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leaves.

MARTIN, H. NEWELL.— The Human Body.
An Account of its Structure and Activities, and
the conditions of its healthy working, by H. New-
ell Martin, D. Sc, M. A., M.B., Professor of Biol-
ogy in the Johns Hopkins University ; Fellow of
University College, London ; late Fellow of Christ's
College, Cambridge. New York : Henry Holt &
Co., 1881. Large i2mo, pp. xvi. 606, Appendix
33. (American Science Series.)

HUMAN BODY, THE.— An Account of its
Structure and Activities, and the conditions of its
healthy working, b}^ H. Newell Martin, D. Sc, M.
A., M. B., Professor of Biology in the Johns Hop-
kins University; Fellow of University College,
London; late Fellow of Christ's College, Cam-
bridge. New York: Henry Holt & Co., iSSi.
Large lamo, pp. xvi. 606, Appendix t^2>- (Ameri-
can Science Series.)

ANATOMY.— The Human Body. An Account
of its Structure and Activities, and the conditions
of its healthy working, by H. Newell IMartin, D.
Sc, M. A., M. B., Professor of Biology in the
Johns Hopkins University; Fellow of University
College, London ; late Fellow of Christ's College,
Cambridge. New York: Henry Holt & Co., 1S81.
Large i2mo, pp. xvi. 606, Appendix 33. (Ameri-
can Science Series.)

PHYSIOLOGY.— The Human Body. An Ac-
count of its Structure and Activities, and the con-
ditions of its healthy working, by H. Newell Mar-
tin, D. Sc, M. A., M. B., Professor of Biology in
the Johns Hopkins University ; Fellow of Univer-
sity College, London ; iate Fellow of Christ's Col-
lege, Cambridge. New York : Henry Holt &
Co., 1881. Large i2mo, pp. xvi. 606, Appendix
33. (American Science Series.)

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THE

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HENRY HOLT & CO., Publishers, NEW YORK.

AMERICAN SCIENCE SERIES

THE HUMAN BODY

AN ACCOUNT OF ITS STRUCTURE AND ACTIVITIES
AND THE CONDITIONS OF ITS HEALTHY WORKING

H. NEWELL MAPiTIA. D.,S< .,, :\l.A., M.B.

Professor of Biology in- the Johns Hopkins University ;

Fellow of University College, London; Late

Fellow of Christ's College, Camhndge

NEW YORK
HEKEY HOLT AXD COMPAXY

1881

\ O /7

Copyright, 1881,

BY

Henry Holt & Co.

A -50.

J. Campbeli-,

PRINTER,

15 VaudewatiT St., N. Y.

PEEFACE.

In the following pages I have endeavored to give an
account of the structure and activities of the Human Body,
which, while intelligible 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 advanced study.
Wherever it seemed to me really profitable, hygienic 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,
however, to examine critically wdiat 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 discussion. 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 responsi-
bility 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

iv PBEFACE.

ol)jcction, thfit too many (lisi»ute(l matters luive been dis-
cussed: tliis was deliberately done as the result of an experi-
ence ill teacliing Physiology which now extends over more
than ten years. It would have been comparatively easy to
slip over things still uncertain and subjects as yet unin-
vestigated, 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 — iotus, tei^es, et rotundus.
Ikit by so doing no adequate idea of the present state of
])hysiological science would have been conveyed; in many
directions it is much farther travelled and more completely
known than in others; and, as ever, exactly the most in-
teresting points are those Avhich lie on the boundary
Ijetween 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 Micro-
scopic Anatomy there are more; but a treatise on Physiology
which would pass by, unnientioned, 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 jiart grown, and
many still represented by unfolded buds. To the teacher,
moreover, no jDupil 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 jiathless for the
pioneers of thought; but the opportunity to arouse the re-
ceptive mental attitude apt to be produced by the recogni-
tion of the fact that much more still remains to be learnt —
to excite the exercise of the reasoning faculties upon dis-
puted matters — and, in some of the better minds, to arouse

PREFACE. V

the longing to assist in adding to knowledge, is an inesti-
mable 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
imjDortant 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 ref-
erences to authorities would have been out of place. I
trust, however, that it will be found throughout 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 Mensclien, and a few from Arendfs
Schulatlas, the publishers of each furnishing electrotypes.
A considerable number, mainly histological, are from
Qiiaiji's Anatomy, and a few figures are after Bernstein,
Carpenter, Frey, Haeckel, Helmholtz, Huxley, McKen-
drick, and Wundt. About thirty, chiefly diagrammatic,
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.

coi^rTEisrTs.

CHAPTER I.

THE GENERAL STRUCTURE AND COMPOSITION OF THE IHIMAN
BODY.

PAGE

Definitions. Tissues and organs. Histology. Zoological posi
lion of man. The vertebrate plan of structure. The mam-
malia. Chemical composition of the Body 1

CHAPTER n.

THE FUNDAMENTAL PHYSIOLOGICAL ACTIONS.

The properties of the living Body. Physiological properties.
Ceils. Cell growth. Cell division. Assimilation and repro-
duction. Contractility. Irritability. Conductivity, Spon-
taneity. Protoplasm. The fundamental physiological proper-
ties 15

CHAPTER HI.

THE DIFFERENTIATION OF THE TISSUES, AND THE PHYSI-
OLOGICAL DIVISION OF EMPLOYMENTS.

Development. The physiological division of labor. Classifica-
tion of the tissues. Undifferentiated tissues. Supporting
tissues. Nutritive tissues. Storage tissues. Irritable tissues.
Co-ordinating and automatic tissues. 3Iotor tissues. Conduc-
tive tissues. Protective tissues. Reproductive tissues. Or-
gans. Physiological mechanisms. Anatomical systems. The
Body as a working whole 26

Vlll CONTENTS.

CHAPTER IV.

TITK INTEKNAIi MEDIUM.

PA(!K

The external medium. Tlie internal medium. The blood. The
lymph. Histology of l)lood. Blood crystals. Histology of
lymph r 39

CHAPTER V.

THE CLOTTING OF THE BLOOD.

The coagulation of blood. Causes of coagulation. Whipped
blood. The Ijuffy coat. Uses of coagulation. The fibrin
factors. Artificial clot. The fibrin ferment. E.\citing causes
of coagulation. Relation of blood-vessels to coagulation.
Composition of the blood. Quantity of blood. Origin and fate
of the blood corpuscles. Chemistry of lymph 50

CHAPTER VI.

THE SKELETON.

Exoskeleton and endoskeleton. The bony skeleton. Segmenta-
tion of the skeleton. Peculiarities of the human bony skele-
ton 62

CHAPTER VII.

THE STRUCTURE AND COMPOSITION OF BONE. .JOINTS.

Gross structure of the bones. Microscopic structure of bone.
Chemical composition of bone. Hygiene of the bony skeleton.
Articulations. Joints. Hygiene of the joints 86

CHAPTER VIII.

CARTILAGE AND CONNECTIVE TISSUE.

Temporary and permanent cartilages. Varieties of cartilage.
The connective ti-ssues. Elastic cartilage and fibro-cartilage.

CONTENTS. ix

PAGE

Homologies of the supportiiiLr tissues. Hygiene of tbe develop-
ing skeleton. Adipose tissue 100

CHAPTER IX.

TIEE STRCCTUKE OF THE JIOTOll ORGANS.

Motion in animals and plants. Ama?boid cells. Ciliated cells.
The muscles. Histology of striped muscle. Uustriped mus-
cles. Cardiac muscular tissue. The chemistry of muscular .
tissue. Beef -tea and Liebig's extract 11;

CHAPTER X.

THE PROPERTIES OP MTSCTTLAR TISSUE.

Contractility. Irritability of muscle. A simple muscular con-
traction. Tetanus. Causes influencing the degree of muscu-
lar contraction. The measurement of muscular work. Mus-
cular elasticity. Physiology of plain muscular tissue. Hy-
giene of the muscles. Exercise 128

CHAPTER XI.

MOTION AND LOCOMOTION.

The special physiology of muscles. Levers in the Body. The
erect posture. "Walking. Running. Leaping 143

CHAPTER XII.

THE ANATOMT OF THE NERVOPS STSTEil.

Nerve-trunks. Plexuses. Xerve-centres. The cerebro-spinal
centre and its membranes. The spinal cord. The spinal
nerves. The brain. The cranial nerves. Ganglia and com-
munications of the cranial nerves. The sympathetic system.
The sporadic ganglia. The histology of nerve-fibres. The
histology of nerve-cells. The structure of the spinal cord 1.54

CONTENTS.

CHAPTER XIII.

THE GENERAL PHYSIOLOGY OF TIIE NERVOUS SYSTEM

PAGK

The properties of nerve tissues. The functions of nerve-centres
and nerve-trunks. Excitant and inhibitory nerves. The clas
sification of nerve libres. Intercentral nerve-fibres. The
stimuli of nerves. General and special nerve stimuli. Specific
nerve energies. All nerve-fibres are fundamentally alike.
The nature of a nervous impulse. The rate of transniis.sion of
a nervous impulse. Functions of the spinal nerve-roots. Tlie
intercommunication of nerve-centres ISO

CHAPTER XIV.

THE ANATOMY OP TIIE HEART AND BLOOD VESSELS.

General statement. Position of the heart. The membranes of
the heart. The cavities of the heart. The heart as viewed
from the outside. The interior of the heart. The valves of
the heart. The arterial system. The capillaries. The veins.
The pulmonary circulation. The course of tlie blood. The
portal circulation. Arterial and venous blood. The structure
of arteries, capillaries, and veins 201

CHAPTER XV.

THE WORKING OF THE HEART AND BLOOD-VESSELS.

The beat of the heart. The cardiac impulse. Events in a cardiac
period. Use of the papillary muscles. Sounds of the heart.
Function of the auricles. The work done by the heart. The
blood flow outside the heart. The circulation as seen with
the microscope. Internal friction in the vessels. The conver-
sion of an intermittent into a continuous flow 219

CHAPTER XVI.

ARTERIAL PRESSDRE AND THE PULSE.

Weber's schema. Arterial pressure. Modifications of arterial
pressure, and how they may be produced. The pulse. The

CONTENTS. xi

PAGE

rate of the blood-flow. Secondary causes of the circulation
Proofs of the circulation of the blood 233

CHAPTER XVII.

THE REGULATION OF THE HEART AND BLOOD \T:SSELS BY
THE NERVOUS SYSTEM.

The need of co-ordination in the vascular sj'stem. The intrinsic
nerves of the heart. Nerves showing the heart's beat. The
cardio-inhibitory mechanism. The accelerator nerves of the
heart. The nerves of the blood-vessels. The vasomotor
centre. Taking cold. Vaso-dilator nerves 247

CHAPTER XVIII.

THJ<; SECRETORY TISSI^S AND ORGANS.

Definition. Organs of secretion. Glands. Physical processes
in secretion. Chemical processes in secretion. The mode of
activity of secretory cells. Influence of the nervous system
upon secretion. Summary 259

CHAPTER XIX.

THE INCOME AND EXPENDITURE OP THE BODY.

The material daily losses of the Body. The daily losses of the
Body in energy. The conservation of energy. Potential and
kinetic energy. The energy of chemical afiinity. The relation
between matter lost by the Body daily and the energy spent by
it. The conditions of oxidation in the living Body. The fuel
of the Body. Oxidation by successive steps. The utilization
of energy in the living Body. Summary 277

CHAPTER XX.

Foods as tissue-formers. The food of plants. Non-oxidizable
foods. Definition of foods. Conditions which a food must

XI 1 CONTENTS.

PAGE

fuUill Alimentary principles. 'J'lie couijjusition of the more
important foods. Cooking. Alcohol. The advantage of a
mixed diet 2*j;i

CHAPTER XXI.

Tnii ALIMKNTAKY ( AN.\fi AND ITS AITENDAOEK.

General arrangement. Tlic teeth. The tongue. The salivary
glands. The fauces. The pharynx. The gullet. The
stomach. The histology of the gastric glands. The small
intestine The large intestine. The liver. The pancreas. . . . 308

CHAPTER XXII.

THE LYMPHATIC SYSTEM AND THE Ul'CTLESS GLjVNDS.

Distribution and structure of lymph vessels. The thoracic duct.
The serous cavities. The lymphatic glands. The movement
of the lymph. The spleen. The thymus. The thyroid body.
The supra-renal capsules 32!)

CHAPTER XXIII.

DIGESTION.

The object of digestion. U.-?es of saliva. Deglutition. The
gastric juice. Gastric digestion. The chyle. The pancreatic
secretion. The bile. The intestinal secretior-s. Intestinal di
geslion. Ab.sorption from the intestines. The digestion of an
ordinary meal. Dyspepsia 334.

CHAPTER XXIV.

THE EESPrRATORY MECHANISM.

Definitions. Respiratory organs. The air-passages and lungs.
The pleura. The respiratory movements. The structure of
the thorax. The mechanism of inspiration. Expiration.
Forced respiration. The respiratory sounds. Tlie capacit}-
of the lungs. Hygiene of respiration. The aspiration of the
thorax. Influence of respiratory movements upon the flow of
blood and lymph 353

CONTENIS. xiii

CHAPTER XXV.

THE CHEMISTEY OF RESPIRATION.

PAGE

Nature of the problems. Changes prockiced m air by being once
breathed. Ventilation. Changes undergone by blood in the
kings. The blood gases. Causes of color of arterial and
venous blood. Laws governing the absorption of gases by
liquids. The absorption of oxygen by the blood. Conse
quences of the way in which oxygen is held in the blood.
The general oxygen interchanges of the blood. The carbon
dioxide of the blood. Internal respiration 3T2

CHAPTER XXVT.

THE NERVOUS FACTORS OF THE RESPIRATORY MECHANISM.
ASPHYXIA.

The respiratory centre. Is the respiratory centre reflex? The
stimulus of the respiratory centre. The cause of the respira-
tory rhythm. The influence of the pneumogastric nerves upon
the respiratory centre. The expiratory centre. Asphyxia.
Carbon - monoxide htnemoglobin. Modified respiratory move-
ments 390

CHAPTER XXVII.

THE KIDNEYS AND THE SKIN.

The general arrangement of the urinary organs. The structure
of the kidneys. The renal secretion. The mechanism of the
renal secretion. The function of the renal epithelium. The
skin. Epidermis and cutis vera. Hairs. Nails. Glands of
the skin. Relation of nerves to sweat secretion. Hygiene of
the skin. Bathing 404

CHAPTER XXVIII.

NUTRITION.

The problems of animal nutrition. The seat of the oxidations of
the Body. Tissue-building and energy-yielding foods. The
source of the energy spent in muscular work. Luxus con-
sumption. The antecedents of urea. Proteid starvation and

XIV COI^TEI^TS.

PAOE

over-feedinc;. The storage tissues. Glycogen. Diabetes. The
history of fats. Dietetics 428

CHAPTER XXIX.

THE PnODUCTION AND REGULATION OF THE HEAT OF THE
BODY.

Cold- and warm-blooded animals. The temperature of the Body.
The sources of animal heat. Energy lost l)y the Body in
twenty-four hours. The superiority of the Body as a machine
for executing mechanical work. The maintenance of an
average temperature. Local temperatures. Thermic nerves.
Clothing 449

CHAPTER XXX.

SENSATION AND SENSE-ORGANS,

The subjective functions of the nervous system. Common sen-
sation and organs of special sense. The peripheral reference
of sensations. Differences between sensations. The essential
structure of a sense organ. The cause of the modality of sensa-
tions. The psycho-physical law. Perceptions. Sensory illu-
sions 461

CHAPTER XXXI.

THE EYE AS AN Ol'TICAL INSTUMENT.

The essential structure of an eye. The appendages of the eye.
The lachrymal apparatus. The muscles of the eyeball. The
anatomy of the eyeball. The structure of the retma. The
refracting media of the eye. The properties of light. Accom-
modation. Short sight and long sight. Hygiene of sight.
Optical defects of the eye 479

CHAPTER XXXII.

THE EYE AS A SENSORY APPARATUS.

The excitation of the visual apparatus. Idio-retinal light. The
parts of the retina on which light directly acts. The vision
purple. Intensity of visual sensations. Duration of luminous
sensations. The localizing power of the retina. Color vision.

CONTENTS. XV

PAGE

Colorblindness. Fatigue of the retina and after-images. Con
tvasts. Heriug's theory of vision. Visual perceptions. Single
vision with two eyes 50(!

CHAPTER XXXIII.

THE EAR AND HEARING.

The external ear. The tympanum. The auditory ossicles. The
internal ear. The organ of Corti. The loudness, pitch, and
timbre of sounds. Sympathetic resonance. Functions of tlie
tympanic membrane, of the auditory ossicles, of the cochlea,
and of the vestibule. Auditory perceptions 535

CHAPTER XXXIV.

TOUCH, THE TEMPER.VTURE SENSE, THE MUSCULAR SENSE,
COrMON SENSATION, SMELL, AND TASTE.

Nerve-endings in the skin. The pressure sense. The localizing
power of the skin. The temperature sense. Modality of skin
sensations. The muscular sense. Pain. Hunger and thirst.
Smell. Taste 556

CHAPTER XXXV

THE FUNCTIONS OF THE BRAIN AND SPINAI, CORD.

The special physiology of nerve-centres. The spinal cord as a
centre. Reflex actions. The least-resistance hypothesis. The
educafiou of the spinal cord. The inhibition of reflexes. Psy-
chical activities of the cord. The cord as a conductor of
nervous impulses. Functions of the brain in general. Func
tions of medulla oblongata. Functions of cerebellum, pons
Varolii, and mid-brain. Sensations of equilibrium and func-
tions of semicircular canals. Functions of the fore-bram
Hygiene of the brain 578

CHAPTER XXX^a.

VOICE .\ N D SPEECH.

Anatomy of larynx. The vocal cords. Causes of the varying
pitch of the voice. Ran^e of the human voice. The produc-
tion of vowels. Consonants 595

xvi VoyTE^TS.

APPENDIX.

REPRODUCTION AND DEVELOPMENT.

Ri^production in general. Sexual and asexual reproduction. Male
reproductive organs. Female reproductive organs. Puberty,
Ovulation. Menstruation. Hygiene of menstruation. Impregna-
tion, Pregnancy. The fatal ai)peiidages. The intra-utcrine
nutrition of the embryo. Parturition. Lactation. Feeding of
infants. The stages of life. Death.

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 especial-
ly examined, and the forms, connections and mode of
growtli of its parts be studied, as also the resemblances or
differences in such respects, which appear when it is com-
pared with other animal bodies. Or the living Body may
be more especially studied as an organism presenting defi-
nite ]n-operties and performing certain actions; and then its
parts will be investigated with a view to discovering what
duty, if any, each fulfills. 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 concerned with the uses — or in tech-
nical language the functions — of each part is known as
Physiology. Closely connected with physiology is the
science of Hygiene, which is concerned with the conditions
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-matters of the sciences of
Pathology and Pathological Anatomy.

Tissues and Organs. Histology. Examined merely
from the outside, our Bodies present a considerable com-
plexity of structure. We easily recognize distinct parts as
head, neck, trunk and limbs ; and in these again smaller

'^ THE HUMAN BODY.

constituent purts, us eyes, nose, ears, mouth; arm, fore-
arm, hand; thigh, leg and foot. We can, with such
an external examination, go even farther and recognize
different materials as entering into the formation of the
larger parts. Skin, hair, nails and teeth are obviously
different substances ; simple examination by pressure
l)roves that internally there are haixler and softer solid
parts; vrliile the blood that flows from a cut finger shows
that liquid constituents also exist in the Body. The con-
ception of complexity, which may be thus arrived at from
external 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 comjiosition of many
different organs. These primary building 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 Anato7ny. 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 windows,
formed by the combination of these, would answer to ana-
tomical organs.

Zoological Position of Man. External examination of
the human Body shows also that it presents certain re-
semblances to the bodies of many other animals: head
and neck, trunk and limbs, and various minor parts enter-
ing 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 su-
perficial 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

THE PRIMATES. 3

a zoological point of view, man's Body marks liim ont as
belonging to the group of Mammalia (see Zoology), wliicli
includes all animals mwhicli 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 Linnaeus, include man with the mon-
keys 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 classifi-
cation, for zoological groups are defined by anatomical and
not by physiological characters ; and mental traits, since we
know that their manifestation depends upon the structural
integrity of certain organs, are essentially phenomena of
function and therefore not available for purposes of zo-
ological arrangement.

Man however walks erect v.'ith the 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, so in considering corre-
sponding parts in such cases confusion is apt to arise
unless a precise meaning be given to such terms as " an-
terior" and ''posterior." To avoid this difficulty anato-
mists give these words definite arbitrary significations in
all cases and these we shall use in future. The head end
is always antei'ior whatever the natural position of the
animal, and the opposite end posteriorj the belly side is
spoken of as ventral, and the opposite side as dorsal; rigid
and left of course present no difficulty. Moreover, that
end of a limb nearer the trunk is spoken of as proximal
Avith reference to the other or distal end. The words
upper and loiver may be conveniently 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 anem-

4 THE HUMAN BODY.

ones, 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 Vertehrata. 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, and in the adults of nearly all vertebrate
animals a hard axis, the vertebral column {hachhone or spine),
develops in this partition and forms a central support for
the rest of the body (Fig. 2, e e). The dorsal cavity is con-
tinued through the neck, when there is one, into the head,
and there widens out. 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 and is thus often called the limmal cavity.

Upon the ventral side of the head is the mouth o^jen-
inff leading into a tube, the alimentary canal, f, which
passes back through tlie 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 ven-
tral cavity is not subdivided, but in the Mammalia it is; a
membranous transverse partition, the midriff or diaphragm
(Fig. 1, z), separating it into an anterior chest or thoracic
cavity, and a posterior or ahdominal cavity. The alimen-
tary canal and whatever else passes from one of these cavi-
ties to the other must therefore perforate the diaphragm.

In the chest, besides part of the alimentary canal, lie
important organs, the heart, h, and lungs, In, the heart
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 intestines. Among the other more prom-
inent organs in it are the 'kid7ieys and the spleen.

In the dorsal cavity lie soft white organs, the hrain and

THE MAMMALIAN TYPE.

spinal cord, the former occupying its anterior enlargement
in the head. Brain and spinal cord together form the
cerebi'o-spinal nervous centre, but in addition to this there
are found in the ventral cavity a number of small nerve cen-

FiG. 1.— The Body opened from the front to show the contents of its ventral
cavity.

tres united together by connecting cords, and. with their
offshoots forming the sympathetic nervous system.

The walls of the three main cavities are lined by smooth,

THE HUMAN BODY.

moist serous membranes. That lining the dorsal cavity
is the arachnoid ; that lining the chest the pleura ; that

lining the abdomen the perito-
netwi ; the abdominal cavity is in
consequence often called the per-
itoneal cavity. Externally the
walls of these cavities are covered
by the sM7i, which consists of two
layers : an outer horny layer called
the epidermis, which is constantly
being shed on the surface and re-
newed from below ; and a deeper
layer, called the dermis and con-
taining blood, which the ej^ider-
mis does not. Between the skin
and the lining serous membranes
are bones, muscles (the lean of
meat), and a great number of
other structure's which we shall
have to consider hereafter. All
cavities inside the body, as the
alimentary canal and the air j^as-
sages, which open directly or indi-
rectly on the surface are lined by
soft and moist prolongations of
the skin known as mucous mem-
branes. In these the same two
layers are found as in the skin,
but the superficial bloodless one
is called epitheUum and the deep-
er one the cerium.

Diagrammatically we may rep-

I'esent the human Body in longi-

, • , 1,- , ■ * *T, tudinal section as in Fig. 2, where

ber, opening behind into the ° '

pharj'nx, from which one tube aa' is the dorsal or luural cavitii,

leads to the lungs, I. and another • i i

to the stomach. /; /i, the heart; and b and c, respectively, the

k. a kidney; s, the sympathetic ,, . ..-, ., ''.-..

nei-\-ous chain. From the stom- tlioracic and abdommal subdivi-

ach, /, the intestinal tube leads . « ,-, , i • , ^

through the abdominal cavity to sious 01 the vcntral cavity: d rep-
mliJtaS'cana?''''"^''' ''"" ""'" rescuts the diaphragm separating

Fig. 2.— Diagrammatic longi-
tudinal section of the body, a,
the neural tube, with its upper
enlargement in the skull cavity
at a'; N, the spinal cord: N',
the brain; ee, vertebrae form-
ing the solid partition between
the dorsal and ventral cavities;
6, the pleural, and c, the abdom-
inal divisions of the ventral cav-
ity, separated from one another
by the diaphragm, d; i, the
nasal, and o, the mouth cham-

CROSS-SECTION OF THE BODY. 7

them; ee is the vertebral cohimn with its modified prolon-
gation 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, / a lung,
s the sympathetic nervous system, and k a kidney.

A transverse section through the chest is represented
diagrammatically in Fig. 3, where x is the neural canal
containing the spinal cord. In the thoracic cavity are seen
the heart, h, the lungs, //, part of the alimentary canal, a,
and the sympathetic nerve centres, sy; the dotted line on
each side covering the inside of the chest wall and the
outside of the lung represents i\i.Q pleura.

Fig. 3.— a diagrammatic section across the Body in the chest region, x. the
dorsal tube, which contains the spinal cord : the black mass surrounding it is
a vertebra; a. the gullet, a part of the alimentary canal; h, the heart; sy, sym-
pathetic nervous system: //, lungs; the dotted lines around them are the
pleurae; rr, ribs; st, the breastbone.

Sections through corresponding parts of any other Mam-
mal would agree in all essential points with those repre-
sented in Figs. 2 and 3.

Tlie Limbs. The limbs present 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 jiarts of one or more
bones (as seen at U 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 enveloj^ed in skin. The only cavities in
the limbs are branching tubes which are filled with liquids
du^^ing life, either hloocl or a watery-looking fluid known as
ly^ph. These tubes, the blood and lymph vessels respec-

8 THE HUMAN BODY.

tively, are not however characteristic of the limbs, for they
are present in abundance in the dorsal and ventral cavities
and in their walls.

Fig. 4.— a section across the forearm a short distance below the elbow-joint.
R and U, its two supporting bones, the radius and ulna; e, the epidermis, and d
the dermis of the skin ; the latter is continuous below with bands of connective
tissue, s, which penetrate between and invest the muscles, which are indicated
by numbers; n, n, nerves and vessels.

Chemical Composition of the Body. In addition to the
study of the Body as composed of tissues and prgans which
are optically recognizable, we may consider it as composed
of a number of different chemical substances. This branch
of knowledge, which is still very incomplete, really j)resents
two classes of joroblems. On the one hand we may limit
ourselves to the examination of the chemical substances
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 Anatotny. But as long as the
Body is alive it is the seat of constant chemical trans-
formations in its material, and these are inseparably con-
nected 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 it is usual to include
all the known facts as to the chemical composition and
metamorphoses of living matter under the name of Physio-
logical Chemistry. For the present we may confine our-
selves to the more important substances derived from or
known to exist in the Body, leaving questions concerning the
chemical changes taking place Avithin it for consideration
along with those functions which are performed in connec-
tion with them.

CHEMISTRY OF THE BODY. 9

Elements Composing the Body. Of the elements known
to chemists only sixteen have been found to take part in
the formation of tlie human Body. These are carbon, hy-
drogen, nitrogen, oxygen, sulphur, iDliosphorus, chlorine,
fluorine, 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.

Uneombined Elements. Only a very small number of
the above elements exist in the body uneombined. 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 contained in these spaces
it can hardly be said to form a part of the Body. Nitro-
gen, also exists uneombined in the lungs and alimentary
canal, and in small quantity in solution in the blood. Free
hydrogen has also been found in the alimentary canal, be-
ing 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 re-
gard to very many of them we do not know that the form
in which we extract them is really that in which .the ele-
ments 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 dehris for examination. We know in
fact, tolerably accurately, what compounds enter the Body
as food and what finally leave it as waste; but the inter-
mediate conditions of the elements contained in these com-
pounds during their sojourn inside the Body we know very
little about; more especially their state of combination dur-
ing 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 com])ounds existing
in or derived from the Body may be classified as organic

10 THE HVMAN BODY.

and inorganic, and the former be subdivided into those
which contam nitrogen and those which do not.

Nitrogenous or Azotized Organic Compounds. These
fall into several main groups: jjroieids, 2^e2)iones, albu-
7ni)ioixh, cnjstaUiae substances, and coloring 'matters.

Proteids are by far the most characteristic substances ob-
tained from the Body, since they are only known as exist-
ing in or derived from living things, either animals or
plants. The type of this class of bodies may be found in
the Avhite of an egg, where it is stored uj) as food for
the developing chick ; from this typical form, which is
called egg albumin, the jsroteids in general are often called
albuminous bodies. Each of them contains carbon, hydro-
gen, oxygen, sulj)hur, and nitrogen united to form a very
complex molecule, and although different members of the
family differ from one another in minor points they all
agree in their broad features and have a similar percentage
composition. The latter in different examples ajoj^ears to
vary within the following limits, but it is almost imj)ossible
to get any one of them pure for analysis:

Carbon 53 to 54 per ceut. ,

Hydrogen 7 to 7.5 " !

Oxj^gen 21 to 24.0 " 1

Nitrogen 15 to 17.0 " j

Sulphur 0.8 to 2.0 " |

Proteids are recognized by the following 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 Avitli ammonia. This is the xa)itho-proteic
test.

2. Boiled with a solution containing subnitratc and per-
nitrate of mercury they give a pink precipitate, or, if in
very small quantity, a pink-colored solution. This is
known as MiUon^s test.

3. If a solution contidning a proteid be acidulated Avitli
strong acetic acid and be boiled after the addition of an
equal bulk of a saturated w^atery solution of sodium sul-
phate, the proteid wall be precipitated.

PR0TEID8. PEPTONES. 11

Among the more important proteids obtained from tlie
human Body are the following:

Serum albumin. This exists in solution in the blood and
is very like egg albumin in its proj^erties. It is coagulated
(like the white of an egg) when boiled, and then passes
into the state of coagulated proteid which is, unlike the
original serum albumin, insoluble in dilute acids or alka-
lies or in water containing neutral salts in solution. All
other proteids can by appropriate treatment be turned into
coagulated proteid.

Fibrin. This forms in blood when it " clots," either in-
side or outside of the body. It is made by the interaction
of two other proteids known SiS fibrinogen and fibrinojjlastin.
It is insoluble in water.

Myosin. This is derived from the muscles, in which it
develops and solidifies after death, causing the ''death-
stift'ening."

Globulin exists in the red globules of the blood and dis-
solved in some other liquids of the body. In the blood
corpuscles it is combined with a colored substance to form
hmmoglobin, which is crystallizable.

Casein is found in milk. It is insoluble in water but
soluble in dilute acids and alkalies. Its solutions, ^^nlike
those of fibrin or myosin, do not coagulate spontaneously,
or like that of serum albumin on boiling. In the milk it
IS held in solution by the free alkali jiresent; when milk be-
comes sour this is neutralized and the casein is precipitated
as the '"curd." Cheese consists mainly of casein.

Peptones. These are formed in the alimentary canal
by the action of some of the digestive liquids upon the
proteids swallowed as food. They contain the same elements
as the proteids and give the xantho-proteic and Millon's
reactions, but are not preci]iitated hj boiling with acetic
acid and sodium sulphate. Their great distinctive charac-
ter is however their difEusibility. The proteids proper will
not dialyze (see Physics), but the peptones in solution pass
readily through moist animal membranes.

Albuminoids. Tliesc contain carbon, hydrogen, oxy-
gen and nitrogen, but rarely any sulphur. Like the

12 THE HUMAN BODY.

j)rotcids, the nearest chemical allies of which they seem
to be, they are only known in or derived from living
beings. Gelatin, obtained from bones and ligaments by
boiling, is a typical albuminoid; as is chondrin, which is
obtained similarly from gristle. 3fucin, which gives their
glairy tenacious character to the secretions of the mouth
and nose, is another albuminoid.

Crystalline Nitrogenous Substances. These are a
heterogeneous group, the great majority of them being
materials which have done their work in the Body and are
about to be got rid of. Nitrogen enters the Body in foods
for tlie most part in the chemically complex form of some
j)rotcid. In the vital processes these proteids are broken
down into simpler substances; their carbon being partly
combined with oxygen and passed out through the lungs
as carbon dioxide; tlieir 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
usually passed out in the form of a crystalline compound,
containing what chemists call an ''ammonium residue."

COl
Of these the most important is urea (Carbamide H^ >■ W),

which is eliminated through the kidneys. U)'ic acid is an-
other nitrogenous waste product, and many others, such as
krcaiin and Tcreatinin, seem to be intermediate stages be-
tween the proteids which enter the body and the urea and
uric acid which leave it.

In the bile or gall, two crystallizable nitrogen-contain-
ing bodies, glycocliolic and taurocliolic acids, are found com-
bined with soda.

Nitrogenous Coloring Matters. These form an arti-
ficial group whose constitution and origin is ill known.
xVmong the most important are the following:

HcBinatin, derived from the red corpuscles of the blood
in which a residue of it is combined with a proteid residue
to form licBinoglohin.

Biliriihin and hiliverdin, which exist in the bile ; the
former predominating in the bile of niau and of carnivo-

FATS. CARBOHYDRATES. 13

rous animals and giving it a reddish yellow color, while
biliverdin predominates in the bile of Herbivora which is
green.

Non-Nitrogenous Organic Compounds. These may
be conveniently grouped as liydrocarbons or fatty bodies;
carbohydrates or amyloids ; and certain non-azotized acids.

Fats. T\\efats 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 (CsiHgsOe), stearin (CsTHuoOe),
and oleifi (C57H104O6). 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 proportions as to be
kept fluid. The total quantity of fats in the Body is sub-
ject to great variations, but its average quantity in a man
weighing 75 kilograms (165 pounds) is about 2.75 kilo-
grams (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 glycerine. The
fatty acid unites with the alkali present to form a soap.

Carbohydrates. These also contain carbon, hydrogen
and oxygen, but there is one atom of oxygen present for
every two of hydrogen in the molecule of each of them.
Chemically they are related to starch. The more impor-
tant of them found in the Body are the following:

Glycogen (CeHioOs) found in large quantities in the
liver, where it seems to be a reserve of material answering
to the starch stored up by many plants. It exists in smaller
quantities in the muscles.

Glucose, or grape sugar (CbHisOb), which exists in the
liver in small quantities ; also in the blood and lymph.
It is largely derived from glycogen which is very readily
converted into it.

Iiiosit, or muscle sugar (CsHiaOs + 2HoO), found in
muscles, liver, spleen, kidneys, etc.

Lactose, or sugar of milh (OiaHooOn + H^O), found in
considerable quantity in milk.

Organic Non-Nitrogenous Acids. Of these the most

14 THE HUMAN BODY.

important is carbonic dioxide (CO*), which is the form in
which by far the 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 also are found in the
Body; stearic, palmitic, and oleic have been above men-
tioned as obtainable from fats. Lactic acid is found in the
stomach and develops in milk when it turns sour. A body
of the same percentage composition, CaHeOa {sarcolac-
tic acid), is formed in muscles when they work or die.

GlycerO'iihosplioric acid (CsHoPOe) is obtained on the
decomposition of lecithin, a complex nitrogenous fat found
in nervous tissue.

Inorganic Constituents. Of the simpler substances en-
tering 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 lbs.), if completely
dried would therefore lose about 50 kilos (110 lbs.) from the
evaporation of water. Of the constituents of the Body the
enamel of the teeth contains least Avater (about two per
cent) and the saliva most (about 99.5 per cent); between
these extremes are all intermediate steps — bones containing
about 23 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 keeping other substances in solution in
water.

Potassium chloride (KCl); 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 jDhosphates, magnesium phosphate, sodium sul-
phate, potassium sulphate and calcium fluoride have been
obtained from the body.

Uncombined Hydrochloric acid (HOI) is found in the
stomach.

CHAPTER II.

THE FUNDAMENTAL PHYSIOLOGICAL
ACTIONS.

The Properties of the Living Body. Wheu we turn
from the structure aud compositiou of the living Body to
consider its powers and properties we meet with the same
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,
ajjertures to receive food and others to get rid of wastes.
We thus perceive that different organs of our Bodies have
Tery different endowments and serve for very distinct pur-
poses; and here again the study of internal organs shows
us that the varieties of quality observed on the exterior are
but slight indications of differences of property which per-
vade the whole, being sometimes dependent on the specific
characters of the tissues concerned and sometimes u]3on the
manner in which these are combined to form various
organs. Some tissues are solid, rigid and of constant
shape, as those composing the bones and teeth; others, as
the muscles, are soft and capable of changing their form;
and still others are capable of working chemical changes
by which such peculiar fluids as the bile or the saliva are
produced. "We find elsewhere a number of tissues com-
bined to form a tube adapted to receive food and carry it
through the Bodv for digestion, and again similar tissues

16 THE HUMAN BODY.

differently arranged to receive tlie air which we breathe-in,
and expel after abstracting from it part of its oxygen and
adding to it certain other things; and in the heart and
blood-YCSsels we find almost the same tissues arranged to
propel and carry the blood over the whole Body. The
working of the Body oiiers clearly even a more comjilex
subject of study than its structure.

Physiological Properties. In common with inanimate
objects the Body possesses many merely physical properties,
as weight, rigidity, elasticity, color, and so on; but in ad-
dition 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 are 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 loug to neighboring objects; neverthe-
less we are at the end of the day as warm as at the begin-
ning, the temperature of the Body in health not varying
much from 37.5° 0. (99° 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 in touching a corpse produces so powerful
an impression as commonly to be described as icy cold.
The other great characteristic of the living Body is its
l^ower of executing movements; so long as life lasts it is
never at rest; even in the deepest slumber the regular
breathing, 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 conscious-
ness by the living Body, Avhether aroused by forces im-
mediately acting upon sense-organs or not, we might de-
scribe it as a heat-producing, moving, conscious organism.

The production of heat in the Body needs fuel of
some kind as much as its jiroduction in a fire; and every
time we move ourselves or external objects some of the
Body is used up to supply the necessary working power, just

CELLS. 17

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 otliers for the supply of new materials to the tis-
sues 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 exterior tempera-
ture, 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 de^Dend 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 siDccial 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 " <M

generalized manifestations as exhibited by '■^

some of the lowest living things or by the .§ '" \

simplest constituents of our own Bodies. f^ .

Cells. Among the anatomical elements \ ^<i|^

which the histologist meets with as entering ^- %*?

into the composition of the human Body ^

are minute granular masses of a soft con- ^ a

sistence, about 0.012 millimeter (gVoo o^ ^^ e0^^: W
inch) in diameter (Fig. 5, e). Imbedded in t'^^^i^S '
each lies a central portion, not granular 'W^^
and therefore different in appearance from ' 0

the rest. These anatomical units are known ^ ^ ^

Fig. 5.— Forms

as cells, the granular substance bemg the of ceUsfromthe
cell body and the imbedded clearer portion
the cell nucleus. Inside the nucleus may often be distin-
guished a still smaller body — the nucleolus. Cells of this
kind exist in abundance in the blood, where they are known
as the lohite hlood corpuscles, and each exhibits of itself

18

THE HUMAN BODY.

certain properties wliicli are distinctive of all living things
as compared with inanimate objects.

Cell Growth. In the first i)lace, each such cell can take
up materials 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 eutirely different way.
The cell takes up chemical elements, either free or com-
bined 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 de-
posited, 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 constituent molecules. The new-
formed molecules therefore contribute to the growth of the
cell not by superficial accretion, but by interstitial deposit
or intussuscej)tion.

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 certain
size, growth of course ceases. In fact the work of the cell in-
creases as its mass, and therefore as the cube of its diame-

FiG. 0.— A white blood corpuscle dividing, as observed at successive intervals
of a few seconds with the microscope.

ter; while the receptive powers, dependent primarily upon
the superficial area, only increase as the square of the di-
ameter. The breaking down in the cell increases Avhen its

ASSIMILATION. 19

work does and so at last equals the reception and construc-
tion. The second limitation to indefinite growth is con-
nected with the power of the cell to give rise to new cells
like itself. Under certain circumstances, which are not
well known, the cell may become narrowed (Fig. 6) at one
zone; the constriction deepens until the parts on each side
of it are merely united by a narrow band which finally
gives way and two cells are formed, each like the parent
but for its smaller size; or the cell may divide into two or
more by flat surfaces of separation, or in
a way intermediate between tliis mode
and the last (Fig. 7). In some cases new
cells form in the interior of the old and
are then set free from it. The new cells
produced in these ways grow as the origi-

1 11 J-J A • , ix- 1 • Fig. r. -Another

nal cell did, and may m turn multiply m mode of ceii division.

1 1 ~rr 1 l^ A rounded cell elon-

the same manner. Very commonly the gates in one diame-
nucleus divides before the rest of the Larrowslni'^me'dian
cell, and its parts then form the nuclei of s°p|^ated hZ\4

the new cells. parts; the nucleus di-

uix^./ ../ y^^ vides at the same

Assimilation: Beproduotion. These *''"^-
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 rejjrodudion, are
possessed by all kinds of living beings, whether animals or
plants. There is, however, this important difference be-
tween 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 each is to continue its exist-
ence. 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 indi-
viduals composing 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 com-

20 TEE HUMAN BODY.

pounds into simpler ones, sometimes called disassifnilation,
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 indirectly with
other elements in the cell, as for exami^le carbon,
giving rise to cai'bon dioxide, 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 w^hen coal is burnt
in a fire, but maybe used in part for other purposes such as
producing movements. The carbon dioxide 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 respiratio7i which occurs in all living things.
Assimilation and disassimilation, going on side by side and
being to a certain extent correlative, are often spoken of
together as the process of nutrition, a term which there-
fore includes all the chemical transformations occurring in
living matter.

Contractility. Nutrition and (with the above-mentioned
partial exception) reproduction characterize all living crea-
tures; and both faculties are possessed by the simple
nucleated cells already referred to as found in our blood.
But these cells possess also certain other jDroperties which,
although not so absolutely diagnostic, are yet very charac-
teristic 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 some-
times show themselves as constrictions ultimately leading
to the division of the cell; but more commonlY(rig. 12*)
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. Th-e portion thus protruded may in turn
be drawn in and u process be thrown out elsewhere : or the
rest of the cell may collect around it, and a fresh protru-

*P. 48.

IRRITABILITY. 21

sion be then made in the same side; and by repeating this
manoeuvre 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 micro-
scopic animal known as the Ammha (see Zoology) are called
amoeboid, and the faculty in the living cell upon which they
depend is known in physiology as contracUlity. It must
be borne in mind that physiological contractility in this
sense is quite diiierent from the so-called contractility of
a stretched Indian- rubber band, which merely tends to re-
assume a form from which it has previously been forcibly
removed.

Irritability. Another property exhibited by these blood-
cells is known as irritahility. An Amoeba coming into
contact with a solid particle calculated to serve it as food
will throw around it processes of its substance, and grad-
ually 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 bur-
rows 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 gunpow-
der or by a slight tap on a piece of gun-cotton. It is this
disproportion between the excitant (known in Physiology as
a stiimdus) and the result, which is the essential character-
istic of irritability when the term is used in a physiological
connection. The granular cells of the blood can take
foreign matters into themselves in exactly the same man-
ner 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 Amoeba or one of these
blood-cells comes into contact with a foreign body and
proceeds to draw it into its own substanQe, the activity ex-
cited is not merely displayed by the parts actually touched.
Distant parts of the cell also co-operate, so that the influ-

23 THE HUMAN BODY.

ence 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, ran-
dom. They are not irregular convulsions, but are adapted
to attain a certain end, being so combined as to bring the
external particle into the interior of the cell. This capa-
city of all the parts to work together in definite strength
and sequence, to fulfil some pur])ose, is known as co-ordi-
nation.

These Properties Characteristic but not Diagnostic.
These four faculties, irritability, conductivity, contractility,
and co-ordination, 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 Avill be produced. Here the feather-
touch {stimulus) has called forth movements 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
contractility, 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 physiological conductivity. And finally
these movements are 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 co-ordination. Sjaeaking broadly, these
properties are more manifest in animals than in plants,
though they are by no means absolutely confined to the
former. In the sensitive plant touching one leaflet will
excite regular movements of the whole leaf, and many of
the lower aquatic plants exhibit movements 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 groivtli hy intussiisceptio7i and reproduction are.
Irritability is but a name for unstable molecular equilibrium,
and is as marked in nitroglycerine as in any living cells; in
the telephone the influence of the voice is conducted as a

SPONTANEITY. 23

molecular change along a wire, and produces results at a
distance; and many inanimate machines alford examples of
the co-ordination of movements for the attainment of
definite ends.

Spontaneity. There is, however, one character belonging
to many of the movements exhil)ited 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 independently of any re-
cognizable 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 conditions, a series of
changes when once a start has been given; but it cannot
initiate them itself. Each living cell in the long-run is but
a complex 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 be-
comes 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 pulling
the trigger will fire off a gun. Once the equilibrium of
the cell has been disturbed, 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 reattainment of this state is
commonly indefinitely postponed by the reception of new
particles, food in one form or another, from the exterior.
The nearest approach to it is probably exhibited by the rest-
ing state into which some of the lower animals, as the wheel

34 THE HUMAN BODY.

animalcules, pass wlien dried slowly at a low temperature;
the drying acting by checking the nutritive processes,
which would otherwise have prevented the reattainment
of molecular equilibrium. 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" move-
ments recommence. If, therefore, we use the term spon-
taneity to express ajDowerina resting system of particles of
initiating changes in itself, it is possessed neither by living
nor not-living things. But if we sim^Dly employ it to desig-
nate changes whose primary cause we do not recognize, and
which 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 Avater 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 dif-
ference between it and the living cell lies not in any spon-
taneity 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. Finding all these properties i^ossessed by
a simple nucleated cell, we are naturally led to inquire upon
what part of it do they depend? It is clear that if they are
exhibited in the absence of any one it cannot be essen-
tial to their manifestation. Now a study of the lower
forms of life shows us that these powers are independent of
the cell nucleus, since we find them all exhibited by cells in
which the nucleus is wanting. Moreover, in many cases
not only the nucleus but all granules are absent, and yet we
find the remaining mass nutritive, reproductive, irritable,
contractile, conductive, co-ordinative. and automatic. We
are thus driven to conclude that in the case of the granular
blood-cells, these faculties are most probably endowments

PE0T0PLA8M. 25

of the transpurent portions of the cell body, in which the
granules lie imbedded. This, the really working part of the
cell, is known as the cell protojjlasm. The role of the
nucleus and granules so often present is not yet well
understood; possibly the granules in many cases represent
incompletely assimilated food.

What the actual chemical constitution of protoplasm is
we do not know, but it is one of great complexity. All
methods of chemical analysis destroy it, and what we analyze
is not protoplasm, which is always alive — which is a form
of matter endowed with those properties which we call
vital — but a mixture of the products of its decomposition
when it ceases to live. Such a mixture is often called dead
protoplasm, but the phrase is objectionable as implying a
contradiction. Wherever there is protoplasm there is life,
and wherever we meet with life we find protoplasm, so that
it has been called the " physical basis of life." The name
protoplasm, too, is rather to be regarded as a general term
for a number of closely allied substances agreeing with one
another chemically in main points, as the proteids do, but
differing in minor details, in consequence of which one cell
differs slightly 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 molecules of protoplasm, and in part probably deposited
in layers between them. Of organic constituents proto-
plasm always yields one or more proteids, some fats, and
some starchy or saccharine body. So that the original
protoplasm is probably to be regarded as containing chemi-
cal " residues" of proteids, fats, and carbohydrates, com-
bined with salts and water.

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

CHAPTER III.

THE DIFFERENTIATION OF THE TISSUES AND
THE PHYSIOLOGICAL DIVISION OF EMPLOY-
MENTS.

Development. Every human Body commences its indi-
vidual existence as a single nucleated cell. This cell,
known as the ovum, divides or segments and gives rise to

Fig. 8. — A, an oviim; B to E, successive stages in its segmentation until the
morula, F. i.> produced.

a mass consisting of a number of similar units and called
the mulberry mass or the morula. At this jDcriod then,
long before birth, there are no distinguishable tissues en-
tering into the structure of the Body, nor are any organs
recognizable.

For a short time the morula increases in size by the
growth and division of its cells, but very soon new pro-
cesses occur which ultimately give rise to the complex

DIVISION OF LABOB. 37

adult body with its many tissues and organs. G-roups 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, forming a complex from a simple whole, is
called developinent; and the process itself in this case is
known as the differentiation of the tissues, since by it they
are, so to speak, separated or specialized from the general
mass of mother-cells forming the morula.

As the difference:^ in the form and structure of the con-
stituent 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 in-
dependent living things, but to form a single human being,
in whom each part, while maintaining 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 co-operation 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 activi-
ties subordinated to tlie good of the entire community.

The Physiological Division of Labor. The fundamen-
tal physiological properties, originally exhibited by all the
cells, become ultimately distributed between the different
modified cells which form the tissues of the fully developed
Body much in the same way as different employments are
distributed in a civilized state; for the difference between
the fully developed human Body and the collection of
amoeboid cells from which it started is essentially the same
as that between a number of wandering savages and a civi-
lized nation. In the former, apart from differences de-
pendent on sex, each individual has no one special occu-
pation different from that of the rest, but has all his own
needs to look after: he must collect his own food and
prepare it for eating, make his own clothes if he wears

'Z8 THE HUMAN BODY.

any, provide liis own shelter, and defend himself from Avild
beasts or his fellow-men. In the civilized countr}', on the
other hand, we find agriculturists to raise food and cooks
to prepare it, tailors to make clothes, and policemen and
soldiers to provide i)rotection. And jnst as we find that
when distribution of employments in it is more minute
the more advanced a nation is in civilization, so is an ani-
mal higher or lower in the scale according to the degree in
Avhich it exhibits a division of physiological duties between
its different tissues.

From the subdivision of labor in advanced communities
several important consequences arise. In the first place,
each man devoting himself to one kind of work mainly and
relying upon others for the supply of his other needs, every
sort of work gets better done. The man who is constantly
making boots becomes more expert than one whose atten-
tion is constantly distracted by other duties, and he will not
only make more boots in a given time, but better ones; and
so with the performance of all other kinds of work. In
the second place, a necessity arises for a new sort of indus-
try, in order to convey the produce of one individual in
excess of the needs of himself and his family to those at a
distance who may want it, and to convey back in return
the excess of their produce which he needs. The carriage
of food from the country to cities, and of city produce
to country districts, and the occupation of shopkeeping,
are instances of these new kinds of labor which arise in
civilized communities. In addition there is developed a
need for arrangements by which the work of individuals
shall be regulated in j^roportion to the wants of the
whole community, such as is in part effected by the agency
of large employers of labor who regulate the activities of
a number of individuals for the production of various
articles in the different quantities required at different
times.

Exactly similar phenomena result from the subdivision
of labor in the human Body. By the distribution of em-
ployments between its different tissues, each one specially
doing one work for the general community and relying on

CLASSIFICATION OF TISSUES. 29

the others for their aid in turn, each necessary work is
better performed. And a need arises for a distributive
mechanism by which the excess products, if any, of various
tissues shall be carried to others which require them, and
for a regulative mechanism by which the activities of the
various tissues shall be rendered proportionate to the needs
of the whole Body at different times and under different
circumstances. Accordingly, as we may classify the in-
habitants of the United States into lawyers, doctors, clergy-
men, merchants, farmers, and so on, we may

Classify the Tissues, by selecting the most distinctive
properties of each of those entering into the construc-
tion of the adult Body and arranging them into physio-
logical 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 arc composed of
cells which have developed along no one special line, but
retain very much the form and properties of the cells form-
ing the very young Body before different tissues were re-
cognizable in it. The lymph corpuscles and the colorless
corpuscles of the blood belong to this class.

2. Supporting Tissues. Including cartilage (gristle),
hone, and contiective tissue. Of the latter there are several
subsidiary varieties, the two more imj)ortant being wliite
fibrous connective tissue, composed mainly of colorless in-
extensible fibres, and yelloiv fibrons tissue, composed mainly
of yellow elastic fibres. All the supi^orting tissues are used
in the Body for mechanical purposes: the bones and carti-
lages form the hard framework by which softer tissues are
supported and protected; and the connective tissues unite
the various bones and cartilages, form investing mem-
branes around different organs, and in the form of fine
networks penetrate their substance and support their con-
stituent cells. The functions of these tissues being for the
most part to passively resist strain or pressure, none of

30 THE HUMAN BODY.

them litis any very marked j^liysiological property; 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. 42,* where
the cells are seen imbedded in cavities in a matrix which
they have formed around them; and which matrix by its
firmness and elasticity forms the functionally im})ortant
part of the tissue.

3. Nutritive Tissues. This is a large group, the mem-
bers of which fall into three main divisions, viz. :

Assimilative tissues, concerned in receiving and prepar-
ing food materials, and including — {a) Secretory tissues,
com])osed of cells Avhich make the digestive liquids poured
into alimentary canal, and bringing about chemical or other
changes in the food, {h) 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 kidneys, 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 gas-
eous interchanges between the Body and the surrounding
air. They arc constituted by the cells lining the lungs
and by the colored corpuscles of the blood.

As regards the nutritive tissues it requires especially to be
borne in mind that although such a classification as is here
given is useful, as helping to show the method i)i!rsued in
the domestic economy of the Body, it is only imperfect
and largely artificial. Every cell of the Body is in itself
assimilative, respiratory, and excretor}^ 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 excre-
tion of its individual tissues to be ultimately louiced after
by themselves, just as even the mandarin described by Robin-
sou Crusoe who found his dignity promoted by having

*P. lui.

STORAGE TISSUES. 31

servants to put the food into his month, 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 formation of cer-
tain substances which are ponred 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
accumulated in it beyond its immediate needs. This lies
in part in the individual cells themselves, just as in a pros-
perous community nearly every one will have some little
pocket-money. But apart from this reserve there are cer-
tain cells, a sort of capitalists, 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 contain in health a reserve
fund for the rest of the Body. Since both of these, to-
gether with secretory and excretory cells, are the seats of
great chemical activity, they are all often called nietaholic
tissues.

5. Irritable Tissues. The maintenance, or at any rate
the best prosperity, of a nation is not fully secured when a
division of labor has taken place in food-supjjly and food-
distribution employments. It is extremely desirable that
means shall be i^rovided by which it may receive infornui-
tion of external changes which may affect it as a whole,
such as the policy of foreign countries ; or which shall en-
able the inhabitants of one jiart to know the needs of an-
other, and direct their activity accordingly. Foreign min-
isters and consuls and newspaper correspondents are em-
ployed to place it in communication with other states and
keep it informed as to its interests ; and we find also orga-
nizations, such as the meteorological department, to warn
distant parts of approaching storms or other climatic
changes which may seriously affect the pursuits carried on
in tliem. In tlie human Body we have a comparable class
of ' ktelligence-gaining tissues lying in the sense organs.

32 THE HUMAN BODY.

"whose business it is to ascerttiin and communicate to the
whole, external changes which occur around it. Since
the usefulness of these tissues depends upon the readiness
with which slight causes excite them to activity, we may
call them the irritahJe tissvef<.

6. Co-ordinating and Automatic Tissues. Such infoi--
mation as that collected by ministers in foreign parts or by
meteorological observers, is usually sent direct to some cen-
tral office from which it is redistributed; this mere redis-
tribution is, however, in many cases but a small j^art of the
work carried on in such offices. Let us suppose informa-
tion to be obtained that an Indian chief is collecting his
men for an attack on some point. The news is probably
first transmitted to Washington, and it becomes the duty
of the executive officers there to employ certain of the con-
stituent units of the society in such definite Avork as is
needed for its protection. Troops have to be sent to the
place threatened; perhaps recruits enlisted; food and
clothes, weapons and ammunition must be provided for
the army; and so on. In other words, the work of the
various classes composing the society has to be organized
for the common good; the mere spreading the news
of the danger would alone be of little avail. So in the
Body: the information forwarded to certain centres from
the irritable tissues is used in such a way as to arouse to
orderly activity other tissues whose services are required; we
find thus in these centres a group of co-ordinating tissues,
represented by nerve-cells and possibly by certain other con-
stituents of the nerve centres. Certain nerve-cells are also
automatic in the physiological sense already pointed out.
The highest manifestation of this latter faculty, shown
objectively by muscular movements, is subjectively known
as the ''will," a state of consciousness; and other mental
phenomena, as sensations and emotions, are also associated
with the activity of nerve-cells lying in the brain. How it
is that any one state of a material cell should give rise to a
particular state of consciousness is a matter quite beyond
our powers of conception; but not really more so than how
it is that every portion of matter attracts every other por-

MOTOR TISSUES. 33

tion according to the law of inverse squares. In the
living Body, as elsewhere in the universe, we can study
phenomena and make out their relations of sequence or
co-existence; but why one phenomenon is accom])anied by
another, why in fact any cause produces an effect, is a
matter quite beyond our reach in every case; whether it be
a sensation accompanying a molecular change in a nerve-
cell, or the fall of a stone to the ground in obedience to the
law of gravitation.

7. Motor Tissiies. These have the contractility of the
original protoplasmic masses highly developed. The more
important are ciliated cells and muscular tissue. The for-
mer 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. 47*) lining the in-
side 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. Mus-
cular tissue occurs in two main varieties. One kind is
found in the muscles attached to the bones, and which are
used in the ordinary voluntary movements of the body.
It is composed of fibres which present cross-stripes when
viewed under the microscope (Fig, 53f), and is hence
known as striped or striated muscular tissue. The other
kind of muscular tissue is found in the walls of the
alimentary canal and some other hollow organs, and con-
sists of elongated cells (Fig. ,55 J) which present no cross
striation. It is known as plain or unstriated muscular
tissue.

The cells enumerated under the heading of ''undiffer-
entiated tissues'' might also be included among the motor
tissues, since they are capable of changing their form.

8. The Conductive Tissues. These are represented by
the nerve fibres, slender threads formed by modification
and fusion of cells, and having the conductivity of the
amoeboid cells of the morula highly developed; that is to
say, they readily transmit molecular disturbances. When
its equilibrium is upset at one end, a nerve-fibre will
transmit to its other a molecular movement known as a

*P. 115. t P^m JP. 134 "

34 TEE HUMAN BODY.

^'nervous impulse,'' and so can excite in turn parts distant
from the original exciting force. Nerve-fibres place, on
the one hand, the irritable tissues in connection with the
automatic, co-ordinating, and sensory; and on the other
put the three latter in communication with the muscular,
secretory, and other tissues.

9. Protective Tissues. These consist of certain cells
lining ca^itics inside the body and called epithelial cells,
and cells covering the whole exterior of the Body and
forming 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
secretory, excretory, or receptive; and ciliated cells have
already been included among the motor tissues, although
from the fact that the movements of their cilia go on
in separated cells and independently of recognizable exter-
nal stimuli, they might well have been put among the au-
tomatic. The protective tissues may be best defined as
including cells which line free surfaces, and whose func-
tions are mainly mechanical or physical.

10. The Reproductive Tissues. These are 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 ovum, or female pro-
duct which directly develops into the new human being,
lies dormant until it has been fertilized or acted ui)on by
the product of the male.

The Combination of Tissues to Form Organs. The va-
rious tissues above enumerated forming the building mate-
rials of the Body, anatomy is primarily concerned with
their structure, and physiology with their properties. If
tliis, however, were the whole matter, the problems
of anatomy and physiology would l)e much simpler
than they actually are. The knowledge about the living
Body obtained by studying only the forms and functions

ORGANS. 35

of the individual tissues would be comparable to that at-
tained about a great factory by studying separately the
boilers, pistons, levers, wheels, etc., found in it, and leav-
ing out 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 tis-
sue, although its essential properties are everywhere the
same, may by its activity subserve very various uses accord-
ing to the manner in which it is combined with others.
For example: A nerve-fibre uniting the eye Avith 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-fibre running from
the brain to the muscles will, also by virtue of its conduc-
tivity, when its ending in the brain is excited by a change
in a nerve-cell connected with it, stir up the muscle to con-
tract under the control of the will. The different results
depend on the different parts connected with the ends of
the nerve-fibres in each case, and not on any difference in
the properties of the nerve-fibres themselves.

It becomes necessary then to study the arrangement and
uses of the tissues as combined to form various organs, and
this is frequently far more difficult than to make out the
structure and properties of the individual tissues. A\\ or-
dinary muscle, such as one sees in the lean of meat, is a
very complex organ, containing not only contractile mus-
cular tissue, but supporting and uniting connective tissue
and conductive nerve-fibres, and in addition a complex
commissariat arrangement, composed in its turn of several
tissues, concerned in the food supply and waste removal of
the whole muscle. The anatomical study of a muscle has

36 THE HUMAN BODY.

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 ac-
count the actions of all these parts working together and
not merely the functions of the muscular fibres 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 Mechanisms. Even the study of organs
added to that of the separate tissues does not exhaust the
whole matter. In a factory avc 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 together 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
physiological cqjparatuses. The circulatory apparatus, for
example, 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 accord-
ing to their needs. Again, in the visual apparatus we find
the co-operation of (a) a set of optical instruments which
bring the light proceeding from external objects to a focus
upon (5) the reifi/^a, which contains highly irritable parts;
these, changed by the light, stimulate (^•) the optic nerve,
which is conductive and transmits a disturbance Avhich
arouses finally (d) sensory parts in the brai7i. In the pro-
duction of ordinary sight sensations all these parts are con-
cerned and work together as a visual apparatus. So, too,
we find a respiratory apparatus, consisting primarily of two
hollow organs, the lungs, which lie in the chest and com-
municate by the ivindpipe with the back of the throat,
from Avhicli air enters them. But to complete the respi-
ratory apparatus are many other organs, bones, muscles,
nerves, and nerve-centres, which work together to renew

ANATOMICAL SYSTEMS. 37

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.

Many similar instances, as the alimentary apparatus, the
auditory apparatus, and so on, will readily be thought of.
The studv of the working of such complicated mechanisms
forms a very important part of physiology.

Anatomical Systems. From the anatomical side the
whole collection of bodily organs agreeing in structure
with one another is often spoken of as a system; all the
muscles, for example, are grouped together as the muscular
system, and all the bones as the osseous system, and so on,
without any reference to the different uses of different
muscles or bones. The term system is, however, often used
as equivalent to " apparatus:" one reads indifferently of the
"circulatory system" or the '^ circulatory apparatus." It
is better, however, to reserve the term system for a collec-
tion of organs classed together on account of similarity of
structure; and "'apparatus" for a collection of organs con-
sidered together on account of their co-operation to execute
one function. The former term will then have an anatomi-
cal, the latter a physiological, significance.

The Body as a Working Whole. Finally it must all
through be borne in mind that not even the most complex
system or apparatus can be considered altogether alone as
an independently living part. All are united to make one
living Body, in which there is throughout a mutual inter-
dependence, so that the whole forms one human being, in
whom the circulatory, respiratory, digestive, sensory, and
other apparatuses are constantly influencing one another,
each modifying the activities of the rest. This interaction
is mainly brought about through the conductive and co-
ordinating tissues of the nervous system, which place all
parts of the Body in communication. But in addition to
this another bond of union is formed by the blood, which
by the circulatory apparatus is carried from tissue to tissue
and organ to organ, and so, bringing materials derived in
one region to distant parts, enables each organ to influence
all the rest for good or ill.

38 THE HUMAN BODY.

Besides the blood another liquid, called lymph, exists in
the Body. It is contained in vessels distinct from those
which carry the blood, but emptying into the blood-vessels
at certain points. This liquid being also in constant move-
ment forms another agency by which products are carried
from part to part, and the welfare or ill-fare of one member
enabled to influence all.

CHAPTER IV.

THE INTERNAL MEDIUM.

The External Medium. During the whole of life inter-
changes of material go on between every living being and
the external world; b}^ these exchanges material particles
that one time constitute 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 cell,
are passed out from the Body in the form of lifeless com-
pounds. As the foods and wastes of various living things
differ more or less, so are more or less different environ-
ments suited for their existence; and there is accordingly
a relationship 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 amceba live only in water or mud containing in solu-
tion certain gases and, in suspension, solid food particles;
and they soon die if the water be changed eitlier by essen-
tially 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 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 liipiid containing only the
solid organic particles on which the amoeba lives; and
the amoeba would die in such solutions as those in which
veast thrives best.

40 TEE HUMAN BODY.

The Internal Medium. The same close relationship
between the living being and its environment, and the
same cyclical interchange between the two which we find
in the amoeba and the yeast-cell, occur 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 so from
immediate contact with the environment, there arises a new
need — a necessity for an internal medium or jjhiftma 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 tlie exterior,
while losing other substances to the exterior at other sur-
faces, thus forms a sort of middleman between the in-
dividual 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 does 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, and 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.

The Blood. In the human Body the internal medium is
primarily furnished by the Mood, 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 in-
tact, no blood will flow from the wound. Hairs and nails
also contain no blood. In the interior of the Body the
epithelial cells lining free surfaces, such as the inside of
the alimentary canal, contain no blood, nor do the hard
parrs of the teeth, the cartilages, and the refracting media

THE INTERNAL MEDIUM. 41

of the eye (see Chap. XXXI.), but these interior parts are
moistened with liquid of some kind, and unlike the epi-
dermis are protected from rapid evaporation. All these
bloodless parts together form a group of non-vascular tis-
sues; they alone excej^ted, wounding any part of the Body
will be followed by bleeding.

In many of the lower animals there is no need that the
liquid representing their blood should l)e renewed very
rapidly in different 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 moyements of the body of the
animal. But in higher animals, especially those with an
elevated temperature, such an arrangement, or rather ab-
sence of arrangement, as this would not suffice. In them
the constituent cells live very fast, making much waste and
using much food, and so alter the blood in their neigh-
borhood very raj^idly. Besides, we have seen that in com-
plex animals certain cells are set apart to get food for the
whole organism, and certain others to finally 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 this is ac-
complished 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 system 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 Lymph. 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

A.

Jb_ic_

42 THE HUMAN BODY.

coats, and through the walls of these capillaries 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.

Dialysis. When two liquids containing different mat-
ters in solution are separated from one another by a moist
animal membrane, an interchange of material will take
place under certain conditions. If A be a
vessel (Fig. 9) completely divided vertically
by such a membrane, and a solution of com-
mon salt in water be placed on the side h,
and a solution of sugar in water on the side
c, it will be found after a time that some
gram of a diaiy- Salt has got into c and some sugar into J, al-
contammg\wo though there are no visible j^ores in the parti-
c,*separated'^by ^ioi^' Sucli an interchange is said to be due
membrane!'™^^ ^^ £?mZys*s or osmosis, and if the process were
allowed to go on for some hours the same
})roportions of salt and sugar would be found in the solu-
tions on each side of the dividing membrane.

The Renewal of the Lymph. Osmotic processes play a
great part in the nutritive i)rocesses of the Body. The
lymph jjresent in any organ gives up things to the cells there
and gets things from them; and so, although it may have
originally been tolerably like the liquid part of the blood, it
soon acquires a different chemical composition. Diffusion
or dialysis then commences between tlie 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 lo^t
and takes from it the waste products it has received from
the tissues. When this blood thus altered by exchanges
with the lymi^h gets again to the neighborhood of the re-
ceptive cells, having lost some food materials it is poorer
in these than the richly sujjplied 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 j^oor in such matters,.

LYMPHATICS. 43

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 lym])h
must vary considerably in composition in various organs of
the Body, and the blood flowing through them will get or
lose different things in different places. But renewing
during its circuit in one what it loses in another, its aver-
age composition is kept pretty constant, and, through in-
terchange with it, the average composition of the lymph
also.

The Lymphatic Vessels. The blood, on the whole,
loses more li(juid 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 ves-
sels is greater than that on the lymph outside, and so a
certain amount of filtration of liquid from Avithin 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 hjmpliatic vessels Avhich carry it
directly back into some of the larger blood-vessels near
where these empty into the heart; and by this flow of lymph,
under pressure from behind, it is renewed in various or-
gans, fresh liquid filtering through the capillaries to take
its place as fast as the old is carried off.

The Lacteals. In the walls of the alimentary canal cer-
tain food materials after passing through the receptive cells
into the lympli are not transferred locally, like the rest, by
dialysis into the blood, but are carrried off' bodilv 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 li(piid during di-
gestion, are called the milky or lacteal vessels.

Summary. To sum up: the blood and lym})]i form the
internal medium in which the tissues of the Body live; the
lymph is primarily derived from the blood and forms the
immediate jdasma for the great majority of the living cells
of the Body; and the excess of it is finally returned to the
blood. The lympli moves but slowly, but is constantly
renovated by the blood, which is kept in rai)id movement,

44 THE nUMAN BODY.

and which, besides contiiining 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 Avhich the wastes of the whole liody are pri-
marily collected.

Microscopic Characters of Blood. If a finger be
pricked, and the drop of blood flowing out be received on
a glass slide, covered, protected from evaporation, and ex-
amined with a microscope magnifying about 400 diameters,
it Avill be seen to consist of innumerable solid bodies float-
ing in a liquid. The solid bodies are the hlood corpuscles,
and the liquid is the Uood pilasma or liquor sanguinis.

The corpuscles are not all alike. While currents still
exist in the freshly spread drop of blood, the great majority
of them are readily carried to and fro; but a certain num-
ber more commonly stick to the glass and remain in one
place. The former are the red, the latter the pale or color-
less hlood corpuscles.

Red Corpuscles. Form and Size. The red corpuscles as
they float about frequently seem to vary in form, but by a
little attention 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 i)rofile.

Sometimes the corpuscle (Fig. 10, 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 cori)uscle is a circu-
lar disk, slightly hollowed in the middle (or biconcave) and
about four times as wide as it is thick. The average trans-
verse diameter is 0.008 millimeter (-g^o-jp inch). — Color.
Seen singly each red corpuscle is of a pale yellow color; it
is only when collected in masses that they appear red.
The blood owes its red color to the great numbers of these
bodies in it; if it be spread out in a very thin layer it, too,
is yellow. The layer must, however, be very thin or the
drop will still look red on account of the immense number

BLOOD.

45

of these corpuscles present; in a cubic millimeter (o^ inch)
of blood there are about five millions of them. — Structure.
Seen from the front the central part of each red corpuscle
in a certain focus of the microscope appears dimmer or
darker than the rest (Fig. 10, B), except a narrow band
near the outer rim. If the lens of the microscope be raised,
however, this previously dimmer central part becomes
brighter, and the previously brighter part obscure {E).

Fig. 10.— Blood corpuscles. A, magnified about 400 diameters. The red cor-
puscles have arranged themselves in rouleaux ; a, a, colorless corpuscles ; B,
red corpuscles more magnified and seen in focus ; E, a red corpuscle slightly
out of focus. At the right-hand top coi'ner is a red corpuscle seen in three-
quarter face, and at C one seen edgewise. F, O, H, I, white corpuscles highly
magnified.

This difference in appearance does not indicate the presence
of a central part or nucleus different from the rest, but is
an optical phenomenon due to the shape of the corpuscle,
in consequence of which it acts like a little biconcave lens
(see Physics). Eays of light passing through near the
centre of the corpuscle are refracted differently from those
passing through elsewhere; and when the microscope is
so focused that the latter reach the eye, the former do not^

46 THE HUMAN BODY.

and vice versa; iliiis when the central parts look bright,
those around them look obscure, and the contrary.

There is no .satisfactory evidence that these corpuscles
have any enveloping sac or cell-wall. All the methods
used to bring one into view under the microscope are such
as Avould coagulate the outer layers of the substance com-
posing the corpuscle and so make an artificial envelope.
ISo far as optical analysis goes, then, each corpuscle is ho-
mogeneous 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 bo then examined with the
microscope, it will be found that the red corpuscles are col-
orless and the plasma colored. The dilution has caujjed
tlie coloring matter to pass out of the corpuscles and dis-
solve in the liquid. This coloring constituent of the cor-
puscle is licemoglohin, and the colorless residue which it
leaves behind and which swells up into a sphere in the di-
luted plasma is the stroma. In the living corpuscle the
two are intimately mingled throughout 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, lahy. The difference may
be very well seen b}' comparing a thin layer of fresh blood
diluted with ten times its volume of tcn-per-cent salt so-
lution with a similar Liver of blood diluted with ten vol-
umes of Avater. 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. — 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 corpuscle immediately reassumes its
pro])er form when the external force is removed. The cor-
puscles are, then, highly elastic; they fretpiently can be seen
much dragged out of shape inside the vessels when the
circulation of the blood is watched in a living animal
(Chap. XA'.), but immediately springing back to their nor-
mal form when they get a chance.

Blood-Crystals. Hcemoglobin is, as above shown, readily

BL00D-CRTSTAL8. 47

soluble in water. In this it soon decomposes if kept in a
warm room, breaking up into a proteid substance called
globulin and a red-colored body, luematin. By keeping
the hemoglobin solution very cold, however, this decompo-
sition can be greatly retarded, and at the same time the
solubility of the haemoglobin in the water much diminished.
In dilute alcohol haemoglobin is still less soluble, and so if
its ice-cold watery solution have one fourth of its volume
of cold alcohol added to it and the mixture be put in a re-
frigerator for twenty-four hours, a part of the hc^moglobin
will often crystallize out and sink to the bottom of the
vessel, where it can be collected for examination. Tlie
haemoglobin of the rat is
less soluble than that of
man, and therefore crys-
tallizes out especially
easily; but these haemo-
globin crvstals, or. as
they are often called,
blood-crystals, can be
obtained from human

blood. In 100 parts of ^^^ „_b1ouc1 -crystals, or hemoglobin

dry human red blood- crystals.

corpuscles there are 90 of haemoglobin. The ha?moglobin
is the essential constituent of the red blood corpuscles,
enabling them to pick up large quantities of oxygen in
the lungs and carry it to all parts of the Body, (See Ees-
piration, )

Haemoglobin contains a considerable quantity of iron,
much more than auv other proximate constituent of the
Body.

The Colorless Blood Corpuscles (Fig, 10, F, H, G).
The colorless, pale, or wJiife corpuscles of the blood are far
less numerous than the red; in health there is on the ave-
rage about one white to three hundred red, but the pro-
portion 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

48

THE HUMAN BODY.

corpuscle, but dilute acetic acid dissolves most of them
and brings the nucleus into view. Tliese ])ale 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 an unicellular animal as an Amaba. Like the
latter, they have the power of sloAvly changing their form
spontaneously, and so have not the definiteness of outline
which belongs to the red corpuscles. At one moment
(Fig. 12) a pale corpuscle will be seen
as a spheroidal mass; a few seconds
later processes will be seen radiating
from this, and soon after these pro-
cesses may be retracted and others
thrust out; and so tlie corpuscle goes
on changing its shape. These slow
amcehoid moi^ements are greatly pro-
FiG. 12.-A white blood nioted by keeping the specimen of

cori^uscle sketched at sue- j i o i

cessive intervals of a few ijlood at the temperature of the Body

seconds to illustrate the ^. . -n t

changes of form due to its wllllc Under examination. By thrust-
am ceboid movements. . , ■ t , i

mg 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 corpuscle can slowly
change its place and creep across the field of the micro-
scope. Inside the blood-vessels these corpuscles execute
quite similar movements; and they sometimes bore right
through the capillary walls and, getting out into the lymph
spaces, creep about among the other tissues. This emigra-
tion is especially frequent in inflamed parts, and the ptis
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 size of the white corpuscles is not
so constant as that of the red; on the whole, however, they
are larger, their average diameter being about 0.0127 milli-
meter (-gifiro i"ch). The general properties of those cor-
puscles have already been described in Chap. II.

Blood of Other Animals. In all animals with blood the
pale corpuscles are pretty much alike, but the red corpus-
cles, which with rare exceptions are found only in Verte-

LYMPH. 49

brates, vary considerably. In all the class of the mammalia
they are circular biconcave disks Avith the exception of the
camel tribe, in which they are oval. They vary in diam-
eter from .002 mm. (musk deer) to .011 mm. (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 corpus-
cles 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 0.02 ram. { -^-^^ inch ) long and .007 mm. broad.

Histology of Lymph. Pure lymph is a colorless watery-
looking liquid; examined with a microscope it is seen to
contain numerous pale corpuscles exactly like those of the
blood, and no doubt largely consisting of pale blood cor-
puscles which have emigrated. It contains none of the
red corpuscles. The lymph flowing from the intestines
durmg digestion is, as already mentioned, not colorless
but white and milky. It is known as chyle and will be
considered with the process of digestion. During fasting
the lymph from the intestines is colorless like that from
other parts of the Body.

CHAPTER V.

THE CLOTTING OF BLOOD.

The Coagulation of the Blood. When blood is first
drawn 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 be-
comes viscid and sticky, and the viscidity becomes more
and more marked until, after the lapse of five or six min-
utes, the Avhole mass sets into a jelly Avhich adheres to
the vessel containing it so that this may be inverted without
any blood whatever being spilled. This stage is known
as that of gehdinization 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 hlood
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. Ulti-
mately we get a solid clot, colored red, and smaller in size
than the vessel in which the blood coagulated but retain-
ing its form, floating in a quantity of pale yellow serum.
If, however, the blood be not allowed to coagulate in per-
fect rest, a certain number of red corpuscles Avill be rubbed
out of the clot into the serum and the latter will be more
or less reddish. Tlie 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 foiind to have shrunk to a smaller
size and to have pressed out more serum; and this goes on
as long as it is kept, until putrefactive changes commence.

CAUSES OF COAGULATION. 51

Cause of Coagulation. If a drop of fresh-drawn blood
be spread out and watched with a microscope magnifying
600 or 700 diameters, it will be seen that the coagulation is
due to the separation of very fine solid threads which run
in every dii-ection through the plasma and form a close
network entangling all the corpuscles. These threads are
composed of a proteid substance known as fibrin. When
they tirst 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 cavities. After the fibrin
tlireads 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 away, and thus the first sign of
the contraction of the fibrin is seen in the cupping of the
surface of the gelatinized lilood 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, j^yressing 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 certain 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 vig-
orously with a bunch of twigs, and the sticky fibrin threads
as they form adhere to these. If tlie 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 l)y washing in water these

52 THE HUMAN BODY.

may be removed, and the piu-e librin thus obtained is per-
fectly white and in the form uf highly elastic tlireads. It
is insoluble in water and in dilute acids, but swells up to a
transparent jelly in the latter. The ''whipped" or "defi-
brinated blood" from which the fibrin has been in this way
removed, looks just like ordinary blood, but has lost its
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 ob-
servation of the process of coagulation; and from the fact
that perfectly formed fibrin can be obtained free from cor-
puscles 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 the
clotting takes place more slowly or the corpuscles sink
more rapidly than usual, a colorless top stratum of plasma,
with no red corpuscles in it, will be left before gelatiniza-
tion occurs and stops the farther sinking of the corpuscles.
The uppermost part of the clot formed under these cir-
cumstances is colorless or pale yellow, and is known as the
huffy coat; it is especially apt to be formed in the blood
drawn from febrile j^atients, and was therefore a point to
which physicians paid much attention in the olden times
when bloodletting was thought a panacea for all ills. In
horse's blood the difference between the specific gravity of
the corpuscles and that of the plasma is greater than in hu-
man blood, and horse's blood also coagulates more slowly,
so that its clot has nearly always a buffy coat. The color-
less buiiy 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 becomes bright scarlet, while the part immersed in the
serum assumes a dark purple-red color. The brightness of
the top layer is due to the action of the oxygen of the air.

USES OF COAGULATION. 53

which forms a bright reel compound with the coloring mat-
ter of the red corpuscles. If the clot be turned upside down
and left for a short time, the previously dark bottom layer,
now exposed to the air, will become bright; and the previ-
ously bright top layer, now immersed in the serum, will
become dark.

Uses of Coagulation. The clotting of the blood is so
important a process that its cause has been frequently in-
vestigated; but as yet it is not perfectly 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 if 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, Avhich would otherwise go on indefinitely,
is stopped. So, too, when a surgeon ties up an artery be-
fore dividing it, and the tight ligature crushes or tears its
delicate inner surface, the blood clots where this 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
gi-eatly diminishes the risk of bleeding.

The Fibrin Factors. As regards the formation of
fibrin the folloAving points seem to be made out with toler-
able certainty. Fresh-drawn blood contains or develops
two substances, fihrinoplastin and fihrinogen, which by
their interaction form fibrin, under the influence of a third
body called the fihrin ferment; moreover, fibrin is only
formed if a certain proportion of neutral mineral salts,
such as are found dissolved in the blood plasma, is present.

Blood serum does not clot of itself at ordinary tempera-
tures: it contains fihrinoplastin and fibrin ferment and
the requisite quantity of salts, but not the fibrinogen; that
which originally existed in the plasma having apparently
been used up with the proper proportion of fihrinoplastin
to form fibrin, leaving over an excess of fibriaoplastin in
solution in the serum.

On the other hand, the liquids found in the cavities of

54 THE HUMAN BODY.

the Body which are lined by serous membraneSj commonly
contain iibrinogen and the salts but no fibrinoplastin, and
therefore they do not coagulate spontaneously. But if a
little blood serum be added to one of these liquids, coagula-
tion takes place.

Artificial Clot. If serum l)e slightly diluted with water
and kept ice-cold while a stream of carbon dioxide gas is
passed through it for some hours, a white precipitate is
thrown down which contains fibrinoplastin and the fibrin
ferment. This precipitate after washing may be dissolved
in cold water containing the merest trace of caustic potash.
If the liquid moistening a serous cavity be treated in a
similar way a precipitate is formed, containing fibrinogen
instead of the fibrinoplastin, and but little of the ferment.
If this precipitate be washed and dissolved and the solution
be added to the solution of the blood-serum precipitate,
no clot is formed; but if about one per cent of sodic car-
bonate or other neutral salt be added to the mixture, then
it clots. This shows the necessity of the salts, which is
perhaps better proved in another way. If serum be put in
a dialyzer (see Physics) with distilled water on the other
side of the membrane, all the salts will gradually pass out
from the serum into the water: as the last portions of
them pass out, the fibrinoplastin and ferment, which are
"colloids''' (that is, bodies which will not dialyze), are pre-
cipitated; they may be redissolved by the addition of a
trace of caustic potash. Similarly the salts may be re-
moved from the liqiiid obtained from a serous cavity, and
the precipitated fibrinogen redissolved. If these solutions
be now mixed no clot is formed; but if the salts which have
been dialyzed ottt, or an equivalent portion of other neu-
tral salts, be added to the mixture, it will clot.

The Fibrin Ferment. Tlie activity of the ferment i.-
proved as follows: If scrum be diluted with a large bulk of
water and then caribou dioxide gas be passed through it,
fibrinoplastin will be precipitated, with little or none of
the ferment. If this fibrinoplastin be dissolved and added
to the liquid from a serous cavity it will not cause it to
clot, or only very slowly, according as no fibrin ferment or

FIBRIN FERMENT. 56

but a little is present. But if some of the ferment be
uclcled, then the mixture co-igulutes nipidl}-. The ferment
may be obtained b}^ adding a large quantity of strong al-
cohol to some fresh blood serum. The alcohol precipi-
tates albumen, fibrinoplastin, and the ferment. The pre-
cipitate is let stay under alcohol for some months, during
which time the albumen and fibrinojilastin are altered so
as to become insoluble in water. The alcohol is then de-
canted off and the residue treated with water which dis-
solves the ferment. This solution added to the above
mixture containing fibrinoplastin, fibi'inogeu, and salts,
will make it clot.

Of these four bodies which play a part in the coagula-
tion of the blood, the fibrinoplastin and fibrinogen jiri-
marily determine the quantity of fibrin formed. The fer-
ment seems to act on them in some way so as to make them
interact, but it does not enter into the fibrin; it is not used
up in the process, and the quantity of fibrin formed is thus
independent of the quantity of the ferment joresent; but
the more of it there is, the more quickly does the coagula-
tion occur. The part the salts play is obscure: probably
part of them are necessary constituents of the fibrin, since
it leaves a large proportion of ash when burnt. But they
seem to act in some other way when present in certain
proportions, since too large a percentage of them stops
coagulation as completely as their total absence. If fresh
blood be mixed with an equal bulk of a saturated solution
of magnesium sulphate (Epsom salts) or of common salt,
it will not clot; but if this mixture be largely diluted with
water, then clotting Avill take place.

Exciting Causes of Coagulation. The above facts show
clearly enough that the coagulation of the blood is a
physico-chemical process, but still leave unexplained why
it does not occur in circulating blood inside healthy blood-
vessels. It is, in fact, much easier to point out what arc
not the proximate reasons of the coagulation of drawn
blood than what are.

Blood when removed from the Body and received in a
vessel comes to rest, cools, and is exposed to the air. from

56 THE HUMAN BODY.

which it may receive or to which it may give off gaseous
bodies. But it is easy to prove that none of these three
things is the cause of coaguhition. Stirring the drawn
blood and so keeping it in movement does not prevent but
hastens its coaguhition; and blood carefully imprisoned in
a living blood-vessel, and so kept at rest, will not clot for a
long time: not until the inner coat of the vessel begins to
change from the "want of fresh blood. Secondly, keeping the
blood at the temperature of the Body hastens coagulation,
and cooling retards it; blood received into an ice-cold vessel
and kept surrounded with ice will clot more slowly than
blood drawn and left exposed to ordinary temperatures.
Finally, if the blood be collected over mercury from a
blood-vessel, without having been exposed to the air even
for an instant, it will still clot perfectly well.

The formation of fibrin is then due to changes taking
place in the blood itself when it is removed from the
blood-vessels; clotting depends upon some rearrangement
of the blood constituents. There is a good deal of reason
to believe that what occurs is a breaking up of a number
of the colorless corpuscles; that these then form fibrino-
plastin and fibrin ferment, and, the fibrinogen and salts
already existing in solution in the blood plasma, fibrin is
formed. \Yhen fluids which contain no red corpuscles
clot, as for instance vaccine lymph, the first threads of
fibrin developed can be seen under the microscope to
radiate from the pale corpuscles present.

Relation of the Blood- Vessels to Coagulation. As to the
role of the vessels with respect to coagulation when the
blood is flowing in them two views are held, between which
the facts at present known do not permit a decisive judg-
ment to be made. One theory is that the vessels actively
prevent coagulation by constantly absorbing from the blood
some substance, as for example the fibrin ferment, which
may be supposed constantly to develop, and the presence
of which is a necessary condition for the formation of
fibrin. The other view is that the blood-vessels are passive
and completely neutral. They simply do not excite those
changes in the blood constituents which give rise to the

COMPOSITION OF THE BLOOD. 57

formation of fibrinoplastin or the ferment, while foreign
bodies in contact with the blood do excite these changes
and so cause coagulation.

Whatever the part which the blood-yessels play, it is only
exhibited when their inner surfaces are healthy and unin-
jured. If this lining be ruptured or diseased the blood
clots. Accordingly, after death, Avhen post-mortem changes
have affected the blood-vessels, the blood clots in them;
but often very slowly, since the vessels only gradually alter.
If the Body be left in one position after death, the clots
furmedinthe heart have often a marked buffy coat, because
the corpuscles have had a long time to sink in the plasma
before coagulation occurred. In medico-legal cases it is
thus sometimes possible to say what was the position of a
corpse for some hours after death, although it has been
subsequently moved. The lymph clots like the blood, but
not so firmly; since it contains no red corpuscles, the clot
formed is of course colorless.

Composition of the Blood. The average specific gravity
of human blood is 1055. It has an alkaline reaction,
which becomes less marked as coagulation occurs. About
one half of its mass consists of moist corpuscles and the
remainder of plasma. Exposed in a vacuum, 100 volumes
of blood yield about 60 of gas consisting of a mixture of
oxygen, carbon dioxide, and nitrogen.

Chemistry of the Serum. The blood plasma cannot well
be examined as to its elieniical constituents, since it clots
under manipulation. The scrum is, however, essentially
blood plasma minus fibrin, and from an analysis of it we
can draw conclusions as to the plasma. In 100 parts of
serum there are about 90 parts of water, 8.5 of proteids, and
1.5 of fats, salts, and other less-known solid bodies. Of
the proteids present the most abtindant is serum albumin,
which agrees with egg albtimin in coagulating when heated:
so that serum when boiled sets into an opaque white mass,
just as the white of an Qgg does. Chemically, serum albu-
min differs from egg albumin in being coagulated by ether;
and physiologically, in the fact that although present in
such large quantities in the blood, it does not pass through

68 THE HUMAN BODY.

tlie kidneys, whereas t-^^g albiiiniu when injected into the
blood-vessels of an animal is rapidly excreted by those
organs. In health the fats are only present in the serum
in small (piantity except after a meal at which fatty sub-
stances have been eaten ; serum obtained from the blood of
an animal soon after such a meal is often milky in appear-
ance from the fats present, instead of being perfectly color-
less or i)alc yellow, and transparent as it is after fasting.
The salts dissolved in the serum are mainly sodium chloride
and carbonate; but small quantities of sodium, calcium,
and magnesium ])h<)sp]i;itos arc also ]iresent.

Chemistry of the Red Corpuscles. In these in the fresh
moist state there are in 100 parts, b% of water and 44 of
solids. Of the solids about one per cent is salts, chiefly
potassium phosphate and chloride. The remaining organic
solids contain, in IdO parts, 90 of haemoglobin and about 8
of other proteids; the residue consists of less well-known
bodies.

Chemistry of the White Corpuscles. These yield be-
sides much water, several proteids, some fats, glycogen
(see Chap. XXVIIL), and salts; and smaller quantities of
other bodies. The predominant salts, like those of the red
corpuscles, are j^otassium phosphates.

Variations in the Composition of the Blood. Hygienic
Jh'Duirl-s. The above statements refer only to the average
comjoosition of the healthy blood, and to its better known
constituents. From what was said in the last chapter it is
clear that the blood flowing from any organ will have lost
or gained, or gained some things and lost others, when
compared with the blood which entered it. But the losses
and gains in particular parts of the Body are in such small
amoimt as, with the exception of the blood gases, to elude
analysis fi)r the most part: and the blood from all parts
being mixed up in the heart, they balance one another and
produce a tolerably constant average. In health, however,
the specific gravity of the blood may vary from 1045 to
1075; the red eorjniscles also are present in greater propor-
tion to the plasma after a meal than before it. Ilealthy
sleep in projjer amount also increases the proportion of red

BLOOD CORPUSCLES. 59

corpuscles, and want of it diminishes tlieir number as may
be recognized in the pallid aspect of a person ^vho has lost
several nights' rest. Fresh air and plenty of it has the
same effect.

The proportion of these corpuscles has a great import-
ance since, as we shall subsequently see, they serve to carry
oxygen, which is necessary for the performance of its func-
tions, all over the Body. Aiicemia is a diseased condition
characterized by pallor due to deficiency of red blood cor-
puscles, and accompanied by languor and listlessness. It is
not unfrequent in young girls on the verge of womanhood,
and in persons overworked and confined within doors. In
such cases the best remedies are oj^en-air exercise and good
food.

Summary. Practically the composition of the blood
may be thus stated: It consists of (1) plasma, consisting
mainly of water containing in solution serum albumin,
sodium salts, smaller amounts of those of other metals, and
extractives of which the most constant are urea, kreatin,
and grape sugar; (2) red corpuscles, containing rather more
than half their weight of water, the remainder being main-
ly haemoglobin, other proteids, and potash salts; (3) white
corpuscles, consisting of water, various proteids, glycogen,
and potash salts; (4) gases, partly dissolved in the plasma
or combined with its sodium salts, and (oxygen) partly
combined with the haemoglobin of the red corpuscles.

Quantity of Blood. The total amount of blood in tlie
Body is difficult of accurate determination. It is, how-
ever, about ^ of the whole weight of the Body, so tlie
(|uantity in a man weighing 75 kilos (165 lbs.) is about 5.8
kilos (12.7 lbs.). Of this at any given moment about one
fourth would be found in the heart and big blood-vessels;
and equal quantities in the capillaries 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.

The Origin and Fate of the Blood Corpuscles. Tlie
white blood corpuscles vary so rapidly and frequently in
number in the blood that they must be constantly in pro-

60 TEE HUMAN BOD T.

cess of alteration or removal, and formation; their number
is largely increased by taking food, even more than that of
the red, so that their i)roportion to tlic red rises, from 1
io 1000 during fasting, to 1 to 250 or 300 after a meal.
They no doubt multiply to a certain extent by division
while circulating in the blood, but the majority come from
liie lymi)hatic glands and similar structures (see Chap.
XXII.) found in many parts of the Body, which con-
tain many cells like pale blood corpuscles, and often in
l)rocess of division. From tliese organs the corpuscles en-
ter the lymph-vessels and are carried on into the blood.
From the capillary blood-vessels many again migrate, and
it is probable that these emigi'ahts take part frequently in
the repair or regeneration of injured tissues. Being un-
differentiated and specialized to no line of work they are
ready to take up any that comes to hand, and may be com-
pared to the young men in a community who have not yet
selected an occupation and are on the lookout for an open-
ing. On the other hand there seems little doubt that a
great many Avhite corpuscles give rise to red ones, and this
is perhaps to be regarded as their special function. The
corpuscles of nearly all invertebrate animals are colorless
only, although the blood plasma of some contains haemo-
globin in solution. Amphioxus, the lowest undoubted
vertebrate animal (see Zoology), also possesses only colorless
corpuscles in its blood. But higher and more complex ani-
mals need more oxygen, and as blood plasma dissolves
very little of that gas, they develop in addition the hgemo-
globin-containing corpuscles which pick it up in the gills
or lungs and carry it to all parts of the Body, leaving it
where wanted (see Chap. XXV. ). In cold-blooded vertebrates
the red corpuscles are not nearly so many in proportion as
in the warm-blooded, Avhich use far more oxygen. The
older view was that the mammalian red corpuscle repre-
sented the nucleus of one of the white, in which hemoglo-
bin had l>cen formed and from about which the rest of the
corpuscle had disappeared. This, however, does not seem
to be the case; but the pale corpuscle develops or forms
I haemoglobin in its cell protoplasm, and flattens and as-

LYMPH. 61

sumes the form of a red corpuscle, while its nucleus disap-
pears. Occasional transitional forms between the pale and
the red corpuscle are seen in blood when examined with
the microscope; and if blood be put fresh on a cold slide
and examined in a cold room these transitional forms are
more numerous, since at ordinary temperatures they very
rapidly break down and fall to pieces when blood is drawn.

How long an individual red corpuscle lasts is not known,
nor with certainty how or when it disappears. There is,
however, some reason to believe that a great many are de-
stroyed in the spleen (see Chap. XXII.).

Chemistry of Lymph. Lymph is a colorless fluid when
pure, feebly alkaline, and with a specific gravity of about
1045. It maybe described as blood minus its red corpuscles
and considera])ly diluted, but of course in various parts of
the Body it will contain minute quantities of substances
derived from neighboring tissues. It contains a considera-
ble quantity of carbon dioxide gas which it gives up in a
vacuum, but no oxygen, since any of that gas which passes
into it by diffusion from the blood is immediately picked
up by the living tissues among which it flows.

CHAPTER V I.

THE SKELETON.

Exoskeleton and Endoskeleton. The skeleton of an
aniiiKil includes all its liard protecting or supporting parts,
and is met with in two main forms in the animal kingdom.
First as an exoskeleton develo23ed in connection with either
the superficial or deeper layer of the skin, and represented
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 al-
though the latter lie within the mouth, the study of devel-
opment 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 structures 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, Avhile the cartilages finish it off at vari-
ous parts. This framework is what is commonly meant by
the skeleton, and it primarily supports 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 skeleton thus formed, however, is completed
and supplemented by another made of the connective tissue,
which not only, in the shape of tough bands or ligaments,
ties the bones and cartilages together, but also in various
forms pervades the whole Body as a sort of subsidiary
skeleton running through all the soft organs, forming net-
works of fibres around their other constituents; so that it
nuikes, as it were, a microscopic skeleton for the individu.tl
modified cells of which the Body is so largely composed.

AXIAL SKELETON. 63

and also forms partitions between tlie muscles, cases for
sncli 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 mcshwork tlie
skeleton of the tissues. Besides forming a support in the
substance of various organs, the connective tissue is also
often laid down as a sort of packing material in the crevices
between them; and so widely is it distributed everywhere
from the skin outside to the lining of the alimentary canal
inside, that if some solvent could be employed which would
corrode away all the rest and leave only this tissue, a very
perfect model of the whole Body would be left; something
like a " skeleton leaf," but far more minute in its tracery.

The Bony Skeleton (Fig. 13). If the hard framework
of the Body were joined together like the joints and beams of
a house, tlie whole mass would be rigid; its parts could not
move with relation to one another, and we would be un-
able 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 are 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 sheleton, consisting of the
bones in the limbs and in the arches (w and s, Fig. 13) by
which these are carried and attached to the trunk.

Axial Skeleton. The axial skeleton consists primarily
of the vertettral column or spine, a side view of which is
represented in Fig. 14. The up^ier part of this column is
composed of twenty-four se])arate bones, each of which is a
vertebra. At the posterior part of the trunk, beneath the

64

THE HUMAN BODY.

'%^

Fig. 13.- The bony and cartilaginous
skeleton.

Fig. 14.— Side view of the
spinal column.

VERTEBRA. 65

movable vertebrae, comes the sacrum (;S' 1), made up of
five vertebrae, which in the adult grow together to form
one bone, and below the sacrum is the coccyx {Co 1-4),
consisting of four very small tail vertebrae, Avhich in ad-
vanced life also unite to form one bone.

On the top of the vertebral column is borne the skull,
made up of two parts, viz., a great box above which in-
closes the brain and is called the cranium, and a large
number of bones on the ventral side of this which form the
skeleton of the face. Attached by ligaments to the under
side of the cranium is the liyoid bone, to which the root of
the tongue is fixed.

Of the twenty -four separate vertebrae of the adult the
seven nearest the skull (Fig. 14, C 1-7) lie in the neck
and are known as the cervical vertebrce. These are fol-
lowed by twelve others which have ribs attached to them
(see Fig. 13) and lie at the back of the chest; they are the
dorsal vertebrce (2)1-12). The ribs (Fig. 35*) are slender
curved bones attached by their dorsal ends, called their
heads, to the dorsal vertebra? and running thence round the
sides of the chest. In the ventral median line of the lat-
ter is the breast-bone or siernum {d, Fig 13), Each rib
near its sternal end ceases to be bony and is composed of
cartilage.

These parts — skull, hyoid bone, vertebral column, ribs,
and sternum — constitute the axial skeleton, and Ave have
now to consider its parts more in detail.

The Dorsal Vertebrae. If a single vertebra, say the
eleventh from the skull, be examined carefully it will be
found to consist of the following parts (Figs. 15 and 16):

First a bony mass, C, rounded on the sides and flattened
on each end where it is turned towards the vertebrjB above
aud below it. This stout bony cylinder is the "body" or
centrum of the vertebra, and the series of vertebral bodies
(Fig. 14) forms in the trunk that bony partition betAveen
the dorsal and ventral cavities of the body spoken of in
Chapter I. To the dorsal side of the body is attached an
tirch — the neural arcli. A, which Avith the back of the body
incloses a space. Fv, the neural ring. In the tube formed

*p773.

66

THE nUMAN BODY.

by the rings of the successive vertebras lies the spinal cord.
Projecting from the dorsal side of the neural arch is a long
bony bar, Fs, i\\Q sjnnous process: and the projections of these
processes from the various vertebras can be felt through the
skin all down the middle of the back. Hence the name of
spinal coliDiin often given to the whole back-bone.

Six other i)rocesses arise from the arcli of the vertebra: two
l)roject forwards, i.e. towards the head; these, Pas, are the
anterior articular processes and have a smooth surface,
covered with cartilage on their dorsal sides. A pair of sim-

^t Paa

Fig. 15. Fig. 16.

Fig. 13.— a dorsal vertebra seen from behind, i.e. the end turned from the
head.

Fig. 16.— Two dorsal vertebrsB viewed from the left side, and in their natural
relative positions. C, the body; A. neural arch; Fi\ the neural ring; Ps. spi-
nous process; Pas, anterior articular process ; Pai, posterior articular process;
Pi, transverse process; Ft, facet for articulation with the tubercle of a rib;
Pes, Fci, articular surfaces on the centrum for articulation with a rib.

ilav posterior a?'ticular processes, Pai, runs back from the
neural arch, and these have smooth surfaces on their ven-
tral aspects. In the natural position of the vertebra, the
smooth surfaces of its anterior' articular processes fit upon
the posterior articular processes of the vertebra next in
front, forming a joint, and the two processes are united
by ligaments. Similarly its posterior articular processes
form joints (Fig. 10) Avith the anterior articular processes
of the vertebra next behind.

SEGMENTATION OF SKELETON. 67

The remaining processes are the transverse, Pt, which
rnn outwards and a little dorsally. Eueli of these has a
smooth articular surface, Ft, near its outer end.

On the •"body" are seen two articular surfaces on each
side: one, Fes, at its anterior, the other, Fri, at its poste-
rior end, and both close to the attachment of the neural
arch. Each of these surfaces formed with corresponding
areas on the vertebrse in front and behind a pit into which
the end of a rib fitted and the rib attached in this way to
the anterior part of the " body" also fitted on, a little way
from its dorsal end, to the articular surface at the end of
the transverse process.

The Segments of the Axial Skeleton. If a dorsal verte-
bra, say the first (Fig. 17), be detached with the pair of ribs,
Cv, belonging to it and the
bit of the sternum, S, to
which these ribs are fixed
ventrally, we would find a
bony partition formed by
the body of the vertebra,
lying betweeij tv/o arches
which surround cavities.
The dorsal cavity inclosed
by the " body" and " neural ^ ,„ ^.

•^ ^ . . ,, Fig. 17.— Diagrammatic representation

arch contained originally of a segment of the axial skeleton. F, a

J, . , . , ? .„, vertebra; C, C'l', ribs articulating above

part Ot the SJJlUal cord, i lie with the body and transverse process of

,1 J i_ J.1 the vertebra; S, the breast-bone. The

other ring, made up by the lighter-shaded part between 5f and C is

body of the vertebra dor- '^"^ "'^ •^^'•"^^^"•
sally, the sternum ventrally, and the ribs on the sides, sur-
rounds the chest cavity with its contents. All of these
parts together form a typical segment of the axial skeleton,
which, however, only attains this completeness in the
thoracic region of the trunk. In the skull it is greatly
modified; and in the neck and the lower part of the trunk
the ribs are either absent or very small, appearing only as
processes of the vertebrae; and the sternal portion is wanting
altogether.

Nevertheless we may regard the whole axial skeleton as
made up of a series of such segments placed one in front of

68

THE HUMAN BODY.

the other, but having ditlerent portions of the complete
segment much niodiliod or rudiuieutarv, or even altogether
wanting in some regions. Parts which in this sort of way
reuUy correspond to one another though they differ in de-
tail, wliich are so to speak different varieties of one thing,
are said in anatomical language to he hoinolocjnus to one an-
other; and when they succeed one another in a row, as the
trunk segments do, the homology is spoken of as serial.

The Cervical Vertebrae. In the cervical region of tlie
vertebral column tlie bodies of the vertebra? are smaller
than in the dorsal, but the arches are larger; the spinous
processes are short and often bifid and the transverse pro-
cesses api)ear perforated by a canal, the vertebral foramen.

The bony bar bounding
this aperture on the ventral
side, however, is in reality
a very small rib which has
grown into continuity with
the body and true transverse
process of the vertebra, al-
though separate in very early
life: the transverse process
proper bounds the vertebral
foramen dorsally. In this latter during life runs an artery,
which ultimately enters the skull cavity.

The Atlas and Axis. The first and second cervical ver-
tebra differ considerably from the rest. The first, or atla>!
(Fig. 19), which carries the head, has a very small body.
Aa, and a large neural ring. This ring is subdivided by a
cord, or the transverse ligament, L, into a dorsal moiety in
which the spinal cord lies and 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. 20). Around titis peg the atlas rotates when the head
is turned from side to side, carrying the skull (which ar-
ticulates with the large hollow surfaces Fas) with it.

The odontoid process really re])resents a large })iece of
the body of the atlas which in early life separates from its
own vertebra and grows on the axis.

Fig. 18.— a cervical vertebra. Frt,
vertebrai foramen ; Pai, anterior artic-
ular process.

SACRUM.

69

The Lumbar Vertebrae (Fig. 21) are the largest of all
the movable vertebrae and have no ribs attached to them.
Their spines are short and stout and lie in a more horizontal

A a Fas

Fig. 19. Fio. 20.

Fig. 19.— The atlas. Fig. 20.— The axis. Aa, body of atlas; D, odontoid pro-
cess; Fas, facet on front of atlas with which the skull articulates; and in Fig. 20,
anterior articular surface of axis; £, transverse ligament; Frt, vertebral fora-
men.

plane than those ot the vertebrae in front. The articular
and transverse processes are also short and stout.

The Sacrum, which is represented along with the last
lumbar vertebra in Fig. 22, consists in the adult of a single
bone; but cross-ridges on its ventral surface indicate the

Fig. 21.— a lumbar vertebra seen from the left side. Ps, spinous process;
Pat. anterior articular process: Pai, posterior articular process.

limits of the five separate vertebrae of which it is composed
in childhood. It is somewhat triangular in form, its base
being directed upv^ards and articulating with the under

70

THE HUMAN BODY

surfiice of the body of lla- lit'th luinbai' vertebra. On its
sides are large surfaces to which the arrli benring the lower

Pas

Fsa

Fig. 22. — The last lumbar vertebra and the sacrum seen from the ventral side.
Fsa. anterior sacral foramina.

limbs is attached (see Fig. 13). Its yentral surface is con-
cave and smooth and presents four pairs of anterior sacral
foramina, Fsa, which communicate "with
the neural canal. Its dorsal surface, convex
and roughened, has fonr similar pairs of pos-
terior sacral foramina.

The coccyx (Fig, 23) calls for no special
remark. The four bones which grow togeth-
er, or anhylose, to form it represent only the
bodies of vertebrae, and even that imperfectly.
It is in reality a short tail, although not visi-
ble as such from the exterior.
The Spinal Column as a Whole. The vertebral column

Fig. 23.— The
coccyx.

SPIJ^^E AS A WHOLE. 71

is ill a man of average height about twenty-eight inches
long. Viewed from one side (Fig. 14) it presents four cur-
vatures ; one with the convexity forwards in the cervical
region is followed, in the dorsal, by a curve with its concav-
ity 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, and this is farther
secured by the presence of compressible elastic pads, the
intervertebral dii^ks, made up of cartilage and connective
tissue, which lie between the bodies of those vertebra?
which are not ankylosed together, and fill up completely
the empty spaces left between the bodies of the vertebriB in
Fig. 14. By means of these pads, moreover, a certain
amount of movement is allowed between each pair of ver-
tebra* ; and so the spinal column can be bent to consider-
able extent in any direction ; while the movement between
any two vertebrre is so limited that no sharp bend can take
place at any one point, such as might tear or injure other-
wise the spinal cord contained in the neural canal. The
amount of movement permitted is greatest in the cervical
region.

In the case of the movable vertebras, where the arch
Joins the body on each side, it is somewhat narrowed; this
narrowed stalk being known as the pedicle [li, Fig. 16),
while the broader remaining portion of the arch is its lam-
ina. Between the pedicles of two contiguous vertebrae
there are in this way left apertures, the intervertebral holes
which form a series on each side of the vertebral column,
and one of which, Fi, is shown between the two dorsal
vcrtebrtB in Fig. 16. Through these foramina nerves run
out from the spinal cord to various regions of the Body.
The sacral foramina, anterior and posterior, are the repre-
sentatives of these apertures, but modified in arrangement,
on account of the fusion of the arches and bodies of the
vertebrjB between which they lie.

Sternum. The sternum or hreast-hone (Fig. 24 and d,
Fig. 13) is wider from side to side than dorso-ventrally. It

72

THE HUMAN BODY.

Icl

Is

y/^'yr'jf^^

consists in the adult of three pieces, and seen from the ven
tral side has somewhat the form of a dagger. The piece
M nearest the head is called the handle or manuhrivyn,
and presents anteriorily a notch, Icl, on each side, with
which the collar-bone articulates {u, Fig. 13); on each side
are two other notches, Ic 1 and Ic 2, to wiiich the sternal
ends of the first and second ribs are attached. The middle
piece, (7, of the sternum is called the body; it completes
the notch for the second rib and has on
its sides others, Ic 3-7, for the third,
fourth, fifth, sixth, and seventh ribs.
The last piece of the sternum, P, is
called the ensiform or xiphoid process;
it is composed of cartilage, and has no
ribs attached to it.

The Ribs (Fig. 25). There are twelve
pairs of ribs, each being 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.
In the case of the anterior seven pairs,
the costal cartilages are attached direct-
ly to the sides of the breast-bone; the
next three cartilages are each attached
to the cartilage of the preceding rib,
Fig. 24.-The sternum while the Cartilages of the tenth and
Kanubrium'^^^^^^^ twelfth rlbs are quite unattached ven-
ijF^^^i^: trally, so these are called the free or
\ i-;j-.notches for the -floaUnq ribs. The convexity of each

nb cartilages. . . •

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 at-
tachment, so that its sternal end is considerably lower than
its dorsal.

The Skull (Fig. 26) consists of twenty-two bones in
the adult, of which eight, forming the cranium, are ar-
ranged so as to inclose the brain-case and protect the
auditory organ, while the remaining fourteen support

THE SKULL. 73

the face and surround the mouth, the nose, and the eye-
sockets.

Fig. 25.— The ribs of the left side, with the dorsal and two lumbar vertebrae,
the rib cartilages and the sternum.

Cranium. The cranium is a box with a thick floor and
thinner walls and roof. Its floor or lase represents in the
head (as is depicted diagrammatically in Fig. 2) that par-

74

TUE HUMAN BODY.

tition between the dorsal and ventral cavities which in the
trunk is made np of the bodies of the vertebrae. In very
early life it presents in the middle line a series of four
bones, the hasi-occipital, basi-sphenoid, presjjhenoid, and

Tsp

Fio. 26 — \ side MOAN of flu skull O, occipital bone; T, temporal; Pr, parie-
tal; F, fitjiital, <S, sphtsiiuid, X, iiia,la,r; Mx, maxilla; N, nasal; E, ethmoid; L,
lachrymal ; Md, inferior maxilla.

hasi-ethmoid, which answer pretty much to the bodies of
four vertebras, and have attached to them the thin bones
which inclose the skull cavity (which may be likened to
an enlarged neural canal) on the sides and top. In the

Cli AXIAL BOXES.

75

Human Body, however, these bones very soon ankylose
with others or with one another; although they remain
distinct throughout life in the skulls of very many lower
animals. On the base of the skull, besides many small
apertures by which nerves and blood-vessels pass in or out,
is a large aperture, i\\e foramen magnum, through which
the spinal cord passes in to join the brain.

The cranial bones are the following:

1. The occipital lone (Fig. 26,
0) unpaired and having in it the
foramen magnum. It is made
up by the fusion of the basi-oc-
cipital (Fig. 27) with other flatter
bones. 2. The frontal hone (Fig.
26, F), also unpaired in the adult,
but in the child each half is a sep-
arate bone. 3. A pair of thin plate-
like imrietal hones (Fig. 26, Pr)
which meet one another along the
middle line in the top of the
skull, and roof in a great part of
the cranial cavity. 4. A pair of
temimral hones (Fig. 26, T), one
on each side of the skull beneath
the parietal. On each temporal
bone is a large ajaerture leading
into the ear cavity, the essential
parts of tlie organs of hearing
))eing contained in these bones.
5. The splienoid hone, made up
hy the union of the hasi-sphe-

noicl Sindpre-sphe7wicl (lying on the base of skull in front
of the basi-occipital) with one another and with flatter
bones, is seen partly (Fig. 26, S) on the sides of the cranium
in front of the temporals. 6. The ethmoid, like the sphe-
noid, single in the adult, is really made up by the union of
a single median hasi-ethmoicl with a pair of lateral bones.
It closes the skull cavity in front, and lies between it and
the top of the nasal chambers, being perforated by many

Fig. 27.— The base of the skull.
The lower jaw has beenremoved.
At the lower part of the figure
is the hard palate forming the
roof of the mouth and surround-
ed by the upper set of teeth.
Above this are the paired open-
ings of the posterior nares, and
a short way above the middle of
the figure is the large median
foramen magnum,v,'ith the bony
convexities (or occipital con-
dyles) which articulate with the
atlas, on its sides.

76 THE HUMAN BODY.

yiiuill holes ihrougli whicli the nerves of smell jiass. A
little bit of it is seen on the inner side of the eye-socket at
E in Fig. 20.

Facial Skeleton. The majority of the face bones are in
pairs ; two only being single and median. One of these is
the lower jaw-bone or inferior maxilla (Fig. 26, Md); the
other is the vomer, which forms part of the partition
between the two nostrils.

The paired face-bones are: 1. The nutxiUa', or upper jaw-
bones {Mx, Fig. 26), one on each side, carrying the upper
row of teeth and forming a great part of the hard palate,
which separates the mouth from the nose. 2. The 2^ala-
tine bones, completing the skeleton of the hard palate, and
behind which the nose communicates by ihe posterior nares
(Fig. 27) with the throat cavity, so that air can pass in or
out in breathing. 3. The malar bones, or cheek-bones,
{Z, Fig. 20), lying beneath and on the outside of the orbit
on each side. 4. The nasal bones {N, Fig. 26), roofing in
the nose. 5. The lachrymal bones {L, Fig. 26), very small
and thin and lying between the nose and orbit. 6. The
inferior turbinate bodies lie inside the nose, one in each
nostril chamber.

The Hyoid. Besides the cranial and facial bones there
is, as already pointed out, one other, the hyoid (Fig. 28),
which really belongs to the skull, although it lies in the
neck. It can be felt in front of the throat, just above
"Adam's apple." The hyoid bone is U-shaped, with its
convexity turned ventrally, and consists
of a body and two pairs of processes
called co7'7iua. The smaller cornua (Fig.
28, 3) are attached to the skull, close to
T in Fig. 26, by long ligaments. These
^'~bodv^-'^*'*^ ligaments in many animals are represent-
cornua; 3, ed bv boucs, SO that the hyoid, witli them,

small cornua. -^ ' it

forms a bony arch attached to the base
of the skull much as the ribs are attached to the bodies of
the vertebrae. In fishes, behind this hyoidean arch come
several others which bear the gills; and in the very young
Human Body these also are represented, though they almost

SHOULDER GIRDLE. 77

entirely disappear long before birth. The hyoid, then,
with its cornua and ligaments answers pretty much to a
gill-arch, or really to parts of two gill-arches, since the
great and small cornna belong to originally separate arches
present at an early stage of development. It is a remnant
of a structure wliich has no longer any use in the Human
Body; but in the young frog tadpole parts answering to
it carry gills and have clefts between them which extend
into the throat just as in fishes. The gills are lost after-
wards and the clefts closed up when the frog gets its lungs
and begins to breathe by them. In the embryonic human
being these gill-clefts are also present and several more
behind them, but the arches between them do not bear
gills, and the clefts themselves are closed long before birth.
As they have no use their presence is hard to account for;
those who accept the doctrine of Evolution regard them as
developmental reminiscences of an extremely remote ances-
tor in which they were of functional importance somewhat
as in the tadpole; of course this does not mean that men
were developed from tadpoles.

