resistance, to meet the action of the former. These movements of the lower
jaw are of three sorts: a vertical or cutting, a lateral or grinding, and a
_to-and-fro_ or gnawing motion.

[Illustration: FIG. 17.--SECTION OF A TOOTH.

_a_, Enamel; _b_, Cavity; _c c_, Roots; _d_, Body of the Tooth.]

[Sidenote: 6. Composition of the teeth? Enamel of the teeth? Interior of
teeth?]

6. The teeth are composed of a bone-like material, and are held in place by
roots running deeply into the jaw. The exposed portion, or "crown," is
protected by a thin layer of enamel (Fig. 17, _a_), the hardest substance
in the body, and, like flint, is capable of striking fire with steel. In
the interior of each tooth is a cavity, containing blood-vessels and a
nerve, which enter it through a minute opening at the point of the root
(Fig. 19).

[Sidenote: 7. The milk teeth? The permanent teeth?]

7. There are two sets of teeth; first, those belonging to the earlier years
of childhood, called the milk teeth, which are twenty in number and small.
At six or eight years of age, when the jaw expands, and when the growing
body requires a more powerful and numerous set, the roots of {83} the milk
teeth are absorbed, and the latter are "shed," or fall out, one after
another (Fig. 18), to make room for the permanent set.

[Illustration: FIG. 18.--SECTION OF THE JAWS.

1' 2' 3' 4' 5', The Milk Teeth; 1" to 8", The Germs of the Permanent Set.]

[Sidenote: 8, 9. Number of teeth? How distributed?]

8. There are thirty-two teeth in the permanent set, as many being in one
jaw as the other. Each half-jaw has eight teeth, similarly shaped and
arranged in the same order: thus, two incisors, one canine, two bicuspids,
and three molars. The front teeth are small, sharp, and chisel-edged, and
are well adapted for cutting purposes; hence their name incisors. The
canines stand next, one on each side of the jaw; these receive their name
from their resemblance to the long, pointed tusks of the dog (Fig. 19).
{84}

[Illustration: FIG. 19.--SECTION OF THE JAWS--RIGHT SIDE.

V, A, N, Veins, Arteries, and Nerves of the Teeth. The root of one tooth in
each jaw is cut vertically to show the cavity and the blood-vessels, etc.,
within it. 1 to 8, Permanent Teeth.]

9. The bicuspids, next in order, are larger and have a broader crown than
the former; while behind them are the molars, the largest and most powerful
of the entire set. These large back teeth, or "grinders," present a broad,
rough surface, suitable for holding and crushing the food. The third molar,
or "wisdom tooth," is the last to be cut, and does not appear until about
the twenty-first year. {85} The order of arrangement of the teeth is
indicated by the following dental formula:--

[Illustration]

[Sidenote: 10. Different forms of teeth? Human teeth? The inference?]

10. It is interesting, at this point, to notice the different forms of
teeth in different animals, and observe how admirably their teeth are
suited to the respective kinds of food upon which they subsist. In the
_carnivora_, or flesh-feeders, the teeth are sharp and pointed, enabling
them both to seize their prey, and tear it in pieces; while the
_herbivora_, or vegetable-feeders, have broad, blunt teeth, with rough
crowns, suitable for grinding the tough grasses and grains upon which they
feed. Human teeth partake of both forms; some of them are sharp, and others
are blunt; they are therefore well adapted for the mastication of both
flesh and vegetables. Hence we argue that, although man may live
exclusively upon either vegetable or animal food, he should, when possible,
choose a diet made up of both varieties.

[Sidenote: 11. Cleaning of teeth? Effects of not cleaning?]

11. PRESERVATION OF THE TEETH.--In order that the teeth shall remain in a
sound and serviceable condition, some care is of course requisite. In the
first place, they require frequent cleansing; for every time we take food,
some particles of it remain in the mouth; and these, on account of the heat
and moisture present, soon begin to putrefy. This not only renders the
breath very offensive, but promotes decay of the teeth. {86}

[Sidenote: 12. Effects upon the saliva? Formation of tartar? How prevented?
How destroyed?]

12. The saliva, or moisture of the mouth, undergoes a putrefactive change,
and becomes the fertile soil in which a certain minute fungus has its
growth. This fluid, too, if allowed to dry in the mouth, collects upon the
teeth in the form of an unsightly, yellow concretion, called tartar. To
prevent this formation, and to remove other offensive substances, the teeth
should be frequently cleaned with water, applied by means of a soft
tooth-brush. The destruction of the tartar fungus is best effected by the
use of a weak solution of carbolic acid.

[Sidenote: 13. Destruction of the enamel? How guarded against?]

13. Again, it should be borne in mind that the enamel, Nature's protection
for the teeth, when once destroyed, is not formed anew; and the body of the
tooth thus exposed, is liable to rapid decay. On this account, certain
articles are to be guarded against; such as sharply acid substances that
corrode the enamel, and hard substances that break or scratch it--as gritty
tooth powders, metal tooth picks, and the shells of hard nuts. Sudden
alternations from heat to cold, when eating or drinking, also tend to crack
the enamel.

[Sidenote: 14. Mixing of food with the saliva? What is the saliva? How
secreted? The salivary glands?]

14. INSALIVATION.--When the morsel of food is cut and ground by the teeth,
it is at the same time also intimately mixed with the saliva, or fluids of
the mouth. This constitutes the second step of digestion, and is called
insalivation. The saliva, the first of the digestive solvents, is a
colorless, watery, and frothy fluid. It is secreted (_i. e._ separated from
the blood) partly by the mucous membrane which lines the mouth; but chiefly
by the salivary glands, of which there are three pairs situated near the
mouth.

[Sidenote: 15. The flow of saliva? The thought of food? Anxiety and grief?
Animals fed upon dry and coarse food?]

{87} 15. These glands consist of clusters of very small pouches, around
which a delicate network of blood-vessels is arranged: they empty into the
mouth by means of little tubes, or ducts. The flow from these glands is at
all times sufficient to maintain a soft and moist condition of the tongue
and mouth; but when they are excited by the presence and taste of food,
they pour forth the saliva more freely. Even the mere thought of food will
at times cause the saliva to flow, as when the appetite is stimulated by
the sight or smell of some savory article; so that the common expression is
correct that "the mouth waters" for the favorite articles of food. Anxiety
and grief prevent its flow, and cause "the tongue to cleave to the roof of
the mouth." In the horse and other animals, that feed upon dry and coarse
fodder, and require an abundant supply of saliva, we find large salivary
glands, as well as powerful muscles of mastication.

[Illustration: FIG. 20.--STRUCTURE OF A SALIVARY GLAND.]

[Illustration: FIG. 21.--THE HEAD OF A HORSE, showing the large salivary
gland (_a_), its duct (_b_), the muscles of mastication (_c_, _d_, _e_,
_f_, and _g_).]

[Sidenote: 16. Importance of the process? The first place? The second? The
third?]

16. The mingling of the saliva with the food seems a simple process, but it
is one that plays an important part {88} in digestion. In the first place,
it facilitates the motions of mastication, by moistening the food and
lubricating the various organs of the mouth. Secondly, it prepares the way
for other digestive acts: by the action of the teeth, the saliva is forced
into the solid food, softens the harder substances, and assists in
converting the whole morsel into a semi-solid, pulpy mass, that can be
easily swallowed, and readily permeated by other digestive fluids. The
saliva also, by dissolving certain substances, as sugar and salt, develops
the peculiar taste of each; whereas, if the tongue be dry and coated, they
are tasteless. Hence, if substances are insoluble, they are devoid of
taste.

[Sidenote: 17. Its final importance? Starch? How effected? Ptyalin?]

17. Finally, the saliva has the property of acting chemically upon the
food. As we have before stated (Chap. IV.), starch, as starch, cannot enter
the tissues of the body; but, in order to become nutriment, must first be
changed to grape sugar. This change is, in part, effected by the saliva,
and takes place almost instantly, whenever it comes in contact with cooked
starch. This important function is due to an organic ingredient of the
saliva called _ptyalin_. This substance has been extracted from the saliva
by the chemist, and has been found, by experiment, to convert into sugar
two thousand times its own weight of starch.

[Sidenote: 18. Each of the processes? Why is a knowledge of the digestive
functions important? How shown?]

18. IMPORTANCE OF MASTICATION AND INSALIVATION.--Each of these processes
complements the other, and makes the entire work available; for, by their
joint action, they prepare the food in the best possible manner for further
digestive changes. The knowledge of these preliminary functions will appear
the more important, when we reflect that they are the only ones which we
can regulate by the will. For, as soon as the act of swallowing begins, the
food not only passes out of sight, but beyond {89} control; and the
subsequent acts of digestion are consequently involuntary and unconsciously
performed.

[Sidenote: 19. Rapid eating? Describe the process and effects.]

19. It is generally known that rapid eating interferes with digestion. How
does this occur? In the first place, in rapid eating, the flow of the
saliva is insufficient to moisten the solid parts of the food, so that they
remain too hard and dry to be easily swallowed. This leads to the free and
frequent use of water, or some other beverage, at meals, to "wash down" the
food,--a most pernicious practice. For these fluids, not only cannot take
the place of the natural digestive juices, but, on the contrary, dilute and
weaken them.

[Sidenote: 20. Loss of taste? Another effect of rapid eating? Mistakes?]

20. Secondly, the saliva being largely the medium of the sense of taste,
the natural flavors of the food are not developed, and consequently it
appears comparatively insipid. Hence the desire for high-seasoned food, and
pungent sauces, that both deprave the taste and over excite the digestive
organs. Rapid eating also permits the entrance of injurious substances
which may escape detection by the taste, and be unconsciously received into
the system. In some instances, the most acrid and poisonous substances have
frequently been swallowed "by mistake," before the sense of taste could
act, and demand their rejection.

[Sidenote: 21. Effect of imperfectly broken food in the stomach? Dyspepsia?
Overeating?]

21. Thirdly, the food, being imperfectly broken up by the teeth, is hurried
onward to the stomach, to be by it more thoroughly divided. But the task
thus imposed upon the stomach, it is not at all adapted to perform; so that
the crude masses of food remain a heavy burden within the stomach, and a
source of distress to that organ, retarding the performance of its
legitimate duty. Hence persons who habitually eat too rapidly, frequently
fall victims to dyspepsia. Rapid eating also conduces to overeating. The
food is introduced so rapidly, that the system has not {90} time to
recognize that its real wants are met, and that its losses have been made
good; and hence the appetite continues, although more nutriment has been
swallowed than the system requires, or can healthfully appropriate.

[Illustration: FIG. 22.--SECTION OF CHEST AND ABDOMEN.

A, Heart. B, The Lungs. C, Stomach. D, The Liver. E, Large Intestine.
G, Small Intestine.]

[Sidenote: 22. Gullet? Describe the stomach and its location. Effects of
gormandizing?]

22. THE STOMACH.--As soon as each separate portion of food is masticated
and insalivated, it is swallowed; that is, it is propelled downward to the
stomach, through a narrow muscular tube about nine inches in length, called
the _oesophagus_, or gullet (Fig. 23). The stomach is the only large
expansion of the digestive canal, and is the most important organ of
digestion. It is a hollow, pear-shaped a pouch, having a capacity of three
pints, in the adult. Its walls are thin and yielding, and may become
unnaturally distended, as in the case of those who subsist on a bulky,
innutritious diet, and of those who habitually gormandize.

[Sidenote: 23. Heart-orifice? Gatekeeper? Coins, etc.? Indication of the
soft and yielding texture of the stomach?]

23. The stomach has also two openings; that by which food enters, being
situated near the heart, is called the _cardiac_, or heart orifice; the
other is the _pylorus_, or "gatekeeper," which guards the entrance to the
intestines, {91} and, under ordinary circumstances, permits only such
matters to pass it as have first been properly acted upon in the stomach.
Coins, buttons, and the like are, however, readily allowed to pass, because
they can be of no use if retained. The soft and yielding texture of this
organ--the stomach--indicates that it is not designed to crush and
comminute solid articles of food.

[Illustration: FIG. 23.--THE ORGANS OF DIGESTION.

O, Oesophagus. S, Stomach. L, Liver. M, Pylorus. C, Large Intestines.
P, Pancreas. I, Small Intestines. N, Spleen. G, Gall-bladder. ]

[Sidenote: 24. What is meant by the gastric juice?]

24. THE GASTRIC JUICE.--We have seen how the presence of food in the mouth
excites the salivary glands, causing the saliva quickly to flow. In the
same manner, when food reaches the stomach, its inner lining, the mucous
membrane, is at once excited to activity. (At first its surface, which
while the stomach is empty presents a pale pink hue, turns to a bright red
color, for the minute blood-vessels which course through it, are filled
with blood. Presently a clear, colorless, and acid fluid exudes, drop by
drop, from its numerous minute glands or "tubules," until finally the
surface is moistened in every part, and the fluid begins to mingle with the
food. This fluid is termed the gastric juice. {92}

[Sidenote: 25. What is the office of the gastric juice? Acidity of the
gastric juice? Quantity of gastric juice used? What becomes of it?]

25. The gastric juice is the proper solvent of certain articles of food,
especially those belonging to the albuminoid class. This solvent power is
due to its peculiar ingredient, _pepsin_; in digestion, this substance acts
like a ferment; that is, it induces changes in the food simply by its
presence, but does not itself undergo change. The acidity of the gastric
juice, which is due to _lactic acid_, is not accidental; for we find that
the pepsin cannot act in an alkaline solution--that is, one which is not
acid or neutral. The quantity of gastric juice secreted daily is very
large, probably not less than three or four pints at each meal. Though this
fluid is at once used in the reduction of the food, it is not lost; since
it is soon re-absorbed by the stomach, together with those parts of the
food which it has digested and holds in solution.

[Sidenote: 26. Muscular coat of the stomach? Expansion and contraction of
its fibres? Action of the fibres?]

26. MOVEMENTS OF THE STOMACH.--The inner coating of the stomach is the
mucous membrane, which, as we have seen, furnishes the gastric juice. Next
to this coating lies another, called the muscular coat, composed of
involuntary muscular fibres, some of which run circularly, and others in a
longitudinal direction. These expand to accommodate the food as it is
introduced, and contract as it passes out. In addition, these fibres are in
continual motion while food remains in the stomach, and they act in such
manner that the contents are gently turned round from side to side, or from
one end of it to the other.

[Sidenote: 27. Peristaltic movements? What is said of our consciousness of
and power over these movements? Describe the movements of the pylorus.]

27. By these incessant movements of the stomach, called the _peristaltic_
movements, the gastric juice comes in contact with all parts of the food.
We are, however, not conscious that these movements take place, nor have we
the power to control them. When such portions of the food as are
sufficiently digested approach the pylorus, it {93} expands to allow them
to pass out, and it closes again to confine the residue for further
preparation.

[Sidenote: 28. How has the knowledge and the workings of the stomach been
ascertained? St. Martin? How else?]

28. The knowledge of these and other interesting and instructive facts has
been obtained by actual observation; the workings of the stomach of a
living human being have been laid open to view and examined--the result of
a remarkable accident. Alexis St. Martin, a Canadian _voyageur_, received a
gun-shot wound which laid open his stomach, and which, in healing, left a
permanent orifice nearly an inch in diameter. Through this opening the
observer could watch the progress of digestion, and experiment with
different articles of food. Since that occurrence, artificial openings into
the stomach of the inferior animals have been repeatedly made, so that the
facts of stomach-digestion are very well ascertained and verified.

[Sidenote: 29. What was formerly thought? What do we now know? What else do
we now know? Water, salt, and sugar? Absorption?]

29. GASTRIC DIGESTION.--What portions of the food are digested in the
stomach? It was formerly thought that all the great changes of digestion
were wrought here, but later investigation has taught us better. We now
know that the first change in digestion takes place in the mouth, in the
partial conversion of starch into sugar. We also know that, of the three
organic food principles (considered in Chapter IV.) two--the fats and the
sugars--are but slightly affected by the stomach; but that its action is
confined to that third and very important class, from which the tissues are
renewed, the albuminoids. A few articles need no preparation before
entering the system, as water, salt, and grape-sugar. These are rapidly
taken up by the blood-vessels of the stomach, which everywhere underlie its
mucous membrane in an intricate and most delicate network. In this way the
function of absorption begins.

[Sidenote: 30. Albuminose? The process? Chyme?]

30. The albuminoid substances are speedily attacked and {94} digested by
the gastric juice. From whatever source they are derived, vegetable or
animal, they are all transformed into the same digestive product, called
_albuminose_. This is very soluble in water, and is readily absorbed by the
blood-vessels of the stomach. After a longer or shorter time, varying from
one to five hours, according to the individual and the quantity and quality
of his food, the stomach will be found empty. Not only has the digested
food passed out, but also those substances which the stomach could not
digest or absorb have passed little by little through the pylorus, to
undergo further action in the intestines. At the time of its exit, the
digested food is of a pulpy consistence, and dark color, and is then known
as the _chyme_.

[Sidenote: 31. What are the intestines? The small intestines? The large
intestines? Their structure?]

31. THE INTESTINES.--The intestines, or "bowels," are continuous with the
stomach, and consist of a fleshy tube, or canal, twenty-five feet in
length. The small intestine, whose diameter is about one inch and a half,
is twenty feet long and very tortuous. The large intestine is much wider
than the former, and five feet long (Fig. 23). The general structure of
these organs resembles that of the stomach. Like it, they are provided with
a mucous membrane, or inner lining, whence flow their digestive juices;
and, just outside of this, a muscular coat, which propels the food onward
from one point to another.

[Sidenote: 32. Peritoneum? The work of digestion?]

32. Moreover, both the intestines and stomach are enveloped in the folds of
the same outer tunic or membrane, called the _peritoneum_. This is so
smooth and so well lubricated, that the intestines have the utmost freedom
of motion. In the small intestines, the work of digestion is completed, the
large intestine receiving from them the indigestible residue of the food,
and in time expelling it from the body.

[Sidenote: 33. The presence of food in the intestines? Bile?]

33. INTESTINAL DIGESTION.--As soon as the food passes the pylorus and
begins to accumulate in the upper {95} part of the intestines, it excites
the flow of a new digestive fluid, which enters through a small tube, or
_duct_, about three inches below the stomach. It is formed by the union of
two distinct fluids--the _bile_ and the _pancreatic_ juice. The bile is
secreted by the liver, which is the largest gland of the body, and is
situated on the right side and upper part of the abdomen (Fig. 22). The
bile is constantly formed, but it flows most rapidly during digestion.
During the intervals of digestion it is stored in the _gall-bladder_, a
small membranous bag attached to the under side of the liver. This fluid is
of a greenish-yellow color, having a peculiar smell, and a very bitter
taste.

[Sidenote: 34. The pancreatic juice? The joint action of these fluids?]

34. The pancreatic juice is the product of a gland called the _pancreas_,
situated behind the stomach. This fluid is colorless, viscid, and without
odor. Like the digestive juices previously described, it owes its solvent
power to its peculiar ferment principle, called _pancreatin_. By the joint
action of these fluids, the fatty parts of the food are prepared for
absorption. By previous steps of digestion the fats are merely separated
from the other components of the food; but here, within the intestines,
they are reduced to a state of minute division, or _emulsion_, resembling
the condition of butter in milk, before it has been churned. There results
from this action a white and milky fluid, termed the _chyle_, which holds
in solution the digestible portions of the food, and is spread over the
extensive absorbent surface of the small intestines.

[Sidenote: 35. The mucous membrane? Experiments on inferior animals?]

35. The mucous membrane of the intestines, also, secretes or produces, a
digestive fluid by means of numerous "follicles," or minute glands; this is
called the intestinal juice. From experiments on the inferior animals, it
has been ascertained that this fluid exerts a solvent influence over each
of the three organic food principles, and in this way may supplement and
complete the action of the {96} fluids previously mentioned, viz.:--of the
saliva in converting starch into sugar, of the gastric juice in digesting
the albuminoids, and of the pancreatic juice and bile in emulsifying the
fats.

[Sidenote: 36, 37. How much thus far has been done with the food? The next
process? Give the first way.]

36. ABSORPTION.--With the elaboration of the chyle, the work of digestion
is completed; but, in a certain sense, the food is yet outside of the body;
that is, the blood is not yet enriched by it, and it is not in a position
to nourish the tissues. The process by which the liquefied food passes out
of the alimentary canal into the blood is called absorption. This is
accomplished in two ways; first, by the _blood-vessels_. We have seen how
the inner membrane of the stomach is underlaid by a tracery of minute and
numerous vessels, and how some portions of the food are by them absorbed.
The supply of blood-vessels to the intestines is even greater; particularly
to the small intestines, where the work of absorption is more actively
carried on.

37. The absorbing surface of the small intestines, if considered as a plane
surface, amounts to not less than half a square yard. Besides, the mucous
membrane is formed in folds with an immense number of thread-like
prolongations, called _villi_, which indefinitely multiply its absorbing
capacity. These minute processes, the villi, give the surface the
appearance and smoothness of velvet; and during digestion, they dip into
the canal, and, by means of their blood-vessels, absorb its fluid contents,
just as the _spongioles_ which terminate the rootlets of plants, imbibe
moisture from the surrounding soil.

[Sidenote: 38. How is absorption effected in another way? Describe it. Name
of the lacteals? Thoracic duct?]

38. Secondly, absorption is also effected by the _lacteals_, a set of
vessels peculiar to the small intestines. These have their beginnings in
the little villi just mentioned, side by side with the blood-vessels. These
two sets of absorbents run in different courses, but their destination is
the same, {97} which is the right side of the heart. The lacteals receive
their name from their milky-white appearance. After a meal containing a
portion of fat, they are then distended with chyle, which they are
specially adapted to receive: at other times they are hardly discernible.
The lacteals all unite to form one tube, the _thoracic duct_, which passes
upward through the _thorax_, or chest, and empties into a large vein,
situated just beneath the left collar-bone.

[Illustration: FIG. 24.--THE LACTEALS.

A, Small Intestine. B, Lacteals. C, Thoracic Duct. D, Absorbents.
E, Blood-vessel. ]

[Sidenote: 39. The absorbents? Lymph? What further of the lymph?]

39. THE ABSORBENTS.--The lacteals belong to a class of vessels known as
absorbents, or lymphatics, which ramify in nearly all parts of the body,
except the brain and spinal cord. The fluid which circulates through the
lymphatics of the limbs, and all the organs not concerned in digestion, is
called _lymph_. This fluid is clear and colorless, like water, and thus
differs from the milky chyle which the lacteals carry after digestion: it
consists chiefly of the watery part of the blood, which was not required by
the tissues, and is returned to the blood by the absorbents or lymphatics.

[Sidenote: 40. What can you state as to the time required for digestion?]

40. CIRCUMSTANCES AFFECTING DIGESTION.--What length of time is required for
the digestion of food? From observations made, in the case of St. Martin,
the Canadian {98} already referred to, it has been ascertained that, at the
end of two hours after a meal, the stomach is ordinarily empty. How much
time is needed to complete the digestion of food, within the small
intestines, is not certain; but, from what we have learned respecting their
functions, it must be evident that it largely depends upon the amount of
starch and fat which the food contains.

[Sidenote: 41. Circumstances affecting duration of digestion? Fresh food?]

41. In addition to the preparations which the food undergoes in cooking,
which we have already considered, many circumstances affect the duration of
digestion; such as the quality, quantity, and temperature of the food; the
condition of the mind and body; sleep, exercise, and habit. Fresh food,
except new bread and the flesh of animals recently slain, is more rapidly
digested than that which is stale; and animal food more rapidly than that
from the vegetable kingdom.

[Sidenote: 42. Food in concentrated form? A large quantity of food?
Experiment on the dog? Ice-water? Variety of articles?]

42. Food should not be taken in too concentrated a form, the action of the
stomach being favored when it is somewhat bulky; but a large quantity in
the stomach often retards digestion. If the white of one egg be given to a
dog, it will be digested in an hour, but if the white of eight eggs be
given it will not disappear in four hours. A wineglassful of ice-water
causes the temperature of the stomach to fall thirty degrees; and it
requires a half-hour before it will recover its natural warmth--about a
hundred degrees--at which the operations of digestion are best conducted. A
variety of articles, if not too large in amount, is more easily disposed of
than a meal made of a single article; although a single indigestible
article may interfere with the reduction of articles that are easily
digested.

[Sidenote: 43. Strong emotion? The tongue of the patient?]

43. Strong emotion, whether of excitement or depression, checks digestion,
as do also a bad temper, anxiety, long fasting, and bodily fatigue. The
majority of these {99} conditions make the mouth dry, that is, they
restrain the flow of the saliva; and without doubt they render the stomach
dry also, by preventing the flow of the gastric juice. And, as a general
rule, we may decide, from a parched and coated tongue, that the condition
of the stomach is not very dissimilar, and that it is unfit for the
performance of digestive labor. This is one of the points which the
physician bears in mind when he examines the tongue of his patient.

[Sidenote: 44. Eating between meals? Severe exercise? Sleep after meals?]

44. The practice of eating at short intervals, or "between meals," as it is
called, has its disadvantage, as well as rapid eating and over-eating,
since it robs the stomach of its needed period of entire rest, and thus
overtasks its power. With the exception of infants and the sick, no persons
require food more frequently than once in four hours. Severe exercise
either directly before or directly after eating retards digestion; a period
of repose is most favorable to the proper action of the stomach. The
natural inclination to rest after a hearty meal may be indulged, but should
not be carried to the extent of sleeping; since in that state the stomach,
as well as the brain and the muscles, seeks release from labor.

{100}

QUESTIONS FOR TOPICAL REVIEW.

                                                                      PAGE

  1. What do you understand by nutrition?                               80
  2. How is the process of nutrition carried on?                        80
  3. What further can you state on the subject?                     80, 81
  4. Describe the general plan of digestion.                            81
  5. How is the process of mastication carried on?                  80, 82
  6. State what you can in relation to the formation of the
        teeth.                                                      82, 86
  7. What, in relation to their arrangement?                        83, 84
  8. What, in relation to the process of "shedding?"            82, 83, 84
  9. In relation to the different forms of teeth in different
        animals?                                                        85
  10. What causes operate to injure or destroy the teeth?           85, 86
  11. What suggestions and directions are given for the
        preservation of the teeth?                                  85, 86
  12. What do you understand by insalivation?                       80, 86
  13. How is the process of insalivation carried on?            86, 87, 88
  14. Of what importance is the saliva to the process?              87, 88
  15. Of what importance are mastication and insalivation?          88, 89
  16. Describe the consequences of rapid eating.                    89, 90
  17. What becomes of the food directly after it has undergone
        mastication and insalivation?                                   90
  18. Describe the location and formation of the stomach.       90, 91, 92
  19. Describe the process by which the gastric juice is formed.        91
  20. What are the properties and uses of the gastric juice?            92
  21. What are the movements of the stomach, and what their
        uses?                                                       92, 93
  22. What further can you state on the subject?                        93
  23. What portions of the food are digested in the stomach?        93, 94
  24. What are the first changes of digestion?                          93
  25. Describe the location and formation of the stomach.               94
  26. What further can you state in relation to the stomach?            94
  27. Describe the process of intestinal digestion.             94, 95, 96
  28. What do you understand by absorption?                         80, 96
  29. How is the process of absorption effected?                    96, 97
  30. What are the lacteals and of what use are they?               96, 97
  31. What length of time is required for the digestion of
        food?                                                       97, 98
  32. What circumstances, of food, affect digestion?                    98
  33. What circumstances, of emotion, affect digestion?             98, 99
  34. What suggestions and directions are given upon the subject of
        eating and drinking?                                        98, 99

       *       *       *       *       *


[Illustration: CIRCULATION OF THE BLOOD.

[Heart, Lungs, Arteries & Veins.]

{101}

CHAPTER VII.

THE CIRCULATION.

_The Blood--Its Plasma and Corpuscles--Coagulation of the Blood--The Uses
of the Blood--Transfusion--Change of Color--The Organs of the
Circulation--The Heart, Arteries, and Veins--The Cavities and Valves of the
Heart--Its Vital Energy--Passage of the Blood through the Heart--The
Frequency and Activity of its Movements--The Pulse--The Sphygmograph--The
Capillary Blood-vessels--The Rate of the
Circulation--Assimilation--Injuries to the Blood-vessels._

[Sidenote: 1. What is required by every living organism? In plants?
Insects? Reptiles? Man?]

1. THE BLOOD.--Every living organism of the higher sort, whether animal or
vegetable, requires for the maintenance of life and activity, a circulatory
fluid, by which nutriment is distributed to all its parts. In plants, this
fluid is the sap; in insects, it is a watery and colorless blood; in
reptiles and fishes, it is red but cold blood; while in the nobler animals
and man, it is the red and warm blood.

[Sidenote: 2. Importance and abundance of blood? Dependence of life? Abel?
Mosaic law? In what part of the body is blood not found? Quantity of blood
in the body?]

2. The blood is the most important, as it is the most abundant, fluid of
the body; and upon its presence, under certain definite conditions, life
depends. On this account it is frequently, and very properly, termed "the
vital fluid." The importance of the blood, as essential to life, was
recognized in the earliest writings. In the narration of the death of the
murdered Abel, it is written, "the voice of his _blood_ crieth from the
ground." In the Mosaic law, proclaimed over thirty centuries ago, the
Israelites were forbidden to eat food that contained blood, for the reason
that "the life of the flesh is in the blood." With the exception of a few
tissues, such as the hair, the nails, and the _cornea_ of the eye, blood
everywhere pervades the body, as may be proven by puncturing any part with
a {102} needle. The total quantity of blood in the body is estimated at
about one-eighth of its weight, or eighteen pounds.

[Sidenote: 3. Color of blood? Its consistence? Odor?]

3. The color of the blood, in man and the higher animals, as is well known,
is red; but it varies from a bright scarlet to a dark purple, according to
the part whence it is taken. "Blood is thicker than water," as the adage
truly states, and has a glutinous quality. It has a faint odor, resembling
that peculiar to the animal from which it is taken.

[Sidenote: 4. What is stated of the blood as viewed under the microscope?]

4. When examined under the microscope, the blood no longer appears a simple
fluid, and its color is no longer red. It is then seen to be made up of two
distinct parts: first, a clear, colorless fluid, called the _plasma_; and
secondly, of a multitude of minute solid bodies, or corpuscles, that float
in the watery plasma. The plasma, or nutritive liquid, is composed of water
richly charged with materials derived from the food, viz., albumen, which
gives it smoothness and swift motion; fibrin; certain fats; traces of
sugar; and various salts.

[Illustration: FIG 25.--THE BLOOD CORPUSCLES.]

[Sidenote: 5. State what you can of the little bodies called corpuscles.]

5. THE BLOOD CORPUSCLES.--In man, these remarkable "little bodies," as the
word _corpuscles_ signifies, are of a yellow color, but by their vast
numbers impart a red hue to the blood. They are very small, having a
diameter of about 1/3500 of an inch, and being one-fourth of that fraction
in thickness; so that if 3,500 of them were placed in line, side by side,
they would only extend one inch; or, if {103} piled one above another, it
would take at least 14,000 of them to stand an inch high. Although so small
in size, they are very regular in form. As seen under the microscope, they
are not globular or spherical, but flat, circular, and disc-like, with
central depressions on each side, somewhat like a pearl button that has not
been perforated. In freshly-drawn blood they show a disposition to arrange
themselves in little rolls like coins (Fig. 25).

[Illustration: FIG. 26.

_a_, Oval Corpuscles of a fowl. _b_, Corpuscles of a frog. _c_, Those of a
shark.

The five small ones at the upper part of the figure, represent the human
corpuscles magnified 400 times.]

[Sidenote: 6. The size and shape of corpuscles? Why is the fact important?]

6. The size and shape of the blood corpuscles vary in different animals, so
that it is possible to discriminate between those of man and the lower
animals (Fig. 26). This is a point of considerable practical importance.
For example, it is sometimes desirable to decide in a court of justice the
source, whether from man or an inferior animal, of blood stains upon the
clothing of an accused person, or upon some deadly weapon. This may be done
by a microscopical examination of a minute portion of the dried stain,
previously refreshed by means of gum-water. Certain celebrated cases are
recorded in which the guilt of criminals has been established, and they
have been condemned and punished upon the evidence which science rendered
on this single point, the detecting of the human from other blood.

[Sidenote: 7. The character of the blood of dead animals? Means of
detecting such blood?]

7. The character of the blood of dead, extinct, and even fossil animals,
such as the mastodon, has been ascertained by obtaining and examining
traces of it which had been shut up, perhaps for ages, in the circulatory
canals of bone. A means of detecting blood in minute quantities is found
{104} in the spectroscope, the same instrument by which the constitution of
the heavenly bodies has been studied. If a solution containing not more
than one-thousandth part of a grain of the coloring matter of the
corpuscle, be examined, this instrument will detect it.

[Sidenote: 8. White corpuscles? Total number of corpuscles in the body?]

8. The corpuscles, just described, are known as the red blood corpuscles.
Besides these, and floating along in the same plasma, are the white
corpuscles. These are fewer in number, but larger and globular in form.
They are colorless, and their motion is less rapid than that of the other
variety. The total number of both varieties of these little bodies in the
blood is enormous. It is calculated that in a cubic inch of that fluid
there are eighty-three millions, and at least five hundred times that
number in the whole body.

[Sidenote: 9. The blood in its natural condition in the body? Describe the
process by which the coagulation of blood takes place?]

9. COAGULATION.--The blood, in its natural condition in the body, remains
perfectly fluid; but, within a few minutes after its removal from its
proper vessels, whether by accident or design, a change takes place. It
begins to coagulate, or assume a semi-solid consistence. If allowed to
stand, after several hours it separates into two distinct parts, one of
them being a dark red jelly, the coagulum, or clot, which is heavy and
sinks; and the other, a clear, straw-colored liquid, called serum, which
covers the clot. This change is dependent upon the presence in the blood of
fibrin, which possesses the property of solidifying under certain
circumstances; one of these circumstances being when the blood is separated
from living tissues. The color of the clot is due to the entanglement of
the corpuscles with the fibrin.

[Sidenote: 10. If coagulation were impossible? How is it in fact?]

10. In this law of the coagulation of the blood is our safeguard against
death by hæmorrhage, or against undue loss of blood. If coagulation were
impossible, the {105} slightest injury in drawing blood would prove fatal.
Whereas now, in vastly the larger proportion of cases, bleeding ceases
spontaneously, because the blood, as it coagulates, stops the mouths of the
injured blood-vessels. In another class of cases, where larger vessels are
cut or torn, it is simply necessary to close them by a temporary pressure;
for in a few minutes the clot will form and seal them up. In still more
serious cases, where the blood-vessel is of large size, the surgeon is
obliged to tie a "ligature" about it, and thus prevent the force of the
blood-current from washing away the clots, which, forming within and around
the vessel, would close it effectually.

[Sidenote: 11. What is worthy of remark? Coagulation of the blood of
inferior animals? Of the blood of birds?]

11. It is worthy of remark that this peculiarity is early implanted in the
blood, even before birth, and in advance of any existing necessity for it;
thus anticipating and guarding against danger. But this is not all. Of most
of the inferior animals, which, as compared with man, are quite helpless,
the blood coagulates more rapidly, and in the case of the birds, almost
instantly. The relative composition of fluid and coagulated blood may be
thus represented:

  _Fluid Blood._              _Coagulated Blood._

    Plasma----------Serum---------Serum
          \
           \
            --------Fibrin--------
                                  \
                                   \
   Corpuscles------Corpuscles-------Clot.
[Sidenote: 12. The blood, as a provider and purifier? What uses does the
blood subserve? Experiments? Transfusion?]

12. THE USES OF THE BLOOD.--The blood is the great provider and purifier of
the body. It both carries new materials to all the tissues, and removes the
worn out particles of matter. This is effected by the plasma. It both
conveys oxygen and removes carbonic acid. This is done through the
corpuscles. Some singular experiments have {106} been tried to illustrate
the life-giving power of the blood. An animal that has bled so freely as to
be at the point of dying, is promptly brought back to life by an operation
called transfusion, by which fresh blood from a living animal is injected
into the blood-vessels of his body.

[Sidenote: 13. The case of the deaf and feeble dog? Horse? Dead dog?]

13. It is related that a dog, deaf and feeble from age, had hearing and
activity restored to him by the introduction into his veins of blood taken
from a young dog; and, that a horse, twenty-six years old, having received
the blood of four lambs acquired new vigor. And further, that a dog, just
dead from an acute disease, was so far revived by transfusion, as to be
able to stand and make a few movements.

[Sidenote: 14. Transfusion, as a fashionable remedy? What further of
transfusion?]

14. Transfusion has been practised upon man. At one time, shortly after
Harvey's discovery of the "Circulation of the Blood," it became quite a
fashionable remedy, it being thought possible by it to cure all forms of
disease, and even to make the old young again. But these claims were soon
found extravagant, and many unhappy accidents occurred in its practice; so
that being forbidden by government and interdicted by the Pope, it rapidly
fell into disuse. At the present time, however, it is sometimes resorted to
in extreme cases, when there has been a great and rapid loss of blood; and
there are upon record several instances where, other means having failed,
life has been restored or prolonged by the operation of transfusion.

[Sidenote: 15. The seat of the reviving power of the blood? What further is
related?]

15. This reviving power of the blood seems to reside in the corpuscles; for
transfusion, when attempted to be performed with the serum alone, has, in
every case, proved fruitless. Now, though so much depends upon the blood
and its corpuscles, it is a mistake to suppose that in them alone is the
seat of life, or that they are, in an exclusive manner, alive. All the
organs and parts of the body are mutually dependent one upon the other; and
the complete usefulness {107} of the blood, or of any other part, flows out
of the harmonious action of all the parts.

[Sidenote: 16. Changes in the blood? What further is stated?]

16. CHANGE OF COLOR.--The blood undergoes a variety of changes in its
journey through the system. As it visits the different organs it both gives
out and takes up materials. In one place it is enriched, in another it is
impoverished. By reason of these alterations in its composition, the blood
also changes its color. In one part of the body it is bright red, or
arterial; in another it is dark blue, or venous. In the former case it is
pure and fit for the support of the tissues; in the latter, it is impure
and charged with effete materials. (The details of the change from dark to
bright will be given in the chapter on Respiration.)

[Sidenote: 17. Motion of the blood? What is meant by the circulation of the
blood? How confined? Discovery made by Harvey?]

17. CIRCULATION.--The blood is in constant motion during life. From the
heart, as a centre, a current is always setting toward the different
organs; and from these organs a current is constantly returning to the
heart. In this way a ceaseless circular movement is kept up, which is
called the Circulation of the Blood. This stream of the vital fluid is
confined to certain fixed channels, the blood-vessels. Those branching from
the heart are the arteries; those converging to it are the veins. The true
course of the blood was unknown before the beginning of the seventeenth
century. In 1619 it was discovered by the illustrious William Harvey. Like
many other great discoverers, he suffered persecution and loss, but unlike
some of them, he was fortunate enough to conquer and survive opposition. He
lived long enough to see his discovery universally accepted, and himself
honored as a benefactor of mankind.

{108}

[Illustration: FIG. 27.--THE ORGANS OF CIRCULATION.]

{109} [Illustration: FIG. 28.--THE HEART AND LARGE VESSELS.

A, Right Ventricle. B, Left Ventricle. C, Right Auricle. D, Left Auricle.
E, Aorta. F, Pulmonary Artery.]

