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ELEMENTS OF
PHYSIOLOGY

HOUGH AND SEDGWICK

ELEMENTS OF PHYSIOLOGY

BEING PART I OF "THE HUMAN MECHANISM

ITS PHYSIOLOGY AND HYGIENE AND

THE SANITATION OF ITS

SURROUNDINGS"

BY
THEODORE HOUGH

Professor of Physiology and Dean of the Department of
Medicine in the University of Virginia

AND

WILLIAM T. SEDGWICK

Professor of Biology and Public Health and Lecturer on Hygiene
and Sanitation, Massachusetts Institute of Technology

R E VIS K I) EDI 77 ON

GINN AND COMPANY

BOSTON • NEW YORK • CHICAGO • LONDON
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THEODORE HOUGH AND WILLIAM T. SEDGWICK

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PRIETORS • BOSTON • U.S.A.

PREFACE TO THE REVISED EDITION

This edition presents a thorough revision, in which the
authors have incorporated / those advances in physiology
which are directly applicable to the fundamental purpose
of this book as stated in the preface to the first edition.
Portions of certain chapters have been entirely rewritten, —
notably those dealing with the work of organs and cells,
internal secretions, digestion, nutrition, and the central
nervous system.

The advances of physiological knowledge in the past
decade have not only given clearer insight into the nature
of the processes which underlie the phenomena of living-
things but have also made the facts of physiology increas-
ingly helpful in the intelligent conduct of life. While this
latter point of view has chiefly determined the selection of
material to be included in this book, it has none the less
been necessary to lay the foundation for the understanding
of what we call the practical knowledge by a clear and suc-
cinct statement of the fundamental principles of physiology.
This first part of " The Human Mechanism " therefore serves
the purpose of those who cannot give the time necessary
for the more extensive study of physiology required of the
physician or the specialist in physiological science.

It is hoped that the interest aroused in the applications
of physiology to the conduct of life may lead many readers of
this book to the subsequent study of hygiene and sanitation.
The second part of " The Human Mechanism" has therefore
been published under the title "Hygiene and Sanitation."

We are indebted to Dr. E. P. Joslin for permission to
reproduce from his work on " The Treatment of Diabetes
Mellitus" the table on page 238.

p iii

518533

PREFACE

The present book is a reprint of the physiological por-
tion of our larger work entitled The Human Mechanism,
together with Chapter XVI (Drugs, Alcohol, and Tobacco),
which has been added to meet the requirements of law
in some states with regard to the teaching of physiology.
For those who desire in compact form the elements of
physiology as a part of general biological training, as an
introduction to the study of psychology, or for other spe-
cial purposes, and for those who, having undertaken the
study of hygiene and sanitation in Elements of Hygiene
and Sanitation (Part II of The Human Mechanism), desire
to acquaint themselves more fully with the fundamental
physiology, the present volume should prove useful.

The references to Part II have been retained in the text,
and apply either to The Human Mechanism or to Elements
of Hygiene and Sanitation.

CONTENTS

CHAPTER PAGE

I. THE HUMAN MECHANISM . , . . . 3

II. THE STRUCTURE (ANATOMY) OF THE HUMAN MECHANISM 6

III. THE FINER STRUCTURE OF Two TYPICAL ORGANS,

GLANDS AND MUSCLES. THE CONNECTIVE TISSUES.

THE LYMPHATIC SYSTEM 28

IV. THE ORGANS AND CELLS OF THE BODY AT WORK . . 43
V. WORK AND FATIGUE ....'...;,..... 55

VI. THE INTERDEPENDENCE OF ORGANS AND OF CELLS.

INTERNAL SECRETIONS. . . ...... . . J . 63

VII. THE ADJUSTMENT OR COORDINATION OF THE WORK OF

ORGANS AND CELLS ...,...*.;. 69

VIII. ALIMENTATION AND DIGESTION . . . . . . , . N . 91

The Supply of Matter and Power to the Human Machine 91

Digestion in the Mouth. Enzymes . 103

Digestion in the Stomach 107

Digestion and Absorption in the Small Intestine and in

the Large Intestine 117

IX. THE CIRCULATION OF THE BLOOD . . 135

Blood and Lymph . ' 135

Mechanics of the Circulation of the Blood and of the

# Flow of Lymph '. 139

The Adjustment of the Circulation to the Needs of

Everyday Life . 152

X. RESPIRATION 165

XL EXCRETION .............'... 180

XII. THERMAL PHENOMENA OF THE BODY 189

The Constant Temperature 189

The Regulation of the Body Temperature 199

1* vii

viii ELEMENTS OF PHYSIOLOGY

CHAPTER PAGE

XIII. NUTRITION 215

The Sources of Power and Heat for the Human

Mechanism 215

The Food Reserve of the Body. Fat. Glycogen. Cell

Proteins 222

Food as the Material for Growth, Repair, and the

Manufacture of Special Products of Cell Activity 229

The Proper Daily Intake of Protein 235

XIV. SENSE ORGANS AND SENSATIONS 240

XV. THE NERVOUS SYSTEM 263

Its Anatomical Basis 263

The Physiology of the Nervous System 269

XVL FOOD ACCESSORIES, DRUGS, ALCOHOL, AND TOBACCO . 286

INDEX . 309

THE HUMAN MECHANISM
PART I

ELEMENTS OF PHYSIOLOGY

PAET I

CHAPTER I
THE HUMAN MECHANISM

1. The human body a living organism. The human body,
as compared with bodies of water such as lakes and seas,
or with heavenly bodies such as the sun, moon, and stars,
is a small mass of matter weighing on the average, when
fully grown, about 150 Ib. and measuring in length about
5 ft. 9 in. It is neither very hot, as is the sun, nor warm
in summer and cold in winter, as are many bodies of
water, but in life and health has always almost exactly the
same moderate temperature, namely, 98.6° F. or 37.5° C. The
human body is not homogeneous, that is to say, alike in all
its parts, as is the substance of a lake, but consists of very
unlike parts — eyes, ears, legs, heart, brain, muscles, etc. —
these parts being known as organs, and the whole body, there-
fore, as the human organism.

The most remarkable peculiarity of the human body, how-
ever, is that it is a living organism. A watch has unlike parts
— spring, dial, hands, case, etc. — which are essentially its
organs, and the watch might therefore be called an organism;
yet it never is so called. We speak of a well-organized army,
navy, government, society, church, or school, but never of a
well-organized automobile, typewriter, printing press, or loco-
motive — apparently for the reason that in army, navy, . or
school living things play a principal part, while in mere
machinery life is wholly wanting. The highest compliment
we can pay to a machine is to say that it seems "almost

: MECHANISM

alive," but it is not a compliment to any human being to
describe him as " a mere machine." What the vital property
is, what we mean by the terms " life " and " living," no one
can exactly tell. About all we know of it is that some of the
commonest elements of matter (carbon, hydrogen, oxygen,
and nitrogen, with a little sulphur, phosphorus, and a few
other elements) frequently occur combined as living matter,
and that this living matter has marvelous powers of growth,
repair, and reproduction, besides a certain spontaneity, origi-
nality, and independence, which lifeless matter never displays.
" While there is life there is hope " for any plant or any ani-
mal, but this saying does not apply to any lifeless machine,
however complex or wonderful.

2. The human body a living machine or mechanism. By
a machine we mean an apparatus, either simple or complex,
and usually composed of unlike parts, by means of which
power received in one form is given out or applied in some
other form. This power may be received, for example, in the
form of heat, or electricity, or muscular effort, or as the poten-
tial energy of fuel ; and it may be given out as heat, or elec-
tricity, or light, or sound, or as mechanical work, or in any
one of many other ways. One of the simplest of all machines
is a stove, an apparatus composed of a few simple parts by
means of which the potential energy or power of fuel —
wood, coal, gas, or oil — is liberated and applied as heat, for
warming or cooking. A lamp is a still simpler machine in
which the potential energy or power of gas or oil is liberated
and converted into useful light. A candle is a lamp so
simple that it almost ceases to be a machine, and yet the
wick is really an apparatus for securing proper combustion
of wax or tallow to provide good light.

Machines of greater complexity are watches or clocks,
pieces of apparatus composed of many unlike parts which
receive power in comparatively large amounts for a short time
during the process of winding, store it as potential energy in

THE HUMAN MECHANISM 5

coiled springs or lifted weights, and liberate it slowly in the
mechanical work of moving the hands of the timepiece over
a dial. Still more complex is a locomotive or an automobile,
machines in which the power of coal, oil, gasoline, or other
fuel or the electricity of a storage battery is applied to swift
locomotion. But the most wonderful of all machines is the
human body, a complicated piece of apparatus in which the
power stored in foods, such as starch, sugar, butter, meat,
milk, eggs, and fish, is transformed into that heat by which
the body is warmed and into that muscular, nervous, diges-
tive, or other work which it performs.

For delicate and intricate machinery the term " mechan-
ism " is often employed, and we may therefore describe
the human body either as the " human organism," or the
" human machine," or, perhaps best of all, as the HUMAN

MECHANISM.

The study or the science of the construction (structure)
of this mechanism is called its anatomy; of its ordinary be-
havior, operation, or working, its physiology; of its proper
management, protection, and care, its hygiene. This textbook
is devoted chiefly to an account of its operation and care,
that is, to its physiology and hygiene ; but as any true com-
prehension of these subjects depends upon some preliminary
knowledge of the parts of the mechanism itself, we shall
begin by considering briefly the structure or anatomy of the
human machine.

CHAPTER II

THE STRUCTURE (ANATOMY) OF THE HUMAN
MECHANISM

Anatomy is studied partly by dissection, which reveals
chiefly those organs which are visible to the naked eye, and
partly by microscopic examination, which gives a deeper in-
sight into the detailed arrangement of the cells and tissues
of which the organs of the mechanism are composed. The
present chapter is devoted to structures or organs shown
by dissection — the gross anatomy of the body — as distin-
guished from its microscopic anatomy (histology).1

1 Further explanation of the structure of the human machine will be
given as it may be needed in subsequent chapters. At this point it is of
the utmost importance that the student thoroughly master the general
relations of the more important organs one to another ; this, however,
is not to be done by extensive reading, and still less by memorizing verbal
descriptions ; the aim should rather be to acquire from figures and dia-
grams, or better yet from actual dissection, where that is possible, a correct
mental picture of the structures involved. Far more can be learned by
constructing drawings or diagrams from memory than by the mere memo-
rizing of text. The drawings may lack finish and may be at first difficult
to execute ; but so long as they represent the relations of the organs one
to another they accomplish their purpose ; beyond this point the more
accurately they are drawn the better.

Moreover, drawing is a great aid to dissection. It not only fixes in the
memory what is seen but it compels close observation ; when one draws an
object he is forced to note details and relations of structure which would
otherwise escape observation. Nor is the freehand drawing which is re-
quired for our purpose so difficult as is often supposed by those who
have never seriously used it. Let the student attempt to reproduce an
object from his memory of its picture ; begin with one which is not too
complicated (such as the figure of the peritoneum and mesentery on
page 14). Where he does not know how to represent a special structure,
let him refer to the original, from which he may get suggestions; then
close the book and draw from memory ; any completed part of the work
may be compared with the original and possible improvements discovered.

STEUCTUKE OF THE HUMAN MECHANISM 7

The human mechanism is composed of different parts, such
as head, neck, trunk, arms, hands, legs, and feet, and each of
these in its turn is composed of lesser parts. Arms and hands,
for example, are covered by skin, which may be moved over
underlying soft parts ; at the ends of the fingers the place of
the skin is taken by nails, while scattered over and emerging
from its surface are hairs. Through the skin may be seen the
veins, which may be emptied of the purplish blood they con-
tain by pressing one finger on a part of the vein near the
finger and pushing another finger along the vein toward the
wrist ; so long as pressure is maintained by both fingers
the vein remains collapsed, but on removing the first finger
it fills again with blood. Finally, through the soft parts
(flesh) may be felt the hard bones. In general these various
parts of which the body is composed are known as its organs,
and because it possesses organs it is called an organism (p. 3).

1. The skin. The body is everywhere covered by a com-
plex protective and sensitive organ, the skin. Only the eyes
and nails seem to be exceptions ; but as a matter of fact the
exposed surface of the eye is covered by a very thin, trans-
parent portion of the skin, and the nails are really modified
portions of skin.

2. Subcutaneous connective tissue. On cutting through the
skin we find that it is bound to the underlying flesh (chiefly
meat or muscle) by what is known as connective tissue, the
structure of which we shall study in the next chapter. Mean-
while we may notice that it contains blood vessels, that at
some places it is more easily stretched than at others, and
that when a flap of skin is pulled away from the muscles,
this subcutaneous tissue fills with air. It often contains large
quantities of fat.

Such practice may well precede drawing from an actual dissection and
will pave the way to the latter. At all events let the student understand
thoroughly that in the present chapter the figures, supplemented if possible
by actual dissections, form the main objects of study ; the text is strictly
subordinate to the figures.

8 THE HUMAN MECHANISM

3. Muscles and deeper connective tissues.1 The subcu-
taneous connective tissue sometimes connects or binds the
skin directly to bone, as in parts of the head ; usually, how-
ever, in the neck, trunk, and limbs the underlying tissue is
the red flesh, or muscle, familiar to us as " lean of meat."
If the skin be removed from the forearm, it at once becomes
evident that this mass of meat or flesh is composed of a
number of muscles which may be separated from one an-
other more or less completely. In doing this it will be found
that the muscles are held together by connective tissue in
most respects quite similar to that immediately under the
skin. Further dissection will show that one or another form
of this tissue is the means of binding other organs together ;
thus the muscles are joined to the bones by a very dense,
compact, and strong form known as tendon:, the bones are
united by a somewhat similar r.rm known as ligament; and
so on. The physical characters of the tissue differ widely,
according to its situation and the use subserved; but one
form shades more or less into another, and we have no diffi-
culty in recognizing the general similarity which leads us to
group them all together in one class.

4. Muscles attached to bones. When a muscle is carefully
dissected away from neighboring muscles and other organs,
it is almost always found that it is attached to one and
usually to two bones; this union is frequently made by
means of a tendon, as in the case of the large muscle of the
calf of the leg, which is attached at one end to the bone
of the thigh and at the other to that of the heel. A good
example of the direct attachment of muscles to bones is
furnished by those muscles which lie between the ribs (see

1 The general appearance and arrangement of muscles, their attachment
by means of tendons to bones, and the action of tendons on bones can be
beautifully shown by a dissection of the leg of a chicken. The difference
between trunk and limbs in the matter of the body cavity may also be
readily demonstrated on the same animal.

STBUCTUBE OF THE HUMAN MECHANISM 9

Fig. 161). In either case the shortening of the muscle brings
closer together the bones to which it is attached.

5. Definition of some anatomical terms. Before proceeding
further we must agree upon the exact meaning of certain
anatomical terms. We often speak of one part of the body as
being "above" or "below," "before" or "behind," another.
Such terms, however, are confusing, because their meaning de-
pends upon the position of the body at the time they are used.
For example, when one is lying on his back the head is in
front of, or before, the
trunk ; but when he is
standing on his feet it
is above the trunk.

Now the body is
certainly divided into
right and left halves,
which are much alike
externally, though this
likeness is not so
marked in the internal
parts. Right and left
then have their ordi-
nary meanings, and
that without regard to

flip vnrinn^ nnditinn<* ^, skin ; 7?, subcutaneous connective tissue, bind-
VailC ing the skin to the muscles D and continuous

with the connective tissue which binds together
the muscles ; C, blood vessels and nerves

FIG. 1. Cross section of arm

the body may take.

To indicate that any
part is nearer the head than another part, we say that the
former is anterior to the latter ; to indicate that the latter is
further away from the head, we say it is posterior to the former.

Finally, the region popularly known as the back is called
dorsal (Latin dorsum, " back "), that opposite the back being
called ventral (Latin venter, "belly"). Thus the nose is on
the ventral side of the head; the toes are at the posterior
extremity of the foot.

THE HUMAN MECHANISM

6. The body cavities. There is one striking and important
structural difference between the trunk and the limbs ; the
former contains a central body cavity, completely filled, how-
ever, with various organs, while the arms and legs are each
composed of a continuous mass of tissues, namely, muscle,

bloodvessels, nerves,
bone, etc., all bound
together by connec-
tive tissue (Figs. 1
and 2).

The cavity of the
trunk, or body cav-
ity, is subdivided
transversely by the
dome-shaped muscle
known as the dia-
phragm into two
cavities — an ante-
rior, known as the
thoracic, or pleural,
cavity ; and a poste-
rior, known as the
abdominal, or perito-
neal, cavity. Both
cavities are lined by
a thin, smooth, shiny
membrane, that of
the thoracic being
known as the pleura, and that of the abdominal as the
peritoneum.

Filling the pleural cavity are found the heart, lungs,
oesophagus, windpipe (or trachea), and many great blood vessels ;
filling the abdominal cavity, the stomach, the small intestine,
the large intestine, the liver, the pancreas, the kidneys, the
spleen, and other organs, together with numerous large and

FIG. 2. The thoracic, or pleural, and the abdomi-
nal, or peritoneal, cavities filled with organs

STRUCTURE OF THE HUMAN MECHANISM 11

important arteries and veins. In both cavities the lining
membrane (pleura or peritoneum) is folded back over the
organs ; that is to say, the organs do not really lie in the
cavities, but only fill them as the hand would fill a bladder
one wall of which it pushes in against the other. The sur-
faces of the organs, like the walls of the cavity, are conse-
quently smoothly covered and glide over one another with
very little friction. The
preservation of these pleu-
ral and peritoneal linings
in their normal condition
is a matter of great impor-
tance ; when inflamed or
otherwise injured their sur-
faces become roughened,
and adhesions of connec-
tive tissue often develop
between them which fas-
ten the organs together or
to the walls of the cavity, FlG- 3- Cross section of the chest ante-

so that surgical interfer-
ence is sometimes neces-

rior to the branching of the trachea

A, a. vertebra of the spinal column ; B, spinal
cord ; (7, the pleural cavity (which is exag-

\ Sary. Pleurisy is Such an gerated for the sake of clearness, the sur-

\ . a , • r , i i face of the lung being actually in contact

\ inflammation Of the pleura, with the body wall). The oesophagus, tra-

\ peritonitis of the peritO- chea» together with several large arteries

, , , and veins, are shown in the mediastinum ven-

\neum ; and both are Very tral to the vertebra and in the order named

serious conditions.

7. Attachment of the organs to the walls of the pleural
and peritoneal cavities. The pleural cavity is completely
divided by a median partition of connective tissue (the
mediastinum), within which are found the trachea, the
oesophagus, the great blood vessels, and — lying within a
special cavity of its own — the heart. Approximately half-
way from the anterior to the posterior border of the medi-
astinum the trachea divides within that membrane into two

THE HUMAN MECHANISM

tubes, or bronchi, which pass through the mediastinum out-
ward, one to the right lung, the other to the left. The pleural
lining of the mediastinum is pushed outward by these tubes
and, as they end in the lungs, forms the pleural covering of
the latter (Fig. 5). Consequently the organs of the pleural
cavity either lie within the mediastinum (heart, oesophagus,

FIG. 4. Cross section of chest posterior to branching of trachea

A, bronchus, entering the lung ; B, the aorta cut at its origin and again at the

descending part of its arch; C, the pericardial space; D, the pleural cavity;

P. A., the pulmonary artery

trachea, etc.) or else are covered by extensions of the
mediastinal pleura (bronchi and lungs).

The abdominal cavity is not similarly separated into right
and left halves ; but a membrane, the mesentery, passes ven-
trally from the dorsal wall to the stomach and intestine,
which are slung in it somewhat as a man lies in a hammock.
The line of attachment of this mesentery to the small intes-
tine is much longer than that of its attachment to the body
wall ; hence it has the general shape of a ruffle, or flounce —
an arrangement which permits the suspension of the very

STRUCTURE OF THE HUMAN MECHANISM 13

long intestine (20 to 25 ft.) from the comparatively short
median dorsal body wall (see Fig. 156). The great arteries
and veins lie in the mesentery near the dorsal body wall,
and branches are distributed from them to the intestine
within this expand-
ing membrane (see
Fig. 163).

The kidneys do
not He movably sus-
pended in the ab-
dominal cavity, as do
the intestines, but
are large organs, one
on each side, situated
near the spinal column
and dorsal to the ab-
dominal cavity from
which they are sep-
arated by the peri-
toneum. Arteries
and veins are sup-
plied to them from
the large median
artery and the me-
dian vein already
referred to (aorta

cava

and vena

Fig. 15), and these

renal arteries and veins are

likewise outside- the abdom-

5. Diagrammatic vertical right-
to-left section of the right thorax

A, muscles, ribs, etc., of the body wall ;
JB, pleura, lining the same ; C, the pleural
space or cavity ; D, the pleural covering
of the lung ; E, connective tissue of the
lung; F, alveoli of the lung; G, dia-
phragm ; H, trachea ; 7, right bronchus,
branching; K, the pericardial space in
which lies the heart. Note the division
of the lung into two lobes

inal cavity.

The relation of the other organs to the peritoneum is more
complicated, notably in the case of the liver ; but in all cases
the organs are inclosed, or wrapped, either in a fold of the
peritoneum, as is the kidney, or in a fold of the mesentery,

THE HUMAN MECHANISM

as is the intestine; and their blood and nerve supplies run

to them in similar folds.

8. The axial skeleton. The bones and cartilages of which

the skeleton is composed may be classified into an axial

skeleton (of the head,
fKidney neck, and trunk) and
an appendicular skele-
ton (of the arms and
legs). The axial skele-
ton comprises (1) the
backbone, or vertebral
column, (2) the ribs
and breastbone, and
(3) the skull.

9. The backbone,
or vertebral (spinal)
column. This is com-

FIG. 6. Diagrammatic cross section of the
abdominal cavity

Showing the relation of the kidneys and great
blood vessels to the peritoneum. The intestine has

been removed, the cut border of the mesentery posed of Separate ir-

regular ringlike bones,

being shown

or vertebrae, placed one above another and bound together by
bands of strong connective tissue known as ligaments. It is
customary to divide the backbone into the following regions :

Cervical, 7 vertebrae of the neck.

Thoracic, 12 vertebrae of the chest, to which ribs are attached.

Lumbar, 5 vertebrae of the " small of the back."

Sacral, 5 vertebrae (fused together) to which the large hip bones
are attached.

Coccygeal, 4 or 5 very small, simple vertebrae (constituting the skele-
ton of a rudimentary tail and corresponding to the tail of lower
animals).

When one looks at the spinal column from behind, the
vertebras are seen to be placed one upon another, but all in
the median dorsoventral plane of the body (see Fig. 7). Seen
from the side, however, several curves come into view, as
shown in Fig. 10. On the ventral side, in the cervical and

FIG. 7. The skeleton entire
15

THE HUMAN MECHANISM

FIG. 8. Sixth thoracic
vertebra

Seen from above

upper thoracic region, the curvature is slightly convex ; in
the thoracic region it is quite concave ; in the lumbar region
slightly convex ; and in the sacral-coccygeal region again

concave. It may well be asked how
these separate vertebrae, piled, as it
were, one above another, maintain
their proper relative positions. This
is partly due to the shape of the
individual vertebrae, partly to the
ligaments (p. 17) which pass from
one vertebra to another and limit
the movements of each, and partly
to the action of muscles which are
placed upon opposite sides of the
vertebrae and by their antagonistic

action hold them in place. The action of muscles and liga-
ments upon the bones may be illustrated by two blocks of
wood held together by two rubber bands (w, m', Fig. 11)
slightly stretched ; so long as each
pair of opposite bands pulls with
the same force, the blocks are kept
iii what we may call their resting
position. Here the rubber bands
represent two of the antagonistic
muscles, which, by maintaining a
steady and equal pull on the oppo-
site sides of the vertebree, keep
them in place. Should one pull
harder than its antagonist, as when
a muscle contracts (see Chap. IV),
the antagonist will be stretched
and the bones become inclined
toward one another, as shown in right portion of Fig. 11.
This principle of muscular antagonism is quite general in
the maintenance of the proper relative positions of bones in

FIG. 9. Sixth thoracic
vertebra

Seen from the side

STRUCTURE OF THE HUMAN MECHANISM 17

Cervical

Thoracic
(or dorsal)

the body. Almost every joint is the theater of such plays
of antagonistic muscles, which serve the double function of
keeping the bones in proper position with regard to one an-
other and of producing movement at the joint, the amount
of this movement being limited by
the slack but inextensible connective-
tissue ligaments which bind the bones
together. In Fig. 11 both the short-
ening of the muscle and the slack-
ness of the ligaments are purposely
exaggerated, in order to represent
more clearly the functions of these
tissues. Ligaments may also guide
the movement of bones by pre-
venting motion in one direction
or another.

10. The ribs. Each rib consists
of a bony and a cartilaginous por-
tion. The former articulates (that
is, forms a joint with) the vertebral
column, while the latter continues
this bony portion to the ventral
median breastbone, to which it is
directly joined. The ribs form the
framework for the thorax and may
be lifted or lowered by muscles
which connect them with the verte-
bral column and other parts of the
skeleton (see Fig. 12).

11. The skeleton and the central

nervous system. The skull consists of the bones of the
face and those of the cranium, the latter holding the brain.
It is supported on the spinal, or vertebral, column, whose
ringlike vertebrae inclose a bony canal continuous with the
cranial cavity. This is known as the spinal, or vertebral.

Lumbar

Sacral

FIG. 10. The vertebral
umn

Seen from the side

THE HUMAN MECHANISM

canal, in which lies the spinal cord1 — the continuation of
the central nervous system posterior to the brain.

FIG. 11

Model showing the action of muscles on two vertebrae and of the ligaments (I, I')

in limiting the amount of movement. The contraction of the muscle m stretches

its antagonist m'. The amount of movement is greatly exaggerated

Nerves, which pass through small openings in the cranium
and between the vertebrae, leave the brain and cord and
end in the muscles, skin, glands,
and other organs of the body (see
Chap. VII).

12. The appendicular skeleton.
The bones of the arm, leg, hand,
and foot may readily be felt and
are sufficiently familiar. We may,
however, call attention to the simi-
larity in the number and form of
the bones of the arms and legs, a

similarity which is not only helpful

FIG. 12. Dorsal view of ver-

!The terms ft spinal cord," "spinal col- tebrse and ribs

limn," and "spinal canal" are sometimes showing some of the muscles
confused by beginners. The spinal column which lift or raise the ribs
is the entire bony framework formed by

the vertebrae — the whole backbone ; it surrounds the spinal canal, which,
in turn, contains that part of the nervous system known as the spinal cord.

STRUCTURE OF THE HUMAN MECHANISM 19

Vertebral
Canal

in mastering their names and arrangement but is also sug-
gestive of the similarity of function in quadrupeds, both
limbs in these animals be-
ing organs of locomotion. /»@JTL ;.Jr .x x.^ Cranium

ARM

Humerus, single long bone of the
upper arm.

Radius and ulna, two nearly par-
allel bones of the forearm.

Eight small irregular bones of
the wrist.

Five parallel bones of the palm.

f Thumb, two bones.
Bones ot I „ ,, ~

4. Other fingers,

finSers[ three bones. Thorax

LEG

Femur, single long bone of the
thigh.

Tibia and fibula, two nearly par-
allel bones of the lower leg.

Seven small irregular bones of
the ankle and heel.

Five parallel bones of the instep.

. f Great toe, two bones.
Bones of ~- ,

•I Other toes,

[ three bones.

The legs are attached to FIG. 13. Median dorsoventral section
the vertebral column by the

large hip bones, which articulate directly and immovably
with the. sacrum 1 ; but the humerus, or bone of the upper
arm, articulates on each side with one of a pair of bones
which form the shoulder girdle, or skeleton of the shoulder
region ; this pair consists of the collar bone (clavicle) ven-
trally and the shoulder blade (scapula^ dorsally. The clavicle
articulates with the head of the breastbone ; otherwise the
shoulder girdle, with the arm attached to it, is connected
1 The sacrum and the two hip bones together form the pelvis.

Sacrum

Pelvis

THE HUMAN MECHANISM

with the axial skeleton by muscles only. A wide range of
movement is thus secured at the shoulder joint.

13. Organs of digestion. The digestive system consists es-
sentially of a long tube, the alimentary canal, passing through
the body.1 Into this tube, at various points, ducts from a
number of glands pour digestive juices. The alimentary
canal begins with the mouth cavity
and its familiar organs, the teeth, the
tongue, etc. ; this cavity opens pos-
teriorly into that of the pharynx,
into which also opens the nasal
cavity, separated- from the mouth
only by the palate (see Fig. 14).

On the ventral side of the pharynx,
just beyond the root of the tongue, is
the slitlike opening of the windpipe
(see sect. 14) ; posteriorly the pharynx
is continued in the long gullet, or
oesophagus, a tube which passes down-
ward through the neck and thorax
FIG. 14. Diagrammatic me- (within the mediastinum) to join the

dian dorsoventral section stomach, which it enters immediately
of the nasal and throat ,, . , , , ,. ,

after passing through the diaphragm.

The stomach is a large pouch with
contractile walls permitting adapta-
tion of its size to the bulk of food
it may contain. Its situation is shown
in Fig. 155, which also shows how it
opens on the right side of the body into the very long,
coiled small intestine. The coils of this part of the tube
may be followed for from twenty to twenty-four feet, to the
large intestine, into one side of which it opens. The large
intestine, or colon, consists of three portions: the first ascend-
ing on the right side to the general level of the stomach, the
1 See Fig. 155 for the general arrangement of the organs of digestion.

passages

C, nasal cavities; M, mouth
cavity; T, tongue; E, epi-
glottis; G, glottis, or opening
from the pharynx into the
trachea; U, the end of the
soft palate ; 0, oesophagus

STRUCTURE OF THE HUMAN MECHANISM 21

second passing transversely at this level from right to left,
and the third descending on the left side to the rectum, the
posterior terminal portion of the digestive tube.

Numerous glands pour secretions through ducts into the
digestive tube, the more important, with their places of dis-
charge, being the following : salivary glands (see Chap. Ill)
— mouth ; liver — beginning of small intestine ; pancreas —
beginning of small intestine (see Fig. 54). Smaller glands
empty into the stomach and intestines at numerous places.

14. The organs of respiration. The organs of respiration
consist of the right and left lungs (see Fig. 5), from each
of which a single bronchus (pi. bronchi) leads to the trachea
(or windpipe). The walls of the trachea and bronchi are kept
from collapsing by successive rings of cartilage. Anteriorly
the trachea opens into the pharynx through the larynx, or
voice box, the cartilages of which may be felt in the throat
at the root of the tongue. The familiar hoarseness which
accompanies inflammatory roughening of the lining of the
larynx shows how important is this organ in the production
of the voice. The respiratory and digestive paths cross in
the pharynx, the former reaching the exterior through the
nose, the latter through the mouth.

15. The organs of circulation. The position of the heart
and the great blood vessels in the thorax has been described
on page 11. The heart is essentially a large mass of muscle
containing a cavity which is divided into right and left
halves, wholly separate from each other. The cavity on each
side is divided into that of the large ventricle, with very thick
walls, and that of the much smaller auricle. The heart is thus
composed of right and left auricles and right and left ven-
tricles. Valves are so placed in the heart as to allow blood
to flow in one direction only (see Fig. 69).

The arteries are tubes which carry the blood to the tis-
sues, and from each side of the heart a single artery takes
its origin — the pulmonary artery from the right ventricle,

22 THE HUMAN MECHANISM

and the aorta from the left ventricle. The pulmonary artery
supplies the lungs with blood, while all other organs are
supplied by the aorta.

The veins are tubes which conduct the blood from the
various organs to the heart. Beginning in the tissues as
microscopic tubes, they unite to form larger and larger tubes
as 'they approach the heart; those visible through the skin
of the hand may be regarded as of medium size ; as the
union goes on, the size of the vessels increases until finally
at the heart there are only two great veins on the right side
(superior vena cava and inferior vena cavd) and four on the
left (pulmonary veins). The venae cavae bring blood back
from those portions of the body which are supplied by the
aorta, that is to say, from all parts of the body except the
lungs ; the pulmonary veins bring blood back only from
the lungs, that is to say, from the organs supplied by the
pulmonary arteries. The venae cavae empty into the right
auricle, the pulmonary veins into the left auricle. The gen-
eral arrangement of heart, arteries, and veins is shown in
Fig. 15, and the figures in Chapter IX (especially 70 and 71)
should also be consulted.

The blood flows in the following circuit:

Right ventricle to

Pulmonary artery to
Pulmonary circulations T

.Lungs to

Pulmonary veins to

Left auricle to

Left ventricle to

Aorta and its branches to

All organs of the body (except the lungs) to

Veins which unite to form the venae cavae to

Right auricle to

Right ventricle

Thus the blood which leaves the left ventricle flows to
the different organs of the body (except the lungs) and
returns by way of the veins to the right side of the heart;

Systemic circulation

FIG. 15. Diagram of the circulation of blood

K.A., right auricle ; L.A., left auricle ; R.V., right ventricle ; L.V., left ventricle ;
P.A., pulmonary artery ; A, pulmonary artery and vein of right lung ; B, pulmo-
nary artery and vein of left lung; (7, carotid artery to head, showing branch
of left subclavian artery ; D, portal vein ; E, hepatic vein ; F, hepatic artery ;
G, jugular vein, bringing blood from head and neck

THE HUMAN MECHANISM

thence it passes through the lungs and again to the left
auricle and ventricle, thus completing the " circulation."
The term " circulation," strictly speaking, is applied to the
entire circuit which the blood must traverse before it returns
again to the point from which it started; it is often con-
venient, however, to use it to denote the course from the
right ventricle to the left auricle, or from the left ventricle
to the right auricle; in this case we speak of the former as

the pulmonary and of
the latter as the sys-
temic, or aortic, circu-
lation. In this sense
there may be said to
be a " double " circu-
lation.

The veins have
thinner walls than the
corresponding arter-
ies, and those of the
systemic circulation
contain purplish or
even bluish blood,
while the arteries of
the same circulation
contain bright-scarlet
blood. The bright
color of the arterial blood is due to the fact that it contains
more oxygen. The change from purple to scarlet occurs in
the lungs, and the reverse change in the organs supplied
by branches of the aorta. Consequently the blood of the
pulmonary arteries is blue, or venous, in color and that of the
pulmonary vein scarlet, or arterial.

16. The course and branches of the pulmonary artery and
vein. Soon after leaving the right ventricle the pulmonary
artery divides into two branches, one going to each lung.

FIG. 16. A network of capillaries, with the
artery a and vein v (highly magnified)

STRUCTURE OF THE HUMAN MECHANISM 25

Each of these further divides as it plunges into the substance
of the lung alongside the bronchus. The course of the four
pulmonary veins may be similarly traced into the lungs, from
which they bring the blood back to the heart (Fig. 15).

17. The course and branches of the aorta. The aorta
passes anteriorly from the left ventricle, but very soon
arches dorsally and posteri-
orly, forming the arch of the
aorta (Fig. 15); the general
course of the artery can be
best understood from the fig-
ures or from actual dissec-
tion. The arch of the aorta is
continued in the large dorsal
aorta, which passes posteriorly
on the left side of the me-
diastinum near the spine,
through the diaphragm, to the
lower portion of the abdomi-
nal cavity, where it divides
into two large arteries which
supply blood to the hips and
legs. From the arch of the
aorta three large arteries pass
to the head, neck, shoulders,
and arms ; from the thoracic
dorsal aorta arise a number of
small arteries which supply
the muscles and other organs FIG. 17. The general arrangement of
of the thoracic wall ; immedi- the nervous system <dorsal view>
ately after passing through the diaphragm two large branches
go to the stomach, spleen, liver, pancreas, and a large part
of the small intestine; posterior to these the renal arteries
pass right and left to the kidneys, and still further down
a large artery supplies the lower small intestine and the

26 THE HUMAN MECHANISM

large intestine. The supply to the legs has already been
mentioned. Other small arteries arise from the abdominal
aorta and are distributed to the muscles
and skin of the back. The arteries to the
stomach and intestine lie in the mesen-
tery (Fig. 163) and their course may
.be readily traced in a dissection.

18. The course and branches of the
venae cavae. The blood which has thus
been distributed from the aorta returns
to the opposite side of the heart through
the veins which ultimately form the two
vense cavae. In general, it may be stated
that the veins of those organs which are
anterior to the diaphragm form the su-
perior vena cava, while those posterior
to the diaphragm form the inferior vena
cava. The larger veins usually run near
and approximately parallel to the larger
arteries. This is the case with those
from the arms and legs, the kidneys,
and the muscles of the trunk. One nota-
ble and very important exception, how-
ever, is found in the venous supply of
the stomach, spleen, and intestines, the
veins of which unite to form a single
large vein (portal vein) which passes
to the liver, where it breaks up into
smaller vessels; the blood which has
thus passed through the liver is finally
collected in the hepatic vein and poured
by this into the inferior vena cava just
before the latter passes through the dia-
phragm on its way to the right ventricle
FIG. 18. Nerve trunks » J £

of the right arm (Fig. 15; see also Ing. 70).

STRUCTURE OF THE HUMAN MECHANISM 27

19. The capillaries. The blood which enters an organ
through the arteries passes to its veins through a system of
microscopic tubes (Fig. 16), the capillaries (Latin capilla, " a
hair") ; these may be readily seen under the microscope in
the web of a frog's foot. From the foregoing description of
the course of the circulation it will be observed that gen-
erally the blood must pass through one set of capillaries in
going from the aorta to the vena3 cavse or from the pul-
monary artery to the pulmonary vein ; but the blood which
flows through the capillaries of most of the abdominal organs
(stomach, intestines, spleen) must pass also through a second
set of capillaries, namely, those of the liver, before it can
return to the heart.

20. Organs of the nervous system. The skull and the
spinal column (p. 18) are chiefly occupied by the brain and
the spinal cord, respectively, and from each of these principal
organs of the nervous system branches consisting of cords
of nervous substance, the nerves, pass out through small
holes in the skull or spinal column and are distributed to
all the other organs, where they terminate in peculiar struc-
tures called end organs. The optic nerve, for example, ends
in the retina, the auditory nerve in the inner ear, and motor
nerves in muscles — the nerve endings in these different
organs differing materially in structure and arrangement.

Fig. 17 gives some idea of the general arrangement of
the nervous system. The nerves to the shoulder, arm, and
hand will be seen to arise from the cervical region of the
spinal cord ; those for the trunk, from the dorsal and lumbar
regions ; those for the legs, from the sacral region. The head
and face receive nerves from the posterior portions of the
brain. The dissection of the arm in Fig. 18 shows more
accurately the main nerve trunks to that region. Further
information with regard to the structure of the nervous
system will be given in Chapters VII, XIV, and XV.

CHAPTER III

THE FINER STRUCTURE OF TWO TYPICAL ORGANS,

GLANDS AND MUSCLES. THE CONNECTIVE TISSUES.

THE LYMPHATIC SYSTEM

In the previous chapter we have examined the general
construction of the human machine as regards its more
conspicuous parts or organs, and especially their location,—
whether internal or external, dorsal or ventral, anterior or
posterior, on the right or on the left, — their relations to
certain important cavities, and their combination to consti-
tute the mechanism which we call the human body. We
must now push our examination further and investigate
the finer structure of some of the more important parts of
the machine. For this purpose we may select two typical
organs, a gland and a muscle, the one unfamiliar, by name
at least, to most people, the other well known in the form
of steaks, chops, roast beef, and other meats.

1. What is a gland? A gland is a mass of tissue, gen-
erally softer than muscle and of no special size or shape,
though often rounded or egg-shaped. The gland most
easily seen is the milk gland or udder of the cow. This
is a large mass of soft tissues devoted to manufactur-
ing or secreting milk. In general, glands are manufac-
turing organs for the preparation of saliva, gastric juice,
bile, tears, sweat, or other secretions. Some have tubes,
or ducts, through which their secretions are carried away;
others have no such outlets and hence are known as duct-
less glands. Glands vary in size from some which are
microscopic to the huge liver, which is the largest single

TYPICAL STRUCTURE OF ORGANS

organ in the human body (see Fig. 2). The pancreas, or
" sweetbread," of the calf is an excellent gland for the
beginner to dissect or study.

2. A typical gland. If we have before us the whole or a
part of any typical gland, we find that we are dealing with
a comparatively soft and sometimes even pulpy mass held
together by a loose mesh or network of harder, tougher, and
more or less fibrous materials.

A pancreas or a liver, if en-
tire, shows conspicuous lobes,
and in the pancreas these
lobes are plainly subdivided
into smaller lobes, or lobules.
In favorable specimens tubes
may be seen connected with
the gland ; some of these are
blood vessels supplying blood
to the gland, and one of them
is a duct draining away from
it the liquid which the gland
has manufactured or secreted.
After a preliminary examina-
tion of this sort of some edible
gland, preferably the pancreas,
we may pass on to consider in greater detail one of our own
salivary glands, of which we have two on each side of the
head, namely, one parotid and one submaxillary gland.

3. The structure of the submaxillary gland. The two sub-
maxillary glands lie, one on each side of the face, embedded
in the tissues between the lower jaw and the upper portion
of the neck. From each gland a duct passes forward in the
tissues forming the floor of the mouth, into which it opens
by one of the small eminences, or papillce, under the tongue.
Through this duct the gland pours into the mouth its
secretion, saliva.

FIG. 19. Diagram of submaxillary
gland

D, its duct ; N, its nerve ; A, its artery ;
V, its vein ; T, tongue

THE HUMAN MECHANISM

If the gland were to be cut in two in any direction with
a sharp knife, we should see at once that it is composed of
separate parts, or lobes, and that these lobes are still further
divided into smaller portions, or lobules. The lobules and
lobes are bound together with a rather loose connective tissue
which is continuous with a somewhat denser layer surround-
ing the gland and forming its capsule ; the connective tissue

between the lobes forms
the primary septa (sing.,
septum) and that between
the lobules the secondary
septa. The relation of
these structures is shown
in Fig. 20. With the aid of
the microscope we find that
each lobule is still further
divided by connective tis-
sue into flask-shaped struc-
tures, or alveoli (sing.,
alveolus) : in these the se-
cretion, saliva, is manufac-
tured and from them it is
discharged into the duct of
which the alveoli are the
blind ends (Fig. 21).

The whole gland may be
compared to a large bunch
of grapes ; the main tubular
duct of the gland branches (in the septa of connective tissue)
very much as the stem of the bunch of grapes branches ; and
just as the branches and subbranches of the stem lead, when
followed up, to the grapes themselves, so the branches of the
duct lead to the alveoli of the gland. If now we pack the
bunch of grapes in a small basket of sawdust or cork waste,
as Malaga grapes are packed, so that the sawdust fills up

FIG. 20. Diagram of a cross section of
a gland

Showing its division by primary septa (S)
into lobes and by secondary septa (s) into
lobules; also the origin of the larger
branches of the duct (D) in the lobes and
lobules. The beginnings of the duct are
shown in Figs. 21 and 22

TYPICAL STRUCTURE OF ORGANS

loosely the spaces between the individual grapes and the
branches of the stem, we shall have something with which
to compare the arrangement of the connective tissue in
relation to the rest of the gland — the sawdust standing
for the connective tissue in which the ducts and alveoli
are embedded, and the basket for the capsule.

Duct

Alveoli

Secreting Cells
Capillary Network

FIG. 21. The origin of the duct of a gland in alveoli, together with the
connective tissue and blood vessels

4. Minute structure of ducts and alveoli. The alveoli are
not, however, empty shells like glass flasks nor solid masses
like grapes, but rather hollow bags lined with a layer of
thickish, closely set cells, all much alike. Each of these cells
consists of two portions — a small central body, the nucleus,
and a larger surrounding mass, the cytoplasm. All organs
of the' body are composed of cells, differing in different
organs or in different parts of the same organ (as in the
duct and the alveolus of the gland), but all consisting of two
never-failing parts — nucleus and cytoplasm.

32 THE HUMAN MECHANISM

The muscle and the gland consist of cells, just as all the
branches of the military service — the infantry, the cavalry,
the artillery, the engineers, etc. — consist of men. The cell
is the anatomical or fundamental unit of these organs, as the
soldier is the fundamental or anatomical unit of the army;
in both cases the anatomical units, differing in equipment
and training, perform different kinds of work, yet have the

FIG. 22. Section of a portion of a salivary gland (magnified 500 diameters)

After Kcelliker

The duct d divides into the two branches d' and d" , one of which ends in the

alveoli, a, a. Neighboring alveoli, a', a', whose ducts are not in the plane of the

section, are also shown. In some cells the section does not include the nucleus,

which would be in the preceding or the succeeding section

same essential structure ; and the cells are combined into
brigades, divisions, or corps, as tissues and organs ; they make
of the body an army organized to fight its way through the
vicissitudes and against the obstacles of life.

5. The structure of the biceps muscle. The biceps muscle
is familiar as the mass of flesh lying on the front of the
upper arm and bulging somewhat when the arm is bent at
the elbow, especially when one "feels his muscle" or when
a weight is being lifted by the hand. Figure 23 shows this
muscle with the bones to which it is attached. It consists of

TYPICAL STRUCTURE OF ORGANS

two portions: a central, thick, red part, known as the belly,
soft when the muscle is at rest, hard when it is contracted ;
and cordlike strings, or tendons, two at the upper end and one
at the lower, by means of which the muscle is attached to two
bones of the shoulder girdle and to one of the forearm. When
the belly of the muscle shortens, the points a and b are brought
closer together and the arm is bent, or flexed, at the elbow.
This drawing together, or contraction, is the special work, or
function, of muscles in general.

Everyone has seen the cross
section of a muscle in a raw
beefsteak. This shows that the
muscle as a whole is surrounded
by a sheath of connective tissue
which contains more or less fat ;
septa pass inwards, dividing the
muscle into lesser red masses
known as fasciculi, or bundles,
and these are further subdivided
into secondary fasciculi by sec-
ondary septa, very much as the
gland is subdivided into lobules.

A longitudinal section shows
that the fasciculi run from ten-
don to tendon, and microscopic
examination proves that the general connective tissues of the
belly of the muscle are continuous with that of the tendon.
The tendon itself is a peculiarly strong and inextensible
variety of connective tissue consisting chiefly of parallel
fibers which are specially fitted to transmit to the bone the
pull of the belly of the muscle.

6. The muscle fibers. Examination of the structure of
one of the finer fasciculi in the belly of the muscle shows
that it is composed of threads, or fibers, which at first sight
differ greatly from the secreting cells of the gland. These

FIG. 23. The biceps muscle of
the arm

The resting condition is shown by
the solid lines, the contracting con-
dition by the dotted lines

THE HUMAN MECHANISM

are the muscle fibers. They are 1 to 1J inches in length and

FIG. 24. Tendon
(highly magnified)

Showing the fiber
bundles separated

from TrrW to ^l"^ of an inch in thickness, thus being from
250 to 2500 times . as long as wide, and
comparable in shape to a long leather shoe-
string rather than to a sausage. Each fascic-
ulus contains hundreds or even thousands
of fibers. The fibers always run lengthwise
of the fasciculus, but, as a usual thing, do
not extend its entire length, as obviously
follows from the fact that a single fasciculus
of the biceps is several inches in length.
The fibers are inclosed in a very thin trans-
parent membrane, the sarcolemma, and are
bound into bundles (or fasciculi) by the
same fine connective tissues seen between
the alveoli of a gland. To the end of
the sarcolemma are attached fine fibers of
connective tissue which pass into the tendon
(Fig. 25). Indeed, the fibers of the tendon are
the collected fibers from the sarcolemmas of all
the muscle fibers. For this reason the part
of the muscle near the tendon is " tough meat,"
while that in the belly of the muscle is tender,
owing to the smaller number of connective-
tissue fibers.

7. The muscle fiber a cell. The muscle fiber
at first sight does not seem like the typical cell
already described, with nucleus and cytoplasm ;
for when examined in the fresh condition the
only obvious points of structure seen in it are showing the at-
striking cross striations consisting of alternate tachment of the
dark and light bands. It has been shown, how- t£g sarco^mnm
ever, by ingenious and careful study that the
cross striations are optical appearances produced by the
peculiar shape of extremely minute longitudinal rods in tho

FIG. 25. One
end of a mus-
cle fiber

TYPICAL STRUCTURE OF ORGANS

cytoplasm of tlie muscle fiber and that, immediately under
the sarcolemma, xdiere are numerous characteristic nuclei
which are easily brbught into view by suitable treatment.
Briefly, then, the muscle fiber is a cell with many nuclei, in
whose cytoplasm are found peculiar structures, the myofibrils ;
upon superficial examination these myofibrils not only obscure
the nuclei but give to the whole fiber the
appearance of cross striation.

8. How far is the structure of glands
and muscles typical of all organs? Both
the gland and the muscle are thus com-
posed of cells. Although differing con-
siderably in the two organs, these cells
possess certain general and fundamental
features in common, for each one contains
a nucleus (or nuclei) and surrounding
cytoplasm. Is the same thing true of all
other organs ? The muscle and the gland
are examples of organs which do active
work, but some other organs perform
purely passive functions. Such are the
bones, which do no work themselves, but
upon which the work of mechanical motion
is done by the muscles ; the tendons, which
transmit the pull of muscles ; the ligaments,
which limit and sometimes guide the mo-
tion of bones; and the connective tissues,
which bind together other parts of the body. None of these
is a working organ in the sense that a muscle or a gland is
a working organ, and we are not surprised to find that their
structure departs from that of the muscle and gland in that,
while nucleated cells are present in all of them, the great mass
of the organ is composed of lifeless matter between the cells. In
a tendon this consists of very strong parallel fibers (Fig. 24) ;
a ligament shows much the same structure ; a bone consists

FIG. 26. Part of a
muscle fiber

Specially prepared to
bring out th

THE HUMAN MECHANISM

chiefly of lifeless material containing large amounts of mineral
matter, with cells lying here and there in spaces which com-
municate with one another by means of minute channels. The
connective tissues, like that which binds the skin to the un-
derlying muscles or that which forms the sheath and septa of
glands and muscles, consist essentially of lifeless fibers run-
ning in all directions and thus ready to limit the extent of any
pull tending to separate unduly the adjacent organs. To organs
and tissues of this kind we may give the name of supporting

organs and tissues,
and they form al-
most the sole excep-
tion to the general
rule that the essen-
tial part of a tissue
consists of its cells.
The latter state-
ment is true of all
organs and tissues
which do work —
the active organs of
the body. In the
case of the support-
ing tissues the cells
which they contain
are the fundamental units of the organ, since they make the
intercellular lifeless substance ; but the part which the organ
plays in the work of the body as a whole is performed
by the lifeless substance (fibers, etc.) which the cells have
manufactured and keep in repair.

9. The blood vessels are closed tubes in connective tissue.
The arrangement of connective tissues is fundamentally the
same in gland and muscle. These tissues serve the obvious
purpose of binding the anatomical units into organs, but
they also perform other functions equally important.

FIG. 27. Longitudinal (A) and transverse (B)
sections of bone

Showing the branching and communicating canals —
in which are blood vessels and nerves — surrounded
by the lifeless bone substance. In this are spaces
connected with one another by very minute channels.
Each of these spaces contains a living cell, shown in O

TYPICAL STRUCTURE OF ORGANS

We have seen (Chap. II, sect. 19) that each organ receives
blood through one or more arteries, and that this blood flows
away from the organ through one or more veins. If a colored
fluid mass which would afterwards set (for example, a warm
solution of gelatin colored with carmine) had been forced
into the arteries before we began our examination, we should
find that this mass would every-
where be confined in a system of
closed tubes which merely lie in
the connective tissue. The artery
entering the muscle branches into
smaller and smaller arteries in the
general sheath of the organ, or in
its branches, the septa; from these
finer arteries an exceedingly rich
network of small thin-walled tubes
is given off to the finest connective
tissue which surrounds the cells
themselves ; these tubes are the
capillaries. They ultimately unite
to form the larger veins, which can
be traced in the septa to those veins
which gross dissection reveals as
leaving the organ (see Fig. 19).
Through these tubes — arteries, capil-
laries, and veins — the blood flows;
and it is important for us to under-

FIG. 28. Three muscle fibers
and an artery breaking up
into capillaries between them

stand that it is everywhere confined to them in its passage
through the organs ; nowhere does it come into direct contact with
the living cells (save those lining the vessels). Whatever ex-
change of matter or energy takes place between the blood and
the living cells must be through the walls of the blood vessels.1

1 The term " blood vessel " is sometimes confusing to the beginner, since
it suggests a utensil for holding liquids. In anatomy "vessel" te a name
for tubes, ducts, or canals through which blood or lymph flows.

THE HUMAN MECHANISM

These walls are relatively thick in the arteries, usually some-
what thinner in the veins; in the capillaries, however, they
are very thin, and it is through these thin capillary walls
that all interchanges of matter take place. That the con-
nective tissue surrounding the capillaries bears an important
relation to the circulation we shall now see.

10. The lymph spaces of the connective tissue ; the lymph.
Careful examination shows that the fine connective tissue

within which the capillaries
are embedded is not a solid or
continuous mass, but rather
a mass or mesh of extremely
fine fibers or bundles of
fibers, with here and there
connective-tissue cells which
keep the fibers in repair.
The connective tissue, there-
fore, is everywhere channeled
by irregular spaces running
between the fibers and other
structures ; these spaces com-
municate freely with each
other and contain a colorless
liquid known as lymph ; the
spaces of the connective tis-
sue may thus be conveniently
described as lymph spaces.
They serve as communicating
channels between the cells and the walls of the capillaries.

11. Origin of the lymph. The lymph which the spaces
contain is derived partly from water and soluble food mate-
rials which have passed through the capillary walls from the
blood and partly from material produced by the neighboring
cells (see the next chapter); on the other hand, the cells
absorb from the lymph substances which the latter has

FIG. 29. Superficial and some deeper
lymphatics of the hand

TYPICAL STRUCTURE OF ORGANS

received from the blood, while the
the lymph substances discharged
from the cells. The lymph thus
becomes the means of communi-
cation, the middleman, between
the living cells of the organs
and the nourishing blood, and
forms the immediate environment
of the cells themselves. In other
words, the cells of muscles, glands,
and other organs live in lymph, just
as the human body as a whole
lives in air, or a fish in water.

12. The lymphatics. Besides
the veins, which convey blood
away from an organ, a second
system of tubes or vessels passes
out through the capsule. These
tubes arise in the lymph spaces
of the connective tissue and
unite with similar tubes from
other regions to form larger and .
larger trunks, known as lym-
phatics, which ultimately form
one or two great trunks and
open into the great veins near
the heart (see Fig. 30). Through
these direct outlets the surplus
lymph of the organ flows in a
varying but for the most part
continuous stream. This flow
of lymph away from an organ
is of the very greatest impor-
tance in maintaining the normal
environment of the cells.

blood, in turn, takes from

FIG. 30. The two main lymphatic

trunks (in white), with their

openings into the great veins

near the heart

The larger of these trunks — that
on the left side and known as
the thoracic duct — returns all the
lymph except that from the right
side of the head and neck and the
right arm and shoulder region

THE HUMAN MECHANISM

13. Function of the lymph flow from an organ. It is clear
from inspection of Fig. 31 that there is a steady flow of
liquid from the capillaries, through the lymph spaces of the
connective tissue, over the surfaces of the living cells or of
any intervening capillaries, to the lymphatics. The cell is
thus bathed not by a stagnant medium but by one which
is in gentle movement — one which brings to all parts of its

FIG. 31. Diagram of the relation of the cells of an organ to its blood

vessels, lymphatics, and connective tissue

A, artery; V, vein ; L, lymphatic

surface the food which it needs and immediately carries
away from all parts of its surface to the adjacent capillaries
the products of its activity. By providing this outlet from
the lymph spaces the lymphatics render possible the move-
ment of lymph within the organ itself, whereby material is
readily transferred from the cell to the capillaries and from
the capillaries to the cell.

14. Distribution of nerves to muscles and glands. The dis-
tribution of nerves resembles that of the arteries, the larger

TYPICAL STRUCTURE OF ORGANS 41

nerve trunks being found in the septa, and their fine ultimate
branches being distributed by way of the connective tissue
which surrounds the cells, in whose neighborhood or even
within whose substance they finally end. As we shall see in
subsequent chapters, it is the function of the nerves to arouse
the gland cells or muscle fibers or other cells to activity.

15. Summary. Disregarding for the moment those pecu-
liarities of arrangement, shape, and structure of the cells
which are connected with the special work of each organ
(for example, the arrangement of gland cells to form a blind
tube or of the connective tissue and fibers of muscle so as
to exert a pull on a bone), we may say that the typical
structure of an organ would be represented in Fig. 31. The
whole is surrounded by a capsule, receives a blood supply
through a system of closed tubes, and contains the special
cells ' upon whose activity its characteristic work depends.
These cells are held together by a fine connective tissue
whose numerous and freely communicating spaces contain
a fluid, the lymph, which is free to flow out through a
second system of tubes, the lymphatics. Nerves from the
brain or spinal cord are also distributed in the connective
tissue to the cells of the organ.

Before concluding this description of the finer structure
of organs, a word may be added with regard to the physical
nature of the cell substance. In its literal meaning the word
"cell" is a misnomer, since it suggests a hollow space inclosed
by solid partitions or walls. Plant cells do, in fact, usually
have such walls around their cytoplasm (Chap. VIII), and
this cytoplasm frequently contains spaces (vacuoles) filled
with a solution of salts, sugar, and other dissolved material ;
but neither the cell wall nor vacuoles are of universal occur-
rence, each being rarely found in the animal cell, and often
absent even in the plant cell. Fifty years of thorough investi-
gation has reduced the number of essential cell constituents
to the cytoplasm and the nucleus, the ultimate structure of

42 THE HUMAN MECHANISM

which is far from being completely understood. It would
seem that the cytoplasm is a mixture of a number of mate-
rials which differ in chemical composition and in physical
properties. Some are dissolved in water, making viscous
solutions comparable to the white of egg or to thick or thin
jellies. Others are known as lipins, or lipoids (from the Greek
lipos, "a fat"), because they resemble fats or oils in physical
characters and to some extent in chemical structure; they
do not mix, or mix only imperfectly, with the viscous aqueous
(that is, watery) solutions, but spread over the outer surface
of the cell, forming a membrane, and probably also penetrate
into the cytoplasm somewhat as the connective-tissue septa
of the gland penetrate the gland. These lipoid membranes
would thus separate the viscous aqueous solutions of the
cytoplasm into separate masses, much as the gland is divided
by its septa into lobes and lobules. The lipins are supposed,
among other functions, to control the passage of material into
and out of the cell. The cytoplasm also frequently contains
granules, one kind of which we have already seen in the
zymogen granules of the gland cells.

The nucleus, on the other hand, is known to contain cer-
tain other compounds peculiar to itself. Some of these at
times are probably in an almost solid state and appear as
denser material within the membrane which usually bounds
the nucleus; at other times they undergo solution, doubt-
less as the result of chemical changes taking place within
them. There are many strong reasons for thinking that the
nucleus bears an important relation to the oxidations of
the cell.

CHAPTER IV
THE OKGANS AND CELLS OF THE BODY AT WOEK

The understanding of a mechanism involves more than a
knowledge of its structure; we must study the mechanism
at work, and the human mechanism, which we are studying,
may be regarded as a factory in which work is done.

The work of some manufacturing establishments consists
in separating useful constituents of the raw material from
useless constituents, as where kerosene is refined from crude
petroleum; that of others consists in producing chemical
changes in the raw material, as where soap is made from fat
or oil; while that of a third class consists in the application
of power by machinery, as where lumber is sawed, planed,
turned, or molded into the material of which houses are
constructed. The work of a factory, in other words, is either
a process of refinement or the production of a new substance or
the application of power.