The Appendicular Skeleton. This consists of the
slioulder 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 represented in Fig. 29.

The Shoulder Girdle, or Pectoral Arch. This is made
up on each side of the scajmla or shoulder -Hade, and the
clavicle or collar-bone.

The scapula [S, Fig. 29) is a flattish triangular bone
which can readily be felt on the back of the thorax. It is
not directly articulated to the axial skeleton, but lies im-
bedded 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. 29) is cylindrical and attached

78

THE HUMAN BODY.

at its inner end to the sternum as shown in the figure, fit-
ting into the notch represented at Id in Fig. 24.

The Fore Limb. In the limb itself (Fig. 30) are thirty
bones. The largest, a, lies in the upper arm, and is called

Fio. 29.— 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 x>ubic symphysis in the ventral median
Ime.

the liumerus. At the elbow the humerus is succeeded by
two bones, the radius and tilna, c and h, which lie side by
side, the radius being on the thumb side. At the distal
ends of these bones come eight small ones, closely packed

PELVIC GIRDLE. 79

and forming the wrist, or carpus. Then come five cylindri-
cal bones which can be felt through the soft parts in the
palm of the hand; one for the thumb, and one for each of
the fingers. These are the metacarpal hones, and are dis-
tinguished as first, second, third, and so on, the first being
that of the thumb. In the thumb itself arc two bones, and
in each finger three, arranged in rows one after the other;
these bones are all called plialanges.

The Pelvic Girdle (Fig. 29). This consists of a large
bone, the on innaminatum, Oc, on each side, which is
firml}^ 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 head
of the thigh-bone fits (see Fig. 13). 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. 29 points.

The Hind Limb. In this there are thirty bones, as in the
fore limb, but not quite similarly arranged; there being one
less at the ankle than in the wrist, and one at the knee
not present at the elbow-joint. The thigh-bone or femur
[a, Fig. 31) is the largest bone in the body and extends
from the hip to the knee-joint. It presents above a large
rounded Jiead. which fits into the acetabulum and, below, it
is also enlarged and presents smooth surfaces which meet
the bones of the leg. These latter are two in number,
known as the tibia, c, or shin-bone, a^nd fibula, d; the tibia
being on the great-toe side. In front of the knee-joint is
the knee-cap, or patella, b.

At the distal end of the leg-bones comes the foot, con-
sisting of tarsus, metatarsus, and phalanges. The tarsus,
which answers to the carpus of the fore limb, is made up of
seven irregular bones, the largest being the heel-bone, or
calcaneum, h. The metatarsus consists of five bones lying
&-*de by side, and each carries a toe at its distal end. In

80

THE HUMAN BODY.

the great toe (or hallux) there are two phalanges, in each

of the others three, arranged as in the fingers, but smaller.

Comparison of the Anterior and Posterior Limbs. It

Fig. 30.

Fig. 31.

Fig. 30.— The bones of the arm. a, humerus; 6. ulna; c. radius; d, the carpus:
e, tlie fifth metacarpal; /. the three phalanges of the fifth digit (little finger),
g, the phalanges of the pollex (thumb).

Fig. 31.— Bones of the leg. a, femur; b. patella; c. tibia; d, fibula; 7i, calca-
neum; e, remaining tarsal bones: /, metatarsal bones; g, phalanges.

is clear that the skeletons of the arm and leg correspond
pretty closely to one another. Both are in fact quite alike
in very early life, and their differences at birth depend upon

HOMOLOGIES OF THE LIMBS.

81

their diverging in different ways as tliey develop from
their primitive simplicity; as both may be regarded as
modifications of the same original structure, they are ho-

o P

Fio. 3-3. —The skeleton of the arm and leg. H. the humerus: Cd, its articular
head which fits into the glenoid fossa of the scapula; £/. the ulna; iJ, the ra-
dius; O, the olecranon; Fe, the femur; P, the patella; Fi, the fibula: T. the
tibia.

mologous. The pelvic girdle clearly corresponds generally
to the pectoral arch, tlie tibia and fibula to the radius and

S2 THE HUMAN BODY.

ulna ; the five metatarsal bones to the five metacarpal, and
the phalanges of the toes to those of the thumb and fingers.
On the other hand, there is in the arm no separate bone
at the elbow-joint corresponding to the patella at the knee,
but the ulna bears above a bony process, the olecranon {0,
Fig. 32), which at first is a separate bone and is the rej)-
resentative of the patella. There are in the carpus eight
bones and in the tarsus but seven. The astragalus of the
tarsus {Ta, Fig. 35) represents two bones Avhich, however,

/>**.

Fig. 33.— Diagram showing the relation of the pectoral arch to the axial skel-
eton.

have grown together. The elbow-joint bends forwards and
the knee-joint backwards.

Comparing the limbs as a whole, greater differences come

to light, differences which are

mainly correlated with the

different uses of the two

limbs. The arms, serving as

( prehensile organs, have all

their parts as movable as is

consistent with the requisite

Fig. »i.-Diagram showing the attach- strength, while the lower

Ston ^^^ '''''''''' ^"""^ **' ^^^ ^^'^^ limbs, having to bear the

whole weight of the Body,
require to liave their parts much more firmly knit together.
Accordingly we find the shoulder girdle, represented red in
the diagram (Fig. 33), only directly attached to the axial
skeleton by the union of the inner ends of the clavicles with
the sternum, and capable of considerable independent move-
ment, as seen, for instance, in " shrugging the shoulders."
The pelvic arch, on the contrary, is firmly and immovably

HAND AND FOOT COMPARED.

83

fixed to the sides of the sacrum. The socket of the scapula,
into which the head of the humerus fits, is very shallow
and allows a far greater range of movement than is per-
mitted by the deeper socket on the pelvis, into which the
head of the femur fits. Further, if we hold the right
humerus tightly in the left hand and do not allow it to
move, we can still move the forearm bones so as to turn
the palm of the hand either up or down: no such move-
ment is possible between the tibia and fibula. Finally, in
the foot the bones are much less movable than in the
hand, and are arranged so as make a springy arch (Fig. 35)
which bears behind on the calcaneum, Ca, and in front on
the distal ends of the tarsal bones, Os] and over the crown
of the arch, at Ta, is the surface with which the leg-bones

\

SfW 3

Fig. 35 —The bones of the foot. Co, calcaneum, ov os calcis; Ta, articular
surface for tibia on the astragalus; Cb. tlie cuboid bone.

articulate and on which the weight of the Body bears in
standing.

The toes, too, are far less movable than the fingers, and
this diiference is especially well marked between the great
toe and the thumb. The latter can be made to meet each
of the finger-tips and so the hand can seize and manipulate
very small objects, while this power of opposing the first
digit to the rest is nearly absent in the foot of civilized
man. In children, however, who have never worn boots,
and in savages, the great toe is far more movable, though it
never forms as complete a thumb as in many apes, which
use their feet, as well as their hands, for prehension. By
practice, however, our own toes can be made much more

84 THE HUMAN BODY.

mobile than they usually are, so that the foot can to a
certain extent replace the hand; as has been illustrated in
the case of persons born without hands who have learned to
write and paint Avith their toes.

Peculiarities of the Human Skeleton. These are largely
connected with the division of labor between the fore and
hind limbs referred to above, which is carried farther in
man than in any other creature. Even the highest apes
frequently use their fore limbs in locomotion and their
hind limbs in prehension, and we find accordingly 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
maintenance 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 pro-
portionately larger, 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 to the ape, and they all combine to give the centre
of gravity of the Body a comparatively very high |)osition
and to render the maintenance of the erect posture difficult
and insecure. 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 Pri-
mates this disproportion between the anterior and posterior
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. 27*), so that but
little effort is needed to keep the head erect. In four-foot-
ed 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 ver-

CHARACTERISTICS OF HUMAN SKELETON. 85

tebrai to tlie 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 part is much
heavier than the back, while in man the face parts are rela-
tively 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
tlie 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 character-
istic. The majority of four-footed beasts, as horses, 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 are plcuitigrade in technical language, have
a much less marked arch there. The vaulted human tar-
sus, 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 admirally 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 justifiably considered a beauty ; it makes
progression easier, and by its springiness gives elasticity to
the step. In London flat-footed candidates for appoint-
ment as policemen are rejected, as they cannot stand the
fatigue of walking the daily "beat."

CHAPTER VII.

THE STRUCTURE AND COMPOSITION OF BONE.
JOINTS.

Gross Structure of the Bones. The bones of the Body
have all a similar structure and composition, but on ac-
count of differences in shape they are divided by anato-
mists into the following groups: (1) Long tones, more
or less cylindrical in form, like the bones of the thigh and
arm, leg and forearm, metacarpus, metatarsus, fingers
and toes. (2) Tabular hones, in the form of exjjanded
plates, like the bones on the roof and sides of the skull, and
the shoulder-blades. (3) Short hones ; rounded or angular
in form and not much greater in one diameter than in
another, like the bones of the tarsus and carpus. (4) Ir-
regular hones, including all which do not get well into any
of the preceding groups,and commonly lying in the middle
line of the Body and divisible into two similar halves, as
the vertebriE. Living bones have a bluish-white color and
possess considerable elasticity, which is best seen in long
slender bones such as the ribs.

To get a general idea of the structure of a bone, we may se-
lect the humerus. Externally in the fresh state it is covered
by a dense white fibrous membrane very closely adherent to
it and containing a good many small blood-vessels.
This membrane is called the periosteum; on its under side
new osseous tissue is formed while the bone is still growing,
and all through life it is concerned in maintaining the nu-
trition of the bone, which dies if it is stripped off. The
periosteum covers the whole surface of the bone except its
ends in the elbow and shoulder joints; the surfaces there
which come in contact with other bones and glide over them

STRUCTURE OF A BONE.

87

in the movements of the joint have no periosteum, but are
covered by a thin layer of
gristle, known as the articu-
lar cartilage. Very early in
the development of the Body
the bone in fact was repre-
sented entirely by cartilage;
but afterwards nearly all this
was replaced by osseous tis-
sue, leaving only a thin carti-
laginous layer at the ends.

The bone itself, Fig. 36,
consists of a central nearly
cylindrical portion or shaft,
extending between the dot-
ted lines X and z in the fig-
ure, and two enlarged artic-
idar extremities.

On the upper articular ex-
tremity is the rounded sur-
face, Cp, which enters into
the shoulder -joint, fitting
against the glenoid cavity of
the scapula; and on the low-
er are the similar surfaces,
Cpl and Tr, which articulate
with the radius and ulna re-
spectively. Besides carry-
ing the articular sui'faces,
each extremity presents sev-
eral j)rominences. On the
upper are those marked Tmj
and Tm (the greater and
smaller trochanters), which
give attachment to muscles;
and similar eminences, the
external and internal con-
dyles, El and Em, are seen
on the lower end. Besides

Fig,
from the front,
text.

•igi]

For description, seo

these, several bony ridges

88

THE HUMAN BODY.

Hiid rough patches on tlie shiiit indicate places to which
muscles of the arm were fixed.

Internal Structure. If the Ijone he divided longitudi-
nally, it will he seen that its shaft is hollow,
the space being known as the medullary
rarity, and in the fresh bone filled with
marrow. Fig. 37 represents a longitudi-
nal section of the femur, which in this re-
spect is quite similar to the humerus.
The marrow cavity does not reach into the
articular extremities, but there the bone
has a loose spongy texture, except a thiii
layer on the surface. In the shaft, on the
other hand, the outer compact layer is
much the thickest, the spongy or cancel-
lated hone forming only a thin stratum im-
mediately around the medullary cavity.
To the naked eye the cancellated bone
appears made up of a trellis-work of thin
bony 2)lates whicli intersect in all direc-
tions and surround cavities rather larger
than the head of an ordinary pin; the
compact bone, on the contrary, ajoiieaj's
to have no cavities in it until it is exam-
ined with a magnifying glass. In the
spaces of the spongy portion lies, during
life, a substance known as the red marroic,
which is quite different from the yellow
fatty marrow lying in the central cavity
of tile shaft.

Microscopic Structure of Bone. Tlic
microscope shows that the compact bone
contains cavities and only differs from the
spongy portion in the fact that these
are much smaller and the hard true bony
plates surrounding them much more nu-
merous in })roportiou than in the spongy
parts. If a thin transverse section of the shaft of the hu-
merus be examined (Fig. 38) with a microscope magnifying

Fig. 37— a longi-
l • idinal section or
the femur, d, dense
bone ; a. cancellated
bone; b, medullary
cavity.

HISTOLOGY OF BONE.

89

twenty diameters, it will be seen that numerous openings
exist III! over the compact parts of the section and gradu-
ally become larger as this passes into the cancellated part,
next the medullary cavity. These openings are the cross-
sections of tubes known as the Haversian canals, which
ramify all through the bone, running mainly in the direc-

FiG. 38. — A, a tranverse section of the ulna, natural size ; showing the medul-
lary cavity. B, the more deeply shaded part of A magnified twenty diameters.

tion of its long axis, but united by numerous cross or ob-
lique branches as seen in the longitudinal section (Fig. 39).
The outermost ones open on the surface of the bone be-
neath the periosteum, and in the living bone blood-vessels
run from this through' the Haversian canals and convey

90 THE HUMAN BODY.

materials for its growth and nourishment. Tlie average
diameter of the Haversian canals is 0.05 mm. {-^^ of an
inch).

Around each Haversian canal lies a set of plates, or
lameU(v, of hard bony substance (see the transverse section
Fig. 38), each canal with its lamellae forming an Haver-
sian system: and the whole bone is made up of a number
of such systems, with the addition of a few lamellse lying
in the corners between them, and a certain number which
run around the whole bone on its outer surface. In the
spongy parts of the bone the Haversian canals are very
Inrge and the intervening lamellse few in number.

Between the lamella? lie small cavities, the lamnm, each
of which is lenticular in form, somewhat like the space which

would be inclosed by two
^^T^l^^'lli^'*^^^^!^ ^.j^|-g|-^_g|j^gggg joined by their
$P r^tii^^ (^^9 ^^'Ses- From the lacuna? many
€^i^^^ ViFI^^^^M ^^^^'^'^^^y ^^^® branching ca-
S/MM ^^ l|^^^Ki~^\^S ^^''^^^' ^^■'^ ccmaliculi, radiate
f T/^V?/ 'll^^ilsV'^ i^^'^^ penetrate the bony la-
' ^ i> ^if^ ^f,PAik\% melL^ in all directions. The
^ L ^^ ^ it J^ i^jt i} i' innermost canaliculi of each

system oiDcn mto the central

Fig. 39— a thin longitudinal sec tV • i i jt c

tion of bone, magnified about 350 Haversian Can al; and those oi

diameters, aa. Haversian canals. • i - ,

various lacunge intercommu-
nicating, these fine tubes form a set of passages through
Avhich liquid which has transuded from the blood-vessels
in the Haversian canals can ooze all through the bone.
The lacunae and canaliculi are well seen in Fig. 39.

In the living bone a granular nucleated cell lies in each
lacuna. These cells, or hone coiyusdes, are the remnants
of those which built up the bone, the hard parts of the lat-
ter being really an intercellular substance or skeleton
formed around and by these cells, much in the same way
as a calcareous skeleton is formed around each Foraminifer
(see Zoology) by the activity of its protoplasm. By the
co-operation of all the bone corpuscles, and the union of
their skeletons, the whole bone is built up.

In other bones we find the same general arrangement of

COMPOSITION OF BONE. 91

the parts, ;in outer dense layer and an inner spongy por-
tion. In the flat and irreguhir bones there is no medullary
cavity, and the whole centre is filled up with cancellated
tissue with red marrow in its spaces. For example, in
the thin bones roofing in the skull we find an outer
and inner hard layer of compact bone known as the outer
and iimer tables respectively, the inner especially being very
dense. Between the two tables lies the spongy bone,
red in color to. the naked eye from the marrow within it,
and called the diploe. The interior of the vertebra also is
entirely occupied by spongy bone. Everywhere, except
where a bone joins some other part of the skeleton, it is
covered with the periosteum

Chemical Composition of Bone. Apart from the bone
corpuscles and the soft contents of the Haversian canals
and of the spaces of the cancellated bone, the bony sub-
stance i)roper. as found in the lamellae, is composed of
earthy and organic portions intimately combined, so that
the smallest distinguishable portion of bone contains both.
The earthy 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 organic portion
left after this treatment constitutes a flexible mass, retain-
ing perfectly the form of the original bone. By long boil-
ing, especially under pressure at a higher temperature
than that at whieli water boils when exposed freely to the
air, the oi'ganic portion of the bone is nearly entirely con-
verted into gelatine which dissolves in the hot water. Much
of the gelatine 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 pressure in a Papin's digester; much nutri-
tious matter being, in the common modes of domestic
cooking, thrown away in the bones.

The earthy salts of bone may be obtained free from or-
ganic matter by calcining a bone in a clear fire, which burns
away the organic matter. The residue forms a white very
brittle mass, retaining perfectly the shape and structural
details of the original bone. It consists mainly of normal

92 THE HUMAN BODY.

calcium phos])liate, or boiif earth (Ctia, 2PO4); but there is
also present a considerable proportion of calcium carbonate
(CaCOa) and smaller ([uantitics of other salts.

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 will be dis-
torted 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 u])on (for no child will continue it volunta-
rily) 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 in
writing, to avoid the risk of producing a lateral curvature
of the spinal column. The facility with which the bones
may be moulded by prolonged pressure in early life is well
seen in the distortion of the feet of Chinese ladies, pro-
duced by kee23ing them in tight shoes; and in the extraor-
dinary forms which some races of man produce in their
skulls, by tying boaixls on the heads of the children.

Throughout the whole of life, moreover, the bones re-
main 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 in-
clined 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 r(>constructed. The expe-
rience of physicians shows that any continued pressure,
such as that of a tumor, will cause the absorption and dis-
appearance of bone almost quicker than that of any other
tissue; and the same is true of any other continued pres-
sure. Moreover, during life the bones ;irc eminently plas-
tic; under abnormal pressures they are found to quickly
assume abnormal shapes, being absorbed and disappearing

ARTICULATION'S. 93

at points where the pressure is most powerful, and increas-
ing at other points; tight hxcingmay 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 been sot up
and the parts have become swollen it is much more diffi-
cult to place the broken ends of the bone togetlier in their
proper position than before this has occurred. Once the
bones are replaced they must be held in position by splints
or bandages, or the muscles attached to them will soon dis-
place them again. With rest, in young and healtliy per-
sons complete union will commonly occur in three or four
weeks; but in old persons the process of cure is slower and
is apt to be imperfect.

Articulations. The bones of the skeleton are joined to-
gether in very various ways; sometimes so as to admit of no
movement at all between them; in other cases so as to per-
mit only a limited range or variety of movement; and else-
where so as to allow of very free movement in many direc-
tions. All kinds of unions between bones are called articu-
lations.

Of articulations permitting no movements, those which
unite the majority of the cranial bones afford a good exam-
ple. Except the lower jaw, and certain tiny bones inside the
temporal bone belonging to the organ of hearing, all
the skull-bones are immovably joined together. This unioji
in the case of most occurs by means of toothed edges which
lit into one another and form jagged lines of union known
as sutures. Some of these can be well seen in Fig. 2G* between
1 he frontal and parietal bones {coronal suture) and between
the parietal and occipital bones {lamMoidal suture); while
another lies along the middle line in the top of the crown
between the two parietal bones, and is known as the sagit-
tal 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 subse-
quently extend over them. These oj^enings are known as
fontanelles. At them a pulsation can often ])e felt syn-
chronous with each beat of the heart, which, driving more

94 THE HUMAN BOBT.

blood into the braiu, distends it and causes it to push out
the skin where bone is absent. Another good example
of an articulation admitting of no movement, is that between
the rough surface on the sides of the sacrum and the in-
nominate bone.

We find good examples of the second class of articula-
tions— those admitting of a slight amount of movement —
in the vertebral column. Between every pair of vertebra^
from the second cervical to the sacrum is an elastic pad,
the intervertebral dish, which adheres by its surfaces to the
bodies of the vertebrae between which it lies, and only per-
mits 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 move-
ments, each individually 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 permitted 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 move-
ment. On the OS innominatum is the cavity of the aceta-
hnhim (Fig. 40), which is lined inside by a thin layer of
articular curtilage which has an extremely 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
witli articular cartilage; and it rolls in the acetabulum like
a ball in a socket. To keep the bones together and limit
the amount of movement, ligaments pass from one to the
other. These are com])osed of white fibrous connective
tissue (Chap. VIII.) and are extremely jDliable but quite
inextensible and very strong and tough. One is the cap-
sular ligament, which forms a sort of loose bag all round

SYJS'^OrTAL JOINTS.

95

the Joint, and another is the rouiid ligament, which passes
from the acetabnhim to tlie head of the femur. Should
the latter rotate above a certain extent in its socket, the
round ligament and one side of the capsular ligament are
put on the stretch, and any further 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

Fia. 40.— Section thi-ough the hip- joint.

the articular cartilages and form the synovial memlrane.
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 white of a raw cg^ in consis-
tency, and playing the part of the oil with which the con-
tiguous moving surfaces in a machine are moistened; it
makes 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.

96 THE HUMAN BODY.

This contact is not nuiintiiiued by the ligaments, which are
too loose and serve only to check excessive movement,
bnt by the numerous stout muscles which pass from the
thigh to the trunk and bind the two firmly togethei-.
Moreover, the atmospheric pressure exerted on the surface
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 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 Ifhat 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 jiexed, or bent so that the knee ap-
proaches the chest; or extended, that is moved in the oppo-
site 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. Fi-
nally 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 tlie
shallowness of the glenoid cavity as compared with the
acetabulum and upon the absence of any ligament answer-
ing to the round ligament of the hip-joint. Another ball-
and-socket joint exists between the carpus and the meta-
carpal bono 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

FORMS OF JOINTS. 97

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 exami)]es 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
articulation 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 per-
mits of a small amount of lateral movement such as occurs
in chewing, and also of a gliding movement by which the
lower jaw can be thrust forward so as to protrude the chin
and bring the lower row of teeth outside the upper.

Pivot- Joints. In this form one bone rotates around an-
other which remains stationary. We have a good example
of it between the first and second cervical vertebrge. The
first cervical vertebra or atlas (Fig. 19*) has a very small
body and a very large arch, and its neural canal is subdi-
vided by a transverse ligament {L, Fig. 19) into a dorsal
and a ventral portion; in the latter the spinal cord lies.
The second vertebra or axis (Fig. 20) has arising from its
body the stout bony peg, D, called the odontoid process.
This projects into the ventral portion of the space sur-
rounded by the atlas, and, kept in place there by the trans-
verse ligament, forms a pivot around which the atlas, car-
rying 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 straight out, with the palm up and the
elbow resting on the table, so that the slioulder-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 upwards and

'■ *P.69.

98

TEE HUMAN BODY.

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 turned over, it will
l)e easily discerned that during the movement this end of
the radius, carrying the hand with it, travels around the
lower end of the nlna 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. 41, and when the palm is down at
B. The former position is known as supination; the lat-
ter as pronation. The elbow
end of the humerus (Fig, 36*)
bears a large articular surface:
on the inner two thirds, of this,
Tr, the ulna fits, and the ridges

R-

■V

and grooves of both bones inter-

locking form a hinge-joint, al-
lowing only of bending or
straightening the forearm on
the arm. The radius fits on
the rounded outer third, Cpl,
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 per-
mit as a rule but little move-
ment: examples are found be-
tween the closely packed bones of the tarsus (Fig. 35 f) and
carpus, which slide a little over one another when subjected
to pressure.

Hygiene of the Joints. AYlien 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 in-
flammation and swelling which make not only the recogni-
tion of the injury but, after diaguo.sis, the replacement
of the bone, or the reduction of the dislocation, difficult.

Fig. 41. — A, arm in supination;
B, arm in pronation; if, humerus;
B, radius; 17, ulna.

» P. 87.

fP. 83.

DISLOCATIONS AND SPRAINS. 99

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 displacement. No injury to a joint should
be neglected. Inflammation once started there is often diffi-
cult to check and runs on, in a chronic way, until the syno-
vial surfaces are destroyed, and the two bones perhaps
grow together, rendering the joint permanently stiff. A
sprained joint should get immediate and complete rest, for
weeks if necessary, and if there be much swelling, or con-
tinued pain, medical advice should be obtained. An im-
properly cared-for sprain is the cause of many a useless
ankle or knee.

CHAPTER VIII.

CARTILAGE AND CONNECTIVE TISSUE.

Temporary and Permanent Cartilages. In earl}^ life
a great m;iiiy parts of the supporting framework of the
Body, which afterwards become boue, consist of cartilage.
Such for example is the case with all the vertebrae, and
with the bones of the limbs. In these cartilages subse-
quently 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, how-
ever, 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 incom-
pletely 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 windpipe
serving to keep it open. These persistent masses are known
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 l^y 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 flexi-
ble 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

CARTILAGE. 101

Joints it is invested by a tongh adherent membrane, the
perichondrium, which resembles in structure and function
the periosteum of the bones. When boiled for a long time
in water such cartilages yield a solution of chondrin, which
differs from gelatin in minor points but agrees with it in
the fact that its hot watery solutions " set" or gelatinize on
cooling. When a thin slice of hyaline cartilage is examined
with a microscope it is found (Fig. 42) to consist of gran-
ular nucleated cells, often collected into groups of two,
four, or more, scattered through a homogeneous or faintly
granular ground substance or mrt/rhr. Essentially, cartilage
resembles bone, being made up of protoplasmic cells and a
proportionately large amount of non-protoplasmic mtercel-

FiG. 43. -Hyaline cartilage, c, a cell with several nuclei, and about to divide ;
h. a cell which has divided into two; a, a group of four cells such as would re-
sult from a repetition of the division of b The granules of the matrix are
represented much too coarse and conspicuous.

lular substance, the cells being the more actively living
part and the matrix their product. Examples of this hya-
line variety (so called from its glassy transj)arent appear-
ance) are found in all the temporary cartilages, and in the
costal and articular among the permanent.

They rarely contain blood-vessels except at those points
where a temporary cartilage is being removed and replaced
by bone; then blood-vessels run in from the loerichondrium
and form loops in the matrix, around which it is absorbed
and bony tissue deposited. In consequence of the usual
absence of blood-vessels the nutritive processes and ex-
changes of material must be small and slow in cartilage, as

102 TEE ETJMAN BODY.

might indeed be expected from the passive and merely
mechanical rdle which this tissue plays.

Hyaline cartilage is the type, or most characteristically
developed form, of a tissue found with modifications else-
where in the Body. One of its other modifications is the
so-called cellular cartilage, which consists of the cells with
hardly any matrix, only just enough to form a thin capsule
around each. This form is that with which all the car-
tilages commence, the hyaline variety being built up by the
increase of the cell capsules and their fusion to form the
matrix. It persists throughout life in the thin cartilaginous
plate of a mouse's external ear. Other varieties of cartilage
are really mixtures of true cartilage and connective tissues,
and will be considered after the latter.

The Connective Tissues. These complete the skeleton,
marked out in its coarser features by the bones and car-
tilages, and constitute the final group of the supporting
tissues. They occur in all forms from broad membranes
and stout cords to the finest threads, forming networks
around the other ultimate histological elements of various
organs. In addition to subsidiary forms, three main varie-
ties of this tissue are readily distinguishable, viz., areolar,
white fibrous, and yelloiv elastic. Each consists of fibres
and cells, the fibres being of two kinds, mixed in nearly
equal proportions in the areolar variety, while one kind
predominates in one and another in the second of the re-
maining chief forms.

Areolar Connective Tissue. This exists abundantly be-
neath the skin, where it forms a loose flocculent layer,
somewhat like raw cotton in appearance but not so white.
It is on account of its loose texture that the skin can every-
where be moved, more or less, to and fro over the subja-
cent parts to which it is united by this tissue. Areolar
tissue consists of innumerable bands and cords interlacing,
in all directions, and can be greatly distended by blowing air
in at any point, from whence it travels widely through the
intercommunicating meshes. In dropsy of the legs or feet
the cavities of this tissue are distended with lymph. From
beneath the skin the areolar tissue extends all through the

CONNECTIVE TISSUES. 103

Body between the muscles and around the blood-vessels and
nerves; and still finer layers of it enter into these and other
organs and unite their various parts together. It consti-
tutes in fact a soft packing material which fills up the
holes and corners of the Body, as for instance around the
blood-vessels and between the muscles in Fig. 4.

Microscopic Structure of Areolar Tissue. When exam-
ined wdth the microscope areolar tissue is seen to consist of
nucleated cells imbedded in a ground substance which is
permeated by fibres. The fibres everywhere form the i)re-
dominant feature of the tissue (the homogeneous matrix
and the cells being inconspicuous) and are of two very dif-
ferent kinds. In a strict sense indeed the areolar tissue
ought to be considered as a mixture of two tissues, one
corresj)onding to each variety of fibres in it. It is charac-
terized as a distinct individual by its loose texture and by
the fact that the two forms of fibres are present in tolera-
bly equal quantities. In many places a tissue containing
the same histological elements as the areolar tissue is found
in the form of dense membranes, as for example periosteum
and perichondrium.

White Fibrous Tissue. One of the varieties of fibres
pervading the matrix of areolar tissue exists almost un-
mixed with the other kind in the cords or tendons which
unite muscles to the bones. This form, known as the white
fibrous connective tissue, also exists fairly pure in the
ligaments around most joints. Physically it is very flexi-
ble but extremely tough and inextensible, so that it w^ill
readily bend in any direction but is very hard to break r
when fresh it has an ojiaijue white color.

White fibrous tissue (Fig, 43) consists of a matrix, con-
taining cavities in which cells lie and pervaded by bundles
of extremely fine fibres. These fibres lie in each bundle
tolerably parallel to one another in a wavy course (Fig, 43)
and never branch or unite. Their diameter varies from
0.0005 to 0.001 millimeter (go Jo¥ to 2yJ-„T o^ an inch).

Chemically this tissue is characterized by the fact that
its fibres swell up and become indistinguishable when
t'Qated with dilute acetic acid, and by the fact that it

104

THE HUMAN BODY.

yields gelatin when boiled in water. The substance in it,
and in tlie bones, which is turned into gelatin by such
treatment is known as collagen. Glue is impure gelatin
obtained from tendons and ligaments, and calf's-foot
jelly, so often recommended to invalids, is a purer form of
the same substance obtained by boiling the feet of calves,
which contain the tendons of many muscles passing from
the k'o; to the foot.

Fig. 43.

Fig. 43a.

Fig. 43.— White fibrous connective tissue, highly magnified. The nucleated
corpuscles, seen edgewise and appearing spindle-shaped, are seen here and
there on the surface of the bundles of fibres.

Fig. 43a.— Yellow elastic tissue, magnified after its fibres have been torn
apart.

Elastic Tissue. This is almost invariably mixed in some
proportion in all specimens of white fibrous tissue, even
the purest, such as the tendons of muscles; but in certain
places it exists almost alone, as for example in the liga-
ments {ligamenta suhjiava) between the arches of the
vertebra?, and in the coats of the larger arteries. In quad-
rupeds it forms the great ligament already referred to (p.
84), which helps to sustain the head. This tissue, in

CONNECTIVE-TISSUE CORPUSCLES. 105

mass, is of a dull yellow color and extremely extensible and
elastic; when purest nearly as much so as a piece of india-
rubber. Sometimes it appears under the microscope to be
made up of delicate membranes, but most often it is in the
form of fibres (Fig. •13rt) which are coarser than those of
white fibrous tissue and frequently branch and unite. It
is unaffected by acetic acid and does not yield gelatin when
boiled.

Connective-Tissue Corpuscles. The fibres of white fi-
brous tissue, wherever it is found, are united into bundles
by a structureless ground material known as the cement sub-
stance, which also invests each bundle, or skein as we may
call it, with a delicate coating. In this ground sub-
stance are numerous cavities, branched and flattened in

Fig. 44.— Connective-tissue corpuscles.

one diameter, and often intercommunicating by their
branches. In these cavities lie nucleated masses of proto-
plasm (Fig. 41), frequently also branched, known as the
connective-tissue coiyuscles. These it is which btiild up
the tissue, each cell in the course of development forming
around it a quantity of intercellular substance, which sub-
sequently becomes fibrillated in great part, the remainder
forming the cement. The cells do not quite fill the cavi-
ties in which they lie, and these opening into others by
their offsets there is formed a set of mintite tl^bes ramify-
ing through the connective tissues ; and (since these in
tttrn permeate nearly all the Body) pervading all the organs.
In these cell cavities and their branches the lymph flows
before it enters definite lymphatic vessels, and they are ac-

106 THE HUMAN BODY.

cordingly known as lymph canaliculi. In addition to the
fixed branched connective-tissue corpuscles lying in the cavi-
ties of the ground substance there are often found other
cells, when living connective tissue is examined by the micro-
scope. These cells much resemble white blood corpuscles,
and probably are such which have bored through the walls
of the finer blood-vessels. They creep about along the
canaliculi by means of their faculty of amoeboid movement,
and are known as the ''wandering cells." During in-
flammation at any point their number in that region is
greatly increased.

Subsidiary Varieties of Connective Tissue. In various
parts of the Body are connective-tissue structures which
have not undergone the typical development, but have de-
parted from it in one way or another. The cells having
formed a non-fibrillated intercellular substance around
them, develoj^ment may go no farther and the mass remain
permanently as the jelly-Uhe connective tissue; or, as in
the vitreous humor of the eye (Chap. XXXI.), the cells
having formed the soft matrix, may disajjpear and leave the
latter only. In other cases the intercellular substance
disappears and the cells branching, and joining by the ends
of their brandies, form a network themselves, nucleated or
not at the points answering to the centre of each originally
separate cell. This adenoid connective tissue is found
widely distributed in the Body especially in connection
with the lymphatic system, and forming a supporting-
framework for the proper nervous elements in the brain
and sj)inal cord. In other cases the cells almost alone con-
stitute the tissue, becoming flattened, closely fitted at their
edges, and united by a very small amount of cement sub-
stance. ]\Iembranes formed in this way lie beneath layers
of epithelium in many places and are known as dasement
memhrcmes, and the flattened cells which line joints and the
serous cavities seem really to be closely apposed flattened
connective- tissue corpuscles.

Elastic Cartilage, and Fibro-Cartilage. We may now
return to cartilages and consider those forms which are
made up of more or less true cartilage mixed with more or

INTEBABTICULAB CARTILAGES.

lor

less connective tissue of one kind or another. The carti-
lages of the ear and nose and some others have their matrix
pervaded by fine branching fibres of yellow elastic tissue,
which form networks around the groups of cartilage cells.
Such cartilages are pliable and tough and possess also con-
siderable extensibility and elasticity. They are known as
elastic or, from their color, as yellow cartilages. Elsewhere,
especially in the cartilages which lie between the bones in
some joints, we find forms which have the matrix pervaded
by white fibrous tissue and known as fibro-vartilages. For
example the articular cartilage on the end of the lower jaw

Fig. 4o. — Section through the joint of the lower jaw showing its interarticular
fibro-cartUage, x, with tlie synovial cavity on each side of it.

does not C(jme into direct contact with that covering its socket
on the skull, but lying between the two in the joint (Fig. 45)
is an interarticular fibro-cartilage : similar cartilages exist
in the knee-joint; and the intervertebral disks are also made
up of this tissue. Both elastic cartilage and fibro-cartihige
often shade off insensibly into pure elastic or pure white
fibrous connective tissue.

Homologies of the Supporting Tissues. Bone, cartilage,
and connective tissue all agree in broad structural charac-
ters, and in the iises to which they are applied in the Body.
In each of them the cells which have built up the tissue,

108 THE HUMAN BODY.

■with rare exceptions, form an inconspicuous part of it in
its fully developed state, tlie chief mass of it consisting
of intercellular substance. In hyaline cartilages this latter
is not fibrillated; but these cartilages pass insensibly in va-
rious regions of the Body into elastic or fibro-cartilages, and
these latter in turn into elastic or fibrous connective tissue.
The lamellse of bone, too, when peeled off a bone softened
in acid and examined with a very high magnifying power,
are seen to be pervaded by fine fibres. Structurally, there-
fore, one can draw no hard and fast line between these tis-
sues. The same is true of their chemical composition; bone
and white fibrous tissue contain a substance (collagen)
which is converted into gelatin when boiled in water; and
in old people many cartilages become hardened by the de-
posit in their matrix of the same lime salts which give its
hardness to bone. Further, the developmental history of
all of them is much alike. In very early life each is repre-
sented by cells only : these form an intermediate substance,
and this subsequently may become fibrillated, or calcified,
or both. Finally they all agree in manifesting in health no
great physiological activity, their use in the Body depend-
ing upon the mechanical properties of their intercellular
substance.

The close alliance of all three is further shown by the
frequency with which they replace one another. All the
bones and cartilages of the adult are at first represented
only by collections of connective tissue. Before or after
birth this is in some cases substituted by bone directly (as
in the case of the collar-bone and the bones on the roof of
the skull), while in other cases cartilage supplants the con-
nective tissue, to be afterwards in many places replaced by
bone, while elsewhere it remains throughout life.

Moreover in different adult animals we often find the
same part bony in one, cartilaginous in a second, and com-
posed of connective tissue in a third: so that these tissues
not only represent one another at different stages in the
life of the same animal Ijut permanently throughout the
whole life of different animals. Low in the animal scale

HYGIENE OF GBOWING SKELETON. 109

we find tliem all represented merely by cells with struc-
tureless intercellular substance: a little higher in the scale
the latter becomes fibrillated and forms distinct connective
tissue. In the highest MoUusks (see Zoology), as the cuttle-
fishes, this is partly replaced by cartilage, and the same
is true of the lowest fishes; while in some other fishes and
the remaining Vertebrates, we find more or less bone ap-
pearing in place of the original connective tissue or carti-
lage.

From the similarity of their modes of development and
fundamental structure, the transitional forms which exist
between them, and the frequency with which they rej^lace
one another, histologists class all three (bone, cartilage, and
connective tissue) together as homologous tissues and re-
gard them as differentiations of the same original struc-
ture.

Hygienic Remarks. Since in the new-born infant many
parts which will ultimately become bone, consist only of car-
tilage, the young child requires food which shall contain a
large proportion of the lime salts which are used in building
up bone. JSTature provides this in the milk, which is rich in
such salts (see Chap. XX. ), and no other food can thoroughly
replace it. If the mother's health be such as to render it
unwise for her to nurse her infant, the best substitute,
apart from a wet-nurse, will be cow's milk diluted with one
fourth its volume of water. Arrowroot, corn-flour, and
other starchy foods will not do alone, since they are all defi-
cient in the required salts, and many infants though given
food abundant in quantity are really starved, since their food
does not contain the substances requisite for their healthy
development.

At birth even those bones of a child which are most ossi-
fied 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 eacli
of the articular extremities there still remains a cartilad-
nous layer, and at those points the bone increases in length,
new cartilage being formed and replaced by it. The bone

110 THE HUMAN BODY.

increases in thickness by new osseous tissue formed beneath
the periosteum. Tlie same tiling is true of tlie 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 j^ermanent distortions. Nevertheless here as else-
where 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 walk beyond its strength.

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.

The final knitting together of the bony articular ends
with the shaft of many bones takes place only compara-
tively late in life, and the age at which it occurs varies
miich 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 examjjles: the lower articular extremity 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 con-
tiuuity in the twentieth year. The u]iper end of the femur
joins the shaft by bone from the seventeeu th to the nine-
teenth 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
eigiiteenth 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 vertebrfe of the
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 iniiominatum 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

FAT-CELLS. Ill

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 race is quite another, and not devoid of
risk.

Adipose Tissue. Fatty substances of several kinds ex-
ist in considerable quantity in tbe human Body in health,
some as minute droplets floating in the bodily liquids or
imbedded in various cells, but most in special cells, nearly
filled with fat, and collected into masses with supporting
and nutritive parts, to form adipose tissue. In fact al-
most in every spot where the widely distributed areo-
lar tissue is found, there is adipose tissue in greater or
less proportion along with it. Considerable quantities ex-
ist for example in the subcutaneous areolar tissue, espe-
cially in the female sex, giving the figure of the woman its
general graceful roundness of contour when compared with
that of the male. Large quantities commonly lie in the
abdominal cavity around the kidneys; in the eye-sockets,
forming a pad for the eyeballs; in
the marrow of bones ; around the
joints, and so on.

Examined with the microscope
(Fig. 46) adipose tissue is found to
consist of small vesicles from 0.2
mm. to 0.09 mm. (jj^ to jJq- inch)
in diameter, clustered together into
little masses and bound to one another

by connective tissue and blood-ves- ^^^ ^^ _^^^ ^^,j^ ^^^
sels which intertwine around them ; suppoi-tmg connective tis-
in this way the little angular masses

which are seen in beef suet are formed, each mass be-
ing separated by a somewhat coarser partition of areo-
lar from its neighbors. The individual fat-cells are
round or oval except when closely packed, when they
become polygoiud. Each consists of a delicate enve-
lope containing oily matter, which in life is liquid
at the temperature of the Body. Besides the oily mat-
ter, a nucleus is commonly present in each fat-cell:

112 THE HUMAN BODY.

and sometimes a thin layer of protoplasm forms a lin-
ing to the cell-wall. The oily matter consists of a mixture
of palmatin, olein and stearin, which are compounds of
palmitic, stearic and oleic acids with glycerine, three
molecules of the acid being combined with one of glycerine,
with the elimination of water; as for example:

o /CisHssO ) r\\ 1 C3H5 ) ^ SfCisHssO) ) /^3 1 QTir\

^[ Hf^j+ h4^^= C3H5 f^ +'^^^^-

stearic acid. Glycerine. Stearin. Water.

CHAPTER IX.

THE STEUOTURE OF THE MOTOR ORGANS.

Motion in Animals and Plants. If one were asked to
point out the most distinctive property of liyiiig animals, the
answer would probably be, their power of executing sijontane-
ous 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 re-
cognizable. Even at their quietest times some kind of mo-
tion is observable in the higher animals. In our own Bod-
ies during the deepest sleep the breathing movements and
the beat of the heart continue; their cessation is to an on-
looker most obvious sign of death. Here however, as else-
where 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 absence of apparently sponta-
neous motility. Many a flower closes in the evening to ex-
pand again in the morning sun; and in many plants compara-
tively 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 flytrap (Dionaea 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 perma-
nently fixed to one spot; and among the lowest plants many
are known which swim about actively through the water in

114 THE HUMAN BODY.

which they live. The lowest animals and plants are in fact
those which have nndcrgone least- differentiation in their
development, and wliich therefore resemble each other in
possessing, in a more or less manifest degree, all the funda-
mental physiological properties of that simple mass of pro-
toplasm which formed the starting point of each individual.
With the johysiological division of labor which takes place
in the higher forms we find that, speaking broadly, plants
especially develop nutritive tissues, while animals are char-
acterized by the high development of tissues with motor
and irritable properties; so that the jDrej^onderance of these
latter is very marked when a comi)lex animal, like a dog
or a man, is compared 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 orfjans of vegetative life), well-devel-
oped spontaneous, irritable 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 sur-
rounding universe, enabling slight external forces to excite
it, and it in turn to act upon external objects, they are also
often spoken of as orgaiis of relatioti. In man they have a
higher develoi)ment on the whole than in any other animal,
and give him liis 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 endure and make the best of what happens to
it ; it has little or no influence m controlling the haj)-
pening.

AmcBboid Cells. The simplest motor tissues in the adult
Human Body are the amoeboid cells (Fig. 12) already de-
scribed, which may be regarded as the slightly modified
descendants of the undifferentiated 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

CILIATED CELLS.

115

can hardly be called motoi- tissues, and so are placed in a
group by themselves under the name 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 c:ivities in its interior undergo a very re-
markable change, by which each cell differentiates itself into
a nutritive, and a highly motile and spontaneous portion.
Such cells are found for example lining the ^\ indpipe, and
a number are represented in Fig. 47. 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 j)rocesses called cilia. During
life these are in constant rapid move-
ment, 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 sur-
face of the windpipe examined with
a microscope, 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 di-
rection of more powerful stroke is the same for all the cilia
on any one surface, the general result 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 of,
instead of accumulating in the deej^er air-passages away
down in the chest.

These cells afford an extremely interesting example of the
division of physiological emijloyments. Each proceeds from
a cell which was primitively equally motile, automatic, 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 automatic and contractile proj)-
erties have been condensed, so to speak, in that modified

Fig. 47.— Ciliated ceU.s.

116 THE HUMAN BODY.

portion of the primitive protoplasmic mass, wliicli forms tlie
cilia. These, being supplied with 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 perform-
ance of automatic movements; which are accordingly far
more rapid and precise than those executed by the whole
cell before any division of kiborhad occurred in it.

That the movements depend upon the structure and com-
position 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 constitution. 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.

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 i:)Osition of nearly all parts of the
Body. With the skeleton and joints, they constitute pre-
eminently the organs of motion and locomotion, and are
governed by the nervous system which regulates their activ-
ity. 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 de-
velopment 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 co-02)eration of will or

VARIETIES OF MUSCLE. 117

consciousness; some moving without our having any direct
knowledge of the fact. This is especially the case with cer-
tain muscles which are not fixed to the skeleton but sur-
round 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 gi'oup, or
skeletal muscles, are also from their microscopic characters
known as striped muscles, while the latter, or visceral mus-
cles^ are called unstriped or plain 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 voluntarij, and the visceral muscles,
which change their form independently of the will, as invol-
untary. The heart-muscle forms a sort of intermediate
link ; it is not directly attached to the skeleton, but forms
a hollow bag which drives on the blood contained in it and
that quite involuntarily; but in its microscopic structure it
resembles the skeletal voluntary muscles. The muscles of
respiration might perhaps be cited as another intermediate
group. They 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 voluntarily hold his breath
long enough to suffocate himself. As we 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 involun-
tary are not good ones. The functional differences be-
tween the two groups depend really more on the nervous
connections of each, than ujion 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
consists of a red soft central part, called the belly, which
tapers at each end and there passes into one or more dense

118 THE HUMAN BODY.

white cords wliich consist nciirly 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
hunj ur cartilaginous skeleton. In Fig. 48 is shuAvn the
bice2}s muscle of the arm, which lies in front of the humerus.
Its fleshy belly, Bh, is seen to divide above and end there
in two tendons, one of which, Bl, is fixed to the scapula,
wliile the other joins the tendon of a neighboring muscle
(Ihe coraco-hrachial) 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 shorten-
ing 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 attached, just as a pull at one end
of 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 that the muscle
would be torn by its own efforts. In the Body, however,
the two ends of a muscle are always attached to two differ-
ent pieces of the skeleton, between which more or less
movement is permitted, and so when the muscle pulls it
alters the relative positions of the parts to which its ten-
dons 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 contrarfs 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

MUSQLE.s OF ARM.

119

^-o>i

130

THE HUMAJf BOJjr.

elbow joints. Where a muscle passes over an articulation
it is nearly always reduced 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 ia
fixed is more easily 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 fore-
arm, bending the elbow-joint as shown in Fig. 49. The

iia. 40. — The biceps muscle and the arm -bones, to illustrate ho\v, imder ordi-
nary circumstances, the elbow joint is flexed when the muscle contracts.

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 insertion. It is clear, how-
ever, that this distinction in the mobility of the points of
fixation of the muscle is only relative for, by changing the
conditions, the insertion may become tlie stationary and
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

FORMS OF MUSCLE.

121

single tendon at each end as a, Fig. 50; but others divide
at one end and are called two-headed or bleeps 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 right up to the point of attachment;
and some have no tendon at either end. In many muscles
a tendon runs along one side and the fibres of the belly are
attached obliquely to it: such muscles {b,
Fig. 50) are coXieA ^^enniform or feath-
er-like; or a tendon runs obliquely down
the middle of the muscle and has the
fibres of the belly fixed obliquely on
each side of it (c, Fig. 50), forming a
hipennifonn muscle: or even two ten-
dons may run down the belly and so form
a tripe im if orm muscle. In a few cases
a tendon is found in the middle of the

terminal teudons;

belly as well as at each end of it; such S^^'-.^oukf not/™n
muscles are called digastric. A muscle uninterruptedly along

'' . Its whole edge as repre-

Of this form (Fig. 51) is found in con- seuted in the figure); c,
. , T I . T • ^ bipennirorm muscle.

nection with the lower jaw. It arises
by a tendon attached 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 directed upwards
towards the chin, where it ends in a tendon
inserted into the lower jaw. Eunning along
the front of the belly from the pelvis to the
chest is a long muscle on each side of the
middle line called the rectus ahdominis : it
is polygastric, consisting of four bellies sep-
arated by short tendons. Many muscles moreover are
not rounded but form wide flat masses, as for example
the muscle Ss seen on the ventral side of the shoulder-blade
in Fig. 48.

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

c

Fig. 51. — a di-
gastric muscle.

123 THE HUMAN BODY.

consists of a proper striped muscular tissue. Avliich is its es-
sential part, but which is supported by connective tissue,
nourished by blood-vessels and lymphatics, and lias its ac-
tivity 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
penmysium, envelops each muscle, and from this parti-
tions run in and subdivide the belly into hnndhs or fasci-
culi which run from tendon to tendon, or for the whole
length of the muscle when it has no tendons. Tlie coarse-
ness or fineness of butcher's moat depends 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 (as shown in Fig. 52,
which represents a few pri-
mary fasciculi of a muscle
and the secondary fasciculi
into which these are di-
vided), each of which cou-
^ sists of a certain number of

Fig. 52.— a small bit ol muscle com-

posed of three pnmaiT fasciculi. A, mUSCUkir JlOreS DOUnd to-

natural size; B. the same magnified, ii t r.

showing the secondary fasciculi of which gether by TCry fine COn-

the primary are composed. nectivB tissue and envel-

oped in a close network of microscopic blood - vessels.
Where a muscle tapers the fibres in the fasciculi become
less numerous, and when a tendon is formed disappear
altogether, leaving little but the connective tissue.

Histology of Muscle. For the present we need only
concern ourselves with the muscular fibres. Each of these is
from eight to thirty-five millimeters (^ to 1\ inch) long,
but only from 0.034 to 0.055 mm. (y|o to j\^ inch) in*
diameter in its widest part, and tapering to a point at each
end. Hence in long muscles witli terminal tendons, no
fibre runs the whole length of a fasciculus, which may be a
foot or more long, but the fasciculus is made up of many
successive fibres, the narrow end of each fitting in between
the ends of those which follow it. In short or penniform

PLAIN MUSCULAR TISSUE.

123

muscles, where the fasciculi are short, the fibres may run
the whole length of each of the latter.

Examined carefully with a good microscope each fibre
is seen to possess a very thin homogeneous sheath or envel-
ope, called the sarcolermna, within which lies the contrac-
tile portion of the fibre, b, which presents a striped appear-
ance as if composed of darker and dimmer alternating
bands (Fig. 53). During life this substance is very soft or
semifluid, but after death it rapidly solidi-
fies and death-stiffening, or rigo?- mortis, is
produced. Besides the contractile sub-
stance a number of oval nuclei, each sur-
rounded by a little unmodified protoplasm,
lie inside the sarcolemma. The latter is
imperforate except at one point where the
central portion (or axis cylinder, see Chap.
XII.) of a nerve-fibre penetrates it, and
ends in an expansion or end plate which is
in immediate contact with the striated sub-
stance.

The larger blood-vessels of a muscle run
in the coarser partitions of the connective
tissue, and the finer ones lie close around
each fibre but entirely outside its sarcolem-
ma.

Structure of the Unstriped Muscles.
Of these the muscular coat of the stomach
(Fig. 54) is a good example. They have no
definite tendons but form expanded mem-
branes surrounding cavities, so that they have
no definite origin or insertion. Like the
skeletal muscles they consist of proper contractile element.-^,
with accessory connective tissue, blood-vessels, and nerves.
Their fibres, however, have a very different microscopic
structure. They present no striation but are made up of
elongated cells (Fig. 55), bound together by a small quan-
tity of cementing material. Each cell is flattened in one
plane and tapers off at each end; in its interior lies an
elongated nucleus with one or two nucleoli. These cells

FiG.53.— Asmall
piece of muscular
fibre highly mag-
nified. At a the
fibre has been
crushedandtwist-
ed so as to tear
its contents while
the tougher sar-
colemma, e 1 s e-
where so closely
applied to the rest
as to be invisible,
remains iintorn
and conspicuous.

124

THE HUMAN BODY.

Fig. 54.— The muscular coat of the stomach.

have the power of shortening in the direction of their long

axis, and so of diminishing the capacity of the cavities in

the walls of which they lie.

Cardiac Muscular Tissue. This con-
sists of flattened branched cells which unite
to form a network, in the interstices of
which blood capillaries and nerve-fibres run.
The cells present transverse striations, but
not so distinct as those of the skeletal mus-
cles, and are said to have no sarcolemma.

The Chemistry of Muscular Tissue. The
chemical structure of the living muscular
fibre is unknown, since all the methods of
chemical analysis at present discovered de-
compose and kill it. It contains 75 per
cent of water; and, among other inorganic
constituents, phosphates and chlorides of
potassium, sodium, and magnesium. When
at rest a living muscle is feebly alkaline, but
after hard work, or when dying, this reaction
is reversed through the formation of sarco-
lactic acid (CaHeO^). Muscles contain small

quantities of grape sugar and glycogen, and of organic

Fig. 55.-Unstriped
muscle-cells.

GHEMISTRT OF MUSCLE. 125

nitrogenous crystalline compounds, especially kreatin
(C4H9N3OJ). As in the case of all other physiologically ac-
tive tissues, however, the main post mortem constituents of
the muscular fibres are proteid substances, and it is proba-
ble that like protoplasm itself (p. 34) the essential con-
tractile part of the tissue consists of a complex body con-
taining proteid, carbohydrate and fatty residues; and that
during muscular work this is broken up yielding proteids,
carbon dioxide, sarcolactic acid, with probably other
things; for this hypothetical substance, which has never
yet been isolated, the name inogeii has been proposed.
The main proteid substance obtained from muscles is that
known as myosin, which is prepared rs follows. Perfectly
fresh and still living muscles are cut out from a frog which
has just been killed by destruction of its brain and spinal
cord, a proceeding which entirely deprives the animal of
consciousness and the power of using its muscles, but
leaves these latter unaltered and alive for some time. The
excised muscles are thrown into a vessel cooled below 0° C.
by a freezing mixture and so are frozen hard before any
great chemical change has had time to occur in them. The
solidified muscles are then cut up into thin slices, the bits
thrown on a cooled filter and let gradually warm up to tlie
freezing point of water, with the addition of some ice-cold
0.5 per cent solution of common salt. Gradually a small
quantity of a tenacious liquid filters through which is at
first alkaline to test-paper but soon sets into a jelly and
becomes acid. The coagulation and the acidity are due to
the breaking up of the muscle substance into the myosin
and other bodies referred to above. At first the jelly is
transparent, but soon the myosin becomes opaque and
shrinks just like blood fibrin, squeezing out a quantity of
muscle serum, and remaining itself as the muscle clot.
Myosin thus prepared is insoluble in water and in saturated
solution of common salt, but soluble in five or ten per cent
watery solutions of the latter substance. When boiled it
IS turned into coagulated proteid (p. 11) and becomes in-
soluble in dilute acids, in which the original ijiyosin was
soluble, being at the same time, however, converted into

1 2 6 THE E UMAN BOD T.

another proteid, called synto7iin, wliicli "vvas formerly con-
sidered to be tlie original proteid yielded by the muscles.
Syntonin is insoluble in water but soluble in dilute acids
and alkalies and its solutions are not coagulated by
boiliug.

Beef Tea and Liebig's Extract. From the above stated
facts it is clear that when a muscle is boiled in water its
myosin is coagulated and left behind in the meat: even if
cooking be commenced by soaking in cold water, the myosin
still remains as it is insoluble in cold water. Beef tea as
ordinarily made, then, contains little but the flavoring
matters and salts of the meat and some gelatin, the former
making it deceptively taste as if it were a strong solution
of the whole meat, whereas it contains but little of the most
nutritious proteid portions, which in an insipid shrunken
form are left when the liquid is strained off. Various pro-
posals 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 myosin; or extracting it
with dilute acids which turn the myosin into syntonin and
dissolve it, at the same time rendering it non-coagulable by
heat when subsequently boiled. Such methods, however,
make unpalatable compounds which invalids, as a rule, will
not take. Beef tea is a slight stimulant but hardly a food
and cannot be relied upon to keep up a jDatient's strength for
any length of time. Liebig's extract of meat is essentially
a very strong beef tea; containing much of the flavoring
substances of the meat, nearly all its salts and the crystal-
line nitrogenous bodies, such as kreatin, which exist in
muscle, but hardly any of its really nutritive joarts. From
its stimulating eflects 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 erroneousness of
the common belief that it is a highly nutritious food can-
not be too strongly insisted upon. Under the name of
liqiiid extracts of meat other substances have been prej^ared

LIQUID EXTRACT OF MEAT. 137

by subjecting meat to chemical processes in which it un-
dergoes cliauges smiilar to those experienced in digestion:
the myosin is thus rendered sohible in water and uncoagu-
lable by heat, and such extracts if properly prepared are
highly nutritious. The flavor may be imi^roved by adding
a little ot Liebio-'s extract if desired.

CHAPTER X.

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 fibres of shortening
forcibly under certain circumstances. The direction in
which this shortenmg occurs is always that of the long axis
of the fibre, m 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
becomes harder and more rigid, especially if it has to over-
come 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 another 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 be-
neath the skin, are made very conspicuous when violent
effort is represented, so as to indicate that they are in vigor-
ous action. In a muscular fibre we find no longer the slow,
irregular, and indefinite changes of form seen m the undif-
ferentiated cells of early development; this is replaced
by a precise, rapid, and definite change of form m one di-
rection only. Muscular tissue represents a group of cells
in the bodily community, which have taken up the one spe-
cial duty of executing changes of form, and m proportion

MUSCULAR IRRITABILITY. 129

as they have fewer other things to do, they do that better.
This contractility of the muscular fibres may be best con-
ceived by considering each to possess two natural shapes;
one, the state of rest, in which the fibres are long and nar-
row; and the other the state of activity, in which they are
shorter and thicker: under certain conditions the fibres
tend to pass, with considerable force, from their resting to
their active form, and in so doing they move parts attached
to their tendons. When the state of activity passes off the
fibres suffer themselves to be passively extended again by
any force pulling upon them, and they so regain their rest-
ing shape; and since in the living Body other parts are
nearly invariably put upon the stretch when any given
muscle contracts, these by their elasticity serve to pull the
latter back again to its primitive shape. No muscular
fibre is known to have the power of actively expanding after
it has contracted: in the active state it forcibly resists ex-
tension, but once the contraction is over it suffers itself
readily to be pulled out to its resting form.

Irritability. With that modification of the primitive
protoplasm of an amoeboid cell which gives rise to a mus-
cular fibre, with its great contractility, there goes a loss of
other properties. All trace of spontaneity seems to disap-
pear; muscles are not automatic like native protoplasm or
ciliated cells; they remain at rest unless directly excited
from without. The amount of external change required
to excite the living muscular fibre at any moment is, how-
ever, very small; in other words, it is highly irritable, a
very little thing being sufficient to call forth a powerful
contraction. In the living Human Body the exciting force,
or stimulus, acting upon a muscle is almost invariably a
nervous impulse, a molecular movement transmitted along
the nerve-fibres attached to it, and upsetting the equili-
brium of the muscle. It is through the nerves that the will
acts upon the muscles, and accordingly injury to the nerve
of a part, as the face or a limb, will cause 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 permanently idle. Although a

130 THE HUMAN BODY.

nervous impulse is the natural physiological muscular
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 tlirough it, or a drop of glycerine 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 ef-
ficacy of all of them is that they shall act with some sud-
denness; a very slowly increased pressure, even if ultimately
very great, or a very slowly raised temperature, or a slowly
increased electrical current passed tlirough it, will not ex-
cite the muscle; although far less pressure, warmth, or
electricity, more rapidly applied would stimulate it power-
fully. It may perhaps still be objected that it is not proved
that any of these stimuli excite the muscular fibres, 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. XIII), and when we ap-
ply 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 fibres.
That the muscular fibres have a piu]ier irritability of
their own, independently of their nerves, is, however, shown
]>y the action of certain drugs — for example curari, 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 snfl'ocation
because of the cessation of the breathing movements. But
the poison does not really act on the muscles themselves:
it kills the muscle nerves, but leaves the muscle intact; and
it kills the very endings of the muscle-nerves right down
\\\ the muscle fibres themselves. Now after its administra-
tion we still find that the various non-physiological stimuli
referred to above make the muscles contract just 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

MUSCULAR IBBIT ABILITY, 131

all their nerve-fibres have been i^oisoned. The experiment
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
l^low and arouses the muscle to manifest a property which
it already possesses. The older physiologists seeing that
muscular paralysis followed when the nervous connection
l^etween a muscle and the brain was interrupted, 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 sjnrUs, and that some of
this was ejected from the brain along the nerve to the
muscle, when the 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 fibre is a collection of highly irrita-
ble material which can have its equilibrium upset in a
definite way, causing it to change its shape, under the influ-
ence of slight disturbing forces, one of which is a ner-
vous impulse ; and since • in the Body no other kind of
stimulus usually reaches the muscles, they remain at rest
when their nervous connections arc severed. But the
muscles paralyzed in this way can still, in the living Body,
be made to contract by sending electrical shocks through
them. Physiologically, then, muscle is a contractile and
irritable, but not automatic 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 animal, 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 ani-
mals 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 it
rapidly shortens and then, if a weight be hanging on it.
rapidly lengthens again. The whole series of phenomena

132 THE HUMAN BODY.

from the moment of stimulation until the muscle regains
its resting form is known as a ' • simple muscular contractioji'
or a " tivitch.'' It occupies in the frog about one tenth of
a second and is separable into three portions. First, there
elaspes a time, after the stimulation and before the com-
mencement of the shortening, which is known as the "lost
time" or the jieriod of latent excitement. This lasts about
one hundredth of a second, and represents the time during
whicii molecular changes preparatory to the contraction are
taking place in the muscle fibres. Then follows the short-
ening, at first slow, then rapid, then slower again up to a
maximum, and occupying rather more than half of the re-
maining time; the elongation occupies the remainder of the
time taken up in the contraction. In warm-blooded ani-
mals, the duration of a simple muscular contraction is even
less than one tenth of a second and all its stages are quick-
ened. In any given animal, cold increases the time taken
in a muscular contraction and also impairs the contractile
power, as we find in our own limbs when " numbed" with
cold. Moderate warmth on the other hand (up to near
the point at which heat-rigor is produced) diminishes the
duration of the contraction; so that the molecular changes
in a muscular fibre are clearly eminently susceptible to slight
changes in its environment. The contractility of a muscle
does not depend upon a vital force, entirely distinct from
ordinary inanimate forces, but upon an arrangement of its
material elements, which is only maintained under cer-
tain conditions and is eminently modifiable by the sur-
roundings.

Physiological Tetanus. It is obvious that the ordinary
movements of the Body are not brought about by such tran-
sient muscular contractions as those described in the last
paragraph. Even a wink lasts longer than one tenth of a
second. Our movements are, m fact, due to more prolonged
contractions which may be described as consisting of several
simple twitches fused together, and known as " tetanic
co7itr actions"; 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

MUSCULAR CONTRACTION. 133

in strychnine jjoisoning and hydrophobia, in which most of
the muscles of the Body are thrown into prolonged and
powerful involuntary contractions.

If, while a frog's muscle is still shortening under the in-
fluence 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 con-
tracting 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 possible to it for that
strength of stimulus. If now the stimuli be repeated at
the proper intervals, each new one will hot 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 maintained, by continuing the stimuli, until
the organ begins to get exhausted or *'' fatigued'' and it
then commences to elongate in spite of the stimulation.
When our muscles are stimulated in the Body, from the
nerve-centres through the nerves, they receive from the lat-
ter about 20 stimuli in a second, and so are thrown into
tetanic contractions. In other words, not even in tlie
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. AVhen 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 so
much us rapidly repeated stimuli of the same strength —
since in the latter case we get, as already explained, scA^eral
simple contractions such as a single stimulus would call
forth, piled on the top of one another. With very ]iower-
ful repeated electrical stimuli a muscle can be made to

134 THE HUMAN BODY.

shorten to one third of its resting length, but in the Body
the strongest effort of the will never produces a contrac-
tion of that extent. Apart from the rate of stimulation,
the strength of the stimulus has some influence, a greater
stimulus causing a greater contraction, but very soon a
point is reached after which increasing the stimulus pro-
duces 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 influ-
ence, 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 milli-
meters {^j inch). Then if we hang one gram (15.5 grains)
on it and give it the same stimulus, it will be found to con-
tract 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 sliortening, puts it into
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 tlie blood be going
on in it at the same time. A frog's muscle cut out of the
body will, however, be considerably restored by a period of
rest, even although no blood flows through it.

The Measure of Muscular Work. The work done by
a muscle m a given contraction, when it lifts a weight verti-
cally against gravity, is measured by the weight moved, mul-
tiplied by the distance through which it is moved. In the
above case when the muscle contracted carrying no load it
did very little work, lifting only its own weight; when
loaded with one gram and lifting it five millimeters it did
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.
Jlven after the weight becomes so great that it is lifted

MUSCULAR WORE. 135

through a less distance, the work done by the muscle goes
on increasing, for the bigger weight lifted more than com-
pensates for the less distance through which it is raised.
For example, if the above muscle were loaded with fifty
grams it would maybe lift that weight only l.o millime-
ters, but it would then do seventy-five 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 docs less work, the lift becoming very
little indeed, until at last the weight becomes so great that
the muscle cannot lift it all and so does no work when
stimulated. Starting 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 Ihat, 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 wdiich 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, how-
ever, 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 stimulation 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.

Influence of the Form of the Muscle on its Working
Power. The amount of work that any muscle can do de-
pends of course largely u])on its physiological state; a
healthy well-nourished muscle can do more than a dis-
eased or starved one; but allowing for such variations the
work which can be done by a muscle varies with its form.
The thicker the muscles, th:it is the greater the number of

136 THE HUMAN BODY

fibres 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 Avliich a muscle can move a weight in-
creases with the length of its fasciculi. A muscle a foot m
length can contract more than a muscle six inches long, and
so would move a bone through a greater distance, jirovided
the resistance were not too 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 together. 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 fibres inserted obliquely
into it. The muscle {gastrocnemius) in the calf of the leg
for instance (Fig. 50, b) is really a short stout muscle, for
its working length depends on the length of its fasciculi
and these are shor.t and oblique, while its true cross-section
is that at right angles to the fasciculi and is very large. The
force with which a muscle can shorten is very great. A
frog's muscle of 1 square centimeter (0.39 inch) m section
can just lift 2800 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
between elasticity and contractility. Elasticity is a physi-
cal property of matter in virtue of which various bodies
tend to assume or retain a certain shape, and when re-
moved from it forcibly, to return to it. When a spiral
steel spring is stretched it will, if let go, "contract" in a
certain sense in virtue of its elasticity, but such a contrac-
tion is clearly quite different from a muscular contraction.
The spring will only contract as a result of previous distor-
tion; it cannot originate a chiinge of form, while the mus-

MUSCULAR ELASTICITY. 137

cle can actively contract or change its shape when a stimnlns
acts npon 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 differ-
ent thing from mere physical elasticity; the essential differ-
ence 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 Avork independ-
ently of any previous mechanical stretching. In addition
to their contractility, however, muscles are highly elastic.
If a 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 indian-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 oi
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 contraction, and this is
always provided for from the first, by having the origin and
insertion of the muscle so far apart as to be pulling on
it Avhen it begins to shorten.

Physiology of Plain Muscular Tissue. What has hith-
erto been said aj^plies especially to the skeletal muscles; but
m the main it is true of the unstriped muscles. These
also are irritable and contractile, but their changes of form
are much more slow than those of the striated fibres.
Upon stimulation, a longer period of latent excitement
elapses before the contraction commences, and when finally
this takes place it is extremely slow, very gradually attain-
ing a maximum and then gi-adually dying away again.

138 THE HU3IAN BODY.

There seems in fact to be some connection 1)etween that ar-
ranffement of tlie contractile substance Avliich sliows itself
under the microscope as striation and the power of rapid
contraction, since we find that the heart, which is not a
skeletal or voluntary muscle but yet one that contracts rap-
idly, agrees with these in having its fibres striated.

Hygiene of the Muscles, The healthy working of the
muscles needs of course a healthy state of the Body gener-
ally, so that they shall be supplied with proper materials
for growth and repair and have their wastes rajndly and
efiiciently 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 exj)osed to any considerable continued j^ressure since
this interferes with the flow of blood and lymph through
them.

As far as the muscles themselves are directly concerned,
exercise is the necessary condition of their best develop-
ment. A muscle Avhich is permanently unused degenerates
and is absorbed, little finally being left but the connective
tissue of the organ and a few muscle fibres filled with oil-
drops. This is well seen in cases of paralysis dependent
on injury to the nerves. In such cases the muscles at first
may themselves be perfectly healthy, but lying unused for
weeks they rapidly alter and,finally, when the nervous in-
jury has been healed, the muscles may be found incapa-
ble 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.
The same fact is illustrated by the feeble and wasted condi-
1 ion 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, l)y judicious and persistent exercise, that the
long idle muscles regain their former size and power. The
great muscles of the '' brawny arm" of tlie blacksmith or
wrestler illustrate the reverse fact, the growth of the mus-
cles by exercise. Exercise, however, must be judicious: re^

MUSCULAR EXERCISE. 139

peatedly continued until exhaustion it does harm; the period
of repair is not suthcient 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 be-
cause muscular work greatly increases the work of the heart,
not only because more blood has to be sent to the muscles
themselves, but they produce great quantities of carbon
dioxide which must be carried off in the blood to the lunsfs
for removal from the Body, and the heart must work harder
to send the blood faster through the lungs and at the same
time the breathing be ha.stened so as to renew the air in
those organs faster. The least evil result of throwing too
violent work on the heart and lungs in this way, is repre-
sented by being '*' out of breath," which is advantageous
insomuch as it may lead to a cessation of the exertion. But
much more serious, and sometimes permanent, injuries of
either the circulatory or respiratory organs may be caused
by violent and prolonged efforts without any previous train-
ing. 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 represented 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 true also of rowing, which when good is
performed much more by the legs than the arms: esjoecially
since the introduction of sliding seats. "Hence any of these
exercises alone is apt to leave the muscles of the chest and
arms imjierfectly exercised. Indeed no one exercise em-
ploys equally or proportionately all the muscles: and hence
gymnnasia in which various feats of agility are practiced,
so as to call different parts into play, have attained a great
popularity. It should be borne in mind howe\'er, that the
legs especially need strength; while the upper limbs, in
which delicacy of movement, as a rule, is more desirable
than power, do not require such constant exertion; and the

140 THE HUM AX BODY.

fact that gymnastic exercises are commonly carried on in-
doors is a great drawback to tlieir value. AVlien 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 introduced, since it not only demands gi'eat
muscular agdity in every part of the Body, but trains the
hand to work with the eye in away 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 con-
vey 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 exer-
cise 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 as much as it wishes; but unwise to tempt
it when disinclined. The bones and muscles are still fee-
ble and may be injured by too much work. The same is
true of commencing walking.

From four or five to twelve years of age almost any form
of exercise should be permitted, or even encouraged. At
this time, however, the epiphyses of many bones are not
firmly united to their shafts and so anything tending to

MUSCULAR EXERCISE. 141

throw too great a strain on the joints should be avoided.
After that up to commencing manhood or maidenhood any
kind of outdoor exercise for healthy persons is good, and
ofirls are all the better for being allowed to ioin in their
brothers' sports. Half of the debility and general ill-health
of so many of our women is dependent upon deficient ex-
ercise during childhood, and the day, which fortunately
seems approaching, which will see dolls as unknown to, or
as despised by, healthy girls as healthy boys, will see the be-
ginning of a great improvement in the stamina of the
female portion of our population.

Exercise in Youth should be regulated largely by sex;
not that women arc 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
adult woman render vigorous muscular work often unad-
visable, especially under conditions where it is apt to b(
followed by a chill.

A healthy boy or young man may do nearly anythin,^,
but until twenty-two or twenty-three very prolonged effort
is unadvisable. The frame is still not firmly knit or as
capable of endurance as it will subsequently become.

Girls should be allowed to ride or play out-door games
m moderation, and in any case should not be cribbed in
tight stays or tight boots. A flannel dress and pro])er
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 base-ball field. Rowing is excel-
lent for girls if there be any one to teach them to do it prop-
erly, with the legs and back and not with the arms only, as
women are so apt to row. Properly practiced 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 ef-
forts 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 dangerous, for with the rigidity of
the cartilages and blood-vessels which begins to show itself
about that time appears a diminished power of meeting a

142 THE IIl'MAK BODY.

sudden violent demand. On the other hand tlie man of
thirty would more safely than the lad of nineuen or twenty
undertake one of the long-distance walking matches which
have lately been in vogue; the jirolonged effort Avould be
less dangerous to him, though a six days' match with its
attendant loss of sleep cannot fail to be more or less dan-
gerous to any one. Probably for one engaged in active
business a Avalk of a couple of miles to it in the morning
and back again in the afternoon is the best and most avail-
able exercise. The habit which Americans have everywhere
acquired, of never walking when they can take a horse-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 ex-
ercise.

In Old Age the needful amount of exercise is less, and it
i> still moi'e important to avoid sudden or violent effort.

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

Training. If any person attempts some unusual exer-
cise he soon finds that he loses breath, gets perhaps a
" stitch in the side," and feels his heart beating with un-
wonted violence. If he perseveres he will probably faint —
or vomit.as is frequently seen in imperfectly trained men
at the end of a hard boat-race. These phenomena are
avoided by careful gradual preparation known as '"train-
ing." The immediate cause of them lies in disturbances
of the circulatory and respiratory organs, on which excessive
work is thrown, and the further discussion of training must
be postponed for the present.

CHAPTER XI.

MOTION AND LOCOMOTION.

The Special Physiology of the Muscles. Having now
considered separately the structure and properties in gen-
eral of the skeleton, the joints, and the muscles, we may
go on to consider how they all work togetlier in the Body.
The properties of a muscle for example are everywhere the
same, but the uses of different 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, and so on. The
determination of tiie exact use of any particular muscle is
known as its special pliijsiologij, as distinguished from its
general fliysiology, or pro^aerties as a muscle without refer-
ence to its use as a muscle in a particular place. The uses
of those muscles forming parts of the physiological mechan-
isms concerned in breathing and swallowing will be studied
hereafter; for the i^resent we may consider the muscles
which co-operate in maintaining postures of the Body; in
])roducing movements of its parts with reference to one
another; and in producing locomotion or movement of the
whole Bod}^ with reference to its environment.

In nearly all case the striped muscles carry out their
functions with the co-operation 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 ])one at the back of
the orbit, but is inserted, not into bone, but into the evelid

144 THE HUMAN BODY.

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
boiies, as for example the muscle [orhicularis oris) which
surrounds the mouth-opening and by its contraction nar-
rows it and purses out the lips; or the orbicularis palpe-
hrarum 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. 56) the
fulcrum or fixed point of support lies between the " weight"

Sli 1 _J^

Fig. 56.— a lever of the first order. 7=^, fulcrum; P. power; W, resistance or
weight.

or 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 " puwer-arm;" the
distance FW is the weight-arm. When power-arm and
weight-arm are equal, as is the case in the beam of an ordi-
nary pair of scales, no mechanical advantage is gained, nor
is there any loss or gain in the distance through which the
Aveight is moved. For every inch through Avhich P is de-
pressed, W Avill 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 propor-
tionate to the difference in the lengths of the arms. For
example if PF is twice as long as FW, then half a kilo-
gram applied at F will balance a whole kilogram at W, and

motio:n and locomotion. 145

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 move-
ments 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 vertebral column to the
occiput; the resistance is the excess in the weight of the
part of the head m 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 asleep
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 artic-
ulation are called into play. Another example of the em-
ployment 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 fulcrum; the weight is that of the trunk
acting as if all concentrated at its centre of gravity, which
lies a little above the sacrum and behind the hip-joints;
and the power is afforded by muscles passing from the thighs
to the front of the pelvis.

Levers of the Second Order. In this form the weiglit
or resistance is lietween the power and the fulcrum. The
power-arm PFh always longer than the weight-arm WF,
and so a comparatively weak force can overcome a consid-
erable resistance. But it is disadvantageous so far as re-
gards rapidity and extent of movement, for it is obvious
that when P is raised a certain distance IFwill be moved a less
distance in the same time. As an exam23le of the employ-

146

THE HUMAN BODY.

ment of such levers (Fig. 57) in tlie Body, we may take the
act of standing on the toes. Here the foot rejj resents the
lever, the fulcrum is at the contact of its fore part with

P

fT

-^

W

Fig. 57.— a lever of the second order. F. fulerum; P, power;
The arrows indicate the direction in which the forces act.

PF, weight.

the ground ; the weight is that of the Body acting down
through the ankle-joints at Ta, Fig. 58; and the power is
the great muscle of the calf acting by its tendon inserted
into the heel-bone {Ca, Fig. 58). Another example is
afforded by holding up the thigh when one foot is kept
raised from the ground, as in hopping on the other. Here
the fulcrum is at the hip-joint, the power is aj^plied at the

em en

Fif}. .'iS.— The skeleton of the foot from the outer side. Ta, surface with
which the leer-bones articulate ; Ca, the calcaneum into which the tendon itendo
Achillis) of the calf muscle is inserted ; Mb, the metatarsal bone of the fifth
diRit.

knee-cap by a great muscle {redus femoris) inserted there
and which arises from the pelvis; and the weight is that
of the whole lower limb acting at its centre of gravity,
which will lie somewhere in the thigh between the hip and

LEVERS IN THE BODY. 147

knee joints, that is between the fulcrum and the point of
application of the power.

Levers of the Third Order. In these (Fig. 59) 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 movement 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. 49*) into the radius
(and of another muscle not represented in the figure, the
hracliialis anticus, into the ulna), and the weight is that

']L

yr

'VfT

Fig. 58.— a lever of the third order. F, fulcrum ; P, power ; W, weight.

of the forearm and hand, with whatever may be contained
in the latter, acting at the centre of gravity of the whole
somewhere on the distal side of the point of application of
the power. In the Body the power-arm is usually very
short so as to gain speed and range of movement, the mus-
cles being powerful enough to still do their work in spite
of the mechanical disadvantage at which they are thus
placed. The limbs are thus made much more shapely than
would be the case were the jjower 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 co-operate.

The Loss to the Muscles from the Direction of their

Pull. It is worthy of note that, owing to the oblique direc-

— — * p. 120. ~

148

THE HUMAN BODY.

tion in which the muscles are coniniunly inserted into the
bones, much of their force is lost so far as producing move-
ment is concerned. Suppose the log of wood in the dia-
gram (Fig. 60) to be raised by pulling 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 line, the more useful will
be the direction of the pull, and the more of it will be spent
on the log and the less lost unavailingly in merely increas-
ing the pressure at the hinge. If we now consider the ac-
tion of the biceps (Fig. 49) in flexing the elbow-joint, we
see similarly that the straighter the joint is, the more of

Fig. 60.— Diagram illustrating the disadvantage of an oblique pull.

the pull of the muscle is wasted. Beginning with the arm
straight, it works at a great disadvantage, but as the fore-
arm is raised the conditions become more and more favor-
able to the muscle. Those who have practiced the gym-
nastic feat of raising one's self by bending the elbows when
hanging by the hands from a horizontal bar, know practi-
cally 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

POSTURES. 149

less; the reason being of course the more adA-untageous
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 short-
ening, the very time at which, we have just seen, it works
at most mechanical 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 working of the skeletal muscles,
strength being sacrificed for variety, ease, rapidity, extent,
and elegance of movement.

Postures. The term posture is applied to those posi-
tions of equilibrium of the Body which can be maintained
for some time> such as standing, sitting, or lying, com-
pared with leaping, running, or falling. In all postures
the condition of stability is that the vertical hne drav\'n
through the centre of gravity of the Body shall fall within
the basis of support afforded by objects with which it is m
contact; and the security of the postitre is proportionate to
the extent of this base, for the wider it is, the less is the
risk of the perpendicular through the centre of gravity fall-
ing outside of it on slight disj^lacement.

The Erect Posture. This is pre-eminently character-
istic of man, his whole skeleton being modified with refer-
ence 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 co-operation of many
muscles, co-ordinated by the nervous system. The influ-
ence of the latter is shown by the fall which follows a
severe blow on the head, which may nevertheless have frac-
tured no bone and injured no muscle: the "concussion" of
the brain, as Ave 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 centre of gravity of the whole adult Body hes in the
articulation between the sacrum and the last lumbar verte-
bra, and the perpendicular drawn from it will reach the

150

THE HUMAN BODY.

I,

7

I

ground between the two feet, within the basis of support
afforded by them. With the feet close together, however,
the i)osture is not very stable, and in standing we com-
monly 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 sway-
ing back and forward will be com pat i
ble with safety, and the greater the
lateral distance separating them, the
greater the lateral sway which is possi-
ble without 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 sol-
dier preparing for the bayonet exercise,
always commences by thrusting one
foot forwards 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 nearly 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 centre of grav-
ity falling rather behind them; so that
just as some muscular effort is needed
to keep the head from falling forwards,
FiGf. 61.— Diagram ii- some is needed to keep the trunk from

lustratiiig the muscles , ■>• ■> •> -i xxii- t

(drawn in thick black topplmg backwards at the hips. In a
^rb^'ehind ^thf YointI similar manner other muscles are called
^cHvi'Jy keep the^S into play at other joints: as between the
rigid and the Body erect, ygrtebral 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 other-
wise be balanced on its feet as a statue can. Beginning
(Fig. 61) at the ankle-joint, we find it kept stiff in standing

W

WALKI^'0. 151

by the combined and balanced contraction of the muscles
passing from 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: and 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 ver-
tebral column to it, in front of and behind the occipital
condyles.

Locomotion includes all movements 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 commencement of each step, propels it.

Suppose a man standing with his heels together to com-
mence 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 cen-
tre of gravity would be thrown in front of the base formed
by the feet and a fall on the face result, were not simulta-
neously 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 comj)leted 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 prevented. MeaiiAvhile 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

U-i THE HUMAN BODY.

cleariug the groimd as the left did before. The Body is
meanwhile sni)ported on the left foot alone, hut wiien the
right completes its steptlie knee of that leg is straightened
and the foot thus placed, heel first, on the ground. Mean-
Avhile the left foot has been gradually leaving the gi'ound,
and its toes alone are at that moment upon it: from these
a push is given, as before Avith the right foot, and the knee
being bent so as to raise the foot, the left leg swings for-
wards at the hij)-joint to make a fresh step.

During each step the Avhole Body sways up and down
and also from side to side. It is highest at the moment
Avhen the advancing trunk is yertically over the foot sup-
porting it, and then sinks until the moment when the ad-
Tancing foot touches the ground, when it is loAvest. 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 SAvay, commonly
more marked in women than in men, and which when ex-
cessive produces an ugly "waddling" gait.

The length of each step is primarily dependent on the
length of the legs; but can be controlled within wide lim-
its by special muscular effort. In easy walking, little mus-
cular work is employed 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 pendulum; but in fast walking
the muscles jjassing in front of the hijj-joint, from the pel-
vis 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 SAving
freely, and since a short pendulum SAvings faster than a
longer, the natural step of short-legged people is quicker
than that of long-legged ones.

In fast Avalking the advanced or supporting leg also
aids in projjulsion; the muscles passing in front of the
ankle-joint contracting so as to pull the Body forwards
OA'er that foot and aid the push from the rear foot. Hence
the fatigue and pain in front of the shin Avhich is felt in

RUNNING AND LEAPING. 153

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 mo-
ment in each step when both feet are off the ground, the
Body being unsupported in the air. The toes alone come in
contact with the ground in each step, and the knee-joint
is not straight when the foot reaches the ground. When
the rear foot is to leave the support, the knee is suddenly
straightened, and at the same time the ankle-joint is ex-
tended 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
lag m each step is violently pulled forwards and not suf-
fered to swing naturally as in walking. By this the rapid-
ity 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 straighten-
ing of the knee of the rear leg just before it leaves the
ground.

Leaping. In this mode of progression the Body is
i-aised 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 centre 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 propul-
sion forwards; much m the same way as a ball rebounds
from a wall. The arms are at the same time swung for-
wards. In leaping back, the Body and arms are Inclined in
that direction; and in jumping vertically there is no lean-
ing either way and the arms are kept by the sides.

CHAPTER XII.

ANATOMY OF THE NERVOUS SYSTEM.

Nerve-Trunks. In dissecting the Human Body nnmer-
ous white cords are found which at tirst sight might be
taken for tendons. That they are something else however
soon becomes clear, since a great many of them have no
connection with muscles at all, and those which have usually
enter somewhere into the belly of the muscle, instead of be-
mg fixed to its ends as most tendons are. These cords are
nerve-trunks: followed in one direction each (Fig. 62) will
be found to break up into finer and finer branches, until
the subdivisions become too small to be followed without
the aid of a microscope. Traced the other way the trunk
will in most cases be found to increase by the union of
others with it, and ultimately to join a much larger mass
of different structure, and from which other trunks also
spring. This mass is a nerve-centre. That end of a nerve
attached to the centre is naturally its central, and the other
its distal or 'peripheral end. Nerve-centres, then, give origin
to nerve-trunks; these latter radiate all over the Body,
usually branching and becoming smaller and smaller as
they proceed from the centre; they finally become very
small, and hoAv they iiltimately end is not in all cases cer-
tain, but it is known that some have sense-organs at their
terminations and others muscular fibres. Tlie general ar-
rangement of the larger nerve-trunks of the Body is shown
in Fig. 62. Physically a nerve is not so tough or strong as
a tendon of the same size; it may readily be split up into
longitudinal strands, each of whicli consists of a number
of microscopic threads, the nerve-fibres, bound together by
connective tissue.

AJVA TOM Y OF NEB VO US S TSTEM. 1 5 o

Fig. 62.— Diagrram illustrating the g'eneral arrangement of the nervous system.

156 THE HUMAN BODY.

Plexuses. Very fn'(nu'ntly several ncigliboring nerve-
trunks send off comimiiiicating branches to one another,
each brunch carrying libres from one trunk to the olhei-.
8uch networks are called j^jfen/.ses (Fig. 05*), and through
the interchanges taking place in them it often happens
that the distal branches of a nerve-trunk contain fibres
which it does not possess as it leaves the centre to which it
is connected.

Nerve-Centres. The great majority of the nerves take
their origin from the hrain and sjnnal cord, which together
form the great cerebro-sjnnal centre. Some, however, com-
mence in rounded or oval masses which vary in size from that
of the kernel of an almond down to microscopic dimensions,
and which are widely distributed in the Body. Each of
these smaller scattered centres is called a ganglion, and the
whole of them are arranged in three sets. A considerable
number of the largest are united directly to one another by
nerve-trunks, and also give off nerves to various organs,
especially to the blood-vessels and the viscera in the thoracic
and abdominal cavities. These ganglia and their branches
form the sympathetic nervous system, as distinguished from
the cerebro-si^inal nervous system consisting of the brain
and spinal cord and the nerves springing from them. Of
the remaining ganglia some are connected with various
cerebro-s2nnal trunks near their origin, while the rest, for
the most part very small and connected with the peripheral
branches of sympathetic or other nerves, are known as the
sporadic ganglia.

The Cerebro-Spinal Centre and its Membranes. Ly-
ing in the skull is the hrain and in the neural canal of the
vertebral column the sjnnal cord or spinal viarrow, the
two being continuous through i\\Q forameii magnum of the
occipital bone and forming the great cerebro-spinal nerve-
centre. This centre 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;
the connective tissue which pervades them being of the deli-
cate retiform variety; accordingly both are placed in nearly

*P. 162.

MEMBRANES OF THE XERVE-CENTRES. 15?

completely closed bony cavities and
are also enveloped by membranes
wliicli give tliem consistency and
support. These membranes are
three in number. Externally is the
dura mater, very tough and strong
and composed of white fibrous and
elastic connective tissues. In the
cranium this dura mater adheres
by its outer surface to the inside of
the skull, 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 sur-
rounding bones, which have a sep-
arate ]3eriosteum of their own. The
innermost membrane of the cerebro-
spinal centre, lying in immediate
contact with the proper nervous
parts, is the j*;ia mater, also made
up of white fibrous tissue inter-
woven with elastic fibres, but less
closely than in the dura mater, so
as to form a less dense and tough
membrane. The pia mater con-
tains many blood-vessels which
break up in it into small branches
before entering the nervous mass
beneath. Covering the outside of
the pia mater is a layer of flat
closely fitting cells, a similar layer
lines the inside of the dura mater,
and these two layers are described
as the third membrane of the cere-
bro-spinal centre, called the arach-

. , T j_i 1 ; ,1 FiR- 63.— The spinal cord

noia. in the space between the an.i meduiia oblongata, a.

(.,„„ 1 „ J? J.1 1 -1 ■ from the ventral, and 5, from

two layers of the arachnoid is con- tue dor.sai aspect ; c to h.
tained a small quantity of watery fevlfs.'^""'"'^ ^^ ^^'''''^

158 THE HUMAN BODY.

cerebro-spi7ial liquid. Part of the surface of the brain is
folded and the pia mater does not dip down and line the
furrows between tlie folds but stretches across them: in the
spaces thus left there is also contained some of the cerebro-
si)inal liquid.

The Spinal Cord (Fig. 03) is nearly cylindrical in form,
being however a little Avider from side to side than dorso-
Tentrally, and tapering off at its posterior end. Its aver-
age 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 sijinal
cord and the brain, the one passing gradually into the
other (Fig. 70*), but the cord is arbitrarily said to com-
mence opposite the outer margin of the foramen magnum :
from there it extends to the articulation between the first
and second lumbar vertebraB, where it narrows off to a
slender filament, the filum terminaU (cut off and rej)re-
sented separately at B' in Fig. 63), which runs back to the
end of the neural canal behind the sacrum. In its course
the cord presents two expansions, an upper, 10, the cer-
vical enlargement, reiichiugiTom. the third cervical to the
first dorsal vertebrae, and a lower or lumhar enlargement,
9, opposite the last dorsal vertebra.

Eunning along the middle line on both the ventral and
the dorsal aspects of the cord is a groove, and a cross-sec-
tion shows tl\at these grooves are the surface indications of
fissures which extend deeply into the cord {C, Fig. 64) and
nearly divide it into right and left halves.

The anterior fissure (1, Fig. 64) is wider and shallower
than the posterior, 2. The transverse section, C, shows
also that the substance of the cord is not alike throughout,
but that its tvJnte superficial layers envelop a central gray
substance arranged somewhat in the form of a capital H.
Each half of the gray matter is crescent-shaped, and the
crescents are turned back to back and united across the
middle line by the grag commissure. The tips of each
crescent are called its horns or cornua, and the ventral, or
anterior cornu, on each side is thicker and larger than the
posterior. In the cervical and lumbar enlargements the

*P. 169.

SPINAL CORD.

159

proportion of white to gray matter is greater than else-
where ; and as the coi-d approaches the meduUa 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 por-

FiG. 64.— The spinal cord and nerve-roots. A. ti small portion of the cord
seen from the ventral side ; B, tlie same seen laterally ; C, a cross-section of the
cord ; D, the two roots of a spinal nerve ; 1, anterior (ventral) fissure ; 2. poste-
rior (dorsal) fissure ; 3, surface groove along the line of attachment of the ante-
rior nerve-roots ; 4, line of origin of the posterior roots : 5, anterior root fila-
ments of a spinal nerve ; 6, posterior root filaments : 6'. ganglion of the poste-
rior root ; 7,7,' the two primary divisions of the nerve-trunic formed by the
miion of the two roots.

tions: one between the anterior fissure and the anterior
cornu, and called the anterior white column ; one between
the posterior fissure and the posterior cornu, and called the
posterior iclrite column; while the remaining one lying in
the hollow of the crescent and between the two horns is the
lateral column. In addition to this a certain amount of
white substance crosses the middle line at the bottom of

160 THE HUMAN BODY.

the iinterior fissure; tliis forms tlie (inferior tvliite commis-
sure. There is no jiosterior white commissure, the bottom
of the posterior fissure being the only portion of the cord
where tlie gray substance is uncovered by white. Running
along the middle of the gray commissure, 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 cen-
tralis).

The Spinal Nerves. Thirty-one pairs of spinal nerve-
trunks enter the neural canal of the vertebral column
through tlie intervertebral foramiiia (p. 71). Each di-
vides in the foramen into a dorsal and ventral portion
known respectively as the posterior and anterior roots of
the nerve (6 and 5, Fig. 64), and these again subdivide into
finer branches which are attached to the sides of the cord,
the posterior root at the point where the posterior and late-
ral Avhite columns meet, and the anterior root at the junc-
tion of the lateral and anterior columns. At the lines on
which the roots are attached there are superficial furrows on
the surface of the cord. On each posterior root is a spinal
ganglion (6', Fig. 64), placed just before it joins the an-
terior root to make up the common nerve-trunk. Imme-
diately after its formation by the mixture of fibres from
both roots, the trunk divides into a small posterior prima rp
and a larger anterior ptrimary hrancli (7' 7 D, Fig. 64).
The former branches of the spinal nerves go for the most
part to the skin and muscles on the back, while the anterior
primary branches form a series of plexuses from which the
nerves for the sides and ventral region of the neck and
trunk, and for the limbs, arise.

The various spinal nerves are named from the jiortions
of the vertebral column through the intervertebral foramina
of which they pass out; and as a general rule each nerve is
named from the vertebra in front of it. For example the
nerve passing out between the fifth and sixtli dorsal verte-
brae is the "fifth dorsal" nerve, and that between the last
dorsal and first lumbar vertebrfe, the "twelfth dorsal."
In the cervical region, however, this rule is not adhered to.
The nerve passing out between the occipital bone and the

THE SPINAL XERVES. lOl

atlas is called the " first cervical" nerve, that between the
atlas and axis the second, and so on; that between seventh
cervical and first dorsal vertebra? being the "eighth cervi-
cal " nerve. The thirty-one pairs of spinal nerves are then
thus distributed: 8 cervical, 13 dorsal, 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 vertebrae 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
oif, occupied by a great bunch of nerve-roots forming the
so-called " horse's tail" or cauda equina.

Distribution of the Spinal Nerves. It would be out
of place here to go into detail as to the exact portions of
the Body supplied hj each spinal nerve, but the following
general statements may be made. The anterior prinuiry
branches of the first four cervical nerves form on each side
the cervical plexus (Fig. 65) from which branches are sup-
plied to the muscles and integument of the neck: also to
the outer ear and the back part of the scalp. The anterior
primary branches of the remaining cervical nerves and the
first dorsal form the bi'achial 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,
small branches arise and unite to make the phrenic nerve
(4, Fig. 65) which runs down through the chest and ends
in the diaphragm.

Tlie anterior primary branches of the dorsal nerves, ex-
cept part of the first which enters the brachial plexus, form
no plexus, 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 anterior primary branches of the four anterior lum-
bar nerves are united by branches to form the lumbar

162

TEE HUMAN BODY.

plexus. It snpi)lies the lower part of the trunk, the but-
tocks, the front of llie thigh, and medial 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 into one 'great cord and so form the

Fig. 65.— The cervical and brachial plexuses of one side of the Body.

xeiatic nerve, which is the largest in the Body and, running
down to 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.

THE BRAIN.

163

The Brain (Fig. 66) is far larger tlian the spinal cord
and more complex in structure. It weighs on the average
about 1415 grams (50 ounces) in the adult male, and about
155 grams (5.5 ounces) less in the female. In its simpler
forms the vertebrate bram consists of three masses, each
with subsidiary parts, following one another in series from
before back, and known as the fore-brain, mid- brain, and
hind-brain respectively. In man the fore-brain, A, weighing

Fig. 66.— Diagram iUustrating the general relationships of the parts of the
brain. A, fore-braiu ; b, mid-brain ; B. cerebellum : C, pons Varolii ; D, me-
dulla oblongata ; B, C, and D together constitute the hind-brain.

about 1215 grams (44: ounces), is much larger than all the
rest put together and laps over them behind. It consists
mainly of two huge convoluted masses, separated from one
"another by a deep median fissure, and known as the cerebral
I/eniisjilieres. The immense proportionate size of these is
very characteristic of the human brain. Beneath each
cerebral hemisphere is an olfactory lobe, inconspicuous in
man but often larger than the cerebral liemisiDheres, as in
most fishes. Buried in the fore-brain on each side are two
large gray masses, the corpora striata and optic thalami.
The mid-brain forms a connecting isthmus between the
two other divisions and presents on its dorsal side four

164

THE IIUMA^ BODY.

hemispherical eminences, the corpora qnadrigemioia. On
its ventral side it exhibits two semicylindrical pillars (seen
iinder the nerve IV in Fig. 70* and known as the crura
cerebri. The hind-brain consists of three main parts: on
its dorsal side is the cerebellum, B, Fig. 66, consisting of a
right, a /(?//, and a median lobe ; on the ventral side is the
pons Varolii, C, Fig. 66, and behind the medulla oblongata,
D, Fig. 66, which is continuous with the spinal cord.

In nature the main divisions of the brain are not sepa-
rated so much as has been represented in the diagram for

Cb

Fig. 67.— The brain from the left side. Cb, the cerebral hemispheres forming
the main bulk of the fore-brain ; Chi, the cerebellum ; Mo, the medulla oblon-
gata ; P, the pons Varolii ; *, the fissure of Sylvius.

the sake of clearness, but lie close together as represented
in Fig. 67, only some folds of the membranes extending
between them; and the mid-brain is entirely covered in on
its dorsal aspect. Xearly everywhere 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 lower animals; and in the human species more
marked in the higher than in the lower races.

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

* P. 169.

CEREBRAL VENTRICLES.

1G5

its interior, a great part of its surface is also covei'ed with
it. By tlie external convolutions of the cerebellum and
the cerebral hemispheres the surface over which this gray
substance is spread is very much increased (see Fig. 68).

The Ventricles of the Brain. The minute central
canal of the spinal cord is continued into the brain and
exjaands there at several points into chambers known as the
ventricles. Entering the medulla oblongata it approaches
its upper surface and dilates into the fourtli ventricle,
which has a very thin roof, lapped over by the cerebellum.
From the front of the fourth ventricle runs a narrow pas-

Ccr

Fig. 68.— a vertical section across the cerebral hemispheres. Ccl^. the corpus
callosum ; IT, the anterior end of the riisrht lateral ventricle: the gray mass on
its exterior is the corpus striatum. On the left side the superficial gray matter
covei'iiig the convolutions is shaded.

sage (ifer a tciiio ad qiiartum vent ricul urn) Avhicli enters
another dilatation lying in the middle line near the under
side of the fore-brain (just above the two small rounded
masses seen between the nerves // and III in Fig. 70) and
known as the tliird ventricle. From the third ventricle
two apertures (the foramens of Monro) lead into the first
and second, or lateral ventricles, one of which lies in each

166 THE HUMAN BODY.

of the cerebral hemispheres. The front ends of these two
ventricles are seen in the vertical transverse section of the
brain reprcscnied in Fig. 68.

The ventricles contain a small amount of cerebrospinal
liquid and are lined by epithelium which is ciliated in
early life.

Note. A frequent cause of apoiilexy is a hemorrhage into
one of the lateral ventricles; the outpoured blood accu-
mulating and pressing upon the cerebral hemispheres their
functions are suppressed and unconsciousness produced.
When a person is found in an apoplectic fit therefore the
best thing to do is to leave him perfectly quiet until medi-
cal aid is obtained: for any movement may start afresh,
bleeding into the ventricle which had been stopped by clots
formed in the mouths of the torn blood-vessels.

Sections of the Brain. Having got a general idea of
the parts composing the brain, the best way to complete a
knowledge of its anatomy is to study sections taken in
various directions. Two such are given in Figs. 08 and GO.
Fig. 69 represents tlie right half of a vertical section of the
brain, taken from before back in the middle line and viewed
from the inner side. Above, the knife has passed between
the two cerebral hemispheres, in the longitudinal fissure,
without cutting either, and the convoluted inner surface of
the right one is seen. The sickle-shaped mass lower down,
CcV to Ccf represents the cut surface of a connecting band
of white nervous tissue called the corinis callosum, which
runs across the middle line from one cerebral hemisjsliere
to the other and puts them in communication. SI, the
septum lucidum, is a thin membrane which forms the inner
wall of the lateral ventricle of the hemisphere. Between
the two septa lucida on the sides (in the natural position
of the parts) and the corpus callosum above is inclosed a
narrow space known as the fifth ventricle. It is, however,
quite different from the remaining cerebral ventricles, not
being a continuation of the canalis centralis of the spinal
cord. Tlie space beneath the septum lucidum and the
back part of the corpus callosum is the third ventricle,
which, lying in the middle line, har^ been laid open in the

MEDIAX SURFACE OF TEE BRAIN.

167

section. It is deep from above down but narrow from side
to side. From its under side a prolongation runs down to
H, the pituitary lodij ; behind, the aqueduct of Sylvius,
A, is seen passing back from the third to the fourth ven-
tricle, Vq. At F3I is the aperture {foramen of Monro)
leading into the right lateral ventricle. Crossing the third
ventricle and putting the two halves of the fore-brain in
direct communication are three small commissures, Coa,

C,l8 Cam sM

Fig. 69.— The i-ight half of the brain as seen on its median side after a section
made througli the organ in the middle line. Vq, fourth ventricle ; Mo, medulla
oblongata ; P. pons Varolii ; II, optic nerve ; H, pituitary body ; Coa. ante-
rior commissure ; FM, foramen of 3Ionro leading from the third ventricle, in
the cavity of which the lower end of the line SM lies, to the right lateral ven-
tricle ; Com, soft commissure, running from side to side of the third ventricle,
divided ; Cop, posterior commissure ; Lq, corpora quadrigemina ; .-1, aque-
duct of Sylvius or iter a tertio ad qnartum ventriculum ; Cbl, cerebeUum ;
Ccl^-Ccl*, corpus callosum ; HI, septum lucidum.

Com, and Coj), known respectively as the anterior, the
median (or soft), and the joos^mor. The mass seen bound-
ing a great part of the side of the third ventricle and
united to its fellow by the soft commissure is the ojJtic
thalamus. Above the aqueduct is the small median body
Cn, called the pineal glajid, which contains no nervous
tissue, but has an interest as being, according to Descartes,

108 THE HUMAN BODY.

the seat of tlic soul. Beliiiul it come the coyjHira qnadri-
(jernlna, Lq, and above the fourth ventricle the cerehcllum,
Ohl, showing the primary and secondary fissures on its
surface which give its section a branched appearance known
as the arhor vitce. Mo is the medulla oUongdta, and P the
})ons A'arolii. The canalis centrahs of the spinal cord is
represented leading back from the fourth ventricle.

Fig. G8 represents a vertical transverse section of the
brain taken through the fore part of the corpus callosum
{CcV) and altogether in front of the third ventricle. It
shows the foldings of the cerebrum and its su})erficial layer
of gray substance; the anterior ends of the lateral ventri-
cles, VI, with a gray mass, the corpus striatum lying be-
neath and on the outer side of each. If the section had
been taken a little farther back the ojjtic tlialavii would
have been found reaching the floor of each ventricle.

The Base of the Brain and the 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. 70. H\\q first p) air, 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 02)tic nerves, II, spring from
the optic thalami and corjDora quadrigemina and, under
the name of the op)tic 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 opitic commissure (seen in
section at //' in Fig. 69) from which an optic nerve pro-
ceeds to each eyeball. Behind the optic commissure is
seen the conical stalk of the pituitary lody or hypojiliysis
cerebri {II in Fig. 69), and still further back a pair of
hemispherical masses, about the size of split peas, known as
the corpora alhicantia.

All the remaining cranial nerves arise from the hind-
brain. The third pair {motores oculi) arise from the front
of tlie pons A'arolii, and are distributed to most of the

THE CRANIAL NERVES.

169

muscles wliich move the eyeball and also to that which
lifts the iipiDcr eyelid. The four-sided space bounded by
the optic tracts and commissure in front and the third
pair of nerves behind, and having on it the pituitary body

Fig. 70.— The base of tlie brain. The cerebral hemispheres are seen over-
lapping all the rest. /, olfactory lobes ; //, optic tract passing to the optic
commissure from whicli the optic nerves proceed ; ///, the third nerve or jho/oc
oculi ; IV, the fourtli nerve or pathcticus ; V, the fifth nerve or trigeminalia ;
FZ, the sixth nerve or abducens ; F/J, the seventh or facial nerve or portio
dura ; VIII, the auditory nerve or portio viollis ; IX, the ninth or glosso-
phaiyngeal ; X, the tenth or pneumogastric or vagus : XI, the spinal acces-
sory ; XII, the hypoglossal ; ncl, the fii'st cervical spinal nerve.

and the corpora albicantia, lies beneath the third ventricle,
so that a probe puslied in there would enter that cavity.

The fourth pair of nerves, IV (pafJieiici), arise from the
front part of the roof of the fourth ventricle. From there.

170 THE HUMA^ BODY.

each curls around a cms 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, V {trigeminales), resemble tlic
spinal nerves in having two roots; one of these is much
larger than the other and possesses a ganglion (the Gasse-
rian ganglion) like the posterior root of a spinal nerve.
Beyond the ganglion the two roots form a common trunk
which divides into three main branches. Of these, the
ophfhcdmic 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 lining the
nose, and to the integument over it. The second division
{superior maxillary nerve) of the trigeminal gives branches
to the skin over the temple, to the cheek between the eye-
brow 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 of the mouth. The third division
{inferior maxillary) is the largest branch of the trigemi-
nal; it receives some fibres 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 thirds of the tongue, the lower teeth, the salivary
glands, and the muscles which move the lower jaw in mas-
tication.

The sixth pair of cranial nerves ( VI, Fig. 70) or ab-
dncentes arise from the posterior margin of the pons Va-
rolii, and each is distributed to one muscle of the eve-
ball.

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 eiglith pair {auditory nerves) arise close to the
facial. They are the nerves of hearing and are distributed
entirely to the internal ear.

The ninth pair {glossojjharyngeals), IX, arising close to

THE CRANIAL NERVES. 171

the auditories, are distributed to the mucous memhrune of
the pharynx, the posterior part of the tongue, and the
middle ear.

The tenth pair {pneumogastric nei'ves 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 2^air {spinal accessory nerves), XI, do not
arise mainly from the brain but by a number of roots at-
tached to the lateral columns of the cervical portion of the
spinal cord, between the anterior and posterior 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 pneumogastric nerves. Out-
side the skull it divides into two branches, one of which
joins the pneumogastric trunk, while the other is distrib-
uted to muscles about the shoulder. v^

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

Deep Origins of the Cranial Nerves. The points re-
ferred to above, at which the various cranial nerves ajipear
on the surface of the brain, are known as their superficial
origins. From them the nerves can be traced for a less or
greater way in the substance of the brain until each is fol-
lowed to one or more masses of gray matter, which con-
stitute its proper starting-point and are known as its deep
origin. The deep origins of all except the first and second
and part of the eleventh lie in the medulla oblongata.

The Ganglia and Communications of the Cranial
Nerves. Besides the Gasserian ganglion above referred
to, many others are found in connection with the cranial
nerves. Thus for example there is one on each of the
main divisions of the trigeminal, two are found on each
pneumogastric and two in connection with the glosso-
pharyngeal. At these ganglia and elsewhere, tlie various

172 Til?: HUMAN BODY.

nerves often receive brunclies from neighboring cr;inial or
spinal nerves, so that very soon after it leaves the brain
hardly any one remains free from fibres derived from other
trunks except the olfactory, optic, and auditory nerves.
This often makes it difficult to say from Avhcre the nerves
of a special part have come; for example, the nerve-fibres
going to the submaxillary salivary gland from the trigemi-
nal leave the brain first in the facial and only afterwards
enter the fifth; and many of the fibres going apparently
from the pneumogastric to tlie heart come originally from
the spinal accessory.

The Sympathetic System. The ganglia which form
the main centres of the sympathetic nervous system lie in
two rows {s, Fig. 2, and sy, Fig. 3), one on either side of
the bodies of the vertebrae. Each ganglion is united by a
nerve-trunk Avith the one in front of it, and so two great
chains are formed reaching from the base of the skull to
the coccyx. In tlie 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 re-
presented on p. 7 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.
Tliey are named from the regions of the vertebral column
near which they lie; there being three cervical, twelve
dorsal, four lumbar, and five sacral pairs.

Each symj^athetic ganglion is united by communicating
Iranches with the neighboring sjjinal nerves, and near the
skull with various cranial nerves also; while from the gan-
glia and their uniting cords arise numerous trunks, many
of which, in the thoracic and abdominal cavities, form
plexuses, from which in turn nerves are given off to the
viscera. These plexuses frequently possess numerous small
ganglia of their own; two of the most important are the
cardiac plexus which lies on the dorsal side of the heart,
and the solar plexus wliicli lies in the abdominal cavity and
supplies nerves to the stomach, liver, kidneys, and intes-
tines. Many of the sympathetic nerves finally end in the
walls of the blood-vessels of various organs. To the naked

HISTOLOGY OF NERVES. 1V3

eye they are commonly grayer in color than the cerebro-
spinal nerves.

The Sporadic Ganglia. These, for the most part very
minute, nerve-centres are found scattered in nearly all
parts of the Body. They are especially abundant in the
neighborhood of secretorv tissues and about blood-vessels,
while a very important set is found in the heart. Serves
unite them with the ccrebro-spinal and sympathetic cen-
tres, and probably many of them belong properly to the
sympathetic system.

The Histology of Nerve-Fibres. The microscope shows
that in addition to connective tissue and other accessory
parts, such as blood-vessels, the nervous organs contain tis-
sues peculiar to themselves and known as nerve-fibres and
nerve-cells. The cells are found in the centres only; while
the fibres, of which there are two main varieties known as
the wliite and the gray, are found in both trunks and cen-
tres; the white variety predominating in the cerebro-spinal
nerves and in the white substance of the centres, and the
gray in the sympathetic trunks and the gray portions of
the central organs.

If an ordinary cerebro-spinal nerve-trunk be examined
it will be found to be enveloped in a loose sheath of areolar
connective tissue, Avhich forms a packing for it and unites
it to neighboring parts. From this sheath, or perineurium,
bands of connective tissite penetrate the nerve and divide
it up into a number of smaller cords or funiculi, much
as a muscle is subdivided into fasciculi; each funiculus
has a sheath of its own called the neurilemma, composed
of several concentric layers of a delicate membrane, with-
in which the true nerve-fibres lie. These, which would be
nearly all of the white kind, consist of extremely delicate
threads, about 0.0125 millm. {-o-^xri) inch) in diameter, but
frequently of a length which is in proportion very great.
Each nerve-fibre in fact is contintious from a nerve-centre
to the organ in which it ends, so that the fibres, e.g. which
pass out through the sacral plexus and then run on through
the sciatic nerve and its branches to the skin of the toes, are
three to four feet long. If a perfectly fresh nerve-fibre

174

THE HUMAN BODY.

be examined with the microscope il presents the appear-
ance of a perfectly homogeneous glassy thread ; but soon it
acquires a characteristic double contour (Fig. 71) from the
coagulation of a portion of its substance. By proper treat-
ment Avith reagents three layers may be brought into view.
Outside is a tine transparent envelope (1, Fig. 72) called
the jjrimitive sheath ; inside this is a fatty substance, 2,

Fig. 71. — White nerve-fihres soon after removal from the Body and when they
have acqmred their double contour.

Fig. 72. — Diag-rani ilhi'^trating the structure of a ivhite or meduUated nerve-
fihre. 1, 1, primitive sheath ; 2, 2, medidlary sheath ; 3, axis cyhnder.

forming the medullarij sheath (the coagulation of which
gives the fibre its double border), and in the centre is a
core, the axis cylinder, 3, which is clearly the essential
part of the fibre, since near its ending the primitive and
medullary sheaths are frequently absent. At intervals of
about one millimeter (^ inch) along the fibre are found
nuclei. These are indications of the primitive cells which
by their elongation, fusion and other modifications have

HI8T0L0Q Y OF NER VE-CELLS. 175

built up the nerve-fibre, and around each there is a small
amount of unmodified jjrotoplasm. The medullary sheath
is interrupted half way between each j)air of nuclei at a
point, called the node, which answers to the original bound-
ary between the two cells. In the course of a nerve-trunk
its fibres rarely divide; when a branch is given off some
fibres merely separate from the rest, much as a skein of silk
miglit be separated out at one end into smaller bundles con-
iaining fewer threads.

Gray Nerve-Fibres. Some of these are merely white
fibres which near their j^eripheral ends have lost their me-
dullary sheaths; but others have no medullary sheath
throughout their whole course, consisting merely of an
axis cylinder (often longitudinally striated) and primitive
sheath, with nuclei. Such fibres are especially abundant in
the sympathetic trunks; and they alone are found in the
olfactory nerve. In the communicating branches between
the sympathetic ganglia and the spinal nerves both white
and gray fibres are found; the former being probably
cerebro-spinal fibres passing into the sympathetic system,
while the gray fibres originate in the sympathetic system
and pass into the spinal cord. Another class of gray nerve-
fibres may be called nerve-fihrils : they are extremely fine
and result from the subdivision of axis cylinders, close to
their final endings in many parts of the Body, after they
have already lost both primitive and medullary sheaths.
Many fine gray fibres exist in the nerve-centres.

The Histology of Nerve-Cells. So far as our knoAvl-
edge at present goes the only structures known with cer-
tainty to be connected with the central ends of nerve-fibres
are nerve-cells, and these latter may therefore be regarded
as the central organs of the nerve-fibres. However, many
nerve-fibres have not yet been traced into continuity with
nerve-cells, and possibly end in the centres in other ways.

