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Title: Disease and Its Causes
Author: Councilman, William Thomas
Language: English
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Professor of Pathology, Harvard University

New York
Henry Holt and Company
Williams and Norgate
The University Press, Cambridge, U.S.A.



In this little volume the author has endeavored to portray disease as
life under conditions which differ from the usual. Life embraces much
that is unknown and in so far as disease is a condition of living
things it too presents many problems which are insoluble with our
present knowledge. Fifty years ago the extent of the unknown, and at
that time insoluble questions of disease, was much greater than at
present, and the problems now are in many ways different from those in
the past. No attempt has been made to simplify the subject by the
presentation of theories as facts.

The limitation as to space has prevented as full a consideration of
the subject as would be desirable for clearness, but a fair division
into the general and concrete phases of disease has been attempted.
Necessarily most attention has been given to the infectious diseases
and their causes. This not only because these diseases are the most
important but they are also the best known and give the simplest
illustrations. The space given to the infectious diseases has allowed
a merely cursory description of the organic diseases and such subjects
as insanity and heredity. Of the organic diseases most space has been
devoted to disease of the heart. There is slight consideration of the
environment and social conditions as causes of disease.

Very few authors are mentioned in the text and no bibliography is
given. There is lack of literature dealing with the general aspects of
disease; the book moreover is not written for physicians, and the list
of investigators from whose work the knowledge of disease has been
derived would be too long to cite.

It has been assumed that the reader has some familiarity with
elementary anatomy and physiology, and these subjects have been
considered only as much as is necessary to set the scene for the
drama. I am indebted to my friend, Mr. W. R. Thayer, for patiently
enduring the reading of the manuscript and for many suggestions as to


CHAPTER                                                           PAGE


CELLS OF THE BLOOD.--THE DUCTLESS GLANDS                             9


DEATH.--THE RECOGNITION OF DEATH                                    40




INFLAMMATION                                                        79


DISEASE                                                             97


DISEASES APPEAR?                                                   116


THE THEORY OF EHRLICH                                              135


EPIDEMIC OF PLAGUE                                                 159




NO INFLUENCE.--EUGENICS                                             197


ORGANIC DISEASE.--NEURASTHENIA                                     219


AND POVERTY AS FACTORS IN DISEASE                                  241

GLOSSARY                                                           250

INDEX                                                              252




There is great difficulty, in the case of a subject so large and
complex as is disease, in giving a definition which will be accurate
and comprehensive. Disease may be defined as "A change produced in
living things in consequence of which they are no longer in harmony
with their environment." It is evident that this conception of disease
is inseparable from the idea of life, since only a living thing can
become diseased. In any dead body there has been a preëxisting disease
or injury, and, in consequence of the change produced, that particular
form of activity which constitutes life has ceased. Changes such as
putrefaction take place in the dead body, but they are changes which
would take place in any mass similarly constituted, and are not
influenced by the fact that the mass was once living. Disease may also
be thought of as the negation of the normal. There is, however, in
living things no definite type for the normal. An ideal normal type
may be constructed by taking the average of a large number of
individuals; but any single individual of the group will, to a greater
or less extent, depart from it. No two individuals have been found in
whom all the Bertillon measurements agree. Disease has reference to
the individual; conditions which in one individual would be regarded
as disease need not be so regarded in another. Comparisons between
health and disease, the normal and the abnormal, must be made not
between the ideal normal and abnormal, but between what constitutes
the normal or usual and the abnormal in a particular individual.

The conception of disease is so inseparably associated with that of
life that a brief review of the structure and properties of living
things is necessary for the comprehension of the definition which has
been given. Living matter is subject to the laws which govern matter,
and like matter of any other sort it is composed of atoms and
molecules. There is no force inherent in living matter, no vital force
independent of and differing from the cosmic forces; the energy which
living matter gives off is counterbalanced by the energy which it
receives. It undergoes constant change, and there is constant
interchange with the environment. The molecules which compose it are
constantly undergoing change in their number, kind and arrangement.
Atom groups as decomposition products are constantly given off from
it, and in return it receives from without other atom groups with
which it regenerates its substance or increases in amount. All
definitions of life convey this idea of activity. Herbert Spencer
says, "Life is the continuous adjustment of internal relations to
external conditions." The molecules of the substances forming the
living material are large, complex and unstable, and as such they
constantly tend to pass from the complex to the simple, from unstable
to stable equilibrium. The elementary substances which form living
material are known, but it has hitherto not been found possible
artificially so to combine these substances that the resulting mass
will exhibit those activities which we call the phenomena of life. The
distinction between living and nonliving matter is manifest only when
the sum of the activities of the living matter is considered; any
single phenomenon of the living may appear also in the non-living
material. Probably the most distinguishing criterion of living matter
is found in its individuality, which undoubtedly depends upon
differences in structure, whether physical or chemical, between the
different units.

Certain conditions are essential for the continued existence of living
matter. It must be surrounded by a fluid or semi-fluid medium in order
that there may be easy interchange with the environment. It must
constantly receive from the outside a supply of energy in the form of
food, and substances formed as the result of the intracellular
chemical activity must be removed. In the case of many animals it
seems as though the necessity of a fluid environment for living matter
did not apply, for the superficial cells of the skin have no fluid
around them; these cells, however, are dead, and serve merely a
mechanical or protective purpose. All the living cells of the skin and
all the cells beneath this have fluid around them.

Living matter occurs always in the form of small masses called
"cells," which are the living units. The cells vary in form, structure
and size, some being so large that they can be seen with the naked
eye, while others are so small that they cannot be distinctly seen
with the highest power of the microscope. The living thing or organism
may be composed of a single cell or, in the case of the higher animals
and plants, may be formed of great numbers of cells, those of a
similar character being combined in masses to form organs such as the
liver and brain.

In each cell there is a differentiated area constituting a special
structure, the nucleus, which contains a peculiar material called
"chromatin." The nucleus has chiefly to do with the multiplication of
the cell and contains the factors which determine heredity. The mass
outside of the nucleus is termed "cytoplasm," and this may be
homogeneous in appearance or may contain granules. On the outside
there is a more or less definite cell membrane. It is generally
believed that the cell material has a semi-fluid or gelatinous
consistency and is contained within an intracellular meshwork. It is
an extraordinarily complex mass, whether regarded from a chemical or
physical point of view. (Fig. 1.)

[Illustration: FIG. 1.--DIAGRAM OF CELL. 1. Cell membrane. 2. Cell
substance or cytoplasm. 3. Nucleus. 4. Nuclear membrane.
5. Nucleolus.]

A simple conception of health and disease can be arrived at by the
study of these conditions in a unicellular animal directly under a
microscope, the animal being placed on a glass slide. For this purpose
a small organism called "Amoeba" (Fig. 2), which is commonly present
in freshwater ponds, may be used. This appears as a small mass,
seemingly of gelatinous consistency with a clear outline, the exterior
part homogeneous, the interior granular. The nucleus, which is seen
with difficulty, appears as a small vesicle in the interior. Many
amoebæ show also in the interior a small clear space, the contractile
vesicle which alternately contracts and expands, through which action
the movement of the intracellular fluid is facilitated and waste
products removed. The interior granules often change their position,
showing that there is motion within the mass. The amoeba slowly moves
along the surface of the glass by the extension of blunt processes
formed from the clear outer portion which adhere to the surface and
into which the interior granular mass flows. This movement does not
take place by chance, but in definite directions, and may be
influenced. The amoeba will move towards certain substances which may
be placed in the fluid around it and away from others. In the water in
which the amoebæ live there are usually other organisms, particularly
bacteria, on which they feed. When such a bacterium comes in contact
with an amoeba, it is taken into its body by becoming enclosed in
processes which the amoeba sends out. The enclosed organism then lies
in a small clear space in the amoeba, surrounded by fluid which has
been shown to differ in its chemical reaction from the general fluid
of the interior. This clear space, which may form at any point in the
body, corresponds to a stomach in a higher animal and the fluid within
it to the digestive fluid or gastric juice. After a time the enclosed
organism disappears, it has undergone solution and is assimilated;
that is, the substances of which its body was composed have been
broken up, the molecules rearranged, and a part has been converted
into the substance of the amoeba. If minute insoluble substances, such
as particles of carmine, are placed in the water, these may also be
taken up by the amoeba; but they undergo no change, and after a time
they are cast out. Under the microscope only the gross vital
phenomena, motion of the mass, motion within the mass, the reception
and disintegration of food particles, and the discharge of inert
substances can be observed. The varied and active chemical changes
which are taking place cannot be observed.

[Illustration: FIG. 2.--AMOEBA. 1. Nucleus. 2. Contractile vesicle.
3. Nutritive vacuole containing a bacillus.]

Up to the present it has been assumed that the environment of the
amoeba is that to which it has become adapted and which is favorable
to its existence. Under these conditions its structure conforms to the
type of the species, as do also the phenomena which it exhibits, and
it can assimilate food, grow and multiply. If, during the observation,
a small crystal of salt be placed in the fluid, changes almost
instantly take place. Motion ceases, the amoebæ appear to shrink into
smaller compass, and they become more granular and opaque. If they
remain a sufficiently long time in this fluid, they do not regain
their usual condition when placed again in fresh water. None of the
phenomena which characterized the living amoebæ appear: we say they
are dead. After a time they begin to disintegrate, and the bacteria
contained in the water and on which the amoebæ fed now invade their
tissue and assist in the disintegration. By varying the duration of
the exposure to the salt water or the amount of salt added, a point
can be reached where some, but not all, of the amoebæ are destroyed.
Whether few or many survive depends upon the degree of injury
produced. Much the same phenomena can be produced by gradually heating
the water in which the amoebæ are contained. It is even possible
gradually to accustom such small organisms to an environment which
would destroy them if suddenly subjected to it, but in the process of
adaptation many individuals will have perished.

It is evident from such an experiment that when a living organism is
subject to an environment to which it has not become adapted and which
is unfavorable, such alterations in its structure may be produced that
it is incapable of living even when it is again returned to the
conditions natural to it. Such alterations of structure or injuries
are called the _lesions_ of disease. We have seen that in certain
individuals the injury was sufficient to inhibit for a time only the
usual manifestations of life; these returned when the organism was
removed from the unfavorable conditions, and with this or preceding it
the organisms, if visibly altered, regained the usual form and
structure. We may regard this as disease and recovery. In the disease
there is both the injury or lesion and the derangement of vital
activity dependent upon this. The cause of the disease acted on the
organism from without, it was external to it. Whether the injurious
external conditions act as in this case by a change in the surrounding
osmotic pressure, or by the destruction of ferments within the cell,
or by the introduction into the cell of substances which form stable
chemical union with certain of its constituents, and thus prevent
chemical processes taking place which are necessary for life, the
result is the same.

The experiments with the amoebæ show also two of the most striking
characteristics of living matter. 1. It is _adaptable_. Under the
influence of unusual conditions, alterations in structure and possibly
in substance, may take place, in consequence of which the organisms
under such external conditions may still exhibit the usual phenomena.
The organism cannot adapt itself to such changes without undergoing
change in structure, although there may be no evidence of such changes
visible. This alteration of structure does not constitute a disease,
provided the harmonious relation of the organism with the environment
be not impaired. An individual without a liver should not be regarded
as diseased, provided there can be such an internal adjustment that
all of the vital phenomena could go on in the usual manner without the
aid of this useful and frequently maligned organ. 2. It is
_individual_. In the varying degrees of exposure to unfavorable
conditions of a more serious nature some, but not all, of the
organisms are destroyed; in the slight exposure, few; in the longer,
many. Unfavorable conditions which will destroy all individuals of a
species exposed to them must be extremely rare.[1] There is no such
individuality in non-living things. In a mass of sugar grains each
grain shows just the same characteristics and reacts in exactly the
same way as all the other grains of the mass. Individuality, however
expressed, is due to structural variation. It is almost impossible to
conceive in the enormous complexity of living things that any two
individuals, whether they be single cells or whether they be formed of
cell masses, can be exactly the same. It is not necessary to assume in
such individual differences that there be any variation in the amount
and character of the component elements, but the individuality may be
due to differences in the atomic or molecular arrangements. There are
two forms of tartaric-acid crystals of precisely the same chemical
formula, one of which reflects polarized light to the left, and the
other to the right. All the left-sided crystals and all the
right-sided are, however, precisely the same. The number of possible
variations in the chemical structure of a substance so complex as is
protoplasm is inconceivable.

In no way is the individuality of living matter more strongly
expressed than in the resistance to disease. The variation in the
degree of resistance to an unfavorable environment is seen in every
tale of shipwreck and exposure. In the most extensive epidemics
certain individuals are spared; but here care must be exercised in
interpreting the immunity, for there must be differences in the degree
of exposure to the cause of the epidemic. It would not do to interpret
the immunity to bullets in battle as due to any individual
peculiarity, save possibly a tendency in certain individuals to remove
the body from the vicinity of the bullets; in battle and in epidemics
the factors of chance and of prudence enter. No other living organism
is so resistant to changes in environment as is man, and to this
resistance he owes his supremacy. By means of his intelligence he can
change the environment. He is able to resist the action of cold by
means of houses, fire and clothing; without such power of intelligent
creation of the immediate environment the climatic area in which man
could live would be very narrow. Just as disease can be acquired by an
unfavorable environment, man can so adjust his environment to an
injury that harmony will result in spite of the injury. The
environment which is necessary to compensate for an injury may become
very narrow. For an individual with a badly working heart more and
more restriction of the free life is necessary, until finally the only
environment in which life is even tolerably harmonious is between
blankets and within the walls of a room.

The various conditions which may act on an organism producing the
changes which are necessary for disease are manifold. Lack of
resistance to injury, incapacity for adaptation, whether it be due to
a congenital defect or to an acquired condition, is not in itself a
disease, but the disease is produced by the action on such an
individual of external conditions which may be nothing more than those
to which the individuals of the species are constantly subject and
which produce no harm.

[Illustration: FIG. 3.--A SECTION OF THE SKIN. 1. A hair. Notice there
is a deep depression of the surface to form a small bulb from which
the hair grows. 2. The superficial or horny layer of the skin; the
cells here are joined to form a dense, smooth, compact layer
impervious to moisture. 3. The lower layer of cells. In this layer new
cells are continually being formed to supply those which as thin
scales are cast off from the surface. 4. Section of a small vein. 9.
Section of an artery. 8. Section of a lymphatic. The magnification is
too low to show the smaller blood vessels. 5. One of the glands
alongside of the hair which furnishes an oily secretion. 6. A sweat
gland. 7. The fat of the skin. Notice that hair, hair glands and sweat
glands are continuous with the surface and represent a downward
extension of this. All the tissue below 2 and 3 is the corium from
which leather is made.]

RELATION OF GLANDS TO THE SURFACE. (_a_) Simple or tubular gland,
(_b_) compound or racemose gland.]

All of the causes of disease act on the body from without, and it is
important to understand the relations which the body of a highly
developed organism such as man has with the world external to him.
This relation is effected by means of the various surfaces of the
body. On the outside is the skin [Fig. 3], which surface is many times
increased by the existence of glands and such appendages to the skin
as the hair and nails. A gland, however complicated its structure, is
nothing more than an extension of the surface into the tissue beneath
[Fig. 4]. In the course of embryonic development all glands are formed
by an ingrowth of the surface. The cells which line the gland surface
undergo a differentiation in structure which enables them to perform
certain definite functions, to take up substances from the same source
of supply and transform them. The largest gland on the external
surface of the body is the mammary gland [Fig. 5] in which milk is
produced; there are two million small, tubular glands, the sweat
glands, which produce a watery fluid which serves the purpose of
cooling the body by evaporation; there are glands at the openings of
the hairs which produce a fatty secretion which lubricates the hair
and prevents drying, and many others.

[Illustration: FIG. 5.--A SECTION OF THE MAMMARY GLAND. (_a_) The
ducts of the gland, by which the milk secreted by the cells which line
all the small openings, is conveyed to the nipple. All these openings
are continuous with the surface of the skin. On each side of the large
ducts is a vein filled with blood corpuscles.]

_x x_ are the air tubes or bronchi which communicate with all of
the small spaces. On the walls of the partitions there is a close
network of blood vessels which are separated from the air in the
spaces by a thin membrane.]

The external surface passes into the interior of the body forming two
surfaces, one of which, the intestinal canal, communicates in two
places, at the mouth and anus, with the external surface; and the
other, the genito-urinary surface, which communicates with the
external surface at one place only. The surface of the intestinal
canal is much greater in extent than the surface on the exterior, and
finds enormous extensions in the lungs and in the great glands such as
the liver and pancreas, which communicate with it by means of their
ducts. The extent of surface within the lungs is estimated at
ninety-eight square yards, which is due to the extensive infoldings of
the surface [Fig 6], just as a large surface of thin cloth can, by
folding, be compressed into a small space. The intestinal canal from
the mouth to the anus is thirty feet long, the circumference varies
greatly, but an average circumference of three inches may safely be
assumed, which would give between seven and eight square feet of
surface, this being many times multiplied by adding the surfaces of
the glands which are connected with it. A diagram of the microscopic
structure of the intestinal wall shows how little appreciation of the
extent of surface the examination with the naked eye gives [Fig. 7].
By means of the intestinal canal food or substances necessary to
provide the energy which the living tissue transforms are introduced.
This food is liquefied and so altered by the action of the various
fluids formed in the glands of the intestine and poured out on the
surface, that it can pass into the interior of the body and become
available for the living cells. Various food residues representing
either excess of material or material incapable of digestion remain in
the intestine, and after undergoing various changes, putrefactive in
character, pass from the anus as feces.

LARGE EXTENT OF SURFACE. (_a_) Internal surface. The small finger-like
projections are the villi, and between these are small depressions
forming tubular glands.]

By means of the lungs, which represent a part of the surface, the
oxygen of the air, which is indispensable for the life of the cells,
is taken into the body and carbonic acid removed. The interchange of
gases is effected by the blood, which, enclosed in innumerable, small,
thin-walled tubes, almost covers the surface, and comes in contact
with the air within the lungs, taking from it oxygen and giving to it
carbonic acid.

The genito-urinary surface is the smallest of the surfaces. In the
male (Fig. 8,--27, 28, 30) this communicates with the general external
surface by the small opening at the extremity of the penis, and in the
female by the opening into the vagina. In its entirety it consists in
a surface of wide extent, comprising in the male the urethra, a long
canal which opens into the bladder, and is continuous with ducts that
lead into the genital glands or testicles. The internal surface of the
bladder is extended by means of two long tubes, the ureters, into the
kidneys, and receives the fluid formed in these organs. In the female
(Fig 9) there is a shallow external orifice which is continued into
the bladder by a short canal, the urethra, the remaining urinary
surface being the same as in the male; the external opening also is
extended into the short, wide tube of the vagina, which is continuous
with the canal of the uterus. This canal is continued on both sides
into the Fallopian tubes or oviducts. There is thus in the female a
more complete separation of the urinary and the genital surfaces than
in the male. Practically all of the waste material of the body which
results from cell activity and is passed from the cells into the fluid
about them is brought by the blood to the kidneys, and removed by
these from the blood, leaving the body as urine.


1.  The skull.
2.  The brain, showing the convolutions of the gray exterior in which
    the nerve cells are most numerous.
3.  The white matter in the interior of the brain formed of nerve
    fibres which connect the various parts of this.
4.  The small brain or cerebellum.
5.  The interior of the nose. Notice the nearness of the upper part of
    this cavity to the brain.
6.  The hard or bony palate forming the roof of the mouth.
7.  The soft palate which hangs as a curtain between the mouth and the
8.  The mouth cavity.
9.  The tongue.
10. The beginning of the gullet or oesophagus.
11. The larynx.
12. The windpipe or trachea.
13. The oesophagus.
14. The thyroid gland.
15. The thymus gland or sweetbread.
16. The large vein, vena cava, which conveys the blood from the brain
    and upper body into the heart.
17-25. Lymph nodes; 17, of the neck; 25, of the abdomen.
18. Cross section of the arch of the aorta or main artery of the body
    after it leaves the heart.
19. The sternum or breast bone.
20. The cavity of the heart.
21. The liver.
22. The descending aorta at the back of the abdominal cavity.
23. The pancreas.
24. The stomach.
26. Cross section of the intestines.
27. The urinary bladder.
28. The entrance into this of the ureter or canal from the kidney.
29. Cross sections of the pubic bone.
30. The canal of the urethra leading into the bladder.
31. The penis.
32. The spinal cord.
33. The bones composing the spinal column.
34. The sacrum. The space between this and No. 29 is the pelvis.
35. The coccyx or extremity of the back bone.
36. The rectum.
37. The testicles.]

Between these various surfaces is the real interior of the body, in
which there are many sorts of living tissues,[2] each, of which, in
addition to maintaining itself, has some function necessary for the
maintenance of the body as a whole. Many of these tissues have for
their main purpose the adjustment and coördination of the activities
of the different organs to the needs of the organism as a whole. The
activity of certain of the organs is essential for the maintenance of
life; without others life can exist for a time only; and others, such
as the genital glands, while essential for the preservation of the
life of the species, are not essential for the individual. There is a
large amount of reciprocity among the tissues; in the case of paired
organs the loss of one can be made good by increased activity of the
remaining, and certain of the organs are so nearly alike in function
that a loss can be compensated for by an increase or modification of
the function of a nearly related organ. The various internal parts are
connected by means of a close meshwork of interlacing fibrils, the
connective tissue, support and strength being given by the various
bones. Everywhere enclosing all living cells and penetrating into the
densest of the tissues there is fluid. We may even consider the body
between the surfaces as a bag filled with fluid into which the various
cells and structures are packed.


1. The Fallopian tube which forms the connection between
the ovary and the uterus.
2. The ovary.
3. The body of the uterus.
4. The uterine canal.
5. The urinary bladder represented as empty.
6. The entrance of the ureter.
7. The pubic bone.
8. The urethra.
9. The vagina.
10. The common external opening or vulva.
11. The rectum and anus.]

[Illustration: FIG. 10.--THE LUNGS AND WINDPIPE. Parts of the lungs
have been removed to show the branching of the air tubes or bronchi
which pass into them. All the tubes and the surfaces of the lungs
communicate with the inner surface of the body through the larynx.]

The nervous system (Fig. 8) represents one of the most important of
the enclosed organs. It serves an important function, not only in
regulating and coördinating all functions, but by means of the special
senses which are a part of it, the relations of the organism as a
whole with the environment are adjusted. It consists of a large
central mass, the brain and spinal cord, which is formed in the embryo
by an infolding of the external surface, much in the same way that a
gland is formed; but the connection with the surface is lost in
further development and it becomes completely enclosed. Connected with
the central nervous mass, forming really a part of it and developing
from it, are the nerves, which appear as white fibrous cords and after
dividing and subdividing, are as extremely fine microscopic filaments
distributed to all parts of the body. By means of the nerves all
impressions are conveyed to the brain and spinal cord; all impulses
from this, whether conscious or unconscious, are conveyed to the
muscles and other parts. The brain is the sole organ of psychical
life; by means of its activity the impressions of the external world
conveyed to it through the sense organs are converted into
consciousness. Whatever consciousness is, and on this much has been
written, it proceeds from or is associated with the activity of the
brain cells just as truly as the secretion of gastric juice is due to
the activity of the cells of the stomach. The activity of the nervous
system is essential for extra-uterine life; life ceases by the
cessation of circulation and respiration when either the whole or
certain small areas of its tissue are destroyed. In intra-uterine
life, with the narrow and unchanging environment of the fluid within
the uterine cavity which encloses the foetus, life is compatible with
the absence or rudimentary development of the nervous system. The
foetus in this condition may be otherwise well developed, and it would
be not a misuse of words to say that it was healthy, since it is
adjusted to and in harmony with its narrow environment, but it would
not be normal. The intra-uterine life of the unborn child, it must be
remembered, is carried out by the transmission of energy from the
mother to the foetus by means of the close relation between the
maternal and foetal circulation. It is only when the free existence
demands activities not necessary in intra-uterine life that existence
without a central nervous system becomes impossible.

It is essential in so complicated a structure as the body that some
apparatus should exist to provide for the interchange of material. The
innumerable cell units of the body must have material to provide
energy, and useless material which results from their activity must be
removed. A household might be almost as much embarrassed by the
accumulation of garbage and ashes as by the absence of food and coal.
The food, which is taken into the alimentary canal and converted by
the digestive fluids into material more directly adapted to the uses
of cells, must be conveyed to them. A supply of oxygen is essential
for the life of the cells, and the supply which is given by
respiration must be carried from the lungs to every cell of the body.
All this is effected by the circulation of the blood, which takes
place in the system of branching closed tubes in which the blood
remains (Fig. 11). Certain of these tubes, the arteries, have strong
and elastic walls and serve to convey and distribute the blood to the
different organs and tissues. From the ultimate branches of the
arteries the blood passes into a close network of tubes, the
capillaries, which in enormous numbers are distributed in the tissues
and have walls so thin that they allow fluid and gaseous interchange
between their contents and the fluid around them to take place. The
blood from the capillaries is then collected into a series of tubes,
the veins, by which it is returned to the heart. This circulation is
maintained by means of a pumping organ or heart, which receives the
blood from the veins and by the contraction of its powerful walls
forces this into the arteries, the direction of flow being determined
as in a pump, by a system of valves. The waste products of cell life
pass from the cells into the fluid about them, and are in part
directly returned into the blood, but for the greater part pass into
it indirectly through another set of vessels, the lymphatics. These
are thin-walled tubes which originate in the tissues, and in which
there is a constant flow towards the heart, maintained by the constant
but varying pressure of the tissue around them, the direction of flow
being maintained by numerous valves. The colorless fluid within these
vessels is termed "lymph." At intervals along these tubes are small
structures termed the lymph nodes, which essentially are filters, and
strain out from the fluid substances which might work great injury if
they passed into the blood. Between the capillary vessels and the
lymphatics is the tissue fluid, in which all the exchange takes place.
It is constantly added to by the blood, and returns fluid to the blood
and lymph; it gives material to the cells and receives material from

artery (_a_) opens into a system of capillaries, (_c_) and
after passing through these collects into a vein (_b_). Notice
that the capillaries connect with other vascular territories at
numerous points (_d_). If the artery (_a_) became closed the
capillaries which it supplies could be filled by blood coming from
other sources.]

In addition to the strength and elasticity of the wall of the
arteries, which enables them to resist the pressure of the blood, they
have the power of varying their calibre by the contraction or
expansion of their muscular walls. Many of the organs of the body
function discontinuously, periods of activity alternating with
comparative repose; during the period of activity a greater blood
supply is demanded, and is furnished by relaxation of the muscle
fibres which allows the calibre to increase, and with this the blood
flow becomes greater in amount. Each part of the body regulates its
supply of blood, the regulation being effected by means of nerves
which control the tension of the muscle fibres. The circulation may be
compared with an irrigation system in which the water supply of each
particular field is regulated not by the engineer, but by an automatic
device connected with the growing crop and responding to its demands.

[Illustration: FIG. 12.--THE VARIOUS CELLS IN THE BLOOD. (_a_) The red
blood cells, single and forming a roll by adhering to one another;
(_b_) different forms of the white blood cells; those marked "1" are
the most numerous and are phagocytic for bacteria.]

The blood consists of a fluid, the blood plasma, in which numerous
cells are contained. The most numerous of these are small cup-shaped
cells which contain a substance called _hæmoglobin_, to which the
red color of the blood is due. There are five million of these cells
in a cubic millimeter (a millimeter is .03937 of an inch), giving a
total number for the average adult of twenty-five trillion. The
surface area of all these, each being one thirty-three hundredth of an
inch in diameter, is about thirty-three hundred square yards. The
hæmoglobin which they contain combines in the lungs with the oxygen in
the inspired air, and they give up this indispensable substance to the
cells everywhere in the body. There are also eight thousand leucocytes
or colorless cells in a cubic millimeter of blood, this giving a total
number of four billion in the average adult, and these vary in
character and in relative numbers (Fig. 12). The most numerous of
these are round and slightly larger than the red cells; they have a
nucleus of peculiar shape and contain granules of a definite
character. These cells serve an important part in infectious diseases
in devouring and destroying parasites. They have power of active
independent motion and somewhat resemble certain of the free living
unicellular organisms. The blood plasma, when taken from the vessels,
clots or passes from a fluid into a gelatinous or semi-solid
condition, which is due to the formation within it of a network of
fine threads termed fibrin. It is by means of the clotting of the
blood that the escape of blood from ruptured vessels is arrested.

Several of the organs of the body, in addition to the formation of
secretions which are discharged on the surfaces by means of their
ducts, produce also substances which pass directly into the blood or
lymph, and have an influence in stimulating or otherwise regulating
the activity of other organs. There are also certain organs of
glandular structure which are called the _ductless glands_; these
are not connected with the surface and all their secretion passes into
the blood. It is a part of recent knowledge that the substances
produced in these glands are of great importance for the body, some of
them even essential for the maintenance of life. In front of the neck
is such an organ, the thyroid gland (Fig. 8, 14). Imperfect
development or absence of this organ, or an inactive condition of it,
produces in the child arrested growth and deficient mental development
known as cretinism, and in the adult the same condition gives rise to
mental deterioration, swelling of the skin, due to a greater content
of water, and loss of hair. This deficiency in the production of
thyroid secretion can be made good and the symptoms removed by feeding
the patient with similar glands removed from animals. The very complex
disease known as exophthalmic goitre, and shown by irregular and rapid
action of the heart, protruding eyeballs and a variety of mental
symptoms, is also associated with this gland, and occasioned not by a
deficiency but by an excess or perversion of its secretion.

Adjoining the thyroid there are four small glands, the parathyroids,
each about the size of a split pea. The removal of these glands in
animals produces a condition resembling acute poisoning accompanied by
spasmodic contraction of the muscles. A small glandular organ at the
base of the brain, the pituitary body, produces a secretion, one of
the most marked properties of which is a control of growth,
particularly that of the bones. Most cases of giantism, combined as
they are with imperfect mentality, are due to disease of this gland.
There are glands near the kidney which regulate the pressure of the
blood in the arteries by causing contraction of their muscular walls.
The sexual characteristics in the male and female are due to an
internal secretion produced by the respective sexual glands which
affects growth, body development and mentality.

