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Zoology.

By A. S. PACKARD, Professor in Brown University.

The Human Body.

By H. NEWELL MARTIN.

Psychology

By WILLIAM JAMES, Professor in Harvard University.

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By JOHN DEWEY, Professor in Columbia University, and
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AMERICAN SCIENCE SERIES— ADVANCED COURSE

THE HUMAN BODY

AN ACCOUNT OF

ITS STRUCTURE AND ACTIVITIES AND THE
CONDITIONS OF ITS HEALTHY WORKING

H. NEWELL MARTIN, D.Sc., M.A., M.D., F.R.S.

Late Professor of Biology in the Johns Hopkins University

and of Physiology in the Medical Faculty

of the same

NINTH EDITION, THOROUGHLY REVISED

BY

ERNEST G. MARTIN, PH.D.

Instructor in Physiology in the Harvard Medical School

NEW YORK

HENRY HOLT AND COMPANY
1910

COPYRIGHT, 1881, 1896,

BY

HENRY HOLT & CO.

COPYRIGHT, 1909, 1910

BY
HENRY HOLT AND COMPANY

PUBLISHERS' NOTE

Copies of the book without the chapter on Reproduction can be
had when specially ordered.

PREFACE TO THE NINTH (REVISED) EDITION

IN making this revision I attempted to preserve, so far as possi-
ble, the spirit and method of the Author. Numerous changes were
necessary, however, as well as the addition of much new material,
because of the great advances in Physiological knowledge during
recent years.

One important departure was made from the former plan of the
book in emphasizing the adaptive character of human activities; a
departure which required an entirely different treatment of the
Nervous System, as well as minor alterations throughout the book.
It involved likewise a rearrangement of the chapters so as to bring
together those dealing with Muscles, Nerves, and Sense-organs.
The conception of the Body as an Adaptive Mechanism is so help-
ful to an understanding of its activities, particularly on the part of
general students, that I felt justified in introducing it even at the
cost of very considerable departures from the original plan.

To make room for the new material without enlarging the vol-
ume unduly some of the purely anatomical portions were con-
densed ; I hope without impairing their value seriously. Likewise
about forty illustrations were omitted, where they could be spared.

For the few new figures introduced, electrotypes were furnished
by the publishers of HowelFs and Kirke's text-books of Physiology,
and of Bailey's and Scymonowicz and MacCallum's text-books of
Histology.

E. G. M.

BOSTON, April, 1910

PREFACE TO THE FIRST EDITION

IN the following pages I have endeavored to give an account of
the structure and activities of the Human Body, which, while in-
telligible to the general reader, shall be accurate, and sufficiently
minute in details to meet the requirements of students who are not
making Human Anatomy and Physiology subjects of special ad-
vanced study. Wherever it seemed to me really profitable, hy-
gienic topics have also been discussed, though at first glance they
may seem less fully treated of than in many School or College
Text-books of Physiology. Whoever will take the trouble, how-
ever, to examine critically what passes for Hygiene in the majority
of such cases will, I think, find that, when correct, much of it is
platitude or truism: since there is so much that is of importance
and interest to be said it seems hardly worth while to occupy space
with insisting on the commonplace or obvious.

It is hard to write a book, not designed for specialists, without
running the risk of being accused of dogmatism, and some readers
will, no doubt, be inclined to think that, in several instances, I have
treated as established facts matters which are still open to discus-
sion. General readers and students are, however, only bewildered
by the production of an array of observations and arguments on
each side of every question, and, in the majority of cases, the chief
responsibility under which the author of a text-book lies is to select
what seem to him the best supported views, and then to state them
simply and concisely : how wise the choice of a side has been in each
case can only be determined by the discoveries of the future.

Others will, I am inclined to think, raise the contrary objection
that too many disputed matters have been discussed : this was de-
liberately done as the result of an experience in teaching Physi-
ology which now extends over more than ten years. It would have
been comparatively easy to slip over things still uncertain and
subjects as yet uninvestigated, and to represent our knowledge of
the workings of the animal body as neatly rounded off at all its
contours and complete in all its details — totus, teres, et rotundus.

viii PREFACE TO THE FIRST EDITION

But by so doing no adequate idea of the present state of physi-
ological science would have been conveyed ; in many directions it is
much farther traveled and more completely known than in others;
and, as ever, exactly the most interesting points are those which lie
on the boundary between what we know and what we hope to
know. In Gross Anatomy there are now but few points calling for
a suspension of judgment; with respect to Microscopic Anatomy
there are more; but a treatise on Physiology which would pass by,
unmentioned, all things not known but sought, would convey an
utterly unfaithful and untrue idea. Physiology has not finished
its course^ It is not cut and dried, and ready to be laid aside for
reference like a specimen in an Herbarium, 'but is comparable
rather to a living, growing plant, with some stout and useful
branches well raised into the light, others but part grown, and
many still represented by unfolded buds. To the teacher, more-
over, no pupil is more discouraging than the one who thinks there
is nothing to learn; and the boy who has "finished" Latin and
" done " Geometry finds sometimes his counterpart in the lad who
has "gone through" Physiology. For this unfortunate state of
mind many Text-books are, I believe, much to blame: difficulties
are too often ignored, or opening vistas of knowledge resolutely
kept out of view: the forbidden regions may be, it is true, too
rough for the young student to be guided through, or as yet path-
less for the pioneers of thought ; but the opportunity to arouse the
receptive mental attitude apt to be produced by the recognition
of the fact that much more still remains to be learned — to excite
the exercise of the reasoning faculties upon disputed matters — and,
in some of the better minds, to arouse the longing to assist in add-
ing to knowledge, is an inestimable advantage, not to be lightly
thrown aside through the desire to make an elegantly symmetrical
book. While I trust, therefore, that this volume contains all the
more important facts at present known about the working of our
Bodies, I as earnestly hope that it makes plain that very much is
yet to be discovered.

A work of the scope of the present volume is, of course, not the
proper medium for the publication of novel facts; but, while the
" Human Body," accordingly, professes to be merely a compilation,
the introduction of constant references to authorities would have
been out of place. I trust, however, that it will be found through-

PREFACE TO THE FIRST EDITION ix

out imbued with the influence of my beloved master, Michael
Foster; and on various hygienic topics I have to acknowledge a
special indebtedness to the excellent series entitled Health Primers.

The majority of the anatomical illustrations are from Henle's
Anatomie des Menschen, and a few from Arendt's Schulatlas, the
publishers of each furnishing electrotypes. A considerable num-
ber, mainly histological, are from Quain's Anatomy, and a few
figures are after Bernstein, Carpenter, Frey, Haeckel, Helmholtz,
Huxley, McKendrick, and Wundt. About thirty, chiefly diagram-
matic, were drawn specially for the work.

Quantities are throughout expressed first on the metric system,
their approximate equivalents in American weights and measures
being added in brackets.

H. NEWELL MARTIN.

BALTIMORE, October, 1880.

CONTENTS

CHAPTER I

THE GENERAL STRUCTURE AND COMPOSITION OF THE HUMAN BODY

PAGE

Definitions. Tissues and organs. Histology. Zoological position of
man. The vertebrate plan of structure. The mammalia. Chem-
ical composition of the Body 1

The properties of the living Body. Physiological properties. Cells.
Cell division. Indirect, karyokinetic or mitotic cell division. As-
similation; reproduction. Contractility. Irritability. Conductiv-
ity. Spontaneity. Protoplasm. The fundamental physiological
properties 10

CHAPfER III

TISSUES, ORGANS, AND PHYSIOLOGICAL SYSTEMS

Development. The physiological division of labor. Classification of the
tissues. Undifferentiated tissues. Supporting tissues. Nutritive
tissues. Storage tissues. Irritable tissues. Conductive tissues.
Motor tissues. Protective tissues. Reproductive tissues. Organs.
Physiological systems. The relation of man to his environment.
Adaptive mechanisms. Maintenance mechanisms 30

CHAPTER IV

THE SUPPORTING TISSUES

Connective tissue. Cartilage. Bone 41

CHAPTER V

THE SKELETON

Exoskeleton and endoskeleton. The bony skeleton. Peculiarities of the
human skeleton. Hygiene of the bony skeleton. Articulations.
Joints. Hygiene of the joints 49

xii CONTENTS

CHAPTER VI

THE STRUCTURE OF THE MOTOR ORGANS

PAGE

Motion in animals and plants. Amoeboid cells. Ciliated cells. The
muscles. Histology of striated muscle. Structure of unstriated

/ muscular tissue. Cardiac muscular tissue. The chemistry of mus-
cular tissue. Beef-tea. Rigor mortis 73

CHAPTER VII

THE PROPERTIES OF MUSCULAR TISSUE

Contractility. Irritability. A simple muscular contraction. Physiolog-
ical tetanus. Causes affecting degree of contraction. Measure of
muscular work. Muscular elasticity. Electrical phenomena of
muscle. Source of muscular energy. Physiology of smooth mus-
cular tissue 87

CHAPTER VIII

MOTION AND LOCOMOTION

Special physiology of the muscles. Levers in the Body. Postures.

Walking. Running. Hygiene of muscles. Exercise. Training . 102

CHAPTER IX

ANATOMY OF THE NERVOUS SYSTEM

General statement. Nerve impulses. Neurons. Synapses. The myelin
sheath. The central nervous system. The spinal cord. The brain.
The spinal nerves. Cranial nerves. White and gray matter. The
sympathetic system 116

CHAPTER X

GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM

Conduction within single neurons. Nature of the nerve impulse. Re-
flexes. Irreversible conduction. Graded synaptic resistance.
Spreading of reflexes. Cerebrum the controlling organ . . . . 134

CHAPTER XI

Cerebrum dependent on the receptor system. Tracing nerve paths.
Paths of the various senses. Structure of the cerebrum. Cortical
localization. Sensory and motor areas. Cortical reflexes. Memory.

CONTENTS xiii

PAGK

Association. Volition. Inhibition. Habit formation. Language.
Consciousness. Emotions. Cerebral functions compared in man
and animals. Nourishment of the brain 142

CHAPTER XII

THE CEREBELLUM. THE MEDULLA AND MIDBRAIN. THE SYMPA-
THETIC SYSTEM

The cerebellum. The medulla and mid brain. The sympathetic system.

Relation of the sympathetic system to emotional states .... 1G1

CHAPTER XIII

THE RECEPTOR SYSTEM. INTERNAL AND CUTANEOUS SENSATIONS

The receptor system. Differences between sensations. Psychophytiical
law. Classification of receptors. Internal senses. Muscle sense.
Hunger and thirst. Fatigue. Cutaneous senses. Pain. Touch.
Temperature sense. Peripheral reference of sensations. Percep-
tions. Sensory illusions 109

CHAPTER XIV

THE EAR. HEARING AND EQUILIBRATION. TASTE AND SMELL

The external ear. The middle ear. Auditory ossicles. Internal ear.
Bony labyrinth. Membranous labyrinth. Organ of Corti. Loud-
ness, pitch, and timbre of sounds. Sympathetic resonance. Func-
tions of tympanic membrane. Functions of auditory ossicles.
Function of the cochlea. Auditory perceptions. Nerve endings
in semicircular canals and vestibule. Equilibrium sense. Smell.
Taste 187

CHAPTER XV

THE EYE AS AN OPTICAL INSTRUMENT

The essential structure of an eye. The appendages of the eye. The lach-
rymal apparatus. The muscles of the eye. Anatomy of the eyeball.
Optic nerves, chiasma, and tracts. The retina. Refracting media
of the eye. The ciliary muscle. Properties of light. Refraction of
light. Accommodation. Short sight and long sight. Optical de-
fects of the eye. Hygiene of the eyes 207

CHAPTER XVI

THE EYE AS A SENSORY APPARATUS

The excitation of the visual apparatus. Intensity of visual sensations.
Function of the rods. Visual purple. Duration of luminous sensa-

xiv CONTENTS

PAGE

tions. Localizing power of retina. Color vision. Function of the
cones. Distribution of color sense over the retina. Color blindness.
After-images. Contrasts. Theories of color vision. Visual percep-
tions. Single vision with two eyes. Perception of solidity . . . 231

CHAPTER XVII

THE STRUCTURE AND FUNCTIONS OF BLOOD AND LYMPH

The external medium. The internal medium. The blood. The lymph.
Filtration, osmosis, and dialysis. Renewal of the lymph. Lym-
phatic vessels. Lacteals. Composition of blood. Red corpuscles.

-r Hemoglobin. Leucocytes. Blood-plates. Blood-plasma. His-
tology and chemistry of lymph 254

CHAPTER XVIII

THE HORMONE-CARRYING AND DISEASE-RESISTING FUNCTIONS OF
THE BLOOD. BLOOD-CLOTTING

Hormones. Ductless glands. Thyroid. Parathyroids. Thymus. Pi-
tuitary. Adrenals. Infection. Resistance to infection. Recovery
from infection. Opsonins, immune bodies, and agglutinins. An-
titoxin. Immunity. Inoculation. Coagulation of blood. Blood
transfusion 270

CHAPTER XIX

THE ANATOMY OF THE HEART AND BLOOD-VESSELS

General statement. Position of heart. Membranes of heart. Anatomy
of heart. Valves of heart. The arterial system. Aorta and its
branches. The capillaries. The veins. The pulmonary circulation.
The portal circulation. Arterial and venous blood. Structure of the
arteries, of the capillaries, and of the veins 288

CHAPTER XX

THE ACTION OF THE HEART. THE REGULATION OF THE HEART-BEAT

The beat of the heart. Cardiac impulse. A cardiac cycle. Use of papil-
lary muscles. Sounds of the heart. Action of the heart valves.
Function of the auricles. Work done by the heart. Relation of
nerve and muscle elements within heart. Peculiarities of heart.
Passage of beat over heart. Neurogenic and myogenic theories of
heart-beat. Nature of automatic rhythmicity. Extrinsic nerves of
heart. Inhibitory and augmentor centers 304

CONTENTS xv

CHAPTER XXI

THE CIRCULATION OF THE BLOOD. BLOOD PRESSURE AND BLOOD
VELOCITY. THE PULSE

PAGE

Circulation seen in frog's web. Resistance to blood-flow. Conversion of
intermittent into continuous flow. Arterial pressure. Weber's
schema. The pulse. Blood-pressure in man. The rate of the blood-
flow. Secondary factors affecting the circulation. Aspiration of
the. thorax. Proofs of the circulation of the blood 318

CHAPTER XXII

THE VA8OMOTOR MECHANISM. SLEEP. THE LYMPHATIC SYSTEM

Distribution of blood in different parts of the body. Nerves of the blood-
vessels. The vasoconstrictor nerves. The vasoconstrictor center.
Depressor nerve. Blushing. Taking cold. Adrenalin. Vasodi-
lator nerves. The vasodilator center. Relation of vasomotor tone
to cerebral activity. Sleep. Lymphatics. Thoracic duct. Lymph
nodes. Movement of lymph 336

CHAPTER XXIII

ANISM OF BREA'
OF BREATHING

Definitions. Respiratory organs. Air-passages and lungs. Trachea
and bronchi. Structure of lungs. Pleura. Respiratory move-
ments. Anatomy of thorax. Expiration. Forced respiration.
Respiratory sounds. Capacity of lungs. Hygiene of respiration.
Aspiration of thorax. Influence of respiratory movements upon
the blood circulation. Influence of respiratory movements on
lymph-flow. Respiratory center. Excitation of respiratory center.
Eupnea, Dyspnea, Apnea. Asphyxia. Artificial respiration.
Modified respiratory movements 348

CHAPTER XXIV

BESPIRATION. THE GASEOUS INTERCHANGES

Nature of the problems. Changes produced in air by being once breathed.
Ventilation. Changes undergone by the blood in the lungs. The
blood gases. The laws governing the absorption of gases by a
liquid. The absorption of oxygen by the blood. The oxygen inter-
changes in the blood. The carbon dioxid of the blood. Hormone
action of carbon dioxid. Tissue respiration 370

xvi CONTENTS

CHAPTER XXV

FOODS: THEIR CLASSIFICATION

PAGE

What constitutes food. Classes of foods. Nutrients. Mixed foods.
Flesh. Eggs. Milk. Vegetable foods. Composition of foods. -
Alcohol. Tea, coffee, cocoa ... .386

CHAPTER XXVI

THE ALIMENTARY CANAL AND ITS APPENDAGES

General arrangement. Subdivisions of the canal. Mouth. Teeth.
Tongue. Salivary glands. Throat. Esophagus. Stomach. In-
testines. Liver. Pancreas. Blood-vessels of canal 397

CHAPTER XXVII

THE CHEMISTRY OF DIGESTION

The object of digestion. Nature of the digestive process. Digestion
products. Salivary digestion. Gastric digestion. Pancreatic di-
gestion. Bile. Intestinal digestion. Summary of digestive process.
Bacterial digestion. Prevention of self-digestion 417

CHAPTER XXVIII

MOVEMENTS OF THE ALIMENTARY CANAL

Mastication. Hygiene' of the mouth. Deglutition. Movements of the
stomach. Control of the pyloric sphincter. Movements of the
small intestine. Extrinsic control of stomach and intestinal move-
ments. Movements of the large intestine . 424

CHAPTER XXIX

THE DIGESTIVE SECRETIONS AND THEIR CONTROL

Organs of secretion. Forms of glands. The secretory process. Control
of the secretory process. Control of the salivary secretion. Control
of the gastric secretion. Control of the pancreatic secretion. Con-
trol of bile-flow. Control of succus entericus. Digestive history of
a meal. Maintenance of good digestion 432

CHAPTER XXX

THE ABSORPTION AND USE OF FOODS

Absorption. Absorption from the stomach. Absorption from the small
intestine. Absorption and storage of carbohydrates. Assimilation

CONTENTS xvii

limit. Alimentary glycosuria. Diabetes. Absorption of proteins.
Absorption of fats. Absorption from large, intestine. Food re-
quirement of t he Body. Liberation of energy in the Body. Rela-
• tive food values. Nitrogen equilibrium. Carbon equilibrium.
Source of body fat ................ 442

CHAPTER XXXI

^/ EXCRETION AND THE KXCRETORY ORGANS

Exogenous and endogenous excreta. Channels of excretion. Liver as an
excretory organ. Arrangement of urinary organs. Structure of
kidney. Blood-flow through kidney. Renal excretion. Compo-
sition of urine. Relation of renal blood-flow to urine secretion.
Skin. Hairs. Nails. Skin glands. Skin secretions. Bathing . . 462

CHAPTER XXXII

THE PRODUCTION AND REGULATION OF THE HEAT OF THE BODY

Cold- and warm-blooded animals. Temperature of Body. Sources of ani-

mal heat. Maintenance of uniform temperature. Fever. Clothing 483

CHAPTER XXXIII

VOICE AND SPEECH
V.

Voice. The larynx. Vocal cords. Muscles of larynx. Vowels. Con-

sonants .................... 491

CHAPTER XXXIV

REPRODUCTION

Reproduction in general. Sexual reproduction. Accessory reproductive
organs. Male reproductive organs. Seminal fluid. Female repro-
ductive organs. Mammalian ovum. Ovulation. Menstruation.
Fertilization. Pregnancy. Parturition. Lactation. Infant feed-
ing. Puberty. Hormones of reproductive system. Stages of life.
Death ..................... 502

Index ....... 531

THE HUMAN BODY

CHAPTER I

THE GENERAL STRUCTURE AND COMPOSITION OF THE

HUMAN BODY

Definitions. The living Human Body may be considered from
either of two aspects. Its structure may be especially examined,
and the forms, connections and mode of growth of its parts be
studied, as also the resemblances or differences in such respects
which appear when it is compared with other animal bodies. Or
the living Body may be more especially studied as an organism
presenting definite properties and performing certain actions ; and
then its parts will be investigated with a view to discovering what
duty, if any, each fulfils. The former group of studies constitutes
the science of Anatomy, and in so far as it deals with the Human
Body alone, of Human Anatomy; while the latter, the science con-
cerned with the uses — or in technical language the functions — of
each part is known as Physiology. Closely connected with physi-
ology is the science of Hygiene, which is concerned with the con-
ditions which are favorable to the healthy action of the various
parts of the Body ; while the activities and structure of the diseased
body form the subject-matter of the science of Pathology.

Tissues and Organs. Histology. Examined merely from the
outside our Bodies present a considerable complexity of structure, i
We easily recognize distinct parts as head, neck, trunk and limbs;!
and in these again smaller constituent parts, as eyes, nose, ears,
mouth; arm, forearm, hand; thigh, leg and foot. We can, with
such an external examination, go even farther and recognize dif-
ferent materials as entering into the formation of the larger parts.
Skin, hair, nails and teeth arc obviously different substances;
sun pie examination by pressure proves that internally there are
harder and softer solid parts ; while the blood that flows from a cut
finger shows that liquid constituents also exist in the Body. The

1

2 THE HUMAN BODY

conception of complexity which may be thus arrived at from ex-
ternal observation of the living, is greatly extended by dissection
of the dead Body, which makes manifest that it consists of a great
number of diverse parts or organs, which in turn are built up of a
limited number of materials; the same material often entering into
the composition of many different organs. These primary build-
ing materials are known as the tissues, and that branch of anatomy
which deals with the characters of the tissues and their arrange-
ment in various organs is known as Histology; or, since it is mainly
carried on with the aid of the microscope, as Microscopic Anatomy.
If, with the poet, we compare the Body to a house, we may go on to
liken the tissues to the bricks, stone, mortar, wood, iron, glass and
so on, used in building; and then walls and floors, stairs and win-
dows, formed by the combination of these, would answer to ana-
tomical organs.

Zoological Position of Man. External examination of the Hu-
man Body shows also that it presents certain resemblances to the
bodies of many other animals: head and neck, trunk and limbs,
and various minor parts entering into them, are not at all peculiar
to it. Closer study and the investigation of internal structure
demonstrates further that these resemblances are in many cases not
superficial only, but that our Bodies may be regarded as built upon
a plan common to them and the bodies of many other creatures:'
and it soon becomes further apparent that this resemblance is
greater between the Human Body and the bodies of ordinary four-
footed beasts, than between it and the bodies of birds, reptiles or
fishes. Hence, from a zoological point of view, man's Body marks
him out as belonging to the group of Mammalia (see Zoology),
which includes all animals in which the female suckles the young;
and among mammals the anatomical resemblances are closer and
the differences less between man and certain apes than between
man and the other mammals; so that zoologists still, with Lin-
naeus, include man with the monkeys and apes in one subdivision
of the Mammalia, known as the Primates. That civilized man is
mentally far superior to any other animal is no valid objection to
such a classification, for zoological groups are defined by ana-
tomical and not by physiological characters; and mental traits,
since we know that their manifestation depends upon the struc-
tural integrity of certain organs, are especially phenomena of

4 THE HUMAN BODY

Upon the ventral side of the head is the mouth-opening leading
into a tube, the alimentary canal, f, which passes back through the
neck and trunk and opens again on the outside at the posterior
part of the latter. In its passage through the trunk-region this
canal lies in the ventral cavity.

The Mammalia. In many vertebrate animals the ventral cavity

FIG. 1. — The Body opened from the front to show the contents of its ventral
cavity, lu, lungs; K, heart, partly covered by other things; le, le', right and left
liver-lobes respectively; ma, stomach; ne, the great omentum, a membrane con-
taining fat which hangs down from the posterior border of the stomach and covers
the intestines.

is not subdivided, but in the Mammalia it is; a membranous trans-
verse partition, the diaphragm (Fig. 1, 2), separating it into an

GENERAL STRUCTURE AND COMPOSITION 3

function and therefore not available for purposes of zoological
arrangement.

As man walks erect with head upward, while the great majority
of Mammals go on all fours with the head forward and the back
upward, and various apes adopt intermediate positions, confusion
is apt to arise in considering corresponding parts in man and other
animals unless a precise meaning be given to such terms as " an-
terior" and "posterior." Anatomists, therefore, give those words
definite arbitrary significations. The head end is always anterior
whatever the natural position of the animal, and the opposite end
posterior; the belly side is spoken of as ventral, and the opposite
side as dorsal; right and left of course present no difficulty: the
terms cephalic and caudal as equivalent, respectively, to anterior
and posterior, are sometimes used. Moreover, that end of a limb
nearer the trunk is spoken of as proximal with reference to the
other or distal end. The words upper and lower may be con-
veniently used for the relative position of parts in the natural
standing position of the animal.

The Vertebrate Plan of Structure. Neglecting such merely
apparent differences as arise from the differences of normal posture
above pointed out, we find that man's own zoological class, the
Mammals, differs very widely in its broad structural plan from the
groups including sea-anemones, insects or oysters, but agrees in
many points with the groups of fishes, amphibians, reptiles "and
birds. These four are therefore placed with man and all other
Mammals in one great division of the animal kingdom known as
the Vertebrata. The main anatomical character of all vertebrate
animals is the presence in the trunk of the body of two cavities, a
dorsal and a ventral, separated by a solid partition; in the adults of
nearly all vertebrate animals, a hard axis, the vertebral column
(backbone or spine) , develops in this partition and forms a central
support for the rest of the Body (Fig. 2, ee). The dorsal cavity is
continued through the neck, when there is one, into the head, and
there widens out. Within it are inclosed the chief organs of the
nervous system. The bony axis is also continued through the
neck and extends into the head in a modified form. The ventral
cavity, on the other hand, is confined to the trunk. It contains the
main organs connected with the blood-flow together with those of
digestion and respiration.

GENERAL STRUCTURE AND COMPOSITION

0

anterior chest or thoracic cavity, and a posterior, or abdominal
cavity. The alimentary canal and whatever else passes from one
of these cavities to the other must there-
fore perforate the diaphragm.

In the chest, besides part of the ali-
mentary canal, lie important organs, the
heart, h, and lungs, lu; the hearlj being
on the ventral side of the alimentary
canal. The abdominal cavity is mainly
occupied by the alimentary canal and
organs connected with it and concerned
in the digestion of food, as the stomach,
ma, the liver, le, the pancreas, and the in-
testines. Among the other more promi-
nent organs in it are the kidneys and the
spleen.

In the dorsal or neural cavity lie the
brain and spinal cord, the former occu-
pying its anterior enlargement in the
head. Brain and spinal cord together
form t^te cerebrospinal nervous center
commonly called the central nervous
system; in addition to this there are
found in the ventral cavity a number
of small nerve-centers united to each
other and to the cerebrospinal center FIG. 2.— Diagrammatic longi-

tudmal section of the .Body, a,

by connecting COrds, and with their off- the neural tube, with its upper
, , . enlargement in the skull-cavity

shoots forming the sympathetic nervous at a'; N, the spinal cord ; N',

tiKiievn *ne brain; ee> vertebrae form-

sysiem. ing the so\{^ partition between

The walls Of the three main cavities the dorsal and ventral cavi-
ties; o, the pleural, and c, the

are lined by Smooth, moist serous mem- abdominal division of the ven-

mi , •,. . ,-, ! i • • tral cavity, separated from one

branes. That lining the dorsal cavity IS another by the diaphragm, d; i,

thp nrnrhnnif?- tVmt linintr trip oViPst, the the nasal, and o, the mouth
ia, tnat \ 3 chamber, opening behind into

pleura; that lining the abdomen the the pharynx, from which one

. . . tube leads to the lungs, Z, and

peritoneum; the abdominal cavity is in another to the stomach, /; h,
consequence often called the peritoneal J^ sympathetic
cavity. Externally the walls of the

* •* .

cavities are covered by the skin, which abdominal cavity to the pos-

. . •• terior opening of the alimen-

consists of two layers: an outer horny tarv canal.

6 THE HUMAN BODY

layer called the epidermis, which is constantly being shed on the
surface and renewed from below; and a deeper layer, called the
dermis and containing blood, which the epidermis does not. Be-
tween the skin and the lining serous membranes are bones, muscles
(the lean of meat), and a great number of other structures which
we shall have to consider hereafter. All cavities inside the Body,
as the alimentary canal and the air-passages, which open directly
or indirectly on the surface are lined by soft and moist prolonga-
tions of the skin known as mucous membranes. In these two layers
are found as in the skin, but the superficial bloodless one is called
epithelium and the deeper vascular one corium.

Diagrammatically we may represent the Human Body in lon-
gitudinal section as in Fig. 2, where aa' is the dorsal or neural
cavity, and 6 and c, respectively, the thoracic and abdominal sub-
divisions of the ventral cavity; d represents the diaphragm separat-
ing them; ee is the vertebral column with its modified prolongation
into the head beneath the anterior enlargement of the dorsal
cavity; / is the alimentary canal opening in front through the
nose, i, and mouth, o; h is the heart, Z, a lung, s the sympathetic
nervous system, and k a kidney.

A transverse section through the chest is represented by the
diagram Fig. 3, where x is the neural canal containing the spinal

cord. In the thoracic cavity are
seen the heart, h, the lungs, II,
part of the alimentary canal,
a, and the sympathetic nerve-
centers, sy; the dotted line on
each side covering the inside of
the chest-wall and the outside of
the lung represents the pleura.

Sections through correspond-

aos th^dv^Tchest regS? mg parts of any other Mammal
x, the dorsal tube which contains the wouid agree in all essential points

spinal cord; the black mass surround- f . .

ing it is a vertebra; a, the gullet, a part with those represented in FigS. 2

of the alimentary canal; h, the heart; , „

sy, sympathetic nervous system- II, and o.

lungs; the dotted lines around them Th*» T imhc TVio limh<3 r>rp«spnt

are the leurse rr ribs st the breast- ine ^H"08-

are the pleurse; rr, ribs; st, the breast-

bone- no such arrangement of cavities

on each side of a bony axis as is seen in the trunk. They have an
axis formed at different parts of one or more bones (as seen at U

GENERAL STRUCTURE AND COMPOSITION 7

and R in Fig. 4, which represents a cross-section of the forearm
near the elbow-joint), but around this are closely-packed soft
parts, chiefly muscles, and the whole is enveloped in skin. The
only cavities in the limbs are branching tubes which are filled with
liquids during life, either blood or a watery-looking fluid known as
lymph. These tubes, the blood and lymph-
vessels respectively, are not, however,
characteristic of the limbs, for they are
present in abundance in the dorsal and
ventral cavities and in their walls.

Chemical Composition of the Body. In
addition to the study of the Body as com-
posed of tissues and organs which are opti- n

. , . , FIG. 4. — A section across

Cally recognizable, We may Consider it as the forearm a short distance

composed of a number of different chemi-

cal substances. This branch of knowl- bo^s, the radius and ulna;

.. e, the epidermis, and a, the

edge, which IS Still Very incomplete, really dermis of the skin; the latter

, - , i /-\ J.U is continuous below with

presents two Classes OI problems. Un the bands of connective tissue,

one hand, we may limit ourselves to the **^vff*&* m£S£

examination Of the chemical Substances which are indicated by num-
... . t i • i <« bers ; n, n, nerves and vessels.

which exist in or may be derived from

the dead Body, or, if such a thing were possible, from the living
Body entirely at rest; such a study is essentially one of structure
and may be called Chemical Anatomy. But as long as the Body
is alive it is the seat of constant chemical transformations in its
material, and these are inseparably connected with its functions,
the great majority of which are in the long run dependent upon
chemical changes. From this point of view, then, the chemical
study of the Body presents physiological problems, and might be
called Chemical Physiology. At present it is customary to include
under the term Biological Chemistry the study of the chemical
structure of living matter and of the chemical changes occurring
in it. At this point we may confine ourselves to the more im-
portant substances derived from or known to exist in the Body
leaving questions concerning the chemical changes taking place
within it for consideration along with those functions which are
performed in connection with them.

Elements Composing the Body. Of the elements known to
chemists only seventeen have been found to take part- in the

8 THE HUMAN BODY

formation of the Human Body. These are carbon, hydrogen,
nitrogen, oxygen, sulphur, phosphorus, chlorin, fluorin, iodin,
silicon, sodium, potassium, lithium, calcium, magnesium, iron, and
manganese. Copper and lead have sometimes been found in small
quantities, but are probably accidental and occasional.

Uncombined Elements. Only a very small number of the
above elements exist in the Body uncombined. Oxygen is found
in small quantity dissolved in the blood ; but even there most of it
is in a state of loose chemical combination. It is also found in the
cavities of the lungs and alimentary canal, being derived from the
inspired air or swallowed with food and saliva; but while con-
tained in these spaces it can hardly be said to form a part of the
Body. Nitrogen also exists uncombined in the lungs and alimen-
tary canal, and in small quantity in solution in the blood. Free
hydrogen has also been found in the alimentary canal, being there
evolved by the fermentation of certain foods.

Chemical Compounds. The number of these which may be
obtained from the Body is very great; but with regard to very
many of them we do not know that the form in which we extract
them is really that in which the elements they contain were united
while in the living Body; since the methods of chemical analysis
are such as always break down the more complex forms of living
matter and leave us only its debris for examination. We know in
fact, tolerably accurately, what compounds enter the Body as
food and what finally leave it as waste; but the intermediate con-
ditions of the elements contained in these compounds during their
sojourn inside the Body we know very little about; more especially
their state of combination during that part of their stay when they
do not exist dissolved in the bodily liquids, but form part of a solid
living tissue.

For present purposes the chemical compounds existing in or de-
rived from the Body may be classified as organic and inorganic,
and the former be subdivided into those which contain nitrogen
and those which do not.

Nitrogenous Organic Compounds. These fall into several main
groups : proteins * — subdivided into simple proteins, conjugated pro-

* The classification of proteins here given is that recommended by the joint
committee on protein nomenclature of the American Physiological Society
and the American Society of Biological Chemists, 1907.

GENERAL STRUCTURE AND COMPOSITION 9

teins, and derived proteins — nitrogenous extractives, and pigments.
The interesting substances known as enzyms probably form like-
wise a group under this head.

Simple Proteins. Under this head are grouped those proteins
whose molecules contain only protein material; in contradistinc-
tion to the conjugated proteins whose molecules contain protein
in combination with a non-protein substance.

Each of them contains carbon, hydrogen, oxygen, and nitrogen;
most of them contain sulphur also, and a few phosphorus in ad-
dition. These elements are united into very complex molecules,
and although different members of the group of simple proteins
differ from one another in minor points they all agree in their
broad features. The common body proteins have a similar per-
centage composition, falling within the limits given in the follow-
ing table:

Carbon 50 to 55 per cent.

Hydrogen 6.5 to 7.3 " "

Oxygen 19 to 24 "

Nitrogen 15 to 17.6 " "

Sulphur 0.3 to 2.4 "

In addition a small quantity of ash is usually left when a protein
is burned, showing that some inorganic salts are held in com-
bination with it.

Recent chemical investigation has shown that the protein
molecule is a complex, made up of a number of simpler molecules
joined together. When a protein is boiled with a dilute acid its
molecules are decomposed, and the resulting solution is found
when examined to contain a mixture of the substances whose in-
dividual molecules were formerly parts of the complex protein
molecules. Sixteen such substances have been obtained from de-
composed proteins; they all contain nitrogen, and they all belong
chemically to the group of amino acids. Some proteins contain
all of them; others only a few. The characteristics of different
proteins are supposed to depend on which of these amino acids
are present in the molecules and also on their arrangement or
grouping therein.

There are a number of chemical tests which may be used in de-
tecting the presence of proteins; but only a few of them apply to

10 THE HUMAN BODY

the entire group. Of these the so-called biuret reaction is the most
easily and most commonly used. It consists in making the pro-
tein solution strongly alkaline with caustic soda or potash and
adding a small amount of a very dilute solution of copper sulphate.
A distinct purple color is evidence of the presence of protein. The
common proteins of the body may also be recognized by the follow-
ing characters:

1. Boiled, either in the solid state or in solution, with strong
nitric acid they give a yellow liquid which becomes orange on
neutralization with ammonia. This is the xanthoproteic test.

2. Boiled with a solution containing subnitrate and pernitrate
of mercury they give a pink precipitate, or, if in very small quan-
tity, a pink-colored solution. This is known as Millon's test.

3. If a solution containing a protein be strongly acidulated with
acetic acid and be boiled after the addition of an equal bulk of a
saturated watery solution of sodium sulphate, the protein will be
precipitated.

The simple proteins which are found in the bodies of man and
the lower animals fall into several groups as follows :

1. Albumins. Several proteins of this group are found in the
body; serum albumin, one of the proteins of the blood, myogen, a
muscle protein, and cell albumin, found in the cellular tissues, are
examples. Egg albumin (white of egg) is perhaps the most
familiar example of an albumin.

The albumins are characterized by being coagulated by heat
(illustrated by boiled white of egg) ; in this respect they are similar
to the proteins of the next group, from which they differ by being
soluble in pure water.

2. Globulins. These proteins, as indicated above, do not differ
greatly from albumins. Like them they are coagulated by heat,
but unlike them, are not soluble in pure water. If a small amount
of an inorganic salt is added to the water they will go into solution.
Two blood proteins, serum globulin or paraglobulin, and fibrinogen
belong to this group ; also myosin, one of the muscle proteins, and
cell globulin, found in cellular tissues.

3. Albuminoids. In chemical structure these simple proteins
are closely similar to those previously described. They are
found, however, exclusively in the supporting and protective
tissues of the body, bone, connective tissue, epidermis, and hair,

GENERAL STRUCTURE AND COMPOSITION 11

and evidently have some important structural difference as com-
pared with the proteins of the cellular tissues since the Body can-
not make use of them in building up its cell proteins in the way it
uses other protein foods.

4. Protamins. These are the simplest proteins known. They
have thus far been found only in the spermatozoa of fishes. Their
molecules consist of a relatively small number of amino acid group-
ings and contain no sulphur.

5. Histons are intermediate in complexity between protamins
and proteins of the albumin class. The one of chief importance
in the body is globin, which is combined with a pigment to form
hemoglobin, the red coloring matter of the blood.

Conjugated Proteins. In addition to the simple proteins de-
scribed above there are present in the Body certain groups of com-
pounds consisting of proteins combined with non-protein sub-
stances. The most important of these are:

Nucleo proteins, consisting of protein combined with nucleic
acid. These are of great interest physiologically since they form
the chief constituents of cell nuclei, to which structures are as-
signed the function of exercising special control over the activities
of living cells.

Glycoproteins, consisting of protein combined with a carbo-
hydrate (see p. 14). Mudn, the substance which gives the secre-
tions of the mouth, nose, and throat their peculiar viscous charac-
ter, is an example of this group.

Phospho proteins, consisting of protein combined with a phos-
phorous-containing substance. The casein of milk, which forms
the curd, is the most familiar member of this group.

Hemoglobins, compounds of protein with a pigment. These are
of great physiological importance on account of the property,
common to all of them, of acting as transporters of oxygen. The
type member of the group, the hemoglobin of Mammalian blood, is
of interest chemically on account of the great size of its molecules,
which are estimated to contain not less than 2,300 atoms each and
to have molecular weight exceeding 16,000.

Derived Proteins. The members of this group are derived, as
their name indicates, from the simple proteins. In the process of
protein digestion, by which the protein portions of the food are
made available for the needs of the Body by being split into simpler

12 THE HUMAN BODY

substances, the first steps in the digestive process give rise to com-
pounds which differ from the simple proteins by a slight degree
only. These are the derived proteins. The members of the group
which occur most commonly in the Body are the proteases and pep-
tones. These are present in the stomach during protein digestion.
They are characterized by greater solubility than simple proteins
possess.

Nitrogenous Extractives. Under this head are grouped various
nitrogen-containing substances most of which represent materials
that have done their work in the Body and are about to be gotten
rid of. Nitrogen is present in the living tissues of the Body chiefly
as a part of their proteins. The vital activities of the tissues in-
volve the breaking down of these complex proteins into simpler
substances. Part of their carbon combines with oxygen and passes
out through the lungs as carbon dioxid; their hydrogen is similarly
in large part combined with oxygen and passed out as water;
while their nitrogen, with some carbon and hydrogen and oxygen,
is passed out in the form of crystalline extractives.

Urea is the most important substance of this class; fully nine-
tenths of all the nitrogen that is eliminated from the Body is in
this form. It is a diamide of carbonic acid, having the formula

CO<MTj2; the relationship of urea to carbonic acid is clear when

OH

the formula for the latter is written thus: CO<QTT. Fully 30

grams of urea are eliminated daily from the Body of an adult man.

Creatinine (C4H7N3O) is an interesting member of the group of
extractives because the amount of it that is eliminated from the
body daily is very constant, regardless of changes in the amount
of food or exercise taken, and seems to depend closely upon the
amount of muscle tissue present in the Body; persons of great
muscular development have a larger daily creatinine output than
those of smaller build.

Creatine (C4H9N302) is closely related chemically to creatinine,
but appears to play a very different part in the Body. Creatinine
is undoubtedly a waste product of protein decomposition, being
merely an incidental product of the vital processes which go on
within the organism. Creatine, on the other hand, seems to be of
use to the muscles, although just what purpose it serves is not clear.
About one per cent of the solid substance of muscle is creatine.

GENERAL STRUCTURE AND COMPOSITION 13

The Purin Bodies, of which Uric Add (C6 H4 N4 O^ is the most
familiar example, are derived chiefly, if not wholly, from the
decomposition of nucleo proteins and are therefore interesting as
being the end products of the vital activities of the cell nuclei.

Pigments. The most important of these which occur in the
Body are:

Hemochromogen, an iron-containing pigment which in com-
bination with the histon ylobin forms hemoglobin, the red coloring
matter of the blood. When hcmochromogen is in the presence of
oxygen it combines with it to form hematin.

Bilirubin and biliverdin are the bile pigments and give to bile its
color. Bilirubin is yellow and biliverdin green. The former
usually predominates in the bile of man and the carnivora, making
such bile yellow; the latter is the dominant color in the bile of
herbivorous animals, which is green. They are closely related
chemically and are derived from the decomposition of hemoglobin.

Urobilin is formed in the intestine as the result of the putrefac-
tion there of the bile pigments. It is absorbed thence into the
blood and excreted by the kidneys, and imparts to the urine its
characteristic yellow color.

Enzyms are a group of substances which seem to be allied in
chemical composition to the true proteins, but it is so difficult to
be sure of the purity of any specimen that their composition is still
in doubt. The enzyms have the power, even when present in very
small quantity, of bringing about extensive changes in other sub-
stances, and they are not themselves necessarily used up or de-
stroyed in the process. Many enzyms of great physiological im-
portance exist in the digestive fluids and play a part in fitting food
for absorption from the alimentary canal. For example, pepsin
found in the gastric juice converts, under suitable conditions, such
complex proteins as albumins into simpler peptones; ptyalin, found
in the saliva, converts starch into sugar. We shall have occasion
later to study a number of enzyms more in detail in connection
with their physiological uses. A characteristic property of all
enzyms is their susceptibility to heat; a temperature of 60° C.
suffices to destroy them completely.

Non-Nitrogenous Organic Compounds. These may be con-
veniently grouped as hydrocarbons or fatty bodies; carbohydrates
or amyloids; and certain non-nitrogenous acids.

14 THE HUMAN BODY

Fats. The fats all contain carbon, hydrogen, and oxygen, the
oxygen being present in small proportion as compared with the
hydrogen. Three fats occur in the Body in large quantities, viz. :
palmatin (C61H98O6), stearin (C61H110O6) , and olein (j^57H104O6).
The two former when pure are solid at the temperature of the
Body, but in it are mixed with olein (which is liquid) in such pro-
portions as to be kept fluid. The total quantity of fat in the Body
is subject to great variations, but its average quantity in a man
weighing 75 kilograms (165 pounds) is about 2.75 kilograms (6
pounds).

Each of these fats when heated with a caustic alkali, in the
presence of water, breaks up into a fatty acid (stearic, palmitic, or
oleic as the case may be), and glycerin. The fatty acid unites with
the alkali present to form a soap.

Carbohydrates. These may be defined as substances composed
of carbon, hydrogen, and oxygen, having the number of carbon
atoms in each molecule usually six or some multiple thereof,
and having the hydrogen and oxygen present in the same propor-
tion as in water. The three chief groups are the sugars, starches,
and cellulose.

Dextrose or grape sugar (C6H1206) is the most important repre-
sentative of the sugars found in the Body. A large part of the food
supply is received from the digestive tract into the blood in this
form. It occurs constantly in small concentration in the blood
and tissues.

Lactose, the sugar of milk, occurs in considerable quantity in
milk.

Glycogen or animal starch (C6H10O5) is the anhydride of grape
sugar. This is the form in which the excess of sugar is stored in the
body to be drawn upon at need. Dextrose is readily converted
into it, and it in turn is easily changed back into sugar. In many
respects it resembles common vegetable starch. It is present in the
muscles of the Body and in the liver, the latter organ alone con-
taining about as much as all the muscles put together.

Cellulose, the woody fiber of plants, is not found in the Human
Body, although a chemically identical substance, tunicin, is found
in the bodies of tunicates.

Organic Non-Nitrogenous Acids. Of these the most important
is carbon dioxid (C0a), which is the form in which by far the

GENERAL STRUCTURE AND COMPOSITION 15

greater part of the carbon taken into the Body ultimately leaves
it. United with calcium it is found in the bones and teeth in large
proportion.

Formic, acetic, and butyric acids arc also found in the Body;
stearic, palmitic, and oloic have been above mentioned as obtain-
able from fats. Lactic, acid is often present in the digestive tract,
and when milk turns sour is formed from lactose. A substance of
the same percentage composition, C3H6O3 (sarcolactic acid), is
formed in muscles when they work or die.

Glycerin phosphoric acid (C3H9PO6) is obtained on the decomposi-
tion of lecithin, a complex nitrogenous fat found in nervous tissue
and to some extent in all living cells.

Inorganic Constituents. Of the simpler substances entering into
the structure of the Body the following are the most important :

Water; in all the tissues in greater or less proportion and forming
about two-thirds of the weight of the whole Body. A man weighing
75 kilos (165 Ibs.), if completely dried would therefore lose about
50 kilos (110 Ibs.) from the evaporation of water. Of the con-
stituents of the Body the enamel of the teeth contains least water
(about 2 per cent), and the saliva most (about 99.5 per cent);
between these extremes are all intermediate steps — bones con-
taining about 22 per cent, muscles 75, blood 79.

Common salt — Sodium chloride — (NaCl) ; found in all the tissues
and liquids, and in many cases playing an important part in keep-
ing other substances in solution in water.

Potassium chloride (KC1) ; in the blood, muscles, nerves and most
liquids.

Calcium phosphate (Ca32P04) ; in the bones and teeth in large
quantity. In less proportion in all the other tissues.

Besides the above, ammonium chloride, sodium and potassium
phosphates, magnesium phosphate, sodium sulphate, potassium
sulphate, and calcium fluoride have been obtained from the Body.

Uncombined hydrochloric acid (HC1) is found in the gastric juice.

CHAPTER II
THE FUNDAMENTAL PHYSIOLOGICAL ACTIONS

The Properties of the Living Body. When we turn from the
structure and composition of the living Body to consider its powers
and properties we meet again with great variety and complexity,
the most superficial examination being sufficient to show that its
parts are endowed with very different faculties. Light falling on
the eye arouses in us a sensation of sight, but falling on the skin
has no such effect; pinching the skin causes pain, but pinching a
hair or a nail does not; when the ears are stopped, sounds arouse
in us no sensation; we readily recognize, too, hard parts formed for
support, joints to admit of movements, apertures to receive food
and others to get rid of waste. We thus perceive that different
organs of our Bodies have very different endowments and serve
for very distinct purposes; and here also the study of internal
organs shows us that the varieties of quality observed on the ex-
terior are but slight indications of differences of property which
pervade the whole, being sometimes dependent on the specific
characters of the tissues concerned and sometimes upon the
manner in which these are combined to form various organs.
Some tissues are solid, rigid and of constant shape, as those com-
posing the bones and teeth; others, as the muscles, are soft and
capable of changing their forms; and still others are capable of
working chemical changes by which such peculiar fluids as the bile
and the saliva are produced. We find elsewhere a number of
tissues combined to form a tube adapted to receive food and carry
it through the Body for digestion, and again similar tissues dif-
ferently arranged to receive the air which we breathe. in, and ex-
pel it after abstracting part of its oxygen and adding to it certain
other things; and in the heart and blood-vessels we find almost
the same tissues arranged to propel and carry the blood over the
whole Body. The working of the Body offers clearly even a more
complex subject of study than its structure.

Physiological Properties. In common with inanimate objects

16

THE FUNDAMENTAL PHYSIOLOGICAL ACTIONS 17

the Body possesses many merely physical properties, as weight,
rigidity, elasticity, color, and so on; but in addition to these we
find in it while alive many others which it ceases to manifest at
death. Of these perhaps the power of executing spontaneous
movements and of maintaining a high bodily temperature arc the
most marked. As long as the Body is alive it is- warm and, since
the surrounding air is nearly always cooler, must be losing heat all
day long to neighboring objects; nevertheless we are at the end of
the day as warm as at the beginning, the temperature of the Body
in health not varying much from 37° C. (98.4° F.), so that clearly
our Bodies must be making heat somehow all the time. After
death this production of heat ceases and the Body cools down to
the temperature in its neighborhood ; but so closely do we associate
with it the idea of warmth that the sensation experienced on touch-
ing a corpse produces so powerful an impression as commonly to
be described as icy cold. The other great characteristic of the liv-
ing Body is its power of executing movements ; so long as life lasts
it is never at rest ; even in the deepest slumber the regular breath-
ing, the tap of the heart against the chest-wall, and the beat of the
pulse tell us that we are watching sleep and not death. If to this
we add the possession of consciousness by the living Body, whether
aroused or not by forces immediately acting upon sense-organs, we
might describe it as a heat-producing, moving, conscious organism.
The production of heat in the Body needs fuel of some kind as
much as its production in a fire ; and every time we move ourselves
or external objects some of the Body is used up to supply the neces-
sary working power, just as some coals are burnt in the furnace of
an engine for every bit of work it does; in the same way every
thought that arises in us is accompanied with the destruction of
some part of the Body. Hence these primary actions of keeping
warm, moving, and being conscious, necessitate many others for
the supply of new materials to the tissues concerned and for the
removal of their wastes; still others are necessary to regulate the
production and loss of heat in accordance with changes in the ex-
terior temperature, to bring the moving tissues into relation with
the thinking, and so on. By such subsidiary arrangements the
working of the whole Body becomes so complex that it would fill
many pages merely to enumerate what is known of the duties of its
various parts. However, all the proper physiological properties

18 THE HUMAN BODY

depend in ultimate analysis on a small number of faculties which
are possessed by all living things, their great variety in the Human
Body depending upon special development and combination in
different tissues and organs ; and before attempting to study them
in their most complex forms it is advantageous to examine them
in their simplest and most generalized manifesta-
tions, as exhibited by some of the lowest living
things or by the simplest constituents of our own
Bodies.

Cells. Among the anatomical elements which
the histologist meets with as entering into the
composition of the Human Body are minute
granular masses of a soft consistence, about 0.012
millimeter (^5 of an inch) in diameter (Fig. 5, 6).
Imbedded in each lies a central portion, not so
granular and therefore different in appearance from
the rest. These anatomical units are known as
cells, the granular substance being the cell-body

FIG. 5.— Forms ' . 6

of cells from the and the imbedded clearer portion the cell-nucleus.
Inside the nucleus may often be distinguished a
still smaller body — the nucleolus. Cells of this kind exist in
abundance in the blood, where they are known as the white blood-
corpuscles, and each exhibits of itself certain properties which are
distinctive of all living things as compared with inanimate objects.
Cell Growth. In the first place, each such cell can take up ma-
terials from its outside and build them up into its own peculiar
substance; and this does not occur by the deposit of new layers
of material like its own on the surface of the cell (as a crystal
might increase in an evaporating solution of the same salt), but in
an entirely different way. The cell takes up chemical elements,
either free or combined in a manner different from that in which
they exist in its own living substance, and works chemical changes
in them by which they are made into part and parcel of itself.
Moreover, the new material thus formed is not deposited, at any
rate necessarily or always, on the surface of the old, but is laid
down in the substance of the already existing cell among its con-
stituent molecules. The new-formed molecules therefore contrib-
ute to the growth of the cell not by superficial accretion, but by
interstitial deposit or intussusception.

THE FUNDAMENTAL PHYSIOLOGICAL ACTIONS 19

Cell Division. The increase of size, which may be brought about
in the above manner, is not indefinite, but is limited in two ways.
Alongside of the formation and deposit of new material there
occurs always in the living cell a breaking down and elimination
of the old; and when this process equals the accumulation of new
material, as it does in all the cells of the Body when they attain a

Fio. 6. — Diagram illustrating direct cell division, a, cell-body; 6, nucleus;
e, nucleolus.

certain size, growth of course ceases. In fact the work of the cell
increases as its mass, and therefore as the cube of its diameter;
while the receptive powers, dependent primarily upon the super-
ficial area, only increase as the square of the diameter. The break-
ing down in the cell increases when its work does, and so comes
at last to equal the reception and construction. The second
limitation to indefinite growth is connected with the power of the
cell to give rise to new cells by division.

Until recently it was believed that cell division was in all cases a
comparatively simple process (Fig. 6). It was thought that the
nucleus, without any important structural change, enlarged some-
what, became elongated, and then divided by simple constriction
into two equal parts, forming two smaller daughter nuclei; and
that the rest of the cell then divided, its halves arranging them-
selves around the new nuclei. The nucleolus when present was
supposed to divide before the nucleus. Such a mode of cell multi-
plication is known as direct division: it possibly occurs in some
cases, but in the great majority of cells division is preceded by
marked changes in the structure of the nucleus and by a rearrange-
ment of its material: such cell division is named indirect, and the
attendant nuclear changes are known as the phenomena of
karyokinesis or mitosis.

Indirect, Karyokinetic or Mitotic Cell Division. Before at-
tempting to describe the phenomena of indirect cell divisions it is
necessary to give some account of the structure of a typical primi-
tive cell as made out in specimens carefully prepared and studied

20

THE HUMAN BODY

with the highest powers of the microscope. The main bulk of the
cell, surrounding the nucleus, is the cell-body, and in some cases

is inclosed in an envelope or sac, which,
however, when present, plays but a
secondary or passive part in cell divi-
sion. The cell-body, known also as the

c \XJ^VX/Mej£*KXl cell-protoplasm (Fig. 7), consists of a net-
work of extremely fine threads, the re-
ticulum or spongioplasm, the meshes of
which are occupied by a different sub-
FIG. 7. — Diagram of an ani- stance, the hyaloplasm: the proportions

mal cell, a, hyaloplasm; b, f , , , , ,

reticuium; c, nucleus, a and of hyaloplasm and spongioplasm vary in

6 together form the cell-body. different ceUs and often in different parts

of the same cell; in fact a layer of hyaloplasm unmixed with
spongioplasm frequently exists on the exterior of the cell, and the
hyaloplasm appears to be the more immediately concerned in the
activities of the living cell. In addition there is to be found, im-
bedded in the cell-body and near the nucleus or attached to it, an
extremely minute particle, the attraction-particle or centrosome,
near which a radial arrangement of the cell-substance may often
be observed.

The nucleus (Fig. 8) of a resting cell, that is of a cell not in proc-
ess of division, consists of an amor-
phous material (nucleoplasm) which
is perhaps similar in composition to the
hyaloplasm, and a filamentous ma-
terial, different from spongioplasm,
and named chromoplasm or karyo-
plasm. As proved by its behavior
with staining fluids and other reagents
karyoplasm is quite different chemic-
ally from the spongioplasm of the ,-,

17 Fio. 8. — Diagram of a resting

cell-body. One or more granules (nu- nucleus, a, nuclear membrane;

7 7 -\ i • i i f i •,! • b, nucleoplasm; c, nucleolus; d.

Cleoll) Which may be found Within chromoplasm ; «, some of the sur-

most nuclei are probably local accu- JC±iSS5P!al^ft
mulations of chromoplasm; a mem- dicated.
brane (nuclear membrane) which surrounds the nucleus of cells
not in process of division is also probably composed of chromo-
plasm.

THE FUNDAMENTAL PHYSIOLOGICAL ACTIONS

21

The first observed step in cell division is binary division of the
attraction-particle: its halves evolve a set of very fine achromatin
j\ laments uniting them, so that each half is one; of the poles of a
spindle-shaped collection of fibers, the nuclear spindle. Mean-
while the nucleolus and nuclear membrane disappear, being prob-
ably taken up into the rest of the chromoplasm, which now, in-
stead of its original reticular arrangement, takes the form of a
single long chromatic filament coiled in the nuclcoplasm. At one

Fio. 9. — Diagrams of a nucleus in an early stage of karyokinesis. A showing
the polar, B the antipolar region; a, nuclear or achromatin spindle; b, part of
general cell-protoplasm around the nucleus; c, looped chromatic filament; d,
nucleoplasm.

portion of the nucleus (pole} the loops of the chromatic filament
leave a space free from them (Fig. 9, A), and in the neighborhood
of this space the nuclear spindle is first seen within the nucleus.
At the opposite side of the nucleus or antipole (Fig. 9, B) the loops
of the chromatic filament leaye no clear space, but cross irregu-
larly. In the next stage the loops at the antipolar end break
through, and in this way the filament is divided into a number of
irregular elongated Vs, each with its closed angle near the pole
and its open end near the antipole. The spindle meanwhile passes
to the center of the nucleus and takes a position in which its long
axis coincides with that joining pole and antipole, and then the
Vs of chromoplasm become shorter and their limbs thicker, and
they also shift position so as to group themselves radially around
the equator of the spindle (A, Fig. 10) with their angles directed
centrally. Each V then divides along its whole length, and one-
half passes towards the pole, the other towards the antipole. The
whole nucleus elongates in the direction of the long axis of the

22

THE HUMAN BODY

spindle; the achromatin filaments disappear, and the nucleus di-
viding in the equatorial plane, two nuclei are formed, each with
nucleoplasm and chromoplasm : the chromoplasm of each is de-

B

FIG. 10. — Diagrams representing more advanced stages of karyokinesis than
those illustrated in Fig. 9. a, polar, and e, antipolar end of nuclear spindle; b and
c, portions of the chromatic filament; d, nucleoplasm; /, cell protoplasm with in-
dications of a radial arrangement in the neighborhood of the pole and antipole.

The nuclear spindle is seen to have lengthened and become placed in the center
of the nucleus, the pole and antipole of which its ends reach. In A the Vs which
resulted from divisions of the chromatic filament at its antipolar loops are seen
to have become much shorter and thicker and to have changed position, so that
instead of lying lengthwise in the nucleus, with their points towards the pole, they
lie equatorially, with their points towards the spindle and their open ends towards
the periphery of the nucleus. For the sake of clearness only two are represented
out of the set of them which surrounds the spindle; b is still uncleft; c lias nearly
completed its longitudinal division into two Vs, the angle of one of which is com-
mencing to travel towards the pole and of the other towards the antipole. In B
the splitting of the Vs and the progress of their halves towards the ends of the
nucleus is more advanced.

rived, as follows from the' preceding description, from both polar
and antipolar regions of the parent nucleus. The chromoplasm in
each daughter nucleus unites into a single convoluted chromatic
filament like that represented for the parent nucleus in Fig. 9, and
this filament breaks up and becomes arranged into reticulum,
nucleolus, and nuclear membrane as in the resting cell (Figs. 7 and
8). Around the new nuclei the cell-protoplasm rearranges itself
and divides to form a new cell-body enveloping each; during its
rearrangement its material frequently presents a radial structure,
the radii converging towards the ends of the nuclear spindle. The
poles of the nuclear spindle, which it will be remembered represent

THE FUNDAMENTAL PHYSIOLOGICAL ACTIONS 23

the halves of the original ccntrosome, probably pass out of the new
nuclei and become the attraction particles of the new cells.

The phenomena of karyokincsis show clearly that in spite of its
small size the animal cell is a complicated structure, made up of
very distinct parts possessing very distinct properties and no
doubt very different functions.

Assimilation : Reproduction. The two powers, that of working
up into their own substance materials derived from outside, known
as assimilation, and that of, in one way or another, giving rise to
new beings like themselves, known as reproduction, are possessed
by all kinds of living beings, whether animals or plants. There is,
however, this important difference between the two : the power of
assimilation is necessary for the maintenance of each individual
cell, plant or animal, since the already existing living material
is constantly breaking down and being removed as long as life
lasts, and the loss must be made good if any of them is to con-
tinue its existence. The power of reproduction, on the other hand,
is necessary only for the continuance of the kind or race, and need
be, and often is, possessed only by some of the individuals com-
posing it. Working bees, for example, cannot reproduce their
kind, that duty being left to the queen bee and the drones of each
hive.

The breaking down of already existing chemical compounds into
simpler ones, sometimes called dissimilation, is as invariable in
living beings as the building up of new complex molecules referred
to above. It is associated with the assumption of uncombined
oxygen from the exterior, which is then combined directly or in-
directly with other elements in the cell, as, for example, carbon,
giving rise to carbon dioxid, or hydrogen, producing water. In
this way the molecule in which the carbon and hydrogen previously
existed is broken down and at the same time energy is liberated,
which in all cases seems to take in part the form of heat just as
when coal is burnt in a fire, but may be used in part for other
purposes, such as producing movements. The carbon dioxid is
usually got rid of by the same mechanism as that which serves to
take up the oxygen, and these two processes constitute the function
of respiration which occurs in all living things. Assimilation and
dissimilation, going on side by side and being to a certain extent
correlative, are often spoken of together as the process of nutri-

24 THE HUMAN BODY

tion: the assimilative or chemically constructive processes are also
named anabolic, and the dissimilative katabolic.

Contractility. Nutrition and (with the above-mentioned partial
exception) reproduction characterize all living creatures ; and both
faculties are possessed by the simple nucleated cells already re-
ferred to as found in our blood. But these cells possess also certain
other properties which, although not so absolutely diagnostic, are
yet very characteristic of living things. Examined carefully with
a microscope in a fresh-drawn drop of blood, they exhibit changes
of form independent of any pressure which might distort them or
otherwise mechanically alter their shape. These changes may
sometimes show themselves as constrictions ultimately leading to
the division of the cell; but more commonly (Fig. 99*) they have
no such result, the cell simply altering its form by drawing in its
substance at one point and thrusting it out at another. The
portion thus protruded may in turn be drawn in and a process be
thrown out elsewhere; or the rest of the cell may collect around
it, and a fresh protrusion be then made on the same side ; and by re-
peating this manoeuver these cells may change their place and creep
across the field of the microscope. Such changes of form from
their close resemblance to those exhibited by the microscopic animal
known as the Amoeba (see Zoology) are called amoeboid, and the fac-
ulty in the living cell upon which they depend is known in physi-
ology as contractility. It must be borne in mind that physiological
contractility in this sense is quite different from the so-called con-
tractility of a stretched india-rubber band, which merely tends to
reassume a form from which it has previously been forcibly removed.

Irritability. Another property exhibited by these blood-cells is
known as irritability. An Amoaba coming into contact with a solid
particle calculated to serve it as food will throw around it processes
of its substance, and gradually carry the foreign mass into its own
body. The amount of energy expended by the animal under these
circumstances is altogether disproportionate to the force of the
external contact. It is not that the swallowed mass pushes in
mechanically the surface of the Amoeba, or burrows into it, but the
mere touch arouses in the animal an activity quite disproportionate
to the exciting force, and comparable to that set free by a spark
falling into gunpowder or by a slight tap on a piece of guncotton.

* P. 266.

THE FUNDAMENTAL PHYSIOLOGICAL ACTIONS 25

It is this disproportion between the excitant (known in Physiology
as a stimulus) and the result, which is the essential characteristic
of irritability when the term is used in a physiological connection.
The granular cells of the blood can take foreign matters into them-
selves in exactly the same manner as an Amoeba does; and in this
and in other ways, as by contracting into rigid spheres under the
influence of electrical shocks, they show that they also are endowed
with irritability.

Conductivity. Further, when an Amceba or one of these blood-
cells comes into contact with a foreign body and proceeds to draw
it into its own substance, the activity excited is not merely dis-
played by the parts actually touched. Distant parts of the cell also
cooperate, so that the influence of the stimulus is not local only,
but in consequence of it a change is brought about in other parts,
arousing them. This property of transmitting disturbances is
known as conductivity.

Finally, the movements excited are not, as a rule, random.
They are not irregular convulsions, but are adapted to attain a cer-
tain end, being so combined as to bring the external particle into
the interior of the cell. This capacity of all the parts to work to-
gether in definite strength and sequence to fulfil some purpose, is
known as coordination.

These Properties Characteristic but not Diagnostic. These
four faculties, irritability, conductivity, contractility, and coordi-
nation, are possessed in a high degree by our Bodies as a whole.
If the inside of the nose be tickled with a feather, a sneeze will
result. Here the feather-touch (stimulus) has called forth move-
ments which are mechanically altogether disproportionate to the
energy of the contact, so that the living Body is clearly irritable.
The movements, which are themselves a manifestation of con-
tractility, are not exhibited at the point touched, but at more or
less distant parts, among which those of abdomen, chest, and face
are visible from the exterior; our Bodies therefore possess physio-
logical conductivity. And finally these movements arc not random,
but combined so as to produce a violent current of air through the
nose tending to remove the irritating object; and in this we have
a manifestation of coordination. Speaking broadly, these proper-
ties arc more manifest in animals than in plants, though they are
by no means absolutely confined to the former. In the sensitive

26 THE HUMAN BODY

plant touching one leaflet will excite regular movements of the
whole leaf, and many of the lower aquatic plants exhibit move-
ments as active as those of animals. On the other hand, no one of
these four faculties is absolutely distinctive of living things in the
way that growth by intussusception and reproduction are. Irritabil-
ity is but a name for unstable molecular equilibrium, and is as
marked in nitroglycerin as in any living cells; in the telephone the
influence of the voice is conducted as a molecular change along a
wire, and produces results at a distance; and many inanimate ma-
chines afford examples of the coordination of movements for the
attainment of definite ends.

Spontaneity. There is, however, one character belonging to
many of the movements exhibited by amoeboid cells, in which they
appear at first sight to differ fundamentally from the movements
of inanimate objects. This character is their apparent spontaneity
or automaticity . The cells frequently change their form inde-
pendently of any recognizable external cause, while a dead mass at
rest and unacted on from outside remains at rest. This difference
is, however, only apparent and depends not upon any faculty of
spontaneous action peculiar to the living cell, but upon its nutritive
powers. It can be proved that any system of material particles in
equilibrium and at rest will forever remain so if not acted upon by
an external force. Such a system can carry on, under certain con-
ditions, a series of changes when once a start has been given ; but it
cannot initiate them. Each living cell in the long run is but a com-
plex aggregate of molecules, composed in their turn of chemical
elements, and if we suppose this whole set of atoms at rest in
equilibrium at any moment, no change can be started in the cell
from inside; in other words, it will possess no real spontaneity.
When, however, we consider the irritability of amoeboid cells, or,
expressed in mechanical terms, the unstable equilibrium of their
particles, it becomes obvious that a very slight external cause,
such as may entirely elude our observation, may serve to set going
in them a very marked series of changes, just as pressing the trigger
will fire off a gun. Once the equilibrium of the cell has been dis-
turbed, movements either of some of its constituent molecules or
of its whole mass will continue until all the molecules have again
settled down into a stable state. But in living cells the reattain-
ment of this state is commonly indefinitely postponed by the re-

THE FUNDAMENTAL PHYSIOLOGICAL ACTIONS 27

ception of new particles, food in one form or another, from the ex-
terior. The nearest approach to it is probably exhibited by the
resting state into which some of the lower animals, as the wheel-
animalcules, pass when dried slowly at a low temperature; the
drying acting by checking the nutritive processes, which would
otherwise have prevented the reattainment of molecular equilib-
rium. All signs of movement or other change disappear under
these circumstances, but as soon as water again soaks into their
substance and disturbs the existing condition, then the so-called
" spontaneous " movements recommence. If, therefore, we use the
term spontaneity to express a power in a resting system of particles
of initiating changes in itself, it is possessed neither by living nor
not-living things. But if we simply employ it to designate changes
whose primary cause we do not recognize, and whose cause was in
many cases long antecedent to the changes which we see, then the
term is unobjectionable and convenient, as it serves to express
briefly a phenomenon presented by many living things and finding
its highest manifestation in many human actions. It then, how-
ever, no longer designates a property peculiar to them. A steam-
engine with its furnace lighted and water in its boiler may be set in
motion ,by opening a valve, and the movements thus started will
continue spontaneously, in the above sense, until the coals or water
are used up. The difference between it and the living cell lies not
in any spontaneity of the latter, but in its nutritive powers, which
enable it to replace continually what answers to the coals and water
of the engine.

Protoplasm. The cell-body-was formerly regarded as essentially
made up of a single substance, which was named protoplasm: and
now that its structure is known to be complex the term is retained
as a convenient one for that mixture of spongioplasm and hyalo-
plasm which constitutes the main bulk of the bodies of most cells.
With the protoplasm other things are frequently present, the most
important of which are either materials undergoing anabolic
changes but not yet completely built up into protoplasm, or
katabolic materials resulting from the chemical degradation of
protoplasm : these secondary matters, mingled with the completed
protoplasm, are conveniently spoken of as the cell deutoplasm or
paraplasm. As between the spongioplasm and hyaloplasm there
are still some differences of opinion as to which is the more im-

28 THE HUMAN BODY

mediate agent in the manifestation of the vital activities of- the
cell. So far as the manifestation of the power of movement is con-
cerned the evidence seems in favor of the hyaloplasm; the outer-
most parts of a white blood-corpuscle, for example, exhibit active
contractile power, yet they contain no spongioplastic filaments;
and many unicellular living things are known in which no reticular
structure can be discovered and which nevertheless nourish them-
selves and are reproductive, irritable, contractile, conductive, co-
ordinative and automatic. It is therefore possible that the fila-
ments when present are to be regarded as secondary in importance
to the hyaloplasm, partly serving as a mechanical support; but in
addition they may play an important part in the internal econ-
omy of the cell. The study of the physiology of individual cells
presents very great difficulties and is yet in its beginnings, so that
we can do little more than speak of the properties of the cell as a
whole, though from the frequent radial arrangement of the cell-
protoplasm in its neighborhood and from the part it plays in the
initiation of cell division, the attraction-particle appears to have
a very important role.

Of the actual chemical composition of living matter we know
only that its molecule is one of great complexity: all methods of
chemical analysis break it up and alter it fundamentally, so that
what is really analyzed is not living matter but a mixture of the
products of its decomposition, among which protein substances
are always prominent.

Cell-protoplasm no doubt varies a little in different cells, so that
the name is to be regarded as a general term designating a number
of closely-allied substances agreeing with one another chemically
in main points, as the proteins do, but differing in minor details, in
consequence of which one cell differs from another in faculty. On
proximate analysis every mass of protoplasm is found to contain
much water and a certain amount of mineral salts; the water being
in part constituent or entering into the structure of the particles of
protoplasm, and in part probably deposited in layers between them.
Of organic constituents protoplasm always yields one or more
proteins, some fats, and some carbohydrates. So that the original
protoplasm is probably to be regarded as containing chemical
"residues" of proteins, fats, and carbohydrates, combined with
salts and water.

THE FUNDAMENTAL PHYSIOLOGICAL ACTIONS 29

The name nudein has been given to a substance which is left be-
hind when the cell-protoplasm has been dissolved away by various
reagents : it is a nucleic acid compound and contains a considerable
quantity of phosphorus. In the living nucleus nuclein is combined
with various proteins to form nucleo proteins.

The Fundamental Physiological Properties. All living animals
possess in greater or less degree the properties considered in this
chapter; and since the science of physiology is virtually concerned
with considering how these properties are acquired, maintained
and manifested, and for what ends they are employed, we may
call them the fundamental physiological properties.

CHAPTER III
TISSUES, ORGANS, AND PHYSIOLOGICAL SYSTEMS

Development. Every Human Body commences its individual
existence as a single nucleated cell. This cell, known as the ovum,
divides or segments and gives rise to a mass consisting of a number

FIG. 11. — A, an ovum; B to E, successive stages in its segmentation until the
morula, F, is produced; a, cell-sac; b, cell contents; c, nucleus.

of similar units and called the mulberry mass or the morula. At
this stage, long before birth, there are no distinguishable tissues
entering into the structure of the Body, nor are any organs recog-
nizable.

For a short time the morula increases in size by the growth and
division of its cells, but very soon new processes occur which ulti-
mately give rise to the complex adult body with its many tissues
and organs. Groups of cells ceasing to grow and multiply like
their parents begin to grow in ways peculiar to themselves, and
so come to differ both from the original cells of the morula and
from the cells of other groups, and this unlikeness becoming more
and more marked, a varied whole is finally built up from one
originally alike in all its parts. Peculiar growth of this kind, form-
ing a complex from a simple whole, is called development; and the

30

TISSUES, ORGANS, AND PHYSIOLOGICAL SYSTEMS 31

process itself in this case is known as the differentiation of the
tissues, since by it they are, so to speak, separated or specialised
from the general muss of mot her-rells forming the morula.

As the differences in the form and structure of the constituent
cells of the morula become marked, differences in property arise,
and it becomes obvious that the whole cell-aggregate is not
destined to give rise to a collection of independent living things,
but to form a single human being, in whom each part, while main-
taining its own life, shall have duties to perform for the good of
the whole. In other words, a single compound individual is to be
built up by the union and cooperation of a number of simple ones
represented by the various cells, each of which thenceforth, while
primarily looking after its own interests and having its own
peculiar faculties, has at the same time its activities subordinated
to the good of the entire community.

The Physiological Division of Labor. As the differentiation
of tissues proceeds the fundamental physiological properties, orig-
inally exhibited in equal degree by all the cells, become distrib-
uted among the various tissues. Thus we find certain tissues
adapted to execute contractions and in these the property of con-
tractility is developed to an especial degree. Other tissues, on the
other hand, show little or no contractility but exhibit a marked
degree of conductivity. The higher we look in the animal scale
the more marked becomes this division of physiological duties
among the tissues. In man it attains its highest development.

Classification of the Tissues. — As we might separate the in-
habitants of the United States' into groups, such as lawyers, doc-
tors, clergymen, merchants, farmers, and so forth, so we may clas-
sify the tissues by selecting the most distinctive properties of each
of those entering into the construction of the adult Body and
arranging them into physiological groups; those of each group
being characterized by some one prominent employment. No
such classification, however, can be more than approximately
accurate, since the same tissue has often more than one well-
marked physiological property. The following arrangement, how-
ever, is practically convenient.

1. UNDIFFERENTIATED TISSUES. These are composed of cells
which have developed along no one special line, but retain very
much the form and properties of the cells forming the very young

32 THE HUMAN BODY

Body before different tissues were recognizable in it. The lymph-
corpuscles and the colorless corpuscles of the blood belong to this
class.

2. SUPPORTING TISSUES. Including cartilage (gristle), bone, and
connective tissue. Of the latter there are several subsidiary vari-
eties, the two more important being white fibrous connective tissue,
composed mainly of colorless inextensible fibers, and yellow fibrous
tissue, composed mainly of yellow elastic fibers. All the supporting
tissues are used in the Body for mechanical purposes: the bones
and cartilages form the haM framework by which softer tissues are
supported and protected; and the connective tissues unite the
various bones and cartilages, form investing membranes around
different organs, and in the form of fine networks penetrate their
substance and support their constituent cells. The functions of
these tissues being for the most part passively to resist strain or
pressure, none of them has any very marked physiological prop-
erty; they are not, for example, irritable or contractile, and their
mass is chiefly made up of an intercellular substance which has
been formed by the actively living cells sparsely scattered through
them, as, for instance, in cartilage, Fig. 14, where the cells are seen
imbedded in cavities in a matrix which they have formed around
them; and this matrix by its firmness and elasticity forms the
functionally important part of the tissue.

3. NUTRITIVE TISSUES. These form a large group, the members
of which fall into three main divisions, viz. :

Assimilative tissues, concerned in receiving and preparing food
materials, and including — (a) Secretory tissues, composed of cells
which make the digestive liquids poured into the alimentary canal
and used to bring about chemical or other changes in the food.
(6) Receptive tissues, represented by cells which line parts of the
alimentary canal and take up the digested food.

Eliminative or excretory tissues, represented by cells in the kid-
neys, skin, and elsewhere, whose main business it is to get rid of the
waste products of the various parts of the Body.

Respiratory tissues. These are concerned in the gaseous inter-
changes between the Body and the surrounding air. They are
constituted by the cells lining the lungs and by the colored cor-
puscles of the blood.

As regards the nutritive tissues it requires especially to be borne

TISSUES, ORGANS, AND PHYSIOLOGICAL SYSTEMS 33

in mind that although such a classification as is here given is use-
ful, as helping to show the method pursued in the domestic econ-
omy of the Body, it is only imperfect and largely artificial. Every
cell of the Body is in itself assimilative, respiratory, and excre-
tory, and the tissues in this class are only those concerned in the
first and last interchanges of material between it and the external
world. They provide or get rid of substances for the whole Body,
leaving the feeding and breathing and excretion of its individual
tissues to be ultimately looked after by themselves, just as even
the mandarin described by Robinson Crusoe who found his dignity
promoted by having servants to put the food into his mouth, had
finally to swallow and digest it for himself. Moreover, there is no
logical distinction between a secretory and an excretory cell: each
of them is characterized by the separation of certain substances
which arc poured out on a free surface on the exterior or interior
of the Body. Many secretory cells, too, have no concern with the
digestion of food, as for example those which form the tears and
sweat.

4. STORAGE TISSUES. The Body does not live from hand to
mouth: it has always in health a supply of food-materials ac-
cumulated in it beyond its immediate needs. This lies in part in
the individual cells themselves, but apart from this reserve there
are certain cells, which store up considerable quantities of material
and constitute what we will call the storage tissues. These are
especially represented by the liver-cells and fat-cells, which con-
tain in health a reserve fund for the rest of the Body.

5. IRRITABLE TISSUES. These include those tissues which are
especially susceptible to changes in their surroundings and are
therefore useful in giving to the Body information of what is going
on around it. Any change in the environment which serves to
arouse response in an irritable tissue constitutes a stimulus.

6. CONDUCTIVE TISSUES. While most, if not all, of the cells of
the Body retain the property of conductivity in some degree, the
nervous tissues exhibit it in very high degree. They serve there-
fore to bring into communication the various parts of the Body.
As an incident in the conveying of messages from one part of the
Body to another certain nervous structures have the power of
modifying the messages which pass through them.

7. MOTOR TISSUES. These have the contractility of the original

34

THE HUMAN BODY

protoplasmic masses highly developed. The more important are
ciliated cells and muscular tissue. The former line certain surfaces
of the Body, and possess on their free surfaces fine threads which
are in constant movement. One finds such cells, for example
(Fig. 40), lining the inside of the windpipe, where their threads or
cilia serve, by their motion, to sweep any fluid formed there
towards the throat, where it can be coughed up and got rid of.
Muscular tissue occurs in two main varieties. One kind is found in
the muscles attached to the bones, and is that used in the ordinary
voluntary movements of the Body. It is composed of fibers which
present cross-stripes when viewed under the microscope (Fig. 45),
and is hence known as striped or striated muscular tissue. Because
the muscles which are made of this sort of tissue are attached to
bones they are often called skeletal muscles. The other kind of
muscular tissue is found in the walls of the alimentary canal, the
arteries, and some other hollow organs, and consists of elongated
cells (Fig. 47) which present no cross-striation. It is known as
smooth or unstriated muscular tissue.

The cells enumerated under the heading of " undifferentiated
tissues" might also be included among the motor tissues, since
they are capable of changing their form.

8. PROTECTIVE TISSUES. These consist of certain cells lining
cavities inside the body and called epithelial cells, and cells cover-
ing the whole exterior of the Body and form-
ing epidermis, hairs, and nails. The enamel
which covers the teeth belongs also to this
group.

The class of protective tissues is, however,
even more artificial than that of the nutritive
tissues, and cannot be defined by positive
characters. Many epithelial cells are secre-
tory, excretory or receptive ; and ciliated cells
have already been included among the motor
tissues. The protective tissues may be best

FIG. 12.— Flat epi-
thelium cells from the
surface of the perito-
neum, a, cell-body; defined as including cells which cover free

c, nucleus; b, nucleoli. . ,

surfaces, and whose junctions are mainly
mechanical or physical. In their simplest form epithelial cells are
flat scales, as, for example, those represented in Fig. 12, from the
lining membrane of the abdominal cavity.

TISSUES, ORGANS, AND PHYSIOLOGICAL SYSTEMS 35

9. THE REPRODUCTIVE TISSUES. These arc concerned in the
production of new individuals, and in the Human Body are of two
kinds, located in different sexes. The conjunction of the products
of each sex is necessary for the origination of offspring, since the
female product, egg-cell or ovum, which directly develops into the
new human being, remains dormant until it has been fertilized, and
fertilization consists essentially in the fusion of its nucleus with the
nucleus of a cell produced by the male.

The Combination of Tissues to Form Organs. The various
tissues above enumerated forming the building materials of the
Body, anatomy is primarily concerned with their structure, and
physiology with their properties. If this, however, were the whole
matter, the problems of anatomy and physiology would be much
simpler than they actually are. The knowledge about the living
Body obtained by studying only the forms and functions of the
individual tissues would be comparable to that attained about a
great factory by studying separately the boilers, pistons, levers,
wheels, etc., found in it, and leaving cut of account altogether the
way in which these are combined to form various machines; for in
the Body the various tissues are for the most part associated to
form organs, each organ answering to a complex machine like a
steam-engine with its numerous constituent parts. And just as in
different machines a cogged wheel may perform very different
duties, dependent upon the way in which it is connected with other
parts, so in the Body any one tissue, although its essential proper-
ties are everywhere the same, may by its activity subserve very
various uses according to the manner in which it is combined with
others. For example: A nerve-fiber uniting the eye with one part
of the brain will, by means of its conductivity, when its end in the
eye is excited by the irritable tissue attached to it on which light
acts, cause changes in the sensory nerve-cells connected with its
other end and so arouse a sight sensation; but an exactly similar
nerve-fiber running from the brain to the muscles will, also by
virtue of its conductivity, when its ending in the brain is excited
by a change in a nerve-cell connected with it, stir up the muscle
to contract under the control of the will. The different results de-
pend on the different parts connected with the ends of the nerve-
fibers in each case, and not on differences in the properties of the
nerve-fibers themselves.

36 THE HUMAN BODY

It becomes necessary then to study the arrangement and uses
of the tissues as combined to form various organs, and this is fre-
quently far more difficult than to make out the structure and
properties of the individual tissues. An ordinary muscle, such as
one sees in the lean of meat, is a very complex organ, containing
not only contractile muscular tissue, but supporting and uniting
connective tissue and conductive nerve-fibers, and in addition a
complex commissariat arrangement, composed in its turn of sev-
eral tissues, concerned in the food-supply and waste removal of the
whole muscle. The anatomical study of a muscle has to take into
account the arrangement of all these parts within it, and also its
connections with other organs of the Body. The physiology of any
muscle must take into account the actions of all these parts work-
ing together and not merely the functions of the muscular fibers
themselves, and has also to make out under what conditions the
muscle is excited to activity by changes in other organs, and what
changes in these it brings about when it works.

Physiological Systems. Even the study of organs added to that
of the separate tissues does not exhaust the matter. In a factory
we frequently find machines arranged so that two or more shall
work together for the performance of some one work: a steam-
engine and a loom may, for example, be connected and used to-
gether to weave carpets. Similarly in the Body several organs are
often arranged to work together so as to attain some one end by
their united actions. Such combinations are known as physi-
ological apparatuses or systems. The circulatory system, for ex-
ample, consists of various organs (each in turn composed of several
tissues) known as heart, arteries, capillaries, and veins. The heart
forms a force-pump by which the blood is kept flowing through
the whole mechanism, and the rest, known together as the blood-
vessels, distribute the blood to the various organs and regulate the
supply according to their needs. Again, in the visual apparatus
we find the cooperation of (a) a set of optical instruments which
bring the light proceeding from external objects to a focus upon
(6) the retina, which contains highly irritable parts ; these, changed
by the light, stimulate (c) the optic nerve, which is conductive and
transmits a disturbance which arouses in turn (d) sensory parts in
the brain. In the production of ordinary sight sensations all these
parts are concerned and work together as a visual apparatus.

TISSUES, ORGANS, AND PHYSIOLOGICAL SYSTEMS 37

So, too, we find a respiratory system consisting primarily of two
hollow organs, the lungs, which lie in the chest and communicate
by the windpipe with the back of the throat, from which air enters
them. But to complete the respiratory apparatus are many other
organs, bones, muscles, nerves, and nerve-centers, which work to-
gether to renew the air in the lungs from time to time; and the
act of breathing is the final result of the activity of the whole
apparatus.

The Relation of Man to His Environment. From infancy the
human organism is confronted with the task of maintaining itself
alive. To this end all the bodily functions bend themselves. The
maintenance of life in man, as in all animals, presents two distinct
problems: first, to obtain the necessary food; and second, to cope
successfully with the innumerable perils with which the organism
is continually confronted. Failure in either of these endeavors
means failure in maintaining life itself.

The labor of obtaining food and the struggle to escape harm take
place in the midst of a world filled with creatures engaged in the
same labor and the same struggle. Indeed it is the very prevalence
of living beings that makes the securing of food labor, and the
avoidance of harm a struggle. All the living beings that belong to
the animal kingdom are in a more or less continuous state of
activity. Each individual, therefore, finds himself surrounded by
a continually shifting world of other beings. Nor is inanimate
Nature stationary; winds and rains, heat and cold, come and go.
To such a constantly changing environment the organism must
adapt itself.

In the complex of systems which together make up the body it
is possible to distinguish between those whose immediate function
is to maintain the necessary adaptation of the organism to its
environment and those which function only indirectly to that end
by keeping the body itself in good working order and each part
well supplied with the energy yielding materials without which
activity is impossible. In making such a distinction, however, it
must be borne in mind that all the bodily functions work together
for the good of the whole body so that no hard and fast line can be
drawn between the two classes of systems. It will be convenient
to consider first the systems which are particularly concerned in
adapting the body to its environment.

38 THE HUMAN BODY

The Motor System. In all members of the animal kingdom
with the exception of certain parasites adaptation to the environ-
ment is secured mainly through movement. Both for obtaining
food and for escaping danger movements either of the whole body
or of parts of it are constantly being resorted to.

In all higher animals the motor mechanism is made up of skeletal
muscles, which by their action upon the movable bones of the
jointed skeleton bring about the various bodily movements.

The Receptor System. It is obvious that the Body cannot
execute movements adapting it to its surroundings unless it knows
what its surroundings are. A blind man, be he never so agile, can-
not escape the onward rush of the approaching car while he is
ignorant of its coming. He will starve in the midst of abundant
food if he does not know where it is to be found.

The Body obtains knowledge of its environment by means of a
set of structures known as the sense-organs. In these the property
of irritability is developed to a high degree, and so long as they all
function properly not much that is important for the organism
to know about need escape its knowledge.

The Conductive System. Organs for making movements and
organs for receiving impressions from the surroundings are not of
themselves adequate to the maintenance of adaptation. It is
necessary that the information gained by the sense-organs be trans-
mitted to the muscles so that their movements may correspond to
the requirements of the situation. This function is performed by
the nervous system. The conduction of stimuli from sense-organs
to muscles is not, however, a simple matter. Impressions are con-
tinually coming into the body by way of a number of different
channels. Movements must be made not in obedience to any one
of these impressions by itself but for the advantage of the whole
Body as indicated by all of them taken together. To this end a
certain part of the nervous system is adapted for receiving all
sorts of incoming-stimuli and before passing them on to the motor-
organs combining and modifying them to produce the best results.

The systems which are not immediately concerned in the adapta-
tion of the Body to its environment but which serve rather to keep
it in proper condition for activity may next be considered. Ac-
tivity in the Body involves the manifestation of energy, but in its
energy relations the Body is on exactly the same plane as any

TISSUES, ORGANS, AND PHYSIOLOGICAL SYSTEMS 39

machine; it is without power to manufacture energy, and must
receive whatever energy it obtains from without. The ultimate
source of the Body's energy is chemical, being received in the com-
plex substances which serve as food. This energy is made avail-
able for the use of the body chiefly through the process of oxida-
tion. Every living cell in the body must share in this process, for
the energy manifestations of the body as a whole are simply the
sum-total of those of its component cells.

The systems which are concerned with the maintenance of ac-
tivity have, then, the task of furnishing to each cell of the body
oxidizable substance and oxygen; they must provide for making
good the wear and tear of the cells themselves; and they must
remove the waste materials which are formed in connection with
the chemical activities of the cells and which would interfere with
their proper working if allowed to accumulate.

The Circulatory System consists of the heart and blood-vessels.
It serves to distribute to all the parts of the Body supplies of
oxidizable material, of repair material, and of oxygen, and to re-
move these from the accumulated waste products. These func-
tions are accomplished through the agency of a circulating me-
dium, the blood.

The Respiratory System consists of the lungs, the bronchial
tubes, and the trachea, together with the respiratory muscles. Its
function is to bring the outside air to a region where the circulating
medium can take up abundant supplies of oxygen, and where it
can get rid of those waste products which are in gaseous form.

The Digestive System consists of the alimentary canal and cer-
tain associated glands (salivary, liver, pancreas) . It serves to bring
the various materials that are taken as food into the forms best
adapted for use as repair materials or as oxidizable substance;
when it has so prepared them it turns them over to the circulating
medium for distribution.

The Excretory System consists of the kidney and bladder with
their connecting tubes, the liver, and the skin. It serves to with-
draw from the circulating medium and to eliminate from the body
those waste products which are in liquid form.

Through these systems provision is made for the activities of the
individual cells. These activities are many and complex. They
include oxidative processes, processes involving waste and repair,

40 THE HUMAN BODY

and doubtless many others of which we know nothing. The study
of these cell activities is comprehended under the head of Me-
tabolism.

The chemical activities which go on in the cells of the body give
rise to much heat. Some cells generate more heat than others.
One of the functions of the circulating medium is to distribute this
heat uniformly over the body. There is constant loss of heat from
the surface of the body. In warm-blooded animals, which have
a nearly constant body temperature, the maintenance of balance
between heat production and heat loss in the face of constantly
varying outside temperatures is a function of great importance.
It is studied under the head of Heat Production and Heat Regula-
tion.

Not immediately concerned with the well-being of the body it-
self, but devoted to the well-being of the race as a whole through
perpetuating the species is the Reproductive System.

CHAPTER IV
THK SUPPORTING TISSUES

Connective Tissue. This is the most widely distributed of the
supporting tissues. It envelopes and pervades all the soft parts of
the Body. The various constituents of individual organs are held
together by it, and the organs themselves, are supported in their
places by the same tissue. Beneath the skin and attaching it
rather loosely to the underlying structures is a layer of connective
tissue known as the fascia. So completely is the entire Body per-
vaded by connective tissue that if a solvent could be found which
would dissolve away all the tissues of the Body except this one
there would still remain in perfect outline not only the whole Body
but also each organ down to minutest detail.

This connective tissue framework is commonly called areolar
tissue. It is composed, in the main, of tough, inelastic strands;
these are arranged, however, in most parts of the Body to form a
rather loose network, so that in removing the skin from an animal
or in separating one muscle from another in making a dissection
a blunt instrument readily tears the strands of areolar tissue apart.

The meshes of areolar tissue are everywhere filled with a fluid,
lymph. Thus the various living tissues of the Body, all of which
are surrounded by areolar tissue, are nourished.

There are in the body connective tissue structures in which the
individual strands, instead of forming a loose network, are in
parallel bundles, forming the toughest and strongest of cords and
bands. These are the tendons, by which muscles are attached to
bones, and the ligaments which hold the different bones of the
skeleton together.

The functional part of connective tissue consists of two sorts of
fibers. In most places the white fibers constitute the bulk of the
tissue. These are flexible, inelastic strands composed of an albu-
minoid substance, collagen. The second sort of fibers are the
elastic fibers. These are intermingled with the white fibers to some
extent in nearly all regions where connective tissue occurs. In

41

42 THE HUMAN BODY

certain structures, where a high degree of elasticity is required,
elastic fibers make up the entire connective tissue content. Ex-
amples of this sort are the walls of the large arteries and the liga-
ments connecting the vertebrae. In quadrupeds these fibers form
the great ligament which helps to sustain the head (see p. 65) .
Elastic tissue is yellow in color, and consists chemically of an al-
buminoid, elastin, which in some important respects differs from
the albuminoid of the white fibers.

Connective tissue fibers are not living structures. They owe
their origin to certain living cells, the so-called connective tissue
cells, which lie irregularly interspersed wherever connective tissue

Fio. 13. — Connective tissue cells: a, from areolar tissue; b, from tendon.

fibers occur (Fig. 13). In areolar tissue many of the cells have
given up their function of forming fibers and have devoted them-
selves instead to storing within their substance masses of fat.
Adipose tissue consists of cells of this sort (Fig. 49), and occurs
in regions where areolar tissue is most abundant, as just under
the skin or in masses about certain internal organs.

Temporary and Permanent Cartilages. In early life a great
many parts of the supporting framework of the Body, which after-
wards become bone, consist of cartilage. Such for example is the
case with all the vertebra?, and with the bones of the limbs. In
these cartilages subsequently the process known as ossification
takes place, by which a great portion of the original cartilaginous
model is removed and replaced by true osseous tissue. Often,
however, some of the primitive cartilage is left throughout the
whole of life at the ends of the bones in joints where it forms the
articular cartilages; and in various other places still larger masses
remain, such as the costal cartilages, those in the external ears
forming their framework, others finishing the skeleton of the nose
which is only incompletely bony, and many in internal parts of
the Body, as the cartilage of "Adam's apple," which can be felt
in the front of the neck, and a number of rings around the wind-
pipe serving to keep it open. These persistent masses are known

THE SUPPORTING TISSUES

43

as the permanent, the others as the temporary cartilages. In old age
many so-called permanent cartilages become calcified — that is,
hardened and made unyielding by deposits of lime-salts in them—
without assuming the histological character of bone, and this
calcification of the permanent cartilages is one chief cause of the
want of pliability and suppleness of the frame in advanced life.

Hyaline Cartilage. In its purest form cartilage is flexible and
elastic, of a pale bluish-white color when alive and seen in large
masses, and cuts readily with a knife. In thin pieces it is quite
transparent. Everywhere except in the joints it is invested by a
tough adherent membrane, the perichondrium.

When a thin slice of hyaline cartilage is examined with a micro-
scope it is found (Fig. 14) to consist of granular nucleated cells,
often collected into groups of two, four, or more, scattered through
a homogeneous or faintly granular ground-substance or matrix.
This matrix is composed of al-
buminoid substances, and owes
its origin to the cells imbedded
within it. At the time the
cartilage was in process of for-
mation these cells laid down
the matrix substance in con-
centric layers about them-
selves; thus they cut them-
selves off from each other and

from Communication with the . JIG- 14.— A thin slice of cartilage, mag-

mi. L 1. nined, to show the cells embedded in the

Outside. I he Substance Of the* homogeneous matrix, a, a cell in which

.• , • rr- • 4.1 , the nucleus has divided; b, a cell in which

matrix IS Sufficiently perme- division is just complete; c,*, a group of

able, however, for a certain f°ur ce-ll8rr^Itiug from f,Vrtuer d,ivisio"

of a pair like b; the new cells have formed

interchange of food materials some matrix between them, separating
., . them from another; d, d, cavities in the

between the cartilage cells and matrix from which cells have dropped out
the blood, SO that the Cells during the preparation of the specimen.

are able to remain alive, although their life is naturally an inac-
tive one.

All temporary cartilages are of the hyaline type as are also the
costal and articular permanent cartilages and the cartilage of the
nose and of the windpipe.

Elastic Cartilage is a tissue whose cartilaginous matrix is inter-
woven with fibers of elastic connective tissue. The result of this

44 THE HUMAN BODY

interweaving is to give the cartilage a yellow color and a high de-
gree of elasticity. Cartilage of this sort is found in the external
ear, the epiglottis, and in certain parts of the larynx.

Fibrocartilage is really a dense fibrous connective tissue within
whose spaces a certain amount of matrix material has been de-
posited. It makes up the intervertebral disks, pads which are
interposed between the bones of the vertebral column, and is
found also in certain joints, notably the knee-joints and the ar-
ticulations of the lower jaw.

Bone. The bones which make up the skeleton vary greatly in
shape and size, ranging from the long cylindrical bones of the
arm and leg to the flat skull bones, and the tiny irregularly shaped
ossicles of the middle ear. They all, however, have a similar
microscopic structure and similar chemical composition.

The bones may be classified according to their origin as mem-
brane bones or cartilage bones. To the first group belong the flat
bones of the skull and the bones of the face (see p. 56) . They do
not replace cartilage but develop upon a foundation of connective
tissue. The so-called cartilage bones replace the temporary
cartilages and make up the whole of the bony skeleton, except the
membrane bones mentioned above.

The Process of Bone Formation is complicated, and can be
described only very briefly here. At the beginning of the develop-
ment of a membrane bone the strands of connective tissue upon
which the bone is to be built become covered with peculiar small
cells which are bone-producing cells or osteoblasts. These osteo-
blasts deposit upon the strands whereon they rest albuminoid
material which constitutes the organic matrix of bone. There is
thus produced a rather open network of bone matrix. By the
deposition within the matrix of lime-salts it takes on the character
of true bone. The original connective tissue is thus replaced by a
network of bony spicules.

The surfaces of this bony mass now become covered with a stout
connective tissue membrane, the periosteum, whose inner surface
is beset with osteoblasts. These deposit upon the underlying mass
a layer of compact bone. Thus the fully formed membrane bone
consists of outer surfaces of compact bone inclosing a mass of
spongy bone.

The replacement of temporary cartilage by bone proceeds from

THE SUPPORTING TISSUES

45

certain points in the cartilage known as centers of ossification.
The cartilage itself becomes surrounded by a periosteum like that
which incloses membrane bones. At the center of ossification the
osteoblast layer of the periosteum begins to force its way into the
cartilage, absorbing much of the latter and leaving only a coarse
network, which is presently converted by the osteoblasts into true
bone. Meanwhile the periosteum has deposited on the surface of
the cartilage a layer of compact bone so that in time the cartilage
bone presents a structure not unlike that of membrane bones, a
spongy interior inclosed in a layer of compact bone. As the
cartilaginous network is being ossified, many osteoblasts are im-
prisoned within the bony substance. The spaces which they
occupy and the tiny canals which radiate therefrom into the bone
substance are among the most characteristic appearances of bone
viewed under the microscope (Fig. 15).

The growth in thickness of bone is accomplished by the addition
of layer after layer of compact bone underneath the periosteum.

FIG. 15. — Cross-section of compact bone from the shaft of the humerus. (Sharpey,
from Bailey's Text Book of Histology.)

During this process blood-vessels of the periosteum often become
imbedded within the bony mass. When this occurs the osteo-
blasts which accompany, the blood-vessel surround it with con-
centric layers of bone. In this manner are formed the so-called
Haversian Systems, each of which consists of the space through

46

THE HUMAN BODY

which the blood-vessel passes with its surrounding rings of bony
material.

The increase in length of the long bones is brought about by
plates of cartilage which persist between the shaft of the bone and
its extremities. There is a continual growth
of bone into these cartilages from both sides,
but they grow in thickness with equal rapidity
until the adult length of the bone is reached
when their growth stops and they are gradually
replaced by bone.

At the same time that the bone is growing
by additions to its outer surfaces a continu-
ous absorption of its inner portions is going
on. This absorption is carried on by large,
multinuclear cells known as osteodasts. It
serves the purpose of preventing the bone from
becoming so heavy as to be unmanageable,
without sacrificing unduly its strength. As
the result of this absorption many adult bones,
especially long ones, contain little or no spongy
bone except at their ends, the shaft being
hollow as shown in Fig. 16.

Chemistry of Bone. Bone is composed of in-
organic and organic portions intimately com-
bined, so that the smallest distinguishable por-
tion contains both. The inorganic matters
form about two-thirds of the total weight of a
dried bone, and may be removed by soaking
the bone in dilute hydrochloric acid. The or-
ganic portion left after this treatment consti-
tutes a flexible mass, retaining the form of the
original bone; it consists chiefly of an albu-
minoid, ossein, which by long boiling, especially
- d°ar- under pressure at a higher temperature than
that at which water boils when exposed freely
to the air, is converted into gelatin, which dissolves in the hot
water. Much of the gelatin of commerce is prepared in this manner
by boiling the bones of slaughtered animals, and even well-picked
bones may be used to form a good thick soup if boiled under

uta

FIG. 16. — The hu-
merus bisected length-
wise, a, marrow-cavi-
ty; b,
c, spongy
ticular cartilage.

THE SUPPORTING TISSUES 47

pressure in a Papin's digester; much nutritious matter being, in
the common modes of domestic cooking, thrown away in the bones.

The inorganic salts of bone may be obtained free from organic
matter by calcining a bone in a clear fire, which burns away the
organic matter. The residue forms a white very brittle mass, re-
taining perfectly the shape and structural details of the original
bone. It consists mainly of normal calcium phosphate, or bone-
earth [Ca3(PO4)2]; but there is also present a eonsiderable propor-
tion of calcium carbonate (CaCO8) and smaller quantities of other
salts.

Hygienic Remarks. Since in the new-born infant many parts
which will ultimately become bone consist only of cartilage, the
young child requires food which shall contain a large proportion of
the lime-salts which are used in building up bone. Nature provides
this in the milk, which is rich in such salts (see Chap. XXXIV),
and no other food can thoroughly replace it. Long after infancy
milk should form a large part of a child's diet. Many children
though given food abundant in quantity are really starved, since
their food does not contain in sufficient amount the mineral salts
requisite for their healthy development.

At birth even those bones of a child which are most ossified are
often not continuous masses of osseous tissue. In the humerus,
for example, the shaft of the bone is well ossified and so is each end,
but between the shafts and each of the articular extremities there
still remains a cartilaginous layer, and at those points the bone in-
creases in length, new cartilage being formed and replaced by bone.
The bone increases in thickness by new osseous tissue formed
beneath the periosteum. The same thing is true of the bones of
the leg. On account of the largely cartilaginous and imperfectly
knit state of its bones, it is cruel to encourage a young child to
walk beyond its strength, and may lead to "bow-legs" or other
permanent distortions. Nevertheless here as elsewhere in the
animal body, moderate exercise promotes the growth of the tissues
concerned, and it is nearly as bad to wheel a child about forever
in a baby-carriage as to force it to overexertion.

The best rule is to let a healthy child use its limbs when it feels
inclined, but not by praise or blame to incite it to efforts which are
beyond its age, and so sacrifice its healthy growth to the vanity of
parent or nurse.

48 THE HUMAN BODY

The final knitting together of the bony articular ends with the
shaft of many bones takes place only comparatively late in life,
and the age at which it occurs varies much in different bones.
Generally speaking, a layer of cartilage remains between the shaft
and the ends of the bone, until the latter has attained its full
adult length. To take a few examples: the lower articular ex-
tremity of the humerus only becomes continuous with the shaft by
bony tissue in the sixteenth or seventeenth year of life. The upper
articular extremity only joins the shaft by bony continuity in
the twentieth year. The upper end of the femur joins the shaft by
bone from the seventeenth to the nineteenth year, and the lower
end during the twentieth. In the tibia the upper extremity and
the shaft unite in the twenty-first year, and .the lower end and the
shaft in the eighteenth or nineteenth : while in the fibula the upper
end joins the shaft in the twenty-fourth year, and the lower end in
the twenty-first. The separate vertebrae of-4fee sacrum are only
united to form one bone in the twenty-fifth year of life; and the
ilium, ischium, and pubis unite to form the os innominatum about
the same period. Up to about twenty-five then the skeleton is not
firmly "knit," and is incapable, without risk of injury, of bearing
strains which it might afterwards meet with impunity. To let
lads of sixteen or seventeen row and take other exercise in plenty
is one thing, and a good one; but to allow them to undergo the
severe and prolonged strain of training for and rowing a long race
is quite another, and not devoid of risk.

CHAPTER V
THE SKELETON

Exoskeleton and Endoskeleton. The •skeleton of an animal in-
cludes all its hard protecting or supporting parts, and is met with
in two main forms. One is an exoskeleton developed in connection
with either the superficial or deeper layer of the skin, and repre-
sented by the shell of a clam, the scales of fishes, the horny plates
of a turtle, the bony plates of an armadillo, and the feathers of
birds. In man the exoskeleton is but slightly developed, but it
is represented by the hairs, nails, and teeth ; for although the latter
lie within the mouth, the study of development shows that they
are developed from an offshoot of the skin which grows in and
lines the mouth long before birth. Hard parts formed from struc-
tures deeper than the skin constitute the endoskeleton, which in
man is highly developed and consists of a great many bones and
cartilages or gristles, the bones forming the mass of the hard frame-
work of the Body, while the cartilages finish it off at various parts.
This framework is what is commonly meant by the skeleton; it
primarily supports all the softer parts and is also arranged so as
to surround cavities in which delicate organs, as the brain, heart,
or spinal cord, may lie with safety. The gross skeleton thus
formed is completed and supplemented by another made of the
connective tissues, which not only, in the shape of tough bands or
ligaments, tie the bones and cartilages together, but also in various
forms pervade the whole Body as a sort of subsidiary skeleton
running through all the soft organs and forming networks of fibers
around their other constituents; they make, as it were, a micro-
scopic skeleton for the individual modified cells of which the Body
is so largely composed, and also form partitions between the muscles,
cases for such organs as the liver and kidneys, and sheaths around
the blood-vessels. The bony and cartilaginous framework with its
ligaments might be called the skeleton of the organs of the Body,
and this finer supporting meshwork the skeleton of the tissues.

The Bony Skeleton (Fig. 17). If the hard framework of the

49

50

THE HUMAN BODY

FIG. 17. — The bony and cartilaginous
skeleton.

Fio. 18. — Side view of
the spinal column. C 1—7,
cervical; D 1-12, dorsal;
L 1-5, lumbar; S 1, sacrum;
Co 1-4, coccygeal.

THE SKELETON 51

Body were joined together like the joists and beams of a house, the
whole mass would be rigid ; its parts could not move with relation
to one another, and we should be unable to raise a hand to the
mouth or put one foot before another. To allow of mobility the
bony skeleton is made of many separate pieces which are joined
together, the points of union being called articulations, and at
many places the bones entering into an articulation are movably
hinged together, forming what are known as joints. The total
number of bones in the Body is more than two hundred in the adult ;
and the number in children is still greater, for various bones which
arc distinct in the child (and remain distinct throughout life in
many lower animals) grow together so as to form one bone in the
full-grown man. The adult bony skeleton may be described as con-
sisting of an axial skeleton, found in the head, neck, and trunk ; and
an appendicular skeleton, consisting of the bones in the limbs and
in the arches (u and s, Fig. 17) by which these are carried and at-
tached to the trunk.

Axial Skeleton. The axial skeleton is made up of the vertebral
column or spine, a side view of which is given in Fig. 18; the skull,
Fig. 28; the sternum, Fig. 31 ; and the ribs, Fig. 32.

The vertebral column is the great supporting center for the whole
skeleton and consists of 33 bones grouped as follows from above
downward: 7 cervical, 12 dorsal or thoracic, 5 lumbar, 5 sacral, in
the adult united into a single bone, the sacrum, and 4 coccygeal, or
rudimentary tail bones.

The vertebral column occupies the mid-dorsal line of the trunk.
On top of it is borne the skull (22 bones) made up of two parts; a
great box above, composed of 8 bones, which incloses the brain
and is called the cranium; and a group of 14 bones on the ventral
side of this which form the skeleton of the face. Attached by liga-
ments to the underside of the cranium is the hyoid bone, to which
the root of the tongue is fixed. There are 12 pairs of ribs, at-
tached dorsally to the 12 thoracic vertebra?, one pair to each ver-
tebra. The sternum, which occupies the mid-ventral line of the
thorax and constitutes the anterior attachment for the ribs is made
up of two bones, the manubrium and the body, and a cartilage, the
ensiform cartilage.

Details of the Vertebral Column. The vertebral column is in a
man of average height about twenty-eight inches long. Viewed

52 THE HUMAN BODY

.

from the side (Fig. 18) it presents four curvatures; one with the
convexity forwards in the cervical region is followed, in the
thoracic, by a curve with its concavity towards the chest. In the
lumbar region the curve has again its convexity turned ventrally,
while in the sacral and coccygeal regions the reverse is the case.
These curvatures give the whole column a good deal of springiness
such as would be absent were it a straight rod.

All the vertebrae are built upon the same plan, although with
modifications in various parts of the column. Each consists: 1, of a
stout bony body or centrum (Fig. 19, C), in shape a cylinder flat-
tened at both ends; 2, a bony arch, the neural arch (Fig. 19, A), at-
tached to the dorsal side of the centrum and inclosing the neural
ring (Fig. 19, Fv). The neural rings of all the vertebrae make up
together a long bony tube, the neural canal, which contains the
spinal cord. Between the bodies of adjoining vertebrae, except in
the sacrum and coccyx, are thick pads of elastic cartilage. These
permit bending movements which, while quite limited at each
joint may be very considerable in the column as a whole. They
also serve to take up a great deal of shock, preventing injury to
the body when one sits down hard or comes down on his heels in
walking or jumping. During the hours when one is on his feet
these intervertebral pads are packed down by the weight of the
body, and especially by the hammering effect of the movements
of walking, running, etc., so that a man may be from a half to three-
quarters of an inch shorter at night than he is in the morning.
Strong ligaments fasten adjoining vertebrae together; there are
also muscles passing from vertebra to vertebra, which by their
contractions assist in bending the body. These muscles are ar-
ranged in antagonistic groups; that is, they are so placed that
whenever the vertebral column is bent through the contraction of
one group the muscles of the antagonistic group are put on the
stretch. The neural arch of each vertebra bears a dorsal spinous
process (Fig. 19, Ps), and a pair of lateral transverse processes
(Fig. 19, Pt}. These serve various purposes; the intervertebral
muscles are attached to them; they also bear articular surfaces
(Pas and Pai Figs. 19 and 20) which sliding upon corresponding
surfaces of adjoining vertebrae serve to limit the movements at
each joint, and also help to prevent dislocation of the vertebral
column. The spinous processes may be felt in the middle of the

THE SKELETON

53

back. The neural arches are notched (Fig. 20, 7s and Fi), adjoin-
ing notches forming rounded openings through which the spinal
nerves pass on their way out from the spinal cord.

r«

FIG. 20.

FIG. 19. — A thoracic vertebra seen from behind, t. e., the end turned from the
head.

FIG. 20. — Two thoracic vertebrae viewed from the left side, and in their natural
relative positions. C, the body; A, neural arch; Ps, spinous process; Pas, anterior
articular process ; Pai, posterior articular process ; Pt, transverse process ; Ft, facet
for articulation with the tubercle of a rib; Fes, Fci, articular surfaces on the centrum
for articulation with a rib.

V

FIG. 21.

FIG. 22.

FIG. 21. — Diagrammatic representation of a segment of the axial skeleton
V, a vertebra; C, Cv, ribs articulating above with the body and transverse process
of the vertebra; <S, the breast-bone. The lighter-shaded part between S and C is
the costal cartilage.

FIG. 22. — A cervical vertebra. Frt, vertebral foramen; Pai, anterior articular
process; R, rudimentary rib.

The Cervical Vertebrae. (Fig. 22), have rather small bodies and
large neural arches; in some of them the spinous process is bifid.
They move more freely upon each other than do the vertebrae
lower down. A rudimentary rib (R, Fig. 22) becomes united

54

THE HUMAN BODY

early in life to the ventral surface of each transverse process; the
foramina (Fig. 22, Frt) thus formed give passage to an important
artery which ultimately passes into the cranial cavity to carry
blood to the brain.

The Atlas and Axis. The first and second cervical vertebrae
differ considerably from the rest. The first, or atlas (Fig. 23),
which carries the head, has a very small body, Aa, and a large
neural ring. This ring is subdivided by a cord, the transverse
ligament, L, into a dorsal moiety in which the spinal cord lies and

Aa Fas

D

D

K.is

Frt

Pal

FIQ. 24.

Fio. 23. — The atlas. FIG. 24. — The axis. Aa, body of atlas; D, odontoid
process; Fas, facet on front of altas with which the skull articulates; and in Fig. 24,
anterior articular surface of axis; L, transverse ligament; Frt, vertebral foramen;
Ap, neural arch ; Tp, spinous process.

a ventral into which the bony process D projects. This is the
odontoid process, and arises from the front of the axis or second
cervical vertebra (Fig. 24). Around this peg the atlas rotates
when the head is turned from side to side, carrying the skull (which
articulates with the large hollow surfaces Fas) with it.

The odontoid process really represents a large piece of the body
of the atlas which in early life separates from its own vertebra and
becomes united to the axis.

The Thoracic Vertebras have larger bodies and longer processes
than do the cervical vertebrae. They are specially modified for
carrying the ribs. Each rib is attached at two points (Fig. 21).
The head of the rib fits into an articulation at the junction of two
vertebrae, a part of the articular surface being on the centrum of
one and a part on the other (Fig. 20, Fes and Fci) . The second
attachment is between a point on the neck of the rib and an artic-
ular surface at the end of the transverse process of the posterior
of the two vertebrae which the rib touches (Fig. 20, Ft) .

THE SKELETON

55

The Lumbar Vertebrae (Fig. 25) arc the largest of all the mov-
able vertebrae and have no ribs attached to them. Their spines are
short and stout and lie in a more horizontal plane than those of the

PHI

Fa

II F.i

FIG. 25. — A lumbar vertebra, seen from the left side. Ps, spinous process;
Pas, anterior articular process; Pai, posterior articular process; Pt, transverse
process.

Fio. 27.
The coccyx.

Fio. 26. — The last lumbar vertebra and the sacrum seen from the ventral side.

vertebrae in front. The articular and transverse processes are also
short and stout.
The Sacrum, which is represented along with the last lumbar

56 THE HUMAN BODY

vertebra in Fig. 26, consists in the adult of a single bone; but
cross-ridges on its ventral surface indicate the limits of the five
separate vertebrae of which it is composed in childhood. It is some-
what triangular in form, its base being directed upwards and
articulating with the under surface of the body of the fifth lumbar
vertebra. On its sides are large surfaces to which the arch bearing
the lower limbs is attached (see Fig. 17). Its ventral surface is
concave and smooth and presents four pairs of anterior sacral
foramina, which communicate with the neural canal. Its dorsal
surface, convex and roughened, has four similar pairs of posterior
sacral foramina.

The coccyx (Fig. 27) calls for no special description. The four
bones which grow together, or ankylose, to form it, represent only
the bodies of vertebrae, and even those incompletely.

Details of the Skull. An account of the bones which make up
the skull can conveniently be given in tabular form. Examination
of the table will show that all the bones are either single or paired.
Single bones are all median, paired bones occupy corresponding
positions on each side of the mid-line. Figs. 28 and 29 will enable
the reader to gain a fairly good notion of the form and relations
of individual bones; for greater detail works on anatomy should
be consulted.
Cranium:

1 Frontal, forehead (Fig. 28, F).

2 Parietal, crown (Fig. 28, Pr).

1 Occipital, base of skull (Fig. 28, 0).

2 Temporal, ear region (Fig. 28, T) .

1 Sphenoid, base of cranium and back of orbit (Fig. 28, £).
1 Ethmoid, between cavities of cranium and nose (Fig. 28, E) .
Face:

1 Inferior maxilla, lower jaw (Fig. 28, Md).

2 Maxillae, upper jaw, front of hard palate (Fig. 28, MX),

2 Palatine, back of hard palate, front of posterior nares
(Fig. 29, PO.
2 Nasal, bridge of nose (Fig. 28, N).

1 Vomer, partition between nostrils (Fig. 29, V).

2 Inferior turbinate, inside nostrils (not shown in Fig.) .
2 Malar, cheeks (Fig. 28, Z).

2 Lachrymal, inside wall of orbit (Fig. 28, L).

THE SKELETON

57

All these bones except the inferior maxilla are immovably
joined together in the adult by irregular, saw-tooth like articu-
lations. The inferior maxilla articulates with the temporal bones
in such a way as to permit not only rotation about the points of
articulation but also a certain amount of sliding from side to side
and from back to front, thus making possible the grinding move-
ments of chewing.

Tsp

E

Md

Fio. 28. — A side view of the skull. O, occipital bone; T, temporal; Pr, parietal;
F, frontal ; S, sphenoid ;Z, malar; MX, maxilla; N, nasal ;E, ethmoid;!/, lachrymal;
Md, inferior maxilla.

There are several features of the skull which call for special
comment. The foramen magnum (Fig. 29) is a large opening into
the cranial cavity through the occipital bone; through it the
spinal cord passes on its way to the brain. On each side of the

58

THE HUMAN BODY

0 c

foramen magnum is an occipital condyle (Fig. 29, oc). These
are the points at which the skull rests upon the atlas. The orbits

or eye sockets are outlined in front
by the frontal, malars, and max-
illaB. The space behind the orbit,
between the malar and temporal
bones, is occupied by a large mus-
cle which closes the jaw. The shape
of the face depends very largely
upon the malar bones. The an-
terior nares, or openings of the nos-
trils are bounded by the maxillae
and nasals. The posterior nares, by
which the nose communicates with
the throat cavity, lie behind the pal-
FIQ. 29.— The base of the skull, ate bones (Fig. 29) . Enlargements

The lower jaw has been removed. . . , , . ,,

At the lower part of the figure is ot the temporal bones contain the

the hard palate forming the roof of «1irlitnrv onnarfltim
the mouth and surrounded by the aua O1T apparatus.

upper set of teeth. Above this are The Hyoid. Besides the cranial

the paired openings of the posterior . ••

nares, and a short way above the and facial bones there is, as already

middle of the figure is the large me- • , j ,1 ,-> ? • j

dian foramen magnum, with the pointed Out, One Other, the kyOld

bony convexities (or occimUd con- (Fig 3Q) which really belongs to

dyles) o. c., which articulate with v ...

the atlas, on its sides; v, the the skull, although it lies in the

vomer; pt, the palatines. , T, . . ,, . ,, .

neck. It can be felt in the front

of the throat, just above "Adam's apple." The hyoid bone
is U-shaped, with its convexity turned ventrally, and con-
sists of a body and two pairs of processes called cornua. The
smaller cornua (Fig. 30, 3) are attached to the
base of the skull by long ligaments. The bone
serves as an attachment for the base of the
tongue. The hyoid is of much interest from the
standpoint of comparative anatomy because in FIG. so.— The hy-

, TT T> j v • r °id bone. 1, body;

the very young Human Body it is part of a 2, great cornua; 3,
structure which corresponds to the gill mechanism sma11
of fish, tadpoles, and similar aquatic animals, consisting of several
gill arches with gill clefts between them. In the human embryo
the gill clefts close before birth, and all the gill arches disappear
except those which persist as the hyoid. It is difficult to explain
the development and subsequent disappearance of this structure

THE SKELETON

59

in the embryo except upon the theory which is part of the doctrine
of evolution that each individual epitomizes in his own develop-

FIG. 31.

Fio. 32.

Fio. 31. — The sternum seen on its ventral aspect. M, manubrium; C, body:
P, ensiform cartilage; Id, notch for the collar-bone; Ic 1-7, notches for the rib-
cartilages.

FIG. 32. — The ribs of the left side, with the dorsal and two lumbar vertebrae,
the rib-cartilages and the sternum.

mental history the evolutionary history of the race to which he
belongs.

The Ribs (Fig. 32) . There are twelve pairs of ribs, each being

60 THE HUMAN BODY

a slender curved bone attached dorsally to the body and transverse
process of a vertebra in the manner already mentioned, and con-
tinued ventrally by a costal cartilage (Fig. 21). In the case of the
anterior seven pairs, the costal cartilages are attached directly to
the sides of the breast-bone; the next three cartilages are each at-
tached to the cartilage of the preceding rib, while the cartilages of
the eleventh and twelfth ribs are quite unattached ventrally, so
these are called the free or floating ribs. The convexity of each
curved rib is turned outwards so as to give roundness to the sides
of the chest and increase its cavity, and each slopes downwards
from its vertebral attachment, so that its sternal end is consider-
ably lower than its dorsal.

Sternum. The sternum or breast-bone (Fig. 31 and d, Fig. 17)
is wider from side to side than dorsoventrally. It consists in the
adult of three pieces, and seen from the ventral side has. somewhat
the form o/ a dagger. At the upper end are notches for the articu-
lations of the collar-bones (Fig. 31, Icl), and along each side notches
for the articulations of the anterior costal cartilages (Fig. 31, Ic,
1-7).

The Appendicular Skeleton. This consists of the shoulder-
girdle and the bones of the fore limbs, and the pelvic girdle and the
bones of the posterior limbs. The two supporting girdles in their
natural position with reference to the trunk skeleton are repre-
sented in Fig. 33.

The Shoulder-girdle, or Pectoral Arch. This is made up, on
each side, of the scapula or shoulder-blade, and the clavicle or collar-
bone.

The scapula (S, Fig. 33) is a flattish triangular bone which can
readily be felt on the back of the thorax. It is not directly articu-
lated to the axial skeleton, but lies imbedded in the muscles and
other parts outside the ribs on each side of the vertebral column.
From its dorsal side arises a crest to which the outer end of the
collar-bone is fixed, and on its outer edge is a shallow cup into
which the top of the arm-bone fits: this hollow is known as the
glenoid fossa.

The collar-bone (C, Fig. 33) is cylindrical and attached at its
inner end to the sternum as shown in the figure, fitting into the ,
notch represented at Icl in Fig. 31.

The Pelvic Girdle (Fig. 33). This consists of a large bone, the

THE SKELETON

61

os innominatum, Oc, on each side, which is firmly fixed dorsally to
the sacrum and meets its fellow in the middle ventral line. In the
child each os innominatum consists of three bones, viz., the ilium,
the ischium, and pubis. Where these three bones meet and finally
ankylose there is a deep socket, the acetabulum, into which the

FIG. 33. — The skeleton of the trunk and the limb arches seen from the front.
C, clavicle; S, scapula; Oc, innominate bone attached to the side of the sacrum
dorsally and meeting its fellow at the pubic sytnphysis in the ventral median line.

head of the thigh-bone fits (see Fig. 17). Between the pubic and
ischial bones is the largest foramen in the whole skeleton, known as
the doorlike or thyroid foramen. The pubic bone lies above and
the ischial below it. The ilium forms the upper expanded portion
of the os innominatum to which the line drawn from Oc in Fig. 33
points.

62 THE HUMAN BODY

Fore and Hind Limbs. Each of these contains thirty bones, and
their arrangement is very similar. This is clearly seen in the
figures (34 and 35) , and is also brought out in the following table
in which the bones of the extremities are enumerated.

Fore Limb Hind Limb

a. Humerus, upper arm. Femur, thigh.

6. Ulna, large bone of forearm. Tibia, shin bone.

c. Radius, smaller bone of forearm. Fibula, small bone of calf.

d. 8 carpals, wrist. 7 tarsals, heel and upper instep.

e. 5 metacarpals, hand. 5 metatarsals, lower instep.

/. 14 phalanges, fingers and thumb. 14 phalanges, toes (2 in great toe, 3

(2 in thumb, 3 in each finger). in others).

g. Patella, knee-cap.

In general the bones of the hind limb are larger and stronger
than the corresponding ones of the fore limb; the femur is the
longest bone in the body. The phalanges, however, are smaller
in the foot than in the hand. The tarsals are one less in num-
ber than the carpals because one of the tarsal bones, the astragalus
(Fig. 38, To), is composed of two bones which have united into one.
A structure of the arm corresponding to the patella is the olecranon
process of the ulna which can be felt at the back of the elbow; in
early life this is a separate bone.

The differences in structure between fore and hind limb corre-
spond to differences of function; the fore limb being a prehensile
organ is capable of great freedom of motion; the hind limb, which
is a supporting and locomotor organ, is adapted rather to main-
tain the weight of the body and to execute the movements of
walking and running to advantage. The special adaptation of the
arm to its purpose is seen particularly in three things: 1, the com-
paratively flexible attachment of the pectoral girdle to the axial
skeleton (Fig. 36), an attachment composed wholly of muscle and
ligament except where the inner ends of the clavicles articulate
with the sternum ; 2, the rotation of the radius over the ulna, an
arrangement which increases very greatly the flexibility of the
hand; 3, the articulation of the thumb, which is of such a sort as
to allow it to be opposed to any of the fingers, thus enabling the
hand to manipulate small objects without difficulty. The leg, on
the other hand, is characterized by much greater firmness, which

THE SKELETON

63

is obtained at the expense of flexibility. The pelvic arch (Figs. 33
and 37) is not only heavy and strong, but is very firmly fixed to
the axial skeleton, the sacrum and os innominatum becoming in
mature life practically one bone. The socket into which the head

Fro. 34. Fio. 35.

FIG. 34. — The bones of the arm. a, humerus; 6, ulna; c, radius; d, the carpus;
e, the fifth metacarpal; /, the three phalanges of the fifth digit (little finger).

FIG. 35. — Bones of the leg. a, femur; 6, tibia; c, fibula; a, tarsal bones; e, meta-
tarsal bones ; /, phalanges ; g, patella.

of the femur fits is much deeper than that which receives the
head of the humerus, rendering the leg much less liable to dislo-
cation than the arm, but at the same time restricting its move-
ments much more. The foot also in becoming adapted to form a

64

THE HUMAN BODY

support for the body has sacrificed its prehensile structure almost
altogether; the toes are less flexible than the fingers and the great

FIG. 36. — Diagram showing the relation of the pectoral arch to the axial skeleton.

toe cannot be opposed to the others. A special modification of
the foot for its particular function is seen in the arching of the
instep. As Fig. 38 shows the bones of the foot form a springy arch,

Othe points of contact with the ground
being at the extremity of the heel
^ bone (os calcis, Ca of figure), and the
^^ distal ends of the metatarsals. The
bones of the leg are mounted upon
the crown of the arch (Ta of figure).

Peculiarities of the Human Skele-
ton. These are largely connected
with the division of labor between the fore and hind limbs re-
ferred to above, which is carried farther in man than in any other
creature. Even the highest apes frequently use their fore limf

FIG. 37. — Diagram showing the
attachment of the pelvic arch to
the axial skeleton.

Cl

cn

Sfhl

FIG. 38. — The bones of the foot. Ca, calcaneum, or os calcis; Ta, articular surface
for tibia on the astragalus; N, scaphoid bone; CI, CII, first and second cuneiform
bones; Cb, cuboid bone; Ml, netatarsal bone of great toe.

in locomotion and their hind limbs in prehension, and we find ac-
cordingly that anatomically they present less differentiation of
hand and foot. The other more important characteristics of the
human skeleton are correlated for the most part with the mainte-

THE SKELETON 65

nance of the erect posture, which is more complete and habitual
in man than in the animals most closely allied to him anatomically.
These peculiarities, however, only appear fully in the adult. In
the infant the head is proportionately larger, which gives the
center of gravity of the Body a comparatively very high position
and renders the maintenance of the erect posture difficult and in-
secure. The curves of the vertebral column are nearly absent,
and the posterior limbs are relatively very short. In all these
points the infant approaches more closely than the adult to the
ape. The subsequent great relative length of the posterior limbs,
which grow disproportionately fast in childhood as compared with
the anterior, makes progression on them more rapid by giving a
longer stride and at the same time makes it almost impossible to
go on " all fours " except by crawling on the hands and knees. In
other Primates this disproportion between the anterior and pos-
terior limbs does not occur to nearly the same extent.

In man the skull is nearly balanced on the top of the vertebral
column, the occipital condyles which articulate with the atlas
being about its middle (Fig. 28), so that but little effort is needed
to keep the head erect. In four-footed beasts, on the contrary, the
skull is carried on the front end of the horizontal vertebral column
and needs special ligaments to sustain it. For instance, in the ox
and sheep there is a great elastic cord running from the cervical
vertebrae to the back of the skull and helping to hold up the head.
Even in the highest apes the skull does not balance on the top of
the spinal column; the face pa.rt is much heavier than the back,
while in man the face parts are relatively smaller and the cranium
larger, so that the two nearly equipoise. To keep the head erect
and look things straight in the face, "like a man," is for the apes
far more fatiguing, and so they cannot long maintain that position.