At 1, Fig. 73, is shown a typical nerve-cell such as may
be found in an anterior horn of the gray matter of the
spinal cord. It consists of the cell body, or cell protoplasm,
in which is a large micleus containing a niicleolns. From
the body of the cell arise several branches, the great ma-

176

THE HUMAN BODY.

jority of which subdivide and form fine nerve-fibres in the
gray sabstanco of tlio spinal cord. One process of the cell,
however, a, does not branch, but is continued into the an-
terior root of a spinal nerve, acquiring a medullary and prim-
itive sheath at J), and becoming thus the axis cylnider of a
nerve-fibre. Other nerve-cells (as shown at 2 and 4) do not
possess the peculiar axis-cylinder process; all their branches
either join the branches of other cells or enter a fine net-

FiG. 73. — Different forms of nerve-cells. ], a cell, one branch of which, a,
becomes the axis cylinder of a nerve-fibre ; 2, two cells united by a process, c;
3, diagram of three" cells united by branches with one another, and each having
an axis-cylinder process ; 4, a multipolar cell without an axis-cylinder process.

work of gray nerve-fibrils. Most nerve-cells are larger
than the majority of the other cells of the Body, their
average diameter in the anterior horns of the gray sub-
stance of the cord being 0.1 millimeter (g^^ inch). In
the posterior horns they are smaller, and in the brain
many minute nerve-cells are found in addition to these
larger ones. In ganglia the cells as a rule are more reg-

MINUTE STBUGTUBE OF SPINAL CORD. 177

ular in outline than those depicted in Fig. 73, and have
fewer branches, most appearing indeed to have but two.
Others have been described as possessing only one process
connected with them, and some with none, but the ex-
istence of these is doubtful, since in separating the cells for
microscopic examination the delicate processes may readily
be broken oif and so escape detection.

The Structure of Nerve-Centres. These consist of white
and gray nerve-fibres, of nerve-cells, and of connective
tissue and blood-vessels arranged in different ways in the
different centres. Ganglia are collections of nerve-cells
and nerve-fibres, some of the latter being connected with
the cells, while others seem merely to pass through the
ganglion on their way to other parts. The whole mass
is enveloped and supported by other tissues. As an illus-
tration of the structure of a more complex nerve-centre
we may study the spinal cord.

Histology of the Spinal Cord. If a thin transverse sec-
tion of the spinal cord be examined with a microscope it
will be found to exhibit the following parts (Fig. 74).
Enveloping the whole and adherent to the rest is the deli-
cate layer of connective tissue forming the pia mater.
This lines the anterior fissure, 1, and an offshoot from it
fills up the posterior fissure, 2. Elsewhere fine bands of it
run in and ramify through the cord, supporting the nerv-
ous elements; some of the coarser of these are represented
at 6, 7, and elsewhere in the figure, but from these still
finer processes arise, as represented at d and e in Fig. 75,
and surround the individual nerve fibres and cells. This
ultimate finest connective tissue supporting the nervous
tissues directly, belongs to the retiform variety (p. 106),
and is called the neuroglia. In the white columns, the
cord (Fig. 75) will be seen to be mainly made up of medul-
lated nerve-fibres which run longitudinally and therefore
appear in the transverse section as circles, with a dot in
the centre, which is the axis cylinder. At h in Fig. 75
these fibres are represented, the intermediate connective
tissue being omitted, while at e this latter alone is repre-
sented in order to show more clearly its arrangement. At

178

THE HUMAN BODY.

the levels of the nerve-ruots liorizoiitul white fibres are
found (1) and 10, Fig. 74, and a, Fig. 75) running into the
gray matter, and others exist at the bottom of the anterior
fissure, running from one side of the cord to the other.
In the gray substance the same supporting network of con-
nective tissue is found, but in it the majority of the nerve-
fibres are non-medullated, and at certain points nerve-cells,

"Fig. 7ir^K thin triansverse section of half of the spinal cord maf^nifled about
10 diameters. 1, anterior fissure ; 2. posterior fls.sure ; 3, caualis centralis :
8, pia mater enveloping the cord ; 6, 7, bands of pia mater penetrating tlie cord
and supporting its nerve elements ; 9, a posterior root ; 10, bundles of an ante
rior root ; a, b, c, d, e, groups of nerve-cells in the gray matter.

such as are totally absent in the white substance, are found.
One collection of these nerve-cells is seen at e in Fig. 74,
and others are represented at a, d, f, and elsewhere. The
nerve-fibres in the gray matter are for the most part
branches of these cells (see Fig. 73), and as they unite
with one another freely they form, a structurally continuous

HISTOLOGY OF SPIRAL COED.

179

network through the whole gray substance. The fibres of
the anterior roots of the spinal nerves enter the gray mat-
ter and there become continuous with the unbranchecl pro-
cess of a nerve-cell; the ending of the posterior root-fibres
is not quite certain, but they appear to break up and join
the gray network directly, without the intervention of a
cell. In any case the fundamental fact remains that every
nerve-fibre joining the spinal cord is directly or indirectly
in continuity Avith the gray network and so with all the

Fig. 75.— a smill bit of the section representee! in Fig. 74 more magnified, a,
a bundle of fibres from an anterior root passing through tlie wliite substance
on its way to the gray. Towai-ds the riglit of the figure the nerve-fibi-es of the
anterior "column tiave been omitted so as to render more conspicuous tlie sup-
porting connective tissue, d and e. Elsewhere the nerve-fibres alone are repre-
sented; c, enveloping i>ja ?wa<er.

other fibres of all the spinal nerves. From the sides of the
gray substance fibres continually pass out into the white
portion and become medullated; some of these enter the
gray network again at another level and so bring parts of
the cord into esi^ecially close union, while others pass on
into the brain. At the top of the neck, moreover, the
gray matter of the cord is continuous with that of the
medulla oblongata and through it with the rest of the
brain, so that nervous disturbances can pass by anatomi-
cally continuous paths from one to the other.

CHAPTER XIII.

THE GENERAL PHYSIOLOGY OF THE NERV-
OUS SYSTEM.

The Properties of the Nervous System. General Con-
siderations. If the linger of any one unexpectedly touches
u very hot object, pain is felt and the hand is suddenly
snatched away; that is to say, sensation is aroused and cer-
tain muscles are caused to contract. If, however, the
nerves jDassing from the arm to the spinal cord have been
divided, or if they have been rendered incapable of activity
by disease, no such results follow. Pain is not then felt on
touching the hot body nor does any movement of the limb
occur; even more, under such circumstances the strongest
effort of the will of the individual will be unable to cause
any movement of his hand. If, again, the nerves of the
limb have uninjured connection with the spinal cord, but
parts of the latter, higher up, between the brain and the
point of junction of the nerves of the brachial plexus with
the cord, are injured, then a sudden contact with the hot
body will cause the arm to be snatched away, but no pain
or other sensation due to the contact will be felt, nor can
the will act upon the muscles of the arm. From the com-
parison of what happens in such cases (which have been
observed over and over again upon wounded or diseased
persons) with what occurs in the natural condition of
things, several important conclusions may be arrived at:

1. The feeling of pain does not reside in the burnt part
itself ; although that may be perfectly normal, no sensa-
tion will be aroused by any external force acting upon it,
if the nervous cords uniting it with the centres be pre-
viously divided.

2. The hot body has origi?iated some change lohich, pro-

GENERAL PROPERTIES OF NERVES. 181

pagated along the nerve-trunks, has excited a condition of
the nei've-centres which is accompanied hy a sensation, in
this particular case a painful one. This is clear from the
fact that the loss of sensation immediately follows division
of the nerves of the limb, but not the injury of any of its
other parts; unless of such a character as to cut off the
supply of blood, when of course the nerves soon die, with
the rest. Even, however, some time after tying the vessels
wliich carry blood to a limb one can observe in experiments
upon the lower animals that sensibility is still retained if
the nerves be not directly injured.

3. ]]lie)i a nerve in the shin is excited by a burn or other-
loise it does not directly call forth muscular contractions;
for if so, touching the hot body would cause the limb to be
moved even when the nerve is divided high nj) in the arm,
and as a matter of observation and experiment we find
that no such result follows if the nerve-fibres have been
cut in any part of their course from the burnt part to the
spinal marrow. It is therefore through the nerve-centres
that the change transmitted from the excited part of tlie
skin is reflected or sent back, to act upon the muscles.

4. The last deduction makes it probable that nerve-fibres
must pass from the centre to muscles as well as from the
fkin to the centre. This is confirmed by the fact that if
the nerves of the limb be divided the will is unable to act
upon its muscles, showing that these are excited to con-
tract through the nerves. That the nerve-fibres concerned
in arousing sensation and muscular contractions are differ-
ent, is shown also by cases of disease in which the sensi-
bility of the limb is lost while the power of voluntarily
moving it remains, and by other cases in which the reverse
is seen, objects touching the hand being felt while it can-
not be moved by the will. We conclude then that cer-
tain nerve-fibres when stimulated convey something (a
nervous impulse) to the centres, and that these when ex-
cited may radiate impulses through other nerve-fibres to
distant parts, the centre serving as a connecting link be-
tween the fibres which carry impulses from without in, and
those which convey them from within out.

182 THE JIUMAA BODY.

5. Fiirtlier we conclude thaf the sjriiial cord can act an
an mtermediari/ between the Jibres carrying in nervous im-
pulses and those carrying tliem out, but that sensations can-
not he aroused by impulses reaching the spinal cord only,
nor has the Will its seat there ; volition and consciousness are
dependent upon states of the brain. Tliis follows from the
unconscious movements of the limb which follow stimula-
tiou of its skin after such injury to the spinal cord as pre-
vents the transmission of nervous impulses farther on
(showing that the cord is a rejlex centre), and fi'om the
absence, in such cases, of sensation in the part whose nerves
have been injured, and the loss of the power of voluntarily
causing its muscles to contract.

6. Finally we conclude that the spinal cord in addition
to being a centre for reflex actions serves also to transmit
onwards tiervous impulses to the brain; a fact which is con-
firmed by the histological observation that in addition to
the nerve-cells, which are the characteristic constituents of
nerve-centres, it contains the simply conductive nerve-
fibres, many of which pass on to the brain. In other
words the spinal cord, besides containing fibres which enter
it from, and pass from it to, peripheral parts contains
many whieli unite it to other centres ; and connect the
various centres in it, as those for the arms and legs, to-
gether. This is true not only of the spinal cord but of the
brain (which contains many fibres uniting different centres
in it), and probably of all nerve-centres.

The Functions of Nerve-Centres and Nerve-Trunks.
Fj-om what has been stated in the previous paragraphs it is
clear that we may distinctly sejDarate the nerve-trunks from
the nerve-centres. The fibres serve simply to convey im-
pulses either from without to a centre or in the opposite
direction, while the centres conduct and do much more.
Some, as the spinal cord,are merely reflex centres, and have
nothing to do with states of consciousness. A man with
his spinal cord cut or diseased in the dorsal region will
kick violently if the soles of his feet be tickled, but will
feel nothing of the tickling, and if he did not see his legs
would not know that they were moving. Reflex centres

FCXCTIOJVS OF JVEE VE- CENTBES. 183

moreoAer do not act, as a rule, iiidiiferently and casually,
but rearrange the impulses reaching them, so as to pro-
duce a protectiye or in some way advantageous result. In
other words, these centres, acting in health, commonly co-
ordinate the incoming impulses and give rise to outward-
going impulses which produce an apparently purposive
result. The burnt hand or the tickled foot, in the absence
of all consciousness, are snatched away from the irritant,
and food chewed in the mouth excites nerves there which
act on a centre which causes certain cells in the salivary
glands to form and pour into the mouth more saliva. In
addition to the reflex centres we have others, placed in the
brain, and which, when excited, cause in us various states
of consciousness, as sensations, emotions, and the will;
concerning these centres of consciousness^, however, our
physiological knowledge is still very incomplete and so they
are still largely grou2)ed by psychologists according to their
individual fancy. The brain also contains a great many
imjjortant reflex centres, as that for the muscles of swallow-
ing, an act which goes on perfectly without our conscious-
ness at all. In fact if we pay attention to our swallowing
we fail to perform it as well as if we let the nervo-muscular
apparatus alone. To complete the statement of the func-
tions of the nerve-centres we must probably add two other
groups. The first of these is that of the automatic centres,
which are centres excited not directly by nerve-fibres con-
veying impulses to them, but in other ways. For example
the breathing movements go on indei)endently of our con-
sciousness, being dependent on stimulation of a nerve-centre
in the brain by the blood which flows through it (see Chap.
XXYI.); and tlie beat of the heart is also dependent (Cbap.
XVII.) upon nerve-centres the excitant of which is un-
l<nown. The final group of nerve-centres is represented
by certain sporadic sympathetic and cerebro-spinal ganglia
Avhich are not known to be either reflex, automatic, or con-
scious in function. Tliey may be called relay and junc-
tion centres, since in them probably an impulse entering
by one nerve-fibre excites a cell, which by its communi-
cating branches arouses many others, and these send out

184 THE HUMAN BODY.

impulses then by the many nerve-fibres connected with
them. By such means a single nerve-fibre can act upon
an extended region of the Body. In other cases it seems
likely tliat a feeble nervous impulse reaching an irritable
nerve-cell excites changes in this comparable to those pro-
duced in a muscle when it is stimulated; and the cell by
its discharge sends on reinforced nerve impulses along its
other branches.

Excitant and Inhibitory Nerves. The great majority
of the nerve-fibres* of the Body when they convey nervous
impulses to a joart arouse it to activity; they are excitant
fibres. There is, however, in the Body another very im-
portant set which arrest the activity of parts and which
are known as inlnhitory nerve-fihres. Some of these check
the action of central nervous organs, and others the work
of peripheral parts. For instance taking a pinch of snufE
will make most persons sneeze; it excites centrally acting
fibres in the nose, these excite a centre in the brain,
and this in turn sends out impulses by efferent fibres which
cause various muscles to contract. But if the skin of the
upper lip be pinched immediately after taking the snuff, in
most cases the reflex act of sneezing, which the "Will alone
could not prevent, will not take place. The afferent im-
pulses conveyed from the skin of the lip have "inhibited"
what we may call the "sneezing centre;" and afford us
therefore an example of inhibitory fibres checking a centre.
On the other hand, the heart is a muscular organ which
goes on beating steadily throughout life; but if the branches
of the pneumogastric nerve going to it be excited, the beat
of the heart will be stojjped; it will cease to work and lie in a
relaxed resting condition: in this we have an instance of an
inhiliitory nerve checking the activity of a peripheral organ.

Classification of Nerve-Fibres. Nearly all the nerve-
fibres of the Body fall into one of two great groups corre-
sponding to those which carry impulses to the centres
and those which carry them out from the centres.
The former are called afferent or centri'petal fibres and the
latter efferent or centrifugal. Since the impulses reaching
the centres through the afferent fibres generally cause sen-

CLASSIFICATION OF NERVE-FIBRES. 185

sations they are often called se7isory fibres; and as many of
those which carry out impulses from the centres excite
movements, they are frequently called motor fibres; but
these last names are bad, since even excluding inhibi-
tory nerves, many afferent fibres are not sensory and many
efferent are not motor.

We may distinguish as subdivisions of afferent fibres — the
following groups. 1. Sensory fibres projjer, the excitement
of which is followed by a sensation when they are con-
nected with their brain-centre, which sensation may or
mav not give rise to a voluntary movement. 2. Reflex
fibres, the excitation of which may be attended with con-
sciousness but gives rise to involuntary efferent impulses.
Thus for example light falling on the eye causes not only
a sensation but also a narrowing of the pupil, which is en-
tirely independent of the control of the Will. No absolute
line can, however, be drawn between these fibres and those
of the last group: any sudden excitation, as an unexpected
noise, will cause an involuntary movement, while the same
sound if expected would cause a movement or not accord-
ing as was loilled. 3. Excito-motor fibres. The excitation
of these when reaching a nerve-centre causes the stimula-
tion of efferent fibres, but without the participation of
consciousness. Dm-ing fasting for instance bile accumu-
lates in the gall-bladder and there remains itntil some
semi-digested food passes from the stomach into the intes-
tine. This is acid, and stimulates nerves in the mucous
membrane lining the intestine, and these convey an im-
pulse to a centre, which in consequence sends out impulses
to the muscular coat of the gall-bladder causing it to con-
tract and expel its contents into the intestine: but of all
this we are entirely unconscious. 4. Centro-inhibitory
fibres. Whether these exist as a distinct class is at present
doubtful. It may be that they are only ordinary sensory
or reflex fibres and that the inhibition is due only to the in-
terference of two impulses reaching a central organ at the
same time and impeding or hindering the full production
of the normal result of either.

In efferent nerve-fibres physiologists also distinguish

186 THE Jli'MAA BODY.

several groups. 1. Motor fibres, Avhich are distributed to
the muscles and govern their contractions. 2. Vaso-motor
fibres. These are not logically separable from other motor
fibres; but they are distributed to the muscles of the blood-
vessels and by governing the blood-supply of various parts,
indirectly produce such secondary results as entirely over-
shadow their primary effect as merely producing muscular
contractions. 3. Secretory fibres. These are distributed
to the cells of the Body which form various liquids used in it,
as the saliva and the gastric juice, and arouse them to ac-
tivity. The salivary glands for instance may be made to
form saliva by stimulating nerves going to them, and the
same is true of the cells which form the sweat poured out
upon the surface of the Body. 4. Trojjhic nerve-fibres.
Under this head are included nerve-fibres which have been
supposed to govern the nutrition of the various tissues, and
so to control their healthy life. It is very doubtful, how-
ever, if any such nerve-fibres exist, most of the facts cited
to prove their existence being otherwise explicable. For
instance shingles is a disease characterized by an erui^tion
on the skin along the line of certain nerves which run be-
tween the ribs; but it may be dependent upon disease of
the vaso-motor nerves which control the blood-supply of
the part. In other cases diseases ascribed to injury of
trophic nerves have been shown to be due to injury
of the sensory nerves of the part, which having lost its
feeling, is exposed to injuries from which it would other-
wise have been protected. On the other hand it may be
said that secretory nerves are trophic nerves in the true
sense of the word, since when excited they cause the se-
cretory cells to live in a special way (p. 269) and produce
substances which when unacted upon by their nerves they
do not form. But if we call secretory nerves trophic we
must include also under that name all other efferent
nerves; the nutritive processes going on in a muscular
fibre Avhen at work are different from those in the same
fibre Avhen at rest, and the same is true of all other cells
the activity of which is governed Ijy nerve-fibres. 5. Feri-
pJierally-actmg inhibitory nerves.

NERVE STIMULI.

187

Intercentral Nerve-Pibres. These, which do not convey
impulses to or from peripheral parts and nerve-centres, bnt
connect one centre with another, form a final group in ad-
dition to efferent and afferent nerve-fibres. Many of them
connect the sporadic and sympathetic ganglia with one
another and with the cerebro-spinal centre, while others
place different parts of this latter in direct communication ;
as for instance different parts of the spinal cord, the brain
and the spinal cord, and the two halves of the brain.
These fibres are of very great importance, but as yet their
course is imperfectly known.

General Table, We may physiologically classify nerve-
fibres as in the following tabular form which is founded
upon the facts above stated.

Nerve-fibres. ^

Peripheral.

Afferent. \

Efferent.

, ( Excitino;.
Intercentral. - y i -i •♦ "

( Inhibitory.

f Sensory.
Reflex.

Excito-motor.
Inhibitory?

r Motor.
I Vaso-motor.
- Secretory.

Trophic ?

Inhibitory.

The Stimuli of Nerve-Fibres, Xerve-fibres, like mus-
cular fibres, possess no automaticity; acted upon by certain
external forces or stimuli they propagate a change, which is
known as a nervous impulse, from the point acted upon to
their ends; but they do not generate nervous impulses when
left entirely to themselves. Normally, in the living Body
the stimulus acts on a nerve-fibre at one of its ends, being
either some change in a nerve-centre with which the fibre
is connected (efferent nerves) or some change in an organ
attached to the outer end of the nerve (afferent fibres).
Experiment shows, however, that a nerve can be stimulated
in any part of its course; that it is irritable all through its

188 THE HUMAN BODY.

extent. If, for exami)le, ihc sciatic of a frog be exiwsed in
the thigh and divided, it will be found that electric shocks
applied at the point of division to the outer half of the
nerve stimulate the motor fibres in it, and cause the mus-
cular fibres of the log to contract: and similarl_y such shocks
applied to the cut end of the central half irritate the affe-
rent fibres in it, as shown by the signs of feeling exhibited
by tiie animal. In ourselves, too, we often have the opjior-
tunity of observing that the sensory fibres can be stimulated
in their course at some distance from their ends. A blow
at the back of the elbow, at the j)oint commonly known as
the ''funny bone" or the "crazy bone," compresses the
ulnar nerve there against the subjacent bone, and starts
nervous impulses which make themselves known by severe
tingling pain referred to the little and ring fingers to which
the nerve is distributed. This shows not only that the
nerve-fibres can be irritated in their course as well as at
their ends, but also that sensations do not directly tell us
where a nerve-fibre has been excited. No matter where in
its course the impulse has been started we unconsciously
refer its origin to the ])eripheral end of the afferent nerve.
General and Special Nerve Stimuli. Certain external
forces excite all nerve-fibres, and in any i^art of their course.
These are known as general nerve stimuli; others act only
on the end organs of nerve-fibres, and often only on one
kind of end organ, and hence cannot be made to excite all
nerves: these latter are commonly known as special nerve
stimuli. In reality they are not properly nerve stimuli at
all; but only things which so affect the irritable tissues at-
tached to the ends of certain nerve-fibres as to make these
tissues in turn excite the nerves. For example light itself
will not stimulate any nerve, not even the optic: but in the
eye it effects changes (apparently of a chemical nature)
by which substances of the nature of general nerve stimuli
are produced and these stimulate the optic nerve-fibres.
The ends of the nerves in the skin are not accessible to
light nor are the proper etui organs on which the light
acts there present, so light does not lead to the production
of nervous impiilses in them: but the optic nerve without

NERVE STIMULI. 189

its peciiliur end organs would be just us insensible to light
as these are. Similarly the aerial vibrations which affect
us as sounds, do not stimulate directly the fibres of the audi-
tory nerve. They act on terminal organs in the ear, and
these then stimulate the fibres of the nerve of hearing, just
as they would any other nerve which happened to be con-
nected with them.

General Nerve Stimuli. Those known are (1) electric
currents : an electric shock passed through any part of
any nerve-fibre, powerfully excites it. A steady current
passing through a nerve does not stimulate it, but any
sudden change in this, whether an increase or a decrease,
does. A very gradual change in the amount of electricity
passing through a nerve in a unit of time will not stimu-
late it. (2) Mechanical stimuli. Any sudden pressure
or traction, as a blow or a pull, will stimulate a nerve-
fibre. On the other hand steady pressure, or pressure very
slowly increased from a minimum, will not excite the
nerve. (3) Thermal stimuli. Any sudden heating or cool-
ing of a nerve, as for instance bringing a hot wire close to
it, will stimulate; slow changes of temperature will not. (4)
Chemical stimuli. Many substances which alter the nerve-
fibre chemically, stimulate before killing it; thus dipping
the cut end of a nerve into strong solution of common salt
will excite it, but very slow chemical change in a nerve
fails to stimulate.

In the case of all these general stimuli it will be seen
that as one condition of their efficacy they must act with
considerable suddenness. On the other hand too transient
influences have no effect. An electric shock sent for only
0.0015 of a second through a nerve does not stimulate
it: apparently the inertia of the nerve molecules is too
great to be overcome by so brief an action. So, also, too
strong sulphuric acid and many other bodies kill nerves
immediately, altering them so rapidly that they die without
being stimulated.

Special Nerve Stimuli. These as already explained
act only on particular nerves, not because one nerve is es-
sentially different from another, but because their influence

100 THE II UMAX BODY.

is excited through spceiul end organs whicli are peculiar to
some nerves. These .stimuli are — (1) Changes occurring in
central organs, of whose nature we know next to nothing,
but wliich excite the efferent nerve-fibres connected Avith
tliem. The remaining special stimuli act on atTcrcut fibres
1 hrough tlic sense-organs. They are — (2) Light, whicli by the
intervention of organs in the eye excites the optic nerve. (2)
Sound, Avliich by the intervention of organs in the ear excites
the auditory nerve. (3) Heat, which through end organs in
the skin is able, by very slight changes, to excite certain
nerve-fibres: such slight changes of temperature being
efficient as would be quite incapable of acting as general
nerve stimuli without the proper end organs. (4) Cliemical
agencies. These when extremely feeble and incapable of
acting as general stimuli, can act as special stimuli through
special end organs in the mouth and nose (as in taste and
smell) and prol)ably in other parts of the alimentary tract,
where very feeble acids and alkalies seem able to excite
certain nerves, and reflexly through them excite movements
or stir up the cells concerned in making the digestive
liquids; for example the contraction of the gall-bladder
already referred to. (5) Mechanical stimuli when so feeble
as to be inefficient as general stimuli. Pressure on the
skin of the forehead or the back of the hand, equal to .002
gram (.03 grain) can be felt through the end organs of
the sensory fibres there, but would be quite incajjable of
acting as a general stimulus in the absence of the.-e.

It "will be noticed as regards the special stimuli of
afferent nerves that many of them are merely less degrees
of general stimuli; the end organs in skin, mouth,and nose
are in fact excited by the same things as the nerve-fibres,
but are far more irritable. In the case of the higher
senses, seeing and hearing, however, the end organs seem
to differ entirely in ]»ro])erty from nerve-fibres, being
excited by sonorous and luminous vibrations which, so far
as we know, will in no degree of intensity directly excite
nerve-fibres. To make an end organ for recognizing very
slight pressures we may imagine all that would be needed

SPECIFIC KERVE ENERGIES. 101

would be to expose directly a very delicate end branch of
the axis cylinder, and this seems in fact to be the case in
the nerves of the transparent exposed part of the eyeball, if
not in many other parts of the external integument of the
Body. But as axis cylinders are quite unirritable by light
or sound a mere exposure of them would be useless in the
eye or ear, and in each case we find accordingly a very
complex apparatus developed, whose function it is to
convert these modes of motion which do not excite nerves
into others which do. AVe might compare it to a
cartridge, which contains not only " irritable" gunpowder
but highly " irritable" detonating powder, and the stimulus
of the blow from the hammer which would not itself ignite
the gunpowder, acting on the detonating powder (which
represents an ''end organ"), causes it to give off a spark
which in turn excites the gunpowder, which answers to the
nerve-fibre.

Specific Nerve Energies. We have already seen that a
nervous impulse propagated along a nerve-fibre will give
rise to different resttlts according as different nerve-fibres
are concerned. Traveling along one fibre it will arouse a
i- ensation, along another a movement, along a third a se-
cretion. In addition we may observe readily that these
different results may be produced by the same physical
force when it acts upon different nerves. Eadiant energy,
for example, falling into the eye cattses a sensation of
sight, but falling tipon the skin one of heat, if any. The
different results which follow the stimulation of different
nerves do not then depend upon differences in the physical
forces exciting them. This is still further shown by the
fact that different physical forces acting tipon the same
nerve arouse the same kind of sensation. Light reaching
the eye causes a sight sensation, but if the optic nerve be
irritated by a blow on the eyeball a sensation of light is
felt just as if actual light had stimulated the nerve ends.
So too when the optic nerve is cut by the surgeon in ex-
tirpating the eyeball, the patient experiences the sensation
of a flash of light; and the same result follows if an electric

192 • THE HUMAN BODY.

shock be sent through the nerve. Different nerves excited
by the same stimulus produce different results, and the
same nerve excited by different stimuli gives the same re-
sult. How are these facts to be explained?

The first explanation which suggests itself is that the
various nerves differ in their projierties: that electricity
applied to a motor nerve causes a muscle to contract, and
to the optic nerve a visual sensation, and to tlie lingual
nerve a sensation of taste, because nervous impulses in
the motor, optic, and lingual nerves differ from one an-
other. This was the view held by the older physiologists;
and that supposed peculiarity of a muscular nerve by which
its irritation caused a muscular contraction, and that of the
optic nerve in consequence of which its excitation caused a
sensation of sight, and so on, they called the S2Jecific energy
of the nerve. Seeing further that when a motor nerve was
cut and its peripheral stump pinched the muscles connected
with it contracted, but that when its central end was
pinched no sensation or other recognizable change followed,
while exactly the reverse was true of a sensory nerve, they
believed that afferent nerves differed essentially from effe-
rent nerves, inasmuch as the latter could only projiagate
impulses centrifugally and the former only centripetally.
Now, however, we have much reason to believe that this
view is wrong and that all nerve-fibres are exactly alike in
their physiological properties, and can carry nervous im-
pulses either way. The differences observed depend in
fact not on any differences in the nerve-fibres, but on the
different parts connected with their ends; in other words
on the different terminal organs excited by the impulses
conveyed by the fibre. A motor fibre is one which has at
its ])eripheral end a muscular fil)re, and a centrifugally trav-
eling mipulse reaching this will cause it to contract; but the
cells connected with its central end are not of such a nature
as to give rise to sensations when centripetally traveling
impulses reach them, or to transmit these to other efferent
fibres and so cause reflex movements; and therefore when a
motor fibre is stimulated in the middle of i1,s course the
outward-going impulse causes a movement, while the cen-

THE SIMILARITY OF NERVE-FIBRES. 193

trally traveling impulse starting at the same time, gives
no direct sign of its existence. Similarly for a sensory
nerve such as the oiatic; if it l^e stimulated somewhere be-
tween the brain and the eyeball the centrally traveling
impulse will cause a sensation of light, by exciting the
brain-centre connected with it, but the outward traveling
impulse not reaching muscular fibres or other parts which
it can arouse to activity, remains concealed from our no-
tice. In other words the so-called specific energy of a
nerve-fibre depends upon the terminal organs on which it
can act and not on any peculiarity of the nerve-fibre itself.
Proofs that all Nerve-Fibres are Physiologically Alike.
(1) The most marked difference between nerve-fibres is
obviously that between efferent and afferent, and the mi-
croscope fails entirely to show any differences between the
two. Some motor and some sensory fibres may be bigger
or less than others, some may be white and others may be
gray, but such differences are secondary and have no cor-
relation with the function of the nerve. We can recognize
no constant difference in structure between the two. (2)
The physical properties and chemical composition of motor
and sensory nerves agree in all known points. (3) When a
nerve, say a motor one, is stimulated half way between
the centre and a muscle, a nervous impulse, as we call it, is
propagated to the muscle, which imjiulse starts at the
point stimulated and travels at a definite rate to the mus-
cle, the contraction of which latter evidences its arrival.
Now starting at the same moment from the same point, and
traveling at the same rate, is a change in the electrical
properties of the nerve which can be detected by a good
galvanometer and wliich is called the "negative vartafioit.'^
Since this negative variation and the nervous impulse co-
exist at any given moment in a particular point of the
nerve and disappear from it together, we conclude that the
negative variation is a change in the electrical properties
of the nerve dependent on that internal movement of its
molecules which constitutes a nervous impulse. It is an
externally recognizable physical sign, and the only known
one, of the existence of the nervous impulse as it travels

194 THE HUMAN BODY.

along tlie libre. If the mu^icle were cut invay from the einl
of the nerve we could still detect that a nervous impulse
had traveled from the point of stimulation to that where
the fibres were divided, by tracking the negative variation.
Now if we examine the part of the nerve on the central
side of the stimulated j)oint we find that a negative varia-
tion (and hence we conclude a nervous impulse) travels that
way too; it starts at the same moment as the efferent nega-
tive variation and travels in the same way, hut the impulse
of which it is a sign produces no more effect than the efferent
impulse would after the muscle had been cut away; for it
does not reach any muscidar fibre, or sensory or reflex centre,
which it can arouse to activity. (4) The folloAving experi-
ment is, however, more conclusive. If a rat's tail be am-
putated close to the body of the animal and be then trans-
planted to the back and sewn into the skin there by its
narrow end, it will grow in this new position, with the
broad end, which was previously attached and nearc st the
spinal cord, now free and farthest from it. The tail, in
other words, will be upside down. After the wound has
healed, the nerve-fibres in the tail, or some of them, attach
themselves to the cut nerve-fibres in the wound of the
back to which it was transplanted, and the tail again be-
comes sensitive if the end now free be pinched. Here one-
of two things must have occurred. Either the afferent
nerve-fibres in the tail which naturally carried impulses
from its tip up, now carry them in the opposite direction
from the broad end to the tip, or the efferent nerve-fibres
which carried motor impulses down the tail, now carry sen-
sory impulses and transmit them to the sensory fibres in
the back with which they have become continuous. If the
first, which is the more i:)robable hypothesis, be true, it
is proved that afferent nerve-fibres can carry imi^ulses in
either direction: if the second be true it is still more clear
that there is no special peculiarity in a sensory nervous
imjiulse when compared with a motor.

Afferent and efferent nerve-fibres then differ in no
observable property. All are alike in faculty and their
different names simply imply that they have different ter-

SIMILARITY OF ALL NERVE-FIBBE8. 195

miniil organs. Ju.st as all muscles are alike iu general
physiological properties, and differ in special function
according to the parts on Avhich they act, so are all nerre-
iibres alike in general physiological properties, and differ
in special function only because they are attached to spe-
cial things. The si)ecial physiology of yarious neryes will
liereafrer l)e considered in connection with the working of
various mechanisms in the Body. If it be true that the
great subdiyisions of afferent and efferent fibres haye
identical properties, it follows that this is a fortiori true
of the minor subdiyisions of each, and that auditory, gus-
tatory, and optic nerve-fibres are all alike, and all identi-
cal with motor and yaso-motor and secretory nerye-fibres;
and that the nervous impulse is in all cases the same thing,
varying in intensity in different cases and in the rate at
which others follow it in the same fibre, but the same in
kind. To put the case more definitely: Light outside the
eye exists as ethereal yibrations, sound outside the ear as
yibrations of the air (commonly). Each kind of vibration
acts on a particular end organ in eye or ear which is
adapted to be acted upon by it, and in turn these end
organs excite the optic and auditory nerve-fibres; these in
consequence transmit impulses, which reaching different
parts of the brain excite them; the excitement of one of
these brain-centres is associated with sonorous and of the
other with yisual sensations. Tlie neryous impulse in the
two cases is quite alike, at least as to quality; though it may
differ in quantity and rhythm, and the resulting difference
in quality of the sensations cannot depend on it. The
quality differences in these cases must be products of the
central neryous system. If we had a set of copper wires we
might by sending precisely similar electric currents through
them produce yery different results if different things were
interposed in their course. In one case the current might
be sent through water and decompose it, doing chemical
work; in another through the coil of an electro-magnet
and raise a weight; in a third through a thin platinum
wire and develop light and heat, and so on; the result
depending on the terminal organs, as we may call them.

lOG THE HUMAN BODY.

of each wire. Or on tlie other luind we might gen-
erate the current in eacli wire differently, in one by a
Daniell's cell, in a second by a thermo-electric machine,
in a third by the rotation of a magnet inside a coil, l)ut the
currents in the wires would be essentially the same, as the
nervous impulses are in a nerve-fibre. No matter how
they have been started and, provided their amount is the
same, whether they produce similar or dissimilar results,
depends only on whether they are connected with similar
or dissimilar end organs.

The Nature of a Nervous Impulse. Since between
sense-organs and sensory centres, and these latter and the
muscles, nervous impulses are the only means of communi-
cation, it is through them that we arrive at our opinions
concerning the external universe and through them that
we are able to act u^^on it; their ultimate nature is there-
fore a matter of great interest, but one about which we
viufortunately know very little. We cannot well ima-
gine it anything but a mode of motion of the molecules of
the nerve-fibres, but beyond this hypothesis we cannot go
far. In many points the phenomena presented by nerve-
fibres as transmitters of disturbances are like the phenom-
ena of wires as transmitters of electricity, and when the
phenomena of current electricity were first observed there
was a great tendency, explaining one unknown by another,
to consider nervous impulses merely as electrical currents.
The increase of our knowledge concerning both nerves and
electric currents, however, has made such an hypothesis
almost if not quite untenable. In the first place, nerve-
fibres are extremely bad conductors of electricity, so bad
that it is impossible to suppose them used in the Body for
that purpose, and in the second place, merely physical con-
tinuity of a nerve-fibre, such as would not interfere with
the passage of an electric current, will not suffice for the
transmission of a nervous impulse. For instance if a damp
string be tied around a nerve, or if it be cut and its two moist
ends placed in contact, no nervous impulse will be trans-
mitted across the constricted or divided point, although
an electrical current would pass readily. An electrical

FUNCTIONS OF THE SPINAL ROOTS. 197

shock may be used like many other stimuli to upset the
equilibrium of the iierve molecules and start a nervous im-
pulse, which then travels along the fibre, but is just as dif-
ferent from the stimulus exciting it, as a muscular contrac-
tion is from the stimulus which calls it forth.

The nerves, however, have certain interesting electrical
properties from the study of which we learn some little
about a nervous impulse. As already mentioned whenever
a nervous impulse is started in a nerve an electrical change,
known as the "negative variation" or "action current," is
started at the same time, from the same point, and travels
along the nerve at the same rate. Hence w^e conclude that
the new internal molecular arrangement in a nerve-fibre
which constitutes its active as compared with its resting
state, is also one which changes the electrical properties of
the fibre. It is an outward sign and the only known one
of the internal change. Now it is found that the action
current travels along the nerve (in the frog) at the rate of 28
meters (92.00 feet) in a second and takes .0007 second to
pass by a given point: accordingly at any one moment it
extends over about 18 mm. (0.720 inch) of the nerve-
fibre. Moreover, when first reaching a point it is very
feeble, then rises to a maximum and gradually fades away
again. Taking it as an indication of what is going on in
the nerve, we may assume that the nervous impulse is a
molecular change of a wavelike character, rising from a
minimum to a maximum, then gradually ceasing, and about
18 millimeters in length.

The Rate of Transmission of a Nervous Impulse.
This can be measured in several ways, and is far slower than
that of electric currents. It agrees as above stated with
that of the negative variation, being 28 meters (92.00 feet)
[)er second in the motor nerves of a frog. In man it is
somewhat quicker, being 33 meters (108. 24 feet) per second,
that is about -^ of the rate of the transmission of sound-
waves in air at zero.

Functions of the Spinal Nerve-Roots. The great ma-
jority of the larger nerve-trunks of the Body contain both
afferent and efferent nerve-fibres. If one be exposed in its

198 THE HUMAN BODY.

course and divided in a living animal, it will be found that
irritating its peripheral stump causes muscular contractions,
and i)incliing its central stum}) causes signs of sensation,
showing that the trunk contained both motor and sensory
fibres. If the trunk be followed away from tlie centre, as
it breaks up into smaller and smaller briuu-hos, it will be
found that these too are mixed until very near their end-
ings, where the very finest terminal branches close to the
end organs, whether muscular fibres, secretory cells, or sen-
sory apparatuses, contain only afferent or efferent fibres.
If the nerve- trunk be one that arises from the spinal cord
and be examined progressively back to its origin, it will
still be found mixed, up to the point where its fibres sepa-
rate to enter either an anterior or a posterior nerve-root.
Each of these latter hoAvever is pure, all the efferent fibres
of the spinal nerves leaving the cord by the anterior roots,
and all the afferent entering it by the posterior. This of
course could not be told from examination of the dead
nerves since the best microscope fails to distinguish an
afferent from an efferent fibre, but is readily proved by
experiments first performed by Sir Charles Bell. If an
anterior root be cut and its outer end stimulated, the mus-
cles of the parts to which the trunk which it helps to form
is distributed, will be made to contract, and the skin will
be made to sweat also if the root haj)pen to be one that
contains secretory fibres for the sweat-glands. On the other
hand, if the central end of the root (that j^art of it attached
to the cord) be stimulated no result will follow, showing that
the root contains no sensory, reflex, or excito-motor fibres.
With the posterior roots the reverse is the case: if one of
them be divided and its outer end stimulated, no observed
result follows, showing the absence of all efferent fibres;
but stimulation of its central end will cause either signs
of feeling, or reflex actions, or both. We might compare a
spinal nerve-trunk to a rope made up of green and red
threads with at one end all the green threads collected into
one skein and the red into another, which would represent
the roots. At its farther end Ave may suppose the rope
divided into finer cords, each of these also containing

COMMUNICATION OF NERVE-CENTRES. 199

red and green threads down to the very finest branches
consisting of only a few threads and those all of one kind
either red or green, one representing efferent, the other
afferent fibres.

The Cranial Nerves. Most of these are mixed also, bnt
with one exception (the fifth pair, the small root of which
is efferent and the large gangliated one afferent) they do not
present distinct motor and sensory roots, like those of the
spinal nerves. At tlieii" origin from the brain most of them
are either purely afferent or efferent, and the mixed char-
acter which their trunks exhibit is due to cross-branches
with neighboring nerves, in which afferent and efferent
fibres are interchanged. The olfactory, optic, and audi-
tory nerves remain, however, purely afferent in all their
course, and others though not quite pure contain mainly
efferent fibres (as the facial) or mainly afferent (as the
glosso-pharyngeal).

The Intercommunication of Nerve-Centres. From the
anatomical arrangement of the nervous system it is clear
that it forms one continuous whole. No subdivision of it
is isolated from the rest, but nerve-trunks proceeding from
the centres in one direction bind them to various tissues
and, proceeding in another, to other nerve-centres; which
in turn are united with other tissues and other centres.
Since the physiological character of a nerve-fibre is its con-
ductivity— its power of propagating a disturbance when
once its molecular equilibrium has been upset at any one
point — it is obvious that through the nervous system any
one part of the Body, supplied with nerves, may react on all
other parts (with the exception of such as hairs and nails
and cartilages, which are not known to possess nerves) and
excite changes in them. Pre-eminently the nervous system
forms a uniting anatomical and physiological bond through
the agency of which unity and order are produced in the
activities of different and distant parts. We may com^Dare
it to the Western Union Telegraph, the head office of
which in New York would represent tlie brain and spinal
cord; the more important central offices in other large
cities, the sympathetic ganglia; and the minor offices in

200 THE HUMAN BODY.

country stations the sporadic ganglia; while the tele-
graph-wires, directly or indirectly uniting all, would corre-
spond to the nerve-trunks. Just as information started along
some outlying Aviro may he transmitted to a central office,
and from it to others, and then, according to what happens
to it in the centre, he stopped there, or spi'ead in all direc-
tions, or in one or two only, so may a nervous disturbance
reaching a centre by one nerve-trunk merely excite changes
in it or he radiated from it through other trunks more
or less widely over the Body and arouse various activi-
ties in its other component tissues. In common life the
very frequency of this uniting activity of the nervous sys-
tem is such that we are apt to entirely overlook it. We
do not wonder how the sight of pleasant food will make the
mouth water and the hand reach out for it; it seems as we
say " natural" and to need no explanation. But the eye
itself can excite no desire, cause the secretion of no saliva,
and the movement of no limb. The whole complex result
depends on the fact that the eye is united by the optic
nerve with the brain, and that again by other nerves with
saliva-forming cells, and with muscular fibres of the arm;
and through these a change excited by light falling into
the eye is enabled to produce changes in far removed or-
gans and excite desire, secretion and movement. In cases
of disease this action exerted at a distance is more apt to ex-
cite our attention: vomiting is a very common symptom of
certain brain diseases and most people know that a disor-
dered stomach will produce a headache; while the pain
conseqxient upon the hip-disease of children is usually felt
not at the hip-joint but at the knee.

CHAPTER XIV.

THE ANATOMY OF THE HEART AND BLOOD-

' VESSELS.

General Statement. During life the blood is kept flow-
ing with gi-eat rapidity tlirough all parts of the Body (ex-
cept the few non-vascular tissues already mentioned) in
definite paths prescribed for it by the heart and blood-
vessels. These paths, which under normal circumstances
it never leaves, constitute a continuous
set of closed tubes (Fig. 76) beginning
at and ending again in the heart, and
simple only close to that organ. Else-
Avhere it is greatly branched, the most
numerous and finest branches {I and a)
being the capillaries. The heart is es-
sentially a bag with muscular walls and
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 remaiii-
ing chambers {g and/) which have very
powerful walls and, forcibly contract-
ing, drive the blcod out into vessels
(i and b) which communicate with
them and are known as the arteries.
The big arteries divide into smaller;
these into smaller again (Fig. 77) until the branches be-
come 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;

Fig. re. —The heart
and blood-vessels dia-
grammatically r e p r e-
sented.

W2

THE nr.uAy body

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
«t one end by the heart and its ejection from the other go

mv8

Fig. 77.— 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 be visible if mag-
nified. R, the radial artei-y on which the pulse is usually felt at the wrist ; U,
the ulnar artery.

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 where the lymph-vessels open into the veins (p. 329);
there some lymph is poured in and mixed directly with the
blood. Accordingly everything which leaves the blood

POSITION OF THE HEART. 20S

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 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 phy-
siological work.

The Position of the Heart. The heart (li, Fig. 1) lies
in the chest immediately above the diaphragm and oppo-
site 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 downwards, projects to the left
of that bone, where it may be felt beating 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 commonly supposed but very nearly in the middle line,
with the upper part inclined to the right, and the lower
(which may be easier 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 connective 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 mem-
brane like that lining the abdominal cavity, and a similar
layer (the visceral layer of the pericardium) covers the out-
side 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.

204

THE HUMAN BODY.

covered witli a single layer of flattened cells, and called the
endocardium. Between the endocardium and the visceral
layer of the pericardium the bulk of the wall of the heart lies
and is made up mainly of striped muscular tissue (differing
somewhat from that of the skeletal muscles); but connective
tissues, blood-vessels, nerve-cells, and nerve-fibres are also
abundant in it.

JVofe. Sometimes the pericardium becomes inflamed,
this affection 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 inflammation the rubbing
surfaces on the outside of the heart and the inside of the
pericardium become roughened, and their friction produces
a sound which can be recognized through the stethoscope.
In later stages great quantities of liquid may accumulate in
the pericardium so as to seriously impede the heart's
beat.

The Cavities of the Heart. On opening the heart (see

diagram, Fig. 78) it is
found to be subdivided
by a longitudinal parti-
tion or sepfuni into
completely separated
right and left halves,
the partition running
Ps 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

Diagram representing a section of the SCptum is agfaiu
lieart from base to anex. . ^ ~

incompletely divided
transversely, into a thinner basal portion into which veins
open, known as the auricle, and a thicker apical por-
tion from which arteries arise, called the ventricle. The
heart thus consists of a right auricle and ventricle and a

Fig. 78.
through the heart from base to apex.

EXTERIOR OF THE HEART.

20o

left auricle and ventricle, each auricle communicating by
an auriculo-ventricular orifice witli the ventricle on its own
side, and there is no direct communication whatever tlirough
the septum between the opposite sides of the heart. To
get from one side to the other the blood must leave the

Fig. 79. — 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. Ats. left auricle : Adx and As, the ri-jht
and left auricular appendages ; Vd, right ventricle ; Vs. left ventricle ; Aa,
aorta : Ab, innominate artery ; Cs, left common carotid artery: Ssi. left sub-
clavian artery ; P. main trunk of the i^ulmonary artery, and Pd and Ps, its
branches to the right and left lungs : <?-•. superior vena cava : Ade and Asi, the
right and left innominate veins ; pd and p.s, the right and left pulmonary veins;
crd and crs, the right and left coronary arteries.

heart and pass through a set of capillaries, as may readily
be seen by tracing tlie course of the vessels in Fig. 76.

The Heart as seen from its Exterior. When the heart
is viewed from the side turned towards the sternum (Fig.
79) the two auricles, Atd andyl.s, are seen to be separated

206

THE HUMAN BODY.

by a deep groove from the ventricles, TV/ and Vs. A more
shallow furrow runs ])etween the vonti'iclos and indicates'
the position of the internal longitudinal septum. On the

Fig. 80.— The heart viewed from its dorsal aspect. Atd, right auricle ; cl,
inferior vena cava; az, azygos vein ; Vc, coronary vein. The remaining letter."
of reference have the same signification as in Fig. 79.

dorsal aspect of the heart (Fig. 80) similar points may be
noted, and on one or other of the two figures the great
vessels opening into the cavities of the heart may l)e seen.
The pulmonary artery, P, arises from the riglit ventricle,

INTERIOR OF THE HEART. 20?

and very soon divides into the right and left pulmonary
arteries, Pel 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. 78), cs and
ci, known respectively as the upper and lower vence cavce,
or *' hollow" veins: so called by the older anatomists be-
cause they are frequently found empty after deatli. Into
the back of the right auricle opens also another vein,
Vc, called the coronary vein or sinus, which biings back
blood that has circulated in the walls of the heart it-
self. Springing from 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, Ah; but before that, close
to the heart, the aorta gives off two coronary arteries,
branches of which are seen at crd and crs lying in the
groove over the partition between the ventricles, and which
carry to the substance of the organ that blood which comes
back through the coronary sinus. Into the left auricle
open two right and two left pidmonary veins, ps and pd,
which are formed by the union of smaller veins proceeding
from the lungs.

In the diagram Fig. 78 from which the branches of the
great vessels near the heart have been omitted for the sake
of clearness, 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 venas cavse
{cs and ci) and proceeding from the right ventricle the
pulmonary a7'tery, P. Opening into the left auricle are
the right and left pulmonary veins {jjd andps) and spring-
ing from the left ventricle the aorta, A.

The Interior of the Heart. The communication of
each auricle with its ventricle is also represented diagram-
matically in Fig. 78, 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 f(jr the most

208 THE HUMAN BODY.

part smooth, but from each a holloAv pouch, the auricidar
appendage, projects over the base of the corresponding ven-
tricle as seen at Adx and As in Figs. 79 and 80. These
pouches have somewhat the sliapc 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 {columnm carnece). On the
inside of the ventricles (Fig. 81) similar fleshy columns are
very prominent.

The Auriculo-Ventricular Valves. These are known
as right and left, or as the tricuspid and mitral valves re-
spectively. The mitral valve (Fig. 81) consists of two flaps
of the endocardium fixed by their bases to the margins of
auriculo-ventricular aperture and with their edges hanging
down into the ventricle when the heart is empty. These
unattached edges are not however free, but have fixed to
them a number of stout connective-tissue cords, the cordm
tendinem, which are fixed below to muscular elevations, the
papillary muscles, 31pm and 3Ipl, 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 be-
tween auricle and ventricle which lies between them, and
passes up behind the opened aorta, Sp, represented in the
figure. The tricuspid 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. 81, 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 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 neighbor doubles 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 pulmo-
nary artery and the great majority of them from the for-

THE ARTERIES.

209

mer 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

Fig. 81.— The left ventricle and the commencement of the aorta laid open
Mnm Mpl. the papillary muscles. From their iipi>er ends are seen the cordce
teiidineoe proceeding to the edges of the tlaps of tlie mitral valve. The open-
ing into the auricle lies between these flaps At the beginning of the aorta are
seen its three pouch-like semilunar valves.

the chest, giving ofi many branches on its way. Piercing
tlie diaphragm it enters the abdomen and after supplying
the parts m and around that cavity with branches, it ends

210 THE HUMAN BODY.

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 consist-
ing of the tlioracic aorta, extending from the end of the
arch to the diaphragm, and the abdominal aorta, extending
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. 79 and 80)
which spring close to the h^art, 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 imiomi7iate artery {Ab, Fig. 79), which is
very short, immediately breaking up into the right subcla-
vian 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 ofE a vertebral artery (which runs up
the neck to the head in the vertebral canal of the transverse
processes of the cervical vertebrse), crosses the arm-pit and
takes there the name of the axillary artery. This con-
tinues 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 a.ndi ulnar
arteries, the lower ends of which are seen at R and U in
Fig. 77.* These supply the forearm and end in the hand
by uniting to form an arch, from which branches are given
ofE to the fingers.

The common carotid arteries pass out of the chest into the
neck, along which they ascend on the sides of the windi^ipe.
Opposite the angle of the lower jaw each divides into an
internal and external carotid artery, right or left as the
case may be. The latter ends ma;inly in branches for the
face, scalio, and salivary glands, one great subdivision of it
with a tortuous course, the temporal artery, being often seen
beating in thin persons on the side of the brow. The in-

* P. 202.

THE CAPILLARIES. 211

ternal caroti.I aitery enters the skull throngli an aperture
in its base and supplies the brain, Avhich it Avill be remem-
bered also gets blood through the vertebral arteries.

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 are 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 cceliac axis which
supplies stomach, spleen, liver, and pancreas; the siqjerior
mesenteric artery which supplies a great part of the intes-
tine; the renal arteries, one for each kidney; and finally
the inferior mesenteric artery which supplies the rest of
the intestine. Besides these the abdominal aorta gives off
very many smaller branches.

Arteries of the Lower Limbs. Each common iliac di-
vides into an intcDial 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 ven-
tral aspect of the limb, but lower down passes back round
the femur, and above the knee-joint, where it is called the
popliteal artery, divides into the anterior and posterior
tibial arteries which supply the leg and foot.

The Capillaries. As the artei'ies ai-e followed from the
heart their branches become smaller and smaller, and finally
cannot be traced without the aid of a microscope. Ulti-
mately they pass into the capillaries, the walls of which
are simpler than those of the arteries, and which form
very close networks m nearly all parts of the Body; their
immense number compensating for their smaller size. The
average diameter of a capillary vessel is .016 mm. (:j-^
inch) so that only two or three l)lood corpuscles can pass
through it abreast, and m many pnrts they are so close

213

THE HUMAN BODY.

that a pin's point could not be inserted between two of
them. 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 net-
works in various organs, and like the last arteries are very
small. They soon increase in size by union and so form

a I ■**

Fig. 82.— 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.

larger and larger trunks. 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

THE VEINS.

213

seen as faint ])lne lines through the skin. Fig. 83 repre-
sents the arm at the front of the elbow-joint after the
skin and subcutaneous areolar tissue and fat have been re-

— rt

has

Fig. 83.— The superficial veins in front of the elbow-joint. B' , tendon of biceps
muscle ; Bi, brachialis intenius muscle ; Pt, pronator teres muscle ; 1, mediau
nerve ; 2, 3, 4, nerve-branches to the skin ; B, brachial artery witli its small
accompanying veins ; cep, cephalic vein ; bas, basilic vein ; tii', median vein ;
*, junction of a deep-lymg vem with the cephalic.

moved. The brachial artery, B, colored red, is seen lying
tolerably deeji and accompanied by two small veins (vence
coviites) which communicate by cross-branches. The great

214 THE HUMAN BODY.

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, as for instance at *, by branches from deeper
parts. Several small nerve-branches Avhich supply the skin
(3, 3, 4) are seen among these veins. It is from the vessel,
cep, called 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 Io)ig saphenous vein, which can be seen running
beneath the skin from the inner side of the ankle to the
top of the thigh. All the blood which leaves the heart by
the aorta, except that flowing through the coronary arte-
ries, is finally collected into the superior and inferior vence
cavcB {cs and ci, Figs. 79 and 80), 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 {snhclavian)
to form on each side an innominate vein {Asi and Ade,
Fig. TO) and the innominates unite to form the superior
cava. The coronary-artery blood after flowing through the
capillaries of the heart itself, also returns to this auricle by
the coronary veins.

The Pulmonary Circulation. Through this the blocd
gets back to the left side of the heart and so into the aorta
again. The pulmonary artery, dividing into branches for
each lung, ends in the capillaries of those organs. From
these it is collected by the pulmonary veins which carry it
back to the left auricle, whence it passes to the left ventricle
to recommence its flow through the Body generally.

Tho Course of the Blood. From what has been said it
is clear that tho 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

PORTAL CIRCULATION. 215

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 f<ystemic circulation, and from the right to the
left, through i\\Q\\xng?,, i\ie iiulmonary circulation. But
since after completing either of these alone the blood is not
again at the point from which it started, but is separated
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 has to pass through three sets of capillaries before it
can again return there. This is the portion which goes
through the stomach, spleen, pancreas, and intestines.
After traversing the capillaries of those organs it is col-
lected 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 inferior vena cava, which carrying it to the right
auricle, 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 thus 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 circulation.

Diagram of the Circulation. Since the two halves of
the heart are actually completely separated from one
another by an impervious partition, although placed in
proximity in the Body, we may conveniently represent the
course of the blood as in the accompanying diagram (Fig.
84) in which the right and left halves of the heart are rep-

216

TEE HUMAN BODY.

resented at different points in the vascular system. Such
an arrangement makes it clear that the heart is really two
pnmi)s working side by side, and each engaged in forcing
the blood to the other. Starting from the left auricle, la,
and following the flow we trace- it through the left ventri-
cle and along the branches of the
aorta into the systemic capillaries, sc;
from thence it passes back through
the systemic veins, vc. Beaching
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, po, car-
ry it to the left auricle, which drives
it into the left ventricle, Iv, and this
again into the aorta.

Arterial and Venoiis Blood. The
blood when flowing in the jDulmo-
nary caj)illaries gives up carbon diox-
ide to the air and receives oxygen
from it; and since its coloring mat-
ter (haemoglobin) forms a scarlet

Fig. 84. — Diagrram of the compound with OXVgen, it flowS to
blood vascular system, show- \ n, • -> . ,

ing that it forms a single the Icit auricle lu the pumionarv

closed circuit with two pumps . » t • i j t i mi •'

in it, consisting of the right VCIUS 01 a bright red COlor. ihlS
and left halves of the heart, i •/ • j. • x-n *i i

which are represented sepa- color it maintains Until it reaches
iTAi^v^i'^^'^-ell-^e, the systemic capillaries, but in these
friS t!aS'tsTstem^ ^^ loscs much oxygeu to the surround-

capillaries ; vc, yenaj cavaB ; i^p- tlSSUCS and gUlUS mucll CarboU
po, pulmonary artery ;pc,pul- o o

monary capillaries ; pv, pul- dioxidc from them. But the blood
monary veins. . t i i i

coloring matter which has lost its
ox3'gen has a dark purple-black color, and since this un-
oxidizcd or "reduced" haemoglobin is now in excess, the
blood returns to the heart by the venae cavse of a dark
purple-red color. This color it keeps until it reaches the
lungs, when the reduced haemoglobin becomes again oxi-
dized. The bright red blood, rich in oxygen and poor in
carbon dioxide, is known as "arterial blood'' and the dark

8TR UCTUBE OF TEE BLOOD- VESSELS. 317

red as " venous blood:" and it must be borne in mind that
the terms have this peculiar teclmical meaning, and that
the pulmonary veins contain arterial blood and the pulmo-
nary arteries, 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 ar-
terial blood are shaded red in Figs. ?9 and 80.

The Structure of the Arteries. A large artery can by
careful dissection be separated into three coats; an internal,
middle and outer. TL\e internal coat tears readily across
the long axis of the artery and consists of an inner lining
of flattened nucleated cells, and of a variable numbefof
layers composed of membranes or networks of elastic tissue,
outside this. The middle coat is made uji of alternating
layers of elastic fibres and jilain muscular tissue; the for-
mer running for the most part longitudinally and the latter
across the long axis of the vessel. The outer coat is the
toughest and strongest of all and is mainly made up of
white fibrous connective tissue but contains a considerable
amount of elastic tissue also. It gradually shades off into
a loose areolar tissue which forms the slieath 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 mus-
cular fibres. As a result the large arteries are highly elas-
tic, the aorta being physically much like a piece of Indian-
rubber tubing, while the smaller arteries are highly con-
tractile, in the physiological 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 adven-
titia. These vessels are thus extremely well adapted to al-
low of filtration or diffusion taking place through their
thin walls.

218 THE HUMAN BODY.

Structure of the Veins. In these the same three pri-
mary coats as in the arteries may be found: the inner and
middle coats are less developed while the outer one remains
thick, and is made up almost entirely of white fibrous tissue.
Hence venous walls are much thinner than those of the
corresponding arteries, and the veins collapse when empty
while the stouter arteries remain ojjen. But the tenacity
and toughness of their outer coats give the veins great
strength.

Excejot the pulmonary artery and the aorta, which pos-
sess the semilunar valves at their cardiac orifices, the
arteries possess no valves. Many veins on the contrary have
such, formed by semilunar pouches of the inner coat, at-
tached by one margin and having that turned towards the
heart free. These valves, sometimes single, oftener in
pairs, and sometimes three at one level, permit blood to
flow only towards the heart, for a current in that direction
(as in the upper diagram, Fig. 85) joresses the valve close
against the side of the vessel and
. meets with no oljstruction from it.

Should any back-flow be attempted,
however, the current closes vc^ the
valve and bars its own passage as
indicated in the lower figure.
These valves are most numerous

tJtfthelSJdf of acuofof "^ Superficial veins and those of
the valves of the veins, c the muscular parts. They are absent

capillary, and H^ the heart , -^ •'

end of the vessel. m the veute cavne 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 sub-
cutaneous veins and cause their dilatation, the points at
which valves are placed can be recognized by their swollen
appearance. They are most frequently found where two
veins communicate.

CHAPTER XV.

THE WORKING OF THE HEART AND BLOOD-
VESSELS.

The Beat of the Heart. It is possible by methods known
to physiologists to open tlie chest of a living narcotized
animal, such as a rabbit, and sec its heart at work, alter-
nately contracting and diminishing the cavities within it
and relaxing and expanding them. It is then 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 commencing to dilate the
moment the ventricles commence to contract. Having
finished their contraction, the ventricles also commence to
dilate and so for some time neither they nor the auricles
are contracting, but the whole heart expanding. The con-
traction of any part of the heart is known as its systole
and the relaxation as its diastole, and since the two sides
of the heart work synchronously, the auricles together and
the ventricles together, we may describe a whole ''cardiac
period" or " heart-beat"' as made up successively of auricu-
lar systole, ventricular systole, and pause. This cycle is
repeated about seventy times a minute; and if the whole
time occupied by it be subdivided into 100 parts, about
9 of these will be occupied by the auricular systole, about
30 by the ventricular systole, and 61 by the pause:
during more than half of life, therefore, the muscles of
the heart are at rest. In the pause the heart if taken be-
tween the finger and thumb feels soft and flabby but dur-
ing the systole it (especially in its ventricular portion) be-
comes hard and rigid.

Change of Form of the Heart. During its systole the

220 THE HUMAN BODY.

heart becomes shorter and rounder, mainly from a change
in the shape of the yentricles. A cross-section of the heart
at the base of these hitter during diastole would be ellipti-
cal in outline, with its long diameter from right to left:
during the systole it is more circular, the long axis of the
ellipse becoming shortened while the dorso-ventral diameter
remains little altered. At the same time the length of the
ventricles 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 ventri-
cles yield to the chest- wall where they touch it, but dur-
ing the systole they become hard and tense and push it out
a little between the ribs, and so cause the a]3ex 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 hoAvever 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 tolerably full, this will be distended not only
transversely but longitudinally. 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 this lengthening of that limb of its arch at-
tached to the heart pushes the latter down towards the dia-
phragm, and compensates for the upward movement of the
apex due to the shortening of the ventricles. Hence if the
exposed living heart be watched it appears as if during the
systole the base of the heart moved towards the tip, rather
than the reverse.

Events occurring -within the Heart during a Cardiac
Period, Let us commence at the end of the ventricular
systole. At this moment the semilunar valves at the orifices

PHENOMENA OF THE HEARTS BEAT. 321

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 venre
cavae; this passes on through the open mitral and tricuspid
valves and fills up the dilating ventricles, as well as 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 contrac-
tion 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 contract-
ing auricles send blood mainly into the ventricles, and
hardly any back into the veins. At the same time the in-
creased direct current into the ventricles produces a greater
back current on the sides, which, as the auricles cease their
contraction and the filled ventricles become tense and press
on the blood inside them, completely close the auriculo-
ventricular valves. That this increased filling of the ven-
tricles, due to auricular contractions, will close the valves is
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
commences. The blood in each ventricle is imprisoned
between the auriculo-ventricular valves behind and the
semilunar valves in front. The former cannot yield on
account of the cordse tendinese 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

222 THE HUMAN BODY.

on each side of them; but of course they might be forced
open without this by applying sufficient power to overcome
the higher water pressure on one side. It is in this latter
way that the semilunar valves are opened. Tlie contracting
ventricle tightens its grip on the blood inside it and becomes
rigid to the touch. As it squeezes harder and harder, at
Icist the pressure on the blood in it becomes greater than
the pressure exerted on the other side of the valves by the
blood in the arteries, the flajas are pushed ojoen, and the
blood begins to pass out: the ventricle continues its con-
traction until it has obliterated its cavity and completely
emptied itself. Then it commences to relax and blood
immediately to flow back into it from the highly stretched
arteries. This back 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 con-
tracting ventricles may not force blood back into the
auricles it is essential that the flaps of the mitral and
tricuspid valves be maintained horizontally across the open-
ings which they close, and be not pushed back into the
auricles. At the commencement of the ventricular sys-
tole this is provided for by the cordse tendine^e, which are
of such a length as to keep the edges of the flaps in appo-
sition, a i^osition which is farther secured by the fact that
each set of cordse tendine« (Fig. 81*) radiating from a
point in the ventricle, is not attached 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 ven-
tricles shorten, the cordse tendineae, 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 distinguish-
able sounds will be heard during each cardiac cycle. They
are known respectiA^ely as the first and second sounds of the

* P. 209.

EVENTS IX A CARDIAC CYCLE.

223

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 lulh, chqj. 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 liAdng animal. The origin of the first sound is
still uncertain: it takes place during the ventricular systole
and is probably due to vibrations of the tense ventricular
wall at that time. It is not due, as has been supposed, to
the auriculo-ventricular valves, since it may still be heard
in a beatmg heart empty of blood, and m which there cotild
be no closure or tension of those valves. In various forms
of heart disease these sounds are modified or cloaked by
additional " murmurs" which arise when the cardiac orifices
are roughened or narrowed or dilated, or the valves ineffi-
cient. 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 impor-
tant information as to its catise.

Diagram of the Events of a Cardiac Cycle. In the
following table the phenomena of the heart's beat are rep-
resented with reference to the changes of form which are
seen in an exposed working heart. Events in the same
vertical column occur simultaneously; on the same horizon-
tal line, from left to right, successively.

Auricular Sys-
tole.

Auricles ' Contracting

and
emptying.

Ventricles | Dilating and

filling.

Impulse

Auriculo - ventric-
ular valves

Semilunar valves.
Sounds

Closing.
Closed.

Commence-
ment of Ven-
tricular Sys-
tole.

Dilating and
filling.

Contracting.

Apex beat.

Closed. I Closed.

Closed. Open.

First sotmd.

Ventricular
Systole.

Dilating and
filling.

Contracting

and
emptying.

Cessation of

Ventricular !
Systole. I

Dilating , Dilating
and filling, and filling.

Dilating. , Dilating
i and filling.

Opening. • Open.
Closing. Closed.
Second
sound, i

224 THE HUMAN BODY.

Function of the Auricles. The ventricles have to do
the work of pumping the blood througli the blood-vessels.
Accordingly their walls are far thicker and more muscular
Llian 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 com-
paratively short pulmonary circuit. The circulation of tlic
blood is in fact maintained by the ventricles, and we
have to inquire what is the use of the auricles. Not un-
frequently 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 con-
traction merely completing their filling and finishing the
closure of the auriculo-ventricular valves. The real use of
the auricles is to afford a reservoir into which the veins may
empty while the comparatively long-lasting ventricular
contraction is taking place: they also largely control the
amount of work done by the heart.

If the heart consisted of the ventricles only, with valves
at the points of entry and exit of the blood, the circulation
could be maintained. During diastole the ventricle would
fill from the veins, and during systole empty into the ar-
teries. But in order to accomplish this, during the systole
the valves at the point of entry must be closed, or the ven-
tricle 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 reser-
voirs at the end of the venous system collecting blood
when the ventricular pump is at work. When the ven-
• tricles relax, the blood entering the auricles flows on
into them: but previously, during the -^^^ of the cardiac
cycle occupied by the ventricular systole, the auricles
have accumulated blood, tmd when thev at last con-

FUNCTION OF THE AURICLES. 225

tract they send on into the ventricles this accnmulation.
Even were the flow from tlie veins stopj^ed 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. For at that mo-
ment the ventricle is relaxing and receives the blood from
the auricles under a less pressure than it enters the latter
from the veins. The heart in fact consists 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 auricles also govern to a certain extent the amount
of work done by the ventricles. These latter contract with
more than sufficient force to completely drive out all the
blood contained in them. If the auricles contract more
})owerfully and empty themselves more completely at any
given time, the ventricles will contain more blood at the
commencement of their systole, and have pumped out more
at its end. Now as we shall see in Chapter XVII., the
contraction of the auncles is under the control of the
nervous system; and through the auricles the whole work
of the heart. In fact the ventricles represent the brute
force concerned in maintaining the circulation, while
the auricles are part of a highly developed co-ordinating
mechanism, by which the rate of the circulation is governed
according to the needs of the whole Body at the time.

The Work Done by the Heart. This can be calculated
witli approximate correctness. At each systole each ven-
tricle sends out the same quantity of blood — about 180
grams (6.3 ounces); the pressure exerted by the blood
in the aorta against the semilunar valves and which the
ventricle has to overcome is about that which would be ex-
erted on the same surface by a column of mercury 200

226 THE HUMAN BODY.

millimeters (8 inches) high. The left ventricle therefore
drives out, seventy times in a minute, 180 grams (6.3
ounces) of blood against this pressure. Since the specific
gravity of mercury is 13,5 and that of blood may for prac-
tical purposes be taken as 1, the work of each stroke of the
ventricle is equivalent to raising 180 grams (6.3 ounces)
of blood 200 X 12.5 = 2500 millim. (8.2 feet); or one
gram 450 meters (one ounce 51.66 feet); or one kilo-
gram 0.45 meters (one lb. 3.23 feet). "Work is measured
by the amount of energy needed to raise a definite weight
a given distance against gravity at the earth's surface, the
unit, called a Icilogr ammeter, being either that necessary to
raise one kilogram one meter, or, called a foot-pound, that
necessary to raise one pound one foot. Expressed thus the
work of the left ventricle in one minute, when the heart'.s
rate is seventy strokes in that time, is 0.45 x 70 = 31.50
kilogrammeters (3.23x70 = 226.1 foot-pounds); in one
hour it is 31.50 X 60 = 1890 kilogrammeters (226.1 X 60
= 13,566 footpounds); and m twenty-four hours 1890 x 24
= 45,360 kilogrammeters (325,584 foot-pounds). The pres-
sure in the pulmonary artery against which the right ventricle
works is about ^ of that in the aorta; hence this ventricle
in twenty-four hours will do one third as much work as the
left, or 15,120 kilogrammeters (108,528 foot pounds) and
adding this to the amount done by the left, we get as the
total work of the ventricles in a day the immense amount
of 60,480 kilogrammeters (434,112 foot-jwunds). If a man
weighing 75 kilograms (165 lbs.) climbed up a mountain
806 meters (2644 feet) high his skeletal muscles would
probably be greatly fatigued at the end of the ascent, and
yet in lifting his Body that height they would only have
performed the amount of work that the ventricles of the
heart do daily without fatigue.

The Flow of the Blood Outside the Heart. The blood
leaves the heart intermittently and not in a regular stream,
a quantity being forced out at each systole of the ventri-
cles: 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 trans-

BLOOD-FLOW AS SEEN WITH THE MICROSCOPE. 327

parent parts of various animals, as the web of a frog's foot,
a mouse's ear, or the tail of a small fish. In consequence
of the steadiness with which the capillaries supi)ly the veins
the flow in these is also unaffected, directly, Ly each beat of
the heart; if a vein be cut the blood wells out uniformly,
while a cut artery spurts out not only with much more
force, but in jets which are much more powerful at regu-
lar intervals corresponding witli the systoles of the ven-
tricles.

The Circulation of the Blood as Seen in the Frog's Web.
There is no more fascinating or instructive phenomenon
than the circulation of the blood as seen with the micro-
scope in the thin membrane between the toes of a frog's
hind limb. Upon focusing beneath the epidermis a net-
work of minute arteries, veins and capillaries, with the blood
flowing through them, comes into view (Fig. 8-?*). The
arteries, a, are readily recognized by the fact that the flow in
ihem is fastest and from larger to smaller branches. The
latter are seen ending in capillaries, which form networks,
rJie channels of which are all nearly equal in size. While
m the veins arising from the capillary the flow is from
smaller to larger 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 m and moves more slowly. The area of the veins
is smaller than that of the capillaries but gi-eater than
that of the arteries, and hence the rate of movement in them
IS also intermediate. Almost always when an arterv
divides, the area of its branches is gi-eater than that of the
mam 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 tlie 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 is greater than that of

*p.ai2-

228 THE HUMAN BODY.

tjie 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
cavee is less than in the pulmonary artery and aorta. We
may represent the vascular system as a double cone, widening
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 form together 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 centre is a very rapid current carrying along all the red
corpuscles and known as the axial dream, while near 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 layer of the
liquid will remain motionless in contact with the tube;
the next layer of molecules will move faster, the next
faster still; and so on until a very rapid current is found
in the centre. If solid bodies, as powdered sealing-wax,
lie suspended in the water, these will all be carried on
in the central faster current or aorial stream, just as the
red corpuscles are in the artery. The white corpuscles, on
account of their powder of executing independent amoeboid
movements and their consequent irregular form, get fre-
quently pushed out of the axial current, so that many of
them are found in the inert layer.

Internal Friction. It follows from the above-stated
facts that there is no noticeable friction between the. blood
and the lining of the vessel through which it flows: since
the outermost blood layer in contact with the wall of the
vessel is almost motionless. But there is very great fric-
tion between the different concentric layers of the liquid,
since each of them is moving at a different rate from those

THE CAPILLARY CIRCULATION. 229

in contact with it on each side. This form of friction is
known in hydro-dynamics as " internal friction" and it is
of great importance in the circulation of the blood. In-
ternal friction increases very fast as the calibre 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 in-
ternal 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 internal friction, but only to a comparatively
slight extent; so that the internal friction, and conse-
quently the resistance to the blood-flow, is far greater in the
capillaries than in the small arteries, and in the small ar-
teries than in the large ones. Practically we may regard
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 inter-
mittently, 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 "j^w^.se," by ap-
plying the finger over the radial artery at the wrist, or 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 obvi-
ous in the more distant vessels. But if this were so, then
when the blood was collected again fi-om the wide capillary
sponge into the great veins near the heart, which together

230

THE HUM AS BODY.

— cL

—d'

cC—

B

are very little bigger tlum the aorta. Ave ought to find a
pulse, but we do not: the venous pulse which sometimes
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 rhythmic 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 suj^jily is transformed

into a continuous flow. Sup-
pose we have two vessels, A
and B (Fig. 86), containing
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 h,
which may be left wide open
or narrowed by the clamp c,
at will. If the apjDaratus be
left at rest the water will lie at the same level, d, in each
vesssel. If now Ave work the pump, at each stroke a cer-
tain amount of water will be conveyed from A to B, and
as a result of the loAvering 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,
Avould represent the venous system, from which the heart
constantly takes blood to pump it into B, representing the
arterial system; and b Avould represent the capillary vessels
through which the return flow takes place: but, so far, we
should have as intermittent a floAv through the capillaries,
b, as through the heart-pump, a. Now imagine b to be
narroAved at one point so as to oppose resistance to the
back-flow, while the pump goes on working steadily. The re-
sult will be an accumulation 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 b to A, and more Avill floAv back, under

CAUSE OF STEADY CAPILLARY BLOOD-FLOW. 331

the greatoi' difference of pressure, in a given time, until al
last, when the water in B has reached a certain level, ct,
and that in A has correspondingly fallen to d/', the current
through b will carry hack in one minute just so much water
as the pump sends the other way, and this back- flow will
be nearly constant; it will not depend directly upon the
strokes of the pump I)ut upon the head of water accumu-
lated 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 driving force; and
its influence will be inappreciable. We thus gain the idea
that an incomplete impediment to the flow from the ar-
teries to the veins (from Bto A in tlie diagram), such as is
afforded by internal friction in the capillaries, may bring
about conditions which will lead to a steady flow through
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 con-
sider if the ''head of water" can be replaced by some other
form of driving force. It is in fact replaced by the elas-
ticity of the large arteries. Suppose an elastic bag in-
stead 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 pum])
sends in a given time more liquid into it than it passes
back through b. But the more it becomes distended the
more will the bag scpieeze the liquid inside and the faster
will it send that back to A, until at last its squeeze is so pow-
erful that in a minute or any other unit of time it sends
tniek into A as much as it receives. Thenceforth the
hack-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 occurring in the blood-vessels.
The highly elastic large arteries are kept stretched with
blood by the heart; and the reaction of their elastic walls.

232 THE HUMAN BODY.

steiidily squeezing on the blood in them, forces it continu-
ously 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 re-
sistance to their emptying, 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.

CHAPTER XVI.

ARTERIAL PRESSURE. THE PULSE.

V7eber's Schema. It is clear from the statements made
in the last chapter that it is the pressure exerted by the elas-
tic arteries upon the blood inside them which keeps up the
flow through the capillaries, the heart serving to keep the
bi(^ arteries tio-htly 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 apjaaratus it is of
great importance to know all 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 the 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 pres-
sure has been ascertained by experiments made upon the
lower animals, from which deductions are then made con-
cerning wliat happens in man, since anatomy shows that the
circulatory organs are arranged upon the same plan in all
the mammalia. A great deal can, however, be learnt by
studying the flow of liquids through ordinary elastic tubes.
Suppose we have a set of such (Fig. 87) supplied at one
point Avith a pump, c, possessing valves of entry and exit
which open only in the direction indicated by the arrows,
and that the whole system is slightly overfilled with liquid so

234

THE HUMAN BODY.

that its elastic 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 J), since
liquid in a set of horizontal tubes communicating freely,
as these do at D, always distributes itself so that the
pressure upon it is everywhere the same. Let the pump
c now contract once, and then dilate: during the contrac-
tion it will empty itself into B and during the dilatation fill
itself from A. Consequently the pressure in B, indicated
by the gauge x, will rise and that in A will fall. But very
rapidly the liquid will redistribute itself from B to A
through B, until it again exists everywhere under the same

Fig. 87. — Diagram of WebPi's Schema.

pressure. Every time the j^ump ^\o]'ks there will occur a
similar serie.s of phenomena, and there will be a disturbance
of equilibrium causing a wave to flow round the tubing;
but there will be no steady maintenance of a pressure on
the side B greater than that in A. Now let the ii})per
tube D lie closed so that the liquid to get from B to A must
flow through the narrow lower tubes D, which opi)Ose con-
siderable resistance to its passage on account of their fre-
quent branchings and the great internal friction in tliem;
then if the pump works frcfpieiitly enough there will be
produced and maintained in B a ])ressiire considerably higher
than that in A, which may even become negative. If, for

WEBERS SCHEMA. 235

example, the pump works 60 times a minute and at each
stroke takes 180 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
oSered by the small branched tubes), have all got back into
A before the next stroke takes place, sending 180 more
cubic centimeters (6 oz. ) into B. Consequently at each
stroke B will become more and more distended and A more
;ind more emptied, and the gauge ./; will indicate a much
higher pressure than that on A. As B is more stretched,
howeyer, 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 oz.) 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 Ions:
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 depends upon the difference in pressure prevailing
between B and A, and only indirectly upon the pump
which serves simjDly to keep the pressure high in B and
low in ^. At each stroke of the pump it is true there will
be a slight increase of pressure in B due to the fresh 180
cub. cent. (6 oz.) forced into it, but this increase will Ije
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 jii^t de-
scribed represents 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 smallest arteries and the capil-
laries the resisttmce at B, the large arteries the elastic
tube B, and the veins the tube A. The ventricles con-
stantly 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 accumu-

236 TEE 3XTMAN BODY.

lates in them until the elastic reaction of the stretched ar-
teries is able to squeeze in a minute through the capillaries
just so much blood as the left yentricle pumps into the
aorta, aud the right into tlie 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 persistent differ-
eucc of pressure, only increased by a small fraction of the
whole at each heart-beat, keeps up 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 these de-
pends 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 pressure, in conse-
quence of this resistance.

The Arterial Pressure. This cannot be directly meas-
ured with accuracy in man, but from measurements made
on other animals it is calculated that in the human aorta
its average is equal to that of a column of mercury 200
millimeters (8 inches) high. During the systole it rises
about 5 millimeters (^ inch) above this and during the pause
falls the same amount below it. The pressure in the venae
cavae on the other hand is often negative, the blood being,
to use ordinary language, often "sucked" out of them into
the heart, and it rarely rises above 5 millimeters {\ inch) of
mercury except under conditions (such as powerful mus-
cular effort accompanied by holding the breath) which
force blood on into the venae cavae and, by impeding the
pulmonary circulation, interfere with the emptying of the
right auricle. Hence to maintain the flow from the aorta
to the vena cava we have an average difference of pressure
equal to 200 — 5 = 195 millimeters. (7f inches) of mercury,
rising to 205 — 5 = 200 mm. (8 inches) during the cardiac
systole and falling to 195 — 5 = 190 mm. (7f inches) dur-
ing the pause; but the slight alterations, only about ^ of
the whole difference of aortic and vena cava pressures which
maintain the blood-flow, are too slight to cause appreciable

ARTERIAL PRESSURE. 237

changes in the rate of the current in the capillaries. The
pressure on the Mood in the pulmonary artery is about \ of
that in the aorta.

Since the blood flows from the aorta to its branches and
from these to the capillaries and thence to the yeins, 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 pro-
portionate to the resistance in front, and since the farther
the blood has gone the less of this, due to impediments at
branchings and to internal friction, it has to overcome in
finishing its round, the pressure on the blood always di-
minishes as we follow it from the aorta to the venas cavae.
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 passed by is (on account of the
great internal friction due to their small calibre) very great,
and consequently the fall of pressure between the medium-
sized arteries and the veins is rapid and considerable.

Modifleations of Arterial Pressure by Changes in the
Rate of the Heart's Beat. A little consideration will make
it clear that the pressure prevailing 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. Returning to the system of elas-
tic tubes with a pump represented in Fig. 87, let us sup-
pose the pump to be driving as before 10.800 cub. cent.
(360 oz.) per minute into the tubes B and that these latter
are so distended that they drive out just that quantity in
the same time. Under such conditions the pressure at'^any
given point in B will remain constant, apart from the small
variations dependent upon each stroke of the pump.
Now, however, let the latter, while still sending in 180 cub.
cent. (6 oz. ) at each stroke, work 80 instead of 60 times a
minute and so send in that time 180 x 80 = 14,400 cub.

2??.<s THE HUMAN BODY.

cent. (480 oz.) instead of the former quantity. Tliis •will
lead to au accumulation in B, since its squeeze is only suf-
ticient, against the resistance opposed to it, to send out 10,-
SOO cub. cent. (360 oz.) in a minute. B consequently will
l)ecome more stretched and the pressure in it will rise. x\s
this takes place, however, it will squeeze more powerfully
on it.s contents until ai la>l it.s distension i> such that \\<
(jiasticiiy is able to force out in a niiiiiite thrcaigh tlit'.^mall
tubes D, 14,400 cub, cent, (480oz,j, Thenceforth, so long
as the pump beats with the same force and at the same rate
and the peripheral resistance remains the same, the mean
pressure in B will neither rise nor fall — B sending into A
in a minute as much as c takes from it, and we would have
a steady condition of things with a higher mean pressure
jn B than before.

On the other hand if the pump begins to work more
slowly while the resistance remains the same, it is clear that
the mean pressure in B will fall. If, for example, the pump
works only forty times a minute and so sends in that time
180 X 40 = 7200 cub. cent. (240 oz.) into B, which is so
stretched that it is squeezing out 10,800 cub. cent. (360
oz.) in that time, it is clear that B will gradually empty
itself and its walls become less stretched and the pressure
in it fall. As this takes place, however, it will force less
liquid in a minute through the small tubes, until at last a
pressure is reached at which the squeeze of B only sends
out 7200 cub. cent. (240 oz.) in a minute; and then the
fall of pressure will cease and a steady one will be main-
tained, but lower than before.

Applying the same reasoning to the vascular system we
see that (when the iieripheral resistance remains unaltered),
if the heart's force remains the same but its rate increases,
arterial i)ressure will rise to a ncAV level, while a sloM'ing of
the lieart's beat will bring about a fall of pressure.

Modifications of Arterial Pressure Dependent on
Changes in the Force of the Heart's Beat. lieturning
again to Tig. 87; suppose that wliile tlie rate of the pumji
remains the same, its jiower alters so that each time it
sends 200 cab. cent. (6.6 oz.) insicndof 180 (6 oz.) andso in

CHANGES Ilf ARTERIAL PRESSURE. 239

a minute 12,000 cub. cent. (396 oz.) instead of 10,800 (360
02.) — the quantity Avhich B is stretched enough to squeeze
out in that time. Water will in consequence accunutlate in
B until it becomes stretched enough to squeeze out 12,000
cub. cent. (306 oz. ) in a minute, and then a steady pressure
at a new and higher level will be maintained. On the
other hand if the pump, still beating sixty times a minute,
^ork* more feebly so as to send out only 160 cub. t-cut. (.").<!
oz.) at each stroke, then B. squeezing out at liiv-fc m<Mv
tlian it receives in a given time, will gradually empty
itself until it only presses hard enough upon its contents
to force 160 X 60 = 9600 cub. cent. (336 oz.) out in a
minute.

Similarly, if while the resistance in the small arteries
and capillaries remains the same and ^ also tlie heart's rate,
the power of the stroke of the latter alters, so that at each
beat it sends more blood out than previously, then arterial
pressure will rise; while if the heart beats more feebly it
will fall.

Modifications of Arterial Pressure by Changes in the
Peripheral Resistance. Let the pnmp c in Fig. 87 still
work steadily sending 10,800 cub. cent. (360 oz.) per min-
ute into B and the resistance increase, it is clear arterial
pressure must rise. For B is only stretched enough to
squeeze out in a minute the above quantity of liquid against
the original resistance and cannot at first send out that
quantity against the greater. Liqitid will consequently ac-
cttmulate in it until at last it becomes stretched enough to
send out 10,800 cub. cent. (360 cubic oz.) in a minute
through the small tubes, 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 otit in a minute more than it receives,
until finally its elastic presstirc is reduced to the point at
Avhicli its receipts and losses balance, and a new and lower
mean pressure will be established in B.

So in the vascular system increase of the peripheral re-
sistance by narrowing of the small arteries will increase ar-

240 THE HUMAN BODY.

terial pressure in all parts nearer the heart, Avhile 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 rate of the heart's beat;
(2) the quantity of blood forced into the arteries at each
beat; (3) the calibre of the smaller vessels. All of these,
and consequently the capillary circulation which depends
upon arterial pressure, are under the control of the nervous
system (see Chap. XVII.).

The Pulse. When the left ventricle contracts it forces
a certain amount of blood into the aorta, Avhich is already
distended and on account of the resistance in front cannot
empty itself so fast as the contracting ventricle fills it. As
a consequence its elastic walls yield still more — it enlarges
both transversely and longitudinally and if ex])osed 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 it emp-
ties (the whole period of the diastole of the heart being
required for emptying the aorta of the blood sent in
during the 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 emj^tying 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, etc. 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 descril)e(l. But after each heart-beat it be-
comes more rigid and dilates a little, the increased disten-
sion and rigidity gradually disaj^pearing as the artery
passes on the excess of blood before the next heart-beat.

THE PULSE. 241

The pulse is then a wave of increased pressure started by
the ventricukir 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 resistance there to the transmission onwards of the
blood sent in by the heart is greatest, and tJie elastic tube
m which it consequently accumulates is shortest, and so the
increase of pressure and the dilatation caused are consider-
able. The aorta, however, gradually squeezes out the ex-
cess blood into its branches and so this becomes distributed
over a wider area, and these branches having less resistance
m front find less and less difficulty in passing it on; conse-
quently 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 sensibly raise
the mean pressure 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 ca-
pillaries.

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 seconds, and in the posterior tibial artery
at the ankle 0.193 seconds, after the ventricular systole.
The blood itself does not of course travel as fast as the
pulse-wave, foi that quantity sent 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 differ-
ent 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 increased tension

242 THE HUMAN BODY.

would he felt immediiituly after each stroke of the pump,
although the li(juid puuijx'd in at the otlier eud would have
remained about its point of entry; it would cause the imi-
tation not by flowing along the tube itself, but by giving a
push to the liquid already in it. If instead of absoluteh
closing the distal end of the tube one brought about a
.4;ite of things more nearly resembling that found in the
.'H't erics by allowing it to empty itself against a resistance,
say tlii-oiigli a narrow ojiening, the phenomena obseiNcd
would not be essentially altered; the increase of pressure
Avould travel along the distended tube far faster than the
liquid in it.

The pulse being dependent on the heart's systole, *' feel-
ing the pulse" of course primaril}" 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" show-
ing a low arterial pressure, or tense and rigid ("a hard
pulse") indicative of high arterial pressure, and so on. In
adults the normal pulse rate may vary from sixty-five to
seventy-five. In the same individual it is faster when
standing than when sitting, and when sitting than when
lying down. Any exercise increases its rate temjiorarily
and so does excitement; a sick person's pulse should not
therefore be felt when he is nervous or excited (as the
])hysician knows when he tries first to get his patient calm
and confident), as it is then difiicult to draw correct
conclusions from it. In children the pulse is quicker than
in adults, and in old age slower than in middle life.

The Rate of the Blood-Flow. As the vascular system
becomes more capacious from the aorta to the capillaries
tlie rate of flow in it becomes proijortionately slower, and
as the total area of the channels diminishes again from the
capillaries to the ven^ cava3, so does the rate of flow quicken
again, just as a river current slackens where it spreads out,
and flows faster where it is confined to a narrower channel;
a fact taken advantage of in the construction of Eads* jetties
at the mouth of the Mississippi, the object of which is to
make the water flow in a narrower channel and so with a

SECOND ART AIDS TO THE CIRCULATION. 243

more ra]ii»l current. Actual nleas^^rements 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 tlic
human carotid the blood flows about 400 millimeters ( Kl
inches) in a second. In the capillaries the cnrrent travels
only from 0.5 to 0,75 mm. (^\^ to -^ inch) in a second.
Till' toi;il time taken liv a |ini-tion of blood in getting from
ihc aorut Lhioiiii-li iho cai-utid and its brunches, and the
capillaries, and then tlirough veins to the right auricle,
that is in going round the systemic circulation, is about
23 seconds — of which time about one second is spent in
the capillaries; each portion of blood on its course from
the last artery to the first vein passes through a length
of capillary Avhicli 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 varia-
tions dependent on the respiratory and other movements
of the Body (see below).

Secondary Causes of 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 whipped blood into the aorta
nnder a less force than that exerted by the left ventricle
during life, is more than suflEicient to drive it round and
back by the venae cavae. Not unfrequently the statement
is made in books that, probably, the systemic cajDillaries
have an attractive force for arterial blood and the pulmonary
capillaries for venous blood, but there is not the slightest
evidence of the correctness of sucli a supposition, nor any
necessity for making it.

The Influence of Gravity. Under ordinary circum-
stances this ma} be neglected, since in parts of the Bodv
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 j)oints in mind. A part
•' congested" or gorged with l)lood should if possible be

244 THE HUMAN BODY.

raised so as to make the back-liow in its veins easier; and
sometimes when the heart is acting feebly it may be able
to drive blood along arteries in which gravity hel])s, but not
otherwise. Accordingly in a tendency to fainting it is best
to lie down, and make it easier for the heart to send blood
up to the brain, bloodlessness of which is the cause of the
loss of consciousness in a fainting-fit. In fact so long as
the breathing continues the aspiration of the thorax will
keep up the Tenons flow (see below), while, in the circum-
stances supposed, a slight diminution in the resistance op-
posed to the arterial flow may be of im2:)ortance. 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 wliom 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 jiressure, as by
a tight garter, is of course bad since it checks all subse-
(pient flow through the vessel, but intermittent pressure,
such as exerted on many veins by muscles in the ordi-
nary movements of the Body, acts as a jiump to force on
the blood in them.

The valves of the veins have another use in diminishing
the pressure on the lower part of those vessels in many
r-egions. If, for instance, there were no valves in the long
saphenous vein (p. 214) of the leg the weight of the Avhole
column of blood in it, which in the erect position would be
about a meter (39 inches) high, would press on the loAver
l)art 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 di-
minished, while that on the other vessels of the Body is un-
affected. In conspquencc bbwd tends to flow into the chest.

PROOFS OF THE CIRCULATION. 345

It cannot, however, flow back from the arteries on account
of the semilunar vulves of the aorta, but it readily is pressed,
or in common language ''sucked," thus 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 influence it, except in so far
as the distension or collapse of the lungs influences the
calibre of their yessels.

The considerable influence of the respiratory movements
upon the venous cu'culation 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 faster than it empties during expiration, thus exhib-
iting a sort of venous pulse. Every one, too, knows that
by making a violent and prolonged expiration, as exhibited
for example by a child with Avhooping-cough, the flow
in all the veins of the head and neck may be checked,
causing them to swell up and hinder the capillary circula-
tion until the person becomes " black in the face," from the
ensforofement of the small vessels with the dark-colored ve-
nous 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 tlie veins.

Proofs of the Circulation of the Blood. The older
physiologists 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 supposed to exist in the
septum. Servetus, who was burnt alive by Calvin in 1553,
first showed 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

246 THE HUMAN BODY.

1616, demonstrated that (lie Tiiovement of (lie lilood was a
continuous circulation as we now know i(, and so laid the
foundation of UKjdern Physiology. In his time, however,
the capillary vessels had not been discovered, so that al-
though he was quite certain that the blood got somehow
from the final branches of the aorta to the radicles of the
^enous system, he did not exactly know how.

Tl.\e 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 puss 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 /ro/« the heart, through the arteries and back ^o 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 whoi
emptied fills only from the end farthest from the lieart.
This experiment 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 a dependent posi-
tion for a few seconds. Select then a vein which runs
for an inch or so without branching, place one 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 vessel will then be found to remain empty as
long as the finger is kept on its lower end, but will fill
immediately when it is removed; which i)roves 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
])roximal side empty, showing again that the veins only
receive blood from their ends turned towards the capilla-
ries. (G) In the lower animals direct observation with the
microscope shows the steady flow of blood from the arte-
ries through the capillaries to the veins, but never in the
()p])osite direction.

CHAPTER XVII.

THE REGULATION OF THE HEAET AND BLOOD-

VESSELS BY THE NERVOUS SYSTEM.

The Need of Co-ordination. For the safe and liarmo-
nious working of the circulatory apparatus it is obviously
necessary that there be some mode of mutual interaction
between the heart and the blood-vessels: if the heart beat
and the arteries relaxed or contracted, each without any
reference to the other, no orderly capillary flow could be
maintained. To secure that, the work done by the heart and
the resistance to the blood-flow offered in the vessels must
at any given moment be correlated : so that the heart shall
not by too powerful action over-distend or perhaps burst
the small arteries, nor the latter contract too much and so,
by increasing the peripheral resistance, raise the aortic pres-
sure to a great height and increase unduly the work to be
done by the left ventricle in forcing open the semilunar
valves. Again, the total amount of blood in the Body is
not sufficient to keep all its organs supplied with the
amount needful for the full exercise of their activity at
one time, and in the Body accordingly we never find all
its parts hard at work at the same moment. If when one
grou)> of muscles was set at work and needed an extra
l)lood-snpply, this was att;iined merely by increasing the
heart's activity and keeping up a faster blood-flow every-
where through the Body, there would be a clear waste of
force, much as if the chandeliers in a house were so ar-
ranged that when a larger flame was Avanted at one burner
it could only be obtained by turning more gas on at all the
rest at the same time: besides the big tap at the gas-meter
regulating the general supply of the house, local taps at

248 TEE HUMAN BODT.

each burner are required which regulate the gas-supply to
each flame independently of tlie rest, A similar arrange-
ment is found in the Body. Certain nerves control the
calibre of the arteries supplying different organs and, when
the Litter are set at work, allow their arteries to dilate and
so increase the amount of blood flowing through them
while the general circulation elsewhere remains practically
unaffected. The resting parts at any moment thus get
Just enough blood to maintain their healthy nutrition and
the working parts get more; and as certain organs come to
rest and others are set in activity, the arteries of the one
narrow and of the other dilate ; in this way the distri-
bution of the blood in the Body is undergoing constant
changes, parts which at one time contain much blood at
another having but little. In addition, then, to nervous
organs regulating the work of the heart and the arteries
with reference to one another, we have to consider another
set of vascular nerves which govern the local blood-supply
of different regions of the Body.

The Nerves of the Heart. The heart gets nerves from
three sources. (1) From nerve-cells buried in its own sub-
stance and known as its intrinsic ganglia. (2) From the
tenth pair (pneumogastrics) of cranial nerves. (3) From
the sympathetic nervous system. The intrinsic ganglia
keep the heart beating, and the other two sets of nerves
control the rate and force of the beat.

The Intrinsic Heart-Nerves. The ganglia of the heart
lie for the most part in the partition between the auricles
and along the line of junction of the auricles and ventricles ;
a few are found also in the upper parts of the latter. From
some of them arise nerve-fibres which go to the muscles of
the heart, while others are connected with the endings of
the extrinsic nerves reaching the organ ; and probably all
communicate by a network of nerve-fibres.

The heart is an automatic organ : its beat, like the
movement of filaments of a ciliated cell, depends on its
own structure and properties and not on anything outside
itself. This is proved by the fact that the heart cut out of
an animal which has just been decapitated, and entirely re-

CAUSE OF THE HEART'S BEAT. 249

moved from all the rest of the body, will go on beating for
some time ; a time which is very short in the case of warm-
blooded animals, the tissues of which die very soon when
the blood-flow through them ceases, but which may extend
to hours or even days in the excised heart of frog or
turtle, if it be kept from drying up. Still, whether the time
of its continuance be shorter or longer, the fact that the
heart-beat continues after complete excision of the organ
proves that it is not dependent on stimuli reaching it from
other parts of the Body. In the ciliated cell we had no
differentiation into muscle and nerve — its contractile and
automatic parts if separated at all were not optically distin-
guishable and we could only speak of the cell as still retain-
ing both of those primitive protoplasmic properties. But
in the heart, where we find distinct muscles and nerves, the
question naturally arises in which of them does the auto-
matic power reside. We have already seen (Chap. X.) that
muscles elsewhere possess no automaticity : they only con-
tract nnder the influence of a recognizable stimulus, and
though the muscular fibres of the heart do differ somewhat
from other muscular fibres in the Body, it is still aj^riori im-
probable that they are automatic and we are accordingly led
rather to suppose that the stimulns resides in the ganglion-
cells of the heart, especially since we know that nerve-cells
elsewhere are automatic. Experiment confirms this suppo-
sition. If a frog's heart, removed from the body and still
beating, be cut into several pieces with a sharp razor it will
be found that, while bits of the auricles and the base of the
ventricle go on beating, the apical portions of the ventricle
lie at rest — not because the muscle there is dead and has
lost its contractility, for these bits if excited by any ex-
traneous muscular stimulus will still l)eat. but because
that part of the heart possesses no automaticity. JSTow this
is just the part of the frog's heart that has no ganglion-cells
of its own, while the parts that go on beating are those
which possess them ; hence we conclude that the stimulus
originates in the nerve-cells of the organ and is from them
carried by its nerve-fibres to the muscles. The excitant of
the nerve-cells being still unknown we call them automatic

350 THE JIUMA.X BODY.

in the restricted ])liy.si()logiciil sense of the word. The
primary cause of the heart's beat lying thus in itself, we have
next to see how this beat is controlled from outside and co-
ordinated with tiie condition of the rest of tlic Body ai any
given moment.

Nerves Slowing the Heart's Beat. Each pneuniogas-
tric trunk sends several brandies to the heart. Certain of
these contain fibres which when excited slow, or even :dto-
gether stop, tlie beat of tlie heart and are hence known as
the cardio-inlnbitory fihrca.

If one pneumogastric trunk be divided as it runs down
the neck and its peripheral, or lower, end be stimulated
feebly the heart's beat becomes less frequent, while a more
powerful stimulation will completely stop it for a few
seconds, as if its muscles were suddenly paralyzed. If the
experiment be performed upon a narcotized animal. the heart
of which is at the same time exposed by opening the chest,
it will be seen that during the stoppage the heart lies flabby
and relaxed in diastole ; the excitation of the nerve does
not stop the heart's beat, as might perhaps be supjiosed, by
keeping it in a state of permanent tetanic contraction, but
it annuls its contractions and throws it into a state of rest ;
the nerve-fibres concerned are not excitant but inhibitory,
stopping instead of calling forth the activity of the part on
which they act. "Whether their influence is exerted di-
rectly on the muscular fibres of the heart or upon iti in-
trinsic ganglia, abolishing their automatic activity and so
cutting off the stimuli which normally radiate from them
to the muscles, is not certainly known, but the latter view
is probably the correct one. In any case the full inhibi-
tory power usually lasts only a short time ; even if the
pneumogastric stimulation be continued the heart will al-
most always after a few seconds recover from its influence
and commence to beat again.

These cardio-inhibitory fibres originate in a collection of
nerve-cells in the medulla oblongata known as tlie cardio-
inhibitory centre. This centre is automatic and always in
a state of slight excitation, feebly stimulating the fibres
proceeding from it and slightly slowing the heart's beat.

CARDIAV lyillDITIOX. 251

This is shown by the fact that if botli pneumogastric nerves
])Q cut in the neck the heart at once begins to beat a little
faster than before; the brake, «n to speak, has been taken
off it.

The Influence upon Arterial Pressure of Inhibiting
the Heart. If tlie heart be entirely stopped arterial jires-
snre Avill of course fall very rapidly, since tlie distended
arterial system Avill go on emptying itself through the capil-
laries into the veins, without receiving any fresh supply at
its cardiac end. So too if the heart be made to beat slower,
but with the same force in each stroke, it follows from the
facts pointed out in the last chapter (p. 238) that arterial
pressure will fall to a new and lower level, at which the
elastic arteries are only stretched enough to squeeze out in
a minute as much as they receive. As a matter of fact,
when the heart is made to beat slower by weak pneumo-
gastric stimulation each beat is usually a little more power-
ful than Ijefore. However, this extra force is not sufficient
to compensate entirely for the slower rate and so the gen-
eral arterial pressure falls.

Use of the Cardio-Inhibitory Mechanism. Although
the cardio-inhiljitory centre is automatic and always in a
state of slight activity it is also greatly under the control of
afferent nerve-fibres reaching it and which can arouse it to a
much greater degree, and soreflexly cozitrol the heart's beat.
If a frog be rendered insensible and its abdominal cavity
opened, it will be found that one or two smart taps on the
intestine Avill cause the heart to stop in diastole. If, how-
ever, the pneumogastric nerves, or the spinal cord, or the
anterior roots of the spinal nerves, or the commimicating
Ijranches between the sympathetic nerves of the abdomen
and the spinal nerves, be cut previously, then striking the in-
testine has no influence upon the heart; nor has it if the
cardio-inhibitory centre in the medulla oblongata be pre-
viously destroyed. We thus get evidence that the mechani-
cal stimulation of the intestinal nerves stops the heart re-
ilexly through the pneumogastrics, the afferent impulses
traveling from the sympathetic into the spinal nerves and
passing then \\\^ the spinal cord to the cardio-inliibitory

252 THE HUMAN BODY.

centre, where they are reilecled as efferent impulses down
the pneumogastric trunks to tlie heart. In man and other
mammals similar arrangements exist, the afferent fibres pass-
ing from the alimentary canal through the solar plexus (p. 172)
which lies behind the stomach. It is by exciting them and
so reflexly stopping the heart, that men are sometimes killed
by a severe blow on the abdomen or even occasionally by a
large draught of very cold water, the sudden cold acting as
a thermal stimulus, through the walls of the stomach, on the
nerve-fibres outside. A hot and very thirsty jierson requir-
ing a big drink should therefore not take too cold water —
or if he does, swallow it only a mouthful at a time.

The blood-vessels of the alimentary canal are very numer-
ous and capacious and form one of the largest vascular tracts
of the whole Body, and through the reflex mechanism above
described we see how they may control the heart's beat.
Probably if the heart is beating too frequently and keeping
up too high a pressure in them, the sympathetic nerve-
fibres in their coats are stimulated and then, reflexly,
through the cardio-inhibitorv centre slow the heart's beat
and lower the general arterial pressure ; and so we get one
co-ordinating mechanism by which the heart and blood-ves-
sels are made to work in unison.

Some other afferent nerves are also known to be in con-
nection with the cardio-inhibitory centre. For instance,
some persons are made to faint by a strong smell, the olfac-
tory nerves exciting the cardio-inhibitory centre and stop-
ping or greatly slowing the heart. Deaths from the admin-
istration of chloroform are also usually brought about in the
same way, the vapor stimulating the sensory nerves of the
air-passages which then excite powerfully the cardio-inhibi-
tory centre and stop the heart.

The Accelerator Nerves of the Heart. These originate
in the spinal cord, from which tliey pass by communicating
branches to the lowest cervical and upper dorsal sym-
pathetic ganglia and thence to the heart. "When stimu-
lated they cause the heart to beat quicker, but under what
conditions they are employed in the physiological working
of the Body is not known.

KERVES OF THE BLOOD-VESSELS. 253

The Nerves of the Blood- Vessels. The arteries, as already
pointed out, possess a muscular coat composed of fibres
arranged across them, so that their contraction will narrow
the vessels. This coat is most prominent in the smaller
vessels, those of the size which go to supply separate organs,
but disappear again in the smallest branches which are about
to divide into capillaries for the individual tissue elements
of an organ. These vascular muscles are under the control
of certain nerves called vaso-motor (p. 186) and these latter
can thus govern the amount of blood reaching any organ at
a given time. The vaso-motor nerves of the arteries are,
like those of the heart, intrinsic and extrinsic. The intrin-
sic fibres originate from ganglion-cells in the coats of the
arteries or lying alongside them, while the extrinsic origi-
nate from cells in the cerebro-spinal centre, from which
they commonly pass into the sympathetic system before
they reach the vessels. The intrinsic ganglia, like those of
the heart, are automatic and tend to keep the muscular
coats of the arteries in a constant state of feeble contrac-
tion so that, apart from their johysical elasticity, the arteries
always hold a certain grip on the blood. The contraction,
however, is as a rule persistent and steady, or tonic, instead
of rhythmic like that of the heart, although slow rhythmic
contractions have been seen to occur in some arteries. The
difference probably depends rather on the kind of muscle
concerned in each case than on the ganglion-cells, since
plain muscular tissue, such as is found in the arteries, con-
tracts so slowly and remains contracted so long when excited,
that stimuli reaching it at intervals which would give a
rhythmic beat in cardiac muscle, would keep the arterial per-
manently contracted or tetanized. As in the heart, the
activity of the arterial intrinsic nervous mechanism is
under the control of extrinsic nerves, certain of which, the
vaso-constrictors, answer to the accelerator nerves of the
heart and increase the activity of the intrinsic ganglia,
while others, corresponding to the cardio-inhibitory fibres,
check the activity of the intrinsic vascular nerves.

The Vaso-Motor Centre. The vaso-constrictor extrinsic
arterial nerves are nearly always in a state of slight activity.

isi

254 THE HUMAN BODY.

keeping the arteries more constricted than they would be
under the influence of their intrinsic nerves alone. Accord-
ingly if they are cut, or paralyzed, in any region of the Body
its arteries dilate and it becomes flu.slied witli blood. Those
of the external ear, for example, run in the cervical sympa-
thetic, from the lower part of the neck where they leave tlie
spinal cord, until they reach the arterial branches for the
ear and run along the smaller twigs to it. If, therefore,
the cervical sympathetic be divided on one side in an
anaesthetized rabbit, the ear on that side becomes red and
warm from the dilatation of its arteries and the extra
(^ amount of blood flowing through it. If, however, that end
of the cut nerve still attached to the ear be g-jccited electri-
cally or otherwise, the ear arteries contract gradually until
their passage is almost closed up, and the whole organ be-
comes cold and very pale. Although these vaso-constrictor
fibres are thus shown to pass through the cervical sympa-
thetic, other experim^ents show that they really originate
in a group of nerve-cells in the medulla oblongata, and
from tliere run down the spinal cord to the lower part of
the neck, where they pass out in the anterior roots of some
spinal nerves and reach the sympathetic system. The same
is true of nearly all extrinsic vaso-constrictor nerve-fibres
in the Body. Some few possibly arise from centres in the
spinal cord, but the great majority come primarily from
the medulla oblongata, and the collection of nerve-cells
there from which they spring is known as the vaso-motor
centre; a better name would be the vaso-constrictor centre.
The Control of the Vaso-Motor Centre. The vaso-
motor centre is automatic; that is to say it maintains a
certain amount of activity of its own, indei^endently of any
stimuli reaching it through afferent nerve-fibres. Never-
theless, like nearly all automatic nerve-centres, it is under
reflex control, so that its activity may be increased or les-
sened by afferent impulses conveyed to it. Nearly every sen-
sory nerve of the Body is in connection with it; any stimu-
lus giving rise to pain, for example, excites it, and so
constricting the arteries, increases the peripheral resist-
ance to the blood-flow and raises arterial pressure. On

VA80-DILAT0R NEBVE8. 255

the other hand, certain fibres conveying impulses from tlie
heart inhibit the centre and dilate the arteries, lower
blood-pressure, and diminish the resistance to be overcome
by the heart. These fibres run in branches of the j)iieumo-
gastric, and are known as the cleiyressor fibres, or in certain
animals, for example the rabbit, where they are all collected
into one branch, as the depressor nerve. If this nerve be
divided and its cardiac end stimulated no effect is pro-
duced, but if its central end (that still connected
with the rest of the pneumogastric trunk and through it
with the medulla oblongata) be stimulated, arterial pressure
gradually falls; this result being dependent upon a dilata-
tion of the small arteries, and consequent diminution of
the peripheral resistance, following an inhibition of the
vaso-motor centre brought about by the depressor nerve.
Through the depressor nerve the heart can therefore influ-
ence the calibre of the small arteries and, by lowering
aortic pressure, diminish its own work if need be.

Blushing. The depressor nerves control a great part of
the vaso-motor centre, and so can bring about dilatation
of a large number of arteries — their influence is called into
play when general arterial pressure is to be lowered, but is
useless for controlling local blood-supply. This is man-
aged by other afferent nerves, each of which inhibits a
small part only of the vaso-motor centre, governing the
arteries of a limited tract of the Body; the dilatation of
these increases the amount of blood flowing through the
particular region to which they are distributed, but does
not affect the total resistance to the blood-flow suflBciently
to influence noticeably the general pressure in the arterial
system. In blushing, for example, under the influence of
an emotion, that part of the vaso-motor centre which sup-
plies constrictor nerves to the arteries of the skin of the
neck and face, is inhibited by nerve-fibres proceeding from
the cerebrum to the medulla oblongata, and the face and
neck consequently become full of blood and flush up.
Quite similar phenomena occur under other conditions in
many parts of the Body, although when not visible on the
surface we do not usually call them blushes. The mucous

256 THE HUMAN BODY.

membrane lining the empty stomacli is pallid and its ar-
teries contracted, but as soon as food enters the organ
it becomes rod and full of blood; the food stimulating
afferent nerve-fibres there, which inhibit tliat part of the
vaso-motor centre which governs the gastric arteries.

Taking Cold. This common disease is not unfrcquent-
ly caused through undue reflex excitement of the vaso-
motor centre. Cold acting upon the skin stimulates, through
the afferent nerves, the region of the vaso-motor centre gov-
erning the skin arteries, and the latter become contracted,
as shown by the jDallor of the surface. This has a two-fold
influence — in the first place, more blood is thrown into in-
ternal parts, and in the second, contraction of the arte-
ries over so much of the Body considerably raises the gen-
eral blood-pressure. Consequently the vessels of internal
parts become overgorged or " congested," a condition which
readily passes into inflammation. Accordingly prolonged
exposure to cold or wet is apt to be followed by catarrh or
inflammation of more or less of the respiratory tract caus-
ing bronchitis, or of the intestines causing diarrhoea. In
fact the common summer diarrhoea is far more often due
to a chill of the surface, causing intestinal catarrh, than to
the fruits eaten in that season which are so often blamed
for it. 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 exter-
nal temperat»re 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 maintain the cutaneous
circulation; for this purpose movement should be persisted
in, or a thick dry outer covering put on, until warm and
dry clothing can be obtained.

For healthy persons a temporary exposure to cold, as a
plunge in a bath, is good, since in them the sudden contrac-
tion of the cutaneous arteries soon jiasses off and is suc-
ceeded by a dilatation causing a warm healthy glow on the
surface. If the bather remain too long in cold water, how-
ever, this reaction passes off and is succeeded by a more

VASO-DILATOR NERVES. 267

persistent chilliness of the surface, which may eyen last
ail 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.

Vaso-Dilator Nerves. We have already seen, 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. Usually, however, in the Body this is managed
in another way; by vaso-dilator nerves which inhibit or
paralyze, not the vaso-motor centre, but the intrinsic nerv-
ous supply of the blood-vessels. The nerves of the skeletal
muscles for example contain two sets of fibres: one motor
proper and the other vaso-dilator. When the muscle con-
tracts in a reflex action or under the influence of the will both
sets of fibres are excited; so that when the organ is set at
work its arteries are simultaneously dilated and more blood
flows through it. Quite a similar thing occurs in the sali-
vary glands. Their cells, which form the saliva, are aroused
to activity by special nerve-fibres; but the gland nerve also
contains vaso-dilator fibres which simultaneously cause a
dilatation of the gland artery. Through such arrange-
ments 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 expansion of a few small local branches but little
influences the total peripheral resistance in the vascular sys-
tem. 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 example,
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. On the other hand, when the brain
is at work its vessels are dilated and often the whole head
flushed: and so excitement or hard thought after a meal
7« tery apt to produce an attack of indigestion, by diverting

258 THE HUMAN BODY.

the blood from the abdominal organs where it ought to be
at that time. Young persons, whose organs have a super-
abundance of energy enabling them to work under unfavor-
able conditions, are less apt to suffer in such ways than
their elders. One sees boys running actively about after
eating, when older people feel a desire to sit quiet and ru-
minate— or even go to sleep.

CHAPTER XVIII.

THE SECEETORY TISSUES AND ORGANS.

Definition. In a strict sense of the terms every pro-
cess in which substances are separated from the blood,
whether they be altered or unaltered, is "secretory" and
every product of such a separation is a " secretion;" in this
sense secretions would be separable into three classes. " (1)
Liquids or gases transuding on free surfaces of the Body,
whether external or internal; (2) the liquids {lymph)
moistening the various tissues of the Body directly, filling
the interstices between them and not contained in definitely
limited cavities ; (3) all the solid tissues of the Body since,
after an early period of embryonic life, they are built up
from materials derived from the blood. Secretions would
thus come to include all constituents of the Body except
the blood itself but, while it is well to bear in mind that
the whole Body is in such a way derived from the blood, in
practice the term secretion is given a narrower connota-
tion, the solid tissues and the lymph being excluded; so that
a secretion is a material (liquid or gaseous) derived from the
blood and poured out on a free surface, whether that of the
general exterior or that of an internal cavity. Such true
secretions fall into two classes; one in which the product
is of no further use in the Body and is merely separated
for removal, as the urine; and one in which the jiroduct is
intended to be used, for instance as a solvent in the diges-
tion of food. The former gi-oup are sometimes distin-
guished as excreiions and the latter as secretions proper, but
there is no real difference between them, the organs and pro-
cesses concerned being fundamentally alike in each case. A
better division is into transiidata tindi secretions, atransuda-

260 THE HUMAN BODY.

tion being a product which contains nothing which did
not previously exist in the blood, and then in such quantity
as might be derivable from it by merely physical processes;
while a secretion in addition to transudation elements con-
tains a specific element, due to the special physiological
activity of the secretory organ; being either something
which does not exist in the blood at all or something wliicli.
existing in the blood in small quantity, exists in the secre-
tion in such a high proportion that it must have ])een
actively picked up and conveyed there by the secretoi-y
tissues concerned. For instance, the gastric juice contains
free hydrochloric acid which docs not exist in the blood;
and the urine contains so much urea that we must sujipose
its cells to have a peculiar power of removing that body
from the liquids flowing near them. This subdivision is
also justifiable on histological grounds ; wherever there is.
a secreting surface it is covered with cells, but these where
transudata are formed (as on the serous membranes) are
mere flat scales, with little or no protoplasm remaining in
them, while the cells which line a true secreting organ are
cuboidal, spherical, or columnar, and still retain, with
their high physiological activity, a good deal of their primi-
tive protoplasm in a but slightly modified state.