The human body is a factory which presents in its activities
examples of all three of these processes. A large part of
digestion is a process of food refinement ; out of the food we
eat the very substance of the body itself is formed; while
all muscular work, including the beat of the heart, consists
in the application of power to accomplish useful ends. This
work is done chiefly by the two kinds of organs whose struc-
ture we have just studied, namely glands and muscles ; and
just as their structure presents a fundamental similarity of
plan, so there is a fundamental similarity in the nature of
their activities. This can best be made clear by a somewhat
detailed study of each organ at work.

44 THE HUMAN MECHANISM

1. Physiology of the salivary glands; working glands and
resting glands. The function of the salivary glands is the
secretion or manufacture of saliva for use in the mouth, and
one of the first things we notice about this act of secretion
is that it is not constant but intermittent. Most organs have
periods of activity, or work, followed by periods of inactivity,
or rest, and these glands are no exception. Physiologists
frequently speak of " working glands " and " resting glands."
We all know that our own salivary glands work more effec-
tively at some times than at others. The mouth " waters "
at the sight of food; when we are in the dentist's chair the
flow of saliva often seems excessive, and at other times our
mouths are " parched " or " dry."

2. The chemical composition of saliva. The saliva is some-
times thin and flows readily, while at other times it is thick
and viscous, or glairy. This difference is caused by the fact
that the amount of water in it varies under different con-
ditions. At all times, however, it is a fluid which consists
of water containing certain solids in solution. The amount
of these solids varies from five to ten parts in a thousand of
saliva, and they consist chiefly of three groups of compounds.
The first is mucin, familiar to us as the chief constituent of
the phlegm or mucus discharged from the nose and throat,
and giving to the fluid its viscous character; the second
group consists of substances known as enzymes, those in the
saliva having the power of changing starch to sugar; these
we shall study in detail in the chapters on digestion; the
third group consists of mineral or inorganic salts, of which
ordinary table salt, or sodium chloride, is the most important.
As we shall see, the salts and water are derived directly from
the blood, while the mucin and enzymes are manufactured
by the gland.

3. Blood supply of the working gland. Whenever a gland is
actively working there is an increased flow of blood through
it. For this reason the resting gland is slightly pink, while

THE WORK OF ORGANS AND CELLS 45

the working gland becomes distinctly red. Since the secretion
of saliva requires water and this can be obtained only from
the blood, it is easy to see why an abundant blood supply is
essential to activity. Other constituents of the saliva, such
as the inorganic salts, likewise come directly from the blood.

4. The relation of nerves to gland work ; irritability. Nerves
pass, as we have seen (p. 29), from the central nervous sys-
tem to the salivary glands. These nerves are essentially
bundles of nerve fibers which are distributed from the brain
and spinal cord to the neighborhood of the gland cells.
Such fibers are the means of conveying to the gland an
influence, called a nervous impulse, and nervous impulses
cause the gland to secrete. It is also a fact that when these
nerves are cut or injured in any way, so that the gland is
no longer in nervous connection with the brain and spinal
cord, saliva is not secreted, even when food is placed in the
mouth. Evidently the activity of the gland is normally
aroused by nervous impulses from the brain and spinal cord,
just as the activity of a receiving instrument in a telegraph
office is aroused by the electric current which comes to it
over a wire, or as a mine is exploded by the same means.
The gland then stands ready for the act of secretion and is
thrown into activity by a nervous impulse from the central
nervous system. We speak of this action of a nerve upon
the organ in which its fibers end as stimulation and that
property of an organ in virtue of which it may be aroused
by a stimulus as irritability.

All the working organs of the body (in contradistinction
to the supporting organs, p. 35) are in this sense irritable,
and most of them receive nerves which set them to work.
Irritable tissues may, however, be stimulated by other means
than by nervous impulses. Of these means an electric shock
is the most familiar; others are the sudden application of
heat, the presence of certain substances in the blood, and
even a sharp blow.

46 THE HUMAN MECHANISM

We have now to inquire what it is that happens in the
gland when it is stimulated by a nervous impulse.

5. The response of the gland to stimulation by its nerve.
The visible result of stimulation of the gland is the discharge
of saliva into the mouth. Something must have happened in
the gland which has led to the passage of water and other
substances from the blood (and lymph) through the gland
cells into the duct. But something more has happened, for
saliva contains several substances which are not found in the
blood. The gland has evidently contributed something to the
saliva. How were these contributions to the secretion made?

FIG. 32. Diagram showing the granules in a resting gland (A) and in a
worked alveolus of a gland (E)

When a gland has been resting for some time microscopic
study shows that the cytoplasm of its cells becomes loaded
with small granules, at times so numerous as to obscure the
nucleus itself. As secretion goes on these granules disappear
from the cell, presumably contributing something to the
secretion. If the secretion continue for several hours, it is
found that the granules have disappeared and that the cell
is often distinctly smaller in size than before secretion began.

The " resting " gland is therefore by no means an idle
gland, but gradually stores within its cytoplasm something
in the form of granules, which under the influence of
nervous impulses or other forms of stimulation more or less
rapidly disappears in the secretion.

6. Activity of the gland involves chemical change within its
cells. It might be supposed that the granules manufactured
during rest are merely dissolved or washed out of the cells

THE WORK OF ORGANS AND CELLS 47

in the copious stream of water and salts which during secre-
tion passes through from the blood and lymph to the duct.
If this were so, it would be possible to dissolve from the
gland a substance exhibiting in general the same properties
as the secretion itself. But this is not generally the case.
Extracts of fresh glands commonly fail to exhibit the char-
acteristic properties of normal secretions, although these ex-
tracts may often be changed by chemical means into the
elements of the secretion. We are therefore compelled to
believe that the activity of a gland means something more
than the mere discharge of previously stored substances ;
that is to say, the material of the granules in the resting
cells is not simply set free when the gland secretes, but is
at the same time chemically changed. In the digestive juices,
for example, we have active substances called enzymes, which,
it has been shown, are derived from other substances, called
zymogens, in the gland cells. The chemical change from the
one into the other is as essential to the process of secretion
as is the visible flow from the duct.

These facts then present to us the picture of the cell as
the working or physiological unit, as we have already seen
that it is the anatomical unit of the gland (p. 32). The
work of the gland is the sum of the work of its constituent
cells. During the period of rest these cells manufacture
from the blood zymogens or other substances which they
store away in the form of granules within their cytoplasm.
When they are stimulated by the nervous impulse a chemical
change takes place in them, the zymogens are changed to
enzymes and other substances, and these, together with the
water, salts, etc., derived from the blood, form the secretion.

7. Physiology of muscular contraction. At first sight mus-
cles and glands seem to differ in action or function no less
than in form and structure. No two acts are apparently
more unlike than lifting a weight by the muscles of the
arm and the secretion of saliva by the salivary glands. But

48 THE HUMAN MECHANISM

beneath obvious and important differences there are profound
and fundamental similarities in the processes which occur in
the two organs during activity. Like the gland, the muscle
is set to work or stimulated by a nervous impulse ; its con-
traction is accompanied by an increased blood supply; and,
most important of all, the work, or contraction, is accom-
panied — indeed, preceded — by chemical changes much more
profound than that of the transformation of zymogen into
enzyme. These chemical changes supply the power for the
work.

That some chemical change has taken place when the
muscle contracts is proved by the fact that certain new sub-
stances then make then* appearance in the muscle and are
given off to the blood flowing through it. The most impor-
tant of these are carbon dioxide, the gas which is formed
whenever wood or coal is burned, and an acid substance
known as lactic acid. These substances were not present
in the resting muscle, or else were present in very small
quantities. With the act of contraction relatively large
quantities of them make their appearance. They are gener-
ally spoken of as waste products, and it is known that they
are the result of a chemical change in the muscle fiber, or cell,
precisely as the enzymes are the result of chemical changes
in gland cells. Just as glandular activity produces an out-
put called a secretion, so muscular activity produces an
output consisting of substances usually described as waste
products.

8. The storage of fuel within the muscle fiber. The source
of the carbon dioxide and lactic acid produced by the active
muscle must in the long run be the matter taken into the
body in the form of food. After undergoing in the stomach
and intestine relatively simple changes, which do not pro-
foundly affect its chemical constitution, this food is absorbed
into the blood and through this channel delivered to the
cells. Thus far, however, the food material does not differ

THE WORK OF ORGANS AND CELLS 49

greatly from the food as swallowed. Especially to be noted
is the fact that it does not undergo sudden and profound
chemical change. When, on the other hand, a muscle is
stimulated to contraction, there occurs in it a chemical
change requiring less than the hundredth of a second for its
completion. This of course suggests the chemical change in
gunpowder or dynamite. Obviously the food delivered by
the blood to the muscle fiber has been transformed in the
fiber into something more unstable, something capable of
a very sudden chemical change. The meat, bread, butter,
potatoes, and the like have been changed into something
comparable to the phosphorus in a match or the gunpowder
in a percussion shell. This unstable material has not been
demonstrated as granules or other visible material within the
cell, as have the zymogen granules of a gland; nor has it
been extracted from the cell, as have mucin and enzymes ;
but the facts force the conclusion that, like the gland cell,
the muscle fiber has used its period of rest to make and store
within itself an unstable compound which undergoes upon
the application of a stimulus a very sudden chemical change.
This unstable compound we may call the fuel substance or
the fuel of the fiber.

9. Available and reserve fuel. The analogy of a match is
useful to make clear these fundamental conceptions of mus-
cular activity. The phosphorus on the head of the match is
the unstable fuel substance ; the friction of the match when
it is rubbed against a rough surface is the stimulus, which
is followed by a sudden chemical change in the fuel when
the match " goes off." At this point, however, the analogy
ends; for when a second stimulus is applied to a muscle
'within one tenth of a second, there is a second contraction,
and in this second contraction there is the same sudden
chemical change in the fuel ; moreover, this stimulation may
be repeated over and over again with like results. Even
more striking is the fact that the same thing is true of a

50 THE HUMAN MECHANISM

muscle removed from the body and consequently shut off
from access to new fuel supply in the blood flowing through
it. Such an excised muscle will give a long series of con-
tractions upon the repeated application of stimuli. With the
match or the percussion cap, on the other hand, such repeated
discharges would not occur, for the entire stock of fuel is
used up with each discharge.

In order to explain these facts, it is commonly assumed
that the fuel substance of the muscle fiber exists in two
forms: the one unstable and ready to be discharged by the
stimulus ; the other and larger part incapable of being dis-
charged by the stimulus, but rapidly providing, after each
discharge, the material to make good the loss of unstable fuel.
We may speak of the one as the available or unstable fuel and
of the other as the reserve fuel.

10. The chemical change of unstable fuel into waste prod-
ucts involves cleavage and oxidation. Although our present
knowledge is inadequate to the full understanding of the
chemical changes in the muscle during activity, it can at
least be stated that changes of two kinds are involved,
namely oxidation arid cleavage.

Oxidation is the union of the material with oxygen, one
of the gases of the atmosphere. When carbon (charcoal) is
burned, for example, it disappears by uniting with oxygen
to form the colorless gas, carbon dioxide ; when hydrogen is
burned, it unites with oxygen to form water; or if a chem-
ical compound of carbon and hydrogen (for example, kero-
sene) is burned, its carbon unites with oxygen to form carbon
dioxide, while its hydrogen unites with oxygen to form water.
Conversely, when we find that the products of any chemical
change contain more oxygen than the original substance, we
infer that the change is a combustion or, as it is generally
called, an oxidation.

The second kind of chemical change, cleavage, takes place
without the addition of oxygen or, indeed, of any other

THE WORK OF ORGANS AND CELLS 51

chemical element, .except that water is often added to the
material changed. In this process the combination of differ-
ent atoms which makes the compound is broken and the
molecule is split into two or more molecules.1 Thus new
compounds or substances are formed.

In the muscle fiber both these changes occur during con-
traction. Many, perhaps the majority of physiologists, now
think that the stimulus to the muscle fiber (nerve impulse,
electric shock) first causes a cleavage of the unstable fuel
of the fiber into lactic acid and possibly other products and
that this cleavage is the cause of the contraction ; under
normal conditions this is followed by an oxidation of the
lactic acid to carbon dioxide and water

C3H6O3 + 3 O2 = 3 CO2 + 3 H2O.

On this view the cleavage takes place very suddenly (per-
haps requiring less than the hundredth of a second), while
the oxidation which follows requires several seconds or even
minutes for completion ; indeed, before it is complete, some
of the lactic acid may have passed out of the muscle fiber
into the lymph and blood. Some of the facts supporting this
view are the following: the lactic acid produced within the
muscle during contraction increases with the intensity of the
work ; the amount of it found after contraction is greater
when the supply of oxygen from the blood is diminished or
cut off ; and, finally, lactic acid disappears from the muscle
more rapidly after contraction, when the blood is well sup-
plied with oxygen, than when it is deficient in that element.

1 Matter is composed of atoms of chemical elements ; these atoms are
combined or bound together to form molecules. A lump of sugar, for ex-
ample, would be composed of an inconceivable number of molecules of sugar,
and each molecule would consist of six atoms of carbon, twelve atoms of
hydrogen, and six atoms of oxygen bound together in chemical combination.
Sugar may undergo cleavage into lactic acid by splitting its molecule of
twenty-four atoms into two molecules of twelve atoms. The chemist ex-
presses this by the following equation :

52 THE HUMAN MECHANISM

Let us not lose sight of the central fact. The activity of a
muscle fiber, like the activity of a gland cell, is the result of a
chemical change within the cell. In both cases the food mate-
rial derived from the blood is transformed into something else
and activity is accompanied by the production of new sub-
stances. In the case of the gland these new substances, or
part of them, go to form essential constituents of the secre-
tion, and we see at once the end secured by the chemical
change. In the case of the muscle the end is, at first sight,
not so clear. The substances formed are not of obvious use
to the body, and we have now to inquire how this chemical
change serves the purpose of producing a muscular contraction.

11. Relation of the chemical changes to the work of muscular
contraction. It is a familiar fact that chemical changes often
yield power for work. The explosion of dynamite (a cleavage
change), for example, will shatter large masses of rock ;
the oxidation of coal in a locomotive engine supplies the
power to move a heavy train of cars. In both cases waste
products are produced, and in the change which produces
them power is liberated; but in order that this power may
be utilized to do work, some mechanism is needed to apply
it to the desired end. The burning of coal in an open grate
liberates power, but in the absence of any mechanism adapted
to that purpose, it does no work ; the same coal burned under
the boiler of an engine, with its mechanism of boiler, piston,
driving rod, and wheels, moves the train of cars.

So it is with the muscle. Within the cytoplasm of the
fiber are the myofibrils (p. 35), and there are convincing
reasons for believing that the combination of myofibril and
sarcoplasm constitutes the mechanism of the muscle fiber.
The power liberated by the cleavage change acts upon this
mechanism, causing the shortening and thickening of the
myofibrils, whereby a pull is exerted on the tendon. Whether
the subsequent oxidative changes also contribute power for
the work or merely produce heat is still an open question.

THE WORK OF ORGANS AND CELLS 53

12. Heat production by the working muscle. One other
point of similarity between the working muscle fiber and the
working steam engine should be pointed out; namely, that
both produce heat. It is a familiar fact that muscular activity
makes us feel warm. This is the direct result of the libera-
tion of heat by the oxidations within the working muscle
fiber. The same thing is true of the steam engine, the liber-
ated heat going in that case to warm the engine or passing
away in the gases which escape from the smokestack, steam
vents, etc. It is important that the student of physiology
bear clearly in mind this feature of muscular action, since
the active muscles not only supply power for work but also
the heat necessary to maintain the temperature of the body,
and no muscle can be thrown into contraction without liber-
ating a certain amount of heat. For a full discussion of this
matter see Chapter XII.

13. The repair and maintenance of the cellular nu

Thus far we have considered only those chemical activities
of gland and muscle cells which are directly concerned with
secretion and contraction or which prepare the cell for the
performance of these functions. This is only a part, how-
ever, of the work of living cells, for, like all machines, cells
may be injured by overwork or by accident, and their parts
(nucleus, cytoplasm, fibrils, etc.) must be kept in working
order. Just here the living mechanism differs from the life-
less engine, for the living mechanism is itself capable of
repairing damage to itself. The locomotive must be sent to
the shops and be repaired by work done upon it by other
machines; if the boiler rusts, it must be taken out and a
new one put in ; if the wheels wear unevenly, they must be
made true again by turning in a lathe or new ones must
be substituted ; when the grate burns out, a new one must be
put in its place. The living cell, on the other hand, itself
makes these repairs from certain constituents of the same
food out of which fuel and zymogen granules are made, and

54 THE HUMAN MECHANISM

it does this by other chemical activities than those we have
described and about which we possess only fragmentary
knowledge at present. In picturing to ourselves the activi-
ties of these living mechanisms we must include all these
chemical processes, those of maintenance and repair as well
as those concerned immediately with the performance by
each cell of its own special functions, such as secretion by
a gland and contraction by a muscle.

14. Recapitulation. We have traced the character of the
work done in the case of the gland and the muscle and have
found that it is fundamentally the work of the cells of which
the organs are composed. The cells of other organs are simi-
larly constructed to do other kinds of work, and the character
of their chemical changes and of the mechanisms for utilizing
power varies accordingly ; all, however, showing the same
fundamental plan of working engines. The body is a com-
munity of groups of cells of different kinds, each kind doing
some work more or less peculiar to itself. In addition to the
two groups (gland and muscle cells) which we have studied,
there are nerve cells in the brain, spinal cord, and elsewhere ;
cells which make blood corpuscles; cells which keep in repair
the connective tissues (bone, gristle, tendon, and ligament) ;
and many more, such as cells which manufacture or them-
selves form the lining of free surfaces, like the skin, the ali-
mentary tract, the air passages, etc. The sum total or net
result of the activities of these and other cells makes up the
work of the body as a whole. The work of the body — the
human organism, the human mechanism — is thus the outcome
or resultant of the work of its different component cells.

CHAPTER V
WORK AND FATIGUE

While it is true, as shown in the last chapter, that ca-
pacity for work is one of the principal characteristics of the
human body, no experience of daily life is more familiar
than that work is followed by fatigue. This is true both
of individual organs and of the organism as a whole ; for
fatigue may be either local, as when some one muscle is
tired from hard work, or general, as when weariness affects
all organs — those which have been resting as well as those
which have been working.

We use the word "fatigue" in two different senses, and it
is important that a distinction be clearly made between them.
In the one sense the word means the diminution of working
capacity due to work. In testing one's strength of grip or of
back a second test, if made immediately, shows less work done
than at the first test, and this is true whether or not we
are conscious of fatigue or of diminished working power. If,
however, a certain time be allowed for rest, the second test
will give as good results as the first.

In the other sense the word refers to the feeling of fatigue
which frequently, though not always, accompanies the dimi-
nution of working power. We may " feel tired " when we
have been doing nothing, and conversely, under the influence
of excitement or other causes we may experience no feeling
of fatigue even when we are near the limit of our working
power. Often in an exciting game the players do not know
at the time that they are tired or even that their working
power is lessened; and stories are told of soldiers in hasty

THE HUMAN MECHANISM

retreat who feel that they must " drop in their tracks "
until the discharge of musketry close behind stimulates
them to move faster than ever.

The feeling of fatigue has its seat in the nervous system,
and its study must be postponed until we have learned some-
thing of the physiology of the brain and spinal cord. In the
present chapter we are not immediately concerned with this
side of the question, but rather with the diminution of work-
ing power produced by
work. Such fatigue
must be measured not
by our sensations but
by the work accom-
plished, whether that
work be physical or
mental. And as we
studied the physiology
of work in its simplest
form in a single work-
ing organ, such as a
muscle or a gland, so we

can best begin our study
FIG. 33. Diagram of apparatus for recording • • ? j •

successive muscular contractions 01 diminished working

power or fatigue in one
of these same organs, namely, the skeletal muscle.

1. Fatigue of an isolated muscle and of a muscle with in-
tact circulation. The course of fatigue in a muscle is best
studied by causing the muscle to contract to its utmost, at
regular intervals of time, against the resistance of a suitable
spring. If now we record the height of each contraction, we
obtain a series which shows at once the effect of the work
on the working power ; that is, the course of fatigue. Fig. 33
gives a diagram of the arrangement of such an experiment
with an isolated muscle ; that is, a living muscle detached
from the rest of the body. One tendon is made fast in a

WORK AND FATIGUE

rigid clamp, while the other is attached to the spring, which

is stretched by the contraction when the muscle is stimulated.

The length of the line written by the lever AB records

what the muscle is capable

of doing at the time ; and

if the records of successive

contractions are made on

the smoked surface of a

slowly revolving drum, as

in the figure, we have at

once a record of the course

of fatigue.

Such fatigue tracings may
also be taken from a muscle
within the body, and hence
with its circulation intact.
Thus the work of the biceps
muscle in bending the arm
at the elbow (Fig. 23) may
be recorded by instruments
essentially similar to that
used with the excised muscle.
In Fig. 34 we have repro-
duced a tracing of this kind.

It is quite evident that a
continuous line joining the
highest points reached by
the several contractions will
represent graphically the
course of fatigue, and in
Fig. 35 the line a represents
this so-called " curve of
fatigue " in the experiment
whose results are given in
Fig. 34. It falls off at first

FIG. 34. Record of the successive con-
tractions of the flexor muscles of the
elbow joint

Showing the gradual decrease in working
power to a fatigue level. The muscle con-
tracted once every three seconds against
the resistance of a strong spring, which
was stretched each time as far as the
strength of the muscle permitted

THE HUMAN MECHANISM

rather rapidly, then more and more slowly, until at last it
becomes parallel with the base line. In other words, the
muscle in this case finally finds a constant level of working
power. This may be called the fatigue level.

The broken line b in Fig. 35 gives the result of a fatigue
tracing with the isolated muscle. It will be seen that the
fall in the height of contraction continues until at last the
muscle no longer responds to stimulation. The contrast
thus brought out between the effect of work upon muscles

FIG. 35. Curves of fatigue
a, from a muscle with intact circulation ; 6, from an isolated muscle

with and those without the circulation suggests that the cir-
culation of the blood through the working organ in some
way maintains the working power.

The height of the fatigue level in the same muscle at
different times is very closely dependent on the rate at
which the muscle works. Thus with a contraction every
four seconds instead of every three seconds the fatigue
level would be higher than in Fig. 34; with a contraction
every second it would be much lower. When the contrac-
tions come every nine or ten seconds there is usually no
falling off in the work done, the time between contractions
being sufficient for the complete recovery of working power.

This picture of fatigue hardly agrees with our feeling of
fatigue, for the decline of working power begins at once, or

WOKK AND FATIGUE 59

at most after a very small number of contractions, whereas
we usually notice fatigue only after work has gone on for
a considerably longer time. One does not feel tired from
walking, for example, during the first ten or twenty minutes
of the walk. We need not discuss here just what makes us
unconscious of the beginnings of fatigue ; but it is important
to understand that whether we are or are not aware of its
presence, fatigue is the invariable and immediate result of
all muscular work.

Weariness is simply the conscious feeling of fatigue, but
fatigue is a physical condition of living cells and organs.
Moreover, its phenomena are by no means confined to
muscular work. When a gland is stimulated to vigorous
secretion a diminution is sooner or later noted in the amount
of the secretion, and there is some reason to believe that
nerve cells may also become tired from continued activity.
Fatigue, then, in one word, is a natural condition of an
organ accompanying work, and we may proceed to inquire
into its exact cause.

2. Waste products as a cause of fatigue. When blood
which has been circulating through a fatigued muscle is
sent through a resting muscle, the resting muscle shows
signs of fatigue, even though it has itself done no work.
Apparently the blood has extracted from the working
muscle something which has the power of lessening the
working capacity of a fresh muscle.

The same thing is illustrated by another experiment. A
muscle which is deprived of its circulation (for example, by
clamping its arteries and veins) is fatigued by vigorous work ;
it is then found that although when left to itself a slight re-
covery takes place, this recovery is much more marked if we
first pass through its blood vessels a weak solution of salt
Here no food is supplied ; the salt solution has only removed
something from the fatigued muscle, which, in consequence of
this treatment, recovers some of its working power.

60 THE HUMAN MECHANISM

Again, the mere exposure of a resting muscle to blood
containing lactic acid or to blood heavily charged with
carbon dioxide (CO2) produces the condition of fatigue.
Now in the last chapter it has been shown that both lactic
acid and carbon dioxide are waste products of muscular
activity; and these and other facts have led to the view,
now generally received, that the waste products of the
active organ interfere with the work of the organ and so
constitute one of the main causes of fatigue. It is ap-
parently for this reason that the injection of an extract of
worked muscle fatigues fresh muscle, for the extract con-
tains waste products. It is for the same reason that wash-
ing out a fatigued muscle with salt solution produces partial
recovery, for the waste products of activity are in this way
partially removed. We can also understand why fatigue
always accompanies vigorous work. Waste products then
necessarily accumulate and clog the living mechanism be-
cause they cannot be removed by the blood as fast as they
are formed by the muscle cells. No fatigue occurs with
only a single contraction every ten seconds or more be-
cause between contractions sufficient time is given to insure
the complete removal of wastes.

3. Loss of fuel in the working muscle as a cause of fatigue.
The blood, however, not only removes the wastes but also
brings new food and oxygen with which the muscle makes
good the loss of fuel; and it may well be — although it is
not absolutely proved — that recovery from fatigue depends
upon both of these good offices of the blood. We have
certainly one well-established cause of fatigue, namely, the
presence of the waste products of activity ; and we recog-
nize the probability that the depletion of fuel may also
contribute to the result. But whether the first of these
causes alone is sufficient to explain it, or whether both
work together, we can understand that the maintenance
of a good blood supply is of the first necessity and that

WORK AND FATIGUE 61

undue fatigue can be avoided only by working at a moder-
ate rate. It is an old and physiologically true saying that
" it is the pace that kills."

4. Explanation of the fatigue level. In the experiment
with the isolated muscle no waste products were removed
nor were new food and oxygen supplied; hence the wastes
in the muscle increased with each contraction, until at last
their accumulation prevented all contraction. In the normal
muscle the wastes likewise accumulate for a time ; and this
is why the curve of work at first falls (Fig. 34). It does
not continue to fall, because as the wastes within the muscle
increase in amount, the blood carries more and more of
them away in a given time. The quantity of waste removed
thus continues to increase until the same quantity is carried
away from the muscle between two contractions as the
muscle produces with each contraction. When this happens
no further accumulation of waste is possible and the fatigue
level is established.

5. General fatigue resulting from muscular activity. Every-
one knows that after a day's tramp it is not simply the
worked muscles which are unfit for good work, but that
the brain, too, is tired, for hard mental work is then dim-
cult or well-nigh impossible ; and it is generally the fact
that long-continued muscular work fatigues the brain more
than brain (mental) work itself. The obvious explanation
of this fact is that the waste products of muscular activity
have accumulated in the blood more rapidly than the body
can get rid of them, and so have fatigued the other tissues,
including the nerve cells of the brain, just as the injection
of the extract of a tired muscle lessens the working power
of a fresh muscle. No doubt these same waste products may
similarly fatigue gland cells; for experience seems to show
that the secretion of digestive juices is not so active when
one is suffering from muscular fatigue and that it is not
wise to eat heavy meals when one is tired out. We can also

62 THE HUMAN MECHANISM

understand why long-continued, vigorous muscular action
produces marked fatigue in nerve cells and gland cells,
while the activity of the latter produces only inappreciable
fatigue in the muscles; for the amount of chemical change
and the production of wastes are far greater in the case of mus-
cular work than in that of nervous or glandular activity.

6. The analogy of the engine. In previous chapters we
have compared the living body with a machine or locomotive
engine ; both do work, and both obtain the power for work
from the chemical changes in food or fuel. What we have
now learned about fatigue suggests an extension of the
same comparison. Every locomotive suffers impairment of
its working power with use, and special measures are taken
to limit this impairment as much as possible; the gases and
smoke are carried away at once by the chimney or smoke-
stack ; the furnace is provided with a grate so that the ashes
shall not accumulate and shut off the draft ; the bearings are
oiled and foreign matters removed; finally, as the consump-
tion of fuel goes on, the loss is made good by stoking.

The continuance of the work of the engine requires two
things — fresh supplies of fuel and the removal of wastes.
Obviously the blood performs these same offices for the cell.
It supplies to the cell fuel (food) from the alimentary canal
and oxygen from the lungs and it carries away the waste.
Provision is thus made to maintain the human machine in
working order and good condition during its activity. If the
blood flows too slowly through the muscle, the same thing
happens as in the locomotive when the fireman neglects to
rake the fire or to put on new fuel ; the efficiency both of the
human engine and of the locomotive may be impaired either
by the undue accumulation of the waste products of its own
activity or by the neglect to supply proper food or fuel.

CHAPTER VI

THE INTEKDEPEKDENCE OF OKGANS AND OF CELLS
INTERNAL SECRETIONS

1. The products of cellular activity not necessarily harm-
ful. We have now learned that the active living cells of
the body are the seat of chemical changes which produce
new substances ; that the accumulation of these products of
activity often limits the working power of the cells in which
they are produced, and may even depress the activity of
other cells to which they are carried by the blood. In the
case of the skeletal muscles we have spoken of the carbon
dioxide, the sarcolactic acid, etc. as "waste products," mean-
ing thereby that they are incapable of serving as sources of
power for the work of the muscle; and this term, together
with the fact that they constitute one cause of fatigue, is apt
to mislead us into supposing that they can be of no further
use to the body or, even more, that they are necessarily
harmful and that their presence in the blood is objectionable.

These conclusions, however, do not necessarily follow from
the facts. It does not even follow that a substance which
produces fatigue for that reason serves no useful purpose.
Most adults can recall times when because of long-continued
application to mental work or because of worry or other
nervous strain they have become overexcitable and restless
and have been unable to obtain the sleep of which the body
as a whole stands in need. At such times sleep is often
best secured by producing general fatigue through muscular
work. The waste products, by their very act of fatiguing
the overexcited nerve cells, may be of service to the body

THE HUMAN MECHANISM

as a whole ; and it is probably true that not only in such
abnormal conditions but also in the daily conduct of life the
fatigue of moderate muscular activity contributes its share
toward inducing healthful and refreshing slumber.

Thus far we have considered the chemical activities of
each organ as contributing to the work of the organ in
which they occur and, because of the accumulation of waste

products, as the occasional
cause of undue interference
with efficient activity, both in
the working organ and else-
where. And yet the familiar
case which we have just cited
suggests another view of the
matter. The products of the
chemical activity of one organ
may be of service to other
organs, and so to the body
as a whole; and while their
too rapid accumulation in the
FIG. 36. Cross section of the thyroid blood maJ be undesirable,

gland

their presence in moderate

The cells secrete into the closed sacs, amounts may be beneficial
^tUS7£±£i££ and may contribute to the
the cells into the lymph spaces of the normal environment of the

connective tissue ^^ f .-, •, •*

cells of the body.

2. The thyroid gland. This view of the case is strikingly
emphasized in the physiology of the thyroid gland — a small
organ in the neck, the two chief lobes of which lie alongside
the trachea. For a long time its use was not understood,
and at times it was even supposed that it plays no important
part in the life of the body as a whole. It has been found
by experiment, however, that removal of the thyroid is fol-
lowed by a disease in all respects similar to one which had
long been observed in human beings, especially in children;

INTERDEPENDENCE OF ORGANS 65

and this fact suggested that the disease is due to the failure
of the thyroid to perform its normal functions.

The subject was further cleared up by the discovery that
after the removal of the thyroid in a lower animal the disease
in question could be prevented by feeding the animal thyroids
or even by giving to it a certain substance extracted from
them. Evidently the thyroid manufactures and discharges
into the blood a peculiar substance necessary to the healthy
life of the cells of the body ; and when the gland fails to
manufacture this substance it can still be supplied artifi-
cially by introducing it into the blood by absorption from
the alimentary canal.

3. Internal secretions. In our study of secretion in Chap-
ter IV (p. 43) we dealt only with glands which discharge
their principal products through a duct into some part of the
alimentary canal; such glands are the salivary glands, the
pancreas, and the liver. Other glands send ducts to the surface
of the body — for example, the sweat glands, which discharge
perspiration upon the skin ; and the lachrymal glands, which
discharge the tears on the eyeball. In the case of the thyroid,
on the other hand, we have an example of an organ which,
like those just mentioned, manufactures a special substance
from the blood, but, having no duct, contributes the products
of its manufacture to the blood, for the use of other cells.
This process is spoken of as internal secretion, to distinguish
it from ordinary secretion, in which case something is dis-
charged on a free surface like the skin or into the alimentary
canal, the nasal cavity, or the air passages.

4. The adrenal glands. Lying immediately above the kid-
neys are two small glandular organs, the adrenals, which, like
the thyroid, were formerly considered of minor importance.
It has been shown, however, that these also contribute to
the blood a most important internal secretion known as
adrenaline. This substance is manufactured by the gland cells
and, during their periods of inactivity, is stored within the

THE HUMAN MECHANISM

cells, from which it is discharged by nervous impulses. Like
the thyroids, the adrenals have no ducts ; but the cells of
the gland come into very close relation with the unusually
rich network of blood capillaries into which the adrenaline is
discharged when the gland is stimulated by its nerves.

Once in the blood, ad-
renaline produces profound
effects in many organs of
the body. Among these may
be mentioned a decrease
in the blood supply to the
digestive organs; a change
in the beat of the heart;
an increased flow of blood
through the brain, the skele-
tal muscles, and, to a less
extent, the skin; the dis-
charge of sugar into the
blood by the liver; and an
increase in the number of
the red blood corpuscles

FIG. 37. Diagrams of external (A) and (see P* 136).

internal (B) secretion It is a most significant

fact that many if not most
of the reactions of the or-
ganism to adrenaline are the
very reactions which are
needed in times of great
muscular exertion. For example, the shifting of the blood
from digestive organs to the working muscles and to the
brain, which is thereby rendered more alert; the supply of
increased quantities of sugar to serve as power for muscular
work ; the assistance to the heart, which is called upon at such
times to pump more blood; the augmented oxygen-carrying
capacity of the blood by increase of its red corpuscles — all

The passage of food material from the
capillaries into the gland cells is repre-
sented by the arrows with broken lines;
the path of discharge of the secretion, in A
into the duct and in B into the blood, is in-
dicated by the arrows with unbroken lines

INTERDEPENDENCE OF ORGANS 67

these reactions place at the disposal of the muscles and nerv-
ous system the conditions for maintaining intense work for
comparatively brief periods of time, and all this is done by
the simple expedient of discharging from one of the organs
of the body an internal secretion on the blood.

Finally, that adrenaline does in fact serve the purpose of
placing the body in condition to perform intense muscular
work is rendered probable by the discovery that conditions
of emotional excitement, especially those of fear or anger,
cause the discharge of nervous impulses to the adrenals.
Among animals it is these very emotions which accompany
or at least precede the most vigorous muscular activity,
fear going along with flight and anger with combat. This
suggests that these emotions serve the purpose of calling
forth the utmost of which the animal is capable in preserving
its very existence.

5. Other examples of internal secretion. An equally re-
markable discovery has shown that the pancreas not only
manufactures an important digestive juice (pancreatic juice)
which it discharges into the intestine through its duct (pan-
creatic duct, see Fig. 54) but also produces another sub-
stance which is necessary, in order that other organs may
use the sugar which is in their food. Here we have an ex-
ample of an organ which produces both an ordinary and
an internal secretion, and the same thing seems to be true
of the kidney, as it certainly is of the liver.

Much attention has recently been given to the study of
another ductless gland, the pituitary body, situated in the
bone between the roof of the nasal cavity and the base of
the brain (see Fig. 14). There is good reason for thinking
that this gland contributes an important internal secretion
to the blood and that certain organs of the body fail to act
normally when this secretion is deficient ; serious results also
follow an excessive secretion. Incidentally it may be men-
tioned that it is widely held that excessive secretion of the

68 THE HUMAN MECHANISM

thyroid leads to a very serious condition, known as Graves's
disease or exophthalmic goiter, just as deficiency of the secre-
tion leads to the entirely different disease to which we have
already referred.

Thus, through the medium of the blood the chemical
activity of one organ may affect the life of other organs
favorably or unfavorably. All the cells of the body help to
make the blood what it is, many of them contributing to it
something useful or even necessary to other cells. The work
of the body is not merely the sum total of the work of its
separate cells, each working for itself alone and performing
a single function; between the cells an exchange of prod-
ucts often takes place, so that cells become both serviceable
to and dependent upon one another for the material needed
to carry out their own special chemical activities. And what
is true of cells is no less true of organs; these also are
interdependent, ministering to one another.

CHAPTER VII

THE ADJUSTMENT OK, COORDINATION OF THE
WORK OF ORGANS AND CELLS

A great physiologist once said, " Science is not a body of
facts ; it is the explanation of facts." Some of the most im-
portant chapters of science are those which seek to explain
facts so well known and obvious that we are apt to forget
that they need explanation. When anything irritates the
lining of the nasal cavity we sneeze ; when it irritates the
larynx we cough; when it irritates the exposed surface of
the eyeball we wink. These three facts are well enough
known ; but it is safe to say that anyone considering the
matter for the first time would find it difficult to explain
how it comes about that anything going " down the wrong
way " does not make us sneeze or wink, but sets us to
coughing. The answer to the general question thus raised
is the subject of this chapter, which considers the adjust-
ment of the work of the individual cells and organs of the
body, each to do its work at the proper time and so to play
its due part in the work of the organism as a whole.

The more we think of it, the more wonderful does this
fact of adjustment appear. The millions of living cells are
in a way individual units, and communities of individuals do
not invariably work together. Let us compare the human
body in this respect with bodies or groups of men or boys.
In a game of football each team is a body of eleven indi-
viduals, and each individual is assigned to a definite posi-
tion to do definite things as occasion arises. Theoretically,
under given conditions of the game it is the work, or
function, of the " left tackle " to prevent a certain player

70 THE HUMAN MECHANISM

of the opposing side from making a certain play. But there
is always a doubt whether he will do this thing or " lose
his head" and do something else, leaving his man free to do
what he pleases. In the latter case that organism which we
call a football eleven would act very much as the human
organism would act if it were to wink and not cough when a
foreign body lodges on the lining membrane of the larynx.

Evidently we have something here to explain. Why are
the actions of the body purposeful; that is, adapted to ac-
complish the right thing at the proper time? And in the
more complicated actions how is the work of the different
units — the organs and the cells — adjusted, or coordinated',

that is to say, how is each
one made to do its proper
share of the work ? Let
us begin with the study
of a very simple action —
that of winking.

1. Winking is caused by

the contraction of muscle
FIG. 38. The muscular mechanism fiberg which mn trang_
of winking

versely across the eyelid

in a curved course. As they are attached most firmly at
the regions A and B (Fig. 38), their shortening straightens
their arched course and so brings the two edges of the eye-
lid into contact. The work of this muscle is obviously pur-
poseful, for the wink takes place when the eyeball needs
protection; it is also coordinated, since the act is executed
by a number of fibers working together, for if only those of
the lower eyelid were to contract the lids could not be closed.
The muscle fibers which work together to produce the
wink do not originate their own activity. They merely do
what they are stimulated to do by the nervous impulse,
which acts upon the muscular fuel substance somewhat as
a fuse acts upon a charge of gunpowder. Even the amount

COORDINATION

of contraction is determined by the strength of the nervous
impulse, a strong impulse producing greater contraction
than a weak impulse. In health the muscle fibers are the
obedient servants of the nerves, and if they act in a pur-
poseful and coordinated manner, it is because the nerves
stimulate them to act in this way. The explanation of
purposeful and coordinated action must therefore be sought

FIG.

Cross section of a nerve

Showing five bundles of nerve fibers bound together by connective tissue con-
taining a few blood vessels. On the right are shown four fibers more highly
magnified, the dark center being the ax on, around which is the white or fatty
sheath, both axon and fatty sheath being inclosed within the fine membrane,
the neurilemma. Cf . Fig. 40

not in the muscles but, behind these, in the nervous system,
to the study of which we now turn.

2. Structure of a nerve. A nerve, like a muscle, may be
separated into long fibers (Fig. 40) which are bound together
by connective tissue containing blood vessels, lymph spaces,
and lymphatics. The nerve fiber, which is the essential
part of the nerve, just as the muscle fiber is of the muscle,
differs somewhat in structure in different nerves; it generally

THE HUMAN MECHANISM

consists of a central threadlike core surrounded by a fatty
sheath, the latter being, therefore, shaped like a hollow
cylinder, — which, however, is interrupted at intervals of
about one millimeter, — and both of these are enveloped in a
delicate membrane comparable to the sarcolemma of the muscle
fiber. Such fibers are from about -5-^5-5- to y-^Q- of an inch
in diameter (compare the diameter of a muscle fiber, p. 34).

There are, however, nerve fibers
which have no fatty sheath, and
others which are destitute of mem-
brane. The essential part of the
fiber is the threadlike portion in
the center; this is never absent
from nerves and is known as the
axon, or axis cylinder.

3. The axon of a nerve fiber is
a branch of a nerve cell. By suit-
able methods these axons may be
traced along the nerve of which
they form part and even into the
brain and spinal cord; it is then
found that they pursue an uninter-
rupted course and ultimately become continuous with the
cytoplasm of a nerve cell. Nerve cells are found in the brain,
in the spinal cord, in enlargements (ganglia) on certain
nerves, and even alone in the connective tissue of many or-
gans of the body, as the heart, the intestine, etc. By far the
greater number are in the brain and spinal cord, and in some
cases the axons to which they give rise are of very consider-
able length; those of the muscles of the foot, for example,
reach from cells in the sacral region of the spinal cord to the
extremity of the foot. Such fibers would be over a yard long
and less than y^o" °^ an mc^ wide, an(^ we mav regard the
cell whose main portion is in the sacral cord as sending out
a branch, or process, from this region to the foot.

FIG. 40. Four nerve fibers
(highly magnified)

R, node of Ranvier at which
the fatty sheath is discon-
tinuous

COORDINATION 73

Furthermore, recent investigations have led to the gener-
ally accepted conclusion that each axon is a part of only one
nerve cell; a single cell may give off more than one axon,
but the axon is never connected with more than one nerve
cell. Of these cells and of their connections with nerve

FIG. 41. Four nerve cells

A and C, from the cerebellum ; B, from the gray matter of the spinal cord ;

D, from the cerebrum ; a, the axon. The cells A and D are stained so that the

main body and the dendrites (p. 75) are a uniform hlack ; B and C are stained so

as to show the nucleus and the cytoplasm

fibers we can get a more definite picture by an examination
of the structure of the spinal cord.

4. Structure of the spinal cord. When the vertebral canal
is opened a whitish cord is found within it, — the spinal
cord, — from each side of which arise thirty-one pairs of
nerves, or, in general, one pair for each vertebra. One
nerve of each pair arises on the ventral side of the cord, the
other on the dorsal side. These nerves are known as the

THE HUMAN MECHANISM

Dorsal

Dorsal Root

Dorsal
Ganglion

ventral and dorsal nerve roots1 respectively. On the dorsal
nerve root, some distance from the cord, there is a slight
enlargement, or ganglion. Just outside this ganglion the
two roots unite, and from their union nerves pass to the
skin, the muscles, the blood vessels, the viscera, etc.

The spinal cord itself in cross section shows a darker
central core, known as the gray matter, surrounded by an
outer lighter portion, the white matter. The white matter

consists essentially of
nerve fibers which run
lengthwise of the cord
and here and there send
branches into the gray
matter; it may be re-
garded as a large nerve.
The gray matter, on the
other hand, contains a
mesh of fibers and, in ad-
dition, numerous nerve
cells. There is the same
difference everywhere
between the white and
gray matter of the nerv-
ous system ; the arrange-
ment in the brain is not
so simple as in the cord, but here also the white matter con-
sists of fibers running from one part of the nervous system
to another, while the masses of gray matter always include
collections of nerve cells.

5. Fibers of the ventral, or anterior, nerve root. These
fibers may be traced into the spinal cord. It is then found
that the nerve cells from which they arise lie in the gray
matter in the immediate neighborhood of the root to which

1 The older anatomical terms and those even to-day more generally used
are "anterior" and "posterior," instead of "ventral" and "dorsal."

Dorsal

Dorsal Root

Ganglion

FIG. 42. The origin of the dorsal and

ventral nerve roots of a segment of the

spinal cord

COOKDINATION 75

they belong; that is, the fibers of the roots do not come from
higher or lower parts of the cord or from the brain. It has
been found that when these roots are stimulated they throw
muscles into contraction and produce effects on the blood
vessels and glands, but they do not give rise to sensations
or produce other effects in the cord itself. In other words,
the fibers of the ventral root conduct impulses from the cells of
the spinal cord outward; they do not conduct impulses from
outside into the spinal cord. Hence they are known as
efferent fibers (Latin ex, " out of " ; ferre, " to carry " ).

The nerve cells from which these fibers arise consist of a
mass of cytoplasm around the nucleus and of one or more
outgrowths of this cytoplasm, usually more or less branched.
These outgrowths of the cytoplasm divide and subdivide, ulti-
mately forming in the gray matter exceedingly fine terminal
branches like those of a tree in the air. Such processes are
known as dendrites (Greek dendron, " a tree " ). The nerve
cells in question have numerous dendritic processes ; in other
nerve cells there may be but one, and still others possess
no den'dritic processes at all. In all cases the general appear-
ance of the cell depends largely upon the number and man-
ner of branching of these dendrites. Thus it happens that
nerve cells differ from one another in appearance just as
a Lombardy poplar, an oak, an elm, and a maple differ,
although all show the fundamental characteristics of a tree
(Fig. 41; see also Figs. 109, 110, and 111).

In subsequent portions of this work it is unnecessary for
us to go into the details of the form of the nerve cells to
any extent ; the student need only understand henceforward
that nerve cells consist of a central mass of nucleated cyto-
plasm from which proceed outgrowths, or processes, which
are of two kinds: (1) those which become axons of nerve
fibers and which form an essential part of all nerve cells ;
and (2) the dendrites, which are usually but not always
present. The whole structure, including the central cell

THE HUMAN MECHANISM

body with its dendrites and axons, is an anatomical unit —
a cell. To this entire cell the term " neurone " is given. The
neurone is the cellular unit of the nervous system, just as
the muscle fiber is the cellular unit of the muscle, and the
gland cell of the gland.

6. Fibers of the dorsal, or posterior, roots. The ventral
roots, as we have seen, are entirely efferent in function ;
that is, they conduct impulses only away from the spinal

FIG. 43. Semidiagrammatic longitudinal section of a ganglion of the dorsal

(posterior) root

cord. The dorsal, or posterior, roots, on the other hand, are
found to be essentially afferent (Latin ad, " to " ; ferre, " to
carry " ) ; that is, they carry impulses from outside toward and
into the spinal cord. This is shown by the fact that when
these roots are destroyed by disease, muscles can still be
thrown into contraction, glands will still secrete, etc., —
that is, there is no interference with efferent impulses, —
but no sensations are received from the part of the body
to which these nerves are distributed; pinching the skin is
riot felt; the flesh may be burned and its owner suffer no

COORDINATION 77

pain. Since these results never follow destruction of the
ventral roots, we must conclude that impulses enter the cord
solely by the dorsal roots precisely as they leave the cord solely
by the ventral roots.

It has been stated above (p. 74) that there is a ganglion
on the dorsal root. Microscopic study of this ganglion shows
that the fibers of the dorsal root pass through it and that
each fiber gives off at right angles to itself a branch which
becomes continuous with a pear-shaped nerve cell of the
ganglion. These cells have no other processes. We may
express the relation between the pear-shaped cells of the
ganglion and the fibers of the dorsal root by saying that the
single axon from the main cell body divides into two in
the ganglion, one branch passing outward to the periphery,
the other passing centrally into the spinal cord (Fig. 43).

7. Endings of the peripheral branches of the neurones of the
dorsal root in sense organs. The peripheral branch ultimately
ends in some " sense organ," one of the most important of
which, so far as the spinal nerves are concerned, is the skin.
The eye, the ear, the nose, the mouth, are examples of other
sense organs, and they all contain the peripheral endings of
afferent neurones. Each is sensitive to some special influence
from without, as the eye to light, the ear to sound, etc. ;
and when stimulated they start nerve impulses moving in-
ward along the nerves toward the brain or cord.

8. Ending in the spinal cord of the central branch of the
neurones of the dorsal root. The other or central branch
passes into the spinal cord. It does not, however, like the
neurones of the ventral root, there become continuous with
the nerve cells of the gray matter,1 but divides, on entering
the cord, into an ascending and a descending branch (Fig. 44),
each of which runs for a longer or shorter distance in the
white matter of the cord. Indeed, many of the ascending

1 It is, as has already been pointed out on this page, part of a nerve cell
in the ganglion of the dorsal root.

THE HUMAN MECHANISM

branches extend as far anteriorly as the lower parts of the
brain. As shown in the figure, these branches give off at
right angles to themselves subbranches (the collaterals), each
of which enters the gray matter and ends there by breaking
up into a tuft of extremely fine fibrils, the synapse. The
synapse is in close proximity to, and possibly in a kind of
anatomical continuity with, the dendrites or the main body

A B

FJG. 44. Relation of afferent (of) to efferent (ef) neurones of the spinal cord

In A the single afferent neurone branches into six collaterals, each of which ends

in a synapse around an efferent cell. In B the connection is made through the

agency of the cell a, as explained in section 13

of a nerve cell of the gray matter. Each afferent neurone,
then, is a cell whose main body is in the ganglion of the
dorsal root and whose branches, or arms, reach out, one of
them to a peripheral sense organ and the other to the gray
matter of the spinal cord and brain, where they end in
synapses. By means of the synapses the afferent neurone
excites or stimulates other neurones.

9. Anatomical relation of afferent to efferent neurones. We
may now put together what we have learned about the
neurones of the ventral and those of the dorsal root; we

COORDINATION

then obtain a plan like that shown in Fig. 44, and such, in
principle at least, represents the manner in which the afferent
neurone is brought into relation with efferent neurones.

Afferent and efferent fibers enter and leave portions of the
brain in much the same way, although the separation into
ventral and dorsal roots is not obvious. We may there-
fore take the above scheme as typical of the relation be-
tween these two kinds of neurones — those of the brain as
well as those of the cord.

10. Application of these facts
of structure in the explanation
of purposeful and coordinated
action. The diagram in Fig. 45
readily explains why the sud-
den appearance of an object in
front of the eye causes us to
wink and not cough ; that is to
say, it explains the purposeful
character of this so-called reflex
action. The formation of the FIG. 45. Diagram of the nervous
image of the object on the mechanism by which a wink is pro-
duced by the sudden appearance of
retina, a sense organ, starts an object in front of the eye

impulses along the fibers of r, afferent neurone of the optic nerve ;

the afferent optic nerve ; these m> m/> m"> m'"> efferent neurones to

. . the muscles of the eyelid

fibers extend into the brain,

and their synapses end around and stimulate those efferent
nerve cells which stimulate the muscles of the eyelid. The
action is purposeful because the fibers of the optic nerve
end around these cells and not around those which, for
example, innervate * the muscles which open the mouth or
flex the finger (Fig. 45).

Our diagram also gives the basis of coordination — the
combination of the work of different muscle fibers in orderly
harmonious action. The system of collaterals on the central
i That is, supply with nerve fibers.

THE HUMAN MECHANISM

branch of the afferent neurone is obviously a mechanism to
combine the action of the efferent neurones in this way. The
diagram also gives a clew, at least, to the explanation of
another element of coordination: when two or more muscles
work together to accomplish a given act, one of the muscles
usually works harder than another; not only must they work
together, but the amount of force exerted by each must be
adjusted to the needs of the movement as a whole. This

adjustment is most
probably effected by
differences in the con-
nection of the , syn-
apses with their cells ;
thus those muscles
which contract most
forcibly are innervated
by neurones whose den-
drites and main cell
body come into more
intimate contact with
the synapses of the
afferent neurone ; or
the number of fibrils
of the synapse may
be greater in their case
than in the others. These, however, are only possibilities ; the
whole subject requires further elucidation.

11. Definition of reflex action. An action such as we have
just been studying is known as a reflex1 action. By this we
mean an action called forth by the more or less direct action of
afferent upon efferent neurones and without the intervention of

1 The word literally suggests the idea of reflection from the afferent to
the efferent neurones, as light is reflected from a surface ; but the student
has already learned enough to understand that efferent impulses are something
more than mere mechanical reflections, or rebounds, of afferent impulses.

FIG. 46. Diagram of the nervous mechanism

represented in Fig. 45, with the addition of

the neuron 6 (see sect. 12)

COORDINATION

the will. The afferent neurone may be stimulated by some
external agent, such as light, heat, sound, pressure, etc., or
by some condition within the body itself, as when diseased
or abnormal conditions of the stomach or some other organ
induce vomiting.

It is a common error to suppose that all actions which
are not called forth by the will are reflex. The essential
feature of a true reflex is the more or less direct action
of the afferent impulses
on efferent neurones and
not merely its nonvoli-
tional character. There
are, in fact, involuntary
actions in which the ef-
ferent neurones are di-
rectly stimulated not by
afferent neurones, but
by the condition of the
blood or in other ways.
Such actions are not re-
flex, though they may
be either involuntary
or unconscious or both.
They are known, in gen-
eral, as automatic actions, and we shall meet examples of them
as we proceed with the study of the various functions of
the body.

12. Actions resulting from stimulation by the will. A wink
is not always a reflex action. We can wink " on purpose,"
or, otherwise expressed, a wink may be called forth by the
will and entirely apart from the sudden appearance of some
object in front of the eye. Here the muscles of the eyelid
act in exactly the same manner as in a reflex wink, which
means that they are stimulated in the same way by the same
efferent neurones. Thus far the mechanism is the same in

FIG. 47. The nervous mechanism shown in

Fig. 46, with the addition of the afferent

neurone c, from the cornea (see sect. 12)

82 THE HUMAN MECHANISM

the two cases, but the source of stimulation of the efferent
neurones must be different.

In later chapters of this book we shall bring forward
evidence to show that the exercise of the will (volition)
requires the cooperation of the highest portion of the brain
or cerebrum. Nerve cells in the gray matter of the cerebrum
send off axons whieh pass downward to those portions of the
brain and spinal cord from which the motor or efferent neu-
rones arise ; with the neurones of these nerves they make ex-
actly the same kind of connections (collaterals and synapses)
as are made by the afferent fibers from the retina which excite
the reflex (see Fig. 46, in which b is the cerebral neurone).

The collaterals and synapses of the cerebral neurone
(which, it will be observed, is entirely confined to the cen-
tral nervous system) simply duplicate those of the afferent
neurone ; hence the two neurones produce the same result.

There is, however, still a third way in which winking may
be stimulated. When the cornea of the eye begins to dry, a
reflex wink spreads tears over the eyeball. In this case we
have to deal with a second reflex, the afferent neurones being
not those in the optic nerve, but those in what is known as
the trigeminal, the sensory nerve of the cornea. Our scheme
thus becomes that shown in Fig. 47.

13. The " master " neurone. The multiplication of collat-
erals and arborizations which this scheme involves would
seem to be largely avoided by the presence of a third neu-
rone between those which stimulate the action and the effer-
ent neurones which directly act on the muscles (Fig. 48).

In this way, when a wink is produced, whether from the
cerebrum or from the retina or from the cornea, the single
cell a is stimulated, and this in turn stimulates the groups
of efferent neurones which immediately innervate the mus-
cles of the eyelids. Many of the nerve fibers of the cord and
brain belong to neurones which perform the same function
as that attributed to the cell a in our diagram. They are

COORDINATION

entirely confined to the brain or cord and group together
those efferent cells which by working together produce a
coordinated action.

The organization of the nervous system is, in fact, much
like that of a large manufacturing establishment. The nerve
cells which send axons to the muscles, glands, blood vessels,
etc. may be compared with the operatives, each with his
special task to perform ; over these are foremen, or " bosses,"
from whom they take their orders or, in physiological
language, who stimulate
them to do their work
and who would corre-
spond to cells like a in
Fig. 48. The foremen in
turn receive orders, now
from one department of
the establishment, now
from another, as the work
of their operatives is
needed in making one or
the other of the products
offered for sale. So the
master neurones receive
stimuli from the brain or from afferent nerves, as the needs
or the desires of the organism as a whole require their
activity. The comparison is instructive and may easily be
carried out in greater detail by the student himself.

14. The coordination of two or more actions to achieve a
definite end. These conceptions will become more definite if
we study the nervous mechanisms represented in Fig. 49,
which represents the combination of the wink with different
physiological actions, according to the nature of the con-
ditions which call it forth. Let us consider the two reflex
winks already referred to, that from the cornea and that from
the retina. The wink from the cornea is for the purpose of

FIG. 48. The master neurone

THE HUMAN MECHANISM

spreading tears over the surface of the eyeball and, to be
effective, must be accompanied by a secretion of tears. We
may suppose that this is accomplished, as in the diagram,
by the afferent neurone (c) from the cornea stimulating two

master neurones, one of
which (mw) produces the
wink, while the other
(nt) stimulates the tear
glands to secrete.

The wink from the
retina, on the other hand,
has the entirely different
purpose of preventing
the contact of foreign
objects with the cornea.
For this purpose tears
are not necessary and
they are not secreted.
But at times this wink
is accompanied by a
sudden starting back of
the body as a whole
to avoid the threatened
danger. In this case we
may suppose that the
afferent neurone from
the retina connects with
the master neurones mw,
for winking, and msb, for
starting back, but that
this afferent neurone
does not connect with m\ for the secretion of tears.

Finally, the volitional neurones vsb and vw, which pass from
the cerebrum to their appropriate master neurones, call forth
these actions of starting back or winking as separate acts.

FIG. 49. Coordinations involved in the com-
bination of the wink with other actions

The afferent neurones r, from the retina, and
c, from the cornea, connect with different com-
binations of efferent neurones as explained in
the text. Efferent master neurones are shown
as follows : mw, for winking ; mf, for secretion
of tears ; m'b, for starting back. Neurones con-
cerned in volitional actions are v*b, for starting
back, and vw, for winking

COORDINATION 85

15. The acquisition of reflexes ; conditioned and uncondi-
tioned reflexes. There can be no doubt that many of these
reflex mechanisms are born with us. A newborn baby, for
example, like the adult, winks and secretes tears when the
cornea dries ; it secretes saliva when a sapid substance is
placed in the mouth ; it swallows when something touches
the throat; if a cane is brought in contact with the palm
of the hand, it is grasped firmly. These and many other
reflex actions take place from the first because the baby in-
herits and hence is born with the complete reflex mechanism
for their execution upon the application of the appropriate
stimuli.

On the other hand, new involuntary reactions can be
acquired in adult life, even reactions which are useless to
the body. The extent to which this is true is illustrated by
the following extreme case : if a piece of ice is applied to a
definite spot of the skin, the amount of blood flowing through
that part of the skin is greatly diminished and the skin be-
comes pale. This is an inherited reflex which (Chap. XII)
protects the body from exposure to cold. A morsel of food
placed on the tongue (where it stimulates the afferent nerves
of taste) will reflexly excite the flow of saliva. In both cases
we see the obvious purposeful relation between the stimulus
and the reaction and in both cases we are dealing with in-
herited reflexes. Moreover, these two reflex mechanisms as
inherited are entirely independent of each other, for the
stimulation of the skin by ice does not excite the flow of
saliva nor does the stimulation of the sense of taste influence
the blood flow through the skin. If, however, every time
that one eats, a piece of ice is applied to the same region of
skin, so that loth these reflexes are simultaneously excited, in
the course of two weeks or more it will be found that the
application of ice * to the skin excites a flow of saliva even
though no food is taken into the mouth. In other words, these
two reflex mechanisms have become associated, so that activity

THE HUMAN MECHANISM

of the one now discharges the other. Evidently some sort
of nervous connection has been established between them.
Fig. 50 gives a diagram of the new association which has

been established
between the two
centers.