So is the body constituted. A series of surfaces, all connected, of
enormous size, which enclose a large number of organs and tissues, the
activities of which differ, but all are coördinated to serve the
purposes of the organism as a whole. We should think of the body not
as an assemblage of more or less independent entities, but as a single
organism in which all parts are firmly knit together both in structure
and in function, as are the components of a single cell.

[1] They do, however, take place, since within comparatively
few years whole species have completely disappeared; for example, the
great auk and the passenger pigeon. In these cases it is not known
what part disease played in the destruction.

[2] A tissue represents an aggregate of similar cells with
the intercellular substances in relation with these as connective
tissue, muscular tissue, etc. Where such cell aggregates are localized
and where the cells are arranged in structures having definite form
and size and performing a definite function, it is customary to
designate such structures as organs, as the brain, liver, etc.



There is no sharp line separating health from disease; changes in the
tissues of the same nature, or closely akin to those which are found
in disease, are constantly occurring in a state of health. The
importance of parasites in causing disease has led to the conception
of disease as almost synonymous with parasitism; but it must be
remembered that the presence of parasites living at the expense of the
body is perfectly consistent with a state of health. Degeneration,
decay and parasitism only become disease factors when the conditions
produced by them interfere with the life which is the normal or usual
for the individual concerned.

All the changes which take place in the cells are of great importance
in conditions of both health and disease, for life consists in
coördinated cell activity. The activities of the cells can be divided
into those which are nutritive, those which are functional and those
which are formative. In the functional activity the cell gives off
energy, this loss being made good by the receipt of new energy in the
form of nutritive material with which the cell renews itself. In
certain cells an exact balance seems to be maintained, but in those
cells whose activity is periodic function takes place at the expense
of the cell substance, the loss being restored by nutrition during the
period of repose. This is shown particularly well in the case of the
nerve cells (Fig. 13). Both the functional and nutritive activity can
be greatly stimulated, but they must balance; otherwise the condition
is that of disease.

[Illustration: FIG 13.--NERVE CELLS OF AN ENGLISH SPARROW (_a_) Cells
after a day's full activity, (_b_) cells after a night's repose. In
(_a_) the cells and nuclei are shrunken and the smaller clear spaces
in the cells are smaller and less evident than in (_b_). (Hodge)]

The formative activity of cells is also essential to the normal state.
Destruction of cells is constantly taking place in the body, and more
rapidly in certain tissues than in others. Dried and dead cells are
constantly and in great numbers thrown off from the surface of the
skin: such epidermic appendages as the hair and nails grow and are
removed, millions of cells are represented in the beard which is daily
removed. Cells are constantly being destroyed on the intestinal
surface and in the glands. There is an enormous destruction of the
blood cells constantly taking place, certain essential pigments, as
that of the bile, being formed from the hæmoglobin which the red blood
corpuscles contain and which becomes available on their destruction.
All such loss of cells must be made good by the formation of new ones
and, as in the case of the nutritive and functional activity, the loss
and renewal must balance. The formative activity of cells is of great
importance, for it is by means of this that wounds heal and diseases
are recovered from. This constant destruction and renewal of the body
is well known, and it is no doubt this which has given rise to the
belief, widely held, that the body renews itself in seven years and
that the changes impressed upon it by vaccination endure for this
period only. The truth is that the destruction and renewal of most
tissues in the body takes place in a much shorter interval, and, as we
shall see, this has nothing to do with the changes concerned in
vaccination. All these activities of the cells vary in different
individuals, in different parts and at different ages.

The lesions or injuries of the body which form so prominent a part of
disease vary in kind, degree and situation, depending upon the
character of the injurious agent, the duration of its action and the
character of the tissue affected. The most obvious injuries are those
produced by violence. By a cut, blood vessels are severed, the
relations of tissues disturbed, and at the gaping edges of the wound
the tissue usually protected by the skin is exposed to the air,
resulting in destruction of the cells contained in a thin layer of the
surface. The discoloration and swelling of the skin following a blow
is due to rupture of vessels and escape of blood and fluid, and
further injury may result from the interruption of the circulation.

By the application of heat the tissue may be charred and the albumen
of the blood and tissue fluids coagulated. Living cells are very
susceptible to the action of heat, a temperature of 130 degrees being
the thermal death point, and even lower temperatures are fatal when
their action is prolonged. The action of the heat may produce definite
coagulation of the fluid within the cells in the same way that the
white of an egg is coagulated. Certain of the albumens of the body
coagulate at a much lower temperature than the white of the egg (as
the myosin, one of the albumens of the muscle which coagulates at 115°
F., egg white coagulating at 158° F.), and in addition to such
coagulation or without it the ferments within the cell and to the
action of which cellular activity is due may be destroyed.

In diseases due to parasites, the parasite produces a change in the
tissue in its immediate vicinity often so great as to result in the
death of the cells. The most general direct cause of lesions is toxic
or poisonous substances, either introduced from without or formed in
the body. In the case of the parasitic diseases the mere presence of
the parasite in the body produces little or no harm, the injury being
caused by poisons which it produces, and which act both locally in the
vicinity of the parasite and at a distance, being absorbed and
entering the blood stream. How certain of the poisonous substances act
is easy to see. Strong caustics act by coagulating the albumen, or by
the withdrawal of water from the cell. Other poisons act by forming
stable chemical compounds with certain of the cell constituents and
thereby preventing the usual chemical processes from taking place.
Death from the inhalation of illuminating gas is due to the carbon
monoxide contained in this, forming a firm chemical union with the
hæmoglobin of the red corpuscles so that the function of these as
oxygen carriers is stopped.

In order that most poisons may act, it is essential that they enter
into the cell, and they cannot do this unless they are able to combine
chemically with certain of the cell constituents. To this is due the
selective action of many poisons. Morphine, for example, acts chiefly
on the cells of the brain; strychnine acts on the cells of the spinal
cord which excite motion and thus causes the characteristic muscular
spasm. The poisonous substances produced by bacteria, as in the case
of diphtheria, act on certain of the organs only. Different animal
species owe their immunity to certain poisons to their cells being so
constituted that a poison cannot gain entrance into them; pigeons, for
example, cannot be poisoned by morphia. Individual variations play an
important part also; thus, shellfish are poisonous for certain
individuals and not so for others. Owing to the variability of living
structures a substance may be poisonous at one time and not at
another, as the following example shows. A man, very fond of crab
meat, was once violently poisoned after eating crabs, being at that
time seemingly in his usual state of health, and no illness resulted
in others who had partaken of the same crabs. Two months later a
hearty meal of crabs produced no ill result. There are also
individuals so constituted that so simple a food as the egg is for
them an active poison.

The lesions produced by the action of injurious conditions are usually
so distinctive in situation and character that by the examination of
the body after death the cause of death can be ascertained. The
lesions of diseases may be very obvious to the naked eye, or in other
cases only the most careful microscopic examination can detect even
the presence of alterations. In the case of poisons the capacity of
the cell for adaptation to unusual conditions is of great importance.
It is probable that certain changes take place within the cells, owing
to which the function can be continued in spite of the unusual
conditions which the presence of the poison brings about. It is in
this way that the habitual use of such poisons as morphine, alcohol
and tobacco, to speak only of those best known, is tolerated. The cell
life can become so accustomed to the presence of poisons that the cell
activities may suffer in their absence.

_Repair_ of the injuries which the body receives is effected in a
variety of ways. We do not know how intracellular repair takes place,
but most probably the cells get rid of the injured areas either by
ejecting them, or chemical changes are produced in the altered cell
substance breaking up and recombining the molecules. When single cells
are destroyed, the loss is made good by new formation of cells, the
cell loss stimulating the formative activity of the cells in the
vicinity. The body maintains a cell and tissue equilibrium, and a loss
is in most cases repaired. The blood fluid lost in a hæmorrhage is
quickly restored by a withdrawal of the fluid from the tissues into
the blood, but the cells lost are restored by new formation of cells
in the blood-forming organs. The blood cells are all formed in bone
marrow and in the lymph nodes, and not from the cells which circulate
in the blood, and the stimulus to new cell formation which the loss of
blood brings about affects this remote tissue.

In general, repair takes place most easily in tissues of a simple
character, and where there is the least differentiation of cell
structure for the purposes of function. A high degree of function in
which the cell produces material of a complex character necessitates a
complex chemical apparatus to carry this out, and a complicated
mechanism is formed less easily than a simple one. In certain tissues
the cells have become so highly differentiated that all formative
activity is lost. Such is the case in the nerve cells of the brain and
spinal cord, a loss in which tissue is never repaired by the formation
of new cells; and in the muscles the same is true. The least
differentiation is seen in those cells which serve the purpose of
mechanical protection only, as the cells of the skin, and in these the
formative activity is very great. Not only must the usual loss be
supplied, but we are all conscious of slight injuries of the surface
which are quickly repaired.

Repair, other things being equal, takes place more easily in the young
than in the old. New formation of cells goes on with great rapidity in
intra-uterine life, the child, beginning its existence as a single
cell one two hundred and fiftieth of an inch in diameter, attains in
nine months a weight of seven pounds. The only similar rapidity of
cell formation is seen in certain tumors; although the body may add a
greater amount of weight and in a shorter time, by deposit of fat,
this in but slight measure represents a new formation of tissue, but
is merely a storage of food material in cells. The remarkable repair
and even the new formation of entire parts of the body in the tadpole
will not take place in the completely developed frog.

Repair will also take place the more readily the less complicated is
the architectural structure of the part affected. When a series of
tissues variously and closely related to one another enter into the
structure of an organ, there may be new formation of cells; but when
the loss involves more than this, the complicated architectural
structure will not be completely replaced. A brick which has been
knocked out of a building can be easily replaced, but the renewal of
an area of the wall is more difficult. In the kidney, for example, the
destruction of single cells is quickly made good by new cell
formation, but the loss of an area of tissue is never restored. In the
liver, on the other hand, which is of much simpler construction, large
areas of tissue can be newly formed. For the formation of new cells in
a part there must be a sufficient amount of formative material; then
the circulation of the blood becomes more active, more blood being
brought to the part by dilatation of the vessels supplying it.

Repair after a loss can be perfect or imperfect. The tissue lost can
be restored so perfectly that no trace of an injury remains; but when
the loss has been extensive, and in a tissue of complex structure,
complete restoration does not take place and a less perfect tissue is
formed which is called a scar. Examination of the skin in almost
anyone will show some such scars which have resulted from wounds. They
are also found in the internal organs of the body as the result of
injuries which have healed. The scar represents a very imperfect
repair. In the skin, for example, the scar tissue never contains such
complicated apparatus as hair and sweat glands; the white area is
composed of an imperfectly vascularized fibrous tissue which is
covered with a modified epidermis. The scar is less resistant than the
normal tissue, injury takes place more easily in it and heals with
more difficulty.

Loss brought about by the injuries of disease can be compensated for,
even when the healing is imperfect, by increased function of similar
tissue in the body. There always seems to be in the body under the
usual conditions a reserve force, no tissue being worked to its full
capacity. Meltzer has compared the reserve force of the body to the
factor of safety in mechanical construction. A bridge is constructed
to sustain the weight of the usual traffic, but is in addition given
strength to meet unusual and unforeseen demands. The stomach provides
secretion to meet the usual demands of digestion, but can take care of
an unusual amount of food. The work of the heart may be doubled by
severe exertions, and it meets this demand by increased force and
rapidity of contraction; and the same is true of the muscles attached
to the skeleton. The constant exercise of this reserve force breaks
down the adjustment. If the weight of the traffic over the bridge be
constantly all that it can carry, there quickly comes a time when some
slight and unforeseen increase of weight brings disaster. The
conditions in the body are rather better than in the case of the
bridge, because with the increased demand for activity the heart, for
example, becomes larger and stronger, and reserve force rises with the
load to be carried, but the ratio of reserve force is diminished.

This discussion of injury and repair leads to the question of old age.
Old age, as such, should not be discussed in a book on disease, for it
is not a disease; it is just as natural to grow old and to die as it
is to be born. Disease, however, differs in many respects in the old
as compared with the young and renders some discussion of the
condition necessary. Changes are constantly taking place in the body
with the advance of years, and in the embryo with the advance of days.
In every period of life in the child, in the adult, in the middle-aged
and in the old we meet with conditions which were not present at
earlier periods. There is no definite period at which the changes
which we are accustomed to regard as those of old age begin. This is
true of both the external appearances of age and the internal changes.
One individual may be fully as old, as far as is indicated by the
changes of age, at fifty as another at eighty.

With advancing age certain organs of the body atrophy; they become
diminished in size, and the microscopic examination shows absence or
diminished numbers of the cells which are peculiar to them. The most
striking example of this is seen in the sexual glands of females, and,
to a less degree, in those of the male. There is a small mass or
glandular tissue at the root of the neck, the thymus, which gradually
grows from birth and reaches its greatest size at the age of fifteen,
when it begins slowly to atrophy and almost disappears at the age of
forty. This is the gland which in the calf is known as the sweetbread
and is a delicious and valued article of food. The tonsils, which in
the child may be so large as to interfere with breathing and
swallowing, have almost disappeared in the adult; and there are other
such examples.

In age atrophy is a prominent change. It is seen in the loss of the
teeth, in the whitening and loss of the hair, in the thinning of the
skin so that it more easily wrinkles, in the thinning and weakening of
the muscles so that there is not only diminished force of muscular
contraction, but weakening of the muscles of support. The back curves
from the action of gravity, the strength of the support of the muscles
at the back not counteracting the pull of the weight of the abdominal
viscera in front. The bones become more porous and more brittle.

The effect of atrophy is also seen in the diminution of all functions,
and in loss of weight in individual organs. That the brain shares in
the general atrophy is evident both anatomically and in function.
Mental activity is more sluggish, impressions are received with more
difficulty, their accuracy may be impaired by accompanying changes in
the sense organs, and the concepts formed from the impressions may
differ from the usual. The slowness of mental action and the
diminution in the range of mental activity excited by impressions, and
the slowness of expression, may give a false idea of the value of the
judgment expressed. The expression changes, the face becomes more
impassive because the facial muscles no longer reflect the constant
and ever changing impressions which the youthful sense organs convey
to a youthful and active brain. That the young should ape the old,
should seek to acquire the gravity of demeanor, to restrain the quick
impulse, is not of advantage. Loss of weight of the body as a whole is
not so apparent, there being a tendency to fat formation owing to the
non-use of fat or fat-forming material which is taken into the body.
One of the most evident alterations is a general diminution in the
fluid of the tissues, to which is chiefly due the lack of plumpness,
the wrinkles of age. The facial appearance of age is given to an
infant when, in consequence of a long-continued diarrhoea, the tissues
become drained of fluid. Every market-man knows that an old animal is
not so available for food, the tissues are tougher, more fibrous, not
so easily disintegrated by chewing. This is due to a relative increase
in the connective tissue which binds all parts together and is
represented in the white fibres of meat.

Senile atrophy is complex in its causes and modes of production. The
atrophy affects different organs in different degree and shows great
variation in situation, in degree and in progress. Atrophic changes of
the blood vessels are of great importance, for this affects the
circulation on which the nutrition of all tissues depends. While there
is undoubted progressive wear of all tissues, this becomes most
evident in the case of the blood vessels of the body. It is rare that
arteries which can be regarded as in all respects normal are found in
individuals over forty, and these changes progress rapidly with
advancing age. So striking and constant are these vascular changes
that they seem almost in themselves sufficient to explain the senile
changes, and this has been frequently expressed in the remark that age
is determined not by years, but by the condition of the arteries.
Comparative studies show the falsity of this view, for animals which
are but little or not at all subject to arterial disease show senile
changes of much the same character as those found in man.

There is another condition which must be considered in a study of
causes of age. In the ordinary course of life slight injuries are
constantly being received and more or less perfectly repaired. An
infection which may but slightly affect the ordinary well-being of the
individual may produce a considerable damage. Excess or deficiency or
improper food, occasional or continued use of alcohol and other
poisons may lead to very definite lesions. Repair after injury is
rarely perfect, the repaired tissue is more susceptible to injury, and
with advancing age there is constant diminution in the ease and
perfection of repair. The effect of the sum of all these changes
becomes operative: a vicious circle is established in which injury
becomes progressively easier to acquire and repair constantly less
perfect. There is some adjustment, however, in that the range of
activities is diminished, the environment becomes narrower and the
organism adapts its life to that environment which makes the least
demands upon it.

Whether there is, entirely apart from all conditions affecting
nutrition and the effect of injuries which disturb the usual cell
activities, an actual senescence of the cells of the body is
uncertain. In the presence of the many factors which influence the
obvious diminution of cell activity in the old, it is impossible to
say whether the loss of cell activity is intrinsic or extrinsic. The
life of the plant cell seems to be immortal; it does not grow old.
Trees die owing to accidents or because the tree acquires in the
course of its growth a mass of tissue in which there is little or no
life, and which becomes the prey of parasites. The growing tissue of a
tree is comprised in a thin layer below the bark, and the life of this
may seemingly be indefinitely prolonged by placing it in a situation
in which it escapes the action of accidental injuries and decay, as by
grafting on young trees. Where the nature of the dead wood is such
that it is immune from parasites and decay, as in the case of the
Sequoias, life seems to be indefinitely prolonged. The growing
branches of one of these trees, whose age has been estimated with
seeming accuracy at six thousand years, are just as fresh and the tree
produces its flowers and fruit in the same degree as a youthful
brother of one thousand years. Nor does old age supervene in the
unicellular organisms. An amoeba assimilates, grows and multiplies
just as long as the environment is favorable.

Old age in itself is seldom a cause of death. In rare cases in the
very old a condition is found in which no change is present to which
death can be attributed, all organs seem to share alike in the
senescence. Death is usually due to some of the accidents of life, a
slight infection to which the less resistant body succumbs, or to the
rupture of a weakened blood vessel in the brain, or to more advanced
decay in some organ whose function is indispensable. The causes and
conditions of age have been a fertile source for speculation. Many of
the hypotheses have been interesting, that of Metschnikoff, for
example, who finds as a dominating influence in causing senescence the
absorption of toxic substances formed in the large intestine by
certain bacteria. He further finds that the cells of the body which
have phagocytic powers turn their activity against cells and tissues
which have become weakened. There may be absorption of injurious
substances from the intestines which the body in a vigorous condition
is able to destroy or to counteract their influence, and these may be
more operative in the weaker condition of the body in the old.
Phagocytes will remove cells which are dead and often cells which are
superfluous in a part, but there is no evidence that this is ever
other than a conservative process. Since it is impossible to single
out any one condition to which old age is due, the hypothesis of
Metschnikoff should have no more regard given it than the many other
hypotheses which have been presented.

Death of the body as a whole takes place from the cessation of the
action of the central nervous system or of the respiratory system or
of the circulation. There are other organs of the body, such as the
intestine, kidney, liver, whose function is essential for life, but
death does not take place immediately on the cessation of their
function. The functions of the heart, the brain and the lungs are
intimately associated. Oxygen is indispensable for the life of the
tissues, and its supply is dependent upon the integrity of the three
organs mentioned, which have been called the tripos of life.
Respiration is brought about by the stimulation of certain nerve cells
in the brain, the most effective stimulus to these cells being a
diminution of oxygen in the blood supplying them. These cells send out
impulses to the muscles concerned in inspiration, the chest expands,
and air is taken into the lungs. Respiration is then a more
complicated process than is the action of the heart, for its
contraction, which causes the blood to circulate, is not immediately
dependent upon extrinsic influences. Death is usually more immediately
due to failure of respiration than to failure of circulation, for the
heart often continues beating for a time after respiration has ceased.
Thus, in cases of drowning and suffocation, by means of artificial
respiration in which air is passively taken into and expelled from the
lungs, giving oxygen to the blood, the heart may continue to beat and
the circulation continue for hours after all evident signs of life and
all sensation has ceased.

By this general death is meant the death of the organism as a whole,
but all parts of the body do not die at the same time. The muscles and
nerves may react, the heart may be kept beating, and organs of the
body when removed and supplied with blood will continue to function.
Certain tissues die early, and the first to succumb to the lack of
oxygenated blood are the nerve cells of the brain. If respiration and
circulation have ceased for as short a time as twelve minutes, life
ceases in certain of these cells and cannot be restored. This is again
an example of the greater vulnerability of the more highly
differentiated structure in which all other forms of cell activity are
subordinated to function. There are, however, pretty well
authenticated cases of resuscitation after immersion in water for a
longer period than twelve minutes, but these cases have not been
carefully timed, and time under such conditions may seem longer than
it actually is; and there is, moreover, the possibility of a slight
gaseous interchange between the blood and the water in the lungs, as
in the case of the fish which uses the water for an oxygen supply as
the mammal does the air. There are also examples of apparent death or
trances which have lasted longer, and the cases of fakirs who have
been buried for prolonged periods and again restored to life. In these
conditions, however, all the activities of the body are reduced to the
utmost, and respiration and circulation, so feeble as to be
imperceptible to ordinary observation, suffice to keep the cells

With the cessation of life the body is subject to the unmodified
action of its physical environment. There is no further production of
heat and the body takes the temperature of the surroundings. The only
exceptions are rare cases in which such active chemical changes take
place in the dead body that heat is generated by chemical action. At a
varying interval after death, usually within twelve hours, there is a
general contraction and hardening of the muscles due to chemical
changes, probably of the nature of coagulation, in them. This begins
in the muscles of the head, extends to the extremities, and usually
disappears in twenty-four hours. It is always most intense and most
rapid in its onset when death is preceded by active muscular exertion.
There have been cases of instantaneous death in battle where the body
has remained in the position it held at the moment of death, this
being due to the instantaneous onset of muscular rigidity. The blood
remains fluid for a time after death and settles in the more dependent
parts of the body, producing bluish red mottled discolorations. Later
the blood coagulates in the vessels. The body loses moisture by
evaporation. Drying of the surface takes place where the epidermis is
thin, as over the transparent part of the eye and over areas deprived
of epidermis. Decomposition and putrefaction of the body due to
bacterial action takes place. The bacteria ever present in the
alimentary canal make their way from this into the dead tissue.
Certain of these bacteria produce gas which accumulates in the tissues
and the body often swells enormously. A greenish discoloration
appears, which is due to the union of the products of decomposition
with the iron in the blood; this is more prominent over the abdomen
and appears in lines along the course of the veins. The rapidity with
which decomposition takes place varies, and is dependent upon many
factors, such as the surrounding temperature, the nutrition of the
body at the time of death, the cause of death. It is usually not
difficult to recognize that a body is dead. In certain cases, however,
the heart's action may be so feeble that no pulse is felt at the
wrist, and the current of the expired air may not move a feather held
to the nostril or cloud the surface of a mirror by the precipitation
of moisture upon it. This condition, combined with unconsciousness and
paralysis of all the voluntary muscles, may very closely simulate
death. The only absolute evidence of death is given by such changes as
loss of body heat, rigor mortis or stiffening of the muscles,
coagulation of the blood and decomposition.



The power of growth is possessed by every living thing, but growth is
not limited to the living. Crystals also will grow, and the rapidity
and character of growth and the maximum size of the crystal depends
upon the character of the substance which forms the crystal. From the
single cell or ovum formed by the union of the male and female sexual
cells, growth is continuous until a size corresponding to the type of
the species is attained. From this time onward growth is limited to
the degree necessary to supply the constant loss of material which the
body undergoes. The rapidity of the growth of the body and of its
component parts differs at different ages, and becomes progressively
less active from its beginning in the ovum until the adult type of the
species is attained. As determined by the volume, the embryo increases
more than ten thousand times in size during the first month of
intra-uterine life. At birth the average weight is six and a half
pounds; at the end of the first year eighteen and a half pounds, a
gain of twelve pounds; at the end of the second year twenty-three
pounds, a gain of four and a half pounds. The growth is coördinated,
the size of the single organs bearing a definite ratio, which varies
within slight limits, to the size of the body, a large individual
having organs of corresponding size. Knowing that the capacity of
growth is one of the inherent properties of living matter, it is much
easier to understand the continuance of growth than its cessation. It
is impossible to avoid the conclusion that there is some internal
mechanism of the body which controls and regulates growth. In the
first chapter reference was made to organs producing substances which
pass directly into the circulation; these substances act by control of
the activities of other parts, stimulating or depressing or altering
their function. Two of these glands, the thymus, lying in front, where
the neck joins the body and which attains its greatest size at
puberty, and the pituitary body, placed beneath the brain but forming
no part of it, have been shown by recent investigations to have a very
definite relation to growth, especially the growth of the skeleton.
The growth energy chiefly resides in the skeleton, and if the growing
animal has a diet sufficient only to maintain the body weight, the
skeleton will continue to grow at the expense of the other tissues,
literally living upon the rest of the body. Disease of the glands
mentioned leading to an increase or diminution or alteration of their
function may not only inhibit or unduly increase the growth of the
skeleton, but may also interfere with the sexual development which
accompanies the skeleton growth.

The difficulties which arise in an endeavor to comprehend normal
growth are greater when the growth of tumors is considered. A tumor is
a mass of newly formed tissue which in structure, in growth, and the
relations which it forms with adjoining tissues departs to a greater
or less degree from the type of the tissue to which it is related in
structure or from which it originates. It is an independent structure
which, like a parasite, grows at the expense of the body, contributing
nothing to it, and its capacity for growth is unlimited. A tumor
cannot be considered as an organ, its activities not being coordinated
with those of the body. A part of the body it certainly is, but in the
household economy it is to be considered as a wild and lawless guest,
not influenced by or conforming with the regulations of the household.
The rapidity of growth varies; certain tumors for years increase but
little in size, while others may be seen to increase from day to day.
The growth is often intermittent, periods of great activity of growth
alternating with periods of quiescence. The nutrition and growth of a
tumor is only slightly influenced by the condition of nutrition of the
bearer. Its cells have a greater avidity for food than have those of
the body, and, like the growing bones of an insufficiently fed animal,
growth in some cases seems to take place at the expense of the body,
the normal cells not obtaining sufficient nutriment to repair their

A tumor may be of any size: so small as to be invisible to the naked
eye, or its weight may exceed that of the individual who bears it. The
limitations to its growth are extrinsic and not intrinsic. There is no
distinct color. Certain tumors have color which depends upon the
presence of a dark brown or black pigment within the cells.
Hæmorrhages within them are not infrequent, and they may be colored by
the blood or by pigments formed from it. Usually they have a gray
color modified by their varying vascularity, or the cut surface may be
mottled due to areas of cell degeneration. The consistency varies;
some tumors are so soft that they can be pressed through a sieve,
others are of stony hardness. There is no distinct shape, this being
influenced by the nature of the tumor, the manner of growth and
situation. When the tumor grows on or near a surface, it may project
from this and be attached by a narrow band only; in the interior of
the body it may be irregular in outline, round or lobular, the shape
being influenced by many factors. Tumors like the tissues of the
normal body are nourished by the blood and contain blood vessels often
in great numbers.

A tumor arises by the cells of a part of the body beginning to grow
and taking on the characteristics of a tumor. Its growth is
independent, the cells of the adjoining tissue taking no part in it.
The tissue in the vicinity of the tumor is partly pushed aside by the
mass, or the tumor grows into it and the tissue disappears as the
tumor advances. The destruction of the surrounding tissue is brought
about partly by the pressure which the tumor exerts, partly by the
compression of the blood vessels or the blood supply of the organs is
diverted to the tumor.

The characteristics of a tumor are due to the cells which it contains
(Fig 14). These often become separated from the main mass and are
carried by the blood into other parts of the body, where they grow and
form tumors similar in character to the parent tumor. In the
extraordinary capacity for growth possessed by tumor cells, they
resemble vegetable rather than animal cells. There is no limit to the
growth of a tumor save by the death of the individual who bears it,
thus cutting off the supply of nutrition. The cells of tumors peculiar
to man show a narrow range of adaptation. They will grow only in the
body of the individual to whom the tumor belongs, and die when grafted
on another individual. In the case of tumors which arise in animals,
pieces of the tumor when grafted on another animal of the same species
will grow, and in this way the growth capacity of the tumor cells has
been estimated. Thus, by transplanting a small section of a mouse
tumor into other mice, the small transplanted fragments will in two
weeks grow to the size of filberts, and each of these will furnish
material to engraft upon ten mice. These new tumors are similar in
character to the original tumor, and really represent parts of it in
the same way that all the Baldwin apples in the world are parts of the
original tree which was found in Baldwinville many years ago, and as
all the Concord grape vines are really parts of the original vine. It
has been estimated that if all the growth capacity of this mouse tumor
were availed of by the successive inoculation of other mice, a mass of
tumor several times the diameter of the sun would grow in two years.
The condition of the individual seems to exert no influence upon the
growth of the tumor. Growth may be as rapid when the bearer is in a
condition of extreme emaciation as it is when the bearer is well
nourished and robust.

CANCER OF THE UTERUS. A large mass of cells is extending into the
tissue of the uterus which is shown as the fibrous structure. Such a
cell mass penetrating into the tissue represents the real cancer, the
tissue about the cell masses bear the blood vessels which nourish the
tumor cells.]

Those tumors which grow rapidly and invade and destroy the surrounding
tissue are called malignant tumors or cancers, but in a strict sense
no tumor can be regarded as benign, for none can serve a useful
purpose. A tumor after a period of slow growth can begin to grow
rapidly. Tumors may arise in any part of the body, but there are
certain places of preference particularly for the more malignant
tumors. These are places where the cells naturally have a marked power
of growth, and especially where growth is intermittent as in the
uterus and mammary gland.

Little is known in regard to the influence of inheritance on the
formation of tumors. Study of the tumors of mice show a slightly
greater susceptibility to tumor formation in the progeny of mice who
have developed tumors. Studies of human families seem to show that
heredity has a slight influence, but in the frequency of tumors such
statistical evidence is of little value. The question of inheritance
has much bearing on the origin of tumors. If the tumor is accidental
and due entirely to extraneous causes, inheritance is not probable;
but if there is some predisposition to tumor formation in certain
individuals due to some peculiarity, then inheritance may exert an

The question as to whether tumors are an increasing cause of disease
is equally difficult of solution. The mortality statistics, if taken
at their face value, show an enormous increase in frequency; but there
are many factors which must be considered and which render the
decision difficult and doubtful. Tumors are largely a prerogative of
age, and the increased duration of life which preventive medicine has
brought about brings more people into the age when tumors are more
common. Owing to the greater skill in the diagnosis of tumors,
especially those of the internal organs, they are now recognized more
frequently and more deaths are correctly ascribed to them. Deaths from
tumors were formerly often purposely concealed and attributed to some
other cause.