Organs of Secretion. The simplest form in which a
secreting organ occurs {A, Fig. 88) is that of a flat membrane
provided with a layer of cells, a, on one side (that on which
the secre':ion is poured out) and with a network of capil-
lary blood-vessels, c, on the other. The dividing mem-
brane, h, is known as the basement memlirane 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

FORMS OF GLANDS.

261

Fig. 88.— Forms of glands. A, a simple secreting surface; a. us epithelium ;
6. basement membraue: c. capillaries: B.a. simple tubular gland : C, a secret-
ing surface increased by protrusions; E. & simple racemose gland ; D and Cr,
compound tubular glands: F.a. compound racemose gland. In all but i^ the
capillaries are omitted for the snke of clearness. H, half of a highly devel-
oped i-acemose gland: c, its main duci. The letter rf by mistake has been usea
both for the basement membrane above, and tor branches of duct below

262 THE HUMAN BODY.

secreting surface in various ways so that a large surface 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 secreting organ as indicated at C, Fig. 88,
and found on some synovial membranes; but much more
commonly the surface extension is attained in another way,
the basement membrane, covered 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 involu-
tions are uniform in diameter, or nearly so, throughout {B,
Fig. 88). Such glands are known as Uihular; examples
are found in the lining coat of the stomach (Fig. 97*); also
in the skin (Fig.l20f), where they form the stveat-glands.
In other cases the involution swells out at its deeper end and
becomes more or less sacculated {B) ; such glands are racemose
or aciiiotcs. The small glands which form the oily matter
poured out on the hairs (Fig. 119;};) are of this t^'pe. In both
kinds 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 dud. 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 considera-
ble length, so that the secretion is poured out at some dis-
tance 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 recesses, ee. The ducts and ductules are
lined with epithelium which is merely protective and differs
in cliurueter from tlie secreting ei^ithelium which lines the

*P. 319. iP. 4ia i P. 416.

PROCESSES CONCERNED IN SECRETION. 263

deepest parts. Surrounding each subdivision and bind-
ing it to its neighbors is the gland stroma formed of con-
nective tissue, a layer of which also commonly envelops
the whole gland, as its capside. Commonly 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 lohes, each
of which answers to a main division of the primar}^ duct;
and the lobes are often similarly divided into smaller parts
or lohules. In the connective tissue between the lobes and
lobules blood-vessels jienetrate, to end in fine capillary
vessels around the terminal recesses. They never pene-
trate the basement membrane. Lymphatics and nerves
take a similar course; but there is reason to believe the
nerve-fibres penetrate the basement membrane and be-
come directly united with the secreting cells.

The Physical Processes in Secretion. From the struc-
ture of a gland it is clear that all matter, derived from the
blood and poured into its cavity, must pass not only through
the walls of the capillary blood-vessels, but also, by filtra-
tion or dialysis, through the basement membrane and the
lining epithelium. By filtration is meant the passage of a
fluid under pressure through tlie coarser mechanical pores
of a membrane, as in the ordinary filtering processes of a
chemical laboratory; and the higher the pressure on the
liquid to be filtered the greater the amount which, other
things bemg equal, will pass through in a given time.
Since in the living Body the liquid pressure in the blood
capillaries is nearly always higher than that outside them,
filtration is apt to take place everywhere to a greater or less
extent, and will be increased in amount in any region by
circumstances raising bloou-pressure there, and diminished
by those lowering it. To a certain extent also the nature
of the liquid filtered has an influence. True solutions, as
those of salt in water, pass through unchanged; but solu-
tions containing substances such as boiled starch or raw
egg albumen, which swell up greatly in water rather than
truly dissolve, are altered by filtration; the filtrate contain-
mg less of the imperfectly dissolved body than the unfil-
tered liquid. The higher the pressure the greater the pro-

264 THE HUMAN BODY.

portion of such substances ■which gets through; and if
the pressure is shght the water or other solvent may alone
pass, leaving all the rest behind on the filter. Under
moderate pressure the blood may thus lose by filtration
such bodies only as water and salines; while an increase of
arterial pressure may lead to the passage of albumen and
fibrinogen. Under healthy conditions, for example, the urine
contains no albumen, but anything increasing the capillary
pressure in the kidneys will cause it to appear. Dialysis
or osmosis has already been considered (p. 42); by it sub-
stances pass through the intermolecular pores of a mem-
brane independently of the joressure on either side, and for
its occurrence two liquids of ditf erent chemical constitution
are required, one on each side of the membrane. At least
if diffusion takes place, as is probable, between two exactly
similar solutions, the amount and character of the sub-
stances passing opposite ways in a given time are exactly
equal, so that no change is produced by the dialysis; which
practically amounts to the same thing as if none occurred.
When a solution is placed on one side of a membrane allow-
ing of dialysis and pure water on the other, it is found that
for every molecule of the dissolved body that passes one
way a definite amount of water, called the endosmotic
equivalent of that body, passes in the opposite direction.
Crystalline bodies as a rule (haemoglobin is an exception)
have a low endosmotic equivalent or are readily dialyzable;
while colloids such as gum and proteids, have a very high
one, so that to get, by dialysis, a small amount of albumen
through a membrane, a practically infinite amount of water
must pass the other way. Accordingly, if we find such
bodies in a secretion we cannot suppose that they have been
derived from the blood by osmosis.

The Chemical Processes of Secretion. As above point-
ed out certain secretions, called transudata, seem to be pro-
ducts of filtration and dialysis alone, containing only such
substances as those which are found in the blood plasma,
more or less altered in relative quantity by the ease or diffi-
culty with which they severally passed through the layers
met with on their way to the sui-face. But in many cases

THE SPECIFIC ELEMENTS OF SECRETIONS. 265

the composition of a secretion cannot be acconntcd for in
this way; it contains some specific element, either a substance
which does not exist in the blood at all and must therefore
have been added by the secreting membrane, or some
body which, although existing in the blood, does so in such
minute proportion compared with that in which it is found
in the secretion, that some special activity of the secreting
cells is indicated; some affinity in them for these bodies by
which they actively pick them up.

Each living cell, we have seen, is the seat of constant
chemical activity, taking u]) materials from the medium
about it, transforming and utilizing them, and sooner or
later restoring their elements, differently combined, to the
medium again. By such means it builds up and maintains
its living substance, and obtains energy to carry on its daily
work. While this is true of all cells in the Body, we find
certain groups in which chemical metabolism is the promi-
nent fact; cells vrhich are specialized for this purpose just
as muscular fibre is for contraction or a nerve-fibre for con-
duction, and certain of these prominently metaholic tissues,
exist in the true glands and produce or collect the specific
elements of their secretions. Their chemical processes are
no doubt primarily directed to their own nutritive mainte-
nance; they live primarily for themselves, but their nutritive
processes are such that the bodies formed in them and sent
. into the secretion are such as to be useful to the rest of the
cells of the community; or the bodies which they specially
collect, and in a certain sense feed on, are those the re-
moval of which from the blood is essential for the general
good. Their individual nutritive peculiarities are utilized
for the welfare of the whole Body.

The Mode of Activity of Secretory Cells. If we con-
sider the modes of activity of living cells in general, it be-
comes clear that secretory cells may produce the specific
element of a secretion in either of two ways. They may,
as a by-result of their living play of forces, produce chemi-
cal changes in the surrounding medium; or they may build
up certain substances in themselves and then set them free
as specific elements. Yeast, for example, in a saccharine

266 THE HUMAN BODY.

solution causes the rearrangement into carbon dioxide,
alcohol, glycerine and succinic acid, of many atoms of car-
bon, hydrogen and oxygen Avhich previously existed as sugar;
and which during the metamorphosis were probably not
j)assed through the living cell. How the latter acts we do
not know with certainty, but most likely by picking certain
atoms out of the sugar molecule, and leaving the rest to
fall down iruto simpler compounds. On the other hand, we
find cells forming and storing up in themselves large quan-
tities of substances, which they afterwards liberate; starch,
for instance, being formed and laid by in many fruit-
cells, and afterwards rendered soluble and passed out to
nourish the young jDlant.

Ghmd-cells might a priori give rise to the specific ele-
ments of secretions in either of these ways and we have to
seek in which manner they work. Do they simply act
as ferments (however that is) ujDon the surrounding
medium; or do they form the special bodies which charac-
terize their secretion, first within their own substance, and
then liberate them, either disintegrating themselves or
not at the same time? At present there is a large and an
increasing mass of evidence in favor of the second view.
There is, no doubt, some reason to believe that every living
cell can act more or less as a ferment upon certain solu-
tions should they come into contact with it. Not always,
of course, as an alcoholic ferment, though even as regards
that one fermentative power it seems very generally pos-
sessed by vegetable cells, and there is some evidence that
alcohol is normally produced in small amount (and presum-
ably by the fermentation of sugar) under the influence of
certain of the living tissues of the Human Body. As re-
gards distinctively secretory cells, however, the evidence is
all the other way, and in many cases we can see the specific
element collecting in the gland-cells before it is set free in
the secretion. For example, in the oil-glands of the skin
(Chap. XXVII.) we find tlie secreting cells, at first granular,
nucleated and protoplasmic, gradually undergoing changes
by which their protoplasm disappears and is rejDlaced by
oil-droplets, until finally the whole cell falls to bits and its

THE ACTION OF GLAND-CELLS. 267

detritus forms the secretion; the cells being replaced by new-
ones constantly formed within the gland. In such cases
the secretion is the ultimate product of the cell life; the
result of degenerative changes of old age occurring in it.

In other cases, however, the liberation of the specific
element is not attended with the destruction of the secret-
ing cell; as an example we may take the pancreas, wdiich
is a large gland lying in the abdomen and forming a secre-
tion used in digestion. Among others, this secretion pos-
sesses the power, under certain conditions, of dissolving
proteids and converting them into dialyzable peptones
(p. 11). This it owes to a specific element known as tryp-
sin, the formation of which within the gland-cells can be '
traced with the microscope.

The pancreas, like the majority of the glands connected
with the alimentary canal, has an intermittent activity;
determined by the presence or absence of food in various
parts of the digestive tract. If the organ be taken from a
recently killed dog which has fasted thirty hours and, after
proper preparation, be stained with carmine and examined
microscopically, we get specimens of what we may call the
" resting gland '' — a gland which has not been secreting for
some time. In these it will be seen that the cells lining the
secreting recesses present two very distinct zones; an outer
next the basement membrane which does not combine wath
the coloring matter and is granular, and an inner which is '^^/c^y
not granular but picks up the carmine. The granules w^e
shall find to be indications of the presence of a trypsin-
vieldino- substance, formed in the cells. ^yl

If another dog be kept fasting until he has a good appe- "^ti,

tite and be then allowed to eat as much meat as he will, he
will commonly take so much that the stomach will only be
emptied at the end of about twenty hours. This period
may, so far as the pancreas is concerned, be divided into
two. From the time the food enters the stomach and on
for about ten hours, the gland secretes abundantly; after
that the secretion dwindles, and by the end of the second
ten hours has nearly ceased. "We have, then, a time during
which the pancreas is working hard, followed by a period

=».>

268 TEE HUMAN BODY.

in which its activity is very little, but during which it is
abundantly supijlied with food materials. The pancreas
taken from an animal at the end of the first period and
prepared for microscopic examination will be found dif-
ferent from that taken from a dog killed at the end of the
second digestion period, and also from the resting gland.
Towards the end of the period of active work, the gland-cells
are diminished in size and the proportions of the granular
and non-granular zones are quite altered. The latter now
occupies most of the cell, while the granular non-staining
inner zone is greatly diminished. During the secretion
there is, therefore, a growth of the non-granular and a de-
struction of the granular zone; and the latter process rather
exceeding the former, the whole secreting cell is diminished
in size. During the second digestive period, when secre-
tion is languid, exactly a reverse process takes place. The
cells increase in size so as to become larger than those of
the resting gland; and this growth is almost entirely due
to the granular zone which now occupies most of the cell.

These facts suggest that during secretion the granular
part of the cells is used up: but that, simultaneously, the
deeper non-granular zone, being formed from materials
yielded by the blood, gradually gives rise to the granular.
During active secretion the breaking doAvn of the lat-
ter to yield the sjiecific elements occurs faster than its re-
generation; in a later period, however, when the secretion
is ceasing, the whole cell grows and, especially, the granular
zone is formed faster than it is disintegrated; hence the
great increase of that part of the cell. If this be so, then
we ought to find some relationship between the diges-
tive activity of an infusion or extract of the gland and the
size of the granular zones of the cells; and it has been
shown that such exists; the quantity of trypsin which can
be obtained from a pancreas being proportionate to the
size of that portion of its cells.

The trypsin, however, does not exist in the cells ready
formed, but only a body which yields it under certain cir-
cumstances, and called zymogen.

If a perfectly fresh pancreas be divided into halves and

INFLTJENCE OF NERVES ON SECRETION. 269

one portion immediately minced and extracted with glyce-
rine, while the other is laid aside for twenty-four hours in
a warm place and then similarly treated, it will be found
that the first glycerine extract has no power of digesting
proteids, while the second is very active. In other words
the fresh gland does not contain trypsin, but only sonu^-
thing which yields it under some conditions; among
others, on being kept. The inactive glycerine extract of
the fresh gland is however rich in zymogen: for if a little
acetic acid be added to it, trypsin is formed and the extract
becomes powerfully digestive.
f We may then sum up the life of pancreas cell in this
way. It grows by materials derived from the blood and
first laid down in the non-granular zone. This latter, in
the ordinary course of the cell-life, gives rise to the granu-
lar zone; and in this is a store of zymogen produced by
the nutritive metabolisms of the cell. When the gland
secretes, the zymogen is converted into try^isin and set free
in the secretion; but in the resting gland this transforma-
tion does not occur. During secretory activity therefore
the chemical processes taking place in the cell, are different
from those at other periods; and we have next to consider
how this change in the mode of life of the cells is brought
about.

Influence of the Nervous System upon Secretion.
When the gland is active it is fuller of blood than when at
rest: its arteries are dilated and its capillaries gorged so
that it gets a brighter pink color; this extra blood-supply
might be the primary cause of the altered metabolism.
Again, the activity of the pancreas is under the influence
of the nervous system, as evinced not only by the reflex
secretion called forth when food enters the stomach, but
also by the fact that electrical stimulation of the medulla
oblongata will causo the gland to secrete. The nervous
system may, however, only act through the nerves governing
the calibre of the gland arteries, and so but indirectly on
the secreting cells; while on the other hand, it is possible
that nerve-fibres act directly upon tlie gland-cells and, con-
trolling their nutritive processes, govern the production of

270 THE HUMAN BODY.

the trypsin. To decide between the relative importance of
these possible agencies we must pass to the consideration of
other glands; since the question can only be decided by
experiment upon the lower animals, and the position of
the pancreas and the difficulty of getting at its nerves with-
out such severe operations as upset the physiological condi-
tion of the animal, furnish obstacles to its study which
have not yet been overcome.

In certain other glands, however, we find conclusive evi-
dence of a direct action of nerve-fibres upon the secreting
elements. If the sciatic nerve of a cat be stimulated elec-
trically the balls of its feet will sweat. Under ordinary
circumstances they become at the same time red and full
of blood; but that this congestion is a factor of subsidiary
importance as regards secretion is proved by the facts that
stimulation of the nerve is still able to excite the gland-
cells and cause sweating in a limb which has been ampu-
tated ten or fifteen minutes (and in which therefore no cir-
culatory changes can occur) and also by the cold sweats,
with a pallid skin, of phthisis and the death agony. It is,
however, with reference to the submaxillary and parotid
salivary glands that our information is most precise.

When the mouth is empty and the jaws at rest the sali-
vary secretion is comparatively small: but a sapid substance
placed on the tongue will cause a copious flow. The phe-
nomenon is closely comparable to the production of a reflex
muscular contraction. A stimulus acting upon an irritable
tissue excites through it certain afferent nerve-fibres; these
excite a nerve-centre, which in turn stimulates efferent
fibres; going to a muscle in the one case, to a gland in the
other. It will be useful to consider again for a moment
what occurs in the case of the muscle, taking account only
of the efferent fibres and the j)arts they act upon.

When a muscle in the Body is made to contract reflexly,
through its nerve, two events occur in it. One is the
shortening of the muscular fibres; the other is the dilata-
tion of the muscular arteries; every muscular nerve con-
tains two sets of fibres, one motor and one vaso-dilator,
and normally botli act together. In tliis case, however,

INFLUENCE OF NERVES ON SECRETION 271

it is clear that the actiYities of both, though correlated, are
essentially independent. The contraction is not due to the
greater blood-flow for, not only can an excised muscle en-
tirely deprived of blood, be made to contract by stimulating
its nerves, but in an animal to which a small dose of curari
— ^the arrow poison of certain South American Indians — has
been given, stimulation of the nerve will cause the vascu-
lar dilatation but no muscular contraction: the curari par-
alyzing the motor fibres, but, unless in large doses, leaving
the vaso-dilators intact. The muscular fibres themselves
are quite unacted upon by the poison, as evinced by their
ready contraction when directly stimulated by an electric
shock.

Now let us return to the salivary glands and see how far
the facts are comparable. The main nerve of the submax-
illary gland is known as the cliorda iijmiiani. If it be di-
vided in a narcotized dog, and a tube placed in the gland-
duct, no saliva will be found to flow. But on stimulating
the peripheral end of the nerve (that end still connected
with the gland) an abundant secretion takes place. At
the same time there is a great dilatation of the arteries of
the organ, much more blood than before flowing through
it in a given time: the cliorda obviously then contains vaso-
dilator fibres. Now in this case it might very well be that
the process was different from that in a muscle. It is con-
ceivable that the secretion may be but a filtration due to
increased pressure in the gland capillaries, consequent
on dilatation of the arteries supplying them. If a greater
filtration into the lymph spaces of the gland took place, this
liquid might then merely ooze on through the secreting cells
into the commencing ducts and, as it passed through, dis-
solve out and carry on from the cells the specific organic
elements of the secretion. Of these, in the submaxillary
of the dog at least, mucin is the most important and
abundant. That, however, the process is quite different,
and that there are in the gland true secretory fibres in ad-
dition to the vaso-dilator, just as in the muscle there are
true motor fibres, is proved by other experiments.

If the flow of liquid from the excited gland were merely

272 THE HUMAN BODY.

the outcome of a filtration dependent on increased blood
pressure in it, then it is clear that the pressure of the
secretion in the duct could never rise above the pressure in
the blood-vessels of the gland. Now it is found, not only
that the gland can be made to secrete in a recently decapi-
tated animal, in which of course there is no blood-pressure,
])ut that, when the circulation is going on, the pressure of
the secretion in the duct can rise far beyond that in the
gland arteries. Obviously, then, the secretion is no ques-
tion of mere filtration, since a liquid cannot filter against a
higher pressure. Finally, the proof that the vascular dila-
tation is quite a subsidiary phenomenon has been com-
pleted by showing that we can produce all the increased
blood-flow through the gland without getting any secretion
— that just as in a muscle nerve we can, by curari, paralyze
the motor fibres and leave the vaso-dilators intact, so we
can by atropin, the active principle of deadly night-shade,
get similar phenomena in the gland. In an atropized
animal stimulation of the chorda jn-oduces vascular dila-
tation but not a droj? of secretion. Bringing blood to
the cells abundantly, will not make them drink; we must
seek something more in the chorda than the vaso-dilator
fibres — some proper secretory fibres; that the poison acts
upon them and not upon the gland-cells, is shown, as in
the muscle, by the fact that the cells still are capable of
activity when stimulated otherwise than through the
chorda tympani. For exami)le, by stimulation of the sym-
pathetic fibres going to the gland.

So far then we seem to have good evidence of a direct
action of nerve-fibres upon the gland-cells. But even that
is not the whole matter. It is extremely probable, if not
certain, that there are two sets of secretory fibres in the
gland-nerves: a set which so acts ujoon the cells as to make
them pass on more abundantly the transudation elements
of the secretion (the water and mineral salts), and another,
quite different, which governs the chemical transformations
of the cells so as to make them produce mucin from matters
previously stored in them, in a comparable way to the ])ro-
duction of trypsin from zymogen in the active pancreas.

INFLUENCE OF NEBVES ON SECRETION. 273

These latter fibres may be called ''trophic," since they
directly control the cell metabolism: while the former may
be called -'transudatory" fibres. Some of the evidence
which leads to this conclusion is a little complex, but it is
worth while to consider it briefly. In the first place, on
stimulation of the chorda of an unexhausted gland (that is
a gland not over-fatigued by previous work) the following
points can be noted: —

With increasing strength of the stimulus the quantity of
the secretion, that is of the water poured out in a unit
of time, increases; at the same time the mineral salts also
increase, but more rapidly, so that their percentage in a
rapidly formed secretion is greater than in a more slowly
formed, up to a certain limit. The percentage of organic
constituents of the secretion also increases \\^ to a limit;
but soon ceases to rise, or even falls again, while the water
and salts still increase. This of course is readily intelligible;
since the water and salts can be derived continually from
the blood, while the specific elements, coming from the
gland-cells, may be soon exhausted; and so far the experi-
ment gives no evidence of the existence of distinct nerve-
fibres for the salts and water, and for the specific elements:
all vary together with the strength of the stimulus applied
to the nerve. But under slightly different circumstances
their quantities do not run parallel. The proportion of
specific elements in the secretion is largely dependent on
whether the gland has been previously excited or not.
Prior stimulation, not carried on of course to exhaustion,
largely increases the percentage of organic matters in the
secretion produced by a subsequent stimulation; but has no
effect whatever on the quantity of water or salts. These
are governed entirely by the strength of the second stimu-
lation. Here, then, we find that under similar circumstances
the transudatory and specific elements of the secretion do
not vary together; and are therefore probably dependent
upon different exciting causes. And the facts might lead
us to suspect that there are in the chorda, besides the vaso-
dilator, two other sets of fibres: one governing the salts
and water, and the other the specific elements of the secre-

274 THE HUMAN BODY.

tiou. The evidence is, perhai)s, not quite conclusive, but
experiments upon the parotid gland of the dog put the
matter beyond a doubt.

The submaxillary gland receives fibres from the sympa-
thetic system, as well as the cliorda tympani from the
cerebro-s])inal. Excitation of the sympathetic filn'es causes
the gland to secrete, but the saliva poured out is differ-
ent from that following chorda stimulation, which is
in the dog abundant and comparatively i)oor in organic
constituents, and accompanied by vascular dilatation: while
the "sympathetic saliva," as it is called, is less abundant,
very rich in mucin, and accompanied with constriction of
the gland arteries. According to the above view we
might suppose that the chorda contains many transuda-
tory and few trophic fibres, and the sympathetic many
trophic and few transudatory. It might, however, well be
objected that the greater richness in organic bodies of the
symiiathetic saliva was really due to the small quantity of
blood reaching the gland, when that nerve was stimulated.
This might alter the nutritive phenomena of the cells and
cause them to form mucin in unusual abundance, in which
case the tro|)hic influence of the nerve would be only in-
direct. Experiments on the parotid preclude this explan-
ation. That gland like the submaxillary gets nerve-fibres
from two sources: a cerebral and a sympathetic. The latter
enter the gland along its artery, while the former, origin-
ating from the glosso-phar^mgeal, run in a roundabout
course to the gland. Stimulation of the cerebral fibres
causes an abundant secreticm, rich in water and salts, but
with hardly any organic constituents. At the same time it
produces dilatation of the gland arteries. Stimulation of the
sympathetic causes contraction of the parotid gland arteries
and no secretion at all. Nevertheless it causes great
changes in the gland-cells. If it be first stimulated for a
Avhile and then the cerebral gland-nerve, the resulting
secretion may be ten times as rich in organic bodies as that
obtained without previous stimulation of the sympathetic;
and a similar phenomenon is observed if the two nerves be
stimulated simultaneously. So that the sympathetic nerve.

INFLUENCE OF NEEVES ON SECRETION. 275

though, unable of itself to cause a secretion, brings about
great chemical changes in the gland-cells. It is a distinct
trophic nerve. This conclusion is confirmed by histology.
Sections of the gland after prolonged stimulation of the sym-
pathetic show its cells to be quite altered in appearance,
and in their tendency to combine with carmine, when com-
pared either with those of the resting gland or of the gland
which has been made to secrete by stimulating its glosso-
pharyngeal branch alone.

We have still to meet the objection that the sympathetic
fibres may be only indirectly trophic, governing the meta-
bolism of the cells through the blood-vessels. If this be
so, cutting off or diminishing the blood-supply of the
gland, in any way. ought to have the same result as stimula-
tion of its sympathetic fibres. Experiment shows that
such is not the case and reduces us to a direct trophic influ-
ence of the nerve. When the arteries are closed and the
cerebral gland-nerve stimulated, it is found that the per-
centage of organic constituents in the secretion is as low
as usual; it remains almost exactly the same whether the
arteries are open or closed or have been previously open or
closed. We must conclude that the peculiar influence of
the sympathetic does not depend upon its vaso-con stricter
fibres.

These observations make it clear that the phenomena of
secretion are dependent on very complex conditions, at least
in the salivary glands and presumably in all others.
Primarily dependent upon filtration and dialysis from the
blood-vessels and the physiological character of the gland-
cells, both of these factors are controlled by the nervous
system, the secretory tissues being no more automatic than
the muscular; and the facts also give us important evidence
of power of the nervous system to influence cell nutrition
directly.

Summary. By secretion is meant the separation of such
substances from the blood as are poured out on free surfaces
of the Body, whether external or internal. In its simplest
form it is merely a physical process dependent on filtra-
tion and dialysis; for example, the elimination of carbon
dioxide from the surfaces of the lungs, and the watery

276 THE HUMAN BODY.

li(iuid poured out on the surfaces of the serous membranes.
Such secretions are known as transiidata and their amount
is only indirectly controlled by the nervous system, through
the influence of the latter upon tlie circulation of the blood.
The cells lining such surfaces are not secretory tissues in
any true sense of the Avord, being merely flat, inactive, thin
scales protecting the surfaces. In other cases the lining
cells are thicker, and actively concerned in the process; they
are then usually spread over the recesses of a much folded
membrane, so that the whole is rolled up into a compact
organ called a gland, the secretion of which may contain
only transudation elements (as for example that of the
lachrymal glands which form the tears) or may contain a
specific element, formed in the gland by its cells, in addition
to transudation elements. In either case the activity of
the organ is directly influenced by the nervous system,
usually in a reflex manner {e.g. the watering of the eyes
when the eyeball is touched and the saliva poured into the
mouth when food is tasted) but may also be otherwise ex-
cited, as for example the flow of tears under the influence
of those changes of the central nervous system which are
associated with sad emotions, or the watering of the mouth
at the thought of dainty food. The nerves going to such
glands, besides controlling their blood-vessels, act upon the
gland-cells; one set governing the amount of transudation
of water and salines which shall take place through them,
and another (in the case of glands producing secretions
with one or more specific elements) controlling the produc-
tion of these, by starting new chemical processes in the cells
by which a substance built up in them during rest is con-
verted into the specific element, which is soluble in and
carried off by the transudation elements. What the speci-
fic element of gland shall be, or whether its secretion con-
tain any, is dependent on the nature of its special cells;
how much transudation and how much specific element
shall be secreted at any time is controlled by the nervous
system; just as the contractility of a muscle depends on
the endowments of muscular tissue, and whether it shall
rest or contract — and if the latter how powerfully — upon
its nerve.

CHAPTER XIX.

THE INCOME AND EXPENDITUKE OF THE
BODY.

The Material Losses of the Body. All day long while
life lasts each of us is losing something from his Body.
The air breathed into the lungs becomes in them laden
with carbon dioxide and water vajoor, which are carried off
with it when it is expired. The skin is as constantly giv-
ing off moisture, the total quantity in twenty-four hours
being a good deal, even when the amount passed out at any
one time is so small as to be evaporated at once and so does
not collect as drops of visible perspiration. The kidneys
again are constantly at work separating water and certain
crystalline nitrogeneous bodies from the blood, along with
some mineral salts. The product of kidney activity, how-
ever, not being forthwith carried to the surface but to
a reservoir, in which it accumulates and Avhich is only
emptied at intervals, the activity of those organs appears
at first sight intermittent. If to these losses we add cer-
tain other waste substances added to the undigested residue
of the food passed out from the alimentary canal, and the
loss of hairs and of dried cells from the surface of the skin,
it is clear that the total amount of matter removed from
the Body daily is considerable. The actual quantity varies
with the individual, with the work done, and with the
nature of the food eaten; but the following table gives
approximately that of the more important daily material
losses of an averairo man.

278

TEE HUMAN BODY.

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THE DAILY LOSSES OF TEE BODY. 379

The living Body thus loses daily in round numbers 4
kilograms of matter (8 lbs.) and, since it is unable to
create new matter, this loss must be compensated for from
the exterior or the tissues would soon dwindle away alto-
gether; or at least until they were so impaired that life
came to an end. After death the losses would be of a differ-
ent kind, and their quantity much more dependent upon
surrounding conditions; but except under very unusual
circumstances the wasting away would still continue in the
dead Body. Finally, the composition of the daily wastes
of the living Body is tolerably constant; it does not simply
lose a quantity of matter weighing so -much, but a certain
amount of definite kinds of matter, carbon, nitrogen, oxygen,
and so on; and these same substances must be restored to
it from outside, in order that life may be continued. To
give one asking for bread a stone might, no doubt, if it were
swallowed, compensate in weight for the matter he lost in
twenty-four hours; but bread would be needed to keep
him alive. In other words, the Body not only requires a
supply of matter from outside, but a supply of certain
definite kinds of matter.

The Losses of the Body in Energy. The daily expendi-
ture of matter by the living Body is not the only one: as
continuously it loses in some form or another energy, or
the power of doing work; often as mechanical work ex-
pended in moving external objects, but even when at rest
energy is constantly being lost to the Body in the form of
lieat, by radiation and conduction to surrounding objects,
by the evaporation of water from the lungs and skin, and
by removal in warm excretions. Unless the Body can
make energy it must therefore receive a certain supply of
it also from the exterior, or it would very soon cease to
carry on any of its vital work ; it would be unable to
move and would cool down to the temperature of surround-
ing objects. The discoveries of this century having shown
that energy is as indestructible and uncreatable (see Pliy-
sics) as matter, we are led to look for the sources of the
supply of it to the Body; and findijig that the living Body
daily receives it and dies when the supply is cut off, we no

•:i80 THE HUMAN BODY.

longer suppose, with the older physiologists, that it works
by means of a mysterious vital force existing in or created
by it; but that getting energy from the outside it utilizes
it for its purposes — for the performance of its nutritive and
other living work — and then returns it to the exterior
in what the physicists know as a degraded state; that is in
a less utilizable condition. While energy like matter is in-
destructible it is, unlike matter, transmutable; iron is always
iron and gold always gold; neither can by any means which
we possess be converted into any other form of matter; and
so the Body, needing carbon, hydrogen, oxygen, and nitro-
gen to build it and. to cover its daily losses, must be sup-
plied with those very substances. As regards energy this
is not the case. While the total amount of it in the uni-
verse is constant, its form is constantly subject to change —
and that one in which it enters the Body need not be that
in which it exists while in it, nor that in which it leaves it.
Daily losing heat and mechanical work the Body does not
need, could not in fact much utilize energy, supplied to it ■
in these forms; but it does need energy of some form and
in amount equivalent to that which it loses.

The Conservation of Energy. The forms of energy
known to us are not nearly so numerous as the kinds of
matter. Still we all know several of them; such as light,
heat, sound, electricity, and mechanical work; and most
people nowadays know that some of these forms are inter-
convertible, so that directly or indirectly we can turn one
into another. In such changes it is found that a definite
amount of one kind ahvays disajijiears to give rise to a
certain quantity of the other; or, in other words, that so
much of the first form is equivalent to so much of the
second. In a steam-engine, heat is produced in the fur-
nace; when the engine is at work all of this energy does
not leave it as heat; some goes as mechanical work, and the
more work the engine does the greater is the difference be-
tween the heat generated in the furnace and that leaving
the machine. If, however, we used the work for rubbing
two rough surfaces together we could get the heat back
again, and if (which of course is impossible in practice)

THE CONSERVATION OF ENERGY. 281

we could avoid all friction in the moving parts of the
machine, the quantity thus restored would be exactly equal
to the excess of the heat generated in the furnace over that
leaving the engine. Having turned some of the heat into
mechanical work we could thus turn the work back into
heat again, and find it yield exactly the amount which
seemed lost. Or we might use the engine to drive an elec-
tro-magnetic machine and so turn part of the heat liber-
ated in its furnace first into mechanical work and this into
electricity; and if we chose to use the latter with the
proper apparatus, we could turn more or less of it into
light, and so have a great part of the energy which first
became conspicuous as heat in the engine furnace, now
manifested in the form of light at some distant point. In
fact, starting with a given quantity of one kind of energy,
we may by proper contrivances turn all or some of it into
one or more other forms; and if we collected all the final
forms and retransformed them into the first, we should
have exactly the amount of it which had disappeared when
the other kinds appeared. This law, that energy can
change its form but that its amount is invariable, that it
cannot be created or destroyed but simply transmuted, is
known as the Icvw of the Conservation of Energy (see Phy-
sics), and, like the indestructibility of matter, lies at the
basis of all scientific conceptions of the universe, whether
concerned with animate or inanimate objects.

Since all forms of energy are interconvertible it is con-
venient in comparing amounts of different kinds to express
thein in terms of some one kind, by saying how much of
that standard form the given amount of the kind spoken
of would give rise to it were all converted into it. Since
the most easily measured form of energy is mechanical
work this is commonly taken as the standard form, and
the quantities of others are expressed by saying how great
a distance against the force of gravity at the earth's sur-
face a given weight could be raised by the energy in ques-
tion, if it were all spent in lifting the weight. The units
of mechanical work being the kilogrammeter, or the foot-
pound, the mechanical equivalent of any given kind of cnei-gv

282 THE HUMAN BODY.

is the number of kilogram meters or foot-pounds of work its
unit quantity would perform, if converted into mechanical
work and used to raise a weight. For exam^jle the unit
quantity of heat is that necessary to raise one kilogram of
water one degree centigrade in temperature; or sometimes,
in books written in English, the quantity necessary to warm
one pound of water one degree Fuhronhcit. When there-
fore we say that the mechanical equivalent of heat is 423
kilogrammeters we mean that the quantity of heat which
would raise one kilogram of water in temperature . from
4° 0. to 5° C. would, if all turned into mechanical work,
be able to raise one kilogram 423 meters against the attrac-
tion of the earth; and conversely that this amount of me-
chanical work if turned into heat would warm a kilogram
of water one degree centigrade. The mechanical equiva-
lent of heat, taking the Fahrenheit thermometric scale and
using feet and pounds as measures, is 772 foot-pounds.

Potential and Kinetic Energy. At times energy seems
to be lost. Ordinarily we only observe it when it is doing
work and producing some change in matter: but sometimes
it is at rest, stored away and producing no changes that
we recognize and thus seems to have been destroyed.
Energy at work is known as hinetic energy; energy at rest,
not producing changes in matter, is called j'^o^eniiV// energy.
Suppose a stone pulled uji by a string and left suspended
in the air. We know a certain amount of energy was
used to lift it; but while it hangs we haA'^e neither heat nor
light nor mechanical work to represent it. Still the energv"
is not lost; we know we have only to cut the string and
the weight will fall, and striking something give rise to
heat. Or we may wind up a spring and keep it so by ii
catch. In winding it up a certain amount of energy in
the form of mechanical work was used to alter the form
of the spring. Until the catch is removed this energy re-
mains stored away as potential energy: but we know it is
not lost. Once the spring is let loose again it may drive a
clock or a watch, and in so doing will jjcrform again just
so much work as was spent in coiling it; and when the
watch has run down this energy will all have been turned

POTENTIAL EimRGT OF CHEMICAL AFFINITY. 283

into other forms — mainly heat deyeloped in the friction of
the parts of the watch against one another: but partly also
in producing movements of the air, a portion of which Vv-e
can readily observe in the sound of its ticking. The law
of the conservation of energy does not say, then, that either
the total potential or the total kinetic energy in the universe
is constant in amount: but that the sum of the two is inva-
riable, while constantly undergoing changes from kinetic
to potential and vice versa: and from one form uf kinetic
to another.

The Energy of Chemical AflQ.mty. Between every two
chemical atoms which are capable of entering into combi-
nation there exists a certain amount of potential energy;
when they unite this energy is liberated, usually in the form
of heat, and once they have combined a certain amount of
kinetic energy must be spent to pull them apart again; this
being exactly the amount which was liberated when they
united. The more stable the compound formed the more
kinetic energy appears during its formation, and the more
must be spent to break it up again. One may imagine the
separated atoms as two balls pushed together by sjjrings,
the strength of the spring being jDroportionate to the de-
gree of their chemical affinity. Once they are let loose
and permitted to strike together the potential energy pre-
viously represented by "the compressed springs disappears,
and in its place we have the kinetic energy, represented by
the heat developed when the balls strike together. To
pull them apart again, against the springs, to their original
positions, just so much mechanical work must be spent as
is the equivalent of that amount of heat which appeared
when they struck; and thus kinetic energy will again be-
come latent in breaking up the compound represented by
the two in contact. The energy liberated in chemical com-
bination is the most important source of that used in our
machines: and also of that spent by the living Body.

The Relation between the Matters Removed from the
Body daily and the Energy Spent by it. A working
locomotive is, we know, constantly losing matter to the
exterior in the form of ashes and gaseous products of com-

284 THE HUMAK BODY.

busliion, tlic latter being mainly carbon dioxide ana* water
vapor. The engine also expends energy, not only in the
form of heat radiated to the air, but as mechanical work
in drawing the cars against the resistance offered ])y fric-
tion or sometimes, up an incline, by gravity. Now the en-
gine-driver knows that there is a close relationship between
the losses of matter and the expenditure of energy, so that
he has to stoke his furnace more frequently and allow a
greater draft of air through it in going up a gradient
than when running on the level. The more work the en-
gine does the more coals and air it needs to make up for
its jri eater waste. If we seek the cause of this relation-
ship between work and waste, the first answer naturally is
that the engine is a machine the special object of which is
to convert heat into mechanical work, and so the more
work it has to do the more heat is required for conversion,
and consequently the more coals must be burnt. This,
however, opens the question of the source of the heat — of
all that vast amount of kinetic energy which is liberated in
the furnace; and to answer this we must consider in what
forms matter and energy enter the furnace, since the
energy liberated there must be carried in somehow from
outside. For present purposes coals may be considered as
consisting of carbon and hydrogen, both of which sub-
stances tend to forcibly combine with oxygen at high tem-
peratures, forming in the one case carbon dioxide and in
the other water. The oxygen necessary to form these com-
])Ounds being supplied by the air entering the furnace, all
the potential energy of chemical affinity which existed be-
tAveen the uncombined elements becomes kinetic, and is
liberated as heat when the combination takes place. The
energy utilized by the engine is therefore supplied to it in
the form of potential energy, associated with the uncom-
bined forms of matter whicn reach the furnace. Once the
carbon and hydrogen have combined with oxygen they are
no longer of any use a-s liberators of energy; and the com-
pounds formed if retained in the furnace would only clog
it and impede farther combustion; they are therefore got
rid of as wastes througli the smoke-stack. Tlie engine.

SOURCES OF EXERGY. 285

in short, receives uncombined elements associated with
})otential energy; and loses combined elements (which have
lost the energy previonsh* associated with them) and kinetic
energy: it so to speak separates the energy from the mat-
ter with which it was connected and, utilizing it, gets rid
of the exhausted matter. The amount of kinetic energy
liberated during such chemical combinations is very great;
a kilogram of carbon uniting with oxygen to form car-
bon dioxide sets free 8080 units of heat, or calories. Dur-
ing the combination of oxygen and hydrogen to form
water even more energy is liberated, one kilogi-am of hydro-
gen when completely burnt liberating more than thirty-four
thousand of the same units. The mechanical equivalent
of this can be calculated if it is remembered that one heat
unit = 423 kilogrammeters.

Turning now to the living Body we find that its income
and expenditure agree very closely with those of the steam-
engine. It receives from the exterior substances capable
of entering into chemical union; these combine in it and
liberate energy; and it loses kinetic energy and the products
of combination. From the outside it takes oxygen through
the lungs, and oxidizable substances (in the form of foods)
through the alimentary canal; these combine under the
conditions prevailing in the living cells just as the carbon
and oxygen, which will not unite at ordinary temperatures,
combine under the conditions existing in the furnace of
the engine; the energy liberated is employed in work of the
Body, while the useless products of combination are got
rid of. To explain, then, the fact that our Bodies go on
working we have no need to invoke some special mysterious
power resident in them and capable of creating energy, a
i'ital force having no relation with other natural forces,
such as the older physiologists used to imagine. The Body
needs and gets a supply of energy from the exterior jnst as
the steam-engine does, food and air being to one what coals
and air are to the other; eacli is a machine in which energy
is liberated by chemical combinations and then used for
special work; the character of which depends upon the
peculiarities of mechanism which utilizes it in each case,

286 777^ HUMAN BODY.

and not upon any peculiarity in the energy utilized or in
its source, "^riic ]5ody is, however, a far more economical
machine than any steam-engine; of all the energy liberated
in the latter only a small fraction, about one eighth, is use-
fully employed, while our Bodies can utilize for the perform-
ance of muscular work alone one fifth of the whole energy
supplied to them; leaving out of account altogether the
nutritive and other work carried on in them, and the heat
lost from them.

The Conditions of Oxidation in the Living Body. Al-
though the general principles uijon which the Body and
the steam-engine get their working power are the same,
still in minor points very obvious differences are found
between them. In the first place the coals of an engine
are oxidized only at a very high temperature, one which
would be instantly fatal to our Bodies which, although
warm when compared with the bulk of inanimate objects,
are very slow fires when compared with a furnace. Chem-
istry and physics, however, teach us that this diffei'ence i.s
quite unimportant so far as concerns the amount of energy
liberated. If magnesium wire be ignited in the air it will
become white-hot, flame, and leave at the end of a few
seconds only a certain amount of incombustible rust or
magnesia, which consists of the metal combined with
oxygen. The heat and light evolved in the process repre-
sent of course the energy which, in a potential form, was
associated with the magnesium and oxygen before their
combination. We can, however, oxidize the metal in a differ-
eut way, attended with no evolution of light and no very per-
ceptible rise of temperature. If, for instance, we leave it in
the air it will become gradually turned into magnesia with-
out having ever been hot to the touch or luminous to the
eye. The process will, however, take days or weeks; anti
while in this slow oxidation just as much energy is liberated
as in the former case it now all takes the form of heat; and
instead of being liberated in a short time is sj)read over a
much longer one. as the gradual chemical combination takes
place. The slowly oxidizing magnesium is, therefore, at no
moment noticeably hot since it loses its heat to surrounding

SOURCE OF BODILY ENERGY. 287

objects as fast as it is generated. The oxidations occurring
in our Bodies are of this slow kind. An ounce of arrow-
root oxidized in a fire, and in the Human Body, would
liberate exactly as much energy in one case as the other,
but the oxidation would take place in a few minutes and
at a high temperature in the former, and slowly, at a lower
temperature, in the latter. In the second place, the engine
differs from the living Body in the fact that the oxidations
in it all take place in a small area, the furnace, and so the
temperature there becomes very high; while in our Bodies
the oxidations take place all over, in each of the living
cells; there is no one furnace or hearth where all the energy
is liberated for the Avhole and transferred thence in one
form or another to distant parts: and this is another reason
why no one part of tlie Body attains a very high temperature.
The Fuel of the Body, This is clearly different from
tbat of an ordinary engine: no one could live by eating
coals. This difference again is subsidiary; a gas-engine
requires different fuel from an ordinary locomotive; and
the Body requires a somewhat different one from either. It
needs as foods, substances which can, in the first place, be
absorbed from the alimentary canal and carried to the
various tissues; and, in the second, can there be oxidized
at a low temperature or, perhaps more probably, can be
converted by the living cells into compounds which can be
so oxidized. With some trivial exceptions, all substances
which fulfill these conditions are complex chemical com-
pounds, and to understand their utilization in the Body
we must extend a little the statements above made as to the
liberation of energy in chemical combinations. The general
law may be stated thus — Energy is liberated tuhenever cJicmi-
ral union takes place : and whenever more stable compounds
are formed from less stable ones, in which the constituent
atoms were less firmhj held toijplhcr. Of the liberation by
simj^le combination we have already seen an instance in the
oxidation of carbon in a furnace: but the union need not
be an oxidation. Everyone knows how hot quicklime
becomes when it is slaked; the water combining strongly
with the lime, and energy being liberated in the form of

288 THE HUMAN BODY.

heat, during the process. Of the liberation of energy by
the breaking down of a complex compound, in which the
atoms are only feebly united, into simpler and stabler ones,
we get an example in alcoholic fermentation. During that
process grape sugar is broken down into more stable com-
])ounds, mainly carbon dioxide and alcohol, while oxygen
is at the same time taken up. To pull apart the carbon,
hydrogen and oxygen of the sugar molecule requires a cer-
tain expenditure of kinetic energy: but in the simultaneous
formation of the new and stabler compounds a greater
amount of energy is set free, and the difference appears as
heat, so that the brewer has to cool his vats with ice. It
is by processes like this latter, rather than by direct com-
binations, that most of the kinetic energy of the Body is
obtained; the complex proteids and fats and starches lyid
sugar taken as food being broken down (usually with con-
comitant oxidation) into simpler and more stable com-
pounds.

Oxidation by Successive Steps. In the furnace of an
engine the oxidation takes place completely at once. The
carbon and hydrogen leaving it, if it is well managed, are
each in the state of their most stable oxygen compound.
But this need not be so: we might first oxidize the carbon
so as to form carbon monoxide, CO, and get a certain
amount of heat; and then oxidize the carbon monoxide
farther so as to form carbon dioxide, CO2, and get more
heat. If w^e add together the amounts of heat liberated in
each stage, the sum will be exactly the quantity which would
have been obtained if the carbon had been completely burnt
to the state of carbon dioxide at first. Every one wdio has
studied chemistry will think of many similar cases. As
the process is important physiologically we may take an-
other example; say the oxidation of alcohol. This may be
burnt completely and directly, giving rise to carbon dioxide
and Avater —

CHeO + 0« = 2C0i + 3H2O

1 Alcohol. C Oxygen. 2 Carbon dioxide. 3 water.

But instead of this we can oxidize the alcohol by stages.

UTILIZATION OF ENERGY IN THE BODY. 289

getting at each stage only a comparatively small amount of
heat evolved. By combining it first with one atom of oxy-
gen, we get aldehyde and water —

C.HeO + 0 = C-'H40 + H=0

1 Alcohol. 1 Oxygen. 1 Aldehyde. 1 Water.

Then we add an atom of oxygen to the aldehyde and get
acetic acid (vinegar) —

CiH40 + 0 = C-2H402

1 Aldehyde. 1 Oxygeu. 1 Acetic acid.

And finally we may oxidize the acetic acid so as to get car-
bon dioxide and water —

C.HiO. + 04 = 2CO2 + 2H2O

We get, in either case, from one molecule of alcohol, two of
carbon dioxide and three of water; and six atoms of oxygen
are taken up. In each stage of the gradual oxidation a
certain amount of heat is evolved; and the sum of these is
exactly the amount which would have been evolved by
burning the alcohol completely at once.

The food taken into the Body is for the most part oxi-
dized in this gradual manner; the products of imperfect
combustion in one set of cells being carried off and more
completely oxidized in another set, until the final pro-
ducts, no longer capable of further oxidation in the Body,
are carried to the lungs, or kidneys, or skin, and got rid of.
A great object of physiology is to trace all intermediate
compounds between the food which enters and the waste
products which leave; to find out Just how far chemical
degradation is carried in each organ; and what substances
are thus formed in various parts: but at present this j^art
of the science is very imperfect.

The Utilization of Energy in the Living Body. In the
steam-engine energy is liberated as heat; some of the heat
is used to evaporate water and expand the resulting steam;
and then the steam to drive a piston. But in the living
Body it is very probable (indeed almost certain) that a
great part of the energy liberated by chemical transfor-

290 THE HUMAN BODY.

mations docs not first take the form of heat; tlioiigli some
of it does. This, again, does not affect the general prin-
ciple: the source of energy is essentially the same in both
cases; it is merely the form which it takes that is dif-
ferent. In a galvanic cell energy is liberated during the
union of zinc and sulphuric acid, and we may so arrange
matters as to get this energy as heat; but on the other
hand we may lead it off, as a so-called galvanic current,
and use it to drive a magneto-electric machine before it
has taken the form of heat at all. In fact, that heat may
be used to do mechanical work we must reduce some of it
to a lower temperature: an engine needs a condenser of
some kind as well as a furnace; and, other things being
equal, the cooler the condenser the greater the proportion
of the whole heat liberated in the furnace which can be
used to do work. Now in a muscle there is no condenser;
its temperature is uniform throughout. So when it contracts
and lifts a weight, the energy employed must be liberated"*
in some other form than heat — some form which the muscu-
lar fibre can use without a condenser.

Summary. The living Body is continually losing mat-
ter and expending energy. So long as we regard it as
working by virtue of some vital force, the power of gener-
ating which it has inherited, the waste is difficult to
account for, since it is far more than we can imagine as due
merely to wear and tear of the working j^arts. When,
however, we consider the nature of the income of the Body,
and of its expenditure, from a chemico-physical point of
view, we get the clue to the puzzle. The Body does not
waste because it works but works because it wastes. The
working power is obtained by chemical changes occurring
in it, associated with the liberation of energy which the
living cells utilize; and the products of these chemical
changes, being no longer available as sources of energy, are
passed out. The chemical changes concerned are mainly
the breaking down of complex and unstable chemical com-
pounds into simpler and more stable ones, with concomi-
tant oxidation. Accordingly the material losses of the

SOURCES OF EXERGY IJV THE BODY. 291

Body are highly or completely oxidized, tolerably simple
chemical compounds; and its material income is mainly
uncombined oxygen, and oxidizable substances, the former
obtained through the lungs, the latter through the alimen-
tary canal. In energy, its income is the potential energy
of uncombined or feebly combined elements, which are
capable of combining or forming more stable combinations;
and its final expenditure, is kinetic energy almost entirely
in the form of mechanical work and heat. Given oxygen,
all oxidizable bodies will not serve to keep the Body alive
and working, but only those which (1) are capable of ab-
sorption from the alimentary canal and (3) those which are
oxidizable at the temperature of the Body under the influ-
ence of protoplasm. Just as carbon and oxj'gen will not
unite in the furnace of an engine unless the "fire be
lighted" by the application of a match but, when once
started, the heat evolved at one point will serve to carry on
the conditions of combination through the rest of the mass,
so the oxidations of the Body only occur under special con-
ditions; and these are transmitted from parent to offspring.
Every new Human Being starts as a portion of protoplasm
separated from a parent and affording the conditions for
those chemical combinations which supply to living matter
its working power : this serves, like the energy of the
burning ])art of a fire, to start similar processes in other
portions of matter. At present we know nothing in physi-
ology answering to the match which lights a furnace; those
manifestations of energy which we call life are handed
down from generation to generation, as the sacred fire in
the temple of Vesta from one watcher to another. Science
may at some time teach us how to bring the chemical con-
stituents of protoplasm into that combination in which
they possess the faculty of starting oxidations under
those conditions which characterize life; then we will have
learnt how to strike the vital match. For the present
we must be content to study the i)ro]ierties of that form
of matter which possesses living faculties; since there is no
satisfactory proof that it has ever been produced, within

292 THE HUMAN BODY.

our experience, apart from the influence of matter already
living. How the vital spark first originated, how mole-
cules of carbon, hydrogen, nitrogen and oxygen first united
with water and salts to form protoplasm, we have no scien-
tific data to ground a positive opinion upon, and such as we
may have must rest upon other grounds.

CHAPTER XX.
FOODS.

Poods as Tissue Formers. Hitherto we have considered
foods merely as sources of energy, but they are also re-
quired to build up the substance of the Body. From birth
to manhood we increase in bnlk and weight, and that, not
merely by accnmnlating water and sncli substances, liut by
forming more bone, more muscle, more brain, and so on,
from materials wdiich are not necessarily bone or muscle
or nerve tissue. Alongside of the processes by which com-
plex substances are broken down and oxidized and energy
liberated, constructive processes take place by Avhicli new
complex bodies are formed from simpler substances taken
as food. A great part of the energy liberated in the Body is
in fact ntilized first for this purpose, since to construct com-
plex unstable molecules, like those of protoplasm, from the
simpler compounds taken into the Body, needs an expendi-
ture of kinetic energy. Even after full growth, when the
Body ceases to gain weight, the same synthetic processes go
on; the living tissues are steadily broken down and con-
stantly reconstructed, as we see illustrated by the condition
of a man who has been starved for some time, and who loses
not only his power of doing work and of maintaining his
bodily temperature but also a great part of his living tissues.
If again fed properly he soon makes new fat and new muscle
and regains his original mass. Another illustration of the
continuance of constructive powers during the whole of life
is afforded by the growth of the muscles when exercised
properly.

Since the tissues, on ultimate analysis, yield mainly car-
bon, hydrogen, nitrogen and oxygen, it might be supposed
a priori tlvdt a supplv of these elements in the uncombined

294 THE HVMAH BODY.

state would serve as material iui- t he constructive forces of
the Body to Avork with. Ex})eriL'uce, however, teaches u^
that this is not the case, hut thai ihe aninuil hody requires,
I'or the most part, highly complex compounds for the con-
struction of new tissue elements. All the active tissues
yield on analysis large quantities of proteids which, as
pointed out in Chapter I., enter always into the structure of
protoplasm. Now, so far as we know at jsresent, the animal
body is unable to build ujj proteids from simpler com-
pounds of nitrogen, although when given one variety of
them it can convert that one into others, and combine them
with other things to form protoplasm. Hence proteids are
an essential article of diet, in order to replace that portion
of the living cells which is daily broken down and elimi-
nated m the form of urea and other waste substances.
Even albuminoids (p. 11), although so nearly allied to pro-
teids, will not serve to replace them entirely in a diet; a
man fed abundantly on gelatin, fats, and starches, would
starve as certainly, though not so quickly, as if he got no
nitrogenous food at all; his tissue waste would not be made
good, and he Avould at last be no more able to utilize the
energy-yielding materials supplied to him, than a worn-out
steam-engine could emjiloy the heat of a fire in its furnace.
So, too, the animal is unable to take the carbon for the
construction of its tissues, from such simple compounds as
carbon dioxide. Its constructive power is limited to the
utilization of the carbon contained in more comjDlex and
less stable compounds, such as proteids, fats, or sugars.

The main bulk of all useful foods must therefore be
made up of complex substances, and of these a part must
be proteids, since the Body can utilize nitrogen for tissue
formation only when supplied with it in that form. The
bodies thus taken in are sooner or later broken down into
simpler and eliminated; some at once in order to yield
energy, others only after having first been built up into
part of a living cell. The partial exceptions afforded by
such losses to the Body as milk for suckling the young,
or the albuminous and fatty bodies stored for the same
purpose in the egg of a bird, are only apparent: the chemi-

FOOD OF PLANTS. 295

cal degradation is only postponed, taking place in the body
of the ofEsi^ring instead of that of the parent. In all cases
animals are tlius, essentially, proteid consumers or wasters,
and breakers down of complex bodies; the carbon, hydro-
gen and nitrogen which they take as foods in the form of
complex unstable bodies, ultimately leaving them in the
simpler compounds, carbon dioxide, water, and urea:
which are incapable of either yielding energy or building-
tissue for any other animal and so of serving it as food.
The question immediately suggests itself. How, since animals
are constantly breaking up these complex bodies and can-
not again build them, is the supply kept up ? For exam-
ple, the supply of proteids, which cannot be made artificially
by any process which we know, and yet are necessary foods
for all animals, and daily destroyed by them.

The Pood of Plants. As regards our own Bodies the
question at the end of the last paragraph might perhaps be
answered by saying that we get our proteids from the flesh
of the other animals which we eat. But, then, we have to
account for the possession of them by those animals; since
they cannot make them from urea and carbon dioxide and
water any more than we can. The animals eaten get them,
in fact, from plants which are the great proteid formers of
the world, so that the most carnivorous animal really de-
pends for its most essential foods upon the vegetable king-
dom; the fox that devours a hare in the long run lives on
the proteids of the herbs that the hare had previously eaten.
All animals are thus, in a certain sense, parasites; they
only do half o£ their own nutritive work, just the final
stages, leaving all the rest to the vegetable kingdom and
using the products of its labor; and plants are able to meet
this demand because they can live on the simple compounds
of carbon, hydrogen, and nitrogen eliminated by animals,
building up out of them new complex substances which
animals can use as food. A green plant, supplied with am-
monia salts, carbon dioxide, water, and some minerals, will
grow and build up large quantities of proteids, fats, starches,
and similar things; it will pull the stable compounds eli-
minated by animals to pieces, and build tliem up into com-

296 THE HUMAN BODY.

plex unstable oodies, capable of yielding energy when again
broken down. However, to do such work, to break up
stable combinations and make from them less stable, needs
a supply of kinetic energy, which disai)pcars in the process
being stored away as potential energy in the new compound;
and we may ask whence it is that the plant gets the suj)})ly
of energy which it thus utilizes for chemical construction,
since its simple and highly oxidized foods can yield it none.
It has been proved that for this purpose the green plant
uses the energy of sunlight: those of its cells which contain
the substance called chlorophyl (leaf green) have the power
of utilizing energy in the form of light for the perform-
ance of chemical work, just as a steam-engine can utilize
heat for the performance of mechanical work. Exposed to
light, and receiving carbon dioxide from the air, and water
and ammonia (which is produced by the decomposition of
urea) from the soil, the plant builds them ujo again, with
the elimination of oxygen, into complex bodies like those
which animals broke down, with fixation of oxygen.
Some of the bodies thus formed it uses for its own growth
and the formation of new protoplasm, just as an animal
does; but in sunlight it forms more than it uses, and the
excess stored up m its tissues is used by animals. In the
long run, then, all the energy spent by our Bodies comes
through millions of miles of space from the sun; but to
seek the source of its supply there would take us far out of
the domain of Physiology (see Astronomy).

Non-Oxidizable Foods. Besides our oxidizable foods, a
large number of necessary food materials are not oxidiza-
ble, or at least are not oxidized in the Body. Typical in-
stances are afforded by water and common salt. The use
of these is in great part physical; the water, for instance,
dissolves materials in the alimentary canal, and carries the
solutions through the walls of the digestive tube into
the blood and lymph vessels, so that they can be carried
from part to part; and it permits interchanges to go on by
diffusion. The salines also influence the solubility and
chemical interchanges of other things present with them.
Serum albumen, tlie chief proteid of the blood, for example,

NON-OXIDIZABLE FOODS. 297

is insoluble in pure water, but dissolves readily if a small
quantity of neutral salts is present. Besides such uses the
non-oxidizable foods have probably others, in vt^hat we may
call machinery formation. In the salts which give their
hardness to the bones and teeth, we have an example of
such an emjiloyment of them: and to a less extent the
same may be true of other tissues. The Body, in fact, is
not a mere store of potential energy but something more —
it is a machine for the disposal of it in certain ways; and,
wherever practicable, it is clearly advantageous to have the
purely energy-expending parts made of non-oxidizable mat-
ters, and so protected from change and the necessity of
frequent renewal. The Body is a self-building and self-
repairing machine, and the material for this building and
repair must be supplied in the food, as well as the fuels, or
oxidizable foods, which yield the energy the machine ex-
pends; and while experience shows us, that even for ma-
chinery construction, oxidizable matters are largely needed,
nevertheless it is a gain to replace them by non-oxidizable
substances when possible; just as if practicable it would be
advantageous to construct an engine out of materials which
would not rust, although other conditions determine the
use of iron for the greater part of it.

Definition of Poods. Foods may be defined as sub-
stances ichich are taken into the alimentary canal, and
which, vjhen absorbed from it, serve either to supply material
for the growtli of the Body, or for the replacement of matter
which has been removed from it, either after oxidation or
without having been oxidized. Foods to replace matters
which have been oxidized must be themselves oxidizable;
they are force generators, but may be and generally are also
tissue formers; and are nearly always complex organic sub-
stances derived from other animals or from plants. Foods
to replace matters not oxidized in the Body are force regu-
lators, and are for the most j^art tolerably simple inorganic
compounds. Among the force regulators we must, how-
ever, include certain organic foods which, although oxidized
in the Body and serving as liberators of energy, yet produce
effects totally disproportionate to the energy they set free.

398 THE UUMAJS- BODY.

iind for which effects they are taken. In other words, theii-
iuiluence as stimuli in exciting certain tissues to liberate
energy, or as inhibitory agents checking the activity of
parts, is more marked than their direct action as force gen-
erators. As examples, we may take condiments: mustard
and pepper are not of much use as sources of energy, al-
though they no doubt yield some; we take them for their
stimulating effect oii the mouth and other parts of the
alimentary canal, by which they promote an increased flow
of the digestive secretions or an increased appetite for
food. Thein, again, the active princiiDle of tea and coffee, is
taken for its stimulating effect on the nervous system, rather
than for the amount of energy which is yielded by its own
oxidation.

Conditions which a Food must Fulfill. ' (1) A food
must contain the elements Avhich it is to replace in the
Body: but that alone is not sufficient. The elements leav-
ing the Body being usually derived from the breaking down
of com^Dlex substances in it, the food must contain them
either in the form of such complex substances, or in forms
which the Body can build up into them. Free nitrogen
and hydrogen are no use as foods, since they are neither cxi-
dizable under the conditions prevailing in the Body (and
consequently cannot yield it energy), nor are they capable
of construction by it into its tissues. (2) Food after it
has been swallowed is still in a strict sense outside the
Body; the alimentary canal is merely a tube running
through it, and so long as food lies there it is not forming
any part of the Body proper. Hence foods must be capa-
ble of absorption from the alimentary canal; either directly,
or after they have been changed by the processes of diges-
tion. Carbon, for example, is no use as a food, not merely
because the Body could not build it n^ into its own tissues,
but because it cannot be absorbed from the alimentary
canal. (3) Neither the substance itself nor any of the
products of its transformation in the Body must be inju-
rious to the structure or activity of any organ. If so it is
di poison, not a food.

Alimentary Principles. What m common language we

ALIMENTARY PRINCIPLES. 299

commonly call foods are, in nearly all cases, mixtures of
several foodstuffs, with substances which are not foods at
all. Bread, for example, contains water, salts, gluten (a
proteid), some fats, much starch, and a little sugar; all true
foodstuffs: but mixed with these is a quantity of cellulose
(the chief chemical constituent of the walls which surround
vegetable cells), and this is not a food since it is incapable
of absorption from the alimentary canal. Chemical exami-
nation of all the common articles of diet shows that the
actual number of important foodstuffs 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 foodstuffs, which are found repeated in so
many different foods, are known as " alimentary principles f
and the physiological value of any article of diet depends
on them far more than on the traces of flavoring matters
which cause certain things to be especially sought after and
so raise their market value. The alimentary principles
may be conveniently classified into proteids, albuminoids,
hydrocarbons, carbohydrates, and inorganic bodies,

Proteid Alimentary Principles. Of the nitrogenous
foodstuffs the most important are proteids: they form an
essential part of all diets, and are obtained both from
animals and plants. The most common and abundant are
myosin and syntonin which exist in the lean of all meats;
egg albumen; casein, found in milk and cheese; gluten and
vegetable casein from various plants.

Albuminoid Alimentary Principles. These also con-
tain nitrogen, but cannot replace the proteids entirely as
foods; though a man can get on with less proteids when he
has some albuminoids in addition. The most important is
gelatin, which is yielded by the white fibrous tissue of
animals when cooked. On the whole the albuminoids are
not foods of high value, and the calf's-foot jelly and such
compounds, often given to invalids, have not nearly the
nutritive value they are commonly supposed to possess.

300 THE HUMAN BODY.

Hydrocarbons {Fals (iiul Oils). The most inii)or1:i,ni.
are stearin, palmatin, margarin and olein, wliicli exist m
various proportions in animal fats and vegetable oils; the
more fluid containing most olein. Butter consists chiefly of
a fat known as butyrin. All are neutral compounds of gly-
cerine and fatty acids and, speaking generally, any such sub-
stance which is fusible at the tem])erature 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
m those foods with so miich 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 Avould be
useful as a food could it be digested. It is convenient to
distinguish fats proper (the adipose tissue of animals
consisting of fatty compounds inclosed in albuminous cell-
walls) from oils, or fatty bodies which are not so surrounded.

Carbohydrates. These arc mainly of vegetable origin.
The most important are starch, found in nearly all vege-
table foods; dextrin; gums; //ra^^e -m^/ar (into which starch
is converted during digestion); and cane sugar. Sugar of
7nilh and glycogen are alimentary principles of this group,
derived from animals. All of them, like the fats, consist
of carbon, hydrogen and oxygen; but the percentage of
oxvgen in them is much higher, there being one atom of
oxvgen for every two of hydrogen in their molecule.

Inorganic Foods. Water; common salts; and the chlo-
rides, phosphates, and sulphates of potassium, magnesium
and calcium. More or less of these bodies, or the materials
for their formation, exists in all ordinary articles of diet,
so that we do not swallow them in a separate form. Phos-
])hates, for example, exist in nearly all animal and yegetabk;
foods; while other foods, as casein, contain phosphorus in
combinations Avhich in the Body yield it up to be oxidized
to form i^hosphoric 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 proteids. Calcium salts are abun-
dant in bread, and arc also found in many drinking waters.

FLESH FOODS. 301

Water and table salt form exceptions to tlie rule that in-
organic bodies are eaten imperceptibly along with other
things, since the Body loses more of each daily than is usu-
ally supplied in that way. It has, however, been main-
tained 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 deprivation.

Mixed Foods. These, as already pointed out, include
nearly all common articles of diet; they contain more than
one alimentary principle. Among them we find great
differences; some being rich in proteids, others in starch,
others in fats, and so on. The formation of a scientific die-
tary depends on a knowledge of these characteristics. 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 fibres,
tendons, fats, blood-vessels, and nerves. It contains several
proteids and especially myosin; gelatin-yielding matters
in the white fibrous tissue; stearin, palmatin, margariu, and
olein among the fats; and a small amount of carbohydrates
in the form of glycogen and grajje sugar; also inosite, a
kind of sugar found only in muscles. Flesh also contains
much water and a considerable number of salines, the most
imi^ortant and abundant being jjotassium phosphate. Os-
mazome is a crystalline nitrogenous body which gives much
of its taste to flesh; and small quantities of various similar
substances exist in different kinds of meat. There is also
more or less yellow elastic tissue in flesh; it is indigestible
and useless 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 much
of the proteid 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, if it is not intended to use

303 THE HUMAN BODY.

; lie broth, by putting the raw meat at once into boiling
•.vater, which coagulates the surface albumin before it dis-
solves out, and this keeps in the rest. 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 proteid, known as vitellin. Also
fats, and a substance known as lecithin (p. 14), which is
important as containing a considerable quantity of phos-
l^horus.

Milk. This contains the proteid known as casein; several
fats forming the butter; a carbohydrate, the milh sugar;
and much water and salts, especially phosphates. Butter is
mainly made of butyrin, a compound of butyric acid and
glycerine. 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 albumi-
nous matter; by churning, these pellicles are broken up and
the fat droplets run together to form the butter. Casein is
insoluble in water, and in the milk it is dissolved by the
alkaline salts present. When milk is kept its sugar fer-
ments and gives rise to lactic acid, which neutralizes the
alkali and precipitates the casein as curds. In cheese-
making the casein is similarly precipitated by the addition
of an acid and (the whey being pressed out) it constitutes
the main bulk of the cheese.

Vegetable Foods. Of these wheat affords the best. In
1000 parts it contains 135 of proteids, 568 of starch, 46 of
dextrin (a carbohydrate), 49 of grape sugar, 19 of fats,
and 32 of cellulose, the remainder being water and salts.
The proteid of wheat is mainly gluten, which when moist-
ened with water forms a tenacious mass, and this it is to
which wh eaten bread owes its superiority. When the
dough is made yeast is added to it, and produces a fermen-
tation by which, among other things, carbon dioxide gas
is produced. This gas, imprisoned in the tenacious dough.

VEGETABLE FOODS. 303

and expauded 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. Other
cereals may contain a larger percentage of starch but none
have so much gluten as wheat; when bread is made from
them the caibon dioxide gas escapes so readily from the less
tenacious dough that it does not expand the mass properly.
Corn contains in 1000 parts, 79 of proteids, 637 of starch, and
from 50 to 87 of fats; much more than any other kind of
grain. Rice is poor in proteids (50 parts in 1000) but very
rich in starch (823 parts in 1000). Peas and beans are rich
in proteids (from 220 to 260 parts in 1000), and contain about
half their weight of starch. Potatoes are a poor food. They
contain a great deal of water and cellulose, and only about
13 parts of proteids and 154 of starch in 1000. 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, proteids,
or fats. Fruits, like most fresh vegetables, are mainly valu-
able for their saline constituents, the other foodstuffs in them
being only j)resent in small proportion. Some form of
fresh vegetables is, however, a necessary article of diet; as
shown by the scurvy which used to prevail among sailors
before fresh vegetables or lime-juice wore supplied to them.
The Cooking of Vegetables. Tliisis of more importance
even than the cooking of flesh, since in most the main ali-
mentary principle is starch, and raw starch is difficult of
digestion. In plants starch is nearly always stored up in
the form of solid granules, which consist of alternating
layers of starch ceUnlose and starch granulose. The diges-
tive fluids turn the starch into grape sugar which is soluble
and can be absorbed from the alimentary canal, while starch
itself cannot. Now these fluids act very slowly and imper-
fectly on raw starch, and then only on the granulose; but
when boiled, the starch gi-anules swell up, and are more
readily converted into grape sugar; and the starch cellulose
is so altered that it too undergoes that change. When starch
is roasted it is turned into a substance known as soluble
starch which is readily dissolved in the alimentary canal.

304 THE HUMAN BODY.

There is therefore a scientific foundation for the common
belief that tlie crust of a loaf is more digestible than the
crumb, and toast than ordinary bread.

Alcohol. Tliere are perhaj^s no common articles of
diet concerning which more contradictory statements have
been made than alcoliolic drinks. This depends upon their
l)eculiar position: according to quantity or circumstances
alcohol may be a poison or a food; and as a food it may be
i-egarded either as a force regulator or a force generator.
There is no doubt that alcohol in certain doses may be pro])-
perly called a food. If not more tlian two ounces (which
would be contained in about four ounces of whiskey or two
quarts of lager beer) are taken in the twenty-four hours, it
is completely oxidized in the Body and excreted as water
and carbon dioxide. In this oxidation energy is of course
liberated and can be utilized. Commonly, however, alco-
hol is not taken for this purpose but, as a force regulator,
for its influence on the nervous system or digestive organs,
and it is in this capacity that it becomes dangerous. For
not only may it be taken in quantities so great that it is
not all oxidized in the Body but is passed through it as alco-
hol, or even that it acts as a narcotic poison instead of a
stimulant, but when taken in what is called moderation
there can be no doubt that the constant " whipping up" of
the flagging organs, if continued, must be dangerous to
their integrity. Hence the daily use of alcohol merely ni
such quantities as to produce slight exhilaration or to facili-
tate work is by no means safe; though in disease when the
system wants rousing to make some special effort, the phy-
sician cannot dispense with it or some other similarly act-
ing substance. In fact, as a force generator alcohol maybe
advantageously replaced by other foods in nearly all cases;
and there is no evidence that it heljis in the construction
of the working tissues, thougli its excessive use often leads
to an abnormal accumulation of fat. Its proper use is as
a '•' Avhip," and, one has no more right to use it to the
healthy Body than the lash to overdrive a Avilling horse.
The jdiysician is the proper jjerson to determine whether it
is wanted under any given circumstances.

ALCOHOL. 305

If alcohol is to be used as a daily article of diet it sliould
he borne in mind that when concentrated it coagulates the
proteids of tlie cells of the stomach witJi Avhich it comes in
contact, in the same sort of way, though of course to a
much less degree, as it shrivels and dries up an animal pre-
served in it. Dilute alcoholic drinks, such as claret and beer,
are therefore far less baneful than whiskey or brandy, and
these are worst of all in the almost undiluted form of most
" mixed drinks." For the same reason alcohol \a far more
injurious on an empty stomach than after a meal. When
the stomach is full it is diluted, is more slowly absorbed,
and, moreover, is largely used up in coagulating the proteids
of the food instead of those of the lining membrane of the
stomach. The old '"three bottle" men who drank their
port-wine after a heavy dinner, got off far more safely than
the modern tippler who is taking ''nips" all day long,
although the latter may imbibe a smaller quantity of alco-
hol in the twenty-four hours. By far the best way, how-
ever, is to avoid alcohol altogether m health. If the facts
lead us to conclude, against the extremists, that it is to a
certain extent a food, it is nevertheless a dangerous one;
even in what we may call "physiological " quantities, or
such amounts as can be totally oxidized in the Body.

The Advantage of a Mixed Diet. The necessary quan-
tity of daily food depends upon that of the material daily
lost from the Body, and this varies both in kind and
amount with the energy expended and the organs most
used. In children a certain excess beyond this is required
to furnish materials for growth. Although it is impossible
to lay down with perfect accuracy how much daily food any
individual requires, still the average quantity may be de-
rived from the table of daily losses given on page 278,
which shows that a healthy man needs daily in assimilable
forms about 274 grams (4220 grains) of carbon and 19
grams (292 grains) of nitrogen. The daily loss of hydrogen
which is very great (352 grams or 5428 grains), is nearly all
that contained in water which has been drunk and, so to
speak, merely filtered through the Body, after having
assisted in the solution and transference through it of other

306 THE HUMAN BODY

substances. About 300 grams (4820 grains) of water
(containing 33.3 grams (513 grains) of hydrogen are, how-
ever, formed in tlie Body by oxidation, and the liydrogenfor
this purpose must be supplied m the form of some oxidiza-
ble foodstuff, wliether proteid, fat, or carbohydrate. The
oxygen wanted is mainly received from the air through the
lungs, but some is taken in the food.

Since protcid foods contain carbon, nitrogen and hydro-
gen, life may be kept up on them alone, with the necessary
salts, Avater and oxygen; but such a form of feeding would
be anything but economical. Ordinary protcids contain in
100 parts (p. 10) about 52 of carbon and 15 of nitrogen, so
a man fed on them alone would get about 3| parts of carbon
for every 1 of nitrogen. His daily losses are not in this
ratio, but about that of 274 grams (4220 grains) of carbon
to 20 grams (308 grains) of nitrogen, or as 13.7 to 1; and
so to get enough carbon from proteids far more tlian the
necessary amount of nitrogen must be taken. Of dry
proteids 527 grams (8116 grains) would yield the necessary
carbon, but would contain 79 gi'ams (1217 grains) of
nitrogen; or four times more than is necessary to cover the
daily losses of that element from the Body. Fed -on a
purely proteid diet a man would, therefore, have to digest a
vast quantity to get enough carbon, and in eating and
absorbing it, as well as in getting rid of the extra nitrogen
which is useless to him, a great deal of unnecessary labor
would be thrown upon the various organs of his Body.
Similarly, if a man were to live on bread alone he would
burden his organs with much useless work. For bread
contains but little nitrogen in proportion to its carbon, and
so, to get enough of the former, far more carbon than was
utilized would have to be eaten, digested, and eliminated
daily.

Accordingly, we find that mankind in general employ a
mixed diet when they can get it, using richly proteid sul)-
stances to supply the nitrogen needed, but deriving the
carbon mainly from non -nitrogenous foods of the fatty or
carbohydrate groups, and so avoiding excess of either. For
instance, lean beef contains about \ of its weight of dry

ADVANTAGE OF A MIXED DIET. 307

proteid, which contaiDS 15 per cent of nitrogen. Conse
quently the 133 grams (2048 grains) of proteid which
would be foiind in 532 grams (1 lb 3 oz.) of lean meat
would supply all the nitrogen needed to compensate for a
day's losses. But the proteid contains 52 per cent of
carbon, so the amount of it in the above weight of fatless
meat would be 69 grams (1062 grains) of carbon, leaving
205 grams (3157 grains) to be got either from fats or car-
bohydrates. The necessary amount would be contained
m about 256 grams (3942 grains) of ordinary fats or 460
grams (7084 grains^ of starch; hence either of these, with
the above quantity of lean meat, would form a far better
diet, both for the purse and the system, than the meat alone.
As already pointed out, nearly all common foods contain
several foodstuffs. Ordinary butcher's meat, for example,
contains nearly half its weight of fat; and bread, besides
proteids, contains starch, fats, and sugar. In none of them,
however, are the foodstuffs mixed m the i)hysiologically
best proportions, and the practice of employing several of
them at each meal or different ones at different meals
during the day, is thus not only agreeable to the palate but
in a high degree advantageous to the Body. The strict
vegetarians who do not employ even such substances as
eggs, cheese, and milk, but confine themselves to a purely
vegetable diet (such as is always poor in proteids), daily take
far more carbon than they require, and are to be congratu-
lated on their excellent digestions which are able to stand
the strain. Those who use eggs, cheese, etc., can of course
get on very well, since such substances are extremely rich
m proteids, and supply the nitrogen needed without the
necessity of swallowing the vast bulk of food which mast
be eaten in order to get it from plants directly.

/

CHAPTER XXI.

THE ALIMENTARY CANAL AND ITS APPEN-
DAGES.

General Arrangement. The alimentary canal is essen-
tially an involuted jiortion of the skin, specially set apart
for absorption, and forming a tube which runs through
the Body (Fig. 2); it communicates with the exterior at
three points (the nose, the mouth, and the anal aperture),
at which this modified skin, or mucous memhrane, is con-
tinuous with the general outer integument. Supporting
the lining a])SorptiYe membrane are other layers which
strengthen the tube, and are also 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
connection with it numerous glands, whose function it is to
pour into it various secretions which exert a solvent influ-
ence on such foodstuffs as would otherwise escape absor])-
tion. Some of these glands are minute and imbedded 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
dilatations on its course; nor is it straight, since, being much
longer than the Body, 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 sue-

MOUTH CAVITY.

309

ceeded by the j)Iiari/nx or throat cavity, which narrows at
the top of the ueck into the gullet or oesophagus; this runs
down through the thorax and, passing through the dia-
phragm, dilates in the upper part of the abdominal cavitr
into the stomach. Beyond
the stomach the channel again
narrows to form a long and
greatlv coiled tube, the small
intestine, which terminates by
opening into the large intes-
tine, much shorter although
wider than the small, and ter-
minating by an opening on
the exterior.

The Mouth Cavity (Fig.
89) is bounded in front and
on the sides by the lips and
cheeks, below by the tongue,
k, and above by the palate;
Avhich latter consists of an an-
terior part, 7, supported by
bone and called the hard pal-
ate, and a jDosterior, /, con-
taining no bone, and called
the soft 2^alate. The two can
readily be distinguished by
applying the tip of the tongue
to the roof of the mouth and
drawing it backwards. The
hard palate forms the parti-
tion between the mouth and
nose. The soft palate arches
down over the back of the
mouth, hanging like a cur-
tain between 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, i\iQ uvula, hangs
doAvn.

The Teeth. Immediately within the cheeks and lips are

Fig. 89.— The mouth, nose and pha-
rynx, with the commencement of the
gullet and larynx, as exposed by a
section, a little to the left of the me-
dian plane of the head. a. vertebral
column : b, gullet : c, windpipe ; c1.
larynx : e, epiglottis ; /. soft palate ;
(J. opening of Eustachian tube ; k.
tongue : ?, hard palate ; m. the sphe-
noid bone on the base of the skull; n.
the fore part of the cranial cavity :
o. p. g, the tubinate bones of the out-
er side of the left nostril chamber.

310 THE HUMAN BODY.

two semicircles, formed by the borders of the upper and
lower jaw-bones, which arc covered by tlie gums, except at
intervals along their edges where they contain sockets in
which the teeth are implanted. During life two sets of
teeth are developed; 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 all three parts are distinguishable; one, seen in the
mouth and called the croiun of the tooth; a second, im-
bedded 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 differences in
their forms and uses the teeth are divided into incisors,
canines, bicnsjjids, 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 are thirty-two in number, eight in each half of each
jaw, viz. — beginning at the middle line — 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 tvis-
doin teeth.

Characters of Individual Teeth. The incisors (Fig.
90) 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. 91) are somewhat larger than the incisors. Their
crowns are thick and somewhat conical, having a central
point or cus}) 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. 93) are rather shorter than the canines and their

TEE TEETH.

311

crowns are somewhat cnboidal. 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
laterally, and has usually a groove partially subdividing it

Fig. •

Fig. 92.

Fig. 93.

Fig. 90.— An incisor tooth.
Fig. 91.— a canine or eye tooth.

Fig. 92.— a bicuspid tooth seen from its outer side; the inner cusp is, accord-
ingly, not visible.
Fig. 93.— a molar tooth.

into two. At its tip the separation is often complete.
The molar teeth or grinders (Fig. 93) have large crowns
with broad surfaces, on which are four or five projecting
tubercles, which roughen them and make them better adapt-
ed to crush the food. Each has usually several fangs. The
milk teeth only differ in subsidiary points from those of
the same names hi 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 filled during Hfe with a soft vascular
pulp, and hence is known as the "pulp cavity" {c, Fig.
94). The hard parts of the tooth disposed around the pulp
cavity consist of three different tissues. Of these one im-
mediately surrounds the cavity and makes uj) most of the
bulk of the tooth; it is dentine (8, Fig. 94); covering the
dentine on the crown is the enamel (1, Fig. 94) and, on the
fang, the cement (3, Fig. 94).

The pulp cavity opens below by a narrow aperture at the
tip of the fang, or at the tip of each if the tooth has more
than one. The pulp consists mainly of connective tissue,
but its surface next the dentine is covered by a layer of

312

THE HUMAN nODT.

columnar cells. Through tlic opening on the fang blood-
vessels and nerves enter the i)ulp.

The dentine vields on clieniical analysis the same mate-

FiG. 0-4. —Section through a premolar tooth of the cat still imbedded iu its
socket. 1. enamel: 2, dentine; 3, cement; 4, the gum, 5, the bone of the lower
jaw ; c, the pulp cavity.

rials as bone but is somewhat harder, earthy matters con-
stituting 72 per cent of it a? against 6G per cent in bone.
Under the microscope it is recognized by the fine dentinal

THE TONGUE. 313

tiidnles which, radiating from the pulp cayity, 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 (j-gVo of an inch) in diameter.
The cement is much like bone in structure and composition,
possessing lacuna? and canaliculi but rarely any Haversian
canals. It is thickest at the tip of the fang and thins
away towards the cervix. Enamel is the hardest tissue m
the Body, yielding on analysis only from two per cent to
three per cent of organic matter, the rest being mainly
calcium. phosi)hate and carbonate. Its histological ele-
ments 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 struc-
tureless horny layer, the enamel cuticle.

The Tongue (Fig- 95) is a muscular organ covered with
a mucous membrane, extremely mobile, and endowed not
only with a delicate tactile sensibility but with the ter-
minal organs of the special sense of taste; it is attached
by its root to the hyoid bone. Its upper surface is covered
with small eminences or j»?«/?i//«9, much like those more
highly developed on the tongue of a cat, where they are
readily felt. On the human tongue there are three kinds
of papillffi, the circum vallate, the fu7igiform, and the fili-
form. The circum vallate papillae, 1 and 2, the largest
and least numerous, 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 forwards. Each is an ele-
vation of the mucous membrane, covered by epithelium,
and surrounded by a trench. On the sides of the papillae,
imbedded in the epithelium, are small oval bodies (Fig. 155)
richly supplied with nerves and supposed to be concerned
in tlie sense of taste, and hence called the taste buds
(Chap. XXXIV.). The fungiform paj)ill(B,d, are rounded
elevations attached by somewhat narrower stalks, and found
all over the middle and fore part of the upper surface of

314

THE HUMAN BODY.

the tongue. They are easily recognized on the living
tongue by their bright red color. The filiforni papiUce,
most numerous and smallest, are scattered all over the dor-

'\3

Fig. 05.— The upper surface of the tongue. 1, 2. oircumvallate papillas; 3, fun-
giform papillae: 4, filiforni papillas; 6, mucous glands ; 7, tonsils; 8, part of epi-
£■10111?.

sum of the tongue except near its base. Each is a conical
eminence covered by a thick horny layer of epithelium.
It is these papillfe which are so highly developed on the
tongues of Carmvoi'a, and serve them to scrape bones clean

TEE SALIVARY GfL4ND8. 315

of even such tongli structures as ligaments. Tamed tigers
have been known to draw blood by licking the'^hand of their
master.

li'ote. 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, ex-
cept 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, a few cells of epithelium shed from the surface, and
numerous microscopic organisms known as bacteria; and
there is frequently a "bad taste" in the mouth. The
whole alimentary mucous membrane is in close physio-
logical relationship; and anything disordering the sto-
mach 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 mibeddcd in its lining membrane, moistens it, is
secreted by three pairs of glands, the parotid, the submaxil-
lary and the snUinguaJ. 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 Stenon^fi
duct, which crosses the cheek and opens opposite the second
upper molar tooth. In the disease known as mumps 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 submaxil-
lary 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 (Fig. 89), leading from it
into the pharynx below the soft palate. It is bounded
above by the soft palate and uvula, below by the root of the
tongue, and on the sides by muscular elevations, covered by

316 77//; HUMAN BODY.

mucous membrane, which reach from the soft palate to tlie
tongue. Tliese elevations are the pillar's of the fauces.
Each bifurcates below, and in the liollow between its divi-
sions lies a tonsil (7, Fig 95), a soft rounded body about the
size of an almond, and containing numerous minute glands
which form mucus.

Note. 1'he tonsils not unfrequently become enlarged
during a cold or sore throat, and then pressing on the
Eustachian tube (Chap. XXXIII. ), which leads from the
pharynx to the middle ear, keep it closed and produce tem-
porary deafness. Sometimes the enlargement is permanent
and causes much annoyance. Tlie tonsils can, however, be
readily removed without danger, and this is the treatment
usually adopted in such cases.

The Pharynx or Throat Cavity (Fig. 89). This por-
tion 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 imi)erfect, presenting apertures
which lead into the nose, the mouth, and (through the
larynx and windpipe) into the lungs. Except when food
is being swallowed the soft palate hangs down between the
mouth and pharynx; during deglutition it is raised into a
horizontal joosition and separates an upper or resjnratory
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; wliile through th3
lower portion both food and air j^ass, one on its way to the
gullet, 1), Fig. 89, 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 num-
ber; the two posterior nares, the two Eustachian tubes, the
fauces, the opening of the larynx, and that of the gul-
let. At the root of the tongue, over the opening of the
larynx, is a plate of cartilage, the ejnglottis, 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

THE STOMACH

317

a s[)Oon. During swallowing the epiglottis is i^ressed clown
like a lid over the air-tube and helps to keep food or saliva
from entering it. In structure the pharynx consists essen-
tially of a bag of connective tissue lined by mucous mem-
brane, and having muscles in its walls which, by their con-
tiactions, drive the food on.

The CEsophagus or Giillet is a tube commencing at the
lower termination of the pharynx and which, passing on
through the neck and chest, ends in the stomach below the
diaphragm. In the neck it lies close behind the windpipe.
It consists of three coats — a mucous membrane within ; next,
a submucous coat of areolar connective tissue; and, outside,
a muscular coat made up of two layers, an inner with trans-
verse and an outer with longitudinally arranged fibres. In
and beneath its mucous membrane are numerous small
glands whose ducts open into the tube.

The Stomach (Fig. 96) is a somewhat conical bag placed
transversely m the upper part of the abdominal cavity. Its
larger end is turned to

y

the left and lies close
beneath the diaphragm;
opening into its upper
border, through the car-
diac orifice at a, is the
gullet, d. The narrower
right end is continuous
at c Avitli the small intes-
tine; the communication
between the two is the
pyloric orifice The py-
loric end of the stomach
lies lower in the abdomen
than the cardiac, and is
separated from the diapliragm by the liver (see Fig. 1).
The concave border between the two orifices is known as
the small curvature, and the convex as the great curvature,
of 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

.^/

Fig. 9R.— The stomach, cf, lower end of
the gullet; a. position of the cardiac aper
tiii'n : 6, the fundus : c, the pylorus: e. the
commencement of the small intestine
along a, 6, c, the great curvature; between
the pylorus and d, the lesser curvature.

318 THE HUMAN BODY.

apron. After middle life much fat frequently accumulates
in the omentum, so that it is largely responsible for the
"fair round belly with good capon hn'd." The protrusion
h to the left side of the cardiac orifice, Fig. 96, is i\\c fun-
dus or (jrctd cul de sac. The size of the stomach varies
greatly with the amount of food in it; just after a mode-
rate meal it is about ten inches long, by five wide at its
Ijroadest part.

Structure of the Stomach. This organ has four coats,
known successively from without in as the serous, the mus-
cular, the submucous, and the mucous. The serous coat is
formed by a reflexion 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. 54*) consists of
un.striped muscilar tissue arranged in three layers: an outer,
longitudinal, most developed about the curvatures; a circu-
lar, 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 suhmicous 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 shrunk, this coat is thrown into folds. During
digestion the arteries supplying the stomach become dilated
and, its capillaries being gorged, its mucous membrane is
then much redder than when the organ is empty.

The blood-vessels of the stomach run to it between the
folds of peritotieuni 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
wiiich twigs proceed to supply the close capillary network
of the mucous membrane. The pneiimogastric nerves
(p. 171) end in the stomach, and it also gets branches from
the sympathetic system.

Histology of the Gastric Mucous Membrane. Exami-
nation of the inner surface of the stomach with a hand

*P. 124.

STRUCTURE OF STOMACH.

319

lens shows it to be covered 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; there being between them only a small amount
of connective tissue, a close netAvork of lymph-channels,
and capillary blood-vessels. The whole surface of the
mucous membrane is lined by a single layer of columnar
epithehum cells (Fig. 97). These dip down and line the
tubular glands, being in some (especially those about the
pyloric end of the stomach) but little modified in appear-
ance (c, Fig. 97). In others the epithelial cells become
shorter and cuboidal, and
have beneath them {a and b.
Fig. 97) a second incomplete
layer of much larger oval
cells, d. The glands of this
second kind are the most
numerous, and have been
called peptic glands from
the idea that they alone
formed pepsin, the essential
digestive ingredient of the
gastric juice; this is how-
ever by no means certain.
The peptic 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 a
spTiincter muscle around the orifice, which in life, by its
contraction, keeps the passage to the small intestines closed
except when portions of food are to be passed on from
the stomach to succeeding divisions of the alimentary
canal.

Xote. The cardiac end of the stomach lying immediately
beneath the diaphragm, which has the heart on its upper
side, over-distension of the stomach, due to indigestion or
flatulence, may impede the action of the thoracic organs, and

Fig. 97.— a thin section through the
gastric mucous membrane, perpendi-
cular to its surface, magnified about
25 diameters, a. a simple peptic gland :
6, a c 'mpoimd peptic gland ; c, a mu-
cous gland; d, oval, chief, or so-called
peptic cells.

32G THE HUMAN BODY.

cauRO feelings of oppression in (lie chest, or palpitation of
the heart.

The Small Intestine, eommeneiiig at the pylorus, ends.
after maiiv Avindiiigs, in the large. It is about six meters
(twenty feet) long, and about five centimeters (two inches)
^vide at its gastric end, narrowing to about two thirds of
t hat width at its lower portion. Externally there are no
lines of subdivision on the small intestine, l)ut anatomists
;;rbitrarily describe it as consisting of three parts; the first
I welve inches being the duodenum, the succeeding two fifths
of the remainder the jejunum, and the rest the ileum.

Like the stomach, the small intestine i)osscsses four coats;
a serous, a muscular, a submucous, and a mucous. The
serous coat is formed by a duplicature of the peritoneum,
but presents nothing answering to the great omentum; this
double fold, slinging the intestine as the small omentum
slings the stomach, is called the mesentery. The muscular
coat is composed of plain muscular tissue arranged in two
strata, an outer longitudinal, and an inner transverse or
circular. The submtccous coat is like that of the stomach;
consisting of loose areolar tissue, binding together the mu-
cous 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
before entering the mucous membrane.

The Mucous Coat of the Small Intestine. This is pink,
soft, and extremely vascular. It does not present tempo-
rary or effaceable folds like those of the stomach, but is.
throughout a great portion of its length, raised up into j)er-
manent transverse folds in the form of crescentic ridges,
each of which runs transversely for a greater or less way
round the tube (Fig. 98). These folds are the valvuhr
C07iniventes. 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 surface of the mucous mem-
brane both for absorption and secretion, and they also de-

SMALL INTESTINE. 321

lay the food somewliat in its passage, since it must collect in
the hollows between them, and so be longer exposed to the
action of the digestive litpiids. Examined closely with the
eye or, better, with a hand lens, the mucous membrane of
the small intestine is seen to be not smooth but shaggy, be-
ing covered everywhere (both over the valvulae conniventetf
Mild between them) 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 millimeters
(^i_ to -3»^ inch) long ; some are conical and rounded, but
the majority are compressed at the base in one diameter
(Fig. 99). In structure a villus is somewhat complex.
Covering it is a single layer of columnar epithelial cells, be-
neath which the villus may be regarded as made up of a

Fig. 98. — A portion of the small intestine opened to show the valvulae conni-
ventes.

framework of connective tissue supporting the more essen-
tial constituents. Near the surface is an incomplete layer
of plain muscular tissue, continuous below with a muscular
layer found on the deep side of the mucous membrane. In
the centre is an offshoot of the lymphatic system; some-
times in the form of a single vessel with a closed dilated
end, and sometimes as a network formed by two main ves-
sels with cross-branches. During digestion these lym-
phatics are filled with a milky white liquid absorbed from
the intestines and they are accordingly called the lacteals.
They communicate with larger branches in the submucous
coat which end in trunks that pass out in the mesentery to
join t'.ie main lymi)hatic system. Finally, in each villus,

322

THE HUMAN BODY.

outside the lacteals find beneath the muscular layer, 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 Lieber-
kilhn. Each is a simple unbranched tube lined by a layer
of columnar cells similar to that wliich covers the villi and
the surface of the mucous membrane between them. In

Fig. 99.— Villi of the small intestine; magnified about 80 diameters. In the
left-hand figure the lacteals, a, b, c, are filled with white injection; (/, blood-ves-
sels. In the right-hand ligure the lacteals alone are represented, filled with a
dark injection. The epith. hum covering the vilh, and their muscular fibres, are
omitted.

structure they greatly resemble the mucous glands of the
stomach {c, Fig. 97). 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, 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). Anatomists describe it as consisting of the ccecum
Avith 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

THE LIVER. 323

upper end, and the caecum is tliut part of the large intes-
tine which extends beyond the communication. From it
projects the vermiform appendix, a narrow tube not thicker
than a cedar 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 {ascend-
ing colon), then crosses the middle line {transverse colon)
below the stomach, and turns down {descending colon)
on the left side and there makes an S-shaped bend known
as the sigmoid flexure; from this the rectum, the termi-
nal straight portion of the intestine, proceeds to the anal
opening, by which the alimentary canal communicates
with the exterior. In structure the large intestine 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 intervals in which it is wanting. These bands being
shorter than the lest of the tube cause it to be puckered, or
sacculated, between them. The mucous coat possesses no
villi or valvule conniventes, but is usually thrown into
effaceable 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 neo-Colic Valve. Where the small intestine joins
the large there is a valve, formed by two flaps of the mucous
membrane sloping down into the colon, and so disposed as
to allow matters to pass readily from the ileum into the
large intestine but not the other way.

The Liver. Besides the secretions formed by the glands
imbedded in its walls, the small intestine receives those of
two other large glands, the liver and 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
1400 to ITOO gi-ams (50 to 64 ounces). It is situated in the
upper part of the abdominal cavity {le, le. Fig. 1), rather
more on the riglit than on the left side and immediately below

3U

THE HmfAiV BODY.

the diaphragm, into the concavity <>f wliich its upper sur-
face fits; it reaches across the middle line above the pyloric
end of the stomach. It is of dark reddish-brown coloi,
and of a soft friable texture. A deep fissure incompletely
divides the organ into r?'_(//// and left lohes, of which the right
is much the larger; on its under surface (Fig. 100) shallower
grooves mark olf several minor lobes. Its upper surface
is smooth and convex. The vessels carr3ing blood to the
liver are i\\e 2)ortal vein, Vp, and the Jiepatic 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.

Fig. 100.- Til e imrler surface of the liver. cJ. right, and s, left lobe ; Vli, hepatic
vein; Vp, portal vein; Fc, vena cava inferior; Dch, common bile-duct; Dc
cystic duct; Dh, hepatic duct; Vf, gall-bladder.

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 di-
gested in the intestine. The common hile-duct, Dch, formed
by the union of the hepatic and cystic ducts, opens into
the duodenum. The blood which enters the liver by the
])ortal vein and hepatic artery passes out by the hepatic
veins, Mt, which leave the posterior border of the organ

HISTOLOGY OF LIVEB.

335

close to the vertebral column, and there open into the infe-
rior vena cava, Vc, jnst 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 areas from one to two millimeters {^
to y^j inch) in diameter. These are the outer sides of the
superficial layer of a vast number of minute polygonal
masses, or lohules, of which the liver is built up; similar
areas are seen on the surface of any section made through
the organ. Each lobule (Fig. 101) consists of a number of

«^H -^o-^tr "Q-^ihy-^ "^^MHi

Fig. 101. — A lobule of the liver, magrnifled, showing the hepatic cells radiately
arrange J around thecenti-al Latralobular vein, and the lobulai- capillaries inter-
laced with them.

liepatic cells supjiorted 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 arrangements for
their nutrition and protection. Each is polygonal, nucle-
ated and very granular, and has a diameter of about .025
millimeter (yo'ito" of ^^^ inch). In each lobule they are ar-
ranged in rows or strings, which intercommunicate and
form a network, in the meshes of which the blood capilla-

326

THE HUMAN BODY.

rios run. Covering the surface of the liver is a layer of the
peritoneum, beneath which is a dense connective-tissue
layer, forming the capsule of Glisson. At the portal fissure
offsets from this capsule run in, and line canals, the portal
canals, which are tunneled through the organs. These,
becoming smaller and smaller as they branch, finally be-
come indistinguishable close to the ultimate lobules. From
their walls and from the external capsule, connective- tissue
partitions radiate in all directions through the organ and
support its other parts. In each portal canal lie three ves-
sels— a branch of the portal vein, a branch of the hepatic

Fig. 102.— a small portion of the liver, injected, and magmifled about twenty
diameters. The blood-vessels are represented wlute ; the large vessel is a sub-
lobnlar vein, receiving the intralobular veins, which m turn are derived from
the capillaries of the lobules.

artery, and a branch of the hepatic duct; the division of
the portal vein being much the largest of the three. These
vessels break up as the portal canals do, and all end in
minute branches around the lobules. The blood carried in
by the portal vein (which has already circulated through
the capillaries of the stomach, spleen, intestines and pan-
creas) is thus conveyed to a fine vascular interlohtilar
plexus around the liver lobules, from which it flows on
through the capillaries {lohular plexus^ of the lobules them-
selves. These (Fig. 101) unite in the centre of the lobule

HEPATIO CIRCULATION.

327

\fO form a small intralohular vein, which carries the blood
out and pours it into one of the branches of origin of the

Fig. 103.— The stomach, pancreas, liver, and duodenum, -with part of the rest
of the small intestine and the mesentery; the stomach and hver have been
turned up so as to expose the pancreas. V, stomach; D. ly . D" , duodenum; L,
spleen ; i^, pancreas ; R. right kidney ; T, jejunum ; Vf, gall-bladder ; /i,
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, superior mesenteric veui ; 7, splenic vein ; Vp, portal
vein.

hepatic vein, called the sublohular vein. Each of the latter
has many lobules emptying blood into it, and if dissected out

328 THE nUMAN BODY.

with them (Fig. 102) would look sonietliing like a branch
of a tree with apples attached to it by short stalks, repre-
sented by the intralobular veins. The blood is finally car-
ried, as above pointed out, by the he^iatic veins into the
inferior vena cava. The hepatic artery, a branch of the
cceliac axis (p. 211), ends mainly in Glisson's capsule and
the walls of the blood-vessels and bile-ducts, but some of
its blood reaches the lobular plexuses; 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 proba])ly communicate with a minute
network of commencing bile-ducts ramifying in the lobule
between the hepatic cells composing it.

The Pancreas or Sweetbread. This is an elongated
soft organ of a pinkish yellow color, lying along the great
curvature of the stomach. Its right end is larger, and
is embraced by the duodenum (Fig. 103), 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 forms a watery-looking secretion wiiich is of
great importance m digestion.

CHAPTER XXII.

THE LYMPHATIC SYSTEM AND THE DUCT-
LESS GLANDS.

The Lymphatics or Absorbents form close networks in
nearly all parts of the Body. Most organs, as has been
pointed out (p. 62), possess a sort of internal skeleton made
up of connective tissue, which consists mainly of bundles of
fibres, united together and coyered in by a "cement" sub-
stance. In this substance are found numerous cavities,
usually branched, and communicating with one another by
their branches. They frequently contain connective-tissue
corpuscles, which, however, do not completely fill them;
and they thus, with their branches, form a set of intercom-
municating channels known as the "serous canalicidij"
these are filled with lymph and constitute the origin of
lymphatic vessels in many organs. Elsewhere the com-
mencing lymphatics seem to be merely interstices {lacunm)
between the constituent tissues of an organ; this is esjie-
cially the case in glands. Such spaces differ from the se-
rous canaliculi in being lined by a definite epithelium.

Structure of Lymph-Vessels. The serous canaliculi and
lym})h-spaces open into better defined channels, lined with
a single layer of wavy-edged flattened epithelial cells.
These form networks in most parts of the Body and are
knowTi as the lymph capillaries. They are usually wider
til an blood capillaries. From the capillary networks
larger vessels arise which in structure resemble veins, and
have similar, but more numerous, valves.

The Thoracic Duct. All the lymphatics end finally in
two main trunks which open into the venous system .on each
side of the neck, at the point of junction of the jugular and

330 THE HUMAN BODY.

subclavian. The trank 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 uj)2oer part of tlie
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, pass-
ing on into the neck, ends on the left side at the point
already indicated; 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 much more lymph
than the right lymphatic duct.

The Serous Cavities. These are great dependencies of
the lymphatic system and may be regarded as large lacunae.
Each of them (peritoneal, pleural, arachnoidal and peri-
cardiac) is lined by a definite epithelioid layer of close-fit-
ting, hexagonal cells. At certain points, however, o^jenings
or stomata occur, surrounded by a ring of smaller cells, and
leading into tubes which open into subjacent lymphatic
vessels. The liquid moistening these cavities is, then, really
lymph.

The Lymphatic Glands. These are roundish masses in-
terposed, at various points, on the course of the lymph- ves-
sels. They are especially numerous in the mesentery, groin,
and neck. In the latter position they often inflame and
give rise to abscesses, especially in scrofulous persons; and
still more often enlarge, harden, and become more or less
tender, so as to attract attention to them. In common par-
lance it is then frequently said that the person's " kertiels
have come down," or that he has " waxing kernels." Each
lymphatic gland is enveloped in a connective-tissue capsule,
and is pervaded by a connective-tissue framework. In the
meshes of this lie numerous lymph corpuscles, which appear
to multiply there by division. "Afferent" lymphatic ves-
sels open into the periphery of the so-called gland, and ef-

MOVEMEWl OF TEE LYMPH. 331

ferent vessels arise in its centre. Hence, the lymiDh in its
flow traverses tlie cellular gland substance, and in its
course picks up extra corpuscles which it carries on to the
blood. In the giand there is a close network of blood capil-
laries. It is clear that these organs are not glands at all, in
the proper sense of the word. They are sometimes called
lymphatic ganglia; but that suggests a false connection with
nerve-centres.

The Movement of the Lymph. This is no doubt some-
what irregular in the commenciug 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 inde-
pendent 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 Avliich will tend to make it flow to
the venous termination of the thoracic or the right lym-
phatic duct. (2) On account of the numerous valves in
the lymphatic vessels (which all only allow the lymph to
flow past them to larger vessels) any movement compress-
ing 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 lymph-vessel, say at the top of the leg of an
animal, it will be seen that the lymph only flows out very
sloMdy when the animal is quiet; but as soon as it moves
its 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. XXIV. ). Accordingly, at that time, lymph
is pressed, or, in common jihrase, is " sucked," into the
thoracic duct. During the succeeding expiration the pres-
sure on the thoracic duct becomes greater again, and some
of its contents are pressed out: 1>ut on account of the valves

332 THE nUMAX BODY.

tliey can only ac forwards, tliat is, towards the ending of
the duct in the veins of the neck.

During digestion, moreover, the contraction of the villi
will press on the lymph or chyle; and in certain parts of the
Body gravity, of course, aids the flow, though it will impede
it in others.

The Spleen. There are in the Body several organs of
such consideraljle size, and of so great constancy in a large
number of vertebrate animals, that they would a j^^'iori
appear to be of considerable functional importance. What
their use may be is still, however, unknown or uncertain.
They are commonly spoken of collectively, along with the
lymphatic ganglia, as the ductless glands; but they are not
glands in the proper sense of the word. The spleen is the
largest of them. It is a red organ situated at the left end
of the stomach and about 170 grams (6 oz.) in weight.
Its size is however very variable; it enlarges during diges-
tion and shrinks again after it until the next meal. In
malarial diseases it also becomes enlarged, frequently to a
very great extent, and then constitutes the so-called '* ague-
cake," In structure, the spleen consists of a connective-
ti?sne capsule, rich in elastic fibres, and giving off processes
whicli ramify through the organ and form a framework for
its pidj). The latter contains numerous blood corpuscles;
and maiiy bodies which seem to be red corpuscles in pro-
cess of decay or destruction. Hence the spleen has been
supposed to be a sort of gi-aveyard for their bodies — a place
where they are broken up and their materials utilized when
they have run their life cycle. Others, however, consider
that in the spleen new red blood corpuscles are pro-
duced from colorless; and others, again, that the main
function of the organ is the formation of substances which
are carried off to the stomach and pancreas, to be there
finally elaborated into digestive ferments. The arteries
of the spleen open directly into the pulp cavities, from
which the veins arise. On their walls are rounded whitish
nodules about the size of a millet-seed, and known as the

THYMUS AND- THYROID. 333

Malpighian corpuscles. They resemble tiny lymphatic
glands in structure.

The Thymus is an organ which only exists in childhood.
At birth it is found lying around the windpipe, in the upper
part of the chest cavity and the lower part of the neck. It
increases in size until the end of the second year, and then
begins to dwindle away. It is grayish pink in color, of a
soft texture, and, in microscopic structui'e, resembles some-
what a lymphatic gland. Hence its function has been sup-
posed to be the formation of ne^v lymph corpuscles. The
"sweetbread" of butchers is sometimes the pancreas and
sometimes the thymus of young animals — neck and belly
sweetbread.

The Thyroid Body and the Suprarenal Capsules.
The former of these structures lies in the neck on the sides
of and below "Adam's apj^le." It is dark red-brown in
color, and sometimes becomes very much enlarged, as in the
disease known as goitre. This enlargement ajjpears to be
often due to drinking water containing magnesian limestone
in solution. In England, for instance, it is known as "Der-
byshire neck" from being especially frequent in parts of that
county, where the hills are mainly composed of magnesian
limestone rocks; and the same geological formation is
found in those districts of Switzerland where cretinism
(one of the symptoms of which is an enlarged thyroid
body) prevails.

The suprarenal bodies lie one over the top of each kid-
ney. Their use, like that of the thyroid, is quite pro-
blematical. In what is known as Addison's disease (in
which the skin becomes of a bronze color) it is said that
these bodies are altered; but it is very improbable that the
change in them is the actual cause, rather than another
symptom, of the disease.

CHAPTER XL

DIGESTION.

The Object of Digestion. Of the various foodstufEe
swallowed, some are already in solution and ready to dialyze
at once into the lymphatics and blood-vessels of the alimen-
tary canal; others, such as a lumjj of sugar, though not
dissolved when put into the mouth, are readily soluble in
the liquids found in the alimentary canal, and need no fur-
ther digestion. In the case of many most imijortant food-
stuffs, however, special chemical changes have to be wrought,
either with the object of converting insoluble bodies into
soluble, or non-dialyzable into dialyzable, or both. The
different secretions poured into the alimentary tube act in
various ways upon different foodstuffs, and at last get them
nito a state in which they can pass into the circulating
medium and be carried to all parts of the Body.

The Saliva. The first solvent 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 oral
mucous membrane. This mixed saliva is a colorless, cloudy,
feebly alkaline liquid, "ropy" from the mucin present in
it, and usually containing air-bubbles. Pure saliva, as ob-
tained by putting a fine tube in the duct of one of the sali-
vary glands, is less tenacious aud contains no imprisoned
air.

The uses of the saliva are for the most part physical and
mechanical. It keeps the mouth moist and allows us to
speak with comfort; most young orators know the distress
occasioned by the suppression of the salivary secretion
through nervousness, and the imperfect efl&cacy under such

USES OF SALIVA. 335

circumstances of the traditional glass of water placed be-
side public speakers. The saliva, also, enables us to swallow
dry food; such a thing as a cracker when chewed would
give rise merely to a heap of dust, impossible to swallow,
were not the mouth cavity kept moist. This fact used to
be taken advantage of in the East Indian rice ordeal for
the detection of criminals. The guilty person, believing
firmly that he cannot swallow the parched rice given him
and sure of detection, is apt to have his salivary glands
paralyzed by fear, and so does actually become unable to
swallow the rice; while in those with clear consciences the
nervous system, acting normally, excites the usual reflex
secretion, and the dry food causes no difficulty of degluti-
tion. The saliva, also, dissolves such bodies as salt and
sugar, when taken into the mouth in a solid form, and
enables us to taste them; undissolved substances are not
tasted, a fact which any one can verify for himself by
Aviping 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, however, exerts a
chemical one on an important foodstuff. Starch (although
it swells up greatly in hot water) is insoluble, and could
not be absorbed from the alimentary canal. The saliva
contains a specific element, ptyalin, which has the power
of turning starch into the readily soluble and dialyzable
grape sugar. In effecting this change the ptyalin is not
altered; at least a very small amount of it can cause the
conversion of a vast amount of starch, and it does not seem
to have its activity impaired in the process, being still ready
at the end of it to act upon more. The starch is made to
combine with the elements of a molecule of water, and the
ptyalin remains behind as it was —

starch. Water. Grape sugar.

Substances acting in this way, producing chemical
changes without being themselves noticeably altered, are
found in many of the digestive secretions, and are called

336 THE TTr.VA y BOD Y.

ferments or enzymes. Tlicy differ from the true ferments,
such as yeast, in tlie fact tliat they are not living organisms,
and do not multiply during the occurrence of the change
whicli tlicy sot up; tlieir activity belongs to the obscure
chemical category of contact actions.

In order that the i^tyalin 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. Boiled starch is changed more rapidly and com-
pletely than raw.

Saliva has another important but indirect influence in
promoting digestion. Weak alkalies stimulate the mucous
membrane of the stomach and cause it to pour forth more
gastric juice. Hence the efficac}' of a little carbonate of
soda, taken before meals, in some forms of dyspepsia. The
saliva by its alkalinity exerts such an action; and this
is one reason why food should be well chewed before being
swallowed; for then its taste, and the movements of the
Jaws, cause the secretion of more saliva.

Deglutition. A mouthful of solid food is broken uj? by
the teeth, and rolled about the mouth by the tongue, until
it is thoroughly mixed with saliva and made into a soft pasty
mass. The muscles of the cheeks keep this from getting
between them and the gums; persons with facial paralysis
have, from time to time, to press out with the finger food
which has collected outside the gums, where it can neither
be chewed nor swallowed. The mass is finally sent on from
the mouth to the stomach by the process of derjlntition,
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 its dorsum is raised from before back,
30 as to press the food mass between it and the palate, and
drive it back through the fauces. This j)ortion of the act of

SWALLOWT^^G. 337

swallowing is volimtary, or at least is under the control of
the will, ulthougli 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
larvnx (or voice-box at the top of the windpipe) is raised, so
as to meet this and thus the passage to the lungs is closed;
muscles around the aperture probably also contract and
narrow the opening. The raising of the larynx can be
readily felt by placing the finger on its large cartilage form-
ing " Adam's apple" in the neck, and then swallowing
something. The soft palate is at the same time raised and
stretched horizontally across the pharynx, thus cutting off
communication with its upper, or respiratory portion, lead-
ing to the nostrils and Eustachian tubes. Finally, the
isthmus of the fauces is closed, as soon as the food has
passed through, by the contraction of the muscles on its
sides and the elevation of the root of the tongue. All pas-
sages out of the pharynx except the gullet are thus blocked,
and when the pharyngeal muscles contract the food can
only he squeezed in+o the a3sophagus. The muscular move-
ments concerned in this part of deglutition are all reflexly
excited; food coming in contact with the mucous mem-
brane of the pharynx stimulates afferent nerve-fibres in it;
these excite the centre of deglutition which is placed in the
medulla ohlongata, and from it efferent nerve-fibres proceed
to the muscles concerned and (under the co-ordinating in-
fluence of the centre) cause them to contract in proper se-
quence. The pharyngeal muscles, although of the striped
variety, are but little under the control of the will; it is ex-
tremely difiicult to go through the movements of swallow-
ing without something (if only a little saliva) to swallow,
and excite the movements reflexly. Many i^ersons 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

338 THE HUMAN BODY.

pliarynx, any food wliich lias oucc entered it must be swal-
lowed: the isthmus of the fauces forms a sort of Kubicon;
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 in
which the food is passing along the gullet, and is compara-
tively slow. Even liquid substances do not fall or flow down
this tube, but have their passage controlled by its muscular
coats, which grip the successive portions swallowed and pass
them on. Hence the possibility of performing the ajiparently
wonderful feat of drinking a glass of water while standing
upon the head, often exhibited by jugglers; people forget-
ting that one sees the same thing done every day by horses,
and other animals, which drink with the pharyngeal end of
the gullet lower than the stomach. The movements of the
oesophagus are of the kind known as vermicular or peri-
staltic. Its circular fibres (p. 317) 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 fibres, at the jioint
where the food-mass is at any moment and immediately in
front of that, contracting, shorten and widen the passage.
The Gastric Juice. The food having entered the sto-
mach is exposed to the action of the gastric juice, which is a
thin, colorless, or pale yellow liquid, of a strongly acid re-
action. It contains as sj^ecific elements free liydrocliloric
acid (about .€)2 per cent), and an enz3"me called pepsin
which, in acid liquids, has the power of converting the or-
dinary non-dialyzablei^roteids which we eat, into the closely
allied but dialyzable bodies called peptones. It also dis-
solves solid j)roteids, changing them too into pej^tones.
Dilute acids will by themselves produce the same changes
in the course of several days, but in the iDresence of pejDsin
and at the temperature of the Body the conversion is far
more rapid. In neutral or alkaline media the pepsin is
inactive; and cold check sits activity. Boiling destro3\s it.
In addition to pepsin, gastric juice contains another enzyme
which has the power of coagulating the casein of milk, as
illustrated by the use of ^'rennet," prejjared from the mu-

DIGESTIoy IN THE STOMACH. 339

cons membrane of the calf's digestive stomach, in cheese-
making. The acid of the natural gastric juice might itself,
it is true, coagulate the casein, but neutralized gastric
juice still possesses this power; and, since pure solutions of
pepsin do not, it must be due to some third body, which has,
however, not yet been isolated. The curdled condition of
the milk regurgitated so often by infants is, therefore, not
any sign of a disordered state of the stomach, as nurses
commonly suppose. It is natural and proper for milk to
undergo this change, before the pepsin and acid of the
gastric juice convert its casein into peptone.