[Sidenote: 18. Office of the heart? Location of the heart? Its beat? Its
shape? Protection to the heart? What else is said in relation to the
heart?]

18. THE HEART.--The heart is the central engine of the circulation. In this
wonderful little organ, hardly larger than a man's fist, resides that
sleepless force by which, during the whole of life, the current of the
blood is kept in motion. It is placed in the middle and front part of the
chest, inclining to the left side. The heartbeat may be felt and heard
between the fifth and sixth ribs, near the breast-bone. The shape of the
heart is conical, with the apex or point downward and in front. The base,
which is upward, is attached so as to hold it securely in its place, while
the apex is freely moveable. In order that loss of power from friction may
be obviated, the heart is enclosed between two layers of serous membrane,
which forms a kind of sac. This membrane is as smooth as satin, and itself
secretes a fluid in sufficient quantities to keep it at all times well
lubricated. The lining membrane of the heart, likewise, is extremely
delicate and smooth.

{110} [Illustration: FIG. 29.--SECTION OF THE HEART.

A, Right Ventricle. B, Left Ventricle. C, Right Auricle. D, Left Auricle.
E, F, Inlets to the Ventricles. G, Pulmonary Artery. H, Aorta.]

[Sidenote: 19. Formation of the heart? Right and left heart?]

19. THE CAVITIES OF THE HEART.--The heart is hollow, and so partitioned as
to contain four chambers or cavities; two at the base, known as the
_auricles_, from a fancied resemblance to the ear of a dog, and two at the
apex or point, called _ventricles_. An auricle and a ventricle on the same
side, communicate with each other, but there is no opening from side to
side. It is customary to regard the heart as a double organ, and to speak
of its division into the right and left heart. For while both halves act
together in point of time, each half sustains an entirely distinct portion
of the labor of the circulation. Thus, the right heart always carries the
dark or venous blood, and the left always circulates the bright or arterial
blood.

[Sidenote: 20. Capacity of the chambers of the heart? What wise provision
is mentioned? The auricles?]

20. If we examine the heart, we at once notice that though its various
chambers have about the same capacity, the walls of the ventricles are
thicker and stronger than those of the auricles. This is a wise provision,
for it is by the powerful action of the former that the blood is forced to
the most remote regions of the body. The auricles, on the contrary, need
much less power, for they simply discharge their contents into the cavities
of the heart near at hand and below them--into the ventricles. {111}

[Sidenote: 21. Substance of the heart? Its fibres? Its movements? The
advantage of such movements? Action of the heart? Its period of repose?]

21. ACTION OF THE HEART.--The substance of the heart is of a deep red
color, and its fibres resemble those of the voluntary muscles by which we
move our bodies. But the heart's movements are entirely involuntary. The
advantage of this is evident; for if it depended upon us to will each
movement, our entire attention would be thus engaged, and we would find no
time for study, pleasure, or even sleep. The action of the heart consists
in alternate contractions and dilatations. During contraction the walls
come forcibly together, and thus drive out the blood. In dilatation, they
expand and receive a renewed supply. These movements are called _systole_
and _diastole_. The latter may be called the heart's period of repose; and
although it lasts only during two-fifths of a heart-beat, or about a third
of a second, yet during the day it amounts to more than nine hours of total
rest.

[Sidenote: 22. Remarkable property of the tissue of the heart? How shown?
How interesting? In cold-blooded animals? Heart of a turtle? Of a frog?
Alligator?]

22. A remarkable property of the tissue of the heart is its intense
vitality. For while it is more constantly active than any other organ of
the body, it is the last to part with its vital energy. This is especially
interesting in view of the fact that after life is apparently extinguished,
as from drowning, or poisoning by chloroform, there yet lingers a spark of
vitality in the heart, which, by continued effort, may be fanned into a
flame so as to revivify the whole body. In cold-blooded animals this
irritability of the heart is especially remarkable. The heart of a turtle
will pulsate, and the blood circulate for a week after its head has been
cut off; and the heart will throb regularly many hours after being cut out
from the creature's chest. The heart of a frog or serpent, separated
entirely from the body, will contract at the end of ten or twelve hours:
that of an alligator has been known to beat twenty-eight hours after the
death of the animal. {112}

[Sidenote: 23. Course of the blood through the heart? Course of
heart-currents?]

23. PASSAGE OF THE BLOOD THROUGH THE HEART.--Let us now trace the course of
the blood through the several cavities of the heart. In the first place,
the venous blood, rendered dark and impure by contact with the changing
tissues of the body, returns to the right heart by the veins. It enters and
fills the right auricle during its dilatation: the auricle then contracts
and fills the right ventricle. Almost instantly, the ventricle contracts
forcibly and hurries the blood along the great artery of the lungs, to be
purified in those organs. Secondly, having completed the circuit of the
lungs, the pure and bright arterial blood enters the left auricle. This now
contracts and fills the left ventricle, which cavity, in its turn,
contracts and sends the blood forth on its journey again through the
system. This general direction from right to left is the uniform and
undeviating course of heart-currents.

[Sidenote: 24. Openings of the ventricles? How guarded? How do the valves
operate? The consequence? Heart-sounds?]

24. The mechanism which enforces and regulates it, is as simple as it is
beautiful. Each ventricle has two openings, an inlet and an outlet, each of
which is guarded by strong curtains, or valves. These valves open freely to
admit the blood entering from the right, but close inflexibly against its
return. Thus, when the auricle contracts, the inlet valve opens; but as
soon as the ventricle begins to contract, it closes promptly. The contents
are then, so to speak, cornered, and have but one avenue of escape, that
through the outlet valve into the arteries beyond. As soon as the ventricle
begins to dilate again, this valve shuts tightly and obstructs the passage.
The closing of these valves occasions the two heart-sounds, which we hear
at the front of the chest.

[Sidenote: 25. Heart-beats? The heart as a susceptible organ? Heat,
exercise, etc.? Posture?]

25. FREQUENCY OF THE HEART'S ACTION.--The alternation of contraction and
dilation constitutes the {113} heartbeats. These follow each other not only
with great regularity, but with great rapidity. The average number in an
adult man is about seventy-two in a minute. But the heart is a susceptible
organ, and many circumstances affect its rate of action. Heat, exercise,
and food will increase its action, as cold, fasting, and sleep will
decrease it. Posture, too, has a curious influence; for if while sitting,
the beats of the heart number seventy-one; standing erect will increase
them to eighty-one, and lying down will lower them to sixty-six.

[Sidenote: 26. Mental emotions? Sudden excitement? Excessive joy? The
heart-beat rate? Bonaparte and Wellington?]

26. The modifying influence of mental emotions is very powerful. Sudden
excitement of feeling will cause the heart to palpitate, or throb
violently. Depressing emotions sometimes temporarily interrupt its
movements, and the person faints in consequence. Excessive joy, grief, or
fear, has occasionally suspended the heart's action entirely, and thus
caused death. The rate of the heart-beat may be naturally above or below
seventy-two. Thus it is stated that the pulse of the savage is always
slower than that of the civilized man. Bonaparte and Wellington were very
much alike in their heart's pulsations, which were less than fifty in the
case of each.

[Sidenote: 27. Average number of heart-beats? In one hour? Year? Lifetime?]

27. ACTIVITY OF THE HEART.--The average number of heart-beats during a
lifetime may be considered as at the rate of seventy-two per minute,
although this estimate is probably low; for during several years of early
life the rate is above one hundred a minute. In one hour, then, the heart
pulsates four thousand times; in a day, one hundred thousand times; and in
a year, nearly thirty-eight million times. If we compute the number during
a lifetime, thirty-nine years being the present average longevity of
civilized mankind, we obtain as the vast aggregate, fourteen hundred
millions of pulsations. {114}

[Sidenote: 28. Amount of blood expelled? Theories of the ancients?]

28. Again, if we estimate the amount of blood expelled by each contraction
of the ventricles, at four ounces, then the weight of the blood moved
during one minute will amount to eighteen pounds. In a day it will be about
twelve tons; in a year, four thousand tons; and in the course of a
lifetime, over one hundred and fifty thousand tons. These large figures
indicate, in some measure, the immense labor necessary to carry on the
interior and vital operations of our bodies. In this connection, we call to
mind the fanciful theories of the ancients in reference to the uses of the
heart. They regarded it as the abode of the soul, and the source of the
nobler emotions--bravery, generosity, mercy, and love. The words courage
and cordiality are derived from a Latin word signifying heart. Many other
words and phrases, as hearty, heart-felt, to learn by heart, and
large-hearted, show how tenaciously these exploded opinions have fastened
themselves upon our language.

[Sidenote: 29. The tendency at the present time? Why is this view
inadequate?]

29. At the present time the tendency is to ascribe purely mechanical
functions to the heart. This view, like the older one, is inadequate; for
it expresses only a small part of our knowledge of this organ. The heart is
unlike a simple machine, because its motive power is not applied from
without, but resides in its own substance. Moreover, it repairs its own
waste, it lubricates its own action, and it modifies its movements
according to the varying needs of the system. It is more than a mere
force-pump, just as the stomach is something more than a crucible, and the
eye something more than an optical instrument.

[Sidenote: 30. What are the arteries? Their walls? Their membrane?]

30. THE ARTERIES.--The tube-like canals which carry the blood away from the
heart are the arteries. Their walls are made of tough, fibrous materials,
so that they sustain the mighty impulse of the heart, and are not ruptured.
In common with the heart, the arteries have a {115} delicately smooth
lining membrane. They are also elastic, and thus re-enforce the action of
the heart: they always remain open when cut across, and after death are
always found empty.

[Sidenote: 31. Early anatomists? The service of the illustration?]

31. The early anatomists observed this phenomenon, and supposing that the
same condition existed during life, came to the conclusion that these tubes
were designed to act as air-vessels, hence the name artery, from a Greek
word which signifies containing air. This circumstance affords us an
illustration of the confused notions of the ancients in reference to the
internal operations of the body. Cicero speaks of the arteries as
"conveying the breath to all parts of the body."

[Sidenote: 32. The arterial system? The branches and sub-branches of the
arteries?]

32. The arterial system springs from the heart by a single trunk, like a
minute and hollow tree, with numberless branches. As these branches leave
the heart they divide and subdivide, continually growing smaller and
smaller, until they can no longer be traced with the naked eye. If, then,
we continue the examination by the aid of a microscope, we see these small
branches sending off still smaller ones, until all the organs of the body
are penetrated by arteries.

[Sidenote: 33. Successive undulations from the heart? Course of the
arteries? Protection of the arteries? General location of the arteries?]

33. THE PULSE.--With each contraction of the left heart, the impulse causes
a wave-like motion to traverse the entire arterial system. If the arteries
were exposed to view, we might see successive undulations speeding from the
heart to the smallest of the branches, in about one-sixth part of a second.
The general course of the arteries is as far as possible from the surface.
This arrangement is certainly wise, as it renders them less liable to
injury, the wounding of an artery being especially dangerous. It also
protects the arteries from external and unequal pressure, by which the
force of the heart would be {116} counteracted and wasted. Accordingly, we
generally find these vessels hugging close to the bones, or hiding behind
the muscles and within the cavities of the body.

[Sidenote: 34. Where do the arteries lie? If we apply the finger? Pulse?
Where felt?]

34. In a few situations, however, the arteries lie near the surface; and if
we apply the finger to any of these parts, we will distinctly feel the
movement described, taking place in harmony with the heart-beat. This is
part of the wave-motion just mentioned, and is known as the pulse. All are
more familiar with the pulse at the wrist, in the _radial_ artery; but the
pulse is not peculiar to that position, for it may be felt in the _carotid_
of the neck, in the _temporal_ at the temple, and elsewhere, especially
near the joints.

[Illustration: FIG. 30.--THE FORM OF THE PULSE.]

[Sidenote: 35. The pulse as an index? Of what does it inform the physician?
Instrument for recording pulsation?]

35. Since the heart-beat makes the pulse, whatever affects the former
affects the latter also. Accordingly, the pulse is a good index of the
state of the health, so far as the health depends upon the action of the
heart. It informs the physician of the condition of the circulation in four
particulars: its rate, regularity, force, and fullness; and nearly every
disease modifies in some respect the condition of the pulse. A very
ingenious instrument, known as the sphygmograph, or pulse-writer, has
recently been invented, by the aid of which the pulse is made to write upon
paper its own signature, or rather to sketch its own profile. This
instrument shows with great accuracy the difference between the pulses of
health and those of disease. In Fig. 30 is traced the form of the pulse in
health, which should be read from left to right. That part of the trace
{117} which is nearly perpendicular coincides with the contraction of the
ventricles; while the wavy portion marks their dilatation.

[Sidenote: 36. What are the veins? How do they form? What do they
resemble?]

36. THE VEINS.--The vessels which convey the blood on its return to the
heart are the veins. They begin in the several organs of the body, and at
first are extremely small; but uniting together as they advance, they
constantly increase in size, reminding us of the way in which the fine
rootlets of the plant join together to form the large roots, or of the
rills and rivulets that flow together to form the large streams and rivers.
In structure, the veins resemble the arteries, but their walls are
comparatively inelastic. They are more numerous, and communicate with each
other freely in their course, by means of interlacing branches.

[Illustration: FIG. 31.--THE VALVES OF THE VEINS.]

[Sidenote: 37. Valves in the veins? What are they? Their position?
Experiment with the cord?]

37. But the chief point of distinction is in the presence of the valves in
the veins. These are little folds of membrane, disposed in such a way, that
they only open to receive blood flowing toward the heart, and close against
a current in the opposite direction. Their position in the veins on the
back of the hand may be readily observed, if we first obstruct the return
of blood by a cord tied around the forearm or wrist. In a few minutes the
veins will appear swollen, and upon them will be seen certain prominences,
about an inch apart. These latter indicate the location of the valves, or,
rather, they show that the vessels in front of the valves are distended by
the blood, which cannot force a passage back through them.

[Sidenote: 38. What will be proved by the experiment? What inference is
drawn?]

38. This simple experiment proves that the true direction of the venous
blood is toward the heart. That the color {118} of the blood is dark, will
be evident, if we compare the hand thus bound by a cord with the hand not
so bound. It also proves that the veins lie superficially, while the
arteries are beneath the muscles, well protected from pressure; and that
free communication exists from one vein to another. If now we test the
temperature of the constricted member by means of a thermometer, we will
find that it is colder than natural, although the amount of blood is larger
than usual. From this fact we infer, that whatever impedes the venous
circulation tends to diminish vitality; and hence, articles of clothing or
constrained postures, that confine the body or limbs, and hinder the
circulation of the blood, are to be avoided as injurious to the health.

[Sidenote: 39. Capillaries? How regarded? Harvey?]

39. THE CAPILLARIES.--A third set of vessels completes the list of the
organs of the circulation, namely, the _capillary_ vessels, so called (from
the Latin word _capillaris_, hair-like), because of their extreme fineness.
They are, however, smaller than any hair, having a diameter of about 1/3000
of an inch, and can only be observed by the use of the microscope. These
vessels may be regarded as the connecting link between the last of the
arteries and the first of the veins. The existence of these vessels was
unknown to Harvey, and was the one step wanting to complete his great work.
The capillaries were not discovered until 1661, a short time after the
invention of the microscope.

[Sidenote: 40. The circulation of the blood in the web of a frog's foot?
Describe it. How general is the existence of the tissues?]

40. The circulation of the blood, as seen under the microscope, in the
transparent web of a frog's foot, is a spectacle of rare beauty, possessing
more than ordinary interest, when we consider that something very similar
is taking place in our own bodies, on a most magnificent scale. It is like
opening a secret page in the history of our own frames. We there see
distinctly the three classes of vessels with their moving contents; first,
the artery, {119} with its torrent of blood rushing down from the heart,
secondly, the vein, with its slow, steady stream flowing in the opposite
direction; and between them lies the network of capillaries, so fine that
the corpuscles can only pass through "in single file." The current has here
an uncertain or swaying motion, hurrying first in one direction, then
hesitating, and then turning back in the opposite direction, and sometimes
the capillaries contract so as to be entirely empty. Certain of the tissues
are destitute of capillaries; such are cartilage, hair, and a few others on
the exterior of the body. In all other structures, networks of these
vessels are spread out in countless numbers: so abundant is the supply,
that it is almost impossible to puncture any part with the point of a
needle without lacerating tens, or even hundreds of these vessels.

[Illustration: FIG. 32.--WEB OF A FROG'S FOOT, slightly magnified.]

[Illustration: FIG. 33.--MARGIN OF FROG'S WEB magnified 30 diameters.]

[Sidenote: 41. Elasticity of the capillaries? Grain of sand in the eye?
Blush? Other cases?]

41. The capillaries are elastic, and may so expand as to produce an effect
visible to the naked eye. If a grain of sand, or some other foreign
particle, lodge in the eye, it will become irritated, and in a short time
the white of the eye will be "blood-shot." This appearance is due to an
{120} increase in the size of these vessels. A blush is another example of
this, but the excitement comes through the nervous system, and the cause is
some transient emotion, either of pleasure or pain. Another example is
sometimes seen in purplish faces of men addicted to drinking brandy; in
them the condition is a congestion of the capillary circulation, and is
permanent, the vessels having lost their power of elastic contraction.

[Sidenote: 42. Show what time is required for a given portion of blood to
travel once around the body.]

42. RAPIDITY OF THE CIRCULATION.--That the blood moves with great rapidity
is evident from the almost instant effects of certain poisons, as prussic
acid, which act through the blood. Experiments upon the horse, dog, and
other inferior animals, have been made to measure its velocity. If a
substance, which is capable of a distinct chemical reaction (as _potassium
ferrocyanide_, or _barium nitrate_), be introduced into a vein of a horse
on one side, and blood be taken from a distant vein on the other side, its
presence may be detected at the end of twenty or thirty-two seconds. In
man, the blood moves with greater speed, and the circuit is completed in
twenty-four seconds.

[Sidenote: 43. Time required for all the blood to circulate completely
around?]

43. What length of time is required for all the blood of the body to make a
complete round of the circulation? This question cannot be answered with
absolute accuracy, since the amount of the blood is subject to continual
variations. But, if we assume this to be one-eighth of the weight of the
body, about eighteen pounds, it will be sufficiently correct for our
purpose. Now to complete the circuit, this blood must pass once through the
left ventricle, the capacity of which is two ounces. Accordingly, we find
that, under ordinary circumstances, all the blood makes one complete
rotation every two minutes; passing successively through the heart, the
capillaries of the lungs, the arteries, the capillaries of the extremities,
and through the veins. {121}

[Sidenote: 44. What is meant by assimilation? What can you say of its use,
etc.? Time?]

44. ASSIMILATION.--The crowning act of the circulation, the furnishing of
supplies to the different parts of the body, is effected by means of the
capillaries. The organs have been wasted by use; the blood has been
enriched by the products of digestion. Here, within the meshes of the
capillary network, the needy tissues and the needed nutriment are brought
together. By some mysterious chemistry, each tissue selects and withdraws
from the blood the materials it requires, and converts them into a
substance like itself. This conversion of lifeless food into living tissue
is called assimilation. The process probably takes place at all times, but
the period especially favorable for it is during sleep. Then the
circulation is slower, and more regular, and most of the functions are at
rest. The body is then like some trusty ship, which after a long voyage is
"hauled up for repairs."

[Sidenote: 45. What is stated of the injuries to the blood-vessels?]

45. INJURIES TO THE BLOOD-VESSELS.--It is important to be able to
discriminate between an artery and a vein, in the case of a wound, and if
we remember the physiology of the circulation we may readily do so. For, as
we have already seen, hæmorrhage from an artery is much more dangerous than
that from a vein. The latter tends to cease spontaneously after a short
time. The arterial blood flows away from the heart with considerable force,
in jets; its color being bright scarlet. The venous blood flows toward the
heart from that side of the wound furthest from the heart; its stream being
continuous and sluggish; its color dark. In an injury to an artery,
pressure should be made between the heart and the wound; and in the case of
a vein that persistently bleeds, it should be made upon the vessel beyond
its point of injury. {122}

QUESTIONS FOR TOPICAL REVIEW.

                                                                       PAGE
  1. In what organisms is the so-called circulatory fluid found?        101
  2. How is it designated in the different organisms?                   101
  3. What can you state of the importance of blood to the body?    101, 105
  4. Of its great abundance, color, and composition?          101, 102, 107
  5. Describe the corpuscles of the human blood.              102, 103, 104
  6. What is said of them in comparison with those of the lower
        animals?                                                        103
  7. Of the importance of sometimes detecting human from other blood?   103
  8. What means have we of detecting blood in spots or stains?     103, 104
  9. What is meant by coagulation of the blood?                         104
  10. What wisdom is there in the law of the blood's coagulation?  104, 105
  11. How is this wisdom made manifest?                                 105
  12. In what cases is the aid of the surgeon required?                 105
  13. What are the two great uses of the blood?                         105
  14. Through what mediums is the blood provided with new material and
        relieved of the old material?                                   105
  15. What do you understand by the operation called transfusion?       106
  16. What cases of transfusion are reported of the lower animals?      106
  17. What can you state of transfusion as practised upon man?          106
  18. What further can you say on the subject?                     106, 107
  19. What changes take place in the color of the blood in its journey
        through the system?                                             107
  20. State all you can in relation to the circulation of the blood.    107
  21. All, in relation to the size, shape, and location of the
        heart.                                                     107, 109
  22. How is the loss of power in the heart movements obviated?         109
  23. Give a description of the formation of the heart.       109, 110, 111
  24. What can you state of the ventricles and auricles of the heart?   110
  25. Describe the action of the heart.                                 111
  26. What special vitality does the tissue of the heart possess?       111
  27. State all you can on the subject.                                 111
  28. Describe the course of the blood through the cavities of the
        heart.                                                          112
  29. Describe the mechanism that regulates the heart-currents.         112
  30. How do you account for the two heart-sounds at the front of
      the chest?                                                        112
  31. State what you can of the frequency of the heart's action.   112, 113
  32. Of the activity of the heart.                                113, 114
  33. What do you understand by the arteries?                      114, 115
  34. State what you can of the arteries and the arterial system.  114, 115
  35. What do you understand by the pulse?                         115, 116
  36. In what part of the body may the pulse be felt?                   116
  37. What further can you state of the pulse?                     116, 117
  38. What are the veins?                                               117
  39. Where do they exist, and how are they formed?                     117
  40. Describe the valves of the veins and their uses.                  117
  41. Now give a full description of the construction of the veins.     117
  42. What further can you state of the veins?                     117, 118
  43. What do you understand by the capillaries?                   118, 119
  44. What service do the capillaries perform?                118, 119, 121
  45. Describe the circulation of the blood in the region of
        the heart.                                                 118, 119
  46. What can you state of the rapidity of the blood's circulation?    120
  47. Of the process known as assimilation?                             121
  48. Of injuries to the blood-vessels?                                 121

       *       *       *       *       *


{123}

CHAPTER VIII.

RESPIRATION.

    _The Objects of Respiration--The Lungs--The Air-Passages--The Movements
    of Respiration--Expiration and Inspiration--The Frequency of
    Respiration--Capacity of the Lungs--The Air we breathe--Changes in the
    Air from Respiration--Changes in the Blood--Interchange of Gases in the
    Lungs--Comparison between Arterial and Venous Blood--Respiratory
    Labor--Impurities of the Air--Dust--Carbonic Acid--Effects of Impure
    Air--Nature's Provision for Purifying the Air--Ventilation--Animal
    Heat--Spontaneous Combustion._

[Sidenote: 1. Difference between the two sets of capillaries? Change
effected by respiration or breathing?]

1. THE OBJECT OF RESPIRATION.--In one set of capillaries, or hair-like
vessels, the blood is impoverished for the support of the different members
and organs of the body. In another capillary system the blood is refreshed
and again made fit to sustain life. The former belongs to the greater or
_systemic_ circulation; the latter to the lesser or _pulmonary_, so called
from _pulmo_, the lungs, in which organs it is situated. The blood, as sent
from the right side of the heart to the lungs, is venous, dark, impure, and
of a nature unfit to circulate again through the tissues. But, when the
blood returns from the lungs to the left side of the heart, it has become
arterial, bright, pure, and no longer hurtful to the tissues. This
marvellous purifying change is effected by means of the very familiar act
of respiration, or breathing.

[Sidenote: 2. What are the lungs? How many lungs are there? Lung-substance?
Its properties? The pleura?]

2. THE LUNGS.--The lungs are the special organs of respiration. There are
two of them, one on each side of the chest, which cavity they, with the
heart, almost wholly occupy. The lung-substance is soft, elastic, and
sponge-like. Under pressure of the finger, it _crepitates_, or crackles,
and floats when thrown into water; these properties being {124} due to the
presence of air in the minute air-cells of the lungs. To facilitate the
movements necessary to these organs, each of them is provided with a double
covering of an exceedingly smooth and delicate membrane, called the
_pleura_. One layer of the pleura is attached to the walls of the chest,
and the other to the lungs; and they glide, one upon the other, with utmost
freedom. Like the membrane which envelops the heart, the pleura secretes
its own lubricating fluid, in quantities sufficient to keep it always
moist.

[Illustration: FIG. 34.--ORGANS OF THE CHEST.

A, Lungs. B, Heart. D, Pulmonary Artery. E, Trachea.]

{125}

[Illustration: FIG. 35.--LARYNX, TRACHEA, AND BRONCHIAL TUBES.]

[Illustration: FIG. 36.--DIAGRAM AND SECTION OF THE AIR-CELLS.]

[Sidenote: 3. Communication of the lungs with the external air? Bronchial
tubes?]

3. THE AIR-PASSAGES. --The lungs communicate with the external air by means
of certain air-tubes, the longest of which, the _trachea_, or windpipe,
runs along the front of the neck (Fig. 34, E, and 35). Within the chest
this tube divides into two branches, one entering each lung; these in turn
give rise to numerous branches, or bronchial tubes, as they are called,
which gradually diminish in size until they are about one-twenty-fifth of
an inch in diameter. Each of these terminates in a cluster of little
pouches, or "air-cells," having very thin walls, and covered with a
capillary network, the most intricate in the body (Fig. 36).

[Sidenote: 4. Office of the bronchial tubes? What further can you state of
them?]

4. These tubes are somewhat flexible, sufficiently so to bend when the
parts move in which they are situated; but they are greatly strengthened by
bands or rings of cartilage which keep the passages always open; otherwise
there would be a constantly-recurring tendency to collapse after every
breath. The lung-substance essentially consists of these bronchial tubes
and terminal air-cells, with the blood-vessels ramifying about them (Fig.
37). At the top of the trachea is the larynx, a sort of {126} box of
cartilage, across which are stretched the vocal cords. Here the voice is
produced chiefly by the passage of the respired air over these cords,
causing them to vibrate.

[Illustration: FIG. 37.--SECTION OF THE LUNGS.]

[Sidenote: 5. The epiglottis? When it does not close in time, what is the
consequence?]

5. Over the opening of the larynx is found the _epiglottis_, which fits
like the lid of a box at the entrance to the lungs, and closes during the
act of swallowing, so that food and drink shall pass backward to the
oesophagus, or gullet (Fig. 38). Occasionally it does not close in time,
and some substance intrudes within the larynx, when we at once discover, by
a choking sensation, that "something has gone the wrong way," and, by
coughing, we attempt to expel the unwelcome intruder. The epiglottis is one
of the many safeguards furnished by nature for our security and {127}
comfort, and is planned and put in place long before these organs are
brought into actual use in breathing and in taking food.

[Illustration: FIG. 38.--SECTION OF MOUTH AND THROAT.

A, The Tongue. B, The Uvula C, Vocal Cord. E, Epiglottis. L, Larynx.
N, Trachea. O, Oesophagus.]

[Sidenote: 6. Lining of the air-passages? Ciliated cells? Their uses? The
three diseases of the lungs?]

{128}

[Illustration: FIG. 39.--CILIATED CELLS.]

6. The air-passages are lined through nearly their whole extent with mucous
membrane, which maintains these parts in a constantly moist condition. This
membrane has a peculiar kind of cells upon its outer surface. If examined
under a powerful microscope, we may see, even for a considerable time after
their removal from the body, that these cells have minute hair-like
processes in motion, which wave like a field of grain under the influence
of a breeze (Fig. 39). This is a truly beautiful sight; and since it is
found that these little _cilia_, as they are called, always produce
currents in one direction, from within outward, it is probable that they
serve a useful purpose in catching and carrying away from the lungs dust
and other small particles drawn in with the breath (Fig. 39). The three
diseases which more commonly affect the lungs, as the result of exposure,
are pneumonia, or inflammation of the lungs, implicating principally the
air-cells; bronchitis, an inflammation of the large bronchial tubes; and
pleurisy, an inflammation of the investing membrane of the lungs, or
pleura. Among the young, an affection of the trachea takes place, known as
croup.

[Sidenote: 7. The act of breathing? Extension of the chest by breathing?]

7. THE MOVEMENTS OF RESPIRATION.--The act of breathing has two parts--(1),
_inspiration_, or drawing air into the lungs, and (2), _expiration_, or
expelling it from the lungs again. In inspiration, the chest extends in its
length, breadth, and height, or width. We can prove that this is the case
as regards the two latter, by observing the effect of a deep breath. The
ribs are elevated by means of numerous muscles, some of which occupy the
entire spaces between those bones. But the increase in length, or
vertically, is not so apparent, as it is caused by a muscle within the body
called the _diaphragm_, it being the thin partition which separates the
chest from the abdomen, rising like a dome within the chest. (Fig. 16).
{129}

[Sidenote: 8. Contraction of the diaphragm? Power of the diaphragm? Effects
of extending the walls of the chest? The habit of taking frequent and deep
inspirations?]

8. With every inspiration, the diaphragm contracts, and in so doing,
approaches more nearly a plane, or horizontal, surface, and thus enlarges
the capacity of the chest. Laughing, sobbing, hiccoughing, and sneezing are
caused by the spasmodic or sudden contraction of the diaphragm. The special
power of this muscle is important in securing endurance, or "long wind," as
it is commonly expressed; which may be obtained mainly by practice. It is
possessed in a marked degree by the mountaineer, the oarsman, and the
trained singer. As the walls of the chest extend, the lungs expand, and the
air rushes in to fill them. This constitutes an inspiration. The habit of
taking frequent and deep inspirations, in the erect position, with the
shoulders thrown back, tends greatly to increase the capacity and power of
the organs of respiration.

[Sidenote: 9. Expiration? The mechanism of expiration?]

9. EXPIRATION is a less powerful act than inspiration. The diaphragm
relaxes its contraction, and ascends in the form of a dome; the ribs
descend and contract the chest; while the lungs themselves, being elastic,
assist to drive out the air. The latter passes out through the same
channels by which it entered. At the end of each expiration there is a
pause, or period of repose, lasting about as long as the period of action.

[Sidenote: 10. Frequency of respiration? Effect of hurried action of the
heart?]

10. FREQUENCY OF RESPIRATION.--It is usually estimated that we breathe once
during every four beats of the heart, or about eighteen times in a minute.
There is, of course, a close relation between the heart and lungs, and
whatever modifies the pulse, in like manner affects the breathing. When the
action of the heart is hurried, a larger amount of blood is sent to the
lungs, and, as the consequence, they must act more rapidly. Occasionally,
the heart beats so very forcibly that the lungs cannot keep pace with it,
and then we experience a peculiar sense of {130} distress from the want of
air. This takes place when we run until we are "out of breath." At the end
of every fifth or sixth breath, the inspiration is generally longer than
usual, the effect being to change more completely the air of the lungs.

[Sidenote: 11. Respiration controlled by the will? Advantage of the
knowledge to us?]

11. Although, as a general rule, the work of respiration goes on
unconsciously and without exertion on our part, it is nevertheless under
the control of the will. We can increase or diminish the frequency of its
acts at pleasure, and we can "hold the breath," or arrest it altogether for
a short time. From twenty to thirty seconds is ordinarily the longest
period in which the breath can be held; but if we first expel all the
impure air from the lungs, by taking several very deep inspirations, the
time may be extended to one and a half or even two minutes. This should be
remembered, and acted upon, before passing through a burning building, or
any place where the air is very foul. The arrest of the respiration may be
still further prolonged by training and habit; thus it is said, the
pearl-fishers of India can remain three or four minutes under water without
being compelled to breathe.

[Sidenote: 12. Capacity of the lungs? Time required to renovate the air in
the lungs? In tranquil respiration? Importance of the provision?]

12. CAPACITY OF THE LUNGS.--The lungs are not filled and emptied by each
respiration. For while their full capacity, in the adult, is three hundred
and twenty cubic inches, or more than a gallon, the ordinary breathing air
is only one-sixteenth part of that volume, or twenty cubic inches, being
two-thirds of a pint. Accordingly, a complete renovation, or rotation, of
the air of the lungs does not take place more frequently than about once in
a minute; and by the gradual introduction of the external air, its
temperature is considerably elevated before it reaches the delicate
pulmonary capillaries. In tranquil respiration, less than two-thirds of the
breathing power is {131} called into exercise, leaving a reserve capacity
of about one hundred and twenty cubic inches, equivalent to three and one
half pints. This provision is indispensable to the continuation of life;
otherwise, a slight embarrassment of respiration, by an ordinary cold, for
instance, would suffice to cut off the necessary air, and the spark of life
would be speedily extinguished.

[Sidenote: 13. The atmosphere? How high or deep? How essential to life?
Marine life in perfectly pure water and air?]

13. THE AIR WE BREATHE.--The earth is enveloped on all sides by an
invisible fluid, called the atmosphere. It forms a vast and shoreless ocean
of air, forty-five miles deep, encircling and pervading all objects on the
earth's surface, which is absolutely essential for the preservation of all
vegetable and animal life,--in the sea, as well as on the land and in the
air. At the bottom, or in the lower strata of this aerial ocean, we move
and have our being. Perfectly pure water will not support marine life, for
a fish may be drowned in water from which the air has been exhausted, just
as certainly as a mouse, or any other land animal, will perish if put
deeply into the water for a length of time. The cause is the same in both
cases: the animal is deprived of the requisite amount of air. It is also
stated, that if the water-supply of the plant be deprived of air, its vital
processes are at once checked.

[Sidenote: 14. Composition of the air? Properties of the two gases?]

14. The air is not a simple element, as the ancients supposed, but is
formed by the mingling of two gases, known to the chemist as oxygen and
nitrogen, in the proportion of one part of the former to four parts of the
latter. These gases are very unlike, being almost opposite in their
properties: nitrogen is weak, inert, and cannot support life; while oxygen
is powerful, and incessantly active; and is the essential element which
gives to the atmosphere its power to support life and combustion. The
discovery of this fact was made by the French chemist, Lavoisier, in 1778.
{132}

[Sidenote: 15. Air once breathed? An animal in it? A candle? Analysis of
expired air? Change in volume?]

15. CHANGES IN THE AIR FROM RESPIRATION.--Air that has been once breathed
is no longer fit for respiration. An animal confined within it will sooner
or later die; so too, a lighted candle placed in it will be at once
extinguished. If we collect a quantity of expired air and analyze it, we
shall find that its composition is not the same as that of the inspired
air. When the air entered the lungs it was rich in oxygen; now it contains
twenty-five per cent. less of that gas. Its volume, however, remains nearly
the same; its loss being replaced by another and very different gas, which
the lungs exhaled, called _carbonic acid_, or, as the chemist terms it,
_carbon dioxide_.

[Sidenote: 16. What else has the expired air gained? When and where
noticed?]

16. The expired air has also gained moisture. This is noticed when we
breathe upon a mirror, or the window-pane, the surface being tarnished by
the condensation of the watery vapor exhaled by the lungs. In cold weather,
this causes the fine cloud which is seen issuing from the nostrils or mouth
with each expiration, and contributes in forming the feathery crystals of
ice which decorate our window-panes on a winter's morning.

[Sidenote: 17. Nature of the watery vapor? Its effects upon animals?]

17. This watery vapor contains a variable quantity of animal matter, the
exact nature of which is unknown; but when collected it speedily putrefies
and becomes highly offensive. From the effects, upon small animals, of
confinement in their own exhalations, having at the same time an abundant
supply of fresh air, it is believed that the organic matters thrown off by
the lungs and skin are direct and active poisons; and that to such
emanations from the body, more than to any other cause, are due the
depressing and even fatal results which follow the crowding of large
numbers of persons into places of limited capacity. {133}

[Sidenote: 18. Give some of the instances furnished by history.]

18. History furnishes many painful instances of the ill effects of
overcrowding. In 1756, of one hundred and forty-six Englishmen imprisoned
in the Black Hole of Calcutta, only twenty-three, at the end of eight
hours, survived. After the battle of Austerlitz, three hundred prisoners
were crowded into a cavern, where, in a few hours, two-thirds of their
number died. On board a steam-ship, during a stormy night, one hundred and
fifty passengers were confined in a small cabin, but when morning came,
only eighty remained alive.

[Sidenote: 19. Change in the blood from blue to red. Upon what does the
change depend? How shown?]

19. CHANGES IN THE BLOOD FROM RESPIRATION.--The most striking change which
the blood undergoes by its passage through the lungs, is the change of
color from a dark blue to bright red. That this change is dependent upon
respiration has been fully proved by experiment. If the trachea, or
windpipe, of a living animal be so compressed as to exclude the air from
the lungs, the blood in the arteries will gradually grow darker, until its
color is the same as that of the venous blood. When the pressure is removed
the blood speedily resumes its bright hue. Again, if the animal be made to
breathe an atmosphere containing more oxygen than atmospheric air, the
color changes from scarlet to vermilion, and becomes even brighter than
arterial blood. This change of color is not of itself a very important
matter, but it indicates a most important change of composition.

[Sidenote: 20. What does the air lose and gain by respiration? What, the
blood? Air as food?]

20. The air, as we have seen, by respiration loses oxygen and gains
carbonic acid: the blood, on the contrary, gains oxygen and loses carbonic
acid. The oxygen is the food of the blood corpuscles; while the articles we
eat and drink belong more particularly to the plasma of the blood. The air,
then, it is plain, is a sort of food, and we should {134} undoubtedly so
regard it, if it were not for the fact that we require it constantly,
instead of taking it at stated intervals, as is the case with our articles
of diet. Again, as the demand of the system for food is expressed by the
sensation of hunger, so the demand for air is marked by a painful sensation
called suffocation.

[Sidenote: 21. Moist animal membranes? How shown with the bladder?]

21. INTERCHANGE OF GASES IN THE LUNGS.--As the air and the blood are not in
contact, they being separated from each other by the walls of the air-cells
and of the blood-vessels, how can the two gases, oxygen and carbonic acid,
exchange places? Moist animal membranes have a property which enables them
to transmit gases through their substance, although they are impervious to
liquids. This may be beautifully shown by suspending a bladder containing
dark blood in a jar of oxygen. At the end of a few hours the oxygen will
have disappeared, the blood will be brighter in color, and carbonic acid
will be found in the jar.

[Sidenote: 22. Gaseous diffusion? If oxygen be not received? If carbonic
acid be retained?]

22. If this interchange takes place outside of the body, how much more
perfectly must it take place within, where it is favored by many additional
circumstances! The walls of the vessels and the air-cells offer no obstacle
to this process, which is known as gaseous diffusion. Both parts of the
process are alike of vital importance. If oxygen be not received, the
organs cease to act; and if carbonic acid be retained in the blood, its
action is that of a poison; unconsciousness, convulsions, and death
following.