No age is immune to tumors. They may be present at birth or develop
shortly afterwards. The age from five to twenty years is the most free
from them, that from forty-five to sixty-five the most susceptible,
particularly to the more malignant forms.

A tumor is a local disease. The growing tissue of the tumor is the
disease, and it is evident that if the entire tumor were removed the
disease would be cured. This is the end sought by surgical
interference, but notwithstanding seemingly thorough removal, the
tumor often reappears after an interval of months or years. There are
many conditions which may render the complete removal of a tumor
difficult or impossible. It is often impossible to ascertain just how
far the tumor cells have invaded the neighboring structures; the
situation of the tumor may be such that an extended removal would
injure organs which are essential for life, or at the time of removal
the tumor cells may have been conveyed elsewhere by the blood or
lymphatic vessels.

Successful removal depends mainly upon the length of time the tumor
has been growing. At an early stage even the most malignant tumor may
be successfully removed. It is evident from this how disastrous may be
the neglect of proper surgical treatment of a tumor. The time may be
very short between the first evidence of the presence of a tumor and
the development of a condition which would render complete removal

The effect of a tumor upon its bearer depends upon its character and
situation. Pain is very commonly present, and is due to the pressure
which the growing tumor exerts upon the sensory nerves. Pain may,
however, not be present or appear only at the last. A condition of
malnutrition and emaciation often results due to the passage into the
blood of injurious substances formed in the tumor, or to the
destruction of important organs by the growing tumor. The growth of a
tumor in the intestine may obstruct or close the canal and thus
interfere with nutrition.

The cause or causes of tumors are unknown. We know that the tumor
represents essentially an abnormal growth, and that this growth is due
to new formation of cells. In certain cases the tumor repeats the
structure of the organ or tissue in which it originates, in others it
departs widely from this; always, however, its structure resembles
structures found in the body at some period of life. The tumor cells,
like all other cells of the body, grow by means of the nutriment which
the body supplies; they have no intrinsic sources of energy. The great
problem is what starts the cells to grow and why the growth differs
from that of normal tissue, why it is not regulated and coördinated as
are other forms of growth. When a small piece of the skin, for
instance, is cut out growth as rapid as that in tumors takes place in
the adjoining cells, _but it ceases when the loss is restored_.
The same is true when a piece of the liver is removed.

Various hypotheses have been formed to explain the tumor, all of them
of interest, and they have had great importance in that the attempt to
prove or disprove the hypothesis by continued observation and
experiment along definite lines has produced new knowledge. The
various theories as to cause may be divided into three heads.

The parasitic theory. This supposes that a living parasite invades the
body, and by its presence excites the cells of certain tissues to grow
in tumor form. It is known that active growth of the cells of the body
can be excited in a number of ways, by chemical substances such as
certain of the coal tar products, and that it often takes place under
the influence of bacteria. It is further known that parasites can
produce tumor-like growths in plants. The large, rough excrescences on
the oaks are produced by a fly which lays its eggs in or beneath the
bark, and the larva which develops from the egg secretes a substance
which causes the cells about it to multiply, and a huge mass is formed
which serves the developing insect for both food and protection. Large
tumor-like masses are formed on the roots and stalk of cabbages as the
result of the invasion of the cells by a minute organism: the tumors
of olive trees are due to a bacterium; the peculiar growths on cedar
trees, the so-called "witches' brooms," are produced by a fungus, and
there are many other such examples. These have many analogies with
tumors in animals. Under the stimulus of the parasite the cells seem
to have unlimited growth capacity and a greater nutritive avidity than
have the normal plant cells; the character of the mass produced
differs as does the tumor, to a greater or less extent, from the
normal growth; on the cedar, for instance, the "witches' broom"
consists of a thick mass of foliage with small stems less green than
the usual foliage, the leaves wider and not so closely applied to the
stems. The entire plant suffers in its nutrition and a condition
resembling tumor cachexia[1] is produced, and there are no fundamental
differences between the plant and animal tumors. Support has also been
given to the parasitic theory by the discovery within tumor cells of
bodies which were supposed to be a peculiar sort of parasite. If the
truth of the parasitic theory could be proved, there would be
justifiable expectation that the tumor disease might be controlled as
are many of the parasitic diseases, but the hypothesis awaits the
demonstration of its correctness. Despite the study of tumors which is
being actively pursued in many places and by the most skilled
investigators, no parasites have been found in animal tumors; the
objects previously described as parasites have been found not to be
such. It is difficult to bring in accord with the parasitic theory the
great variation in tumor structure, the relation of certain tumors, as
the malignant tumors of the breast and uterus, with the age of the
bearer, the congenital tumors which develop in intra-uterine life, and
there are many other conditions which oppose the theory.

The traumatic[2] theory. There is much in favor of this. In a certain
number of cases tumors do develop at the site of injuries. The
coincidence of injury and tumor is apt to be overestimated because of
the strong tendency to connect succeeding events. Tumors are not most
common on those parts of the body which are most exposed to injury.
They are rare, for instance, on the hands and feet, and very rarely do
they appear at the site of wounds caused by surgical operations. For
those tumors which develop in intra-uterine life it is difficult to
assign injury as a cause. There does, however, seem to be a relation
between tumors and injuries of a certain character. The natives of
Cashmere use in winter for purposes of heat a small charcoal stove
which they bind on the front of the body; burns often result and
tumors not infrequently develop at the site of such burns. Injuries of
tissue which are produced by the X-ray not infrequently result in
tumor formation and years may elapse between the receipt of the injury
and the development of the tumor. These X-ray injuries are of a
peculiar character, their nature but imperfectly understood, and the
injured tissues seem to have lost the capacity for perfect repair.

In regard to the possible action of both injuries and parasites in
causing tumors, the possibility that their effects on different
individuals may not be the same must be considered. In addition to the
trauma or the parasite which may be considered as extrinsic factors,
there may be conditions of the body, intrinsic factors, which favor
their action in tumor development. The peculiar tissue growth within
the uterus called decidua, which occurs normally in pregnancy and
serves to fasten the developing ovum to the inner lining of the
uterus, may be produced experimentally. This growth depends upon two
factors, an internal secretion derived from the ovary and the
introduction into the uterus of a foreign body of some sort; in the
case of pregnancy the developing embryo acts as the foreign body. It
is not impossible that some variation in the complex relations which
determine normal growth may be one factor, possibly the most
important, in tumor formation.

Another theory is that the tumor is the result of imperfect embryonic
development. The development of the child from the ovum is the result
of a continued formation and differentiation of cells. A cell mass is
first produced, and the cells in this differentiate into three layers
called ectoderm, entoderm and mesoderm, from which the external and
internal surfaces and the enclosed tissues respectively develop, and
the different organs are produced by growth of the cells of certain
areas of these layers. The embryonic theory assumes that in the course
of embryonic development not all the cell material destined for the
formation of individual organs is used up for this purpose, that
certain of the embryonic cells become enclosed in the developing
organs, they retain the embryonic capacity for growth and tumors arise
from them. There is no doubt that something like this does take place.
There is a relation between malformations due to imperfect development
of the embryo and tumors, the two conditions occurring together too
frequently to be regarded as mere coincidence. Also tumors may occur
in parts of the body in which there is no tissue capable of forming
structures which may be present in the tumors. The theory, however, is
not adequate, but it may be among the factors.

The problems concerned in the nature and cause of tumors are the most
important in medicine at the present time. No other form of disease
causes a similar amount of suffering and anxiety, which often extends
over years and makes a terrible drain on the sympathy and resources of
the family. The only efficient treatment for tumors at the present
time is removal by surgical operation, and the success of the
operation is in direct ratio to the age of the tumor, the time which
elapses from its beginning development. It is of the utmost importance
that this should be generally recognized, and the facts relating to
tumors become general knowledge. Tumors form one of the most common
causes of death (after the age of thirty-five one in every ten
individuals dies of tumor); medical and surgical resources are, in
many cases, powerless to afford relief and the tumor stands as a bar
to the attainment of the utopia represented by a happy and comfortable
old age, and a quiet passing. Every possible resource should be placed
at the disposal of the scientific investigation of the subject, for
with knowledge will come power to relieve.

[1] By cachexia is understood a condition of malnutrition and
emaciation which is usually accompanied by a pale sallow color of the

[2] By trauma is understood a wound or injury of any sort.



Injury and repair have already been briefly considered in their
relation to the normal body and to old age; there are, however,
certain phenomena included under the term inflammation which follow
the more extensive injuries and demand a closer consideration than was
given in Chapter II. These phenomena differ in degree and character;
they are affected by the nature of the injurious agent and the
intensity of its action, by the character of the tissue which is
affected and by variations in individual resistance to injury. A blow
which would have no effect upon the general surface of the body may
produce serious results if it fall upon the eye, and less serious
results for a robust than for a weak individual.

Most of the changes which take place after an injury and their
sequence can be followed under the microscope. If the thin membrane
between the toes of a living frog be placed under the microscope the
blood vessels and the circulating blood can be distinctly seen in the
thin tissue between the transparent surfaces. The arteries, the
capillaries and veins can be distinguished, the arteries by the
changing rapidity of the blood stream within them, there being a
quickening of the flow corresponding with each contraction of the
heart; the veins appear as large vessels in which the blood flows
regularly (Fig. 11). Between the veins and arteries is a large number
of capillaries with thin transparent walls and a diameter no greater
than that of the single blood corpuscles; they receive the blood from
the arteries and the flow in them is continuous. The white and red
blood corpuscles can be distinguished, the red appearing as oval discs
and the white as colorless spheres. In the arteries and veins the red
corpuscles remain in the centre of the vessels appearing as a rapidly
moving red core, and between this core and the wall of the vessels is
a layer of clear fluid in which the white corpuscles move more slowly,
often turning over and over as a ball rolls along the table.

If, now, the web be injured by pricking it or placing some irritating
substance upon it, a change takes place in the circulation. The
arteries and the veins become dilated and the flow of blood more
rapid, so rapid, indeed, that it is difficult to distinguish the
single corpuscles. In a short while the rapidity of flow in the
dilated vessels diminishes, becoming slower than the normal, and the
separation between the red and white corpuscles is not so evident. In
the slowly moving stream the white corpuscles move much more slowly
than do the red, and hence accumulate in the vessels lining the inner
surface and later become attached to this and cease to move forward.
The attached corpuscles then begin to move as does an amoeba, sending
out projections, some one of which penetrates the wall, and following
this the corpuscles creep through. Red corpuscles also pass out of the
vessels, this taking place in the capillaries; the white corpuscles,
on the other hand, pass through the small veins. Not only do the white
corpuscles pass through the vessels, but the blood fluid also passes
out. The corpuscles which have passed into the tissue around the
vessels are carried away by the outstreaming fluid, and the web
becomes swollen from the increased amount of fluid which it contains.
The injured area of the web is more sensitive than a corresponding
uninjured area and the foot is more quickly moved if it be touched. If
the injury has been very slight, observation of the area on the
following day will show no change beyond a slight dilatation of the
vessels and a great accumulation of cells in the tissue.

Everyone has experienced the effect of such changes as have been
described in this simple experiment. An inflamed part on the surface
of the body is redder than the normal, swollen, hot and painful. The
usual red tinge of the skin is due to the red blood contained in the
vessels, and the color is intensified when, owing to the dilatation,
the vessels contain more blood. The inflamed area feels hot, and if
the temperature be taken it may be two or three degrees warmer than a
corresponding area. The increased heat is due to the richer
circulation. Heat is produced in the interior of the body chiefly in
the muscles and great glands, and the increased afflux of blood brings
more heat to the surface. A certain degree of swelling of the tissue
is due to the dilatation of the vessels; but this is a negligible
factor as compared with the effect of the presence of the fluid and
cells of the exudate.[1] The fluid distends the tissue spaces, and it
may pass from the tissue and accumulate on surfaces or in the large
cavities within the body. The greatly increased discharge from the
nose in a "cold in the head" is due to the exudation formed in the
acutely inflamed tissue, and which readily passes through the thin
epithelial covering. Various degrees of inflammation of the skin may
be produced by the action of the sun, the injury being due not to the
heat but to the actinic rays. In a mild degree of exposure only
redness and a strong sense of heat are produced, but in prolonged
exposure an exudate is formed which causes the skin to swell and
blisters to form, these being due to the exudate which passes through
the lower layers of the cells of the epidermis and collects beneath
the impervious upper layer, detaching this from its connections. If a
small wad of cotton, soaked in strong ammonia, be placed on the skin
and covered with a thimble and removed after two minutes, minute
blisters of exudate slowly form at the spot.

The pain in an inflamed part is due to a number of factors, but
chiefly to the increased pressure upon the sensory nerves caused by
the exudate. The pain varies so greatly in degree and character that
parts which ordinarily have little sensation may become exquisitely
painful when inflamed. The pain is usually greater when the affected
part is dense and unyielding, as the membranes around bones and teeth.
The pain is often intermittent, there being acute paroxysms
synchronous with the pulse, this being due to momentary increase of
pressure when more blood is forced into the part at each contraction
of the heart. The pain may also be due to the direct action of an
injurious substance upon the sensory nerves, as in the case of the
sting of an insect where the pain is immediate and most intense before
the exudate has begun to appear.

When an inflamed area is examined, after twenty-four hours, by
hardening the tissue in some of the fluids used for this purpose and
cutting it into very thin slices by means of an instrument called a
microtome, the microscope shows a series of changes which were not
apparent on naked eye examination. The texture is looser, due to the
exudate which has dilated all the spaces in the tissue. Red and white
corpuscles in varying numbers and proportions infiltrate the tissue;
all the cells which belong to the part, even those forming the walls
of the vessels, are swollen, the nuclei contain more chromatin, and
the changes in the nuclei which indicate that the cells are
multiplying appear. The blood vessels are dilated, and the part in
every way gives the indication of a more active life within it. There
are also evidences of the tissue injury which has called forth all the
changes which we have considered. (Fig. 15.)

EXUDATE WITHIN THE AIR SPACES. Compare this with Fig 6. Fig 15 is from
the human lung, in which the air spaces are much larger than in the

The microscopic examination of any normal tissue of the body shows
within it a variable number of cells which have no intimate
association with the structure of the part and do not seem to
participate in its function. They are found in situations which
indicate that these cells have power of active independent motion. In
the inflamed tissue a greatly increased number of these cells is
found, but they do not appear until the height of the process has
passed, usually not before thirty-six or forty-eight hours after the
injury has been received. The numbers present depend much upon the
character of the agent which has produced the injury, and they may be
more numerous than the ordinary leucocytes which migrate from the
blood vessels.

All these changes which an injured part undergoes are found when
closely analyzed to be purposeful; that is, they are in accord with
the conditions under which the living matter acts, and they seem to
facilitate the operation of these conditions. It has been said that
the life of the organism depends upon the coördinated activity of the
living units or cells of which it is composed. The cells receive from
the blood material for the purpose of function, for cell repair and
renewal, and the products of waste must be removed. In the injury
which has been produced in the tissue all the cells have suffered,
some possibly displaced from their connections, others may have been
completely destroyed, others have sustained varying degrees of injury.
If the injury be of an infectious character, that is, produced by
bacteria, these may be present in the part and continue to exert
injury by the poisonous substances which they produce, or if the
injury has been produced by the action of some other sort of poison,
this may be present in concentrated form, or the injury may have been
the result of the presence of a foreign body in the part. Under these
conditions, since the usual activities of the cells in the injured
part will not suffice to restore the integrity of the tissue, repair
and cell formation must be more active than usual, any injurious
substances must be removed or such changes must take place in the
tissue that the cell life adapts itself to new conditions.

[Illustration: FIG. 16.--PHAGOCYTOSIS. _a_, _b_, _c_ are the
microphages or the bacterial phagocytes. (_a_) Contains a number of
round bacteria, and (_b_) similar bacteria arranged in chains, and
(_c_) a number of rod-shaped bacteria (_d_) Is a cell phagocyte or
macrophage which contains five red blood corpuscles.]

All life in the tissues depends upon the circulation of the blood.
There is definite relation between the activity of cells and the blood
supply; a part, for instance, which is in active function receives a
greater supply of blood by means of dilatation of the arteries which
supply it. If the body be exactly balanced longitudinally on a
platform, reading or any exercise of the brain causes the head end to
sink owing to the relatively greater amount of blood which the brain
receives when in active function. The regulation of the blood supply
is effected by means of nerves which act upon the muscular walls of
the arteries causing, by the contraction or the relaxation of the
muscle, diminution or dilatation of the calibre of the vessel. After
injury the dilatation of the vessels with the greater afflux of blood
to the part is the effect of the greatly increased cell activity, and
is a necessity for this. In many forms of disease it has been found
that by increasing the blood flow to a part and producing an active
circulation in it, that recovery more readily takes place and many of
the procedures which have been found useful in inflammation, such as
hot applications, act by increasing the blood flow. So intimate is the
association between cell activity, as shown in repair and new
formation of cells, and the blood flow, that new blood vessels
frequently develop by means of which the capacity for nutrition is
still more increased. The cornea or transparent part of the eye
contains no blood vessels, the cells which it contains being nourished
by the tissue fluid which comes from the outside and circulates in
small communicating spaces. If the centre of the cornea be injured,
the cells of the blood vessels in the tissue around the cornea
multiply and form new vessels which grow into the cornea and appear as
a pink fringe around the periphery; when repair has taken place the
newly formed vessels disappear.

The exudate from the blood vessels in various ways assists in repair.
An injurious substance in the tissue may be so diluted by the fluid
that its action is minimized. A small crystal of salt is irritating to
the eye, but a much greater amount of the same substance in dilute
solution causes no irritation. The poisonous substances produced by
bacteria are diluted and washed away from the part by the exudate. Not
only is there a greater amount of tissue fluid in the inflamed part,
but the circulation of this is also increased, as is shown by
comparing the outflow in the lymphatic vessels with the normal. The
fluid exudate which has come from the blood and differs but slightly
from the blood fluid exerts not only the purely physical action of
removing and diluting injurious substances, but in many cases has a
remarkable power, exercised particularly on bacterial poisons, of
neutralizing poisons or so changing their character that they cease to
be injurious.

We have learned, chiefly from the work of Metschnikoff, that those
white corpuscles or leucocytes which migrate from the vessels in the
greatest numbers have marked phagocytic properties, that is, they can
devour other living things and thus destroy them just as do the
amoebæ. In inflammations produced by bacteria there is a very active
migration of these cells from the vessels; they accumulate in the
tissue and devour the bacteria. They may be present in such masses as
to form a dense wall around the bacteria, thus acting as a physical
bar to their further extension. The other form of amoeboid cell, which
Metschnikoff calls the macrophage, has more feeble phagocytic action
towards bacteria, and these are rarely found enclosed within them. It
is chiefly by means of their activity that other sorts of substances
are removed. They often contain dead cells or cell fragments, and when
hæmorrhage takes place in a tissue they enclose and remove the
granules of blood pigment which result. They often join together,
forming connected masses, and surround such a foreign body as a hair,
or a thread which the surgeon places in a wound to close it. They may
destroy living cells, and do this seemingly when certain cells are in
too great numbers and superfluous in a part, their action tending to
restore the cell equilibrium. The foreign cells do even more than
this: they themselves may be devoured by the growing cells of the
tissue, seemingly being actuated by the same supreme idea of sacrifice
which led Buddha to give himself to the tigress.

The explanation of most of the changes which take place in
inflammation is obvious. It is a definite property of all living
things that repair takes place after injury, and certain of the
changes are only an accentuation of those which take place in the
usual life; but others, such as the formation of the exudate, are
unusual; not only is the outpouring of fluid greatly increased, but
its character is changed. In the normal transudation[2] the substances
on which the coagulation of the blood depends pass through the vessel
wall to a very slight extent, but the exudate may contain the
coagulable material in such amounts that it easily clots. The
interchange between the fluid outside the vessels and the blood fluid
takes place by means of filtration and osmosis. There is a greater
pressure in the vessels than in the fluid outside of them, and the
fluid filters through the wall as fluid filters through a thin
membrane outside of the body. Osmosis takes place when two fluids of
different osmotic pressure are separated by animal membrane.
Difference in osmotic pressure is due to differences in molecular
concentration, the greater the number of molecules the greater is the
pressure, and the greater rapidity of flow is from the fluid of less
pressure to the fluid of greater pressure. The molecular concentration
of tissue and blood fluid is constantly being equalized by the process
of osmosis. In the injured tissue the conditions are more favorable
for the fluid of the blood to pass from the vessels: by filtration,
because owing to the dilatation of the arteries there is increased
amount of blood and greater pressure within the vessels, and the
filtering membrane is also thinner because the same amount of membrane
(here the wall of the vessel) must cover the larger surface produced
by the dilatation. It is, moreover, very generally believed that there
are minute openings in the walls of the capillaries, and these would
become larger in the dilated vessel just as openings in a sheet of
rubber become larger when this is stretched. Osmosis towards the
tissue is favored because, owing to destructive processes the
molecular pressure in the injured area is increased; an injured tissue
has been shown to take up fluid more readily outside of the body than
a corresponding uninjured tissue. The slowing of the blood stream, in
spite of the dilatation of the vessels, is due to the greater friction
of the suspended corpuscles on the walls of the vessels. This is due
to the loss from the blood of the outstreaming fluid and the relative
increase in the number of corpuscles, added to by the unevenness of
surface which the attached corpuscles produce.

The wonderful migration of the leucocytes, which seems to show a
conscious protective action on their part, takes place under the
action of conditions which influence the movement of cells. When an
actively moving amoeba is observed it is seen that the motion is not
the result of chance, for it is influenced by conditions external to
the organism; certain substances are found to attract the amoebae
towards them and other substances to repel them. These influences or
forces affecting the movements of organisms are known as
_tropisms_, and play a large part in nature; the attraction of
various organisms towards a source of light is known as
_heliotropism_, and there are many other instances of such
attraction. The leucocytes as free moving cells also come under the
influence of such tropisms. When a small capillary tube having one end
sealed is partially filled with the bacteria which produce abscess and
placed beneath the skin it quickly becomes filled with leucocytes,
these being attracted by the bacteria it contains. Dead cells exert a
similar attraction for the large phagocytes. Such attraction is called
_chemotropism_ and is supposed to be due in the cases mentioned,
to the action of chemical substances such as are given off by the
bacteria or the dead cells. The direction of motion is due to
stimulation of that part of the body of the leucocyte which is towards
the source of the stimulus. The presence in the injured part of
bacteria or of injured and dead cells exerts an attraction for the
leucocytes within the vessels causing their migration. When the centre
of the cornea is injured, this tissue having no vessels, all the
vascular phenomena take place in the white part of the eye immediately
around the cornea, this becoming red and congested. The migration of
leucocytes from the vessels takes place chiefly on the side towards
the cornea, and the migrated cells make their way along the devious
tracts of the communicating lymph spaces to the area of injury. The
objection may be raised that it is difficult to think of a chemical
substance produced in an injured area no larger than a millimeter,
diffusing through the cornea and reaching the vessels outside this in
such quantity and concentration as to affect their contents, nor has
there been any evidence presented that definite chemical substances
are produced in injured tissues; but there is no difficulty in view of
the possibilities. It is not necessary to assume that an actual
substance so diffuses itself, but the influence exerted may be thought
of as a force, possibly some form of molecular motion, which is set in
action at the area of injury and extends from this. No actual
substance passes along a nerve when it conveys an impulse.

We have left the injured area with an increased amount of fluid and
cells within it, with the blood vessels dilated and with both cells
and fluid streaming through their walls, and the cells belonging to
the area actively repairing damages and multiplying. The process will
continue as long as the cause which produces the injury continues to
act, and will gradually cease with the discontinuance of this action,
and this may be brought about in various ways. A foreign body may be
mechanically removed, as when a thorn is plucked out; or bacteria may
be destroyed by the leucocytes; or a poison, such as the sting of an
insect, may be diluted by the exudate until it be no longer injurious,
or it may be neutralized. Even without the removal of the cause the
power of adaptation will enable the life of the affected part to go
on, less perfectly perhaps, in the new environment. The excess of
fluid is removed by the outflow exceeding the inflow, or it may pass
to some one of the surfaces of the body, or in other cases an incision
favors its escape. The excess of cells is in part removed with the
fluid, in part they disappear by undergoing solution and in part they
are devoured by other cells. With the diminishing cell activity the
blood vessels resume their usual calibre, and when the newly formed
vessels become redundant they disappear by undergoing atrophy in the
same way as other tissues which have become useless.

When these changes take place rapidly the inflammation is said to be
acute, and chronic when they take place slowly. Chronic inflammation
is more complex than is the acute, and there is more variation in the
single conditions. The chronicity may be due to a number of
conditions, as the persistence of a cause, or to incompleteness of
repair which renders the part once affected more vulnerable, to such a
degree even that the ordinary conditions to which it is subjected
become injurious. A chronic inflammation may be little more than an
almost continuous series of acute inflammations, with repair
continuously less perfect. Chronic imflammations are a prerogative of
the old as compared with the young, of the weak rather than the

[1] The term exudation is used to designate the
passing of cells and fluid from the vessels in inflammation; the
material is the exudate.

[2] By transudation is meant the constant interchange between
the blood and the tissue fluid.



These are diseases which are caused by living things which enter the
tissues of the body and, living at the expense of the body, produce
injury. Such diseases play an important part in the life of man; the
majority of deaths are caused directly or indirectly by infection. No
other diseases have been so much studied, and in no other department
of science has knowledge been capable of such direct application in
promoting the health, the efficiency and the happiness of man. This
knowledge has added years to the average length of life, it has
rendered possible such great engineering works as the Panama Canal,
and has contributed to the food supply by making habitation possible
over large and productive regions of the earth, formerly uninhabitable
owing to the prevalence of disease. It is not too much to say that our
modern civilization is dependent upon this knowledge. The massing of
the people in large cities, the factory life, the much greater social
life, which are all prominent features of modern civilization, would
be difficult or impossible without control of the infectious diseases.
The rapidity of communication and the increased general movement of
people, which have developed in equal ratio with the massing, would
serve to extend widely every local outbreak of infection. The
principles underlying fermentation and putrefaction which have been
applied with great economic advantage to the preservation of food were
many of them developed in the course of the study of the infectious
diseases. Whether the development of the present civilization is for
the ultimate advantage of man may perhaps be disputed, but medicine
has made it possible.

The infectious diseases appearing in the form of great epidemics have
been important factors in determining historical events, for they have
led to the defeat of armies, the fall of cities and of nations. War is
properly regarded as one of the greatest evils that can afflict a
nation, since it destroys men in the bloom of youth, at the age of
greatest service, and brings sorrow and care and poverty to many. But
the most potent factor in the losses of war is not the deaths in
battle but the deaths from disease. If we designate the lives lost in
battle, the killed and the wounded who die, as 1, the loss of the
German army from disease in 1870-71 was 1.5, that of the Russians in
1877-78 was 2.7, that of the French in Mexico was 2.8, that of the
French in the Crimea 3.7, that of the English in Egypt 4.2. The total
loss of the German army in 1870-71 from wounds and disease was 43,182
officers and men, and this seems a small number compared with the
129,128 deaths from smallpox in the same period in Prussia alone. In
the Spanish American war there were 20,178 cases of typhoid fever with
1,580 deaths. In the South African war there were in the British
troops 31,118 cases of typhoid with 5,877 deaths, and 5,149 deaths
from other diseases while the loss in battle was 7,582. The Athenian
plague which prevailed during the Peloponnesian war, 431-405 B.C., not
only caused the death of Pericles, but according to Thucydides a loss
of 4,800 Athenian soldiers, and brought about the downfall of the
Athenian hegemony in Greece. In the Crimean war between 1853-56,
16,000 English, 80,000 French and 800,000 Russians died of typhus
fever. The plague contributed as much as did the arms of the Turks to
the downfall of Constantinople and the Eastern Empire in 1453. It was
the plague which in 1348 overthrew Siena from her proud position as
one of the first of the Italian cities and the rival of Florence, and
broke the city forever, leaving it as a phantom of its former glory
and prosperity. The work on the great cathedral which had progressed
for ten years was suspended, and when it was resumed it was upon a
scale adjusted to the diminished wealth of the city, and the plan
restricted to the present dimensions. As a little relief to the
darkness the same plague saw the birth of the novel in the tales of
Boccaccio, which were related to a delighted audience of the women who
had fled from the plague in Florence to a rural retreat.

The knowledge which has come from the study of infectious disease has
served also to broaden our conception of disease and has created
preventive medicine; it has linked more closely to medicine such
sciences as zoölogy and botany; it has given birth to the sciences of
bacteriology and protozoölogy and in a way has brought all sciences
more closely together. Above all it has made medicine scientific, and
never has knowledge obtained been more quickening and stimulating to
its pursuit.

Although the dimensions of this book forbid much reference to the
historical development of a subject, some mention must still be made
of the development of knowledge of the infectious diseases. It was
early recognized that there were diseases which differed in character
from those generally prevalent; large numbers of people were affected
in the same way; the disease beginning with a few cases gradually
increased in intensity until an acme was reached which prevailed for a
time and the disease gradually disappeared. Such diseases were
attributed to changes in the air, to the influence of planets or to
the action of offended gods. The priests and charlatans who sought to
excuse their inability to treat epidemics successfully were quick to
affirm supernatural causes. Hippocrates (400 B.C.), with whom medicine
may be said to begin, thought such diseases, even then called
epidemics, were caused by the air; he says, "When many individuals are
attacked by a disease at the same time, the cause must be sought in
some agent which is common to all, something which everyone uses, and
that is the air which must contain at this time something injurious."
Aristotle recognized that disease was often conveyed by contact, and
Varro (116-27 B.C.) advanced the idea that disease might be caused by
minute organisms. He says, "Certain minute organisms develop which the
eye cannot see, and which being disseminated in the air enter into the
body by means of the mouth and nostrils and give rise to serious
ailments." In spite of this hypothesis, which has proved to be
correct, the belief became general that epidemics were due to
putrefaction of the air brought about by decaying animal bodies, (this
explaining the frequent association of epidemics and wars,) by
emanations from swamps, by periods of unusual heat, etc.

