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LESSONS
ELEMENTARY PHYSIOLOGY
•?&&&■
LESSONS
IN
ELEMENTARY PHYSIOLOGY
BY
THOMAS H. HUXLEY, LL.D., F.R.S.
EDITED
FOR THE USE OF AMERICAN SCHOOLS AND COLLEGES BY
FREDERIC S. LEE, Ph.D.
Adjunct Professor of Physiology in Columbia University
WITH NUMEROUS ILLUSTRATIONS
THE MACMILLAN COMPANY
LONDON: MACMILLAN & CO., Ltd.
1916
AH rights reserved
Copyright, 1900,
By THE MACMILLAN COMPANY.
Sit up and electrotyped February, 1900. Reprinted October,
1902; April, 1903; October, 1904 ; October, 1905; October, 1906:
July, 1907; July, 1909 ; September, 1910; January, 1912;
September, 1913; October, 1914; March, 1916.
QP36
NotrjjanU $K00
J. S. Cushing & Co. — Berwick & Smith
Norwood Maas U.S.A.
EXPLANATION OF THE PLATE
The Human. Skeleton in Profile
Na.
Fr.
Pa.
Oc.
Mn.
St.
R.
R>.
S.
Cx.
Sep.
CI.
H.
Ra.
U.
CP.
Mc.
D.
I, If,
II.
Pb.
Is.
F.
Tb.
Fb.
T.
Mt.
D.
The Nasal bones.
The Frontal bone.
The Parietal bones. \ In the Skull.
The Occipital bone.
The Mandible, or Lower Jaw.
The Sternum, or Breast-bone.
The Ribs. In the Thorax.
The Cartilages of the Ribs.
The Sacrum.
The Coccyx.
The Scapula, or Shoulder-blade.
The Clavicle, or Collar-bone.
The Humerus.
The Radius.
The Ulna.
The Carpus, or Wrist-bones,
The Metacarpus.
The Phalanges of the Fingers, or Digits of the
Hand.
Ill, IV, V. The Pollex, or Thumb, and the succeeding Fingers.
The mum. j h ther form the Hip-bone,
The Pubis.
The Ischium.
The Femur.
The Tibia.
The Fibula. I- In the Leg.
The Tarsus, or Ankle-bones.
The Metatarsus.
The Phalanges of the Toes, or Digits of the Foot. J
In the Arm.
or Os innominatum.
PREFACE
Huxley's " Lessons in Elementary Physiology " was pub-
lished first in 1866. The last edition which the author
himself brought out, was the revised edition of 1885. The
book has recently undergone an extensive and careful
revision at the hands of Professor Michael Foster and Dr.
Sheridan Lea. In the preface to this English edition
Professor Foster writes as follows : —
" My friend, Dr. [Sheridan] Lea, and I have undertaken
a task of great difficulty.
"The progress which has taken place in science and in
education, since the last revision of this work, has rendered
it desirable to make considerable changes and additions in
order to maintain for the book the usefulness which has
been for so long a time its conspicuous feature.
" At the same time a pious feeling has led us to preserve
as far as possible the original author's own form of exposi-
tion and, indeed, his own words. We have done our best
to secure both of these ends.
" Although I share with Dr. Lea the responsibility of all
the changes which have been made, the main labour has
fallen on him ; and I may say that both his larger and my
vi PREFACE
smaller share in the work have been truly labours of love,
small tributes of affection to the master who is no more."
In the preparation of the present edition, which the pub-
lishers desired for use in America, it has been deemed
advisable to make many alterations of the latest English
text, in order to adapt the book to the needs of those classes
of American students who most demand it. The present
writer has performed his task with a long-standing feeling
of affection for the pages which introduced him to the
study of Physiology, and first gave him a clear insight into
the nature of scientific conceptions and scientific reasoning.
FREDERIC S. LEE.
Columbia University, New York,
January, 190x3.
CONTENTS
LESSON I
A GENERAL VIEW OF THE STRUCTURE AND FUNC-
TIONS OF THE HUMAN BODY
PAGE
SEC.
I
The Work of the Body as a Whole
10
2. The General Build of the Body
2. The Tissues
4. The Skeleton
C. The Erect Position I?
20
6. Sensory Organs . . • ••
7. The Renewal of the Tissues 2I
8. Alimentary Organs
9. Circulatory Organs
10. Excretory Organs 23
11. Respiratory Organs 3
12. Coordinating Action of the Nervous System ... 25
13. Life and Death
(i) Local Death 2°
26
27
2$
u-
(ii) General Death
Modes of Death 27
15. Decomposition of the Body
LESSON II
THE MINUTE STRUCTURE OF THE TISSUES
Every Tissue a Compound Structure 3°
The Embryonic Tissues and the Cells. Protoplasm . • 31
The Body starts as a Single < 'ell, the Ovum, which then
divides into Primitive Cells 32
Tii
CONTENTS
4. The Differentiation of the Primitive Cells
5. The Chief Tissues of the Body •
6. The Structure of the Epidermis .
7. The Growth of the Epidermis
8. The Unit used in Histological Measurement
9. The Epithelium of Mucous Membrane
10. The Structure of Cartilage .
(i) Hyaline Cartilage .
(ii) White Fibro-cartilage
(iii) Yellow or Elastic Fibro-cartilage
11. The Development of Cartilage
12. The Structure of the Connective Tissues
(i) Areolar Tissue
(ii) Other Varieties of Connective Tissue
13. The Development of Connective Tissue
PAGE
33
34
35
38
40
4i
42
42
46
46
47
49
49
5i
53
LESSON III
THE VASCULAR SYSTEM AND THE CIRCULATION
Part I. The Blood Vascular System and the
Circulation of Blood
1. The Capillaries . . ' 55
2. The Arteries and Veins 56
(i) The Structure of an Artery . . . . -57
(ii) The Structure of a Vein ...... 59
3. The General Arrangement of Blood-vessels in the Body . 61
4. The Heart .......... 66
5. The Valves of the Heart ....... 69
6. The Structure of the Heart ...... 74
7. The Beat of the Heart 75
8. The Action of the Valves 76
9. The Working of the Arteries ...... 80
10. The Cardiac Impulse ........ 81
11. The Sounds of the Heart 82
12. Blood-pressure . . . . . . . ' . . 83
13. The Pulse .85
CONTENTS u
SEC. PAGE
14. The Rate of Blood Flow . . . ' . . . .89
15. The Nervous Control of the Arteries. Vaso-motor Nerves . 90
16. The Vaso-motor Centre 94
17. Vaso-dilator Nerves . . . . . . .96
18. The Nervous Control of the Heart. Cardiac Nerves . . 98
19. The Proofs of the Circulation 103
20. The Capillary Circulation 105
21. Inflammation ......... 107
Part II. The Lymphatic System and the Circula-
tion of Lymph
1. The General Arrangement of the Lymphatics
2. The Oiigin and Structure of Lymphatics
3. The Structure and Function of Lymphatic Glands
4. Causes which lead to the Movements of Lymph .
109
112
"5
117
LESSON IV
THE BLOOD AND THE LYMPH
1 . Microscopic Examination of Blood
2. The Red Corpuscles .
3. The White Corpuscles
4. Blood Platelets ....
5. The Origin and Fate of the Corpuscles
6. The Physical Qualities of Blood .
7. The General Composition of Blood
8. The Proteids of Plasma
9. The Clotting of Blood
10. The Quantity and Distribution of Blood in the Body
11. The Functions of the Blood
12. Lymph: its Character and Composition
13. The Mode of Formation of Lymph
14. The Functions of the Lymph
119
122
126
130
130
131
132
134
136
140
141
142
144
147
CONTENTS
LESSON V
RESPIRATION
SEC.
1. The Gases of Arterial and Venous Blood
2. The Nature and Essence of Respiration
3. The Organs of Respiration .
4. The Thorax and Pleura
5. The Movements of Respiration
6. The Amount of Air respired
7. The Changes of Air in Respiration
8. The Amount of Waste which leaves the Lungs
9. The Nature of the Respiratory Changes in the Lung:
Tissues . . .
10. The Nervous Mechanism ot Respiration
11. Influence of Blood-supply on the Respiratory Centre
pncea and Asphyxia .....
12. The Influence of Respiration on the Circulation .
13. Ventilation .......
s and
Dys-
152
J54
160
162
172
174
175
177
179
191
LESSON VI
THE SOURCES OF LOSS AND OF GAIN TO
THE BLOOD
1. General Review of the Gain and Loss
2. Secretion in General .
3. The Urinary Organs .
4. The Structure of a Kidney
5. The Urine .
6. The Secretion of Urine
7. The History of Urea .
8. The Structure of the Skin. Nails and Hairs
9. The Composition and Quantity of Sweat
10. The Secretion of Sweat and its Nervous Control
11. A Comparison of the Lungs, Kidneys, and Skin
12. Animal Meat: its Production and Distribution
13. Regulation of Body-temperature by Altered Loss of Heat
193
199
2b 1
203
208
211
213
215
223
224
226
227
229
CONTENTS
14. Regulation of Body-temperature by Altered Production of
Heat .... ... ... 232
15. The Temperature of Fever . . . . ... . 233
16. The Structure of the Liver ....... 233
1 7. The Work of the Liver . 239
18. The Spleen ......... 244
19. The Thymus Gland ........ 246
20. The Thyroid Body or Gland ...... 246
21. The Suprarenal Bodies ....... 247
LESSON VII
THE SOURCES OF LOSS AND OF GAIN TO THE
BLOOD {Continued): THE FUNCTION
OF ALIMENTATION
Part I. Digestion and Absorption
1. Waste made Good by Food ...... 249
2. Food and Food-stuffs ........ 250
A. Nitrogenous Food-stuffs ...... 250
B. Non-nitrogenous Food-stuffs . . . . .251
(i) Fats 251
(ii) Carbohydrates . . . . . ,251
(iii) Salts and Water . . . . .251
3. The Purpose and Means of Digestion ..... 252
4. The Mouth and the Teeth ....... 252
5. The Development of the Teeth . . . . . .257
6. Mastication ......... 260
7. The GEsophagus and Swallowing . . . . .261
8. The Salivary Glands ........ 262
9. Saliva and its Secretion ....... 265
10. The Action of Saliva ........ 267
11. Soluble Ferments or Enzymes ...... 267
12. The Structure of the Stomach ...... 268
13. Gastric Juice and its Secretion ...... 270
14. The Action of Gastric Juice . . . . . .271
15. The General Arrangement and Structure of the Intestines . 274
CONTENTS
SEC. PAGE
1 6. The Structure of the Villi ....... 280
17. Succus Entericus ........ 282
18. The Structure of the Pancreas and its Changes during Secretion 282
19. The Nature and Action of Pancreatic Juice .... 283
20. The Function of Bile ........ 285
21. The Changes undergone by Food in the Intestines . . 285
22. Absorption from the Intestines ...... 288
Part II. Food and Nutrition
1. Some Aspects of Nutrition . . . . . . .291
2. Some Statistics of Nutrition . . . . . .292
3- Diet 295
4. The Economy of a Mixed Diet . . . . . 297
5. The Effects of the Several Food-stuffs ..... 300
6. The Erroneous Division of Food-stuffs into Heat producers
and Tissue-formers . . ..... 302
7. The Income and Expenditure of Energy . 303
LESSON VIII
MOTION AND LOCOMOTION
1. The Source of Active Power and the Organs of Motion
2. Ciliated Epithelium and the Action of Cilia
3. The Structure of Unstriated Muscle
4. The Structure of Striated Muscle
5. The Chemistry of Muscle .
6. The Phenomena of Muscular Contraction
7. The Tetanic Contraction of Muscles .
8. The Various Kinds of Muscles .
Muscles not attached to Solid Levers
Muscles attached to Definite Levers
9. The Structure of Bone
10. The Development of Bone .
11. The Mechanics of Motion. Levers
1 2. The Joints of the Body
13. The Various Movements of the Body .
307
308
310
3"
3i8
320
323
324
324
325
328
335
34i
345
353
CONTENTS xiii
SEC. PAGE
14. The Mechanics of Locomotion ...... 354
15. The Mechanism of the Larynx ...... 356
16. The Voice .......... 361
17. Speech .......... 363
LESSON IX
SENSATIONS AND SENSORY ORGANS
Movement the Result of Reflex Action
Sensations and Consciousness ....
The Special Senses ......
The General Plan of a Sense-organ
The Skin as a Sense-organ ....
(i) The Sensation of Pressure
(ii) The Sensations of Temperature
(iii) The Sensation of Pain ....
(iv) The Localisation of Tactile Sensations .
The Muscular Sense ......
The Sense of Taste ......
The Sense of Smell ......
The Ear and the Sense of Hearing in General
The Membranous Labyrinth ....
(i) The Utricle, the Saccule, and the Membranous
circular Canals .....
(ii) The Membranous Cochlea
(iii) The Organ of Corti ....
The Bony Labyrinth ......
The Middle Ear
(i) The Auditory Ossicles ....
(ii) The Muscles of the Tympanum
The External Ear ......
The Transmission of Sound Waves to the Inner Ear
The Conversion of Sonorous Vibrations into Sensations o
Sound .......
The Mode of Action of the Auditory End-organs
Localisation of Sound .....
Semi
367
369
37°
371
373
378
378
380
38i
382
383
387
392
395
395
399
402
405
406
407
409
410
410
414
4i5
419
xiv CONTENTS
SEC PAGE
1 8. The Functions of the Tympanic Muscles and Eustachian Tube 420
19. The Functions of the Semicircular Canals, the Utricle, and
the Saccule . ' . . . . . . 421
LESSON X
THE ORGAN OF SIGHT
1. The General Structure of the Eye 423
2. The Eye as a Water Camera ...... 428
3. The Mechanism of Accommodation ..... 433
4. The Limits of Accommodation. Use of Spectacles . . 437
5. The Muscles of the Eyeball 439
6. The Protective Appendages of the Eye .... 440
7. The Structure of the Retina . . . . . -441
8. The Sensation of Light ....... 448
9. The " Blind Spot " 449
10. The Duration of a Luminous Impression .... 450
11. Sensations of Light produced without the Action of Light . 451
12. The Functions of the Rods and Cones . . . . .452
13. Sensations of Colour and Colour-blindness .... 453
Colour-blindness . . . . . . . -457
LESSON XI
THE COALESCENCE OF SENSATIONS WITH ONE
ANOTHER AND WITH OTHER STATES
OF CONSCIOUSNESS
1. Sensations may be Simple or Composite
2. Judgments, not Sensations, are Delusive
3. Subjective Sensations ....
4. Delusions of Judgment
t;. The Inversion of the Visual Image
(>. Every Image referred U> an Object
7. The judgment of Distance and Size by the Brightness an
Size of Visual Images ......
459
462
463
465
466
467
CONTENTS
8. The Judgment of Form by Shadows . . . . .470
9. The Judgment of Changes of Form ..... 471
[O. Single Vision with Two Eyes. Corresponding Points . . 472
ti. The Judgment of Solidity ....... 473
LESSON XII
THE NERVOUS SYSTEM AND INNERVATION
1. The General Arrangement of the Nervous System
2. The Investing Membranes of the Cerebro-spinal System
3. The Anatomy of the Spinal Cord and the Roots of the Spinal
Nerves ........