[Sidenote: 23. Difference in the appearance and composition of the blood?
Temperature of the blood? The blood while passing through the lungs? The
consequence?]

23. DIFFERENCE BETWEEN ARTERIAL AND VENOUS BLOOD.--The following table
presents the essential points of difference in the appearance and
composition of the blood, before and after its passage through the lungs:--
{135}

                  _Venous Blood._      _Arterial Blood._

  Color,          Dark blue,           Scarlet.
  Oxygen,         8 per cent.,         18 per cent.
  Carbonic Acid,  15 to 20 per cent.,  6 per cent., or less.
  Water,          More,                Less.

The temperature of the blood varies considerably; but the arterial stream
is generally warmer than the venous. The blood imparts heat to the air
while passing through the lungs, and consequently the contents of the right
side of the heart has a higher temperature than the contents on the left
side.

[Sidenote: 24. What do we learn by means of the spectroscope? "Carriers of
oxygen?" Blue blood in the system?]

24. By means of the spectroscope, we learn that the change of color in the
blood has its seat in the corpuscles; and that, according as they retain
oxygen, or release it, they present the spectrum of arterial or venous
blood. There evidently exists, on the part of these little bodies, an
affinity for this gas, and hence they have been called "carriers of
oxygen." It was long ago thought that blue blood was a trait peculiar to
persons of princely and royal descent, and boastful allusions to the "_sang
azure_" of kings and nobles are quite often met with. Physiology, however,
informs us that blue blood flows in the veins of the low as well as the
high, and that so far from its presence indicating a mark of purity, it, in
reality, represents the waste and decay of the system.

[Sidenote: 25. The amount of air that passes in and out of the lungs?]

25. AMOUNT OF RESPIRATORY LABOR.--During ordinary calm respiration, we
breathe eighteen times in a minute; and twenty cubic inches of air pass in
and out of the lungs with every breath. This is equivalent to the use of
three hundred and sixty cubic inches, or more than ten pints of air each
minute. From this we calculate that the quantity of air which hourly
traverses the lungs is about thirteen cubic feet, or seventy-eight gallons;
and daily, not {136} less than three hundred cubic feet, an amount nearly
equal to the contents of sixty barrels.

[Sidenote: 26. Air absorbed in its transit through the lungs? The loss?
Carbonic acid exhaled? Effect of excitement or exertion? What estimate?]

26. Of this large volume of air five per cent. is absorbed in its transit
through the lungs. The loss thus sustained is almost wholly of oxygen, and
amounts to fifteen cubic feet daily. The quantity of carbonic acid exhaled
by the lungs during the day is somewhat less, being twelve cubic feet.
Under the influence of excitement or exertion, the breathing becomes more
frequent and more profound; and then the internal respiratory work
increases proportionately, and may even be double that of the above
estimate. It has been estimated that in drawing a full breath, a man exerts
a muscular force equal to raising two hundred pounds placed upon the chest.

[Sidenote: 27. Importance of the oxygen in the atmosphere? Injurious
character of gases?]

27. IMPURITIES OF THE AIR.--The oxygen in the atmosphere is of such prime
importance, and its proportion is so nicely adjusted to the wants of man,
that any gas or volatile substance which supplants it must be regarded as a
hurtful impurity. All gases, however, are not alike injurious. Some, if
inhaled, are necessarily fatal; _arsenuretted hydrogen_ being one of these,
a single bubble of which destroyed the life of its discoverer, Gehlen.
Others are not directly dangerous, but by taking the place of oxygen, and
excluding it from the lungs, they become so. Into this latter class we
place carbonic acid.

[Sidenote: 28. Pungency of gases? The inference? Our safeguard?]

28. Most of the actively poisonous gases have a pungent or offensive odor;
and, as may be inferred, most repugnant odors indicate the presence of
substances unfit for respiration. Accordingly, as we cannot see or taste
these impurities, the sense of smell is our principal safeguard against
them; and we recognize the design which has planted this sense, like a
sentinel at the proper entrance of the {137} air-passages, the nostrils, to
give us warning of approaching harm. Take, as an example, the ordinary
illuminating gas of cities, from which so many accidents happen. How many
more deaths would it cause if, when a leak occurs, we were not able to
discover the escape of the gas by means of its disagreeable odor.

[Sidenote: 29. The air of rooms in which fever-sick persons are confined?]

29. Organic matters exist in increased measure in the expired breath of
sick persons, and impart to it, at times, a putrid odor. This is especially
true in diseases which, like typhus and scarlet fever, are referable to a
blood poison. In such cases the breath is one of the means by which nature
seeks to expel the offending material from the system. Hence, those who
visit or administer to fever-sick persons should obey the oft-repeated
direction, "not to take the breath of the sick." At such times, if ever,
fresh air is demanded, not alone for the sick, but as well for those who
are in attendance.

[Sidenote: 30. Animalcula in the water? Dust in the air?]

30. DUST IN THE AIR.--Attention has lately been directed to the dust, or
haze, that marks the ray of sunshine across a shaded room. Just as, many
years ago, it was discovered that myriads of animalcula infested much of
the water we drank, so now the microscope reveals "the gay motes that dance
along a sunbeam" to be, in part, composed of multitudes of animal and
vegetable forms of a very low grade, the germs of fermentation and
putrefaction, and the probable sources of disease.

[Sidenote: 31. The best air filter? The remarks of Prof. Tyndall?]

31. It is found that the best filter by which to separate this floating
dust from the air is cotton wool, although a handkerchief will imperfectly
answer the same purpose. In a lecture on this subject by Prof. Tyndall, he
remarks that, "by breathing through a cotton wool respirator, the noxious
air of the sick room is restored to practical purity. Thus filtered,
attendants may breathe the air unharmed. {138} In all probability, the
protection of the lungs will be the protection of the whole system. For it
is exceedingly probable that the germs which lodge in the air-passages are
those which sow epidemic disease in the body. If this be so, then disease
can certainly be warded off by filters of cotton wool. By this means, so
far as the germs are concerned, the air of the highest Alps may be brought
into the chamber of the invalid."

[Sidenote: 32. Carbonic acid in volcanic regions? In Java? At Lake Avernus?
In mines?]

32. CARBONIC ACID IN THE AIR.--We have already spoken of this gas as an
exhalation from the lungs, and a source of impurity; but it exists
naturally in the atmosphere in the proportion of one half part per
thousand. In volcanic regions it is poured forth in enormous quantities
from fissures in the earth's surface. Being heavier than air, it sometimes
settles into caves and depressions in the surface. It is stated that in the
island of Java, there is a place called the "Valley of Poison," where the
ground is covered with the bones of birds, tigers, and other wild animals,
which were suffocated by carbonic acid while passing. The Lake Avernus, the
fabled entrance to the infernal regions, was, as its name implies,
bird-less, because the birds, while flying over it, were poisoned by the
gas and fell dead into its waters. In mines, carbonic acid forms the
dreaded _choke-damp_, while carburetted hydrogen is the _fire-damp_.

[Sidenote: 33. In the open air? Amount of carbonic acid exhaled by a man? A
gas-burner? A room fire? From furnaces?]

33. In the open air, men seldom suffer from carbonic acid, for, as we shall
see presently, nature provides for its rapid distribution, and even turns
it to profitable use. But its ill effects are painfully evident in the
abodes of men, in which it is liable to collect as the waste product of
respiration and of that combustion which is necessary for lighting and
warming our homes. A man exhales, during repose, not less than one-half
cubic foot of carbonic acid per hour. One gas-burner liberates five cubic
feet in the {139} same time, and spoils about as much air as ten men. A
fire burning in a grate or stove emits some gaseous impurity, and at the
same time abstracts from the air as much oxygen as twelve men would consume
in the same period, thus increasing the relative amount of carbonic acid in
the air. From furnaces, as ordinarily constructed, this gas, with other
products of combustion, is constantly leaking and vitiating the air of
tightly-closed apartments.

[Sidenote: 34. Effects of inhaling carbonic acid alone? In small
quantities?]

34. EFFECTS OF IMPURE AIR.--Carbonic acid, in its pure form, is
irrespirable, causing rapid death by suffocation. Air containing forty
parts per thousand of this gas (the composition of the expired breath)
extinguishes a lighted candle, and is fatal to birds; when containing one
hundred parts, it no longer yields oxygen to man and other warm-blooded
animals; and is of course at once fatal to them. In smaller quantities,
this gas causes headache, labored respiration, palpitation,
unconsciousness, and convulsions.

[Sidenote: 35. Effects of the air in crowded and badly ventilated rooms?]

35. In crowded and badly ventilated apartments, where the atmosphere
relatively contains from six to ten times the natural amount of carbonic
acid, the contaminated air causes dulness, drowsiness, and faintness; the
dark, impure blood circulating through the brain, oppressing that organ and
causing it to act like a blunted tool. This is a condition not uncommon in
our schools, churches, court-rooms, and the like, the places of all others
where it is desirable that the mind should be alert and free to act; but,
unhappily, an unseen physiological cause is at work, dispensing weariness
and stupor over juries, audience, and pupils.

[Sidenote: 36. A cause of consumption? How was the fact illustrated?]

36. Another unmistakable result of living in and breathing foul air is
found in certain diseases of the lungs, especially consumption. For many
years the barracks of {140} the British army were constructed without any
regard to ventilation; and during those years the statistics showed that
consumption was the cause of a very large proportion of deaths. At last the
government began to improve the condition of the buildings, giving larger
space and air-supply; and as a consequence, the mortality from consumption
has diminished more than one-third.

[Sidenote: 37. How, in the case of the lower animals? Tendency of certain
occupations?]

37. The lower animals confined in the impure atmosphere of menageries,
contract the same diseases as man. Those brought from a tropical climate,
and requiring artificial warmth, generally die of consumption. In the
Zoological gardens of Paris, this disease affected nearly all monkeys,
until care was taken to introduce fresh air by ventilation; and then it
almost wholly disappeared. The tendency of certain occupations to shorten
life is well known; disease being occasioned by the fumes and dust which
arise from the material employed, in addition to the unhealthful condition
of the workshop or factory where many hours are passed daily.

[Sidenote: 38. Give the fact as set forth in the table.]

38. The following table shows the comparative amount of carbonic acid in
the air under different conditions and the effects sometimes produced:--

  PROPORTION OF CARBONIC ACID.             In 1000 parts of Air.

  Air of country.                                       .4
   "   "  city.                                         .5
  In hospital, well ventilated.                         .6
  In school, church, etc., fairly ventilated.          1.2 to 2.5
  In court-house, factory, etc., without ventilation.  4. to 40.
  In bedroom, before being aired.                      4.5
  In bedroom, after being aired.                       1.5
  Constantly breathed, causing ill health.             2.
  Occasionally breathed, causing discomfort.           3.
  Occasionally breathed, causing distress.            10.
  Expired air.                                        40.
  Air no longer yielding oxygen                      100.

{141}

[Sidenote: 39. What can you state of the diffusive power of gases? The
added influence of the winds?]

39. NATURE'S PROVISION FOR PURIFYING THE AIR.--We have seen that carbonic
acid is heavier than air, and is poisonous. Why, then, does it not sink
upon and overwhelm mankind with a silent, invisible wave of death? Among
the gases there is a more potent force than gravity, which forever
precludes such a tragedy. It is known as the diffusive power of gases. It
acts according to a definite law, and with a resistless energy compelling
these gases, when in contact, to mingle until they are thoroughly diffused.
The added influence of the winds is useful, by insuring more rapid changes
in the air; air in motion being perfectly wholesome. The rains also wash
the air.

[Sidenote: 40. How is the constant purity of the air secured? Explain the
process?]

40. We have seen that the whole animal creation is constantly abstracting
oxygen from the atmosphere, and as constantly adding to it vast volumes of
a gas injurious alike to all, even in small quantities. How, then, does the
air retain, unchanged, its life-giving properties? The constant purity of
the air is secured by means of the vegetable creation. Carbonic acid is the
food of the plants, and oxygen is its waste product. The leaves are its
lungs, and under the stimulus of sunlight a vegetable respiration is set in
motion, the effects of which are just the reverse of the function we have
been considering. Thus nature purifies the air, and at the same time builds
up beautiful and useful forms of life from elements of decay.

[Sidenote: 41. What process occurs in the sea? How is the fact
illustrated?]

41. In the sea, as in the air, the same circle of changes is observed.
Marine animals consume oxygen and give off carbonic acid; while marine
plants consume carbonic acid and liberate oxygen. Taking advantage of this
fact, we may so arrange aquaria with fishes and sea-plants, in their proper
combinations, so that each supplies the needs of the other, and the water
requires seldom to be renewed. This {142} affords us, on a small scale, an
illustration of the mutual dependence of the two great kingdoms of nature;
as well as of those compensating changes which are taking place on such a
grand scale in the world about us.

[Sidenote: 42. Character of the external air? Of the air in our dwellings?
What becomes imperative? Imperfect ventilation of our dwellings?]

42. VENTILATION.--Since the external atmosphere, as provided by nature, is
always pure, and since the air in our dwellings and other buildings is
almost always impure, it becomes imperative that there should be a free
communication from the one to the other. This we aim to accomplish by
ventilation. As our houses are ordinarily constructed, the theory of
ventilation, "to make the internal as pure as the external air," is seldom
carried out. Doors, windows, and flues, the natural means of replenishing
the air, are too often closed, almost hermetically, against the precious
element. Special means, or special attention, must therefore be used to
secure even a fair supply of fresh air. This is still more true of those
places of public resort, where many persons are crowded together.

[Sidenote: 43. What hints are given for the ventilation of our dwellings?]

43. If there are two openings in a room, one as a vent for foul air, and
the other an inlet for atmospheric air, and if the openings be large, in
proportion to the number of air consumers, the principal object will be
attained. Thus, a door and window, each opening into the outer air, will
ordinarily ventilate a small apartment; or a window alone will answer, if
it be open both above and below, and the open space at each end be not less
than one inch for each occupant of the room, when the window is about a
yard wide. The direction of the current is generally from below upward,
since the foul, heated air tends to rise; but this is not essential. Its
rate need not be rapid; a "draught," or perceptible current, is never
necessary to good ventilation. The temperature of the air admitted may be
warm or cold. It is thought by many that if the {143} air is cold, it is
pure; but this is an error, since cold air will receive and retain the same
impurities as warm air.

[Sidenote: 44. State what Florence Nightingale says about inhaling night
air?]

44. Shall we open our bedrooms to the night air? Florence Nightingale says,
in effect, that night air is the only air that we can then breathe. "The
choice is between pure air without and impure air within. Most people
prefer the latter,--an unaccountable choice. An open window, most nights in
the year, can hurt no one. In great cities, night air is the best and
purest to be had in twenty-four hours. I could better understand, in towns,
shutting the windows during the day than during the night."

[Sidenote: 45. Warmth of the bird as compared with that of the air? Of the
fish and the water? Heat in animals and plants? How illustrated with the
thermometer?]

45. ANIMAL HEAT.--Intimately connected with respiration is the production
of animal heat, or the power of maintaining the temperature of the body
above that of the medium in which the creature moves; thus, the bird is
warmer than the air, and the fish than the water. This elevation of
temperature is a result of the various chemical changes which are
constantly taking place in the system. Although common to all animals, in a
greater or less degree, heat is not peculiar to them; since plants also
generate it, especially at the time of sprouting and flowering. If a
thermometer be placed in a cluster of geranium flowers, it will indicate a
temperature several degrees above that of the surrounding air.

[Sidenote: 46. Amount of heat in animals, how apportioned? As regards the
birds? Frogs, and other sluggish animals? Arrangement made by zoologists?]

46. Among animals great differences are noticed in this respect, but the
degree of heat produced is always proportional to the activity of
respiration and the amount of oxygen consumed. Accordingly, the birds,
whose habits are extremely active, and whose breathing capacity is the
greatest, have uniformly the highest temperature. Sluggish animals, on the
contrary, as frogs, lizards, and snakes, have little need for oxygen, and
have incompletely {144} developed lungs; these animals are cold to the
touch, that is, they have relatively a lower temperature than man, and
their positive temperature is but little above that of the external air.
Accordingly, zoologists have so arranged the animal kingdom that
_warm-blooded_ animals, including man, the birds, and the quadrupeds, are
classified together; while the _cold-blooded_ animals, such as the fish,
tortoise, frog, and all that have no vertebral column, are classed by
themselves.

[Sidenote: 47. State what is said respecting the temperature of the human
body.]

47. The temperature of the human body is about 100° Fahrenheit, and remains
about the same through winter and summer, in the tropics as well as in the
frozen regions of the north. It may change temporarily within the range of
about twelve degrees; but any considerable, or long-continued elevation or
diminution of the bodily heat is certain to result disastrously.

[Sidenote: 48. Ability of man to adapt himself to different climates? In
what does the power to resist cold consist? What is said about warm
clothing?]

48. Man is able to adapt himself to all extremes of climate; and, in fact,
by means of clothing, shelter, and food, is able to create for himself an
artificial climate where-ever he choses to reside. The power to resist cold
consists chiefly in preventing the heat which is generated by the vital
processes of the body from being lost by radiation. Warm clothing, such as
we wear in winter, has, in reality, the same temperature as that which is
worn in summer; but, by reason of being thick and porous, it is a bad
conductor of heat, and thus prevents the escape of that produced by the
body. If woollen fabrics were intrinsically warm, no one would wrap a piece
of flannel, or blanket, around a block of ice to prevent its melting in
summer.

[Sidenote: 49. Men in an atmosphere above the boiling-point? In foundries
and glass works?]

49. The faculty of generating heat explains how it is that we are enabled
to resist the effects of cold; but how does the body withstand a
temperature higher than its {145} own? Men have been known to remain
several minutes in an atmosphere heated above the boiling-point of water,
and yet the temperature of their own bodies was not greatly elevated. Those
who labor in foundries and glass-works are habitually subjected to very
high degrees of temperature, but they do not suffer in health more than
those engaged in many other occupations.

[Sidenote: 50. The regulation of the temperature of the body. Give the
explanation.]

50. The regulation of the temperature of the body is effected by means of
perspiration, and by its evaporation. So long as the skin acts freely and
the air freely absorbs the moisture, the heat of the body does not
increase, for whenever evaporation takes place, it is attended by the
abstraction of heat--that is, the part becomes relatively colder. This may
be tested by moistening some part of the surface with cologne, ether, or
other volatile liquid, and then causing it to evaporate rapidly by fanning.
The principle that evaporation produces cold has been ingeniously and
practically employed, in the manufacture of ice, by means of freezing
machines.

[Sidenote: 51, 52. State what is said of spontaneous combustion.]

51. SPONTANEOUS COMBUSTION.--Is it possible that the temperature of the
living body can be so increased, that its tissues will burn spontaneously?
From time to time, cases have been reported in which, by some mysterious
means, considerable portions of the human body have been consumed,
apparently by fire, the victim being found dead, or incapable of explaining
the occurrence. Hence, the theory has been current that, under certain
conditions, the tissues of the body might become self-ignited; and the fact
that this so-called _spontaneous combustion_ has ordinarily taken place in
those who had been addicted to the use of alcoholic drinks, has given a
color of probability to the opinion. It has been supposed that the flesh of
these unfortunate persons becoming saturated with the inflammable
properties of the alcohol thus taken into the {146} system, took fire upon
being exposed to a flame, as of a lighted candle, or, indeed, without any
external cause. But, whether this be possible or not, one thing is certain,
this strange kind of combustion has never been actually witnessed by any
one competent to give a satisfactory account of it.

52. The results that have been observed may be satisfactorily explained by
the accidental ignition of the clothes, or other articles near the body,
and by the supposition that the individual was at the time too much
stupefied by intoxication, to notice the source of danger, and provide for
his safety. The highest temperature that has been observed in the body,
about 112° Fahrenheit, is too low to ignite the vapor of alcohol; much less
will it cause the burning of animal tissues. It is undoubtedly true that
when the tissues are filled with alcohol, combustion will more easily take
place than when the body is in a normal state; but, under any condition,
the combustion of the body requires a higher degree of heat than can be
generated by the body itself, or the mere _proximity_ of a lighted candle,
or any cause of a similar character. {147}

QUESTIONS FOR TOPICAL REVIEW.

                                                                       PAGE
  1. What is the object of respiration?                                 123
  2. What are the special organs of respiration?                        123
  3. In what organs does a change in the blood take place?              123
  4. What is the nature of the change?                             123, 133
  5. Where are the lungs situated, and what is the character of the
        substance of which they are composed?                      123, 125
  6. Describe the facilities provided for the lung movements.           124
  7. Describe the trachea, or windpipe.                  124, 125, 127, 128
  8. Describe the bronchial tubes, and their uses.                 125, 126
  9. What can you state in relation to the epiglottis?             126, 127
  10. What are the cilia and what use do they probably serve?           128
  11. How may the lungs be affected by not being properly protected?    128
  12. Describe the movements necessary to the act of perfect
        respiration.                                               128, 129
  13. What is the diaphragm, and what is its office?               128, 129
  14. How may the organs of respiration be so improved as to
        increase their capacity and power?                         129, 137
  15. What is stated in relation to the frequency of respiration?  129, 130
  16. To what extent may the act of respiration be subjected to
        our wills?                                                      130
  17. What may be said to be the capacity of the lungs?            130, 131
  18. How long does it take every particle of air in the lungs to
        be expelled and new air to take its place?                      130
  19. What would be the consequences, if the entire capacity of
        the lungs were constantly used?                            130, 131
  20. What would be the consequences to a fish put into water from
        which the air had been completely exhausted? Why?               131
  21. What is the air, and what are its parts?                131, 136, 138
  22. What is the character of the air that has been just breathed?     132
  23. Why is it that such air is not fit for respiration?          132, 139
  24. What are the effects, as recorded in notable cases, of confinement
        in places the air of which has been breathed "over and over?"   133
  25. What can you state of changes in the blood from respiration?      133
  26. What of the air, as an article of food?                      133, 134
  27. What, on the subject of interchange of gases in the lungs?        134
  28. Explain the difference between arterial and venous blood.    134, 135
  29. Explain, if you can, the cause of the difference.                 135
  30. State what you can in relation to blue blood.                     135
  31. In relation to the amount of labor exerted in respiration.   135, 136
  32. In relation to the deleterious properties of different
        gases.                                                     136, 137
  33. In relation to the dust that floats in the air.              137, 138
  34. What are the properties of carbonic acid gas?           132, 138, 141
  35. In what places is carbonic acid gas commonly found?     132, 138, 139
  36. Describe the effects of carbonic acid gas.         132, 138, 139, 141
  37. What are the general effects of breathing any impure
        atmosphere?                                                139, 140
  38. What are Nature's provisions for purifying the air?          141, 142
  39. What hints and directions are given on the subject of
        ventilation?                                               142, 143
  40. How does the temperature of the body compare with the medium
        in which it lives?                                              143
  41. How is temperature of the body regulated and sustained? 143, 144, 145
  42. State what you can on the subject of spontaneous combustion. 145, 146

       *       *       *       *       *


{148}

CHAPTER IX.

THE NERVOUS SYSTEM.

    _Animal and Vegetative Functions--Sensation, Motion, and Volition--The
    Structure of the Nervous System--The White and Gray Substances--The
    Brain--Its Convolutions--The Cerebellum--The Spinal Cord and its System
    of Nerves--The Anterior and Posterior Roots--The Sympathetic System of
    Nerves--The Properties of Nervous Tissue--Excitability of Nervous
    Tissues--The Functions of the Spinal Nerves and Cord--The Direction of
    the Fibres of the Cord--Reflex Activity, and its Uses--The Functions of
    the Medulla Oblongata and the Cranial Ganglia--The Reflex Action of the
    Brain._

[Sidenote: 1. What processes are known as the vegetative functions? Why so
called? What properties and functions does the plant possess? Their
object?]

1. ANIMAL FUNCTIONS.--The vital processes which we have been considering,
in the three previous chapters, of digestion, circulation, and
respiration--belong to the class of functions known as _vegetative_
functions. That is, they are common to vegetables as well as animals; for
the plant, like the animal, can originate nothing, not even the smallest
particle of matter; and yet it grows, blossoms, and bears fruit, by reason
of obtaining and digesting the nutriment which the air and soil provide.
The plant has its circulatory fluid and channels, by which the nutriment is
distributed to all its parts. It has, also, a curious apparatus in its
foliage, by which it abstracts from the air those gaseous elements so
necessary to its support; and thus it accomplishes vegetable respiration.
These vegetative functions have their beginning and end within the organism
of the plant; and their object is the preservation of the plant itself, as
well as of the entire species.

[Sidenote: 2. What second set of powers has the animal? What functions are
mentioned? The advantage they give?]

2. The animal, in addition to these vegetative functions, has another set
of powers, by the use of which he becomes conscious of a world external to
himself, and brings {149} himself into active relations with it. By means
of the vegetative processes, his life and species are maintained; while, by
means of certain animal functions, he feels, acts, and thinks. These
functions, among which are sensation, motion, and volition, not only
distinguish the animal from the plant, but, in proportion to their
development, elevate one creature above another; and it is by virtue of his
pre-eminent endowment, in these respects, that man holds his position at
the head of the animal creation.

[Sidenote: 3. Animals whose structure is simple? As we approach man?
Dependence of the animal functions of man?]

3. Among animals whose structure is very simple, the hydra, or fresh-water
polyp, being an example, no special organs are empowered to perform
separate functions; but every part is endowed alike, so that if the animal
be cut into pieces, each portion has all the properties of the entire
original; and, if the circumstances be favorable, each of the pieces will
soon become a complete hydra. As we approach man, in the scale of beings,
we find that the organs multiply, and the functions become more complete.
The function of motion, the instruments of which--the muscles and
bones--have been considered in former chapters, and all the other animal
functions of man, depend upon the set of organs known as the nervous
system.

[Sidenote: 4. The nervous tissues, of what composed? When examined by the
aid of the microscope? The white substance? The gray substance?]

4. THE NERVOUS SYSTEM.--The intimate structure of this system differs from
any tissue which we have before examined. It is composed of a soft, pulpy
substance, which, early in life, is almost fluid, but which gradually
hardens with the growth of the body. When examined under the microscope, it
is found to be composed of two distinct elements:--(1) the white substance,
composing the larger proportion of the nervous organs of the body, which is
formed of delicate cylindrical filaments, about 1/6000 of an inch in
diameter, termed the nerve-fibres; and (2) the gray substance, composed of
grayish-red, or {150} ashen-colored cells, of various sizes, generally
possessing one or more off-shoots, which are continuous with the
nerve-fibres just mentioned.

[Sidenote: 5. Nervous centres and ganglia? Nerves? What do they serve?
Cerebro-spinal system?]

5. The gray, cellular substance constitutes the larger portion of those
important masses, which bear the name of _nervous centres_ and _ganglia_
(from _ganglion_, a knot), and in which all the nerve-fibres unite. These
white nerve-fibres are found combined together in long and dense cords,
called _nerves_ (from _neuron_, a cord), which serve to connect the nervous
centres with each other, and to place them in communication with all the
other parts of the body which have sensibility or power of motion. That
part of the nervous system which is concerned in the animal functions,
comprises the brain, the spinal cord, and the nerves which are derived
therefrom; these are, together, called the _cerebro-spinal_ system (Fig.
40); while that other set of organs, which presides over, and regulates the
vegetative functions, is called the sympathetic system of nerves.

[Sidenote: 6. Location of the brain? Its weight? Its shape? Of what it
consists? What organs at the base?]

6. THE BRAIN.--The brain is the great volume of nervous tissue that is
lodged within the skull. It is the largest and most complex of the nervous
centres, its weight, in the adult, being about fifty ounces, or
one-fortieth of that of the whole body. The shape of the brain is oval, or
egg-shaped, with one extremity larger than the other, which is placed
posteriorly in the skull, to the concavity of which it very closely
conforms. The brain consists chiefly of two parts; the _cerebrum_, or brain
proper, and the _cerebellum_, or "little brain." In addition to these,
there are several smaller organs at the base, among which is the
commencement or expansion of the spinal cord, termed the _medulla
oblongata_, or oblong marrow.

{151}

[Illustration: FIG. 40.--THE CEREBRO-SPINAL SYSTEM.]

{152} [Sidenote: 7. The tissue of the brain? What, therefore, is required?
Blows on the head? Membranes of the brain? Blood sent to the brain?]

7. The tissue of the brain is soft and easily altered in shape by pressure;
it therefore requires to be placed in a well-protected position, such as is
afforded by the skull, or _cranium_, which is strong without being
cumbrous. In the course of an ordinary lifetime, this bony box sustains
many blows, with little inconvenience; while, if they fell directly upon
the brain, they would at once, and completely, disorganize that structure.
Within the skull, the brain is enveloped by certain membranes, which at
once protect it from friction, and furnish it with a supply of nutrient
vessels; they are called the _arachnoid_, or "spider's web," the _dura
mater_ and the _pia mater_, or the "tough" and "delicate coverings." The
supply of blood sent to the brain is very liberal, amounting to one-fifth
of all that the entire body possesses. The brain of man is heavier than
that of any other animal, except the elephant and whale.

[Sidenote: 8. Size of the brain proper? How divided? The exterior of the
hemispheres? The interior?]

8. THE CEREBRUM.--The brain proper, or _cerebrum_, is the largest of the
intracranial organs, and occupies the entire upper and front portion of the
skull. It is almost completely bisected, by a fissure, or cleft, running
through it lengthwise, into two equal parts called _hemispheres_. The
exterior of these hemispheres is gray in color, consisting chiefly of
nerve-cells, arranged so as to form a layer of gray matter one-fifth of an
inch in thickness, and is abundantly supplied with blood-vessels. The
interior of the brain, however, is composed almost wholly of white
substance, or nerve-fibres.

[Sidenote: 9. The surface of the cerebrum, how marked? The gray matter of
the surface? Extent of the entire brain surface? Source of nervous power?
What further?]

9. The surface of the cerebrum is divided by a considerable number of
tortuous and irregular furrows, about an inch deep, into "convolutions," as
shown in Fig. 41. Into these furrows the gray matter of the surface is
extended, and, in this manner, its quantity is vastly increased. The extent
of the entire surface of the brain, {153} with the convolutions unfolded,
is computed to be equal to four square feet; and yet it is easily enclosed
within the narrow limits of the skull. When it is stated that the gray
matter is the true source of nervous power, it becomes evident that this
arrangement has an important bearing on the mental capacity of the
individual. And it is noticed that in children, before the mind is brought
into vigorous use, these markings or furrows on the surface are
comparatively shallow and indistinct; the same fact is true of the brain in
the less civilized races of mankind and in the lower animals. It is also
noticeable, that among animals, those are the most capable of being
educated which have the best development of the cerebrum.

[Illustration: FIG. 41.--UPPER SURFACE OF THE CEREBRUM.

A, Longitudinal Fissure. B, The Hemispheres.]

[Sidenote: 10. Location of the "little brain?" How divided? Its surface and
interior? Its subdivisions? Its size?]

10. THE CEREBELLUM.--The "little brain" is placed beneath the posterior
part of the cerebrum, and, like the latter, is divided into hemispheres.
Like it, also, the surface of the cerebellum is composed of gray matter,
and its interior is chiefly white matter. It has, however, no convolutions,
but is subdivided by many crescentic, parallel ridges, which, sending down
gray matter deeply into the {154} white, central portion, gives the latter
a somewhat branched appearance. This peculiar appearance has been called
the _arbor vitæ_, or the "tree of life," from the fact that when a section
of the organ is made, it bears some resemblance to the trunk and branches
of a tree (Fig. 42, F). In size, this cerebellum, or "little brain," is
less than one-eighth of the cerebrum.

[Illustration: FIG. 42.--VERTICAL SECTION OF THE BRAIN.

A, Left Hemisphere of Cerebrum. B, Corpus Callosum. C, Optic Thalamus.
D, The Pons Varolii. E, Upper extremity of the Spinal Cord. F, The Arbor
Vitæ.]

[Sidenote: 11. Medulla oblongata? Cranial nerves? Their shape and
position?]

11. From the under surface of the cerebrum, and from the front margin of
the cerebellum, fibres collect together to form the _medulla oblongata_
(Fig. 43, MA), which, on issuing from the skull, enters the spinal column,
and then becomes known as the spinal cord. From the base of the brain, and
from the sides of the medulla originate, also, the _cranial nerves_, of
which there are twelve pairs. These nerves are round cords of glistening
white appearance, and, {155} like the arteries, generally lie remote from
the surface of the body, and are well protected from injury.

[Illustration: FIG. 43.--THE BASE OF THE BRAIN.]

[Sidenote: 12. The spinal cord? Of what composed? How divided? Each half?]

12. THE SPINAL CORD.--The spinal cord, or "marrow," is a cylindrical mass
of soft nervous tissue, which occupies a chamber, or tunnel, fashioned for
it in the spinal column (Fig. 44). It is composed of the same substances as
the brain; but the arrangement is exactly reversed, the white matter
encompassing or surrounding the gray matter instead of being encompassed by
it. The amount of the white substance is also greatly in excess of the
other material. A vertical fissure partly separates the cord into two
lateral halves, and each half is composed of two separate bundles of
fibres, which are named the anterior and posterior columns.

{156} [Illustration: FIG. 44.

A, Cerebrum. B, Cerebellum. D, D, Spinal Cord.]

[Sidenote: 13. Uses of these columns? Importance of this part of the
nervous system? How protected?]

13. These columns have entirely different uses, and each of them unites
with a different portion of the nerves which have their origin in the
spinal cord. The importance of this part of the nervous system is apparent
from the extreme care taken to protect it from external injury. For, while
a very slight disturbance of its structure suffices to disarm it of its
power, yet so staunch is its bony enclosure, that only by very severe
injuries is it put in peril. The three membranes that cover the brain are
continued downward so as to envelope and still further shield this delicate
organism.

[Sidenote: 14. The spinal nerves? The posterior root? The nerves, how
arranged? Their office?]

14. THE SPINAL NERVES.--The spinal nerves, thirty-one pairs in number,
spring from each side of the cord by two roots, an anterior and a posterior
root, which have the same functions as the columns bearing similar names.
The posterior root is distinguished by possessing a ganglion of gray
matter, and by a somewhat larger size. The successive points of departure,
or the off-shooting of these nerves, occur at short and nearly regular
intervals along the course of the spinal cord. Soon after leaving these
points, {157} the anterior and posterior roots unite to form the trunk of a
nerve, which is distributed, by means of branches, to the various organs of
that part of the body which this nerve is designed to serve. The spinal
nerves supply chiefly the muscles of the trunk and limbs and the external
surface of the body.

[Sidenote: 15. The nerve tissue? Its character? Course of each nerve
fibre?]

15. The tissue composing the nerves is entirely of the white variety, or,
in other words, the nerve-fibres; the same as we have observed forming a
part of the brain. But the nerves, instead of being soft and pulpy, as in
the case of the brain, are dense in structure, being hardened and
strengthened by means of a fibrous tissue which surrounds each of these
delicate fibres, and binds them together in glistening, silvery bundles.
Delicate and minutely fine as are these nerve-fibres, it is probable that
each of them pursues an unbroken, isolated course, from its origin, in the
brain or elsewhere, to that particular point which it is intended to serve.
For, although their extremities are often only a hair's breadth distant
from each other, the impression which any one of them communicates is
perfectly distinct, and is referred to the exact point whence it came.

[Sidenote: 16. How may we illustrate the fact? The fibre connecting the
brain with a point in the foot?]

16. This may be illustrated in a simple manner, thus: if two fingers be
pressed closely together, and the point of a pin be carried lightly across
from one to the other, the eyes may be closed, and yet we can easily note
the precise instant when the pin passes from one finger to the other. If
the nerve-fibres were less independent, and if it were necessary that they
should blend with and support each other, all accuracy of perception would
be lost, and all information thus afforded would be pointless and confused.
These silvery threads must, therefore, be spun out with an infinite degree
of nicety. Imagine, for instance, the fibre which {158} connects the brain
with some point on the foot,--its length cannot be less than one hundred
thousand times greater than its diameter; and yet it performs its work with
as much precision as fibres that are comparatively much stronger and less
exposed.

[Sidenote: 17. The sympathetic system of nerves? Of what does it consist?]

17. THE SYMPATHETIC SYSTEM.--The _sympathetic system_ of nerves remains to
be described. It consists of a double chain of ganglia, situated on each
side of the spinal column, and extending through the cavities of the trunk,
and along the neck into the head. These ganglia are made up for the most
part of small collections of gray nerve-cells, and are the nerve-centres of
this system. From these, numerous small nerves are derived, which connect
the ganglia together, send out branches to the cranial and spinal nerves,
and form networks in the vicinity of the stomach and other large organs. A
considerable portion of them also follows the distribution of the large and
small blood-vessels, in which the muscular tunic appears. Branches also
ascend into the head, and supply the muscles of the eye and ear, and other
organs of sense.

[Sidenote: 18. Association of the various regions of the body? If one
member suffers? Blushing?]

18. In this manner, the various regions of the body are associated with
each other by a nervous apparatus, which is only indirectly connected with
the brain and spinal cord; and thus it is arranged that the most widely
separated organs of the body are brought into close and active sympathy
with each other, so that, "if one member suffers, all the other members
suffer with it." From this fact, the name _sympathetic system_, or the
_great sympathetic nerve_, has been given to the complicated apparatus we
have briefly described. Blushing and pallor are caused by mental emotions,
as modesty and fear, which produce opposite conditions of the capillaries
of the face by means of these sympathetic nerves. {159}

[Sidenote: 19. Properties of nervous tissue? Office of the gray substance?
Of the white? The nervous centres? White fibres?]

19. THE PROPERTIES OF NERVOUS TISSUE.--We have seen that in all parts of
this system, there are only two forms of nervous tissue; namely, the gray
substance and the white substance, so called from their difference of color
as seen by the naked eye; or the nerve-cell, and the nerve-fibre, so called
from their microscopic appearance. Now these two tissues are not commonly
mingled together, but either form separate organs, or distinct parts of the
same organs. This leads us to the conclusion that their respective uses are
distinct. And this proves to be the simple fact; wherever we find the gray
substance, we must look upon it as performing an active part in the system,
that is, it originates nervous impulses; the white matter, on the contrary,
is a passive agent, and serves merely as a conductor of nervous influences.
Accordingly, the nervous centres, composed so largely of the gray cells,
are the great centres of power, and the white fibres are simply the
instruments by which the former communicate with the near and distant
regions of the body under their control.

[Sidenote: 20. What comparison is made between the brain and the nation's
capitol? The vital property, excitability? What example is given?]

20. We may compare the brain, then, to the capital, or seat of government,
while the various ganglia, including the gray matter of the cord, like so
many subordinate official posts, are invested with authority over the
outlying provinces; and the nerves, with the white matter of the cord, are
the highways over which messages go and return between these provinces and
the local or central governments. But both forms of nervous tissue possess
the same vital property, called excitability; by which term is meant, that
when a nerve-cell or fibre is stimulated by some external agent, it is
capable of receiving an impression and of being by it excited into
activity. A ray of light, for example, falling upon one extremity of a
fibre in the eye, excites it throughout its whole length; and its {160}
other extremity, within the brain, communicating with a nerve-cell, the
latter, in its turn, is excited, and the sensation of sight is produced.

[Sidenote: 21. Change in the nervous tissues? Nerve force and electricity?]