The human spinal column, gradually widening from the neck to
the sacrum, is well fitted to sustain the weight of the head, upper
limbs, etc., carried by it; and its curvatures, which are peculiarly
human, give it considerable elasticity combined with strength.
The pelvis, to the sides of which the lower limbs are attached, is
proportionately very broad in man, so that the balance can be
more readily maintained during lateral bending of the trunk. The
arched instep and broad sole of the human foot are also very
characteristic. The majority of four-footed beasts, as horses,

66 THE HUMAN BODY

walk on the tips of their toes and fingers; and those animals, as
bears and apes, which like man place the tarsus also on the ground,
or in technical language are plantigrade, have a much less marked
arch there. The vaulted human tarsus, composed of a number of
small bones, each of which can glide a little over its neighbors, but
none of .which can move much, is admirably calculated to break
any jar which might be transmitted to the spinal column by the
contact of the sole with the ground at each step. A well-arched
instep is therefore rightly considered a beauty; it makes progres-
sion easier, and by its springiness gives elasticity to the step. In
London flat-footed candidates for appointment as policemen are
rejected, as they cannot stand the fatigue of walking the daily
"beat."

Hygiene of the Bony Skeleton. In early life the bones are less
rigid, from the fact that the earthy matters then present in them
bear a less proportion to the softer organic parts. Hence the bones
of an aged person are more brittle and easily broken than those of a
child. The bones of a young child are in fact tolerably flexible
and may be distorted by any continued strain; therefore children
should never be kept sitting for hours, in school or elsewhere, on a
bench which is so high that the feet are not supported. If this be
insisted upon (for no child will continue it voluntarily) the thigh-
bones will almost certainly be bent over the edge of the seat by the
weight of the legs and feet, and a permanent distortion may be
produced. For the same reason it is important that a child be
made to sit straight while writing, to avoid the risk of producing
a lateral curvature of the spinal column. The facility with which
the bones may be molded by prolonged pressure in early life is
well seen in the distortion of the feet of Chinese ladies, produced
by keeping them in tight shoes; and in the extraordinary forms
which some races of man produce in their skulls, by tying boards on
the heads of the children.

Throughout the whole of life, moreover, the bones remain among
the most easily modified parts of the Body; although judging from
the fact that dead bones are the most permanent parts of fossil
animals we might be inclined to think otherwise. The living bone,
however, is constantly undergoing changes under the influence of
the protoplasmic cells imbedded in it, and in the living Body is
constantly being absorbed and reconstructed. The experience of

THE SKELETON 67

physicians showa that any continued pressure, such as that of a
tumor, will cause the absorption and disappearance of bone almost
quicker than that of any other tissue; and the same is true of any
other continued pressure. Moreover, during life the bones are
eminently plastic; under abnormal pressures they are found to
quickly assume abnormal shapes, being absorbed and disappear-
ing at points where the pressure is most powerful, and increasing
at other points; tight lacing may in this way produce a permanent
distortion of the ribs.

When a bone is fractured a surgeon should be called in as soon
as possible, for once inflammation has set in and the parts have be-
come swollen it is much more difficult to place the broken ends of
the bone together in their proper position than before this has
occurred. Once the bones are replaced they must be held in posi-
tion by splints or bandages, or the muscles attached to them will
soon displace them again. With rest, in young and healthy per-
sons complete union will commonly occur in three or four weeks;
but in old persons the process of healing is slower and is apt to be
imperfect.

Articulations. The bones of the skeleton are joined together in
very various ways; sometimes so as to admit of no movement at
all between them; in other cases so as to permit only a limited
range or variety of movement; and elsewhere so as to allow of very
free movement in many directions. All kinds of unions between
bones are called articulations.

Of articulations permitting no movements, those which unite
the majority of the cranial bones afford a good example. Except
the lower jaw, and certain tiny bones inside the temporal bone be-
longing to the organ of hearing, all the skull-bones are immovably
joined together. This union in most cases occurs by means of
toothed edges which fit into one another and form jagged lines of
union known as sutures. Some of these can be well seen in Fig. 28
between the frontal and parietal bones (coronal suture) and be-
tween the parietal and occipital bones (lambdoidal suture) ; while
another lies along the middle line in the top of the crown between
the two parietal bones, and is known as the sagittal suture. In new-
born children where the sagittal meets the coronal and lambdoidal
sutures there are large spaces not yet covered in by the neighboring
bones, which subsequently extend over them. These openings

68 THE HUMAN BODY

are known as fontanelles. At them a pulsation can often be felt
synchronous with each beat of the heart, which, driving more blood
into the brain, distends it and causes it to push out the skin where
bone is absent. Another good example of an articulation admit-
ting of no movement is that between the rough surfaces on the
sides of the sacrum and the innominate bones.

We find good examples of the second class of articulations —
those admitting of a slight amount of movement — in the vertebral
column. Between every pair of vertebrae from the second cervical
to the sacrum is an elastic pad, the intervertebral disk, which ad-
heres by its surfaces to the bodies of the vertebrsB between which it
lies, and only permits so much movement between them as can
be brought about by its own compression or stretching. When
the back-bone is curved to the right, for instance, each of the
intervertebral disks is compressed on its right side and stretched
a little on its left, and this combination of movements, each in-
dividually but slight, gives considerable flexibility to the spinal
column as a whole.

Joints. Articulations permitting of movement by the gliding of
one bone over another are known as joints, and all have the same
fundamental structure, although the amount of movement per-
mitted in different joints is very different.

Hip- joint. We may take this as a good example of a true joint
permitting a great amount and variety of movement. On the
os innominatum is the cavity of the acetabulum (Fig. 39) , which is
lined inside by a thin layer of articular cartilage which has an ex-
tremely smooth surface. The bony cup is also deepened a little by
a cartilaginous rim. The proximal end of the femur consists of a
nearly spherical smooth head, borne on a somewhat narrower neck,
and fitting into the acetabulum. This head also is covered with
articular cartilage; and it rolls in the acetabulum like a ball in a
socket. To keep the bones together and limit the amount of move-
ment, ligaments pass from one to the other. These are composed
of white fibrous connective tissue (Chap. IV) and are extremely
pliable, but quite inextensible and very strong and tough. One is
the capsular ligament, which forms a sort of loose bag all round the
joint, and another is the round ligament, which passes from the
acetabulum to the head of the femur. Should the latter rotate
above a certain extent in its socket, the round ligament and one

THE SKELETON

69

side of the capsular ligament are put on the stretch, and any fur-
ther movement which might dislocate the femur (that is, remove
the head from its socket) is checked. Covering the inside of the
capsular ligament and the outside of the round ligament is a layer
of flat cells, which are continued in a modified form over the ar-
ticular cartilages and form the synovial membrane. This, which
thus forms the lining of the joint, is always moistened in health
by a small quantity of glairy synovial fluid, something like the

FIG. 39. — Section through the hip-joint, a and 6, articular cartilages; c, capsu-
lar ligament.

white of a raw egg in consistency, and playing the part of the oil
with which the contiguous moving surfaces of a machine are mois-
tened ; it maizes all run smoothly with very little friction.

In the natural state of the parts, the head of the femur and the
bottom and sides of the acetabulum lie in close contact, the two
synovial membranes rubbing together. This contact is not main-
tained by the ligaments, which are too loose and serve only to
check excessive movement, but by the numerous stout muscles
which pass from the thigh to the trunk and bind the two firmly
together. Moreover, the atmospheric pressure exerted on the sur-
face of the Body and transmitted through the soft parts to the
outside of the air-tight joint helps also to keep the parts in contact.
If all the muscles and ligaments around the joint be cut away, it is

70 THE HUMAN BODY

still found in the dead Body that the head of the femur will be kept
in its socket by this pressure, and so firmly as to bear the weight
of the whole limb without dislocation, just as the pressure of the
air will enable a boy's " sucker " to lift a tolerably heavy stone.

Ball-and-socket Joints. Such a joint as that at the hip is called
a ball-and-socket joint and allows of more free movement than any
other. Through movements occurring in it the thigh can be flexed,
or bent so that the knee approaches the chest; or extended, that is,
moved in the opposite direction. It can be abducted, so that the
knee moves outwards; and adducted, or moved back towards the
other knee again. The limb can also by movements at the hip-
joint be circumducted, that is, made to describe a cone of which the
base is at the foot and the apex at the hip. Finally, rotation can
occur in the joint, so that with knee and foot joints held rigid the
toes can be turned in or out, to a certain extent, by a rolling around
of the femur in its socket.

At the junction of the humerus with the scapula is another ball-
and-socket joint permitting all the above movements to even a
greater extent. This greater range of motion at the shoulder-joint
depends mainly on the shallowness of the glenoid cavity as com-
pared with the acetabulum, and upon the absence of any ligament
answering to the round ligament of the hip-joint. Another
ball-and-socket joint exists between the carpus and the metacarpal
bone of the thumb ; and others with the same variety, but a much
less range, of movement between each of the remaining metacarpal
bones and the proximal phalanx of the finger which articulates
with it.

Hinge- joints. Another form of synovial joint is known as a hinge-
joint. In it the articulating bony surfaces are of such shape as to
permit of movement, to and fro, in one plane only, like a door on
its hinges. The joints between the phalanges of the fingers are
good examples of hinge-joints. If no movement be allowed where
the finger joins the palm of the hand it will be found that each can
be bent and straightened at its own two joints^but not moved in
any other way. The knee is also a hinge-joint, as is the articula-
tion between the lower jaw and the base of the skull which allows
us to open and close our mouths. The latter is, however, not a
perfect hinge-joint, since it permits of a small amount of lateral
movement such as occurs in chewing, and also of a gliding move-

THE SKELETON

71

fr-

mcnt by which the lower jaw can be thrust forward so as to pro-
trude the chin and bring the lower row of teeth outside the upper.

Pivot-joints. In this form one bone rotates around another
which remains stationary. We have a good example of it between
the first and second cervical vertebrae. The first cervical vertebra
or atlas (Fig. 23) has a very small body and a very large arch, and
its neural canal is subdivided by a transverse ligament (L, Fig. 23)
into a dorsal and a ventral portion; in
the former the spinal cord lies. The
second vertebra or axis (Fig. 24) has
arising from its body the stout bony
peg, D, called the odontoid process.
This projects into the ventral portion
of the space surrounded by the atlas,
and, kept in place there by the trans-
verse ligament, forms a pivot around
which the atlas, carrying the skull
with it, rotates when we turn the head
from side to side. The joints on each
side between the atlas and the skull
are hinge-joints and permit only the
movements of nodding and raising the
head. When the head is leaned over
to one side, the cervical part of the
spinal column is bent.

Another kind of pivot-joint is seen

in the forearm. If the limb be held humerus;«, radius; u, ulna,
straight out, with the palm up and the elbow resting on the table,
so that the shoulder-joint be kept steady while the hand is
rotated until its back is turned upwards, it will be found that the
radius has partly rolled round the ulna. When the palm is up-
wards and the thumb outwards, the lower end of the radius can
be felt on the outer side of the forearm just above the wrist, and if
this be done while the hand is turning over, it will be easily dis-
cerned that during the movement this end of the radius, carrying
the hand with it, travels around the lower end of the ulna so as to
get to its inner side. The relative position of the bones when the
palm is upwards is shown at A in Fig. 40, and when the palm is
down at B. The former position is known as supination; the latter

72 THE HUMAN BODY

as pronation. The elbow end of the humerus (Fig. 40) bears a
large articular surface: on the inner two-thirds of this, the ulna fits,
and the ridges and grooves of both bones interlocking form a hinge-
joint, allowing only of bending or straightening the forearm on the
arm. The radius fits on the rounded outer third, and forms there
a ball-and-socket joint at which the movement takes place when
the hand is turned from the supine to the prone position; the ulna
forming a fixed bar around which the lower end of the radius is
moved.

Gliding Joints. These permit as a rule but little movement:
examples are found between the closely packed bones of the tarsus
and carpus, (Figs. 34 and 35) which slide a little over one another
when subjected to pressure.

Hygiene of the Joints. When a bone is displaced or dislocated
the ligaments around the joint are more or less torn and other
soft parts injured. This soon leads to inflammation and swelling
which make not only the recognition of the injury but, after
diagnosis, the replacement of the bone, or the reduction of the dis-
location, difficult. Moreover, the muscles attached to it constantly
pull on the displaced bone and drag it still farther out of place; so
that it is of great importance that a dislocation be reduced as soon
as possible. In most cases this can only be attempted with safety
by one who knows the form of the bones, and possesses sufficient
anatomical knowledge to recognize the direction of the displace-
ment. No injury to a joint should be neglected. Inflammation
once started there is often difficult to check and runs on, in a
chronic way, until the synovial surfaces are destroyed, and the
two bones perhaps grow together, rendering the joint permanently
stiff.

Immediate and complete rest has been commonly supposed to
be the only proper treatment for sprained joints, but it has been
shown recently that massage, properly applied by one expert in its
use, has a remarkably beneficial effect upon sprains. Injuries of
this sort so severe that under the rest treatment they would re-
quire weeks for recovery yield so completely in a few days to
massage treatment that the injured individual can participate in
athletic contests. It should be borne in mind that massage to be
effective must be applied by an expert in its use.

CHAPTER VI
THE STRUCTURE OF THE MOTOR ORGANS

Motion in Animals and Plants. If one were asked to point out
the most distinctive property of living animals, the answer would
probably be, their power of executing spontaneous movements.
Animals as we commonly know them are rarely at rest, while trees
and stones move only when acted upon by external forces, which
are in most cases readily recognizable. Even at their quietest
times some kind of motion is observable in the higher animals. In
our own Bodies during the deepest sleep the breathing movements
and the beat of the heart continue; their cessation is to an onlooker
the most obvious sign of death. Here, however, as elsewhere in
Biology, we find that precise boundaries do not exist; at any rate
so far as animals and plants are concerned we cannot draw a hard
and fast line between them with reference to the presence or ab-
sence of apparently spontaneous motility. Many a flower closes
in the evening to expand again in the morning sun; and in many
plants comparatively rapid and extensive movements can be
called forth by a slight touch, which in itself is quite insufficient
to produce mechanically that amount of motion in the mass. The
Venus's fly-trap (Dioncea muscipuLa) for example has fine hairs on
its leaves, and when these are touched by an insect the leaf closes
up so as to imprison the animal, which is subsequently digested
and absorbed by the leaf. The higher plants it is true have not the
power of locomotion, they cannot change their place as the higher
animals can ; but on the other hand some of the lower animals are
permanently fixed to one spot ; and among the lowest plants many
are known which swim about actively through the water in which
they live. The lowest animals and plants are in fact those which
have undergone least differentiation in their development, and
which therefore resemble each other in possessing, in a more or less
manifest degree, all the fundamental physiological properties of
that simple mass of protoplasm which formed the starting-point of
each individual. With the physiological division of labor which

73

74 THE HUMAN BODY

takes place in the higher forms we find that, speaking broadly,
plants especially develop nutritive tissues, while animals are
characterized by the high development of tissues with motor and
irritable properties; so that the preponderance of these latter is
very marked when a complex animal, like a dog or a man, is com-
pared with a complex plant, like a pine or a hickory. The higher
animal possesses in addition to greatly developed nutritive tissues
(which differ only in detail from those of the plant, and constitute
what are therefore often called organs of vegetative life) well-
developed irritable conducting, and contractile tissues, found
mainly in the nervous and muscular systems, and forming what
have been called the organs of animal life. Since these place the
animal in close relationship with the surrounding universe, en-
abling slight external forces to excite it, and it in turn to act upon
external objects, they are also often spoken of as organs of relation.
In man they have a higher development on the whole than in any
other animal, and give him his leading place in the animate world,
and his power of so largely controlling and directing natural forces
for his own good, while the plant can only passively strive to en-
dure and make the best of what happens to it ; it has little or no in-
fluence in controlling the happening.

Amoeboid Cells. The simplest motor tissues in the adult Human
Body are the amoeboid cells (Fig. 99) already described, which
may be regarded as the slightly modified descendants of the un-
differentiated cells which at one time made up the whole Body.
In the adult they are not attached to other parts, so that their
changes of form only affect themselves and produce no movements
in the rest of the Body. Hence with regard to the whole frame they
can hardly be called motor tissues, and are classed in the group of
undifferentiated tissues.

Ciliated Cells. As the growing Body develops from its primitive
simplicity we find that the cells lining some of the tubes and
cavities in its interior undergo a very remarkable change, by which
each cell differentiates itself into a nutritive and a highly motile
portion. Such cells are found for example lining the windpipe, and
are represented in Fig. 41. Each has a conical form, the base of
the cone being turned to the cavity of the air-tube, and contains
an oval nucleus with a nucleolus. On the broader free end are a
number (about thirty on the average) of extremely fine processes

THE STRUCTURE OF THE MOTOR ORGANS 75

called cilia. During life these are in constant rapid movement,
lashing to and fro in the liquid which moistens the interior of the
passage; and ag the cells are very closely packed, a bit of the inner
surface of the windpipe, examined with a mi-
croscope, looks like a field of wheat or barley
when the wind blows over it. Each cilium
strikes with more force in one direction than
in the opposite, and as this direction of more
powerful stroke is the same for all the cilia on
any one surface, the resultant effect is that the Flo 41 _cjiiated
liquid in which they move is driven one way. cells-
In the case of the windpipe for example it is driven up towards the
throat, and the tenacious liquid or mucus which is thus swept
along is finally coughed or " hawked " up and got rid off, instead
of accumulating in the deeper air-passages away down in the chest.

These cells afford an extremely interesting example of the di-
vision of physiological employments. Each proceeds from a cell
which was primitively equally motile and nutritive in all its parts.
But in the fully developed state the nutritive duties have been
especially assumed by the conical cell-body, while the contractile
properties have been condensed, so to speak, in that modified
portion of the primitive protoplasmic mass which forms the cilia.
These, being supplied with elaborated food by the rest of the cell,
are raised above the vulgar cares of life and have the opportunity
to devote their whole attention to the performance of automatic
movements; which are accordingly far more rapid and precise
than those executed by the whole cell before any division of labor
had occurred in it.

That the movements depend upon the structure and composi-
tion of the cells themselves, and not upon influences reaching them
from the nervous or other tissues, is proved by the fact that they
continue for a long time in isolated cells, removed and placed in a
liquid, as blood-serum, which does not alter their physical consti-
tution. In cold-blooded animals, as turtles, whose constituent
tissues frequently retain their individual vitality long after that
bond of union has been destroyed which constitutes the life of the
whole animal as distinct from the lives of its different tissues, the
ciliated cells in the windpipe have been found still at work three
weeks after the general death of the animal.

76 THE HUMAN BODY

The Muscles. These are the main motor organs; their general
appearance is well known to every one in the lean of butcher's
meat. While amceboid cells can only move themselves, and (at
least in the Human Body) ciliated cells the layer of liquid with
which they may happen to be in contact, the majority of the
muscles, being fixed to the skeleton, can, by alterations in their
form, bring about changes in the form and position of nearly all
parts of the Body. With the skeleton and joints, they constitute
preeminently the organs of motion and locomotion, and are gov-
erned by the nervous system which regulates their activity. In
fact skeleton, muscles, and nervous system are correlated parts:
the degree of usefulness of any one of them largely depends upon
the more or less complete development of the others. Man's
highly endowed senses and his powers of reflection and reason
would be of little use to him, were his muscles less fitted to carry
out the dictates of his will or his joints less numerous or mobile.
All the muscles are under the control of the nervous system, but
all are not governed by it with the cooperation of will or con-
sciousness; some move without our having any direct knowledge
of the fact. This is especially the case with certain muscles which
are not fixed to the skeleton but surround cavities or tubes in the
Body, as the blood-vessels and the alimentary canal, and by their
movements control the passage of substances through them. The
former group, or skeletal muscles, are also from their microscopic
characters known as striped muscles, while the latter, or visceral
muscles, are called unstriped or smooth muscles. The skeletal
muscles being generally more or less subject to the control of the
will (as for example those moving the limbs) are frequently spoken
of as voluntary, and the visceral muscles, which change their form
independently of the will, as involuntary. The heart muscle forms
a sort of intermediate link ; it is not directly attached to the skele-
ton, but forms a hollow bag which drives on the blood contained
in it and that quite- involuntarily ; but in its microscopic struc-
ture it resembles somewhat the skeletal voluntary muscles. The
muscles of respiration are striped skeletal muscles and, as we all
know, are to a certain extent subject to the will; any one can draw
a deep breath when he chooses. But in ordinary quiet breathing
we are quite unconscious of their working, and even when attention
is turned to them the power of control is limited; no one can

THE STRUCTURE OF THE MOTOR ORGANS 75

called cilia. During life these are in constant rapid movement,
lashing to and fro in the liquid which moistens the interior of the
passage; and as the cells are very closely packed, a bit of the inner
surface of the windpipe, examined with a mi-
croscope, looks like a field of wheat or barley
when the wind blows over it. Each cilium
strikes with more force*in one direction than
in the opposite, and as this direction of more
powerful stroke is the same for all the cilia on
any one surface, the resultant effect is that the
liquid in which they move is driven one way.
In the case of the windpipe for example it is driven up towards the
throat, and the tenacious liquid or mucus which is thus swept
along is finally coughed or " hawked " up and got rid off, instead
of accumulating in the deeper air-passages away down in the chest.

These cells afford an extremely interesting example of the di-
vision of physiological employments. Each proceeds from a cell
which was primitively equally motile and nutritive in all its parts.
But in the fully developed state the nutritive duties have been
especially assumed by the conical cell-body, while the contractile
properties have been condensed, so to speak, in that modified
portion of the primitive protoplasmic mass which forms the cilia.
These, being supplied with elaborated food by the rest of the cell,
are raised above the vulgar cares of life and have the opportunity
to devote their whole attention to the performance of automatic
movements; which are accordingly far more rapid and precise
than those executed by the whole cell before any division of labor
had occurred in it.

That the movements depend upon the structure and composi-
tion of the cells themselves, and not upon influences reaching them
from the nervous or other tissues, is proved by the fact that they
continue for a long time in isolated cells, removed and placed in a
liquid, as blood-serum, which does not alter their physical consti-
tution. In cold-blooded animals, as turtles, whose constituent
tissues frequently retain their individual vitality long after that
bond of union has been destroyed which constitutes the life of the
whole animal as distinct from the lives of its different tissues, the
ciliated cells in the windpipe have been found still at work three
weeks after the general death of the animal.

76 THE HUMAN BODY

The Muscles. These are the main motor organs; their general
appearance is well known to every one in the lean of butcher's
meat. While amoeboid cells can only move themselves, and (at
least in the Human Body) ciliated cells the layer of liquid with
which they may happen to be in contact, the majority of the
muscles, being fixed to the skeleton, can, by alterations in their
form, bring about changes in the form atid position of nearly all
parts of the Body. With the skeleton and joints, they constitute
preeminently the organs of motion and locomotion, and are gov-
erned by the nervous system which regulates their activity. In
fact skeleton, muscles, and nervous system are correlated parts:
the degree of usefulness of any one of them largely depends upon
the more or less complete development of the others. Man's
highly endowed senses and his powers of reflection and reason
would be of little use to him, were his muscles less fitted to carry
out the dictates of his will or his joints less numerous or mobile.
All the muscles are under the control of the nervous system, but
all are not governed by it with the cooperation of will or con-
sciousness; some move without our having any direct knowledge
of the fact. This is especially the case with certain muscles which
are not fixed to the skeleton but surround cavities or tubes in the
Body, as the blood-vessels and the alimentary canal, and by their
movements control the passage of substances through them. The
former group, or skeletal muscles, are also from their microscopic
characters known as striped muscles, while the latter, or visceral
muscles, are called unstriped or smooth muscles. The skeletal
muscles being generally more or less subject to the control of the
will (as for example those moving the limbs) are frequently spoken
of as voluntary, and the visceral muscles, which change their form
independently of the will, as involuntary. The heart muscle forms
a sort of intermediate link ; it is not directly attached to the skele-
ton, but forms a hollow bag which drives on the blood contained
in it and that quite involuntarily; but in its microscopic struc-
ture it resembles somewhat the skeletal voluntary muscles. The
muscles of respiration are striped skeletal muscles and, as we all
know, are to a certain extent subject to the will; any one can draw
a deep breath when he chooses. But in ordinary quiet breathing
we are quite unconscious of their working, and even when attention
is turned to them the power of control is limited; no one can

THE STRUCTURE OF THE MOTOR ORGANS 77

voluntarily hold his breath long enough to suffocate himself. As
wr shall see hereafter, moreover, any one or all of the striped
muscles of the Body may be thrown into activity independently
of or even against the will, as, to cite no other instances, is seen in
the "fidgets" of nervousness and the irrepressible trembling of
extreme terror; so that the names voluntary and involuntary are
not good ones. The functional differences between the two groups
depend really more on the nervous connections of each than upon
any essential difference in the properties of the so-called voluntary
or involuntary muscular tissues themselves.

The Skeletal Muscles. In its simplest form a skeletal muscle
consists of a red soft central part, the belly, which tapers at each
end and there passes into one or more dense white cords which
consist almost entirely of white fibrous connective tissue. These
terminal cords are called the tendons of the muscle and serve to
attach it to parts of the bony or cartilaginous skeleton. In
Fig. 42 is shown the biceps muscle of the arm, which lies in front of
the humerus. Its fleshy belly is seen to divide above and end there
in two tendons, one of which, Bl, is fixed to the scapula, while the
other, Bb, joins the tendon of a neighboring muscle (the coraco-
brachial, Cb), and is also fixed above to the shoulder-blade. Near
the elbow-joint the muscle is continued into a single tendon, B',
which is fixed to the radius, but gives an offshoot, B", to the
connective-tissue membranes lying around the elbow-joint.

The belly of every muscle possesses the power of shortening
forcibly under certain conditions. In so doing it pulls upon the
tendons, which being composed of inextensible white fibrous tissue
transmit the movement to the hard parts to which they are at-
tached, just as a pull at one end of a rope may be made to act upon
distant objects to which the other end is tied. The tendons are
merely passive cords and are sometimes very long, as for instance
in the case of the muscles of the fingers, the bellies of many of
which lie away in the forearm.

If the tendons at each end of a muscle were fixed to the same
bone the muscle would clearly be able to produce no movement,
unless by bending or breaking the bone ; the probable result in such
a case would be the tearing of the muscle by its own efforts. In
the Body, however, the two ends of a muscle are always attached
to different parts, usually two bones, between which more or less

78

THE HUMAN BODY

THE STRUCTURE OF THE MOTOR ORGANS 79

movement is permitted, and so when the muscle pulls it alters
the relative positions of the parts to which its tendons are fixed.
In the great majority of cases a true joint lies between the bones on
which the muscle can pull, and when the latter contracts it produces
movement at the joint. Many muscles even pass over two joints
and can produce movement at either, as the biceps of the arm
which, fixed at one end to the scapula and at the other to the
radius, can move the bones at either the shoulder or elbow-joint.
Where a muscle passes over an articulation it is nearly always re-
duced to a narrow tendon; otherwise the bulky bellies lying around
the joints would make them extremely clumsy and limit their
mobility.

Origin and Insertion of Muscles. Almost invariably that part of
the skeleton to which one end of a muscle is fixed is more easily

FIG. 43. — The biceps muscle and the" arm-bones, to illustrate how, under ordi-
nary circumstances, the elbow-joint is flexed when the muscle contracts.

moved than the part on which it pulls by its other tendon. The
less movable attachment of a muscle is called its origin, the more
movable its insertion. Taking for example the biceps of the arm,
we find that when the belly of the muscle contracts and pulls on its
upper and lower tendons, it commonly moves only the forearm,
bending the elbow-joint as shown in Fig. 43. The shoulder is so
much more firm that it serves as a fixed point, and so that end is
the origin of the muscle, and the forearm attachment, P, the in-
sertion. It is clear, however, that this distinction in the mobility
of the points of fixation of the muscle is only relative, for, by chang-
ing the conditions, the insertion may become the stationary and

80

THE HUMAN BODY

the origin the moved point; as for instance in going up a rope
"hand over hand." In that case the radial end of the muscle is
fixed and the shoulder is moved through space by its contraction.
Different Forms of Muscles. Many muscles of the Body have
the simple typical form of a belly tapering to a single tendon at
each end as A, Fig. 44; but others divide at one end and are called
two-headed or biceps muscles ; while some are even three-headed or
triceps muscles. On the other hand, some muscles have no tendon
at all at one end, the belly running quite up to the point of attach-
ment; and some have no tendon at either end. In many muscles
a tendon runs along one side and the fibers of the
belly are attached obliquely to it : such muscles
(B, Fig. 44) are called penniform or featherlike;
or a tendon runs obliquely down the middle of
the muscle and has the fibers of the belly fixed
obliquely on each side of it (C, Fig. 44) , forming
a bipenniform muscle: or even two tendons may
run down the belly and so form a tripenniform
muscle. In a few cases a tendon is found in the
iiiuttratin'" middle °f the belly as well as at each end of it;
typical muscles with such muscles are called digastric. A muscle of

a central belly and . . . . . . . . ,

two terminal ten- this form (Fig. 45) is found in connection with
fl30rmmuscie;c,eabi: the lower Jaw. It arises by a tendon attached
penniform muscle, to the base of the skull; from there its first belly
runs downwards and forwards to the neck by the side of the
hyoid bone, where it ends in a tendon which passes through a
loop serving as a pulley. This is succeeded by a second belly di-
rected upwards towards the chin, where it ends in a tendon in-
serted into the lower jaw. Running along the
front of the abdomen from the pelvis to the chest
is a long muscle on each side of the middle line
called the rectus abdominis: it is poly gastric, con-
sisting of four bellies separated by short tendons.
Many muscles moreover are not rounded but form gastric muscle,
wide flat masses, as for example the muscle Ss seen on the ventral
side of the shoulder-blade in Fig. 42.

Gross Structure of a Muscle. However the form of the skeletal
muscles and the arrangement of their tendons may vary, the
essential structure of all is the same. Each consists of a proper

FIG.
grams

FIG. 45. — A di-

THE STRUCTURE OF THE MOTOR ORGANS 81

striped muscular tissue, which is its essential part, but which is
supported by connective tissue, nourished by blood-vessels and
lymphatics, and has its activity governed by nerves; so that a
great variety of things go to form the complete organ.

A loose sheath of areolar connective tissue, called the peri-
mysiuin, envelops each muscle, and from this partitions run
in and subdivide the belly into bundles or fasciculi which
run from tendon to tendon, or for the whole
length of the muscle when it has no tendons.
The coarseness or fineness of butcher's meat de-
pends upon the size of these primary fasciculi,
which differs in different muscles of the same
animal. These larger fasciculi are subdivided
by finer connective tissue membranes into smaller
ones, each of which consists of a certain number
of microscopic muscular fibers bound together by
very fine connective tissue and enveloped in a
close network of blood-vessels. Where a muscle smaiiG part ' oF a
tapers the fibers in the fasciculi become less nu- nified^fh'ow'in^fs
merous,-and when a tendon is formed disappear al- cross-striationand

' . . . . a couple of nuclei.

together, leaving little but the connective tissue.

Histology of Skeletal Muscle. Each muscle-fiber is developed
from a single cell and so constitutes a single histological element.
In the adult form, however, a muscle-fiber differs from an ordinary
cell in that it contains several nuclei. Muscle-fibers vary greatly in
size; ranging in length from 1 up to 35 mm. (gV in. to 3 in.), and in
diameter from 0.034 to 0.055 mm. (TTTT to ?^ in.) . Each fiber con-
sists of a certain amount of muscle substance, the muscle plasma,
inclosed in a transparent connective tissue sheath, the sarco-
lemma. This latter structure serves not only to hold the semi-
fluid muscle plasma in place, but also to transmit the pull of the
contracting fiber to the point of attachment of the muscle. The
most striking characteristic of a fiber's appearance is the series
of alternating light and dim transverse bands of nearly equal
width with which it is marked, and from which its designation as
striped muscle is derived (Fig. 46). Under the high power of the
microscope the muscle plasma is seen to be made up of a number
of longitudinal fibrils, the sarcostyles, surrounded by a homogene-
ous medium, the sarcoplasm.

82 THE HUMAN BODY

Not all histologists are agreed as to the details of structure of
the fibrils; they are so small that only the highest powers of the
microscope can be used in studying them; they occur in ordinary
muscle surrounded always by sarcoplasm and in company with
many others. These circumstances combine to present to the eye
of the observer a more or less distorted picture. It is no wonder,
therefore, that differences of opinion as to the real structure of the
fibrils have arisen.

Certain insects' muscles happen to be so constituted that the
fibrils can be separated one from another and isolated ones gotten
under the field of the microscope for study. When examined thus
singly and free from surrounding media which distort the view,
these fibrils are seen to be tiny cylinders divided at regular inter-
vals by transverse partitions, made, apparently, of delicate mem-
brane. Many biologists think it likely that the fibrils of ordinary
skeletal muscle have really this same structure; that the position
of the transverse membranes is indicated by faint dark lines in the
middle of the light bands and that the appearance of light and dim
bands of nearly equal width is an optical illusion due to the un-
favorable conditions of observation. Since the fiber as a whole
contains many fibrils and since the cross striations are regular
throughout the entire fiber it follojvs that all the fibrils of any
fiber must have their partitions at corresponding levels. The
fibrils are probably kept in place by an interfibrillar network of
some sort.

The blood-vessels and nerve-fibers supplied to the skeletal
muscles are numerous. The larger blood-vessels run in the coarser
partitions of the connective tissue lying between the fasciculi and
give off fine branches which form a network between the individual
fibers but never penetrate the sarcolemma.

Connected with each muscle-fiber is a nerve-fiber. The central
core of the nerve-fiber ends in an oval expansion (end plate) which
contains many nuclei and lies close under the sarcolemma, its
deeper side being in immediate contact and possibly continuous
with the striated contents. These nerve-fibers are motor or con-
cerned in exciting a contraction of the muscle-fiber. Other nerve-
fibers are connected with very peculiar bodies found scattered
throughout the muscle, but especially numerous near the tendons.
They are usually of a size just visible to the unaided eye 'and from

THE STRUCTURE OF THE MOTOR ORGANS

83

their form have been named muscle-spindles. They are doubtless
sensory in function. Somewhat similar bodies (Golgi's tendon-

FIG. 47. — The muscular coat of the stomach.

organs) are found in the tendons and are also richly supplied with
nerve-fibers.

Structure of the Unstriped Muscles. Of these
the muscular coat of the stomach (Fig. 47) is a
good example. They have no definite tendons,
but form expanded membranes surrounding cavi-
ties, so that they have no definite origin or inser-
tion. Like the skeletal muscles they consist of
proper contractile elements, .with accessory con-
nective tissue, blood-vessels, and nerves. Their
fibers, however, have a very different microscopic
structure. They present a slightly marked longi-
tudinal but no cross striation and are made up of'
elongated cells (Fig. 48), bound together by a
small quantity of cementing material. The cells
vary considerably in size, but on the average are
about ^ mm. (F(^ in.) in length. Each is flat-
tened in one plane, tapers off at each end, and pos-
sesses a very thin enveloping membrane; in its
interior lies an elongated nucleus with one or two
nucleoli. These cells have the power of shorten-
ing in the direction of their long axes, and so of diminishing the ca-
pacity of the cavities in the walls of which they lie.

48.— Un-
muscle-

84 THE HUMAN BODY

Cardiac Muscular Tissue. This consists of nucleated branched
cells which unite to form a network, in the interstices of which
blood-capillaries and nerve-fibers run. The
cells present transverse striations, but not so
distinct as those of the skeletal muscles, and
are said to have no sarcolemma (Fig. 49).

The Chemistry of Muscular Tissue. The
chemical structure of the living muscular fiber
is unknown, but some idea as to it may be ob-
tained from examination of the substances it
yields on proximate analysis. Muscle contains
75 per cent of water; and, among other inor-

FIG. 49. — Cardiac \ .

muscular tissue, mag- game constituents, phosphates and chlorides

nified about 400 diam- . • ,. , . ,T7,

eters. The cell-bound- °f potassium, sodium, and magnesium. When
n athe at rest a livinS muscle is feebly alkaline, but

right-hand portion of after hard work, or when dying, this reaction
is reversed through the formation of sarco-
lactic acid (C3H6O3) . Muscles contain small quantities of grape-
sugar and glycogen, and of nitrogenous extractives, especially
creatine (C4H9NSO2). As in the case of all other physiologically
active tissues, however, the main post-mortem constituents of the
muscular fibers are protein substances.

At least three proteins have been obtained from mammalian
striped muscle, myogen, a globulin, myosin, an albumin, both
coagulable by heat, and a protein which is insoluble in pure water
or dilute saline solution and which appears to form a protein
framework within the fiber. This latter is called the muscle
stroma and constitutes nine per cent of the weight of striated
muscle. Muscle tissue contains three or four times as much
myogen as myosin. Both of these proteins possess the property
of passing over into insoluble forms known respectively as myogen
fibrin and myosin fibrin. It has been commonly supposed that
the death stiffening, rigor mortis, which is such a marked feature
of the death of muscle tissue is due to this change of the muscle
proteins from a soluble to an insoluble form.

Heart muscle contains relatively much less myogen and myosin
and much more stroma than does ordinary striated muscle, its
stroma constituting 56 per cent of its weight. Smooth muscle con-
tains an even larger proportion of stroma, 72 per cent.

THE STRUCTURE OF THE MOTOR ORGANS

83

their form have been named muscle-spindles. They are doubtless
sensory in function. Somewhat similar bodies (Golgi's tendon-

FIG. 47. — The muscular coat of the stomach.

organs) are found in the tendons and are also richly supplied with
nerve-fibers.

Structure of the Unstriped Muscles. Of these
the muscular coat of the stomach (Fig. 47) is a
good example. They have no definite tendons,
but form expanded membranes surrounding cavi-
ties, so that they have no definite origin or inser-
tion. Like the skeletal muscles they consist of
proper contractile elements, with accessory con-
nective tissue, blood-vessels, and nerves. Their
fibers, however, have a very different microscopic
structure. They present a slightly marked longi-
tudinal" but no cross striation and are made up of
elongated cells (Fig. 48), bound together by a
small quantity of cementing material. The cells
vary considerably in size, but on the average are
about ^ mm. (^ in.) in length. Each is flat-
tened in one plane, tapers off at each end, and pos-
sesses a very thin enveloping membrane; in its
interior lies an elongated nucleus with one or two
nucleoli. These cells have the power of shorten-
ing in the direction of their long axes, and so of diminishing the ca-
pacity of the cavities in the walls of which they lie.

Fio. 48.— Un-
muscle-

84 THE HUMAN BODY

Cardiac Muscular Tissue. This consists of nucleated branched
cells which unite to form a network, in the interstices of which
blood-capillaries and nerve-fibers run. The
cells present transverse striations, but not so
distinct as those of the skeletal muscles, and
are said to have no sarcolemma (Fig. 49).

The Chemistry of Muscular Tissue. The
chemical structure of the living muscular fiber
is unknown, but some idea as to it may be ob-
tained from examination of the substances it
yields on proximate analysis. Muscle contains
75 per cent of water; and, among other inor-

FIG. 49. — Cardiac . .

muscular tissue, mag- game constituents, phosphates and chlorides

nified about 400 diam- . • , . , . ,,„

eters. The cell-bound- °f potassium, sodium, and magnesium. When

ln Ihe at rest a living muscle » feebly alkaline, but
right-hand portion of after hard work, or when dying. this reaction

the figure. r 1' r

is reversed through the formation of sarco-
lactic acid (0SH603) . Muscles contain small quantities of grape-
sugar and glycogen, and of nitrogenous extractives, especially
creatine (C4H9N3O2). As in the case of all other physiologically
active tissues, however, the main post-mortem constituents of the
muscular fibers are protein substances.

At least three proteins have been obtained from mammalian
striped muscle, myogen, a globulin, myosin, an albumin, both
coagulable by heat, and a protein which is insoluble in pure water
or dilute saline solution and which appears to form a protein
framework within the fiber. This latter is called the muscle
stroma and constitutes nine per cent of the weight of striated
muscle. Muscle tissue contains three or four times as much
myogen as myosin. Both of these proteins possess the property
of passing over into insoluble forms known respectively as myogen
fibrin and myosin fibrin. It has been commonly supposed that
the death stiffening, rigor mortis, which is such a marked feature
of the death of muscle tissue is due to this change of the muscle
proteins from a soluble to an insoluble form.

Heart muscle contains relatively much less myogen and myosin
and much more stroma than does ordinary striated muscle, its
stroma constituting 56 per cent of its weight. Smooth muscle con-
tains an even larger proportion of stroma, 72 per cent.

THE STRUCTURE OF THE MOTOR ORGANS 85

Beef Tea. From the above-stated facts it is clear that when a
muscle is boiled in water its myogcn and myosin arc coagulated
and left behind in the meat: oven ircooJdng bc_gtrmmcnced by
soaking in cold water the myogen still remains, as it is as insoluble
in cold water as in hot. Beef tea as ordinarily made, then, con-
tains little but the flavoring matters and salts of the meat, traces
of some albumins and some gelatin, the latter derived from the
connective tissues of the muscle. The flavoring matters and salts
make it deceptively taste as if it were a strong solution of the whole
meat, and the gelatin causes it to " set " on cooling, so the cook
feels quite sure she has got out "all the strength of the meat,"
whereas the beef tea so prepared contains but little of the most
nutritious protein portions, which in an insipid shrunken form are
left when the liquid is strained off. Various proposals have been
made with the object of avoiding this and getting a really nutritive
beef tea; as for example chopping the raw meat fine and soaking it
in strong brine for some hours to dissolve out the myogen; or ex-
tracting it with dilute acids which dissolves the myogen and
myosin and at the same time render it non-coagulable by heat
when subsequently boiled. Such methods, however, make un-
palatable compounds \vhich invalids will not take. Beef tea is a
slight stimulant, and often extremely useful in temporarily main-
taining the strength and in preparing the stomach for other food,
but its direct value as a food is slight, and it cannot be relied upon
to keep up a patient's strength for any length of time. There can
be no doubt that thousands of sick persons have in the past and
are being to-day starved to death on it. Liebig's extract of meat is
essentially a very strong beef tea; containing much of the flavor-
ing substances of the meat, nearly all its salts and the crystalline
nitrogenous bodies, such as crcatine, which exist in muscle, but
hardly any of its really nutritive parts, as was pointed out by
Liebig himself. From its stimulating effects it is often useful to
persons in feeble health, but other food should be given with it. It
may also be used on account of its flavor to add to the " stock " of
soup and for similar purposes; but the crroneousness of the com-
mon belief that it is a highly nutritious food cannot be too strongly
insisted upon. Under the name of liquid extracts of meat other
substances have been prepared by subjecting meat to chemical
processes in which it undergoes changes similar to those experienced

86 THE HUMAN BODY

in digestion : the myosin is thus rendered soluble in water and un-
coagulable by heat, and such extracts if properly prepared are
nutritious and can often be absorbed when meat in the solid form
cannot be digested : they may thus help the stomach over a crisis,
but are not, even the best of them, to be depended on as anything
but temporary substitutes for other food ; or in some cases as use-
ful additions to it.

Rigor Mortis. During life and for a certain time after general
death the muscles are soft, translucent, extensible and elastic, and
neutral or feebly alkaline in reaction; after a period which in warm-
blooded animals is brief (varying from a few minutes to three or
four hours) they gradually become harder, more opaque, less ex-
tensible and less elastic, and distinctly acid in reaction. The
result of these changes is the well-known cadaveric rigidity or
rigor mortis. It was formerly very generally believed that the
cause of rigor is the change of soluble myogen and myosin to in-
soluble myogen fibrin and myosin fibrin. Quite recently, how-
ever, some physiologists have called attention to the strong
probability that death stiffening may be due to the considerable
production of sarcolactic acid which is known to accompany the
death process. In support of their view may be cited the well-
known tendency of animals or men killed suddenly in the midst of
violent exertion to stiffen very quickly. Men killed in battle often
retain the postures in which death overtook them. Hard muscular
work involves a large production of sarcolactic acid, a condition
favorable according to the view quoted, to a prompt onset of rigor.

CHAPTER VII
THE PROPERTIES OF MUSCULAR TISSUE

Contractility. The characteristic physiological property of
muscular tissue, and that for which it is employed in the Body, is
the faculty possessed by its fibers of shortening forcibly under
certain circumstances. The direction in which this shortening
occurs is always that of the long axis of the fiber in both plain and
striped muscles, and it is accompanied by an almost equivalent
thickening in other diameters, so that when a muscle contracts it
does not shrivel up or diminish its bulk in any appreciable way; it
simply changes its form. When a muscle contracts it also be-
comes harder and more rigid, especially if it has to overcome any
resistance. This and the change of form can be well felt by placing
the fingers of one hand over the biceps muscle lying in front of the
humerus, of the other arm. When the muscle is contracted so as
to bend the elbow it can be felt to swell out and harden as it
shortens. Every schoolboy knows that when he appeals to an-
other to " feel his muscle " he contracts the latter so as to make it
thicker and apparently more massive as well as harder. In statues
the prominences on the surface indicating the muscles beneath the
skin are made very conspicuous when violent effort is represented,
so as to indicate that the muscles are in vigorous action. In a
muscular fiber we find no longer the slow, irregular, and indefinite
changes of form seen in amreboid slightly differentiated cells;
they are replaced by a precise, rapid and definite change of form.
Muscular tissue represents a group of cells in the bodily com-
munity which have taken up the, one special duty of executing
changes of form, and in proportion as these cells have fewer other
things to do, they do that one better. This contractility of the
muscular fibers may be briefly described as a passage from the
state of rest, in which the fibers are long and narrow, into the
state of activity, in which they are shorter and thicker: this
change is made with considerable force, and thus the muscles
move parts attached to their tendons. When the state of activity

87

88 THE HUMAN BODY

has passed off the fibers suffer themselves to be extended again by
any force pulling upon them, and so regain their resting shape;
and since in the living Body almost invariably other parts are put
upon the stretch when any muscle contracts, these by their
elasticity serve to pull the latter back again to its primitive shape.
No muscular fiber is known to have the power of actively expand-
ing after it has contracted: in the active state it forcibly resists
extension, but once the contraction is completely over, it suffers
itself readily to be pulled back to its resting form. The contracted
state lasts always longer, however, than the mere time occupied
by the muscle in shortening : as will be seen later, the full state of
contraction is gradually attained and gradually disappears.

Irritability. With that modification of the primitive protoplasm
of an amoeboid embryonic cell which gives rise to a muscular fiber
with its great contractility, there goes a loss of other properties.
Nearly all spontaneity disappears; muscles are not automatic like
primitive protoplasm or ciliated cells; except under certain very
special conditions they remain at rest unless excited from without.
The amount of external change required to excite the living mus-
cular fiber is, however, very small ; muscle tissue is highly irritable,
a very little thing being sufficient to call forth a powerful contrac-
tion. In the living Human Body the exciting force, or stimulus,
acting upon a muscle is almost invariably a nervous impulse, trans-
mitted along the nerve-fiber attached to it, and upsetting the
molecular equilibrium of the muscle. It is through the nerves that
the will acts upon the muscle-fiber, and accordingly injury to the
nerves of a part, as the face or a limb, causes paralysis of its
muscles. They may still be there, intact and quite ready to work,
but there are no means of sending commands to them, and so they
remain idle.

Although a nervous impulse is the natural physiological muscu-
lar stimulus it is not the only one known. If a muscle be exposed
in a living animal and a slight but sudden tap be given to it, or a
hot bar be suddenly brought near it, or an electric shock be sent
through it, or a drop of glycerin or of solution of ammonia be placed
on it, it will contract; so that in addition to the natural nervous
stimulus, muscles are irritable under the influence of mechanical,
thermal, electrical, and chemical stimuli. One condition of the
efficacy of each of them is that it shall act with some suddenness;

THE PROPERTIES OF MUSCULAR TISSUE 89

a very slowly increased pressure, even if ultimately very great, or
a very slowly raised temperature, or a slowly increased electrical
current passed through it, will not excite the muscle; although far
less pressure, warmth, or electricity more rapidly applied would
stimulate it powerfully. Once an electric current has been set up
through a muscle, its steady passage does not act as a stimulus;
but a sudden diminution or increase of it does. It may perhaps
still be objected that it is not proved that any of these stimuli ex-
cite the muscular fibers, and that in all these cases it is possible
that the muscle is only excited through its nerves. For the various
stimuli named above also excite nerves (see Chap. X), and when
we apply them to the muscle we may really be acting first upon
the fine nerve-endings there, and only indirectly and through the
mediation of these upon the muscular fibers. That the muscular
fibers have a proper irritability of their own, independently of their
nerves, is, however, shown by the action of certain drugs — for ex-
ample curare, a South American Indian arrow poison. When this
substance is introduced into a wound all the striped muscles are
apparently poisoned, and the animal dies of suffocation because of
the cessation of the breathing movements. But the poison does
not really act on the muscles themselves: it has been proved to
paralyze the very endings of the muscle-nerves right down in the
muscle-fibers themselves. Yet after its administration we still
find that the various non-physiological stimuli referred to above
make the muscles contract as powerfully as before the poisoning,
so we must conclude that the muscles themselves are irritable in
the absence of all nerve stimuli — or, what amounts to the same
thing, when all their nerve-fibers have been poisoned. The ex-
periment also shows that the contractility of a muscle is a property
belonging to itself, and that its contracting force is not something
derived from the nerves attached to it. The nerve stimulus simply
acts like the electric shock or sudden blow and arouses the muscle
to manifest a property which it already possesses. The older
physiologists observing that muscular paralysis followed when the
nervous connection between a muscle and the brain was inter-
rupted, concluded that the nerves gave the muscles the power of
contracting. They believed that in the brain there was a great
store of a mysterious thing called vital spirits, and that some of this
was ejected from the brain along the nerve to the muscle, when the

90 THE HUMAN BODY

latter was to be set at work, and gave it its working power. We
now know that such is not the case, but that a muscle-fiber is a
collection of highly irritable material which can have its equilib-
rium upset in a definite way, causing it to change its shape, under
the influence of certain slight disturbing forces, one of which is a
nervous impulse; and since in the Body no other kind of stimulus
usually reaches the muscles, they remain at rest when their nervous
connections are severed. But the muscles paralyzed in this way
can still, in the living Body, be made to contract by sending elec-
trical shocks through them. Physiologically, then, muscle is a con-
tractile and irritable tissue.

A Simple Muscular Contraction. Most of the details concerning
the physiological properties of muscles have been studied on those
of cold-blooded animals." A frog's muscle will retain all its living
properties for some time after removal from the body of the ani-
mal, and so can be experimented on with ease, while the muscles
of a rabbit or cat soon die under those circumstances. Enough
has, however, been observed on the muscles of the higher animals
to show that in all essentials they agree with those of the frog or
terrapin.

When a single electric shock is sent through a muscle, the nerves
of which have been thrown out of action by curare, it rapidly
shortens and then, if a weight be hanging on it, rapidly lengthens
again. The whole series of phenomena from the moment of stimu-
lation until the muscle regains its resting form is known as a simple
muscular contraction or a "twitch": it occupies in frog's muscle
about one-tenth of a second. So brief a movement as this cannot
be followed in its details by direct observation, but it is possible
to record it and study its phases at leisure. This may be done by
firmly fixing the upper tendon of an isolated muscle, M , Fig. 50,
and attaching the other end at d to a lever, /, which can move about
the fulcrum/: the end of the long arm of the lever bears a point, p,
which scratches on a smooth smoked surface, S. Suppose the
surface to be placed so that the writing point of the lever is at a;
if the muscle now contracts it will raise the point of the lever, and a
line ac will be drawn on the smoked surface, its vertical height, cm,
being dependent, first, on the extent of the shortening of the mus-
cle, and second, on the proportion between the long and short
arms of the lever: the longer fp is as compared with/d, the more

THE PROPERTIES OF MUSCULAR TISSUE

91

will the actual shortening of the muscle be magnified. With the
lever shown in the figure this magnification would be about ten

CO

OJ

Ml

.5

times, so that one-tenth of cm would be the extent of the shorten-
ing of the muscle. Suppose, next, the smoked surface to be moved

92 THE HUMAN BODY

to such position that the writing point of the lever touches it at i,
and, the muscle being left at rest, the surface to be moved evenly
from left to right; the horizontal Jine io would then be traced, its
length depending on the distance through which S moved during
the time the lever was marking on it: and it is clear that if S move
uniformly, and we know its rate of movement, we can very readily
calculate from the length of io how long S was moving while that
line was being traced : for example, if we know the rate of move-
ment to be ten inches per second, and on measurement find io to
be an inch long, the time during which the surface was moving
must have been ^ of a second ; and each tenth of w correspond to
i55 of a second.

If we set the recording surface in motion and while the lever
point is tracing a horizontal line cause the muscle to contract, the
point will be raised as long as the muscle is contracted, and the
line drawn by it will be due to a combination of two simultaneous
movements — a horizontal, due to the motion of S, a nearly verti-
cal, due to the shortening of the muscle; the resulting line is a
curve known as the curve of a simple muscular contraction. Let the
surface S be placed so that the writing point is at q and then be set
in uniform motion from left to right at the same rate as before
(ten inches per second). When the point is opposite t, stimulate
the muscle by an electric shock; the result, until the muscle has
fully lengthened again, will be the curve tuvwxy, from which many
things may be learned. In the first place we see that the muscle
does not commence to contract at the very instant of stimulation,
but at an appreciably later time, and during the interval the lever
draws the horizontal line tu; this period, occupied by preparatory
changes within the muscle, is known as the latent period. Then the
muscle begins to shorten and the lever to rise, at first slowly from
u to v, then more rapidly, and again more slowly until the summit
of the contraction is reached at w. The muscle does not now in-
stantly relax, but only gradually passes back to the resting state:
beginning at w, we see the descent of the curve is for a time slow,
then more rapid, and finally slow again from x to y, when the con-
traction is completed and the lever once more traces only the
horizontal line yp, due to the continued movement of the record-
ing surface. The curve then shows three distinct phases in the
contraction: the latent period; the period of shortening; the period

THE PROPERTIES OF MUSCULAR TISSUE 93

of elongating. Knowing the rate of horizontal movement, we can
measure off the time occupied by each phase. The horizontal
distance from t to u represents the time taken by the latent period;
from u to z, the time occupied in shortening; from z to y, the time
taken in elongation: in a fresh frog's muscle these times are re-
spectively 1^5, j^, -^ of a second. In the muscles of warm-blooded
animals they are all shorter, but the difficulties in the way of
accurate experiment are very great. If we know the relative
lengths of the arms of the lever we can of course readily calculate
from the height, wz, of the curve the extent of shortening of the
muscle. With a single electrical stimulation this is never more
than one-fourth the total length of the muscle.

In Fig. 50 the accessory apparatus used in practice to indicate
on the moving surface the exact instant of stimulation and to
measure the rate at which S moves have been omitted.

Physiological Tetanus. It is obvious that the ordinary move-
ments of the Body are not brought about by such transient mus-
cular contractions as those just described. Even a wink lasts
longer than one-tenth of a second. Our movements are, in fact,
due to more prolonged contractions which may be described as
consisting of several simple contractions fused together, and
known as "tetanic contractions"; it might be better to call them
"compound contractions," since the word tetanus has long been
used by pathologists to signify a diseased state, such as occurs in
strychnine poisoning and hydrophobia, in which most of the
muscles of the Body are thrqwn into prolonged and powerful in-
voluntary contractions.

If, while a frog's muscle is still shortening under the influence
of one electric shock, another stimulus be given it, it will contract
again and the new contraction will be added on to that already
existing, without any period of elongation occurring between
them. While the muscle is still contracting under the influence of
the second stimulus a third electric shock will make it contract
more, and so on, until the muscle is shortened as much as is possi-
ble to it for that strength of stimulus. If now the stimuli be re-
peated at the proper intervals, each new one will not produce any
further shortening, but, each acting on the muscle before the
effect of the last has begun to pass off, the muscle will be kept in a
state of permanent or "tetanic" contraction; and this can be

94 THE HUMAN BODY

maintained, by continuing the application of the stimuli, until the
organ begins to get exhausted or " fatigued " ; elongation then com-
mences in spite of the stimulation. When our muscles are stimu-
lated in the Body, from the nerve-centers through the nerves,
they receive from the latter a sufficient number of stimuli in a
second (the exact number is still doubtful) to throw them into
tetanic contractions. In other words, not even in the most rapid
movements of the Body is a muscle made to execute a simple
muscular contraction; it is always a longer or a shorter tetanus.
When very quick movements are executed, as in performing rapid
passages on the piano, the result is obtained by contracting two
opposing muscles and alternately strengthening and weakening
a little the tetanus of each.

Causes affecting the Degree of Muscular Contraction. The
extent of shortening which can be called forth in a muscle varies
with the stimulus. In the first place, a single stimulus can never
cause a muscle to contract as much as rapidly repeated stimuli
of the same strength — since in the latter case we get, as already
explained, several simple contractions such as a single stimulus
would call forth, piled on the top of one another. With powerful
repeated electrical stimuli a muscle can be made to shorten to one-
third of its resting length, but in the Body the strongest effort of
the will never produces a contraction of that extent. Apart from
the rate of stimulation, the strength of the stimulus has some in-
fluence, a greater stimulus causing a greater contraction; but very
soon a point is reached beyond which increase of stimulus produces
no increased contraction; the muscle has reached its limit. The
amount of load carried by the muscle (or the resistance opposed to
its shortening) has also an influence, and that in a very remarkable
way. Suppose we have a frog's calf-muscle, carrying no weight,
and find that with a stimulus of a certain strength it shortens two
millimeters (^ inch). Then if we hang one gram (15.5 grains) on
it and give it the same stimulus, it will be found to contract more,
say four or five millimeters, and so on, up to the point when it
carries eight or ten grams. After that an increased weight will,
with the same stimulus, cause a less contraction. So that up to a
certain limit, resistance to the shortening of the muscle makes it
more able to shorten: the mere greater extension of the muscle
due to the greater resistance opposed to its shortening, puts it into

THE PROPERTIES OF MUSCULAR TISSUE 95

a state in which it is able to contract more powerfully. Fatigue
diminishes the working power of a muscle and rest restores it,
especially if the circulation of the blood be going on in it at the
same time. A frog's muscle cut out of the body will, however, be
considerably restored during a period of rest, even although no
blood flow through it.

Cold increases the time occupied by a simple muscular contrac-
tion, and also impairs the contractile power, as we find in our own
limbs when " numbed " with cold, though in that case the hurtful
influence of the cold on the nerves no doubt also plays a part.
Moderate warmth on the other hand, up to near the point at which
death stiffening (often in this case spoken of as heat rigor) occurs,
diminishes the time taken by a contraction, and increases its
height. Heat rigor is produced in excised frog's muscle by heating
it to about 40.° C. (104° F.) The required temperature is higher
in warm-blooded animals, especially while the circulation through
the muscle is maintained: in fevers temperatures considerably
greater than the above have been observed without the occurrence
of muscular rigor.

The Measure of Muscular Work. The work done by a muscle in
a given contraction, when it lifts a weight vertically against grav-
ity, is measured by the weight moved, multiplied by the distance
through which it is moved. When a muscle contracts carrying no
load it does very little work, lifting only its own weight; when
loaded with one gram and lifting it five millimeters it does five
gram-millimeters of work, just as an engineer would say an engine
had done so many kilogrammeters or foot-pounds. If loaded with
ten grams and lifting it six millimeters it would do sixty gram-
millimeters of work. Even after the weight becomes so great that
it is lifted through a less distance, the work done by the muscle
goes on increasing, for the heavier weight lifted more than com-
pensates for the less distance through which it is raised. For ex-
ample, if the above muscle were loaded with fifty grams it would
maybe lift that weight only 1.5 millimeters, but it would then do
75 gram-millimeters of work, which is more than when it lifted
ten grams six millimeters. A load is, however, at last reached
with which the muscle does less work, the lift becoming very little
indeed, until at last the weight becomes so great that the muscle
cannot lift it at all and so does no work when stimulated. Starting

96 THE HUMAN BODY

then from the time when the muscle carried no load and did no
work, we pass with increasing weights, through phases in which
it does more and more work, until with one particular load it does
the greatest amount possible to it with that stimulus: after that,
with increasing loads less work is done, until finally a load is
reached with which the muscle again does no work. What is true
of one muscle is of course true of all, and what is true of work done
against gravity is true of all muscular work, so that there is one
precise load with which a beast of burden or a man can do the
greatest possible amount of work in a day. With a lighter or
heavier load the distance through which it can be moved will be
more or less, but the actual work done always less. In the living
Body, however, the working of the muscles depends so much on
other things, as the due action of the circulatory and respiratory
systems and the nervous energy or " grit " (upon which the stimu-
lation of the muscles depends) of the individual man or beast, that
the greatest amount of work obtainable is not a simple mechanical
problem as it is with the excised muscle.

From what precedes it is clear that the molecular changes which
take place in a contracting muscle-fiber are eminently susceptible
of modification by slight changes in its environment. The evidence
indicates that the contractility of a muscle depends, not upon a
vital force entirely distinct from ordinary inanimate forces, but
upon an arrangement of its material elements which is only main-
tained under certain conditions and is eminently modifiable by
changes in the surroundings.

Influence of the Form of the Muscle on its Working Power.
The amount of work that any muscle can do depends of course
largely upon its physiological state; a healthy well-nourished
muscle can do more than a diseased or starved one; but allowing
for such variations the work which can be done by a muscle varies
with its form. The thicker the muscle, that is the greater the
number of fibers present in a section made across the long axes
of the fasciculi, the greater the load that can be lifted or the other
resistance that can be overcome. On the other hand, the extent
through which a muscle can move a weight increases with the
length of its fasciculi. A muscle a foot in length can contract
more than a muscle six inches long, and so would move a bone
through a greater distance, provided the resistance were not too

THE PROPERTIES OF MUSCULAR TISSUE 97

great for its strength. But if the shorter muscle had double the
thickness, then it could lift twice the weight that the longer
muscle could. We find in the Body muscles constructed on both
plans; some to have a great range of movement, others to
overcome great resistance, besides numerous intermediate forms
which cannot be called either long and slender or short and thick ;
many short muscles for example are not specially thick, but are
short merely because the parts on which they act lie near to-
gether. It must be borne in mind, too, that many apparently
long muscles are really short stout ones — those namely in which
a tendon runs down the side or middle of the muscle, and has
the fibers inserted obliquely into it. The muscle (gastrocnemius)
in the calf of the leg, for instance (Fig. 44, B), is really a short stout
muscle, for its working length depends on the length of its fasciculi
and these are short and oblique, while its true cross-section is that
at right angles to the fasciculi and is considerable. The force
with which a muscle can shorten is very great. A frog's muscle
of 1 square centimeter (0.39 inch) in section can just lift 2,800
grams (98.5 ounces), and a human muscle of the same area more
than twice as much.

Muscular Elasticity. A clear distinction must be made be-
tween elasticity and contractility. Elasticity is a physical prop-
erty of matter in virtue of which various bodies tend to assume
or retain a certain shape, and when removed from it, forcibly to
return to it. When a spiral steel spring is stretched it will, if let
go, " contract " in a certain sense, by virtue of its elasticity, but
such a contraction is clearly quite different from a muscular con-
traction. The spring will only contract as a result of previous
distortion; it cannot originate a change of form, while the muscle
can actively contract or change its shape when a stimulus acts
upon it, and that without being previously stretched. It does
not merely tend to return to a natural shape from which it has
been removed, but it assumes a quite new natural shape, so that
physiological contractility is a different thing from mere physical
elasticity; the essential difference being that the coiled spring or
a stretched band only gives back mechanical work which has
already been spent on it, while the muscle originates work inde-
pendently of any previous mechanical stretching. In addition
to their contractility, however, muscles are highly elastic. If a

98 THE HUMAN BODY

fresh muscle be hung up and its length measured, and then a
weight be hung upon it, it will stretch just like a piece of india-
rubber, and when the weight is removed, provided it has not been
so great as to injure the muscle, the latter will return passively,
without any stimulus or active contraction, to its original form.
In the Body all the muscles are so attached that they are usually
a little stretched beyond their natural resting length; so that
when a limb is amputated the muscles divided in the stump
shrink away considerably. By this stretched state of the resting
elastic muscles two things are gained. In the first place when
the muscle contracts it is already taut, there is no " slack " to be
hauled in before it pulls on the parts attached to its tendons;
and, secondly, as we have already seen the working power of a
muscle is increased by the presence of some resistance to its con-
traction, and this is always provided for from the first, by having
the origin and insertion of the muscles so far apart as to be pull-
ing on it when it begins to shorten.

The Electrical Phenomena of Muscle. When a living muscle
is carefully exposed and suitable electrodes connected with a
sensitive galvanometer or electrometer are applied to its surface
the entire surface is found to be isoelectric, i. e., having a uniform
electric potential. If, however, an injury such as cutting or
burning is inflicted upon any part of the muscle the injured sur-
face is found to possess a different potential from the surround-
ing uninjured surfaces. This difference of potential is shown by
movements of the indicator of the galvanometer or electrometer.
These movements are usually in such a direction as to indicate
that the injured region has a lower potential than uninjured parts
of the same tissue. This difference of potential existing between
injured and uninjured living tissue is often referred to as the
current of injury, although no current actually flows unless the
two regions are connected by an electrical conductor. No cur-
rent of injury can be obtained by connecting living tissue with
dead tissue. Only while the injured tissue is in act of dying does
it exhibit the altered potential which may give rise to an injury
current.

The explanation of the change of electric potential accompany-
ing an injury to living tissue is found in the fact that the death
process which follows injury involves extensive chemical changes

THE PROPERTIES OF MUSCULAR TISSUE 99

in the tissue. This disturbance in chemical relationship brings
about corresponding disturbance in the electric equilibrium which
finds expression in an altered electric potential in the part where
the chemical activity is going on.

Just as the chemical changes which follow injury to the tissue
give rise to the change of electrical potential which we call the
current of injury, so the chemical changes which accompany
normal activity in the tissue give rise to electrical changes which
are designated currents of action. Action currents cannot easily
be demonstrated in an ordinary contracting muscle because the
whole muscle goes into contraction at once and so the electric
potential of its entire surface rises and falls uniformly. In the
heart we have a muscle, however, which does not contract all at
once, the contraction sweeping over it from base to apex. The
action currents of the heart, therefore, can be demonstrated with-
out difficulty if the apparatus used for detecting them is able to
respond quickly enough to recurrent changes of potential in
opposite directions. An ordinary galvanometer cannot do this
because of its too great inertia, but the capillary electrometer
answers admirably for the purpose. Another interesting method
of demonstrating the action currents of the heart is by causing
them to act as stimuli for an irritable tissue. If in a recently
killed frog the sciatic nerve is dissected out as far as the knee
and cut away from its connection with the spinal cord, being left
in connection with the leg below, and if this nerve is laid on the
exposed beating heart of the same frog or some other recently
killed animal, often the muscles of the lower leg and foot which
are connected with the nerve will contract at each beat of the
heart. The nerve where it lies on the heart serves as a conductor
for the action currents as they are generated in the heart, and
the action currents in turn stimulate the nerve during their flow
through it.

The Source of Muscular Energy. In the physical sense a
muscle is a machine. By this we mean that whatever energy it
gives out must have been supplied to it previously from the
outside. The work which a muscle does in contracting is at the
expense of its available store of energy. We know that the
energy exhibited by a steam-engine is derived from the combus-
tion or oxidation of the fuel under the boiler. We know also that

100 THE HUMAN BODY

the energy exhibited by a contracting muscle is derived from
the oxidation of fuel substances within it. The physical accom-
paniments of oxidation are not the same in the two cases; the
fuel under the boiler burns with flame and at a high temperature;
the fuel substance within the muscle burns without flame and at
a temperature only slightly higher than that of the body. The
energy yield, however, for corresponding amounts of fuel is as
great in one case as in the other. The fuel substance used by
contracting muscle is probably for the most part a certain sugar,
dextrose, or its anhydride, glycogen. When dextrose is completely
oxidized it yields carbon dioxid and water. This reaction is rep-
resented by the equation C6H12O6+12O=6CO2+6H2O.

The manner in which this energy of oxidation is converted
within the muscle to energy of motion is not certainly known,
although many interesting theories have been proposed to
explain it. Most physiologists agree that the mechanism of
skeletal muscle is quite different from that present in smooth
muscle.

The longitudinal fibrils which form such a characteristic feature
of skeletal muscle are believed by many physiologists to repre-
sent the actual contractile elements of this type of muscular
tissue. The precise way in which they perform their function is
at present a matter of conjecture. It has been suggested that
contraction is due to a rush of fluid into the fibrils from the sur-
rounding sarcoplasm. Those who hold this view believe that the
oxidation of dextrose to carbon dioxid and water bring about
conditions within the muscle which result in movement of fluid
of the sort indicated.

Physiology of Smooth Muscular Tissue. What has hitherto
been said applies especially to the skeletal muscles; but in the
main it is true of the unstriped muscles. These also are irritable
and contractile, but their changes of form are much slower than
those of the striated fibers. Upon stimulation, a longer period
elapses before the contraction commences and when, finally, this
takes place it is comparatively very slow, gradually attaining a
maximum and gradually passing away.

Unstriped muscular tissue has a remarkable power of remain-
ing in the contracted state for long periods: the muscular coats
of many small arteries, for example, are rarely relaxed; some-

THE PROPERTIES OF MUSCULAR TISSUE

101

times they may be more contracted, sometimes less, but in health
seldom if ever completely relaxed.

There are in the body a number of sphincters, circular bands
of smooth muscle which guard the openings of various organs
such as the stomach, large intestine, and bladder. These are
strongly contracted the greater part of the time, relaxation being
for them only an occasional occurrence. They maintain their
condition of strong contraction without fatigue and apparently
without much expenditure of energy, offering in this regard a
sharp contrast to skeletal muscle.

CHAPTER VIII
MOTION AND LOCOMOTION

The Special Physiology of the Muscles. Having now con-
sidered separately the structure and properties in general of the
skeleton, the joints, and the muscles, we may go on to consider
how they all work together in the Body. Although the properties
of muscular tissue are everywhere the same, the uses of dif-
ferent muscles are very varied, by reason of the different parts
with which they are connected. Some are muscles of respiration,
others of deglutition; many are known as flexors because they
bend joints, others as extensors because they straighten them.
The exact use of any particular muscle, acting alone or in concert
with others, is known as its special physiology, as distinguished
from its general physiology, or properties as a muscle without refer-
ence to its use as a muscle in a particular place. The functions
of those muscles forming parts of the physiological mechanisms
concerned in breathing and swallowing will be studied here-
after; for the present we may consider the muscles which co-
operate in maintaining postures of the Body; in producing move-
ments of its larger parts with reference to one another; and in
producing locomotion or movement of the whole Body in space.

In nearly all cases the striped muscles carry out their functions
with the cooperation of the skeleton, since nearly all are fixed
to bones at each end, and when they contract primarily move
these, and only secondarily the soft parts attached to them. To
this general rule there are, however, exceptions. The muscle for
example which lifts the upper eyelid and opens the eye arises
from bone at the back of the orbit, but is inserted, not into bone,
but into the eyelid directly; and similarly other muscles arising
at the back of the orbit are directly fixed to the eyeball in front
and serve to rotate it on the pad of fat on which it lies. Many
facial muscles again have no direct attachment whatever to bones,
as for example the muscle (orbicularis oris) which surrounds the
mouth-opening, and by its contraction narrows it and purses out

102

MOTION AND LOCOMOTION 103

the lips; or the orbicularis palpebrarum which similarly surrounds
the eyes and when it contracts closes them.

Levers in the Body. When the muscles serve to move bones
the latter are in nearly all cases to be regarded as levers whose
fulcra lie at the joint where the movement takes place. Examples
of all the three forms of levers recognized in mechanics are found
in the Human Body.

Levers of the First Order. In this form (Fig. 51) the ful-
crum or fixed point of support lies between the "weight" or

i F J

P J^ W

FIG. 51. — A lever of the first order. F, fulcrum; P, power; W, resistance or
weight.

resistance to be overcome and the "power" or moving force,
as shown in the diagram. The distance PF, from the power to
the fulcrum, is called the "power-arm"; the distance FW is the
"weightrarm." When power-arm and weight-arm are equal, as
is the case in the beam of an ordinary pair of scales, no mechanical
advantage is gained, nor is there any loss or gain in the distance
through which the weight is moved. For every inch through
which P is depressed, W will be raised an equal distance. When
the power-arm is longer than the other, then a smaller force at P
will raise a larger weight at W,- the gain being proportionate to the
difference in the lengths of the arms. For example if PF is twice
as long as FW, then half a kilogram applied at P will balance a
whole kilogram at W, and just more than half a kilogram would
lift it; but for every centimeter through which P descended, W
would only be lifted half a centimeter. On the other hand, when
the weight-arm in a lever is longer than the power-arm, there is
loss in force but a gain in the distance through which the weight
is moved.

Examples of the first form of lever are not numerous in the
Human Body. One is afforded in the nodding movements of the
head, the fulcrum being the articulations between the skull and
the atlas. When the chin is elevated the power is applied to the
skull, behind the fulcrum, by small muscles passing from the

104 THE HUMAN BODY

vertebral column to the occiput; the resistance is the excess in
the weight of the part of the head in front of the fulcrum over
that behind it, and is not great. To depress the chin as in nodding
does not necessarily call for any muscular effort, as the head will
fall forward of itself if the muscles keeping it erect cease to work,
as those of us who have fallen alseep during a dull discourse on a
hot day have learnt. If the chin however be depressed forcibly,
as in the athletic feat of suspending one's self by the chin, the
muscles passing from the chest to the skull in front of the atlanto-
occipital articulation are called into play. Another example of
the employment of the first form of lever in the Body is afforded
by the curtsey with which a lady salutes another. In curtseying
the trunk is bent forward at the hip-joints, which form the ful-
crum ; the weight is that of the trunk acting as if all concentrated
at its center of gravity, which lies a little above the sacrum and
behind the hip-joints; and the power is afforded by muscles pass-
ing from the thighs to the front of the pelvis.

Levers of the Second Order. In this form the weight or re-
sistance is between the power and the fulcrum. The power-
arm PF is always longer than the weight-arm WF, and so a com-
paratively weak force can overcome a considerable resistance.
But it is disadvantageous so far as regards rapidity and extent
of movement, for it is obvious that when P is raised a certain
distance W will be moved a less -distance in the same time. As
an example of the employment of such levers (Fig. 52) in the

W

FIG. 52. — A lever of the second order. F, fulcrum; P, power; W, weight. The
arrows indicate the direction in which the forces act.

Body, we may take the act of standing on the toes. Here the
foot represents the lever, the fulcrum is at the contact of its fore
part with the ground ; the weight is that of the Body acting down
through the ankle-joints at Ta, Fig. 53 ; and the power is the great
muscle of the calf acting by its tendon inserted into the heel-
bone (Ca, Fig. 53) . Another example is afforded by holding up the
thigh when one foot is kept raised from the ground, as in hopping

MOTION AND LOCOMOTION

105

on the other. Here the fulcrum is at the hip-joint, the power is
applied at the knee-cap by a great muscle (rectus femoris) which
is inserted there and arises from the pelvis; and the weight is

cm en CI

FIG. 53. — The skeleton of the foot from the outer side. Ta, surface with which
the leg-bones articulate; Ca, the calcaneum into which the tendon (tendo Achillis)
of the calf muscle is inserted: M 5, the metatarsal bone of the fifth digit; N, the
scaphoid bone; CI, CII, CHI, first, second, and third cuneiform bones; Cb, the
cuboid bone.

that of the whole lower limb acting at its center of gravity, which
lies somewhere in the thigh between the hip and knee-joints, that
is, between the fulcrum and the point of application of the power.
Levers of the Third Order. In these (Fig. 54) the power is
between the fulcrum and the weight. In such levers the weight-
arm is always longer than the power-arm, so the power works at
a mechanical disadvantage, but swiftness and range of move-
ment are gained. It is the lever most commonly used in the
Human Body. For example, when the forearm is bent up towards
the arm, the fulcrum is the- .elbow-joint, the power is applied at
the insertion of the biceps muscle (Fig. 43) into the radius and of

Fio. 54. — A lever of the third order. F, fulcrum; P, power; W, weight.

another muscle (not represented in the figure^the brachialis
anticus, into the ulna), and the weight is that of the (forearm and
hand, with whatever may be contained in the latter, acting at
the center of gravity of the whole somewhere on the distal side
of the point of application of the power. In the Body the power-

106 THE HUMAN BODY

arm is usually very short so as to gain speed and range of move-
ment, the muscles being powerful enough to do their work in spite
of the mechanical disadvantage at which they are placed. The
limbs are thus made much more shapely than would be the case
were the power applied near or beyond the weight.

It is of course only rarely that simple movements as those
described above take place. In the great majority of those
executed several or many muscles cooperate.

The Loss to the Muscles from the Direction of their Pull. It
is worthy of note that, owing to the oblique direction in which
the muscles are commonly inserted into the bones, much of their
force is lost so far as producing movement is concerned. Sup-
pose the log of wood in the diagram (Fig. 55) to be raised by pull-
ing on the rope in the direction a; it is clear at first that the rope
will act at a great disadvantage; most of the pull transmitted
by it will be exerted against the pivot on which the log hinges,
and only a small fraction be available for elevating the latter.
But the more the log is lifted, as for example into the position
indicated by the dotted lines, the more useful will be the direction
of the pull, and the more of it will be spent on the log and the

FIG. 55. — Diagram illustrating the disadvantage of an oblique pull.

less lost unavailingly in merely increasing the pressure at the
hinge. If we now consider the action of the biceps (Fig. 43) in
flexing the elbow-joint, we see similarly that the straighter the
joint is, the more of the pull of the muscle is wasted. Beginning
with the arm straight, it works at a great disadvantage, but
as the forearm is raised the conditions become more and more

MOTION AND LOCOMOTION 107

favorable to the muscle. Those who have practised the gym-
nastic feat of raising one's self by bending the elbows when hang-
ing by the hands from a horizontal bar know practically that if
the elbow-joints are quite straight it is very hard to start; and
that, on the other hand, if they are kept a little flexed at the
beginning the effort needed is much less; the reason being of
course the more advantageous direction of traction by the biceps
in the latter case.

Experiment proves that the power with which a muscle can
contract is greatest at the commencement of its shortening, the
very time at which, we have just seen, it works at most me-
chanical disadvantage; in proportion as its force becomes less
the conditions become more favorable to it. There is, however,
it is clear, nearly always a considerable loss of power in the work-
ing of the skeletal muscles, strength being sacrificed for variety,
ease, rapidity, extent, and elegance of movement.

Postures. The term posture is applied to those positions of
equilibrium of the Body which can be maintained for some time,
such as standing, sitting, or lying, compared with leaping, run-
ning, or. falling. In all postures the condition of stability is that
the vertical line drawn through the center of gravity of the Body
shall fall within the basis of support afforded by objects with
which it is in contact; and the security of the posture is propor-
tionate to the extent of this base, for the wider it is the less is the
risk of the perpendicular through the center of gravity falling
outside of it on slight displacement.

The Erect Posture. This is preeminently characteristic of
man, his whole skeleton being modified with reference to it.
Nevertheless the power of maintaining it is only slowly learnt
in the first years after birth, and for a long while it is unsafe.
And though finally we learn to stand erect without conscious
attention, the maintenance of that posture always requires the
cooperation of many muscles, coordinated by the nervous system.
The influence of the latter is shown by the fall which follows a
severe blow on the head, which may nevertheless have fractured
no bone nor injured any muscle: the concussion of the brain, as
we say, "stuns" the man, and until its effects have passed off
he cannot stand upright. In standing with the arms straight
by the sides and the feet together the center of gravity of the

108

THE HUMAN BODY

whole adult Body lies in the articulation between the sacrum and
the last lumbar vertebra, and the perpendicular drawn from it
will reach the ground between the two feet, within the basis of
support afforded by them. With the feet close together, how-
ever, the posture is not very stable, and in
standing we commonly make it more so by
slightly separating them so as to increase the
base. The more one foot is in front of the other
the more swaying back and forward will be
compatible with safety; and the greater the
lateral distance separating them the greater
will be the lateral sway which is possible with-
out falling. Consequently we see that a man
about to make great movements with the upper
part of his Body, as in fencing or boxing, or a
soldier preparing for the bayonet exercise, al-
ways commences by thrusting one foot for-
wards obliquely, so as to increase his basis of
support in both directions.

The ease with which we can stand is largely
dependent upon the way in which the head is
almost balanced on the top of the vertebral
column, so that but little muscular effort is
needed to keep it upright. In the same way
the trunk is almost balanced on the hip-joints,
but not quite, its center of gravity falling rather
behind them; so that just as some muscular
effort is needed to keep the head from falling

FIG. 56. — Diagram

illustrating the mus- forwards, some is needed to keep the trunk
bieaSck(drHnens)in wS from toppling backwards at the hips. In a
pass before and be- smiiiar manner other muscles are called into

hind the joints and bv

their balanced activ- play at other joints: as between the vertebral
rigid and the Jbody column and the pelvis, and at the knees and
ankles; and thus a certain rigidity, due to
muscular effort, extends all along the erect Body: which, on
account of the flexibility of its joints, could not otherwise be
balanced on its feet, as a statue can. Beginning (Fig. 56) at
the ankle-joint, we find it kept stiff in standing by the com-
bined and balanced contraction of the muscles passing from

MOTION AND LOCOMOTION 109

the heel to the thigh, and from the dorsum of the foot to
the shin-bone (tibia). Others passing before and behind the
knee-joint keep it from yielding; and so at the hip-joints: the
others again, lying in the walls of the abdomen and along the
vertebral column, keep the latter rigid and erect on the pelvis;
and finally the skull is kept in position by muscles passing from
the sternum and vertebral column to it, in front of and behind
the occipital condyles.

Locomotion includes all the motions of the whole Body in
space, dependent on its own muscular efforts: such as walking,
running, leaping, and swimming.

Walking. In walking the Body never entirely quits the
ground, the heel of the advanced foot touching the ground in
each step before the toe of the rear foot leaves it. The advanced
limb supports the Body, and the foot in the rear at the com-
mencement of each step propels it.

Suppose a man standing with his heels together to commence
to walk, stepping out with the left foot ; the whole Body is at first
inclined forwards, the movement taking place mainly at the
ankle-joints. By this means the center of gravity would be
thrown in front of the base formed by the feet and a fall on the
face result, were not simultaneously the left foot slightly raised
by bending the knee and then swung forwards, the toes just clear
of the ground and, in good walking, the sole nearly parallel to it.
When the step is completed the left knee is straightened and the
sole placed on the ground, the heel touching it first, and the base
of support being thus widened from before back, a fall is pre-
vented. Meanwhile the right leg is kept straight, but inclines
forwards above with the trunk when the latter advances, and as
this occurs the sole gradually leaves the ground, commencing
with the heel. When the step of the left leg is completed the
great toe of the right alone is in contact with the support. With
this a push is given which sends the trunk on over the left leg,
which is now kept rigid, except at the ankle-joint; and the right
knee being bent that limb swings forwards, its foot just clearing
the ground as the left did before. The Body is meanwhile sup-
ported on the left foot alone, but when the right completes its
step the knee of that leg is straightened and the foot thus placed,
heel first, on the ground. Meanwhile the left foot has been gradu-

110 THE HUMAN BODY

ally leaving the ground, and its toes only are at that moment
upon it : from these a push is given, as before, with the right foot,
and the knee being bent so as to raise the foot, the left leg swings
forwards at the hip-joint to make a fresh step.

During each step the whole Body sways up and down and also
from side to side. It is highest at the moment when the advanc-
ing trunk is vertically over the foot supporting it, and then sinks
until the moment when the advancing foot touches the ground,
when it is lowest. From this moment it rises as it swings forward
on this foot, until it is vertically over it, and then sinks again
until the other touches the ground; and so on. At the same time,
as its weight is alternately transferred from the right to the left
foot and vice versa, there is a slight lateral sway, commonly more
marked in women than in men, and which when excessive pro-
duces an ugly " waddling " gait.

The length of each step is primarily dependent on the length
of the legs; but can be controlled within wide limits by special
muscular effort. In easy walking little muscular work is em-
ployed to carry the rear leg forwards after it has given its push.
When its foot is raised from the ground it swings on, like a pendu-
lum; but in fast walking the muscles, passing in front of the hip-
joint, from the pelvis to the limb, by their contraction forcibly
carry the leg forwards. The easiest step, that in which there is
most economy of labor, is that in which the limb is let swing
freely, and since a short pendulum swings faster than a longer,
the natural step of short-legged people is quicker than that of
long-legged ones.

In fast walking the advanced or supporting leg also aids in
propulsion; the muscles passing in front of the ankle-joint con-
tracting so as to pull the Body forwards over that foot and aid
the push from the rear foot. Hence the fatigue and pain in front
of the shin which is felt in prolonged, very fast walking. From
the fact that each foot reaches the ground heel first, but leaves
it toe last, the length of each stride is increased by the length of
the foot.

Running. In this mode of progression there is a moment in
each step when both feet are off the ground, the Body being un-
supported in the air. The toes alone come in contact with the
ground at each step, and the knee-joint is not straight when the

MOTION AND LOCOMOTION 111

foot reaches the ground. When the rear foot is to leave the sup-
port, the knee is suddenly straightened, and at the same time the
ankle-joint is extended so as to push the toes forcibly on the
ground and give the whole Body a powerful push forwards and
upwards. Immediately after this the knee is greatly flexed and
the foot raised from the ground, and this occurs before the toes of
the forward foot reach the latter. The swinging leg in each step
is violently pulled forwards and not suffered to swing naturally,
as in walking. By this the rapidity of the succession of steps is
increased, and at the same time the stride is made greater by
the sort of one-legged leap that occurs through the jerk given by
the straightening of the knee of the rear leg just before it leaves
the ground.

Leaping. In this mode of progression the Body is raised
completely from the ground for a considerable period. In a
powerful leap the ankles, knees, and hip-joints are all flexed as a
preparatory measure, so that the Body assumes a crouching
attitude. The heels, next, are raised from the ground and the
Body balanced on the toes. The center of gravity of the Body
is then thrown forwards, and simultaneously the flexed joints
are straightened, and by the resistance of the ground, the Body
receives a propulsion forwards; much in the same way as a ball
rebounds from a wall. The arms are at the same time thrown
forwards. In leaping backwards, the Body and arms are in-
clined in that direction; and in jumping vertically there is no
leaning either way and the arms are kept by the sides.

Hygiene of the Muscles. The healthy working of the muscles
needs of course a healthy state of the Body generally, so that
they shall be supplied with proper materials for growth and re-
pair, and have their wastes rapidly and efficiently removed. In
other words, good food and pure air are necessary for a vigorous
muscular system, a fact which trainers recognize in insisting upon
a strict dietary, and in supervising generally the mode of life of
those who are to engage in athletic contests. The muscles should
also not be exposed to any considerable continued pressure, since
this interferes with the flow of blood and lymph through them.

As far as the muscles themselves are directly concerned, exer-
cise is the necessary condition of their best development. A
muscle which is permanently unused degenerates and is absorbed,

1 1 2 THE H UMAN BOD Y

little finally being left but the connective tissue of the organ and
a few muscle-fibers filled with oil-drops. This is well seen in cases
of paralysis dependent on injury to the nerves. In such cases
the muscles may themselves be perfectly healthy at first, but ly-
ing unused for weeks they become altered, and finally, when the
nervous injury has been healed, the muscles may be found in-
capable of functional activity. The physician therefore is often
careful to avoid this by exercising the paralyzed muscles daily
by means of electrical shocks sent through the part; passive exer-
cise, as by proper massage, is frequently of great use in such cases.
The same fact is illustrated by the feeble and wasted condition of
the muscles of a limb which has been kept for some time in splints.
After the latter have been removed it is only slowly, by judicious
and persistent exercise, that the long-idle muscles regain their
former size and power. The great muscles of the " brawny " arm
of the blacksmith or wrestler illustrate the reverse fact, the
growth of the muscles by exercise. Exercise, however, must be
judicious; repeated frequently to the point of exhaustion it does
harm; the period of repair is not sufficient to allow replacement
of the parts used in work, and the muscles thus waste under too
violent exercise as with too little. Rest should alternate with
work, and that regularly, if benefit is to be obtained. Moreover,
violent exercise should never be suddenly undertaken by one
unused to it, not only lest the muscles suffer, but because muscular
effort greatly increases the work of the heart. No general rule
can be laid down as to the amount of exercise to be taken; for a
healthy man in business the minimum would perhaps be repre-
sented by a daily walk of five miles.

Varieties of Exercise. In walking and running the muscles
chiefly employed are those of the lower limbs and trunk. This is
in part true of rowing, which when good is performed much more
by the legs than the arms: especially since the introduction of
sliding seats. Hence any of these exercises alone is apt to leave
the muscles of the chest and arms imperfectly exercised. Indeed,
no one exercise employs equally or proportionately all the muscles :
therefore gymnasia in which various feats of agility are practised,
so as to call different parts into play, have very great utility. It
should be borne in mind, however, that the legs especially need
strength; while the upper limbs, in which delicacy of movement,

MOTION AND LOCOMOTION 113

as a rule, is more desirable than power, do not require so much
exercise; and the fact that gymnastic exercises are commonly
carried on indoors is a great drawback to their value. When the
weather permits, out-of-door exercise is far better than that
carried on in even the best ventilated and lighted gymnasium.
For those who are so fortunate as to possess a garden there is no
better exercise, at suitable seasons, than an hour's daily digging
in it; since this calls into play nearly all the muscles of the Body;
while of games, the modern one of lawn-tennis is perhaps the best
from a hygienic view that has ever been invented, since it not
only demands great muscular agility in every part of the Body,
but trains the hand to work with the eye in a way that walking,
running, rowing, and similar pursuits do not. For the same
reasons baseball, cricket, and boxing are excellent.

Exercise in Infancy and Childhood. Young children have
not only to strengthen their muscles by exercise, but also to learn
to use them. Watch an infant trying to convey something to its
mouth, and you will see how little control it has over its muscles.
On the other hand, the healthy infant is never at rest when awake;
it constantly throws its limbs around, grasps at all objects within
its reach, coils itself about, and so gradually learns to exercise its
powers. It is a good plan to leave every healthy child more than
a few months old several times daily on a large bed, or even on a
rug or carpeted floor, with as little covering as is safe, and that
as loose as possible, and let it wriggle about as it pleases. In this
way it will not only enjoy itself thoroughly, but gain strength and
a knowledge of how to use its limbs. To keep a healthy child
swathed all day in tight and heavy clothes is cruelty.

When a little later the infant commences to crawl it is safe to
permit it to as much as it wishes, but unwise to tempt it to do
so when disinclined: the bones and muscles are still feeble and
may be injured by too much work. The same is true of learning
to walk.

From four or five to twelve years of age almost any form of
exercise should be permitted, or even encouraged. During this
time, however, the epiphyses of many bones are not firmly united
to their shafts, and so anything tending to throw too great a
strain on the joints should be avoided. After that up to com-
mencing manhood or maidenhood any kind of outdoor exercise

114 THE HUMAN BODY

for healthy persons is good, and girls are all the better for being
allowed to join in their brothers' sports. Half of the debility and
general ill-health of so many of our women is the consequence of
deficient exercise during early life.

Exercise in Youth should be regulated largely by sex; not that
women are to be shut up and made pale, delicate, and unfit to
share the duties or participate fully in the pleasures of life; but
the other calls on the strength of the young woman render vig-
orous muscular work often unadvisable, especially under con-
ditions where it is apt to be followed by a chill.

A healthy boy or young man may do nearly anything; but
until twenty-two or twenty-three very prolonged effort is un-f
advisable. The frame is still not firmly knit or as capable of en-
durance as it will subsequently become.

Girls should be allowed to ride or play outdoor games in mod-
eration, and in any case should not be cribbed in tight stays or
tight boots. A flannel dress and proper lawn tennis shoes are as
necessary for the healthy and safe enjoyment of an afternoon at
that game by a girl as they are for her brother in the baseball
field. Rowing is excellent for girls if there be any one to teach
them to do it properly with the legs and back, and not with the
arms only, as women are so apt to row. Properly practised it
strengthens the back and improves the carriage.

Exercise in Adult Life. Up to forty a man may carry on safely
the exercises of youth, but after that sudden efforts should be
avoided. A lad of twenty-one or so may, if trained, safely run a
quarter-mile race, but to a man of forty-five it would be dan-
gerous, for with the rigidity of the cartilages and blood-vessels
which begins to show itself about that time comes a diminished
power of meeting a sudden violent demand. On the other hand,
the man of thirty would more safely than the lad of nineteen or
twenty undertake one of the long-distance walking matches which
have lately been in vogue; the prolonged effort would be less
dangerous to him, though a six-days' match, with its attendant
loss of sleep, cannot fail to be more or less dangerous t6 any one.
Probably for one engaged in active business a walk of two or
three miles to it in the morning and back again in the afternoon is
the best and most available exercise. The habit which Americans
have everywhere acquired, of never walking when they can take

MOTION AND LOCOMOTION 115

a street car, is certainly detrimental to the general health; though
the extremes of heat and cold to which we are subject often
render it unavoidable.

For women during middle life the same rules apply: there
should be some regular but not violent daily exercise.

In Old Age the needful amount of exercise is less, and it is
still more important to avoid sudden or violent effort.

Exercise for Invalids. This should be regulated under med-
ical advice. For feeble persons gymnastic exercises are especially
valuable, since from their variety they permit of selection accord-
ing to the condition of the individual; and their amount can be
conveniently controlled.

Training. If any person attempt some unusual exercise he
soon finds that he loses breath, gets perhaps a " stitch in the side,"
and feels his heart beating with unwonted violence. If he perse-
vere he will probably faint — or vomit, as is frequently seen in the
case of imperfectly trained men at the end of a hard boat-race.
These phenomena are avoided by careful gradual preparation
known as "training." The immediate cause of them lies in dis-
turbances of the circulatory and respiratory organs, on which
excessive work is thrown.

CHAPTER IX
ANATOMY OF THE NERVOUS SYSTEM

General Statement. In Chapter III the special function of
the nervous system was outlined, and was shown to involve the
transmission of stimuli from the sensory regions of the body to
the muscles, and in the course of such transmission to make what-
ever modifications are necessary to the production of the best
results. The sensory regions of the body are numerous; there are
likewise many muscles. Successful adaptation of the individual
to his surroundings may call at one time or another for the pas-
sage of stimuli from any sensory region to any muscle, or for the
combination of stimuli from several sensory regions to form
stimuli to go to any group of muscles. A somewhat analogous
situation occurs in the telephone systems with which the country
is dotted. In these communication may be desired between any
pair of instruments in the system. To make this possible all the
telephones in any one system are led into a central exchange
where provision is made for connecting any instrument with any
other. Flexibility of communication between sensory and motor
regions in the Body is secured in somewhat similar fashion. All
nerves from sensory regions are led into a central " exchange"
from which start all nerves to the motor organs.

Nerve Impulses. Since it is impossible to describe the nervous
system without frequent reference to the messages which nerves
carry it is desirable before proceeding farther to state that it has
become the custom to call these messages nerve impulses. When
we speak of a nerve impulse we have in mind the process by which
the message is transmitted along the nerve. The situation cor-
responds to that in a telephone wire. When the latter is trans-
mitting a message the words spoken into the transmitter are
not carried along, but an electrical disturbance which they set
up. So the nerve does not transmit the exact stimulus which acts
upon it, but a nerve impulse which the stimulus arouses.

Neurons. The nervous system as a whole is made up of struc-

116

ANATOMY OF THE NERVOUS SYSTEM 117

tures called neurons, each of which seems to be a single nerve-
cell.

A typical neuron consists of a cell-body containing a nucleus and
from whose surface project many rather short branching processes
called dendrites, and a single long process having few if any
branches and known as the axon. Neurons which convey impulses
to muscles (motor neurons) have this structure (d, Fig. 65).

The neurons which convey impulses from sensory regions to
the center (sensory neurons) have a structure which appears at
first view, to be altogether different from that of the typical
neuron just described. They have cell-bodies with nuclei but in-
stead of a single axon and numerous much-branched dendrites the
cell-body gives rise to two long axon-like processes, one of which
may have a comparatively small number of branches. The bipolar
character of these .neurons, moreover, is concealed in many through
the union of the two processes for a short distance from the cell-
body, giving an appearance as though the latter were on a side
branch of a long axon (6, Fig. 65). The underlying similarity of
these to the type neuron appears if we consider that the dendrites
of the typical neuron are replaced in the sensory neuron by one
of the axon-like processes mentioned above.

A third sort of neurons occurring in the Body resembles the
first or motor type in the possession of cell-body and many branch-
ing dendrites. Instead of long, slightly branched axons, however,
neurons of this sort have short and very much-branched ones.
These neurons occur interposed .in the pathway of impulses from
sensory to motor neurons and are often called association neurons
(c, Fig. 65) : they are not, however, the only sort of association neu-
rons; many neurons which belong physiologically to the group of
association neurons in that they form communicating paths be-
tween sensory and motor neurons are anatomically of the type to
which all motor neurons belong.

If we adopt the usual view that each single neuron represents
one nerve-cell, neurons are the largest cells known. Although
axons are so small in cross-section as to be microscopic they may
have a length of three feet or more, as in the nerve trunks which
extend down the legs to the feet.

Synapses. Communication between neuron and neuron is
always according to a certain scheme. The axons of all except

118 THE HUMAN BODY

motor neurons end in masses of fine branches known as end
arborizations. These are in contact with the branching dendrites
of some other neuron. The surfaces of contact between the end
arborization of one neuron and the dendrites of another constitute
what is called a synapse. In order for a nerve impulse to pass
from one neuron to another it must cross this synapse.

The Myelin Sheath. All true nerve tissue has a characteristic
gray color. This statement applies equally to cell-bodies, den-
drites, and axons. Most, but not all, of the long axons of the
body are inclosed within sheaths composed chiefly of a substance,
myelin, which has a characteristic glossy white color. The myelin
sheath where present does not inclose the axon throughout its
entire length; near the cell-body and again near its termination
the axon is not inclosed. Surrounding the myelin sheath, or,
where it is absent, the axon itself, is a delicate membrane, the
neurilemma. The myelin sheath is made up of short segments
which are separated one from another by the nodes of Ranvier.

The myelin sheath is not composed of living cells and so does
not contain nuclei. The neurilemma, however, is a living mem-
brane; scattered along it at intervals are nuclei. The function
of the myelin sheath is not known. Perhaps the most satisfactory
suggestion that has been offered is that it serves as an insulator
to keep the nerve impulse within its own axon and prevent its
escape to adjacent ones.

Axons which are inclosed in myelin sheaths are spoken of as
medullated or myelinated nerve-fibers.

It is the presence of myelin sheaths that gives to certain parts
of the nervous system their characteristic white appearance.
All "white matter" is made up of medullated axons. "Gray
matter," on the other hand, is made of cell-bodies and dendrites,
together with some non-medullated axons.

The Central and Peripheral Nervous Systems. In a preced-
ing paragraph was pointed out the analogy between the nervous
system and a telephone system. That part of the nervous system
corresponding to the telephone "exchange," to which sensory
neurons lead and from which motor neurons spring is called the
central nervous system. It consists of the brain and spinal cord.
(The analogy between the central nervous system and a telephone
exchange should not be pushed too far, for the central nervous

. ANATOMY OF THE NERVOUS SYSTEM 119

system has numerous functions in addition to the simple one of
making connections between sensory and motor neurons. These
special functions have to do with the modification of the impulses
passing through it for the best advantage of the organism as a
whole.)

Springing from the central nervous system and corresponding
to the cables bearing wires to individual telephones are forty-
three pairs of nerve-trunks. Twelve pairs arise from the brain and
are called cranial nerves; the remaining thirty-one pairs arise from
the spinal cord and are called spinal nerves. Each nerve-trunk
contains a large number of axons, and in most nerve-trunks the
axons of both motor and sensory neurons are present. These forty-
three pairs of nerve-trunks with their ramifications to all parts
of the Body constitute the peripheral nervous system (Fig. 57).

There are in the Body a set of neurons which though part of
the peripheral nervous system are specially adapted for a certain
function and are therefore usually considered independently.
These constitute the sympathetic or autonomic system.

The Central Nervous System and its Membranes. Lying in-
side the skull is the brain and in the neural canal of the verte-
bral column the spinal cord, the two being continuous through
the foramen magnum of the occipital bone. The central nervous
system is bilaterally symmetrical throughout except for slight
differences on the surfaces of parts of the brain, which are often
found in the higher races of mankind. Both brain and spinal cord
are very soft and easily crushed; nervous tissue as well as the con-
nective tissue and a peculiar supporting tissue (neuroglid) which
pervades it being delicate; accordingly both organs are placed in
nearly completely closed bony cavities and are also enveloped by
membranes which give them support. These membranes are
three in number. Externally is the dura mater, very tough and
strong and composed of white fibrous and elastic connective tis-
sues. In the cranium the dura mater adheres by its outer surface
to the inside of the skull chamber, serving as the periosteum of its
bones; this is not the case in the vertebral column, where the
dura mater forms a loose sheath around the spinal cord and is
only attached here and there to the surrounding bones, which
have a separate periosteum of their own. The innermost mem-
brane of the cerebrospinal center, lying in immediate contact

120

THE HUMAN BODY

FIG. 57. — Diagram illustrating the general arrangement of the nervous system.

ANATOMY OF THE NERVOUS SYSTE\f

121

with the proper nervous parts, is the pia mater, also made up
of white fibrous tissue interwoven with
elastic fibers, but less closely than in
the dura mater, so as to form a less
dense and tough membrane. The pia
mater contains many blood-vessels which
break up in it into small branches be-
fore entering1 the nervous mass beneath.
Covering the outside of the pia mater is
a layer of flat closely fitting cells; a simi-
lar layer lines the inside of the dura
mater, and these two layers are described
as the third membrane of the cerebro-
spinal center, called the arachnoid. In
the space between the two layers of the
arachnoid is contained a small quantity
of watery cerebrospinal liquid. The sur-
face of the brain is folded and the pia
mater follows closely these folds; the
arachnoid often stretches across them:
in the spaces thus left between it and
the pia mater is contained some of the
cerebrospinal liquid.

The Spinal Cord (Fig. 58) is nearly
cylindrical in form, being however a lit-
tle wider from side to side than dorsiven-
trally, and tapering off at its posterior
end. Its average diameter is about 19
millimeters (f inch) and its length 0.43
meter (17 inches). It weighs 42.5 grams
(1£ ounces). There is no marked limit
between the spinal cord and the brain,
the one passing gradually into the other
(Fig. 64) , but the cord is arbitrarily said
to commence opposite the outer margin
of the foramen magnum of the occipital
bone: from there it extends to the articu- fron* the ventral, ami

...... „ the dorsal aspect ;C to a cross-

lation between the first and second lum- sections at different levels,
bar vertebra, where it narrows off to a slender non-nervous fila-

122

THE HUMAN BODY

ment, the filum terminate (cut off and represented separately at
B' in Fig. 58), which runs back to the end of the neural canal
behind the sacrum. In its course the cord presents two expan-
sions, an upper, 10, the cervical enlargement, reaching from the
third cervical to the first dorsal vertebra, and a lower or lumbar
enlargement, 9, opposite the last dorsal vertebra.

Running along the middle line on both the ventral and the
dorsal aspects of the cord is a groove, and a cross-section showa

FIG. 59. — The spinal cord and nerve-roots. A, a small portion of the cord seen
from the ventral side; B, the same seen laterally; C, a cross-section of the cord;
D, the two roots of a spinal nerve; 1, ventral fissure; 2, dorsal fissure; 3, surface
groove along the line of attachment of the ventral nerve-roots; 4, line of origin of
the dorsal roots; 5, ventral root filaments of spinal nerve; 6, dorsal root filaments;
6', ganglion of the dorsal root; 7, 7', the first two divisions of the nerve-trunk after
its formation by the union of the two roots. The grooves are much exaggerated.

that these grooves are the surface indications of fissures which
extend deeply into the cord (C, Fig. 59) and nearly divide it into
right and left halves.

The ventral fissure (1, Fig. 59) is wider and shallower than the
dorsal, 2, which indeed is hardly a true fissure, being completely
filled up by an ingrowth of pia mater. The transverse section,
C, shows also that the substance of the cord is not alike through-
out, but that its white superficial layers envelop a central gray
substance arranged somewhat in the form of a capital H. Each

ANATOMY OF THE NERVOUS SYSTEM 123

half of the gray matter is crescent-shaped, and the crescents are
turned back to back and united across the middle line by the
gray commissure. The tips of each crescent are called its horns or
cornua, and the ventral horn on each side is thicker and larger
than the dorsal. In the cervical and lumbar enlargements the
proportion of white to gray matter is greater than elsewhere ; and
as the cord approaches the medulla oblongata its central gray
mass becomes irregular in form and begins to break up into
smaller portions. If lines be drawn on the transverse section of
the cord from the tip of each horn of the gray matter to the
nearest point of the surface, the white substance in each half will
be divided into three portions: one between the ventral fissure
and the ventral cornu, and called the ventral white column; one

Fio. 60. — Diagram illustrating the general relationships of the parts of the brain.

A, fore-brain; b, midbrain; B, cerebellum; C{ pons Varolii; D, medulla oblongata;

B, C, and D together constitute the hind-brain.

between the dorsal fissure and the dorsal cornu, and called the
dorsal white column; while the remaining one lying in the hollow
of the crescent and between the two horns is the lateral column:
the ventral and lateral columns of the same side are frequently
named the ventrolatcral column. A certain amount of white
substance crosses the middle line at the bottom of the ventral
fissure; this forms the ventral white commissure. There is no
dorsal white commissure, the bottom of the dorsal fissure being

124 THE HUMAN BODY

the only portion of the cord where the gray substance is un-
covered by white. Running along the middle of the gray com-
missure, for the whole length of the cord, is a tiny channel, just
visible to the unaided eye; it is known as the central canal (canalis
centralist .

The Brain (Fig. 60) is far larger than the spinal cord and more
complex in structure. It weighs on the average about 1,415 grams
(50 ounces) in the adult male, and about 155 grams (5.5 ounces)
less in the female. In its simpler forms the vertebrate brain con-

Cb Ro

Po

Cbl

FIG. 61. — The brain from the left side. Cb, the cerebral hemispheres forming
the main bulk of the fore-brain; Cbl, the cerebellum; Mo, the medulla oblongata;
P, the pons Varolii; *, the fissure of Sylvius; Ro, the fissure of Rolando; Po, the
Parieto-occipital fissure.

sists of three masses, each with subsidiary parts, following one
another in series from before back, and known as the fore-brain,
midbrain, and hind-brain respectively. In man the fore-brain,
A, weighing about 1,245 grams (44 ounces), is much larger than
all the rest put together and laps over them behind. It consists
mainly of two large convoluted masses, separated from one an-
other by a deep median fissure, and known as the cerebral hemi-
spheres. The immense proportionate size of these is very char-
acteristic of the human brain. Beneath each cerebral hemisphere
is an olfactory lobe, inconspicuous in man but in many animals
larger than the cerebral hemispheres. Buried in the fore-brain
on each side are two large gray masses, the corpora striata and
optic thalami. The midbrain forms a connecting isthmus between

ANATOMY OF THE NERVOUS SYSTEM

125

the two other divisions and presents on its dorsal side four hemi-
spherical eminences, the corpora quadrigemina or colliculi. On
its ventral side it exhibits two semicylindrical pillars (seen under
the nerve IV in Fig. 64), known as the-crwra cerebri. The hind-
brain consists of three main parts: on its dorsal side is the cere-
bellum, B (Fig. 60), consisting of a right, a left, and a median lobe;
on the ventral side is the pans Varolii, C (Fig. 60), and behind
that the medulla oblongata, D (Fig. 60), which is continuous with
the spinal cord.

In nature, the main divisions of the brain are not separated so
much as has been represented in the diagram for the sake of clear-
ness, but lie close together, as represented in Fig. 61, only some

FIG. 62. — Diagram of the right half of a vertical median section of the brain.
H, H, convoluted inner surface of right cerebral hemisphere; Cc, corpus callosum;
pt, the pituitary body; the mass on which the figure 3 is placed is the inner side
of the right optic thalamus; o, d, the anterior and posterior corpora quadriaemina
of the right side; .!/«, the medulla oblongata; Cr, right crus cerrbri; P, pans Varolii;
Cb, cerebellum; where it is divided in the middle line the radial arrangement of its
central white matter forming the so-called arbor vitae is seen; op, right optic nerve
proceeding from the optic chiasma; oc, the third cranial nerve arising from the
cms cerebri.

folds of the membranes extending between them; and the mid-
brain is entirely covered in on its dorsal aspect. Nearly every-
where the surface of the brain is folded, the folds, known as gyri
or convolutions being deeper and more numerous in the brain of
man than in that of the animals nearest allied to him; and in the

126 THE HUMAN BODY

human species more marked in the higher than in the lower races.
It should however be added that some species of animals which
are not markedly intelligent have much convoluted cerebral
hemispheres.

The brain like the spinal cord consists of gray and white nervous
matter, but somewhat differently arranged, for while the brain,
like the cord, contains gray matter in its interior, a great part of
its surface is also covered with it. By the external convolutions
of the cerebellum and the cerebral hemispheres the surface over
which this gray substance is spread is very much increased (see
Fig. 61).

The Spinal Nerves. Thirty-one pairs of spinal nerve-trunks
enter the neural canal of the vertebral column through the in-
tervertebral foramina (p. 53). Each divides in the foramen
into a dorsal and ventral portion known respectively as the dorsal
and ventral roots of the nerve (6 and 5, Fig. 59), and these again
subdivide into finer branches which are attached to the sides of
the cord, the dorsal root at the point where the dorsal and lateral
white columns meet, and the ventral root at the junction of the
lateral and ventral columns. Although the nerve-trunks contain
both sensory and motor neurons these are completely separated
in the roots; the dorsal root contains only sensory neurons; the
ventral only motor. At the lines on which the roots are attached
there are superficial furrows on the surface of the cord. On each
dorsal root is a spinal ganglion (6', Fig. 59), placed just before
it joins the ventral root to make up the common nerve-trunk.
This spinal ganglion contains the cell-bodies of the bipolar sensory
neurons. Immediately after its formation by the mixture of
fibers from both roots, the trunk divides (Z), Fig. 59), into a
dorsal primary, a ventral primary, and a communicating branch.
The branches of the first set go for the most part to the skin and
muscles on the back; from the second the nerves for the sides
and ventral region of the neck and trunk and for the limbs
arise; the communicating branches form part of the sympathetic
system.

The various spinal nerves are named from the portions of the
vertebral column through the intervertebral openings of which
they pass out ; and as a general rule each nerve is named from the
voadfcbra in front of it. For example, the nerve passing out be-

ANATOMY OF THE NERVOUS SYSTEM 127

tween the fifth and sixth thoracic vertebra) is the " fifth thoracic "
nerve, and that between the last thoracic and first lumbar verte-
bra), the "twelfth thoracic." In the cervical region, however,
this rule is not adhered to. The nerve passing out between th?
occipital bone and the atlas is called the "first cervical" nerve,
that between the atlas and axis the second, and so on; that be-
tween seventh cervical and first thoracic vertebrae being the
"eighth cervical" nerve. The thirty-one pairs of spinal nerves
are then thus distributed: 8 cervical, 12 thoracic, 5 lumbar,
5 sacral, and 1 coccygeal; the latter passing out between the
sacrum and coccyx. Since the spinal cord ends opposite the
upper lumbar vertebras while the sacral and coccygeal nerves pass
out from the neural canal much farther back, it is clear that the
roots of those nerves, on their way to unite in the foramina of
exit and form nerve-trunks, must run obliquely backwards in the
spinal canal for a considerable distance. One finds in fact the
neural canal in the lumbar and sacral regions, behind the point
where the spinal cord has tapered off to form ihefilum terminate,
occupied chiefly by a great bunch of nerve-roots forming the so-
called " horse's tail " or cauda equina.

Plexuses. Very frequently several neighboring nerve-trunks
send off communicating branches to one another, each branch
carrying fibers from one trunk to the other. Such networks are
called plexuses (Fig. 63), and through the interchanges taking
place in them it often happens that the distal branches of a nerve-
trunk contain fibers which it' docs not possess as it leaves the
central nervous system.

Distribution of the Spinal Nerves. It would be out of place
here to go into detail as to the exact portions of the Body sup-
plied by each spinal nerve, but the following general statements
may be made. The ventral primary branches of the first four
cervical nerves form on each side the cervical plexus (Fig. 63)
from which branches are supplied to the muscles and integument
of the neck: also to the outer ear and the back part of the scalp.
The ventral primary branches of the remaining cervical nerves
and the first dorsal form the brachial plexus, from which the upper
limb is supplied. The roots of the trunks which form this plexus
arise from the cervical enlargement of the spinal cord.

From the fourth and fifth cervical nerves on each side,

128

THE HUMAN BODY

branches arise and unite to make the phrenic nerve (4', Fig. 63)
which runs down through the chest and ends in the diaphragm.

The ventral primary branches of the thoracic nerves, except
part of the first which enters the brachial plexus, form no plexus,

IU

FIG. 63. — The cervical and brachial plexuses of the left side of the Body.

but each runs along the posterior border of a rib and supplies
branches to the chest-walls, and the lower ones to those of the
abdomen also.

The ventral primary branches of the four anterior lumbar
nerves are united by branches to form the lumbar plexus. It sup-

ANATOMY OF THE NERVOUS SYSTEM 129

plies the lower part of the trunk, the buttocks, the front of the
thigh, and inner side of the leg.

The sacral plexus is formed by the anterior primary branches
of the fifth lumbar and the first four sacral nerves, which unite
in one great cord and so form the sciatic nerve, which is the largest
in the Body and, running down the back of the thigh, ends in
branches for the lower limb. The roots of the trunks which form
the sacral plexus arise from the lumbar enlargement of the cord.

Cranial Nerves. Twelve pairs of nerves leave the skull by
apertures in its base, and are known as the cranial nerves. Most
of them spring from the under side of the brain, and so they are
best studied in connection with the base of that organ, which is
represented in Fig. 64. The first pair, or olfactory nerves, spring
from the under sides of the olfactory lobes, /, and pass out through
the roof of the nose. They are the nerves of smell. The second
pair, or optic nerves, II, spring from the optic thalami and corpora
quadrigemina, and, under the name of optic tracts, run down to
the base of the brain, where they appear passing around the crura
cerebri, as represented in the figure. In the middle line the two
optic tracts unite to form the optic chiasma, from which an optic
nerve proceeds to each eyeball.

All the remaining cranial nerves arise from the hind-brain.
The third pair (motores oculi) arise from the front of the pons
Varolii, and are distributed to most of the muscles which move
the eyeball and also to that which lifts the upper eyelid.

The fourth pair of nerves, (pathetici) IV, arise from behind the
crura cerebri. From there, each curls around a crus cerebri (the
cylindrical mass seen beneath it in the figure, running from the
pons Varolii to enter the under surface of the cerebral hemispheres)
and appears on the base of the brain. Each goes to one muscle of
the eyeball.

The fifth pair of nerves (t rigeminales) , V, resemble the spinal
nerves in having two roots; one of these is much larger than the
other and possesses a ganglion (the Gasserian or semilunar gan-
glion) like the dorsal root of a spinal nerve. Beyond the ganglion
the two roots form a common trunk which divides into three
main branches. Of these, the ophthalmic is the smallest and is
mainly distributed to the muscles and skin over the forehead and
upper eyelid; but also gives branches to the mucous membrane

130

THE HUMAN BODY

lining the nose, and to the integument over it. The second di-
vision (superior maxillary nerve) of the trigeminal gives branches
to the skin over the temple, to the cheek between the eyebrow
and the angle of the mouth, and to the upper teeth; as well as to
the mucous membrane of the nose, pharynx, soft palate and roof

nel

Fio. 64. — The base of the brain. The cerebral hemispheres are seen overlapping
all the rest. /, olfactory lobes ; II, optic tract passing to the optic chiasma from
which the optic nerves proceed; ///, the third nerve or motor oculi; IV, the fourth
nerve or patheticus; V, the fifth nerve or trigeminalis; VI, the sixth nerve or ab-
ducens; VII, the seventh or facial nerve or portio dura; VIII, the auditory nerve
or portio mollis; IX, the ninth or glossopharyngeal ; X, the tenth or pneumogastric
or vagus; XI, the spinal accessory; XI I, the hypoglossal; ncl, the first cervical
spinal nerve.

of the mouth. The third division (inferior maxillary) is the
largest branch of the trigeminal; it receives some fibers from the
larger root and all of the smaller. It is distributed to the side of
the head and the external ear, the lower lip and lower part of the
face, the mucous membrane of the mouth and the anterior two-

ANATOMY OF THE NERVOUS SYSTEM 131

thirds of the tongue, the lower teeth, the salivary glands, and
the muscles which move the lower jaw in mastication.

The sixth pair of cranial nerves VI, or abducentes arises from
the posterior margin of the pons Varolii, and each is distributed
to one muscle of the eyeball.

The seventh pair (facial nerves) , VII, appear also at the posterior
margin of the pons. They are distributed to most of the muscles
of the face and scalp.

The eighth pair (auditory nerves), VIII, arise close to the facial.
They are the nerves of hearing and are distributed entirely to the
internal ear.

The ninth pair (glossopharyngeals) , IX, arising close to the
auditories, are distributed to the mucous membrane of the pharynx
the posterior part of the tongue, and the middle ear.

The tenth pair (pneumogastric nerves or vagi), X, arise from the
sides of the medulla oblongata. Each gives branches to the
pharynx, gullet, and stomach, the larynx, windpipe, and lungs,
and to the heart. The vagus runs farther through the body than
any other cranial nerve.

The eleventh pair (spinal accessory nerves), XI, do not arise
mainly from the brain but by a number of roots attached to the
lateral columns of the cervical portion of the spinal cord, be-
tween the ventral and dorsal roots of the proper cervical spinal
nerves. Each, however, runs into the skull cavity alongside of
the spinal cord and, getting a few filaments from the medulla
oblongata, passes out along with the glossopharyngeal and pneu-
mogastric nerves. Outside the skull it divides into two branches,
one of which joins the pneumogastric trunk, while the other is
distributed to muscles about the shoulder.

The twelfth pair of cranial nerves (hypoglossa) , XII, arise from
the sides of the medulla oblongata; they are distributed mainly
to the muscles of the tongue and hyoid bone.

It must be remembered that the cranial nerves, like the spinal
nerves, are really bundles containing hundreds of axons having
various destinations. Just as in the spinal nerve plexuses bundles
of axons cross over from one nerve-trunk to another, so in many
of the cranial nerves, especially the fifth and seventh, there
are branchings from one nerve to another, making it difficult to
tell in many cases from what part of the brain the nerves to a

132 THE HUMAN BODY

special part have come; for example, it was believed for a long
time that the axons mediating the sense of taste enter the brain
as part of the trigeminal nerve. It is now practically certain that
they enter instead by way of the facial and glossopharyngeal.

White and Gray Matter. In preceding paragraphs the occur-
rence of white and gray matter in the central nervous system has
been mentioned. In the paragraph on myelin sheaths (p. 118)
the difference between them was described. It may be worth
while, for emphasis, to state again this difference before discussing
more specifically their distribution in the nervous system. White
matter consists of medullated axons, and is concerned function-
ally, therefore, with the conduction of impulses from point to
point. Gray matter consists of cell-bodies, dendrites, and parts of
axons, and in it and it alone are the synapses found over which
impulses pass across from one neuron to another. Gray matter,
therefore, is concerned with the distribution of nerve impulses
among the neurons. In it also, as we shall see, take place the
modifications which nerve impulses undergo during their passage
through the central nervous system.

Most of the gray matter of the body is found in three special
regions. These are: (1) the gray columns of the spinal cord;
(2) a layer about 2 mm. (^ in.) thick over the entire outer surface
of the cerebral hemispheres, including the mesial surface of each,
and (3) a similar layer over the surface of the cerebellum. In
addition to these chief gray regions there are a number of small
masses of gray matter distributed in various parts of the body.
Some of these are imbedded in the brain; others are outside the
central nervous system altogether. Those within the central nerv-
ous system are known as nuclei,* those outside it as ganglia.

The gray nuclei are found in the following regions: (1) The
base of the cerebrum; these are known as the basal nuclei and
include the optic thalami, the caudate, and the lenticular nuclei;
(2) the base of the cerebellum; here are several pairs of nuclei,
including the dentate nuclei; (3) the midbrain; here are several
small nuclei, the superior and inferior colliculi (corpora quadri-
gemina), the external and internal geniculate bodies, and the red

* It must be understood that the term nucleus as applied to a mass of gray
nervous matter has an entirely different significance than when applied to a
part of a single cell.

ANATOMY OF THE NERVOUS SYSTEM 133

nucleus; (4) the medulla; all the gray matter of the medulla is
contained within its nuclei. They constitute the so-called deep-
origins of those cranial nerves which arise in the medulla.

All nerve-ganglia in the Body, using the term ganglia in the
restricted sense suggested above, fall into two groups: (1) Those
which contain the cell-bodies of sensory neurons; in this group
belong all dorsal root-ganglia of spinal nerves (see p. 126), like-
wise the ganglia which are found on some of the cranial nerves;
(2) the so-called sympathetic ganglia which are described in the
next paragraph.

The Sympathetic System. The ganglia which form the
main centers of the sympathetic nervous system lie in two rows
(5, Fig. 2, and sy, Fig. 3), one on either side of the bodies of the
vertebrae. Each ganglion is united by a nerve-trunk with the one
in front of it, and so two great chains are formed reaching from
the base of the skull to the coccyx. In the trunk region these
chains lie in the ventral cavity, their relative position in which is
indicated by the dots sy in the diagrammatic transverse section
represented on p. 6 in Fig. 3. The ganglia on these chains are
forty-nine in number, viz., twenty-four pairs, and a single one in
front of the coccyx in which both chains terminate. They are
named from the regions of the vertebral column near which they
lie; there being three cervical, twelve thoracic, four lumbar, and
five sacral pairs.

Each sympathetic ganglion is united by communicating branches
with the neighboring spinal nerves, and near the skull with various
cranial nerves also ; while from the ganglia and their uniting cords
arise numerous trunks, many of which, in the thoracic and abdom-
inal cavities, form plexuses, from which in turn nerves are given
off to the viscera. These plexuses frequently possess numerous
ganglia of their own; two of the most important are the cardiac
plexus which lies on the dorsal side of the heart, and the so/ar
plexus which lies in the abdominal cavity and supplies nerves
to the stomach, liver, kidneys, and intestines. Many of the
sympathetic nerves finally end in the walls of the blood-vessels
of various organs. To the naked eye they are commonly grayer
in color than the cerebro-spinal nerves.

CHAPTER X
GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM

Conduction within Single Neurons. Since the nervous sys-
tem, whose function as a whole is the conduction of impulses
from sensory regions to motor ones, is made up of individual
neurons, the study of its physiology can best be begun by con-
sidering the phenomenon of conduction as exhibited in single
neurons, passing later to conduction as it involves more neurons
than one.

The passage of an impulse along a nerve is attended by no
visible alteration of the nerve itself; it is impossible to tell by
looking at a nerve whether it is carrying impulses or not. For
this reason nerve impulses can only be studied indirectly. If
as the result of stimulating a motor nerve at some point along
its course the muscle in which it terminates is thrown into con-
traction the obvious conclusion is that nerve impulses are passing
from the point of stimulation to the muscle. When the prick of a
finger gives rise within the brain to a conscious sensation of pain
we know that a nerve impulse must have passed between the
finger and the brain, although we would be unable to detect any
sign of its passage if the nerve were visible throughout its length.
In addition to these methods of studying nerve impulses, in which
the passage of the impulse is made known through its effect on
some other part of the Body, we have a method which depends
upon the fact that activity of nerve, like activity of muscle or
any other living tissue, is accompanied by changes of electrical
potential which may give rise to action currents. Every time an
impulse passes along a nerve it is accompanied by this electrical
alteration. Sensitive electrometers applied to nerves will indi-
cate the passage of impulses under their points of contact.

By the use of these methods of studying nerve impulses we have
learned many things about them, although much more remains
unknown.

How Nerve Impulses Are Aroused. We know that nerve im-

134

GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM 135

pulses may be started in various ways. A sharp blow on a living
nerve starts impulses traveling along it; a good example of this is
the effect of striking the "funny" bone. Nerves may be stimu-
lated by heat or by cold, by chemical agents or by an electric
spark. Whatever the nature of the stimulus the nerve impulse
which it arouses is, so far as we can tell, the same in all cases.

Speed of Nerve Impulses. The nerve impulse travels from
the point of stimulation over the neuron at a regular and rather
slow rate which probably varies somewhat in different animals
and in different nerves of the same animal, but at ordinary tem-
peratures approximates 30 meters (97 ft.) per second in most ver-
tebrate nerves in which it has been measured.

Spread of Impulses in Both Directions. Through observations
of the action currents of nerves it has been shown that the impulse
spreads from the point of stimulation in both directions along
the neuron, finally traversing all parts of it. This fact could
never have been learned if the existence of the action currents
were unknown because our only other method of detecting the
presence of nerve impulses depends upon the production of effects
in the organs to which the neurons lead; and in the body each
neuron has such connection only at one end; a nerve impulse
imparted to a motor nerve will cause contraction in its connected
muscle but produces no effect whatever at its other end.

Nerve Impulses Vary in Intensity. It is part of our every
day knowledge that nerve impulses may vary greatly in intensity;
slight contractions of the muscles are produced by feeble impulses;
when stronger contractions are 'desired impulses of greater in-
tensity must be sent in.

Fatigue. Finally, it has been proven beyond question that
the passage of impulses over nerve-fibers does not fatigue them
to an appreciable degree. In this respect the nerve is comparable
to a telephone wire; in each case the message is transmitted with-
out impairing the ability of the structure to transmit other mes-
sages.

Nature of the Nerve Impulse. Although we know these
things about nerve impulses, we do not know what the nerve
impulse itself really is. There have been many interesting and
ingenious theories of its nature proposed. Some of these attempt
,to describe it as a purely physical process, the transmission of a

136 THE HUMAN BODY

physical stress from particle to particle along the nerve; others
would consider it as a chemical process, too delicate and transi-
tory to be detected. None of them is sufficiently well established
to be considered here.

Conduction Involving More Than One Neuron. Reflexes. In
the actual passage of nerve impulses through the Body more
neurons than one are always involved. Let us examine a simple
case of conduction by which the Body adapts itself to its surround-
ings. Accidentally my finger comes in contact with a hot surface.
Quite involuntarily I jerk my hand away. The chain of events is
as follows: the skin of the hand is stimulated by the heat; the
sensory neurons in the nerve supplying that part of the hand
convey the nerve impulses thus aroused to the central nervous
system; here the impulses are conveyed over one or more asso-
ciation neurons to the motor neuron leading to the muscle which
retracts the arm; upon the arrival of the impulses within the
muscle the latter is stimulated to contract. The whole process is
entirely mechanical; none of the structures involved has any

FIG. 65. — Diagram of the simple reflex arc. a, receptor; b, sensory neuron;
c, association neuron; d, motor neuron; e, muscle; /, spinal cord.

knowledge that the hand is in danger of being severely burnt, or
that retraction of the arm will save it. It is an example of an
adaptive mechanism. Such a chain of events as the one described
constitutes a simple reflex. The neurons involved in the trans-
mission of the impulse from receptor to muscle make up the
reflex arc. The simplest imaginable reflex arc must include at
least two neurons, the sensory neuron which brings the impulse
from the receptor to the center, and the motor neuron which
conveys the impulse from the center to the motor organ. Many
facts indicate that reflex arcs as they actually exist in the Body
always include at least three neurons, an association neuron

GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM 137

being interposed in every case between the sensory and the motor
neuron.

The cell-body of the sensory neuron always lies outside the
central nervous system in a dorsal root ganglion or a homologous
ganglion of a cranial nerve; its axon extends thence into the cen-
tral nervous system, and finally enters gray matter where it comes
into synaptic connection with the dendrites of an association
neuron. This makes connection in turn with a motor neuron
whose cell-body lies in the ventral horn of gray matter of the
cord, or in a corresponding gray region in the brain (Fig. 65).

Reflex Arcs Not Rigidly Fixed Paths. Although a given sen-
sory stimulus usually arouses the same sort of reflex response
every time it is applied, this does not mean that the reflex path
followed in such a case is the only one into which that sensory
neuron leads. Very different reflex responses may originate in
the same receptor. A good illustration of this is furnished by
certain reflexes through the eye. If I sec that a small floating
particle threatens my eye I am apt to wink ; if a flying insect ap-
proaches I am more likely to turn my head to one side; if the
threatening object is a swiftly thrown baseball I will probably
bring the hands before the face , or perhaps dodge to one side.
All these actions are performed mechanically and are therefore
true simple reflexes. The originating sensory impulse travels in
each case over the same sensory neurons, those of the optic nerves.
It is evident, then, that impulses coming in over the sensory
neurons of the optic nerve do not have to pass over to any par-
ticular motor neuron, such as the one which leads to the muscle
of winking, but may follow any one of various courses, finally
terminating in muscles far distant from the eye. In fact, and this
is one of the most important things to remember about the
nervous system, there is such an extraordinary richness of con-
nection among the various neurons within the central nervous
system that any sensory neuron may be brought into commu-
nication with any motor neuron.