The connec-
tion thus newly
established be-
tween the affer-
ent neurones of
cold (c) and the
efferent neu-
rones to the sal-
ivary glands (s)
differs in sev-
eral ways from
the connection
between the af-
ferent and ef-
ferent sides of
an inherited
nervous mech-
anism. Such ac-
quired reactions
are not evoked
with the same
certainty as the

inherited and, once acquired, they are more readily lost by
disuse. Whether we get the reaction or not depends upon
the condition of the body at the time we apply the stimulus.
Hence they are spoken of as conditioned reflexes, to distinguish
them from the unconditioned (or inherited) reflexes. Undoubt-
edly many of our involuntary actions, especially acquired
habits in general, are conditioned reflexes acquired since birth

FIG. 50. The acquisition of a conditioned reflex

A, reflex mechanism for constriction of cutaneous vessels
when cold is applied to the skin; B, reflex mechanism of
the secretion of saliva when a sapid suhstance comes in
contact with the tongue. Above is shown the usual normal
condition with no connection between the two mechanisms ;
below, the condition after both have been repeatedly in
simultaneous action

COORDINATION 87

and thus added on to the stock of inherited reflexes which
make part of the equipment with which we begin life.

16. The complexity of the mechanisms of the nervous
system. Such actions as we have been studying — whether
the inherited reflex of winking, even when this is combined
with other acts like the secretion of tears, or the acquired
conditioned reflex secretion of saliva from the stimulation
of the skin by cold — are comparatively simple, as compared
with many other actions of daily life, such, for example,
as the throwing of a stone. Here not only muscles which
produce motion at the shoulder, elbow, wrist, and finger
joints are called into play, but also muscles which maintain
the erect position and balance of the body as a whole.
The entire nervous mechanism involved baffles the imagi-
nation to conceive ; and yet any boy can perform the act.
He can do it, however, because his motor neurones are
grouped together into a perfectly well-organized army which
executes at once the bidding of its commander in chief —
the will.

We have given in the foregoing pages a mere glimpse
into the complexity of one part of the wonderful nervous
mechanism. No watch, no machine which man has ever
invented or constructed can for a moment compare with this
living machine in complexity or in perfection. Yet, like all
machines, this one can be abused ; it can get out of order ; it
can even break down. And we have already learned enough
to understand why this is so. Some neurones may be injured
by overwork or may degenerate from disuse ; indulgence in
stimulants or narcotics may poison the governing nerve cells ;
above all, constant failure to lead a normal life may deprive
these cells of their sole means of repair. The human body
is a machine designed for use, even for hard use, and it
thrives upon right use ; but it is a machine too delicate and
too complex to be abused with impunity.

88 THE HUMAN MECHANISM

When one thinks of the hundreds, perhaps thousands, of
movements which the body makes, and of the combination
of these movements into definite actions or work, and then
reflects that the muscle fibers which execute any movement
are thrown into orderly contraction by nerve cells which are
themselves commanded by higher nerve cells; that these in
turn are marshaled, as it were, by still higher cells when
the separate movements they evoke are to be combined into
a still more complicated action — one begins to appreciate
the complexity of the organization of the nervous system.
The number of the nerve cells is measured by hundreds of
thousands, and their efficiency in directing the working
organs of the body, so as to meet the demands of life, de-
pends not only upon the integrity of the neurones but also
upon the perfection of their organization, that is, their
grouping into squads, companies, regiments, brigades, divi-
sions, and corps, ready to yield instant and obedient response
to the command of the higher officers of the will or to the
signals of those pickets — the sense organs and their afferent
neurones — which everywhere guard the outposts and give
information of the need for action.

Moreover, this army of neurones, like any other army,
becomes efficient by work, by drilling, by practice, even by
battle. Like the soldiers of a regular army the neurones
may be overworked and their efficiency as a military body
may suffer thereby, but they may also work too little ;
the perfection of their development and of then- organi-
zation depends on the practice they get with reasonable
activity. To this point we shall return ; but meantime
the student can safely make the application for himself.
Such comparison and such application are not only in-
structive but intensely practical in their bearing upon
the affairs of everyday life — upon that right conduct of
life which is the first duty of every man, svery woman,
every child. .

COORDINATION 89

17. Stimulation and coordination by chemical means ; hor-
mones. In previous chapters we have dealt chiefly with
examples of stimulation of muscle and gland cells by nervous
impulses and of the coordination of the work of organs
through the central nervous system; but there is another
way by which both stimulation and coordination are effected.
An irritable cell will respond to other stimuli than nervous
impulses ; among these are a sharp blow, sudden heating,
make or break of an electric current, and exposure to the
action of certain substances. The last is generally spoken
of as chemical stimulation, and we shall meet with examples
of this in our subsequent study. One will suffice for the
present. After the food has undergone a preliminary diges-
tion in the stomach by the acid gastric juice, it is passed
into the small intestine, where its digestion is completed.
The first requisite for this purpose is the secretion of pan-
creatic juice, and this is secured as follows: the acid of the
stomach contents liberates from the lining cells of the first
part of the intestine a substance known as secretin, which
enters the blood and chemically excites the cells of the pan-
creas to secrete pancreatic juice. By this means the pancre-
atic juice is secreted into the intestine at precisely the time
that it is needed there ; that is, as each consignment of acid
food is discharged from the stomach (see Chap. VIII, p. 113).
A substance thus liberated in one organ and stimulating
another organ to activity at the time when such activity is
needed is known as a hormone (Greek hormao, " I arouse " ).

The action of secretin evidently presents, in addition to its
feature of stimulation, an element of purposeful coordination,
since it insures the proper cooperation of the stomach and
pancreas in the work of digestion ; and other examples of the
same thing might be cited. We have, however, only to refer
the student to the case of adrenaline, already described in
Chapter VI, for the most striking example of coordination
produced by chemical means.

90 THE HUMAN MECHANISM

•Cooperation, adjustment, and coordination are thus brought
about in the body by two means : first, through the chemical
action of hormones ; and, second, through the mechanisms of
the central nervous system. The first provides for situations
where no great delicacy of adjustment is required; in the
secretion of the pancreatic juice, for example, it is not neces-
sary that a definite quantity, no more and no less, be secreted ;
in such a muscular movement as writing, on the other hand,
it is necessary that each muscle taking part shall act in a very
exact manner. For such coordinations the action of the nerv-
ous system is generally necessary. Finally, as suggested by
our consideration of the conditioned reflex, the nervous system
is the chief means whereby we can acquire new mechanisms
of coordination, thereby increasing our power of adjustment
to new conditions of life.

CHAPTER VIII
ALIMENTATION AND DIGESTION

A. THE SUPPLY OF MATTER AND POWEK TO THE
HUMAN MACHINE

1. Power and the materials for repair supplied separately
to lifeless machines. Living and lifeless machines are alike
in that worn-out parts must be renewed and that power
must be supplied to do work. In the lifeless machine these
two requirements are supplied separately. A factory and
its equipment of machinery are kept in repair and enlarged
(grow) by means of bricks, lumber, steel, belting, new pieces
of machinery, etc., which are brought into the building,
while the power which runs the machinery comes in quite
separately as fuel, or water power, or electric power.

2. Power and the materials for growth and repair supplied
to the human machine in the one form of foods. With the
human mechanism this is not so. Materials for growth and
repair, and power for running, are introduced from without
not separately, but together, both being supplied in the one
form of food. As it does its life work the human mechanism,
like a lifeless machine, not only consumes power but its parts
deteriorate, and it is the double function of the food we eat
to make good this double loss. Some foods possibly serve
only as means of power; others merely make good the loss
of essential parts of the mechanism; while still others may
serve both purposes.

3. Food as a source of power. Experiment and experience
alike prove that foods are the source of power for work.
Bread, butter, starch, sugar, beef, and the like may be dried

92 THE HUMAN MECHANISM

and then burned as fuel, giving power to an engine. The
occasional use of Indian corn or wheat for fuel, in the West,
the employment of hams and bacon as fuel by steamers short
of coal, the explosion of flour dust in mills, and similar phe-
nomena further illustrate by the teachings of experience the
fact that these foods are rich in energy, or power.

When we say that the food must supply power to the
body, we mean that the power which it contains must be
available to the body. A lump of coal may be a source of
power, as is shown by its use in a locomotive ; but a lump
of coal would be of no use as food, because the body has
no such means of burning it as has the engine. Again,
nitroglycerin contains chemical elements needed in the food;
but although when exploded in a dynamite cartridge it
may furnish power enough to shatter heavy armor plate,
its energy is not available to the body.

Thus, to recapitulate, (a) food makes good the loss of
living substance in the body; (&) it supplies material for
growth and for the manufacture of the secretions of the
body ; and (c?) it supplies power for the work which the body
is to do. It also performs one more important function,
which will be more clearly understood hereafter; for (d) by
its oxidation food provides the heat usually required to keep
up the body temperature. The detailed consideration of this
subject, however, must be postponed to Chapter XII.

4. Chemical composition of foods ; nutrients. The human
race has learned by long experience that certain things meet
the demands of the body for food, and that other things
do not. Perhaps no animal uses so many different materials
as man in satisfying sensations of hunger and thirst. Some
foods are taken from the animal and some from the vegetable
kingdom, and their variety is greatly increased by special
modes of preparation. But however numerous the foods
from which we prepare the dishes served at different meals,
chemical analysis shows that the essential constituents of all

ALIMENTATION AND DIGESTION 93

foods belong to a comparatively small number of chemical
groups. These classes, or groups, may be called nutrients;
and as all the members of the same group undergo practi-
cally the same processes of digestion and perform similar
functions in nourishing the body, it will be equally accurate
and more convenient, in treating of this part of physiology,
to speak of the different nutrients, and not of beef, mutton,
fish, eggs, bread, milk, butter, etc.

From the point of view of digestion the most important
nutrients are the proteins, the carbohydrates, the fats, the
inorganic salts, and water-, and the student must at this
point become thoroughly familiar with what is meant by
these fundamental terms.

5. The group of proteins. We may obtain a working idea
of what a protein is by recalling some of the foods in which
protein preponderates or is easily seen. Such foods are the
white of egg, the lean of tender meat (muscle fibers), the
curd of milk, the tenacious gluten of wheat. Proteins also
exist in relatively large quantities, though not so readily
seen, in yolk of egg, beans, peas, oats, and other grains.

Proteins contain carbon, hydrogen, nitrogen, oxygen, and
sulphur. Some contain phosphorus and some contain iron.
Chemically they are exceedingly complex substances. It
should be noted that the proteins are the most important
nutrients which contain nitrogen and sulphur.

Many proteins readily become insoluble. Examples of this
are the hardening of the white of egg or the lean of meat
by cooking and of the casein or curd of milk by rennet or
" junket tablets." This change is known as coagulation, and
most of our protein food is eaten after having been coagulated
in the process of cooking.

Proteins occur only within the living cells of plants and
animals or as the products of these living cells. They form,
as we shall more clearly see later, an essential part of the
basis of the living cell and are constantly disintegrating

94 THE HUMAN MECHANISM

within the cell into simpler substances. Hence there is a
constant cellular loss of protein, which in the animal body
can be made good only from protein in the food. Plants, on
the other hand, have the power of manufacturing proteins
from sugars and certain mineral salts, the latter supplying
the needed nitrogen and sulphur. The plant kingdom is,
therefore, in the long run the sole source of protein food for
animals ; for while some animals (carnivores) get their pro-
tein entirely by eating the flesh of other animals, the latter
(herbivorous animals) in turn have obtained their protein
from plants.

Unlike fats and carbohydrates, protein is an absolute
essential of animal diet; that is to say, protein food per-
forms certain functions in the animal body which cannot be
performed by fats or carbohydrates, while the two latter
nutrients perform no functions which cannot also, when
necessary, be met by proteins. Some proteins, however, are
incapable of meeting all the protein requirements of the
organism, although they may meet some of them. Of these
the most important in use as food is the fibrous connective
tissue (pp. 7, 8), whose fibers in the uncooked state consist
of the insoluble protein substance collagen, which by heating
in the presence of water is converted into the closely related
but soluble gelatin. Collagen and gelatin belong to the
albuminoids, one of the subclasses of proteins. The chief
protein of Indian corn is similarly incapable of meeting all
the protein requirements of the organism.

6. The group of carbohydrates ; the plant cell as a food
factory. The carbohydrates constitute a very large chemical
group, although comparatively few members of it (starch
and sugars) are of importance as food. They are all com-
pounds of the elements carbon, hydrogen, and oxygen, and
contain no nitrogen or sulphur; those used as food are
manufactured in the cells of green plants. This production
of carbohydrates by the plant cell is another example of the

ALIMENTATION AND DIGESTION 95

work of cells as chemical factories, which we studied in
Chapter IV. The cells of the green parts of plants, espe-
cially of the leaves, take in carbon dioxide from the air and
water from the soil, and from these plant foods, with the
aid of sunlight, manufacture sugar, which is transported in
the sap from one part of the plant to the other and is used
as a source of power for plant work. The excess of sugar
is converted by certain cells into starch and is stored in the
form of small granules in the cytoplasm for future use.
A potato or a grain of wheat consists of cells loaded with
these starch granules. When the plant is not manufacturing
sugar directly from carbon dioxide and water, its cells again
transform the starch granules into sugar. The presence of
sugar in sugar beets, apples, pears, and peaches and in the sap
of sugar maples are familiar examples of this manufacture
and transport of sugar by plants.

It will be noticed that only green plants have this power
of manufacturing carbohydrates from carbon dioxide and
water; hence we do not find large quantities of sugar and
starch in mushrooms and other fungi. The cells of green
plants, in short, are the starch factories of the world, the
factories from which we purchase our supplies of starch be-
ing only refineries, that is, places where starch is separated
from other constituents of plant cells.

All plants, however, possess the power of manufacturing
proteins from carbohydrates and certain salts, which salts
they get from the soil. The carbohydrates furnish carbon,
hydrogen, and some of the oxygen, while the salts furnish
nitrogen, sulphur, phosphorus, etc. One great difference
between plants and animals is this power of protein manu-
facture by the cells from material which is not protein. The
animal cell can manufacture protein only from protein itself
or from certain decomposition products of protein.

7. The group of fats. Fats are familiar to us in such
forms as butter, lard, olive oil, and the fat of meat. Like

96 THE HUMAN MECHANISM

the carbohydrates they are compounds of carbon, hydrogen,
and oxygen, although the oxygen is always present in small
quantities. The formula for one of the fats is C51H98O6, and
this composition is typical of all of them.

Fats may be split up into certain acids (fatty acids) and
glycerin, and when treated with alkalies, like caustic soda or
caustic potash, they form soaps. They are insoluble in water.
Like the carbohydrates they contain no nitrogen.

8. Oxidizable and nonoxidizable nutrients. All the above
nutrients may and do combine with oxygen within the cells
of the body, although the way in which this chemical union is
brought about is one of the unsolved problems of physiology.
While all the nutrients may be burned after being dried,
such combustion requires a high temperature. Within the
body they are not only burned (that is, combined with oxy-
gen) at a temperature rarely exceeding 39° C. (100° F.), but
they undergo oxidation while in a moist state or even in
solution. However this oxidation may be effected within the
cell, there can be no doubt that it yields the heat for keeping
the body warm and possibly the power for its work.

The remaining groups of nutrients — the inorganic salts
and water — are, for the most part, not oxidized in the body.

9. The groups of inorganic salts and water. These nutrients
are absolutely necessary for the proper nourishment of the
body, their presence in the blood and lymph and in the liv-
ing cells being indispensable to the processes of life. The
salts are taken in small quantities, partly as salt itself, partly
as portions of the various foods we eat. During growth they
furnish much of the mineral matter of bones, and since the
body is daily losing salt, it is necessary that salt be supplied
in the food. Salts, however, are not acted on to any large
extent in the alimentary canal by the processes of digestion;
they are largely absorbed in the same form as eaten. Hence
they do not concern us at present to the same extent as do the
oxidizable nutrients, which generally have to be chemically

ALIMENTATION AND DIGESTION

changed, or digested, before they can be absorbed for use in
the body. The same thing is true of water.

10. Composition of some common foods. The following table
gives the percentage composition of some of the more common
foods (see also p. 238).

WATER

PROTEIN

STARCH

SUGAR

FAT

SALTS

Bread . .

Wheat flour ....
Oatmeal

15
15

11
12.6

4.2
5 4

2
5.6

1.7
3

Rice

0 7

0 5

Peas

Potatoes

0 2

0 7

Milk

0 8

Cheese

Lean beef . ''. . . v

Fat beef

Veal . . .

White fish

Salmon ... . .

5 5

1.5

Eo-ff

10 5

1 5

Butter

11. Indigestible material in food. When we say that "a food
is digestible we generally mean that when taken into the
alimentary canal, if not already in solution, it is chemically
acted upon by the digestive juices so as to be dissolved and
made capable of being absorbed into the blood. The greater
part of the food we eat consists in this sense of digestible sub-
stances, but many foods contain a certain amount of indigest-
ible material, and some contain a very considerable amount.

The most conspicuous example of such material is cellu-
lose, a member of the same group of carbohydrates to which
starch belongs. It occurs in almost all vegetable foods ; and
since, in the human alimentary canal, cellulose is for the
most part unaffected, it cannot be absorbed and necessarily

THE HUMAN MECHANISM

forms an important part of the feces. Other indigestible
substances are the outer skin of animals (for example, the
skin of fowls), and certain portions of the connective tissue
of meat.

12. Animal and vegetable foods. The classification of foods
into animal and vegetable not only describes the origin of
foods from the two great kingdoms of living things, but
also defines important differences between them with refer-
ence to digestion. These differences may be summed up as
follows : Animal foods are generally rich in proteins and poor

FIG. 51. Part of the seed of the bean

Showing the larger starch granules and

the finer protein granules inclosed

within the cellulose cell walls

FIG. 52. Section of potato

Showing starch granules inclosed
within the cellulose cell walls

in carbohydrates, while vegetable foods are generally poor in
proteins and very rich in carbohydrates, especially starch.
In the second place, animal foods contain relatively little
indigestible material, while vegetable foods, as they occur
in nature, contain large amounts of indigestible cellulose.
In the third place, the digestible materials of vegetable
foods (the proteins, carbohydrates, and fats) are often con-
tained within a plant cell which is surrounded by a cellu-
lose membrane impermeable to the digestive juices; before
they can be digested this membrane must be ruptured in
one way or another. In the case of many animal foods, on

ALIMENTATION AND DIGESTION 99

the other hand, especially meat and fat, the cells (muscle
fibers and fat cells) which contain the essential nutrients
are held together by connective tissue made up largely of
fibers of an albuminoid nature. These fibers are soluble in
the juices of the stomach, in which the cellulose which
holds together the vegetable foods is insoluble. The full
importance of these differences will be evident before we
have finished the study of digestion.

13. The process of alimentation. Before corn, wheat, meat,
vegetables, and other food materials can be taken into the
body and made to yield up to it the material and power
which they contain, they must, in most cases, undergo
various preparatory or preliminary processes or treatments
which shall make them easier or better to eat or more
attractive. The most familiar of these processes is cooking,
but it is by no means the only one. In the case of animal
food the animal must be captured, if wild, or raised, if
domesticated. It must be killed, skinned, dressed, cut up,
and the meat in many cases " ripened " by keeping, or
" cured " by smoking, salting, drying, or corning. So, also,
with plant food, such as cereals, vegetables, fruits, nuts,
and the like ; these must first be found, if wild, or grown,
if domesticated. They must then be separated from the
rest of the plant — threshed, if wheat, rye, oats, or barley;
husked and shelled, if corn ; dug up or removed from the
earth, if vegetables like potatoes, celery, radishes, or lettuce.
Fruits and nuts must be separated or picked from vine or
tree ; milk must be drawn from animals ; and even salt, water,
and condiments like mustard and pepper must be separated
from the earth or the sea or from plants. After collection
and further preparation by winnowing, grinding, or cleaning,
elaborate cooking is applied to many forms of food before it
is put upon the table ; and even then, at the last moment
before it is eaten, a further separation, as of meat from bone,
must be made either by the carver or by the eater himself.

100 THE HUMAN MECHANISM

To this entire process of the supply and preparation of
food for eating, the term "alimentation" may be conveniently
applied. Reflection will show that it is largely a process of
food refining, the principal result being a concentration of the
nutrients at every step. It is also a separation of the com-
paratively useful from the comparatively worthless (as food) ;
and just here, and in these points, — concentration and the
separation of good from poor materials, — we may recognize a
true process of digestion, but one external rather than in-
ternal : a refining in the field, the mill, and the kitchen
rather than in the stomach; in the environment rather than
within the organism.

14. The ends accomplished by digestion. The processes of
digestion accomplish three chief results: First, they separate
the nutritious and therefore important part of the food from
the innutritious and therefore useless. This process, so con-
spicuous in the case of external digestion, is continued within
the alimentary canal. Second, digestion brings the solid part
of the food into solution by changing insoluble into soluble
substances. This is necessary, since food is received into the
body proper (that is, into the blood) through the lining
membranes of the alimentary canal, and in order that it may
pass through these membranes it must be dissolved. In the
third place, digestion transforms the food as eaten into com-
pounds which can be used by the cells of the body. Common
cane sugar, for example, is very soluble and can be absorbed
into the blood, but the cells of the body cannot use it. In
the intestine it is split into grape sugar and fruit sugar, both
of which can be used. Similarly, the white of egg (a protein),
though soluble, would be of little, if any, use if injected
unchanged into the blood; in the alimentary canal it is
transformed into available compounds. It will be helpful to
acquire at this time a general idea of the chemical structure
of two of our most important foods and of the chemical
changes which they undergo in the alimentary canal.

101

15. The chemical structure of the starch and protein mole-
cules ; cleavage changes during digestion. The huge molecules
of starch and protein are believed by chemists to consist of
a large number of much smaller molecules linked together

FIG. 53. Diagram of the structure of molecules of starch and protein

Starch is represented as formed by the chemical linking together of many like
molecules of dextrose; protein, by the linking together of many unlike mole-
cules of amino-acids. Some of these chemical links (indicated by the arrows)
are broken by cleavage more easily than others. Hence cleavage first forms
smaller molecules of dextrines from starch and of polypeptids from proteins.
Ultimately each may be broken up into its constituent molecules of dextrose or
amino-acids respectively

in chemical combination (see Fig. 53). By boiling in water
containing acid, these large molecules undergo a very simple
cleavage into their component molecules. Starch treated in
this manner yields only one substance, namely dextrose
(glucose, or grape sugar (C6H12O6)). Protein, on the other
hand, yields a much greater variety of compounds, some

102 THE ti u MAN MECHANISM

twenty or more in number, which, though differing greatly
from one another in most respects, have in common one
point of structure in virtue of which they are known as
amino-acids. In the chemical laboratory amino-acids are
readily bound together to form peptids, and we speak of
dipeptids, tripeptids, tetrapeptids, and polypeptids accord-
ing as two, three, four, or many amino-acids enter into their
formation. It is now thought that protein, as it occurs in
nature, is essentially a^very complex polypeptid.

In the body the enzymes of the digestive juices produce
virtually the same cleavage in starch and protein as that
caused by boiling with acids, and the chemical action upon
the food within the stomach and intestine consists essentially
in breaking up the starch and protein into their component
molecules — dextrose in the one case, amino-acids or small
peptids in the other. We accordingly find that as the result
of digestion the starch we eat supplies the blood (and so the
body cells) with only one substance, namely dextrose (grape
sugar), and the value of starch in nutrition is limited to the
nutritional value of this single substance, dextrose, of which
it is composed. Protein, on the other hand, yields twenty
or more different chemical compounds, each with its own
possibilities of chemical action in the cell. Moreover, indi-
vidual proteins differ in their constituent amino-acids; a given
protein may be entirely lacking in one or more amino-acids,
or it may have one or more present in very small or very
large proportions. The nutritional value of the protein is con-
sequently determined by the possibilities of chemical action
of its constituent amino-acids and by the quantity of each
amino-acid yielded by the digestive cleavage. From this we
can readily understand why protein food meets a wider variety
of nutritional requirements than does starch or fat, which also
yields only a few cleavage products upon digestion.

16. Digestion a chain of events. Before entering upon the
study of the details of digestion in the different parts of the

ALIMENTATION AND DIGESTION 103

alimentary canal, a suggestion as to the proper point of view
will be helpful. While it is true that each part of the diges-
tive system performs functions of its own, it is also true that
what takes place in one part is dependent on what takes
place in others ; digestion in the mouth has reference largely
to subsequent work in the stomach; gastric digestion, in
turn, carries one step further the refinement of the food,
which it thereby prepares for what is to take place in the
small intestine ; finally, the digestive processes of the large
intestine are carried out normally only when preceded by the
proper completion of those of the small intestine. Digestion
is a chain of events, each one depending upon those which have
gone before and, to a large extent, upon others which are tak-
ing place at the same time. The student is urged to keep
this in view in the study of all the digestive processes.

B. DIGESTION IN THE MOUTH. ENZYMES

17. Stimulation of the sense of taste a reflex excitant of
the flow of gastric juice. Digestion in the mouth prepares for
digestion in the stomach, in the first place, by stimulating
the sense of taste through the flavor of the food, for the
afferent impulses thus aroused play a very important r61e in
evoking the secretion of gastric juice. This point will be
more fully discussed in our studies of gastric digestion. It
is referred to here that the student may understand that far
more is to be accomplished by the stay of food in the mouth
than its mastication and mixture with saliva preparatory to
the act of swallowing. We might imagine a meal composed
of food already well moistened and requiring no chewing,
so that it could be swallowed immediately. Such a meal
might have all the nutrients in the proper proportions, and
yet, from the very fact that it stays so short a time in the
mouth, it may not sufficiently arouse sensations of taste to
evoke an adequate reflex secretion of gastric juice. It is

104 THE HUMAN MECHANISM

perhaps here that we have the strongest argument against
hasty eating.

18. Mastication. Digestion in the mouth prepares for
digestion in the stomach, in the second place, by the com-
minution, or grinding down, of the food in the act of chew-
ing. When this is properly done the larger food masses are
broken up into smaller ones, so that the whole is made more
readily accessible to the subsequent action of digestive secre-
tions. The small intestine has almost no means of accom-
plishing this subdivision of the food ; the stomach can do it
for some foods easily, for others with difficulty, while against
others it is virtually powerless. Only in the mouth can all
foods be thoroughly comminuted. For this purpose it is
necessary to keep the teeth sound.1

19. Chemical action of saliva. Digestion in the mouth
presents a feature which is characteristic of all the digestive
processes ; namely, a combination of the mechanical action of
some form of muscular movement with the physical and
chemical action of some digestive juice. The muscular act
of chewing and the secretion of saliva, which moistens and
acts chemically upon the food, cooperate to reduce the food
to smaller particles and to change part of it into other sub-
stances. Neither mastication nor insalivation, acting alone,
would be as effective as are both when acting together.
We shall see the same thing more strikingly illustrated in
our studies of gastric and intestinal digestion.

The chemical action of saliva is much less important than
that of other digestive juices, but it is typical of the charac-
ter of all of them, so that it is profitable to consider it at
some length. Upon proteins and fats saliva has no action
whatever, but upon starch it exerts a striking and readily
demonstrable influence. To demonstrate the effect in ques-
tion some starch paste should be prepared. This is not a

1 The structure and care of the teeth will be described in Part II,
Chap. XXIII.

ALIMENTATION AND DIGESTION 105

clear solution, like salt or sugar, but an opalescent liquid,
which does not become clear by passing through ordinary
filter paper. A characteristic test for starch — the blue color
produced when a few drops of a solution of iodine * are
added to it — may be used to detect its presence in the
following experiments :

EXPERIMENT I

Two test tubes or small beakers containing starch paste are prepared.
Collect some saliva and boil half of it. To one portion of the starch
paste add the boiled saliva (after it has again cooled to the room tem-
perature) ; to the other add the unboiled saliva. Mere observation will
show that while the first test tube remains opalescent, the second soon
becomes clear. A few minutes after this change has occurred, a little
of the second starch-saliva mixture may be removed, diluted with water,
and tested with iodine ; the color produced is no longer pure blue, but
purplish ; that is, a mixture of red and blue. Some minutes later the
iodine test gives a port-wine red color, and stiil later no color at all.
This change of reaction is due to the fact that the saliva has changed
the starch into dextrine, which gives the red color, and then has changed
the dextrine into a substance which gives no color with iodine.2 Mean-
while the starch in the first test tube shows no change either in its
opalescent appearance or in its original blue reaction with iodine.

Boiling the saliva has destroyed its power of acting on
starch, and it is known that this is due to the fact that
the heat has destroyed the enzyme, known as ptyalin, or
salivary diastase, which has the power of changing starch
to sugar.

1 Made by dissolving a few flakes of iodine in alcohol or in an aqueous
solution of potassium iodide.

2 The cleavage of the starch molecule does not take place by splitting off
successive molecules of dextrose, but by splitting into two molecules, each,
let us say, approximately half as large as the original molecule. By some
such process first one, then another, dextrine successively appears. Con-
tinuation of the cleavage ultimately gives a substance, maltose, which
consists of two molecules of dextrose bound together. Finally, the maltose
is split into two molecules of grape sugar. We speak of the dextrines and
maltose as intermediate products, and of the dextrose as the end product, of
the cleavage.

106 THE HUMAN MECHANISM

EXPERIMENT II

Let us now inquire what has become of the starch in the second test
tube. The solution is clear and has a sweetish taste. Moreover, if boiled
with a mixture of sodium hydroxide and a few drops of copper sulphate,
it gives a red precipitate, indicating the presence of sugar. These sim-
ple tests then prove that saliva first changes starch into dextrine and
subsequently changes dextrine into sugar.

EXPERIMENT III

Dilute some starch paste with an equal volume of 0.4 per cent
hydrochloric acid (which will, of course, make a 0.2 per cent solution
of the acid). Now add a few drops of saliva. It will be found that no
reaction takes place. Saliva will not act in an acid medium of this
strength, and it can be easily shown that it acts most vigorously in a
neutral or faintly alkaline medium. This result is of much practical
importance, because the gastric juice contains approximately 0.2 per
cent of hydrochloric acid and may therefore be expected to interfere
with salivary digestion.

EXPERIMENT IV

Prepare five or more small beakers of starch paste and add (best
with a medicine dropper) to the first a drop of filtered saliva, to the sec-
ond two drops, to the third three drops, and so on; then observe the
time required in each case for the disappearance of the opalescence
and also of the iodine reaction. This experiment will show that while
a very small amount of saliva will transform an indefinite amount of
starch into sugar, the more saliva there is present the more rapidly will
the transformation occur ; and the same thing is true of all enzymes.
If the result is not perfectly clear with the undiluted saliva, repeat,
but use saliva diluted two or three times with water.

While we are eating, the food obviously stays too short
a time in the mouth to allow the conversion of any large
amount of its starch into sugar before it is swallowed.
Whatever actual work the saliva may do in bringing about
this chemical change must evidently be done chiefly in the
stomach, and this will be studied in the next section.

We have dwelt at length upon the enzyme action of saliva
not merely for its own sake but rather because the behavior

ALIMENTATION AND DIGESTION 107

of the salivary juice is typical of the action of other of the
digestive juices and of enzyme action in general. All the
other juices of the alimentary canal, with the single excep-
tion of the bile, contain enzymes, and it will greatly help
our understanding of the digestive action of these enzymes
if that of the salivary enzyme be first mastered.

Digestion in the mouth, then, consists first, of a mechanical
process of chewing, by which food is crushed or comminuted;
second, of a physical process of moistening, by which dry
foods are prepared for the act of swallowing; and third, of
a chemical process, the chief part of which is the conversion
of starch into sugar by enzyme action. In addition to this
the stimulation of the sense of taste reflexly starts the
secretion of the gastric juice, which now becomes the main
chemical agent in carrying on the work of digestion. To
the consideration of the digestive processes in the stomach
we may now devote our attention.

C. DIGESTION IN THE STOMACH

According to popular ideas the stomach is the chief organ
of digestion; in fact, however, it is an organ in which the
food which has been swallowed is temporarily stored while
undergoing a preliminary preparation for the more impor-
tant changes which are to take place in the intestine. In this
preparatory process, to be sure, some of the food is inciden-
tally changed into those forms in which it passes into the
blood, but this action is incidental and subordinate to the
main function.

20. Form and structure of the stomach. The stomach is a
large pouch into which open two tubes — the oesophagus
(gullet) toward the left side and the intestine on the right
(see Fig. 54). The two regions into which these tubes open
are different in structure and are known as the cardiac (left)
and pyloric (right) portions of the stomach; the cardiac

108

THE HUMAN MECHANISM

portion differs from the pyloric portion in having greater di-
ameter and thinner walls. The entire inner surface is lined by
the mucous membrane some three or more millimeters in thick-
ness, crowded with comparatively simple glands which pour
their secretion, the gastric juice, into the stomach very much as
sweat glands discharge perspiration on the skin (see Fig. 55).

CEsophagus IT-

Cardiac \
Muscle
Gall_
Bladder

Bile Duct

Intestine

From the Liver
\ \ Pylorus

'Pancreatic Duct

FIG. 54. Stomach, beginning of small intestine, and entrance of bile and
pancreatic ducts

During digestion the bile flows directly from the liver into the intestine ; at other

times the opening of the bile duct is closed and the bile passes into the gall

bladder, where it is stored

The glandular membrane is one of the two principal
components of the stomach wall ; the other is the muscular
or contractile tissue, which forms a second coat outside the
other, arid closely united to it by connective tissue con-
taining the larger blood vessels, lymphatics, nerves, etc.1
The muscular coat is comparatively thin in the cardiac region

1 Fig. 63 (large intestine) shows in cross section somewhat the same
arrangement of mucous and muscular coats as in the wall of the stomach.

ALIMENTATION AND DIGESTION

109

and comparatively thick in the pyloric, the thickening in the
latter region being caused chiefly by muscle fibers circularly
arranged.

21. The gastric juice. The gastric juice is a clear, thin,
colorless liquid which contains, among other things, about
0.2-0.3 per cent of hydrochloric
acid and certain enzymes. Upon
starch it has no action whatever,
nor has it any action on fats, unless
the fat is in the form of an emulsion
(that is, very fine drops of oil sus-
pended in water, as in milk or may-
onnaise dressing) ; indeed, the very
limited power of gastric juice to at-
tack fat is a matter of considerable
importance in dietetics. Its main
chemical action is upon the proteins,
which under its influence undergo
cleavage into proteases and peptones.
The proteoses and peptones, like

the original protein, are polypeptids (p. 102), but of smaller
molecular size. They are not coagulated by heat, and most
of them are soluble.

EXPERIMENTS

Prepare some artificial gastric juice as follows : To one quart of water
add 7 or 8 cc. of concentrated hydrochloric acid and to this a little active
pepsin, which may be obtained at any drug store. Pepsin is extracted
from the stomach and is the most important of its enzymes. A solution
of pepsin in the given strength of hydrochloric acid is virtually gastric
juice. Try the effect of this on the following substances by placing each
in a half tumblerful of the juice. To get the complete effect the mixture
should be set aside for twenty-four hours and tests made the next day.
Observations should be made during the first hour or two. If the digest-
ing mixture can be kept in a warm place (90°-100° F.), the action will
be more rapid and the results more satisfactory. The digestions can
best be carried out in corked 4-ounce bottles, which should be shaken

FIG. 55. The inner surface of

the stomach (magnified about

20 diameters)

Showing the openings of the
glands. The lining glandular
membrane is thrown into folds

110 THE HUMAN MECHANISM

occasionally to secure better contact of the digestive juice with the
material undergoing digestion.

1. The white of soft-boiled (3-4 minutes) egg. This is composed
mostly of protein ; it will be dissolved. Into what is the egg white
changed ?

2. A piece of tendon, which can be obtained from any butcher.
This is composed of the kind of fibers which are found in the con-
nective tissues holding the cells together (see Chap. III). The tendon
first swells, then gradually disintegrates, its protein (albuminoid, p. 94)
fibers going into solution. A small residue will be left.

3. A piece of the lean of rare meat cut or chopped into small pieces.
The meat will disintegrate, owing to the solution of its connective tissue
fibers ; then the protein muscle fibers will go into solution, being changed
into soluble peptids.

4. A piece of lean of well-cooked meat. The result will be much like
that in (3) except that it will probably take longer to bring the muscle
fibers into solution.

5. Some jelly (made from gelatin) which has set. This will be
gradually dissolved.

6. Some fat (not gristle) of beef. The mass will disintegrate for the
same reason as in the case of meat. The fat itself will be unacted on,
but will rise to the top, where it may form a layer of fat or oil.

7. A piece of bread. This consists of starch, fat, etc. held togethei
by the tenacious gluten (a protein). As the gluten is dissolved by the
gastric juice the undissolved starch, fat, etc. is set free.

8. Some starch paste. No action.

9. Some fried steak. Note the prolongation of the period of digestion.

Instructive experiments may also be made with cheese,
sweetbreads, potatoes, peas, etc. They would all bring out
the main points in the action of the gastric juice. These
may be summed up as follows: Gastric juice has no effect
upon pure fats (although it plays an important part in the
digestion of adipose tissue1), nor upon carbohydrates, such
as starch or sugar. Its part in digestion consists in its action

1 The fat of meat consists of connective tissue whose cells are greatly
swollen with drops of fat. In typical adipose tissue the connective-tissue
cell becomes one large fat droplet surrounded by the thin layer of the cell
cytoplasm with its nucleus. These fffat cells," like the muscle fibers of
meat, are thus held together by the fibers of connective tissue and are set
free when the latter are digested and dissolved away by the gastric juice
(see Figs. 90-92).

ALIMENTATION AND DIGESTION 111

upon the proteins of the food and especially upon those
proteins (albuminoids) which make up the connective tissue
of animal foods. By dissolving this connective tissue, which
holds together the muscle fibers, fat cells, etc., animal food
is considerably subdivided and made to present a greatly in-
creased surface to the further action of digestive juices. It
is also well to remember that the gastric juice dissolves con-
nective tissue much more rapidly than does, any other of the
digestive juices and that this action upon connective tissue
is really more important than that upon other proteins, al-
though the latter is usually more emphasized. Other proteins
not acted on in the stomach are rapidly digested by the
pancreatic juice in the intestine; connective tissue, on the
contrary, escaping solution in the stomach, is dissolved but
.slowly in the intestine.

The student is, however, warned against supposing that
because gastric juice is able to transform the proteins of the
food to peptids, it actually does exert this action upon all
the protein eaten. In point of fact, as protein foods are
divided into smaller and smaller particles in the stomach,
they are discharged into the intestine, where their digestion
is completed by the pancreatic juice. In man the pancreatic,
and not the gastric, juice is the main agent of protein digestion.

22. The stomach at work. Having now gained a general
idea of the chemical changes which occur in the stomach, we
may proceed to consider what actually happens when food
enters that organ. And here our knowledge has been gained
partly by examining the gastric contents at different periods
of digestion, partly by observing the movements of the
stomach by the aid of the Rbntgen rays, and partly by
other means.

As soon as food enters the stomach, and even while it is
still in the mouth, the gastric glands begin to discharge the
gastric juice, and continue to do so during the four or more
hours of gastric digestion. When the meal is fluid or is small

112

THE HUMAN MECHANISM

in amount, this gastric juice is thoroughly mixed with it;
when, however, the food is more or less solid and bulky, only
the outer layers, which are in immediate contact with the
walls of the stomach, are mixed with the juice. At least this

is true at the cardiac end ; the
cavity of the pyloric portion is so
small and the amount of move-
ment there so great that all por-
tions of the pyloric contents are
thoroughly mixed with gastric
juice ; in the much larger cardiac
portion the central mass of the
food may receive no gastric juice
and thus remain, for an hour or
more after the meal, neutral or
alkaline in reaction. Under these
circumstances very considerable
amounts of starch may continue
to undergo the salivary digestion
begun in the mouth.

Any chemical action is aided by
agitation, since the reacting com-
pounds are thus brought into more
intimate union ; and observation
of the working stomach shows
that while the cardiac portion
makes no movements, but merely
keeps up a steady contraction
and thereby exerts a moderate
pressure upon its contents, the pyloric portion executes, from
a very early stage of digestion and throughout the whole proc-
ess, a series of contractions which gradually bring about a
thorough mixture of the contents and rub down the softened
food into smaller and smaller masses. These contractions
consist of rings of constriction which arise at the beginning

FIG. 56. Outline of the contents

of the stomach of a cat at three

stages of the digestion of a meal

taken about 11 A.M.

Showing the peristaltic constric-
tions which pass over the pyloric
portion and the diminution of the
quantity of food in the cardiac
end. (Full description given in
sect. 22)

ALIMENTATION AND DIGESTION 113

of the pyloric portion and pass onward to the pylorus itself,
a new ring beginning about once every ten seconds and con-
suming from thirty to forty seconds in passing to the pylorus.
Consequently there are always two or more slowly moving
rings in the pyloric end of the stomach at one time.1

The pyloric end of the stomach is thus the seat of a combined
chemical and mechanical action on the food. The vegetable
foods are softened, while the connective tissue of the animal
foods is dissolved away ; in addition, the food is mixed with
a considerable amount of liquid supplied by the secretion of
gastric juice. The contents of the pyloric end of the stomach
thus ultimately come to consist of minute solid masses sus-
pended in a liquid, the consistency of the whole being that
of moderately thick pea soup. This product of the work of
the stomach is known as chyme.

23. The expulsion of chyme into the intestine. The open-
ings of the oesophagus and intestine into the stomach are
usually closed ; the former is opened normally only during
the act of swallowing, while the latter opens at irregular
intervals during the process of gastric digestion. The open-
ing of the pylorus allows the rings of constriction moving
over that region of the stomach to discharge the semifluid
chyme into the intestine. If, however, a large mass of solid
food arrives and is driven against the walls, the pylorus
reflexly closes, thus guarding the entrance of the intestine
from the passage of food not yet ready for intestinal diges-
tion. The pressure exerted by the sustained contraction of
the walls of the cardiac end of the stomach adds to the
food in the pyloric region new portions from time to time,
and the same combined chemical and mechanical process
already described is continued until the whole mass is
reduced to chyme and driven into the intestine.

1 These movements of the stomach and intestine are well shown in
zoetrope figures, which may be obtained from the Harvard Apparatus
Company, Back Bay Post Office, Boston.

114 THE HUMAN MECHANISM

This brief sketch of the working of the stomach shows
that this organ serves the two main functions of storing the
food and of making it more accessible to the digestive fluids
of the intestine. When the chyme is delivered to the intes-
tine, the mechanical difficulties in the way of absorption are
practically gone ; the surface of the food exposed to diges-
tive action is now immensely increased by its subdivision,
and the work remaining for the intestine is almost wholly
the chemical duty of changing the constituents of the chyme
into substances which are soluble and ready for absorption.

Serious troubles arise when, for one reason or another,
gastric digestion goes wrong, because the subsequent proc-
esses of digestion are largely dependent upon the preparation
which the food receives in the stomach. Gastric digestion
may be impaired in one of three ways : first, the gastric
juice may not be secreted in proper amount or proper
strength; second, the stomach may not execute its move-
ments efficiently ; third, the gastric juice secreted may not
be able to get at the food readily, owing to improper cook-
ing or insufficient mastication. The study of the conditions
which produce these troubles — which taken together consti-
tute one form of indigestion, or dyspepsia — will be postponed
to the chapter on the Hygiene of Feeding (Part II).

24. The stimulus to the secretion of the gastric juice. The
first requirement for the work of the stomach is the secre-
tion of sufficient gastric juice. Of late years the brilliant
researches of physiologists have shown that the secretion of
gastric juice is called forth in three ways :

1. The "psychic" secretion. When agreeable or appetizing
food is offered to an animal, and especially when such food
is taken into the mouth, a secretion of gastric juice follows,
which may continue for fifteen minutes or more. This secre-
tion occurs when the food has been in the mouth only ten
or fifteen seconds and even when it is merely offered to a
hungry animal and not taken into the mouth at all. Again,

ALIMENTATION AND DIGESTION

115

1 23456789 10

it occurs only when the animal is conscious; for if food be
introduced into the stomach of a sleeping dog, it evokes only
the most scanty secretion of gastric juice after the animal
has awakened. Moreover, both the amount and the efficiency
of the juice secreted vary directly with the enjoyment of the
meal. When meat is given to a dog which is not hungry,
no such abundant secretion of
gastric juice occurs as during
hunger.

It is clear that we have here
to deal with a nervous process
more complicated than the sim-
ple reflex, and that the efferent
discharge to the stomach occurs
as the result of nervous proc-
esses taking place in the brain
in connection with the enjoy-
ment of food. In other words,
the more the food is desired
or enjoyed, the more efficient
will be this secretion of the
gastric juice.

It is known that this " psy-
chic" secretion will continue
for several hours after an ordi-
nary meal, increasing in amount during the first hour or more
and gradually diminishing from that time onward (Fig. 57).

2. Stimulation of the stomach by constituents of certain foods.
We have seen that the direct introduction of food into the
stomach (for example, into the stomach of a sleeping animal)
does not of itself evoke a secretion of gastric juice. Some
foods, however, contain substances which do evoke such a
secretion, the most important of these being certain con-
stituents of meat. Bouillon, for example, which is an extract
of meat, directly excites the wall of the stomach to secrete.

FIG. 57. The curve of the "psychic"
secretion of gastric juice

Vertical lines represent half-hour pe-
riods after taking the meal ; horizon-
tal lines, relative amounts of gastric
juice secreted

116 THE HUMAN MECHANISM

This is a reason for introducing the soup early at a course
dinner. Meat extracts and meat juices are the most effective
food constituents for this purpose ; milk and water are far
less effective, while most foods, notably bread, white of eggs,
etc., have no such effect at all.

3. Stimulation of the stomach itself by the products of protein
digestion. Although the mere contact of most foods with the
lining of the stomach does not evoke a secretion of gastric
juice, yet it is known that after digestion has been begun
by the action of the " psychic " secretion, certain of the
products of protein digestion arouse a second secretion ,by
acting directly on the lining of the stomach. This second
secretion increases in amount as the first (or " psychic " )
secretion diminishes, and continues throughout the remaining
period of gastric digestion.

To sum up : The secretion of the gastric juice is initiated
by a complicated series of nervous processes connected with
the enjoyment of the food while it is being taken and masti-
cated ; this is aided to some extent by direct stimulation of
the lining of the stomach by a few food constituents, notably
the extractives of meat. The gastric juice thus secreted acts
upon the proteins of the food and produces from them diges-
tive products which directly stimulate the stomach to secrete
and, in fact, maintain the secretion to the end of the period
of gastric digestion. Without the "psychic" secretion pro-
teins are not digested fast enough to induce sufficient sub-
sequent secretion ; without the stimulus of the products of
protein digestion the " psychic " secretion does not suffice to
complete the digestion of a hearty meal — a labor which may
require four or five hours.1

1 What we have called the "psychic" secretion is probably an uncondi-
tioned reflex from the mouth, reenforced by a conditioned reflex involving the
action of the cerebrum ; the stimulation by the products of protein digestion
and possibly that by meat extracts, on the other hand, is probably due to
a hormone (p. 89) liberated in the mucous membrane of the pyloric region,
thence passing into the blood, and so stimulating the gastric glands to secrete.

ALIMENTATION AND DIGESTION 117

D. DIGESTION AND ABSORPTION IN THE SMALL INTESTINE

AND IN THE LARGE INTESTINE

Every few minutes during the process of gastric digestion
the pylorus opens and the stomach forces a few cubic centi-
meters of chyme into the intestine. Chyme, which consists
of water holding in solution certain products of digestion,
and carrying in suspension larger quantities of undissolved
matter, has the consistency of moderately thick pea soup.
The suspended matter consists, among other things, of small
bundles of muscle fibers (from meat), fat melted by the
heat of the body and set free from adipose tissue by the
digestion of its connective tissue, bits of coagulated protein,
such as casein from milk or the white of egg, together with
starches, fats, and proteins of animal or vegetable foods.
Thus far the digestive processes in the mouth and stomach
have been essentially preparatory to the main chemical work
of digestion, which takes place in the small intestine. The finely
subdivided food is now attacked by the digestive juices of
the small intestine brought into solution, and otherwise made
ready for absorption into the blood.

25. The general structure of the intestine; the pancreas
and the liver. The main functions of the intestine, like those
of the stomach, are indicated in the structure of two of its
coats, the muscular coat and the glandular mucous mem-
brane. The fibers of the former are arranged in two layers —
an inner layer in which they are circularly disposed around
the mucous membrane (see Fig. 58), and a much thinner
outer layer in which they run lengthwise. The contraction,
or shortening, of the circular fibers constricts the bore, or
lumen, of the tube, and this constriction of the intestinal
tube is the most important work of the muscular coat.
Sometimes the constriction is confined to one place ; at other
times it moves along the tube, pushing before it the contents.
(See under Peristalsis, p. 125.)

118 THE HUMAN MECHANISM

In the structure of the inner or mucous membrane two
points are of importance to us. In the first place, numerous
simple tubular glands discharge into the intestinal tube an
important digestive juice, the intestinal juice ; in the second
place, fingerlike processes, or villi (0.5—0.7 mm. long by
0.1 mm. thick), arise from its surface and project into the
intestinal cavity. These are important organs of absorp-
tion. The entire surface of the villi, the glands, and the
plane surface of the intestine between these structures is
lined with a continuous membrane composed of colum-
nar cells, which separates blood vessels and lymphatics • in
the intestinal wall from the cavity of the intestine (see
Fig. 59). The products of digestion must therefore pass either
through these cells or between them to enter the blood
or lymph.

The intestine is some twenty or twenty-five feet in length,
and the intestinal glands (Fig. 58) constantly secrete intes-
tinal juice upon the contents as they are slowly moved along
the tube. Two other juices are added to the intestinal con-
tents almost immediately after then* entrance to the upper
part of the small intestine. These are the pancreatic juice
and the bile, which are secreted, respectively, by the pancreas
and the liver. The entrance of the ducts of these glands is
shown in Fig. 54. It is not necessary for our present purpose
to describe the minute structure of these organs; it is enough
for the student to understand that they are glands (p. 29)
which pour their secretions through ducts into the intestine
very much as the salivary glands pour their secretions into
the mouth.

26. The mechanism of secretion of pancreatic juice, bile,
and intestinal juice. The mechanism which evokes the secre-
tion of the pancreatic juice has already been described
(p. 89). It will be recalled that the lining cells of the
intestine immediately beyond the pylorus (duodenum) con-
tain a material which when acted upon by the hydrochloric

ALIMENTATION AND DIGESTION

119

acid of the chyme is transformed into the hormone secretin.
This is absorbed into the blood and chemically excites the
pancreas to secrete.

The secretion of bile by the liver is continuous, although
it is greater at one time than at another. Circular muscle
fibers at the mouth of the
bile duct close the opening
into the intestine when bile
is not needed there ; at such
times the bile secreted accu-
mulates in the gall bladder.
During active digestion the
mouth of the bile duct re-
mains open and the bile flows
immediately into the intestine.

Little is known of the fac-
tors determining the secretion
of intestinal juice, but it prob-
ably is continuously secreted,
at least so long as food is in
the intestine. Thus each con-
signment of chyme from the
stomach receives its share of
pancreatic juice and bile soon
after it enters the duodenum,
and then subsequently re-
ceives continuous additions of
intestinal juice as it is passed
along the intestinal tube by
the action of the muscular
coat presently to be described.

27. The pancreatic juice is a strongly alkaline liquid and
consequently, when mixed with the acid chyme, neutralizes
most, if not all, of the hydrochloric acid of the chyme. Thus it
happens that while the food in the stomach is strongly acid,

FIG. 58. Longitudinal section of the
small intestine

The submucous coat consists of con-
nective tissue and contains the larger
blood vessels from which the mucous
and muscular coats are supplied with
blood

120

THE HUMAN MECHANISM

in the intestine it becomes at once more nearly neutral or
even alkaline. Since pepsin acts only in an acid medium,
the gastric juice now becomes inactive and is soon de-
stroyed by the pancreatic juice, so that it plays no further

r61e in protein digestion. This
is henceforward carried on by
an enzyme of the pancreatic
juice, trypsin, which acts most
vigorously in a neutral or
slightly alkaline medium. It
forms from the proteins of the
food the same general class of
peptone-like substances pro-
duced by the action of the
gastric juice, but carries this
cleavage further into smaller
peptids and even to some ex-
tent to the constituent amino-
acids. Trypsin continues the
digestion of proteins begun by
FIG. 59. Longitudinal section of pepsin. Indeed, in some cases
the tip of a villas tjie preliminary action of pep-

Showing the columnar lining cells sin • necessary, since trypsin

B through which the products of J' J^

digestion must pass on their way to does not act SO readily Upon

the °"inal «>tein as it does

columnar cells and the vessels is in- upon the earlier products of
dicated diagrammatically and with- t:p rli option • nnon thpsp

out showing its structure. A, cell P6Pt]

which manufactures mucus ; C, cap- cleavage products, however, its

illaries; D. lacteal, or lymphatic ^^ jg mog(. vigorous.

In addition to trypsin the pancreatic juice contains at least
two other important enzymes. One of them, amylopsin, is
practically identical with the ptyalin of the saliva and
changes starch into sugar much as happens in salivary diges-
tion. The other enzyme, lipase, acts upon fats, changing
them into fatty acids and glycerin. We cannot go into the

ALIMENTATION AND DIGESTION 121

details of the somewhat complicated digestion of fats. The
change, like that of proteins into peptones and of starches
into sugar, involves the formation of a smaller molecule,
either of fatty acids or soaps, or both, and it is probably
in these forms that fats are received from the intestine by
the villi.

The pancreatic juice thus contains a special enzyme for
each of the three great classes of nutrients — proteins, fats,
and carbohydrates — and thoroughly completes their diges-
tion after they have undergone the preparatory processes
effected by cooking, mastication, and gastric digestion.
Pancreatic juice is by far the most important of the digestive
juices in producing the chemical changes of digestion. In this
respect, also, we may say it is of primary importance in the
work of intestinal digestion, the -other two juices, the bile
and the intestinal juice, acting as aids in its work.

28. The bile contains no enzymes of importance in diges-
tion. It is in fact partly an excretion, some of its con-
stituents being waste products which are poured into the
intestine only to be ultimately discharged from the rectum.
Other constituents of the bile play an important role in the
digestion and absorption of fats, as is shown by the fact that
if bile be prevented from entering the intestine, from forty
to sixty per cent of the fat eaten fails of absorption and is
discharged with the feces. It is probable that this is because
certain soaps formed in pancreatic digestion are not soluble
unless bile is present. When these soaps are not dissolved,
they are not only themselves not absorbed, but, by being
precipitated and adhering to other still undigested food, pre-
vent ready access of enzymes and so greatly retard digestion.

29. The intestinal juice contains two kinds of enzymes, one
acting on protein, the other on carbohydrate material. The
former class, represented by the single enzyme erepsin, has
no action on the proteins of the food, but splits peptones and
other products of gastric and pancreatic digestion into very

122 THE HUMAN MECHANISM

small peptids and amino-acids. A similar thing is true of the
carbohydrate enzymes — they have no action on starch nor on
dextrines (p. 105), but disaccharides (that is, sugars formed
by the chemical combination of two simple sugars, as dipep-
tids are combinations of two amino-acids) are readily split
into their component simple sugars. Cane sugar (sucrose)
and milk sugar (lactose) are two carbohydrate foods which
belong to the disaccharides ; a third is maltose, which is the
stage in the cleavage of starch preceding the final separation
into its component molecules of grape sugar (dextrose).
These inverting enzymes insure the complete cleavage of the
larger carbohydrate molecules into their component sugars,
precisely as erepsin insures the complete cleavage of the
large protein molecule into its component amino-acids or
smaller peptids.

Another most important character of the intestinal juice
is its large content of alkaline salts, especially sodium car-
bonate (soda). Two processes constantly occurring in the
intestine produce acid ; these are (1) the splitting of the
fats into fatty acids and glycerin by lipase and (2) the bac-
terial decomposition of carbohydrates and (to some extent)
of proteins. The sodium carbonate of the intestinal juice,
which, it will be remembered, is being secreted along the
entire length of the intestine, neutralizes these acids arid so
maintains the reaction of the contents at an approximately
neutral point. This reaction is most favorable for the action
of the enzymes present. The combination of sodium carbon-
ate with fatty acids, moreover, forms soaps, which are more
readily soluble than the fatty acids. In this way no doubt
the products of fat digestion are more promptly absorbed
than would otherwise be the case.

30. Action of the muscular coat of the small intestine.
The object of the movements of the intestine is not the
grinding down of the food into smaller masses, but, in the
first place, the agitation of the digesting mixture so that, on

ALIMENTATION AND DIGESTION

123

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the one hand, good contact is secured between food particles
and digestive juices, while, on the other hand, the products
of digestion are quickly brought into contact with the villi for
absorption ; and, in the second place, the slow movement
of the food onwards in the intestinal tube. To accomplish
these ends there are two kinds of intestinal movements.

FIG. 60. The divisive, or segmenting, movements of the small intestine

A, surface view of a portion of the intestine, showing six constrictions which
divide the contents into five segments, as shown in B; as these constrictions
pass away, new ones come in hetween them and divide each segment of the con-
tents into two, the adjoining halves of neighboring segments fusing to make the
new segments shown in C. Repetition of this process results in the condition

shown in D

31. Divisive, or segmenting, movements. The food is not
distributed continuously along the entire length of the intes-
tine, but is subdivided into a number of separate portions
which lie in different loops of the tube. This is partly ex-
plained by the intermittent character of the discharge of the
chyme from the stomach. The number of these portions
varies at different times, but may be as many as twenty
or even more. A certain number, sometimes all, of these

ALIMENTATION AND DIGESTION 125

masses of food will be seen to undergo division into small
segments, obviously produced by a series of constrictions of
the walls, as shown in Fig. 60. The next moment these are
replaced by a second series of constrictions between the first.
Each segment is thus divided into two, and the neighboring
halves of these segments fuse. The next moment the second
series of constrictions is replaced by the first, and this process
continues at times for many minutes with no change in the
general position of the food mass. These divisive, or segment-
ing, movements occur from twenty to thirty times a minute,
and it has been estimated " that a slender string of food
may commonly undergo division into small particles more
than a thousand times while scarcely changing its position
in the intestine."

32. Peristalsis. Every now and then a ring of constriction,
instead of being confined to one place, moves onward, push-
ing the contents of the tube before it for a short distance
(two or more inches). A contraction of this kind is called
peristaltic. The effect produced is much the same as when
the contents of a rubber tube are emptied by squeezing it
along between the thumb and finger.

Thus each consignment of chyme delivered from the
stomach immediately receives its share of pancreatic juice
and of bile, and the final transformation of the digestible
foods takes place as the whole is driven from time to time
along the intestine by peristaltic contractions, the efficiency
of the contact of the food with the digestive juices, as well
as its exposure to the absorbing surfaces, being greatly
enhanced by the agitation produced by the movements of
constrictive division carried out by the circular muscles be-
tween periods of peristaltic activity. The efficiency of digestion
and absorption depends as much on the movements carried out
by the muscular coat as on the chemical processes effected by
enzymes and other constituents of the digestive juices. Digestion
is always a cooperation of chemical and mechanical work.

126

THE HUMAN MECHANISM

So far as is known, these movements are aroused by the
distention of the intestine with food and possibly by chemical
stimulation of the muscular coat by substances formed within

the tube. The presence of solid in-
digestible material also favors the
movements.