21. What sort of change takes place in the nervous tissue when its
excitability is aroused, is not known; certainly none is visible. On this
account, it has been thought by some, that the nerve-fibre acts after the
manner of a telegraph wire; that is, it transmits its messages without
undergoing any material change of form. But, though the comparison is a
convenient one, it is far from being strictly applicable; and the notion
that nerve-force is identical with electricity has been fully proved to be
incorrect.

[Sidenote: 22. Functions of the nerves? In the case of the nerve of a
living animal? Of the human body?]

22. THE FUNCTIONS OF THE NERVES.--The nerves are the instruments of the two
grand functions of the nervous system, Sensation and Motion. They are not
the true centres of either function, but they are the conductors of
influences which occasion both. If the nerve in a limb of a living animal
be laid bare, and irritated by pinching, galvanizing, or the like, two
results follow, namely: the animal experiences a sensation, that of pain,
in the part to which the nerve is distributed, and the limb is thrown into
convulsive action. When a nerve in a human body is cut by accident, or
destroyed by disease, the part in which it ramifies loses both sensation
and power of motion; or, in other words, it is paralyzed. We accordingly
say that the nerves have a twofold use, a _sensory_ and _motor_ function.

[Sidenote: 23. If an exposed nerve be divided? What is proved? The course
of the sensory set of fibres? Of the motor set? To what are they likened?]

23. If a nerve that has been exposed be divided, and the inner end, or that
still in connection with the nerve-centres, be irritated, sensation is
produced, but no movement takes place. But if the outer end, or that still
connected with the limb, be irritated, then no pain is felt, but {161}
muscular contractions are produced. Thus we prove that there are two
distinct sets of fibres in the nerves; one of which, the _sensory_ fibres,
conduct toward the brain, and another, the _motor_ fibres, conduct to the
muscles. The former may be said to begin in the skin and other organs, and
end in the brain; while the latter begin in the nervous centres and end in
the muscles. They are like a double line of telegraph wires, one for
inquiries, the other for responses.

[Sidenote: 24. The two roots of the spinal nerves? What has been found?
Difference of the two sorts of fibres? Result of their union?]

24. We have already spoken of the two roots of the spinal nerves, called
from their points of origin in the spinal cord, the anterior and posterior
roots. These have been separately cut and irritated in the living animal,
and it has been found that the posterior root contains only sensory fibres,
and the anterior root has only motor fibres. So that the nerves of a limb
may be injured in such a way that it will retain power of motion and yet
lose sensation; or the reverse condition, feeling without motion, may
exist. Between these two sorts of fibres, no difference of structure can be
found; and where they have joined to form a nerve it is impossible to
distinguish one sort from the other.

[Sidenote: 25. Transient paralysis? When such is the case with the leg?
What other fact is observed?]

25. Occasionally a nerve is so compressed as to be temporarily unable to
perform its functions: a transient paralysis then takes place. This is the
case when the leg or arm "gets asleep," as it is expressed. When such is
the condition with the leg, and the person suddenly attempts to walk, he is
liable to fall, inasmuch as the motor fibres cannot convey orders to the
muscles of the limb. Another fact is observed: there is no sensation in
this nerve at the point of its compression; but the whole limb is numb, and
tingling sensations are felt in the foot, the point from which the sensory
fibres arise.

[Sidenote: 26. What does this illustrate? Sensation? The feeling after a
limb has been amputated? Striking of the "funny bone?"]

26. This illustrates the manner in which the brain {162} interprets all
injuries of the trunk of a nerve. Sensation or pain is not felt at the
point of injury, but is referred to the outer extremities of the nerve,
where impressions are habitually received. This is the reason why, after a
limb has been amputated by the surgeon, the patient appears to suffer pain
in the member that has been severed from the body; while some form of
irritation at the end of the nerve in the wound, or stump, is the real
source of his distress. Again, when the "funny-bone"--that is, the ulnar
nerve at the elbow,--is accidentally struck, the tingling sensations thus
produced are referred to the outer side of the hand and the little finger,
the parts to which that nerve is distributed.

[Sidenote: 27. The spinal nerves, and two from the brain? Of the remainder?
Difference in the nerves? How accounted for? The rate of conduction along a
nerve? As compared with electricity?]

27. All the spinal nerves, and two from the brain, are concerned in both
sensation and motion. Of the remainder of the cranial nerves, some are
exclusively motor, others exclusively sensory; and still others convey, not
ordinary sensations, but special impressions, such as sight, hearing, and
smell, which we have yet to consider. However much the functions of the
nerves seem to vary, there is but little difference discoverable in the
nerves themselves, when examined under the microscope. Whatever difference
exists must be accounted for in consequence of the nerves communicating
with different portions of the gray matter of the brain. The rate of motion
of a message, to or from the brain along a nerve, has been measured by
experiment upon the lower animals, and estimated in the case of man at
about two hundred feet per second. As compared with that of electricity,
this is a very slow rate, but, in respect to the size of the human body, it
is practically instantaneous.

[Sidenote: 28. Functions of the anterior and posterior columns of the cord?
If the cord be divided?]

28. THE FUNCTIONS OF THE SPINAL CORD.--As the {163} anterior and posterior
roots of the spinal nerves have separate functions, so the anterior and
posterior columns of the cord are distinct in function. The former are
concerned in the production of motion, the latter in sensation. If the cord
be divided, as before in the case of the nerve, it is found that the parts
below the point of injury are deprived of sensation and of the power of
voluntary motion on both sides of the body, a form of paralysis which is
called _paraplegia_.

[Sidenote: 29. Paraplegia? Result and danger to life? When the injury
occurs in the neck?]

29. This form of disease, paraplegia, is sometimes seen among men,
generally as the result of a fall, or some other severe accident, by which
the bones of the spine are broken, and the cord is crushed, or pierced by
fragments of bone. The parts which are supplied by nerves from the cord
above the point of injury are as sensitive and mobile as before. The
results are similar, whether the division happens at a higher or lower
portion of the spinal cord; but the danger to life increases proportionally
as the injury approaches the brain. When it occurs in the neck, the muscles
of inspiration are paralyzed, since they are supplied by nerves issuing
from that region; and as a result of this paralysis, the lungs are unable
to act, and life is speedily brought to a close.

[Sidenote: 30. Experiment of cutting the spinal cord of an animal? What
inference is drawn?]

30. When the spinal cord of an animal has been cut, in experiment, it may
be irritated in a manner similar to that alluded to when considering the
nerves. If, then, the upper cut surface be excited, it is found that pain,
referable to the parts below the cut, is produced; but when the lower cut
surface is irritated, no feeling is manifested. So we conclude that in
respect to sensation, the spinal cord is not its true centre, but that it
is merely a conductor, and is therefore the great sensory nerve of the
body. When the lower surface of the cut is irritated, the muscles of the
{164} parts below the section are violently contracted. Hence, we conclude
that, in respect to the movements ordered by the will, the spinal cord is
not their source; but that it acts only as a conductor, and is,
accordingly, the great motor nerve of the body.

[Sidenote: 31. What singular fact is noticed? What does the result show?]

31. DIRECTION OF THE FIBRES OF THE CORD.--If one lateral half of the spinal
cord be cut, or injured, a very singular fact is observed. All voluntary
power over the muscles of the corresponding half of the body is lost, but
the sensibility of that side remains undiminished. This result seems to
show that the motor fibres of the cord pursue a direct course, while its
sensory fibres are bent from their course. And this has been proved to be
the fact; for immediately after the posterior roots--the conductors of
sensory impressions--join the posterior columns, they enter the gray matter
of the cord, and passing over, ascend to the brain on the opposite side.
Accordingly, the sensory fibres from the right and left sides interlace
each other in the gray matter; this arrangement has been termed the
_decussation_, or crossing of these fibres. This condition serves to
explain how a disease or injury of the cord may cause a paralysis of motion
in one leg, and a loss of sensation in the other.

[Sidenote: 32. Direction of the anterior or motor columns? In the cord
itself? In the medulla oblongata? The decussation?]

32. The direction of the anterior, or motor columns of the cord, is
downward from the brain. In the cord itself, the course of the motor fibres
is for the most part, a direct one; but in the medulla oblongata, or upper
extremity of the cord, and therefore early in their career, these fibres
decussate, or cross from side to side in a mass; and not separately, as in
the case of the posterior fibres just mentioned. This arrangement is termed
the _decussation_ of the anterior columns of the medulla.

[Sidenote: 33. Result of the double interlacing of fibres? Where is the
seat of pain when the right hand is hurt? The moving of the foot? Loss of
sensation in one side of the body?]

33. From this double interlacing of fibres results a {165} crossed action
between the original and terminal extremity of all nerve-fibres which pass
through the medulla; namely, those of all the spinal nerves. Consequently,
if the right hand be hurt, the left side of the brain feels the pain; and
if the left foot move, it is the right hemisphere which dictates its
movement. For the same reason, when a loss of sensation and power of motion
affecting the right side of the body alone is observed, the physiologist
understands that the brain has been invaded by disease upon its left side.
This affection is termed _hemiplegia_, or the "half-stroke." The
full-stroke, which often follows the rupture of a blood-vessel in the
brain, is commonly called _paralysis_.

[Sidenote: 34. What other important use has the cord? What is the activity
denominated?]

34. THE REFLEX ACTION OF THE CORD.--We have already considered the cord as
the great motor and sensory nerve of the body, but it has another and
extremely important use. By virtue of the gray matter, which occupies its
central portion, it plays the part of an independent nerve centre. The
spinal cord not only conducts some impressions to the brain, but it also
arrests others; and, as it is expressed, "reflects" them into movements by
its own power. This mode of nervous activity is denominated the _Reflex
Action_ of the cord.

[Sidenote: 35. Example of the fowl? Centipede? Frog? What do they prove?]

35. A familiar example of this power of the cord is found in the violent
movements which agitate a fowl after its head has been cut off. The
cold-blooded animals also exhibit reflex movements in an astonishing
degree. A decapitated centipede will run rapidly forward, and will
seemingly strive to overturn, or else climb over obstacles placed in its
way. A frog similarly mutilated will sustain its headless body upon its
feet, in the standing posture, just as it might do if it were still alive.
If pushed over, it will regain its feet; and if the feet are irritated, it
will {166} jump forward. There can be no doubt that, in the lower animals,
movements may take place which are completely divorced from the will,
sensation, and consciousness; for in those animals, as well as in man,
these faculties have their principal seat within the brain.

[Sidenote: 36. What is necessary in most cases to awaken reflex movements?
In the case of the fowl? Convulsions which follow decapitation?]

36. An irritation is necessary, in most instances, to awaken reflex
movements. In the case of the decapitated fowl, its muscles are excited to
convulsive action by reason of its being thrown upon the hard ground and
roughly handled. Let it be treated differently, and the convulsions will
not take place: let it be laid gently upon soft cotton, and the body will
remain comparatively quiet. It may comfort some people to know that the
convulsions which follow decapitation are not attended with pain; nor are
they a necessary part of the "act of death," as some suppose.

[Sidenote: 37. Actions in the human body distinct from voluntary efforts?]

37. In the human body, likewise, actions are excited that are entirely
distinct from the ordinary voluntary efforts. It is not permissible,
desirable, nor even necessary to decapitate a man that the body may be
disconnected from his brain, in order to test the effect of irritation upon
the spinal cord; although the bodies of beheaded criminals have been
experimented upon, and caused to move by powerful galvanic batteries. The
resort to such means of experiment is rendered unnecessary by the
occurrence of certain deplorable cases of disease and injury, which
effectually sever all communication between the brain and a large part of
the body.

[Sidenote: 38. Reflex action after injury of the cord? Why not due to the
muscles?]

38. Thus, the cord may be so far injured, as the result of accident, as to
terminate all sensation and voluntary motion in the lower half of the body,
the patient seemingly becoming lifeless and powerless from the waist
downward. And yet, by tickling or pinching either foot, the leg {167} of
the same side may be made to jerk, or even to kick with considerable force;
but, unless the patient is observing his limbs, he is wholly unconscious of
these movements, which are, therefore, performed independently of the
brain. And they are in nowise due to the muscles of the limb; for, if the
cord itself becomes diseased below the point of injury, the muscles cease
to contract.

[Sidenote: 39. What are the requisites for the production of this form of
nervous action?]

39. For the production of this form of nervous action three things are
requisite--(1) a nerve to conduct messages from the surface of the body,
one of that variety formerly described as sensory, but which are now
incapable of awakening sensation; (2) a portion of uninjured spinal cord
which shall reflect or convert impressions into impulses; and (3) a motor
nerve to conduct impulses outward to the muscles. The power of the cord to
enforce reflex acts resides in the gray matter, into which the reflex
nerves enter and from which they depart, by means of their posterior and
anterior roots respectively.

[Sidenote: 40. Why do we not readily recognize the reflex activity of the
cord in our own bodies? How best studied in others? Example?]

40. THE USES OF THE REFLEX ACTION.--The reflex activity of the cord is
exhibited in the healthy body in many ways, but since it is never
accompanied with sensation, we do not readily recognize it in our own
bodies. Reflex movements are best studied in the cases of other persons,
when the conditions enable us to distinguish between acts that are
consciously, and those that are unconsciously performed. For example, if
the foot of a person soundly asleep be tickled or pinched, it will be
quickly withdrawn from the irritation.

[Sidenote: 41. Similar movements? Arm of a person? Melted wax or heated
coin on the hand?]

41. Similar movements may be observed in cases where the consciousness and
sensation are temporarily obliterated by disease, or by means of narcotic
poisons. If the arm of a person who has been rendered insensible by {168}
chloroform, be raised, and then allowed to fall, it will be noticed that
the limb does not drop instantly, like a lifeless member, but a certain
amount of rigidity remains in its muscles, which resists or breaks the
force of its descent. Again, when a substance like melted sealing-wax, or a
heated coin, falls upon the hand, the limb is snatched away at once, even
before the feeling of pain has been recognized by the brain. When jolted in
a rapidly moving car, we involuntarily step forward or backward, so as to
preserve the centre of gravity of the body.

[Sidenote: 42. Result of healthful reflex activity? When may the reflex
energy be deficient?]

42. These and similar acts are executed by the same mechanism as that
previously described in the case of paralysis from an injury of the spinal
cord. The muscles thus called into play, are those which are ordinarily
under the sway of the will, but which in these cases act through this
reflex action of the cord, altogether independently of the will. A
healthful reflex activity produces an elasticity, or "tone," of the
voluntary muscular system, which, in a great measure, explains the
existence in the young and vigorous of a feeling of buoyancy and reserve
power. Its possessor is restlessly active, and it may appropriately be said
of him, "he rejoiceth as a strong man to run a race." But this reflex
energy may be deficient. This is true when the blood is poor and wanting in
its solid ingredients, or the circulation is feeble; the muscles, then, are
flabby and weak, and the person himself is said to be "nerveless," or
indisposed to exertion. Shivering from cold, and trembling from fear, may,
in part, be referred to a temporary loss of tone, resulting from a powerful
impression upon the brain.

[Sidenote: 43. Excess of this activity in disease? Hydrophobia, etc.? The
difference in severity of the convulsions?]

43. An excess of this activity may also be observed in disease. In this
condition, the excitability of the cord is unnaturally aroused, and
frequent and violent movements {169} of the limbs and body, called
convulsions, are the result. The convulsions of young children, and the
nervous agitation of _chorea_, or St. Vitus's dance, are reflex in
character; as are also the symptoms attending poisoning by strychnine, and
those terrible diseases, _tetanus_, or "locked jaw," and _hydrophobia_. The
severity of the convulsions is not the same in all cases of these
disorders; but, in those last mentioned the most violent spasmodic
movements are provoked by the slightest form of irritation--such as the
sound of pouring water, the sight of any glittering object, the glancing of
a mirror, the contact of cool air, or even the touch of the bedclothes.

[Sidenote: 44. Another variety of reflex motions? What are they? What is
stated of the mind in connection with these movements?]

44. Another variety of reflex motions takes place in certain involuntary
muscles, and over these the cord exercises supreme control. They are
principally those movements which aid the performance of digestion and
nutrition, the valve-action of the pylorus, and other movements of the
stomach and intestines. In these movements the mind shares no part. And it
is well that this is so; for since the mind is largely occupied with
affairs external to the body, it acts irregularly, becomes fatigued, and
needs frequent rest. The spinal cord, on the contrary, is well fitted for
the form of work on which depends the growth and support of the body, as it
acts uniformly, and with a machine-like regularity.

[Sidenote: 45. Consciousness in these operations? Physical wants?]

45. These operations are not accompanied by consciousness; for, as a
general rule, the attention is only called to them when they become
disordered. Many a person does not know where his stomach is situated,
until he discovers its position by reason of a feeling of distress within
it, produced by giving that organ improper work to perform. In this manner
the higher and nobler faculties of the mind are liberated from the simply
routine duties of the {170} body; and we are thus left to direct the
attention, the reason, and the will to the accomplishment of the great ends
of our existence. If it were otherwise, we could only find time to attend
to our ordinary physical wants.

[Sidenote: 46. How many objects may the reflex activity be said to have?
State the first. The second. The third.]

46. The objects of the reflex activity of the cord are threefold. In the
first place, it acts as the protector of man, in his unconscious moments.
It is his unseen guardian, always ready to act, never growing weary, and
never requiring sleep. Nor does its faithful action wholly cease with the
cessation of life in other parts. In the second place, it is the regulator
of numerous involuntary motions that are necessary to the nutrition of the
body. Here its actions are entirely independent of the brain, and are
performed in a secret and automatic manner. And, thirdly, it acts as a
substitute, and regulates involuntary movements in the muscles usually
under the influence of the will. It thus takes the place of the higher
faculties in performing habitual acts, and permits them to extend their
operations more and more beyond the body and its material wants.

[Sidenote: 47. How does the medulla oblongata resemble the cord?]

47. THE FUNCTIONS OF THE MEDULLA OBLONGATA.--The prolongation of the spinal
cord, within the skull, has been previously spoken of as the medulla
oblongata. It resembles the cord, in being composed of both white and gray
matter, and in conducting sensory and motor influences. It likewise gives
rise to certain nerves, which are here called cranial nerves (from
_cranium_, the skull). All except two of these important nerves spring from
the medulla, or the parts immediately adjoining it; the exceptions are the
two nerves taking part in the special senses of sight and smell, which
nerves have their origin at the base of the cerebrum.

[Sidenote: 48. What final fact is observed in the crossing of the motor
columns?]

48. The decussation, or crossing of the motor columns, has been previously
described, when treating of the {171} direction of the nerve-fibres of the
cord; and the singular fact has been alluded to, that when one side of the
brain is injured, its effects are limited to the opposite side of the body.
One more fact remains to be observed in this connection, namely, this
crossed action does not usually take place in the cranial nerves.
Accordingly, when apoplexy, or the rupture of a blood-vessel, occurs in the
right hemisphere of the cerebrum, the left side of the body is paralyzed,
but the right side of the face is affected; this is because that part of
the body is supplied by the cranial nerves.

[Sidenote: 49. The pneumogastric nerve? The feelings aroused by it? The
"vital knot?"]

49. A portion of the medulla presides over the important function of
respiration, and from it arises the _pneumogastric_ nerve, so called
because its branches serve both the lungs and stomach. The feelings of
hunger, thirst, and the desire for air are aroused by means of this nerve.
The wounding of the gray matter of the medulla, even of a small portion of
it, near the origin of the pneumogastric nerve, at once stops the action of
the lungs and causes death. In consequence of the importance of this part,
it has been termed the "vital knot." We find, also, that its location
within the skull is exceedingly well protected, it being quite beyond the
reach of any ordinary form of harm from without.

[Sidenote: 50. The uses of the smaller gray masses at the base of the
brain?]

50. THE FUNCTIONS OF THE CRANIAL GANGLIA.--The uses of the smaller gray
masses lying at the base of the brain are not well ascertained; and, on
account of their position, so remote from the surface, it would, at first,
seem well-nigh impossible to study them. But, from the results following
diseases in these parts, and from experiments upon inferior animals, they
are becoming gradually better understood; and there is reason to believe
that eventually the physiological office of each part will be clearly
ascertained and defined. It is believed, however, but not {172} absolutely
proven, that the anterior masses, like the anterior roots of the spinal
nerves and the anterior columns of the cord, are concerned in the
production of motion; in fact, that they are the central organs of that
function. The posterior gray masses are, on the contrary, supposed to be
the seat of sensation.

[Sidenote: 51. Function of the cerebellum? When it is diseased?]

51. THE FUNCTION OF THE CEREBELLUM.--The function of the cerebellum, or
"little brain," is the direction of the movements of the voluntary muscles.
When this organ is the seat of disease or injury, it is usually observed
that the person is unable to execute orderly and regular acts, but moves in
a confused manner as if in a state of intoxication. Like the larger brain,
or cerebrum, it appears to be devoid of feeling; but it takes no part in
the operations of the mind.

[Sidenote: 52. Where is the seat of the mind? The subordination of the
other organs? The gray matter?]

52. THE FUNCTION OF THE CEREBRUM.--The cerebrum, or brain proper, is the
seat of the mind; or, speaking more exactly, it is the material instrument
by which the mind acts; and, as it occupies the highest position in the
body, so it fulfils the loftiest uses. All the other organs are subordinate
to it: the senses are its messengers, which bring it information from the
outer world, and the organs of motion are its servants, which execute its
commands. Here, as in the nervous apparatus of lower grade already
considered, the gray matter is the element of power; and, in proportion as
this substance increases in extent, and in proportion to the number of
convolutions in the hemispheres, do the mental faculties expand.

[Sidenote: 53. What is stated of men in connection with the size of their
brain? With the brains of other animals?]

53. There have been a few, but only a few, men of distinguished ability
whose brains have been comparatively small in size; the rule being that
great men possess large brains. The relative weight of the brain of man, as
{173} compared with the weight of the body, does not, in all instances,
exceed that of the inferior animals; the canary and other singing-birds
have a greater relative amount of nervous matter than man; but man
surpasses all other creatures in the size of the hemispheres of the
cerebrum, and in the amount of gray substance which they contain.

[Sidenote: 54. Sensitiveness of the brain substance? The removal of a
portion of the brain? State the remarkable case mentioned?]

54. It is a singular fact that this cerebral substance is insensitive, and
may be cut without causing pain. The removal of a considerable quantity of
the brain has taken place, as the result of accident, without causing
death, and without even affecting seriously the intellect. A remarkable
case of injury of the brain is recorded, in which, from the accidental
explosion of gunpowder used in blasting a rock, the "tamping-iron" was
driven directly through the skull of a man. This iron rod, three feet and
seven inches long, an inch and a quarter in diameter, and weighing more
than thirteen pounds, entered the head below the ear and passed out at the
top of the skull, carrying with it portions of the brain and fragments of
bone. The man sustained the loss of sight on one side, but otherwise
recovered his health and the use of his faculties. Moreover, disease has
occurred, compromising a large portion of the brain, without impairing the
faculties of the mind, when the disease was limited to one side only.

[Sidenote: 55. Thought, emotion, and will? What power do they give us?]

55. Impressions conveyed to the hemispheres from the external world arouse
the mental operations called thought, emotion, and the will. These are the
godlike attributes which enable man to subjugate a world, and afterward
cause him to "sigh for other worlds to conquer;" which enable him to
acquaint himself with the properties of planets millions of miles distant
from him, and which give him that creative power by which he builds and
peoples the new worlds of poetry and art. {174}

[Sidenote: 56. Are the brain and the mind identical?]

56. All these mental acts, and many others, are developed through the
action of the brain; not that the brain and the mind are the same, or that
the brain secretes memory, imagination, or the ideas of truth and justice,
as the stomach secretes the gastric juice. But rather, as the nerve of the
eye, stimulated by the subtile waves of light, occasions the notion of
color, so the brain, called into action by the mysterious influences of the
immaterial soul, gives rise to all the intellectual, emotional, and
voluntary activities of mankind.

[Sidenote: 57. What do we know of the cerebrum and its powers?]

57. The cerebrum, according to our present knowledge of it, must be
regarded as a single organ, which produces different results, according as
it is acted upon by the immaterial mind in different ways. Recent
investigations, however, seem to prove that the faculty of language is
dependent upon a small part of the left hemisphere of the cerebrum, near
the temple. At least, in almost every instance where this part is diseased,
the patient can no longer express himself in speech and writing.

[Sidenote: 58. The reflex function of the organs within the skull? The
reflex power of the medulla? Respiration?]

58. THE REFLEX ACTION OF THE BRAIN.--The reflex function of the organs
within the skull is very active and important. Like that of the cord, it
protects the body by involuntary movements, it regulates the so-called
vegetative acts, and it takes the place of the will in controlling the
voluntary muscles, when the attention is turned in other directions. The
reflex power of the medulla governs the acts of respiration, which are
absolutely and continuously essential to life. Respiration is, as we have
seen, partly under the influence of the will; but this is due in part to
the fact that respiration is indirectly concerned in one of the animal
functions, that of speech.

[Sidenote: 59. What else does reflex action occasion? Winking? Other
examples?]

59. Reflex action also occasions coughing and sneezing, {175} whenever
improper substances enter the air-passages. Winking is an act of the same
sort, and serves both to shield the eyes from too great glare of light, and
to preserve them by keeping the cornea moist. Looking at the sun or other
strong light, causes sneezing by reflex action. Laughing, whether caused by
tickling the feet or by some happy thought, and also sobbing, are reflex
acts, taking place by means of the respiratory muscles.

[Sidenote: 60. Muscles called into play by certain reflex movements? The
somnambulist?]

60. Certain of the protective reflex movements call into play a large
number of muscles, as in the balancing of the body when walking along a
narrow ledge, or on a slippery pavement. The dodging motion of the recruit,
when the first cannon ball passes over his head, is reflex and involuntary.
The fact that these involuntary, reflex acts are performed with great
precision, will explain why it is that accidents seldom befall the
somnambulist, or sleep-walker, although he often ventures in most perilous
places.

[Sidenote: 61. What is said of walking and other acts in connection with
the office performed by the medulla and spinal cord?]

61. Walking, sitting, and other acts of daily life, become automatic, or
reflex, from habit: the mind is seldom directed to them, but delegates
their control to the medulla and spinal cord. Thus a person in walking, may
traverse several miles while absorbed in thought, or in argument with a
companion, and yet be conscious of scarcely one in a thousand of the acts
that have been necessary to carry his body from one point to another. By
this admirable and beautiful provision, the mind is released from the
charge of the ordinary mechanical acts of life, and may devote itself to
the exercise of its nobler faculties. And it is worthy of notice, that the
greater the use of these faculties, the more work does the reflex function
assume and perform; and thus the employment of the one insures the
improvement of the other. {176}

QUESTIONS FOR TOPICAL REVIEW.

                                                                       PAGE
  1. State fully what is meant by the term vegetable function.          148
  2. To what is man indebted for his position as the head of the
        animal creation?                                           148, 149
  3. What can you state on the subject of special organs for
        separate functions?                                             149
  4. Describe, as fully as you can, the structure of the nervous
        system.                                                    149, 150
  5. Describe the brain, its location, size, shape, and structure. 150, 152
  6. Describe the brain proper, or cerebrum.                  152, 153, 174
  7. What connection is noticed between the cerebrum and mental
        power?                                                153, 172, 174
  8. Describe the little brain, or cerebellum.                153, 154, 172
  9. Describe the spinal cord.                                154, 155, 156
  10. What are the spinal nerves, and how are they arranged?       156, 157
  11. What is the character and substance of their tissues?             157
  12. State how the nerve-fibres perform their office, and give
        the illustration.                                          157, 158
  13. Describe the sympathetic system of nerves.                        158
  14. State what is meant by the properties of nervous tissue,
        and give the illustration.                                 159, 160
  15. Explain what is meant by the functions of the nerves, and
        give the illustration.                                160, 161, 162
  16. What is meant by a transient paralysis of a nerve? Give the
        illustration.                                              161, 162
  17. What can you state of the rate of message-motion along a nerve?   162
  18. What are the functions of the spinal cord?         162, 163, 164, 165
  19. State what you can of the form of paralysis known as
        paraplegia.                                                     163
  20. What experiments, with results, upon the spinal cord are
        noted?                                                     163, 164
  21. Explain how injury of the cord may produce paralysis of
        motion in one leg, and at the same time a loss of sensation
        in the other.                                                   164
  22. Explain how, if the right hand be hurt, the left side of
        the brain is made to feel the pain.                             165
  23. Now, explain as fully as you can the direction of the fibres
        of the cord.                                               164, 165
  24. What is understood by the reflex action of the cord?              165
  25. What experiments are mentioned to prove this power of the
        cord?                                                      165, 166
  26. What are the uses of the reflex action of the cord?           167-170
  27. What illustrations are mentioned to show such uses?           167-170
  28. What is the medulla oblongata?                               154, 170
  29. What are the functions of the medulla oblongata?             170, 171
  30. What can you state of the functions of the cranial ganglia?  171, 172
  31. What are the functions of the cerebellum?                         172
  32. What is the function of the cerebrum?                        172, 174
  33. In what way does the size of the brain generally indicate
        the character of the man?                                  172, 173
  34. What facts show that the gray substance of the brain is
        insensitive?                                                    173
  35. Upon what does the faculty of language seem to depend?            174
  36. What has been observed in support of this statement?              174
  37. Of what importance is the reflex action of the brain?        174, 175
  38. In what ways is this importance made manifest?               174, 175

       *       *       *       *       *


{177}

CHAPTER X.

THE SPECIAL SENSES.

    _The Production of Sensations--Variety of Sensations--General
    Sensibility--Pain and its Function--Special Sensation, Touch, Taste,
    Smell, Sight, and Hearing--The Hand, the Organ of Touch--The Sense of
    Touch--Delicacy of Touch--Sensation of Temperature and Weight--The
    Tongue the Organ of Taste--The Nerves of Taste--The Sense of Taste and
    its Relations with the other Senses--The Influence of Education on the
    Taste--The Nasal Cavities, or the organs of Smell--The Olfactory
    Nerve--The Uses of the Sense of Smell--The Sense of Sight--Light--The
    Optic Nerve--The Eyeball and its Coverings--The Function of the
    Iris--The Sclerotic, Choroid, and Retina--The Tears and their
    Function--The Movements of the Eyeball--The Function of
    Accommodation--The Sense of Hearing and Sound--The Ear, or the organ of
    Hearing--The External, Middle, and Internal Ear._

[Sidenote: 1. True centre of sensation? Place of the mind's impressions?
What is it convenient to say? What further is stated?]

1. PRODUCTION OF SENSATIONS.--We have already seen that the true centre of
sensation is some organ within the skull, probably among the gray masses at
the base of the brain; but the mind never perceives impressions at that
point; but, on the contrary, always refers them to the external organs of
sensation. Hence, it is convenient to say, that those outer parts possess
the property of sensibility. For instance, we say that we hear with the
ear, taste with the tongue, and feel with the fingers. That this is not the
exact truth is proven by the fact, that whenever the nerve connecting one
of these organs with the brain is severed, it at once loses its capacity
for sensation.

[Sidenote: 2. Consciousness? During sleep? In profound insensibility?]

2. Consciousness, another faculty of the brain, is necessary to complete a
sensation. During sleep, and in other unconscious states, the usual
impressions are presented to {178} the ear, the nose, and the skin, but
they fail to excite sensations, because the nerve-centres are inactive. In
profound insensibility, from chloroform or ether, a limb may be removed
without occasioning the least feeling.

[Sidenote: 3. Sensibility in animals? In the earth-worm? In man?]

3. VARIETY OF SENSATIONS.--All animals have some degree of sensibility. It
is of course feeble and indistinct in the lower forms of life, but
increases in power and variety as we ascend the scale. In the earth-worm,
the nervous system is very simple, the sensibility being moderate and alike
in all parts: hence, if its body be cut into two pieces, each piece will
have the same degree of feeling as before. As we approach man, however, the
sensations multiply and become more acute; the organs are more complex, and
special parts are endowed with special gifts. These special organs cannot
be separated from the rest of the body without the loss of the functions
they are designed to exercise.

[Sidenote: 4. The lowest form of sensation? The highest? Sensations, how
modified? What further can you state as to habitual impressions?]

4. The lowest form of sensation, that of simple contact, is possessed by
the lowest of the animal creation. The highest forms are those by which we
are enabled to know the properties of external objects, such as shape,
size, sound, and color. A variety of means of communicating with the outer
world is the necessary possession of a high intelligence. Sensations are
modified by use. They become more acute and powerful by moderate exercise;
or, they are dulled by undue excitement. The former is shown by the acute
hearing of the Indian, by the sharp sight of the sailor, and by the
delicate touch of the blind. The latter is exemplified by the impaired
hearing of the boiler-maker, and the depraved taste of him who uses pungent
condiments with his food. Again, impressions habitually presented may not
be consciously felt; as is the case with the rumbling of carriages in a
neighboring {179} street, or the regular ticking of a clock. All sensations
become less vivid with the advance of age, especially hearing and vision.

[Sidenote: 5. General sensibility? What have we seen as regards the brain?
Of what other structures is the same true?]

5. GENERAL SENSIBILITY.--There is a property possessed by nearly all parts
of the human body which we call general sensibility. We have recently seen
that the brain is wholly insensitive, and may be cut or pinched without
pain. The same is true of the nails, hair, the scarf-skin or external
covering of the body, and a few other structures. In these parts no nerves
are found. On the other hand, the sensibility of the true skin, and of
mucous membranes, as of the eye and nose, is exquisite, these organs having
a large supply of sensory nerve-fibres. The bones and tendons have less of
these fibres, and are only moderately sensitive.

[Sidenote: 6. The cause of sensibility? Painful part in a surgical
operation? Benumbing the surface? How done by ether?]

6. The sensibility of any part of the body, then, depends upon the number
of nerves present; and, as a rule, the nervous supply is proportional to
the importance of the part, and to its liability to injury. When,
therefore, a surgical operation is performed, the most painful part of it
is the incision through the skin; the muscles, cartilage, and bone being
comparatively without sensation. Hence, if we could benumb the surface,
certain of the lesser operations might be undergone without great
inconvenience. This is, in fact, very successfully accomplished by means of
the cold produced by throwing a spray of ether, or of some other rapidly
evaporating liquid, upon the part to be cut.

[Sidenote: 7. Tickling? Internal sensations? The nerves of general
sensibility?]

7. Tickling is a modification of general sensibility. At first, it excites
a pleasurable sensation, but this soon passes into pain. It is only present
in those parts where the sense of touch is feeble. But all impressions are
not received from without: there are, also, certain internal {180}
sensations, as they are called, which depend upon the condition of the
internal organs, such as appetite, hunger, thirst, the sense of
satisfaction after taking food, dizziness when looking down from some lofty
position, lassitude, drowsiness, fatigue, and other feelings of comfort or
discomfort. General sensibility, whether of the internal or external organs
of the body, chiefly depends upon the sensory fibres of the spinal nerve.
The face, however, is supplied by the sensory cranial nerves. The
sympathetic system has a low grade of feeling in health; but disease in the
parts served by it arouses an intense degree of pain.

[Sidenote: 8. Connection between pain and sensibility?]

8. THE SENSATION OF PAIN.--What then is _pain_? Is it identical with
ordinary sensibility? There seems to be some necessary connection between
the two feelings, for they take place through the same channels, and they
are alike intense in the same situations. But sensibility habitually
contributes to our sources of pleasure, the very opposite of pain; hence,
these feelings cannot be identical.

[Sidenote: 9. Explain the difference between pain and sensibility.]

9. Pain must, therefore, be a modification of the general sensibility,
which follows an excessive degree of excitement of the nerves; there being
a natural limit to the amount of stimulation which they will sustain. So
long as this limit is observed, the part excited may be said to be simply
sensitive; but when it is exceeded, the impression becomes painful. This
difference between sensibility and pain is well shown by the effects of
sunlight upon the eye. The indirect illumination of the sun arouses only
the former feeling, and is indispensable to our comfort and existence;
while the direct ray received into the eye occasions great pain.

[Sidenote: 10. Dread of pain? How may its value be appreciated? Example.]

10. THE USES OF PAIN.--The dread of pain is a valuable monitor to the body.
It puts us on our guard in the presence of danger; teaches moderation in
the use of our {181} powers; indicates the approach of disease; and calls
attention to it when present. The word disease, in fact, according to its
original use, had reference simply to the pain, or want of ease, which
commonly attends disordered health. When we observe the serious mishaps
which occur when sensibility and pain are absent, we cannot fail to
appreciate its value. For example, a paralytic in taking a foot-bath,
forgets to test its temperature, and putting his limbs into water while it
is too hot, is severely scalded without knowing it.

[Sidenote: 11. The case of the traveller? Grain of sand? The sun and
child?]

11. A traveller, overcome by cold and fatigue, lies down and falls asleep
near a large fire, and when he is aroused in the morning, it is discovered
that one of his feet has been insensibly destroyed. A grain of sand,
lodging in an insensitive eye, may cause inflammation and even the loss of
sight. If intense light were not painful to the eye, many a child would
innocently gaze upon the glories of the sun to the ruin of his sight.

[Sidenote: 12. Mission of pain? Painful impressions compared with those of
pleasure?]

12. Pain is, indeed, a present evil, but its relations with the future
prove its mission merciful. Painful impressions cannot be recollected from
past experience; and they cannot be called into existence by the fancy.
Considered in the light of results, pain has a use above that of pleasure;
for while the immoderate pursuit of the latter leads to harm, the tendency
of pain is to restrict the hurtful courses of life, and in this manner to
protect the body.

[Sidenote: 13. What does Magendie say of the relation of pain to pleasure?]

13. The relations of pain to pleasure are thus described by the eminent
physiologist, Magendie:--"By these sensations Nature induces us to concur
in the order which she has established among organized beings. Though it
may appear like sophistry to say that pain is the shadow of pleasure, yet
it is certain that those who have exhausted the ordinary sources of
pleasure have recourse to the {182} causes of pain, and gratify themselves
by their effects. Do we not see in all large cities, that men who are
debauched and depraved find agreeable sensations, where others experience
only intolerable pain?"

[Sidenote: 14. The law of Nature as regards painful sensations among
animals?]

14. As to painful sensation among the inferior animals, the plan of Nature
seems to be, that the higher the intelligence of the creature, and the more
complete its power of defence, the more acute is its sensibility. We infer,
therefore, that animals low in the scale of existence, and helpless, are
not very liable to suffer pain.

[Sidenote: 15. The sensation of contact and pain? Special sensations of
man? How regarded?]

15. SPECIAL SENSATION.--The sensations of simple contact and pain are felt
by nearly all parts of the system, whether external or internal, and are
the necessary consequence of the general sensibility; but, so far as the
objects which surround us are concerned, these impressions are vague and
passive in character, and inform the mind of none of the properties or
powers of these objects. Besides these feelings, therefore, man is endowed
with certain special sensations, which are positive and distinct in
character, and which he can call into exercise at will, and employ in the
pursuit of knowledge. For reasons relating to the original constitution of
the body, these sensations are to be regarded as modifications of the
general sensibility already alluded to, constructed with special reference
to the different forces of Nature, of which we have any knowledge, such as
heat, motion, gravity, sunlight, and the like.

[Sidenote: 16. What are the special senses? Special organs for them?]

16. These distinct and active faculties are termed the special senses, and
are five in number, viz., Touch, Taste, Smell, Sight, and Hearing. For the
exercise of these senses, special organs are furnished, such as the hand,
the tongue, the nose, the eye, and the ear. The manner in which the nerves
of special sense terminate, varies in the {183} case of each organ, so that
each is adapted to one set of sensations alone, and is incapable of
perceiving any other. Thus the nerve of hearing is excited by the
undulations of sound, and not by those of light, while the reverse is true
of the nerve of sight; and the nerve of smell can appreciate neither of
them, being capable only of taking cognizance of the odorous properties of
bodies.