With the continued study of epidemics the importance of contagion was
recognized; it was found that epidemics differed in character and in
the modes of extension. Some seemed to extend by contact with the
sick, and in others this seemed to play no part; it was further found
impossible in many cases to show evidence of air contamination, and
contamination of the air by putrefactive material did not always
produce disease. Most important was the recognition that single cases
of diseases which often occurred in epidemic form might be present and
no further extension follow; this led to the assumption in epidemics
of the existence of some condition in addition to the cause, and which
made the cause operative. In this way arose the theory of the epidemic
constitution, a supposed peculiar condition of the body due to changes
in the character of the air, or to the climate, or to changes in the
interior of the earth as shown by earthquakes, or to the movements of
planets; in consequence of this peculiar constitution there was a
greater susceptibility to disease, but the direct cause might arise in
the interior of the body or enter the body from without. The character
of the disease which appeared in epidemic form, the "Genius
epidemicus," was determined not by differences in the intrinsic cause,
but by the type of constitution which prevailed at that time. The
first epidemic of cholera which visited Europe in 1830-37 was for the
most part referred to the existence of a peculiar epidemic
constitution for which various causes were assigned. It was only when
the second epidemic of this disease appeared in 1840 that the
existence of some special virus or poison which entered the body was

Meanwhile, by the study of the material of disease knowledge was being
slowly acquired which had much bearing on the causes. The first
observations which tended to show that the causes were living were
made by a learned Jesuit, Athanasius, in 1659. He found in milk,
cheese, vinegar, decayed vegetables, and in the blood and secretions
of cases of plague bodies, which he described as tiny worms and which
he thought were due to putrefaction. He studied these objects with the
simple lenses in use at that time, and there is little doubt that he
did see certain of the larger organisms which are present in vinegar,
cheese and decaying vegetables, and it is not impossible that he may
have seen the animal and vegetable cells.

The first description of bacteria with illustrations showing their
forms was given by Loewenhoeck, a linen dealer in Amsterdam in 1675.
The fineness of the linen being determined by the number of threads in
a given area, it is necessary to examine it with a magnifying lens,
and he succeeded in perfecting a simple lens with which objects
smaller than had been seen up to that time became visible. It must be
added that he was probably endowed with very unusual acuteness of
vision. He found in a drop of water, in the fluid in the intestines of
frogs and birds, and in his evacuations, objects of great minuteness
which differed from each other in form and size and in the peculiar
motion which some of them possessed. In the year 1683 he presented to
the Royal Society of London a paper describing a certain minute
organism which he found in the tartar of his teeth. After these
observations of Loewenhoeck became known to the world they quickly
found application in disease, although the author had expressed
himself very cautiously in this regard. The strongest exponent of the
view of a living contagion was Plenciz, 1762, a physician of Vienna,
basing his belief not only on the demonstration of minute organisms by
Loewenhoeck which he was able to verify, but on certain shrewdly
conceived theoretical considerations. He was the first to recognize
the specificity of the epidemic diseases, and argued from this that
each disease must have a specific cause. "Just as a certain plant
comes from the seed of the same plant and not from any plant at will,
so each contagious disease must be propagated from a similar disease
and cannot be the result of any other disease." Further he says, "It
is necessary to assume that during the prevalence of an epidemic the
contagious material undergoes an enormous increase, and this is
compatible only with the assumption that it is a living substance."
But as is so often the case, speculation ran far ahead of the
observations on which it is based. There was a long gap between the
observations of Loewenhoeck and the theories of Plenciz, justified as
these have been by present knowledge. In the spirit of speculation
which was dominant in Europe and particularly in Germany in the latter
half of the eighteenth and the first half of the nineteenth centuries,
hypotheses did not stimulate research, but led to further
speculations. As late as 1820 Ozanam expressed himself as follows:
"Many authors have written concerning the animal nature of the
contagion of disease; many have assumed it to be developed from animal
substance, and that it is itself animal and possesses the property of
life. I shall not waste time in refuting these absurd hypotheses." The
theory of a living contagion was too simple, and not sufficiently
related to the problems of the universe to serve the medical

Knowledge of the minute organisms was slowly accumulating. The first
questions to be determined were as to their nature and origin. How
were they produced? Did they come from bodies of the same sort
according to the general laws governing the production of living
things, or did they arise spontaneously? a question which could not be
solved by speculation but by experiment. The first experiments, by
Needham, 1745, pointed to the spontaneous origin of the organisms. He
enclosed various substances in carefully sealed watch crystals from
which the air was excluded, and found that animalculi appeared in the
substance, and argued from this that they developed spontaneously. In
1769, Spallanzani, a skilled experimental physiologist, in a brilliant
series of experiments showed the imperfect character of Needham's work
and the fallacy of his conclusions. Spallanzani placed fluids, which
easily became putrid, in glass tubes, which he then hermetically
sealed and boiled. He found that the fluid remained clear and
unchanged; if, however, he broke the sealed point of such a tube and
allowed the air to enter, putrefaction, or in some cases fermentation,
of the contents took place. He concluded that boiling the substances
destroyed the living germs which they contained, the sealed tubes
prevented the air from entering, and when putrefaction or fermentation
of the contents took place the organisms to which this was due, being
contained in the air, entered from without. Objection was made to the
conclusions of Spallanzani that heating the air in the closed tubes so
changed its character as to prevent development of organisms in the
contents. This objection was finally set aside by Pasteur, who showed
that it was not necessary to seal the end of the tube before boiling,
but it could be closed by a plug of cotton wool, which mechanically
removed the organisms from the air which entered the tube, or if the
tube were bent in the shape of a _U_ and the end left open,
organisms from the air could not pass into the tube against gravity
when air movement within the tube was prevented by bending. The
possibility of spontaneous generation cannot be denied, but that it
takes place is against all human experience.

It was not possible to attain any considerable knowledge of the
bacteria discovered by Loewenhoeck until more perfect instruments for
studying them were devised. Lenses for studying objects were used in
remote antiquity, but the compound microscope in which the image made
by the lens is further magnified was not discovered until 1605, and
when first made was so imperfect that the best simple lenses gave
clearer definition. With the betterment of the microscope, increasing
the magnifying power and the sharpness of the image of the object
seen, it became possible to classify the minute organisms according to
size and form and to study the separate species. The microscope has
now reached such a degree of perfection that objects smaller than one
one hundred thousandth of an inch in diameter can be clearly seen and

Great impetus was given to the biological investigation of disease by
the discoveries which led to the formulation of the cell theory in
1840 and the brilliant work of Pasteur on fermentation,[1] but it was
not until 1878 that it was definitely proved that a disease of cattle
called anthrax was due to a species of bacteria. What should be
regarded as such proof had been formulated by Henle in 1840. To prove
that a certain sort of organism when found associated with a disease
is the cause of the disease, three things are necessary:

1. The organism must always be found in the diseased animal and
associated with the changes produced by the disease.

2. The organism so found must be grown outside of the body in what is
termed pure cultures, that is, not associated with any other
organisms, and for so long a time with constant transfers or new
seedings that there can be no admixture of other products of the
disease in the material in which it is grown.

3. The disease must be produced by inoculating a susceptible animal
with a small portion of such a culture, and the organism shown in
relation to the lesions so produced.

It is worth while to devote some attention to the disease anthrax.
This occupies a unique position, in that it was the first of the
infectious diseases to be scientifically investigated. In this
investigation one fact after another was discovered and confirmed;
some of these facts seemed to give clearer conceptions of the disease,
others served to make it more obscure; new questions arose with each
extension of knowledge; in the course of the work new methods of
investigation were discovered; the sides of the arch were slowly and
painfully erected by the work of many men, and finally one man placed
the keystone and anthrax was for a long time the best known of
diseases. Men whose reputation is now worldwide first became known by
their work in this disease. It was a favorable disease for
investigation, being a disease primarily of cattle, but occasionally
appearing in man, and the susceptibility of laboratory animals made
possible experimental study.

Anthrax is a disease of domestic cattle affecting particularly bovine
cattle, horses and sheep, swine more rarely. The disease exists in
practically all countries and has caused great economic losses. There
are no characteristic symptoms of the disease; the affected cattle
have high fever, refuse to eat, their pulse and respiration are rapid,
they become progressively weaker, unable to walk and finally fall. The
disease lasts a variable time; in the most acute cases animals may die
in less than twenty-four hours, or the disease may last ten or
fourteen days; recovery from the disease is rare and treatment has no
effect. It does not appear in the form of epidemics, but single cases
appear frequently or rarely, and there is seemingly no extension from
case to case, animals in adjoining stalls to the sick are not more
prone to infection than others of the herd. On examination after death
the blood is dark and fluid, the spleen is greatly enlarged (one of
the names of the disease "splenic fever" indicates the relation to the
spleen) and there is often bloody fluid in the tissues.

Where the disease is prevalent there are numbers of human cases. Only
those become infected who come into close relations with cattle, the
infection most commonly taking place from small wounds or scratches
made in skinning dead cattle or in handling hides. The wool of sheep
who die of the disease finds its way into commerce, and those employed
in handling the wool have a form of anthrax known as wool-sorters'
disease in which lesions are found in the lungs, the organisms being
mingled with the wool dust and inspired. In Boston occasional cases of
anthrax appear in teamsters who are employed in handling and carrying
hides. The disease in man is not so fatal as in cattle, for it remains
local for a time at the site of infection, and this local disease can
be successfully treated.

The beginning of our knowledge of the cause dates from 1851, when
small rod-shaped bodies (Fig. 17) were found in the blood of the
affected cattle, and by the work of a number of observers it was
established that these bodies were constantly present. Nothing was
known of their nature; some held that they were living organisms,
others that they were formed in the body as a result of the disease.
Next the causal relation of these bodies with the disease was shown
and in several ways. The disease could be caused in other cattle by
injecting blood containing the rods beneath the skin, certainly no
proof, for the blood might have contained in addition to the rods
something which was the real cause of the disease. Next it was shown
that the blood of the unborn calf of a cow who died of the disease did
not contain the rods, and the disease could not be produced by
inoculating with the calf's blood although the blood of the mother was
infectious. This was a very strong indication that the rods were the
cause; the maternal and foetal blood are separated by a membrane
through which fluids and substances in solution pass; but insoluble
substances, even when very minutely subdivided, do not pass the
membrane. If the cause were a poison in solution, the foetal blood
would have been as toxic as the maternal. The blood of infected cattle
was filtered through filters made of unbaked porcelain and having very
fine pores which allowed only the blood fluid to pass, holding back
both the blood corpuscles and the rods, and such filtered blood was
found to be innocuous. It was further shown that the rods increased
enormously in number in the infected animal, for the blood contained
them in great numbers when but a fraction of a drop was used for
inoculation. Attempts were also made with a greater or less degree of
success to grow the rod shaped organisms or bacilli in various fluids,
and the characteristic disease was produced by inoculating animals
with these cultures; but it remained for Koch, 1878, who was at that
time an obscure young country physician, to show the life history of
the organism and to clear up the obscurity of the disease. Up to that
time, although it had been shown that the rods or bacilli contained in
the blood were living organisms and the cause of the disease, this did
not explain the mode of infection; how the organisms contained in the
blood passed to another animal, why the disease occurred on certain
farms and the adjoining farms, particularly if they lay higher, were
free. Koch showed that in the cultures the organisms grew out into
long interlacing threads, and that in these threads spores which were
very difficult to destroy developed at intervals; that the organisms
grew easily in bouillon, in milk, in blood, and even in an infusion of
hay made by soaking this in water. This explained, what had been an
enigma before, how the fields became sources of infection. The
infection did not spread from animal to animal by contact, but
infection took place from eating grass or hay which contained either
the bacilli or their spores. When a dead animal was skinned on the
field, the bacilli contained in the blood escaped and became mingled
with the various fluids which flowed from the body and in which they
grew and developed spores. It was shown by Pasteur that even when a
carcass was buried the earthworms brought spores developed in the body
to the surface and deposited them in their casts, and in this way also
the fields became infected. From such a spot of infected earth the
spores could be washed by the rains over greater areas and would find
opportunity to develop further and form new spores in puddles of water
left on the fields, which became a culture medium by the soaking of
the dead grass. The contamination of the fields was also brought about
by spreading over them the accumulations of stable manure which
contained the discharges of the sick cattle. The tendency of the
disease to extend to lower-lying adjacent fields was due to the spores
being washed from the upper fields to the lower by the spring
freshets. Meanwhile Pasteur had discovered that by growing the
organisms at higher temperatures than the animal body, it was possible
to attenuate the virulence of the bacilli so that inoculations with
these produced a mild form of the disease which rendered the
inoculated animals immune to the fatal disease. The description of
Pasteur's work on the disease as given in the account of his life by
his son-in-law is fascinating.

Hides and wool taken from dead animals invariably contained the spores
which could pass unharmed through some of the curing processes, and
were responsible for some of the cases in man. Owing to the
introduction of regulations which were based on the knowledge of the
cause of the disease and the life history of the organism, together
with the prophylactic inoculation devised by Pasteur, the incidence of
the disease has been very greatly lessened. Looking at the matter from
the lowest point of view, the money which has been saved by the
control of the disease, as shown in its decline, has been many times
the cost of all the work of the investigations which made the control
possible. It is a greater satisfaction to know that many human lives
have been saved, and that small farmers and shepherds have been the
chief sharers in the economic benefits. The indirect benefits,
however, which have resulted from the application of the knowledge of
this disease, and the methods of investigation developed here, to the
study of the infections more peculiar to man, are very much greater.

[1] The interesting analogy between fermentation and infectious
disease did not escape attention. A clear fluid containing in solution
sugar and other constituents necessary for the life of the yeast cells
will remain clear provided all living things within it have been
destroyed and those in the air prevented from entering. If it be
inoculated with a minute fragment of yeast culture containing a few
yeast cells, for a time no change takes place; but gradually the fluid
becomes cloudy, bubbles of gas appear in it and its taste changes.
Finally it again becomes clear, a sediment forms at the bottom, and on
re-inoculating it with yeast culture no fermentation takes place. The
analogy is obvious, the fluid in the first instance corresponds with
an individual susceptible to the disease, the inoculated yeast to the
contagion from a case of transmissible disease, the fermentation to
the illness with fever, etc., which constitutes the disease, the
returning clearness of the fluid to the recovery, and like the
fermenting fluid the individual is not susceptible to a new attack of
the disease. It will be observed that during the process both the
yeast and the material which produced the disease have enormously
increased. Fermentation of immense quantities of fluid could be
produced by the sediment of yeast cells at the bottom of the vessel
and a single case of smallpox would be capable of infecting



The living organisms which cause the infectious diseases are
classified under bacteria, protozoa, yeasts, moulds, and
ultra-microscopic organisms. It is necessary to place in a separate
class the organisms whose existence is known, but which are not
visible under the highest powers of the microscope, and have not been
classified. The yeasts and moulds play a minor part in the production
of disease and cannot be considered in the necessary limitation of

[Illustration: FIG. 17.--VARIOUS FORMS OF BACTERIA, _a_, _b_, _c_,
_d_, Round bacteria or cocci: (_a_) Staphylococci, organisms which
occur in groups and a common cause of boils; (_b_) streptococci,
organisms which occur in chains and produce erysipelas and more severe
forms of inflammation; (_c_) diplococci, or paired organisms with a
capsule, which cause acute pneumonia; (_d_) gonococci, with the
opposed surfaces flattened, which cause gonorrhoea. _e_, _f_, _g_,
_h_, Rod-shaped bacteria or bacilli: (_e_) diphtheria bacilli; (_f_)
tubercle bacilli; (_g_) anthrax bacilli; (_h_) the same bacilli in
cultures and producing spores; a small group of spores is shown. (_i_)
Cholera spirillæ. (_j_) Typhoid bacilli. (_k_) Tetanus bacillus;
_i_, _j_, _k_ are actively motile, motion being effected by the small
attached threads. (_l_) The screw-shaped spirochite which is the cause
of syphilis.]

The bacteria (Fig. 17) are unicellular organisms and vary greatly in
size, shape and capacity of growth. The smallest of the pathogenic or
disease-producing bacteria is the influenza bacillus, 1/51000 of an
inch in length and 1/102000 of an inch in thickness; and among the
largest is a bacillus causing an animal disease which is 1/2000 of an
inch in length and 1/25000 of an inch in diameter. Among the
free-living non-pathogenic forms much larger examples are found. In
shape bacteria are round, or rod-shaped, or spiral; the round forms
are called micrococci, the rod-shaped bacilli and the spiral forms are
called spirilli. A clearer idea of the size is possibly given by the
calculation that a drop of water would contain one billion micrococci
of the usual size. Their structure in a general way conforms with that
of other cells. On the outside is a cell membrane which encloses
cytoplasm and nucleus; the latter, however, is not in a single mass,
but the nuclear material is distributed through the cell. Many of the
bacteria have the power of motion, this being effected by small
hair-like appendages or flagellæ which may be numerous, projecting
from all parts of the organisms or from one or both ends, the movement
being produced by rapid lashing of these hairs. A bacterium grows
until it attains the size of the species, when it divides by simple
cleavage at right angles to the long axis forming two individuals. In
some of the spherical forms division takes place alternately in two
planes, and not infrequently the single individuals adhere, forming
figures of long threads or chains or double forms. The rate of growth
varies with the species and with the environment, and under the best
conditions may be very rapid. A generation, that is, the interval
between divisions, has been seen to take place in twenty minutes. At
this rate of growth from a single cholera bacillus sixteen quadrillion
might arise in a single day. Such a rate of growth is extremely
improbable under either natural or artificial conditions, both from
lack of food and from the accumulation in the fluid of waste products
which check growth. Many species of bacteria in addition to this
simple mode of multiplication form spores which are in a way analogous
to the seeds of higher plants and are much more resistant than the
simple or vegetative forms; they endure boiling water and even higher
degrees of dry heat for a considerable time before they are destroyed.
When these spores are placed in conditions favorable for bacterial
life, the bacterial cells grow out from them and the usual mode of
multiplication continues. This capacity for spore formation is of
great importance, and until it was discovered by Cohn in 1876, many of
the conditions of disease and putrefaction could not be explained.
Spores, as the seeds of plants, often seem to be produced when the
conditions are unfavorable; the bacterium then changes into this form,
which under natural conditions is almost indestructible and awaits
better days.

The bacteria are divided into species, the classification being based
on their forms, on the mode of growth, the various substances which
they produce and their capacity for producing disease. The
differentiation of species in bacteria is based chiefly upon their
properties, there being too little difference in form and size to
distinguish species. The introduction of methods of culture was
followed by an immediate advance of our knowledge concerning them.
This method consists in the use of fluid and solid substances which
contain the necessary salts and other ingredients for their food, and
in or on which they are planted. The use of a solid or gelatinous
medium for growth has greatly facilitated the separation of single
species from a mixture of bacteria; a culture fluid containing
sufficient gelatine to render it solid when cooled is sown with the
bacteria to be tested by placing in it while warm and fluid, a small
portion of material containing the bacteria, and after being
thoroughly mixed the fluid is poured on a glass plate and allowed to
cool. The bacteria are in this way separated, and each by its growth
forms a single colony which can be further tested. It is self-evident
that all culture material must be sterilized by heat before using, and
in the manipulations care must be exercised to avoid contamination
from the air. The refraction index of the bacterial cell is so slight
that the microscopic study is facilitated or made possible by staining
them with various aniline dyes. Owing to differences in the cell
material the different species of bacteria show differences in the
facility with which they take the color and the tenacity with which
they retain it, and this also forms a means of species differentiation.
The interrelation of science is well shown in this, for it was the
discovery of the aniline dyes in the latter half of the nineteenth
century which made the fruitful study of bacteria possible.

From the simplicity of structure it is not improbable that the
bacteria are among the oldest forms of life, and all life has become
adapted to their presence. They are of universal distribution; they
play such an important part in the inter-relations of living things
that it is probable life could not continue without them, at least not
in the present way. They form important food for other unicellular
organisms which are important links in the chain; they are the agents
of decomposition, by which the complex substances of living things are
reduced to elementary substances and made available for use; without
them plant life would be impossible, for it is by their
instrumentality that material in the soil is so changed as to be
available as plant food; by their action many of the important foods
of man, often those especially delectable, are produced; they are
constantly with us on all the surfaces of the body; masses live on the
intestinal surfaces and the excrement is largely composed of bacteria.
It has been said that life would be impossible without bacteria, for
the accumulation of the carcasses of all animals which have died would
so encumber the earth as to prevent its use; but the folly of such
speculation is shown by the fact that animals would not have been
there without bacteria. It has been shown, however, that the presence
of bacteria in the intestine of the higher animals is not essential
for life. The coldest parts of the ocean are free from those forms
which live in the intestines, and fish and birds inhabiting these
regions have been found free from bacteria; it has also been found
possible to remove small animals from their mother by Cæsarian section
and to rear them for a few weeks on sterilized food, showing that
digestion and nutrition may go on without bacteria.

Certain species of bacteria are aërobic, that is, they need free
oxygen for their growth; others are anaërobic and will not grow in the
presence of oxygen. Most of the bacteria which produce disease are
facultative, that is, they grow either with or without oxygen; but
certain of them, as the bacillus of tetanus, are anaërobic. There is,
of course, abundance of oxygen in the blood and tissues, but it is so
combined as to be unavailable for the bacteria. Bacteria may further
be divided into those which are saprophytic or which find favorable
conditions for life outside of the body, and the parasitic. Many are
exclusively parasitic or saprophytic, and many are facultative, both
conditions of living being possible. It has been found possible by
varying in many ways the character of the culture medium and
temperature to grow under artificial conditions outside of the body
most, if not all, of the bacteria which cause disease. Thus, such
bacteria as tubercle bacilli and the influenza bacillus can be
cultivated, but they certainly would not find natural conditions which
would make saprophytic growth possible.

Bacteria may be very sensitive to the presence of certain substances
in the fluid in which they are growing. Growth may be inhibited by the
smallest trace of some of the metallic salts, as corrosive sublimate,
although the bacteria themselves are not destroyed. If small pieces of
gold foil be placed on the surface of prepared jelly on which bacteria
have been planted, no growth will take place in the vicinity of the
gold foil.

Variations can easily be produced in bacteria, but they do not tend to
become established. In certain of the bacterial species there are
strains which represent slight variations from the type but which are
not sufficient to constitute new species. If the environment in which
bacteria are living be unusual and to a greater or less degree
unfavorable, those individuals in the mass with the least power of
adaptibility will perish, those more resistant and with greater
adaptability will survive and propagate; and the peculiarity being
transmitted a new strain will arise characterized by this
adaptability. Bacteria with slight adaptability to the environment of
the tissues and fluids of the animal body can, by repeated
inoculations, become so adapted to the new environment as to be in a
high degree pathogenic. In such a process the organisms with the least
power of adaptation are destroyed and new generations are formed from
those of greater power of adaptation. When bacteria are caused to grow
in a new environment they may acquire new characteristics. The anthrax
bacilli find the optimum conditions for growth at the temperature of
the animal body, but they will grow at temperatures both above and
below this. Pasteur found that by gradually increasing the temperature
they could be grown at one hundred and ten degrees. When grown at this
temperature they were no longer so virulent and produced in animals a
mild non-fatal form of anthrax which protected the animal when
inoculated with the virulent strain. The well known variations in the
character of disease, shown in differences in severity and ease of
transmission, seen in different years and in different epidemics, may
be due to many conditions, but probably variation in the infecting
organisms is the most important.

The protozoa, like the bacteria, are unicellular organisms and contain
a nucleus as do all cells. They vary in size from forms seen with
difficulty under the highest power of the microscope to forms readily
seen with the unaided eye. Their structure in general is more complex
than is the structure of bacteria, and many show extreme
differentiation of parts of the single cells, as a firm exterior
surface or cuticle, an internal skeleton, organs of locomotion, mouth
and digestive organs and organs of excretion. They are more widely
distributed than are the bacteria, and found from pole to pole in all
oceans and in all fresh water. There are many modes of multiplication,
and these are often extremely complicated. The most general mode and
one which is common to all is by simple division; a modification of
this is by budding in which projections or buds form on the body and
after separation become new organisms. In other cases spores form
within the cell which become free and develop further into complete
organisms. These simple modes of multiplication often alternate in the
same organism with sexual differentiation and conjugation. There is
never a permanent sexual differentiation, but the sexual forms develop
from a simple and non-sexual organism. Usually the sexual forms
develop only in a special environment; thus the protozoon which in man
is the cause of malaria, multiplies in the human blood by simple
division, but in the body of the mosquito multiplication by sexual
differentiation takes place. Under no conditions is multiplication so
rapid as with the bacteria, and in general the simpler the form of
organism the more rapid is the multiplication. It is common to all of
the protozoa to develop forms which have great powers of resistance,
this being due in some cases to encystment, in which condition a
resistant membrane is formed on the outside, in others to the
production of spores. A fluid environment is essential to the life of
the protozoa, but the resistant forms can endure long periods of
dryness or other unfavorable environmental conditions. The universal
distribution of the protozoa is due to this; the spores or cysts can
be carried long distances by the wind and develop into active forms
when they reach an environment which is favorable. Their distribution
in water depends upon the amount of organic material this contains. In
pure drinking water there may be very few, but in stagnant water they
are very numerous, living not on the organic material in solution in
this, but on the bacteria which find in such fluid favorable
conditions for existence. The food of protozoa consists chiefly of
other organisms, particularly bacteria, and they are classed with the
animals. The protozoa are the most widely distributed and the most
universal of the parasites. The infectious diseases which they produce
in man, although among the most serious are less in number than those
produced by bacteria. So marked is the tendency to parasitism that
they are often parasitic for each other, smaller forms entering into
and living upon the larger. Variation does not seem to be so marked in
the protozoa as in the bacteria, though this is possibly due to our
greater ignorance of them as a class. We are not able, except in rare
instances, to grow them in pure culture, and study innumerable
generations under changes in the environment, as the bacteria have
been studied.

If we regard the living things on earth from the narrow point of view
as to whether they are necessary or useless or hostile to man, the
protozoa must be regarded as about the least useful members of the
biological society. It is very possible that such a conclusion is due
to ignorance; so closely are all living things united, so dependent is
one form of cell activity upon other forms that it is impossible to
foretell the result of the removal of a link. The protozoa do not seem
to be as necessary for the life of man as are the bacteria; they
produce many of the diseases of man, many of the diseases of animals
on which man depends for food; they cause great destruction in plant
life, and in the soil they feed upon the useful bacteria. It is well
to remember, however, that fifty years ago several of the organs of
the body whose activity we now recognize as furnishing substances
necessary for life were regarded as useless members and, since they
became the seat of tumors, as dangerous members of the body. The only
organ which now seems to come into such a class is the vermiform
appendix, and its lowly position among organs is due merely to an
unhappy accident of development.

The class of organisms known as the filterable viruses or the
ultra-microscopic or the invisible organisms have a special interest
in many ways. The limitation in the power of the microscope for the
study of minute objects is due not to a defect in the instrument but
to the length of the wave of light. It is impossible to see clearly
under the microscope using white light, objects which are smaller in
diameter than the length of the wave which gives a limit of 0.5µ. or
1/125,000 of an inch. By using waves of shorter length, as the
ultra-violet light, objects of 0.1µ. or 1/250000 of an inch can be
seen; but as these methods depend upon photography for the
demonstration of the object the study is difficult. The presence of
objects still smaller than 0.1 m. can be detected in a fluid by the
use of the dark field illumination and the ultra-microscope, the
principle of which is the direction of a powerful oblique ray of light
into the field of the microscope. The objects are not visible as such,
but the dispersion of the light by their presence is seen.

The demonstration that infectious diseases were produced by organisms
so small as to be beyond demonstration with the best microscopes was
made possible by showing, that some fluid from a diseased animal was
infectious; and capable of producing the disease when inoculated into
a susceptible animal. The fluid was then filtered through porcelain
filters which were known to hold back all objects of the size of the
smallest bacteria and the disease produced by inoculating with the
clear filtrate. There are a number of such filters of different
degrees of porosity manufactured, and they are often used to procure
pure water for drinking, for which use they are more or less,
generally however, less efficacious. The filter has the form of a
hollow cylinder and the liquid to be filtered is forced through it
under pressure. For domestic use the filter is attached by its open
end to the water tap and the pressure from the mains forces the water
through it. In laboratory uses, denser filters of smaller diameters
are used, and the filter is surrounded by the fluid to be tested. The
open end of the filter passes into a vessel from which the air is
exhausted and filtration takes place from without inward. The test of
the effectiveness of the filter is made by adding to the filtering
fluid some very minute and easily recognizable bacteria and testing
the filtrate for their presence. These filters have been studied
microscopically by grinding very thin sections and measuring the
diameter of the spaces in the material. These are very numerous, and
from 1/25000 to 1/1000 of an inch in diameter, spaces which would
allow bacteria to pass through, but they are held back by the very
fine openings between the spaces and by the tortuosity of the
intercommunications. When the coarser of such filters have been long
in domestic service in filtering drinking water, bacteria may grow in
and through them giving greater bacterial content to the supposed
bacteria-free filtrate than in the filtering water.

That an animal disease was due to such a minute and filterable
organism was first shown by Loeffler in 1898 for the foot and mouth
disease of cattle. This is one of the most infectious and easily
communicable diseases. The lesions of the disease take the form of
blisters which form on the lips and feet and in the mouths of cattle,
and inoculation with minute quantities of the fluid in the blisters
produces the disease. Loeffler filtered the fluid through porcelain
filters, hoping to obtain a material which inoculated into other
cattle would render them immune, and to his surprise found that the
typical disease was produced by inoculating with the filtrate.
Naturally the first idea was that the disease was caused by some
soluble poison and not by a living organism, but this was disproved in
a number of ways. The most powerful poison known is obtained from
cultures of the tetanus bacillus of which 0.000,000,1 of a gram (one
gram is 15.43 grains) kills a mouse, or one gram kills ten million
mice. Loeffler found that 1/30 gram of the contents of the vesicles
killed a calf of two hundred kilograms weight, and assuming that the
essential poison was present in the fluid in one part to five hundred
it would be several hundred times more powerful than the tetanus
poison. Further, the disease produced by inoculation of the filtrate
was itself inoculable and could be transmitted from animal to animal.
It was also found that when the virus was filtered several times it
ceased to be inoculable, showing that each time the fluid was passed
through the filter some of the minute organisms contained in it were
held back.