This richness of connection is afforded anatomically through
two rather simple arrangements. In the first place the axons of
sensory neurons after entering the spinal cord continue along it
for some distance, giving off branches, called collaterals, at va-
rious levels. Each collateral terminates in an end arborization

138 THE HUMAN BODY

which communicates in turn with the dendrites of an association
neuron. Thus each sensory neuron has possible connection with
various association neurons located in different parts of the cen-
tral nervous system. The association neurons likewise are richly
branched, each branch terminating in a synaptic connection with
another neuron, and this in turn may be an association neuron,
or may be a motor neuron. In the second place the dendrites of
all association and motor neurons doubtless have synaptic con-
nection with end arborizations of numerous neurons, sensory or
association as the case may be. Thus a sensory neuron has a
wide choice of paths over which to send its impulses ; and a motor
neuron may receive impulses from a great variety of sources.

Irreversible Conduction. In all this maze of connections and
interconnections within the central nervous system, how is it
that the impulse coming in at a sensory neuron always comes
out finally at a motor neuron instead of becoming switched
sometimes to another sensory neuron? The orderly progress of
impulses is insured by a very simple arrangement, namely, that
impulses can pass freely across a synapse from end arborization
to dendrites but can never pass in the reverse direction, from
dendrites to end arborization. When a sensory neuron delivers
its impulse to an association neuron the impulse doubtless spreads
to all parts of* the latter. It can leave it, however, only by way
of its end arborizations, and these communicate only with the
dendrites of motor neurons or of other association neurons. The
final outcome is bound to be a motor neuron since all association
neurons lead ultimately to them. Sensory neurons never receive
impulses from other neurons because they have no dendrites
within the central nervous system by which impulses might be
received. The portion of a sensory neuron which corresponds to
the dendrites of a motor neuron is the long axon-like process
communicating with the receptor.

Graded Synaptic Resistance. Another question which nat-
urally arises when one considers the innumerable courses which
an impulse may take within the central nervous system is what
determines the course it actually does take? Why, for instance,
when my eye is threatened do I wink instead of opening my
mouth, or why do I sometimes wink and sometimes dodge? A
complete answer to this question cannot be made in the present

GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM 139

state of our knowledge, but we have a fairly good general idea of
the way in which nerve impulses are probably guided. A sensory
neuron has several collaterals, each with its synaptic connection
with another neuron. If we suppose these synapses are not all
alike, but that certain ones transmit impulses more readily than
do the others, an impulse spreading over the sensory neuron will
pass most easily to that connecting neuron whose synapse offers
least resistance to its passage. Thus we may imagine an impulse
spreading from neuron to neuron following always the path of
least resistance until it finally terminates in a muscle which it
arouses to activity. In the central nervous system the various
paths of least resistance are so blocked out as to lead to adapt-
ive motions; a prick on the finger causes retraction of the hurt
hand; irritation in the nose causes the convulsive movements of
the respiratory muscles which constitute a sneeze; in each case the
motions are calculated to get rid of the source of irritation.

That adaptive reflexes are due to paths of least resistance
blocked out from an infinite number of possible paths is strik-
ingly illustrated by the effects of strychnine poisoning. This
drug acts on the central nervous system in such a way as to
abolish differences of synaptic resistance. When one suffering
from the drug receives a stimulus by way of any sensory nerve
the impulse, instead of following the usual path, spreads over the
whole central nervous system;' all the muscles are stimulated
simultaneously and the well-known strychnine convulsion results.

The Orderly Spreading of Reflexes. The conception of graded
synaptic resistances explains also in a very satisfactory way the
phenomenon of the orderly spreading of reflexes. A feeble stim-
ulus produces reflex movement in those muscles only which are
immediately concerned in the adaptive response; stronger stimuli
involve more muscles, but only such as by their movement make
the response more effective. For example, if a frog's hind leg is
touched gently it will be drawn away from the source of irrita-
tion; a stronger stimulus is likely to cause contractions of such
additional muscles as are required for jumping away from the
point of danger. If we assume that the reflex paths to the first
set of muscles have such low resistances as to allow feeble impulses
to pass them, and that stronger impulses can overcome enough
additional resistance to enter the paths of higher resistance lead-

140 THE HUMAN BODY

ing to the jumping muscles, while the paths to muscles not con-
cerned in any way in an adaptive response have too high resistance
to be passed at all, we can account for reflex actions of very great
complexity.

Simple Reflexes Mediated by the Spinal Cord. The simple re-
flexes described in the preceding paragraphs are all of a sort that
can be carried on through the lowest part of the central nervous
system, the spinal cord. A frog whose brain has been destroyed
and which is therefore wholly devoid of feeling and consciousness
can still perform highly complicated reflex acts; he will retract a
foot which is pinched; he will wipe off a bit of acid-soaked paper
from his flank, and if unable to reach it with one foot will bring
the other into service. All his acts, however, are purely mechan-
ical, and are determined by the spread of impulses over reflex
paths of less or greater complexity. A very striking thing about
such a " reflex " frog is the accuracy with which the character of
his responses to various sorts of stimuli can be predicted. He is
a pure automaton. The most apparent difference between the
responses of such a " reflex " frog and those of a normal one, with
brain intact, is that in the case of the latter one cannot predict
certainly what his response to any particular stimulus will be.
Looking at the nervous system purely from the standpoint thus
far assumed, namely, as an adaptive conducting mechanism, this
property that the brain has of interfering with the orderly progress
of reflex actions is the first of its properties to require considera-
tion.

The Cerebrum the Controlling Organ. In the preceding par-
agraph the "reflex" frog was described as having its entire brain
destroyed. Such a procedure reduces the animal to the " reflex "
state, but does not enable us to judge whether the brain acts as
a whole in modifying the reflex activities of the spinal cord or
whether this function is confined to certain parts of the organ.
Studies upon higher animals, especially upon pigeons, have dem-
onstrated that the particular part of the brain which has the power
to modify reflexes is the cerebrum. A pigeon whose cerebrum has
been removed is as truly a " reflex " animal as is a frog whose en-
tire brain has been destroyed. Since the cerebrum is also the seat
of consciousness an animal treated in this way is wholly insensi-
tive to pain.

GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM 141

A Normal Animal Compared with a " Reflex " One. Let us

imagine that we have side by side before us two living animals of
the same species, one normal in every respect, the other in the
"reflex" condition; that is, having had the cerebrum destroyed.
Disregarding for the present the phenomenon of consciousness and
looking at both animals simply as pieces of machinery three strik-
ing differences between them are manifest: 1. The "reflex" animal
always responds to adequate stimulation by a predictable response;
the intact animal sometimes responds and sometimes does not.
2. The "reflex" animal does not move except when stimulated,
while the intact animal often moves without any apparent rea-
son. 3. The amount of response given by the "reflex" animal
bears some relation to the intensity of the exciting stimulus,
whereas in the normal animal an apparently feeble stimulus may
arouse a vigorous and long-continued response. An example of
this last is the running of a dog to its master upon hearing his
whistle. The stimulus may be a very faint one, the motions
which it arouses are exceedingly vigorous and complicated.

All these differences depend at bottom upon a single funda-
mental difference between the two animals which is this: in the
"reflex" animal the immediate stimulus dominates the situation
completely; in the intact animal the immediate stimulus is only
one factor of many which together determine what the response
shall be. The superior practical efficiency of the intact animal
as an adaptive organism depends upon this power, resident in the
cerebrum, of modifying immediate stimuli in accordance with the
demands of less obvious considerations. To illustrate: a hungry
man perceiving food would inevitably respond to the double
stimulus of hunger and the sight of food by taking the food and
eating it if he acted upon a purely reflex basis ; his actual response
to these stimuli will depend, however, upon whether they are in
harmony with or opposed to certain more remote factors, such as
the question whether the food is of a sort that will agree with him,
or whether he is likely to need it more urgently at some future
time than at present.

Before entering upon a fuller discussion of the functions of the
cerebrum, its structure and its connections with lower nerve-cen-
ters must be described.

CHAPTER XI

STRUCTURE, NERVE CONNECTIONS, AND FUNCTIONS OF
THE CEREBRUM

The Cerebrum Dependent on the Receptor System. If the

cerebrum is to introduce remote considerations as factors in de-
termining the nature of reflex responses it must have within it the
knowledge upon which these remote considerations are based.
That the cerebrum has little original endowment of knowledge is
evident from study of infants, who during the first months are
perfect examples of "reflex" organisms. The equipment which
the cerebrum finally obtains must be gotten bit by bit by ex-
perience or the teaching of others. Since the receptor system is
the organism's only means of acquiring information, the cere-
brum must be in communication with this system if it is to learn
anything whatsoever.

Afferent Paths of the Cerebrum. We have learned in previous
paragraphs that all sensory neurons lead directly into the central
nervous system and there have numerous synaptic connections
with association neurons. These connections are all, however,
with the possible exception of those of the sense of smell, made
in gray matter of the spinal cord, the medulla, or the midbrain.
In order for impulses coming in over these sensory neurons to
reach the cerebrum there must be communication by association
neurons between the terminations of the sensory neurons and the
cerebrum. As a matter of fact such connections are richly sup-
plied. Some of the most conspicuous tracts of white matter in
the central nervous system consist of the myelinated axons of
association neurons which form connecting links between sensory
neurons and the cerebrum. Since the cerebrum is the crown of
the entire nervous system it is used as a landmark in describing
other nervous structures. Thus nerve paths which convey im-
pulses toward the cerebrum are called afferent paths; those carry-
ing impulses away from it are efferent paths. According to this
classification all sensory neurons are afferent and all motor ones

142

STRUCTURE AND FUNCTIONS OF THE CEREBRUM 143

efferent, while association neurons are either afferent or efferent
according as they carry impulses toward the cerebrum or away
from it.

Tracing Nerve Paths. Wallerian Degeneration. One of the
very satisfactory achievements of biologists has been the reso-
lution of the apparently inextricable tangle of gray and white
matter of the central nervous system into a system of fairly
definite nerve tracts whose origins, courses, and terminations are
known. Our present knowledge is the result of various methods
of study, but the most fruitful one of all has rested upon recog-
nition of three facts: first, that white matter always consists of
myelinated axons; second, that axons always are outgrowths of
cell-bodies which are to be looked for in gray matter; and third,
the fact discovered by the English physiologist, Waller, in 1852
that axons cut off from connection with their cell-bodies undergo
degeneration in a few days. Because of this latter fact if a cut
be made anywhere in the central nervous system of an animal,
and the animal be killed a few days later and its spinal cord and
brain examined microscopically, the direction and extent of de-
generation reveal the relation of the severed axons to the rest
of the nervous system. If the degeneration is all toward the head
the severed tract must be an afferent one with cell-bodies some-
where below the cut. Backward degeneration would signify an
efferent tract with its origin somewhere forward of the point of
injury. Wallerian degeneration is not difficult to follow because
it is fatty and the drops of fat in the degenerated region can be
plainly revealed by the application of osmic acid, which turns
them black.

Paths of the Various Senses. For convenience in describing
the paths by which information is conveyed from the various re-
ceptors to the cerebrum, the receptors will be classified as body
sense receptors and head sense receptors. The group of body
senses includes all those senses such as touch, pain, muscle sense,
etc., whose receptors are for the most part in parts of the Body
other than the head, and which therefore communicate with the
central nervous system by way of spinal nerves. The head senses,
sight, hearing, taste, and smell, are those from which stimuli are
carried over cranial nerves to the medulla or midbrain, or in the
case of the sense of smell directly into the cerebrum.

144 THE HUMAN BODY

Tracts of Body Sense. Sensory neurons of body sense enter
the spinal cord all along its length. Afferent paths within the
cord begin, therefore, at its extreme end. These are to be looked
for, as previously stated, in the columns of white matter which
make up the greater part of the substance of the cord. Two dis-
tinct regions of white matter in each half of the cord have been
shown to consist chiefly of afferent neurons leading toward the
cerebrum. These are: first, the dorsal columns, each of which con-
sists of two rather well-marked bundles of axons, the so-called fas-
ciculus gracilis (Column of Goll) next the dorsal fissure, and the
fasciculus cuneatus (Column of Burdach) next to the dorsal horn
of gray matter; second, the ventrolateral tracts which lie next to
the ventral horns of gray matter, surrounding them on the sides
and below (Fig. 66). It is thought that the dorsal columns con-
sists chiefly if not wholly of the axons of sensory neurons which,
entering the cord by the dorsal roots of spinal nerves, extend for-
ward within the dorsal columns, giving off collaterals into the gray
matter at various levels. Only a part of the sensory axons which
enter the dorsal columns continue along them as far as the medulla;
the others after extending a short distance plunge into the gray
matter and terminate in synaptic connection with association
neurons. The ventrolateral afferent columns consist wholly of
association neurons which communicate, presumably, with those
sensory neurons which do not themselves extend all the way to
the medulla; these columns serve, therefore, to afford cerebral
communication to those sensory neurons which terminate within
the gray matter of the cord.

None of the afferent axons coming up the cord by the tracts
just described extend further than the medulla; they all termi-
nate there in masses of gray matter known as the gracile and
cuneate nuclei; here they form synaptic connections with a new
set of association neurons which continue the path toward the
cerebrum. These tracts, which from their ribbon-like appearance
have been named the fillets, cross the mid-line at a point in the
medulla known as the sensory decussation; so that sensory stimuli
from the right half of the Body are carried to the left cerebral
hemisphere, and those from the left half of the Body to the right
hemisphere.

Tracts of the Head Senses. The senses of sight and hearing

STRUCTURE AND FUNCTIONS OF THE CEREBRUM 145

are the head senses whose central connections are best known.
The central connections of the sense of smell are imperfectly
known; those of taste practically not at all. Axons conveying
visual impulses enter the midbrain by way of the optic nerves
and optic tracts and terminate for the most part in nuclei of the

Entering posterior
root

Lissauer's tract

' Emerging anterior root

FIG. 66. — Diagrammatic transverse, section of the spinal cord showing the con-
duction paths. (Cunningham.)

midbrain, the external geniculates and superior colliculi; some
of them appear to terminate in basal nuclei of the cerebrum, the
optic thalami. In all these nuclei synaptic connection is made
with new neurons which carry the impulses into the cerebrum.

Auditory impulses enter the medulla by way of the auditory
nerves. The axons of the nerves themselves terminate in nuclei
of the medulla, the auditory nuclei; new neurons continue the
path thence across the mid-line of the medulla and forward into
the midbrain, terminating in the internal geniculate nuclei and
the inferior colliculi. From these nuclei a third set of neurons
continue the path to the cerebrum.

General Structure of the Cerebrum. This organ consists, as
previously stated, of an outer surface of gray matter, two milli-
meters thick, overlying a mass of white matter; the whole held
together by neuroglia and connective tissue, and mounted upon

146 THE HUMAN BODY

the midbrain as upon a stalk. Because of the convoluted surface
of the cerebrum the total amount of superficial gray matter is
much greater than it would be if the cerebrum were smooth. This
layer of gray matter is the region wherein occur those special
activities which set the cerebrum above the rest of the nervous
system. It is called the cortex cerebri, or for convenience simply
the cortex.

Structure of the Cortex. The cortex cerebri consists for the
most part of neurons with small cell-bodies having much branched
processes, signifying rich synaptic connections. Many of these
neurons appear to be confined altogether within the cortex; others
give off myclinated axons into the underlying white matter.
Interspersed with these small cell-bodies are others which are
much larger, which are pyramidal in shape, and which always
give off a large axon into the white matter. From their shape
and size these are known as large pyramidal cells. In a certain
region of the cortex, known as the motor area, the pyramidal
cells are relatively gigantic, being just at the limit of naked eye
visibility.

The White Matter of the Cerebrum. This consists of myelin-
ated axons classified according to their course and distribution
into three groups. The so-called projection fibers are the axons
by which the cortex is brought into connection with the other
parts of the nervous system. These include afferent projection
fibers, which are the continuations within the cerebrum of the
various sensory paths described in previous paragraphs (see
p. 144), and efferent projection fibers, which convey impulses from
the cortex to the rest of the Body.

At the base of the cerebrum, where it rests upon the midbrain,
all the projection fibers, both afferent and efferent, are crowded
together into a restricted space between two of the basal nuclei.
This region is known as the internal capsule. As the fibers emerge
thence into the roomy cerebrum they spread apart on their way
to the different parts of the cortex forming the corona radiata.

The second group of cerebral axons are the association fibers.
These pass between one part of the cortex and another within
the same hemisphere, enabling impulses to travel freely among
the cortical cells. The third group of cerebral axons are the
commissural fibers which pass between cortical areas in opposite

STRUCTURE AND FUNCTIONS OF THE CEREBRUM 147

hemispheres; these serve to unify the anatomically double cere-
brum into a single physiological organ; the corpus callosum (Cc,
Fig. 62) is made up of commissural fibers.

Lobes of the Cerebrum. The convolutions of the cerebrum are
sufficiently constant in number and position to serve as land-
marks in locating particular regions. The individual convolutions,
or gyri, have been given specific names, as have also the fissures,
or sulci, which separate them. For our purposes' it is necessary
to mention by name only those fissures which mark off the grand
divisions, or lobes, of the cerebrum. The division of the cerebrum
into lobes is purely arbitrary, and is made for greater ease in
describing it. In general the lobes correspond in position to the
overlying skull bones for which they are named. The fissures
which mark the boundaries of the lobes are indicated in Fig. 61.
They are the fissure of Sylvius, the fissure of Rolando, and the
Parieto-occipital fissure. The frontal lobe is that part of the
cerebrum above the fissure of Sylvius and in front of the fissure
of Rolando; the parietal lobe is between the fissure of Rolando and
the parieto-occipital fissure ; the occipital lobe is the wedge-shaped
portion behind the parieto-occipital fissure; the temporal lobe is
below the fissure of Sylvius; it is the only one of the lobes which
is sharply set off as a distinct region.

Cortical Localization. A problem of much interest in connec-
tion with the study of cerebral functions is whether there is di-
vision of labor among the various parts of the cortex. Do certain
groups of cells perform certain special functions, or are all cortical
activities shared in by all the cells? This is not the place for a
history of the solution of this problem. Suffice it to say that we
now have positive proof of a high degree of specialization of
function in the cortex.

Sensory Areas. In previous paragraphs the paths of the
various senses were traced as far as their entrance into the cere-
brum by way of the internal capsule. We must now continue
the paths to their cortical terminations. The body sense-fibers
pass to that part of the parietal lobe just behind the fissure of
Rolando; the region where they terminate is the body sense area.
The visual tracts end in the occipital lobes in the visual areas.
The auditory tracts terminate in the temporal lobes in a region
just below and within the fissure of Sylvius; this region con-

148 THE HUMAN BODY

stitutes the auditory area. Although the paths of smell and taste
are imperfectly known, their cortical terminations have been
fairly well established. The olfactory area is supposed to be in
the temporal lobe, and possibly at its very tip. The area for
taste, gustatory area, is thought to be also in the temporal lobe,
probably adjacent to the area for smell. Since the nerve-paths
of the various senses lead directly to these areas, and since de-
struction of any one of them, by accident or disease, results in
loss of the particular sense whose area is involved, we must con-
clude that the sensory areas are the receiving stations of the
cerebrum. All afferent projection fibers entering the cerebrum
terminate in one or another of the sensory areas. Within these
areas they have synaptic connection with the association neurons
of the region.

The Motor Area and the Pyramidal Tracts. In each hemi-
sphere a region of the frontal lobe just in front of the fissure of
Rolando contains numerous giant pyramidal cells whose axons
extend into the white matter and are grouped together in the
internal capsule as a conspicuous nerve tract. Because all the
axons of this tract arise from pyramidal cells it is called the
pyramidal tract. It extends through the midbrain to the medulla
and appears upon the ventral surface of the latter as a well-marked
anatomical feature. About midway of the medulla the pyramidal
tracts cross the mid-line in the decussation of the pyramids. This
decussation is not complete; part of the fibers of each pyramidal
tract continue along the same side of the medulla to the spinal
cord and down the latter in the ventral column, forming the direct
pyramidal tract. That part of each pyramidal tract which crosses
over at the "decussation" proceeds along the spinal cord in the
lateral column as the crossed pyramidal tract (Fig. 66). It appears
that most of the fibers of the direct pyramidal tracts cross the
mid-line in the spinal cord before reaching their terminations; so
that the pyramidal tracts are finally crossed tracts. All the py-
ramidal axons have synaptic connection, either direct or through
intervening association neurons, with the cells of motor neurons
in the ventral horns of gray matter of the cord.

Since the pyramidal axons arise from cell-bodies within the
cortex it is evident that the pyramidal tracts must be efferent
paths. The intimate way in which the pyramidal fibers connect

STRUCTURE AND FUNCTIONS OF THE CEREBRUM 149

with the cell-bodies of motor neurons indicates that they form
the paths by which the cerebrum exercises control over bodily
movements. The anatomical evidence for that view has been
corroborated and strengthened by physiological evidence. The
German physiologists, Fritsch and Hitzig, showed that in dogs
electrical excitation of those areas of the brain from which spring
the pyramidal tracts is followed by movements of the muscles of
the Body. They showed also that these are the only areas from
which such movements can be elicited.

Upon the basis of all this evidence we are justified in looking
upon the regions immediately in front of the Rolandic fissures
as motor areas. These areas have been much studied physiologi-
cally in recent years. The brains of the higher apes have been
preferred in these studies to those of lower animals because of
their greater similarity to the human brain.

There have been a few observations upon the brains of human
beings in cases where the surgical treatment of certain diseases
has involved removal of portions of the skull overlying the Ro-
landic areas.

These recent studies have shown that there is a considerable
localization within the motor areas themselves; stimulation of one
point causes movements of the hand, of another the foot, of still
another the head. They have shown incidentally, also, that the
cerebral cortex is not painfully sensitive to direct stimulation.
The men whose brains were excited electrically in the observa-
tions cited above were conscious throughout the procedure and
reported no sensations of pain or discomfort at any stage.

Cortical Reflex Paths. The various sensory areas with their
afferent nerve-paths afford means whereby impulses may enter
the cerebrum from the different receptors; the motor areas, one
in each hemisphere, with their efferent paths, provide for the
passage of impulses from the cerebrum to the motor organs of the
Body; the abundant equipment of association fibers within the
cerebrum makes possible the passage of impulses across from
sensory areas to motor areas. We can picture, then, reflex arcs
involving the cerebrum. Such arcs are necessarily complex, in-
volving many more neurons than do the simple spinal cord reflex
arcs already described. In a previous paragraph (p. 136) we saw
that the simplest reflex arc through the cord involves at least

150 THE HUMAN BODY

three neurons, one sensory, one association, and one motor. If
we trace a reflex arc involving the cortex from a receptor in the
skin of the right hand, for example, to a retractor muscle of the
right arm, we find in it at least five neurons and possibly many
more. The five which are necessarily included are: 1, the sensory-
neuron which we suppose extends all the way from the receptor
into the cord and up the dorsal column to a termination in the cu-
neate or gracile nucleus ; 2, a neuron of the fillet tract, having its
cell-body in the cuneate or gracile nucleus, and its axon extending
through the medulla and midbrain and the white matter of the
cerebrum, crossing the mid-line in the " sensory decussation"
of the fillet, and terminating in synaptic connection with a neuron
of the body sense area in the left cerebral hemisphere; 3, the neu-
ron just mentioned, having its cell-body in the body sense area
and an axon which passes by way of the cerebral white matter to
the motor area; 4, a pyramidal neuron of the motor area whose
dendrites receive the impulse from the body sense neuron (3),
and whose axon forms part of the pyramidal tract, crossing back
to the right side of the body in the decussation of the pyramids,
and terminating in synaptic connection with the cell-body of a
motor neuron in the ventral horn of gray matter of the cord; 5, the
motor neuron which forms the last link in the reflex chain, con-
veying the impulse from the pyramidal neuron to the muscle. It
is doubtful whether any cortical reflex arcs are actually composed
of as few neurons as five ; probably the simplest ones contain sev-
eral additional association neurons within the cerebrum.

Cortical Reflexes Compared with Spinal Reflexes. As an ex-
ample of a simple spinal reflex was cited the involuntary with-
drawal of the hand from accidental contact with a hot body.
To illustrate a simple cortical reflex suppose that my finger rests
upon the terminals of an apparatus for generating electric shocks;
I am told that when I feel the shock I must withdraw my hand.
The shock may be so feeble as to be barely perceptible. Under
such circumstances the withdrawal must be voluntary and the re-
sponse, therefore, must involve the cerebrum. The chief objective
difference between voluntary withdrawal of the hand in response
to feeble stimulation, and its involuntary retraction in response
to strongly painful stimulation is that the former reaction requires
a noticeably longer time than does the latter. The only simple

STRUCTURE AND FUNCTIONS OF THE CEREBRUM 151

reflex whose time has been satisfactorily measured in man is
the winking reflex; this requires about 0.06 second for its com-
pletion. The quickest cortical reflexes take about 0.15 second.
This difference in time is much greater than can be accounted for
by supposing the cortical reflex to involve a greater length of
nerve-fibers, and therefore must be due to the fact that the cor-
tical reflex involves a greater number of neurons and consequently
more synapses to be crossed.

An additional difference which we recognize subjectively be-
tween spinal and cortical reflexes is that while the former are
involuntary and unconscious, the latter are voluntary responses
to stimuli consciously perceived. This difference will be discussed
more fully in a later paragraph, when the meaning of the terms
"voluntary" and "consciously" shall have been considered.

Memory. We have seen that the primary function of the
cerebrum is to introduce remote considerations as determining
factors in the responses of the organisms. We have seen also that
in order to do this the cerebrum must have an equipment of
knowledge, which can be gained only through the receptor chan-
nels of the Body. The information which reaches the brain, to
be of service, must be retained there until needed, and must be
held in such a way as to be available when required.

The neurons of the nervous system generally act, in the main,
as conductors pure and simple. When they are stimulated nerve
impulses are aroused; these spread over them and escape by
those synapses whose resistance is not too high ; thus other neurons
are involved and so the impulses advance to a motor termination.

The cortical neurons of the cerebrum owe their dominant po-
sition in the nervous system chiefly to a peculiar ability which
they possess of "holding up" impulses which come to them, re-
taining them indefinitely, and giving them out again in the future.
This storing of impulses constitutes memory. The "reflex"
animal, because he is deprived of this property, must always re-
spond immediately to adequate stimulation; the intact animal
may respond immediately or may retain the stimulus as a memory
to modify his future activities. Since the intact animal has
within his cerebrum a store of impulses "held in leash," he may
at any time become active through the liberation of some of them,
without immediate external stimulation.

152 THE HUMAN BODY

Association Areas. The different sensory areas and the motor
areas occupy only a small part of the whole cerebral cortex.
Most of the frontal lobes and large areas of the parietal and tem-
poral lobes are not involved in the immediate reception of im-
pulses, nor in their transmission to the Body. These areas are
as richly supplied with interconnecting neurons as any part of the
cortex. They are assumed, without very positive proof, to be
the seat of a function we know the cerebrum to possess, that of
association.

The Nature and Mechanism of Association. At birth the
brain of the infant may be compared to a clean page. It bears no
impressions of any sort. Such activities as the infant shows are
purely reflex. In course of time sense impressions begin to come
into the sensory areas of the cortex. These register themselves
more or less definitely as memories, and presently the child is in
possession of a considerable store of memories of various sorts.
He may know the sound of his mother's voice or may recognize
her face. As yet, however, there is no connection between these
independent impressions. When in the child's mind that voice is
associated with that face, so that he knows them as parts of a
single whole, he has performed an act of association. From this
time throughout his life his memory is not alone of the simple
sound of the voice or the appearance of the face but of the mother
whom he has learned to know by these associated impressions.

Acts of association are supposed to be carried on within the
association areas of the cortex. We may picture the process in
the example cited above somewhat as follows: The impression of
the voice is stored in the auditory area; that of the appearance
of the face is in the visual area; both these sensory areas have
rich communications with neurons of the association areas. By
some means impulses from the sensory cells where these impres-
sions are stored meet in a cell of an association area. That cell
builds from these single related sense impressions, a composite,
which is stored in turn as a memory. As additional related in-
formation is gained the composite, or concept, is enlarged.

The union of related impressions into concepts does not nec-
essarily involve loss or impairment of the fundamental impres-
sions themselves; the child in whose mind is a definite concept
of his mother retains also clear memories of her voice and her

STRUCTURE AND FUNCTIONS OF THE CEREBRUM 153

face. The paths of communication between the cells where are
stored the primary sense impressions and those where the resulting
concepts are formed seem to remain always very easy of passage.
The sound of the mother's voice calls up the entire concept of the
mother with great clearness, even though years may have elapsed
since it was heard.

Since concepts are stored as memories they may serve in their
turn as bases for more complex associations; these again by be-
coming memories may contribute to the associative process, and
so the complex structure of the mind is built up, resting at bot-
tom always upon primary sense impressions.

The act of association is essentially one of combining related
memories ; the formed associations become memories in their turn.
For these reasons the term associative memory is used as more
truly describing the nature of associative processes than the older
expression " the association of ideas."

The use of a memory in forming one association does not inter-
fere with, its use in the formation of others. This ability of the
cerebrum to use memories over and over again is a very valuable
property since it enables us to make the utmost of all our knowl-
edge.

Development of the Cortex. The increase in intellectual
power which accompanies the growth of the child is not the re-
sult of any increase in the number of nerve-cells, for the child
is born with his full number. It is, however, based upon their
continuous development; this development consisting chiefly of
greater and greater branching with correspondingly richer synaptic
connections. At birth scarcely any cortical cells are sufficiently
developed to be functional. The sensory areas first become so.
The association areas reach their highest point of development at
about the thirty-fifth year. At this age the anatomical progress of
the brain comes to an end; all possible paths of association have
been laid down. This does not mean, however, that all possible
associations have been formed. These continue to be formed so
long as the brain continues active. It is probably true, however,
that with advancing years there is a diminution in the freedom
of associative activity; the brain no longer accomplishes daring
feats of thought, such as constitute creative genius, but plods
along in the ruts established by its earlier activities. This fact

154 THE HUMAN BODY

explains why conservative tendencies usually become more pro-
nounced as age advances.

The Functions of Associative Memory. It is because the cere-
brum is able to form associative memories that the organism can
adjust its responses with due regard to remote as well as to im-
mediate considerations. Incoming stimuli, which in a "reflex"
animal would produce a definite response of a certain kind, are
in an intact animal balanced against such related associative
memories as the animal possesses; if these indicate that the
natural reflex response is the proper one to make, the animal re-
sponds as does the "reflex" one; if, however, they indicate a
different line of action as more advantageous, the animal sub-
stitutes for the natural reflex response a different one, suited to
the situation.

Associative memory also forms the basis for the execution of
complex movements from feeble, immediate stimuli, or in their
absence; the young puppy responds to his master's whistle only
by a pricking of the ears ; in the older dog the sound of the whistle
arouses a chain of associative memories and under their impelling
force he executes the complex movements which carry him to his
master's feet.

In order that associative memory may influence bodily activ-
ities it must have access to the efferent nerve-paths of the cere-
brum. This access it has through rich connections from the
association areas to the motor areas. It must have also the
power to stimulate the efferent nerves. This power it exercises
through the function of volition.

Volition. Although all voluntary acts result from nerve im-
pulses which have come from the motor areas of the cerebrum
by way of the pyramidal tracts, we cannot suppose that they
originate in the cells of the motor cortex. There is no evidence
that these or any cortical cells are able to originate any activities
whatever. All voluntary acts, as a matter of fact, are based upon
associative memory; the immediate stimulus to the performance
of the voluntary act comes, not from the motor areas, but from
that part of the association areas where the exciting memory is
stored. All memories, as we have seen, are at bottom stored sen-
sory impressions. What happens, then, when we perform volun-
tary acts is that we cause to pass on to the motor areas stimuli

STRUCTURE AND FUNCTIONS OF THE CEREBRUM 155

which originally entered the nervous system by way of the re-
ceptors, and which have since been combined in various ways,
and the resulting associations stored as memories. Voluntary acts
are, therefore, the completion of reflexes.

The Usefulness of Associative Memory Depends on its Orderli-
ness. It is perfectly obvious that associations to be of value must
be formed from related impressions or related concepts. We
know that our brains normally form associations in this orderly
way. How the brain is guided in its selection of material for
making associations so as to include what is relevant and exclude
the rest is quite beyond our knowledge or even imagination. That
in the highly complex associative processes which we call think-
ing there may be a conscious selection or rejection of memories
we know from our own experience.

It is true, of course, that the brain, being an imperfect instru-
ment, often makes mistakes and forms associations that instead
of being useful give rise to harmful activities. The resulting
disaster, through the additional knowledge it affords, may enable
the brain to form correct associations next time. Thus we profit
by our mistakes.

The Interaction of Associative Memories. Inhibition. The hu-
man brain acquires in the course of years such a wealth of associa-
tive memories, based upon so many phases of experience, that the
determination of the conduct to be employed in any particular
situation becomes often a matter of much difficulty. One set of
memories point toward one course and another set toward quite
the opposite course. When this happens it is necessary to call
in more and more remote considerations until the balance tips
unmistakably in one way or the other. When even this pro-
cedure fails to be decisive, or when the mind wishes to avoid the
labor of deciding by this method recourse is often had to a se-
lective external stimulus. Deciding a course of action by the
flip of a coin is a case in point.

Associative memories also come into conflict when immediate
considerations point toward one course and remote considerations
toward a different one. Associative memories are classified by
placing those of remote bearing higher than those of immediate
bearing. Highest of all, because most remote, are abstract con-
ceptions of right and wrong; conceptions of altruism, care for

156 THE HUMAN BODY

mankind, are higher than conceptions of family love; these in
turn rank above purely personal considerations. Personal con-
siderations which have regard to the future are higher than those
dealing only with the immediate present. The progress of civiliza-
tion is largely measured by the degree to which remote considera-
tions outweigh immediate ones in determining conduct.

Because the cerebrum rests upon an underlying reflex mechan-
ism the tendency of the organism is always toward immediate
response to sensory stimulation; the hungry man tends to take
the first food that comes to hand ; the cold man tends to seek the
nearest available shelter. The action of associative memory,
when higher considerations dictate a different course, is to pre-
vent or inhibit the carrying out of the immediate response. In-
hibition is, then, one of the important functions of associative
memory. The man who deliberately does what he knows to be
wrong, acts as he does because his conceptions of right are not
powerful enough to inhibit the response to the lower stimulus.
The importance of inculcating the highest principles of right
living by training and example, during the receptive period of
the brain's development, is therefore clearly manifest.

Habit Formation. Just as a single sensory impression re-
peated over and over becomes more firmly fixed in memory than
does one received only once, there seeming to be some sort of
impression upon the remembering nerve-cell which becomes
deeper at each repetition of the stimulus; so every interaction of
associative memories in determining a course of conduct leaves
a track upon the cells involved, which is deepened by repetition
of the same series of memories leading to the same conduct. One
of the strong tendencies of the brain is to arrange its associative
memories thus in groups leading to certain definite responses.
It is this tendency which lies at the basis of habit formations.*

Habits which are formed in this way, by repeated following
of the same line of thought to the same actions, take on much of
the character of simple reflexes. A stimulus which arouses the
chain of associative memories results in immediate carrying out

* The term habit is used here with reference to those acts which we do
habitually because of having done them so often before, not with reference
to habits which are based upon perverted or diseased conditions of the body,
such as drug or liquor habits.

STRUCTURE AND FUNCTIONS OF THE CEREBRUM 157

of the habitual action. A definite act of inhibition on the part
of other associative memories is necessary to prevent the response.
All of us have many habits of this sort and they are of the greatest
value in our daily lives because on account of them many things
that we have to do are more easily done than they would be if
the whole mental process upon which the acts depend had to be
gone through with at each repetition.

The tendency to habit formation can be used very effectively
in training the child to right actions. It can be used as well in
training to right thoughts, since thoughts are associative proc-
esses, and these tend to follow the lines laid down by habit.

Language. Of all the powers of the human mind its power
to use language has had as much to do with the progress of the
race as any other property it possesses. Language, from the
standpoint of cerebral function, is simply a special sort of asso-
ciation; the association of arbitrarily selected sounds or written
symbols with objects and concepts. Let us, for illustration, con-
sider again the case of the child forming the concept mother. Co-
incident with the association of her voice, appearance, and other
personal attributes into a definite concept is the repeated auditory
stimulus of the word mother, heard when she is present or when
she is indicated in some way. In course of time this particular
succession of syllables is included as part of the concept. Several
years later the group of symbols making up the word mother is
included in the same concept. Thus language, spoken and writ-
ten, becomes indissolubly included in our whole mental equipment.

It is a curious fact that in man the use of language seems to be
not optional, but a necessary factor in his mental development.
Two lines of evidence favor this view. The first is the common
experience of all of us that we are incapable of thought except
in terms of words; coupled with the observation that no race of
men exists or is known to have existed without some form of
language. The second, and more striking, fact is that certain
regions in the association areas of the cerebrum are specially
devoted to language associations. Four such regions are known,
having been revealed by the physiological effects of their impair-
ment through accident or disease. Two of these areas have to
do with spoken, and two with written, language. One of the two
areas for each form of language is sensory and the other motor.

158 THE HUMAN BODY

An interesting thing about these language association areas is
that they seem to be confined to one of the cerebral hemispheres;
in right-handed people the left hemisphere contains them, and
in left-handed people they occur in the right hemisphere. It has
been observed, moreover, that the development of right or left-
handedness in infants is coincident with their learning to use
language. Just what the relationship between the two properties
may be is not clear.

Impairment of a sensory language area results in word-deafness
or word-blindness; the sounds are heard, or the words are seen,
but they are without meaning because the power to associate
language with concepts is affected. When motor language areas
are injured the power of expression is lost. The commonest of all
these abnormalities is the loss of power to use spoken language, a
condition known as motor aphasia. The sufferer from this con-
dition knows what he wants to say but is unable to recall the
words by which to express his ideas. Embarrassment often gives
rise to a momentary inhibition of the motor aphasia region, re-
sulting in the same inability to recall the needful words.

Since all our mental processes are dependent on language, im-
pairment of the language areas would be expected to lower the
whole mental power. This appears to be the case in most sufferers
from this condition.

There is no evidence that any species of animals except the
human species possesses the power to use language. This differ-
ence sets man sharply apart from the animals most nearly ap-
proaching him in intelligence.

Consciousness. This is a phenomenon that we all recognize
as existing in ourselves and as accompanying most if not all of
our cerebral activities. That it is present when the cortical cells
are actively functioning and absent when they are inactive is
indicated by the fact that any treatment, such as anaesthesia,
which depresses nerve-cells, tends to abolish consciousness. It
is a phenomenon whose nature is wholly unknown, and for whose
existence, even, there is no objective evidence. We cannot prove
that any lower animal has the same sort of consciousness that we
have. We can only suppose from the general similarity of their
cerebral processes to ours that this particular phenomenon is
also in them as in us. As we go down the animal scale where

STRUCTURE AND FUNCTIONS OF THE CEREBRUM 159

mental processes become simpler and simpler does it follow that
consciousness becomes dimmer and dimmer? We ordinarily as-
sume this to be true, but without any positive evidence upon
which to base the assumption.

Emotions. Another set of phenomena accompanying cerebral
activity, but known chiefly by subjective experience, are the
emotions. . We know that certain sensory stimuli give us pleasure,
others arouse disgust. Love and hate, sorrow and joy, are mental
states which are associated with certain sense impressions im-
mediate or remembered. Emotion, like consciousness, does not
lend itself to objective study, and therefore does not come within
the realm of physiology beyond simple recognition of the exist-
ence of the phenomenon. It is true that emotional states are
usually accompanied by reactions of other parts of the body, the
blush which accompanies embarrassment being an example, but
this allows us only to judge whether an emotion is present, and
tells us nothing about its actual nature.

Cerebral Functions Compared in Man and Animals. In the
higher animals as well as in man associative memory is the rep-
resentative cerebral activity. So far as we can judge it represents
in animals the climax of intellectual achievement. No animal
has ever been seen to perform any act, not purely reflex, as are all
" instinctive" actions, which associative memory cannot account
for. The activities of man are for that matter based upon as-
sociative memory almost as fully as are those of animals. The
important intellectual difference between man and animals is
the possession by man of the faculty of reason, which is denied
to animals. The power to reason is itself, however, based upon
associative memory. It may be roughly explained as the as-
sociation of concepts whose relationship is not obvious. An
animal, according to this idea, cannot reason because he cannot
form associations except of concepts that manifestly belong to-
gether. The man reasons by perceiving relationships in appar-
ently unrelated facts or ideas. We may illustrate the difference
between man and the higher animals by comparing the actions
of a man and a monkey in opening a specially fastened door.
Suppose the door is held by a catch which is released by pull-
ing a pin upward. Both man and monkey will learn to raise the
pin and so release the catch; one can open the door as readily

160 THE HUMAN BODY

as the other. Now let us turn the catch around so that the pin
must be drawn out horizontally. When the man first sees the
new arrangement he is able to associate the new appearance of
the catch with horizontal pin with his previous experience of
releasing the catch by pulling on the pin; he therefore opens the
door at the first trial. Here we have an act of reason, since the
association is of things not obviously related. When the monkey
comes to the door with rearranged fastening he finds himself con-
fronted with a new problem. He has no previous experience with
catches held by horizontal pins and he cannot associate this new
appearance with his previous experience of catches with vertical
pins. He therefore must learn this fastening as he learned the
other, by working at it in random fashion until he opens it acci-
dentally and then repeating as many of the motions of the suc-
cessful attempt as he can remember until he has the trick learned.
In all this there is no indication of reason.

The power of reasoning is the only cerebral function possessed
by man and not by animals of which we have positive objective
proof. It is therefore the only one of which physiology can take
account at the present time. Physiology does not thereby deny,
however, the existence of many activities in the human brain which
are without counterpart in the brains of lower animals. While
associative memory accounts completely for all non-instinctive
actions of the lower animals, the history of the human race and
the experience of individuals contain much that baffles explanation
in terms of associative memory or of reason. The factors which
lead the race always onward and upward to greater and greater
heights of spiritual achievement are beyond the power of present-
day physiology to analyze or even discuss.

Nourishment of the Brain. The cells of the cerebral cortex
are very dependent upon their blood supply. A slight diminution
in the rate of blood flow through the brain may depress the cor-
tical cells to such an extent that consciousness is lost. The prob-
lem of retaining consciousness is, then, the problem of keeping
the cerebral circulation up to the proper level. How this is ac-
complished during our waking hours, and how its falling off af-
fords opportunity for needed intervals of sleep will be discussed
in connection with the circulation of the blood (Chap. XXII).

CHAPTER XII

THE CEREBELLUM. THE MEDULLA AND MIDBRAIN. THE
SYMPATHETIC SYSTEM

The Cerebellum. This organ, as shown in Figs. 60, 61, is a dis-
tinct portion of the brain, lying underneath the posterior part of
the cerebrum, and behind and above the midbrain and medulla.
It consists, as does the cerebrum, of cortical gray matter super-
posed upon white matter, and having imbedded within the white
matter at its base gray masses, the nuclei of the cerebellum. Like
the cerebrum also, its cortex has afferent and efferent nerve con-
nections with the rest of the central nervous system. From this
anatomical structure we are led to conclude that the cerebellum
is probably as is the cerebrum, the seat of complicated reflex
actions. When we examine the afferent tracts leading to the
cerebrum we are struck with the high degree to which afferent
tracts from certain senses are developed and the smallness or ab-
sence of tracts from other senses. The most conspicuous afferent
tracts of the cerebellum are the tracts of body sense; these pass
up the spinal cord along its lateral margins (Fig. 66). They arise
from cell-bodies lying along the cord in its gray matter and hav-
ing dendrites doubtless in synaptic connection with collateral ter-
minations of sensory neurons. As these tracts reach the medulla
there is a branching on each side into two bundles of fibers; the
dorsal bundle passes directly from the medulla to the cerebellum
by way of the lower supporting stalk of the cerebellum, the inferior
peduncle; the ventral bundle passes further along to the upper
supporting stalk, the superior peduncle, through which it reaches
the cerebellum.

There is reason to think that two of the body senses, muscle
sense and touch (Chap. XIII), are in specially close communi-
cation with the cerebellum through these tracts.

Another sense which seems to have particularly abundant
communication with the cerebellum is the equilibrium sense
(Chap. XIV). This sense is mediated by the semicircular canals

161

162 THE HUMAN BODY

and the vestibule of the ear. Its sensory nerve-trunk is the ves-
tibular portion of the auditory nerve. Probably the sense of sight
has communication also with the cerebellum through its connec-
tions with the midbrain nuclei. So far as we know the other
senses do not come into afferent connection with the cerebellum.
The only afferent tract of the cerebellum remaining to be men-
tioned is the one by which it may receive impulses from the
cerebrum. This consists of pyramidal fibers from the motor cor-
tex which end in the pons Varolii in synaptic connection with
neurons which pass from this structure to the cerebellum by way
of its middle peduncles.

The efferent paths from the cerebellum consist at the beginning
of fibers which arise from large characteristic cell-bodies of the
cerebellar cortex, the so-called " cells of Purkinje." These fibers
terminate in the dentate nuclei in synaptic connection with neurons
which continue the path to the red nuclei of the midbrain. From
the red nuclei proceed efferent fibers which terminate in the
same manner as do the pyramidal fibers from the cerebrum, in
synaptic connection with the motor cells of the ventral horn.
The efferent tracts from the red nuclei are interesting as forming
practically the only means of communication between the brain
and the motor organs aside from the pyramidal tracts. Since the
latter are presumably wholly given over to voluntary activities
all our involuntary motions must receive their impulses through
the tracts from the red nuclei. These tracts extend along the
cord in the lateral columns on the ventral side of the crossed
pyramidal tracts. They are often called the pre-pyramidal tracts.

We have in the cerebellum a complex organ receiving impulses
specially from the senses of touch, equilibrium, muscle sense,
and sight, in connection, on its efferent side with the motor or-
gans of the Body, and subject through its afferent tract from the
cerebrum, to voluntary control. The most marked result of ex-
tensive injury of the cerebellum is muscular incoordination; this
is especially marked in acts of locomotion, and similar acts re-
quiring delicate interaction of many muscles. The senses which
act most directly on the cerebellum are the very ones which have
most to do with such an act as walking, where the movements
must be adjusted according to the guidance of the pressure of the
feet upon the ground, the condition of equilibrium of the Body,

THE CEREBELLUM, MEDULLA AND MIDBRAIN 163

the position of the parts of the Body as indicated by the muscle
sense, and the nature of the footing as shown by the sense of sight.

We can safely conclude, then, that the cerebellum is an organ
through which are carried on complicated muscular reflexes.
The reflexes which it- performs are all continually subject to
volitional modification; they are of a sort which the cerebrum is
competent to carry on. We may view the cerebellum, therefore,
as an organ which by taking up complicated but not highly in-
tellectual tasks leaves the cerebrum free for higher forms of ac-
tivity.

Every one has to learn to stand, walk, run, and so on; at first
all are difficult, but after a time become easy and are performed
unconsciously. In standing or walking very many muscles are
concerned, and if the mind had all the time to look directly after
them we could do nothing else at the same time; we have for-
gotten how we learnt to walk, but in acquiring a new mode of
progression in later years, as skating, we find that at first it needs
all our 'attention, but when once learnt we have only to start the
series of movements and they are almost unconsciously carried
on for us. At first we had to learn to contract certain muscle
groups when we got particular sensations, either tactile, from the
soles, or muscular, from the general position of the limbs, or visual,
or equilibrium sensations from the semicircular canals. But the
oftener a given group of sensations has been followed by a given
muscular contraction the more close becomes the association of
the two ; the path of connection between the afferent and efferent
fibers becomes easier the more it is traveled, and at last the af-
ferent impulses arouse the proper movement without volitional
interference at all, and while hardly exciting any consciousness;
we can then walk or skate without thinking about it. The will,
which had at first to excite the proper motor neurons in accord-
ance with the felt directing sensations, now has no more trouble
in the matter; the afferent impulses stimulate the proper motor
centers in an unconscious and unheeded way. Injury or disease
of the cerebellum produces great disturbances of locomotion and
insecurity in maintaining various postures. After a time the
animals (birds, which bear the operation best) can walk again, and
fly, but they soon become fatigued, perhaps because the move-
ments require close mental attention and direction all the time.

164 THE HUMAN BODY

The Medulla and Midbrain. If our attention had been called
to the matter when the courses of the various afferent and ef-
ferent pathways of the cerebrum and cerebellum were being de-
scribed, we should have noted that the medulla and midbrain
form a great highway through which pass virtually all impulses
on their way to or from the higher brain structures. Moreover,
most of the nerve tracts leading through these structures do not
pass directly through them, but suffer interruption in one or the
other of the many nuclei which occur therein. Wherever a nerve
tract is interrupted by a nucleus the axons leading into the nu-
cleus terminate in synaptic connection with new neurons by
which the tract is continued. There is always the possibility,
where such connections are being formed, of a certain amount of
diversion from the main channel into side channels. The medulla
and midbrain, then, are strategically located for concentrating
into small areas influences from all the receptors of the Body.
This region has also its own efferent pathway in the tract from
the red nucleus. It affords, therefore, an additional field for the
establishment of reflex arcs, but, as we shall see, of a somewhat less
specialized sort than are afforded by the cerebrum and cerebellum.

There are a number of so-called "vital processes" going on in
the Body. These are activities whose continuance is essential
to the maintenance of life, and which must, therefore, go on quite
independently of the will; they are of a sort, however, to require
modification in accordance with the demands of the Body. Such
activities are the beating of the heart, breathing, the secretion of
sweat, and some others.

Most of these so-called "vital" activities are really as purely
reflex as any of the ordinary reflex acts of the Body, and the few
that are truly automatic are subject to constant reflex influence.
Their immediate control is vested in certain centers located in
the medulla. This location for the centers insures that they shall
never be wholly free from sensory stimulation, for no matter how
quiet the surroundings of the Body may be the processes going
on within it give rise to sensory stimuli, and, as we have seen,
whatever impulses are aroused are sure to pass through the
medulla. Detailed consideration of the various centers of the
medulla is not necessary here as each will be treated in connec-
tion with the vital process with which it is related.

THE CEREBELLUM, MEDULLA AND MIDBRAIN 165

The Sympathetic System. This system is treated as a distinct
portion of the nervous system because to a rather special physio-
logical function it adds peculiar anatomical relationships. In
spite of its anatomical and physiological peculiarities, however,
it forms an integral part of the whole nervous system, and inter-
acts with other parts as completely as though nothing distin-
guished it from them. Its name has no present significance,
having been given to it in the erroneous belief that its function is
to bring remote organs into sympathy with each other. The
special physiological function of the sympathetic system may be
stated in a sentence: it forms the efferent connection between the
central nervous system and all the smooth muscles and glands
of the body, and the heart.

It will be recalled that the skeletal muscles have motor con-
nection with the central nervous system by means of motor
neurons, structures whose cell-bodies lie in the ventral horns of
gray matter and whose axons extend directly to the muscles.
The sympathetic system differs from the motor system to skeletal
muscles in that each pathway from the central nervous system
to a smooth muscle or to a gland is made up of a succession of
two neurons. The first neuron has its cell-body in the ventral
horn of gray matter; its axon passes out by way of the ventral
root of the spinal nerve and the communicating branch (see
p. 126) to one of the sympathetic ganglia where it forms synaptic
connection with the second neuron of the chain. This neuron sends
its axon back over the communicating branch to the spinal nerve
along which it passes to its destination in a smooth muscle or a
gland. Because of their positions with regard to sympathetic
ganglia the first and second neurons are known respectively as
pre-ganglionic and post-ganglionic neurons. The latter present
the anatomical peculiarity of being for the most part devoid of
myelin sheaths; nerve-trunks made up of post-ganglionic fibers
can therefore be distinguished from other nerve-trunks by their
gray color.

The structures innervated by the sympathetic system perform
their functions by acting to a considerable extent in groups
together; not individually as do skeletal muscles. To enable them
to be stimulated in groups single sympathetic pathways com-
monly involve numerous end structures, This is accomplished

166 THE HUMAN BODY

by rich branching of the pre-ganglionic fibers, enabling each to
have synaptic connection with a number of post-ganglionic
neurons, and so to influence simultaneously numerous end organs.

The heart and most of the abdominal organs receive part of
their innervation by way of the tenth cranial nerves, the vagi.
Although here the pathway is not by the sympathetic system
proper, it is constructed on the true sympathetic plan; there
being in each case two neurons, pre-ganglionic and post-ganglionic.
Connection between the two occurs in ganglia placed conveniently
to the organs affected. Those of the heart, for example, are im-
bedded within its mass, and are known as the cardiac ganglia.
The sympathetic system of the head is also in large part not
connected with the sympathetic system proper; here again, how-
ever, the true sympathetic structure obtains.

The Effect of Nicotine. Much of our knowledge of the sym-
pathetic system has resulted from the discovery that application
of the drug nicotine to sympathetic ganglia prevents the passage
of impulses over whatever synapses may be contained therein.
By the use of this drug, therefore, the point of contact of pre-
ganglionic with post-ganglionic fibers in the pathway to any
particular organ can be determined. To illustrate how its use
brings out these points of contact we may take the sympathetic
innervation of the eye. The size of the pupil is regulated by
opposing sympathetic fibers; one set tending to constrict it, the
other to dilate it. By the use of nicotine it has been shown that
the contact of pre-ganglionic with post-ganglionic fibers in the
constrictor pathway is in the ciliary ganglion, which is in the
orbit, while for the dilator pathway the connection between pre-
ganglionic and post-ganglionic fibers is in one of the sympathetic
ganglia of the neck.

Reflex Control of the Sympathetic System. The sympathetic
system, as we have seen, forms only the last step in the conduct-
ing pathway by which influences are brought to bear on the
structures it innervates. Like the motor system for the skeletal
muscles it conveys only those impulses which are imparted to it
from without. It is, in other words, the efferent portion of a
reflex mechanism.

The so-called "vital" processes of the Body are, with the ex-
ception of respiration, largely carried on through the agency of

THE CEREBELLUM, MEDULLA AND M1DBRA1N 167

smooth muscles and glands. The sympathetic system is, there-
fore, the system through which these processes have their nervous
control. In the paragraph dealing with the medulla the exist-
ence therein of reflex " centers " for the various " vital " processes
was mentioned. On the afferent side these centers are subject to
all sensory stimulations which affect the Body. On the efferent
side they act through the sympathetic system.

This reflex mechanism is not subject to voluntary control ex-
cept for the single case of the muscle of accommodation of the
eye, the ciliary muscle. This muscle is innervated through the
sympathetic system, but can be voluntarily controlled as com-
pletely as any muscle in the Body. When we say that the sym-
pathetic system is not under voluntary control we are simply
stating in other words that the motor area of the cerebrum is not
able to establish connection through the pyramidal tracts with
the neurons of the sympathetic system. Since this system is
outside the control of the motor area all reflexes which affect it
must be immediate ones. Only present stimuli can arouse it to
activity. When we bear in mind that the proper functioning of
the Body requires its vital activities to be adjusted to its im-
mediate circumstances and not to its circumstances of a week or
a year ago, the necessity that sympathetic reflexes be immediate
ones is manifest.

The Relation of the Sympathetic System to Emotional States.
A curious feature of the Body's functioning is the way in which
emotional states affect the sympathetic system. Nearly all of the
well-recognized reactions which the Body makes under emotional
stress are reactions of smooth muscles or glands. Blushing and
pallor result from changes in the smooth muscles of the blood-
vessels; the hair stands on end under the influence of fright, an
effect of smooth muscles in the skin; in conditions of embarrass-
ment the mouth becomes dry through inhibition of the salivary
gland, or the Body is drenched with sweat from stimulation of
the sweat glands.

Although these effects are extremely well marked the mechan-
ism by which they are produced is unknown. Their fuller under-
standing waits our increased knowledge of the nature of emotion
itself.

A striking feature of emotional reactions is that they do not

168 THE HUMAN BODY

appear, except in a very few cases, to be adaptive. It is difficult
to see where any advantage arises from the pallor of fright, or
from the dry mouth of embarrassment. In fact some of the
emotional reactions involving the digestive mechanism are, as
we shall see, distinctly harmful. This is all the more curious
when we remember that the ordinary reflexes of the sympathetic
system, which do not involve emotional states, are as adaptive as
any of the reactions of the Body.

Emotional reactions differ, also, from the other reactions of
the sympathetic system in that they may be based upon memory.
To the extent that memories of previous experiences may arouse
definite emotional states within us, may the Body show the
characteristic reactions which accompany them.

CHAPTER XIII

THE RECEPTOR SYSTEM. INTERNAL AND CUTANEOUS
SENSATIONS

The Receptor System constitutes the Body's means of gam-
ing information of its surroundings and of such internal condi-
tions as it needs to know about. Since the surroundings may play
upon the Body in many different ways and through the operation
of many forms of energy, receptors are provided which respond
to all sorts of stimuli. Inasmuch as proper adaptation requires
that different sorts of stimuli affect the Body differently partic-
ular receptors are specialized to respond most readily to partic-
ular kinds of stimulation.

An interesting thing about the responses of the different re-
ceptors is that while their adaptation to special forms of stimula-
tion does not exclude the possibility of their being aroused by
other sorts of stimuli than the normal ones, when so aroused the
effect in consciousness is as though the normal stimulus had been
applied. Pressure on the eyes gives rise to sensations of light;
electrical stimulation of the tongue may cause sensations of taste.
This fact has led physiologists to take the view that the quality
of any sensation depends on the region of the cerebrum to which
it comes, and that it is quite independent of the structure of the
receptor or the manner of its stimulation. If this is true it ac-
cords well with another conception which most physiologists
find very attractive, that the nerve impulse, whatever it may be,
is the same sort of process wherever it occurs. It is, of course,
evident that this idea, the so-called "doctrine of specific nerve
energies," cannot be true if the quality of sensation depends in
any manner upon the nature of the receptor or the way in which
it is stimulated. It must be confessed that many known facts
about the senses, that of sight particularly, cannot at present be
explained upon any basis which excludes differences in the re-
ceptor as determining factors of the quality of sensation.

The Differences between Sensations. We distinguish among

169

170 THE HUMAN BODY

our sensations kinds which are absolutely distinct for our con-
sciousness, and not comparable mentally. We can never get con-
fused between a sight, a sound, and a touch, nor between pain
and hunger; nor can we compare them with one another: each is
sui generis. The fundamental difference which thus separates one
sensation from another is its modality. Sensations of the same
modality may differ; but they shade imperceptibly into one an-
other, and are comparable between themselves in two ways.
First, as regards quality: while a high and a low pitched note are
both auditory sensations, they are nevertheless different and yet
intelligibly comparable; and so are blue, purple, and red objects.
In the second place, sensations of the same modality are distin-
guishable and comparable as to amount or intensity: we readily
recognize and compare a loud and a weak sound of the same pitch;
a bright and feeble light of the same color; an acute and a slight
pain of the same general character. Our sensations thus differ in
the three aspects of modality, quality within the same modality,
and intensity. Certain sensations also differ in what is known as
the " local signs," a difference by which we tell a touch on one part
of the skin from a similar touch on another; or an object exciting
one part of the eye from an object like it, but in a different location
in space and exciting another part of the visual surface.

As regards modality, we commonly distinguish five senses, those
of sight, sound, touch, taste, and smell; to these at least seven
others must be added to make the list approximately complete.
The additional senses are temperature, pain, hunger, thirst, fatigue,
muscle sense, and equilibrium sense. The last six of this list were
formerly set apart as common sensations, but there seems to be no
good reason for viewing them in any different light from the others.

The Psychophysical Law. Although our sensations are, in
modality or kind, independent of the force exciting them, they are
not so in degree or intensity, at least within certain limits. We
cannot measure the amount of a sensation and express it in foot-
pounds or calories, but we can get a sort of unit by determining
how small a difference in sensation can be perceived. This smallest
perceptible difference varies in the different senses and for different
amounts of stimulation in the same sense. Its variation in any
single sense follows, however, a certain law. The increase of stimu-
lus necessary to produce the smallest perceptible change in a sensation

THE RECEPTOR SYSTEM 171

ts proportional to the strength of the stimulus already acting; for ex-
ample, the heavier a pressure already acting on the skin the more
must it be increased or diminished in order that the increase or
diminution may be felt. Examples of this, which is known as
" Weber's " or " Fechner's psychophysical law " will be hereafter
pointed out, and are readily observable in daily life; we have, for
example, a luminous sensation of certain intensity when a lighted
candle is brought into a dark room; this sensation is not doubled
when a second candle is brought in ; and is hardly affected at all by
a third. The law is only true, however (and then but approxi-
mately), for sensations of medium intensity; it is applicable, for
example, to light sensations of all degrees between those aroused
by the light of a candle and ordinary clear daylight: but it is not
true for luminosities so feeble as only to be seen at all with diffi-
culty, or so bright as to be dazzling.

Besides their variations in intensity, dependent on variations
in the strength of the stimulus, our sensations also vary with the
irritability of the sensory apparatus itself; which is not constant
from time to time or from person to person. In the above state-
ments the condition of the sense-organ and its nervous connections
is presumed to remain the same throughout.

Classification of Receptors. It is possible to group the sense-
organs in several different ways according to the properties upon
which the classification is based. If we group them according to
the forms of energy to which they respond they fall into four
classes: 1, the senses aroused by mechanical stimulation, touch,
pain, muscle sense, equilibrium, and hearing; 2, those aroused by
chemical stimulation, taste, smell, and probably the sensations of
hunger, thirst, and fatigue; 3, the temperature sense, aroused by
thermal stimuli; 4, the sense of sight, aroused by stimuli of light.

Another classification, and a more convenient one to follow in
describing the receptors, is based upon their position in the body.
This classification gives us two main groups: 1, the internal senses,
whose receptors lie within the body; here belong muscle sense,
equilibrium, pain, hunger, thirst, and fatigue; 2, the external
senses, whose receptors are on the surface of the body and which
therefore obtain information of the outside world. These senses
fall again into two subgroups ; the first includes the contact senses
which are stimulated only by things in immediate contact with the

172 THE HUMAN BODY

body; the second includes the projecting senses which tell us -of the
surroundings not immediately touching us.

The group of contact senses includes the cutaneous senses, touch,
temperature, and pain, the latter being both external and internal,
and the sense of taste. The group of projecting senses includes
hearing, smell, and sight.

It is not desirable to follow this classification exactly in the
discussion of the various senses, but it represents in the main the
order of their consideration.

The Internal Senses. Of these only muscle sense, hunger, thirst,
and fatigue will be considered here. The sense of pain is treated
more satisfactorily in connection with the cutaneous senses. The
equilibrium sense requires an account of the structure of the ear
and will be given in connection with the sense of hearing. The
functions of these senses are to inform the body of its own con-
dition. They are recognized in consciousness as bodily states,
being in this respect very different from the external senses, which
we interpret altogether in terms of the sources from which the
stimuli arise. The difference in consciousness between internal and
external senses may be illustrated by supposing that a kntfe is
held in the hand. The sensations we have are referred in our
consciousness to the knife. It is hard, cold, etc. Let the knife now
cut through the skin. The stimulus arises from the knife as much
as before, but it is to the hand and not to the knife that we refer
the feeling of pain.

The Muscular Sense. From the muscles arise sensations of great
importance, although they do not often become so obtrusive in
consciousness as to arouse separate attention. They are due to the
excitation of sensory nerves ending within the muscles themselves,
or in the tendons or joints with which the muscles are connected.

We have at any moment a fairly accurate knowledge of the
position of various parts of our Bodies, even when we do not see
them; and we can also judge fairly accurately the extent of a
movement made with the eyes shut. The afferent nerve impulses
concerned in the development of such judgments may be various;
different parts of the skin are pressed or creased; different joints
are subjected to pressure; different tendons are put on the stretch
and different muscles are in different states of contraction, and it
is by no means easy to determine the part played in each case by

THE RECEPTOR SYSTEM 173

the sensory nerves of the different organs. Moreover, when we
push against an object, or lift it, we are able to form a judgment
as to the amount of effort exerted; but here again pressure on
skin and joints and tension of tendons come in. Although under
normal circumstances the skin sensations are undoubtedly of im-
portance, they are not necessary : persons with cutaneous paralysis
can, apart from sight, judge truly the position of a limb and the
extent of movement made by it ; and in many movements change
in joint pressure must be very little if any. We have then to look
to muscles and tendons themselves for an important part of the
sensations, and in both muscles and tendons there are organs in
connection with nerve-fibers which are certainly sensory in nature:
moreover, muscle sensory nerves appear to be excited by mere
passive change of form in the muscle; with the eyes closed each of
us can tell how much another person has lifted one of our arms.

The sensations by which we judge the extent of a muscular
movement enable us to determine very minute differences of con-
traction; the ocular determination of the distance of an object not
too far off to have its absolute distance determined with con-
siderable accuracy, depends almost entirely upon judgments based
upon very small changes in the degree of contraction of the internal
and external straight (recti) muscles, converging or diverging the
eyeballs. A singer, too, must be able to judge with great minute-
ness the degree of contraction of the small muscles of the larynx
necessary to produce a certain tension of the vocal cords. It may
be well to point out that we do not refer a muscular sensation to
any given muscle or muscles; it is merely associated with a certain
movement or position, and a person who knows nothing about his
ocular muscles can judge distance through sensations derived from
them, quite as well as any anatomist. This fact is of course cor-
related with the fact that in voluntary movement we do not make
a conscious effort to contract any particular muscles: the higher
nerve-centers are merely concerned with the initiation of a given
movement of a given extent, and all the details are carried out by
lower coordinating centers. In ordinary daily life in fact we have
no interest whatever in a muscular contraction per se; all we are
concerned with is the result, and consciousness has never had need
to trouble itself, if it could, with associating a particular feeling or
a particular movement with any individual muscle.

174 THE HUMAN BODY

Muscular feelings are, as already pointed out, frequently and
closely combined not only with visual but also with tactile, in pro-
viding sensations on which to base judgments: in the dark, when
an object is of such size and form that it cannot be felt all over by
any one region of the skin, we deduce its shape and extent by com-
bining the tactile feelings it gives rise to, with the muscular feelings
accompanying the movements of the hands over it. Even when
the eyes are used the sensations attained through them mainly
serve as short-cuts which we have learned by experience to inter-
pret, as telling us what tactile and muscular feelings the object
seen would give us if felt; and, in regard to distant points, although
we have learnt to apply arbitrarily selected standards of measure-
ment, it is probable that distance, in relation to perception, is
primarily a judgment as to how much muscular effort would be
needed to come into contact with the thing looked at.

When we wish to estimate the weight of an object we always,
when possible, lift it, and so combine muscular with tactile sensa-
tions. By this means we can form much better judgments. While
with touch alone just perceptibly different pressures have the
ratio 1 : 3, with the muscular sense added differences of -^ can be
perceived.

Hunger and Thirst. These sensations, which regulate the taking
of food, are peripherally localized in consciousness, the former in
the stomach and the latter in the throat, and local conditions no
doubt play a part in their production ; though general states of the
Body are also concerned.

Hunger in its first stages may be due to a condition of the gastric
mucous membrane which comes on when the stomach is empty,
since it is temporarily stilled by filling the organ with indigestible
substances. But soon the feeling comes back intensified and can
only be allayed by the ingestion of nutritive substances; provided
these are absorbed and reach the blood, their mode of entry is un-
essential; the hunger may be stayed by injections of food into the
rectum as well as by putting it into the stomach.

Similarly, thirst may be temporarily relieved by moistening the
throat without swallowing, but then soon returns; while it may be
permanently relieved by water injections into the veins, without
wetting the throat.

While both sensations depend in part on local peripheral con-

THE RECEPTOR SYSTEM 175

ditions, they may also be, and more powerfully, excited by poverty
of the blood in foods and water; such deficiency probably acts, in
the case of thirst, at least, by bringing about stimulation of many
afferent nerves which are normally not concerned in the sense of
thirst. This stimulation may become so powerful as to upset the
higher brain structures. Loss of reason is said to be the inevitable
result of too prolonged deprivation of water.

That hunger is not altogether determined by the degree of empti-
ness of the stomach is shown by the experience of nearly all adults,
who are less hungry in the morning, after a fast of such length that
the stomach is practically certain to be empty, than at noon or
night, when the interval after the preceding meal is so short as to
make it often unlikely that the stomach has emptied itself. We
must recognize, however, that the so-called hunger of well nour-
ished human beings is rather appetite than true hunger, and de-
pends more on habit than on real bodily demands. So that true
physiological hunger cannot be explained in terms of it.

Fatigue is the least definitely localized of the senses. It is felt
as a state of the whole Body, rather than as arising from any par-
ticular region. It is not definitely proven that fatigue is a sense so
far as that term implies the possession of receptors with their in-
dividual afferent nerves. Many physiologists think that it .arises
from the presence in the blood of "fatigue products," chemical
substances given off from active tissues, and that these may pro-
duce the feeling of fatigue by direct chemical stimulation of the
central nervous system itself, or in some other way not involving
a particular set of fatigue receptors.

The Cutaneous Senses. These occur over the entire body, not
uniformly distributed but scattered in fine dots over the surface.
This punctiform arrangement can be demonstrated by exploring
the skin with fine needles. Such a procedure shows that the dif-
ferent cutaneous senses occur in distinct spots which do not over-
lap, but which in most parts of the Body are so intermingled as to
leave no area of any size devoid of any one of the senses. Sensory
spots are much more numerous and more closely packed together
in such regions as the hands and face which are liable to come in
contact with foreign bodies, than they are in the better protected
surfaces of the trunk and limbs. Four sorts of cutaneous sense
spots are recognized; those of pain, touch, warmth, and cold.

176 THE HUMAN BODY

Pain spots are more numerous than any of the others; touch spots
rank next in number, it being estimated that on the trunk and
limbs there are a half million of them; cold spots are only half as
numerous as touch spots; warmth spots are fewest of all, their
number being estimated at thirty thousand for the entire Body.

Pain. When the skin is powerfully stimulated by heat, cold or
pressure, or is inflamed, we get a sensation which we call pain.
This is something quite different from the unpleasantness caused
by a dazzling light or a musical discord or a disagreeable odor or
taste. We recognize these as being still sight or sound or smell
or taste sensations. Pain, however, is always recognized as a
distinct sensation having its own modality. Its function seems
to be wholly one of warning; only when something is amiss 'do
we feel it. Since danger results from strong stimulation but not
from feeble stimulation pain receptors are less irritable than other
sorts ; it is estimated that the sense of touch is one thousand times
as delicate as the sense of pain. Harm may result from excessive
stimulation of any sort. Pain receptors, therefore, are irritable
to all forms of energy except that of light.

Because pain results from any sort of stimulation, but only
when excessive, it was formerly thought to be not a distinct sense
but the result of overstimulation of the other senses. On this
theory it would be hard to account for the fact that skin pain is
so very different in modality from a touch or temperature feeling,
and to understand why it gives rise in consciousness to concep-
tions concerning a condition of the Body and not of some external
object: it is not extrinsically referred by the mind to a quality of
anything but the painful part itself, as a dazzling light sensation
or a fetid odor is. There is also experimental and pathological
evidence that the paths taken in the spinal cord by nerve impulses
causing pain are different from those leading to a consciousness
of touch. If certain parts of the cord are cut in the thoracic
region of a rabbit, gentle touches on the hind limb appear to be
felt; the animal erects its ears or moves its head: but powerful
stimulation of the sciatic nerve causes no signs of pain, while if
the dorsal white columns be cut the animal still can feel stimuli
applied to the hind limb and sufficient to cause pain under normal
conditions, but it appears insensible to gentle pressure on the
skin. In human beings very similar phenomena have been ob-

THE RECEPTOR SYSTEM 177

served In cases of spinal cord disease: and in a certain stage of
chloroform or ether narcosis the patient feels the surgeon's hand
or his knife where it touches the skin, but he experiences no pain
when deeper parts are cut.

Such considerations seem to lead to the conclusion that the
nerve-fibers and receptors concerned with painful sensations are
quite distinct from those of the other senses. If that be so we
must assume that there are " pain " fibers very widely distributed
over the skin and through most other parts of the Body. In
accident or disease these are stimulated powerfully enough to
arouse perception and imperiously call attention to danger.

The pain nerves of the skin do not seem to be provided with
spebial end organs but to end nakedly among the cells of the
epidermis. Such a mode of termination accords with the low
irritability of the pain mechanism and with its absence of adapta-
tion to particular forms of energy, since nerve-tissue proper ex-
hibits these same qualities.

The interior of the Body, in certain regions at least, seems to
be provided with special pain receptors. These are the Pacinian
corpuscles (see Fig. 67). They are specially numerous in the
mesentery, the connective tissue membrane which supports the
abdominal viscera.

Pains can be localized, though only imperfectly, and the less
perfectly the more severe they are. The exact place of a needle
prick after removal of the needle (so that there is no guiding
concomitant touch sensation) cannot be recognized as well as a
pin touch on the same region of the skin, but still fairly well;
while the acute pain caused by a small abscess (bone felon) under
the periosteum of a finger bone is often felt all over the forearm;
and a single diseased tooth may cause pain felt over the whole
of that side of the face.

Many internal pains instead of being felt as coming from the
organ where they originate are referred to areas of the skin. So
constant is this misreference that the physician is able to judge
of the seat of many disturbances from the particular skin areas
that exhibit tenderness. The explanation of this misreference
of internal pain to the skin is not easy to make. It has been sug-
gested that the nerve-paths over which internal pain reach the
body sense-area of the cortex lie close to those of pains from cer-

178

THE HUMAN BODY

tain skin areas; and that since painful skin stimulation is much
more common than internal pains, the brain interprets all im-
pulses reaching it over a restricted nerve-path as coming from

the particular skin area whose nerve-
path forms part of the whole nerve-
path in question.

Touch, or the Pressure Sense.
Through touch proper we recognize
pressure or traction exerted on the
skin, and the force of the pressure;
the softness or hardness, roughness or
smoothness, of the body producing
it ; and the form of this, when not too
large to be felt all over. When to
learn the form of an object we move
the hand over it, muscular sensations
are combined with proper tactile, and
such a combination of the two sen-
sations is frequent; moreover, we
rarely touch anything without at the
same time getting temperature sensations; therefore pure tactile
feelings are rare.

From an evolution point of view, touch is probably the first
distinctly differentiated sensation, and this primary position
it still largely holds in our mental life; we mainly think of the
things about us as objects which would give us certain tactile
sensations if we were in contact with them. Though the eye
tells us much quicker, and at a greater range, what are the shapes
of objects and whether they are smooth, rough, and so on, our
real conceptions of round and square and rough bodies are de-
rived through touch, and we largely translate unconsciously the
teachings of the eye into mental terms of the tactile sense.

The delicacy of the pressure sense varies on different parts
of the skin; it is greatest on the forehead, temples, and back of
the forearm, where a weight of 2 milligr. (0.03 grain) pressing on
an area of 9 sq. millim. (0.0139 sq. inch) can be felt. On the front
of the forearm 3 milligr. (0.036 grain) can be similarly felt, and
on the front of the forefinger 5 to 15 milligr. (0.07-0.23 grain) .
In order that the sense of touch may be excited neighboring

FIG. 67. — A Pacinian corpus-
cle, magnified.

THE RECEPTOR SYSTEM 179

skin areas must be differently pressed; when we lay the hand
on a table this is secured by the inequalities of the skin, which
prevent end organs, lying near together, from being equally com-
pressed. When, however, the hand is immersed in a liquid, as
mercury, which fits into all its inequalities and presses with
practically the same weight on all neighboring immersed areas,
the sense of pressure is only felt at a line along the surface, where
the immersed and non-immersed parts of the skin meet.

It was in connection with the tactile sense that the facts on
which the so-called psychophysical law (p. 170) is based, were first
observed. The smallest perceptible difference of pressure recog-
nizable when touch alone is used, is about £, i. e., we can just tell
a weight of 20 grams (310 grains) from one of 30 (465 grains) or
of 40 grams (620 grains) from one of 60 (930 grains) ; the change
which can just be recognized being thus the same fraction of that
already acting as a stimulus. The rate only holds good, how-
ever, for a certain mean range of pressure; it is not true for very
small or very great pressures. The experimental difficulties in
determining the question are considerable; muscular sensations
must be rigidly excluded; the time elapsing between laying the
different weights on the skin must always be equal; the same
region and area of the skin must be used; the weights must have
the same temperature; and fatigue of the organs must be elimi-
nated. Considerable individual variations are also observed, the
least perceptible difference not being the same in all persons.

The Localizing Power of the Skin. When the eyes are closed
and a point of the skin is touched we can with some accuracy
indicate the region stimulated; although tactile feelings are in
general characters alike, they differ in something (local sign)
besides intensity by which we can distinguish them; some sensa-
tion quality must be present enabling us to tell from one another
two precisely similar contacts of an external object when ap-
plied, say, to the tips of the fore and ring fingers respectively.
The accuracy of the localizing power is not nearly so great as in
the eye and varies widely in different skin regions; it may be
measured by observing the least distance which must separate
two objects (as the blunted points of a pair of compasses) in order
that they may be felt as two. The following table illustrates
some of the differences observed:

180 THE HUMAN BODY

Tongue-tip 1.1 mm. (0.04 inch)

Palm side of last phalanx of finger 2.2 mm. (0.08 inch)

Red part of lips 4.4 mm. (0.16 inch)

Tip of nose . 6.6 mm. (0.24 inch)

Back of second phalanx of finger 1 1.0 mm. (0.44 inch)

Heel 22.0 mm. (0.88 inch)

Back of hand 30.8 mm. (1.23 inches)

Forearm 39.6 mm. (1.58 inches)

Sternum 44.0 mm. (1.76 inches)

Back of neck 52.8 mm. (2.11 inches)

Middle of back 66.0 mm. (2.64 inches)

The localizing power is a little more acute across the long axis
of a limb, and is better when the pressure is only strong enough
just to cause a distinct tactile sensation, than when it is more
powerful; it is also very readily and rapidly improvable by practice.
It might be thought that this localizing power depended di-
rectly on nerve distribution; that each touch nerve had connec-
tion with a special brain-center at one end (the excitation of
which caused a sensation with a characteristic local sign), and
at the other end was distributed over a certain skin area, and
that the larger this area the farther apart
might two points be and still give rise to
only one sensation. If this were so, how-
ever, the peripheral tactile areas (each be-

e /rj-H-jriiujuiAJUUjj mg determined by the anatomical distribu-
tion of a nerve-fiber) must have definite
unchangeable limits, which . experiment
shows that they do not possess. Suppose
each of the small areas in Fig. 68 to repre-
sent a peripheral area of nerve distribu-
tion. If any two points in c were touched
we would according to the theory get but
a single sensation; but if, while the compass
points remained the same distance apart, or were even approxi-
mated, one were placed in c and the other on a contiguous area,
two fibers would be stimulated and we ought to get two sensa-
tions; but such is not the case; on the same skin region the
points must be always the same distance apart, no matter how
they be shifted, in order to give rise to two just distinguishable
sensations.

THE RECEPTOR SYSTEM 181

It is probable that the nerve areas are much smaller than the
tactile; and that several unstimulated must intervene between
the excited, in order to produce sensations which shall be dis-
tinct. If we suppose twelve unexcited nerve areas must inter-
vene, then, in Fig. 67, a and 6 will be just on the limits of a single
tactile area; and no matter how the points are moved, so long as
eleven, or fewer, unexcited areas come between, we would get a
single tactile sensation; in this way we can explain the fact that
tactile areas have no fixed boundaries in the skin, although the
nerve distribution in any part must be constant. We also see
why the back of a knife laid on the surface causes a continuous
linear sensation, although it touches many distinct nerve areas;
if we could discriminate the excitations of each of these from that
of its immediate neighbors we would get the sensation of a series
of points touching us, one for each nerve region excited; but in
the absence of intervening unexcited nerve areas the sensations
are fused together.

The Temperature Sense. By this we mean our faculty of
perceiving cold and warmth; and, with the help of these sensa-
tions, of perceiving temperature differences in external objects.
Its organ is the whole skin, the mucous membrane of mouth and
fauces, pharynx and upper part of gullet, and the entry of the
nares. Direct heating or cooling of a sensory nerve may stimulate
it and cause pain, but not a true temperature sensation; and the
amount of heat or cold requisite is much greater than that neces-
sary when a temperature-perceiving surface is acted upon; hence
we must assume the presence of temperature receptors. As
previously stated these are of two kinds, those that are stimulated
by cold, and those that are stimulated by warmth.

In a comfortable room we feel at no part of the Body either
heat or cold, although different parts of its surface are at differ-
ent temperatures; the fingers and nose being cooler than the
trunk which is covered by clothes, and this, in turn, cooler than
the interior of the mouth. The temperature which a given region
of the temperature organ has (as measured by a thermometer)
when it feels neither hot nor cold is its temperature-sensation zero
for that time, and is not associated with any one objective tem-
perature; for not only, as we have just seen, does it vary in dif-
ferent parts of the organ, but also on the same part from time to

182 THE HUMAN BODY

time. Whenever a skin region passes with a certain rapidity to
a temperature above its sensation zero we feel warmth; and vice
versa: the sensation is more marked the greater the difference,
and the more suddenly it is produced; touching a metallic body,
which conducts heat rapidly to or from the skin, causes a more
marked hot or cold sensation than touching a worse conductor,
as a piece of wood, of the same temperature.

The change of temperature in the organ may be brought about
by changes in the circulatory apparatus (more blood flowing
through the skin warms it and less leads to its cooling), or by
temperature changes in gases, liquids, or solids in contact with it.
Sometimes we fail to distinguish clearly whether the cause is
external or internal; a person coming in from a windy walk often
feels a room uncomfortably warm which is not really so; the
exercise has accelerated his circulation and tended to warm his
skin, but the moving outer air has rapidly conducted off the extra
heat; on entering the house the stationary air there does this less
quickly, the skin becomes hotter, and the cause is supposed to be
oppressive heat of the room. Hence, frequently, opening of win-
dows and sitting in a draught, with its concomitant risks; whereas
keeping quiet for five or ten minutes, until the circulation had
returned to its normal rate, would attain the same end without
danger.

The acuteness of the temperature sense is greatest at tem-
peratures within a few degrees of 30° C. (86° F.) ; at these differ-
ences of less than 0.1° C. can be discriminated. As a means of
measuring absolute temperature, however, the skin is very un-
reliable, on account of the changeability of its sensation zero.
We can localize temperature sensations much as tactile, but not
so accurately.

The receptors for cold are near the surface of the skin; those
for warmth are imbedded deeply within it. While the latter
respond only to temperatures above their own, the cold receptors
are stimulated not only by temperatures below their own but
also by temperatures above 45° C. (140° F.). It is for this reason
that a sensation of cold is felt when one first steps into a hot bath;
the receptors for cold being nearer the surface than those for
warmth are stimulated an instant before them. It is said that the
sensation of "hot" as distinguished from "warm" results from

THE RECEPTOR SYSTEM 183

simultaneous stimulation of warmth and cold spots by tempera-
tures above 45° C.

The Peripheral Reference of Our Sensations. Repeated men-
tion has been made of the fact that we refer our external sensa-
tions to the outside world; this is only one case of a more general
law, in accordance with which we do not ascribe our sensations,
as regards their locality, to the brain, where the sensation is,
actually aroused, but to a peripheral part. With respect to the
brain, other parts of the Body are external objects as much as
the rest of the material universe, yet we locate the majority of
our internal sensations at the places where the sensory nerves
concerned are irritated, and not in the brain. Even if a nerve-
trunk be stimulated in the middle of its course, we refer the re-
sulting sensation to its outer endings. A blow on the inside of
the elbow-joint, injuring the ulnar nerve, produces not only a
local pain, but a sense of tingling ascribed to the fingers to which
the ends of the fibers go. Persons with amputated limbs have
feelings in their fingers and toes long after they have been lost,
if the nerve-trunks in the stump be irritated. This persistent
reference is commonly ascribed to the results of experience. The
events of life have taught us that in the great majority of in-
stances the sensory impulses which excite a given tactile sensa-
tion, for example, have acted upon the tip of a finger. The sen-
sation goes when the finger is removed, and returns when it is
replaced; and the eye confirms the contact of the external object
with the finger-tip when we get the tactile sensation in question.
We thus come firmly to associate a particular region of the skin
with a given sensation, and whenever afterwards the nerve-fibers
coming from the finger are stimulated, no matter where in their
course, we ascribe the origin of the sensation to something acting
on the finger tip.

Perceptions. In every sensation we have to distinguish care-
fully between the pure sensation and certain judgments founded
upon it; we have to distinguish between what we really feel and
what we think we feel; and very often firmly believe we do feel
when we do not.

The most important of these judgments is that which leads us
to ascribe certain sensations, those aroused through organs of
special sense, to external objects — that outer reference of our

184 THE HUMAN BODY

sensations which leads us to form ideas concerning the existence,
form, position, and properties of external things. Such represen-
tations as these, founded on our senses, are called perceptions.
Since these always imply some mental activity in addition to a
mere feeling, their full discussion belongs to the domain of Psy-
chology. Physiology, however, is concerned with them so far as
it can determine the conditions of stimulation under which a
given mental representation concerning a sensation is made.
It is quite certain that we can feel nothing but states of ourselves,
but, as already pointed out, we have no hesitation in saying we
feel a hard or a cold, a rough or smooth body. When we look at
a distant object we usually make no demur to saying that we
perceive it. What we really feel is, however, the change produced
by it in our eyes. There are no parts of our Bodies reaching to
a tree or a house a mile off — and yet we seem to feel all the while
that we are looking at the tree or the house and feeling them, and
not merely experiencing modifications of our own eyes or brains.
When reading we feel that what we really see is the book ; and yet
the existence of the book is a judgment founded on a state of our
Body, which alone is what we truly feel.

We have the same experience in other cases, for example with
regard to touch.

Hairs are quite insensible, but are imbedded in the sensitive
skin, which is excited when they are moved. But if the tip of a
hair be touched by some external object we believe we feel the
contact at its insensible end, and not in the sensitive skin at its
root. So, the hard parts of the teeth are insensible; yet when we
rub them together we refer the seat of the sensation aroused to the
points where they touch one another, and not to the sensitive parts
around the sockets where the sensory nerve impulse is really started.

Still more, we may refer tactile sensations, not merely to the
distal ends of insensible bodies implanted in the skin, but to the
far ends of things which are not parts of our Bodies at all; for
instance, the distant end of a rod held between the finger and a
table while the finger is moved a little from side to side. We then
believe we feel touch or pressure in two places; one where the rod
touches our finger, and the other where it comes in contact with
the table. A blind man gropes his way along by feeling at the end
of his stick.

THE RECEPTOR SYSTEM 185

This irresistible mental tendency to refer certain of our states
of feeling to causes outside of our Bodies, whether in contact with
them or separated from them by a certain space, is known as the
phenomenon of the extrinsic reference of our sensations. It seems
largely to depend on the fact that the sensations cxtrinsically
referred can be modified by movements of our Bodies. Hunger,
thirst, and toothache all remain the same whether we turn to the
right or left, or move away from the place we are standing in.
But a sound is altered. We may find that in a certain position
of the head it is heard more by the right ear than the left ; but on
turning round the reverse is the case; and half-way round the
loudness in each ear is the same. Hence we are led, by mental
laws outside of the physiological domain, to suspect that its
cause is not in our Body, but outside of it; and depends not on a
condition of the Body but on something else.

Sensory Illusions. "I must believe my own eyes" and "we
can't always believe our senses" are two expressions frequently
heard, and each expressing a truth. No doubt a sensation in
itself is an absolute incontrovertible fact : if I feel redness or hot-
ness I do feel it, and that is an end of the matter: but if I go be-
yond the fact of my having a certain sensation and conclude from
it as to properties of something else — if I form a judgment from
my sensation — I may be totally wrong; and in so far be unable to
believe my eyes or skin. Such judgments are almost inextricably
woven up with many of our sensations, and so closely that we
cannot readily separate the two; not even when we know that
the judgment is erroneous.

For example, the moon when rising or setting appears bigger
than when high in the heavens — we seem to feel directly that it
arouses more sensation, and yet we know certainly that it does
not. With a body of a given brightness the amount of change
produced in the end organs of the eye will depend on the size of
the image formed in the eye, provided the same part of its sensory
surface is acted upon. Now the size of this image depends on the
distance of the object; it is smaller the farther off it is and greater
the nearer, and measurements show that the area of the sensitive
surface affected by the image of the rising moon is no larger than
that affected by it when overhead. WThy then do we, even after
we know this, see it bigger? The reason is that when the moon is

186 THE HUMAN BODY

near the horizon we imagine, unconsciously and irresistibly, that
it is farther off; even astronomers who know perfectly well that
it is not, cannot help forming this unconscious and erroneous
judgment — and to them the moon appears in consequence larger
when near the horizon, just as it does to less well-informed mor-
tals. In fact we have a conception of the sky over which the moon
seems to travel, not as a half sphere but as somewhat flattened,
and hence when the moon is at the horizon we unconsciously
judge that it is farther off than when overhead. But any body
which excites the same extent of the sensitive surface of the eye
at a great distance that another does at less, must be larger than
the latter; and so we conclude that the moon at the horizon is
larger than the moon in the zenith, and are ready to declare that
we see it so.

Erroneous perceptions of this sort are known as sensory illu-
sions; and we ought to be constantly on guard against them.

CHAPTER XIV

THE EAR. HEARING AND EQUILIBRATION. TASTE AND

SMELL

The External Ear. The auditory organ in man consists of
three portions, known respectively as the external ear, the middle
ear or tympanic cavity, and the internal ear or labyrinth; the latter
contains the end organs of the auditory nerve. The external ear
consists of the expansion seen on the exterior of the head, called
the concha, M, Fig. 69, and a passage leading in from it, the ex-

FIG. 69. — Semidiagrammatic section through the right ear (Czennak). M , con-
cha; G, external auditory meatus; T, tympanic membrane; P, middle ear; o, oval
foramen; r, round foramen; R, pharyngeal opening of Eustachian tube; V, vesti-
bule; B, a semicircular canal; S, the cochlea; Vt, scala vestibuli; Pt, scala tympani;
A, auditory nerve.

ternal auditory meatus, G. This passage is closed at its inner end
by the tympanic or drum membrane, T. It is lined by skin, through
which numerous small glands, secreting the wax of the ear, open.
The Middle Ear (P, Fig. 69) is an irregular cavity in the tem-
poral bone, closed externally by the drum membrane. From its

187

188 THE HUMAN BODY

inner side the Eustachian tube (R) proceeds to the pharynx, and
the mucous membrane of that cavity is continued up the tube to
line the middle ear; the proper tympanic membrane composed
of connective tissue is therefore covered by mucous membrane on
its inner, as it is by very thin skin on its outer, side. In the bony
inner wall of the middle ear are two small apertures, the oval and
round foramens, o and r, which lead into the labyrinth. During
life the round aperture is closed by the lining mucous membrane,
and the oval in another way, to be described presently. The
tympanic membrane, T, stretched across the outer side of the
middle ear, forms a shallow funnel with its concavity outwards.
It is pressed by the external air on its exterior, and by air enter-
ing the tympanic cavity through the Eustachian tube on its inner
side. If the middle ear were closed the pressures on the inner and
outer sides of the drum membrane would not be always equal
when barometric pressure varied, and the membrane would be
bulged in or out according as the external or internal pressure on
it were the greater. On the other hand, were the Eustachian tube
always open the sounds of our own voices would be loud and dis-
concerting, so it is usually closed ; but every time we swallow it is
opened, and thus the air-pressure in the cavity is kept equal to
that in the external auditory meatus. By holding the nose, keep-
ing the mouth shut, and forcibly expiring, air may be forced un-
der pressure into the middle ear, and will be held in part im-
prisoned there until the next act of swallowing. On making a
balloon ascent or going rapidly down a deep mine, the sudden
and great change of aerial pressure outside frequently causes
painful tension of the drum membrane, which may be greatly
alleviated by frequent swallowing movements.

The Auditory Ossicles. Three small bones lie in the middle
ear forming a chain from the drum membrane to the oval fora-
men. The external bone (Fig. 70) is the malleus or hammer; the
middle one, the incus or anvil; and the internal, the stapes or
stirrup. The malleus, M, has an upper enlargement or head,
which carries on its inner side an articular surface for the incus;
below the head is a constriction, the neck, and below this two
processes complete the bone; one, the long or slender process, is
imbedded in a ligament which reaches from it to the front wall of
the tympanic cavity; the other process, the handle, reaches down

THE EAR, HEARING, TASTE AND SMELL 189

between the mucous membrane lining the inside of the drum

membrane and the membrane proper, and is firmly attached to

the latter near its center and keeps the membrane dragged in

there so as to give it its peculiar

concave form, as seen from the

outside. The incus has a body

and two processes, and is much

like a molar tooth with two fangs.

On its body is an articular hollow

to receive the head of the malleus ;

its short process («/&) is attached by

ligament to the back wall of the

tympanum; the long process (JZ)

is directed inwards to the stapes;

On the tip of this process is a little .. Fip. 70.— The auditory ossicles of

r the right ear, seen from the front. M,

knob, which represents a bone (OS malleus; J, incus; S, stapes; Mcp,

j.7 \ i • j. • • i i'f head of the malleus ; Me, neck of

OrOlCUlare) distinct in early life, ditto -Ml, long process; Mm, handle;

<?tnnoej (K} i« p-v-tromplv liko a Jc' body> Jb> short, and Jl, long

' a process of incus; J'pl, os orbiculare;

stirrup, and its base (the footpiece SCP> head of stapes.
of the stirrup) fits into the oval foramen, to the margin of which
its edge is united by a fibrous membrane, allowing of a little play
in and out.

From the posterior side of the neck of the malleus a ligament
passes to the back wall of the middle ear: this, with the ligament
imbedding the slender process and fixed to the front wall of the
cavity, forms an anteroposterior axial ligament, on which the
malleus can slightly rotate, so that the handle can be pushed in
and the head out and vice versa. If a pin be driven through
Fig. 70 just below the neck of the malleus and perpendicular to
the paper it will very fairly represent this axis of rotation. Con-
nected with the malleus is a tiny muscle, called the tensor tympani;
it is inserted on the handle of the bone below the axis of rotation,
and when it contracts pulls the handle in and tightens the drum
membrane. Another muscle (the stapedius) is inserted into the
outer end of the stapes, and when it contracts fixes the bone so
as to limit its range of movement in and out of the fenestra ovalis.

The Internal Ear. The labyrinth consists primarily of cham-
bers and tubes hollowed out in the temporal bone and inclosed
by it on all sides, except for the oval and round foramina on its

190

THE HUMAN BODY

exterior, and certain apertures on its inner side by which blood-
vessels and branches of the auditory nerve enter; during life all
these are closed water-tight in one way or another. Lying in the

FIG. 71. — Casts of the bony labyrinth. A, left labyrinth seen from the outer
side; B, right labyrinth from the inner side; C, left labyrinth from above; Fc, round
foramen; Fv, oval foramen; h, horizontal semicircular canal; ha, its ampulla;
Vaa, ampulla of anterior vertical semicircular canal; Vpa, ampulla of posterior
vertical semicircular canal; Vc, conjoined portion of the two vertical canals.

bony labyrinth thus constituted, are membranous parts, of the
same general form but smaller, so that between the two a space is
left; this is filled with a watery fluid, called the perilymph; and
the membranous internal ear is filled by a similar liquid, the endo-
lymph.

The Bony Labyrinth. The bony labyrinth is described in three
portions, the vestibule, the semicircular canals, and the cochlea;
casts of its interior are represented from different aspects in
Fig. 71. The vestibule is the central part and has on its exterior
the oval foramen (Fv) into which the base of the stirrup-bone fits.
Behind the vestibule are three bony semicircular canals, com-
municating with the back of the vestibule at each end, and dilated
near one end to form an ampulla (Vpa, Vaa, and ha) . The horizon-
tal canal lies in the plane which its name implies, and has its am-
pulla at the front end. The two other canals lie vertically, the
anterior at right angles, and the posterior parallel, to the median
anteroposterior vertical plane of the head. Their ampullary ends
are turned forwards and open close together into the vestibule;
their posterior ends unite (Vc) and have a common vestibular
opening.

The bony cochlea is a tube coiled on itself somewhat like a snail's
shell, and lying in front of the vestibule.

The Membranous Labyrinth. The membranous vestibule, lying

THE EAR, HEARING, TASTE AND SMELL 191

in the bony one, consists of two sacs communicating by a narrow
aperture. The posterior is called the utriculus, and into it the
membranous semicircular canals open. The anterior, called the
sacculus, communicates by a
tube with the membranous coch-
lea. The membranous semicir-
cular canals much resemble the
bony, and each has an ampulla;
in most of their extent they are
only united by a few irregular
connective-tissue bands with the
periosteum lining the bony ca-
nals ; but in the ampulla one side

Of the membranous tube is FIG. 72.— A section through the cochlea

closely adherent to its bony pro- in the line of its axis-
tector; at this point nerves enter the former. The relations
of the membranous to the bony cochlea are more complicated.
A section through this part of the auditory apparatus (Fig. 72)
shows that its osseous portion consists of a tube wound two and
a half times (from left to right in the right ear and vice versa)
around a central bony axis, the modiolus. From the axis
a shelf, the lamina spiralis, projects and partially subdivides
the tube, extending farthest across in its lower coils. At-
tached to the outer edge of this bony plate js the membranous
cochlea (scala media), a tube triangular in cross-section and at-
tached by its base to the outer side of the bony cochlear spiral.
The spiral lamina and the membranous cochlea thus subdivide the
cavity of the bony tube (Fig. 73) into an upper portion, the scala
vestibuli, SV, and a lower, the scala tympani, ST. Between these
lie the lamina spiralis (Iso) and the membranous cochlea (CC) , the
latter being bounded above by the membrane of Reissner (R) and
below by the basilar membrane (6) . The free edge of the lamina
spiralis is thickened and covered with connective tissue which is
hollowed out so as to form a spiral groove (the sulcus spiralis, ss)
along the whole length of the membranous cochlea. The latter
does not extend to the tip of the bony cochlea; above its apex the
scala vestibuli and scala tympani join; both are filled with peri-
lymph, and the former communicates below with the perilymph
cavity of the vestibule, while the scala tympani abuts below on the

192

THE HUMAN BODY

round foramen, which, as has already been pointed out, is closed by
a membrane. The membranous cochlea contains certain solid

FIG. 73. — Section of one coil of the cochlea, magnified. SV, scala vestibuli;
R, membrane of Reissner; CC, membranous cochlea (scala media); Us, limbus
laminae spiralis; t, tectorial membrane; ST, scala tympani; Iso, spiral lamina;
Co, rods of Corti; b, basilar membrane.

structures seated on the basilar membrane and forming the organ
of Corti; the rest of its cavity is filled with endolymph, which has
free passage to that in the sacculus.

The Organ of Corti. This contains the end organs of the cochlear
nerves. Lining the sulcus spiralis are cuboidal cells; on the inner
margin of the basilar membrane the cells become columnar, and
then are succeeded by a row which bear on their upper ends a set of

A B

4' **

Fio. 74.— The rods of Corti. A, a pair of rods separated from the rest; B, a bit
of the basilar membrane with several rods on it, snowing how they cover in the
tunnel of Corti; i, inner, and e, outer rods ; b, basilar membrane : r, recticular mem-
brane.

short stiff hairs, and constitute the inner hair-cells, which are fixed
below by a narrow apex to the basilar membrane; nerve-fibers
enter them. To the inner hair-cells succeed the rods of Corti (Co,
Fig. 73), which are represented much magnified in Fig. 74. These

THE EAR, HEARING, TASTE AND SMELL 193

rods arc stiff and arranged side by side in two rows, leaned against
one another by their upper ends so as to cover in a tunnel; they are
known respectively as the inner and outer rods, the former being
nearer the lamina spiralis. Each has a somewhat dilated base,
firmly fixed to the basilar membrane; an expanded head where it
meets its fellow (the inner rod presenting there a concavity into
which the rounded head of the outer fits); and a slender shaft
uniting the two, slightly curved like an italic/. The inner rods are
more slender and more numerous than the outer, the numbers be-
ing about 6,000 and 4,500 respectively. Attached to the external
sides of the head of the outer rods is the reticular membrane (r,
Fig. 74), which is stiff and perforated by holes. External to the
outer rods come four rows of outer hair-cells, connected like the
inner row with nerve-fibers; their bristles project into the holes of
the reticular membrane. Beyond the outer hair-cells is ordinary
columnar epithelium, which passes gradually into cuboidal cells
lining most of the membranous cochlea. The upper lip of the
sulcus spiralis is uncovered by epithelium, and is known as
the limbus lamina spiralis; from it projects the tectorial mem-
brane (t, Fig. 73) which extends over the rods of Corti and
the hair-cells.

The Loudness, Pitch, and Timbre of Sounds. Sounds, as sensa-
tions, fall into two groups — notes and noises. Physically, sounds
consist of vibrations, and these, under most circumstances, when
they first reach our auditory organs, are alternating rarefactions
and condensations of the air, or aerial waves. When the waves fol-
low one another uniformly, or periodically, the resulting sensation
(if any) is a note; when the vibrations are aperiodic it is a noise.
In notes we recognize (1) loudness or intensity; (2) pitch; (3) qual-
ity or timbre, or, as it has been called, tone color; a note of a given
loudness and pitch produced by a flute and by a violin has a dif-
ferent character or individuality in each case; this quality is its
timbre. Before understanding the working of the auditory mech-
anism we must get some idea of the physical qualities in ob-
jective sound of which the subjective differences of auditory
sensations are signs.

The loudness of a sound depends on the force of the aerial waves;
the greater the intensity of the alternating condensations and rare-
factions of these in the external auditory meatus, the louder the

194 THE HUMAN BODY

sound. The pitch of a note depends on the length of the waves,
that is, the distance from one point of greatest condensation to the
next, or (what amounts to the same thing) on the number of waves
reaching the ear in given time, say a second. The shorter the
waves the more rapidly they follow one another, and the higher the
pitch of the note. When audible vibrations bear the ratio 1 : 2 to
one another, we hear the musical interval called an octave. The
middle C of the musical scale is due to 256 vibrations per second.
Its octave has 512 vibrations.

Sound vibrations may be too rapid or too slow in succession to
produce sonorous sensations. The highest-pitched audible note
answers to about 38,000 vibrations in a second, but it differs in
individuals; many persons cannot hear the cry of a bat nor the
chirp of a cricket, which lie near this upper audible limit. On the
other hand, sounds of vibrational rate about 40 per second are not
well heard, and a little below this become inaudible. The highest
note used in orchestras is the cT of the fifth accented octave, pro-
duced by the piccolo flute, due to 4,752 vibrations in a second;
and the lowest-pitched is the Ev of the contra octave, produced
by the double bass. Modern grand pianos and organs go down to
C, in the contra octave (33 vibrations per second) or even A",
(27J), but the musical quality of such notes is imperfect; they pro-
duce rather a " hum " than a true tone sensation, and are only used
along with notes of higher octaves to which they give a character
of greater depth.

Timbre. Since the loudness of a tone depends on the vibrational
amplitude of its physical antecedent, and its pitch on the vibra-
tional rate, we have still to seek the cause of timbre; the quality by
which we recognize the human voice, the violin, the piano, and the
flute, even when all sound the same note and of the same loudness.
Helmholtz showed that the quality of any tone is determined by
the particular overtones or harmonic partials that are combined in
it with the fundamental tone. Most vibrating bodies are able to
vibrate both as a whole and in sections. Since the sections are
smaller than the whole body their vibrations are more rapid than
those of the body as a whole. The vibrating sections may be
halves, thirds, fourths, or any other fraction of the whole body.
Also one and the same body may be vibrating at once in halves,
quarters, and several other smaller divisions. These vibrations

THE EAR, HEARING, TASTE AND SMELL 195

in parts are the sources of overtones, the pilch of the tone being
determined by its vibration as a whole, the so-called fundamental
vibration.

The air waves set in motion by a body vibrating in such com-
plex fashion must necessarily be themselves very complex. Since
they are periodic, however, they produce audible notes, if rapid
and intense enough. The actual form of air wave which proceeds
from a body vibrating thus depends upon the particular com-
ponents which make it, and it has been shown that any complex
periodic vibration can be analyzed mathematically into its con-
stituents, and these unerringly determined. The timbre of a tone
depends, then, according to our former definition, upon the form of
air wave which enters the ear. A tone composed of a fundamental
and three overtones will come to the ear as a wave having quite a
different form from one having in addition to the fundamental
five partials.

Whereas we ordinarily hear compound tones merely as tones of
certain quality, the trained ear is able to hear and pick out the
overtones by which the quality is determined. It is evident,
therefore, that the ear is able to analyze compound tones into their
individual constituents.

Sympathetic Resonance. Imagine slight taps to be given to a
pendulum; if these be repeated at such intervals of time as al-
ways to help the swing and never to retard it, the pendulum will
soon be set in powerful movement. If the taps are irregular, or
when regular come at such intervals as sometimes to promote and
sometimes retard the movement, no great swing will be produced;
but if they always push the pendulum in the way it is going at that
instant, they need not come every swing in order to set up a pow-
erful vibration; once in two, three, or four swings will do. A
stretched string, such as that of a piano, is so far like a pendulum
that it tends to vibrate at one rate and no other; if aerial waves hit
it at exactly the right times they soon set it in sufficiently power-
ful vibrations to cause it to emit an audible note. By using such
strings we can analyze compound tones and thus prove objectively
that they are made up of partials. If the dampers of a piano be
raised and a note be sung loudly to it, it will be found that several
strings are set in vibration, such vibrations being called sympa-
thetic. The human voice emits compound tones which can be

196 THE HUMAN BODY

mathematically analyzed into simple vibrations, and if the piano
strings set in movement by it be examined, they will be found to
be exactly those which answer to these vibrations and to no
others. We thus get experimental grounds for believing that com-
pound tones are really made up of a number of simple vibrations,
and get an additional justification for the supposition that in
the ear each note is analyzed into its components; and that the
difference of sensation which we call timbre is due to the effect of
the secondary partial tones thus perceived. If so, the ear must
have in it an apparatus adapted for sympathetic resonance.

The Functions of the Tympanic Membrane. If a stretched
membrane, such as a drumhead, be struck, it will be thrown into
periodic vibration and emit for a time a note of a determined pitch.
The smaller the membrane and the tighter it is stretched the higher
the pitch of its note ; every stretched membrane thus has a rate of
its own at which it tends to vibrate, just as a piano or violin string
has. When a note is sounded in the air near such a membrane, the
alternating waves of aerial condensation and rarefaction will move
it ; and if the waves succeed at the vibrational rate of the membrane
the latter will be set in powerful sympathetic vibration; if they do
not push the membrane at the proper times, their effects will
neutralize one another: hence such membranes respond well to
only one note. The tympanic membrane, however, responds
equally well to a large number of notes ; at the least for those due
to aerial vibrations of rates from 60 to 4,000 per second, running
over eight octaves and constituting those commonly used in
music. This faculty depends on two things: (1) the membrane is
comparatively loosely and not uniformly stretched ; (2) it is loaded
by the tympanic bones.

The drum-membrane is a shallow funnel with its sides convex
towards the external auditory meatus; something like an umbrella
turned inside out ; in such a membrane the tension is not uniform
but increases towards the center, and it has accordingly no proper
note of its own. Further, whatever tendency such a membrane
may have to vibrate rather at one rate than another, is almost com-
pletely removed by "damping" it, i.e. placing in contact with it
something comparatively heavy and which has to be moved when
the membrane vibrates. This is effected by the tympanic bones,
fixed to the drum-membrane by the handle of the malleus. An-

THE EAR, HEARING, TASTE AND SMELL 197

other advantage is gained by the damping; once a stretched mem-
brane is set vibrating it continues so doing for some time; but if
loaded its movements cease almost as soon as the moving impulses.
The dampers of a piano are for this purpose; and violin-players
have to " damp " with the fingers the strings they have used when
they wish the note to cease. The tympanic bones act as dampers.

Functions of the Auditory Ossicles. When the air in the ex-
ternal auditory meatus is condensed it pushes in the tympanic
membrane which carries with it the handle of the malleus. This
bone then slightly rotates on the axial ligament and, locking
into the incus where the two bones articulate, causes the long
process (Jl, Fig. 70) of the latter to move inwards. The incus
thus pushes in the stapes; the reverse occurs when air in the au-
ditory passage is rarefied. Aerial vibrations thus set the chain of
bones swinging, and push in and pull out the base of the stapes,
which sets up waves in the perilymph of the labyrinth, and these
are transmitted through the membranous labyrinth to the endo-
lymph. • These liquids being chiefly water, and practically incom-
pressible, the end of the stapes could not work in and out at the
oval foramen, were the labyrinth elsewhere completely surrounded
by bone : but the membrane covering the round foramen bulges out
when the base of the stapes is pushed in, and vice versa; and so
allows of waves being set up in the labyrinthic liquids. These
correspond in period and form to those in the auditory meatus;
their amplitude is determined by the extent of the vibrations of
the drum membrane.

The form of the tympanic membrane causes it to transmit to its
center, where the malleus is attached, vibrations of its lateral
parts in diminished amplitude but increased power; so that the
tympanic bones are pushed only a little way but with considerable
force. Its area, too, is about twenty times as great as that of the
oval foramen, so that force collected on the large area is, by push-
ing the tympanic bones, all concentrated on the smaller. The
ossicles also form a bent lever (Fig. 70) of which the fulcrum is at
the axial ligament and the effective outer arm of this lever is about
half as long again as the inner, and so the movements transmitted
by the drum-membrane to the handle of the malleus are com-
municated with diminished range, but increased power, to the base
of the stapes.

198 THE HUMAN BODY

Ordinarily, sound-waves reach the labyrinth through the
tympanum, but they may also be transmitted through the bones
of the head ; if the handle of a vibrating tuning-fork be placed on
the vertex, the sounds heard by the person experimented upon
seem to have their origin inside his own cranium. Similarly, when
a vibrating body is held between the teeth, sound reaches the end
organs of the auditory nerve through the skull-bones ; and persons
who are deaf from disease or injury of the tympanum can thus
be made to hear, as with the audiphone. Of course if deafness be
due to disease of the proper nervous auditory apparatus no device
can make the person hear.

Function of the Cochlea. We have already seen reason to be-
lieve that in the ear there is an apparatus adapted for sympathetic
resonance, by which we recognize different musical tone colors; the
minute structure of the membranous cochlea is such as to lead us to
look for it there. Of the various structures making up the mem-
branous cochlea the basilar membrane seems to satisfy best the
requirements of an apparatus for registering sounds by sympa-
thetic resonance. It increases in breadth twelve times from the
base of the cochlea to its tip (the less width of the lamina spiralis
at the apex more than compensating for the less size of the bony
tube there) . Careful histological examination has shown that in-
stead of being a true membrane it is really made up of a large
number of transverse strands tightly stretched, and varying in
length as the space between the lamina spiralis and the wall of the
bony cochlea varies.

Probably each strand vibrates to simple tones of its own period,
and excites the hair-cells which lie on it, and through them the
nerve-fibers. Perhaps the rods of Corti, being stiff, and carrying
the reticular membrane, rub that against the upper ends of the
hair-cells which project into its apertures and so help in a sub-
siduary way, each pair of rods being especially moved when the
band of basilar membrane carrying it is set in vibration. The
tectorial membrane is probably a " damper " ; it is soft and in-
clastic, and suppresses the vibrations as soon as the moving force
ceases.

It is said that eleven thousand different tones can be distin-
guished in the whole range of the ear. The basilar membrane is
more than adequate to distinguish this number as it consists of

THE EAR, HEARING, TASTE AND SMELL 199

twenty-four thousand strands. Fourteen thousand nerve-fibers
communicate with the hair-cells of the organ of Corti.

We must suppose that compound tones entering the ear set the
fluids of the cochlea into vibrations whose form depends upon the
make-up of the tone producing them. These vibrations are
analyzed by the basilar membrane, the particular strands having
the vibration rates of the fundamental and the partials which are
present being set into sympathetic vibration and stimulating the
nerve-fibers with which they communicate.

Auditory Perceptions. Sounds, as a general rule, do not seem
to us to originate within the auditory apparatus; we refer them to
an external source, and to a certain extent can judge the distance
and direction of this. As already mentioned, the extrinsic reference
of sounds which reach the labyrinth through the general skull-
bones instead of through the tympanic chain is imperfect or
absent. The recognition of the distance of a sounding body is pos-
sible only when the sound is well known, and then not very accu-
rately; from its faintness or loudness we may make in some cases a
pretty good guess. Judgments as to the direction of a sound are
also liable to be grossly wrong, as most persons have experienced.
However, when a sound is heard louder by the left than the right
ear we can recognize that its source is on the left; when equally
with both ears, that it is straight in front or behind; and so on.
The concha has perhaps something to do with enabling us to detect
whether a sound originates before or behind the ear, since it col-
lects, and turns with more intensity into the external auditory
meatus, sound-waves coming from the front. By turning the head
and noting the accompanying changes of sensation in each ear we
can localize sounds better than if the head be kept motionless. The
large movable concha of many animals, as a rabbit or a horse,
which can be turned in several directions, is probably an important
aid to them in detecting the position of the source of a sound. That
the recognition of the direction of sounds is not a true sensation,
but a judgment, founded on experience, is illustrated by the fact
that we can estimate much more accurately the direction of the
human voice, which we hear and heed most, than that of any other
sound.

Nerve-Endings in the Semicircular Canals and the Vestibule.
Medullated fibers (/, Fig. 75) from the vestibular branch of the

THE HUMAN BODY

auditory nerve are distributed along a line across the ampulla of
each semicircular canal. They lose their medullary sheath close
to the basement membrane, a, which the axons pierce. The

axons branch among the epithelium
cells, which at this place are sev-
eral rows thick, but have not yet
been traced into direct continuity
with any of them. The cells of the
epithelium are of two varieties.
The columnar cells or hair-cells, c,
do not reach the basement mem-
brane, are nucleated or slightly
granular: from the free end of each
projects a rigid hair process, d.
The remaining cells, rod-cells, b,

. FIG. 75.-Diagram of epithelium are in Several r°WS: each has a

in nervous region of ampulla of a slender inner process extending to

semicircular canal.

the basement membrane and an

outer which reaches to the bases of the columnar cells and appears
there to end in a rigid membrane, e, which is perforated for the
passage of the hairs. They probably are mere supporting struc-
tures.

In some parts of the utricle and saccule are regions of epithelium
very similar to that above described, and also supplied with nerve-
fibers. In connection with them are found minute calcareous
particles, — otoliths or ear-stones.

The Equilibrium Sense. An important group of afferent im-
pulses concerned with the maintenance of bodily equilibrium is
derived through the semicircular canals and vestibule of the ear,
which are supplied by the vestibular portion of the auditory nerve.

Experiment shows that cutting a semicircular canal is followed
by violent movements of the head in the plane of the canal di-
vided; the animal staggers, also, if made to walk; and, if a pigeon
and thrown into the air, cannot fly. All its muscles can contract
as before, but they are no longer so coordinated as to enable the
animal to maintain or regain a position of equilibrium. It is like
a creature suffering from giddiness; and similar phenomena fol-
low, in man, electrical stimulation of the regions of the skull in
which the semicircular canals lie.

THE EAR, HEARING, TASTE AND SMELL 201

If, moreover, a person lie perfectly quiet with closed eyes on
a table which can be rotated, he is able to tell when the table is
turned and in which direction, and often with considerable ac-
curacy through what angle. If the rotation be continued for a
time the feeling of it is lost, and then when the movement ceases
there is a sense of rotation in the opposite direction. In such
case neither tactile, muscular, nor visual sensations can help, and
in the semicircular canals we seem to have a mechanism through
which rotation of the head could give origin to afferent impulses,
whether the head be passively moved with the rest of the Body
or independently by its own muscles. Movements of endolymph
in relation to the walls of the canals may act as stimuli by caus-
ing a swaying of the projecting hairs of the ampullsB (Fig. 75).
Place a few small bits of cork in a tumbler of water, and rotate
the tumbler; at first the water does not move with it; then it be-
gins to go in the same direction, but more slowly; and, finally,
moves at the same angular velocity as the tumbler. Then stop
the tumbler, and the water will go on rotating for some time.
Now if the head be turned or rotated in a horizontal plane simi-
lar phenomena will occur in the endolymph of the horizontal
canal; if it be bent sidewise in the vertical plane, in the ante-
rior vertical canal; and if nodded, in the posterior vertical; the
hairs moving with the canal, would meet the more stationary
water and be pushed and so, possibly, excite the nerves at the
deep ends of the cells which bear them, and generate afferent
impulses which will cause the general nerve-centers of bodily
equilibration to be differently acted upon in each case. Under
ordinary circumstances the results of these impulses do not be-
come prominent in consciousness as definite sensations; but they
are probably always present. If one spins round for a time, the
endolymph takes up the movement of the canals, as the water in
the tumbler does that of the glass; on stopping, the liquid still
goes on moving and stimulates the hairs which are now stationary;
and we feel giddy, from the ears telling us we are rotating and the
eyes that we are not; hence difficulty in standing erect or walking
straight. A common trick illustrates this very well: make a per-
son place his forehead on the handle of an umbrella, the other
end of which is on the floor, and then walk three or four times
round it, rise, and try to go out of a door; he will nearly always

202 THE HUMAN BODY

fail, being unable to combine his muscles properly on account of
the conflicting afferent impulses. This and the feeling of rotation
in the contrary direction when a previous rotation ceases become
readily intelligible if we suppose feelings to be excited by relative
movements of the endolymph and the canals inclosing it.

The sense of equilibrium as mediated by the semicircular canals
is a dynamic sense, one dealing with equilibrium of motion. That
we have also a static sense of equilibrium, which tells us our posi-
tion when at rest is well known. The swimmer immersed in
water knows perfectly whether he is on his face or on his back;
whether his head is up or down. This static equilibrium sense is
thought to be mediated by structures of the vestibule, the utricle
and saccule. These are hollow structures having stiff hairs pro-
jecting into their cavities and tiny stones caught among the hairs.
The weight of the stones will affect the hairs among which it rests
in one way when the head is erect, in quite another way when
the head is horizontal. Thus the nerves may be stimulated dif-
ferently for different positions of the head, fulfilling the conditions
that the sense requires. In many invertebrate animals structures
similar to the utricle and saccule represent their only organs re-
sembling our ears in any way. Experiments upon these animals
have shown that in them these structures are not hearing organs
but organs of equilibrium.

Smell. The region of the nostril nearest its outer end possesses
the sense of touch : the olfactory organ proper consists of the upper
portions of the two nasal cavities, over which the endings of the
olfactory nerves are spread and where the mucous membrane has
a brownish-yellow color, This region (regio olfactoria) covers the
upper and lower turbinate bones, which are expansions of the
ethmoid on the outer wall of the nostril chamber, the opposite
part of the partition between the nares, and the part of the roof
of the nose separating it from the cranial cavity. The epithelium
covering the mucous membrane contains three varieties of cells
(2, Fig. 76) . The cells of one set are much like ordinary columnar
epithelium, but with long branched processes attached to their
deeper ends; mixed with these are peculiar cells, each of which
has a large nucleus surrounded by a little protoplasm; a slender
external process reaching to the surface; and a very slender deep
one. The latter cells have been supposed to be the proper olfac-

THE EAR, HEARING, TASTE AND SMELL

203

tory end organs, and to be connected with the fibers of the ol-
factory nerve, which enter the deeper strata of the epithelium
and there divide. In Amphibia the corresponding cells have fine
filaments on their free ends. The cells
of the third kind are irregular in form
and lie in several rows in the deeper
parts of the epithelium. It may be
that the cylindrical cells if not (as is
possible) directly concerned in olfac-
tion, have important functions in re-
gard to the nourishment of the olfac-
tory cells which they surround; they
may supply them with needful ma-
terial.

Odorous substances, the stimuli of
the olfactory apparatus, are always
gaseous and frequently act powerfully
when present in very small amount.
We cannot, however, classify them by
the sensations they arouse, or arrange
them in series: and smells are but

, . FIG. 76. — Cells from the ol-

mmor sensory factors in our mental factory epithelium, i, from the
life, although very powerful* associa- Sf &VK3SF3Z
tions of memory are often aroused by process; b, so-called olfactory

J cell ; c, its narrow outer process ;

Odors. We Commonly refer them to d, its slender central process,

i , . . f, , ,1 , ,, 3, gray nerve-fibers of the olfac-

CXternal Objects, Since We find that the tory nerve, seen dividing into

Sensation is intensified by "sniffing" fine peripheral branches at a.

air into the nose, and ceases when the nostrils are closed. Their
peripheral localization is, however, imperfect, for we confound
many smells with tastes (see below) ; nor can we well judge of
the direction of an odorous body through the olfactory sensations
which it arouses.

Although the sense of smell in man is aroused by inconceivably
small amounts of odoriferous substance, one part of mercaptan
to thirty billion of air being detectible, it is much less keen than
the sense of smell in many animals, canines in particular. In
such animals the sense of smell as a source of information seems
to be of the first importance, approaching our eyes in rank.

A striking thing about the sense of smell is the ease with which

204

THE HUMAN BODY

it is fatigued. One may notice a bad odor upon entering a room,
but in a few minutes ceases to perceive it because his olfactory
apparatus has become fatigued. For this reason the sense of
smell is wholly untrustworthy as a guide by which to regulate
the ventilation of a room.

Taste. The organ of taste is the mucous membrane on the
dorsum of the tongue * and, in some persons, of the soft palate
and fauces. The nerves concerned are the glossopharyngeals,
distributed over the hind part of the tongue, and the lingual
branches of the. inferior maxillary division of the trigeminals
on its anterior two-thirds. It has been shown that the nerves
of taste which reach the tongue by way of the trigeminal
nerve spring from the medulla as part of the sensory branch
of the facial.

On the tongue most of the sensory nerves run to papillae; the
circumvallate have the richest supply, and on these are peculiar

FIG. 77.— Taste-buds.

end organs (Fig. 77) known as taste-buds; they are oval and em-
bedded in the epidermis covering the side of the papilla. Each
consists, externally, of a number of flat, fusiform, nucleated cells
and, internally, of six or eight so-called taste-cells. The latter are
much like the olfactory cells of the nose, and are probably con-
nected with nerve-fibers at their deeper ends. The capsule formed
by the enveloping cells has a small opening on the surface; each
taste-cell terminates in a very fine thread which there protrudes.
Taste-buds are also found on some of the fungiform papillae, and
it is possible that simpler structures, not yet recognized, and con-

* A description of the tongue will be found on p. 400.

THE EAR, HEARING, TASTE AND SMELL 205

sisting of single taste-cells are widely spread over the tongue,
since the sense of taste exists where no taste-buds can be found.
The filiform papillae are probably tactile.

In order for substances to be tasted they must be in solution:
wipe the tongue dry and put a crystal of sugar on it; no taste will
be felt until exuding moisture has dissolved some of the crystal.
Excluding the feelings aroused by acid substances, tastes proper
may be divided into sweet, bitter, acid, and saline. Although con-
tributing much to the pleasures of life, they are intellectually of
small value ; the perceptions we attain through them as to quali-
ties of external objects being of little use, except as aiding in the
selection of food, and for that purpose they are not safe guides at
all times.

Many so-called tastes (flavors) are really smells; odoriferous
particles of substances which are being eaten reach the olfactory
region through the posterior nares and arouse sensations which,
since they accompany the presence of objects in the mouth, we
take for tastes. Such is the case, e. g., with most spices; when the
nasal chambers are blocked or inflamed by a cold in the head, or
closed by compressing the nose, the so-called taste of spices is not
perceived when they are eaten; all that is felt, when cinnamon,
e. g., is chewed under such circumstances is a certain pungency
due to its stimulating nerves of touch in the tongue. This fact
is sometimes taken advantage of in the practice of domestic
medicine when a nauseous dose, as rhubarb, is to be given to a
child.

As the tongue, in addition to taste functions, possesses tactile
and temperature sensibility, its nerve apparatus must be complex;
and there is even reason to believe that different nerve-fibers
with presumably different end organs are concerned in the differ-
ent true tastes. Most persons taste bitter things better with the
back part of the tongue and sweet things with the tip, and in
some persons the separation of function is quite complete. Chem-
ical compounds are known which in such persons cause a pure
sweet sensation if placed on the tongue tip and a pure bitter sen-
sation if placed in the region of the circumvallate papillae; these
facts seem to show that the fibers concerned in bitter and sweet
sensation are distinct. Again, if leaves of a certain plant (Gym-
nema sylvestre) be chewed, the capacity to taste sweet or bitter

206 THE HUMAN BODY

things is lost for some time, but salts and acids are tasted as well
as usual; and most persons taste salines better at the sides of the
tongue than elsewhere; so that the salt and acid sensations seem
to have a different apparatus, not only from the sweet and bitter,
but from one another.

CHAPTER XV
THE EYE AS AN OPTICAL INSTRUMENT

The Essential Structure of an Eye. Every visual organ con-
sists primarily of a nervous expansion, provided with end organs
by means of which light is enabled to excite nervous impulses,
and exposed to the access of objective light; such an expansion is
called a retina. By itself, however, a retina would give no visual
sensations referable to distinctly limited external objects; it
would enable its possessor to tell light from darkness, more light
from less light, and (at least in its highly developed forms) light
of one color from light of another color; but that would be all.
Were our eyes merely retinas we could only tell a printed page
from a blank one by the fact that, being partly covered with
black letters (which reflect less light), it would excite our visual
organ less powerfully than the spotless white page would. In
order that distinct objects and not merely degrees of luminosity
may be seen, some arrangement is needed which shall bring all
light entering the eye from one point of a luminous surface to a
focus again on one point of the sensitive surface. If A and B
(Fig. 78) be two red spots on a black surface, K, and rr be a ret-
ina, then rays of light diverging from A would fall equally on all
parts of the retina and excite it all a little; so with rays starting
from B. The sensation aroused, supposing the retina in connec-
tion with the rest of the nervous visual apparatus, would be one
of a certain amount of red light reaching the eye; the red spots,
as definite objects, would be indistinguishable. If, however, a
convex glass lens L (Fig. 79) be put in front of the retina, it will
cause to converge again to a single point all the rays from A fall-
ing upon it; so, too, with the rays from B: and if the focal distance
of the lens be properly adjusted these points of convergence will
both lie on the retina, that for rays from A at a, and that for rays
from B at b. The sensitive surface would then only be excited at
two limited and separated points by the red light emanating from
the spots; consequently only some of its end organs and nerve-

207

208

THE HUMAN BODY

fibers would be stimulated and the result would be the recognition
of two separate red objects. In our eyes there are certain refract-

Fia. 78. — Diagram illustrating the indistinctness of vision with a retina alone.
K, a surface on which are two spots, A and B; r r, the retina. The diverging lines
represent rays of light spread uniformly over the retina from each spot.

ing media which lie in front of the retina and take the place of the
lens L in Fig. 79. That portion of physiology which treats of the

FIG. 79. — Illustrating the use of a lens in giving definite retinal images. A, B, K,
r r, as in Fig. 78. L, a biconvex lens so placed that it brings to a focus on the
points a and b of the retina, rays of light diverging from A and B respectively.

physical action of these media or, in other words, of the eye as an
optical instrument, is known as the dioptrics of the eye.

The Appendages of the Eye. The eyeball itself consists of the
retina and refracting media, together with supporting and nutri-
tive structures and other accessory apparatuses, as, for example,
some controlling the light-converging power of the media, and
others regulating the size of the aperture (pupil) by which light
enters. Outside the ball lie muscles which bring about its move-
ments, and other parts serving to protect it.

Each orbit is a pyramidal cavity occupied by connective tissue,
muscles, blood-vessels, and nerves, and in great part by fat, which
forms a soft cushion on which the back of the eyeball lies and
rolls during its movements. The contents of the orbit being for
the most part incompressible, the eye cannot be drawn into its

THE EYE AS AN OPTICAL INSTRUMENT . 209

socket. It simply rotates there, as the head of the femur does in
the acetabulum. When the orbital blood-vessels are gorged,
however, the eyeball may protude (as in strangulation) ; and when
these vessels empty it recedes somewhat, as is commonly seen
after death. The front of the eye is exposed for the purpose of
allowing light to reach it, but can be covered up by the eyelids,
which are folds of integument, movable by muscles and strength-
ened by plates of fibrocartilage. At the edge of each eyelid the
skin which covers its outside is turned in, and becomes continu-
ous with a mucous membrane, the conjunctiva, which lines the
inside of each lid, and also covers all the front of the eyeball as a
closely adherent layer.