Gastric Digestion. The jDrocess of swallowing is con-
tinuous, but in the stomach the onward progress of the
food is stayed for some time. The pyloric sphincter, re-
maining contracted, closes the aperture leading into the
intestine, and the irregularly disposed muscular layers of
the stomach keep its semi-liquid contents in constant
movement, maintaining a sort of churning by which all
portions are brought into contact with the mucous mem-
brane and thoroughly mixed with the secretion of its glands.
The gelatin-yielding connective tissue of meats is dissolved
away, and the proteid-containing fibres, left loose, are dis-
solved and turned into peptones. The albuminous walls
of the fat-cells are dissolved and their oily contents set
free; but the gastric juice does not act upon the latter.
Certain mineral salts (as phosphate of lime, of which there
is always some in bread) which are insoluble in water but
soluble in dilate acids, are also dissolved in the stomach.
On the other hand the gastric juice has itself no action
upon starch, and since ptyalin does not act at all, or only
imperfectly, in an acid medium, the activity of the saliva
in converting starch is stayed in the stomach. By the solu-
tion of the white fibrous connective tissue, that disintegra-
tion of animal foods commenced by the teeth, is carried
much farther in the stomach, and the food-mass, mixed
with much gastric secretion, becomes reduced to the con-
sistency of a thick soup, usually of a grayish color. In
this state it is called chyme. This contains, after an ordi-
nary meal, a considerable quantity of peptones which are

340 THE HUMAN BODY.

in great part gradually dialyzod into the blood and lympha-
tic vessels of the gastric mucous membrane, and carried off,
along with other dissolved dialyzable bodies, such as salts
and sugar. After the food has remained in the stomach
some time (one and a half to two hours) the chyme begins
to be passed on into the intestine in successive portions.
The pyloric sphincter relaxes at intervals, and tlie rest
of the stomach, contracting at the same moment, injects a
((uantity of chyme into the duodenum ; this is repeated
frequently, the larger undigested fragments being at first
unable to pass the orifice. At the end of about three or
four hours after a meal the stomach is again quite emptied,
the pyloric sphincter finally relaxing to a greater extent
and allowing any larger indigestible masses, which the gas-
tric juice cannot break down, to be squeezed into the intes-
tine.

The Chyle. When tlie chyme passes into the duodenum
it finds preparation made for it. The jiancreas is in reflex
connection with the stomach, and its nerves cause it to
commence secreting so soon as food enters the latter; hence
a quantity of its secretion is already accumulated in the
intestine when food enters. The gall-bladder is distended
with bile, secreted since the last meal; this passing down
the hepatic duct has been turned back up the cystic duct
{Dc, Fig. 100*) on account of the closure of the common
bile-duct. The acid chyme, stimulating nerve-endings in
the duodenal mucous meml)rane, causes reflex contrac-
tion of the muscular coat of the gall-bladder, and a relaxa-
tion of the orifice of the common bile-duct; and so a gush
of bile is poured out on the chyme. From this time on,
both liver and pancreas continue secreting actively for
some hours, and pour their products into the intestine.
The glands of Brunner and the crypts of Lieberkiihn are
also set at work, but concerning their physiology we know
very little. All of these secretions are alkaline, and they
suffice very soon to more than neutralize the acidity of the
gastric juice, and so to convert the acid chyme into alka-
line chyle, which, after an ordinary meal, will contain a
great variety of things: mucus derived from the alimen-

*P. 324

PANCREATIC DIGESTION. 341

tary canal; ptyalin from the saliva; pepsin from the sto-
mach; water, partly swallowed and partly derived from the
salivary and other secretions; the .peculiar constitiients of
the bile and pancreatic Juice and of the intestinal secretions;
some undigested proteids; unchanged starch; oils from the
fats eaten; peptones formed in tlie stomacli but not yet
absorbed; possibly salines and sugar which have also escaped
absorption in the stomach; and indigestible substances
taken with the food.

The Pancreatic Secretion is clear, watery, alkaline, and
much like saliva in appearance. The Germans call the
pancreas the '• abdominal salivary gland." In digestive
properties, however, the pancreatic secretion is far more
important than the saliva, acting not only on starch but,
also, on proteids and fats. On starch it acts like the saliva,
but more energetically. It produces changes in jiroteids
similar to those effected in the stomach, but by the agency
of a different ferment, trypsin; which differs from pepsin in
acting only in an alkaline instead of an acid medium. On
fats it has a double action. To a certain extent it breaks
them up, with hydration, into free fatty acids and glycerine;
for examjDle —

(C"H.^O)> I 0. + 3H.0 = 3(C-H-.0 | o ) +C'II; | 0.

1 Stearin + 3 Water = 3 Stearic acid + 1 Glycerine.

The fatty acid then combines with some of the alkali pres-
ent to make a soap, which being soluble in water is cai)able
of absorption. Glycerine, also, is soluble in water and diah^-
zable. The greater part of the fats are not, however, so
broken up, but are simply mechanically separated into little
droplets, which remain suspended in the chyle and give it a
whitish color, just as the cream-drops are suspended in
milk, or the olive-oil in mayonnaise sauce. This is effected
by the help of a quantity of albumin which exists dissolved
in the pancreatic secretion. In the stomach, the animal
fats eaten have lost their cell-walls, and have become melted
by the temperature to which they are exposed. Hence
their oily part floats free in the chyme when it enters the

342 THE HUMAN liOBY.

duodenum. If oil be shaken up with water, the two can-
not be got to mix; immediately the shaking ceases the oil
floats up to the top; but if some raw Q'^g be added, a creamy
mixLurc is readily formed, in which the oil remains for a
long time evenly suspended in the watery menstruum.
The reason of this is that eacli oil-droplet becomes sur-
rounded by a delicate pellicle of albumen, and is thus pre-
vented from fusing with its neighbors to make large drops,
which would soon float to the toji. Such a mixture is called
an emulsion, and the albumin of the pancreatic secretion
emulsifies the oils in the chyle, which becomes white (for the
same reason as milk is that color) because the innumerable
tiny oil-drops floating in it reflect all the light which falls
on its surface.

The pancreatic secretion thus converts starch into grape
sugar, dissolves proteids (if necessary) and converts them
into peptones, emulsifies fats, and, to a certain extent, breaks
them up into glycerine and fatty acids, which latter are
saponified by the alkalies present.

The Bile. Human bile when quite fresh is a golden
brown liquid; it becomes green when kept. As formed in
the liver it contains hardly any mucin, but if it makes any
stay in the gall-bladder it acquires a great deal from, the
lining membrane of that sac, and becomes very ''ropy."
It is alkaline in reaction and, besides coloring matters, min-
eral salts, and water, contains the sodium salts of two nitro-
genized acids, taurocJiolic and glycliocholic, the former j^re-
dominating in human bile.

Pettenkofer's Bile Test. If a small fragment of cane
sugar be added to some bile, and then a large quantity of
strong sulphuric acid, a brilliant purple color is develo^ied,
by certain products of the decomposition of its acids; the
physician can in this way, in disease, detect their presence
in the urine or other secretions of the Body. Gmelvii's
Bile Test. The bile-coloring matters, treated with yellow
nitric acid, go through a series of oxidations, accompanied
with changes of color from yellow-brown to green, then to
blue, violet, purple, red, and dirty yellow, in succession.

Bile has no digestive action upon starch or proteids. It

DIGESTIVE ACTION OF BILE. 343

does not break up fats, but to a limited extent emulsifies
them, though far less perfectly tlian the iDancreatic secre-
tion. It is even doubtful if this action is exerted in the
intestines at all. In many animals, as in man, the bile and
pancreatic ducts open together iuto the duodenum, so that,
on killing them during digestion and finding emulsified
fats in the chyle, it is impossible to say whether or no the
bile had a share in the process. In the rabbit, however, the
pancreatic duct opens into the intestine about a foot farther
from the stomach than the bile-duct, and it is fonnd that
if a rabbit be killed after being fed with oil, no milky chyle
is found down to the point where the pancreatic duct opens.
In this animal, therefore, the bile alone does not emulsify
fats, and, since the bile is j^retty much the same in it and
other mammals, it probably does not emulsify fats in them
either. From the inertness of bile with respect to most
foodstuffs it has been doubted if it is of any digestive use
at all, and whether it should not be regarded merely as an
excretion, poured into the alimentary canal to be got rid of.
But there are many reasons against such a view. In the
first place, the entry of the bile into the upper end of the
small intestine where it has to traverse a course of more
than twenty feet before getting out of the Body, instead of
its being sent into the rectum close to the final opening of
the alimentary canal, makes it probable that it has some
function to fulfill in the intestine. Moreover, a great part
of the bile poured into the intestines is again absorbed from
them, only a small part being finally excreted; and this also
seems to sIioav that part of it at least, is secreted for some
other purpose than mere elimination from the Body. One
use is, no doubt, to assist, by its alkalinity, in overcoming the
acidity of the chyme, and so to allow the trypsin of the
pancreatic secretion to act upon proteids. Constipation is,
also, apt to occur in cases where the bile-duct is temporarily
stopped, so that the bile probal^ly helps to excite the con-
tractions of the muscular coats of the intestines; and it is
said that under similar circumstances putrefactive decom-
positions are extremely apt to occur in the intestinal con-
tents. Apart from such secondary actions, however, the

344 THE nUMAN BODY.

bile probably has some iiifliieiice in proiiioting the absorp-
tion of fats. If one end of a capillary glass tube, moistened
with water, be dipped in oil, the latter will not ascend in it,
or but a short way; but if the tube be moistened with bile,
instead of water, the oil will ascend higher in it. So, too,
oil i)asses through a plug of porous clay kept moist with bile,
under a much lower pressure than tlirough one wet with
water. Hence bile, by soaking the epithelial cells lining the
intestine, may facilitate the passage into the villi of oily sub-
stances. At any rate, experiment shows that if the bile be
prevented from entering the intestine of a dog, the animal
eats an enormous amount of food compared with that
amount which it needed previously; and that of this food
a great proportion of the fatty parts passes out of the ali-
mentary canal unabsorbed. There is no doubt, therefore,
that the bile somehow aids in the absorption of fats, but
exactly how is uncertain. Its possible action in exciting
the muscles of the villi to contract will be referred to jires-
ently. Bile precipitates from solution, not only pepsin, but
any peptones contained in the chyme which enters the in-
testine from the stomach.

The Intestinal Secretions or Succus Entericus. This
consists of the secretions of the glands of Brunner and the
crypts of Lieberkiihn. It is difficult to obtain pure; in-
deed the product of Brunner's glands has never been ob-
tained unmixed. That of the crypts of Lieberkiihn is
watery and alkaline, and poured out more abundantly dur-
ing digestion than at other times. It has no special action
on starches, most proteids, or on fats; but is said to dissolve
blood fibrin and convert it into peptone, and to change cane
into grape sugar, a transformation the object of which is
not very clear, since cane sugar is itself readily soluble and
diffusible.

Intestinal Digestion. Having considered separately the
actions of the secretions with which the food meets in the
small intestine we may now consider their combined effect.

The neutralization of the chyme, followed by its conver-
sion into alkaline cliyle. will prevent any further action of
the pepsin on pi'oteids, but v,i!l allow tlic pty;;lin of the

IXTESTiyAL digestion: 345

saliva (the actiyity of which was stopped by the acidity of
the gastric Juice) to recommence its action upon starch.
Moreover, in the .stomach there is produced, alongside of the
true pe^Jtones, a body called parapeptone, which agrees very
closely with syntonin (p. 126) in its projierties, and this
passes into the duodenum in the chyme. As soon as the
bile meets the chyme it precipitates the parapeptone, and
this carries down with it any peptones which, having es-
caped absorj)tion in the stomach, may be present; it also
precipitates the pepsin. In consequence, one commonly
finds, during digestion, a sticky granular precipitate over
the villi, and in the folds between the valvulse conniventes
of the duodenum. This is soon redissolved by the pancre-
atic secretion, which also changes into peptones the pro-
teids (usually a considerable proportion of those eaten at a
meal) which have passed through the stomach unchanged,
or in the form of joaraiieptones. The conversion of starch
into grape sugar will go on rapidly under the influence of
the pancreatic secretion. Fats will be split up and saponi-
fied to a certain extent, but a far larger proportion will be
emulsified and give the chyle a whitish appearance. Cane
sugar, which may have escaped absorjition in the stomach,
will be converted into grape sugar and absorbed, along with
any salines which may, also, have hitherto escaped. Elastic
tissue from animal substances eaten, cellulose from plants,
and mucin from the secretions of the alimentary tract, will
all remain unchanged.

Absorption from the Small Intestine. The chyme leav-
ing the stomach is a semi-liquid mass which, being mixed
in the duodenum with considerable quantities of pancreatic
secretion and bile, is still further diluted. Thenceforth it
gets the intestinal secretion added to it but, the absorjition
more than counterbalancing the addition of liquid, the food-
mass becomes more and more solid as it approaches the
ileo-colic valve. At the same time it becomes poorer in
nutritive constituents, these being gradually removed from
it in its progress; most dialyze through the epithelium into
the subjacent blood and lymphatic vessels, and are carried
off. Those passing into the blood capillaries are taken by

346 THE HUMAN BODY.

the portal vein to the liver; while tliose entering the hicteuls
are carried into the left jugular vein by the thoracic duct.
As to which foodstuffs go one road and which the other,
there is still much doubt; sugars jirobably go by the portal
system, while the fats, mainly, if not entirely, go through
the lacteals. How the fats are absorbed is not clear,
since oils will not dialyze through membranes, such as that
lining the intestine, moistened with Avatery liquids. Most
of them, however, certainly get into the lacteals as oils and
not as solulilo soaps; for one finds these vessels, in a digest-
ing animal, filled with a beautifully white milky chyle; Avhile
at other jDeriods their contents are watery and colorless like
the lymph elsewhere in the Body. The little fat-drops of
the emulsion formed in the intestine, go through the epi-
thelial cells and not between them, for during digestion
one finds these cells loaded wnth oil-droplets. Now the
free ends of these cells arc striated and probably devoid of
any definite cell-wall, and it is jiossible that the intestinal
movements squeeze oil-drops into them, which the cell then
passes on to its deeper end and, thence, out into the sub-
jacent lymph-spaces, which communicate with the central
lacteal of a villus. Possibly, too, these cells are amoeboid
and can thrust out processes from their free ends and
actively pick up the oil-drops. In the villus there are all
the anatomical arrangements for a mechanism which shall
actively suck u]? substances into it. Each is more or less
elastic, and, moreover, its capillary network when filled with
blood wall distend it. If therefore the muscular stratum
(p. 321) contracts and compresses it, emptying its lacteals
into the vessels lying deeper in the intestinal wall, the villus
will actively expand again so soon as its muscles relax.
In so doing it could not fill its lacteals from the deeper ves-
sels, on account of the valves in the latter, and, accordingly,
would tend to draw into itself materials from the intestines;
much like a sponge re-expanding in Avater, after having been
squeezed dry. The liquid tlius sucked up may draw oil-
drops with it, into the free ends of the cells and between
them; and by repetitions of the process it is possible that
considerable quantities of liquid, with suspended oil-drops.

DI9ESTI02ir IN THE LARGE INTESTINE. 347

might be carried into the vilhis iudependently of any pro-
cess of dialysis. Tlie bile moistening the surface of the
Yillns may facilitate the passage of oil, as it does through a
paper filter or a plate of plaster-of-Paris, and it is also said
to stimulate the contractions of the villi; if so, its efficacy
in promoting the absorption of fats will be explained, in
si)ite of its chemical inertness with respect to those bodies.

Digestion in the Large Intestine. The contractions of
the small intestine drive on its continually diminishing
contents, until they reach the ileo-colic valve, through which
they are ultimately pressed. As a rule, when the mass
enters the large intestine its nutritive portions have been
almost entirely absorbed, and it consists merely of some
water, with the indigestible portion of the food and of the
secretions of the alimentary canal. It contains cellulose,
elastic tissue, mucin, and somewhat altered bile pigments;
commonly some fat if a large quantity has been eaten; and
some starch, if raw vegetables have formed part of the diet.
In its progress through the large intestine it loses more
water, and the digestion of starch and the absorj)tion of
fats is continued. Finally the residue, with some excretory
matters added to it in the large intestine, collects in the
sigmoid flexure of the colon and in the rectum, and is
finally sent out of the Body from the latter.

The Digestion of an Ordinary Meal. We may best sum
up the facts stated in this chaj^ter by considering the diges-
tion of a common meal; say a breakfast consisting of bread
and butter, beefsteak, potatoes and milk. Many of these
substances contain several alimentary principles, and, since
these are digested in different ways and in different parts
of the alimentary tract, the first thing to be done is to con-
sider what are the proximate constituents of each. We
then separate the materials of the breakfast as in the fol-
lowing table —

348

THE HUMAN BODY.

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DIGESTION OF A MEAL. 349

From such a meal we may first separate the elastin, cel-
lulose, and calcium sulphate, as indigestible and passed out
of the Body m the same state and in the same quantity as
they entered it. Then come the salines which need no
si)ecial digestion, and, either taken in solution or dis-
solved in the saliva or gastric juice, are absorbed from the
mouth, stomach and intestines without further change.
Cane and grape sugars experience the same fate, except
that any cane sugar reaching the intestines before absorp-
tion, is liable to be changed into grape sugar by the succus
entericus. Calcium phosphate will be dissolved by the
free acid in the stomach, yielding calcium chloride, which
will be absorbed there or in the intestine. Starch will be
partially converted into grape sugar during mastication and
deglutition, and the sugar will be absorbed from the sto-
mach. A great part of the starch will, however, be passed
on into the intestine unchanged, since the action of the
saliva is suspended in the stomach; iind its conversion will
be completed by the pancreatic secretion, and by the ptyalin
of the saliva, which will recommence its activity when the
chyle becomes alkaline. The various proteids will be par-
tially dissolved in the stomach and converted into peptones,
which will in part be absorbed there; the residue, with the
undigested proteids, will be passed on to the intestines.
There the bile will precipitate the peptones and parapep-
tones and, with the pancreatic secretion, render the chyme
idkaline, and so stop the activity of the gastric pepsin. The
pancreatic secretion will, however, redissolve the precipi-
tated peptone, and the unchanged proteids and parapeptone,
and turn the latter into peptones; these will be absorbed as
they pass along the small intestine; a small quantity perhaps
passing into the large intestine, to be taken up there.
The fats will remain unchanged until they enter the small
intestine, except that the proteid cell-walls of the fats of
the beefsteak will be dissolved away. In the small intes-
tine these bodies will be partially saponified, but most will
be emulsified and taken up into the lacteals in that condi-
tion. Gelatin, from the white fibrous tissue of the beef-

350 THE HUMAN BODY.

steak, will undergo changes in the stomach and intestine
and be dissolved and absorbed.

The substances leaving the alimentary canal after such a
meal would be, primarily, the indigestible cellulose and
elastin, together with some Avater. But there might be in
addition some unabsorbed fats, starch, and salts. To this
would be added, in the alimentary canal, mucin, some of
the ferments of the digestive secretions, some slightly
altered bile pigments, and other bodies excreted by the
large intestine.

Dyspepsia is the common name of a number of diseased
conditions attended with loss of appetite or troublesome
digestion. Being usually unattended with acute pain, and
if it kills at all doing so very slowly, it is pre-eminently
suited for treatment by domestic quackery. In reality,
however, the immediate cause of the symptoms, and the
treatment called for, may vary widely; and its detection
and the choice of the proper remedial agents often call for
more than ordinary medical skill. A few of the more com-
mon forms of dysjDepsia may be mentioned here, with their
Xn'oximate causes, not in order to enable jieople to under-
take the rash experiment of dosing themselves, but to show
how wide a chance there is for any unskilled treatment to
miss its end, and do more harm than good.

Appetite is primarily due to a condition of the mucous
membrane of the stomach which, in health, comes on after
a short fast, and stimulates its sensory nerves; and loss of
appetite may be due to either of several causes. The sto-
mach may be apathetic and lack its normal sensibility, so
that the empty condition does not act, as it normally does,
as a sufficient excitant. When food is taken it is a farther
stimulus and may be enough; in such cases "appetite
comes with eating." A bitter before a meal is useful as an
appetizer to patients of this sort. On the other hand, the
stomach may be too sensitive, and a voracious appetite be
felt before a meal, which is replaced by nausea, or even
vomiting, as soon as a few mouthfuls have been swallowed;
the extra stimulus of the food then over-stimulates tiie too
irritable stomach, just as a draught of mustard and warm

mDIQESTION. 351

water will a healthy one. The proper treatment in such
cases is a soothing one. When food is taken it ought to
stimulate the sensory gastric nerves, so as to excite the reflex
centres for the secretory nerves, and for the dilatation of
the blood-vessels, of the organ; if it does not, iV.Q gastric
juice will be imperfectly secreted. In such cases one may
stimuhi,te the secretory nerves by weak alkalies (p. 33(3), as
Apollinaris water or a little carbonate of soda, before meals;
or give drugs, as strychnine, which increase the irritability
of reflex nerve-centres. The vascular dilatation may be
lielped by warm drinks, and this is probably the rationale
of the glass of hot water after eating which has recently
been in vogue; the usual cup of hot coffee after dinner (the
desirability of which is proved by the consensus of civilized
mankind) is a more agreeable foi-m of the same aid to
digestion. In states of general debility, when the stomach
is too feeble to secrete under any stimulation, the adminis-
tration of weak acids and artificially prepared pepsin is
needed, so as to supply gastric juice from outside, until the
improved digestion strengthens tlie stomacli up to the
point of being able to do its own work.

Enough has probably been said to show that dyspepsia
is not a disease, but a symptom accompanying many patho-
logical conditions, requiring special knowledge for their
treatment. From its nature — depriving the Body of its
proper nourishment — it tends to intensify itself, and so
should never be neglected; a stitch in time saves nine.

CHAPTER XXIV.

THE EESPIRATORY MECHANISM.

Definitions. The blood us it flows from the right Ycn-
tricle of the heart, through the lungs, to the left auricle,
loses curLon dioxide and gains oxygen. In the systemic
circulation exactly the reverse changes take place, oxygen
leaving the blood to supi:)ly the living tissues and carbon
dioxide, generated in them, passing back into the blood
capillaries. The oxygen loss and carbon dioxide gain are
associated with a cliange 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 becomes venous in the systemic circulation and
arterial in the pulmonary — in other words the processes
concerned in the gaseous reception, distribution and elimi-
nation of the Body — constitute the function of resjnration;
so much of this as is concerned in the interchanges between
the blood and air being known as external respiration;
while the interchanges occurring in the systemic capillaries,
and the processes in general by which oxygen is fixed and
carbon dioxide formed by the living tissues, are known as
internal respiration. When the term respiration is used
alone, without any limiting adjective, the external respira-
tion only, is commonly meant.

Respiratory Organs. The blood being kept poor in
oxygen and rich in carbon dioxide by the action of the liA'-
ing 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 atmos-
phere itself or water containing air in solution. When an

BESPIRATOET ORGANS. 353

animal i.s small there are often no special organs for its ex-
ternal respiration, its general surface being sufficient (espe-
cially in aquatic animals with a moist skin) to permit of all
the gaseous exchange that is necessary. In the simplest
creatures, indeed, there is even no blood, the cell or cells
composing them taking up for themselves from their en-
vn-onment the oxygen which they need, and passing out
into it their carbon dioxide waste; in other words, there is
no differentiation of the external and internal rfSpir.iUons.
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 surrounding 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 or-
gans developed, into which the blood is brought to replace
its oxygen loss and get rid of its excess of carbon dioxide.
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 solu-
tion, is kejst up; and in which blood capillaries form a
close network immediately beneath the surface. In air-
breathing animals a different arrangement is usually found.
Ill some, as frogs, it is true, the skin is kept moist and
serves as an important respiratory organ, large quanti-
ties 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 compensate for evaporation; and accordingly in
land animals the air is usually carried into the body by
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 disapjiear 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 gaseous exchanges

354

THE HUMAN BODY.

without the intervention of the blood. But in the great
majority of air-bretithing 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 m whose walls it flows through
a close capillary network. Such respiratory sacs are called
lungs, and it is a highly developed form of them which is
employed in the Human Body.

The Air-Passages and Lungs. In our own Bodies some
small amount of respiration is carried on in the alimentary

canal, the air swallowed with
food or saliva undergoing gas-
eous exchanges with the blood
in the gastric and intestinal mu-
cous membranes. The amount
of oxygen thus obtained by the
blood is however very trivial, as
is that absorbed through the
skin, covered as it is by its dry
horny non-vascular epidermis.
All the really essential gaseous
interchanges between the Body
and the atmosphere take place
in the lungs, two large sacs {In,
Fig. 1) lying in the thoracic
cavity, one on each side of the
heart. To tliese sacs the air is
conveyed through a series of
passages. Entering the pharynx
through the nostrils or mouth,
it passes out of this by the open-
ing leading into the larynx, or
voice-box [a. Fig. 104), lying in
the upper part of the neck (tho communication of the two
is seen in Fig. 80); from the larynx passes back the
?frr<c/^m or windpipe, ^, which, after entering the chest cavity,
divides into the riglit and left hronclii, d. Each bronchus
divides up into smaller and smaller branches, called Jron-

FiG. 104.— The lungs and air-pas-
sages seen from the front. On the
left of the figure the pulmonary
tissue has been dissected away to
sliow the ramifications of the bron-
chial tubes, a, larynx ; b. tracliea ;
d, right bronclius. The left bron-
chus is seen entering the root of its
lung.

STRUCTURE OF THE LUNGS.

355

Fig. 105. — A small bronchial tube,
a, dividing into its terminal branch-
es, c; these have pouched or saccu-
lated walls and end iu the saccu-
lated alveoli, b.

chial tubes, "within the king on its own side; and the smallest
bronchial tubes end in sacculated dilatations, the alveoli of
the lungs, the sacculations (Fig. 106) being the air-cells:
the word ''cell" being here used
in its primitive sense of a small
cavity, and not in its later tech-
nical signification of a morpho-
logical unit of the Body. On
the walls of the air-cells the
pulmonary capillaries ramify,
and it is in them that the inter-
changes of the external respira-
tion take place.

Structure of the Trachea
and. Bronchi. The windpipe
may readily be felt in the middle line of the neck, a little
below Adam's apple, as a rigid cylindrical mass. It con-
sists fundamentally of a fibrous tube in which cartilages
are imbedded, so as to keep it from collapsing; and is lined
internally by a mucous membrane covered by several layers
of epithelium cells, of which the superficial is ciliated (Fig.
47). The cartilages imbedded in its walls are imperfect
rings, 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 thereof the cartilages no doubt facilitates swal-
lowing. The bronchi resemble the windpipe in structure.

The Structure of the Lungs. These consist of the
bronchial tubes and their terminal dilatations; numerous
Ijlood-vessels, nerves and lymphatics; and an abundance of
'connective tissue, rich in elastic fibres, binding all together.
The bronchial tubes ramify in a tree-like manner (Fig. 104).
In structure the larger ones resemble the trachea, excej^t
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 epitlieliuni
is reduced to a single layer of cells which, though still cili-

356

THE HUMAN BODY.

ated, are much sliortcr tiian the cohimnar superficial cell
layer of the larger tubes. The terminal alveoli {a, a, Fig.
lUG,) and the air-cells, h, Avhich open into tliem, have Avails
composed mainly of elastic tissue and lined by a single

layer of flat, non-ciliated c\)\-
thelium,immediately l)cneatli
which is a very close network
of capillary blood - vessels.
The air entering by the bi-ou-
chial tube is thus only sepa-
rated from the blood by the
thin capillary wall and the
thin epithelium, both of
which are moist, and avcII
adapted to permit gaseous
diffusion.

The Pleura. Each lung
is covered, except at one
point, by an elastic serous
membrane which adheres
tightly to it and is called the
pleura; that point at which
the pleura is wanting is called the root of the lung and is
on its inner 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 rep-
resented by the dotted line in the diagram Fig. 1. The
part of the pleura attached to each lung is its visceral, and
that attached to the chest-wall its 2)arietal layer. Each
pleura thus forms a closed sac surrounding ?l pleural cavity,
in which, during health, there are found a few drops of
lymph, keeping its surfaces moist. This lessens friction
betAveen the two layers during the movements of the chest-
Avalls and the lungs; for although, to insure distinctness,
the visceral and j)arietal layers of the pleura are represented
in the diagram as not in contact, that is not the natural
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 brought about by

Fig. 106.- Two alveoli of the lung
highly magnifled. 6, 6, the air-cells,
orhoilow protrusions of tlie alveolus,
opening into itscentral cavity; c, ter-
minal branches of a bronchial tube.

WHY THE irXGS DO ^'0T COLLAPSE.

35 r

the pressure of the atmosphere on the inside of the hmgs,
through the air-passuge.s. The lungs are extremely elastic
and distensible, and when the chest cavity is perforated
each shrivels up just as an nidian-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 neutralizing one an-
other, there is nothing to overcome the tendency of the
lungs to collapse. So long as the chest-walls are whole,
however, the lungs remain distended. The pleural sac is
air-tiglit 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 reaching the lungs; consequently no
atmospheric pressure is exerted on their outsides. On their
interior, however, the atmosphere presses with its full
weight, equal (see Physics) to about 90 centigrams on a
square centimeter (1-1.5 lbs. 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. 107) to be a l)>)ttle closed air-tight
by a cork through which two tubes pass,
one of which, b, leads into an elastic bag,
d, and the other, c, provided with a stop-
cock, opens freely below into the bottle.
If 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 in-
side through b. The bag will therefore
'ollapse, as the lungs do when the chest cavity is opened.
But if some air be sucked out of c the pressure of that remain-
ing in the bottle will diminish, while that inside the bag
will be the same, and r.he hni will thus be blown up, because
the atmospheric pressure on its interior will not be balanced
by that on its exterioiv At last, when all the air is sucked
out of the bottle and the ,^top-cock on c closed, the bag, if
sufficiently distensible, will be expanded until it completely
61ls the bottle and presses against its inside, and the state

Ftg. 107.— Diagram
illustrating the pres-
sure relationships of
the lungs in the tho
rax.

3^S THE HUMAN BODY.

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 exjiand still more, so as to
fill it, and in so doing would receive air from outside
through b\ and if the bottle then returned to its original
size, its walls would press on the bag and cause it to shrink
and expel some of its air through b. Exactly the same
must of course happen, under similar circumstances, in the
chest, the windpipe answering to the tube b through which
air enters or leaves the elastic sac.

The Respiratory Movements. The air taken into the
lungs soon becomes laden in them with carbon dioxide, and
at the same time loses much of its oxygen; these inter-
changes taking 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 to any longer convert venous blood into
arterial, could only very slowly be renewed by gaseous dif-
fusion with the outer air through the long air-passages —
not nearly fast enough for the requirements of the Body, as
any one readily experiences through the sensation of suffo-
cation which follows holding the breath for a very short
time. Consequently, added on to the breathing lungs is a
respiratory mechanism, 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 Avails of the air-cells.
This mixing is brought about by the breathing movements,
consisting of regularly alternating inspiratiofis, during
Avhich 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 s})ace; in so doing it become 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 and enters the lungs.
In expiration the reverse takes place. The chest cavity,
liminishing, presses on the lungs and makes the air inside

MOVEMENTS OF THE THORAX. 359

them denser than the external air, and so some passes out
until an equilibrium of pressure is restored. The chest, in
fact, acts very much like a bellows. When the bellows are
opened air enters in con-
sequence of the rarefaction
of that in the interior,
which is expanding to fill
the larger space; and when
the bellows are closed

... n 1 m Fig. 108.— Diagram to illustrate the en-

again it is expelled. iO try of air to the lungs when the thoracic

make the bellows quite ^^^^y ^^^^^s^^-
like the lungs we must, however, as in Fig 108, 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
lying 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 cavi-
ties.

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 tlie .shape of its skeleton (Fig.
109), its narrower end being turned upwards. Dorsally,
ventrally, and on the sides, it is supported by the rigid
framework afforded by the dorsal 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 pie urge 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
diaiJtliragm, which forms a movable bottom to the, other-
v/ise, tolerably rigid box. In inspiration this box is in-
creased in all its diameters — dorso-ventrally, laterally, and
from above down.

The Vertical Enlargement of the Thorax. This is
!>rought about by the contraction of the diaphragm which
(Figs. 1 and 110) is a thin sheet-like muscle, with a fibrous
membrane, serving as a tendon, in its centre. In rest, the

3G0

THE HUMAN BODY.

diaphragm is dome-shaped, its concavity being turned
towards the ahdoinen. From the tendon on the crown of
the dome stri])ed muscuhir fibres radiate, downwards and
outwards, to all sides; and are fixed by their inferior ends
to the lower ribs, the breast-bone, and the vertebral column.
In expiration the lower lateral portions of the diaphraj:;m
lie close against the chest-walls, no lung inten'cning between
them. In inspiration the muscular fibres, shortening, flat-

FiG. 109.— The skeleton of the thorax,
clavicle; d, third rib; i, glenoid fossa.

a, g, vertebral coliunn; h, first rib; c.

ten the dome and so enlarge the thoracic cavity at the ex-
pense of the abdominal; and 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 Dorso-Ventral Enlargement of the Thorax. The
ribs on the whole slope downwards (t, Fig. 25) from the
vertebral column to the breast-bone, the slope being most

2HE EyLARGEMENT OF THE THORAX.

361

marked in the lower ones. During inspiration the breast-
bone and the sternal ends of the ribs attached to it are

' imm\ ^,

4
Fig. 110.— The diaphragm seen from below.

raised, and so the distance between the sternum and the
vertebral column is increased. That this must be so will
readily be seen by examining the diagram Fig. Ill, where
ah represents the vertebral column,
c and d two ribs, and st the ster-
num. The continuous lines repre-
sent the natural position of the ribs
at rest in expiration, and the dotted
lines the position in insi^iration. It
is clear that when their lower ends
are raised, so as to make the bars lie
in a more horizontal plane, the ster-
num is pushed away from the spine.
and so the chest cavity is increased
dorso- ventrally. The inspiratory
elevation of the ribs is mainly due to
the action of the scalene and exter-
nal intercostal muscles. The scalene muscles, three on each
side, arise from the cervical vertebra and are inserted into

Fig. 111.— Diagram illus-
trating the dorso-ventral in-
crease in the diameter of the
thorax when the ribs are
raised.

3g;>

TlIK II UMAX BODY.

tlie nppcr ribs. Tlie oxternul iutercostuls (Fig. 11'3, A)h(i
between the ribs and extend from the vertebral eolunm to the
costal cartilages; their fibres slope downwards and forwards.
Dui'ing an inspiration the scalenes contract and fix the
u])per ribs firmly; then the external intercostals shorten
and each raises the rib below it. The mnscle, in fact,
tends to pull together the pair of ribs between which it
lies, but as the upper one of these is held tight by the

Fig. 112. — 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, b, which are fixed to
cartilaginous portions, d. of the sternum. A, external intercostal muscle, ceas-
ing between tlie 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 displaj' the internal which was covered by it.

scalenes and 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 upjoer, for the
whole external intercostal muscles on one side may be re-
garded as one great muscle with many bellies, each belly
separated from the next by a tendon, represented by the
rib. When the whole muscular sheet is fixed above and

MECHANISM OF EXPIRATION. 363

contracts, it is clear that its lower end will be raised more
than any intermediate point, since there is a greater length
of contractile tissue above it. The elevation of the ribs
tends 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 mainly due
to the diaphragm, which, when it contracts, adds to the
lowest and widest part of the conical chest cavity. Some
small widening is, however, brought about by a rotation of
some of the middle ribs which, as they are raised, roll round
a little at their vertebral a-rticulations and twist their car-
tilages. Each rib is curved and, if the bones be examined
in their natural position iu 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 j^art is raised and turned out, and the chest widened.
The mechanism can be understood by clasping the hands
opposite the lower end of the sternum and a few inches in
front of it, with the elbows bent and pointing downwards.
Each arm will then answer, in an exaggerated way, to a
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
dorso-ventral enlargement 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 tlie space surrounded by the
two arms is considerably increased from side to side, as the
chest cavity is in inspiration by the similar elevation of the
most curved part or " angle" of the middle ribs.

Expiration. To produce an inspiration requires con-
siderable 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 ab-
dominal wall pushed out to make room for them. In ex-
piration, on the contrary, but little, if any, muscular effort

364 THE nUMAN BODY.

is needed. As soon as the muscles which have raised the
ribs and sternum rehix, these tend to return to their natural
unconstrained position, and the rib cartilages, also, to un-
twist 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 fibres cease contract-
ing. By these means the chest cavity is restored to its
original capacity and the air sent out of the lungs, rather
by the elasticity of the parts which were stretched in inspir-
ation, than 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 pro-
ducing the respiratory movements, and exjiiration then be-
comes, in part, an actively muscular act. The main expira-
tory muscles are the internal intercostals which lie beneath
the external between each pair of ribs (Fig. 112, B), and
have an opposite direction, their fibres running upwards
and forwards. In forced exijiration 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 in-
ternal intercostals then contracting, pull down the upper
ribs and the sternum, and so diminish the thoracic cavity
dorso-ventrally. At the same time, the contracted abdomi-
nal muscles press the walls of that cavity against the viscera
within it, and pushing these up forcibly against the dia-
phragm 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 dimin-
ish 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 verte-
bral column by the muscles going between the two, and
then other muscles, which pass from the collar-bone and

CAPACITY OF TEE LUNGS. 365

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 res-
piration.

The Respiratory Sounds. The entry and exit of air
are accompanied by resi)iratory sounds or innrmurs,
which can be heard on a])plying the ear to the chest wall.
The character of these sounds is different and characteristic
over the trachea, the larger bronchial tubes, and portions
of lung from which large bronchial tubes are absent. They
are yariously modified in pulmonary affections and hence
the value of auscultation of the lungs in assisting the phy-
sician to form a diagnosis.

The Capacity of the Lungs. Since the chest cavity
never even approximately collaiDses, 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 exjDiration they still con-
tain a large amount of air which can only be expelled from
them by opening the pleural cavities; then they entirely
collapse, just as the bag in Fig. 107 would if the bottle in-
closing it were broken. The capacity of the chest, and
therefore of the lungs, varies much in different individuals,
but in a man of medium height there remains in the lungs
after the most violent possible expiration, about 16-iO cub.
cent. (100 cub. inches) of air, called the residual air.
After an ordinary expiration there will be in addition to
this about as much more supplemental air; the residual and
supplemental together forming the stationary air, which
remains in the chest during quiet breathing. In an ordi-
nary inspiration 500 cub. cent, (30 cub. inches) of tidal air
are taken in, and about the same amount is expelled in nat-
ural expiration. By a forced inspiration about 1600 cub.
cent. (98 cub. inches) of complemental air can be added to
the tidal air. After a forced inspiration therefore the chest
will contain 1640 + 1640 + 500 + 1600 = 5380 cubic centi-
meters (328 cubic 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,

366 rilE HUMAN BODY.

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 3700 cub. cent. (225 cub. inches)
and increases GO cub. cent, for each additional centime-
ter of stature; or about 9 cubic 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 mixture 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 respirations 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 of these half a liter (30 cubic inches)
of air is concerned; therefore 0.5 x 15 X 60 X 24 = 10,800
liters (374 cubic feet) is the quantity of air breathed under
ordinary circumstances by each person in a day.

Hygienic Remarks. Since the diaphragm when it con-
tracts pu-slies down the abdominal viscera beneath it, these
have to make room for themselves by pushing out the soft
front of the abdomen which, accordingly, protrudes when
the diaphragm descends. Hence breathing by the dia-
phragm, being indicated on the exterior 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 ex})and and contract in
healthy breathing anything which impedes their free move-
ment is to be avoided; and the tight lacing which used to
be thought elegant a few years 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 in-
capacity for muscular exertion. In extreme cases of tight
lacing some organs are often directly injured, weals of

ASPIRATION OF THE THORAX. 3G7

fibrous tissue being, for exam^ile, uot uufretjueutl}' found
developed on tlie liver, from the pressure of the lower ribs
forced against it by a tight corset.

The Aspiration of the Thorax, As already pointed out,
in consccpience of the rigid framework which supjiorts its
walls, the external air cannot jjress directly \x\)0\\ the con-
tents of the thoracic cavity. It still, however, presses on
them indirectly, through the lungs. Acting on the interior
of these with a pressure ecpial to that exerted on the same
area by a column of mercury 760 mm. (30 inches) high, it
distends them, and pushes them against the inside of the
chest-walls, the heart, the great thoracic blood-vessels, the
thoracic duct, and the other contents of the cavity. The
pressure thus exerted is not equal to that of the external
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 about equal to that ex-
erted by a column of mercury 754 mm. (| inch less than be-
fore) high. On most parts of the Body the atmospheric
pressure can, however, act with full force. Pressing on a
limb it pushes the skin against the soft parts beneath, and
these against 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 in
the chest, and consequently these liquids tend to flow into
the thorax, until the extra distension of the vessels in which
they there accumulate compensates for the less external
pressure to which those vessels are exposed. An e(|uili-
brium would thus very soon be brought about were it not
for the respiratory movements, in consequence of which the
mtra-thoracic pressure is alternately increased and dimin-
ished, 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 respiratory movements come to influence the cir-
culation of the blood and the flow of the lymph.

368 THE HUMAN BODY.

Influence of the Respiratory Movements upon the
Circulation. Suppose the chest in a condition oi' natural
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 paragi-aph, into equili-
brium with the atmospheric pressure on the blood-vessels of
the neck and abdomen. If an insjoiration now occurs, the
chest cavity being enlarged the pressure on all of its con-
tents will be diminished. In consequence, air enters the
lungs from the windpipe, and blood enters the venae cavae
and the right auricle of the heart. Not only the lungs,
then, but the right side of the heart, and the intra-thoracic
portions of the systemic veins leading to it, are expanded
during an inspiration; but the lungs being much the most
distensible take far the greatest part in filling up the in-
creased space. The left side of the heart is not much in-
fluenced as it is filled from the pulmonary veins; and the
Avhole vessels of the lesser circulation lying within the
chest, and being all affected in the same way at the same
time, the blood-fiow in them is not influenced by the aspi-
ration of the thorax. Distension of the lungs seems, how-
ever, to diminish the capacity of their vessels, and so to a
certain extent the flow is influenced; as the lungs expand
blood is forced out of their vessels into the left auricle, and
when they again contract their vessels fill up from the right
ventricle. The pressure on the thoracic aorta being dimin-
ished in inspiration, blood tends to flow back into it from
the abdominal portion of the vessel, but cannot enter the
heart on account of the semilunar valves; and the back-flow
does not in any case equal the onflow due to the beat of the
heart; so the what happens in the aorta is but a slight
slowing of the current. The general result of all this is
that the circulation is considerably assisted. When the
next expiration occnrs, and the pressure in the thorax again
rises, air and blood both tend to be expelled from the cavity.
The aorta thus regains what it lost during insjiiration; the
pressure on it is increased and it empties itself faster into
its abdominal portion. The semilunar valves having pre-

EFFECT OF RESPIRATION ON BLOOD-FLOW. 369

vented any regurgitation into the heart, there is neither
gam noi loss so far as it is concerned. With the systemic
intrathoracic veins, however, this is not the case; the extra
blood entering them has already in great par*" gone on be-
vond the tricuspid valve, and cannot flow back during ex-
piration; and the pressure in the auricle being constantly
kept low by its emptying into the ventricle, the increased
pressure on the venee cava? tends as much to send the blood
on into the heart, as back into the extra-thoracic veins.
Moreover, whatever blood tends to take the latter course
cannot do it effectually since, although the vense cava?
themselves contain no valves, the more distant veins which
open into them do. Consequently, whatever extra blood
has, to use the common phrase, been ''sucked" into the
intra-tlioracic veiife cav^e in inspiration and has not been
sent already on into the right ventricle before expiration
occurs, is, on account of the venous valves, imprisoned in
the cavge under an increased pressure during expiration;
and this tends to make it flow faster into the auricle during
the diastole of the lattei". How much the alternating res^
piratory movements assist the venous flow is shown by the
dilation of the veins of the head and neck which occur.s
when a person is holding his breath; and the blackness for
the face, from distension 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 in-
spirations.

In still another way the aspiration of the thorax assists
the heart. The heart and lungs are both extensible, though
in different degrees, and each is stretched in the chest
somewhat beyond its natural size; the one by the atmos-
pheric pressure directly, the other by that pressure in-
directly, exerted through the blood exposed to it in the
extra-thoracic veins. Supposing, therefore, the heart sud-
denly to shrink it would leave more space in the chest to be
filled by the lungs, and these, accordingly, at each cardiac
systole expand a little to fill the extra room, just as thevdo
when the space around them is otherwise enlarged during

370

THE HUMAN BODY,

an inspiration. Tlie elasticity of the lungs, however,
causes them to resist this distension and oppose the cardiac
systole. The matter may be made clear by an arrangement
like that in Fig. 113. A is an air-tight vessel with a tube,
e, provided with a stop-cock, leading from it; J is a highly
distensible elastic bag in free communication through d
with the exterior; and c, representing
the heart, is a less extensible sac, from
which a tube leads and dips under
water in the vessel B. If air be
pumped out through e both bags will
dilate, h filling with air, and c with
water driven up by atmospheric pres-
sure. Ultimately, if sufficiently ex-
tensible, they would fill the whole
space, the thinner walled, h, occupy-
ing most of it. If then the stop-cock
be closed, things will remain in equi-
librium, each bag striving to collapse
and so exerting a pull on the other,
for if h shrinks c must expand and vice
versa. If c suddenly shrink, as the
heart does in its systole, b will dilate; but as soon as the
systole of c ceases, h will shrink again and pull c out to
its previous size. In the same way, after the cardiac sys-
tole, when the heart-walls relax, the lungs pull them out
again and dilate the organ. The contracting heart thus
expends some of its work in overcoming the elasticity of the
lungs, which ppposes their expansion to fill the space left
by the smaller heart; but during the diastole of the heart
this work is utilized to pull out its walls again, and draw
blood into it. Since the normal heart has muscular power,
and to spare, for its systole, this arrangement, by which
some of the work then spent is stored away to assist the
diastole, which cannot be directly performed by cardiac
muscles, is of service to it on the whole. It is a physio-
logical though not a mechanical advantage; no work power
is gained, but what there is, is better distributed.

Fig. 113.— Diagram illus-
trating the influence of as-
piration of the thorax on
the circulation of the
blood.

INFLUENCE OF RESPIBATION OX I.T^IPHFLO^V. 371

Influence of the Respiration on the Lymph-Flow.

During inspiration, when intra-thoracic pressure is lowered,
lympli is pressed mto the thoracic duct from the abdomi-
nal lymphatics. In expiration, when thoracic pressure rises
again, the extra IjTuph cannot flow back on account of the
valves in the lymphatic vessels, and \t ^s consequently
driven on to ths cervical ending of the thoracic duct. The
breathing movements thus pump the lymph on.

CHAPTER XXV.

THE CHEMISTllY OF EESPIEATION.

Nature of the Problems. Tlie study of the respira-
tory process from a chemical standpoint has for its object
to discover, first, wliat are, in kind and extent, tlie inter-
changes between the air in the kings and the blood in pul-
monary cajDillaries; and, in the second place, the nature
and amount of the corresponding gaseous changes between
the various living tissues and the blood in the systemic
capillaries. These processes are the reverse of one another
and in the long run balance, the blood losing as much car-
bon dioxide gas in the pulmonary circulation as it gains in
the systemic, and gaining as much oxygen in the former
as it loses in the latter. To thoroughly comprehend the
matter it is, moreover, necessary to know the jjliysical and
chemical conditions of these gases in the lungs, in the
blood, and in the tissues genei'ally; 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 prob-
lems connected with the external respiration are concerned
our knowledge is tolerably complete; but as regards the
internal respiration, taking place all through tlie Body,
much has yet to be learnt; for example, we know that a
muscle at work gives more carbon dioxide to the blood than
one at rest and takes more oxygen from it, but exactly how
much of the one it gives and of the other it takes is only
known approximately; as are also the conditions 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

CHANGES PB on UCED IM AIB ONCE BREATHED. 373

find how great its gaseous interchauges 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 comj)letely answered.

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 usuially about 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 calculated as about equal to*3.5 calories
in twenty-four hours; a calorie (see Physics) being as much
heat as will raise the temperature of one kilogram (2.2 lbs)
of water one degree centigrade (1.8° F.).

The inspired air always contains more or less w^ater 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 readily shoAvn 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 witli the tem-
perature and saturation of the inspired air: the cooler and
drier this is, the more water will it gain when breathed.
On an average 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 apj^ear again
outside it when the water vapor condenses (see Physics).

374 THE HUMAN BODY.

The amount of heat taken oil in ihis way during the day is
about 7.2 calories. The total daily loss of heat from the
Body througli tlie lungs is therefore 10.7 calories, 3.5 in
warming the inspired air and 7.2 in evaporating water.

The most important changes brought about in the
breathed air are those in its chemical composition. Pure
air when completely dried consist in 100 parts of —

By Volume. By Weight.

Oxygen 20.8 23

Nitrogen 79.2 77

Ordinary atmospheric air contains in addition 4 volumes
of carbon dioxide 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 dioxide, and loses rather more than 5 of oxygen.
More accurately, 100 volumes of expired air when dried con-
sist of —

Oxygen 15.4

Nitrogen 79.2

Carbon dioxide 4.3

The expired air also contains volatile organic substances
in quantities too minute for chemical analysi-s, but readily
detected by the nose upon coming into a close room in
which a number of persons have been collected.

Since 10,800 liters (346 cubic feet) of air are breathed in
twenty-four hours and lose 5.4 per cent of oxygen, the
total quantity of this gas taken up in the lungs daily is
10,800 X 5.4-f- 100 =583.2 liters (20.4 cubic feet). One
liter of oxvgen measured at 0°C (32° F.) and under a pressure
equal to one atmosphere, weighs 1.43 grams (see Chemistry),
so the total weight of oxygen taken up l)y the lungs daily is
583.2 X 1.43 = 833.9 grams. Or, using inches and grains as
standards, 44.5 cubic inches of oxygen at the above tem-
perature and pressure weigh almost exactly 16 grains, so
the 20,4 culjic feet absorbed in the lungs daily weigh 20,4
X 1728 --44,5X16 = 12,818 grains.

The amount of carbon dioxide excreted from the luno^s

VEJS^TILA riON. 3 To

being 4.3 ^jer cent of the volume of the air breathed
daily, is— 10,800 X 4.3 -j- 100 = 4G4.4 liters (1G.,:25 cubic
feet) measured at the normal temperature and pressure.
This volume -weighs 910 grams, or 14,li35 grains.

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 5.4 volumes per cent of oxygen and
gained only 4.3 of carbon dioxide. In round numbers, 100
volumes of dry inspired air at zero, give 99 volumes of dry
expired air measured at the same temperature and pres-
sure.

Ventilation. Since at every breath some oxygen is taken
from the an" and some carbon dioxide given to it, were the
atmosphere around a living man not renewed he would, at
last, be una'ole to get from the air the oxygen he required;
he would die of oxygen 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 receiver of an air-pump and all
the air around him removed. Hence the necessity of ven-
tilation to supply fresh air in place of that breathed, and
clearly the amount of fresh air requisite must be deter-
mined by the number of persons collected in a room; the
supply which would be ample for one person would be in-
sufficient for two. Moreover fires, gas, and lamps, all use
up the oxygen of the air and give carbon dioxide to it, and
hence calculation must be made for them in arranging for
the ventilation of a building in which they are to be em-
ployed.

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
rooms for, as we shall see in the next paragraph, the blood
can take what oxygen it wants from air containing compara-

376 THE limfAX BODY.

lively little of that gas. The lieadache and drowsiness
wliich come on from silling in a badly venlihitcd room, and
the want of energy and general ill-health wliich result from
permanently living in such, are dependent on a slow ijoison^
ing of the Body by the reabsorption of the things elimi-
nated from the lungs in previous respirations. What these
are is not accurately known; they doubtless belong to those
volatile bodies mentioned above, as carried off in minute
quantities in each breath; since observation shows that the
air becomes injurious long before the amount of carbon
dioxide in it is sufficient to do any harm. Breathing air
containing one or two per cent of that gas produced
by ordinary chemical methods does no particular injury,
but breathing air containing one per cent of it produced
by respiration is decidedly injurious, because of the other
things sent out of the lungs at the same time. Carbon
dioxide itself, at least in any such percentage as is com-
monly found in a room, is not poisonous, as used to be
believed, but, since it is tolerably easily estimated in air,
while the actually injurious substances evolved in breath-
ing are not, the purity or foulness of the air in a room is
usually determined by finding the percentage of carbon
dioxide in it; but it must be borne in mind that to mean
much this must have been produced by breathing; other-
wise the amount of it present is no guide to the cpumtity of
really important injurious substances jjresent. Of course
when a great deal of carbon dioxide is present the air is
irrespirable: as for examj)le sometimes at the bottom of
wells or brewing-vats.

In one minute as we have seen (p. 3GG) .5 X 15 = 7.5
liters (0.25-4 cubic feet) of air are breathed and vitiated with
carbon dioxide to the extent of rather more than four per
cent; this, mixed with three times its volume of extermxl
air, would give thirty liters (a little over one cubic foot)
vitiated to the extent of one per cent, and such air is no
longer respirable for any length of time with safety. The
result of breathing it for an evening is headache and gen-
eral malaise; of breathing it for weeks or months a lowered
tone of the whole Body — less power of work, physical or

VENTILA nON. 377

mental, and less power of resisting disease; the ill effects
may not show themselves at once, and may accordinciy be
overlooked, or considered scientific fancies, by the careless;
but they are there ready to manifest themselves neverthe-
less. In order to have air to breathe in a fairly pure
state every man should have for his own allowance at least
•^3,000 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. The nose is, however, the best guide, and it
is found that at least live times this supply of fresh air is
necessary to keep free from any odor the room inhabited bv
one adult. 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, is also provided for.

Ventilation does not necessarily mean draughts of cold
air, as is too often supposed. In warming by indirect radia-
tion it may readily be secured by fixing, in addition to the
registers from which the new warmed air reaches the room,
corresponding 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 one of the healthiest, though not the most economi-
cal, methods of warming an apartment.

Stoves in a room, unless constantly supplied with fresh air
from without, dry its air to an unwholesome extent. If no
a^jpliance 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 open-
ing between the two sashes and in a current directed
upwards, which gradually diffuses itself over the room with-
out being felt as a draught at any one point. In the
method of heating by direct radiation, the apparatus em-
ployed 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
openings are very necessary. Since few doors and windows,

378 THE lir}rA^ BODY.

rurtunaicly, fit quite tight, fresh air gets even into closed
rooms, in tolerable abundance for one or two inhabitants,
if there be outlets for the air already in them.

Changes undergone by the Blood in the Lungs. These
are the exact reverse of tliose exliiljited by the breathed air
— what the air gains the blood Uiscs, and vice versa. Con-
sequently, the blood loses heat, and water, nnd carbon
dioxide in the pulmonary capillaries; and gains oxygen.
These gains and losses are accompanied by a change of color
from the dark purple wliich the blood exhibits in the pul-
monary artery, to the bright scarlet it jDOSsesses in the pul-
monary veins.

The dependence of this color change upon the access of
fresh air to the lungs while the blood is flowing througli them,
can be 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 pulmo-
nary 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, unless
the bellows be again set at work.

The Blood Gases. If fresh blood be rapidly exposed to
as complete 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 arteriiil
blood; there being more carbon dioxide 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
pressure and at the normal temperature (see Physics),

THE BLOOD GASES. 379

amouTit to about 72 Yolumes for every 100 volumes of bloody
and iu the two cases are as follows —

Venous Blood. Arterial Blood.

Oxygen 10 20

Carbon dioxide 60 50

Nitrogen 2 2

It is important to bear in mind that Avhilo arterial blood
contains some carbon dioxide that can be removed by the
air-pumj), venous blood also contains some ox3'gen, remova-
ble in the same way; so that the difference between the
two is only one of degree. When an animal is killed by
f'lffocation, 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 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 chanige in its red corpuscles. Now, minute solid
bodies suspended in a liquid reflect more light when they
are more dense, other things being equal; and the first
thing that suggests itself as the cause of the change in
color of the blood is that, its red corpuscles have shrunk in
the pulmonary circulation, and so reflect more light and
give the blood a brighter look. This idea gains some
support from the fact that, as seen under the microscope,
the red blood corpuscles of some animals, as the frog, do
expand somewhat when exposed to carbon dioxide gas and
shrink up a little in oxygen. But that this is not the chief
cause of the color change is readily proved. By diluting
blood with water the coloring matter of the red corpuscles
can be made to pass out of them and go into solution in the
plasma (p. 46) and it is found that such a solution, in which
there can be no question as to the reflecting powers of
colored solid bodies suspended in it, is brighter red when
supplied with oxygen than when deprived of that gas.

380 THE HUMAN BODY.

This suggests that tlio c )loring iiiiittcr or hmmoglohi n of
the red corpuscles combines Avith oxygen to form a scarlet
compound, and when deprived of that gas has a darker and
more purple color; and further experiments confirm this.
Hannoglohin combined with oxygen is known as oxylimmo-
(jjiihin and it is on its predominance that the color of
arterial blood dei)ends. H;emoglol)in uncombined with oxy-
gen is reduced hoenioglobin; it predominates in venous blood,
and is alone found in the blood of a suffocated 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 temperatnTe
if it absorbs any of a gas to which it is ex2:)0sed, and yet does
not combine chemically with it, takes up a definite volume
of the gas. If the gas be compressed the liquid will still,
at the same temperature, take ujt the same volume asljefore,
but now it takes up a greater weight; and a weight exactly
as much greater as the pressure is greater, since one volume
of a gas under any pressure contains exactly twice as much
of the gas by weight as the same volume under half the
pressure, and so on. A liter or a quart of water, for example,
exposed to the air will dissolve a certain amount of oxygen.
If the air (and therefore the oxygen in it) he compressed to
one fourth its bulk tlien the water will dissolve exactly the
same volume of oxygen as before, but this volume of the
compressed gas will contain exactly four times as much
oxygen as did the same volume of the gas under the origi-
nal pressure; and if now the pressure be again diminished
the oxygen Avill be given off exactly in proportion as its
pressure on the surface of the water decreases. Finally,
when a complete vacuum is formed above the surface of the
water it will be found that the latter has given off all its
dissolved oxygen. This law, that the quantity of a gas dis-
solved by a lifjuid varies directly as the pressure of that gas
on the surface of the liquid is known as Dalton's law (see
Physics).

THE ABSORPTION OF GASES BY LIQUIDS. "-SI

, 2. The amount of a gas dissolyed 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 particular gas in question. For example,
the total 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 pressure 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 removed from the air and its total pressure there-
fore reduced to -t of 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 absorbed by water at any
given temperature. So, too, of all the atmospheric pressure
4 is due to nitrogen, and all the oxj'gen 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 f of 760
mm. of mercury, or 608 mm, (24 inches), but it would all
be due to nitrogen ga? — 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 to-
gether the fpaction -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
individual 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 nitrogen, 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 \
more nitrogen, since the whole gaseous pressure on its sur-
face was now due to that gas while before only \ of the
total was exerted by it. If, on the contrary, the liquid were
exposed to pure hydrogen under a pressure of one atmos-

382 THE HUMAN BODY.

pliere it would give ofE nil its previously dissolved oxygen
and nitrogen, since none of ihj pressure on its surface woulcT
now bo due to tlioso gases; and would take up as much
liydrogen as corresponded to a pressure of that gas equal to
7G0 mm. of mercury (30 inches).

3. A liquid may be such as to combine chemically with
a gas. Tlien the amount of the gas absorbed is indepen-
dent of the partial jjressure of the gas on the surface of tlie
liquid. The quantity absorbed will depend upon how much
the liquid can combine with. Or, a liquid may partly be
composed of things which simply dissolve a gas and partly
of things which chemically combine with it. Then the
amount of the gas taken uj? under a given partial pressure
will depend on two things; a certain portion, that merely
dissolved, will vary with the pressure of the gas in question;
but another portion, that chemically combined, will remain
the same under difEerent pressures. The amount of this
second portion depends only on the amount of the sub-
stance in the liquid which can chemically combine with it,
and is totally independent of the partial pressure of the
gas.

4. Bodies are known which chemically combine with
certain gases when the partial pressure of these is consider-
able; but the compounds thus formed are broken up, and
the gas liberated, when its partial pressure on the surface
of the liquid falls below a certain limit.

5. 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 physical and chemical facts stated in the joreceding
paragraph to the blood, we find that the blood contains (1 )
plasma, which simply dissolves oxygen, and (2) licemoglohin.
which combines with it under 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

ABSOBPTIOY OF OXYGEy BY THE BLOOD. 383

much water, that is about 3 volumes of the gas for every
100 of the liquid. This quantity obeys Dalton's law.

If instead of blood serum fresh whipped blood be em-
ployed, the quantity of oxygen taken up is much greater;
this extra quantity must therefore be taken up by the red
corpuscles (in possessing which whipped blood alone differs
from blood serum) and it does not obey Dalton's law. If
the partial pressure of oxygen on the surface of the
whipped blood be doubled, only as much more oxygen will
be taken up as corresponds to that dissolved in the plasma;
and if the partial pressure of oxygen on its surface be re-
duced to one half only a very small amount of oxygen (^
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 made up by h^Bmoglobin, and this body when pre-
pared pure is found capable of combining with oxygen, there
is no doubt that it is the haemoglobin in the circulating blood
which carries around nearly all the oxygen found in it.
The red corpuscles are so many little packages in which
oxygen is stowed away.

The compound formed between oxygen and hsemoglobin
is, however, a very feeble one; the two easily separate, and
always do so when the oxygen pressure in the liquid or gas
to which the oxyhoemoglobin is exposed falls below 25 mil-
limeters of mercury. Hence, in an air-pump, the blood only
gives off some of its small portion of merely dissolved oxy-
gen, until the pressure falls to about ^ of an atmosphere,
that is to -^Q- = 125 mm. (5 inches) of mercury, of which
total pressure one fifth (25 millimeters or 1 inch) is due to
the oxygen present. As soon as this limit is reached the
haemoglobin gives up its oxygen.

Consequences of the Peculiar "Way in which the
Oxygen of the Blood is Held. The first, and most im-
portant, is that the blood can take up far more oxygen in
the lungs than would otherwise be possible. Since blood
serum exposed to pure oxygen takes up only 3 volumes for
100, blood exposed to the air would take up \ only of that

384 THE HUMAN BODY.

amount at ordinary temperatures, and still less at the tcm-
l)erature of the Body, were it not for its haemoglobin. In
the lungs even less would be taken up, since tlie 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 one examined the pulmonary air the
less oxygen and more carbon dioxide would be found,
till, in the layers farthest from the exterior and only re-
newed by difEusion with the air of the large bronchi, it is
estimated that the oxygen only exists in such quantity that
its partial pressure is equal to 130 millimeters of mercury,
instead of 153 as in ordinary air. In the second place, on
account of the way in which haemoglobin 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 gi'eatly diminished, but still we can breathe freely and get
all the oxygen we want. So long as the partial pressure of
tliat gas remains above 25 millimeters (1 inch) of mercury,
the amount of it taken up by the blood will depend on how
much haemoglobin there is in that liquid and not on how
much oxygen there is in the air. So, too, breathing pure
oxygen under a pressure of one atmosphere, or air com-
l^ressed to ^ or \ its bulk, does not increase the quantity of
that gas taken up by the blood, apart from the very small
extra quantity which would be dissolved by the i")lasma.
All the widespread statements found as to the exhilaration
and excitement caused by breathing pure oxygen are, as a
matter of fact, erroneous, being founded, on early experi-
ments made with impure gas, and corrected by many com-
petent observers since.

The General Oxygen Interchanges in the Blood. We
may now try to depict what hap})ens to the blood oxygen
in a complete circulation. Sup}iose we have a quantity of
arterial blood in the aorta. This, fresh from the lungs, will
have its haemoglobin almost fully combined with oxygen

THE BLOOD GASES. 385

and in lii-c state of oxylisaiogiobiu. lu the blood jjlusma
some more oxygen will be dissolved and so much as answers
to a pressure of that gas ec^ual to 130 mm. {o.'Z inches) of
mercury, which is the partial pressure of oxygen in the
pulmonary air-cells. This teyisiou of the gas in the jjlasma
will be more than sufficient to keep the hgemogiobin 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 almost nil, since the tissue elements
are steadily taking the gas up from the lymph around them.
Consequently, through the capillary walls, the plasma will
give oft' oxygen until the tension of tliat gas m it falls below
35 millimeters of mercury. Immediately some of the oxy-
liJBmoglobin is decomposed, and the oxygen liberated is dis-
solved in the plasma, and from there again passed on to the
lymph outside; and so the tension in the plasma is once
more lowered and more oxyhasmoglobin decomposed. This
goes on so long as the blood is in the capillaries of tlie
muscle, or at any rate so long as the muscular fibres kee})
on taking oxygen from the lymph bathing them; if they
cease to do so of course the tension of that gas in the lymph
will soon come to equal that in the plasma: the latter will
therefore cease to yield oxygen to the former; and so main-
tain its tension (by the oxygen received from the last de-
composed oxyhemoglobin) at a point Avhich will preven'
the liberation of anymore oxygen from such red cori^uscles
as have not yet given all theirs up. 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 oxygen dissolved in it at a tension of about 25 milli-
meters (1 inch) of mercury. (2) A number of red cor-
puscles containing reduced hajmoglobin. (3) A number of
rod corpuscles containing oxyha^mogiobin. Or perhaps all
of the red corpuscles will contain some reduced and some
oxidized haemoglobin. The relative proportion of reduced
aud unreduced haemoglobin will depend on how active the
muscle was; if it worked while the blood flowed through it
it Avill have used up more oxygen, and the blood leaving it
will consequently be more venous, than if it rested. This

386 THE HUMAN BODY.

venous blood, returning to the heart, is sent on to the pul-
monary capillaries. Here, Lhe partial pressure of oxygen
in the air-cells being loU mm. (5.;i inches) and that in the
blood plasma much less, oxygen will be taken \\\) by the
latter, and the tension of that gas in the jjlasma tend to be
raised above tiie limit at which haemoglobin combines with
it. Hence, as fast as the plasma gets oxygen those red cor-
puscles which contain any reduced haemoglobin rob it, and
so its oxygen tension is kept down below that in the air-
cells until all the hasmogiobin is satisfied. 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 fol-
low it.

The Carbon Dioxide of the Blood. The same general
laws ajjply to this as to the blood oxygen. The gas is
partly merely dissolved and partly in a loose chemical com-
bination much like that of oxygen with haemoglobin, but
the body Avith which it combines in this way exists in the
plasma and not in the red corpuscles; what it may be is
not certainly known. Besides this, some more carbon
dioxide is stably combined and is only given off on the
addition of a stronger acid. The partial pressure of carbon
dioxide in the pulmonary air-cells is about 40mm. (1.6
inches) of mercury. Therefore the tension of that gas in
the pulmonary capillaries must be more than this. On
the other hand its tension in arterial blood must be less
than that in the lymph around the tissues; otherwise it
could not enter the blood in the systemic circulation, which
it does, as proved by the fact that 100 vols, of venous blood
give off 60 of this gas, and 100 vols, of arterial only 50.

The nitrogen contained in the blood is, so far as we know,
quite unimportant.

Internal Respiration. As to the amount of oxygen
used by each tissue and the quantity of carbon dioxide pro-
duced by it we know but little; the following points seem,
however, tolerably certain: —

1. The amount of carbon dioxide produced in an organ

mTERNAL RESPIRATION. 387

in a given time bears no constant ratio to the amount of
oxygen taken np by it simultaneously. This is certainly
true of muscle, for experiment shows that muscular work,
while it continues, leads to an elimination of carbon dioxide
containing more oxygen than the total oxygen taken up
from the lungs in the same time. The balance is of course
made up in subsequent periods of rest, when more free
oxygen is taken up than is eliminated in combination
during the same time. Moreover, a frog's muscle excised
from the body and put in an atmosphere containing no
oxygen and made there to contract, will evolve with each
contraction considerable quantities of carbon dioxide —
although from the conditions of the experiment it can
receive from outside no uncombined oxygen, and other
experiments show that it contains none. Hence the living
muscular fibre must contain a substance which is decom-
posed during activity and yields carbon dioxide as one pro-
duct of decomposition; and this quite independent of any
simultaneous direct oxidation.

2. What is true of muscle is probably true of most of
the tissues. During rest they take up oxygen and fix it
in the form of complex compounds, bodies which, like gun-
powder, are readily decomposed into simpler, and in such
decompositions liberate energy which is used by the work-
ing tissue. One product of the decomposition is the
highly oxidized carbon dioxide, c^nd this is eliminated;
other products are less oxidized, and possibly are not elimi-
nated but built up again, with fresh oxygen taken from the
blood and fresh carbon from the food, into the decomposa-
ble substance.

3. During the day a man gives off from his lungs more
oxygen in carbon dioxide, tlian he takes up by the same
organs from the air. During the night the reverse is the
case. This, however, has nothing to do with the alternating
jjeriods of light and darkness, as it has in the case of a
green plant, which in the light evolves more oxygen than
it consumes and in the dark the contrary. It depends,
rather, on the fact that during the day more muscular effort
is exerted than at night, and the meals are then taken

388 TITM HUMAN BODY.

and digested. The activity of tlic muscles and the digestive
glands is dependent on processes which give rise to a large
production of carbon dioxide and, during the night, when
botli are at rest, more oxygen is taken up than is contained
in the carbon dioxide eliminated. If a man works and
takes his meals at night, and slecj^s in the d:iy, the usual
ratios of his gaseous exchanges with the exterior are entirely
reversed.

4. 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. The quantity of oxygen supj)lied must
of course always be, at least, that required to prevent suffoca-
tion; but an excess above this limit will not make the tissues
work. Just as a man must have a certain amount of food
to keej^ him alive, so he must have a certain amount of
oxygen; but as extra food will not make his tissues or Mm
(who is physiologically the sum of all his tissues) work,
apart from some stimulus to exertion, so it is with oxygen.
Highly arterialized blood, or an abnormal amount of blood,
flowing through an organ will not arouse it to activity;
the working organ, muscle (p. 257) or gland (p. 269), for ex-
ample, usually gets more blood to supply its extra needs —
just as a healthy man who works will have a better appe-
tite than an idle one; but as taking more food by an idle
man will not of itself make him more energetic, so neither
will sending more arterial blood through an organ excite
it to activity.

5. The preceding statement is confirmed by experiments
which show that an aninud uses no more oxygen in an hour
when made to breathe that gas in a pure state, than when
allowed to breathe ordinary air. In other words, the
amount of oxygen an animal uses (provided it gets the
minimum necessary for health) is dependent only on how
much it uses its tissues. These (the rest in most cases sul>-
Ject to a certain amount of control from the nervous) de-
termine their own activity, and this, in turn, how much
oxygen shall be used in the systemic circulation and re-

INTERNAL RESPIRATION. 389

stored in the pulmonary. In other words, the physiological
work of an animal, which in turn is largely dependent upon
how external forces act upon it, determines how much
oxygen it uses daily; and not the supjily of oxygen how
much its tissue activity shall be, unless the supply sinks
below the starvation limit.

CHAPTER XXVI.

THE NERVOUS FACTORS OF THE RESPIRA-
TORY MECHANISM. ASPHYXIA.

The Respiratory Centre. The respiratory movements
are to a certain extent under the control of the will; we
can breathe faster or slower, shallower or more deejAj, 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 involun-
tary; they go on perfectly without the least attention on
our part, and, not only in sleep, but during the unconscious-
ness of fainting or of an apoplectic fit. The natural breath-
ing movements are 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 con-
tract in the Body under the influence of the nerves going
to them; the nerves of the diaphragm are the two phrenic
nerves (p. 161), one for each side of it; the external inter-
costal muscles are supplied by certain branches of the dor-
sal 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 intercostal 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 tlie 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 RESPIRATORY CENTRE. 391

the medulla oblongata only, the rest of the brain being un-
dereloped, and yet they breathe perfectly well. 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 respira-
tory muscles arise in the medulla oblongata, and travel
down the spinal cord and thence out along the phrenic and
intercostal nerves. This is conlirmed by the fact that if
the spinal cord be cut across below the origin of the fourth
]-)air of cervical spinal nerves (from which the phrenics
mainly arise) but above the first dorsal spinal nerves,
the respiratory movements of the diaphragm continue but
those of the intercostal muscles cease; this phenomenon
has sometimes been observed in men stabbed in the back, so
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
forever, although all the rest of the brain and spinal cord
may be left uninjured. This part of the medulla is known
as the respiratory centre.

In the above statements, for the sake of simplicity, atten-
tion has been chiefly confined to the diaphragm and the
intercostal muscles; but what is said of them is true of the
respiratory innervation of all other breathing muscles,
whether expiratory or insjoiratory, normal or extraordinary;
in all cases the impulse giving rise to a resjDiratory move-
ment starts from the centre placed in the medulla oblon-
gata.

Is the Respiratory Centre Reflex ? Since this centre
goes on working independently of the will we have next to
inquire is it a reflex centre or not; are the efferent dis-
charges it sends along the respiratory nerves due to afferent
impulses reaching it by centripetal nerve-fibres; or does it
originate efferent nervous impulses independently of excita-
tion through afferent nerves?

We know, in the first place, that the respiratory centre is

392 ■ THE HUMAN BODY.

largely tinder reflex control; ji dash of coldwatei on the
skin, the irritation of the nasal mucous membrane by
snuff, or of the larynx by a foreign body, Avill eacli cause a
modification in the respiratory movements — along indrawn
breath, a sneeze, or a cough. But, although tluis subject
to influences reaching it by afferent nerves, tlie respiratory
centre 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 im-
pulses to the respiratory centre except such as may reach
it through cranial nerves, the resjiiratory 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,
all afferent cranial nerves be divided, so as to cut off the
respiratory centre from all possible afferent nervous im-
pulses, the regular breathing movements of the nostrils
continue. It is, therefoi'e, obvious that the activity of the
respiratory centre, however much it may be capable of
modification through sensory nerves, is essentially inde-
pendent of them; in other words the normal respiratory
movements are not reflex.

What it is that Excites the Respiratory Centre. The
thing that, above all others, influences the respiratory centre
is the greater or less venosity of the blood flowing through
it. If this blood be very rich in oxygen and comparatively
poor in carbon dioxide the inspiratory centre acts but feebly,
and the respirations are shallow. If, on the other hand, this
blood be highly venous the respiratory movements are more
rapid than normal, and forced, the extraordinary muscles of
respiration being called into play; this state of violent
labored respiration, due to deficient aeration of the blood, is
called dyspncea. IsTormal quiet breathing is eupncea. If the
blood be highly aerated, as by keeping up forced artificial
respiration for a time, all respiratory movements cease; the
highly oxygenated blood does not excite the respiratory

STIMULATIOX OF THE RESPIRATOBT CENTRE. 393

centre, tiiid the auimal therefore remains without breathing
at all for some time; tliis condition is apncea, though phy-
sicians by the word apno^a commonly mean merely extreme
dyspnoea. If an animal be made apnoeic and the artificial
respiration stopped, its blood, during the cessation of the
respiratory movements, gradually losing oxygen and receiv-
ing carbon dioxide, passes into the state of ordinary blood
and again stimulates the respiratory centre, and the breath-
ing movements then recommence.

How it is that highly venous blood causes great excita-
tion of the respiratory centre, and highly arterial cessation
of its activity, is not certainly known; but we may make the
following provisional hypothesis. The chemical changes
occurring in the. respiratory centre give rise to a substance
or substances which stimulate its nerve-cells. When the
blood is richly supplied with oxygen this substance is oxi-
dized and removed as fast as it is formed, and so the centre
is not excited. When the blood, on the other hand, is un-
usually poor in ox3'gen, this stimulating body accumulates
and the respiratory discharges become more powerful.
Under normal circumstances the blood oxygen is not kept
quite up to the point of entirely removing this exciting
substance, and the centre is stimulated so far as to pro-
duce the natural breathing movements but not the more
forced ones of dyspna3a. That the stimulating cause, what-
ever it is, acts upon the respiratory centre and not upon the
various organs of the Body and through their sensory
nerves, in turn, upon the medulla, is proved not only by the
facts above cited showing that the resjjiratory centre con-
tinues to act when all afferent nerves are cut off from it,
but also by experiments which show that the circulation of
venous blood through the body of an animal, while at the
same time its respiratory centre is supplied with arterial
blood, does not produce dyspnoea; while sending venous
blood to the medulla and arterial to all the rest of the Body
does cause dyspncea.

Why are the Respiratory Discharges Rhythmic ? Every
complete resj)iratory act consists of an inspiration, an ex2)ira-
tion and a pause; and then follows the inspiration of the

394 THE HUMAN BOD I*.

next act. In natural quiet breathing there is no essential
difference between the expiration and the pause. The in-
spiration is the only active part (p. 303); the expiration and
the pause are dependent on muscular inactivity and, there-
fore, on the cessation of the discharge of nervous impulses
from the resjjiratory centre. But then, we may ask, if in
accordance with the hypothesis made in the last paragrai)h,
the respiratory centre is constantly being excited, why is it
not always discharging? why does it only send out nervous
impulses at intervals? This question, which is essentially the
same as that why the heart beats rhythmically, belongs to
the higher regions of Ph}' siology and can only at present be
hypothetically answered. Let us consider, for a moment,
ordinary mechanical circumstances under which a steady
supply is turned into an intermittent discharge. Suppose
a tube closed water-tight below by a hinged plate, which is
kept shut by a spring. If a steady stream of water is poured
into the tube from above, the water will rise until its weight
is able to overcome the pressure of the spring, and the
plate will then be forced down and some water flow out.
Tlie spring will then press the plate up again, and the water
accumulate until its weight again forces open the bottom of
the tube, and there is another outrush; and so on. By
opposing a certain resistance to the exit we could thus
turn a steady inflow into a rhythmic outflow. Or, take the
case of a tube with one end immersed in water and a steady
stream of air sent into its other end. The air will emerge
from the immersed end, not in a steady current, but in a
series of bubbles. Its pressure in the tube must rise
until it is able to overcome the cohesive force of the water,
and then a bubble bursts forth; after this the air has again
to get up the requisite pressure in the tube before another
bubble is ejected; and so the continuous suj^ply is trans^
formed into an intermittent delivery. Physiologists sup-
pose something of the same kind to occur in the respira-
tory centre. Its nerve-cells are always, under usual
circumstances, being excited; but, to discharge a nervous
impulse along the efferent respiratory nerves, they have to
overcome a certain resistance. The nervous impulses have

CAUSE OF THE RESPIRATOIiT RHYTHM. 395

to accumulate, or ''gain a head," before they travel out
from the centre, and, after their discharge, time is required
to attain once more the necessary level of irruption before a
fresh innervation is sent to the muscles. This method of
accounting for the respiratory rhythm is known as the
"resistance theory " If not altogether satisfactory it is at
least far preferable to the older mode of considering the
question solved by assuming a rhythmic character or prop-
erty of the respiratory centre. It gives a definite hypothe-
sis, which accords with what is known of general natural
laws outside of the Body, and the truth or falsity of which
can be tested by experiment: and so serves very well to
show how scientific differs from pre-scientific, or mediseval,
physiology. The latter was content with observing things
in the Body and considered it explained a phenomenon
when it gave it a ijame. Now we call a phenomenon ex-
plained, when we have found to what general catego^'y
of natural laws it can be reduced as a special example;
and this reducing a special case to a particular manifesta-
tion of some one or more general properties of matter
already known is, of course, all that we ever mean wlien we
say we explain anything. We explain the fall of an apple
and the rise of the tides by referring them to the class of
general results of the Law of Gravitation; but the why of
the law of gravitation we do not know at all; it is merely a
fact which we have found out. So with regard to Physi-
ology; we are working scientifically when we try to reduce
the activities of the living Body to special instances of
mechanical, physical, or chemical laws otherwise known to
us, and unscientifically when we lose sight of that aim.
Cerfeain vital phenomena, as those of blood-pressure, we can
thus explain, as much as we can explain anything; others, as
the rhythm of the respiratory movements, we can provision-
ally explain, although not yet certain that our explana-
tion is the right one; and still others, as the phenomena of
consciousness, jwo_cannot explain at all, and possibly _never
will, by referring them to general properties of matter,
since they may be properties only of that particular kind-of

306 THE HUMAX BODY.

matter called protoplasm, and i)erlia])s only of some varie-
ties of it.

The Relation of the Pneumogastric Nerves to tlie Res-
piratory Centre, ^^'e have iiexi lo CDiisidcr if any })lieno-
mena presented by the living Body give support to the
resistance theory of the res})iratoi"y rhythm. A very ini-
porti^nt collateral proj) to it is given by the relation of the
pneumogastric nerves to the rate and force of the respira-
tory movements. These nerves give branches to the larynx,
the windpipe, and the lungs, in addition to numerous other
parts, and might therefore be suspected to have something
to do with breathing. That they are not concerned in in-
fluencing the resjiiratory muscles directly is shown by the
fact that all of these muscles (except certain small ones in
the larynx) contract as usual in breathing after both
pneumogastric nerves have been divided. Still, the section
of both nerves has a considerable influence on the respira-
tory movements; i\iQj\)eGOXi\e sloiver and deeper. We may
understand this by supjjosing that the resistance to the dis-
charges of the respiratory centre is liable to variation. It
may be increased, and then the discharges will be fewer and
larger; or diminished, and then they will be more frequent
but each one less powerful. If the spring, in the illustra-
tion used in the preceding paragrajih, be made stronger,
while the inflow of water to the tube remains the same, the
outflows will be less frequent but each one greater; and vice
verna. The effect of section of the pneumogastric trunk ^
may, therefore, be explained if we suppose that, normally, it
carries up, from its lung branches, nervous impulses which
diminish the resistance to the discharges of the respiratory
centre; when the nerves are cut these helping impul:;es are
lost to the centre, and its impulses must gather more head
before they break out, but will be greater when they do.
This view is confirmed by the fact that stimulation of the
central ends of the divided pneumogastrics, if weak, brings
back the respirations to their normal rate and force; if
strcaiger makes them more rapid and shallower; and when
stronger still, abolishes the respiratory rhythm altogether,
with the inspiratory muscles in a steady state of feeble con-

THE EXPTRArORY CENTRE. 397

traction. That is to say, tlie resistance to the discharges of
the centre being entirely taken away (which is equivalent
to tlie total removal of the spring m our example), the cen-
tre sends out uninterrupted and non-rhythmic stimuli to
tlio ins])iratory muscles.

The pneumogastric nerve gives two branches to the larynx:
known respectively as the superior and inferior {recurrent)
laryngeal nerves; the action of these on the respiratory
centre is opposite to that of the fibres from the lungs
coming up in the main pneumogastric trunk. If the
superior laryngeal branch be divided and its central end
stimulated, the respirations become less frequent but each
one more powerful; hence this nerve is supposed to increase
the resistance to the discharges of the respiratory centre.
The same, but to a less degree, is true of the inferior laryn-
geal branch.

The Expiratory Centre. Hitherto we have considered
Ijreathing as due to the rhythmically alternating activity
and rest of an inspiratory centre — and such is the case in
normal quiet breathing, in which the expirations are pas-
sive. But in dyspnoea expiration is a muscular act, and
so there must be a section of the respiratory centre control-
ling the expiratory muscles. This part of the respiratory
centre, howe^'er, is less irritable than the inspiratory part,
and hence when the blood is in a normal state of aeration
never gets stimulated up to the discharging ]3oint. In dysp-
ntea the stimulus becomes sufficient to cause it also to
discharge, but only after the more irritable inspiratory
centre; hence the expiration follows the inspiration. This
alternation of activity is, moreover, promoted by the fact
that the pneumogastric nerve-fibres coming ^^.^ from the
lungs are of two kinds. The predominant sort are those
already referred to, which diminish the resistance to dis-
charge of the inspiratory centre, and perhaps also increase
the resistance to the expiratory discharge. This set is ex-
cited when the lungs diminish in bulk, as in expiration;
and when the whole nerve is stimulated electrically they
usually get the better of the other set, which carry up to
the medulla impulses which increase the resistance to in-

398 THE Hi' MAX BODY.

spiratory discliarges and diminish that to expiratory, and
are stimulated when the lungs expand. Hence, every ex-
pansion of the lungs (inspiration) tends to promote an
expiration, and every collapse of the lungs (expiration) tends
to produce an inspiration; and so, through the pneumo-
gastric nerves, the respiratory mechanism is largely self-
regulating.

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 con-
taining no oxygen; or by putting an animal in a vacuum;
or by making it breathe air containing a gas which has a
stronger affinity for haemoglobin than oxygen has, and
Avhich, 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 round by the
haemoglobin of the red corpuscles; that amount dissolved
in the blood plasma being insufficient for its needs. Of
such gases carbon monoxide is the most important and best
studied; in the favorite French mode of committing suicide
by stopping up all the ventilation holes of a room and
burning charcoal in it, it is poisoning by carbon monoxide
which causes death.

The Relations of Carbon Monoxide to Hsemoglobin.
If aerated whipped blood, or a solution of oxyhemoglobin,
be exposed to a gaseous mixture containing carbon mon-
oxide, the liquid will absorb the latter gas and give off
oxygen. The amount of carbon monoxide taken up will
(apart from a small amount dissolved in the plasma) be inde-
pendent of the partial pressure of ihat gas in the gaseous
mixture to which the blood is exposed; the quantity absorbed
depends on the quantity of haemoglobin in the liquid,
and is replaced by an equal volume of oxygen liberated.
This equivalence of volume, of itself, proves that the phe-
nomenon is due to the chemical rej)lacement of oxygen
in some compound, by the carbon monoxide; for if the
carbon monoxide were merely dissolved in the liquid in

CABBOX-MOyOXIDE HAEMOGLOBIN. 399

proportion to its partial pressure on the surface, it would
turn out no oxygen; the quantity of dissolved gases held
by a liquid being dependent only on the partial pressure of
each individual gas on its surface, and unaffected by
that of all others. During the taking up of carbon
monoxide the blood changes color in a way that can be
recognized by a practiced eye; it becomes cherry red instead
of scarlet. This shows that some new chemical compound
has been formed in it; examination with the spectroscope
confi]-ms this, and shows the color change to be due to the
formation of carbon-monoxide ha?moglol)in which has a
different color from oxyhsemogiobin. A dilute solution
of reduced hsemogiobin absorbs all the rays of light in one
region about the green of the solar spectrum (see Physics),
and so produces there a dark band; a thin layer of the blood
of an asphyxiated animal does the same. Dilute solution
of oxyhemoglobin absorbs the rays in two narrow regions
of the solar spectrum at the confines of the yellow
and green, and arterial blood does the same. Dilute
solution of carbon-monoxide haemoglobin, or blood which
has been exposed to this gas, also absorbs the light in two
narrow bands of the solar spectrum; but these are nearer
the blue end of the spectrum than the absorption bands
of oxyha3moglobin. Pure blood serum saturated with oxy-
gen gas or with carbon monoxide does not specially absorb
any part of the spectrum; therefore the absorjitions when
haemoglobin is present, must be due to chemical compounds
of those gases with that body.

Since carbon-monoxide-hremoglobin has a bright red color,
we find in tlie Bodies of persons poisoned by that gas, the
blood all through the Body cherry red; the tissues being
unable to take carbon monoxide from haemoglobin in the sys-
temic circulation. Hence the curious fact that, while death
is really due to asphyxia, the blood is almost the color of
arterial blood, instead of very dark purple, as in ordinary
cases of death by suffocation. Experiments with animals
show that in poisoning by carbon monoxide persistent ex-
posure of the blood to oxygen, by means of artificial respi-
ration, will cause the poisonous gas to be slowly replaced

400 THE HmrAN BODY.

again by oxygen; hence if the heart has not yet quite
stopped beating, artificial respiration, kept up patiently,
should be emi)loyed for the restoration of i^ersous poisoned
l)y carlion monoxide.

The Phenomena of Asphyxia. As soon as the oxygen
in the blood falls below the normal amount the breathing
becomes hurried and deeper, and tlie extraordinary muscles
of respiration are called into activity. The dyspnoea Ijc-
comes 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 contrac-
tions of the expiratory muscles are more violent than those
of the inspiratory. Thus undue want of oxygen at first
merely brings about an increased activity of the respiratory
centre, and especially of its expiratory division which is not
excited in normal breathing. Then it stimulates other jior-
tions (the convuUive centre) of the medulla oblongata also,
and gives rise to violent and irregular muscular spasms.
That the convulsions are due to excitation of nerve-centres
in the medulla (;ind not, as might be supposed, to poisoning
of the muscles by the extremely venous blood) is shown by
the facts (1) that they do not occur in the trunk of an animal
when the spinal cord has been divided in the neck so as to
cut off the muscles from the medulla; and (2) that they still
occur if (the si)inal cord remaining undivided) all the
parts of the brain in front of the medulla have been re-
moved.

The violent excitation of the nerve-centres 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 gradu-
ally cease, and the animal becomes calm again, save for an
occasional act of breathing when the oxygen want becomes
so gi-eat as to cause efficient stimulation even of the dying
respiratory centre: these final movements are inspira-
tions and, becoming less and less frequent, at last cease.

ASPHYXIA. 401

and the animal appears dead. If, however, its chest be
opened the heart will be found gorged with extremely dark
venous blood and making its last few slow feeble pulsations.
So long as it beats artificial respiration can restore the ani-
mal, but once the heart has finally stopped restoration is
impossible. There are thus three distinguishable stages
in death from asphyxia. (1) The stage of dyspna?a. (2)
The stage of convulsions. (3) The stage of exhaustion; the
convulsions having ceased but there being from time to
time an inspiration. 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 oxygen of course all the stages
take, longer.

Circulatory Changes in Asphyxia. During death by
suffocation characteristic changes occur in the working of
the heart and blood-vessels. The heart at first beats
quicker, btit very soon, before the end of the dyspnceic stage,
more slowly, though, at first, more powerftilly. This slowing
is due to the fact that the itnusual want of oxygen leads to
stimulation of the cardio-inhibitory centre in tlie medulla
(p. 250) and this, through the pneumogastric nerves, slows
the heart's beat. Soon, however, the want of oxygen affects
the heart itself and it begins to beat more feebly, and also
more slowly, from exhaustion, until its final stoppage.
During the second and third stages the heart and the venge
cavfB become greatly overfilled with blood, because the
violent mttscular contractions facilitate the flow of blood
to the heart, while its beats become too feeble to send it
oitt again. The overfilling is most marked on the right
side of the heart which receives the venous blood from the
Body generally.

During the first and second stages of asphyxia arterial
pressure rises in a marked degree. This is due to excitation
of the vaso-motor centre (p. 254) by the venous blood, and
the consequent constriction of the nmscular coats of the
arteries and increase of the peripheral resistance. In the
third stage the blood-pressure falls very rapidly, becaitse
the feebly acting heart then fails to keep the arteries

402 THE HUMA^ BODY.

tense, even although their diminished cahbre greatly slows
the rate at which they empty themselves into the capilla-
ries.

Another medullary centre unduly excited during asphyxia
IS that from which j)roceed the nerve-fibres governing
(hose muscular fibres of the eye which enlarge the pupil.
During suffocation, therefore, the pupils become widely
dilated. At the same time all reflex irritability is lost, and
touching the eyeball causes no wink; the reflex centres all
over the Body being rendered, through want of oxygen, in-
capable of activity. The same is true of the higher nerve-
centres; unconsciousness comes on during the convulsive
stage, whicli, liorrible as it looks, is unattended with suffer-
ing.

Modified Bespiratory Movements. SigMng is a deep
long-drawn inspiration followed by a shorter but corresj^ond-
ingly large expiration. Yuuming 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. Hic-
cough depends upon a sudden contraction of the dia])hragm,
while the aperture of the larynx closes; the entering air,
drawn through the narrowing opening, causes the peculiar
sound. Coug]ii7ig 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 sud-
denly 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 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, bat 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. XXXVI.) are set in vibration. Crying is, phy-
siologically, much like laughing and, as we all know, one
often jjasses into the other. The accomjjanyiug contrac-

MODIFIED RmPIRATORT MOVEMENTS. 403

tions of the face muscles giving expression to the counten-
ance are, however, different in the two.

All these modified resjiiratory acts are essentially reflex,
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 XXVII.

THE KIDNEYS AND SKIN.

General Arrangement of the Urinary Organs. These
consist of (1) the Tculneys, the ghmds which secrete the
urine; (2) the ureters or ducts of the kidneys, which carry
their secretion to (3) the urinary bladder, a reservoir in
Avhich 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
shown in the figure opposite. The kidneys, i?, 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 outer and a concave inner border, and its
upper end a little larger than the lower. From the
abdominal aorta. A, a renal artery, Ar, enters the inner
border of each kidney, to break up within it into finer
branches, ultimately ending in capillaries. The blood is
collected from these into the renal veins, Vr, one of which
leaves each kidney and opens into the inferior vena cava,
Vc. From the concave border of each kidney proceeds
also the tcreter, 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 T^all 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

THE RENAL OBGANS.

405

Fig. 114.— The renal organs, viewed from behind. R, right kidney; A, aorta;
Ar, right renal artery; Vc, inferior vena cava; TV, right renal vein; U, i-ight
ureter; Fm, bladder; C/a, commencement of ui'ethra.

406 THE HUMAN BODY.

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. The commencement of the urethra being kept
closed by clastic tissue around it (which can voluntarily be
reinforced by muscles which compress the tube) the urine
accumulates in the bladder. When this latter contracts and
presses on its contents, the ureters are closed in the way
above indicated, the elastic fibres closing the urethral exit
from the bladder are overcome, and the liquid forced
out.

Naked Eye Structure of the Kidneys. These organs
have externally a red-brown color, which can be seen
through the transparent capsule of peritoneum which en-
velops them. When a section is carried through a kidney
from its outer to its inner border (Fig. 115) it is seen that
a deep fissure, the hilus, leads into the latter. In theJiilus
the ureter widens out to form the ^;eZy/.s, 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 portio7i, 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 conical portions, the pyramids of MalpigJii,
2', each of which is separated from its neighbors Ijy an in-
ward prolongation,*, of the cortical substance. This, how-
ever, does not reach to the inner end of the pyramid,
which projects, as the papilla, into a calyx of the ureter.
At its outer end each pyramid separates into smaller i^or-
tions, the pyramids of Ferrein, 2", separated by thin layers
of cortex and gradually spreading everywhere into the lat-
ter. The cortical substance is redder and more granular
looking and less shiny than the medullary, and forms every-
where the outer layer of the organ next its capsule, besides
dipping in between the j^yramids 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

HISTOLOGY OF THE KIDyETS.

401

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 compound tubular glands, composed essentially of

Fig. 115— Section througrh the li-htkidnev from its-outer to its inner border
1. cortex; 2, medulla; e. pyramirt of Blalpighi: 2'. pyramid of Ferrein: ^- small
branches of the renal artery entering between the pyramids; A, a branch or the
renal artery ; C, the pelvis of the kidney ; U, ureter.

branched microscopic iiriniferous tubules, lined by epithe-
lium. 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. There they have a diameter of about

408 THE HUMAN BODY.

0.125 mm. (^-^ inch). Running into the i^jTamid from
this point each tuhule divides several times. At first the
hranclies are smaller than the main tube; but as soon as
they have come down to about 0.04 mm. (-g^ inch) this
diminution in size ceases, and the division continuing
while the tubules retain the same diameter, the pyramid
thus gets, in part, its conical form. Ultimately each branch
runs somewhere out of the pyramid, either from its base or
side, into the cortex and there dilates and is twisted. It
then narrows and doubles back again into the pyramid and
runs as a straight tube towards the papilla, but before reach-
ing it makes a loop, and turns back again as a straight tube
to the base of the pp'amid;, where it once more enters the
cortex, dilates and becomes contorted, and then ends in
a sioherical capsule, containing a tuft of small blood-vessels.
Or, followed the other way, each tubule commences in the
cortex with a globular dilatation, the MaljngMan capsule.
From this it continues as a convoluted tubule in the
cortex; this passes into a pyramid, becomes straight, and
runs on as the descending limb of a loop of Henle. Turn-
ing at the loop, it continues as its ascending limb, and this
passes out again into the cortex and becomes the convoluted
junctional tubule, which passes as a straight collecting
tubule into the pyramid and there joins others to form an
excretory tnbule which opens on the papilla. Throughout
its course the tubule is lined by a single layer of epithelium
cells differing in character in its different sections. All the
tubes are bound together by connective tissue and blood-
vessels to form the gland.

The Blood-Plow through the Kidney. 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, are continued as the afferent vessels of Malpi-
ghian capsules, the walls of which are doubled in before them
(Fig. 116); there each breaks up into a little knot of
capillary vessels called the glomerulus, from which ulti-
mately an efferent vessel proceeds, and outside the capsule
this breaks up into a close capillary network among the con-
voluted tubes. From the capillaries the blood is collected

THE RENAL SECRETION.

409

into the renal vein. Most of the blood flowing through the
kidney thus goes through two sets of capillaries; one in the
capsules, and a second formed
by the breaking uj) of the
efferent vein of the latter.
The capillary network in the
pyramids is much less close
than that in the cortex, which
gives reason to suspect that
most of the secretory work of
the kidneys is done in the cap-
sules and convoluted tubules.
The pyramidal blood flows
only through one set of capil-
laries, there being no glome-
ruli in the kidney medulla.

The Renal Secretion. The
amount of this carried off
from the Body in 24 hours is
subject to considerable varia-
tion, being especially dimin-
ished by anything which pro-
motes perspiration, and increased by conditions, as cold to
the surface, which diminish the skin excretion, its average
daily quantity varies from 1200 to 1750 cub. cent. (40
to 60 fluid ounces). The urine is a clear amber-colored
liquid, of a slightly acid reaction; its specific gravity is
about 1040, being higher when the total quantity excreted
is small than when it is greater, since the amount of solids
dissolved in 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.

Normal urine consists, in 1000 parts, of about 9G0 water
and 40 solids. The' latter are mainly crystalline nitro-
genous bodies {iirea and tiric acid), but small quantities of
pigments and of non- nitrogenous organic bodies are also
present, and a considerable quantity of mineral salts. The
following table gives approximately, in the first column, the

Fig. 116.— The termination of a
uriniferous tubule, with its glomeru-
lus, a. the glomerulus or Malphighian
corpuscle; 6. the convoluted ending
of the tubule :d, its lining epithelium;
/, the afferent blood-vessel of the
glomerulus; (/, the efferent vessel ; c,
h, the blood-vessels forming the tuft
in the glomerulus.

410

THE HUMAN BODY.

average composition of the urine excreted in twenty-four
hours expressed in grams; in the second column the same
expressed in grains. The third column gives the composi-
tion of 1000 parts of urine.

The solids consisls of —

Urea (CN2H4O)

Uric acid (C5H4N4O:0.

Hippuric acid ^. .

Kreatinin T . .

Pigments and fats. . . .

Sulphiu'ic acid

Pliosplioric acid

Chlorine

Ammonia

Potassium

Sodium

Calcium

Magnesium

Urine in 24 hours.

1500 grams.

23,250 grains.

In'lOOO parts.

Water

Solids

142S.00
73.00

23,134.00
1116.00

952.00

48 00

33.00
0.50
0.40
1.00

10.00
2.00
3.00
7.00
0.75
2.50

11.00
0.25
0.20

511.50

7.75

6.20

15.50

155.00

31.00

46.50

108.50

13.00

38.75

170.50

3.80

3.00

71.60

1110.00

22.00
0.83
0.27

10.33
6.66
1.33
2.00
4.70
0.50
1.70
7.33
0.16
0.13

47.44

The urine, however, even in health is subject to consid-
erable variation in composition; not only as regards the
amount of water in it, but also in its solid constituents;
the latter are especially modified by the quantity and nature
of the food taken.

100 volumes of urine contain in solution about 14 vol-
umes of gas, consisting of about 13 of carbon dioxide, 1
of nitrogen, and mere traces of oxygen.

Mechanism of the Benal Secretion. The kidneys, as
secreting organs, consist of two distinct parts; (1) the glome-
ruli through which a filtration of water, probably with salts
in solution, takes 2:)lace; and (2) an actively secretory appa-
ratus, formed by all parts of the urinif erous tubules between
their terminal capsules and the collecting tubes. Accord-
inglv, we find in the urine bodies, as water and common salt,
which already exist in the blood and can be removed from it

SOURCES OF UREA. 411

merely by dialysis or filtration ; and others (the specific ele-
ments of the secretion), especially urea, which are selected
or made by a special activity of the renal gland-cells. The
total quantity of the twenty-four hours' urine thus depends
largely on the pressure in the renal arteries, since the
higlier this is the greater will the amount of filtered liquid
be. Under ordinary pressures such substances as albumen
will not filter, but they do under higher; accordingly in
healthy conditions none of the albumen of the blood jjlasma
passes into the urine, but if the pressure in the capillaries of
the glomeruli is considerably raised it does; its presence in
the urine being the most prominent symjotom of that danger-
ous class of maladies grouped together under the name of
Briglifs disease. Filtration in the glomeruli is largely pro-
moted by the fact that the calibre of the efferent vessel of each
is rather less than that of the afferent; and thus the pressure
m the thin-walled vessels of the vascular tuft is raised.

The Role of the Renal Epithelium. Water and salines
being passed out mainly through the glomeruli, we have
now to consider what part the secreting cells of the kidney
play; and especially as regards urea, the most important
constituent of the urine. Urea represents the final state
in which the proteids taken into the Body from the alimen-
tary canal (or at- least their nitrogen) leave, after having
3delded up, by chemical changes, a certain amount of energy.
In this process the proteids are oxidized and broken down
into carbon dioxide and water and urea; and the kidneys
get rid of the latter.

Since the life and activity of every tissue is accompanied
by a breaking down of proteids (though not necessarily at
once into urea, as many intermediate stages may, and doubt-
less do, occur in different tissues), there is no doubt that the
main chemical degradation of albuminous compounds takes
place outside the kidneys. Whether the final steps by
which urea is formed occur in those organs or elsewhere is
not yet certainly known. According to one view the urea is
carried to the kidney m the blood of the renal artery, and
there merely picked up and passed on by the excreting cells;
while, according to another, not urea, but the penul-

412 THE HUMAN BODY.

timate products of protcid degradiition from which urea
is made, are carried to the kidneys, and tlie final formation
of urea occurs in tliese organs. The results of hlood analy-
sis are conflicting, but on the whole it seems proved that
more urea exists in renal-artery blood than in renal-vein
blood, which indicates that urea is not made in the kidneys.
In death, too, from suppression of the renal reaction, urea
is found to accumulate in the blood which would not be the
case unless it were normally formed elsewhere and car-
ried off by the kidneys. The whole urea question, which
is one of great importance, will l)o more fully considered in
Chapter XXVIII., in connection with the chemistry of
nutrition in general.

The Skin» which covers the whole exterior of the Body,
consists everywhere of two distinct layers; an outer, the cuticle
or ejjulet'rnis, 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 develoj)ed
parts of the epidermis.

The Epidermis, Fig. 117, 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 strata of
roundish cells, h, which in the outer layers become more
and more flattened in a plane parallel to the surface. The
outermost epidermic stratum is comi)osed of many layers of
extremely flattened cells from which the nuclei (conspicu-
ous in the deeper layers) have disappeared. These super-
ficial 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 outwards is accompanied l)y
chemical changes, and they finally constitute a semitrans-
parent dry lior^iy stratmn, a, distinct from the deeper, more
ojiaque and softer Ifalpighian or mucoiis layer, h and
d, of the epidermis. The cells of this latter are soluble in
acetic acid; those of the horny stratum, not.

THE EPIDERMIS.

413

The rolls of material which are peeled off the skin in the
" shampooing" 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.

Fig. 117.— a section through the epidermis, somewhat diagrammatic, highly
magnified. Below is seen a papilla of the dermis, with its artery, /, and veins.
Of/; a, the horny layer of the epidermis; b, the rete mucosum or Malpighian
layer; d, the layer of columnar ei)idermic cells in immediate contact with the
dermis; A, the duct of a sweat-glaud.

In dark races the color of the skin depends mainly on
minute pigment granules lying in the deeper cells of the
Malpighian layer.

414 THE HUMAN BODY.

H^o blood-vessels or lymphatics enter the epidermis, which
is entirely nourished by matters derived from the subjacent
corium. Fine nerve-fibres run into it and end there
among the cells, in various ways.

The Corium, Cutis Vera, or True Skin, Fig. 118, con-
sists fundamentally of a close feltwork of elastic and white
fibrous tissue, which, becoming wider nie.'shed below, passes

.'<«

L.^

...d

Fig. 118. — A sectiuu through the skin and subcutaneous areolar tissue, a, norny
stratum, and 6, Malpighian layer of the epidermis; c, dermis, passing below
into, d, loose areolar tissue, with fat,/, in its meslies: above, dermic papillae are
seen, projecting into the epidermis which is moulded on them, i, opening of a
sweat-gland ; /i, duct of ditto ; g, the gland itself.

gradually into the subcutaneous areolar tissue (p. 102)
which attaches the skin loosely to parts beneath. In
tanning it is the cutis vera which is turned into leather, its
white fibrous tissue forming an insoluble and tough com-
pound with the tannin of the oak-bark employed. Wherever
there are hairs, bundles of plain muscular tissue are found
in the corium; it contains also a close capillary network

HAIRS. 415

and numerous lymphatics and neryes. In shaving, so
long as the razor keejss in the epidermis there is no bleed-
ing; but a deeper cut shows at once the vascularity of the
true skin.

The outer surface of the corium is almost everywhere
raised mto minute elevations, called the papillm, on which
the epidermis is moulded, so that its deep side presents pits
corresponding to the projections of the dermis. In Fig. 117
is a papilla of tho, 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 papillse
contain no capillary loops but special organs connected
with nerve-fibres, and supposed to be concerned in the sense
of touch. On the palmar surface of the hand the dermic
papillae are especially well developed (as they are in most
parts where the sense of touch is acute) and are frequently
comjjoioid or branched at the tip. On the front of the
hand, they are arranged in rows; the epidermis fills up the
hollows between the papillop, of the same row, but dips down
between adjacent rows, and thus are produced the epidermic
ridges seen on the palms. In many places the corium is
furrowed, as opposite the finger-joints and on the palm.
Elsewhere such furrows are commonly less marked, but
they exist over the whole skin. The epidermis closely
follows all the hollows, and thus they are made visible from
tlie surface. The wrinkles of old persons are due to the
absorption of subcutaneous fat and of other soft parts
beneath the skin, which, not shrinking itself at the same
rate, becomes thrown into folds.

Hairs. Each hair is a long filament of epidermis devel-
oped on the top of a special dermic papi,lla, seated at the
bottom of a depression reaching down from the skin into the
tissue beneath and called the Mir follicle. The portion of
a hair buried m the skin is called its root, this is succeeded
by a stem which, in an uncut hair, tapers off to apoint. The
stem is covered by a single layer of overlapping scales form-
ing the Jiair cuticle; the projecting edges of these scales
are directed towards the top of the hair. Beneath the hair
cuticle comes the cortex, made up of greatly elongated cells

416

THE HUMAN BODY.

united to form fibres; and in the centre of the shaft there
is found, in many hairs, a meduUa, ni.ide np of more or less
rounded cells. The color of hair is mainly dependent upon
pigment granules lying lietween the fibres of the cortex.
All hairs contain some air cavities, especially in the medulla.
They are very abundant in white hairs and cause the white-
ness 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. 119) is a narrow pit of the dermis,
projecting down into the subcutaneous areolar tissue, and
lined by an involution of the epidermis. At the bottom of the

follicle is a j^apilla and tlie
3, ^.^ •• ^.^„' . epidermis, turning up over
this, becomes continuous
with the hair. On the
papilla epidermic cells
multi})ly rapidly so long as
the hair is growing, and
the whole hair is there
made up of roundish cells.

Firr. 119.— Parts of two hairs imbedded As thcSC get pushed Up by
in their follicles, o, the skin, which is seen « -i „,„^ ^„ „n v^ „„j-i

to dip down and line tiie follicle; 6, the trcsh oucs tomied beneath

subontaneous tissue; c, the muscles of the ^-i,„„-, thp nntprmnsf livor
hair follicle, which by their contraction inem, tlie OLlieimoSL laytr

can erect the hair. become flattened and form

the hair cuticle; several succeeding layers elongate and
form the cortex; while, in hairs with a medulla, the middle
cells retain pretty much their original form and size.
Pulled apart by tlie 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 overlapping edges turned down-
wards. Accordingly these interlock with the upward
directed edges of the cells of the hair cuticle; consequently
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 a new hair will be formed, but not otherwise.
Slender bundles of unstriped muscle {c, Fig. 119) run from

NAILS. 417

the dermis to the side of the hair follicles. The latter are
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, tlioy 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 glands (p. 418). Hairs arc
found on all regions of the skin except 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 yisible part consists of a iody, fixed to the dermis
beneath (which forms the led 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 yascular.

The portion of the corium on which the nail is formed
is called its matrix. Behind, 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, is highly vascular and
raised up into papillfe which, except in the region of the
lunula, 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 i)ushed 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, ortornoif, a new one is produced,
provided the matrix is left.

The Glands of the Skin are of two kinds, the sudori-
jjarous or sweat glands, and the sebaceous or oil glands.
The former belong to the tubular, the latter to the race-
mose type. The sweat-glands, Fig. 120, lie in the subcu-

418

THE HUMAN BODY.

taneons tissue, where they form little globular masses com-
posed of a coiled tube. From the coil a duct (sometimes
double) leads to the surface, being
usually spirally coiled as it passes
througli the epidermis. The secret-
ing part of the gland consists of a
connective-tissue tube, continuous
along the duct with the dermis;
Avithin this is a basement membrane^
and the final secretory lining consists
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
the brow, than elsewhere. There arc
altogether about two and a half millions
of them opening on the surface of the
Body.

The sebaceous glands nearly always
open into hair follicles, and are found
wherever there are hairs. Each con-
sists of a duct opening near the mouth of a hair follicle and
branching at its other end : the final branches lead into globu-
lar 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 disappearing;
and finally the whole cell falls to pieces, its detritus constitut-
ing the secretion. New cells are, meanwhile, formed to take
the place of those destroyed. Usually two glands are con-
nected with each hair follicle, but there may be three or
only one. A jiair of sebaceous glands are represented on
the sides of each of the hair follicles in Fig. 119.

The Skin Secretions. The skin besides forming a pro-
tective covering and serving as a sense-organ (Chap.
XXXIV.) also plays an important part in regulating the
temperature of the Body, and, as an excretory organ, in
carrying off certain waste products from it.

Fig. 120.— a sweat
gland, d, horny layer of
cuticle; c, Malpighian
layer: h. dermis. The
coils of the gland proper,
imbedded in the suljcu-
taneous f at,areseenbelow
the dermis.

PERSPIRATIOX. . 419

The sweat poured out by tlie sudoriparous glands is a
transparent colorless liquid, with a peculiar odor, yaryiug
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
'lOQO grams (10,850 and 31,000 grains). The amount is in-
fluenced mainly by the surrounding temperature, being
greater when this is high; but it is also increased 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
insensiUe, in the latter as sensible jperspiration. By far
the most passes off in the insensible form, drops of sweat
onh' accumulating when the secretion is very profuse, or the
surrounding atmosphere so humid that it does not readily
take up more moisture. The perspiration is acid, and in
1000 parts contains 990 of water to 10 of solids. Among
the latter are found urea (1.5 in 1000), fatty acids, sodium
chloride, and other salts. In diseased conditions of the
kidneys the urea may be greatly increased, the skin supple-
menting 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 ot
blood through the skin would suffice of itself to cause in-
creased 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.

Eecent experiments show that the secretory activity of
the sweat-glands is under the direct control of nerve-fibres,
and is only indirectly dependent on the blood-supply in
their neighborhood. 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 skin arteries di-
late and the foot gets more blood and becomes warmer;
but it does not sweat. The sweat-fibres originate in certain

430 THE HUMAN BODY.

sweat-centres in the spinal cord, which may either be di-
rectly excited by blood of a higher temperature than usual
flowing through thcni or, reflexly, by Avarmth acting on the
exterior of the Body and stimulating the sensory nerves
there. Both of these agencies commonly also excite the
vaso-dilator nerves of the sweating part, and so the increased
blood-supply goes along with the secretion; but the two
phenomena are fundamentally independent.