4. The Structural Elements of Nervous Tissue .
5. The Structure of Nerves ......
6. The Minute Structure of the Spinal Cord and Spinal Ganglia 490
The Cells of the Grey Matter. . . . . . 491
The Differences in -Structure of the Spinal Cord at Various
Levels ......... 492
The Structure of a Spinal Ganglion ..... 494
7. The Functions of the Roots of the Spinal Nerves. . . 495
8. The Physiological Properties of a Nerve .... 499
The Electrical Properties of a Nerve . . .. . 502
The Rate of Transmission of a Nervous Impulse . . 503
9. The Functions of the Spinal Cord ..... 506
Reflex Action through the Spinal Cord .... 506
The Paths of Conduction of Impulses along the Spinal Cord 5 1 1
10. The Sympathetic Nervous System
11. The Anatomy of the Brain .
The Corpora Quadrigemina
The Optic Thalami .
The Corpora Striata
The Membranes of the Brain .
12. The Minute Structure of the Brain
The Cerebellum
The Cerebral Cortex
I 3. The Cranial Nerves
475
476
477
481
483
5H
5*7
526
526
528
528
529
530
532
535
xvi CONTENTS
SEC. PAGE
14. The Functions of the Spinal Bulb or Medulla Oblongata . 538
15. The Functions of the Cerebellum ..... 540
.16. The Functions of the Cerebral Hemispheres . . . 542
The Hemispheres the Seat of Intelligence and Will . . 542
Reflex Actions of the Brain ...... 545
Localisation of Function in the Cortex of the Cerebral
Hemispheres ........ 547
17. The Paths of Conduction of Impulses in the Brain . . 551
APPENDIX
ANATOMICAL AND PHYSIOLOGICAL CONSTANTS
I. General Statistics ........ 555
II. Nutrition 557
III. Circulation ......... 557
IV. Respiration 558
V. Cutaneous Excretion ....... 559
VI. Renal Excretion 560
VII. Nervous Action 560
VIII. Histology 560
LESSONS
IN
ELEMENTARY PHYSIOLOGY
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LESSONS
IN
ELEMENTARY PHYSIOLOGY
LESSON I
A GENERAL VIEW OF THE STRUCTURE AND
FUNCTIONS OF THE HUMAN BODY
1. The Work of the Body as a Whole. — The body of
a living man performs a great diversity of actions, some of
which are quite obvious ; others require more or less careful
observation; and yet others can be detected only by the
employment of the most delicate appliances of science.
Thus, some part of the body of a living man is plainly
always in motion. Even in sleep, when the limbs, head, and
eyelids may be still, the incessant rise and fall of the chest
continue to remind us that we are viewing slumber and not
death.
More careful observation, however, is needed to detect
the motion of the heart ; or the pulsation of the arteries ; or
the changes in the size of the pupil of the eye with varying
light ; or to ascertain that the air which is breathed out of
the body is hotter and damper than the air which is taken
in by breathing.
And lastly : when we try to ascertain what happens in the
eye when that organ is adjusted to different distances; or
B I
2 ELEMENTARY PHYSIOLOGY less,
what in a nerve when it is excited ; or of what materials flesh
and blood are made ; or in virtue of what mechanism it is
that a sudden pain makes one start — we have to call into
operation all the methods of inductive and deductive logic ;
all the resources of physics and chemistry ; and all the deli-
cacies of the art of experiment.
The sum of the facts and generalisations at which we
arrive by these various modes of inquiry, be they simple or
be they refined, concerning the actions of the body and the
manner in which those actions are brought about, constitutes
the science of Human Physiology. An elementary outline
of this science, and of so much anatomy as is incidentally
necessary, is the subject of the following Lessons ; of which
we shall devote the present to an account of so much of the
structure and such of the actions (or, as they are technically
called, " functions ") of the body, as can be ascertained by
easy observation ; or might be so ascertained if the bodies
of men were as easily procured, examined, and subjected to
experiment, as those of animals.
Suppose a chamber with walls of ice, through which a cur-
rent of pure ice-cold air passes ; the walls of the chamber will
of course remain unmelted.
Now, having weighed a healthy living man with great care,
let him walk up and down the chamber for an hour. In do-
ing this he will obviously do a considerable amount of work
and use up a proportionate quantity of energy ; as much, at
least, as would be required to lift his weight as high and as
often as he has raised himself at every step. But, in addi-
tion, a certain quantity of the ice will be melted, or converted
into water ; showing that the man has given off heat in abun-
dance. Furthermore, if the air which enters the chamber be
made to pass through lime-water, it will cause no cloudy
white precipitate of carbonate of lime, because the quantity
I WORK AND WASTE 3
of carbonic acid1 in ordinary air is so small as to be inappre-
ciable in this way. But if the air which passes out is made
to take the same course, the lime-water will soon become
milky, from the precipitation of carbonate of lime, showing
the presence of carbonic acid, which, like the heat, is given
off by the man.
Again, even if the air be quite dry as it enters the chamber
(and the chamber be lined with some material so as to shut
out all vapour from the melting ice walls), that which is
breathed out of the man, and that which is given off from
his skin, will exhibit clouds of vapour ; which vapour, there-
fore, is derived from the body.
After the expiration of the hour during which the experi-
ment has lasted, let the man be released and weighed once
more. He will be found to have lost weight.
Thus a living, active man constantly does mechanical
work, gives off heat, evolves carbonic acid and water, and
undergoes a loss of substance.
Plainly, this state of things could not continue for an un-
limited period, or the man would dwindle to nothing. But
long before the effects of this gradual diminution of substance
become apparent to a bystander, they are felt by the subject
of the experiment in the form of the two imperious sensa-
tions called hunger and thirst. To still these cravings, to
restore the weight of the body to its former amount, to ena-
ble it to continue giving out heat, water, and carbonic acid,
at the same rate, for an indefinite period, it is absolutely
necessary that the body should be supplied with each of
1 By " carbonic acid " we mean " carbonic acid gas." This should in
strictness be called carbon dioxide (COo), carbonic acid being the com-
pound of this with water, H2C03. But for simplicity's sake, and because
the expression "carbonic acid" is in general use and is generally under-
stood to stand for carbon dioxide, we shall use it throughout this book.
+ ELEMENTARY PHYSIOLOGY less.
three things, and with three only. These are, first, fresh
air ; secondly, drink — consisting of water in some shape or
other, however much it may be adulterated ; thirdly, food.
That compound known to chemists as proteid matter
(p. 134), and which contains carbon, hydrogen, oxygen,
and nitrogen, must form a part of this food, if it is to sustain
life indefinitely ; and fatty, starchy, or saccharine, i.e. car-
bohydrate matters, together with a certain amount of salts,
ought to be contained in the food, if it is to sustain life
conveniently.
A certain proportion of the matter taken in as food either
cannot be, or at any rate is not, used ; and leaves the body
as excrementitious matter, having simply passed through the
alimentary canal without undergoing much change, and with-
out ever being incorporated into the actual substance of the
body. But, under healthy conditions, and when only so
much food as is necessary is taken, no important proportion
of either proteid matter, or fat, or starchy or saccharine
food, passes out of the body as such. Almost all real food
ultimately leaves the body as waste in the form either of
•water, or of carbonic acid, or of a third substance called
urea, or of certain saline compounds or salts.
Chemists have determined that these products, which are
thrown out of the body and are called excretions, contain,
if taken together, far more oxygen than the food and water
taken into the body. Now, the only possible source whence
the body can obtain oxygen, except from food and water,
is the air which surrounds it.1 And careful investigation of
1 Fresh country air contains in every 100 parts nearly 21 of oxygen
and 79 of nitrogen gas, together with a small fraction of a part (.04) of
carbonic acid, and a variable quantity of watery vapour. The recently
discovered constituent of the atmosphere, argon, is here reckoned in with
the nitrogen.
i WORK AND WASTE 5
the air which leaves the chamber in the imaginary experi-
ment described above would show, not only that it has
gained carbonic acid from the man, but that it has lost
oxygen in equal or rather greater amount to him.
Thus, if a man is neither gaining nor losing weight, the
sum of the weights of all the substances above enumerated
which leave the body ought to be exactly equal to the
weight of the food and water which enter it, together with
that of the oxygen which it absorbs from the air. And this
is proved to be the case.
Hence it follows that a man in health, and " neither gain-
ing nor losing flesh," is incessantly oxidating and wasting
away, and periodically making good the loss. So that if,
in his average condition, he could be confined in the scale-
pan of a delicate spring balance, like that used for weighing
letters, the scale-pan would descend at every meal, and as-
cend in the intervals, oscillating to equal distances on each
side of the average position, which would never be main-
tained for longer than a few minutes. There is, therefore,
no such thing as a stationary condition of the weight of the
body, and what we call such is simply a condition of varia-
tion within narrow limits — a condition in which the gains
and losses of the numerous daily transactions of the econ-
omy balance one another.
Suppose this diurnally-balanced physiological state to be
reached, it can be maintained only so long as the quantity
of the mechanical work done, and of heat, or other force
evolved, remains absolutely unchanged.
Let such a physiologically-balanced man lift a heavy body
from the ground, and the loss of weight which he would
have undergone without that exertion will be increased by
a definite amount, which cannot be made good unless a pro-
portionate amount of extra food be supplied to him. Let
6 ELEMENTARY PHYSIOLOGY less,
the temperature of the surrounding air fall, and the same
result will occur, if his body remains as warm as before.
On the other hand, diminish his exertion and lower his
production of heat, and either he will gain weight, or some
of his food will remain unused.
Thus, in a properly nourished man, a stream of food is
constantly entering the body in the shape of complex com-
pounds containing comparatively little oxygen ; as con-
stantly, the elements of the food (whether before or after
they have formed part of the living substance) are leaving
the body, combined with more oxygen. And the incessant
breaking down and oxidation of the complex compounds
which enter the body are definitely proportioned to the
amount of energy the body gives out, whether in the shape
of heat or otherwise ; just in the same way as the amount
of work to be got out of a steam-engine, and the amount of
heat it and its furnace give off, bear a strict proportion to
its consumption of fuel.
From these general considerations regarding the nature
of life, considered as physiological work, we may turn for
the purpose of taking a like broad survey of the apparatus
which does the work. We have seen the general perfor-
mance of the engine, we may now look at its build.
2. The General Build of the Body. — The human body
is obviously separable into head, trunk, and limbs. In the
head, the brain-case or skull is distinguishable from the
face. The trunk is naturally divided into the chest or
thorax, and the belly or abdomen. Of the limbs there are
two pairs - — the upper, or arms, and the lower, or legs ; and
legs and arms again are subdivided by their joints into parts
which obviously exhibit a rough correspondence — thigh
and upper arm, leg and fore -arm, ankle and wrist, toes and
fingers, plainly answering to one another. And the two last,
1 THE BUILD OF THE BODY 7
in fact, are so similar that they receive the same name of
digits ; while the several joints of the fingers and toes have
the common denomination of phalanges.
The whole body thus composed (without the viscera or
organs which fill the cavities of the trunk) is seen to be
bilaterally symmetrical ; that is to say, if it were split length-
wise by a great knife, which should be made to pass along
the middle line of both the dorsal and ventral (or back and
front) aspects, the two halves would almost exactly resemble
one another.
One-half of the body, divided in the manner described
(Fig. 1, A), would exhibit, in the trunk, the cut faces of
thirty-three bones, joined together by a very strong and
tough substance into a long column, which lies much nearer
the dorsal (or back) than the ventral (or front) aspect of
the body. The bones thus cut through are called the bodies
of the vertebrae. They separate a long, narrow canal, called
the spinal canal, which is placed upon their dorsal side,
from the spacious chamber of the chest and abdomen, which
lies upon their ventral side. There is no direct communi-
cation between the dorsal canal and the ventral cavity.
The spinal canal contains a long white cord — the spinal
cord — which is an important part of the nervous system.
The ventral chamber is divided into the two subordinate
cavities of the thorax and abdomen by a remarkable, partly
fleshy and partly membranous, partition, the diaphragm
(Fig. 1, D), which is concave towards the abdomen, and
convex towards the thorax. The alimentary canal (Fig. 1,
A/.) traverses these cavities from one end to the other,
piercing the diaphragm. So does a long double series ot
distinct masses of nervous substance, which are called
ganglia : these are connected together by nervous cords,
and constitute the so-called sympathetic system (Fig. 1, Sy).
8 ELEMENTARY PHYSIOLOGY less.
The abdomen contains, in addition to these parts, the
two kidneys, one placed against each side of the vertebral
column and connected each by a tube, the ureter, to a
muscular bag, the bladder, lying at the bottom of the abdo-
men; the liver, the pancreas or "sweetbread," and the
spleen. The thorax incloses, besides its segment of the ali-
mentary canal and of the sympathetic system, the heart
and the two lungs. The latter are placed one on each side
of the heart, which lies nearly in the middle of the thorax.
Where the body is succeeded by the head, the upper-
most of the thirty-three vertebral bodies is followed by a
continuous mass of bone, which extends through the whole
length of the head, and, like the spinal column, separates a
dorsal chamber from a ventral one. The dorsal chamber,
or cavity of the skull, opens into the spinal canal. It con-
tains a mass of nervous matter called the brain, which is con-
tinuous with the spinal cord, the brain and the spinal cord
together constituting what is termed the cerebro-spinal sys-
tem (Fig. i, C.S., C.S.). The ventral chamber, or cavity
of the face, is almost entirely occupied by the mouth and
pharynx, into which last the upper end of the alimentary
canal (called gullet or oesophagus) opens.
Thus, the study of a longitudinal section shows us that
the human body is a double tube, the two tubes being com-
pletely separated by the spinal column and the bony axis of
the skull, which form the floor of the one tube and the roof
of the other. The dorsal tube contains the cerebro-spinal
axis ; the ventral tube contains the. alimentary canal, the
sympathetic nervous system, the heart, and the lungs, besides
other organs.
Transverse sections, taken perpendicularly to the axis of
the vertebral column or to that of the skull, show still more
clearly that this is the fundamental structure of the human
THE BUILD OF THE BODY
Fig. i.
A. A diagrammatic section of the human body, taken vertically through the
median plane. C.S. , the cerebro-spinal nervous system; JV, the cavity of the nose;
M, that of the mouth; Al.,AL, the alimentary canal represented as a simple tube;
H, the heart; D, the diaphragm; Sy, the sympathetic ganglia.
B. A transverse vertical section of the head taken along the line a b; letters as
before.
C. A transverse section taken along the line c d; letters as before.
io ELEMENTARY PHYSIOLOGY less.
body, and that the great apparent difference between the
head and the trunk is due to the different size of the dorsal
cavity relatively to the ventral. In the head the former
cavity is very large in proportion to the size of the latter
(Fig. i, B) ; in the thorax or abdomen it is very small
(Fig. i, C).
The limbs contain no such chambers as are found in the
body and the head ; but with the exception of certain
branching tubes filled with fluid, which are called blood-
vessels and lymphatics, are solid or semi-solid, throughout.
3. The Tissues. — Such being the general character and
arrangement of the parts of the human body, it will next be
well to consider into what constituents it may be separated
by the aid of no better means of discrimination than the
eye and the anatomist's knife.
With no more elaborate aids than these, it becomes easy
to separate that tough membrane which invests the whole
body, and is called the skin, or integument, from the parts
which lie beneath it. Furthermore, it is readily enough
ascertained that this integument consists of two portions :
a superficial layer, which is constantly being shed in the
form of powder or scales, composed of minute particles of
horny matter, and is called the epidermis ; and the deeper
part, the dermis, which is dense and fibrous (p. 215).