The upper eyelid is larger and more mobile than the lower, and
when the eye is closed covers all its transparent part. It has a
special muscle to raise it, the levator palpebrce superioris. The
eyes are closed by a flat circular muscle, the orbicularis palpebra-
rum which, lying on and around the lids, immediately beneath
the skhij surrounds the aperture between them. At their outer
and inner angles (canthi) the eyelids are united, and the apparent
size of the eye depends upon the interval between the canthi, the
eyeball itself being nearly of the same size in all persons. Near
the inner canthus the line of the edge of each eyelid changes its
direction and becomes more horizontal. At this point is found a
small eminence, the lachrymal' papilla, on each lid. For most of
their extent the inner surfaces of the eyelids are in contact with
the outside of the eyeball, but near their inner ends a red vertical
fold of conjunctiva, the semilunar fold (plica semilunaris) inter-
venes. This is a representative of the third eyelid, or nictitating
membrane, found largely developed in many animals, as birds, in
which it can be drawn all over the exposed part of the eyeball.
At the inner or nasal corner is a reddish elevation, the caruncula
lachrymalis, caused by a collection of sebaceous glands * embedded
in the semilunar fold. Opening along the edge of each eyelid are
from twenty to thirty minute compound sebaceous glands, named
the Meibomian follicles. Their secretion is sometimes abnormally
abundant, and then appears as a yellowish matter along the edges
of the eyelids, which often dries in the night and causes the lids
to be glued together in the morning. The eyelashes are short
* For a description of the glands see p. 479.

210 THE HUMAN BODY

curved hairs, arranged in one or two rows along each lid where
the skin joins the conjunctiva.

The Lachrymal Apparatus consists of the tear-gland in each
orbit, the ducts which carry its secretion to the upper eyelid,
and the canals by which the tears, unless when excessive, are
carried off from the front of the eye without running down over
the face. The lachrymal or tear-gland, about the size of an almond,
lies in the upper and outer part of the orbit, near the front end.
It is a compound racemose gland, (see Chap. XXIX) from which
twelve or fourteen ducts run and open in a row at the outer corner
of the upper eyelid. The secretion there poured out, is spread
evenly over the exposed part of the eye by the movements of
winking, and keeps it moist; finally the tear is drained off by two
lachrymal canals, one of which opens by a small pore (punctum
lachrymalis) on each lachrymal papilla. The aperture of the lower
canal can be readily seen by examining the corresponding papilla
by the aid of a looking-glass. The canals run inwards and open
into the lachrymal sac, which lies just outside the nose, in a hollow
where the lachrymal and superior maxillary bones (L and MX,
Fig. 28) meet. From the sac the nasal duct proceeds to open into
the nose-chamber, below the inferior turbinate bone and within
the nostril.

Tears are constantly being secreted, but ordinarily in such
quantity as to be drained off into the nose, from which they flow
into the pharynx and are swallowed. When the lachrymal ducts
are stopped up, however, their continual presence makes itself
unpleasantly felt, and may need the aid of a surgeon to clear the
passage. In weeping the secretion is increased, and then not only
more of it enters the nose, but some flows down the cheeks. The
frequent swallowing movements of a crying child, sometimes
spoken of as " gulping down his passion, " are due to the need
of swallowing the extra tears which reach the pharynx.

The Muscles of the Eye (Fig. 80). The eyeball is spheroidal
in form and attached behind to the optic nerve, n, somewhat as
a cherry might be to a thick stalk. On its exterior are inserted
the tendons of six muscles, four straight and two oblique. The
straight muscles lie, one (superior rectus}, s, above, one (inferior
rectus), not appearing in the figure, below, one (external rectus},
a, outside, and one (internal rectus), i, inside the eyeball. Each

THE EYE AS AN OPTICAL INSTRUMENT 211

arises behind from the bony margin of the foramen through which
the optic nerve enters the orbit. In the figure, which represents
the orbits opened from above, the superior rectus of the right
side has been removed. The superior oblique or pulley (trochlear)
muscle, t, arises behind near the straight muscles and forms an-
teriorly a tendon, u, which passes through a fibrocartilaginous
ring, or pulley, placed at the notch in the frontal bone where it

Fio. 80. — The eyeballs and their muscles as seen when the roof of the orbit
has been removed and the fat in the cavity has been partly cleared away. On the
right side the superior rectus muscle has been cut away, a, external rectus; s, su-
perior rectus; i, internal rectus; t, superior oblique.

bounds superiorly the front end of the orbit. The tendon then
turns back and is inserted into the eyeball between the upper and
outer recti muscles. The inferior oblique muscle does not arise,
like the rest, at the back of the orbit, but near its front at the
inner side, close to the lachrymal sac. It passes thence outwards
and backwards beneath the eyeball to be inserted into its outer
and posterior part.

The inner, upper, and lower straight muscles, the inferior
oblique, and the elevator of the upper lid are supplied by branches
of the third cranial nerve. The sixth cranial nerve goes to the
outer rectus ; and the fourth to the superior oblique.

The eye may be moved from side to side; up or down; obliquely,

212 THE HUMAN BODY

that is, neither truly vertically nor horizontally, but partly both;
or, finally, it may be rotated on its' anteroposterior axis. The
oblique movements are always accompanied by a slight amount
of rotation. When the glance is turned to the left, the left external
rectus and the right internal contract, and vice versa; when up,
both superior recti; when down, both the inferior. The superior
oblique muscle acting alone will roll the front of the eye down-
wards and outwards with a certain amount of rotation; the infe-
rior oblique does the reverse. In oblique movements two of the
recti are concerned, an upper or lower with an inner or outer; at
the same time one of the oblique also always contracts. Move-
ments of rotation rarely, if ever, occur alone.

The natural combined movements of the eyes by which both
are directed simultaneously towards the' same point depends on
the accurate adjustment of all its nervo-muscular apparatus.
When the coordination is deficient the person is said to squint.
A left external squint would be caused by paralysis of the inner
rectus of that eye, for then, after the eyeball had been turned
out by the external rectus, it would not be brought back again
to its median position. A left internal squint would be caused,
similarly, by paralysis of the left external rectus; and probably
by disease of the sixth cranial nerve or its brain-centers. Drop-
ping of the upper eyelid (ptosis) indicates paralysis of its special
elevator muscle and is often a serious symptom, pointing to
disease of the brain-parts from which it is innervated.

The Globe of the Eye is on the whole spherical, but consists of
segments of two spheres (see Fig. 81), a portion of a sphere of
smaller radius forming its anterior transparent part and being
set on to the front of its posterior segment, which is part of a
larger sphere. From before back it measures about 22.5 milli-
meters (j5 inch), and from side to side about 25 millimeters (1
inch). Except when looking at near objects, the anteroposterior
axes of the eyeballs are nearly parallel, though the optic nerves
diverge considerably (Fig. 80); each nerve joins its eyeball, not
at the center, but about 2.5 mm. (^ inch) on the nasal side of the
posterior end of its anteroposterior axis. In general terms the
eyeball may be described as consisting of three coats and three
refracting media.

The outer coat, 1 and 3, Fig. 81, consists of the sclerotic and the

THE EYE AS AN OPTICAL INSTRUMENT 213

cornea, the latter being transparent and situated in front; the
former is opaque and white and covers the back and sides of the
globe and part of the front, where it is seen between the eyelids

FIG. 81. — The left eyeball in horizontal section from before back. 1, sclerotic;
2, junction of sclerotic and cornea; 3, cornea; 4, 5, conjunctiva; 7, ciliary muscle;
10, choroid; 11, 13, ciliary processes; 14, iris; 15, retina; 16, optic nerve; 17, artery
entering retina in optic nerve; 18, fovea centralis; 19, 20, region where sensory part
of retina ends; 22, suspensory ligament; 24, the anterior part of the hyaloid mem-
brane; 26, the lens; 29, vitreous numor; 30, aqueous humor.

as the white of the eye. Both are tough and strong, being com-
posed of dense connective tissue.

The second coat consists of the choroid, 10, the ciliary proc-
esses, 11, 13, and the iris, 14. The choroid is made up of blood-
vessels supported by loose connective tissue containing numerous
corpuscles, which in its inner layers are richly filled with dark-
brown or black pigment granules. Towards the front of the eye-
ball, where it begins to diminish in diameter, the choroid is thrown
into plaits, the ciliary processes, 11, 13. Beyond these it con-
tinues as the ins, which forms the colored part of the eye seen
through the cornea; and in the center of the iris is a circular aper-
ture, the pupil: so its second coat does not, like the outer one,
completely envelop the eyeball. In the iris is a ring of plain mus-
cular tissue encircling the aperture of the pupil: when its fibers
contract they narrow the pupil.

214 THE HUMAN BODY

Radiating from this ring to the edges of the iris are muscle-
fibers which by their contraction enlarge the pupil. Both sets of
muscles are under the control of sympathetic nerves. Those to
the constrictor-fibers reach the eye by way of the third cranial
nerve; those innervating the dilator-fibers enter by way of the
ophthalmic branch of the fifth nerve. These latter fibers have a
rather tortuous connection with the central nervous system. The
pathway starts in the midbrain and passes down the cord to the
upper thoracic region where the cell-body of the preganglionic
neuron lies. The axon of this neuron passes out from the cord to
the sympathetic chain and in this chain back up the neck to the
superior cervical ganglion at the base of the skull. Here the
preganglionic neuron terminates in connection with a postgan-
glionic one. The axon of the latter passes to the fifth nerve and
along the latter to its termination in the pupillo-dilator muscle.

The iris contains pigment which is yellow, or of lighter or
darker brown, according to the color of the eye, and more or less
abundant according as the eye is black, brown, or gray. In blue
eyes the pigment is confined to the deeper layers, and modified in
tint by light absorption in the anterior colorless strata through
which the light passes.

The third coat of the eye, the retina, 15, is its essential portion,
being the part in which the light produces those changes that give
rise to impulses in the optic nerve. It is a still less complete en-
velope than the second tunic, extending forwards only as far as
the commencement of the ciliary processes, at least in its typical
form. It is extremely soft and delicate; and, when fresh, trans-
parent. Usually when an eye is opened the retina is colorless;
but when the eye has been cut open in faint yellow light and the
exposed retina quickly examined in white light it is seen to be
purple. The coloring substance (visual purple) very rapidly
bleaches when a dead eye is exposed to daylight. On the front or
inner surface of the human retina two special areas can be dis-
tinguished in a fresh eye. One is the point of entry of the optic
nerve, 16, the fibers of which, penetrating the sclerotic and cho-
roid, spread out in the retina. At this place the retina is whiter
than elsewhere and presents an elevation, the optic disk. The
other peculiar region is the fovea centralis, 18, which lies nearly
at the posterior end of the axis of the eyeball and therefore out-

THE EYE AS AN OPTICAL INSTRUMENT 215

side the optic disk; in it the retina is thinner than elsewhere
and so a pit is formed. This appears black, the thinned retina
there allowing the choroid to be seen through it more clearly than
elsewhere. In Fig. 82 is represented the left retina as seen from
the front, the elliptical darker patch about the center indicating
the fovea and the white circle on one side, the optic disk. The
vessels of the retina arise from an artery (17, Fig. 81) which runs
in with the optic nerve and from which branches diverge as shown
in Fig. 82.

The Optic Nerves, Chiasma, and Tracts. The optic nerves
converge to meet in the optic chiasma (m, Fig. 80), from which
the optic tracts pass to the region of the midbrain. They termi-
nate mainly in the anterior corpora quadrigemina, (superior col-
liculi) (Chap. IX) and in the corpora geniculata. The behavior
of the nerve-fibers in the chiasma is interesting in that part of
them cross to the opposite side and part continue into the tract
of the same side. The fibers which cross over in each optic nerve
are those, coming from the inner half of the retina, the right half
of the left retina and the left half of the right retina. The effect
of this arrangement is to include in the right optic tract, behind
the chiasma, the nerve-fibers from the right halves of both retinas,
and in the left optic tract those from the left halves of both
retinas. Cutting the right optic nerve, therefore, causes total
blindness of the right eye, but cutting the right optic tract blind-
ness of the right half of each retina (hemianopia) .

The half crossing of the optic nerve-fibers in man is correllated
with the fact that his eyes are so placed that most of the field of
vision is common to both. In mammals whose eyes are so lat-
erally placed that at any given moment the objects seen by the
two eyes are quite different, the crossing at the commissure is
complete; this condition obtains also in birds with the exception
of owls, whose eyes like those of man have their visual axes
parallel; in owls the crossing is only partial. It should be noted
that the fovea centralis, which is the center of distinct vision, has
nerve connections from both eyes with both optic tracts. For
this reason unilateral injury to the visual mechanism back of
the chiasma interferes practically not at all with ordinary vision,
and sufferers from hemianopia may be unaware of their infirmity
until careful examination by a physician reveals it.

216 THE HUMAN BODY

The Microscopic Structure of the Retina. This, the sensitive
portion of the eye, has the form of a thin membrane lining the
entire back part of the cavity of the eyeball as far forward as
the ciliary processes. Although only 0.15 millimeter (0.006 inch)

thick it presents a very complex
structure, ten distinct layers ap-
pearing upon microscopic exam-
ination. The membrane as a
whole includes supporting tissues
as well as sensitive and nervous
tissues proper; we are concerned
only with the latter and shall
confine our discussion to them.
The retina develops in such a
way that the actual sensitive
structures instead of being on
its front surface where light
would strike them immediately

FIG. 82. — The right retina as it would linnn rponViinD. fh.p rptina nrp rm
be seen if the front part of the eyeball UP°n reading tne etina, are
with the lens and vitreous humor were its posterior Surf ace, next to the

choroid coat, and interposing be-
tween themselves and the source of light the nerve structures
which connect them with the optic nerve, and the supporting
tissues and blood-vessels of the retina. Fortunately all these
structures are so transparent or so placed as not to interfere ser-
iously with vision. In the fovea, where all clear sight is located,
blood-vessels are absent and the other structures are much reduced.
The sensitive elements of the eye are called, from their shape,
rods and cones. The rods consist of basal enlarged portions from
which slender rod-like processes project toward the choroid coat.
These processes contain a peculiar reddish substance (visual pur-
ple), which has the property of bleaching out when exposed to
light (R, Fig. 83). The cones have somewhat thicker basal por-
tions than the rods and much shorter processes containing no
visual purple (C, Fig. 83). Rods and cones make up layer number
two of the ten retinal layers. The first layer, which is between
the rods and cones and the choroid coat, is a layer of pigment
cells which send processes in among the rods, and seem to have
something to do with forming the visual purple.

THE EYE AS AN OPTICAL INSTRUMENT

217

The rods and cones appear to constitute the peripheral or
dendritic portions of bipolar sensory neurons. They communi-
cate with cell-bodies from which in turn pass typical, though
very short, axons. The third retinal layer is composed of these
cell-bodies with their axons. The axons of the rod and cone
neurons come into synaptic connection with dendrites of a second

FIG. 83. — Diagram of the structure of the human retina (Greeff) : /, pigment
layer;//, rod and cone layer; R, rods; C, cones ;III-IX, intraretinal nerve-elements;
X, axons which pass to optic nerve.

set of retinal neurons, the synapses making up the fourth retinal
layer. The fifth, sixth, and seventh retinal layers contain the
cell-bodies and short axons of these second retinal neurons; in
the eighth layer these come into synaptic connection with the
dendrites of the third set of retinal neurons. The large cell-
bodies of these neurons make up the ninth retinal layer, and
their axons, converging from all parts of the retina upon the optic

218 THE HUMAN BODY

disk, constitute the tenth and front layer of the retina. These
axons continue uninterrupted to terminations in the midbrain
ganglia. The relations of the three sets of retinal neurons are
shown in the diagram (Fig. 83).

Rods and cones are not uniformly distributed over the retina.
The fovea, where distinct vision is centered, contains only cones.
The peripheral portions of the retina contain a larger and larger
proportion of rods as the margin is approached, until the outer-
most regions contain only rods. This difference of distribution
indicates a differentiation of function between the two sorts of
sensitive structures. The probability of such differentiation is
strengthened by the observation that each cone communicates
through the intervening retinal neuron with a single and sepa-
rate neuron of the optic nerve, whereas the connection of the rods
is such that several of them may send impulses into a single optic
neuron.

The blood-vessels of the retina lie almost entirely in the ninth
and tenth retinal layers.

The Refracting Media of the Eye are, in succession from before
back, the aqueous humor, the crystalline lens, and the vitreous humor.

The aqueous humor fills the space between the front of the lens,
and the back of the cornea (30, Fig. 81). Chemically, it consists
of water holding in solution a small amount of solid matters,
mainly common salt.

The crystalline lens (26, Fig. 81) is colorless, transparent, and bi-
convex, with its anterior surface less curved than the posterior. It
is surrounded by a capsule, and the inner edge of the iris lies in
contact with it in front. In consistence it is soft, but its central
layers are rather more dense than the outer.

The capsule is continuous at the margin of the lens with the
suspensory ligament which in turn is attached all around to the
ciliary processes. The suspensory ligament is stretched and its
pull upon the capsule keeps the lens more flattened than it would
be if free.

The vitreous humor (29, Fig. 81) is a soft jelly enveloped in a thin
capsule, the hyaloid membrane. It consists mainly of water and
contains some salts, a little albumin, and some mucin. It is di-
vided up, by delicate membranes, into compartments in which its
more liquid portions are imprisoned.

THE EYE AS AN OPTICAL INSTRUMENT 219

The Ciliary Muscle. (7, Fig. 81.) Between the sclerotic and
choroid coats, just where the former merges into the cornea, are
small masses of smooth muscle-fibers which make up the ciliary
muscle. These fibers are attached in front to the sclerotic coat and
pass back a short distance to an insertion in the choroid coat just
in front of the ciliary processes. The contraction of the ciliary
muscle pulls the margin of the choroid coat forward and inward.
The effect of this is to bring the ciliary processes nearer together
and loosen the suspensory ligament, which is attached to them.
The tension upon the capsule of the crystalline lens is thus di-
minished.

The ciliary muscle is interesting as being the only voluntary
muscle in the Body which is innervated through the sympathetic
system.

The Properties of Light. Before proceeding to the study of
the eye as an optical instrument, it is necessary to recall briefly
certain properties of light.

Light 'is considered as a form of movement of the particles of
an hypothetical medium, or ether, the vibrations being in planes
at right angles to the line of propagation of the light. Starting
from a luminous point light travels in all directions along the
radii of a sphere of which the point is the center; the light propa-
gated along one such radius is called a ray, and in each ray the
ethereal particles vibrate from side to side in a plane perpendic-
ular to the direction of the ray.

Any ray, all of whose particles are vibrating at the same rate,
is a ray of monochromatic light. It has a pure spectral color. The
wave length of a beam of monochromatic light is measured by the
distance between any ethereal particle of the beam and the next
one which is in precisely the same phase of vibration. Since the
rate at which light travels is nearly fixed, the wave length must
vary inversely as the vibration rate. Light of high vibration rate
has short wave length and vice versa. The color of monochro-
matic light depends upon its wave length. Where lights of va-
rious wave lengths are mixed together in a beam a compound light
results. To the eye such a beam gives a definite color sensation
but not one of the pure spectral colors.

Refraction. When light passes obliquely from one transpar-
ent medium into another of different density it is bent from ita.

220

THE HUMAN BODY

course, or refracted. The amount of refraction depends upon
the optical nature of the two media and also upon the angle at

which the ray strikes the surface
of separation. This angle, meas-
ured between the incident ray and
a line drawn at right angles to
the surface between the media, is
known as the angle of incidence.
The angle which the refracted ray
makes with this same perpendicu-
lar is the angle of refraction. If

the ray is Passing from a less re-

f

D

FIG. 84.-Diagram illustrating the

refraction of light. A B, surface of fractive to a more refractive me-

separation between two transparent .

media; C D, the perpendicular to the dlUm it IS bent toward the normal;
surface at the point of incidence; £,•»_«.» i

ax, incident ray; x d, refracted ray, " paSSHlg from a more to a leSS

tfthf^econd medium be denser than refractive medium it is bent away

the first; x g, refracted ray, if the t J

second medium is less refractive than from the normal (Fig. 84). The

amount of bending is determined

by the law of refraction which is : the ratio of the sine of the angle
of incidence to that of the angle of refraction is always constant for
the same two media and for light of the same wave length.
This ratio of sines is the index of refraction. It is usually ex-

S F^X B ^D

FIG. 85. — Diagram illustrating the dispersion of mixed light by a prism.

pressed for various refractive media with air as the second and
less refractive one.

Dispersion of Mixed Light. The shorter the vibration periods
of light-rays the more they are deviated by refraction. Hence

THE EYE AS AN OPTICAL INSTRUMENT 221

mixed light when sent through a prism is spread out, and decom-
posed into its simple constituents. For let ax (Fig. 85) be a ray
of mixed light composed of a set of short and a set of long ethereal
waves. When it falls on the surface AB of the prism, that por-
tion which enters will be refracted towards the normal ED, but
the short waves more than the longer. Hence the former will
take the direction xy, and the latter the direction xz. On emerg-
ing from the prism both rays will again be refracted, but now
from the normals Fy and Gz, since the light is passing from a
more to a less refracting medium. Again the ray xy, made up
of shorter waves, will be most deviated, as in the direction yv,
and the long waves less, in the direction zr. If a screen were put
at SS', we would receive on it at separate points, v and r, the two
simple lights which were mixed together in the compound inci-
dent ray ax. Such a separation of light-rays is called dispersion.

Ordinary white light, such as that of the sun, is composed of
ethereal vibrations of every rate, mixed together. When such
light is sent through a prism it gives a continuous band of light-
rays, known as the solar spectrum, reaching from the least refracted
to the most refracted and shortest waves. The exceptions to this
statement due to Frauenhofer's lines (see Physics) are unessential
for our present purpose. Not all the rays of the solar spectrum
are visible to the human eye. The least refracted ones, called
the ultra red, and the most refracted ones, the ultra violet, do not
stimulate the retina; they are determined by their physical and
chemical effects. The visible spectrum includes in order of in-
creasing refrangibility the seven spectral colors red, orange,
yellow, green, blue, indigo, and violet. These merge insensibly
into one another, showing the sun's light to be a mixture of all
possible wave lengths, and not of certain selected ones.

Refraction of Light by Lenses. In the eye the refracting
media have the form of lenses thicker in the center than towards
the periphery; and we may here confine ourselves, therefore, to
such convex lenses. If simple light from a point A, Fig. 79,
fall on such a lens its rays, emerging on the other side, will take
new directions after refraction and meet anew at a point, a, after
which they again diverge. If a screen, rr, be held at a it will
therefore receive an image of the luminous point A. For every
convex lens there is such a point behind it at which the rays

222 THE HUMAN BODY

from a given point in front of it meet: the point of meeting is
called the conjugate focus of the point from which the rays start.
If instead of a luminous point a luminous object be placed in
front of the lens an image of the object will be formed at a certain

distance behind it, for all rays proceed-
ing from one point of the object will
meet in the conjugate focus of that
point behind. The image is inverted,
as can be readily seen from Fig. 86.

A11 ravs from the p°int A of the obJect

by a convex lens. meet at the point a of the image; those

from B at 6, and those from intermediate points at intermediate
positions. If the single lens were replaced by several combined
so as to form an optical system the general result would be the
same, provided the system were thicker in the center than at the
periphery.

A moment's consideration of the diagram (Fig. 86) shows us
that the nearer any luminous point is to the lens the further be-
hind the lens its conjugate focus will be. The rays from near
points are more divergent when they strike the lens than are those
from far points, they are therefore not so much bent toward each
other upon emerging, and their point of meeting is further back.
There must be some point near the lens from which rays are so
divergent that after emerging they do not meet at all, but con-
tinue to diverge or form a parallel beam. A plane so located with
reference to a lens that rays from any point in it striking the lens
emerge in a parallel beam is the principal focal plane of the lens.
The thicker a lens the nearer to it is its principal focal plane.

The Ordinary Photographic Camera is an instrument which
serves to illustrate the formation of images by converging sys-
tems of lenses. It consists of a box blackened inside and having
on its front face a tube containing the lenses; the posterior wall
is made of ground glass. If the front of the instrument be di-
rected on exterior objects, in verted .and diminished images of
them will be formed on the ground glass; those images only are
well defined, at any one time, which are at such a distance in
front of the instrument that the conjugate foci of points on them
fall exactly on the glass behind the lens: objects nearer or farther
off give confused and indistinct images; but by altering the dis-

THE EYE AS AN OPTICAL INSTRUMENT 223

tance between the lenses and the ground glass, in common lan-
guage "focussing the instrument," either can be made distinct.
For near objects the lenses must be farther from the surface on
which the image is to be received, and for distant nearer.

The Refracting Media of the Eye Form a Convergent Optical
System, made up of cornea, aqueous humor, lens, and vitreous
humor. These four media are reduced to three practically, by
the fact that the indices of refraction of the cornea and aqueous
humor are the same, so that they act together as one converging
lens. The surfaces at which refraction occurs are: (1) that be-
tween the air and the cornea; (2) that between the aqueous humor
and the front of the lens; (3) that between the vitreous humor
and the back of the lens. The refractive indices of those media
are: the air, 1; the aqueous humor, 1.3379; the lens (average),
1.4545; the vitreous humor, 1.3379. From the laws of the re-
fraction of light it therefore follows that (Fig. 87) the rays Cd
will at the corneal surface be refracted towards the normals N, N,
and tak6 the course de. At the front of the lens they will again
be refracted towards the normals to that surface and take the
course ef; at the back of the lens, passing from a more refracting
to a less refracting medium, they will be bent from the normals
N" and take the course fg. If the retina be there, these parallel
rays will therefore be brought to a focus on it. In the resting
condition of the natural eye this is what happens to parallel rays
entering it: and, since distant objects send into the eye rays which
are practically parallel, such objects are seen distinctly without
any effort, because all rays emanating from a point of the object
meet again in one point on the retina.

Accommodation. Points on near objects send into the eye
diverging rays: these therefore would not come to a focus on the
retina but behind it, and would not be seen distinctly, did not
some change occur in the eye ; since we can see them quite plainly
if we choose (unless they be very near indeed), there must exist
some means by which the eye is focussed or accommodated for
looking at objects at different distances. That some change does
occur one can, also, readily prove by observing that we cannot
see distinctly, at the same moment, both near and distant objects.
For example, standing behind a lace curtain, at a window, we can
as we choose look at the threads of the lace or at the houses across

224 THE HUMAN BODY

the street; but when we look at the one we see the other only
indistinctly; and if, after looking at the more distant object, we
look at the nearer we experience a distinct sense of effort. It is
clear, then, that something in the eye is different in the two cases.
The resting eye, suited for distinctly seeing distant objects,
might conceivably be accommodated for near vision in several
ways. The refracting indices of its media might be increased;
that of course does not happen; the physical properties of the
media are the same in both cases: or the distance of the retina

FIG. 87. — Diagram illustrating the surfaces at which light is refracted in the
eye.

from the refracting surfaces might be increased, for example, by
compression of the eyeball by the muscles around it; however,
experiment shows that changes of accommodation can, by stim-
ulating the third cranial nerve, be brought about in the fresh
excised eyes of animals from which the muscles lying outside the
eyeball have been removed, in which no such compression is
possible; we arc thus reduced to the third explanation, that the
refracting surfaces, or some of them, become more curved, and
so bring diverging rays sooner to a focus. Observation shows
that this is what actually happens: the corneal surface remains
unchanged when a near object is looked at after a distant one,
but the anterior surface of the lens becomes considerably more
convex and the posterior slightly so. When light meets the sep-
arating surface of two media some is reflected and some refracted.

THE EYE AS AN OPTICAL INSTRUMENT

225

If, therefore, a person be taken into a dark room and a candle be

held on one side of his eye while he looks at a distant object, an

observer can see three images of the flame in his pupil, due to

that portion of the light reflected from the surfaces between the

media. One image (a, Fig. 88) is erect and

bright, reflected from the convex mirror formed

by the cornea; the next, b, is dimmer and also

erect; it comes from the front of the lens. The

third, c, is dim and inverted, being reflected from

the concave mirror (see Physics) formed by the

back of the lens. When the curvature of a

, ,, , ,, r ,, FIG. 88. — Theim-

curved mirror is altered the size of the image ages Of a candle-
reflected from it is also altered, becoming small %£^d fronfthe re^
when the radius of curvature of the mirror is fracting media of
lessened and vice versa. If the three images be
carefully watched while the observed eye looks at a near object
in the same line as the distant point previously looked at, it is
seen that the image due to corneal reflection remains unchanged ;
that due to light from the front of the lens becomes smaller and
brighter; the image from the back of the lens also becomes very

ch

Fio. 80. — Diagram to illustrate the mechanism of accommodation; on the
right half of the figure for a near, on the left for a distant, object; r/, ciliary mus-
cle; ch, ciliary process of choroicl; si, suspensory ligament; i, iris.

slightly smaller. The change in the curvature of the front of the
lens can be calculated from the change in size of the image re-
flected from it when the eye changes from distant to near ac-
commodation. When a distant object is looked at the radius of

226 THE HUMAN BODY

curvature is 10 mm. (f inch), when a very near object about
6 mm. (-^5 inch), and this change is sufficient to account for the
range of accommodation of the normal eye.

When the eye is focussed for seeing a near object the circular
muscle of the iris contracts, narrowing the pupil, but this has
nothing directly to do with the accommodation.

Accommodation is brought about mainly by the ciliary muscle
(Fig. 89). In the resting eye it is relaxed and the suspensory
ligament of the lens is taut, and, pulling on its edge, drags it out
laterally a little and flattens its surfaces, especially the anterior,
since the ligament is attached a little in front of the edge. To
see a nearer object the ciliary muscle is contracted, and accord-
ing to the degree of its contraction slackens the suspensory liga-
ment, and then the elastic lens, relieved from the lateral drag,
bulges out a little in the center.

Short Sight and Long Sight. In the eye the range of accommo-
dation is very great, allowing the rays from points infinitely distant

up to those from points about eight
inches in front of the eye to be brought
to a focus on the retina. In the normal
eye parallel rays meet on the retina
when the ciliary muscle is completely

^^-- **^ relaxed (A, Fig. 90). Such eyes are em-
metropic. In other eyes the eyeball is
too long from before back ; in the resting
state parallel rays meet in front of the
retina (B), Persons with such eyes,

FIG. 90.— Diagram iiiustrat- therefore, cannot see distant objects dis-
ing the path of parallel rays tinctly without the aid of diverging

after entering an emmetropic °

(4), a myopic (B), and a hy- (concave) spectacles; they are short-

permetropic (C) eye. -7.7 • /-\ , i in

sighted or myopic. Or the eyeball may

be too short from before back; then, in the resting state, par-
allel rays are brought to a focus behind the retina (C) . To see
even infinitely distant objects, such persons must therefore use
their accommodating apparatus to increase the converging power
of the lens; and when objects are near they cannot, with the
greatest effort, bring the divergent rays proceeding from them
to a focus soon enough. To get distinct retinal images of
near objects they therefore need converging (convex) spectacles.

THE EYE AS AN OPTICAL INSTRUMENT 227

Such eyes are called hypermetropic, or in common language long-
sighted.

Optical Defects of the Eye. The eye, though it answers ad-
mirably as a physiological instrument, is by no means perfect
optically; not nearly so good, for example, as a good microscope
objective. The main defects in it are due to:

1. Chromatic Aberration. As already pointed out, the rays at
the violet end of the solar spectrum are more refrangible than those
at the rear end. Hence they are brought to a focus sooner. The
light emanating from a point on a white abject does not, therefore,
all meet in one point on the retina; but the violet rays come to a
focus first, then the indigo, and so on to the red, farthest back of
all. If the eye is accommodated so as to bring to a focus on the
retina parallel red rays, then violet rays from the same source will
meet half a millimeter in front of it, and crossing and diverging
there make a little violet circle of diffusion around the red point on
the retina. In optical instruments this defect is remedied by com-
bining tbgether lenses made of different kinds of glass ; such com-
pound lenses are called achromatic.

The general result of chromatic aberration, as may be seen in a
bad opera-glass, is to cause colored borders to appear around the
edges of the images of objects. In the eye we usually do not notice
such borders unless we especially look for them; but if, while a
white surface is looked at, the edge of an opaque body be brought
in front of the eye so as to cover half the pupil, colorations will be
seen at its margin. If accommodation be inexact they appear also
when the boundary between a white and a black surface is ob-
served. The phenomena due to chromatic aberration are much
more easily seen if light containing only red and violet rays be
used instead of white light containing all the rays of intermediate
refrangibility. Ordinary blue glass only lets through these two
kinds of rays. If a bit of it be placed over a very small hole in an
opaque shutter and sunlight be admitted through the hole, it will
be found that with one accommodation (that for the red rays) a
red point is seen with a violet border, and with another (that at
which violet rays are brought to a focus on the retina) a violet
point is seen with a red aureole.

2. Spherical Aberration. It is not quite correct to state that
ordinary lenses bring to a focus in one point behind them rays

228 THE HUMAN BODY

proceeding from a point in front, even when these are all of the
same refrangibility. Convex lenses whose surfaces are segments
of spheres, as are those of the eye, bring to a focus sooner the rays
which pass through their marginal than those passing through their
central parts. If rays proceeding from a point and traversing the
lateral part of a lens be brought to a focus at any point, then those
passing through the center of the lens will not meet until a little
beyond that point. If the retina receive the image formed by the
peripheral rays the others will form around this a small luminous
circle of light — such as would be formed by sections of the cones
of converging rays in Fig. 78, taken a little in front of r r. This
defect exists in all glass lenses, as it is found impossible in practice
to grind them of the non-spherical curvatures necessary to avoid
it. In our eyes its effect is to a large extent corrected in the
following ways: (a) The opaque iris cuts off many of the ex-
ternal and more strongly refracted rays, preventing them from
reaching the retina. (6) The outer layers of the lens are less re-
fracting than the central; hence the rays passing through its
peripheral parts are less refracted than those passing nearer its
axis.

3. Irregularities in Curvature. The refracting surfaces of our
eyes are not even truly spherical; this is especially the case with

the cornea, which is very rarely
curved to the same extent in its
vertical and horizontal, diameters.
Suppose the vertical meridian to
be the most curved ; then the rays
proceeding from points along a ver-
tical line will be brought to a
focus sooner than those from points
on a horizontal line. If the eye is
accommodated to see distinctly
the vertical line, it will see indis-
F g. tinctly the horizontal and vice

versa. Few people therefore see

equally clearly at once two lines crossing one another at right
angles. The phenomenon is most obvious, however, when a series
of concentric circles (Fig. 91) is looked at: then when the lines
appear sharp along some sectors, they are dim along the rest.

THE EYE AS AN OPTICAL INSTRUMENT 229

When this defect, known as astigmatism, is marked it causes seri-
ous troubles of vision and requires peculiarly shaped glasses to
counteract it.

4. Opaque Bodies in the Refracting Media. In diseased eyes the
lens may be opaque (cataract) and need removal ; or opacities from
ulcers or wounds may exist on the cornea. But even in the best
eye there are apt to be small opaque bodies in the vitreous humor
causing muscce volitantes; that is, the appearance of minute bodies
floating in space outside the eye, but changing their position when
the position of the eye changes, by which fact their origin in in-
ternal causes may be recognized. Many persons never see them
until their attention is called to their sight by some weakness of it,
and then they think they are new phenomena. Visual phenomena
due to causes in the eye itself are called entoptic; the most interest-
ing are those due to the retinal blood-vessels (Chap. XVI). Tears,
or bits of the secretion of the Meibomian glands, on the front of the
eyeball often cause distant luminous objects to look like ill-defined
luminous bands or patches of various shape. The cause of such
appearances is readily recognized, since they disappear or are
changed after winking.

Hygienic Remarks. Since muscular effort is needed by the
normal eye to see near objects, it is clear why the prolonged con-
templation of such is more fatiguing than looking at more distant
things. If the eye be hypermetropic still more is this apt to be the
case, for then the ciliary muscle has no rest when the eye is used,
and to read a book at a distance such that enough light is reflected
from it into the eye in order to enable the letters to be seen by all,
requires an extraordinary effort of accommodation. Such persons
complain that they can read well enough for a time, but soon fail
to be able to see distinctly. This kind of weak sight should always
lead to examination of the eyes by an oculist, to see if glasses are
needed ; otherwise severe neuralgic pains about the eyes are apt to
come on, and the overstrained organ may be permanently injured.
Old persons are apt to have such eyes; but young children fre-
quently also possess them, and if so should at once be provided
with spectacles. The occurrence of headache at frequent inter-
vals, particularly in connection with use of the eyes, as in read-
ing or sewing, is more often than not an indication of visual
defects which proper glasses would overcome. Sufferers from

230 THE HUMAN BODY

such headaches should therefore have their eyes examined and
if glasses are necessary should wear them.

Short-sighted eyes appear to be much more common now than
formerly, especially in those given to literary pursuits. Myopia
is rare among those who cannot read or who live mainly out of
doors. It is not so apt to lead to permanent injury of the eye as is
the opposite condition, but the effort to see distinctly objects a
little distant is apt to produce headaches and other symptoms of
nervous exhaustion. If the myopia become gradually worse the
eyes should be rested for several months. Short-sighted persons
are apt to have, or acquire, peculiarities of appearance : their eyes
are often prominent, indicative of the abnormal length of the eye-
ball. They also get a habit of " screwing " up the eyelids, probably
an indication of an effort to compress the eyeball from before back
so that distant objects may be better seen. They often stoop, too,
from the necessity of getting their eyes near objects they want to
see. The acquirement of such habits may be usually prevented
by the use of proper glasses. On the other hand, " it is said that
myopia even induces peculiarities of character, and that myopes
are usually unsuspicious and easily pleased; being unable to ob-
serve many little matters in the demeanor or expression of those
with whom they converse, which, being noticed by those of quicker
sight, might induce feelings of distrust or annoyance."

In old age the lens loses some of its elasticity and becomes more
rigid. This leads to the long-sightedness of old people, known as
presbyopia. The stiffer lens does not become as convex as it did
in early life, when the ciliary muscle contracts and the suspensory
ligament is relaxed. In order to adapt the eye to see near objects
distinctly, therefore, convex glasses are required.

In all forms of defective vision too strong glasses will injure
the eyes irreparably, increasing the defects they are intended to
relieve. Skilled advice should therefore be invariably obtained
in their selection, except perhaps in the long-sightedness of old
age, when the sufferer may tolerably safely select for himself any
glasses that allow him to read easily a book about 30 centimeters
(12 inches) from the eye. AS age advances stronger lenses must
usually be obtained.

CHAPTER XVI
THE EYE AS A SENSORY APPARATUS

The Excitation of the Visual Apparatus. The excitable visual
apparatus for each eye consists of the retina, the optic nerve, and
the brain-centers connected with the latter; however stimulated,
if intact, it causes visual sensations. In the great majority of
cases its excitant is objective light, and so we refer all stimula-
tions of it to that cause, unless we have special reason to know
the contrary. As already pointed out pressure on the eyeball
causes a luminous sensation (phosphene), which suggests itself
to us as dependent on a luminous body situated in space where
such an, object must be in order to excite the same part of the
retina. Since all rays of light penetrating the eye, except in the
line of its long axis, cross that axis, if we press the outer side of
the eyeball we get a visual sensation referred to a luminous body
on the nasal side; if we press below we see the luminous patch
above, and so on.

Of course different rays entering the eye take different paths
through it, but on general optical principles, which cannot here be
detailed, we may trace all oblique rays through the organ by
assuming that they meet and leave the optic axis at what are
known as the nodal points of the system; these (kk', Fig. 92) lie
near together in the lens. If we want to find where rays of light
from A will meet the retina (the eye being properly accommodated
for seeing an object at that distance) we draw a line from A to k
(the first nodal point) and then another, parallel to the first, from
k' (the second nodal point) to the retina. The nodal points of
the eye lie so near together that for practical purposes we may
treat them as one (k, Fig. 93), placed near the back of the lens.
By manifold experience we have learnt that a luminous body
(A, Fig. 93) which we see, always lies on the prolongation of the
line joining the excited part of the retina, a, and the nodal point k.
Hence any excitation of that part of the retina makes us think
of a luminous body somewhere on the line a A, and, similarly, any

231

232

THE HUMAN BODY

excitation of b, of a body on the line b B or its prolongation. It is
only other conflicting experiences, as that with the eyes closed

FIG. 92. — Diagram illustrating the points at which incident rays meet the
retina, xx, optic axis; k, first nodal point; k', second nodal point; b, point where
the image of B would be formed, were the eye properly accommodated for it;
a, the retinal point where the image of A would be formed.

external bodies do not excite visual sensations, and the constant
connection of the pressure felt on the eyelid with the visual sen-

FIG. 93. — Diagrammatic section through the eyeball, xx, optic axis; k, nodal
point.

sation, that enable us when we press the eyeball to conclude that,
in spite of what we seem to see, the luminous sensation is not due
to objective light from outside the eye.

The Excitation of the Visual Apparatus by Light. Light only
excites the retina when it reaches its nerve end organs, the rods
and cones. The proofs of this are several.

1. Light does not arouse visual sensations when it falls directly on
the fibers of the optic nerve. Where this nerve enters there is a
retinal part possessing only nerve-fibers, and this part is blind.
Close the left eye and look steadily with the right at the cross in

THE EYE AS A SENSORY APPARATUS

233

Fig. 94, holding the book vertically in front of the face, and mov-
ing it to and fro. It will be found that at about 25 centimeters

Fio. 94.

(10 inches) off the white circle disappears; but when the page is
nearer or farther, it is seen. During the experiment the gaze must
be kept fixed on the cross. There is thus in the field of vision a
blind spot, and it is easy to show by measurement that it lies where
the optic nerve enters.

When the right eye is fixed on the cross, it is so directed that
rays from this fall on the fovea (y, Fig. 95). The rays from the
circle then cross the visual axis at the nodal point,
n, and meet the retina at o. If the distance of
the nodal point of the eye from the paper be /,
and from the retina (which is, 15 mm.) be F, then
the distance, on the paper, of the cross from the
circle will be to the distance of y from o as / is to F.
Measurements made in this way show that the
circle disappears when its image is thrown on the
entry of the optic nerve, which lies to the nasal
side of the fovea.

2. The above experiment having shown that
light does not act directly on the optic nerve-
fibers any more than it does on any other nerve-
fibers, we have next to see in what part of the
retina those changes do first occur which form
the link between light and nervous impulses.
They occur in the outer part of the retina, in the rods and cones.
This is proved by what is called Purkinje's experiment. Take
a candle into a dark room and look at a surface not covered
with any special pattern, say a whitewashed wall or a plain

234 THE HUMAN BODY

window-shade. Hold the candle to the side of one eye and close
to it, but so far back that no light enters the pupil from
it; that is, so far back that the flame just cannot be seen, but
so that a strong light is thrown on the white of the eye as far back
as possible. Then move the candle a little to and fro. The sur-
face looked at will appear luminous with reddish-yellow light,
and on it will be seen dark branching lines which are the shadows
of the retinal vessels. Now in order that these shadows may be
seen the parts on which the light acts must be behind the vessels,
that is, in the layers of the retina next the choroid since the blood-
vessels lie in its front strata.

If the light be kept steady the vascular shadows soon disappear;
in order to continue to see them the candle must be kept moving.
The explanation of this fact may readily be made clear by fixing
the eyes for ten or fifteen seconds on the dot of an " i " somewhere
about the middle of this page : at first the distinction between the
slightly luminous black letters and the highly luminous white
page is very obvious; in other words, the different sensations
arising from the strongly and the feebly excited areas of the
retina. But if the glance be not allowed to wander, very soon
the letters become indistinct and at last disappear altogether;
the whole page looks uniformly grayish. The reason of this is
that the powerful stimulation of the retina by the light reflected
from the white part of the page soon fatigues the part of the
visual apparatus it acts upon; and as this fatigue progresses the
stimulus produces less and less effect. The parts of the retina,
on the other hand, which receive light only from the black letters
are but little stimulated and retain much of their original ex-
citability, so that, at last, the feebler excitation acting upon
these more irritable parts produces as much sensation as the
stronger stimulus acting upon the fatigued parts; and the letters
become indistinguishable. To see them continuously we must
keep shifting the eyes so that the parts of the visual apparatus are
alternately fatigued and rested, and the general irritability of
the whole is kept about the same. So, in Purkinje's experiments
if the position of the shadows remain the same, the shaded part
of the retina soon becomes more irritable than the more excited
unshaded parts, and its relative increase of irritability makes up
for the less light falling on it, so that the shadows cease to be per-

THE EYE AS A SENSORY APPARATUS 235

ceivecl. It is for this reason that we do not see the retinal vessels
under ordinary circumstances. When light, as usual, enters the
eye from the front through the pupil the shadows always fall on
the same parts of the retina, and these parts are thus kept suffi-
ciently more excitable than the rest to make up for the less light
reaching them through the vessels.

Further evidence that the rod and cone layer is the true recep-
tor of the eye is furnished by the fact that the seat of most acute
vision is the fovea centralis, where only this layer and the cone-
fibers diverging from it are present. When we want to see any-
thing distinctly we always turn our eyes so that its image shall
fall on the foveae.

The Intensity of Visual Sensations. Light considered as a
form of energy may vary in quantity; physiologically, also, we
distinguish quantitative differences in light as degrees of bright-
ness, but the connection between the intensity of the sensation
excited and the quantity of energy represented by the stimulat-
ing light is not a direct one. In the first place, some rays excite
our visual apparatus more powerfully than others : a given amount
of energy in the form of yellow light, for example, causes more
powerful visual sensations than the same quantity of energy in
the form of violet light.

Furthermore, the sense of vision, like all the other senses, obeys
the psychophysical law (Chap. XIII). That is, differences of
sensation are proportional not to absolute but to relative changes
in the amount of stimulating energy. If a room is lighted by one
candle and another is brought in we perceive an increase of il-
lumination, but if it is lighted by an arc light the bringing in of a
single lighted candle makes no perceptible difference in the il-
lumination. Another illustration of the application of the psycho-
physical law to the visual sense is found in the fact that the stars
which are ordinarily invisible in the daytime can be seen from
the bottom of deep wells or from deep and narrow canons. The
explanation is that in open day the general illumination of the
sky is so intense that the additional light of the stars is unper-
ceived. To one in the bottom of a well, however, the general
illumination is cut down so much as to bring the additional light
from the stars within the limits of perception. The smallest dif-
ference in luminous intensity which we can perceive is about

236 THE HUMAN BODY

^5 of the whole, for all the range of lights we use in carrying
on our ordinary occupations. For strong lights the smallest per-
ceptible fraction is considerably greater; finally we reach a limit
where no increase in brightness is felt. For weak illumination the
sensation is more nearly proportioned to the total differences of
the objective light. Thus in a dark room an object reflecting all
the little light that reaches it appears almost twice as bright as
one reflecting only half; in a stronger light it would not so appear.
Bright objects in general obscurity thus appear unnaturally
bright when compared with things about them, and indeed often
look self-luminous. A cat's eyes, for example, are said to " shine
in the dark"; and painters to produce moonlight effects always
make the bright parts of a picture relatively brighter, when com-
pared with things about them, than would be the case if a sunny
scene were to be represented; by a relatively excessive use of
white pigment they produce the relatively great brightness of
those things which are seen at all in the general obscurity of a
moonlight landscape.

Function of the Rods. Inasmuch as the rods are absent from
the fovese, they cannot be concerned with ordinary conscious vi-
sion since clear vision, as we know from experience, is confined to
these areas. It is easy to demonstrate by a simple experiment
that the parts of the retina containing rods are more susceptible
to feeble lights than is the fovea, which is devoid of them. The
constellation of the Pleiades consists of seven stars ; one of these is
so faint, however, as to be invisible to most eyes when the con-
stellation is looked at directly. If the gaze be turned to a point
in the sky a degree or two to one side of the constellation, so as
to throw its image off the fovea unto a rod-containing area, the
seventh star becomes immediately visible.

This evidence indicates that the function of the rods is some
how related to the reception of light stimuli of feeble intensity.
The portions of the retina outside the fovea seem to function for
the most part more reflexly than consciously; stimuli striking
these portions of the retina bring about reflex movements of the
eyes and head so that the source of stimulation throws its light
upon the fovese, and we derive conscious perceptions as to its
nature. For such reflex activity a high degree of irritability is
desirable.

THE EYE AS A SENSORY APPARATUS 237

It is said that some animals, such as snakes, have no rods in
their retinas, while the retinas of others of nocturnal habits, such
as owls, consist exclusively of rods.

Visual Purple. If a perfectly fresh retina be excised rapidly,
its outer layers will be found of a rich purple color. In daylight
this rapidly bleaches, but in the dark persists even when putre-
faction has set in. In pure yellow light it also remains unbleached
a long time, but in other lights disappears at different rates. If a
rabbit's eye be fixed immovably and exposed so that an image
of a window is focussed on the same part of its retina for some
time, and then the eye be rapidly excised in the dark and placed
in solution of potash alum, a colorless image of the window is
found on the retina, surrounded by the visual purple of the rest
which is, through the alum, fixed or rendered incapable of change
by light. • Photographs, or optograms, are thus obtained which
differ from the photographer's only in the nature of the chemical
substances and processes involved. Both depend on the produc-
tion of a chemical reaction by light. If the eye be not rapidly ex-
cised and put in the alum after its exposure, the optogram will
disappear; the vision purple being rapidly regenerated at the
bleached part. This reproduction of it is due mainly to the cells
of the pigmentary layer of the retina, which in living eyes ex-
posed to light thrust long processes between the rods and cones.
Portion of frogs' retinas raised from this, bleach more rapidly
than those left in contact with it, but become soon purple again
if let fall back upon the pigment-cells.

The visual purple, as stated previously, occurs only in the outer
segments of the rods. Whatever function it has is probably con-
nected, therefore, with their special property of reacting to feeble
lights. The nature of its function is, further than this, unknown.

The Duration of Luminous Sensations. This is greater than
that of the stimulus, a fact taken advantage of in making fire-
works: an ascending rocket produces the sensation of a trail of
light extending far behind the position of the bright part of the
rocket itself at the moment, because the sensation aroused by it
in a lower part of its course still persists. So, shooting stars ap-
pear to have luminous tails behind them. By rotating rapidly
before the eye a disk with alternate white and black sectors we
get for each point of the retina on which a part of its image falls,

238 THE HUMAN BODY

e*

alternating stimulation (due to the passage of white sector) and
rest (when a black sector is passing). If the rotation be rapid
enough the sensation aroused is that of a uniform gray, such as
would be produced if the white and black were mixed and spread
evenly over the disk. In each revolution the eye gets as much
light as if that were the case, and is unable to distinguish that this
light is made up of separate portions reaching it at intervals:
the stimulation due to each lasts until the next begins and so
all are fused together. If, while looking at the flame, one turns
out suddenly the gas in a room containing no other light, the image
of the flame persists a short time after the flame itself is extin-
guished.

The Localizing Power of the Retina. As already pointed out
a necessary condition of seeing definite objects, as distinguished
from the power of recognizing differences of light and darkness, is
that all light entering the eye from one point of an object shall be
focussed on one point of the retina. This, however, would not be
of any use had we not the faculty of distinguishing the stimula-
tion of one part of the retina from that of another part. This

FIG. 96.

power the visual apparatus possesses in a very high degree; while
with the skin we cannot distinguish from one, two points touching
it less than 1 mm. (^ inch) apart, with our eyes we can distin-
guish two points whose retinal images are not more than .004 mm.
(.00016 inch) apart. The distance between the retinal images of
two points is determined by the "visual angle" under which
they are seen; this angle is that included between lines drawn
from them to the nodal point of the eye. If a and b (Fig. 96) are
luminous points, the image of a will be formed at a' on the pro-
longation of the line a n joining a with the node, n. Similarly the
image of 6 will be formed at b'. If a and b still remaining the same
distance apart, be moved nearer the eye to c and d, then the
visual angle under which they are seen will be greater and their
retinal images will be farther apart, at c' and d'. If a and 6 are

THE EYE AS A SENSORY APPARATUS 239

the highest and lowest parts of an object, the distance between
their retinal images will then depend, clearly, not only on the
size of the object, but on its distance from the eye; to know the
discriminating power of the retina we must therefore measure
the visual angle in each case. In the fovea centralis two objects
seen under a visual angle of 50 to 70 seconds can be distinguished
from one another; this gives for the distance between the retinal
images that above mentioned, and corresponds pretty accurately
to the diameter of a cone in that part of the retina. We may
conclude, therefore, that when two images fall on the same cone
or on two contiguous cones they are not discriminated; but that
if one or more unstimulated cones intervene between the stimu-
lated, the points may be perceived as distinct. The diameter of
a rod or cone, in fact, marks the anatomical limit up to which
we can by practice raise our acuteness of visual discrimination;
and in the fovea which we constantly use all our lives in looking
at things which we want to see distinctly, we have educated the
visual apparatus up to about its highest power. Elsewhere on
the retina our discriminating power is much less and diminishes
as the distance from the fovea increases.

While we can Jfell the stimulation of an upper part of the retina
from a lower, or a right region from a left, it must be borne in
mind that we have no direct knowledge of which is upper or lower
or right or left in the ocular image. All our visual sensations tell
us is that they are aroused at different points, and nothing at all
about the actual positions of these on the retina. There is no
other eye behind the retina looking at it to see the inversion of the
image formed on it. Suppose I am looking at a pane in a second-
story window of a distant house: its image will then fall on the
fovea centralis; the line joining this with the pane is called the
visual axis. The image of the roof will be formed on a part of the
retina below the fovea, and that of the front door above it. I
distinguish that the images of all these fall on different parts of
the retina in certain relative positions, and have learnt, by the
experience of all my life, that when the image of anything arouses
the sensation due to excitation of part of the retina below the
fovea the object is above my visual axis, and vice versa; similarly
with right and left. Consequently I interpret the stimulation of
lower retinal regions as meaning high objects, and of right retinal

240 THE HUMAN BODY

regions as meaning left objects, and never get confused by the
inverted retinal image about which directly I know nothing. A
new-born child, even supposing it could use its muscles perfectly,
could not, except by mere chance, reach towards an object which
it saw; it would grasp at random, not yet having learnt that to
reach an object exciting a part of the retina above the fovea
needed movement of the hand towards a position in space below
the visual axis; but very soon it learns that things near its brow,
that is up, excite certain visual sensations, and objects below its
eyes others, and similarly with regard to right and left; in time
it learns to interpret 'retinal stimuli so as to localize accurately
the direction, with reference to its eyes, of outer objects, and
never thenceforth is puzzled by retinal inversion.

Color Vision. Sunlight reflected from snow gives us a sensa-
tion which we call white. The same light sent through a prism
and reflected from a white surface excites in us no white sensation
but a number of color sensations, gradating insensibly from red to
violet, through orange, yellow, green, blue-green, blue, and indigo.
The prism separates from one another light-rays of different
periods of oscillation and each ray excites in us a colored visual
sensation, while all mixed together, as in sunlight, they arouse
the entirely different sensation of white. If the light fall on a
piece of black velvet we get still another sensation, that of black ;
in this case the light-rays are so absorbed that but few are reflected
to the eye and the visual apparatus is left at rest. Physically
black represents nothing: it is a mere zero — the absence of ethereal
vibrations; but, in consciousness, it is as definite a sensation as
white, red, or any other color. We do not feel blackness or dark-
ness except over the region of the possible visual field of our eyes.
In a perfectly dark room we only feel the darkness in front of our
eyes, and in the light there is no such sensation associated with
the back of our heads or the palms of our hands, though through
these we get no visual sensations. It is obvious, therefore, that
the sensation of blackness is not due to the -mere absence of lu-
minous stimuli, but to the unexcited state of the retinas, which are
alone capable of being excited by such stimuli when present.
This fact is a very remarkable one, and is not paralleled in any
other sense. Physically, complete stillness is to the ear what
darkness is to the eye ; but silence impresses itself on us as the ab-

THE EYE AS A SENSORY APPARATUS 241

sence of sensation, while darkness causes a definite feeling of
"blackness."

Our color sensations insensibly fade into one another; starting
with black we can insensibly pass through lighter and lighter
shades of gray to white: or beginning with green through darker
and darker shades of it to black or through lighter and lighter
to white: or beginning with red we can by imperceptible steps
pass to orange, from that to yellow and so on to the end of the
solar spectrum: and from the violet, through purple and carmine,
we may get back again to red. Black and white appear to be
fundamental color sensations mixed up with all the rest: we
never imagine a color but as light or dark, that is, as more or less
near white or black; and it is found that as the light thrown on
any given colored surface weakens, the shade becomes deeper
until it passes into black; and if the illumination be increased, the
color becomes "lighter" until it passes into white. Of all the
colors of the spectrum yellow most easily passes into white with
strong illumination. Black and white, with the grays which are
mixtures of the two, thus seem to stand apart from all the rest as
the fundamental visual sensations, and the others alone are in
common parlance named "colors." It has even been suggested
that the power of differentiating them in sensation has only lately
been acquired by man, and a certain amount of evidence has been
adduced from passages in the Iliad to prove that the Greeks in
Homer's time confused together colors that are very different to
most modern eyes; at any rate there seems to be no doubt that
the color sense can be greatly improved by practice; women whose
mode of dress causes them to pay more attention to the matter,
have, as a general rule, a more acute color sense than men.

Leaving aside black, white, gray, and the various browns
(which are only dark tints of other colors), we may enumerate
our color sensations as red, orange, yellow, green, blue, violet,
and purple; between each there are, however, numerous transi-
tion shades, as yellow-green, blue-green, etc., so that the number
which shall have definite names given to them is to a large extent
arbitrary. Of the above, all but purple are found in the spec-
trum given when sunlight is separated by a prism into its rays
of different refrangibility; rays of a certain wave-length cause in
us the feeling red; others yellow, and so on; for convenience we

242 THE HUMAN BODY

may speak of these as red, yellow, blue, etc., rays; all together,
in about equal proportions, they arouse the sensation of white.

Peculiarities of Color Vision. A remarkable fact is that most
color feelings can be aroused in several ways. White, for ex-
ample, not only by the above general mixture, but red and blue-
green rays, or orange and blue, or yellow and violet, taken in pairs
in certain proportions, and acting simultaneously or in very rapid
succession on the same part of the retina, cause the sensation of
white: such colors are called complementary to one another. The
mixture may be made in several ways; as, for example, by caus-
ing the red and blue-green parts of the spectrum to overlap, or by
painting red and blue-green sectors on a disk and rotating it
rapidly; they cannot be made, however, by mixing pigments,
since what happens in such cases is a very complex phenomenon.
Painters, for example, are accustomed to produce green by mix-
ing blue and yellow paints, and some may be inclined to ridicule
the statement that yellow and blue when mixed give white.
When, however, we mix the pigments we do not combine the
sensations of the same name, which is the matter in question. Blue
paint is blue because it absorbs all the rays of the sunlight except
the blue and some of the green ; yellow is yellow because it absorbs
all but the yellow and some of the green, and when blue and yel-
low are mixed the blue absorbs all the distinctive part of the
yellow and the yellow does the same for the blue; and so only the
green is left over to reflect light to the eye, and the mixture has
that color. Grass-green has no complementary color in the solar
spectrum ; but with purple, which is made by mixing red and blue,
it gives white. Several other colors taken three together, give
also the sensation of white. If then we call the light-rays which
arouse in us the sensation red, a, those giving us the sensation
orange b, yellow c, and so on, we find that we get the sensation
white with a, 6, c, d, e, f, and g all together; or with 6 and c, or
with c and /, or with a, d, and e; our sensation white has no deter-
minate relation to ethereal oscillations of a given period, and the
same is true for several other colors; yellow feeling, for example,
may be excited by ethereal vibrations of one given wave-length
(spectral yellow), or by a light containing only such waves as
taken separately cause the sensations red and grass-green; in
other words, a physical light in which there are no waves of the

THE EYE AS A SENSORY APPARATUS 243

"yellow" length may cause in us the sensation yellow, which is
only one more instance of the general fact that our sensations,
as such, give us no direct information as to the nature of external
forces; they are but signs which we have to interpret.

Function of the Cones. These structures, since they are the
only sensitive elements of the fovea centralis, must be the recep-
tors for all ordinary conscious vision. Their special function is
doubtless the perception of color. This perception is a part of
all our conscious visual sensations. We never think of a luminous
object as being merely light; but always as having some color.

Distribution of Color Sense over the Retina. By means of an
apparatus called the perimeter it is possible to determine the
boundaries of visual sensation in the retina. In using this ap-
paratus the subject with head supported in one position looks
fixedly at a point straight in front; the observer then brings
small squares of paper from the side toward the front and the sub-
ject reports the instant the square of paper comes into his field
of vision. The angle is marked on a specially prepared chart and
the observation repeated along different radii. By this means
the field of vision is mapped out. The visual field for any par-
ticular color can be determined similarly, the subject in this case
being required to report as -soon as he is certain what the color
of the square of paper is. Such studies have brought out the in-
teresting fact that ability to perceive the different colors is un-
equally distributed over the retina. The margins of the visual
field are sensitive only to white and black, and to their mixtures
of gray ; the fields for blue and yellow cover the whole area except
the margins ; the fields for red and green sensation arc the smallest
of all, occupying only the central part of the field and covering
about half its entire surface. According to most determinations
the boundaries of the yellow and blue fields do not coincide ex-
actly, nor those of green and red; but it is quite probable that
they do coincide exactly in reality, and that experimental errors
account for their apparent divergences.

It is clear from these observations that the cones in the central
part of the visual field are sensitive to all colors ; that those further
out are sensitive to all except red and green ; and that the marginal
ones are insensitive to color as such, and distinguish only degrees
of light and darkness.

244 THE HUMAN BODY

Color Blindness. This is a deficiency in color vision whereby
certain colors fail to produce the characteristic color sensations
that they do in normal eyes. The commonest sort of color blind-
ness is so-called red-green blindness. In- it neither red nor green
has the same value as in normal eyes. Both colors seem to give
the sensation of " neutral " tints, grays and browns. Two varieties
of red-green blindness are recognized; the difference between
them is, however, apparently one of perception of luminosity
rather than of color. To the red-blind person a red object looks
dim as well as of neutral tint; to the green-blind person a red ob-
ject appears to be bright, although in color of neutral tint likewise.

Red-green blindness is the common form. It is usually con-
genital and occurs more frequently in males than in females.
One male in twenty-five, on the average, is color blind, and less
than one female in a hundred. It has been suggested that this
difference is at bottom one of training; women have from time
immemorial used brighter colors and more colors in their clothing
than have men, and have therefore become more accustomed to
making nice color discriminations.

A form of violet blindness has been described as occurring in
rare pathological conditions. It can be brought on temporarily,
it is said, by taking the drug santonin. This form of color blind-
ness has not been thoroughly studied. Monochromatic blindness,
in which the only sensation is of degrees of grayness, shading at
one end into white, at the other into black, is also described. This
is accompanied in most cases by blindness of the fovea, and is
probably therefore the result of complete loss of cone function.

A full explanation of red-green blindness cannot be had, of
course, until the mechanism of color vision is understood. From
what was said about the distribution of color perception in the
retina it is clear, however, that in all eyes there is an area of red-
green blindness between the area of complete color perception
and the area of white-black vision. If we suppose the cones in
the central area to be undifferentiated from those of this im-
mediately surrounding zone we have a condition of red-green
blindness involving the whole eye and corresponding to that of the
color-blind person.

The detection of color blindness is often a matter of considerable
importance, especially in sailors and railroad operators since the

THE EYE AS A SENSORY APPARATUS 243

two colors most commonly confounded, red and green, are those
used in maritime and railroad signals. Persons attach such dif-
ferent names to colors that a decision as to color blindness can-
not be safely arrived at by simply showing a color and asking its
name. The best plan is to take a heap of worsted of all tints,
select one, say a red, and tell the man to put alongside it all those
of the same color, whether of a lighter or a darker shade; if red
blind he will select not only the reds but the greens, especially
the paler tints, as well as the grays and browns. This test, which
is almost universally used, was devised by the Sweedish physiol-
ogist, Holmgren.

After-images and Contrasts. These are well-marked visual
phenomena, and have to be taken into account in attempting to
explain the mechanism of color vision. After-images are visual
sensations which remain after the withdrawal of the stimulus.
They are best seen after looking at bright objects, or fixedly for
several seconds at the same object. After-images are of two
sorts, positive and negative. Positive after-images are always
the same color as the object looked at; if one looks for an instant
at an incandescent filament and then shuts his eyes he perceives
a positive after image of the filament. This is due, probably, to
the persistence of the chemical process in the retina after the light
which causes it is withdrawn. Negative after-images, instead of
being the color of the object looked at, are always of its compli-
mentary color; if a red paper is looked at fixedly for several
seconds and the eyes then turned to a white wall, a bluish green
after-image is seen, instead of a red one. Negative after-images
can also be seen by closing the eyes after looking fixedly at a
bright object for some seconds.

Contrasts are effects produced by bringing side by side different
colors ; blue appears bluer when near yellow than when near other
shades or the same shade of blue. Red and green heighten each
other in a similar way. If a large black square and a large white
square are placed side by side the black square looks blacker on
the edge next the white than elsewhere, and the white looks
whiter next the black than elsewhere.

Theories of Color Vision. A theory of color vision to be ac-
ceptable must explain first the fundamental facts of color per-
ception, our ability to distinguish innumerable shades of color,

246 THE HUMAN BODY

and the fact that pairs or groups of fused colors give rise to sen-
sations entirely unrelated to any of the constituent colors. The
theory must account for the distribution of color perception over
the retina and for the facts of color blindness ; it must also explain
after-images and contrasts. The fact that black, the absence of
stimulation, has all the subjective qualities of a true sensation is
also to be explained in some way. No theory yet proposed is
satisfactory in accounting for all the known facts. Each one
lays special emphasis on some group of visual phenomena and
disregards such facts as cannot be harmonized with it. Three
interesting theories will be briefly summarized for the sake of
showing how such a problem is attacked. Each of them assumes
that the excitation of the visual nerve endings depends upon the
action of light upon certain photochemical substances in the cones.

The Young-Helmholtz Theory. This theory, proposed by
Young in 1807 and elaborated by Helmholtz many years later,
may be described rather as an attempt to apply the doctrine of
specific nerve energies to color vision than as an attempt to ex-
plain the facts of color vision as we know them.

It is an interesting illustration of the extent to which this doc-
trine has come to physiologists to seem fundamental in forming
conceptions of the nervous system that the theory of Young,
manifestly impossible as it is, because of the numerous facts with
which it cannot be harmonized, has received much more atten-
tion and consideration than other theories, agreeing with many of
the facts as we know them, but not in accord with the doctrine
of specific nerve energies.

The theory assumes all our color sensations to be based on three
primary ones, red, green, and violet, each of which is aroused by
the decomposition of its special photochemical substance, and
each having distinct nervous connection with the visual area of
the cerebrum. Since anatomical study shows that each cone has
a single nerve-fiber leading from it we must either suppose that
there are three sorts of cones, one red-perceiving, one green-
perceiving, and one violet-perceiving, and that these are scattered
in groups of three over the retina; or we must conclude that the
nerve-fiber is not the unit of nervous conduction but that it is
made up of smaller units in the same way that the nerve-trunk is
made up of fibers. The originators of the theory held the first

THE EYE AS A SENSORY APPARATUS 247

of these views; they assumed that any method of stimulating a
red-perceiving cone would give rise to red sensations; if red- and
green-perceiving cones were stimulated simultaneously the effect
in consciousness would be very different from that of stimulating
either one alone, the red cone and the green cone together, giving
yellow; and if all three sorts were stimulated at once in equal
amounts the effect would be a sensation of white. All our color
perceptions are supposed to be based on proper combinations of
stimuli acting on the groups of three cones. To explain some facts,
such as that pure red light as it becomes brighter and brighter
approaches and finally becomes white, the theory supposes that
no light stimulates only one cone; all three of the group are stim-
ulated by light of any color, and the effect in consciousness de-
pends on which is more strongly stimulated.

It is easy to demonstrate that the color of a spot of light whose
retinal image is of such a size as to fall within the boundaries of a
single cone can be accurately distinguished. According to the
theory white light should seem to be one or the other fundamental
color under such circumstances, instead of looking white as it
actually does.

When the theory was proposed it was thought that red-blindness
and green-blindness were entirely distinct forms of color blind-
ness. The theory fits that idea very well, since it supposes dis-
tinct red-perceiving and green-perceiving cones. Now that we
know that both red and green blindness are really forms of red-
green blindness in which neither red nor green gives normal color
sensations the theory does not agree at all with the facts in this
regard. The theory also fails to explain the distribution of color
vision over the retina or the fact that black is a true sensation.
It explains very well on the basis of fatigue the negative after-
images that one sees when the eyes are turned to a white surface
after looking at a colored body; for if one particular set of cones is
fatigued by looking steadily at any color, white light coming upon
the retina stimulates the unfatigued ones more powerfully than
the fatigued ones, and instead of the sensation of white which
follows equal stimulation of all cones, the complimentary color
to that one which fatigued the cones in the first place is seen.
The theory does not explain well the negative after-images seen
with closed eyes, nor does it explain the phenomena of contrast.

248 THE HUMAN BODY

The Hering Theory. This theory frankly makes no attempt to
accord with the doctrine of specific nerve energies, but seeks
rather to explain on a rational basis those visual phenomena
which the Young-Helmholtz theory explains poorly or not at all.
It is based upon the observation that whereas we recognize certain
colors as being combinations of two others, as bluish-green, or
reddish-yellow, there are no colors which we recognize as com-
binations of complementary colors ; greenish-red or yellowish-blue
do not occur. The existence of these mutually exclusive colors
suggested to the author of this theory that there might be two
opposing processes going on in the retina, one a process of chemical
breaking down, or disassimilation; the other a process of building
up, or assimilation.

He therefore postulated three photochemical substances, a
white-black substance, a yellow-blue substance, and a red-green
substance. He supposed that white light falling on the retina
breaks down the white-black substance and gives rise to the
sensation of white ; whenever no white light is falling on the retina
this substance is building itself up; this gives rise to the sensation
of black. Similarly the sensation of red is the result of breaking
down the red-green substance, and green of its assimilation. The
white sensation resulting from stimulation of complementary colors
is explained as due to neutralization of opposing effects. When red
and green light come together into the retina the red-green sub-
stance is neither broken down nor built up. Both red and green
light have a disassimilatory effect on the white-black substance as
do rays of all colors, according to the theory. The only effect,
therefore, of the complementary colors, is to produce a sensation
of white. Contrast is explained as due to the maintenance of a
sort of chemical balance in the retina whereby a breaking down of
one of the elements in part of it is accompanied by building up of
the same element in neighboring areas. So, if the yellow-blue
substance is being broken down in part of the retina by yellow
light, and built up in adjoining part by blue light, at the border
between them each process is heightened by the near presence of
the other.

The theory explains very well, also, the facts of negative after-
images, of color blindness, and of the distribution of color vision
in the retina. The chief criticism that has been offered against it,

THE EYE AS A SENSORY APPARATUS 249

apart from its failure to accord with the doctrine of specific nerve
energies, is that its assumption of similar nervous activities result-
ing from opposing chemical processes is unwarranted by any
knowledge that we have of the relation between chemical processes
and nervous activities in other parts of the body.

The Franklin Theory is based on the idea that the peculiar dis-
tribution of color vision over the retina is significant as suggesting
that the more complex color perceptions are evolved from simpler
ones. According to this theory the primary photochemical sub-
stance is a gray-perceiving substance; white and black represent-
ing the ends of the gray color series. This substance is in all the
rods, and in the cones of the retinal margin where only gray per-
ception occurs. In the cones of the yellow-blue field, the funda-
mental gray-perceiving photochemical substance is supposed to
be dissociated into two different photochemical substances, one
yellow-perceiving, the other blue-perceiving. Since these are
products of the gray-perceiving substance when both are stimu-
lated together the effect is the same as when the gray-perceiving
substance itself is stimulated, namely, a shade of gray.

In the central cones of the retina a still further decomposition
is assumed to have occurred, of the yellow-perceiving substance
into red and green-perceiving substances. The central cones, then,
contain three photochemical substances, a red-perceiving one, a
green-perceiving one, and a blue-perceiving one. Since all are
ultimately derived from the gray-perceiving substance their com-
bined stimulation produces gray sensations; simultaneous stimu-
lation of the red and green substances gives the same result as
stimulation of their parent substance, that for perceiving yellow.

This theory puts the distribution of color vision in the retina and
the phenomenon of color blindness, which it explains as due to
failure of dissociation of the yellow-perceiving substance, upon a
more rational basis than do either of the other theories described.
In most other respects it offers little advantage over them.

While we must admit that at present a full understanding of
color vision is beyond us we may properly look forward to its ulti-
mate mastery, as physiology is able to penetrate more deeply the
processes which underly it.

Visual Perceptions. The sensations which light excites in us we
interpret as indications of the existence, form, and position of ex-

250 THE HUMAN BODY

ternal objects. The conceptions which we arrive at in this way are
known as visual perceptions. The full treatment of perceptions be-
longs to the domain of Psychology, but Physiology is concerned
with the conditions under which they are produced.

The Visual Perception of Distance. With one eye our perception
of distance is very imperfect, as illustrated by the common trick of
holding a ring suspended by a string in front of a person's face, and
telling him to shut one eye and pass a rod from one side through
the ring. If a penholder be held erect before one eye, while the
other is closed, and an attempt be made to touch it with a finger
moved across towards it, an error will nearly always be made. (If
the finger be moved straight on towards the pen it will be touched
because with one eye we can estimate direction accurately and
have only to go on moving the finger in the proper direction till it
meets the object.) In such cases we get the only clue from the
amount of effort needed to "accommodate" the eye to see the
object distinctly. When we use both eyes our perception of dis-
tance is much better; when we look at an object with two eyes the
visual axes are converged on it, and the nearer the object the
greater the convergence. We have a pretty accurate knowledge
of the degree of muscular effort required to converge the eyes on
all tolerably near points. When objects are farther off, their
apparent size, and the modifications of their retinal images brought
about by aerial perspective, come in to help. The relative distance
of objects is easiest determined by moving the eyes; all stationary
objects then appear displaced in the opposite direction (as for ex-
ample when we look out of the window of a railway car) and those
nearest most rapidly; from the different apparent rates of move-
ment we can tell which are farther and nearer. We so inseparably
and unconsciously bind up perceptions of distance with the sensa-
tions aroused by objects looked at, that we seem to see distance;
it seems at first thought as definite a sensation as color. That it is
not is shown by cases of persons born blind, who have had sight
restored later in life by surgical operations. Such persons have at
first no visual perceptions of distance: all objects seem spread out
on a flat surface in contact with the eyes, and they only learn
gradually to interpret their sensations so as to form judgments
about distances, as the rest of us did unconsciously in childhood
before we thought about such things.

THE EYE AS A SENSORY APPARATUS 251

The Visual Perception of Size. The dimensions of the retinal
image determine primarily the sensations on which conclusions
as to size are based; and the larger the visual angle the larger the
retinal image; since the visual angle depends on the distance of an
object the correct perception of size depends largely upon a correct
perception of distance; having formed a judgment, conscious or
unconscious, as to that, we conclude as to size from the extent of
the retinal region affected. Most people have been surprised now
and then to find that what appeared a large bird in the clouds was
only a small insect close to the eye; the large apparent size being
due to the previous incorrect judgment as to the distance of the
object. The presence of an object of tolerably well-known height,
as a man, also assists in forming conceptions (by comparison) as to
size; artists for this purpose frequently introduce human figures to
assist in giving an idea of the size of other objects represented.

The Visual Perception of a Third Dimension of Space. This
is very imperfect with one eye; still we can thus arrive at conclu-
sions from the distribution of light and shade on an object, and
from that amount of knowledge as to the relative distance of dif-
ferent points which is attainable monocularly ; the different visual
angles under which objects are seen also assist us in concluding that
objects are farther and nearer, and so are not spread out on a plane
before the eye, but occupy depth also. Painters depend mainly
on devices of these kinds for representing solid bodies, and objects
spread over the visual field in the third dimension of space.

Single Vision with Two Eyes. When we look at a flat object
with both eyes we get a similar retinal image in each. Under ordi-
nary circumstances we see, however, not two objects but one. In
the habitual use of the eyes we move them so that the images of
the object looked at fall on the two foveae. A point to the left
of this object forms its image on the inner (right) side of the left
eye and the outer (right) side of the right. An object vertically
above that looked at would form an image straight below the
fovea of each eye; an object to the left and above, its image to
the inner side and below in the left eye and to the outer side and
below in the right eye; and so on. We have learned that similar
simultaneous excitations of these corresponding points mean single
objects, and so interpret our sensations. When the eyes do not
work together, as in the muscular ^coordination of one stage of

252

THE HUMAN BODY

intoxication, then they are not turned so that images of the same
objects fall on corresponding retinal points, and the person sees
double. When a squint comes on, as from paralysis of the external
rectus of one eye, the sufferer at first sees double for the same
reason, but after a time he makes new associations of correspond-
ing retinal points.

When a given object is looked at, lines drawn from it through
the nodal points reach the fovea centralis in each eye. Lines so
drawn at the same time from a more distant object diverge less and
meet each retina on the inner side of its fovea; but as above pointed
out the corresponding points for each retinal region on the inside
of the left eye, are on the outside of the right, and vice versa.
Hence the more distant object is seen double. So, also, is a nearer
object, because the more diverging lines drawn from it through the
nodal points lie outside of the fovea in each eye. Most people go
through life unobservant of this fact; we only pay attention to
what we are looking at, and nearly always this makes its images

FIG. 97.

on the two fovese. That the fact is as above stated may, however,
be readily observed. Hold one finger a short way from the face
and the other a little farther off; looking at one, observe the other
without moving the eyes; it will be seen double. For every given
position of the eyes there is a surface in space, all objects on which
produce images on corresponding points of the two retinas: this
surface is called the horopter for that position of the eyes: all ob-
jects in it are seen single; all others in the visual field, double.

The Perception of Solidity. When a solid object is looked at the
two retinal images are different. If a truncated pyramid be held
in front of one eye its image will be that represented at P, Fig. 97.
If, however, it be held midway between the eyes, and looked at
with both, then the left-eye image will be that in the middle of the
figure, and the right-eye image that to the right. The small sur-

THE EYE AS A SENSORY APPARATUS 253

face, 6 d c a, in one answers to the large surface, b' d' c' a', in the
other. This may be readily observed by holding a small cube in
front of the nose and alternately looking at it with each eye. In
such cases, then, the retinal images do not correspond, and yet we
combine them in consciousness so as to see one solid object. This
is known as stereoscopic vision, and the illusion of the common
stereoscope depends on it. Two photographs are taken of the same
object from two different points of view, one as it appears when
seen from the left, and the other when seen from the right. These
are then mounted for the stereoscope so that each is looked at by
its proper eye, and the object appears in distinct relief, as if, in-
stead of flat pictures, solid objects, occupying three dimensions of
space, were looked at.

CHAPTER XVII
THE STRUCTURE AND FUNCTIONS OF BLOOD AND LYMPH

Introductory. We turn at this point from study of the mech-
anism by which the Body adapts itself to its surroundings to a
consideration of the structures and processes engaged in body
maintenance. These have the task of providing the living tissues
of the Body with supplies of energy-yielding material and of
keeping them in good working order. Their dependence upon
the environment is not so obvious perhaps as is that of the adapt-
ive mechanism proper, but as we shall see, changes in the en-
vironment do very often have immediate influence upon the
maintenance mechanism. For example, variations in the sur-
rounding temperature bring about adaptive responses in the
mechanism for keeping the Body at the proper degree of warmth.
We shall have constant occasion, therefore, to recall the facts
brought out in preceding chapters.

The External Medium. During the whole of life interchanges
of material go on between every living being and the external
world; by these exchanges material particles that one time con-
stitute parts of inanimate objects come at another to form part
of a living being; and later on these same atoms, after having
been a part of a living thing, are passed out from it in the form
of lifeless compounds. As the foods and wastes of various or-
ganisms differ more or less, so are more or less different environ-
ments suited for their existence; and there is accordingly a re-
lationship between the plants and animals living in any one place
and the conditions of air, earth, and water prevailing there. Even
such simple unicellular animals as the amoebae live only in water
or mud containing in solution certain gases, and in suspension
solid food-particles; and they soon die if the water be changed
either by essentially altering its gases or by taking out of it the
solid food. So in yeast we find a unicellular plant which thrives
and multiplies only in liquids of certain composition, and which
in the absence of organic compounds of carbon in solution will not

254

STRUCTURE AND FUNCTIONS OF BLOOD AND LYMPH 255

grow at all. Each of these simple living things, which corresponds
to one only of the innumerable cells composing the full-grown
Human Body, thus requires for the manifestation of its vital
properties the presence of a surrounding medium suited to itself:
the yeast would die, or at the best lie dormant, in a liquid con-
taining only the solid organic particles on which the amoeba lives ;
and the amoeba would die in such solutions as those in which yeast
thrives best.

The Internal Medium. A similar close relationship between
the living being and its environment, and an interchange between
the two like that which we find in the amoeba and the yeast-cell,
we find also in even the most complex living beings. When,
however, an animal comes to be composed of many cells, some
of which are placed far away from the surface of its body and
from immediate contact with the environment, there arises a new
need — a necessity for an internal medium or plasma which shall
play the same part toward the individual cells as the surrounding
air, water and food to the whole animal. This internal medium
kept in movement and receiving at some regions of the bodily
surfaces materials from the exterior, while losing substances to
the exterior at the same or other surfaces, forms a sort of middle-
man between the individual tissues and the surrounding world,
and stands in the same relationship to each of the cells of the Body
as the water in which an amoeba lives docs to that animal, or
beer-wort does to a yeast-cell. We find accordingly the Human
Body pervaded by a liquid plasma, containing gases and food-
material in solution, the presence of which is necessary for the
maintenance of the life of the tissues. Any great change in this
medium will affect injuriously few or many of the groups of cells
in the Body, or may even cause their death; just as altering the
media in which they live will kill an amoeba or a yeast-cell.

In a body so large and complex as that of man, moreover, the
internal medium must do more than merely bring food to the
individual cells and carry waste materials away from them. All
the cells have to be kept at just the right degree of warmth, but
some produce more heat than others; so part of its work is to
maintain an even distribution of heat over the whole Body or
when excess is generated to provide for its escape. Many bodily
processes, particularly the slower ones, such as growth, are not

256 THE HUMAN BODY

of a nature to be conveniently controlled by the nervous system.
Their control is vested in substances known as hormones, which
are produced in special tissues differentiated for the purpose, and
are conveyed by the internal medium to all parts of the Body,
being thus sure of reaching the structures upon which their in-
fluence is to be exerted. Finally, the environment which is fa-
vorable for the life and growth of the body-cells is also favorable
for the life and growth of foreign and harmful organisms. That
the Body is subject to invasion by such organisms is only too
well known, and but for the system of defense which the internal
medium affords these invasions could not fail to be even more
disastrous than they are.

We can summarize the functions of the internal medium as
follows :

— 1. To convey to all the living cells their needed supplies of
food material and gases.

2. To convey away from the body-cells the waste materials gen-
erated by their activities.

3. To distribute heat uniformly over the Body and provide for
getting rid of the excess.

4. To convey from the regions where they are produced to those
where they are used the special substances, hormones, which
regulate many bodily processes.

. .5. To defend the Body against the inroads of disease-producing
micro-organisms.

The Blood. In the Human Body the internal medium is
primarily furnished by the blood which, as every one knows, is a
red liquid very widely distributed over the frame, since it flows
from any part when the skin is cut through. There are in fact
very few portions of the Body into which the blood is not carried.
One of the exceptions is the epidermis or outer layer of the skin:
if a cut be made through it only, leaving the deeper skin-layers
intact, no blood will flow from the wound. Hairs and nails also
contain no blood. In the interior of the Body the epithelial layers
lining free surfaces, such as the inside of the alimentary canal,
contain no blood, nor do the hard parts of the teeth, the cartilages,
and the refracting media of the eye (see Chap. XV), but these
interior parts are moistened with liquid of some kind, and unlike
the epidermis are protected from rapid evaporation. All these

STRUCTURE AND FUNCTIONS OF BLOOD AND LYMPH 257

bloodless parts together form a group of non-vascular tissues;
they alone excepted, a wound of any part of the Body will cause
bleeding.

In many of the lower animals^ there is no need that the liquid
representing their blood should be renewed very rapidly in dif-
ferent parts. Their cells live slowly, and so require but little food
and produce but little waste. In a sea-anemone, for example,
there is no special arrangement to keep the blood moving; it is
just pushed about from part to part by the general movements
of the body of the animal. But in higher animals, especially
warm-blooded ones, such an arrangement, or rather absence of
arrangement, as this would not suffice. In them the constituent
cells live very fast, making much waste and using much food, and
altering the blood in their neighborhood very rapidly. Besides,
we have seen that in complex animals certain cells are set apart
to get food for the whole organism and certain others to remove
its wastes, and there must be a sure and rapid interchange of
material between the feeding and excreting tissues and all the
others. This can only be brought about by a rapid movement
of the blood in a definite course, and that is accomplished by
shutting it up in a closed set of tubes, and placing somewhere a
pump, which constantly takes in blood from one end of the sys-
tem of tubes and forces it out again into the other. Sent by this
pump, the heart, through all parts of the Body and back to the
heart again, the blood gets food from the receptive cells, takes
it to the working cells, carries off the waste of these latter to the
excreting cells; and so the round goes on.

The LympE. The blood, however, lies everywhere in closed
tubes formed by the vascular system, and does not come into
direct contact with any cells of the Body except those which float
in it and those which line the interior of the blood-vessels. At
one part of its course, however, the vessels through which it passes
have extremely thin coats, and through the walls of these capil-
laries liquid transudes from the blood and bathes the various
tissues. The transuded liquid is the lymph, and it is this which
forms the immediate nutrient plasma of the tissues except the
few which the blood moistens directly.

Filtration, Osmosis, and Dialysis. In the transudation of
liquid from the capillaries to the lymph spaces we encounter a

258 THE HUMAN BODY

phenomenon which is of great importance in the working of the
Body. At every step in the complex process of supplying the
living cells with nourishment and removing from them their
harmful waste products membranes stand in the way of the
liquids involved and must be traversed by them. Thus the di-
gested food must pass through the membranous lining of the
digestive tract before it can enter the blood; the oxygen of the
air must pass through a membrane in the lungs on its way to the
same medium. The juices which are secreted or excreted have
to be forced through membranes in passing out from the organs
from which they come. The movements of liquids through the
membranes of the Body take place for the most part in accordance
with certain physical principles which may conveniently be stated
at this point.

FILTRATION. If a membranous bag such as an ox bladder be
filled with a liquid and pressure be applied to the liquid in the
bag a point may be reached where the liquid is squeezed through
the membrane and appears in drops on its outer surface. This is
an example of filtration. When a liquid is filtered in this way any
solid particles which may have been suspended in it are left be-
hind, but any substances which may be dissolved in it pass through
as part of the liquid. Thus a salt solution which contained some
particles of sand might be filtered and the sand removed, but the
solution would have just as much salt dissolved in it after filtra-
tion as before.

OSMOSIS. If we should take such a membranous bag as de-
scribed above filled with salt solution and dip it into a vessel of
pure water, so that the surfaces within and without the bag are
at the same level and taking care that no water could run over
the edge into the bag, it would be seen after a while that the level
of liquid within the bag had risen while that in the vessel outside
had correspondingly fallen. That is, there would have been an
actual movement of water into the bag with sufficient force to
overcome the pressure resulting from the change of water level
on the two sides of the membrane. Whenever two solutions of
different concentrations are separated by a membrane which is per-
meable to water there will be a flow of water through the membrane
in the direction of the greater concentration. This phenomenon is
known as osmosis. The force which drives the water is called

STRUCTURE AND FUNCTIONS OF BLOOD AND LYMPH 259

osmotic pressure and is said to be exerted by any solution of
higher concentration toward any of lower concentration.

DIALYSIS. A membrane which is permeable to water but not
to any particles which may be dissolved in it is known as a semi-
permeable membrane; one which allows dissolved substances as
well as water to pass is a permeable membrane. When two so-
lutions of different concentration are separated by a membrane
of this latter class we have in addition to the movement of water
under the driving force of osmotic pressure a movement of dis-
solved particles through the membrane. This is a special case of
the general phenomenon of diffusion. Diffusion may be defined
as the tendency of substances in solution to distribute themselves
evenly throughout the solvent. Where this distribution necessi-
tates the passage of particles through permeable membranes the
phenomenon is called dialysis. The effect of both osmosis and
dialysis is to equalize the concentrations of the solutions on the
two sides of the membrane, but it must be remembered that
they are entirely distinct phenomena. To illustrate: suppose we
have on the two sides of a permeable membrane solutions re-
spectively of sugar and salt of the same concentration, that is,
having the same number of particles in solution; there would then
be no flow of water in either direction since the osmotic pressure
of both solutions is the same, but since neither the sugar nor the
salt is evenly distributed throughout the solvent there will be
dialysis of both substances until an even distribution is obtained.

Again, both osmosis and dialysis bring about changes in the
concentration of the solutions affected by them whereas filtration
does not. In considering the influence of the membranes of
the Body upon its liquid contents these facts must be borne in
mind.

The Renewal of the Lymph. Osmotic phenomena play a great
part in the nutritive processes of the Body. The lymph present
in any organ gives up things to the cells there and gets things
from them; and thus, although it may have originally been tol-
erably like the liquid part of the blood, it soon acquires a different
chemical composition. Diffusion or dialysis then commences be-
tween the lymph outside and the blood inside the capillaries, and
the latter gives up to the lymph new materials in place of those
which it has lost and takes from it the waste products it has re-

260 THE HUMAN BODY

ceivcd from the tissues. When this blood, altered by exchanges
with the lymph, gets again to the neighborhood of the receptive
cells, having lost some food-materials it is poorer in these than
the richly supplied lymph around those cells, and takes up a
supply by dialysis from it. When it reaches the excretory organs
it has previously picked up a quantity of waste matters and loses
these by dialysis to the lymph there present, which is specially
poor in such matters, since the excretory cells constantly deprive
it of them. In consequence of the different wants and wastes of
various cells, and of the same cells at different times, the lymph
must vary considerably in composition in various organs of the
Body, and the blood flowing through them will gain or lose dif-
ferent things in different places. But renewing during its circuit
in one what it loses in another, its average composition is kept
pretty constant, and, through interchange with it, the average
composition of the lymph also.

The Lymphatic Vessels. The blood, on the whole, loses more
liquid to the lymph through the capillary walls than it receives
back the same way. This depends mainly on the fact that the
pressure on the blood inside the vessels is greater than that on
the lymph outside, and so a certain amount of filtration of liquid
from within out occurs through the vascular wall in addition to
the dialysis proper. The excess is collected from the various
organs of the Body into a set of lymphatic vessels which carry it
directly back into some of the larger blood-vessels near where
these empty into the heart ; in this way the liquid which is forced
out of the blood stream in the capillaries gets back into it again.

The Lacteals. In the walls of the alimentary canal certain
food-materials after passing through the receptive cells into the
lymph are not transferred locally, like the rest, by dialysis into
the blood, but are carried off bodily in the lymph-vessels and
poured into the veins of a distant part of the Body. The lymphatic
vessels concerned in this work, being frequently filled with a white
liquid during digestion, are called the milky or lacteal vessels.

Summary. To sum up : the bipod and lymph form the internal
medium in which the tissues of the Body live; the lymph is pri-
marily derived from the blood and forms the immediate plasma
for the great majority of the living cells of the Body; and the ex-
cess of it is finally returned to the blood. The lymph moves but

STRUCTURE AND FUNCTIONS OF BLOOD AND LYMPH 261

slowly, but is constantly renovated by the blood, which is kept in
rapid movement, and which, besides containing a store of new
food-matters for the lymph, carries off the wastes which the
various cells have poured into the latter, and thus is also a sort
of sewage stream into which the wastes of the whole Body are
primarily collected.

Composition of the Blood. The average specific gravity of
human blood is 1,055. It has an alkaline reaction to litmus.
About one-third its mass consists of moist corpuscles and the re-
mainder of the liquid part or plasma. Exposed in a vacuum,
100 volumes of blood yield about 60 of gas consisting of a mixture
of oxygen, carbon dioxid, and nitrogen.

Microscopic Characters of Blood. If a finger be pricked, and
the drop of blood flowing out be spread on a glass slide, covered,
protected from evaporation, and examined with a microscope
magnifying about 400 diameters, it will be seen to consist of in-
numerable solid bodies floating in a liquid. The solid bodies are
the blood-corpuscles, and the liquid is the blood-plasma.

The corpuscles are not all .alike.. While currents still exist in
the freshly-spreacj drop of blood, the great majority of them are
readily carried to and fro ; but a certain number more commonly
stick to the glass and remain in one place. The former are the
red, the latter the pale or colorless blood-corpuscles. With proper
precautions a third sort of corpuscles, the blood-plates, may also
be seen.

Red Corpuscles. Form and Size. The red corpuscles as they
float about frequently seem to vary in form, but by a little at-
tention it can be made out that this appearance is due to their
turning round as they float, and so presenting different aspects to
view; just as a silver dollar presents a different outline according
as it is looked at from the front or edgewise or in three-quarter
profile.

Sometimes the corpuscle (Fig. 98, B) appears circular; then it is
seen in full face; sometimes linear (C), and slightly narrowed in
the middle; sometimes oval, as the dollar when half-way between
a full and a side view. These appearances show that each red
corpuscle is a circular disk, slightly hollowed in the middle (or
biconcave) and about four times as wide as it is thick. The av-
erage transverse diameter is 0.008 millimeter (^ inch). Shortly

262

THE HUMAN BODY

after blood is drawn the corpuscles arrange themselves in rows,
or rouleaux, adhering to one another by their broader surfaces.

Color. Seen singly each red corpuscle is of a pale yellow color;
it is only when collected in masses that they appear red. .The

FIG. 98. — Blood-corpuscles. A, magnified about 400 diameters. The red corpus-
cles have arranged themselves in rouleaux; a, a, colorless corpuscles; B, red cor-
puscles more magnified and seen in focus; E, a red corpuscle slightly out of focus.
Near the right-hand top corner is a red corpuscle seen in three-quarter face, and at
C one is seen edgewise. F, G, H, I, white corpuscles highly magnified.

blood owes its red color to the great numbers of these bodies in it;
if it is spread out in a very thin layer it, too, is yellow.

Structure. There is no satisfactory evidence that these cor-
puscles have any enveloping sac or cell-wall. All the methods
used to bring one into view under the microscope are such as
would coagulate the outer layers of the substance composing the
corpuscle and so make an artificial envelope. So far as optical
analysis goes, then, each corpuscle is homogeneous throughout.
By other means we can, however, show that at least two materials
enter into the structure of each red corpuscle. If the blood be
diluted with several times its own bulk of water and examined
with the microscope, it will be found that the formerly red cor-
puscles are now colorless and the plasma colored. The dilution
has caused the coloring matter to pass out of the corpuscles and
dissolve in the liquid. This coloring constituent of the corpuscle is

STRUCTURE AND FUNCTIONS OF BLOOD AND LYMPH 263

hemoglobin, and the colorless residue which it leaves behind and
which swells jgp-into a sphere in the diluted plasma is the stroma.
In the living corpuscle the two are intimately mingled through-
out it, and so long as this is the case the blood is opaque; but
when the coloring matter dissolves in the plasma, then the blood
becomes transparent, or, as it is called, laky. The difference may
be very well seen by comparing a thin layer of fresh blood diluted
with ten times its volume of ten per cent salt solution with a
similar layer of blood diluted with ten volumes of water. The
watery mixture is a dark transparent red ; the other, in which the
coloring matter still lies in the corpuscles, is a brighter opaque red.

Red corpuscles do not possess nuclei; they are not, therefore,
living cells in the ordinary sense. Whether they contain any
living protoplasm cannot be told certainly. So far as we can
judge their activities they are purely mechanical and do not re-
quire the participation of living substance.

Consistency. Each red corpuscle is a soft jelly-like mass which
can be readily crushed out of shape. Unless the pressure be such
as to rupture it, the corpuscfc immediately reassumes its proper
form when the external force is removed. The corpuscles are,
then, highly elastic; they frequently can be seen much dragged
out of shape inside the vessels when the circulation of the blood is
watched in a living animal (Chap. XX), but immediately spring-
ing back to their normal form when they get a chance.

Composition. In the fresh moist state there are in 100 parts
of red corpuscles 57 to 64 of water and 36 to 43 of solids. Of the
solids nearly ninety per cent is hemoglobin, about one per cent
inorganic salts, chiefly phosphates and chlorides of potassium, the
residue the proteins and other materials of the stroma.

Number. There is considerable variation in the number of red
corpuscles in any given volume of the blood. The average for the
adult male is stated at 5,000,000 per cubic millimeter (^ cubic
inch) ; for the adult female the figure is half a million less. The
method of determining this number is to draw from the ear or
finger tip an accurately measured volume of blood ; this is diluted
to a known volume and the number of corpuscles in a known
amount of this diluted blood is counted under the microscope.
From this the total number in any volume of undiluted blood
can easily be calculated.

264 THE HUMAN BODY

It must be remembered that the liquid part of the blood is
subject to changes of volume, either in the way of increase as
liquid is received into it from the digestive tract, or decrease as
liquid passes from it into the lymph; therefore a variation in the
number of red corpuscles per cubic millimeter does not necessarily
mean a corresponding variation in the total number in the Body.

There is a pathological condition known as anemia in which
there is a considerable reduction in the number of red corpuscles.
This is usually accompanied by a diminution in the amount of
hemoglobin contained in each corpuscle, so that as a result there
is a serious shortage in the hemoglobin content of the Body. Per-
sons suffering from this condition usually have little or no color,
and because the oxygen-carrying mechanism of the Body is be-
low normal there is a loss of bodily strength and endurance. The
condition is more common between the ages of twelve and twenty
years than at other periods, and in girls than in boys. It is not
always easily overcome and should have the care of a physician.
An outdoor life and plenty of nourishing food, in which iron con-
taining substances are included, are beneficial in such cases.

Hemoglobin. This substance, which is a compound of a pig-
ment with a protein (see Chap. I), is the functionally important
part of the red corpuscle, the stroma serving merely as a frame-
work upon which it is carried.. Its importance lies in the fact
that it combines readily with oxygen, forming a loose combina-
tion which can easily be broken up, thus it serves to transport
oxygen from the lungs to the tissues of the Body (see Respira-
tion). This property seems to be associated with the presence
of iron in the pigment part of the hemoglobin molecule.
• In the adult male about fourteen parts in the hundred by
weight of the blood are hemoglobin. It has been estimated that
a man weighing 68 kilograms (150 Ibs.) has in his blood 750 grams
(1.64 Ibs.) of hemoglobin, which is distributed among some
25,000,000,000^0 red corpuscles, giving a total superficial area
of about 3,200 sq. meters (3,800 sq. yds.) of hemoglobin. On ac-
count of the very rapid circulation of the blood (see Circulation)
practically the whole of this great area of hemoglobin is poured
through the capillaries of the lungs every thirty seconds, so it is
apparent that we have here a remarkably efficient arrangement
for supplying the Body with oxygen.

STRUCTURE AND FUNCTIONS OF BLOOD AND LYMPH 265

Origin and Fate of the Red Corpuscles. Mammalian red
corpuscles are cells which have lost their nuclei. In the red mar-
row of certain bones is the so-called hematopoietic (corpuscle-
forming) tissue where red corpuscles are constantly being formed.
The cells of this corpuscle-forming tissue are continually multi-
plying by mitotic division (see Chap. II), and the daughter cells
thus formed store up within themselves hemoglobin, lose their
nuclei, either by disintegration or extrusion, and are cast off into
the blood stream. It is not known how rapidly they are formed,
nor how long any individual corpuscle remains actively at work
in the blood stream; but it is known that sooner or later the red
corpuscles become worn out, and disintegrate; the hemoglobin is
decomposed in the liver in such fashion as to save the iron, and
the residue is converted into the bile pigments and excreted (see
Chap. XXXI).

After hemorrhage or as the result of certain pathological con-
ditions the rate of production of red corpuscles may be much
increased. When this occurs some corpuscles are liberated into
the blood stream in an immature condition, and so the blood will
be found at such times to contain nucleated as well as non-
nucleated red corpuscles.

In the human embryo the labor of making red corpuscles is
shared by many of the organs of the Body, notably the liver and
spleen.

The Colorless Blood-Corpuscles or Leucocytes (Fig. 98, F, H, G).
The colorless or white corpuscles of the blood are far less numerous
than the red; in health there is on the average about one white
to three hundred red, but the proportion may vary considerably.
Each is finely granular and consists of a soft mass of protoplasm
enveloped in no definite cell-wall, but containing a nucleus. The
granules in the protoplasm commonly hide the nucleus in a fresh
corpuscle, but dilute acetic acid dissolves most of them and brings
the nucleus into view. These colorless corpuscles belong to the
group of undifferentiated tissues, and differ in no important
recognizable character from the cells which make up the whole
very young Human Body, nor indeed from such a unicellular
animal as an Amoeba. They have the power of slowly changing
their form spontaneously. At one moment a leucocyte will be
seen as a spheroidal mass; a few seconds later (Fig. 99) processes

266 THE HUMAN BODY

will be seen radiating from this, and soon after these processes
may be retracted and others thrust out; and so the corpuscle goes
on changing its shape. These slow amoeboid movements are greatly
promoted by keeping the specimen of blood at the temperature
of the Body. By thrusting out a process
on one side, then drawing the rest of its
body up to it, and then sending out a
process again on the same side, the corpus-
cle can slowly change its place and creep
across the field of the microscope. Inside
the blood-vessels these corpuscles often ex-

Fio. 99.— A white blood- ecute similar movements: and they some-
corpuscle sketched at sue- .

cessive intervals of a few times bore right through the capillary walls

fon^du* to its and, getting out into the lymph-spaces,

movements. creep about among the other tissues. This
migration is especially frequent in inflamed parts, and the pus
or "matter" which collects in abscesses is largely made up of
white blood-corpuscles which have in this way got out of the
blood-vessels. The average diameter of the white corpuscles is
one-third greater than that of the red.

The colorless corpuscles, or some of them, are capable of tak-
ing into themselves foreign particles present in the blood; this
they do in a manner similar to that in which an amoeba feeds:
the process is known as phagocytosis and the cells exhibiting it as
phagocytes. Among the substances observed to be taken up by
white corpuscles are the minute organisms known as Bacteria,
certain species of which have been proved to be the causes of some
diseases. The white corpuscles doubtless in this way play an
important part in the cure of such diseases, or in their preven-
tion in persons exposed to infection. The accumulation of white
corpuscles in inflamed or injured parts is probably primarily as-
sociated with the removal of dead and broken-down tissues,
though it may be carried to excess as in the case of purulent ac-
cumulations.

The Blood-Plates. These are a third kind of corpuscle which
remained undiscovered for a long time after the others were known
because they break up and disappear very soon after the blood
is shed unless special precautions are taken to preserve them.
They are smaller than the red corpuscles; in structure and com-

STRUCTURE AND FUNCTIONS OF BLOOD AND LYMPH 267

position they appear to resemble somewhat the colorless corpus-
cles, although they do not possess a well-marked nucleus. They
are said to exhibit amoeboid movements under certain conditions.
The only function that is known for them is in connection with
the process of bloodH3lotting (see page 283). They are fairly
numerous, the blood containing perhaps one-tenth as many plate-
lets as red corpuscles. The promptness with which they disin-
tegrate when exposed to a foreign environment is their most
marked characteristic.

The Blood-Plasma. This is a very complex liquid, containing
as it does all the varied substances which are associated in the
carrying out of the blood's many functions. The plasma is 90
per cent water; of the remaining 10 per cent over 8.5 per cent con-
sists of the various food stuffs which the blood carries for the
nourishment of the body-cells. Nearly all of this is protein, serum
albumin, serum globulin, and fibrinogen; the remainder is sugar,
about 0.15 per cent, and fat; the latter constituent varies greatly,
being considerable after a meal rich in fats, and slight at other
times. About 0.8 of 1 per cent of the plasma consists of sodium,
potassium, and calcium salts, the sodium salts constituting by
far the greatest part of the inprganic content. The small residue
is made up mostly of the waste materials which have been received
into the blood from the tissues, and which are to be gotten rid of
in the excretory organs. The most important of these are urea,
creatinine, uric acid, and similar bodies. The plasma contains
also the various hormones, mentioned in a previous paragraph,
and a group of substances, known as biological reagents, which are
part of the disease-resisting mechanism. These will be considered
in detail in later paragraphs.

The plasma carries in solution a certain amount of oxygen,
nitrogen, and carbon dioxid, but no more than a similar amount
of pure water would dissolve at the same temperature. Most of
the oxygen carried by the blood is in combination with the hemo-
globin of the red corpuscles ; most of the carbon dioxid is in com-
bination with sodium, forming sodium carbonate or bicarbonate,
to which the alkilinity of the blood is due.

Summary. Practically the composition of the blood may be
thus stated: It consists of (1) plasma, consisting of watery solu-
tions of serum-albumin, paraglobulin, fibrinogen, sodium and

268 THE HUMAN BODY

other salts, and extractives of which the most constant are urea,
creatinine, uric acid, and grape-sugar; (2) red corpuscles, contain-
ing rather more than half their weight of water, the remainder
being mainly hemoglobin, other proteins, and potash salts; (3)
white corpuscles, consisting of water, various proteins, glycogen,
and potash salts; (4) the platelets; (5) gases, partly dissolved in
the plasma or combined with its sodium salts, and partly com-
bined (oxygen) with the hemoglobin of the red corpuscles.

Quantity of Blood. The total amount of blood in the Body is
difficult of accurate determination. It is about ^ of the whole
weight of the Body, so the quantity in a man weighing 75 kilos
(165 Ibs.) is about 5.8 kilos (12.7 Ibs.). Of this at any given
moment about one-fourth would be found in the heart, lungs, and
larger blood-vessels; and equal quantities in the vessels of the
liver, and in those of the muscles which move the skeleton; while
the remaining fourth is distributed among the remaining parts
of the Body.

Blood of Other Animals. In all animals with blood the white
corpuscles are pretty much alike, but the red corpuscles, which
with rare exceptions are found only in Vertebrates, vary con-
siderably. In all the classes of the mammalia they are circular
biconcave disks, with the exception of the camel tribe, in which
they are oval. They vary in diameter from 0.02 mm. (^ inch)
(musk deer) to 0.011 mm. (^5 inch) (elephant). In the dog
they are nearly the same size as those of man. In no mammals
do the fully-developed red corpuscles possess a nucleus. In all
other vertebrate classes the red corpuscles possess a central
nucleus, and are oval slightly biconvex disks, except in a few
fishes in which they are circular. They are largest of all in the
amphibia. Those of the frog are 0.022 mm. (j^ inch) long and
0.015 mm. (^ inch) broad.

The blood of certain crustaceans contains instead of hemo-
globin a substance of similar physiological action, hemocyanin,
which is blue instead of red, and contains copper in place of iron.

Histology and Chemistry of Lymph. Pure lymph is a color-
less watery -looking liquid ; examined with a microscope it is seen
to contain numerous white corpuscles closely resembling those of
the blood, and no doubt many are leucocytes which have mi-
grated. These lymph-corpuscles or leucocytes have, however,

STRUCTURE AND FUNCTIONS OF BLOOD AND LYMPH 2G9

another more important origin. In many parts of the Body
there are collections of a peculiar lymphoid or adenoid tissue,
sometimes in nodular masses (lymphatic glands). This tissue
consists essentially of a fine network, the meshes of which are
occupied with leucocytes which frequently show signs of division.
The meshes of the network communicate with lymphatic vessels
and the lymph flowing through picks up and carries off the new-
formed leucocytes. The lymph being ultimately poured into the
blood, the leucocytes become the colorless corpuscles of the latter;
and the migrating cells of the lymph are therefore but lymph-
corpuscles restored to it, perhaps somewhat changed during their
life in the blood-plasma.

The lymph flowing from the intestines during digestion is, as
already mentioned, not colorless, but white and milky. It is
known, as chyle, and will be considered with the process of diges-
tion. During fasting the lymph from the intestines is colorless,
like that from other parts of the Body.

Lymph is feebly alkaline, and has a specific gravity of about
1,045. The chief chemical difference between lymph and blood-
plasma is that the former contains somewhat more waste ma-
terials and less food stuffs than the latter. This is because the
consumption of food by the cells and their production of waste
keep slightly ahead of the interchange of these substances between
blood and lymph by the processes of filtratiop. and dialysis.
Lymph contains carbon dioxid in solution but no uncombined
oxygen, the latter substance being taken up by the living cells
as fast as it enters the lymph from the blood.

CHAPTER XVIII

THE HORMONE-CARRYING AND DISEASE-RESISTING FUNC-
TIONS OF THE BLOOD. BLOOD-CLOTTING

Hormones. The chemical control of bodily processes by
means of hormones has assumed great importance of recent years
and is at present the subject of active investigation. For a long
time it has been recognized that many processes are subject to
hormone influence, but the number of such processes is being
constantly added to as our knowledge advances. Although a
few of the hormones have been isolated and their chemistry
studied, by far the greater number are known only by their
physiological effects. Most of the hormones are special substances,
formed in organs whose sole function, so far as we can judge, is
their production. A few of them exercise their hormone function
only incidentally to their chief bodily destiny.

The functions of many hormones are associated with the proper
carrying out of ordinary bodily processes, as, for example, the
digestive process. Such will be considered in connection with
the study of those processes. Only those which are riot so asso-
ciated will be taken up here.

The Ductless Glands. There are in the Body several organs
of such considerable size and so constantly present in vertebrate
animals that a priori they would seem to be of functional impor-
tance. Until quite recently, however, the functions of nearly
all of them were quite problematical, although it has long been
known that pathological changes in some of them were associated
with grave conditions of general disease. Even yet their physi-
ology is very incompletely known.

When we speak of a true gland we mean an organ that forms
some definite secretion which it pours out in a separate form, but
the organs we are about to consider have no secreting recesses
and no ducts:. nevertheless they undoubtedly. make, and pass into
the lymph and blood, substances of great importance to the
healthy working of the Body, which we call hormones.

270

FUNCTIONS OF THE BLOOD 271

The Thyroid Gland. This organ lies in the neck on the sides
of the windpipe and consists usually of a right and a left lobe
united by a narrow isthmus across the front of the air-tube. It is
about thirty grams (two ounces) in weight; in the disease known
as goiter it is greatly enlarged and its structure altered. The
thyroid is dark red in color and very vascular, richly supplied
with nerves, and is subdivided by connective tissue into cavities
or alveoli, the largest of which are just visible to the unaided eye.
Each alveolus is lined by a single layer of cuboidal cells, and filled
by a glairy fluid known as the thyroid colloid.

From the gland can be obtained, in addition to the usual or-
ganic compounds, a peculiar substance containing a large percen-
tage of iodine, and known as iodothyrin. This compound was
thought, when first discovered, to be the hormone of the gland,
but fuller study showed that iodothyrin as such is not the hormone
although it probably has to do in some way with it. Although
the chemistry of the hormone is not known its physiology can be
studied indirectly by observing the effect of depriving the Body
of it through removal of the gland, either experimentally or as
the result of disease.

These studies show that the hormone of the thyroid gland has
a great deal to do with the proper carrying on of those chemical
activities of living cells which constitute their "vital" processes
and which are grouped together under the term metabolism.
Lack of the hormone results in a condition of general malnutrition,
known as cachexia. Beside this general symptom certain special
tissues show very strikingly the effects of being deprived of the
thyroid hormone; these are the connective and nervous tissues.
In adults a condition known as myxedema is the result of such
deprivation; its most marked signs are swellings of the subcu-
taneous connective tissues, whereby the outlines of the Body are
much distorted; and distressing mental deterioration. Some-
times children are bom in whom the thyroid gland fails to de-
velop properly; they grow into dwarfish, misshapen idiots. To
such a condition the name cretinism is applied. The sufferers
are called cretins. Thanks to the discovery that by simple feeding
of thyroid material the hormone can be supplied in ample quan-
tity, sufferers from myxedema and cretinism are now restored to
perfectly normal condition; although it is said that for the treat-

272 THE HUMAN BODY

ment to be wholly successful for cretins it must be begun quite
early in life.

The Para Thyroids. These are small bodies, usually four in
number, which are found near or on the thyroids, sometimes em-
bedded within them. The effect of their complete removal is very
striking, consisting in the onset of acute toxic symptoms, with
muscular convulsions, ending in death. Recently the interesting
discovery has been made that the characteristic symptoms fol-
lowing extirpation of the para thyroids do not develop if calcium
salts are introduced into the blood. It has long been recognized
that the element calcium plays a very important, though ill-
understood, role in the Body. On the basis of the above observa-
tion the suggestion has been made that the para thyroids may
function in some way to control the use of calcium by the Body..
It is impossible at present to judge finally as to the value of this
suggestion.

The Thymus is a large and conspicuous organ in the new-born
mammal. As obtained from calves it constitutes the "neck-
sweetbread." Its structure places it unmistakably among the
ductless glands. Up to the present its function remains unknown
although some evidence has been presented to. indicate that it
may be associated with the development of the lymphatic system
(Chap. XXII). It can be removed without fatal or even tem-
porary effect. It was formerly thought to atrophy completely
soon after birth, but careful study has shown that some true thy-
mus tissue persists in man for years and perhaps throughout life.

The Pituitary Body (Fig. 62) consists of two lobes, a posterior
one, which is an offshoot of the brain, and an anterior one which
develops from the pharynx. It seems quite likely that the two
lobes are really independent organs physiologically, producing
hormones having different effects. A substance has been ob-
tained from the posterior lobe which acts specifically upon the
kidneys, increasing their output of urine. Effects upon the heart
and blood-vessels following injection of extracts of this lobe are
also reported.

Complete removal of the pituitary body in the case of cats and
dogs causes a lowering of temperature, muscular twitchings and
spasms, difficulty in breathing, general lassitude, and death within
a fortnight. The organ has therefore been supposed to form a

FUNCTIONS OF THE BLOOD 273

hormone useful in maintaining the nutrition of the muscular and
nervous systems. Over-development of the anterior lobe of the
pituitary body in man has been found to be associated with the
curious condition named acromegaly, in which there is hypertrophy
of the bones of the limbs and face. It will be noted that this
disease, acromegaly, is essentially one of overgrowth, and since
it is associated with an enlargement of the gland is probably the
result of over-stimulation of the growth process through too
abundant production of the hormone. This idea is borne out by
observations of conditions in which the hormone is deficient.
When this occurs there is an excessive tendency to lay on fat,
suggesting that materials which would normally be used for
tissue growth are not called for by the tissues and so are stored in
the form of fat. Failure of development of certain organs, not-
ably those of generation, also results from a deficiency of the
hormone.

The Suprarenal Capsules or Adrenals are a pair of small organs,
weighing together about 12 grams (f oz.) placed one on the top
of each kidney. They have, however, no intimate connection
with the kidneys, and in many animals are placed at some dis-
tance from them. Each consists of a denser less colored external
cortex, and a central deep yellow-brown softer medulla. The cor-
tex is subdivided into chambers by connective tissue, and the
chambers are filled by closely packed, polygonal nucleated cells.
Similar cells are found in the medulla, which is, moreover, closely
connected with the sympathetic system and is richly supplied
with nerves.

It was noticed some seventy years ago by a physician named
Addison that certain obscure diseased conditions characterized by
great debility and by the appearance of bronzed patches on the
skin, and leading to death, were found on post-mortem examina-
tion to be accompanied by disease of the adrenals. The disease
has since been named Addison's disease. When the suprarenal
capsules are completely removed from animals a similar fatal
diseased condition results, death taking place in warm-blooded
animals within two or three days, and being preceded by muscu-
lar weakness, dilatation of the arteries, mental feebleness and
general prostration. A substance of comparatively simple chem-
ical composition, known as adrenalin, can be obtained from the

274 THE HUMAN BODY

adrenal bodies. It exhibits very characteristic properties and is
thought to be the hormone secreted by them. The physiological
action of adrenalin appears to be very specific, and to be that of
stimulating the post-ganglionic fibers of the sympathetic system
proper at their terminations in smooth muscles or in glands.
Curiously those post-ganglionic fibers that arise in other than true
sympathetic ganglia, as the ones supplying the ciliary muscle of
the eye, appear to be unaffected by adrenalin.

The widest distribution of smooth muscles receiving innerva-
tion from the sympathetic system proper is in the circular coats
of the small arteries. Accordingly, the most conspicuous effect of
the administration of adrenalin is upon the circulation, through
changes in the caliber of these vessels. This effect will be con-
sidered in detail in connection with the circulation (Chap. XXII).

Infection. Bacteriology has taught us that we are continu-
ally surrounded by myriads of micro-organisms of various kinds.
They are on the skin and mucous membranes; they are breathed
in with the air and swallowed in the food and water; colonies of
them flourish in the intestinal tracts. Not all of them are disease
producing (pathogenic), but these are always present along with
the harmless varieties.

Not only are these organisms always present, but small num-
bers of them are always finding their way into the lymph spaces
of the Body, whence they get into the blood. The entry of patho-
genic organisms into the Body does not constitute infection. It is
only when they gain a foothold and begin to multiply that the
infection is established and the disease under way.

It is recognized that the ill effects of infection are not due to
the presence of the bacteria, merely, but to poisonous substances,
or toxins, which the bacteria produce as incidents in their vital
processes. Some sorts of bacteria give off this poison to the blood,
themselves remaining out of the blood stream; the diphtheria
organism is of this sort. Others retain the toxin within them-
selves, and it is only when they die and decompose that the poison
is liberated.

Resistance to Infection. It is obvious that the Body does not
become infected every time pathogenic organisms gain entry into
it. If it did infection would be our hourly fate. The tissues of
the Body form, however, excellent culture media; bacteria that

FUNCTIONS OF THE BLOOD 275

do establish themselves flourish mightily, at least for a time.
It follows, therefore, that bacteria are ordinarily forcibly pre-
vented from establishing themselves. This prevention of infection
is one, of the functions of the blood. It must be sharply differen-
tiated from an additional disease-resisting function possessed also
by the blood, which is the overcoming of infection after it is once
established. In the absence of this latter function every infec-
tion would result fatally.

The Infection-Resisting Mechanism. Two sorts of structures
in the blood are engaged in the destruction of invading micro-
organisms. They work independently but in cooperation. The
first of these are the phagocytes previously mentioned (p. 266)
which engulf and thus dispose of the invading foreign bodies.
The second sort is not made up of formed elements like the phago-
cytes, but is in solution in the plasma. It attacks and destroys
bacteria by what seems to be an enzym action. To this sub-
stance is given the name alexin. It has been shown to be made
up of two other substances, the complements, which are probably
formed as part of the leucocytes and given off to the blood when
they disintegrate, and the intermediary bodies which are constant
constituents of the plasma. The complements are thought to be
the actively destructive agents, but to be unable to attack foreign
cells except when in combination with the intermediary bodies.
Since the complements are derived from disintegrating leucocytes
their number is variable, depending on whether the blood is rich
or poor in these structures.

Why Does Infection Ever Occur? The establishment of an
infection in the face of this double protective mechanism can be
explained in one of two ways. Either the mechanism falls off in
efficiency, which is the condition present when we say "the re-
sistance of the body is lowered," or the invading organisms are so
virulent that the Body is unable to overcome them. Lowered
body resistance may result from a number of. conditions; under-
nutrition, prolonged exposure to extremes of heat or cold, alco-
holism, severe local inflammations, all of these may diminish the
number of phagocytes or the quantity of alexins, or may lessen
their activity. Bacteria may vary from time to time in virulence.
It appears that the virulence of most sorts is much increased by a
period of growth in a living Body. It is because of this increase

276 THE HUMAN BODY

of virulence that " exposure " to an infected individual is so often
followed by infection. The fact of increased virulence explains
also the occurrence of "epidemics."

Recovery from Infection involves two processes: 1. Destroying
and getting rid of the enormous numbers of bacteria which de-
velop during the progress of the disease; 2. Getting rid of or
neutralizing the poison, or toxin, which the bacteria produce and
which is the real cause of trouble. The course of every infection
is a struggle between the Body on one hand and the micro-
organisms on the other. The outcome is recovery or death ac-
cording as one side or the other proves victorious.

For destroying and getting rid of the bacteria the Body makes
use of the same structures, the complements and phagocytes,
that it uses in resisting infection in the first place; but the effi-
ciency of these is enormously increased through the development
of special aids to their activity.

Opsonins, Immune Bodies, and Agglutinins. The presence and
growth of foreign organisms stimulate the cells of the Body to
produce and set free in the blood large numbers of bodies of prob-
ably at least three sorts. The first of these, called opsonins, act
upon the invading bacteria in such a way as to increase very
greatly the " appetite " of the phagocytes for them. It is possible
to obtain living phagocytes in salt solution, free from the other
elements of blood. If to a slide containing some of these a num-
ber of bacteria be added and the whole kept at body temperature,
the average number of bacteria ingested by each phagocyte can
be determined by actual observation. It is found that if the
bacteria, before being placed on the slide, are treated with a
liquid containing the proper opsonin, the average ingestion per
phagocyte is multiplied many fold. It is necessary, for this effect
to be produced, that the opsonin be applied to the bacteria; treat-
ment of the phagocytes with opsonin, with subsequent washing,
does not increase at all their tendency to ingest bacteria.

The second sort of bodies produced by the living cells under
the stimulus of foreign organisms are the immune bodies. These
function, as do the intermediary bodies described above, to enable
the complements to attack and destroy the invading bacteria,
but many times more powerfully.

In addition to opsonins and immune bodies, the cells under

FUNCTIONS OF THE BLOOD 277

bacterial stimulation produce what is thought to be a third sub-
stance, agglutinin, which causes the bacteria to clump together,
becoming thus more subject to the action of the phagocytes or
complements. The development of these various bodies is the
process of immunization.

Specific Nature of Opsonins and Immune Bodies. A very
interesting fact about the bodies which the living cells develop
under bacterial stimulation is that their action is very specific.
The opsonins which are developed in an individual under the
stimulus of invading typhoid bacilli increase the tendency of the
phagocytes to ingest those particular organisms and no others.
Similarly immune bodies developed by the pneumococcus enable
the complements of the blood to attack those bacteria and no
others. For this reason immunity against one kind of infection
does nqt protect the Body against other kinds.

Antitoxin. Beside the destruction of the invading bacteria
it is necessary, before the Body is cured of an infection, that
the toxins produced by the rapidly multiplying organisms be neu-
tralized. This neutralization of poison is a simpler process than
the destruction of formed elements as carried on by the phago-
cytes and complements. It is brought about in the Body, how-
ever, in much the same way. The foreign toxin stimulates the
cells of the Body to produce and pour into the blood an antitoxin
which neutralizes the toxin. Antitoxins, like opsonins and im-
mune bodies, are specific for the toxin which stimulated their
development.

Immunity. An individual who has gone through an infection,
and by the cooperation of the forces described above has over-
come and destroyed the invaders with their harmful toxins, re-
tains for a long time afterward in his blood the special opsonins,
immune bodies, and antitoxins which were developed therein
during the course of the infection. He is, during this time, im-
mune toward that particular disease. The existence of this im-
munity has been known for centuries ; its explanation is the result
of the work of the last fifteen years.

The Use of Antitoxin in Disease. In some diseases, of which
diphtheria is the best known example, the bacteria do not spread
through the Body but take up their abode on a convenient sur-
face where they develop and whence they discharge their toxin

278 THE HUMAN BODY

into the blood. Successful combating of such diseases requires
only that the toxin be neutralized. In course of time the bacteria
will reach the end of their development and die.

The antitoxin for any particular kind of toxin will neutralize
it whether produced in the body which is infected or in some
other body from which it is transferred to the infected one. This
fact has made possible the development of the well-known anti-
toxin treatment. Animals, usually horses, receive doses of toxin
obtained by growing the bacteria on culture media in proper
vessels. These doses are small at first, but are gradually increased
as the animal acquires immunity. In course of time the blood of
an animal so treated contains large quantities of antitoxin. Con-
siderable amounts of blood can be withdrawn from animals the
size of horses without their suffering the slightest inconvenience.
It is thus possible to obtain abundant supplies of antitoxin.

The methods of purifying antitoxin-containing solutions are
so perfect at the present time that no one should feel the least
hesitation at the prospect of its use. The percentage of deaths
from diphtheria has fallen from more than fifty to about two
since its introduction. The distressing symptoms which some-
times follow its administration are not effects of the antitoxin,
but are due to imperfect neutralization of the toxin, and appear
because the life of the patient, thanks to the partial overcoming
of the poison, is saved long enough to give them time to become
manifest.

Protective Inoculation. It has been found practicable in some
diseases, notably smallpox, to develop immunity by infecting the
Body with an organism which is not virulent enough to endanger
life but which produces immune substances that protect the Body
against the more virulent infection. On account of the specific
character of immunity this method can only be used where vir-
tually the same organism occurs in virulent and non-virulent
forms.

The most hopeful path of progress at present toward the mas-
tery of disease is along the lines here indicated. We may look
forward confidently to a time when most if not all the acute in-
fections will be brought under medical control through applica-
tion of the principles of immunity.

The Coagulation of the Blood. When blood is first drawn

FUNCTIONS OF THE BLOOD 279

from the living Body it is perfectly liquid, flowing in any direction
as readily as water. This condition is, however, only temporary;
in a few minutes the blood becomes viscid and sticky, and the
viscidity becomes more and more marked until, after the lapse
of five or six minutes, the whole mass sets into a jelly which ad-
heres to the vessel containing it, so that this may be inverted
without any blood whatever being spilled. This stage is known as
that of gelatinization and is also not permanent. In a few minutes
the top of the jelly-like mass will be seen to be hollowed or
"cupped" and in the concavity will be seen a small quantity of
nearly colorless liquid, the blood-serum. The jelly next shrinks
so as to pull itself loose from the sides and bottom of the vessel
containing it, and as it shrinks squeezes out more and more serum.
Ultimately we get a solid clot, colored red and smaller in size than
the vessel in which the blood coagulated though retaining its
form, floating in a quantity of pale yellow serum. If, however,
the blood be not allowed to coagulate in perfect rest, a certain
number of red corpuscles will be rubbed out of the clot into the
serum and the latter will be more or less reddish. The longer the
clot is kept the more serum will be obtained : if the first quantity
exuded be decanted off and the clot put aside and protected from
evaporation, it will in a short time be found to have shrunk to a
smaller size and to have pressed out more serum; and this goes on
until putrefactive changes commence.

Cause of Coagulation. If a drop of fresh-drawn blood be spread
out very thin and watched for a few minutes with a microscope
magnifying 600 or 700 diameters, it will be seen that the coagu-
lation is due to the separation of very fine solid threads which
run in every direction through the plasma and form a close net-
work entangling all the corpuscles. These threads are composed
of the protein substance fibrin. When they first form, the whole
drop is much like a sponge soaked full of water (represented by
the serum) and having solid bodies (the corpuscles) in its cavi-
ties. After the fibrin threads have been formed they tend to
shorten; hence when blood clots in mass in a vessel, the fibrinous
network tends to shrink in every direction just as a network
formed of stretched india-rubber bands would, and this shrinkage
is greater the longer the clotted blood is kept. At first the threads
stick too firmly to the bottom and sides of the vessel to be pulled

280 THE HUMAN BODY

away, and thus the first sign of the contraction of the fibrin is
seen in the cupping of the surface of the gelatinized blood where
the threads have no solid attachment, and there the contracting
mass presses out from its meshes the first drops of serum. Finally
the contraction of the fibrin overcomes its adhesion to the vessel
and the clot pulls itself loose on all sides, pressing out more and
more serum, in which it ultimately floats. The great majority
of the red corpuscles are held back in the meshes of the fibrin,
but a good many pale corpuscles, by their amoeboid movements,
work their way out and get into the serum.