33. Absorption is the name given
to the passage of digested food
materials from the cavity of the
intestine into the blood. The word
itself perhaps suggests that the
products of digestion are received
into the blood without change, as
a sponge might absorb a mixture
of peptids, amino-acids, sugar, fatty
acids, soaps, and inorganic salts.
Such, however, is by no means the
case, and the actual physical and
chemical processes of absorption are
complicated — far too complicated
to be discussed here. Suffice it to
say that the intestine is not lined
by a dead membrane but by living
cells, and through these guardian
cells the products of digestion must
pass to enter the blood (see Fig. 59).
In their passage through these cells
teal. Observe that the products some of the digestive products are

of digestion must first be ex- .. , . .. ,, t,

acted upon chemically so that they
enter the blood in forms more
available to the tissues of the
body. The object of the whole process of alimentation,
digestion, and absorption would seem to be that of sup-
plying food to the muscle fiber, the gland cell, the nerve
cell, etc., through the blood as an internal medium or

FIG. 61. The intestinal struc-
tures concerned in absorp-
tion

In one villas is shown the close
network of blood vessels im-
mediately under the lining
membrane ; in the other villus,
the central lymphatic, or lac-

posed to absorption by the

blood vessels before they can

enter the lacteal

ALIMENTATION AND DIGESTION

127

middleman, in that form which is best fitted for the use
of the tissues.

34. Digestion in the large intestine. The large intestine
contains no villi, and its glands secrete an intestinal juice
characterized by a large content of mucin (p. 44).

In the small intestine the amount of water added by secre-
tion balances that absorbed, so that the consistency of the
contents undergoes but little change from the stomach to the
beginning of the large intestine. This consistency, it will be

FIG. 62. The paths by which the products of digestion enter the general

circulation

Those which are absorbed by the blood vessels ((7) of the intestine pass by the
portal vein (P. V.) to the liver before they can enter the right auricle (R.A.) through
the hepatic vein (H. V.) and the inferior vena cava (I. V.C.). Those products which
are absorbed by the lacteals pass directly to the superior vena cava (S.V.C.)
through the thoracic duct

remembered, was (approximately) that of moderately thick
pea soup. During the passage through the small intestine
the digested portions of the food are being removed by
absorption, while the indigestible elements are left behind.
Among the indigestible elements of food are certain connec-
tive tissues of the animal foods, but especially the cellulose
(p. 97), which forms the cell wall of plant tissues. The large
intestine receives from the small this indigestible material,
together with a certain variable but usually comparatively
small proportion of the proteins, fats, and carbohydrates

128

THE HUMAN MECHANISM

which have thus far escaped digestion ; in addition there are
certain constituents of the digestive juices which are not
absorbed and some (for example, certain constituents of the
bile) which are distinctly excretory products.

Special provision seems to be made to insure the approxi-
mately complete digestion and absorption of proteins, carbo-
hydrates, and fats before the food enters the large intestine.
The opening from the small into the large intestine is guarded

by a circular muscle, the ileo-
colic sphincter, which ordinarily
prevents the passage of food out
of the small intestine much as
the passage of food from the
stomach is regulated at the
pylorus (p. 113). In this man-
ner considerable accumulations
of material may occur at the
end of the small intestine and
remain there for two hours or
more while the combined ac-
tion of enzymes and segmenting
movements completes the diges-
tion and absorption of the nu-
trients. Recent work indicates
that this material is discharged
periodically into the large intestine by a relaxation of the
ileocolic sphincter and a vigorous peristalsis in the terminal
portion of the small intestine. It would also seem that this
discharge is especially apt to occur when food is taken into
the stomach, as if there is a reflex to this discharging
mechanism. Obviously the end attained is the more complete
digestion of the food in the small intestine.

Reference to Fig. 154 will show that the large intestine
consists of four parts, the ascending, transverse, and de-
scending colons and the rectum, there being an S-like bend

FIG. 63. Longitudinal section of
the large intestine

Note the absence of villi

FIG. 64. Action of the museular coat of the large intestine, as shown by
the X-rays. After Hertz

The lower border of the ribs and the upper border of the pelvis are sketched.
Black shadows are the food masses in the lower small and the large intestine.
Breakfast about 7 A. M. For some time before noon the food shadows showed
no change (12 M). Shortly after 12 luncheon was taken. At 12.20 the food
accumulated in the lower small intestine had been discharged into the ascend-
ing colon, which it distends. At 12.23 the distal end of this food mass was con-
stricted off and later (12.25) passed along the transverse colon, where divisive
movements take place (12.26) ; but at 12.31 the distal part of this mass is sepa-
rated and rapidly passed through the descending colon (12.31 +) to the sigmoid
flexure (12.31++)

129

130 THE HUMAN MECHANISM

(sigmoid flexure) between the descending colon and the
rectum. The ascending colon is always filled, while the rest
of the tube may be empty. It is chiefly in this first part of
the tube that the abstraction of water occurs. When, as the
result of the discharge of new material from the small intes-
tine into the large, the ascending colon becomes distended,
some of its contents are pushed into the transverse colon,
and this material is rather rapidly passed by peristalsis
through the descending colon, in the lower part of which it
accumulates, being prevented from entering the rectum by
the sigmoid flexure. Finally, with sufficient accumulation of
this more solid material at the sigmoid flexure, stronger peri-
staltic contractions move the mass on into the rectum, which
thereby becomes distended, and this gives the desire to empty
the bowels. From this it will be seen why the bowels are
more readily emptied after meals. It is also highly advisable
to empty the bowels when this desire comes on, since other-
wise the distending stimulus loses its effectiveness and the
continued abstraction of water hardens the feces.

35. Microbic life in the intestine. Occurring simultaneously
with the chemical changes produced by the digestive juices
are others produced by microbes (Part II), which are always
found in the intestine in large quantities. The acidity of the
gastric juice keeps down the numbers of these germs in the
stomach and, under healthy conditions, greatly limits their
activity in that organ. We have seen, however, that some
portions of the contents of the stomach are not acid in reac-
tion during certain periods of digestion, and it not infre-
quently happens for this reason that unhealthy living and,
especially, improper feeding may result in serious gastric
indigestion with excessive bacterial decomposition of the food.
The production of gas, leading to flatulence or belching, is
one of the most familiar results of such bacterial action.

In the intestine the less strongly acid (or even neutral or
slightly alkaline) reaction is much more favorable to bacterial

ALIMENTATION AND DIGESTION 131

life and growth, and we accordingly find that the number of
microbes is much greater in the small and large intestines.
It is not the microbe itself, however, which is of importance
to the organism as a whole, but the substances which it pro-
duces from the foods. Most of these substances are either
harmless themselves or else are readily changed into harm-
less substances either before or soon after entering the
blood; others are poisons, but are normally present in such
minute quantities as to be entirely negligible; more rarely
they are produced in large quantities and may cause various
ill effects either locally or upon the body as a whole.

The production of undue quantities of such harmful sub-
stances, most of which are derived from proteins, is chiefly
dependent upon the food supply of the bacteria. This is
normally kept low by the speedy and efficient removal of the
peptones. Native1 proteins are acted on comparatively slowly
by bacteria and, in any case, must first be changed into pep-
tones or simpler peptids before they can be further broken
down into harmful bodies. If, however, the processes of absorp-
tion quickly and efficiently remove the digestive products, sub-
sequent harmful decomposition of the food is prevented, for
there are normally no bacteria in the blood. It is therefore of
great importance to maintain the efficiency of absorption. This
can be done in general only by leading a normal life — by tak-
ing sufficient muscular exercise, by proper habits of sleep and
rest, by proper feeding, and so on. The hygienic conduct of
life tends to maintain all functions of the body in proper work-
ing condition, those of the digestive organs included; and
nothing else can be depended on, in the long run, to do this.
To this subject we shall return in the chapters on hygiene,
when dealing directly with the personal conduct of life.

1 A "native" protein is a protein as it occurs in nature before being
changed by digestion or other chemical action. The proteins in food are
largely native proteins or else, what amounts to the same thing, as far
as the action of bacteria is concerned, native proteins coagulated by heat

132 THE HUMAN MECHANISM

The chief seat of the putrefactive decomposition of pro-
teins is in the large intestine, where conditions are favorable
for the activity of the special bacteria responsible for this
food change. The reader will recall the provisions for com-
pleting the digestion of proteins and carbohydrates in the
small intestine, and these certainly play a very important
role in limiting harmful microbic action in the large intestine.
It often happens, especially in middle life, that the quantity
of food eaten, and of protein food in particular, must be con-
siderably diminished to insure complete digestion of these
nutrients in the small intestine and thus deprive the putre-
factive bacteria of the large intestine of the material out of
which to make deleterious substances.

We have thus far been dealing only with those microbes
commonly found in the intestine. At times foreign microbes
find entrance, some of which cause such diseases as typhoid
fever, dysentery, cholera, etc. The action of these occasional
intruders will be more fully dealt with in Part II.

36. The elimination of intestinal waste. Those who are
" blessed with a good digestion " sometimes find it hard to
realize that the preparation of food for absorption through the
delicate membranes lining the alimentary canal is a difficult
and complex process, requiring much delicate physical and
physiological apparatus and involving various and important
chemical reactions. Even when they realize this, they rarely
appreciate the indispensable cooperation and fine adjustment
of the several parts and processes concerned. It is just here,
however, that a clear understanding is important, for without
this it is not easy to see how disorders of digestion arise.

Let us then remember that the efficient handling of the food
in the stomach is aided by the preparatory crushing it receives
in the process of mastication ; that in the stomach an adequate
and efficient secretion of gastric juice must take place, and that
this begins as the result of nervous events connected with
our enjoyment of the food when eaten ; that the continued

ALIMENTATION AND DIGESTION 133

secretion of gastric juice is secured, in turn, by stimulation of
the mucous membrane of the stomach by the peptones which
the psychic secretion has formed from the proteins of the food ;
and, finally, that the chemical action of the gastric juice is
aided by the peculiar contractions of the muscular coat of the
stomach. All these agencies working together deliver the food
to the intestine in a finely divided state, well adapted and in-
deed absolutely necessary to secure the proper contact of the
food with the pancreatic juice, the bile, and the intestinal juice.

The flow of pancreatic juice, in turn, is partly the result
of the action of the hydrochloric acid of the chyme on the
walls of the intestine, while the efficiency of the action of the
pancreatic enzymes depends upon the simultaneous action of
the bile and the intestinal juice ; lastly, the chemical action
of these juices, as well as the final act of absorption, requires
the cooperation of the muscular coat. Healthy conditions
with respect to bacterial action similarly depend upon all
else occurring as it should. Digestion, in short, is a chain of
events, each depending upon those which have gone before
and, to a large extent, upon those which are taking place at
the same time.

Keeping these facts in mind, it is easy to appreciate the
possibility of diarrhea or constipation, the latter consisting
in the retention of wastes, the poisonous constituents of which
may be absorbed into the body and cause discomfort, head-
aches, and malaise. When all the digestive processes work
together properly there should be a perfectly natural and
regular evacuation of the bowels. The frequency of such
evacuation varies somewhat and is largely a matter of habit ;
with some 'people it is twice a day, with others once every
other day, but with the vast majority it is normally once every
day and at about the same time. Where this is not the case
there is reason to believe that some part of the work of diges-
tion is not being properly performed. The trouble is not
ordinarily in the mechanism governing the actual discharge

134 THE HUMAN MECHANISM

of the feces from the rectum, but in a derangement some-
where else; it may be entirely the fault of the mechanism
of peristalsis, or it may be due to imperfect secretion. In all
cases it means that something is wrong, &nd .the remedy should
be sought not in drugs or pills but in search for and removal
of the cause. A moment's consideration will show the reason-
ableness of this position. If a watch loses time because it
needs cleaning, we do not seek a remedy in drugs, but in its
cleaning, better adjustment, and good care ; and the remedy
for diarrhea or constipation should in all cases be sought for
in the better conduct of life. Is enough muscular exercise
being taken? Is the diet properly chosen? Are we drinking
enough water? Especially, is the food of sufficient bulk and
does it contain enough laxative material (such as fruit)?
Above all, are we getting enough sleep ? Are we over-
working, or do we work too long at a time without resting?
Is our clothing warm enough, or are we overclad? Such are
the questions which should be seriously asked. The student
of personal hygiene cannot lay to heart too seriously the
truth that the man who goes from day to day, from week
to week, from year to year, neglecting the warnings of diar-
rhea or constipation, only reaps the harvest of his folly when
in later years he suffers loss of health and at times bodily
discomfort; and it is nothing short of impiety to marvel
under such circumstances at the " mysterious " ways of a
Providence which so " afflicts " his creatures. It is no ex-
aggeration to say that the regular discharge of the wastes is
quite as important as the regular feeding of the body and
that no less pains should be taken to form good habits in the
one case than in the other. Many of the headaches, many of
the bad feelings, and many of the bad tempers of the world
are due to neglect of this simple fact. No city, however well
fed or beautiful, the drains of which are choked with filth,
can long remain either wholesome or attractive — and the
human body is essentially a city teeming with living cells.

CHAPTER IX

THE CIRCULATION OF THE BLOOD
A. BLOOD AND LYMPH

1. The blood as a common carrier. In previous chapters
some of the more general features of the circulation have
already been touched upon. In studying the parts of the
body the student has become somewhat acquainted with the
heart, the arteries, and the veins; in considering the typical
structure of the organs (Chap. Ill) he has seen how the
arteries are connected with the veins by a system of com-
municating tubes, the capillaries, through the thin walls of
which interchange takes place between the lymph and the
blood ; and in studying the interdependence and cooperation
of the cells and organs (Chap. VI) he has learned how the
blood leaving each organ returns to the heart, there to be
mixed with that coming from all other organs and thence
pumped first to the lungs and then to the rest of the body.
The need of a circulation is obvious, for the food received
from the alimentary canal and the oxygen received from the
lungs must somehow be carried to the muscle fibers, the
nerve cells, the gland cells ; the cellular wastes must be
taken away to the organs of excretion ; and the internal
secretions of the body must be transported from the organs
in which they are made to those in which they are to be
used. In other words, it is a necessary corollary to the fact
that no cell or organ " liveth unto itself " that there should
be some common carrier of matter and of energy from one
organ to another. Such a common carrier is the blood. The
analogy of the blood system of the body with the railway

135

136

THE HUMAN MECHANISM

system of a country is instructive. As different persons and
different communities in any country make different prod-
ucts and have different needs, it becomes more and more
necessary that the means of communication between them

be extensive and efficient.
Hence the remarkable
growth of the railroads,
or " common carriers," of
any country in which in-
dustrial development pro-
duces increasing division
of labor.

The blood, which is thus
the common carrier first
between the various or-
gans and second between
each organ and the outer
environment, is the net
product of the united
work of all the organs:
from the alimentary canal
it receives water and the
products of digestion ;
from the lungs it receives
oxygen ; each organ con-
tributes its share of waste
products or of internal
secretion, while some in-
fluence the composition of
the blood by removing
from it certain things that it contains.

2. The microscopic structure of the blood. Examined under
the microscope the blood is seen to consist of a liquid portion,
the plasma, crowded with small solid bodies, the corpuscles.
These are of two kinds : the red corpuscles — biconcave disks

FIG. 65. Structure of a drop of blood as
seen under the microscope

Above are shown nine red corpuscles highly
magnified ; below, less highly magnified,
the appearance of the blood soon after being
drawn. Two white corpuscles are shown,
and the red corpuscles stick together, form-
ing " rouleaux." Size of red corpuscle, 7.7 n
wide, 2-4 /^ thick. Diameter of white cor-
puscle, 5-10 M. Number of red corpuscles,
4,500,000-5,000,000 per cubic millimeter;
number of white corpuscles, 4500-13,000
per cubic millimeter, according to the state
of digestion, etc. Surface area of all the
red corpuscles of the blood, 3000 square
meters (30,000 square feet or approximately
four times the size of a baseball diamond).
(!M, or micron — 0.001 millimeter)

THE CIRCULATION OF THE BLOOD

137

containing a pigment, hemoglobin, which gives the red color
to the blood ; and the white corpuscles, which are colorless,
nucleated cells.

Important data on the number, size, and surface area of
the corpuscles will be found in connection with Fig. 65.

3. The white blood corpuscles. The white blood corpuscles
really comprise several different kinds of cells, having differ-
ent functions, the study and explanation of which belong to
advanced rather than to
elementary physiology. It
is enough for our purpose
to state that these cells are
not confined to the blood,
but work their way out of
the blood vessels between
the cells of the capillary
walls and are often found
in the lymph spaces of the
tissues as wandering cells.
The latter term refers to
their movement from place FIG. 66. Amoeboid movement of a white
to place. The cytoplasm of corpuscle

the white Corpuscle is a Showing four consecutive positions among
. . , (. . , . . a group of red corpuscles

thick, viscous fluid without

constant or definite form. In locomotion the cytoplasm flows
slowly from some part of the surface in the direction of
motion, forming what is known as a pseudopodium (from the
Greek, meaning a false foot), as shown in Fig. 66 ; the rest
of the body of the corpuscle then flows into the pseudo-
podium. By the continuation of this process the white cor-
puscles make their way through the spaces of the connective
tissue. Locomotion by means of pseudopodia is frequently
spoken of as amoeboid, from the amoeba, a unicellular animal
which moves in the same manner. (See Chapter XXIII for
examples of the functions of white blood corpuscles.)

138 • THE HUMAN MECHANISM

4. The red blood corpuscles. The red corpuscles are pig-
mented, biconcave disks with no nucleus ; they are normally
confined to the blood vessels and are carried around passively
in the blood current without active movements of their own.
The main function of these corpuscles is to carry oxygen
from the lungs to the tissues, a function which will be fur-
ther studied in connection with respiration. They contain a
pigment, hemoglobin, which gives to the blood its red color
and carries the oxygen.

5. The blood plasma is an exceedingly complex fluid
whose general composition is represented as follows: water,
90 parts; solids, 10 parts (proteins, 8 parts; inorganic salts,
1 part; extractives, 1 part).

Under the extractives are included a very large number
of substances which, though present in small quantities, are
interesting to the physiologist because they are largely prod-
ucts of the chemical activities of the body and as such
give information about the nature of the chemical changes
occurring in the organs.

Finally, it should be remembered that the cells of the
body generally are bathed with lymph, not with blood ;
in other words, that the lymph and not the blood is the
immediate environment of the cells. Lymph is sometimes
described as blood minus its red corpuscles; but this state-
ment, though convenient, is not strictly correct, since the
amount of waste products in lymph must be greater than
in blood, while the amount of food material must be less
(see Chap. IV). Much as the blood is a product of the
united chemical activity of all the organs of the body, so
the lymph of each organ is derived from the cells of that
organ and from the blood flowing through it. Lymph thus
has a double origin and of course shows very considerable
differences of composition in different organs.

THE CIRCULATION OF THE BLOOD

139

B. MECHANICS OF THE CIRCULATION OF THE BLOOD

AND OF THE FLOW OF LYMPH

The greatest discovery ever made in physiology was that
of the circulation of the blood. As late as the settlement of
the earliest English colonies in America it was thought that
the blood moved back and forth in the blood vessels, as the
waters in the sea ebb and flow ; but of any circulation, in the
sense of a steady stream returning to its source, there was
no idea; and it was not until 1621 that William Harvey, an
English physician, proved be-
yond the shadow of a doubt
that the blood in the body of
all the higher animals flows like
a stream always in one direc-
tion, ultimately returning to its
source.

Tests made upon various
animals have shown that this
circulation is accomplished in
the surprisingly short time of
from twenty to thirty seconds ;
which means that the whole
mass of the blood (in man about twelve pints) passes between
three and four thousand times a day through the various
organs of the body, bringing to them their food, carrying
away their wastes, and in general helping to maintain normal
conditions. By what hydraulic machinery is this marvelous
work done ?

6. The motive power of the circulation as a whole ; the beat
of the heart. Whenever a mass of liquid is kept in motion
we naturally look first for the motive power. In answering
the question, What makes the blood circulate ? we shall
find that while there are several causes, one of these, namely
the beat of the heart, is vastly more important than all the

FIG. 67. The circulation of the

blood as seen in the small arteries

and capillaries of the web of a

frog's foot

140 THE HUMAN MECHANISM

others combined. This fact is now so familiar that it is hard
to realize that we owe to Harvey not only the discovery of
the circulation but also the discovery of the meaning of the
heart beat. Before his time, to be sure, the living heart had
been seen at work, alternately shrinking in size and then
swelling, the shrinking being called systole and the swelling
diastole ; but these changes in size were regarded as the
results of the contraction and expansion of certain "vital
spirits " which the arterial blood was then supposed to contain,
and not as muscular contractions and relaxations. Harvey
showed that the heart is a powerful muscle and that its systole
is a muscular contraction ; that during systole it becomes
hard, just as the biceps muscle does when it shortens, and
during diastole soft and flabby; he also proved that with
each systole the heart drives or spouts blood into the large
arteries (the aorta and the pulmonary artery), and that this
blood is prevented from flowing back into the heart during
diastole by membranous valves at the very beginning of the
large arteries in question.

7. The heart a muscular force pump. The beat of the
heart, even to its most minute detail, is one of the most
important as well as one of the most interesting subjects in
physiology; everything in the body hangs on its proper effi-
ciency and regulation, and it cannot be too thoroughly
studied. For our present purposes it will suffice to describe
the heart as composed essentially of a pair of muscular force
pumps. Dissection shows that it is divided into right and
left halves (see Fig. 70), completely separated from each
other, and that each half consists of two chambers — an
auricle and a ventricle. The auricles, into which the great
veins open, have thin muscular walls and are comparatively
small in size ; the ventricles, on the other hand, from which
the great arteries arise, have thick muscular walls, especially
the left ventricle. The ventricles, indeed, constitute the prin-
cipal part of the force pump ; the auricles merely facilitate

THE CIRCULATION OF THE BLOOD

141

the work of the ventricles and for purposes of elementary
study may be mostly neglected. The student should, if pos-
sible, examine for himself and actually handle the auricles,
ventricles, and great blood vessels of a sheep's heart, which
in size and structure sufficiently resembles the human heart.
Figs. 15 and 162 should also be consulted.

8. The mechanics of the heart beat. All force pumps con-
sist of two indispensable parts — some device for pressing
upon a liquid within a chamber, and valves at the openings
of the chamber so arranged as to allow the passage of the
liquid in one direction
only. Each ventricle of
the heart is really such
a pump and is pro-
vided with two sets of
valves — one set at the
inlet, between the auri-
cles and the ventricles,
and the other at the
arterial outlet. These
valves permit blood to
pass only from the
great veins through the

auricles and 011 through the ventricles to the great arteries.
The contraction of the muscular wall of the ventricles pro-
duces pressure on the blood within their cavities ; this
pressure quickly and easily closes the auriculo-ventricular
valves and finally forces open the shut valves at the open-
ings of the great arteries. In this way the right ventricle
drives venous blood into the pulmonary artery, and the left
ventricle arterial blood into the aorta. With the relaxa-
tion of the ventricles (diastole) pressure falls within their
cavities, and were it not for the valves at the mouths of
the aorta and the pulmonary artery, blood would regurgitate,
or flow back, into the heart ; but this " slip " (as it is

FIG. 68. Diagram of the action of
a force pump

142

THE HUMAN MECHANISM

called in hydraulics) the valves prevent, and the ventricles
again fill through the only open channel, that is, the one
leading from the great veins and the auricles. Thus by
contractions rhythmically repeated the heart continues to
spout or deliver blood from the two sets of great veins into
the two sets of great arteries. It is plainly a double force
pump or, better, a pair of force pumps lying and working
side by side.

9. The arterial and the venous reservoirs. To understand
the exact nature and result of the work of the heart we must

now consider the rela-
tion of this living pump
to the pipe system (ar-
teries, capillaries, and
veins) with which it
is connected. The stu-
dent should first trace
the general course of
the circulation in the
simple diagrammatic
representation given in
Fig. 70. This shows
that the blood which
enters the aorta from
the left ventricle must
return to the right side of the heart and pass through the
lungs before it can again reach the aorta. As the physical
principles of the circulation are the same for the systemic
and the pulmonary vessels, we shall confine our attention
to the former.

In the first place, we may observe that the heart pumps the
blood into what is practically a large reservoir (the larger
arteries) and that the blood flows from this reservoir to a sec-
ond reservoir (the larger veins) by various routes ; for the ves-
sels of the different organs represent many alternative courses

FIG. 69. The force-pump action of a ventricle
of the heart

On the left is shown the condition during dias-
tole; on the right, during systole

THE CIRCULATION OF THE BLOOD

143

which the blood may take
in flowing from the arte-
rial to the venous reser-
voir. The blood stream,
indeed, may be compared
with a stream supplying
water power to a series of
mills in a manufacturing
town. The larger arteries
from the main source of
pressure (the heart) cor-
respond to the headrace
from above the dam, while
the larger veins correspond
to the tailrace. The water
flows from the one into
the other only through the
smaller sluices, or pen-
stocks, which supply the
mills. So in the vascular
system a part of the blood
pumped into the arterial
reservoir, or aorta, finds
its way into the venous
reservoir by way of the
skin, another part by way
of the digestive organs,
another by way of the
brain, still another by
way of the kidneys, and
so on ; but the flow in
every case is essentially
the same, namely from a
reservoir of high pressure
to one of lower pressure.

FIG. 70.

Diagram

of the organs

of the circulation

L, pulmonary circulation; M, circulation
through the organs suspended by the mes-
entery, the blood being carried to the liver
P before it returns to the heart. The circu-
lation through other organs, such as brain,
muscles, skin, and kidneys, is indicated.
Lymphatics are represented by dotted lines

144 THE HUMAN MECHANISM

10. The driving force for the flow of blood from the aorta ;
pressure in arteries and veins. The hydraulic conditions in
the aorta may be illustrated by means of the following sim-
ple piece of apparatus : To an ordinary rubber syringe attach
a piece of elastic rubber tubing, the other end of which is
closed by a detachable nozzle. If now the nozzle be removed
and water pumped into the tube, it will be found that the
flow from the open end consists of squirts or spouts and
continues only during the stroke of the pump; if, however,
we attach the nozzle and again pump water into the .tube,
the resistance caused by the small orifice of the nozzle pre-
vents the water from flowing out of the tube as fast as the
syringe pumps it in. The tubing becomes distended with water.
Since, however, the tube is elastic,1 and so tends to return
to its original size, it forces the liquid out through the
nozzle even between the strokes of the pump. The imme-
diate cause of the steady flow from the nozzle is therefore
the elastic squeeze of the rubber tube. The intermittent
stroke of the pump produces distention of the tube, and the
elasticity of the distended tube constantly forces the water
out of the nozzle.

Closely similar conditions obtain in the arterial reservoir.
Here the outlet is also through very small tubes, the small
arteries, whose bore is not greater than -^ or yl ^ of an inch ;
which fact introduces the same condition as does the nozzle
of our apparatus, that is, a resistance to the outward flow of
the blood. Consequently the blood cannot flow out of the
aorta as rapidly as it is driven in, and the extensible and
elastic walls are necessarily stretched. The immediate effect

1 An elastic body is one which returns to its original shape when it has
been stretched, compressed, or otherwise deformed. Elasticity must not be
confounded with " extensibility," or the property of allowing stretching.
Thus when we "pull" taffy we deal with a body which is very extensible
but which is practically inelastic. A body, indeed, may be extensible only
with difficulty, but possess a very high degree of elasticity ; ivory is a good
example of this kind.

(Esophagus

To head and
neck

To shoulder and
arm

To stomach,
intestine, etc.

To kidney

To leg

FIG. 71. The aorta and its main branches

At the beginning are shown the three pocket valves which prevent regurgitation
of blood during diastole

145

146 THE HUMAN MECHANISM

of the heart beat is to keep the arterial reservoir overfilled
or distended, so that the elastic reaction of its walls is brought
into play; and it is this elastic reaction of the arterial walls
which is the immediate cause of the steady outflow through
the small arteries and capillaries.

The force of compression, or pressure, exerted by the
clastic arterial walls is primarily exerted upon the blood
within them ; and the more the arteries are distended the
greater will be the pressure exerted on the blood. A liquid
thus under pressure tends to find an outlet; should any
part of the arterial wall be weak, as sometimes happens in
diseased conditions, it is bulged outward ; and, for the same
reason, a flow of blood will take place through such outlets as
are presented by the smaller arteries and capillaries. More-
over, the greater the pressure of the blood in the arteries, the
more rapid will be the flow into the capillaries. Hence it is
customary to use the arterial blood pressure as a measure of
the force of elasticity exerted by the distended arterial wall.

The veins, on the other hand, are less elastic than the
arteries ; they are, indeed, more like mere conducting tubes
through which the blood can flow back to the heart. They
are not overfilled (since, for one reason, there is no resistance
to the flow of blood out of them into the heart) and hence
venous blood pressure is low.

Thus we have the conditions favorable for the flow from
the aorta to the great veins — a high pressure in the arte-
rial reservoir and a low pressure in the venous reservoir.
It is the function of the heart, by continually pumping the
blood from the veins into the arteries, to keep the arterial
reservoir distended, thus maintaining a difference of pressure
in the two reservoirs. It is this difference of pressure which
drives the blood through the organs.

11. The distribution of the blood among the organs. Some
organs require more blood than others, and the same organ
often requires more blood at one time than at another. Thus

THE CIRCULATION OF THE BLOOD 147

muscles and glands, the seat of very active chemical changes,
require more blood than a tendon ; and a gland requires
more blood during the process of secretion than during rest.
How is the supply of blood to the organs regulated to meet
their varying needs ? In the first place, some organs are
more vascular than others; those requiring a larger supply
of blood receive a greater number of arteries from the arte-
rial reservoir and have a closer network of capillaries. But
in addition to this, these smaller arteries contain circular
muscle fibers whose contraction diminishes the bore of the
tube. When an organ needs more blood the muscle fibers of
its small arteries relax, thus permitting the arterial tubes to
widen or dilate — just as when we want the water to flow
faster from a faucet, we widen the outlet from the pipe by
turning the spigot a little further. When less blood is needed
the small arteries are caused to constrict, just as a spigot
may be partially turned off (see sects. 25-27). In this way
the flow of blood to any organ is regulated to meet the
varying needs of the organ in question.1

12. Secondary aids to the circulation. In the preceding
discussion we have seen that the cause of the flow of blood
through the organs is the difference of pressure in the two
reservoirs. We have further seen that this difference of
pressure is maintained by the heart beat in pumping blood
from the venous into the arterial reservoir. A moment's
consideration will show that anything which hastens the flow
of blood from the veins into the heart and so lowers pres-
sure within the veins would similarly aid the circulation,

1 In order that the student may become more familiar with these funda-
mental hydraulic principles of the circulation, such questions as the follow-
ing should be answered : (1) What are the two principal factors whose
variations change the amount of arterial pressure ? Illustrate by an example
or model. (2) How would the dilation of all the arteries of the intestine
affect the general arterial pressure ? (3) What would be the effect upon the
amount of blood flowing through the skin under this condition ? (4) How
would dilation of the arteries of the skin affect the blood flow through
the brain?

148

THE HUMAN MECHANISM

since, with the same arterial pressure, more blood will flow

into an empty vein than into one which is partially filled.

1. The breathing movements. There are two factors which

thus tend to empty the veins. The first is the suction exerted

on the blood within the veins
by breathing movements. The
exact mechanism by which
this is accomplished must
be left for consideration in
the chapter on respiration.
Suffice it to say here that
just as the enlargement of
the thorax, when we take
in a breath, sucks air into
the lungs, so it also sucks
blood from the large veins
outside the thorax into those
which lie within it ; because
of the thickness of the
walls of the arteries the
same effect occurs to only
a very slight extent in the
arterial reservoir. During
expiration, on the other
hand, the reduction in size
of the thorax forces air out
of the lungs, arid we might
expect that it would simi-
larly force blood from the

veins within the thorax into those without. And this it cer-
tainly would do if the veins were not provided with valves
which allow the blood to flow only toward the heart. In
general, therefore, both inspiration and expiration aid the cir-
culation, the former by sucking blood into the thoracic veins
and so emptying those outside, the latter by making this

FIG. 72. Cross sections of portions of

the wall of a smaller artery (a) and a

smaller vein (»)

A, internal coat; B, middle coat, with
muscle fibers ; C, outer coat of connective
tissue. The contraction of the circularly
disposed muscle fibers narrows the bore
of the tube

THE CIRCULATION OF THE BLOOD

149

blood in the intrathoracic veins flow on more rapidly to the
heart, whence it is pumped into the arteries. In a word,
deep breathing greatly promotes a good circulation.

2. Intermittent compression of the veins. The other second-
ary factor of the circulation is intermittent compression of the
veins, and in ordinary life this is brought about in two ways :
(1) Whenever a muscle contracts it thickens and hardens;
the veins and capilla-
ries which are between
the fibers and fiber bun-
dles, or in the con-
nective tissue between
two contracting mus-
cles, will thus have the
blood squeezed out of
them into the large
veins ; when the mus-
cle relaxes, the empty
veins and capillaries
will readily fill from
the arteries, since the valves of the veins will prevent any
backward flow of the blood from the larger veins. Alternate
contractions and relaxations of muscles therefore aid the
flow of blood through this so-called " pumping " action on
the veins. (2) A similar pumping action on the veins is ex-
erted by alternate flexions (bendings) and extensions at any
joint. In general, flexions force the blood out of the veins,
while extensions allow them to fill. When we remember how
largely most of our usual muscular actions consist of alter-
nate flexions and extensions of joints and alternate contrac-
tions and relaxations of muscles (for example, in walking
and running), we can at once appreciate how greatly mus-
cular activities must aid the circulation. When to the effect
of these we add the suction action of the deepened breathing
movements, the effect upon the circulation becomes very great.

FIG. 73. The pocket valves in the veins

On the right is shown the external appearance
of the vein at the valves when the latter are
closed ; on the left, a vein slit lengthwise and
opened ; in the middle, a longitudinal section
of a vein

150 THE HUMAN MECHANISM

13. Massage. The action of massage is only another illus-
tration of the same principle. By rubbing the legs and arms
in the direction of the heart, the blood contained in their
veins is forced onward and the circulation aided, precisely
as when a muscle contracts or one member of a limb is
flexed upon another.

14. The lymphatics. Important as are the suction action
of the breathing movements and the pumping action of con-
tracting muscles as aids to the circulation of the blood, they
are even more important as causes of the flow of lymph along
the greater lymphatic trunks toward the heart. Reference to
the general method of origin of lymphatics, as described in
Chapter III, will show that the lymph in the lymph spaces,
unlike the blood in the capillaries, has not behind it a high-
pressure reservoir ; there is no such force from behind to send
it onward, since the lymphatics arise blindly in the tissues.
What, then, makes the lymph flow along the lymphatics
toward the heart?

The lymphatics resemble the veins in structure, having
thin walls and pocket valves; like the veins, most of them
originate in extrathoracic organs and join or combine to
form larger trunks as they proceed toward the thorax. All
of them finally unite in two large lymphatics within the
thoracic cavity, and these open into the great veins near the
heart. (Figs. 30 and 70 should be consulted in this con-
nection.) It is at once clear that the breathing movements
must exert on the lymph within these thin-walled vessels
exactly the same suction action as they exert on the blood
in the veins, and anything which increases this suction action,
such as the deepened breathing movements during muscular
activity, must necessarily increase the flow of lymph from
every organ of the body. On the other hand, a pumping
action on the lymph in the organs results from all rhythmic
movements of parts of the body with reference to one another,
since each change of position carries with it some change of

THE CIRCULATION OF THE BLOOD 151

external pressure on lymphatics. Familiar examples are the
movements of arms and legs in locomotion, of the diaphragm
in breathing, and of the lungs in respiration.

It has also been supposed that a third cause of the
lymph flow is the passage of waves of constriction (peri-
stalsis, cf. p. 125) over the larger lymphatics. This, however,
probably plays only a minor part.

Finally, in the formation of lymph from the blood, more
water generally passes from the capillaries to the lymph
spaces than from the lymph spaces into the capillaries.
Under these circumstances, at least at certain times, the
lymph spaces become distended and a certain low pressure
obtains in them. This we may speak of as the " active
force " of lymph formation, and it constitutes a fourth factor
in causing the lymph flow.

We have already pointed out the importance of the lymph
flow in maintaining the lymph currents about the living
cells ; we are now able to appreciate the importance of those
agents which secure this flow. As enumerated above, they
are four in number: (1) suction action of the breathing
movements ; (2) pumping action of muscular or passive
movements ; (3) active force of lymph formation ; (4) peri-
staltic contractions of the large lymphatics.

Of these the fourth is at least doubtful and in no case of
great importance ; the other three may therefore be regarded
as the chief causes of the lymph flow, and of these the first
arid second are brought into most effective action by mus-
cular activity; this deepens the breathing movements and
so increases their suction action on the lymph, while the
movements of the body exert on the lymphatics a pumping
action which is largely lacking during complete inactivity.
The great practical importance of this aspect of the subject
will be discussed beyond in those chapters which deal with
the hygiene of muscular activity (Part II).

152 THE HUMAN MECHANISM

O. THE ADJUSTMENT OF THE CIRCULATION TO THE
NEEDS OF EVERYDAY LIFE

The total quantity of. blood in the body (ten to fourteen
pints) is riot enough to furnish a working supply to all
organs at the same time ; and since, in general, whenever
an organ works it receives more blood, and when it is at rest
it receives less, our daily life with its changes of activity
among the organs makes necessary frequent adjustments of
the circulation to the needs of the organs at various times.

Some of these adjustments are matters of familiar expe-
rience. The increased flow of blood to the skin on a warm
day makes the veins stand out and the face red, and we are
conscious of the more rapid heart beat during muscular
activity, even in an act so simple as running upstairs. Other
adjustments are not so evident, but betray themselves by
their results, as happens after a hearty meal when the
demand of the digestive organs for blood lessens the supply
to the brain and we feel disinclined to hard mental work.
We may begin our study of these adjustments by learning
what occurs in the circulation during some of the more
common activities and events of daily life.

15. The circulation during exposure to heat and cold. When
the skin is exposed to cold its blood supply is greatly dimin-
ished ; the veins no longer stand out prominently on the hand,
and if a small area of skin be made pale by pressing upon it
(thus driving the blood out of its capillaries), the pallor passes
off very slowly. This simple experiment shows that blood is
flowing but slowly from the arterial reservoir into the skin.
Conversely, on a warm day the veins stand out prominently
and the red color instantly returns upon the removal of pres-
sure. These variations in the supply to the skin are due, as
we have already seen (p. 147), to changes in the diameter of
the arteries of the skin, which changes serve, like the spigot
of an ordinary water faucet, to regulate the flow of liquid.

THE CIRCULATION OF THE BLOOD 153

The changes in the blood flow through the skin are
accompanied by corresponding but inverse changes in the
internal organs. On a cold day the stomach and intestines,
the pancreas, the liver, the kidneys, etc. are richly supplied
with blood, while on a warm day their blood supply is
diminished. In the former case the blood withheld from the
skin finds its way into the internal organs ; in the latter case
the skin draws upon these organs for its needed supply.
The circulation in the internal organs compensates for that
in the skin.

16. The reason for compensatory changes. We have seen
that it is the function of the heart to keep the arterial reser-
voir adequately distended with blood, thus supplying a steady
driving force for the flow of blood through the organs. When
the small arteries of the skin widen on a warm day, blood
escapes more rapidly into the skin from the arterial reservoir.
This alone would diminish the amount of blood in the reser-
voir unless the heart pumped more blood or unless the dila-
tion or widening of the cutaneous arterioles were compensated
by a constriction elsewhere, so that the total drain on the
reservoir remained the same. In the case in question it is
the latter of these alternatives which is adopted, and the
reservoir is kept filled without calling on the heart to pump
more blood.

Conversely, on a cold day the diminution of the outflow
into the skin would lead to a backing up or accumulation
of blood in the great arteries, and so to their increased and
perhaps undesirable distention, if the dilation of the arte-
rioles of internal organs did not provide an outlet for the
surplus blood.

Nowhere, perhaps, is this principle of compensatory dila-
tion or constriction of arteries in one region, to allow for the
effect of the opposite change in some other region, so highly
developed or so fully applied as in the reactions of the body
to changes in external temperature.

154 THE HUMAN MECHANISM

17. The circulation during muscular activity. During mus
cular activity the arterioles of the muscles and of the skin
are dilated, the former in order to supply more blood to the
working organ, the latter to aid in the discharge of the ex-
cess of heat produced by the contracting muscles. The heavy
drain upon the arterial reservoir by these two large areas
(among the largest in the body) is compensated to some
extent by the constriction of the arteries of the digestive and
other internal organs. This alone, however, would not suffice
to keep the arterial reservoir filled ; and we accordingly find
that the heart beats more rapidly and more powerfully,
pumping more blood into the aorta in a given time.

It is very important to remember that muscular activity
is the one condition of life which materially increases the
work of the heart ; at other times the greater demand of
blood for the working organ is met more or less success-
fully by withdrawing blood from a resting organ, while the
supply to the whole arterial system, and hence the work
of the heart, remains approximately unchanged. During
muscular exercise, and then only, is the heart called upon
for decidedly increased work ; and, as with skeletal muscles,
its" strength, its ability to meet strain and emergencies and
to withstand fatigue, depend to a great extent upon the
training given it in this way.

Muscular activity also influences the circulation indirectly
by increasing the action of its secondary driving forces —
the suction action of the respiratory movements and the
pumping action of the contracting muscles on the veins.
These are among the most important effects of this agent
upon the flow of blood, but they are too complicated for
detailed discussion here.

It is sometimes stated that muscular exercise " quickens "
the circulation. This is true in the sense that the heart
pumps more blood into the pulmonary artery and the aorta
than during rest. From this it follows that during exercise

THE CIRCULATION OF THE BLOOD

155

more blood flows through the lungs and that blood flows
more rapidly out of the arterial reservoir, but it does not
mean that blood flows more rapidly through all organs, for
the digestive and other internal organs at such times actually
receive less blood. Indeed, we may say that the quickening

FIG. 74. Simple apparatus to illustrate the relation between the output of
the heart, the peripheral resistance, and the general arterial pressure

The amount delivered by the faucet represents the output of the heart, and is one
factor in keeping up arterial pressure ; two alternative routes of outflow, each
capable of regulation, represent the arterioles to different organs. Compensatory
constrictions and dilations and other hydraulic conditions described in the text
may readily be imitated

of the circulation during exercise is chiefly confined to three
important organs — the muscles, the skin, and the lungs ; in
other organs the change is relatively slight, as, for example, in
the brain ; while in still others, notably those of the digestive
system and the kidneys, the speed is diminished.

18. The circulation during sleep. An adequate blood sup-
ply is necessary to the full activity of the brain; when the

156

THE HUMAN MECHANISM

circulation in this organ is seriously interfered with, imperfect
mental action or even unconsciousness is a result. Thus
when all the arterioles of the body dilate, or the heart beat
is slowed down, in consequence of some sudden " shock," so

that pressure in the arterial
reservoir falls too far, the
driving force for the flow
of blood through the brain,
as well as through other or-
gans, is diminished, and. the
person loses consciousness,
or faints. Most cases of
fainting are traceable to
one or the other of these
causes.

The most familiar and
most common example of
unconsciousness, however,
is that of sleep, which in
so many respects resembles
fainting as to suggest that
the unconsciousness in both
cases is due to the same
cause, namely a lessened
blood supply to the brain.
Unquestionably, the amount
of blood flowing through
the brain is greatly lessened
during sleep. The evidence
for this statement cannot
be given here in full, but it
is known that where accident has destroyed a part of the
rigid bone of the skull, and the wound has been covered
over by connective tissue and skin, the scar sinks in dur-
ing sleep — indicating less blood in the brain — and returns

FIG. 75. Showing the relation between

general arterial tone and the supply of

blood to the brain

In A the arterioles of the organs m, n, s
are constricted, raising general arterial
pressure, which forces a large amount of
blood through the brain b. In B the ar-
terioles of m, n, and s are dilated, general
arterial pressure is low, and less blood is
forced through the brain. //, heart

THE CIRCULATION OF THE BLOOD 157

to the level of the general surface of the head when the
subject awakens.

Upon this point of diminished blood supply to the brain
during sleep almost all physiologists are agreed; there is also
general agreement that the arm and the leg increase in vol-
ume when we go to sleep, and this is thought to be due to
a dilation of the arteries of the skin. It is very significant,
on the other hand, that the arm shrinks in volume when the
brain is active in mental work, and especially in mental work
involving the personal interest or mental concentration of the
subject of the experiment.

It is thought by some that other vascular areas — that of
the abdominal cavity, for example — behave in this respect in
the same way as the skin, but on this point the evidence is
not conclusive. It is, indeed, not improbable that these other
vascular areas play some part in the regulation of the flow
to the brain, but it is not likely that they stand in the same
intimate relation to it as does the skin.

The fact is clear, however, that a close relation exists
between cutaneous circulation and the maintenance of proper
vascular conditions in the brain. Mental work, for example,
is more difficult for most people in very warm weather
because at that time the cutaneous arterioles are widely
dilated ; and, on the other hand, it is easy to understand
why the constriction of the vessels of the skin by cold makes
it difficult to go to sleep without sufficient bedclothing.

19. The circulation during the digestion of a meal. After
eating a meal more blood is needed in the secreting diges-
tive glands (especially the stomach and pancreas) and also
in the intestinal organs of absorption, the villi. This need
is greatest during the first hour or two, when there is the
largest amount of food to be worked upon. We find, accord-
ingly, that the arteries of these organs then dilate to such
an extent that the mucous membrane of the stomach and
intestine, which is pale pink while those organs are at rest,

158 THE HUMAN MECHANISM

now becomes very red on account of the large amount of
blood flowing through them.

There is probably some compensation for this in other
organs, but it is an imperfect compensation. The drowsiness
which is apt to come on after a hearty meal is probably an
indication that these compensations are not complete and
that owing to the fall of arterial pressure the brain is not
receiving its normal blood supply.

20. Some practical applications. We may pause here to
consider some important practical applications of these facts.
While the most active secretion is in progress nothing should
be done which will take blood away in large quantities from
the stomach. Muscular exercise, for example, then as always,
dilates the arterioles of the muscles and skin and constricts
those of the digestive organs ; this is obviously an unfavorable
vascular condition for the act of secretion. If the meal be a
light one, so that comparatively little of the digestive juices
are required, no harm may be done by taking exercise after
a meal ; but where the meal is heavier it is almost always
unwise, especially in warm weather. Similar considerations,
which are likewise in full accord with experience, indicate
that it is unwise to eat as large meals in very warm weather
as in cooler weather. The larger the meal, the greater the
amount of gastric juice required to start its digestion; but
in warm weather the arteries of the stomach and intestine
tend to be constricted (see p. 152), so that it is difficult to
secure an adequate blood flow through these organs, and
their efficiency is to this extent impaired.

It is sometimes stated that mental work immediately after
meals causes indigestion by taking blood away from the
digestive organs and sending it to the brain. It is very
doubtful, however, whether the increased blood flow to the
brain is secured largely at the expense of that to the diges-
tive organs. While instances might be cited of indigestion
among people who do mental work upon a "full stomach,"

THE CIRCULATION OF THE BLOOD 159

it must be remembered that these are usually people who
fail to take proper exercise or sufficient sleep and rest; the
indigestion from which they too frequently suffer is more
often attributable to these causes than to the fact that the
digestive organs are deprived of their proper blood supply.

21. The mechanism of the regulation of the flow of blood.
Having thus considered exactly what takes place in the circu-
lation during some of the more important events of daily life,
we may next inquire briefly into the physiological mechanism
by which these adjustments are secured. Its most important
features are the regulation of the inflow from the heart into
the arterial reservoir and the regulation of the outflow
through the arterioles and capillaries of the organs. These
two must be adjusted to each other in order that the reser-
voir may remain full and thus the driving force for the flow
through the organs be maintained. We shall go into the de-
tails of this very beautiful but complicated mechanism only
far enough to enable the student to appreciate certain prin-
ciples of fundamental importance in the practical conduct
of life.

22. The regulation of the pumping action of the heart. The
amount of blood which the heart pumps varies considerably
from time to time. At times it may be as low as three
quarts a minute and at other times as high as twelve quarts,
the quantity being largely determined by the drain made at
the time upon the arterial reservoir. It will be seen at once
that this involves a wide range of adjustment.

The beat of the heart is primarily due to events which
take place within the heart itself. We have seen that this
beat is a muscular contraction. But the cardiac muscle dif-
fers from the skeletal muscle in that it does not require an
impulse from the central nervous system to throw it into
activity. When the heart is cut off from connection with
the rest of the body, it continues to beat for a time and, if
supplied with warm blood, it may be kept beating for hours.

160 THE HUMAN MECHANISM

We express this by saying that the heart beat is automatic,
by which we mean that the heart contains within itself a
complete mechanism for doing its own work.

23. The augmentor and the inhibitory nerves of the heart.
Nevertheless the heart receives from the central nervous
system two pairs of nerves which are able to influence the
rate and the force of the automatic beats. One pair of these
nerves carries from the spinal cord to the heart impulses
which stimulate that organ to beat more rapidly or more
forcibly, or both. Hence these are known as the augmentor,
or accelerator, nerves.

The fibers of the other pair of nerves produce exactly
the opposite effect. Running from the lower part of the
brain, they carry to the heart impulses which slow the beat
or lessen its force, or they may produce both effects at the
same time. They act, as it were, like a brake on a wheel,
checking the activity of the automatic beat. These fibers
are known as inhibitory fibers, and their action is a case of
inhibition.

24. Inhibition. In the examples of nervous action which
we have thus far studied, the nervous impulse has uniformly
thrown some cell into activity. The stimulation of muscle
fibers to contract, of gland cells to secrete, and of nerve cells
in the execution of reflexes will be readily recalled. To this
same class of nervous actions must now be added that of
the augmentor nerves of the heart, for they excite the heart
to greater activity.

In the inhibitory nerves, on the other hand, the nervous
impulse produces exactly the opposite result. Instead of set-
ting organs to work or stimulating them to more vigorous
action, they diminish activity and in extreme cases check or
stop it altogether. In our subsequent studies we shall meet
with many examples of this effect; but we may say at once
that inhibition is as characteristic and as important a feature
of the nervous system as is excitation (see p. 281).

THE CIRCULATION OF THE BLOOD 161

25. The regulation of the outflow from the arterial reservoir;
arterial tone. Wound around the walls of the arterial tubes,
especially the smaller arteries (arterioles) which deliver blood
from the arterial reservoir to the organs, are peculiar muscle
fibers. Their contraction diminishes the size and bore of the
tube, and, when they relax, the tube and its lumen become
wider. As a usual thing these smaller arteries are kept some-
where midway between extreme constriction and extreme
dilation. On a day of moderate temperature, for example,
the arterioles of the skin are moderately narrowed by this
action of their muscle fibers. During colder weather these
fibers contract more than usual and so lessen the size of the
tube, while during warm weather they relax somewhat and
widen it; but ordinarily they are never contracted to their
utmost nor are they often completely relaxed.

This condition of sustained activity of the arterial muscles
is known as arterial tone, and in general any sustained activity
of a living cell is spoken of as tonic activity, or tone. Since,
as we have seen, the total quantity of blood in the body is
not enough to fill completely and distend all the blood ves-
sels when they are widened to their utmost, it follows that
the maintenance of arterial tone is essential to that overfilling
of the great arteries which supplies the driving force for the
flow of blood through the organs. If every arteriole were to
lose its tone, blood would flow out of the reservoir more
rapidly than the heart could possibly pump it in ; we should
have somewhat the same condition of affairs as if, in our arti-
ficial model (p. 144), the small nozzle which affords resist-
ance to the outflow were removed. Arterial pressure would
fall and, the driving force being thus removed, the blood
would remain at rest in the capillaries and veins of the
organs; the circulation would cease because blood would
not return to the heart to be pumped. The maintenance
of arterial tone is consequently no less essential to the
circulation than is the beat of the heart itself.

162 THE HUMAN MECHANISM

Two means are known by which the contraction of the
circular muscle fibers of the arterioles is regulated: first,
impulses from the central nervous system over the vasomotor
nerves; and, second, direct excitation of the arterioles by
hormones in the circulating blood. The vasomotor nerves
are of two kinds, the vasoconstrictors and the vasodilators;
the best-known hormone acting on the arterioles is adre-
naline, the action of which has already been referred to in
Chapters VI and VII.

26. Vasoconstrictor nerves. The muscle fibers of the arter-
ies receive nerves which stimulate them to contract, for if
these nerves are cut, the arteries lose their tone (dilate).
We conclude, therefore, that the ordinary maintenance of
arterial tone is, in part at least, a function of the nervous
system. The muscle fibers of the arteries, in other words,
remain in tonic activity because the neurones which supply
them with nerve fibers are in tonic activity; and we can
understand how general arterial tone may be increased or
decreased by the condition of the central nervous system, by
reflexes, by the nervous " shock " of surgical operations, etc.

Neurones which maintain the proper amount of arterial
tone are known as vasoconstrictor neurones. They obviously do
for the muscles of the arteries what the motor nerves do for
the skeletal muscles, and the augmentors do for the heart.

27. Vasodilator nerves. Many arteries, however, receive a
second set of nerves, which have exactly the opposite func-
tion, that is, to make their muscle fibers relax and so lead to
a widening or dilation of the artery. These nerves do for the
tonic contraction of the arteries what the inhibitory nerves of
the heart do for the heart beat — they diminish or abolish
an existing activity and thus give us our second example
of inhibitory nerves. They are known as the vasodilators.

The vasodilators are not regularly in tonic activity like
the vasoconstrictors. They are called into action, reflexly or
otherwise, when it is necessary that an organ receive more

THE CIRCULATION OF THE BLOOD 163

blood than usual; at other times the vasoconstrictors are
free to exert their tonic stimulation and so regulate the flow
of blood to the organs.

28. The regulation of arterial tone by hormones ; adrenaline.
This has already been described on page 65. It will be
recalled that the presence of adrenaline in the circulating
blood directly excites the arterioles to constrict; that this
action on the arterioles is greater in some regions (for ex-
ample, the abdominal organs) than in others (for example,
the skeletal muscles and skin) ; that the rate and force of
the heart beat are influenced; that the adrenal glands are
excited to secrete by nervous impulses which are dispatched
from the central nervous system during states of emotional
excitement (fear and anger) and, we may now add, when-
ever the blood is deficient in oxygen. There are also reasons
for thinking that the internal secretion of the pituitary body
(p. 67) may likewise play some role in regulating arterial
tone and possibly in the distribution of the blood among the
organs. This is a new field of physiology and the present
state of our knowledge justifies only this brief reference
to it. Enough is known, however, to show that hormones
cooperate with the vasomotor nerves in regulating the flow
of blood to the organs.

29. Importance of the vascular adjustments in daily life.
It is not possible within the limits of the present work to
enter further into the mode of action of these factors of vas-
cular coordination. Our main purpose is to show the student
that proper coordination is as important in adapting the work
of the heart and blood vessels to the hourly needs of daily
life as it is in producing purposeful movements of the skeletal
muscles. Every change of occupation and activity, every
change of surrounding conditions of temperature, moisture,
wind, etc., necessitates some special adjustment of the vas-
cular system; and this adjustment is dependent upon the
same sort of coordinating action which we have already

164 THE HUMAN MECHANISM

compared with the operations of a large army. In spite of
the fact that we are for the most part uriconscious of it, it is
none the less a part of our daily life; and the fatigue induced
within these vasomotor and hormone mechanisms by their
continued activity probably contributes a large share to that
general bodily fatigue which leads us to seek recuperation
in rest and sleep.

The apparatus, the operation, and the regulation of the
flow of blood and lymph afford an excellent illustration of
the fact that the human body, at least in this particular, is
a complex machine. But while we of to-day look upon it
with somewhat less of awe than did our ancestors, and
while there is for us less of mystery and more of mechanism
in it, we gain, on the other hand, a wholly new revelation
of its intricacy and a fresh sense of its marvelous delicacy,
beauty, and perfection of adjustment. The mere fact that
everyone of us carries in his bosom a powerful double
force-pump of remarkable design, original construction, and
extraordinary power, capable in many instances of success-
ful and unremitting service for more than three quarters of
a century, should be, in itself alone, enough to excite admi-
ration and respect for the entire mechanism of which it is
only one part and to awaken within us a desire to use that
mechanism " as not abusing it."

CHAPTER X
BESPIEATION

1. The fundamental act of respiration. We have found in
studying the chemical changes which underlie cellular activity
(Chap. IV) that muscle fibers and gland cells and, we may
now add, nerve cells take in oxygen and give out carbon
dioxide. This cell breathing is the essential act of respiration,
for respiration is only another name for the oxidative proc-
esses of the living body. Respiration of this kind (and of
this kind only) is universal among living things. The one-
celled animal, for example, takes its oxygen directly from
the free oxygen of the water in which it lives, and discharges
its carbon dioxide into the same surrounding medium. Every
one of the thousands of cells of which the human body is
composed repeats this same process, taking its oxygen from
and discharging its carbon dioxide into its surrounding
medium — in this case the lymph. The breathing move-
ments, which renew the air in the lungs, and the circulation
of blood, which affords the channel of communication between
the Inngs and the tissues, are merely accessory mechanisms
rendered necessary by the distance of the cells and the lymph
from the surface of the body. Their principal function is to
keep the lymph supplied with oxygen and to remove from it
the carbon dioxide. In other words, breathing, though minis-
tering to respiration, is not respiration itself.

2. The quantity of oxygen and of carbon dioxide in the
lymph surrounding the cells of the body. The cell is the true
seat of oxidation. Within its imperfectly understood mecha-
nism are found the conditions which lead to the union of

165

166 THE HUMAN MECHANISM

oxygen with the proteins, the carbohydrates, and the fats
of the food.

The cell draws oxygen from the surrounding lymph very
much as a burning match draws oxygen from the surround-
ing air. Consequently the amount of oxygen dissolved in
the lymph is generally comparatively small and would be
removed altogether were it not constantly renewed from
the blood.

For similar reasons the lymph must be relatively rich in
carbon dioxide, since it is this fluid which directly receives
the gas (in solution) from its source of manufacture, the
working cell.1

3. The quantity of oxygen and of carbon dioxide in arterial
blood. It is through the lungs that the body as a whole
receives its oxygen and discharges its excess of carbon
dioxide. Consequently arterial blood contains more oxygen
and less carbon dioxide than venous blood. The actual
figures are as follows:

OXYGEN CARBON DIOXIDE NITROGEN

100 cc. of arterial blood contain 20 cc. 38 cc. 1-2 cc.

100 cc. of venous blood contain 8-12 cc. 45-50 cc. 1-2 cc.

These figures apply to the whole blood, that is, to plasma
and corpuscles; but what is true of the whole blood is true
in a general way also of the circulating plasma, which con-
sequently enters the capillaries2 relatively rich in oxygen
and poor in carbon dioxide, thus presenting exactly the
reverse composition, in respect to these gases, of that found
in the lymph surrounding the living cells.

1 The gases oxygen and carbon dioxide are, of course, dissolved in the
liquid lymph and blood plasma. A liquid exposed to a gas absorbs or dis-
solves the gas. Thus 100 cc. of water when exposed to atmospheric air at
0°C. dissolves 4 cc. of oxygen and 2 cc. of nitrogen.

2 The total time consumed by the blood in passing from the capillaries of
the lungs through the heart to those of the rest of the body seldom exceeds
five or six seconds. Hence the amount of the gases in the blood entering
the capillaries, for example, of a muscle is practically the same as in the
blood leaving the lungs.