The Sebaceous Secretion. This is oily, semifluid, and of
a sjjecial odor. It contains about 50 per cent of fats (olein
and palmatin). It lubricates the hairs and usually renders
them glossy, even in persons who use none of the various
compounds sold as "hair-oil." No doubt, too, it gets
S})read 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
up the mouths of the sweat-glands (the so-called "pores" of
the skin) and impede their activity. Hence the value to
health of keeping the skin clean: a daily bath should be
taken by every one. Women cannot well wash their hair
daily as it takes so long to dry, but a man should immerse
his head when he takes his bath. As a general rule, soap
should only be used occasionally; it is quite unnecessary for
cleanliness, except on exposed parts of the Body, if frequent
bathing is a habit and the skin be well rubbed afterwards
until dry. Soap nearly always contains an excess of alkali
which in itself injures some skins, and, besides, is apt to com-
bine chemically with the sebaceous secretion and carry it
too freely away. Persons whose skin will not stand soap
can find a good substitute, for washing the hands and face,
in a little cornmeal. ISTo doubt many folk go about in
very good health with very little washing ; contact with
the clothes and other external objects keeps its excretions
from accumulating on the skin to any very great extent.
But apart from the duty of personal cleanliness imposed on

BATHII^G. 421

man as a social animal in daily intercourse with others,
the mere fact that the healthy Body can manage to get
along under unfavorable conditions is no reason for expos-
ing it to them. A clogged skin throws more work on the
lungs and kidneys than their fair share, and the evil con-
sequences may be experienced any day when something else
throws another extra strain on them.

Animals, a considerable portion of whose skin has been
varnished, die within a few hours. This used to be thought
due to poisoning by retained ingredients of the sweat.
But the real cause of death seems to be an excessive radia-
tion of heat from the surface of the body, which the vital
oxidative processes cannot keep up with, so the bodily tem-
perature falls until it reaches a fatal point, about 20° C.
(68° F.) for rabbits. If the animal be packed in raw cotton
or kef)t in an atmosphere at a temperature of 30° C. (86° F.)
it will not die from the varnishing.

Bathing. The general subject of bathing may be con-
sidered 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 contract all
the skin -vessels and make the surface pallid. This is soon
followed by a reaction, in which the skin becomes red and
congested, 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 again becomes pallid, and the person
probably feels cold, uncomfortable, and depressed all d[iy.
Then bathing is injurious instead of beneficial; it lowers
instead of stimulating the activities of the Body. Hov/
long a stay in the cold water may be made with benefit,
depends greatly on the individual; a vigorous man can bear
it and set uj) a healthy reaction after much longer immer-
sion than a feeble one; moreover, being used to cold bathing

433 THE HUMAN BODY.

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 indi-
cated 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 dur-
ing 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 pi-omote 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 jiurpose
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 pre-
valent to the contrary notwithstanding. On the other
hand, no one should enter a cold bath when feeling chilly,
or in a depressed vital condition. 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 ordinary cold bath and, at the same time, gives it a
greater stimulus: hence it has certain advantages.

Persons in feeble health may diminish the shock to the
system by raising the temi3erature of the water they bathe
in up to any point at which it still feels cool to the skin.
Bathing in water which feels warm is not advisable: it tends
generally to diminish the vital activity of the Body. Hence
Avarm baths should only be taken occasionally and for
special purposes.

CHAPTER XXVIII.

NUTEITION.

The Problems of Animal Nutrition. We have in pre-
ceding chapters traced certain materials, consisting of foods
more or less changed by digestion, into the Body from the
alimentary canal, and oxygen into it from the lungs. We
have also detected the elements thus taken into the Body
in their passage out of it again by lungs, kidneys and skin;
and found that for the most part their chemical state was
different from that in which they entered; the difference
being expressible in general terms by saying that more
oxidized forms of matter leave the Body than enter it.
We have now to consider what hapj)ens to each food during
the journey through the Body: is it changed at all? is it
oxidized? if so where? what products does its oxidation
give rise to? Is the oxidation direct and complete at once
or does it occur in successive steps? Has the food been
used first to make part of a living tissue and is this then
oxidized; or has it been oxidized without forming part of a
living tissue? if so, where? in the blood stream or outside
of it? Finally, if the chemical changes undergone in
the Body are such as to liberate energy, how has this energy
been utilized? to maintain the temperature of the Body or
to give rise to muscular work, or for other purposes? This
is a long string of questions, the answers to many of which
Physiology has still to seek.

The Seat of the Oxidations of the Body. According
to older views oxidation mainly took place in the blood
while flowing through the lungs. Those organs were con-
sidered a sort of furnace in which heat was liberated by
blood oxidation, and then distributed bv the circulation.

4:24 THE HUMAN BODY.

But if this were so the lungs ouglit U> be the hottest part.-
of the Body, and the blood leaving them by the pulmonale
veins much hotter than that brought to them by the pulmo-
nary artery after it had been cooled by warming all the tis-
sues; and neither of these things is true. A small amount
of heat is liberated when haemoglobin combines with oxygen
in the pulmonary capillaries, but the affinities thus satisfied
are so feeble that the energy liberated is trivial in amount
when compared with that set free when this oxygen subse-
quently forms stabler compounds elsewhei-e. It is now, more-
over, tolerably certain that hardly any of this latter class of
oxidations occurs in the living circulating blood at all; its
cells do, no doubt, use up some oxygen and set free some car-
bon dioxide; but not enough to be detected by ordinary
methods of analysis. The percentage of oxygen liberated in a
vacuum by two specimens of the blood of an animal, taken
one from an artery near the heart, and the other from a distant
one, are practically the same; showing that during the time
occupied in flowing two or three feet through an artery the
blood uses up no appreciable quantity of its own oxygen;
while in the short time occu2:)ied in its brief capillary transit
it loses so much oxygen as to become venous. The differ-
ence is explained by the fact that the blood gives off oxygen
gas through the thin capillary walls to the surrounding
tissues; and in them the oxidation takes place. As we
have already seen, a freshly excised muscle deprived of
blood can still be made to contract; and for some consider-
able time if it be the muscle of a cold-blooded animal.
During its contraction it evolves large amounts of carbon
dioxide, although the resting fresh muscle contains hardly
any of that gas. Here Ave have direct evidence of oxidation
taking place in a living tissue and in connection with its
fuuctional activity; and what is true of a muscle is prob-
ably true of all tissues; the oxidations which supply them
with energ)' take place withm the living cells themselves.
The statement frequently made that the oxygen m the cir-
culating blood exists as ozone, rests on no sufficient basis;
decomposing haemoglobin does seem to form ozone when
exposed to the air, but fresh blood yields no sign of it.

SYS TRESES IX THE BODY. 425

Experiments made by adding yarious combustible sub-
stances, as sugar, to fresh blood, also fail to prove the oc-
currence of any oxidation of such bodies in tbat liquid.

Tissu.e-Building and Energy- Yielding Foods. The
Human Body, like that of other animals, is, on the whole,
chemically destructive; it takes in highly complex sub-
stances as food, and eliminates their elements in much
simpler compounds, which can again be built up to their
original condition by plants. Nevertheless the Body has
certain constructive powers; it, at least, builds up protoplasm
from proteids and other substances received from the
exterior; and there is reason to believe it does a good deal
more of the same kind of work, though never an amount
equaling its chemical destructions. Given one single pro-
teid in its food, say egg albumen, the Body can do very
well; making serum albumen and fibrin factors out of it
for the blood, myosin for the muscles, and so on: in such
cases the original proteid must have been taken more or
less to pieces, remodeled, and built up again by the living
tissues; and it is extremely doubtful if anything different
occurs in other cases, when the proteid eaten happens to be
one found in the Body. In fact, during digestion the pro-
teids are broken down somewhat, and turned into peptones;
in this state they enter the blood and must again be built
up into proteids, either there or in the solid tissues.

The constructive powers of the Body used to be rather
too much ignored. Foods were divided into assimilahle and
combustible, the former serviug directly to renew the organs
or tissues as they were used up, or to supply materials for
growth; these were mainly proteids and fats; no special
chemical synthesis was thus supposed to take place, the
living cells being nourished by the reception from outside of
molecules similar to those they had lost. Fat-cells grew
by picking up fatty molecules, like their contents, received
from the food; and protoplasmic tissues by the reception
of ready-made proteid molecules, needing no further
manufacture in the cell. The combustible foods, on the
other hand, were the carbohydrates and some fats: these,
according to the Inqiothesis, were incapable of being made

4:■^6 THE HUMAN BODY.

into parts of a living tissue, and were simply burnt at once
in order to maintain the bodily warmth. It having been
proved, however, that more fat might accumulate in the
body of an animal than was taken in its food, this excess
was accounted for by supposing it was due to excess of com-
bustible foods, converted into fats and stored away as oil-
droplets in various cells; but not actually built up into true
living adipose tissue. Liebig, somewhat similarly, classed
all foods into plastic, concerned in making new tissues, and
respiratory, directly oxidized before they ever constituted
a tissue. The plastic foods were the proteids, but these
also indirectly gave rise to the energy expended in muscu-
lar work, and to some heat: the proteid muscular fibre
being broken first into a highly nitrogenous part (urea, or
some body well on the road to become urea) and a noii-
nitrogenized richly hydrocarbonous part; and this latter
was then oxidized and gave rise to heat. Several facts may
be urged against this view — (1) Men in tropical climates
live mainly on non-jDroteid foods, yet their chief needs are
not heat production, but tissue formation and muscular
work : according to Liebig's view their diet should be
mainly nitrogenous. (2) Carnivorous animals live on a
diet very rich in proteids, nevertheless develop plenty of
animal heat, and that without doing the excessive muscu-
lar work which, on Liebig's theory, must first be gone
through in order to break vq) the proteids, with the jiroduc-
tion of a non-azotized part which could then be oxidized for
heat-jDroduction. (3) Great muscular work can be done on
a diet poor in proteids; beasts of burden are for the most
part herbivorous. (4) Further, we know exactly how much
energy can be liberated by the oxidation of proteids to that
stage which occurs in the Body; and it is perfectly possible
to estimate pretty accurately the amount of urea and uric
acid excreted in a given time; from their sum the amount
of proteid oxidized and the amount of energy liberated in
that oxidation can be calculated; if this be done it is found
that, nearly always, the muscular work done during the
same period represents far more energy expended than could
be yielded by the proteids broken down.

SOURCE OF MUSCULAR WORK. 427

The Source of the Energy Expended in Muscular
Work. This question, which was postponed in tlie chap-
ters dealing with the muscular tissues, on account of its
importance demands here a discussion. It may be put thus :
— Does a muscle-fibre work by the oxidation of its proteids,
i.e. by breaking them down into compounds which are
then removed from it and conveyed out of the Body? or
does it work by the energy liberated by the oxidation of
carbon and hydrogen compounds only? The problem mav
be attacked in two ways; first, by examining the excretions
of a man, or other animal, during work and rest; second,
by examining directly the chemical changes produced in a
muscle when it contracts. Both methods point to the same
conclusion, viz. that proteid oxidation is not the source of
the mechanical energy expended by the Body.

One gram (15.5 grains) of pure albumen when completely
burnt liberates, as heat, an amount of energy equal to 2117
kilogrammeters (15,270 foot-pounds). But in the Body
proteids are not fully oxidized; part of their carbon is, to
form carbon dioxide, and part of the hydrogen, to form
water; but some carbon and hydrogen pass out, combined
with nitrogen and oxygen, in the incompletely oxidized
state of urea. Therefore all of the energy theoretically ob-
tainable is not derived from proteids in the Body: from
the above full amount for each gram of proteid we must
take the quantity carried off in the urea, which will be the
amount liberated when that urea is completely oxidized.
Each gram (15.5 grains) of proteid oxidized in the Body gives
^ of a gram (5.14 grains) of urea; and since one gram of
urea liberates, on oxidation, energy amounting to 934 kilo-
grammeters (6740 foot-pounds) each gram of proteid
oxidized, so far as is possible in the Body, yields during the
proccess 2117- ^^ = 1805.7 kilogrammeters (13..037 foot-
pounds) of energy. Knowing that urea carries off practi-
cally all the nitrogen of proteids broken up in the Body, and
contains 46.6 per cent of nitrogen, while proteids contain
16 per cent, it is easy to find that each gram of urea repre-
sents the decomposition of about 2.80 grams of proteid and,
therefore, the liberation of 5060.00 kilogrammeters (36,533.0

438 THE nUMAN BODY.

foot-ponnds) of cnerg}'. If, therefore, we know how much
urea a man excretes during a given time, and iiow much
mechanical work lie does during the same time, we can
readily discoyer if the latter could possibly have been done
by the energy set free by proteid decomposition. Let us
take a sj^ecial case. Fick and Wisleccnus, two German
observers, climbed the Faulhorn mountain, which is 1956
meters (about 6415 feet) high. Fick weighed 06 kilograms
and, therefore, in lifting his Body alone, did during the
ascent 129,096 kilogrammeters (932,073 foot-pounds) of
work. "Wislecenus, who weighed 76 kilograms, did similarly
148,656 kilogrameters (1,073,296 foot-pounds) of work.
But during the ascent, and for five hours afterwards, Fick
secreted urine containing urea answering only to 37.17
grams of proteid, and Wislecenus urea answering to 37
grams. Since each gram of jjroteid broken up in the Body
liberates 1805.7 kilogrammeters (13,037 foot-pounds) of
energy, the amount that Fick could possibly have obtained
from such a source is 1805.7 X 37.17 = 07,117 kilogram-
meters (484,584 foot-pounds), and Wislecenus 1805.7 X 37
= 66,810 kilogrammeters. If to the muscular work done
in actually raising their bodies, we add that done simul-
taneously by the heart and the respiratory muscles, and in
such movements of the limbs as were not actually concerned
in litiug their weight, we should have, at least, to double
the above total muscular Avork done; and the amount of
energy liberated meanwhile by proteid oxidation, becomes
utterly inadequate for its execution. It is thus clear that \
muscular work is not wholly done at the expense of the 1
oxidation of muscle proteid, and it is very probable that I
none is so done under ordinary circumstances, for the urea
excretion during rest is about as great as that during
work, if the diet remain the same; if the work is very |
violent, as in long-distance walking matches, the urea quan- 1
, tity is sometimes temporarily raised but this increase,
/ which no doubt represents an abnormal wear and tear of
muscle-fibre, is probably independent of the liberation of
energy in the form in which a muscle can use it, more
likely taking the form of heat; and is, moreover, compen-

UltEA AXD MUSCULAR WOBK. 429

sated for afterwards by a diminished urea excretion. Thus,
hourly, before the ascent Fick and Wislecenus each ex-
creted on the average about 4 grams (63 grains) of urea;
during the ascent between 7 and 8 grams (108-124 grains);
but during the subsequent 16 hours, when any urea formed
in the work would certainly have reached the urine, only
an average of about 3 grams (46. 5 grains) per hour.

It may still be objected, however, that a good deal of the
muscle work may be done by the energy of oxidized muscle
proteid; that the amount of this oxidation occurring in a
muscle during rest or ordinary work is pretty constant and
simply takes different forms in the two cases, much as a
steam-engine with its furnance in full blast will burn as
much coal when working as when resting, but in the former
case lose all the energy generated in the form of heat, and
in the latter partly as mechanical work. Thus the want of
increase in urea during muscular activity would be explained,
while still a good deal of utilizable energy might come from
proteid degradation. But if this were so, then the work-
ing Body should eliminate no more carbon dioxide than the
resting; the amount of chemical changes in its muscles
being by hypothesis the same, the carbon dioxide eliminated
should not be increased. Experiment, however, shows
that it is, and that to a very large extent, even when the
work done is quite moderate and falls within the limits
which could be performed by the normal proteid degrada-
tion of the Body. Quite easy work doubles the carbon
dioxide excreted in twenty-four hours, and in a short period
of very hard work it may rise to five times the amount
eliminated during rest. Since the urea is not increased, or but
very slightly increased, at the same time, this carbon dioxide
cannot be due to increased joroteid metamorphosis; and
it therefore indicates that a muscle works by the oxidation
of carbonaceous non-nitrogenous compounds. Since all
the carbon compounds oxidized in the Body contain hydro-
gen this element is also no doubt oxidized during muscu-
lar work; but its estimation is difficult and has not been
attempted, because the Body contains so much water ready
formed that a large -quantity is always ready for increased

430 TEE HUMAN BODY.

evaporation from the lungs and skin, whenever the respira-
tions are quickened, as they are by exercise. It, thus, is very
difficult to say how much of the extra water eliminated
from the Body during work is due merely to this cause and
how much to increased hydrogen oxidation.

The conclusion we are led to is, then, that a muscle works
hy the oxidation mainlj^ if not entirely, of carbon and
hydrogen; much as a steam-engine does: the j)roteid con-
stituents of the muscle answer roughly to the metallic parts
of the engine, to the machinery using the energy liberated
by the oxidations, but in itself only suffering wear and tear
bearing no direct proportion to the work done; as an engine
may rust, so muscle proteid may and does oxidize, but not
to supply the organ with energy for use. This conclusion,
arrived at by a study of the excretions of the whole Body, is
confirmed by the results obtained by the chemical study of
a single muscle. A fresh frog's muscle (which agrees in all
essential points with a man's) contains practically no car-
bon dioxide, yet, made to work in a vacuum, gives off that
gas, and more the more it works. Some carbon dioxide is
therefore formed in the working muscle. If the muscle,
after contracting as long as it will, be thrown into death
rigor it gives off more carbon dioxide; and if taken per-
fectly fresh and sent into rigor mortis without contracting
it gives off carbon dioxide also, in amount exactly equal to
the sum of that which it would have given off in two stages,
if first worked and then sent into rigor. The muscle must,
therefore, contain a certain store of a carbon-dioxide-yield-
ing body, and the decomjiosition of this is associated with
the occurrence both of muscular activity and death stiffen-
ing. Similar things are true of the acid simultaneously
developed ; the muscle when it works produces some sarco-
lactic acid, and when thrown into rigor mortis still more.
N"o increase of urea or kreatin or any similar product of
nitrogenous decomposition is found in a worked muscle
when compared with a rested one, but the total carbohy-
drates are rather less in the former. These facts make it
clear that muscular work is not done at the expense of
proteid oxidation; and we have already seen {]). 387) that

CHEMISTRY OF WORKING MUSCLE. 431

the oxygen a muscle uses in contracting is not taken up by
it at the time it is used, since a muscle containing no oxy-
gen will still contract and evolve carbon dioxide in a vacuum.
It is probable that the chemical phenomena occurring in con-
traction and rigor are essentially the same; the death stiffen-
ing results when they occur to an extreme degi'ee. Pro-
visionally one may explain the facts as follows — A muscle in
the Body takes up from the blood, oxygen, proteids, and
non-nitrogenous (carbohydrate or fatty) substances. These
it builds up into a highly complex and very unstable com-
pound, comparable, for example, to nitro-glycerine. When
the muscle is stimulated this falls down into simpler sub-
stances in which stronger affinities are satisfied; among
these are carbon dioxide and sarcolactic acid and a proteid
(myosin). The energy liberated is thus independent of
any simultaneous taking up of oxygen; the amount possible
depends only on how much of the decomposable body exists
in the muscle. Under natural conditions the carbon
dioxide is carried off in the blood and perhaps the sarco-
lactic acid also, the latter to be elsewhere oxidized further
to form water and more carbon dioxide. The myosin
remains in the muscle-fibre and is combined with more
oxygen, and compounds of carbon and hydrogen taken from
the blood, and built up into the unstable energy-yielding
body again; none of it, under ordinary circumstances,
leaves the muscle. If, however, the blood-supply be defi-
cient, the myosin clots (p. 125) before this restitution takes
place and it cannot then be rebuilt; and in excessive work
the same thing partially occurs, the decomposition occur-
ring faster than the recomposition; the clotted myosin is
then broken up into simpler bodies and yields a certain
mcrease of the urea excreted. In rigor mortis all the myosin
passes into the clotted state and causes the rigidity. A
working muscle takes up more oxygen from the blood than a
resting one, as is shown by a comparison of the gases of its
venous blood in the two cases; this oxygen assumption is
not necessarily proportionate to the carbon-dioxide elimina-
tion at the same time; for the latter depends on the break-
ing down of a body already accumulated in the muscle dur-

432 THE HUMAN BODY.

ing rest, and this breaking down may occur faster than the
reconstruction. We are thus enabled, also, to understand
how, during exercise, the carbon dioxide evolved from the
lungs may contain more oxygen than that taken up at the
same time; for it is largely oxygen previously stored dur-
ing rest, that then appears in the carbon dioxide of the ex-
pired air.

Are any Foods Respiratory in Liebig's Sense of the
Term? AVo find, then, tliut Liebig's classification of food.'i
cannot be accepted in an absolute sense. There is no doubt
that the substance broken down in muscular contraction is
proper living muscular tissue; and if this (its proteid con-
stituent being retained) be reconstructed from foods con-
taining no nitrogen (whether carbohydrates or fats) then
the term plastic or tissue-forming cannot be restricted to
the proteids of the diet. We must rather conclude that
any alimentary princi^ile containing carbon may be used to
replace the oxidized carbon, and any containing hydrogen
to replace the oxidized hydrogen, of a tissue; and so even
non-proteid foods may be plastic. A certain proportion of
the foods digested may i:)erhai)s be oxidized to yield energy,
before they ever form part of a tissue; and so correspond
jiretty much to Liebig's respiratory foods; but no hard and
fast line can be drawn, making all proteid foods jolastic
and all oxidizaljle non-proteid foods respiratory.

Luxus Consumption. Not only, as above pointed out, may
non-nitrogenous foods be ])lastic, but it is certain that if
any foods are oxidized at once before being organized into
ji tissue, proteids are under certain circumstances; namely,
when -they are contained in excess in a diet. If an tinimal
be starved it is found that its non-nitrogenous tissues go
first; an insufficiently fed animal loses its fat first, and if it
ultimately dies of starvation, is found to have lost 97 per cent
of its adipose tissue and only about 30 per cent of its
proteid-rich muscular tissue, and almost none of its brain
and spinal cord; all of course reckoned by their dry weight.
It is thus clear that the proteids of the tissues resist oxida-
tion much better than fat does. But, on the other hand, if
a well-fed animal be given a very rich proteid diet all the

SOURCES OF UREA. 433

nitrogen of its food reappears in its urine, and that when
it is laying up fat; so that then we get a state of things in
which proteids are broken up more easily than fats. This
indicates that proteid in the Body may exist under two
conditions; one, when it forms part of a living tissue and is
protected to a great extent from oxidation, and another, in
which it is oxidized with readiness and is presumably in ;!
different condition from the first, and not yet built up into
part of a living cell. The use of proteids for direct oxidation
is known as luxus consumption ; how far ic occurs under
ordinary circumstances will be considered presently. The
main point now to be borne in mind is that while all organic
non-nitrogenous foods cannot be called respiratory, neither
can proteids under all circumstances be called ^j/as/ic, in
Liebig's sense.

The Antecedents of Urea. In the long run the pro-
genitors of the urea excreted from the Body are the proteids
taken in the food; but it remains still to be considered
what intermediate steps these take before excretion in the
urine; and whether urea itself is finally formed in the kid-
neys or merely separated by them from the blood.

In seeking antecedents of urea one naturally turns first
to the muscles, which form by far the largest mass of pro-
teid tissues in the Body. Analysis shows that they always
contain kreatin, a body intermediate chemically between
proteids and urea. The quantity of this in muscles is prac-
tically unaffected by work, and is from 0.2 to 0.4 per cent.
Since it is readily soluble and dialyzable, and therefore fit-
ted to pass rapidly out of the muscles into the blood stream,
it is a fair conclusion that a good deal of it is formed in
the muscles daily and carried off from them. Kreatin,
too, exists in the brain, and probably there, and elsewhere
in the nervous system, is produced by chemical degrada-
tion of protoplasm; the spleen also contains a good deal of
kreatin, and so do many glands. This substance would
therefore seem to be constantly produced in considerable
quantities by the protoplasmic tissues generally; and since
it belongs to a group of nitrogenous compounds which the
Body is unable to utilize for reconstruction into proteids.

434 THE HUMAN BODY.

it must be carried off somehow. The urine, however, con-
tains very little kreatin, or its immediate derivative, krea-
tinin, and what it does contain depends mainly on the
feeding, since it varies with the diet and vanishes during
starvation; so it is probable that this substance is con-
verted into urea and excreted in that form. This conver-
sion must occur elsewhere than in the muscles, which con-
tain no urea; also, very little, if any, exists in the brain.

Where the kreatin is finally changed into urea is doubt-
ful. It may be in the kidneys by the renal epithelium,
or it may be elsewhere, and the urea produced be merely
picked up from the blood and passed out by the kidney-
cells; or both may occur; histologically the distinctly
secretory epitheliums of the convoluted parts of the tubules
and of Henle's loops, differ so much as to suggest an en-
tirely different function for them.

On the whole, the evidence seems to show that urea is
merely separated and not produced in the kidneys; a priori
this is more probable, since in the degradation of kreatin to
yield urea energy is liberated and this might very well be
utilized in some organ; while if the process took place in
the kidney tubules the force set free would be wasted.
The blood always contains urea, and renal-artery blood
aj)parently more than renal- vein blood, which shows that
urea is removed from the blood in the kidneys. Moreover,
if a mammal's kidneys be extirpated urea accumulates in
its blood, which could not be the case if urea were nor-
mally only produced in the kidneys; and if urea be injected
into a vein it is rapidly picked up and carried off in the
urine, showing that the kidney-cells have a selective power
with respect to it.

While the urea resulting from further changes in the
kreatin formed in the tissues is a measure of the wear
and tear of their protoplasm, part of the urea excreted
has probably a different source; being due to the oxidation
of proteids, as energy liberators or respiratory foods, before
they have ever formed a tissue. When plenty of proteid
food is taken the urea excretion is largely increased and
ihat very rapidly, within a couple of hours for example,

LUXUS CONSUMPTION. 435

and before we can well suppose the proteids eaten to have
been built up into tissues, and these in turn broken down; in
fact there need be, and usually is, under such circumstances
no sign of any special activity of any group of tissues, such
as one would expect to see if the urea always came from the
breaking down of formed histological elements. This urea
is thus indicative of a utilization of proteids for other than
plastic purposes; and the same fact is indicated by the
storage of carbon and elimination of all the nitrogen of the
food (p. 444) when a diet very rich in proteid alimentary
principles is taken. This hixus consumption may be com-
pared to the paying out of gold by a merchant instead of
greenbacks when he has an abundance of both. Only the
gold can be used for certain purposes, as settling foreign
debts, but any quantity above that needed for such a pur-
pose is harder to store than the paper money and not so
convenient to handle; so it is paid out in preference to
the paper money, which is really somewhat less valuable, as
available at par only for the settlement of domestic debts.
In artificial pancreatic digestions, when long carried on,
two bodies, called lencin and tyrosin, are produced from
proteids. It is found that when leucin is given to an
animal in its food it reappears in the urine as urea; so
the Body can turn leucin into that substance. Hence a
possible source of some of the luxus-consumjition urea is
leucin produced during intestinal digestion; and this is
very likely turned into urea in the liver. At any rate
the liver, to which the portal vein might carry all leucin
thus formed, contains urea, which no other gland does; and
when the liver is greatly altered, as in phosphorus poison-
ing and the disease known as acute yellow atrophy, urea
almost entirely disappears from the urine. This latter fact
seems to point to a final production of urea in the liver, what-
ever its immediate antecedents may be; whether muscle
kreatin, or intestinal leucin, or excess of peptones in the
diet. The latter might perhaps be broken up there into a
nitrogenous part (urea) and a non-nitrogenous part; and we
shall find that a non-nitrogenous substance (glycogen) is
stored in the liver.

436 THE HUMAN BODY.

Proteid Starvation and Overfeeding. When an ani-
mal is fed on food deficient in proteids, or containing none
of them at all, its urea exci'etion falls very rapidly during
the first day or two, but then much more slowly until
death: there is thus indicated a double source of urea, a part
resulting from tissue wear and tear, and always present; and
a part resulting from the breaking down of proteids not
built up into tissue, and ceasing when the amount of this
2)roteid in the Body (in tlie blood for example) falls below
a certain limit as a result of the starvation. As tlie
nitrogen-starved Body wastes, its bulk of proteid tissues
is slowdy reduced and the urea I'esulting from their degrad-
ation diminishes also. How well jirotcid built up into a
tissue resists removal is shown by the facts already men-
tioned (p. 432) as to the relative losses of the proteid-rich
and proteid-poor tissues in starvation.

On tlie other hand, if an animal be taken while starving
and losing weight and have a small amount of flesh giA^en
it, it will continue to lose weight, and more urea than
before will appear in the urine; increased proteid diet in-
creases the proteid metamorphosis, and the animal still
loses, though less rapidly than it did. A little more proteid
still increases proteid metamorphosis in the body, and
the urea elimination, and so on for some time; but each
increment of proteid in the food increases the nitrogcnou^'
metamorphosis somewhat less than the last did, until,
finally, a point is reached at which the nitrogeii egesta and
ingesta balance: in a dog this occurs when it gets daily
gijj- its weight of meat, and no other solid food. More food
if then given is at first stored up and the annual increases in
Aveight; but very soon the greater wear and tear of the
larger mass of tissues shows itself as increased urea ex-
cretion; again the egesta and ingesta balance, and the ani-
mal comes to a new weight equilibrium at the higher level.
More meat now causes a repetition of the phenomenon: at
first increase of tissue, and nitrogen storage; and then a
cessation of the gain in weight, and an excretion m twenty-
four hours of all the nitrogen taken. And so on, until the
animal refuses to eat any more.

STORAGE TISSUES. 43?

These facts seem, very clearly, to show that proteids can-
not be built up quickly into tissues. Meat given to the
starving animal has its proteids, at first, used up mainly in
luxus cofisumption — while a little is stored as tissue, though
at first not enough to counterbalance the daily tissue waste.
When a good deal more proteid is given than answers to
the nitrogen excretion during starvation, the animal builds
up as much into living tissues as it breaks down in the vital
vrocessesot these, therest going ml H.rus consnmpf ion ; it thus
neither gains nor loses. More proteid does not all appear in
the urine at once; some is used to build up new tissue, but
only slowly; then, after some days, the increased metabolism
of the increased flesh balances the excess of nitrogen in the
diet, and equilibrium is again attained. But, all through, it
seems clear that the tissue formation is slow and gradual;
and so it becomes additionally probable that the increased
urea excretion soon after a meal is not due to rapidly in-
creased tissue formation and degradation, but to a more
direct proteid oxidation.

The Storage Tissues. Every healthy cell of the Body
contains at any moment some little excess of material laid
by in itself, above what is required for its immediate neces-
sities. The capacity of contracting, and the concomitant
evolution of carbon dioxide, exhibited by an excised muscle
in a vacuum, seem to show that even oxygen, of which
warm-blooded animals have but a small reserve, may be
stored up in the living tissues in such forms that they can
utilize it, even when the air-pump fails to extract any from
them. But in addition to the supplies for immediate spend-
ing, contained in all the cells, we find special food reserves
in the Body, on which any of the tissues can call at need.
These, especially the oxygen and proteid reserves, are found
largely in the blood. Special oxygen storage is, however,
rendered unnecessary by the fact that the Body can,
except under very unusual circumstances, get more from
the air at any time, so the quantity of this substance laid
by is only small; hence death from asphyxia follows very
rapidly when the air-passages are stopped, while, on account
of the reserves laid up, death from other forms of starva-

438 THE HUMAN BODY.

tion is a much slower occurrence. Proteids, also, we have
learnt from the study of muscle, are probably but little con-
cerned in energy-production in the tissues. Speaking
broadly, the work of the Body is carried on by the oxidation
of carbon and hydrogen, and we find in the Body, in corre-
spondence with this fact, two great storehouses of fatty and
carl)ohydrate foods, which serve to supply the materials for
the performance of work and the maintenance of the bodily
temperature in the intervals between meals, and during
longer periods of starvation. One such store, that of car-
bohydrate material, is found in the liver-cells; the other,
or fatty reserve, is found in the adipose tissue. That such
substances are true reserves, not for any special local purpose
but for the use of the Body generally, is shown by the way
they disappear in starvation; the liver reserve in a few days,
and the fat somewhat later and more slowly, but very largely
before any of the other tissues have been seriously affected.
By using these accumulated matters the Body can work
and keep warm during several days of more or less deficient
feeding; and the fatter an animal is at the beginning of a
starvation period the longer will it live; which would not
be the case could not its fat be utilized by the working
tissues. Hybernating animals prove the same thing; bears,
before their winter sleep, are very fat, and at the end of it
commonly very thin; while their muscular and nervous
systems are not noticeably diminished in mass. During the
whole winter, then, the energy needed to keep the heart and
respiratory muscles at work, and to maintain the tempera-
ture of the body, must have been obtained from the oxida-
tion of the fat reserve with which the animal started.

Glycogen. It may perhaps have struck the reader as
curious that so large an organ as the liver should be set
apart for the formation of so comparatively unimportant a
digestive secretion as the bile; and were this the sole use of
the liver the size of the organ would certainly be anomalous.
The main function of the liver is, however, quite a different
one, the formation and storage of a carbohydrate called
glycogen, from the abundant food materials carried through
it by the portal vein after a meal; in the times between

GLYCOOEN. 439

meals this substance is then doled out gradually, and sent
round tlie Body in the blood. If a liver be cut up two or
three hours after removal from the body of a healthy well-
fed animal, and thoroughly extracted with water, it will
yield up much grape sugar. If, on the other hand, a per-
fectly fresh liver be heated rapidly to the temperature of
boiling water, and then pounded up and extracted, it will
yield a milky solution, containing little grape sugar but
much glycogen; a substance which chemically has the same
empirical formula as starch (CeHioOs), and in other ways
is closely allied to that body. The salivary and pancreatic
secretions rapidly convert it into sugar, as they do starch,
the elements of a molecule of water being taken up at the
same time —

CeHioOs + H.2O = CeHiiOe
Glycogen. Water. Glucose.

The same transformation is rapidly effected by ferments
present in the blood and liver, and hence the first thing to
be done in preparing glycogen is to heat the organ at once
to a temperature high enougli to destroy these ferments.
Pure glycogen is a white amorphous inodorous powder,
readily soluble in water, forming an opalescent milky solu-
tion; insoluble in alcohol, and giving with iodine a red
coloration which disappears on heating and reappears on
cooling again.

About four per cent of glycogen can be obtained from
the liver of a well-nourished animal (dog or rabbit). This
for the human liver, which weighs about 1500 grams (53 oz.),
would give about 60 grams (3.1 oz.) of glycogen at any one
moment. The quantity actually formed daily is, however,
much in excess of that, since glycogen is constantly being
removed from the liver and carried elsewhere, while a fresh
supply is formed in the organ. Its quantity is subject, also,
to considerable fluctuations; being greatest about two hours
after a good meal, and falling from that time until the next
digestion period commences, when it begins to rise until it
reattains its maximum. "When a warm-blooded animal
is starved the glycogen entirely disappears from its liver in
the course of four or five days. Glycogen is, thus, clearly

440 THE HUMAN BODY.

being constantly used up, and its maintenance in normal
quantity depends on food.

The Source and Destination of Liver Glycogen. All

foods arc not equally efficacious in keeping up the stock of
glycogen in the liver; fats by themselves ai-e useless; pro-
teids by themselves give a little; but by far the most is
formed on a diet rich in starch and sugar; so it would seem
that glycogen is mainly formed from cai-bohydrate materials
absorbed from the alimentary canal and carried to the
hepatic cells by the portal vein. These materials are
mainly glucose, since the starch eaten is changed into that
substance before absorption. This view of' the matter is
supported by several facts. (1) Grape sugar if it exist
in the blood in above a certain small percentage passes out
by the kidneys and appears in the urine, constituting the
characteristic symptom of the disease called diahetes. In
health, however, even after a meal very rich in carbohy-
drates, no sugar appears in the urine; so that the large
quantity of it absorbed from the alimentary canal, within a
brief time under such circumstances, must be stopped
somewhere before it reaches the general blood current. (2)
Glucose injected into one of the general veins of an animal,
if in any quantity, soon appears in the urine; but the same
amount injected into the portal vein, or one of its radicles,
causes no diabetes, but an accumulation of glycogen in
the liver. We may therefore conclude that the grape sugar
absorbed from the alimentary canal is taken by the portal
vein to the liver; there stayed and converted into glycogen;
which is then more slowly passed on into the hepatic
veins during the intervals between meals. Thus in spite
of the intervals which elapse between meals the carbo-
hydrate content of the blood is kept pretty constant: dur-
ing digestion it is not suffered to rise very high, nor dur-
ing ordinary periods of fasting to fall very much ])elow the
average.

In what form glycogen leaves the liver is not certain;
it might be dissolved out and carried off as such, or
previously turned again into glucose and sent on in that
form: since the blood and the liver both seem to contain

GLYCOGEN. 441

ferments capable of changing glycogen into^^cose the
latter view is the more probable. Analj'ses'of portal
and hepatic bloods, made with the yiew of determming
whether more sugar was carried out of the liver during
fasting than into it, are conflicting. The main fact, how-
ever, remains that somehow this carbohydrate reserve in
the liver is steadily carried off to be used elsewhere: and
animal glycogen thus answers pretty much to vegetable
starch, which, made in the green leaves, is dissolved and
carried away by the sap currents to distant and not green
parts (as the grains of corn or tubers of a potato, which
cannot make starch for themselves) and in them is again
laid down in the form of solid starch grains, which are
subsequently dissolved and used for the growth of tlie ger-
minating seed or potato. Reasons have already been given
(p. 423) for believing that the carbohydrate leaving the
liver is not oxidized in the blood, but first after it has jiassed
out of that into a living tissue. Among these the muscles at
least seem to get some, since a fresh muscle always contains
glycogen, and even in normal amount when an animal is
starved for some time; the muscle-fibres then, so to sj^eak,
calling on the balance with their banker (the liver) so long
as there is any. When a muscle contracts this glycogen
disappears and some glucose appears, but not an amount
equivalent to the glycogen used up; so that the working
muscle would appear, probably for its repair after each con-
traction (see p. 431), to utilize this substance.

How it is that the glycogen, which is so rapidly con-
verted into grape sugar by the liver ferment after death,
escapes such rapid conversion during life has not been
satisfactorily answered. Two possible reasons readily sug-
gest themselves; the liver ferment may be only produced
by dying hepatic cells; or the glycogen in the living cell
may not exist free, but combined with other portions of the
cell substance so as to be protected; while, after death,
post-mortem changes may rapidly liberate it in a condition
to be acted upon by the ferment.

Diabetes. The study of this disease throws some light
upon the history of glycogen. Two distinct varieties of it

442 THE HUMAN BODY.

arc known; one in which sngar appears in the urine only
when the patient takes carbohydrate foods; the other in
which it is still excreted when he takes no such foods, and
must therefore form sugar in his Body from substances not at
all chemically allied to it. The most probable source of the
sugar in the latter case is proteids; since some glycogen is
found in the livers of animals fed on proteids only, while fats
alone give none of it. In some complex way the proteid mole-
cule would appear to split up in the liver into a highly nitro-
genized part (urea or an antecedent of urea) and a non-
azotized part, glycogen. On this view the more severe form
of diabetes would be due to an increased activity of a normal
proteid-decomposing function of the hepatic cells; and
sometimes the urea and sugar in the urine of diabetics
rise and fall together, thus seeming to indicate a com-
munity of origin. Diabetes dependent on carbohydrate
food might be produced in several ways. The liver-cells
might cease to stop the sugar and, letting it all pass on into
tlie general circulation, suffer it to rise to such a percentage
in the blood after a meal, that it attained the proportion in
which the kidneys pass it out; or the tissues might cease to
use their natural amount of sugar, and this, sent on steadily
out of the liver, at last rise in the blood to the point of ex-
cretion. Or the liver might transform (into glucose) and
pass on its glycogen faster than the other tissues used it.
and so diabetes might arise; but this would only be tem-
porary, lasting until the liver stock was used up by the
rapid conversion. Artificially we can, in fact, produce
diabetes in several of these ways; curari poisoning, for ex-
ample, joaralyzing the motor nerves, makes the skeletal
muscles lie completely at rest, and so diminishes the glyco-
gen consumption of the Body and produces diabetes. Car-
bon monoxide poisoning produces diabetes also, presumably
l)y checking bodily oxidation. Finally, pricking a certain
spot in the medulla oblongata causes a temporary diabetes.
This may be due to the fact that the operation injures
that part of the vaso-motor centre which controls the mus-
cular coat of the hepatic artery; this artery, then dilating,
carries so much blood through the liver that an excess of

FATS. 443

glycogen is turned into glucose in a given time, and carried off
by the hepatic veins. If the sj^lanchnic nerves be cut the
whole arteries of the abdominal viscera dilate and no diabetes
follows, because so many vessels being dilated a great par-
of the blood of the Body accumulates in them, and there is
no noticeably increased flow through the liver. Others,
however, maintain that the " j^iqfire" diabetes (as that due
to pricking the medulla is called) is due to irritation of
trophic nerve-fibres originating there, and governing the
rate at which the liver-cells produce glycogen or convert it
into glucose. This latter view, though perhaps the less
commonly accepted, is probably the more correct. The
hepatic cells do not merely hold back glucose carried
through the liver so that it is there to be washed out by a
greater blood-flow, but they feed on glucose and proteids
and make glycogen; and this is later converted into glucose
and carried off. Glycogen is thus comparable to the zymo-
gen of the pancreas and other glands (Cluq). XVIII. ); and
the transformation of such bodies into the specific element
of a secretion we have already seen to be directly under the
control of the nervous system, and almost entirely or quite
independent of the blood-flow.

The History of Fats. "While glycogen forms a reserve
store of material that is subject to rapid alterations, deter-
mined by meal-times, the fats are much more stable; their
periods of fluctuation are regulated by days, weeks, or
months of good or bad nutrition, and during starvation they
are not so readily, or at least so rapidly, called upon as the
hepatic glycogen. If we carry on the simile by which we
compared the reserve in each cell to pocket-money (p. 31),
tlie glycogen would answer somewhat to a balance on the
right side with a man's banker ; while the fat would
represent assets or securities not so rapidly realizable; as
capital in business, or the cargoes afloat in the argosies of
Antonio, the "Merchant of Venice." Fat, in fact, is
slowly laid down in fat-cells and surrounded in these by a
cell-wall, and, being itself insoluble in blood plasma or
lymph, it must undergo chemical changes, which no doubt

444 THE HUMAN BODY.

require some time, before it cuu be taken into the blood
and carried off to other parts.

When adipose tissue is developing it is seen that undif-
ferentiated cells in the connective tissues (especially areolar)
show minute oil-drops in their protoplasms. These increase
in size and, ultimately, fuse together and form one larger
oil-droplet, while most of the original protoplasm dis-
ap])cars.

The oily matter would thus seem due to a chemical
metamorphosis of the cell protoplasm, during which it gives
rise to a non-azotized fatty residue which remains behind,
and a highly nitrogenous part which is carried off. In
many parts of the Body protoi)lasmic masses are subject to
a similar but less complete metamorphosis; fatty degenera-
tion of the heart, for example, is a more or less extensive
replacement of the proper substance of its muscular fibres
by fat-droplets; and the cream of milk and the oily matter
of the sebaceous secretion are due to a similar fatty
degeneration in gland-cells. Moreover, careful feeding ex-
periments undoubtedly show that fat can come from pro-
teids; when an animal is very richly supplied with these
all the nitrogen taken in them reappears in its execretions,
but all the carbon does not; it is in part stored in the Body:
and, since such feeding produces but little glycogen, this
carbon can only be stored as fat.

While there is, then, no doubt that some fat may have a
proteid origin, it is not certain that all has such. During
digestion a great deal of fat is ordinarily absorbed, in a
chemically unchanged state, from the alimentary canal; it
is merely emulsified and carried off in minute drops by the
chyle to be poured into the blood: and this fat might be
directly deposited, as such, in adipose tissue. There are,
however, good reasons for supposing that all the fat in the
Body is manufactured. The fat of a man, of a dog, and of a
cat varies in the proportions of palmatin, stearin, margarin,
iuid olein in it; and varies in just the same way if all be fed on
the same kind of food, which could not be the case if the
fat eaten were simply deposited unchanged. Moreover, if
an animal be fed on a diet containing one kind of fat only.

ORIGIX OF FATS. 445

say olein, but a very slightly increased percentage of that
particular fatty substance is found in its adij^ose tissue,
which goes to show that if fats come from fats eaten, these
latter are first pulled to bits by the living cells and built up
again into the forms normal to the animal; so that, even with
fatty food, the fats stored up seem to be in most part
manufactured in the Body.

In still another way it is proved that fats can be constructed
in the Body. In animals fed for slaughter, the total fat
stored up in them during the process is greatly in excess
of that taken with their food during the same time. For ex-
ample, a fattening pig may store up nearly five hundred
parts of fat for every hundred in its food, and this fat
must be made from proteids or carbohydrates. Whether it
can come from the latter is still perhaps an open question;
for, while all fattening foods are rich in starch or similar
bodies, there are considerable chemical difficulties in sup-
/ posing an origin of fats from such; and it is on the whole
I more probable that they simply act by sparing from use
fats simultaneously formed or stored in the body, and
which would have otherwise been called upon. They make
glycogen, and this shelters the fats. Liebig, indeed, in a very
celebrated discussion, maintained that fats were formed
from carbohydrates. He showed that a cow gave out more
butter in its milk than it received fats in its food; and
Huber, the blind naturalist, showed that bees still made
wax (a fatty body) for a time when fed on pure sugar; and
indefinitely when fed on honey. Consequently, for a long
time, an origin of fats from carbohydrates was supposed to
be proved; but their possible origin from proteids (a possi-
bility now shown to be a certainty) was neglected, and the
validity of the above proofs of their carbohydrate origin is
thus upset. The cow may have made its butter from proteids;
the bees, fed on sugar, their wax for a time from proteids
in their bodies already; and, indefinitely, when fed on lioney,
from the proteids in that substance. Moreover, animals
(ducks) fed on abundant rice, which contains much carbo-
hydrate but very little proteid or fat, remain lean; while if
some fat be added they lay up fat.

446 THE HUMAN BODY.

Persons who fatten cattle for the butcher find that the

foods useful for the purpose all contain proteids, carbohy-
drates, and fats, and that rapid fattening is only obtained
with foods containing a good deal of fat; as oilcake, milk,
or Indian corn. Taking all the facts into account we shall
probably not be wrong in concluding that nearly all the
bodily fat is manufactured either from fats or proteid.--;
from fats easier than from anything else, but when much
proteid is eaten some is made from it also. Carbohydrates
alone do not fatten; the animal body cannot make its pal-
matin, etc., out of them. Nevertheless they are, indirectly,
important fattening foods when given with others, since,
being oxidized instead of it, they protect the fat formed.

Dietetics. That ^' one man's meat may be another
man's poison" is a familiar saying, and one that, no doubt,
expresses a certain amount of truth; but the difference
probably depends on the varying digestive powers of indi-
viduals rather than on peculiarities in their laws of cell
nutrition: all need pretty much the same amount of pro-
teids, fats, and carbohydrates for each kilogram of body
weight; but all cannot digest the same varieties of them
equally well : while many foods have peculiar, almost
poisonous, effects on some persons. A good many people
are made ill by mutton, which the majority digest better
than beef.

The proper diet, too, will necessarily vary, at least as to
amount,with the work done; whether it should vary in kind
with the nature of the work is not so certain. Provided a
man gets enough proteids to balance those lost in the wear and
tear of his tissues, it probably matters little whether he gets
for oxidation and the liberation of energy either fats or car-
bohydrates, or even excess of proteids themselves; any one of
the three will allow him to work either his brain or his muscles,
and to maintain his temperature. Proteids, however, are
wasteful foods for mere energy-yielding23uri:)oses: in the first
place, they are more costly than the others; secondly, they
are incompletely oxidized in the Body; and, thirdly, it is
probably more laborious to the system to get rid of urea than
of the carbon dioxide and water, which alone are yielded by

DIETETICS. 447

the oxidation of fats arid carbohydrates. Between fats and
carboliydrates simihir considerations lead to a use of the latter
when practicable: starch is more easily utilized in the Body
than fats, as shown by the manner in which it protects the
latter from oxidation; and a given weight of starch fully
oxidized in the Body will liberate one and a half times as much
energy as the same amount of butter, while it costs consider- * • \
ably less than half the money. Probably, too, starch is . •.,
more easily digested than fats; at least by many people: - "^
children especially are apt to be fond of starchy or sac-
charine foods and to loathe fats; and the appetite in such
cases is a good guide. As a race, too, the American people
differ markedly from the English in their love of sweet
foods of all kinds; whether this is correlated with their
characteristic activity, calling for some food that can be
rapidly used, is an interesting question, to which, however,
it would be rash to give at present an affirmative answer.

It is clear, therefore, that no general rules for every one's
diet can be laid down; but still on broad principles the
best diet would be that which contained just the amount of
proteid necessary for tissue repair, and so much carbo-
hydrates as could be well digested, the balance needed, if
any, being made up by fats. Such a food would be the
cheapest; that is the supplying of it would call for less of
the time and energy of the nation using it, and leave more
work to spare for other pursuits than food production —
for all the arts which make life agreeable and worth living,
and which elevate civilized man above the merely material
life of the savage whose time is devoted to catching and eat-
ing. We have high authority for saying that man does
not live by bread alone; m other words his highest develop-
ment is impossible when he is totally absorbed in *' keeping
body and soul together," and the more labor that can be
spared from getting enough food the better chance has he,
if he use his leisure rightly, of becoming a more worthy
man. While there is, thus, a theoretically best diet, it is
nevertheless impossible to say what that is for each indi-
vidual; but what the general experience is may be approxi-
mately gathered by taking an average of the dietaries of a

448 THE HUMAN BODY.

number of iniblic institutions in which the health of many
people is maintained as economically as ])ossible. Such an
examination made by Moleschott, gives us as its result a diet
containing daily —

Proteids /SO grams or 465 grains.

Fats 84 " or 1,300

Amyloids 404 " or 6,263 "

Salts 30 " or 465

Water.... 2800 " or 43,400 "

People in easy circumstances take as a rule more proteids
and fats and less amyloids; and this selection, when a
choice is possible, probably indicates that such a diet is the
better one: the proteids in the above table seem especially
deficient.

CHAPTER XXIX.

THE PRODUCTION AND RECfULATIOX OF THE
HEAT OF THE BODY.

Cold- and Warm-Blooded Animals. All animals, so
long as they are alive, are tiie seat of ehemical changes by
"which heat is liberated; hence all tend to be somewhat warmer
than their ordinary 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 pro-
duces 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 cliangeable-temperatured
{poihilo-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 at any given time balancing the loss a nor-
mal body temperature is maintained, and usually one con-
siderably higher than that of the medium in which they
live; such animals are therefore known as animals of con-
stant temperature {Jiomo-thermous), or more commonly
" warm-blooded" animals. The latter name, however, does
not properly express the facts; a lizard basking in the sun
on a Avarm summer's day may be nearly 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

450 THE HUMAN BODY.

is, within a degree, the same in winter or summer; within
the iirctic circle or on the equator.

Moderate warmth accelerates protoplasmic activity; com-
pare 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 warmed, and the white corpuscles of its
blood when heated up to the temperature of the Human
Body are seen (with the microscope) to exhibit much more
active amoeboid movements than they do at the tempera-
ture of frog's blood. In summer a frog or other cold-
blooded animal uses much more oxygen and evolves much
more carbon dioxide than in winter, as shown not only by
direct measurements 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 sufficieiitly
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 tem-
perature of the medium in which it is living. With the
warm-blooded animal the reverse is the case. It always
keeps up its temperature to that at which its tissues live
best, and accordingly in cold weather uses more oxygen and
sets free more carbon dioxide because it needs a more active
internal combustion to compensate for its greater loss of
heat to the exterior. In fact the living tissues of a man
may be compared to hothouse plants, living in an artifici-
ally maintained temperature; but they differ from the
])lants 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 tem-
]ierature no matter whether it be in cold or warm surround-
ings, it is clear that it must jDOSsess an accurate arrangement
for heat regulation; 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 eitlior in contact with one another directly or, if

SOURCES OF BODILY HEAT. 451

not, at least indirectly 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
ma}' 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, howeyer, 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 protected parts, as the hollow of the armpit. In
internal organs, as the liver and brain, the temjierature is
higher; about 42° 0. (107° F.) in health. In the lungs,
though there is a certain quantity of heat liberated when
oxygen combines with haemoglobin, this is more than
counterbalanced by loss of the heat carried out by the ex-
pired air, and that used up in evaporating the water carried
out in the breath; so 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. These are two-fold;
direct and indirect. Heat is directly produced wherever
oxidation is taking place; so that all the living tissues at
rest produce heat as the result of the chemical changes sup-
plying them with energy for the maintenance of their
vitality; and whenever an organ is active and its chemical
metamorphoses are increased it becomes hotter: a secreting
gland or a contracting muscle is warmer than a resting one.
Indirectly, heat is developed by the transformation of other
forms of energy; mainly mechanical work, but, to a less
extent, also of electricity. All movements of parts of the
Body which do not move it in space or move external objects,
are 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, ex-
cept in so far as they expel matters from the Body; and

452 THE HUMxiN BODY.

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 muscles and nerves are also the seats
of manifestations of electricity, which, though small in
amount, for the most part do not leave the Body in that
form but are first converted into heat. A certain amount
of heat is also carried into the Body with hot foods and
drinks.

The Energy Lost by the Body in Twenty -four Hours.
Practically speaking, the Body only loses energy in two
forms; as heat and mechanical work: by applying conduc-
tors to different parts of its surface small amounts of elec-
tricity can be carried off, but the amount is quite trivial in
comparison with the total daily energy expenditure. Dur-
ing complete rest, that is when no more work is done than
that necessary for the maintenance of life, nearly all the
loss takes the form of heat. The absolute amount of this
will vary with the surrounding temperature and other con-
ditions, but on an average a man loses, during a day of
rest, 2700 calories; that is enough to raise 2700 kilograms
(5-940 lbs.) of water from 0° to 1° C. (from 32° to 33.8° F.);
otherwise expressed, this amount of heat would boil 27 kilos
(59.4 lbs.) of ice-cold water. This does not quite represent
all the energy lost by the Body in that time: since a small
proportion is lost as mechanical work in moving the clothes
and air by the respiratory movements, and even by the
beat of the heart, which at each systole pushes out the
chest-wall a little and moves the things in contact with it.
The working Body liberates and loses much more energy;
part as mechanical work done on external objects, part as
increased heat radiated or conducted from the surface, or
carried off by the expired air in the quickened respirations.
Every one knows that he becomes warmer Avhen he takes
exercise, and measurements made on men show that the
heat produced and lost in a day of moderate work is about
one third greater than that in a day of rest. The follow-
ing table gives more accurate numbers —

BODILY ENERGY LOST PER DAY. 453

Day of Rest. Day of Work.

Rest 16 hrs. Sleep 8 hrs. Rest 8 hrs. Work 8 hrs. Sleep 8 hrs!
Heat units (calor- 1 oj^q^ goQ jggg g 2169.6 320.

les) produced.. )

o-on ^ / 10,885l^ah.-lb. \ q^^ « /^ 14,528 Fall. -lb. \

^^■^ \ heat units. } '^''^■^ \ heat units. )

The mechanical work done on the working day, repre-
sented in addition an expenditure of energy of 213,344
kilogrammeters, which is equal to 502 calories. Of the ex-
cess heat in the working day, part is directly produced by
the increased chemical changes in the quicker working heart
and respiratory muscles, and the other muscles set at work;
while part is indirectly due to heat arising from increased
friction in the blood-vessels as the blood is driven faster
around them, and to friction of the various muscles used.
The average cardiac work in twenty-four hours is about
60,000 kilogrammeters ; that of the respiratory muscles
about 14,000 ; and since nearly all of both is turned finally
into heat within the Body, we have 74,000 kilogrammeters
of energy answering to about 174 calories (6786 Fah.-lb.
units) indirectly produced in the resting Body daily from
these sources.

Of 100 parts of heat lost from the resting Body, about
73 are carried off in radiation or conduction from the skin.
14.5 are carried off m evaporation from the skin.

7.2 " *' " " '^ '^ lungs.

3.0 " " '* expired air.

1.8 " " " in the excretions.

In a day of average work, of every 100 parts of energy lost
i 11 any form from the Body —
1-2 go as heat in the excreta.
3-4 in heating the expired air.
20-30 in evaporating water from the lungs and skin.
60-75 in heat radiated or conducted from the surfaces and
in external mechanical work.

The Superiority of the Body as a Working Machine.

During eight hours of work, we find (table at top of page)
the Body loses 2169.6 calories of energy as heat; and can do

454 THE HUMAN BODY.

simnltaneonsly work equivalent to 502 calories. So of all
the energy lost from it in that time tibont \ may take the
form of mechanical work; this is a very large proportion
of the total energy expended, being a much liiglier per-
centage than that given by ordinary machines. The best
steam-engines can utilize as mechanical work only libout
■^ of the total energy liberated in them and lost from
them m a given time; the remainder is transmitted directly
as heat to the exterior, and is lost to the engine for all useful
purpo.-es.

The Maintenance of an Average Temperature. This is
necessary for the continuance of the life of a warm-blooded
animal; should the temperature rise above certain limits
chemical changes, incompatible with life, occur in the tis-
sues ; for example at about 49° C. (120° F.) the muscles be-
gin to become rigid. On the other hand death ensues if the
Body be cooled down to about 19° C. (66° F.). Hence the
need for means of getting rid of excess heat, and of protec-
tion from excessive cooling. Either end may be gained in
two ways; by altering the rate at which heat, is lost or that
at which it is produced. As regards heat-loss, by far the
most important regulating organ is the skin: under ordi'
nary circumstances nearly 90 per cent of the total heat given
off from the Body in 24 hours goes by the skin (73 by radi-
ation and conduction, 14.5 by evaporation; see above table).
This loss may be controlled —

1. Jiy clothing; we naturally wear more in cold and less in
warm weather ; the effect of clothes being, of course, not to
Avarm the Body but to diminish the rate at which the heat
formed in it is lost.

2. Increased temperature of the surrounding medium in-
creases the activity of the heart and lungs. A hastened
circulation by itself does not as already pointed out (p. 388)
increase tlie general tissue activity of the Body, or the oxida-
tions occurring in it, and, so. a})art from the harder working
heart itself, does not influence the amount of heat liberated in
the Body during a given time: but the more rapid blood-flow
through the skin carries more of that fluid through this cool
surface and increases the loss of heat in that way. The

TEMPEBATURE-REQULATION IK 2 HE BODY. 455

quickened respirations, too, increase the evaporation of water
in the lungs and, thus, the loss of heat,

3. Warmth directly dilates the skin-vessels and cold con-
tracts them. In a warm room the vessels on the surface
dilate as shown by its redness, while in a cold atmosphere
tliey 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.

4. Heat induces sweating and cold checks it ; the heat
appears to act, partly, reflexly in exciting the sweat-centres
from which the secretory nerves for the sudoriparous glands
arise, and, partly, directly on those centres, which are thrown
into activity, at least in health, as soon as the temperature of
the blood is raised. In fever of course we may have a high
temperature with a dry non-sweating skin. The more sweat
there is poured out, the more heat is used up in evaporating
it and the more the Body is cooled.

5. Our sensations induce us to add to or diminish the
lieat 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 processes which liberate a tol-
erably 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 limits, the heat production may b'^ con-
trolled m several ways —

1. Cold increases hunger; and increased ingestion of food
increases bodily oxidation as shown by the greater amount
of carbon dioxide excreted in the hours succeeding a meal.
This increase is probably due to the activity into which the
digestive organs are throAvn.

2. Cold inclines to voluntary exercise; warmth to muscu-
lar idleness; and the more the muscles are worked the more
heat is produced in the Body.

456 THE nUMAN BODY.

3. Cold tends to produce involuntary muscular move-
ments, and so increased lieat production; as chattering
of the teeth and shivering.

4. Cold applied to the skin increases the bodily chemical
metamorphoses and so heat production. At least the tem-
perature in the armpit rises at first on entering a cold bath,
though the heat carried off from the surface soon overbalances
its increased production. The phenomenon may, however,
be explained in another way, the rise being attributed to a
sudden diminution of loss from more exposed parts of the
skin, dependent on contraction of the cutaneous arteries.
In some cases, however, the temjiorary rise is accompanied
by an increased excretion of carbon dioxide, which would
indicate that the surface cooling does really increase the
oxidations of the Body.

5. Certain drugs, as salicylic acid, and perhaps quinine,
diminish the heat production of the Body. Their mode of
action is still obscure.

On the whole, however, the direct heat-regulating me-
chanisms of the Human Body itself are not very efficient,
especially as protections against excessive cooling. Man
needs to supplement them 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 equalization 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 always
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 lo ibcm; while the reverse is the

TRERMIG NERVES. 457

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 tem-
perature in the skin of the face in blushing.

Thermic Nerves. All nerves, such as motor or secre-
tory, which can throw working tissues into activity are in
a certain sense thermic nerves: since they excite increased
oxidation and heat production in the parts under their con-
trol. A true, purely thermic nerve would be one which
increased the heat production in a tissue without otherwise
throwing it into activity; and whether such exist is still
undecided. Certain phenomena of disease, however, seem
to render their existence probable. If we return for a
moment to our former comparison of the working Body to
a steam-engine, such nerves might be regarded as agencies
increasing its rate of rusting without setting it at work.
The oxidation of the iron would develop some heat, but by
processes useless to the steam-engine, although such are,
in moderation, essential to living cells; the vitality of
these even when at rest, seems to necessitate a constant,
if small, breaking down of its substance. In an amoeboid
cell no doubt such processes occur quite independently of
the nervous system; but in more differentiated tissues they
may be controlled by it. Just as a muscle does not nor-
mally contract unless excited through its nerve, although
a white blood corpuscle does, so may the natural nutritive
processes of the muscle-fibre in its resting condition be de-
pendent on the nerves going to it. If these be abnormally
excited the muscle Avill break down its protoplasm faster
than it constructs it, and consequently waste; at the
same time the increased chemical degradation of its sub-
stance will elevate its temperature. Febrile conditions, in
which many tissues waste, without any unusual manifesta-
tion of their normal physiological activity, would thus be
readily accounted for as due to superexcitation of the
thermic nerves. Moreover, it is found that lesions or sec-
tions of the spinal cord are followed by a rise in the tem-
perature of those parts of the Body sujjplied with nerves
arising below the diseased or divided portion. Now

458 THE HUMAN BODY.

division of the spinal cord in two ways tends to lower the
temperature of parts below the injury: in the first place,
the muscles are paralyzed and so a great source of heat is
cut otf ; and in the second, the yaso-motor nerves traveling
down from the medulla-«-y centre are cut, and hence the
skin arteries behind the section dilate and carry more blood
to the surface to be cooled. To explain the rise of tem-
perature it has therefore been concluded that there are
true thermic centres in the spinal cord, which centres, like
others in that organ (Chap. XXXV.), are held in check or
inliibitecl by brain-centres; when the controlling influence
of the latter is removed the former may excite excessive oxida-
tions in the tissues to which they are distributed, and so
produce the rise of temperature. The proof, however, is
not complete; for the raised temperature may, after all, be
due merely to an excessive supply of blood, warmed else-
where m the Body, to the dilated skin-vessels.

Clothing. To man, as social animal, endowed with
moral feelings, clothing has certain uses in the interests
of morality; but for such purposes the amount necessary is
not great, as we find in many tribes living in warm climates.
Except in tropical regions, liowever, clothing has in addi-
tion an important physiological use in regulating the bodily
temperature. 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 capillary coating is merely rudi-
mentary and has lost all i)hysiological importance; and so
lie has to protect himself by artificial garments, which
Ills gesthetic sense has led him to utilize also for purposes of
;idornment. 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 I'egions,
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 tem-
peratures, it should vary with the season. If the surface

CLOTHING. 459

of the Body be exposed so tliat ciirrents of air can freely
traverse it much more lieat 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 con-
tact with the surface. As every one knows, a "draught"
cools much faster thaa air of the same temperature not in
motion. All clothing, tlierefore, 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 contact 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, a condition which readily jxasses into the
diseased state known as inflatmnation. 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 sud-
denly cooled. 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 temperature 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 re-
turn 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 many persons, especially of the female
sex, often acquire far more serious diseases. When sudden
changes of temperature are at all probable, even if the pro-
vailing weather be warm, the trunk of the Body should
be always protected by some tolerably close-fitting garment

460 THE HUMAN BODY.

of non-conducting material. Those whose skins are irri-
tated by anything but linen should wear immediately out-
side the under-garments a jacket of silken or woolen
material. In midwinter comparatively few joeople take
cold, because all then wear thick and nonconducting cloth-
ins: of some kind.

CHAPTER XXX.

SENSATION AND SENSE-ORGANS.

The Subjective Functions of the Nervous System.

Changes in many parts of our Bodies are accompanied or
followed by those states of consciousness which we call sew-
sations. All such sensitive parts are in connection, direct
or indirect, with the brain, by certain afferent nerve-fibres
called sensory. Since all feeling is lost in any region of the
Body when this connecting path is severed, it is clear that
all sensations, whatever their primary exciting cause, are
finally dependent on conditions of the central nervous sys-
tem. Hitherto we have studied this as its activities are
revealed through movements which it excites or prevents;
we have seen it, directly or reflexly, cause muscles to con-
tract, glands to secrete, or the pulsations of the heart to
cease; we have viewed it ohjedively, as a motion-regulating
apparatus. Now we have to turn to another side and con-
sider it (or parts of it) as influencing the states of conscious-
ness of its possessor: this study of the subjective activities
of the nervous system is one of much gi'eater difficulty.

It may be objected that considerations concerning states
of feeling h-ave no proper place in a treatise on Anatomy
and Physiology; that, since we cannot form the beginning'
of a conception how a certain state of the nervous system
causing the feeling redness, another the feeling blueness, and
a third the emotion anger, all examination of mental phe-
nomena should be excluded from the sciences dealing with
the structure and properties of living things. But, although
we cannot imagine how a nervous state {neurosis) gives rise
to a conscious state (psychosis), we do know this, that dis-
tinct phenomena of consciousness never come under our

462 THE HUMA^' BODY.

observation apart from a nervous system, and so are pre-
sumably, in some way, endowments of it; we are, therefore,
justilied in calling them 2)ro2)erties of the nervous system;
and their examination, especially with respect to what
nerve-parts are concerned with different mental states, and
what changes in the former are associated with given phe-
nomena in the latter, forms properly a part of Physiology.
Whether masses of protoplasm, before the differentiation of
definite nerve-tissues, possess some ill-defined sort of con-
sciousness, as they possess an indefinite contractility before
they have been modified into muscular fibres, may for the
present be left undecided: though those who accept the doc-
trine of evolution will be inclined to assent to the proposition.

While, however, the Physiologist has a right to be heard
on questions relating to our mental faculties, it is never-
theless true that many laws of thought have been esta-
blished, concerning which our present knowledge of the
laws of the nervous system gives us no clue; the science of
Psychology has thus a well-founded claim to an independent
existence. But, in so far as its results are confined merely
to the successions and connections of mental states, as estali-
lished by observation, they are merely descriptions, and noi
explanations in a scientific sense: we know that so many
mental phenomena have necessary material antecedents and
concomitants in nervous changes, that we are justified in
believing that all have such, and in continuing to seek for
tliem. We do not know at all how an electric current sent
round a bar of soft iron makes it magnetic; we only know
that the one change is accompanied by the other; h\xi we
say we have explained the magnetism of a piece of iron if
we have found an electric current circulating around it.
Similarly, we do not know how a nervous change causes a
mental state, but we have not explained the mental state
until we have found the nervous state associated with it
and how that nervous state was produced.

As yet it is only with respect to some of the simplest
riates of consciousness that we know much of the necessary
physiological antecedents, and among these our sensations
are the best investigated. As regards siich mental pheno-

sensatto:n' and organs of special sense. 4G3

meua a,s the Association of Ideas, and Memory, physio-
logy can give ns some light; but so far as others, such as the
Will and the Emotions, are concerned, it has at present little
to offer. The phenomena of Sensation, therefore, occupy
at present a much larger portion of physiological works
than all other mental facts put together.

Common Sensation and Organs of Special Sense. A
sensory nerve is one which, when stimulated, arouses, or
may arouse, a sensation in its possessor. The stimulant is
in all cases some form of motion, molar [e.g. mechanical
pressure) or molecular (as ethereal vibrations or chemical
changes). Since all our nerves lie within our Bodies as
circumscribed by the skin, and are excited within them, one
might, a priori be inclined to suppose that the cause of all
sensations would appear to be within our Bodies themselves;
that the tlwwg felt would be a modified i)ortiou of fhe feeler.
This is the case with regard to many sensations; a head-
ache, toothache, or earache gives us no idea of any external
object; it merely suggests to each of us a particuhir
state of a sensitive portion of myself. As regards many
sensations, however, this is not so; they suggest to us ex-
ternal causes, to properties of which and not to states of our
Bodies, we ascribe them ; and so they lead us to the concejition
of an external universe. A knife laid on the skin produces
changes in it which lead us to think not of a state of our
skin, but of states of some object outside the skin; we
believe Ave feel a cold heavy hard thing in contact with it.
Nevertheless we have no sensory nerves going into the knife
and informing us directly of its condition; what we really
feel are the modifications of our Body produced by it,
although we irresistibly think of them as properties of the
knife — of some object that is no part of our Body, and not as
states of the latter itself. Let now the knife cut through
the skin; we feel no more koiife, but experience pain,
which we thmk of as a condition of ourselves. We do not
say the knife is painful, but that our fiiiger is, and yet we
have, so far as sensation goes, as much reason to call the
knife painful as cold. Applied one way it produced local
changes arousing a sensation of cold, and in another local

464 THE HUMAN BODY.

changes causing a sensation of pain. Nevertheless in the
one case we speak of the cold as being in the knife, and in
the other of the pain as being in the finger.

Sensitive parts, such as the surface of the skin, through
which we get, or believe Ave get, information about outer
things, are of far more intellectual value to us than sensitive
parts, such as the subcutaneous tissue into which the knife
may cut, which give us only sensations referred to conditions
of our Bodies. The former are called Sense-organs proper,
or Organs of Special Sense; the latter are sensitive paris, or
Organs of Common Sensation.

The Peripheral Reference of our Sensations. The
fact that we refer certain sensations to external causes is
only one case of a more general law, in accordance with
which we do not ascribe our sensations, as regards their lo-
cality, to the brain, where the neurosis is accomi^anied by
the sensation, but to a peripheral part. With respect to
the brain, other parts of the Body are external ol)jects as
much as the rest of the material universe, yet the majority
of our common sensations are felt 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 resulting 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 ting-
ling ascribed to the fingers to which the ends of the fibres
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. To explain such facts we
must trench on the ground of Psychology, and so they can-
not be fully discussed here; but they are commonly ascribed
to the results of experience. The events of life have taught
us that in the great majority of instances the sensory im-
])ulses which excite a given tactile sensation, for example,
have acted upon the tip of a finger. The sensation goes when
tbe finger is removed, and returns when it is rejilaced; and
the eye confirms the contact of the external object with the
tinger-tip wlien we get tlie tactile sensation in question.
Wc thus come firmly to assocuite a particular region of the

QUALITIES OF SENSATIONS. 465

skin with a given sensation, and whenever afterwards the
nerve-fibres coming from the finger are stimulated, no mat-
ter where, we ascribe the origin of the sensation to some-
thing acting on the finger-tip.

The Differences between Sensations. In both groups
of sensations, those derived through organs of special
sense and those due to organs of common sensation, we dis-
tinguish kinds which are absolutely distinct for our con-
sciousness, and not comparable mentally. We can never
get confused between a sight, a sound, and a touch, nor be-
tween pain, hunger, and nausea; nor can we compare them
with one another; eacli is sai generis. The fundamental
difference which_thvis separates one sensation from another
is its modality. Sensations of the same modality may differ;
but they shade imperceptibly into one another, and are com-
parable between themselves in two ways. First, as regards
qualityj while a high and a low pitched note are both
auditory sensations, they are nevertheless different and yet
intelligibly comparable; and so are blue and red objects.
In the second place, sensations of the same modality are
distinguishable and comparable as to amount or intensitij:
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 char-
acter. Our sensations thus differ in the three aspects of
onodality, quality untJtin the same modality, and intensity.
Certain sensations also differ in what is known as the
*' local sign," a difference by which we tell a touch on one
part of the skin from a similar touch on another; or an ob-
ject exciting one part of the eye from an object like it, but
in a different location in sjoace and exciting another part of
the visual surface.

As regards modality, we commonly distinguish five
senses, those of sight, sound, touch, taste, and smell; it is
doubtful whether temperature should not be added. The
varieties of common sensation are also several; for example,
pain, hunger, satiety, thirst, nausea, malaise, hien-etre (feel-
ing "good"), fatigue. The muscular sense stands on the
intermediate line between special and common sensations;

466 THE HUMAN BODY.

we gather by it how much our various muscles are con-
tracted, and so learn the position of various parts of the
Body, on the one hand, and the resistance opposed to bodily
movement by external objects, on the other. In fact, we
cannot draw a sharp line between the special senses and
common sensations: all the Body, we conclude from ob-
servations on the lower animals, is, at an early stage of its
development, sensitive; very soon its cells sej^arate them-
selves into an outer layer exposed to the action of external
forces and an inner layer protected from them: and some
of the former cells become especially sensitive. From them,
as development proceeds, some are sej)arated and buried be-
neath the surface to become the brain and spinal cord; of
those which remain superficial, some are modified so that
they (in the eye) become especially excited by ethereal vi-
brations; others (in the ear) become especially responsive
to sound vibrations; others to slight chemical changes (in
mouth and nose), and others (in the skin) to variations
in pressure or temperature.

All our sensations are thus modifications of one common
primary sensibility, represented by that of the skin, or
rather by the primitive rej^resentative of the skin in such
an animal as the Hydra (see Zoology). The cutaneous sen-
sations, being less differentiated, shade off more readily into
the common sensibility of the other living tissues than do
the activities of the highly differentiated cells in the e3'e
and ear. We find, accordingly, that while a powerful pres-
sure or a high temperature acting on the skin readily
arouses a sensation of pain, that this is not the case with the
more specialized visual and auditory organs. Their super-ex-
citement maybe disagreeable, but never passes into j^ai'w, in
the ordinary sense of the word. Similarly the special skin
sensations, touch and temperature, may sometimes be con-
founded, while a sound and a sight cannot be: the mo-
dality of the less modified skin-senses is less comj)lete. The
study of comparative anatomy and development thus shows
that the irritable parts of our sense-organs are but special dif-
ferentiations of the primary external layer of cells covering
the Body when it is very young. Some of these become nerve

STRUCTURE OF SENSE-ORGANS. -iG?

end organs in the eye, others end organs in the ear, and so
on; while others, less changed, remain in the skin as organs
of touch and temperature; and so, frona a general exterior
surface responding equally readily to many external natural
forces, we get a surface modified so that its various parts
resj^oud with different degrees of readiness to different ex-
ternal forces; and these modified parts constitute the essen-
tial portions of our organs of special sense. Every sense
organ thus comes to have a special relationship to some one
natural force or form of energy — is a specially irritable
mechanism by which such a force is enabled to excite sen-
sory nerves; and is, moreover, commonly supplemented by
arrangements which, in the ordinary circumstances of life,
prevent other forces from stimulating the nerves connected
with it. Not all natural forces have sense-organs with ref-
erence to them developed in the Human Body; for exam-
ple, we have no organ standing to electrical changes in the
same relation that the eye does to light or the ear to
sound.

The Essential Structure of a Sense-Organ. In every
sense-organ the fundamental part is thus one or more end
organs^ which are highly irritable tissues (p. 31), so con-
structed and so placed as to be normally acted on by some
one of the modes of motion met with in the external world.
A sensory apparatus requires in addition at least a brain-cen-
tre, and a sensory nerve-fibre connecting this with the ter-
minal ap2)aratus; but one commonly finds accessory parts
added. In the eye, e.g., we have arrangements for bringing
to a focus the light rays which are to act on the end
organs of the uerve-fibres; and in the ear are found similar
subsidiary parts, to conduct sonorous vibrations to the end
apparatus of the auditory nerve.

Seeing and hearing are the two most specialized senses;
the stimuli usually arousing tliem are peculiar and quite
distinct from the group of general nerve stimuli (see p. 188),
while those most frequently, or naturally, acting upon our
other sense-organs are not so peculiar; they are forces wJaich
act as general nerve stmiuli when directly applied to nerve-
fibres. The end organs, however, as already pointed out

468 THE HUM AX BODY.

(p, 100), so increase the sensitiveness of the parts contain-
ing them that degrees of change in the exciting forces,
which would be totally unable to stimulate neiTe-fibres
themselves are appreciated. These terminal apparatuses
are therefore as truly mechanisms enabling changes, which
would not otherwise stimulate nerves, to excite them, as
are the end organs in the eye or ear.

The Cause of the Modality of our Sensations. Seeing
that the external forces usually exciting our difl'erent sen-
sations differ, and that the sensations do also, we might at
first be inclined to believe that the latter difference de-
pended on the former: that brightness differed from loud-
ness because light was different from sound. In other
words, we are apt to think that each sensation derives its
specific character from some property of its external physi-
cal antecedent, and that our sensations answer in some way
to, and represent more or less accurately, properties of the
forms of energy arousing them. It is, however, quite easy
to show that we have no sufficient logical warrant for such
a belief. Light falling into the eye causes a sensation of
luminosity, a feeling belonging to the visual group or
modality; and, since usually nothing else excites such
feelings and light entering the healthy eye always does,
we come to believe that the physical agent light is some-
thing like our sensation of luminosity. But, as we have
already seen (p. 191), no matter how we stimulate the optic
nerve we still get visual sensations; close the eyes and press
with a finger-nail on one eyelid; a sensation of touch is
aroused where the finger meets the skin; but the pressure
on the eyeball distorts it and stimulates the optic nerve-
fibres in it also, and the result is a luminous patch seen
in front of the eye in such a position as a bright body must
occupy in space to radiate light to that part of the expan-
sion of the optic nerve. Finding, then, the same kind of
sensation, a visual one, produced by the totally different
causes, pressure and light, we are led to doubt if the dif-
ferences of modality in our sensations depend upon the dif-
ferences of the natural forces arousing them; and this
doubt is strengthened when we find still other forces (p. 191)

WHY OUR SEirSATIONS DIFFEB. 469

giving rise to visual sensations. But then, since light and
pressure, electricity and cutting, all cause visual sensations,
we have no valid reason for supposing that light, more than
either of the others, is really in any way like our sensation
of light: or that sight-feeling differs from sound-feeling
because objectively light differs from sound. The eye is an
organ specially set apart to be excited by light, and accord-
ingly so fixed as to have its nerve-fibres far more often ex-
cited by that form of force than by any other; but the fact
that light sensations can be otherwise aroused shows plainly
that their kind or character has nothing directly to do witli
any property of light. Just as by pinching or heating or
galvanizing a motor nerve we can make the muscles attached
to it contract, and the contraction has nothing in common
with the excitant, so the visual sensation, as such, is inde-
pendent of the stimulus arousing it and, of itself, tells
us nothing concerning the kind of stimulus which has
operated.

Differences in kind between external forces being thus
eliminated as possible causes of the modalities of our sen-
sations, we next naturally fall back upon differences in the
sense-organs themselves. They do undoubtedly differ both
in gross and microscopic structure, and the fact that pres-
sure on the closed eye arouses a touch-feeling where the
skin is compressed, and a sight-feeling where the optic nerve
is, might well be due to the fact that a peripheral touch-
organ was different from a peripheral sight-organ, and the
same force might therefore produce totally different effects
on them and so cause different kinds of feelings. However,
here also closer examination shows that we must seek far-
ther. Sensation is not produced in a sense-organ, but far
away from it in the brain; the organ is merely an apparatus
for generating nervous impulses. If the optic nerves be
divided, no matter how perfect the eyeballs, no amount of
light will arouse visual sensations; if the spinal cord be cut
in the middle of the back no pressure on the feet will cause
a tactile or other feeling; though the skin, and its nerves
and the lower half of the spinal cord be all intact. In all
cases we find that if the nerve-paths between a sense-organ

470 THE HVMAN BODY.

and the brain be severed no stimidation of the organ will
call forth a sensation. The final production of this clearly
depends, then, on something occurring in the brain, and so
tliekind of a sensation is presumably dependent upon brain
events rather than on occurrences in sense-organs. Still it
might be that something in the sense-organ caused one sen-
sation to differ from another. Each organ might excite the
brain in a different way and cause a different sensation, and
so our sensations differ because our sense-organs did. Such a
view is, however, negatived by observations which show that
perfectly characteristic sensations can be felt in the absence
of the sense-organs through which they are normally ex-
cited. Persons whose eyeballs have been removed by the
surgeon, or completely destroyed by disease, have frequently
afterwards definite and unmistakable visual sensations, quite
as characteristic as those which they had while still possess-
ing the visual end organs. The tactile sensations felt in
amputated limbs, referred to above, afford another example
of the same fact. The persons still feel things touching their
legs or lying between their long-lost toes; and the sen-
sations are distinctly tactile and not in any way less different
from visual or auditory sensations than are the touch-feel-
ings following stimulation of those parts of the skin which
are still possessed. It is, then, clear that the modality of our
sensations is to be sought deeper than in properties of the
end organs of the nerves of each sense.

Properties of external forces and properties of periphe-
ral nerve-organs being excluded as causes of differences in
kind of sensation, we come next to the sensory nerve-fibres
themselves. Is it because optic nerve-fibres are different
from auditory nerve-fibres that luminous sensations aie
different from sonorous? This question must be answered
in the negative, for we have already (p. 193) seen reason to
believe that all nerve-fibres are alike in essential structure
and that their properties are everywhere the same; that all
they do is to transmit "nervous impulses" when excited,
and that, no matter what the c::ritant, these impulses are
molecular movements, always alike in kind, though they
may differ in amount and in rate of succession. Since,

WHY OUR 8EXSATI0N8 DIFFER. 471

then, all that the optic nerve does is to send nervous impulses
to the brain, and all that the auditory and gustatory and
touch and olfactory nerve-fibres do is the same, and these
impulses are all alike in kind, we cannot explain the differ-
ence in quality of visual and other sensations by any dif-
ferences in property of the nerve-trunks concerned, any more
than we could attempt to explain the facts that, in one case,
an electric current sent through a thin platinum wire heats
it, and, in another, sent through a solution of a salt decom-
poses it, by assuming that the different results depend on
differences in the conducting copper wires, which may be
absolutely the same in the two cases.

We are thus driven to conclude that our sensations
primarily differ because different central nerve-organs in
the brain are concerned in their production. That just as
an efferent nerve-fibre will, when stimulated, cause a secre-
tion if it go to a gland-cell, and a contraction of it to go
to a muscle-fibre, so an optic nerve-fibre, carrying impulses to
one brain apparatus and exciting it, will cause a visual
sensation, and a gustatory nerve-fibre, connected with
another brain-centre, a taste sensation. In other words,
our kinds of sensation depend fundamentally on the proper-
ties of our own cerebral nervous system. For each special
sense we have a nervous apparatus with its peripheral
terminal organs, nerve-fibres, and brain-centres; and the
excitement of this apparatus, no matter in what way, causes
a sensation of a given modality, determined by the proper-
ties of its central portion. Usually the apparatus is excited
by one particular force acting first on its peripheral organs,
but it may be aroused by stimulating its nerve-fibres
directly or, as in certain diseased states (delirium), or under
the action of certain drugs, by direct excitation of the centres.
The sensations of dreams, frequently so vivid, and halluci-
nations, a,re also probably in many cases due to direct
excitation of the central organs of sensory apparatuses,
though no doubt also often due to peripheral stimulation.
But no matter how or where the apparatus is excited, pro-
vided a sensation is produced it is always of the modality
of that sense apparatus.

472 THE HUM AN BODY.

It is possible that the general statement in the last para-
graph needs some limitation, Wliile in senses of distinct
modality, this character can be ascribed only to brain projjer-
ties (so that Ave may be pretty sure that a man, the inner end
of whose optic nerve was in physiological continuity with
the outer end of his auditory, and the inner end of his
auditory with the outer end of his optic, might hear a pic-
ture and see a symphony), yet, perhaps, differences in the
rhythm or intensity of afferent nervous impulses may cause
differences in modality in less differentiated senses. Thus
contact with a cold soft object may be felt as heat,
thought to be due to the approach of a warm body; and
from such cases we must perhaps conclude that touch and
temperature depend on excitations in different ways of one
and the same brain-centre; impulses of a certain rhythm
producing a sensation of heat, and those of another (deter-
mined by the different heat and touch end organs) causing
a tactile sensation. If this be so, however, heat and touch
would be but extreme varieties of one kind of sensation,
and comparable to yellow and blue. Again, a heavy pres-
sure, gradually increased, arouses sensations which pass im-
perceptibly from touch to pain, and the result may be due
to the fact that regular and orderly afferent impulses,
determined through tactile r erve-endings, excite the centre
in one Avay; while irregular, disorderly, and violent, excited
Avhen the nerve-trunks beneath the skin are directly stimu-
lated, may cause a different sensation; much as the same
musical notes combined in one order cause pleasure but in
another are disagreeable, causing a sort of pain, although
the same brain-centres are stimulated in the two cases.
The pain from a heavy weight may, however, be merely due
to the fact that it excites the nerves very powerfully and
gives rise to impulses which radiate farther in the brain than
those causing touch sensations, and so excite new centres,
the modality of Avliich is a pain sensation.

However differences in nervous rhythm may account for
minor differences in sensation, it remains clear that the
characters of our sensations are creations of our own organ-

FECHNER8 LAW. 473

ism; they depend on properties of our Bodies and not on
properties of external things, except in so far as these may
or may not be adajjted to arouse our different sensory
apparatuses to activity. From the kind of the sensation we
cannot, therefore, argue as to the nature of the excitant: we
have no more warrant for supposing that hght is like our
sensation of light than that the knife that cuts us is like
our sensation of pain. All that we know with certainty is
states of our own consciousness, and although from these we
form working hypotheses as to an external universe, yet,
granting it, we have no means of acquiring any real knowl-
edge as to the properties of things about us. What we
want to know, however, for the practical purposes of life
is, not what things are, but how to use them for our advan-
tage, or to prevent them from acting to our disadvantage;
and our senses enable us to do this sufficiently well.

The Psycho-Physical 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 cer-
tain 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 sen-
sation can be perceived. Supposing this smallest perceptible
difference to be constant within the range of the same sense,
(which is not proved,) it is found that it is produced by dif-
ferent amounts of stimuli, measured objectively as forces;
and that there exists in some cases a relation between the two
which can be expressed in numbers. The increase of stim-
ulus necessary to produce the smallest perceptible change in
a sensation is proportional to the strength of the stimulus
already acting; for example, the heavier a pressure already
acting on the skin the more must it be increased or dimin-
ished in order that the increase or diminution may be felt.
Expressed in another way the facts may be put thus: sup-
pose three degrees of stimulation to bear to one another ob-
jectively the ratios 10, 100, 1000, then their subjective ef-
fects, or the amounts of sensation aroused by them, will be
respectively as 1, 2, 3; in other words, the sensation in-

474 THE HVMAX BODY.

creases proportionately to the logaritlim of tlie strength of the
stimulus. Examples of this, which is known as "■ Weber's "
or "Fechner's psycho-physical Icciv" will be hereafter
pointed out, and are readily observable in daily life; wo
have, for example, a luminous sensation of certain intensity
when a lighted candle is brought into a darkroom; this sen-
sation 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 api^roximately), for sensations
of medium intensity; it is applicable, for example, to light
sensations of all degrees between those aroused by the light
of a caudle and ordinary clear daylight: but it is not ti'ue
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 varia-
tions 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 statements the condition of the sense-
organ and its nervous connections is presumed to remain
the same throughout.

Perceptions. In every sensation we have to carefully
distinguish between the pure sensation and certain judg-
ments 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 sensations which leads us to form ideas
concerning the existence, form, position, and properties of
external things. Such representations as these, founded on
our senses, are called pierceptions. Since these always imply
some mental activity in addition to a mere feeling, their
f nil discussion belongs to the domain of Psychology. Phy-
siology, however, is concerned with them so far as it can
determine the conditions of stimulation and neurosis
under which a given mental representation concerning a
sensation is made. It is quite certain that we can feel

PERCEPTIONS. 475

nothing biTt states of ourselves, but, as already pointed
out, we have no hesitation in saving 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
oar 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 sen-
sitive skin, which is excited when they are moved. But if
the tip of a hair be touched by some external object we be-
lieve 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 merelv 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. 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 witli
the table. We have, simultaneously, sensations at two
places separated by the length of the rod. If we hold the rod
immovably on the table we feel only its end next the fin-
ger. If we could fix it immovably on the finger while the
other end was movable on the talkie, we would lose the sen-
sation at the finger and only belie-tte we felt the pressure
where the rod touched the table. When a tooth is touched
with a rod we only feel the contact at its end, unless it is

476 TEE HUMAN BOD Y.

loose in its socket; and then we get two sensations on
touching its free end with a foreign body.

This irresistible mental tendency to refer certain of our
states of feeling to causes outside of our Bodies, and either
in contact with them or separated from them by a certain
space, is known as the phenomenon of the extrmsic refer-
ence of our sensations.

The discussion of its origin belongs jjroperly to Psychol-
ogy, and it will suffice here to point out that it seems largely
to depend on the fact that the sensations extrinsically 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 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. And this is confirmed
when going in one direction we find the sound inci'eased,
and in the other that it is diminished. This implies that
we have a knowledge of our movements, and this we gain
through the muscular sense. It constitutes the reactive side
of our sensory life, associated with the changes,we produce
in external things; and is correlated and contrasted with
the passive side, in which other things produce sensations
by acting upon us.

As regards our common sensations we find something of
the same kind. The more readily they can be modified by
movement the more definitely do we localize them in space,
though in this case within the Body instead of outside it.
Hunger and nausea can be altered by jiressure on the pit
of the stomach; thirst by moistening the throat with water;
the desire for oxygen (respiration-hunger) by movements of
the chest; and so we more or less definitely ascribe these
sensations to conditions of those parts of the Body. Other
general sensations, as depression, anxiety, and so on, are not

SENSORY ILLUSIONS. 477

modifiable by any particular movement, and so appear to
us rather as mental states, pure and simple, than bodily
sensations.

Sensory Illusions. "I must believe my own eyes" and
"we can't always believe our senses" are two expres-
sions frequently heard, a^id, each expressing a truth. No
doubt a sensation in itself is an absolute incontrovertible
fact: if I feel redness or hotness I do feel it and that is an
end of the m'atter: but if I go beyond the fact of my having
a certain sensation and conclude from it as to j)roperties
of something else — if I form adjudgment from my sensatio7i
— 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
U23on. Now the size of this image depends on the distance
of the object; it is smaller the farther off it is and bigger
the nearer, and measurements show that the area of the
sensitive surface affected by the image of the rising moon
is no bigger than that affected by it when overhead. Why
then do we, even after we know this, see it bigger ? The
reason is that when the moon is near the horizon we imagine,
unconsciously and irresistibly, that it is farther off; even
astronomers who know perfectly 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 mortals. In fact
we have a conception of the sky over which the moon trav-
els, 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

478 THE HUMAN BODY.

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 tlie horizon is larger than the moon in the zenith, and
are ready to declare that we see it so.

So, again, a small bit of light gray paper on a white sheet
looks gray: but j^laced. on a large bright green surface it
looks purple; and on a bright red surface looks blue-green.
As the same bit of gray paper is shifted from one to the
other we see it change its color: it arouses in us different
feelings, or feelings which we interpret differently, although
objectively the light reflected from it remains the same.
Similarly a medium-sized man alongside of a very tall one
appears short, but when walking with a very short one, tall.

Such erroneous perceptions as these are known as sensory
illusions; and we ought to be constantly on guard against
rhem.

CHAPTER XXXI.

THE EYE AS AN OPTICAL INSTRUMENT.

The Essential Structure of an Eye. Every visual organ
consists 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 jjartly 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 fomis again on one
point of the sensitive surface. If A and B (Fig. 121) be
two red spots on a black surface, K, and rr be a retina,
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 connection with the rest of the nervous visual
apparatus, would be one of a certain amount of red light
reachmg the eye; the red spots, as definite objects, would
be mdislinguishable. If, however, a convex glass lens
L (Fig. 122) be put in front of the retina, it will cause to
converge again to a single point all the rays from A falling

480

THE HUMAN BODY.

upon it; so, too, with the raj^s from B: and if the focal dis-
tance (see Physics) of the lens be properly adjusted these
points of convergence will both lie on the retina^ that for

Fig. 121.— Diagram illustrating the indistinctness of vision with a retina alone.
K, a surface on which are two spots, A and B; r r, tlie retina. The diverging
lines represent rays of light spread uniformly over the retina from each spot.

rays from A at a, and that for rays from B at b. The
sensitive surface could 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-

FiG. 122.— Illustrating the use of a lens in giving definite retinal images. A, B,
K, r r, as in Fig. l~'l. L, a biconvex lens so placed that it brings to a focus on
the points a and 6 of the retina, rays of light diverging from A and B re-
spectively.

fibres would be stimulated and the result would be the
recognition of two separate red objects. In our eyes are
found certain refracting media which lie in front of the
retina and take the place of the lens L in Fig 122. That
portion of physiology which treats of the physical action of
these media, or in other words of the eye as an optical in-
strument, is known as the dioptrics of the eye.

The Appendages of the Eye. The eyeball itself con-
sists of the retina and refracting media, together with
supporting and nutritive structures and other accessory

THE EYELIDS. 481

apparatuses, as, for example, some controlling the light-
converging power of the media, and others regulating the
size of the aperture (jjupil) by which light enters. Out-
side the hall lie muscles which bring about its movements,
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 socket. It simply rotates there,
as the head of the femur does in the acetabulum. When
the blood-vessels are gorged, however, the eyeballs may be
caused to protrude (as in strangulation), and when the ves-
sels 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 strengthened by plates of fibro-cartilage. At the edge
of each eyelid the skin which covers its outside is turned
in, and becomes continuous 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 palpe-
hrcB superioris. The eyes are closed by a flat circular mus-
cle, the orbicularis palpebrarum, which, lying on and around
the lids, immediately beneath the skin, surrounds the ajjer-
ture between them. At their outer and inner angles {can-
f/ii) 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 j^ersons. 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 lacJirpnial pajnlla,
on each lid. For most of their extert the i?iner surfaces
of the eyelids are in contact with the outside of the eye-

482 THE HUMAN BODY.

biill but, near their inner ends, a red vertical fold of con-
junctiva, the semilunar fold {plica semilunaris) intervenes.
This is a remnant of the third eyelid, or nictitating
membrane, found largely developed in many animals, as
birds, in wliich it can bo drawn all over the exposed part
of the eyeball. Quite in the inner corner is a reddish ele-
vation, the caruncula laclirymnlis, caused by a collection of
sebaceous glands imbedded in the semilunar fold. Opening
along the edge of each eyelid are from twenty to thirty
minute compound sebaceous glands, called the Meibomian
follicles. Their secretion is sometimes abnormally abun-
dant, and then appears as a yellowish matter along the
edges of the e3'elids, which often dries in the night and
causes the lids to be glued together in the morning. The
eyelaslies are short 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 this, unless when excessive,
is carried off fi'om 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, from which twelve or fourteen ducts run and open
in a row at the outer corner of the upper eyelid. The se-
cretion there poured out is spread evenly over the exposed
part of the eye by the movements of winking, and keeps it
moist; finally it is drained off by two lachrymal cancds, one
of which oj^ens by a small pore {punctum lachrymalis) on
each lachrymal papilla. The aperture of the lower canal
can be readily seen l)y examining the corresponding papilla
in front of a looking-glass. The canals run inwards and
ojoen into the lachrymal sac, which lies just outside the nose,
in a holloAV where the lachrymal and superior maxillary
bones {L and M.v, Fig. 26)* meet. From the sac the nasal
duct proceeds to open into the nose-chamber below the in-
ferior turbinate bone {q. Fig. 89, p. 309).

Tears are constantly being secreted, but ordinarily in

* Page 74

MUSCLES OF THE EYEBALL.

483

such quantity as to be drained off into tlie 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 iveeping
the secretion is increased, and then not only more of it en-
ters the nose, but some flows down the cheeks. The fre-
quent 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.

Fig. 123.— 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 awaj'. a, external rec-
tus; s, superior rectus; i, internal rectus; t, superior oblique.

The Muscles of the Eye (Fig. V2o). The eyeball is
spheroidal in form and attached behind to the optic nerve, 11,
somewhat as a cherry might be to a thick stalk. On its outside
are inserted the tendons of six muscles, ionv straight and two
oblique. The straight muscles lie, one {superior rectns), s,
above, one {inferior rectus) below, one {external rectus), a,
outside, and one {internal rectus), i, inside the eyeball. Each
ai'ises behind from the bony margin of the foramen through
which the optic nerve enters the orbit. In the figure.

484 THE HUMAN BODY.

which represents tlie orbits opened from ahove, the superior
rectus of tlie right side has been removed. The superior
oblique or pulley {trochlear) muscle, t, arises behind near
the straight muscles and forms anteriorly a tendon,
■u, which passes through a fibro-cartilaginous ring, or
pulley, placed at the notch in the frontal bone, Avhere it
bounds superiorly the front end of the orbit. The tendon
then turns back and is inserted into the eyeball between
the ujiper and outer recti mu.-cles. 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 lachiymal
sac. It passes thence outwards and backwards beneath the
eyeball to be inserted into its outer and j^osterior 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 (see p. 168). 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, that is neither truly vertically nor horizontally,
but partly both; or, finally, it may be rotated on its antero-
posterior axis. The oblique movements are always accom-
panied by a slight amount of rotation. When the glance is
turned to the left, the left external rectus and the right in-
ternal 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 downwards and
outwards with a certain amount of rotation; the inferior
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. Movements 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-muscu-
lar apparatus. When the co-ordination 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,

AJ!^ATO^rY OF EYEBALL.

485

when the eyeball had been turned out by the external rec-
tus, it would not be brought back again to its median
position. A left internal squint would be caused, similarly,
by pai'alysis of the left external rectus; and probably by
disease of the sixth cranial nerve or its brain-centres.
Dropping of the upper eyelid (ptosis) indicates paralysis of

Fig. 134. — The left eyeball iu horizontal section from before back. 1, sclerotic;
2, junction of sclerotic and cornea: 3, cornea; 4, 5. conjunctiva: 6 posterior
elastic layer of cornea: 7, ciliary muscle: 10. choroid; 11. 13 ciliary processes;
14, iris; 15, retina; 16, optic nerve; 17, artery entering retina m optic nerve; 18,
fovea centralis: 10. region where sensory part of retina ends; •^•^, suspensory
ligament; 23 is placed in the canal of Petit and the line from 25 points to it; 24,
the anterior part of the hyaloid membrane: 26, 27, 28 are placed on the lens;
28 points to the line of attachment aroimd it of the suspensory ligament: 29,
vitreous himior; 30, anterior chamber of aqueous humor; 31, posterior chamber
of aqueous humor.

its elevator muscle (p. 481), and is often a serious syinjDtom,
as pointing to disease of the brain-parts from which it is
innervated.

The Globe of the Eye is on the whole spheroidal, but
consists of segments of two spheres (see Fig. 124). 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

486 THE HUMAN BODY.

measures about 22.5 milliTnetens (^ inch), and from side
to side about 25 millimeters (1 inch). Except when looking
a-t near objects, the antero-posterior axes of the eyeballs are
nearly parallel, but the optic nerves diverge considerably
(Fig. 123); each joins its eyeball, not at the centre, but about
2.5 mm. (^V inch) on the nasal side of the posterior end of
its antero-posterior axis. In general terms the eyeball may
be described as consisting of three coats and three refrad-
irifj media.

The outer coat, 1 and 3. Fig. 124, consists of the sderofic
and the cornea, the latter being transparent and situated in
front; the former ie opaque and white and covers the back
and sides of the globe and part of the front, where it is seen
between the eyelids as the white of the eye. Both are
tough and strong, being composed of dense connective tis-
sue. The white of the eye and the cornea are also covered
over by a thin layer of the conjunctiva, 4 and 5. Behind
the proper connective-tissue layer, 3, of the cornea is a thin
structureless membrane, 6, lined inside by a single layer of
epithelial cells; it is called the membrane of Descemst, or
posterior elastic layer.

The second coat consists of the clioroid, 9, 10, the ciliary
processes, 11, 13, and the iris, 14. The choroid consists
mainly 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 eyeball, where it begins to dimin-
isli in diameter, the choroid is thrown into plaits, the ciliary
processes, 11, 12, 13. Beyond these it continues as the iris,
which forms the colored part of the eye which is seen
through the cornea; and in the centre of it this a circular
aperture, the pupil: so the second coat does not, like the
outer one, comj)letely envelop the ball. In the iris are
two sets of plain muscular fibres; a circular around the
margin of the i)upil and narrowing it when they contract;
the other set radiate from the inner to the outer margin of
the iris and by their contraction dilate the pupil. The
pigment in the iris is yellow, or of lighter or darker
brown, according to the color of the eve, and more or less

HISTOLOGY OF RETINA. 487

abundant according as the eye is black, brown, or gray. In
blue eyes tlie pigment i;3 confined to the deeper layers and
modified in tint by light absorption in the anterior color-
less strata through which the light passes.

The third coat of the eye, the retina, 15, is its essential
portion, being the part m which the light produces those
changes that give rise to impulses in the optic nerve. It
is a still less complete envelope than the second tunic, ex-
tending 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, transparent. Usually
when an eye is opened it looks colorless; but by taking
proper precautions the natural purjDle color of some of its
outer layers can be seen. Its most external layer, more-
over, is composed of black pigment cells. On its inner
surface two parts, different from the rest, can be seen in a
fresh eye. One is the point of entry of the optic nerve,
16, the fibres of which, penetrating the sclerotic and
choroid, spread out in the retina. At this jolace the retina
is whiter than elsewhere and presents an elevation, the
02)tic mound. The other peculiar region is the yellow sjjot
{macula lutea), 18, which lies nearly at the posterior end of
the axis of the eyeball and therefore outside the optic mound;
m its centre the retma is thinner than elsewhere and so a
•!^\i{ fovea centralis), 18, is formed. This appears black, the
thinned retina there allowing the choroid to be seen through
it more clearly than elsewhere. In Fig. 125 is represented
the left retina as seen from the front, the elliptical darker
patch about the centre being the yellow spot, and the
white circle on one side, the optic mound. The vessels of
the retina arise from an artery (17, Fig. 124) which runs
m with the optic nerve and from which branches diverge
as shown in Fig. 125.

The Microscopic Structure of the Retina. A simplified
stratum, continuous with the proper retina, and formed of
a layer of nucleated columnar cells is continued over the
ciliary processes; elsewhere the membrane has a very com^
plex structure and a section taken, except at tlie yellow
spot or the optic mound, shoM's ten layers, partly sensory

488

THE HUMAN BODY

apparatuses and nerve-tissues, and partly accessory struc-
tures.

Beginning (Fig. 12G) on the inner side we find, first, the
itUernal llmUuig monhrane, 1, a thin structureless layer.
Next comes the nerve-fibre layer, 2, formed by radiating
fibres of the optic nerve; third, the nerve-cell layer, 3;

Fig. 135.— The left retina as it would be seen if the front part of the eyeball
with the lens and vitreous humor were removed.

fourth, the i7iner molecular layer, 4, consisting partly of
very fine nerve-fibrils, and largely of connective tissue;
fifth, the inner granular layer, 5, composed of nucleated
cells, with a small amount of protoplasm at each end, and
a nucleolus. These granules, or at any rate the majority
of tliem, have an tjiner process running to the inner
molecular layer and an outer running to, 6, the outer
molecular layer, which is thinner than the inner. Then
comes, seventh, the rod and cone fibre layer, 7, or outer
granular layer; composed of thick and thin fibres on each
of which is a conspicuous nucleus witli a nucleolus. Next
is the thin external limiting membrane, 8, perforated by
apertures through which the rods and. cones, 9, of the ninth
layer join the fibres of the seventh. Outside of all, next
tlie choroid, is the pigmentary layer, 10. In addition, cer-

HISTOLOGY OF RETINA.

489

taiu libres rim vertically through the retina from the inner
to the outer limiting membrane; they are known as the
radial fibres of Miiller and give off lateral branches, which
are especially numerous in the molecular layers.

Fig. 136. — a section througlti the retina from its anterior or inner surface,
1, in contact witli the hyaloid memh ane, to its outer. 10, in contact witli the
choroid. 1, internal limitina: membrane; 2, nerve fibre layer: 3, nerve -eel I
layer; 4, inner molecular layer; 5, inner granular layer; 6, outer molecular
layer; 7, outer granular layer; 8, external luniting membrane; 9, rod and cone
layer; 10, pigment-cell layer.

On account of the way in which the supporting and essen-
tial parts are interwoven in the retina it is not easy to track

490 THE UUMAN BODY.

the latter through it. We shall find, however (Cha]).
XXXri.). tliat light ih-st acts u])on the rod and cone layer,
traversing all the thickness of inner strata of tlie retina to
reach this, before it can start those changes which result in
visual sensations; and it is thereiore probable that the rods
and cones are m direct continuity with the optic nervc-
tibres. The limiting membranes, with the fibres of Miiller
and their branches, are undoubtedly accessory.

Each rod and cone consists of an outer and an i7iner
segment. The outer segments of both tend to split up trans-
versely into disks and are very similar, except that those of
the rods are longer than those of the cones and do not taper
as the latter do. The inner segments of the cones are
swollen, while those of the rods are narrow and nearly cy-
Imdrical. Over most of the retina the rods are longer and
much more numerous than the cones, but near the ciliary
processes they cease before the cones do, and in the yellow
spot elongated cones alone are found. In this region the
whole retina is much modified; at its margin all the layers
are thickened but especially the nerve-cell layer, Avliich is
here six or seven thick, while elscAvhere the cells are found
m but one or two strata. All the fibres also are oblique,
reaching in to become continuous with the cones of the
central pit, Avhich are long, slender and very closely packed.
In the fovea itself all the layers, except that of the cones,
thin away, and so the depression is produced. The fovea is
the seat of most acute vision; when we look at an object
we always turn our eyes so that the light proceeding fi'om
it shall be focussed on this spot. Where the optic neiwe
enters, all the layers but the nerve-fibre layer, which is very
thick, and the internal limiting membrane, are absent.

The blood-vessels of the retina lie in the nerve-fibre and
nerve-cell layers.

The Refracting Media of the Ejne are, in succession from
before back, the cornea, the aqueous humor, the crystalline
lens, and the vitreous humor.

The aqueous humor fills the space between the front of
the lens, 28, and the back of the cornea. This space is in-
completely divided by the iris into an anterior chamber.

THE REFR ACTING MEDIA OF THE EYE. 491

30. and a posterior, 31 (Fig. 124). Chemicalh', the aque-
ous humor consists of water holding in solution a small
amount' of solid matters, mainly common salt.

The crystalline lens (?-S, 26, 27) is colorless, transparent,
and biconvex, 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 consist-
ence it is soft, but its central layers are rather more dense
than the outer.

The vitreous liumor is a soft jelly, enveloped in a thin
cai)sule, the hyaloid memhrane. In front, this membrane
splits into two layers, one of which, 22, passes on to be
fixed to the lens a little in front of its edge. This layer is
known as the susjjcnsory ligament of the lens; its line of at-
tachment around that organ is not straight but sinuous
as represented by the curved line between 28 and 26 in Fig.
124. The space between the two layers into which the
hyaloid splits is the canal of Petit. The vitreous humor
consists mainly of water and contains some salts, a little
albumin, and some mucin. It is divided up, by delicate
membranes, into compartments in which its more liquid
portions are imprisoned.

The Ciliary Muscle. Eunning around the eyeball
where the cornea joins the sclerotic is a little vein called
the canal of Schlenim; it is seen in section at 8 in Fig. 124.
Lying on the inner side of this canal, just where the iris
and the ciliary processes meet, there is some 2:ilain muscular
tissue, imbedded mainly in the middle coat of the eyeball
and forming the ciliary muscle, which consists of a radial
and a circular portion. The radial part is much the larger,
and arises m front from the inner surface of the sclerotic; the
fibres pass back, spreading out as they go, and are inserted
into the front of the choroid opposite the ciliary processes.
The circular part of the muscle lies around the outer rim of
t he iris. The contraction of the ciliary muscle tends to pull
forward (radial fibres) and press inward (circular fibres) the
front part of the choroid, to which the back part of the sus-
pensory ligament of the lens is closely attached. In this way
the tension exerted on the lens bv its ligament is diminished.

492 THE HUMAN BODY.

The Properties of Light. Before proceeding to the
study of the eye as tin opticul instrument, it is necessary to
recall brieiiy certain properties of liglit.

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. When a stone is thrown into a pond a series of
circular waves travel from that point in a horizontal direc-
tion over the water, while the particles of water themselves
move up and down, and cause the surface inequalities
which we see as the waves. Somewhat similarly, light-Avaves
spread out from a luminous point, but in the same medium
travel equally in all directions so that the point is surrounded
by shells of spherical waves, instead of rings of circular
waves traveling in one plane only, as those on the surface
of the water. Starting from a luminous point light
would travel in all directions along the radii of a sphere of
which the point is the centre; the light propagated along
one such radius is called a ray, and in each ray the ethereal
particles swing from side to side in a jilane perpendicular
to the direction of the ray. Taking a particle on any ray
it would swing aside a certain distance from it, then back
to it again, and across for a certain distance on the other
side; and then back to its original position on the line of
the ray. Such a movement is an oscillation, and takes a
certain time; in lights of certain kinds the periods of oscilla-
tion are all the same, no matter how great the extent or
arrqjlitude of the oscillation; just as a given pendulum will
always comj)lete its swing in the same time no matter
whether its swings be great or small. Light composed of
rays in which the periods of oscillation are all equal is
called monochromatic or simple light, while light made of a
mixture of oscillations of different periods is called mixed
or compound light.

If monochromatic light is steadily emitted from a point,
then, at certain distances along a ray, we come to particles
in the s,2imQ phase ot oscillation, say at their greatest dis-
tance from their position of rest; just as in" the concentric
waves seen on the water after thi'owing in a stone we would

PROPERTIES OF LIGHT.

493

along uny radius meet, at intervals, with water raised most
above its horizontal ])lane as the crest of a wave, or depressed
most below it as the holloiv of a wave. The distance along
the ray from crest to crest is called a ivave-lengtli and is
always the same in any given simple light; but differs in
different-colored lights; the briefer the time of an oscillation
the less the wave-length.

When light falls on a polished surface separating two
transparent media, as air and glass, part of it is reflected
or turned back into the first medium; part goes on into
tlie second medium, and is commonly deviated from its
original course or refracted. The original ray falling in the
surface is the incident ray.

Let A B (Fig. 127) be the
surface of separation ; a x the
incident ray; and CD the
perpendicular or normal to
the surface at the point of
incidence: ax Cwill then be
the angle of incidence. Then
the reflected ray makes an
angle of reflection with the
normal which is equal to
the angle of incidence: and
the reflected ray lies in
the same plane as the inci-
dent ray and the normal to
the surface at x. The re-
fracted ray lies also in the
same plane as the normal and
the incident ray, but does not continue in its original direc-
tion, xf; if the medium below A B be denser than that
above it, the refracted ray is bent in the direction xd
nearer the normal, and making with it an angle of refrac-
tion, Dxd, smaller than the angle of incidence, ax C. If,
on the contrary, the second medium be less dense than the
first, the refracted ray xg is bent away from the normal,
and makes an angle of refraction, Dxg, greater than the
angle of incidence. The ratio of the sine of the angle of

Fio. 127.— Diagram illustrating the
refraction of light. A B. surface of
separation between two transparent
media; CD. the perpendicular to the
surf'ace at the point of incidence, .c;
a X. incident ray ; .r d. refracted ray. if
the second medium be denser than the
first; xg, refracted ray. if the second
medium is less refractive than the
first. The reflected ray is not repre-
sented, but would make an angle with
Cx equal to the angle a x C.

494

THE HUMAN BODY.

incidence to that of the angle of refraction is always the
same for the same two media with light of the same wave-
length. When the first medinm is air the ratio of the sine
of the angle of refraction to that of the angle of incidence
is called the refractive index of the second medium. The
greater this refractive index the more is the refracted r;iy
deviated from its original course. Rays which fall perpen-
dicularly on the surface of separation of two media pass on
without refraction.

The shorter the oscillation periods of light-rays the mxore
they are deviated by refraction. Hence mixed light when

Fig. 128.— Diagram illustrating the dispersion of mixed light by a prism.

sent through a prism is spread out, and decomposed into
its simple constituents. For let a x (Fig. 128) 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 A B of
the prism, that portion which enters will be refracted
towards the normal ED, but the short waves more than the
longer. Hence the former will take the direction x y, and
the latter the direction x z. On emerging from the prism
l)oth rays will again be refracted, bat now from the no»--
mals Fy and G z, since the light is passing from a denser to
a rarer medium. But again the ray x y, made up of shorter
waves, would be most deviated, as in the direction y v, and

REFRACTION BY LENSES. 495

the long waves less, in the direction z r. If a screen were
pnt at S S, we would receive on it at separate points, v and
r, the two simple lights which were mixed together in the
compound incident ray a x. Such a separation of light-rays
is called disjjersion.

Ordinary white light, such as that of the sun, is com-
posed of ethereal vibrations of all possible lengths. Hence
when such light is sent through a prism it gives a contin-
uous band of light-rays, known as the solar spectrum,
reaching from the least refracted to the most refracted and
shortest. The exceptions to this statement due to Frauen-
hofer's lines (see Physics) are unessential for our present
purpose. All of the simple lights into which the compound
solar light is thus separated do not, however, excite in us
visual sensations when they fall into the eye, but only cer-
tain middle ones. If solar light were used with the prism,
Fig. 125, certain least refracted rays between r and 8 would
not be seen, nor the most refracted between v and S; while
between v and r would stretch a luminous band exciting in
us the series of colors red (due to the least refracted visible
rays), successively through orange, yellow, green, bright
blue, and indigo, to violet, which latter is the sensation
aroused by the most refrangible visible rays. The still
shorter waves beyond the violet, are known mainly by their
chemical eifects and make up what are called the actinic
rays; the longer invisible waves, beyond the red, exert a
powerful heating influence and compose the thermal or darh
heat rays. The eye, as an organ for making known to us
the existence of ethereal vibrations, has, therefore, only a
limited range.

Refraction of Light by Lenses. In the eye the refract-
ing media have the form of lenses thicker in the centre
tlian towards the periphery; and we may here confine our-
selves therefore to such converging lenses. If simple light
from a point A, Fig. 122, 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, r r, be held at a it will therefore
receive an image of the luminous point A. For every con-

496

THE HUMAN BODY.