Whipped Blood. The essential point in coagulation being the
formation of fibrin in the plasma, and blood only forming a cer-
tain amount of fibrin, if this be removed as fast as it forms the
remaining blood will not clot. The fibrin may be separated by
what is known as "whipping" the blood. For this purpose fresh-
drawn blood is stirred up vigorously with a bunch of twigs, and
to these the sticky fibrin threads as they form, adhere. If the
twigs be withdrawn after a few minutes a quantity of stringy
material will be found attached to them. This is at first colored
red by adhering blood-corpuscles: but by washing in water they
may be removed, and the pure fibrin thus obtained is perfectly
white and in the form of highly elastic threads. It is insoluble in
water and in dilute acids, but swells up to a transparent jelly in
the latter. The " whipped " or " defibrinated blood " from which
the fibrin has been in this way removed, looks just like ordinary
blood, but has lost the power of coagulating spontaneously.

The Buffy Coat. That the red corpuscles are not an essential
part of the clot, but are merely mechanically caught up in it,
seems clear from the microscopic observation of the process of
coagulation; and from the fact that perfectly formed fibrin can
be obtained free from corpuscles by whipping the blood and
washing the threads which adhere to the twigs. Under certain
conditions, moreover, one gets a naturally formed clot containing
no red corpuscles in one part of it. The corpuscles of human blood
are a little heavier, bulk for bulk, than the plasma in which they
float; hence, when the blood is drawn and left at rest they sink
slowly in it; and if for any reason clotting take place more slowly
or the corpuscles sink more rapidly than usual, a colorless top
stratum of plasma, with no red corpuscles in it, is left before

FUNCTIONS OF THE BLOOD 281

gelatinization occurs and stops the further sinking of the cor-
puscles. The uppermost part of the clot formed under such cir-
cumstances is colorless or pale yellow, and is known as the buffy
coat; it is especially apt to be formed in the blood drawn from
febrile patients, and was therefore a point to which physicians
paid much attention in the olden times when blood-letting was
thought to be almost a panacea. In horse's blood the difference
between the specific gravity of the corpuscles and that of the
plasma is greater than in human blood, and horse's blood also
coagulates more slowly, so that its clot has nearly always a buffy
coat. The colorless buffy coat seen sometimes on the top of the
clot must, however, not be confounded with another phenomenon.
When a blood-clot is left floating exposed to the air its top be-
comes bright scarlet, while the part immersed in the serum has
a dark purple-red color. The brightness of the top layer is due
to the action of the oxygen of the air, which forms a scarlet com-
pound with the coloring matter of the red corpuscles. If the
clot be turned upside down and left for a short time, the pre-
viously dark red bottom layer, now exposed to the air, becomes
bright.

Uses of Coagulation. The clotting of the blood is so important
a process that its cause has been frequently investigated ; but it is
not yet completely understood. The living circulating blood in
the healthy blood-vessels does not clot; it contains no solid fibrin,
but this forms in it, sooner or later, when the blood gets by any
means out of the vessels or when the lining of these is injured.
In this way the mouths of the small vessels opened in a cut are
clogged up, and the bleeding, which would otherwise go on in-
definitely, is stopped. So, too, when a surgeon ties up an artery
before dividing it, the tight ligature crushes or tears its delicate
inner surface, and the blood clots where that is injured, and from
there a coagulum is formed reaching up to the next highest branch
of the vessel. This becomes more and more solid, and by the time
the ligature is removed has formed a firm plug in the cut end of
the artery, which greatly diminishes the risk of bleeding.

The Source of Blood-Fibrin. Since fresh blood-plasma contains
no fibrin but does contain considerable quantities of other pro-
teins, we look first to these as a possible source of the fibrin formed
during coagulation. If horse's blood be drawn directly from the

282 THE HUMAN BODY

living animal into a cold vessel and kept just above freezing
temperature it does not clot and after a time the corpuscles settle
to the bottom leaving a supernatant portion of clear plasma. This
plasma retains the power of clotting, as is shown when it is
warmed ; but if before it clots it be saturated with sodium chlorid
and filtered, the liquid that remains will no longer clot. The
precipitate formed by the saturation with sodium chlorid must
contain, therefore, some essential in the process of clotting. This
precipitate if examined will be found to be a mixture containing
all the fibrinogen of the plasma and part of the para globulin.
These two substances may be separated by proper treatment,
and after this has been done it is found that a solution of the
fibrinogen can be made to clot, while one containing only para
globulin cannot. During the clotting of the fibrinogen solution
all the fibrinogen disappears, giving place to fibrin.

We are thus led to the conclusion that the natural clotting of
fresh blood is due to the formation of fibrin from fibrinogen which
existed in solution in the plasma of the circulating blood and has
been altered in the clotted, giving origin to fibrin. But as normal
blood circulating in healthy uninjured blood-vessels does not clot
nor do pure solutions of fibrinogen, we have still to seek the ex-
citing cause of the change.

If to a solution of fibrinogen there be added a few drops of
blood or of blood-serum, or of the washings of a blood-clot, fibrin
will be formed; therefore drawn blood and serum and natural
clot each contain something which can effect the conversion of
fibrinogen into fibrin. This substance is thrombin, frequently
called also the fibrin ferment.

Thrombin. When blood-serum is treated with several times
its volume of strong alcohol its various proteins and most of its
salts are precipitated : if the precipitate be left standing in alcohol
for some days the proteins become almost entirely insoluble in
water, but a few drops of the watery extract cause clotting in a
saline solution of fibrinogen, and clearly contain some of the
thrombin. This substance was for a long time believed to be
an enzym, hence its name of " fibrin ferment." Recent careful
study shows, however, that it does not correspond to enzyms in
either of their two cardinal characteristics, namely, the ability
of a small amount of the substance to produce a very large amount

FUNCTIONS OF THE BLOOD 283

of chemical activity, and the destruction of the substance by
heating above 60° C. It has been definitely proven that the
amount of fibrinogen that is converted to fibrin bears a direct
relationship to the amount of thrombin present, and that throm-
bin solutions free from protein impurities can be boiled without
destroying the thrombin.

Source of Thrombin. If fresh blood is drawn directly from
the veins of an animal into strong alcohol, and the resulting pre-
cipitate treated as described above for preparing thrombin from
serum, it yields no thrombin; this substance, therefore, which is
present in blood-serum, is absent from the blood within the Body
and must be formed after the blood is shed -and before the forma-
tion of the clot. When the process of clotting is watched under
the microscope the fibrin threads will usually be seen to form
about certain centers. These centers consist of disintegrating
blood-opiates, and the observation that fibrin formation proceeds
from them in this fashion led to the idea that the blood-plates are
in some way associated with the process.

The natural conclusion drawn from this observation was that
the blood-plates contain thrombin which is inactive so long as
they are intact, and is liberated by their disintegration. This
simple conclusion was upset" by the further observation that fresh
blood drawn into a solution of sodium oxalate will not clot.
Sodium oxalate does not hinder the process of blood-plate disin-
tegration. In fact its sole effect upon blood, so far as can be de-
termined, is to precipitate out its calcium, as calcium oxalate.
That the prevention of clotting is due to this precipitation of cal-
cium is shown by the fact that addition of excess of a soluble cal-
cium salt to " oxalate " blood causes it to clot with great prompt-
ness. The formation of active thrombin is dependent, then, upon
the presence of calcium in the blood, and the substance contained
in the blood-plates is not true thrombin, but a preparatory sub-
stance which we may call prothrombin.

Summary of the Process of Coagulation. We may picture the
entire process of blood-clotting somewhat as follows:

1. As the result of exposure to a foreign environment the blood-
plates disintegrate, yielding prothrombin.

2. The prothrombin thus set free reacts with the calcium of the
blood and forms thrombin.

284 THE HUMAN BODY

3. By a reaction between thrombin and fibrinogen insoluble
fibrin is precipitated in the form of a sticky network.

4. The fibrin network entangles corpuscles within it, forming
a typical clot.

It should be stated in this connection that some physiologists
have supposed the process of blood-clotting to be more complex
than here outlined. Their view has been based upon such observa-
tions as that blood drawn carefully from blood-vessels without
coming into contact with the tissues of the wound clots much
more slowly than does blood which has flowed over raw tissue
surfaces. Reptiles and birds show this difference very strikingly.
This phenomenon has been interpreted as indicating that a third
substance, furnished by tissues generally, is involved in the coag-
ulation process. The tendency of the most recent work seems,
however, to be against the necessity of there being any such
third substance. In the case of birds, for instance, it has been
shown that clotting is brought on as quickly by throwing a little
dust into the blood as by allowing it to come in contact with raw
tissues; a result that removes the probability of the tissue yield-
ing a specific substance to the coagulation process.

Why Coagulation Does Not Occur within the Blood- Vessels.
In the process of clotting outlined above the initial step is the
disintegration of the blood-plates; unless this takes place there
will be no clotting. It is only in the presence of a foreign surface,
such as air or the injured wall of a blood-vessel, that there is any
considerable breaking down of these structures. There is, doubt-
less, a certain amount of disintegration occurring normally in the
blood-stream, for there is no reason to suppose that the blood-
plates are specially endowed with longevity, but the small amounts
of prothrombin thus liberated and the thrombin which may be
formed therefrom are probably taken care of either by the cells
forming the lining of the blood-vessels or by some other struc-
tures adapted to that purpose; there is some evidence that the
liver functions in this way.

The formation of blood-clots (thrombi) within the vessels is
likely to be followed by serious effects, due to the plugging of im-
portant vessels by the clotted blood, but the occurrence of thrombi
in the intact healthy circulation is unknown; their formation pre-
supposes either some injury to the walls of the blood-vessels, as

FUNCTIONS OF THE BLOOD 1's:,

by crushing them or tying ligatures about them, or the introduc-
tion of some foreign substance into the circulation.

Methods of Hastening or Retarding Coagulation. Since the
process of clotting is in several steps there are a corresponding
number of points at which its normal course may be broken into,
either with the effect of hastening the result or of retarding it
or even preventing it altogether. Anything which quickens the
disintegration of the blood-plates, as the application of a hand-
kerchief to a wound, which acts by increasing the foreign surface
hi contact with the blood, makes the blood clot more quickly.
The application of heat has this same effect; probably it acts both
by increasing the rate of destruction of blood-plates and by has-
tening the chemical reactions involved in the process as a whole.
Cold, as would be expected, has the converse effect. An increase
in the calcium content of the blood shortens the coagulation time.
Coagulation may be retarded, as we have seen, by cold or by de-
priving the blood of its calcium content. Blood drawn into a
strong solution of sodium or magnesium sulphate and well mixed
will not clot, these salts appearing to interfere in some way with
the formation of the thrombin; such "salted" blood will clot if
thrombin is added or if diluted sufficiently with water. There are
a number of other substances which either retard or prevent
coagulation; their mode of action is for the most part obscure.
Among them are sodium fluoride, leech extract, snake venom, and
commercial peptone solution. This latter is of interest because
it only produces its effect of destroying the coagulating power of
the blood when introduced into the veins of the animal. When
added to shed blood it has no effect on coagulation. It has been
shown that this property does not reside in the peptone itself,
but in some unknown impurity which is associated with it in the
process of preparation. The blood of persons suffering from fever
does not clot as quickly as that of healthy persons. This fact has
been known since the days when bloodletting was a common
practice. Its explanation remains obscure.

" Bleeders." There is a pathological condition, fortunately not
very common, known as hemophilia, in which the blood will not
clot. Persons suffering from this disease are called bleeders. Such
persons are in danger of bleeding to death from slight wounds; a
nosebleed, or the bleeding which follows the extraction of a tooth,

286 THE HUMAN BODY

becomes in such persons an affair of the utmost gravity. Just
what is lacking in the blood of " bleeders " is not known, it may
be any one or a combination of several of the elements whose
united action is essential to the process of clotting.

This condition is usually hereditary. An interesting fact in
connection with it is that whereas the disease itself appears only
in males, its transmission seems to be confined wholly to females;
a father who was a "bleeder" would have no children suffering
from the condition nor would his sons, but if his daughters had
sons they would probably be bleeders.

Blood Transfusion. The restoration of blood lost in severe
hemorrhage, or the replacement of diseased blood by healthy
blood through transfusion from the veins of one individual to
those of another has long been a dream of physicians and physi-
ologists. The early attempts to treat disease by this method were
more often fatal than not because the blood to be introduced into
the circulation had to be defibrinated. This process, as we have
seen, preserves the blood in a liquid condition, but it leaves in it
large quantities of the exciting agent to coagulation, thrombin.
When such blood was introduced into the circulation it usually
induced prompt clotting of the blood already there, with im-
mediately fatal results. The fuller knowledge of the mechanism
of blood-clotting gained of late years has made it clear that blood
transfusion need not be followed by clotting if the transfer of
blood be made without exposing it at any time to a foreign sur-
face, such as favors the disintegration of the blood-plates. In
accordance with this idea an ingenious method has recently been
developed whereby an artery of one individual can be brought
into communication with a vein of another and the blood allowed
to flow naturally across the living channel thus formed. Many
lives have been saved by this method during the few years since
its first application, and it promises to fulfil in some degree, at
least, the early hopes of the medical world. It should be noted
that successful blood transfusion requires that the blood to be
introduced be taken from an individual of the same species as the
one who is to receive it; hence human beings who require blood
must receive it from other human beings, and not from animals.
One of the most curious facts brought out in connection with the
study of the disease-resisting mechanism of the Body is that to

FUNCTIONS OF THE BLOOD 287

this mechanism the red corpuscles of animals of a different species
are as much foreign bodies to be attacked and destroyed as are
the most malignant bacteria. The introduction of foreign blood,
even if not attended by coagulation, is therefore more apt than
not to be fatal, through the destruction of each kind of corpuscles
by the liquid portion of the other sort of blood. Fortunately the
operation of transfusing blood is neither excessively painful nor
accompanied by untoward after effects to the donor, and persons
can always be found who are willing to undergo the discomfort
involved for the sake of restoring a fellow-being to health.

CHAPTER XIX

THE ANATOMY OF THE HEART AND BLOOD-VESSELS

General Statement. During life the blood is kept flowing with
great rapidity through all parts of the Body (except the few non-
vascular tissues already mentioned) in definite paths prescribed
for it by the heart and blood-vessels. These p^ths, which under
normal circumstances it never leaves, consti-
tute a continous set of closed tubes (Fig. 100)
beginning at and ending again in the heart,
and simple only close to that organ. Else-
where it is greatly branched, the most numer-
m ous and finest branches (/ and a) being the
capillaries. The heart is essentially a bag
with muscular walls, internally divided into
four chambers (d, g, e, /). Those at one end
(d and e) receive blood from vessels opening
into them and known as the veins. From
there the blood passes on to the remaining
chambers (g and /) which have very power-
ful walls and, forcibly contracting, drive the
FIG 100— The heart blood out into vessels (m and 6) which com-
and blood-vessels dia- municate with them and are known as the

grammatically repre- . . .....

sented. arteries. The big arteries divide into smaller;

these into smaller again (Fig. 101) until the branches become too
small to be traced by the unaided eye, and these smallest branches
end in the capillaries, through which the blood flows and enters
the commencements of the veins; and these convey it again to
the heart. At certain points in the course of the blood-paths
valves are placed, which prevent a back-flow. This alternating
reception of blood at one end by the heart and its ejection from
the other go on during life steadily about seventy times in a
minute, and so keep the liquid constantly in motion.

The vascular system is completely closed except at two points
in the neck where lymph-vessels open into the veins; there some

288

ANATOMY OF THE HEART AND BLOOD-VESSELS 289

lymph is poured in and mixed directly with the blood. Accord-
ingly everything which leaves the bloo$J must do so by oozing
through the walls of the blood-vessels, and everything which enters
it must do the same, except matters conveyed in by the lymph

• lu

del

mvs

mvp

Fio. 101. — The arteries of the hand, showing the communications or anasto-
moses of different arteries and the fine terminal twigs given off from the larger
trunks; these twigs end in the capillaries which would only become visible if
magnified. R, the radial artery on which the pulse is usually felt at the wrist;
U, the ulnar artery.

at the points above mentioned. This interchange through the
walls of the vessels takes place only in the capillaries, which form
a sort of irrigation system all through the Body. The heart,
arteries, and veins are all merely arrangements for keeping the
capillaries full and renewing the blood within them. It is in the
capillaries alone that the blood does its physiological work.

The Position of the Heart. The heart (h, Fig. 1) lies in the

290 THE HUMAN BODY

chest immediately above the diaphragm and opposite the lower
two-thirds of the breast-bone. It is conical in form with its base
or broader end turned upwards and projecting a little on the
right of the sternum, while its narrow end or apex, turned down-
wards, projects to the left of that bone, where it may be felt beat-
ing between the cartilages of the fifth and sixth ribs. The position
of the organ in the Body is therefore oblique with reference to its
long axis. It does not, however, lie on the left side as is so com-
monly supposed but very nearly in the middle line, with the upper
part inclined to the right, and the lower (which may be more
easily felt beating — hence the common belief) to the left.

The Membranes of the Heart. The heart does not lie bare in
the chest but is surrounded by a loose bag composed of connect-
ive tissue and called the pericardium. This bag, like the heart,
is conical but turned the other way, its broad part being lowest
and attached to the upper surface of the diaphragm. Internally
it is lined by a smooth serous membrane like that lining the ab-
dominal cavity, and a similar layer (the visceral layer of the
pericardium) covers the outside of the heart itself, adhering closely
to it. Each of the serous layers is covered by a stratum of flat
cells, and in the space between them is found a small quantity of
liquid which moistens the contiguous surfaces, and diminishes the
friction which would otherwise occur during the movements of
the heart.

Internally the heart is also lined by a fibrous membrane, covered
with a single layer of flattened cells, and called the endocardium.
Between the endocardium and the visceral layer of the pericar-
dium the bulk of the wall of the heart lies and is made up mainly
of striped muscular tissue (myocardium) differing from that of
the skeletal muscles; but connective tissues, blood-vessels, nerve-
cells, and nerve-fibers are also abundant in it.

Note. Sometimes the pericardium becomes inflamed, this af-
fection being known as pericarditis. It is extremely apt to occur
in acute rheumatism, and great care should be taken never, even
for a moment, except under medical advice, to expose a patient
to cold during that disease, since any chill is then especially apt
to set up pericarditis. In the earlier stages of pericardiac inflam-
mation the rubbing surfaces on the outside of the heart and the
inside of the pericardium become roughened, and their friction

ANATOMY OF THE HEART AND BLOOD-VESSELS 291

FIG. 102. — Diagram representing a sec-

produces a sound which can be recognized through the stethoscope.
In later stages great quantities of liquid may accumulate in the
pericardium so as seriously to impede the heart's beat.

The Cavities of the Heart. On opening the heart (see diagram
Fig. 102) it is found to be subdivided by a longitudinal partition
or septum into completely separated right and left halves, the
partition running from about
the middle of the base to a point
a little on the right of the apex.
Each of the chambers on the
sides of the septum is again in-
completely divided transversely,
into a thinner basal portion into
which veins open, known as
the auricle, and a thicker ap-
ical portion from which arteries
arise, called the ventricle. The
heart thus consists of a right

auricle and ventricle and a left ti °ugh the Heart fr°m base * apex"
auricle and ventricle, each auricle communicating by an auric-
uloventricular orifice with the ventricle on its own side, and
there is no direct communication whatever through the septum
between the opposite sides of the heart. To get from one side to
the other the blood must leave the heart and pass through a set
of capillaries, as may readily be seen by tracing the course of the
vessels in Fig. 100.

The Heart as seen from its Exterior. When the heart is viewed
from the side turned towards the sternum (Fig. 103) the two
auricles, Aid and As, are seen to be separated by a deep groove
from the ventricles, Vd and Vs. A more shallow furrow runs
between the ventricles and indicates the position of the internal
longitudinal septum. On the dorsal aspect of the heart (Fig. 104)
similar furrows may be noted, and on one or other of the two fig-
ures the great vessels opening into the cavities of the heart may be
seen. The pulmonary artery, P, arises from the right ventricle,
and very soon divides into the right and left pulmonary arteries,
Pd and Ps, which break up into smaller branches and enter the
corresponding lungs. Opening into the right auricle are two
great veins (see also Fig. 102), cs and ci, known respectively as

292

THE HUMAN BODY

the upper and lower vence cavce, or "hollow" veins; so called by the
older anatomists because they are frequently found empty after
death. Into the back of the right auricle opens also another vein,
Vc, called the coronary vein or sinus, which brings back blood
that has circulated in the walls of the heart itself. Springing from

ABi

ori

Vd

FIQ. 103. — The heart and the great blood-vessel attached to it, seen from the
side towards the sternum. The left cavities and the vessels connected with them
are colored red; the right black. Atd, right auricle; Adx and As, the right and
left auricular appendages; Vd, right ventricle; Vs, left ventricle; Aa, aorta; Ab, in-
nominate artery ; Cs, left common carotid artery ; Ssi, left subclavin artery ; P, main
trunk of the pulmonary artery, and Pd and Ps, its branches to the right and left
lungs; cs, superior vena cava; Ade and A si, the right and left innominate veins;
pd and ps, the right and left pulmonary veins ; crd and crs, the right and left coro-
nary arteries.

the left ventricle, and appearing from beneath the pulmonary
artery when the heart is looked at from the ventral side, is a great
artery, the aorta, Aa. It forms an arch over the base of the heart
and then runs down behind it at the back of the chest. From the
convexity of the arch of the aorta several great branches are
given off, Ssi, Cs, Ab; but before that, close to the heart, the aorta

ANATOMY OF THE HEART AND BLOOD-VESSELS 293

gives off two coronary arteries, branches of which are seen at crd
and crs lying in the groove over the partition between the ventri-
cles, and which carry to the substance of the organ that blood
which comes back through the coronary sinus. Into the left au-

FIG. 104. — The heart viewed from its dorsal aspect. Atd, right auricle; ct, in-
ferior vena caya; Vc, coronary vein. The remaining letters of reference have the
same signification as in Fig. 103.

ricle open two right and two left pulmonary veins, ps and pd,
which are formed by the union of smaller veins proceeding from
the lungs.

In the diagram Fig. 102 from which the branches of the great
vessels near the heart have been omitted for the sake of clearness,

294 THE HUMAN BODY

the connection of the various vessels with the chambers of the
heart can be better seen. Opening into the right auricle are the
superior and inferior vense cavse (cs and ct) and proceeding from
the right ventricle the pulmonary artery, P. Opening into the
left auricle are the right and left pulmonary veins (pd and ps) and
springing from the left ventricle the aorta, A .

The Interior of the Heart. The communication of each auricle
with its ventricle is also represented in the diagram Fig. 102, and
the valves which are present at those points and at the origin of
the pulmonary artery and that of the aorta. Internally the auricles
are for the most part smooth, but from each a hollow pouch, the
auricular appendage, projects over the base of the corresponding
ventricle as seen at Adx and ^.s in Figs. 103 and 104. These
pouches have somewhat the shape of a dog's ear and have given
their name to the whole auricle. Their interior is roughened by
muscular elevations, covered by endocardium, known as the fleshy
columns (columnce carnaz). On the inside of the ventricles (Fig.
105) similar fleshy columns are very prominent.

The Auriculoventricular Valves. These are known as right
and left, or as the tricuspid and mitral valves respectively. The
mitral valve (Fig. 105) consists of two flaps of the endocardium
fixed by their bases to the margins of the auriculoventricular
aperture and with their edges hanging down into the ventricle
when the heart is empty. These unattached edges are not, how-
ever, free, but have fixed to them a number of stout connective-
tissue cords, the cordce tendinece, which are fixed below to muscular
elevations, the papillary muscles, Mpm and Mpl, on the interior
of the ventricle. The cords are long enough to let the valve flaps
rise into a horizontal position and so close the opening between
auricle and ventricle which lies between them, and passes up be-
hind the opened aorta, Sp, represented in the figure. The tricus-
pid valve is like the mitral, but with three flaps instead of two.

Semilunar Valves. These are six in number: three at the mouth
of the aorta, Fig. 105, and three, quite like them, at the mouth
of the pulmonary artery. Each is a strong crescentic pouch fixed
by its more curved border, and with its free edge turned away
from the heart. When the valves are in action these free edges
meet across the vessel and prevent blood from flowing back into
the ventricle. In the middle of the free border of each valve is a

ANATOMY OF THE HEART AND BLOOD-VESSELS 295

little cartilaginous nodule, the corpus Arantii, and on each side of
this the edge of the valve is very thin and when it meets its neigh-
bor turns up against it and so secures the closure.

The Arterial System. All the arteries of the Body arise either
directly or indirectly from the aorta or pulmonary artery, and the

Mpl

Mpra

Fio. 105. — The left ventricle and the commencement of the aorta laid open.
Mpm, Mpl, the papillary muscles. From their upper ends are seen the corda
tendinixz proceeding to the edges of the flaps of the mitral valve. The opening
into the auricle lies between these flaps. At the beginning of the aorta are seen its
three pouch-like semilunar valves.

great majority of them from the former vessel. The pulmonary
artery only carries blood to the lungs, to undergo exchanges with
the air in them after it has circulated through the Body generally.
After making its arch the aorta continues back through the
chest, giving off many branches on its way. Piercing the dia-

296 THE HUMAN BODY

phragm it enters the abdomen and after supplying the parts in
and around that cavity with branches, it ends opposite the last
lumbar vertebra by dividing into the right and left common iliac
arteries, which carry blood to the lower limbs. We have then to
consider the branches of the arch of the aorta, and those of the
descending aorta, which latter is for convenience described by
anatomists as consisting of the thoracic aorta, extending from the
end of the arch to the diaphragm, and the abdominal aorta, extend-
ing from the diaphragm to the final subdivision of the vessel.

Branches of the Arch of the Aorta. From this arise first the
coronary arteries (crd and crs, Figs. 103 and 104) which spring
close to the heart, just above two of the pouches of the semilunar
valve, and carry blood into the substance of that organ. The
remaining branches of the arch are three in number, and all arise
from its convexity. The first is the innominate artery (Ab, Fig.
103), which is very short, immediately breaking up into the right
subclavian artery, and the right common carotid. Then comes the
left common carotid, Cs, and finally the left subclavian, Ssi.

Each subclavian artery runs out to the arm on its own side and
after giving off a vertebral artery (which runs up the neck to the
head in the vertebral canal of the transverse processes of the
cervical vertebrae), crosses the armpit and takes there the name of
the axillary artery. This continues down the arm as the brachial
artery, which, giving off branches on its way, runs to the front of
the arm, and just below the elbow-joint divides into the radial
and ulnar arteries, the lower ends of which are seen at R and U in
Fig. 101. These supply the forearm and end in the hand by unit-
ing to form an arch, from which branches are given off to the
fingers.

The common carotid arteries pass out of the chest into the neck,
along which they ascend on the sides of the windpipe. Opposite
the angle of the lower jaw each divides into an internal and exter-
nal carotid artery, right or left as the case may be. The external
ends mainly in branches for the face, scalp, and salivary glands,
one great subdivision of it with a tortuous course, the temporal
artery, being often seen in thin persons beating on the side of the
brow. The internal carotid artery enters the skull through an
aperture in its base and supplies the brain, which it will be re-
membered also gets blood through the vertebral arteries.

ANATOMY OF THE HEART AND BLOOD-VESSELS 297

Branches of the Thoracic Aorta. These are numerous but small.
Some, the intercostal arteries, run out between the ribs and supply
the chest-walls; others, the bronchial arteries, carry blood to the
lungs for their nourishment, that carried to them by the pulmonary
arteries being brought there for another purpose; and a few other
small branches arc given to other neighboring parts.

Branches of the Abdominal Aorta. These are both large and
numerous, supplying not only the wall of the posterior part of the
trunk, but the important organs in the abdominal cavity. The
larger are : the cosliac axis which supplies stomach, spleen, liver, and
pancreas ; the superior mesenteric artery, which supplies a great part
of the intestine ; the renal arteries, one for each kidney ; and finally
the inferior mesenteric artery, which supplies the rest of the in-
testine. Besides these the abdominal aorta gives off very many
smaller branches.

Arteries of the Lower Limbs. Each common iliac divides into
an internal and external iliac artery. The former mainly ends in
branches to parts lying in the pelvis, but the latter passes into the
thighs and there takes the name of the femoral artery. At first this
lies on the ventral aspect of the limb, but lower down passes to the
back of the femur, and above the knee-joint (where it is called the
popliteal artery] divides into the anterior and posterior tibial ar-
teries, which supply the leg and foot.

The Capillaries. As the arteries are followed from the heart,
their branches become smaller and smaller, and finally cannot be
traced without the aid of a microscope. The smallest arteries are
called arterioles. These pass into the capillaries, the walls of
which are simpler than those of the arterioles, and which form very
close networks in nearly all parts of the Body ; their immense num-
ber compensating for their smaller size. The average diameter of a
capillary vessel is .016 mm. (^ inch) so that only two or three
blood-corpuscles can pass through it abreast, and in many parts
they are close so that a pin's point could not be inserted between
two of them (Fig. 106). It is while flowing in these delicate tubes
that the blood does its nutritive work, the arteries being merely
supply-tubes for the capillaries.

The Veins. The first veins arise from the capillary networks in
various organs, and like the last arteries are very small. They soon
increase in size by union, and so form larger and larger trunks.

298

THE HUMAN BODY

These in many places lie near or alongside the main artery of the
part, but there are many more large veins just beneath the skin
than there are large arteries. This is especially the case in the
limbs, the main veins of which are superficial, and can in many
persons be seen as faint blue marks through the skin. Fig. 107
represents the arm at the front of the elbow-joint after the skin
and subcutaneous areolar tissue and fat have been removed. The

Fia. 106. — A small portion of the capillary network as seen in the frog's web
when magnified about 25 diameters, .a, a small artery feeding the capillaries;
v, v, small veins carrying blood back from the latter.

brachial artery, B, colored red, is seen lying tolerably deep, and
accompanied by two small veins (vence comites) which communi-
cate by cross-branches. The great median nerve, 1, a branch of the
brachial plexus which supplies several muscles of the forearm and
hand, the skin over a great part of the palm and the three inner
fingers, is seen alongside the artery. The larger veins of the part
are seen to form a more superficial network, joined here and there,

ANATOMY OF THE HEART AND BLOOD-VESSELS 299

as for instance at *, by branches from deeper parts. Several
small nerve-branches which supply the skin (2, 3, 4) are seen
among these veins. It is from the vessel, cep, called the cephalic

cop

bas

3. 107. — The superficial veins in front of the elbow-joint. B', tendon of biceps
le; Bi, brachialis internus muscle; Pt, pronator tares muscle; 1, median nerve;

Fio.

muscle; , ...

2, 3, 4, nerve-branches to the skin; B, brachial artery, with its small accompany-
ing veins; ctp, cephalic vein; bos, basilic vein; m', median vein; *, junction of a
deep-lying vein with the cephalic.

vein, just above the point where it crosses the median nerve, that
surgeons usually bleed a patient.

A great part of the blood of the lower limb is brought back by the
long saphenous vein, which. can be seen in thin persons running
from the inner side of the ankle to the top of the thigh. All the

300 THE HUMAN BODY

blood which leaves the heart by the aorta, except that flowing
through the coronary arteries, is finally collected into the superior
and inferior vence cavce (cs and ci, Figs. 103 and 104), and poured
into the right auricle. The jugular veins which run down the neck,
carrying back the blood which went out along the carotid arteries,
unite below with the arm-vein (subclaviari) to form on each side an
innominate vein (Asi and Ade, Fig. 103) and the innominates unite
to form the superior cava. The coronary-artery blood after flow-
ing through the capillaries of the heart itself also returns to this
auricle by the coronary veins and sinus.

The Pulmonary Circulation. Through this the blood gets back
to the left side of the heart and so into the aorta again. The pul-
monary artery, dividing into branches for each lung, ends in the
capillaries of those organs. From these it is collected by the pul-
monary veins, which carry it back to the left auricle, whence it
passes to the left ventricle to recommence its flow through the
Body generally.

The Course of the Blood. From what has been said it is clear
that the movement of the blood is a circulation. Starting from any
one chamber of the heart it will in time return to it ; but to do this
it must pass through at least two sets of capillaries; one of these
is connected with the aorta and the other with the pulmonary
artery, and in its circuit the blood returns to the heart twice.
Leaving the left side it returns to the right, and leaving the right it
returns to the left; and there is no road for it from one side of the
heart to the other except through a capillary network. Moreover,
it always leaves from a ventricle through an artery, and returns to
an auricle through a vein.

There is then really only one circulation; but it is not uncommon
to speak of two, the flow from the left side of the heart to the right,
through the Body generally, being called the systemic circulation,
and from the right to the left, through the lungs, the pulmonary
•circulation. But since after completing either of these alone the
blood is not back at the point from which it started, but is sepa-
rated from it by the septum of the heart, neither is a " circulation "
in the proper sense of the word. .

The Portal Circulation. A certain portion of the blood which
leaves the left ventricle of the heart through the aorta hag to pass
through three sets of capillaries before it can again return there.

ANATOMY OF THE HEART AXD BLOOD-VESSELS

301

This is the portion which goes through the stomach, spleen, pan-

CIVMS, and intestines. After traversing the capillaries of those

organs it is collected into the portal vein which enters the liver, and

breaking up in it into finer and finer branches like an artery, ends

in the capillaries of that organ, forming the second set which this

blood passes through on its course. From these it is collected by

the hepatic veins, which pour it into the in-

ferior vena cava, which carries it to the

right auricle, so that it has still to pass

through the pulmonary capillaries to get

back to the left side of the heart. The

portal vein is the only one in the Human

Body which like an artery feeds a capillary

network, and the flow from the stomach

and intestines through the liver to the vena

cava is often spoken of as the portal circu-

lation. .

Diagram of the Circulation. Since the
two halves of the heart are actually com-
pletely separated from one another by an
impervious partition, although placed in
proximity in the Body, we may conven-
iently represent the course of the blood ., Fl°- los.— Diagram of

' . . the blood vascular system,

as in the accompanying diagram (Fig. 108), showing that it forms a

, • , ., • , ~ i i ... , i r xi_ single closed circuit with

in which the right and left halves of the two pumps in it, consisting
heart are represented at different points °[ £ ^ ™&f JStS
in the vascular system. Such an arrange- resented separate in the

, diagram, ra and rv, right

ment makes it clear that the heart is really auricle and ventricle; la and

i • -iv -i v Iv, left auricle and ventri-

two pumps working side by side, each en- ci'e. ao> aorta. SC( 8y8temic

gaged in forcing the blood to the other.

Starting from the left auricle, la, and fol- pulmonary capillaries; pv,

. . , _ . , , , pulmonary veins.

lowing the flow, we trace it through the

left ventricle and along the branches of the aorta into the sys-
temic capillaries, sc; from thence it passes back through the sys-
temic veins, vc. Reaching the right auricle, ra, it is sent into the
right ventricle, rv, and thence through the pulmonary artery, pa,
to the lung capillaries, pc, from which the pulmonary veins, pv,
carry it to the left auricle, which drives it into the left ventricle, Iv,
and this again into the aorta.

302 THE HUMAN BODY

Arterial and Venous Blood. The blood when flowing in the
pulmonary capillaries gives up carbon dioxid to the air and re-
ceives oxygen from it; and since its coloring matter (hemoglobin)
forms a scarlet compound with oxygen, it flows to the left auricle
through the pulmonary veins of a bright red color. This color it
maintains until it reaches the systemic capillaries, but in these it
loses much oxygen to the surrounding tissues and gains much
carbon dioxid from them. But the blood coloring-matter which
has lost its oxygen has a dark purple color, and since this unoxi-
dized or " reduced " hemoglobin is now in excess, the blood returns
to the heart by the venss cavse of a dark purple-red color. This
hue it keeps until it reaches the lungs, when the reduced hemo-
globin becomes again oxidized. The bright red blood, rich in
oxygen and poor in carbon dioxid, is known as " arterial blood "
and the dark red as " venous blood " : and it must be borne in mind
that the terms have this peculiar technical meaning, and that the
pulmonary veins contain arterial blood, and the pulmonary ar-
teries, venous blood; the change from arterial to venous taking
place in the systemic capillaries, and from venous to arterial in the
pulmonary capillaries. The chambers of the heart and the great
vessels containing arterial blood are shaded red in Figs. 103 and 104.

The Structure of the Arteries. A large artery can by careful
dissection be separated into three coats : an internal, a middle, and
an outer. The internal coat tears readily across the long axis of the
artery and consists of an inner lining of flattened nucleated cells,
enveloped by a variable number of layers composed of membranes
or networks of elastic tissue. The middle coat is made up of
alternating layers of elastic fibers and plain muscular tissue; the
former running for the most part longitudinally and the latter
across the long axis of the vessel. The outer coat is the toughest
and strongest because it is mainly made up of white fibrous con-
nective tissue; it contains a considerable amount of elastic tissue
also, and gradually shades off into a loose areolar tissue which
forms the sheath of the artery, or the tunica adventitia, and packs it
between surrounding parts. The smaller arteries have all the
elastic elements less developed. The internal coat is consequently
thinner, and the middle coat is made up mainly of involuntary
muscular fibers. As a result the large arteries are highly elastic,
the aorta being physically much like a piece of india-rubber tubing,

ANATOMY OF THE HEART AND BLOOD-VESSELS 303

while the smaller arteries are highly contractile, in the physio-
logical sense of the word.

Structure of the Capillaries. In the smaller arteries the outer
and middle coats gradually disappear, and the elastic layers of the
inner coat also go. Finally, in the capillaries the lining epithelium
alone is left, with a more or less developed layer of connective-
tissue corpuscles around it, representing the remnant of the tunica
adventitia. These vessels are thus extremely well adapted to
allow of filtration or diffusion taking place through their thin walls.

Structure of the Veins. In these the same three primary coats
as in the arteries are found ; the inner and middle coats are less de-
veloped, while the outer one remains thick, and is made up almost
entirely of white fibrous tissue. Hence the venous walls are much
thinner than those of the corresponding arteries, and the veins
collapse -when empty while the stouter arteries remain open. But
the toughness of their outer coats gives the veins great strength.

Except the pulmonary artery and the aorta, which possess the
semilunar valves at their cardiac orifices, the arteries possess no
valves. Many veins, on the contrary, have such, formed by semi-
lunar pouches of the inner coat, attached by one margin and hav-
ing the edge turned towards the heart free.
These valves, sometimes single, oftener in
pairs, and rarely three at one level, permit
blood to flow only towards the heart, for a
current in that direction (as in the upper c
diagram, Fig. 109) presses the valve close

,,.,,.,, , , FIG. 109. — Diagram to

against the side of the vessel and meets illustrate the mode of ac-
with no obstruction from it. Should any

back-flow be attempted, however, the CUr- and H, the heart end of the

rent closes up the valve and bars its own

passage as indicated in the lower figure. These valves are most
numerous in superficial veins and those of muscular parts. They
are absent in the venae cavae and the portal and pulmonary veins.
Usually the vein is a little dilated opposite a valve, and hence in
parts where the valves are numerous gets a knotted look. On
compressing the forearm so as to stop the flow in its subcutaneous
veins and cause their dilatation, the points at which valves are
placed can be recognized by their swollen appearance. They are
most frequently situated where two veins communicate.

CHAPTER XX

THE ACTION QF THE HEART. THE REGULATION OF THE

HEART-BEAT

The Beat of the Heart. It is possible with some little skill and
care to open the chest of a living narcotized animal, such as a
rabbit, and see its heart at work, alternately contracting and re-
laxing. As observed under ordinary conditions these phases fol-
low one another so rapidly as seemingly to defy analysis. When
Harvey, the discoverer of the circulation, first looked upon the
beating heart of a mammal he was so impressed by the complex-
ity and rapidity of its action as to believe for the moment that
the human mind could never fathom it.

By proper treatment the beat of the heart can be much slowed.
When this has been done it is observed that each beat commences
at the mouths of the great veins; from there runs over the rest
of the auricles, and then over the ventricles; the auricles dilating
the moment the ventricles commence to contract. Having fin-
ished their contraction the ventricles also dilate, and so for some
time neither they nor the auricles are contracting, but the whole
heart is at rest. The contraction of any part of the heart is
known as its systole and the relaxation as its diastole.

The average heart-rate in man is 72 beats per minute, giving
for each beat 0.8 second. The two sides of the heart work syn-
chronously, the auricles together and the ventricles together. In
describing the " cardiac cycle," therefore, the auricles are treated
as one organ and the ventricles as one. The auricular systole
occupies about 0.1 second, its diastole lasts 0.7 second. The
ventricular systole begins at the end of the auricular contraction;
it occupies about 0.3 second; the diastole of the ventricle lasts
about 0.5 second. During fully half of each cardiac cycle, then,
there is no muscular activity going on in any part of the heart.
During diastole the heart if taken between the finger and thumb
feels soft and flabby, but during systole it (especially its ventric-
ular portion) becomes hard and rigid.

304

THE ACTION OF THE HEART 305

Change of Form of the Heart. During its systole the heart
becomes shorter and rounder, mainly from a change in the shape
of the ventricles, which from having an elliptical cross-section
take on a circular one. At the same time the length of the ven-
tricles is lessened, the apex of the heart approaching the base and
becoming blunter and rounder.

The Cardiac Impulse. The human heart lies with its apex
touching the chest-wall between the fifth and sixth ribs on the
left side of the breast-bone. At every beat a sort of tap, known
as the "cardiac impulse" or "apex beat/' may be felt by the
finger at that point. There is, however, no actual "tapping,
since the heart's apex never leaves the chest-wall. During the
diastole the soft ventricles yield to the chest-wall where they
touch it, but during the systole they become hard and tense and
push it out a little between the ribs, and so cause the apex beat.
Since the heart becomes shorter during the ventricular systole, it
might be supposed that at that time the apex would move up a
little in the chest. This, however, is not the case, the ascent of
the apex towards the base of the ventricles being compensated
for by a movement of the whole heart in the opposite direction.
If water be pumped into an elastic tube, already moderately full,
the tube will be distended not only transversely but longitudi-
nally. This is what happens in the aorta: when the left ventricle
contracts and pumps blood forcibly into it, the elastic artery is
elongated as well as widened, and the lengthening of that limb of
its arch attached to the heart pushes the latter down towards the
diaphragm, and compensates for the upward movement of the
apex due to the shortening of the ventricles. Hence if the ex-
posed living heart be watched it appears as if during the systole
the base of the heart moved towards the tip, rather than the re-
verse.

Events occurring within the Heart during a Cardiac Cycle.
Let us commence at the end of the ventricular systole. At this
moment the semilunar valves at the orifices of the aorta and the
pulmonary artery are closed, so that no blood can flow back from
those vessels. The whole heart, however, is soft and distensible
and yields readily to blood flowing into it from the pulmonary
veins and the venjs cavae; this" passes on through the open mitral
and tricuspid valves and fills up the dilating ventricles, as well as

306 THE HUMAN BODY

the auricles. As the ventricles fill, back currents are set up along
their walls and these carry up the flaps of the valves so that by
the end of the pause they are nearly closed. At this moment the
auricles contract, and since this contraction commences at and
narrows the mouths of the veins opening into them, and at the
same time the blood in those vessels opposes some resistance to
a back-flow into them, while the still flabby and dilating ventricles
oppose much less resistance, the general result is that the con-
tracting auricles send blood into the ventricles, and not back into
the veins. At the same time the increased direct current into the
ventricles produces a greater back current on the sides, which,
when the auricles cease their contraction and the filled ventricles
become tense and press on the blood inside them, completely closes
the auriculoventricular valves. That this increased filling of the
ventricles, due to auricular contractions will close the valves may
be seen easily in a sheep's heart. If the auricles be carefully cut
away from this so as to expose the mitral and tricuspid valves,
and water be then poured from a little height into the ventricles,
it will be seen that as these cavities are filled the valve-flaps are
floated up and close the orifices.

The auricular contraction now ceases and the ventricular com-
mences. The blood in each ventricle is imprisoned between the
auriculoventricular valves behind and the semilunar valves in
front. The former cannot yield on account of the cordse tendineae
fixed to their edges : the semilunar valves, on the other hand, can
open outwards from the ventricle and let the blood pass on, but
they are kept tightly 'shut by the pressure of the blood on their
other sides, just as the lock-gates of a canal are by the pressure of
the water on them. In order to open the canal-gates water is let
in or out of the lock until it stands at the same level on each side
of them; but of course they might be forced open without this
by applying sufficient power to overcome the higher water pres-
sure on one side. It is in this latter way that the semilunar valves
are opened. The contracting ventricle tightens its grip on the
blood inside it and becomes rigid to the touch. As it squeezes
harder and harder, at last the pressure on the blood within it be-
comes greater than the pressure exerted on the other side of the
valves by the blood in the arteries, the flaps are forced open and
the blood begins to pass* out : the ventricle continues its contrac-

THE ACTION OF THE HEART 307

tion until it has obliterated its cavity and completely emptied
itself; this total emptying appears, at least, to occur in the nor-
mally beating heart, but in some pathological conditions and
under the influence of certain drugs the emptying of the ventri-
cles is incomplete. After the systole the ventricle commences to
relax and blood immediately to flow back towards it from the
highly stretched arteries. This return current, however, catches
the pockets of the semilunar valves, drives them back and closes
the valve so as to form an impassable barrier; and so the blood
which has been forced out of either ventricle cannot flow directly
back into it.

Use of the Papillary Muscles. In order that the contracting
ventricles may not force blood back into the auricles it is essential
that the flaps of the mitral and tricuspid valves be maintained
in position across the openings which they close, and be not
pushed back into the auricles. At the commencement of the
ventricular systole this is provided for by the corda3 tendineae,
which are of such a length as to keep the edges of the flaps in ap-
position, a position which is further secured by the fact that each
set of cordse tendineaB (Fig. 105) radiating from a point in
the ventricle, is not attache'd around the edges of one flap but
on the contiguous edges of two flaps, and so tends to pull them
together. But as the contracting ventricles shorten, the cordae
tendinese, if directly fixed to their interior, would be slackened
and the valve-flaps pushed up into the auricle. The little
papillary muscles prevent this. Shortening as the ventricular
systole proceeds, they keep the cordse taut and the valves
closed.

Sounds of the Heart. If the ear be placed on the chest over
the region of the heart during life, two distinguishable sounds
will be heard during each cardiac cycle. They are known re-
spectively as the first and second sounds of the heart. The first is
of lower pitch and lasts longer than the second and sharper sound :
vocally their character may be tolerably imitated by the words
lubb, dup. The cause of the second sound is the closure, or, as one
might say, the "clicking up," of the semilunar valves, since it
occurs at the moment of their closure and ceases if they be hooked
back in a living animal. The origin of the first sound is still un-
certain: it takes place during the ventricular systole and is prob-

308 THE HUMAN BODY

ably due to vibrations of the tense ventricular wall at that time.
It is not due, at least not entirely, to the auriculoventricular
valves, since it may still be heard in a beating heart empty of
blood, and in which there could be no closure or tension of those
valves. In various forms of heart disease these sounds are mod-
ified or cloaked by additional "murmurs" which arise when the
cardiac orifices are roughened or narrowed or dilated, or the
valves inefficient. By paying attention to the character of the
new sound then heard, the exact period in the cardiac cycle at
which it occurs, and the region of the chest-wall at which it is
heard most distinctly, the physician can often get important in-
formation as to its cause.

Action of the Heart Valves. The valves of the heart are en-
tirely without rigidity. They consist of tough, but perfectly
flaccid membranes, so that they respond perfectly to the forces
which act upon them. This structure makes it inevitable that the
valves will open whenever the pressure behind them is greater
than that in front, and will close whenever the pressure in front
is greater than that behind. During the whole diastole of the heart
the pressure behind the auriculoventricular valves is greater than
that in front of them; for in front is only the gradually filling
cavity of the ventricle, while behind is the onward flow of blood
from the great veins. During this time, therefore, these valves
stand open. The systole of the auricle, by increasing the pressure
behind, keeps them open until its end. During this same time the
aortic valves are shut, because in front of them are arteries whose
walls are stretched with their load of blood and which, therefore,
exert high pressure upon the valves, while behind are only the
ventricular cavities, filling with blood. At the instant the ven-
tricles begin to contract the situation with regard to the auriculo-
ventricular valves changes. The relaxing auricles make room for
the blood coming in from the great veins and so release the pressure
behind these valves; the contracting ventricle exerts pressure in
front of them ; they therefore close instantly. Since the semilunar
valves remain closed until the rising pressure in the ventricle be-
comes greater than that in the aorta there is an instant at the be-
ginning of ventricular systole when all the valves are shut. Again,
at the beginning of ventricular diastole there is an instant when the
ventricular pressure has fallen below that in the aorta but is still

THE ACTION OF THE HEART

309

above the pressure in the auricles; during this time, again, the
valves of the heart are all shut.

Diagram of the Events of a Cardiac Cycle. In the following
table the phenomena of the heart's beat are represented with
reference to the changes of form which are seen on an exposed
working heart. Events in the same vertical column occur simul-
taneously; on the same horizontal line, from left to right, succes-
sively.

Auricular
Systole

Commence-
ment of
Ventricular
Systole

Ventricular
Systole

Cessat ion
of Ven-
tricular
Systole

Pause

Auricles ... ....

Contracting

Dilating and

Dilating and

Dilating and

Dilating and

Ventricles

and
emptying.
Dilating and

filling.
Contracting.

filling.
Contracting

filling.
Dilating.

filling.
Dilating and

Impulse

filling.

Apex beat.

and
emptying.

filling.

Auriculpventricular

Open

Closed.

Closed.

Closed.

Open.

Semilunar valves. . . .
Sounds

Closed.

Closed.
First sound.

Open.

Closed.
Second

Closed.

sound.

Function of the Auricles. The ventricles have to do the work of
pumping the blood through the blood-vessels. Accordingly their
walls are far thicker and more muscular than those of the auricles;
and the left ventricle, which has to force the blood over the Body
generally, is stouter than the right, which has only to send blood
around the comparatively short pulmonary circuit. Th'e circu-
lation of the blood is in fact maintained by the ventricles, and we
have to inquire what is the use of the auricles. Not unfrequently
the heart's action is described as if the auricles first filled with
blood and then contracted and filled the ventricles; and then the
latter contracted and drove the blood into the arteries. From the
account given above, however, it will be seen that the events are
not accurately so represented, but that during all the pause blood
flows on through the' auricles into the ventricles, which latter are
already nearly full when the auricles contract; this contraction
merely completing their filling. The real use of the auricles'is to
afford a reservoir into which the veins may empty while the com-
paratively long-lasting ventricular contraction is taking place.

If the heart consisted of the ventricles only, with valves at the
points of entry and exit of the blood, the circulation could be

310 THE HUMAN BODY

maintained. During diastole the ventricle would fill from the
veins, and during systole empty into the arteries. But in order
to accomplish this, during the systole the valves at the point of
entry must be closed, or the ventricle would empty itself into the
veins as well as into the arteries ; and this closure would necessitate
a great loss of time which might be utilized for feeding the pump.
This is avoided by the auricles, which are really reservoirs at the
end of the venous system, collecting blood wrhen the ventricular
pump is at work. When the ventricles relax, the blood entering
the auricles flows on into them ; but previously, during the part of
the cardiac cycle occupied by the ventricular systole, the auricles
have accumulated blood, and when they at last contract they send
on into the ventricles this accumulation. Even were the flow from
the veins stopped during the auricular contraction this would be of
comparatively little consequence, since that event occupies so
brief a time. But, although no doubt somewhat lessened, the
emptying of the veins into the heart does not seem to be, in health,
stopped while the auricle is contracting. The heart in fact con-
sists of a couple of "feed-pumps" — the auricles — and a couple of
"force-pumps" — the ventricles; and so wonderfully perfect is the
mechanism that the supply to the feed-pumps is never stopped.
The auricles are never empty, being supplied all the time of their
contraction, which is never so great as to obliterate their cavities;
while the ventricles contain no blood at the end of their systole.

The Work Done by the Heart. According to the physical
definition work is measured by the weight lifted times the height
to which it is raised. In estimating the work of the heart we sub-
stitute for the height the resistance against which the heart works.
This resistance is equivalent in the case of the left ventricle to that
of a column of blood about 2 meters high, and for the right ven-
tricle about 0.8 meter. The mass of blood ejected from each ven-
tricle during systole probably averages about 100 gms. The
work done by the left ventricle per beat equals, then, about
100x2=200 grammeters, and that by the right ventricle equals
about 100x0.8=80 grammeters. Since the heart in addition to
moving the weight of blood imparts to it a considerable velocity, it
is necessary to add to the amounts of work calculated above an
additional amount to represent that required to impart to the
blood its velocity. This latter amount approximates 3 gram-

THE ACTION OF THE HEART 311

meters. The total work output of the heart per beat is, therefore,
roughly 283 grammeters, equivalent in the English scale to about
2 foot-pounds. When the heart is beating at the rate of 70 per
minute it does 140 foot-pounds per minute, making it a 240th
horse-power engine. If it maintained this rate throughout the
entire twenty-four hours of the day it would do in that time
200,000 foot-pounds of work, an amount equivalent to that done
by the leg muscles of a man weighing 150 pounds in climbing a
mountain 1,300 feet high.

That the heart is able to do this amount of work daily without
fatigue, and keep it up day in and day out for seventy or more
years, is due to its ability to recover quickly from the effects of its
activity, coupled with the fact that in a whole day its resting time
considerably outweighs the time during which it is active. The
heart-beat is ordinarily much slower during sleep than during
bodily activity; as the result the heart enjoys an " eight hour day "
if only its actual contraction time be counted.

Relations of Nerve and Muscle Elements within the Heart.
The heart-muscle consists, as previously stated, of muscle-cells of
small size, intimately communicating with one another through
their branches, and showing signs of cross-striation. At the junc-
tion of the great veins with the heart, a region, as we shall see, of
great importance in the heart's activity, these muscular elements
form thin sheets; in the auricles the heart-muscle is somewhat
heavier and thicker; but it attains its greatest development in the
ventricles, where the muscular walls are exceedingly heavy, and
very stout. In mammals the only pulsating heart structures are
the auricles and ventricles; in lower vertebrates, such as the frog,
the great veins near the heart are differentiated into a pulsating
structure, the sinus venosus, and the outlet from the ventricle, the
bulbus arteriosus, also pulsates. Although in mammals these
structures no longer pulsate, the region of the great veins which
corresponds to the sinus venosus still seems to preserve to some
degree the physiological properties it has in lower animals, and
observations made upon frogs' hearts are interpreted for mammals'
hearts upon that basis.

Embedded within the tissue of the heart are numerous nerve-
cells. These are most numerous in the region of the sinus venosus
and auricles; the base of the ventricles contains some of them, but

312 THE HUMAN BODY

the apex of the ventricles is said to be wholly free from them.
Nerve-fibers, communicating with these cells, penetrate all parts
of the cardiac musculature. It has not been possible by histologic
means to show that these fibers are dendrites and axons such as
occur in the general nervous system, and many histologists and
physiologists believe that they form a continuous network or
plexus involving all parts of the heart and so constituted that a
stimulus applied at any point spreads over the whole organ. Ac-
cording to this view the nervous mechanism of the heart does not
show the irreversibility of conduction which is a cardinal feature
of the general nervous system. Some support for this idea is had
in the fact that certain other viscera, notably the stomach and in-
testines, have within their walls nerve plexuses showing similar
physiological properties.

Physiological Peculiarities of the Heart. The most striking of
these is its automatic rhythmitity. The heart may be removed com-
pletely from the Body without its regular beating being at all in-
terfered with. In cold-blooded animals such as frogs or turtles
this activity outside the Body may continue for hours. While we
refer to this activity as automatic we do not mean by the word
anything more than the fact just stated, that the heart continues
to beat independently of the rest of the Body. The rhythmic na-
ture of the heart's activity is as characteristic as its automaticity.
The regular succession of contractions and relaxations is its normal
response to continuous or rapidly recurring stimulation. In this
respect it differs strikingly from skeletal muscle, which remains
strongly contracted throughout the period of such stimulation un-
less fatigue sets in to release it.

Another peculiarity of heart-muscle, and one that probably ex-
plains in part its rhythmic property, is that its contractions are
always maximal. By this is meant that whenever heart-muscle
contracts it always does so to the full extent of its ability at the
time. In this respect we may compare its energy liberation with
the discharge of a gun. When the trigger is pulled all the powder
in the cartridge is exploded ; similarly whenever the heart contracts
it uses up all the energy available at the time. Because of this
it is necessary that the contraction be followed by a relaxation
during which an accumulation of energy may prepare for the next
contraction.

THE ACTION OF THE HEART 313

The evidence that all the available energy of the heart-muscle is
used up at each systole is furnished by the existence of the refrac-
tory period. During this period, which coincides with the systole,
external stimulation of the heart-muscle is altogether ineffective,
although during diastole the heart responds to adequate stimu-
lation by contraction. It is observed, also, that the irritability of
the heart increases steadily from the end of the refractory period
to the beginning of the next systole. We may assume, then, that
during diastole there is a gradual replacement of the energy
supply used up during the preceding systole, and that the more
energy has accumulated the more irritable is the tissue.

The Passage of the Beat over the Heart. In the first paragraph
of the chapter it was stated that the beat of the heart takes a
certain course, beginning at the mouths of the great veins, spread-
ing thence over the auricles, and passing from them to the ven-
tricles. In all vertebrates there is a distinct pause between the
contraction of the auricles and of the ventricles. In animals, such
as the frog and turtle that have a pulsating sinus, there is likewise
a pause between the contraction of the sinus and of the auricles.

If in a beating heart a cut be made between the sinus and the
auricles so that they are completely separated, the sinus con-
tinues to beat exactly as before; the other chambers of the heart
may not beat for a moment, but after a short interval usually
resume activity. The rate of beat of these chambers under such
circumstances is slower than that of the sinus. Similarly the ven-
tricles may be separated from the auricles without affecting the
auricular beat, but with the result that the ventricles either fail to
beat at all, or beat at a much slower rate than the auricles. Such
experiments as these show that the rhythmic power increases the
nearer we go toward the venous end of the heart, and also that in
the normal heart the most rhythmic portion imposes its rate on
the rest of the organ. In order for the heart-rate to be determined
as a whole by the beat of the venous end it is evident that then-
must be a conduction of the impulse to activity from one chamber
to the next throughout the heart. This conduction moves over
the heart in the form of a wave.

There are in the frog's heart two places and in that of the mam-
mal one place where there is a delay in the passage of the con-
traction wave. These are, as already noted, at the junction of the

314 THE HUMAN BODY

sinus with the auricles and of the auricles with the ventricles.
Anatomical study shows that at these junctions most of the
cardiac tissue proper is replaced by connective tissue, so that
physiological communication between one chamber and another
is restricted to small bundles of conducting heart tissue. The de-
lay at the junctions is usually explained as resulting from the
small size of these conducting paths, which offer on that account
considerable resistance to the passage of the contraction wave.

Neurogenic and Myrogenic Theories of the Heart Beat. There
are two questions of fundamental importance to an understanding
of the mechanism of the heart's action. These are: (1) Does the
rhythmic property of the heart reside in its muscular elements
or in its nervous elements? and (2) Is the contraction wave con-
ducted over the heart by muscle or by nerve-tissue? By the
early students of the heart both these properties were attributed
to its nervous elements as being more like nerve activities in gen-
eral than like those of muscle; and also because the venous end of
the heart, where the beat originates, contains more nervous matter
than do the other chambers. More recently the view that both
rhythmicity and conductivity are cardinal functions of the heart's
musculature began to receive considerable attention, chiefly
through such observations as that the apex of the ventricle, which
is devoid of nerve-cells, may be made to show true rhythmicity,
and that a series of zigzag cuts, sufficient to sever all direct nerve
paths although leaving ample muscular connections, can be made
in the ventricle without preventing the passage of the contraction
wave over it. With recognition of the probability that the nervous
elements of the heart form, not a synaptic system with irreversible
conduction, but an intercommunicating plexus which may con-
duct in all directions, most of the evidence in favor of the myogenic
theory seems less conclusive than it did at first, so that the prob-
lems of which is the rhythmic and conducting tissue, or whether
both properties are possessed by both tissues, are still far from
settled.

The Nature of Automatic Rhythmicity. It should be clearly
understood that the question whether rhythmicity is a property of
cardiac muscle or of cardiac nerve-tissue is quite distinct from the
question of the underlying nature of rhythmicity itself. Much
study has been given to this latter problem and here again two

THE ACTION OF THE HEART 315

opposing views are held. One of these is that the heart is sub-
ject to the influence of a constant stimulus, its property of " maxi-
mal" contractions with their accompanying refractory periods
sufficing to bring about rhythmic responses to such constant
stimulation. The other view is that the heart is a truly automatic
organ, the metabolic processes going on within the heart tissue
being of such a nature as to produce rhythmic activity quite
independently of "stimulation " as we ordinarily understand it.

Those who believe the heart to be under the influence of a
constant stimulus .look to the blood as its source, and especially
to the inorganic blood-salts, it having been shown that the heart-
beat can be maintained for an astonishing length of time when
the heart is fed solutions containing only inorganic salts of sodium,
potassium, and calcium in proper proportion. Those who look
upon the heart as a truly automatic organ take the position that
their view is more in accordance with general physiological prin-
ciples than the other, and that no evidence yet brought forth
disproves their claim. They put the burden of proof upon the
supporters of the "constant stimulus" theory. It must be ad-
mitted that at present no conclusive evidence for either view is
available, nor are the supporters of either able to picture a satisfac-
tory mechanism of rhythmicity in terms of their particular theory.

The Extrinsic Nerves of the Heart. The heart, as stated pre-
viously, is under the control of the sympathetic system. It
receives nerve-fibers over two pathways. One of these is by way
of the tenth cranial nerves, the vagi, the other by way of the sym-
pathetic system proper. The vagus nerves give off their cardiac
branches in the neck; the cardiac nerves from the sympathetic
system arise from the inferior cervical ganglion, a sympathetic
ganglion lying in the lower neck region. Both anatomically and
physiologically the two sets of nerve-fibers are distinct. Anatomic-
ally the vagus fibers arc prc-ganglionic ; they arise from cell-bodies
in the nucleus of the tenth nerve in the medulla and are myelin-
ated. They terminate about nerve-cells lying on or within the
heart itself. The fibers from the sympathetic system are post-
ganglionic ; they arise from cell-bodies in sympathetic ganglia, the
inferior cervical for the most part, and are non-myelinated.
They terminate in the tissues of the heart directly. Since nicotine
cuts the connection between pre- and post-ganglionic fibers, appli-

316 THE HUMAN BODY

cation of that drug to the nerve-cells of the heart abolishes the influ-
ence of the vagi, but does not affect the sympathetic control at all.

Physiologically the vagus fibers are inhibitory; their stimula-
tion slows and weakens the heart-beat. When very strongly
stimulated they may bring the heart to a complete standstill,
although in mammals the standstill is maintained for a few
seconds only, the heart soon "breaking through" the inhibition.
The sympathetic fibers have precisely the opposite function, be-
ing augmentor; their stimulation accelerates and strengthens the
beat of the heart.

In addition to the efferent sympathetic innervation just de-
scribed the heart is provided with a set of afferent nerve-fibers.
These reach the central nervous system either by way of the
vagus nerves, or in some species of animals, rabbits for example,
as separate nerve-trunks known as the depressor nerves. The
function of these afferent fibers will be discussed in Chap. XXII
in connection with the nervous control of the blood-vessels.

The Inhibitory and Augmentor Centers. The control of the
heart-beat is reflex in its nature, and like most other "vital"
processes which are subject to reflex control is vested in certain
"centers" of the medulla. Two heart-regulating centers are
recognized, the cardio-inhibitory center and the cardio-aug mentor
center. The inhibitory center is in the nuclei of the tenth nerve.
It is bilateral, each side containing half of it. The exact position
of the augmentor center has not been determined. It is probably
not a compact mass of cells as is the inhibitory center, but is
scattered diffusely through the medulla.

Both these centers are in the path of all incoming impulses,
and there is evidence that both of them are kept in constant
" tonic " activity through the incessant play of stimuli upon them.

The heart is thus constantly receiving both inhibitory and
augmentor impulses, the former tending to diminish its activity,
the latter to increase it. The actual heart-beat, is the expression
therefore, of the balance between two opposing tendencies, and
its increase or decrease indicates that one or the other has gained
the advantage.

In attempting to analyze the causes of changes in the heart-
rate it must be remembered that an increase in rate may mean
either an increase in the activity of the augmentor center, or a

THE ACTION OF THE HEART 317

depression of the inhibitory center. Conversely, a decrease in
rate may mean either a depression of the augmentor center or an
increase in the activity of the inhibitory center. We may illus-
trate all these effects by specific cases. The a.ugmentor center
seems to be to a peculiar degree subject to impulses of muscle
sense. Running, therefore, or any other form of violent muscular
exercise, stimulates this center strongly and so the heart-rate is
increased. When one lies down the stream of muscle sense im-
pulses is greatly diminished; the augmentor center is therefore
less active, and the heart beats more slowly. Successive swallow-
ing, as in sipping water, increases the heart-rate by depressing
the inhibitory center. A blow over the stomach (the solar plexus)
gives rise to afferent impulses which stimulate the inhibitory
center; the heart-rate is therefore diminished.

These are specific illustrations of the general rule that the heart-
beat may be modified by all sorts of sensory stimulations. It is
a matter of ordinary observation that many experiences, particu-
larly those involving sensory impressions of high intensity, are
accompanied by marked changes in heart-rate. In common with
the structures of the body generally that are innervated by the
sympathetic system the heart-rate is also much affected by
emotional states. Excitement, fear, almost any strong emotion,
is reflected in the conduct of the heart.

In connection with this analysis of the control of the heart-
beat the importance of obtaining the proper viewpoint for con-
sidering physiological processes may well be emphasized. If one
who has not studied the subject particularly be asked why run-
ning makes the heart beat faster he will probably answer that
exercising muscles require more blood than resting ones, and that
the heart beats faster to furnish this extra amount. A moment's
thought shows that this statement, though quite true, does not
really answer the question. It implies that the heart has knowl-
edge of the needs of the tissues, which, of course, it cannot have.
The increased heart-rate which accompanies exercise is undoubtedly
an adaptive response, as are most reflex responses, but its explana-
tion resides, not in the adaptation, but in the reflex mechanism
which brings it about. We should be continually on guard again ,-t
the tendency to explain physiological processes by their results
rather than by the means by which the results are accomplished.

CHAPTER XXI

THE CIRCULATION OF THE BLOOD. BLOOD PRESSURE
AND BLOOD- VELOCITY. THE PULSE

The Flow of the Blood Outside of the Heart. The blood leaves
the heart intermittently and not in a regular stream, a quantity
being forced out at each systole of the ventricles : before it reaches
the capillaries, however, this rhythmic movement is transformed
into a steady flow, as may readily be seen by examining under the
microscope thin transparent parts of various animals, as the web
of a frog's foot, a mouse's ear, or the tail of a small fish. In conse-
quence of the steadiness with which the capillaries supply the
veins the flow in these is also unaffected, directly, by each beat
of the heart ; if a vein be cut the blood wells out uniformly, while
from a cut artery the blood spurts out not only with much greater
force, but in jets which are much more powerful at regular inter-
vals corresponding with the systoles of the ventricles.

The Circulation of the Blood as seen in the Frog's Web. There
is no more fascinating or instructive phenomenon than the circu-
lation of the blood as seen with the microscope in the thin mem-
brane between the toes of a frog's hind limb. Upon focusing
beneath the epidermis a network of minute arteries, veins, and
capillaries, with the blood flowing through them, comes into view
(Fig. 106). The arteries, a, are readily recognized by the fact
that the flow in them is fastest and from larger to smaller branches.
The latter are seen ending in capillaries, which form networks,
the channels of which are all nearly equal in size. While in the
veins arising from the capillaries the flow is from smaller toJarger
trunks, and slower than in the arteries, but faster than in the
capillaries.

The reason of the slower flow of the capillaries is that their
united area is considerably greater than that of the arteries
supplying them, so that the same quantity of blood flowing
through them in a given time has a wider channel to flow in and
moves slowly. The area of the veins is smaller than that of the

318

THE CIRCULATION OF THE BLOOD 319

capillaries but greater than that of the arteries, and hence the
rate of movement in them is also intermediate. Almost always
when an artery divides, the area of its branches is greater than
that of the main trunk, and so the arterial current becomes
slower and slower from the heart onwards. In the veins, on the
other hand, the area of a trunk formed by the union of two or
more branches is less than that of the branches together, and the
flow becomes quicker and quicker towards the heart. But even
at the heart the united cross-sections of the veins entering the
auricles are greater than those of the arteries leaving the ventricles,
so that, since as much blood returns to the heart in a given time
as leaves it, the rate of the current in the pulmonary veins and
the venae cavae is less than in the pulmonary artery and aorta.
We may represent the vascular system as a double cone, widen-
ing from -the ventricles to the capillaries and narrowing from the
latter to the auricles. Just as water forced in at a narrow end of
this would flow quickest there and slowest at the widest part, so
the blood flows quickest in the aorta and slowest in the capillaries,
which taken together form a much wider channel.

The Axial Current and the Inert Layer. If a small artery in
the frog's web be closely examined it will be seen that the rate of
flow is not the same in all parts of it. In the center is a very
rapid current carrying along all the red corpuscles and known as
the axial stream, while hear the wall of the vessel the flow is much
slower, as indicated by the rate at which the pale blood-corpuscles
are carried along in it. This is a purely physical phenomenon.
If any liquid be forcibly driven through a fine tube which it wets,
water for instance through a glass tube, the outermost layers of
the liquid will remain nearly motionless in contact with the tube;
the next layers of molecules will move a little, the next faster
still; and so on until a rapid current is found in the center. If
solid bodies, as powdered sealing-wax, be suspended in the water,
these will all be carried on in the central faster current or axial
stream, just as the red corpuscles are in the artery. The white
corpuscles, partly because of their less specific gravity, and partly
because of their sometimes irregular form, due to amoeboid move-
ments, get frequently pushed out of the axial current, so that
many of them are found in the inert layer.

The Resistance to the Blood-Flow. As liquid flows through a

320 THE HUMAN BODY

tube there is a certain amount of friction between the moving
liquid and the walls of the tube. There is also friction between
the different concentric layers of the liquid, since each of them is
moving at a different rate from that in contact with it on each
side. This form of friction is known in hydrodynamics as "in-
ternal friction," and it is of great importance in the circulation
of the blood. The friction increases very fast as the caliber of the
tube through which the liquid flows diminishes: so that with the
same rate of flow it is disproportionately much greater in a small
tube than in a larger one. Hence a given quantity of liquid forced
in a minute through one large tube would experience much less
resistance from friction than if sent in the same time through
four or five smaller tubes, the united transverse sections of which
were together equal to that of the single larger one. In the blood-
vessels the increased total area, and consequently slower flow, in
the smaller channels partly counteracts this increase of friction,
but only to a comparatively slight extent; so that the friction,
and consequently the resistance to the blood-flow, is far greater
in the capillaries and arterioles than in the small arteries, and in
the small arteries than in the large ones. Practically we may re-
gard the arteries as tubes ending in a sponge: the united areas of
all the channels in the latter might be considerably larger than
that of the supplying tubes, but the friction to be overcome in
the flow through them would be much greater.

The Conversion of the Intermittent into a Continuous Flow.
Since the heart sends blood into the aorta intermittently, we
have still to inquire how it is that the flow in the capillaries is
continuous. In the larger arteries it is not, since we can feel
them dilating as the "pulse,'1 on applying the finger over the
radial artery at the wrist, or over the temporal artery on the side
of the brow.

The first explanation which suggests itself is that since the
capacity of the blood-vessels increases from the heart to the
capillaries, an acceleration of the flow during the ventricular
contraction which might be very manifest in the vessels near the
heart would become less and less obvious in the more distant
vessels. But if this were so, then when the blood was collected
again from the wide capillary sponge into the great veins near
the heart, which together are but little bigger than the aorta, we

THE CIRCULATION OF THE BLOOD

321

— aL

-d'

FIG. 110.

ought to find a pulse, but we do not : the venous pulse which some-
times occurs having quite a different cause, being due to a back-
flow from the auricles, or a checking of the on-flow into them,
during the cardiac systole. The rhythm of the flow caused by
the heart is therefore not merely cloaked in the small arteries and
capillaries, but abolished in them.

We can, however, readily contrive conditions outside the Body
under which an intermittent supply is transformed into a con-
tinuous flow. Suppose we have two
vessels, A and B (Fig. 110) contain-
ing water and connected below in
two ways: through the 'tube a on
which there is a pump provided with
valves so that it can only drive liquid
from A to B; and through b, which
may be left wide open or narrowed by
the clamp c , at will. If the apparatus
be left at rest the water will lie at
the same level, d, in each vessel.
If now we work the pump, at each stroke a certain amount of
water will be conveyed from A to B, and as result of the lower-
ing of the level of liquid in A and its rise in B, there will be
immediately a return flow from B to A through the tube b. A, in
these circumstances, would represent the venous system, from
which the heart constantly takes blood to pump it into B, repre-
senting the arterial system; and b would represent the capillary
vessels through which the return flow takes place; but, so far, we
should have as intermittent a flow through the capillaries, 6, as
through the heart-pump, a. Now imagine b to be narrowed at
one point so as to oppose resistance to the back-flow, while the
pump goes on working steadily. The result will be an accumula-
tion of water in B, and a fall of its level in A. But the more the
difference of level in the two vessels increases, the greater is the
force tending to drive water back through 6 to A, and more will
flow back, under the greater difference of pressure, in a given time,
until at last, when the water in B has reached a certain level, d',
and that in A has correspondingly fallen to d", the current through
b will carry back in one minute just so much -water as the pump
sends the other way, and this back-flow will be nearly constant;

322 THE HUMAN BODY

it will not depend directly upon the strokes of the pump, but
upon the head of water accumulated in B; which head of water
will, it is true, be slightly increased at each stroke of the pump,
but the increase will be very small compared with the whole driv-
ing force, and its influence will be inappreciable. We thus gain
the idea that an incomplete impediment to the flow from the
arteries to the veins (from B to A in the diagram), such as is
afforded by friction in the capillaries, may bring about conditions
which will lead to a steady flow along the latter vessels.

But in the arterial system there can be no accumulation of
blood at a higher level than that in the veins, such as is supposed in
the above apparatus; and we must next consider if the "head of
water" can be replaced by some other form of driving force. It
is in fact replaced by the elasticity of the large arteries. Suppose
an elastic bag instead of the vessel B connected with the pump
"a." If there be no resistance to the back-flow the current
through b will be discontinuous. But if resistance be interposed,
then the elastic bag will become distended, since the pump sends
in a given time more liquid into it than it passes back through 6.
But the more it becomes distended the more will the bag squeeze
the liquid inside and the faster will it send that back to A, until
at last its squeeze is so powerful that each minute or two or five
minutes it sends back into A as much as it receives. Thenceforth
the back-flow through b will be practically constant, being im-
mediately dependent upon the elastic reaction of the bag, and only
indirectly upon the action of the pump which keeps it distended.
Such a state of things represents very closely the phenomena oc-
curring in the blood-vessels. The highly elastic large arteries are
kept stretched with blood by the heart ; and the reaction of their
elastic walls, steadily squeezing on the blood in them, forces it con-
tinuously through the small arteries and capillaries. The steady
flow' in the latter depends thus on two factors : first, the elasticity
of the large arteries; and secondly, the resistance to their empty-
ing, dependent upon internal friction in the small arteries and the
capillaries, which calls into play the elasticity of the large vessels.
Were the capillary resistance or the arterial elasticity absent the
blood-flow in the capillaries would be rhythmic.

Weber's Schema. It is clear from the statements made in the
last paragraph that it is the pressure exerted by the elastic arteries

THE CIRCULATION OF THE BLOOD 323

upon the blood inside them which keeps up the flow through the
capillaries, the heart serving to keep the big arteries tightly filled
and so to call the elastic reaction of their walls into play. The
whole circulation depends primarily, of course, upon the beat of
the heart, but this only indirectly governs the capillary flow, and
since the latter is the aim of the whole vascular apparatus, it
is of great importance to know as much as possible about arterial
pressure; not only how great it is on the average, but how it is
altered in different vessels in various circumstances so as to make
the flow through the capillaries of a given part greater or less
according to circumstances; for, as blushing and pallor of the face
(which frequently occur without any change in the skin elsewhere)
prove, the quantity of blood flowing through a given part is not
always the same, nor is it always increased or diminished in all
parts of the Body at the same time. Most of what we know about
arterial pressure has been ascertained by experiments made upon
the lower animals, from which deductions arc then made concern-
ing what happens in man, since Anatomy shows that the circula-
tory organs are arranged upon the same plan in all the mammalia.
A great deal can, however, be learnt by studying the flow of liq-
uids through ordinary elastic tubes. Suppose we have a set of
such (Fig. Ill) supplied at one point with a pump, c, possessing
valves of entry and exit which open only in the direction indi-
cated by the arrows, and that the whole system is slightly over-
filled with liquid so that its clastic walls are slightly stretched.
These will in consequence press upon the liquid inside them and
the amount of this pressure will be indicated by the gauges; so
long as the pump is at rest it will be the same everywhere (and
therefore equal in the gauges on B and A), since liquid in a set of
horizontal tubes communicating freely, as these do at /), always
distributes itself so that the pressure upon it is everywhere the
same. Let the pump c now contract once, and then dilate: dur-
ing the contraction it will empty itself into B and during the dila-
tation fill itself from A. Consequently the pressure in B, indi-
cated by the gauge x, will rise and that in A will full. But very
rapidly the liquid will redistribute itself from B to .4 through D,
until it again exists everywhere under the same pressure. Every
time the pump works there will occur a similar series of phenom-
ena, and there will be a disturbance of equilibrium causing a

324 THE HUMAN BODY

wave to flow round the tubing; but there will be no steady main-
tenance of a pressure on the side B greater than that in A . Now
let the upper tube D be closed so that the liquid to get from B to
A must flow through the narrow lower tubes D' ', which oppose
considerable resistance to its passage on account of their frequent
branchings and the great friction in them ; then if the pump works
frequently enough there will be produced and maintained in B a
pressure considerably higher than that in A. If, for example,
the pump works 60 times a minute and at each stroke takes 180

FIG. 111. — Diagram of Weber's Schema.

cubic centimeters of liquid (6 ounces) from A and drives it into
B, the quantity sent in at the first stroke will not (on account of
the resistance to its flow offered by the small branched tubes),
have all got back into A before the next stroke takes place, send-
ing 180 more cubic centimeters (6 ounces) into B. Consequently
at each stroke B will become more and more distended and A more
and more emptied, and the gauge x will indicate a much higher
pressure than that on A. As -B is more stretched, however, it
squeezes harder upon its contents, until at last a time comes when
this squeeze is powerful enough to force through the small tubes
just 180 cubic centimeters (6 ounces) in a second. Then further
accumulation in B ceases. The pump sends into it 10,800 cubic
centimeters (360 ounces) in a minute at one end and it squeezes
out exactly that amount in the same time from its other end ; and
so long as the pump works steadily the pressure in B will not rise,
nor that in A fall, any more. But under such circumstances the
flow through the small tubes will be nearly constant since it de-
pends upon the difference in pressure prevailing between B and

THE CIRCULATION OF THE BLOOD 325

A, and only indirectly upon the pump which serves simply to
keep the pressure high in B and low in A. At each stroke of the
pump it is true there will be a slight increase of pressure in B due
to the fresh 180 cubic centimeters (6 ounces) forced into it, but
this increase will be but a small fraction of the total pressure and
so have but an insignificant influence upon the rate of flow through
the small connecting tubes.

Arterial Pressure. The condition of things just described repre-
sents very closely the phenomena presented in the blood-vascular
system, in which the ventricles of the heart, with their auriculo-
ventricular and semilunar valves, represent the pump, the small-
est arteries and the capillaries the resistance at /)', the large
arteries the elastic tube B, and the veins the tube A . The ventri-
cles constantly receiving blood through the auricles from the
veins, send it into the arteries, which find a difficulty in emptying
themselves through the capillaries, and so blood accumulates in
them until the elastic reaction of the stretched arteries is able to
squeeze in a minute through the capillaries just so much blood as
the left ventricle pumps into the aorta, and the right into the
pulmonary artery, in the same time. Accordingly in a living
animal a pressure-gauge connected with an artery shows a much
higher pressure than one connected with a vein, and this persist-
ing difference of pressure, only increased by a small fraction of
the whole at each heart-beat, brings about a steady flow from the
arteries to the veins. The heart keeps the arteries stretched and
the stretched arteries maintain the flow through the capillaries,
and the constancy of the current in them depends on two factors:
(1) the resistance experienced by the blood in its flow from the
ventricles to the veins, and (2) the elasticity of the larger arteries
which allows the blood to accumulate in them under a high pres-
sure, in consequence of this resistance.

Since the blood flows from the aorta to its branches and from
these to the capillaries and thence to the veins, and liquids in a
set of continuous tubes flow from points of greater to those of
less pressure, it is clear that the blood-pressure must constantly
diminish from the aorta to the right auricle; and similarly from
the pulmonary artery to the left auricle. At any point, in fact,
the pressure is proportionate to the resistance in front, and since
the farther the blood has gone the less of this, due to impediments

326 THE HUMAN BODY

at branchings and to internal friction, it has to overcome in finish-
ing its round, the pressure on the blood diminishes as we follow
it from the aorta to the venae cavse. In the larger arteries the fall
of pressure is gradual and small, since the amount of resistance
met with in the flow through them is but little. In the small
arteries and capillaries the resistance overcome and left behind
is (on account of the great internal friction due to their small
caliber) very great, and consequently the fall of pressure between
the medium-sized arteries and the veins is rapid and considerable.
Modifications of Arterial Pressure by Changes in the Heart-beat.
A little consideration will make it clear that the pressure prevail-
ing at any time in a given artery depends on two things — the rate
at which the vessel is filled, i. e., upon the amount of work done
by the heart; and the ease or difficulty with which it is emptied,
that is, upon the resistance in front. A third factor has to be
taken into account in some cases; namely, that when the muscular
coats of the small arteries contract the local capacity of the vas-
cular system is diminished, and has to be compensated for by
greater distention elsewhere, and vice versa. This would of itself
of course bring about changes in the pressure exerted on the
contained liquid, but for the present it may be left out of con-
sideration. If we suppose a system such as represented in Fig. Ill,
to be in equilibrium, with the pump injecting into B a certain
volume of liquid per minute, and the elastic tension of the tube B
just sufficient to force that volume through the resistance D' in
the same time, it is clear that the pressure indicated on the
gauge x will be very nearly constant. If, now, the volume of
liquid forced into B in a minute be increased, either by the pump
working faster or by its pumping more at each stroke, there will
evidently be an accumulation in B, since its tension is adjusted
to force out the less volume per minute, but this accumulation,
by stretching the tube still more, increases its elastic tension, so
that this is presently great enough to force out the added volume
as fast as it comes in. The pressure-gauge will now stand at a
higher point, showing that the contents of the tube are under
greater pressure than before. Similarly, a diminution in the in-
flux of liquid into B will be followed by a fall of pressure within
it as the walls of the tube adjust themselves to the smaller volume
to be forced out per minute. Precisely the same reasoning may

THE CIRCULATION OF THE BLOOD 327

be applied to the vascular system for determining the effects upon
arterial pressure of changes in the heart-beat.

Modifications of Arterial Pressure by Changes in the Peripheral
Resistance. If while the pump c in Fig. Ill is steadily sending
a given volume of liquid per minute into B the resistance at D'
increase, it is clear arterial pressure must rise. For B is only
stretched enough to squeeze out in a minute the given quantity
of liquid against the original resistance, and cannot at first send
out that quantity against the greater. Liquid will consequently
accumulate in it until at last it becomes stretched enough to send
out as much in a minute as before in spite of the greater resistance
to be overcome. A new mean pressure at a higher level will then
be established. If, on the contrary, the resistance diminishes
while the pump's work remains the same, then B will at first
squeeze out in a minute more than it receives, until finally its
elastic pressure is reduced to the point at which its receipts and
losses balance, and a new and lower mean pressure will be estab-
lished in B.

Similarly in the vascular system, increase of the peripheral
resistance by narrowing of the small arteries will increase arterial
pressure in all parts nearer the heart, while dilatation of the small
arteries will have the contrary effect.

Summary. We find then that arterial pressure at any moment
is dependent upon: (1) the quantity of blood forced into the ar-
teries in a given time; (2) the caliber of the smaller vessels. Both
of these and consequently the capillary circulation which depends
upon arterial pressure, are under the control of the nervous sys-
tem (see Chaps. XX and XXII).

The Pulse. When the left ventricle contracts it forces a cer-
tain amount of blood into the aorta, which is already distended
and on .account of the resistance in front cannot empty itself as
fast as the contracting ventricle fills it. As a consequence its
elastic walls yield still more — it enlarges both transversely and
longitudinally and if exposed in a living animal can be seen and
felt to pulsate, swelling out at each systole of the heart, and
shrinking and getting rid of the excess during the pause. A
similar phenomenon can be observed in all the other large arteries,
for just as the contracting ventricle fills the aorta faster than the
latter empties (the whole period of diastole and systole being

328 THE HUMAN BODY

required by the aorta to pass on the blood sent in during systole) ,
so the increased tension in the aorta immediately after the cardiac
contraction drives on some of its contents into its branches, and
fills these faster than they are emptying, and so causes a dilatation
of them also, which only gradually disappears as the aortic tension
falls before the next systole. Hence after each beat of the heart
there is a sensible dilatation of all the larger arteries, known as
the pulse, which becomes less and less marked at points on the
smaller branches farther from the heart, but which in health can
readily be recognized on any artery large enough to be felt by
the finger through the skin. The radial artery near the wrist,
for example, will always be felt tense by the finger, since it is
kept overfilled by the heart in the way already described. But
after each heart-beat it becomes more rigid and dilates a little,
the increased distension and rigidity gradually disappearing as
the artery passes on the excess of blood before the next heart-
beat. The pulse is then a wave of increased pressure started by
the ventricular systole, radiating from the semilunar valves over
the arterial system, and gradually disappearing in the smaller
branches. In the aorta the pulse is most marked, for the resist-
ance there to the transmission onwards of the blood sent in by
the heart is greatest, and the clastic tube in which it consequently
accumulates is shortest, and so the increase of pressure and the
dilatation caused are considerable. The aorta, however, gradually
squeezes out the excess blood into its branches, and so this be-
comes distributed over a wider area, and these branches having
less resistance in front find less and less difficulty in passing it on;
consequently the pulse-wave becomes less and less conspicuous
and finally altogether disappears before the capillaries are reached,
the excess of liquid in the whole arterial system after a ventricular
systole being too small to raise the mean pressure sensibly once it
has been widely distributed over the elastic vessels, which is the
case by the time the wave has reached the small branches which
supply the capillaries.

The pulse-wave travels over the arterial system at the rate of
about 9 meters (29.5 feet) in a second, commencing at the wrist
0.159 second, and in the posterior tibial artery at the ankle 0.193
second, after the ventricular systole. The blood itself does not
of course travel as fast as the pulse-wave, for that quantity sent

THE CIRCULATION OF THE BLOOD 329

into the aorta at each heart-beat does not immediately rush on
over the whole arterial system, but by raising the local pressure
causes the vessel to squeeze out faster than before some of the
blood it already contains, and this entering its branches raises
the pressure in them and causes them to more quickly fill their
branches and raise the pressure in them ; the pulse-wave or wave
of increased pressure is transmitted in this way much faster than
any given portion of the blood. How the wave of increased
pressure and the liquid travel at different rates may be made
clearer perhaps by picturing what would happen if liquid were
pumped into one end of an already full elastic tube, closed at
the other end. At the closed end of the tube a dilatation and in-
creased tension would be felt immediately after each stroke of
the pump, although the liquid pumped in at the other end would
have remained about its point of entry ; it would cause the pulsa-
tion not by flowing along the tube itself, but by giving a push to
the liquid already in it. If instead of absolutely closing the distal
end of the tube one brought about a state of things more nearly
resembling that found in the arteries by allowing it to empty
itself against a resistance, say through a narrow opening, the phe-
nomena observed would not* be essentially altered ; the increase
of pressure would travel along the distended tube far faster than
the liquid itself.

The pulse being dependent on the heart's systole, "feeling the
pulse" of course primarily gives a convenient means of counting
the rate of beat of that organ. To the skilled touch, however, it
may tell a great deal more, as for example whether it is a readily
compressible or "soft pulse" showing a low arterial pressure, or
tense and rigid ("a hard pulse") indicative of high arterial pres-
sure, and so on. In adults the normal pulse-rate may vary from
sixty-five to seventy-five, the most common number being seventy-
two. In the same individual it is faster when standing than when
sitting, and when sitting than when lying down. Any exercise
increases its rate temporarily, and so does excitement; a sick
person's pulse should not therefore be felt when he is nervous or
excited (as the physician knows when he tries first to get his
patient calm and confident), as it is then difficult to draw correct
conclusions from it. In children the pulse is quicker than in
adults, and in old age slower than in middle life.

330

THE HUMAN BODY

The Measurement of • Blood- Pressure. Direct determinations
of arterial and venous pressures are made in living, anesthetized
animals by inserting into a large artery or vein a glass tube con-
nected with a pressure-gauge. The usual form of gauge for such
work is the mercury manometer represented in Fig. 112. This

FIG. 112. — Mercury manometer for recording blood pressure, dg, glass JJ-tube
partly filled with mercury. In one limb is borne a float, e, bearing a recording de-
vice ; the other limb is filled with a suitable liquid and connected water-tight with
the heart end of a divided artery. Changes in the mercury level indicate changes
of arterial pressure.

instrument, on account of the great inertia of mercury, follows
only slightly the rapid fluctuations of pressure due to the beats
of the heart. It therefore gives mean or average pressures. Re-
sults obtained with mercury manometers are expressed in terms
of the height of the mercury column sustained by the blood-
pressure. To reduce them to columns of blood they must be
multiplied by 13.6, the number of times mercury is heavier than
blood. The mean aortic pressure in average-sized dogs is ordi-

THE CIRCULATION OF THE BWOD 331

narily not far from 170 millimeters of mercury. The pressure in
the veins diminishes from 3 or 4 millimeters of mercury in the
large veins of the front leg to zero at the entrance to the auricle
(see p. 325).

Blood-Pressure in Man. In man it is necessary to determine
blood-pressures by methods that do not involve operative pro-
cedure. Various devices are in use for this purpose. Most of
them depend on the fact that bodily tissues, being for the most
part liquid, are virtually incompressible and so transmit through-
out their extent pressures applied to them. For determining
arterial pressures the upper arm is inclosed in a cuff of hollow
rubber tubing so arranged that its inflation presses from all sides
on the arm. The cuff is inflated until its pressure on the arm is
just sufficient to squeeze shut the brachial artery. By means of
a manometer attached to the cuff the amount of pressure applied
can be determined. The differences between the various forms of
instruments depend chiefly on their methods for determining
exactly when the artery is occluded. These instruments do not
give mean blood-pressures, as does the mercury manometer, but
maximum (systolic) and minimum (diastolic) pressures. It is
found that in man the systolic pressure averages from 110 to
120 mms. of mercury, and the diastolic about 65 mms. of mercury.

Determinations of capillary and venous pressures in man can
be made more easily than determinations of arterial pressure
because there are superficial capillaries and veins whose occlusion
can be observed directly; in capillaries by whitening of the skin,
in veins by the disappearance of the vein-ridge along it. The
basis of the method is the same as for arterial pressure, namely,
determination of the pressure necessary to occlude the vessel.
Capillary pressures measured by this method average about
30 mms. of mercury ; venous pressures 10 mms. or less.

The Rate of the Blood-Flow. As the vascular system be-
comes more capacious from the aorta to the capillaries the rate
of flow in it becomes proportionately slower, and as the total
area of the channels diminishes again from the capillaries to the
vensB cavae, so does the rate of flow quicken, just as a river current
slackens when it spreads out, and flows faster when it is confined
to a narrower channel ; a fact taken advantage of in the construc-
tion of Eads' jetties at the mouth of the Mississippi, the object

332 THE HUMAN BODY

of which is to make the water flow in a narrower channel and so
with a more rapid current in that part of the river. Actual meas-
urements as to the rate of flow in the arteries cannot be made on
man, but from experiments on lower animals it is calculated that
in the human carotid the blood flows about 400 millimeters
(16 inches) in a second. In the capillaries the current travels only
from 0.5 to 0.75 mm. (^ to -^ inch) in a second. The total time
taken by a portion of blood in making a complete circulation has
been measured by injecting some easily detected substance into
an artery on one side of the body and noting the time which
elapses before it can be found in a corresponding vein on the
opposite side. In dogs this time is 15 seconds, and it is calcu-
lated for man at about 23 seconds. Of this total time about half
a second is spent in the systemic and another half second in the
pulmonary capillaries, as each portion of blood on its course
from the last artery to the first vein passes through a length of
capillary which on the average is 0.5 mm. (•£$ inch). The rate of
flow in the great veins is about 100 mm. (4 inches) in a second,
but is subject to considerable variations dependent on the respira-
tory and other movements of the Body; in the small veins it is
much slowrer.

Secondary Factors Affecting the Circulation. While the heart's
beat is the great driving force of the circulation, certain other
things help more or less — viz., gravity, compression of the veins,
and aspiration of the thorax. All of them are, however, quite
subsidiary; experiment on the dead Body shows that the injection
of defibrinated blood into the aorta under a less force than that
exerted by the left ventricle during life is more than sufficient
to drive it round and back by the venae cavse.

The Influence of Gravity. Under ordinary circumstances this
may be neglected, since in parts of the Body below the level of
the heart it will assist the flow in the arteries and impede it equally
in the veins, while the reverse is the case in the upper parts of the
Body. In certain cases, however, it is well to bear these points in
mind. A part " congested " or gorged with blood should if possi-
ble be raised so as to make the back-flow in its veins easier; and
sometimes when the heart is acting feebly it may be able to drive
blood along arteries in which gravity helps, but not otherwise.
Accordingly in a tendency to fainting it is best to lie down, and

THE CIRCULATION OF THE BLOOD 333

make it easier for the heart to send blood up to the brain, defi-
ciency in its blood-supply being the cause of the loss of conscious-
ness in a fainting-fit. In fact, so long as the breathing continues,
the aspiration of the thorax will keep up the venous flow (see
below), while, in the circumstances supposed, a slight diminution
in the resistance opposed to the arterial flow may be of impor-
tance. The head of a person who has fainted should accordingly
never be raised until he has undoubtedly recovered, a fact rarely
borne in mind by spectators, who commonly rush at once to lift
any one whom they see fall in the street or elsewhere.

The Influence of Transient Compression of the Veins. The
valves of the veins being so disposed as to permit only a flow
towards the heart, when external pressure empties a vein it assists
the circulation. Continuous pressure, as by a tight garter, is of
course bad, since it checks all subsequent flow through the vessel ;
but intermittent pressure, such as is exerted on many veins by
muscles in the ordinary movements of the Body, acts as a pump
to force on the blood in them.

The value of this pumping of the blood out of the veins by
muscular movements is well illustrated by comparing two classes
of workers whose occupations require that they be upon their
feet continuously for hours. The condition of varicose veins,
which is a stasis of blood in the superficial veins of the lower
extremities, is very prevalent among motormen, and others who
must stand still for long periods, but is virtually unknown among
postmen, who are walking during the time spent on their feet.

The valves of the veins have another use in diminishing the
pressure on the lower part of those vessels in many regions. If,
for instance, there were no valves in the long saphenous vein of
the leg the considerable weight of the column of blood in it,
which in the erect position would be about a meter (39 inches)
high, would press on the lower part of the vessel. But each set of
valves in it carries the weight of the column of blood between it
and the next set of valves above, and relieves parts below, and
so the weight of the column of blood is distributed and does not
all bear on any one point.

Aspiration of the Thorax. Whenever a breath is drawn the
pressure of the air on the vessels inside the chest is diminished,
while that on the other vessels of the Body is unaffected. In

334 THE HUMAN BODY

consequence blood tends to flow into the chest. It cannot, how-
ever, flow back from the arteries on account of the semilunar
valves of the aorta, but it can readily be pressed, or in common
language "sucked," into the great veins close to the heart and
into the right auricle of the latter. The details of this action must
be omitted until the respiratory mechanism has been considered.
All parts of the pulmonary circuit being within the thorax, the
respiratory movements do not directly influence it, except in so
far as the distention or collapse of the lungs alters the caliber of
their vessels.

The considerable influence of the respiratory movements upon
the venous circulation can be readily observed. In thin persons
the jugular vein in the neck can often be seen to empty rapidly
and collapse during inspiration, and fill up in a very noticeable
way during expiration, exhibiting a sort of venous pulse. Every
one, too, knows that by making a violent and prolonged expira-
tion, as exhibited for example by a child with whooping-cough,
the flow in all the veins of the head and neck may be checked,
causing them to swell up and hinder the capillary circulation until
the person becomes " black in the face," from the engorgement of
the small vessels with dark-colored venous blood.

In diseases of the tricuspid valve another form of venous pulse
is often seen in the superficial veins of the neck, since at each
contraction of the right ventricle some blood is driven back
through the right auricle into the veins.

Proofs of the Circulation of the Blood. The ancient physiolo-
gists believed that the movement of the blood was an ebb and
flow, to and from each side of the heart, and out and in by both
arteries and veins. They had no idea of a circulation, but thought
pure blood was formed in the lungs and impure in the liver, and
that these partially mixed in the heart through minute pores sup-
posed to exist in the septum. Servetus, who was burnt alive by
Calvin in 1553, first stated that there was a continuous passage
through the lungs from the pulmonary artery to the pulmonary
veins, but the great Englishman Harvey first, in lectures delivered
in the College of Physicians of London about 1616, demonstrated
that the movement of the blood was a continuous circulation as
we now know it, and so laid the foundation of modern Physi-
ology. In his time,- however, the capillary vessels had not been

THE CIRCULATION OF THE BLOOD 335

discovered, so that although he was quite certain that the blood
got somehow from the final branches of the aorta to the radicles
of the venous system, he did not exactly know how.

The proofs of the course of the circulation are at present quite
conclusive, and may be summed up as follows: (1) Blood injected
into an artery in the dead Body will return by a vein; but injected
into a vein will not pass back by an artery. (2) The anatomical
arrangement of the valves of the heart and of the veins shows
that the blood can only flow from the heart, through the arteries
and back to the heart by the veins. (3) A cut artery spurts from
the end next the heart, a- cut vein bleeds most from the end
farthest from the heart. (4) A portion of a vein when emptied
fills only from the end farthest from the heart. This observation
can be made on the veins on the back of the hand of any thin
person, especially if the vessels be first gorged by holding the
hand in u dependent position for a few seconds. Select then a
vein which runs for an inch or so without branching, place a finger
on its distal end, and then empty it up to its next branch (where
valves usually exist) by compressing it from below up. The ves-
sel will then be found to remain empty as long as the finger is kept
on its lower end, but to fill immediately when it is removed;
which proves that the valves prevent any filling of the vein from
its heart-end backwards. (5) If a bandage be placed around the
arm, so as to close the superficial veins, but not tight enough to
occlude the deeper-seated arteries, the veins on the distal side
of the bandage will become gorged and those on its proximal side
empty, showing again that the veins only receive blood from their
ends turned towards the capillaries. (6) In the lower animals
direct observation with the microscope shows the steady flow
of blood from the arteries through the capillaries to the veins, but
never in the opposite direction.

CHAPTER XXII

THE VASOMOTOR MECHANISM. SLEEP. THE LYMPHATIC

SYSTEM

The Distribution of Blood Among Various Parts of the Body.
In the nervous control of the heart-beat we have, as already
noted, a mechanism whereby the blood-flow through the Body as
a whole can be modified in accordance with the needs of the
organism. In the vasomotor mechanism we have an arrangement,
equally important, whereby individual organs or regions can be
furnished with more or less blood as their activities require with-
out the necessity of involving the whole circulation.

The Nerves of the Blood-Vessels. The arteries, as already
pointed out, possess a muscular coat composed of fibers arranged
around them, so that their contraction can narrow the vessels.
This coat is most prominent in the smaller vessels, the arterioles.
These vascular muscles are under the control of certain special
nerves called vasomotor, and these latter can thus govern the
amount of blood reaching any organ at a given time. The vaso-
motor nerves belong to the sympathetic system. Their physi-
ology is therefore the application to special structures of the
general principles laid down in connection with that system
(Chap. XII).

In the heart we had to consider a rhythmically contracting
organ the force of whose contractions could be increased or dimin-
ished by the influence of extrinsic nerves; in the arteries, speak-
ing broadly, we have to deal with muscle in a condition of tonic
or constant contraction, which contraction can be increased by
impulses coming through excitor or vasoconstrictor nerves, and
diminished through the activity of inhibitory or vasodilator
nerves. The general tonic contraction of the arterial muscle is,
however, much more dependent on the vasoconstrictor nerve-
fibers than is the beat of the heart on the cardio-excitor nerves.
The inhibitory (dilator) set of vasomotor nerves have a much less
extensive distribution over the arterial system than the constrictor.

336

THE VASOMOTOR MECHANISM 337

The Vasoconstrictor Nerves. If the car of a white rabbit be
held up against the light while the animal is kept quiet and not
alarmed, the red central artery can be seen coursing along the
translucent organ, giving off branches which by subdivision be-
come too small to be separately visible, and the whole ear has a
pink color and is warm from the abundant blood flowing through
it. Attentive observation will show also that the caliber of the
main artery is not constant; at somewhat irregular periods of a
minute or more it dilates and contracts a little.

If the sympathetic trunk have been previously divided on the
other side of the neck of the animal, the ear on that side will pre-
sent a very different appearance. Its arteries will be much dilated
and the whole ear fuller of blood, redder, and distinctly warmer;
the slow alternating variations in arterial diameter also have
disappeared. We get thus evidence that the normal mean caliber
of the artery is maintained by influences reaching its muscular
coat through the cervical sympathetic. Stimulation of the upper
end of the cut nerve confirms this opinion. It is then seen that
the arteries of the corresponding ear gradually contract until
even the main vessel can hardly be seen, and in consequence the
whole ear becomes pale and coid. After the stimulation is stopped
the arteries again slowly dilate until they have regained their
full paralytic size.

Quite similar phenomena can be observed in transparent parts
of other living animals, as in the web of a frog's foot, the arteries
of which dilate after section of the sciatic nerve and constrict
when the peripheral end of the nerve is stimulated. In the case
of other parts changes in temperature may be used to detect
alterations in the flow of blood. In a dog or cat, for example, a
sensitive thermometer placed between the toes indicates a rise
of temperature, owing to increased flow of warm blood through
the skin, after section of the chief nerve of the limb, and a fall of
temperature (usually) during stimulation of the peripheral end
of the divided nerve.

When the vasoconstrictor nerves cut are those controlling a
large number of arteries, the dilatation of the latter so much
diminishes peripheral resistance to the blood-flow as to lead to a
marked fall of general arterial pressure; and, due care being taken
to avoid or to allow for concomitant variations in the rate or

338 THE HUMAN BODY

force of the heart's beat, this gives us another useful method of
studying the distribution of the nerves concerned. For example,
the splanchnic nerves are branches which spring from the thoracic
portion of the sympathetic chain and pass through the diaphragm
to end in the solar plexus from which nerves pass to the arteries
of most of the abdominal viscera. The region whose blood-vessels
are innervated by these nerves is often spoken of as the splanchnic
region. When the splanchnic nerves are cut on both sides arterial
pressure falls enormously, from say 120 millimeters of mercury in
the carotid of a dog to 15 or 20 millimeters, most of the blood of
the Body lying almost stagnant in the dilated blood-vessels of
the abdomen. On the other hand, stimulation of the splanchnic
nerves so diminishes the paths open for the circulation of the
blood as to increase general blood-pressure enormously.

The skin and the abdominal organs seem to be the predomi-
nant localities of distribution of the vasoconstrictor nerves: other
parts have them, but not in quantity sufficient to bring about
any great general change in the blood-flow.

The Vasoconstrictor Center. This, one of the most important
of the "vital" centers of the medulla, has not been identified
anatomically with any particular group of nerve-cells, but its
location is quite sharply defined physiologically. There is a small
region of the medulla, known as the " vital knot, " whose destruc-
tion is promptly fatal to the life of the organism. This region
includes, in addition to at least one other " center, " the vaso-
constrictor center. From this center there is a constant outflow
of impulses to all those arterioles of the Body whose muscles
contain vasoconstrictor nerve-endings. This constant stream of
constrictor impulses is the chief factor in the maintenance of so-
called vasomotor tone, a condition of continuous moderate con-
striction of the arterioles by which general arterial pressure is kept
at the proper level.

It is probable that the vasoconstrictor center consists physi-
ologically of a number of associated centers which may act as a
unit or separately. These "partial" centers are in connection
with restricted vasomotor areas, and thus are enabled to bring
about local vasomotor effects.

The Control of the Vasoconstrictor Center. This center, like
the other "vital" centers of the medulla, is kept in activity re-

THE VASOMOTOR MECHANISM 339

flexly. The whole stream of afferent impulses passing through
the medulla plays upon it. Like the centers for controlling the
heart-beat its activity may be increased through the influx of
stimuli into it, or it may suffer depression for the same cause.
We divide afferent impulses affecting the vasoconstrictor center,
therefore, into two groups: those increasing its activity, pressor
impulses, and those diminishing it, depressor impulses. Certain
sorts of stimuli are uniformly pressor in effect; pain, for example,
always brings about a reflex rise of blood-pressure through stim-
ulating the vasoconstrictor center. It is possible that other stimuli
may be pressor or depressor according to circumstances.

The Depressor Nerve. The best-known nerve-tract which car-
ries depressor impulses uniformly has already been mentioned.
It is the afferent tract from the heart known, in animals where it
is present as a separate trunk, as the depressor nerve. Stimu-
lation of this nerve brings about, always, a reflex fall of blood-
pressure, which is due solely to vasodilatation resulting from de-
pression of the vasoconstrictor center. This nerve rises, not in
heart tissue proper, but in the walls of the aorta near where that
vessel springs from the heart. An undue increase in blood-
pressure, such as might affect the heart injurously, subjects
the aortic wall to unusual tension. This seems to stimulate
the depressor nerve mechanically. Thus the heart is protected
against injury arising from working against too great resist-
ance.

Blushing is a vasodilatation involving chiefly the artcrioles of
the skin of the face and head. It, and similar local vasodilatations
of skin or visceral regions, are due to depression of one or more
of the subordinate centers, which together make up the vaso-
constrictor center as a whole. The result of such local vaso-
dilatation is to increase very much the blood supply of the region
involved, without much effect on the rest of the Body. Blushing
is one of the most familiar examples of an emotional reaction in-
volving the sympathetic system. A case of an adaptive vaso-
dilatation is the reddening of the hand when placed in hot water.
In this reflex the sensory stimulus is the heat applied to the skin.
It depresses that part of the vasoconstrictor center which controls
the arterioles of the hand and their relaxation follows. The result
of the increased blood-supply to the hand is a more rapid loss of

340 THE HUMAN BODY

heat from it, thus counteracting the effects of the higher surround-
ing temperature.

Quite similar phenomena occur under proper conditions in many
parts of the Body. The mucous membrane lining the empty
stomach is pallid and its arteries contracted, but as soon as food
enters the organ it becomes red and full of blood ; the food stimu-
lating afferent nerve-fibers there, which inhibit that part of the
vasomotor center which governs the gastric arteries.

Taking Cold. This common condition is not unfrequently the
indirect result of undue reflex excitement of the vasomotor center.
Cold acting upon the skin stimulates, through the afferent nerves,
the portion of the vasomotor center governing the skin arteries,
and the latter become contracted, as shown by the pallor of the
surface. This has a twofold influence — in the first place, more
blood is thrown into internal parts, and in the second, contraction
of the arteries over so much of the Body considerably raises the
general blood-pressure. Consequently the vessels of internal parts
become overgorged or " congested," a condition which is espe-
cially favorable to inflammation. The best preventive is to wear,
when exposed to great changes of temperature, a woolen or at
least a cotton garment over the trunk of the Body; linen is so good
a conductor of heat that it permits any change in the external
temperature to act almost at once upon the surface of the Body.
After an unavoidable exposure to cold or wet the thing to be done
is of course to restore the cutaneous circulation; for this purpose
movement should be persisted in, and a thick dry outer covering
put on, until warm arid dry underclothing can be obtained.

For healthy persons a temporary exposure to cold, as a plunge
in a bath, is good, since in them the sudden contraction of the
cutaneous arteries soon passes off and is succeeded by a dilatation
causing a warm healthy glow on the surface. If the bather remain
too long in cold water, however, this reaction passes off and is suc-
ceeded by a more persistent chilliness of the surface, which may
even last all day. The bath should therefore be left before this
occurs, but no absolute time can be stated, as the reaction is more
marked and lasts longer in strong persons, and in those used to
cold bathing, than in others.

Adrenalin. The effect of this hormone upon the vascular system,
as stated previously (Chap. XVIII), is to stimulate the vaso-

THE VASOMOTOR MECHANISM 341

constrictor fibers at their terminations in the muscles of the
arterioles. The constant presence of this hormone in the blood is
doubtless an important factor in maintaining that degree of vaso-
motor tone upon which the well-being of the Body depends. The
substance adrenalin as used experimentally shows several striking
characteristics. In the first place a very small concentration of it
(one part in ten thousand), introduced into a capillary region,
brings about so strong a constriction in the immediate neighbor-
hood as to stop the flow of blood completely through that region.
It is possible thus to prevent troublesome bleeding in small
operations. The effect of adrenalin used in this way is, however,
very transient; repeated injections are necessary to maintain the
constricted state. Adrenalin injected into the general circulation,
even in small doses, causes a marked rise of arterial pressure due to
general vasoconstriction. This again is a very momentary effect.

Vasodilator Nerves. We have already noticed, in the case of
the stomach, one method by which a locally increased blood-
supply may be brought about in an organ while it is at work, viz.,
by inhibition of local vasoconstrictor fibers. Frequently, how-
ever, in the Body this is managed in another way; by efferent vaso-
dilator nerves which inhibit, not the vasoconstrictor center, but
the muscles of the blood-vessels directly. The nerves of the
skeletal muscles for example contain two sets of efferent fibers : one
motor proper and the other vasodilator. When the muscle con-
tracts in a reflex action or under the influence of the will both sets
of fibers are excited; so that when the organ is set at work its
arteries are simultaneously dilated and more blood flows through
it. But if the animal have previously administered to it such a
dose of curare as just to throw out of function the true motor-
fibers, stimulation of the nerve produces dilatation of the arteries
without a corresponding muscular contraction. Quite a similar
thing occurs in the salivary glands. Their cells, which form the
saliva, are aroused to activity by special nerve-fibers; but the
gland-nerve also contains a quite distinct set of vasodilator fibers
which normally cause a simultaneous dilatation of the gland-
artery, though either can be artificially stimulated by itself and
produce its effect alone.

Since the effect of stimulating vasodilator nerves is the same as
inhibiting the constrictor mechanism we might ask why there

342 THE HUMAN BODY

should be two distinct means thus provided for securing the same
result. As a matter of fact the two mechanisms do not seem to
overlap to any great extent; they rather supplement each other.
The vasoconstrictor mechanism is confined, in the main, to the
blood-vessels of the skin and viscera; the dilator mechanism is
distributed chiefly to the muscles, the glands, and the genital
organs.

Through such arrangements the distribution of the blood in the
Body at any moment is governed : so that working parts shall have
abundance and other parts less, while at the same time the general
arterial pressure remains the same on the average; since the ex-
pansion of a few small local branches but little influences the total
peripheral resistance in the vascular system. Moreover, commonly
when one set of organs is at work with its vessels dilated, others
are at rest with their arteries comparatively contracted, and so a
general average blood-pressure is maintained. Few persons, for ex-
ample, feel inclined to do brain-work after a heavy meal; for then a
great part of the blood of the whole Body is led off into the dilated
vessels of the digestive organs, and the brain gets a smaller supply.

The Vasodilator Center. There is reason to believe that the
vasodilator nerves are under the control of a center in the medulla,
which is in turn subject to the influence of afferent impulses of
various sorts. The exact location of this center has not been de-
termined. So far as can be judged from observation of vaso-
dilator phenomena the vasodilator center is probably not in con-
stant tonic activity, as is the constrictor center, but is aroused to
activity only when afferent stimuli come to it from certain par-
ticular regions.

The Relation of Vasomotor Tone to Cerebral Activity. The
circulation through the brain differs in some important respects
from that of the rest of the body. The differences arise from the
fact that the brain, a fluid and therefore incompressible mass, is
inclosed in an unyielding receptacle, the skull, which it fills com-
pletely. The result is that the cerebral blood-vessels occupy their
allotted space, which cannot be either increased or diminished ap-
preciably. The total volume of blood in the brain at any time is
therefore practically constant, and the circulation through the
brain can only be altered by changing the rate at which the blood
flows through it. In such an arrangement as this, where local

THE VASOMOTOR MECHANISM 3-13

vasodilatation cannot occur, the only way in which the rate of
blood-flow can be altered is by changes in the pressure at which
the blood is forced into the region. The arteries feeding the brain
spring directly from the aorta; it follows, therefore, that variations
in aortic pressure, in other words, in general blood-pressure, are
reflected exactly in the rate of cerebral blood-flow.

General blood-pressure, as we have seen, is maintained by vaso-
motor tone, the state of moderate constriction of arterioles gener-
ally. Variations in the tone of restricted areas, such as occur in
connection with the functioning of individual organs do not ordi-
narily affect general blood-pressure enough to alter the circulation
through the brain to any extent.

There is good evidence that the degree of activity of the cells of
the cerebral cortex is directly and immediately dependent upon
the rate of blood-flow through the organ. A rapid circulation
means alertness and efficiency of mental processes; as the flow
becomes slower and slower the cells work less and less actively;
when a certain point of sluggishness is reached consciousness dis-
appears, the cells, if not altogether quiescent, working too freely to
arouse that state.

The phenomenon of fainting, which has already been men-
tioned, is the result usually of a sudden inhibition of the vaso-
constrictor center whereby over a large area, the whole splanchnic
region, for instance, there is general vasodilatation and a resulting
fall in blood-pressure. The rate of cerebral blood-flow falls to a
point below that required for the maintenance of consciousness
and the individual falls in a faint.

Sleep. This periodic loss of consciousness, so important for the
proper restoration of the fatigued organs and tissues of the Body,
has been the subject of considerable attention and investigation.
Its explanation is not simple, involving as it does a number of
questions, as, for instance, why fatigue, which ordinarily induces
sleep, may, if extreme, prevent it; and what it is that causes one
to awake after the proper number of hours of sleep.

Objectively sleep is marked by its well-known signs, which are
not very instructive as to its cause, and also by certain vaso-
motor changes which have been looked upon as very instructive;
and as affording us, indeed, our only satisfactory method of study-
ing sleep experimentally. Observations upon sleeping individuals

344 THE HUMAN BODY

have shown that normal sleep is frequently accompanied by a con-
siderable fall in general blood-pressure, resulting from extensive
vasodilatation. This is itself sufficient to account for the dimin-
ished cerebral activity with its accompanying loss of conscious-
ness which constitutes sleep, and many physiologists are inclined
to believe, therefore, that the vasomotor changes may form the
underlying basis for the phenomenon. A theory which expresses
this view looks upon the vasoconstrictor center as the controlling
mechanism of sleep. When this center is in good condition the
constant stream of afferent impulses playing upon it maintains it
in strong activity, and vasomotor tone is kept high. With the
passage of hours of such ceaseless activity the center becomes
fatigued and tends to respond less strongly to the afferent impulses
coming to it. The result will be a falling off of vasomotor tone,
unless by an effort of the will or an increase in the stream of af-
ferent impulses, such as follows muscular exercise, for example,
the center is whipped up to renewed activity. " Keeping awake "
when one is sleepy is, according to this view, a matter of stimu-
lating the tired vasoconstrictor center to continued effort. The
effect may be produced by an artificial stimulant, such as coffee,
or by an act of the will. The usual preparations for sleep are such
as favor diminished activity of the vasoconstrictor center by
lessening the afferent impulses coming to it. Lying in a comfort-
able position removes most of the impulses of muscle sense; by
closing the eyes visual stimuli are gotten rid of. Thus unless the
center is so irritable that the small stream of inevitable afferent
impulses keeps it up to the mark the essential condition for sleep,
loss of vasomotor tone, is fulfilled. The act of waking, according
to this theory, results either from an undue stimulation of the vaso-
constrictor center, as when one is waked by being violently shaken,
or from a gradual restoration of the irritability of the center
during its period of rest, to a point where the minimal stream
of afferent impulses, inseparable from the living Body, is sufficient
to stimulate it to the maintenance of waking vasomotor tone.

It must be admitted that not all experiments upon sleep have
shown marked loss of vasomotor tone, but even if we consider
vasomotor fatigue the primary factor we must grant, of course,
that there are numerous additional factors modifying sleep. The
condition of the cerebral cells and the nature of their activity

THE VASOMOTOR MECHANISM 345

doubtless have much to do with the phenomenon. These, how-
ever, are factors which physiology at present is unable to ana-
lyze completely, so that the vasomotor theory affords our most
satisfactory explanation of sleep from the physiological standpoint.

The Lymphatics. The living cells of the Body, as previously
pointed out (Chap. XVII), are bathed in lymph, a liquid derived
from the blood and serving as the intermediary by which inter-
changes of food materials, gases and waste substances between
it and the cells are carried on. At the same place it was shown
that there is a continuous movement of liquid from the blood into
the lymph spaces, necessitating a system whereby the accumu-
lation can be drained away from the tissues and carried back to
the blood. This drainage is afforded by the lymphatic system. At
its beginning this system is without definite structure, consisting
simply of intercellular spaces. These communicate with one an-
other, and at intervals with minute vessels having definite walls.
These latter are the beginnings of definite lymph-channels.

The Structure of Lymph- Vessels. The smallest lymph-vessels
proper are the lymph-capillaries; tubes rather wider than the blood-
capillaries, but like them having a wall consisting of a single layer
of flattened epithelium cells*. The cells have, however, a wavy
margin and are not as a rule much longer in one diameter than an-
other, in both of which respects they differ from the cells of the
corresponding blood-vessels. In some regions, as in many glands,
the lymph-capillaries are much dilated and form irregular lymph
lacunae, everywhere bounded by their peculiar wavy cells, lying in
the interstices of organs; and sometimes they form tubes around
small blood-vessels, as in the brain (periwucidar lymph-channel).
In some places they commence by blind ends as in the lacteal
vessels of the villi of the small intestine (Fig. 132) which are lymph-
capillaries; but usually they branch and join to form networks.
Lymph from the intercellular spaces enters them (probably by
passing through their boundary cells) and is passed on to larger
vessels which much resemble veins of corresponding size, having
the same three coats, and being abundantly provided with valves.

The Thoracic Duct. The lymph-vessels proceeding from the
capillaries in various organs become larger and fewer by joining
together, and all end finally in two main trunks which open into
the venous system on the sides of the neck, at the point of junction

346 THE HUMAN BODY

of the jugular and subclavian veins. The trunk on the right side is
much smaller than the other and is known as the " right lymphatic
duct." It collects lymph from the right side of the thorax, from
the right side of the head and neck, and the right arm. The lymph
from all the rest of the Body is collected into the thoracic duct. It
commences at the upper part of the abdominal cavity in a dilated
reservoir (the receptaculum chyli) , into which the lacteals from the
intestines, and the lymphatics of the rest of the lower part of the
Body, open. From thence the thoracic duct, receiving tributaries
on its course, runs up the thorax alongside of the aorta and, passing
on into the neck, ends on the left side at the point already indi-
cated; receiving on its way the main stems from the left arm and
the left side of the head and neck. The thoracic duct, thus, brings
back to the blood much more lymph than the right lymphatic duct.

Lymph-Nodes. At intervals along the course of various
lymphatic vessels are structures consisting of cells so arranged
as to leave interspaces among them, through which interspaces
the lymph is forced to flow. These structures are the lymph-nodes
or lymph-glands and the peculiar tissue of which they are com-
posed is lymphoid or adenoid tissue. Lymph-nodes occur in the
neck, the groin, the axilla and in various other regions of the
Body. Certain structures in the wall of the small intestine near
its lower end, the so-called Peyer's Patches, are composed of
lymphoid tissue as are also the structures in the throat making
up the tonsillar ring.

Functions of Lymph-Nodes. Two quite different functions
are attributed to the lymph-nodes. They are supposed in the
first place to be the seat of leucocyte production. When any
lymph-node is examined its spaces are usually found filled with
leucocytes, some of which are in process of division. This process
has been observed in leucocytes in lymph-nodes but not else-
where, and since it is agreed that leucocytes are produced by the
division of parent ones, the lymph-nodes are looked upon as the
regions where this goes on.

The lymph-nodes have also the additional function of filtering
the lymph that passes through them. This filtering action is
probably of great importance in confining micro-organisms to the
region which they first enter, since if they get into the lymph
stream they are arrested at the first lymph-node. It is thought

THE VASOMOTOR MECHANISM 347

that the lymph-nodes are able also to arrest, for a time at least,
the spread of cancer-cells over the body. The lymph-nodes located
on the channels draining the lungs become filled with dust that
has worked its way through the pulmonary walls into the lymph,
and that is prevented thus from spreading throughout the Body.

The Movement of the Lymph. This is no doubt somewhat
irregular in the commencing vessels, but, on the whole, sets on
to the larger trunks and through them to the veins. In many
animals (as the frog) at points where the lymphatics communicate
with the veins, there are found regularly contractile "lymph-
hearts" which beat with a rhythm independent of that of the
blood-heart, and pump the lymph into a vein. In the Human
Body, however, there are no such hearts, and the flow of the
lymph is dependent on less definite arrangements. It seems to
be maintained mainly by three things: (1) The pressure on the
blood-plasma in the capillaries is greater than that in the great
veins of the neck; hence any plasma filtered through the capillary-
walls will be under a pressure which will tend to make it flow to
the venous termination of the thoracic or the right lymphatic
duct. (2) On account of the numerous valves in the lymphatic
vessels (which all only allow the lymph to flow past them to
larger trunks) any movement compressing a lymph-vessel will
cause an onward flow of its contents. The influence thus exerted
is very important. If a tube be put in a large lymphatic, say at
the top of the leg of an animal, it will be seen that the lymph only
flows out very slowly while the animal is quiet; but as soon as it
moves the leg the flow is greatly accelerated. (3) During each
inspiration the pressure on the thoracic duct is less than that in
the lymphatics in parts of the Body outside the thorax (see
Chap. XXIII). Accordingly, at that time, lymph is pressed, or,
in common phrase, is "sucked," into the thoracic duct. During
the succeeding expiration the pressure on the thoracic duct be-
comes greater again, and some of its contents are pressed out; but
on account of the valves of the vessels which unite to form the
duct, they can only go towards the veins of the neck.

During digestion, moreover, contractions of the villi and of
the intestinal walls press on the lymph or chyle within them and
force it on; and in certain parts of the Body gravity, of course,
aids the flow, though it will impede it in others.

CHAPTER XXIII

RESPIRATION. THE MECHANISM OF BREATHING. THE
REGULATION OF BREATHING.

Definitions. The blood as it flows from the right ventricle of
the heart, through the lungs, to the left auricle, loses carbon
dioxid and gains oxygen. In the systemic circulation exactly
the reverse changes take place, oxygen leaving the blood to supply
the living tissues; and carbon dioxid, generated in them, passing
back into the blood capillaries. The oxygen loss and carbon
dioxid gain are associated with a change in the color of the blood
from bright scarlet to purple-red, or from arterial to venous; and
the opposite changes in the lungs restore to the dark blood its
bright tint. The whole set of processes through which blood be-
comes venous in the systemic circulation and arterial in the
pulmonary — in other words, the processes concerned in the gaseous
, reception, distribution, and slimination of the Body — constitute
the function of respiration; so much of this as is concerned in the
"interchanges between the blood and air being known as external
respiration; while the interchanges occurring between the tissues
and the systemic capillaries through the lymph, constitute internal
respiration, and the processes in general by which oxygen is fixed
and carbon dioxid formed by the living tissues, are known as
tissue respiration. When the term respiration is used alone,
without any limiting adjective, the external respiration only, is
commonly meant.

Respiratory Organs. The blood being kept poor in oxygen
and rich in carbon dioxid by the action of the living tissues, a
certain amount of gaseous interchange will nearly always take
place when it comes into close proximity to the surrounding
medium; whether this be the atmosphere itself or water contain-
ing air in solution. When an animal is small there are often no
special organs for its external respiration, its general surface being
sufficient (especially in aquatic animals with a moist skin) to
permit of all the gaseous exchange that is necessary. In the

348

RESPIRATION: THE MECHANISM OF BREATHING 349

simplest creatures, indeed, there is even no blood, the cell or cells
composing them taking up for themselves from their environ-
ment the oxygen which they need, and passing out into it their
carbon dioxid waste; in other words, there is no differentiation
of the external and internal respirations. When, however, an
animal is larger many of its cells are so far from a free surface
that they cannot transact this give-and-take with the surround-
ing medium directly, and the blood, or some liquid representing
it in this respect, serves as a middleman between the living tissues
and the external oxygen; and then one usually finds special
respiratory organs developed, to which the blood is brought to
make good its oxygen loss and get rid of its excess of carbon
dioxid. In aquatic animals such organs take commonly the form
of gills; these are protrusions of the body over which a constant
current of water, containing oxygen in solution, is kept up; and
in which blood capillaries form a close network immediately be-
neath the surface. In air-breathing animals a different arrange-
ment is usually found. In some, as frogs, it is true, the skin is
always moist and serves as an important respiratory organ, large
quantities of venous blood being sent to it for aeration. But for
the occurrence of the necessary gaseous diffusion, the skin must
be kept very moist, and this, in a terrestrial animal, necessitates
a great amount of secretion by the cutaneous glands to com-
pensate for evaporation; accordingly in most land animals the |
air is carried into the body through tubes with narrow external
orifices and so the drying up of the breathing surfaces is greatly
diminished; just as water in a bottle with a narrow neck will
evaporate much more slowly than the same amount exposed
in an open dish. In insects (as bees, butterflies, and beetles)
the air is carried by tubes which split up into extremely fine
branches and ramify all through the body, even down to the
individual tissue elements, which thus carry on their gase-
ous exchanges without the intervention of blood. But in the
great majority of air-breathing animals the arrangement is dif-
ferent; the air-tubes leading from the exterior of the body
do not subdivide into branches which ramify all through it,
but open into one or more large sacs to which the venous
blood is brought, and in whose walls it flows through a close
capillary network. Such respiratory sacs are called lungs, and it

350

THE HUMAN BODY

is a highly developed form of them which is employed in the

Human Body.

The Air-Passages and Lungs. In our own Bodies the es-
sential gaseous interchanges between
the Body and the atmosphere take
place in the lungs, two large sacs (lu,
Fig. 1) lying in the thoracic cavity, one
on each side of the heart. To these
sacs the air is conveyed through a series
of passages. Entering tne pharynx
through the nostrils or mouth, it passes
out of this by the opening leading into
the larynx, or voice-box (a, Fig. 113),
lying in the upper part of the neck (the
communication of the two is seen in
Fig. 122) ; from the larynx passes back
the trachea or windpipe, b, which, after
entering the chest cavity, divides into

FiV/llS.-The lungs and air- the ridht and lefl bronchi, d, 6. Each

passages seen from the front, bronchus divides up into smaller and

On the left of the figure the c

pulmonary tissue has been dis- smaller branches, called bronchial tubes,

sected away to show the rami- . ,, . ., , .. . ,

fications of the bronchial tubes, within the lung on its own side; and

a, larynx; b, trachea; d, right fu omnllpcf hrnnnViial tiihpq pnH in

bronchus. The left bronchus is tfte Smallest)

seen entering the root of its lung. saCCulated dilatations, the infuudibula of

the lungs, the sacculations (Fig. 115) being the alveoli. On the
walls of the alveoli the pulmonary capillaries ramify, and it is
in them that the interchanges of the
external respiration take place.

Structure of the Trachea and Bronchi.
The windpipe may readily be felt in
the middle line of the neck, a little be-
low Adam's apple, as a rigid cylindrical
mass. It consists fundamentally of a
fibrous tube in which cartilages are FJQ m_A gmall bronchial

embedded, SO as to keep it from COl- tube, a, dividing into its terminal
.... 11 i branches, c; these have pouched

lapsing; and is lined internally by a or saccuiated walls and end in
mucous membrane covered by several the sacculated infundibuia, b.
layers of epithelium cells, of which the superficial is ciliated.
The elastic cartilages embedded in its walls are imperfect rings,

RESPIRATION. THE MECHANISM OF BREATHING 351

each somewhat the shape of a horseshoe, and the deficient part
of each ring being turned backwards, it comes to pass that
the deeper or dorsal side of the windpipe has no hard parts in
it. Against this side the gullet lies, and the absence there of the
cartilages no doubt facilitates swallowing. The bronchi resem-
ble the windpipe in structure.