RESPIRATION

167

4. The exchange of oxygen and carbon dioxide between the
lymph and the blood plasma. In the capillary regions of all
parts of the body except the lungs we have two fluids, the
lymph and the blood plasma, containing very different amounts
of oxygen and carbon dioxide and separated from each other
by the exceedingly thin membrane of the capillary wall. Under
such conditions both gases will tend to equalize, and each gas
will pass through the membrane from that liquid in which it
is more abundant to that

in which it is less abun-
dant; that is to say, the
oxygen will pass from the
blood plasma in which it
abounds to the lymph in
which it is scarce ; and
the carbon dioxide, in the
other direction, from the
lymph to the blood plasma
(see Fig. 76). Hence the
blood enters the veins
richer in carbon dioxide
and poorer in oxygen than
it left the arteries.

5. The red corpuscle as a carrier of oxygen. The blood
plasma under the conditions of temperature and pressure to
which it is exposed can hold only a small amount of oxygen,
too little to meet satisfactorily the demands of the resting
tissues and utterly inadequate for the much greater needs
of the working tissues. This difficulty is met and the oxygen-
carrying capacity of the blood vastly increased by the peculiar
properties of the coloring matter, or pigment, of the red cor-
puscles. This substance, known as hemoglobin, readily forms
with oxygen a compound (oxyhemoglobiri) whenever the amount
of oxygen is high in the medium surrounding it ; if, however,
much oxygen is removed from its surrounding medium, the

FIG. 76. The exchange of oxygen and

carbon dioxide between the blood and

the lymph in the tissues

168 THE HUMAN MECHANISM

oxyhemoglobin breaks up or dissociates into hemoglobin and
free oxygen. Applying this to the conditions in the capil-
laries, we find that 100 cc. of arterial blood contain less than
1 cc. of free oxygen in the plasma, but about 19 cc. of oxy-
gen combined in the oxyhemoglobin of the red corpuscles.
When the blood enters the capillaries of living tissues, oxy-
gen passes, as we have seen, from the plasma into the lymph,
so that the oxygen content of the plasma is reduced. When
this reduction goes below a point which is quickly reached,
dissociation of the oxyhemoglobin occurs, and the oxygen
thus set free in the plasma is drawn away by the lymph,
from which it is in turn drawn by the cell, the real seat
of oxidation.

The amount of oxygen given above (20 cc. to 100 cc. of
blood) is all that the blood can hold under the usual con-
ditions of atmospheric pressure and at the temperature of the
body. Moreover, the oxygen content of the blood leaving
the lungs (arterial blood) is usually kept remarkably con-
stant by the accurate adjustment of the breathing movements
to the needs of the body. Neither by deeper nor by more
rapid breathing is it possible to increase appreciably the
amount of oxygen absorbed by the same volume of blood
flowing through the lungs. Only by increasing the quantity
of blood pumped through the lungs can we increase the
amount of oxygen carried to the organs and tissues; and,
for the same reason, only by increasing the quantity of
blood flowing through an organ can we increase the oxygen
supplied to that organ.

6. The consumption of oxygen in the tissues. The quantity
of material oxidized in the cells of the body depends chiefly,
indeed under ordinary conditions of life it depends entirely,
on the amount of work these cells are doing. To put the
matter in another way, the cells always contain a certain
quantity of oxidizable material formed by the chemical
changes going on within them; during work, or activity,

EESPIRATION 169

there is a marked increase of oxidizable material (possibly
the result of the cleavages described in Chapter IV), and
for this reason there is a corresponding increase of oxidation
in the cell. It follows that, in general, cell oxidation can
be increased only by increasing cell work ; it cannot be in-
creased by the mere act of deep breathing. " We may lead
a horse to water or fetch water to a horse, but we cannot
make him drink." The assertion, too frequently heard, that
some special form of breathing movement leads to more
efficient oxidation of wastes throughout
the body betrays lamentable ignorance of
this fundamental fact of physiology. This,
however, is not denying that one type of
breathing movement may still be prefer-
able to another, nor affirming that deep-
ened breathing may not sometimes be
desirable. Breathing movements accom- FlG 77 Two adja_
plish other things than oxygenation of the cent alveoli of the
blood, and we may now proceed to study lun&

their physiology. " Showing the air cells

7. Structure of the lungs. In Chapter II the anatomical
relations of the air passages (trachea, bronchi, etc.) and lungs
have been described. The student at this point should con-
sult especially Fig. 5 (p. 13) in order to obtain a clear idea
of the structure of the lungs. The bronchi which enter the
lungs branch, much as the ducts of a gland, and their ulti-
mate branches end in the alveoli, which, like those of a gland,
consist of a single layer of cells, but in this case of very thin,
flattened cells. Fig. 77 shows two of these alveoli dissected,
and Fig. 78 a section taken lengthwise through the same.
Connective tissue binds together the alveoli and bronchial
tubes, thus forming the lobes of the lungs. In this connective
tissue — and hence between the alveoli — are the larger blood
vessels, branches of the pulmonary artery and pulmonary
veins. The arterioles supply an exceedingly close network

1TO

THE HUMAN MECHANISM

of capillaries (Fig. 159), which are in direct contact with the
lining cells of the alveolus, so that the blood in these capil-
laries is separated from the air in the alveolus only by the
thin capillary wall and the equally thin layer of flattened
alveolar cells. Under these circumstances the exchange of
oxygen and carbon dioxide takes place readily between the

air in the lungs
and the blood
in the capillaries.
Finally, the ab-
sorbing surface
of the alveolar
wall is greatly
increased by be-
ing arranged in
the form of pits,
or air cells?- as
shown in Figs. 5,
77, 78, and 159.
8. Purpose of
breathing move-
ments. As the
blood is con-
stantly giving up
carbon dioxide to,
and taking oxy-
gen from, the air
of the lungs, this air would soon cease to be of use in
purifying the blood were it not for the breathing move-
ments, whose function is to replace the vitiated air within the
lung with pure air from without. Breathing is, accordingly,
an act of ventilation of the lungs, and it is the stoppage of
this ventilation which produces suffocation, or asphyxia.

1 The word "cell" is here used to represent a hollow space and not
with its usual histological meaning.

FIG. 78. Diagram of a longitudinal section of two
alveoli with their common bronchiole, and show-
ing, in black, the larger blood vessels in the con-
nective tissue

The capillary network belonging to these vessels is
shown in Fig. 159

RESPIRATION

171

9. Mechanics of the breathing movements. A knowledge of
the mechanism of the breathing movements is of much practi-
cal importance, especially in hygiene, and may be understood
without great difficulty by the study of the model shown in
Fig. 79. The trachea and the bronchi are represented by the
glass tube, and the lungs by an elastic bag, Z, at the end of
the tube. The lungs lie in the large air-tight thorax, which in-
closes the pleural, or thoracic, cavity (p. 10). This thoracic wall
is represented in the model by a glass bell jar closed beneath
by a sheet of thick rubber, D. The cavity
of the bell jar represents the pleural
cavity, and the rubber represents the dia-
phragm (see Fig. 154). The condition of
the lung in the pleural cavity may be
still further imitated in the model by fas-
tening the .inflated rubber bag tightly into
the jar.1 The rubber bag remains moder-
ately inflated within the air-tight cavity
of the bell jar. In the body the distended
lungs virtually fill those portions of the
thoracic cavity not occupied by the heart,
great blood vessels, and other organs.2

Now enlarge the "thoracic cavity" of the model by pulling
downwards on the sheet of rubber which represents the dia-
phragm. The " lungs " within will expand while air is sucked
through the glass " trachea " and mixes with that in the model
" lungs." When the pull is released, the " diaphragm " rises,
thus diminishing the size of the " thorax " and so forcing air
out of the " lungs." In this way the mechanism of the venti-
lation of the lungs may be imitated in essential particulars.

1 Loosen the rubber stopper and, while the neck of the bell jar is open,
inflate the rubber bag through the tube ; while the bag is thus inflated,
push the rubber stopper down into the neck of the bottle.

2 The student is again warned against supposing that the pleural cavity
is a large space filled with air ; in this respect the model is misleading, since
the lungs and other organs completely fill the thoracic cavity.

FIG. 79. Model of the
action of the thoracic
walls and lungs in res-
piration (see sect. 9)

172 THE HUMAN MECHANISM

In life the pleural cavity is enlarged during inspiration by
the contraction of the diaphragm and the elevation of the
ribs. Both of these are movements effected by the action of
skeletal muscles. The understanding of the elevation of the
ribs need give no difficulty ; muscles, some of which are
shown in Fig. 12, pull upwards on the ribs; and the attach-
ment of the ribs to the vertebral column and the breastbone
(sternum) is such that when they are raised the diameter of
the thorax is increased dorsoventrally and from side to side.
The diaphragm, on the other hand, is a kind of circular .mus-
cle with a central fibrous or tendinous portion from which
the bundles of muscle fibers radiate outwards to its edges.
Any shortening of these fibers evidently diminishes the
diameter of the diaphragm ; and because of its form (that
of a dome directed upwards into the thoracic cavity),
contraction of this muscle must increase the size of the
lower thorax.1

There are three typical modes of breathing : (1) The pre^
dominantly costal, or " rib," breathing. Here the diaphragm
is but little used. It is the type characteristic of those who
impede movements of the lower ribs and abdomen with
constricting clothing, such as tight corsets. (2) The pre-
dominantly abdominal. Here the ribs are little used, while
the diaphragm does most of the work, the abdominal mus-
cles being relaxed so that the belly wall has its maximum
of movement. This type of breathing .involves great relax-
ation of tone of the abdominal muscles, which is a serious

1 The action of the diaphragm is often described as increasing the antero-
posterior (head to foot) dimension of the thorax ; but this can happen only
when the diaphragm is free to descend, and it can descend only when, by
displacing downwards the contents of the abdominal cavity, it causes the
well-known respiratory movements of the abdominal walls. These "abdomi-
nal movements" may, however, be prevented by the simultaneous contrac-
tion of the abdominal muscles. In this case the diaphragm cannot descend,
and its contraction can only raise the lower ribs to which it is attached.
The mechanism in these two methods of using the diaphragm is clear
from Fig. 80.

RESPIRATION

173

disadvantage. (3) The lateral costal. Here the abdominal
muscles act at the same time as the ribs and the diaphragm.
This form of breathing produces the highest pressures
on the contents of the abdominal cavity and maintains the
tone of the abdominal walls without diminishing the effi-
ciency of the oxygenation of the blood. It also forces the

FIG. 80. Action of the diaphragm in abdominal and in lateral
costal breathing

Solid lines represent position of body wall, diaphragm, and ribs during expira-
tion; dotted lines, the same during inspiration. The left-hand figure represents
abdominal breathing, the diaphragm becoming more convex, displacing down-
ward the abdominal viscera and forcing outward the abdominal body wall. In
the lateral costal type the diaphragm raises the lower ribs, and the abdominal
walls may actually move inward, owing to the contraction of their muscles

use of the upper ribs to a much greater extent than does
the predominantly abdominal type of breathing (Fig. 80).

It is seldom that one or another of these types is used
in its entirety, and the advantages of one form over another
are often greatly exaggerated. The following statements
may, however, be taken as summing up the essential
practical points.

174 THE HUMAN MECHANISM

1. The breathing movements should be such as to use all
portions of the lungs. In the abdominal type there is little
or no movement of the. upper thorax. The result is that
the apical, or upper, Jobes of the lungs do not share in the
enlargement and contraction of the lungs; they are poorly
ventilated, their lymph current — which largely depends
upon these movements — becomes sluggish, and because of
these unfavorable physiological conditions there is greater
liability to disease. More than 60 per cent (some observers
claim, as many as 80 per cent) of the beginnings of the
lung ravages of pulmonary consumption are found in this
portion of the lung, and this is believed to be due to the
lack of movement which results from the failure to use
the upper thorax.

2. Actual study of the breathing movements in people
who have not worn constricting clothing indicates that the
enlargement of the thorax in inspiration is effected by
the approximately equal action of the diaphragm and of
the muscles which elevate the ribs.

3. The abdominal muscles should to some extent contract
with the diaphragm. This is especially important in those
whose occupation is more or less sedentary, as it is the most
convenient means of giving to these muscles the use which
is essential to the maintenance of their strength and the
consequent prevention of that loss of tone which takes away
from the organs of the abdominal cavity one of their chief
supports (consult Part II, Chap. XVIII).

4. There are good reasons for thinking that it is important
to develop properly the muscles of the upper thorax and
especially those which lie in the triangle between the root
of the neck, the collar bone, and the shoulder blade. When
these muscles are not developed, especially in thin people,
the wall of the thorax in this region sinks inward during
inspiration ; under these circumstances this portion of the
thorax is not enlarged during inspiration, the apical lobes no

EESPIRATION" 175

longer share in the expansions and contractions of the lungs,
and imperfect ventilation of this part of the lung results.

10. Secondary effects of the breathing movements. The
student will now be better able to- understand the part
taken by the breathing movements in facilitating the return
of blood and lymph to the heart. The enlargement of the
thorax during inspiration sucks blood and lymph in toward
the great veins by the same process that it sucks air into
the lungs. Especially in the case of the lymph flow is this
a most important factor. Moreover, in the lymphatics of the
lungs, situated as they are entirely within the thorax, the
movements of the lungs during respiration pump the lymph
onwards and are of special importance in this respect. Much
of the invigorating effect of muscular exercise, popularly
ascribed to better oxygenation of the blood and tissues, is
really attributable to the greatly improved lymph flow from
all organs which results from the deepened respiration in
muscular activity.

11. The automatic respiratory center and its regulation by
the carbon dioxide of the blood. The muscles of the dia-
phragm and those of the ribs, like the biceps and other
muscles which act upon the skeleton, are stimulated to con-
traction by nervous impulses from the brain and spinal cord.
Every movement of respiration is called forth and regulated,
in accordance with the needs of the body at the time, by the
coordinated action of a number of nerve cells. Those which
are most intimately concerned with respiration are found in
different parts of the central nervous system, from the lower
portion of the brain to the end of the first half of the spinal
cord, inclusive ; and there is good reason for thinking that
a group of nerve cells, usually known as the respiratory center,
in the lower portion of the brain, send out stimuli to those
of the cord and through them excite the muscles to contract.

The respiratory center, like the heart (see p. 159), is auto-
matic. This means that its nerve cells periodically (usually

176 THE HUMAN MECHANISM

eight to twenty times a minute) discharge impulses to the
respiratory muscles independently of any stimulation either
by afferent nerves or by other means. Like the beat of the
heart, however, this automatic action is regulated in various
ways. A dash of cold water on the skin reflexly changes
the character of respiration ; coughing and sneezing are simi-
larly examples of reflex modification of the breathing move-
ments ; during vigorous muscular activity the change in
composition of the blood by the addition of waste products
deepens and quickens the breathing ; last, but not least, one
of the most important discoveries of recent years has shown
that the carbon dioxide of the arterial blood going to the
respiratory center is a most important agent in regulating
the automatic activity of the center. No sooner does the
carbon dioxide of the blood increase than the center dis-
charges more powerfully, thus deepening the breathing. An
increase of from 3 to 4 per cent in the carbon dioxide
of the arterial blood doubles the quantity of air breathed
per minute. From this it is evident that the high content
of this gas in arterial blood (see p. 166) serves the very
important function of adjusting the work of the center to
the needs of the body. Whenever, for any cause, the respira-
tory movements no longer adequately ventilate the lungs —
so that carbon dioxide discharged upon the blood in its
course through the body is not completely removed in the
lungs — the consequent increase of this gas in the arterial
blood excites the center to greater activity, with a resulting
increase of breathing and more efficient ventilation of the
lungs. We may recall, in this connection, the warning given
in Chapter VI against supposing that a " waste product "
of the activity of one organ is necessarily harmful, for
carbon dioxide is the chief waste of the body; yet it is
most important that the amount usually present in arterial
blood be maintained. Only the excess above this amount
is injurious.

RESPIRATION 177

12. The circulation as an essential part of the mechanism
of respiration. The consumption of oxygen and the produc-
tion of carbon dioxide thus involve an interchange of these
gases between the blood and the tissues (internal respiration)
on the one hand, and between the blood and the air in the
lungs (external respiration) on the other. But to carry out
these gaseous exchanges a third factor is obviously necessary,
namely, a means of communication between the two, so that
the oxygen absorbed in the lungs may be carried to the
tissues, and the carbon dioxide produced in the tissues be
carried back to the lungs. This communication is provided,
as has been shown in earlier chapters, by the circulation,
which thus becomes an essential part of the respiratory
mechanism.

We have already seen that under the most varying con-
ditions 100 cc. of arterial blood always contain approxi-
mately 20 cc. of oxygen and 38 cc. of carbon dioxide and
that this is practically all the oxygen this amount of blood
can hold. From this it follows that so long as the amount
of blood pumped by the heart in a given time remains con-
stant, no more oxygen will be carried to the tissues, even
if we breathe more deeply. In other words, increased ventila-
tion of the lungs without any accompanying increase in the rate
and force of the heart beat will not supply more -oxygen to the
tissues. The beat of the heart is as important to proper tis-
sue respiration as are the breathing movements ; and we find
accordingly that these two events are closely coordinated.
Greatly increased tissue respiration invariably carries along
with it increased work on the part of the heart.

A large number of measurements of the respiratory ex-
changes l under different conditions and activities of our
life has shown that these are increased by the taking of
food, by exposure to cold, by awaking from sleep, and,
above all, by muscular activity. Exposure to cold acts by

1 That is, oxygen absorbed and carbon dioxide discharged in a given time.

178 THE HUMAN MECHANISM

causing us to move about more briskly, or, if we do not,
by causing us to shiver, so that this really becomes a case
of muscular activity. The same thing is true of awakening
from sleep. We may therefore make the general statement
that muscular activity is the one important agent of life
which increases tissue respiration.

And this increase is at times very great. Even the mus-
cular activity necessary to maintain the erect position in
sitting and standing, as compared with the complete relax-
ation of sleep, doubles the gaseous exchange ; gentle exercise
(a walk of three miles an hour) more than doubles that of
rest ; and vigorous, yet by no means excessive, exercise will
increase it tenfold. These increases mean corresponding,
though not absolutely proportionate, demands on the heart
and emphasize the importance of keeping that organ in an
efficient working condition. Breathlessness, for example,
usually indicates, in part at least, that the heart fails to
respond properly to the demands made upon it, these de-
mands being greater than it can meet without undue fatigue;
it is a warning that we are pushing the heart too hard, a
warning which we will do well to heed. Generally it is
also a warning that we are not getting sufficient muscular
activity; the heart fails to meet the emergency of some
unusual exertion because all along it has not been kept in
proper training; so that while we should, as stated, heed
the warning not to push the heart so hard for the time
being, we should also act upon the equally important warn-
ing that it needs practice or training — a training which can
be given only by reasonable, regular, muscular activity.

The training of muscular activity is therefore not only a
training of the muscles but also of the heart. But this is
not all. The work of the circulatory and respiratory mecha-
nisms must be adjusted or coordinated, the one to the other.
When, for example, the deepened breathing movements
accompanying muscular activity rush the blood back more

EESPIRATION 179

rapidly to the heart (p. 148), it becomes necessary for the
heart to adjust the character of its beat to the new condi-
tions ; and this adjustment is the work of the nervous sys-
tem. Time is, however, required to make the adjustment, so
that it is wise to " warm up " gradually to more vigorous
work. We can also understand how by physical training
this process of adjustment comes to be shortened, for we
have not only trained the heart by giving it more work to
do but we have also trained those portions of the nervous
system which regulate its beat.

CHAPTER XI
EXCBETION

1. The organs of excretion. The student now realizes that
the work of the body is accompanied by the production of
wastes and also understands the necessity for their removal.
The most abundant waste product of the body, carbon dioxide,
is a gas and is excreted by the lungs ; others, notably urea
and other waste products of the proteins, are dissolved solids
and are removed from the blood to some extent by the
intestine and the sweat glands of the skin, but chiefly by
the kidneys.

A number of organs thus perform the work of excretion,
but four of them — namely, the lungs, the kidneys, the in-
testine, and the skin — are of greater importance than all
others. Of these four the lungs and kidneys are far more
important than the intestine, and all three of these are more
important than the skin.

2. Essential and incidental excretion by organs. An organ
may be essential to the proper removal of a given waste, or
it may remove the waste product only incidentally in per-
forming its essential functions. Thus the skin removes a
small amount of carbon dioxide from the body merely be-
cause a certain amount of this gas diffuses from the blood
as it flows through the skin. It is not necessary to the
health of the body that the skin should excrete this carbon
dioxide, for the lungs are quite capable of doing the work
and would do so if for any reason such excretion through
the skin were prevented. Without the lungs, on the other
hand, the carbon dioxide would rapidly accumulate in the

180

EXCKETION 181

blood and cause death. The lungs are essential to the re-
moval of this waste; the skin is not. Similarly, the perspi-
ration contains small amounts of urea and other wastes
which are removed in large quantities by the kidneys. It is
not necessary that the skin should remove any of these, for
the healthy kidney can and does, when necessary, remove
them. Small quantities of them appear in the perspiration
because they are in the blood from which the perspiration
is formed and because the cells of the sweat glands allow
them to pass through, just as the skin allows the passage
of carbon dioxide. .

These considerations are of practical importance in the
hygiene of the skin. It is not necessary to induce perspi-
ration merely to remove waste products from the body. If
the human skin, like that of the cat and the dog, contained
no sweat glands, the waste products would be thoroughly
removed; and in cold weather, when no perspiration is
secreted, the excretion of waste is as complete as when in
warm weather perspiration is abundantly secreted. On the
other hand, perspiration, though not secreted to rid the body
of wastes, nevertheless contains wastes which accumulate
upon the skin. Hence the need of bathing, both as a matter
of health and of decency.

The chief wastes leaving the body and their main chan-
nels of excretion are given in the following table, incidental
excretions being given in italics:

Lungs : carbon dioxide, water.

Kidneys : urea, uric acid, and other compounds, salts, water.
Intestine : bile pigments, nitrogenous compounds, etc.
Skin : urea, etc., salts, water.

The structure and action of the lungs and intestine have
already been described, so that we have left for study the
kidneys and the skin.

3. Structure of the kidneys. Each kidney is a bean-shaped
gland whose duct, the ureter, runs to the urinary bladder. As

182

THE HUMAN MECHANISM

- Vena Cava

- Ureter

the ureter enters the kidney at the center of the depression
in that organ it expands to form a basin, known as the pelvis
of the ureter. Into this basin open the hundreds of glandular
tubules of which the bulk of the kidney is composed. Each
tubule, like the alveolus and ducts of the gland described in
Chapter III, consists of a single layer of cells, which separate
the blood and lymph from the lumen of the tubule ; and the

formation of urine by the kidney
is essentially an act of secretion.
4. The secretion of urine.
The urine is secreted continu-
ously from the blood, at one
time more rapidly than at an-
other, but under normal condi-
tions never ceasing altogether.
Passing down the tubules, it
collects in the upper portion of
the ureter, and successive peri-
staltic waves carry it from this
point to the urinary bladder, an
organ with muscular walls in
which the urine accumulates
and from which it is from time
to time discharged.

In one very important re-
spect, however, secretion by the
kidney presents a sharp contrast to secretion by the stomach
and the submaxillary gland. While an adequate blood supply
to the two latter glands accompanies secretion and, indeed,
is necessary to maintain the secretion for any length of time,
yet these glands secrete only as they are stimulated to ac-
tivity by their nerves ; merely increasing their blood supply
does not produce increased secretion. In the case of the
kidney there seem to be no secretory nerves, and the activity
of the gland seems to be determined to a large extent ly the

FIG. 81. Dorsal aspect of the kid-
neys, ureter, urinary bladder, and
abdominal aorta and vena cava

EXCRETION

183

quantity of blood flowing through it. Anything which increases
this quantity of blood increases the quantity of urine secreted;
anything which diminishes it lessens the amount of urine
secreted.

In the everyday experience of healthy people the activity
of the kidneys is chiefly affected by three things ; namely,
(1) external temperature — more
urine is secreted on a cold than on
a warm day; (2) the quantity of
water drunk ; and (3) the quantity
of food, and especially of protein
food, eaten. All three of these
agents, however, produce their re-
sults, largely if not entirely, because
of their influence upon the blood
flow through the kidney. Thus ex-
posure of the skin to cold causes a
constriction of the arterioles of the
skin and a compensating dilation of
those of internal organs, the kid-
neys included. More blood flows
through the kidneys and more urine

is Secreted. Much the Same thing open, on the papillae (B, B}, into

is true of the absorption of water " pel™(c'> o£ the uretl
and of protein food, for both these conditions cause a widen-
ing of the arterioles of the kidney.

Changes in the quantity of the urine secreted are, generally
speaking, only changes in the amount of water rather than in
the amount of urea and other dissolved wastes. Certain con-
stituents of the urine, however, are not very soluble, so that it
is not well to have water, the only solvent of these substances
in the urine, unduly diminished. A scanty secretion of urine
during the day is, in general, a distinct indication, especially
in warm weather, that insufficient water is being taken. Many
persons drink too little water rather than too much.

FIG. 82. Vertical section of
the kidney. Diagrammatic

The tubules (A) of the gland

184

THE HUMAN MECHANISM

5. The structure of the skin. The skin is an organ which
performs several functions, the most important being (1) that
of protecting the underlying structures
from drying and mechanical injury ;
(2) that of assisting in maintaining the
constant internal temperature of the
body ; and (3) that of receiving the ex-
ternal stimuli of pressure, heat, and cold.
Incidentally, as we have seen, the skin
is an organ of excretion. We may there-
fore describe its structure and excretory
function in this connection, reserving
the study of its other functions for
Chapters XII and XIV.

The skin consists of an outer layer,
the epidermis, and an inner layer, the
dermis, cutis, or corium. The clermis
consists of connective tissue richly sup-
plied with blood vessels, lymphatics,
and nerve fibers, together with sense
organs of touch. The fiber bundles of
the connective tissue are most dense
near the epidermis; in the deeper por-
tions the network is loose and the lymph
FIG. 83. Cross section spaces larger, the connective tissue of
the dermis passing insensibly into that of
the subcutaneous connective tissue.

The cells of the more open portions
sweat gland; D, dermis; of the dermal network, and especially

E, subcutaneous connec- , ,

The those of the subcutaneous tissue, store

within their CVtO-
. J

plasm. The subcutaneous tissue, indeed,
is one of the most important organs in the body for the
storage of fat. Connective tissue in which large amounts of
fat are stored is known as adipose tissue (see p. 223).

of skin

A, horny layer of epi-
dermis; B, deeper layer
of epidermis ; C, duct of

tive tissue (p. 7).

blood vessels are injected up more or }egs
to show black. Cf.Fig.89

EXCRETION

185

The outer surface of the dermis is not flat, but contains
rooundlike projections known as papillce, which project into
the overlying epidermis. Some of these papillae contain nerve
endings of the sense of touch, while others contain capillaries,
which are found also in other portions of the dermis. The
dermis is the vascular organ of the skin, blood vessels being
entirely absent from the epider-
mis (see Figs. 86, 89).

The epidermis consists of
many layers of cells, the num-
ber of layers being very great —
a hundred or more on the palms
of the hands and the soles of
the feet; in other places less
exposed to pressure or friction
they may not exceed twenty.
The deeper cells (that is, those
nearer the dermis) are alive and
in process of active growth and
multiplication. The outer layers,
which are further from the der-
mis with its blood supply and
nearer the surface with its ex-
posure to drying, degenerate and
are gradually transformed into
dead, flattened horny scales
which, packed together, form the
horny layer. These scales are
being constantly rubbed off and their loss made good by the
growth and multiplication of the living cells beneath. Such a
covering or lining is well fitted for surfaces which are exposed
to friction or drying, and we accordingly find that the mouth,
the part of the pharynx used in swallowing, the oesophagus,
and the rectum are lined with the same tissue. The endings
of nerve fibers are found in the lower layers of the epidermis.

FIG. 84. Hair and hair follicle

A, horny layer of epidermis. B,
layer of living, growing cells ex-
tending (B') into the hair follicle, at
the bottom of which it forms the
mass of growing cells E over the
papilla (P) with its knot of capil-
laries; the growth, multiplication,
and transformation of these cells
into horny fibers forms the shaft of
the hair, D. C, capillaries in the
dermis. S, a sebaceous gland dis-
charging its oily secretion (0) into
the follicle to lubricate the hair and
the horny layer of the skin

186

THE HUMAN MECHANISM

The hairs, the sweat glands, and the nails are modified
portions of the epidermis. Of these the hairs and the sweat
glands are of sufficient importance to m^rit some description.

6. Structure of a hair and a hair follicle. A hair grows from
the bottom of a pit, the hair follicle, which extends downward

into the dermis or even into the
subcutaneous tissue. Microscopic
examination shows that this fol-
licle is lined with a continuation
of the epidermis, just as a gland
of the stomach or intestine is
lined by an ingrowth of the cells
of its surface. At the bottom of
the follicle is a papilla, and the
hair which grows out from this
papilla to the surface bears to
the cells of the papilla the same
relation that the horny layer of
the epidermis bears to the similar
underlying cells. We accord-
ingly find that the hair is com-
posed of horny scales closely
pressed together into the well-
known threadlike structure.

Opening into the hair follicle,
one or more sebaceous glands dis-
charge an oily secretion which
lubricates the hair and the horny layer of the epidermis, and
so prevents drying and chapping (Figs. 84 and 85).

7. The sweat glands are tubular prolongations of the epi-
dermis through the dermis into the subcutaneous tissue. Here
the tube becomes much coiled, forming the secreting recess,
which is richly supplied with blood vessels and also receives
nerves. It is a simple tubular gland formed as an ingrowth
from the epidermis (see Figs. 86 and 89).

FIG. 85. Magnified section of the

lower portion of a hair and hair

follicle

A, membrane of the hair follicle,
cells with nuclei and pigmentary
granules ; B, external lining of the
root sheath; C, internal lining of
the root sheath; D, cortical or
fibrous portion of the hair shaft;
JS, medullary portion (pith) of
shaft; F, hair bulb, showing its
development from cells from A

EXCRETION

187

8. The secretion of the perspiration, like the secretion of
the gastric juice, is under the control of the nervous system.
When the nerves going to the sweat glands of a given
area of skin are cut or otherwise injured, the secretion of
perspiration ceases over that area; and the appearance of
cold beads of perspiration as the result of fright shows
how events taking place in the nervous

system may excite these glands to activity
apart from the presence of their usual
stimuli — the application of heat to the
skin and the liberation of heat within the
body by muscular and other activities.
The distinction should be made between
the so-called " sensible " and " insensible "
perspiration, the latter name being given
to the perspiration the water of which
evaporates as fast as secreted ; the former
to that which does not evaporate so rapidly
and hence remains for a time on the sur-
face of the skin. When the water evapo-
rates, the dissolved solids (salts, urea, and
other compounds) remain behind on the
skin.

9. Value of profuse perspiration in the
care of the skin. While the skin is not

.-, p ,. ,-, Note the coiled form

primarily an organ of excretion, the per- of the tube in the
spiration contains a certain amount of
waste substances and salts, which are left
by the evaporation of the water upon the surface and, to
some extent, in the mouths of the ducts of the sweat glands ;
this is especially the case when evaporation takes place about
as rapidly as the perspiration is discharged. When the secre-
tion of perspiration is more abundant, as during muscular
work, or at very high temperatures, or, in general, where it
does not evaporate as rapidly as discharged, the accumulation

FIG. 86. Sweat gland
(slightly magnified)

subcutaneous tissue.

188 THE HUMAN MECHANISM

of solids in the ducts of the glands is washed out. For this
reason a vigorous perspiration followed by a bath is a useful
hygienic measure in the care of the skin, although it is not
necessary, as is sometimes supposed, in order to secure the
efficient elimination of wastes from the blood.

10. The skin as an organ of absorption. While it is true
that water as perspiration may readily find its way out
through the skin, such escape is effected chiefly by the
sweat glands, which are under the strict control of the nerv-
ous system. Apart from this the skin is virtually water-
tight; and, oiled as it is by the secretion of the sebaceous
glands, it serves both to keep in the water, which forms so
important a part of the tissues, and also to keep out water
which might otherwise soak into the body, as, for example,
during bathing. This waterproof characteristic also makes it
next to impossible for us to absorb food materials by way of
the skin. A " milk bath" may be at times useful in the care
of the skin, because the fat or oil of the milk may supply
any deficiency in the sebaceous secretion and so insure
lubrication of the epidermis ; but it cannot be regarded as
a means of supplying food to the body.

CHAPTER XII

THERMAL PHENOMENA OF THE BODY
A. THE 'CONSTANT TEMPERATURE

1. The normal temperature. No characteristic of the
human mechanism is more remarkable than its constant
temperature. Whether we are awake or asleep, by night or
by day, at work or at rest, at home or abroad, in summer
or in winter, in the tropics or in the polar regions, in subter-
ranean caves or on lofty mountain peaks, the temperature of
healthy human beings is always nearly the same. So steady
is this temperature that an increase or decrease of two or
three degrees gives just cause for anxiety, and a change of
seven or eight degrees is looked upon with alarm.

In many modern laboratories constant temperatures are
obtained by the use of a thermostat, the apparatus of which
is visible and easily understood ; but no such special appa-
ratus regulates the constant temperature of the human body,
and we have rather to seek an explanation in the coordi-
nated activities of organs already familiar, such as muscles,
skin, blood vessels, and especially the all-controlling nervous
system.

2. Temperature and chemical changes. Every chemical
reaction takes place more readily under some external
physical conditions than under others, and among these
conditions none is more important than temperature. This
fact is illustrated in the case of the enzymes. At the freez-
ing point saliva exerts no action upon starch paste ; as the
temperature rises, the activity of the enzyme increases up
to a certain point and then diminishes more or less rapidly

189

190 THE HUMAN MECHANISM

until a point is finally reached at which its peculiar chemi-
cal properties are destroyed.

3. Temperature and vital activities. When we come to
the activities of living cells — activities which, it will be
recalled, depend on chemical changes — precisely the same
thing holds true and in so striking a manner as to create a
widespread but erroneous impression that this dependence
upon temperature is peculiarly characteristic of living things.
The one-celled animal, amoeba, moves about more actively
and digests more food at 20° C. than at 10° C. ; bacteria
grow more rapidly at the room temperature than near the
freezing point; the pitch of the note made by a cricket rises
with the temperature, indicating that the movements of the
wing covers which produce the sound are being made more
rapidly ; and in the winter sleep of hibernating animals we
have a beautiful example of the decline of vital activities
with the fall of external temperature.

Nor are the living cells of the human body exceptions to
this rule. The rate of the heart beat varies directly with the
temperature of the blood, and the character of the breathing
movements is influenced by the same cause; a cooled muscle
contracts more slowly, a cooled gland secretes less abun-
dantly. If the temperature of the body itself falls, every
vital activity is depressed, and death itself may result from
undue cooling.

4. The constant temperature of the body. This depression
of nervous, muscular, and glandular activity results, how-
ever, only from a fall of the temperature of the body, not of
that of the surrounding air or other medium. These two
things are by no means the same, as may be readily seen
from the fact that a thermometer placed in the mouth indi-
cates almost the same temperature of the body on warm and
on cold days ; even while we are shivering with cold the
thermometer gives about the same reading as when we are
enjoying the warmest summer weather. The temperature of

THERMAL PHENOMENA OF THE BODY 191

the body remains nearly constant, regardless of changes in
the temperature of the air around it.

We have only to appeal to experience to see that this is
not the way in which lifeless matter generally behaves; a
stone, the earth, a piece of iron is warmer on a warm day
and colder on a cold day; in general, lifeless things take the
temperature of the medium in which they are placed, and this
is one of the fundamental principles of physics. Nor do most
living things act differently; the temperature of a plant or
a tree, of an earthworm, a frog, a turtle, a snake, does not
differ greatly from that of its surroundings. It is only birds
and mammals which show this remarkable power of maintain-
ing an approximately constant body temperature notwith-
standing wide limits of change in that of the surrounding air.
Such animals are known as warm-blooded because they are
usually warmer than surrounding objects; those animals
which do not thus maintain a constant temperature, on the
other hand, are known as cold-blooded.1

It is clear that the power to maintain a constant body
temperature is of the utmost importance in enabling an ani-
mal to counteract the varying conditions of climate. Were
it not for this power, man would be a hibernating animal;
with the coming of winter all his activities would gradually
be slowed down and, long before our rivers and ponds had
begun to freeze, all business, industrial life, and intellectual
life would come to a standstill; it would not be possible
for the human race to people every zone of the earth — the
shores of Alaska or Iceland as well as the banks of the
Ganges or the Amazon.

5. The temperature of the body not absolutely constant.
The term " constant " as applied to the temperature of

1 A cold-blooded animal exposed to a temperature of 99° F. is as warm
as a warm-blooded animal. Such animals are so called because they usually
feel colder when handled than do warm-blooded animals ; but this is merely
because the temperature of the air (which is also their temperature) is usually
lower than the temperature of warm-blooded animals.

192 THE HUMAN MECHANISM

warm-blooded animals is not, however, to be taken too literally.
No animal has an absolutely constant temperature. In the first
place, there are slight variations from time to time under the
changing conditions of life. The temperature is higher by
from one to four degrees during muscular activity than
during rest; it varies during the day, being highest in the
afternoon and lowest in the small hours of the morning; it
is often raised half a degree or more by taking food, and
marked changes of surrounding temperature may cause a
change of one degree or even more in that of the body.
These changes between 97.5° and 99.5° F. are of everyday
occurrence and are entirely normal; so that when we speak
of the temperature of the body being constant we mean that
it varies only within narrow limits or that it is constant in
comparison with that observed in cold-blooded animals.

6. The temperature of different organs. Nor is this all;
some parts of the body have a higher temperature than
others. Thus the temperature of the liver is often as high
as 107° F. ; that of the muscles varies between 99° and
105° F. ; that of the blood in the right side of the heart is
usually a degree or so higher than that of the blood in the
left side. But it is in the skin that we meet with the widest
variations from the general average. Everyone knows that
on a very cold day the temperature of the skin may be far
below 98.6° F. ; indeed, the experience of " frosted " ears or
feet shows that at times cutaneous temperature may descend
to, or even below, the freezing point itself; and it is very
exceptional indeed when the skin temperature is above 92°
or 93° F., even on very hot summer days. These variations
are due to the fact that the skin is the organ which is
immediately exposed to the changing environment and
hence peculiarly subject to cooling influences. It is there-
fore customary to distinguish between an outer body zone
of variable temperature and the more constant temperature
of internal organs.

THERMAL PHENOMENA OF THE BODY 193

7. Measurement of the body temperature. The great equal-
izer of the body temperature is the blood. Blood which has
flowed through the skin comes away cooled ; that which
comes from an organ like the liver or a working muscle, in
which active oxidations or other chemical changes have taken
place, is heated. In the great veins and in the heart the
warmer blood is mixed with the cooler, and an average
temperature of the arterial blood results. It is this average
temperature of the arterial blood flowing to the organs that
is approximately constant.

When this blood flows for a time through an organ which
is itself not producing heat and is at the same time protected
from loss of heat, the organ ultimately takes on the tempera-
ture of the blood; so that by measuring the temperature of
such an organ we get the temperature of the blood itself.
It is customary to take the temperature in the mouth, the
bulb of the thermometer being placed under the tongue and
the lips kept closed. Subject to the variations mentioned
above, the temperature of the mouth is 98.6 F.

8. The feeling of cold or warmth not a true test of the
body temperature. It is well at this point to warn the stu-
dent against confusing the body temperature with sensations
of cold or warmth. Just as visual sensations are aroused
only by that light which falls upon the sense organ espe-
cially adapted to respond to its stimulation, namely the eye,
while light falling upon the skin arouses no such sensation,
so heat and cold can excite the corresponding sensations
only when they act on special end organs adapted to receive
these stimuli, and these end organs are found only in the
skin, the mouth, and perhaps the nose, pharynx, and upper
oesophagus. We are therefore conscious only of the tempera-
ture of these organs ; we are not and cannot be conscious of
the temperature of the blood or of internal organs generally.
It is therefore clear that our feelings give us no reliable in-
formation as to the temperature of the internal parts of the

194 THE HUMAN MECHANISM

body. This fact is strikingly illustrated in the case of a "chill,"
when the internal temperature is almost always really above,
and not below, the normal, and the feeling of warmth pro-
duced by muscular activity or by warming one's self at a fire
merely indicates a higher temperature of the skin, not a higher
temperature of internal organs.

Having now learned the more obvious facts about the
constant temperature of the body, we have next to inquire
by what means this constant temperature is maintained.

9. The production and the loss of heat. We must first
remember that the body produces or liberates heat. The
chemical changes, largely oxidative in character, which are
at the basis of the work of its muscles, glands, nerve cells,
etc., liberate heat just as truly as the burning of coal in the
furnace of an engine liberates heat. Heat production is there-
fore an indispensable result of cellular and organic activity,
and it is greatest in those organs, like the muscles and liver,
which carry out the most active chemical processes. The
body is warm for the same reason that a stove is warm;
that is, because heat-producing chemical changes, largely of
an oxidative character, are going on within it. In the second
place, the body is always losing heat, and this in two ways:
(1) by the transfer of heat by conduction, convection, and
radiation * to colder objects or to the colder air with which
the body is surrounded, and (2) by the evaporation of water
from the surfaces of the body — especially by the evaporation
of water of perspiration.

Everyone knows in a general way that when a warm
body is brought near a colder one, the former becomes colder
and the latter warmer; heat is transferred from the warmer
body to the colder. In this way the clothing is warmed by

1 Those not familiar with the meaning of the terms "conduction," "con-
vection," and "radiation" will find them explained in section 26 of this
chapter (p. 211). In the following discussion we have arbitrarily adopted
the term "heat transfer" to include these three means of heat loss, in order
to distinguish them from the loss of heat by evaporation.

THEKMAL PHENOMENA OF THE BODY 195

contact with the body; so is the air in immediate contact
with the skin ; and conversely the body may be warmed by
contact with anything warmer than itself, a hot-water bottle,
for example. It is not, however, necessary that two solid
bodies be in actual contact in order that heat may pass from
one to the other. A stove warms all the objects in a room,
although few of them are touching it; and the human body
may lose heat to, or gain heat from, objects at a greater or
less distance. The heating of the body by the sun, millions
of miles away, clearly shows this fact.

The loss of heat by evaporation of water or other liquid
from the skin may be readily illustrated by the simple experi-
ment of blowing a gentle current of cool, dry air over the
dry hand and comparing the cooling thus produced with
that which results from blowing a similar current against
the moistened hand. In the latter case the cooling will be
much greater than in the former. Liquids, like ether, which
evaporate more rapidly than water will produce even greater
feeling of cold on the skin.

10. The heat account of the body. The body is therefore
constantly receiving and constantly giving out heat, just as
a bank is constantly receiving and paying out cash. In the
bank a cash account is kept, on one side of which is entered
the cash received and on the other the cash paid out. The
difference between the two sides, known in business as the
balance of the account, shows how much cash is on hand at
the time of taking the balance. Should the cash unduly
accumulate, efforts are made to keep down the balance by
increasing loans; should the cash on hand fall below a
desired level, active efforts to encourage loans are lessened
and the normal desired balance is restored; finally, should
there be an unusual demand for cash at the window of the
paying teller, for example, a " run on the bank," the bank
will borrow from other banks and in this way keep income
and outgo of cash approximately equal.

196 THE HUMAN MECHANISM

In what follows the student will learn that this is precisely
what the body is doing with regard to heat. We may, in-
deed, imagine a heat account of the body, the two sides of
which would be as follows:

DEBIT CREDIT

(Heat received) (Output of heat)

1. Heat produced within the body. 1. Heat transferred to surround-

2. Heat transferred to the body ing objects colder than the

from warmer objects without body (by conduction, convec-

(by conduction, convection, tion, and radiation).

and radiation). 2. Heat lost in evaporating water

of perspiration, etc.

The balance of this heat account at any one time is the
amount of heat in the body, and this determines the temper-
ature of the body. When the output of heat exactly equals
the heat received, the balance of the account remains the
same; that is to say, the temperature is constant. A con-
stant temperature, therefore, means that the two sides of
the heat account are being kept equal to each other. If
the balance increases, either by the production of more heat
or by the loss of less, the temperature of the body rises,
and we have fever.

11. Transfer of heat dependent upon the nature of the
vehicle of transfer. The rate at which heat may be trans-
ferred depends upon the nature of the substance through
which the transfer occurs and which we may speak of as the
vehicle of transfer. We cannot go minutely into the factors
here concerned, but would call attention to the following
points, which will be readily verified from experience:

1. A gas is in general a poorer vehicle of heat transfer than
a liquid or a solid. We make use of this fact in the manu-
facture of fabrics for our warmer clothing, for these fabrics
are warm according to the quantity of air within their meshes.
A woolen garment is warmer than a cotton garment because
it contains within the fabric so large a quantity of the poorly

THERMAL PHENOMENA OF THE BODY 197

conducting air; or, of two woolen garments of the same
thickness, one of which is rather loosely and the other tightly
woven, the loosely woven garment will be much the warmer
because so large a proportion of its thickness consists of the
poorly conducting air rather than of the rather rapidly
conducting solid woolen fibers (see p. 423) „ Or, again,
air of 70° F. is very comfortable ; it feels neither cold nor
warm to the skin ; but water of 70° F0 feels distinctly cool.
This is because heat is conducted away from the skin more
rapidly by water than by air. For this reason we may feel
chilly when our clothing has become drenched with rain.

2. Moist air is a better vehicle of heat transfer than dry air.
This becomes obvious when one is exposed to damp air at a
temperature of less than 70°, and the familiar difference be-
tween dry and damp winds in winter illustrates the same fact,
for a damp wind at 50° F. chills the skin more than a dry
wind at 40° F. The student is cautioned, however, against
supposing that dampness always favors the output of heat
from the body; it favors only one method of heat output,
namely the transfer of heat. On the other hand, dampness
hinders the output of heat by evaporation. Hence at those
temperatures (above 80°) where the output is chiefly by
evaporation, a damp atmosphere is close, warm, and muggy;
where the output is chiefly by heat transfer (below 70°), a
damp atmosphere is chilly.

12. The evaporation and not the secretion of perspiration
cools the body. The student should understand clearly that
it is the evaporation of the perspiration, not the secretion of
it, which abstracts heat from the body. Perspiration may be
secreted in large quantities, but if it does not evaporate, —
as happens on a very moist, humid, muggy day, when the
atmosphere already contains about as much aqueous vapor as
it can hold, — it takes little or no heat from the skin. Nor
is the efficiency of the perspiration as a cooling agent meas-
ured by the amount of visible or " sensible " perspiration, for

198 THE HUMAN MECHANISM

this is only the perspiration which has not evaporated ; the
true measure of the cooling effect would be the perspiration
which has evaporated and of which we are not conscious.

It is important to note that the evaporation of perspira-
tion (or of water from the lungs and air passages) is the
only means of cooling the body when objects around it are
warmer than the body itself. In this case the agents of heat
transfer only add heat to the body, but even their combined
action may often be overcome by an abundant evaporation
of perspiration. Men have remained for some time in rooms
whose temperature was as high as 260° F., or 48° above the
boiling point of water, without any marked rise of the body
temperature and without severe discomfort, the temperature
of the body being kept down solely by the evaporation of
perspiration from the skin. In order to make this means of
cooling possible, it is absolutely essential that the air be dry
and capable of taking up moisture. No one can survive long
at such temperatures in moist air.

13. The effect of stagnant versus moving air ; the aerial
blanket. On a perfectly still day the layer of air about the
body becomes warmed by the skin and, so long as it is not
removed, forms an air-blanket which goes far to keep the
skin warm; for air is a poor conductor of heat. As soon,
however, as a breeze springs up, convection comes into play
and the skin is cooled more rapidly. In stagnant air, more-
over, the evaporation of the perspiration tends to saturate
this air-blanket with water vapor, so that further evaporation
is rendered difficult. Accordingly, when perspiration is not
being secreted, moving air cools the body by increasing con-
vection ; and when the skin is moist it cools the body both
by increasing convection and by facilitating the evaporation
of perspiration. The breeze which in winter is an unwhole-
some draft, in summer is often absolutely essential to working
power as well as to bodily comfort, for without it we are
clothed in this aerial blanket.

THERMAL PHENOMENA OF THE BODY 199

B. THE REGULATION OF THE BODY TEMPERATURE

14. How the balance of the heat account may be disturbed.

Events both within the body and in its immediate surround-
ings tend to change the balance of the heat account ; that is,
to upset the equilibrium previously existing between heat
loss and heat production. The most important of these
events are (1) muscular activity and the digestion of food
within, and (2) changes of atmospheric or weather condi-
tions without. Let us consider how each of these acts.

Muscular activity, by producing more heat within the body,
would tend to increase the heat balance ; and, unless measures
were taken at the same time to increase heat output, the tem-
perature of the body would rise. Muscular activity may double
or even treble the heat produced. The digestion of a meal
similarly liberates heat within the body and so tends to raise
its temperature, but the heat produced in this case is far less
in amount than that produced during muscular activity.

Changes of atmospheric or weather conditions act by changing
the ease with which heat is lost ; and, remembering that heat
is lost in two ways, — by transfer to colder surroundings and
by evaporation of perspiration, — we must inquire how various
weather conditions influence each of these agents of heat out-
put. The three main weather conditions are the temperature,
movement, and moisture of the air, and the following tabular
form will aid in understanding the relation of each of these
conditions to the heat output of the body.

I. TEMPERATURE OF AIR

A. INFLUENCE ON HEAT B. INFLUENCE ON EVAPO-

TRANSFER RATION

Heat is transferred more rapidly The warmer the air, the more

to colder surroundings than to water vapor it can take up. This
surroundings which are near the facilitates the evaporation of per-
temperature of the body. spiration on a warm day, when

this is most needed to cool the body.

200

THE HUMAN MECHANISM

II. MOVEMENT OF AIR

A. INFLUENCE ON HEAT
TRANSFER

Movement of air increases heat
transfer to the atmosphere by re-
placing the " aerial blanket " of
warmed air with colder air, to
which heat is transferred more
rapidly.

B. INFLUENCE ON EVAPO-
RATION

When perspiration is evapo-
rating into stagnant air in contact
with the skin, this air becomes
more nearly saturated with water
vapor, and its power of absorbing
water vapor is lessened. By re-
placing the "aerial blanket" of
muggy air with dry air, the out-
put of heat by evaporation is
greatly favored.

III. HUMIDITY OF THE ATMOSPHERE

A. INFLUENCE ON HEAT
TRANSFER

Humidity increases the rate of
transfer of heat, as explained on
page 197. This is of little impor-
tance on warm days, because little
heat is then transferred either by
dry or by moist air. On cooler
days it is of great importance.

B. INFLUENCE ON EVAPO-
RATION

Humidity diminishes the out-
put of heat by evaporation, because
the water vapor which the atmos-
phere can take up is limited and
a humid atmosphere is one already
largely saturated. This influence
of humidity is of no consequence
unless perspiration is being se-
creted, but it is a very important
matter on warm days.

15. How the heat balance when disturbed is restored by
the body. In these ways changes in the activities of daily
life and changes of weather tend to change the heat balance
of the body — that is to say, they tend to change the tem-
perature of the body. And they would do this, did not the
body possess the power, within certain limits, of changing
both its rate of heat loss and its rate of heat production.

The rate of heat loss may be changed in two ways:
(1) by changing the quantity of blood flowing through the
skin. Obviously the more the warmed blood is kept within

THERMAL PHENOMENA OF THE BODY 201

the internal organs, the smaller will be the amount of heat
transferred from the surface of the body to surrounding
objects. The student now understands the reason for the
reactions of the circulation to changes of surrounding tem-
perature. The entire vasomotor mechanism with its vaso-
constrictor and vasodilator nerves thus forms part of the
mechanism of temperature regulation. The rate of heat loss
may also be changed (2) by producing a secretion of per-
spiration. This secretion begins at about 68° or 70° F. in
the body at rest and increases in amount as the external
temperature rises. The sweat glands are thrown into action
by nervous impulses. Hence the nervous system through its
nerves to the arterioles and the sweat glands controls the
output of heat from the body.

The nervous system also controls the rate of heat produc-
tion, for this is changed by increasing or diminishing the
activity of the skeletal muscles. We are more active on cold
than on warm days, and this apart from any conscious adjust-
ment of muscular activity to the temperature needs of the
body. We shall return to several interesting features of this
part of our subject in later paragraphs.

16. Reactions of the body at rest and lightly clad to changes
of external temperature. Having learned the more important
principles concerned in maintaining the constant heat balance,
let us now observe the actual behavior of the body as the
external temperature changes, assuming that the air remains
of moderate humidity and that there is little or no wind.1
To do this let us suppose that the body at rest and lightly
clad is exposed, to begin with, to a temperature of 90° F.
At this point but little heat is transferred by conduction, con-
vection, and radiation from the skin to surrounding objects,
since both are so nearly of the same temperature. Hence the
main reliance for getting rid of the heat constantly being
liberated is upon the evaporation of the perspiration, which
1 Consult Fig. 87 when reading this section.

202

THE HUMAN MECHANISM

is abundantly secreted ; the cutaneous arterioles are also
widely dilated. Let us now suppose the day becomes cooler
and the temperature falls to 80° F. Heat production remains
unchanged; but more heat is now transferred to the cooler
surrounding objects, and less is lost by evaporation because
less perspiration is secreted. As the external temperature
falls further, still more heat is transferred to colder objects

ANGER

—

•

_^—

— — ~^

i
l

]

•

100° 90° 80° 7fi° 72° 70°. 68° 64° 60° 50° 40° 30° 20° 10° 0° -10*

Heat loss

checked

solely by

vasomotor

Problem is to get
rid of the heat

means
Heat productior

Problem is to produce heat

enough to compensate for

the rapid loss

——————* = Heat lost by transfer (conduction, convection, radiation)

................ •ss: Heat lost by evaporation of perspiration

FIG. 87. Production and output of heat at different temperatures

and correspondingly less is lost by evaporation of the per-
spiration until, somewhere about 68° to 70° F., exactly the
same amount of heat is lost by conduction, convection, and
radiation as is produced. At this point the secretion of the
perspiration ceases.

Thus far the difficulty in maintaining a constant tempera-
ture has been that of getting rid of heat under atmospheric
conditions which are unfavorable for the ready conduction,
convection, and radiation of heat from the skin. Blood is

THERMAL PHENOMENA OF THE BODY 203

brought in large quantities to the skin and correspondingly
drawn away from internal organs, and the evaporation of
perspiration becomes increasingly important as the external
temperature rises from 70° F. to 90° and 100° F. The
organism is striving against a rise of its body temperature.

About 68° or 70° F., however, the situation changes ; for,
as the external temperature continues to fall, heat begins to
be transferred to surrounding objects more rapidly than it is
produced. The temperature of the body would fall if no
means were taken to prevent the result. Even during the
fall from 90° to 70° the cutaneous arterioles, widely dilated
at the higher temperature, have been gradually increasing
their tone and so sending diminishing quantities of blood
through the skin. Below 68° to 70° this tone rapidly increases ;
the veins are no longer conspicuous on the hand and arm;
if the blood is forced out of a portion of the skin by gentle
compression with the finger, the color returns slowly, indi-
cating considerable constriction of the cutaneous arterioles.
At the same time the arterioles of internal organs are dilat-
ing (see p. 152) so that the liver, the kidneys, the mucous
membranes of the alimentary canal and of the air passages
contain an increasing quantity of blood. The body is now
striving against a fall of its internal temperature by driving the
blood from the skin back upon internal organs.

By the time the temperature has fallen to 60° F., or there-
abouts, the cutaneous arterioles have constricted to their
utmost, the blood flow through the skin has nearly ceased,
and the organism has no means at command by which to
restrict the further output of heat. If in this emergency
heat production were to remain constant while external tem-
perature continued to fall, the temperature of the body would
be lowered, for the transfer of heat would not only continue
but increase. That it is not usually lowered is due solely
to the fact that more heat is then produced within the body ;
the oxidations (and hence heat production) which have

204 THE HUMAN MECHANISM

remained fairly constant in amount between 90° F. and 65° F.
now increase to compensate the inevitable loss, and continue to
increase as the atmospheric temperature continues to fall. The
body is now striving against the effects of a rapid and inevitable
loss of heat by producing more heat, and continues to do so until
somewhere near the freezing point (32° F.) it can no longer pro-
duce enough heat to balance the loss ; the temperature of the
body then falls and the man ultimately freezes to death.1

Briefly, then, at an external temperature somewhere be-
tween 65° and 70° heat production exactly equals heat trans-
fer, and it is not necessary that the body make any special
effort to get rid of heat or to compensate for heat loss.
The blood is properly distributed between the skin and internal
organs, and there is no excess in either. This we may call the
ideal or optimum temperature, for the given conditions.
Above this point measures must be taken to provide for an
adequate heat output by sending a larger quantity of blood
to the skin and by the secretion of perspiration ; below this
point measures of the opposite kind must be taken to check
heat loss or even to increase heat production.

17. Changes of the optimum temperature with high humid-
ity, with wind, and with muscular activity. High humidity,
by facilitating the transfer of heat from the body, raises the
optimum temperature a few degrees; a room is comfortable
at 65° when the air is dry ; it is too cool when the air is
moist. Wind may raise the optimum temperature still more,
and for the same reason; it may be safe to sit in a breeze
at 75° when it is decidedly unsafe to do so at 65° or 70°.

Muscular activity on the other hand, because of the produc-
tion of larger quantities of heat, lowers the optimum tem-
perature, for at the lower temperature the agencies of heat
transfer can get rid of the excess of heat without a large
blood flow to the skin and without inducing perspiration.

1 In all this it must be remembered that the body is still lightly clad and
at rest.

THERMAL PHENOMENA OF THE BODY

205

In all cases, — rest or muscular activity, high or low
humidity, wind or calm, — wherever the point of optimum
external temperature may be, we always find above this
point the region of active measures for heat dissipation,
and below it the region of active heat production. This is
graphically shown in Fig. 88.

18. The "danger zone " of atmospheric temperature. We have
seen that, as the temperature falls from 70° to 60°, the main
agency employed for temperature regulation is the diminution
of the blood flow through the skin, with its compensating in-
crease of the, blood flow within internal organs, thereby retaining

100° 90° 80° 70° 60° 50' 40°

Rest, normal humidity,

no wind
Rest, high humidity

Rest, wind
Muscular activity

I = Optimum temperature

I = Point at which increased heat production begins

Blank space between the two indicates region of the " danger zone"

FIG. 88. Variations in the optimum temperature

as far as possible the heat within the body. This threatens
serious congestions and other unhealthful conditions, which
we shall consider at length in our study of hygiene (see
Chap. XXI). It is because the temperature of a room may
fall from 66° to 60° so gradually that we do not notice it
until the internal damage is done, whereas it could not fall
to 50° or 40° without our noticing it and correcting the
trouble, that more colds are taken in the former case than
in the latter. In other words, as the temperature goes below
65° the body seems at first to rely wholly on the vascular
mechanism of temperature regulation, and does not begin to
produce more heat until this resource has been utilized not
only to its utmost, but even to an extent inconsistent with
health. The "danger zone" temperature may then be defined

206 THE HUMAN MECHANISM

as beginning a degree or two below the ideal or optimum
temperature and extending about five degrees below this
point. Like the optimum temperature, its exact position
varies with atmospheric conditions and with the amount
of muscular activity.

19. The influence of clothing. In the discussion above we
have assumed that the clothing has not been changed with
the change of external temperature, etc. Clothing, however,
may modify greatly the figures given above, for it interferes
with the loss of heat from the skin, and the obvious effect
of increasing its weight is to lower the optimum temperature
and the region of dangerous temperature. By changes of
clothing, by muscular activity, and by the use of fans, man
has it in his power to supplement the unconscious reflex
adjustments which we have thus far been studying by a
conscious adaptation to changing conditions of climate or
weather. The hygienic use of clothing will be discussed
in Chapter XXVI.