The epidermis, if wounded, neither gives rise to pain nor
bleeds. The dermis, under like circumstances, is very ten-
der, and bleeds freely. A practical distinction is drawn
between the two in shaving, in the course of which opera-
tion the razor ought to cut only epidermal structures ; for if
it go a shade deeper, it gives rise to pain and bleeding.
The skin can be readily enough removed from all parts
of the exterior, but at the margins of the apertures of the
body it seems to stop, and to be replaced by a layer which
I THE TISSUES "
is much redder, more sensitive, bleeds more readily, and
which keeps itself continually moist by giving out a more or
less tenacious fluid, called mucus. Hence, at these aper-
tures, the skin is said to stop, and to be replaced by mucous
membrane, which lines all those interior cavities, such as the
alimentary canal, into which the apertures open. But, in
truth, the skin does not really come to an end at these points,
but is directly continued into the mucous membrane, which
last is simply an integument of greater delicacy, but consist-
ing fundamentally of the same two layers — a deep, fibrous
layer, called also dermis, and containing blood-vessels, and a
superficial, bloodless one, now called the epithelium. Thus
every part of the body might be said to be contained between
the walls of a double bag, formed by the epidermis, which
invests the outside of the body, and the epithelium, its con-
tinuation, which lines the alimentary canal.
The dermis of the skin, and that of the mucous mem-
branes, are chiefly made up of a filamentous substance,
which yields abundant gelatine on being boiled, and is the
matter which tans when hide is made into leather. This is
called connective tissue,1 because it is the great connecting
medium by which the different parts of the body are held
together. Thus it passes from the dermis between all the
other organs, ensheathing the muscles, coating the bones and
cartilages, and eventually reaching and entering into the
mucous membranes. And so completely and thoroughly
does the connective tissue permeate almost all parts of the
body, that, if every other tissue could be dissected away, a
complete model of all the organs would be left composed of
this tissue. Connective tissue varies very much in character ;
in some places being very soft and tender, at others — as in
1 Every such constituent of the body, as epidermis, cartilage, or muscle,
is called a " tissue." (See Lesson II.)
12 ELEMENTARY PHYSIOLOGY less.
the tendons and ligaments, which are almost wholly com-
posed of it — attaining great strength and density.
Among the most important of the tissues imbedded in
and ensheathed by the connective tissue, are some the pres-
ence and action of which can be readily determined during
life.
If the upper arm of a man whose arm is stretched out
be tightly grasped by another person, the latter, as the
former bends up his fore-arm, will feel a great soft mass,
which lies at the fore part of the upper arm, swell, harden,
and become prominent. As the arm is extended again, the
swelling and hardness vanish.
On removing the skin, the body which thus changes its
configuration is found to be a mass of red flesh, sheathed in
connective tissue. The sheath is continued at each end into
a tendon, by which the muscle is attached, on the one hand,
to the shoulder-bone, and, on the other, to one of the bones
of the fore-arm. This mass of flesh is the muscle called
biceps, and it has the peculiar property of changing its dimen-
sions — shortening and becoming thick in proportion to its
decrease in length — when influenced by the will as well as
by some other causes, called stimuli, and of returning to its
original form when let alone. This temporary change in the
dimensions of a muscle, this shortening and thickening, is
spoken of as its contraction. It is by reason of this property
that muscular tissue becomes the great motor agent of the
body ; the muscles being so disposed between the systems
of levers which support the body, that their contraction
necessitates the motion of one lever upon another.
4. The Skeleton. — These levers form part of the system
of hard tissues which constitute the skeleton. The less hard
of these are the cartilages, composed of a dense, firm sub-
stance, ordinarily known as "gristle." The harder are the
THE SKELETON
*3
Fig. 2. — The Vertebral Column.
A, side view, left side; B, back view; C 1-7, cervical vertebra:; D 1-12, dors?!
(thoracic) vertebrae; L 1-5, lumbar vertebras; S, sacrum; C, coccyx; sp, spinous
processes; tr, transverse processes.
14
ELEM ENTARY PHYSIOLOGY
bones, which are masses of tissue, hardened by being impreg-
nated with phosphate and carbonate of lime. They are ani-
mal tissues which have become, in a manner, naturally
petrified ; and when the salts of lime are extracted, as they
may be, by the action of acids, a model of the bone in
soft and flexible animal matter remains.
Fig. 3. — Side View of the Skull.
/, frontal bone; /, parietal; o, occipital; a, wing of sphenoid; s, flat part of tem-
poral; c, vt, st, other parts of temporal; an, opening of ear or external auditory
canal; z, process of temporal passing to./', the cheek-bone; mx, the upper jaw-bone;
■n, nasal bone; /, lacrymal; pi, part of sphenoid. The lower jaw-bone is drawn
downwards; cy, its process which articulates with the temporal; cr, its process to
which muscles of mastication are attached; th, ty, hyoid bone, the dotted line indi-
cating its attachment by a ligament to the temporal.
More than 200 separate bones are ordinarily reckoned in
the human body, though the actual number of distinct bones
varies at different periods of life, many bones which are
separate in youth becoming united together in old age
THE SKELETON
15
Thus there are originally, as we have seen, thirty-three
separate bodies of vertebrae in the spinal column (Fig. 2),
and the upper twenty-four of these commonly remain dis-
tinct throughout life. But the twenty-fifth, twenty-sixth,
twenty-seventh, twenty-eighth, and twenty-ninth early unite
into one great bone, called the sacrum; and the four remain-
ing vertebrae often run into one bony mass called the
coccyx.
SLIT
disc
Fig.
- The Pelvis
Sac, sacrum; Cocc, coccyx; il, is, pu, ilium, ischium, pubis, three parts of ths
innominate or hip-bone; acet, acetabulum or cup for head of" femur; j L. F, 5th lum-
bar vertebra; (fee, disc of cartilage between vertebrae; R, right; L, left.
In early adult life, the skull contains twenty-two naturally
separate bones, but in youth the number is much greater,
and in old age far less.
Twenty-four ribs bound the chest laterally, twelve on each
side, and most of them are connected by cartilages with the
breast-bone or sternum (Fig. 50, p. 161). In the girdle
n6
ELEMENTARY PHYSIOLOGY
cl
hum
.rad.
Fig. 5. — The Bones ok -iiiic Limbs. Front View. Left Limbs.
A, the innominate and bones of the leg; inn, innominate or hip-bone ;fem, femur;
pat, patella or knee-cap; tit>, tibia;y?/<, fibula; tar, (seven) tarsal bones; metat, (five)
metatarsal bones; phi, (fourteen) phalanges. B, the scapula, clavicle, and bones ol
the arm; cl, clavicle or collar-bone; scap, scapula or shoulder-bone; hum, humerus;
rdd, radius; iilu, ulna; car, (eight) carpal bones; metar, (five) metacarpal bones;
phi. (fourteen) phalanges.
i THE ERECT POSITION 17
which supports the shoulder, two bones are always distin-
guishable as the scapula, or shoulder-blade, and the clavicle,
or collar-bone (Fig. 5, B). The pelvis (Fig. 4), to which
the legs are attached, consists of two separate bones called
the ossa innominata, or hip-bones, in the adult ; but each os
innominatum is separable into three (called pubis, ischium,
and ilium) in the young.
There are thirty bones in each of the arms, and the same
number in each of the legs, counting the patella, or knee-
pan (Fig. 5).
All these bones are fastened together by ligaments, or by
cartilages ; and where they play freely over one another, a
coat of cartilage furnishes the surfaces which come into con-
tact. The cartilages which thus form part of a joint are
called articular cartilages, and their free surfaces, by which
they rub against each other, are lined by a delicate syno-
vial membrane, which secretes a lubricating fluid — the
synovia.
5. The Erect Position. — Though the bones of the skele-
ton are all strongly enough connected together by ligaments
and cartilages, the joints play so freely, and the centre of grav-
ity of the body, when erect, is so high up, that it is impossi-
ble to make a skeleton or a dead body support itself in the
upright position. That position, easy as it seems, is the
result of the contraction of a multitude of muscles which
oppose and balance one another. Thus, the foot affording
the surface of support, the muscles of the calf (Fig. 6, I)
must contract, or the legs and body would fall forward. But
this action tends to bend the leg ; and to neutralise this and
keep the leg straight, the great muscles in front of the thigh
(Fig. 6, 2) must come into play. But these, by the same
action, tend to bend the body forward on the legs ; and if
the body is to be kept straight, they must be neutralised by
G
i8 ELEMENTARY PHYSIOLOGY less.
the action of the muscles of the buttocks and of the back
(Fig. 6, III).
The erect position, then, which we assume so easily and
without thinking about it, is the result of the combined and
i*'io. 6. — A Diagram illustrating the Attachments of some of the most
important Muscles which keep the Body in the Erect Posture.
I. The muscles of the calf. II. Those of the back of the thigh. III. Those of
the spine. These tend to keep the body from falling forward.
r. The muscles of the front of the leg. 2. Those of the front of the thigh.
3. Those of the front of the abdomen. 4, 5. Those of the front of the neck. These
tend to keep the body from falling backward. The arrows indicate the direction o(
action of the muscles, the foot being fixed.
I THE ERECT POSITION 19
accurately proportioned action of a vast number of muscles.
What is it that makes them work together in this way ?
Let any person in the erect position receive a violent
blow on the head, and you know what occurs. On the
instant he drops prostrate, in a heap, with his limbs relaxed
and powerless. What has happened to him? The blow
may have been so inflicted as not to touch a single muscle
of the body; it may not cause the loss of a drop of blood ;
and, indeed, if the " concussion," as it is called, has not been
too severe, the sufferer, after a few moments of unconscious-
ness, will come to himself, and be as well as ever again.
Clearly, therefore, no permanent injury has been done to
any part of the body, least of all to the muscles, but an influ-
ence has been exerted upon a something which governs the
muscles. And a similar influence may be the effect of very
subtle causes. A strong mental emotion, and even a very
bad smell, will, in some people, produce the same effect as
a blow.
These observations might lead to the conclusion that it is
the mind which directly governs the muscles, but a little
further inquiry will show that such is not the case. For
people have been so stabbed, or shot in the back, as to cut
the spinal cord, without any considerable injury to other
parts : and then they have lost the power of standing
upright as much as before, though their minds may have
remained perfectly clear. And not only have they lost the
power of standing upright under these circumstances, but
they no longer retain any power of either feeling what is
going on in their legs, or, by an act of their own will, causing
motion in them.
And yet, though the mind is thus cut off from the lower
limbs, a controlling and governing power over them still re-
mains in the body. For if the soles of the disabled feet be
20 ELEMENTARY PHYSIOLOGY less.
tickled, though the mind does not feel the tickling, the legs
will be jerked up, just as would be the case in an uninjured
person. Again, if a series of galvanic shocks be sent into
the spinal cord, the legs will perform movements even more
powerful than those which the will could produce in an unin-
jured person. And, finally, if the injury is of such a nature
as not simply to divide or injure the spinal cord in one
place only, but to crush or profoundly disorganise it, all
these phenomena cease ; tickling the soles, or sending gal-
vanic shocks along the spine, will produce no effect upon
the legs.
By examinations of this kind carried still further, we arrive
at the remarkable result that, while the brain is the seat of
all sensation and mental action, and the primary source of
all voluntary muscular contractions, the spinal cord is by
itself capable of receiving an impression from the exterior,
and converting it, not only into a simple muscular contrac-
tion, but into a combination of such actions.
Thus, in general terms, we may say of the cerebro-spinal
nervous centres, that they have the power, when they receive
certain impressions from without, of giving rise to simple or
combined muscular contractions.
6. Sensory Organs. — But you will further note that these
impressions from without are of very different characters.
Any part of the surface of the body may be so affected as to
give rise to the sensations of contact, or of heat or cold ;
and any or every substance is able, under certain circum-
stances, to produce these sensations. But only very few
and comparatively small portions of the bodily framework
are competent to be affected in such a manner as to cause
the sensations of taste or of smell, of sight or of hearing :
and only a few substances, or particular kinds of vibrations,
are able so to affect those regions. These very limited parts
I THE ORGANS ?i
of the body, which put us in relation with particular kinds
of substances, or forms of force, are what are termed sen-
sory organs. There are two such organs for sight, two for
hearing, two for smell, and one, or more strictly speaking
two, for taste.
7. The Renewal of the Tissues. — And now that we have
taken this brief view of the structure of the body, of the
organs which support it, of the organs which move it, and
of the organs which put it in relation with the surrounding
world, or, in other words, enable it to move in harmony
with influences from without, we must consider the means
by which all this wonderful apparatus is kept in working
order.
All work, as we have seen, implies waste. The work of
the nervous system and that of the muscles, therefore, im-
plies consumption either of their own substance or of some-
thing else. And as the organism can make nothing, it must
possess the means of obtaining from without that which it
wants, and of throwing off from itself that which it wastes;
and we have seen that, in the gross, it does these things. The
body feeds, and it excretes. But we must now pass from the
broad fact to the mechanism by which the fact is brought
about. The'organs which convert food into nutriment are
the organs of alimentation ; those which distribute nutri-
ment all over the body are organs of circulation ; those
which get rid of the waste products are organs of excretion.
8. Alimentary Organs, — The organs of alimentation are
the mouth, pharynx, gullet, stomach, and intestines, with
their appendages, the pancreas and the liver. What they
do is, first, to receive and grind the food. They then act
upon it with chemical agents, of which they possess a store
which is renewed as fast as it is used ; and in this way
convert the food by processes of digestion into a fluid con-
22 ELEMENTARY PHYSIOLOGY less.
taining nutritious matters in solution or suspension, and
innutritious dregs or faeces.
9. Circulatory Organs. — A system of minute tubes,
with very thin walls, termed capillaries, is distributed
through the whole organism except the epidermis and its
products, the epithelium, the cartilages, and the substance
of the teeth. On all sides, these tubes pass into others,
which are called arteries and veins ; while these, becoming
larger and larger, at length open into the heart, an organ
which, as we have seen, is placed in the thorax. During
life, these tubes and the chambers of the heart, with which
they are connected, are all full of liquid, which is, for the
most part, that red fluid with which we are all familiar as
blood.
The walls of the heart are muscular, and contract rhyth-
mically, or at regular intervals. By means of these contrac-
tions the blood which its cavities contain is driven in jets
out of these cavities, into the arteries, and thence into the
capillaries, whence it returns by the veins back into the
heart.
This is the circulation of the blood.
Now the fluid containing the dissolved or suspended nu-
tritive matters which are the result of the process of diges-
tion, traverses the very thin layer of soft and permeable
tissue which separates the cavity of the alimentary canal
from the cavities of the innumerable capillary vessels which
lie in the walls of that canal, and so enters the blood, with
which those capillaries are filled. Whirled away by the tor-
rent of the circulation, the blood, thus charged with nutri-
tive matter, enters the heart, and is thence propelled into
the organs of the body. To these organs it supplies the
nutriment with which it is charged ; from them it takes their
waste products, and, finally, returns by the veins to the
I THE ORGANS 23
heart, loaded with useless and injurious excretions, which
sooner or later take the form of water, carbonic acid, and
urea.