[Sidenote: 17. What is said in relation to one more than the five senses?]

17. By some writers six senses are accorded to man; the additional one
being either the sense of temperature, for as we shall presently see this
is not the same as touch; or according to others, the muscular sense by
which we are enabled to estimate the weights of bodies. The latter also
differs in some respects from the sense of touch.

[Sidenote: 18. The sense of touch, how prevalent? What is said of the
hand?]

18. ORGANS OF TOUCH.--The sense of touch is possessed by nearly all
portions of the general surface of the body, but it finds its highest
development in the hands. The human hand is properly regarded as the model
organ of touch. The minute structure of the skin fits it admirably for this
form of sensation: the cuticle, or scarf-skin, is fine and flexible, while
the cutis, or true skin, contains multitudes of nerve-filaments, arranged
in rows of _papillæ_, or cone-like projections, about one-hundredth of an
inch in length. It is estimated that there are 20,000 of these papillæ in a
square inch of the palmar surface of the hand. Now, although the nerves of
the cutis are the instruments by which impressions are received and
transmitted to the brain, yet the cuticle is essential to the sensation of
touch. This is shown by the fact that whenever the true skin is laid bare,
as by a burn or blister, the only feeling that it experiences from contact
is one of pain, not that of touch.

[Sidenote: 19. Office of the cuticle? Tips of the fingers? The fingers with
thumb?]

19. The office of the cuticle is thus made evident: it is to shield the
nerve filaments from direct contact with {184} external objects. At the
tips of the fingers, where touch is most delicate, the skin rests upon a
cushion of elastic material, and receives firmness and permanence of shape
by means of the nail placed upon the less sensitive side. Besides these
favorable conditions, the form of the arm is such, and its motions are so
easy and varied, that we are able to apply the test of touch in a great
number of directions. The slender, tapering fingers, with their pliant
joints, together with the strong opposable thumb, enable the hand to mold
itself upon and grasp a great variety of objects; so that great as are the
delicacy and grace of the hand, it is not wanting in the elements of power.

[Sidenote: 20. What special importance is attributed to the hand?]

20. Its beauty and adaptation to the wants of man have made the hand an
attractive theme for philosophers. They do not, however, always agree in
their conclusions. One has the opinion that man has acquired his
intelligence and achieved his place as "lord of creation," because he has
this organ. Buffon, in effect, declares that with fingers twice as numerous
and twice as long, we would become proportionally wiser; but Galen long ago
took a more reasonable view, when he taught that "man is the wisest of
animals, not because he possesses the hand; but because he is the wisest
and understands its use, the hand has been given to him; for his mind, not
his hand has taught him the arts." Another has well said, that "no one can
study carefully the human hand and fail to be convinced of the existence of
the Deity."

[Sidenote: 21. The simplicity of touch? What does it teach us?]

21. THE SENSE OF TOUCH.--Touch is the simplest of the senses. It is that
which the child first calls into exercise in solving the early problems of
existence; and it is that which is in the most constant use throughout
life. We are brought by the touch into the most intimate relations with
external objects, and by it we learn the greater number, if not the most
important, of the properties of {185} these objects; such as size, figure,
solidity, motion, and smoothness or roughness of surface.

[Sidenote: 22. Importance of the sense of touch to the development of the
other senses?]

22. The sense of touch assists the other senses, especially that of sight,
giving foundation and reality to their perceptions. Without it, the
impressions received by the eye would be as vague and unreal as the figures
that float through our dreams. A boy who had been blind from birth, at the
age of twelve years received sight by means of a surgical operation: at
first, he was unable to distinguish between a globe and a circular card, of
the same color, before he had touched them. After that, he at once
recognized the difference in their form. He knew the peculiarities of a dog
and a cat by feeling, but not by sight, until one day, happening to take up
the cat, he recognized the connection of the two sorts of impressions,
those of touch and sight; and then, putting the cat down, he said: "So,
puss, I shall know you next time."

[Illustration: FIG. 45.]

[Sidenote: 23. Liability of touch to err? Describe the illustration.]

23. Touch is considered the least liable to error of all the senses; yet,
if that part of the skin by which the sense is exercised is removed from
its customary position, a false impression may be created in the mind. This
is well illustrated by an experiment, which dates from the time of
Aristotle. If we cross the middle finger behind the {186} forefinger, and
then roll a marble, or some small object, upon the tips of the fingers (see
Fig. 45), the impression will be that two marbles are felt. If the fingers,
thus transposed, be applied to the end of the tongue, two tongues will be
felt. When the nose is accidentally destroyed, the surgeon sometimes
performs an operation for the purpose of forming a new one, by
transplanting a partially removed piece of the skin of the forehead upon
the injured part: then, if the new nose be touched or pinched, the feeling
is referred to the forehead. This fact illustrates one important truth,
that the nerves will re-unite after they have been cut, and feeling will be
restored: if it were otherwise, a succession of slight cuts upon the
fingers would seriously impair their tactile sensibility.

[Sidenote: 24. The delicacy of touch? Experiments with a pair of
compasses?]

24. THE DELICACY OF TOUCH.--Although the hand is the proper organ of this
sense, yet it is exercised by various parts of the body, their degree of
sensibility being proportional to the number of papillæ they contain. The
varying degrees of tactile delicacy of the different parts of the surface
have been measured, in an ingenious manner, by means of a pair of
compasses, tipped with small pieces of cork. The two points of the
compasses are touched at the same moment to the skin, the eyes being
closed, and it is found that, in sensitive parts, the distance between the
points may be quite slight, and yet each be plainly felt; while, in less
sensitive parts, the points of the compasses are felt as a single point,
although they are separated one or two inches.

[Sidenote: 25. Further experiments and results?]

25. At the tips of the fingers, the distance between the points being
one-twentieth of an inch, a double impression is felt. The distance must be
twice as great, for the palm; four times as great, for the lips; and, on
the forehead, it must be twenty times greater. At the middle of the back,
where the touch is least acute, the points must be {187} separated more
than two inches before they can be separately felt. Therefore, the sense of
touch in the fingers is said to be fifty times more delicate than upon the
posterior surface of the body.

[Sidenote: 26. Exquisite delicacy of touch? The same among the blind?]

26. Exquisite delicacy of touch is attained by practice. This is shown in
many of the lighter and more graceful employments of daily life. Without
it, the skill of the painter, sculptor, and musician would be rude indeed.
By training, also, the physician acquires the _tactus eruditus_, or
discriminating touch; but among the blind, delicacy of touch is most
remarkable, and it here finds its highest value; for its possession, in a
measure, compensates the loss of sight by enabling them to read, by means
of raised letters, to work with certain tools, and even to play upon
musical instruments. A person born without sight, and without hearing or
voice, may, by the education of the touch, be rescued from apparent
imbecility, and be taught not only to read and write, but even to perform
household and other useful labors.

[Sidenote: 27. Rival candidates for the sixth sense? Give the two reasons
on the subject.]

27. SENSATIONS OF TEMPERATURE AND WEIGHT.--Each of these sensations has
been described by the physiologists as a special sense, and they are rival
candidates, so to speak, for the position and title of the sixth sense. In
the sensation of temperature, or the thermal sense, touch bears a part, but
the two feelings appear to be distinct. In proof of this, we observe,
firstly, that they are not alike intense in the same situations; as, for
example, the skin of the face and elbow, where the sense of touch is
feeble, is very sensitive to impressions of heat and cold. Secondly, the
ability to recognize temperature may be lost by paralysis, while the
sensibility of touch remains unaffected. When the skin comes in contact
with a very hot substance, the sensation felt is that of pain, not of
touch. In like manner, a {188} very cold substance causes pain, not the
feeling of cold. So that a red-hot iron, and solid carbonic acid (the
temperature of which is 108° below zero), feel alike; and each, if pressed
slightly, will produce a blister.

[Sidenote: 28. The muscular sense? State what is said to illustrate the
subject.]

28. The _muscular sense_, by some considered distinct from touch, gives
rise to the sensations of weight, and other forms of external resistance.
That this feeling exists, is shown by the following simple experiment. If
the hand be placed flat upon a table, and a somewhat heavy weight be put
into it, touch alone is exercised and a feeling of pressure results; but if
the hand be raised, a certain amount of muscular effort must be put forth,
and thus the sensation of weight is recognized. Through the muscular sense,
precision of effort is rendered possible; for by it we learn to adjust the
force exerted to the weight of the object to be lifted, moved, or carried.
Without it, all our movements would necessarily become ill-regulated and
spasmodic. In cases of disease, where the sensibility of the lower limbs is
lost, while power of motion remains, the patient is able to stand erect so
long as he can see his limbs; but just as soon as his eyes are closed, he
begins to waver, and will fall unless supported.

[Sidenote: 29. The organ of taste? The tongue? Its powers of motion?]

29. THE ORGAN OF TASTE.--The _tongue_ is the special organ of the sense of
taste; but the back part of the mouth also possesses this faculty. The
tongue is a muscular organ, the muscles composing it being so numerous and
interwoven as to give it the freedom and variety of motion which it
possesses. It can curve itself upward or downward; it can extend or
contract itself; and, with its point, can sweep the cavity of the mouth, in
all directions, in the search for scattered particles of food.

[Sidenote: 30. Peculiarities of the tongue? Uses of the papillæ?]

30. The upper surface of the tongue is peculiar, being marked by the
presence of innumerable _papillæ_, some of {189} which are of microscopic
size, resembling those that abound in the fingers, and in other parts of
the body that have the sense of touch. Others are much larger, and give to
the tongue its roughness of feeling and appearance. Through the medium of
these papillæ, the tongue receives impressions of touch and temperature, as
well as taste: indeed, its extremity is fully as delicate, in respect to
tactile sensations, as the tips of the fingers themselves. It can recognize
the two points of the compasses when separated not more than
one-twenty-fourth of an inch; the back of it is much less sensitive to
touch, while at the same time it is more highly sensitive to impressions of
taste.

[Sidenote: 31. Resemblance to the parts of the tongue? Powers and functions
of the parts?]

31. Each lateral half of the tongue resembles the other in structure, and
each receives the same number of nerves--three. One of these regulates
motion, the other two are nerves of special sense. One of the latter
supplies the front half of the tongue, and is called the _gustatory_ nerve.
This is a branch of the great cranial nerve, called the "fifth pair," which
ramifies in all parts of the face. The back of the tongue is endowed with
the power of taste through a nerve known as the _glosso-pharyngeal_,
because it is distributed both to the tongue and throat. This difference in
the nervous supply of the tongue becomes significant, when we learn, as we
shall presently, that each part of it perceives a different class of
flavors.

[Sidenote: 32. Taste? What are the requisites to taste?]

32. THE SENSE OF TASTE.--Taste is the special sense by means of which we
discover the savors, or flavoring properties of the substances, which come
in contact with the tongue. Mere contact with the surface of the tongue,
however, is not sufficient, but contact with the extremities of the nerves
of taste within the papillæ is required. In order that the substance to be
tasted may penetrate the {190} cells covering the nerves, it must either be
liquid in form, or readily soluble in the watery secretion of the mouth,
the saliva. The tongue must be moist also. If the substance be insoluble,
as glass or sand, or the tongue dry, the sense of taste is not awakened. In
sickness, when the tongue is heavily coated, the taste is very defective,
or, as is frequently expressed, "nothing tastes aright."

[Sidenote: 33. Portions of the tongue endowed with taste? Where else does
the sense lodge? What is stated in respect to sweet and bitter flavors?
Reflex effects mentioned?]

33. All portions of the tongue are not alike endowed with the sense of
taste, that function being limited to the posterior third, and to the
margin and tip of this organ. The soft palate, also, possesses the sense of
taste; hence, an article that has an agreeable flavor may very properly be
spoken of as palatable, as is often done. All parts of the tongue do not
perceive equally well the same flavors. Thus, the front extremity and
margin, which is the portion supplied by the "fifth pair" of nerves,
perceives more acutely sweet and sour tastes; but the base of the tongue,
supplied by the _glosso-pharyngeal_ nerve, is especially sensitive to salt
and bitter substances. The nerve of the front part of the tongue, as before
stated, is in active sympathy with those of the face, while the relations
of the other nerve are chiefly with the throat and stomach; so that when an
intensely sour taste is perceived, the countenance is involuntarily
distorted, and is said to wear an acid expression. On the other hand, a
very bitter taste affects certain internal organs, and occasions a
sensation of nausea, or sickness of the stomach.

[Sidenote: 34. What is stated of the relations of taste with other senses?]

34. RELATIONS OF TASTE WITH OTHER SENSES.--Taste is not a simple sense.
Certain other sensations, as those of touch, temperature, smell, and pain,
are blended and confused with it; and certain so-called tastes are really
sensations of another kind. Thus an astringent taste, like that of alum, is
more properly an astringent feeling, and {191} results from an impression
made upon the nerves of touch, that ramify in the tongue. In like manner,
the qualities known as smooth, oily, watery, and mealy tastes, are
dependent upon these same nerves of touch. A burning or pungent taste is a
sensation of pain, having its seat in the tongue and throat. A cooling
taste, like that of mint, pertains to that modification of touch called the
sense of temperature.

[Sidenote: 35. Its dependence on smell? on sight?]

35. Taste is largely dependent upon the sense of smell. A considerable
number of substances, like vanilla, coffee, and garlic, which appear to
possess a strong and distinct flavor, have in reality a powerful odor, but
only a feeble taste. When the sense of smell is interfered with by holding
the nose, it becomes difficult to distinguish between substances of this
class. The same effect is frequently observed when smell is blunted during
an ordinary cold in the head. Sight also contributes to taste. With the
eyes closed, food appears comparatively insipid; and a person smoking
tobacco in the dark is unable to determine by the taste whether his cigar
is lighted or not. Accordingly, it is not a bad plan to close the nose and
shut the eyes when about to swallow some disagreeable medicine.

[Sidenote: 36. The chief use of the sense of taste? The position of its
organs? The rule as regards wholesome and unwholesome food? Remarks
respecting the rule?]

36. INFLUENCE OF EDUCATION ON THE TASTE.--The chief use of the sense of
taste appears to be to act as a guide in the selection of proper food.
Hence its organs are properly placed at the entrance of the digestive
canal. As a general rule, those articles which gratify the taste are
wholesome; while the opposite is true of those which impress it
disagreeably. This statement is more exact in reference to the early years
of life than to later years, when, by reason of mischievous habits, the
sense of taste has become dulled or perverted. The desires of a child are
simple; he is fully satisfied with plain and wholesome articles of diet,
and must usually "learn to like" those {192} which have a strongly marked
flavor. Accordingly, it is far easier at this age to encourage the
preference for plain food, and thus establish healthful habits, than later
in life to uproot habits of indulgence in stimulating substances, after
their ill effects begin to manifest themselves.

[Sidenote: 37. Diversity in tastes of men? How shown? The education of the
sense of taste?]

37. The tastes of men present the most singular diversities, partly the
result of necessity and partly of habit or education. The Esquimaux like
the rank smell of whale oil, which is a kind of food admirably suited to
the requirements of their icy climate; and travellers who go from our
climate to theirs are not slow to develop a liking for the same articles
that the natives themselves enjoy. The sense of taste is rendered very
acute by education, as is shown in an especial manner by those who become
professional "tasters" of tea and wine.

[Sidenote: 38. Location of the sense of smell? The nose? "Roof of the
mouth?"]

38. THE SENSE OF SMELL--THE NASAL CAVITIES.--The sense of smell is located
in the delicate mucous membrane which lines the interior of the nose. That
prominent feature of the face, the nose, which is merely the front boundary
of the true nasal organ, is composed partly of bone and partly of
cartilage. The upper part of it is united with the skull by means of a few
small bones; to which circumstance is due its permanence of shape. The
lower portion, or tip of the nose, contains several thin pieces of
cartilage, which render it flexible and better able to resist the effects
of blows and pressure. Behind the nose we find quite a spacious chamber,
separated from the mouth by the hard palate, forming the "roof of the
mouth," and by the soft palate (see Fig. 46); and divided into two cavities
by a central partition running from before backward.

[Sidenote: 39. Cavities of the nose? Obstruction of the passage of air
through them?]

39. These nasal cavities, constituting the true beginning of the
air-passages, extend from the nose backward to the {193} upper opening of
the throat, and rise as high as the junction of the nose with the forehead.
The inner wall of each cavity is straight and smooth; but from the outer
wall there jut into each cavity three small scroll-like bones. The
structure of these bones is very light, and hence they have been called the
"spongy" bones of the nose. In this manner, while the extent of surface is
greatly increased by the formation of these winding passages, the cavities
are rendered extremely narrow; so much so, in fact, that a moderate
swelling of the mucous membrane which lines them, as from a cold, is
sufficient to obstruct the passage of air through them.

[Illustration: FIG. 46.--SECTION OF THE RIGHT NASAL CAVITY.]

[Sidenote: 40. The special nerve of smell? Its location?]

40. THE NERVE OF SMELL.--The internal surface of the nasal passages is
covered by a delicate and sensitive mucous membrane. Its surface is quite
extensive, following as it does, all the inequalities produced by the
curved spongy bones of the nose. The upper portion of it alone is the seat
of smell, since that part alone receives branches from the "first pair" of
cranial nerves, or the olfactory nerve, which is the special nerve of smell
(see Fig. 43). In Fig. 46 is shown the distribution of this nerve, in the
form of an intricate network upon the two upper spongy bones. The nerve
itself (1) does not issue from the skull, but rests upon a thin bone which
separates it from the cavity of the nose; and the branches which proceed
from it pass through this bone by means of numerous small openings. The
{194} engraving represents the outer surface of the right nasal cavity; the
three wave-like inequalities, upon which the nervous network is spread out,
are due to the spongy bones. The left cavity is supplied in the same
manner.

[Sidenote: 41. Branches of the "fifth pair" of nerves? Nasal mucus? Birds?]

41. The nerves which ramify over the lower part of the membrane, and which
endow it with sensibility to touch and pain, are branches of the "fifth
pair" of nerves. An irritation applied to the parts where this nerve is
distributed occasions sneezing, that is, a spasmodic contraction of the
diaphragm; the object of which is the expulsion of the irritating cause.
The manner in which the olfactory nerve-fibres terminate is peculiar.
Unlike the extremities of other nerves, which are covered in by a greater
or less thickness of tissue, these come directly to the surface of the
mucous membrane, and thus come into very close contact with the odorous
particles that are carried along by the respired air. The surface is at all
times kept in a moist condition by an abundant flow of nasal mucus;
otherwise it would become dry, hard, and insensitive from the continual
passage of air to and fro in breathing. Birds, which respire more actively
than men, have a special gland, for secreting a lubricating fluid, located
in the air-passages of the head.

[Sidenote: 42. Smell? Touch? Taste? Design of smell? Invisible and gaseous
particles? The extreme fineness of the particles? Musk? In other cases?]

42. THE USES OF THE SENSE OF SMELL.--Smell is the special sense which
enables us to appreciate odors. Touch, as we have seen, is largely
concerned with solid bodies; and taste, with fluids, or with solids in
solution. Smell, on the other hand, is designed to afford us information in
reference to substances in a volatile or gaseous form. Invisible and
subtile particles emanate from odorous bodies, and are brought by the
respired air in contact with the terminal filaments of the olfactory nerve,
upon which an agreeable or disagreeable impression is produced. {195} The
fineness of the particles that constitute odors is often so extreme, that
they elude all attempts to measure or weigh them. A piece of musk, for
instance, may be kept for several years, constantly emitting perfume,
without any appreciable loss of weight. In other cases, a loss of substance
is perceptible, such as the essential oils, which enter into the
composition of the ordinary perfumes.

[Sidenote: 43. Aid given by smell? The highest use of the sense? Explain
the manner.]

43. Smell, like taste, aids us in the choice of proper food, leading us to
reject such articles as have a rank or putrid odor, and which are, as a
rule, unfit to be eaten. The highest usefulness of this sense, however,
consists in the protection it affords to the organs of respiration.
Stationed at the gateways of the air-passages, it examines the current of
air as it enters, and warns us of the presence of noxious gases, and of
other and generally invisible enemies to health. Not all dangerous vapors
are offensive, but almost all offensive vapors are unfit to be breathed. A
number of small stiff hairs grow from the margin of the nostrils to prevent
the entrance of dust and other atmospheric impurities, which would be alike
injurious to the olfactory mucous membrane and to the lungs. The benevolent
design of the Maker of our bodies may be observed in all parts of their
mechanism; but, probably, in none is it more clearly displayed than in
connection with the sense of smell.

[Sidenote: 44. Sense of smell in the inferior animals? How, and in what
cases, illustrated?]

44. The sense of smell is developed in a remarkable degree in certain of
the inferior animals, and is especially acute in reference to the peculiar
emanations that appear to characterize the different animals. The lion and
other carnivorous beasts scent their prey from a great distance; and the
fox-hound is able to track the fox through thickets and over open country
for many miles; while the timid, helpless herbivora, such as the deer and
sheep, find in the {196} sense of smell a means of protection against their
natural enemies, of whose approach they are in this manner warned. By
training this sense in the dog, and making it subservient to his use, man
is able to hunt with success certain shy and very fleet animals, which
otherwise he could but seldom approach. Among men, individuals differ
greatly in respect to the development of this sense; and especially in
certain savage tribes it is found to be extremely delicate. Humboldt states
that the natives of Peru can, by this sense, distinguish in the dark
between persons of different races.

[Sidenote: 45. What is sight? What information does it furnish? Composite
visual sensations?]

45. THE SENSE OF SIGHT.--Sight, or Vision, is the special sense by means of
which we appreciate the color, form, size, distance, and other physical
properties of the objects of external nature. Primarily, this sense
furnishes us with information concerning the different shades of color and
the different degrees of brightness: these are the simple sensations of
sight, such as the yellowness and glitter of a gold coin. In addition to
these, there are composite visual sensations, produced by the joint action
of the other senses and by the use of the memory and judgment; such as, in
the case of the coin, its roundness, solidity, size, its distance and
direction from us. So that many of our sensations, commonly considered as
due to sight, are in reality the results of intellectual processes which
take place instantaneously and unconsciously.

[Sidenote: 46. Comparison between sight and hearing? Relative capacity of
deaf and blind?]

46. This faculty not only has value in the practical every-day affairs of
life, but it contributes so largely to the culture of the intellect and to
our higher forms of pleasure, that some writers are disposed to rate it as
the first and most valuable of the senses. Others, however, maintain that
the sense of hearing does not yield in importance to that of sight; and
they cite in support of their position {197} the fact that the blind are
commonly cheerful and gay, while the deaf are inclined to be morose and
melancholy. In respect to the relative capacity for receiving education in
the deaf and blind, it is found that the former learn more quickly, but
their attainments are not profound; while the blind acquire more slowly,
but are able to study more thoroughly.

[Sidenote: 47. Sight, unlike the other senses? In the case of the stars?]

47. LIGHT.--THE OPTIC NERVE.--Unlike the senses previously
considered--touch, taste, and smell--sight does not bring us into immediate
contact with the bodies that are examined; but, by it, we perceive the
existence and qualities of objects that are at a greater or less distance
from us. In the case of the stars, the distance is incalculable, while the
book we read is removed but a few inches. Light is the agent which gives to
this sense its wide range. The nature of this mysterious force is not
known, and it is not here to be discussed; since its study belongs more
properly to the province of natural philosophy.

[Sidenote: 48. The undulatory theory of light? What does the theory
suppose?]

48. It is sufficient, in this connection, to state that the theory of light
now generally accepted, and which best explains the facts of optics, is
that known as the undulatory theory. This theory supposes that there exists
an intangible, elastic medium, which fills all space, and penetrates all
transparent substances, and which is thrown into exceedingly rapid
undulations or waves, by the sun and every other luminous body; the
undulations being propagated with extreme rapidity, and moving not less
than 186,000 miles in a second.

[Sidenote: 49. The sensation of light? Optic nerve?]

49. These waves are thought to produce in the eye the sensation of light,
in the same manner as the sonorous vibrations of the air produce in the ear
the sensation of sound. That part of the eye which is sensitive to these
waves is the expansion of the _optic nerve_. It is sensitive {198} to no
other impression than that of light, and it is the only nerve which is
acted upon by this agent. The optic nerve, also called the "second pair" of
cranial nerves, is the means of communication between the eye and the
brain.

[Sidenote: 50. The two nerves constituting the pair of nerves?]

50. The two nerves constituting the pair, arise from ganglia lying at the
base of the cerebrum, one of them on each side; from which points they
advance to the eyes, being united together in the middle of their course in
the form of the letter X (Fig. 43--2). By this union the two eyes are
enabled to act harmoniously, and in some respects to serve as a double
organ. And by reason of this same intimate nervous communication, when
serious disease affects one eye, the fellow-eye is extremely liable to
become the seat of _sympathetic_ inflammation; and this, if neglected,
almost certainly results in hopeless blindness.

[Sidenote: 51. Why is the eye called the "window of the soul?" Why, the
subject of enthusiastic study?]

51. THE ORGAN OF SIGHT.--THE EYE.--The proximity of the eye to the brain,
and the important part it performs in giving expression to the emotions,
have given it the name of "the window of the soul." The exceeding beauty of
its external parts, and the high value of its function, have long made this
organ the subject of enthusiastic study. It is chiefly within the last
twenty years, however, that this study has been successful and fruitful of
practical results. Several ingenious instruments have been invented for the
examination of the eye in health and disease, and new operations have been
devised for the relief of blindness and of impaired vision. As a result, it
is now a well-marked fact that, in civilized lands, the number of those who
suffer from loss of sight is proportionally much less than in countries
where science is less known and cultivated.

[Sidenote: 52. The most obvious fact? The consequence? The next thing
noticed? Its range of view? Of what does the organ of vision consist?]

52. The most obvious fact in respect to the apparatus of {199} sight is
that there are two eyes, which may either act together as one, and be fixed
upon one object, or one eye may be used independently of the other. In
consequence of this arrangement the loss of one eye does not necessitate
blindness, and, in fact, it not infrequently happens that the sight of one
eye may be long impaired or lost before the fact is discovered. We next
notice that it is placed at the most elevated part of the body, in front,
and near the brain. It also commands a wide range of view, being itself
moved with great rapidity, and being further aided by the free motion of
the head and neck. The organ of vision consists essentially of two parts:
the optical instrument itself--the eyeball--and its enveloping parts, or
the case in which the instrument is kept free from harm. The latter, which
are external, and which we shall first consider, are chiefly the _Orbits_,
the _Eyelids_, and the apparatus for the _Tears_.

[Sidenote: 53. The protection of the eyeball against injury? The
overhanging brow? The opening for the optic nerve?]

53. THE ORBITS.--The eyeball, which is a delicate organ, is well defended
against external injury within the orbits or bony sockets of the head.
These are deep conical hollows, bounded in part by the bones of the skull,
and in part by those of the nose and cheek. The orbit juts out beyond the
most exposed portion of the eyeball, as may be seen by laying a book over
the eye, when it will be found that no part of the eyeball, unless it be
very prominent, will be touched by the book; so that the only direction in
which an injury is liable to be received is immediately in front of the
eye. The overhanging brow is itself covered by a layer of thick skin,
studded with short, stout hairs, which are so bent as to prevent the
perspiration from running into the eye and obscuring vision. Through a hole
in the bottom of the orbit, the nerve of sight passes outward from the
brain. The orbit also contains a considerable amount of a fatty tissue,
upon which, as upon an elastic cushion, the eye rests. {200}

[Illustration: FIG. 47.--FRONT VIEW OF RIGHT EYE. (Natural Size.)

1. The Lachrymal, or tear gland, lying beneath the upper eyelid.

2. The Nasal Duct is shown by the dotted line. The * marks the orifice in
the lower lid.

The central black spot is the _pupil_; surrounding it is the _iris_; and
the triangular white spaces are the visible portion of the _sclerotic_.]

[Sidenote: 54. What are the eyelids? The upper lid? The lower one? The
mucous membrane of the eye?]

54. THE EYELIDS.--The eyelids are two movable curtains, or folds, which,
when shut, cover the front part of the orbit, and hide the eye from view.
The upper lid is the larger, has a curved margin, and moves freely, while
the lower lid is comparatively short and straight, and has but a slight
degree of motion (Fig. 47). Skin covers the exterior of the lids, while a
fine mucous membrane lines their inner surface, and is likewise spread out
over the entire front of the eyeball. This membrane, which is called the
_Conjunctiva_, is highly sensitive, and thus plays an important part in
protecting the eye against the lodgment of sand, ashes, chaff, and other
foreign particles that are blown about in the air. This sensitive membrane
will not endure the presence of these particles. If any find access, it
causes a constant winking, a flow of tears, and other signs of irritation,
until it is removed.

[Sidenote: 55. The eyelashes? The little points within the line of the
lashes? Of what use are these glands?]

55. The long, silky eyelashes, which garnish the edges of the lids, act
like a sieve to prevent the entry of dust and the like; and together with
the lids, they regulate the amount of light which is permitted to enter the
eye, so that it is shielded from a sudden flood or glare of light. The
little points seen in the figure just within the line of the lashes,
especially on the lower lid, represent the mouths of numerous little
sebaceous glands (Fig. 48, D,D), such as are always {201} found in the
neighborhood of hairs. These glands supply a thick, oily material which
greases the edges of the lids and prevents their adhering together, and
likewise prevents the overflow of the tears upon the cheek.

[Sidenote: 56. The location of the lachrymal gland? The use of the gland?]

56. THE LACHRYMAL FLUID, OR THE TEARS.--Just within the outer part of the
bony arch of the brow, where the bone may be felt to be sharper than in
other positions, is lodged a little organ called the lachrymal gland, the
situation of which is indicated in Fig. 47, 1. This is the gland whence
flows the watery secretion, commonly called the _tears_, which is designed
to perform an exceedingly important duty in lubricating the lids, and in
keeping the exposed surface of the eyeball moist and transparent. For,
without this or some similar liquid, the front of the eye would speedily
become dry and lustreless, like that of a fish which has been removed from
the water: the simple exposure of the eye to the air would then suffice to
destroy vision.

[Sidenote: 57. When does the secretion of the tears occur? The secretion
not used for the eye? Location of the nasal duct? Its use? The overflow of
tears in old people?]

57. This secretion of the tears takes place at all times, during the night
as well as the day; but it is seldom noticed, except when under the
influence of some strong mental emotion, whether of sorrow or happiness, it
is poured forth in excess, so as to overflow the lids. Strong light or a
rapid breeze will, among many other causes, excite the flow of the tears.
That portion of this secretion which is not used in moistening the eye is
carried off into the nose by a canal situated near the inner angle of the
eye, called the _nasal duct_. This duct is shown in Fig. 47, 2, and is
connected with each lid by delicate tubes, which are indicated by dotted
lines in the figure; the asterisk marks the little opening in the lower
lid, by which the tears enter the nasal duct. By gently turning the inner
part of that lid downward, and looking in a mirror, {202} this small
"lachrymal point" may be seen in your own eye. In old people, these points
become everted, and do not conduct the tears to the nasal cavity, so that
they are inconvenienced by an overflow of tears upon the face.

[Illustration: FIG. 48.--VERTICAL SECTION OF THE EYE. (Enlarged.)

C, The Cornea. A, The Aqueous Humor. I, The Iris. P, The Pupil. L, The
Crystalline Lens. H, The Ligament of the Lens. B, The Ciliary Process.
V, The Cavity containing the Vitreous Humor. S, The Sclerotic. Ch, The
Choroid. R, The Retina. N, The Optic Nerve. DD, The Eyelids. X, The Levator
Muscle of the Upper Lid. Y, The Upper Straight Muscle of the Eye. Z, The
Lower Straight Muscle.]

[Sidenote: 58. The watery fluid passing over the eyeball? Design of the
arrangement? Winking?]

58. Thus we observe that the gland which forms the tears is placed at the
outer part of the eye, while their means of exit is at the inner angle of
the eye; which fact renders it necessary that this watery fluid shall pass
over the surface of the eyeball before it can escape. This arrangement
cannot be accidental, but evinces design, for it thus secures the perfect
lubrication of the surface of the eye, and cleanses it from the smaller
particles of dust {203} which may enter it, in spite of the vigilance of
the lids and lashes. The act of winking, which is generally unconsciously
performed, and which takes place six or more times in a minute, assists
this passage of the tears across the eye, and is especially frequent when
the secretion is most abundant.

[Sidenote: 59. Describe the shape of the eyeball. Its structure.]

59. THE EYEBALL.--The remarkable optical instrument called the eyeball, or
the globe of the eye, upon which sight depends, is, as the name indicates,
spherical in shape. It is not a perfect sphere, since the front part
projects somewhat beyond the rest, and at the posterior part the optic
nerve (Fig. 48, N) is united to it, resembling the junction of the stem
with a fruit. In its long diameter, that is, the horizontal or from side to
side, it measures a little more than an inch; in other directions it is
rather less than an inch. In structure, the ball of the eye is firm, and
its tense round contour may in part be felt by pressing the fingers over
the closed lids.

[Sidenote: 60. Of what is the eyeball composed? State how.]

60. The eyeball is composed chiefly of three internal, transparent media,
called _humors_; and three investing coats, or _tunics_. The former are the
_aqueous humor_, Fig. 48, A, the _crystalline lens_ L, and the _vitreous
humor_ V. Of these the lens alone is solid. The three coats of the eyeball
are called the _sclerotic_ S, the _choroid_ CH, and the _retina_ R. This
arrangement exists in respect to five-sixths of the globe of the eye, but
in the anterior one-sixth, these coats are replaced by the _cornea_ C,
which is thin and transparent, so that the rays of light pass freely
through it, as through a clear window-pane.

[Sidenote: 61. The shape of the cornea? Its structure? The "white of the
eye?"]

61. In shape, the cornea is circular and prominent, resembling a miniature
watch-glass, about 1/25 of an inch thick. In structure, it resembles horn
(as the name signifies), or the nail of the finger, and is destitute of
{204} blood-vessels. The _Sclerotic_ (from _scleros_, hard) is composed of
dense, white fibrous tissue, and gives to the eyeball its firmness of
figure and its white color; in front, it constitutes the part commonly
called "the white of the eye." It is one of the strongest tissues in the
body; it possesses very few vessels, and is not very sensitive. It affords
protection to the extremely delicate interior parts of the eye; and the
little muscles which effect its movements are inserted into the sclerotic a
short distance behind the cornea (see Fig. 48, Y, Z). It is perforated
posteriorly to admit the optic nerve.

[Sidenote: 62. The second or middle coat of the eyeball? Its dark color?]

62. The _Choroid_ is the second or middle coat of the eyeball, and lies
closely attached to the inner surface of the sclerotic. Unlike the latter
tunic, its structure is soft and tender, it is dark in color, and possesses
a great abundance of blood-vessels. Its dark color is due to a layer of
dark brown or chocolate-colored cells spread out over its inner surface.
This dark layer serves to absorb the rays of light after they have
traversed the transparent structures in front of it; if the rays were
reflected from side to side within the eye, instead of being thus absorbed,
confused vision would result from the multitude of images which would be
impressed upon the optic nerve.

[Sidenote: 63. Similar mechanism in microscopes? The albinos? White
rabbits?]

63. This mechanism has been unconsciously imitated by the opticians, who,
when they make a microscope or telescope, take care that the interior of
its tube shall be coated with a thick layer of black paint or lamp-black;
for without it, a clear delineation of the object to be viewed is
impossible. The albinos, in whom these dark cells of the choroid are
wanting, have imperfect vision, especially in the daytime and in strong
lights. The dark cells are also wanting in white rabbits, and other animals
that have red or pink eyes; their vision appears to be imperfect in the
presence of a bright light. {205}

[Sidenote: 64. What is the iris? Its construction? How is the size of the
pupil regulated?]

64. THE IRIS.--Continuous with the choroid, in the front part of the globe
of the eye, is a thin, circular curtain, which occasions the brown, blue,
or gray color of the eye in different individuals. On account of the
varieties of its color, this membrane has received the name _Iris_, which
is the Greek word for "rainbow" (see Fig. 48, I). A front view of it is
shown in Fig. 47. The iris is pierced in its centre by a round opening,
called the _pupil_ (P), which is constantly varying in size. In olden times
it was spoken of as the "apple of the eye." The hinder surface of the iris,
except in albinos, has a layer of dark coloring matter resembling that of
the choroid. The iris is a muscular organ, and contains two distinct sets
of fibres; one of which is circular, while the other radiates outward from
the pupil. The action of these sets of fibres regulates the size of the
pupil; for when the circular set acts, the pupil contracts, and when the
other set acts, the opening expands. Their action is involuntary, and
depends on the reflex system of nerves, which causes the contraction of the
pupil when a strong light falls upon the eye, and its expansion when the
illumination is feeble.

[Sidenote: 65. The admission of light to the eye? The action of the iris
under different circumstances? The lustre of the eye, how affected in youth
and old age?]

65. The iris, accordingly, serves a very useful purpose in regulating the
admission of light to the eye. It, however, does not act instantaneously;
and hence, when we pass quickly from a dark room into the bright sunlight,
the vision is at first confused by the glare of light, but as soon as the
pupil contracts, the ability to see becomes perfect. On the other hand,
when we enter a dark apartment, such as a cellar, for a short time we can
see nothing clearly; but as soon as the pupil expands and admits more
light, we are enabled to distinguish the surrounding objects. Animals of
the cat species, and others which prowl around after nightfall, are enabled
to see in the dark by {206} having the iris very dilatable. The size of the
pupil affects the lustre of the eye. When it is large, as it usually is
during youth, the eye appears clear and brilliant; while in old age the
pupil is small and the eye is dull. The brilliancy of the eye is in part,
at least, dependent upon the reflection of light from the front surface of
the crystalline lens.

[Sidenote: 66. Means used to increase the beauty of the eye? The injurious
consequences?]

66. Certain poisonous vegetables have the property of causing the pupil to
dilate, and have been used in small doses to increase the beauty of the
eye. One of these drugs has been so largely used by the ladies for this
purpose, that it has received the name _belladonna_, from the Italian words
meaning "beautiful lady." This hazardous practice has resulted more than
once in the death of the person desiring thus to increase her personal
attraction. The common English name for belladonna is "deadly nightshade."
(In the diagram on page 214 the shape and relations of the iris are more
accurately shown than in the figure referred to above.)

[Sidenote: 67. What part does the retina constitute? How formed? Its
texture? Color? Sensitiveness?]

67. THE RETINA constitutes the third and inner coat of the globe of the
eye. This, the important part of the eye that is sensitive to light, is a
kind of nervous membrane, formed by the expansion of the ultimate filaments
of the optic nerve. Its texture is soft, smooth, and very thin; it is
translucent and of an opaline, or grayish-white color. It is sensitive to
light alone; and if any form of mechanical irritation be applied to it, the
sensations of touch and pain are not experienced, but flashes of fire,
sparks, and other luminous appearances are perceived. Thus an electric
shock given to the eye-ball occasions a flash of light; and a sudden fall,
or a blow upon the eye, is often apparently accompanied by the vision of
"stars."

[Sidenote: 68. Specific energy of the optic nerve? Trial in Germany?]