20. Temperature regulation and muscular activity. The
reactions of the body to maintain its constant temperature
during muscular activity are familiar to everyone, and it
is only necessary to sum them up and to point out some
practical applications. The arterioles of the skin are dilated
(while those of internal organs are constricted) and perspi-
ration is secreted. These are the same reactions which are
noticed when the body is exposed to external warmth, and
their purpose is the same in both cases — to facilitate the
escape of heat. But in the one case they are made necessary
by the fact that climatic conditions interfere with the out-
put of heat, in the other by the fact that more heat is being
liberated and hence more must be got rid of.

Seldom indeed is so severe a strain imposed upon the
mechanism of heat dissipation as during vigorous muscular
exertion, and especially when the external conditions are not
favorable for the output of heat. Caution is then urgently

THERMAL PHENOMENA OF THE BODY 207

indicated lest we make the strain too great. It is a practical
point to remember in this connection that some forms of
muscular exertion introduce conditions for getting rid of the
surplus heat much more readily than others; this is especially
true of those which involve movement of the body as a
whole. Bicycle or horseback riding, by creating a • breeze,
renders the cooling of the body a much easier matter than
does sawing wood, or swinging Indian clubs, or gymnastic
work in general; again, a particular form of exercise on a
dry day, when the perspiration can evaporate readily, may
be safe, while it would be decidedly inadvisable on a muggy
day, even though the temperature were somewhat lower.
Indeed, by this time the student must have learned that
the thermometer alone is no safe indicator of the difficulty
of heat elimination in warm weather.

21. Relations of climatic conditions to mental work and
sleep. During mental work the brain requires an increased
supply of blood, and this is obtained partly by diminishing
the supply to the skin (constriction of cutaneous arteries) ;
during sleep, on the other hand, the supply to the brain is
diminished, and this is ordinarily effected by dilating the
arteries of the skin (see p. 155). Mental work is difficult
on very warm days, partly because it is difficult to bring
about cutaneous constriction ; and it is especially difficult
on warm, muggy days, since the maintenance of the constant
temperature then requires an excessive cutaneous dilation, and
the brain is quite unable to command its needed blood supply.

It is also clear that since the arterioles of the skin should
dilate during sleep, and since they cannot readily do this
when the skin is exposed to cold, to " sleep warm " is good
advice, based on sound physiological principles.

22. Digestion and the maintenance of the constant temper-
ature. During digestion, and especially during its earlier
stages, when secretion is at its maximum, a large supply
of blood is needed in the stomach, the pancreas, and the

208 THE HUMAN MECHANISM

intestine. This cannot readily be secured when blood is
being sent in large quantities to the skin in order to cool
the body. We have seen all along that the two great vas-
cular areas of the skin and digestive organs are more or less
antagonistic or compensating in their vasomotor reactions.
When the blood is present in large quantities in the skin,
it is present in smaller quantities in the stomach, the intes-
tine, the pancreas, the liver; and, vice versa, these organs
can best obtain an adequate blood supply when the, de-
mands of the skin are not excessive. Consequently diges-
tion is more difficult in warm than in cold weather, and we
should then eat less at a time, even if we have to eat
somewhat more frequently.

During the digestion of a meal the chemical activities of
secretion, the peristaltic muscular movements, etc., somewhat
increase heat production in the body; and this increase,
though not great, is at times great enough to make us feel
distinctly warmer. When one is slightly chilly, for example,
he often feels warmer after eating something, even though
the meal be cold; and on a very warm, muggy day, when
the blood flow through the skin is already excessive and its
temperature unduly high, the digestion of a meal often adds
to the discomfort, because the larger production of heat leads
to further dilation of the skin vessels.

23. The mechanism of temperature regulation. The pre-
ceding pages have shown us that temperature regulation
depends chiefly on three physiological mechanisms: (1) the
vasomotor system, which controls the distribution of blood
between the skin and the internal organs ; (2) the sweat
glands; (3) the mechanism of heat production. The first
of these has already been described in the study of the
circulation. The heating of the skin stimulates afferent
nerves which reflexly dilate the arteries of the skin and
also simultaneously constrict those of internal organs. This
reflex, then, is dependent on the temperature of the skin ;

THEEMAL PHENOMENA OF THE BODY

209

anything which heats the skin causes a reflex dilation of its
arterioles and lessens the supply of blood to internal organs.
The secretion of perspiration is also under the control
of the nervous system. The sweat glands, like the salivary
glands, receive nerves, and secrete only in response to their
stimulation. When the nerves going to the sweat glands of
any region are injured, exposure of these glands to external

Nerve Endings affected
by Warmth

FIG. 89. Diagram of the cutaneous reflexes of temperature regulation

Showing the epidermis, blood vessels of the dermis, a sweat gland, and the
nervous mechanism governing blood vessels and sweat glands

warmth produces no perspiration; stimulation of their nerves,
however, produces a copious secretion.

24. The skeletal muscles the main organs in the regulation
of heat production. The third mechanism of heat regulation
is that whereby the amount of heat produced is increased
as it is needed. The main organs here concerned are the
skeletal muscles. As the afferent impulses started in the
skin by the stimulation of cold become stronger, they ulti-
mately stimulate reflexly the skeletal muscles to contraction,
and so to the production of heat. This contraction does not

210 THE HUMAN MECHANISM

ordinarily produce motion, because antagonistic muscles are
stimulated equally; but in another way we are often con-
scious of this increased muscular action. Everyone knows
the difference between the " bracing " effects of a cool or
cold day and the " relaxed," " slack-twisted " feeling on a
warm day ; and this is largely traceable to the sensations
which come from the contracting muscles in the former case
and to the absence of such sensations from the inactive
muscles in the latter. To put it in another way, cold
increases the tone of the skeletal muscles (see p. 161). A
skeletal muscle on a cold day is never completely relaxed ;
like the unstriped muscles of the arteries, it is in a con-
dition somewhere between extreme contraction and extreme
relaxation.

This muscular reflex also betrays itself in shivering.
Ordinarily the reflex contraction consists of an even, steady
tone, but at times it becomes more or less incoordinated,
and shivering results.

25. The regulation of the body temperature a function of the
nervous system. We may close this brief account of thermal
phenomena of the body by recalling to the attention of the
student what must now be obvious at a glance ; namely, that
a constant temperature is maintained by the coordinating
action of very many nervous reflexes. The action of the
vasomotors of the skin and of the internal organs, of the
nerves of the sweat glands and of the motor nerves of
the skeletal muscles must all be so adjusted with regard
to one another that exactly the right balance is preserved
amid all the variations of heat production and of climatic
conditions which affect heat loss. Success in this adjustment
depends upon the skill with which the coordinating nervous
system does its part. With the single exception of muscular
exertion, no condition of life makes such far-reaching or
such imperious demands upon the system as a whole as does
the maintenance of the proper internal temperature. Mental

THERMAL PHENOMENA OF THE BODY 211

work and the efficiency of digestion are examples we have
already studied — and more could easily be cited — of func-
tions which, important as they are, are subordinated, even
sacrificed, to prevent a marked rise or fall in the temperature
of the blood.

To such an extent is the nervous system as a whole
adapted to maintain the constant temperature, that the failure
to do this, as shown by the presence of fever or by the even
more serious subnormal temperature, becomes one of the
most important indications that something has gone wrong.
We know already how the nervous system intervenes in
every function of our lives, and how the well-being of the
body as a whole depends upon the adjustments which it
brings about. It is for these reasons that, when it is no
longer able to exercise that firm control of the constant tem-
perature which is one of its most characteristic features in
health, the physician's orders usually are to " go to bed and
be perfectly quiet." The body is then in no condition to
make demands on the nervous system for action ; and a per-
son who refuses to heed the plain warning which his tempera-
ture holds out has nothing but his own foolishness to blame
if he suffers serious consequences.

26. Definitions. Those not familiar with the exact meaning
of the terms " conduction," " convection," and " radiation "
will find the following helpful.

Conduction. Whenever heat is transferred directly from one
mass of matter to another with which it is in contact, such
transfer is known as conduction. A good example is the heating
of a poker in a fire ; the heat of burning coal is communicated
directly to the outer particles of iron and then from one particle
of the iron to another. The particles of iron do not move up and
down the length of the poker ; each one simply passes on to the
next the heat it has received, and finally those of the handle com-
municate their heat to the hand. All transfer of heat along solid
objects, or from one mass of matter to another with which it is in
immediate contact, is by means of conduction.

212 THE HUMAN MECHANISM

Solids and liquids are much better conductors of heat than
gases, and air when perfectly still is one of the poorest con-
ductors of heat with which we have to deal. It is a familiar
fact that the skin is chilled much more rapidly by water than
by air of the same temperature (why?); and we shall learn in
hygiene that warm fabrics owe their warmth mainly to the
amount of poor-conducting air stagnant within their meshes.

Convection. When a warm body is surrounded by a fluid such
as water or air, heat is similarly conducted to the adjacent layer
of water or air, which thus becomes warmer ; but, unlike the case
of the solid, this heated layer now moves off, carrying its heat
with it to other parts of the gas or liquid, and so communicating
it to other matter with which it subsequently comes in contact.
This method of heat transfer is known as convection, which, it
will be seen, depends at bottom upon conduction, but which is at
the same time conduction modified by the movement of a heated
gas or liquid. So long as the air around us is at rest, it does not
remove heat readily from the skin, since air is a poor conductor.
Air in motion, on the other hand (as in fanning), cools the skin
more rapidly, because as each part of the air is heated, it is moved
away and replaced by colder air. In this case the air cools the
skin by convection (Latin con, " with" ; veliere, "to carry" ).

The transfer of heat from the internal heat-producing organs
to the skin affords an excellent example of the difference between
conduction and convection, for some of this heat passes by direct
conduction through the subcutaneous tissue to the overlying skin,
while some of it is carried to the surface by convection in the blood
stream. When 'the arterioles of the skin are dilated, convection is
an important means of heat transfer to the surface ; when, in the
reverse case, the cutaneous arterioles are constricted to their
utmost, convection becomes relatively unimportant and direct
conduction alone remains as the chief means of heat transfer to
the skin. Moreover, when the subcutaneous tissue contains large
amounts of fat, it is a poor conductor of heat, and for this reason
fat people when sitting still on cold days often feel colder than
lean people do.

Radiation. Heat is thus removed from the skin by conduction,
and at times to an even greater extent by convection. But there
is still a third method of heat loss, known as radiation, by which

THERMAL PHENOMENA OF THE BODY 213

heat can be transferred across a space in which there is neither
solid, liquid, nor gas, and in which conduction and convection are
consequently impossible. The most familiar and striking example
of radiation is the transfer of heat from the sun to the earth, since
there is no atmosphere in the greater part of the more than ninety
millions of miles of space which separate us from that intensely
heated body.

Any detailed consideration of radiation belongs to the domain
of physics rather than physiology and would be out of place
here. It is enough for our present purposes to understand that,
whether a solid body be in an atmosphere of air, or in a trans-
parent liquid, or even in a vacuum, it transfers or loses heat by
direct radiation to colder objects about it. From an open fire heat
may be transferred by conduction to andirons or walls in direct
contact with it; or by convection through heated air currents to
the chimney top ; or, finally, by radiation to persons standing in
front of it. In the latter case the heating is chiefly by radiation,
since there is no contact with the fire, and such air currents as
exist are mostly composed of cool air sucked towards and into
the chimney by its draft. It is for these reasons that open fires
are said to " roast people in front and freeze them behind." Con-
versely, the human body, if warmer than its surroundings, may
lose heat by conduction, convection, and radiation to cooler objects
in the vicinity.

The practical importance of these facts is seldom realized. It
often happens that the air in contact with the skin is of the proper
room temperature ; and yet, if one is sitting too near a cold wall
or window, enough heat may be lost by radiation from the skin
to the cold wall, through the warm air, to chill the skin materially,
causing a loss of heat and a " cold."

Laws of conduction and radiation. For our purposes the two
most important factors which determine the loss of heat by con-
duction and radiation are (1) the difference in the temperature of
the two objects and (2) the distance between them. In general,
the greater the difference of temperature, the more heat will be
lost from the warmer to the colder object ; thus the skin loses
heat rapidly by these means when surrounding objects are at
0° F., but only slowly when they are at 90°. It is also clear that
as soon as the temperature of surrounding objects and of the

214 THE HUMAN MECHANISM

atmosphere is as high as that of the body (98.6° F.), no further
heat can be lost by conduction and radiation; and that above
98.6° F. heat is conducted and radiated to the body, not from it.
Furthermore, the greater the distance of the colder object from
the body, the less heat will the body lose to it. Here heat loss
takes place inversely as the square of the distance ; that is, when
we are twice as far away from a cold (closed) window, we lose
only one fourth as much heat through it by radiation ; if we are
three times as far away, we lose only one ninth as much, and
so on. Consequently we rapidly diminish radiation from our
bodies by sitting farther away from the walls of a room ; and it
is important to have our living rooms large enough to make it
unnecessary to sit near the windows or near a cold outer wall
in very cold weather.

CHAPTER XIII

NUTEITION

A. THE SOURCES OF POWER AND HEAT FOR THE
HUMAN MECHANISM

1 . Food and nutrition. In general food must meet the
following fundamental needs of the body: first, it must
supply power for the work of muscles, heart, etc. ; second,
it must give, through oxidative or other chemical change,
the heat necessary to maintain the body temperature ; third,
it must supply all the material needed for the manufacture
of everything that enters into the structure of the living
cell (growth and repair) and also of the secretions, internal
and external, the hormones, and all other special compounds
which play any r61e in the working of the human machine.
Since the first two of these functions are met by the same
food material and in much the same way, we may consider
first this aspect of nutrition.

2. The fuel value of food. In any locomotive engine the
same amount of a given fuel will enable the engine to pull
a train of the same weight for the same distance over the
same track, provided, of course, the engine itself, the bear-
ings of the wheels, etc., are in the same condition. When a
ton of coal is put into the tender, it is with the expectation
that it will move the train a certain distance. Thus there is
a definite relation between the fuel burned and the work
done. Every engineer knows also that the same weight
of different fuels will carry the train different distances;
a thousand pounds of wood, of bituminous coal, and of
anthracite coal have different fuel values.

215

216 THE HUMAN MECHANISM

The same weight of a given fuel when burned will always
yield exactly the same amount of heat, as is proved by
burning the fuel under conditions which enable us to meas-
ure the heat given off. The simplest means of doing this is
perhaps with the ice calorimeter — a metal box within which
the fuel is burned, the box being everywhere surrounded
by a thick layer of ice. The heat produced in burning the
fuel is measured by the amount of ice melted.

In this way we may find the relative amounts of work
which can be done with two different fuels, for it has been
discovered by actual experiment that if one kind of. fuel
produces twice as much heat as another, it will also do
twice as much work.

Now food is the fuel for the muscular work of the body
and also for the liberation of heat. Consequently, if we
determine how much heat is liberated when a certain amount
of protein, or fat, or carbohydrate is burned in a calorimeter,
we know how much work it may do in the body ; or at least
we know that it can do no more than the amount indicated
by the calorimetric experiment.

3. Units of heat and work. In order to measure we must
have units of measurement. Common units of length are the
inch or centimeter ; units of area are the square yard, the
square meter, or the acre ; units of volume, the quart or
peck ; units of weight, the pound or kilogram. We express
the results of these measurements by saying that a thing is
so many inches long or of so many pounds weight. What are
the units of heat and work ?

Like all units, these are arbitrarily chosen. The unit of
heat, known as the calorie, is the amount of heat necessary
to raise one kilogram of water one degree Centigrade. The
unit of work is the amount of work done in lifting a kilogram
(2.2 Ib.) to the height of one meter (39.37 in.) from the
surface of the earth against the attraction of gravitation.
This is known as the kilogrammeter. Thus, when a man

NUTRITION 217

weighing sixty kilograms goes up a flight of stairs ten meters
high, his muscles do 600 kilogrammeters of work.1

Finally, it has been found that the same fuel which when
burned will liberate one calorie of heat will supply the power
to do 423.985 kilogrammeters of work. By this we mean
that not more than 423.985 kilogrammeters of work can be
obtained from it. Not every engine is so perfectly con-
structed as to get from a certain fuel its full working
capacity ; indeed, most engines transform only a small frac-
tion of the power of their fuel into work, the rest escaping
as heat — in the smoke, or by radiation, conduction, and
convection from the furnace, boiler, etc. But by the method
above outlined it is possible to find the maximum amount
of work which can be obtained from a given weight of fuel.

Applying the same methods to food, we find that

1 gram of dried protein yields 4.1 calories.

1 gram of dried carbohydrate yields 4.1 calories.

1 gram of fat yields 9.3 calories.

These figures are known as the fuel values of proteins,
carbohydrates, and fats.

But the total possible power which may be obtained by
actually burning a certain substance under the most favor-
able conditions is one thing, and the amount of power which
the muscles may obtain from it is quite another. When coal
is burned in an engine it does work, but the human body
would get no energy for its muscular work from eating coal.
So that we have now to inquire from what nutrients the
muscles get their energy for work and from what nutrients
the body derives its heat.

4. The power for muscular work. Few questions in physi-
ology have been more thoroughly investigated than this.
In the first half of the nineteenth century many investi-
gators, impressed with the fact that the muscle fiber yields,

1 Work may also be expressed in foot-pounds. (How many foot-pounds
equal one kilogrammeter ?)
p

218 THE HUMAN MECHANISM

on chemical -analysis, large quantities of protein and only
traces of carbohydrates and fats, believed that the energy
for muscular contraction comes entirely from the consump-
tion of the protein of the muscle substance. If this were
true, it would necessarily follow that protein is the proper
food to yield the energy for muscular contraction, while fats
and carbohydrates would simply be oxidized to give heat.

This view was disproved by the following epoch-making
experiment of physiology: Two observers determined for
three successive days the nitrogen excreted by themselves;
since almost all this nitrogen comes from protein, this gave
the amount of protein consumed by the body. On the first
and third days no vigorous muscular work was done ; on the
second day they climbed a mountain 1956 meters (6415 ft.)
high. As one man weighed 66 kilograms and the other
76 kilograms, the work done in lifting the body to the top
of the mountain in the two cases was 129,096 and 148,656
kilogrammeters respectively. The protein which was oxidized
in this time could in the two cases have yielded power for
only 68,690 and 68,376 kilogrammeters of work. In other
words, the protein did not begin to yield sufficient power
for the muscular work done in lifting the body to the top
of the mountain ; something else than protein must have
been oxidized for that purpose, and that something must
evidently have been carbohydrate or fat, or both.

Again, it was noticed that there was no increase of pro-
tein disintegration on the day of work; this remained
practically unaffected by muscular contraction. Numerous
other experiments made since that time have shown the
same thing. Muscular exercise does not necessarily increase
protein disintegration, and the power for it can be obtained
from fats and carbohydrates.

In the experiment above referred to no determinations
were made of the excretion of carbon dioxide. Since then
numerous experiments have been made in which, on an

NUTKITION 219

abundant mixed diet, both the nitrogen and the carbon of
the excretions were accurately determined. These have
shown that while muscular exercise does not necessarily increase
protein disintegration, it invariably increases the production of
carbon dioxide. If the carbon of the carbon dioxide came
from protein, it would be accompanied by increased excretion
of nitrogen derived from the broken-down protein. The fact
that it is not so accompanied can only mean that it came
from the oxidation of something which did not contain
nitrogen, that is, from fat or carbohydrate, or both.

But while muscular work does not necessarily or even
usually increase protein decomposition, and the power for
the same may be derived largely, if not entirely, from car-
bohydrates and fats, it has been shown conclusively that
under certain conditions this power may come entirely from
protein. In one experiment a large and very lean dog was
fed for several weeks on an abundant diet of lean meat,
containing practically no carbohydrate or fat; during this
time the dog did large amounts of work in a treadmill and
in other ways; and since it was found that this work could
be done for weeks •at a time on the meat diet, we conclude
that the protein must have been the sole source of power
for the work ; it must also have served as the source of
heat production, for the normal temperature of the animal
was maintained.

5. Summary of considerations on the supply of power for
work. . These and other experiments show (1) that the
animal body can get its energy for mechanical work and for
the production of heat from protein, or from carbohydrate,
or from fat; (2) that when the animal is on an abundant
mixed diet, even vigorous muscular work does not increase
the oxidation of protein,1 but does enormously increase that

1 Under the abnormal conditions of excessive muscular work (for ex-
ample, six-day walking matches or bicycle races) the protein oxidation is
often increased.

220 THE HUMAN MECHANISM

of carbohydrates and fats. The probable meaning of this is
to be sought in the fact that protein decomposition depends
primarily not on muscular work but, as we shall see later,
on the amount of protein eaten; while the oxidation of fats
and carbohydrates depends almost entirely on the demands of
the body for energy and is largely independent of the amount
of these foods eaten.

6. The supply of energy for heat production; " heating "
foods. In studying the phenomena of heat production in the
body we found that when the body needs more heat in order
to maintain its normal temperature, this heat is supplied
chiefly by greater chemical activity in the muscles (p. 209).
The contraction or tone of the muscles increases in response
to stimuli from the same motor nerves which stimulate them
to activity when they do external mechanical work. Heat
production in the body, from the standpoint of nutrition, is
therefore, as far as we know, largely a case of increased
muscular activity ; and here, as in the case of mechanical
work, the energy can be obtained from one kind of food as
well as from another. Contrary to popular ideas we have
no conclusive evidence that one kind of food supplies heat
more readily than another. What is required in cold weather
is more food, whether protein or carbohydrate or fat. We
shall see that there are good reasons for not unduly increas-
ing the protein of the diet under any conditions, and hence
in this special case it is probably better to increase the non-
nitrogenous foods to a greater extent than the proteins,
though not because they are better " heating " foods.

7. The daily requirement of the body for power and heat.
How many calories must be furnished the body to cover its
daily needs for work and for the maintenance of its temper-
ature ? This question has been studied by several methods,
but we must content ourselves with a statement of some of
the most important results. Healthy people whose choice
of food is not restricted by financial considerations, but is

NUTKITION 221

determined solely by appetite and the feeding customs of
their home or community, usually consume each day food
of a fuel value of 20 calories per pound of body weight.
It is exceptional to find less than 16 calories or more than
24 so long as only moderate amounts of exercise are taken;
and many students of this subject have assumed that one
requirement of diet is that the daily intake of food should
have approximately this fuel value.

This view has, however, been seriously questioned by
careful and competent observers, and their work seems to
show that a fuel value of 13.5 calories per pound of body
weight more nearly represents the actual needs of the body.
In other words, the usual diet, with its three hearty meals a day,
has a fuel value one and one half times as great as the minimum
requirement of the body. Whether the excess is or is not
harmful to the body will be discussed later (see p. 239).

The chief factor which influences the amount of this
minimum fuel value is the amount of muscular exercise
taken. Men at hard labor require from 20 to 25 calories per
pound of body weight, or even more; on the other hand,
during the marked muscular relaxation of sleep the require-
ment is reduced to from 10 to 11 calories. Exposure to cold,
when not counteracted by warm clothing, similarly increases
the fuel requirement.

If, then, we generally eat more food than the fuel needs
of the body require, what becomes of the excess ? This
question can be answered only incompletely in the present
state of our knowledge. A portion of the food eaten leaves
the body undigested in the feces ; and the more abundant
the diet, the greater is the amount lost in this way. Part
of the food also is destroyed in the alimentary canal, es-
pecially in the large intestine, by microbic action, and this
similarly increases with the diet. This microbic food destruc-
tion involves the liberation of heat within the body but does
not yield power for work, the excess of heat being dissipated

222 THE HUMAN MECHANISM

from the skin. Again, the absorption of some foods, notably
proteins or their cleavage products, the ammo-acids, leads to
their increased destruction in the cells of the body, just as
an open fire burns more vigorously when new fuel is added.
Finally, food may be stored within the body. That this is
true is shown by the histories of prolonged fasts in which
men and women have abstained entirely from food for more
than a month. Such fasters steadily lose weight, showing
that the body is consuming its own substance. We may
therefore pass to the consideration of the storage of material
capable of meeting future nutritional needs.

B. THE FOOD RESERVE OF THE BODY. FAT.
GLYCOGEN. CELL PROTEINS

8. The hoarding of inactive food material. I. The storage
of fat. The most obvious reserve food stored in the animal
body is fat, which may appear as drops of oil in the cyto-
plasm of any cell. Muscle fibers, for example, contain at
times large quantities of this substance, and are then said
to have undergone fatty degeneration. Under normal condi-
tions, however, the presence of considerable quantities of fat
in muscle fibers or nerve cells or most gland cells is unusual.
In the cells of connective tissue, on the other hand, fat is
readily stored under normal conditions, and the adipose tis-
sue or fat of the body is simply connective tissue whose cells
are loaded with droplets of fat. Figs. 90-92, with their ex-
planation, will show how this takes place. But while fat
may be normally stored in any of the more open connective
tissues, it is especially in the subcutaneous tissue, the great
omentum, the mesentery, and some other situations that its
chief storage takes place. From these storehouses it is drawn
upon as a reserve food material when the immediate supply
of food from the alimentary canal becomes inadequate for
the work of the body. The exact mechanism by which it is

NUTRITION

223

stored in a cell at one time and
discharged at another is not
fully understood. Some of the
conditions under which it is
accumulated, and some of those
under which it disappears, are
known ; but we do not know
the whole story. Some
people lay up fat in
larger quantities than
others on the same diet
and apparently while do-
ing the same amount of
work, and some keep
lean under conditions ap-
parently the most favor-
able for growing fat.

It was formerly be-
lieved, and is still some-
times supposed, that the
animal body forms fat
only from the fat of the
food; that to get fat we
must eat fat. This was
disproved by a number
of experiments, especially
one by Liebig, who kept
account over a long period
of the fat in the food
supplied to a cow, and
found that the fat given
off in the cow's milk
far exceeded that in her
food. In another experi-
ment four pigs out of a

FIG. 90

FIG. 91

FIG. 92

FIGS. 90-92. Three successive stages in

the transformation of ordinary connective

tissue into adipose tissue

A portion of the capillary network is shown,
surrounded hy the fibers, among which are
several cells. The accumulation of fat droplets
in the cell cytoplasm is shown in Fig. 91, and
the fusion of these upon their increase in size
and number to form one large droplet, sur-
rounded hy the cytoplasm, is seen in Fig. 92

224 THE HUMAN MECHANISM

litter of eight were killed and the total amount of fat in
their bodies determined. The other four pigs were fattened
for a time, then killed, and the fat in their bodies eventually
determined. Assuming that the second set of four pigs
originally had the same quantity of fat as the first four, the
difference between the two quantities of fat found would
give the quantity of fat the last four had stored up. Mean-
time, strict account had been kept of the fat supplied in the
food of the last four, and it was shown that for every 100
parts of fat fed to them these pigs had laid up 472 parts of
fat. They had evidently manufactured fat from some substance
other than the fat in their food.

9. Fats can be stored from fats and carbohydrates in food.
There is no doubt that fat may be both stored away and
manufactured from the fats in the food. There is also no
doubt that large quantities of fat may be and often are
manufactured and stored from the carbohydrates (sugars,
starches, etc.) of the food; so that, while there is some
truth in the idea that one may get fat by eating fat, it is
equally true that we can get fat by eating other foods.

10. Are proteins a source of fat? Whether fats are nor-
mally made in the body from proteins is a more difficult
question. There is no undisputed case on record of such
manufacture and storage ; and while the facts do not yet
justify us in denying the possibility, it is very doubtful
whether such transformation takes place to any great ex-
tent, and it is possible that in the mammalian body it does
not normally occur at all.

Fats, then, are manufactured readily from fats and car-
bohydrates and sparingly, if at all, from proteins. Their
disappearance during starvation, when they are drawn upon
to supply power and heat for the body, shows that they
serve as a true reserve food material. They are a kind
of food capital or hoard, saved and laid up by the body
against a rainy day.

NUTRITION 225

11. The hoarding of inactive food material. II. The storage
of glycogen. In many cells of the body, but especially in
those of the liver and to a less extent in those of the skeletal
muscles, there is found a carbohydrate substance known as
glycogen. This substance belongs to the same group of car-
bohydrates as starch and dextrines (see Chap. VIII), and is
sometimes called animal starch. Like them it is changed into
sugar by the action of saliva and pancreatic juice, whence its
name (7X1^5, "sweet"; -yevrjs, "former"). The same change
occurs on the death of the cells in which it is contained,
the sugar thus formed giving to such tissues a sweetish
taste. This is often noticed, for example, in liver and in
scallops (the shell muscle of Pecteri). The total amount
of glyeogen in the human body may exceed 700 grams
(13 ounces), one half of which is concentrated in the
liver and the other half scattered about in the other tissues
of the body.

Experiments have shown that glycogen is not formed from
the fat in food ; that it is formed in small quantities from pro-
tein; while its chief source is the carbohydrates of the food.

The blood may be said to be always sweet, its constant
percentage of sugar (1 to 2 grams per 1000 cubic centimeters
or 0.05 ounce per quart of blood plasma) being a striking
fact, and one that we should hardly have anticipated. One
might suppose that the sugar in the blood would increase,
as does the amount of fat, during active digestion and ab-
sorption, and that after digestion had ended, it would
diminish. As a matter of fact the amount of sugar in the
blood remains practically constant both during and after
the completion of digestion, and this despite the fact that
the tissues are constantly abstracting sugar from the blood.
Evidently the blood must be supplied with sugar from some
other source than the alimentary canal, and there must be
somewhere in the body a compensating mechanism controlling
the sugar supply of the blood.

226 THE HUMAN MECHANISM

Experiments have shown that sugar is absorbed from the
alimentary canal entirely by the intestinal blood vessels. It
must pass, therefore, through the liver by the portal vein
(see Fig. 62) before going to the rest of the body. The
liver, thus standing at this great gateway to the circulation,
would seem to act as the carbohydrate storehouse, or savings
bank, of the body. Any excess of sugar in the portal blood
is there transformed into glycogen and deposited, saved until
it is needed, and then " paid out," as sugar, when the ready
supply is exhausted. Other tissues doubtless aid in prevent-
ing an undue richness of sugar in the blood, acting likewise
as temporary storehouses for this form of food.

12. Protein a source of sugar to the body. It has been
stated that glycogen may be formed from protein. This is
because the body can and does constantly form sugar
(dextrose) from protein. Experiments have shown that
about half of the protein may in this way be transformed
into sugar, the greater part of which is ordinarily oxidized
as fuel; but in case there is an excess over and above fuel
needs, this excess of sugar is stored as glyccgen by the liver
and other organs, just as the excess of sugar absorbed from
the alimentary canal is so stored. The formation of sugar,
and consequently of glycogen from fat, on the other hand,
is negligible.

In this formation of sugar from protein the body obviously
has an additional means of supplying its carbohydrate needs
when the sugar delivered to the blood by absorption from
the alimentary canal is inadequate.

13. The protein reserve. Provision is thus made for a
continuous supply of fat and carbohydrate (sugar) between
periods of absorption of these foods and even during starva-
tion. How are the protein needs of the body met under
similar conditions? There is no visible supply of inactive
protein in the body similar to fat or glycogen. It is true
that analysis of the lifeless cell shows that proteins make

NUTBITION 227

up by far its largest constituent, 1 but there is no ground
for thinking that this cell protein exists in any other form
than as an active constituent of the cell substance. There
is no evidence of protein stored simply as reserve to meet
future possible protein needs.

And yet, during starvation, protein is steadily lost from
the body, as is shown by the fact that urea and other protein
derivatives continue to be eliminated by the kidneys. Nor
can this loss of protein be checked by feeding carbohydrates
and fats; these may be provided in the food in amounts
abundantly sufficient to meet the fuel demands of the body,
but without checking the loss of protein. We can only con-
clude that the disintegration of protein within the body is
an inevitable part of the chemical activity of the cells, and
that in the absence of a supply of the protein products of
digestion the body takes protein from its own living sub-
stance. Hence protein becomes an indispensable article of
diet. The student will, moreover, recall the fact that while
carbohydrates and possibly fats may be made from pro-
tein, protein cannot be manufactured from the non-protein
nutrients. This obviously follows from the fact that fats
and carbohydrates are lacking in nitrogen and sulphur, two
essential elements of the protein molecule.

14. Increase of protein in the food increases protein destruc-
tion by the body. One peculiarity of the behavior of protein
in the body of itself limits the accumulation of any large
amount of storage protein. As soon as we increase the protein
of the food, there is an increase of protein disintegration in
the body, and in a few days protein disintegration equals

1 The following analysis of muscle cells (lean of meat) is typical :

Water 75 parts

Solids 25 parts

Protein 21 parts

Salts 1 part

Fat, connective tissue, etc . . . 2 parts

Other extractives ,,,,,,, 1 part

228 THE HUMAN MECHANISM

the greater protein consumption. Instead of storing the
additional food protein or even part of it over any great
length of time, the body soon comes to destroy all the pro-
tein eaten. It is for this reason that while animals may be
fattened to a remarkable extent by proper feeding, it is not
possible to secure a corresponding increase of protein material
of the muscle, or lean meat. The accumulation of protein is
self-limited.

In two physiological states the increase of protein is much
more marked; namely, during growth and during convales-
cence from wasting disease (or after a period of prolonged
fasting). It would seem that there is a certain maximum
content of protein-like substances in the cell, and that it is
not possible by the most abundant feeding to increase
this amount.

It follows from the above that very abundant protein
feeding must result in the production of increased protein
waste within the body. In the first place, the greater the
quantity of protein fed, the greater will be the microbic
destruction of protein within the intestine and especially in
the large intestine. Not only is the protein so destroyed
largely useless to the body, but, in so far as its microbic
destruction involves putrefactive changes, harmful products
may be formed from it. In the second place, that portion of
the protein which escapes microbic action and is absorbed
into the blood in the form of digestive products (amino-acids
and peptids) disintegrates in the cells with the formation of
wastes. Both these processes increase the amount of waste
to be eliminated, chiefly by the kidneys. It has been urged
that this overburdens the kidneys and causes disease of
these organs. While convincing proof has perhaps not been
given that a healthy kidney may be injured in this way,
it is certain that a diseased or even a temporarily impaired
kidney may suffer when such excessive work is thrown
upon it.

NUTRITION 229

C. FOOD AS THE MATERIAL FOR GROWTH, REPAIR, AND

THE MANUFACTURE OF SPECIAL PRODUCTS OF

CELL ACTIVITY

15. Complexity of the chemical composition of living cells
and of the products of their manufacture. In the first sub-
division of this chapter we have considered food as fuel.
We are now in a position to consider some of the more
important features of the other great function of food,
namely as the material for the growth and repair of living
cells and for the manufacture of the special products of
cell life — the secretions (internal and external), the hor-
mones, and all other substances produced by the body for
special purposes.

The living cell is an extremely complex machine into the
construction of which enter numerous compounds of diverse
chemical nature. Formerly there was a tendency to regard
the cell as composed essentially of protein; but the increase
of our knowledge has shown that there are other essential
constituents, notably (in addition to water and inorganic
salts) a group of compounds known as lipoids, or lipins,
substances which more or less resemble fats in their physi-
cal characters, although not always in chemical structure.
The cell nucleus also contains special material of still dif-
ferent chemical composition. The chemical properties ' and
the physiological significance of these cell components are
far too complicated subjects for discussion in this work ; we
merely wish to emphasize the complexity of the mixture and
the variety of chemical compounds concerned. (See p. 42.)

We are impressed with the same diversity of chemical
structure in the secretions, hormones, and other material
manufactured by the body for special purposes. The stu-
dent has only to recall the examples of these already men-
tioned — the enzymes of the digestive juices ; the internal
secretions of the adrenals, thyroids, pituitary, and pancreas ;

230 THE HUMAN MECHANISM

secretin and other hormones; mucin; hemoglobin — to realize
that the food must furnish material out of which to manu-
facture compounds of the greatest variety of chemical struc-
ture ; and for this purpose the greatest variety of material
must be furnished in the food.

16. The unique position of protein. Considerations such as
the above give a glimpse into the unique value of protein
food. While all forms of carbohydrate yield the body, for
the greater part, only dextrose, and the fats yield only fatty
acids and soaps, all of them closely similar in structure, pro-
tein yields amino-acids of the greatest diversity of chemical
structure. The possibilities of chemical construction, or syn-
thesis (as it is generally called), are thereby greatly increased.
Only a chemically complex food like protein could serve for
the construction of the proteins of the living cell and for the
formation of the varied products of cell manufacture. Review
in this connection section 15 of Chapter VIII.

Protein is also unique among the nutrients in the fact
that the body can make other nutrients from it. It is a well-
established fact that large quantities of sugar (dextrose) may
be made from protein, and we can therefore understand how
a dog living on fat and the leanest sort of meat (protein)
can do without carbohydrate in the diet. It is also possible
that at least small amounts of fat may be derived from pro-
tein through this intermediate stage of sugar, for fat may be
made from sugar.

17. Variations in the nutritional value of individual pro-
teins. Until comparatively recent times all food proteins
were regarded as having equal value in nutrition, with the
single exception of gelatin, which has long been known to
be incapable of meeting the protein needs of the body. The
discovery that some food proteins are lacking in one or more
of the amino-acids, and that the same amino-acid may occur
in very small amounts in one protein and very large amounts
in another, suggested to two American physiologists, Mendel

NUTRITION 231

and Osborne, the idea that different proteins may have very
different values in nutrition. They therefore fed rats and
mice on diets of abundant fuel value and containing all the
non-protein constituents of the diet, but containing only one
protein. It was found that some proteins failed entirely to
nourish the animal, as shown by the steady loss of weight ;
others would keep an adult animal in good condition with
no loss of weight, but did not provide the material for the
normal growth of young animals ; other proteins not only
maintained the normal weight of the adult but a young
animal fed on them would grow in a perfectly normal man-
ner. We must therefore distinguish between (1) proteins
which may provide for both growth and maintenance, (2) pro-
teins which will provide for maintenance but not for growth,
and (3) proteins which will provide for neither maintenance
nor growth.

Further study showed that the nutritional limitations of
the last two classes of proteins are due to the fact that they
are lacking in certain amino-acids or else contain them in
very small amounts; for if these amino-acids were added to
the diet, growth and maintenance became normal. The rea-
son for this becomes clear on the assumption already made
in our discussions of digestion and nutrition, that the value
of protein as food lies in the fact that it yields a great
variety of amino-acids, each necessary to some chemical
manufacturing process of the living cell.

18. The value of the mixed diet. As a matter of fact no
one tries to live on a single protein. Meat contains at least
two ; eggs, three or more ; milk, two ; the cereals, two or
more each. By taking a mixture of these in our food, the
deficiency of one protein in amino-acids is made up by the
excess of the same acid in another. For this reason we can
completely meet the protein needs of the body on a mixed
diet with a far smaller total intake of protein than if the
diet contained only one protein.

232 THE HUMAN MECHANISM

The same consideration applies in a larger way to the food
as a whole. Some foods, like meat, are chiefly protein;
others, like the cereals, have an excess of starch, while
others, like butter or olive oil, are chiefly fat. A diet com-
posed of several kinds of food, that is, a mixed diet, is more
likely to avoid an excess of any one nutrient than when
any one food unduly preponderates.

19. Other indispensable constituents of the food. I. Inor-
ganic salts. In addition to the proteins, fats, and carbo-
hydrates, which together make up almost the whole (96 to 98
per cent or even more) of the food we eat, two other groups
of substances are required in much smaller quantities, but
they are none the less absolutely indispensable. The first
of these is the group of inorganic salts. In the body are
found chlorides, carbonates, and phosphates of sodium, potas-
sium, calcium, and magnesium. These occur both in the
living cells and in the blood and lymph, and they are con-
stantly being removed from the blood in the urine and per-
spiration. This loss must be made good by the food. Most
foods contain salts, and our usual food contains most of the
inorganic salt necessary for making good the loss. The table
salt used in cooking and to develop the flavor of food at
tablets for the greater part in excess of the actual needs of
the body, the excess being promptly excreted by the kidneys.
The addition of some salt, however, seems to be necessary.
The craving of herbivorous animals for salt in which their
food is deficient is well known, and in parts of India salt
famines have occurred during which the price of salt was
higher than that of gold.

20. Other indispensable constituents of the food. II. " Vita-
mines. " Finally, it is known that certain other compounds,
found in small quantities in many foods, are necessary
for adequate nourishment. The exact chemical nature of
these substances is still a matter of investigation, but it is
known that they are neither protein, fat, carbohydrate, nor

NUTRITION 233

inorganic salts. They occur in the outer layers of cereal
grains, such as wheat, rice, oats, etc. ; they are also present
in most fresh vegetables and, in smaller quantities, in meats,
eggs, and milk; and they are produced by the yeast plant
during its active growth. Hence they may be extracted from
yeast cakes. To them the general name of vitamines has
been given.

In many Eastern countries, where rice forms the chief
article of diet, a severe disease known as beriberi is more
or less common. It is characterized by grave disorders of
nutrition, and in severe forms the nerve fibers undergo
degeneration, so that paralysis of the skeletal muscles
develops. It was found that beriberi occurred chiefly among
those who used polished rice, that is, rice from which the
dark outer portion of the grain had been removed in the
process of milling, in order to give a whiter rice grain,;
it seldom developed in those using the whole rice grain
(that is, the unpolished rice). It was furthermore found
that from the rice polishings something could be extracted
which when administered in very small quantities would
cure the disease. Finally, it was found that a similar disease
(polyneuritis) could easily be induced in fowls by feeding
them on a diet consisting solely of polished rice, but that
it did not develop when the extract of rice polishings was
administered to the fowls even though their food other-
wise consisted wholly of polished rice. This extract would,
moreover, cure the disease when it had once developed.

Whether one group or more than one group of compounds
is concerned here is not known. It is clear, however, that
we have in the above facts proof of some essential constitu-
ent or constituents of the diet other than the usual nutrients.
These vitamines seem to occur abundantly in most fresh
fruits and freshly cooked vegetables and in the outer por-
tion of most cereal grains. They are destroyed by very high
temperatures, especially those used in sterilizing canned

234 THE HUMAN MECHANISM

foods, and they are largely removed from the cereal grains
in the attempt of the miller to produce the whitest possible
flour or rice, for this means the removal of the outer por-
tion (bran) of the grain with its vitamines. It follows that
" whole wheat " flour or graham flour contains these sub-
stances, while very white flour is deficient in them; and
we accordingly find that the same disease (beriberi) has
occurred in Newfoundland, where a community was shut
off during whiter from its usual food supply and white bread
constituted for too long a time the chief food. A similar
and probably identical disease has been found among people
living exclusively upon canned goods, the sterilization by
high temperatures having destroyed the vitamines.

In the days of sailing vessels, scurvy, a disease of mal-
nutrition, often developed on long voyages, despite a diet
which contained an abundance of protein, fat, carbohydrate,
and salt; and it was found that this disease could be pre-
vented by the use of fresh limes or freshly cooked vege-
tables. There is little doubt that here again we are dealing
with a disease analogous to beriberi.

In all the above cases it must be clearly understood that
there is no harmful constituent in the foods mentioned —
canned foods, polished rice, white bread, and the like. The
trouble lies in the absence from the food of an essential con-
stituent of the diet. No harm would result, for example,
from a diet of canned meat, white bread, and fresh vege-
tables ; for the fresh vegetables would supply the necessary
vitamines. It is only when the diet consists almost exclu-
sively of foods deficient in vitamines that trouble results.

The physiological action of these vitamines is not yet clear,
but we are probably not far from the truth if we regard them
as furnishing the body with some material indispensable for
carrying out the processes of chemical manufacture. Though
required in much smaller quantities, they are as necessary to
these processes as the amino-acids themselves.

NUTEITION 235

D. THE PROPER DAILY INTAKE OF PROTEIN

21. The economic and the physiological question. The
proper amount of protein in the diet is both economically
and physiologically important. Since foods rich in protein —
meats, eggs, dairy products, etc. — are among the more ex-
pensive foods, it is often for a family with limited income
a practical question how much of these foods must be used
to assure proper nourishment of the body. In this work we
are more directly concerned with the physiological effects of
low, moderate, and abundant protein diet, but the answer
to this question also gives the answer to the economic ques-
tion, since the problem in the latter case is to keep down
the consumption of the more expensive foods to the level
which is consistent with adequate nutrition.

It is comparatively easy to determine whether the fuel
value of the diet is adequate. If it is insufficient, loss of
weight inevitably results ; if it is excessive, and especially
if it is excessive in fat and carbohydrate, there is apt to be
increase of weight. An equilibrium of total intake and out-
put for months usually indicates that the fuel needs of the
body are being met. Equality of intake and output of pro-
tein, on the other hand, does not prove that the protein of
the diet is what it should be, for the body breaks down all
the protein it consumes whether the amount be excessive or
not. We can, however, determine by dietary studies what
is the usual consumption of protein by different classes of
people and also what is the lowest intake upon which
protein equilibrium may be maintained in the body.

22. The usual and the minimum intake of protein. When
the choice of food is not restricted by economic or other
consideration, but is determined solely by appetite or the
feeding customs of one's home or community, the protein
intake of an adult healthy man usually varies between 100
and 150 grams daily. This is equivalent to from 500 to

236 THE HUMAN MECHANISM

750 grams (1 to li pounds) of lean meat, although of course
the protein is not all taken in the form of meat. On the
other hand, experiments have shown that men may live for
years in good health on a protein intake of from 40 to
50 grams daily without loss of protein from the body.

If then an adult man can maintain puotein equilibrium on
from 40 to 50 grams of protein daily, but ordinarily con-
sumes two to three times this quantity, the question arises
whether the additional 50 to 100 grams are in any way
harmful. Many students of this subject have strongly taken
the position that such is the case, and there can be no
question that the health of many people, especially when
leading sedentary lives, has been greatly improved by re-
ducing the consumption of protein to 60 or 70 grams, or
even to 40 or 50 grams daily. To what is this improvement
due ? Is it because the handling of so much protein by the
adult is necessarily harmful ? (See p. 239.)

23. Possible harm and possible advantage in an abundant
protein diet. We can readily see at least two ways in. which
the greater protein intake may be harmful. In the first place,
it involves greater danger of incomplete protein digestion in
the small intestine and the consequent delivery by peristalsis
of excessive amounts of protein to undergo microbic putre-
faction in the large intestine. In general the presence of a
decidedly offensive odor to the feces suggests that more pro-
tein1 is being eaten than can be properly digested, and justi-
fies at least an experimental reduction in the protein of the
food. It must, however, be remembered that putrefactive
odors may be due to other causes than excessive protein
diet (impaired digestion of fats, for example) and, on the
other hand, there may be excessive putrefaction and yet the
feces have no very offensive odor because the compounds
responsible therefor have been destroyed within the body.

1 The substances responsible for the offensive odor are almost entirely
derivatives of protein.

NUTRITION 237

In the second place, the larger protein diet with its in-
crease of protein wastes in the body itself (as contrasted
with the alimentary canal) involves a greater burden on the
organs of excretion. This burden may fall not alone on the
kidneys, which finally discharge these wastes from the body,
but also upon other organs in which the waste products are
prepared for final removal from the blood by the kidneys.
Convincing proof has, however, not been given that these
organs, when in a healthy condition, are injured by the work
of caring for more than the waste of a low protein diet.
A somewhat analogous case is that of muscular activity.
This, too, must be limited or even given up altogether in
some diseased conditions lest some undue burden be placed
upon the organism ; but in health the body is actually bene-
fited by the " burden " of even vigorous muscular activity.

The further question then arises whether there is any
possible advantage in a liberal protein diet. It is certainly
not needed for power or for fuel; it may, however, be plau-
sibly urged that thereby we insure an abundance of each
amino-acid needed for the formation of the innumerable
products of chemical manufacture in the body. When an
engineer builds a bridge, he does not build it just strong
enough to sustain the expected load ; he allows a liberal
" margin of safety." Similarly, it is not a desirable economic
condition when one's income each week is just enough to
meet necessary expenses, for this does not allow for the
unexpected emergency which we cannot foresee. So it has
been urged and, it would seem, reasonably urged that it is
better not to diminish protein intake, as a rule, to the irre-
ducible minimum of 40 to 50 grams daily. While 100 to 150
grams is almost certainly far more than is necessary to secure
the proper margin of safety, it may well be wiser to use 20
or more grams above the minimum ; that is to say, a protein
intake of 70 grams corresponds with what, in the present state
of our knowledge, may be regarded as a conservative estimate.

238

THE HUMAN MECHANISM

Infants and rapidly growing children need relatively more
protein than adults. The protein of the usual adult diet
makes about 13 to 15 per cent of the total fuel value of the
food ; in milk, the sole diet of a baby, it makes 20 to 25 per
cent. A similar thing is true of the diet during convalescence
from wasting diseases ; such a diet should be as rich in pro-
tein as is consistent with its proper digestion and utilization
by the body.

24. Food values of some common foods. The folio whig
table (from Joslin) will be found useful in forming an esti-
mate of the content of certain foods in protein, fat, and
carbohydrate, and also of the fuel value of these foods.

30 GRAMS (OR 1 OUNCE) CONTAIN
APPROXIMATELY

PROTEIN

FAT

CARBOHY-
DRATE

CALORIES

Oatmeal, dry weight ....
Cream, 40 per cent ....
Cream, 20 per cent ....
Milk

Grams
5
1
1
1

Grams
2
12
6
1

Grams
20
1
1
1 5

120
120
60
20

Brazil nuts ....

210

Oysters (six)

Meat (uncooked, lean) . .
Meat (cooked, lean) ....
Bacon

6
8
5

3
5
15

0
0
0

50
75
155

Ego- (one) .

Vegetables (5 and 10 per cent

rrpOUDS)

0 5

1 or 2

6 or 10

Potato

Bread

Butter

225

Fish

Broth

0 7

Small orange or half a grape-
fruit

An individual "at rest" requires about 25 calories per kilogram
(2.2 Ib.) body weight per 24 hours, equivalent to approximately 1 calorie
per kilogram per hour.

NUTRITION 239

25. Example of a diet of moderately low protein and fuel
value. The following table gives an example of three meals
which would give the moderate protein intake referred to on
page 237. The fuel value also corresponds approximately,
for a man of 150 to 160 pounds, to the fuel value of 13.5
calories per pound of body weight referred to on page 221.

Breakfast. Bread, 38.7 grams ; tea, 146 grams.

Lunch. Bread, 97.5 grams; butter, 31. 5 grams ; sweet potato, 108.7 grams;

spaghetti, 82.5 grams ; peaches, 89.4 grams ; coffee, 210 grams ; sugar,

21 grams.
Dinner. Bread, 75 grams ; butter, 21.5 grams ; roast beef, 116 grams ;

lemon pie, 188.5 grams ; coffee, 210 grams ; sugar, 21 grams.

Protein in food 70 grams

Fuel value 2334 calories

30 grams = 1 ounce, or yL pint.

CHAPTER XIV

SENSE OKGANS AND SENSATIONS

1. The human mechanism a conscious mechanism. Thus
far we have repeatedly compared the human mechanism
with lifeless mechanisms, and the points of similarity are
most interesting and instructive. In the supply of power,
the elimination of wastes, the interdependence and coopera-
tion of parts, the adjustment to the changing conditions
of work, and in many other respects the resemblance holds
good. But in one respect there is no likeness whatever.
When a human mechanism is not in good working order or
is tired, it may be aware of the fact; when an engine is
damaged in any way, the engine does not know it. Events
taking place in the living animal body arouse in it, and in
it only, conscious sensations.

Sensations are always called forth by the condition of
some organ or by the condition of the body as a whole.
When several hours have passed since the taking of food,
we feel hungry; or of drink, we feel thirsty; when any-
thing touches the skin a sensation of touch is aroused; if
it presses very hard, that part of the skin feels painful ; if
the tongue is acted upon by sugar or salt, we get a sensa-
tion of taste ; if light enters the eye, it produces conditions
hi that organ which arouse in us sensations of color. In all
these cases the conscious sensation is due to the condition of
some part of the body.

2. The reference of sensations. Sometimes we refer the
sensation to the part of the body which is first affected,
or to the body as a whole, and sometimes we refer it to

240

SENSE ORGANS AND SENSATIONS 241

external objects. Thus, if in driving a nail the hammer
misses the nail and hits a finger, we refer the pain to the
finger and not to the hammer ; and we similarly refer sensa-
tions of hunger and thirst to the body and not to external
objects. If, on the other hand, the skin is cooled by a
piece of ice, we do not say that the skin is cold, but that
the ice is cold; we refer the sensation to the external object
which causes it, not to the skin in which it actually origi-
nates. In the case of the sense of sight, this reference of
the sensation to the external object which sends light into
the eye is so complete that unless we stop and reflect upon
it we do not realize that it is the condition of the eye of
which we are conscious rather than the condition of the
external object at which we are looking.

3. Sense organs. A few sensations, like pain, are aroused
by the condition of most, if not all, parts of the body;
there is no one organ set apart to produce them. Some, like
hunger, although at times more or less general in origin,
are commonly aroused by the condition of some one organ 1
which ordinarily performs other functions. Other sensations
arise in organs set apart for the purpose and constructed to
react to only one kind of stimulus (special sense organs, or
organs of special sensation). To this latter class belong tl.e
eye, the ear, the olfactory mucous membrane of the nose,
the touch organs in the skin, etc. We therefore speak of
general sensations and special senses, although no sharp line
of division can be drawn between the two.

4. The brain the seat of sensation. In all cases, however,
the sensation, although originating elsewhere, is developed
in the brain and not in the sense organ. If the optic nerve
be cut, blindness ensues, although light falling on the retina
produces the same effect in the eye itself as when the nerve
is intact ; it even starts nervous impulses toward the brain ;
but, since these impulses go no farther than the cut, they

1 In the case of hunger, the stomach.

242 THE HUMAN MECHANISM

excite no sensation of light. And the same thing is true of
other sensations. Conversely, after the amputation of a limb
it often happens that sensations are felt, as if they came
from the lost member. In this case the stump of the cut
nerve is stimulated in some way, and the impulses thus
sent to the brain excite the same sensations as if they came
from the usual endings of the nerve. When one hits his
" funny " or " crazy " bone (that is, directly stimulates the
ulnar nerve) the sensations developed in the brain may be
referred to the fingers in which the nerve originates.

In the development of every sensation, therefore, we have
to distinguish between (1) what takes place in the sense
organ or end organ, (2) the passage of a nervous impulse
from this organ to the central nervous system, and (3) the
events which the arrival of the nervous impulse excites in
the brain. It is only the last (3) that, strictly speaking, we
can call sensation. The sense organs and their afferent fibers
are merely tributary mechanisms which serve to excite the
sensations in the brain. We are not aware that it is the
brain which is thus active, for we refer the sensation either
to the organ or to some external object.

5. The sense of sight ; the eye. Sight is one of the most
highly specialized of the senses. The eye is the only organ
in which originate sensations of light or color, and it is a
wonderfully constructed apparatus, the function of which is
to stimulate the optic nerve by rays of light. It is essen-
tially a living camera in which, by means of a lens, an
image of things around us is formed upon the retina; just
as in the photographer's camera the lens forms an image on
the ground glass or on the sensitive plate or film.

6. Structure of the eye. The eyeball consists of three
concentric coats surrounding and inclosing transparent sub-
stances through which rays of light pass to the retina. The
outer, or sclerotic, coat (the white of the eye) is composed of
very tough, dense connective tissue, and forms the protecting

SENSE ORGANS AND SENSATIONS 243

covering of the eye. Over a small area in front this coat is
transparent, and this part of it is known as the cornea. In-
side the sclerotic is the middle coat, or choroid, richly sup-
plied with blood vessels and containing in its connective
tissue large quantities of black pigment, which prevents the
passage of light into the eyeball except through the cornea.
The choroid lines the sclerotic everywhere except in front,
where in the region of the cornea it leaves the sclerotic and
projects toward the long axis of the eye as a kind of cur-
tain, the iris — that part of the eye which is black or gray
or blue. The pupil is the dark round opening, or hole, in
the iris. Immediately inside the choroid is the third and
innermost coat, the retina. This is a thin membrane, not
more than one eightieth of an inch in thickness, and lining
the chamber of the vitreous humor as far forward as the
ciliary region (Fig. 93). The retina is the part of the eye
sensitive to the stimulation of light. Here also begin the
fibers of the optic nerve, which passes through and perforates
the choroid and sclerotic coats behind on its way from the
retina to the brain. These and other parts of the eye may
be easily seen by dissecting the eye of an ox or sheep.

7. The lens and the muscle of accommodation. Immedi-
ately behind the pupil is the lens, a biconvex, transparent,
compressible, and elastic body fastened by a circular liga-
mentous sheet to the choroid coat immediately above and
behind the iris. The lens and its suspensory ligamentous
sheet thus divide the eye into two distinct chambers: the
one, in front of the lens and behind the cornea, filled with
a watery fluid, the aqueous humor; the other, behind the
lens and surrounded by the retina, filled with a jellylike,
transparent substance, the vitreous humor (Figs. 93, 96).

The elastic choroid coat is not long enough to reach
around and inclose the vitreous humor without stretching,
and hence it constantly exerts a steady, elastic pull or ten-
sion on the ligament of the lens. This tension flattens the

244

THE HUMAN MECHANISM

compressible lens (that is, makes it less convex), and
the lens is always in this flattened condition in the resting
eye; for example, when one is asleep. The same condition
should obtain, as we shall learn, whenever we are looking
at distant objects.

The pull of the tense choroid on the lens is, however,
overcome at times by the action of the sheetlike ciliary

muscle. The fibers of this pe-
culiar muscle originate in the
sclerotic coat around and just
outside the cornea, and diverge
radially outward and backward
to end in the choroid beyond
the attachment of the suspen-
sory ligament of the lens.
^Suspensory Fig. 94 shows how the con-
traction of this muscle, fixed
as it is near the cornea, must
draw the choroid forward and
so ease the pull of the latter
FIG. 93. Vertical section through on the ligament of the lens,
the ciliary region of the eye When this happens, the lens,

Showing the structures concerned in owing to its OW11 elasticity,
accommodation (see sect. 7). This . . , ,

should be compared with the perspec- assumes its independent (more

tive view into the hemisphere of the convex^) shape

The curvature of the lens is

Ligament

eyeball, shown in Fig. 167

thus variable, and is determined by the action of this muscle
of accommodation. When the ciliary muscle is relaxed, the
lens is kept flattened by the pull of the choroid on the liga-
ment; when the muscle contracts, this pull is eased off (or
slacked) and the lens becomes more convex. The entire
operation is known as accommodation, and we may now
inquire what part accommodation plays in vision.

8. The formation of an image by a lens. The eye is a
camera, in that it forms on the retina an image of objects

SENSE OKGANS AND SENSATIONS 245

in front of the cornea; and it is the first essential of clear
vision, just as it is the first essential of photography, that
this image be sharp, or at least distinct. A simple experi-
ment will show that clear vision of near and of distant ob-
jects cannot be had by the eye at the same time. Hold up
a pencil or a pen about ten inches from the eye and look
first at it and then at some object far away. Both can be
seen, but only one at a time clearly,
and often an effort is required to shift
from the far to the near object.

The change which occurs in the
eye in the act of accommodation is
illustrated in the following experi-
ment: A wooden or pasteboard box
(approximately 8 by 5 by 4 inches)
is fitted with a piece of ground glass
on one side and provided with a con-
vex lens on the opposite side. This
is a rude camera, and some object is
now placed at such a distance that
the lens forms an image of it on the FIG. 94. Diagram of the
ground glass, which is now in focus mechanism of accommoda-
for the object. If, later, the object ,

J The ciliary muscle is repre-

be moved nearer to the lens, the sented as three fibers passing

focus is changed; the image on the obliquely from the sclerotic

to the choroid

glass becomes blurred, and m order

to make it distinct it will be found necessary to use a

more convex lens.