10. Excretory Organs. — These excretionary matters are
separated from the blood by the excretory organs, of which
there are three — the skin, the lungs, and the kidneys.
Different as these organs may be in appearance, they are
constructed upon one and the same principle. Each, in
ultimate analysis, consists of a very thin sheet of tissue,
like so much delicate blotting-paper, the one face of which
is free, or lines a cavity in communication with the exterior
of the body, while the other is in contact with the blood
which has to be purified.
The excreted matters are, as it were (though, as we shall
see, in a peculiar way), strained from the blood, through this
delicate layer of tissue, and on to its free surface, whence
they make their escape.
Each of these organs is especially concerned in the elimi-
nation of one of the chief waste products — water, carbonic
acid, and urea — though it may at the same time be a means
of escape for the others. Thus, the lungs are especially
busied in getting rid of carbonic acid, but at the same time
they give off a good deal of water. The duty of the kidneys
is to excrete urea (together with other substances, chiefly
salts), but at the same time they pass away a large quan-
tity of water and a trifling amount of carbonic acid ;
while the skin gives off much water, some carbonic acid,
and a certain quantity of saline matter, among which a
trace of urea may be, sometimes, though very doubtfully,
present.
11. Respiratory Organs. — Finally, the lungs play a
double part, being not merely eliminators of waste, or
excretionary products, but importers into the economy of
24 ELEMENTARY PHYSIOLOGY less
a substance which is not exactly either food or drink, but
something as important as either, — to wit, oxygen.
As the carbonic acid (and water) is passing from the
blood through the lungs into the external air, oxygen is pass-
ing from the air through the lungs into the blood, and is
carried, as we shall see, by the blood lo all parts of the body.
We have seen (p. 4) that the waste which leaves the body
contains more oxygen than the food which enters the
body. Indeed oxidation, the oxygen being supplied by the
blood, is going on all over the body. All parts of the body are
thus continually being oxidised, or, in other words, are con-
tinually burning, some more rapidly and fiercely than others.
And this burning, though it is carried on in a peculiar man-
ner, so as never to give rise to a flame, yet nevertheless pro-
duces an amount of heat which is as efficient as a fire to
raise the blood to a temperature of about 370 C. (98. 6° F.) ;
and this hot fluid, incessantly renewed in all parts of the
body by the torrent of the circulation, warms the body, as a
house is warmed by hot-water apparatus. Nor is it alone
the heat of the body which is provided by this oxidation ;
the energy which appears in the muscular work done by the
body has the same source. Just as the burning of the coal
in a steam-engine supplies the motive power which drives
the wheels, so, though in a peculiar way, the oxidation of the
muscles (and thus ultimately of the food) supplies the motive
power of those muscular contractions which carry out the
movements of the body. The food, like coal combustible or
capable of oxidation, is built up into the living body, which,
in like manner combustible, is continually being oxidised by
the oxygen from the blood, thus doing work and giving out
heat. Some of the food perhaps may be oxidised without
ever actually forming part of the body or after it has already
become waste matter, but this does not concern us now.
I LIFE AND DEATH 25
12. Coordinating Action of the Nervous System. —
These alimentary, circulatory or distributive, excretory, and
respiratory (oxidational) processes would however be worse
than useless if they were not kept in strict proportion one to
another. If the state of physiological balance is to be main-
tained, not only must the quantity of food taken be at least
equivalent to the quantity of matter excreted ; but that food
must be distributed with due rapidity to the seat of each
local waste. The circulatory system is the commissariat of
the physiological army.
Again, if the body is to be maintained at a tolerably even
temperature, while that of the air is constantly varying, the
condition of the hot-water apparatus must be most carefully
regulated.
In other words, a coordinating organ must be added
to the organs already mentioned, and this is found in the
nervous system, which not only possesses the function
already described of enabling us to move our bodies and to
know what is going on in the external world ; but makes us
aware of the need of food, enables us to discriminate nutri-
tious from innutritious matters, and to exert the muscular
actions needful for seizing, killing, and cooking ; guides the
hand to the mouth, governs all the movements of the jaws
and of the alimentary canal, and determines the due supply
of the juices necessary for digestion. By it, the working of
the heart is properly adjusted and the calibers of the dis-
tributing pipes are regulated, so as indirectly to govern the
excretory and oxidational processes, which are also addi-
tionally and more directly affected by other actions of the
nervous system.
13. Life and Death. — The various functions which have
been thus briefly indicated constitute the greater part of what
are called the vital actions of the human body, and so long
26 ELEMENTARY PHYSIOLOGY less.
as they are performed, the body is said to possess life. The
cessation of the performance of these functions is what is
ordinarily called death.
But there are really several kinds of death, which may, in
the first place, be distinguished from one another under the
two heads of local and of general death.
(i) Local death is going on at every moment, and in most,
if not in all, parts of the living body. Individual cells of the
epidermis and of the epithelium are incessantly dying and
being cast off, to be replaced by others which are, as con-
stantly, coming into separate existence. The like is true of
blood- corpuscles, and probably of many other elements of
the tissues.
This form of local death is insensible to ourselves, and is
essential to the due maintenance of life. But, occasionally,
local death occurs on a larger scale, as the result of injury,
or as the consequence of disease. A burn, for example, may
suddenly kill more or less of the skin ; or part of the tissues
of the skin may die, as in the case of the slough which lies
in the midst of a boil ; or a whole limb may die, and exhibit
the strange phenomena of mortification.
The local death of some tissues is followed by their regen-
eration. Not only all the forms of epidermis and epithe-
lium, but nerves, connective tissue, bone, and at any rate,
some muscles, may be thus reproduced, even on a large
scale.
(ii) General death is of two kinds, death of the body as a
whole, and death of the tissues. By the former term is im-
plied the absolute cessation of the functions of the brain, of
the circulatory, and of the respiratory organs ; by the latter,
the entire disappearance of the vital actions of the ultimate
structural constituents of the body. When death takes place,
the body, as a whole, dies first, the death of the tissues not
I LIFE AND DEATH 27
occurring until after an interval, which is sometimes consid-
erable.
Hence it is that, for some little time after what is ordi-
narily called death, the muscles of an executed criminal
may be made to contract by the application of proper
stimuli. The muscles are not dead, though the man is.
14. Modes of Death. — The modes in which death is
brought about appear at first sight to be extremely varied.
We speak of natural death by old age, or by some of the
endless forms of disease ; of violent death by starvation, or
by the innumerable varieties of injury, or poison. But, in
reality, the immediate cause of death is always the stoppage
of the functions of one of three organs : the cerebro-spinal
nervous system, the lungs, or the heart. Thus, a man may
be instantly killed by such an injury to a part of the brain
which is called the spinal bulb or medulla oblongata (see
p. 538) as may be produced by hanging, or breaking the
neck.
Or death may be the immediate result of suffocation by
strangulation, smothering, or drowning, — or, in other words,
of stoppage of the respiratory functions.
Or, finally, death ensues at once when the heart ceases to
propel blood. These three organs — the brain, the lungs,
and the heart — have been fancifully termed the tripod of
life.
In ultimate analysis, however, life has but two legs to
stand upon, the lungs and the heart, for death through
the brain is always the effect of the secondary action of
the injury to that organ upon the lungs or the heart. The
functions of the brain cease when either respiration or cir-
culation is at an end. But if circulation and respiration be
kept up artificially, the brain may be removed without caus-
ing death. On the other hand, if the blood be not aerated,
28 ELEMENTARY PHYSIOLOGY less.
its circulation by the heart cannot preserve life ; and, if the
circulation be at an end, mere aeration of the blood in the
lungs is equally ineffectual for the prevention of death.
15. Decomposition of the Body. — With the cessation of
life, the everyday forces of the inorganic world no longer
remain the servants of the bodily frame, as they were during
life, but become its masters. Oxygen, the slave of the liv-
ing organism, becomes the lord of the dead body. Atom
by atom, the complex molecules of the tissues are taken to
pieces and reduced to simpler and more oxidised substances,
until the soft parts are dissipated chiefly in the form of car-
bonic acid, ammonia, water, and soluble salts, and the bones
and teeth alone remain. But not even these dense and earthy
structures are competent to offer a permanent resistance to
water and air. Sooner or later the animal basis which holds
together the earthy salts decomposes and dissolves — the
solid structures become friable, and break down into powder.
Finally, they dissolve and are diffused among the waters
of the surface of the globe, just as the gaseous products of
decomposition are dissipated through its atmosphere.
It is impossible to follow, with any degree of certainty,
wanderings more varied and more extensive than those
imagined by the ancient sages who held the doctrine of
transmigration ; but the chances are, that, sooner or later,
some, if not all, of the scattered atoms will be gathered into
new forms of life.
The sun's rays, acting through the vegetable world, build
up some of the wandering molecules of carbonic acid, of
water, of ammonia, and of salts, into the fabric of plants.
The plants are devoured by animals, animals devour one
another, man devours both plants and other animals ; and
hence it is very possible that atoms which once formed an
integral part of the busy brain of Julius Caesar may now
I CHANGES OF MATTER 29
enter into the composition of Caesar the negro in Alabama,
and of Caesar the house-dog in an English homestead.
And thus there is sober truth in the words which Shake-
speare puts into the mouth of Hamlet —
" Imperious Caesar, dead and turn'd to clay,
Might stop a hole to keep the wind away :
O, that that earth, which kept the world in awe,
Should patch a wall to expel the winter's flaw ! "
LESSON II
THE MINUTE STRUCTURE OF THE TISSUES
1. Every Tissue a Compound Structure. — In the first
chapter attention was directed to the obvious fact that the
substance of which the body of a man or other of the
higher animals is composed, is not of uniform texture
throughout ; but that, on the contrary, it is distinguishable
into a variety of components, which differ very widely from
one another, not only in their general appearance, their
colour, and their hardness or softness, but also in their
chemical composition, and in the properties which they
exhibit in the living state.
In dissecting a limb there is no difficulty in distinguish-
ing the bones, the cartilages, the muscles, the nerves, and
so forth, from one another ; and it is obvious that the other
limbs, the trunk, and the head, are chiefly made up of simi-
lar structures. Hence, when the foundations of anatomical
science were laid, more than two thousand years ago, these
"like " structures which occur in different parts of the
organism were termed homoiomcra, " similar parts." In
modern times they have been termed tissues, and the
branch of biology which is concerned with the investigation
of the nature of these tissues is called Histology.
Histology is a very large and difficult subject, and this
whole book might well be taken up with a thorough dis-
cussion of even its elements. But physiology is, in ultimate
analysis, the investigation of the vital properties of the his-
3°
less, ii THE TISSUES 31
tulogical units of which the body is composed. And even
the elements of physiology cannot be thoroughly compre-
hended without a clear apprehension of the nature and
properties of the principal tissues.
A good deal may be learned about the tissues without
other aid than that of the ordinary methods of anatomy,
and it is extremely desirable that the student should acquire
this knowledge as a preliminary to further inquiry. But the
chief part of modern histology is the product of the appli-
cation of the microscope to the elucidation of the minute
structure of the tissues ; and this has had the remarkable
result of proving that these tissues themselves are made
up of extremely small homoiomera, or similar parts, which
are primitively alike in form in all the tissues.
Every tissue therefore is a compound structure : a mul-
tiple of histological units, or an aggregation of histological
elements ; and the properties of the tissue are the sum of
the properties of its components. The distinctive charac-
ter of every fully- formed tissue depends on the structure,
mode of union, and vital properties of its histological ele-
ments when they are fully formed.
2. The Embryonic Tissues and the Cells. Protoplasm.
— Each tissue can be traced back to a young or embryonic
condition, in which it has no characteristic properties, and
in which its histological elements are so similar in structure,
mode of union, and vital properties to those of every other
embryonic tissue, that our present means of investigation
do not enable us to discover any difference among them.
These embryonic, undifferentiated, histological elements,
of which every tissue is primitively composed, or, as it
would be more correct to say, which, in the embryonic
condition, occupy the place of the tissues, are technically
named cells. The colourless blood-corpuscle (p. 126) is
32 ELEMENTARY PHYSIOLOGY les:.
a typical representative of such a cell. And it is substan-
tially correct to say (i) that the histological elements of
every tissue are modifications or products of such cells ;
(2) that every tissue was once a mass of such cells more
or less closely packed together; and (3) that the whole
embryonic body was at one time nothing but an aggrega-
tion of such cells. In its undifferentiated condition, each
cell consists mainly of a soft, colourless mass of living sub-
stance, in consistency somewhat thicker than raw white of
egg, and containing more or less non-living material. There
is imbedded in it a somewhat denser body, also mainly
living, termed the nucleus. The living substance, whether
occurring in the body of the cell or in the nucleus, is called
protoplasm. This substance is the material basis of life
wherever life occurs, whether in the human body, in the
bodies of lower animals, or in plants.
Besides the living cells, every tissue contains a greater
or less quantity of lifeless substance, lying among the cells,
and hence called intercellular sub-
stance. This is produced at some
time by the living cells. As will be
seen, it varies greatly in quantity and
characteristics in the different tissues.
3. The Body starts as a Single
Cell, the Ovum, which then divides
FlG' _IoCumAM °F ™E into Primitive Cells. — The body of
a, granular protoplasm; a man or of any of the higher ani-
ves?de""Sc,Cnucieoi'uff raffed mals commences as an ovum or egg.
"germinal spot." Thig ^ pig_ ^ ;g R minute spheroidal
body 200 fx (T|-g- of an inch) in diameter in man, consisting
of protoplasm, in which a single large nucleus is imbedded,
and covered by a transparent membrane.
The first step towards the production of all the com-
ii THE TISSUES 33
plex organisation of a mammal out of this simple budy is
the division of the nucleus into two new nuclei, which re-
cede from one another, while at the same time the proto-
plasmic body becomes divided, by a narrow cleft which
runs between the two nuclei, into two masses, or blasto-
meres (Fig. 8, a), one for each nucleus. By the repetition
of the process the two blastomeres give rise to four, the
four to eight, the eight to sixteen, and so on, until the
embryo is an aggregate of numerous small blastomeres, or
nucleated cells. These grow at the expense of the nutri-
ment supplied from without, and continue to multiply
by division according to the tendencies inherent in each
until, long before any definite tissue has made its appear-
ance, they build themselves up into a kind of sketch
model of the developing animal, in which model many of,
if not all, the future organs are represented by mere aggre-
gates of undifferentiated cells.
4. The Differentiation of the Primitive Cells. —
Gradually, these undifferentiated cells become changed,
as regards their shape, size, and structure, into groups or
sets of differentiated cells, the cells in one set being like
each other, but unlike those of other sets. Each set of
differentiated cells constitutes a " tissue," and each tissue
is variously distributed among the several organs, each
organ generally consisting of more than one tissue.
And this differentiation of form is accompanied by a
change of properties. The undifferentiated cells are, as
far as we can see, alike in function and properties as they
are alike in form. But coincident with their differentiation
into tissues, a division of labour takes place, so that in one
tissue the cells manifest special properties and carry on
a special work ; in another they have other properties, and
other work ; and so on.
D
34
ELEMENTARY PHYSIOLOGY
5. The Chief Tissues of the Body. — The principal
tissues into which the undifferentiated cells of the embryo
become differentiated, and which are variously built up into
the organs and parts of the adult body, may be arranged as
follows.