It is not known whether these organisms belong to the bacteria or
protozoa, and naturally nothing is known as to their form, size and
structure. Up to the present about twenty diseases are known to be due
to a filterable virus, and among these are some of the most important
for animals and for man. Among the human diseases, yellow fever,
poliomyelitis, and dengue are so produced; of the animal diseases in
addition to foot and mouth disease, pleuropneumonia, cattle plague,
African horse sickness, several diseases of fowls and the mosaic
disease of the tobacco plant have all been shown to be due to a
filterable virus. Of these organisms the largest is that which
produces pleuropneumonia in cattle, and this alone has been
cultivated. It gives a slight opacity to the culture fluids, and when
magnified two thousand diameters appears as a minute spiral or round
or stellate organism having a variety of forms. Its size is such that
it passes the coarse, but is held back by the finer, filters and it is
possible that this does not belong to the same class with the
others.[1] The diseases produced by the filterable viruses taken as a
class show much similarity. They run an acute course, are severe, and
the immunity produced by the attack endures for a long time.

Considered in its biological relations, infection is the adaptation of
an organism to the environment which the body of the host offers. It
is rather singular that variations in organisms represented by such
adaptation do not more frequently arise, in which case new diseases
would frequently occur. It cannot be denied that new diseases appear,
but there is no certain evidence that they do, and there is equally no
evidence that diseases disappear. From the meagre descriptions of
diseases, usually of the epidemic type, which have come down to us
from the past, it is difficult to recognize many of the diseases
described. The single diseases are recognized by comparing the causes,
the lesions and the symptoms with those of other diseases, and new
diseases are constantly being separated off from other diseases having
more or less common features. Many new diseases have been recognized
and named, but it is always more than probable that previously they
were confounded with other diseases. Smallpox is such a characteristic
disease that one would think it would have been recognized as an
entity from the beginning, but although the description of some of the
epidemics in remote times conform more or less to the disease as we
know it, the first accurate description is in the eighth century by
the Arabian physician Rhazes. Cerebro-spinal meningitis was not
recognized as a separate disease until 1803, diphtheria not until
1826, and the separation between typhoid and typhus fever was not made
before 1840. Nor is it sure that any diseases have disappeared,
although there seems to have been a change in the character of many.
It is difficult to reconcile leprosy as it appears now with the
universal horror felt towards it, due to the persistence of the old
traditions. It is possible, however, that the disease has not changed
its character, but that such diseases as smallpox, syphilis, and
certain forms of tuberculosis were formerly confounded with leprosy,
thus giving a false idea of its prevalence.

In certain cases the adaptation of the organism is for a narrow
environment; for example, the parasitism may extend to a simple
species only, in others the adaptation may extend to a number of
genera. In certain cases the adaptation is mutual, extending to both
parasite and host and resulting in symbiosis, and this condition may
be advantageous for both. Certain of the protozoa harbor within them
cells of algæ utilizing to their own advantage the green chlorophil of
the algæ in obtaining energy from sunlight and in turn giving
sustenance to the algæ. Although the algæ are useful guests, when they
become too numerous the protozoan devours them. It is evident that
symbiosis is the most favorable condition for the existence of the
parasite, and an injurious action exerted by the parasite on the host
unfavorable. The death of the host is an unfortunate incident from the
parasite's point of view in that it is deprived of habitation and food
supply, being placed in the same unfortunate situation as may befall a
social parasite by the death of his host.

[1] Flexner has recently succeeded in isolating and cultivating the
organism of poliomyelitis, but the organism is so small that its
classification is not possible.



As has been said, infection consists in the injury of the body by
living organisms which enter it. The body is in relation to the
external world by its surfaces only, and organisms must enter it by
some one of these surfaces. It is true that the bacteria in the
intestine--either those normally present or unusual varieties--may,
under certain circumstances, produce substances which are injurious
when absorbed; but this is not infection, and is analogous to any
other sort of poisoning. Each surface of the body has its own
bacterial flora. Organisms live on the surface either on matter which
is secreted by the surface or they use up an inappreciable amount of
body material. Many of these bacteria are harmless, some are
protective, producing by their growth such changes in the surface
fluids that these become hostile to the existence of other and
pathogenic forms. The surfaces also frequently harbor pathogenic
organisms which await some condition to arise which will permit them
to effect entrance into the tissues.

The surfaces of the body protect from invasion to a greater or less
degree. The skin protects by the impervious horny layer on the
outside, the external cells of which are dead and constantly being
thrown off. Bacteria are always found on and in this layer, but the
conditions for growth here are not very favorable and the surface is
constantly cleansed by desquamation. The new cells to supply the loss
are produced in the deepest layer of the epidermis, and the movement
of cells and fluids takes place from within outwards. The protection
is less perfect about the hairs and the sweat glands. Infection by the
route of the sweat glands is, however, uncommon, for the sweat is a
fluid unfavorable for bacterial growth and the flow acts mechanically
in washing away organisms which may have entered the ducts. Infection
by the route of the hair follicles is common. There is no mechanical
cleansing as by the sweat, the space around the hair is large and the
accumulated secretion of the hair glands and the desquamated cells
furnish a material in which bacteria may grow. Growing as a mass in
this situation, they may produce sufficient toxic material to destroy
adjacent living cells and thus effect entrance. Infection from the eye
is not common, the surface, though moist, is smooth; the eyelashes
around the margin of the lids give some mechanical protection from the
entrance of bacteria contained in dust, and the movements of the lids
and the constant and easily accelerated secretion of tears act
mechanically in removing foreign substances. It is possible that the
mechanical cleansing of the skin by the daily bath may have some
action in preventing infection.

The internal surfaces are much more exposed to attack and the
protection is not so efficient. The moisture of these surfaces is both
a protection and a source of danger. It protects by favoring the
lodgment near the orifices of organisms which are in the inspired air,
for when bacteria touch a moist surface they cannot be raised from
this and carried further by air currents. The moisture is a source of
danger in that it favors the growth of bacteria which lodge on the
surface. The respiratory surface which is most exposed to infection
from the air is further protected by the cilia, which are fine
hair-like processes covering the cells of the surface and which by
their constant motion sweep out fine particles of all sorts which
lodge upon them. The cavity of the mouth harbors large numbers of
organisms, many of them pathogenic. It forms a depot from which
bacteria may pass to communicating surfaces and infection from these
may result. Food particles collect in the mouth and provide culture
material, and there are many crypts and irregularities of surface
which oppose mechanical cleaning. Infection of the middle ear, the
most common cause of deafness, takes place by means of the Eustachian
tube which connects the cavity of the ear with the mouth. Organisms
from the mouth can extend into the various large salivary glands by
means of the ducts and give rise to infections. The tonsils,
particularly in children, provide a favorable surface for infection.
The mucous surface extends into these forming deep pockets lined with
very thin epithelium, and in these débris of all sorts accumulates and
provides material favorable for bacterial growth.

The lungs at first sight seem to offer the most favorable surface for
infection. The surface, ninety-seven square yards, is enormous; it is
moist, the epithelial covering is so thin as to give practically no
mechanical protection, large amounts of air constantly pass in and
out, and the surface is in contact with this. They are protected from
infection in many ways. The tubes or bronchi by which the air passes
into and from the lungs are covered with cilia; the surface area of
these tubes constantly enlarges as they branch, the sum of the
diameters of the small tubes being many times greater than that of the
windpipe, and this enlargement by retarding the motion of the air
favors the lodgment of particles on the surface whence they are
removed by the action of the cilia. The entering air is also brought
closely in contact with a moist surface at the narrow opening of the
larynx. That bacteria and other foreign substances can enter the lungs
in spite of these guards is shown not only by the infections which
take place here, but also by the large amount of black carbon
deposited in them from the soot contained in the air.

Infection rarely takes place from the surface of the gullet or
oesophagus which leads from the mouth to the stomach. This is due to
the smoothness of the surface and to the rapidity with which food
passes over it. Infection by the stomach also is rare, for this
contains a strong acid secretion which destroys many of the bacteria
which are taken in with the food. It is found impossible to infect
animals with cholera unless the acidity of the stomach contents be
neutralized by an alkali. Many organisms, although their growth in the
stomach is inhibited, are not destroyed there and pass into the
intestines, where the conditions for infection are more favorable.
This large and very irregular surface is bathed in fluid which is a
good culture medium and but a single layer of cells covers it. The
organisms which cause many of the infectious diseases in both man and
animals find entrance by means of the alimentary canal, as cholera,
dysentery, typhoid fever, chicken cholera, hog cholera.

Infection by the genito-urinary surface is comparatively rare. The
surface openings are usually closed, and the discharge of urine has a
mechanical cleansing effect. The wide tube of the vagina is further
protected by a normal bacterial flora which produces conditions
hostile to other and pathogenic bacteria. The most common infections
are the sexual diseases, which are due to organisms which find
favorable conditions for growth in and on the surface and which are
conveyed from a similar surface by sexual contact.

It remains a question whether bacteria can penetrate an intact surface
producing no injury at the point of entrance and be carried by the
lymph or blood into internal organs where they produce disease.
Internal infections are often found with seemingly intact body
surfaces, but it is impossible to exclude the presence of minute or
microscopic surface injuries by which the organisms may have entered.
It is also possible that a slight injury at the point of entrance may
heal so completely as to leave no trace.

The chief danger from wounds is that their surfaces may become
infected. Death from wounds is due more frequently to infection than
to the actual injury represented by the wounds. Much depends upon the
character of the wound. Infection of clean wounds which are made by a
sharp cutting instrument and from which there is abundant hæmorrhage
with sealing of the edges of the wound by clotted blood, rarely
happens. Typical wounds of this sort are often made in shaving, and
infection of such wounds is extraordinarily rare. If, with the wound,
pathogenic organisms are placed in the tissue, or foreign substances
such as bits of clothing are carried in with a bullet, for example, or
if the instrument causing the wound be of such a character as to
produce extensive lacerations of tissue, infection is more apt to
occur. The less frequency of infection in modern wars is in part due
to the simpler character of the wounds and in part to the fact that
modern fixed ammunition is practically free from germs. The old
spear-head, the arrow, the cross bow bolt, had little regard for the
probabilities of infection. Whether infection follows a wound depends
both upon the entry of pathogenic organisms and upon these finding in
the tissues suitable opportunities for growth. In wounds in which
there is much laceration of tissue organisms find the most favorable
conditions for development. The very slight wounds produced by the
exploded cap in the toy pistol give suitable conditions for the
development of the bacillus which produces tetanus or lockjaw. The
deaths of children from lockjaw following a Fourth of July celebration
have often exceeded the total deaths in a Central American revolution.
The tetanus bacillus is a widely distributed organism, whose normal
habitat is in the soil and which is usually present on the dirty hands
of little boys. The toy-pistol wounds are made by small bits of paper
or metal being driven into the skin by the explosion of the cap. The
wound is of little moment, the surface becomes closed, and a bit of
foreign substance, a few dead cells and the tetanus bacilli from the
surface remain enclosed and in a few days the fatal disease develops.
Infection of the surfaces of old wounds such as the surface of an
ulcer takes place with difficulty. Large numbers of leucocytes which
give protection by phagocytosis are constantly passing to the surface,
and there is also a constant stream of fluid towards the surface. On
such a surface there may be an abundant growth of pathogenic
organisms, but no infection results.

In most infections there is a focus where the infectious organisms are
localized; this may correspond to the point of entrance on a surface
or it may be in the interior of the body, the organisms being
deposited there after entrance. At this primary localization, the
_atrium_ of infection,[1] the organisms multiply and from this
point further invasion takes place. Many secondary foci may be formed
in the organs by distribution of the organisms, or there may be
infection of the blood and fluids of the body. The injuries which are
produced depend upon the nature of the infecting organisms. The most
common lesion consists in the death of the tissue about the infecting
organisms. In most cases the sum of the changes are so characteristic
that from them the nature of the infection is easily determined, and
these changes often give names to the disease; thus tuberculosis is a
disease characterized by the formation of tubercles or little nodules
in the body. The situation of the foci of disease is determined by
many conditions, the most important being the varying resistance of
the different organs of the body to the growth of bacteria. Certain
organs, such as the central nervous system, the muscles, the testicles
and the ovaries, have a high resistance to the growth of bacteria. The
disease may be localized in certain organs because only in these do
the bacteria find favorable conditions for growth. In spite of a high
general resistance to infection the lesions in chronic glanders are
most marked in the muscles, those of poliomyelitis in the spinal cord.
There are few bacterial diseases which are localized in the blood, but
many of the diseases caused by protozoa have this localization. In
every infection some organisms enter the blood, which acts as a
carrier and deposits them in the organs.

Bacteria cause disease by producing substances called toxines which
are poisonous to the cells, and of which two sorts are distinguished.
One form of toxines is produced by the bacteria as a sort of
secretion, and is formed both in the body and when the bacteria are
growing in cultures. Substances of this character, many of them highly
poisonous, are produced both by animals and plants. They may serve the
purpose both of offence and defence, as in the case of the snake
venom, and in other cases they seem to benefit their producers in no
way whatever, and may even be injurious to them. After the different
cereals have been grown for succeeding years in the same place, growth
finally diminishes not from the exhaustion of the soil, but from the
accumulation in it of substances produced by the plants. Beneath
certain trees, as the Norway maple, grass will not grow, and it has
been shown that the tree produces substances which inhibit the growth
of grass. When bacteria are grown in a culture flask, growth ceases
long before the nutritive material has been consumed, from the
accumulation of waste products in the fluid. The other class of toxic
substances, called endotoxines, are not secretion products, but are
contained in the bacterial substance and become active by the
destruction and disintegration of the bacteria. They can be
artificially produced by grinding up masses of bacteria, and in the
body the destruction and solution of bacteria which is constantly
taking place sets them free. The toxines and the endotoxines are of an
albuminous nature, and act only when they come in contact with the
living cells within the body. When taken into the alimentary canal
they are either not absorbed or so changed by the digestive fluids as
to be innocuous. Many of the ordinary food substances, even a material
apparently so simple as the white of an egg, are highly injurious if
they reach the tissues in an unchanged form.

By means of these substances the bacteria produce such changes in
their environment within the body that this becomes adapted to their
parasitic existence. In symbiosis the bacteria probably undergo
changes by which they become adapted to the environment, and in
parasitism the environment becomes adapted to them. In the same way
man can change his immediate environment by means of clothing,
artificial heating, etc., and adapt it to his needs; or by hardening
his body he can adapt it to the environment. The pathogenic bacterium
finds the living tissue hostile, its cells devour him, the tissue
fluids destroy him, and by means of the toxines he changes the
environment from that of living to dead tissue, or in other ways so
alters it that it is no longer hostile. The parasite has also means of
passive defence comparable to the armor of the warrior in the past. It
may form a protective mantle called a capsule around itself, which
serves to protect it from the action of the body fluids. Such capsule
formation is a very common thing in the pathogenic organisms, and they
are found only when these are growing in the body and do not appear in
cultures (Fig. 17-c).

It is evident that just as the parasite has his weapons of offence and
defence so has the host, otherwise there would be no recovery from
infectious diseases. Although many of the infectious diseases have a
high mortality, which in rare instances reaches one hundred per cent,
the majority do recover. In certain cases the recovery is attended by
immunity, the individual being protected to a greater or less degree
from a recurrence of the same disease. The immunity is never absolute;
it may last for a number of years only, and usually, if the disease be
again acquired, the second attack is milder than the primary. Probably
the most enduring immunity is in smallpox, although cases are known of
two and even three attacks; the immunity is high in scarlet fever,
measles, mumps and typhoid fever. The immunity from diphtheria is
short, and in pneumonia, although there must be a temporary immunity,
future susceptibility to the disease is probably increased. In certain
cases the immunity is only local; the focus of disease heals because
the tissue there has evolved means of protection from the parasite,
but if any other part of the body be infected, the disease pursues the
usual course. A boil, for example, is frequently followed by the
appearance of similar boils in the vicinity due to the infection of
the skin by the micrococci from the first boil, which by dressings,
etc., have become spread over the surface.

The natural methods of defence of the host against the parasites have
formed the main subject in the study of the infectious diseases for
the last twenty years. Speculation in this territory has been rife and
most of it fruitless, but by patient study of disease in man and by
animal experimentation there has been gradually evolved a sum of
knowledge which has been applied in many cases to the treatment of
infectious diseases with immense benefit. Research was naturally
turned to this subject, for it was evident that the processes by which
the protection of the body was brought about must be known before
there could be a really rational method of treatment directed towards
the artificial induction of such processes, or hastening and
strengthening those which were taking place. Previous to knowledge of
the bacteria, their mode of life, their methods of infection and
knowledge of the defences of the body, most of the methods of
prevention and treatment of the infectious diseases was based largely
on conjecture, the one brilliant exception being the discovery of
vaccination by Jenner in 1798.

The host possesses the passive defences of the surfaces which have
already been considered. The first theories advanced in explanation of
immunity were influenced by what was known of fermentation. One, the
exhaustion theory, assumed that in the course of disease substances
contained in the body and necessary for the growth of the bacteria
became exhausted and the bacteria died in consequence. Another, the
theory of addition, assumed that in the course of the disease
substances inimical to the bacteria were formed. Both these theories
were inadequate and not in accord with what was known of the
physiology of the body. The most general mode of defence is by
phagocytosis, the property which many cells have of devouring and
digesting solid substances (Fig. 16-p). Although this had been known
to take place in the amoebæ and other unicellular organisms, the wide
extent of the process and its importance in immunity was first
recognized by Metschnikoff in 1884 and the phagocytic theory of
immunity advanced and defended by a brilliant series of experiments by
Metschnikoff and his pupils conducted in the Pasteur Institute.
Metschnikoff's first observations were made on the daphnea, a small
animalcule just visible to the naked eye which lives in fresh water.
The structure of the organism is simple, consisting of an external and
internal surface between which there is a space, the body cavity;
daphneæ are transparent and can be studied under the microscope while
living. Metschnikoff observed that certain of them in the aquarium
gradually lost their transparency and died, and examining these he
found they were attacked by a species of fungus having long, thin
spores. These spores were taken into the intestine with other food;
they penetrated the thin wall of the intestine, passed into the body
cavity, multiplied there, and in consequence the animal died. In many
cases, however, those penetrating became enclosed in cells which the
body cavity contains and which correspond with the leucocytes of the
blood; in these the spores were digested and destroyed. The daphneæ in
which this took place recovered from the infection. Here was a case in
which all the stages of an infectious disease could be directly
followed under the microscope, and the whole process was simple in
comparison with infections in the higher animals. The pathogenic
organism was known, the manner and site of invasion was clear, it was
also evident that if the multiplication of the parasite was unchecked
the animal died, but if the parasite was opposed by the body cells and
destroyed the animal recovered. The studies were carried further into
the diseases of the higher animals, and it was found the leucocytes in
these played the same part as did the cells in the body cavity of the
daphnea. The introduction of bacteria into certain animals was
followed by their destruction within cells and no disease resulted; if
this did not take place, the bacteria multiplied and produced disease.
Support also was given the theory by the demonstration at about the
same time that in most of the infectious diseases the leucocytes of
the blood became increased in number,--that in pneumonia, for
instance, instead of the usual number of eight thousand in a cubic
millimeter of blood, there were often thirty thousand or even fifty
thousand. At about the same time also chemotaxis, or the action of
chemical substances in attracting or repelling organisms, excited
attention, and all these facts together became woven into the theory.
It was soon seen, however, that this theory, based as it was on
observation and supported by the facts observed, was not, at least in
its first crude form, capable of general application. Many animals
have natural immunity to certain diseases; they do not have the
disease under natural conditions, nor do they acquire the disease when
the organisms causing it are artificially introduced into their
tissues by inoculation. Such natural immunity seemed to be unconnected
with defence by phagocytosis, for the leucocytes of the animal might
or might not have phagocytic reaction to the particular organisms to
which the animal was immune. It was also seen that recovery from
infection in certain diseases was unconnected with phagocytosis. It
had also been demonstrated, by German observers chiefly, that the
serum of the blood, the colorless fluid in which the corpuscles float,
was itself destructive, and that in an animal rendered immune to a
special bacterium the destructive action of the serum on that organism
was greatly increased. In this hostile serum the bacteria often became
clumped together in masses, the bodies became swollen, broken up, and
finally disintegrated. This property of the serum was described as due
to a substance in the serum called _alexine_, which in the immune
animal became greatly increased in amount. It was even denied by some
that phagocytosis of living bacteria took place, and that all those
included in the cells were dead, having been destroyed in the first
instance by the serum. The strife became a national one between the
French and Germans,--on the one side in France the phagocytic theory
was defended, and in Germany, on the other, the theory of serum
immunity. The mass of experimental work which poured from the
laboratories of the two countries in attack and defence became so
great that it could not easily be followed. It had a good influence
because, without the stimulation of this national rivalry, the
knowledge which gradually arose from this work would not have been so
quickly acquired. It is interesting that the mode of action of the
serum in destroying bacteria was demonstrated not by a German but by
Bordet, a French observer and a pupil of Metschnikoff. He showed that
the serum contained two distinct substances, each necessary for the
destructive action. The separate action of these substances can be
studied since one is _thermolabile_, or destroyed by heating the
serum to one hundred and thirty-three degrees; the other
_thermostabile_, or capable of withstanding a greater degree of
heat. These substances are known only by their effect, they have never
been separated from the serum. The thermostabile substance, or
_amboceptor_, as it is generally called, has in itself no
destructive action on the bacteria; but in some way so alters them
that they can be acted on by the thermolabile substance called
_complement_ whose action is destructive. The amount of
amboceptor may increase in the course of infection and its formation
stimulated, the amount of complement remains unchanged. The action of
the amboceptor is specific, that is, directed against a single species
of bacterium only; the destructive power of the blood may be very
great against a single bacterium species and have no effect on others.
There seem naturally to be many different amboceptors in the blood,
and the number may be very greatly increased. It has been shown as a
result of the work of many investigators that the shield has two
faces,--there is destruction both by cells and fluids and there is
interaction by both. The amboceptors so necessary for the destructive
action of the serum are produced by the body cells, particularly the
leucocytes. The serum assists in pagocytosis by the action on bacteria
of substances called _opsonins_ which are contained in it, and
the formation of which can be very greatly stimulated. Again, not all
inclusion of bacteria within leucocytes is indicative of phagocytosis;
in many cases the bacteria seem to find the best conditions for
existence within the leucocytes, and these and not the bacteria are

So far it has been shown that the best defence of the body is, as is
the best defence in war, by offensive measures, as illustrated by
phagocytosis and destruction by the serum. Both of these actions can
be increased by their exercise just as the strength of muscular
contraction can be increased by exercise, and the facility for doing
everything increased by habit. Certain of the infectious diseases are,
as has been said, essentially toxic in their nature, and in cultures
the organisms produce poisonous substances. By the injection into the
tissues of such substances the same disturbances are produced as when
the bacteria are injected. Such a disease is diphtheria. In this there
is only a superficial invasion of the tissues. The diphtheria bacilli
are located on the surface of the tonsils or pharynx or windpipe,
where, as a result of their action, the membrane so characteristic of
the disease is produced. The membrane may be the cause of death when
it is so extensively formed as to occlude the air passages, but the
prominent symptoms of the disease, the fever, the weakness of the
heart and the great prostration are due not to the presence of the
membrane, but to the action of toxic substances which are formed by
the bacteria growing in the superficial lesions and absorbed. Tetanus,
or lockjaw, is another example of these essentially toxic diseases.
The body must find some means of counteracting or destroying these
injurious toxic substances. It does this by forming antagonistic
substances called antitoxines, which act not by destroying the
toxines, but by uniting with them, the compound substance being
harmless. It has been found that the production of antitoxine can be
so stimulated by the injection of toxine that the blood of the animal
used for the purpose contains large amounts of antitoxine. The horse
is used in this way to manufacture antitoxine, and the serum injected
into a patient with diphtheria has a curative action, a greater amount
being thus introduced than the patient can manufacture.

ANTITOXINE FORMATION. The surface of the cell (_n_) is covered with
receptors some of which (_b_) fit the toxine molecule, (_a_) allowing
the toxine to act upon the cell. Under the stimulus of this the cell
produces these receptors in excess which enter into the blood and
there combine with the toxine as in _a^1 b^1_, thus anchoring it and
preventing it from acting upon the cells. The receptors _c_ and _d_ do
not fit the toxine molecule.]

A very ingenious theory which well accords with the facts has been
given by Ehrlich in explanation of the production of antitoxine and of
the reaction between toxine and antitoxine (Fig. 18). This is based on
the hypothesis, which is in accord with all facts and generally
accepted, that the molecules which enter into the structure of any
chemical substance have in each particular substance a definite
arrangement, and that in a compound substance each elementary
substance entering into the compound molecule has chemical affinities,
most of which may be satisfied by finding a suitable mate. Ehrlich
assumes that the very complex chemical substances which form the
living cells have many unsatisfied chemical affinities, and that it is
due to this that molecules of substances adapted for food can enter
the cells and unite with them; but there must be some coincidence of
molecular structure to enable the union to take place, the comparison
being made of the fitting of a key into a lock. The toxines--that
produced by the diphtheria bacillus being the best example--are
substances whose molecular structure enables them to combine with the
cells of the body, the combination being effected through certain
chemical affinities belonging to the cells termed _receptors_.
Unless the living cells have receptors which will enable the
combination with the toxine to take place, no effect can be produced
by the toxine and the cells are not injured. This is the case in an
animal naturally immune to the action of the diphtheria bacillus or
its toxines. In the case of the susceptible animal the receptors of
the cells of the different organs combine with the toxine to a greater
or less extent, which explains the fact that different degrees of
injury are produced in the different tissues; the toxine of tetanus,
or lockjaw, for example, combines by preference with the nervous
tissue, that of diphtheria with the lymphatic tissue. It is known that
in accordance with the general law of injury and repair, a loss in any
part of the body stimulates the tissue of the same kind to new growth
and the loss is thus repaired; it is assumed that the cell receptors
which combine with the toxine are lost for the cell which then
produces them in excess. The receptors so produced pass into the
blood, where they combine with the toxine which has been absorbed; the
combination is a stable one, and the toxine is thus prevented from
combining with the tissue cells. The antitoxine which is formed during
the disease, and the production of which in the horse can be
enormously stimulated by the injection of toxine, represents merely
the excess of cell receptors, and when the serum of the horse
containing them is injected in a case of diphtheria the same
combination takes place as in the case of receptors provided by the
patient. In the case of the destruction of bacteria in the blood by
the action of amboceptor and complement, the amboceptor must be able
to combine with both the bacterial cell and the complement which
brings about its destruction, and just as antitoxine is formed so new
amboceptors may be formed.

Few hypotheses have been advanced in science which are more ingenious,
in better accord with the facts, have had greater importance in
enabling the student to grasp the intricacies of an obscure problem,
and which have had an equal influence in stimulating research. The
immunity which results from disease in accordance with this theory, is
due not to conditions preventing the entrance of organisms into the
body, but to greater aptitude on the part of the cells to produce
these protective substances having once learned to do so. An
individual need not practise for many years, having once learned them,
those combinations of muscular action used in swimming; but the habit
at once returns when he falls into the water.

Infectious diseases and recovery are phases of the struggle for
existence between parasite and host, and illustrate the power of
adaptation to environment which is so striking a characteristic of
living matter.


[1] The comparison here is with the atrium of a Pompeiian house.



The infectious diseases are often complicated by secondary infections,
some other organism finding opportunity for invasion in the presence
of the injuries produced in the primary disease. In many diseases,
such as diphtheria, scarlet fever and smallpox, death is frequently
due to the secondary infection. The secondary invaders not only find
local conditions favoring a successful attack, but the activity of the
tissue cells on which the production of protective substances
essentially depends has suffered by the primary infection, or the
cells are occupied in meeting the exigencies of this. The body is in
the position of a state invaded by a second power where all its forces
and resources are engaged in repelling the first attack.

What are known as terminal infections occur shortly before death. No
matter what the disease which causes death, in the last hours of life
the body usually becomes invaded by organisms which find their
opportunity in the then defenceless tissues, and the end is often
hastened by this invasion.

There are also mixed infections in which two different organisms unite
in attack, each in some way assisting in the action of the other. The
best known example of this is in the highly infectious disease of
swine known as hog cholera. It has been shown that in this disease two
organisms are associated,--one an invisible and filterable organism,
and the other a bacillus. It was first supposed that the bacillus was
the specific organism; it was found in the lesions and certain, but
not all, the features of the disease were produced by inoculating hogs
with pure cultures. The disease so produced is not contagious, and the
contagious element seems to be due to the filterable virus.

The modes of transmission of infectious diseases are of great
importance and are the foundation of measures of public health. In the
preceding chapter we have seen that in the infected individual the
disease extends from one part of the body to another. There is a
primary focus of disease from which the extension takes place, and the
study of the modes of extension in the individual throws some light on
the much more difficult subject of the transmission of disease from
one individual to another. There are four ways by which extension in
the individual may take place.