Essentially the same change occurs in the eye in accom-
modating for near objects : the lens must be made more con-
vex; and this, it will be remembered, involves work on the
part of the muscle of accommodation (see p. 244). We can
thus understand why, in general, it is too much of "near
work," and especially near work necessitating very distinct
vision, that tires the eye. The ideal condition of the eye,

246

THE HUMAN MECHANISM

regarded merely as a camera, is that in which distant objects
are focused on the retina when the muscle of accommoda-
tion is completely relaxed and the lens is thus flattened to
its utmost by the elastic pull of the choroid coat (p. 243),
for iii this case the eye is rested by looking at distant ob-
jects, and works only when looking at near objects. Such
an eye is known as an emmetropic eye (Fig. 97, E).

Unfortunately, not all eyes meet this requirement. The
eyeball may be either too short or too long ; so that, with
the muscle of accommodation relaxed, the position of the

.-A.

^"""_'I'-- -;-''":J--

;

~";i'-*.— .

"~-~»»^ >>kx

*— ..^ *

FIG. 95. Action of a convex lens in bringing to a focus the rays of light

diverging from a single point
The rays from A are focused at a ; those from B, at 6

perfect focus for distant objects is either before or behind
the retina; the eye no longer sees distant objects distinctly
when it is at rest, because then the retinal image is blurred.
To understand more fully the undesirable consequences of
this condition, we must learn how convex lenses produce
images of objects.

9. The action of a convex lens on rays of light. The rays
of light diverging from a single point and entering a convex
lens are bent so that all come together again in a point be-
hind the lens, or, as it is said, are brought to a focus. This
is shown in Fig. 95, as is also the fact that rays of light
diverging from more distant points come to a focus behind
the lens sooner than those diverging from nearer points.

SENSE ORGANS AND SENSATIONS 247

Now a lens forms an image of an object because all the
rays of light from each point of the object are focused in
corresponding points behind the lens. This is shown in
Fig. 96, where all the rays diverging from 1 are focused
at 1', all those from 2 at 2', and those from intermediate
points of the object at intermediate points of the image.

If the rays from each point meet in front of the retina and
then diverge before reaching the retina, the retinal image is
blurred; and the image is also blurred if the retina is so

FIG. 96. Diagram showing the formation of an image on the retina

Jf, 2, the object ; 1', 2' , the image of the object ; c, cornea ; i, iris ; I, lens ; v, vitreous
humor; w, sclerotic; ch, choroid; o.n., optic nerve

near the lens that the rays from each point have not yet
come to a focus. The more convex the lens, the more will
the rays of light be bent; consequently we use the muscle
of accommodation (which makes the lens more convex) to
get clear images of near objects (see Fig. 95).

10. Myopia, hypermetropia, and presbyopia. In the em-
metropic eye (Fig. 97, E) the distance between the retina
and the lens is such that light from distant points comes
to a focus on the retina without any active muscular ac-
commodation; to see near objects the lens is made more
convex.

248

THE HUMAN MECHANISM

When the retina is so far away from the lens that, with
the muscle of accommodation completely relaxed and there-
fore the lens flattened to its utmost, light from distant
points comes to a focus in front of the retina, the retinal
image is blurred, and it is impossible for such an eye to see
distant objects clearly. To correct such vision it would be

necessary to make the
lens still less convex,
and this the eye is un-
able to do. (Why?)
Such an eye is known
as myopic, or near-
sighted, and its defect
must be corrected by
the use of concave
glasses, which act as
if the lens were made
flatter, and so throw
the focus farther back
upon the retina. A
myopic eye generally
has clear sight for
very near objects be-
cause, as stated above,

Course of the rays of light from a
distant point

Through the emmetropic(^), the myopic (M), and
the hypermetropic (H) eye, the muscle of accom-
modation being relaxed. (The rays diverging
from a distant point would enter the eye practi-
cally parallel)

FIG. 97.

the nearer the object
the farther back is the
image formed.

On the other hand,
the eyeball may be
too short, fore and aft (Fig. 97, ^T), so that, when the ciliary
muscle is relaxed, light from distant points has not yet been
brought to a focus when it reaches the retina (Jiypermetropia).
Such an eye must accommodate not only for near but also
for distant objects, and its muscle of accommodation can
never rest so long as the eye is being used. Moreover, to see

SENSE ORGANS AND SENSATIONS 249

near objects the ciliary muscle must work much harder
than in the normal eye, and it often happens that, even
with its utmost effort, the rays are not sufficiently bent to
focus them on the retina ; so that a book, for example, must
be held at arm's length to be read. Persons having such
eyes form one class of those said to be " far-sighted," and
their trouble can be corrected by the use of convex glasses.

As old age ap-
proaches, changes oc-
cur in the lens; in
consequence, it no
longer becomes as
convex as formerly
in response to the ac-
tion of the muscle
of accommodation
(presbyopia, from
irplffflvt, " old ").
Some, though not
all, results of this
condition resemble

those of hyperme-

i , i FIG. 98. A test for astigmatism

tropia; but the two

differ in cause. Hypermetropia is due to shortness of eyeball ;
presbyopia, to failure of accommodation.

11. Astigmatism. We have thus far been dealing with
those optical imperfections due to improper distance between
the lens and the retina. Another and frequently more seri-
ous trouble, known as astigmatism, results when the cur-
vature of the cornea (and sometimes of the lens) is not
perfectly regular; that is, when these surfaces are not seg-
ments of perfect spheres, but resemble in curvature the
side of a lemon. In this case the rays of light from a point
are not brought to a focus again in a point behind the
lens; and remembering the importance of sharp focusing in

250 THE HUMAN MECHANISM

securing distinct retinal images, the student will see that this
defect must seriously interfere with clear vision. The optics
of astigmatism are too complicated to be explained in an
elementary work, but the defect reveals itself generally in
an inability to see with equal clearness lines running in dif-
ferent directions. Thus some of the lines in Fig. 98 will be
sharply defined and black while one is looking with one
eye at the white center, and others will be blurred and
lighter in color.

Astigmatism is of special importance in reading, because
the lines of printed letters run in different directions. The
effort to see clearly the printed page is often severe, and
results in headaches and other general disturbances of health,
the true cause of which is often unsuspected. The trouble
m&y usually be corrected by the use of so-called " cylindri-
cal " glasses ; that is, glasses which compensate the defects
of curvature in lens and cornea.

12. Accommodation and "near" work. The above-described
defects of the eye as an optical instrument may usually be
successfully corrected by the use of proper glasses, which
should, generally speaking, be prescribed by a good oculist
and not by an optician. Glasses may be used for various
reasons — as a matter of convenience, as where a person
with slight myopia wears them merely to see distant objects
clearly; or of necessity, as when the myopia is more pro-
nounced; or they may serve the much more important pur-
pose of relieving the muscle of accommodation of undue
work in reading or sewing, and thus of avoiding " eye-
strain." A hypermetropic eye should always be provided
with glasses, since otherwise its muscle of accommodation
cannot be rested by looking at distant objects. But since it
is near work which requires the greatest effort of accom-
modation, it is in reading, writing, drawing, sewing, etc.
that the eyestrain is apt to be greatest. As this kind of
work is constantly increasing in modern life, the need for

SENSE ORGANS AND SENSATIONS 251

the complete correction of such defects becomes more and
more necessary. Those whose occupations require long-
continued use of the eyes should see to it that these
precious organs are used only under the most favorable
conditions and that all strain is as far as possible relieved.
13. Accommodation involves nervous as well as muscular
work ; the importance of sharp contrast. The work of the
muscle of accommodation is controlled by the nervous sys-
tem, and accurate accommodation involves an unusually high
degree of nervous coordination. The strain thus imposed
may be lessened not only by the use of proper lenses and
by giving the mechanism of accommodation periods of rest
(by looking for a time at distant objects) but also by using
the eyes in near work under the most favorable conditions.
Perhaps the most important principle involved here is to
secure the greatest possible contrast between the light and
dark parts of objects at which we are looking. When the
contrast is marked, the objects can be seen easily and recog-
nized even though the accommodation is not absolutely per-
fect. When, on the other hand, the contrast is not great,
very accurate accommodation is necessary. Important means
of securing the maximum contrast are the following:

1. The avoidance of too little and of too great illumination
of the object. Let the student examine any printed page with
different degrees of illumination. The contrast of white and
black will be poor in dim and in very bright lights, and
greatest with a certain moderate illumination. Hence read-
ing in twilight or with sunlight falling directly on the page
means greater eyestrain.

2. The avoidance of a flickering light. A steady light —
one free from nicker — is of the highest importance for near
work. In this respect a good kerosene lamp (student's lamp
or Rochester burner) is perhaps the best of all lights for
reading, provided the heat which it gives off is not too
great. Electric lights are good if steady, but too frequently

252 THE HUMAN MECHANISM

they are not. Gas from an ordinary fishtail burner is one
of the poorest lights for reading and sewing. The flicker of
gas lights may, however, be largely avoided by the use of
mantles.

3. If the printed matter is not held steady, the effort of
accommodation becomes much more difficult. Consequently
it is in general a bad thing to read, and especially to read
fine or poorly printed matter, on any but the steadiest
railroad train.

4. The use of very fine type should be reduced to a mini-
mum. When such printed matter is held at the ordinary
distance of eighteen inches from the eye, very accurate
accommodation is needed, and this, we have just seen,
involves nervous strain ; if it is held closer to the eye (so as
to make a larger image on the retina) the lens must be made
much more convex to focus it, and this means excessive
work on the part of the muscle of accommodation. This is
very undesirable, and especially so in youth, since then the
tissues of the eye are more plastic, and excessive strain of
the muscle of accommodation, pulling as it does on the scle-
rotic and the choroid coats, may lead to permanent deforma
tion of the curved surfaces. The marked increase of myopia
within the past forty or fifty years is generally explained in
this way.

5. Highly calendered paper objectionable. Closely connected
with the size of the type is the character of the paper on
which it is printed. This should be as dull as possible in
order to avoid the confusing effect of a glossy surface. The
use of highly calendered paper in many books and serial
publications, because such paper lends itself more readily
to the reproduction of pictures in half tone, is a sacrifice
of hygienic considerations to cheapness.

14. Visual sensations. We have shown (p. 241) that the
sensation of sight does not develop in the eye, but in the
brain, as the result of nervous impulses sent thither over

SENSE ORGANS AND SENSATIONS 253

the fibers of the optic nerve from the retina. Just how the
light falling upon the retina originates these impulses cannot
be discussed here ; suffice it to say that the character of the
impulse differs according to the color of the light1 stimulat-
ing the retina ; the lens focuses upon the retina a flat, colored
picture of the objects at which it is looking, just as a pho-
tographic camera does, or as the painter represents a scene
on canvas. One part of the retina is thus stimulated by light
of one color, and another part by light of another color or
by another shade of the same color ; and the different kinds
of impulses started in the fibers of the optic nerve ultimately,
upon their arrival in the brain, excite in consciousness what
we know as visual sensations. The sensations which we get
from the retina are therefore primarily sensations of color.

15. Visual judgments. But when we look at an object we
get more than mere sensations of color. The world does
not appear to us as a flat surface, of different colors, like
the painter's canvas. When we look at the wall of a room
we know that it is a flat surface, and when we look at a box
we know that it has not only length and breadth but also
thickness. If we were dependent entirely upon the retinal
image for our idea of the box, it would look as flat as the
wall ; that it does not appear so is because we receive other
information about the box than that which comes from the
retina. We have to accommodate the lens differently for
the near and the far edges, and we have learned by experi-
ence that this necessity indicates depth, or different distances
of different parts of the object. Again, we see the box with
both eyes, and the images formed on the two retinas are not
exactly the same. One eye sees more of one side, the other
eye more of another side ; and while we are not conscious
of this fact, we have really learned by experience and by
the actual handling of objects that this slight difference in

1 In this and the following paragraphs white, black, and gray are
regarded as colors.

254

THE HUMAN MECHANISM

sensations from the two eyes are produced only by solid
objects. Again, when we look at any point on the near edge
of a box the two eyes are converged by their muscles to a
greater extent than when we look at a point on the far edge,
and we have learned that these different pulls of muscles and
positions of eyeballs indicate that the object is not flat, but
has depth. The importance of binocular vision in the estima-
tion of depth or distance from the eye is most strikingly
illustrated by attempting, with one eye closed, to bring
together the points of two pencils held in the hands and
moved from side to side at arm's length.

Consequently when we look at anything we get a number
of sensations ; from the retina, those of color and the posi-
tion of the color spots with
reference to one another ; from
the muscular efforts of accom-
modation and of convergence
of the eyeballs, those which
reveal the property of depth
in what we see. And from
all of these, fused together
and interpreted in the light
of experience, we construct a

' * * <

• *

! s

> ^

: :

FIG. 99

visual judgment of the nature

of the object.

16. Optical illusions. That

our vision is essentially the

result of unconscious judgments is strikingly shown by the
fact that these sometimes deceive us. Thus the parallel
vertical lines in Fig. 99, when crossed by the oblique lines, seem
to be inclined toward each other. The retinal images of the
lines are parallel, and we falsely judge them inclined, this
error of judgment arising from the presence of the oblique
lines. In other words, our final idea of the lines does not
correspond to their image on the retina.

SENSE ORGANS AND SENSATIONS 255

Many other examples might be given showing that our
visual idea of the world around us is not a simple sensa-
tion or impression, but an unconscious inference, judgment,
or conclusion built up from a number of simple sensations
taken separately or blended together and compounded with
results of lifelong experience. In looking at a piece of fine
silk or cloth we seldom stop to think that its tissue may be
resolved into many simple component threads; and in quite
the same way we fail to realize that even our quickly formed
judgments of the size, distance, form, or color of objects are
likewise tissues woven out of many threads, most of which we
have been slowly and laboriously spinning since childhood's
days in the hidden factory of individual experience.

17. Sound and hearing. When the string of a violin,
piano, or harp " sounds," one can observe that it is in rapid
vibration ; and the same thing is true of all sounding bodies.
These vibrations are imparted to the air, water, or other
surrounding medium, and through this medium they are
transmitted as waves of sound. It is these waves, or vibra-
tions, which, on entering the ear, excite the sensation of sound.
The more rapid the vibrations, the higher is the pitch of the
note ; and the greater their amplitude, the louder the sound. .

The ear is an organ specially adapted to receive these vibra-
tions of air and to transform them into nervous impulses.
It is subdivided by anatomists into the outer ear, the middle
ear, and the inner ear.

18. The outer ear. The outer ear consists of the expanded
pinna (or that part which we commonly call " the ear ") and
a tube along which the vibrations of sound pass inward to
the tympanic membrane, or drum. Glands along this canal
secrete wax which guards the approach to the drum. It is a
bad habit to pick at this wax, and especially to dig into the
ear with any pointed instrument, for there is always danger
of perforating the drum. If trouble is suspected, a physician
should be consulted.

256

THE HUMAN MECHANISM

19. The middle ear ; the Eustachian tube. The tympanic
membrane separates the outer from the middle ear, or tym-
panum, a small cavity lying in the temporal bone of the
skull and communicating with the throat or pharynx by
means of the Eustachian tube. The air which it contains
is consequently under the same pressure as that of the

FIG. 100. Diagram of the ear

A, the auditory canal, leading to the tympanic membrane B; C, cavity of the
tympanum, communicating by the Eustachian tube with the pharynx D \ E, semi-
circular canals ; F, cochlea ; G, auditory nerve

atmosphere without, and the tympanic membrane is not
normally bulged inward or outward by inequality of pres-
sure on its two sides. The opening of the Eustachian tube
into the pharynx is, however, closed except when one swal-
lows, and hence swallowing often relieves the drum from
undue pressure of air in the middle ear.

The cavity of the tympanum also communicates with a
network of spaces, or sinuses, in the temporal bone. Because

SENSE ORGANS AND SENSATIONS

257

of these connections of the middle ear with the throat, on
the one hand, and with the temporal sinuses on the other,
inflammatory processes in. the nose and throat during a cold
sometimes extend into the Eustachian tube, the tympanum,
and even into the temporal sinuses, causing serious trouble
and occasionally deafness.

Passing directly across the tympanum, from the drum on
its outer side to the cochlea on its inner side, is a chain of
three very small bones, the ear
ossicles (hammer, anvil, and stir-
rup). These bones are bound
together and attached to the
walls of the tympanum by liga-
ments, and are so arranged that
when sound waves set the tym-
panic membrane in vibration
this motion is transmitted by
the ossicles to a portion of the
inner ear known as the cochlea.

20. The inner ear. The struc-
tures, of the inner ear lie in
the temporal bone, on the side
of the tympanum opposite the
drum. They consist of a system
of small bony spaces and tubes,
the ~bony labyrinth, within which lies a corresponding membra-
nous labyrinth. Forming part of the lining of the membranous
labyrinth are very sensitive cells, and between these cells
are the endings of the nerve fibers which connect the ear
with the brain. The cells of the inner ear are sensitive to
the vibrations which have been transmitted across the tym-
panum by the ossicles, just as the retina is sensitive to
light; and as the retina is the origin of the fibers of the
optic nerve, so the inner ear is the origin of those of the
auditory nerve.

FIG. 101. The bony labyrinth, its

actual size being shown in the

smaller figure

B, (7, D, the semicircular canals;
A, the oval window, by means of
which the vibrations of the stirrup
bone are transmitted to the cochlea ;
E, F, G, the whorls of the cochlea.
Cf . Fig. 102

258

THE HUMAN MECHANISM

Vestibule with Openings
of Semicircular Canals

Scala Vestibuli

Cochlea

Eustachian Tube*^-.

FIG. 102. Diagrammatic representation of the

membranous labyrinth of the cochlea in relation

to the structures shown in Figs. 100 and 101

The scala vestibuli and scala tympani are the two

portions of the bony cochlea which inclose the

membranous cochlea

21. Taste and smell. The end organs of taste are small
rounded eminences, or papillce, on the dorsal surface of the
tongue, and from these the fibers of the nerves of taste pass
to the brain. The end organs of the nerve of smell are situ-
ated in the upper portion of the nasal cavity and consist
of delicate cells very sensitive to the presence of odors.
Sensations of taste are frequently confounded with those of
smell. An onion, for example, has little or no taste, as can

be shown by placing
a bit on the tongue
when one is holding
the breath ; none of
the flavor of the
onion is perceived.
On the other hand,
sour, sweet, bitter.,
Scala'Tympani and salt are true

sensations of taste.
This unconscious
blending of tastes
with odors in form-
ing our ideas of the
nature of objects re-
calls the formation of visual judgments by the combination
of retinal sensations with those aroused by the muscular
act of converging the eyeballs.

22. Cutaneous sensations. The skin is the place of origin
of at least three sensations — touch, cold, and warmth. These
sensations are distinct, as is shown by the observation that
on certain points of the skin some*of them may be felt, but
not others. This fact is usually interpreted to mean that
each sensation has its own set of end organs and nerve
fibers. Especially striking is the fact that warmth and cold
are not felt by the same spot of skin, which seems to prove
conclusively that they are separate sensations.

SENSE OKGANS AND SENSATIONS

259

The afferent nerves of cold and warmth not only carry
into the brain those impulses which give rise to the corre-
sponding sensations but also serve as one important means
of stimulating the reflexes which help to regulate heat
production and heat output (see Chap. XII).

23. The sense of position. The expression "the five senses"
has become proverbial, and comes from the time when sight,
hearing, taste, smell, and touch were the recognized special
senses. To-day, however, we must add to
these not only warmth and cold but still
others, most conspicuous among which is the
sense of position. When the eyes are closed
we are aware of the position of the various
parts of the body. We know whether the
arm is bent at the elbow or straight ; whether
the head is looking forward or is turned to
one side or the other. And while we are

aware of these things, partly from tactile

. 8 ' F . J . . FIG. 103. A tac-

sensations, there is conclusive evidence that tiie corpuscle in

afferent impulses from the muscles, tendons, one of the papil-

and joints also play an important part in lse of t?e dermis ]

, , an end organ of

the result the sense of touch

When one is blindfolded and lies flat on a
revolving table which can be turned noiselessly in one direc-
tion or the other, the subject of experiment can form fairly
correct judgments as to the angle and direction through
which the table is turned. Here there is no change of char-
acter either in the tactile impulses or in those from the mus-
cles, tendons, and joints, for the subject of experiment lies
still and is only passively moved. It is believed that in this
case the sensations in question come from the movements of
the lymph in portions of the inner ear. One part of this,
the cochlea, is undoubtedly concerned with the perception of
sound ; but another part, the three semicircular canals, are
now believed to be end organs of this sense of position.

260 THE HUMAN MECHANISM

The impulses which make us aware of the position of parts
of our bodies also play a very important role in reflexly
guiding our movements. Upon this we shall dwell at greater
length in subsequent chapters (see especially Chap. XV).

24. Sensations of pain. Most organs of the body may also
give rise to impulses which, on their arrival in the brain,
cause sensations of pain. It is still, perhaps, an open ques-
tion whether this sensation, like sight, smell, and hearing, is
aroused by its own mechanism of end organs and afferent
nerves or whether it is called forth by the excessive stimu-
lation of the nerves of the other senses, but for the discus-
sion of this question the reader must consult more advanced
works on physiology.

Pain is a useful danger signal, since it effectively calls
attention to abnormal conditions and incites us to the adop-
tion of active remedial measures. Remedies, however, should
not be confined to the abolition of unpleasant sensations, but
should be directed to the removal of their cause. A tooth-
ache from a decaying tooth may often be stopped, for a time
at least, by the use of chloroform or other anesthetic drugs,
but the drug only stops the pain ; it does not check the
progress of decay or repair the damage. Again, a bronchial
cough may be unpleasant and even painful, but we should
not rest content with the use of some drug or cough medicine
which merely lessens the irritability of the inflamed surface
of the air passages, and so, perhaps, stops the cough without
curing the disease.

Pain is a warning that some abnormal condition needs
attention. Sometimes that attention may be supplied by the
sufferer himself, or by his friends, but often skilled medical
advice is needed. Too frequently, for the sake of economy
or from feelings of modesty, or even because of an unwilling-
ness to acknowledge illness either to the world or to one's self,
the mistake is made of postponing the visit to the physician,
the patient meanwhile bearing discomfort and perhaps actual

SENSE ORGANS AND SENSATIONS 261

suffering in the hope that he will soon be better and that the
trouble will " cure itself." Sometimes, of course, it does cure
itself ; but sometimes it does not ; and remediable disease
has too frequently been allowed to run on in this way until
some vital spot is attacked or the trouble has become too
grave for medical skill to overcome. Many diseases, like a
fire, may be extinguished at the start, but if not attended to,
grow rapidly into a conflagration beyond control. Pain is
one of the most trustworthy warnings that attention to the
mechanism itself or to our operation of it is necessary; and
we have no right, either for our own sake or that of our
friends, to neglect its warnings. While there are times when
it is an act of heroism to endure suffering and to keep the
knowledge of it to one's self, there are other times when to
do this is not only foolish but wrong.

25. Hunger and thirst. No account of the physiology of
sensations would be complete without some reference to
those very common experiences of life — hunger and thirst.
We have already spoken of them as sensations which are
referred to the body and never to external objects, thirst
usually being referred to the mouth and throat, and hunger
frequently to the stomach ; but hunger and even thirst may
sometimes affect us as sensations coming from the body as
a whole, in which case they are usually indistinguishable
from certain forms of general fatigue.

Hunger is excited by automatic rhythmic contractions of
the musculature of the cardiac end of the stomach. The
stomach, like the heart, executes rhythmic contractions, and
we may speak of the " beat " of the stomach just as we
speak of the " beat " of the heart, although each stomach
contraction is much slower than those of the heart. When
food is in the stomach, these contractions or " beats " are
inhibited in the cardiac end or else are reduced to very in-
significant proportions, and we have the inactive condition
of this portion of the stomach described in Chapter VIII ;

262 THE HUMAN MECHANISM

but when the cardiac pouch is again empty, the inhibiting
check is removed and the automatic " beats " become quite
powerful. These contractions start impulses up the sensory
nerves of the stomach, and these impulses excite in our con-
sciousness sensations of hunger. Often the " beats " occur
in rhythmic periods, a group of strong contractions alter-
nating with groups of weak contractions or even total
quiescence. In this case we have the " griping " hunger
pangs coincident with the strong contractions. In certain
abnormal conditions the presence of food in the stomach
fails to exert its inhibiting effect and we have a continual
" gnawing " hunger.

Thirst is aroused by the dryness of the mouth and throat,
probably by the reduction of the amount of water in cells
and tissues of this organ.

Hunger and thirst are definite sensations, as truly adapted
to guide us in the choice of food as sight is adapted to
picture to us the world in which we live. So long as the
body is normally occupied and healthy they may usually
be trusted; but there are abnormal conditions of sedentary
life, in the midst of a superabundance of tempting food,
when they become less trustworthy, and in some forms of
dyspepsia the sensation of hunger is never absent, no matter
how often one eats. In such cases the very effort to satisfy
hunger only aggravates disease. Conditions of this sort
should not prevail if proper attention be paid to the general
hygienic conduct of life. Broadly speaking, appetites, like
fire and dynamite, are good servants but bad masters.

CHAPTER XV

THE NERVOUS SYSTEM
A. ITS ANATOMICAL BASIS

In the preceding chapter we have repeatedly emphasized
the fact that sensations of all kinds are developed in the
brain from nervous
impulses coming
from the sense or-
gans, and in a pre-
vious chapter (VII)
we have seen that
Avithout reaching
the brain, or at
least without af-
fecting conscious-
ness, these affer-
ent impulses may
give rise to reflex
action. A reflex ac-
tion or a conscious
sensation, or both a
reflex action and

FlG- 104- Tne human brain viewed from above.

The cerebral hemispheres completely cover the

rest of the brain

a conscous sensa-

tion, may therefore

„ -., j- ,1

trance of a nervous
impulse into the central nervous system, and we have now
to inquire what is known of the mechanism by which these
results are brought about.

263

264

THE HUMAN MECHANISM

1. Fundamental structure of the nervous system; the brain
of a frog. The human spinal cord and brain are so com-

plicated that it is best to

Forebrain

' Tweenbrain
Midbrain

Hindbrain

Spinal
Cord

EIG. 105. The brain and spinal cord of the frog

On the left is a longitudinal, right-to-left section,
showing the central canal and the ventricles of the
brain ; on the right the dorsal view of the brain and
cord. A, the c'erebral hemispheres ; B, the optic lobes ;
C, the cerebellum; D, the bulb; E, the spinal cord

study first the nervous system
of a simple verte-
brate like the frog,
for the fundamen-
tal plan of struc-
ture is the same in
both. The spinal
cord is a relatively
thick-walled tube,
« I p the walls of which

J I I are composed of

1 white and gray

matter, the minute
bore, or lumen, of
the tube being
known as the cen-
tral canal. The ar-
rangement in the
brain is similar, but
here the central
space is no longer
a small tube of even
bore, but consists
for the greater part
of irregular cavities
known as the ven-
tricles of the brain,
while the walls
consist of masses
of gray and white

matter varying in size, shape, and relation to each other.

Fig. 105 will assist the student in understanding this plan
of structure. Anteriorly the spinal cord is continued in the

THE KEKVOUS SYSTEM 265

bulb, * whose central cavity is the fourth ventricle. Part of
the dorsal wall of this ventricle forms the cerebellum, which
in the frog is only slightly developed, but which in higher
vertebrates (birds and mammals) becomes a large and con-
spicuous organ. Anteriorly the fourth ventricle is connected
with the third by a tube, the aqueduct of Sylvius. The thick
walls of this aqueduct contain various masses of gray matter
whose names need not detain us ; the walls of the third ven-
tricle are similarly composed of large masses of gray matter

FIG. 106. Diagrammatic median longitudinal section of a mammalian brain

After Edinger

For convenience the cerebrum, with its lateral ventricle, is represented as a single

organ in the median plane instead of two hemispheres on either side of this

plane and each with its own lateral ventricle. The division into forebrain,

'tweenbrain, midbrain, and hindbrain is marked by the broken lines

scattered among the fibers of the white matter. Still farther
forward two openings from the third ventricle, one on the
right and one on the left side, lead into the large lateral
ventricles, the nervous tissue of whose walls is the cerebrum,
or the cerebral hemispheres. It is convenient to divide the brain
into the forebrain, surrounding the lateral ventricles ; the
'tweenbrain, surrounding -the third ventricle ; the midbrain,
surrounding the aqueduct of Sylvius ; and the hindbrain,
surrounding the fourth ventricle.

1 The older term for the bulb is the medulla oblongata, to distinguish
it from the medulla spinalis, or spinal cord.

266

THE HUMAN MECHANISM

2. The brain of the mammal is built on the same funda-
mental plan as that of the frog, and differs from it mainly
in the greater number of neurones and in the complexity

FIG. 107. The base of the human brain, showing the cranial nerves

A, the cms cerebri, composed largely of nerve fibers which connect the hind-
brain with the 'tweenbrain and forebrain ; B, the pans Varolii, the anterior floor
of the fourth ventricle, connected laterally with the cerebellum; (7, the bulb;
D, the cerebellum; E, the spinal cord

of their connections with one another. This results in
great thickening of the ventricular walls and the formation
of a very complicated anatomical structure. Mammals are
especially characterized by an enormous development of the

THE NEKVOUS SYSTEM

267

cerebral hemispheres, which in man grow to such proportions
upwards and backwards as to overhang and completely cover
the other structures on the dorsal side. But even these large
masses of nervous tissue, no less than the smaller cerebrum
of the frog, are composed entirely of the gray and white
matter forming the walls of the lateral ventricles.

By comparing the brain of a frog (Fig. 105) with those
of the rabbit, cat, and monkey (Fig. 166), and finally with

FIG. 108. Median longitudinal section of the human brain

A, B, C, D, L, convolutions of the median surface of the cerebrum ; E, F, the

cerebellum, showing in the plane of section the inner white matter and the outer

gray matter ; H, the pons Varolii ; If, the bulb

the human brain (Figs. 104, 107, 108), a fairly good idea
may be had of the increasing complexity of the brain as we
pass from the lower to the higher animals. Especially note-
worthy is the greater relative prominence of the cerebrum.
In the frog this organ is small and inconspicuous ; in the
rabbit it is much larger, but its surface is smooth ; in the cat
there is a further increase in size, and the surface is thrown

268

THE HUMAN MECHANISM

into folds, or convolutions-, and this increase in size and
surface folding — carried yet farther in the monkey — reaches

its highest development
in the human brain.

3. The cranial nerves.
Nerves enter the 'tween-
brain, midbrain, and hind-
brain somewhat as they
enter the spinal cord ;
and although their sepa-
ration into dorsal and
ventral roots is not ob-
vious, the neurones to
which their nerve fibers
belong are in all respects
analogous to the neu-
rones of the spinal nerves.
They may serve as the
paths of reflexes (for ex-
ample, a wink is a re-
flex from the optic or the
trigeminal nerve to the
facial nerve), and their
relation to the cells of
the cerebrum and other
higher portions of the

FIG. 109. A portion of the gray matter bram * essentially the
(cortex) of the cerebrum (highly magni- same as that of the spinal

fied). After Kolliker

nerves. Fig. 107 will give

Note the large number of dendrites. The the pointg Qf entrance or

axons are the fibers of uniform diameter

running lengthwise of the drawing. One of

these cells is shown in Fig. 41, D

exit of these nerves from
the human brain.
4. Histological structure of the brain. Microscopic study
of the brain shows an aggregation of neurones similar to
that seen in the spinal cord. These neurones differ greatly

THE KEKVOUS SYSTEM 269

in shape (see Chap. VII, p. 73), in the number of their
dendrites, and in the abundance of their connections with
other neurones. The regular arrangement in the cord of
central gray matter surrounded by white matter is wanting ;
instead, masses of gray matter occur here and there among
the bundles of nerve fibers of which the white matter is
composed. In the cerebrum and cerebellum the external
surface consists of gray matter and is known as the cortex
of the cerebrum and cerebellum respectively. These cortical
structures form the most complicated system of nervous
tissue in the body, and the cerebral cortex is intimately
concerned with the highest functions of the brain. (See
Figs. 109, 110, and 111.)

The figures give some idea of the variety and complexity
of the neurones of the brain. But however different, at first
sight, the brain may be from the spinal cord, the anatomical
plan of organization is the same in both; the brain as well
as the cord does its work because the connections of its neu-
rones with one another bring about coordinated action. The
secret of the structure of the brain, as of the cord, lies in
the nature of the connections of its units, the neurones, one
with another.

B. THE PHYSIOLOGY OF THE NERVOUS SYSTEM

Whenever through accident, disease, or otherwise, some
portion of the nervous system is destroyed, functions depend-
ent upon it are no longer performed, or at least are not per-
formed normally. A very large number of observations have
been made upon both animals and men in this condition, and
these have made it possible for us to obtain some idea of the
part played in normal life by each part of the brain and cord.
We shall attempt here to sketch only a few of the more im-
portant outlines of the picture, which the reader may com-
plete by more extensive study of physiology and psychology.

270

THE HUMAN MECHANISM

We shall choose for study the case of a single animal, the
frog, the anatomical structure of whose brain has been given
in this chapter. The phenomena shown by the frog are,
however, as far as we shall describe them, in general true
of higher vertebrate animals.

We shall therefore study (1) the behavior of a frog whose
brain has been destroyed, that is, a frog which possesses no

part of its central
nervous system ex-
cept the spinal
cord ; (2) the be-
havior of a frog
with spinal cord
and bulb intact,
but destitute of
midbrain, 'tween-
brain, and cere-
brum ; (3) the
behavior of a frog
with spinal cord,
bulb, midbrain, and
'tweenbrain, but
destitute of the
cerebrum.

•The behavior of
these incomplete
animals will each
be compared with that of a normal frog, which, of course,
possesses a complete nervous system.

5. The behavior of a brainless frog ; that is, a frog which
possesses of its nervous system only the spinal cord. Such
a frog can carry out only reflex actions of a comparatively
simple character. It lies flat upon its belly and, like the
normal frog, bends its hind legs under its flank, but does
not sit erect by supporting the head and upper trunk on the

FIG. 110. Transverse section of a convolution of
the cerebellum. After Ramon y Cajal

The figure represents only a few of each kind of nerve

cells and nerve endings. A, D, E, cells ; B, C, nerve

endings (synapses)

THE NERVOUS SYSTEM

271

fore legs. There are no respiratory movements; the vaso-
constrictor tone of the blood vessels is impaired or absent,
as are also many other of the most important reflexes.

But if one leg be pulled gently backward, the animal will
bend it again to its normal position under the body. If the
toe be pinched, the leg will suddenly be drawn away; and
if the skin of the flank be irritated by a bit of filter paper
moistened with acid, the paper will be kicked off by the leg
of the same side.

These are all pur-
poseful 1 and coordi-
nated actions, and
make upon the inex-
perienced observer
the impression that
the frog is aware
of the stimulus and
acts intelligently.
But the mere fact
that an act is pur-
poseful and coordi-
nated does not show

FIG. 111. Section of the cortex of the cerebellum

(at right angles to that shown in Fig. 110). After

Ramon y Cajal

that it is a conscious
act ; our movements
of respiration, winking, coughing, and sneezing are purpose-
ful and coordinated, but we know well enough that they,
as well as more complicated actions, may and often do occur
in the complete absence of consciousness. One of the first
lessons that the student of animal behavior must learn is not
to make the mistake of regarding an action as conscious
merely because " it looks so" or is purposeful and more or
less highly coordinated.

1 The word f f purposeful " is used here in the same sense as in Chapter VII
(p. 70) and does not include conscious purpose in its meaning. We shall see
that conscious purpose involves the cooperation of the cerebrum.

272 THE HUMAN MECHANISM

The spinal cord alone, then, and without the help of the
brain, is capable of maintaining a small part of the normal
posture of the resting frog and also of executing some of
the simple reflexes, especially those involving movements
of the hind legs, but it does not seem to be capable of
originating actions or of doing any except reflex actions.

6. The behavior of a frog with spinal cord and bulb only.
In this case there is no new feature in the maintenance of
posture ; the frog lies on its belly and executes the same
reflexes as before. The respiratory movements, however, go
on in a normal manner; the vasomotor tone of the arteries
is maintained, most vasomotor reflexes may be produced
with ease, and the heart may be reflexly inhibited. As com-
pared with the brainless frog, the number of actions which
the animal can execute is increased, and the reflex move-
ments become somewhat more complicated; but the differ-
ences are slight as compared with those seen in the animal
which has the 'tweenbrain and midbrain in addition to the
hindbrain and cord.

7. The behavior of a frog with spinal cord, bulb, midbrain,
and 'tweenbrain ; that is to say, a frog with the entire nerv-
ous system exclusive of the forebrain, or cerebrum. The
following points are especially noteworthy : (1) the sitting
posture maintained at rest; (2) balancing movements; and
(3) more complicated movements of locomotion.

(1) Such a frog, unlike those already described, sits erect
exactly like a normal frog; and this fact shows that com-
plete maintenance of the normal posture requires the coopera-
tion of higher portions of the nervous system than the bulb
and spinal cord, but does not involve the cooperation of the
cerebrum.

(2) If the frog be placed on a rectangular block of wood,
and the block slowly turned so that the frog tends to slip
off backwards, it will crawl up and over the descending
edge, keeping itself perfectly balanced. By continuing to

THE NERVOUS SYSTEM 273

turn the block the frog can be made to creep around it
almost indefinitely. Thus it not only maintains the erect
position but also corrects loss of equilibrium by appropriate
balancing movements.

(3) If the frog be stroked upon its belly, it will croak;
if its lips be touched with a blunt pin, it will brush the pin
away with its forefoot. Most important of all, if it be thrown
into the water, it will swim; and when it reaches a solid
object it will crawl out upon it and come to rest. In short,
the animal will carry out almost any movement of which a
normal frog is capable, provided the proper stimulus is applied ;
but without this it will do nothing, though capable of doing
so much.

The facts thus far brought forward show that the neurones
of the 'tween, mid, and hind brains and of the spinal cord
constitute nervous mechanisms which can maintain the nor-
mal posture, correct loss of balance, and even carry out the
usual acts of locomotion. The more of the nervous system
which the animal retains, the more complicated are the move-
ments, as we should expect when we remember the increase
in the number of neurones and the greater complexity of
coordination thereby rendered possible.

8. Comparison with the normal frog. The behavior of a
frog lacking only the forebrain, or cerebrum, differs from
that of a normal frog in two most significant respects. In
the first place, the animal rarely makes any movement with-
out obvious external stimulation ; if protected from drying,
it will often sit motionless for days, or even weeks. Such is
not the conduct of an animal which is aware of what is going
on around it or of its own sensations or feelings, that is, of
a conscious animal. In the second place, the frog shows the
most remarkable regularity and persistency in making re-
peatedly the same response to the same stimulus ; if its lips
be touched thirty times with a blunt needle, it will brush
at the offending object every time in the same way with the

274 THE HUMAN MECHANISM

same forefoot. We should certainly not expect a conscious
animal to do this ; for, after trying one plan of action a few
times, it would realize that its efforts were unavailing, and
would try something else, such as jumping away. This same
peculiarity is met with in all animals deprived of the cere-
brum. They act like mere complicated and faithful machines ;
they do not act as if they were thoughtful, original, or wise.

Especially striking is the avoidance of objects during
locomotion. This fact looks at first sight as if the animal
were aware of the presence of the obstacle in its path ; but a
dog without a cerebrum, even when it has been without food
for a day or more, will go to one side of a piece of meat and
pass it by. He acts as if unaware of the nature of the object,
of its use as food, etc. The image of the piece of meat
formed on his retina seems to generate nervous impulses
which pass to the brain by way of the optic nerve and re-
flexly guide the movements of the dog, but these impulses
do not inform the animal of the nature of the object, and
we have no reason to believe that the dog is aware of the
existence of the meat.

When we consider our own experience we find that we
too, as we walk along a crowded street, avoid objects, not
only without noticing them but without even being aware
of their presence. Here again the afferent impulses from
the retina pass to the nervous system and reflexly guide our
walking without affecting consciousness at all. And the
wonderful feats of somnambulism, where the " eyes are
open " but " their sense is shut," where the sleeper main-
tains his balance and avoids stumbling in situations where
he would almost inevitably fall if he were aware of his
surroundings, show how perfect is this very complicated
mechanism of locomotion, which seems to be complete even
in the absence of the cerebrum.

We are, indeed, so accustomed to regard our actions as
volitional and conscious that we rarely consider the large

THE NERVOUS SYSTEM 275

part which reflexes from the eye, the ear, the skin, the
muscles, and the joints play in guiding them. We will to
do a certain thing, to walk to a certain point, for example ;
perhaps the first step is a volitional act, but subsequent
steps, the suiting of these steps to slight unevenness of .the
path, the avoidance of many obstacles, the maintenance of
the balance of the body as a whole, — for we walk not only
with the legs but with the entire body, — all these things
take place apart from any exercise of the will and, for
the greater part, in the entire absence of consciousness,
although consciousness may, of course, at any time inter-
vene. Reflex actions thus play a most important part
even in the execution of those movements which we think
of as distinctly conscious acts.

9. Connections of the cerebrum with lower portions of the
nervous system; "the way out." Granting that the nerv-
ous events at the basis of consciousness occur within the
cerebrum, how do these events influence the muscles, the
glands, and other organs which do the bidding of the will?
What is the way out from this seat of consciousness ? This
path has already been referred to in Chapter VII (p. 81).
Cells in the gray matter of the cerebrum give off axons
which pass downward through the structures of the 'tween,
mid, and hind brain into the white matter of the spinal cord.
These axons give off along their course collaterals which end
in arborizations around nerve cells of the lower portions of
the nervous system and, by bringing groups of these cells
into coordinated activity, produce definite volitional move-
ments. The student should review carefully in this connec-
tion what has already been said with reference to these
neurones (see Fig. 165, v).

10. Connections of the cerebrum with lower portions of the
nervous system; "the way in." The fact that afferent
impulses from our sense organs of sight, hearing, etc. may
affect consciousness indicates that there must be some

276 THE HUMAN MECHANISM

connection between afferent neurones and the cerebral hemi-
spheres, since only when the latter are present does a nervous
impulse produce a conscious sensation. The connection is not,
however, so direct as in the case of efferent impulses. The
neurone of the dorsal root may be traced as far as the bulb,
but no farther ; from this point the impulse can find its way
to the cerebrum only by new neurones, and of these it would
seem that there are several. These relations are indicated in
Fig. 165, where the efferent neurones are represented in black,
and the afferent in red.

This diagram brings out the fact of increasing complexity
of reflexes as we proceed to the more anterior portions of
the nervous system. In the spinal cord the collaterals of
the afferent neurone act upon the efferent neurones; in the
structures of the midbrain and the 'tweenbrain the afferent
tract makes connection with more and more complicated and
extensive systems of these efferent neurones or motor mecha-
nisms. The range of possible movement is increased to in-
clude most of the usual actions of the animal, and some of
these actions represent a very high degree of coordination.
Finally, in the cerebrum the highest of all these connections
is made; here take place those events of whose nature we
have thus far been quite unable to form any conception, but
which play some part in the genesis of conscious sensations
and in the closely related dispatch of volitional impulses.
We can now understand why it is that removing this high-
est portion of the nervous system leaves untouched not only
the simpler reflexes but even the more complicated reflexes
of locomotion, of swimming, of flight, etc.

11. The nervous factors in locomotion ; automatic and
reflex elements. It is clear from the considerations given
above that walking, running, and other forms of locomotion
are essentially nonvolitional acts, and it is also clear that
there must be a nervous mechanism capable of carrying
them out without the aid of and in the complete absence

THE NEKVOUS SYSTEM 277

of consciousness. What is the nature of this mechanism?
In answer to this question we can only make a suggestion
without pretending to give a final explanation.

In the first place, walking obviously involves alternate
steps, or forward thrusts of the body by the two legs ; that
is, while one leg is pushing the body forward by straighten-
ing at hip, knee, and ankle joints, the other leg is bending
at these joints, the flexion of each leg at the hip joint bring-
ing it forward in preparation for its next forward thrust.
In each leg, then, we have an alternation of "forward swing"
(flexion at hip, knee, and ankle) and of " extensor thrust "
(extension at hip, knee, and ankle). In the same leg the
flexors of the hip, knee, and ankle obviously contract at
approximately the same time, and the extensors at the three
joints similarly act together ; furthermore, the extensor action
in one leg is simultaneous with the flexor action in the
opposite leg. These actions may be represented in diagram
as follows:

Hip

Ex.

Fl.

Ex.

Fl.

Right leg

Knee
Ankle

Ex.
Ex.

Fl.
Fl.

Ex.
Ex.

Fl.

£H
PH
O>
!H
O3

rt
<o

•£,

[

Toes1

Ex.

Fl.

Ex.

t»

(

Hip

Fl.

Ex.

Fl.

Ex.

'o
o

9
g

Left leg

Knee
Ankle

Fl.

Ex.
Ex.

Fl.
Fl.

Ex.
Ex.

^
«

"S3

-1-3

{

Toes

Ex.

Fl.

Ex.

Fl.

•J

Now it has been shown that in an animal made unconscious
by ether anesthesia, and in which no afferent impulses may
enter the cord or brain, — because of the depth of anesthesia
or even because of cutting the dorsal nerve roots, — similar
movements of the hind legs spontaneously arise and may

1 Flexion of the toes in each leg occurs simultaneously with extension at
the other three joints, and vice versa. With most people, owing to the use
of improperly shaped shoes, the toes are little used in walking. See Chapter
XXIV on the Hygiene of the Feet.

278 THE HUMAN MECHANISM

be kept up for long periods of time. Evidently there is
a mechanism consisting entirely of motor or efferent neurones
which by itself, independently of any afferent (that is, reflex)
or volitional stimulation, can automatically carry out a large
part of the act of locomotion. Locomotion becomes funda-
mentally the act of an automatic mechanism and is com-
parable to the alternate automatic contractions of inspiration
and expiration.

The parallel between the automatism of respiration and
the automatism of locomotion becomes still more striking
when we find in both cases that afferent impulses do actually
intervene to guide and so make more exact and efficient
the fundamental automatic movements. Thus we know that
afferent impulses started by the expansion of the lungs
during inspiration check the inspiratory effort then in prog-
ress and so bring on the next expiration sooner than it would
automatically occur. Similarly, the pressure upon the sole of
the foot as it touches the ground reflexly guides and proba-
bly strengthens the automatic extensor thrust; and many
other reflexes through the cord are known to serve similar
functions.

To sum up, then: The action of the legs in locomotion
seems to be fundamentally an automatic action, but these
automatic movements are guided by afferent impulses which
stream in from skin, muscles, and joints as the act pro-
gresses. Just as we can volitionally hold the breath, so we
can volitionally start or stop walking; or just as we can
volitionally change the depth and rhythm of respiration,
so we can volitionally change the pace or length of stride;
or just as a dash of cold water on the skin or the presence
of an irrespirable gas reflexly changes the character of the
breathing movements, so unevenness of the path or visual
impulses from an object in the way changes the character of
the locomotion. In all cases reflex and volitional interference
acts on a fundamental automatic nervous mechanism.

THE NERVOUS SYSTEM 279

12. The maintenance of balance and the regulation of mus-
cular tone. Walking, however, involves more than the action
of the neuromuscular mechanisms of the legs ; for here, as
weU as in complicated volitional actions, the balance of the
body must be preserved. For this reason we swing the arms
and execute ever-changing contractions of the muscles of the
trunk. Moreover, a proper state of tonic contraction in each
muscle is necessary to the proper execution not only of the
act of walking but of other acts as well, whether these are
volitional or nonvolitional. Into the mechanism of this won-
derfully perfect function of the body we cannot go within
the limits of the present book; but there is good ground
for thinking that, at least in the mammals, the cerebellum
is a very important and probably the all-important organ
concerned in effecting these coordinations.

13. Actions resulting from nervous processes originating
within the cerebrum. A very large part of the activities of
the body are thus fundamentally reflex actions ; they do not
require the aid of consciousness for then* execution. And it
is fortunate for us that this is the case; one has only to
imagine a human being who has to give his attention, or
"his mind," as we often say, to every adjustment of the
digestive, respiratory, and vascular systems required to meet
the changing necessities of life ; who has to keep his thoughts
on every movement of walking or running; who has to be
constantly on guard against loss of balance even when sitting
still. Such a being is almost inconceivable ; he would " go
crazy " in a single day ; but we can in this way realize to
what extent the reflex mechanisms of the body perform the
menial offices of life, leaving the mind free for higher things.

Speech is the result of movements in which the muscles
of respiration, those of the larynx, those of the tongue,
and those of the lips cooperate to produce articulate and
intelligible sound. The act of writing also consists of a series
of movements in which the muscles of the arm and hand

280 THE HUMAN MECHANISM

cooperate to make thought visible; performing on a musical
instrument, modeling a figure in clay or marble or bronze,
painting a picture — all these things occur to us as examples
of movements which are fundamentally neither reflex nor
automatic. Such are the highest actions of the body, and
the movements of which these actions are made up are
chosen and directed by the will.

These higher actions, like consciousness, depend upon the
presence of the forebrain. When a certain area of the cere-
brum is destroyed by disease, the power of speech is lost;
when another part is destroyed, the skilled use of the hand
is lost ; destruction of other portions affects in the same way
others of these skilled movements. In such cases locomotion,
the maintenance of balance, the movements of respiration,
etc. may be and usually are unaffected; the patient merely
loses the power of doing one or more of those things which
involve the selection of disconnected and to some extent
independent movements giving expression to some original
thought, sentiment, or idea.

The neurones of the cerebrum and their connections thus
constitute nervous mechanisms whose activity is essential to
consciousness, — to our seeing, our hearing, our smelling, and,
more than this, to our understanding of what we see, or hear,
or smell, — nervous mechanisms whose activity is also neces-
sary to the expression of our thought in action. It is because
of this fact that, when the cerebrum is removed, the animal
becomes merely a complicated reflex machine, acting only
as it is immediately stimulated from without or by events
taking place within its own body.

14. Effects of anesthetics on the nervous system. When
a person passes under the influence of an anesthetic, the
first function to disappear is consciousness; the ether or the
chloroform first paralyzes this highest and most complex
connection between the afferent and the efferent sides of
the nervous system. In this condition the patient may groan

THE NERVOUS SYSTEM 281

and struggle, for he is in somewhat the same state as the
animal without cerebral hemispheres. The use of the sur-
geon's knife will still produce movements; respiration may
be affected so as to result in groans and other movements
which the inexpert observer, perhaps in alarm, attributes to
severe suffering ; and yet when the patient awakes he tells
us he knew nothing of what passed and felt no pain. It is
important to realize that the signs of pain are never reliable
evidence of its existence.

If the anesthesia be pushed further, even these more
complicated reflexes disappear. In the ordinary major opera-
tions of surgery the ether or the chloroform is given until
it interrupts not only the cerebral connections between the
afferent and efferent paths but also those of the lower por-
tions of the brain; it is even administered until only a few
reflexes are left, such as the wink when the cornea is touched,
the contraction of the pupil when the eye is exposed to light,
etc. — these serving as useful tests of the condition of the
patient. If, for example, the pupil no longer contracts to
light, it is an indication that the anesthesia is going too far
— too near the point where the nervous mechanism of respi-
ration, etc., will be paralyzed. The giving of ether is then
suspended until these reflexes are again well established.

After the operation, as the ether or chloroform is elimi-
nated from the system, the reflexes return in the reverse
order; and the unconscious movements, groans, incoherent,
or even more or less coherent, talking (comparable with
talking in one's sleep) are sometimes most harrowing to the
feelings of those who do not understand that they are all
unconscious acts. The physician and nurse who remain un-
moved may even be wrongly charged with lack of feeling
because they do not waste sympathy where they know there
is neither suffering nor consciousness.

15. Inhibitory phenomena in the nervous system. We have
learned that some nerves excite organs to activity, while

282 THE HUMAN MECHANISM

others diminish activity or abolish it altogether (p. 160).
The beat of the heart is quickened by one set of nerves
and slowed by another; the circular muscular fibers of the
arterioles are excited to contract by vasomotor nerves, their
tonic constriction is paralyzed or inhibited by vasodilators,
and many other examples might be drawn from the action
of neurones on peripheral organs of the body.

Precisely the same thing is true in the brain and spinal
cord. Afferent impulses may not only reflexly excite neu-,
rones to activity but may also inhibit the existing or threat-
ened activity of other neurones, as when a sneeze is stopped
by biting the upper lip or by pinching the nose; or an
action may be inhibited by a volitional impulse from the
cerebrum, as when the breathing movements are voluntarily
stopped for a while, or when we similarly stop a wink or
a sneeze. These are all examples of inhibition, not of the
skeletal muscles concerned but of the neurones which inner-
vate them — in other words, of the inhibition of one neurone
by another.

It must be understood that inhibition is as essential a
part of the activity of the nervous system as is excitation.
Just as the driver of a team must urge on one horse while
he restrains another, so in all more complicated actions,
probably in all actions, reflex or volitional, the orderly
movement is as much the result of holding one neurone in
check as of stimulating another one to work, or to work
harder. Consciousness proves its presence most conclusively
by suppressing reflexes which would otherwise inevitably
occur and by bringing about new movements to meet the
desired end. Even in the highest processes of the most
highly organized of nervous systems, namely, those in which
human action originates, the man reveals his character and
influences the world around him by what he does not do —
by what he refrains from doing, sometimes at the cost of
severe struggle against impulse, instinct, or passion — quite

THE NERVOUS SYSTEM 283

as much as by what he does. Education, even, has been
partially denned as the "training of inhibitions and the
control of reflexes."

16. The cerebrum the chief organ for the acquisition of new
coordinations and associations. It would, however, be taking
too narrow a view of the functions of the cerebrum to regard
it simply as the seat of consciousness and volition. In
Chapter VII, § 15, we saw that hi addition to definite in-
herited reflex mechanisms, such as those of winking, — the
so-called unconditioned reflexes, — new paths of conduction
from the afferent to the efferent side are acquired during
life by the repeated association of two acts. Doubtless all
parts of the brain and spinal cord possess in some degree
this power of making new associations like those concerned
in the conditioned reflexes ; but the cerebrum is certainly
the organ in which they are made most readily, and there
can be no doubt that one of its chief functions is the acqui-
sition of such new paths of conduction as the experience
and activities of life first blaze within its nervous substance
and subsequently, by the repeated passage of nervous im-
pulses over the " blazed trail," change to " beaten paths " of
easy conduction. Here every act and experience of life may
leave its record, and here good and bad habits are acquired.

17. Use and disuse as factors in individual development,
training, and efficiency. When we consider the marvelously
complicated character of the nervous mechanisms which
control our actions, we naturally wonder how this intricate
machinery can be built and why it does not more frequently
get out of order. We cannot say that a simple and compre-
hensive answer will not some day be given to these ques-
tions, but to-day we have no adequate answer whatever.
The neurones with which we must work in life are born with
us ; but in most cases efficient connections must subsequently
be made between them, thus perfecting the mechanisms they
compose ; and this perfecting of the nervous machine comes

284 THE HUMAN MECHANISM

with use. The use of a nervous mechanism is generally
essential to its proper development, just as the use of a
muscle is essential to its strength. If the child never tried to
walk, the neurones which carry out the movements of walk-
ing would not develop ; not only do the muscles of an arm
strapped down to the side of the body waste away and be-
come practically bands of connective tissue, but the neurones
concerned in the actions which the arms should execute
degenerate and may ultimately be irreparably injured.

Provision is made from earliest life for the proper develop-
ment of these neurones and the establishment of irritable
connections between them by use; out of the first aimless
movements of the head and eyes and hands and legs of the
baby the simpler coordinating nervous mechanisms are one
by one brought to perfection ; then comes the training of
those reflexes which maintain the erect position and of those
nervous mechanisms which govern locomotion; then play
comes in, with its ceaseless activity, increasing still further
the number of movements which the nervous system can
make and correspondingly enlarging the possibility of human
achievement. As the child grows older the family calls
upon him to contribute some share to its life or support;
new activities, in the shape of chores about the house or the
farm, now share with play the work of the nervous system ;
activity becomes less general, more special. Finally the
youth settles down to some definite occupation or pursuit,
and the more strictly this is adhered to, the narrower be-
comes the range of activity; the more constantly a few sys-
tems of neurones are used, the more rarely are others called
into play.

18. The physical basis of habits. All this indelibly writes
its history in the nervous system. No fact is more significant
or of greater physical and moral import than that the doing
of any act so affects the connections of neurones with one
another as to make it easier to do the same act again under

THE NERVOUS SYSTEM 285

the same conditions ; that refraining from doing something
toward which we are inclined similarly renders more easy
the inhibitory processes concerned when the same conditions
impel us toward it again. We are largely what we make
ourselves by the training which our actions give to the
nervous system.

And what activity thus does for the development of power
it does also for the maintenance of power. An efficient nerv-
ous mechanism of any kind once acquired does not remain
efficient without use. The man who has developed a rugged
constitution in colder climates and then lives for years in
the tropics, constantly exposed to a warm climate, finds on
return to the home of his youth that the mechanism of heat
regulation does not readily adjust itself to cold damp winds
and blizzards; the athlete who has learned to execute the
greatest variety of " tricks " in the gymnasium and then
settles down to a sedentary life finds after some years that
he is almost as helpless as the man who gave no attention
to such training. It is unnecessary to multiply examples.
Efficiency in any direction is the result of continued use of
organs and especially of continued training of the nervous
system. As we fit ourselves to do some few things, and to
do them well, we have not time to conserve by use the effi-
ciency of all the nervous mechanisms we have acquired ; we
must to some extent sacrifice the more general actions for
those which are more special and useful. But it must not
be forgotten that this can be carried too far ; that a certain
amount of general activity is a condition of healthy living and
that one of the problems of life to solve, and to solve aright,
is how to distribute our activity between the two. To the
consideration of these questions we shall return in our study
of personal hygiene.

CHAPTER XVI
FOOD ACCESSORIES, DKUGS, ALCOHOL, AND TOBACCO

1. Food accessories and drugs. Through the alimentary
and respiratory tracts there are received into the blood not
only substances such as proteins, gelatin, fats, carbohydrates,
salts, and water, which we have described as supplying the
material for power and for growth and repair, but also other
substances capable of modifying in one way or another the
course of events within the body. The flavors which con-
tribute to the enjoyment of foods play an important r61e in
the secretion of the gastric juice, and yet the substances
which cause these flavors are negligible as sources of power.
Salt belongs under the same head, for we use in cooking
more salt than is needed to make good the daily loss from
the body, and we do this to develop an agreeable flavor in
our food. Substances of this kind are spoken of as food
accessories, and among them must be included coffee and tea,
for their effect is not chiefly a matter of nutrition ; certain
constituents of tea and coffee absorbed into the blood affect
the nervous system, and it is largely for this reason that we
use them.

We may pass in this way from the necessary food acces-
sories through those, like coffee and tea, which, while not
essential, may still be regarded as part of the food of a large
portion of mankind, to the great number of chemical com-
pounds known as drugs, which also act by changing the
course of events within the body ; and it is difficult to draw
any sharp line of distinction between those which occasionally
serve as medicine or " stimulants " and those of which daily
use is made as food accessories.

286

FOOD ACCESSORIES AND DRUGS 287

Animals as a rule take substances into their bodies only
to satisfy hunger or thirst or appetite ; man alone takes, in
addition to his nutriment, food accessories and drugs for
the sake of their special effect upon the nervous system or
other organs. Many of the numerous food accessories which
human ingenuity has discovered or devised are harmless
enough in the form used, but others contain substances
which are capable of poisoning the body. It is an important
part of the study of personal hygiene to learn of what
these substances consist, what is their action on the human
organism, and wherein lie their special dangers.