68. These phenomena are due to what is termed the {207} "specific energy"
of the optic nerve, which nerve, in common with the other nerves of special
sense, obeys a general law of nature, which requires that, whenever one of
these nerves is stimulated, it shall respond with the sensation peculiar to
itself. These flashes of retinal light have no power to illuminate external
objects, although the opposite of this statement has been maintained. On
the occasion of a remarkable trial in Germany, it was claimed by a person
who had been severely assaulted on a very dark night, that the flashes of
light caused by repeated blows upon the head enabled him to see with
sufficient distinctness to recognize his assailant. But the evidence of
scientific men entirely refuted this claim, by pronouncing that the eye,
under the circumstances named, was incapacitated for vision. Too intense
light occasions a feeling of pain, but it is of a peculiar kind, and is
termed "dazzling."

[Illustration: FIG. 49.]

[Sidenote: 69. Sensitiveness of all parts of the retina? Experiment to
prove the existence of the "blind spot."]

69. All parts of the retina are not equally sensitive, and singularly
enough, the point of entry of the nerve of sight, in the back part of the
eyeball, is entirely insensible to light, and is called the "blind spot."
The existence of this point may be proven by a simple experiment. Hold the
accompanying figure, on page 207, directly in front of and parallel with
the eyes. Close the left eye, and fix the sight steadily on the left-hand
circle; then, by gradually varying the distance of the figure from the eye,
at a certain distance (about six inches), the right-hand circle will
disappear, {208} but nearer or further than that, it will be plainly seen.
The other eye may be also tried, with a similar result: if the gaze be
directed to the right-hand circle, the left one will seem to disappear. The
experiment may be repeated by using two black buttons on the marble top of
a bureau, or on some other white surface. The blind spot does not
practically interfere with vision, since the eye is seldom fixed immovably
on an object, and the insensitive parts of the two eyes can never be
directed upon the same object at the same time.

[Sidenote: 70. Duration of impressions upon the retina? How illustrated?]

70. Impressions made upon the retina are not at once lost, but persist a
measurable length of time, and then gradually fade away. Thus, a bright
light or color, gazed at intently, cannot be immediately dismissed from
sight by closing or turning away the eyes. A stick lighted at one end, if
whirled around rapidly in the dark, presents the appearance of an unbroken
luminous ring; and the spokes of a rapidly revolving carriage-wheel seem to
be merged into a plane surface. If an object move too rapidly to produce
this sort of lasting impression, it is invisible, as in the case of a
cannon-ball passing through the air in front of us.

[Sidenote: 71. What further illustration? Winking, why it is not noticed.
Ease with which the retina is fatigued or deprived of sensibility? How
shown?]

71. If a card, painted with two primary colors--as red and yellow--be made
to rotate swiftly, the eye perceives neither of them distinctly; but the
card appears painted with their secondary color--orange. The average
duration of retinal images is estimated at one-eighth of a second; and it
is because they thus endure, that the act of winking, which takes place so
frequently, but so quickly, is not noticed and does not interrupt the
vision. The retina is easily fatigued or deprived of its sensibility. After
looking steadfastly at a bright light, or at a white object on a black
ground, a dark spot, corresponding in shape to the bright object, {209}
presents itself in whatever direction we look. This spot passes away as the
retina resumes its activity.

[Sidenote: 72. How further shown? How is the result accounted for?
"Color-blindness?"]

72. If a bright color be gazed at intently, and the eyes then be turned to
a white surface, a spot will appear; but its color will be the complement
of that of the object. Fix the eye upon a red wafer upon a white ground,
and on removing the wafer a greenish spot of the same shape takes its
place. This result happens because a certain portion of the retina has
exhausted its power to perceive the red ray, and perceives only its
complementary ray, which is green. The color thus substituted by the
exhausted retina is called a physiological or accidental color. In some
persons the retina is incapable of distinguishing different colors, when
they are said to be affected with "color-blindness." Thus, red and green
may appear alike, and then a cherry-tree, full of ripe fruit, will seem of
the same color in every part. Railroad accidents have occurred because the
engineer of the train, who was color-blind, has mistaken the color of a
signal.

[Sidenote: 73. The location of the crystalline lens? How supported? Its
color and texture? Shape? Size?]

73. THE CRYSTALLINE LENS.--Across the front of the eye, just behind the
iris, is situated the _Crystalline lens_, enclosed within its own capsule.
It is supported in its place partly by a delicate circular ligament, and
partly by the pressure of adjacent structures. It is colorless and
perfectly transparent, and has a firm but elastic texture. In shape it is
doubly convex, and may be rudely compared to a small lemon-drop. The front
face of the lens is flatter than the other, and is in contact with the iris
near its pupillary margin, as is represented in the diagram on page 214. It
is only one-fourth of an inch thick.

[Sidenote: 74. Cataract? Aqueous humor? Vitreous humor?]

74. When this little body becomes opaque, and no longer affords free
passage to the rays of light, as often happens {210} with the advance of
age, an affection termed "cataract" is produced. Between the crystalline
lens and the cornea is a small space which contains the _aqueous humor_
(see Fig. 48, A). This humor consists of five or six drops of a clear,
colorless liquid very much like water, as its name implies. That part of
the globe of the eye lying behind the lens is occupied by the _vitreous
humor_, so called from its fancied resemblance to melted glass (Fig. 48,
V). This humor is a transparent, jelly-like mass, enclosed within an
exceedingly thin membrane. It lies very closely applied to the retina, or
nervous membrane of the eye, and constitutes fully two-thirds of the bulk
of the eyeball.

[Illustration: FIG. 50.--THE RETINAL IMAGE.]

[Sidenote: 75. What is a lens and its focus? The miniature image, how
produced?]

75. THE USES OF THE CRYSTALLINE LENS.--A convex lens has the property of
converging the rays of light which pass through it; and the point at which
it causes them to meet is termed its focus. If a lens of this description,
such as a magnifying or burning-glass, be held in front of an open window,
in such a position as to allow its focus to fall upon a piece of paper, it
will be found to depict upon the paper a miniature image of the scene
outside of the window. It will be further noticed that the image is
inverted, or upside down, and that the paper {211} at the place upon which
the image is thrown is much brighter than any other part.

[Sidenote: 76. How are figures painted upon the retina? How proved?]

76. Now all the transparent structures of the eye, but especially the
crystalline lens, operate upon its posterior part, or retina, as the convex
lens acts upon the paper; that is, they paint upon the retina a bright
inverted miniature of the objects that appear in front of the eye (Fig.
50). That this actually takes place may be proved by experiment. If the
eyeball of a white rabbit, the walls of which are transparent, be examined
while a lighted candle is held before the cornea, an image of the
candle-flame may be seen upon the retina.

[Sidenote: 77. What can be said in respect to the form and structure of the
crystalline lens?]

77. The form and structure of the crystalline lens endow it with a
remarkable degree of refractive power, and enable it to converge all the
rays of light that enter it through the pupil, to a focus exactly at the
surface of the retina. When this lens is removed from the eye, as is
frequently done for the cure of cataract, it is found that the rays of
light then have their focus three-eighths of an inch behind the retina;
that the image is four times larger than in the healthy eye, that it is
less brilliant, and that its outline is very indistinct. From this we learn
that one of the uses of the crystalline lens is to make the retinal image
bright and sharply-defined, at the same time that it reduces its size.
Indeed, the small size of the image is a great advantage, as it enables the
limited surface of the retina to receive, at a glance, impressions from a
considerable field of vision.

[Sidenote: 78. How is the inverted image upon the retina presented in its
true position to the mind?]

78. As the image upon the retina is inverted, how does the mind perceive
the object in its true, erect position? Many explanations have been
advanced, but the simplest and most satisfactory appears to be found in the
fact that {212} the retina observes no difference, so to speak, between the
right and left or the upper and lower positions of objects. In fact, the
mind is never conscious of the formation of a retinal image, and until
instructed, has no knowledge that it exists. Consequently, our knowledge of
the relative location of external objects must be obtained from some other
source than the retina. The probable source of this knowledge is the
habitual comparison of those objects with the position of our own bodies:
thus, to see an elevated object, we know we must raise the head and eyes;
and to see one at our right hand, we must turn the head and eyes to the
right.

[Illustration: FIG. 51.--THE DIFFERENT SHAPES OF THE GLOBE OF THE EYE.

N, The Natural Eye. M, The Short-sighted Eye. H, The Long-sighted Eye.
S, Parallel Rays from the Sun.]

[Sidenote: 79. The uniform perfection of the eye? Examples? The most common
imperfection?]

79. LONG-SIGHT OR HYPEROPIA, AND SHORT-SIGHT OR MYOPIA.--The eye is not in
all cases perfectly formed. For example, persons may from birth have the
cornea too prominent or too flat, or the lens may be too thick or too thin.
In either of these conditions sight will be more or less defective from the
first, and the defect will not tend to disappear as life advances. The most
common imperfection, however, is in the shape of the globe; which may be
short (Fig. 51, H), as compared with the natural eye, N, or it may be too
long, M.

[Sidenote: 80. How is "long-sight" explained? "Short-sight?"]

80. When the globe is short, objects can only be clearly {213} seen that
are at a distance, and the condition of the vision is known as
"long-sight," or hyperopia. It will be observed, by reference to Fig. 51,
that the focus of the rays of light would fall behind the retina of this
eye. When the globe is too long, objects can only be clearly seen that are
very near to the eye; and the condition resulting from this defect is
termed "short-sight," or myopia. The focus of the rays of light is, in this
case, formed in the interior of the eye in front of the retina.

[Sidenote: 81. Long-sight, how common? With what must it not be confounded?
Kind of glasses for short-sight? Why? Squint?]

81. Long-sight, or hyperopia, is common among schoolchildren, nearly as
much so as short-sight, and must not be confounded with the defect known as
the "far sight" of old people; although in both affections the sight is
improved by the use of convex glasses. Children not infrequently discover
that they see much better when they chance to put on the spectacles of old
persons. For the relief of short-sight, concave glasses should be employed;
as they so scatter the rays of light as to bring the focus to the retina,
and thus cause the vision of remote objects to become at once distinct.
That form of "squint," in which the eyes are turned inward, is generally
dependent upon long-sight, while that rarer form, when they turn outward,
is due to short-sight.

[Sidenote: 82. What is stated in connection with the opera-glass?
Experiment with pencil and distant object?]

82. THE FUNCTION OF ACCOMMODATION.--If, after looking through an
opera-glass at a very distant object, it is desired to view another nearer
at hand, it will be found impossible to obtain a clear vision of the second
object unless the adjustment of the instrument is altered; which is
effected by means of the screw. If an object, like the end of a pencil, be
held near the eye, in a line with another object at the other side of the
room, or out of the window, and the eye be fixed first upon one and then
upon the other, it will be found that when the pencil is clearly seen, the
{214} further object is indistinct; and when the latter is seen clearly,
the pencil appears indistinct; and that it is impossible to see both
clearly at the same time. Accordingly, the eye must have the capacity of
adjusting itself to distances, which is in some manner comparable to the
action of the screw of the opera-glass.

[Illustration: FIG. 52.--THE FUNCTION OF ACCOMMODATION.

The right half of the diagram shows the eye at rest. The left half shows
the lens accommodated for near vision.]

[Sidenote: 83. Function of accommodation? In what does it consist? How is
the function explained?]

83. This, which has been called the function of accommodation, is one of
the most admirable of all the powers of the eye, and is exercised by the
crystalline lens. It consists essentially in a change in the curvature of
the front surface of the lens, partly through its own elasticity, and
partly through the action of the ciliary muscle. When the eye is at rest,
that is, when accommodated for a distant object, the lens is flatter and
its curvature diminished (see Fig. 52); but when strongly accommodated for
near vision the lens becomes thicker, its curvature increases, and the
image on the retina is made more sharp and distinct. Since a strong light
is not required in viewing near objects, the pupil contracts, as is shown
in the left-hand half of the diagram. {215}

[Sidenote: 84. Change of sight with the approach of old age? Explain the
change?]

84. OLD-SIGHT, OR PRESBYOPIA.--But this marvellously beautiful mechanism
becomes worn with use; or, more strictly speaking, the lens, like other
structures of the body, becomes harder with the approach of old age. The
material composing the lens becomes less elastic, the power to increase its
curvature is gradually lost, and as a consequence, the person is obliged to
hold the book further away when reading, and to seek a stronger light. In a
word, the function of accommodation begins to fail, and is about the first
evidence that marks the decline of life. By looking at the last preceding
diagram, and remembering that the increased curvature of the lens cannot
take place, it will be at once understood why old-sight is benefitted in
near vision by the convex lens, such as the spectacles of old people
contain. It acts as a substitute for the deficiency of the crystalline
lens.

[Sidenote: 85. Hearing or audition? What is sound? How propagated commonly?
Stone thrown in water?]

85. THE SENSE OF HEARING.--SOUND.--Hearing, or audition, is the special
sense by means of which we are made acquainted with _sound_. What is sound?
It is an impression made upon the organs of hearing, by the vibrations of
elastic bodies. This impression is commonly propagated by means of the air,
which is thrown into delicate undulations, in all directions from the
vibrating substance. When a stone is thrown into smooth water, a wave of
circular form is set in motion, from the point where the stone struck,
which constantly increases in size and diminishes in force, as it advances.

[Sidenote: 86. Sound-wave in the atmosphere? Its shape? Rate of motion?
Sound, in water, air, and solid bodies?]

86. Somewhat resembling this, is the undulation, or sound-wave, which is
imparted by a sonorous vibration to the surrounding atmosphere. Its shape,
however, is spherical, rather than circular, since it radiates upward,
downward, and obliquely as well as horizontally, like the wave {216} in
water. The rate of motion of this spherical wave of air is about 1050 feet
per second, or one mile in five seconds. In water, sound travels four times
as fast as in air, and still more rapidly through solid bodies; along an
iron rod, its velocity is equal to two miles per second.

[Sidenote: 87. The earth as a conductor of sound? To what has the western
Indian been taught? Solid substances as conductors? As regards sound, in
what respect is air necessary? Sound in a vacuum?]

87. The earth, likewise, is a good conductor of sound. It is said that the
Indian of our western prairies can, by listening at the surface of the
ground, hear the advance of a troop of cavalry, while they are still out of
sight, and can even discriminate between their tread and that of a herd of
buffaloes. Solid substances also convey sounds with greater power than air.
If the ear be pressed against one end of a long beam, the scratching of a
pin at the other extremity may be distinctly heard, which will not be at
all audible when the ear is removed from the beam. Although air is not the
best medium for conveying sound, it is necessary for its production. Sound
cannot be produced in a vacuum, as is shown by ringing a bell in the
exhausted receiver of an air-pump, for it is then entirely inaudible. But
let the air be readmitted gradually, then the tones become more and more
distinct, and when the receiver is again full of air, they will be as clear
as usual.

88. All sonorous bodies do not vibrate with the same degree of rapidity,
and upon this fact depends the _pitch_ of the sounds that they respectively
produce. The more frequent the number of vibrations within a given time,
the higher will be the pitch; and the fewer their number, the lower or
graver will it be. Now, the rate of the successive vibrations of different
notes has been measured, and it has thus been found that if they are less
than sixteen in a second, no sound is audible; while if they exceed 60,000
per second the sound is very faint, and is painful to the {217} ear. The
extreme limit of the capacity of the human ear may be considered as
included between these points; but the sounds which we ordinarily hear are
embraced between 100 and 3,000 vibrations per second.

89. The _ear_, which is the proper organ of hearing, is the most
complicated of all the structures that are employed in the reception of
external impressions. The parts of which it is composed are numerous, and
some of them are extremely small and delicate. Nearly all these parts are
located in an irregularly shaped cavity hollowed out in the temporal, or
"temple," bone of each side of the head. That part of the bone in which the
auditory cavity is placed has the densest structure of all bones of the
body, and has therefore been called the "petrous," or rocky part of the
temporal bone. In studying the ear, it is necessary to consider it as
divided into three portions, which are called, from their relative
positions, the _external_ ear, the _middle_ ear, and the _internal_ ear.
(In the diagram, Fig. 53, A, the first is not shaded, the second is lightly
shaded, and the last has a dark background.)

[Sidenote: 90. Of what does the external portion of the organ of hearing
consist? Describe the portal of that organ known as the ear. Its use?]

90. THE EXTERNAL EAR.--The external portion of the organ of hearing,
designated in Fig. 53, A, includes, first, that outer part (_a_), which is
commonly spoken of as "the ear," but which in fact is only the portal of
that organ; and, secondly, the _auditory canal_ (_b_). The former consists
of a flat flexible piece of cartilage, projecting slightly from the side of
the head, attached to it by ligaments, and supplied with a few weak
muscles. Its surface is uneven, and curiously curved, and from its
resemblance to a shell, it has been called the _concha_. It probably serves
to collect sounds, and to give them an inward direction; although its
removal is said not to impair the acuteness of hearing more than a few
days. {218}

[Sidenote: 91. The ear in the animals of delicate hearing? Rabbit? Fishes?]

91. In those animals whose hearing is more delicate than that of man, the
corresponding organ is of greater importance, it being larger and supplied
with muscles of greater power, so that it serves as a natural kind of
ear-trumpet, which is easily movable in the direction of any sound that
attracts the attention of the animal. Bold, predaceous animals generally
have the concavity of this organ directed forward, while in timorous
animals, like the rabbit, it is directed backward. Fishes have no outer
ear, but sounds are transmitted directly through the solid bones of the
head, to the internal organ of hearing.

[Illustration: FIG. 53.--THE EAR AND ITS DIFFERENT PARTS.

A, Diagram of the Ear.

_a_, _b_, External Ear. _c_, Membrane tympani. _d_, Middle Ear.
_e_, Internal Ear.

B to B''', Bones of the Middle Ear (magnified).

C, The Labyrinths, or Internal Ear (highly magnified).]

[Sidenote: 92. What is the auditory canal? Describe it.]

92. The _auditory canal_ (Fig. 53, A, _b_), which is continuous with the
outer opening of the ear, is a blind passage, an inch and a quarter in
length, its inner extremity being bounded by a closely-fitting, circular
membrane. This canal is of oval form, is directed forward and inward, {219}
and is slightly curved; so that the inner end is ordinarily concealed from
view. The pouch of the skin which lines this passage is smooth and thin,
especially at the lower end, where it covers the membrane just mentioned.

[Sidenote: 93. How is it guarded and protected? Ear-wax?]

93. As in the case of the nostrils, a number of small, stiff hairs garnish
the margin of the auditory canal, and guard it, to some extent, against the
entrance of insects and other foreign objects. The skin, too, covering its
outer half, is furnished with a belt of little glands which secrete a
yellow, viscid, and bitter substance, called "ear-wax," which is especially
obnoxious to small insects. As the outer layer of this wax-like material
loses its useful properties, it becomes dry, and falls out of the ear in
the form of minute, thin scales, a fresh supply being furnished from the
little glands beneath. In its form, the auditory canal resembles the tube
of an ear-trumpet, and serves to convey the waves of sound to the middle
portion of the ear.

[Sidenote: 94. What is the middle ear? Why called tympanum?]

94. THE MIDDLE EAR, OR TYMPANUM.--The middle ear is a small cavity, or
chamber, of irregular shape, about one-fourth of an inch across from side
to side, and half an inch long (see Fig. 53, A, _d_). From the peculiar
arrangement of its various parts it has very properly been called the
_tympanum_, or the "drum of the ear." The middle ear, like the external
canal, contains air.

[Sidenote: 95. What is the membrana tympani? Describe it.]

95. The circular membrane, already mentioned as closing the auditory canal,
is the partition which separates the middle from the external ear, and is
called the _membrana tympani_ (_c_), and may be considered as the outer
head of the drum of the ear. It is sometimes itself spoken of as the
"drum," but this is incorrect; since a drum is not a membrane, but is the
hollow space across which the membrane is stretched. This membranous
drum-head is very tense and elastic, and so thin as to be almost
transparent; {220} its margin is fastened into a circular groove in the
adjacent bone. Each wave of sound that impinges against this delicate
membrane causes it to vibrate, and it, in turn, excites movements in the
parts beyond.

[Sidenote: 96. What are the ossicles? Their number and names? Their
arrangement?]

96. Within the tympanum is arranged a chain of remarkable "little bones,"
or _ossicles_. They are chiefly three in number, and from their peculiar
shapes bear the following names: _malleus_, or the mallet; _incus_, or the
anvil; and _stapes_, or the stirrup. A fourth, the smallest bone in the
body, in early life intervenes between the incus and stapes, but at a later
period it becomes a part of the incus. It is called the _orbicular_ bone.
Small as are these ossicles--and they, together, weigh only a few
grains--they have their little muscles, cartilages, and blood-vessels, as
perfectly arranged as the larger bones of the body. One end of the chain of
ossicles, the mallet, is attached to the membrane of the tympanum, or outer
drum-head, while the other end, the stirrup, is firmly joined by its
foot-piece to a membrane in the opposite side of the cavity. The chain,
accordingly, hangs suspended across the drum between the two membranes; and
when the outer one vibrates under the influence of the sound-wave, the
chain swings inward and transmits the vibration to the entrance of the
inner ear.

[Sidenote: 97. The Eustachian tube? Describe it, and state its use.]

97. The musical instrument, the drum, is not complete if the air within be
perfectly confined: we therefore find in all instruments of this kind a
small opening in the side, through which air may pass freely. By this means
the pressure of the air upon the vellum which forms the head of the drum is
made equal upon all sides, and the resonance of the drum remains unaffected
by the varying density of the atmosphere. It will, therefore, emit its
proper sound, whether it be struck in the rarified air of the mountain-top,
or in the condensed air of a mine. The tympanum, {221} or drum of the ear,
in like manner has an opening by means of which it communicates freely with
the external air. This opening is a narrow canal, about an inch and a half
long, called the _Eustachian tube_, after the name of its discoverer,
Eustachius.

[Illustration: FIG. 54.--SECTION OF THE RIGHT EAR.

A, The Concha. B, Auditory Canal. C, Membrane of the Drum, (the lower
half.) D, A small muscle. E, Incus, or Anvil. M, Malleus, or Mallet.
I, Eustachian Tube. G, Semicircular Canals. H, Cochlea, or snail's shell.]

[Sidenote: 98. What can you state of the action of the Eustachian tube?]

98. The course of this passage is indicated in Fig. 54, I, directed
downward and inward: its other extremity opens into the upper part of the
throat. The passage itself is ordinarily closed, but whenever the act of
swallowing or gaping takes place, the orifice in the throat is stretched
open, and the air of the cavity of the tympanum may then be renewed. Air
may at will be made to enter through this tube, by closing the mouth and
nose, and then trying {222} to force air through the latter. When this is
done, a distinct crackle or clicking sound is perceived, due to the
movement of the membranes, and of the little bones of the ear.

[Sidenote: 99. What other purpose does the Eustachian tube serve? How is
this shown? "Throat-deafness?" Primary use of the Eustachian tube?]

99. The Eustachian tube serves, also, as an escape-pipe for the fluids
which form within the middle ear; and hence, when its lining membrane
becomes thickened, in consequence of a cold, or sore throat, and the
passage is thus more or less choked up, the fluids are unable to escape as
usual, and therefore accumulate within the ear. When this takes place, the
vibrations of the membrane are interfered with; the sounds heard appear
muffled and indistinct; and a temporary difficulty of hearing, which is
known as "throat-deafness," is the result. This result resembles the effect
produced by interrupting the vibrations of a sonorous body, such as all are
familiar with; if the finger be placed upon a piano-string or bell when it
is struck, the proper sound is no longer fully and clearly emitted. But the
primary use of this tube is to afford a free communication between the
middle ear and the external atmosphere, and thus secure an equal pressure
upon both sides of the membrane of the drum of the ear, however the density
of the atmosphere may vary. If, from undue tension of the membranes, pain
is experienced in the ears, when ascending into a rare atmosphere, as in a
balloon, or descending into a dense one, as in a diving-bell, it may be
relieved by repeating the act of swallowing, from time to time, in order
that the inner and outer pressure may thus be promptly equalized.

[Sidenote: 100. The essential part of the organ of hearing? Its location?
Formation?]

100. THE INTERNAL EAR, OR LABYRINTH.--The most essential part of the organ
of hearing is the distribution of the _auditory nerve_. This is found
within the cavity of the internal ear, which, from its exceedingly tortuous
shape, {223} has been termed the _labyrinth_ (see Fig. 53, C). This cavity
is hollowed out in dense bone, and consists of three parts; the _vestibule_
(_a_), or ante-chamber, which is connected with the other two; the
_cochlea_ (_b_), or snail's shell; and the three _semicircular canals_
(_c_). The manner in which the nerve of hearing is distributed is
remarkable, and is peculiar to this nerve. In the vestibule and the canals
its fibres are spread out over the inner surface, not of the bony cavity
but of a membranous bag, which conforms to and partially fills that cavity;
and which floats in it, being both filled and surrounded with a clear,
limpid fluid.

[Sidenote: 101. Where is the "ear-sand" found? Give the theory as to its
use.]

101. A singular addition to the mechanism of hearing is observed within
this membranous bag of the labyrinth. This consists of two small oval
ear-stones, and a quantity of fine powder of a calcareous nature, which is
called "ear-sand." When examined under the microscope, these sandy
particles are seen to lie scattered upon and among the delicate filaments
of the auditory nerve; and it is probable, that as the tremulous sound-wave
traverses the fluid of the vestibule, the sand rises and falls upon the
nerve filaments, and thus intensifies the sonorous impression.

[Sidenote: 102. In the cochlea or snail's shell? "Key-board" in the
internal ear? The vestibules? Semicircular canals?]

102. In the cochlea, or snail's shell, which contains the fluid, but no
membrane, the nerve ramifies upon a spiral shelf, which, like the cochlea
itself, takes two and a half turns, growing continuously smaller as it
winds upward. As many as three thousand nerve fibres of different lengths
have been counted therein; which, it has been thought, form the grand, yet
minutely small key-board, upon which strike all the musical tones that are
destined to be conveyed to the brain. The vestibule, it is also supposed,
takes cognizance of noise as distinguished from musical sounds; while the
office of the semicircular canals is, in part at least, to prevent internal
echoes, or reverberations. {224}

[Sidenote: 103. With what does the vestibule communicate? What is the
theory by which sound is conducted to the brain?]

103. The vestibule communicates with the chain of bones of the middle ear
by means of a small opening, called the "oval window," or _fenestra
ovalis_. Across this window is stretched the membrane, which has already
been alluded to as being joined to the stirrup-bone of the middle ear.
Through this window, then, the sound-wave, which traverses the external and
middle ear, arrives at last at the labyrinth. The limpid fluid which the
latter contains, and which bathes the terminal fibres of the nerve of
hearing, is thus agitated, the nerve-fibres are excited, and a sonorous
impression is conducted to the brain, or, as we say, a sound is heard.

[Sidenote: 104. The formation of the organ of hearing with a view to its
protection?]

104. PROTECTION OF THE SENSE OF HEARING.--From what has been seen of the
complicated parts which compose the organ of hearing, it is evident that
while many of them possess an exquisite delicacy of structure, Nature has
well and amply provided for their protection. We have observed the
concealed situation of the most important parts of the mechanism of the
ear, the length of its cavity, its partitions, the hardness of its walls,
and its communication with the atmosphere; all these provisions rendering
unnecessary any supervision or care on our part in reference to the
interior of the ear. But in respect to its external parts, which are under
our control and within the reach of harm, it is otherwise. We may both
observe the dangers which threaten them, and learn the means necessary to
protect them.

[Sidenote: 105. Danger to which the hearing may be subjected? Advice?]

105. One source of danger to the hearing consists in lowering the
temperature of the ear, especially by the introduction of cold water into
the auditory canal. Every one is familiar with the unpleasant sensation of
distension and the confusion of sounds which accompany the filling of the
ear with water when bathing: the weight of the {225} water within it really
distends the membrane, and the cold chills the adjacent sensitive parts. It
is not surprising, therefore, that the frequent introduction of cold water
and its continued presence in the ear enfeeble the sense of hearing. Care
should be taken to remove water from the ear after bathing, by holding the
head on one side, and, at the same time, slightly expanding the outer
orifice, so that the fluid may run out. For a like reason, the hair about
the ears should not be allowed to remain wet, but should be thoroughly
dried as soon as possible.

[Sidenote: 106. The general rule as to the use of water for the ear?]

106. It may be stated as a general rule, to which there are but few
exceptions, that no cold liquid should ever be allowed to enter the ear.
When a wash or injection is rendered necessary, it should always be warmed
before use. The introduction of cold air is likewise hurtful, especially
when it pours through a crevice directly into the ear, as it may often do
through the broken or partially closed window of a car. The avoidance of
this evil gives rise to another almost as great; namely, the introduction
of cotton or other soft substances into the ear to prevent it from
"catching cold." This kind of protection tends to make the part unnaturally
susceptible to changes of temperature, and its security seems to demand the
continued presence of the "warm" covering. As a consequence of its
presence, sounds are not naturally conveyed, and the sensitiveness of the
nerve of hearing is gradually impaired.

[Sidenote: 107. Chief source of injury to the ear? Directions for removing
foreign objects from the ear? Of a live insect?]

107. The chief source of injury, however, to the ear is from the
introduction of solid substances into the auditory canal, with the design
of removing insects or other foreign objects that have found their way into
the ear; or with the design of scraping out the ear-wax. For displacing a
foreign object, it is usually sufficient to syringe the ear gently with
warm water, the head being so held that the {226} fluid easily escapes. If
a live insect has gained entrance to the ear, it may first be suffocated by
pouring a little oil upon it, and afterward removed by syringing the ear as
just mentioned.

108. The removal of ear-wax is generally unnecessary; for, as we have
before seen, Nature provides that the excess of it shall become dry, and
then spontaneously fall out in the form of fine scales. The danger from the
introduction of solid implements into the outer ear is chiefly found in the
fact that the membrane which lies at the bottom of it is very fragile, and
that any injury of it is liable to be permanent, and to permanently impair
the hearing of the injured ear.

QUESTIONS FOR TOPICAL REVIEW.

_Give as full statements as you can on the following subjects_:

  1. Production of sensation                       177, 178
  2. Variety of sensations                         178, 179
  3. General sensibility                           179, 180
  4. The sensation of pain                              180
  5. The uses of pain                         180, 181, 182
  6. Special sensation                             182, 183
  7. Organs of touch                               183, 184
  8. The sense of touch                       184, 185, 186
  9. The delicacy of touch                         186, 187
  10. Sensations of temperature and weight         187, 188
  11. The organ of taste                           188, 189
  12. The sense of taste                           189, 190
  13. Relations of taste, etc.                     190, 191
  14. Influence of education, etc.                 191, 192
  15. The sense of smell                           192, 193
  16. The nerve of smell                           193, 194
  17. Uses of the sense of smell                   194, 195
  18. The sense of sight                           196, 197
  19. Light, and the optic nerve                   197, 198
  20. The organ of sight                           198, 199
  21. The orbits                                        199
  22. The eyelids                                  200, 201
  23. The lachrymal fluid                     201, 202, 203
  24. The eyeball                                  203, 204
  25. The iris                                     205, 206
  26. The retina                         206, 207, 208, 209
  27. The crystalline lens                         209, 210
  28. Uses of the lens                        210, 211, 212
  29. Long and short sight                         212, 213
  30. Function of accommodation                    213, 214
  31. Old sight, or presbyopia                          215
  32. Hearing and sound                       215, 216, 217
  33. The external ear                        217, 218, 219
  34. The middle ear                                219-222
  35. The internal ear                        222, 223, 224
  36. Protection of the sense of hearing      224, 225, 226

       *       *       *       *       *


{227}

CHAPTER XI.

THE VOICE.

    _Voice and Speech--The Larynx, or the Organ of the Voice--The Vocal
    Cords--The Laryngoscope--The Production of the Voice--The Use of the
    Tongue--The different Varieties of Voice--The Change of Voice--Its
    Compass--Purity of Tone--Ventriloquy._

[Sidenote: 1. The uttering of sounds by animals? How produced?]

1. VOICE AND SPEECH.--In common with the majority of the nobler animals,
man possesses the power of uttering sounds, which are employed as a means
of communication and expression. In man, these sounds constitute the voice;
in the animals, they are designated as the cry. The song of the bird is a
modification of its cry, which is rendered possible from the fact that its
respiratory function is remarkably active. The sounds of the animals are
generally, but not always, produced by means of their breathing organs.
Among the insects, they are sometimes produced by the extremely rapid
vibrations of the wings in the act of flight, as in the case of the
musquito; or they are produced by the rubbing together of hard portions of
the external covering of the body, as in the cricket. Almost all kinds of
marine animals are voiceless. The tambour-fish and a few others have,
however, the power of making a sort of noise in the water.

[Sidenote: 2. The evidence of man's superior endowment? What is stated of
the idiot? Parrot? Raven?]

2. But man alone possesses the faculty of speech, or the power to use
articulate sounds in the expression of ideas, and in the communication of
mind with mind. Speech is thus an evidence of the superior endowment of
man, and involves the culture of the intellect. An idiot, while he {228}
may have complete vocal organs and full power of uttering sounds or cries,
is entirely incapable of speech; and, as a rule, the excellence of the
language of any people will be found to be proportional to their
development of brain. Man, however, is not the only being that has the
power to form articulate sounds, for the parrot and the raven may also be
taught to speak by rote; but man alone attaches meaning to the words and
phrases he employs.

[Sidenote: 3. Speech and hearing? A deaf child? Person having "no ear for
music?" Impaired hearing? What do the examples show?]

3. Speech is intimately related to the sense of hearing. A child born deaf
is, of necessity, dumb also; not because the organs of speech are
imperfect, for he can utter cries and may be taught to speak, and even to
converse in a rude and harsh kind of language; but because he can form no
accurate notion of sound. And a person, whose hearing is not delicate, or
as it is commonly expressed, who "has no ear for music," cannot sing
correctly. A person who has impaired hearing commonly talks in an
unnaturally loud and monotonous voice. These examples show the necessary
relation of intelligence and the sense of hearing with that form of
articulate voice, which is termed speech.

[Sidenote: 4. Organ of the voice? Where situated? Of what is its framework
composed?]

4. THE ORGAN OF THE VOICE.--The essential organ of the voice is the Larynx.
This has been previously alluded to in its relation to the function of
respiration; and, in the chapter on that subject, are figured the front
view of that organ (Fig. 35), and its connection with the trachea, tongue,
and other neighboring parts (Fig. 38). It is situated at the upper part of
the neck, at the top of the trachea, or tube by which air passes into and
out of the lungs. The framework of the larynx is composed of four
cartilages, which render it at once very strong and sufficiently flexible
to enable it to move according to the requirements of the voice. {229}

[Illustration: FIG. 55. SECTION OF THE LARYNX AND TRACHEA.

A, The Epiglottis. B, The Thyroid Cartilage. C, Arytenoid Cartilage.
D, Ventricle of the Larynx. E, Cricoid Cartilage. F, Right Vocal Cord.
H, The Trachea.]

[Sidenote: 5. Names, formation, and situation of the cartilages?]

5. The names of the cartilages are (1) the _thyroid_, which is a broad thin
plate, bent in the middle and placed in the central line of the front part
of the neck, where it is known as the _pomum Adami_, or Adam's apple (Fig.
55, B), and where it may be felt moving up and down with each act of
swallowing; (2) the _cricoid_, which is shaped like a seal ring, with the
broad part placed posteriorly (Fig. 55, E). At the top of the cricoid
cartilage are situated the two small _arytenoid_ cartilages, the right one
of which is shown in Fig. 55, C. These latter little organs are much more
movable than the other two, and are very important in the production of the
voice. They have a true ball and socket joint, and several small muscles
which contract and relax with as perfect regularity and accuracy as any of
the larger muscles of the body.

[Sidenote: 6. Lining of the interior of the larynx? The epiglottis?]

6. The interior of the larynx is lined with a very sensitive mucous
membrane, which is much more closely adherent to the parts beneath than is
usually the case with membranes of this description. The epiglottis (A),
consisting of a single leaf-shaped piece of cartilage, is attached to the
front part of the larynx. It is elastic, easily moved, and fits accurately
over the entrance to the air-passages below it. Its office is to guard
these delicate passages and the lungs against the intrusion of food and
other foreign {230} articles, when the act of swallowing takes place. It
also assists in modifying the voice.

[Sidenote: 7. Where are the vocal cords? The false cords? The true cords?]

7. THE VOCAL CORDS.--Within the larynx, and stretched across it from the
thyroid cartilage in front to the arytenoid cartilages behind, are placed
the two sets of folds, called the vocal cords. The upper of these, one on
each side, are the false cords, which are comparatively fixed and
inflexible. These are not at all essential to the formation of vocal
sounds, for they have been injured, in those lower animals whose larynx
resembles that of man, without materially affecting their characteristic
cries. Below these, one on each side, are the true vocal cords (Fig. 55,
F), which pursue a similar direction to the false cords, namely, from
before backward. But they are composed of a highly elastic, though strong
tissue, and are covered with a thin, tightly-fitting layer of mucous
membrane. Their edges are smooth and sharply defined, and when they meet,
as they do in the formation of sounds, they exactly match each other.

[Sidenote: 8. Where is the ventricle of the larynx? The essentials to the
formation of the tones and modulation of the voice?]

8. Between the true and false vocal cords is a depression on each side,
which is termed the ventricle of the larynx (Fig. 55, D). The integrity of
these true cords, and their free vibration, are essential to the formation
of the tones and the modulation of the natural voice. This is shown by the
fact that, if one or both of these cords are injured or become diseased,
voice and speech are compromised; or when the mucous membrane covering them
becomes thickened, in consequence of a cold, the vocal sounds are rendered
husky and indistinct. When an opening is made in the throat below the
cords, as not infrequently happens in consequence of an attempt to commit
suicide, voice is impossible except when the opening is closed by external
pressure. {231}

[Illustration: FIG. 56. A VIEW OF THE VOCAL CORDS BY MEANS OF THE
LARYNGOSCOPE.]

[Sidenote: 9. Variation in the interval between the true cords of the
voice? Experiment with the mirror?]

9. The interval or space between the true cords of the voice is constantly
varying, not only when their vocal function is in exercise, but also during
the act of respiration. Every time the lungs are inflated, the space
increases to make wide the entrance for the air; and diminishes slightly
during expiration. So that these little cords move gently to and fro in
rhythm with the expansion and contraction of the chest in breathing. These
movements and others may be seen to take place, if a small mirror attached
to a long handle be placed back into the upper part of the throat; the
handle near the mirror must be bent at an angle of 45°, so that we may look
"around the corner," so to speak, behind the tongue. The position which the
mirror must assume will be understood by reference to Fig. 38. A view of
what may be seen under favorable circumstances, during tranquil inspiration
is represented in Fig. 56. The vocal cords are there shown as narrow, white
bands, on each side of the central opening, and since the image is
inverted, the epiglottis appears uppermost. The rings partly seen through
the opening belong to the trachea. This little mirror is the essential part
{232} of an instrument, which is called the laryngoscope, and, simple as it
may seem, it is accounted one of the most valuable of the recently invented
appliances of the medical art.

[Sidenote: 10. The formation of true vocal tones?]