The Structure of the Lungs. These consist of the bronchial
tubes and their terminal dilatations; numerous blood-vessels,
nerves, arid lymphatics; and an abundance of connective tissue,
rich in elastic fibers, binding all together. The bronchial tubes
ramify in a tree-like manner (Fig. 113). In structure the larger
ones resemble the trachea, except that the cartilage rings are not
regularly arranged so as to have their open parts all turned one
way. As the tubes become smaller their constituents thin away;
the cartilages become less frequent and finally disappear; the
epithelium is reduced to a single layer of cells which, though
still ciliated, are much shorter than the columnar superficial cell-
layer of the larger tubes. The terminal alveoli (a, a, P"ig. 115)
have walls composed mainly of elastic
tissue and lined by a single layer of
flat, non-ciliated epithelium, immedi-
ately beneath which is a very close
network of capillary blood-vessels.
The air entering by the bronchial tube
is thus only separated from the blood
by the thin capillary wall and the thin
epithelium, both of which are moist,
and well adapted to permit gaseous
diffusion.

The Pleura. Each lung is covered,
except at one point, by an elastic se-
rous membrane which adheres tightly ^S^^ag^^SS^. k, b,
to it and is called the pleura; that |lollow protrusions of the aiveo-

r , . jus, opening into its central cav-

point at which the pleura is Wanting ity; c, terminal branches of a
11 i .1 <« ,i i i • bronchial tul i-.

is called the root of the lung and is on

its median side; it is there that its bronchus, blood-vessels and
nerves enter it. At the root of the lung the pleura turns back
and lines the inside of the chest cavity, as represented by the
dotted line in the diagram Fig. 3. The part of the pleura at-

352 THE HUMAN BODY

tached to each lung is its visceral, and that attached to the
chest-wall its parietal layer. Each pleura thus forms a closed
sac surrounding a pleural cavity, in which, during health, there
are found a few drops of lymph, keeping its surfaces moist. This
lessens friction between the two layers during the movements
of the chest-walls and the lungs; for although, to insure dis-
tinctness, the visceral and parietal layers of the pleura are rep-
resented in the diagram as not in contact, that is not the nat-
ural condition of things; the lungs are in life distended so that
the visceral pleura rubs against the parietal, and the pleural
cavity is practically obliterated. This is due to the pressure of
the atmosphere exerted through the air-passages on the interior
of the lungs. The lungs are extremely elastic and distensible, and
when the chest cavity is perforated each shrivels up just as an
india-rubber bladder does when its neck is opened; the reason
being that then the air presses on the outside of each with as
much force as it does on the inside. These two pressures neutral-
izing one another, there is nothing to overcome the tendency of
the lungs to collapse. So long as the chest-walls are whole, how-
ever, the lungs remain distended. The pleural sac is air-tight
and contains no air, and the pressure of the air around the Body
is borne by the rigid walls of the chest and prevented from reach-
ing the lungs; consequently no atmospheric pressure is exerted
on their outside. On their interior, however, the atmosphere
presses with its full weight, equal to about 90 centigrams on a
square centimeter (14.5 Ibs. on the square inch), and this is far
more than sufficient to distend the lungs so as to make them
completely fill all the parts of the thoracic cavity not occupied by
other organs. Suppose A (Fig. 116) to be a bottle closed air-
tight .by a cork through which two tubes pass, one of which, 6,
leads into an elastic bag, d, and the other, c, provided with a stop-
cock, opens freely below into the bottle. When the stop-cock, c,
is open the air will enter the bottle and press there on the outside
of the bag, as well as on its inside through b. The bag will there-
fore collapse, as the lungs do when the chest cavity is opened.
But if some air be sucked out through c the pressure of that re-
maining in the bottle will diminish, and of that inside the bag
will be unchanged, and the bag will thus be blown up; because
the atmospheric pressure on its interior will not be balanced by

RESPIRATION: THE MECHANISM OF BREATHING 353

that on its exterior. At last, when all the air is sucked out of the
bottle and the stop-cock on c closed, the bag, if sufficiently dis-
tensible, will be expanded so as to completely fill the bottle and
press against its inside, and the state of things
will then answer to that naturally found in the
chest. If the bottle were now increased in size
without letting air into it, the bag would ex-
pand still more, so as to fill it, and in so doing
would receive air from outside through 6; and
if the bottle then returned to its original size,

rio. llo. — Iiia-

its walls would press on the bag and cause it to gram iliustratinK the

v • i i , ... .., T, pressure relation-

Shrink and expel some of its air through 6. Ex- ships of the lungs in

actly the same must of course happen, under the thorax-
similar circumstances, in the chest, the windpipe answering to
the tube ,6 through which air enters or leaves this elastic sac.

The Respiratory Movements. The air taken into the lungs
soon becomes laden in them with carbon dioxid, and at the same
time loses much of its oxygen; these interchanges take place
mainly in the deep recesses of the alveoli, far from the exterior
and only communicating with it through a long tract of narrow
tubes. The alveolar air, thus become unfit any longer to convert
venous blood into arterial, could only very slowly be renewed by
gaseous diffusion with the atmosphere through the long air-
passages — not nearly fast enough for the requirements of the
Body, as one learns by the sensation of suffocation which follows
holding the breath for a short time with mouth and larynx open.
Consequently cooperating with the lungs is a respiratory mechan-
ism, by which the air within them is periodically mixed with fresh
air taken from the outside, and also the air in the alveoli is stirred
up so as to bring fresh layers of it in contact with the walls of the
air-cells. This mixing is brought about by the breathing move-
ments, consisting of regularly alternating inspirations, during
which the chest cavity is enlarged and fresh air enters the lungs,
and expirations, in which the cavity is diminished and air expelled
from the lungs. When the chest is enlarged the air the lungs
contain immediately distends them so as to fill the larger space;
in so doing it becomes rarefied and less dense than the external
air; and since gases flow from points of greater to those of less
pressure, some outside air at once flows in by the air-passages

354 THE HUMAN BODY

and enters the lungs. In expiration the reverse takes place. The
chest cavity, diminishing, presses on the lungs and makes the
air inside them denser than the external air, and so some passes

out until an equilibrium of pres-
sure is restored. The chest, in fact,
acts very much like a bellows.
When the bellows are opened air
FIG. 117.— Diagram to illustrate enters in consequence of the rare-

the entry of air to the lungs when the factjon Qf that m the mterior,
thoracic cavity enlarges.

which is expanding to fill the larger

space; and when the bellows are closed again it is expelled. To
make the bellows quite like the lungs we must, however, as in
Fig. 117, have only one opening in them, that of the nozzle, for
both the entry and exit of the air; and this opening should lead,
not directly into the bellows-cavity, but into an elastic bag ly-
ing in it, and tied to the inner end of the nozzle-pipe. This sac
would represent the lungs and the space between its outside and
the inside of the bellows, the pleural cavities.

We have next to see how the expansion and contraction of the
chest cavity are brought about.

The Structure of the Thorax. The thoracic cavity has a conical
form determined by the shape of its skeleton (Fig. 118), its nar-
rower end being turned upwards. Dorsally, ventrally, and on the
sides, it is supported by the rigid framework afforded by the
thoracic vertebrae, the breast-bone, and the ribs. Between and
over these lie muscles, and the whole is covered in, air-tight, by
the skin externally, and the parietal layers of the pleurae inside.
Above, its aperture is closed by muscles and by various organs
passing between the thorax and the neck; and below it is bounded
by the diaphragm, which forms a movable bottom to the, other-
wise, tolerably rigid box. In inspiration this box is increased in
all its diameters — dorsiventrally, laterally, and from above down.

The Vertical Enlargement of the Thorax. This is brought
about by the contraction of the diaphragm which (Figs. 1 and 119)
is a thin muscular sheet, writh a fibrous membrane, serving as a
tendon, in its center. In rest, the diaphragm is dome-shaped,
with its concavity towards the abdomen, being supported in that
position by the pressure of the underlying abdominal organs.
From the tendon on the crown of the dome striped muscular fibers

RESPIRATION: THE MECHANISM OF BREATHING 355

radiate, downwards and outwards, to all sides; and are fixed by
their inferior ends to the lower ribs, the breast-bone, and the ver-
tebral column. In expiration the lower lateral portions of the
diaphragm lie close against the chest-walls, no lung intervening
between them. In inspiration the muscular fibers, shortening,
flatten the dome and enlarge the thoracic cavity, room for the

i

Fio. 118. — The skeleton of the thorax, a, g, vertebral column; 6, first rib; c,
clavicle; e, seventh rib; i, glenoid fossa.

viscera thus displaced being secured by stretching the abdominal
walls; at the same time its lateral portions are pulled away from
the chest-walls, leaving a space into which the lower ends of the
lungs expand. The contraction of the diaphragm thus increases
greatly the size of the thorax chamber by adding to its lowest and
widest part.

The Dorsiventral Enlargement of the Thorax. The ribs on the
whole slope downwards from the vertebral column to the breast-
bone, the slope being most marked in the lower ones. During
inspiration the breast-bone and the sternal ends of the ribs at-
tached to it are raised, and so the distance between the sternum
and the vertebral column is increased. That this must be so will
readily be seen on considering the diagram Fig. 120, where ab
represents the vertebral column, c and d two ribs, and st the ster-

356

THE HUMAN BODY

num. The continuous lines represent the natural position of the
ribs at rest in expiration, and the dotted lines the position in
inspiration. It is clear that when their lower ends are raised, so

Ql

FIG. 119. — The diaphragm seen from below.

as to make the bars lie in a more horizontal plane, the sternum is
pushed away from the spine, and so the chest cavity is increased
dorsiventrally. The inspiratory elevation of the ribs is mainly
due to the action of the scalene and external intercostal muscles.
The scalene muscles, three on each side, arise
from the cervical vertebrae, and are inserted
into the upper ribs. The external intercos-
tals (Fig. 121, A) lie between the ribs and
extend from the vertebral column to the
costal cartilages; their fibers slope downwards
and forwards. During an inspiration the
scalenes contract and fix the upper ribs
firmly ; then the external intercostals shorten
, and each raises the rib below it. The muscle,

rio. 120. — Diagram il- _ .

lustrating the dorsi- in f act, tends to pull together the pair of ribs

ventral increase in the . i • t •* T i ji

diameter of the thorax between which it lies, but as the upper one

when the ribs are raised. of thege ig held tjght by the scalenes ancj

other muscles above, the result is that the lower rib is pulled up,
and not the upper down. In this way the lower ribs are raised
much more than the upper, for the whole external intercostal
muscles on each side may be regarded as one great muscle with
many bellies, each belly separated from the next by a tendon,

RESPIRATION: THE MECHANISM OF BREATHING 357

represented by the rib. When the whole muscular sheet is fixed
aboVe and contracts, it is clear that its lower end will be raised
more than any intermediate point, since there is a greater length
of contracting muscle above it. The elevation of the ribs tends

FIG. 121. — Portions of four ribs of a dog with the muscles between them, a, a,
ventral ends of the ribs, joining at c the rib cartilages, 6, which are fixed to carti-
laginous portions, d, of the sternum. A, external Intercostal muscle, ceasing be-
tween the rib cartilages, where the internal intercostal, B, is seen. Between the
middle two ribs the external intercostal muscle has been dissected away, so as to
display the internal which was covered by it.

to diminish the vertical diameter of the chest; this is more than
compensated for by the simultaneous descent of the diaphragm.
The Lateral Enlargement of the Chest is brought about by a
rotation of the middle ribs which, as they are raised, roll round a
little at their vertebral articulations and twist their cartilages.
Each rib is curved and, if the bones be examined in their natural
position in a skeleton, it will be seen that the most curved part
lies below the level of a straight line drawn from the vertebral to
the sternal attachment of the bone. By the rotation of the rib,
during inspiration, this curved part is raised and turned out, and
the chest widened. The mechanism can be understood by clasp-
ing the hands opposite the lower end of the sternum and a feu-
inches in front of it, with the elbows bent and pointing down-
wards. Each arm will then answer, in an exaggerated way, to a

358 THE HUMAN BODY

curved rib, and the clasped hands to the breast-bone. If the
hands be simply raised a few inches by movement at the shoulder-
joints only, they will be separated farther from the front of the
Body, and rib elevation and the consequent dorsiventral en-
largement of the cavity surrounded will be represented. But if,
simultaneously, the arms be rotated at the shoulder-joints so as
to raise the elbows and turn them out a little, it will be seen that
the space surrounded by the two arms is considerably increased
from side to side, as the chest cavity is in inspiration by the simi-
lar elevation of the most curved part or "angle" of the middle
ribs.

Expiration. To produce an inspiration requires considerable
muscular effort. The ribs and sternum have to be raised; the
elastic rib cartilages bent and somewhat twisted; the abdominal
viscera pushed down; and the abdominal wall pushed out to
make room for them. In expiration, on the contrary, no muscu-
lar effort is needed. As soon as the muscles which have raised
the ribs and sternum relax, these tend to return to their natural
unconstrained position, and the rib cartilages, also, to untwist
themselves and bring the ribs back to their position of rest; the
elastic abdominal wall presses the contained viscera against the
under side of the diaphragm, and pushes that up again as soon
as its muscular fibers cease contracting. By these means the
chest cavity is restored to its original capacity and the air sent
out of the lungs, by the elasticity of the parts which were stretched
or twisted in inspiration, and not by any special expiratory
muscles.

Forced Respiration. When a very deep breath is drawn or
expelled, or when there is some impediment to the entry or exit
of the air, a great many muscles take part in producing the respir-
atory movements; and expiration then becomes, in part, an ac-
tively muscular act. The main expiratory muscles are the internal
intercostals which lie beneath the external between each pair of
ribs (Fig. 121, B), and have an opposite direction, their fibers
running upwards and forwards. In forced expiration the lower
ribs are fixed or pulled down by muscles running in the abdominal
wall from the pelvis to them and to the breast-bone. The internal
intercostals, contracting, pull down the upper ribs and the ster-
num, and so diminish the thoracic cavity dorsiventrally. At

RESPIRATION: THE MECHANISM OF BREATHING 359

the same time, the contracted abdominal muscles press the walls
of that cavity against the viscera within it, and pushing these up
forcibly against the diaphragm make it very convex towards the
chest, and so diminish the latter in its vertical diameter. In very
violent expiration many other muscles may co-operate, tending
to fix points on which those muscles which can directly diminish
the thoracic cavity, pull. In violent inspiration, also, many extra
muscles are called into play. The neck is held rigid to give the
scalenes a firm attachment; the shoulder-joint is held fixed and
muscles going from it to the chest-wall, and commonly serving
to move the arm, are then used to elevate the ribs; the head is
held firm on the vertebral column by the muscles going between
the two, and then other muscles, which pass from the collar-bone
and sternum to the skull, are used to pull up the former. The
muscles which are thus called into play in labored but not in
quiet breathing are called extraordinary muscles of respiration.

The Respiratory Sounds. The entry and exit of air are accom-
panied by respiratory sounds or murmurs, which can be heard on
applying the ear to the chest-wall. The character of these sounds
is different and characteristic over the trachea, the larger bron-
chial tubes, and portions of lung from which large bronchial tubes
are absent. They are variously modified in pulmonary affections,
and hence the value of auscultation of the lungs in assisting the
physician to form a diagnosis.

The Capacity of the Lungs. Since the chest cavity never even
approximately collapses, the lungs are never completely emptied
of air: the space they have to occupy is larger in inspiration than
during expiration, but is always considerable, so that after a
forced expiration they still contain a large amount of air which
can only be expelled from them by opening the pleural cavities;
then they collapse almost completely, retaining within them only a
small quantity of air imprisoned within the alveoli by the collapse
of the small bronchi.

The capacity of the chest, and therefore of the lungs, varies
much in different individuals, but in a man of medium height
there remain in the lungs after the most violent possible expira-
tion, about 1,000 cub. cent, of air, called the residual air. Aft.-r
an ordinary expiration there will be in addition to this about
1,000 cub. cent, of supplemental air; the residual and supplemental

360 THE HUMAN BODY

together forming the stationary air, which remains in the chest
during quiet breathing. In an ordinary inspiration 500 cub. cent.
(30 cub. inches) of tidal air are taken in, and about the same
amount is expelled in natural expiration. By a forced inspira-
tion about 1,600 cub. cent. (98 cub. inches) of complemental air
can be added to the tidal air. After a forced inspiration, therefore,
the chest will contain 1,000+1,600+500+1,600=4,700 cub. cent.
(300 cub. inches) of air. The amount which can be taken in by
the most violent possible inspiration after the strongest possible
expiration, that is, the supplemental, tidal, and complemental
air together, is known as the vital capacity. For a healthy man
1.7 meters (5 feet 8 inches) high it is about 3,700 cub. cent. (225
cub. inches) and increases 60 cub. cent, for each additional centi-
meter of stature; or about 9 cub. inches for each inch of height.

The Quantity of Air Breathed Daily. Knowing the quantity
of air taken in at each breath and expelled again (after more or
less thorough admixture with the stationary air) we have only to
know, in addition, the rate at which the breathing movements
occur, to be able to calculate how much air passes through the
lungs in twenty-four hours. The average number of respira-
tions in a minute is found by counting on persons sitting quietly,
and not knowing that their breathing rate is under observation,
to be fifteen in a minute. In each respiration half a liter (30 cub.
inches) of air is concerned; therefore 0.5X15x60x24=10,800
liters (375 cub. feet) is the quantity of air breathed under ordi-
nary circumstances by each person in a day.

Hygienic Remarks. Since the diaphragm when it contracts
pushes down the abdominal viscera beneath it, these have to make
room for themselves by pushing out the soft front of the abdomen
which, accordingly, protudes when the diaphragm descends.
Hence breathing by the diaphragm, being indicated on the exte-
rior by movements of the abdomen, is often called "abdominal
respiration," as distinguished from breathing by the ribs, called
"costal" or "chest breathing." In both sexes the diaphragmatic
breathing is the most important, but, as a rule, men and children
use the ribs less than adult women. Since both abdomen and
chest alternately expand and contract in healthy breathing, any-
thing which impedes their free movement is to be avoided; and
the tight lacing which used to be thought elegant a few years

RESPIRATION: THE MECHANISM OF BREATHING 361

back, and is still indulged in by some who think a distorted form
beautiful, seriously impedes one of the most important functions
of the Body, leading, if nothing worse, to shortness of breath and
an incapacity for muscular exertion. In extreme cases of tight
lacing some organs are often directly injured, weals of fibrous
tissue being, for example, not unfrequently found developed on
the liver, from the pressure of the lower ribs forced against it by
a tight corset.

The Aspiration of the Thorax. As already pointed out, the
external air cannot press directly upon the contents of the thoracic
cavity, on account of the rigid framework which supports its
walls; it still, however, presses on them indirectly through the
lungs. Pushing on the interior of these with a pressure equal to
that exerted on the same area by a column of mecury 760 mm.
(30 inches) high, it distends them and forces them against the in-
side of the chest-walls, the heart, the great thoracic blood-vessels,
the thoracic duct, and the. other contents of the chest cavity.
The pressure against these organs is not equal to that of the ex-
ternal air, since some of the total air-pressure on the inside of the
lungs is used up in overcoming their elasticity, and it is only the
residue which pushes them against the things outside them. In
expiration this residue is equal to that exerted by a column of
mercury 754 mm. (29.8 inches) high. On most parts of the Body
the atmospheric pressure acts, however, with full force. Pressing
on a limb it pushes the skin against the soft parts beneath, and
these compress the blood- and lymph-vessels among them ; and the
yielding abdominal walls do not, like the rigid thoracic walls,
carry the atmospheric pressure themselves, but transmit it to the
contents of the cavity. It thus comes to pass that the blood and
lymph in most parts of the Body are under a higher atmospheric
pressure than they are exposed to in the chest, and consequently
these liquids tend to flow into the thorax, until the extra disten-
tion of the vessels in which they there accumulate compensates
for the less external pressure to which those vessels are exposed.
An equilibrium would thus very soon be brought about were it
not for the respiratoiy movements, in consequence of which the
intrathoracic pressure is alternately increased and diminished,
and the thorax comes to act as a sort of suction-pump on the
contents of the vessels of the Body outside it; thus the respira-

362 THE HUMAN BODY

tory movements influence the circulation of the blood and the
flow of the lymph.

Influence of the Respiratory Movements upon the Circulation.

Suppose the chest in a condition of normal expiration and the
external pressure on the blood in the blood-vessels within it and
in the heart, to have come, in the manner pointed out in the last
paragraph, into equilibrium with the atmospheric pressure exerted
on the blood-vessels of the neck and abdomen. If an inspiration
now occurs, the chest cavity being enlarged the pressure on all of
its contents will be diminished. In consequence, air enters the
lungs from the windpipe, and blood enters the venae cavse and the
right auricle of the heart from the outlying veins. When the
next expiration occurs, and the pressure in the thorax again rises,
air and blood both tend to be expelled from the cavity. What-
ever extra blood has, to use the common phrase, been " sucked "
into the intrathoracic venae cavae in inspiration and has not been
sent already on into the right ventricle before expiration occurs,
is, however, on account of the venous valves, prevented from
flowing back whence it came, and is imprisoned in the cavae under
an increased pressure during expiration; and this tends to make
it flow faster into the auricle during the diastole of the latter. How
much the alternating respiratory movements assist the venous
flow is shown by the dilatation of the veins of the head and neck
which occurs when a person is holding his breath; and the black-
ness of the face, from distention of the veins and stagnation of
the capillary flow, which occurs during a prolonged fit of cough-
ing, which is a series of expiratory efforts without any inspira-
tions.

The ventricles and arteries are not directly affected to any
appreciable extent by the respiratory movements; their walls
are too thick and the arterial pressure too great to respond to
these small variations of intrathoracic pressure. The increase
in venous flow which occurs during inspiration does, however,
by supplying the heart with more blood at that time, bring about
a small increase in arterial pressure during each inspiration. The
increased blood-supply is handled by the heart through a reflex
augmentation of its beat. This arises from the stimulation of the
nerves of muscle sense in the muscles of inspiration during their
contraction.

RESPIRATION: THE MECHANISM OF BREATHING 363

Influence of the Respiration on the Lymph-Flow. During
inspiration, when intrathoracic pressure is lowered, lymph is
pressed into the thoracic duct from the abdominal lymphatics.
In expiration, when thoracic pressure rises again, the extra lymph
cannot flow back on account of the valves in the lymphatic ves-
sels, and it is consequently driven on to the cervical ending of the
thoracic duct. The breathing movements thus pump the lymph
on.

The Respiratory Center. The respiratory movements are to a
certain extent under the control of the will ; we can breathe faster
or slower, shallower or more deeply, as we wish, and can also " hold
the breath " for some time — but the voluntary control thus exerted
is limited in extent; no one can commit suicide by holding his
breath. In ordinary quiet breathing the movements are quite in-
voluntary; they go on perfectly without the least attention on our
part, and, not only in sleep, but during the unconsciousness of
fainting or of an apoplectic fit. The natural breathing movements
arc therefore either reflex or automatic.

The muscles concerned in producing the changes in the chest
which lead to the entry or exit of air are of the ordinary striped
kind; and these, as we have seen, only contract in the Body under
the influence of the nerves going to them; the nerves of the dia-
phragm are the two phrenic nerves, one for each side of it ; the ex-
ternal intercostal muscles are supplied by certain branches of the
thoracic spinal nerves, called the intercostal nerves. If the phrenic
nerves be cut the diaphragm ceases its contractions, and a similar
paralysis of the external intercostals follows section of the inter-
costal nerves.

Since the inspiratory muscles only act when stimulated by
nervous impulses reaching them, we have next to seek where these
impulses originate; and experiment shows that it is in the medulla
oblongata. All the brain of a cat or a rabbit in front of the medulla
can be removed, and it will still go on breathing; and children are
sometimes born with the medulla oblongata only, the rest of the
brain being undeveloped, and yet they breathe for a time. If, on
the other hand, the spinal cord be divided immediately below the
medulla of an animal, all breathing movements of the chest cease
at once. We conclude, therefore, that the nervous impulses calling
forth contractions of the respiratory muscles arise in the medulla

364 THE HUMAN BODY

oblongata, and travel down the spinal cord and thence out along
the phrenic and intercostal nerves. * This is confirmed by the fact
that if the spinal cord be cut across below the origin of the fourth
pair of cervical spinal nerves (from which the phrenics mainly
arise) but above the first thoracic spinal nerves, the respiratory
movements of the diaphragm continue, but those of the intercostal
muscles cease; this phenomenon has sometimes been observed on
men so stabbed in the back as to divide the spinal cord in the
region indicated. Finally, that the nervous impulses exciting the
inspiratory muscles originate in the medulla, is proved by the fact
that if a small portion of that organ, the so-called vital point, be
destroyed, all the respiratory movements cease at once and for-
ever, although all the rest of the brain and spinal cord may be left
uninjured. This part of the medulla is known as the respiratory
center.

Is the Respiratory Center Reflex? Since this center goes on
working independently of the will, we have next to inquire, Is it a
reflex center or not? Are the efferent discharges it sends along the
respiratory nerves due to afferent impulses reaching it by centrip-
etal nerve-fibers? Or does it originate efferent nervous impulses
independently of excitation through afferent nerves?

We know, in the first place, that the respiratory center is largely
under reflex control ; a dash of cold water on the skin, the irritation
of the nasal mucous membrane by snuff, or of the larynx by a
foreign body, will each cause a modification in the respiratory
movements — a long indrawn breath, a sneeze, or a cough. But,
although thus very subject to influences reaching it by afferent
nerves, the respiratory center seems essentially independent of
such. In many animals, as rabbits (and in some men), marked
breathing movements take place in the nostrils, which dilate during
inspiration ; and when the spinal cord of a rabbit is cut close to the
medulla, thus cutting off all afferent nervous impulses to the re-
spiratory center except such as may reach it through cranial
nerves, the respiratory movements of the nostrils still continue
until death. The movements of the ribs and diaphragm of course
cease, and so the animal dies very soon unless artificial respiration
be maintained. Moreover, if after cutting the spinal cord as above
described, the chief sensory cranial nerves be divided, so as to cut
off the respiratory center from almost all possible afferent nervous

RESPIRATION: THE MECHANISM OF BREATHING 3G5

impulses, regular breathing movements of the nostrils continue.
It is, therefore, nearly certain that the activity of the respiratory
center, however much it may be capable of modification through
sensory nerves, is essentially independent of them.

What it is that Excites the Respiratory Center. It has long been
recognized that the activity of the respiratory center is related to
the condition of the blood flowing through it; arterial blootl ex-
cites it feebly or not at all; venous blood excites it powerfully, and
more and more strongly as its venosity increases. The difference
between arterial and venous blood is wholly a difference in the
relative amounts of oxygen and of carbon dioxid present therein.
The question is : Does venous blood owe its ability to stimulate the
respiratory center to its low oxygen content or to its high content
of carbon dioxid? Experiment has shown that the carbon dioxid
of venous blood is the source of its stimulating power. We may
look upon carbon dioxid, then, as a hormone whose function is the
chemical stimulation of the respiratory center.

Why are the Respiratory Discharges Rhythmic? If carbon
dioxid is the stimulus for the respiratory center, why does that
center act rhythmically? Does the carbon dioxid content of the
circulating blood increase and decrease fifteen times or more a
minute? The answer to this question is afforded by a simple ex-
periment. If in an animal breathing naturally under anesthesia
both vagus nerves are cut there is an immediate change in the
character of the respirations. From being rapid and shallow they
become very deep and take on a much slower rate. Under this
condition we may properly assume that the respirations do follow
the carbon dioxid content of the blood; the center begins to dis-
charge when the blood contains enough carbon dioxid to stimulate
it, and continues its discharge until the aeration of the blood, re-
sulting from the inspiration, lowers the carbon dioxid below the
point of stimulation. There follows a period of expiration and
rest which continues until sufficient carbon dioxid has again ac-
cumulated to start the action anew.

Since with the vagus nerves cut the respirations follow the car-
bon dioxid concentration of the blood, but with the nerves intact
do not, being much more shallow and rapid, we must determine
the influence of the vagus nerves upon the center in order to un-
derstand ordinary breathing. It has been shown that the influence

366 THE HUMAN BODY

of the vagus nerves is a simple reflex one. These nerves contain
sensory fibers arising in the lung tissue and so situated as to be
stimulated mechanically every time the lung is inflated. The im-
pulses conveyed over these fibers to the central nervous system
are inhibitory to the respiratory center. Bearing this action of the
vagus fibers in mind we may account for normal breathing thus;
the blood contains enough carbon dioxid all the time, under ordi-
nary circumstances, to stimulate the respiratory center; when-
ever the center discharges under this stimulus it brings about
the movements of inspiration which result in expansion of the
lungs; whenever the lungs expand the sensory fibers contained in
their walls are stimulated and so inhibitory influences are sent to
the respiratory center. Inspiration proceeds, then, until the in-
hibitory impulses from the lungs overcome the stimulus of carbon
dioxid, when it comes to an end and the thorax falls back to the
position of rest. This falling back, which constitutes normal ex-
piration, collapses the lungs somewhat; the inhibitory impulses
diminish or disappear; and the stimulating action of the carbon
dioxid again becomes effective. Thus in normal breathing in-
spiration and expiration follow one another without any pause
between, and the respirations are shallow because the inhibition
cuts them off almost as soon as started.

Eupnea, Dyspnosa, Apnrea. Ordinary quiet breathing is known
as eupnea. When the breathing is forced, and especially when
forced expiration enters, we have the condition called dyspnoea.
This results from abnormal excitement of the respiratory center
either reflexly, as from stimulation of pain nerves, or by an increase
in the carbon dioxid content of the blood. The dyspnoea of the
early stages of suffocation arises from this latter cause. Apnoea, or
absence of breathing, may result from one of two conditions or
from both acting together. The first of these is a deficiency of
carbon dioxid in the blood, so that the respiratory center is not
stimulated. The second is inhibition of the center through vigor-
ous and repeated inflation of the lungs. Since inflation of the lungs
with ordinary air brings about both conditions the apnoea which
results from this treatment is partly chemical and partly inhibi-
tory. That inhibition enters in the production of apncea in this
way is shown by the greater difficulty of producing the condition
in animals with both vagi cut.

RESPIRATION: THE MECHANISM OF BREATHING 367

Asphyxia. Asphyxia is death from suffocation, or want of
oxygen by the tissues. It may be brought about in various ways;
as by strangulation, which prevents the entry of air into the lungs;
or by exposure in an atmosphere containing no oxygen; or by
putting an animal in a vacuum; or by making it breathe air con-
taining a gas which has a stronger affinity for hemoglobin than
oxygen has, and which, therefore, turns the oxygen out of the red
corpuscles and takes its place. The gases which do the latter are
very interesting since they serve to prove conclusively that the
Body can only live by the oxygen carried around by the hemo-
globin of the red corpuscles; the amount dissolved in the blood-
plasma being insufficient for its needs. Of such gases carbon
monoxid is the most important and best studied ; in the frequent
mode of committing suicide by stopping up all the ventilation
holes of. a room and turning on the gas, it is poisoning by carbon
monoxid which causes death.

The Phenomena of Asphyxia. As soon as the oxygen in the
blood falls below the normal amount the breathing becomes
hurried and deeper, and the extraordinary muscles of respiration
are called into activity. The dyspnoea becomes more and more
marked, and this is especially -the case with the expirations which,
almost or quite passively performed in natural breathing, become
violently muscular. At last nearly all the muscles in the Body are
set at work; the rhythmic character of the respiratory acts is lost,
and general convulsions occur, but, on the whole,- the contractions
of the expiratory muscles are more violent than those of the in-
spiratory.

The violent excitation of the nerve-centers soon exhausts them,
and all the more readily since their oxygen supply (which they like
all other tissues need in order to continue their activity) is cut off.
The convulsions therefore gradually cease, and the animal be-
comes calm again, save for an occasional act of breathing: these
final movements are inspirations and, becoming less and less fre-
quent, at last cease, and the animal appears dead. Its heart, how-
ever, though gorged with extremely dark venous blood still makes
some slow feeble pulsations. So long as it beats artificial respira-
tion can restore the animal, but once the heart has finally stopped
restoration is impossible. There are thus three distinguishable
stages in death from asphyxia. (1) The stage of dyspnoea. (2)

368 THE HUMAN BODY

The stage of convulsions. (3) The stage of exhaustion; the con-
vulsions having ceased but there being from time to time an in-
spiration. The end of the third stage occurs in a mammal about
five minutes after the oxygen supply has been totally cut off. If
the asphyxia be due to deficiency, and not absolute want of oxy-
gen, of course all the stages take longer.

Artificial Respiration. Asphyxia from drowning and other
causes occurs with lamentable frequency these days, and there is
no doubt that many lives are sacrificed through ignorance on the
part of bystanders of the proper restorative procedures. There
are several methods of applying artificial respiration to human
beings. The method of Schaefer is as effective as any. The follow-
ing description is quoted from his paper on the subject: "The
method consists in laying the subject in the prone posture, prefer-
ably on tke ground, with a thick folded garment underneath the
chest and epigastrium. The operator puts himself athwart or at
the side of the subject, facing his head and places his hands on
each side over the lower part of the back (lowest ribs). He then
slowly throws the weight of his Body forward to bear upon his own
arms, and thus presses upon the thorax of the subject and forces
air out of the lungs. This being effected, he gradually relaxes the
pressure by bringing his own Body up again to a more erect posi-
tion, but without moving the hands." These movements should
be repeated about fifteen times a minute until normal breathing is
resumed, and should not be given up for at least a half hour if re-
covery does not occur sooner.

Modified Respiratory Movements. Sighing is a deep long-drawn
inspiration followed by a shorter but correspondingly large ex-
piration. Yawning is similar, but the air is mainly taken in by the
mouth instead of the nose, and the lower jaw is drawn down in a
characteristic manner. Hiccough depends upon a sudden contrac-
tion of the diaphragm, while the aperture of the larynx closes; the
entering air, drawn through the narrowing opening, causes the
peculiar sound. Coughing consists of a full inspiration followed by
a violent and rapid expiration, during the first part of which the
laryngeal opening is kept closed; being afterwards suddenly
opened, the air issues forth with a rush, tending to carry out with
it anything lodged in the windpipe or larynx. Sneezing. is much
like coughing, except that, while in a cough the isthmus of the

RESPIRATION: THE MECHANISM OF BREATHING 369

fauces is held open and the air mainly passes out through the
mouth, in sneezing the fauces are closed and the blast is driven
through the nostrils. It is commonly excited by irritation of the
nasal mucous membrane, but in many persons a sudden bright
light falling into the eye will produce a sneeze. Laughing consists
of a series of short expirations following a single inspiration; the
larynx is open all the time, and the vocal cords (Chap. XXXIII)
are set in vibration. Crying is, physiologically, much like laughing
and, as we all know, one often passes into the other. The accom-
panying contractions of the face muscles giving expression to the
countenance are, however, different in the two.

All these modified respiratory acts are essentially reflex and
they serve to show to what a great extent the discharges of the
respiratory center can be modified by afferent nerve impulses; but,
with the exception of hiccough, they are to a certain extent, like
natural breathing, under the control of the will. Most of them,
too, can be imitated more or less perfectly by voluntary muscular
movements ; though a good stage sneeze or cough is rare.

CHAPTER XXIV
RESPIRATION. THE GASEOUS INTERCHANGES

Nature of the Problems. The study of the respiratory process
from a chemical standpoint has for its object to discover what are,
in kind and extent, the interchanges between the air in the lungs
and the blood in the pulmonary capillaries; and the nature and
amount of the corresponding gaseous changes between the living
tissues, and the blood in the systemic capillaries. Neglecting some
oxygen used up otherwise than in forming carbon dioxid, and some
carbon dioxid eliminated by other organs than the lungs, these
processes in the long run balance, the blood losing as much carbon
dioxid gas in the lungs as it gains elsewhere, and gaining as
much oxygen in the lungs as it loses in the systemic capillaries.
To comprehend the matter it is necessary to know the physical
and chemical conditions of these gases in the lungs, in the blood,
and in the tissues generally; for only so can we understand how
it is that in different localities of the Body such exactly contrary
processes occur. So far as the problems connected with the exter-
nal respiration are concerned our knowledge is tolerably complete;
but as regards the internal respiration, taking place all through
the Body, much has yet to be learnt; we know that a muscle at
work gives more carbon dioxid to the blood than one at rest and
takes more oxygen from it, but how much of the one it gives and
of the other it takes is only known approximately ; as are the con-
ditions under which this greater interchange during the activity
of the muscular tissue is effected: and concerning nearly all the
other tissues we know even less than about muscle. In fact, as
regards the Body as a whole, it is comparatively easy to find how
great its gaseous interchanges with the air are during work and
rest, waking and sleeping, while fasting or digesting, and so on;
but when it comes to be decided what organs are concerned in
each case in producing the greater or less exchange, and how
much of the whole is due to each of them, the question is one far
more difficult to settle and still very far from completely answered.

370

RESPIRATION: THE GASEOUS INTERCHANGES 371

The Changes Produced in Air by Being Once Breathed. These
are fourfold — changes in its temperature, in its moisture, in its
chemical composition, and its volume.

The air taken into the lungs is nearly always cooler than that
expired, which has a temperature of about 36° C. (97° F.). The
temperature of a room is usually less than 21° C. (70° F.). The
warmer the inspired air the less, of course, the heat which is lost
to the Body in the breathing process ; its average amount is calcu-
lated as about equal to 50 Calories in twenty-four hours; a Calory
being as much heat as will raise the temperature of one kilogram
(2.2 Ibs.) of water one degree centigrade (1.8° F.).

The inspired air always contains more or less water vapor, but
is rarely saturated; that is, rarely contains so much but it can
take up more without showing it as mist; the warmer air is, the
more water vapor it requires to saturate it. The expired air is
nearly saturated for the temperature at which it leaves the Body,
as is already shown by the water deposited when it is slightly
cooled, as when a mirror is breathed upon; or by the clouds seen
issuing from the nostrils on a frosty day, these being due to the
fact that the air, as soon as it is cooled, cannot hold all the water
vapor which it took up when warmed in the Body. Air, therefore,
when breathed once, gains water vapor and carries it off from the
lungs; the actual amount being subject to variation with the
temperature and saturation of the inspired air: the cooler and drier
that is, the more water will it gain when breathed. On an aver-
age the amount thus carried off in twenty-four hours is about 255
grams (9 ounces). To evaporate this water in the lungs an amount
of heat is required, which disappears for this purpose in the Body,
to reappear again outside it when the water vapor condenses.
The amount of heat taken off in this way during the day is about
148 Calories. The total daily loss of heat from the Body through
the lungs is therefore 198 Calories, 50 in wanning the inspired air
and 148 in the evaporation of water.

The most important changes brought about in the breathed
air are those in its chemical composition. Pure air when com-
pletely dried consists in each 100 parts of:

By Volume. By Weight.

Oxygen 21

Nitrogen 79 77

372 THE HUMAN BODY

Ordinary atmospheric air contains in addition 4 volumes of
carbon dioxid in 10,000, or 0.04 in 100, a quantity which, for
practical purposes, may be neglected. When breathed once,
such air gains rather more than 4 volumes in 100 of carbon di-
oxid, and loses a little less than 5 of oxygen. More accurately,
100 volumes of expired air after drying contain :

Oxygen 16.

Nitrogen 79.

Carbon dioxid 4.4

Since 10,800 liters (375 cubic feet) of air are breathed in twenty-
four hours and lose 5 per cent of oxygen, the total quantity of
this gas taken up in the lungs daily is 10,800X5-i-100=540 liters.
One liter of oxygen measured at 0° C. (32° F.) and under a pres-
sure equal to one atmosphere, weighs 1.43 grams, so the total
weight of oxygen taken up by the lungs daily is 540X1.43=772
grams (27 ounces).

The amount of carbon dioxid excreted from the lungs being
4.4 per cent of the volume of the air breathed daily, is 10,800 X
4.4^100 = 475 liters measured at the normal temperature and
pressure. This volume weighs 930 grams, or 32.5 ounces. If all
the oxygen taken in were breathed out again as carbon dioxid
the volume of the latter should equal that of the oxygen breathed
in. The discrepancy results from the fact that not all the oxygen
combines with carbon; some of it unites with hydrogen to form
water.

If the expired air be measured as it leaves the Body its bulk
will be found greater than that of the inspired air, since it not
only has water vapor added to it, but is expanded in consequence
of its higher temperature. If, however, it be dried and reduced
to the same temperature as the inspired air its volume will be
found diminished, since it has lost 5 volumes per cent of oxygen
and gained only 4.4 of carbon dioxid. In round numbers, 100
volumes of dry inspired air at zero, give 99 volumes of dry expired
air measured at the same temperature and pressure.

Ventilation. Since at every breath some oxygen is taken from
the air and some carbon dioxid given to it, were the atmosphere
around a living man not renewed he would, at last, be unable to
get from the air the oxygen he required ; he would die of oxygen

RESPIRATION : THE GASEOUS INTERCHANGES 373

starvation or be suffocated, as such a mode of death is called, as
surely, though not quite so fast, as if he were put under the re-
ceiver of an air-pump and all the air around him removed. Hence
the necessity of ventilation to supply fresh air in place of that
breathed, and clearly the amount of fresh air requisite must be
determined by the number of persons collected in a room; the
supply which would be ample for one person would be insufficient
for two. Moreover, fires, gas, and oil lamps, all use up the oxygen
of the air and give carbon dioxid to it, and hence calculation
must be made for them in arranging for the ventilation of a
building in which they are to be employed.

In order that air be unwholesome to breathe, it is by no means
necessary that it have lost so much of its oxygen as to make it
difficult for the Body to get what it wants of that gas. The evil
results. of insufficient air-supply are rarely, if ever, due to that
cause even in the worst-ventilated room for, as we shall see here-
after, the blood is able to take what oxygen it wants from air
containing comparatively little of that gas. The headache and
drowsiness which come on from sitting in a badly ventilated room
appear to be due chiefly to the high percentage of water-vapor
present under such circumstances, and the want of energy and
general ill-health which result from permanently living in such
surroundings are probably the result of a slow poisoning of the
body by absorption of gaseous substances given off to the air, not
from the lungs, but from the skin in evaporating sweat and from
the alimentary tract. The idea, formerly held very generally,
that volatile poisons are given off by the lungs in quantities too
small for chemical detection, has been largely abandoned partly
because of the failure of the most careful experiments to demon-
strate any such substances, but more because there are enough
injurious materials given off from other channels of the body to
explain all the ill effects of insufficient ventilation.

That the air of rooms occupied by persons becomes injurious
long before the amount of carbon dioxid in it is sufficient to do
any harm has been abundantly demonstrated. Breathing air
containing one or two per cent of that gas produced by ordinary
chemical methods does no particular injury, but air containing
one per cent of it produced by respiration is decidedly injurious,
because of the other things present in it at the same time. Carbon

374 THE HUMAN BODY

dioxid itself, at least in any such percentage as is commonly
found in a room, is not poisonous, as used to be believed, but,
since it is tolerably easily estimated in air, while the actually in-
jurious substances also present are not, the purity or foulness of
the air in a room is usually determined by finding the percentage
of carbon dioxid in it: it must be borne in mind that to mean
much this carbon dioxid must have been produced by breathing;
the amount of it found is in itself no guide to the quantity of
really important injurious substances present. Of course when a
great deal of carbon dioxid is present the air is irrespirable : as
for example sometimes at the bottom of we^ls or brewing-vats.

In one minute .5X15 = 7.5 liters (0.254 cubic feet) of air are
breathed and this is vitiated with carbon dioxid to the extent of
rather more than four per cent; mixed with three times its volume
of external air, it would give thirty liters (a little over one cubic
foot) vitiated to the extent of one per cent, and such air is not
respirable for any length of time with safety. The result of breath-
ing it for an evening is headache and general malaise; of breath-
ing it weeks or months a lowered tone of the whole Body — less
power of work, physical or mental, and less power of resisting
disease; the ill effects may not show themselves at once, and may
accordingly be overlooked, or considered scientific fancies, by
the careless; but they are nevertheless there ready to manifest
themselves. In order to have air to breathe in an even moder-
ately pure state every man should get for his own allowance at
least 23,000 liters of space to begin with (about 800 cubic feet)
and the arrangements for ventilation should, at the very least,
renew this at the rate of 30 liters (one cubic foot) per minute. In
the more recently constructed hospitals, as a result of experience,
twice the above minimum cubic space is allowed for each bed in a
ward, and the replacement of the old air at a far more rapid rate,
100,000 liters per hour per person, is also provided for.

Ventilation does not necessarily imply draughts of cold air, as
is often supposed. In warming by indirect radiation (the ordi-
nary hot-air furnace) it may readily be secured by arranging, in
addition to the registers from which the warmed air reaches the
room, proper openings at the opposite side, by which the old air
may pass off to make room for the fresh. An open fire in a room
will always keep up a current of air through it, and is the healthiest,

RESPIRATION: THE GASEOUS INTERCHANGE 375

though not the most economical, method of warming an apart-
ment.

Stoves in a room, unless constantly supplied with fresh air
from without, vitiate its air to an unwholesome extent. If no
appliance for providing this supply exists in a room, it can usually
be got, without a draught, by fixing a board about four inches
wide under the lower sash and shutting the window down on it.
Fresh air then comes in by the opening between the two sashes
and in a current directed upwards, which gradually diffuses itself
over the room without being felt as a draught at any one point.
In the method of heating by direct radiation (steam or hot water),
the apparatus employed provides of itself no means of drawing
fresh air into a room, as the draught up the chimney of an open
fireplace or of a stove does; and therefore special inlet and outlet
opening!? are very necessary.

In severe weather, when there is a tendency to keep rooms
rather tightly closed, a good plan is to open widely all doors and
windows for a few minutes each day, allowing fresh air to penetrate
to every corner, sweeping out the old air before it. This daily
renewing, helped out by such ventilation as is afforded by ill-
fitting doors and windows, usually keeps the air of rooms in
respirable condition when not occupied by too many persons.
The modern habit of sleeping summer and winter in rooms with
open windows is to be highly commended, and should be even
more generally adopted. In fact the more outdoor air one can
have, and at the same time keep warm, the better for the bodily
well-being. The beneficial effects of fresh air and sunshine,
especially in pulmonary tuberculosis, cannot be too strongly
emphasized.

Changes undergone by the Blood in the Lungs. These are the
exact reverse of those undergone by the breathed air— what the
air gains the blood loses, and vice versa. Consequently, the blood
loses heat, and water, and carbon dioxid in the pulmonary capil-
laries; and gains oxygen. These gains and losses are accompanied
by a change of color from the dark purple which the blood ex-
hibits in the pulmonary artery, to the bright scarlet it possesses in
the pulmonary veins.

The dependence of this color change upon the access of fresh
air to the lungs while the blood is flowing through them, can be

376 THE HUMAN BODY

readily demonstrated. If a rabbit be rendered unconscious by
chloroform, and its chest be opened, after a pair of bellows has
been connected with its windpipe, it is seen that, so long as the
bellows are worked to keep up artificial respiration, the blood in the
right side of the heart (as seen through the thin auricle) and that in
the pulmonary artery, is dark colored, while that in the pulmonary
veins and the left auricle is bright red. Let, however, the artificial
respiration be stopped for a few seconds and, consequently, the
renewal of the air in the lungs (since an animal cannot breathe for
itself when its chest is opened) , and very soon the blood returns to
the left auricle as dark as it left the right. In a very short time
symptoms of suffocation show themselves and the animal dies, un-
less the bellows be again set at work.

The Blood Gases. If fresh blood be rapidly exposed to as com-
plete a vacuum as can be obtained, it gives off certain gases, known
as the gases of the blood. These are the same in kind, but differ in
proportion, in venous and arterial blood ; there being more carbon
dioxid and less oxygen obtainable from the venous blood going to
the lungs by the pulmonary artery, than from the arterial blood
coming back to the heart by the pulmonary veins. The gases given
off by venous and arterial blood, measured under the normal pres-
sure and at the normal temperature, amount to from 58 to 60
volumes for every 100 volumes of blood, and in the two cases are
about as follows:

Venous Blood. Arterial Blood.

Oxygen 12 20

Carbon dioxid 45 38

Nitrogen 1.7 1.7

It is important to bear in mind that while arterial blood contains
some carbon dioxid that can be removed by the air-pump, venous
blood also contains some oxygen removable in the same way; so
that the difference between the two is only one of degree. When
an animal is killed by suffocation, however, the last trace of oxygen
which can be yielded up in a vacuum disappears from the blood
before the heart ceases to beat. All the blood of such an animal
is what might be called suffocation blood, and has a far darker
color than ordinary venous blood.
The Cause of the Bright Color of Arterial Blood. The color of

RESPIRATION: THE GASEOUS INTERCHANGES 377

the blood depends on its red corpuscles, since pure blood-plasma
or blood-serum is colorless, or at most a very faint straw yellow.
Hence the color change which the blood experiences in circulating
through the lungs must be due to some change in its red corpuscles.
We have already seen (Chap. XVII) that the functional sub-
stance of the red corpuscles is hemoglobin, which has the prop-
erty of combining with oxygen. Hemoglobin itself is of a dark
purplish color, when combined with oxygen the resulting com-
pound is a bright scarlet. Hemoglobin combined with oxygen is
known as oxyhemoglobin, and it is on its predominance that the
color of arterial blood depends. Hemoglobin uncombined with
oxygen, sometimes named reduced hemoglobin, predominates in
venous blood, and is the only kind found in the blood of a suffo-
cated mammal.

The Laws Governing the Absorption of Gases by a Liquid. In
order to understand the condition of the gases in the blood liquid
it is necessary to recall the general laws in accordance with which
liquids absorb gases. They are as follows:

1. A given volume of a liquid at a definite temperature if it
absorbs any of a gas to which it is exposed, and yet does not com-
bine chemically with it, takes up an amount of the gas which de-
pends upon two things: (1) the solubility of the gas in the liquid;
and (2) the pressure of the gas upon the surface of the liquid. As
the pressure of the gas is increased the amount of it which goes in
solution in the liquid is increased in exactly the same proportion.
If a complete vacuum be formed above a liquid all the gas con-
tained within it is given off. This law, that the quantity of a gas
dissolved by a liquid varies directly as the pressure of that gas on
the surface of -the liquid is known as Henry's law.

2. The amount of a gas dissolved by a liquid depends, not on the
total pressure exerted by all the gases pressing on its surface, but
on the fraction of the total pressure which is exerted by the par-
ticular gas in question. For example, the average atmospheric
pressure is equal to that of a column of mercury 760 mm. (30
inches) high. But 100 volumes of air contain approximately 80
volumes of nitrogen and 20 of oxygen; therefore | of the total pres-
sure is due to oxygen and £ to nitrogen.: and the amount of oxygen
absorbed by water is just the same as if all the nitrogen were re-
moved from the air and its total pressure therefore reduced to £ of

378 THE HUMAN BODY

760 mm. (30 inches) of mercury; that is, to 152 mm. (6 inches) of
mercury pressure. It is only the fraction of the total pressure
exerted by the oxygen itself which affects the quantity of oxygen
absorbed by water at any given temperature. So, too, of all the at-
mospheric pressure -f- is due to nitrogen, and all the oxygen might
be removed from the air without affecting the quantity of nitrogen
which would be absorbed from it by a given volume of water. The
atmospheric pressure would then be £ of 760 mm. of mercury, or
608 mm. (24 inches), but it would all be due to nitrogen gas — and
be exactly equal to the fraction of the total pressure due to that
gas before the oxygen was removed from the air. When several
gases are mixed together the fraction of the total pressure exerted
by each one is known as the partial pressure of that gas; and it is
this partial pressure which determines the amount of each in-
dividual gas dissolved by a liquid. If a liquid exposed to the air
for some time had taken up all the oxygen and nitrogen it could at
the partial pressures of those gases in the air, and were then put in
an atmosphere in which the oxygen had all been replaced by nitro-
gen, it would now give off all its oxygen, since, although the total
gaseous pressure on it was the same, no part of it was any longer
due to oxygen; and at the same time it would take up one-fifth
more nitrogen, since the whole gaseous pressure on its surface was
now due to that gas, while before only four-fifths of the total was
exerted by it. If, on the contrary, the liquid were exposed to pure
hydrogen under a pressure of one atmosphere it would give off all
its previously dissolved oxygen and nitrogen, since none of the
pressure on its surface would now be due to those gases ; and would
take up as much hydrogen as corresponded to a pressure of that
gas equal to 760 mm. of mercury (30 inches).

3. The amount of gas taken up by a liquid varies, other things
being equal, inversely as the temperature.

4. A liquid may be such as to combine chemically with a gas.
Then the amount of the gas absorbed is independent of the partial
pressure of the gas on the surface of the liquid. The quantity ab-
sorbed will depend upon how much the liquid can combine with.
Or, a liquid may partly be composed of things which simply dis-
solve a gas and partly of things which chemically combine with it.
Then the amount of the gas taken up under a given partial pres-
sure will depend on two things; a certain portion, that merely dis-

KEXl'llfATION: THE GASEOUS INTERCHANGES 37<J

solved, will vary with the pressure of the gas in question; but
another portion, that chemically combined, will remain the same
under different pressures. The amount of this second portion de-
pends only on the amount of the substance in the liquid which can
chemically combine with it, and is totally independent of the
partial pressure of the gas.

5. Bodies are known which chemically combine with certain
gases when the partial pressure of these is considerable, forming
compounds which break up, or dissociate, liberating the gas, when
its partial pressure falls below a certain limit. Oxygen forms such
a compound with hemoglobin.

6. A membrane, moistened by a liquid in which a gas is soluble,
does not essentially alter the laws of absorption, by a liquid on one
side of it of a gas present on its other side, whether the absorption
be due to mere solution or to chemical combinations or to both.

The Absorption of Oxygen by the Blood. Applying the physi-
cal and chemical facts stated in the preceding paragraph to the
blood, we find that the blood contains (1) plasma, which simply
dissolves oxygen, and (2) hemoglobin, which combines with it un-
der some partial pressures of that gas, but gives it up under lower.

Blood-plasma or, what comes to the same thing, fresh serum,
exposed to the air, takes up no more oxygen than so much water:
about 0.56 volumes of the gas for every 100 of the liquid, at a
temperature of 20° C. At the temperature of the Body the volume
absorbed would be still less. This quantity obeys Henry's law.

If fresh defibrinated blood be employed, the quantity of oxygen
taken up is much greater; this extra quantity must be taken up
by the red corpuscles and it does not obey Henry's law. If the
partial pressure of oxygen on the surface of the defibrinated blood
be doubled, only as much more oxygen will be taken up as corre-
sponds to that dissolved in the serum; and if the partial pressure
of oxygen on its surface be reduced to one-half, only a very small
amount of oxygen (one-half of that dissolved by the serum) will
be given off. All the much larger quantity taken up by the red
corpuscles will be unaffected and must therefore be chemically
combined with something in them. Since 90 per cent of their
dry weight is hemoglobin, and this body when prepared pure is
found capable of combining with oxygen, there is no doubt that it
is the hemoglobin in the circulating blood which carries around

380 THE HUMAN BODY

most of its oxygen. The red corpuscles are so many little packages
in which oxygen is stowed away.

The compound formed between oxygen and hemoglobin is, how-
ever, a very feeble one; the two easily separate, and always do
so completely when the oxygen pressure in the liquid or gas to
which the oxyhemoglobin is exposed falls below 25 mm. of mer-
cury. There is some slight dissociation at pressures of 70 mm.
of mercury. Hence, in an air-pump, the blood only gives off a
little of its oxygen, until the pressure falls to about £ of an at-
mosphere, that is to •1|JL=125 mm. (5 inches) of mercury, of
which total pressure one-fifth (25 mm. or 1 inch) is due to the
oxygen present. As soon as this limit is- passed the hemoglobin
gives up its remaining oxygen with a rush.

Consequences of the Peculiar Way in Which the Oxygen of the
Blood is Held. The first, and most important, is that the blood
can take up far more oxygen in the lungs than would otherwise be
possible. Blood-serum exposed to the air would take up only one-
half volume of oxygen per hundred of liquid at ordinary tempera-
tures, and still less at the temperature of the Body, were 't not for
its hemoglobin. In the lungs even less would be taken up, since
the air in the air-cells of those organs is poorer in oxygen than the
external air; and consequently the partial pressure of that gas in
it is lower. The tidal air taken in at each breath serves merely to
renew directly the air in the big bronchi; the deeper we examine
the pulmonary air the less oxygen and more carbon dioxid wrould
be found; in the layers farthest from the exterior and only re-
newed by diffusion with the air of the large bronchi, it is estimated
that the oxygen only exists in such quantity that its partial pres-
sure is equal to about 100 mm. of mercury, instead of 152 as
in ordinary air. In the second place, on account of the way in
which hemoglobin combines with oxygen, the quantity of that
gas taken up by the blood is independent of such variations of its
partial pressure in the atmosphere as we are subjected to in daily
life. At the top of a high mountain, for example, the atmospheric
pressure is greatly diminished, but still mountaineers can breathe
freely and get all the oxygen they want ; the distress felt for a time
by persons unused to living in high altitudes is due in part to cir-
culatory disturbances resulting from the low atmospheric pressure
and in part to another condition to be described presently, but not

RESPIRATION: THE GASEOUS INTERCHANGES 381

at all to deficiency of oxygen. So long as the partial pressure
of that gas in the lung air-cells is above 25 mm. of mercury,
the amount of it taken up by the blood depends on how much
hemoglobin there is in that liquid and not on how much oxygen
there is in the air. So, too, breathing pure oxygen under a pres-
sure of one atmosphere, or air compressed to one-half or a fourth
its normal bulk, does not increase the quantity of oxygen ab-
sorbed by the blood, apart from the small extra quantity dissolved
by the plasma. The widespread statements as to the exhilaration
caused by breathing pure oxygen are erroneous, being founded on
experiments made with impure gas.

The General Oxygen Interchanges in the Blood. Suppose we
have a quantity of arterial blood in the aorta. This, fresh from the
lungs, will have its hemoglobin practically saturated with oxygen
and in the state of oxyhemoglobin. In the blood-plasma some
more oxygen will be dissolved, viz., so much as answers to a pres-
sure of that gas equal to 100 mm. of mercury, which is the partial
pressure of oxygen in the pulmonary air-cells. This tension of the
gas in the plasma will be more than sufficient to keep the hemo-
globin from giving off its oxygen. Suppose the blood now enters
the capillaries of a muscle. In- the liquid moistening this organ the
oxygen tension is practically nil, since the tissue elements are
steadily taking the gas up from the lymph around them. Conse-
quently, through the capillary walls, the plasma will give off
oxygen until the tension of that gas in it falls below 25 mm. of
mercury. Immediately some of the oxyhemoglobin is decom-
posed, and the oxygen liberated is dissolved in the plasma, and
from there next passed on to the lymph outside; and so the tension
in the plasma is once more lowered and more oxyhemoglobin
decomposed. This goes on so long as the blood is in the capillaries
of the muscle, but on account of the shortness of this interval,
about one second, not all the oxyhemoglobin has time to decom-
pose before the blood has passed on into the veins. Here further
decomposition is quickly brought to an end by the rising tension of
the oxygen dissolved in the plasma, the last oxygen given off from
the corpuscles not being taken up by the lymph because of the
passage of the blood on out of the capillaries. The blood will now
go on as ordinary venous blood into the veins of the muscle and
so back to the lungs. ' It will consist of (1) plasma with oxy-

382 THE HUMAN BODY

gen dissolved in it at a tension of about 25 mm. (1 inch) of
mercury. (2) A number of red corpuscles containing reduced
hemoglobin. (3) A number of red corpuscles containing oxy-
hemoglobin. Or perhaps all of the red corpuscles will contain
some reduced and some oxidized hemoglobin. This venous blood,
returning to the heart, is sent on to the pulmonary capillaries.
Here, the partial pressure of oxygen in the air-cells being 100
mm. and that in the blood-plasma much less, oxygen will be
taken up by the latter, and the tension of that gas in the plasma
tend to be raised above the limit at which hemoglobin combines
with it. Hence, as fast as the plasma gets oxygen those red cor-
puscles which contain any reduced hemoglobin rob it, and so its
oxygen tension is kept down below that in the air-cells until all the
hemoglobin is saturated. Then the oxygen tension of the plasma
rises to that of the gas in the air-cells; no more oxygen is absorbed,
and the blood returns to the left auricle of the heart in the same
condition, so far as oxygen is concerned, as when we commenced
to follow it.

The Carbon Dioxid of the Blood. The same general laws apply
to this as to the blood oxygen. The gas is partly merely dissolved
and partly in a loose chemical combination with some one or more
of the constituents of blood. Carbon dioxid is about twenty times
as soluble in blood-plasma as is oxygen under equivalent conditions
of temperature and pressure. We can therefore account for more
of it than of oxygen in the state of simple solution. Not more
than 6 per cent of the total amount present in venous blood can be
accounted for, however, in this way. The remainder must be in
some form of easily dissociable chemical compound. The nature
of this combination is not certainly known, but it is thought that
it may be for the most part simple sodium bicarbonate. The
large amount of sodium in the blood, and the ease with which it
combines with carbon dioxid, favor such a view. Sodium bicar-
bonate under ordinary circumstances is not easily enough dissoci-
ated to serve as a carbon dioxid carrier for the Body, but there
is reason to believe that in the presence of the blood proteins it
breaks down whenever the carbon dioxid tension of the plasma
falls below a certain point.

We may summarize the carbon dioxid interchanges as follows:
1. The tissues constantly produce and give off to the lymph

RESPIRATION: THE CASEOUS INTERCHANGES 383

carbon dioxid. It is present in lymph, therefore, at all times in
considerable quantity, probably amounting to 70 mm. of mercury
carbon dioxid tension.

2. The blood entering the capillaries contains carbon dioxid
under much less tension than this (about 35 mm.), there is there-
fore a movement of carbon dioxid from lymph to blood. This
movement, by raising the tension of carbon dioxid in the blood
brings about conditions under which chemical combination may
take place, forming, perhaps, sodium bicarbonate.

3. The venous blood as it enters the lungs contains carbon
dioxid under a higher tension than that of alveolar air, 70 mm.
for venous blood, 35 mm. for the alveoli; there is therefore a
movement of carbon dioxid from the blood to the alveoli. This
movement, by lowering the carbon dioxid tension of the blood,
favors the dissociation of the chemical compound formed during
the passage of the blood through the tissue capillaries; thus the
carbon dioxid taken up in the systemic capillaries is gotten rid of
in the lung capillaries.

The Hormone Action of Carbon Dioxid. We have already
learned (Chap. XXIII) that carbon dioxid has an important
hormone action in connection* with maintaining the activity of
the respiratory center. Recent work has shown that it has other
functions as well. The carbon dioxid tension of alveolar air is
ordinarily about 35 mm. of mercury. The carbon dioxid tension
of the blood docs not, of course, fall below that of the alveoli,
so that arterial blood under normal conditions contains a con-
siderable amount of carbon dioxid. Under exceptional circum-
stances, as at high altitudes, where the atmospheric pressure as
a whole is less than at the earth's surface, the tension of car-
bon dioxid in the alveoli may be considerably less than 35 mm.,
and that of the blood correspondingly diminished. There is a
condition known as mountain sickness, characterized by nausea
and other distressing symptoms, which is due to this diminution
of the carbon dioxid content of the blood. Any one, by taking
a number of deep breaths in rapid succession, can lower the car-
bon dioxid tension of his alveolar air, and consequently of his
blood, to a point where very disagreeable sensations are felt.
Just how the carbon dioxid of the blood prevents these symptoms
is not clear. That it has the power to do so is, however, well

384 THE HUMAN BODY

demonstrated. In view of these facts the physiological value
of deep breathing as a habit, so much insisted upon by certain
physical culturists, is, to say the least, problematical.

The normal breathing mechanism is an adaptation by which
the blood is continuously provided with all the oxygen it is able
to carry, and by which also its carbon dioxid content, while
never allowed to become excessive, is kept high enough for the
proper performance of its hormone function. It would seem,
therefore, that before adopting a breathing habit different from
that established by nature one should require a very convincing
justification of the proposed change.

Tissue Respiration. Our knowledge of the use of oxygen and
the production of carbon dioxid by the tissues is not very com-
plete. The following general facts may be stated here: (1) Al-
though the tissues take up oxygen from the lymph as fast as it is
brought by the blood they do not necessarily use it in oxidative
processes at the same rate; most of the oxygen appears to enter
some sort of chemical combination whereby it is stored until re-
quired by the tissues. (2) Tissue oxidations differ from ordinary
oxidative processes, such as occur when fuel is burned in a fur-
nace, for example, in that they are carried on through the agency
of enzyms known as oxidases. The chemical process of oxidation
carried on thus is not direct as in ordinary burning; it occurs at
a lower temperature, and requires a longer time; but it must be
remembered that the amount of heat produced by the oxidation
of a given weight of fuel is always the same whether the process
be rapid or slow, direct or indirect. Tissue oxidations, therefore,
are not necessarily wasteful because they go on indirectly and
slowly. (3) The amount of work that a man's organs do, is not
dependent on the amount of oxygen supplied to them, but the
amount of oxygen used by him depends on how much he uses his
organs. It is necessary to emphasize this fact because of the
notion, which seems to be rather wide-spread, that bodily proc-
esses are augmented by increasing the supply of oxygen to them.
The man who goes from his ill-ventilated office to the open coun-
try, and feels the impulse to vigorous exercise as he breathes the
pure country air, is apt to attribute his sensations of virility to
an imagined augmentation of all his bodily processes through the
increased amount of oxygen breathed in. The fact is that what-

RESPIRATION. THE GASEOUS INTERCHANGES

ever augmentation of activity he may experience is the result of
the agreeable sensory stimulations coming to him, which arouse
his tissues to activity, either reflexly or voluntarily. Increased
oxygen consumption is, therefore, never the cause, but always
the result of augmented tissue activity.

Coal Gas Poisoning. In the paragraph on asphyxia (Chap.
XXIII) the possibility of suffocation by carbon monoxid was
mentioned. This substance, which is an important constituent
of illuminating gas, has a greater affinity for hemoglobin than
has oxygen, and forms with it a very stable compound, carbon
monoxid hemoglobin. The result of breathing illuminating gas is,
then, the conversion of the hemoglobin of the blood into undis-
sociable carbon monoxid hemoglobin, and the consequent abol-
ishment of the oxygen-carrying function of the red corpuscles. If
the breathing of carbon monoxid has gone on long enough for
practically all the hemoglobin of the blood to be combined with
it, death from lack of oxygen is inevitable unless by the prompt
performance of blood transfusion a fresh supply of properly
functioning red corpuscles be introduced into the circulation.
Exposure to the gas for a shorter time, not enough to prove
fatal, but to the point of unconsciousness, is usually followed by
a long period, weeks or months, of serious functional impairment
of the tissues of the Body, due to the injury suffered by them
during the period of oxygen deficiency.

CHAPTER XXV
FOODS: THEIR CLASSIFICATION

What Constitutes Food. Material is taken into the Body in
three physical states: solid, liquid, gaseous. We have considered
the gaseous intake under the head of respiration, and turn now
to the use by the Body of solid and liquid substances. From the
standpoint of physiology we may include under the head of food
everything, either solid or liquid which is taken into the Body and
used there for its normal functioning. This classification includes
with the foods liquid substances, such as milk and water, which
we ordinarily classify separately as drinks. It is clear, however,
that from the standpoint of the Body a classification on this
basis, the physical nature of the substance taken, is not very
helpful, and we shall therefore disregard the distinction commonly
made between liquid and solid foods.

The Function of Food. If we have gotten the viewpoint which
the earlier chapters of this book have attempted to instil, and
are able to look upon the Body as a piece of machinery, we appre-
ciate that materials must be furnished to the Body for two pur-
poses: (1) to supply what it needs for the liberation of energy; and
(2) to provide for its maintenance and repair. The first of these
requirements is a simple fuel demand; anything that the Body is
able to burn can be used if its burning or mere presence does not
injure the delicate machinery. The second requirement is not so
simple; the repair of the complex body mechanism calls for
particular repair materials; in the carrying on of the Body's func-
tions there is a continuous loss from it of substances, such as water,
which must be continuously replaced; moreover, we often see fit
to introduce substances which we think will aid the Body in carry-
ing out its functions, as coffee, tea, spices, and condiments.

Classes of Foods. We would naturally, on the basis of the
discussion in the last paragraph, divide all foods into two groups,
energy-yielding foods, and repair and maintenance foods. While
we shall use this classification in the main in our discussion of

386

FOODS: THEIR CLASSIFICATION .>7

foods it cannot be sharply adhered to for the simple reason that
some foods may function either as repair materials or for yielding
energy.

The substances we take into our Bodies belong in one or the
other of the two great chemical groups: they are either organic or
inorganic. The inorganic foods are included without exception,
in the class of maintenance foods. They are taken into the Body
to replace materials which are essential to it, but which arc neces-
sarily lost from it in the course of its functioning. Of the various
organic foods a great number form what are commonly known as
accessory articles of diet. By imparting flavor to the food they aid
the digestive process, or they may exert influence upon other
bodily processes, as tea and coffee do upon nervous activity ; they
are therefore to be included as maintenance foods.

The organic constituents of our food not included among the
group of accessory articles of diet belong chemically to one or
other of three great subdivisions. They arc either carbohydrates,
fats, or proteins. The entire supply of energy for the Body, and
its repair and maintenance to great extent are derived from these
three classes of food-stuffs. Because of their prime importance
they are usually set apart from the other foods as nutrients proper.

Occurrence of Nutrients in Food. The articles which in com-
mon language we call foods are, in most cases, mixtures of several
nutrients with inorganic and accessory substances and with sub-
stances which are not foods at all. Bread, for example, contains
water, salts, gluten (a protein), some fats, much starch, and a little
sugar; all true food-stuffs: but mixed with these is a quantity of
cellulose (the chief chemical constituent of the walls which sur-
round vegetable cells) , and this is not a food since it is incapable of
digestion. Chemical examination of all the common articles of
diet shows that the actual number of important food-stuffs is but
small: they are repeated in various proportions in the different
things we eat, mixed with small quantities of different flavoring
substances, and so give us a pleasing variety in our meals; but the
essential substances are much the same in the fare of the workman
and in the "delicacies of the season." These primary food-stuffs,
which are found repeated in so many different foods, belong to one
or the other of the classes of nutrients mentioned above; and the
food value of any article of diet depends on them far more than on

388 THE HUMAN BODY

the traces of flavoring matters which cause certain things to be
especially sought after and so raise their market value. We can-
not, however, conclude that the possession of flavor by foods is
wholly unnecessary. We shall see that the accessories play a very
real and very important role in our use of foods in general.

Occurrence of Inorganic Materials in Food. Two inorganic sub-
stances, water and sodium chlorid (common salt), are taken
separately and consciously as constituents of the diet. We re-
quire such large amounts of these substances that they have to be
taken thus purposely to insure that enough be gotten. The other
inorganic materials, the chlorids, phosphates, and sulphates of
potassium, magnesium, and calcium, occur in most ordinary arti-
cles of diet, so that we do not swallow them in a separate form.
Phosphates, for example, exist in nearly all animal and vegetable
foods; while other foods, as casein, contain phosphorus in com-
binations which in the Body yield it up to be oxidized to form
phosphoric acid. The same is true of sulphates, which are partially
swallowed as such in various articles of diet, and are partly formed
in the Body by the oxidation of the sulphur of various proteins.
Calcium salts are abundant in bread and milk, and are also found
in many drinking-waters. It has been maintained that salt, as
such, is an unnecessary luxury; and there seems some evidence
that certain savage tribes live without more than they get in
the meat and vegetables they eat. Such tribes are, however,
said to suffer especially from intestinal parasites; and there is
no doubt that to civilized man the absence of salt is a great
privation.

Occurrence of Accessories in Food. Variety in the diet depends
practically altogether upon the accessories, for the primary food-
stuffs are few in number and for the most part without very pro-
nounced tastes or flavors, with the single exception of sugar, whose
sweet taste makes it, to the eyes of most children at least, the
most desirable of all foods. To civilized man variety of diet is a
virtual necessity; the accessories, therefore, are to him of great im-
portance. Both meats and vegetables owe their characteristic
flavors, in the main, to organic substances present in them. We
do not, however, depend wholly on these substances for securing
the needed variety in our food. Condiments, pepper and mustard
for example, and spices are used very largely in all civilized coun-

FOODS: THEIR CLASSIFICATION 389

tries. Chocolate, coffee, and tea are taken by most people more
for their agreeable flavor than for their stimulating properties.

The Nutrients. The actual nourishment of the Body depends,
as stated above, primarily upon the taking of sufficient quantities
of the nutrients proper. Of the three groups of nutrients two,
carbohydrates and fats, are exclusively energy-yielders. Their
function is to be oxidized in the Body and thus to furnish the
energy by which the machine does its work. The third nutrient
group, the proteins, furnishes all the material by which waste of
living tissues is made good, and provides likewise a very con-
siderable proportion of the fuel supply of the Body. Because of
the twofold function of proteins it is possible for a person or
animal to live for a long time upon an exclusively protein diet.
Since repair of tissue waste can be made only by proteins, an ani-
mal or a man would starve to death upon a protein-free diet, no
matter how much of the other food-stuffs he might have. For
that matter not all proteins are tissue-formers; reference to the
classification of proteins in Chap. I shows that only the first
two classes, the albumins and globulins, are sufficiently complex
to yield all the constituents needed for the formation or repair of
living tissues. Albuminoids form a constant part of all flesh food,
but they can be used by the Body, in the long run, only as it uses
carbohydrates and fats, for fuel.

Carbohydrates. These are mainly of vegetable origin. The
most important are starch, found in nearly all vegetable foods, and
having the chemical formula (C6Hi006)n; the dextrins, or gums;
and two classes of sugars; double sugars, having the formula
C12H22OU, and represented by cane-sugar, sucrose, and milk-sugar,
lactose, and single sugars, having the formula C,H13O6, and repre-
sented by grape-sugar, dextrose. Glycogen, animal starch, is a
constituent of muscle tissue and is eaten as a part of flesh. It and
milk-sugar are the only carbohydrates commonly eaten which are
of animal origin. Cellulose, a very abundant vegetable carbo-
hydrate, is to the human alimentary tract practically indigestible.

Fats. The most important are stearin, palmatin, and olein,
which exist in various proportions in animal fats and vegetable
oils; the more fluid containing more olcin. Butter contains also a
little of a fat named butyrin. Fats are compounds of glycerin and
fatty acids, and any such substance which is fusible at the temper-

390 THE HUMAN BODY

ature of the Body will serve as a food. The stearin of beef and
mutton fats is not by itself fusible at the body temperature, but
is mixed in those foods with so much olein as to be melted in the
alimentary canal. Beeswax, on the other hand, is a fatty body
which will not melt in the intestines and so passes on unabsorbed;
although from its composition it would be useful as a food could it
be digested. A distinction is sometimes made between fats proper
(the adipose tissue of animals consisting of fatty compounds in-
closed in albuminous cell-walls) and oils, or fatty bodies which are
not so organized.

Proteins occur as the chief constituent of animal foods, lean
meat for example being 90 per cent protein after its large water
content is removed. Eggs and milk contain considerable amounts
of protein also. Proteins occur to a greater or lesser degree in
most vegetable foods. The gluten of wheat is protein; beans and
peas contain a larger percentage of protein than any other food
except cheese.

The albuminoid of connective tissue, which is present in
all meat, is by cooking converted into gelatin, a digestible
protein.

Mixed Foods. These, as already pointed out, include nearly all
common articles of diet; they contain more than one nutrient.
Among them we find great differences; some being rich in pro-
teins, others in starch, others in fats, and so on. The formation
of a scientific dietary depends on a knowledge of these charac-
teristics. The foods eaten by man are, however, so varied that we
cannot do more than consider the most important.

Flesh. This, whether derived from bird, beast, or fish, consists
essentially of the same things — muscular fibers, connective tissue
and tendons, fats, blood-vessels, and nerves. It contains several
proteins, especially myosin and myogen; gelatin-yielding matters
in the white fibrous tissue; stearin, palmatin, and olein as repre-
sentatives of the fats ; and a small amount of carbohydrates in the
form of glycogen and grape-sugar, or some chemically allied sub-
stances. Flesh also contains much water and a considerable
number of salines, the most important and abundant being
potassium phosphate. The nitrogenous extractives (Chap. I)
give much of its taste to flesh; and small quantities of various of
these substances exist in different kinds of meat. There is also

FOODS: THEIR CLASSIFICATION 391

more or less yellow clastic tissue in flesh; it is indigestible and use-
less as food.

When meat is cooked its white fibrous tissue is turned into
gelatin, and the whole mass becomes thus softer and more easily
disintegrated by the teeth. When boiled some of the protein
matters of the meat pass out into the broth, and there in part
coagulate and form the scum: this loss may be prevented in great
part by putting the raw meat at once into boiling water which
coagulates the surface albumen before it dissolves out, and this
keeps in the rest, while the subsequent cooking is continued
slowly. In any case the myosin, being insoluble in water, remains
behind in the boiled meat. In baking or roasting, all the solid
parts of the flesh are preserved and certain agreeably flavored
bodies are produced, as to the nature of which little is known.

Eggs. These contain a large amount of egg albumen and, in the
yolk, another protein, known as vitellin. Also fats, and a sub-
stance known as lecithin, which is important as containing a con-
siderable quantity of phosphorus. Lecithin, or rather a substance
yielding it, is an important constituent of the nervous tissues.

Milk contains at least two proteins, lactalbumin and casein;
several fats in the butter; a carbohydrate, milk-sugar; much water;
and salts, especially potassium and calcium phosphates. Butter
consists mainly of the same fats as those in beef and mutton; but
has in it about one per cent of a special fat, butyrin. In the milk
it is disseminated in the form of minute globules which, for the
most part, float up to the top when the milk is let stand and then
form the cream. In this each fat-droplet is surrounded by a
pellicle of albuminous matter; by churning, these pellicles are
broken up and the fat-droplets then run together to form the
butter. Casein is insoluble in water; in milk it is dissolved by the
alkaline salts present. When milk is kept, its sugar ferments and
gives rise to lactic acid, which neutralizes the alkali and precipi-
tates the casein as curds. In cheese-making the casein is acted
upon by a ferment present in the extract of stomach used, and
converted into tyrein which is precipitated: this clotting does not
take place unless a calcium salt be present. Tyrein, which forms
the main bulk of a true cheese, is different from the curd pre-
cipitated from milk by acids; cheese made from the latter does
not "ripen."

392 THE HUMAN BODY

Vegetable Foods. Of these wheat affords the best; not that it
contains more of any particular nutrient but because of a peculiar
property of its protein. The protein of wheat is mainly gluten,
which when moistened with water forms a tenacious mass, and
this it is to which wheaten bread owes its superiority. When the
dough is made, yeast is added to it, and produces a fermentation by
which, among other things, carbon dioxid gas is produced. This
gas, imprisoned in the tenacious dough, and expanded during
baking, forms cavities in it and causes it to "rise" and make
"light bread," which is not only more pleasant to eat but more
digestible than heavy bread. Other cereals may contain a larger
percentage of starch, but none have so much gluten as wheat;
when bread is made from them the carbon dioxid gas escapes so
readily from the less tenacious dough that it does not expand the
mass properly. Corn and rice are valuable chiefly for their high
carbohydrate content; beans and peas, on the other hand, have
a high per cent of protein. Potatoes contain less actual nutri-
ment for their weight than do any of the other important foods.
Their cheapness and digestibility have combined to give them a
place in the average dietary out of all proportion to their real
value. Other fresh vegetables, as carrots, turnips, and cabbages,
are valuable mainly for the salts they contain; their weight is
mainly due to water, and they contain but little starch, proteins,
or fats. Fruits, like most fresh vegetables, are mainly valuable
for their saline constituents, the other food-stuffs in them being
only present in small proportion. Some fruit or vegetable is,
however, a necessary article of diet ; as shown by the scurvy which
used to prevail among sailors before fresh or canned vegetables
and lime-juice were supplied to them.

The Cooking of Vegetables. This is of more importance even
than the cooking of flesh, since in most the main alimentary
principle is starch, and raw starch is difficult of digestion. In
plants starch is stored up within the walls of the plant-cells,
which are of cellulose and therefore indigestible. When vege-
tables are cooked the contents of the cells swell, the cellulose walls
are ruptured and the starch is set free to be acted upon by the
digestive mechanism of the Body.

Composition of Foods. The following table gives the per-
centage composition of some of the common foods.

FOODS: THEIR CLASSIFICATION

3U3

In 100 Parts

Water

Protein

Fat

I" ice*tiMp
Carbohydrate

IlKirg i:..'-

Material

Meat

7(5.7

20.8

1.5

0.3

1.3

Eggs

73.7

12.6

12.1

1. 1

Cheese

36-60

25-33

7-30

3-7

3^4

Cow's Milk

87.7

3.4

3.2

! x

0.7

Human Milk

89.7

2.0

3.1

5.0

0.2

Wheat Flour

13.3

10.2

0.9

74.8

0.5

Wheat Bread

35.6

7.1

0.2

55.5

1.1

Rye Flour

13.7

11.5

2.1

69.7

1.4

Rye Bread

42.3

6.1

0.4

49.2

1.5

Rice

13.1

7.0

0.9

77.4

1.0

Corn

13.1

9.9

4.6

68.4

1.5

Macaroni

10.1

9.0

0.3

79.0

0.5

Peas and Beans

12-15

23-26

H-2

49-54

2-3

Potatoes

75.5

2.0

0.2

20.6

1.0

Carrots

87.1

1.0

0.2

9.3

0.9

Cabbages

90.0

2-3

0.5

4-6

1.3

Mushrooms

73-91

4-8

0.5

3-12

1.2

Fruit

84.0

o :,

10.0

0.5

In a bulletin of the U. S. Department of Agriculture more de-
tailed analyses can be found. (Bull. 28.)

Alcohol. Perhaps no single question in physiology has aroused
more discussion than that of the physiological position of alcohol.
Its use from time immemorial as a beverage, and the long history
of misery and crime which has followed its use to excess, make the
problem of its true place one of very great practical importance.

We must recognize at the outset that alcohol has very diverse
immediate effects according as it is taken in large or small
amounts. The miserable spectacle presented by an intoxicated
man emphasizes only too clearly the harm of excessive indulgence;
on the other hand, the taking of a small quantity often leads to an
appearance of heightened mental and physical ability. Both Mind
and Body seem more alert than commonly. No one questions
the injurious effects of large amounts of alcohol; the diversity of
opinion is with reference to its use in small doses.

It has been demonstrated that alcohol in moderate amounts is
oxidized in the Body with the liberation of energy, and is there-
fore a fuel in the true sense of the word. That it may serve
as fuel is not in itself, however, justification for its use, even in
small quantities. It must be shown that its direct physiological
effects are not harmful to the Body, before it can be accepted as a
food.

The action of alcohol in small doses appears to be chiefly upon
the nervous system, and particularly upon the higher portions of

394 THE HUMAN BODY

the central nervous system. Its effect upon nerve-centers seems
to be a depressing one; the generally accepted view that alcohol
is a stimulant being based upon bodily effects which follow nerve-
center depression rather than stimulation. For example, cutane-
ous vasodilatation, with flushing of the skin, such as is commonly
seen after taking alcohol, is the result of depression of the vaso-
constrictor center. The rapid heart-beat, which is another usual
phenomenon, results from depression of the cardio-inhibitory
center. Even the sparkle of wit and repartee, which is reputed
to be very marked after partaking of wine, is the result of removal
of the brakes of judgment and caution through depression of those
regions of the brain where these functions reside. It is intimated,
in fact, that after-dinner wit is ordinarily appreciated at more
than its due desert, because of the depression of judgment in the
brains of the hearers.

The depressing effect of alcohol upon the brain appears to be
progressively from higher to lower centers. The first traits to be
dulled are those acquired through precept and moral training;
therefore the individual is apt to reveal his " true self," stripped
of the veneer of education. With increasing indulgence in alcohol
lower and lower tendencies come to the fore, set free by the de-
pression of the higher, and ordinarily controlling ones. Thus it
comes to pass that man may sink to the level of the beast.

The question of the moderate use of alcohol resolves itself, then,
from a physiological standpoint, into one of the desirability of
setting free the lower mental traits and activities through depres-
sion of the higher inhibitory ones. It is sometimes argued that in
America where the dominant mental obsession of a considerable
proportion of the population is in affairs of business the setting
free of the brain from business cares during leisure hours is a
virtual necessity, and that the use of alcohol is the most direct
method of bringing this about. Even though we grant the first
part of the argument it does not necessarily follow that the second
part is to be accepted also. For the real objection to the use of
alcohol, even in small quantities, is that the desire for alcohol,
unlike the desire for food, increases as more is taken instead of
decreasing when satiety is reached. It thus requires a stronger
effort of will to leave off as more is taken, and since the alcohol
at the same time depresses the power of the will the danger of

FOODS: THEIR CLASSIFICATION 395

overindulgence is continually present. Only where the will is
sufficiently strong to set a limit and adhere rigidly to it is the con-
tinuous moderate use of alcohol in any degree safe.

Returning to the question of the desirability of the practice of
removing the brakes from the brain periodically, it may be said
that the opinion seems to be becoming more and more prevalent
among neurologists that the use of alcohol for such a purpose,
particularly in early and middle life, is more of an injury than a
benefit. The normal interactions among the different parts of the
mental apparatus should be permitted, according to these ob-
servers, to proceed without artificial interference, at least during
the period of the most active associative processes. There seems
to be no vital objection to the moderate use of alcohol on the part
of persons who have passed the age of fifty or thereabouts. The
danger of acquiring the alcohol habit is practically nil at that age,
and the predominant mental traits are by that time so completely
in control that occasional release from them may operate as
a distinct advantage. This is particularly true in the case of
those elderly persons who find themselves disposed to a somewhat
gloomy outlook upon life. The temperate use of alcohol may
make life more enjoyable for themselves and also for those about
them.

Tea, Coffee, and Cocoa. These beverages all owe their special
physiological properties to certain alkaloids present in them.
The active principle of tea and coffee is the same, caffein; that of
cocoa, and its derivative, chocolate, is a closely related substance,
theobromin. Caffein and theobromin appear to be direct nerve-
stimulants. They cause a rise of blood-pressure through stimu-
lation of the vasoconstrictor center. Their use, like that of
alcohol, constitutes an artificial interference with normal proc-
esses, and is subject, therefore, to the general objections which
arise against such interference. Their effects are of varying in-
tensity; cocoa is an exceedingly mild stimulant; tea, properly
made, is somewhat stronger; and coffee, properly made, is stronger
yet. Their use is borne much better by some persons than by
others. They are not dangerous in the sense that alcohol is,
through an increasing craving which readily leads to overindul-
gence and resulting disaster, although they, like alcohol, are often
taken to excess. Temperance in the use of these beverages is as

396 THE HUMAN BODY

much the part of wisdom as in the use of alcohol. Again, like
alcohol, they are best left alone during early life.

The improper preparation of tea and coffee, by boiling them in
water, carries into solution, in addition to the stimulating prin-
ciple, a substance, tannin, whose effect upon the system is apt to be
distinctly harmful. These beverages should therefore always be
prepared by methods which do not involve prolonged or even
brief boiling while the tea leaves or coffee grounds are actually
present in the liquid.

CHAPTER XXVI
THE ALIMENTARY CANAL AND ITS APPENDAGES

General Arrangement. The alimentary canal is essentially a
tube running through the Body (Fig. 2) and lined by a vascular
membrane, most of which is specially adapted for absorption; it
communicates with the exterior at three points (the nose, the
mouth, and the anal aperture), at which the lining mucous mem-
brane is continuous with the general outer integument. Support-
ing the absorbent membrane arc layers which strengthen the tube,
and are in part muscular and, by their contractions, serve to pass
materials along it from one end to the other. In the walls of the
canal are numerous blood and lymphatic vessels which carry off
the matters absorbed from its cavity; and there also exist in con-
nection with it numerous glands, whose function it is to pour into
it various secretions by which the chemical act of digestion is
carried on. Some of these glands arc minute and embedded in the
walls of the alimentary tube itself, but others (such as the salivary
glands) are larger and lie away from the main channel, into which
their products are carried by ducts of various lengths.

The alimentary tube is not uniform but presents several dilata-
tions on its course; nor is it straight, since, being much longer than
the Body, a large part of it is packed away by being coiled up in
the abdominal cavity.

Subdivisions of the Alimentary Canal. The mouth-opening
leads into a chamber containing the teeth and tongue, the mouth-
chamber or buccal cavity. This is succeeded by the pharynx or
throat-cavity, which narrows at the top of the neck into the gullet or
esophagus; this runs down through the thorax and, passing
through the diaphragm, dilates in the upper part of the abdominal
cavity into the stomach. Beyond the stomach the channel again
narrows to form a long and greatly coiled tube, the small intestine,
which terminates by opening into the large intestine, much shorter
although wider than the small, and terminating by an opening on
the exterior.

397

398

THE HUMAN BODY

m

The Mouth-Cavity (Fig. 122) is bounded in front and on the
sides by the lips and cheeks, below by the tongue, k, and above

by the palate; which latter consists
of an anterior part, /, supported
by bone and called the hard palate,
and a posterior, /, containing no
bone, and called the soft palate.
The two can readily be distinguished
by applying the tip of the tongue
to the roof of the mouth and draw-
ing it backwards. The hard palate
forms the partition between the
mouth and nose. The soft palate
arches down over the back of the
mouth, hanging like a curtain be-
tween it and the pharynx, as can
be seen by holding the mouth open
in front of a looking-glass. From
the middle of its free border a
conical process, the uvula, hangs
down.
The Teeth. Immediately within

FIG. 122.— The mouth, nose and the cheeks and lips are two Semi-
pharynx, with the commencement . *

of the gullet and larynx, as exposed Circles, formed by the borders of

by a section, a little to the left of ,, , •• ,

the median plane of the head, a, the upper - and lower jaw-bones,

vertebralcolumnjh, gullet ;c wind- hih covered by the QUmS,

pipe ; d, larynx ; e, epiglottis , /, soft *

palate; g, opening of Eustachian except at intervals along their edges

tube; k, tongue ;l, hard palate; m, . . . .

the sphenoid bone on the base of where they contain sockets in which

the skull; n, the fore part of the .•> fppth nrp imnlnnfprl Dnrino-
cranial cavity; o, p, q, the turbinate .*

bones of the outer side of the left \tfe two sets of teeth are developed:

nostril-chamber.

the first or milk set appears soon

after birth and is shed during childhood, when the second or
permanent set appears.

The teeth differ in minor points from one another, but in each
three parts are distinguishable; one, seen in the mouth and called
the crown of the tooth; a second, embedded in the jaw-bone and
called the root or fang; and between the two, embraced by the edge
of the gum, is a narrowed portion, the neck or cervix. From dif-
ferences in their forms and uses the teeth are divided into incisors,

ALIMENTARY CANAL AND ITS APPENDAGES

3'J«J

canines, bicuspids, and molars, arranged in a definite order in each
jaw. Beginning at the middle line we meet in each half of each
jaw with, successively, two incisors, one canine, and two molars
in the milk set; making twenty altogether in the two jaws. The
teeth of the permanent set arc thirty-two in number, eight in each
half of each jaw, viz. — beginning at the middle lino — two incisors,
one canine, two bicuspids, and three molars. The bicuspids, or
premolars, of the permanent set replace the milk-molars, while the
permanent molars are new teeth added on as the jaw grows, and
not substituting any of the milk-teeth. The hindmost permanent
molars are often called the wisdom-teeth.

Characters of Individual Teeth. The incisors (Fig. 123) are
adapted for cutting the food. Their crowns are chisel-shaped and
have sharp horizontal cutting edges, which become worn away by
use so that they are beveled off behind in the upper row, and in the
opposite direction in the lower. Each has a single long fang. The
canines (Fig. 124) are somewhat larger than the incisors. Their
crowns are thick and somewhat conical, having a central point or

FIG. 123

FIG. 124

FIG. 125

FIG. 126

FIG. 123. — An incisor tooth.
FIG. 124. — A canine or eye-tooth.

FIG. 125. — A bicuspid tooth seen from its outer side; the inner cusp is, accord-
ingly, not visible.

FIG. 126. — A molar tooth.

cusp on the cutting edge. In dogs, cats, and other carnivora the
canines are very large and adapted for seizing and holding prey.
The bicuspids or premolars (Fig. 125) are rather shorter than the
canines and their crowns are somewhat cuboidal. Each has two
cusps, an outer towards the cheek, and an inner on the side turned
towards the interior of the mouth. The fang is compressed later-
ally, and has usually a groove partially subdividing it into two.
At its tip the separation is often complete. The molar teeth or
grinders (Fig. 126) have large crowns with broad surfaces, on which

400 THE HUMAN BODY

are four or five projecting tubercles, which roughen them and
make them better adapted to crush the food. Each has usually
several fangs. The milk-teeth differ only in subsidiary points from
those of the same names in the permanent set.

The Structure of a Tooth. If a tooth be broken open, a cavity
extending through both crown and fang will be found in it. This
is rilled during life with a soft vascular pulp, and hence is known
as the "pulp-cavity" (c, Fig. 127). The hard parts of the tooth
disposed around the pulp-cavity consist of three different tissues.
Of these one immediately surrounds the cavity and makes up most
of the bulk of the tooth; it is dentine (2, Fig. 127); covering the
dentine on the crown is the enamel (I, Fig. 127) and, on the fang,
the cement (3, Fig. 127).

The pulp-cavity opens below by a narrow aperture at the tip of
the fang, or at the tip of each if the tooth have more than one.
The pulp consists mainly of connective tissue, but its surface next
the dentine is covered by a layer of columnar cells. Through the
opening on the fang blood-vessels and nerves enter the pulp.

The dentine (ivory) yields on analysis the same materials as
bone but is somewhat harder, earthy matters constituting 72 per
cent of it as against 66 per cent in bone. Under the microscope it
is recognized by the fine dentinal tubules which, radiating from the
pulp-cavity, perforate it throughout, finally ending in minute
branches which open into irregular cavities, the interglobular
spaces, which lie just beneath the enamel or cement. At their
widest ends, close to the pulp-cavity, the dentinal tubules are only
about 0.005 millimeter (^V<r °f an mch) in diameter. The cement is
much like bone in structure and composition. It is thickest at the
tip of the fang and thins away towards the cervix. Enamel is the
hardest tissue in the Body, yielding on analysis only from two per
cent to three per cent of organic matter, the rest being mainly
calcium phosphate and carbonate. Its histological elements are
minute hexagonal prisms, closely packed, and set on vertically to
the surface of the subjacent dentine. It is thickest over the free
end of the crown, until worn away by use. Covering the enamel in
unworn teeth is a thin structureless horny layer, the enamel cuticle.

The Tongue (Fig. 128) is a muscular organ covered by mucous
membrane, extremely mobile, and endowed not only with a deli-
cate tactile sensibility but with the terminal organs of the special

ALIMENTARY CANAL AND ITS APPENDAGES 401

sense of taste; it is attached by its root to the hyoid bone. On its
upper surface are numerous small eminences or papilla, such as are

Fio. 127. — Section through a premolar tooth of the oat still embedded in ita
socket. 1, enamel; 2, dentine; 3, cement; 4, the gum; 5, the bone of the lower
jaw; c, the pulp-cavity.

found more highly developed on the tongue of a cat, where they
may be readily felt. On the human tongue there are three forms
of papilla, the circumvallate, the fungiform, and the filiform. The
circumvallate papillae, 1 and 2, (Fig. 128) the largest and least nu-
merous, are from seven to twelve in number and lie near the root of
the tongue arranged in the form of a V with its open angle turned

402

THE HUMAN BODY

forwards. Each is an elevation of the mucous membrane, covered
by epithelium, and surrounded by a trench. On the sides of these
papilla?, embedded in the epithelium, are many small oval bodies
richly supplied with nerves and supposed to be concerned in the

FIG. 128. — The upper surface of the tongue with part of the pillars of the fauces
and the tonsils. 1, 2, circumvallate papillae; 3, fungiform papillae; 4, filiform
papillae; 6, mucous glands; 7, tonsils; 8, tip of epiglottis.

sense of taste, and hence called the taste-buds (Chap. XIV) . The
fungiform papillce, 3, are rounded elevations attached by somewhat
narrowed stalks, and found all over the middle and fore part of the
upper surface of the tongue. They are easily recognized on the
living tongue by their bright red color. The filiform papillce, 4,

ALIMENTARY CANAL AND ITS APPENDAGES 403

most numerous and smallest, are scattered all over the dorsum of
the tongue except near its base. Each is a conical eminence
covered by a thick horny layer of epithelium. It is these papilla,'
which are so highly developed on the tongues of Carnivora, and
serve them to scrape bones clean of even such tough structures as
ligaments.

In health the surface of the tongue is moist, covered by little
"fur," and in childhood of a red color. In adult life the natural
color of the tongue is less red, except around the edges and tip; a
bright-red glistening tongue being then, usually a symptom of
disease. When the digestive organs are deranged the tongue is
commonly covered with a thick yellowish coat, composed of a little
mucus, some cells of epithelium shed from the surface, and numer-
ous microscopic organisms known as bacteria; and there is fre-
quently a "bad taste in the mouth." The whole alimentary
mucous membrane is in close physiological relationship; and any-
thing disordering the stomach is likely to produce a "furred
tongue."

The Salivary Glands. The saliva, which is poured into the
mouth and which, mixed with the secretion of minute glands em-
bedded in its lining membrane, moistens it, is secreted by three
pairs of glands, the parotid, the sub maxillary, and the sublinyual.
The parotid glands lie in front of the ear behind the ramus of the
lower jaw; each sends its secretion into the mouth by a tube known
as Stenson's duct, which crosses the check and opens oppoa||e the
second upper molar tooth. In the disease known as nntinp& the
parotid glands are inflamed and enlarged. The submaxillary
glands lie between the halves of the lower jaw-bone, near its angles,
and their ducts open beneath the tongue near the middle line. The
sublingual glands lie beneath the floor of the mouth, covered by
its mucous membrane, between the back part of the tongue and
the lower jaw-bone. Each has many ducts (8 to 20) , some of which
join the submaxillary duct, while the rest open separately in the
floor of the mouth.

The Fauces is the name given to the aperture which can be seen

at the back of the mouth below the soft palate (Fig. 122), and

leading into the pharynx. It is bounded above by the soft palate

and uvula, below by the root of the tongue, and on the sides by

* Parotitis, in technical language.

404 THE HUMAN BODY

muscular elevations covered by mucous membrane, which reach
from the soft palate to the tongue. These elevations are the pillars
of the fauces. Each bifurcates below, and in the hollow between its
divisions lies a tonsil (7, Fig. 128), a soft rounded body about the
size of an almond, composed of lymphoid tissue (Chap. XXII).
The tonsils not unfrequently become enlarged during a cold or sore
throat, giving rise to considerable discomfort.

The Pharynx or Throat-Cavity (Fig. 122). This portion of the
alimentary canal may be described as a conical bag with its broad
end turned upwards towards the base of the skull, and its narrow
end downwards and passing into the gullet. Its front is imperfect,
presenting openings which lead into the nose, the mouth, and
(through the larynx and windpipe) the lungs. Except during
swallowing or speech the soft palate hangs down between the
mouth and pharynx ; during deglutition it is raised into a horizon-
tal position and separates an upper or respiratory portion of the
pharynx from the rest. Through this upper part, therefore, air
alone passes, entering it from the posterior ends of the two nostril-
chambers ; while through the lower portion both food and air pass,
one on its way to the gullet, 6, Fig. 122, the other through the
larynx, d, to the windpipe, c; when a morsel of food "goes the
wrong way" it takes the latter course. Opening into the upper
portion of the pharynx on each side is an Eustachian tube, g; so
that the apertures leading out of it are seven in number; the two
posterior nares, the two Eustachian tubes, the fauces, the opening
of the larynx, and that of the gullet. At the root of the tongue,
over the opening of the larynx, is a plate of cartilage, the epiglottis,
e, which can be seen if the mouth is widely opened and the back
of the tongue pressed down by some such thing as the handle of a
spoon. During swallowing the epiglottis is pressed down like a lid
over the air-tube and helps to keep food or saliva from entering it.
In structure the pharynx consists essentially of a bag of connective
tissue lined by mucous membrane, and having muscles in its walls
which drive the food on.

The Esophagus or Gullet is a tube commencing at the lower
termination of the pharynx and which, passing on through the
neck and chest, ends below the diaphragm by joining the stomach.
In the neck it lies close behind the windpipe. It consists of three
coats — a mucous membrane within; next, a submucous coat of

ALIMENTARY CANAL AND ITS APPENDAGES 405

arcolar connective tissue: and, outside, a muscular coat made up of
two layers, an inner with transversely and an outer with longi-
tudinally arranged fibers. In and beneath its mucous membrane
are numerous small mucous glands whose ducts open into the
tube.

The Stomach (Fig. 129) is a somewhat conical bag placed trans-
versely in the upper part of the abdominal cavity. Its larger end is
turned to the left and lies close beneath the diaphragm ; opening
into its upper border, through the
cardiac orifice at a, is the gullet d. ^JfT
The narrower right end is con-
tinuous at c with the small intes-
tine; the aperture between the
two is the pyloric orifice. The
pyloric .end of the stomach lies
lower in the abdomen than the
cardiac, and is separated from
the diaphragm by the liver (see ~

. rio. 129. — riio stomach, d, lower

Flg.l). The concave border be- end of the gullet ; «, position of the

,i •/» • i cardiac aperture; 6, the fundus; c.

tWCCn the tWO OriflCCS IS known the pylorus; e, the commencement of

as the small rumvifi/rp nnrl -tho tne 8ma11 intestine; alonj? a, 6, c, the
re, an C grpat curvaturo; between the pylorus

convex as the great curvature, of and d> the Ie88er curvature,
the stomach. From the latter hangs down a fold of peritoneum
(ne, Fig. 1) known as the great omentum. It is spread over the rest
of the abdominal contents like an apron. After middle life much
fat frequently accumulates in the omentum, so that it is largely re-
sponsible for the "fair round belly with good capon lin'd." The
protrusion b to the left side of the cardiac orifice, Fig. 129, is the
fundus. The size of the stomach varies greatly with the amount
of food in it; just after a moderate meal it is about ten inches long,
by five wide at its broadest part.

Structure of the Stomach. This organ has four coats, known
successively from without in as the serous, the muscular, the sub-
mucous, and the mucous. The serous coat is formed by a reflection
of the peritoneum, a double fold of which slings the stomach; after
separating to envelop it the two layers again unite and, hanging
down beyond it, form the great omentum. The muscular coat
(Fig. 47) consists of unstriped muscular tissue arranged in three
layers: an outer, longitudinal, most developed about the curva-

406 THE HUMAN BODY

tures; a circular, evenly spread over the whole organ, except
around the pyloric orifice where it forms a thick ring; and an inner,
oblique and very incomplete, radiating from the cardiac orifice.
The submucous coat is made up of lax areolar tissue and binds
loosely the mucous coat to the muscular. The mucous coat is a
moist pink membrane which is inelastic, and large enough to line
the stomach evenly when it is fully distended. Accordingly, when
the organ is empty and shrunken, this coat is thrown into folds,
which disappear when the organ is distended. During digestion
the arteries supplying the stomach become dilated and, its capil-
laries being gorged, its mucous membrane is then much redder
than during hunger.

The blood-vessels of the stomach run to it between the folds of
peritoneum which sling it. After giving off a few branches to the
outer layers, most of the arteries break up into small branches in
the submucous coat, from which twigs proceed to supply the close
capillary network of the mucous membrane.

The nerves of the stomach are chiefly derived from the vagi.
In the lower part of the thorax these nerves consist mainly of non-
medullated fibers, and lie on the sides of the gullet, across which
they interchange fibers by means of several branches. On entering
the abdomen the left vagus passes to the ventral side of the
stomach, in which it ends : the right supplies the dorsal side of the
stomach, but a considerable portion of it passes on to enter the
solar plexus, which lies behind the stomach and contains several
large ganglia. The sympathetic also supplies gastric nerves which
mainly go to the blood-vessels. In the muscular coat of the stom-
ach are many nerve-cells.

Histology of the Gastric Mucous Membrane. Examination of
the inner surface of the stomach with a hand lens shows it to be
covered, except in the fundic region, with minute shallow pits.
Into these open the mouths of minute tubes, the gastric glands,
which are closely packed side by side in the mucous membrane;
something like the cells of a honeycomb, except that each is open
at one end. Between them lie a small amount of connective tis-
sue, a close network of lymph-channels, and capillary blood-
vessels. The whole surface of the mucous membrane is lined by a
single layer of columnar mucus-making epithelium cells (Fig. 130).
These dip down and line the necks of the tubular glands. The

ALIMENTARY CANAL AND ITS APPENDAGES 407

deeper portions of the glands are lined by a layer of shorter and
somewhat cuboidal cells, the central or chief cells. In s|>eciinens
taken from a healthy animal killed dur-
ing digestion these cells arc large and do
not stain .deeply with carmine. Similar
specimens taken from an animal an hour
or two after a good meal has been swal-
lowed show the chief cells shrunken and
staining more deeply. They, thus, store
up during rest a material which they get

• i r L xu ^ • • • • , • FIG. 130.—.'

rid of when the gastric juice is being throuKh ti»

cor»rr>torl membrane, ixTix-iulicular to

QCLICtCU. f

its surface, magnified about

In the pyloric end of the stomach only 25 diameters, a, a simple

,,..,,,.,. gastric gland ;b, a com pound

the chief cells line the glands, but else- gastric gland • c. & Kiand con-

where there is found outside of them, in

most of the glands, an incomplete layer nective tissue.

of larger oval cells (d, Fig. 130). The glands frequently branch

at their deeper ends.

The Pylorus. If the stomach be opened it is seen that the
mucous membrane projects in a fold around the pyloric orifice and
narrows it. This is due to a thick ring of the circular muscular
layer there developed, and forming around the orifice a sphincter
muscle, which, by its contraction, keeps the passage to the small
intestine closed except when portions of food are to be passed on
from the stomach to succeeding divisions of the alimentary canal.

Since the cardiac end of the stomach lies immediately l>cneath
the diaphragm, which has the heart on its upper side, its over-
distension, due to indigestion or flatulence, may impede the action
of the thoracic organs, and cause feelings of oppression in the
chest, or palpitation of the heart.

The Small Intestine (Fig. 136), commencing at the pylorus, ends,
after many windings, in the large intestine. It is about six metere
(twenty feet) long, and about five centimeters (two inches) wide
at its gastric end, narrowing to about two-thirds of that width at
its lower portion. Externally there are no lines of subdivision on
the small intestine, but anatomists arbitrarily describe it as con-
sisting of three parts; the first twelve inches being the duodenum,
D, the succeeding two-fifths of the remainder the jejunum, J, and
the rest the ileum, I.

408 THE HUMAN BODY

Like the stomach, the small intestine possesses four coats; a
serous, a muscular, a submucous, and a mucous. The serous coat
is formed by a duplicature of the peritoneum, but presents noth-
ing answering to the great omentum ; this double fold slinging the
intestine is named the mesentery. The muscular coat is composed
of plain muscular tissue arranged in two strata, an outer longitu-
dinal, and an inner transverse or circular. The submucous coat is
like that of the stomach ; consisting of loose areolar tissue, binding
together the mucous and muscular coats, and forming a bed in
which the blood and lymphatic vessels (which reach the intestine
in the fold of the mesentery) break up into minute branches be-
fore entering the mucous membrane.

The Mucous Coat of the Small Intestine. This is pink, soft and
extremely vascular. It does not present temporary or effaceable
folds like those of the stomach, but is, throughout a great portion
of its length, raised up into permanent transverse folds in the form
of crescentic ridges, each of which runs transversely for a greater
or less way round the tube (Fig. 131). These folds are the valvulce
conniventes. They are first found about two inches from the
pylorus, and are most thickly set and largest in the upper half of
the jejunum, in the lower half of which they become gradually
less conspicuous; and they finally disappear altogether about the
middle of the ileum. The folds serve greatly to increase the sur-
face of the mucous membrane both for absorption and secretion,
and they also delay the food somewhat in its passage, since it must
collect in the hollows between them, and so be longer exposed to
the action of the digestive liquids. Examined closely with the eye

FIG. 131. — A portion of the small intestine oponed to show the valvulce conniventes.

or, better, with aid of a lens, the mucous membrane of the small
intestine is seen to be not smooth but shaggy, being covered every-
where (both over the valvula? conniventes and between them)

ALIMENTARY CANAL AND ITS APPENDAGES

10!)

with closely packed minute processes, standing up somewhat like
the "pile" on velvet, and known as the villi. Each villus is from
0.5 to 0.7 millimeter CsV to ^ inch) in length; some are conical and
rounded, but the majority are compressed at the base in one di-
ameter (Fig. 132). In structure a villus is somewhat complex.
Covering it is a single layer of columnar epithelial cells, the ex-
posed ends of the majority having a peculiar bright striated border
and being probably of great importance in absorption. Mixed with
these cells arc others in which most of the cell has become filled
with a clear mass which does not stain readily with reagents; the
deep narrow end of the cell stains easily and contains the nucleus.
From time to time the clear substance (mucigen) is converted into
mucus and discharged into the intestine, leaving behind only the
nucleus and the protoplasm around it. These reconstruct the cell
and form more mucigen. These mucus-forming cells arc named
goblet-cells, from their shape. Beneath the epithelium the villus
may be regarded as made up of a framework of connective tissue,
supporting the more essential constituents. Near the surface is an

Fia 132. — Villi of the small intestine; magnified about 80 diameters. In the
right-hand figun- tin- hu-tcals, a, b, c, are filled with white injection; d blood-
vessels. In tin- left-hand figure the lacteals alone are represented, filled with a
dark injection. The epithelium covering the villi, and their muscular fibers, are
omitted.

incomplete layer of plain muscular tissue, continuous below with a
muscular stratum forming the deepest layer of the mucous mem-
brane and named the muscularis mucosce. In the center is an off-

'410 THE HUMAN BODY

shoot of the lymphatic system; sometimes in the form of a single
vessel with a closed dilated end, and sometimes as a network formed
by two main vessels with cross-branches. During digestion these
lymphatics are filled with a milky-white liquid absorbed from the
intestines, and they are accordingly called the lacteals. They com-
municate with larger branches in the submucous coat, which end
in trunks that pass out through the mesentery to join the main
lymphatic system. Finally, in each villus, outside the lacteals and
beneath the muscular layer of the villus, is a close network of
blood-vessels.

Opening on the surface of the small intestine, between the bases
of the villi, are small glands, the crypts of Lieberkuhn. Each is a
simple unbranched tube lined by a layer of columnar cells some of
which have a striated free border, though less marked than that on
the corresponding cells of the villi, and others are goblet-cells. The
crypts of Lieberkuhn are closely packed, side by side, like the
glands of the stomach. In the duodenum are found other minute
glands, the glands of Brunner. They lie in the submucous coat
and send their ducts through the mucous membrane to open on its
inner side.

The Large Intestine (Fig. 136), forming the final portion of the
alimentary canal, is about 1.5 meters (5 feet) long, and varies in
diameter from about 6 to 4 centimeters (2£ to 1£ inches). Anato-
mists describe it as consisting of the caecum with the vermiform
appendix, the colon, and the rectum. The small intestine does not
open into the commencement of the large but into its side, some
distance from its closed upper end, and the caecum, CC, is that part
of the large intestine which extends beyond the communication.
From it projects the vermiform appendix, a narrow tube not
thicker than a lead pencil, and about 10 centimeters (4 inches)
long. The colon commences on the right side of the abdominal
cavity where the small intestine communicates with the large,
runs up for some way on that side (ascending colon, AC], then
crosses the middle line (transverse colon, TC) below the stomach,
and turns down (descending colon, DC) on the left side and there
makes an S-shaped bend known as the sigmoid flexure, SF; from
this the rectum, R, the terminal straight portion of the intestine,
proceeds to the anal opening, by which the alimentary canal com-
municates with the exterior. In structure the large intestine

ALIMENTARY CANAL AND ITS APPENDAGES 411

presents the same coats as the small. The external stratum of the
muscular coat is not, however, developed uniformly around it,
except on the rectum, but occurs in three bands separated by in-
tervals in which it is wanting. These bands being shorter than the
rest of the tube cause it to be puckered, or sacculated, between
them. The mucous coat possesses no villi or valvuhc conniventcs,
but is usually thrown into cffaceable folds, like those of the
stomach but smaller. It contains numerous closely set glands
much like the crypts of Lieberkiihn of the small intestine.

The Ileocolic Valve. Where the small intestine joins the large
there is a valve, formed by two flaps of the mucous membrane slop-
ing down into the colon, and so disposed as to allow matters to
pass readily from the ilcum into the large intestine but not the
other way.

The Nerves of the Intestines. The intestines arc innervated
through sympathetic nerves. These come to them by way of two
main channels; from the sympathetic system proper the splanchnic
nerves pass to the intestines; their other chief channel of inncrva-
tion is by way of the vagi. Both these sets of nerves ramify in the
solar plexus; from here nerve strands pass to the intestine, as well
as to the stomach, along the mesentery. There are also fibers
passing to the intestines from the mescnteric plexus lying in the
lower part of the abdomen, these fibers reach that plexus from the
posterior thoracic and anterior lumbar sympathetic ganglia.

The intestines are provided, in addition, with an intrinsic inncr-
vation consisting of two nervous networks or plexuses lying, one
between the mucosa and the muscular coat, the plexus of Meissner,
and the other between the circular and longitudinal muscle layers,
the plexus of Auerbach.

The Liver. Besides the secretions formed by the glands em-
bedded in its walls, the small intestine receives those of two large
glands, the liver and the pancreas, which lie in the abdominal
cavity. The ducts of both open by a common aperture into the
duodenum about 10 centimeters (4 inches) from the pylorus.

The liver is the largest gland in the Body, weighing from 1,400 to
1,700 grams (50 to (>1 ounces). It is situated in the upper part of
the abdominal cavity (/.-. /»•'. Fig. 1), rather more on the right than
on the left side and immediately below the diaphragm, into the
concavity of which its upper surface fits, and reaches across the

412

THE HUMAN BODY

middle line above the pyloric end of the stomach. It is of dark
reddish-brown color, and of a soft friable texture. A deep fissure
incompletely divides the organ into right and left lobes, of which
_VT ^ a.. "Lfl

Dch

FIG. 133. — The under surface of the liver, d, right, and s, left lobe; Vh, hepatic
vein; Vp, portal vein; Vc, vena cava inferior; Dch, common bile-duct; DC, cystic
duct; Dh, hepatic duct; Vf, gall-bladder.

the right is much the larger; on its under surface (Fig. 133) shal-
lower grooves mark off several minor lobes. Its upper surface is
smooth and convex. The vessels carrying blood to the liver are
the portal vein, Vp, and the hepatic artery; both enter it at a fissure
(the portal fissure) on its under side, and there also a duct passes
out from each half of the organ. The ducts unite to form the
hepatic duct, Dh, which meets at an acute angle, the cystic duct, DC,
proceeding from the gall-bladder, Vf, a pear-shaped sac in which
the bile, or gall, formed by the liver, accumulates when food is not
being digested in the intestine. The common bile-duct, Dch, formed
by the union of the hepatic and cystic ducts, opens into the duode-
num. The blood which enters the liver by the portal vein and
hepatic artery passes out by the hepatic veins, Vh, which leave the
posterior border of the organ close to the vertebral column, and
there open into the inferior vena cava just before it passes up
through the diaphragm.

The Structure of the Liver. On closely examining the surface
of the liver, it will be seen to be marked out into small angular

ALIMENTARY CANAL AND ITS APPENDAGES 413

areas from one to two millimeters (^ to ± inch) in diameter. These
are the outer sides of the superficial layer of a vast number of
minute polygonal masses, or lobules, of which the liver is built up;
similar areas are seen on the surface of any section made through
the organ. Each lobule (Fig. 134) consists of a number of hepatic

Fia. 134. — A lobule of the liver (pig), magnified, showing the hepatic cells
radiately arranged around the central intralobular vein, and the connective ti.-v-.in-
surrounding the lobule. (Scymonowicz.)

cells supported by a close network of capillaries; and is separated
from neighboring lobules by connective tissue, larger blood-vessels,
and branches of the hepatic duct. The hepatic cells are the proper
tissue elements of the liver, all the rest being subsidiary arrange-
ments for their nutrition and protection. Each is polygonal,
nucleated and very granular, and has a diameter of about ().< )_'.">
millimeter (^ of an inch). In each lobule they are arranged in
rows or strings, which form a network, in the meshes of which the
blood-capillaries and bile-capillaries run. The blood carried in by
the portal vein (which has Already circulated through the capil-
laries of the stomach, spleen, intestines and pancreas) is conveyed
to a fine vascular interlobular plexus around the liver-lobules, from

414

which it flows on through the capillaries of the lobules themselves.
These (Fig. 134) unite in the center of the lobule to form a small
intralobular vein, which carries the blood out and pours it into one

Flo. 135. — The stomach, pancreas, liver, and duodenum, with part of the rest,
of the small intestine and the mesentery; the stomach and liver have been turned
up so as to expose the pancreas. V, stomach; D, D', D", duodenum; L, spleen;
P, pancreas; R, right kidney; T, jejunum; Vf, gall-bladder; h, hepatic duct;
c, cystic duct; ch, common bile-duct; 1, aorta, 2, an artery (left coronary) of the
stomach; 3, hepatic artery; 4, splenic artery; 5, superior mesenteric artery; 6, su-
perior mesenteric vein; 7, splenic vein; Vp, portal vein.

of the branches of origin of the hepatic vein, called the sublobular
vein. Each of the latter has many lobules emptying blood into it,
and if dissected out with them would look something like a branch
of a tree with apples attached to it by short stalks, represented by

ALIMENTARY CANAL AND ITS APPENDAGES

415

the intralobular veins. The blood is finally carried, as already
pointed out, by the hepatic veins into the inferior vena cava. The
hepatic artery, a direct offshoot of the celiac axis (p. 416) supplies
some blood to the lobular plexuses, but by no means so much as
the portal vein; it all finally leaves the liver by the hepatic veins.

The bile-ducts can be readily traced to the periphery of the
lobules, and there communicate with a network of extremely
minute commencing bile-capillaries, ramifying in the lobule between
the hepatic cells composing
it. The relation of the
bile-capillaries to the blood
capillaries within the lobule
is such that there is always
a liver-cell interposed be-
tween them.

From the arrangement of
blood-capillaries and bile-
capillaries with their con-
nections we can picture the
movement of blood and bile
through the lobules; the
blood, both from the portal
vein and the hepatic artery,
is delivered to the lobule at
its periphery and flows
thence from all sides to-
ward the center, where it
enters the interlobular vein
and is conveyed away. The
bile, on the other hand, is

Secreted by the liver-cells FIO. 136.— Diagram of abdominal part of al-
1 f™ n )!,,.,, nac ir»l intn imentiiry canal. C, the cardiac, and P, the

and from them passed into pylori(. ;>lul of t|ic ?t^marh. Dt tho duodenum;

the bile-capillaries; it flows -A I, the convolutions of the small int.-Mm, •;

CC, the cax-urn with the vermiform apiM-n.liv;

along these toward thc pe- AC, uMvnding. TC, transverse, and DC, de-
. , , sccnding colon; K, thc rectum.

nphcry where it enters

small bile-ducts, and so is carried toward thc great outlet of the

gland, thc hepatic duct.

The Pancreas or Sweetbread. This is an elongated soft organ of
a pinkish-yellow color, lying along thc great curvature of the

sr

416 THE HUMAN BODY

stomach. Its right end is the larger, and is embraced by the
duodenum (Fig. 135), which there makes a curve to the left. A
duct traverses the gland and joins the common bile-duct close to
its intestinal opening. The pancreas produces a watery-looking
secretion which is of great importance in digestion; the gland also
secretes a hormone which exerts an important influence on the
general nutritional processes of the Body (Chap. XXX).

The Blood- Vessels of Alimentary Canal, Liver, Spleen, and Pan-
creas. The portal vein (Vp, Fig. 135) has already been referred
to as differing from all other veins in that it not only receives blood
from a system of capillaries but ends in a second set of capillaries,
which lie in the liver. The quantity of blood brought to supply the
hepatic capillaries by the hepatic artery is in fact much less than
that brought by the portal vein. The stomach, the intestines, the
pancreas, and the spleen are supplied with arterial blood from three
great branches of the aorta. The most anterior of these, the celiac
axis, springs from the aorta close beneath the diaphragm and
divides into the hepatic artery, splenic artery, and arteries for the
stomach; some of these divisions may be seen in Fig. 135. The
pancreas is supplied partly from the hepatic, partly from the
splenic artery. The two other branches (superior and inferior
mesenteric artery) are given off from the aorta lower down in the
abdominal cavity; the former (5, Fig. 135) supplies the small in-
testine and half of the large, the latter the remainder.of the large.
The blood passing through all these arteries becomes venous in the
capillaries of the organs they supply, and is gathered into corre-
sponding veins (Fig. 135) which unite near the liver to form the
portal vein. The further course of the blood carried to the liver
(partly arterial from the hepatic artery, partly venous from the
portal system) has been described already (p. 412).

CHAPTER XXVII
THE CHEMISTRY OF DIGESTION

The Object of Digestion is twofold ; to prepare the various foods
for absorption by the lining of the digestive tract, which means
that they must be made soluble if not already so; and to convert
them into forms in which the Body can make use of them after
they have been absorbed. Digestion is confined to the nutrients;
the inorganic salts of the food are soluble, and are used by the
Body in essentially the same form as eaten, they therefore need no
digestion.. The accessories either perform their function in con-
nection with the process of digestion itself, or are absorbed and
used by the Body in the form in which they are taken.

Nature of the Digestive Process. Although the foods requiring
digestion are of very different sorts chemically, the method of
digestion is at bottom the same for all of them. It consists of the
process known in chemistry as hydrolysis. Hydrolysis is a chemi-
cal reaction in which one molecule of the substance involved com-
bines with one molecule of water and the resulting compound splits
into two or more simpler molecules. By repeated hydrolyses very
complex substances may be split into comparatively simple
ones.

Hydrolysis is a common reaction of organic chemistry. It is
probable that it is the most frequently occurring reaction of the
living Body. Not only the digestive processes, but many of the ac-
tivities of living cells are of this nature. The digestive hydrolyses
are all carried on through the agency of enzyms. There is a special
and specific enzym for each particular reaction; the cnzym that
splits starch is without effect on protein or fat. These enzym
reactions are all simple chemical reactions; they are carried on in
the alimentary tract as in a chemical laboratory, anil will go on
just as well in test-tubes kept at Body temperature as in tin- Body
itself. They are not therefore " vital " processes in the sense that
they cannot occur except in the presence of living cells.

Digestion Products. Before beginning a detailed description

417

418 THE HUMAN BODY

of the digestive process as it affects the different food-stuffs it
will perhaps be helpful to call attention to the comparatively few
and simple substances which are finally produced as the result of
the numerous reactions that go on in the alimentary tract. All
carbohydrates (except the single sugars), the starches, gums, and
double sugars, are hydrolyzed into single sugars during their di-
gestion, so that absorption of carbohydrates is altogether in the
form of single sugars. All fats are split into fatty acid and glycerin,
in which state they are ready to be taken up by the intestinal
walls. The proteins, as we saw in Chap. I, are complexes built up
of a large number of amino acids. The digestive process splits
them into simpler molecules each of which is composed either of a
single amino acid, or of two or three of them together. We may
say, in general, that proteins are split into their constituent
amino acids.

Tabulating the digestion products we have :

from carbohydrates, single sugars;

from fats, fatty acid and glycerin ;

from proteins, amino acids.

The Saliva. The first digestive fluid that the food meets with
is the saliva, which, as found in the mouth, is a mixture of pure
saliva, formed in parotid, submaxillary, and sublingual glands,
with the mucus secreted by small glands of the buccal mucous
membrane. This mixed saliva is a colorless, cloudy, feebly alkaline
liquid, " ropy " from the mucin present in it, and usually contain-
ing air-bubbles. Pure saliva, as obtained by putting a fine tube in
the duct of one of the salivary glands, is more fluid and contains
no imprisoned air.

The uses of the saliva are in part physical and mechanical. It
keeps the mouth moist and allows us to speak with comfort; it
also dissolves such bodies as salt, and sugar, when they are taken
into the mouth in solid form, and enables us to taste them; undis-
solved substances are not tasted, a fact which any one can verify
for himself by wiping his tongue dry and placing a fragment of
sugar upon it. No sweetness will be felt until a little moisture has
exuded and dissolved part of the sugar.

In addition to such actions the saliva exerts a chemical one on
an important food-stuff. It contains an enzym, ptydlin, which
has the power of turning starch into a double sugar, maltose. This

THE CHEMISTRY OF DIGESTION 41Q

change, like all digestive reactions, is a hydrolysis. It does not
occur in a single stage; that is, the starch molecule is not split
directly into maltose, but first into a dextrin which is hydrolyzed
into a simpler dextrin, and this in turn into maltose. In effecting
the change the ptyalin is not altered; a very small amount of it can
convert a vast amount of starch, and does not seem to have its
activity impaired in the process.

In order that the ptyalin may act upon starch certain conditions
are essential. Water must be present, and the liquid must be
neutral or feebly alkaline; acids retard, or if stronger, entirely
stop the process. The change takes place most quickly at about
the temperature of the Human Body, and is greatly checked by
cold. Boiling the saliva destroys its ptyalin and renders it quite
incapable of converting starch. Cooked starch is changed more
rapidly and completely than raw.

It will be noted that salivary digestion is only a stage in the
preparation of starch for the use of the Body, since starch, in
common with the other carbohydrates taken as food, is finally
converted to single sugar before it is absorbed.

The Gastric Juice. The food having entered the stomach is
subjected to the action of the* gastric juice, which is a thin, color-
less or pale yellow liquid, of a strongly acid reaction. It contains
as specific elements free hydrochloric acid (about 0.2 per cent), and
an cnzym called pepsin which, in acid liquids, has the power of
converting the ordinaiy proteins which we eat by hydrolysis, into
closely allied bodies, proteases and peptones.

In neutral or alkaline media the pepsin is inactive; and cold
checks its activity. Boiling destroys it. In addition to pepsin,
gastric juice contains another enzym (rennin) which coagulates
the casein of milk, as illustrated by the use of "rennet," prepared
from the mucous membrane of the calf's digestive stomach, in
cheese-making. The acid of the natural gastric juice would, it is
true, precipitate the casein, but such precipitate is quite different
from the true tyrein, and neutralized gastric juice still possesses
this power; moreover, boiled gastric juice loses the milk-clotting
property, and a very little normal juice can coagulate a great
quantity of milk. The curdled condition of the milk regurgitated
by infants is, therefore, not any sign of a disordered state of the
stomach, as nurses commonly suppose. It is proper for milk to

420 THE HUMAN BODY

undergo this change, before the pepsin and acid of the gastric
juice digest it.

Although rennin has always been looked upon as an inde-
pendent enzym having the specific function attributed to it, some
evidence has been brought forward recently to indicate that pep-
sin is the only enzym of gastric juice, and that it exercises a two-
fold function, that of splitting proteins to proteoses and peptones,
and that of coagulating casein.

Since muscle-fibers are enclosed within connective tissue (al-
buminoid) envelopes, it is necessary that the albuminoid cover-
ings be digested off before the protein contents are exposed to the
action of the digestive enzyms. There is reason to think that
pepsin, which converts proteins, including albuminoids, into pro-
teoses and peptones, soluble substances, but does not carry the
digestion to completion, has as an important part of its function
this removal from animal proteins of their albuminoid coverings.

The Pancreatic Juice. In the intestine the food is subjected
to the action of the pancreatic juice. This is clear, watery, alka-
line, and much like saliva in appearance. The Germans call the
pancreas the "abdominal salivary gland.'.' In digestive prop-
erties, however, the pancreatic secretion is far more important
than the saliva, or even the gastric juice. It contains three di-
gestive enzyms; amylopsin a starch-splitting enzym whose action
is identical with that of salivary ptyalin, and which is thought to
be, perhaps, itself identical with ptyalin; lipase, a fat-splitting
enzym, converting fats to fatty acid and glycerin; trypsin, a
protein-splitting (proteolytic) enzym whose action is much more
powerful than that of pepsin, as it is able to carry the process of
protein hydrolysis clear to the amino acid stage. It acts upon
such proteins as escape the influence of pepsin in the stomach.