1. By continuity of tissue, an adjoining tissue or organ becoming
infected by the extension of a focus of infection.

2. By means of lymphatics. Organisms easily enter these vessels which
are in continuity with the tissue spaces and receive the exudate from
the focus of infection. The organisms are carried to the lymph nodes,
which, acting as filters, retain them and for a time prevent a further
extension. The following illustrates the importance of the part the
nodes may play in mechanically holding back a flood of infection. A
physician examined after death the body of a person who died from
infection with a very virulent micrococcus and in the course of the
examination slightly scratched a finger. One of the organs of the body
was removed, sent to a laboratory and received by a laboratory worker,
a woman physician, who had slight abrasions and fissures in the skin
of the hands from contact with irritating chemicals. In the course of
a few hours the wound on the finger of the man became inflamed,
intensely painful, and red lines extended up the arm in the course of
the lymphatic vessels, showing that the organisms were in the
lymphatics and causing inflammation in their course. The lymph nodes
in the armpit into which these vessels empty became greatly inflamed,
swollen, and an abscess formed in them which was opened. There was
high fever, great prostration, a serious illness from which the man
did not recover for several months. The woman only handled the organ
which was sent to the laboratory in order to place it in a fluid for
preservation. She also had a focus of infection of a finger with the
same red lines on the arm, showing extension by the lymphatics; but
there was no halt of the infection in the armpit, for all the lymph
nodes there had been removed several years before in the course of an
operation for a tumor of the breast. A general infection of the blood
took place, there was very high fever, and death followed in a few
days. The halt of the infection is important in allowing time for the
body to make ready its means of defence. One cannot avoid comparing
the lymph node with a strong fortress thrown in the path of a
victorious invading army behind which the defenders may gather and
which affords them time to renovate their strength.

3. By means of the blood. The blood vessels are universally
distributed, the smaller vessels have thin walls easily ruptured and
easily penetrated. It is probable that in every infection some
organisms enter the blood which, under usual conditions, is peculiarly
hostile to bacteria. These may, however, be carried by the blood to
other organs and start foci of infection in these.

4. By means of continuous surfaces. The bacteria may either grow along
such surfaces forming a continuous or more or less broken layer, or
may be carried from place to place in the fluids which bathe them.

All these modes of extension are well shown in tuberculosis. This
disease is caused by a small bacillus which does not produce spores,
has no power of saphrophytic growth under natural conditions, and is
easily destroyed. Moisture and darkness are favorable conditions for
its existence, sunlight and dryness the reverse. There are three
varieties or strains of the tubercle bacilli which infect respectively
man, cattle and birds, and each class of animals shows considerable
resistance to the varieties of the bacillus which are most infectious
for the others.

The primary seat of the infection in man is generally in the upper
part of the lung. The organisms settle on the surface here and cause
multiplication of the cells and an inflammatory exudate in a small
area. With the continuous growth of the bacilli in the focus,
adjoining areas of the lung become affected, and there is further
extension in the immediate vicinity by means of the lymphatics. Small
nodules are formed and larger areas by their coalescence. Infection
with tuberculosis is so common that at least three-fourths of all
individuals over forty show evidences of it. The examination of two
hundred and twenty-five children of the average age of five years who
had died of diphtheria showed tuberculous infection in one-fifth of
the cases and the frequency of infection increases with age. The
defence on the part of the body is chiefly by the formation of dense
masses of cicatricial tissue which walls off the affected area and in
which the bacilli do not find favorable conditions for growth. This
mode of defence, which is probably combined with the production of
substances antagonistic to the toxines produced by the bacilli, is so
efficacious that in the great majority of cases no further extension
of the process takes place. In certain cases, however, the growth of
the bacilli in the focus is unchecked, the tissue about them is killed
and becomes converted into a soft semi-fluid material; further
extension then takes place. All parts of the enormous surface of the
lungs are connected by means of the system of air tubes or bronchi,
and the bacilli have favorable opportunity for distribution, which is
facilitated by sudden movements of the air currents in the lung
produced by coughing. The defence of the body can still keep pace with
the attack, and even in an advanced stage the infection can be checked
in some cases permanently; in others the check is but temporary, the
process of softening continues, and large cavities are produced by the
destruction of the tissue. On the inner surface of these cavities
there may be a rapid growth of bacilli.

From the lungs the bacilli are carried by the lymphatics to the lymph
nodes at the root of the lungs, in which a similar process takes
place; this, on the whole, is favorable, because further extension by
this route is for a time blocked. The extension by means of surfaces
continues, the abundant sputum which is formed in the lungs and which
contains large numbers of bacilli, becomes the vehicle of
transportation. The windpipe and larynx may become infected, the back
parts of each are more closely in contact with the sputum and are the
parts most generally infected. A large part of the sputum is swallowed
and infection of the intestine takes place, the lesions taking the
form of large ulcers. From the intestinal ulcers there is further
extension by means of the lymphatics, to the large lymph nodes in the
back of the abdominal cavity (Fig. 8-25); the bacilli may also pass
from the ulcers into the abdominal cavity and be distributed over the
surface of the peritoneum resulting in tuberculous peritonitis. When
the disease has reached an advanced stage, bacilli in small numbers
continually pass into the blood and are distributed by this over the
body, producing small nodules in many places. In rare cases
distribution by the blood is the principal method of extension, and
immense numbers of small foci of disease are produced, the form of
disease being known as acute miliary tuberculosis. Although the
bacilli are distributed everywhere, certain organs, as the brain and
muscles, are usually exempt, because in these the conditions are not
favorable to further growth of the bacilli. Tuberculosis, although
frequently a very acute disease, is usually one of the best types of a
chronic disease and may last for many years. The chronic form is
characterized by periods of slow or rapid advance when conditions
arise in the body favorable for the growth of the bacilli, and periods
when the disease is checked and quiescent, the defensive forces of the
body having gained the upper hand. Often the intervention of some
other disease so weakens the defences of the body that the bacilli
again find their opportunity. Thus typhoid fever, scarlet fever and
other diseases may be followed by a rapidly fatal advance of the
tuberculosis, starting from some old and quiescent focus of the

Tuberculosis is also one of the best examples of what is known as
latent infection. In this the infectious organisms enter the body and
produce primary lesions in which the organisms persist but do not
extend owing to their being enclosed in a dense and resistant tissue,
or to the production of a local immunity to their action. Dr. Head has
recently examined the children of households in which there was open
tuberculosis in some member of the household. By open tuberculosis is
understood a case from which bacilli are being discharged. He found
with scarcely an exception that all the children in such families
showed evidences of infection. The detection of slight degrees of
tuberculous infection is now made easy by certain skin reactions on
inoculation of the skin with a substance derived from the tubercle
bacilli. Such latent infections may never become active and in the
majority of cases do not. When, however, in consequence of some
intercurrent disease or conditions of malnutrition the general
defences of the body become weakened extension follows. Such latent
infections explain the enormous frequency of tuberculosis in prisons.
Under the general prison conditions infection in the prisons probably
does not take place to any extent, and the disease is as common when
the prisoners are kept in individual cells as in common prisons. It is
probable that in these cases the prisoners have latent tuberculosis
when entering, and the disease becomes active under the moral and
physical depression which prison life entails.

For the extension of infection from one individual to another the
infecting organisms must in some way be transferred. The most
important of the conditions influencing this are the localization of
the disease and the character of the infectious organisms,
particularly with regard to their resistance to the conditions met
with outside of the body. The seat of disease influences the discharge
of organisms; thus, if the disease involve any of the surfaces the
organisms become mingled with the secretions of the surface and are
discharged with these. If the seat of disease be in the lungs, the
throat or the mouth, the sputum forms the medium of extension, which
can take place in many ways. The sputum may become dried, forms part
of the dust and the organisms enter with the inspired air. The
organisms which cause most of the diseases in which the sputum becomes
infectious are quickly destroyed by conditions in the open, such as
the sunlight and drying; street dust does not play so prominent a part
in extension as is generally supposed. Organisms find much more
favorable conditions within houses. It is now generally recognized
that infection with tuberculosis does not take place in the open, but
in houses in which the bacilli on being discharged are not destroyed.
The hands, the clothing and surroundings even with the exercise of the
greatest care may become soiled with the saliva.

It has been shown that in coughing and speaking very fine particles of
spray are formed by the intermingling of air and saliva, which may be
projected a considerable distance and remain floating in the air for
some time. These particles are so fine as to be invisible; they may be
inspired, and their presence in the air forms an area of indeterminate
extent around the infected person within which such infection is
possible. Such spray formation is also an important means of the
extension of infection in the sick individual, for it is continually
formed and inspired. It is in this way that the extreme prevalence of
broncho-pneumonia in infants and young children is to be explained. No
matter what the essential disease, an almost constant finding in young
children after death is small areas of inflammation in the lungs in
and around the terminations of the air tubes. The situation renders it
evident that the organisms which caused the lesions entered the lung
by the air tubes. The mouth of the child is unclean and harbors
numbers of the same sort of organisms as those causing the lung
inflammation; but in the absence of such a mode of infection as is
given by spray formation it is difficult to see how the extension from
the mouth to the lungs could take place. The weakened condition of the
body in these cases favors the secondary infection.

If the disease be located in the intestines, as in typhoid fever and
dysentery, the organisms are contained in the fecal discharges, and by
means of these the infection is extended. In typhoid fever, dysentery
and cholera massive infections of the populace may take place from the
contamination of a water supply and the disease be extended over an
entire city. One of the most striking instances of this mode of
extension was in the epidemic of cholera in Hamburg in 1892. There
were two sources of water supply, one of which was infected, and the
cases were distributed in the city in the track of the infected
supply. Many such instances have been seen in typhoid fever. Certain
articles of food, particularly milk, serve as sources of infection.
This is more apt to happen when the organism causing the infection
grows easily outside of the body. A few such organisms entering into
the milk can multiply enormously in a few hours and increase the
amount of infectious material. In all these cases the sick individual
remains a source of infection, for it is almost impossible to avoid
some contamination of the body and the immediate surroundings with the
organisms contained in the discharges.

Transmission by air plays but little part in the extension of
infection. In such a disease as smallpox, where the localization is on
the surface of the body, the organisms are contained in or on the thin
epithelial scales which are constantly given off. These are light, and
may remain floating in the air and carried by air currents just as is
the pollen of plants. There seem to have been cases of smallpox where
other modes of more direct transmission could be excluded and in which
the organisms were carried in the air over a considerable space. All
sorts of intermediate objects, both living and inanimate, such as
persons, domestic animals, toys, books, money, etc., can serve as
conveyors of infection.

Insects play a most important part in the transmission of disease, and
in certain cases, as when a disease is localized in the blood, this is
the only means of transmission. There are three ways in which the
insect plays the rôle of conveyor.

1. The insect may play a purely passive part in that its exterior
surface becomes contaminated with the discharges of the sick person,
and in this way the organisms of disease may be conveyed to articles
of food, etc. The ordinary house fly conveys in this way the organisms
of typhoid and dysentery. Flies seek the discharges not only for food,
but for the purpose of depositing their eggs, and the hairy and
irregular surface of their feet facilitates contamination and
conveyance. When flies eat such discharges the organisms may pass
through the alimentary canal unchanged and be deposited with their
feces; they also often vomit or regurgitate food, and in this way also
contaminate objects. Flies very greedily devour the sputum of
tuberculous patients, and the tubercle bacilli contained in this pass
through them unchanged and are deposited in their feces.

[Illustration: FIG. 19.--TRYPANOSOMES FROM BIRDS. All the trypanosomes
are very much alike. They contain a nucleus represented by the dark
area in the centre and a fur-like membrane terminating in a long
whip-like flagellum. They have the power of very active motion within
the blood.]

2. Diseases which are localized in the blood are transmitted by biting
flies. The biting apparatus becomes contaminated with the organisms
contained in the blood, and these are directly inoculated into the
blood of the next victim. The trypanosome diseases form the best
example of this mode of transmission. The trypanosomes are widely
distributed, exclusively parasitic, flagellated protozoa which live in
the blood of a large number of animals and birds (Fig. 19). They may
give rise to fatal diseases, but in most cases there is mutual
adaptation of host and parasite and they seem to do no harm. One of
the most dangerous diseases in man, the African sleeping sickness, is
caused by a trypanosome, and the disease of domestic cattle in Africa,
nagana, or tsetse fly disease, is also so produced. In certain regions
of Africa where a biting fly, the _Glossina morsitans_, occurs in
large numbers, it has long been known that cattle bitten by these
flies sickened and died, and this prevented the settling and use of
the land. In the blood of the sick cattle swarms of trypanosomes are
found. The source from which the tsetse fly obtained the trypanosomes
which it conveyed to the cattle was unknown until it was discovered
that similar trypanosomes exist in the blood of the wild animals which
inhabit the region, but these have acquired by long residence in the
region immunity or adaptation to the parasite and no disease is
produced. With the gradual extension of settlement of the country and
the accompanying destruction of wild life the disease is diminishing.
Some of the inter-relations of infections are interesting. The
destruction of wild animals in South Africa which, by removing the
sources of nagana, rendered the settlement of the country possible was
due chiefly to the introduction of another infectious disease,
rinderpest, which not only destroyed the wild animals but produced
great destruction of the domestic cattle as well.

The _sleeping sickness_ has many features of interest. In the old
slavery days it was found that the negroes from the Congo region in
the course of the voyage or after they were landed sometimes were
affected with a peculiar disease. They were lethargic, took little
notice of their surroundings, slept easily and finally passed into a
condition of somnolence in which they took no food and gradually died.
There was no extension of the disease and it was attributed to extreme
homesickness and depression. A similar disease has been known for more
than one hundred years on the west coast of Africa, and attracted a
good deal of interest and curiosity on account of the peculiar
lethargy which it produced and from which it has received the name of
"sleeping sickness." Although apparently infectious in its native
haunts, it lost the power of spreading from man upon removal to
regions where it did not prevail. At first confined to a very small
region on the Niger river, it gradually extended with the development
of trade routes and the general increase of communications which trade
brings, until it prevails in the entire Congo basin, in the British
and German possessions in East Africa, and is extending north and
south of these regions. The cause of the disease and its mode of
conveyance was discovered in 1903. The fly _Glossina palpalis_
which conveys the disease is a biting fly about the size of the common
house fly and lives chiefly in the vicinity of water. When such a fly
bites an individual who has sleeping sickness its bite can convey the
disease to monkeys, on whom the transmission experiments were made.
After biting the fly is infectious for a period of two days. After
this it is harmless, unless it again obtains a supply of living
trypanosomes. There is quite a period in which there are no symptoms
of the disease, although trypanosomes are found in the blood and in
the lymph nodes, and the individual is a source of infection. The
peculiar lethargy which has given the disease its name does not appear
until the nervous system is invaded by the parasites. It is impossible
to compute accurately the numbers of deaths from this disease--in the
region of Victoria Nyanza alone the estimates extend to hundreds of

3. In the third mode of insect conveyance the insect does not play a
merely passive rôle, but becomes a part of the disease, itself
undergoing infection, and a period in the life cycle of the organism
takes place within it. In all these cases quite a period of time must
elapse before the insect is capable of transmitting the disease; in
malaria, which is the best type of such a disease, this period is ten
days. Malaria is due to a small protozoan, the _Plasmodium
malariæ_, which was discovered by Lavaran, a French investigator,
in 1882. The organism lives within or on the surface of the red blood
corpuscles. It first appears as a very minute colorless body with
active amoeboid movements, and increases in size, attacks a succession
of corpuscles, and finally attains a size as large as or larger than a
corpuscle. The corpuscles attacked become pale by the destruction of
hæmoglobin, swell up and disintegrate, the hæmoglobin becoming
converted into granules of black pigment inside the parasite. Having
attained a definite size the organism forms a rosette and divides into
a number of forms similar to the smallest seen inside the corpuscles;
these small forms enter other corpuscles and the cycle again begins.
This cycle of development takes place in forty-eight hours, and
segmentation is always accompanied by a paroxysm of the disease shown
in a chill followed by fever and sweating which is due to the effect
of substances liberated by the organism at the time of segmentation. A
patient may have two crops of the parasite developing independently in
the blood, and the two periods of segmentation give a paroxysm for
each, so that the paroxysms may appear at intervals of twenty-four
hours instead of forty-eight (Fig. 20). This cycle of development may
continue for an indefinite time, and there may be such a rapid
increase in the parasites as to bring about the death of the
individual; but with him the parasite would also perish, for there
would be no way of extending the infection and providing a new crop.
The disease has been transmitted by injecting the infected blood into
a normal individual.

ORGANISM OF MALARIA, _a-g_, Cycle of forty-eight hour development, the
period of chill coinciding with the appearance of _f_ and _g_ in the
blood. The organisms _g_, which result from segmentation, attack other
corpuscles and a new cycle begins. _h_, The male form or
microgametocyte, with the protruding and actively moving spermatozoa,
one of which is shown free. _i_ and _j_ are the macrogametes or female
forms. _k_ shows one of these in the act of being fertilized by the
entering spermatozoön. The differentiation into male and female forms
takes place in the blood, the further development of the sexual cycle
within the mosquito.]

If a mosquito of the species _anopheles_ bites the affected
person, it obtains a large amount of blood which contains many
parasites. Within the mosquito the parasite undergoes a further
development into male and female sexual forms, which may also form in
the blood, termed respectively _microgametocyte_ and _macrogamete_.
From the microgametocyte small flagellate bodies, the male sexual
elements _microgametes_ or _spermatozoa_, develop and fertilize the
_macrogametes_; after fertilization this develops into a large body,
the _oöcyst_ which is attached to the wall of the stomach of the
mosquito. Within the oöcyst, innumerable small bodies, the
sporozoites, develop, make their way into the salivary glands and are
injected into the individual who becomes the prey of the mosquito,
and again the cycle of development begins. The presence of the
parasite within the mosquito does not constitute a disease. So far as
can be determined, life goes on in the usual way, and its duration in
the insect is not shortened.

The nature of the parasite which produces yellow fever is unknown, for
it belongs to the filterable viruses; the infectious material,
however, has been shown by inoculation to exist in the blood, and the
disease is transmitted by a mosquito of another species, the
_stegomyia_. The development cycle within this takes a period of
twelve days, which time must elapse after the mosquito has bitten
before it can transmit the disease. Here again the mutual
interdependence of knowledge is shown. Nothing could have seemed less
useful than the study of mosquitoes, the differentiation of the
different species, their mode of life, etc., and yet without this
knowledge discoveries so beneficial and of such far-reaching
importance to the whole human race as that of the cause and mode of
transmission of malaria and yellow fever would have been impossible;
for it could easily have been shown that the ordinary _culex_
mosquito played no rôle. The rôle which insects may play in the
transmission of disease was first shown by Theobald Smith in this
country, in the transmission by a tick of the disease of cattle known
as Texas fever. The infecting organism _pyrosoma bigenimum_ is a
tiny pear-shaped parasite of the red corpuscles. Smith's
investigations on the disease, published in 1893, is one of the
classics in medicine, and one of the few examples of an investigation
which has not been changed or added to by further work.

One of the most interesting methods of extension of infection, showing
on what small circumstances infection may depend, is seen in the case
of the hookworm disease, which causes such devastation in the Southern
States. The organism which produces the disease, the _Uncinaria_,
belongs to the more highly developed parasites, and is a small round
worm one-third of an inch long. The worms which inhabit the intestines
have a sharp biting mouth by which they fasten themselves to the
mucous membrane and devour the blood. The most prominent symptom of
the disease is anæmia, or loss of blood, due not only to the direct
eating of the parasite, but to bleeding from the small wounds caused
by its bite. Large numbers of eggs are produced by the parasite which
are passed out with the feces, which becomes the only infectious
material. In a city provided with water-closets and a system of
sewerage there would be no means of extension of infection. The eggs
in the feces in conditions of warmth and moisture develop into small
crawling larvæ which can penetrate the skin, producing inflammation of
this, known in the region as the ground itch. The larvæ enter the
circulation and are carried to the lungs, where they perforate the
capillaries and reach the inner surface; from this they pass along the
windpipe, and then by way of the gullet and stomach reach their
habitat, the small intestine. Unfortunately, the habits and poverty of
the people in every way facilitate the extension of the infection.
There is no proper disposal of the feces, few of the houses have even
a privy attached to them, and the feces are distributed in the
vicinity of the houses. This leads to contamination of the soil over
wide areas. Most of the inhabitants of the country go barefoot the
greater part of the year, and this gives ready means of contact with
the larvæ which crawl over the surface of the ground. The disease is
necessarily associated with poverty and ignorance, the amount of blood
is reduced to a low point, and industry, energy and ambition fall with
the blood reduction; the schools are few and inefficient; the children
are backward, for no child can learn whose brain cells receive but a
small proportion of the necessary oxygen; and a general condition of
apathy and hopelessness prevails in the effected communities. The
control of the disease depends upon the disinfection of the feces, or
at least their disposal in some hygienic method, the wearing of shoes,
and the better education of the people, all of which conditions seem
almost hopeless of attainment. The infection is also extended by means
of the negroes who harbor the parasite, but who have acquired a high
degree of immunity to its effects and whose hygienic habits are even
worse than those of the whites. The organism was probably imported
with the negroes from Africa and is one of the legacies of slavery.

The diseases of animals are in many ways closely linked with those of
man. In the case of the larger parasites, such as the tapeworms and
the trichina, there is a direct interchange of disease with animals,
certain phases of the life cycle of the organisms are passed in man
and others in various of the domestic animals. A small inconspicuous
tapeworm inhabits the intestine of dogs and seems to produce no ill
effects. The eggs are passed from the dog, taken into man, and result
in the formation of large cystic tumors which not infrequently cause
death. Where the companionship between dog and man is very close, as
in Iceland, the cases are numerous.

Most of the diseases in animals caused by bacteria and protozoa are
not transmitted to man, but there is a conspicuous exception. Plague
is now recognized as essentially an animal disease affecting rats and
other small rodents, and from these the disease from time to time
makes excursions to the human family with dire results. The greatest
epidemics of which we have any knowledge are of plague. In the time of
Justinian, 542 B.C., a great epidemic of plague extended over what was
then regarded as the inhabited earth. This pandemic lasted for fifty
years, the disease disappeared and appeared again in many places and
caused frightful destruction of life. Cities were depopulated, the
land in many places reverted to a wilderness, and the works of man
disappeared. The actual mortality cannot be known, but has been
estimated at fifty millions. Plague played a large part in the
epidemics of the Middle Ages. An epidemic started in 1346 and had as
great an extension as the Justinian plague, destroying a fourth of the
inhabitants of the places attacked; and during the fifteenth and
sixteenth and seventeenth centuries the disease repeatedly raised its
head, producing smaller and greater epidemics, the best known of
which, from the wonderful description of De Foe, is that of London in
1665, and called the Black Death. Little was heard of the disease in
the nineteenth century, although its existence in Asia was known. In
1894 it appeared in Hong Kong, extended to Canton, thence to India,
Japan, San Francisco, Mexico, and, in fact, few parts of the tropics
or temperate regions of the earth have been free from it. Mortality
has varied greatly, being greatest in China and in India; in the last
the estimate since 1900 is seven million five hundred thousand deaths.
The disease is caused by a small bacillus discovered in 1894 which
forms no spores and is easily destroyed by sunlight, but in the dark
is capable of living with undiminished virulence for an indefinite
time. The disease in man appears in two forms, the most common known
as bubonic plague, from the great enlargement of the lymph nodes,
those of the groin being most frequently affected. The more fatal form
is known as pneumonic plague, and in this the lungs are the seat of
the disease.

In the old descriptions of the disease it was frequently mentioned
that large numbers of dead rats were found when it was prevalent, and
the most striking fact of the recent investigations is the
demonstration that the infection in man is due to transference of the
bacillus from infected rats. There are endemic foci of the disease
where it exists in animals, the present epidemic having started from
such a focus in Northern China, in which region the _Tarabagan_,
a small fur-bearing animal of the squirrel species, was infected. Rats
are easily infected, the close social habits of the animal, the vermin
which they harbor, and the habits of devouring their dead fellows
favor the extension of infection. The disease extends from the rat to
man chiefly by means of the fleas which contain the bacilli, and in
cases of pneumonic plague from man to man by means of sputum
infection. The disease once established in animals tends to remain,
the virus being kept alive by transmission from animal to animal, and
the persistence of the infection is favored by mild and chronic cases.



We have seen that insects serve as carriers of disease in two ways: in
one, by becoming contaminated with organisms they serve as passive
carriers, and in the other they undergo infection and form a link in
the disease. The more recent investigations of modes of transmission
of infectious diseases have shown that man, in addition to serving
while sick as a source of infection, may serve as a passive carrier in
two ways. For infection to take place not only must the pathogenic
organism be present, but it must be able to overcome the passive and
active defences of the body and produce injury. Pathogenic organisms
may find conditions favorable for growth on the surfaces of the body,
and may live there, but be unable to produce infection, and the
individual who simply harbors the organisms can transmit them to
others. Such an individual may be a greater source of infection than
one with the disease, because there is no suspicion of danger. The
organisms which thus grow on the surfaces have in some cases been
shown to be of diminished virulence, but in others have full
pathogenic power. Such passive carriers of infection have been found
for a number of diseases, as cerebro-spinal meningitis, diphtheria,
poliomyelitis and cholera. In all these cases the organisms are most
frequently found in those individuals who have been exposed to
infection as members of a family in which there have been cases of
disease. The other sort of carrier has had and overcome the disease,
but mutual relations have been established with the organism which
continues to live in the body cavity. Diphtheria bacilli usually
linger in the throat after convalescence is established, and until
they have disappeared the individual is more dangerous than one
actually sick with the disease. Health officers have recognized this
in continuing the quarantine against the disease until the organism
disappears. In typhoid fever bacilli may remain in the body for a long
time and be continually discharged, as in the well-known case of
"typhoid Mary."[1]

Single cases of certain infectious diseases may appear in a community
year after year, and at intervals the cases become so numerous that
the disease is said to be epidemic. Such a disease is smallpox. This
is a highly infectious disease, towards which all mankind is
susceptible. Complete protection against the disease can be conferred
by Jenner's discovery of vaccination. The disease becomes modified
when transferred to cattle, producing what is known as cowpox, in
which vesicles similar to those of smallpox appear on the skin. The
inoculation of man with the contents of such a vesicle produces a mild
form of disease known as vaccinia, which protects the individual from
smallpox. This protection is fully as adequate as that produced by an
attack of smallpox, and we are warranted in saying that if thorough
vaccination, or the inoculation with vaccinia, were carried out
smallpox would disappear. There are great difficulties in the way of
carrying out effective vaccination of the whole population, which are
accentuated by the active opposition of people who are ignorant and
wilfully remain so. There exists in every state a number of people
unprotected by vaccination, and among these single cases of smallpox
appear. The unprotected individuals gradually increase in number,
forming an inflammable material awaiting the spark or infection which
produces a conflagration in the one case and an epidemic in the other.

Cerebro-spinal meningitis is another example of a disease which exists
in sporadic and epidemic form. This disease is caused by a small
micrococcus, the organisms joined in pairs. The seat of the disease is
in the meninges or membranes around the brain and spinal cord. The
micrococci enter the body from the throat and nose, and either pass
directly from here into the meninges, or they enter into the blood and
are carried by this into the meninges. The organisms are easily
destroyed and cannot long survive the conditions outside the body, so
that for infection to take place the transmission must be very direct.
Carriers who have the organisms in the throat, but who do not have the
disease, are the principal agents in dissemination. The mortality is
high, and even in recovery permanent damage is often done to the brain
or to the organs of special sense. Sporadic cases constantly occur in
small numbers, and it is difficult or impossible to trace any
connection between these cases. At varying intervals, often twenty
years intervening, an epidemic appears which sometimes remains local
in a city or state, sometimes extends to adjoining cities or states,
and may even extend over a very large area. In the epidemics the
mortality is much higher than in the sporadic cases. The same
explanation given for smallpox cannot apply here, for there is not a
similar accumulation of susceptible material. We know there is a great
deal of variation in the virulence of the different pathogenic
organisms, and the virulence can be artificially increased and
diminished. In epidemics of meningitis the virulence of the organisms
is increased, as is shown by the greater mortality. It is highly
probable that such epidemics are due to changes which arise in the
organisms from causes we do not know and which increase their capacity
for harm. It is possible that such a change would convert a carrier
into a case of disease, the organism acquiring greater powers of
invasion. Such a strain of organisms arising in one place and
producing an epidemic could be transported to another locality and
exert the same action, or similar changes in the organisms could arise
simultaneously in a number of places. Analogies to such conditions are
given in plants. In certain plants it has been shown that from unknown
causes there appears a tendency to the production of variations. A
very beautiful herbaceous peony known as "Bridesmaid" after having
grown for a number of years in single form, in one year wherever grown
suddenly became double. The peculiar thing with the lower unicellular
organisms is that the changes which so arise do not tend to become
permanent, the organism reverts to its usual character, the disease to
its sporadic type.

A very fatal form of poliomyelitis has for a number of years prevailed
in Sweden. In the United States there have been continually a number
of single cases of the disease, and it is not impossible that a more
pathogenic strain of the organism has developed in Sweden and has been
imported into this country, giving rise to the much greater extension
of the disease in a number of places.

The most cursory study of the infectious diseases shows that there is
great variation in the susceptibility of individuals. Even in the most
severe epidemics all are not equally affected, some escape the
infection, others have the disease lightly, others severely, some die.
Chance enters into this, but plays a small part, for the same varying
individual susceptibility is shown experimentally. If a given number
of animals of the same species, age and weight, even those from the
same litter, be inoculated with a given number of bacteria shown to be
pathogenic for that species, the results differ. If the dose be
necessarily fatal, death will take place at intervals; if a dose
smaller than the fatal be used, some animals will die, others will
recover. The defences of the organism being centred in the activity of
the living tissue, any condition which depresses cell activity may
have an effect in increasing susceptibility to infection. Animals
which ordinarily are not susceptible to infection with a certain
organism may be made so by prolonged hunger, or fatigue, by the
influence of narcotics, by reduction of the body temperature, by loss
of blood. In man prolonged fatigue, cold, the use of alcohol to excess
and even psychic depression increases susceptibility. It has been
shown that such conditions are accompanied by a diminution in the
power of the blood to destroy bacteria.

There is variation in the susceptibility to infection in the different
races of man. If a race be confined to one habitat with close
intercourse between the people, such a race may acquire a high degree
of immunity to local diseases by a gradual weeding out of the
individuals who are most susceptible. A degree of comparative harmony
may be gradually established between host and parasite, as is the case
in wild animals. These have few diseases, the weak die, the resistant
breed; they harbor, it is true, large numbers of parasites, but there
is mutual adjustment between parasite and host. Diseases in animals
greatly increase under the artificial conditions of domestication.
Certain highly specialized breeds of cattle, as the Alderneys, are
much more susceptible to tuberculosis than the less specialized. The
high development of the variation which consists in a marked ability
to produce milk fat is probably combined with other qualities, shown
in diminished resistance to disease, and under natural conditions the
variation would not have persisted. The introduction of a new disease
into an isolated people has often been attended with dire
consequences. It is much the same thing with the introduction of
disease of plants. In Europe the brown-tail moth and the gypsy moth
produce continuously a certain amount of damage to the trees, but
their parasitic enemies have developed with them and check their
increase. These pests were brought to this country in which there were
no conditions retarding their increase and have produced great damage.