Fig. 129.— Diagrram illustrating
the formation of an image by a
converging lens.

verging lens there is such a point behind it at which the
rays 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 proceeding from one point of the object will meet
m the conjugate focus of that point behind. The image
is inverted, as can be readily seen from Fig. 129. All rays

from the point A of the object
meet at the point a of the
image; those from B at l, and
those from intermediate points
at intermediate positions. If
the single lens were replaced
by several combined so as to
form an ojitical system the
general result would be the same, provided the system
were thicker in the centre than at its perijohery.

The Camera Obscura, as used by photographers, is an
instrument which serves to illustrate the formation of
images by converging systems of lenses. It consists of a
box blackened inside and having on its front face a tuJje
containing the lenses; the posterior wall is made of ground
glass. If the front of the instrument be directed on ex-
terior objects, inverted and diminished images of them will
be formed on the ground glass; those images are only 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 distance between the lenses and the
ground glass, in common language "focusing the instru-
ment," 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
reason of this may readily be seen from Fig. 130. If the
system of lenses brings the parallel rays a c and h d, pro-
ceeding from an infinitely distant object, to a focus at x,

REFRACTION IN THE ETE. 497

then the diverging rays/c and fcl, proceeding from a nearer
point, will be harder to bend round, so to speak, and will
not meet until a point y, farther behind the system than
X IS. The more divergent the rays, or what amounts to the
same thing, the nearer the points they proceed from, the
farther behind the refracting system will y be.

F<*

Fig. 130,— Diagram illustrating the need of " focusing" in an optical instru
nient.

The eye is such a system, made up of the four refracting
media, cornea, aqueous humor, lens, and vitreous humor.
These four media are, however, reduced to three prac-
tically, 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 between the air and ihe
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
refraction of light it therefore follows that (Fig. 131) the
rays c d will at the corneal surface be refracted towards the
normals N, N, and take the course cl e. At the front of
lenses they will again be refracted towards the normals to
that surface and take the course e/; at the back of the lens,
])assing from a more refracting to a less refracting medium,
they will be bent from the normals N" and take the course
/ g. If the retina be there, these jjarallel rays will therefore
be brought to a focus on it. In the resting condition of
the natural eye that is what happens to jDarallel rays entering
it; and, since distant objects send into the eye rays which
are practicallv parallel, such o1)jects are seen distinctly

498

THE HUMAN BOD 7.

without any effort; all rays emanating from a point of the
object meet again in one point on the retina.

Accommodation. Points on near ob ject s send into the eye
divei'ging rays: those therefore would not come lo 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 mdeed),
there must exist some means by which the eye is adapted
or accommodated for looking at objects at different distances.
That some change does occur one can, also, readily prove

Fig. 131.— Diagram illustrating the .surfaces at which light is refracted in the
eye.

by observing that we cannot see distinctly, at the same
moment, both near and distant objects. For example
standing at a window, behind a lace curtain, we can if we
choose look at the threads of the lace or the houses across
the street; but when we look at the one we only see the
other indistinctly; and if, after looking at the more distant
object, we look at the nearer we ex2:)erience 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 dis-
tinctly seeing distant objects, might conceivably be accom-
modated for near vision in several ways. The refracting

ACCOMMODATION. 499

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 from
the refracting surfaces might be increased, for example by
compression of the eyeball by the muscles around it; how-
ever, experiment shows that changes of accommodation can
be brought about in the fresh excised eyes of animals, in
which no such compression is jDossible; we are thus reduced
to the third explanation, that the refracting surfaces, or
some of them, become more curved, and so bring more
diverging rays sooner to a focus; since a lens of smaller
curvature is more converging than one of greater curvature
composed of the same material. 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 sur-
face of the lens becomes considerably more
convex and the posterior slightly so. As
already pointed out when light meets the
separating surface of two media some is
reflected and some refracted (p. 493). If,
therefore, a person be taken into a dark
room and a candle held on one side of his i-I^^'Bhz!^}^^

images ot a candle-
eye, while he looks at a distant object an 5^"^^ as seen re-

•J ' . _ "> fleeted from the re-

observer can see three images of its flame fracting media of
in his pupil, due to that part of the light
reflected from the surfaces between the media. One (a.
Fig. 133) is erect and bright, reflected from the convex
mirror formed by the cornea; the next, h, 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. If now 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,
indicating (see Ph3'sics) a greater convexity of the reflecting
surface; the image from the back of the lens also becomes very
slightly smaller, indicating a feebly increased curvature.

a 6 d

500

THE HUMAN BODY.

Accommodation is brouglit about mainly by the ciliavy
muscle. In tlie 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 according to the degree of its contraction
slackens the suspensory ligament (p. 491), and then the
elastic lens, relieved from the lateral drag, bulges out a
little in the centre.

Short Sight and Long Sight. In the eye the range of
accommodation is very great, allowing the rays from jioints

infinitely distant up to those
from points about eight inches
in front of the eye to be
brought to a focus on the re-
tina. In the normal eye par-
allel rays meet on the retina
when the ciliary muscle is
completely relaxed {A, Fig.
133). Such eyes are emme-
tropic. In other eyes the eye-
ball IS too long from before
back; in the resting state par-
allel rays meet in front of the
retina (B). Persons with such
eyes, therefore, cannot see distant objects distinctly without
the aid of diverging (concave) spectacles; they are short-
sighted or myopic. Or the eyeball may be too short from
before back; then, in the resting state, parallel 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 (con-
vex) spectacles. Such eyes are called hypermetropic, or in
common language long-sighted.

Fig. 133— Diagram illustrating the
path of parallel ravs after entering
an emmetropic {A), a myopic {B),
and a hypermetropic (C) eye.

HYGIENE OF THE EYE. 501

Hygienic Remarks. Since muscular effort is needed by
the normul eye to see near objects, it is clear why the pro-
longed contemplation of such is more fatiguing than look-
ing 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 tliat enough light is reflected from it into the
eye in order to enable the letters to be seen at 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 neu-
ralgic pains about the eyes are apt to come on, and the
overstrained organ may be permanently injured. Old per-
sons are apt to have such eyes; but young childen frequently
also possess tliem, and if so should at once be provided
with spectacles.

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 per-
manent 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 mj^opia become gradually worse the
eyes should be rested for several months. Short-sighted
persons are apt to have, or acquire, peculiarities of appear-
ance: their eyes are often prominent, indicative of the
abnormal length of the eyeball. 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 peculiari-
ties of character, and that myopes are usually unsuspicious
and easil}^ pleased; being unable to observe many little

502 THE HUMAN BODY.

matters in the demeanor or expression of those with
whom they converse, which, being noticed by those of
quiclvcr siglit, might induce feelings of distrust or annoy-
ance."

In old age the eyeball tends to become flattened; hence
cmmetro])ic eyes become hypermetropic and old persons
are Ui^ually " long-sighted " and need convex glasses. 8nch
a flattening of the eyeball is of course a relief to the
myopic eye; and so short-sighted persons can frequently,
when old, still read without glasses. But this is poor com-
pensation for the mistiness with which everything around
them, except very near objects, has been seen throughout
their previous life.

In all forms of deficient accommodation too strong glasses
will injure the eyes irreparably, increasing the defects
they are intended to relieve. Skilled advice should there-
foi-e be invariably obtained in their selection, except per-
haps 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 of course be obtained.

Optical Defects of the Eye. The eye, though it an-
swers admirably 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 re-
frangible than those at the red end. Hence they are
brought to a focus sooner. The light emanating from a
point on a white object 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 j^arallel red rays, then violet rays from the same
source will meet half a millimeter in front of it, and cross-
ing and diverging there make a little violet circle of diffu-
sion around the red point on the retina. In optical instru-

OPTICAL DEFECTS OF THE EYE. 503

nients this defect is remedied h\ combining together lenses
made of different kinds of glass; such compound 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 ap-
pear 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 is inexact they appear
also when the boundary between a white and a black sur-
face is observed. The phenomena due to chromatic
aberration are much more easily seen if light containing
only red and violet rays be used instead of Avhite light con-
taining all the rays of intermediate refrangibility. Ordi-
dinary 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 the sunlight be suffered to enter 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 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 through their central parts. If the
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 centre 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 Avill form
around this a small luminous circle of light — such as would
be formed by sections of the cones of converging rays in Fig.
122, taken a little in front of r r. This defect is found in all

504

THE HUMAN BODY.

optical instruments, as it is impossible in practice to grind
lenses 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 external and more strongly refracted rays, preventing
them from reaching the retina, (b) The outer layers of
the lens are less refracting than its central; lience the rays
passing through its peripheral parts are less refracted than
those passing nearer its axis.

3. Ii'Tegularities 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. Sup-
pose the vertical meridian to be tlie most curved; then the
rays proceeding from points along a vertical line will be
brought to a focus sooner than those from points on a hori-
zontal line. If the eye is accommodated to see distinctly
the vertical line, it will see indistinctly the horizontal and
vice versa. Few people therefore see equally clearly at once
two lines crossing one another at right angles. The pheno-
menon is most obvious, however, when a series of concentric
circles (Fig. 134) is looked at: then when the lines appear

sharp along some sectors,
they are dim along the
rest. When this defect,
known as astigmatism, is
marked it causes serious
troubles of vision and re-
quires 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
\\Vimox caxi?>mgr)iuscm volitantes; that is, the appearance of

Fig. 134.

OPTICAL DEFECTS OF THE EYE. 505

minute bodies floating in space outside tlie eye, but chang-
ing their position when the position of the eye changes,
by which fact their origin in interr al causes may be recog-
nized. 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 entopficj the most
interesting are those due to the retinal blood-vessels (ChajD.
XXXII. ). Tears, or bits of the secretion of the Meibo-
mian 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.

CHAPTER XXXII.

THE EYE AS A SENSORY APPARATUS.

The Excitation of the Visual Apparatus. The excita-
ble visual apparatus for each eye consists of the retina, the
optic nerve, and the brain-centres 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 stimulations of it to that cause, unless
we have special reason to know the contrary. As already
pointed out (p. 468) pressure on the eyeball causes a lumi-
nous sensation (phospheite), 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 m the line of its long axis, cross that axis, if
Ave press the outer side of the eyeball we get a visual sensa-
tion 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 ijomts of
the system; these {kh'. Fig. 135) 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
^ to ^ (the first nodal point) and then another, parallel to
the first, from h' (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 {Jc, Fig. 136),

POSITION OF RETIXAL IMAGES.

507

placed near the back of the lens. By manifold experience
we have learnt that a lumiuous body {A Fig. 136) which we
see, always lies on the prolongation of the hue joining the
excited part of the retina, a, and the nodal point, k. Hence

Fig. 135.— Diagram illustrating the points at which incident rays in the eye
meet the retina, x x, optic axis; k. first nodal point; k\ second uodal point;
6. point where the image of B would be formed, were the eye properly accom-
modated for it; a, tlie retinal point where the image of A would be formed.

any excitation of that part of the retina makes us think of a
luminous body somewhere on the line a A, and, similarly,
any excitation of h, of a body on the line h B or its pro-
longation. It is only other conflicting experiences, as that

Fig. 136.— Diagrammatic section through the eyeball,
nodal point.

a; a-, optic axis; fc,

with the eyes closed external bodies do not excite visual
sensations, and the constant connection of the ])ressure felt
on the eyelid with the visual sensation, that enable us
when we press the eyeball to conclude that, in spite of what

508 TEE HUMAN BODY.

we seem to see, the luminous sensation is not due to objective
light from outside the eye.

The Idio-Retinal Light. The eyelids are not by any
means perfectly opaque; in ordinary daylight they still allow
a considerable quantity of light to penetrate the eye, as any
one may observe by passing his hand in front of the closed
eyes. But even in a dark room with the eyes completely
covered up so that no objective light can enter them, there
is still experienced a small amount of visual sensation due to
internal causes. The field of vision is not absolutely dark
but slightly lummous, with brighter fleeting patches trav-
ersing it. These are especially noticeable, for example, in
trying to see and grope one's way with the eyes open up a
perfectly dark staircase. Then the luminous patches attract
special attention because they are apt to be taken for the
signs of objective realities; they become very manifest when
any sudden jar of the Body, due for example to knocking
against something, occurs; and have no doubt given rise to
many ghost stories. These visual sensations felt in the
absence of all external stimulation of the eyes, may for con-
venience be spoken of as due to the idio-retinal light.

The Excitation, of the Visual Apparatus by Light.
Light only excites the retina when it reaches its nerve end

Fig. 137.

organs, (he rods and cones. The proofs of this are several.
1. Light does not aronse visval sensations when it falls
directly on tl\e fibres of the optic nerve. Where this nerve
enters (p. 490) is a retinal part possessing only nerve-fibres,
and this part is blind. Close the left eye and look steadily
with the right at the cross in Fig. 137, holding the book

FUNCTION OF RODS AND CONES.

509

vertically in front of the face, and moving it to and fro.
It will be found that at about 25 centimeters (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 ojDtic nerve centres.

When the right eye is fixed on the cross, it is so directed
that rays from this fall on the yellow spot, y, Fig. 138.
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 eye from the paper
be /, and that of the nodal point from the
retina (which is 15 mm. ) be F, then the dis-
tance, 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 yellow spot (p. 487).

3. The above experiment having shown
that light does not act directly on the
optic nerve-fibres any more than it does
on any other nerve-fibres, 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
tlie outer part of the retina, in the rods and cones. This is
jn'oved 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
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 surface looked at will appear
luminous with reddish-yellow light, and on it will be seen

Fig. 138.

510 THE HUMAN BODT.

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 them, and
therefore in the outer layers of the retina since the vessels
lie (p. 490) in its inner strata.

If the light is kept steady the vascular shadows soon dis-
appear; in order to continue to see them the candle roust
I^e 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 thi.s
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 do not be 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 very little stimu-
lated and retain their original excitability 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 indistin-
guishable. To see them continuously we must keep shift-
ing the eyes so that the same parts of the visual apparatus
are alternately fatigued and rested, and the general irrita-
Ijility of the whole is kept about the same. So, in
Purkinje's experiment, if the position of the shadows
I'cmain the same, the shaded part of the retina soon be-
comes 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
perceived. It is for this same reason that we do not see
the vessels under ordinary circumstances. When light, as
usual, enters the eye from front through the pupil the

PURKiyjE'S EXPERIMENT.

511

Fig. 1S9.

.shadows always protect the same jDarts of the retina, and
these parts are thus kept sufficiently more excitable than the
rest to make up for the less light reaching them through
the vessels. To see the latter we must throw the light into
the eye in an unusual direction, not through the pupil but
laterally through the sclerotic. If v, Fig. 139, be the
section of a retinal vessel, ordi-
narily its shadow will fall at
some point on the line prolonged
through it from the centre of
the pupil. If a candle be held
opposite h it illuminates that
part of the sclerotic and from
there light radiates and illumines
the eye. The sensation we refer
to light entering the eye in the
usual manner through the pupil,
and accordingly see the surface
we look at as if it were illuminated. The shadow of v is
now cast on an unusual spot c, and we see it as if at the point
d on the wall, on the prolongation of the line joining the
nodal point, I:, of the eye with c. If the candle be moved
so as to illuminate the point b' of the sclerotic, the shadow
of V will be cast on c' and will accordingly seem on the
wall to move from d to d\ It is clear that if we know how
far h is from V , how far the wall is from the eye, and how
far the nodal point is from the retina (15 mm. or 0.6 inch),
and measure the distance on the wall from d to d' , we can
calculate how far c is from c': and then how far the vessel
throwing the shadow must be in front of the retinal jiarts
perceiving it. In this way it is found that the part seeing
the shadow, that is the layer on which light acts, is just
about as far behind the retinal vessels as the main vascular
trunks of the retina are in front of the rod and cone layer.
It is, therefore, in that layer that the light initiates those
changes which give rise to nervous impulses; which is
further made obvious by the fact that the seat of most
acute vision is the fovea centralis, where this layer and the
cone-fibres diverging from it alone are found (p. 490).

512 THE HUMAN BODY.

When we want to see anything distinctly we always turn
our eyes so that its image shall fall on the centre of the
yellow spot.

The Vision Purple. How light acts in the retina so as to
produce nerve stimuli is still uncertain. Recent observa-
tions show that it produces chemical changes in the rod and
cone layer, and seemed at first to indicate that its action
was to produce substances which were chemical excitants
of nerve-fibres; but although there can be little doubt that
these chemical changes play some important part in vision,
what their role may be is at present quite obscure. If a
perfectly fresh retina be excised raj)idly, 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 focused 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 jiurple 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 in that he uses light
to produce chemical changes which give rise to colored
bodies, while here the reverse is the case. If the eye be
not rapidly excised 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. Portions of frogs' retinas raised from this bleach
more rapidly than those left in contact Avith it, but become
soon purple again if let fall back upon the pigment-cells.
It thus seemed as if we had got a clue to the physiological
action of light in the eye: but experiments show that
animals (frogs) exposed for a long time to a bright light
may have their retinas completely bleached and still see
very well; they can still unerringly catch flics that come

VISION PURPLE. 513

withm their reach; and they can also distinguish colors, or
at least some colors, as green. Moreover, the vision purple
is only found in the outer segments of the rods; there is
none in the cones, and yet these alone exist in the yellow
spot of the human eye, which is the seat of most acute
vision; and animals, such as snakes, which have only cones
in the retina, possess no vision purple and nevertheless see
very well.

It may be that other bodies exist in the retina which are
also chemically changed by light, but the changes of which
are not accompanied by alterations in color which we can
see; and m the absence of the vision purple seeing might be
carried on by means of these, which may be less quickly
destroyed by light and so still persist in the bleached
retinas of the frogs above mentioned. For the present,
however, the question of the part, if any, played in vision
by such bodies must be left an open one.

The Intensity of Visual Sensations. Light considered
as a form of energy may vary in cpiantity; physiologically,
also, we distinguish quantitative differences in light as
degrees of brightness, but the connection between the in-
tensity of the sensation excited and the quantity of energy
represented by the stimulating light is jiot 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
yisual sensations than the same quantity of energy in the
form of violet light. The ultra-violet rays only become
visible, and then very faintly, when all others are suj^pressed;
but if they be passed through some fluorescent substance
(see Physics), such as an acid solution of quinine sulphate,
which, without altering the amount of energy, turns it into
ethereal oscillations of a longer period, then the light be-
comes readibly perceptible.

Even with light-rays of the same oscillation period our
sensation is not proportioned to the amount of energy in
the light; to the amount of heat, for example, to which it
would give rise if all transformed into it. If objective
light increase gradually in amount our sensation increases

514 THE HUMAN BODY.

also, up to a limit beyond which it does not go, no matter
how strong the light becomes; but the increase of sensation
takes place far more slowly than that of the light, in accord-
ance with the psycho-physical law mentioned on page 473.
If we call the amount of light given out by a single candle
a, then that emitted by two candles will be 2a; and so on.
If the amount of sensation excited by the single candle be
A, then that due to two candles will not be %A, and that
by three will be far less than ^A. If a white surface, F,
Fig. 140, be illuminated by a candle at c and another else-
where, and a rod, o, be placed
so as to intercept the light
from c, we see clearly a shadow,
since our eyes recognize the
difference in luminosity of
this part of the paper, reflect-
^^' ' ing light from one caudle only.,

from that of the rest which is illuminated by two: that is
we tell the sensation due to the stimulus a from that due
to the stimulus 2rt. If now a bright lamp be brought in
and placed alongside, and its light be physically equal to
that of 10 candles, we cease to joerceive the shadow s.
That is the sensation aroused by objective light = 12«
(due to the lamp and tAVO candles) cannot be told from that
due to light = 11«; although the difference of objective
light is still la as before. Most persons must have ob-
served illustrations of this. Sitting in a room with three
lights not un frequently some object so intercepts the light
from two as to cast on the Avail two shadoAvs which partly
overlap. Where the shadoAvs overlap the Avail gets light
only from the third candle; around that,Avhere each shadow
is separate, it is illuminated by this and one other candle:
and the wall in the neighborhood of the shadows by all
three. Objectively, therefore, the difference betAveen the
deep shadoAv and half shadoAv is that between the light
of one candle and that of two. The difference between
the half shadoAvs and the Avail around is that between the
light of tAVo and three candles. But as a matter of sensa-
tion the difference betAvecn the half shadoAv and the full

INTENSITY OF VISUAL SENSATIONS. 515

shadow seems mucli greater than that between the Iialf
shadow and the rest of the wall; in other words the differ-
ence, a, between a and 2a, is a more efficient stimuhis than
the same difference, a, between 2« and So. When the
total stimulus increases the same absolute difference is less
felt or may be entirely unperceived. An example of this
which every one will recognize is afforded by the invisibility
of the stars in daytime.

On the other hand, as the total stimulus increases or de-
creases the same fractional difference of the whole is per-
ceived with the same ease; i.e. excites the same amount
of sensation. In reading a book by lamplight w^e perceive
clearly the difference between the amount of light reflected
from the black letters and the white page. If we call the
total lamplight reflected by the blank parts lOrt and that
by the letters 2a, we may say we perceive with a certain
distinctness a luminous difference equal to one fifth of
the whole. If we now take the book into the daylight the
total light reflected from both the letters and the unprinted
part of the page nicreases, but in the same proportion. Say
the one now is 50a and the other 10a; although the
absolute difference between the two is now 40a instead of
8« we do not see the letters any more plainly than before.
The smallest difference in luminous intensity which we
can perceive is about -^ of the whole, for all the range of
lights we use m carrying on our ordinary occupations.
For strong lights the smallest perceptible fraction is con-
siderably 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; which in a stronger
light it would not do. Bright objects in general obscurity
thus appear unnaturally bright when compared wath things
about chem, 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 compared

516 THE HUMAN BODY.

with things about them, than would be the case if a sunny
scene were to be represented; by an 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.

The Duration of Luminous Sensations. This is greater
than that of the stimulus, a fact taken advantage of in
making fireworks: an ascending rocket produces the sen-
sation 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 appear to
have luminous tails behind them. By rotating rajjidly
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, 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 Ijlack 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 one turns out suddenly the gas
in a room containing no other light, tlie image of the flame
persists a short time after the flame itself is extinquished.

The Localizing Power of the Retina. As already
pointed out a necessary condition of seeing definite ob-
jects, as distinguished from the power of recognizing dif-
ferences of light and darkness, is that all light entering
the eye from one point of an object shall be focused 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 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 distinguish two points whose retinal images

LOCALIZING POWER OF RETINA. 517

are not more than .004 mm. (.00016 inch) apart. The
distance between the retinal images of two points is deter-
mined 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 h (Fig. Ill) are

4

Fig. 141.

two 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. Sim-
ilarly the image of b will be formed at h'. If a and h still
remaining the same distance apart, be moved nearer the
eye to c and (Z,theu 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 h are 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 un-
stimulated cones intervene between the stimulated, 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 discrimina-
tion; and in the yellow spot which we constantly use all
our lives in looking at things which we want to see dis-
tinctly, we have educated the visual apparatus up to about

518 THE HUMAN BODY.

its highest power. Elsewhere on the retina our discrimi-
nating power is miicli less and diminishes as the distance
from the yellow spot increases. This is partly due, no doubt,
to a less sensibility of those retinal regions, such as, by other
facts, is proved to exist, but in part no doubt is also due to
a want of practice. The more peripheral the retinal region
the less we have used it for such purposes. It is probable,
therefore, that outlying portions of the retina are capable
of education to a higher discriminating power, just as we
shall find the skin to be for tactile stimuli.

While we can tell 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 difEerent 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 (p. 496) 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 mean-
ing high objects, and of right retinal 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 seize a reachable object which it saw; it would
not yet have learnt that attaining a point exciting that part
of the retina above the fovea, meant reaching a position in
space below the visual axis; but very soon it learns that things

COLOR VIS 10 2^. 519

near its brow, that is up, excite certain visnal sensations, and
objects below its eyes others, and learns to interpret retinal
stimuli so as to localize accurately the direction, with ref-
erence to its eyes, of outer objects, and never thenceforth
gets puzzled by retinal inversion.

Color Vision. Sunlight reflected from snow gives us
a sensation which we call white. The same light sent
through a prism and reflected from a white surface excites
in us no iviiite sensation but a number of color sensations,
graduating 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 (p. 494) 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 vis-
ual apparatus is left at rest. Physically black repre-
sents 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 darkness except over the region of the jDossible 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 sensa-
tions. It is obvious, therefore, that the sensation of blackness
is not due to the mere absence of luminous 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 absence of sensation, while darkness causes a definite
feeling of '^'blackness."

Young's Theory of Color Vision. Our color sensations
insensibly fade into one another; starting with black we can
insensibly pass through lighter and lighter shades of gray

520 THE HUMAN BODY.

to white: or bt3ginning willi green through darker and
darker shades of it to black or through lighter and lighter
to white: or beginning with red we can b}' impercej)tiblc
Rtejas i)ass to orange, from that to yellow and so on to the
end of the solar spectrum: and from the violet, througli
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 sur-
face weakens, the shade becomes deeper until it joasses into
black; and if the illumination is 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 jDower of differentiating
them in sensation has only lately been acquired ])y 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 begreatly improved by practice; women
Avhose mode of dress causes them to pay more attention to
tlie 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 enun'-
erate our color sensations as red, orange, yellow, green, blue,
violet and purple; between each there are, however, numer-
ous transition shades, as yellow-green, blue-green, etc., so
that the number which shall have definite names given to
them is to a large extent arbitraiy. Of the above, all but
purple are found in the spectrum given when sunlight is
separated by a prism into its rays of different refrangibility;
rays of a certain Avave-length or period of oscillation cause in
us the feeling red; others yellow, and so on; for convenience

COLOR VISION. 521

we may speak of these as red, yellow, blue, etc., rays; all
together, iu about equal proportions, they arouse the sen-
sation of white. A remarkable fact is that most color feel-
ings can be aroused in several ways. White, for example,
not only by the above general mixture, but red and blue-
green rays, or orange and blue, or yellow and violet, taken
together in pairs, cause the sensation of white: such colors
are called complermntary to one another. The mixture
may be made in several ways; as, for example, by causing
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 mix-
ing pigments, since what happens in such cases is a very
complex phenomenon. Painters, for example, are accus-
tomed to produce green by mixing blue and yellow paints,
and some may be inclined to ridicule the statement that yel-
low 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 hand. 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 yellow are mixed the blue absorbs all the distinc-
tive 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 5, yellow c, and so on, we find that we
get the sensation white with a, h, c, d, e, /and g all together;
or with 5 and e, or with c and /, or with a, d, and e\ our
sensation ivhite has no determinate 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 mixing red and grass-green, which

522 THE HUMAN BODY.

are duo to ethereal vibrations of totally different wave-
lengths; in other words a physical light in which there are
no waves of the "yellow" length may cause in us the sen-
sation yellow, which is only one more instance of the gen-
eral 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. The modern view of
specific nerve energies (p. 191) makes it highly improbable
that our different color sensations can all be due to different
modes of excitation of exactly the same nerve-fibres; a fibre
which when excited alone gives us the sensation red will
always give us that feeling when so excited. The simplest
method of explaining our color sensations would therefore
be to assume that for each there exists in the retina a set
of nerve-fibres with appropriate terminal oi'gans, each ex-
citable by its own proper stimulus. But we can distinguish
so innumerable and so finely graded colors, that, on such a
supposition, there must be an almost infinite number of
different end organs in the retina, and it is more reasonable
to suppose that there are a limited number of primary color
sensations, and that the rest are due to combinations of these.
That a compound color sensation may be very different
from its components when these are regarded apart, is
clearly shown by the sensation Avhite aroused either by
what we may call red and blue-green, or green and purple,
stimuli acting together; or of yellow due to grass-green and
red. To account for our various color sensations we may,
therefore, assume a much smaller number of primary sen-
sations than the total number of color sensations we expe-
rience; all can in fact be explained by assuming any three
jDrimary color sensations which together give white, and
regarding all the rest as due to mixtures of these in various
proportions; there may be more than three, but three will
account for all the phenomena, black being a sensation
experienced when all visual stimuli are absent. This is
known as Young's theory of color vision, and is that at
present most commonly accepted. The selection of the
three primary sensations is somewhat arbitrary, but they
are usually regarded as red, green, and violet. It is

COLOR BLINDNESS. 523

assumed that all kinds of light stimulating the end appa-
ratuses give rise to all three sensations, but not necessarily
in the same proportion. When all are equally aroused the
sensation is -white; when the red and green are tolerably
powerfully excited and the violet little, the sensation is
yellow; when the green powerfully and the red and violet
little, the sensation is green, and so on. In this way we
can also explain the fact that all colored surfaces when
intensely illuminated pass into white. A red light, for
example, excites the primary red sensation most, but green
and violet a little; as the light becomes stronger a limit is
reached beyond which the red sensation cannot go, but the
green and violet go on increasing with the intensity of the
light, until they too reach their limits; and all three pri-
mary sensations being then equally aroused, the sensation
white is produced.

Color Blindness. Some jiersons fail to distinguish colors
which are to others quite different; when such a de-
ficiency is well marked it is known as "color blindness,"
and, assuming Young's theory to be correct, it may be ex-
plained by an absence of one or more of the three primary
color sensations; observation of color-blind persons thus
helps in deciding which these are. The most common
form is red color blindness; persons afflicted with it con-
fuse reds and greens. Red to the normal eye is red because
it excites red sensation much, green some, and violet
less; and a white page white, because it excites red, green,
and violet sensations about equally. In a person without
]-ed sensation a red object would arouse only some green
and violet sensation and so would be indistinguishable from
a bluish green; in practice it is found that many persons
confound these colors. Cases of green and violet color
blindness are also met with, but they are much rarer than
the red color blindness or "Daltonism."

The detection of colorblindness is often a matter of con-
siderable importance, especially in sailors and railroad
officials, since the two colors most commonly confounded,
red and green, are those used in maritime and railroad
signals. Persons attach such different names to colors that

534 THE HUMAN BODY.

a decision as to color blindness cannot 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. Aliout one man in eight is red
blind. The defect is much rarer in women.

Fatigue of the Retina. The nervous visual apparatus
is easily fatigued. Usually we do not observe this be-
cause its restoration is also rapid, and in ordinary life our
eyes, when open, are never at rest; we move them to and
fro, so that parts of the retina receive light alternately
from brighter and darker objects and are alternately excited
and rested. How constant and habitual the movement of
the eyes is can be readily observed by trying to fix for a
short time a small spot without deviating the glance; to
do so for even a few seconds is impossible without practice.
If any small object is steadily "fixed" for twenty or
thirty seconds it will be found that the whole field of
vision becomes grayish and obscure, because the parts of
the retina receiving most light get fatigued, and arouse
no more sensation than those less fatigued and stimulated
by light from less illuminated objects. Or look steadily at
a black object, say a blot on a white page, for twenty
seconds, and then turn the eye on a white wall; the latter
will seem dark gray, with a white patch on it; an effect
due to the greater excitability of the retinal parts previously
rested by the black, when compared with the sensation
aroused elsewhere by light from the white wall acting on
the previously stimulated parts of the visual surface.
All persons will recall many instances of such phenomena,
which are especially noticeable soon after rising m the
morning. Similar things maybe noticed with colors; after
looking at a red patch the eye turned on a white wall sees
a blue-green patch ; the elements causing red sensations
having been fatigued, the white mixed light from the wall
now excites on that region of the retina only the other
primary color sensations. The blending of colors so as to

COLOR CONTRASTS. 52b

secure their greatest effect depends on this fact; red and
green go well together because each rests the parts of the
visual apparatus most excited by the otlier, and so each
appears bright and vivid as the eye wanders to and fro;
while red and orange together, each exciting and exhaust-
ing mainly the same visual elements, render dull, or in
popular phrase "kill," one another.

Contrasts. If a well-defined black surface be looked at on
a la-i'ger white one the j^arts of the latter close to the black
look whiter than the rest, and the parts of the black near
the white blacker than the rest; so, also, if a green patch be
looked at on a red surface each color is heightened near
where they meet. This jihenomenon is largely due to fatigue
and deficient fixation: a region of the eye rested by the
black or the green is brought by a movement of the organ
so as to receive light from the white or red surface; phe-
nomena due to this cause are known as those of successive
contrast. Even m the case of perfect fixation, however,
something of the sanjie kind is seen; black looks blacker
near white, and green greener near red when the eye has not
moved in the least from one to the other. A small piece of
light gray paper jDut on a sheet of red, which latter is
then covered accurately with a sheet of semi-transparent
tissue-paper, assumes the complementary color of the red,
I.e. looks bluish green; and gray on a green sheet under
similar circumstances looks pink. Such phenomena are
known as those of simultaneous contrast, and are explained
on psychological grounds by those who accept Young's
theory of color vision. Just as a medium-sized man looks
short beside a tall one, so, it is said, a black surface looks
blacker near a white one, or a gray (slightly luminous
white) surface, vv'hich feebly excites red, green, and violet
sensations, looks deficient in red (and so bluish green)
near a deeper red surface. There are, however, certain
phenomena of simultaneous contrast which cannot be satis-
factorily so explained, and these have led to other theories
of color vision, the most important of which is that de-
scribed in the next paragraph.

Hering's Theory of Vision. Contrasts can be seen with

52G THE HUMAN BODY.

the eyes closed and covered. If we look a short time at a
bright object and then rapidly exclude light from the eye,
we see for a moment a posiiive afier-image of the object,
e.g. a window Avith its frame and panes after a glance at it
and then closing the eyes. In these positive after-images
the bright and dark parts of the object which was looked at
retain their original relationship; they depend on the persist-
ence of retinal excitement after the cessation of the stimulus
and usually soon disappear. If an object be looked at
steadily for some time, say twenty seconds, and the eyes be
then closed a, negative after-image is seen. In this the lights
and shades of the object looked at are reversed. Frequently
a positive after-image becomes negative before disappearing.
The negative images are explained commonly by fatigue;
when the eye is closed some light still enters through the
lids and excites less those parts of the retina previously
exhausted by prolonged looking at the brighter parts of the
Held of vision; or, Avhen all light is rigorously excluded, the
proper stimulation of the visual apparatus itself, causing
the idio-retinal light, affects less the exhausted portions, and
so a negative image is produced. If Ave fix steadily for
thirty seconds a point between two white squares about 4
mm. {\ inch) apart on a large black sheet, and then close
and coA'er our eyes, we get a negative after-image in Avhich are
seen two dark squares on a brighter surface; this surface is
brighter close around the negative after-image of each square,
and brightest of all between them. This luminous bound-
ary is called the corona, and is explained usually as an effect
of simultaneous contrast; the dark after-image of the square
it is said makes us mentally err in judgment and think the
clear surface close to it brighter than elsewhere; and it is
brightest between the two dark squares, just as a middle-
sized man between tAvo tall ones looks shorter than if along-
side one only. If, however, the after-image be watched it
will often be noticed not only that the light band betAveen
the squares is intensely white, mucli more so than the normal
idio-retinal light, bat, as the image fades aways, often the
tAvo dark after-images of the squares disappear entirely with
all of the corona, except that part between them which is

BERING'S THEORY OF COLOR VISION. b21t

still seen as a bright band on a uniform grayish field.
Here there is no contrast to produce the error of judgment,
and from this and other experiments Hering concludes that
light acting on one j^art of the retina i^roduces inverse
changes in all the rest, and that this has an important j^art
in producing the phenomena of contrasts. Similar pheno-
mena may be observed with colored objects; m their nega-
tive af ter-im-ages each tint is reiDresented by its complemen-
tary, as black is by white in colorless vision.

Endeavoring to exclude such loose general explanations
us " errors of judgment," Hering proposes a theory of vision
which can only be briefly sketched here. We may put all
our colorless sensations in a continuous series, passing
through grays from the deepest black to the brightest
white; somewhere half-way between will be a neutral
gray which is as black as it is white. We may do some-
tliing similar with our color sensations; as in gray we see
black and white so in purple we see red and blue, and all
colors containing red and blue may be put in a series of
which one end is pure red, the other pure blue. So with red
and yellow, blue and green, yellow and green. If we call to
mind the whole solar spectrum from yellow to blue, through
the yellow-greens, green, and blue-greens, we get a series in
which all but the terminals have this in common that they
contain some green. Green itself forms, however, a special
point; it differs from all tints on one side of it in contain-
ing no yellow, and from all on the other in containing no
blue. In ordinary language this is recognized: we give
it a definite name of its own and call it green. Its sim-
plicity compared with the doubleness of its immediate
neighbors entitles it to a distinct place in the color-sensa-
tion series. There are three other color sensations which
like green are simple and must have specific names of their
own; they are red, blue, and yellow. Green may be pure
green or yellow green or blue green, but never yellow
and bluish at once, or reddish. Eed may be pure or
yellowish or bluish, but never greenish. Red and green
are thus mutually exclusive; yellow and blue stand in a
similar relationship. All other color sensations, as orange

528 THE EUMAN BODY.

suggest tAvo of the above, and may be described as mixtures
of them; but they themselves stand out as fundamental color
sensations. Moreover, it follows from the above, that more
than two simple colored sensations are never combined in
a compound color sensation.

Since red always excludes green, and yellow blue, we may
call them anti-colors (the complementary colors of Young's
theory), and are led to suspect that in the visual organ there
must occur, in the production of each, processes which
prevent the simultaneous production of the other, since
there is no a priori reason in the nature of things why
we should not see red and green simultaneously, as well as
red and yellow. Along with our color sensations there is
always some colorless from the black-white series; which
we recognize m speaking of lighter and darker shades of
the same color.

Hering assumes, then, in the retina or some part of the
nervous visual apparatus, three substances answering to the
black-white, red-green, and yellow-blue sensational series,
the construction of each substance being attended with
one sensation of its pair, and its destruction with the other.
Thus, when construction of the black-white substance ex-
ceeds destruction, we get a blackish-gray sensation; when
the processes are equal the neutral gray; when destruction
exceeds construction a light-gray, and so on. In the
other color series similar things would occur; when con-
struction of red-green substance exceeded destruction
in any point of the retina we would get, say, a red feeling;
if so, then excess of destruction would give green sensa-
tion. The intensity of any given simple sensation would
depend on the ratio of the difPerence between the construc-
tion and destruction of the corresponding substance, to the
sum of all the constructions and destructions of visual su))-
stances going on in that part of the visual apparatus. A
little thought will show that this can hardly be reconciled
with the results expressed in Fechner's law. The intensity of
a mixed color sensation would l)e the sum of the intensities
of its factors, and its tint and shade dependent on the rela-
tive proportion of these factors. When the construction

HERING'S THEORY OF COLOR VISION. 529

and destruction of tlie red-greeii substance are equal no
color sensation is aroused by it; and we get gray, due to
those simultaneously occurriug changes in the black-white
substance which are always present, but were i)reviously
more or less cloaked by the results of the changes in
the red-green substance. Red and green in certain pro-
portions cause then a white or gray sensation, not because
they supplement one another, as on Young's theory, but
because they mutually cancel; and so for other comple-
mentary colors.

Moreover, according to Hering, destruction of a visual
substance going on in one region of the retina promotes
construction and accumulation of that substance elsewhere,
but especially in the neighborhood of the excited spot.
Hence, when a white square on a black ground is looked
at, destruction of the black-white substance overbalances
construction in the place on which the image of the square
falls, but around this construction occurs in a high degree.
When the eyes are shut, this latter retinal region, with its
great accumulation of decomposable material, is highly
irritable and, under the internal stimuli causing the idio-
retinal light, breaks down comparatively fast, causing the
corona, which may be intensely himinous; for with the
closed eye the total constructive and destructive processes
in the visual apparatus are small, and so the excess of de-
struction in the coronal region bears a large ratio to the
sum of the whole jn-ocesses. The student must apply this
theory for himself to the other phenomena of contrasts and
negative images, as also to the gradual disappearance of
differences between light and dark objects when looked at
for a time with steady fixation; the general key being the
pi'inciple that anything leading to the accumulation of a
visual substance increases its decompositions under stimu-
lation, and vice versa. The main value of Hering's theory
is that it attempts to account physiologically for phenomena
previously indefinitely ex[)lained psychologically by such
terms as '•'errors of judgment,'' which really leave the
whole matter whei'e it was, since if (as we must believe)
mind is a function of brain, the errors of judgment have

530 THE HUMAN BODY.

still to be accounted for on i^hysiological grounds, as due
to conditions of the nervous system.

Visual Perceptions. The sensations which light excites
in us we interpret as indications of the existence, form, and
position of external objects. The conceptions which we
arrive at in this way are known as visual perceptions.
The full treatment of perceptions belongs to the domain of
Psychology, but Physiology is concerned with the condi-
tions under which they are produced.

The Visual Perception of Distance. With one eye our
perception of distance is very imperfect, as illustrated by
the commcm trick of holding a ring suspended by a string
a short way in front of a person's face, and telling him
to shut one eye and pass a rod through the ring. If a pen-
holder 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 out 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, however, we get some 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 modifi-
cations their retinal images experience by aerial perspective,
come in to help. The relative distance of objects is easiest
Tictcrmined by moving the eyes; all stationary objects then
a})pear displaced in the opposite direction (as for example
when we look out of the window of a railway car) and those
nearest most rapidly; from the different ajoparent rates of
movement we can tell which are farther and nearer. We
•TO inseparably and unconsciously bind up perceptions of dis-
tance with the sensations aroused by objects looked at, that

VISUAL PERCEPTION OF SIZE. 531

we seem to see distance; it seems at first thought as definite
a sensation as color, Tliat it is not is sliown 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 ns did uncon-
sciously in childhood before we thought about such things.

The Visual Perception of Size. The dimensions of the
retinal image determine primarily the sensations on which
conclusions as to its size are based; 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 a^^parent size
being due to the previous incorrect judgment as to the dis-
tance of the object. The presence of an object of tolerably
well-known height, as a man, also assists in forming con-
ceptions (by comparison)as to size; artists for this jDurpose
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 conclusions from the distribution of light and shade on
an object, and from that amount of knowledge as to the
relative distance of different 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

532 THE HUMAN BOD T.

flat object with both eyes we get a similar retinal image in
each. Under ordinary 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 yellow spots. A point to tlie left of this'
forms its image on the inner (right) side of the left eye
and the outer (riglit) side of tlic right. An object verti-
cally above that looked at would form an image straight
l)elow the 3'ellow spot 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 learnt that similar simultaneous excita-
tions of these corresponding pointn mean single objects, and
so interpret our sensations. This at least is the theory of
the experiential or empirical school of psychologists, thougJi
others believe we have a sort of intuition on the subject.
When the eyes do not work together, as in the muscular
incoordination of one stage of intoxication, then they are
not turned so that images of the same objects fall on cor-
responding retinal points, and the person sees double.
AVhen a squint comes on, as from paralysis of the external
rectus of one eye, the sufferer at first sees double for the
same reason.

If 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, be-
cause the more diverging lines drawn from it through the
nodal jooints 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 on the two foveae. 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

STEREOSCOPIC VISION.

533

the eyes; it will be seen double. For any 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 lioropter for that position of the
eyes: all objects in it are seen single; all others in the visual
field, double.

The Perception of Solidity. When a solid o))ject is
looked at the two retinal images arc different. If a trun-
cated pyramid be held in front of one eye its image will be
that represented at P, Fig. 142. 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 surface, bdca, in one answers to the large surface,
b' d' d a, in the other. This may be readily observed by

I

^

\

/

\

d

/

V

I

p

/

c

\

/ ■

a

fl.

Fig. l-W

holding a small cube in front of the face and alternately
looking at it with each eye. In such cases, then, the
retinal images do not correspond, and yet we combine them
so as to see one solid object. This is known as sfereoscojJi'c
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 by the left, and the other Avhen looked at with the
right eye. These are then mounted for the stereoscope so
that each is seen by its proper eye, and the scene or object
is seen in distinct relief, as if, instead of flat pictures,
solid objects were looked at. Of course in many stereo-
scopic views the distribution of light and shade, etc., assist,
but these are quite unessential, as may be readily observed
by enlarging the middle and right outline drawings of Fig.

534 THE HUMAN BODY.

14:3 to the ordinary size of a stereoscopic slide, and placing
them in the instrument. A solid pyramid standing out
into space will be distinctly perceived; if the pictures be
reversed the pyramid appears hollow. The pictures must
not be too diiferent, or their combination to give the idea
of a single solid body will not take place. Many persons,
indeed, fail entirely to get the illusion with ordinary stereo-
scopic slides. The phenomena of stereoscopic vision mili-
tate strongly against the view that there are any pre-
arranged corresponding points in the two retinas.

The Perception of Shine. When we look at a rip2')led
lake in the moonlight, we get the perception of a " shiny"
or brilliant surface. The moonlight is reflected from the
waves to the eyes in a number of bright points: these are
not exactly the same for both eyes, since the lines of light-
reflection from the surface of the water to each are
different. The perception of brilliancy seems largely to
depend on this slight non-agi'eement of the light and dark
points on the two retinas. A rapid change of luminous
points, to and fro between neighboring points on one
retina, seems also to produce it.

CHAPTER XXXIV.

THE EAR AND HEARING.

The External Ear. The auditory organ in man con-
sists of three portions, known respectively as the external
ear, the middle ear or tympanum, and the internal ear or
labyrinth; of these the latter is the essential one, contain-
ing the end organs of the auditory nerve. The external
ear consists of the expansion seen on the exterior of the
head, called the concha, and a passage leading in from it,
the external auditory meatus {D to c, Fig. 143). This pas-

FiG. 143.— The right ear (excluding the concha) seen from the front. D to c,
external auditory meatus ; c c, tympanic membrane ; B, the tympanum with
the auditory ossicles in it ; B to E, Eustachian tube ; A, labyrinth.

sage is closed at its inner end by tlie tympanic or drum
mernbrane. It is lined by a prolongation of the skin,
through which numerous small glands, secreting the 2vax
of the ear. open.

The Tympanum, or drum-chamber of tlie ear (B, Fig.
143), is an irregular cavity in the temporal bone, closed ex-
ternally by the drum membrane. From its inner side the
Eustachian tube {B to E) proceeds and opens into the

536 THE HUM AX BODY.

pharynx (//, Fig. 80)*, and the mncons membrane of that cav-
ity is continued up the tube to line the tympanum; between
this inside, and tlie skin of the external auditory meatus
outside, is the proper tympanic membrane composed of
connective tissue. The inner wall of the tympanum is
bony except for two small ai)ortures, the oval and round
foramens, which lead into the labyrinth. Daring life the
round aperture is closed by the lining mucous membrane,
and the oval in another way, to be described presently.
The ti/mpanic memirane, c c, stretched like a drum-head
across the outer side of the tympanum, forms a shallow
funnel with its concavity outwards. If a sheet of indian-
rubber be stretched over a ring and pulled down in the
centre, its form will be very like that of the membrane in
question. It is pressed by the external air on its exterior, and
by air entering the tympanic cavity through the Eustachian
tube on its inner side. If the tympanum were closed these
pressures would not be always equal when barometric pres-
sure 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
extremely loud and disconcerting, 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, keeping the mouth
shut, and forcibly expiring, air may be forced under j)res-
sure into the tympanum, and will be held in part impris-
oned 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.

The Auditory Ossicles. Three small bones lie in the
tympanum forming a chain (Fig. 144) from the drum
membrane to the oval foramen. The external bone is the
malleus OY hammer ; the middle one, ihe incus or an nl; and
the internal, the stapes or stirrup. The malleus, M, has

* Paire 309.

TYMPANIC BONES.

537

an upper enlargement or head, which carries on its inner
side ail 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 slendei' process, is imbedded in a
ligament which reaches from it to the front wall of the
tympanum; the other process, the handle, reaches down
between the mucous membrane lining the inside of the
drum membrane and the
membrane proper, and is
firmly attached to the lat-
ter near its centre and
keeps t h e membrane
dragged in there so as to
give it its peculiar concave
form, as seen from the out-
side. 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
{Jh) is attached by ligament
to the back wall of the tym-
panum; the long jirocess {Jl) is directed inwards to the
stapes; on the tip of this process is a little knob, which
represents a bone {osorhiculare), distinct in early life. The
stapes {S) is extremely like a stirrup, and its base (the foot-
l)iece of the stirrup) fits into the oval foramen, to the mar-
ghi 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 tympanum: this,
with the ligament imbedding the slender process and fixed
to the front wall of the tympanum, forms an antero-pos-
terior 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. 144:
just below the neck of the malleus and perpendicular to the

Mm

Fig. iM.— The auditory ossicles of the
right ear, seen from the front. ..V, mal
lens; J. incus; -S', stapes: Mcj), head of
the malleus; Mc, neck of ditto; Ml, long
process; 31m, handle: Jc,b(^d.y, Jb. short,
and Jl, long process, of incus; Jpl, os
orbiculare: Sep. head of stapes.

538

TUB HUMAN BODY.

paper it will very fairly rci)rcsent this axis of rotation.
Connected with the malleus is a tiny muscle, called the
tensoi' tympani; it is inserted in 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 moment in and out of the fenestra ovalis.

The Internal Ear. The labyrinth consists primarily of
cham])ers and tubes hollowed out in the temporal bone and
inclosed by it on all sides, except for the oval and round

Fc vj)a

Fig. 145.— Casts of the bony labyrinth. A. left labyrinth seen from the outer
side; B, riglit labyrinth from the inner side; C, left labyrinth from above; Fc,
round foramen; Fv, "val foramen; A, horizontal semicircular canal; /la, its
ampulla; raa, ampulla of anterior vertical semicircular canal; Vj^ju, ampulla
of posterior vertical semicircular canal ; vc, conjoined portion of Uie two ver-
tical cauals.

foramens on its 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 lony lahyrinth thus consti-
tuted, 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 perilymjili; and the mem-
hranoiis 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

THE INTERNAL EAR.

53'J

different aspects in Fig. 145. 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, communicating 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 ampulla at the front end. The two other canals
lie vertically, the anterior at right angles, and the pos-
terior parallel, to the median antero-posterior 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 vesti-
bule, lying iu the bony, consists of two sacs communicating
by a narrow aperture.
The posterior is called
the utriculus, and into
it the membranous
semicircular canals
open. The piiste^kir,
called the sac cuius,
communicates by a tube
with the mcmbranoits
cochlea. The mem-
branous semicircular
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 canals; but in the amj^ulla one side of the mem-
branous tube is closely adherent to its bony protector; 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.
146) shows that its osseous portion consists of a tube

Fig. 146.— a section through the cochlea
iu tlie line of its axis.

540 THE ITU MAN BODY.

woumi two and a half tiiuus (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 par-
tially subdivides the tube, extending farthest across in its
lower coils. Attached to the outer end of this bony
plate is the membranous cochlea [scala media), a tube
triangular' in cross-section and attached 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 tu^e (Fig, 147) into an upper portion,
the scala vestibicli, S V, and a lo\^•cr, the scala tympani, ST.
Between these lie the lamina spiralis {ho) and the mem-

FiG. 147.— Section of one coil of the cochlea, niaj?nified. SV, scala vestibuli'
B, membrane of Reissner; CC, membranous cochlea (scato media); Us, limbus
tamince spiralis, t, tectorial membrane; ST, scala tympani', Iso, spiral lamina ;
Co. rods of Corti; b, basilar membrane.

branous cochlea (CC), the latter being bounded above by
the membrane of Reissner {R) and below by the basilar
membrane {b). The inner edge of the lamina spiralis is
thickened and covered with connective tissue which is hol-
lowed 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 com-
municate; both are filled with perilymph, and the former
communicates below with the perilymph cavity of the ves-
tibule, while the scala tympani abuts below on the round
foramen, which, as has already been pointed out, is closed
by a membrane. The membranous cochlea contains cer-

ORGAN OF CORTI.

541

tain solid structures seated on the basilar membrane and
forming the organ of Corti; the rest of its cavity is filled
with endolymph, which communicates with that in the
sacculus.

The Organ of Corti. This contains the end organs of
the cochlear nerves. Lining the sulcus spiralis are cuboi-
dal cells; on the inner edge of the basilar membrane they
become cclumnar, and these are succeeded by a row which
bear on their upper ends a set of short stiff hairs, and con-
stitute the inner hair-cells, which are fixed below by a
narrow aiiex to the basilar membrane; nerve-fibres enter
them. To the inner hair-cells succeed the rods of Corti

A B

Fig. 148. — 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, showing: how they cover iu
the tunnel of Corti; i, inner, and e, outer rods; b, basilar membrane; r, reti-
cular membrane.

{Co, Fig. 147), which are represented highly magnified in
Fig. 148. These rods are 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 rod has a somewhat dilated base,
firmly fixed to the basilar membrane; an expanded head
where it meets its felloAV (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 S. The inner rods are more slender and more
numerous than the outer, their numbers being about 6000
and 4500 respectively. Attached to the external sides of
the heads of the outer rods is the reticular vienibrane [r,

i42

THE HUMAN BODY.

Fig. 148), which is stiff and perforated by holes. External
to the outer rods come four rows of oiiter hair-cells, con-
nected like the inner row with nerve-
fibres; their bristles project into the
holes of the reticular membrane. Be-
yond 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 lamince spi-
ralis; from it projects the tectorial
membrane {t, Fig. 147) which extends
over the rods of Corti and the hair-
cells.

Nerve-Endings in the Semicir-
cular Canals and the Vestibule.
Nerves reach the ampulla of each
semicii'cular canal, and, perforating
its Avail, enter the epithelium lining
it which is there several layers thick
(Fig. 149). Some of the cells {sjj)
are fusiform and have large nuclei; a
slender external 2Jrocess runs from
each to the cavity of the ampulla and
is then continued as slender stiff hair
[li), which projects into the endo-
lymph. The deeper ends of these
cells have been described as joining
the terminal branches of nerve-
fibres, so that they must be regarded
as efid organs. In the utricle and saccule are somewhat
similar structures; but collected among the hairs are
minute calcareous particles, the ear-stones or otoliths.

Fig. 149— The epithelium
at the point where tlie
nerves enter an ampulla.
n, nerves; c, superficial
columnar epithelium: sp,
hair -ceils; /i, hairs; 6,
basal cells.

The Loudness, Pitch, and Timbre of Sounds. Sounds,
as sensations, fall into two groups— ^zo/es and noises. Physi-

PROPERTIES OF SOUND. 543

cally, 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 follow one another uni-
formly, or periodicalli/, 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) quality or timbre, or, as it has been called, to7ie color;
a note of a given loudness and pitch produced by a trum-
pet and by a violin has a different character or individu-
ality in each case; this cpiality is its timbre. Before un-
derstanding the working of the auditory mechanism we
must get some idea of the physical qualities in objective
sound which the subjective differences of auditory sensa-
tions are signs of.

The loudness of a sound depends on the force of the
aerial waves; the greater the intensity of the alternating
condensations and rarefactions of these in the external
auditory meatus, the louder the sound. The ^jitch of a
note depends on the length of the waves, that is the dis-
tance from one point of greatest condensation to the next,
or (what amounts to the same thing) on the numljer 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 note c on the un-
accented octave is due to 132 vibrations in a second. The
note c', the next higher octave of this, is produced by 264
vibrations in a second; the next lower octave (great octave,
C), by 60; and so on. Sound vibrations may be too rapid
or too slow in succession to produce sonorous sensations,
just as the ultra-violet and ultra-red rays of the solar
spectrum fail to excite the retina. The highest-pitched
audible note answers to about 38,016 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

544 THE HUMAN BODY.

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 tV of the fifth accented
octave, produced by the piccolo flute, due to 4753 vibra-
tions in a second; and the lowest-pitched is the E, of the
contra octave, jiroduced by the double bass. Modern grand
pianos and organs go down to C,, in the contra octave (33
vibrations per second) or even A,„ \ll\), but the musical
quality of such notes is imperfect; they produce rather a
''buzz" than a true tone sensation, and are only used
along with notes of higher octaves to which they give a
character of greater depth.

Pendular Vibrations. Since the loudness of a tone de-
pends on the vibrational amplitude of its physical antece-
dent, and its pitch on the vibrational rate, we have still
to seek the cause of timbre; the quality by which we recog-
nize the human voice, the violin, the piano, and the flute,
even when all sound the same note and of the same
loudness. The only quality of periodic vibrations left to
account for this, is what we may call wave-fomi. Think of
the movement of a pendulum; starting slowly from its
highest point, it sweeps faster and faster to its lowest, and
then slower and slower to its highest point on the opposite
side; and then repeats the movements in the reverse direc-
tion. Graphically we may represent such vibrations by the
outer continuous curved line in Fig. 150. Suj^pose the
lower end of the pendulum to bear a writing point which
marked on a sheet of paper traveling down uniformly
behind it, and at such a rate as to travel the distance 0-1-
in two seconds. If the pendulum were at rest the straight
vertical line would be draAvn. But if the pendulum were
swinging we would get a curved line, compounded of the
vertical movement of the jiapcr and the to-and-fro move-
ment of the pendulum, writing sometimes on one side
of the line 0-1-2 and sometimes on the other: Starting
at a moment when the pendulum crosses the middle, 0, we
would get described the curve 0 a^ a^, at first sejDaratingfast
from the vertical line, then slower, then returning, at first

PENDULAR VIBRATIONS.

545

gradually then faster, until it met the vertical again, at the
end of 1" and commenced an exactly similar excursion on its
other side, at the end of which it would be back at 1,
and in just the same position, and ready to repeat exactly
the swing, with which we commenced. A pendulum
thus executes similar movements in equal
periods of time, or its vibrations are
periodic. A full swing on each side of
the position of rest constitutes a complete
vibration, so the vibrational period of a
second's pendulum is two seconds: at the
end of that time it is precisely where it
was two seconds before, and moving in
the same direction and at the same rate.
It is clear that by examining such a curve
we could tell exactly how the pendulum
moved, and also in what period if we knew
the rate at which the paper on which its
point wrote was moving. The vertical
line 0-1-2 is called the abscissa; per-
pendiculars drawn from it and meeting
the curve are ordinates: equal lengths on
the abscissa represent equal times; where
an ordinate from a given point of the
abscissa meets the curve, there the writing
point was at that moment; where succes-
sive ordinates increase or decrease rapidly
the pendulum moved fast from or towards
its position of rest, and vice versa. Simi-
Jarly, any other periodic movement may be
perfectly represented by curves ; and since
the form of the curve tells us all about the
movement, it iscommon to speak of the "form of a vibration,"
meaning the form of the curve which indicates its charac-
ters. Periodic vibrations like those in Fig. 150, where the
ordinates at first grow fast, then more slowly, then dimin-
ish slowly and then faster, and represented by a symme-
trical curve on one side the abscissa, Avhich is repeated

Fig. 150.

546 THE HUMAN BODY.

exactly on the other side of the abscissa, are known as
pendular vibrations.

The Composition of Vibrations. The vibrations of a
second's ponduluni set tlie air-particles in contact with it
in similar movement, bnt the aerial waves succeed one
another too slowly to produce in us the sensation of a
musical note. If for the pendulum we substitute a tuning-
fork (the prongs of which move in a like way), and the
fork vibrates 132 times per 1", then 132 aerial waves will fall
on the tympanic membrane in that time, and we will hear
the note c of the unaccented octave. If the larger con-
tinuous curve in Fig. 150 represent the aerial vibrations in
this case, the distance 0 to 1 on the abscissa will represent
Y^ of a second. Let, simultaneously, the air be set in
movement by a fork of the next higher octave, c', making
264 vibrations per 1"; under the influence of this second fork
alone, the aerial particles would move as represented by the
smaller continuous curved line, the waves being half as long
and cutting the abscissa twice as often. But when both
forks act together the aerial movement will be the algel)raic
sum of the movements due to each fork; when both drive the
air one way they will reinforce one another, and vice verso;
the result will be the movement represented by the dotted
line, which is still periodic, repeating itself at equal intervals
of time, but no longer pendular, since it is not alike on the
ascending and descending limbs of the curves. We thus
get at the fact that non-pendular vibrations may be pro-
duced by the fusion of pendular, or, in technical phrase, by
their compositioii.

Suppose several musical instruments, as those of an or-
chestra, to be sounded together. Each produces its own
effect on the air-particles, whose movements, being the
algebraical sum of those due to all, must at any given in-
stant be very complex; yet the ear can pick out at will and
follow the tones of any one instrument. From the com-
plex aerial movement it can select that fraction of it which
one vibrating body produces. The air in the extei'nal
auditory meatus at any given moment can only be in
one state of rarefaction or condensation and at one rate

ANALYSIS OF VIBRATIONS. 547

and in one direction of movement, this being the resultant
of all the forces acting upon it; all clashing, and some push-
ing one way and others another. If the resultant move-
ment be not periodic it will be recognized as due to noises
or to several simultaneous inharmonic musical tones; this
is commonly the case when musical tones are not united
designedly, and the ear thus get one criterion for distin-
guishing movements of the air due to several simultaneous
musical tones. However, a composite set of tones will give
rise to periodic vibrations when all are due to vibrations of
rates which are multiples of the same whole number. In
such cases the movement of the air in the auditory meatus
has no property except vibrational form by which the ear
could distinguish it from a simple tone; when the two
tuning-forks giving the forms of vibration (with rates as
1 to 3), represented in Fig. 150 by continuous lines, are
sounded together, we get the new form of vibration repre-
sented by the dotted line, and this has the same period as
that of the lower-pitched fork; yet the ear can clearly dis-
tinguish the resultant sound from that of this fork alone,
as a note of the same pitch but of different timbre; and
with practice can recognize exactly what simple vibrations
go to make it up.

The Analysis of Non-Pendular Vibrations. If a per-
son Avith a trained ear listens attentively to any ordinary
musical tone, such as that of a piano, he hears, not only
the note whose vibrational rate determines the pitch of the
tone as a whole, but a whole series of higher notes, in
harmony with the general or fundamental tone; this latter is
the 'primary partial tone, and the others are secondary
pai'tial tones; nearly all tones used in music contain both.
If the prime tone be due to 132 vibrations a second (c),
its first upper partial is c' (= 264 vibrations j^er second);
the next is the fifth of this octave {g' = 396 = 132 X 3
vibrations per 1'); the next is the second octave, c" (132 X
4 = 528 vibrations per 1'); the next is the major third of the
c" (=132 X 5 = 660 vibrations per second = e"), and so on.
The only form of vibration which gives no upper partial
tones is the pendular; we may coll notes due to such vibra-

548 THE HUMAN BODY.

tions simple tones; and we, consequently, recognize in music
tones which arc simple (such as those of tuning-forks) and
those which are compound; these latter are non-pendular
in form.

We find, then, that the form of aerial vibrations deter-
mines in our sensations the occurrence or non-occurrence
of upper partial tones. It also, as we have seen, deter-
mines the quality or tinihre of the tone, since vibrational
amplitude and rate are otherwise accounted for in sensa-
tion by loudness and pitch.

It can be proved, by the employment of the higher
mathematics, that every periodic non-pendular movement
can be analyzed (as the dotted curve of Fig. 150 may be)
into a given number of pendular vibrations, that is, every
compound vibration into a set of simple ones; and that
every periodic non-pendular vibration can be made by the
combination of pendular. Moreover, any given comj)ound
vibration can be analyzed into but one set of simple ones;
no other combination will produce it. Consequently a vibra-
tional movement of the air in the external auditory passage,
producing a compound musical tone sensation, can be ex-
hibited always, but only in one way, as the sum of a num-
ber of simple vibrations, whose rates are multiples of that
which determines the pitch of the tone.

Xow when the trained ear listens to a tone with the
object of detecting upper partials if present, it hears them
only when the vibrations are non-pendular {i.e. when
theoretically they ought to be present), and those it hears
are exactly those demanded by theory. By the help of
certain instruments their detection is made easy even to
the untrained ear. But in ordinary circumstances we do
not heed secondary partial tones; we hear a note of the
pitch of the primary partial and of a certain timbre; and
whenever the upper partials present are different, or of
different relative intensities, the timbre of the note varies.
Hence it becomes probable that, just as the ear can at will
follow any instrument in an orchestra, analyzing the aerial
movement so as to select and follow the fraction of the
whole due to that one, so it cm and does analyze compound

SYMPATHETIC RESONANCE. 549

tones when proceeding from one instrument, and that the
npper partials, not rising into consciousness as definite
tones but present as subdued sensations, give its char-
acter to the whole tone and determine its timbre. It
might be, however, that the composition of non-pendular
vibrations from pendular was a mere mathematical fiction,
liaving no real existence in nature; before we can accept
the above explanation of timbre, we must see if there is
any evidence that, as a matter of fact, non-pendular vibra-
tions, not only may be, but are made up by the combination
of pendular.

Sympathetic Besonance. Imagine slight taps to be
given to a jiendulum; if these be repeated at such intervals
of time as to always 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 move-
ment, 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 power-
ful vibration; once in two, three, or four swings will do.
A stretched string, such as that of a piano, is in so far like a
given 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 powerful vibrations to cause
it to emit an audible note. By using such strings we
might hope to detect the separate pendular vibrations in
any non-pendular aerial jDcriodic movement if such really
existed; certain strings would pick out the pendular com-
ponent agreeing in rate with their own vibrational period
and be soon set in powerful movement; while those not
^■ibrating in the same period as any of the pendular compo-
nents, would remain jjractically at rest, like the pendulum
getting taps which sometimes helped and sometimes impeded
its swing. If the dampers of a piano be raised and a note
be sung to it, it will be found that several strings are set in
vibration, such vibrations being called sympathetic. The
human voice emits compound tones which can be mathe-
matically analyzed into simple vibrations, and if the piano

550 THE HUMAN BODY.

strings set in movement l)y it be examined, they will be
found to be exactly those which answer to these pendular
vil)rations and to no others. We tims get ex}ierimental
grouiul.s for believing that compound tones are really made
up of a number of sim})le vibrations, and get an additional
justification for the supi)osition that in the ear each note is
analyzed into its pendular comjioiients; and that the differ-
ence of sensation which we call timbre is due to the effect of
the secondary j)artial tones thus perceived. If so, the
ear must have in it an apparatus adapted for sympathetic
resonance.

It may be asked why, if the car analyzes yibrations in
this way, do we not commonly perceive it? How is it that
what we ordinarily hear is the timbre of a given tone and not
the separate upper partials which give it this character? The
explanation is more jisychological than physiological, and
belongs to the same series as the reason why we do not
ordinarily notice the blind spot in the eye, or the double-
ness of objects out of the horopter, or the duplicity of
stereoscopic images. We only use our senses in daily life
when they can tell us something that may be useful to us,
and we neglect so habitually all sensations which would be
useless or confusing, that at last it needs special attention
to observe them at all. The way in which tones are com-
bined to give timbre to a note is a matter of no importance
in the daily use of them, and so we fail entirely to observe
the components and note only the resultant, until we make
a careful and scientific examination of our sensations.

The Functions of the Tympanic Membrane. If a
stretched membrane, such as a drum-head, 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 stretcbcd tlie higher the pitch of its note; every
stretched membrane tlins 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 sympafJietic

USES OF DRUM-MEMBRANE. 651

vibration; if tlioy do not push the membrane at the
proper times, their eiiects will neutralize one another: hence
such membranes respond well to only one note. The tym-
panic membrane, however, responds equally well to a large
number of notes; at the least for those due to aerial vibra-
tions of rates from 60 to 4000 per second, running over
eight octaves and constituting those commonly used in
music. This faculty depends on two things; (1) the mem-
brane is comparatively loosely and not uniformly stretched;
(2) it is loaded by the tympanic bones.

The drum-membrane is (p. 536) in the form of a shallow
funnel with its sides convex towards its cavity; in such a
membrane the tension is not uniform but increases towards
the centre, 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 yet has
to be moved when the membrane does. This is effected
by the tympanic bones, fixed to the drum-membrane by the
handle of the malleus. Another advantage is gained by the
damping; once a stretched membrane is set vibrating it
continues so doing for some time; but if loaded its move-
ments 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. When the
aerial waves cease the loaded drum-membrane comes to rest
almost immediately, and is ready to respond to the next
set of waves reaching it.

Functions of the Auditory Ossicles. When the air in
the external auditory meatus is condensed it pushes in the
handle of the malleus. This bone then slightly rotates on
the axial ligament (p. 537) and drags out the incus and
with it the base of the stapes; the reverse occurs when air in
the auditory passage is rarefied. Aerial vibrations thus set
the chain of bones swinging, and pull out and push in the
base of the stapes, which sets up waves in the perilymph of
the labyrinth, and these are transmitted through the mem-

552 THE HUMAN BODY.

brauoiis labyrinth to the endolymi)h. These liquids being
chiefly water, and practically incompressible, 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
]i(|uids. 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 trans-
mit to its centre, 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 larger area is, by pushing the tym-
panic bones, all concentrated on the smaller. The ossicles
also form a bent lever (Fig, 144) 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 move-
ments transmitted by the drum-membrane to the handle of
the malleus are communicated with diminished range, but
increased j^iower, to the base of the stapes.

Ordinarily sound-waves reach the labyrinth in this way
through the tympanum, but they may also be transmitted
through the bones of the head in general; if the handle of
a vibrating tuning-fork be placed on the vertex, for exam-
ample. Such sounds seem to have their origin in the head
itself. 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 audiplione. Of course if deafness be
due to disease of the proper nervous auditory apparatus no
device can make the ])erson hear.

Function of the Cochlea. We have already seen reason
to believe that in the ear there is an apparatus adapted for
sympathetic resonance, by which we recognize different

FUNCTIONS OF COCHLEA. 553

musical tone-colors; the minute structure of the membra-
nous cochlea is such as to lead us to look for it there. An
old view was that the rods of Corti, which yary in length,
were like so many piano-strings, each tending to vibrate at
a given rate and picking out and responding to peudular
aerial vibrations of its own period, and exciting a nerve
Avhich gave rise to a particular tone sensation. "When the
labyrinthic fluids were set in non-pendular vibrations, the
rods of Oorti were thought to analyze these into their peudu-
lar components, all rods of the vibrational rate of these be-
ing set in sympathetic movement, but that rod most wh-ose
period was that of the primary partial tone; this would
determine the pitch of the note, and the less-marked sen-
sations due to the others affected would give it its timbre.
The rods, however, do not differ in size sufficiently to
account for the range of notes which we hear, and they are
absent in birds, which undoubtedly distinguish different
musical notes; and the nerve-fibres of the cochlea are not
connected with them but with the hair-cells.

On the whole it seems probable that the basilar mem-
brane is to be looked upon as the primary arrangement for
sympathetic resonance in the ear. 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)
and is stretched tight across, but loosely in the other direc-
tion. A membrane so stretched behaves as a set of separate
strings placed side by side, somewhat as those of a harp
but much closer together; and each string would vibrate at
its own period without influencing much those on each side
of it. Probably, then, each transverse band vibrates to sim-
ple tones of its own jieriod, and excites the hair-cells which
lie on it, and through them the nerve-fibres. Perhaps the
rods of Corti, being stiff, and carrying the reticular mem-
brane, rub that against the upper ends of the hair-cells which
project into its apertures and so help in a subsidiary way,
each pair of rods being especially moved when the band of
basilar membrane carrying it is set in vibration. The tec-
torial membrane is probably a ''damper;" it is soft and

554 THE HUMAN BODY.

inelastic, and suppresses the vibrations as soon as the mov-
ing force ceases.

Function of the Vestibule and Semicircular Canals.
Many noises are merely spoiled music; they are due to tones
so combined as not to give rise to periodic vibrations; these
are probably heard by the cochlea. If a single violent
air-wave ever cause a sound sensation (which is doubtful
since any violent push of an elastic substance, such as the
air, will cause it to make several rebounds before coming to
rest) we j)erhapshear it by the vestibule; the otoliths, there
in contact with the auditory hairs, are imbedded in a
tenacious gummy mass quite distinct from the projier
endolymph, and are not adapted for executing regular
vibrations, but they might yield to a single powerful impulse
and transmit it to the hair -cells, and through them
stimulate the nerves. There is reason to believe that the
semicircular canals have nothing to do with hearing; their
supposed function is described in Chapter XXXV.

Auditory Perceptions. Sounds, as a general rule, do
not seem to us to originate within the auditory apparatus;
we refer them to an exteriud source, and to a certain extent
can judge the distance and direction of this. As already
mer.tioned, 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 recogni-
tion of the distance of a sounding body is j^ossible only when
the sound is well known, and then not very accurately; 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 per-
haps something to do with enabling us to detect whether
a sound originates before or behind the ear, since it collects,
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

AUDITORY PERCEPTIONS. 555

in each ear we can localize sounds better than if the head be
kept motionless. The large movable concha of many ani-
mals, as a rabbit or a horse, which can be turned in several
directions, is probably an important aid to them in de-
tecting the position of the source of a sound. That the
recognition of the direction of sounds is not a true sensation,
buta judgment, founded on experience, is illustrated by the
fact that we can estimate much more accurately the direc-
tion of the human voice, which we hear and heed most,
than that of any other sound.

CHAPTER XXXIV.

TOUCH, THE TEMPERATURE SENSE, THE
MUSCULAR SENSE, COMMON SENSA-
TION, SMELL, AND TASTE.

Nerve-Endings in the Skin. Many of the afEerent
skin-nerves end in connection with hair-bulbs; the fine
hairs over most of the cutaneous surface, projecting from
the skin, transmit any movement impressed on them, with

increased force, to the nerve-
fibres at their fixed ends. In
many animals, as cats, large,
specially tactile, hairs are de-
veloped on the face, and
these have a very rich nerve-
supply. Fine branches of
axis cylinders have also been
described as penetrating be-
tween epidermic cells and
ending there without termi-
nal organs. In or immedi-
ately beneath the skin several
peculiar forms of nerve end
organs have also been de-
scribed; they are known as
(1) Touch-cells; (2) Pacinian
corpuscles; (3) Tactile co7'-
jmscles; (4) End-hulbs.
The Pacinian corpuscles (Fig. 151) lie in the subcutane-
ous tissue of the hand and foot, and about the knee-joint;
but also away from the skin on branches of the solar plexus
(p. 172), so that it is doubtful if they are touch-organs.

Fig. 151. — A Pacinian corpuscle,
magnifled.

NERVE END ORGANS IN SKIN

557

They are oval, from 1.5 to 3.5 mm. [^ to ^ inch) long,
by about half that width, and have a whitish translucent
appearance, with a more opaque centre. When magnified
each is found to consist of a core, surrounded by many
concentric capsules, h. A nerve-fibre, a, enters at one end,
and its axis cylinder, c, runs along the core to the other,
where it terminates in one or two little knobs, or a number
of fine branches.

The tactile corpuscles lie in papillae of the dermis, aiid
are oval and about .08 mm. (-j-J-g- inch) in length. They
contain a soft core, enveloped by a connective-tissue cap-

' ''1

Fig. 152.— Dermic papillae with tactile corpuscles, A. a corpuscle with four
nerve-fibres; «, corpuscle; 6 and c. nerve-fibres, i?, papilla made transparent
with acetic acid to show tactile eorpvisele within ; o. proper tissue of the papilla ;
6, tactile corpuscle; c, entering nerve, d, e, nerve-fibres twining round the cor-
puscle. C, a papilla, containiug a tactile corpuscle, seen in optical transverse
section.

sule, and separated into several masses. Two, three, or
more nerve-fibres go to each corpuscle and appear to end
in plates lying between each of the segments of the core.
Tactile corpuscles are numerous in the skin of the hand
and foot, but are rare elsewhere. This limited distribution
over the surface militated against the belief that they were
tactile end organs; but it has lately been found that simpler
bodies, the touch-cells, of the same essential structure but
receiving only one nerve-fibre each, are distributed all over
the skin; the more complex, and probably more irritable,
form being found where the epidermis is especially thick.

558 THE HUMAN BODY.

The end-hulbs are spheroidal and about .04 mm. (^5-^^ inch)
in diameter. Each consists of a core, with a connective-
tissue capsule, to which two or three nerve-fibres run; the
axis cylinders penetrate the core. End-bulbs are found on
one or two regions of the skin, as that on the red part of
the lips, in the conjunctiva, and the mucous membrane
covering the soft palate, and the tongue.

Touch, or the Pressure Sense. Through the skin we
get several kinds of sensation; touch proper, heat and cold,
and pain; and we can with more or less accuracy localize
them on the surface of the Body. The interior of the
mouth jDossesses also these sensibilities. Through touch
proper we recognize pressure or traction exerted on the
skin, and the force of the pressure; the softness or hard-
ness, roughness or smoothness, of the body producing it;
and the form of this, when not too large to be felt all over.
In the latter case, as when we move the hand over an
object to study its shape, muscular sensations are com-
bined with proper tactile, and such a combination of the
two sensations is frequent; moreover, we rarely touch any-
thing without at the same time getting temperature sen-
sations; so that 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 tac-
tile 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 derived through touch, and we trans-
late unconsciously the teachings of the eye into mental
terms of the tactile sense. A person who saw but had no
touch-sense would conceive solid objects very differently
from the rest of mankind.

The delicacy of the tactile sense varies on different parts
of the skin; it is greatest on the forehead, temples, and

TOUCH. 559

buck of the forearm, where a weight of 2 milligr. (.03
grain) j^ressing on an area of 9 sq. millim. (.0139 sq. inch)
can be felt. On the front of the forearm 3 milHgr, (.036
grain) can be simihirly felt, and on the front of the fore-
finger 5 to 15 milligr. (.07-0.23 grain).

In order that the sense of touch may be excited neigh-
boring 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 compressed. 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 so-called iisycho-physical law (p. 473) is based,
were first observed. The smallest perceptible difference of
pressure recognizable 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 act-
ing as a stimulus. The ratio only holds good, however,
for a certain mean range of pressures; and its existence for
any has lately been denied. The experimental difficulties in
determining the question are considerable; muscular sensa-
tions 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 eliminated. 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 jioint of the skin is touched we can Avitli some
accuracy indicate the region stimulated; although tactile
feelings are in general characters alike, they differ in some-
thing [loccd sifffi) besides intensity by which we can distin-

500 TUE nUMAN BODY.

guisli tlieni; some sub-sensation quality not rising definitely
into i)roniinence in consciousness must be present, com-
parable to the upper joartials determining the timbre of
a tone. Tlie accuracy of the localizing power A'aries widely
in different skin regions and is measured by observing
the least distance which must separate two objects (as the
blunted points of a joair of compasses) in order that they
may be felt as two. The following table illustrates some
of the differences observed —

Tongue-tip 1.1 mm. (.04 inch)

Palm side of Itist phaluux of linger 2.2 mm. (.08 inch)

Red part of lips 4.4 mm. (.16 inch)

Tip of nose 6.G mm. (.24 inch)

Back of second pU;i];inx of linger 11.0 mm. (.44 inch)

Heel ;>2.0 mm. (.88 inch)

Buck 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 inclic.'.)

Middle of hack 66.0 mm. (2.64 inches)

The localizing power is a little moi'e acute across the long
axis of a limb than in it; and is better when the pressure
is only strong enough to just cause a distinct tactile sensa-
tion, tluiu when it is more power-

'^"""/fiW^'N W?*^ ^'^^^' ^^ ^^ ^^^'^ ^^^'^ readily and

nnW rapidly improvable by j^ractice.

fW|YiT^^ ^^ might be thought that this

\UJwU-Wt¥w localizing power dej^ended directly

'^ — 'uSuuXuuuYv ^^^ nerve distribution; that each

; ulUM touch-nerve had connection with

I \}JfJ^}'}^M>{ ^ special brain-centre on the one

\ \ti\jVW/ I hand (the excitation of which

) j caused a sensation with a charac-

I / teristic 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 tliis
were so, however, the peripheral tactile areas (each being
determined by the anatomical distribution of a nerve-fibre)

LOCALIZATION OF TACTILE SENSATIONS. 561

must have definite unchangeable limits, which experiment
shows that they do not possess. Suppose the small areas in
Fig. 153 to each represent a peripheral area of nerve distribu-
tion. If any two points in c were touched we would accord-
ing to the theory get but a single sensation; but if, while
the compass points remained the same distance apart, or were
even approximated, one were placed in c and the other on a
contiguous area, two fibres would be stimulated and we ought
to get two sensations; 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.

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 distinct. If we suppose twelve unexcited nerve
areas must intervene, then, in Fig. 153, a and b will be just
on the limits of a single tactile area; and no matter how
the pomts 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 distri-
bution 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 ultimate differentiation of tactile areas takes place in
the central organs, as will be more fully pointed out in the
next chapter. Afferent nerve impulses reaching the spinal
cord from a finger-tip enter the gray matter and tend
to radiate some way in it; from the gray region through
which they spread, impulses are sent on to perceptive
tactile centres in the brain; if two skin-points are so close
that their regions of irradiation in the cord overlap, then
the two points touched cannot be discriminated in con-

6fi2 THE HUMAN BODY.

sciousness, since the stime brain regions are excited. Tlie
more poAverful tlie stimulus the wider the irradiation in
the cord, and hence the less accurate the discriminating
power. The more often an impulse has traveled, the more
does it tend to keep its own proper tract through the gray
matter of the cord, and get on to its own proper brain-
centre alone, hence the increase of tactile discrimination
with practice, for we cannot suppose it to be due to a growth
of more nerve-fibres down to the skin, and a rearrangement
of the old, with smaller areas of anatomical distribution.
As a general rule, more movable parts have smaller tactile
areas; this probably depends on practice, since they are the
parts which get the greatest number of different tactile
stimulations.

The Temperature Sense. By this we mean our faculty
of perceiving cold and warmth; and, with the help of these
sensations, of perceiving temperature differences in external
objects. Its organ is the whole skin, the mucous membrane
of mouth and fauces, pharynx and 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 degree of heat and cold requisite is much
greater than that necessary when a temperature-perceiving
surface is acted upon; hence we must assume the presence
of temperature end organs.

In a comfortable room we feel at no part of the Body
either heat or cold, although different parts of its surface
are at different 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 tem-
perature which a given region of the temperature organ
has (as measured by a thermometer) when it feels neither
heat nor cold is its temjx'r (dure- sensation zero, and is not
associated Avitli any one objective temperature; for not only,
as we have just seen, does it vary in different parts of the
organ, but also on the same part from time to time.
Whenever a skin region has a temperature above its sensa-
sation zero we feel warmth and vice versa; the sensation is
more marked the greater the difference, and the more

TEMPERATURE SENSATIONS. 563

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 con-
ductor, as a piece of wood, at 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 cool-
ing), 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 circula-
tion and tended to warm his skin, but the moving outer air
has rapidly conducted off the extra heat; on entering tlie
house the stationary air there does this less qiiickly, the
skin gets hot, 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
temperatures within a few degrees of 30° 0. (86° F.); at
these differences of less than .1°C. can be discriminated.
As a means of measuring absolute temperatures, however,
the skin is very unreliable, on account of the changeability
of its sensation zero. We can localize temperature sensa-
tions much as tactile, but not so accurately.

Are Toucli and Temperature Sensations of Different
Modality ? Tactile and temperature feelings are ordina-
rily so very diffei'ent that we can no more compare them
than luminous and auditory; and if we accept the modern
modified form of the doctrine of specific nerve energies
(p. 191), in accordance with which the same sensory fibre
when excited always arouses a sensation of the same quality
because it excites the same brain-centre, it is hard to con-
ceive how the same fibre could at one time arouse a tactile,
and at another a temperature sensation. It has, however,
been maintained that touch and temperature feelings

564 THE HUMAN BODY.

sometimes pass into one another insensibly, and that
they are really extreme varieties of one kind of sensation,
comparable to yellow and blue in the visual series. If a
half dollar cooled to 5° 0. (41° F.) be placed on a person's
brow, and ihen two (one on the other) warmed to 37° C.
(98.5° F.), he commonly thinks the weight in the two cases
is equal; i.e. the temperature difference leads to errors in
his pressure perceptions, the cold body seeming heavier.
But this does not prove an essential identity in the sensa-
tions; it is conceivable, e.g., that the cold half dollar pro-
duces contraction of the cutaneous tissues, leading to com-
pression of the tactile end organs, which is mistaken, in
mental interpretation, for a heavier external object. When
sensations are combined in other cases, as red and blue-
green to produce white, or partial tones to form a com-
pound, we either cannot at all, or only with difficulty,
recognize the components; while in this case the person
feels both the cold and pressure distinctly when the half
dollar is laid on him.

It has also been shown that in certain cases a person mis-
takes the contact of a piece of raw cotton with a sm^all ex-
posed area of his skin, for the approach of a warm object;
and this has been taken to prove that touch and temperature
feelings may gradate into one another. However, the
feeble touch of the raw cotton might well be less felt than the
increased temperature of the skin, due to diminished radia-
tion when it was covered by this non-conducting substance;
and the constancy with which, in the ordinary circumstances
of life, we feel and discriminate clearly, on the same skin
region, both temperature and touch sensations, is very
strongly against any transitional passage of one into the
other.

In favor of the view that touch and temperature are sen-
sations of distinct modality, with different end organs,
afferent nerve-fibres, and brain-centres, are several weighty
facts. The regions of most acute discrimination for each
sensation are different; and cases of disease are recorded in
which persons have been extremely sensitive to variations
in temperature, while their tactile sensibility was unaltered ;

MUSCULAR FEELINGS. 565

and, conversely, cases in which the patient could feel that
he had been touched but was unable to say whether with a
hot or cold object.

The Muscular Sense. In connection with our muscles
we have sensations of great importance, although they do
not often become so obtrusive in consciousness as to arouse
our attention until we look for them. Certain of these
feelings {muscle sensations proijer) are due to the excitation
of sensory nerves ending within the muscles themselves:
the others {innervation sensations) have probably a central
origin and accompany the starting of volitional impulses
from brain-cells; they are only felt in connection with the
voluntary skeletal muscles.

The proper muscle sensations only become marked on
powerful or long-continued muscular effort (cramp, fatigue),
but a lower grade of them, not distinctly perceived, proba-
bly accompanies all muscular activity.

The innervation feelings are of far more consequence.
They accompany the slightest movement of a skeletal mus-
cle, and we derive from them means of determining with
great accuracy the force and extent of the contraction
willed. The belief that their origin is central mainly rests
on the fact that we have sensations, not merely of executed
but of intended movements. The actual nature of the
movement performed is probably characterized by other
contemporary sensations, as of the muscle sense proper,
from pressure and folding of the skin, and so on.

The innervation feelings thus stand apart and opposed
to all our others as primary factors in our mental life; they
represent the reactive work of the organism with respect
to its environment. Some distinguished physiologists,
however, deny their existence, ascribing them all to a
peripheral origin, cither in sensory muscle-nerves, or in
skin-nerves affected when a part of the Body is moved.
As, however, we can determine more accurately the differ-
ence between two weights when we lift them than we can
when they are merely laid on a hand supported by a table
(see below), there are undoubtedly (apart from cramp and
fatigue) true muscular sensations distinct from tactile; and

566 THE HUMAN BODY.

tliut these, or some of tlium, are central in origin seems
proved by certain phenomena observed in disease. Persons
suffering ivom. jjarcsis, i.e. muscuhir weakness not amount-
ing to complete paralysis, make (until they have learnt to
interpret correctly their sensations under the new condi-
tions) entirely wrong judgments as to the extent of their
movements; they think they have contracted their muscles
much more than is really the case. If the muscular sense
depended on stimulation of sensory nerves in the mus-
cles this error could not arise, for the intensity of the sen-
sation would be determined by the amount of contraction
which actually occurred. Under the circumstances, how-
ever, a stronger volition than previously is needed to cause
a movement of given extent or power; and we can readily
explain the mistakes if we suppose the sensation is of cen-
tral origin, and determined in intensity by the will-iiower
exerted.

We have already seen how closely muscular sensations
are combined with visual in enabling us to form judgments
as to the distance, size, and movements of objects. They
are as closely combined in ordinary life with tactile sensa-
tions; 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 combining the tactile
feelings it gives rise to, with the muscular feelings accom-
panying 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
interpret, 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 neasurement, 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 con-
tact with the thing looked at.

"When w^e wish to estimate accurately the weight of an
object we always, when possible, lift it, and so combine mus-
cular with tactile sensations. By this means we can form
much better judgments. While with touch alone just percej)-

CO MM ox SENSATIONS. 56?

tibly different pressures have the ratio 1 : 3, with the mus-
cular sense differences of y^ can be perceived.

Common Sensations. Under tliis name are included the
sensations Avluch we do not mentally attribute to the prop-
erties of external objects, but to conditions of our own
Bodies; of them we may here consider pain, hunger, and
thirst.

Pain arises when powerful mechanical or thermal stimuli
act on the skin, and when a sensory nerve-trunk (except the
optic, auditory, or olfactory) is directly excited. Most
commonly we derive the feeling through cutaneous or sub-
cutaneous nerves, and hence it has been supposed that pain
is not a sensation of distinct modality due to the excitation
of special nerve-fibres, but is dependent on excessive excite-
ment of ordinary tactile fibres; and pressure or temperature
sensations do undoubtedly gradate into painful as the
stimuli increase. If this be so, pain is a sort of incoordi-
nate or " convulsive" sensation. We shall see in Chapter
XXXV. that, when a sensory nerve is normally excited in
a decapitated animal, regular purpose-like reflex movements
result: but if the stimulus be very powerful, or a nerve-
trunk be directly excited, then inco-ordinate convulsions
occur- the afferent impulses radiate farther in the centre
and produce a new and useless result. We may supj^ose
something similar to occur in the cutaneous nerves of an
animal still possessing sensory brain-centres, if the stimuli
acting on the skin are such as to excite the end organs very
powerfully, or the sensory fibres directly without the inter-
mediation of end organs; that a new sensation should be
thus aroused, different from tactile though gradually shad-
ing off into them, is a phenomenon comparable with the
production of new color sensations by combinations of the
fundamental ones. In such case, too, we could understand
the difference of kinds of "pain" in a more general sense
of the word; muscular cramp, dazzling, and disagreeably
shrill or inharmouiously combined tones, might all be
looked upon as inco-ordinate sensations, each with a charac-
acter of its own determined by the central apparatus ex-
cited.

5 (J 8 THE HUMAN BODY.

The niiilter cuniiot, however, bout present decided. The
skin seems indubitably to contain many nerve-fibres which
terminate in free axis cylinders without end organs; and
these may well be true pain-fibres, fitted to respond to
stronger stimuli than those which excite the tactile and
thermic end organs. Certain pathological and experi-
mental phenomena tend also to prove that the brain-centres
concerned in the production of tactile and painful sensa-
tions arc different. Persons sometimes lose pain sensations
and keej) tactile; a gentle touch is felt as well as usual, but
a powerful pinch causes no pain: in one stage of ether and
chloroform narcosis the same thing is observed; the sur-
geon's hand and knife are felt where they touch the skin,
but cutting deeper into the tissues produces no pain. In
animals a similar state of things may be produced by divid-
ing the gray matter of the cord, leaving the posterior white
columns intact; while, if the latter be divided and the gray
substance loft uninjured, there is increased sensitiveness to
l)ain, and probably touch proper is lost, though that is im-
possible to say with certainty. Such experiments make it
pretty certain that when sensory afferent impulses reach
the cord at any level and there enter its gray matter with
the posterior i-oot-fibres, they travel on in different tracks
to conscious centres; the tactile coming soon out of the
gray network and coursing on in a readily conducting white
fibre, while the painful first travel on farther in the gray sub-
stance. It is still uncertain if both impulses reach the cord
in the same fibres. The gray network conducts nerve im-
pulses, but not easily; they tend soon to be blocked in it.
A feeble (tactile) impulse reaching it by an afferent fibre
might only spread a short way and then pass out into
a single good conducting fibre in a white column, and
])roceed to the brain; while a stronger (painful) impulse
would radiate farther in the gray matter, and perhaps
break out of it by many fibres leading to the brain through
the white columns, and so give rise to an inco-ordinate and
ill localized sensation. That pains are badly localized, and
worse the more intense they are, is a well-known fact,
which would thus receive an explanation (see p. 579).

SMELL. 569

Hunger and Thirst. These sensations, which regulate
the taking of food, are peripherally localized in conscious-
ness, the former in the stomach and the latter in the throat,
and local conditions no doubt i^lay a part in their produc-
tion; though general states of the Body are also concerned.
Hunger in its first stages is probably due to a condition
of the gastric mucous membrane which comes on when the
stomach has been empty some time, and may be 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; pro-
vided these are absorbed and reach the blood, their mode of
entry is unessential; the hunger may be stayed by injec-
tions of food into the rectum as well as by putting it into
the stomach.

Similarly, thirst may be temporarily relieved by moisten-
ing the throat without swallowing, but then soon returns;
while it may be permanently relieved by water injections
into the veins, without wetting the throat at all.

While both sensations thus depend in part on local
peripheral conditions of afferent nerves (pneumogastric
and glossopharyngeal), they may be also, and more power-
fully, excited by poverty of the blood in foods and water;
this probably directly stimulates the hunger and thirst
brain-centres.

Smell. The olfactory organ consists of the upper por-
tions 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
{regto oJfactoria) covers the upper and lower turbinate
bones {o, p, Fig. 89*), which are expansions of the ethmoid
(p. 75) on the outer wall of the nostril chamber, the oppo-
site part of the iiartition between the nares, and the part of
the roof of the nose {n, Fig. 89) separating it from the
cranial cavity. The epithelium covering the mucous mem-
brane contains two varieties of cells arranged in several
layers (2, Fig. 154). Some cells are much like ordinary
columnar epithelium but with long branched processes
attached to their deeper ends; the others have a large

* P. 309.

570

THE HUMAN BODY.

nucleus surrounded by a little protoplasm; a slender exter-
nal process reaching to the surface; and a very fine deep one.
The latter cells have been supj^oscd to be the proper olfac-
tory end organs, and to be connected with the fibres of the
olfactory nerve, which enter the deeper strata of the epithe-
lium and there divide; but it is doubtful whether both
kinds of cells are not so connected.

Odorous substances, the
stimuli of the olfactory appa-
ratus, are always gaseous and
frequently act powerfully
Avhen present in very small
amount. We cannot, how-
ever, classify them by the sen-
sations they arouse, or arrange
them in series; and smells are
but minor sensory factors in
our mental life. We commonly
refer them to external objects
since we find that the sensa-
tion is intensified by " sniff-
ing" air powerfully into the
nose, and ceases when the
nostrils are closed. Their
peripheral localization is, how-
ever, imperfect, for we con-
found many smells with tastes
(see below); nor can we judge
well of the direction of an
odorous body through the
olfactory sensations which it
arouses.

nerves of common sensation,
such substances as ammonia

Fig. 15i.— Cells from the olfactory
epithelium. 1, from the frog; 2.
from man ; a, columnar cell, with its
bi-anched deep process; 6, so-called
olfactory cell; c, its narrow outer
p:-ocess; d, its slender central pro-
cess. 3, gray nerve-fibres of the ol-
factory nerve, seen dividing into
fine peripheral branches at a.

also

by

The nose possesses
which are stimulated
vapor.

Taste. The organ of taste is the mucous membrane on
the dorsum of the tongue and possibly other parts of the
boundary of the mouth cavity. The nerves concerned are
the glossopharyngeals, distributed over the hind part of the

TASTE. 571

tongue, and the lingual branches of the inferior maxillary
division (p. 170) of the trigeminals on its anterior two
thirds.

On the tongue the nerves run to papillae; the circumval-
late (p. 313) have the richest supply, and on these are cer-
tain peculiar end organs (Fig. 155) known as taste-buds,
which are oval and imbedded 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 connected with
nerve-fibres at their deeper ends. The capsule formed by
the enveloping cells has a small opening on the surface;

Fig. 155.— Taste-buds.

each taste-cell terminates in a very fine thread which pro-
trudes there. Taste-buds are also found on some of the
fungiform papillae, and it is possible that simpler struc-
tures, not yet recognized, consisting of single taste-cells
are widely spread on the tongue, since the sense of taste
exists where no taste-buds can be found. The filiform
papillae are probably tactile.

In order that substances be tasted they must be in solu-
tion: 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. Tastes proper may be divided into
sweet, bitter, acid, and saline. Intellectually they are, like
smells, of small value; the perceptions we attain through
them as to qualities of external objects being of little use.

572 THE HUMAN BODY.

except as aiding in the selection of food, and for that pur-
pose they are not by any means safe guides at all times.

Many so-called tastes (flavors) are really smells; odorif-
erous particles of substances which are being eaten reach the
olfactory region through the posterior nares and arouse sen-
sations 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 common sensation in the tongue.
This fact is sometimes taken advantage of in the jiractice
of domestic medicine when a nauseous dose, as rhubarb, is
to be given to a child. Tactile sensations play also a part
in many so-called tastes.

Most persons taste bitters best with the back of the
tongue and sweets towards the tip; but this is not constant.
The curious interference of tastes which takes place when
the acidity of a sour body is covered by adding a sweet one,
which does not in any way chemically neutralize the acid
(when sugar is put on a lemon for example), needs explana-
tion.

CHAPTER XXXV.

THE FUNCTIONS OF THE BKAm AND
SPINAL CORD.

The Special Physiology of Nerve-Centres. We have
already studied the general physiological properties of
nerves and nerve-centres (Chaj^. XIII.) and found that
while the former are mere transmitters of nervous impulses,
the latter do much more. In some cases the centres are auto-
matic; they originate nerve impulses, as illustrated by tlie
beat of the heart under the influence of its ganglia. In
other cases a feeble impulse reaching the centre gives rise
to a great discharge of energy from it (as when an unex-
pected noise produces a violent start, due to many impulses
sent out from the excited centre to numerous muscles), so
that certain centres are irritable; they contain a store of
energy-liberating material which only needs a slight dis-
turbance to upset its equilibrium and produce many efferent
impulses as the result of one afferent. Further, the im-
pulses thus liberated are often co-ordinated. When mucus
in the windpipe tickles the throat and excites afferent nerve
impulses, these, reaching a centre, cause discharges along
many efferent fibres, so combined in sequence and power
as to produce, not a mere aimless spasm, but a cough
which clears the passage. In still other cases the excita-
tion of centres, with or without at the same time some of
the above phenomena, is associated with sensations or other
states of consciousness. We have now to study which of
these powers are manifested by different parts of the cen-
tral cerebro-spinal neiwous system, and under what circum-
stances and m what degree: what is known of the general
functions of the sympathetic and sporadic ganglia has al-
ready been stated (p. 183).

574 THE nmr AN BODY.

The Spinal Cord as a Centre. The spinal cord, forming
(except tlio slender sympathetic) the only direct communi-
cation between the brain and most of the nerves of the Body,
was considered by the older physiologists as merely a huge
nerve-trunk, into which the various spinal nerves were
collected on their way to the encephalon. It does, it is
true, contain the paths for the conduction of all those im-
pulses which, originating in the cerebrum, give rise to
voluntary movements of the trunk and limbs; also for all
the centrally traveling impulses which give rise to sensa-
tions ascribed to those parts; and it is also the path for cer-
tain impulses giving rise to involuntary movements as, for
example, those which, originating in the respiratory centre,
travel to the phrenic and intercostal nerves.

If, however, the cord were merely collected and con-
tinued nerve-roots it ought to increase considerably in bulk
as it approached the skull, and this it does not do in any-
thing like the required proportion; a histological examina-
tion, too, shows that few, if any, of the entering fibres pro-
ceed directly to the brain; they pass to the central core of
gray substance, containing nerve-cells and different from
anything in a true nerve-trunk. In the gray core some
fibres (anterior roots) end in cells of the anterior horns, and
others (posterior roots) in that fine network which pervades
the whole gray substance (p. 179); the nerve-cells by their
branches are also in continuity with this network and so,
ultimately, with all the nerve-fibres entering the cord.
From the network fibres arise at different levels, and pass
out into the white columns; some place two regions of the
gray matter in intimate connection, while others go on
to the brain. In order to understand physiological facts
we must assume, first, that a nervous impulse entering the
gray network at any point may, under certain conditions,
travel all through it, and give rise to efferent impulses
emerging at any level; and, on the other hand, that there
are certain lines or paths of easiest propagation between
different points in this network, which the impulses keep
to under ordinary conditions.

Keflex Actions. When a frog is decapitated it lies down

BEFLEX ACTIONS OF THE COBD. 575

squat on its belly instead of assuming the more erect jjosi-
tion of the uninjured animal; its respiratory moyements
cease (their centre being removed with the medulla); the
hind legs at first remain sprawled out in any position into
wliich they may happen to fall, but after a time are drawn up
into their usual j^osition, with the hip and knee joints flexed;
having made this movement the animal, if protected from
external stimuli, makes no other by its skeletal muscles; it
has lost all spontaneity, and only stirs under the influence
of immediate excitation. Nevertheless the heart goes on
beating for hours; the muscles and nerves, when examined,
are found to still have all their usual i^hysiological proper-
ties; and, by suitable irritation, the animal can be made to
execute a great variety of complex movements. But it
is no longer a creature with a will, doing things which we
cannot predict; it is an instrument Avhich can be played
upon, giving different responses to different stimuli (as dif-
ferent notes are produced when different keys of a piano are
struck), and always the same reaction to the same stimulus;
so that we can say beforehand what will happen when we
touch it. Such actions are called 7'eflex or excito-motor and
fall into two groups; (1) orderly or inirpose-like reflexes,
which are correlated to the stimulus and are often defensive,
tending, for instance, to remove an irritated part from the
irritating object; (2) disorderly or convulsive reflexes, not
tending to produce any definite result, and affecting either a
limited region or all the muscles of the body.

In higher animals similar phenomena may be observed.
If a rabbit's spinal cord be divided at the bottom of the neck
the animal is at first thrown into a flaccid limp condition
like the frog, but it soon recovers. Voluntary movements
in muscles supplied from the spinal cord behind the section
are never seen again; but on pinching the hind foot it is
forcibly withdrawn. Men, whose spinal cord has been
divided by stabs or disease below the level of the fifth cervical
spinal roots (above which the fibres of the phrenic nerve,
which are necessary for breathing, jiass out), sometimes live
for a time, but can no longer move their legs by any
effort of the will, nor do they feel touches, pinches, or hot

576 THE HUMAN BODY.

things applied to them; if, however, the soles of the feet
he tickled the legs are thrown into vigorous movement.
As a rule, however, orderly reflexes are less marked and
less numerous in the higher animals; in them the organiza-
tion is less machine-like, the sj)inal cord being more the
servant of the larger brain, and less capable of working
without directions. Such animals, when intact, can to a
greater extent control the muscular responses which shall
be made to stimuli under various conditions; they have less
automatic protection in the ordinary risks of life, but a
greater range of possil:)le protection. The human spinal
cord, controlled by the Ijrain, can adapt the reactions of the
Body, with great nicety, to a vast variety of conditions; the
frog's cord by itself does this for a smaller number of possi-
ble emergencies without troul)ling at all such brain as the
animal has, but is less completely under the control of the
higher centres for adajDtation to other conditions. The
difference being, however, but one of degree and not of
kind, it is best to approach the study of the reflex actions
of the human spinal cord through an examination of those
cxhiljited by the frog.

The Orderly Reflex Movements of a Decapitated Frog.
For the occurrence of these the following parts must Ije in-
tact; {a) the end organs of sensory nerve-fibres; {h) afferent
filu'cs from these to the cord; {(•) efferent fil)rcs from the
cord to the muscles; {d) the part of the spinal cord l^etween
the afferent and efferent fibres; {e) the muscles concerned
in the particular movement. If the animal be suspended
vertically after the shock of the operation is over, it makes
a few attempts to hold its hind legs in their usual flexed
position; these soon cease, the legs hang down, and the
creature comes to rest. If one flank be now gently scratched
with the point of a ])encil a reflex movement occurs,
limited to the muscles of that region; they twitch, some-
wdiat as a horse's neck when tickled by flies. If a jiinch be
given at the same spot, more muscles on the same side
come into play; a harder pinch causes also the hind leg of
that side to be raised to push away the offending object;
more violent and prolonged irritation causes all the muscles

DISORDEELY REFLEX MOVEMENTS. 577

of the body to contract, and the animal is convulsed. Here
then we see that a feeble stimulation causes a limited and
purpose-like response; stronger causes a wider radiation of
efferent impulses from the cord and the contraction of more
muscles, but still the movements are co-ordinated to an
end; while abnormally powerful stimulation of the sensory
nerves throws all the motor fibres arising from the cord into
activity, and calls forth inco-ordinate spasmodic action.
The orderly movements are very uniform for a given stimu-
lation; if the anal region be j)inched, both hind legs are
raised to push aAvay the forceps; if a tiny bit of bibulous
paper moistened with dilute vinegar be put on the thigh,
the lower part of that leg is raised to wipe it off; if on the
middle of the back near the head, both feet are wiped over
the spot; if on one flank, the leg and foot of that side are
used, and so on; in fact, by careful working, the frog's skin
can be mapped into many regions, the application of acidu-
lated water to each causing one particular movement,
due to the co-ordinated contractions of muscles in different
combinations, and never, under ordinary circumstances,
any but that one movement. The above purpose-like
reflex movements may all be characterized as defensive,
Init all orderly reflexes are not so. For example, in the
breeding season the male frog clasps the female for several
days with his fore limbs. If a male at this season be
decapitated and left to recover from the shock, it will be
found that gently rubbing his sternal region with the finger
causes him to clasp it vigorously.

Disorderly Reflexes or Reflex Convulsions. These
come on when an afferent nerve-trunk is stimulated instead
of the tactile end organs in the skin; or when the skin is
very powerfully excited; or. with feeble stimuli, in certain
diseased states {pafhologlcal tetanus), and under the in-
fluence of certain poisons, especially strychnine. If a frog
or a warm-blooded animal be given a dose of tlie latter drug,
a stimulus, such as normally would excite only limited
orderly reflexes, will excite the whole cord, and lead to
discharges along all the efferent fil^res so that general con-
vulsions result. It has been clearly proved that, in such

578 THE HUMAN BODY.

cases, not the skin, or afferent or efferent nerves, or the
muscles, but ilie spinal cord itself is affected by the poison
(at least iirimarily), unless unnecessarily large doses have
been given.

The Least-Resistance Hypothesis. In order to com-
prehend reflex acts we must assume a manifold union of
sensory with efferent nerve-fibres; this is anatomically
afforded by the minute plexus of the gray network, which
is continuous through the whole cord and in which the fibres
of the anterior and posterior nerve-roots, directly or indirect-
ly end. The continuity of this network serves to exj^lain
general reflex convulsions, and the spread of an afferent
impulse, or its results, through the whole cord, Avith the
consequent emission of efferent impulses through many or
all the anterior roots; but, on the other hand, it renders it
difficult to understand limited and orderly reflexes, in
which only a few efferent fibres are stimulated. To explain
them we have to assume a great resistance to conduction in
the gray network, so that a nerve impulse entering it is
soon blocked and transmuted into some other form of
energy; hence it only reaches efferent fibres originating near
the point at which it enters, or fibres placed in siiecially
easy communication with that. When the frog's flank is
tickled, only muscles innervated from anterior roots on the
same side of the body, and springing from the same level
of the cord, are made to contract; when the stimulus is
more powerful the stronger afferent impulse radiates farther,
but mainly in directions determined by lines of conduc-
tivity in the cord; e.g., to the origin of the efferent fibres
which cause lifting of the hind leg to the irritated spot.
These paths of easiest conduction, or of least resistance, in
some cases lie in the gray matter itself, in others in inter-
central or commissural fibres of the highly conductive
medullated kind, which, passing out of the gray substance
at one level, run in the white columns to it at another,
where the efferent fibres of the muscles called into play
originate. A still stronger afferent impulse radiates wider
still, and, liberating energy from all the nerve-cells in the
gray matter, produces a useless general convulsion. Under

CONDUCTIOX IN THE CORD. 579

the influence of stryclmine and in pathological tetanus (as
observed, for example, in liydrophobia) the conductivity of
the whole gray matter is so increased that all paths
through it are easy, and so a feeble afferent impulse spreads
in all directions.

To account for the phenomena of localized skin sensa-
tions (p. 559) and of limited voluntary movements we must
make a similar hypothesis. If the nervous impulses enter-
ing the gray network when the tip of a finger is touched
sjiread all through it irregularly, we could not tell what
region of the skin had been stimulated, for the central
results of stimulating the most varied peripheral parts
would be the same. From each region of the gray network
of the cord where a sensory skin-nerve enters there must,
therefore, be a special path of conduction to a given brain-
region, producing results which differ recognizably in con-
sciousness from those following the stimulation of a differ-
ent skin region. The acuteness of the localizing power will
largely depend on the definiteness of the path of least resis-
tance in the gray matter, since while traveling in a medul-
lated nerve-fibre from the skin to the cord, or (in the
white columns) from the gi'ay matter of the latter to the
brain, the nervous imjnilse is confined to a definite track.
Hence anything tending to let the afferent impulse radiate
when it enters the cord «'ill diminish the accuracy with
which its peripheral origin can be located. This we see in
violent pains; a whitlow on the finger affects only a few
nerve-fibres, but gives rise to so powerful ner-ve impulses
that when they reach the cord they spread widely and, break-
ing out of the usual track of propagation to the brain, give
rise to ill-localized feelings of pain often referred ail the
way up the arm to the elbow. So the pain from one diseased
tooth is often felt along half a dozen, or all over one side
of the head. Such cases are comparable to the transforma-
tion of an orderly reflex into a general convulsion when the
stimulus increases.

As an animal shows no spontaneous movements when its
cerebral hemispheres are removed, we conclude that the
nerve impulses giving rise to such movements start in those

680 THE HUMAN BOLT.

parts of the brain. Thence they travel down the white
columns of the cord to its gray matter, which they enter at
different levels, each in the neighborhood of a centre for
producing a given movement. If they there radiated far
and wide no definite movement could result, for all tlie
muscles supplied from the cord would be made to contract,
and not merely those necessary to bend tlie index finger,
for example. We must here again, therefore, assume a path
of least resistance for the propagation of nerve impulses from
a given fibre coming down from the brain, to the efferent
fibres going to a certain muscle or group of muscles. The
path between the two is almost certainly not direct; a
co-ordinating spinal centre intervenes, and all that the
brain has to do is to excite this centre, which then secures
the proper muscular co-ordination. If the hand be laid flat
on the table and its palm be rolled over, many muscles, in-
cluding thousands of muscular fibres, have to contract in
definite order and sequence. Persons who have not studied
anatomy and who are quite ignorant of the muscles to be
used can perform the movement perfectly; and even a
sl-cilled anatomist and physiologist, if he knew them all
and their actions, could not by conscious effort combine
them so well as the cord does without such direct interfer-
ence. We have then to look on the cord as containing a
host of co-ordinating centres for different muscles. These
centres are put in nervous connection, on the one hand,
with certain regions of the skin, and, on the other, with
regions of the brain, and may be excited from either; in
the former case the movement is called reflex; in tlie latter
it may be reflex, or may be accompanied by a feeling of
" will" and is then called voluntary. The more accurately
the required centre, and no other, is excited, the more
definite and precise the movement.

The Education of the Cord- Much of what is called
educating our touch or our muscles is really education of
the spinal cord. A person who begins to play the piano
finds at first much difficulty in moving his fingers inde-
pendently; the nervous impulses from tlio l)rainto the cord
radiate from the spinal centres of the muscle which it is

THE lyni^TION OF REFLEX ACTIONS. 581

desired to move, to others. But with practice the indepen-
dent movements become easy. So, too, the localizing power
of the skin can be greatly increased by exercise (p. 5(30) as
one sees in blind persons, who often can distinguish two
stimuli on parts of the skin which are so near together as to
give only one sensation to other peojile. Such phenomena
depend on the fact that the more often a nervous impulse has
traveled along a given road in the gray matter, the easier
does its path become, and the less does it tend to wander
from it into others. We may compare the gray matter to a
thicket; persons seeking to beat a road through from one
point to another would keep the same general direction,
determined by the larger obstacles in the way, but all would
diverge more or less from the straight path on account of
undergrowth, tree trunks, etc., and would meet with cojisid-
erable difficulty in their progress. After some hundreds had
passed, however, a tolerably beaten track would be marked
out, along which travel was easy and all after-comers would
take it. If instead of one entry and one exit we imagine
thousands of each, and that the paths between certain have
been often traveled, others less, and some hardly at all, we
get a pretty good mental picture of what happens in the
passage of nervous impulses through the gray matter of the
cord; the clearing of the more trodden paths answering to
the effects of use and practice. The human cord and that
of the frog must not, however, be looked upon as pathless
thickets at the commencement; each individual inherits
certain paths of least resistance determined by the structure
of the cord, which is the transmitted material result of the
life experience of a long line of ancestry.

The Inhibition of Reflexes. Since it is possible, as by
strychnine, to diminish the resistance in the gray matter, it
is conceivably also possible to increase it, and diminish or pre-
vent reflexes. Such is found to be actually the case. We can
to a great extent control reflexes by the will; for example,
the jerking of the muscles which tends to follow tickling.
and it is found that after a frog's brain is removed it is much
easier to get reflex actions out of the spmal cord. Certain
drugs, as bromide of potassium, also diminish reflex excita-

582 777^ HUMAN BODY..

bility. If a frog's brain be remoyed and the animal's toe
be dij^ped into very dilute acid, it will be removed after a
few seconds; the time elapsing between the immersion and
tlie lifting of the foot is known as the reflex time; any-
thing diminishing reflex excitability increases this, as the
stimulus (which has a cumulative effect on the centre) has
to act longer before it arouses the cord to the discharging
point. If the sciatic nerve of the other leg be stimulated
while the toe is in the acid the reflex time is increased, or
the reflex may fail entirely to apj)ear. This is one case of
a general law, that any powerful stimulation of one sen-
sory nerve tends to inhibit orderly reflexes due to the ex-
citation of another. A common example is the well-known
trick of pressing on the upper lip to prevent a sneeze.
The whole question of reflex inhibition is at present very
obscure. It may be due to the excitation of special fibres
which inhibit reflex centres, as pneumogastric fibres do the
cardio-motor; or to the fact that one nerve impulse in the
cord in some cases blocks or interferes with another; or
l^artly to both.