The Bile. This fluid, which is poured into the intestine from
the liver does not contain any digestive enzym, but it does have
an important role in connection with fat digestion; it has been
shown that pancreatic lipase splits fats several times as rapidly
when bile is present as when it is absent.

The Succus Entericus (Intestinal juice). This fluid, which is
secreted by the minute glands of the intestinal wall, is the last of
the digestive fluids to come in contact with the food, and by its
enzyms whatever foods are not completely digested must be

THE CHEMISTRY OF DIGESTION

421

finally prepared for absorption. By the cnzyms thus far de-
scribed none of the carbohydrate digestion is carried to comple-
tion, and only part of the proteins are made ready for use, for
proteose and peptone are not end products, but only intenncdiate
.products of digestion. Fats are the only foods which do not re-
quire the aid of the succus entericus for their complete digestion.

The digestive enzyms of the succus entericus are four; one pro-
teolytic, erepsin, which acts particularly on proteoses and pep-
tones, thus completing the work of the gastric pepsin; and three
so-called inverting enzyms, which change double sugars to single
sugars. These enzyms are specific in their action, each affecting
only its particular sugar. Maltase inverts maltose, thus com-
pleting the starch digestion begun by ptyalin and amylopsin;
invertaSe splits cane-sugar, and lactase converts milk-sugar, lac-
tose, to- single sugar. The result of the action of these three en-
zyms is to bring all the carbohydrates of the food, except cellu-
lose, into the condition of single sugars, in which form they are
ready for the use of the Body.

Summary of the Digestive Process. The chemical reactions by
which the various food-stuffs are made ready for absorption and
use by the Body can be conveniently summarized in tabular form :

Region

Secretion

Enzyms

Substances
Affected

Products Formed

Mouth

Saliva

Ptyalin

Starch

Maltose >

Stomach

Gastric Juice

Pepsin

Albuminoid

Proteoses *

Protein

Peptones l

Small

Pancreatic

Amylopsin

Starch

Maltose >

Intestine

Juice

Lipasc

Fats

Fatty acid '

(llycerin *

Trypsin

Proteins

Amino Acids '

Succus
Entericus

Erepsin

Proteoses
Peptones

ii .«

Maltase

Maltose

Single Sugar 3

Invertasc

Cane-Sugar

« >i

Lactaae

Milk-Sugar

« it

1 Intermediate products.

2 Final products.

Bacterial Digestion. The human intestines normally contain
enormous numbers of bacteria. In the small intrstim- these an-
for the most part fermentative bacteria; organisms having the

422 THE HUMAN BODY

power to ferment carbohydrates with the production of carbon
dioxid, alcohol, and acetic and lactic acids. There is no doubt
that even in perfect health a considerable fermentation goes on
in the intestine. So far as appears it is neither particularly harm-
ful nor beneficial. The fermentation products are probably ab-
sorbed and used by the Body, but they would be used equally
well if absorbed as sugar without fermentation. In the case of one
particular carbohydrate, however, cellulose, bacterial fermenta-
tion affords the only means by which it can be made available
in man for the use of the Body. It seems to be well established
that tender cellulose, such as is eaten in lettuce, for example, may
be digested by bacteria to a considerable extent; where it is less
tender, as in most fruits and vegetables, it remains, as stated
earlier, practically undigested.

Intestinal fermentation is not essential to health as is shown
by the possibility of living normally in arctic regions, where, it is
said, intestinal bacteria are often wholly wanting. When the
fermentation becomes excessive intestinal disturbances may
readily result. The production of fermentation acids in too great
concentration leads to irritation of the intestinal wall and causes
diarrhea.

The Prevention of Self-Digestion. A question of much interest
to physiologists has been why the stomach and intestinal walls
and the gastric and pancreatic glands are not themselves digested
by the powerful proteolytic enzyms which they produce, in the
case of the glands, or which are poured out unto them in the case
of the walls of the digestive organs. It has been shown that the
prevention of self -digestion of stomach and intestine depends upon
the continuance of life, for animals killed in the midst of digest-
ing a meal often do digest great parts of their stomach and in-
testinal walls. Just how self-digestion of these structures is
normally prevented is not clear, except in so far as the mechan-
ism to be described presently (Chap. XXIX), which limits the
outpouring of the secretions to periods when food is present, may
be efficacious. The self-digestion of the pancreatic and gastric
glands is, however, prevented by an interesting arrangement
which has been recently analyzed. It appears that neither pepsin
nor trypsin is formed in the gland as an active enzym but in an
inactive pro-enzym or zymogen form, pepsinogen or trypsinogen,

THE CHEMISTRY OF DIGESTION 423

which becomes active only when converted into pepsin or trypsin
by some activating agent. It has been shown that the conversion
of trypsinogen to trypsin occurs only when the pancreatic juice is
poured into the small intestine, and that it is brought about
through a constituent of the succus entericus, enterokinase. This
substance is believed to be an enzym having the sole function of
activating trypsinogen to trypsin. The conversion of pepsinogen
to pepsin is a similar activation, but whether it is carried on by
an enzym, gastric kinase, or by the hydrochloric acid of gastric
juice, cannot be said at the present time.

CHAPTER XXVIII
MOVEMENTS OF THE ALIMENTARY CANAL

Mastication serves to break the food into fine particles and by,
mixing it intimately with saliva to reduce it to a semi-liquid state.
It consists primarily of cutting and grinding the food between
the upper and lower teeth, a process which is performed by move-
ments of the lower jaw. The articulation of the lower jaw with
the skull and its equipment of muscles permit both up and down
cutting movements and sidewise grinding movements. The ac-
tual chewing process involves, in addition, motions of the lips,
cheeks, and tongue in holding the food in position for the teeth
to act upon it. The whole process is carried on by skeletal muscles
and is, therefore, under the control of the will.

It ought not to be necessary to emphasize the importance of
thorough mastication of the food. Salivary digestion depends
wholly, of course, upon the bringing of saliva into contact with
the starch particles, and it can easily be shown experimentally
that gastric digestion is several times more rapid when the ma-
terial exposed to the action of gastric juice is finely divided than
when it is in large masses.

The interesting fact has recently been brought out that the
more the process of masticating each mouthful is prolonged the
less food is required to satisfy the appetite. Since many people
doubtless eat too much there is here a suggestion as to a way of
reducing the amount taken without serious sacrifice of appetite.

Hygiene of the Mouth. The mouth cavity is almost never free
from micro-organisms. The alkaline reaction of saliva is favor-
able to their growth, and they scarcely ever lack for food. The
irregularly shaped teeth, packed closely along the jaw, have be-
tween them spaces where material that is being chewed readily
lodges, and where it stays unless special care is taken to remove
it. Such lodged food-masses shortly harbor flourishing colonies
of bacteria. These in connection with their growth and multi-
plication produce substances which attack the protective enamel

424

MOVEMENTS OF THE ALIMENTARY CANAL 425

of the teeth and so gain foothold within the tooth substance itself,
and we have under way the too-familiar process of tooth decay.
Good teeth are so important for efficient mastication, as well as
for the appearance of the face, that no pains should be spared to
preserve them. Evidently the way to do this is to prevent the
accumulation of bacteria in the spaces between them. Thorough
cleaning, desirably after each meal, with the occasional use of an
antiseptic mouth-wash is fairly but not completely satisfactory.
Half yearly inspection and cleaning by a dentist are usually nec-
essary to supplement one's own efforts, because of the practical
impossibility of keeping every one of the small mouth spaces clear.
Such inspection also insures the discover}' of decay while the cavi-
ties are still small, and makes possible the preservation of the
teeth in approximately normal condition for many years.

Deglutition. A mouthful of solid food is broken up by the
teeth, and rolled about the mouth by the tongue, until it is thor-
oughly mixed with saliva and made into a soft pasty mass. This
mass is sent on from the mouth to the stomach by the process of
deglutition, which is described as occurring in three stages. The
first stage includes the passage from the mouth into the pharynx.
The food being collected into a heap on the tongue, the tip of that
organ is placed against the front of the hard palate, and then the
rest of the tongue is raised from before back, so as to press the
food mass between it and the palate, and drive it back through
the fauces. This portion of the act of swallowing is voluntary, or
at least is under the control of the will, although it commonly
takes place unconsciously. The second stage of deglutition is that
in which the food passes through the pharynx; it is the most rapid
part of its progress, since the pharynx has to be emptied quickly
so as to clear the opening of the air-passages for breathing pur-
poses. The food mass, passing back over the root of the tongue,
pushes down the epiglottis; at the same time the larynx (or voice-
box at the top of the windpipe) is raised, so as to meet it, and thus
the passage to the lungs is closed; muscles around the apcrt im-
probably also contract and narrow the opening. The raising of
the larynx can be readily felt by placing the finger on the large
cartilage forming "Adam's apple" in the neck, and then swallow-
ing something. The soft palate is at the same time raised and
stretched horizontally across the pharynx, thus cutting off com-

426 THE HUMAN BODY

munication with its upper, or respiratory portion, leading to the
nostrils and Eustachian tubes. Finally, the isthmus of the fauces
is closed as soon as the food has passed through, by the contrac-
tion of the muscles on its sides and the elevation of the root of the
tongue. All passages out of the pharynx except the gullet are
thus blocked, and by a sharp contraction of the mylohyoid
muscles, in the floor of the mouth, such great pressure is put upon
the food-mass as to shoot it clear through the pharynx into the
opening of the esophagus. Liquids or very soft foods, under the
impetus given by the contraction of these muscles, are propelled
the whole length of the gullet to the sphincter which guards the
entrance to the stomach; more solid masses are thrown only into
the entrance of the gullet whence the third stage of swallowing
conveys them to the stomach. The muscular movements con-
cerned in this part of deglutition are all reflexly excited; food
coming in contact with the mucous membrane of the pharynx
stimulates afferent nerve-fibers in it; these excite efferent nerve-
fibers proceeding to the muscles concerned and cause them to
contract in proper sequence. The pharyngeal muscles, although
of the striped variety, are but little under the control of the will;
it is extremely difficult to go through the movements of swallow-
ing without something (if only a little saliva) to swallow and thus
excite the movements reflexly. Many persons, after having got
the mouth completely empty cannot perform the movements of
the second stage of deglutition at all. On account of the reflex
nature of the contractions of the pharynx, any food which has
once entered it must be swallowed : the isthmus of the fauces is a
sort of Rubicon; food that has passed it must continue its course
to the stomach, although the swallower learnt immediately that
he was taking poison. The third stage of deglutition is that by
which solid food is passed along the gullet, and is comparatively
slow. The movements of the esophagus are of the kind known
as peristaltic. Its circular muscular fibers contract behind the
morsel and narrow the passage there; and the constriction then
travels along to the stomach, pushing the food in front of it.
Simultaneously the longitudinal fibers, at the point where the
food-mass is at any moment and immediately in front of that,
contracting, shorten and widen the passage. This peristaltic
wave requires about six seconds in man for its passage along the

MOVEMENTS OF THE ALIMENTARY CANAL 427

esophagus. It is part of the reflex act of swallowing and takes
place whenever the act occurs, whether there be any food-mass
to be conveyed to the stomach or not. The sphincter muscle at
the entrance of the stomach is ordinarily tightly contracted, hold-
ing the esophagus shut, and only opens when the peristaltic wave
coming down crowds it open, forcing the food-mass through into
the stomach. Liquids, which pass very quickly down the esoph-
agus (in 0.1 sec.), usually do not get into the stomach at once, but
are held by the sphincter until the arrival of the peristaltic wave
forces a passage for them. A curious fact is that two peristaltic
waves cannot be moving along the gullet simultaneously. If one
swallows a second time within six seconds of a former swallowing
the peristaltic wave started by the first is inhibited, promptly
fades out, and whatever food is at the sphincter must wait for the
arrival of the second wave to enter the stomach.

Movements of the Stomach. When the stomach is empty its
muscular walls are so strongly contracted as to bring the mucus
layers into contact, leaving no cmp^y space. As food enters the
stomach it makes room for itself by stretching the stomach walls,
and the more food is taken, the more the stomach is distended.
One result of this manner of filling the stomach is that the food is
deposited in it in layers, the first food taken being next to the
walls, subsequent amounts being toward the center, and further
from the walls the more has entered before them.

The gastric glands are located in the middle and pyloric regions
of the stomach. Such food as is in the fundus is not exposed di-
rectly, therefore, to the action of gastric juice, and so is not very
rapidly acidified. The action of salivary ptyalin, which is brought
to an end when the food becomes acid, may thus continue in the
fundic region for a considerable time after the food is swallowed,
especially in those portions of food which are swallowed late in
the meal.

The movements of the stomach have been watched by means
of the X-rays. Food which has been mixed with bismuth subni-
trate is opaque to these rays and its movements in response to the
movements of the stomach walls can be readily followed. By
this means it has been learned that the walls of the stomach show
peristaltic waves; these begin at about the middle, in a strong
contraction of a ring of circular muscles at that point, and sweep

428 THE HUMAN BODY

to the pylorus. The fundic end is not involved at all in them.
In man they recur regularly, so long as food is in the stomach, at
intervals of about twenty seconds. For a considerable period
after food enters the stomach the pyloric sphincter, which guards
the exit into the small intestine, remains perfectly tight. During
this time the peristaltic waves crowd the food caught by them up
to the pylorus but cannot force any through. As the constriction
approaches the pylorus the food mass in front of it escapes back
through the opening at its center, the waves not being deep enough
to close this entirely, and so the food in the central and pyloric
portions of the stomach is thoroughly churned.

During this churning the food, already semi-liquid from the
mixture with saliva and with such liquid as was taken with the
meal, is mixed with the gastric juice and made still more liquid,
being called at this stage chyme. The effect of the gastric juice is
to give the food an acid reaction, stopping the action of ptyalin
and permitting that of the pepsin which it also pours out upon
the food.

The Control of the Pyloric Sphincter. The way in which the
sphincter of the pylorus is regulated so that after the food has been
thoroughly mixed with gastric juice it opens and allows a small
amount to pass, and then promptly closes to give opportunity for
this to be influenced by the intestinal secretions before more is ad-
mitted, is one of the most interesting adaptations that we know
of in the Body. The mechanism of this action is a special case
of a peculiar reflex which apparently obtains throughout the ali-
mentary canal, and is probably dependent on special properties
of the nerve plexus which is embedded therein. This so-called
myenteric reflex, is of such a sort that a stimulus applied to any
point along the alimentary canal causes a contraction of the
muscles immediately in front of the stimulated point, and a re-
laxation of those immediately behind it. The reflex was worked
out first for the small intestine, and has since been shown to apply
to the other parts of the canal, except perhaps, the esophagus. It
is a so-called "local reflex," as the central nervous system has
nothing whatever to do with it.

The adequate stimulus for arousing the reflex in the pyloric
sphincter is the presence of free hydrochloric acid. When there-
fore the originally alkaline food in the pyloric part of the stomach

MOVEMENTS OF THE ALIMENTARY CANAL 429

has been completely neutralized by the acid of the gastric juice,
and excess acid begins to accumulate the pyloric sphincter is stim-
ulated, but from the stomach side, that is, in front, and accord-
ing to the working of the myenteric reflex a stimulus from that
side produces relaxation. As soon as the sphincter relaxes under
this stimulation that part of the food lying in the pylorus is forced
through into the intestine, but it carries with it the free acid with
which the food is mixed and stimulates the sphincter from the in-
testinal side, namely, from behind, and therefore tends to cause it
to close. A feature of the myenteric reflex is that where, as just
described, a point is simultaneously stimulated from in front and
from behind, the stimulus causing contraction, that from behind, is
dominant. Therefore as soon as food enters the intestine the
sphincter of the pylorus contracts and prevents more from passing.
Before 'it will relax again the acid on its intestinal side must be
neutralized; but this is rapidly done by the strongly alkaline bile
and pancreatic juice, and so as fast as the food in the intestine is
mixed with these juices more is admitted from the stomach.

The fundus of the stomach, which stores the bulk of the food
while that in the pylorus is being thus treated and passed on to the
intestine, is on the stretch all the time, so that as fast as food is
passed out through the pyloric sphincter more is pushed to the
pylorus from the fundus until at last the stomach is wholly
emptied. The tune required for emptying the stomach com-
pletely varies with different foods and under different bodily con-
ditions. An average meal is probably all out of the stomach about
six hours after eating.

Movements of the Small Intestine. The food entering the
small intestine is subjected to two sorts of movements whose
combined effect is to churn it very thoroughly and to move it
slowly along the gut so as to make room for more to come in from
the stomach. The churning is effected mainly by movements of
the intestine known as rhythmic segmentation. In these move-
ments rings of the circular muscle coat about an inch apart con-
strict simultaneously, splitting the contained food into a scries of
segments; an instant later these constrictions disappear, and now
ones, midway between the first, are formed, by which the food is
again segmented, but in a shifted position. These rhythmic move-
ments may recur as often as thirty times a minute. Their effort i.*

430 THE HUMAN BODY

to bring every particle of the contained food into intimate contact
with the intestinal walls, insuring thorough mixing with the in-
testinal secretions, and also favoring absorption. The liquid,
usually milky-looking, food mass in the small intestine is called
chyle.

The onward movement of the food is secured by peristaltic
waves which start at the pylorus and run rather slowly along the
intestine. They are normally gentle movements, which do not
carry the chyle bodily before them, but move it forward little by
little.

The mechanism of these intestinal movements is not entirely
clear, although it is believed that the peristaltic waves, and pos-
sibly also the segmentations, are special manifestations of the
my enteric reflex described above.

Extrinsic Control of Stomach and Intestinal Movements. It
has been shown that normal movements of both stomach and in-
testine may go on in animals in which the nerves leading to these
organs from the central nervous system are cut. To a certain ex-
tent, therefore, they, like the heart, contain within themselves the
essential requirements for normal activity. Like the heart, how-
ever, they are subject to reflex control through the central nervous
system.

The vagus nerves carry sympathetic fibers which when stimu-
lated arouse the stomach and intestine to activity. Stimulation
of the splanchnics inhibits their activity; hence these nerves must
contain inhibitory fibers. Both these sets of nerves are under
reflex control. Violent emotions, as of anger or anxiety, may bring
about reflex inhibition of the stomach and intestinal movements,
with serious impairment of the digestive process.

Movements of the Large Intestine. During the passage of the
chyle through the small intestine the greater part of its nutritive
content is absorbed, but practically none of the water, so that it
is delivered through the ileocolic valve to the large intestine in a
very watery condition. The parts of the large intestine next to
the small intestine, the ascending and transverse colon, show an
interesting movement in the form of an antiperistalsis. This is a
peristaltic wave which begins in the transverse colon and sweeps
toward the ileocolic valve. It would tend to force the material
within the colon back into the small intestine did not the ileocolic

MOVEMENTS OF THE ALIMENTARY CANAL

431

valve prevent. The result of this movement is a churning and
mixing of the contents whereby the absorption of the last useful
materials, including the water, is promoted. As the large in-
testine is filled more and more from the small, some of its contents
are crowded, in spite of the antiperistalsis, into the descending
colon, where regular peristaltic waves carry them on to the sigmoid
flexure and the rectum, whence they are discharged from the Body.

CHAPTER XXIX
THE DIGESTIVE SECRETIONS AND THEIR CONTROL

Organs of Secretion. The simplest form in which a secreting
organ occurs (A, Fig. 137) is that of a flat membrane provided with
a layer of cells, a, on one side (that on which the secretion is poured
out) and with a network of capillary blood-vessels, c, on the other.
The dividing membrane, b, is known as the basement membrane
and is usually made up of flat, closely fitting connective-tissue
corpuscles; supporting it on its deep side is a layer of connective
tissue, d, in which the blood-vessels and lymphatics are supported.
Such simple forms of secreting surfaces are found on the serous
membranes, but are not common; in most cases an extended area
is required to form the necessary amount of secretion, and if this
were attained simply by spreading out plane surfaces, these from
their number and extent would be hard to pack conveniently in the
Body. Accordingly in most cases, the greater area is attained by
folding the secreting surface in various ways so that a large area
can be packed in a small bulk, just as a Chinese lantern when shut
up occupies much less space than when extended, although its
actual surface remains of the same extent. In a few cases the
folding takes the form of protrusions into the cavity of the se-
creting organ as indicated at C, Fig. 137, and found on some
synovial membranes; but much more commonly the surface ex-
tension is attained in another way, the basement membrane, cov-
ered by its epithelium, being pitted in or involuted as at B. Such
a secreting organ is known as a gland.

Forms of Glands. In some cases the surface involutions are uni-
form in diameter, or nearly so, throughout (B, Fig. 137). Such
glands are known as tubular; examples are found in the lining coat
of the stomach (Fig. 130) ; also in the skin (Fig. 141), where they
form the sweat-glands. In other cases the involution swells out at
its deeper end and becomes more or less sacculated; (E) such
glands arc racemose or acinous. The small glands which form the
oily matter poured out on the hairs are of this type. In both kinds

432

DIGESTIVE SECRETIONS AND THEIR CONTROL 433

Fio. 137. — Forms of glands. A, a simple secreting surface; a, its epithelium;
6, basement membrane; c, capillaries; B, a simple tubular gland; C, a secrvtmn
surface increased by protrusions; E, a simple nirrmosr glimd ; 1> and <;. r«>m-
pound tubular glands; /•'. a compound racemose gland. In all hut .1, li. and C
the capillaries are omitted for the sake of clearness. //, half of a highly developed
racemose gland; c, its main duct.

434 THE HUMAN BODY

the lining cells near the deeper end are commonly different in
character from the rest; and around that part of the gland the
blood-vessels form a closer network. These deeper cells form the
true secreting elements of the gland, and the passage, lined with
different cells, leading from them to the surface, and serving merely
to carry off the secretion, is known as the gland-duct. When the
duct is undivided the gland is simple; but when, as is more usual,
it is branched and each branch has a true secreting part at its
end, we get a compound gland, tubular (G) or racemose (F, H)
as the case may be. In such cases the main duct, into which
the rest open, is often of considerable length, so that the se-
cretion is poured out at some distance from the main mass of the
gland.

A fully formed gland, H, thus comes to be a complex structure,
consisting primarily of a duct, c, ductules, dd, and secreting re-
cesses, ee. The ducts and ductules are lined with epithelium which
is merely protective and differs in character from the secreting
epithelium which lines the deepest parts. Surrounding each sub-
division and binding it to its neighbors is the gland stroma formed
of connective tissue, a layer of which also commonly envelops the
whole gland, as its capsule. Usually on looking at the surface of a
large gland it is seen to be separated by partitions of its stroma,
coarser than the rest, into lobes, each of which answers to a main
division of the primary duct; and the lobes are often similarly di-
vided into smaller parts or lobules. In the connective tissue be-
tween the lobes and lobules blood-vessels penetrate, to end in fine
capillary vessels around the terminal recesses. They never pene-
trate the basement membrane. Lymphatics and nerves take a
similar course; there is reason to believe that the nerve-fibers
penetrate the basement membrane and become directly united
with the secreting cells of some glands.

The Secretory Process. The function of glands is to elaborate
and pour out a liquid, the secretion. It is obvious that the ulti-
mate source of the secretion is the blood circulating through the
gland. The digestive secretions, as we have already seen, contain,
in addition to water, and inorganic salts, special chemical sub-
stances, the enzyms, which are different in different glands. It
is easy to believe that the water and salts of the gland, since they
are precisely the same as occur in blood, may be withdrawn from

DIGESTIVE SECRETIONS AND THEIR CONTROL ].',',

the blood through simple physical processes, filtration and dialysis
(Chap. XVII). The special constituents of each secretion, be-
ing different from anything contained in the blood, must, on the
other hand, be produced by chemical processes within the gland
itself. It is easy to show microscopically that the cells of most
glands during rest become filled with small granules, and that
when the gland is active these granules for the most part disap-
pear. We can picture the entire secretory process as occurring
in two stages : the first, a chemical stage, during which the peculiar
constituents of the secretion are elaborated and deposited within
the cells of the gland ; and a second physical stage consisting of a
rapid flow of water with its dissolved salts from the blood through
the gland into its duct, carrying with it the special materials pre-
viously prepared by the gland.

Nervpus Control of the Secretory Process. Considerable evi-
dence has accumulated indicating that gland tissue, like skeletal
muscle tissue, carries on its function only when stimulated to do
so, and that the stimulus is in many glands nervous. It has been
shown for the salivary glands of dogs, for example, that proper
stimulation of certain nerve-fibers leading to them causes them to
produce and store within thejnselves granules, while stimulation
of quite different nerve-fibers causes them to pour out their secre-
tion. The chemical part of secretion is thus controlled by one set
of nerves, often called trophic nerves, and the physical part by
another. It is interesting to note that the nerves which cause the
gland to pour out its secretion usually cause also vasodilatation
within it; an increased flow of blood through the gland therefore
usually accompanies the physical part of secretion. That this in-
creased blood-flow is not the sole cause of the outpouring of the
secretion, as might easily be supposed, is proved by the possibility
under proper conditions of stimulating a gland to pour out its fluid
without any accompanying vasodilatation. We must recognize,
then, that the physical act of secretion is the result of the action of
definite secretory nerves, as distinct from vasodilator nerves. Just
how these function to bring about the more rapid passage of water
and salts through the gland-cell is not clear. As we should expect,
continued stimulation of the secretory fibers leading to a gland,
without accompanying stimulation of the trophic fibers, results
soon in the production of a secretion which is very watery, virtu-

436 THE HUMAN BODY

ally free from the special chemical substances that usually are
present in the secretion.

The efferent nerves to glands belong, without exception, to the
sympathetic system. Glands are, therefore, under reflex control,
and not subject to the will.

Hormone Control of Gland Activity. Some of the digestive
glands, notably the pancreas, appear to be wholly, or at least
chiefly, independent of nervous influences. Their control is vested
in hormones. The details of this method of control will be de-
scribed in connection with the glands themselves. It may be
noted here, however, that in general those glands whose secre-
tions are needed early in the digestive process are under reflex
control, and those whose secretions may not be required for some
time after are under hormone control.

Control of the Salivary Secretion. The salivary glands are sub-
ject to reflex stimulation. We must inquire, therefore, what sen-
sory stimuli may excite the reflex. At least three sorts of stimuli
are effective to this end; mechanical, the presence of dry sub-
stances in the mouth, or merely the rubbing of the tongue against
the palate and jaws; chemical, the presence of sapid substances
upon the tongue; and psychic, the thought of savory food, as
when the mouth "waters." It is an interesting fact that the
character of the saliva varies somewhat with the nature of the
exciting stimulus; mechanical stimulation causes the production
of an abundant but very watery secretion; whereas the chemical
stimulus of food in the mouth calls forth a secretion rich in ptyalin.
By this mechanism the character of the secretion is adapted to
the need which excites it. The mucous lining of the mouth and
throat requires constant moistening. For this a watery saliva is
adequate, and such a saliva is poured out whenever the dryness
of the mouth becomes pronounced enough to act as a stimulus.
When food is taken, on the other hand, the proper functioning of
saliva requires that it be rich in ptyalin. Chemical stimulation,
therefore, excites a secretion containing this substance.

The watering of the mouth at the thought of food is an example
of an emotional reflex through the sympathetic system such as
was discussed earlier (Chap. XII). Inhibition of the salivary
glands, leading to dryness of the mouth, may occur also under
the influence of certain emotions, as in stage fright.

DIGESTIVE SECRETIONS AND THEIR CONTROL 437

The Control of the Gastric Secretion. Our present knowledge
of the mechanism for controlling the secretion of gastric juice is
the result of some of the most interesting investigations of modern
Physiology. Many workers have had a share in the solution of
the problem but the name of one of them, the Russian physiologist
Pawlow (Pavloff), is more closely associated with it than that of
any other one man. Pawlow's chief contribution was the demon-
stration that the secretion of gastric juice is in its early stages ex-
cited reflexly, and by only one particular sort of stimulus, namely,
the psychical state accompanying the eating of food which is
enjoyed. Pawlow gained this information through feeding ex-
periments on dogs which had been prepared in a special way for
the study. The preparation consisted of making a fistulous open-
ing into .the stomach, through which the secretion of gastric juico
could be followed, and of cutting the esophagus in the neck and
bringing the cut ends to the surface in such fashion that all the
food swallowed reappeared at the upper csophageal opening, and
none reached the stomach unless it was placed within the lower
section of the esophagus through its opening. Dogs thus operated
upon recovered promptly and completely and could be studied
very satisfactorily. It was found that one of these dogs would
eat with the greatest enjoyment, although none of the food
reached the stomach, and that within a few minutes of the be-
ginning of eating a secretion of gastric juice began to be poured
into the stomach. That this secretion was excited reflexly was
proved by cutting the vagus nerves, after which it never appeared.
That it depends upon a certain psychical state, and not upon the
mere eating of food was shown in various ways. Dogs which were
not hungry would chew and swallow food, but without signs of
much interest in it; no secretion was evoked. Meat which had
been boiled till it was tasteless was eaten without the production
of a secretion. These results make it clear that the stimulus is a
psychical one, and that it depends upon active enjoyment of food.
Equally important is the observation, made upon these same dogs,
that unfavorable emotional states prevent the secretion of the
juice. If the dog was angered while eating no juice appeared;
even the presence of an attendant for whom he had an aversion
sufficed to prevent the secretion. All these facts, established first
upon dogs, have been proved true likewise for human beings.

438 THE HUMAN BODY

Pawlow's studies showed, moreover, that the psychical secre-
tion is not the only secretion of gastric juice which occurs during
the digestion of a meal. This was proved by the simple observa-
tion that the amount of juice secreted during the eating of a
"fictitious meal" is much less than that produced if the food
eaten enters the stomach. We must look, then, for some other
stimulating agency additional to the psychical one. In such a
search the attention turns naturally to the foods swallowed. Do
they serve as chemical stimuli for the production of the additional
secretion? It has been shown that some foods, milk and water
very slightly, the juices of meat more, do excite the secreting
mechanism somewhat, but the really effective excitant appears
to be something produced during the process of gastric digestion
itself. Thus if the taking of food is attended with pleasure, so that
a psychical secretion is produced, the digestive process is started
and itself furnishes the stimulating agent for the additional se-
cretion needed to complete the digestion. On the other hand,
food eaten under conditions not favorable to the production of a
psychical secretion may fail of digestion completely, through the
absence of all factors which may lead to an outpouring of the juice.

Nature of the Chemical Stimulus to Gastric Secretion. It has
been shown that the substances mentioned in the last paragraph
as chemical excitants of gastric secretion do not stimulate the
glands directly but indirectly through a hormone, gastric secretin.
This hormone is apparently derived from some substance in the
mucous membrane of the pyloric region, which reacts with the
exciting substances derived from the food in such fashion as to
produce the hormone, which is then taken up by the blood and
carried to the gastric glands.

Control of the Pancreatic Secretion. Proper regulation of the
outpouring of pancreatic juice requires that it begin about the
time food begins to pass from the stomach into the small intes-
tine. Since this may occur at a variable time after the eating of
the meal, it would seem to call for a regulating mechanism quite
independent of the act of eating. It has been shown that this
requirement is fulfilled through the action of a hormone which
is produced in active form during the time that food is passing
from the stomach into the small intestine, and only then. The
mucous membrane of the small intestine at its upper end contains

DIGESTIVE SECRETIONS AND THEIR CONTROL 439

a substance which has been named prosecretin. This substance
reacts with hydrochloric acid to form pancreatic secretin, the
hormone for exciting the pancreas to secrete. But this region of
the small intestine comes in contact with hydrochloric acid only
at the moment when a mass of food is entering it from the stomach;
we have previously seen that this passage of food occurs only
when the food is mixed with excess of hydrochloric acid. Thus
the production of the hormone is confined to the time when its
stimulating function is required.

The Control of the Bile Flow. It has been shown recently
that the bile, which, although secreted continuously, is poured
out only when food enters the small intestine, is controlled by the
same hormone, secretin, which excites the flow of pancreatic juice.
Under the stimulation of this hormone the gall bladder contracts,
forcing its contents through the bile-duct into the intestine.

The Control of the Succus Entericus is at present wholly un-
known. Whether it is constantly present in the intestine or
whether its secretion is controlled by a hormone remains to be
determined. It is worth noting, however, that there are probably
not many periods, except during prolonged fasting, when intestinal
digestion is not going on, so that a continuous secretion of intes-
tinal juice would be less wasteful than of the other digestive juices.

Digestive History of a Meal. We can summarize the whole
process of digestion as well, perhaps, by following the course of
an ordinary meal through the digestive tract as in any other way.
We shall disregard the accessories, and consider only the nutrients
proper, since, as we have seen, the digestive process concerns it-
self with these alone. The meal, then, is a mixture of carbo-
hydrates, proteins, albuminoids, and fats.

In the mouth the food is reduced to a semi-liquid alkaline mass,
containing no large particles, by the combined action of chewing
and mixing with the saliva. The salivary glands are reflexly
excited to secrete their juice by the presence of the food in the
mouth. The enzym of saliva, ptyalin, begins its digestive action
on the starch, converting it to maltose. By the act of deglutition
the food, when sufficiently mixed with saliva, is passed on to the
stomach. If the chewing and swallowing of the food is attended
with agreeable emotions, there is aroused a reflex secretion of
gastric juice; the so-called "psychical" secretion.

440 THE HUMAN BODY

The food enters the stomach in very much the same condition
chemically as when taken into the mouth; a small amount of
maltose added to it through the action of salivary ptyalin, and a
correspondingly diminished amount of starch, being the only
differences. That part of the food which is crowded down into
the pyloric region begins at once to be churned by the peristaltic
waves which sweep over that region; by the churning it is mixed
with gastric juice. The food which remains in the fundic end of
the stomach does not come into contact with the gastric juice;
its reaction, therefore, continues alkaline, and the splitting of
starch by pytalin goes on uninterruptedly. In the portion of
food (chyme) which becomes impregnated with gastric juice there
is an acid reaction and the changes which the gastric enzyms,
pepsin and rennin, are capable of producing take place. Rennin
clots any milk that may be present; pepsin attacks albuminoids
and proteins, converting them into proteoses and peptones. Any
fats present are liquefied, not by enzyms but by the stomach
warmth. Some of the substances produced during this peptic
digestion react with other substances in the mucosa of the pyloric
region, forming a hormone, gastric secretin. This hormone is
taken up by the blood, passes in the blood-stream to the gastric
glands, and stimulates them to further outpouring of juice; thus
enough for the whole meal is secured. Finally as the hydrochloric
acid of the gastric juice accumulates in excess the pyloric sphincter
is stimulated to relax; the mass of chyme next to it is pushed
through; and more material from the fundic end comes down to
fill its place. Too much chyme is prevented from passing the
sphincter at once by the powerful stimulus to contraction which is
exerted on the sphincter by the acid chyme in contact with the
upper intestine. The acid of this same chyme reacts with the prose-
cretin of the intestinal mucosa to form secretin, a hormone which
is carried by the blood to the pancreas and excites it to activity.

The chyme which enters the intestine contains some, at least, of
all the food-stuffs originally making up the meal, and in addition
maltose, proteose, and peptone. The strongly alkaline bile and
pancreatic juice quickly neutralize its acid and the various en-
zyms of the intestinal tract act upon it. The amylopsin of the
pancreatic juice converts to maltose all starch not affected by
ptyalin; the lipase of the same secretion splits the fats to fatty

DIGESTIVE SECRETIONS AND THEIR CONTROL 441

acid and glycerin; the trypsin of pancreatic juice, in cooperation
with erepsin of the succus entericus reduces all proteins, includ-
ing proteoses and peptones, to amino acids; the inverting enzyms,
maltase, invertase, and lactase, change all the double sugars, and
therefore all the carbohydrates of the meal, to single sugars. The
intestinal contents (chyle) are churned and kept in onward
progress by movements of segmentation and peristalsis performed
by the muscular walls of the gut.

The Maintenance of Good Digestion. In the preceding para-
graph the various activities essential to the proper performance
of the digestive function have been outlined. If they are reviewed
carefully it will be seen that most of them, after the food reaches
the stomach, are affected, directly or indirectly, by the conditions
upon which depend the proper production of the psychical secre-
tion of gastric juice. If, through anxiety or anger at meal-time,
this secretion is inhibited, the whole sequence of the digestive
process is upset. Without a psychic secretion little or no chemical
secretion of gastric juice will appear; there is therefore not the
necessary hydrochloric acid to stimulate the pyloric sphincter to
relax, nor to react with prosecretin to form pancreatic secretin,
should any food by any means get through into the intestine.
Moreover, the same conditions which inhibit the psychical secre-
tion inhibit also, as stated previously, the motions of the stomach
and intestines. That indigestion usually follows the eating of
meals under unfavorable emotional conditions is well known to all;
the reason for it we have just seen. Of as great importance,
though not so generally recognized, is that the psychical secretion,
and hence good digestion, depends upon an active emotional state
of enjoyment of the meal. Preoccupation, allowing the mind to
dwell upon business or household cares, may interfere with the di-
gestive processes only less seriously than worry or angry discussion.

The value of soups in aiding digestion is twofold. By exciting
the appetite they help to arouse the psychical secretion ; their con-
tent of meat juice is itself in some measure an excitant of the
hormone to chemical gastric secretion, thus they are usually effect-
ive in starting the chain of events which make up the digestion of
a meal. The practice of using them at the beginning rather than
elsewhere in the meal, although long antedating our knowledge of
their real value is thus seen to be physiologically sound.

CHAPTER XXX
THE ABSORPTION AND USE OF FOODS

General Statement. The digestive process, as we have con-
sidered it in preceding chapters, is purely one of preparation. Its
completion finds the food still within the alimentary tract, but
ready for the use of the Body. It is conveyed to the tissues, as we
have seen (Chap. XVII), by the blood. The passage of digested
food from the alimentary tract, through its walls, into the blood
or lymph, is known as absorption. The use of the food by the
tissues, since it involves chemical activities on the part of the
tissues themselves, is spoken of as metabolism. The discussion of
these two processes is the purpose of the present chapter.

Absorption from the Stomach. Although the food remains in
the stomach for several hours after each meal, in fact is often not
wholly discharged before the taking of another one, it appears that
absorption from the stomach into the blood normally occurs to a
very limited degree, if at all. The fact that the digestive process
is for no foods completed in the stomach affords sufficient reason
why absorption should not take place there. We might suppose
that the single great group of food-stuffs not requiring digestion,
the single sugars, could advantageously be absorbed from the
stomach, but experiment shows that even they are absorbed very
slightly unless in rather high concentration, 5 per cent, in which
case the walls of the stomach do take them up rather rapidly. The
presence of alcohol in the stomach is said to increase markedly its
absorptive power, but this is at best a doubtful benefit, since the
single sugars form ordinarily a minor part of the meal, and the
other food-stuffs are not ready for the use of the Body, and are
wasted, therefore, if they arc absorbed.

Absorption in the Small Intestine. The small intestine, being
the chief and final digestive laboratory of the Body, is naturally
the place from which absorption most largely goes on. It is, in
fact, specially adapted structurally, as is no other region of the

442

THE ABSORPTION AND USE OF FOODS 443

alimentary tract, for the absorptive processes. The innumerable
projecting villi, each containing a capillary network and a lymph-
channel, afford a total absorbing surface many times greater than
would the same area if lined with ordinary mucous membrane;
they also, by projecting into the intestinal cavity, are brought
more readily into intimate contact with the intestinal contents.

Nature of the Absorptive Process. There is very good reason
to believe that the process of absorption is not a simple physical
one, involving only filtration, osmosis, and dialysis, but that it is
carried on actively by the living cells which form the innermost
intestinal lining, the columnar epithelium (Chap. XXVI). The
support for this idea is partly experimental: the observation that
blood-serum placed in the intestine is absorbed completely through
its wall's into the blood, a phenomenon inexplicable on physical
grounds; and partly based on clear indications that none of the
digested foods, except the single sugars, pass through these cells
without undergoing chemical modification during the passage, so
that they enter the blood as different compounds than as they left
the intestine.

The entire phenomenon of 'absorption from the small intestine
presents so many phases that it will be convenient to consider it in
sections, one class of nutrients at a time.

The Absorption and Temporary Storage of Carbohydrates.
Carbohydrate digestion reduces all foods of the class to single
sugars. It is in this form, then, that they undergo absorption.
However the process may be carried on it results in a flow of single
sugars from the intestinal cavity into the blood-capillaries of the
villi. These capillaries all drain, as previously stated (Chap. XIX),
into the portal vein, which in turn passes to the liver and breaks up
therein into the liver-capillaries (Chap. XXVI) ; so that all blood
from the intestine, with whatever it may have taken up there, is
forced to traverse the liver, and to come into intimate contact with
the liver-cells, before it reaches any of the other living tissues of
the Body.

The amount of sugar present in the blood of the portal vein is, of
course, variable, there being a higher concentration at times when
sugar is being actively absorbed from the intestine than at other
times. Curiously, the blood flowing away from the liver, in the
hepatic vein, is always found, normally, to contain a certain small

444 THE HUMAN BODY

percentage, about 0.15 per cent, of sugar, whether the sugar con-
tent of the portal vein is high or low.

It is evident that the liver must be able to store within itself the
excess sugar that comes to it during active absorption from the
intestine, and to give this out again between times. The sugar
is retained in the liver, not as such, but in the form of glycogen or
animal starch. The conversion of sugar into glycogen is a simple
dehydration (C6H12O6— H20=C6H10O5), and is doubtless easily ef-
fected by the liver-cells. The purpose of the change from sugar
to starch seems to be to make the retention by the liver easier;
sugar is too soluble to be held readily, whereas the liver can hold
the glycogen without trouble. The liver is said to be able to hold
10 per cent of its weight of glycogen.

The use of the sugar is, as we have already seen, for fuel for the
Body. Oxidations are constantly going on in the living-tissues,
therefore there is a steady withdrawal of sugar from the blood,
and the liver must be continually making good the depletion by
reconverting some of its glycogen into sugar. That the sugar
content of the blood is kept up at the expense 'of liver-glycogen is
proven by observations on fasting animals. A comparatively
short period of starvation results in the complete disappearance
of glycogen from the liver. That in fact is the first fuel supply to
be drawn upon in the absence of food.

Just how the chemical process of converting glycogen to sugar
is performed is not certain ; although an enzym capable of effect-
ing the transformation is said to be present in the liver. If the
process is carried on by an enzym it is under closer control than
the enzym reactions we have studied in connection with diges-
tion, for it does not go on rapidly till all the glycogen is used up,
but only so fast as is necessary to make good the loss of sugar from
the blood.

Storage of Glycogen in the Muscles. These organs, as we learned
when studying them (Chap. VII), perform their work through the
oxidation of sugar, and since they are likely to be called upon for
prolonged activity need to have immediately available a supply of
their special fuel. Such a supply they have, in the form of glyco-
gen, which makes up about 1 per cent of the weight of muscle
tissue. This glycogen is, of course, derived from the sugar of the
blood, so that the muscle-cells must have the same power that

THE ABSORPTION AND USE OF FOODS 445

liver-cells have of changing sugar to glycogen and glycogen back
to sugar. Whether the muscles are able to use the sugar of the
blood directly or whether they always convert it into glycogen
first is not certainly known.

The Relation of the Kidney to the Concentration of Sugar in the
Blood. As we have seen the sugar content of the blood remains
practically constant all the time at a relatively low concentration,
about 0.15 per cent. It is an interesting fact that the kidney, the
great excretory organ of the Body, is so constructed that if for any
reason the sugar content of the blood rises much above normal, to
0.2 per cent or more, the excess of sugar is withdrawn from the
blood by the kidney and appears in the urine. The kidney stands
to the sugar of the blood in the relation of a spillway; it allows the
concentration to rise just so high, but no higher. This property
of the kidney makes such a storage mechanism for sugar as we
have described virtually necessary to the Body, since without it
the tissues could not be provided with fuel at once continuously
and economically.

The Assimilation Limit. Alimentary Glycosuria. The ability
of the liver to convert into glycogen the sugar delivered to it by
the portal vein is not without limit. If the absorption from the
intestine is so rapid as to raise the sugar content of the portal
blood to an abnormally high point, the liver is not able to handle
all the sugar; and the excess escapes into the hepatic vein and so
into the general circulation. Should this excess be sufficient to
raise the sugar percentage of the blood above 0.2 per cent there is
excretion of sugar from the kidney, a condition known as gly-
cosuria. It is found that the rate of absorption of sugar depends
chiefly on how much of it is present at one time in absorbable
form in the intestine. Thus if large amounts of single sugar are
eaten the essential condition for excessive absorption is likely to
be fulfilled. Honey, a sweet containing considerable single sugar,
is thus apt to cause glycosuria if too freely eaten. The greatest
amount that can be eaten without causing glycosuria marks the
assimilation limit. The other carbohydrates, since they require
digestion before they are absorbed, are less apt to give rise to too
rapid absorption. It is found, however, that there is a great dif-
ference in the rate of digestion of the different carbohydrates, and
a corresponding difference in the amounts that can be taken with-

446 THE HUMAN BODY

out exceeding the assimilation limit. Milk -sugar is split very
rapidly by its enzym, lactase, and its assimilation limit is corre-
spondingly low. Starch is digested so slowly that the assimila-
tion limit for it is quite difficult to exceed. Glycosuria resulting,
not from disease, but merely from overconsumption of carbo-
hydrates, is called alimentary glycosuria.

Diabetes. When there is excretion of sugar from the kidney
because of disease, the condition is called diabetes. An analysis
of the carbohydrate-storage mechanism just described reveals
three points where an upset of the normal sequence might give
rise to glycosuria; and three corresponding varieties of diabetes
are known. The three conditions which may cause glycosuria are:
(1) A disturbance of the mechanism which controls the rate of
conversion of liver-glycogen into sugar, so that more is poured
into the blood than the tissues are able to use; (2) a diminution
in the consumption of sugar by the tissues, so that more accumu-
lates than the liver can store ; (3) an alteration of the kidney such
that it excretes all the sugar that comes to it, and thus drains
sugar from the blood continuously. Much insight into the work-
ing of the carbohydrate-storing mechanism, as well as the use of
carbohydrates by the Body, has been gained by study of these
three forms of diabetes.

Diabetes from Disturbance of the Liver Function. It has been
shown that injury to a definite point in the medulla destroys the
coordination between the output of sugar from the liver and the
use of sugar by the tissues, with resulting glycosuria. This sug-
gests, of course, that the liver carries on its function of storing
and delivering sugar under the control of a reflex "center." Such
a method of control seems reasonable inasmuch as increased ac-
tivity of the tissues involves increased consumption of sugar,
with a greater call upon the liver for supplies, and, as we know,
the tissues most involved, the muscles, send into the medulla
streams of afferent impulses whenever they are active, which
would serve to excite the center. In corroboration of this idea it
may be stated that certain diseases of the central nervous system
in man result in an upset of the liver function of precisely this
sort. It must be admitted, however, that more evidence is re-
quired before the reflex control of the liver's storage function can
be said to be wholly demonstrated.

THE ABSORPTION AND USE OF FOODS 447

Diabetes from Inability of the Tissues to Use Sugar. Diabetes
Mellitus. This condition, the usual form of diabetes, and un-
fortunately not of rare occurrence, has been much studied, chiefly
because it involves the relation of the tissues to their chief fuel
supply, sugar, and a complete understanding of the disease should
throw much light on the mechanism of the consumption of fuel by
them. The presence of sugar in the urine is only one of the symp-
toms of diabetes mellitus. A symptom of equal importance is the
muscular weakness, and particularly the lack of endurance, which
results from the failure of the tissues to make use of their fuel
supply to advantage.

A very interesting feature of this condition is that it can be in-
duced experimentally in a quite unexpected way, namely, by in-
juring or removing the pancreas. Complete destruction of this
organ is followed by an apparent total loss of the power of the
tissues to use sugar; there is excessive muscular weakness, and
death occurs in a few days after the operation. The effects of
partial destruction are less severe; in fact no symptoms appear
unless fully three-fourths of the gland are destroyed. The func-
tion of the gland in connection with the prevention of diabetes is
wholly independent of its function as a digestive gland. The
duct of the pancreas may be tied without the production of dia-
betes, or the gland may be transplanted from its usual location
to some other, quite abnormal one, where, if it lives and estab-
lishes connections with the circulation, it suffices to prevent
diabetes perfectly.

The interpretation of this function of the pancreas is that it is
a hormone action. The gland produces the hormone, and this,
when carried by the blood to the tissues, in some way enables
them to use sugar; perhaps by activating some tissue enzym or
enzyms upon which the oxidation of sugar depends. It is not
thought that the ordinary secreting cells of the pancreas produce
the hormone, but that certain peculiar groups of cells embedded
in the gland, the Islands of Langerhans, have this function. Al-
though not all physiologists agree in assigning the production of
the hormone to the Islands of Langerhans, the general trend of
opinion seems to be that that is their function.

Diabetes mellitus in man not only shows symptoms agreeing
precisely with those seen in animals with injuries to the pancreas,

448 THE HUMAN BODY

but many cases show on autopsy very well marked lesions of the
Islands of Langerhans. We may thus conclude with fair cer-
tainty that the disease is one affecting these Islands, and that its
symptoms are the result of more or less complete failure of the
hormone formed by them.

Diabetes from Increased Permeability of the Kidney-Cells to
Sugar. The injection of a certain drug, phlorhizin, into the cir-
culation is followed by a glycosuria which is due to alterations in
the kidney. These are of such a sort that the kidney-cells, instead
of removing only sugar in excess of 0.2 per cent, take all that
comes to them. The result, of course, is a great waste of this
valuable fuel, requiring greatly increased consumption of carbo-
hydrates to make it good. This form of diabetes has been pro-
duced experimentally in animals, for purposes of study, but
occurs rarely, if at all, as a disease of man.

The Absorption of Proteins. The nature of protein absorption
has been until recently one of the great unsolved problems of
physiology. Even at present we know much less about it than
about most other physiological problems of equal importance; but
within recent years physiologists have succeeded in putting to-
gether the various facts that have been gathered on the subject,
and have based on them a very interesting theory of protein ab-
sorption, which seems very likely to be proved true.

The great difficulty which has always confronted those who
have tried to analyze the process has been that no matter how
much protein was being absorbed from the alimentary tract, it
was never possible to demonstrate any perceptible increase in the
protein content of the blood in the portal vein or of the lymph in
the lacteals. In other words, as soon as the protein passed through
the walls of the alimentary canal it disappeared. When it became
known that proteins are split in digestion into amino acids there
were many attempts made to find these in the blood or lymph
draining away from the gut, but without success. We know, of
course, that the proteins must be absorbed somehow, and must be
taken up either by blood-capillaries or lacteals. The latter all
drain into the thoracic duct, and it is easy to open this in such
fashion as to secure every drop of lymph that comes from the
intestine. Most careful analyses of lymph thus secured have
failed to show any change in the protein content of lymph during

THE ABSORPTION AND USE OF FOODS 449

the absorption of a meal, or any increase of nitrogen in it, such
as would occur if protein derivatives were absorbed. We are
therefore compelled to believe that the portal blood, which
cannot all be collected and analyzed, since such a procedure
would bleed the animal to death, and which can, therefore, only
be examined in occasional samples, must receive the absorbed
protein.

The modern ideas as to the way in which the proteins are ab-
sorbed have arisen chiefly as the result of a somewhat changed
conception of the use of protein in the Body. It will be recalled
that protein as a food is supposed to have a twofold function: as
a repairer of the waste of living tissues, and as part of the fuel
supply of the Body (Chap. XXV).

It was formerly taken for granted that the wear and tear on
the living cells is so great as to require the bulk, if not the whole,
of the ordinary protein intake for the restoration of the broken-
down tissues. Of recent years, however, physiologists have come
more and more to believe that the cells work with comparatively
little injury to themselves, and that therefore only a small part of
the usual protein intake is required for tissue repair; the rest
serving for fuel.

The fuel value of protein lies, not in its nitrogen content, but
in its carbon and hydrogen, since these are the elements which
oxidize readily with evolution of heat. It is therefore quite un-
necessary that the protein that is to serve as fuel be present in the
tissues as protein, having its elements combined into the complex
structure of the protein molecule; all that is necessary is to have
its carbon and hydrogen in available form for ready oxidation.
For tissue repair, however, it seems to be essential that the com-
plex protein molecule be preserved.

The theory of protein absorption that we shall consider here
accounts for our inability to demonstrate the presence of ab-
sorbed protein in the portal blood on the basis of the possible
chemical alteration in the fuel protein suggested above. Accord-
ing to this theory the amino acids which are in the intestine as
the end products of protein digestion, all undergo chemical al-
teration during their passage through the walls of the intestine;
those that are destined for tissue repair being rebuilt into the
blood proteins; and those that are to serve for fuel being so split

450 THE HUMAN BODY

as to set their nitrogen-containing parts off from their more readily
oxidizable carbon and hydrogen-containing portions. We may
readily suppose, from what has been assumed as to the small
actual protein requirement of the tissues, that the amount of
protein absorbed as such into the portal vein at any one time is
too small to be detected, in the presence of the normally rather
high protein content of blood. If, however, it be true that the
protein which is destined to serve as fuel is split in such fashion
as to separate the nitrogenous from the non-nitrogenous portion
we ought to be able to find in the portal blood some of these split
products. Very fortunately for the theory this we are able to do.
It has been shown that the blood of the portal vein contains am-
monia compounds which have been absorbed from the intestine.
That proteins can be so split as to yield a large part of their nitro-
gen as ammonia has long been known, so that this observation
accords perfectly with the theory. Special non-nitrogenous deriv-
atives of protein have not been demonstrated in portal blood, but
it is assumed that they are so similar to the carbohydrates in it
as to be indistinguishable as separate substances; and that they
undergo conversion into glycogen in the liver in common with
the carbohydrates.

The Absorption of Fats. The result of fat digestion is to split
the fats to fatty acid and glycerin. It is believed that they are
taken up by the cells of the intestinal lining partly in this form;
but not wholly so, since free fatty acid in the presence of free
alkali, such as is furnished by the bile and pancreatic juice, reacts
with the alkali to form soap. That there is in the small intestine
a certain amount of soap formation cannot be doubted. The ad-
vantage of soap formation is one of increased solubility; fatty
acids arc insoluble in water, soap quite soluble. There is reason
to believe, however, that only part of the fatty acid is combined
into soap, and that the remainder is absorbed, as stated above,
as fatty acid. This direct fatty acid absorption seems to be ef-
fected largely through the agency of the bile. It is known that
fatty acids are soluble in bile, and can thus be brought in solution
into contact with the absorbing cells; and a very common observa-
tion of physicians is that stoppage of the flow of bile into the in-
testine, as by occlusion of the bile-duct, is followed by an almost
complete failure of fat absorption. The glycerin part of the de-

THE ABSORPTION AND USE OF FOODS 451

composed fat is quite soluble in water and is doubtless absorbed
readily.

After the absorbing cells of the intestinal wall have taken up
the fatty acid and glycerin, these are recombined within the cells
into fat. The presence of fat droplets in the absorbing cells can be
demonstrated microscopically. We know that the fat droplets are
not absorbed as such, but arc formed after their constituents have
been separately taken up, because these fat droplets are always
observed in the part of the cells away from the intestinal cavity,
and never in the part next to it; also because we know that the
digestive splitting to acid and glycerin takes place, a meaning-
less process if not necessary to absorption.

The fat finds its way into the circulation by way of the lymph-
channels of the villi, the lacteals, and the thoracic duct, entering
the blood stream at the point of emptying of the thoracic duct in
the large vein of the shoulder. The fats alone, of all the food-
stuffs, take this course, and we may suppose the difference to
mean that the liver has no special function to carry out in con-
nection with the fats as it has for carbohydrates and proteins.
Therefore the fats arc shunted into another course which carries
them into the blood stream without having first to traverse the
liver.

Absorption from the Large Intestine. The chyle that passes
through the ileocolic valve into the large intestine contains com-
paratively little absorbable food material. The carbohydrates and
fats are very completely removed during the passage of the small
intestine, and fully ninety per cent of the proteins as well. There
remains for the large intestine, then, only the absorption of the
protein residue and the absorption of water. It is probable that
this latter function, that of absorbing water, is in reality the chief
one possessed by the large intestine. There is virtually no ab-
sorption of water in the small intestine ; the chyle passes the ileo-
colic valve as liquid as it is when leaving the stomach. This
maintenance of a liquid consistency is, of course, essential to the
absorptive processes, and it is only after all absorbable food has
been removed that the water, which is also needed by the Body,
is taken up.

The Food Requirement of the Body. If we know how much
energy the Body liberates in a day, and how much tissue break-

452 THE HUMAN BODY

down it suffers, we ought to be able to estimate how much energy-
yielding food, and how much tissue-repair food is required daily;
assuming, of course, that we know the amount of energy yielded
by definite weights of food-stuffs. By the use of devices called
calorimeters the total energy liberation of the Body per day has
been determined under various conditions, and the energy content
of the various foods has also been found. It is usual to express
these energy values in terms of the heat energy they represent,
since oxidation processes lie at the basis of the Body's activities.
The unit of heat energy commonly used in physiology is the
Calory; the amount of heat required to raise 1,000 grams of water
through 1° centigrade. In terms of this unit the energy output
of man in 24 hours averages from about 2,400 Calories for men of
sedentary occupation to 5,000 Calories for those doing heavy
manual labor. The energy yield of the various foods is as follows :

Carbohydrates 4 . 1 C. per gram.

Proteins 4 . 1 C. " "

Fats 9.3C. " "

It is therefore a matter of simple calculation to determine how
much of any one food-stuff is needed to supply the required en-
ergy, or to arrange suitable mixtures of the three. By reference
to the table of food compositions (Chap. XXV), the amounts of
actual food materials needed can be found.

The Protein Requirement of the Body. We have next to con-
sider the tissue-maintenance requirement, which as we have seen,
is wholly a protein need. We do not know just how much protein
is used every twenty-four hours for repair of the tissues. It is
thought, however, that it is not large, probably less than twenty
grams per day. The reason why the exact requirement cannot be
determined is that the Body seems to demand that some of its
fuel be furnished as protein, and unless enough protein is eaten
to supply this fuel demand in addition to the tissue-repair demand
the Body breaks down its own tissues until it gets enough protein
for the purpose. The evidence that there is this fuel demand
which can only be satisfied by protein will be presented in a future
paragraph (p. 458). The reason for its existence is quite inex-
plicable, since it seems almost certain that the protein substance

THE ABSORPTION AND USE OF FOODS 453

actually used for fuel loses its nitrogen and becomes indistinguish-
able from carbohydrate long before it is used. It has been sug-
gested that the animal world has always had more protein in its
diet than was necessary for tissue repair, and so has developed
the rather complex machinery for making use of the surplus as
fuel; till now the mechanism works so well that if the food does
not contain protein upon which it can expend itself it performs its
function at the expense of the body tissues themselves. This
idea, while interesting, cannot be proven, and must not, therefore,
be viewed as having much weight.

Although physiologists are virtually agreed that the minimum
protein requirement of the Body is in excess of the tissue-repair
requirement, they are not agreed as to its actual amount. The
earlier physiologists and many modern ones place it between
100 and 120 grams per day. Others consider those figures exces-
sive, and place the requirement lower, the extreme low limit
being placed at 30 to 40 grams per day. In view of these di-
verging opinions we may well inquire what is the amount of pro-
tein ordinarily taken per day by civilized man. A large scries of
estimations made some years .ago in Europe gave the average
daily protein consumption as 118 grams. Some recent estima-
tions by American college students indicate that in this country
the average is somewhat lower, probably about 90 grams. That
all the actual protein needs of the Body can be satisfied with 30 or
40 grams daily intake has been demonstrated by several observ-
ers. Whether it is therefore desirable to reduce the protein to
that amount is quite another question, and one which is at present
under active discussion. In the absence of more information on
the subject the safe practical course is probably to continue our
present habits so far as amounts of the various foods to be eaten
are concerned, but bearing in mind that moderation in the use
of proteins is certainly not harmful and may be distinctly ad-
vantageous.

It must be noted that in estimating the amount of food to be
taken to satisfy the energy requirements of the Body it is per-
missible to compute all the protein as though it served as fuel,
disregarding the fact that part of it is used for tissue repair. This
can be done because although the work of restoring the worn-out
tissues is part of the total work of the Body, and the energy re-

454 THE HUMAN BODY

quired for it is included as part of the Body's energy liberation,
this energy is probably furnished in part, at least, by the tissue-
repair protein itself.

The Liberation of Energy in the Body. We have seen that all
the energy liberated by the Body can be expressed in terms of heat-
units, but it is not to be concluded, therefore, that heat energy is
the only form manifested by the Body. As a matter of fact the
Body undoubtedly converts the potential energy of the food into
at least three forms of kinetic energy ; chemical, the carrying on of
the digestive and other chemical processes of the Body; mechanical,
the working of the skeletal muscles, as well as of the heart, the
muscles of respiration, and the muscles of the viscera; and thermal,
the direct production of heat by oxidation processes. This latter
form of energy, although far exceeding in amount both the others
together, may be looked upon as in large degree a by-product of
the mechanical work of the Body, and arising through the ineffi-
ciency of the body machinery. We know that most of the heat of
the Body is produced in the muscles, and that though these are
producing some heat even when at rest, they produce enormously
more when they are active. A characteristic of all machines is
that they work more or less wastefully ; not all the energy imparted
to them appears again as useful work; the part that is lost, more-
over, appears always as heat. In the Body there is this same in-
ability to convert food energy into mechanical energy without
there being at the same time a large heat production. This is
brought out strikingly in calorimetric experiments with resting
and working men. It is found in such experiments that a man
making only ordinary movements gives out in a day energy
equivalent to 2,400 Calories, of which at least 2,000 represent
probably direct heat production by oxidation within the Body.
If the same man does in a day additional muscular work equivalent
to 600 Calories, which by the way is about all a man can do in one
day, being equivalent to 250,000 kilogrammcters (1,825,000 foot-
pounds), his direct heat production jumps from 2,000 to 4,800
Calories. In other words, the machine in converting the energy
of 600 Calories of food into mechanical work is obliged to oxidize
nearly 3,000 Calories additional.

We shall see in the chapter on Heat Regulation (Chap. XXXII)
that the Body makes very good use of this by-product of

THE ABSORPTION AND USE OF FOODS 455

heat in keeping itself at a proper temperature the year round,
and so the extra amounts of food we have to eat on account of
the inefficiency of our bodily machines are not wholly wasted
after all.

The Relative Food Values of Proteins, Carbohydrates and Fats.
Disregarding the use of protein as a tissue-repairer, and consider-
ing all three varieties of foods simply as furnishers of energy, we
may inquire whether any one of them is superior to the others, or
whether any particular proportion of the three food-stuffs is
specially desirable. From the purely mechanical standpoint there
is evidently no choice among them; the Body requires 2,400 or
more Calories' of energy each day; each food-stuff yields definite
amounts of energy; therefore all we have to do to supply the Body's
requirement is to eat enough grams of one or the other food-stuff,
or of a mixture of them. The answer to the question goes back,
then, to other considerations than that of the energy content of the
foods. The first of these is the matter of relative digestibility and
absorbability ; it is of little avail to eat a food if it fails to be prop-
erly digested and absorbed. Experiments have shown that carbo-
hydrates, exclusive, of course, of cellulose, are the most completely
absorbed of all foods, 97 per cent of the amount eaten finding its
way into the Body; fats come next in order, 94.4 per cent being
absorbed; proteins are taken up least completely of all, the Body
getting only 92.6 per cent of the protein eaten. There are also dif-
ferences of digestibility and absorbability of different foods within
the same class; the protein of lean meat, for example, being more
readily digested and absorbed than that of beans and peas. Cheese,
which contains the highest per cent of protein of any common food,
has a reputation, perhaps undeserved, for indigestibility. Graham
bread is, by many, supposed to be more nutritious than white.
It is true that graham flour contains a higher percentage of protein
than does white flour, but the extra protein of the graham flour is
in the bran, whence the human digestive process fails to extract it;
so as a matter of fact white bread yields more actual nourishment
to the Body than docs graham. Some fats are much more di-
gestible than others; olive oil and pork fat, for example, are more
completely utilized by the Body than is mutton fat. Fat of any
sort, taken in the meal with other foods, seems for some reason to
delay the whole digestive process, and the delay is greater the

456 THE HUMAN BODY

more fat is present. For this reason it is desirable to limit some-
what the amount of fat used.

Another question which may affect the choice of foods is the
degree to which they tax the excretory organs of the Body. We
have seen that fuel proteins yield an ammonia residue in absorp-
tion, which must be gotten rid of by the excretory organs. There
seems to be a rather general belief that this task constitutes a
somewhat serious strain upon these organs, and if it does tend to
throw upon them excessive labor it is clear that the consumption
of proteins ought on this account to be kept as low as possible.
The idea that the excretory organs are endangered by ordinary
amounts of protein in the diet is not sustained by any very con-
vincing evidence, although that excessive protein consumption
may be harmful is quite well established.

In the matter of cost, which must also be taken into considera-
tion, carbohydrates have a marked advantage over the other food-
stuffs. For example, bread, which is chiefly carbohydrate, yields,
dollar for dollar, about ten times as many Calories as lean beef, a
protein. The cheapest proteins are the vegetable ones; a given
weight of protein costing about five times as much when bought
as beef as when purchased in the form of beans.

The final factor to be taken into account is the appetizing
quality of the different foods. The dependence of the whole di-
gestive process upon a. proper initial psychic secretion of gastric
juice emphasizes the importance of the use of appetizing foods.
Boiled meat contains as much nourishment as the same weight
of roasted meat, but the former is less desirable as a food because
the process of boiling extracts from it the substances which impart
to meat its flavor. Eggs are exceedingly nutritious, but to some
people they are practically valueless as food, because they inspire
aversion rather than appetite.

From these various considerations we may summarize the gen-
eral rule that the choice of food should be such as to yield sufficient
protein for the Body's protein requirement, without containing an
amount so excessive as to throw an undue burden on the excretory
organs; that the amount of fat should be somewhat limited; and
that enough carbohydrate should be added to bring the sum total
up to the Body's energy requirement; finally, that the most ap-
petizing foods obtainable within a reasonable limit of cost should

THE ABSORPTION AND USE OF FOODS 457

be selected. Fortunately for the well-being of the race, mankind
has always selected just such a diet under no other guidance than
his appetite and his means, and these, to a healthy person, make
trustworthy guides, so long as they arc accompanied by temper-
ance as a third.

The importance of dietetics as a science is chiefly in connection
with the feeding of the sick, or providing for the maintenance of
large numbers of individuals, as in armies or public institutions,
where a slight error in selecting food, in greater amounts, or at
greater cost than needed, amounts in the aggregate to a very large
waste.

The Nutritive Value of Albuminoids. These proteins lack some
of the essential constituents of cell proteins, and cannot, therefore,
serve as tissue-restorers. We can imagine, however, that they
ought to satisfy the Body's demand for protein fuel, and so be
substituted for the major part of the protein of the diet. Various
attempts have been made to substitute gelatin for proteins in this
way, and it seems to be highly efficacious in satisfying the Body's
protein-fuel demand. But curiously gelatin can be used thus for
only a few meals; presently there is a revolt of the appetite against
it and no more can be eaten. Experiments have shown that dogs
will starve rather than take continuously a diet whose chief con-
stituent is gelatin.

The Maintenance of Constant Weight. It is the experience of
most adults that during periods of unbroken health the body
weight remains practically unchanged day in and day out. It is
clear that this condition depends on the maintenance of an exact
balance between the intake and outgo of the Body, since if more
is taken in than is given out there must be a gain in weight, and
vice versa. It is customary to consider the question of weight
maintenance under three heads: water equilibrium, nitrogen equilib-
rium, and carbon equilibrium.

Water Equilibrium. For a Body to be in water equilibrium
the amount of water lost per day must be exactly replaced by
the amount drank. In large measure the sudden and transient
changes of weight which occur are due to upsets of water equi-
librium. Any violent exercise in hot weather reduces the weight
by inducing a profuse perspiration with resulting loss of
water. The intense thirst which follows the exercise leads to

458 THE HUMAN BODY

abundant ingestion of water and a speedy restoration of the lost
weight.

Nitrogen Equilibrium. Those metabolic activities of living
tissues which result in tissue breakdown are particularly associ-
ated with the use of protein foods, since, as we have seen, their
repair can be accomplished only by proteins. The characteristic
constituent of protein is nitrogen; and the simplest way to esti-
mate the amount of protein contained in any food mass, or repre-
sented by any particular amount of excretion, is to determine the
nitrogen and multiply the weight of it present by 6.25, the fraction
of protein which is nitrogen. We shall learn in the chapter on
Excretion (Chap. XXXI), that in the healthy Body an accumula-
tion of nitrogen-containing excretory products never occurs; as
fast as wastes are formed they are gotten rid of. It follows,
then, that if there is less nitrogen being given off than taken
in, the living tissues of the Body must be increasing in amount,
and if more is given off than is obtained in the food the living
tissues must be wasting away. In the healthy adult Body,
neither of these conditions is at all usual; the intake and outgo
of nitrogen balance each other and the Body is in nitrogen equi-
librium.

It has been chiefly through experimental studies of nitrogen
equilibrium that our ideas of the twofold function of protein, as
tissue-restorer and as fuel, have been gained. If an animal be fed
large enough quantities of protein he requires no other food, and if
healthy maintains nitrogen equilibrium upon this high level, the
large nitrogen intake being exactly balanced by an equally large
outgo. Now by substituting other foods, as carbohydrates or
fats, for part of the protein, the nitrogen intake and outgo are each
less in quantity, but they still balance; the animal is in nitrogen
equilibrium upon a lower level. If the substitution of other foods
for protein is increased a point is presently reached when the
nitrogen outgo exceeds its intake; the animal is not getting enough
protein for his needs, and so his own tissues are breaking down.
It was stated in an earlier paragraph (p. 452) that this breakdown
begins while the protein intake is still somewhat in excess of the
tissue-repair requirement, indicating that there is a fuel-protein
requirement which must be met. Two lines of evidence point to
this conclusion. The first is gained by starvation studies. If an

THE ABSORPTION AND USE OF FOODS l.V.i

animal be deprived completely of protein food, although supplied
abundantly with fats and carbohydrates, he continues to lose
nitrogen from the Body, since tissue breakdown continues, but
presently the daily loss of nitrogen becomes constant at a low
" starvation level." If this daily loss of nitrogen be measured and
an amount of protein sufficient to replace it exactly be added to
the food, nitrogen equilibrium is not thereby restored; there is still
loss of nitrogen, and to get the animal back into nitrogen equi-
librium more protein must be fed. It is hard to explain this except
upon the assumption of fuel-proteins already made. The second
line of evidence for fuel-proteins as essential to the diet has been
gained in feeding experiments with gelatin. Although animals
will refuse to eat this albuminoid after a few days, it has been
shown that during the period when gelatin is being eaten nitrogen
equilibrium can be maintained upon very small quantities of true
protein, much less than suffices under any other diet. Since it is
certain that albuminoids cannot function as tissue-restorers the
conclusion is that the amount of protein required for tissue-
restoration is considerably less than the total amount demanded
by the Body, and that therefore the Body has a fuel-protein re-
quirement which must be met.

Carbon Equilibrium. For an animal to be in carbon equilibrium
only needs that all the fuel taken in be burned, and that no reserve
store be called upon. Aside from the temporary storage of carbo-
hydrate food as glycogen all the fuel taken into the Body must
look forward to one of two fates, either to be oxidized promptly or
to be stored in the form of fat for future use. Just as nitrogen
equilibrium may be established on a high or a low level so carbon
equilibrium can be maintained in the face of variations in the in-
take of fuel. It is easily seen, however, that the limits of carbon
equilibrium must be narrower than of nitrogen equilibrium. The
actual protein requirement of the Body is so much less than the
usual protein intake that considerable variations in the protein
consumed can be made without affecting the nitrogen equilibrium ;
but the energy requirement of the Body is quite definite, varying
with the work done rather than with the food eaten. Thus it fol-
lows that the fuel intake and the energy requirement are harder to
keep balanced than are the nitrogen intake and outgo. It may
easily be a matter of astonishment how successfully the Body, un-

460 THE HUMAN BODY

der the guidance of the appetite, manages to make its fuel con-
sumption balance its fuel need.

Since only such fuel as is not oxidized is stored as fat we might
conclude that those people who show a tendency to lay on fat are
the ones who habitually overeat. But overeating from the stand-
point of the energy requirement of the Body is quite distinct from
overeating in the sense of throwing upon the digestive organs too
heavy a burden. It appears that most of us, particularly when not
exercising vigorously, eat more than is necessary for the require-
ments of the Body, and so our bodies accommodatingly burn up
the surplus rather than to store it. In those whose bodies refuse to
get rid of the surplus in this way there is a deposition of fat under
similar circumstances. The degree of vigor of the bodily oxida-
tions is in large measure an idiosyncrasy, and one which is trans-
mitted in heredity from generation to generation, hence the
tendency of certain families to be fleshy and of others to be spare.
Changes in environment or in habit of life seem often to affect the
oxidation vigor, and so to change the tendency of the Body toward
fat-formation. We all know persons who in middle life have
changed from a spare to a fleshy tendency or the reverse, in a very
striking way.

The Treatment for Obesity is obviously to make the energy re-
quirement equal, or even exceed the fuel intake. Vigorous mus-
cular exercise accompanied by strict dietary limitation may pro-
duce the desired result, but the good effects continue only so long
as the flesh-reducing measures are persisted in. Exercise and
dieting are both conducive to good appetite, therefore as soon as
the treatment is relaxed a return to the former condition is vir-
tually inevitable. An ingenious treatment for obesity, and one
which appears to be effective, is based upon the observation
quoted in the paragraph on Mastication (Chap. XXVIII), that
prolonged chewing of the food diminishes the appetite and allows
one to conclude a meal with comfort when much less has been
taken than the ordinary individual would demand. Thus the
fuel intake may be reduced to the minimum energy requirement,
or for a time below this, without conscious self-denial.

Source of the Body Fat. For a long time there was much dis-
cussion as to which of the three sorts of food-stuffs, proteins,
carbohydrates, or fats, is the source of the fat which is stored in

THE ABSORPTION AND USE OF FOODS 461

the Body. The natural conclusion that body fat is derived from
food fat is shown to be not universally true, at any rate, by the
ability of cattle to produce milk, with its abundant fat content,
upon a diet of hay and grain in which no trace of fat occurs. The
question whether in these animals the protein or the carbohydrate
of the food gives rise to the fat was formerly much studied; but
with the rise of the modern view of normal protein absorption,
according to which all but a small percentage of the protein taken
in the food is absorbed virtually as carbohydrate, the question has
lost much of its force. There can be little doubt that body fat
represents stored fuel, and since the whole fuel supply of the
bovine Body is represented, after absorption, by carbohydrates,
these must be the source of the fat which the Body elaborates.

It seems to be the general opinion that even in animals whose
diet includes some fat the normal source of the body fat is for the
most part carbohydrate. It is supposed, without very definite
evidence to prove it, that the fat absorbed after a meal is retained
in the blood till taken up by the tissues and burned, and that the
somewhat leisurely process of fat deposition is carried on in con-
nection with the carbohydrate, which is transferred from its
temporary storehouse in the liver to a more permanent one in the
adipose tissues. There is no reason to doubt that when large
amounts of fat are included in the diet there may be direct storage
of some of the fat absorbed. In fact it has been shown that under
these circumstances foreign fats, such as linseed-oil, for example,
can be deposited in the adipose tissues of animals.

CHAPTER XXXI
EXCRETION AND THE EXCRETORY ORGANS

Exogenous and Endogenous Excreta. It is usual to include un-
der the general head of excreta all waste materials of any kind that
are given out from the Body. We shall see, however, that under
this general definition come two very distinct classes of materials.
Many substances are taken into the Body with the food which
have of themselves no food value, and escape absorption during
the passage of the food through the alimentary tract ; these appear,
of course, among the excreta. Other substances have an accessory
food value, in arousing appetite, or in stimulating some of the
bodily processes; these may be absorbed from the alimentary
tra»ct into the blood, but they do not enter in any intimate fashion
into the metabolic activities of the living tissues, and after a longer
or shorter sojourn in the blood they appear among the excreta.
The third substances to be grouped with those just described are
the ammonia compounds which arc split off from the fuel-proteins
during absorption. These, from the moment of their separation,
are waste products, to be conveyed as rapidly as possible to the
excretory organs and gotten rid of. All these excretory materials
are grouped together as exogenous excreta, the term suggesting
that they are derived from sources outside the actual life processes
of the tissues.

The second group of excreta, the endogenous excreta, includes
those substances that are produced by the living cells of the Body
in the course of their metabolic activities. Most of our knowledge
of cell metabolism has been gained through studies of the en-
dogenous excreta.

The Channels of Excretion. Four channels are recognized
through which the body discharges waste materials ; these are : the
lungs, the skin, the urinary system, the rectum. The lungs, are the
channel for the discharge of gaseous wastes, carbon dioxid, and
water vapor; the skin and urinary system together dispose of the
major part of the endogenous excreta other than gaseous, and

462

EXCRETION AND THE EXCRETORY ORGANS 463

also of those exogenous excreta that are absorbed from the ali-
mentary tract into the blood. From the rectum are discharged
all exogenous excreta that fail of absorption, and likewise a num-
ber of endogenous excretory substances received into the intestine
from the liver, by way of the bile duct. The chapter on Respira-
tion contains the discussion of the excretory function of the lungs.
It is not necessary, therefore, to consider it here.

The Liver as an Excretory Organ. To the functions previously
described of aiding the digestive and absorptive processes, and of
serving as a temporary storehouse for carbohydrates, the liver
adds a very important excretory function. This is in part direct,
the separation from the blood of waste materials contained in it,
and in part the working over of harmful excretory substances into
harmless ones which it does not excrete but returns to the blood
to be discharged through the urinary system and skin. This latter
function will be considered before the direct excretions of the liver
are discussed. It will be recalled that in the process of protein
absorption the " fuel-protein " is supposed to be split into a nitrog-
enous waste portion, and a non-nitrogenous oxidizable portion.
The nitrogenous part probably consists largely of ammonia com-
pounds, chief of which is ammonium carbonate (NH4),COS. It
is well known that ammonia compounds are very poisonous to
animals into whose circulating blood they are introduced, and it
has been proven that an animal would be seriously affected if the
ammonia content of the general circulation should ever reach that
which is normal to the portal vein during protein absorption. It
is through the action of the liver that the Body is protected from
the harmful effects of this portal ammonia. During the passage
of the portal blood through the liver its ammonia is converted by
dehydration into urea, a compound harmless to the Body if not
present in the blood in too great concentration. The conversion of
ammonium carbonate by dehydration to urea is made clear if we
compare the chemical formula? of the two substances:

NH40-H,0_ NH,

D<NH«0-H,0 ^NH,

(ammonium (urea)
carbonate)

The urea formed thus from the ammonia compounds of the portal
blood belongs to the group of exogenous excreta, HIM •<• it does not

464 THE HUMAN BODY

represent a product of true cell metabolism in the Body. From
the liver it is delivered to the blood of the general circulation
where it floats about till filtered out by the kidneys.

The direct excretory function of the liver consists in the with-
drawal from the blood and the delivery to the intestine through the
bile of certain endogenous excretory substances. The most marked
of these are the bile-pigments, which, as stated in Chap. XVII, are
derived from the wornout red corpuscles of the blood, and consist
essentially of the pigment portion of hemoglobin minus its iron.
Two bile-pigments occur, of very similar chemical constitution;
bilirubin, golden-brown in color, is the predominating pigment of
carnivorous bile, and of human bile on a mixed diet; biliverdin, a
green pigment, predominates in the bile of herbiverous animals.

Beside the bile-pigments the liver excretes small amounts of
various substances which are interesting chiefly on account of
their insolubility in the ordinary fluids of the Body, and the fact
that they are soluble in bile. These are found in the Body for the
most part in nervous tissues, and they may be excretory products
of nerve-cell metabolism. The most abundant of them is the non-
nitrogenous substance cholesterin.

The chief constituents of bile not heretofore mentioned are the
bile salts, sodium salts of peculiar acids found only in bile, glyco-
cholic acid and taurocholic add. These do not appear to be excreta
pure and simple, inasmuch as they are reabsorbed in part by the
intestinal walls, and returned by the portal vein to the liver whence
they again appear as constituents of the bile. They are thought
to give to bile its special ability to promote fat absorption by dis-
solving the fatty acids, and it is also by virtue of their presence
that the bile is able to dissolve cholesterin.

General Arrangement of the Urinary Organs. These consist
of (1) the kidneys, the glands which secrete the urine; (2) the
ureters or ducts of the kidneys, which carry their secretion to
(3) the urinary bladder, a reservoir in which it accumulates and
from which it is expelled from time to time through (4) an exit
tube, the urethra. The general arrangement of these parts, as
seen from behind, is represented in Fig. 138. The two kidneys,
R, lie in the dorsal part of the lumbar region of the abdominal
cavity, one on each side of the middle line. Each is a solid mass,
with a convex puter and a concave inner border, and its upper end

EXCRETION AND THE EXCRETORY ORGANS 465

a little larger than the lower. From the abdominal aorta, A, a
renal artery, Ar, enters the inner border of each kidney, to break

Vr

u»

Fiq. 138. — The renal organs, viewed from behind. R, right kidney; A, aorta;
Ar, right renal artery; Vc, inferior veme cavw; Vr, right renal vein; U, right
ureter; Vu, bladder; Ua, commencement of urethra.

up within it into finer branches, ultimately ending in capillaries.
The blood is collected from these into the renal veins, Vr, one of

466 THE HUMAN BODY

which leaves each kidney and opens into the inferior vena cava,
Vc. From the concave border of each kidney proceeds also the
ureter, U, a slender tube from 28 to 34 cm. (11 to 13.5 inches)
long, opening below into the bladder, Vu, on its dorsal aspect, and
near its lower end. From the bladder proceeds the urethra, at
Ua. The channel of each ureter passes very obliquely through
the wall of the bladder to open into it ; accordingly if the pressure
inside the latter organ rises above that of the liquid in the ureter,
the walls of the oblique passage are pressed together and it is
closed. Usually the bladder, which has a thick coat of unstriped
muscular tissue lined by a mucous membrane, is relaxed, and
the urine flows readily into it from the ureters. While urine is
collecting, the beginning of the urethra is kept closed, in part .at
least, by bands of elastic tissue around it: some of the muscles
which surround the commencement of the urethra assist, being
kept in reflex contraction; it is found that in a dog the urinary
bladder can retain liquid under considerably higher pressure when
the spinal cord is intact than after destruction of its lumbar por-
tion. The contraction of these urethra constricting muscles can
be reinforced voluntarily. When some amount of urine has ac-
cumulated in the bladder, it contracts and presses on its contents;
the ureters being closed in the way above indicated, the elastic
fibers closing the urethral exit are overcome, and the urethral
muscles simultaneously relaxing, the liquid is forced out.

Naked Eye Structure of the Kidneys. These organs have ex-
ternally a red-brown color, which can be seen through the trans-
parent capsule of peritoneum which envelops them. When a
section is carried through a kidney from its outer to its inner
border (Fig. 139) it is seen that a deep fissure, the hilus, leads into
the latter. In the hilus the ureter widens out to form the pelvis,
D, which breaks up again into a number of smaller divisions, the
cups or calices. The cut surface of the kidney proper is seen to
consist of two distinct parts; an outer or cortical portion, and an
inner or medullary. The medullary portion is less red and more
glistening to the eye, is finely striated in a radial direction, and
does not consist of one continuous mass but of a number of con-
ical portions, the pyramids of Malpighi, 2', each of which is sep-
arated from its neighbors by an inward prolongation, 4, of the
cortical substance: this, however, does not reach to the inner end

EXCRETION AND THE EXCRETORY ORGANS 467

of the pyramid, which projects, as the papilla, into a calyx of the
ureter. At its outer end each pyramid separates into smaller
portions, the pyramids of Ferrein, 2", separated by thin layers of
cortex and gradually spreading everywhere into the latter. The
cortical substance is redder and more granular looking and less
shiny than the medullary, and forms everywhere the outer layer
of the organ next its capsule, besides dipping in between the
pryamids in the way described.

The renal artery divides in the hilus into branches (5) which
run into the kidney between the pyramids, giving off a few twigs
to the latter and ending finally in a much richer vascular network
in the cortex. The branches of the renal vein have a similar
course.

The Minute Structure of the Kidney. The kidneys are com-
pound tubular glands, composed essentially of branched micro-
scopic uriniferous tubules, lined by epithelium. Each tubule
commences at a small opening on a papilla and from thence has
a very complex course to its other extremity: usually about
twenty open, side by side, on one papilla, where they have a diam-
eter of about 0.125 mm. (^-J-g- inch). Running from this place into
the pyramid each tubule divides repeatedly ; the ultimate branches,
which are the secreting tubules, pursue a tortuous course to ter-
minations in the cortex of the kidney in peculiar spherical dila-
tations, the Malpighian capsules, each containing a tuft of capil-
laries, the glomerulus. Throughout its course the tubule is lined
by a single layer of epithelium cells differing in character in its
different sections : they are flat and clear in the capsules, and very
granular in the convoluted parts, where their appearance suggests
that they are not mere lining cells but cells with active work to
do; in the collecting and discharging tubules they are somewhat
cuboidal in form and have no active secretory function. All the
tubes are bound together tiy a sparse amount of connective tissue
and by blood-vessels to form the gland. The lymph-spaces are
large and numerous, especially about the convoluted portions of
the tubules.

The Blood-Flow Through the Kidney. The amount of blood
brought to the kidney is large relatively to the size of the organ
and enters under a very high pressure almost direct from the aorta,
and leaves under a very low, into the inferior cava (Fig. 138).

468

THE HUMAN BODY

The final twigs of the renal artery in the cortex, giving off a few
branches which end in a capillary network around the convoluted
tubules and in the pyramids, are continued as the afferent ves-
sels of Malpighian capsules, the walls of which are doubled in be-
fore them (Fig. 140) ; there each breaks up into a little knot of

FIG. 139. — Section through the right kidney from its outer to its inner border,
1, cortex; 2, medulla; 2', pyramid of Malpighi;t2", pyramid of Ferrein; 5, small
branches of the renal artery entering between the pyramids; A, a branch of the
renal artery; D, the pelvis of the kidney; U, ureter; C, a calyx.

capillary vessels called the glomerulus, from which ultimately
an efferent vessel proceeds. Where the wall of the capsule, w,
Fig. 140, is doubled in before the blood-vessels, its lining cells
continue as a covering, c, to the latter, closely adhering to the
vascular walls. A space, A, is left between the epithelial cells of
the outside of the capsule and those involuted on the vessels, as

EXCRETION AND THE EXCRETORY ORGANS [<;;,

there would be in the interior of a rubber ball one side of which

was pushed in so as to nearly meet the other; this cleft, into which

any liquid transuded from the vessels must enter, opens by a

narrow neck, d, into the commencement of the first contorted

part of an uriniferous tubule. The ef-

ferent vein, carrying blood away from

the glomerulus, breaks up into a close

capillary network around the neighbor-

ing tubules of the cortex. From these

capillaries the blood is collected into the

renal vein. Most of the blood flowing

through the kidney thus goes through

two sets of capillaries; one found in the

capsules, and the second formed by the

breaking up of their efferent veins. The

capillary network in the pyramids is

much less close than that in the cortex,

... . FIG. 140. — Diagram showirig

which gives reason to suspect that most a kidney glomerulua and the
of the secretory work of the kidneys is ''

done in the capsules and convoluted

tubules. The pyramidal blood flows ?ndinK »> the capillary tuft

, , , . from which the vein c issue* ;

only through one set of capillaries, there ?, involuted epithelium cover-
being no glomeruli in the kidney me- IX'of S^fiJ ^

dulla. resented as a general wrapping

' for the whole tuft, but in na-

The Renal Excretion. The amount ture it forms a close in vestment

e ,, • . , _ , . around each vessel of the glom-

Ot thlS Carried Off from the Body in emlu.s; A. space in capsule into

OA li,>nr • io aiiKI,™* ±.rv ,C 1^««U1^ which liquid transuded from

24 hOUrS IS Subject tO Considerable the vessels of the glomerulus

variation, being especially diminished pa*i«i;rf. ncck °f capsule pj».

. ~ . J . ing into commencement of first

by anything which promotes pcrspira- convoluted portion, //, of an

.• j • i , ,.,. uriniferous tubule: o, granular

tion, and increased by conditions, as epithelial cells; 6, basement

cold to the surface, which diminish the membrane-
skin excretion. Its average daily quantity varies from 1,200
to 1,750 cub. cent. (40 to 60 fluid ounces). The urine is a clear
amber-colored liquid, of a slightly acid reaction; its specific grav-
ity is about 1,022, being higher when the total quantity excreted
is small than when it is greater, since the amount of solids dis-
solved hi it remains nearly the same in health ; the changes in its
bulk being dependent mainly on changes in the amount of water
separated from the blood by the kidneys.

470 THE HUMAN BODY

Normal Urine consists of about 96 per cent water and 4 per cent
dissolved solids. Chemically it is a very complex liquid, the 4
per cent of dissolved materials including a large variety of dif-
ferent substances. This is to be expected when we recall that the
kidney is the excretory channel, not only for the chief part of the
endogenous excreta, but also for virtually all the exogenous waste
materials that are absorbed into the blood stream. Among these
latter are found the substances that lend flavor to our food ; like-
wise most drugs that are taken find their way ultimately into the
urine. One group of exogenous urinary substances, the ethereal
sulphates, are interesting since they are derived from compounds
formed in the large intestine in the course of the putrefactive
processes which normally go on there; these compounds are ab-
sorbed into the blood stream and are excreted by the kidney.
The extent of their occurrence in the urine measures the amount
of putrefaction in the large intestine. These substances are toxic
if present in quantity and it may be that the ill feeling which
often accompanies constipation is the result of their presence in
considerable concentration in the blood.

Urea is the constituent of urine most abundant next to the
water. About two per cent of urine, half of all the dissolved ma-
terials, is urea. The greater part of this is of exogenous origin,
being formed in the liver from the ammonia residues of fuel-
protein. The amount of exogenous urea varies from time to time
according as the amount of protein undergoing absorption varies.
It is thought that a certain amount of endogenous urea is pro-
duced during the course of cell metabolism. How much of the
total urea of the excretion is of this origin cannot be told.

Creatinin. In some respects the most interesting of the en-
dogenous excreta found in the urine is the compound creatinin.
This substance, as stated in Chap. I, is excreted during health at
a rate which is practically constant for a given individual, and
which appears to be determined chiefly by the amount of muscle
tissue present in the Body. The conclusion with regard to creat-
inin which has been drawn from these facts is that it is a product
of the life of muscles as distinct from their special function. In
other words, the muscle in doing its work uses up sugar and pro-
duces carbon dioxid and water, but in living it uses up protein
and produces, among other things, creatinin. Since the amount

EXCRETION AND THE EXCRETORY ORGANS 471

of creatinin is constant, regardless of the extent to which the
muscles are used, unless they are used to excess, it is believed that
muscle-cells, and perhaps other cells as well, live at a rate which
varies scarcely at all from day to day, and is independent of their
functional activity. The interesting observation that the amount
of creatinin excreted is roughly proportional to the bulk of the
muscle tissues may be taken to indicate that all muscle-cells live
at about the same rate, the temperamental differences noted in
different individuals not involving differences in the metabolic
activities of their muscle tissues.

The Purin Bodies, of which uric acid is the best known, are
other endogenous excreta found in urine. They show chemical
characteristics which indicate that they represent probably the
end products of the metabolism of cell nuclei. Caffein, the active
principle of coffee and tea, and theobromin, the active principle
of cocoa, are very closely related chemically to the purin bodies
excreted from the kidney.

Since all the endogenous excreta are produced in the living
tissues they occur in the flesn of animals eaten for food. In fact
the flavor of meat is largely the result of their presence. When
eaten with meat they are, of course, absorbed into the blood from
the intestine and become part of the exogenous excreta. For
this reason it is often necessary, when studying metabolism ex-
perimentally, to exclude meat from the diet, so that the endoge-
nous excreta may be obtained pure.

The Urinary Salts are chiefly sodium chlorid, and the sulphates
and acid phosphates of sodium, potassium, calcium, and magne-
sium. Whatever salt is taken with the food, unless stored perma-
nently in the Body, as in bone formation, finally is excreted by the
kidneys. The acid phosphates of sodium and potassium are in
part responsible for the acid reaction of urine.

In various diseases abnormal substances are found in the urine:
the more important are albumens in albuminuria or Bright's
disease; grape-sugar or glucose in diabetes; bile-salts; bile-pigments.

The Secretory Actions of Different Parts of a Uriniferous Tubule.
The miscroscopic structure of the kidneys is such as to suggest
that in those organs we have to do with two essentially distinct
secretory apparatuses: one represented by the glomenili, with
their capillaries separated only by a single layer of flat epithelial

472 THE HUMAN BODY

cells from the cavity of the capsule and especially adapted for
filtration and dialysis; the other represented by the contorted por-
tions of the tubules, with their large granular cells, which clearly
have some more active part to play than that of a mere passive
transudation membrane. And we find in the urine substances
which like the water and mineral salts may easily be accounted for
by mere physical processes, and others, urea especially, which are
present in such proportion as must be due to some active physio-
logical work of the kidney. More direct evidence does, in fact,
justify us in saying that in general the glomcruli are transudation
organs, the contorted portions of the tubuli secretory organs,
while the collecting and discharging tubules are merely passive
channels for the gathering and transmission of liquid. In calling
the capsules transudation organs we do not intend to assert that
the passage of water and salts through them is necessarily a phys-
ical process pure and simple. Although many physiologists have
supposed it to be nothing more, there is abundant evidence that
here, as elsewhere in the Body where the passage of liquids is
through membranes composed of living cells, the cells of the cap-
sule exercise a controlling function over the passage of the water
and salts through them.

Several lines of evidence indicate that the organic constituents
of urine are excreted through the secretory portions of the tubules.
One of the best of these has come from work on frogs. Urea, the
most important and most abundant of the characteristic ingre-
dients of urine, has a very marked influence on kidney activity, the
injection of some of it into blood causing a greatly increased se-
cretion of urine, in which the injected urea is quickly passed out.
In amphibia the blood carried to the kidney, like that supply-
ing the mammalian liver, has two sources, one venous and one
arterial; the arterial supply comes from the renal arteries, the
venous from the veins of the leg by the* reniportal vein. Both
bloods leave the organ by the renal veins, but their distribution
in it is in great part distinct; the arteries supply the glomeruli,
the reniportal vein the tubules of the cortex, though mixed there
with blood from the efferent vessels of the glomeruli. On tying the
renal arteries of one of these animals urinary secretion ceases, there
being then no blood-pressure in the glomeruli to cause' the tran-
sudation of liquid; but if some urea be now injected into the blood

EXCRETION AND THE EXCRETORY ORGANS 473

the epithelial cells of the tubules arc stimulated to secrete, and
urine rich in urea is formed ; but in these circumstances it cannot
come from the Malpighian bodies. It would seem then that urea
is a special stimulant to some cells of the tubules, and that an
excess of it in the blood can stir them up to its elimination along
with some water, quite independently of any formation of tran-
sudation urine.

The Relation of Renal Blood-Flow to the Secretion of Urine.
The kidneys have probably a richer blood supply than any other
organs of the Body. It has been estimated that under proper
circumstances their own weight of blood may flow through them
each minute. This rich blood supply is, of course, an adaptation
to secure the withdrawal of waste substances from the blood at a
rapid rate. From the structure of the glomeruli and the fact that
most of the water of the urine is derived from them it is a priori
probable that anything tending to increase the pressure of blood
in them will increase the bulk of urine secreted, and anything
diminishing that pressure will decrease the urine. This is con-
firmed by experiment. The kidney is supplied with both vaso-
constrictor and vasodilator nerves which reach it mainly through
the solar plexus. When the spinal cord is cut in the neck region
of a dog the kidney vessels as well as those of the rest of its Body
dilate and blood-pressure everywhere is very low. Under these
circumstances the secretion of urine is suppressed. If the lower
end of the cut cord be stimulated the vessels all over the Body of
the animal contract, and blood-pressure cvciy where becomes very
high. But the kidney vessels being constricted with the rest allow
very little blood to enter the glomeruli in spite of the high aortic
pressure, and little or no urine is secreted. If, however, the vaso-
constrictor nerves of the kidney be cut before the stimulation of
the cord, we get a dilatation of the kidney vessels with a constric-
tion of vessels elsewhere, and abundant blood flows through the
glomeruli under high pressure: the whole kidney swells and abun-
dant urine is formed. When the skin vessels contract on exposure
to cold, more blood flows through internal organs, the kidneys
included, and the blood-pressure in these is if anything increased,
the expansion of internal arteries not at the most more than
counterbalancing the constriction of the cutaneous. Hence tin-
greater secretion of urine in cold weather.

474 THE HUMAN BODY

Diuretics. Various substances, caffein, digitalis, urea, salts,
and even water, stimulate the kidney to increased activity. Sub-
stances which have this effect are known as diuretics. It appears
that these act for the most part by stimulating the secreting cells
of the tubules to greater activity, although some of them, notably
the salts, may bring about an increased pressure in the glomeruli
and so an increased transudation through the capsule.

The Skin, which covers the whole exterior of the Body, consists
everywhere of two distinct layers; an outer, the cuticle or epider-
mis, and a deeper, the dermis, cutis vera, or corium. A blister is
due to the accumulation of liquid between these two layers. The
hairs and nails are excessively developed parts of the epidermis.

The Epidermis, Fig. 141, consists of cells, arranged in many
layers, and united by a small amount of cementing substance.
The deepest layer, d, is composed of elongated or columnar cells,
set on with their long axes perpendicular to the corium beneath.
To it succeed several layers of roundish cells, b, the deepest of
which, prickle-cells, are covered by minute processes (not indicated
in the figure) which do not interlock but join end to end so as to
leave narrow spaces between the cells; in more external layers the
cells become more and more flattened in a plane parallel to the
surface. The outermost epidermic stratum is composed of many
layers of extremely flattened cells from which the nuclei (conspic-
uous in the deeper layers) have disappeared. These superficial
cells are dead and are constantly being shed from the surface of
the Body, while their place is taken by new cells, formed in the
deeper layers, and pushed up to the surface and flattened in their
progress. The change in the form of the cells as they travel out-
wards is accompanied by chemical changes, and they finally con-
stitute a semitransparent dry horny stratum, a, distinct from the
deeper, more opaque and softer Malpighian or mucous layer, b and
d, of the epidermis.

The rolls of material which are peeled off the skin in the "sham-
pooing" of the Turkish bath, or by rubbing with a rough towel
after an ordinary warm bath, are the dead outer scales of the
horny stratum of the epidermis.

In dark races the color of the skin depends mainly on minute
pigment-granules lying in the cells of the deeper part of the Mal-
pighian layer.

EXCRETION AND THE EXCRETORY ORGANS 475

No blood or lymphatic vessels enter the epidermis, which is en-
tirely nourished by matters derived from the subjacent corium.
Fine nerve-fibers run into it and end there among the cells.

x— a

Fia. 141. — A section through the epidermis, somewhat diagrammatic, highly
magnified. Below is seen a papilla of the dermis, with its artery, /, and veins, g g;
a, the horny layer of the epidermis; b, the rrtc mucosum or Malpighian layer; d, the
layer of columnar epidermic cells in immediate contact with the dermis; h, the
duct of a sweat-gland.

The Corium, Dermis, or True Skin, Fig. 142, consists funda-
mentally of a close feltwork of elastic and white fibrous tissue,
which, becoming wider meshed below, passes gradually into the
subcutaneous areolar tissue (Chap. IV) which attaches the skin
loosely to parts beneath. In tanning it is the dermis which is
turned into leather, its white fibrous tissue forming an insoluble
and tough compound with the tannin of the oak-bark employed.

476

THE HUMAN BODY

Wherever there are hairs, bundles of smooth muscular tissue are
found in the corium; it contains also a close capillary network
and numerous lymphatics and nerves. In shaving, so long as the
razor keeps in the epidermis there is no bleeding; but a deeper cut
shows at once the vascularity of the true skin.

The outer surface of the corium is almost everywhere raised into
minute elevations, called the papillce, on which the epidermis is

a

FIQ. 142. — A section through the skin and subcutaneous aerolar tissue, h,
horny stratum, and m, deeper more opaque layer of the epidermis; d, dermis
passing below into sc, loose areolar tissue, with fat, /, in its meshes; above, dermic
papillae are seen, projecting into the epidermis which is molded on them, a,
opening of a sweat-gland; gl, the gland itself.

molded, so that its. deep side presents pits corresponding to the
projections of the dermis. In Fig. 141 is shown a papilla of the
corium containing a knot of blood-vessels, supplied by the small
artery, /, and having the blood carried off from them by the two
little veins, g g. Other papillae contain no capillary loops but
special organs connected with nerve-fibers, and supposed to be
concerned in the cutaneous senses (Chap. XIII). On the pal-
mar surface of the hand the dermic papillae are especially well de-

EXCRETION AND THE EXCRETORY ORGANS 477

vclopcd (as they are in most parts where the sense of touch is
acute) and are frequently compound, or branched at the tip. On
the front of the hand, they are arranged in rows; the epidermis fills
up the hollows between the papilla; of the same row, but dips down
between adjacent rows, and thus an; produced the finer ridges seen
4on the palms. In many places the corium is also furrowed, as op-
posite the finger-joints and on the palm. Elsewhere such furrows
are less marked, but they exist over the whole skin. The epidermis
closely follows all the hollows, and thus they are made visible
from the surface. The wrinkles of old persons are due to the ab-
sorption of subcutaneous fat and of other soft parts beneath the skin,
which, not shrinking itself at the same rate, is thrown into folds.

Hairs. Each hair is a long filament of epidermis developed on
the top of a special dcrmic papilla, seated at the bottom of a de-
pression reaching down from the skin into the tissue beneath, and
called the hair-follicle. The portion of a hair buried in the skin is
called its root; this is succeeded by a stern which, in an uncut hair,
tapers off to a point. The stem is covered by a single layer of over-
lapping scales forming the hair-cuticle; the projecting edges of these
scales are directed towards the iop of the hair. Beneath the hair-
cuticle comes the cortex, made up of greatly elongated cells united
to form fibers; and in the center of the shaft there is found, in
many hairs, a medulla, made up of more or less rounded cells. The
color of hair is mainly dependent upon pigment-granules lying
between the fibers of the cortex. All hairs contain some air cavi-
ties, especially in the medulla. They are very abundant in white
hairs and cause the whiteness by reflecting all the incident light,
just as a liquid beaten into fine foam looks white because of the
light reflected from the walls of all the little air cavities in it. In
dark hairs the air cavities are few.

The hair-follicle (Fig. 143) is a narrow pit of the dermis, pro-
jecting down into the subcutaneous arcolar tissue, and lined by an
involution of the epidermis. At the bottom of the follicle is a pap-
illa, and the epidermis, turning up over this, becomes continuous
with the hair. On the papilla epidermic cells multiply rapidly so
long as the hair is growing, and the whole hair is there made up »f
roundish cells. As these are pushed up by fresh ones formed be-
neath them, the outermost layer become flattened and form the
hair-cuticle; several succeeding layers elongate and form the cor-

478 THE HUMAN BODY

tex; while, in hairs with a medulla, the middle cells retain pretty
much their original form and size. Pulled apart by the elongating
cortical cells, these central ones then form the medulla with its air-
cavities. The innermost layer of the epidermis lining the follicle,

has its cells projecting, with over-
lapping edges turned downwards.
Accordingly these interlock with
the upward directed edges of the
cells of the hair-cuticle; conse-
quently when a hair is pulled out
the epidermic lining of the follicle
is usually brought with it. So long
as the dermic papilla is left intact
O a new hair will be formed, but not

FIG. 143. — Parts of two hairs em- ,, . 01 , , ,, f

bedded in their follicles, a, the skin, otherwise, blender bundles ot un-

which is seen to dip down and line afrinprl miT?plp '^ T?i0- 14^O nin frrvm
the follicle; 6, the subcutaneous tis- £ l6 (C'^1S- 1*>J ^

sue; c, the muscles of the hair-follicle, the dermis to the side of the hair-

which by their contraction can erect „ ,,. , _. ,

the hair; o, sebaceous gland. follicles. The latter are in most re-

gions obliquely implanted in the skin so that the hairs lie down on
the surface of the Body, and the muscles are so fixed that when
they shorten, they erect the hair and cause it to bristle, as may be
seen in an angry cat, or sometimes in a greatly terrified man.
Opening into each hair-follicle are usually a couple of sebaceous or
oil-glands. Hairs are found all over the skin except on the palms of
the hands and the soles of the feet ; the back of the last phalanx of
the fingers and toes, the upper eyelids, and one or two other regions.

Nails. Each nail is a part of the epidermis, with its horny
stratum greatly developed. The back part of the nail fits behind
into a furrow of the dermis and is called its root. The visible part
consists of a body, fixed to the dermis beneath (which forms the bed
of the nail), and of a, free edge. Near the root is a little area whiter
than the rest of the nail and called the lunula. The whiteness is
due in part to the nail being really more opaque there and partly to
the fact that its bed, which seen through the nail causes its pink
color, is in this region less vascular.

The portion of the corium on which the nail is formed is called its
matrix. Posteriorly this forms a furrow lodging the root, and it is
by new cells added on there that the nail grows in length.' The part
of the matrix lying beneath the body of the nail, and called its bed,

EXCRETION AND THE EXCRETORY ORGANS 479

is highly vascular and raised up into papillie which, except in the
region of the lunular, are arranged in longitudinal rows, slightly
diverging as they run towards the tip of the finger or toe. It is by
new cells formed on its bed and added to its under surface that the
nail grows in thickness,. as it is pushed forward by the new growth
in length at its root. The free end of a nail is therefore its thickest
part. If a nail is " cast " in consequence of an injury, or torn off, a
new one is produced, provided the matrix is left.

The Glands of the Skin are of two kinds, the sudoriparous or
sweat-glands, and the sebaceous or oil-ylands. The former belong to
the tubular, the latter to the racemose type. The sweat-glands,
Fig. 144, lie in the subcutaneous tissue, where
.they form little globular masses composed of a
coiled tube. From the coil a duct (sometimes
double) leads to the surface, being usually spi-
rally twisted as it passes through the epidermis.
The secreting part of the gland consists of a
connective-tissue tube, continuous along the
duct with the dermis; within this is a basement
membrane; and the final secretory lining con-
sists of several layers of gland-cells. A close
capillary network intertwines with the coils of
the gland. Sweat-glands are found on all regions
of the skin, but more closely set in some places,
as the palms of the hands and on the brow, than
elsewhere: there are altogether about two and a FIO. 144.— A sweat-
half millions of them opening on the surface of *l*ml ,.".- i>°myiawr

of cuticle; c, Malpi-

the Body. ghian layer; b, der-

_,. , , , , , . mis. The coils of the

Ihe sebaceous glands nearly always open into Kiand proper, em-

hair-follicles, and are found wherever there are
hairs. Each consists of a duct opening near ^low the denim.
the mouth of a hair-follicle and branching at its other end : the
final branches lead into globular secreting saccules, which, like
the ducts, are lined with epithelium. In the saccules the substance
of the cells becomes charged with oil-drops, the protoplasm disap-
pearing; and finally the whole cell falls to pieces, its detritus con-
stituting the secretion. NV\v cells arc, meanwhile, formed to take
the place of those destroyed. Usually two glands are connected
with each hair-follicle, but there may be three or only one. A pair

480 THE HUMAN BODY

of sebaceous glands are represented on the sides of each of the hair-
follicles in Fig. 143.

The Skin Secretions. The skin besides forming a protective
covering and serving as a sense-organ (Chap. XIII) also plays an
important part in regulating the temperature of the Body, and, as
an excretory organ, in carrying off certain waste products.

The sweat poured out by the sudoriparous glands is a trans-
parent colorless liquid, with a peculiar odor, varying in different
races and, in the same individual, in different regions of the Body.
Its quantity in twenty-four hours is subject to great variations,
but usually lies between 700 and 2,000 grams (10,850 and 31,000
grains). The amount is influenced mainly by the surrounding
temperature, being greater when this is high; but it is also in-
creased by other things tending to raise the temperature of the
Body, as muscular exercise. The sweat may or may not evaporate
as fast as it is secreted; in the former case it is known as insensible,
in the latter as sensible perspiration. By far the most passes off in
the insensible form, drops of sweat only accumulating when the se-
cretion is very profuse, or the surrounding atmosphere so humid
that it does not readily take up more moisture. The perspiration
is acid, and in 1,000 parts contains 990 of water to 10 of solids.
Among the latter are found urea (1.5 in 1,000), fatty acids, sodium
chlorid, and other salts. In diseased conditions of the kidneys the
urea may be greatly increased, the skin supplementing to a certain
extent deficiencies of those organs.

The Nervous and Circulatory Factors in the Sweat Secretion.
It used to be believed that an increased flow of blood through the
skin would suffice of itself to cause increased perspiration; but
against this view are the facts that, in terror for example, there
may be profuse sweating with a cold pallid skin ; and that in many
febrile states the skin may be hot and its vessels full of blood, and
yet there may be no sweating.

Direct experiment shows that the secretory activity of the
sweat-glands is under immediate control of nerve-fibers, and is
only indirectly dependent on the blood-supply in their neighbor-
hood. Stimulating the sciatic nerve of the freshly amputated
leg of a cat will cause the balls of its feet to sweat, although there
is no blood flowing through the limb. . On the other hand, if the
sciatic nerve be cut so as to paralyze it, in a living animal, the

EXCRETION AND THE EXCRETORY ORGANS 481

skin arteries dilate and the foofrgets more blood and becomes
warmer; but it does not sweat. The sweat-fibers doubtless com-
municate with sweat-centers in the medulla, which may either be
directly excited by blood of a higher temperature than usual flow-
ing through them or, reflexly, by warmth acting on the exterior
of the Body and stimulating the sensory nerves there. Both of
these agencies commonly also excite the vasodilator nerves of the
sweating part, and so the increased blood-supply goes along with
the secretion; but the two phenomena are fundamentally inde-
pendent. Since the sweat-glands are innervated through the
sympathetic system they share in the emotional reactions which
are characteristic of this system. The effect of embarrassment to
cause profuse sweating is too well known to require comment.

The Sebaceous Secretion. This is oily, semifluid, and of a
special odor. It contains about 50 per cent of fats (olein and
palmatin). It lubricates the hairs and usually renders them
glossy. No doubt, too, it gets spread more or less over the skin
and makes the cuticle less permeable by water. Water poured
on a healthy skin does not wet it readily but runs off it, as " off
a duck's back" though to a less marked degree.

Hygiene of the Skin. The sebaceous secretion, and the solid
residue left by evaporating sweat, constantly form a solid film
over the skin, which must tend to choke the mouths of the sweat-
glands (the so-called "pores" of the skin) and impede their ac-
tivity. Hence the value to health of keeping the skin clean: a
daily bath should be taken by every one.

Bathing. The general subject of bathing may be considered
here. One object of it is that above mentioned — to cleanse the
skin; but it is also useful to strengthen and invigorate the whole
frame. For strong healthy persons a cold bath is the best, except
in extremely severe weather, when the temperature of the water
should be raised to 15° C. (about 60° F.), at which it still feels
quite cold to the surface. The first effect of a cold bath is to con-
tract all the skin-vessels and make the surface pallid. This is soon
followed by a reaction, in which the skin becomes red and con-
gested, and a glow of warmth is felt in it. The proper time to
come out is while this reaction lasts, and after emersion it should
be promoted by a good rub. If the stay in the cold water be too
prolonged the state of reaction passes off, the skin becomes cold

482 THE HUMAN BODY

and pale and the person feels chilly, uncomfortable, and depressed
all day. Then bathing is injurious instead of beneficial; it lowers
instead of stimulating the activities of the Body. How long a
stay in the cold water may be made with benefit depends greatly
on the individual: a vigorous man can bear and set up a healthy
reaction after much longer immersion than a feeble one; moreover,
being used to cold bathing renders a longer stay safe, and, of
course, the temperature of the water has a great influence : water
called " cold " may vary within very wide limits of temperature,
as indicated by the thermometer; and the colder it is the shorter
is the time which it is wise to remain in it. Persons who in the
comparatively warm water of Narragansett during the summer
months stay with benefit and pleasure in the sea, have to content
themselves with a single plunge on parts of the coast where the
water is colder. The nature of the water has some influence; the
salts contained in sea-water stimulate the skin-nerves and pro-
mote the afterglow. Many persons who cannot stand a simple
cold fresh-water bath take one with benefit when some salines are
previously dissolved in the water. The best for this purpose are
probably those sold in the shops under the name of "sea-salts."

It is perfectly safe to bathe when warm, provided the skin is
not perspiring profusely, the notion commonly prevalent to the
contrary notwithstanding. On the other hand, no one should
enter a cold bath when feeling chilly, or in a depressed vital con-
dition. It is not wise to take a bath immediately after a meal,
since the afterglow tends to draw away too much blood from the
digestive organs, which are then actively at work. The best time
for a long bath is about three hours after breakfast; but for an
ordinary daily dip, lasting but a short time, there is no better
period than on rising and while still warm from bed.

The shower-bath abstracts less heat from the skin than an or-
dinary cold bath and, at the same time, gives it a greater stimu-
lus: hence it has certain advantages.

Persons in feeble health may diminish the shock to the system
by raising the temperature of the water they bathe in up to any
point at which it still feels cool to the skin. The very hot bath
is occasionally useful as the most efficient means for cleansing the
skin. There is no doubt, however, that its effect tends to be ener-
vating, and it should not be indulged in too frequently.

CHAPTER XXXII

THE PRODUCTION AND REGULATION OF THE HEAT OF

THE BODY

Cold- and Warm-Blooded Animals. All animals, so long- as
they are alive, are the seat of chemical changes by which heat is
liberated; hence all tend to be somewhat warmer than their or-
dinary surroundings, though the difference may not be noticeable
unless the heat production is considerable. A frog or a fish is a
little hotter than the air or water in which it lives, but not much;
the little -heat that it produces is lost, by radiation or conduction,
almost at once. Hence such animals have no proper temperature
of their own; on a warm day they are warm, on a cold day cold,
and are accordingly known as changeable-temperatured (poikilo-
thermous) or, in ordinary language, "cold-blooded" animals.
Man and other mammals, as well as birds, on the contrary, are
the seat of very active chemical changes by which much heat is
produced, and so maintain a tolerably uniform temperature of
their own, much as a fire does whether it be burning in a warm or
a cold room; the heat production during any given time balancing
the loss, a normal body temperature is maintained, and usually
one considerably higher than that of the medium in which they
live; such animals are commonly named "warm-blooded." This
name, however, does not properly express the facts; a lizard
basking in the sun on a warm summer's day may be quite as hot
as a man usually is; but on the cold day the lizard becomes cold,
while the average temperature of the healthy Human Body is,
within a degree, the same in winter or summer; within the arctic
circle or on the equator. Hence it is better to call such animals
" homothermous " or of uniform temperature.

Moderate warmth accelerates protoplasmic activity; compare
a frog dormant in the winter with the same animal active in the
warm months: what is true of the whole frog is true of each of its
living cells. Its muscles contract more rapidly when wanned.
and the white corpuscles of its blood when heated up to the tem-

483

484 THE HUMAN BODY

pcrature of the Human Body arc seen (with the microscope) to
exhibit much more active amoeboid movements than they do at
the temperature of frog's blood. In summer a frog or other cold-
blooded animal uses much more oxygen and evolves much more
carbon dioxid than in winter, as shown not only by direct meas-
urements of its gaseous exchanges, but by the fact that in winter
a frog can live a long time after its lungs have been removed
(being able to breathe sufficiently through its moist skin), while
in warm weather it dies of asphyxia very soon after the same loss.
The warmer weather puts its tissues in a more active state; and
so the amount of work the animal does, and therefore the amount
of oxygen it needs, depend to a great extent upon the temperature
of the medium in which it is living. With the warm-blooded
animal the reverse is the case. Within very wide limits of expo-
sure to heat or cold it maintains its temperature at that at which
its tissues live best; accordingly in cold weather it uses more
oxygen and sets free more carbon dioxid because it needs a more
active internal combustion to compensate for its greater loss of
heat to the exterior. And it does not become warmer in warm
weather, partly because its oxidations are less than in cold (other
things being equal), and partly because of physiological arrange-
ments by which it loses heat faster from its body. In fact the
living tissues of a man may be compared to hothouse plants,
living in an artificially maintained temperature; but they differ
from the plants in the fact that they themselves are the seats of
the combustions by which the temperature is kept up. Since,
within wide limits, the Human Body retains the same tempera-
ture no matter whether it be in cold or warm surroundings, it is
clear that it must possess an accurate arrangement for heat reg-
ulation; either by controlling the production of heat in it, or the
loss of heat from it, or both.

The Temperature of the Body. The parts of the Body are all
either in contact with one another directly or, if not, at least in-
directly through the blood, which, flowing from part to part,
carries heat from warmer to colder regions. Thus, although at
one time one group of muscles may especially work, liberating
heat, and at other times another, or the muscles may be at rest
and the glands the seat of active oxidation, the temperature of
the whole Body is kept pretty much the same. The skin, however,

THE HEAT OF THE BODY

which is in direct contact with external bodies, usually colder than
itself, is cooler than the internal organs; its temperature in health
is from 36° to 37° C. (96.8-98.5° F.), being warmer in more pro-
tected parts, as the hollow of the armpit. In internal organs, as
the liver and brain, the temperature is somewhat higher. In the
lungs there is loss of the heat carried out by the expired air and
that used up in evaporating the water carried out in the breath, so
the blood returned to the heart by the pulmonary veins is slightly
colder than that carried from the right side of the heart to the
lungs.

The Sources of Animal Heat. Apart from heat received from
its surroundings in hot food and drink the sources of heat in the
Body are twofold — direct and indirect. Heat is directly pro-
duced wherever oxidation is taking place; the living tissues at
rest produce heat as one result of the chemical changes supplying
them with energy for the maintenance of their vitality: and when-
ever an organ is active and its chemical metamorphoses arc in-
creased it becomes hotter: a secreting gland or a contracting
muscle is warmer than a resting one, and the venous blood leav-
ing noticeably warmer than the arterial supplied to it. Indirectly,
heat is developed within the Body by the transformation of other
forms of energy: mainly mechanical work. All movements of
parts of the Body which do not move it in space or move ex-
ternal objects, arc transformed into heat within it; and the energy
they represent is lost in that form. Every cardiac contraction
sets the blood in movement, and this motion is for the most part
turned into heat within the Body by friction within the blood-
vessels. The same transformation of energy occurs with respect
to the movements of the alimentary canal, except in so far as they
expel matters from the Body; and every muscle in contracting
has part of the mechanical energy expended by it turned into heat
by friction against neighboring parts. Similarly the movements
of cilia and of amoeboid cells are for the most part converted in the
Body into heat.

The Maintenance of a Uniform Temperature. Obviously if
the Body is to preserve the same temperature during any period
of time the production of heat \vitliin it must exactly balance t la-
loss of heat from it during that time. In ourx-lves this balan
actually maintained within narrow limits of fluctuation through-

486 THE HUMAN BODY

out healthy life. Only in fevers, or as the result of prolonged ex-
posure to cold, is the balance upset. In fact its preservation is
necessary for the continuance of the life of a warm-blooded ani-
mal; should the temperature rise above certain limits chemical
changes, incompatible with life, occur in the tissues; for example,
at about 49° C. (120° F.) the muscles begin to become rigid. On
the other hand, death ensues if the Body be cooled down to about
19° C. (66° F,).

Since we live in an environment of constantly varying tem-
perature a rather delicate adjustment between heat production
and heat loss is required.

This adjustment is attained through the interaction of two sorts
of regulatory devices, one for controlling the loss of heat from the
Body, the other its production in the Body. As regards heat-
loss, by far the most important regulating organ is the skin: un-
der ordinary circumstances nearly 90 per cent of the total heat
given off from the Body in 24 hours goes by the skin (73 by ra-
diation and conduction, 14.5 by evaporation). This loss may be
controlled :

1. By clothing; we naturally wear more in cold and less in warm
weather; the effect of clothes being, of course, not to warm the
Body but to diminish the rate at which the heat produced in it is
lost.

2. Warmth through reflex vasomotor actions leads to dila-
tation of the skin-vessels and cold to contraction. In a warm
room the vessels on the surface dilate as shown by its redness,
while in a cold atmosphere they contract and the skin becomes
pale. But the more blood that flows through the skin the greater
will be the heat lost from the surface — and vice versa.

3. Heat induces sweating and cold checks it; the heat appears
to act, for the most part, reflexly through afferent cutaneous nerve-
fibers exciting the sweat-centers from which the secretory nerves
for the sudoriparous glands arise ; it may also act to some extent
directly on those centers, as they are thrown into activity, at least
in health, as soon as the temperature of the blood flowing through
the spinal cord is raised. In fever of course we may have a high
temperature with a dry non-sweating skin. The more sweat is
poured out, the more heat is used up in evaporating it and the
more the Body is cooled.

THE HEAT OF THE BODY 487

Of less importance in man, but of great importance in fur-
bearing animals, is the loss of heat through the lungs. In warm
weather there is quickened respiration, brought about reflexly
through the play of cutaneous sensory impulses of warmth upon
the respiratory center. This quickened respiration carries off
heat more rapidly both by increasing the amount of air warmed to
body temperature in a given time, and by increasing the evapora-
tion of water from the lungs.

Our sensations induce us to add to or diminish the heat in the
Body according to circumstances; as by cold or warm baths, and
iced or hot drinks.

As regards temperature regulation by modifying the rate of heat
production in the Body, the following points may be noted; on the
whole, .such regulation is far less important than that brought
about by changes in the rate of loss, since the necessary vital work
of the Body always necessitates the continuance of oxidative proc-
esses which liberate a tolerably large quantity of heat. The Body
cannot therefore be cooled by diminishing such oxidations; nor, on
the other hand, can it be safely warmed by largely increasing
them. Still, within certain Hmits, the heat production may be
controlled in several ways :

1. Cold increases hunger; and increased ingestion of food in-
creases bodily oxidation, as shown by the greater amount of car-
bon dioxid excreted in the hours succeeding a meal. This in-
crease is probably due to the activity into which the digestive
organs and such metabolic organs as the liver arc thrown; hepatic-
vein blood is about one degree centigrade (nearly two degrees
Fahrenheit) warmer than portal-vein blood, and during digestion
much more blood flows through the liver.

2. Cold inclines us to voluntary exercise; warmth to muscular
idleness; and the more the muscles are worked the more heat is
produced in the Body.

3. Cold tends to produce reflex muscular movements, and so in-
creased heat production; as chattering of the teeth and shivering.

4. Certain drugs, as salicylic acid, and perhaps quinine, diminish
the heat production of the Body. Their mode of action is ftill
obscure.

On the whole, however, the direct heat-regulating mechanisms
of the Human Body itself are not very eHirient, especially as

488 THE HUMAN BODY

protections against excessive cooling. Man needs to supplement
them in cold climates by the use of clothing, fuel, and exercise.

Local Temperatures. Although, by the means above described,
a wonderfully uniform bodily temperature is maintained, and by
the circulating blood all parts are kept at nearly the same warmth,
variations in both respects do occur. The arrangements for equal-
ization are not by any means fully efficient. External parts, as the
skin, the lungs (which are really external in the sense of being in
contact with the air), the mouth, and the nose chambers, are al-
ways cooler than internal; and even all parts of the skin have not
the same temperature, such hollows as the armpit being warmer
than more exposed regions. On the other hand, a secreting gland
or a working muscle becomes warmer, for the time, than the rest
of the Body, because more heat is liberated in it than is carried off
by the blood flowing through. In such organs the venous blood
leaving is warmer than the arterial coming to them; while the
reverse is the case with parts, like the skin, in which the blood is
cooled. An organ colder than the blood is of course warmed by an
increase in its circulation, as seen in the local rise of temperature
in the skin of the face in blushing.

Fever. The condition of fever or pyrexia, as an abnormally high
temperature is named, could conceivably be brought about by
increased heat production, decreased heat loss, or both; or by a
greater increase of production than of loss. Direct experiments
on animals prove that there is always increased production of
heat, in febrile diseases. This is shown by the fact that the animal
uses more oxygen and gives off more carbon dioxid in a given time
than when in health. It also usually gives off more heat, but not
enough to compensate for the increase of oxidative processes going
on in its body, and so its temperature rises. The regulating mech-
anism which in health keeps heat production and heat dissipa-
tion proportionate is out of gear. The increased heat production
during fever is usually attributed to stimulation of the oxidative
processes of the Body by toxins in the blood, but the mechanism
of their action is not known. It has been suggested that fever is a
protective reaction in that it raises the body temperature above
that which is most favorable to the growth of the invading organ-
isms, while at the same time favoring the development of the re-
sisting mechanism of the Body itself.

THE HEAT OF THE BODY 439

Clothing. While the majority of other warm-blooded animals
have coats of their own, formed of hairs or feathers, over most of
man's Body his hairy coating is merely rudimentary and has lost
nearly all physiological importance as a protection from cold; ex-
cept in tropical regions he has to protect himself by artificial gar-
ments, which his esthetic sense had led him to utilize also for pur-
poses of adornment. Here, however, we must confine ourselves to
clothes from a physiological point of view. In civilized societies
every one is required to cover most of his Body with something,
and the question is what is the best covering; the answer will vary,
of course, with the climatic conditions of the country dwelt in.
In warm countries, clothing, in general terms, should allow free
radiation or conduction of heat from the surface; in cold it should
do the reverse; and in temperate climates, with varying temper-
atures, it should vary with the season. If the surface of the
Body be exposed so that currents of air can freely traverse it
much more heat will be carried off (under those usual conditions
in which the air is cooler than the skin) than if a stationary layer
of air be maintained in contact with the surface. As every one
knows, a " draught " cools much faster than air of the same temper-
ature not in motion. All clothing, therefore, tends to keep up the
temperature of the Body by checking the renewal of the layer of
air in contact with it. Apart from this, however, clothes fall into
two great groups: those which are good, and those which are bad,
conductors of heat. The former allow changes in the external
temperature to cool or heat rapidly the air stratum in actual con-
tact with the Body, while the latter only permit these changes to
act more slowly. Of the materials used for clothes, linen is a good
conductor; calico not quite so good; and silk, wool, and fur are bad
conductors.

Whenever the surface of the Body is suddenly chilled the skin-
vessels-are contracted and those of internal parts reflexly dilated;
hence internal organs tend to become congested; this within limits
is a protective physiological process, but if excessive it is danger-
ous since the congested membranes of the nose, throat, and lungs
are especially liable to fall victims to the agencies which pro-
duce colds, influenza or even pneumonia. When hot, therefore,
the most unadvisable thing to do is to sit in a draught, throw off
the clothing, or in other ways to strive to get suddenly cooled.

490 THE HUMAN BODY

Moreover, while in the American summer it is tolerably safe to
wear good-conducting garments, and few people take cold then,
this is by no means safe in the spring or autumn, when the tem-
perature of the air is apt to vary considerably within the course of
a day. A person going out, clad only for a warm morning, may
have to return in a very much colder evening; and if his clothes be
not such as to prevent a sudden surface chill, will get off lightly if
he only " take " one of the colds so prevalent at those seasons. In
the great majority of cases, no doubt, he suffers nothing worse, but
persons, especially of the female sex, often thus acquire far more
serious diseases. When sudden changes of temperature are at all
probable, even if the prevailing weather be warm, the trunk of the
Body should be always protected by some tolerably close-fitting
garment of non-conducting material. Those whose skins are
irritated by anything but linen should wear immediately outside
the under-garments a jacket of silken or woolen material. In mid-
winter comparatively few people take cold, because all then wear
thick and non-conducting clothing of some kind.