It is very difficult to estimate the degree of racial susceptibility.
The negro race seems to be more susceptible to certain diseases, such
as tuberculosis and smallpox, less so to others, as yellow fever,
malaria and uncinariasis. What are apparently differences in
susceptibility may be explained by racial customs. A statistical
inquiry into death in India from poisonous snakes might be interpreted
as showing a marked resistance on the part of the white to the action
of the venom, but it is merely a question of the boots of the whites
and the naked feet and legs of the natives. The relatively greater
frequency of smallpox in the blacks is due to the greater difficulties
in carrying out vaccination measures among them and the greater
opportunity for infection which results from their less hygienic life.
It has always been noted that when plague prevails in Oriental cities,
the natives are more frequently attacked than are Europeans. This does
not depend upon differences in susceptibility, but on the better
hygienic conditions of the whites which prevent the close relation to
rats and vermin by which infection is extended. There would be but
little extension of the hookworm disease in a community where shoes
were worn and the habits were cleanly.

It is by no means improbable that the formation of the habits of
civilization was influenced by infection. Most of these habits, such
as personal cleanliness, the avoidance of close contact, the demand
for individual utensils for eating and drinking, are all of distinct
advantage in opposing infection. Certain habits, on the other hand,
such as kissing, which probably represents the extension of a habit of
sexual origin, are disadvantageous and infection is often transmitted
in this way. In syphilitic infection the mouth forms one of the most
common localizations of the disease and may contain the causal
organisms in great numbers. This, the _spirochæta pallida_, is an
organism of great virulence, and man is the most susceptible animal.
The disease, like gonorrhoea, is essentially a sexual disease, the
primary location is in the sexual organs, and it is transmitted
chiefly by sexual contact. Of all the infectious diseases, it is the
one most frequently transmitted to the unborn child; in certain cases
the disease is transmitted, in others the developing foetus may be so
injured by the toxic products of the disease that various
imperfections of development result, as is shown in deformities, or in
conditions which render the entire organism or individual organs,
particularly the nervous system, more susceptible to injury. Following
the primary localization of the acquired form of the disease, there is
usually secondary localization in the mucous membrane of the mouth,
and the disease may be transmitted by kissing or by the use of
contaminated utensils. The habit of indiscriminate kissing is one
which might with great benefit be given up.

There is definite relation between age and the infectious diseases. In
general, susceptibility is increased in the young; young animals can
be successfully inoculated with diseases to which the adults of the
species are immune, and certain human diseases, such as scarlet fever,
measles and whooping cough, seem to be the prerogatives of the child.
It must be remembered, however, that one attack of these diseases
confers a strong and lasting immunity and children represent a raw
material unprotected by previous disease. Where measles has been
introduced into an island population for the first time, all ages seem
equally susceptible. All ages are equally susceptible to smallpox, and
yet in the general prevalence of the disease in the prevaccination
period it was almost confined to children, the adults being protected
by a previous attack. The habits and environment at different ages
have an influence on the opportunities for infection. There is
comparatively little opportunity for infection during the first year,
in which period the infant is nursed and has a narrow environment
within which infection is easily controlled. With increasing years the
opportunities for infection increase. When the child begins to move
and crawl on hands and knees the hands become contaminated, and the
habit of putting objects handled into the mouth makes infection by
this route possible. Food also becomes more varied, milk forms an
important part of the diet, and we are now appreciating the
possibilities of raw milk in conveying infection. With the enlarging
environment, with the school age bringing greater contact of the child
with others, there come greater opportunities for infection which are
partly offset by the increase in cleanliness. The dangers of infection
in the school period are now greatly lessened by medical inspection
and care of the school children. In the small epidemic of smallpox
which prevailed in Boston from 1881 to 1883, there was a sharp decline
in the incidence of the disease in children as soon as the school age
was reached, this being due to the demand of vaccination as a
condition for entrance into the schools. Many of the infectious
diseases are much milder in children than in adults. This is the case
in typhoid fever, malaria and yellow fever. The comparative immunity
of the natives to yellow fever in regions where this prevails seems to
be due to their having acquired the disease in infancy in so mild a
form that it was not recognized as such.

The infectious diseases are preëminently the diseases of the first
third of life. After the age of forty man represents a select
material. He has acquired immunity to many infections by having
experienced them. Habits of life have become fixed and there is a
general adjustment to environment. The only infectious disease which
shows no abatement in its incidence is pneumonia, and the mortality in
this increases with age. Between thirty-five and fifty-five man stands
on a tolerably firm foundation regarding health; after this the age
atrophies begin, the effects of previous damage begin to be apparent,
and the tumor incidence increases.

[1] This was the case of a woman, by occupation a cook, whose numerous
exchanges of service were accompanied by the appearance of cases of
typhoid fever in the families. This became so marked that an
examination was made and she was found to be a typhoid carrier and as
such constantly discharging typhoid bacilli. She is now isolated.



The question of inheritance of disease is closely associated with the
study of infection, and the general subject of heredity in its bearing
on disease can be considered here. By heredity is understood the
transference of similar characteristics from one generation of
organisms to another. The formation of the sexual cells is a much more
complex process than that of the formation of single differentiated
cells, for the properties of all the cells of the body are represented
in the sexual cells, to the union of which the heredity transmission
of the qualities of the parents is due. In the nucleus of all the
cells in the body there is a material called _chromatin_, which
in the process of cell division forms a convoluted thread; this
afterwards divides into a number of loops called _chromosomes_,
the number of which are constant for each animal species. In cell
division these loops divide longitudinally, one-half of each going to
the two new cells which result from the division; each new cell has
one-half of all the chromatin contained in the old and also one-half
of the cytoplasm or the cell material outside of the nucleus. The
process of sexual fertilization consists in the union of the male and
female sex cells and an equal blending of the chromatin contained in
each (Fig. 22). In the process of formation of the sexual cells a
diminution of the number of chromosomes contained in them takes place,
but this is preceded by such an intimate intermingling of the
chromatin that the sexual cells contain part of all the chromosomes of
the undifferentiated cells from which they were formed. The new cell
which is formed by the union of the male and female sexual cells and
which constitutes a new organism, contains the number of chromosomes
characteristic of the species and parts of all the chromatin of the
undifferentiated cells of male and female ancestors. As a result of
this the most complicated mechanism in nature, it is evident that in a
strict sense there can be no heredity of a disease because heredity in
the mammal is solely a matter of the chromosomes and these could not
convey a parasite. The new organism can, however, quickly become
diseased and, by the transference of disease to it and by either
parent, there is the appearance of hereditary transmission of disease,
though in reality it is not such. The ovum itself can become the site
of infection; this, which was first discovered by Pasteur in the eggs
of silkworms, takes place not infrequently in the infection of insects
with protozoa. In Texas fever the ticks which transmit the disease,
after filling with the infected blood, drop off and lay eggs which
contain the parasites, and the disease is propagated by the young
ticks in whom the parasites have multiplied. The same thing is true in
regard to the African relapsing or tick fever, which is also
transferred by a tick. In the white diarrhoea of chickens the eggs
become infected before they are laid and the young chick is infected
before it emerges from the shell. It is highly improbable, and there
is no certain evidence for it, that the extremely small amount of
material contributed by the male can become infected and bring
infection to the new organism. In the cases in which disease of the
male parent is transferred to the offspring, it is either by an
infection of the female by the male, with transference of the
infection from her to the developing organism, or with the male sexual
cells there may be a transference to the female of the infectious
material and the new organism may be directly infected. No other
disease in man is so easily and directly transferred from either
parent to offspring as is syphilis, and the disease is extremely
malignant for the foetus, usually causing death before the normal
period of intra-uterine development is reached.

cells are represented to the left of the line at the bottom of diagram
and are black. From the fertilized ovum at the top there is a
continuous cell development, with differentiation represented in the
cell groups of the bottom row. It is seen that the sexual cells are
formed directly from the germ cell and contain no admixture from the
cells of the body.]

The mother gives the protection of a narrow and unchanging environment
and food to the new organism which develops within the uterus, and
there is always a membranous separation between them. Disease of the
mother may affect the foetus in a number of ways. In most cases the
membrane of separation is an efficient guard preventing pathogenic
organisms reaching the foetus from the mother. In certain cases,
however, the guard can be passed. In smallpox, not infrequently, the
disease extends from the mother to the foetus, and the child may die
of the infection or be born at term with the scars resulting from the
disease upon it. Syphilis in the mother in an active stage is
practically always extended to the foetus. We have said that in an
infectious disease substances of an injurious character are produced
by bacteria, and such substances being in solution in the blood of the
infected mother can pass through the membranous barrier and may
destroy the foetus although the mother recovers from the infection.

FERTILIZATION. (Boveri.) In the first cell (_a_) the ovum is shown in
process of fertilization by the entering spermatozoon or male sexual
element. In the following cells there is shown the increase in amount
of the male material and the final intimate commingling in _g_ which
precedes the first segmentation. _g_ represents a new organism formed
by the union of the male or female cell but differing from either of

Living matter is always individual, and this individuality is
expressed in slight structural variations from the type of the species
as shown in an average of measurements, and also in slight variations
in function or the reactions which living tissue shows towards the
conditions acting upon it. The anatomical variations are more striking
because they can be demonstrated by weight and measure, but the
functional variations are equally numerous. Thus, no two brains react
in exactly the same way to the impressions received by the sense
organs; there are differences in muscular action, differences in
digestion; these variations in function are due to variations in the
structure of living material which are too minute for our
comparatively coarse methods of detection. In the enormous complexity
of living matter it is impossible that there should not be minute
differences in molecular arrangement and to this such functional
variations may be due. Chemistry gives us a number of examples of
variations in the reaction of substances which with the same
composition differ in the molecular arrangement. Even in so simple a
mechanism as a watch there are slight differences in structure which
gives to each watch certain individual characteristics, but the type
as an instrument constructed for recording time remains. In the fusion
of the chromosomes of the male and female sexual cells, to which the
hereditary transmission of the ancestral qualities to the new
offspring is due, there are differences in the qualities of each, for
the individuality of the parents is expressed in the germ cells, and
the varying way in which these may fuse gives to the new cell
qualities of its own in addition to qualities which come from each
ancestor, and from remote ancestors through these. The qualities with
which the new organism starts are those which it has received from its
ancestors plus its individuality. The fact that the sexual cells are
formed from the early formed cells of the new organism which represent
all of the qualities of the fertilized ovum or primordial cell,
renders it unlikely that the new offspring will contain qualities
which the parents have acquired. The question of the inheritance of
characteristics which the parents have acquired as the result of the
action of environment upon them is one which is still actively
investigated by the students of heredity, but the weight of evidence
is opposed to this belief.

In the new organism the type of the species is preserved and the
variations from the mean to which individuality is due are slight. We
are accustomed to regard as variations somewhat greater departures
from the species type than is represented in individuality, but there
is no sharp dividing line between them.

Very much wider departures from the species type are known as
mutations. Such variations and mutations, like individuality, may be
expressed in qualities which can be weighed and measured, or in
function, and all these can be inherited; certain of them known as
dominant characteristics more readily than others, which are known as
recessive. If these variations from the type are advantageous, they
may be preserved and become the property of the species, and it is in
this way that the characteristics of the different races have arisen.
Certain of the variations are unfavorable to the race. The varying
predisposition to infection which undoubtedly exists and may be
inherited represents such a variation. Tuberculosis is an instance of
this; for, while the cause of the disease is the tubercle bacillus,
there is enormous difference in the resistance of the body to its
action in different individuals. The disease is to a considerable
extent one of families, but while this is true the degree of the
influence exerted by heredity can be greatly overestimated. The
disease is so common that in tracing the ancestry of tuberculous
patients it is rare to find the disease not represented in the
ancestors. A further difficulty is that the environment is also
inherited. The child of a tuberculous parent has much better
opportunity to acquire the infection than a child without such an
environment [page 167]. Other diseases than the infectious seem to be
inherited, of which gout is an example. In gout there is an unusual
action of the cells of the body which leads to the formation and the
retention in the body of substances which are injurious. Here it is
not the disease which is inherited, but the variation in structure to
which the unusual and injurious action of the cells is due.

While tuberculosis and gout represent instances in which, although the
disease itself is not inherited yet the presence of the disease in the
ascendants so affects the germinal material that the offspring is more
susceptible to these particular diseases, much more common are the
cases in which disease in the parents produces a defective offspring,
the defect consisting in a general loss of resistance manifested in a
variety of ways, but not necessarily repeating the diseased condition
of the parent. In these cases the disease in the parents affects all
the cells of the body including the germinal cells, and the defective
qualities in the germ cells will affect the cells of the offspring
which are derived from these. There is a tendency in these cases to
the repetition in the offspring of the disease of the parents, because
the particular form of the parental disease may have been due to or
influenced by variation of structure. One of the best examples of
affection of the offspring by diseased conditions of the parents
produced by a toxic agent which directly or indirectly affects all the
cells of the body is afforded by alcohol when used in excess. Since
drunkenness has become a medical rather than a moral question, a great
deal of reliable data has accumulated in regard to it as a factor in
the heredity of disease. Grotjahn gives the following examples: Six
families were investigated in which there were thirty-one children. In
all these families the father and grandfather on the father's side
were chronic alcoholics, and in certain of the families drunkenness
prevailed in the more remote ancestors. The following was the fate of
the children: eight died shortly after birth of general weakness,
seven died of convulsions in the first month, three were malformed,
three were idiotic, three were feeble-minded, three were dwarfs, three
were epileptics, two were normal. In a second group of three families
there were twenty children. The fathers were drunkards, but their
immediate ancestors were free: four children died of general weakness,
three of convulsions in the first month, two were feeble-minded, one
was a dwarf, one was an epileptic, seven were normal. In a family in
which both father and mother and their ancestors were drunkards there
were six children: three died of convulsions within six months, one
was an idiot, one a dwarf, and one an epileptic. For comparison there
were taken from the same station in life ten families in which there
was no drunkenness: three children died from general weakness, three
from intestinal troubles, two of nervous affection, two were
feeble-minded, two were malformed, fifty were normal. Legrain has
studied on a larger scale the descendants of two hundred and fifteen
families of drunkards in which there were eight hundred and nineteen
children. One hundred and forty-five of these were insane, sixty-two
were criminals, and one hundred and ninety-seven drunkards. Of course
all this cannot be attributed to alcohol alone. There is first to be
considered a probable variation in the nervous system which is
expressed in the alcoholic habit; second, the environment consisting
in poverty, bad associates, etc., which the alcoholic habit brings;
third, the alcohol alone. That defective inheritance so frequently
takes the form of alcoholism is largely due to the environment. There
has never been the opportunity to study on a large scale the effect of
the complete deprivation of alcohol from a people living in the
environment of modern civilization. There is a possibility, and even
probability, that the defective nervous organization which predisposes
to alcoholism would seek satisfaction in the use of some other
sedative drug. So complex are all the interrelations of the social
system that it would be possible to regard alcohol as an agent useful
in removing the defective, were it not for its long-enduring action
and its effects on the descendants, procreation not being affected by
its use.

Diseases of the nervous system are particularly apt to affect the
offspring, and often the inherited condition repeats that of the
parents. This is due to the fact that most of the nervous diseases
depend both upon intrinsic factors which consist in some defective
condition of the nervous system representing a variation, and
extrinsic factors due to environment or occupation which make the
basal condition operative. The definite relation between alcoholism
and insanity is due to alcohol acting not as an intrinsic but an
extrinsic factor, bringing into effectiveness the hereditary weakness
of the nervous system. The influence of heredity in producing insanity
is variously estimated at from twenty-six per cent to sixty per cent
of all cases. This great difference in the estimation of the
hereditary influence is due to the personal equation of the
statistician, and the care with which other factors are eliminated. In
the more severe form of the hereditary degeneration the same
pathological conditions are repeated in the descendants. In certain
cases the severity of the condition increases from generation to
generation. According to Morel there may be merely what is recognized
as a nervous temperament often associated with moral depravity and
various excesses in the first generation; in the second, severe
neuroses, a tendency to apoplexy and alcoholism; in the third, psychic
disturbances, suicidal tendencies and intellectual incapacity; and in
the fourth, congenital idiocy, malformations and arrests of
development. There are some very definite data with regard to
inheritance in the nervous disease known as epilepsy. The essential
condition in this consists in attacks of unconsciousness, usually
accompanied by a discharge of nerve force shown in convulsions, the
attack being often preceded by peculiar sensations of some sort known
as the aura. In the most marked forms of the affection heredity plays
but little part, owing to the early supervention of imbecility and
helplessness, and it is a greater factor in the better classes of
society than in the proletariat. In the better classes, owing to the
greater care of the cases and the avoidance of exciting causes of the
attacks, the disease is better controlled and rarely advances to the
extent that it does among the poor. The association of epilepsy and
alcoholism is especially dangerous, for a slight amount of alcohol may
greatly accentuate the disease. In five hundred and thirty-five
children in whose parentage there were sixty-two male and seventy-four
female epileptics, twenty-two were born dead, one hundred and
ninety-five died from convulsions in infancy, twenty-seven died in
infancy from other causes, seventy-eight were epileptics, eleven were
insane, thirty-nine were paralyzed, forty-five were hysterical, six
had St. Vitus's dance, one hundred and five were ordinarily healthy.
That variations in the nervous system which produce more or less
unusual mental peculiarities and which do not take the form of nervous
disease are inherited, the most superficial consideration shows. A
child in its mental characteristics is said to take after one or the
other of its parents, certain habits and mental traits are the same,
often even the handwriting of a child resembles that of a parent.

In certain cases the inheritance is transmitted by the female alone.
This is the case in the hæmophilia, the unfortunate subjects of which
are known as bleeders. There is in this a marked tendency to
hæmorrhage which depends upon an alteration in the character of the
blood which prevents clotting. This, the natural means of stopping
bleeding from small wounds, being in abeyance, fatal hæmorrhage may
result from pulling a tooth or from an insignificant wound. There is a
seeming injustice in the inheritance, for the females do not suffer
from the disease although they transmit it, while the males who have
the disease cannot even create additional sympathy by transmitting it.

The most obvious inheritance is seen in the case of malformations.
These represent wide departures from the type of the species as
represented in the form. There is no hard and fast line separating the
slight departures from the normal type known as variations and
mutations, from the malformations. Certain of the malformations known
as monstrosities hardly represent the human type. These are the cases
in which the foetus is represented in a formless mass of tissue, or
there is absence of development of important parts such as the nervous
system or there is more or less extensive duplication of the body.
There has always been a great deal of popular interest attached to the
malformations owing to the part which maternal impressions are
supposed to play in their production. In this, some striking
impression made on the pregnant woman is supposed to affect in a
definite way the structure of the child. The cases, for instance, in
which a woman sees an accident involving a wound or a loss of an arm
and the child at birth shows a malformation involving the same part.
There is no association between maternal impressions and
malformations, although there have been many striking coincidences.
All malformations arise during the first six weeks of pregnancy known
as the embryonic period, in which the development of the form of the
child is taking place, and during which time there is little
consciousness of pregnancy. Maternal impressions are usually received
at a later period, when the form of the child is complete and it is
merely growing. It must be remembered also that there is neither
nervous nor vascular connection between the child in the uterus and
the mother, the child being from the period of conception an
independent entity to which the mother gives nutriment merely. Of
course, as has been said, the mother may transmit to the child
substances which are injurious, and in certain cases parasites may
pass from the mother to the foetus. The same types of malformations
which occur in man are also seen in birds, and it would require a more
vigorous imagination than is usual to believe that a brooding hen
could transmit an impression to an egg and that a headless chick could
result from witnessing the sacrifice of an associate. The idea of the
importance of maternal impressions in influencing the character of the
offspring is a very old one, a well-known instance being the sharp
practice of Jacob's using peeled wands to influence the color of his
cattle. In regard to coincidences the great number of cases in which
strong impressions made on the mind of the pregnant mother without
result on the offspring are forgotten. The belief has been productive
of great anxiety and even unhappiness during a period which is
necessarily a trying one, and should be dismissed as being both
theoretically impossible and unsupported by fact.

The malformations are divided anatomically into those characterized,
first, by excess formation, second, by deficient formation, third, by
abnormal displacement of parts. They are due to intrinsic causes which
are in the germ, and which may be due to some unusual conditions in
either the male or female germ cell or an imperfect commingling of the
germinal material, and to extrinsic causes which physically, as in the
nature of a shock or chemically as by the action of a poison, may
affect the embryo through the mother. Malformations are made more
numerous in chickens by shaking the eggs before brooding. A number of
malformations are produced by accidental conditions arising in the
environment; for instance, the vascular cord connecting mother and
child may become wound around parts constricting them or even cutting
them off, and the membrane around the child may become adherent to
certain parts and prevent the development of these. The extrinsic
causes are more operative the more unfavorable is the environment of
the mother. Malformations are more common in illegitimate children
than in legitimate and more common in alcoholic mothers; there is an
unfavorable environment of poverty in both cases, added to in the
latter and usually in the former by the injurious action of the

The more extensive malformations have no effect on heredity, because
the subjects of them are incapable of procreation. The malformations
which arise from the accidents of pregnancy and which are compatible
with a perfectly normal germ are in the nature of acquired
characteristics and are not inherited. Those malformations, however,
which are due to qualities in the germinal material itself are
inherited, and certain of them with remarkable persistence. There are
instances in which the slight malformation consisting in an excess of
fingers or toes has persisted through many generations. It may
occasionally lapse in a generation to reappear later. In certain
cases, notably in the bleeders, the inheritance is transmitted by the
female alone, in other cases by the sexes equally, but there are no
cases of transmission by the male line only. It is evident that when
the same malformation affects both the male and the female line the
hereditary influence is much stronger. A case has been related to me
in which most of the inhabitants in a remote mountain valley in
Virginia where there has been much intermarriage have one of the
joints of the fingers missing. There is a very prevalent idea that in
close intermarriage in families variations and malformations often
unfortunate for the individual are more common. All experimental
evidence obtained by interbreeding of animals shows that close
interbreeding is not productive of variation, but that variations
existing in the breed become accentuated. Variations either
advantageous or disadvantageous for the race or individual may either
of them become more prevalent by close intermarriage. It seems,
however, to have been shown by the customs of the human race that very
close intermarriage is disadvantageous.

Eugenics, which signifies an attempt at the betterment of the race by
the avoidance of bad heredity, has within recent years attracted much
attention and is of importance. Some of its advocates have become so
enthusiastic as to believe that it will be possible to breed men as
cattle and ultimately to produce a race ideally perfect. It is true
that by careful selection and regulation of marriage certain
variations, whether relating to coarse bodily form or to the less
obvious changes denoted by function, can be perpetuated and
strengthened. That the Semitic race excels in commerce is probably due
to the fact that the variation of the brain which affected favorably
the mental action conducive to this form of activity, was favorable
for the race in the hostile environment in which it was usually placed
and transmitted and strengthened by close intermarriage. It is
impossible, however, to form a conception of what may be regarded as
an ideal type of the human species. The type which might be ideal in a
certain environment might not be ideal in another, and environment is
probably of equal importance with the material. The eugenics movement
has enormously stimulated research into heredity by the methods both
of animal experimentation and observation, and study of heredity in
man. As in all of the beginning sciences there is not the close
inter-relation of observed facts and theory, but there is excess of
theory and dearth of facts. Certain considerations, however, seem to
be evident. It would seem to be evident that individuals should be
healthy and enabled to maintain themselves in the environment in which
they are placed, but the qualities which may enable an individual
successfully to adapt himself to factory life, or life in the crowds
and strong competition of the city, may not be, and probably are not,
qualities which are good for the race in general or for his immediate
descendants. At present our attempts to influence heredity should be
limited to the heredity of disease only. We can certainly say that
intermarriage between persons who have tuberculosis or in whose
families the disease has prevailed is disadvantageous for the
offspring; the same holds true for insanity and for nervous diseases
of all sorts, for forms of criminality, for alcoholism, and for those
diseases which are long enduring and transmitted by sexual contact
such as syphilis and gonorrhoea. It is of importance that the facts
bearing on the hereditary transmission of disease should become of
general knowledge, in order that the dangers may be known and
voluntarily avoided. No measures of preventive medicine are successful
which are not supported by a public educated to appreciate their
importance, and the same holds true of eugenics. How successful will
be public measures leading to the prevention of offspring in the
obviously unfit by sterilization of both males and females is
uncertain. It is doubtful whether public sentiment at the present time
will allow the measure to be thoroughly carried out. Some results in
preventing unfit heredity may be attained by the greater extension of
asylum life, but the additional burden of this upon the labor of the
people would be difficult to bear. At best such measures would only be
carried out in the lower class of society.



Chronic diseases are diseases of long duration and which do not tend
to result in complete recovery; in certain cases a cause of disease
persists in the body producing constant damage, or in the course of
disease some organ or organs of the body are damaged beyond the
capacity of repair, and the imperfect action of such damaged organs
interferes with the harmonious inter-relation of organs and the
general well-being of the body. The effect of damage in producing
chronic disease may not appear at once, for the great power of
adaptation of organs and the exercise of reserve force may for a time
render the damage imperceptible; when, however, age or the
supervention of further injury diminishes the power of adaptation the
condition of disease becomes evident. Chronic disease may be caused by
parasites when the relation between host and parasite is not in high
degree inimical, as in tuberculosis, gonorrhoea, syphilis, most of the
trypanosome diseases and the diseases produced by the higher
parasites. In certain cases the chronic disease represents really a
series of acute onsets; thus in the case of the parasites there may be
periods of complete quiescence of infection but not recovery, the
parasites remaining in the body and attacking when the defences of the
body are in some way weakened. In such cases there may be temporary
immunity produced by each excursion of the disease, but the immunity
is not permanent nor is the parasite destroyed. There is a further
connection between chronic disease and infection in that the damage to
the organs, which is the great factor underlying chronic disease, is
so often the result of an infection.

The infectious diseases are those of early life; chronic disease, on
the other hand, is most common in the latter third of life. This is
due to the fact that in consequence of the general wear of the body
this becomes less resistant, less capable of adaptation, and organic
injury, which in the younger individual would be in some way
compensated for, becomes operative. The territory of chronic disease
is so vast that not even a superficial review of the diseases coming
under this category can be attempted in the limits of this book, and
it will be best to give single examples only, for the same general
principles apply to all. One of the best examples is given in chronic
disease of the heart.

The heart is a hollow organ forming a part of the blood vascular
system and serving to give motion to the blood within the vessels by
the contraction of its strong muscular walls. It is essentially a
pump, and, as in a pump, the direction which the fluid takes when
forced out of its cavity by the contraction of the walls diminishing
or closing the cavity space, is determined by valves. The contraction
of the heart, which takes place seventy to eighty times in a minute,
is automatic and is due to the essential quality of the muscle which
composes it. The character, frequency and force of contraction,
however, can be influenced by the nervous system and by the direct
action of substances upon the heart muscle. The heart is divided by a
longitudinal partition into a right and left cavity, and these
cavities are divided by transverse septa, with openings in them
controlled by valves, each into two chambers termed _auricle_ and
_ventricle_. The auricle and ventricle on each side are
completely separated.

The circulation of the blood through the heart is as follows: The
blood, which in the veins of the body is flowing towards the heart,
passes by two channels, which respectively receive the blood from the
upper and lower part of the body, into the right auricle. When this
becomes distended it contracts, forcing the blood into the right
ventricle; the ventricle then contracts and sends the blood into the
arteries of the lungs, the passage of blood into the auricle being
prevented by valves which close the opening between auricle and
ventricle when the latter contracts upon its contents. When the
ventricle empties by its contraction the wall relaxes and the back
flow from the artery is prevented by crescentic-shaped valves placed
where the artery joins the ventricle. A similar arrangement of valves
is on the left side of the heart. The pressure given the blood by the
contraction of the right ventricle sends it through the lungs; from
these, after it has been oxygenated, it passes into the left auricle,
then into the left ventricle and from this into the great artery of
the body, the aorta, which gives off branches supplying the
capillaries of all parts of the body. Both of the auricles and both of
the ventricles contract at the same, time, the ventricular contraction
following closely upon the contraction of the auricles. Contraction or
systole is followed by a pause or diastole during which the blood
flows from the veins into the auricles. The work which the right
ventricle accomplishes is very much less than that of the left, and
the right ventricle has a correspondingly thinner wall. The size of
the heart is influenced by the size and the occupation of the
individual being larger in the large individual than in the small, and
larger in the active and vigorous than in the inactive. Generally
speaking, the heart is about as large as the closed fist of its

Imperfections of the heart which interfere with its action may be the
result of failure of development or disease. An imperfect heart which
can, however, fully meet the limited demands made upon it in
intra-uterine life, may be incapable of the work placed upon it in
extra-uterine life. Children with imperfectly formed hearts may be
otherwise perfect at birth, but they have a bluish color due to the
imperfect supply of the blood with oxygen, and are known as blue
babies. The condition becomes progressively worse due to the
progressive demands made upon the heart, and death takes place after
some days or months or years, the time depending upon the degree of
the imperfection.