Fig. 8. — The Successive Division of the Mammalian Ovum into Blasto-
meres. Somewhat diagrammatic.
a, division into two, b, into four, c, into eight, and d, into many blastomeres.
The clear ring seen in each case is the zona pellucida, or membrane investing the
ovum.
(i) The most important tissues are the muscular and ner-
vous tissues, for it is by these that the active life of the
individual is carried on.
(ii) Next come the epithelial tissues, which, on the one
hand, afford a covering for the surface of the body as well as
a lining for the various internal cavities, and, on the other
hand, carry on a great deal of the chemical work of the
body, inasmuch as they form the essential part of the various
glandular organs.
ii THE EPIDERMIS 35
(iii) The remaining principal tissues of the body, namely
the so-called connective tissue, cartilaginous tissue, and
osseous or bony tissue, form a group by themselves, being
all three similar in their fundamental structure, and all three
being, for the most part, of use to the body for iheir passive
rather than for their active qualities. They chiefly serve to
support and connect the other tissues.
These principal or fundamental tissues are often arranged
together to form more complex parts of the body, which are
sometimes spoken of, though in a different sense, as tissues.
Thus, various forms of connective tissue are built up, with
some muscular tissue and nervous tissue, to form the blood-
vessels (see Lesson III.), which are sometimes spoken of as
"vascular tissue." So, again, a certain kind of epithelial
tissue, known as " epidermis," together with connective
tissue, blood-vessels, and nerves, forms the skin or tegument-
ary tissue ; a similar combination of epithelium with other
tissues constitutes the mucous membrane lining the ali-
mentary canal, and also occurs in the so-called "glandular"
tissue. The structure of these, as also of muscle and nerve
and bone, will be described later, and we may confine our
attention here to the other principal tissues : epithelial
tissues, the connective tissues, and cartilage.
6. The Structure of the Epidermis. — A good example
of this tissue is to be found in the skin, which consists of the
superficial epidermis, which is non-vascular and epithelial in
nature, and of the deep dermis, which is vascular, and is, in-
deed, chiefly composed of connective tissue carrying blood-
vessels and nerves (Fig. 65, p. 216). And in all the mucous
membranes there is a similar superficial epithelial layer,
which is there simply called epithelium, and a deep layer,
which similarly consists of connective tissue carrying blood
vessels and nerves and may also be spoken of as dermis.
36 ELEMENTARY PHYSIOLOGY less
If a piece of fresh skin is macerated for some time in
water, it is easy to strip off the epidermis from the dermis.
The outer part of the epidermis which has been detached
by maceration will be found to be tolerably dense and cohe-
rent, while its deep or inner substance is soft and almost
gelatinous. Moreover, this softer substance fills up all the
irregularities of the surface of the dermis to which it adheres,
and hence, where the dermis is raised up into papilla?, the
deep or under surface of the epidermis presents innumer-
able depressions, into which the papilla? fit, giving it an
irregular appearance, somewhat like a network. Hence it
used not unfrequently to be called the network of Malpighi
(rete Malpighii) , after a great Italian anatomist of the sev-
enteenth century, who first properly described it. On the
other hand, its soft and gelatinous character led to its being
called mucous layer {stratum mucosuni). Its common
name is Malpighian layer. Chemical analysis shows that
the firm outer layer of the epidermis differs from the
deep soft part by containing a great deal of horny matter.
Hence this is distinguished as the horny layer {stratum
corneum).
In the living subject the superficial layers of the epi-
dermis become separated from the lower layers and
the dermis, when friction or other irritation produces a
" blister." Fluid is poured out from the vessels of the
dermis, and, accumulating between the upper and lower
layers of the epidermis, detaches the former.
The epidermis is constantly growing upon the deep or
dermal side in such a manner that the horny layer is con-
tinually being shed and replaced. The "scurf" which
collects between the hairs and on the whole surface of the
body, and is removed by our daily brushing and washing,
is nothing but shed epidermis. When a limb has been
II THE EPIDERMIS 37
bandaged up and left undisturbed for weeks, as in case of
a fracture, the shed epidermis collects on the surface of the
skin in the shape of scales and flakes, which break up into
a fine white powder when rubbed. Thus we " shed our
skins " just as snakes do, only that the snake sheds all his
dead epidermis as a coherent sheet at once, while we shed
ours bit by bit, and hour by hour.
What is the nature of the process by which the epidermis
is continually removed ?
If a little of the epidermal scurf is mixed with water and
examined under a power magnifying 300 or 400 diameters, it
will seem to consist of nothing but irregular particles of very
various sizes and with no definite structure. But if a little
caustic potash or soda is previously added to the water, the
appearance will be changed. The caustic alkali causes the
horny substance to swell up and become transparent ; and
this is now seen to consist of minute separable plates, some
of which contain a rounded body in the interior of the plate,
though in many this is no longer recognisable. In fact, so
far as their form is concerned, these bodies have the char-
acter of nucleated cells, in which the protoplasmic body
has been more or less extensively converted into horny
substance.
Thus the cast-off epidermis in reality consists of more or
less coherent masses of cornified nucleated cells.
There is a yet simpler method of demonstrating this truth.
At the margins of the lips the epidermis is continued into
the interior of the mouth, and, though it now receives the
name of epithelium, it differs from the rest of the skin in no
essential respect except that it is very thin, and allows the
blood in the vessels of the subjacent dermis to shine through.
Let the lower lip be turned down, its surface very gently
scraped with a blunt-edged knife, and the substance removed
38 ELEMENTARY PHYSIOLOGY less.
be spread out, covered with a thin glass, and examined as
before. The whole field of view will then be seen to be
spread over with flat irregular bodies very like the epidermal
scales, but more transparent, and each provided with a
nucleus in its centre (Fig. 9).
Fig. 9. — Two Epithelial Scales from the Interior of the Mouth.
A small nucleus n is seen in each, as well as fine granulations in the body of the
cell. The edges of the cells are irregular from pressure. Magnified about 400
times.
Since these detached scales are always to be found on the
inner surface of the lip, it follows that they are always being
thrown off.
7. The Growth of the Epidermis. — The horny external
layer of the epidermis is composed of coherent cornified
flattened cells, which are constantly becoming detached
from the soft internal layer, and must needs be, in some
way, derived from it. But in what way ? Here microscopic
investigation furnishes the answer. For, if the soft layer is
properly macerated, it breaks up into small masses of nucle-
ated protoplasmic substance, that is, into nucleated cells,
which in the innermost or deepest part of the layer are colum-
nar in form, being elongated perpendicularly to the face of
the dermis, on which they rest, and which in the interme-
diate region present transitions in form and othei respects
between these and the shed scales.
II GROWTH OF EPIDERMIS 39
A thin vertical section of epidermis (see Fig. 65, p. 216),
in undisturbed relation with the subjacent dermis, leaves not
the smallest doubt (a) that the epidermis consists of noth-
ing but nucleated cells, with perhaps an infinitesimal amount
of cementing substance between them ; {b) that, from the
deep to the superficial part of the epidermis, the cells always
present a succession from columnar or subcylindrical, proto-
plasmic forms to flattened, completely cornified forms. And,
since we know that the latter are constantly being thrown
off, it follows (Y) that these gradations of form represent
cells of the deep layer which are continually passing to the
surface and there being thrown off.
What is the cause of this constant succession ? To this
question, also, microscopic investigation furnishes a clear
answer. The deeper cells are constantly growing and then
multiplying by a process of division in such a manner that
the nucleus of a cell divides into two new nuclei, around
each of which one-half of the protoplasmic body disposes
itself. Thus one cell becomes two, and each of these grows
until it acquires its full size at the expense of the nutritive
matters which exude from the vessels with which the dermis
is abundantly supplied ; such a cell, in fact, possesses the
vital properties of a primitive embryonic cell.
The cells nearer the dermis are more immediately and
abundantly supplied with nourishment from the dermal
blood-vessels, and serve as the focus of growth and multi-
plication for the whole epidermis ; they are, in fact, the pro-
genitors of the superficial cells, which, as they are thrust
away by the intercalation of new cells between the last
formed and the progenitors, become metamorphosed in
form and chemical character, and at last die and are cast off.
And it follows that the epidermis is to be regarded as a
compound organism made up of myriads of cells, each of
40 ELEMENTARY PHYSIOLOGY less.
which follows its own laws of growth and multiplication, and
is dependent upon nothing save the due supply of nutriment
from the dermal vessels. The epidermis, so far, stands in
the same relation to the dermis as does the turf of a meadow
to the subjacent soil.
8. The Unit used in Histological Measurement. —
Structures which are rendered clearly distinguishable only
by a magnifying power of 300 or 400 diameters must needs
be very small, and it is desirable that, before going any fur-
ther, the learner should try to form a definite notion of their
actual and relative dimensions by comparison with more
familiar objects. A hair of the human head of ordinary
fineness has a diameter of about ¥^~q (say 0-003) of an inch,
or 0-08 mm. (millimetre). The hairs which constitute the
fur of a rabbit, on the other hand, are very much finer, and
the thickest part of the shaft usually does not exceed 10100
of an inch, i.e. o-ooi inch, or about 0-025 mnl- J while the
fine point of such a hair may be as little as sjinru °^ an
inch, about o-ooi mm., or even less, in diameter.
In microscopic histological investigations the range of
the magnitudes with which we have to deal ordinarily lies
between o-i and o-ooi millimetre; that is to say, roughly,
between one two hundred and fiftieth and one twenty-five
thousandth of an inch. It is, therefore, extremely conven-
ient to adopt, as a unit of measurement, o-ooi millimetre,
called a micro-millimetre, and indicated by the symbol /*,
of which all greater magnitudes are multiples.1 Thus, if the
extreme point of a rabbit's hair has a diameter of i/x, the
middle of the shaft will be 25^, and the shaft of a hair of
the human head 80 fx.
Adopting this system, the deep cells of epidermis have on
1 Since 1 millimetre is very nearly equal to jfe of an inch, ^ = jeJoo of an
inch.
a EPITHELIUM 41
an average a diameter of 12/x or more, the nuclei of 4/^ to
5jh, while the superficial cells are plates of about 25/A, the
nuclei retaining about the same dimensions.
9. The Epithelium of Mucous Membrane. — The mucous
membrane lining the alimentary canal, as has been stated, is
framed on the plan of the skin, inasmuch as it consists of a
vascular dermis, and a non-vascular epithelium, the latter
being composed of cells in juxtaposition. But, except in
the region of the mouth, where the epithelium, like the
epidermis, is composed of many layers of cells, arranged as
a soft Malpighian layer and a horny layer, and the oesopha-
gus, where the structure is similar, the epithelium of the ali-
mentary canal and the continuations of that epithelium into
the various glands, such as the salivary glands, glands of
the stomach, the pancreas, the liver, etc., consist of hardly
more than a single layer of cells placed side by side.
Hence in a vertical section of the mucous membrane the
vascular part is seen to be covered by a single row of soft,
nucleated cells ; though sometimes a second row of incon-
spicuous small cells may be seen below the latter. The
cells constituting this single layer vary in shape, being cylin-
drical or conical or, as especially in the glands, cubical or
sphenoidal ; but they always are delicate masses of proto-
plasm, each containing a nucleus.
In the air passages of the lungs and in certain other places
the epithelium of the mucous membrane consists again of
several layers of cells, but all are soft and protoplasmic nucle-
ated masses, the uppermost layer being cylindrical in form.
The exposed ends of the cells in the uppermost layer are cov-
ered by innumerable minute, hair-like projections from the
body of the cells, like the nap of velvet, and called cilia ;
such epithelium is called ciliated epithelium (see Fig. 90,
p. 308). During life the cilia are in constant waving
42 ELEMENTARY PHYSIOLOGY less.
motion, sweeping along foreign matter that comes in contact
vith them, and thus protecting the cells.
Lastly, the blood-vessels and lymphatic vessels and the
large cavities, such as the chest and abdomen, are lined by
a peculiar epithelium, different in origin from the epithelium
of the skin and mucous membranes. It consists of a single
layer of flat, nucleated plates, cemented together at their
edges. The form of the plate or cell varies, being some-
times polygonal, sometimes spindle-shaped, sometimes quite
irregular.
10. The Structure of Cartilage. — A second group of
tissues, of which cartilage may be taken as the simplest form
and the type, differs from epithelium in a very essential
feature. In epithelium, wherever it is found, the cells are
placed close together, and the amount of material existing
between the cells, or intercellular material, is exceedingly
small. In the group of tissues, however, to which cartilage
belongs, a very considerable quantity of intercellular mate-
rial is, as we shall see, developed between the individual
nucleated protoplasmic cells. Hence the cells are, more or
less, distinctly imbedded in a substance different from them-
selves and called a matrix. In epithelium, though the cells
are sometimes joined together by a cement material, this is
never abundant enough to deserve the name of matrix.
(i) Hyaline Cartilage. — Characteristic specimens of this
tissue are to be found in the " sterno-costal cartilages,"
which unite many of the ribs with the breast-bone. A thin
but tough layer of vascular connective tissue invests, and
closely adheres to, the surface of the cartilage. It is termed
the perichondrium. The substance of the cartilage itself is
devoid of vessels; it is hard, but not very brittle, for it will
bend under pressure ; and, moreover, it is elastic, returning
to its original shape when the pressure is removed. It may
II CARTILAGE 43
easily be cut into very thin slices, which are as transparent
as glass, and to the naked eye appear homogeneous. Dilute
acids and alkalies have no effect upon it in the cold ; but, if
it is boiled in water, it yields a substance similar to gelatin
but somewhat different from it, which is called chondrin.
The sterno-costal cartilages of an adult man are many
times larger than those of an infant. It follows that these
cartilages must grow. The only source from whence they
can derive the necessary nutritive material is the plasma
exuded from the blood contained in the vessels of the peri-
chondrium. The vascular perichondrium, therefore, stands
in the same relation to the non-vascular cartilaginous tissue
as the vascular dermis does to the non-vascular epidermis.
But, since the cartilage is invested on all sides by the peri-
chondrium, it is clear that no part of the cartilage can be
shed in the fashion that the superficial layers of epidermis
are got rid of. As the nutritive materials, at the expense of
which the cartilage grows, are supplied from the perichon-
drium, it might be concluded that the cartilage grows only
at its surface. But, if a piece of cartilage is placed in a
staining fluid, it will be found that it soon becomes more or
less coloured throughout. In spite of its density, therefore,
cartilage is very permeable, and hence the nutritive plasma
also may permeate it, and enable every part to grow.
If a thin section of perfectly fresh and living cartilage is
placed on a glass slide, either without addition or with only
a little serum, it appears to the naked eye, as has been said,
to be as homogeneous as a piece of glass. But the employ-
ment of an ordinary hand magnifier is sufficient to show that
it is not really homogeneous, inasmuch as minute points of
less transparency are seen to be scattered singly or in groups
throughout the thickness of the section. When the section
is examined with the microscope (Fig. 10) these points
44 ELEMENTARY PHYSIOLOGY less*
prove to be nucleated cells, the cartilage corpuscles, vary,
ing in shape, but generally more or less spheroidal, some-
times far apart, sometimes very near, or in groups in contact
with one another, in which last case the applied sides are flat.
Usually each cell has a single nucleus, but sometimes there
are two nuclei in a cell. And sometimes globules of fat
appear in the protoplasmic bodies of the cells, and may
completely fill them.