CHAPTER XXXIII
VOICE AND SPEECH

Voice consists of sounds produced by the vibrations of two
elastic bands, the true vocal cords, placed in the larynx, an upper
modified portion of the passage which leads from the pharynx to
the lungs. When the vocal cords are put in a certain position, air
driven past them sets them in vibration, and they emit a musical
note; the lungs and respiratory muscles are, therefore, accessory
parts of the vocal apparatus: the strength of the blast produced
by them determines the loudness of the voice. The larynx itself
is the essential voice-organ : its size primarily determines the pitch
of the voice, which is lower the longer the vocal cords; and, hence,
shrill in children, and usually higher pitched in women than in
men; the male larynx grows rapidly at commencing manhood,
causing the change commonly known as the "breaking of the
.voice." Every voice, while its general pitch is dependent on the
length of the vocal cords, has, however, a certain range, within
limits which determine whether it shall be soprano, mezzo-soprano,
alto, tenor, baritone, or bass. This variety is produced by muscles
within the larynx which alter the tension of the vocal cords. Those
characters of voice which we express by such phrases as harsh,
sweet, or sympathetic, depend on the structure of the vocal cords
of the individual ; cords which in vibrating emit only harmonic
partial tones (Chap. XIV) are pleasant; while those in which in-
harmonic partials are conspicuous are disagreeable.

The vocal cords alone would produce but feeble sounds; those
that they emit are strengthened by sympathetic resonance of the
air in the pharynx and mouth, the action of which may be com-
pared to that of the sounding-board of a violin. By movements
of throat, soft palate, tongue, cheeks, and lips the sounds emitted
from the larynx are altered or supplemented in various ways, and
converted into articulate language or speech.

The Larynx lies in front of the neck, beneath the hyoid bone
and above the windpipe; in many persons it is prominent, caus-

491

THE. HUMAN BODY

ing the projection known as "Adam's apple." It consists of a
framework of cartilages, partly joined by true synovial joints
and partly bound together by membranes; muscles are added

Cs which move the cartilages with
reference to one another; and
the whole is lined by a mucous
membrane.

The cartilages of the larynx
(Fig. 145) are nine in number;
three single and median, and
Pv three pairs. The largest (£) is
called the thyroid, and consists
'•Pm of two halves which meet at an
angle in front, but separate be-
hind so as to inclose a V-shaped
space, in which most of the re-
maining cartilages lie. The
epiglottis (not represented in the

Ci

•~&^Mr

FIG. 145. — The more important carti- c \ • c -, f

lages of the larynx from behind, t, thy- Hgure) IS tlXed to the top Ot
roid; Cs, its superior, and Ci, its inferior, fhp fVnrrmYl r>nrtilao-<3 onrl rviror
horn of the right side; **, cricoid carti- l tn^r( <-ar™age and OVC1
lage;t,arytenoid cartilage ;Pv, the corner hangS the entry from the phar-
to which the posterior end of a vocal cord , .

is attached; Pm, corner on which the ynx tO the larynx; it may be
muscles which approximate or separate _ .„ j v

the vocal cords are inserted; co, cartilage Seen> covered by niUCOUS mcm-

of Santorini. brane, projecting at the base of

the tongue, if the latter be pushed down while the mouth is held
open in front of a mirror; and is, similarly covered, represented, as
seen from behind, at a in Fig. 146. The cricoid, the last of the un-
paired cartilages, has the shape of a signet-ring; its broad part
(**, Fig. 145) is on the posterior side and lies at the lower part of
the opening between the halves of the thyroid; in front and on
the sides it is narrow, and a space, occupied by the cricothyroid
membrane, intervenes between its upper border and the lower
edge of the thyroid cartilage. The angles of the latter are
produced above and below into projecting horns (Cs and Ci,
Fig. 145), and the lower horn on each side forms a joint with the
cricoid. The thyroid can be rotated on an axis, passing through
the joints on each side, and rolled down so that its lower front
edge shall come nearer the cricoid cartilage, the membrane there
intervening being folded. The arytenoids (f, Fig. 145) are the

VOICE AND SPEECH

493

largest of the paired cartilages; they are seated on the upper

edge of the posterior wide portion of the cricoid, and form

true joints with it. Each is

pyramidal with a triangular

base, and has on its tip a small

nodule (co, Fig. 145), the carti-

lage of Santorini. From the tip

of each arytcnoid cartilage the

aryteno-epiglottic fold of mucous

membrane (10, Fig. 146) extends

to the epiglottis; the cartilage of

Santorini causes a projection

(8, Fig. 146) in this, and a little

farther on (9) is a similar emi-

nence on each side, caused by

the remaining pair of cartilages,

known as the cuneiform, or car-

tilages of Wrisberg.

The Vocal Cords are bands of
elastic tissue which reach from
the inner angle (Pv, Fig. 145) of
the base of each arytenoid carti-
lage to the angle on the inside
of the thyroid where the sides
of the V unite; they thus meet
in front but are separated at of, "™ ^f^P* ha? bw.Ill dividedand its

r edges (11) turned aside. 1, body of

their Other ends. The COrds hyoid; 2, its small, and 3, its great, hunts;

, , . 4, upper and lower horns of thyroid car-

are not, however, bare Strings, tilage; 5, mucous membrane of front of

likp thrxso of n Vinrn hilt pnvprprl Pharvnx. covering the back of the cricoid

)i a narp, out coveieu cartifagt.; 6> upper cnd of Kunet. 7f

Over With the lining mUCOUS windpipe, lying in front of the gullet;
rii T ' 9mmence caused by cartilage of San-

membrane of the larynx, a slit, torini; 9, emineaoe cannd b\- cartilage

,1 •• ,r in-/ T\' i Af>\ of Wrisberg; both lie in, 10, tlie aryteno-

Called the glOttlS (C, Jblg. 140), epigloUic foldoi mucous membrane, sur-

being left between them. It is

the projecting cushions formed t'p of epiglottis; c, the glottis, the lines

. . leading from the latter point to the free

by them On each Side Of thlS vibratory edges of the vocal cords. /,',

slit which are set in vibration edges? marking them ^M&m *•"!££
during phonation. Above each nences 6- are the fabe voral l'linls-
vocal cord is a depression, the ventricle of the larynx (b', Fig. 146);
this is bounded above by a somewhat prominent edge, the false

494 THE HUMAN BODY

vocal cord. Over most of the interior of the larynx its mucous
membrane is thick and covered by ciliated epithelium, and has
many mucous glands embedded in it. Over the vocal cords,
however, it is represented only by a thin layer of flat non-
ciliated cells, and contains no glands. In quiet breathing, and
after death, the free inner edges of the vocal cords are thick and
rounded, and seem very unsuitable for being readily set in vibra-
tion. They are also tolerably widely separated behind, the aryte-
noid cartilages, to which their posterior ends are attached, being
separated. Air under these conditions passes through without pro-
ducing voice. If they are watched with the laryngoscope during
phonation, it is seen that the cords approximate behind so as to
narrow the glottis; at the same time they become more tense, and
their inner edges project more sharply and form a better-defined
margin to the glottis, and their vibrations can be seen. These
changes are brought about by the delicately coordinated activity
of a number of small muscles, which move the cartilages to which
the cords are fixed.

The Muscles of the Larynx. In describing the direction and
action of these it is convenient to use the words front or anterior
and back or posterior with reference to the larynx itself (that is,
as equivalent to ventral and dorsal) and not with reference to the
head, as usual. The base of each arytenoid cartilage is triangular
and fits on a surface of the cricoid, on which it can slip to and fro
to some extent, the ligaments of the joint being lax. One corner
of the triangular base is directed inwards and forwards (i. e., to-
wards the thyroid) and is called the vocal process (Pv, Fig. 145) , as
to it the vocal cords are fixed. The outer posterior angle (Pm,
Fig. 145) has several muscles inserted on it and is called the mus-
cular process. If it be pulled back and towards the middle line
the arytenoid cartilage will rotate on its vertical axis, and roll
its vocal processes forwards and outwards, and so widen the
glottis; the reverse will happen if the muscular process be drawn
forwards. The muscle producing the former movement is the
posterior crico-arytenoid (Cap, Fig. 147); it arises from the back
of the cricoid cartilage, and narrows to its insertion into the mus-
cular process of the arytenoid on the same side. The opponent
of this muscle is the lateral crico-arytenoid, which arises from the
side of the cricoid cartilage, on its inner surface, and passes up-

VOICE AND SPEECH 495

wards and backwards to the muscular process. The posterior
crico-arytenoids, working alone, pull inwards and downwards the
muscular processes, turn upwards and outwards the vocal proc-
ess.es, and separate the posterior ends of the vocal cords. The
Jateral cricothyroid, working alone, pulls downwards and for-
wards the muscular process, and rotates inwards and upwards
the vocal process, and narrows the glottis; it is the chief agent in
producing the approximation of the cords necessary for the pro-
duction of voice. When both pairs of muscles act together, how-
ever, each neutralizes the tendency of the other to rotate the

Fio. 147. — The larynx seen from behind and dissected BO as to display some of
its muscles. The mucous membrane of the front of the pharynx (5, Fig! 146) has
been dissected away, so as to display the laryngeal muscles beneath it. Part of
the left half of the thyroid cartilage has been cut away, co, cartilage of San-
tori ni ; cu, cartilage of Wrisberg.

arytenoid cartilage; the downward part of the pull of each is, thus,
alone left, and this causes the arytenoid to slip downwards and
outwards, off the eminence on the cricoid with which it articu-
lates, as far as the loose capsular ligament of the joint will allow.
The arytenoid cartilages are thus moved apart and the glottis
greatly widened and brought into its state in deep quiet breathing.

496 THE HUMAN BODY

Other muscles approximate the arytenoid cartilages after the car-
tilages have been separated. The most important is the transverse
arytenoid (A, Fig. 147), which runs across from one arytenoid car-
tilage to the other. Another is the oblique arytenoid (Taep), which
runs across the middle line from the base of one arytenoid to the
tip of the other; thence certain fibers continue in the aryteno-
epiglottic fold (10, Fig. 146) to the base of the epiglottis; this,
with its fellow, embraces the whole entry to the larynx; when
they contract they bend inwards the tips of the arytenoid carti-
lages, approximate the edges of the aryteno-epiglottic fold, and
draw down the epiglottis, and so close the passage from the
pharynx to the larynx. When the epiglottis has been removed,
food and drink rarely enter the larynx in swallowing, the folds of
mucous membrane being so brought together as to effectually close
the aperture between them.

Increased tension of the vocal cords is produced by the crico-
thyroid muscles, one of which lies on each side of the larynx, over
the cricothyroid membrane. Their action may be understood
by help of the diagram, Fig. 148, in which t represents the thyroid

cartilage, c the cricoid, a an arytenoid,
and vc a vocal cord. The muscle passes
obliquely backwards and upwards from
about d near the front end of c, to t,
about I, near the pivot (which represents
the joint between the cricoid cartilage
.^~-^ *'*'*-./ /*/ and the inferior horn of the thyroid).
^"--^V^' / When the muscle contracts it pulls to-

-pi gether the anterior ends of t and c; either

by depressing the thyroid (as represented
by the dotted lines) or by raising the front

end of the cricoid; and thus stretches the vocal cord, if the aryte-
noid cartilages be held from slipping forwards. The antagonist of
the cricothyroid is the thyro-arytenoid muscle; it lies, on each side,
embedded in the fold of elastic tissue forming the vocal cord, and
passes from the inside of the angle of the thyroid cartilage in front,
to the anterior angle and front surface of the arytenoid behind.
If the latter be held firm, the muscle raises the thyroid cartilage
from the position into which the cricothyroid pulls it down, and so
slackens the vocal cords. If the thyroid be held fixed by the

VOICE AND SPEECH 497

cricothyroid muscle, the thyro-arytenoicl will help to approxi-
mate the vocal cords, rotating inwards the vocal processes of the
arytenoids.

The lengthening of the vocal cords when the thyroid cartilage
is depressed tends to lower their pitch ; the increased tension, how-
ever, more than compensates for this and raises it. There seems,
however, still another method by which high notes are produced.
Beginning at the bottom of his register, a singer can go on up the
scale some distance without a break ; but, then, to reach his higher
notes, must pause, rearrange his larynx, and begin again. What
happens is that, at first, the vocal processes arc turned in, so as to
approximate but not to meet ; the whole length of each edge of the
glottis then vibrates, and its tension is increased, and the pitch
of the note raised, by increasing contraction of the cricothyroid.
At last this attains its limit and a new method has to be adopted.
The vocal processes are more rolled in, until they touch. This
produces a node (see Physics) at that point and shortens the
length of vocal cord which vibrates. The shorter string emits a
higher note; so the cricothyroid is relaxed, and then again gradu-
ally tightened as the notes sung are raised in pitch from the new
starting-point. To pass easily ,and imperceptibly from one such
arrangement of the larynx to another is a great art in singing.
There is some reason to believe that a second node may, for still
higher notes, be produced at a more anterior point on the vocal
cords.

The method of production of falsetto notes is uncertain; dur-
ing their emission the free border of the vocal cords alone vi-
brates.

The range of the human voice is about three octaves, from
e (80 vib. per 1") on the unaccented octave, in male voices,
to c on the thrice-accented octave (1,024 vib. per 1"), in fe-
male. Great singers of course go beyond this range; basses
have been known to take a on the great octave (55 vib. per 1") ;
and Nilsson in "II Flauto Magico" used to take /on the
fourth accented octave (1,408 vib. per 1"). Mozart heard at
Parma, in 1770, an Italian songstress whose voice had the ex-
traordinary range from g in the first accented octave (198 vib.
per 1") to c on the fifth accented octave (2,112 vib. per 1").
An ordinary good bass voice has a compass from / (88 vib.

498 THE HUMAN BODY

per 1") to d" (297 vib. per 1"); and a soprano from b' (248 vib.
per 1") to g"f (792).

Vowels are, primarily, compound musical tones produced in
the larynx. Accompanying the primary partial of each, which
determines its pitch when said or sung, are a number of upper
partials, the first five or six being recognizable in good full voices.
Certain of these upper partials are reinforced in the mouth to
produce one vowel, and others for other vowels; so that the va-
rious vowel sounds are really musical notes differing from one an-
other in timbre. The mouth and throat cavities form an air-
chamber above the larnyx, and this has a note of its own which
varies with its size and form, as may be observed by opening the
mouth widely, with the lips retracted and the cheeks tense; then
gradually closing it and protruding the lips, meanwhile tapping
the cheek. As the mouth changes its form the note produced
changes, tending in general to pass from a higher to a lower pitch
and suggesting to the ear at the same time a change from the
sound of a (father) through o (more) to oo (moor). When the
mouth and throat chambers are so arranged that the air in them
has a vibratory rate in unison with any partial in the laryngeal
tone, it will be set in sympathetic vibration, that partial will be
strengthened, and the vowel characterized by it uttered. As the
mouth alters its form, although the same note be still sung, the
vowel changes. In the above series (a, o, oo) the tongue is de-
pressed and the cavity forms one chamber; for a this has a wide
mouth opening ; for o it is narrowed ; for oo still more narrowed,
and the lips protruded so as to increase the length of the resonance
chamber. The partial tones reinforced in each case are, accord-
ing to Helmholtz —

In other cases the mouth and throat cavity is partially subdi-
vided, by elevating the tongue, into a wide posterior and a nar-
row anterior part, each of which has its own note ; and the vowels
thus produced owe their character to two reinforced partials.

VOICE AND SPEECH 1,,

This is the case with the series a" (man), c (there), and i (machine),
the tones reinforced by resonance in the mouth being—

The usual I of English, as in spire, is not a true simple vowel
but a diphthong, consisting of & (pad) followed by e (feet), as
may be observed by trying to sing a sustained note to the sound » ;
it will then be seen that it begins as & and ends as ee. A simple
vowel can be maintained pure as long as the breath holds out.

In uttering true vowel sounds the soft palate is raised so as to
cut off the air in the nose, which, thus, does not take part in the
sympathetic resonance. For some other sounds (the semi-vowels
or resonants) the initial step is, as in the case of the true vowels,
the production of a laryngeal tone; but the soft palate is not
raised, and the mouth exit is more or less closed by the lips or the
tongue; hence the blast partly issues through the nose, and the
air there takes part in the vibrations and gives them a special
character; this is the case with m, n, and ng.

Consonants are sounds produced not mainly by the vocal cords,
but by modifications of the expiratory blast on its way through
the mouth. The current may be interrupted and the sound
changed by the lips (labials) ; or, at or near the teeth, by the tip
of the tongue (dentals) ; or, in the throat, by the root of the tongue
and the soft palate (gutturals). Consonants are also characterized
by the kind of movement which gives rise to them. In explosives
an interruption to the passage of the air-current is suddenly in-
terposed or removed (P, T, B, D, K, G). Other consonants are
continuous (as F, S, R), and may be subdivided into: (1) Aspirates,
characterized by the sound produced by a rush of air through a
narrow passage, as when the lips are approximated (F), or the
teeth (S), or the tongue is brought near the palate (Sh), or its tip
against the two rows of teeth, they not being quite in contact
(Th). For L the tongue is put against the hard palate and the

500 THE HUMAN BODY

air escapes on its sides. For Ch (as in the proper Scotch pronun-
ciation of loch) the passage between the back of the tongue and
the soft palate is narrowed. To many of the above pure conso-
nants answer others, in whose production true vocalization (i. e.,
a laryngeal tone) takes a part. F with some voice becomes V;
S becomes Z, Th soft (teei/i) becomes Th hard; and Ch becomes
Gh. (2) Resonants; these have been referred to above. (3) Vibra-
tories (the different forms of R), which are due to vibrations of
parts bounding a constriction put in the course of the air-current.
Ordinary R is due to vibrations of the tip of the tongue held near
the hard palate; and guttural R to vibrations of the uvula and
parts of the pharynx.

The consonants may physiologically be classified as in the fol-
lowing table (Foster) :

Explosives. Labials, without voice P.

" with voice B.

Dentals, without voice T.

" with voice D.

Gutturals, without voice K.

" with voice G (hard).

Aspirates. Labials, without voice F.

" with voice V.

Dentals, without voice S, L, Sh, Th (hard).

with voice Z, Zh (azure), Th (soft).

Gutturals, without voice Ch (loch).

" with voice Ch.

Resonants. Labial M.

Dental N.

Gutteral NG.

Vibratories. Labial — not used in European languages.

Dental R (common).

Guttural R (guttural).

H is a laryngeal sound: the vocal cords are separated for its
production, yet not so far as in quiet breathing. The air-current
then produces a friction sound but not a true note, as it passes
the glottis; and this is again modified when the current strikes
the wall of the pharynx. Simple sudden closure of the glottis,
attended with no sound, is also a speech element, though we do
not indicate it with a special letter, since it is always understood
when a word begins with a vowel, and only rarely is used at other
times. The Greeks had a special sign for it, ', the soft breathing;

VOICE AND SPEECH 501

and another, ', the hard breathing, answering somewhat to our h
and indicating that the larynx was to be held open, so as to give
a friction sound, but not voice.

In whispering there is no true voice; the latter implies true
tones, and these are only produced by periodic vibrations ; whisper-
ing is a noise. To produce it the glottis is considerably narrowed
but the cords are not so stretched as to produce a sharply denned
edge on them, and the air driven past is then thrown into irregular
vibrations. Such vibrations as coincide in period with the air
in the mouth and throat are always present in sufficient number
to characterize the vowels; and the consonants are produced in
the ordinary way, though the distinction between such letters as
P and B, F and V, remains imperfect.

CHAPTER XXXIV
REPRODUCTION

Reproduction in General. In all cases reproduction consists,
essentially, in the separation of a portion of living matter from a
parent; the separated part bearing with it, or inheriting, certain
tendencies to repeat, with more or less variation, the life history
of its progenitor. In the more simple cases a parent merely di-
vides into two or more pieces, each resembling itself except in
size; these then grow and repeat the process; as, for instance, in
the case of Amceba and our own white blood corpuscles (p. 19).
Such a process may be summed up in two words as discontinu-
ous growth; the mass, instead of increasing in size without seg-
mentation, divides as it grows, and so forms independent living
beings. In some tolerably complex multicellular animals we find
essentially the same thing; at times certain cells of the fresh-
water Polyp multiply by simple division in the manner above
described, but there is a certain concert between them : they build
up a tube projecting from the side of the parent, a mouth-opening
forms at the distal end of this, tentacles sprout out around it,
and only when thus completely built up and equipped is the young
Hydra set loose on its own career. How closely such a mode of
multiplication is allied to mere growth is shown by other polyps
in which the young, thus formed, remain permanently attached
to the parent stem, so that a compound animal results. This
mode of reproduction (known as gemmation or budding) may be
compared to the method in which many of the ancient Greek
colonies were founded ; carefully organized and prepared at home,
they were sent out with a due proportion of artificers of various
kinds; so that the new commonwealth had from its first separa-
tion a considerable division of employments in it, and was, on a
small scale, a repetition of the parent community. In the great
majority of animals, however (even those which at times multi-
ply by budding), a different mode of reproduction occurs, one
more like that by which our western lands were settled and grad-

502

REPRODUCTION %%

ually built up into Territories and States. The new individual
in the political world began with little differentiation; it consisted
of units, separated from older and highly organized societies, and
these units at first did pretty much everything, each man for him-
self, with more or less efficiency. As growth took place develop-
ment also occurred; persons assumed different duties and per-
formed different work until, finally, a fully organized State was
formed. Similarly, the body of one of the higher animals is, at
an early stage of life, merely a collection of undifferentiated cells,
each capable of multiplication by division, and more or less re-
taining all its original protoplasmic properties; and with no spe-
cific individual endowment or function. The mass (Chap. Ill)
then slowly differentiates into the various tissues, each with a
predominant character and duty; at the same time the majority
of the cells lose their primitive powers of reproduction, though
exactly how completely is a problem not yet sufficiently studied.
In adult Vertebrates it seems certain that the white blood cor-
puscles multiply by division: and in some cases (in the newts or
tritons, for example) a limb is reproduced after amputation.
But exactly what cells take^ part in such restorative processes is
uncertain ; we do not know whether or not the old bone corpuscles
left form new bones, old muscle-fibers new muscles, and so on.
In Mammals no such restoration occurs; an amputated leg may
heal at the stump but does not form again. In the healing proc-
esses the connective tissues play the main part, as we might
expect; their cellular elements being but little modified from
their primitive state can still multiply and develop. New blood-
capillaries, however, sprout out from the sides of old, and new
epidermis seems only to be formed by the multiplication of
epidermic cells; hence the practice, frequently adopted by sur-
geons, of transplanting little bits of skin to points on the surface
of an extensive burn or ulcer. In blood-capillaries and epidermis
the departure from the primary undifferentiated cell is but slight ;
and, as regards the cuticle, one of the permanent physiological
characters of the cells of the rete mucosum is their multiplication
throughout the whole of life; that is a main physiological char-
acteristic of the tissue: the same is very probably true of the pro-
toplasmic cells forming the walls of the capillaries. When a highly
differentiated tissue is replaced in the body of mammals after

504 THE HUMAN BODY

breaking down or removal, it is usually by the activity of special
cells set apart for that purpose, or by repair or outgrowth of the
cells affected and not by their division. The red blood-corpuscles
are constantly being broken down and replaced, but the new ones
are not formed by the division of already fully formed corpuscles
but by certain special hematoblastic cells retained throughout
life in the red marrow of bone. The nervous tissues are highly
differentiated and a nerve is often regenerated after division, but
this is by outgrowth of the ends of axons still attached to their
cells and by secondary formation of a myelin sheath around these,
and not by division or multiplication of already existing fibers.
A striped muscle when cut across is healed by the formation of a
band of connective tissue; after a very long time it is said that
true muscular fibers may be found in the cicatrix, but their origin
is not known ; it is probably not from previously developed muscle-
fibers. On the other hand, the less differentiated unstriated
muscle has been observed to be repaired in some cases after
injury by true karyokinetic division of previously formed muscle-
cells. Although many gland-cells in the performance of their phys-
iological work are partially broken down and lost in their secre-
tion, and then repaired by the residue of the cell, multiplication by
division of fully differentiated gland-cells does not appear to occur,
if we except such organs as the testes, the secretion of which con-
sists essentially of cells. An excised portion of a salivary or pa-
rotid gland is never regenerated: the wound is repaired by con-
nective tissues.

We find, then, as we ascend in the animal scale a diminishing
reproductive power in the tissues generally: with the increasing
division of physiological labor, with the changes that fit pre-
eminently for one work, there is a loss of other faculties, and this
one among them. The more specialized a tissue the less the re-
productive power of its elements, and the most differentiated
tissues are either not reproduced at all after injury, or only
by the specialization of amoeboid cells, and not by a progeni-
tive activity of survivors of the same kind as those destroyed.
In none of the higher animals, therefore, do we find multi-
plication by simple division, or by budding: no one cell, and
no group of cells used for the physiological maintenance of
the individual, can build up a new complete living being; but

REPRODUCTION 505

the continuance of the race is specially provided for by setting
apart certain cells which shall have this one property— cells
who.se duty is to the species and not to any one representative of
it— an essentially altruistic element in the otherwise egoistic whole.

Sexual Reproduction. In some cases, especially among insects,
the specialized reproductive cells can develop, each for itself,'
under suitable conditions, and give rise to new individuals; such
a mode of reproduction is called parthenogenesis: but in the major-
ity of cases, and always in the higher animals, this is not so; the
fusion of two cells, or of products of two cells, is a necessary pre-
liminary to development. Commonly the coalescing cells differ
considerably in size and form, and one takes a more direct share
in the developmental processes; this is the egg-cell or ovum; the
other is the sperm-cell or spermatozoon. The fusion of the two is
known as fertilization. Animals producing both ova and sperma-
tozoa are hermaphrodite; those bearing ova only, female; and those
spermatozoa only, male: hermaphroditism is not found in Verte-
brates, except in rare and doubtful cases of -monstrosity.

Accessory Reproductive Organs. The organ in which ova are
produced is known as the ovary, that forming spermatozoa, as
the testis or testicle; but in different groups of animals many addi-
tional accessory parts may be developed. Thus, in all but the
very lowest Mammalia, the offspring is nourished for a consid-
erable portion of its early life within the body of its mother, a
special cavity, the uterus or womb, being provided for this purpose:
the womb communicates with the exterior by a passage, the va-
gina; and two tubes, the oviducts or Fallopian tubes, convey the
eggs to it from the ovaries. In addition, mammon/ glands provide
milk for the nourishment of the young in the first months after
birth. In the male mammal we find as accessory reproductive
organs, vasa deferentia which convey from the testes the seminal
fluid containing spermatozoa; vesiculce seminales (not present in
all Mammalia), glands whose secretion is mixed with that of the
testes or is expelled after it in the sexual act; a prostate gland,
whose secretion is added to the semen; and an erectile organ, the
penis, by which the fertilizing liquid is conveyed into the vagina
of the female.

The Male Reproductive Organs. The testes in man are paired
tubular glands, which lie in a pouch of skin called the scrotum.

506

THE HUMAN BODY

This pouch is subdivided internally by a partition into right and
left chambers, in each of which a testicle lies. The chambers are
lined inside by a serous membrane, the tunica vaginalis, and this
doubles back (like the pleura round the lung) and covers the ex-
terior of the gland. Between the external and reflected layers of the
tunica vaginalis is a space containing a small quantity of lymph.
The testicles develop in the abdominal cavity, and only later
(though commonly before birth) descend into the scrotum,
passing through apertures in the muscles, etc., of the abdominal
wall, and then sliding down over the front of the pubes, beneath
the skin. The cavity of the tunica vaginalis at first is a mere
offshoot of the peritoneal cavity, and its
serous membrane is originally a part of the
peritoneum. In the early years of life the
passage along which the testis passes usually
becomes nearly closed up, and the com-
munication between the peritoneal cavity
and that of the tunica vaginalis is also ob-
literated. Traces of this passage can, how-
ever, readily be observed in male infants;
if the skin inside the thigh be tickled a
muscle lying beneath the skin of the scrotum
is made to contract reflexly, and the testis
is ierked up some way towards the abdo-

FJG. 149.— Diagram of J *. J,

a vertical section through men and quite out of the scrotum. Some-

semimferi; Cvasa Seta'; times *ne passage remains permanently open
d, yasa efferentia ending an(j a co]\ of intestine may descend along

in the com vasculosi ; e, e,

epididymis. h, vas def- it and enter the scrotum, constituting an
inguinal hernia or rupture. A hydrocele is
an excessive accumulation of liquid in the serous cavity of the
tunica vaginalis.

Beneath its covering of serous membrane each testis has a
proper fibrous tunic of its own. This forms a thick mass on
the posterior side of the gland, from which partitions or septa
(i, Fig. 149) radiate, subdividing the gland into many chambers.
In each chamber lie several greatly coiled seminiferous tubules, a,
a, averaging in length 0.68 meter (27 inches) and in diameter only
0.14 mm. (y^ inch). Their total number in each gland is about
800. Near the posterior side of the testis the tubules unite to

REPRODUCTION 507

form about 20 rasa recta (b), and these pass out of the gland at its
upper end, as the vasa efferentia (d), which become coiled up into
conical masses, the coni vasculosi; these, when unrolled, are tubes
from 15 to 20 cm. (6-8 inches) in length; they taper somewhat
from their commencements at the vasa efferentia, where they arc
0.5 mm. (^ inch) in diameter, to the other end where they ter-
minate in the epididymis (e, e, Fig. 149). The latter is a narrow
mass, slightly longer than the testicle, which lies along the posterior
side of that organ, near the lower end of which it passes (g) into the
vas deferens, h. If the epididymis be carefully unravelled it is
found to consist of a tube about 6 meters (20 feet) in length, and
varying in diameter from 0.35 to 0.25 mm. (^ to ^0 inch).

The vas deferens (h, Fig. 149) commences at the lower part of
the epididymis as a coiled tube, but it soon ceases to be convo-
luted and passes up beneath the skin covering the inner part of
the groin, till it gets above the pelvis and then, passing through
the abdominal walls, turns inwards, backwards, and downwards,
to the under side of the urinary bladder, where it joins the duct
of the seminal vesicle; it is about 0.6 meter (2 feet) in length and
2.5 mm. (yV inch) in diameter. Its lining epithelium is ciliated.

The vesiculcc seminales, two in number, are membranous recepta-
cles which lie, one on each side, beneath the bladder, between it and
the rectum. They are commonly about 5 cm. (2 inches) long and a
little more than a centimeter wide (or about 0.5 inch) at their broad-
est part. The narrowed end of each enters the vas deferens on its
own side, the tube formed by the union being the ejaculatory duct,
which, after a course of about an inch, enters the urethra near
the neck of the bladder. In some animals the vesicular seminales
form a liquid which is added to the secretion of the testis. In man
they appear to be merely reservoirs in which the semen collects.

The prostate gland is a dense body, about the size of a large
chestnut, which surrounds the commencement of the urethra;
the ejaculatory ducts pass through it. It is largely made up of
fibrous and unstriped muscular tissues, but contains also a num-
ber of small secreting saccules whose ducts open into the urethra.
The prostatic secretion though small in amount would appear to
be of import unco: at least the gland remains undeveloped in per-
sons who have been cast rated in childhood; and atrophies after
removal of the testicles later in life.

508 THE HUMAN BODY

The male urethra leads from the bladder to the end of the penis,
where it terminates in an opening, the meatus urinarius. It is de-
scribed by anatomists as made up of three portions, the prostatic,
the membranous, and the spongy. The first is surrounded by
the prostate gland and receives the ejaculatory ducts. On its
posterior wall, . close to the bladder, is an elevation containing
erectile tissues (see below) and supposed to be dilated during
sexual congress, so as to cut off the passage to the urinary recep-
tacle. On this crest is an opening leading into a small recess, the
utricle, which is of interest, since the study of its embryology
shows it to be an undeveloped male uterus. The succeeding mem-
branous portion of the urethra is about 1.8 cm. (£ inch) long; the
spongy portion lies in the penis.

The penis is composed mainly of erectile tissue, i. e., tissues so
arranged as to inclose cavities which can be distended by blood.
Covered outside by the skin, internally it is made up of three
elongated cylindrical masses, two of which, the corpora cavernosa,
lie on its anterior side ; the third, the corpus spongiosum, surrounds
the urethra and lies on the posterior side of the organ for most of
its length; it, however, alono forms the terminal dilatation, or
glans, of the penis. Each corpus cavernosum is closely united to
its fellow in the middle line and extends from the, pubic bones,
to which it is attached behind, to the glans penis in front. It is
enveloped in a dense connective-tissue capsule from which nu-
merous bars, containing white fibrous, elastic, and unstriped
muscular tissues, radiate and intersect in all directions, dividing
its interior into many irregular chambers called venous sinuses.
Into these blood is conveyed partly through open capillaries,
partly directly by the open ends of small arteries; this blood is
carried off by veins proceeding from the sinuses.

The arteries of the penis are supplied with vasodilator nerves,
the nervi erigentes, derived from the sacral plexus. Under cer-
tain conditions these are stimulated and, the arteries expanding,
blood is poured into the venous sinuses faster than the veins drain
it off; the latter are probably also at the same time compressed
where they leave the penis by the contraction of certain muscles
passing over them. Simultaneously the involuntary muscular
tissue of the bars ramifying through the erectile masses relaxes.
As a result the whole organ becomes distended and finally rigid

REPRODUCTION 509

and erect. The coordinating center of erection lies in the lumbar
region of the spinal cord, and may be excited reflexly by mechan-
ical stimulation of the penis, or under the influence of nervous
impulses originating in the brain and associated with sexual emo-
tions. The corpus spongiosum resembles the corpora cavernosa
in essential structure and function.

The skin of the penis is thin and forms a simple layer for some
distance; towards the end of the organ it separates and forms a
fold, the foreskin or prepuce, which doubles back, and, becoming
soft, moist, red, and very vascular, covers the glans to the mcalu*
urinarius, where it becomes continuous with the mucous mem-
brane of the urethra; in it, near the projecting posterior rim of
the glans, are embedded many sebaceous glands. It possesses
nerve end organs (genital corpuscles) which much resemble end
bulbs iii structure.

The Seminal Fluid. The essential elements of the testicular
secretion are much modified cells, the spermatozoa, which are
passed out with some albuminous liquid. The spermatozoa
(Fig. 150) are motile bodies about 0.04 mm. (g^ inch) in length.
They have a flattened clear body or head and a
long vibratile tail or cilium; the portion of the
tail nearest to the head is thicker than the rest,
and is known as the neck. The mode of develop-
ment of a spermatozoon shows that the head is a
cell-nucleus and the neck and tail a modified cell-
body. Flo. 150.— Sper-

On cross-section a seminiferous tubule prc-

scnts externally a well-marked basement mem- side view, a, head;
I-!, it ,6, neck; c, tail.

branc, upon which arc borne several layers of
cells; the lumen or bore of the tubule is in great part occu-
pied by the tails of spermatozoa projecting from some of the
lining cells. The outer cells, those next the basement membrane,
are arranged in a single layer, and are usually found in one or
other stage of active karyokinetic division (p. 19). The result of
the division is an outer cell, which remains next the basement
membrane to repeat the process, and an inner, which is the mother-
cell of spermatozoa. The laticr cell by repeated mitotic division
gives rise to four cells lying side by side and each having a rela-
tively large nucleus and small cell-body. These cells elongate,

510 THE HUMAN BODY

the nucleus remaining near the deeper end and the protoplasm
extending towards the lumen of the tubule, into which it ulti-
mately projects. Such cells are spermatids. Interlaced among
them are other granular supporting cells of the epithelium, which
are probably concerned with the nutrition of the essential cells.
Each spermatid develops directly into a functional spermatozoon
The spermatozoa appear frequently to be cast off before their de-
velopment is completed : at least many spermatids which have not
gone through the final stages are found in the vasa recta, and
even in the vas deferens. Probably the secretion normally collects
in the vesiculce seminales, and there undergoes its final elaboration.
The Reproductive Organs of the Female. Each ovary (o,
Fig. 151) is a dense oval mass about 3.25 cm. (1.5 inches) in
length, 2 cm. (0.75 inch) in width, and 1.27 cm. (0.5 inch) in
thickness; it weighs from 4 to 7 grams (60-100 grains). The
organs lie in the pelvic cavity enveloped in a fold of peritoneum
(the broad ligament), and receive blood-vessels and nerves along
one border. From time to time ova reach the surface, burst
through the enveloping peritoneum, and are received by. the wide
fringed aperture, fi, of the oviduct or Fallopian tube, od. This
tube narrows towards its inner end, where it communicates with
the uterus, and is lined by a mucous membrane, covered by
ciliated epithelium; plain muscular tissue is also developed in its
wall. The uterus (u, c, Fig. 151) is a hollow organ, with relatively
thick muscular walls (left unshaded in the figure) ; it contains the
fetus during pregnancy and expels it at birth; it lies in the pelvis
between the urinary bladder and the rectum (Fig. 152) ; the Fal-
lopian tubes open into its anterior corners. It is free above, but
its lower end is attached to and projects into the vagina. In the
fully developed virgin state the organ is somewhat pear-shaped,
but flattened from before back; about 7.5 cm. (3 inches) in length,
5 cm. (2 inches) in breadth at its upper widest part, and 2.5 cm.
(1 inch) in thickness; it weighs from 25 to 42 grams Q- to 1£ oz.).
The upper wider portion of the womb is known as its body; the
cavity of this is produced at each side to meet the openings of the
Fallopian tubes, and narrows below to the neck, or cervix uteri,
opposite c (Fig. 151), the communication between neck, and body
cavities being known as the os internum. Below this the neck
dilates somewhat: it forms no part of the cavity in which the em-

REPRODUCTION

oil

bryo is retained and nourished. The lowest part of the cervix
reaches into the vagina and communicates with it by a transverse
aperture, the os uteri. During gestation or pregnancy the fetus
develops in the body of the womb, which becomes greatly enlarged
and rises high into the abdomen: the virgin womb lies mainly
below the level of the bones of the pelvis.

The chief bulk of the non-gravid uterus consists of a coat of
plain muscular tissue, arranged in a thin outer longitudinal layer,

FIG. 151. — The uterus, in section, with the right Fallopian tube nnd ovary, as
seen from behind, about 4 the natural size. 11, umjer part of uterus; c, cervix;
v, upper part of vagina; od, Fallopian tube; fi, its nmbriated extremity; o, ovary;
po, parovarium.

and an inner, thicker, consisting of oblique and circular fibers.
Between the layers is an extensive vascular network, with many
dilated veins or venous sinuses. The muscular coat is lined in-
ternally by a ciliated mucous membrane, and is covered externally
by the peritoneum, bands of which project from each side of it
as the broad ligaments (II, Fig. 151). The outer layer of the mucous
membrane presents a very well developed muscularis mucosfr,
much thicker than the corresponding layer in the gastric or intes-
tinal mucous membranes and much less sharply marked off from
the true muscular coat outside it. The main thickness of the
mucous membrane consists of closely set, simple or slightly
branched, tubular glands; between these is a close blood-vascular
and lymphatic network. The glands open on the interior of the
womb; they and the mucous membrane between their mouths are
lined by a single layer of columnar ciliated cells, with some gob-

512

THE HUMAN BODY

let cells between them. In the cervix the glands are shorter, and
many of the epithelial cells not ciliated. The viscid mucus se-
creted by the uterine glands is alkaline or neutral.

The vagina is a distensible passage, extending from the uterus
to the exterior; dorsally it rests on the rectum, and ventrally is
in contact with the bladder and urethra. It is lined by mucous
membrane, the epithelium of which is much like the epidermis

FIG. 152. — The viscera of the female pelvis as exposed by a dorsiventral me-
dian section, s, symphysis pubis; v, v', urinary bladder; n, urethra; u, uterus;
va, vagina; r, r1, rectum; a, anal opening; I, right labium major; n, right nympha;
h, hymen; cl, divided cilitoris.

but thinner; outside the mucous membrane the vagina is made
up of areolar, erectile, and unstriped muscular tissues. Around
its lower end is a ring of striated muscular tissue, the sphincter
vagince.

The vulva is a general term for all the portions of the female gen-
erative organs visible from the exterior. Over the front of the pel-
vis the skin is elevated by adipose tissue beneath it, and forms the
mons Veneris. From this two folds of skin (/, Fig. 152), the labia

REPRODUCTION

513

majora, extend downwards and backwards on each side of a median
cleft, beyond which they again unite. On separating the labia
majora a shallow genito-urinary sinus, into which the urethra and
vagina open, is exposed. At the upper portion of this sinus lies the
clitoris, a small and very sensitive erectile organ, resembling a
miniature penis in structure, except that it has no corpus spon-
giosum and is not traversed by the uretha. From the clitoris de-
scend two folds of mucous membrane, the nympha or labia interna,
between which is the vestibule, a recess containing above, the open-
ing of the short female urethra, and, below, the aperture of the
vagina, which is in the virgin more or less closed by a thin dupli-
cature of mucous membrane, the hymen.

Fia. 153. — A section of a Mammalian ovary, considerably magnified. 1, outer
capsule of ovary; 2, 3, 3', stroma; 4, blood-vessels; 5, rudimentary Graafian fol-
licles; 6, 7, 8, follicles beginning to enlarge and mature, and receding from the sur-
face; 9, a nearly ripe follicle which is extending towards the surface preparatory to
discharging the ovum; a, membrana granulosa; 6, discus proligerus; c, ovum, with
d, germinal vesicle, and • . germinal spot. The general cavity of the follicle (in
which 9 is printed) is filled with lymph-like transudation liquid during life.

Microscopic Structure of the Ovary. The main mass of the
ovary consists of a dense connective-tissue stroma, containing un-
striped muscle, blood-vessels, and nerves: it is covered externally
by a peculiar germinal epithelium, and contains embedded in it
many minute cavities, the Graafian follicle*, in which ora lie. If a
thin section of an ovary be examined with the microscope many
hundreds of small Graafian follicles, each about 0.25 mm.
inch) in diameter, will be found embedded in it near the surface.

514 THE HUMAN BODY

These are lined by cells, and each contains a single ovum. In a
woman of child-bearing age there will be found also, deeper in,
larger follicles (7, 8, 9, Fig. 153), their cavities being distended,
during life, by liquid ; in these the essential structure may be more
readily made out. Each has an external fibrous coat constituted
by a dense and vascular layer of the ovarian stroma; within this
come several layers of lining cells (9, a, Fig. 153) constituting the
membrana granulosa. At one point, b, the cells of this layer are
heaped up, forming the discus proligerus, which projects into the
liquid filling the cavity of the follicle. Buried among the cells of
the discus proligerus the ovum, c, lies.

The Mammalian Ovum. As the Graafian follicles enlarge the
ova grow but not proportionately, so that they occupy relatively
less of the cavities of the larger follicles : the cells of the discus pro-
ligerus probably elaborate food for the egg-cell from material de-
rived from the blood-vessels which form a close network around
most of each enlarging Graafian follicle and transude crude nutri-
tive matter into the liquid filling most of the follicle. The fully
formed ovum (Fig. 154) is about 0.2 mm. (^~ inch) in diameter:
it has a well-marked outer coat or sac, a, the zona radiata, zona
pellucida or vitelline membrane, surrounding a very granular cell-
body or vitellus, b, in which is a conspicuous nucleus, c, here named
the germinal vesicle and possessing a nu-
cleolus or germinal spot. The main bulk of
the vitellus or yolk consists of highly re-
fracting spheroidal particles of nutritive
matter (deutoplasm) embedded in and
concealing a true protoplasmic reticulum.
In the eggs of birds and reptiles the deuto-
plasm is in very large amount and forms
FIG. 154. — A human nearly all of the yolk, the protoplasm being

ovum; somewhat diagram- .,

matic. a, zona pellucida; for the most part aggregated around the

b, vitellus; c, germinal vesi- 0^™;™! Vp=!irlp nf a small nrpa rm nr>P <?irlp
cle, with distinct reticulum Ser

of karyopiasm and a nu- of the yolk. It is in this area that new

cleolus or germinal spot.

cell-formation occurs and the embryo is

built up, the rest of the yolk being gradually absorbed by it;
such eggs are known as mesoblastic or partly dividing eggs. In
all the higher mammalia the deutoplasm is relatively sparse and
tolerably evenly mingled with the protoplasm, and the whole

REPRODUCTION

515

fertilized ovum divides to form the first cells df the embryo:
such eggs are named holoblastic.

The Maturation of the Ovum. From time to time, usually at
intervals of about four weeks, in a woman of child-bearing age,
certain ova after attaining the size and structure described in the
preceding paragraph undergo further changes by which the egg-
cell is rendered capable of fertilization. These phenomena, known
as the maturation of the ovum, result in separation of small parts of
the nucleus or germinal vesicle and cell protoplasm from the rest.
They are essentially typical cases of indirect cell division (p. 19).
The cell-body shrinks a little so as not quite to fill the zona pellu-
cida, and the germinal vesicle approaches one side. Meanwhile the
nuclear membrane and karyoplasm form the chromatic loop and
this divides into the usual two sets of Vs. One set of these, with
part of the nuclear plasm, now separates a little of the cell with
protoplasm to form a small cell, the first polar globule (c, Fig. 155).
The much larger cell resulting from the
division and representing the remainder
of the vitellus and nucleus now repeats
the process, and gives rise to the second
polar globule. In Fig. 155 the first
polar globule is shown at c, as already
separated, and the nucleus, d, is divid-
ing, preparatory to the formation of the
second one. The stage of karyokinesis
is more advanced than those represented Fir,. IBS.— An ovum

ii-i re to form the second polar glo-

in Fig. 10. The two polar globules lie lor bule a< 20na peiluoida; 6.
a time (Fig. 156) within the zona pcllu- g
cida in the space left by the shrinkage of

the vitellus. but take no part 111 the preparatory to the separatum

j ,!:»„ of the second polar globule;

formation of the embryo and soon clisap- t, vitonus> showing radial ar-

pear. The rest of the original ovum is

now mature and ready for fertilization;

its nucleus is known as the female pronudes, fn, Fig. 156.

passes towards the center of the ovum and forms the usual

ticulum of karyoplasm found in normal resting nuclei

Ovulation. From puberty, during the whole chil,
period of life, certain comparatively very large Graafian folh
may nearlv always bo found either close to the surface ot th, ovary

516 THE HUMAN BODY

or projecting on its exterior. These, by accumulation of liquid
within them, have become distended to a diameter of about 4 mm.
(^ inch) ; finally, the thinned projecting portion of the wall of the
follicle, which differs from the rest in containing few blood-vessels,
gives way and the ovum is discharged, surrounded by some cells of
the discus proligerus. The emptied follicle becomes filled up with
a reddish-yellow mass of cells, and constitutes the corpus luteum,
which recedes again to the interior of the ovary and disappears in
three or four weeks, unless pregnancy occur; in that case the corpus
luteum increases for a time, and persists during the greater part of
the gestation period.

Menstruation. Ovulation occurs during the sexual life of a
healthy woman, at intervals of about four weeks, and is attended
with important changes in other portions of the generative appara-
tus. The ovaries and Fallopian tubes become congested, and the
fimbriae of the latter are erected and come into contact with the
ovary so as to receive any ova discharged. Whether the fimbriae
embrace the ovary and catch the ovum, or merely touch it at vari-
ous points and the ova are swept along them by their cilia to the
cavity to the oviduct, is not certain. Having entered the Fallo-
pian tube the egg slowly passes on to the uterus, probably moved
by the cilia lining the oviduct; its descent probably takes about
four or five days; if not fertilized, it dies and is passed out. In the
womb important changes occur at or just before the periods of
ovulation; its mucuous membrane becomes swollen and soft, and
minute hemorrhages occur in its substance. The superficial layers
of the uterine mucous membrane are broken down, and discharged
along with more or less blood, constituting the menses, or monthly
sickness, which commonly lasts from three to five days. During
this time the vaginal secretion is also increased, and, mixed with the
blood discharged, more or less alters its color and usually destroys
its coagulating power. Except during pregnancy and while suck-
ling, menstruation occurs at the above intervals, from puberty up
to about the forty-fifth year; the periods then become irregular,
and finally the discharges cease; this is an indication that ovulation
has come to an end, and that the sexual life of the woman is com-
pleted. This time, the climacteric or " turn of life," is a critical one;
various local disorders are apt to supervene, and even mental de-
rangement.

REPRODUCTION ;,17

Hygiene of Menstruation. During menstruation there is apt to
be more or less general discomfort and nervous irritability; the
woman is not quite herself, and those responsible for her happiness
ought to watch and tend her with special solicitude, forbearance,
and tenderness, and protect her from anxiety and agitation. Any
strong emotion, especially of a disagreeable character, is apt to
check the flow, and this is always liable to be followed by serious
consequences. A sudden chill often has the same effect; hence a
menstruating woman ought always to be warmly clad, and take
more than usual care to avoid draughts or getting wet. At these
periods, also, the uterus is enlarged and heavy, and being (as may
be seen in Fig. 152) but slightly supported, and that near its lower
end, it is especially apt to be displaced or distorted; it may tilt
forwards or sideways (versions of the uterus,) or be bent where the
neck and body of the organ meet (flexion) . Hence violent exercise
at this time should be avoided, though there is no reason why a
properly clad woman should not take her usual daily walk.

The absence of the menstrual flow (amenorrhea) is normal dur-
ing pregnancy and while suckling; and in some rare cases it never
occurs throughout life, even in healthy women capable of child-
bearing. Usually, however, the non-appearance of the menses at
the proper periods is a serious symptom, and one which calls for
prompt measures. In all such cases it cannot be too strongly im-
pressed upon women that the most dangerous thing to do is to take
drugs tending to induce the discharge, except under skilled ad-
vice; to excite the flow, in many cases, as for example occlusion of
the os uteri, or in general debility (when its absence is a conserva-
tive effort of the system), may have the most disastrous results.

Fertilization. As the ovum descends the Fallopian tube the
changes of menstruation are taking place in the uterus. Fertiliza-
tion usually takes place in a Fallopian tube. The spermatozoa are
carried along partly, perhaps, by the contractions of the muscular
walls of the female cavities, but mainly by their own activity.
Occasionally the ovum is fertilized before reaching the Fallopian
tube and fails to enter it, giving rise to an extra-uttrinc i>r«jn<inry.

The actual process of the fertilization of the ovum has only boon
observed in the lower animals, but there is no doubt that the phe-
nomena are the same in all essentials in all cases. Some of the
spermatozoa penetrate the zona pellucida and the head of one of

518

THE HUMAN BODY

them enters the ovum, when it grows and forms the male pronu-
cleus (mn, Fig. 156) . This travels towards the nucleus of the ma-
tured ovum or female pronucleus, fn, and in each pronucleus a
karyoplastic filament forms and breaks up into a set of V's; in the
pronuclei represented in Fig. 156 this has not yet taken place, the
karyoplasm being still arranged in a reticulum. The tail of the
spermatozoon (which represents, it will be remembered, the pro-
toplasm of a male cell) disappears ; whether it is cast off when the
head enters the vitellus or mingles with the protoplasm of the latter
is not known. As the pronuclei approach one another two attrac-
tion particles, p, p, appear in the protoplasm of the ovum ; around

these the granules of the vitellus show
a radial arrangement and a nuclear
spindle (p. 21) unites them. The
spindle lies with its long axis at right,
angles to a line joining the pronuclei.
The latter next completely fuse
across the middle of the spindle and
form a new single nucleus. Fertili-
zation is then complete, and the ovum
capable of dividing or segmenting
(Fig. 11) to form the cells which
FIG. 156.— An ovum shortly be- by multiplication and differentiation

fore the fusion of the pronuclei. * . f

a, zona peiiucida; b, polar giobu- build up the embryo. The zona pel-

les: fn, female pronucleus: mn, -, . , . •, , • ,,

male pronucleus; PP, attraction lucida takes no part in the segmen-

bodies, with the nuclear spindle fofinn nnH i«? D-rnrhiflllv nb<?f>rbpd

tatl(

Impregnation. The fertilized ovum

...

continues its descent to the uterine
cavity, but, instead of lying dormant like the unfertilized, seg-
ments (p. 30), and forms a morula. This, entering the womb,
becomes embedded in the soft, vascular mucous membrane from
which it imbibes nourishment, and which, instead of being cast off
in subsequent menstrual discharges, is retained and grows during
the whole of pregnancy, having important duties to discharge in
connection with the nutrition of the embryo.

Sexual congress is most apt to be followed by pregnancy if it
occur immediately after a menstrual period ; at those times a ripe
ovum is usually in the Fallopian tube, near the upper end of which
it is .probably fertilized in the majority of cases. There is some

lying between them ; s, spermatozoa

which have not taken part in fer-

tilization.

REPRODUCTION 519

difference of opinion as to whether the rupture of the Graafian
follicle occurs most frequently immediately before the appearance
of the menstrual flow, or towards its close; but the preponderance
of evidence favors the latter view. The menstrual process probably
is a special preparation of the womb for the reception of an embryo
and its nourishment. There is, however, evidence that ova arc
occasionally discharged at other than the regular monthly periods
of ovulation and may be fertilized and cause a pregnancy.

Pregnancy. When the mulberry mass reaches the uterine cav-
ity the mucous membrancc lining the latter grows rapidly and
forms a new, thick, very vascular lining to the womb, known as the
decidua. At one point on this the morula becomes attached, the
decidua growing up around it. As pregnancy advances and the
embryo grows, it bulges out into the uterine cavity and pushes
before it that part of the decidua which has grown over it (the
decidua reflexa) ; at about the end of the third month this coalesces
with the decidua lining the opposite sides of the uterine cavity so
that the two can no longer be separated. That part of the decidua
(decidua serotina) against which the morula is first attached sub-
sequently undergoes a great development in connection with the
formation of the placenta (see below). Meanwhile the whole
uterus enlarges; its muscular coat especially thickens. At first the
organ still lies within the pelvis, where there is but little room for
it; it accordingly presses on the bladder and rectum (see Fig. 152)
and the nerves in the neighborhood, frequently causing consider-
able discomfort or pain; and, reflexly, often exciting nausea or
vomiting (the morning sickness of pregnancy). Later on, the preg-
nant womb escapes higher into the abdominal cavity, and although
then larger, the soft abdominal walls more readily make room for
it, and less discomfort is usually felt, though there may be short-
ness of breath and palpitation of the heart from interference with
the diaphragmatic movements. All tight garments should at this
time be especially avoided; the woman's breathing is already suffi-
ciently impeded, and the pressure may also injure the developing
child. Meanwhile, changes occur elsewhere in the Body. The
breasts enlarge and hard masses of developing glandular tissue
can be felt in them; and there may be mental symptoms: depres-
sion, anxiety, and an emotional nervous state.

During the whole period of gestation the woman is not merely

520 THE HUMAN BODY

supplying from her blood nutriment for the fetus, but also,
through her lungs and kidneys, getting rid of its wastes; the result
is a strain on her whole system which, it is true, she is constructed
to bear and will carry well if in good health, but which is severely
felt is she be feeble or suffering from disease. The healthy married
woman who endeavors to evade motherhood because she thinks
she will thus preserve her personal appearance, or because she dis-
likes the trouble of a family, deserves but little sympathy; she is
trying to escape a duty voluntarily undertaken, and owed to her
husband, her country, and her race; but she whose strength is un-
dermined and whose life is made one long discomfort for the sexual
gratification of her husband deserves eveiy consideration, and the
family physician ought perhaps to warn the husband more fre-
quently than he does of the risk to a delicate wife's health, or even
life, of frequent pregnancies : and the husband should control him-
self accordingly.

The Intra-Uterine Nutrition of the Embryo. At first the em-
bryo is nourished by absorption of materials from the soft vas-
cular lining of the womb; as it increases in size this is not suffi-
cient, and a new organ, the placenta, is formed for the purpose.
A fetal outgrowth, the allantois, plants itself firmly against the
decidua serotina, and villi developed on it burrow from its surface
into the uterine mucous membrane. In the deeper layer of this
latter are large sinuses through which the maternal blood flows,
and into which the allantoic villi project. Blood is brought from
the fetus to the allantois by arteries and carried back by veins
after traversing the capillaries of the villi, and while flowing
through these receives, by dialysis, oxygen and food materials
from the maternal blood, and gives up to it carbon dioxid, urea,
and other wastes. There is thus no direct intermixture of the two
bloods; the embryo is from the first an essentially separate and
independent organism. The allantois and decidua serotina be-
coming inseparably united together form the placenta, which in
the human species is, when fully developed, a round thick mass
about the size of a large saucer, connected to the embryo by a
narrow stalk, the umbilical cord, in which blood-vessels run to and
from the placenta.

Parturition. At the end of from 275 to 280 days from fertiliza-
tion of the ovum (conception) pregnancy terminates, and the child

REPRODUCTION .YJl

is expelled by powerful contractions of the uterus, assisted by
those of the muscles in the abdominal walls. When the child is
born, it has attached to its navel the umbilical cord, which is
then usually ligatured and cut across: some good authorities,
however, maintain that this should not be done until after the
contractions which expel the placenta, as otherwise a quantity
of the infant's blood remains in that organ; the loss of which
might be serious to a feeble infant. Shortly after the birth of the
child renewed uterine contractions detach and expel the placenta,
both its fetal or allantoic and maternal or decidual part, as the
afterbirth. Where it is torn loose from the uterine wall large blood
sinuses are left open; hence a certain amount of bleeding occurs,
but in normal labor this is speedily checked by firm contraction
of the uterus. Should this fail to take place profuse hemorrhage
occurs' (flooding) and the mother may bleed to death in a few
minutes unless prompt measures are adopted.

For a few days after delivery there is some discharge (the
lochia) from the uterine cavity: the whole decidua being broken
down and carried off, to be subsequently replaced by new mucous
membrane. The muscular fibers developed in the uterine wall in
such large quantities during* pregnancy undergo rapid fatty de-
generation and are absorbed, and in a few weeks the organ re-
turns almost to its original size. The parturient woman is es-
pecially apt to take infectious diseases; and these, should they
attack her, are fatal in a very large percentage of cases. Very
special care should therefore he taken to keep all contagion from
her.

There is a current impression that a pregnancy, once com-
menced, can be brought to a premature end, especially in its early
stages, without any serious risk to the woman. That belief is
erroneous. Premature delivery, early or late in pregnancy, is
always more dangerous than natural labor at the proper term;
the physician has sometimes to induce it, as when a malformed
pelvis makes normal parturition impossible, or the general de-
rangement of health accompanying the pregnancy is such as to
threaten the mother's life; but the occasional necessity of .!
ing whether it 4s his duty to procure an abortion is one of the most
snious responsibilities he meets with in the course of his pro-
fessional work.

522 THE HUMAN BODY

The production of abortion, even in the first stages of preg-
nancy, by the taking of drugs, the so-called abortifacients, a prac-
tice which seems to have gained considerable headway through
the widespread advertisement of their wares by unscrupulous
vendors of "patent medicines," is so dangerous to the health,
and even the life, of the woman who practices it that no consid-
eration sanctions it.

Lactation. The mammary glands for several years after birth
remain small, and alike in both sexes. Towards puberty they be-
gin to enlarge in the female, and when fully developed form in
that sex two rounded eminences, the breasts, placed on the thorax.
A little below the center of each* projects a small eminence, the
nipple, and the skin around this forms a colored circle, the areola.
In virgins the areolse are pink; they darken in tint and enlarge
during the first pregnancy and never quite regain their original
hue. The mammary glands are constructed on the compound
racemose type. Each consists of from fifteen to twenty distinct
lobes, made up of smaller divisions; from each main lobe a sep-
arate galadophorous duct, made by the union of smaller branches
from the lobules, runs towards the nipple, all converging beneath
the areola. There each dilates and forms a small elongated reser-
voir in which the milk may temporarily collect. Beyond this the
ducts narrow again, and each continues to a separate, opening on
the nipple. Embedding and enveloping the lobes of the gland is a
quantity of firm adipose tissue which gives the whole breast its
rounded form.

During maidenhood the glandular tissue remains imperfectly
developed and dormant. Early in pregnancy it begins to increase
in bulk, and the gland-lobes can be felt as hard masses through
the super] acent skin and fat. Even at parturition, however, their
functional activity is not fully established. The oil-globules of
the milk are formed by a sort of fatty degeneration of the gland-
cells, which finally fall to pieces; the cream is thus set free in the
watery and albuminous secretion formed simultaneously, while
newly developed gland-cells take the place of those destroyed.
In the milk first secreted after accouchment (the colostrum) the
cell destruction is incomplete, and many cells still float in the
liquid, which has a yellowish color; this first milk acts' as a pur-
gative on the infant, and probably thus serves a useful purpose,

REPRODUCTION 523

as a certain amount of substances (biliary and other), excreted
by its organs during development, are found in the intestines at
birth.

Human milk is undoubtedly the best food for an infant in the
early months of life; and to suckle her child is useful to the mother
if she be a healthy woman. Many women refuse to suckle their
children from a belief that so doing will injure their personal ap-
pearance, but skilled medical opinion is to the contrary effect; the
natural course of events is the best for this purpose, unless lac-
tation be too prolonged. Of course in many cases there are justi-
fiable grounds for a mother's not undertaking this part of her
duties; a physician is the proper person to decide.

In a healthy woman, not suckling her child, ovulation and
menstruation recommence about six weeks after childbirth; a
nursing mother usually does not menstruate for ten or twelve
months; the infant should then be weaned.

When an infant cannot be suckled by its mother or a wet-nurse
an important matter is to decide what is the best food to substi-
tute. Good cow's milk contains rather more fats than that of a
woman, and much more casein; the following table gives averages
in 1,000 parts of milk:

Woman Cow

Casein 28.0 54.0

Butter - 33.5 43.0

Milk-sugar 44.5 42.5

Inorganic matters _ - 4.75 7.75

The inorganic matters of human milk yield, on analysis, in
100 parts — calcium carbonate 6.9; calcium phosphate, 70.G;
sodium chlorid, 9.8; sodium sulphate, 7.4; other salts, 5.3. The
lime salts are of especial importance to the child, which has still
to build up nearly all its bony skeleton.

When undiluted cow's milk is given to infants they rarely bear
it well; the too abundant casein is vomited in loose coagula.
The milk should therefore be diluted with half or, for very young
children, even two-thirds its bulk of water. This, however, brings
down the percentage of sugar and fat below the proper amount.
The sugar is commonly replaced by adding cane-sugar; but sugar
of milk is readily obtainable and is better for the purpose. If
used at all it should, however, be employed from the first; it

524 THE HUMAN BODY

sweetens much less than cane-sugar, and infants used to the latter
often refuse milk in which milk-sugar is substituted. In order to
bring the percentage of fat up to normal it is usual to dilute, not
"whole milk" but "top milk." The latter, after the milk has
stood for a few hours, contains enough of the rising cream to
supply the needed fat. As the infant grows older less diluted
cow's milk may gradually be given; after the seventh or eighth
month no water need be added.

It should not be necessary to emphasize the vital importance of
giving to infants only the purest milk obtainable. It is unfor-
tunately true that the milk produced in the average dairy is not
only dirty but swarming with micro-organisms. In cities it has
become the practice for medical societies to inspect various dairies
and set their seal of approval upon those that fulfil the sanitary
conditions essential to the production of pure, clean milk. The
slightly higher cost of such " certified " milk should not be allowed
to bar it from homes where children are to be fed except where
extreme poverty makes its procurement impossible. In small
towns and in the country personal inspection of the source of
the milk supply on the part of parent or physician should give
assurance of its cleanliness. Where it is impossible to procure
milk free from suspicion, pasteurization (heating to 120° F. for
20 minutes) should be resorted to. This destroys most of the
dangerous organisms, but of course is not a complete substitute
for cleanliness and care in the production of the milk in the be-
ginning.

In the first weeks after birth it is no use to give an infant starchy
foods, as arrowroot. The greater part of the starch passes through
the bowels unchanged; apparently because the pancreas has not
yet fully developed, and has not commenced to make its starch-
converting enzym. Later on, starchy substances may be added to
the diet with advantage, but it should be borne in mind that they
cannot form the chief part of the child's food ; it needs proteins for
the formation of its tissues, and amyloid foods contain none of
these. Many infants are, ignorantly, half starved by being fed
almost entirely on such things as corn-flour or arrowroot.

Puberty. The condition of the reproductive organs of- each sex
described in preceding pages is that found in adults; although
mapped out, and, to a certain extent, developed before birth and

REPRODUCTION

during childhood, these parts grow but slowly and remain func-
tionally incapable during the early years of life; then they com-
paratively rapidly increase in size and become physiologically ac-
tive; the boy or girl becomes man or woman.

This period of attaining sexual maturity, known as puberty,
takes place from the eleventh to the sixteenth year, and is accom-
panied by changes in many parts of the Body. Hair grows more
abundantly on the pubes and genital organs, and in the armpits,
in the male also on various parts of the face. The lad's shoulders
broaden; his larynx enlarges, and lengthening of the vocal cords
causes a fall in the pitch of his voice; all the reproductive organs
increase in size; fully formed seminal fluid is secreted, and erections
of the penis occur. As these changes are completed spontaneous
nocturnal seminal emissions take place from time to time during
sleep, being usually associated with voluptuous dreams. Many a
young man is alarmed by these; he has been kept in ignorance of
the whole matter, is too bashful to speak of it. and getting some
quack advertisement thrust into his hand in the street is alarmed to
learn that his strength is being drained off, and that he is on the
highroad to idiocy and impotence unless he place himself in the
hands of the advertiser. Lads at this period of life should have
been taught that such emissions, when not too frequent and not
excited by any voluntary act of their own, are natural and healthy.
They may, however, occur too often; if there is any reason to sus-
pect this, the family physician should be consulted, as the healthy
activity of the sexual organs varies so much in individuals as to
make it impossible to lay down numerical rules on the subject.
The best preventives in any case are, however, not drugs, but an
avoidance of too warm and soft a bed, plenty of muscular exercise,
and keeping out of the way of anything likely to excite the sexual
instincts.

In the woman the pelvis enlarges considerably at puberty, and,
commonly, more subcutaneous adipose tissue develops over the
Body generally, but especially on the breasts and hips; conse-
quently the contours become more rounded. The external genera-
tive organs increase in size, and the clitoris and nymph® become
erectile. The uterus grows considerably, the ovaries enlarge, some
Graafian follicles ripen, and menstruation commences.

Hormones of the Reproductive System. The interrelations of

526 THE HUMAN BODY

various processes in the functioning of the reproductive mechan-
ism are many of them very striking and they have long been the
subject of investigation. The development of the so-called second-
ary sexual characters at puberty, where in a few weeks the vocal
cords change and hair develops over various parts of the body, is a
good example of the sort of interrelations that occur in this sys-
tem. The fact, known for centuries, that castration in early life
prevents the appearance of the secondary sexual characters, shows
that they are directly dependent on the reproductive organs. Be-
fore the idea of hormone action had crystallized to its present form
some such mechanism had been postulated for the reproductive
system. For it is difficult to explain such effects as those of castra-
tion on any other basis than that the generative organs elaborate
some control-exercising substance of which the body is deprived
by castration. Attempts have been made to demonstrate the pro-
duction of hormones by the testes and ovaries by experimental in-
troduction of extracts of them into the blood ; but as yet such ex-
periments have failed to yield very positive results. There can be
little doubt, however, that the control of the secondary sexual
characters, the stimulation of the mammary glands to activity at
the end of pregnancy, and other reproductive functions are medi-
ated by hormones, produced by the generative organs, . or in cer-
tain instances by the developing fetus.

The Stages of Life. Starting from the ovum each human being,
apart from accident or disease, runs through a life-cycle which
terminates on the average after a course of from 75 to 80 years.
The earliest years are marked not only by rapid growth but by
differentiating growth or development; then comes a more station-
ary period, and finally one of degeneration. The life of various
tissues and of many organs is not, however, coextensive with that
of the individual. During life all the formed elements of the Body
are constantly being broken down and removed; either molecularly
(i. e., bit by bit while the general size and form of the cell or fiber
remains unaltered), or in mass, as when hairs and the cuticle are
shed. The life of many organs, also, does not extend from birth to
death, at least in a functionally active state. At the former period
numerous bones are represented mainly by cartilage. The pan-
creas has not attained its full development; and some of the sense-
organs seem to be in the same case; at least new-born infants

REPRODUCTION 527

appear to hear very imperfectly. The reproductive organs only
attain full development at puberty, and degenerate and lose all or
much of their functional importance as years accumulate. Cer-
tain organs have even a still shorter range of physiological life; the
thymus, for example, attains its fullest development at the end of
the second year and then gradually dwindles away, so that in the
adult scarcely a trace of it is to be found. The milk-teeth are shed
in childhood, and their so-called permanent successors rarely last
to ripe old age.

During early life the Body increases in mass, at first very rapidly,
and then more slowly, till the full size is attained, except that girls
make a sudden advance in this respect at puberty. Henceforth the
woman's weight (excluding exceptional cases of accumulation of
non-working adipose tissue) remains about the same until the
climacteric. After that there is often an increase of weight for
several years due mainly to increased formation of fat; a man's
weight usually slowly increases until forty.

As old age comes on a general decline sets in, the rib cartilages
become calcified, and lime salts arc laid down in the arterial walls,
which thus lose their elasticity; the refracting media of the eye be-
come more or less opaque; the physiological irritability of the
sense-organs in general diminishes; and fatty degeneration, di-
minishing their working power, occurs in many tissues. In the
brain we find signs of less plasticity; the youth in whom few lines
of least resistance have been firmly established is ready to accept
novelties and form new associations; but the longer he lives, the
more difficult does this become to him. A man past middle life
may do good, or even his best work, but almost invariably in
some line of thought which he has already accepted; it is ex-
tremely .rare for an old man to take up a new study or change his
views, philosophical, scientific, or other. Hence, as we live, we all
tend to lag behind the rising generation.

Death. After the prime of life the tissues dwindle (or at least
the most important ones) as they increased in childhood.

Before any great diminution takes place, however, a breakdown
occurs somewhere, the enfeebled community of organs and ti
forming the man is unable to inert the rontingrnrirs of life, and
death supervenes. " It is as natural to die as to be born," Bacon
wrote long since; but though we all know it, few reali/.e the fact

528 THE HUMAN BODY

until the summons comes. To the popular imagination the pros-
pect of dying is often associated with thoughts of extreme suffer-
ing; personifying life, men picture a forcible and agonizing rending
of it, as an entity, from the bodily frame with which it is associated.
As a matter of fact, death is probably rarely associated with any
immediate suffering. The sensibilities are gradually dulled as the
end approaches ; the nervous tissues, with the rest, lose their func-
tional capacity, and, before the heart ceases to beat, the individual
has commonly lost consciousness.

The actual moment of death is hard to define : that of the Body
generally, of the mass as a whole, may be taken to be the moment
when the heart makes its last beat; arterial pressure then falls ir-
retrievably, the capillary circulation ceases, and the tissues, no
longer nourished from the blood, gradually die, not all at one in-
stant, but one after another, according as their individual respira-
tory or other needs are great or little.

While death is the natural end of life, it is not its aim — we should
not live to die, but live prepared to die. Life has its duties and its
legitimate pleasures, and we better play our part by attending to
the fulfilment of the one and the enjoyment of the other, than by
concentrating a morbid and paralyzing attention on the inevitable,
with the too frequent result of producing indifference to the work
which lies at hand for each. Our organs and faculties are not tal-
ents which we may justifiably leave unemployed; each is bound to
do his best with them, and so to live that he may most utilize them.
An active, vigorous, dutiful, unselfish life is a good preparation for
death ; when that time, at which we must pass from the realm con-
trolled by physiological laws, approaches, when the hands tremble
and the eyes grow dim, when " the grasshopper shall be a burden
and desire shall fail," then, surely, the consciousness of having
quitted us like men in the employment of our faculties while they
were ours to use, will be no mean consolation.

INDEX

INDEX

Abdomen, contents of,* 5.

Abdominal respiration, 360.

Abducens nerve, 130.

Aberration, chromatic, 227.

Aberration, spherical, 227.

Abortion, danger of, 521.

Absorption, 442; of carbohydrates,
443; of fats, 450; from large intes-
tine, 451; nature of, 443; of pro-
teins, 448; from small intestine,
442; from stomach, 442.

Accessories of diet, 387, 388.

Accessory reproductive organs, 505.

Accommodation, 223; mechanism of,
226. ,

Acetabulum, 61.

Achromatic lenses, 227.

Achromatin filaments, 21.

Acid, acetic, 15; amino, 9, 418;
butyric, 15; fatty, 418; formic, 15;
glycocholic, 464; hydrochloric, 15,
419; lactic, 15; sarcolactic, 15;
taurocholic, 464; uric, 13, 471.

Acrpmegaly, 273. .

Action currents, 99.

Adam's apple, 492.

Addison's Disease, 273.

Adenoid tissue, 269, 346.

Adipose tissue, 42.

Adrenalin, 273; relation of to circu-
lation, 340.

Adrenals (suprarenal capsules), 273.

Afferent nerve paths. 142.

Afterbirth (placenta), 520.

After-images, 245.

Agglutinins, 277.

Air, changes in when breathed, 371;
composition of, 371; expired, com-
position of, 372; quantity breathed
daily, 360.

Albumin, 10; serum, 267.

Albuminoid, 10; nutritive value of,
457.

Albuminuria, 471.

Alcohol, effect of on Body, 393.

Alexin, 275.

Alimentary canal, general arrange-
ment of, 397; blood-vessels of, 416;
subdivisions of, 397.

Alimentary glycosuria, 445.

Allantois, 520.

Alveoli of lungs, 350.

Amino acid, 9, 418.

Amoeboid cells, 74; movements, 24,
266.

Amylopsin, 420.

Anabolic processes, 24.

Anatomy, definition of, 1.

Anatomy of alimentary canal, 397;
of brain, 124; of ear, 187; of eye,
212; of joints, 6H; of lymphatic sys-
tem, 345; of muscular system, 73;
of nervous system, 116; of respira-
tory organs, 348; of skeleton, 49;
of skin, 474 ; of urinary organs, 464 ;
of vascular system, 288.

Anemia, 264.

Angle, visual, 238.

Animal heat, source of, 485.

Ankle bones, 62.

Antiperistalsis, 430.

Antitoxin, 277.

Anus. 410.

Anvil bone, 188.

Aorta. 292; branches of, 296.

Apex beat of heart, 305.

Aphasia, 158.

Apnea, 366.

Appendicular skeleton, 60.

Appendix vermiformis, 410.

Appetite, importance of, 441.

Aqueous humor, 218.

Arachnoid, 5, 121.

Areas of cerebrum, association, 152;
motor, 148; for language, 157;
sensory, 147.

Areola, 522.

Areolar tissue, 41.

Arm, skeleton of, 62.

Arterial blood, 302; color of, 878.

Arterial pressure, 325; determining
factors of, 326; influence of capil-
lary resistance on, 327; influence
of neart rate on, 326.

Arterial system, 295.

Arterioles, 297.

Artery, axillary. SMi; hniohial, 296;
bronchial. 2"7; ramtid. L*.Mi; ediac.
•J'.i7; coronary f.-tnonil.

297; hepatic.' tr_'; iliac.
innominate, 296; intercostal,
meeenteric, 297, 410; popliteal,

681

532

INDEX

297; pulmonary, 291; radial, 296;
renal, 297; splenic, 416; subclavian,
296; temporal, 296; tibial, 297;
ulnar, 296; vertebral, 296.
Artery, structure of, 302.
Articular cartilage, 68.
Articulations, 67.
Aryteno-epiglottic membrane, 493.
Arytenoid cartilage, 492; muscles,

496.

Asphyxia, 367.
Aspirates, 499.

Aspiration of thorax, 333, 361.
Assimilation, 23.
Assimilation limit, 445.
Assimilative tissues, 32.
Association areas of cerebrum, 152;

fibers of cerebrum, 147.
Association, nature of, 152.
Association neurons, 117.
Associative memory, 153; functions

of, 154; interactions of, 155.
Astigmatism, 228.
Atlas, 54.

Attraction particle, 20.
Auditory areas of cerebrum, 148.
Auditory meatus, external, 187.
Auditory nerve, 131.
Auditory nuclei, 146.
Auditory ossicles, 188; functions of,

197.

Auditory perceptions, 199.
Auerbach's plexus, 411.
Augmentor center of heart, 316.
Augmentor nerves of heart, 315.
Auricle, 291 ; functions of, 309.
Auriculoventricular valves, 294.
Auscultation of lungs, 359.
Automaticity, 26.
Autonomic nervous system, 119.
Axial stream, in artery, 319.
Axillary artery, 296.
Axis, 54.
Axis, visual, 239.
Axon, 117.

Bacterial digestion, 421.

Ball-and-socket joints, 70.

Basement membrane, 432.

Basilar membrane, 191.

Bathing, 481.

Beat of heart, 304.

Beef tea, 85.

Biceps muscles, 80.

Bicuspid teeth, 399.

Bile, 420; capillaries, 415; control of,

439; duct, 412; pigments, 13, 464;

salts, 464.
Bilirubin, 13, 464.

Biliverdin, 13, 464.
Binocular vision, 251.
Blackness, sensation of, 240.
Bladder, urinary, 464; control of.

466.

Bleeders, 285.
Blind spot, 2^3.

Blood, 256; of animals other than
man, 268; arterial, 302; carbon-
dioxid of, 382; changes in, in lungs,
375; chemical composition of, 267;
coagulation of, 278; course of, 300 :
distribution of, in Body, 256; fibrin
source of, 281; functions of, 256;
gases, 376; microscopic character
of, 261; oxygen interchanges in,
381; plates, 266; plasma, 267;
quantity of, 268; reaction of, 261-
serum, 279; specific gravity of,
261; structure of, 261; transfusion,
286; venous, 302; whipped, 280.
Blood-corpuscles, 263, 265.
Blood-flow, rate of, 331.
Blood-flow, see Circulation.
Blood-pressure, 325; in man, de-
termination of, 331; measurement
of, 330.

Blood-vessels, anatomy of, 288.
Blood-vessels of alimentary tract

416.

Blood-vessels, nerves of, 336.
Blushing, 339.

Body, development of, 30; elements
present in, 7; physical properties
of, 17; physiological properties of,
17; food requirements of, 451;
liberation of energy in, 454; protein
requirement of, 452; temperature
of, 484.

Body fat, source of, 460.
Body sense areas of cortex, 148.
Body sense, nerve tracts of, 144.
Bone, 44; chemistry of, 46; formation
of, 44; hygiene of, 47; structure of,
45.

Bones of cranium, 56; of face, 56; of
limbs, 62; of pectoral arch, 60; of
pelvic girdle, 60; of skull, 56; of
vertebral column, 51.
Bony labyrinth, 190.
Bow legs, cause of, 47.
Brachial artery, 296.
Brachial plexus, 127.
Brain, anatomy of, 124; arrangement
of white and gray matter in, 126;
convolutions of, 125; nourishment
of, 160; membranes of, 119.
Bread, 392.
Breast-bone, 60.

INDEX

533

Bright's disease, 471.
Broad ligament, 510.
Bronchial artery, 297.
Bronchial tubes. 350.
Brunner's glands, 410.
Buccal cavity, 398.
Buffy coat, on blood-clot, 288.
Burdach, columns of, 144.
Butter, 391.

Cachexia, 271.

Caffein, 395.

Calcium phosphate, 15.

Calcium salts, relation of to blood-
clotting, 283.

Calorimeter, 452.

Calory, 452.

Camera, photographic, 222.

Canals, lachrymal, 210; semicircular,
190, 199.

Canthi of eyelids, 209.

Capacity of lungs, 359.

Capillaries, bile, 415; blood, 297;
structure of, 303.

Capillary blood-flow, 320.

Capsule, internal, 147.

Carbohydrates, 14; absorption of,
443; food value of, 455; storage of,
443.

Carbohydrate foods, 389.

Carbon dioxid, 14; of blood, 382;
influence of on respiratory center,
365; hormone action of, 383.

Carbon equilibrium, 459.

Carbon monoxid hemoglobin, 385;
poisoning, 385.

Cardiac cycle, diagram of, 309;
events of, 305; time relations of,
304.

Cardiac ganglia, 166.

Cardiac impulse, 305.

Cardiac muscle, 84.

Cardiac orifice of stomach, 405.

Cardiac plexus, 133.

Cardio-augmentor center, 316;
nerves, 315.

Cardio-inhibitory center, 316; nerves,
315.

Carotid artery, 296.

Carpals, 62.

Cartilage, articular, 68; arytenoid,
492; costal, 60; cricoid, 492; cunei-
form, 493; elastic, 43; fibro-, 44;
hyaline, 43; of Santorini, 493;
structure of, 43; temporary and
permanent, 42; thyroid, 492; of
Wrisberg, 493.

Canmcula lachrymalis, 209.

Casein, 11, 391.

Castration, 527.

Cataract, 228.

Cauda equina, 127.

Caudate nucleus, 132.

Celiac axis, 297, 416.

Cells, 18; amoeboid, 74; ciliated, 74;
division of, direct, 19; indirect, 19;
growth of, 18; nucleus, 18.

Cellulose, 14; digestion of, 422; oc-
currence or, in food, 387.

Cement of tooth, 400.

Center of gravity of Body, 108.

Centers, cardio-augmentor, 316;
cardio-inhibitory, 316; of medulla,
164; respiratory, 363; vasocon-
strictor, 338; vasodilator, 342.

Central nervous system, 118.

Centrosome, 20.

Cephalic vein, 299.

Cerebellum, 125, 161; functions, of,
162; relation of to red nucleus. 162.

Cerebral activity, dependence of on
circulation, 342.

Cerebral circulation, relation of to
consciousness, 160.

Cerebral functions compared in man
and animals, 159.

Cerebrospinal liquid, 121.

Cerebrum, 124; afferent paths of,
1*42; cortex of, 146; development
of, 153; function of, in modifying
reflexes, 140; general structure of,
146; lobes of, 147; motor areas of,
148; relation of, to receptor sys-
tem, 142; white matter of, 146.

Cervical plexus, 127.

Cervical vertebrae, 53.

Characteristics of human skeleton,
64.

Cheeks, bones of, 56.

Cheese, 391.

Chemical changes in respired air, 371.

Chemical composition of Body, 7.

Chemistry, biological, definition of,
7.

Chemistry, of bile, 464; of blood, 267;
of bone, 46; of fats, 14; of gastric
juice, 419; of lymph, 269; of mus-
cle. 84; of pancreatic juice, 420; of
saliva, 418; of teeth, 400; of urine,
470.

Chest, see Thorax.

Childbirth, 520.

Cholesterin, 464.

Choroid, 213.

Chromatic aberration, 227. f

Chromoplasm, 20.

Chyle, 430. %

Chyme, 428.

534

INDEX

Cilia, 75.

Ciliary muscle, 167, 218.

Ciliary processes, 213.

Ciliated cells, 74.

Circulation, appearance under micro-
scope, 318; diagram of, 301; in-
fluence of gravity on, 332; of vein
compression on, 333 ; of respiratory
movements on, 333, 361; portal,
300; proofs of, 334; pulmonary,
300; resistance to, 319; renal, 467.

Circulation scheme, 322.

Circulatory system, anatomy of, 288.

Circumvallate papillae, 401.

Classification of tissues, 31.

Classification of neurons, 117.

Clavicle, 60.

Climacteric, 516.

Clitoris, 512.

Clot, of blood, 279.

Clothing, 489.

Coagulation of blood, 278; cause of,
279; summary of, 283; use of, 281;
methods of hastening and retard-
ing, 285; within blood-vessels, 284.

Coal gas poisoning, 385.

Coccyx, 56.

Cochlea, bony, 190; membranous,
191; functions of, 198.

Cocoa, 395.

Caecum, 410.

Coffee, 395.

Cold-blooded animals, 483.

Cold spots, location of, 182.

Colds, 340; protection against, 489.

Collar-bone, 60.

Colliculi, 125, 132.

Colon, 410.

Color blindness, 244; tests for, 245.

Color sensations, 241.

Color sense, distribution of, in ret-
ina, 243.

Color vision, 240; peculiarities of,
242; theories of, 245.

Colostrum, 522.

Column of Burdach, 144; of Goll, 144.

Columnee carnse, 294.

Commissural fibers of cerebrum, 147.

Common bile duct, 412.

Common sensations, 170.

Complemental air, 360.

Complemental colors, 242.

Complements of blood, 275.

Conception, 518.

Concepts, 152.

Concha, 187.

Conduction, nervous, irreversibility
of, 138.

Conductive system, 38.

Conductive tissues, 33.

Conductivity, 25.

Cones, of retina, 216; functions of,
243.

Coni vasculosi, 506.

Conjugate focus, 221.

Conjunctiva, 209.

Connective tissue, 32, 41.

Consciousness, 158; dependence of
on cerebrum, 160.

Consonants, 499.

Contact senses, 171.

Contractile tissues, see Motor tissues.

Contractility, 24, 87.

Contraction of muscle, 90; effect of
temperature on, 95; relation of to
fatigue, 95; to strength of stimu-
lus, 94; graphic record of, 92;
tetanic, 93.

Contrasts, visual, 245.

Convolutions of brain, 125, 147.

Cooking, of meats, 391 ; of vegetables,
392.

Coordination, 25.

Cordse tendinese, 294.

Cord, spinal, 121.

Cords, vocal, 493.

Corium, 475.

Cornea, 213.

Corona radiata, 147.

Coronary artery, 293, 296; vein, 292.

Corpora quadrigemina, 125, 132.

Corpora striata, 124.

Corpus Arantii, 295.

Corpus callosum, 147.

Corpus cavernosum, 508.

Corpus luteum, 516.

Corpus spongiosum, 508.

Corpuscles of blood, colorless, 265;
red, 261.

Corpuscles, pacinian, 177.

Corresponding points of retina, 251.

Cortex of cerebrum, 146; develop-
ment of, 153.

Corti, organ of, 192.

Cortical localization, 147.

Cortical reflex paths, 149.

Cortical reflexes, nature of, 150.

Costal breathing, 360.

Costal cartilages, 60.

Coughing, 368.

Course of blood, 300.

Cranial nerves, 119, 129.

Cranium, 56.

Creatin, 12.

Creatinin, 12, 470.

Cretinism, 271.

Crico-arytenoid muscle, 494.

Cricoid cartilage, 492.

INDEX

Cricothyroid membrane, 492.

Cricothyroid muscle, 496.

Crossed pyramidal tracts, 148.

Crura cerebri, 125.

Crying, 369.

Crypts of Lieberkiihn, 410.

Crystalline lens, 218.

Cuneate nucleus, 144.

Cuneiform cartilage, 493.

Curare, effect of, 89.

Currents of action, 99; of injury, 98.

Curve of muscular contraction, 92.

Cutaneous senses, 175.

Cuticle, 474.

Cystic duct, 412.

Death, 527; rigor, 86.

Decidua, 519.

Decussation, of pyramids, 148; sen-
sory, 144.

Degeneration of nerves, 143.

Deglutition, 425.

Dendrites, 117.

Dentals, 499.

Dentate nucleus, 132, 162.

Dentine, 400.

Depressor nerve, 316, 339.

Depth, perception of, 251.

Dermis, 6, 475.

Development, 30.

Dextrin, 389.

Dextrose, 14, 389.

Diabetes, 446.

Dialysis, 259.

Diaphragm, 4, 354.

Diastole of heart, 304.

Dietetics, 457.

Diffusion, 259.

Digastric muscles, 80.

Digestion, 417; auto, 422; bacterial,
421; of cellulose, 422; good, main-
tenance of, 441; in intestine, 440;
in mouth, 439; object of, 417;
products, 417; in stomach, 440;
summary of, 421.

Dioptrics of eye, 208.

Direct pyramidal tract, 148.

Discus proligerus, 514.

Dislocations, 72.

Dispersion of light, 220.

Dissimilation, 23.

Distance, perception of, 250.

Diuretics, 474.

Division of labor, physiological, 31.

Doctrine of specific nerve energies,
169.

Dorsal (neural) cavity, 5.

Dorsal (thoracic) vertebrae, 54.

Drum of ear, 187.

Duct, bile, 412; cystic, 412; hepatic,
412; of pancreas, 416; of salivary
glands, 403; of Stenson, 408.

Ductless glands, 270.

Duodenum, 407.

Dura mater, 119.

Dyspnea, 366.

Ear, 187; drum, 187.

Efferent nerve patlis, 142.

Eggs, 391.

Elastic cartilage, 43.

Elastic connective tissue, 42.

Elasticity of muscle, 97.

Electrical phenomena of muscle, 98.

Elements found in Body, 8.

Embryo, nutrition of, 520.

Emmetropia, 226.

Emotional reactions of Body, 167.

Emotions, 159.

Enamel, 400.

End arborization, 118.

End plate, 82.

Endocardium, 290.

Endogenous excreta, 462.

Endolymph, 190.

Endoskeleton, 49.

Energy, consumption of in Body, 454;
furnished by food, 386; muscular,
source of. 99.

Energy-yielding foods, 386.
Enterokinase, 423.
Entoptic phenomena, 229.
Environment, relation of man to, 37.
Enzvms, 13; digestive, 417.
Epidermis, 6, 34, 474.
Epididymis, 507.
Epiglottis, 404, 492.
Epithelium, 6, 34; olfactory, 202.
Equilibrium, maintenance of, 108.
Equilibrium sense, 200.
Erectile tissue, SOS.
Erect posture, 107.
Erepsin, 421.
Esophagus, 404.
Ethmoid bone, 56.
Eupnea, 366.
Eustachian tube, 188.
Excreta, endogenous and exogenous,

462.
Excretion, channels of, 462; from

lungs, 462; renal, 469.
Excretory function of liver, 463.
Exercise, 112.
Exogenous excreta, 4C.J.

Exoski-Ii'toll, }'.».

Expiration

Expired air, c<>!ii[><»ition of, 372.

Extensor muscles, 1<

536

INDEX

External auditory meatus, 187.

External ear, 187.

External rectus muscle, 210.

External respiration, 348.

External senses, 171.

Extractives, nitrogenous, 12.

Extrinsic reference of sensations. 185.

Eye, 207; appendages of, 208; defects
of, 226; hygiene of, 229; motions
of, 211; muscles of, 210; nodal
points of, 231; physiology of, 231;
refracting media of, 218; structure
of, 212.

Eyelashes, 209.

Eyelids, 209; muscles of, 209.

Eyestrain, 229.

Face, bones of, 56.

Facial nerve, 131.

Fallopian tube, 510.

False vocal cords, 494.

Falsetto, 497.

Farsightedness, 226.

Farjiculus gracilis, 144; cuneatus,
144.

Fat, absorption of, 450; food value
of, 455; food, 389; of Body, source
of, 460; structure of, 14.

Fatigue, 175; effect of, on contrac-
tion, 95; of nerve- fibers, 135.

Fatty tissue, 42.

Fauces, 403.

Fechner's law, 171.

Feeding of infants, 523.

Female, 505.

Femoral artery, 297.

Femur, 62.

Ferments, see Enzyms.

Ferrein, pyramids of, 467.

Fertilization, 517.

Fetus, nutrition of, 520.

Fever, 488.

Fibrin, 279; ferment, 282; source of,
281.

Fibrinogen, 10, 282.

Fibrocartilage, 44.

Fibula, 62.

Filiform papillae, 402.

Fillet, 144.

Filtration, 258.

Filum terminale, 122.

First order levers in body, 103.

Fissures of cerebrum, 147.

Flavor, 205.

Flesh foods, 390.

Flexors, 102.

Flexure, sigmoid, 410.

Focus, of lens, 222.

Follicle, Graafian, 513.

Follicle, of hair, 477.

Food, carbohydrate, 389; classes of,
386; definition of, 386; energy-
yielding, 386; fat, 389; flesh, 390;
functions of, 386; inorganic, 388;
maintenance, 386; protein, 390;
requirement of Body, 451; values,
455; vegetable, 392.

Foods, common, composition of, 393.

Foot, skeleton of, 64.

Foramen magnum, 57; oval, 188;
round, 188.

Fore-brain, 124.

Fore limb, 62.

Fovea centralis, 214.

Franklin theory of color vision, 249.

Frontal bone, 56.

Frontal lobe, 147.

Fuel of Body, 386.

Fundamental vibrations, 195.

Fundus of stomach, 405.

Fungiform papillae, 402.

Fur on tongue, 403.

Gall bladder, 412.

Ganglia, cardiac, 166; spinal, 126;
sympathetic, 133.

Ganglion, definition of, 132; Gas-
serian, 129; semilunar, 129.

Gas, absorption of, by liquid, 377;
partial pressure of. 378.

Gases of blood, 376.

Gasserian ganglion, 129.

Gastric digestion, 440.

Gastric glands, 406.

Gastric juice, 419.

Gastric secretin, 438.

Gastric secretion, control of, 437.

Gelatin, 46.

Gemmation, 502.

Geniculate bodies, 132.

Gestation, 519.

Gland-duct, 434.

Glands, 432; of Brunner, 410; duct-
less, 270; gastric, 406; mammary,
521; prostate, 507; pancreatic, 415;
salivary, 403; sebaceous, 479; of
skin, 479; sweat, 479; thyroid, 271.

Glenoid fossa, 60.

Gliding joints, 72.

Globe of eye, 212.

Globin, 11.

Globule, polar, 515.

Globulin, 10.

Glomerulus, 467.

Glossopharyngeal nerve, 131.

Glottis, 493.

Glucose, see Dextrose.

Gluten, 392.

INDEX

537

Glycerine, 418.

Glycocholic acid, 464.

Glycogen, 14, 389; storage of in liver,

443; in muscles, 444.
Glycoprotein, 11.
Glycosuria, alimentary, 445.
Goiter, 271.
Goll, column of, 144.
Graafian follicle, 513.
Gracile nucleus, 144.
Grape-sugar (dextrose), 14, 389.
Graphic record, 90.
Gray matter, 118; definition of, 132;

distribution of, 132; of spinal cord

122.

Growth of cells, 18.
Gullet, 404.
Gums, 398.

Gustatory areas of cerebrum, 148.
Gutterals, 499.

Habit formation, 156.
Haemoglobin, see Hemoglobin.
Hairs, 477.

Hair-cells of cochlea, 192; of semi-
circular canals, 200.
Hammer bone, 188.
Hand, see Fore limb.
Harmonic partials, 194.
Haversian system, 45.
Hearing, 187; nerve paths of, 145;

range of, 194.

Heart, 289; augmentor center of,
316; augmentor nerves of, 316;
automaticity of, 312; cavities of,
291; change in form of, 305; con-
tractions, maximal, 312; extrinsic
nerves of, 315; ganglia of, 166;
influence of salts on, 315; inhibi-
tory center of, 316; inhibitory
nerves of, 316; interior of, 294;
membranes of, 290; passage of
beat over, 313; physiological pecul-
iarities of, 312; position of, 289;
rate, 304; refractory period, 313;
relation of nerve and muscle ele-
ments within, 31 1 ; rhythmic action
of, 312; sounds of, 307; valves of,
294; work of, 310.
Heart-beat, 304; theories of, 314.
Heart valves, action of, 306, 308.
Heat, animal, sources of, 485.
Heat, distinguished from warmth,

182.

Heat loss, regulation of, 486.
Heat production, control of, 487.
Hematin, 13.
Hematopoietic tissue, 265.
Hemianopia, 215.

Hemochromogen, 13.
Hemocyanin, 268.

Hemoglobin, 11, 264; absorption of
oxygen by, 379; amount of in Body,
264; carbon monoxid, 385.
Hemophilia, 285.
Hepatic artery, 412; cells, 413; duct

412; vein, 301, 412.
Hering's theory of color vision, 248.
Hermaphrodite, 505.
Hernia, inguinal, 506,
Hiccough, 368,
Hilus of kidney, 466,
Hind-brain, 125.
Hind limb, structure of, 62.
Hinge-joints, 70,
Hip-joint, 68.

Histology, definition, 2; of adipose
tissue, 42; of adenoid tissue, 269;
of areolar tissue, 41; of blood,
261; of bone, 45; of cardiac muscle,
84; of cartilage, 43; of connective
tissue, 41; of ear, 191, 200; of
elastic tissue, 42; of hairs, 477;
of heart, 84; of kidney, 467, of
liver, 412; of lungs, 351; of lymph,
268; of lymph-glands, 269, 346;
of nails, 478; of nervous tissue, 117;
of retina, 216; of skeletal muscle,
*81; of small intestine, 408; of
smooth muscle, 83; of stomach,
406; of tongue, 401.
Histon, 11,
Holmgren test for color blindness,

245,

Holoblastic ova, 514.
Homothermous animals, 483.
Hormones, definition, 266; produc-
tion of, 270.
Horopter, 252,
Humerus, 62,

Humor, aqueous and vitreous, 218.
Hunger, 174,
Hyaline cartilage, 43.
Hyaloid membrane, 218.
Hydrocarbons, 13.
Hydrocele, 506.
Hydrochloric acid, 419.
Hydrolysis, 417.

Hygiene, definition, 1; of bones, 47;
of clothing, 489; of digestion, 441;
of exercise, 112; of eye, 229; of
joints, 72; of muscles, 111; of
mouth, 424; of respiration, 360;
of skeleton, 66; of skin, 481.
Hymen, 513.
Hyoid bone, 58,
Hypermetropia, 226.
Hypoglossal nerve, 131.

538

INDEX

Ileocolic valve, 411,

Ileum, 407,

Iliac artery, 296,

Ilium, 61,

Illusions, sensory, 185,

Images, after, 245,

Immune bodies, 276,

Immunity, 277; specific nature of,
277,

Immunization, 277,

Impregnation, 518,

Impulse, cardiac, 305,

Impulse, nervous, 116.

Incisor teeth, 399.

Incus, 189.

Index of refraction, 220.

Infant feeding, 523.

Infection, 274; recovery from, 276.

Infection-resisting mechanism, 275.

Inferior maxillary nerve, 130.

Inferior mesenteric artery, 297.

Inferior oblique muscle, 211.

Inferior rectus muscle, 210.

Infundibulum, 350.

Inhibition, 155.

Inhibitory center of heart, 316.

Inhibitory nerves of heart, 316.

Injury currents, 98.

Innervation of iris, 214.

Innominate artery, 296; vein, 300.

Inoculation, protective, 278.

Inorganic food, 388.

Insertion of muscles, 79.

Inspiration, 353.

Intensity of sensations, 170.

Intercostal arteries, 297; muscles, ex-
ternal, 356; internal, 358.

Intermediary bodies, 275.

Internal capsule, 147.

Internal ear, 189.

Internal medium, 255.

Internal rectus muscle, 210.

Internal senses, 171 ; effect of, in con-
sciousness, 172.

Intestinal digestion, 440.

Intestinal juice, 420.

Intestine, large, 410; absorption
from, 451 ; movements of, 430.

Intestine, small, 407; absorption
from, 442; movements of, 429;
nervous control of, 430; mucous
coat of, 408.

Intestines, nerves of, 411.

Invertase, 421.

lodothyrin, 271.

Iris, 213; innervation of, 214; muscles
of, 214; pigment of, 214.

Irritability, 24, 88.

Irritable tissues, 33.

Ischium, 61.

Islands of Langerhans, 447.

Jaw, 56.

Jejunum, 407.

Joints, 68; ball-and-socket, 70; glid-
ing, 72; hinge, 70; hygiene of, 72;
pivot, 71.

Judgments, 185.

Jugular vein, 300.

Karyokinetic division, 19.

Katabolic processes, 24.

Kidney, 466; blood-flow through,

467; blood-supply of, 465; minute

structure of, 467.
Kidney secretion, mechanism of, 471 ;

relation of to blood-flow, 473.
Knee-cap, 62.

Labials, 499.

Labyrinth, 190.

Lachrymal apparatus, 210.

Lachrymal bone, 56.

Lachrymal canals, 210.

Lachrymal papilla, 209.

Lachrymal sac, 210.

Lactase, 421.

Lactation, 521.

Lacteals, 260, 410.

Lactose, 14, 389.

Lamina spiralis, 191.

Langerhans, Islands of, 447.

Language, 157.

Large intestine, 410; absorption from,
451 ; movements of, 430.

Larynx, 491; cartilages of, 492;
muscles of, 494.

Latent period, 92.

Laughing, 369.

Leaping, 111.

Lecithin, 15, 391.

Leg bones, 62.

Lens, crystalline, 218.

Lens, refraction by, 221.

Lenticular nucleus, 132.

Leucocytes, 265; movements of, 266.

Levator palpebrae superioris, 209.

Levers in the body, 103.

Lieberkiihn, crypts of, 410.

Life, stages of, 526.

Ligament, broad, 510, capsular, 68;
round, 68.

Light, 219; dispersion of, 220; mono-
chromatic, 219; refraction of, 219;
wave-length of, 219.

Limbs, 6; skeleton of, 62.

Lipase, 420.

Liver, 411; excretory function of,

INDEX

639

463; glycogenetic function of, 444;
histology of, 412.

Lobes of cerebrum, 147.

Lobules of liver, 413.

Local sign in sensation, 170.

Localization of function in the cere-
brum, 147.

Localizing power of retina, 238; of
skin, 179.

Lochia, 521.

Locomotion, 109.

Lumbar plexus, 128.

Lumbar vertebra, 55.

Lungs, 351 ; capacity of, 359; changes
of blood in, 375; excretory func-
tions of, 462.

Lymph, 257; chemistry of, 269; his-
tology of, 268; movements of, 347;
nodes, 346; relation of to blood,
259; vessels, 260, 345.

Lymphatics, 260,- 345.

Lymph-nodes, 346; functions of, 346.

Lymphoid tissue, 269, 346.

Maintenance food, 386.

Malar bone, 56.

Male, 505.

Malleus, 188.

Malpighian capsule, 467.

Malpighian layer of epidermis, 474.

Malpighian pyramids of kidney, 466.

Maltase, 421.

Maltose, 418.

Mammal, characteristics of, 4.

Mammary gland, 521.

Man, zoological position, 2.

Manometer, 330.

Mastication, 422.

Maturation of ovum, 514.

Maxilla, 56.

Measurement of arterial pressure,
330.

Meatus, external auditory, 187.

Meatus urinarius, 507.

Media, refracting, of eye, 218; re-
fractive indices of, 223.

Median nerve, 298.

Medulla oblongata, 125, 163; centers
of, 164.

Medullated nerve-fibers, 118.

Meibomian follicles, 209.

Meissner's plexus, 411.

Membrane, aryteno-epiglottic, 4
basilar, 191; cricothyroid, 4! Hi;
mucous, 397; nictitating, 20!) ; per-
meable, 259; of Reissncr, 191;
semipermeable, 259; synovia I, 69;
tectorial, 193; tympanic, 187, 190;
vitelline, 514.

Membranous labyrinth, 190.

Membranes of central nervous sys-
tem, 119.

Memory, 151; associative, 153.

Menstruation, 516.

Mesenteric artery, 297, 416.

Mesentery, 408.

Mesoblastic ova, 514*

Metabolism, 442.

Metacarpals, 62.

Metatarsals, 62.

Microscopic anatomy, see Histology.

Midbrain, 124, 163.

Middle ear, 187.

Milk, composition of, 391, 523; for
infants, 523; Pasteurization of,
524; pure, importance of, 523.

Millon's test for proteins, 10.

Mitotic division, 19.

Mitral valve, 294.

Modality of sensations, 170.

Modiolus, 191

Molar teeth, 399.

Monochromatic light, 219.

Morula, 30.

Motion, 102; in plants, 73.

Motor area of cortex, 148.

Motor neurons, 117.

Motor tissues, 33.

Motores oculi, 129

Mountain sickness, 383.

Mouth, 398; digestion in, 439; hy-
giene of, 424.

Mucin, 11.

Mucous layer of epidermis, <
intestine', 408; of stomach, 406.

Mucous membrane, 6, 397.

Mulberry mass, 30.

Mumps, 403.

Muscse volitantes, 229.

Muscle, anatomy of, 80; biceps, 9
cardiac, 84; chemistry of, 84;
ciliary, 167, 218; electrical phe-
nomena of, 98; fuel of 100; hw-
tology of, 81; independent
tability of, 89; plasma, 81; rela-
tion of form to working power, W;
spindles, 83; stroma, 84; stapcdiiw,
189; tensor tympani, 189.

Muscles, classification of, <6;

eastric, 80; end plates ol

tensor, 102; of eyeball. Jl«>tl,v,.r.

102- forms of, 80; glycogen in, 41
hygiene of, 111; intiwwtal, 3

uuuroi « • i ~

83; special physiology of

540

INDEX

Muscle sense, 172.

Muscle spindle, 83.

Muscle stroma, 84.

Muscular contraction, 90; extent of,

93.

Muscular elasticity, 97.
Muscular energy, source of, 99.
Muscular sense, 172.
Muscular tissue, 34, 80.
Muscular work, 95.
Muscularis mucosse, 409.
Myelin sheath, 118.
Myenteric reflex, 428.
Myogen, 84.

Myogenic theory of heart-beat, 314.
Myopia, 226.
Myosin, 84.
Myxedema, 271.

Nails, 478.

Nasal bone, 56.

Nasal duct, 210.

Nerve, 116; abducens, 131; auditory,
131; depressor, 316; facial, 131;
glossopharyngeal, 131; hypoglos-
sal, 131; inferior maxillary, 130;
median, 298; oculomotor 129; ol-
factory, 129; ophthalmic, 129;
optic, 129, 215; patheticus, 129;
phrenic, 128; pneumogastric, 131;
sciatic, 129; spinal accessory, 131;
splanchnic, 338, 411; superior
maxillary, 130; trigeminal, 129;
vagus, 131.

Nerve-cells, 117; sensory, cell-bodies
of, 137.

Nerve-fibers, fatigue of, 135.

Nerve impulse, definition, 116; how
aroused, 134; nature of, 135;
speed of, 136; spread of, in both
directions, 135; as stimulus to
muscle, 88; methods of studying,
134; variations in intensity, 135.

Nerve paths, afferent, 142; efferent,
142; method of tracing, 143.

Nerves, cardiac, 315; cranial, 119,
129; of respiration, 364; secretory,
435; spinal, 119, 126; sympathetic,
119, 165; trophic, 435; vasocon-
strictor, 337; vasodilator, 341 ; vaso-
motor, 336.

Nervi erigentes, 508.

Nervous system, anatomy ofj 116;
central and peripheral, 118; mem-
branes of, 119; physiology of, 134;
sympathetic, 119, 165.

Neurilemma, 118.

Neurogenic theory of heart-beat, 314.

Neuroglia, 119.

Neurons, 116; association, 117; bi-
polar, 117; motor, 117; post-
ganglionic, 165; pre-ganglionic, 165;
sensory, 117.

Nicotine, effect of, on nerves, 166.

Nictitating membrane, 209.

Nipple, 522.

Nitrogen equilibrium, 458.

Nitrogenous extractives, 12.

Nodal points of eye, 231.

Nodes of Ranvier, 118.

Noise, 193.

Nose, bones of, 56.

Notes, musical, 193.

Nuclear spindle, 21.

Nuclei, nervous, 132.

Nuclein, 29.

Nucleolus, 18, 20.

Nucleo protein, 11.

Nucleus of cell, 18.

Nucleus, caudate, 132; cuneate, 144;
dentate, 132, 162; gracile, 144;
lenticular, 132; red, 132.

Nutrients, 387; function of, 389.

Nutrition, see Metabolism.

Nutrition of embryo, 520.

Nutritive tissues, 32.

Nutritive value, of albuminoids, 457 ;
of carbohydrates, fats and pro-
teins, 455.

Obesity, treatment for, 460.

Oblique muscles of eye, 211.

Occipital bone, 56.

Occipital lobe of cerebrum, 147.-

Oculomotor nerve, 129.

Odontoid process, 54.

Odors, nature of, 203.

Old-sightedness, 230.

Olecranon, 62.

Olein, 14.

Olfactory areas of cerebrum, 148.

Olfactory lobes, 124.

Olfactory nerves, 129.

Olfactory organ, 202.

Omentum, 405.

Ophthalmic nerve, 129.

Opsonins, 276.

Optical defects, 226.

Optic chiasma, 129, 215.

Optic disk, 214.

Optic nerve, 129, 215.

Optic thalami, 124, 132.

Optic tracts, 129, 215.

Optogram, 237.

Orbicularis oris, 102.

Orbicularis palpebrarum, 103, 209.

Organ of Corti, 192.

Organs, 1, 35; of circulation, 288; of

INDEX

541

digestion, 397; of excretion, 462;
of movement, 73; of nervous sys-
tem, 116; of reproduction, 502;
• of respiration, 348; of secretion,
432; of sensation, 169, 187, 207.

Origin of muscles, 79.

Os innominatum, 61.

Osmosis, 258.

Osmotic pressure, 259.

Os orbiculare, 189.

Ossein, 46.

Ossicles, auditory, 188: functions of.
197.

Osteoblast, 44.

Osteoclast, 46.

Otoliths, 200.

Oval foramen, 188.

Ovary, 510; structure of, 513.

Overtones, 194.

Oviduct, 510.

Ovulation, 515.

Ovum, 30, 514; fertilization of, 517;
maturation, 514.

Oxidase, 384.

Oxidation, in muscle, 100; as source
of animal heat, 485.

Oxygen, absorption of by blood, 379;
interchanges in blood, 381.

Oxyhemoglobin, 379.

Pacinian corpuscles, 177.

Pain, 176.

Palate, 398.

Palatine bones, 56.

Palmatin, 14.

Pancreas, 415.

Pancreatic juice, 420.

Pancreatic secretin, 439.

Pancreatic secretion, control of, 438.

Papillae of skin, 476.

Papillae of tongue, 401.

Papillary muscles, 294; use of, 307.

Paraglobulin, 10.

Parathyroids, 272.

Parietal bones, 56.

Parietal lobe, 147.

Parieto-occipital fissure, 147.

Parotid gland, 403.

Parthenogenesis, 505.

Partial tones, 194.

Parturition, 520.

Pasteurization of milk, 524.

Patella, 62.

Patheticus nerve, 129.

Pathogenic organisms, 274.

Pathology, definition, 1.

Peas, 455.

Pectoral arch, 60.

Pelvic girdle, 60.

Pelvis of kidney, 466.

Penis, 508.

Pepsin, 419.

Pepsinogen, 422.

Peptone, 12, 419.

Perceptions, 183; auditory, 199;
visual, 249.

Pericarditis, 290.

Pericardium, 290.

Perilymph, 190.

Perimeter, 243.

Perimysium, 81.

Periosteum, 44.

Peripheral nervous system, 119.

Peripheral reference of sensations,
183.

Peristalsis, 426.

Peritoneum, 5, 406.

Permeable membrane, 259.

Perspiration, 480.

Peyer's patches, 346.

Phagocytes, 266; action of in resisting
infection, 275.

Phagocytosis, 266.

Phalanges, 62.

Pharynx, 404.

Phlorhizin, 448.

Phospho proteins, 11.

Phrenic nerve, 128.

Physiological division of labor, 31.

Physiological properties, 16.

Physiological systems, 36.

Physiology, 1; of bruin, 142; of di-
gestion, 417; of ear, 196; of eye,
231; of heart, 304; of kidney, 469;
of metabolism, 442; of muscle, 87;
of nerve, 134; of respiration, 348;
of sensation, 169; of skin, 479;
of smell, 202; of the spinal cord,
136; of taste, 204; of touch, 178.

Pia mater, 121.

Pigment, 13; of iris, 214.

Pitch, audible limits of, 194; defini-
tion of, 194; range of, in human
voice, 497.

Pituitary body, 272.

Pivot joints, 71.

Placenta, 520.

Plain muscular tissue, see Smooth
muscle.

Plasma, 267.

Platelets, or placques of blood.

Pleura, 5, 351.

Plexus, 127; of Auerbach, 411 ; brach-
ial, 127; cardiac, 133; cervical,
127; lumbar, 128; of Meissner, 411 ;
sacral, 129; solar, 411.

Pneumogastric nerves, 131.

Poikilothermous animals, 483.

542

INDEX

Polar globules, 515.

Pons Varolii, 125.

Popliteal artery, 297.

Portal circulation, 300.

Portal vein, 301, 412.

Post-ganglionic neurons, 165.

Postures, 107.

Potassium chlorid, 15.

Pre-ganglionic neurons, 165.

Pregnancy, 518.

Prepuce, 509.

Pre-pyramidal tracts, 162.

Presbyopia, 230.

Pressor impulses, 339.

Pressure, arterial, 325; intrathoracic,
361; osmotic, 259; partial, of gases,
378.

Pressure sense, 178.

Primates, 2.

Production of heat in body, 485.

Projecting senses, 172.

Projection fibers of cerebrum, 146.

Pronation, 72.

Proofs of circulation, 334.

Properties of body, physical and
physiological, 17.

Prosecretin, 439.

Prostate, 507.

Protamin, 11.

Protective tissues, 34.

Proteins, 9; absorption of, 448; con-
jugated, 11; derived, 11; foods,
390; food value of, 455; require-
ment of Body for, 452; subdivisions
of, 8; tests for, 10; use of in body,
449.

Proteose, 12, 419.

Prothrombin, 283.

Protoplasm, 20, 28.

Psychic secretion, 437.

Psychophysical law, 170.

Ptosis, 212.

Ptyalin, 418.

Puberty, 524.

Pubis, 61.

Pulmonary artery, 291.

Pulmonary circulation, 300.

Pulmonary veins, 293.

Pulse, 327; use of in diagnosis, 392.

Pulse-wave, rate of movement, 328.

Pupil, 213.

Purin bodies, 13, 471.

Purkinje's experiment, 233.

Pus, 266.

Pyloric sphincter, control of, 428.

Pylorus, 405, 407.

Pyramidal nerve-cells, 146.

Pyramidal tracts, 148.

Pyramids, decussation of, 148.

Pyramids of Ferrein, 467; of Mal-

pighi, 466.
Pyrexia, 488.

Qualities of sensation, 170.
Quantity of air breathed daily, 360.
Quantity of blood, 268.
Quantity of food needed daily, 451,
456.

Racemose glands, 432.

Radial artery, 296.

Radio-ulnar articulation, 71.

Radius, 62.

Range of voice, 497.

Rate of blood-flow, 331.

Reaction of blood, 261.

Reactions, emotional, 167.

Reason, faculty of, 159.

Receptaculum chyli, 346.

Receptive tissues, see Irritable tis-
sues.

Receptor system, 169.

Receptors, classification of, 171.

Record, graphic, 90.

Rectum, 410.

Rectus muscles of eye, 210.

Red blood-corpuscles, 261.

Red nucleus, 132.

Reduced hemoglobin, see Hemo-
globin.

Reflex animal, characteristics of, 141;
compared with normal, 141.

Reflex arcs, 136.

Reflex paths, including cortex, 149;
variations in, 137.

Reflex time, 151.

Reflexes, 136; function of spinal cord
in, 140; inhibition of, 155; spread-
ing of, 139.

Refracting media of eye, 218.

Refraction, index of, 220; law of, 220.

Refraction of lenses, 221.

Refraction of light, 219.

Refraction in the eye, 223.

Refractory period of heart, 313.

Regeneration, 503.

Regulation of temperature, 485.

Renal artery, 297, 465.

Renal organs, 464.

Renal secretion, 469.

Rennin, 419.

Reproduction, 23, 502; sexual, 505.

Reproductive organs, accessory, 505;
female, 500; male, 505.

Reproductive system, hormones of,
525.

Reproductive tissues, 35.

Residual air, 359.

INDEX

543

Resistance, synaptic, 138.

Resonance, sympathetic, 195.

Resonants, 500.

Respiration, 23, 348; abdominal, 360;
artificial, 368; chemistry of, 370;
costal, 360; external, 348; forced,
358; hygiene of, 360; influence of,
on circulation, 362; on lymph-flow,
363; internal, 348; nerves of, 364;
rhythmic character of, 365; tissue,
384.

Respiratory center, 363.

Respiratory movements, 353; modi-
fied, 368.

Respiratory organs, 348.

Respiratory sounds, 359.

Reticular membrane, 193.

Reticulum of cell, 20.

Retina, 214; blood-vessels of, 215;
localizing power of, 238; micro-
scopic structure of, 216; nervous
elements of, 217.

Rheumatism, pericarditis in, 290.

Rib cartilage, 60.

Ribs, 59.

Rigor mortis, 84, 86.

Rods of Corti, 192.

Rods of retina, 216; function of, 236.

Rolando, fissure of, 147.

Roots of spinal nerves, 126.

Round foramen, 188.

Running, 110.

Rupture, 506.

Sacculus, 191.

Sacral plexus, 129.

Sacral vertebra, 56.

Sacrum, 55.

Saliva, 418.

Salivary glands, 403.

Salivary secretion, control of, 436.

Salts of body, 15.

Santorini, cartilages of, 493.

Saphenous vein, 299.

Sarcolactic acid, 15.

Sarcolemma, 81.

Sarcoplasm, 81.

Sarcostyle, 81.

Scalse of cochlea, 191.

Scalene muscles, 356.

Scapula, 60.

Sciatic nerve, 129.

Sclerotic, 212.

Scrotum, 505.

Sebaceous glands, 479.

Sebaceous secretion, 481.

Secretin, gastric, 438; pancreatic,

439.
Secretion, 434; cutaneous, 480; gas-

tric, control of, 437; organs of, 432;
pancreatic, control of, 438; psychic.
437; renal, 469; salivary, control
of, 436; sebaceous, 481.

Secretory nerves, 435.

Secretory process, 434 ; hormone con-
trol of, 436; nervous control of,
435.

Secretory tissues, 32.

Sections of body, 6.

Segmentation of ovum, 30.

Segmentation, rhythmic, of intestine,
429.

Self-digestion, prevention of, 422.

Semicircular canals, bony, 1!M); epi-
thelium of, 200; membranous,
191; nerve-endings in, 199.

Semilunar ganglion, 129.

Semilunar valves, 294.

Seminal fluid, 509.

Seminal vesicle, 507.

Seminiferous tubule, 506.

Semipermeable membrane, 259.

Sensations, 169; of color, 241; com-
mon, 170; differences between, 169;
extrinsic reference of, 1S5; in-
tensity of, 170; modality of, 170;
peripheral reference of, iS3; qual-
ity of, 170; visual, duration of, 237;
intensity of, 235.

Sense, of equilibrium, 200; of hearing,
187; of hunger, 174; muscular, 172;
of pain, 176; of sight, 231 ; of smell,
202; of taste, 204; of thirst, 174;
of touch, 178; of temperature,
181.

Senses, 170; classification of, 171;
contact, 171; cutaneous, 175; ex-
ternal, 171; internal, 171; nerve
paths of, 143; projecting, 172.

Sensory areas of cortex, 147.

Sensory decussation, 144.

Sensory illusions, 185.

Sensory neurons, 117.

Septum of heart, 291.

Serous membranes, 5, 405.

Serum, 279.

Serum albumin, 10, 267.

Shivering, 487.

Short sight, 226.

Shoulder-blade, 60.

Shoulder-girdle, 60.
Sighing, 368.

Sight, 207; nerve paths of, 145; hy-
giene of, 2~i.
Sigmoid flexure, 410.
Size, perception of, 251.
Sk.-lrtal muscles, 77.
Skeleton, 49; appendicular, 60; axial,

INDEX

51; of face, 56; hygiene of, 66;

peculiarities of, 64; of skull, 56;

of thorax, 59.
Skin, 5, 474; glands, 479; hygiene of,

481; localizing power of, 179;

papillae of, 476.
Skull, 56.
Sleep, 343.
Small intestine, 407 ; absorption from,

442; movements of, 429.
Smell, 202; fatigue of, 203; keenness

of, 203.
Smooth muscle, 83; physiology of,

100.

Sneezing, 368.
Soap, production of, from fat, 14; in

intestine, 450.
Sodium chlorid, 15.
Solar plexus, 133, 338, 406, 411.
Solidity, perception of, 252.
Sound, 193; loudness of, 193.
Sounds of heart, 307.
Source of animal heat, 485.
Source of body fat, 460.
Source of glycogen, 443.
Source of muscular energy, 99.
Source of urea, 463.
Special senses, 170.
Specific gravity of blood, 261.
Specific nerve energies, 169.
Spectacles, 229.
Speech, 491.
Spermatid, 509.
Spermatozoa, 509.
Sphenoid bone, 56.
Spherical aberration, 227.
Spinal accessory nerve, 131.
Spinal column, see Vertebral column.
Spinal cord, 121; central canal of,

124; columns of, 123; fissures of,

122; functions of, 140, 144; gray

matter of, 122; membranes of, 119;

white matter of, 123.
Spinal ganglia, 126.
Spinal nerve-roots, 126.
Spinal nerves, 119, 126; distribution

of, 127.

Spindle, nuclear, 21.
Sphincter muscles, 101.
Sphincter of pylorus, 407.
Splanchnic nerves, 338, 411.
Splanchnic region, 338.
Splenic artery, 416.
Spontaneity, 26.
Sprains, 72.
Squinting, 212.
Stages of life, 526.
Stapedius muscle, 189.
Stapes, 189.

Starch, animal, 14; digestion of, 418,
421, 439; as food, 389.

Stearin, 14.

Stenson's duct, 403.

Stereoscopic vision, 253.

Sternum, 60.

Stimulus, 25, 88.

Stirrup bone, 189.

Stomach, 405; absorption from, 442;
digestion in, 440; histology of, 406;
movements of, 427; nervous con-
trol of, 430.

Storage, of carbohydrates, 443; of
glycogen in muscles, 444.

Storage tissues, 33.

Strabismus (squinting), 212.

Structure, vertebrate, 3.

Strychnine poisoning, 139.

Subclavian artery, 296; vein, 300.

Subcutaneous areolar tissue, 475.

Sublingual gland, 403.

Submaxillary gland, 403.

Succus entericus, 420; control of, 439.

Sucrose, 389.

Sudoriparous glands, 479.

Sugar, 14, 418; as food, 389; as fuel
for muscles, 100.

Sulcus spiralis, 191.

Superior maxillary nerve, 130.

Superior mesenteric artery, 416.

Superior oblique muscle, 211.

Superior rectus muscle, 210.

Supination, 72.

Supplemental air, 359.

Supporting tissue, 32.

Suprarenal capsules, 273.

Suspensory ligament of eye, 218.

Sutures, 67.

Swallowing, 425.

Sweat, 480.

Sweat center, 481.

Sweat-glands, 479.

Sweat, nervous control of, 480.

Sweating, relation of to heat loss,
486.

Sweetbread, 415.

Sylvius, fissure of, 147.

Sympathetic ganglion, 133.

Sympathetic resonance, 195.

Sympathetic system, 119, 133, 164;
relation of to emotional states, 167;
reflex control of, 166.

Synapse, 117.

Synaptic resistance, 138.

Synovial membrane, 69.

System, alimentary, 39, 397; circu-
latory, 39, 288; conductive, 38,
116; excretory, 39, 462; motor,
38, 73; nervous, 38, 116; receptor,

INDEX

38, 169; respiratory, 39, 348;

sympathetic, 119, 133, 164.
Systems, physiological, 36.
Systole of heart, 304.

Taking cold, 340.

Tannin, 396.

Tarsus, 62.

Taste, 204.

Taste-buds, 204, 402.

Taurocholic acid, 464.

Tea, 395.

Tear-glands, 210.

Tectorial membrane, 193.

Teeth, 398.

Teeth, structure of, 398.

Temperature of Body, 484.

Temperature, effect of on muscular
contraction, 95.

Temperature sensation zero, 181.

Temperature sense, 181.

Temperature, bodily, regulation of,
485.

Temperatures, local, 488.

Temporal artery, 296.

Temporal bone, 56.

Temporal lobe, 147.

Tendons, 41.

Tension of blood gases, 380.

Tensor tympani muscle, 189.

Testis, 506.

Tests for proteins, 10.

Tetanus, 93.

Thalami, optic, 132.

Theobromin, 395.

Theories of color vision, 245; of
heart-beat, 314; of sleep, 344.

Thirst, 174.

Thoracic duct, 345.

Thoracic vertebrae, 54.

Thorax, aspiration of, 333, 361;
contents of, 5; movements of, in
respiration, 354; skeleton of, 354.

Throat, 404.

Thrombin, 282; source of, 283.

Thymus, 272.
• Thyro-arytenoid muscle, 496.

Thyroid cartilage, 492.

Thyroid gland, 271.

Tibia, 62.

Tibial artery, 297.

Tidal air, 360.

Timbre, 194.

Tissue respiration, 384.

Tissues, adenoid, 269, 346; adipose,
42; areolar, 41; assimilative, 32;
bony, 45; cartilaginous, 43; clas-
sification of, 31; conductive, 33;
connective, 41; contractile, 33, 76;

elastic, 42; erectile, 508; excretory
32, 462; irritable, 33; lymphoid,
269, 346; motor, 33, 76; nervous,
116; nutritive, 32; protective, 34;
reproductive, 35; respiratory, 32,
351; secretory, 32, 432; storage,
33; supporting, 32; undifferen-
tiated, 31, 74.

Tones, number distinguishable. 198.

Tongue, 400.

Tonsil, 346, 404.

Touch, sensations of, 178.

Toxins, bacterial, 274.

Trachea, 350.

Tracts, of spinal cord, 144; optic, 129.

Training, 115.

Transfusion of blood, 286.

Tricuspid valve, 294.

Trigeminal nerve, 129.

Trochlear muscle, 211.

Trophic nerves, 435.

Trypsin, 420.

Trypsinogen, 422.

Tubular glands, 432.

Tubules, uriniferous, 467.

Tunica adventitia, 302.

Tunica vaginalis, 505.

Turbinate bones, 56.

Tympanic membrane, 187; functions
of, 196.

Tyrein, 39.

Ulna, 62.

Ulnar artery, 296.

Undifferentiated tissues, 31, 74.

Unstriped muscles, 83.

Upper maxilla, 56.

Urea, 12, 463; in urine, 470.

Ureter, 464.

Urethra, 464; male, 507.

Uric acid, 13, 471.

Urinary organs, 464.

Urinary salts, 471.

Urine, 470.

Uriniferous tubules, 467.

Urobilin, 13.

Uterus. 510.

Utriculus, 191.

Uvula, 398.

Vaccination, 278.

Vagina, 511.

Vagus nerve, 131.

Valve, ileocolic, 411.

Valves of heart, 294; action of, 306,

308.

Valves of veins, 303.
Valvulsc conniventes, 408.
Varicose veins, 333.

546

INDEX

Vasa efferentia, 506.

Vasa recta, 506.

Vas deferens, 507.

Vasoconstrictor center, 338; control
of, 338.'

Vasoconstrictor nerves, 337.

Vasodilator center, 342.

Vasodilator nerves, 341.

Vasomotor tone, 338.

Vegetable foods, 392.

Vegetables, cooking of, 392.

Vegetative organs, 74.

Veins, 297; cephalic, 299; coronary,
292; hepatic, 301, 412; innomi-
nate, 300; jugular, 300; portal,
301, 412; pulmonary, 293; saphe-
nous, 299; structure of, 303; sub-
clavian, 300; valves of, 303; var-
icose, 333.

Vena cava, 292, 300.

Venous blood, 302.

Ventilation, 342.

Ventricle of heart, 291; functions of,
309.

Vermiform appendix, 410.

Vertebrae, 51; cervical, 53; lumbar,
55; sacral, 56; structure of, 52;
thoracic, 54.

Vertebral artery, 296.

Vertebral column, 3, 51.

Vertebrata, 3.

Vesicle, seminal, 507.

Vessels, blood, 288; lymphatic, 260.

Vestibule of ear, 190; nerve endings
in, 200.

Vibrations, analysis of, 194; the
basis of sound, 193.

Vibratories, 500.

Villi of intestine, 409.

Vision, 231; binocular, 251; color,
240; stereoscopic, 253.

Visual angle, 238.

Visual areas of cerebrum, 148.

Visual axis, 239.

Visual contrasts, 245.

Visual defects, 226; treatment of,
229.

Visual perceptions, 249.

Visual purple, 214, 237.

Visual sensations, duration of, 237;
intensity of, 235.

Vital capacity, 360.

Vital point, 364.

Vital processes, 164.

Vital spirits, 89.

Vitelline membrane, 514.

Vitreous humor, 218.

Vocal cords, 493; false, 494; relation

of, to pitch, 497.
Voice, 491; range of, 497.
Volition, 154.
Voluntary acts, reflex at bottom,

154.

Vomer, 56.
Vowels, 498.
Vulva, 512.

Walking, 109.

Wallerian degeneration, 143.

Wandering cells, see Phagocytes.

Warm-blooded animals, 483.

Warmth spots, 182.

Water equilibrium, 457.

Water, proportion of, in Body, 15.

Wax of ear, 187.

Weber's law, 171.

Weber's scheme of circulation, 322.

Weeping, 210.

Weight, maintenance of, 457.

Whipped blood, 280.

Whispering, 501.

White, 240.

White blood-corpuscles, 265.

White of eye, 213.

White fibrous connective tissue, 40.

White matter, 118, 132; of cerebrum,

146; of spinal cord, 123.
Windpipe, 350.
Work of heart, 310.
Work, muscular, measure of, 95.
Wrisberg, cartilage of, 493.
Wrist bones, 62.

Xanthoproteic test, 10.

Yawning, 368.
Yolk, 514.

Young-Helmholtz theory of color
vision, 246.

Zoological position of man, 2.
Zymogen, 422.

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