Much of the damage of the heart in later life is due to infection. The
valves of the heart are a favorite place for attack by certain sorts
of bacteria which get into the blood. This is due to the prominent
position of the valves which brings them in contact with all the blood
in the body, the large extent and unevenness of the surface and to the
rubbing together and contact of their edges when closed. At the site
of infection there is a slight destruction of tissue and on this the
blood clots producing rough wart-like projections. The valves in some
cases are to a greater or less extent destroyed, they may become
greatly thickened and by the deposit of lime salts converted into
hard, stony masses. Essentially two conditions are produced. In one
the thickened, unyielding valves project across the openings they
should guard, and thus by constricting the opening interfere with the
passage of blood either through the heart or from it. In the other the
valves are so damaged that they cannot properly close the orifices
they guard, and on or after the contraction of the cavities there is
back flow or regurgitation of the blood. If, for instance, the orifice
of the heart into the aorta is narrowed, then the left ventricle can
only accomplish its work of projecting into the aorta a given amount
of blood in a given time by contracting with greater force and giving
a greater rapidity to the stream passing through the narrow orifice.
This the heart can do because, like all other organs of the body, it
has a large reserve force which enables it, even suddenly, to meet
demands double the usual, and like all other muscles of the body it
becomes larger and stronger by increased work. The condition here is
much simpler than when the same valve is incapable of perfect closure,
or when both obstruction and imperfect closure, are combined as they
not infrequently are. In such cases the ventricle must do more than in
the first case. It must force through the orifice, which may be
narrowed, the amount of blood which is necessary to keep up the
pressure within the aorta and give to the circulation the necessary
rapidity of flow, and also the amount which flows back into the heart
through the imperfectly acting valve. This it can do by contracting
with greater force upon a larger amount of blood, the cavity becoming
enlarged to receive this. Not only may such damage to the valves be
produced, but the muscular tissue of the heart may suffer from
defective nutrition or from the effect of poisons, whether these are
formed in the body as the effect of disease or introduced from
without; or in consequence of disease in the lungs the flow of blood
through them may be impeded, or disease elsewhere in the body, as in
the kidneys may, by increasing the pressure of the blood within the
arteries, throw more than the usual amount of work upon the heart.

The power of the heart in meeting these conditions, however various
they are and however variously they act, seems little short of
marvellous, and it goes on throwing three and one-third ounces of
blood seventy or eighty times a minute into a tube against nine feet
of water pressure, working often perfectly under conditions which
would be fatal to a machine. As long as this goes on the injury is
said to be compensated for; the increased work which the heart is able
to accomplish by the exercise of its reserve force and by becoming
larger and stronger enables it to cope with the adverse conditions.
With increased demand for work there is a gradual diminution of the
reserve force. An individual may be able to carry easily forty pounds
up a hill and by exerting all his force may carry eighty pounds, but
if he habitually carries the eighty pounds, even though the muscles
become stronger by exercise the load cannot be again doubled. The
dilatation of the heart which is so important in compensation is
fraught with danger, because any weakening of the muscle increases the
dilatation, until a point is reached when, owing to the dilatation of
the orifices between auricles and ventricles, the valves become
incompetent to close them.

When the heart is not able to accomplish its work, the effect of the
condition becomes apparent by the accumulation of blood within the
veins and a less active circulation. This affects the nutrition and
the capacity for work of all the organs of the body, and the imperfect
function of the organs may in a variety of ways make still greater
demands upon an already overloaded heart. Other conditions supervene.
The increased pressure within the veins and capillaries due to the
impossibility of the blood in the usual amount passing through or from
the heart increases the amount of fluid in the tissues. There is
always an interchange between the blood within the vessels and the
fluid outside of them; the passage of fluid from the vessels is
facilitated by the increased pressure within them, just as pressure
upon a filtering fluid increases the rapidity of filtration, and the
increase of pressure within veins and capillaries impedes passage of
tissue fluid into them. The fluid accumulates within the tissues
leading to dropsy, or the accumulation may take place in some of the
cavities of the body. The diminished flow of blood through the lungs
prevents its proper oxygenation; this may also be interfered with by
the accumulation of fluid within the air spaces of the lungs.

Every additional burden thrown upon the heart increases the evil. In
women the additional burden of pregnancy may suffice to overcome a
compensation which has been perfect, and the same may result from an
acute attack of disease. Age, diminishing as it does the capacity for
work in all organs, diminishes the compensation capacity of the heart,
and a heart which at the age of forty acts perfectly may break down at
the age of fifty. Compensation may be gained in other ways, as by
reducing the demand made upon the heart by changing the mode of life,
by leading an inactive rather than an active life, by avoiding
excitement or any condition which entails work of the heart. Social
conditions are of great importance; it makes a great difference
whether the unfortunate possessor of such a heart be a stevedore whose
capital lies in the strength of his muscles, or a more fortunately
placed member of society for whom the stevedore works and whose
occupation or lack of occupation does not interfere with the
adjustment of his external relations to the condition of his heart.

Disease of the nervous system does not differ from disease elsewhere.
The system is complex in structure and in function. It consists in
nerves which are composed of very fine fibrils distributed in all
parts of the body and serve the purpose of conduction, and a central
body composed of the brain and spinal cord which is largely cellular
in character; it receives impressions by means of the nerves and sends
out impulses which produce or affect action in all parts. By means of
the organs of special sense, the brain receives impressions from the
outer world which it transforms into the concepts of consciousness.
Many of the impressions which the central nervous system receives from
nerves other than those of special sense and even many of the
impressions from these and the impulses which it sends out do not
affect consciousness. The memory faculty is seated in the brain and
all parts of the brain are closely connected by means of small nerve
fibres. The nervous system plays an important part in the internal
regulation and coordination of all parts of the body, and it is by
means of this that the general adjustment of man with his environment
is effected.

Malformations of the brain, except very gross conditions which are
incompatible with extra-uterine existence, are not very common. At
birth those parts of the brain which are the seat of memory and what
are understood as the higher faculties are very imperfectly developed.
Variations in structure are extremely common, there are differences in
different individuals in the nerves and in the number, size, form and
arrangement of the nerve cells, and so complex is the structure that
considerable variation can exist without detection. The tissue of the
central nervous system has a considerable degree of resistance to the
action of bacteria, but is, however, very susceptible to injury by
means of poisons. Serious injury or destruction of tissue of the brain
and spinal cord is never regenerated or repaired, but adjustment to
such conditions may be effected by reciprocity of function, other
cells taking up the functions of those which were destroyed.

Certain parts of the brain are associated with definite functions;
thus, there are areas which influence or control speech and motion of
parts as the arm or leg, and there are large areas known as the silent
areas whose function we do not know. All activity of the central
nervous system, however expressed, is due to cell activity and is
associated with consumption of cell material which is renewed in
periods of repose and sleep. Fig. 13 shows a nerve cell of a sparrow
at the end of a day's activity and the same after the repose of a

Diseases of the nervous system have a special interest in that they so
often interfere with man in his relations with his fellows. In
diseases of other organs the disturbances set up concern the
individual only. Thus, others need not be disturbed save by the
demands made on their sympathies by an individual with a cold in the
head or a cancer of the stomach. Disease of the nervous system is
another affair, instead of those reactions and expressions of activity
to which we are accustomed and to which society is adjusted, the
reactions and activities are unusual and the individual in consequence
does not fit into the social state and is said to be anti-social.
There are all possible grades of this, from mere unpleasantness in the
social relations with such an individual, to states in which he is
dangerous to society and must be isolated from it. Insanity is an
extreme case. There is no disease signified in the expression, but it
is merely a legal term to designate those individuals whose actions
are opposed to the social state and who are not responsible for them.
In insanity there is falsity in impressions, in conceptions, in
judgment, a defective power of will and an uncontrollable violence of
emotion. The individual is prevented from thinking the thoughts or
feeling the feelings and doing the duties of the social body in the
community in which he lives. The insane are out of harmony with their
social environment, but not necessarily in opposition to it.

There is no very sharp line between insanity and criminality. The
criminal is in direct antagonism to the laws of social life. An insane
person may cause the same injury to society as a criminal, but his
actions are not voluntary, whereas the criminal is one who can control
his actions, but does not. Mentally degenerated persons, however, can
be both insane and criminal. Whatever the state of society, this
reprobates the actions of one opposed to it; in a society in which it
were usual to appropriate the possessions of others or to devour
unpleasant or useless relatives, virtue and lack of appetite would be
reprobated as unsocial.

The symptoms of insanity or the manner in which the defective action
of the brain expresses itself and the various underlying pathological
changes vary, and by combining these it has been possible to subdivide
insanity into a number of distinct forms. There are both intrinsic and
extrinsic causes of insanity. The intrinsic are the structural
differences in the brain as compared with the normal or usual, whether
these are due to imperfection in development or to defective heredity
or to the injury of disease; the extrinsic causes are those which come
from without and bring the intrinsic into activity. Syphilis is a
frequent cause of insanity, and probably the only cause of the
condition known as general paralysis of the insane, acting by means of
the injury which it produces in the cortex of the brain. The abuse of
alcohol is another fertile cause, but the changes produced in this are
not so obvious as in the case of syphilis. Tumors of the brain are not
infrequently a cause, and the same is true of infections, even those
not located in the brain. How susceptible the brain is to the effects
of the toxines of the infectious diseases is shown in the frequency of
delirium in these diseases. There is an interesting relation between
this and alcoholism. Alcohol abuse may produce injury, but not
sufficient to manifest itself under ordinary conditions; when,
however, the action of toxic substance is superadded to the effect of
the alcohol the delirium of fever is more marked.

Probably of greater importance than the acquired pathological
conditions of the brain in producing insanity is a congenital
condition in which the nervous system is defective. The most fertile
cause of insanity lies in the inheritance; by this it must not be
understood that insane parents produce insane offsprings, but that
conditions inherited from immediate or remote ancestors appear in a
diminished resistance of the nervous system which is sooner or later
expressed as insanity. Given such a defective nervous system,
extrinsic conditions which would have no effect on another individual
or would be felt in different ways may produce insanity. In these
cases occupation plays a great role. The excitement and privations of
war especially in the tropics and the ennui of camps leads to insanity
in soldiers; occupations such as that of the baker in which there is
loss of sleep and the mental strain of students can all act in the
same way. A woman who gives no sign of nervous defect may become
insane under the strain of pregnancy.

Although insanity is determined by the social relations of man, that
part of the social organization which is termed _Society_, and
which has been developed by the idle as a diverting game, is a fertile
source of nervous disease and even of insanity, affecting particularly
females. The strenuosity of the life, the nervous excitement
alternating with ennui, the lack and improper times of sleep, the lack
of rest and particularly of restful occupation, the not infrequent use
of alcohol in injurious amounts, are all factors calculated to make a
defect operative. The so-called "coming out" of young girls is an
important element in the game, and their headlong plunge into such a
life at a period under any conditions full of danger to the nervous
system is especially to be reprobated. If we consider the influence of
the game in other respects as conducing to lack of moral sense, to
alcoholic abuse (for without the seeming stimulation, but which is
really the blunting of impressions which alcohol brings, the game
would not be possible), to discontent, to mental enfeeblement, it is
all bad. Curiously enough the game is one which in all periods has
been played by the idle, but its evil influence is greater now than
before when it was the game of royalty chiefly, because there are now
more people living from the work of others.

The unusual mental action of the insane not infrequently expresses
itself by suicide. The analysis of three hundred deaths from suicide
showed pathological changes in the brain in forty-three per cent, and
when we think that mental disturbances are very often without
recognizable anatomical changes after death, the percentage is very
large. In another analysis of one hundred and twenty-four suicides
forty-four of these were mentally affected to various degrees. Five of
the men and seven women were epileptics, in ten of the families there
was hysteria, twenty-four of the men and four of the women were
chronic alcoholics.

It is extremely difficult at the present time to say whether insanity
is increasing. Statistics in all lands giving the numbers committed to
insane hospitals show on their face a great increase, but so many
factors enter into these statistics that their value is uncertain.
There is now an ever-increasing provision for the care of the insane.
Owing to the recognition of insanity as a part of nervous disease and
its separation from criminality there is no longer the same attempt to
conceal it as was formerly the case, and hospitals for the insane are
no longer associated with ideas of Bedlam. It is generally believed
that modern conditions in the hurry and excitement of life, and the
extreme social differences, the greater urban life, the greater
extension of factory life, all tend to an increase in insanity, but
there is no absolute proof that this is true. We know very little
about insanity in the Middle Ages, but the conditions then were not
conducive to a quiet life. There prevailed then as now excess and
want, luxury and poverty, enjoyment and deprivations, balls and dinner
parties and other features of the social game. There were factions in
the cities, public executions, not infrequent sieges, scenes of
horror, epidemics, famines, and all these combined with religious
superstition and the often unjust and cruel laws should have been
factors for insanity. There were actual epidemics of insanity
affecting masses of the population, as shown in the children's
crusade, the Jewish massacres and the dancing mania in the Rhine
provinces. Where civilization seems to be the highest, statistics show
the most insane, but this most probably depends upon better
recognition of the condition and better provision for asylum care.

The so-called functional diseases have a close relation with diseases
of the nervous system, for they chiefly concern the reactions of nerve
tissue. Disease expressing itself in disturbance of function only,
does not seem to fit in with the conceptions of disease which have
been expressed, nor can we imagine a disturbance of function which
does not depend upon a change of material. Living matter does not
differ intrinsically from any other sort of matter; like other matter
its reactions depend upon its composition structure[1] and the
character of the action exerted upon it. By functional disease there
is expressed merely that no anatomical or chemical change is
discoverable in the material which gives the unusual reaction. The
further our researches into the nature of disease extend, particularly
the researches into the physiology and chemistry of disease, the
smaller is the area of functional disease. In functional disease there
may be either vague discomfort or actual pain under conditions when
usually such would not be experienced, and on examination no condition
is found which in the vast majority of cases would alone give rise to
that impression on the nervous system which is interpreted as pain. In
the production of the sensations of disease there can be change at any
place along the line, in the sense organs, in the conducting paths or
in the central organ. Thus there may be false visual impressions which
may be due to changes in the retina or in the optic nerve or in the
brain matter to which the nerve is distributed. It is perfectly
possible that substances of an unusual character or an excess or
deficiency of usual substances in the fluids around brain cells may so
change them that such unusual reactions appear. There may be, of
course, very marked individual susceptibility, which may be congenital
or acquired. The perception of every stimulus involves activity of the
nerve cells, and it is possible that the constant repetition of
stimuli of an ordinary character may produce sufficient change to give
rise to unusual reactions, and this particularly when there is lack of
the restoration which repose and sleep bring. We know into what a
condition one's nervous system may be thrown by the incessant noise
attending the erection of a building in the vicinity of one's house or
the pounding of a plumber working within the house, this being
accentuated in the latter case by the thought of impending financial
disaster. Even the confused and disagreeable sound due to the clatter
of high-pitched women's voices at teas and receptions may, when
frequently repeated, be productive of changes in the nerve cells
sufficiently marked to give rise to the unusual reactions which are
evidence of disease.

In the condition known as neurasthenia, which is often taken as a type
of a functional disease, the basal and intrinsic cause is activity of
the nervous system with the using up of material which is not
compensated for by the renewal which comes in repose and sleep.
Neurasthenia is one of the common conditions of our civilization,
found among children and adults, the poor and rich, the idle and the
factory worker; it is rife in the scholastic professions and among
those who earn their living by brain work. It seems to be more common
in the upper classes and particularly in the women, but this is
because these are more subject to medical care and the condition is
more in evidence. There are all sorts of symptoms attached to the
condition, for the unusual mental action can be variously expressed.
The cerebral form has been thus described by a well-known medical
writer: "One of the most characteristic features of cerebral
neurasthenia is a weary brain. The sensation is familiar enough to any
fagged man, especially if he fall short of sleep. Impressions seem to
go half into one's head and there sink into a woolly bed and die.
Voices sound far off, the lines of a book run into one another and the
meaning of them passes unperceived. Doors bang and windows rattle as
they never did before; if a shoestring breaks, an imprecation is upon
the lips. Business matters are in a conspiracy to go wrong. Letters
are left unopened partly from want of will, partly from a senseless
dread lest they contain bad news. At night the patient tosses on his
bed possessed by all the cares which blacken with darkness. Headache
is common, loss of memory is distressing, and in severe cases it is
wider and deeper than mere inattention can explain. There is often the
torture of acute hearing, or an inability to suppress attention; the
hater of clocks and crowing cocks is a neurasthenic." The disease is
especially common in the women players of the social game, and its
unhappy victims too often seek relief from the nervous irritability
which is a common early symptom in still greater nervous excitement.
It is a sad commentary on our civilization that one of the means of
treatment for these persons which has been found efficacious is to
supply them with some restful household occupation such as knitting or
plain sewing, and there are institutions which combine refuge from
social activities, often called duties, with simple occupation.

[1] By structure as used in this wide sense, there must be
understood not merely the anatomical structure, which is revealed by
the dissecting knife and microscope, but molecular structure, or the
manner in which elements are arranged to form the molecule, as well.



Certain conditions have arisen in the past fifty years which have
profoundly affected the thoughts, the beliefs and the activities of
man. Within this period what is generally known as Darwinism,
including under this evolution, has developed. Unlike theories which
came from philosophical speculation only, the theory of evolution was
one which could be subjected to observation and experiment. It freed
man's mind from dogmas, it stimulated the imagination, it enlarged the
territory in which it seemed possible to extend knowledge by the
methods of science, and has resulted in an enormous increase of
knowledge. This has been more striking in medical science than
elsewhere, and in this of more far-reaching influence. Evolution
coincided with another important development. History shows that all
great periods of civilization have at their back sources of energy. In
the civilizations of the past such sources of energy have come from
the enslavement of conquered peoples or from commerce, or more direct
forms of robbery, which have enabled a favored class to appropriate
for its purposes the results of the work of others. While these
sources have not been absent in the development of our civilization,
the great source of energy has come from the rapid, and usually
wasteful and reckless, utilization of the stored energy of the earth.
The almost incredible advance in medical and other forms of scientific
knowledge and the utilization of this knowledge is largely due to the
greater forces which we have become possessed of.

Disease plays such a large part in the life of man and is so closely
related to all of his activities that the changes in this period must
have exerted an influence on disease. We have already seen that within
the period we have obtained knowledge of the causes of disease and the
conditions under which these causes became operative. The mystery
which formerly enveloped disease is gone; disease is recognized as due
to conditions which for the most part are within the control of man,
and like gravity and chemical attraction it follows the operation of
definite laws. There has been developed within the period what is
known as preventive medicine, which aims rather at prevention than
cure, and the resources of prevention are capable of much greater

Have there been new conditions developed within the period, or an
increase of existing conditions which can be regarded as disease
factors and which counterbalance the results which have come from the
knowledge of prevention and cure? There has been an increase of
certain factors of immense importance in the extension of disease.
These are:

1. The increase in industrialism, involving as this does an increase
in factory life. In many ways this is a factor in disease. (_a_) By
favoring the extension of infection, particularly in such diseases as
tuberculosis. (_b_) The life indoors, and frequently with the
combination of insufficient air and space, produces a condition of
malnutrition and deficient general resistance. (_c_) The family life
is interfered with by the mothers, whose primary duty is the care of
home and children, working in factories, and the too frequent
conversion of the house into a factory. (_d_) The influence of factory
life is towards a loss of moral stamina rendering more easy of
operation the conditions of alcoholism and general immorality. How
great has been this increase in industrialism, fostered as it has been
by conditions both natural and artificially created by unwise
legislation, is shown in the figures from the last census. The number
of factory operatives increased forty per cent between 1899 and 1909
and the total population of the country in the period between 1900 and
1910 increased twenty per cent. It is probable that the future will
see an extension rather than a diminution of mass labor.

2. The increase in urban life is as conspicuous as the increase in
industrialism. In 1880, twenty-nine and five-tenths per cent of the
population was urban and seventy and five-tenths per cent was rural;
in 1910, forty-six and three-tenths per cent was urban and fifty-three
and seven-tenths was rural, the increase being most marked in cities
of over five hundred thousand inhabitants. Of the total increase in
population between 1900 and 1910, seven-tenths per cent was in the
cities and three-tenths per cent in the country. City life in itself
is not necessarily unhealthy and there are many advantages associated
with it. The conditions which have chiefly fostered it are the
immigration of people who are accustomed to community life, the
increase in factory life and the increased number of people of wealth
who seek the advantages which the city gives them. The city has always
been the favored playground for the social game. The unhealthy
conditions of city life are due to the crowding, the more uncertain
means of livelihood, the greater influence of vice and alcoholism.
Prostitution and the sexual diseases are almost the prerogatives of
the cities.

3. All means of transportation have increased and communication
between peoples has become more extended and more rapid. In the past
isolation was one of the safeguards of the people against disease.
With the increase and greater rapidity of communication there is a
tendency not only to loss of individuality in nations as expressed in
dress, customs, traditions and beliefs, but many diseases are no
longer so strictly local as formerly--pellagra, for example. Only
those diseases which are transmitted by insects which have a strictly
local habitat remain endemic, although the region of endemic
prevalence may become greatly extended, as is seen in the distribution
of sleeping sickness. Diseases of plants and of animals have become
disseminated. Any plants desirable for economic use or for beauty of
foliage and flower become generally distributed, their parasites are
removed from the regions where harmonious parasitic inter-relations
have been established, and in new regions the parasites may not find
the former restrictions to their growth. There have been many examples
of this, such as the ravages of the brown-tail and gypsy moths which
were introduced into New England and of the San Jose scale which was
introduced into California. There have been many other examples of the
almost incredible power of multiplication of an animal or plant when
taken into a new environment, removed from conditions which held it in
check, as the introduction of the mongoose into Jamaica, the rabbit
into Australia, the thistle into New South Wales and the water-plant
chara into England.

It is very difficult to say, but it seems as though there is an
increasing unevenness in the distribution of wealth, an increase in
the number of persons who live at the expense of the laboring class.
Mass labor, effective though it be, makes it easier to divert the
proceeds of labor from the laborers. The evidence of this is seen in
the increase in number and the prosperity of those pursuits which
purvey to luxury, as the automobile industry and the florists' trade
and the greatly increased scope and activity of the social game. On
the other hand, there is an increase in the number of people who are
to a greater or less extent dependent upon extraneous aid, evinced
among other ways by the increase in the asylum populations. Both these
conditions, wealth and poverty, are important disease factors.
Tuberculosis is now a disease of the proletariat chiefly. The measures
both of prevention and cure can be and are carried out by the
well-to-do, but the disease must remain where there are the conditions
of the slums. Of all the conditions favoring infant mortality poverty
comes first. In Erfurt, a small city of Germany, of one thousand
infants born in each of the different classes, there died of the
illegitimate children three hundren and fifty-two; of those of the
laboring class, three hundred and five; of those in the medium station
(official class largely), one hundred and seventy-three; of those in
higher station, eighty-nine. The same relation of infant mortality to
poverty becomes apparent when estimated in other ways. In Berlin, with
an average infant mortality of one hundred and ninety-six per
thousand, the deaths in the best districts of the city were fifty-two
and in the poorer quarters four hundred and twenty. The effect of
poverty is seen particularly in the bottle-fed infants; with natural
nursing the child of poverty has almost as good a chance as the child
of wealth. From reasons which are almost self-evident, the mortality
in illegitimate infants is almost double that of the legitimate. The
greater infant mortality in poverty is due to the more numerous
children preventing individual care, the separation of the mother from
the nursing child in consequence of the demand made upon her earning
capacity, and the decline in breast nursing. Wealth is on the whole
more advantageous from the narrow point of view of disease than is
poverty, but if we regard its influence on the race its advantages are
not so evident. Nothing can be worse for a race than that it should
die out, and wealthy families have never reproduced themselves.
Conditions always tending to destruction are a necessary part of the
environment of poverty; wealth voluntarily creates these conditions,
and chiefly by the pernicious influence of its amusements on the

A new and in many respects a nobler conception of medicine has been
developed. Formerly medical practice was almost exclusively a personal
service to the sick individual, and measures looking toward the
general relief of disease and its prevention received scanty
consideration. The idea of a wider service to the city, to the state,
to the nation, to humanity rather than the personal service to the
individual, is becoming dominant in medicine. This is seen in the
establishment of laboratories by boards of health in cities and states
in which knowledge obtained by exact investigations can be made of
direct service to the people; in the medical inspection of schools and
factories; in promulgating laws directed against conditions which
affect health, in the extension of hospitals, and in divers other
ways. The idea of public service and of returning to the people in an
effective way some of the results of their labor also underlies the
large donations which have been given for the creation of special
laboratories and institutes in which, through research, greater
knowledge of disease may be obtained and made available. The
researches which have been made on the nutrition of man and the
nutritive value of different foods are of great importance, and this
knowledge has not yet begun to be applied as it should be.

There seems to be a balance maintained between the restriction of
disease by prevention and the increased influence of social conditions
which are in themselves factors of disease. Preventive medicine seems
to have made possible, by restricting their harmful influence, the
increase in industrialism, in urban life, and in the
intercommunications of peoples. The most important aid in the future
to the influence of preventive medicine must be the education of the
people so that the conditions of disease, the intrinsic and the
extrinsic causes and the manner in which these act, shall all become a
part of general knowledge, and the sympathy of the people with health
legislation and their active assistance in carrying out measures of
prevention may be obtained. The effect of social conditions on disease
must become more generally recognized.


ATROPHY--A condition of imperfect nutrition producing diminution in
size and loss of function of parts.

BERTILLON--A French anthropologist who devised a system of
measurements of the human body for purposes of identification.

BLOOD-PLASMA--The fluid of the blood.

CELL--The unit of living matter. Living things may be unicellular or
composed of a multitude of cells which are interdependent. The
general mass of material forming the cell is termed cytoplasm. In this
there is a differentiated area termed nucleus which governs the
multiplication of cells. In the nucleus is a material termed chromatin
which bears the factors of heredity.

CHEMOTROPISM--The influence of chemical substances in directing the
movement of organisms.

EXUDATE--The material which passes from the blood into an injured part
and causes the swelling.

FIBRIN--The gelatinous material formed in the blood when it clots.

HÆMOGLOBIN--A substance which gives the red color to the blood; by
means of its ready combination with the oxygen of the air in the lungs
this necessary element is carried to all parts of the body.

INFLAMMATION--Literally a "burning"; the changes which take place in a
part after injury.

LYMPH--The fluid which is contained in the lymphatic vessels--nodes.
Circumscribed masses of cells connected with the lymphatic vessels.

OSMOSIS--The process of diffusion between fluids of different
molecular pressures.

SPORE FORMATION--A mode of reproduction in lower forms of life by
which resistant bodies, _spores_, are formed. These have many
analogies with the seed of higher plants.

SYMBIOSIS--A mutual adaptation between parasite and host.

TRANSUDATION--The normal interchange of fluid between the blood and
the tissue fluids. The material interchanged is the transudate.

TROPISM--The influence of forces which direct the movement of cells.

ULTRA-MICROSCOPE--A form of microscope which by means of oblique
illumination renders visible objects so small as to be invisible with
the ordinary microscope.

VIRUS--A substance either living or formed by living things which may
cause disease.


Amoeba, 13

Anthrax, 109

Antitoxin, 154

Bacteria, 116
 adaptation in, 123
  ærobic, 122
  anærobic, 122
  artificial cultivation of, 119
  distribution in nature, 121
  growth and reproduction, 118
  mode of action in disease, 144
  size, 117
  spore formation, 118
  substances affecting growth of, 123
  toxin production by, 144
  variations in, 123

Blood, 35
  circulation of, 33, 80
  vessels, 32

Body, 22
  defenses of, 146
  organs of, 28
  reserve force of, 50
  surfaces of, 22

Brain, 31

Cerebro-spinal meningitis, 188

Chemotropism, 93

Cretinism, 37

Darwinism, 240

Death, 57
  decomposition after, 51
  rigor after, 60
  signs of, 59

Disease, 1
  action of poisons, 44
  acute and chronic, 219
  industrialism as factor in, 243
  lesions of, 46
  superstitions concerning, 10
  urban life as factor in, 244
  wealth and poverty as factors in, 246

Ductless glands, 37

Embryo, 77

Epilepsy, 209

Eugenics, 215

Foetus, 32
  infection of, 200

Foot and Mouth Disease, 129

Glands, 22

Growth, 62

Heart, 33, 221
  disease of, 223

Heliotropism, 93

Heredity, 197
  influence of alcohol, 206
  of insanity, 209
  variations and imitations, 204

Hookworm disease, 179

Immunity, 148
  theories of, 149
  natural, 150

Infection, 135
  from external surface, 136
  from genito-urinary surface, 137
  from lungs, 138
  from mouth, 138
  from stomach and intestines, 139
  from wounds, 141
  in children, 195
  in wild animals, 191
  latent, 166
  mixed, 160
  racial susceptibility to, 191
  resistance to, 143
  by air, 170
  by insects, 171

Infectious diseases, 97
  carriers of, 185
  comparison with fermentation, 108
  epidemics of, 98
  endemic, epidemic and sporadic forms, 188
  modes of transmission, 161

Inflammation, 80
 acute and chronic, 95

Injury, 54-74

Insanity, 231
  causes of, 232
  question of increase, 235

Lesion, 17

Leucocytes, 36
  migration of, 92

Living matter, 10

Malaria, 175
  rôle of mosquito in transmitting, 178

Malformations, 211
  heredity of, 215

Maternal impressions, 212

Nervous system, 228
  disease of, 230
  effect of social life on, 233

Neurasthenia, 238

Old age, 51
  atrophy in, 51
  blood vessels in, 54
  causes of death in, 56
  in animals and plants, 55
  mental activity in, 53

Osmosis, 91

Opsonius, 153

Ovum, 201
  fertilization of, 198
  infection of, 199

Phagocytosis, 86

Plague, 182
  transmission by animals, 183

Plasmodium Malariae, 175

Preventive medicine, 242

Protozoa, 124
  distribution in nature, 125
  mode of growth, 125
  sexual differentiation, 125
  spore formation, 125

Polyomyelitis, 190

Repair, 46
  conditions influencing, 47

Scar, 49

Skin, 21

Sleeping sickness, 173

Smallpox, 187

Spontaneous generation, 106

Sunburn, 83

Syphilis, 193

Tetanus, 142

Thymus, 52

Thyroid, 37

Tonsils, 52

Toxins, 144

Tropisms, 93

Trypanosomes, 172

Tuberculosis, 163
 infection by sputum, 169
 modes of extension, 163

Tumors, 64
  benign and malignant, 69
  cells of, 66
  color, size and shape, 65
  growth of, 65
  importance of, 77
  origin of, 66
  question of increase, 69
  theories of cause, 71
  treatment of, 77

Typhoid fever, 170

Ultra-microscopic organisms, 128

Virus, 128

Yellow fever, 178

*** End of this Doctrine Publishing Corporation Digital Book "Disease and Its Causes" ***

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