Psychical Activities of the Cord. Since we can get
quite marked reflex movements in the lower part of the
Body of a man whose cord is divided and who cannot
voluntarily move his lower limbs, and on questioning him
find that he feels nothing and is quite ignorant of his
movements unless he sees his legs, it is most probable that
the sjiinal cord in all cases is devoid of centres of conscious-
ness and volition: this is not certain, however; for tliere
might well be a less division of physiological labor between
llic cord and brain of a frog, than between those of a man.
Still we are entitled to good evidence before we admit that
two things so similar as the human cord and the frog's pos-
sess different properties. Co-ordinated movements follow-
i:iga given stimulus, or cries emitted by an animal, will not
suffice to prove that it is conscious, since these we know may
occur entirely unconsciously in men, who alone can tell us of
their feelings. We must look for something that resembles
actions only done by men consciously. In the frog it has
been maintained that we have evidence of such. If a bit of

PSYCHICAL ACTIVITIES OF TEE SPIHAL CORD 583

acidulated paper be put on a decapitated frog's thigh, the
animal will bend its knee and use its leg to brush off the
irritant; always using this same leg if the stimulus be not
so strong as to produce disorderly reflexes. If now the
foot be tied down so that the frog cannot raise it, after a
few ineffectual efforts it will move the other leg, and may
wipe the paper off with it. This it has been said shows a
true psychical activity in the cord; a conscious and volun-
tary employment of new procedures under unusual circum-
stances. But a close observation of the phenomenon shows
that it will hardly bear this interpretation; the movements
of the other leg are very irregular and inco-ordinate, and
much resemble reflex convulsions stirred up by the pro-
longed action of the acid, which goes on stimulating the
skin nerves more and more powerfully. Even if new
muscles came, in an orderly way, into play under the
stronger stimulus, that would not prove a volitional con-
scious use of them; we see quite similar phenomena when
there is nothing purpose-like in the movement. Many
dogs reflexly kick violently the hind leg of the same
side when one flank is tickled. If this leg be held and
the tickling continued, very frequently the opposite hind
leg will take on the movements, which it never does in
ordinary circumstances. This is quite comparable to the
frog's use of its other leg under the circumstances above
described, but here it would be obviously absurd to talk of
a volitional source for such a senseless movement.
/ Putting together, then, the testimony of persons with
/ injured spinal cords, and the observations made on them
1 and on animals, we may tolerably safely conclude that the
\ cord contains no centres of consciousness. There are, how-
' ever, persons who maintain that in such cases the cord itself
I feels though the individual does not, whatever that may
I mean; if the statement is used merely to imply that the
I cord is irritable (just as a muscle is) no one denies it; but
it is an unnecessarily confusing method of stating the fact.
The Cord as a Transmitter of Nervous Impulses. In
the gray substance, as we have seen, there is reason to
believe that nervous impulses can pass in all directions,

584 THE HUMAN BODY.

though they usually take certain easiest roads. The main
tracks between bram and cord, however, lie in the white
columns; nerve-roots enter the gray matter, and from each
region of the latter nicdullated fibres arise, pass out into a
white column, and continue to the brain. Once in these
fibres the impulse has to keep to a definitely marked out
anatomical path, which leads to a brain centre or centres.
These paths lie mainly in the lateral columns, the affe-
rent (sensory) impulses tending, however, to spread into
the posterior columns, and the efferent (volitional) into
the anterior; the main bulk of the posterior and anterior
columns seems to be made up of commissural fibres join-
ing different levels of the cord. The sensory fibres for the
most part cross soon after they enter the gray matter, and
proceed onwards mainly on the side opposite to that of the
nerve-root which conveyed them to the cord, while the effe-
rent cross mainly in the medulla before they reach the cord;
the crossing seems in neither case complete. Hemisection
of the cord, therefore, causes marked, but not absolute, loss of
voluntary movement in muscles of the same side supplied
with nerves from a part of the cord below the level of the
section; and a considerable, but not entire, loss of sensi-
bility on the opposite side. Impulses so powerful as to lead
to feelings of pain travel mainly in the gray matter (p. 568),
and they also for the most part cross the middle line soon
after their entry.

The Functions of the Brain in General. The brain, at
least in man and the higher animals, is the seat of con-
sciousness and intelligence; these disappear when its blood-
supply is cut off, as in fainting; pressure on parts of it, as
by a tumor or by an effusion of blood in apoplexy, has the
same result; inflammation of it causes delirium; and when
the cerebral hemispheres are unusually small idiotcy is
observed. The brain has, however, many other important
functions; it is the seat of many reflex, automatic, and
co-ordinating centres, which may act as entirely apart from
consciousness as those of the spinal cord; experiment makes
it probable that psychical faculties are dependent on the
fore-brain, while the rest of the complex mass has other,

CEREBRAL FUNCTIONS. 585

non-mental, duties. If the cerebral hemispheres be re-
moved from a frog, the animal can still perform every
movement as well as before; but it no longer performs
any spontaneously; it must be aroused by an immediately
acting stimulus, and its response to this is as invariable
and predicable as that of a frog with its spinal cord only.
The movements which can be educed are, however, far
more complex; instead of mere kicks in various directions
the animal can walk, leap, swim, get off its back on to its
feet, and so on. Similar results are observable in pigeons
whose fore-brain has been removed; mammals bear the
operation badly, but some, as rats, survive it several hours
and then exhibit like phenomena. The creatures can move,
but do not unless directly stimulated; all their volitional
spontaneity is lost, and, apparently, all perceptions also;
they start at a loud noise, but do not run away as if they
conceived danger; they follow a light with the eyes, but do
not attempt to escape a hand stretched forth to catch them;
they can and do swallow food placed in the mouth, but
would die of starvation if left alone with plenty of it al^out
them, the sight of edible things seeming to arouse no idea
or conception. It may be doubted, perhaps, whether the
animals have any true sensations; they start at sounds,
avoid opaque objects in their road, and cry Avhen pinched;
but all these may be unconscious reflex acts: on the whole
it seems more probable, however, that they have sensations
but not perceptions; they feel redness and blueness, hard-
ness and softness, and so on; but sensations, as already
pointed out, tell in themselves nothing; they are but signs
which have to be mentally interpreted as indications of ex-
ternal objects: it is this interpreting power which seems
deficient in the animal deprived of its fore brain.

Functions of the Medulla Oblongata. This contains
the paths of conduction between the parts of the brain in
front of it and the spinal cord. It is also the seat of
many important reflex and automatic centres, especially
those governing the organs immediately concerned in the
maintenance of life; as the respirator \% circulatory, and
masticatory. It may therefore be called the "nerve cen-

586 THE HUMAN BODY.

tral organ of tlie nutritive processes." The physiological
action of most of the medullary centres has already been
described; the more important are — 1. The respiratory
centre (p. 390). 2. The cardio-inhibitory centre (p. 250);
(there is also some reason to believe that the centre of the
accelerator heart-fibres (p. 252) lies in the medulla). 3.
Tlie vaso-motor centre (p. 253). 4. The centre for the
dilator muscle-fibres of the pupil (p. 486). 5. The centre
for the muscles of chewing and swallowing (p. 337), which
are commonly thrown into action reflexly, though they may
be made to contract voluntarily. 6. The convulsive centre
(p. 400). 7. The diabetic centre (p. 442). 8. The centre
reflexly exciting activity in the salivary glands, when sensory
nerves in the mouth are stimulated. 9. Certain centres
for complex bodily movements; an animal with its medulla
oblongata can execute much more complicated reflex acts
than one with its spinal cord alone.

Functions of the Cerebellum, Pons Varolii, and Mid-
Brain. These contain paths of conduction between the
fore-brain and parts behind, and many important centres,
especially those concerned with the maintenance of the
equilibrium of tlie Body in various postures and modes of
locomotion. If, as has been above suggested, an animal
without its cerebral hemispheres has sensations which
remain untranslated into terms of external things, these
sensations must have their seats in these brain-regions.

The crura cerebri (p. 164) are essentially paths of
conduction between the cerebral hemispheres and parts
behind. Injury of one produces partial loss of sensibility
and incomplete muscular paralysis on the ojoposite side of
the Body. The anterior pair of eminences of the corpora
quadrigemvna are concerned with sight; stimuli reaching
thcra through the optic nerve, there, probably, first cause
visual sensations, which it is left to the fore-brain to interjiret.
If the latter is removed from an animal liglit brought in
front of the eye still causes contraction of the j^upil; direct
irritation of the eminences in question has the same effect,
and destruction of one of them causes blindness of the oppo-
site eye. The functions of the posterior pair of eminences

FUNCTIONS OF CEREBELLUM. 587

of the corpora quadrigemina in man are uncertain. The
cerebellum is the chief organ of combined muscular move-
ments; it is the main seat of what we may call acquired
reflexes. 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 forgotten 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 atten-
tion, bnt 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 jDosition of
the limbs, or visual, or others (equilibrium sensations, see
below) 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 associ-
tion of the two; the path of connection between the
afferent and efferent fibres becomes easier the more it is
traveled, and at last the sensations arouse the proper move-
ment 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 muscular nerve-centres in accordance
with the felt directing sensations, now has no more trouble
iu the matter; the afferent impulses stimulate the proper
motor centres in an unconscious and unheeded way. Injury
or disease of the cerebellum produces great disturbances
of locomotion and insecurity in maintaining various pos-
tures. After a time the animals (birds, which bear the
operation best) can walk again, and fly, but they soon become
fatigued, probably because the movements require close
mental attention and direction all the time.

Sensations of Equilibrium. The semicircular canals
have probably nothing to do with hearing. An old view was
that, lying in three planes at right angles to one another.

588 THE HUMAN BODY.

they served to distinguish tlic direction of sound-waves reach-
ing the ear; but as the direction of oscillation of the tym-
panic ossicles is the same, no matter what that of the sound-
waves entering the external auditory meatus may be, such an
hypothesis has no foundation. The cochlea abundantly
accounts for the ap})reciation of notes, and such noises as are
due to inharmonically combined tones; while the vestibule
will suffice for other noises: and it is found that disease of the
semicircular canals does not interfere with hearing, but often
causes uncertainty in movements and feelings of giddiness.

Experiment shows that cutting a semicircular canal is
followed by violent movements of the head in the plane of
the canal divided; 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 co-ordinated as to enable the animal to maintain
or regain a position of equilibrium. It is like a creature
sufl'ering from giddiness; and similar phenomena follow,
m man, electrical stimulation of the regions of the skull in
which the semicircular canals lie.

Such facts suggest that the semicircular canals are organs
in which sensory afilerent impulses, assisting in the pre-
servation of bodily equilibrium, arise. The unconscious
maintenance of the erect position depends on thQ excitation
of many co-ordinating motor centres through tactile, mus-
cular, visual, and aural sensations; all acting together in
normal combination these enable us to stand without
thought; but loss of any one, or its abnormal state, will
throw the whole mechanism out of gear. Persons who have
lost muscular or tactile sensibility stand and walk with diffi-
culty; those who have nystagmus (jerking unconscious
movements of the eyeballs which cause the visual field to
seem to move in space) do the same and feel giddy; and,
as we have Just seen, similar phenomena follow injury of
the semicircular canals.

How the nerves in the latter are stimulated is not certain;
being filled with liquid the pressure of this on any ampulla
will be increased when the head is bent so as to place that
one below; and this may be the excitant; giving rise to

EQUILIBRIUM SENSATIONS. 589

afferent impulses, which change the condition of the
co-ordinating locomotor centres, with every position of the
liead. Or, movements of the endolymph m relation to the
wall of the canal may be the stimulus, the current swaying
the projecting hairs (Fig. 149).* 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 begins to go m
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 in a horizontal plane similar
phenomena will occur in the endolymi^h of the horizontal
canal; if it be bent sidewise in the vertical plane, in the
anterior vertical canal; and if nodded, in the posterior verti-
cal; 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 gen-
erate afferent impulses which will cause the general nerve-
centres of bodily equilibration to be differently acted upon
in each case. Under ordinary circumstances the results of
these impulses do not become prominent in consciousness
as sensations; but they sometimes may. If one spins round
for a time, the endolymph takes up the movement of the
canals, as the water m the tumbler does that of the glass;
on stopping, the lic[uid 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 person 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 fail, being unable to combine his
muscles properly on account of the conflicting afferent
impulses. If a person, with eyes shut, be laid on a hori-
zontal table which is turned, he can at first feel and tell the
direction of the rotation; as it continues he loses the feelinsr,
and when the movement stops feels as if he were being turned

* Page 543.

590 THE HUMAN BODY.

in the opposite direction. All this becomes readily intelli-
gible if we sup})Ose feelings to be excited by relative move-
ments of the endolymph a,nd the canals inclosing it.

The so-called "auditory sacs" of many Mollnsks (see
Zoology) are probably organs for equilibration sensations
and comparable to our semicircular canals.

Functions of the Fore-Brain. Beyond the broad fact
that this part of the nervous system is essentially volitional
and intellectual in function, we know very little. It is
clearly not the seat of the centres of muscular co-ordination;
for, after its removal, an animal can still, if properly stimu-
lated, execute j^erfectly all its usual movements. The true
motor centres lie farther back; those for the less compli-
cated combinations, as bending a limb, in the spinal cord,
and those for more complex, as standing, walking, or
breathing, in the mid- and hind-brains: and are not auto-
matic. They may be excited to activity either, reflexly,
by afferent impulses, traveling in from sensory regions and
associated or not with consciousness; or, directly, by im-
pulses, associated with those states of consciousness which
we call volitions, passing back from the fore-brain. The
fore -brain, also, frequently inhibits movements which,
in its absence, would be caused by discharges, reflex in
nature, of the lower centres; the will is as often employed
in restraining as in exciting muscular contractions. For
instance, after the cerebral hemispheres have been removed
from a frog, stroking the animal gently on its back will,
each time, cause it to croak; the skin stimulation origi-
nates afferent impulses which excite the "croaking centre"'
to discharge: but if the creature has its fore-brain it croaks
or not as it pleases; it can then allow the co-ordinating
mechanism to work freely under the stimulus, or check it;
and it can also, independently of any immediate stimulus,
excite voluntarily the same motor nerve-centre and croak if it
chooses. We constantly meet with similar phenomena in
ourselves; afferent impulses are all the time at work, tend-
ing to produce one action or another; and a gi-eat part of
our mental activity consists in deciding which we shall
jn-event and which we shall permit. The restraint thus

FUNCTIOXS OF THE FORE-BRAIN. 591

exercised by the fore-brain on the lower centres is at least
as important as its power of exciting them; strength of
character depends, perhaps, more on gTeat inhibitory power
in the fore-brain than on its initiating facnlty.

The intellectnal powers seem mainly, if not entirely, de-
pendent for their necessary material antecedents or con-
comitants on the surface convolutions of the cerebral
hemispheres; if these alone be remoyod from an animal its
mental condition is much the same as if the whole fore-
brain be taken away. Some simple and fundamental
perceptions seem, however, to remain, having, perhaps,
their seats in the deeper gray masses constituting the
optic thalami and corpora striata (p. 167); a dog, from
which the greater part of the cerebral surfaces had been re-
moved, after a time learnt to w^ilk about, apparently volun-
tarily, and to find and eat his food; he even learnt not to
take the bones of other dogs after he had several times been
severely bitten for so doing. But more complex perceptions
were lost; before the operation, for example, he was violently
terrified by seeing a man fantastically dressed; but after-
wards no such things seemed to arouse in him so complex
a conception as that of a strange or dangerous object.

Although the fore-brain is the seat of consciousness it is
itself insensible to cutting or wounding; and was long
sui)posed to be entirely inexcitable by general nerve stimuli.
It has, however, been found that tolerably powerful electri-
cal currents applied to the convolutions produce, in many
cases, definite movements; the nature of the movement
depending upon the area stimulated. Hence an attempt
has been made to detect the functions of different j)arts of
the cerebral hemispheres by observing the results of stimu-
lating each; and provisionally we may, perhaps, assume that
the brain-centres, from which volitional impulses proceed to
the co-ordinating centres for the muscle groups called into
play, lie in the cerebral regions whose stimulation is followed
by the movement. The animals, however, so often recover
the power of executing the movement spontaneously after
its supposed volitional centre has been removed that the
proper interpretation of the experimental results is still

592 TUE HUMAN BODY.

doubtful. Stimulation of muuy regions of tlie brain is
followed by no results; and that of others by movements the
])o\ver of voluntarily executing which is not, even tempo-
rarily, lost when that brain-part is removed; these cerebral
areas have been supposed to be concerned Avith mental
faculties other than volition, the stimulation exciting sen-
sations, perceptions, or emotions; but this is still very
doubtful. Localized disease of regions of the human brain
lias, so far, given better results than physiological experi-
ment on the lower animals. The power of using words to
express ideas seems intimately connected with a small area
on the fore part of the left cerebral hemisj)here, and to be lost
(producing the condition known as aphasia) when that
part IS diseased; and many cases have been recorded in
which a wound of the skull, has been followed exactly by
loss of the power of voluntarily moving those muscle groups
which (accepting the results of electrical stimulation in
the lower animals) might be supposed to l)e normally excited,
through the will, from the cerebral area injured. Absence of
recovery unless the brain injury is cured seems, moreover,
to be the rule in man; while as we have seen this is not the
case in lower animals; this may, perhaps, indicate a more
precise division of physiological labor in the human brain ;
which is a priori probable, considering its great superiority
as a mental ajiparatus.

What the use of two cerebral hemispheres is, cannot at
present be said. Injury of one produces its main effects,
so far as sensation and motion are concerned, on the oppo-
site side of the Body; but other faculties, as that of using
speech, seem located on one side only; and in the brains of
the higher human races the surface markings on the two
sides are not perfectly symmetrical, which may indicate
some difference in function. It has been suggested that in
many cases we only learn to use one side of the brain, and
that the other is in reserve in case of injury or disease; but
the evidence is inconclusive: a good deal may, however, be
said for the view that a good deal of brain in every one's
skull IS never used. It is there untaught but ready to be
educated.

IMPORTANCE OF EXERCISING THE BRAIN. 593

MoYements which are commonly executed together tend
to become so associated that it is difficult to perform one
alone; many persons, e.g., cannot close one eye and keep
the other open. From frequent use, the paths of con-
duction between the co-ordinating centres for both groups
of muscles have become so easy that a volitional imjDulse
reaching one centre spreads to the other and excites both.
This association of movements, dependent on the modifica-
tion of brain structure by use, finds an interesting parallel
in the psychological phenomenon known as the association
of ideas; and all education is largely based on the fact that
the more often brain regions have acted together the more
readily, until finally almost indissolubly, do they so act. If
we always tram up the child to associate feelings of disgust
with wrong actions and of approbation with right, when he
is old he will find it very hard to do otherwise: such an
organic nexus will have been established that the activity
of the one set of centres will lead to an excitation of that
which habit has always associated with it. The nerve-centres
are throughout eminently plastic; every thought leaves its
trace for good or ill; and the moral truism that the more
often we yield to temptation — the more often an evil solici-
tation, sensory or otherwise, has resulted in a wrong act —
the harder it is to resist it, has its parallel (and we can
hardly doubt its physical antecedent) in the marking out of
a path of easier conduction from perceptive to volitional
centres in the brain. The knowledge that every weak
yielding degrades our brain structure and leaves its trail in
that organ through which man is the " paragon of animals,"
while every resistance makes less close the bond between
the thought and the act for all future time, ought surely to
"give us pause:" on the other hand, every right action
helps to establish a ''path of least resistance," and makes
its subsequent performance easier.

The brain, like the muscles, is improved and strengthened
by exercise and injured by overwork or idleness; and just
as a man may specially develop one set of muscles and
neglect the rest until they degenerate, so he may do with
his brain; developing one set of intellectual faculties and

594 THE HUMAN BODY.

leaving the rest to lie fallow until, at last, he almost loses
the power of using tliem at all. The fierceness of the battle
of life nowadays especially tends to produce such lopsided
mental developments; how often does one meet the business
man, so absorbed m money-getting that he has lost all
})ower of aj)preciating any but the lower sensual pleasures;
the intellectual joys of art, science, and literature have no
charm for him; he is a mere money-making machine. One,
also, not unfrequently meets the scientific man with no
appreciation of art or literature; and literary men utterly in-
capable of sympathy with science. A good collegiate
education in early life, on a broad basis of mathematics,
languages, and the natural sciences, is a great security against
such imperfect mental growth; one danger in American
life is the tendency to put lads in a technical college, or to
start them in business before they have attained any broad
general education. Another danger, no doubt, is the oppo-
site one of making the training too broad; a man who knows
one or two literatures fairly well, and who has mastered the
elements of mathematics and of one of the observational or
experimental sciences, is likely to have a better and more
utilizable brain than he who has a smattering of half a
dozen languages and a confused idea of all the '^ologies."
The habits of mental slovenliness, the illogical thinking,
and the incapacity to know when a thing really is mastered
and understood, which, one so often finds as the results of
such an education, are far worse than the narrowness apt
to follow the opposite error, which is often associated
with the power of accurate logical thought. Those who
are deprived of the advantages of a general collegiate educa-
tion may now, more easily than at any jDrevious period,
cultivate mental breadth by reading some of the many
excellent general reviews and magazines, and the readable
but exact popular expositions now available on nearly all
subjects, which are such a feature of our age. Associating,
o:it of working hours, with those whose special pursuits aie
different from our own is almost necessary to those who
would avoid such an asymmetrical development as almost
amounts to intellectual deformity.

CHAPTER XXXVI.

VOICE AND SPEECH.

■Voice consists of sounds produced by the vibrations of
two elastic bands, the U'lie 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
periodic 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 pro-
duced 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 com-
monly 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
l)y 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
(p. 547) are pleasant; while those in which inharmonic
partials are conspicuous are disagreeable.

The vocal cords alone would produce but feeble sounds;
those that they emit are strengthened by sympathetic re-
sonance of the air in the pharynx and mouth, the action of

59G

THE HUMAN BODY.

wliich may bo compared to that of the sounding-board of a
viohn. 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 tbe Avindpipe; in many persons it is
prominent, causing the projection known as " Adam's
apple." It consists of a framework of cartilages, partly
joined by true synovial joints and partly bound together

Cs

Fig. 156. — The more important cartilages of fhe laryiix from behind. <,
th>Toid; Cs, its superior, and Ci, its inferior, honi of tlie rightside; **, cricoid
cartilage; t, arytenoid cartilage; Pv, the corner to wliich the posterior end of
a vocal cord is attached; Prn, corner on which the muscles which approximate
or separate the vocal cords are inserted ; co, cartilage of Santorini.

by membranes; muscles are added which move the car-
tilages with reference to one another; and the whole is lined
by a mucous membrane.

The cartilages of the larynx (Fig. 156) are nine in number;
three single and median, and three pairs. The largest {t)
is called the thyroid, and consists of two halves which meet
at an angle in front, but separate behind so as to inclose a
V-shaped space, in which most of the remaining cartilages
lie. The epiglottis (not represented in the figure) is fixed
to the top of the thyroid cartilage and overhangs the entry

ANATOMY OF LARYNX. 597

from the pharynx to the larynx {c, Fig. 89);* it may be
seen, covered by mucous membrane, projecting at the base
of the tongue, if the latter be pushed down while the
mouth is held open in front of a glass; and is, similarly
covered, represented, as seen from behind, at a in Fig. 15T.
The cricoid, the last of the unpaired cartilages, is the shape
of a signet-ring; its broad part (^*, Fig. 156) is on the pos-
terior 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 crico-thyroid mem-
hrane, 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. 156), and the lower horn on each side
forms a joint with the cricoid. The thyroid can be ro-
tated 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 {\, Fig. 156) are the 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. 156), the cartilage of
Santorini. From the tip of each arytenoid cartilage the
aryteno-ejnglottidean fold of mucous membrane (10, Fig.
157) extends to the epiglottis; the cartilage of Santorini
causes a projection (8, Fig. 157) in this; and a little
farther on (9) is a similar eminence on each side, caused
by the remaining pair of cartilages, known as the cuneiform,
or cartilages of Wrisierg.

The Vocal Cords are bands of elastic tissue which reach
from the inner angle {Pv, Fig. 156) of the base of each
arytenoid cartilage to the angle on the inside of the thyroid
where the sides of the F unite; they thus meet in front but
are separated at their other ends. The cords are not, how-
ever, bare strings, like those of a harp, but covered over with
the lining mucous membrane of the larynx, a slit, called
the glottis {c, Fig. 157), being left between them. It is the
* Page 309.

598

THE HUMAN BODY.

projecting cushions formed h\ them on each side of this
slit which are set in vibration during phonation. Above
each vocal cord is a depression, tlie ventricle of the larynx^
[b' , Fig. 157); this is bounded above by a somewhat pronii-

FiG. 157.— The larynx -viewed from its pharyngeal opening. The back wall of
the piiarynx has been divided and its edges (11 ) turned aside. 1, body of hyoid ;
2, its small, and 3. its great, horns: 4, upper and lower horns of thyroid carti-
lage; 5, mucous membrane of front of pharynx, covering the back of the cricoid
cartilage; 6, upper end of gullet; T, windpipe, lying in front of the gullet; 8,
eminence caused by cartilage of Santorini; !t. eminence caused by cartilage of
"Wrisberg; both lie in, 10, the ari/tcno-cpir/lottidean /oW of mucous membrane,
surrounding the opening (aditn^ /fn//7((//.s) from pharynx to larjnx. o, project-
ing tip of epiglottis; c, the glotiis, tlie lines leading from the letter point to the
free vibrating edges of the vocal cords. 6, the ventricles of the larynx: their
upper edges, marking them off from the eminences b, are the false vocal cords.

nent edge, the false 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 im-
bedded in it. Over the vocal cords, however, it is repre-
sented only by a thin layer of flat non-ciliated cells, and

MOVEMENTS OF LARYNX. 599

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 vibration.
They are also tolerably widely separated behind, the aryten-
oid cartilages, to which their posterior ends are attached,
being separated. Air under these conditions passes through
without producing 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 co-ordinated 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 direc-
tion 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 ex-
tent, the ligaments of the joint being lax. One corner of
the triangular base is directed inwards and forwards {i.e.
towards the thyroid) and is called the vocal process {Pv,
Fig. 156). as to it the vocal cords are fixed. The outer
posterior angle {Pm, Fig. 15G) has several muscles inserted
on it and is called the muscnlar 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 re-
verse will happen if the muscular process be drawn forwards.
The muscle producing the former movement is the posterior
crico-arytenoid {Cap, Fig. 1^8); it arises from the back of
the cricoid cartilage, and narrows to its insertion into the
muscular 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 upwards and backwards to the

600

THE HUMAX JiODT.

muscular process. The posterior crico-arytenoids, working
alone, pull inwards and downwards the muscular processes,
turn upwards and outwards tlie vocal processes, and sepa-
rate the posterior ends of the vocal cords. The lateral
crico-thyroid, working alone, pulls downwards and forwards
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

Taep

Fig. 158.— The larj'nx seen from behind and dissected so as to display some of
its muscles. The mucous membrane of the front of the pharynx (5, Fig. 157)
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
Santorini ; cii, cartilage of Wrisberg.

the production of voice. When both pairs of muscles act
together, however, each neutralizes the tendency of the other
to rotate the 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 articulates, as far as the loose capsu-
lar ligament of the joint will aWow. The arytenoid car-
tilages are thus moved apart and the glottis greatly widened

TE^'SIO^' CHAXGES lA VOCAL CORDS. 601

and brongiit into its state in deej^ quiet breathing. Other
muscles approximate the arytenoid cartilages after they
have been separated. The most important is the transverse
arytenoid {A, Fig. 158), which runs across from one ary-
tenoid cartilage to the other. Another is the oUique arij-
tenoid {Taep), which runs across the middle line from the
base of one arytenoid to the ti}) of the other; thence cer-
tain fibres continue in the aryteno-epiglottidean fold (10,
Fig. 157) to the base of the epiglottis; this, with its fellou',
thus embraces the whole entry to the larynx ; when they
contract they bend inwards the tips of the arytenoid car-
tilages, approximate the edges of the aryteno-epiglottidean
fold, and draw down the epiglottis, and so close the pas-
sage from the pharynx to the larynx; this is probably their
chief function. When the epiglottis has been removed,
food and drink rarely enter the larynx in swallowing, the
eds^es of the folds of mucous membrane on its sides being:
so brought together as to effectually close the aperture be-
tween them.

Increased tension of the vocal cords is produced mainly
by the crico-tliyroid muscles, one of which lies on each side
of the larynx, over the crico-thyroid membrane. Their
action may be understood by help of the diagram, Fig. 159,
in which t represents the thy-
roid cartilage, c the cricoid, a
an arytenoid, and vc a vocal
cord. The muscle in question
passes obliquely backwards and
upwalrds from near the front
end of c (to the right in the
diagram) to t, near the pivot
(which represents the joint be-
tween the cricoid cartilage and

the inferior horn of the thyroid).

When the muscle contracts it ^^^" ^^^'

[)ulls t down into the position indicated by the dotted lines
and stretches the vocal cord, if the arytenoid cartilages be
kept, by the muscles behind, from slipping forwards at the
<ame time. The antagonist of the crico-thyroid is the

t

-I

n^^-

!.«-,

I

002 THE HUMAN BODY.

thijro-anjienoid muscle; it lies, on each side, imbedded in
tlie fold of elastic tissue forming the vocal cord, and passes
f]-om the inside of the angle of the thyroid cartilage in
front, to the anterior angle and front surface of the ary-
tenoid behind. If the latter be held firm, the muscle
raises the thyroid cartilage from the position into which
the crico-thyroid pulls it down, and so slackens the vocal
cords. If the thyroid be held fixed by the crico-thyroid
muscle, the thyro-arytenoid will help to approximate 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 in-
ci"eased tension, however, 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 are
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, while the pitch of the note is raised, by
increasing contraction of the crico-thyroid. At last this at-
tains 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 crico-thyroid is relaxed,
and then again gradually 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 jiroduction oi falsetto oiotes is uncertain;
during their emission the free border of the vocal cords
alone vibrates.

VOWEL SOUyDS. 603

The range of the human voice is about three octayes,
from /(176 vib. per 1") on the unaccented octave, in male
voices, to ^on the thrice accented octave (1584 vib. per 1"),
in female. Great singers of course go beyond this range;
basses have been known to take a on the great octave (110
vib. per 1") ; and Nilsson in " II Flauto Magico" used to take
f on the fourth accented octave (2816 vib. perl"). Mozart
heard at Parma, in 1770, an Italian songstress whose voice
had the extraordinary range from g in the first accented
octave (396 vib. per 1") to c on the fifth accented octave
(4224 vib. per 1"). An ordinary good bass voice has a com-
pass from/ (176 vib. per 1") to a" (594 vib. per 1"); and
a soprano from b' (495 vib. per 1") to g'" (1584).

Vowels are, primarily, compound musical tones (p. 548)
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 recogni-
zable 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 various vowel sounds are really
musical notes differing from one another in timbre. The
mouth and throat cavities form an air-chamber above the
larynx, 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 reti-acted and the cheeks tense; then
gradually closing it and protruding the lij)s, 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 (farther) through o
(more) to oo (moor). When the mouth and throat cham-
bers 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, 0, oo) the tongue is depressed and the cavity forms one
chamber; for a this has a wide mouth opening; for o it is

604 THE nUMAK BODY.

narrowed; for oo still more narrowed, and the lijis protruded
so as to increase the length of the resonance chamber. The
partial tones reinforced in each case are, according to
Ilclmholtz —

In other cases the mouth and throat cavity is i^artially suh-
divided, by elevating the tongue, into a wide posterior and
a narrow anterior part, each of which has its own note; and
the vowels thus produced owe their character to two rein-
forced partials. This is the case with the series a (man),
6 (there) and i (mach/ne). The tones reinforced by reson-
ance in the mouth being —

M^=

e n

The usual i of English, as in sp/re, is not a true simple
vowel but a di})ththong, consisting of a (pad) followed by
e (feet) ; as may be observed by trying to sing a sustained
note to the sound I; it will then be seen that it begins as a
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 symjiathetic resonance. For some other sounds
(the semi-vo7vels or resonanfs) 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.

CO^^SONAyTS. 605

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 inter-
rupted 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
{guthirals). Consonants are also characterized by the kind
of movement which gives rise to them. In explosives an
interruption to the passage of the" air-cuiTent is suddenly
interposed or removed (P, T, B, D, K, G). Other con-
sonants are contimions (as E, S, E), 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 j)alate and the air escapes on its
sides. For Ch (as in the proper Scotch pronunciation of
loch) the passage between the back of the tongue and the
soft palate is narrowed. To many of the above pure
consonants answer others, in whose production true vocali-
zation {i.e. a laryngeal tone) takes a part. F with some
voice becomes V; S becomes Z, Th soft {teeth) becomes Th
hard; and Ch becomes Gh. (2) Reso7iants; these have
been referred to above. (3) Vibratories (the different
forms of E), which are due to vibrations of parts bounding
a constriction j^ut in the course of the air-current. Ordi-
nary E is due to vibrations of the tip of the tongue held near
the hard palate; and guttural E to vibrations of the uvula
and parts of the pharynx.

The consonants may ijliysiologically be classified as in the
following table (Foster).

Explosives. Labials, without voice P.

" with voice ,B.

Dentah, without voice . . . .T.

with voice D.

Gutturals, vrithout voice K.

" with voice G (hard).

606 THE HUMAN BODY.

Aspirates. Lnhinh, without voice F.

" witii voice V.

Bentah, witiiout voice... S, L, Sh, Th (hard).

" with voice Z, Zh (asure), Th (soft).

Gutturals, without voice . Ch (locA).
witli voice, . . .Ch.

Resonants. Liihlal M.

Dental N.

Guttural NG.

Vibratories. Labial — not used in European languages.

Dental R (common).

Guttui'al 11 (guttural).

II i.s a lar}' ngeal sound : the vocal cords are separated for its
production, yet not so far as in quiet breathing. The air-
ciirrent 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
Avith a vowel, and only rarely is used at other times. The
Greeks had a special sign for it, , the soft breathing; and
another, ', the hard hreathing, 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 j^eriodic vibra-
tions; whispering is a worse. To jiroduce it the glottis is
tolerably narrowed but the cords are not so stretched as to
produce a sharply defined edge on them, and the air driven
jiast is then thrown into irregular vibrations. Such vibra-
tions as coincide in period witli 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 l)etween such letters as P and
13, F and V, remains imperfect.

INDEX

Abdomen, contents of, 4.

Abdominal respiration, 366.

Abducens nerve, 170.

Aberration, chomatic, 503.

Aberration, spherical, 503.

Absorbents, 329.

Absorption from intestines, 345

Absorption of gases, 380.

Absorption of oxygen by blood,
382.

Accelerator nerves of heart, 253.

Accommodation, 498.

Acetabulum, 79.

Acid, acetic, 14; butyric, 14; car-
bonic, see carbon dioxide ; for-
mic, 14; glycero-phosphoric,
14; glycocholic, 342; lactic. 14;
oleic, 13; palmitic, 13; sarco-
lactic, 14, 125; stearic, 13;
taurocholic, 342.

Acquired (secondary) reflexes,
587.

Action current (negative varia-
tion), 193, 197.

Actions, reflex, 183, 574.

Addison's disease, 333.

Adenoid tissue, 106.

Adipose tissue. 111.

Adrenals (supra-renal capsules),
383.

Advantage of mixed diet, 305,
446.

After images, 526.

Ague cake, 336.

Air, chemical composition of,
374.

Air cells, 354.

Air, changes produced in by
breathing, 373.

Air, complemental, tidal, etc.,
365.

Air passages, 353.

Albumin, serum, 57.

Alliuminoids, 11.

Albuminous bodies, 10.

Alcohol, 304.

Alimentary canal, 308.

Alimentary principles, 398.

Amoeboid cells, 114.

Amoeboid movements, 21, 48.

Amyloids (carbohydrates), 13,
300.

Amyloids, digestion of, 33), 341,
349.

Anaemia, 59.

Anatomical systems, 37.

Anatomy of alimentary canal,
308; of ear, 585; of eye, 479;
of joints, 94; of lymphatic sys-
tem, 339; of muscular system,
116; of nervous system, 154; of
respiratory organs, 352; of
skeleton, 63; of skin, 413; of
urinary organs, 403; of vascu-
lar system, 201.

Animal heat, source of, 451.

Anterior tibial nerve, 311.

Anvil bone, 537.

fi08

INDEX.

Aorta, 209.

Apex beat of heart, 220.

Appendicular skeleton, 77.

Appetite, 350.

Appendix vermiformis, 323.

Apoplexy, 166.

Aqueduct of Sylvius, 137.

Aqueous humor, 490.

Arachnoid, 6, 157.

Arbor vitaj, 168.

Areolar tissue, 102.

Areolar tissue, subcutaneous, 414.

Arm, skeleton of, 78.

Arterial blood, 216, 379.

Arterial pressure, 235, 251.

Arteries, distribution of, 208.

Arteries, structure of, 217.

Artery, axillary, 210; brachial,
210; bronchial, 211; carotid,
210; ccEliac, 210; coronary, 207,
210; iliac, 210; innominate, 210;
intercostal, 211; radial, 210;
subclavian, 210; ulnar, 210;
vertebral, 210,

Articular cartilage, 94.

Articulations, 63, 93.

Arytenoid cartilages, 597.

Asphyxia, 398.

Aspiration of thorax, 244, 237.

Assimilation, 19.

Assimilative tissues, 30.

Associated movements, 593.

Association of ideas, 593.

Astigmatism, 503.

Astragalus, 82.

Atlanto-axial articulation, 97.

Atlas vertebra, 68.

Auditory nerve, 170.

Auditory ossicles, 536.

Auditory perceptions, 554.

Auriculo-ventricular valves, 208.

Automatic centres, 183.

Automatic movements, 23.

Automatic tissues, 32.

Axial current 228.

Axial ligament, T)?,"!.

Axial skeleton, 63, 67.

Axillary artery, 210.

Axis, vertebra, 68.

Axis, visual, 518.

Ball and socket joints, 96.

Basement membrane, 106, 260.

Basilar membrane, 540, 553.

Bathing, 421.

Beans (as food), 303.

Beat of heart, 219.

Beef tea, 126.

Biceps muscle of arm, 118.

Bile, 342.

Blackness, sensation of, 519.

Bladder, urinary, 402.

Blind spot, 508.

Blood, 40; arterial and venous,
216, 379; composition of, 57
clotting of, 50; crystals, 47
gases of 378; histology of, 44
laky, 46; quantity of, 59; se-
rum, 50.

Bloodflow in capillaries, 227; in
kidneys, 408; in liver, 325;
rate of, 242.

Blood-vessels, anatomy of, 208.

Blood-vessels, nerves of, 253.

Blood-vessels, structure of, 218.

Blushing, 255.

Bone, composition of, 91; his-
tology of, 88; gross structure
of, 80.

Bones of face, 64; of fore-limb,
78; of hind-limb, 79; of pec-
toral arch, 77; of pelvic arch,
79; of skull, 75.

Brachial artery, 210.

Brachial plexus, 161.

Brain, anatomy of, 163; physi-
ology of, 584; membranes of,
157.

Bread, 302.

INDEX.

609

Breast-bone, 71.

Bronchial arteries, 211.

Bronchial tubes, 354.

Bronchus, 354.

Brunner's glands, 322.

Buccal cavity, 308.

Buffy coat on blood clot, 52.

Calcaneum, 79.

Camera obscura, 496.

Canals, semicircular, 589, 587

Capacity of lungs, 365.

Capillaries, blood, 211, 217.

Capillaries, lymphatic, 329.

Capillary circulation, 227.

Capsule of Glisson, 326.

Carbohydrates, see amyloids.

Carbon dioxide, 14; in blood,
886 ; production of in muscle,
430.

Carbon monoxide haemoglobin,
398.

Cardiac muscular tissue, 124.

Cardiac impulse, 220.

Cardiac nerves, 248.

Cardiac orifice of stomach, 317.

Cardiac plexus, 172.

Cardio-inhibitory nerves 250.

Carotid artery, 210.

Carpus, 79.

Casein, 11.

Cartilage, 100; articular, 94; elas-
tic, 106; fibro-, 107; histology
of, 101; inter-articular, 107.

Cartilages of larynx, 596.

Cataract, 504.

Catarrh, 256.

Cauda equina, 161.

Cells, 17; amoeboid, 114; ciliated,
33, 115; division of, 18; diffe-
rentiation of, 26; growth of, 18.

Cement, of tooth, 311.

Centre, cardio-inhibitory, 250;
cerebro-spinal, 156; convulsive,
400; respiratory, 391.

Centre of gravity of body, 149.

Centres, nerve, general functions
of, 182.

Cephalic vein, 214.

Cerebellum, 164, 586.

Cerebral hemispheres, 163.

Cerebral hemispheres, functions
of, 590.

Cerebro-spinal centre, 5, 156.

Cerebro-spinal liquid, 158.

Cervical plexus, 161.

Cervical vertebrae, 68.

Characteristics of human skele-
ton, 84.

Chemical combinations, energy
liberated in, 283.

Chemical composition of body, 8.

Chemical changes in breathed
air, 373.

Chemistry, of bile, 842; of blood,
57; of bone, 91; of fats, 112;
of gastric juice, 338; of lymph,
61; of muscle, 124, 429; of
pancreatic secretion, 264; of
respiration, 872; of secretion,
264; of teeth, 312; of urine,
410; of white librous tissue,
103 ; of working muscle, 427.

Chest. See Thorax.

Chondrin, 107.

Chorda tympani nerve, 271.

Choroid, 486.

Chromatic aberration, 502.

Chyle, 340.

Chyme, 389.

Ciliary muscle, 491.

Ciliary processes, 486.

Ciliated cells, 33, 115.

Circulation, 201, 214; during as-
phyxia, 401; influence of re-
spiratory movements on, 368;
influence of nerves on, 347;
portal, 216, 825; renal, 408.

Circulatory organs, 201.

610

INDEX

Circum vallate papillae, 313.

Classification of the tissues, 29.

Classification of nerve-fibres, 184.

Clavicle, 77.

Clothing, 458.

Coagulated proteid, 11.

Coagulation of blood, 50.

Coccyx, 70.

Cochlea, 539.

Cochlea, functions of, 553.

Caecum, 323.

Cceliac axis, 211.

Cold-blooded animals, 449.

Collagen, 104.

Collar-bone, 77.

Colon, 322.

Color blindness, 523.

Color mixing, 521.

Color vision, 519.

Combustible foods, 425.

Commissures, cerebral, 167.

Common bile duct, 324.

Common sensation, 463, 567.

Complemental air, 365.

Complementary colors, 521.

Concha, 535.

Conduction in spinal cord, 578,
583.

Conductive tissues, 33.

Conductivity, physiological, 21.

Connective tissue, 62, 102, 106.

Connective-tissue corpuscles, 105.

Conservation of energy, 280.

Consonants, 605.

Contractile tissues, 33, 113.

Contractility, 20, 128.

Contrasts, visual, 525,

Convulsive centre, 400.

Cooking of meats, 301: of vege-
tables, 303.

Co-ordinating tissues, 32.

Coordination, 22, 183.

Cords, vocal, 567.

Corium, 6, 413.

Corn, 303.

Cornea, 486.

Coronary artery, 207; sinus, 207.

Corpora albicantia, 168.

Corpora quadrigemina, 164.

Corpora striata, 163, 591.

Corpus callosum, 166.

Corresponding retinal points, 532.

Corti, organ of, 541.

Costal cartilages, 72.

Costal respiration, 366.

Cranial nerves, 168, 168.

Cranium, 73.-

Crazy bone, 188.

Cream, 302.

Cretinism, 333.

Cricoid cartilage, 597.

Crura cerebri, 164.

Crying, 402.

Crypts of Lieberklihn, 322.

Crystalline lens, 491.

Curari poisoning, 130.

Cutaneous organs, 412.

Cutaneous secretions, 417.

Cutis vera, 413.

Cystic duct, 324.

Daltonism, 523.

Death stiffening, 430.

Defects (optical) of eye, 500, 502.

Deglutition, 336.

Dentine, 311.

Depressor nerve, 255.

Derbyshire neck, 333.

Dermis, 6, 413.

Descemet, membrane of, 486.

Development, 26.

Diabetes, 441.

Dialysis, 42.

Diaphragm, 4, 359.

Dietetics, 446.

Diet, mixed, advantages of, 305.

Diiferentiation of the tissues, 20.

Digestion, 334.

Digestion of a typical meal, 347.

INDEX.

611

Biploe, 91.

Direction, perception of, 530,
554.

Disassimilation, 20.

Dislocation, 98.

Dispersion of light, 495.

Distance, perception of, 530, 554.

Division of pliysiological employ-
ments, 27.

Dorsal (neural) cavity, 5.

Dorsal vertebrae, 65.

Drum of ear, 535.

Ductless glandf-, 33?

Duodenum, 320.

Dura mater, 157.

Duration of luminous sensations,
516,

Dyspepsia, 350.

Ear, 535.

Eggs, 302.

Elastic tissue, 104.

Elastic cartilage, 106.

Elements found in body, 9.

Eliminative (excretory) tissues,
30.

Emulsification, 341.

Enamel, 311.

Endbulbs, 556

Endocardium, 204

Endo-lympli, 538.

Endo-skeleton, 62.

Energy, conservation of, 280;
kinetic, 282; lost from body
daily, 279, 452; of chemical
affinity, 283; potential, 282;
muscular, source of, 427;
source of in body, 283; utili-
zation of in body, 289.

Energy-yielding foods, 425.

Enzymes, 336.

Epidermis, 6, 412.

Epiglottis, 316, 596.

Epithelium, 6, 34.

Epithelium, ciliated, 115.

Equilibrium sensations, 587.

Erect posture, 149.

Ethmoid bone, 75.

Eustachian tube, 535.

Excretion, 259.

Exercise, 138.

Exoskeleton, 62.

Expiration, 863.

Expiratory centre, 397.

External auditory meatus, 535.

External ear, 535,

External respiration, 352.

Extrinsic reference of sensations,
476.

Eye, anatomy of, 479; append-
ages of, 480; optical defects of,
502; physiology of, 506; re-
fraction of light in, 497.

Eyeball, 485.

Eyeball, muscles of, 483.

Eyelids, 481.

Facial nerve, 170.

False vocal cords, 598.

Fat, 13, 111.

Fat, source of in body, 443.

Fatigue of retina, 524.

Fatty tissue. 111.

Fauces, 315.

Fechner's law, 473.

Femoral artery, 211.

Femur, 79, 88.

Ferments, 336.

Fibrin, 11, 51, 53.

Fibrin ferment, 54.

Fibrinogen, 53.

Fibrinoplastin, 53.

Fibro-cartilage, 106.

Fibula, 79.

Fick and Wislecenus, 427.

Filiform papillae, 313.

Flesh foods, 301.

Follicles of hairs, 415.

Fontanelles, 93.

Foods, definition of, 297 ; energy-

613

INDEX.

yieldinj;, 425; fleshy, 301;non-
oxidisahle, 290, J500; tissue-
forming, 293, 495; vegetable,
303.

Food of plants, 295.

Foot, skeleton of, 79.

Foramen, inter vertebral, 71; mag-
num, 75; of Monro, 167; oval,
536; round, 536; thyroid, 79;
vertebral, 68.

Forebrain, 163, 591.

Forelimb, skeleton of, 78.

Frontal bone, 75.

Fuel of Body, 287.

Fundamental phy.siological ac-
tions, 15.

Fungiform papillae, 313.

Funny bone, 188.

Fur on tongue, 315.

Gall bladder, 324.

Ganglia, 156, 177; of cranial
nerves, 171; of heart, 248; of
spinal nerve-roots, 160; spora-
dic, 173.

Ganglion, Gas.serian, 170.

Gases of blood, 378.

Gasserian ganglion, 170.

Gastric digestion, 339.

Gastric juice, 338.

Glands, 262; cutaneous, 417;
ductless, 332; gastric, 319
lachrymal, 482; lymphatic, 330;
of "Brunner, 332 of Lieberkiihn ;
parotid, 274; salivary, 270, 315;
submaxillary, 271.

Glenoid fossa, 77.

Gliding joints, 98.

Glisson, capsule of, 326.

Globe of eye, 485.

Globulin, 11, 47.

Glossopharyngeal nerve, 170.

Glottis, 597.

Glucose (grape sugar), 11.

Gluten, 302.

Glycerine, 13.

Glycogen, 14, 438.

Gmelin's test, 342.

Goitre, 333.

Grape sugar, 13.

Great omentum, 317.

Growth, 18.

Gullet, 317.

Hsemal (ventral) cavity, 4, 6.

Ilaamatin, 12, 47.

Haemoglobin, 46, 379.

Hairs, 415.

Hair-cells, 542.

Hammer-bone, 537.

Hand, skeleton of, 78.

Haversian canals, 89.

Heart, 203; beat of, 219; nerves
of, 248.

Heat production and regulation
in Body, 449.

Heat lost from lungs, 374.

Heel-bone, 79.

Hemispheres, cerebral, 163; func-
tions of, 590.

Hepatic artery, 324.

Hepatic cells, 325.

Hepatic duct, 324.

Hepatic veins, 215, 324.

Hering's theory of color vision,
524.

Hiccough, 402.

Hind limb, skeleton of, 79.

Hinge-joints, 94.

Hip-joint, 94.

Histology, 1; of adipose tissue,
111; of areolar tissue, 103; of
blood, 44; of bone, 88; of car-
diac muscle, 124; of cartilage,
101; of connective tissues, 103;
of ear, 540; of elastic tissue,
104; of hair.s, 415; of kidney,
407; of liver, 325; of lungs,
355; of lymph, 49; of nails,
417; of nerve cells, 175; of

INDEX.

613

nerve-fibres, 173; of nose, 569;
of plain muscular tissue, 123;
of retina, 487; of skin, 412; of
small mtestine, 320; of spinal
cord, 177; of stomach, 318; of
striped muscle, 122; of teeth,
311; of tactile organs, 556; of
tongue, 570; of white fibrous
tissue, 103

Hollow veins, 207.

Homologies of supporting tissues
107.

Homology, 58; of limbs. 80.

Horopter, 533.

Humerus, 78. 86.

Humor, aqueous, 490; vitreous,
491.

Hunger, 569.

Hydrocarbons. See Fats.

Hydrogen, 9.

Hygiene, 1; of blood, 58; of
brain, 593; of clothing, 366,
458; of exercise, 138; of grow-
ing skeleton, 109; of joints, 98;
of muscles, 138; of respiration,
366, 375; of sight, 501; of
skeleton, 92; of skin, 420; of
supporting tissues, 109.

Hyoid bone, 76.

Hypermetropia, 500.

Hypoglossal nerve, 171.

Ideas, association of, 593.

Idio-retinal light, 508.

Ileum, 320.

Ileo-colic valve, 323.

Iliac artery, 210, 211.

Ilium, 79.

Illusions, sensory, 477.

Impulse, cardiac, 220.

Impulse, nervous, 196.

Incus, 537,

Indigestion, 350.

Inert layer, 228.

Inferior laryngeal nerve, 397.

Inferior maxillary nerve, 170.
Inferior mesenteric artery, 211.
Inferior vena cava, 207.
Inhibition of reflexes, 581.
Inliibitory nerves, 184.
Innervation sensations, 565.
Innominate artery, 210.
Innominate bone, 79.
Innominate vein, 214.
Inogen, 125.
Inorganic constituents of Body,

14.
Inosit, 13.

Inspiration, how effected, 359.
Intensity of sensations, 473.
Interarticular cartilage, 107.
Intercostal arteries, 211.
Intercostal muscles, 361.
Internal ear, 538.
Internal medium, 40.
Internal respiration, 352, 380.
Intervertebral disks, 71, 94.
Intervertebral foramina, 71.
Intestinal digestion, 340.
Intestines, 320.
Intra-thoracic pressure, 357.
Intrinsic heart-nerves, 248.
Iris, 491.
Irritability, 21.
Irritability, muscular, 129.
Irritable tissues, 31.
Ischium, 79.
Jaw-bones, 76.
Jejunum, 320.

Jelly-like connective tissue, 106.
Joints, 94.
Jugular vein, 214.
Kidneys, 402.
Kinetic energy, 282.
Knee-cap or knee-pan, 79.
Kreatin, 12, 434
Labyrinth, 528,
Lachrymal apparatus, 483.
Lachrymal bone, 76

614

INDEX.

Lacteal s, 43, 321.

Lacunse, lymphatic, 329.

Lamina spiralis, 540.

Large intestine, 322.

Laryngeal nerves, 397.

Larynx, 596.

Laughing, 402.

Law, the psycho-physical, 473.

Leaping, 153,

Least-resistance hypothesis, 578.

Lecithin, 14.

Leg, skeleton of, 79.

Lens, crystalline, 491.

Lenses, refraction of light by,

495.
Levers in the Body, 144.
Lieberkiihn, crypts of, 322.
Liebig's classification of foods,

426.
Liebig's extract, 126.
Ligament, suspensory, of lens,

491.
Ligament, round, 95.
Ligaments, 94.
Light, dispersion of, 495.
Light, properties of, 492.
Light, refraction of, 493.
Limbs, 7.

Liml)s, skeleton of, 77.
Liquid extract of meat, 126.
Liquor sanguinis, 44.
Liver, 323.

Local sign of sensations, 465.
Local temperatures, 455.
Localization of cerebral func

tions, 591.
Localizing powers of retina, 516.
Localizing power of skin, 559.
Locomotion, 151.
Long saphenous vein, 214.
Long sight, 500.
Losses of material from Body,

277.
Losses of energy daily, 452.

Lower maxilla, 76.

Lumbar plexus, 162.

Lumbar vertebrae, 69.

Lungs, 353.

Lungs, capacity of, 365.

Luxus consumption, 432.

Lymph, 41 ; canaliculi, 106 ; chem-
istry of, 61: heart's, 331; histo
logy of, 49; lacunse, 329;
movement of, 331, 371 ; renewal
of, 42; vessels, 43, 329.

Lymphatic glands, 330.

Lymphatic system, 329.

IMalar bone, 76.

Malleus, 536.

Malpighian corpuscles of spleen,
333.

Malpighian layer of epidermis,
412.

Malpighian pyramids of kidney,
406.

Mammalia, 4.

Mandible, 76.

Marrow of bone, 88.

Marrow, spinal (spinal cord),
177, 182, 574.

Material daily losses of Body,
277.

IMaxilla, 76.

Meatus, external auditory, 535.

Mechanisms, physiological, 36.

Medulla oblongata, 164, 585.

Media, refracting, in eye, 490.

Medullary cavity, 88.

Membrane, basilar, 540, 533; of
Descemet, 486; reticular, 541;
tectorial, 542; tympanic, 536.

Metabolic tissues, 31, 265.

Metacarpus, 79.

Metatarsus, 79.

Microscopic anatomy. See His-
tology.

Mid-brain, functions of, 586

Midriff. See Diaphragm.

INDEX.

615

Milk, 302.

Millon's test, 10.

Mixed diet, advantage of, 305.

Modality of sensation, 465, 468.

Modiolus, 540.

Morula, 26.

Motion, 143.

Motor organs, 118.

Motor tissues, 33.

Motores oculi, 168.

Mouth, 308.

Movements, associated, 593.

Movements, respiratory, 358.

Mucin, 12.

Mucous membranes, 6.

Mucous layer of epidermis, 412.

Mulberry mass, 36.

Mumps, 315.

Muscoe volutantes, 504.

Muscle, biceps, 118; cardiac, 124;
ciliary, 491; stapedius, 538;
tensor tympani, 538.

Muscles, chemistry of, 124; histo-
logy of, 122; of eyeball, 483;
of larynx, 599; of respiration,
359 ; physiology of, 128 ; skele-
tal, 117; structure of, 116;
visceral, 123.

Muscular sense, 565.

Muscular tissue, 33, 122.

Muscular work, 134, 428.

Myopia, 500.

Myosin, 11, 125.

Nails, 417.

Nasal bone, 76.

Negative variation, 193, 197.

Nerve-cells, 175.

Nerve-centres, 156, 177, 182, 573.

Nerve-fibres, 33, 173.

Nerve-fibres, classification of, 184.

Nerve plexuses, 156.

Nerve stimuli, 187.

Nerves, 154; cranial, 168, 199;
cardiac. 248; laryngeal, 397;

respiratory, 390 ; secretory, 269,
271; spinal, 160; sympathetic,
156; thermic, 457; trophic,
273; vaso-dilator, 257, 271;
vaso-motor, 253.

Nervous impulses, 196.

Nervous system, anatomy of,
154.

Nervous system, physiology of,
180.

Neural tube (dorsal cavity), 5.

Neurilemma, 173.

Nitrogenous compounds in Body,
10.

Nodal points of eye, 506.

Noises, 542.

Non-vascular tissues, 41.

Notes, musical, 542.

Nucleolus, 17.

Nucleus, 17.

Nutrition, 423.

Nutritive tissues, 30.

Occipital bone, 75.

Oculo-motor nerves, 168.

Odorous bodies, 570.

CEsophagus, 317.

Olecranon, 82.

Olein, 13.

Olfactory lobe, 163.

Olfactory nerves, 168.

Olfactory organs, 569.

Omentum, 317.

Ophthalmic nerve, 170.

Optical defects of eye, 500, 502.

Optic nerves, 168.

Optic thalami, 163.

Organ of Corti, 541.

Organs, 2, 34; of animal life,
114; of circulation, 201; of
common sensation, 463; of
digestion, 308; of movement,
113; of relation, 114; of respi
ration, 352; of secretion, 260;
of special sense, 463, 467; urin-

616

INDEX.

ary, 404; of vegetative life,

114.
Os calcis, 79.
Os inuominatum, 79.
Os orbiculare, 537.
Os pubis, 79.
Osmazome, 301.
Ossicles, auditory, 53G.
Otoliths, 542.
Oval foramen, 536.
Over-feeding, proteid, 436.
Over-tones (upper partial tones),

547.
Ovum, 26.

Oxidation by stages, 288.
Oxidations in the Body, 286, 423,

427.
Oxygen in the blood, 382,
Oxygen consumed daily, 374.
Oxyhfemoglobin, 379.
Pacinian bodies, 556.
Pain, 567.
Palate, 809.
Palate bones, 76.
Pancreas, 267, 328.
Pancreatic secretion, 341.
Papillary muscles, 208, 221.
Papillae of tongue, 313.
Parapcptone, 345.
Parietal bone, 75.
Parotid gland, 274, 315.
Partial tones, 547.
Patella, 79.
Patheticus, 169.
Pathology, 1.
Peas, 303.
Pectoral arch, 77.
Pelvic arch, 79.
Pepsin, 838.
Peptic glands, 319.
Peptones, 11.
Perceptions, 474; visual, 530;

auditory, 554.
Pericardium, 203.

Perichondrium, 101.

Perilymph, 538.

Perimysium, 122.

Perineurium, 173.

Periosteum, 86.

Peripheral reference of sensa-
tions, 464, 476.

Peristaltic movements, 338.

Peritoneum, 6.

Pettenkofer's test, 342.

Phalanges of fingers and toes, 79,
80.

Pharynx, 316.

Phrenic nerve, 161, 390.

Pliysiology, 1; of blood-vessels.
219; of brain, 584; of connec-
tive tissues, 102; of digestion,
334; of ear, 550; of eye, 506;
of heart, 219; of kidneys, 409;
of muscles, 128; of nerves, 180;
of nerve-centres, 180, 573; of
nutrition, 423; of respiration,
358; of skin, 418, 558; of smell,
569; of spinal cord, 574; of
taste, 570; of touch, 558.

Physiological chemistry, 8.

Physiological mechanisms, 36.

Physiological properties, 16.

Pia mater, 157.

Pineal gland, 167.

Pitch of notes, 543.

Pitch of voice, 603.

Pituitary body, 167.

Pivot-joints, 97.

Plastic foods, 426.

Pleura, 6, 356.

Plexus, 156; brachial, 161; car-
diac, 172; cervical, 161; lum-
bar, 162; sacral, 162; solar,
172.

Pneumogastric nerves, 171, 396.

Pons Varolii, 164, 586.

Popliteal artery, 211.

Portal circulation, 215, 326.

INDEX.

617

Portal vein, 324.

Posterior tibial artery, 211.

Postures, 149.

Potatoes, 303.

Potential energy, 282.

Pressure, intra- thoracic, 357.

Pressure sense, 558.

Primates, 3.

Production of lieat in Bodj', 449.

Pronation, 98.

Proofs of cirv Illation, 205.

Protective tissues, 34.

Proteids, 10.

Proteids, oxidation of, 427.

Protoplasm, 24.

Psychical activities of cord, 582.

Psycho-phj^sical law, 473.

Ptosis, 485.

Ptyalin, 335.

Pulmonary circulation, 214.

Pulmonary artery, 206.

Pulmonary veins, 207.

Pulse, 240.

Purkinje's experiment, 509.

Pus, 48.

Pylorus, 317, 319.

Pyramids of Malpighi, 406.

Qualities of sensation, 405.

Quantity of blood, 50.

Quantity of food needed daily,

305, 448.
Radial artery, 210.
Radioulnar articulation, 97.
Radius, 78.
Range of voice, 603.
Rate of blood flow, 242.
Receptaculum chyli, 330.
Receptive tissues, 30.
Rectum, 322.

Red blood corpuscles, 44, 59
Reduced haemoglobin, 379
Reflex actions, 182, 574.
Reflex convulsions, 577.
Reflexes, acquired, 587.

Refracting media of eye, 490.
Refraction by lenses, 495.
Refraction of light, 493.
Refraction in the eye, 497.
Regulation of temperature. 454.
Renal artery, 211.
Renal organs, 402.
Renal secretion, 409.
Rennet, 338.
Reproduction, 19.
Reproductive tissues, 34.
Residual air, 365.
Resistance theory, 395.
Resonance, sympathetic, 549.
Respiration, 20, 352.
Respiration, chemistry of, 379.
Respiration, nerves of, 390.
Respiratory centre, 391.
Respiratory foods, 426.
Respiratory movements, 357.
Respiratory sounds, 865.
Reticular membrane, 541.
Retiform (adenoid) connective

tissue, 106.
Retina, 487.

Rhythmic movements, 393.
Ribs, 72.
Rib cartilage, 72.
Rice, 303.

Right lymphatic duct, 330.
Rigor mortis, 430.
Rods and cones, 488, 568.
Round foramen, 536.
Running, 153.
Sacculus, 539.
Sacral plexus, 162.
Sacral vertebra?, 69.
Saliva, uses of, 334.
Salivary glands, 315.
Salivary glands, nerves of, 270.
Salivin (ptyalin), 335.
Santorini, cartilages of, 597
Sarcolactic acid, 14, 125,
Sarcolemma, 123.

618

INDEX.

Scala of coclilea, 540.
Scalene muscles, 361.
Scapula, 77.
Sciatic nerve, 162.
Sclerotic, 486.
Sebaceous glands, 418.
Secondary (acquired) reflexes,

587.
Secretion, 259.
Secretion, cutaneous, 417-
Secretion, renal, 409.
Secretory tissues, 30, 265.
Secretory nerves, 271.
Sections of Body, 6, 7.
Segmentation of ovum, 26.
Segmentation of skeleton, G7.
Semicircular canals, 539, 587.
Semilunar valves, 208.
Sensations, 461; color, 519; com-
mon, 463, 567; intensity of,
645; of equilibrium, 587; of
hunger, 567; of thirst, 567;
peripheral reference of, 464,
476; qualities of, 465.
Sense-organs, 467.
Sense, muscular, 565; of hearing,
585; of pain, 567; of sight,
506; of smell, 569; of taste,
570; of temperature, 562; of
touch, 558.
Sensory illusions, 477.
Serous canaliculi, 339.
Serous cavities, 330.
Serous membranes, 6.
Scrum, 50, 57.
Serum albumin, 11, 57.
Shin-bone, 79.
Shingles, 186.
Short sight, 500.
Shoulder-blade, 77.
Shoulder-girdle, 77.
Sighing, 402.
Siglit, sense of, 506.
Sight, h.ygiene of, 501.

Sigmoid flexure, 823.

Size, perception of, 531.

Skeleton, 62; appendicular, 76;
axial, 63, 67; of face, 72; of
skull, 76, peculiarities of hu-
man, 84; of thorax, 359.

Skin, 6, 412; glands of, 417; hy-
giene of, 420; nerve endings
in, 556.

Skull, 72.

Small intestine, 320.

Smell, 567.

Sneezing, 402.

Solar plexus, 172.

Solar spectrum, 495,

Solidity, visual perception of,
533,

Sounds, 542.

Sounds of the heart, 222.

Sounds, respiratory, 365.

Source of animal heat, 451

Source of fats, 441.

Source of glycogen, 440.

Source of muscular work, 427

Source of urea, 433.

Sources of energy to Body, 3!::3,

Special senses, 463.

Specific elements, 260.

Specific nerve energies, 191.

Spectacles, 501.

Sphenoid bone, 75.

Spherical aberration, 503.

Spinal cord, 158; conduction in,
578, 583; functions of, 182, 574;
histology of, 177; membranes
of 157; psychical activities of,
582.

Spinal accessory nerve, 171.

Spinal marrow. See Spinal cord.

Spinal nerves, 160.

Spinal nerve-roots, 160, 197.

Spleen, 332.

Spontaneity, 23.

Sporadic ganglia, 173.

INDEX.

619

Sprains, 99.

Squinting, 484

Stapedius muscle, 538.

Stapes, 537.

Starcli, 303; digestion of, 335.

Starvation, proteid, 43G.

Stationary air, 3G5.

Stereoscopic vision, 533.

Sternum, 71.

Stimuli, muscular, 129; nervous,

187.
Stimulus, 21.
Stirrup-bone, 537.
Stomach, 317.
Stomata, lymphatic, 330.
Storage tissues, 31, 437.
Strabismus (squinting), 484.
Structure of bone, 86.
Strychnine poisoning, 577.
Subclavian artery, 210.
Subcutaneous areolar tissue, 414.
Sublingual glaiiil, 315.
Submaxillary gland, 271, 315.
Succus entericus, 344.
Sudoriparous glands, 417.
Superior laryngeal nerve, 397.
Superior maxillary nerve, 170.
Superior mesenteric arterj', 211.
Supination, 98.
Supplemental air, 365.
Supporting tissues, 29.
Supra-renal capsules, 338.
Sutures, 93.
Swallowing, 336.
Sweat, 418.
Sweat-glands, 417.
Sweat-glands, nerves of, 270.
Sweetbread, 333.
Sj^mpathetic nervous system, 5,

15G, 172.
Sympathetic resonance, 549.
Sympathetic resonance in ear,

553.
Synovial membranes, 95.

Syntonin, 126.

System, alimentary, 308; circula-
tory, 201; muscular, 116, 128;
nervous, 128, 154; osseous, G3;
respiratory, 354; renal, 405.

Systemic circulation, 315.

Systems, anatomical, 37.

Tactile organs, 55G.

Taking cold, 459.

Tarsus, 79.

Taste, 570.

Taste-buds, 571.

Tear-glands, 482.

Tectorial membrane, 542.

Teeth, 309.

Teeth, structure of, 311.

Temperature of Bod}', 450.

Temperature, bodily, regulation
of, 454.

Temperature sense, 562.

Temperatures, local, 455.

Temporal artery, 210.

Temporal bone, 75.

Tension of blood gases.

Tendons, 118.

Tensor tympani muscle

Tests for proteids, 10.

Tetanus, 132.

Theory, resistance, 395.

Theory of color vision, 519.

Thermic nerves, 457.

Thigh-bone, 79.

Thirst, 569.

Thoracic duct, 329.

Thorax, aspiration of, 3o7; con-
tents of, 4; movements of in
respiration, 359; slceleton of,
359.

Tliroat, 316.

Thyroid body, 333.

Thyroid cartilage, 596.

Thyroid foramen, 79.

Thjnnus, 333.

Tibia, 79.

382.

538.

620

INDEX.

Timbre, 550.

Tissues, 2, adenoid, 106; adipose,
lll;areohir, 102; assimilative,
30; automatic, 82; cartilagin-
ous, 100; conductive, 33, 182;
connective, 62, 102; contractile,
33, 128; co-ordinating, 32,573;
elastic, 104; eliminative, 30;
excretory, 30; irritable, 31;
jelly-lilie connective, 106, me-
tabolic, 31, 265; motor, 33,
122; muscular, 33, 128; ner-
vous, 173; nutritive, 30; pro-
tective, 34; receptive, 30; re-
productive, 34; respiratory, 30;
retiform or adenoid, 106; se-
cretory, 30, 265; storage, 31, 437
supporting, 29; undifferenti-
ated, 29; white fibrous, 103.

Tissue-forming foods, 425.

Toast, 304.

Tone, sensations of, 542.

Tone color (timbre), 550.

Tongue, 313.

Tonsil, 316.

Touch organs, 556.

Touch, sensations of, 558.

Trachea, 354.

Transudata, 259.

Trigeminal nerve, 170.

Trophic nerves, 273.

Trypsin, 267, 341.

Tunica adventitia, 217.

Turbinate bones, 76.

Tympanic bones, 536, 551.

Tympanic membrane, 536. 550.

Ulna, 78.

Ulnar artery, 210.

Undifferentiated tissues, 29.

Upper maxilla, 76.

Urea, 12, 409, 433.

Ureter, 402.

Uric acid, 13.

Urinary organs, 402.

Urine, 409.

Utilization of energy in Body,

289.
Utriculus, 539.
Uvula, 309.
Vagus nerve, 171, 396.
Valve, ileocolic, 323.
Valves, auriculo-ventricular, 208,

of veins, 218; semilunar, 208.
Valvulaj conniventes, 320.
Vaso-dilator nerves, 257, 271.
Vasomotor centre, 253.
Vaso motor nerves, 253.
Vegetable foods, 302.
Veins, 212, 218; cephalic, 214;

coronary, 207; hepatic, 324;

hollow, 207; innominate, 214;

jugular, 214; long saphenous,

214; portal, 324; pulmonary,

207.
Vena cava, 207.
Venous blood, 216.
Ventilation, 375.
Ventral cavity, 4, 6.
Ventricles of brain, 165; of

larynx, 598.
Vermicular (peristaltic) move

ments, 338.
Vermiform appendix, 322.
Vertebrae, 63; cervical, 68; coccy

geal, 70; dorsal, 65; lumbar,

69; sacral, 69.
Vertebral artery, 210.
Vertebral column, 63, 70.
Vertebral foramen, 68.
Vertebrata, 3.
Vestibule, 538.

Vibrations, analysis of, 547; com-
position of, 546; pendular, 544;

sonorous, 542.
Villi of intestine, 321.
Vision, color, 519.
Vision purple, 512.
Vision, stereoscopic, 533.

INDEX

621

Visual axis, 518

Visual contrasts, 625.

Visual perceptions, 530.

Visual sensations, 506, 519 ; dura-
tion of, 516; intensity of, 513.

Vital capacity, 366.

Vitreous humor, 491.

Vocal cords, 597.

Vocal cords, false, 598. '

Voice, 595.

Vomer, 76.

Vowels, 603.

AValking, 151.

Wandering cells, 106.

Warm-blooded animals, 449.

Water, constituent, 25.

Water, percentage of, in Body,
14.

Waxing kernels, 330.

Weber's schema, 233.

Weber's law, 473.

Wheat, 302.

Whipped blood, 51.

White blood corpuscles, 17, 47,

59.
White fibrous tissue, 103.
Windpipe, 354.
Wrisberg, cartilage of, 597.
Wrist, 79.

Xantho-proteic test, 10.
Yawning, 402.
Young's theory of color vision,

519.
Zoological position of man, 2.
Zymogen, 268.

APPENDIX.

EEPRODUCTION AND DEVELOPMENT.

Beproduction 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 divides into two or
more pieces, each resembling itself exce^Dt in size; these
then grow and repeat the process; as, for instance, in
the case of Amoeba (see Zoology), and our own white
blood corpuscles (p. 18). Such a process may be summed
up m two words as discontinuous groiotJi; the mass, instead
of increasing in size without segmentation, divides as it
grows, and so forms independent living beings. In some
tolerably complex multicellular animals we find essentially
the same thmg; at times certain cells of the fresh- water Po-
lype (see Zoology) 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 com-
pletely built up and equipped is the young Hydra set loose
on its own career. How closely such a mode of multiplica-
tion is allied to mere growth is shown by other polypes 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 hud-
ding') may be compared to the method in which many of
the ancient Greek colonies were founded; carefully organ-
ized and prepared at home, they were sent out with a due

3 THE HUMAN BODY.

proportion of artificers of various kinds; so that the new
commonwealth had from its first separation a consider-
able division of em}»l()yments in it, and was, on a small
scale, a repetition of the i)arent community. In the great
majority of animals, however, (even those which at times
multiply by budding,) a different mode of ' reproduction
occurs, one more like that by Avhicli our western lands are
now settled and gradually built up into Territories and
States. The new individual in the political world begins
with little differentiation ; it consists of units, sej^arated from
older and highly organized societies, and these units at first
do pretty much everything, each man for himself, with more
or less efficiency. As growth takes place devclojiment also
occurs; persons assume different duties and perform differ-
ent work until, finally, a fully organized State is 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 multii^lication by division, retaining
tolerably evenly all its original protoplasmic properties,
and with no specific individual endowment or function.
The mass (Chap. III.) tlien 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 com-
pletely is a problem not yet sufficiently studied. In adult
Vertebrates it seems certain that the white blood corpuscles
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 i)ro-
cesses is uncertain; we do not know if the old bone corpus-
cles left form new bones, old muscle-fibres new muscles,
and so on; though it is probable that the undifferen-
tiated tissues (which we have compared — p. 60 — to youths on
the look-out for an opening in life) build up the new limb.
In Mammals no such restoration occurs; an amputated leg
may heal at the stumj) but does not grow again; in the
healing processes the connective tissues jjlay the main part,
as we might expect; their cellular elements being but little
modified from their primitive state (p. 105) can still multi-

REPRODUCTION OF TISSUES. 3

ply aud develop. New blood capillaries, however, sjjrout
out from the sides of old, and new epidermis seems only
to be formed by the multiplication of epidermic cells; hence
the practice, recently adopted by surgeons, of transplanting
little bits of skin to points on the surface of an extensive
burn or ulcer. In both blood capillaries and epidermis the
departure from the primary undiSerentiated 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 characteristic of the tissue: the same
is very probably true of the protoplasmic cells forming the
walls of the capillaries. Nerve-fibres are highly differenti-
atedj yet nerves rapidly unite after division; this, how-
ever, occurs by modification of amoeboid wandering cells
(p. 106) which align themselves with them. In Mammals,
muscular and glandular tissues seem never to be reproduced
after removal.

We find, then, as we ascend in the animal scale a dimin-
ishing reproductive power m the tissues generally: with tlie
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 reproductive power of its elements, and
the most differentiated tissues are either not reproduced at all
after mjury, or only by the specialization of amoeboid cells,
and not by a progenitive activity of survivors of the same
kind as those destroyed. In none of the higher animals,
therefore, do we find multiplication 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 the continuance of the
race is specially provided for by setting apart certain
cells which shall have this one property — ^cells whose 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

4 THE HUMAN BODY.

individuals; such a mode of reproduction is called parthen-
ogenesis: but in the majority of cases, and always in the
higher animals, this is not so; the fusion of two cells, or of
l^roducts of two cells, is a necessary preliminary to develoji-
ment. 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 &?, fertilization. Animals producing both
ova and spermatozoa are hermaphrodite; those bearing ova
on\y, female; and those spermatozoa only, male: hermaphro-
ditism is not found in Vertebrates, except in rare and
doubtful cases of monstrosity.

Accessory Reproductive Organs. The organ in which
ova are 2)roduced is known as the ovanj, that forming
spermatozoa, as the testis or testicle; but in different groups
of animals many additional accessory parts may be devel-
oped. Thus, in Mammalia, the offspring is nourished for a
considerable portion of its early life within the body of the
mother, a special cavity, the uterus or womh, being provided
for this purpose: the womb communicates with the exterior
_jDy a passage, the vagina, and two tubes, the oviducts
or Fallopian tnles, convey the eggs to it from the ovaries.
In addition, mammary glands provide milk for the nour-
ishment of the young in the first months after birth. In
the male mammal we find as accessory reproductive organs,
the vasa deferentia, which convey off from the testes the
seminal fluid, containing spermatozoa; vesiculce seminales
(not present in all Mammalia), in which the semen accumu-
lates before expulsion; a prostate gland, which forms a
secretion added to the semen; and an erectile organ, the
pe7iis, by which the fertilizing liquid is conveyed into the
body 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.. This pouch is subdivided internally by a par-
tition 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

STRUCTURE OF THE TESTIS.

— p. 356 — round the lung) and covers the exterior 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 tlio tunica
vaginalis at first is a mere offshoot of the peritoneal cavity,
and its serous membrane is originally a part of the perito-
neum. In the early years of life the passage along which
the testis passes usually becomes nearly closed up, and the
communication between the peritoneal cavity and that of
the tunica vaginalis is also obliterated. Traces of this
passage can, however, readily be observed in male children ;
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 jerked up some way towards
the abdomen and quite out of the
scrotum. Sometimes the passage
remains permanently open and a coil
of intestine may descend along it
and enter the scrotum, constituting
an inguinal hernia or rnpUire. A
hydrocele is an excessive accumula-
tion 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
sejjta {i, Fig. 160) radiate, subdivid-
ing the gland into many cliambers.
several greatly coiled seminiferous tubules, a, a, averaging
in length 0.68 meters (27 inches) and in diameter only
0.14 mm. (yIo- inch). Their total number in each gland is

Fig. 160.— Diagram of a
vertical section through tlie
testis, a, a, tubuli semini-
feri; fo, vasa recta; d, vasa
efferentia ending in the
coni vasculosi; e, e, epidi-
dymis. h,va.s deferens.

In each chamber lie

6 TlIM HUMAN BODY.

about 800. Near the posterior side of tlie testis the tubules
unite to form about 20 vasa recta {b), and these pass out of
tlie gland at its u])per end, as tlie vam 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 com-
mencements at the vasa efferentia, where they are 0.5 mm.
(■jV inch) in diameter, to the other end where they termi-
nate in the epididymis {e, e, Fig. 160). The latter is a nar-
row mass, slightly longer than the testicle, which lies
along the posterior side of that orgaii, near the lower end
of which [g) it passes into the vas deferens, h. If the
epididymis be carefully unraveled 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 -gig- inch).

The vas deferens {Ji, Fig. 160) commences at the lower
part of the epididymis as a slightly coiled tube, but it soon
becomes straight 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 meters (2 feet) in length and 2.5 mm. (yV inch)
in diameter.

The vcsiculcB seminales, two in number, are membranous
receptacles which lie, one on each side, beneath the bladder,
between it and the rectum. Tliey are commonly about 5
cm. (2 inches) long and a little more than a centimeter
wide (or about 0.5 inch) at their broadest part. The nar-
rowed 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.

The prostate gland is a dense l)ody about the size of a
chestnut which surrounds the commencement of the ure-
thra; the ejaculatory ducts pass through it. It is largely
made up of fibrous and unstriped muscular tissues, but
contains also a number of small secreting saccules whose
ducts open into the urethra.

3TALE REPRODUCTIVE ORGANS. 7

The male urethra leads from tlie bladder to the end of
the penis, where it terminates in an opening, the meatus
^trinarius. It is described by anatomists as made \\]) of three
portions, the prostatic, the membranous, and the spongy.
The first is surrounded by the prostate gland and receives
the ejacuhitory ducts. On its posterior wall, close to the
bUidder, is an elevation containing erectile tissue (see below)
and supposed to be dilated during sexual congress, so as to
cut off the jiassage to the urinary receptacle. On this crest
is an opening leading into a small recess, the utricle, which
is of interest, since the study of Embryology shows it to be
an undeveloped male uterus. The succeeding membranous
l^ortion of the urethra is about 1.8 cm. (f 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 dis-
tended by blood. Covered outside by the skin, interualJy
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, alone forms the terminal dilatation, ov 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 m a dense connective-tissue capsule from
which numerous bars, containing white fibrous, elastic, and
unstriped muscular tissues, radiate and intersect in all
directions, dividing its interior into many irregular cham-
bers called venous sinuses. Into these arteries convey blood,
which is carried off by veins springing from them.

The arteries of the penis are supplied with vaso-dilator
nerves (p. 257), the nervi erigentes, from the sacral plexus
([). 1G2). Under certain 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; the organ then becomes distended, rigid, and

8 THE HUM AX BODY.

erect. The co-ordinating centre of erection lies in the lumbar
region of the spinal cord, and may be excited rcflcxly by,
mechanical stimulation of the penis, or nnder the influence
of nervous impulses originating in the brain and associated
with sexual emotions. 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, but towards the end of the organ separates
and forms a fold, the prepuce, which doubles back, and,
becoming soft, moist, red, and very vascular, covers the
glans to the meatus tirinarius, where it becomes continu-
ous with the mucous membrane of the urethra; in it, near
the projecting posterior rim of the glans, are imbedded
many scljaceous glands.

Histology of the Testis and its Secretion. Each semi-
niferous tubule consists of a basement membrane, sup-
porting an epithelium, which in early life forms a definite
lining to the tube; in the adult the cells multiply so as to
nearly fill tlie cavity. From these cells the spermatozoa
are formed. The seminal fluid is an albuminous liquid con-
taining granules and spermatozoa; exam-
ined with the microscope the latter are seen
as rapidly moving minute objects (Fig. ICl)
each consisting of a broad flattened body
and a slender vibratile tail or cilium; they
are about 0.04 mm. {j\^ inch) in length.
The seminal fluid is not fully formed when
it leaves the seminiferous tubules; com-
matoio^a^" migni- P^etcly developed spermatozoa are first
fieci 350 diameters, found in the vasa recta, and even in the

«, viewed from the

side; 6, seen from contents of the vas deferens cells are often
found which have not yet finished the pro-
duction of a spermatozoon; probably, therefore, the fertil-
izing liquid is first fully elaborated in the vesiculae semi-
nales.

The Reproductive Organs of the Female. Each ovary
(o, Fig. 102) 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

FEMALE BEPBODUCTIVE ORGANS.

9

(60-100 grains). The organs lie in the pelyic cavity
enveloped in a fold of peritonenm (the hroad ligament), and
receive blood-vessels and nerves along one border. From
time to time ova reach the surface, burst through the envel-
oping peritoneum, and are received by the wide fringed
aperture, Ji, of the oviduct or Fallopian tube, od. This
tube narrows towards its inner end, where it communicates
Avith the uterus, and is lined Ity a mucous membrane, cov-
ered by ciliated epithelium; plain muscular tissue is also
developed in its wall. The uterus {a, c, Fig. 163) is a hol-
low organ, with relatively tliick muscular walls (left un-
shaded in the figure) which contains the foetus during

Fig. 102.— The uterus, in section, witti the right Fallopian tube and ovary, as
seen from behind, about % the natural size, w, upper part of uterus: c, cervix,
opposite the ns infernum; r, upper part of vagina; od. Fallopian tube; Ji, its
fimbriated extremity ; po, parovarium.

pregnancy and exjiels it at birth; it lies in the pelvi.s between
the urinary bladder and the rectum (Fig. 163); the Fallo-
jnan tubes open into its upper 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 incites) in breadth at
its upper widest part, and 2.5 cm. (1 inch) in thickness;
it weighs from 25 to 42 grams (|-to 1^ oz. ). The upper wider
portion of the womb is known as its bodi/; the cavity of this
is produced at each side to meet the openings of the Fallo-

10

THE nUMAN BODY.

pian tubes, and narrows below to the necTc, or cervix uteris
opposite e (1^'ig. 1(3;^), the cuniniunication between neck and
body cavities being known as the os inter7ium. Below this
the neck dilates somewhat. The lowest part of the cervix
reaches into the vagina and communicates with it by a
transverse aperture, the os uterx. On account of the thick-
ness of the uterine walls the cavity of the organ is very small

\

Fig. 163.— The viscera of the female pelvis as exposed by a dorso-ventral me-
diau section, j), upper end of sacrum; s, sympliysis pubis; i', f ', urinary blad
der; n, ui-ethra; ii, uterus; va, vagina; ?•, r', rectum: a, anal opening; I, right
labiimi major; n, right nympha ; /t, hymen; ci, divided clitoris.

ill the unimj)regnated state. During parturition thefretus
develops in the body of the womb, which becomes greatly
enlarged and rises high into the abdomen: the virgin womb
lies entirely below the level of the bones of the pelvis.

The chief bulk of tlie uterus consists of plain muscular
tissue; but it is lined internally by a ciliated mucous mem-

THE 0VAR7. U

brane, and is covered, externally by the peritoneum, bands
of which project from each side of it as the broad ligaments
{II, Fig. 162). Opening on the internal mucous membrane
are the mouths of closely set, simple or slightly branched,
tubular glands.

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, containing mucous glands, and
outside this is made up of areolar*, erectile, and unstriped
muscular tissues. Around its lower end is a ring of striated
muscular tissue, the spliincter vaginm.

The vulva is a general term for all the portions of the
female generative organs visible from the exterior. Over
the front of the pelvis the skin is elevated by adipose tissue
beneath it, and forms the mons Veneris. From this two folds
of skin (/, Fig. 163), the labia 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-iirinary 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, resem-
bling a miniature penis in structure, except that it has no
corpus spongiosum and is not traversed by the urethra.
From the clitoris descend two folds of mucous membrane,
the nymphce or labia interna, between which is the vestibule,
a recess containing, above, the opening of the short female
"itiithra, and, below, the aj)erture of the vagina, which is in
the virgin more or less closed by a thin duplicature of mu-
cous membrane, the hymen.

Microscopic Structure of the Ovary. The main mass
of the ovary consists of a close connective-tissue stroma,
containing unstriiDed muscle, blood-vessels, and nerves: it
is covered externally by a peculiar germinal epithelium,
and contains imbedded m it many minute cavities, the
Graafian follicles, in which ova lie. If a thin section of an
ovary be examined with the microscope many hundreds of
small ^^Graafian follicles, each about 0.25 mm. (j-}-q inch) in
diameter, will be found imbedded in it near the surface.

12

THE HUMAN BODY

These arc lined by cells, and each contains a single OTum.
Deeper in, larger follicles (7, 8, 9, Fig. 1G4) are seen, 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 more dense
layer of the ovarian stroma; within this come several layers
of lining cells (9, a, Fig. 164) constituting the memb'rana
granulosa. At one point, b, the cells of this layer are heaped
up, forming the discus proligerus, which projects into the

Fig. 164.— a section of a Mammalian ovary, considerably magnified. 1, outer
capsule of ovary; 2, 3, 3'. stroma; 4, blood-vessels: .5. nirtimentary Graafian fol
licles: 6, 7, 8, follicles beginning to enlarge and matnre. and rece'ding from the
surface: 9, a nearly ripe follicle which is again traveling towards the surface
preparatory to discharging the ovum; a. membrana granulosa: ?>. discus pro-
ligerus; c, ovimi. with, d, germinal vesicle and. e. geiminal spot. The general
cavity of the follicle (in which 9 is printed) is flUed with liquid during life.

liquid filling the cavity of the follicle. In the discus proli-
gerus the ovum, c, lies, having in it a nucleus or germinal
vesicle, d, and a nucleolus or germinal spot, e. The ovum
is about 0.2 mm. (y^g inch) in diameter; its structure is
better represented at A, Fig. 8*, where it is seen to consist
of a thick outer coat or cell-wall, a, called the viteUine
membrane or zona pellucida; within Avhich is the granular
cell protoplasm, called here the vitellus or yelk; and in that
again the germinal vesicle and spot.

* Page 26.

PUBERTY. 13

Puberty. The condition of tlie reproductive organs of
each sex described above is that found in adults; altliough
mapped out, and, to a certain extent, developed, before
birth and during childhood, these parts grow but slowly and
remain functionally incapable during the early years of life:
then they comparatively rapidly increase in size and become
physiologically active: the boy or girl becomes man or
woman.

This period of attaniing sexual maturity, known as pu-
berty, takes place from the eleventh to the sixteenth year
commonly, and is accompanied 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 associ-
ated with voluptuous dreams. Many a young man is alarmed
by these; he has been kept in ignorance of the Avhole mat-
ter, 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 high-road 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 suspect 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 any thing-
likely to excite the sexual instincts.

In the woman the pelvis enlarges considerably at puberty,

14 THE HUM AX BOB 7.

and, commonly, more snbcntaneous adipopo tissue develops
over the Body generally, bnt especially on the breasts and
hips; consequently the contours become more rounded.
The external generative organs increase in size, and the
clitoris and nymphfe become erectile. The uterus grows
considerably, the ovaries enlarge, some Graafian follicles
ripen, and the events treated of in the succeeding paragrai)hs
occur.

Ovulation. From puberty, during the whole child-
Ijcaring period of life, certain comparatively very large
Graafian follicles may nearly always be found either close
to the surface of the ovary 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 wall of the follicle gives way and the ovum is dis-
charged, 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 hiteu7n, which
recedes again to the interior of the ovary and disajipears in
three or four weeks, unless pregnancy occurs; then 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 Aveeks, and is
attended with important changes in other portions of the
generative apparatus. The ovaries and Fallopian tubes
becomes 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 various points
and the ova are swept along them by their cilia to tlie
cavity of the oviduct, is not certain. Having entered the
Fallopian tube the egg slowly passes on to the uterus,
prol)ably moved by the cilia lining the oviduct. In the
woml), also, important changes occur at this time; it be-
comes swollen and gorged with blood, especially its lining
mucous membrane and its glands. If the ovum be not
fertilized it dies and is passed out. At the same time the
gVipcrficial part of the uterine mucous membrane is softened,

MENSTRUATION. 15

broken down, and discharged along with more or less blood,
constituting the menses, or monthly sickness, which com-
monly 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 daring pregnancy
and while suckling, 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 tlie sexual life of the woman is completed.
This time, the diniucteric or ''tarn of life," is a critical
one; various local disorders are apt to supervene, and even
mental derangement.

Hygiene of Menstruation. During menstruation there
IS apt to be more or less general discomfort and nervous irri-
tability; the woman is not quite herself, and those respon-
sible 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 con-
sequences. 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. 163) but slightly supported,
and that near its lower end, it is especially apt to be dis-
placed or distorted; it may tilt forwards or sideways (fer-
sions 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.

Painful menstruation {dysmenorrlicea) may be due to very
many causes, but it is only within recent years that physi-
cians have come to recognize how often it depends on uterine
displacements, and in such cases how readily it may usually
be remedied by restoring the organ to its proper position,
and supporting it there if necessary. A flexion of the organ

16 THE nUMAN BODY.

closes the passage of the cervix and is especially apt to cause
pain at the menstrual period. The accumulated blood
distends the uterus, which makes violent contractions to
expel it; finally, if the resistance is overcome, the blood has
probably already clotted and its expulsion causes more suf-
fering. All this might usually be soon remedied, but the
sufferer, ignorant of what is wrong, often goes on montli
after month until her health is undermined, hoping that
the trouble will get better of itself, which it never will.
To submit to the necessary examination and treatment from
one of the other sex is, to a refined woman, hoAvever, apt to
be a more severe trial than all the jDhysical pain; and there
is no recent social movement more deserving of every en-
couragement and support than that whose aim is to provide
proj)erly trained female medical attendance for women in
the diseases peculiar to their sex; such as may now, fortu-
nately, be found in most of our large cities. Few except
physicians, and perhaps few physicians, know what an
amount of relievable pain women endure in silence rather
than run the risk of being forced to consult a male doc-
tor. If no skilled person of her own sex is at hand the
sufferer, if she do anything, is only too apt to take some of
the nostrums advertised in such number for '' female com-
plaints," or to consult a half -educated lady quack of some
novel "school" with a taking title. The result of doing
this, or doing nothing, is often permanent valetudinarianism
and a life of uselessness, to those who might be active and
happy wives and mothers.

The absence of the menstrual flow {amenorrhcBa) is nor-
mal during pregnancy and Avhile suckling; and in some rare
cases it never occurs throughout life, even in healthy women
capable of child-ljcaring. 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 impressed upon
women that the most dangerous thing to do is to take drugs
tending to induce the discharge, except under skilled advice;
to excite the floAV, in many cases, as for example occlusion
of the OS uteri, or in general debility (when its absence is

CONCEPTION. 17

a conservative effort of the system), may be the most dis-
astrous thing to do.

Impregnation. As the ovum descends the Fallopian tube
the changes preliminary to menstruation are taking place
in the uterus. Its mucous membrane is thickened, all the
generative organs of the woman are more or less congested,
and her sexual emotions are commonly more easily aroused.
Unless the act of coition occur all this passes off with the
menstrual flux, and the organs return to a quiet state until
the period of the next ovulation. If sexual congress takes
place the vagina, uterus, and oviducts are thrown into
reflex peristaltic contractions; and there is frequently an
increased secretion by the vaginal mucous membrane.
Some of the seminal fluid is received into the uterine cavity,
and there, or, more probably, in the Fallopian tube, meets
the ovum. The spermatozoa are carried along partly,
perhaps, by the contractions of the muscular walls of the
female cavities, but mainly by their own activity. From
observations made on various lower animals it appears that
their movements cease immediately on coming into contact
with the ovum, and that one only takes part in fertilization.
How this lattei occurs in the mammalian ovum is not cer-
tain; observations in other groups make it probable that
the male element directly fuses m whole or part with the
protoplasmic mass of the ovum, but no opening has been de-
tected in the zona pellucidaof the mammalian ovum, which
IS so thick and firm that it is hard to imagine a spermato-
zoon otherwise penetrating it; some, therefore, are inclined
to suppose that material is merely passed by dialysis from
the spermatozoon into the egg-cell.

The fertilized ovum continues its descent to the uterine
cavity, but, instead of lying dormant like the tmfertilized,
segments (p. 26), and forms a morula. This, entering the
womb, becomes imbedded in the soft, thickened, vascular
mucous membrane there, from which it imbibes nourish-
ment, and which, instead of being cast off in a menstrual
discharge, is now retained and grows during the whole of
pregnane^ , having important dtities to discharge in connec-
tion with the nutrition of the embryo.

18 THE HUM Ay BODY.

Sexual congress is most apt to be followed by impregnation
if it occur Just before the menstrual flow is due, while
a ripe and receptive ovum is in the Fallopian tube. This
time is about four days, that being the period the ovum
ajipears to take in its passage from the ovary to the womlj.
The spermatozoa, however, retain their vitality in the
female cavities for some five or six days, so that coition
during the ten or twelve days preceding the date when the
next menstruation may be expected, is a23t to be followed
by pregnancy. There seems reason to believe, however, that
ova are occasionally discharged at other than the regular
monthly periods of ovulation, especially under the influence
of the erection and congestion of the parts which takes
place during the sexual act. There is, therefore, no time
during a woman's life from jouberty to the climacteric,
except during pregnancy or lactation, when sexual congress
may not result in impregnation.

The exact parts taken by the male and female elements
respectively in the formation of the embryo are not known
with certainty; seeing that the ovum is much larger than
the spermatozoon it is not uncommon to speak of the latter
as a mere stimulant or excitant, arousing the egg-cell to
developmental activity; but the definite characteristics of ten
inherited by children from the father show that the sperma-
tozoon IS much more than that; materials derived from it
are no doubt an essential constituent of the compound mass
Avhich develops into the new human being.

Pregnancy. When the mulberry mass reaches the uterine
cavity the mucous membrane 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 gi'ows, it bulges out
into the uterine cavity and pushes before it that part of the
decidua which has grown over it (the decidna rejiexa); at
about the end of the third month this meets the decidua
lining the opposite sides of the uterine cavity and the two
grow together. That part of the decidua (decidua serotina)
against which the morula is first attached subsequently un-

GESTATION. 19

dergoes a great developineiit iu connection with the forma-
tion 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
Httle room for it; it accordingly presses on the bladder and
rectum (see Fig. 163) and the nerves in the neighborhood,
frequently causing considerable discomfort or pain; and,
reflexly, often exciting nausea or vomiting (the morning
siclcness of pregnancy). Later on, the pregnant 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
shortness of breath and palpitation of the heart from inter-
ference with the diaphragmatic movements. All tight
garments should at this time be especially avoided; the
woman's breathing is already sufficiently 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: depression,
anxiety, and aii emotional nervoits state.

During the whole period of gestation the woman is not
merely sttpplying from her blood nutriment for the foetus,
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 iu
good health, but which is severely felt if she be feeble or
suffering from disease. Many a wife who might have led a
long and happy life is made an invalid or brought to jjre-
mature death, through being kept in a chronic state of
pregnancy. There is a general agreement that sexual con-
tinence is j)ossible and a duty in unmarried men, but the
husband rarely considers that he should put any bounds on
himself beyond those indicated by his own passions; consid-
eration for his wife's health rarely enters his head in this
connection. The healthy, married woman who endeavors
to evade motherhood because she thinks she will thus pre-
serve her personal appearance, or because she dislikes the
trouble of a family, deserves but little sympathy; she is

30 THE HUMAN BODY.

trying to escape a duty voluutarily undertaken, and owed to
her husband, her country, and her race; but she whoso
strength is undermined and whose life is made one long
discomfort for the sexual gratification of her husband calls
for all aid, and it is wrong to keep silent on the subject.
The professor of gynajcology in a leading medical school,
gives it as his deliberate opinion that the majority of
American women must at some periods of their lives choose
between freedom from pregnancy or early death. He
further says that he does not believe that healthy men are
so organized that this matter can be regulated by them;
but that the question depends on the tact and prudence of
the wife. Men, however, as a rule, are not utterly selfish;
they commonly err through ignorance or thoughtlessness,
not knowing or realizing what a strain frequent pregnancy
is on the strength of a delicate woman. A social custom so
deep-rooted and ancient as the usual American and English
one of married couples constantly occupying the same bed is
not easily changed, yet it probably leads to much harm, and
especially in this country. Whatever the reason be, there is
no doubt that the physical stamina of the average English
woman is considerably greater than that of her American
cousin, and she bears and brings up large families with
greater safety. For a husband, who has reason to believe
that child-bearing will injure his wife's health, to always
share her couch is a deliberate walking into temptation.

Apart from pregnancy, moreover, a woman's health is
often injured by frequent sexual intercourse. A johysician
who has unusual opportunities of knowing states that he
has reason to believe that not only is the act of sexual con-
gress at best, from a physical jDoint of view, a mere nuisance
to the majority of women belonging to the more luxurious
classes of society after they attain the age of twenty-two
or twenty-three, but that a very considerable proportion suf-
fer acute pain from it such as, if frequent, breaks down the
general health. A loving woman, finding her highest happi-
ness in suffering for those dear to her, is very unlikely to let
her husband know this, so long as she can bear it; but if
the possibility is known it will not, perhaps, need much

NUTRITION OF THE EMBRYO.

31

acuteness in him to discover sucli suffering when it exists,
nor very much real affection to control himself accordingly.
In the class of cases referred to, rest of the over-irritable
and congested female organs is above all essential. The
cause is frequently removable by simple, but skilled, treat-
ment; the desirability of rendering this available to a
woman in members of her own sex has already been insisted
upon.

Even when no pain is caused harm may be done: the presi-
dent of the Gynaecological Society, in an address delivered
before that body, lately stated that if either party to a coi-
tion fails of the orgasm damage is apt to ensue, but espe-
cially to the woman if she fail; the organs are congested from
the stimulus of the sexual act and the normal final orgasm
is required for their healthy relief and return to the resting
condition. If this be so, it is
clear that coition should be
restricted to times when the
woman's general state encour-
ages the orgasm, and unless
she generally experiences it
sexual congress should be
avoided until her health is
restored.

The Foetal Appendages. In
the earliest stages of life, those
occurring in the days imme-
diately after fertilization of the
ovum, there is little or no
growth. The ovum segments
into a number of cells, but the morula thus formed is little
larger than the original egg-cell itself. At first it is a solid
mass (F, Fig. 8, p. 26), but its cells soon recede from the
centre and become arranged (Fig, 165) around a central
cavity containing mainly some absorbed liquid; at one point,
c (the embryonal disk), the layer thickens, and from thence
the thickening spreads, by cell growth and multiplication,
over the whole sac, which is known as the embryonal
vesicle; the membrane thus formed is the blastoderm, and

Fig. 165.— The embryonal vesicle.
a, thinned and distended zona pel-
lucida: this soon after disappears
altogether; b, tlie blastoderm; c,
the embryonal disk.

22

TEE HUMAN BODY.

consists of three cell layers; au outer or epihlast, a middle
or mesohlast, and an inner or hypoblast. From this simple
sac, presenting no resemblance not merely to a human being
but to a vertebrate animal, the foetus is built up by cell
division and modification. The general history of intra-
uterine development is much too long and too complex to
enter upon here, but the formation of certain structures
lost at or before birth, and associated with the jn-otection and
nourishment of the embryo, may be attempted: they are the
yelh sac, the amnion, and the allantois. The developing
blastoderm especially thickens in the neighborhood of the
embryonal disk and there the outlines of the Body are first
laid down. Along the thickening a groove appears, which

A

B

Fig. 166.— a, an early blastoderm with the first traces of the primitive groove;
B, the same a little later ; /, primitive groove ; d, thickened region of the blas-
toderm which directlj- builds up the embryo.

marks out the future longitudinal axis and dorsal side of the
bod}^, (A, Fig. IGG). This groove elongates, its edges rise (B),
and finally arch over, meet, and fuse together above it. The
tube thus closed in is the rudiment of the cerebro-spinal
axis; from its lining epiblastic cells the brain and spinal
cord are developed, and its cavity remains throughout life-
as the central canal of the spinal cord and as the cerebral ven-
tricles, excluding the fifth (p. 165). Some way outside this
dorsal tube the mesoblast splits into an outer leaf, adherent
to the epiblast, and an inner adherent to the hypoblast; the
conjoined meso-epiblastic layer is the somatopleure; the
meso-hypoblastic the splanchnopleiire. The proximal j^arts
of the somatopleure {i.e., the regions nearest the central

THE FCETAL APPENDAGES. 23

axis) develoj) into the walls of chest and abdomen; farther
out it turns up and arches over the back of the embryo, and
its edges, there meeting, grow together and form a bag, the
amnion, enveloping the foetus. Into this a considerable
quantity of liquid is secreted, m which the fcetus floats. At
birth the contractions of the uterus, pressing on the amnion,
drive part of it down like a wedge into the neck of the
uterus, and through its liquid contents an equable pressure
is exerted there, until the os uteri is tolerably dilated; the
sac then normally ruptures and the "waters" escape. Some-
times, however, an infant is born still enveloped in the
amnion, which is then popularly known as a caul. While
the amnion is develoi)ing, a semi-cartilaginous rod forms
along the axis of the Body beneath the floor of the dorsal
tube; this is the notochord; when it appears the young being
is marked out distinctly as a vertebrate animal, having a dor-
sal neural tube above an axial skeleton, and a ventral hcemal
tube (p. 4), formed by the proximal regions of the somato-
pleure, beneath it. The ventral tube, however, is still widely
open, the points where the amniotic folds turn back being
far from meeting in the future middle line of the chest and
abdomen.

Tlie proximal portions of the splanchnopleure incurve to
inclose the alimentary tube, which is at first straight and
simple. Beyond the point where it bends in for this purpose
the splanchnopleure again diverges, and incloses a small
globular bag, the yelk sac, which is, thus, attached to the
ventral side of the alimentary canal; it at first projects
through the opening where the amniotic folds turn back,
but has little importance in the mammalian embryo and is
soon absorbed.

The aUa)Lfois is primarily an outgrowth from the ali-
mentary canal, containing blood-vessels. It passes out from
the Body on the ventral side where the somatopleures have
not yet met, and reaching the inside of the uterus, its dist:il
end expands there to make the main part of the placenta (see
below). Its narrow proximal portion forms the umbilical
cord, around which the somatojDleures, incurving to in-
close the belly, meet at the navel some time before birth.

24 TUE HUMAN BODY.

The Intra-Uterine Nutrition of the Embryo. At tirst
the embryo is uuuri.slied by tibsor])tion of niateriuls from
Llie soft vascular lining of the womb; as it increases in size
this is not sufficient, and a new organ, the placenta, is
formed for the purpose. The allantois plants itself firmly
against the decidua serotina (p. 18), 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. Into
the allantoic villi foetal blood-vessels run and form capil-
lary networks; the blood flowing through these receives, by
dialysis, oxygen and food materials from the maternal
blood, and gives up to it carbon dioxide, urea, and other
wastes. There is thus no direct intermixture of the two
bloods; the embryo is from the first an essentially sei)arate
and independent organism. The allantois and decidua
serotina becoming closely 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.

Parturition. At the end of fj-om 275 to 280 days from
fertilization of the ovum [conception) pregnancy terminates,
and the child 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, through which arteries run to, and
veins from, the placenta. Shortly afterwards the latter is
detached and expelled (both its allantoic and decidual
parts), and where it is torn loose from the uterine wall
large blood sinuses are left open and exposed. 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 hiBmorrhage occurs
{flooding) and the mother may bleed to death in a few min-
utes 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 subsetpiently replaced by
new mucous membrane. The muscular fibres developed in
the uterine wall in such large quantities during pregnancy.

PARTURITION. 25

undergo rapid fatty degeneration and are absorbed, and
in a few weeks the organ returns almost to its original
size. The parturient woman is especially apt to take infec-
tious diseases; and these, should they attack her, are fatal
in a very large percentage of cases. Very special care
should therefore be taken to keeja all contagion from
her.

There is a current impression that a pregnancy, once
commenced, can be brought to a premature end, especially
m its early stages, without any serious risk to the woman.
It ought to be widely made known that such a belief is
erroneous. Premature delivery, early or late in pregnancy,
is always more dangerous than natural labor at the jiroi^er
term; the physician has sometimes to induce it, as when a
malformed pelvis makes normal parturition impossible, or
the general derangement of health accompanying the preg-
nancy IS such as to threaten the mother's life; but the occa-
sional necessity of deciding whether it is liis duty to pro-
cure an abortion is one of the most serious responsibilities
he meets with in the course of his professional work.

Dr. Storer, an eminent gynaecologist, states emphatically,
from extended observation, that "despite aiij)arent and
isolated instances to the contrary —

1. A larger proportion of women die during or in con-
sequence of an abortion, than during or in consequence of
child-bed at the full term of i)regnancy.

3. A very much larger number of women become con-
firmed invalids, perhaps for life; and —

3. The tendency to serious and often fatal organic disease,
as cancer, is rendered very much greater at the so-called
"turn of life,"' by previous artificially induced premature
delivery.

During pregnancy there is a close connection between
the placenta and uterus; nature makes preparation for the
safe dissolutioii of this at the end of the normal period,
but " its premature rupture is usually attended by pro-
fuse haemorrhage, often fatal, often persistent to a greater or
less degree for many months after the act is completed, and
always attended with more or less shock to the maternal
system, even though the full effect of this is not noted for

26 THE HUMAN BODY.

years." Tlie sume authority states again: "Any deviation
from this process at the full term" {i.e., the process, asso-
ciated with lactation, by which the uterus is restored to its
small non-gravid dimensions) "lays the foundation of. and
causes, a wide range of uterine accidents and disease, dis-
[dacements of various kinds; falling of the womb downwards
or forwards or backwards, with the long list of neuralgic
pains in the l)ack, groin, thighs and elsewhere that they
occasion; constant and inordinate leucorrhoea; sympathetic
attacks of ovarian irritation, running even into dropsy,"
etc. etc. There is, thus, abundant reason for bearing most
things rather than the risks of an avoidable abortion; among
these is one not mentioned above, but more terrible than
all, insanity.

Lactation. The mammary glands for several years aftei
birtli remain small, and alike in both sexes. Towards
puberty they begin to enlarge in the female, and when fully
developed form in that sex two rounded eminences, tho
breasts, placed on the thorax. A little below the centre of
each projects a small eminence, the nipple, and the skin
around this forms a colored circle, the areola. In virgma
the areola? are pink; they darken m tint and enlarge during
the first pregnancy and never quite regain their original
hue. The mammary glands are constructed on the com
pound racemose type (p. 2G2). Each consists of from
fifteen to twenty distinct lobes, made up of smaller divisions^
from each main lobe a separate galadophorous duct, made
by the union of smaller branches from the lobules, runs
towards the nipjole, all converging beneath the areola.
There each dilates and forms a small elongated reservoir
m which the milk may temporarily collect. Beyond this
the ducts narrow again, and each continues to a separate
opening on the nipple. Imbedding 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 imper-
fectly developed and dormant. Early in pregnancy it begins
to increase in bulk, and the gland lobes can be felt as hard
masses through the superjacent skin and fat. Even at par-
turition, however, their functional activity is not fully

LACTATION. 27

established. The oil-globixles 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 m 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
purgative on the infant, and probably thus serves a useful
purpose, as a certain amount of substances (bdiary 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. There is reason
to believe that the processes of involution by which the large
mass of muscular and other tissues developed in the uterine
walls during pregnancy are broken down and absorbed,
take place more safely to health if the natural milk secretion
is encouraged. Many women refuse to suckle their children
from a belief that so doing will injure their personal appear-
ance, but skilled medical opinion is to the co itrary effect;
the natural course of events is the best for this purpose,
unless lactation be too prolonged. Of course in many cases
there are justifiable grounds for a mother's not undertaking
this part of her duties; a physician is the pioper 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 substitute. Grood cow's milk contains rather more fats
than that of a woman, and much more casein; the following
table gives averages in 1000 parts of milk:

Woman. Cow.

Casein 28.0 54.0

Butter 8:5 5 43.0

Milk sugar 44.5 42.5

Inorganic matters 4.75 7.75

28 THE HUMA^' BODY.

The inorganic matters of human milk yield, on analysis,
m 100 parts — calcium carbonate 0.9; calcium phosphate 70.6;
sodium chloride D.8; sodium sulphate 7.4; other salts 5.3.
The lime salts arc of especial importance to the child, which
lias still to build up nearly all its bony skeleton.

When undiluted covv'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
butter 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 sweetens
much less than cane sugar, and infants used to the latter
refuse milk in which milk sugar is substituted. Cream from
cow's milk may be added to raise the percentage of fats to
the normal, but must be perfectly fresh and only added to the
milk immediately before it is given to the child. While
milk is standing for the cream to rise it is very apt to turn
a little sour; the amount of this sour milk carried off with the
cream is itself no harm when mixed with a large bulk of fresh
milk; it carries with it, however, some of the fungus whose
development causes the souring, and this will rapidly de-
velop and sour all the milk it is added to if the mixture be
let stand. As the infant grows older less diluted cow's milk
may gradually be given; after the seventh or eighth month
no addition of water is necessary.

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 ferment
(p. 311). Later on, starchy substances may be added to
tlie diet with advantage, but it should be borne in mind
that they cannot form the chief part of the child's food; it
needs proteids for the formation of its tissues, and amyloid
foods contain none of these. Many infants are, ignorautly,
half starved by being fed almost entirely on such things as
corn-flour or arrowroot.

STAGES OF LIFE. 29

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 raj^id growth but by differentiating growth or de-
velopment; then comes a more stationary 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 Avhile the general size
and form of the cell or fibre 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 jieriod
numerous bones are represented mainly by cartilage. The
pancreas has not attained its full development; and some
of the sense-organs seem to be in the sarne case; at least
new-born infants appear to hear very imi^erfectly. Tlie
reproductive organs only attain full development at pu-
berty, and degenerate and lose all or much of their func-
tional importance as years accumulate. Certain organs
have even a still shorter range of physiological life; the
thymus, for example (p. 833), attains its fullest develop-
ment at the end of the second year and then gradually
dwindles away, so that m 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 m this respect at
puberty. Henceforth the woman's weight (excluding 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; a man's weight
usually slowdy increases until forty.

As old age comes on a general decline sets in, the rib
cartilages become calcified, and lime salts are laid down

30 THE HUMAN BODY.

in the ai'terial walls, wliieli thus lose their elasticity; the
refracting media of the eye become more or less opaqne;
the pln'siological irritability of the sense-organs in general
diminishes; and fatty degeneration, diminishing their work-
ing 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 of ideas; 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 extremely 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 child-
hood; it is conceivable that, without death, this })rocess
might occur until the Body was reduced to its original
microscopic dimensions.

Before any great diminution takes place, however, a
breakdown occurs somewhere, the enfeebled community of
organs and tissues forming the man is unable to meet the
contingencies 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 realize the fact until the sum-
mons comes. To the popular imagination the prospect of
dying is often associated with thoughts of extreme suffering;
]iersonifying life people 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 prob-
ably rarely associated with any immediate suffering. The
sensibilities are gradually dulled as the end approaches; the
nervous tissues, with the rest, lose their functional 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

DEATH. 31

pressure then falls irretrievably, the capillary circulation
ceases, and the tissues, no longer nourished from the blood,
gradually die, not all at one instant, but one after another,
according as their individual resjiiratory 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 rather 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 arc
not talents which we may justifiably leave unem])loyed; each
is bound to do his best with them, and so to live that he
may most utilize them. An active, vigorous, dutiful, un-
selfish life is a good preparation for death; when that time,
at which we must pass from the realm controlled by physi-
ological 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 cm])loyment of our
faculties while they were ours to use, will be no mean conso-
lation.

INDEX TO APPENDIX.

Abortion, 25.
Allantois, 23.
Amenorrhoea, 16.
Amnion, 2.3.
Blastoderm. 21.
Breasts, 26.
Budding, 1.
Caul, 23.
Cervix uteri, 10.
Childbirth, 24.
Climacteric, 15.
Clitoris, 11.
Coni vasculosi, 6.
Corpora cavernosa, 7.
Corpus luteum, 14.
Corpus spongiosum, 7.
Death, 30.
Decidua, 18.

Development of embryo, 21.
Discontinuous growth, 1.
Discus proligerus, 12.
Displacement of uterus, 15.
Dysmenorrhea, 15.
Egg-cell, 4.
Emliryonal disk, 21.
Embryonal vesicle, 21.
Epi])last, 22.
Epididymis, 6.
Erectile tissues, 7.
Fallopian tube, 4, 9.
Feeding of infants, 27.
Fertilization, 4, 17.
Foetal appendages, 21.
Galactophorous ducts. 2fl

Gemmation, 1.

Germinal epithelium, 11.

Germinal spot, 12.

Germinal vesicle, 12.

Gland, maiiunary, 26.

Graafian follicles, 11.

Hermaphroditism, 4.

Hernia, inguinal, 5.

Hi.stology of ovary, 11; of testis,

8.
Hydrocele, 5.
Hygiene of menstruation, 15- of

pregnancy, 19.
Hymen, 11.
Hypoblast, 22.
Impregnation, 17.
Inguinal hernia, 5.
Intra uterine development, 21.
Intrauterine nutrition of embryo,

24.
Lactation, 26.
Lochia, 24.
Mammary gland, 26.
Membrana grauulo.sa, 12.
Menstruation, 14.
Mesoblast, 22.
Milk, 27.

Nervi erigentes, 7.
Notochord, 23.
Organs of reproduction, 4.
Ovary, 4, 8.
Oviduct, 4, 9.
Ovulation, 14.
Ovum, 4, 12.

INDEX rO APPENDIX.

Parthenogenesis, 4.

Parturition, 24.

Penis, 7.

Placenta, 24.

Pregnancy, 18.

Prepuce, 8.

Prostate gl;incl, (i.

Puberty, 1:5.

Reproduction in general, 1.

Reproductive organs, accessory,

4; male, 4; female, 8.
Ruplure, 5.
Seminal fluid, 8.
Seminiferous tubules, 5.
Sexual reproduction, 3.
Somatopleure, 22.
Spermatozoon, 4, 8.
Sperm-cell, 4.
Splanchnopleure, 22.

Stages of life, 29.

Testes, 4, 8.

Tunica vaginalis, 4.

Umbilical cord, 23.

Urethra, 7.

Uterus, 4, 9.

Utricle, 7.

Vagina, 10.

Vas deferens, 6.

Vasa recta, 6.

Vasa efferentia, G.

Vesiculoe seminales, 6.

Vitelline membrane, 12.

Vitellus, 12.

Vulva, 10.

Yelk, 12.

Yelk-sac, 22.

Zona pellucida, 12

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