& : I
-/S: V
Fig. io. — Hyaline Cartilage. A Thin Section highly Magnified.
m, matrix; a, a group of two cartilage cells; b, a group of four cells; c, a cell;
n, nucleus.
As a rule each cell lies in, and exactly fills, a cavity in
the transparent matrix, or intercellular substance, which
constitutes the chief mass of the tissue. But a pair of
closely opposed flattened cells may occupy only one cavity,
and all sorts of gradations may be found between hemi-
spheroidal cells in contact, and hemi-spheroidal cells sepa-
rated by a mere film of intercellular substance, and widely
separate spheroidal, ellipsoidal, or otherwise shaped cells.
In size, the cells vary very much, some being as small as
io/a, and others as large as 50/x., or even larger.
As the cartilage dies, and especially if water is added to
it, the protoplasmic bodies of the cells shrink and become
irregularly drawn away from the walls of the cavities which
CARTILAGE
45
contain them, and the appearance of the tissue is greatly
altered.
No structure is discernible in the matrix or intercellular
substance under ordinary circumstances ; but it may be
split up into thin sheets or laminae. The portions of matrix
immediately surrounding the several cavities sometimes
differ in appearance and nature from the rest of the matrix,
so as to constitute distinct capsules (Fig. ii, r) for the
Fig. ii. — A Small Portion of a Section of Articular Cartilage (Frog), verv
highly magnified (6oodiam.).
j, matrix or intercellular substance; /, the protoplasmic body of a cartilage cor-
puscle; n, its nucleus; «', nucleoli; c, the capsule, or wall of the cavity in which
the cartilage corpuscle lies. The four cells here figured seem to have arisen from a
single cell, by division, first into two and then into four. The shading of the matrix
in an oblique line indicates the earlier division into two.
cells ; and, at times, the matrix may by appropriate methods
be split up into pieces, each belonging to and surrounding
a cell, or group of cells, and often disposed in concentric
layers.
Close to the perichondrial surface of the cartilage the
cells become smaller and separated by less intercellular
46
ELEMENTARY PHYSIOLOGY
substance, until at length the transparent chondrigenous
material is replaced by the fibrous collagenous substance of
connective tissue (p. 51), and the cartilage cells take on
the form of " connective tissue corpuscles."
(ii) White Fibro-cartilage. — Since cartilage is a tissue
which serves chiefly for the purposes of supporting and con-
necting other structures of the body, it requires, in certain
positions, to be somewhat more tough and resistant, less
brittle and more flexible, than in others. Thus, in some
joints, as, for instance, the knee, there are little pads or
discs of cartilage between the ordinary articular cartilage
rr^ry^-
Fig. 12. - Section of White Fibro Cartilage. (Hardy.)
(see Fig. 104, c). Similar discs lie in between and are at-
tached to the bodies of the vertebrae. They act not only
as a sort of cushion to break the "jar" arising from a sud-
den concussion of the vertebral column, but also bind the
vertebrae into a column which is resistant but at the same
time flexible. The additional strength required by the car-
tilages of these discs is provided by the introduction into
their matrix of bundles of white fibrous connective tissue ;
hence the name, white fibro-cartilage (Fig. 12).
(iii) Yellow or Elastic Fibro-cartilage. — In certain other
parts of the body cartilage is required to be peculiarly
elastic and flexible, as in the epiglottis and cartilage of the
external ear. In this case the requisite elasticity is given to
GROWTH OF CARTILAGE
47
it by the introduction into the matrix of a dense feltwork of
fibres of yellow or elastic connective tissue (Fig. .13).
11. The Development of Cartilage. — In a very young
embryo we find in the place of a sterno-costal cartilage noth-
ing but a mass of closely applied, undifferentiated, nucleated
cells, having the same essential characters as the deepest
epidermal cells. The rudiment, or embryonic model of
the future cartilage thus constituted, increases in size by
the growth and division of the cells. But, after a time, the
characteristic intercellular substance appears, at first in small
Fig. 13. — Section of Yellow Elastic Cartilage. (Hardy.)
quantity, between the central cells of the mass, and a deli-
cate sterno-costal cartilage is thus formed. This is con-
verted into the full-grown cartilage (a) by the continual
division and subsequent growth to full size of all its cells,
and especially of those which lie at the surface ; (/>) by the
constant increase in the quantity of intercellular substance,
especially in the case of the deeper part of the cartilage.
The manner in which this intercellular substance is in-
creased is not certainly rriade out. If the outermost layer
only of each of the protoplasmic bodies of adjacent cells of
the epidermis were to become cornified and fused together
into one mass, while the remainder of each cell continued
to grow and divide and its progeny threw off fresh outer
48 ELEMENTARY PHYSIOLOGY less.
cornified layers, we should have an epidermal structure
which would resemble cartilage except that the "intercellular
substance " would be corneous and not chondrigenous. And
it is possible that the intercellular substance of cartilage may
be formed in this way. But it is possible that the chondrig-
enous material may be, as it were, secreted by and thrown
out between the cells, as we shall find the cells of glands to
secrete the gland products, or at all events manufactured
in some way by the agency of the cells, without the sub-
stance of the cells being actually transformed into it. Thus,
the capsule, of each cell may be such a secretion, which
then fuses into the adjacent matrix. Our knowledge will
not at present permit us to form a definite judgment on this
point. One thing, however, seems certain, viz., that the
cells are in some way concerned in the matter ; the matrix
is unable to increase itself in the entire absence of cells.
The embryonic cells which give rise to cartilage are
not distinguishable, by any means we at present possess,
in any important respect from those which give rise to
epidermis.
Nevertheless, the similar form must disguise a different
molecular machinery, inasmuch as the two, when developing
under the conditions of temperature, oxygen, and nutriment
to which they are exposed in the living body, produce tissues
which differ so widely as cartilage and epidermis.
The embryonic cartilage cells, like the embryonic epider-
mal cells, are living organisms in which certain definitely
limited possibilities of growth and metamorphosis are
inherent, as they are in those equally simple organisms, the
spores of the common moulds, Penicillium and Mucor.
Given the proper external conditions, the latter grow into
moulds of two different kinds, while the former grow into
cartilage and horny plates.
CONNECTIVE TISSUES
49
12. The Structure of Connective Tissues.
(i) Areolar Tissue. — If a specimen of the loose subcuta-
neous tissue which binds the skin to the body, or of the
similar tissue from between the muscles of a limb, be exam-
ined, it is found to be a soft stringy substance. If a small
portion is carefully spread out in fluid on a glass slide and
examined without the aid of any microscope, it is seen to
consist of semi-transparent whitish bands and fibres, of very
various thicknesses, interlaced so as to form a network, the
c—
JsAm
iftJ
Fig. 14. — Connective Tissue Fibres.
a. small bundles of white fibres: b, larger bundles: c, single elastic fibres.
meshes of which are extremely irregular. Hence the oldei
anatomists termed this tissue areolar or cellular.
When a specimen of fresh connective tissue is prepared
for the microscope in its own fluid, it is seen to consist
largely of strings and threads varying extremely in thickness,
which cross one another in all directions and are often
wavy (Fig. 14). A few of the threads are seen to be sharply
defined fibres of a strongly refracting substance (Fig. 15).
E
5°
ELEMENTARY PHYSIOLOGY
When occurring in mass the latter appear yellowish in
color. They are very elastic and are unaffected by even
strong acids or alkalies or by prolonged boiling. They are
called elastic fibres.
The majority of the threads are pale and not darkly con-
toured. All the thicker strings present a fine longitudinal
striation, due to their being composed of extremely fine
fibrilke (Fig. 16, A). These pale threads, whether occur-
ring singly or in bundles, are known as white fibres. They
differ from the elastic fibres in being smaller and of a differ-
ent chemical nature. When subjected to acids or alkalies,
Fig. 15. — Elastic Fibres of Connective Tissue, forming a Luose Xetwork.
Obtained by special preparation from subcutaneous tissue. Magnified 800
diameters.
they swell up and acquire a glassy transparency (Fig. 16, B).
When boiled in water, they dissolve into gelatin, from which
fact they are sometimes called collagenous fibres.
With care certain cells may also be seen in fresh, living,
connective tissue, but they are most distinctly visible when
the tissue is treated with dilute acetic acid. These cells, or
connective tissue corpuscles, as they are called (just as
cartilage cells are called cartilage corpuscles), when seen in
fresh tissue, care being taken to prevent the post-mortem
changes which they readily undergo, are found to be flat-
CONNECTIVE TISSUES
51
tened plates, almost like epithelial scales, but with very
irregular contours (Fig. 17). They closely adhere to and
are, as it were, bent round the convex faces of the larger
bundles of white fibres.
Thus, connective tissue resembles cartilage in so far as it
consists of cells separated by a large quantity of intercellular
substance ; but this intercellular substance is soft, areolated,
fibrous, and, for the most part, either collagenous or elastic,
in contradistinction from that of cartilage, which is hard,
solid, laminated, and chondrigenous.
A. A small bundle of connective tissue, showing longitudinal fibrillation, and at
g and b encircling (annular, spiral) fibres Magnified 400 diameters.
B. A similar bundle swollen and rendered transparent by dilute acid. The en-
circling fibres are seen at a, a, a.
Besides these fixed connective tissue corpuscles as they
are called, white blood corpuscles (p. 126), or lymph cor-
puscles, or bodies exceedingly like them, are found lying
loose in the fluid which occupies the meshes of the network
of fibres, and appear to wander or travel through the spaces
of the network by virtue of their power of amoeboid move-
ment. Such cells are spoken of as wandering or migratory
cells.
(ii) Other Varieties of Connective Tissue. — Such are the
characters of that which may be regarded as a typical speci-
men of connective tissue. But in different parts of the
52 ELEMENTARY PHYSIOLOGY less.
body this tissue presents great differences, alL of which,
however, are dependent upon the different relative extent to
which the various elements of the tissue are developed.
Thus, {a) the intercellular substance may be very much
reduced in amount in proportion to the cells, as is the case
in the superficial layer of the dermis and some other places,
(b) The intercellular substance may be abundant, and
the white fibres strongly marked and arranged in close-set
parallel bundles, leaving mere clefts in place of the wide
meshes of ordinary connective tissue. This structure is
seen in tendons and most ligaments and is called fibrous
tissue.
V
Pft-:.
%
Fig. 17. — Two Connective Tissue Cokpuscles.
Each is seen to consist of a branched protoplasmic body, containing a nucleus.
Very highly magnified.
(c) The elastic fibres may predominate, as in the vocal
cords, and the strong ligament {ligamentum nuchce) at the
back of the neck (Fig. 109, b), which is so highly developed
in long-necked animals, such as the horse. Such tissue is
called elastic tissue.
(d) The white fibrous or elastic elements may abound,
but a greater or less amount of chondrigenous substance
may be developed around the corpuscles. These are the
fibro- cartilages which we have already described, and which
present every transition between ordinary cartilage and
ordinary connective tissue (epiglottis, intervertebral liga-
VARIETIES OF CONNECTIVE TISSUE
53
merits) . Where a tendon is inserted into a cartilage, as in
the case of the tendo Achi/lis, the powerful tendon that
stretches from the calf-muscle of the leg into the bone of
the heel, the passage of the cartilage into the tendon is
beautifully displayed. The intercellular substance of the
cartilage gradually takes on the characters of that of the
tendon, and the corpuscles of the cartilage become connec-
tive tissue corpuscles.
( rney
are sometimes disposed in loops, sometimes in long, some-
times in wide, sometimes in narrow meshes ; and the diam-
eters of these meshes, or, in other words, the interspaces
between the capillaries, are sometimes hardly wider than
the diameter of a capillary, sometimes many times as wide.
(See Figs. 36, 48, 66, and 72.) These interspaces are occu-
pied by the substance of the tissue which the capillaries per-
meate, so that the ultimate anatomical components of every
part of the body are, strictly speaking, outside the vessels,
or extra-vascular.
But there are certain parts of the body in which these
blood-capillaries are absent. These are the epidermis and
epithelium, the nails and hairs, the substance of the teeth,
and to a certain extent the cartilages and the transparent
coat (cornea) of the eye in front ; which may and do attain
a very considerable thickness or length, and yet contain no
blood-vessels. However, since we have seen that all the
tissues are really extra- vascular, these differ only in degree
from the rest. The circumstance that all the tissues are
outside the vessels by no means interferes with their being
bathed by the fluid which is inside the vessels. In fact, the
walls of the capillaries are so exceedingly thin that their
fluid contents readily exude through the delicate membrane
of which they are composed, and irrigate the tissues in
which they lie.
2. The Arteries and Veins. — The capillary tubes so far
described contain, during life, the red fluid, blood, and are
continued, on opposite sides, into somewhat larger tubes,
with thicker walls, which are the smallest arteries, on the
one side, and veins, on the other, and these again join
on to larger arteries and veins, which ultimately communi-
IU STRUCTURE OF ARTERIES 57
cate by a few principal arterial and venous trunks with the
heart.
The mere fact that the walls of these vessels are thicker
than those of the capillaries constitutes an important differ-
ence between the capillaries and the small arteries and
veins ; for the walls of the latter are thus rendered far less
permeable to fluids, and that thorough irrigation of the tis-
sues, which is effected by the capillaries, cannot be performed
by them.
The most important difference between these vessels and
the capillaries, however, lies in the circumstance that their
walls are not only thicker, but also more complex, being
composed of several coats, one, at least, of which is muscu-
lar. The number, arrangement, and even nature of these
coats differ according to the size of the vessels, and are not
the same in the veins as in the arteries, though the smallest
veins and arteries tend to resemble each other.
(i) The Structure of an Artery. — If we take one of the
smallest arteries, we find, first, a very delicate lining of cells
constituting a sort of epithelium continuous with the celk
which form the entire thickness of the wall of the capillaries
(Figs. 20, 21). Outside this comes the muscular coat, con-
sisting of a thin layer of muscle fibres of the kind called plain
or non- striated (p. 310), made up of flattened spindle-shaped
cells with an elongated nucleus, wrapped round the vessel at
right angles to its length. Outside this muscular coat is a
thin layer of fibrous connective tissue, intermixed with a
variable amount of fibres of elastic tissue. The larger arte-
ries are similarly composed of three layers or coats, which
are, however, thicker and more complex in structure. The
muscular layer is very greatly thickened, and elastic tissue
permeates all the layers.
We thus see that arteries are strong, muscular, and elastic.
58
ELEMENTARY PHYSIOLOGY
The largest arteries are, as a rule, characteristically more
elastic than the smaller, while in the latter the muscular tis-
sue is present in large amount relatively to the elastic tissue.
The significance of this difference will become apparent
later on (see pp. 87 and 91).
The plain muscular fibres in the arterial wall possess that
same power of contraction, or shortening in the long, and
broadening in the narrow, directions, which, as was stated
in the first Lesson, is the special
property of muscular tissue. And
when they exercise this power, they,
of course, narrow the calibre of the
vessel, just as squeezing it with the
hand or in any other way would do ;
and this contraction may go so far,
as, in some cases, to reduce the cav-
ity of the vessel almost to nothing,
and to render it practically imper-
vious.