2. The drug habit. It is a lamentable fact that large
amounts of drugs are swallowed by men and women apart
from any medical need which compels their use. In a sub-
sequent chapter we shall show reasons for avoiding an undue
dependence upon drugs as a remedy for various minor ills.
Bad as this practice is, with its tendency to rely upon the
uncertain action of a drug instead of taking proper hygienic
care of the body, it is far worse to make habitual use of
drugs for their special effects upon the healthy body, for
the habit is one which is only too easily cultivated. There
is no reason why a healthy human being, living a normal
lif e amid healthful surroundings, should need to use drugs
habitually, and a little consideration will show that the
practice is dangerous.

3. Dangers of the drug habit. When we eat meat or vege-
tables, or when we breathe air, we take into the body
materials needed for normal living. These things have
always formed part of the food of the race and, unless
wrongly taken, do good and not harm. When, on the other
hand, we take a drug, such as chloroform, or cocaine, or
opium, or alcohol, or coffee, or tea, we take something which
is foreign to the body, in so far as it has not been a regular
constituent of animal food in the past. It is not needed, as
protein and salt and water are needed; there is no special

288 THE HUMAN MECHANISM

preparation for its reception ; and while it may do good,
there is danger that it may do harm.

In the second place, the exact action of many drugs is
only imperfectly understood. In an emergency the physi-
cian uses them temporarily, for some effect which he desires
to produce, thus tiding over a difficulty. He uses the drug
only a few times, at most, and is consequently not greatly
concerned about unfavorable attendant effects ; it accom-
plishes some needed purpose, and if it does any harm, the
organism may be trusted to recover from it. It is very dif-
ferent, however, with the habitual use of any drug. The very
fact that it gives some new direction to the events taking
place within the body means that abnormal conditions of life
are being maintained, and we have already learned that
abnormal conditions of life are apt to be unhygienic.

Again, the use of drugs is only too apt to be substituted
for the hygienic conduct of life. We may, for example,
take drugs to accomplish something which the healthy body
should accomplish for itself without outside help. When
one drinks a cup of black coffee to facilitate mental work
which his fatigued condition would not otherwise allow
him to do, he is trying to get from a drug the power which
he could and probably should secure by normal sleep. The
coffee acts like a whip to a tired horse ; the same work
is done as might have been done had the horse been allowed
a little rest, but the horse is not as well off when he does
the work under the lash as when he does it in a properly
rested condition. Similarly, persons suffering from sleepless-
ness often take drugs used to produce sleep (hypnotics),
and, superficially at least, the sleep thus secured resembles
normal sleep; but experience shows that few if any hypnotics
can be used for any length of time without bad effects.
Here again a drug is being depended upon to do what the
normal body should do for itself. Pepsin tablets may be
taken to aid digestion, and thereby an attack of indigestion

FOOD ACCESSORIES AND DRUGS 289

may sometimes be prevented or relieved; but a healthy
stomach should furnish its own pepsin, and the fact that it
does not do so is a sure warning that something is wrong
in the conduct of life. It is irrational to neglect the duty
of attending to the cause of the ailment, and it is foolish to
substitute temporary relief for permanent cure. Perhaps if
the drug did all that the proper care of the body does, and
did no more, no serious objection could be made to its use ;
but there is probably no drug of w^hich this is true, and for
this reason it is foolish and rash to try to substitute the use
of drugs for the hygienic conduct of life.

Lastly, if the drugs do not accomplish in the long run
what should be done by the hygienic conduct of life, their
extensive use becomes all the more dangerous in view of
the unquestioned fact that we are apt thereby to become
their slaves. Every man is the slave, broadly speaking, of
the habits he forms, and it is only a question as to whether
he will be the willing slave of good habits or the abject
slave of bad habits. The man who leads a hygienic life is
the slave of muscular activity, of correct feeding, of proper
clothing, of rest, etc. ; that is to say, these things become
necessary to his life ; he cannot get along without them.
If for these proper agents of health he persistently sub-
stitutes some drug, whether it be alcohol, or tobacco, or
coffee, or tea, or chocolate, or opium, the habit of using
the drug is substituted for that of maintaining normal
conditions. But since drugs cannot entirely take the place
of such conditions, the constitution goes from bad to worse,
and increasing dependence must be placed upon the drug.
It is a safe rule that whenever we are uncomfortable or
unhappy without the use of a certain drug we should cease
using it until, with the help of hygienic living, we can get
along without it.

There are people who are slaves of coffee, of tea, of
chocolate, of patent medicines, of candy, and of soda water

290 THE HUMAN MECHANISM

just as truly as there are slaves of tobacco, or of alcohol,
or of opium. It is worse to be the slave of alcohol than of
coffee, because the evil consequences of alcohol are greater
than those produced by the corresponding use of coffee;
but it is by the same process in both cases that the man
or woman becomes a slave to the drug, and that process is
the formation of bad habits.

With these practical considerations about the use of drugs
— by which term it will be seen that we mean not simply
the medicines purchased from the apothecary but all those
substances which are taken into the body in order to give
some new or abnormal direction to the course of events in
the organism — we may pass on to the discussion of those
in common use.

4. Tea and coffee. Different as are these drinks in taste
and appearance, their most important physiological effects
are due essentially to the same substances ; namely, caffeine
(or theine) and tannic acid (or tannin). Caffeine is a very
powerful stimulant, especially of the nervous system and
also of the heart, although probably to a lesser degree ;
tannin, on the other hand, is a bitter, astringent substance,
which may considerably hinder digestion and directly injure
the mucous membrane of the stomach. Tea contains about
twice as much tannin as an equal weight of coffee, but as
coffee is frequently made much stronger than tea, the actual
amount per cup may often be more nearly equal in the two
drinks than these figures indicate. The amount of tannin
dissolved in tea varies greatly with the method of prepara-
tion, and largely for this reason tea should not be boiled
nor allowed to steep too long. The proper method of making
tea is to pour over the dry leaves water which has been
brought just to the boiling point and then to allow the
infusion to stand, without further heating, for not more
than a few minutes.

Both tea and coffee seem to have a slightly retarding

FOOD ACCESSORIES AND DRUGS 291

influence upon gastric digestion. In healthy people this
is of little consequence, but when the digestive powers are
in any way impaired, the use of these beverages may be
inadvisable. The more important effect, however, of both
tea and coffee is in their stimulating action on the nervous
system. No satisfactory explanation has yet been given of
the fact that some people can use tea and not coffee, while
with others the reverse is true. It is probably safe to say
that when used in moderation, tea and coffee are usually
harmless to those leading an otherwise hygienic life. They
should be used sparingly by nervous people and by those in
whom digestion is feeble and slow (Hutchinson). Even by
the perfectly healthy they should not be used to excess, nor
should the habit be acquired of using them as the whip to
the tired horse. Drinking strong coffee in order to keep awake
for evening study is objectionable, and the substitution of
afternoon tea for a little rest or sleep is also unwise.

5. Cocoa is made from the seeds of trees of the genus
Theobroma, and chocolate is prepared . from cocoa. In the
solid form both are highly nutritious, as shown by the
following average results of analyses:

PROTEIN FAT CARBOHYDRATE

Cocoa 21.6% 28.9% 37.7%

Chocolate 12.9% 48.7% 30.3%

When used as a beverage, however, the nutriment derived
from them is small. In addition, cocoa and chocolate both
contain theobromine, a substance closely related chemically to
caffeine and possessing much the same stimulating proper-
ties. In general, the same hygienic considerations which
apply to the use of tea and coffee should guide us also in
the use of chocolate and cocoa.

6. Soda water and similar beverages. Of these little need
be said. In general, they are harmless enough, especially to
those enjoying perfect digestion. The large amount of sugar

292 ' THE HUMAN MECHANISM

which they contain is apt to make matters worse in many
cases of dyspepsia; by taking them frequently between
meals the appetite for wholesome food is impaired, and
excessive indulgence in them under any circumstances is
needless and foolish.

7. Alcoholic beverages. In the case of an alcoholic drink
we have to deal with something which, like tea and coffee
and cocoa and " temperance drinks," is used as a beverage,
and to that extent must be classed in the same group.
Alcoholic drinks are, however, taken as stimulants and so
resemble tea and coffee and cocoa, but they differ from all
of these in their action upon the body. Moreover, their
abuse gives rise not only to degraded moral and social
conditions, but is also attended with bad hygienic effects.
Everyone should be informed of their nature and of the
dangers attending their use.

The common alcoholic beverages consist of (1) malt
liquors, including beer and ale; (2) wines, such as hock,
claret, Burgundy, sherry, and champagne ; (3) distilled
liquors, including brandy, whisky, rum, and gin ; and
(4) liqueurs and cordials. These groups are distinguished
from one another largely by the method of preparation and
by the amount of alcohol they contain. Malt liquors are
fermented liquors which contain from three to eight per cent
of alcohol; wines are also fermented liquors, but contain
from seven to twenty per cent of alcohol; distilled liquors,
on the other hand, are first fermented and then concen-
trated by distillation, and contain from thirty to sixty-five
per cent of alcohol. In all these the most important con-
stituent, so far as their physiological action upon the body
is concerned, is the chemical compound known as ethyl
alcohol (C2H6O or C2H6 . OH).

8. Fermentation. The ethyl alcohol in each of these bev-
erages is produced by the action of yeast on sugar, and
this action is known as alcoholic fermentation. Yeast is a

FOOD ACCESSORIES AND DRUGS 293

unicellular plant, and when a small amount of it is added
to a solution of grape sugar or fruit sugar, it breaks up these
substances, chiefly into alcohol and carbon dioxide gas. The
latter passes off, while the alcohol remains behind in the
solution. In addition to these chief products of fermenta-
tion there are always formed other products in small quan-
tities, and to these, in part, the flavor of the fermented
mixture is due. Different varieties of yeast produce dif-
ferent kinds of fermentation. Thus one variety (domesti-
cated yeast) is used in making beer, and another (wild
yeast) in making wine. The amount of alcohol produced
differs with the yeast used, as do
also the character and quantity of
the secondary products. The growth
of yeast, like that of all living fer-
ments, is checked by the accumu-
lation of the products of its own
activity. Consequently when the al-
cohol produced reaches a certain per-
centage (usually less than ten per FIG. 117. Yeast cells
cent) the fermentation ceases. Alco-
holic drinks which contain higher percentages of alcohol are
prepared by special processes, which will be described later.
9. Malt liquors. Malt consists of sprouted grains (chiefly
barley). The grains contain a large amount of starch which
during the process of germination is converted into sugar
by diastase, an enzyme produced by the living cells of the
plant — the action of diastase being essentially similar to
that of the ptyalin of the saliva. The germinating plant
thus comes to contain considerable quantities of sugar, to-
gether with salts, proteins, and other substances. The
watery extract of malt is known as wort, and it is this
which, after being boiled with hops, is acted upon by the
yeast. The liquid thus produced from wort by fermentation
is known as ale, beer, stout, porter, etc., according to the

294 THE HUMAN MECHANISM

conditions under which the fermentation takes place and
the character of the malt and the yeast employed. German
beers contain from three to four per cent of alcohol; ale
contains from four to six per cent.

10. Wines. Wine is produced by the fermentation of the
juice obtained by crushing grapes, and the yeast comes from
the "bloom" on the skin of the grapes. The juice, or "must,"
thus extracted is allowed to undergo fermentation, and the
fermented liquid is wine. Most wines, however, are sub-
jected to subsequent treatment. Some are allowed to ripen
in wooden casks, during which process there take place
chemical changes which give to each wine its peculiar flavor.
In other cases the wine is "fortified" by the direct addition
of alcohol. Wines differ from one another according to the
variety of the grape used in making the must, according to
the variety of yeast used for fermentation, and according to
other circumstances.

11. Distilled liquors and spirits. This group of alcoholic
beverages contains the highest percentage of alcohol, and
includes whisky, brandy, rum, and gin. In the making of
all of these the essential procedure is the same ; namely,
first to produce fermentation in some sugary liquid and
afterwards to distill from the products of this fermentation
its alcohol and some other volatile constituents. Whisky is
made by distilling fermented corn or rye ; brandy may be
spoken of as distilled wine; rum is distilled from fermented
molasses, and gin from a fermented mixture of rye and
malt — juniper berries and other substances being added to
the distilled product. In general, distilled liquors contain
from thirty to sixty per cent of alcohol.

With these differences of preparation, alcoholic beverages
differ greatly among themselves, independently of the quan-
tity of alcohol they contain, and some of their special effects
are due to other constituents. The chief danger of most of

FOOD ACCESSORIES AND DRUGS 295

them, however, lies in the action of the ethyl alcohol upon
the system, and we shall confine our discussion to the effects
of this substance. The problem is by no means a simple
one, because these beverages are used in so many different
ways by different people. Moreover, the results of their use
differ according to the constitution of the person using them
and according to his other habits of life. Sweeping assertions
are too frequently made, in good faith, only to be found false
by experience in special cases, and in this way harm is done
where good was intended. For example, it is often asserted
that alcohol used in any amount whatever is a poison to the
healthy organism. If this be so, it is the only known drug
of which this is true. Dr. John J. Abel, from whom we
shall extensively quote, says on this subject: "All poisons
are capable of being taken without demonstrable injury in
a certain quantity, which is for each of them a special
though sometimes very minute fraction of their toxic or
lethal dose. , There is no substance which is always and
everywhere a poison." Alcohol is a drug and, like many
drugs, may be and frequently is used in poisonous doses,
but it must not be supposed that its real danger lies
in the fact that it always exerts a poisonous effect on
the body.

12. The physiological action of alcohol. As to the imme-
diate action of alcohol on the body we may say that it
belongs in the same general class of drugs as the ether and
chloroform used for anesthesia; in other words, its general
action is that of a hypnotic or anesthetic. To quote again
from Dr. Abel:

An exhilarating action is an inherent property of these substances
in certain doses. Occasionally the physician meets with persons who
have formed the habit of inhaling chloroform from the palm of the
hand or from a lightly saturated handkerchief. The inhalation is
usually carried on for a short time only, and its object is to induce a
pleasant form of mental stimulation. Only occasionally is the inhalation
of chloroform carried on until helpless intoxication occurs.

296 THE HUMAN MECHANISM

And again:

That alcohol can produce as profound anesthesia as any of the sub-
stances named is also well known. In the days before anesthesia it was
the custom of bone setters to ply their patients with alcohol in order to
facilitate the reduction of difficult dislocations. . . . The anesthesia pro-
duced by alcohol is, however, not commendable, since it cannot safely
be induced in a short time and is too prolonged. The quantity needed
for surgical anesthesia would in many cases lead to a fatal result.

13. Is alcohol a stimulant? The view of the action of
alcohol just stated is, of course, borne out by the condition
of a thoroughly intoxicated person ; but it is opposed to the
very general idea that alcohol, except in large doses, is to be
regarded as a stimulant. Whether we shall call it a * stimu-
lant" or not depends upon how we use that term. Some of
the exhilarating effects of alcoholic drinks might lead us to
speak of it hi this way. People who have drunk wine often
become more talkative, so that the first effects of intoxica-
tion often resemble those of stimulation. There is, however,
strong reason for thinking that this action is only super-
ficially, and not fundamentally, a case of stimulation, as we
shall now see.

In studying the physiology of the nervous system we
lound that processes of inhibition are as important in its
operation as are those of excitation ; and in mental opera-
tions the course of our thinking is constantly checked or
inhibited by the knowledge of facts opposed to the con-
clusions towards which we are tending. Probably it is this
essential feature of all accurate and valuable mental work
which is the first to be paralyzed by alcohol. The man who
takes alcohol becomes fluent not because he is stimulated
but because of the removal of checks whose presence may
make him talk less fluently, but which at the same time
make him speak more accurately. He may become witty,
and may say some brilliant things, but he will almost
always do and say some very erratic things.

FOOD ACCESSOBIES AND DEUGS 297

The following (by Dr. Abel) appears to be a sound state-
ment of our present knowledge of this important subject:

Alcohol is not found by psychologists to increase the quantity or
vigor of mental operations ; in fact, it clearly tends to lessen the power of
clear and consecutive reasoning. In many respects its action on the higher
functions of the mind resembles that of fatigue of the brain, though with
this action is associated a tendency to greater motor energy and ease.

In speaking of a certain type of individual James says : " It is the
absence of scruples, of consequences, of considerations, the extraordinary
simplification of each moment's outlook, that gives to the explosive in-
dividual such motor energy and ease." This description aptly applies
to the individual who is under the influence of a " moderate " quantity
of alcohol. It tends to turn the inhibitive type of mind into the " hair-
trigger " type. We have said that the speech and the bearing of men,
the play of their features, all bear witness to the action of alcohol on
the brain ; that it removes restraints, blunts too acute sensibilities, dis-
pels sensations of fatigue, causes a certain type of ideas and mental
images to follow each other with greater rapidity, and gives a "cerebral
sense of richness."

Larger quantities, such as are for most individuals represented by
one or two bottles of wine (ten per cent of alcohol), may, according to
the resistance and type of individual in question, cause a lack of con-
trol of the emotions ; noticeably affect the power of attention, of clear
judgment and reason ; and decidedly lower the acuteness of the several
senses. In many individuals such quantities will develop so marked an
anesthetic action that all phenomena of intoxication may be seen to
follow each other in due sequence, finally to end in the sleep of
drunkenness.

There has been much discussion as to whether alcohol is in any
sense a stimulant for the brain. We have seen that pharmacologists of
high repute deny that it has this action, holding that alcohol is a seda-
tive or narcotic substance which belongs to the same class as paralde-
hyde and chloroform ; that its stimulating action is but fictitious ; and
that even the earlier phenomena of its action are to be referred to a
paralyzing action on cerebral (inhibitory) functions. This theory as-
sumes an unequal action on cerebral functions in the order of time.
Kraepelin, however, holds that this is a purely subjective analysis, and
that in the early stages of its action alcohol truly stimulates the motor
functions of the brain ; that a state of mental exhilaration, of " motor
excitability," may coexist with undiminished power of perception and
judgment. His psychological experiments on the action of alcohol,
taken all in all, do not, however, entirely prove his position.

298 THE HUMAN MECHANISM

Some cases of apparent stimulation are really due to the
fact that alcohol, when taken in the form of wines and dis-
tilled liquors, sets up an irritation in the mucous membrane
of the mouth, oesophagus, and stomach, which reflexly excites
the heart to greater activity or for the time being reflexly
stimulates the nervous system. Such stimulation is, however,
transient and, as the alcohol is absorbed into the blood, gives
way to depression and even stupor.

It is neither possible nor necessary to state here in full
the reasons which have led to what seems to the authors
the erroneous view that alcohol in small doses is a -stimu-
lant and only in larger doses a depressant and hypnotic.
Enough has been said to show that there are at least two
opinions about the matter: that even if alcohol is at times
a stimulant, it is an uncertain stimulant, and that its excita-
tion is liable to give way at any time to depressing effects.
A critical examination of the literature on the subject has
failed to demonstrate to us a direct stimulating action of
alcohol on any of the functions, such as the beat of the
heart, respiration, digestion, etc. At tunes, especially in sick-
ness, alcohol may be useful; but the evidence tends to the
conclusion that where it exerts any physiological action
on the healthy body at all, that action is usually depress-
ing. This is notably true as to the beat of the heart, as to
respiration, and as to the ability to do muscular work.

We have dwelt at length upon this question in order to
disabuse the student's mind of the idea that alcoholic drinks
can be safely depended upon as an aid in the performance
of work. Few causes are more effective in leading to the
abuse of alcohol than the idea that when one finds difficulty
in doing a thing it may be accomplished more easily by
having recourse to beer or wine or whisky for their " stimu-
lating" effect. In general, so far is this from being the
truth that the person seeking such aid is really using a hyp-
notic and depressant. Obviously he would be acting more

FOOD ACCESSORIES AND DRUGS 299

wisely to adopt other methods of accomplishing his end.
Nor is this conclusion merely theoretical. Brain workers
who wish to " keep a clear head " almost universally avoid
alcoholic drinks, at least until work is over.

14. Alcohol in muscular work. That the general effect of
alcoholic drinks is to depress rather than stimulate the
powers of the body is furthermore indicated by the results
of experiments on men doing heavy work, as, for example,
soldiers on forced marches. In the Ashanti campaign the
effect of alcohol as compared with beef tea was tested. To
quote from Sir Lauder Brunton :

It was found that when a ration of rum was served out, the soldier
at first marched more briskly, but after about three miles had been
traversed the effect of it seemed to be worn off, and then he lagged
more than before. If a second ration were given, its effect was less
marked, and wore off sooner than that of the first. A ration of beef tea,
however, seemed to have as great a stimulating power as one of rum, and
not to be followed by any secondary depression.

The results of these and other experiments lead us to
the conclusion that alcohol cannot be depended upon to in-
crease the capacity for hard muscular work and that in the
great majority of cases it actually diminishes it.

15. The dilation of cutaneous arteries by alcohol. One of
the most important effects of alcoholic drinks is the dilation
of the arteries of the skin, thus sending more warm blood
to the surface. It is a common experience among persons
not accustomed to alcoholic drinks that even a small amount
" makes the face hot " and flushed, and the red face of the
toper is proverbial. The result of this dilating effect is that
the temperature of the skin rises and the individual feels
warmer. Congested states of internal organs may thus be re-
lieved, and this is probably one reason why men leading an
exclusively sedentary life often use alcoholic drinks apparently
to some advantage. But even these would do infinitely better
to secure the same result by proper muscular activity.

300 THE HUMAN MECHANISM

Even if a temporary advantage appears to be gained
in some cases or at some times, this has often to be
paid for by bad secondary effects, such as impaired ca-
pacity for good work some hours later; and in mental
work of the highest kind, such as original writing or
composition, the after effects of alcoholic drinks are some-
times prolonged and easily detected by the subject of
the experiment.

16. Alcohol as a defense against exposure to cold. Because
of this effect upon the cutaneous circulation alcoholic drinks
are frequently used by men exposed to cold, with the mis-
taken idea that the conditions within the body are thereby
improved. The student has, however, learned (p. 193) that
a feeling or sensation of warmth does not necessarily indicate
greater heat production within the body; and he also knows
that bringing the blood to the skin when the body is ex-
posed to cold serves to increase the loss of heat. As a
matter of fact the internal temperature often falls when
alcohol is taken under these conditions. The story is told
of some woodsmen who were overtaken by a severe snow-
storm and had to spend the night away from camp; they
had with them a bottle of whisky, and, chilled to the bone,
some imbibed freely, while others refused to drink. Those
who drank soon felt comfortable and went to sleep in their
improvised shelter; those who did not drink felt very un-
comfortable throughout the night and could get no sleep,
but in the morning they were alive and able to struggle
back to camp, while their companions who had used alco-
holic drinks were found frozen to death. They had pur-
chased relief from their unpleasant sensations of cold at the
cost of lowering their body temperature below the safety
point. This, if true, was, of course, an extreme case; but
it accords with the universal experience of arctic travelers
and of lumbermen and hunters in northern woods, that the
use of alcohol during exposure to cold, although contributing

FOOD ACCESSOEIES AND DKUGS 301

greatly to one's comfort for the time being, is generally
followed by undesirable or dangerous after effects.

17. Alcohol as a food. There has been much discussion
as to whether alcohol is or is not a food; that is, whether
its oxidation within the body may supply energy. This
question must now be answered in the affirmative, although
whether it can do more than supply heat to maintain the
body temperature, — that is, whether it can also supply the
power for muscular work, as do fats and carbohydrates, — we
cannot in the present state of our knowledge positively say.
In many cases of sickness the oxidation of alcohol is prob-
ably a useful source of heat production, since it is absorbed
quickly and without digestion, but the healthy man does
not and should not use it in this way. The amounts which
would be required to be of any considerable service as food
are far beyond those in which it may be used with safety.
In other words, in using alcohol for food one would be ob-
taining heat at the cost of direct injury to many organs and
also at the cost of impaired working power. Moreover, men
do not use alcohol as a food ; they use it as a drug. So that
while the action of alcohol as a food is of practical impor-
tance to the physician, who must deal with the abnormal
conditions of disease, its action as a food is not a matter of
practical importance to healthy people.

18. Pathological conditions due to the use of alcohol.
When alcoholic beverages are taken in excessive amounts
we have the sad and degrading spectacle of a " drunken
spree." Whether or not the drinker at first appears bright
or witty, sooner or later there is presented the pitiable pic-
ture of complete loss of nervous coordination and control.
The man becomes silly, or maudlin, or pugnacious, as the
case may be, but always irrational; he staggers, stumbles,
or falls ; and finally passes into a drunken stupor. In this
event the victim of his own indulgence is said to be
"dead" drunk, or "intoxicated," being as it were thoroughly

302 THE HUMAN MECHANISM

poisoned. If such intoxication is frequently repeated, there
is a complete breakdown of the nervous system ; the victim
of alcoholic indulgence becomes a raving maniac and, with
disordered vision, thinks he sees all about him snakes or
foul vermin (delirium tr emeus). The silly or foolish stage
of this poisoning sometimes provokes smiles or laughter in
thoughtless observers, but none can witness the more serious
consequences of repeated intoxication by alcoholic drinks
without disgust and horror.

Many steady drinkers, even though they have never been
drunk in their lives, are apt ultimately to acquire various
diseased conditions of the body, into which we cannot enter
in detail. The heart may be injured, or the arteries become
diseased ; the repeated irritation of the stomach may produce
chronic gastritis ; or the connective tissue of the liver and
kidneys may increase, thus crowding upon the living cells
and ultimately throwing a large part of them entirely out of
use. While it must not be supposed that drinking alcohol is
the sole cause of these troubles, — for some or all of them
may come from other causes, — the frequency of their occur-
rence in steady drinkers is suspiciously high, and this has
led to the very strong conviction among medical men that
alcohol plays a large role in producing them.

19. Summary of the action of alcohol as a drug. In small
doses alcohol may be completely oxidized within the body
without exerting any pharmacological action. In the forms
and amounts usually employed in alcoholic beverages it
exerts, in general, a hypnotic or anesthetic action ; the result
on the system as a whole depends on the amount taken,
and varies from the paralysis of inhibitory processes to the
depression of all nervous functions, ending in drunken
stupor. Continued excess may produce exaggerated forms
of temporary insanity, among which delirium tremens may
be mentioned. There is, moreover, good reason for believing
that steady drinking is very frequently an important agent

FOOD ACCESSORIES AOT> DRUGS 303

in preparing the way for many other diseases, and is hence
a serious menace to health.

20. The seat of the danger in alcoholic drink. The regular
use of alcoholic beverages is dangerous for the same reason
that the regular use of any drug is dangerous. We are too
apt to rely upon the drug to do for us what we ought to
accomplish only by the hygienic conduct of life ; the drug
never satisfactorily does the work, and we go from bad to
worse, and become its slave. But there is certainly greater
danger in hypnotic drugs, like alcohol, than in true stimu-
lants, like coffee, and cocoa, and tea. We need to have
ourselves well under control when we use any drug; the
highest faculties of the mind must keep tight rein or we
may lose control of ourselves. With hypnotic drugs — to
which class belong not only alcohol but ether, chloroform,
opium, chloral, etc. — there is special danger that these
powers of control (inhibition) may be stealthily paralyzed
before we know it. Of course thousands of people use
alcohol in moderation and never become drunkards ; but
thousands also, with no intention of using it to excess, do
unconsciously let the reins drop, and before they know it
the drug gets the better of them. Experience shows that
it is with the hypnotic drugs that this most frequently
happens.

Again, if we make a habit of taking alcoholic drinks, we
are specially exposed to temptation from our fellow men to
go too far. For the most part, people take coffee and tea
or do not take them, as they please ; no one urges them to
use these drinks when they are disinclined to do so. To a
less degree the same thing is true of tobacco, although here
the force of fashion and example is stronger. But with
alcoholic beverages the custom of " treating " makes the
exercise of self-restraint more difficult than it would other-
wise be, for here we are dealing with a drug which is
capable of impairing self-control. Some one "treats" a friend

304 THE HUMAN MECHANISM

to a drink; the friend wishes to return the compliment and
so they drink again; the person with deficient self-control —
and what little he has now lessened — insists upon a third,
and so on, perhaps to intoxication. This, of course, does
not always happen; thousands are strong and escape the
danger, but thousands are weak or do not know better, and
many a week's wages has gone in this way, leaving behind
poverty and misery and impaired capacity before the close
of Saturday night.

21. Concluding remarks on the use of alcoholic beverages.
In the foregoing pages we have stated the salient facts
concerning the physiological action of alcohol and alcoholic
drinks. It only remains to point out for the student the
obvious conclusions to be drawn from them and from the
long and, on the whole, very sad experience of the race with
alcoholic drinks. The first is that except in sickness and
under the advice of a physician, alcoholic drinks are wholly
unnecessary and much more likely to prove harmful than
beneficial. The second is that their frequent and especially
their constant use is attended with the gravest danger to
the user, no matter how strong or self-controlled he may be.

It is true that history and romance and poetry contain
many attractive allusions to wine and other alcoholic drinks,
and it may also be true that such drinks, by loosening
tongues and breaking down social, political, or other barriers
(removing inhibitions), may tend towards conviviality and
good-fellowship ; but it is no less true that the path of his-
tory is strewn with human wreckage directly due to alcohol;
that many a promising career has been drowned in wine;
and that indescribable misery follows in the trail of drunken-
ness. The only absolutely safe attitude toward alcoholic
drinks is that of total abstinence from their use as beverages.

22. Opium, morphine, and the opium habit. The danger
of the use of drugs as a regular habit of life is perhaps
most painfully illustrated by what is known as the opium

FOOD ACCESSORIES AND DRUGS 305

habit. Among the most valuable remedies at the physician's
disposal is opium or its active principle, •morphine, which
possesses remarkable power to produce insensibility to pain.
It sometimes happens, however, that by incautiously using
this drug for this purpose men and women become addicted
to the habit. They finally cannot do without the drug, and
its constant use causes an appalling moral and physical de-
generation ; so far indeed does this often go that the victim
will commit crime in order to obtain the drug. It should
be clearly understood that it is unsafe for anyone to use
opiates to relieve pain; indeed, these should never be used
except when prescribed by a careful physician.

23. Chloral, cocaine, etc. Men and women may become
slaves to the use of other drugs and in much the same
way as they become slaves to alcohol and morphine. Among
these drugs are chloral and cocaine. They belong in the
same general group of hypnotics or anesthetics, and the
habit acquired is perhaps no worse than the opium habit.
It is certainly very little better. Let the student remember
that the root of the evil here, as elsewhere, is the substitution
of the use of the drug for normal habits of healthful living.

24. Tobacco. The physiological effects of tobacco are quite
complicated, so complicated that it is difficult to make
general statements with regard to them. The effects of
chewing are quite different from those of smoking, and
those of smoking, no doubt, vary according as the smoke
is or is not drawn into the lungs (inhaled).

The leaf of tobacco contains a poison (nicotine) which
exerts a powerful action on the heart and on nerve cells.
It is not, however, proved that the bad effects of the use
of tobacco are due entirely or even chiefly to this substance,
but it unquestionably contributes to the physiological effects.

The smoke from tobacco also contains ammonia vapor
which locally irritates the mucous membrane of the mouth,
throat, nose, etc., and this irritating action at times acts.

306 THE HUMAN MECHANISM

as a stimulant to the whole system in much the same
manner as do " smelling salts."

It has been recently suggested that, owing to the incom-
plete character of the combustion, tobacco smoke contains
a small amount of the poisonous gas carbon monoxide (CO),
and it is quite possible that some effects of smoking —
especially where the smoke is drawn into the lungs (inhaled)
— may be attributed to this gas; but the suggestion has
not yet been submitted to the test of actual experiment.

Indeed, the physiological action of tobacco probably not
only varies with the form in which the tobacco is used but
is in any case the result of a combination of a number of
factors partly physiological and partly psychical. We must
here, however, confine our attention to the purely hygienic
aspects of the matter.

Human experience shows that the unwise use of tobacco
may unfavorably affect digestion, cause serious disorders of
the heart, and impair the work of the nervous system. Those
training for athletic events are usually forbidden the use of
tobacco because it " takes the wind " ; that is, makes impossi-
ble the most efficient training of the heart. Many employers
have found that youths who smoke cigarettes are less reliable
in their work ; and this is only one instance of the effect
upon the nervous system already referred to, the same result
being observed in a diminished steadiness of the hand, often
amounting to actual tremor.

These effects do not, of course, manifest themselves in
their extreme form whenever tobacco is used, but it is
probable that they are always present in some degree.
Whether they are noticeable or not depends largely upon
the ability of the constitution to resist them. Tobacco is
thus often used without demonstrable bad effects when one
is leading a hygienic life ; but very often the habit, formed
under these conditions, persists after the increasing intensity
of occupation and the attendant cares and responsibilities

FOOD ACCESSORIES AND DRUGS 307

of life result in neglect of muscular exercise and improperly
directed nervous activity. As this neglect begins to tell on
general health it is found that the unfavorable effects of
tobacco become more pronounced.

Especially to be condemned is its use by those who have
not attained their full growth. During youth nothing should
be allowed to interfere with the best development of the
heart and nervous system, and the use of tobacco endangers
the proper development of both of these most important
parts of the human mechanism. It can hardly be doubted
that many a young man has failed to make the most out of
life because the habit contracted in youth has struck in this
way at the foundations upon which he had subsequently
to build.

FIG, 154. The thoracic and abdominal cavities, after the removal of
the organs shown in Fig. 2

The diaphragm has been drawn somewhat forward

I —

FIG. 155. General view of the digestive tract. After Spalteholz

A, mouth cavity ; B, pharynx; (7, oesophagus ; D, diaphragm; E, stomach;
FI small intestine ; G, ascending colon ; H, descending colon ; /, rectum.
The transverse colon has been cut away, its position being indicated by
dotted lines

FIG. 156. The flouncelike folding of the mesentery, as seen after
removing the small intestine. After Spalteholz

•TV* •"• "• • i *.
:* •••" :'•":"•: :/•:

FIG. 157. Median dorso-ventral section of the trunk in the abdominal
region, showing the suspension of the stomach and intestine by the
mesentery. After Spalteholz

A, liver; B, stomach; C, transverse colon; D, mesentery; E, rectum;
r F, urinary bladder

FIG. 158. The permanent teeth in the jaw-bones, viewed
from the right. After Spalteholz

FIG. 159. The network of capillaries on the lining of the
air cells of the lungs. After Kolliker

See page 169

A— 4

FIG. 160. First layer of muscles of the breast and shoulder region.
After Spalteholz

A, biceps of the arm (p. 33) ; B, deltoid; C, portion of the trapezius (see
Figs. 113 and 114); D, clavicle; E, sternum or breastbone; F, pecto-
ralis major (see p. 316 and Fig. 114)

— H

— K

FIG. 161. Second layer of muscles of the breast, exposed by dissecting
away the pectoralis major in Fig. 160. After Spalteholz

A, B, the two " heads " of the biceps ; C, cut end of the pectoralis major ;
D, deltoid ; E, pectoralis minor ; F, trapezius ; G, clavicle ; H, first rib ;
X, sternum. Note the direct attachment of the intercostal muscles to
the ribs (p. 8). Compare Fig. 160

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The parts of the nervous system
represented are the cerebrum,
cerebellum,bulb,and segment
of the spinal cord. Afferent
nerves in red, efferent nerves
in black, m, m, motor neu-
rones to some of the muscles
of the leg. These may be
stimulated to coordinate ac-
tion by neurones (v) from
the cerebrum, neurones (cb)
from the cerebellum, or by
the afferent neurones (a/1)
from the tendons, etc. In the
bulb this afferent neurone
connects with a second neu-
rone (a/2), and this with a
third (a/3), thus providing
the path to the cerebrum and exciting
in consciousness sensations of position
of the leg (muscular sense). The same
neurones connect with the cerebellum,
as do also neurones from the inner ear.
For further explanations see Chapter
XV, pp. 275-279.

Dia^am jTfJtlie Nervous mechanism of walking

FIG. 166. Side view of the brains; of; i^bfc£t,\c
See pa?e 267 "

INDEX

Abdominal breathing, 172

Abdominal cavity. See Peritoneal
cavity

Abdominal muscles, action of, in
breathing, 174

Absorption from the intestine, 126

Accommodation, in vision, for near
objects, 243 ; muscle of, 244 (fig.)

Adipose tissue, 184, 222

Adrenal glands, 64

Adrenalin, action of, 64, 163

Aerial blanket, 198

Afferent impulses, 76 ff . ; reflex and
conscious effects of, 275 ff.

Afferent neurones, 76, 78

Air, stagnant, 198

Air cell, 169

Albuminoids, 94

Alcohol, physiological action of,
295 ; as a stimulant, 296 ; in mus-
cular work, 299 ; as a defense
against cold, 300 ; as a food, 301 ;
pathological conditions due to,
301 ; influence on self-control,
303-304

Alcoholic beverages, composition of,
292

Alimentary canal, structure of, 20,
107, 118, 128

Alimentation, 91, 98

Alveolus of gland, 30, 31 (fig.), 32
(fig.) ; of lungs, 169, 170 (fig.)

Amino-acids, 102

Amoeba, amoeboid movement, 137

Arnylopsin, 120

Anesthetics, 280

Animal foods, 97, 111

Ankle, bones of, 19

Anterior, definition of term, 9

Aorta, 13, 22, 23 (fig.), 25, 145

Apical lobes of lungs. See Lungs

Appendicular skeleton, 18

Aqueduct of Sylvius, 265

Aqueous humor,, 243

Arborization. See Synapse

Arterial reservoir, 142

Arterial tone, 161
Arteries, 21, 37, 148 (fig.)
Astigmatism, 249
Auditory nerve, 27, 256
Augmentor nerves of heart, 160
Auricle, 21, 23 (fig.), 140
Auriculo-ventricular valves, 141
Automatic nervous actions, 81
Axial skeleton, 14
Axon, or axis cylinder, 72, 75

Bile, 121

Bile duct, 108 (fig.)

Bladder, urinary, 181, 182 (fig.)

Blood, arterial and venous, 24, 166 ;
as a common carrier, 135 ; micro-
scopic structure of, 136 ; distribu-
tion among organs, 146 ; gases of,
166

Blood corpuscles, red, 136-138 ; as
carriers of oxygen, 167

Blood corpuscles, white, 136-137

Blood plasma, 138 ; gases of, 166

Blood vessels, 37. See also Arteries,
Capillaries, and Veins

Body cavity, 10

Bone, 36. See also Skeleton

Brain, 27 ; the seat of sensations,
241 ; of frog, 264 ; of mammal,
265 ; histological structure of, 268 ;
functions of, 270 ff.

Breastbone, 17

Breathing movements, 171 ff. ; effect
on circulation, 148, 175 ; effect on
flow of lymph, 150, 175 ; hygiene
of, 174

Breathlessness, 178

Bronchiole, 170 (fig.)

Bronchus, 12, 21, 169

Bulb, 264 (fig.), 265, 267 (fig.);
functions of, 272

Caffeine, 290

Calorie, 216

Canal, spinal, or vertebral, 17

Capillaries, 24 (fig.), 27, 31 (fig.), 37

309

310

ELEMENTS OF PHYSIOLOGY

Capsule of gland, 30

Carbohydrates, 94; digestion of,
104, 112, 120 ; as source of power
for work, 217 ff. ; fuel value of,
217; as food in cold climates,
220 ; as source of fat, 224

Carbon dioxide (carbonic acid),
formed during muscular work,
48, 60, 178, 299; in lymph, 167;
in blood plasma, 168 ; action of,
on respiratory center, 176

Cardiac region of stomach, 107

Cell walls in plants, 98

Cells, 31, 32, 34, 36, 39, 40 (fig.), 41 ;
as chemical factories, 46 ff . ; waste
and repair of, 229 ff.

Cellulose, 97

Central canal of spinal cord, 264

Cerebellum, 264 (fig.), 265, 267, 270
(fig.), 279

Cerebrum, 82, 264 (fig.), 266 (fig.),
267 (fig.), 268 (fig.); connections
with other parts of the nervous
system, 275 ; functions of, 279 ff.

Cervical vertebrae, 14

Chloral, 305

Chloroform, 280

Chocolate, 291

Choroid, 243, 244, 247 (fig.)

Chyme, 117

Ciliary muscle, 244

Ciliary region of eye, 244 (fig.)

Cinders removed from eye, 394

Circulation, organs of, 21, 143 (fig.) ;
time of, 136 ; mechanics of, 139 ;
in warm and cold weather, 152,
201 ; during muscular activity,
154 ; during mental work and
sleep, 155 ; during digestion, 157,
158 ; nervous factors of, 159 ff. ;
essential to respiration, 177

Clavicle, 19

Cleavage, chemical, in muscular con-
traction, 50 ; of starch and protein
in digestion, 102

Climate, and mental work, 207 ; and
feeding, 220

Coagulation of proteins, 93

Cocaine, 305

Coccygeal vertebrae, 14

Cochlea, 257

Cocoa, 291

Coffee, 290

Cold, effect on circulation of the
blood, 152 ; effect on body as

a whole, 201 ff. ; sensations of,
258

Cold-blooded animals, 191

Collagen, 94

Collar bone. See Clavicle

Collaterals, 78

Colon, 20

Color, sensations of, 252

Compensatory adjustments of the
circulation, 153

Conditioned reflexes, 85

Conduction of heat, 211

Connective tissues, structure of, 7, 8,
37, 222 ; of glands, 30, 31 ; of
muscles, 33-34 ; relation of, to
blood vessels, 36 ; relation of, to
lymphatics, 39 ; of nerves, 71 ; di-
gestion of, 110; of lungs, 169; of
skin, 184

Consciousness, 240, 273, 279

Constant temperature of the body,
190 ; maintenance of, 201 ff.

Constipation, 132

Consumption. See Tuberculosis

Contagious diseases. See Diseases

Contraction of musclB, 33, 47, 56

Convection of heat, 212

Convolutions of cerebrum, 266 (fig.),
267

Coordination, 70, 83, 271 ; training
in, 87 , 283 ; by chemical means, 89

Corium, 184

Cornea, 243, 244 (fig.), 247 (fig.)

Corpuscles. See Blood corpuscles

Cortex of cerebrum and cerebellum,
269

Costal breathing, 172

Cranial nerves, 268

Cranium, 17

Curd of milk, 93

Curvatures of vertebral column,
14 ff., 321

Curve of fatigue, 56

Cutaneous sensations, 258

Cutis, 184

Cytoplasm, 31, 35, 41, 75

" Danger zone " of atmospheric tem-
perature, 205

Dendrites, 75, 268, 270, 271

Dermis, 184

Dextrines, 101 (fig.), 105

Dextrose, 101

Diaphragm, 10 ; action in respira-
tion, 171-173

INDEX

311

Diarrhea, 133

Diastase, 293

Diastole, 140, 142 (fig.)

Diet, value of a mixed, 231

Digestion, organs of, 20 ; nature of,
100 ; external and internal, 100 ;
in the mouth, 103 ; in the stomach,
107; in the intestine, 117; sum-
mary of chemical processes of,
123 ; cooperation of processes of,
132 ; and the circulation, 157 ;
and temperature regulation, 207

Distilled liquors, 294

Divisive movements of intestine, 124

Dorsal, definition of term, 9

Driving force for circulation through
capillaries, 144

Drug habit, 287

Drugs, 286 ff .

Ducts of glands, 21, 28, 31 (fig.),
32 (fig.)

Dyspepsia, 114

Ear, structure of, 255

Efferent nerve fibers and impulses,
75, 78 (fig.)

Elasticity of arteries, 144

Elimination of intestinal waste, 132

Emmetropic eye, 246

End organs of nerves, 27, 74-82,
240

Enjoyment of food, hygienic value
of, 115

Enzyme, 44, 47 ; action of, 101 ff. ;
of saliva, 104 ; of gastric juice, 109 ;
of small intestine, 119-123

Epidermis, 185

Epiglottis, 20 (fig.)

Equilibrium, nervous factors in, 272

Esophagus. See (Esophagus

Ether, 280

Eustachian tube, 256

Evaporation a cooling process, 195

Excitation, 282, 296

Excretion, 180 ; in relation to feed-
ing, 228

Extractive, 138, 232

Eye, structure of, 242

Face, bones of, 17
Farsightedness, 248
Fasciculus of muscle, 33, 34
Fatigue, 55 ff. ; hygienic value of,

63
Fatigue level, 58, 61

Fats, 95, 222; of meat, 110; diges-
tion of, 120-123; fuel value of,
218 ; storage of, 222, 224

Fatty acids, 96, 120

Feces, 130, 132

Feeding, muscular activity after,
158

Femur, 19

Fermentation, 292

Fever, 211

Fibula, 19

Fires, open, 213

Food accessories, 286 ff.

Foods, as source of energy or power,
91, 215; as material for growth
and repair, 91, 229 ; chemical com-
position of, 92, 99, 238 ; animal and
vegetable, 97; fuel value of, 215;
heating, 220

Foodstuffs. See Nutrients

Force-pump action of heart, 140,
142 (fig.)

Forebrain, 264 (fig.), 279

Fuel requirements of the body, 220

Fuel substances, storage of, in mus-
cle, 48

Fuel value of food, 215

Gall bladder, 108 (fig.)

Ganglion, 72 ; of the dorsal root,

Gas as a conductor of heat, 197
Gaseous exchange in capillaries, 167 ;

during muscular activity, 178
Gastric juice, 110; secretion of,

114

Gelatin, 94
Glands, 21, 28, 30, 31 (fig.), 32 (fig.) ;

working and resting, 44, 46 (fig.) ;

blood supply during activity, 44 ;

ductless, 28, 64 ff .
Glottis, 20 (fig.)
Gluten, 93, 110
Glycerin, 96
Glycogen, 225
Granules, storage of, in gland cells,

46
Gray matter of spinal cord and brain,

74, 264 ff .

Habits, physical basis of, 284

Hair and hair follicle, 185, 186

(fig-)

Harvey, William, 139
Headaches, 134

312

ELEMENTS OF PHYSIOLOGY

Hearing, 255

Heart, 10, 12, 21 ; force-pump action
of, 140 ff . ; valves of, 141 ; regula-
tion of, 159 ; nerves of, 160

Heart beat, 139

Heat, produced in working muscles,
52 ; effect of, on the circulation of
the blood, 152 ; production and
transfer of, 194, 204 ; transfer of,
from internal organs to the skin,
212; unit of, 216; supply of en-
ergy for production of, 220

Heat balance, 199 ff .

Heating foods, 220

Hemoglobin, 137, 138, 167

Hepatic artery, 24 (fig.)

Hepatic vein, 24 (fig.), 26

Hindbrain, 264

Hip bones, 19

Hoarseness, 21

Hormones, 89

Horny layer of skin, 185

Humerus, 19

Humidity, influence on temperature
regulation, 200 ; influence on men-
tal work, 207

Hunger, 261

Hypermetropia, 248, 250

Ileocolic sphincter, 128
Illumination for near work, 251
Illusions, optical, 254
Indigestible material in food, 97
Indigestion, 114, 132 ff.
Inhibition, 160, 296 ; in the nervous

system, 281

Inhibitory nerves of heart, 160
Inorganic salts. See Salts
Instep, bones of, 19
Interdependence of organs, 63
Internal secretion, 64
Intestinal juice, 118-121
Intestinal waste, elimination of,

132
Intestine, small, 10, 20, 23 (fig.), 25,

117 ; large, 10, 20, 127 ; action of

muscular coat of, 122, 128
Iris, 243, 244 (fig.), 247 (fig.)
Irritability, 45

Jugular vein, 23 (fig.)

Kidneys, 10, 13, 14 (fig.), 23, 25;

structure of, 181 ff.
Kilogrammeter, 216

Labyrinth of ear, 256-258

Lactic acid, 48, 59

Large intestine. See Intestine

Larynx, 20

Lateral costal breathing, 173

Lens of eye, 243, 247 (fig.) ; forma-
tion of image by, 244

Ligaments, 8, 14, 16, 18

Lipase, 120

Lipoids, or lipins, 229

Liver, 10, 21, 23 (fig.), 25, 26, 108
(fig.), 118 ff.

Lobes and lobules of the lung, 13,
174; of glands, 29, 30 (fig.)

Locomotion, nervous factors in, 276

Lumbar vertebrae, 14

Lungs, 10, 12, 21, 23 (fig.) ; structure
of, 169 ; apical lobes of. 174

Lymph, 38 ; origin of, 38 ; environ-
ment of cells, 40, 138 ; gases of,
167

Lymph flow, function of, 40 ; cause
of, 150 ; influenced by respira-
tory movements, 175

Lymph spaces, 38

Lymphatics, 39, 150

Malt liquors, 293

Massage, 150

"Master neurones," 82

Mastication, 104

Meat a protein food, 93, 97

Mediastinum, 11, 20

Medulla oblongata. See Bulb

Mental work, and the circulation,
155 ; after meals, 158 ; as influ-
enced by climatic conditions, 207

Mesentery, 12

Microbic life in the intestine, 130,
132

Micromillimeter, or micron, 136 (fig.)

Midbrain, 264 (fig.)

Moisture, influence on temperature
regulation, 197

Morphine, 304

Motor nerves, 74

Movements, active or passive, effect
on circulation, 149 ; respiratory.
See also Breathing movements

Mucin, 44, 127

Mucous coat, of stomach, 108 ; of
intestine, 117, 128 (fig.)

Muscle fibers, 34, 35 ; of stomach
and intestine, 108, 117, 128; of
arteries and veins, 148

INDEX

313

Muscles, 8, 16; antagonistic action of,
16-18, 209 ; structure of, 32 ; physi-
ology of, 47, 55 ; isolated, 49, 56 ;
and temperature regulation, 209

Muscular activity, effect on circula-
tion, 149, 154, 158; after meals,
158, 348 ; effect on respiration,
178; and the regulation of the
temperature of the body, 206

Muscular sense, 259 ; relations of,
to locomotion and maintenance of
equilibrium, 278-279

Muscular work, power for, 217 ;
feeding for, 220

Myofibrils, 35, 52

Myopia, 248

Nasal cavity, 20

Near vision, 245, 250

Nearsightedness, 248

Nerve cells, 72 ff., 268 ff.

Nerve fibers, structure of, 71 ; affer-
ent and efferent, 75, 76 ; of spinal
cord and brain, 74, 268

Nerve roots, 74

Nerves, 27 ; distribution to organs,
40 ; structure of, 71 ; cranial, 268

Nervous system, general anatomy of,
27, 71, 263 ; training by practice,
88, 283 ; physiology of, 69, 269 ;
and the circulation, 159-162 ; and
respiration, 175 ; and secretion of
• perspiration, 187 ; and regulation
of body temperature, 208

Neurones, 76; "master," 82; of
brain, 268

Nitrogenous equilibrium, 235

Nucleus, 31, 35

Nutrients, 92 ; classification of, 93

Nutrition, 215 ff.

CEsophagus, 10, 12, 20 (fig.)

Opium, 304

Optic lobes, 264

Optic nerve, 28, 243, 247 (fig.)

Organs, typical structure of, 40 (fig.)

Oxidation, 50 ff., 91, 165

Oxygen, r61e of, in cell life, 50 ff.,
165 ; in lymph, 165 ; in blood
plasma, 165 ff. ; absorbed during
muscular activity, 178

Pain, sensations of, 260 ff. ; signs

of, 281
Palate, 20
p

Pancreas, anatomical relations, 10,
20, 25, 29, 108 (fig.), 118 ; the
source of an internal secretion, 67

Pancreatic duct, 108 ; stimulus to
secretion of, 89

Pancreatic juice, 111, 119

Papillae of skin, 185

Parotid gland, 29

Pelvis, of skeleton, 19 (fig.) ; of
ureter, 182, 183 (fig.)

Pepsin, 109

Peptids, 102

Peptones, 109

Peristalsis, 125

Peritoneal cavity, 10, 11, 12

Peritoneum, 10, 11,_13

Peritonitis, 11

Perspective, idea of, 253

Perspiration, composition of, 181,
187 ; secretion of, 187 ; sensible
and insensible, 187 ; and the out-
put of heat, 194-198

Pharynx, 20, 185, 256

Pituitary body, 67

Plasma. See Blood plasma

Play, 284

Pleura, 10, 11

Pleural cavity, 10, 11 ; pressure
in, 171

Pleurisy, 11

Pons Varolii, 266 (fig.)

Portal vein, 23 (fig.), 26, 127 (fig.)

Position, sense of, 259, 276 ff.

Posterior, definition of term, 9

Posture, nervous factors in, 279

Presbyopia, 249

Pressure in arteries and veins, 144 ff.,
155 ff. ; in pleural space, 171

Processes of nerve cells, 72

Proteins, nature of, 93 ; chemical
structure of, 101 ; digestion of, in
stomach and intestine, 109, 120 ;
in blood plasma, 138 ; influence of,
on secretion of urine, 183 ; fuel
value of, 217 ; as a source of fat,
224 ; of sugar, 226 ; in living cells,
229 ff.; daily requirement of, 235

Proteoses, 109

Pseudopodium, 137

Psychic secretion of gastric juice,
114

Ptyalin, 105

Pulmonary arteries, 22, 24 (fig.),
25, 167

Pulmonary circulation, 23, 140 (fig.)

314

ELEMENTS OF PHYSIOLOGY

Pulmonary veins, 21, 22, 23 (fig.),

24, 169

Pupil of eye, 243
Purposeful character of reflex and

volitional actions, 70, 79, 271
Pylorus, 108 (fig.)

Radiation of heat, 212

Radius, 19

Rectum, 123, 185

Reflexes, 80 ; conditioned and un-
conditioned, 85 ; of locomotion,
etc., 276 ; disappearance of, dur-
ing anesthesia, 280

Renal arteries, 25, 182 (fig.)

Repair of cells, 229 ff .

Reservoirs, arterial and venous,
142

Resistance to the flow of blood, 144,
156 (fig.)

Respiration, organs of, 21 ; of the
cells, 165 ; nervous factors in, 175 ;
and muscular activity, 178

Respiratory center, 175

Respiratory movements. See Breath-
ing movements

Retina, 243, 247 (fig.), 253

Ribs, 17 ; action in respiration, 172 ff.

Rice, polished, 233

Sacrum, 14, 19

Saliva, chemical composition of, 44 ;
secretion of, 44-47 ; action in di-
gestion, 104 ff.

Salivary glands, 21, 29, 44

Salts, inorganic, 44, 96, 232

Sarcolemma, 34

Sarcostyles. See Myofibrils

Scapula, 19

Sclerotic coat, 242 ff .

Scurvy, 234

Sebaceous glands, 185 (fig.), 186

Secondary aids to the circulation,
147

Secretin, 89

Secretion, general physiology of,
44-47 ; internal, 64 ; of gastric
juice, 108 ; of pancreatic juice,
bile, and intestinal juice, 119-122;
of urine, 182 ; of perspiration, 187

Segmenting movements of intestine,
124

Semicircular canals, 256-258, 259

Sensations, 240 ff . ; reference of, 240

Sense organs, 77, 242

Septum, of gland, 30 ; of muscle, 33

Shivering, 210

Shoulder blade, 19

Shoulder girdle, 19

Sigmoid flexure, 130

Sinuses of skull, infections of, 257

Skeleton, 14-19

Skin, 7 ; structure and functions of,
184 ; care of, 187 ; as an organ of
absorption, 188 ; regulator of the
output of heat, 208

Skull, 17

Sleep, circulation during, 155

Smell, sensations of, 258

Soaps, 96, 121

Soda water, 291

Solidity, ideas of, 253

Somnambulism, 274

Speech, 279

Spinal column, 14, 17

Spinal cord, gross anatomy of, 18,
27; structure of, 73, 264; func-
tions of, 270

Spleen, 10, 25

Starch, 94, 97 (fig.) ; chemical struc-
ture of, 101 ; digestion of, 104,
112, 120. See also Carbohydrates

Steapsin. See Lipase

Sternum. See Breastbone

Stimulants, 296 ff .

Stimulation, 45, 79

Stomach, anatomical relations of,
10, 20, 25 ; structure of, 107 ff . ;
digestive work of, 109-116

Storage of material in the cell, 46,
48

Submaxillary gland, 29, 44

Submucous coat of intestine, 117
(fig.), 128 (fig.)

Suction action of breathing move-
ments, 148

Sugars, 95, 105 ff., 120. See also
Carbohydrates

Supporting organs and tissues, 36

Suprarenal glands. See Adrenal
glands

Suspensory ligament of eye, 244

Sweat glands, structure of, 186

Synapse, 78, 270 (fig.)

Systemic circulation, 22

Systole, 140, 142 (fig.)

Tannic acid, 290
Taste sensations, 258
Tea, 290

INDEX

315

Temperature, influence of external,
on secretion of urine, 183 ; influ-
ence of, on chemical change and
vital activities, 189, 190; of the
body, 190 ff. ; reactions of body
to changes of, 201 ; "danger zone "
of, 202 (fig.), 205, 205 (fig.)

Temperature sensations, 193, 258

Tendon, 8, 33, 34

Theine, 290

Thermal phenomena of the body,
189 ff.

Thirst, 261

Thoracic cavity. See Pleural cavity

Thyroid gland, 64

Tibia, 19

Tobacco, 305

Toes, bones of, 19

Tone, arterial, 161 ; of skeletal
muscle, 209 ff .

Touch, 258

Trachea, 10, 11, 21

Trypsin, 120

'Tweenbrain, 264 (fig.)

Tympanic membrane, 255, 258 (fig.)

Tympanum, 256

Ulna, 19
Urea, 181

Ureter, 181, 182 (fig.)
Uric acid, 181
Urine, secretion of, 182
Use and disuse in the training of
the nervous system, 284

Valves, of heart, 21, 141, 144 ; in
veins, 149

Vasoconstrictor nerves, 162

Vasodilator nerves, 162

Vasomotor nerves. See Vasoconstric-
tor nerves and Vasodilator nerves

Vegetable foods, 97, 113

Veins, 7, 23, 26, 38, 39, 144 (fig.) ;

intermittent compression of, in

muscular activity, 146
Venae cavse, 13, 22, 23 (fig.), 24 (fig.),

Venous reservoir, 142
Ventral, definition of term, 9
Ventricles, of heart, 21, 23, 140 ; of

brain, 264, 265
Vertebra, 14, 16

Vertebral column. See Spinal column
Villus of intestine, 117 (fig.), 118,

120 (fig.), 126 (fig.)
Visual judgments, 253
Visual sensations, 252
Vitamines, 232

Vitreous humor, 243, 247 (fig.)
Volitional actions, or movements,

81, 275, 279

Wandering cells, 137

Warm weather, circulation in, 152 ;
and feeding, 158

Warm-blooded animals, 191

Warmth, sensations of, 258

Waste products, 48, 58, 63 ; excre-
tion of, 180

Water, as a food, 96 ; and the secre-
tion of urine, 183

White matter of spinal cord and
brain, 74

Will, 81, 279

Wind and temperature regulation,
200

Wines, 294

Winking, muscular and nervous
mechanism of, 69

Work, unit of, 216

Wrist, bones of, 19

Writing, 279

Yeast, 292
Zymogen, 46

THIS BOOK IS DUE ON THE LAST BATE
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THIS BOOK ON THE DATE DUE. THE PENALTY
WILL INCREASE TO SO CENTS ON THE FOURTH
DAY AND TO «I.OO ON THE SEVENTH DAY
OVERDUE.

JAN 13 1933
APR 13 1933

27 1933

PO1936

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NOV 18 1939

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UNIVERSITY OF CALIFORNIA LIBRARY

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Full text of "Elements of physiology; being part I of "The human mechanism: its physiology and hygiene and the sanitation of its surroundings""

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