10. THE PRODUCTION OF THE VOICE.--During ordinary tranquil breathing no
sound is produced in the larynx, true vocal tones being formed only during
forcible expiration, when, by an effort of the will, the cords are brought
close together, and are stretched so as to be very tense. The space between
them is then reduced to a narrow slit, at times not more than 1/100 of an
inch in width; and the column of expired air being forced through it causes
the cords to vibrate rapidly, like the strings of a musical instrument.
Thus the voice is produced in its many varieties of tone and pitch; its
intensity, or loudness, depending chiefly upon the power exerted in
expelling the air from the lungs. When the note is high, the space is
diminished both in length and width; but when it is low, the space is wider
and longer (Fig. 57, B, C), and the number of vibrations is fewer within
the same period of time.

[Illustration: FIG. 57. THE DIFFERENT POSITIONS OF THE VOCAL CORDS.

A, The position during inspiration. B, In the formation of low notes. C, In
the formation of high notes.]

[Sidenote: 11. To what is the personal quality of the voice mainly due?
What aids are there?]

11. The personal quality of the voice, or that which enables us to
recognize a person by his speech, is mainly due to the peculiar shape of
the throat, nose, and mouth, and {233} the resonance of the air contained
within those cavities. The walls of the chest and the trachea take part in
the resonance of the voice, the air within them vibrating at the same time
with the parts above them. This may be tested by touching the throat or
breast-bone, when a strong vocal effort is made. The teeth and the lips
also are important, as is shown by the unnatural tones emitted by a person
who has lost the former, or by one who is affected with the deformity known
as "hare-lip." The tongue is useful, but not indispensable to speech; the
case of a woman is reported, from whom nearly the whole tongue had been
torn out, but who could, nevertheless, speak distinctly and even sing.

[Sidenote: 12. Varieties of voice? The baritone? The voice in early youth?]

12. THE VARIETIES of voice are said to be four in number; two, the bass and
tenor, belonging to the male sex; and two, the contralto or alto, and
soprano, peculiar to the female. The baritone voice is the name given to a
variety intervening between the bass and tenor. In man, the voice is strong
and grave; in woman, soft and high. In infancy and early youth, the voice
is the same in both sexes, being of the soprano variety: that of boys is
both clear and loud, and being susceptible of considerable training, is
highly prized in the choral services of the church and cathedral. At about
fourteen years of age the voice is said to change; that is, it becomes
hoarse and unsteady by reason of the rapid growth of the larynx. In the
case of the girl, the change is not very marked, except that the voice
becomes stronger and has a wider compass; but in the boy, the larynx nearly
doubles its size in a single year, the vocal cords grow thicker, longer,
and coarser, and the voice becomes masculine in character. During the
progress of this change, the use of the voice in singing is injudicious.

[Sidenote: 13. The range of the voice? Result of careful training of the
vocal organs?]

13. The ordinary range of each of the four varieties of {234} the voice is
about two octaves; but this is exceeded in the case of several celebrated
vocalists. Madame Parepa-Rosa has a compass of three full octaves. When the
vocal organs have been subjected to careful training, and are brought under
complete control of the will, the tension of the cords become exact, and
their vibrations become exceedingly precise and true. Under these
circumstances the voice is said to possess "purity" of tone, and can be
heard at a great distance, and above a multitude of other sounds. The power
of a pure voice to make itself heard was recently exemplified in a striking
manner: at a musical festival held in an audience-room of extraordinary
size, and amid an orchestra of a thousand instruments and a chorus of
twelve thousand voices, the artist named above also sang; yet such was the
purity and strength of her voice that its notes could be clearly heard
rising above the vast waves of sound produced by the full accompaniment of
chorus and orchestra.

[Sidenote: 14. The production of the articulate sounds? What experiment is
mentioned?]

14. In the production of the articulate sounds of speech, the larynx is not
directly concerned, but those sounds really depend upon alterations in the
shape of the air-passages above that organ. That speech is not necessarily
due to the action of the larynx is proven by the following simple
experiment. Let an elastic tube be passed through the nostril to the back
of the mouth. Then, while the breath is held, cause the tongue, teeth, and
lips to go through the form of pronouncing words, and at the same time, let
a second person blow through the tube into the mouth. Speech, pure and
simple, or, in other words, a whisper is produced. Still further continue
the experiment, while permitting vocal sounds to be made, and there will be
produced a loud and whispering speech at the same moment; thus showing that
voice and speech are the result of two distinct acts. Sighing, in like
manner, is {235} produced in the mouth and throat; if, however, a vocal
sound be added, the sigh is changed into a groan.

[Sidenote: 15. What is ventriloquism? Indication of the original meaning of
the word? How are the ventriloquous sounds produced?]

15. VENTRILOQUISM is a peculiar modification of natural speech, which
consists in so managing the voice that words and sounds appear to issue,
not from the person, but from some distant place, as from the chimney,
cellar, or the interior of a chest. The original meaning of the word
ventriloquism (that is, speaking from the belly), indicates the early
belief that this mode of speech was dependent upon the possession and use
of some special organ besides the larynx and mouth; but at the present
time, it is known that it is produced by these organs alone, and that the
sources of deception consist on the part of the performer, in the dexterous
management of the voice, together with a talent for mimicry; and, on the
part of the auditory, in the liability of the sense of hearing to error in
respect to the direction of sounds. The ventriloquist not only seems to
"throw his voice," as it is said, or simulates the sound as it usually
appears at a distance with but little motion of the lips and face, but he
imitates the voices of an infant and of a feeble old man, of a drunken man
disputing with an exasperated wife, the broken language of a foreigner, the
cry of an animal in distress, demonstrating that the performer must be
proficient in the art of mimicry. Ventriloquism was known to the ancient
Romans and Greeks; and it is thought that the mysterious responses that
were said to issue from the sacred trees and shrines of the oracles at
Dodona and Delphi, were really uttered by priests who had the power of
producing this form of speech.

       *       *       *       *       *


{236}

CHAPTER XII.

THE USE OF THE MICROSCOPE IN THE STUDY OF PHYSIOLOGY.

[Sidenote: 1. The will of the Creator, by what obeyed? The power of a
muscle? Amount of duty performed by the liver?]

1. THE LAW OF THE TISSUES.--The will of an infinite Creator is obeyed by
atoms as well as by worlds. He has seen fit to commit all the functions of
life to structures or tissues so small as to be invisible to the naked eye.
A muscle, for example, as we have already learned, is composed of
innumerable filaments, visible only by the aid of the microscope; and the
power of the muscular mass is but the sum of the contractile power of the
filaments which enter into its composition. Again, each cell of the liver,
invisible to unassisted sight, is a secreting organ, and the liver performs
as much duty as the sum of these minute organs renders possible and no
more.

[Sidenote: 2. Necessity for using the microscope? The advantages gained by
its use?]

2. THE NECESSITY OF THE MICROSCOPE.--If, therefore, we would know the real
structure of the human body, we must make use of the microscope. We are not
at liberty either to use it or not; we _must_ have recourse to it in order
to obtain a real knowledge of the human body. Our eyes are constructed for
the common offices of life, to provide for our wants and guard us from the
ordinary sources of danger; but by arming them with _lenses_, the real
structure of plants and animals is revealed to our intelligence; and
enemies, otherwise invisible, that lie in wait in the air we breathe, and
in our daily food and drink, to destroy life, are guarded against.

[Sidenote: 3. What are convex lenses? Kind of lenses used in microscopes?
Experiment? Picture thrown upon the eye? Derivation of the word
microscope?]

3. CONVEX LENSES, or magnifying glasses, are disks of glass or other
transparent substance, which have the {237} property of picturing upon the
retina of the eye an image of an object larger than the image produced
there without their aid. The glasses used in microscopes are either double
convex lenses (_a_) or plano-convex lenses (_b_). If a double convex lens
or a plano-convex one be placed over a hole in the shutter of a darkened
room, or over the key-hole of a door, and a piece of paper be held at a
proper distance, a picture of all objects in front of the lens will be
thrown on the paper, as in the camera-obscura or the magic-lantern. Now, in
the same manner, a lens throws a picture of objects to which it is directed
on the retina of the eye, and when that picture is larger than the image
made in the eye by the object, without the aid of the lens, it is
magnified, or the lens has served as a _microscope_, so called, from its
use in seeing small objects, from _mikros_, small, and _skopeo_, to see.

[Illustration]

[Sidenote: 4. Kinds of microscope? What are simple microscopes?]

4. DIFFERENT KINDS OF MICROSCOPES.--Microscopes are either _simple_ or
_compound_. The glasses of magnifying spectacles, like those commonly used
by aged persons, are simple microscopes. Magnifying glasses, mounted in
frames such as are for sale by opticians and others, for the detection of
counterfeit money, are simple microscopes, and are useful in studying the
coarser structure of plants and animals.

[Sidenote: 5. Construction of the most powerful simple microscopes? In
practice? A doublet? Triplet? Why are compound microscopes superior to
simple ones?]

5. The most powerful simple microscopes are made by melting in a flame a
thread of spun glass, so as to form a {238} minute globule or bead, which,
when set in a piece of metal and used to examine objects on a plate of
glass held up to the light, gives a high magnifying power. In practice,
however, it is found better to use several magnifying glasses of moderate
power, than a simple lens alone of high power. A combination of two lenses
is called a _doublet_--of three, a _triplet_. All _simple_ microscopes
throw an enlarged image of the object upon the retina. _Compound_
microscopes are so constructed that the enlarged image of an object is
again magnified by a second lens, and hence their magnifying power is
vastly superior to that of simple microscopes.

[Sidenote: 6. Explain, by means of the diagram, the action of the compound
microscope.]

6. The accompanying diagrams will explain the action of the compound
microscope compared with that of the simple microscope. In Fig. 58, which
represents the working of the simple microscope, the rays from the object
(_a b_), passing through the lens (L), form an image (_a' b'_) in the
retina of the eye (E), and as all images are inverted in the eye, the
object is seen as all other objects are, and appears erect. In Fig. 59 is
seen the action of the compound microscope. An inverted image (_a' b'_) of
the object (_a b_) is magnified by the second lens (L'), and an erect image
is thrown upon the retina, which, as all other objects seen erect with the
naked eye are inverted, gives to the image a contrary direction, or inverts
it to the mind.

[Illustration: FIG. 58.--SIMPLE MICROSCOPE.]

{239}

[Sidenote: 7. Portions, in a compound microscope? The glasses?]

7. A COMPOUND MICROSCOPE consists of two portions: the optical portion, or
the lenses, and the mechanical portion, or the instrument which bears the
lenses. The glasses of a compound microscope are two: the _object-glass_
(D), Fig. 60, and the lower lens of Fig. 59, and the _ocular_ or
_eye-piece_ (A), Fig. 60, and the upper piece of Fig. 59. Both the
object-glass and the eye-piece may, and usually do, consist of more than
one lens, for, as previously mentioned, better results are obtained by a
combination of lenses of moderate power than by single lenses of high power
and great curvature.

[Sidenote: 8. How to choose a microscope? How to use it?]

8. HOW TO CHOOSE AND USE A MICROSCOPE.--No attractiveness in the mechanical
part of a microscope can compensate for inferior lenses; and the very first
consideration in the choice of an instrument should be the excellence of
the optical part of the instrument. In the use of the instrument, care
should be exercised to keep the lenses clean, free from dust, not to press
the object-glass upon the object under observation, and not to wet it in
the water in which most objects are examined. A good microscope requires
its own table; and when not in use should be covered by a bell glass or a
clean linen cloth.

[Illustration: FIG. 59. COMPOUND MICROSCOPE.]

{240} [Illustration: FIG. 60.

A, Eye-piece. B, Body. C, Collar. D, Object-glass. E, Stage. F, Hinge.
G, Mirror. H, Stand.]

[Sidenote: 9. The characteristics of the best instrument? What special
requisites should be insisted upon? Why, as to a horizontal stage?]

9. The mechanical portion of the instrument varies greatly in different
instruments. That one is the best which is simplest, the most solid and
easily managed. The stage (E), upon which the object is placed, should not
be movable: it should be solid and firm. The screw by which the focal
distance is adjusted, and which {241} is in constant use, should be so
placed that it can be worked by the hand resting on the table: otherwise
fatigue is soon induced. The direction of the tube carrying the glasses
should be perpendicular, and the stage therefore horizontal. Most objects
in human anatomy are examined in water or in other liquids, or they are
themselves liquids; hence an oblique stage is often inconvenient.

[Illustration: FIG. 61.]

{242}

[Sidenote: 10. Slides? Covers, square and circular? How kept?]

10. ADDITIONAL APPARATUS.--As almost all objects in human anatomy are
examined by transmitted light thrown up from the mirror (G, Fig. 60)
beneath the stage through the object to the eye, they must be placed upon
strips of clear glass about three inches long and one inch wide, commonly
called "slides." These should be procured with the microscope. Again, most
objects seen with high powers require to be covered with a thin plate of
glass, very properly called a "cover," that the moisture of the specimen
may not tarnish the object-glass. Square or circular covers of very thin
glass are therefore provided; and a good supply of these should be always
on hand. These glasses should be kept in a covered dish filled with a
mixture of alcohol and water. Simple water will not remove the fatty matter
which exists in all animal tissues, and, therefore, the glasses cannot be
thoroughly cleaned with it alone.

[Sidenote: 11. Cleaning the glasses? Knives, scissors, etc.? Various
liquids?]

11. When glasses are required for use, they should be removed from the
liquid and wiped clean and dry with a soft linen handkerchief. Delicate
knives, scissors, needles mounted in handles, forceps, pipettes or little
tubes for taking up water, should be obtained; these are essential to all
microscopical study. The table should be supplied with glass-stoppered
bottles containing the various liquids ordinarily used in the study of
physiology. Thus, tincture of iodine is indispensable in studying vegetable
structure, acetic acid in the study of animal tissues; and other articles
will have to be added from time to time, as your progress in study demands
them. {243}

[Sidenote: 12. Bodies, in air and water? The examination of starch?]

12. PRELIMINARY STUDIES.--In order to prepare the way for the study of any
department of science with the aid of the microscope--for the microscope is
but an eye, and can be turned in almost any direction for purposes of
investigation--it is necessary to become acquainted with the many objects
which are liable to complicate the examination of particular structures.
Both air and water are full of floating bodies, and the most common of
these should first occupy the attention. In the city, particles of starch
are always floating in the air. Take a very minute portion of wheat flour,
place it in the middle of a clean glass "slide," drop upon it a drop of
pure water, cover it with a plate of thin glass, and examine it with a
power of from one hundred to six hundred diameters. It will be found to be
composed of minute grains or granules, the largest of which are made up of
coats or layers, like an onion, arranged around a central spot called the
_hilum_.

[Sidenote: 13. The examination with solution of iodine? Advice respecting
other articles?]

13. Make another preparation in the same manner, and, after adding the
water and before covering with the thin glass cover, add a small drop of a
solution of iodine. Now, upon examining the specimen, every grain will be
seen to be of a beautiful deep blue color. After thus studying wheat
starch, the starch of Indian corn, of arrowroot, and of various grains
should be examined in like manner, and their resemblances and differences
noted. The granules of potato-starch are as distinctly marked as any. (See
Fig. 15, page 61.)

[Sidenote: 14. Directions for examining cotton and other fibres? Vegetable
hairs?]

14. Fibres of cotton, lint, and wool are liable to be found in every
specimen prepared for microscopical examination. In order to study these,
any cotton, woollen, or linen fabric, or garment, may be scraped, and the
scrapings placed on a piece of glass moistened with water, covered with the
thin glass plate or cover as before, and {244} examined with the same
magnifying power, namely, from one hundred to six hundred diameters.
Vegetable hairs or down are constantly floating in air and water. These are
of very various forms, are simple or grouped, and form very interesting
objects of study. They are readily procured from the epidermis or outer
membrane of the leaves or stems of plants, by section with a delicate
knife.

[Sidenote: 15. Directions for examining various tissues? Down of moths, and
other structures?]

15. The tissues of plants, epidermis, ducts, and woody fibres are
constantly found in microscopic preparations. They may be studied in
delicate sections made with a sharp knife, or by tearing vegetable tissues
apart with needles. The down of moths, the hairs of different animals, the
fibres of paper, the most common animalcules in water, the dust of shelves,
and generally the structures found in all vegetable and animal substances
by which we are surrounded, should be studied as a preliminary to any
special line of microscopical investigation.

[Sidenote: 16. Directions for examining a drop of blood?]

16. THE STUDY OF HUMAN TISSUES.--When this has been done and familiarity
with the use of the instrument has been obtained, proceed to the study of
the human body, for human physiology is our subject. If the end of the
finger be pricked with a pin, a drop of blood may be procured for
examination. Place this on one of the glass slides, cover it with a thin
piece of glass, press down the cover so as to make a thin layer, and then
examine with the magnifying power just mentioned. Do not add water, for
that will cause the blood corpuscles to disappear. If the drop of blood is
placed under the microscope at once after being drawn from the finger, most
interesting phenomena will be observed. The red corpuscles will be seen to
arrange themselves in rows, like piles of coin, while the blood is
coagulating. The spherical, white corpuscles will {245} be left out of the
rows of red disks, and, if the highest power be used, will be seen to
change their shape constantly.

[Sidenote: 17. Examination of the scales of the mouth? Dandruff?]

17. If you scrape with a dull knife the inside of the cheek, the flattened
scales of "pavement epithelium," or of the insensible covering which,
analogous to the scarf-skin on the outer surface of the body, lines the
cavities of its interior, may be readily studied. They have the appearance
of transparent tiles, each enclosing a round or oval body, called its
nucleus. Dandruff and the scrapings from the skin of the body are composed
of scales like those of the mouth, but they differ somewhat in being
hardened by horny matter, and in having a very faint central body or
nucleus.

[Sidenote: 18. In what, as respects the tissues, do the warm blooded
animals differ? Statement of Milne Edwards?]

18. THE TISSUES OF THE INFERIOR ANIMALS.--The warm-blooded animals do not
differ in the tissues, or microscopic structures, that compose them, but
only in the amount and arrangement of these tissues. Milne Edwards says
these tissues "do not differ much in different animals, but their mode of
association varies; and it is chiefly by reason of the differences in the
combination of these associations in various degrees, that each species
possesses the anatomical properties and characters which are peculiar to
it."

[Sidenote: 19. How to procure materials for the study of the tissues of
man?]

19. Hence the butcher's stall will furnish all the materials for the study
of the microscopic tissues. The structure of the heart, lungs, liver,
brain, and muscle may all be studied, and well studied, by using minute
pieces of the flesh of the lower animals, especially of the quadrupeds.
Such portions of these animals as are not exposed for sale can be readily
obtained by order from the slaughter-house. To examine with the powers of
which we have been speaking, it is only necessary to cut off {246}
exceedingly small pieces, tear them apart with needles, or make very
delicate sections with a sharp knife.

20. INCENTIVES TO STUDY.--A complete knowledge of all minute structures is
not to be expected at once, for you are here introduced into a new realm of
Nature, a world of little things as vast, as wonderful, and as carefully
constructed as the starry firmament,--that other realm of grand objects
which the astronomer nightly scans with the telescope. It will not appear
singular, therefore, if, at first, you feel strange and awkward in this new
creation. With a little perseverance, however, and with the attention
directed toward simple objects at the outset, it will not be long before an
increasing experience will engender confidence.

21. If to all this there be added an enthusiastic study of the standard
authorities on the subject, the rate of progress will be by so much the
more rapid. As compared with similar studies, few possess more interest
than microscopy, and to the one who pursues it with fondness, it constantly
affords sources of pleasure and agreeable surprises; and in the end, often
leads to new and valuable additions to the sum of human knowledge. The
depths which the microscope is employed to fathom are no more completely
known, than are the heights above us explored and comprehended by the
astronomer.

QUESTIONS FOR TOPICAL REVIEW.

_State what you can on the following subjects_:

  1. Voice and speech                   227, 228
  2. The organ of the voice             228, 229
  3. The vocal cords                    230, 231
  4. The production of the voice        232, 233
  5. The varieties of voice             233, 234
  6. Ventriloquism                           235
  7. The law of the tissues                  236
  8. Necessity of the microscope             236
  9. Convex lenses                      236, 237
  10. Kinds of microscope               237, 238
  11. Choosing a microscope             239, 242
  12. Preliminary studies               243, 244
  13. The study of human tissues             244
  14. The study of the inferior animals      245

       *       *       *       *       *


{247}

APPENDIX.

------o------

POISONS AND THEIR ANTIDOTES.

Accidents from poisoning are of such frequent occurrence, that every one
should be able to administer the more common antidotes, until the _services
of a physician can be obtained_. As many poisons bear a close resemblance
to articles in common use, no dangerous substance should be brought into
the household without having the word _poison_ plainly written or printed
on the label; and any package, box, or vial, without a label, should be at
once destroyed, if the contents are not positively known.

When a healthy person is taken severely and _suddenly_ ill _soon after some
substance has been swallowed_, we may suspect that he has been poisoned. In
all cases where poison has been taken into the stomach, it should be
quickly and thoroughly evacuated by some active emetic, which can be
speedily obtained. This may be accomplished by drinking a tumblerful of
warm water, containing either a tablespoonful of powdered mustard or of
common salt, or two teaspoonfuls of powdered alum in two tablespoonfuls of
syrup. When vomiting has already taken place, it should be maintained by
copious draughts of warm water or mucilaginous drinks, such as gum-water or
flaxseed tea, and tickling the throat with the finger until there is reason
to believe that all the poisonous substance has been expelled from the
stomach.

The following list embraces only the more common poisons, together with
such antidotes as are usually at hand, to be used until the physician
arrives.

POISONS.

ACIDS.--_Hydrochloric acid_; _muriatic acid_ (spirits of salt); _nitric
acid_ (aqua fortis); _sulphuric acid_ (oil of vitriol).

ANTIDOTE.--An antidote should be given at once to neutralize the acid.
Strong soapsuds is an efficient remedy, and can always be obtained. It
should be followed by copious draughts of warm water or flaxseed tea.
Chalk, magnesia, soda or saleratus (with water) or {248} lime-water, are
the best remedies. When sulphuric acid has been taken, water should be
given sparingly, because, when water unites with this acid, intense heat is
produced.

_Oxalic acid._

ANTIDOTE.--Oxalic acid resembles Epsom salts in appearance, and may easily
be mistaken for it. The antidotes are magnesia, or chalk mixed with water.

PRUSSIC ACID; _oil of bitter almonds_; _laurel water_; _cyanide of
potassium_ (used in electrotyping).

ANTIDOTE.--Cold douche to the spine. Chlorine water, or water of ammonia
largely diluted, should be given, and the vapor arising from them may be
inhaled.

ALKALIES AND THEIR SALTS.--AMMONIA (hartshorn), _liquor or water of
ammonia_. POTASSA:--_caustic potash_, _strong ley_, _carbonate of potassa_
(pearlash), _nitrate of potassa_ (saltpetre).

ANTIDOTE.--Give the vegetable acids diluted, as weak vinegar, acetic,
citric, or tartaric acids dissolved in water. Castor oil, linseed oil, and
sweet oil may also be used; they form soaps when mixed with the free
alkalies, which they thus render harmless. The poisonous effects of
saltpetre must be counteracted by taking mucilaginous drinks freely, so as
to produce vomiting.

ALCOHOL.--_Brandy_, _wine_; _all spirituous liquors_.

ANTIDOTE.--Give as an emetic ground mustard or tartar emetic. If the
patient cannot swallow, introduce a stomach pump; pour cold water on the
head.

GASES.--_Chlorine_, _carbonic acid gas_, _carbonic oxide_, _fumes of
burning charcoal_, _sulphuretted hydrogen_, _illuminating or coal-gas_.

ANTIDOTE.--For poisoning by chlorine, inhale, cautiously, ammonia
(hartshorn). For the other gases, cold water should be poured upon the
head, and stimulants cautiously administered; artificial respiration. (See
_Marshall Hall's Ready Method_, page 250.)

METALS.--_Antimony_, _tartar emetic_, _wine of antimony_, etc.

ANTIDOTE.--If vomiting has not occurred, it should be produced by tickling
the throat with the finger or a feather, and the abundant use of warm
water. Astringent infusions, such as common tea, oak bark, and solution of
tannin, act as antidotes.

ARSENIC.--_White arsenic_, _Fowler's solution_, _fly-powder_, _cobalt_,
_Paris green_, etc.

ANTIDOTE.--Produce vomiting at once with a tablespoonful or two of powdered
mustard in a glass of warm water, or with ipecac. The antidote is hydrated
peroxide of iron. If Fowler's solution has been taken, lime-water must be
given. {249}

COPPER.--_Acetate of copper_ (verdigris), _sulphate of copper_ (blue
vitriol), food cooked in dirty _copper vessels_, or pickles made green by
_copper_.

ANTIDOTE.--Milk or white of eggs, with mucilaginous drinks (flaxseed tea,
etc.), should be freely given.

IRON.--_Sulphate of iron_ (copperas), etc.

ANTIDOTE.--Carbonate of soda in some mucilaginous drink, or in water, is an
excellent antidote.

LEAD.--_Acetate of lead_ (sugar of lead), _carbonate of lead_ (white lead),
water kept in _leaden pipes_ or _vessels_, food cooked in _vessels_ glazed
with _lead_.

ANTIDOTE.--Induce vomiting with ground mustard or common salt in warm
water. The antidote for soluble preparations of lead is Epsom salts; for
the insoluble forms, sulphuric acid largely diluted.

MERCURY.--_Bichloride of mercury_ (corrosive sublimate), _ammoniated
mercury_ (white precipitate), _red oxide of mercury_ (red precipitate),
_red sulphuret of mercury_ (vermilion).

ANTIDOTE.--The white of eggs, or wheat flour beaten up with water and milk,
are the best antidotes.

SILVER.--_Nitrate of silver_ (lunar caustic).

ANTIDOTE.--Give a teaspoonful of common salt in a tumbler of water. It
decomposes the salts of silver and destroys their activity.

ZINC.--_Sulphate of zinc_, etc. (white vitriol).

ANTIDOTE.--The vomiting may be relieved by copious draughts of warm water.
The antidote is carbonate of soda administered in water.

NARCOTIC POISONS.--_Opium_ (laudanum, paregoric, salts of morphia,
Godfrey's cordial, Dalby's carminative, soothing syrup, cholera mixtures),
_aconite_, _belladonna_, _hemlock_, _stramonium_, _digitalis_, _tobacco_,
_hyosciamus_, _nux vomica_, _strychnine_.

ANTIDOTE.--Evacuate the stomach by the most active emetics, as mustard,
alum, or sulphate of zinc. The patient should be kept in motion, and cold
water dashed on the head and shoulders. Strong coffee must be given. The
physician will use the stomach pump and electricity. In poisoning by nux
vomica or strychnine, etc., chloroform or ether should be inhaled to quiet
the spasms.

IRRITANT VEGETABLE POISONS.--_Croton oil_, _oil of savine_, _poke_, _oil of
tansy_, etc.

ANTIDOTE.--If vomiting has taken place, it may be rendered easier by
copious draughts of warm water. But if symptoms of insensibility have come
on without vomiting, it ought to be immediately excited by ground mustard
mixed with warm water, or some other active emetic {250} and after its
operation an active purgative should be given. After evacuating as much of
the poison as possible, strong coffee or vinegar and water may be given
with advantage.

POISONOUS FISH.--_Conger eel_, _mussels_, _crabs_, etc.

ANTIDOTE.--Evacuate, as soon as possible, the contents of the stomach and
bowels by emetics (ground mustard mixed with warm water or powdered alum),
and castor oil, drinking freely at the same time of vinegar and water.
Ether, with a few drops of laudanum mixed with sugar and water, may
afterward be taken freely.

POISONOUS SERPENTS.--ANTIDOTE.--A ligature or handkerchief should be
applied moderately tight above the bite, and a cupping-glass over the
wound. The patient should drink freely of alcoholic stimulants containing a
small quantity of ammonia. The physician may inject ammonia into the veins.

POISONOUS INSECTS.--_Stings of scorpion_, _hornet_, _wasp_, _bee_, etc.

ANTIDOTE.--A piece of rag moistened with a solution of carbolic acid may be
kept on the affected part until the pain is relieved; and a few drops of
carbolic acid may be given frequently in a little water. The sting may be
removed by making strong pressure around it with the barrel of a small
watch-key.

DROWNING.

MARSHALL HALL'S "READY METHOD" of treatment in asphyxia from drowning,
chloroform, coal gas, etc.

1st. Treat the patient _instantly on the spot_, in the _open air_, freely
exposing the face, neck, and chest to the breeze, except in severe weather.

2d. In order _to clear the throat_, place the patient gently on the face,
with one wrist under the forehead, that all fluid, and the tongue itself,
may fall forward, and leave the entrance into the windpipe free.

3d. _To excite respiration_, turn the patient slightly on his side, and
apply some irritating or stimulating agent to the nostrils, as _veratrine_,
_dilute ammonia_, etc.

4th. Make the face warm by brisk friction; then dash cold water upon it.

5th. If not successful, lose no time; but, _to imitate respiration_, place
the patient on his face, and turn the body gently, but completely _on the
side, and a little beyond_; then again on the face, and so on, alternately.
Repeat these movements, deliberately and perseveringly, {251} _fifteen
times only_ in a minute. (When the patient lies on the thorax, this cavity
is _compressed_ by the weight of the body, and _ex_piration takes place.
When he is turned on the side, this pressure is removed, and _in_spiration
occurs.)

6th. When the prone position is resumed, make a uniform and efficient
pressure _along the spine_, removing the pressure immediately, before
rotation on the side. (The pressure augments the _ex_piration: the rotation
commences _in_spiration.) Continue these measures.

7th. Rub the limbs _upward_, with _firm pressure_ and with _energy_. (The
object being to aid the return of venous blood to the heart.)

8th. Substitute for the patient's wet clothing, if possible, such other
covering as can be instantly procured, each bystander supplying a coat or
cloak, etc. Meantime, and from time to time, _to excite inspiration_, let
the surface of the body be _slapped_ briskly with the hand.

9th. Rub the body briskly till it is dry and warm, then dash _cold_ water
upon it, and repeat the rubbing.

Avoid the immediate removal of the patient, as it involves a _dangerous
loss of time_--also, the use of bellows, or any _forcing_ instrument; also,
the _warm bath_, and _all rough treatment_.

       *       *       *       *       *


{252}

GLOSSARY.

------o------

AB-DO´MEN (Latin _abdo_, to conceal). The largest cavity of the body
containing the liver, stomach, intestines, etc.; the belly.

AB-SOR´BENTS (L. _ab_ and _sorbeo_, to suck up). The vessels which take
part in the process of absorption.

AB-SORP´TION. The process of sucking up fluids by means of an animal
membrane.

AC-COM-MO-DA´TION of the Eye. The alteration in the shape of the
crystalline lens, which accommodates or adjusts the eye for near and remote
vision.

AC´ID, LACTIC (L. _lac_, milk). The acid ingredient of sour milk; the
gastric juice also contains it.

AL-BU´MEN, or Albumin (L. _albus_, white). An animal substance resembling
white of egg.

AL-BU´MI-NOSE (from _albumen_). A soluble animal substance produced in the
stomach by the digestion of the albuminoid substances.

AL-BU´MIN-OID substances. A class of proximate principles resembling
albumen; they may be derived from either the animal or vegetable kingdoms.

AL´I-MENT (L. _alo_, to nourish). That which affords nourishment; food.

AL-I-MENT´A-RY CA-NAL (from _aliment_). A long tube in which the food is
digested, or prepared for reception into the system.

AN-ÆS-THET´ICS (Greek, [Greek: an], _an_, without, [Greek: aisthesia],
_aisthesia_, feeling).--Those medicinal agents which prevent the feeling of
pain, such as chloroform, laughing-gas, etc.

AN-I-MAL´CULE (L. _animal´culum_, a small animal). Applied to animals which
can only be seen with the aid of the microscope. Animalculum (plural,
animalcula) is used with the same meaning.

A-OR´TA (Gr. [Greek: aorteomai], _aorteomai_, to be lifted up). The largest
artery of the body, and main trunk of all the arteries. It arises from the
left ventricle of the heart. The name was first applied to the two large
branches of the trachea, which appear to be lifted up by the heart.

{253} A´QUE-OUS HUMOR (L. _aqua_, water). A few drops of watery colorless
fluid occupying the space between the cornea and crystalline lens.

A-RACH´NOID MEM´BRANE (Gr. [Greek: arachne], _arachne_, a cobweb, and
[Greek: eidos], _eidos_, like). An extremely thin covering of the brain and
spinal cord. It lies between the _dura mater_ and the _pia mater_.

AR´BOR VI´TÆ (L.). Literally, "the tree of life;" a name given to the
peculiar appearance presented by a section of the cerebellum.

AR´TER-Y (Gr. [Greek: aer], _aer_, air, and [Greek: terein], _terein_, to
contain). A vessel by which blood is conveyed away from the heart. It was
supposed by the ancients to contain air; hence the name.

AR-TIC-U-LA´TION (L. _articulo_, to form a joint). The more or less movable
union of bones, etc.; a joint.

A-RYT´E-NOID CAR´TI-LA-GES (Gr. [Greek: arutaina], _arutaina_, a pitcher).
Two small cartilages of the larynx, resembling the mouth of a pitcher.

AS-SIM-I-LA´TION (L. _ad_, to, and _similis_, like). The conversion of food
into living tissue.

AU-DI´TION (L. _audio_, to hear). The act of hearing sounds.

AU´DI-TO-RY NERVE. One of the cranial nerves; it is the special nerve of
hearing.

AU´RI-CLE (L. _auris_, the ear). A cavity of the heart.

BAR´I-TONE (Gr. [Greek: barus], _barus_, heavy, and [Greek: tonos],
_tonos_, tone). A variety of male voice between the bass and tenor.

BEL-LA-DON´NA (It. beautiful lady). A vegetable narcotic poison. It has the
property of enlarging the pupil, and thus increasing the brilliancy of the
eye; so called from its use by Italian ladies.

BI-CUS´PID (L. _bi_, two, and _cuspis_, prominence). The name of the fourth
and fifth teeth on each side of the jaw; possessing two prominences.

BILE. The gall, or peculiar secretion of the liver; a viscid, yellowish
fluid, and very bitter to the taste.

BRONCH´I (Gr. [Greek: bronkos], _bronkos_, the windpipe). The two first
divisions or branches of the trachea; one enters each lung.

BRONCH´I-AL TUBES. The smaller branches of the trachea within the substance
of the lungs, terminating in the air-cells.

BRONCH-I´TIS (from _bronchia_, and _itis_, a suffix signifying
inflammation). An inflammation of the larger bronchial tubes; a "cold"
affecting the lungs.

CAL-CA´RE-OUS (L. _calx_, lime). Containing lime.

CA-NAL´ (L.). In the body, any tube or passage.

CA-NINE´ (L. _canis_, a dog). Name given to the third tooth on each {254}
side of the jaw; in the upper jaw it is also known as the eye-tooth:
pointed like the tusks of a dog.

CAP´IL-LA-RY (L. _capil´lus_, a hair, _capilla´ris_ hair-like). The name of
the extremely minute blood-vessels which connect the arteries with the
veins.

CAR´BON DIOX-IDE (CO_{2}). Chemical name for carbonic acid gas.

CAR-BON´IC A-CID. The gas which is present in the air expired from the
lungs; a waste product of the animal kingdom, and a food of the vegetable
kingdom.

CAR´DI-AC (Gr. [Greek: kardia], _cardia_, the heart). The cardiac orifice
of the stomach is the upper one, and is near the heart; hence its name.

CAR-NIV´O-ROUS (L. _ca´ro_, flesh, and _vo´ro_, to devour). Subsisting upon
flesh.

CA-ROT´ID AR-TE-RY. The large artery of the neck, supplying the head and
brain.

CAR´TI-LAGE. A solid but flexible material, forming a part of the joints,
air-passages, nostrils, etc.; gristle.

CA´SE-INE (L. _ca´seus_, cheese). The albuminoid substance of milk; it
forms the basis of cheese.

CER-E-BEL´LUM (diminutive for _cer´ebrum_, the brain). The little brain,
situated beneath the posterior third of the cerebrum.

CER´E-BRUM (L.). The brain proper, occupying the entire upper portion of
the skull. It is nearly divided into two equal parts, called "hemispheres,"
by a cleft extending from before backward.

CHO´ROID (Gr. [Greek: chorion], _chorion_, a membrane or covering). The
middle tunic or coat of the eyeball.

CHYLE (Gr. [Greek: chulos], chulos, juice). The milk-like fluid formed by
the digestion of fatty articles of food in the intestines.

CHYME (Gr. [Greek: chumos], _chumos_, juice). The pulpy liquid formed by
digestion within the stomach.

CIL´I-A (pl. of _cil´i-um_, an eyelash). Minute, vibratile, hair-like
processes found upon the cells of the air-passages, and other parts that
are habitually moist.

CIR-CU-LA´TION (L. _cir´culus_, a ring). The circuit, or course of the
blood through the blood-vessels of the body, from the heart to the
arteries, through the capillaries into the veins, and from the veins back
to the heart.

CO-AG-U-LA´TION (L. _coag´ulo_, to curdle). Applied to the process by which
the blood clots or solidifies.

COCH´LE-A (L. _coch´lea_, a snail-shell). The spiral cavity of the internal
ear.

{255} CONCH´A (Gr. [Greek: konchê], _konche_, a mussel-shell). The external
shell-shaped portion of the external ear.

CON-JUNC-TI´VA (L. _con_ and _jun´go_, to join together). A thin layer of
mucous membrane which lines the eyelids and covers the front of the
eyeball; thus joining the latter to the lids.

CON-TRAC-TIL´I-TY (L. _con_ and _tra´ho_, to draw together). The property
of a muscle which enables it to contract, or draw its extremities closer
together.

CON-VO-LU´TIONS (L. _con_ and _vol´vo_, to roll together). The tortuous
foldings of the external surface of the brain.

CON-VUL´SION (L. _convel´lo_, to pull together). A more or less violent
agitation of the limbs or body.

COR´NE-A (L. _cor´nu_, a horn). The transparent, horn-like substance which
covers the anterior fifth of the eyeball.

COR´PUS-CLES, BLOOD (L. dim. of _cor´pus_, a body). The small biconcave
disks which give to the blood its red color; the _white_ corpuscles are
globular and larger.

COS-MET´IC (Gr. [Greek: kosmeô], _kosmeo_, to adorn). Beautifying; applied
to articles which are supposed to increase the beauty of the skin, etc.

CRA´NI-AL (L. _cra´nium_, the skull). Pertaining to the skull. The nerves
which arise from the brain are called cranial nerves.

CRI´COID (Gr. [Greek: krikos], _kri´kos_, a ring). A cartilage of the
larynx, resembling a seal-ring in shape.

CRYS´TAL-LINE LENS (L. _crystal´lum_, a crystal). One of the so-called
humors of the eye; a double convex body situated in the front part of the
eyeball.

CU´TI-CLE (L. dim. of _cu´tis_, the skin). The scarf-skin; also called the
_epider´mis_.

CU´TIS (Gr. [Greek: skutos], _skutos_, a skin or hide). The true skin,
lying beneath the cuticle; also called the _der´ma_.

DE-CUS-SA´TION (L. _decus´sis_, the Roman numeral ten, X). A reciprocal
crossing of fibres from side to side.

DI´A-PHRAGM (Gr. [Greek: diaphrassô], _diaphrasso_, to divide by a
partition). A large, thin muscle which separates the cavity of the chest
from the abdomen; a muscle of respiration.