The state of contraction of these
muscles of the small arteries is regu-
lated, like that of other muscles, by
their nerves ; or, in other words, the
nerves supplied to the vessels deter-
mine whether the passage through
these tubes shall be wide and free,
or narrow and obstructed. Thus, while the small arteries
lack the function, which the capillaries possess, of directly
irrigating the tissues by transudation, they possess that of
regulating the supply of fluid to the irrigators or capil-
laries themselves. The contraction, or dilation, of the
arteries which supply a set of capillaries, comes to the
same result as lowering or raising the sluice-gates of a sys-
Fig. 20. — Diagram illus-
trating the Structure
of an Artery.
e, inner coat of epithelium;
m, middle coat of smooth
muscle, here shown for the
sake of simplicity as a single
layer of cells; c, outer coat
of connective tissue, showing
fibres and cells.
in THE VALVES OF THE VEINS 59
tern of irrigation-canals. Thus the one great and all-impor-
tant use of the muscular tissue of the smaller arteries is to
determine and control the supply of blood to each part of the
body, according to the varying needs of that part.
The smaller arteries and veins severally unite into, or are
branches of, larger arterial or venous trunks, which again
spring from 'or unite into still larger ones, and these, at
length, communicate by a few principal arterial and venous
trunks with the heart.
(ii) The Structure of a Vein. — The wall of a vein is struc-
turally similar to that of an artery in so far that it consists
essentially of the same three layers or coats, but the distinc-
tion between the middle and outer coats, so easily made
out in an artery, is usually very obscure in a vein or even
altogether wanting in some veins (Fig. 21). It differs from
that of an artery chiefly in the fact that it is thinner, less
muscular and less elastic, and contains relatively more
connective tissue. Hence the walls of a vein collapse or
fall together when the vessel is empty, whereas those of
an artery do not.
This is one great difference between the arteries and the
veins ; the other is the presence of what are termed valves
in a great many of the veins, especially in those which lie in
muscular parts of the body. They are absent in the largest
trunks, such as the superior and inferior vena cava, and in
the smallest branches, as also in the portal, pulmonary, and
cerebral veins, and in those of the bones.
These valves (Fig. 22) are pouch-like folds of the inner
wall of the vein. The bottom of the pouch is turned
towards those capillaries from which the vein springs. The
free edge of the pouch is directed the other way, or towards
the heart. The action of these pouches is to impede the
passage of any fluid from the heart towards the capillaries,
6o
ELEMENTARY PHYSIOLOGY
while they do not interfere with fluid passing in the oppo-
site direction. The working of some of these valves may be
very easily demonstrated in the living body. When the arm
is bared, blue veins may be seen running from the hand,
under the skin, to the upper arm. The diameter of these
veins is pretty even, and diminishes regularly towards the
hand, so long as the current of the blood, which is running
in them, from the hand to the upper arm, is uninterrupted.
Fig. 21. — Transverse Section of an Artery and of a Corresponding
Vein.
A, artery; V, vein: e.c, epithelial cells; m, muscular (middle) coat; c, connective
tissue (outer) coat; «, nuclei of epithelial cells.
But if a finger be pressed upon the upper part of one of
these veins, and then passed downwards along it, so as to
drive the blood which it contains backwards, sundry swell-
ings, like little knots, will suddenly make their appearance at
several points in the length of the vein, where nothing of
the kind was visible before. These swellings are simply
dilatations of the wall of the vein, caused by the pressure
of the blood on that wall, above a valve which opposes its
backward progress. The moment the backward impulse
ceases the blood flows on again ; the valve, swinging
in GENERAL ARRANGEMENT OF BLOOD-VESSELS 61
back towards the wall of the vein, affords no obstacle to
its progress, and the distension caused by its pressure dis-
appears.
These valves play an important part in determining the
flow of blood along the veins from the capillaries towards
the heart. This they do, not in virtue of any propulsive
power of their own, but in response to pressure applied to the
Fig. 22. — The Valves of Veins.
C, H, C, H, diagrammatic sections of veins with valves. In the upper figure the
blood is supposed to be flowing in the direction of the arrow, towards the heart; in
the lower, back towards the capillaries; C, capillary side; H, heart side. A, a vein
laid open to show a pair of pouch-shaped valves.
veins f?'om their exterior. Such pressure tends to squeeze
the blood out of that part of the vein on which it is brought
"to bear ; but since the valves only open towards the heart,
the blood is thereby driven on in the desired direction.
Hence it is that the valves are most numerous in those veins
which are most subject to muscular pressure, such as those
of the arms and legs.
The only arteries which possess valves are the primary
trunks — the aorta and pulmonary artery — which spring
from the heart, but these valves, since they really belong to
the heart, will be best considered with that organ.
3. The General Arrangement of Blood-vessels in the
Body. — It will now be desirable to take a general view of
the arrangement of all these different vessels, and of their
62
ELEMENTARY PHYSIOLOGY
-IhJk
TTCI
Fig. 23 — Diagram of the Heart and Vessels, with the Course of the
Circulation, viewed from behind, so that the proper left of the
Observer corresponds with the left side of the Heart in the Diagram.
L.A. left auricle; L.V. left ventricle; Ao. aorta; A1, arteries to the upper part
of the body; A2, arteries to the lower part of the body; H.A. hepatic artery, which
supplies the liver with part of its blood: I'', veins of the upper part of the body;
V'1, veins of the lower part of the body; V.P. portal vein; H.V. hepatic vein;
V.C.I, inferior vena cava; V.C.S. superior vena cava; R.A. right auricle; R.V.
right ventricle; P. A. pulmonary artery; Lg. lung; P. V. pulmonary vein; Let,
lacteals; Ly, lymphatics; Th.D. thoracic duct: Al. alimentary canal: Lr. liver.
The arrows indicate the course of the blood, lymph, and chyle. The vessels which
contain arterial blood have dark contours, while those which carry venous blood have
light contours.
HI HEART AND VESSELS 6j
relations to the great central organ of the vascular system — ■
the heart (Fig. 23).
All the veins of every part of the body, except the lungs,
the heart itself, and certain viscera of the abdomen, join
together into larger veins, which, sooner or later, open into
one of two great trunks (Fig. 23, V.C.S., V.C.I.) , termed
the superior and the inferior vena cava ; these in turn open
into the upper or broad end of the right half of the heart.
All the arteries of every part of the body, except the lungs,
are more or less remote branches of one great trunk — the
aorta (Fig. 23, Ac), which springs from the lower division
of the left half of the heart.
The arteries of the lungs are branches of a great trunk,
the pulmonary artery (Fig. 23, P.A.), springing from the
lower division of the right side of the heart. The veins of
the lungs, on the contrary, open by four trunks, the pulmo-
nary veins (Fig. 23, P.V.), into the upper part of the left
side of the heart.
Thus, the venous trunks open into the upper division of
each half of the heart : those of the body in general into
that of the right half, those of the lungs into that of the left
half; while the arterial trunks spring from the lower moieties
of each half of the heart : that for the body in general from
the left side, and that for the lungs from the right side.
Hence it follows that the great artery of the body, and the
great veins of the body, are connected with opposite sides
of the heart ; and the great artery of the lungs and the
great veins of the lungs also with opposite sides of that
organ. On the other hand, the veins of the body open into
the same side of the heart as the artery of the lungs, and
the veins of the lungs open into the same side of the heart
as the artery of the body.
The arteries which open into the capillaries of the sub-
64
ELEMENTARY PHYSIOLOGY
Fig. 24. — Heart of Sheep, as seen aftek Removal from the Body, lying
upon the two Lungs. The Pericardium has been cut away, but no other
dissection made.
R.A. auricular appendage of right auricle; L.A. auricular appendage of left
auricle; R.V. right ventricle; L.V. left ventricle; S.V.C. superior vena cava;
I.V.C. inferior vena cava: P. A. pulmonary artery; Ao, aorta; A'o', innominate
branch from aorta dividing into subclavian and carotid arteries; L. lung; TV. trachea,
1, solid cord often present, the remnant of a communication, open in the embryo,
between the pulmonary artery and aorta. 2, masses of fat at the bases of the
ventricle hiding from view the greater part of the auricles. 3, line of fat marking
the division between the two ventricles. 4, mass of fat covering end of trachea.
in BLOOD-VESSELS 65
stance of the heart are called coronary arteries, and arise,
like the' other arteries, from the aorta, but quite close to its
origin, just beyond the semilunar valves. But the coronary
vein, which is formed by the union of the small veins which
arise from the capillaries of the heart, does not open into
either of the venae cavge, but pours the blood which it con-
tains directly into the division of the heart into which these
venae cavae open — that is to say, into the right upper division
(Fig. 30, b).
The abdominal viscera referred to above, the veins of
which do not take the usual course, are the stomach, the
intestines, the spleen, and the pancreas. These veins all
combine into a single trunk, which is termed the portal vein
(Fig. 23, V.P.), but this trunk does not open into the infe-
rior vena cava. On the contrary, having reached the liver,
it enters the substance of that organ, and breaks up into
an immense multitude of capillaries, which ramify through
the liver, and become connected with those into which the
artery of the liver, called the hepatic artery (Fig. 23, H.A.),
branches. From this common capillary meshwork veins
arise, and unite, at length, into a single trunk, the hepatic
vein (Fig. 23, H.V.), which emerges from the liver, and
opens into the inferior vena cava. The flow of blood from
the abdominal viscera through the liver to the hepatic vein
is called the portal circulation. The portal vein is the only
great. vein in the body which branches out and becomes con-
tinuous with the capillaries of an organ, like an artery. But
certain small veins in the kidney are similarly arranged
(p. 205).
The shortest possible course which any particle of the
blood can take in order to pass from one side of the heart
to the other, is to leave the aorta by one of the coronary
arteries, and return to the right auricle by the coronary
F
66
ELEMENTARY PHYSIOLOGY
vein. And in order to pass through the greatest possible
number of capillaries and return to the point from which
it started, a particle of blood must leave the heart by the
aorta and traverse the arteries which supply the alimentary
canal, spleen, and pancreas. It then enters, first, the capil-
laries of these organs; secondly, the capillaries of the liver;
and, thirdly, after passing through the right side of the heart,
the capillaries of the lungs, from which it returns to the left
side and eventually to the aorta.
Fig.
25-
Transverse Section of the Chest, with Heart and Lungs in
Place. (A little diagrammatic.)
D. V. dorsal vertebra, or joint of the backbone; Ao, Ao' , aorta, the top of its arch
being cut away in this section; S.C. superior vena cava; P. A. pulmonary artery,
divided into a branch for each lung; L.P., R.P. left and right pulmonary veins; Br,
bronchi; R.L., L.L. right and left lungs; CE, the gullet or oesophagus; p, outer bag
of pericardium; //, the two layers of pleura; v, azygos vein.
4. The Heart. — The heart (Figs. 24 and 26), to which
all the vessels in the body have now been directly or indi-
rectly traced, is an organ, the size of which is usually roughly
estimated as equal to that of the closed fist of the person to
whom it belongs, and which has a broad end turned upwards
and backwards, and rather to the right side, called its base ;
in THE HEART 6?
and a pointed end which is called its apex, turned down-
wards and forwards, and to the left side, so as to lie oppo-
site the interval between the fifth and sixth ribs.
It is lodged between the lungs, nearer the front than the
back wall of the chest, and is inclosed in a sort of double
bag, the pericardium (Fig. 25, /). One-half of the
double bag is closely adherent to the heart itself, forming a
thin coat upon its outer surface. At the base of the heart,
this half of the bag passes on to the great vessels which
spring from, or open into, that organ ; and becomes con-
tinuous with the other half, which loosely envelopes both
the heart and the adherent half of the bag. Between the
two layers of the pericardium, consequently, there is a com-
pletely closed, narrow cavity, lined by an epithelium, and
containing in its interior a small quantity of clear fluid, the
pericardial fluid.1
The outer layer of the pericardium is firmly connected
below with the upper surface of the diaphragm.
But the heart cannot be said to depend greatly upon the
diaphragm for support, inasmuch as the great vessels which
issue from or enter it — and for the most part pass upwards
from its base — help to suspend and keep it in place.
Thus the heart is coated, outside, by one layer of the
pericardium. Inside, it contains two great cavities or
" divisions," as they have been termed above, a right and
a left cavity, completely separated by a fixed partition, which
extends from the base to the apex of the heart ; and con-
sequently, having no direct communication with one another.
1 This fluid, like that contained in the peritoneum, pleura, and other
shut sacs of a similar character to the pericardium, used to be called serum ;
whence the membranes forming the walls of these sacs are frequently termed
serous membranes. The fluid is, however, in reality a form of lymph. (See
p. 142.)
68
ELEMENTARY PHYSIOLOGY
Each of these two great cavities is further subdivided, not
longitudinally but transversely, by a movable partition. The
cavity above the transverse partition on each side is called
the auricle ; the cavity below, the ventricle — right or left
as the case may be.
Each of the four cavities has the same capacity, and is
capable of containing from four to six cubic inches of water
(70 to 100 cubic centimetres). The walls of the auricles
are much thinner than those of the ventricles. The wall
RJlVi n£ T Sil^*^ J*&
Fig. 26. — The Heakt, Great Vessels, and Lungs. (Front View.)
R.V. right ventricle; L.V. left ventricle; R.A. right auricle; L.A. left auricle;
Ao, aorta; P. A. pulmonary artery; P. V. pulmonary veins; R.L. right lung; L.L.
left lung; V.S. vena cava superior; S.C. subclavian vessels; C. carotid arteries;
R.J.V. and L.J.V. right and left jugular veins; V.I, vena cava inferior; T. trachea;
B. bronchi.
All the great vessels but those of the lungs are cut.
of the left ventricle is much thicker than that of the right
ventricle ; but no such difference is perceptible between the
two auricles (Figs. 27 and 28, 1 and 3).
In fact, as we shall see, the ventricles have more work
to do than the auricles, and the left ventricle more to do
in THE VALVES OF THE HEART 69
than the right. Hence the ventricles have more muscular
substance than the auricles, and the left ventricle more than
the right ; and it is this excess of muscular substance' which
gives rise to the excess of thickness observed in the left
ventricle.
At the junction between the auricles and ventricles, the
apertures of communication between their cavities, called the
auriculo-ventricular apertures, are strengthened by fibrous
rings of connective tissue. To these rings the movable par-
titions, or valves, between the auricles and ventricles, the
arrangement of which must next be considered, are attached.
5. The Valves of the Heart. — There are three of these
partitions attached to the circumference of the right auric-
ulo-ventricular aperture, and two to that of the left (Figs.
27, 28, 29, 30, tv, mv). Each is a broad, thin, but very
tough and strong triangular fold of connective tissue, at-
tached by its base, which joins on to its fellow, to the
auriculo-ventricular fibrous ring, and hanging with its point
downwards into the ventricular cavity. On the right side
there are, therefore, three of these broad, pointed mem-
branes, whence the whole apparatus is called the tricuspid
valve. On the left side there are but two, which, when
detached from all their connections but the auriculo-ven-
tricular ring, look something like a bishop's mitre, and
hence bear the name of the mitral valve.
The edges and apices of the valves are not completely
free and loose. On the contrary, a number of fine, but
strong, tendinous cords, called chordae tendiiieae, connect
them with some column-like elevations of the fleshy sub-
stance of the walls of the ventricle, which are termed papil-
lary muscles (Figs. 27 and 28, pp) ; similar column-like
elevations of the walls of the ventricles, with no chordse
tendinese attached to them, are called columnae carneae.