DIF-FUS´ION OF GASES. The power of gases to become intimately mingled,
without reference to the force of gravity.

DUCT (L. _du´co_, to lead). A narrow tube; the _thoracic duct_ is the main
trunk of the absorbent vessels.

DU-O-DE´NUM (L. _duode´ni_, twelve). The first division of the small
intestines, about twelve fingers-breadth long.

{256} DU´RA MA´TER (L.). Literally, the hard mother; the tough membrane
which envelops the brain.

DYS-PEP´SI-A (Gr. [Greek: dus], _dus_, difficult, and [Greek: peptô],
_pep´to_, to digest). Difficult or painful digestion; a disordered
condition of the stomach.

E-MUL´SION (L. _emul´geo_, to milk). Oil in a finely divided state
suspended in water.

EN-AM´EL (Fr. _email_). The dense material which covers the crown of the
tooth.

EN´ER-GY, Specific, of a Nerve. When a nerve of special sense is excited,
whatever be the cause, the sensation experienced is that peculiar to the
nerve; this is said to be the law of the specific energy of the nerves.

EP-I-GLOT´TIS (Gr. [Greek: epi], _epi_, upon, and [Greek: glôttis],
_glottis_, the entrance to the windpipe). A leaf-shaped piece of cartilage
which covers the top of the larynx during the act of swallowing.

EX-CRE´TION (L. _excer´no_, to separate). The separation from the blood of
the waste particles of the body; also the materials excreted.

EX-PI-RA´TION (L. _expi´ro_, to breathe out). The act of forcing air out of
the lungs.

EX-TEN´SION (L. _ex_, out, and _ten´do_, to stretch). The act of restoring
a limb, etc., to its natural position after it has been flexed, or bent;
the opposite of _Flexion_.

FE-NES´TRA (L.). Literally, a window; the opening between the middle and
internal ear.

FI´BRIN (L. _fi´bra_, a fibre). An albuminoid substance found in the blood;
in coagulating it assumes a fibrous form.

FLEX´ION (L. _flec´to_, to bend). The act of bending a limb, etc.

FOL´LI-CLE (L. dim. of _fol´lis_, a bag). A little pouch or depression in a
membrane; it has generally a secretory function.

FUN´GOUS GROWTHS (L. _fun´gus_, a mushroom). A low grade of vegetable life.

GAN´GLI-ON (Gr. [Greek: ganglion], _ganglion_, a knot). A knot-like
swelling in the course of a nerve; a smaller nerve-centre.

GAS´TRIC (Gr. [Greek: gastêr], _gaster_, stomach). Pertaining to the
stomach.

GLAND (L. _glans_, an acorn). An organ consisting of follicles and ducts,
with numerous blood-vessels interwoven; it separates some particular fluid
from the blood.

GLOS´SO-PHAR-YN-GE´AL NERVE (Gr. [Greek: glôssa], _glossa_, the tongue, and
[Greek: pharunx], _pharunx_, the throat). The nerve of taste supplying the
posterior third of the tongue; it also supplies the throat.

GLU´TEN (L.). Literally, glue: the glutinous albuminoid ingredient of
wheat.

{257} GRAN´ULE (L. dim. of _gra´num_, a grain). A little grain; a
microscopic object.

GUS-TA´TION (L. _gus´to_, to taste) The sense of taste.

GUS´TA-TO-RY NERVE. The nerve of taste supplying the front part of the
tongue; a branch of the "fifth" pair.

HÆM´OR-RHAGE (Gr. [Greek: haima], _hai´ma_, blood, and [Greek: rhêgnumi],
_regnumi_, to burst). Bleeding, or the loss of blood.

HEM-I-PLE´GIA (Gr. [Greek: hêmisus], _hemisus_, half, and [Greek: plêssô],
_plesso_, to strike). Paralysis, or loss of power, affecting one side of
the body.

HEM´I-SPHERES (Gr. [Greek: sphaira], _sphaira_, a sphere). Half a sphere,
the lateral halves of the cerebrum, or brain proper.

HE-PAT´IC (Gr. [Greek: hêpar], _hepar_, the liver). Pertaining to the
liver.

HER-BIV´O-ROUS (L. _her´ba_, an herb, and _vo´ro_ to devour). Applied to
animals that subsist upon vegetable food.

HU´MOR (L.). Moisture: the humors are transparent contents of the eyeball.

HY-DRO-PHO´BI-A (Gr. [Greek: hudor], _hudor_, water, and [Greek: phobeô],
_phobeo_, to fear). A disease caused by the bite of a rabid dog or other
animal. In a person affected with it, convulsions are occasioned by the
sight of a glittering object, like water, by the sound of running water,
and by almost any external impression.

HY´GI-ENE (Gr. [Greek: hugieia], _huygieia_, health). The art of preserving
health and preventing disease.

HY´PER-O-PI-A. Abbreviated from HY´PER-MET-RO´PI-A (Gr. [Greek: huper],
_huper_, beyond, [Greek: metron], _metron_, the measure, and [Greek: ôps],
_ops_, the eye). A defect of vision dependent upon a too short eyeball; so
called because the rays of light are brought to a focus at a point behind
the retina; the true "far sight."

IN-CI´SOR (L. _inci´do_, to cut). Applied to the four front teeth of both
jaws, which have sharp cutting edges.

IN´CUS (L). An anvil; the name of one of the bones of the middle ear.

IN-SAL-I-VA´TION (L. _in_, and _sali´va_, the fluid of the mouth). The
mingling of the saliva with the food during the act of chewing.

IN-SPI-RA´TION (L. _in_, and _spi´ro_, _spira´tum_, to breathe). The act of
drawing in the breath.

IN-TEG´U-MENT (L. _in_, and _te´go_, to cover). The skin, or outer covering
of the body.

IN-TES´TINE (L. _in´tus_, within). The part of the alimentary canal which
is continuous with the lower end of the stomach; also called the
intestines, or the bowels.

I´RIS (L. _i´ris_, the rainbow). The thin muscular ring which lies {258}
between the cornea and crystalline lens, and which gives the eye its brown,
blue, or other color.

JU´GU-LAR (L. _ju´gulum_, the throat). The name of the large veins which
run along the front of the neck.

LAB´Y-RINTH (Gr. [Greek: laburinthos], _laburin´thos_, a building with many
winding passages). The very tortuous cavity of the inner ear, comprising
the vestibule, semicircular canals, and the cochlea.

LACH´RY-MAL APPARATUS (L. _lach´ryma_, a tear). The organs for forming and
conveying away the _tears_.

LAC´TE-ALS (L. _lac_, _lac´tis_, milk). The absorbent vessels of the small
intestines; during digestion they are filled with chyle, which has a milky
appearance.

LA-RYN´GO-SCOPE (Gr. [Greek: larunx], _larunx_, the larynx, and [Greek:
skopeô], _skopeo_, to look at). The instrument by which the larynx may be
examined in the living subject.

LAR´YNX (Gr.). The cartilaginous tube situated at the top of the windpipe,
or trachea; the organ of the voice.

LENS (L.). Literally, a lentil; a piece of transparent glass or other
substance so shaped as either to converge or disperse the rays of light.

LIG´A-MENT (L. _li´go_, to bind). A strong, fibrous material binding bones
or other solid parts together; it is especially necessary to give strength
to joints.

LIG´A-TURE. A thread of silk or other material used in tying around an
artery.

LYMPH (L. _lym´pha_, spring-water). The colorless, watery fluid conveyed by
the lymphatic vessels.

LYM-PHAT´IC VESSELS. A system of absorbent vessels.

MAL´LE-US (L.). Literally, the mallet; one of the small bones of the middle
ear.

MAR´ROW. The soft, fatty substance contained in the central cavities of the
bones: the spinal marrow, however, is composed of nervous tissue.

MAS-TI-CA´TION (L. _mas´tico_, to chew). The act of cutting and grinding
the food to pieces by means of the teeth.

ME-DUL´LA OB-LON-GA´TA. The "oblong marrow," or nervous cord, which is
continuous with the spinal cord within the skull.

MEM-BRA´NA TYM´PAN-I (L.). Literally, the membrane of the drum; a delicate
partition separating the outer from the middle ear; it is sometimes
incorrectly called the drum of the ear.

MEM´BRANE. A thin layer of tissue serving to cover some part of the body.

MI´CRO-SCOPE (Gr. [Greek: mikros], _mikros_, small, and [Greek: skopeô],
_skopeo_, to {259} look at). An optical instrument which assists in the
examination of minute objects.

MO´LAR (L. _mo´la_, a mill). The name applied to the three back teeth of
each side of the jaw; the grinders, or mill-like teeth.

MO´TOR (L. _mo´veo_, _mo´tum_, to move). Causing motion; the name of those
nerves which conduct to the muscles the stimulus which causes them to
contract.

MU´COUS MEMBRANE. The thin layer of tissue which covers those internal
cavities or passages which communicate with the external air.

MU´CUS. The glairy fluid which is secreted by mucous membranes, and which
serves to keep them in a moist condition.

MY-O´PI-A (Gr. [Greek: muô], _muo_, to contract, and [Greek: ôps], _ops_,
the eye). A defect of vision dependent upon an eyeball that is too long,
rendering distant objects indistinct; near-sight.

NA´SAL (L. _na´sus_, the nose). Pertaining to the nose; the _nasal
cavities_ contain the distribution of the special nerve of smell.

NERVE (Gr. [Greek: neuron], _neuron_, a cord or string). A glistening,
white cord of cylindrical shape, connecting the brain or spinal cord with
some other organ of the body.

NERVE CELL. A minute, round and ashen-gray cell found in the brain and
other nervous centres.

NERVE FI´BRE. An exceedingly slender thread of nervous tissue found in the
various nervous organs, but especially in the nerves; it is of a white
color.

NU-TRI´TION (L. _nu´trio_, to nourish). The processes by which the
nourishment of the body is accomplished.

OE-SOPH´A-GUS (Gr.). Literally, that which carries food. The tube leading
from the throat to the stomach; the gullet.

O-LE-AG´I-NOUS (L. _o´leum_, oil). Of the nature of oil: applied to an
important group of food-principles--the fats.

OL-FAC´TO-RY (L. _olfa´cio_, to smell). Pertaining to the sense of smell.

OPH-THAL´MO-SCOPE (Gr. [Greek: ophthalmos], _ophthalmos_, the eye, and
[Greek: skopeô], _skopeo_, to look at). An instrument devised for examining
the interior of the globe of the eye.

OP´TIC (Gr. [Greek: optô], _opto_, to see). Pertaining to the sense of
sight.

OR´BIT (L. _or´bis_, the socket). The bony socket or cavity in which the
eyeball is situated.

OS´MOSE (Gr. [Greek: ôsmos], _osmos_, a thrusting or impulsion). The
process by which liquids are impelled through a moist membrane.

OS´SE-OUS (L. _os_, a bone). Consisting of, or resembling bone.

PAL´ATE (L. _pala´tum_, the palate). The roof of the mouth, consisting of
the hard and soft palate.

{260} PAL´MAR. Relating to the palm of the hand.

PAN´CRE-AS (Gr. [Greek: pas], [Greek: pantos], _pas_, _pantos_, all, and
[Greek: kreas], _kreas_, flesh). A long, flat gland situated near the
stomach; in the lower animals the analogous organ is called the
sweet-bread.

PA-PIL´LÆ (L. _papil´la_). The minute prominences in which terminate the
ultimate fibres of the nerves of touch and taste.

PA-RAL´Y-SIS. A disease of the nervous system marked by the loss of
sensation, or voluntary motion, or both; palsy.

PAR-A-PLE´GI-A (Gr. [Greek: paraplêssô], _paraplesso_, to strike amiss). A
form of paralysis affecting the lower half of the body.

PA-TEL´LA (L. dim. of _pat´ina_, a pan). The knee-pan; a small bone.

PEL´VIS (L.). Literally a basin; the bony cavity at the lower part of the
trunk.

PEP´SIN (Gr. [Greek: peptô], _pepto_, to digest). The organic principle of
the gastric juice.

PER-I-STAL´TIC MOVE´MENTS (Gr. [Greek: peristellô], _peristello_, to
contract). The slow, wave-like movements of the stomach and intestines.

PER-I-TO-NE´UM (Gr. [Greek: periteinô], _periteino_, to stretch around).
The investing membrane of the stomach, intestines, and other abdominal
organs.

PER-SPI-RA´TION (L. _perspi´ro_, to breathe through). The sweat, or watery
exhalation of the skin; when visible, it is called _sensible_ perspiration,
when invisible, it is called _insensible_ perspiration.

PE´TROUS (Gr. [Greek: petra], _petra_, a rock). The name of the hard
portion of the temporal bone, in which is situated the drum of the ear and
labyrinth.

PHAR´YNX (Gr. [Greek: pharunx], _pharunx_, the throat). The cavity between
the back of the mouth and gullet.

PHYS-I-OL´O-GY (Gr. [Greek: phusis], _phusis_, nature, and [Greek: logos],
_logos_, a discourse). The science of the functions of living, organized
beings.

PI´A MA´TER (L.). Literally, the tender mother; the innermost of the three
coverings of the brain. It is thin and delicate; hence the name.

PLEU´RA (Gr. [Greek: pleura], a rib). A membrane covering the lung and
lining the chest. There is one for each lung.

PLEU´RI-SY. An inflammation affecting the pleura.

PNEU-MO-GAS´TRIC (Gr. [Greek: pneumôn], _pneumon_, the lungs, and [Greek:
gastêr], _gaster_, the stomach). The name of a nerve distributed to the
lungs and stomach; it is the principal nerve of respiration.

PNEU-MO´NIA (Gr.). An inflammation affecting the air-cells of the lungs.

{261} PRES-BY-O´PI-A (Gr. [Greek: presbus], _presbus_, old, and [Greek:
ôps], _ops_, the eye). A defect of the accommodation of the eye, caused by
the hardening of the crystalline lens; the "far-sight" of adults and aged
persons.

PROC´ESS (L. _proce´do_, _proces´sus_, to proceed, to go forth). Any
projection from a surface. Also, a method of performance; a procedure.

PTY´A-LIN (Gr. [Greek: ptualon], _ptualon_, saliva). The peculiar organic
ingredient of the saliva.

PUL´MO-NA-RY (L. _pul´mo_, _pulmo´nis_, the lungs). Pertaining to the
lungs.

PULSE (L. _pel´lo_, _pul´sum_, to beat). The striking of an artery against
the finger, occasioned by the contraction of the heart, commonly felt at
the wrist.

PU´PIL (L. _pupil´la_). The central, round opening in the iris, through
which light passes into the depths of the eye.

PY-LO´RUS (Gr. [Greek: pulôros], _puloros_, a gate-keeper). The lower
opening of the stomach, at the beginning of the small intestine.

RE´FLEX ACTION. An involuntary action of the nervous system, by which an
external impression conducted by a sensory nerve is reflected, or converted
into a motor impulse.

RES-PI-RA´TION (L. _res´piro_, to breathe frequently). The function of
breathing, comprising two acts: _inspiration_, or breathing in, and
_expiration_, or breathing out.

RET´I-NA (L. _re´te_, a net). The innermost of the three tunics or coats of
the eyeball, being an expansion of the optic nerve.

SAC´CHA-RINE (L. _sac´charum_, sugar). Of the nature of sugar; applied to
the important group of food substances which embraces the different
varieties of sugar, starch, and gum.

SA-LI´VA (L.). The moisture or fluids of the mouth, secreted by the
salivary glands, etc.

SCLE-ROT´IC (Gr. [Greek: sklêros], _skleros_, hard). The tough, fibrous
outer tunic of the eyeball.

SE-BA´CEOUS (L. _sebum_, fat). Resembling fat, the name of the oily
secretion by which the skin is kept flexible and soft.

SE-CRE´TION (L. _secer´no_, _secre´tum_, to separate). The process of
separating from the blood some essential important fluid; which fluid is
also called a secretion.

SEM-I-CIR´CU-LAR CANALS. A portion of the internal ear.

SEN-SA´TION. The perception of an external impression by the nervous
system; a function of the brain.

SEN-SI-BIL´I-TY, GENERAL. The power possessed by nearly all parts {262} of
the human body of recognizing the presence of foreign objects that come in
contact with them.

SE´RUM (L.). The watery constituent of the blood, which separates from the
clot during the process of coagulation.

SKEL´E-TON (Gr.). The bony framework of an animal, the different parts of
which are maintained in their proper relative positions.

SPEC´TRO-SCOPE (from _spec´trum_ and [Greek: skopeô], _scopeo_, to examine
the spectrum). An instrument employed in the examination of the spectrum of
the sun or any other luminous body.

SPHYG´MO-GRAPH (Gr. [Greek: sphugmos], _sphugmos_, the pulse, and [Greek:
graphô], _grapho_, to write). An ingenious instrument by means of which the
pulse is delineated upon paper.

STA´PES (L.). Literally, a stirrup; one of the small bones of the tympanum,
or middle ear, resembling somewhat a stirrup in shape.

SYM-PA-THET´IC SYSTEM OF NERVES. A double chain of nervous ganglia,
connected together by numerous small nerves, situated chiefly in front of
and on each side of the spinal column.

SYN-O´VI-A (Gr. [Greek: sun], _sun_, and [Greek: ôon], _oon_, resembling an
egg). The lubricating fluid of joints, so called because it resembles the
white of egg.

SYS´TO-LE (Gr. [Greek: sustellô], _sustello_, to contract). The contraction
of the heart, by which the blood is expelled from that organ.

TAC´TILE (L. _tac´tus_, touch). Relating to the sense of touch.

TEM´PO-RAL (L. _tem´pus_, time, and _tem´pora_, the temples). Pertaining to
the temples; the name of an artery: so called, because the hair begins to
turn white with age in that portion of the scalp.

TEN´DON (L. _ten´do_, to stretch). The white, fibrous cord or band by which
a muscle is attached to a bone; a sinew.

TET´A-NUS (Gr. [Greek: teinô], _teino_, to stretch). A disease marked by
persistent contractions of all or some of the voluntary muscles; those of
the jaw are sometimes solely affected: the disorder is then termed
locked-jaw.

THO´RAX (Gr. [Greek: thôrax], _thorax_, a breastplate). The upper cavity of
the trunk of the body, containing the lungs, heart, etc.; the chest.

THY´ROID (Gr. [Greek: thureos], _thureos_, a shield). The largest of the
cartilages of the larynx; its angular projection in the front of the neck
is called "Adam's apple."

TRA´CHE-A (Gr. [Greek: trachus], _trachus_, rough). The windpipe, or the
largest of the air-passages; composed in part of cartilaginous rings, which
render its surface rough and uneven.

TRANS-FU´SION (L. _transfun´do_, to pour from one vessel to another). {263}
The operation of injecting blood taken from one person into the veins of
another; other fluids than blood are sometimes used.

TRICH-I´NA SPI-RA´LIS. (L.) A minute species of parasite or worm, which
infests the flesh of the hog, and which may be introduced into the human
system by eating pork not thoroughly cooked.

TYM´PA-NUM (Gr. [Greek: tumpanon], _tumpanon_, a drum). The cavity of the
middle ear, resembling a drum in being closed by two membranes, and in
having communication with the atmosphere.

U´VU-LA (L. _uva_, a grape). The small pendulous body attached to the back
part of the palate.

VAS´CU-LAR (L. _vas´culum_, a little vessel). Pertaining to, or containing
blood-vessels.

VE´NOUS (L. _ve´na_, a vein). Pertaining to, or contained within a vein.

VEN-TI-LA´TION. The introduction of fresh air into a room or building, in
such a manner as to keep the air within it in a pure condition.

VEN-TRIL´O-QUISM (L. _ven´ter_, the belly, and _lo´quor_, to speak). A
modification of natural speech by which the voice is made to appear to come
from a distance. The ancients supposed that the voice was formed in the
belly; hence the name.

VEN´TRI-CLES of the heart. The two largest cavities of the heart, situated
at its apex or point.

VER´TE-BRAL COLUMN (L. _ver´tebra_, a joint). The back-bone, consisting of
twenty-four separate bones, called vertebræ, firmly jointed together; also
called the spinal column and spine.

VES´TI-BULE. A portion of the internal ear, communicating with the
semicircular canals and the cochlea; so called from its fancied resemblance
to the vestibule or porch of a house.

VIL´LI (L. _vil´lus_, the nap of cloth). Minute thread-like projections
found upon the internal surface of the small intestine, giving it a velvety
appearance.

VIT´RE-OUS (L. _vi´trum_, glass). Having the appearance of glass; applied
to the humor occupying the largest part of the cavity of the eyeball.

VIV-I-SEC´TION (L. _vi´vus_, alive, and _se´co_, to cut). The practice of
operating upon living animals, for the purpose of studying some
physiological process.

VOCAL CORDS. Two elastic bands or ridges situated in the larynx; they are
the essential parts of the organs of the voice.

       *       *       *       *       *


{265}

INDEX.

------o------

                     A.
                                      PAGE
  Absorbent vessels,                    97
  Absorption,                           96
    by blood-vessels,                   96
    by the lacteals,                    96
    of the food,                        96
  Accommodation, function of,          213
  Achilles, tendon of,                  27
  Adam's apple,                        229
  Air, atmospheric,                    131
    Changes in, in respiration,        132
    Carbonic acid in,                  138
    Composition of,                    131
    Dust in the,                       137
    Effects of impure,                 139
    Impurities in,                     136
    Matters in the expired,            132
    Provision for purifying,           141
    Renovation by ventilation,         142
  Air-cells of the lungs,              125
  Air-passages,                        125
  Albinos,                              44
  Albumen,                              58
    of the blood,                      102
  Albuminoid substance,                 57
    Varieties of,                       57
    Properties of,                   57-58
  Albuminose,                           94
  Alcoholic liquors,                    77
    Physiological action of,            78
  Alimentary canal,                     81
  Animal functions,                    143
  Animal heat,                         143
    how produced,                      143
    regulated by perspiration,         145
  Animals, relative strength of,        28
  Apoplexy,                            171
  Aqueous humor,                       210
  Arachnoid membrane,                  152
  Arbor vitæ,                          154
  Arterial blood,                  107-135
    differs from venous,               135
  Arteries,                            114
    Arrangement of,                    115
    Carotid,                           116
    Distribution of,                   115
    Pulsation of,                      115
    Radial,                            116
    Temporal,                          116
  Arytenoid cartilage,                 229
  Asphyxia,                            250
  Assimilation,                    80, 121
  Audition,                            215
  Auditory canal,                      218
    nerve,                             222
  Auricles of the heart,               109

                     B.

  Back-bone,                            21
  Bathing,                              47
    Importance of,                      47
    Time and manner of,                 49
  Baths,                                48
    Different kinds of,                 48
  Belladonna,                          206
    dilates the pupil,                 206
    Use as a cosmetic,                 206
  Bile,                                 95
    Secretion of, in the liver,         95
    Accumulation of, in the
      gall-bladder,                     95
    Uses of,                            95
  Biliary duct,                         95
  Bladder, Gall-,                       95
  Bleeding, how stopped,               121
  Blind-spot,                          207
  Blood,                               101
    Arterial,                      107-135
    Change of color,                   107
    Circulation of,               107, 133
    Coagulation of,                    105
    Composition of,                    102
    corpuscles,                   102, 103
    fluid,                             105
    Microscopic appearance of,         102
    Respiratory changes in,            133
    Uses of the,                       105
    Venous,                       107, 135
  Blood-vessels,                  114, 118
    Absorption by,                      96
    Injuries to the,                   121
  Body, renovation of the,              66
  Bones,                                15
    Form and composition of,            16
    Growth of,                          22
    Microscopic structure of,           17
    Repair of,                          23
    Structure of,                       17
    Uses of,                            15
  Bowels,                               94
  Brain,                               150
    Anatomical structure of,           152
    Function of the,                   172
    Injuries of the,                   173
    Membranes of the,                  152
    Reflex action of the,              174
  {266}
  Bread,                                72
  Bronchial tubes,                     125
  Bronchitis,                          128

                     C.

  Canals, Semicircular,                223
  Capillary blood-vessels,             118
    Circulation in the,                118
  Carbonic acid,                       132
    exhaled from the lungs,            132
    in the air,                        138
    retention in the blood,            134
  Cartilage,                            20
    Arytenoid,                         229
    Cricoid,                           229
    Thyroid,                           229
  Casein,                               58
  Cataract,                            210
  Cells, Nerve,                   150, 159
    Ciliated,                          128
  Cerebellum,                          153
    Function of the,                   172
  Cerebro-spinal nervous system,       150
  Cerebrum,                            152
    Function of the,                   172
  Cheese,                               58
  Chest, Framework of,                  19
    Contents of the,                    19
  Chocolate,                            77
  Chorea,                              169
  Choroid coat of the eye,             204
  Chyle,                                95
  Chyme,                                94
  Cilia,                               128
  Circulation,                         107
    in the frog's foot,                119
    of the blood,                      107
    Rapidity of,                       120
    through the heart,                 112
    through the lungs,                 123
  Clothing,                             51
  Coagulation of milk,                  58
    of the blood,                      104
  Cochlea,                             223
  Coffee,                               75
    Effects of,                     75, 76
  Collar-bone,                          19
  Color-blindness,                     209
  Column, Spinal,                       21
  Combustion, Spontaneous,             145
  Complexion,                           44
  Concha of the ear,                   217
  Conjunctiva,                         200
  Contraction of heart,                111
    of muscles,                         27
  Convulsions,                         169
  Cooking,                              70
  Cords, Vocal,                   126, 230
  Cornea,                              203
  Corpuscles, Blood,                   102
  Cosmetics,                            51
  Cranial ganglia,                     150
    Functions of,                      171
  Cranial nerves,                      154
  Cranium,                              19
  Cricoid cartilage,                   229
  Crystalline lens,                    209
    Uses of,                           210
  Cuticle,                              41
    Function of,                       183
  Cutis,                                42

                     D.

  Decussation of motor and sensory
    fibres of spinal cord,             164
  Dentition of infancy,                 82
  Diaphragm, Movements of the, in
    respiration,                  128, 129
  Diastole of the heart,               111
  Diet, Mixed,                      66, 85
    Necessity for changing,             67
    Necessity of a regulated,           62
    The best,                           63
  Digestion,                            80
    Circumstances affecting,            97
    Gastric,                            93
    Intestinal,                         94
    Nature of,                          81
    Organs of,                       81-91
  Drowning,                            250
  Duct, Biliary,                        95
    Nasal,                             201
    Pancreatic,                         95
    Thoracic,                           97
  Dura Mater,                          152

                     E.

  Ear,                                 217
    External,                          217
    Internal,                          222
    Middle,                            219
    Foreign bodies in,                 225
    Drum of the,                       219
    Bones of the,                      220
  Ear-sand,                            223
  Ear-stones,                          223
  Ear-wax,                             219
  Eggs,                                 68
    Composition of,                     68
  Emulsion of fats, in digestion,       95
  Enamel of the teeth,                  82
  Epiglottis,                     126, 229
    Uses of,                      126, 220
  Eustachian tube,                     221
  Exercise,                             30
    Different modes of,                 31
    Effects of,                         30
    Importance of,                      30
    Open-air,                           33
  Expiration,                          128
    Movements of,                      129
  Extensor muscles,                     26
  Eye,                                 198
  Eyeball,                             203
  Eyelashes,                           200
  Eyelids,                             200

                     F.

  Fats,                                 59
    Emulsion of,                        59
    Source of, in food,                 59
  Fenestra ovalis,                     224
  Fibres, Muscular,                     25
  {267}
  Fibres, Nervous,                     149
  Fibrine in food,                      58
    of the blood,                      102
  Fish, as food,                        71
  Flexor muscles,                       26
  Food,                                 53
    Animal,                             67
    Daily quantity of,                  65
    Ingredients of,                  54-62
    Necessity for,                      64
    Source of,                          53
    Vegetable,                          71

                     G.

  Gall-bladder,                         95
  Ganglia, cranial, Functions of the,  171
  Gases, Interchange of, in the lungs, 134
  Gastric digestion,                    93
  Gastric juice,                        91
    Action of,                          93
    Daily quantity of,                  92
  General sensibility,                 179
  Glands, Perspiratory,                 45
    Salivary,                           86
    Sebaceous,                          44
  Glossary,                            252
  Glosso-pharyngeal nerve,             189
  Gullet,                               90
  Gum,                                  61
    as food,                            62
  Gustatory nerve,                     189
  Gymnastics,                        33-38
    for schools and colleges,           33

                     H.

  Hair,                                 42
    Uses of,                            44
  Hearing, Sense of,                   215
    Protection of,                     224
  Heart,                               107
    Cavities of the,              109, 110
    Circulation through the,           112
    Frequency of action,               112
    Movements of the,                  111
    Valves of the,                     112
  Heat, Animal,                        143
    Production of,                     143
    Regulation of,                     145
  Hemiplegia,                          165
  Humor, Aqueous,                      210
    Crystalline,                       209
    Vitreous,                          210
  Hunger,                               65
    Seat of the sensation of,           65
  Hydra,                               149
  Hydrophobia,                         169
  Hygiene,                              13
  Hyperopia,                           212

                     I.

  Incus,                               220
  Inorganic substances in food,         54
  Insalivation,                     86, 88
  Insensible perspiration,              46
  Inspiration,                         128
  Intestinal juice,                     95
    Action of,                          96
  Intestines,                           94
    Complete digestion in the small,    94
    Villi of the,                       96
  Iris,                                205
    Function of,                       205
  Iron,                                 56
    Proportion in the blood,            57
    Proportion in the food,             57

                     J.

  Joints,                               19
    Varieties of,                       20
  Juice, gastric,                       91
    Intestinal,                         95
    Pancreatic,                         95

                     L.

  Labyrinth,                           222
  Lachrymal canals,                    201
    gland,                             201
  Lacteals,                             96
    Absorption by,                      96
  Lactic acid in gastric juice,         92
  Lactometer,                           68
  Large intestines,                     94
  Laryngoscope,                        231
  Larynx,                         125, 228
    Production of the voice in
      the,                        126, 228
  Lens, crystalline,                   209
  Ligaments,                            19
  Light, theory of,                    197
  Lime in the bones,                    16
    in the food,                        56
    Importance of,                      56
  Liver,                                95
    Secretion of the,                   95
  Locked jaw,                          169
  Long-sight,                          212
  Lungs,                               123
    Capacity of,                       130
    Structure of,                      125
  Lymph,                                97
  Lymphatic vessels,                    97

                     M.

  Magendie, on pain,                   181
  Magnesia, Compounds of, in food,      57
  Malleus,                             220
  Marrow of the bones,                  17
  Mastication,                          82
    Importance of,                  88, 89
  Meats,                                68
    The cooking of,                     69
    The preservation of,                69
  Membrane of the tympanum,            219
  Medulla oblongata,                   154
    Function of the,                   171
  Microscope,                          236
    The value of the,                  236
    Simple,                            237
    Compound,                          239
    The use of the,                    239
  {268}
  Milk,                                 68
    Composition of,                     68
    Specific gravity of,                68
  Milk-teeth,                           82
  Mucous membrane of air passages,     127
  Muscles,                              25
    Function of the,                    25
    Flexion and extension of,           26
    Voluntary and involuntary,          26
  Muscular contraction,                 27
    fibres,                             25
    sense,                             188
  Myopia                               212

                     N.

  Nails,                                42
    Uses of the,                        44
  Nasal cavities,                      192
    duct,                              201
  Nerve, Auditory,                     222
    Glossopharyngeal,                  189
    Gustatory,                         189
    Olfactory,                         193
    Optic,                             197
    Sympathetic,                       158
  Nerve cells,                    150, 159
  Nerve fibres,                        149
  Nerves, Cranial,                     154
    Spinal,                            156
    Functions of the,                  160
    Sensory, functions of the,         160
    Motor, functions of the,           160
    Sympathetic system of,             158
  Nervous system,                 148, 149
    Cerebro-spinal,                    150
  Nervous tissue, Properties of,       159
  Nose,                                192
  Nutrition, Processes of,              80

                     O.

  Oesophagus,                           90
  Oil, Sources of, in food,             59
  Old-sight,                           215
  Olfactory nerve,                     193
  Optic nerve,                         197
  Orbicular bone,                      220
  Orbit of the eye,                    199
  Organic substances as food,        57-62
  Organs of circulation,               107
    Digestion,                       81-91
    Respiration,                       123
    Sight,                             198
    Voice,                             228
  Oxygen,                              131
    Amount of, consumed in
      respiration,                     132
    Continually supplied to the
      atmosphere,                      141

                     P.

  Pain, Relations of, to pleasure,     181
    Sensation of,                      180
    Uses of,                           180
  Pancreatic juice,                     95
    Uses of,                            95
  Pancreatin,                           95
  Paraplegia,                          163
  Parlor gymnasium,                     36
  Passages, Air,                       125
  Pelvis,                               19
  Pepsin,                               92
  Peristaltic action of the stomach,    92
  Peritoneum,                           94
  Perspiration, Daily amount of,        46
    Sensible and insensible,            46
    Uses of,                       46, 145
  Perspiratory glands,                  45
  Physical strength,                    29
    Culture,                            33
  Physiology,                           11
    Animal,                             11
    Comparative,                        11
    Human,                              11
    Vegetable,                          11
  Pia mater,                           152
  Plasma of the blood,                 102
  Pleura,                              124
  Pleurisy,                            128
  Pneumo gastric nerve,                171
  Pneumonia,                           128
  Poisons and their antidotes,         247
  Potash in the blood,                  57
  Potato,                               73
  Presbyopia,                          215
  Preservation of the teeth,            85
  Ptyalin,                              88
  Pulsation of the heart,              113
    of the arteries,                   116
  Pulse,                               115
    Form of the,                       116
    Writer,                            116
  Pylorus,                              90

                     R.

  Radial artery,                       116
  Red corpuscles of the blood,         102
  Reflex action of the spinal cord,    165
    Requisites for,                    167
    Uses of,                      167, 170
    Causing convulsions,               169
    Objects of,                        170
    of the brain,                 174, 175
  Rennet,                               58
  Respiration,                         123
    Change of blood in,            123-133
    Frequency of,                      129
    Movements of,                      128
    Object of,                         123
    Organs of,                         123
  Respiratory labor,                   135
  Rest, necessity for,                  38
  Retina,                              206
  Retinal light,                       207
  Ribs, Movements of, in respiration,  128

                     S.

  Saccharine substances,                60
  Saliva,                               86
    Importance of,                      88
    Secretion of,                       86
  Salivary glands,                  86, 87
  {269}
  Salt, Common,                         55
    Importance of,                      56
  Sclerotic coat of the eyeball,       204
  Sebaceous glands,                     44
    Secretion of,                       45
  Semicircular canals,                 223
  Sensation of pain,                   180
    Relations of, to pleasure,         181
    of temperature,                    187
    of weight,                         188
    Modification of,                   178
    Production of,                     177
    Variety of,                        178
  Sense of hearing,                    215
    sight,                             196
    smell,                             192
    taste,                             189
    touch,                             184
  Sense, muscular,                     188
    thermal,                           187
  Senses, Special,                     177
  Sensibility, General,                179
  Short-sight,                         212
  Sinews,                               27
  Sight, Sense of,                     196
    Organs of,                         198
  Skeleton,                             19
  Skin,                                 41
    Structure of,                       41
  Skull,                                19
    Uses of the,                        19
  Sleep, Necessity for,                 38
    Amount required,                    39
  Small intestines,                     94
  Smell, Sense of,                     192
    Nerve of,                          193
    Uses of,                           194
  Soda in the food,                     57
  Sound, Production of,                215
  Special senses,                      177
  Spectroscope,                        104
  Speech,                              227
    Relation of, to the sense of
      hearing,                         228
  Sphygmograph,                        116
  Spinal column,                        21
  Spinal cord,                         155
    Decussation of the,                164
    Direction of fibres in,            164
    Functions of the,                  162
    Nerves of,                         156
    Reflex action of,                  165
  Spontaneous combustion,              145
  Stapes,                              220
  Starch,                               61
    Its change into sugar,              61
    Different kinds,                    61
    Effect of boiling,                  61
    Microscopic appearance,             61
  Stimulating substances,               62
  Stomach,                          90, 92
    Digestion,                          93
    Movements of,                       92
    Secretion of,                       92
  St. Vitus' dance,                    169
  Sugar,                                60
    Varieties,                          60
    Sources of,                         61
  Sun-bath,                             50
  Sympathetic system of nerves,        158
  Synovia,                              20
  Systole of the heart,                111

                     T.

  Taste, Association of,               190
    Education of,                      191
    Organ of,                          188
    Sense of,                          189
  Tea, Effect of,                       76
    Kinds of,                           76
  Tears,                               201
    Escape of the,                     201
  Teeth,                                82
    Temporary set of,                   82
    Permanent set of,                   83
    Bicuspid,                           83
    Canine,                             83
    Incisor,                            83
    Molar,                              84
    Arrangement of,                     85
    of different animals,               85
    Preservation of,                    85
  Temperature of the body,             146
    Extremes of,                       146
    Sensations of,                     187
  Tendon of Achilles,                   27
  Tendons,                              27
  Tetanus,                             169
  Thermal,                              50
  Thermæ sense,                        187
  Thirst,                               65
  Thoracic duct,                        97
  Thorax,                               19
  Thyroid cartilage,                   229
  Tissues, intimate structure of the,  236
    Human,                             244
    of the lower animals,              245
  Tongue,                              188
    Nerves of,                         189
    Sensibility,                       189
  Touch, Delicacy of,                  186
    Organs of,                         183
    Sense of,                          184
  Trachea,                             125
  Transfusion,                         106
  Trichina spiralis,                    71
  Trunk,                                19
  Tympanum of the ear,                 219
    Membrane of,                       219

                     V.

  Valves of the heart,                 112
    of the veins,                      117
  Vapor, Animal, in breath,            132
  Vegetable food,                       71
  Vegetative functions,                148
  Veins,                               117
    Valves of,                         117
  Venous blood,                        135
    Changes of, in respiration,        133
  Ventilation,                         142
  Ventricles of the larynx,            229
    of the heart,                      110
  Ventriloquism,                       235
  {270}
  Vertebrae,                            21
  Vestibule of the internal ear,       223
  Villi of the intestines,              96
    Absorption by,                      96
  Vital knot,                          171
  Vitreous humor,                      210
  Vocal cords,                    126, 230
    Observation of, with laryngoscope, 231
  Voice,                               227
    Organ of,                          228
    Production of,                     232
    Varieties of,                      233

                     W.

  Water,                                74
    Action of, on lead,                 75
    Chemically pure,                    74
    Croton,                             74
    exhaled with the breath,           132
    from springs and wells,             74
    Proportion of, in the blood         55
        "      of, in the tissues and
      fluids of the body,               54
    Ridgewood,                          74
  Walking, as a means of exercise,      31
  White corpuscles of the blood,       104
  Wisdom teeth,                         84

       *       *       *       *       *


Changes made to printed original

Page 45, Sect. 12. "(4, Fig. 14)": '(4, Fig. 13)' in original.

Page 20, Sect. 20, note. "philosophy": 'philosphy' in original.

Page 101, heading. "Sphygmograph": 'Spygmograph' in original. So also in
Table of Contents, but cf. p. 116 and Index.

Page 144, Sect. 46, note. "zoologists": 'zooligsts' in original.

Page 199, Sect. 53. "considerable": 'considera-ale' (on line break) in
original.

Page 255, s.v. Convolutions. "external": 'extenal' in original.





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