7o
ELEMENTARY PHYSIOLOGY
It follows, from this arrangement, that the valves oppose
no obstacle to the passage of fluid from the auricles to
Fig 27. — Right Side of the Heart of a Sheep (laid open).
R.A. cavity of right auricle; S.V.C. superior vena cava; I V.C. inferior vena
cava (a style has been passed through each of these) ; «, a style passed from the
auricle to the ventricle through the auriculo-ventricular orifice; b, a style passed into
the coronary vein
R.V. cavity of the right ventricle; tv, tv, two flaps of the tricuspid valve; the
third is dimly seen behind them, the style a passing between the three. Between the
two flaps, and attached to them by chorda tendinece, is seen a papillary muscle,//,
cut away from its attachment to that portion of the wall of the ventricle which has
been removed. Above, the ventricle terminates somewhat like a funnel in the pul-
monary artery, P A. One of the pockets in the semilunar valve, sv, is seen in its
entirety, another partially.
1, the wall of the ventricle cut across; 2, the position of the auriculo-ventricular
ring; 3 the wall of the auricle; 4, masses of fat lodged between the auricle and pul-
monary artery.
the ventricles ; but if any should be forced the other way,
it will at once get between the valve and the wall of the
THE VALVES OF THE HEART
7i
heart, and drive the valve backwards and upwards. Partly
because they soon meet in the middle and oppose one
another's action, and partly because the chordce tendinece
Ac
Ao
ijJN
Fig. 28. — Left Side of the Heart of a Sheep (laid open).
P. V. pulmonary veins opening into the left auricle by four openings, as shown by
the styles', a, a style passed from auricle into ventricle through the auriculo-ventricu-
lar orifice; 6, a style passed into the coronary vein, which, though it has no connec-
tion with the left auricle, is, from its position, necessarily cut across in thus laying
open the auricle.
mv, the two flaps of the mitral valve (drawn somewhat diagrammatically) ; //,
papillary muscles, belonging as before to the part of the ventricle cut away; c, a style
passed from ventricle into Ao, aorta; Ao1, branch of aorta (see Fig. 24, A V) ; P. A.
pulmonary artery; S.l'.C. superior vena cava.
i, wall of ventricle cut across; 2, wall of auricle cut away around auriculo-ventricu-
lar orifice; 3, other portions of auricular wall cut across; 4, mass of fat around base
of ventricle (see Fig. 24, 2).
72
ELEMENTARY PHYSIOLOGY
hold their edges and prevent them from going back too
far, the valves, thus forced back, give rise to the formation
of a complete transverse partition between the ventricle
and the auricle, through which no fluid can pass.
Ao
RAV
Fig. 29. — View of the Orifices of the Heart from below, the Whole of
the Ventricles having been cut away.
R.A.V. right auriculo-ventricular orifice surrounded by the three flaps, t.v. 1,
t.v. 2, t.v. 3, of the tricuspid valve; these are stretched by cords attached to the
chorda te?tdinece.
L A.V. left auriculo-ventricular orifice surrounded in the same way by the two
flaps, m.v. 1, m.v. 2, of mitral valve; P. A. the orifice of the pulmonary artery, the
semilunar valves having met and closed together; Ao, the orifice of the aorta with
its semilunar valves. The shaded portion, leading from R.A.V. to P. A., represents
the funnel seen in Fig. 27.
Where the aorta opens into the left ventricle, and where
the pulmonary artery opens into the right ventricle, another
valvular apparatus is placed, consisting in each case of
three pouch-like valves called the semilunar valves (Fig.
27, s.v. ; Figs. 29 and 30, Ao, P. A.), which are similar to
those of the veins. Since they are placed on the same
Ill
THE VALVES OF THE HEART
73
level and meet in the middle line, they completely stop
the passage when any fluid is forced along the artery
towards the heart. On the other hand, these valves flap
back and allow any fluid to pass from the heart into the
artery, with the utmost readiness.
The action of the auriculo-ventricular valves may be
tvz
Fig. 30. — The Orifices of the Heart seen from above, the Auricles and
Great Vessels being cut away.
P. A. pulmonary artery, with its semilunar valves; Ao, aorta, ditto.
R.A.V. right auriculo-ventricular orifice with the three flaps ((.v. 1, 2, 3) of tri-
cuspid valve.
L.A.V. left auriculo-ventricular orifice, with m.v. 1 and 2, flaps of mitral valve:
b, style passed into coronary vein. On the left part of L.A.V. the section of the
auricle is carried through the auricular appendage: hence the toothed appearance
due to the portions in relief being cut across.
demonstrated with great ease on a sheep's heart, in which
the aorta and pulmonary artery have been tied and the
greater part of the auricles cut away, by pouring water
into the ventricles through the auriculo-ventricular aperture.
The tricuspid and mitral valves then usually become closed
74
ELEMENTARY PHYSIOLOGY
by the upward pressure of the water which gets behind
them. Or, if the ventricles be nearly filled, the valves may
be made to come together at once by gently squeezing the
ventricles. In like manner, if the base of the aorta, or
pulmonary artery, be cut out of the heart, so as not to
injure the semilunar valves, water poured into the upper
ends of the vessel will cause its valves to close tightly, and
allow nothing to flow out after the first moment.
Thus, the arrangement of the auriculo-ventricular valves
is such, that any fluid contained in the chambers of the
heart can be made to pass through the auriculo-ventricular
apertures in one direction only : that is to say, from the
auricles to the ventricles. On the other hand, the arrange-
ment of the semilunar valves is such
that the fluid contents of the ventri-
cles pass easily into the aorta and
pulmonary artery, while none can be
made to travel the other way from
the arterial trunks to the ventricles.
6. The Structure of the Heart. —
The heart is a muscular organ, and
the substance of its walls is mainly
muscular tissue. Like all other mus-
cles this tissue is composed of cells,
and these cells resemble those of
non-striated muscle, as it occurs, for
example, in the arteries and veins, in
containing each a single nucleus, and
possessing no cell-wall, or sarcolemma
(Fig. 31). But the cells are generally
short and broad, frequently branched
or irregular in shape, and their substance is more or less dis-
tinctly striated, like the substance of a striated muscular fibre
Fig. 31. — Cardiac Muscle
Cells.
Two cells isolated from
the heart. «, nucleus; /,
line of junction between the
two cells; p, process joining
a similar process of another
cell. (Magnified 400 diame-
ters.)
Hi THE BEAT OF THE HEART 75
(p. 311). Cardiac muscle is hence intermediate in charac-
ter between non-striated and striated muscle, representing
a higher stage of differentiation from the primitive cells than
the muscle of the arteries and veins, but not so high a stage
as the muscles of the limbs. The cells are joined by inter-
cellular cement substance into sets of anastomosing fibres,
which are built up in a complex interwoven manner into the
walls of the ventricles and auricles.
The cavities of the heart are lined, and the valves are
covered, by a smooth, shiny membrane called the endocar-
dium, which consists of a layer of connective tissue covered
with thin flattened cells continuous with and similar to those
which form the wall of the capillaries and which line the
arteries and veins.
7. The Beat of the Heart. — Like all other muscular
tissues, the substance of the heart is contractile ; but, unlike
most muscles, the heart contains within itself something
which causes its different parts to contract in a definite
succession and at regular intervals.
If the heart of a living animal be removed from the body,
it will, though in most cases for a very short time only,
unless the animal be " cold-blooded " like a frog, go on
beating much as it did while in the body. And careful
attention to these beats will show that they consist of : —
(1) A simultaneous contraction of the walls of both auricles.
(2) Immediately following this, a simultaneous contraction
of the walls of both ventricles. (3) Then comes a pause,
or state of rest, after which the auricles and ventricles con-
tract again in the same order as before, and their contrac-
tions are followed by the same pause as before.
The state of contraction of the ventricle or auricle is
called its systole ; the state of relaxation, during which it
undergoes dilation, its diastole.
j6 ELEMENTARY PHYSIOLOGY less.
If the auricular contraction be represented by A", the
ventricular by V, and the pauses by — , the series of actions
will be as follows: A~V" — ; AVV" — ; A"VV — ; etc.
Thus, the contraction of the heart is rhythmical, two short
contractions of its upper and lower halves respectively being
followed by a pause of the whole, which occupies nearly as
much time as the two contractions.
The period occupied by one complete beat and the pause
is usually spoken of as a "cardiac cycle." This cycle is
repeated, or as we more ordinarily say, " the heart beats "
in an average healthy adult person about 72 times in a min-
ute. From this it follows that the ordinary duration of each
beat is T8^ of a second. Of this period the contraction of
the auricles occupies y^ and that of the ventricles T3T, the
remaining fL being taken up by the pause of the heart as a
whole. During each cycle or beat the heart undergoes cer-
tain changes of shape and position, as to the details of which
there is some uncertainty, but which are, on the whole, as fol-
lows. During each systole the width of the heart from side
to side and probably also the depth from back to front
becomes less. The result of this is that, whereas during
diastole the shape of a section of the base of the ventricles
is elliptical, during systole it becomes much more nearly
circular.
The length of the heart is perhaps lessened, but very
slightly, if at all, during systole, and the heart as a whole is
twisted to a certain extent on its long axis, from the left and
behind towards the front and right. The apex is at the
same time tilted slightly forward and is hence pressed rather
more firmly against the wall of the thorax, a fact of some
importance in connection with what we shall describe pres-
ently as the " cardiac impulse " (see p. 81).
8. The Action of the Valves. — Having now acquired a
Ill THE ACTION OF THE VALVES 77
notion of the arrangement of the different pipes and reser-
voirs of the circulatory system, of the position of the valves,
and of the rhythmical contractions of the heart, it will be
easy to comprehend what must happen if, when the whole
apparatus is full of blood, the first step in the pulsation of
the heart occurs and the auricles contract.
By this action each auricle tends to squeeze the fluid
which it contains out of itself in two directions, — the one
towards the great veins, the other towards the ventricles ;
and the direction which the blood, as a whole, will take,
will depend upon the relative resistance offered to it in
these two directions. Towards the great veins it is resisted
by the mass of the blood contained in the veins. Towards
the ventricles, on the contrary, there is no resistance worth
mentioning, inasmuch as the valves are open, the walls of
the ventricles, in their uncontracted state, are flaccid and
easily distended, and the entire pressure of the arterial blood
is taken off by the semilunar valves, which are necessarily
closed. The return of blood into the veins is further checked
by a contraction of the great veins at their point of junction
with the heart, which immediately precedes the systole of
the auricles, and is practically continuous with it.
Therefore, when the auricles contract, little or none of the
fluid which they contain will flow back into the veins ; all
the contents, or nearly all, will pass into and distend the
ventricles. As the ventricles fill and begin to resist further
distension, the blood, getting behind the auriculo-ventricu-
lar valves, will push them towards one another, and indeed
almost shut them. The auricles now cease to contract, and,
immediately that their walls relax, fresh blood flows from the
great veins and slowly distends them again.
But the moment the auricular systole is over, the ventric-
ular systole begins. The walls of each ventricle contract
78
ELEMENTARY PHYSIOLOGY
vigorously, and the first effect of that contraction is to com-
plete the closure of the auriculo-ventricular valves and so to
stop all egress towards the auricle (Fig. 32). The pressure
upon the valves becomes very considerable, and they might
even be driven upwards, if it were not for the chordce ten-
dinece which hold down their edges.
Fig. 32. — Diagram to illustrate the Action of the Heart.
aur. auricle; vent, ventricle; v, v, veins; a, aorta; m, mitral valve; s, semi-
lunar valve.
In A the auricle is contracting, ventricle dilated, mitral valve open, semilunar
valves closed. In B the auricle is dilated, ventricle contracting, mitral valve closed,
semilunar valves open.
As the contraction continues and the capacities of the
ventricle become diminished, the points of the wall of the
heart to which the chordce tendineaz are attached approach
the edges of the valves ; and thus there is a tendency to
allow of a slackening of these cords, which, if it really took
place, might permit the edges of the valves to flap back and
so destroy their utility. This tendency, however, is counter-
acted by the chordce tendinecu being connected, not directly
to the walls of the heart, but to those muscular pillars, the
papillary muscles, which stand out from its substance. These
Ill THE ACTION OF THE VALVES 79
muscular pillars shorten at the same time as the substance
of the heart contracts; and thus, just so far as the contrac-
tion of the walls of the ventricles brings the papillary muscles
nearer the valves, do they, by their own contraction, pull
the chorda tendincce as tight as before.
By the means which have now been described, the fluid
in the ventricle is debarred from passing back into the auri-
cle ; the whole force of the contraction of the ventricular
walls, therefore, is expended in overcoming the resistance
presented by the semilunar valves (Fig. 32). This resist-
ance is partly the result of the mere weight of the vertical
column of blood which the valves support ; but is chiefly
due to the reaction of the distended elastic walls of the
great arteries, for, as we shall see, these arteries are already
so full that the blood within them is pressing on their walls
with great force.
It now becomes obvious why the ventricles have so much
more to do than the auricles, and why valves are needed
between the auricles and ventricles, while none are wanted
between the auricles and the veins'.
All that the auricles have to do is to fill the ventricles,
which offer no active resistance to that process. Hence the
thinness of the walls of the auricles, and hence the need-
lessness of any auriculo-venous valve, the resistance on the
side of the ventricle being so insignificant that it gives way,
at once, before the pressure of the blood in the veins.
On the other hand, the ventricles have to overcome a
great resistance in order to force fluid into elastic tubes
which are already full ; and if there were no auriculo-ven-
tricular valves, the fluid in the ventricles would meet with
less obstacle in pushing its way backward into the auricles
and thence into the veins, than in separating the semilunar
valves. Hence the necessity, first, of the auriculo-ventricu-
So ELEMENTARY PHYSIOLOGY less,
lar valves ; and, secondly, of the thickness and strength ot
the walls of the ventricles. And since the aorta, systemic
arteries, capillaries, and veins form a system of tubes, which,
from a variety of causes, offer more resistance than do the
pulmonary arteries, capillaries, and veins, it follows that the
left ventricle needs a thicker muscular wall than the right.
Thus, at every systole of the auricles, the ventricles are
filled and the auricles emptied, the latter being slowly re-
filled by the pressure of the fluid in the great veins, which
is amply sufficient to overcome the passive resistance of the
relaxed auricular walls. And, at every systole of the ventri-
cles, the arterial systems of the body and lungs receive the
contents of these ventricles, and the emptied ventricles
remain ready to be filled by the auricles.
9. The Working of the Arteries. — We must now con-
sider what happens in the arteries when the contents of the
ventricles are suddenly forced into' these tubes (which, it
must be recollected, are already full).
If the vessels were tubes of a rigid material, like gas-
pipes, the forcible discharge of the contents of the left ven-
tricle into the beginning of the aorta would send a shock,
travelling with great rapidity, right along the whole system
of tubes, through the arteries into the capillaries, through
the capillaries into the veins, and through these into the
right auricle ; and just as much blood would be driven from
the end of the veins into the right auricle as had escaped
from the left ventricle into the beginning of the aorta ; and
that, at almost the same instant of time. And the same
would take place in the pulmonary vessels between the
right ventricle and left auricle.
However, the vessels are not rigid, but, on the contrary,
very yielding tubes ; and the great arteries, as we have
seen, have especially elastic walls. On the other hand, the
in THE CARDIAC IMPULSE Si
friction in the small arteries and capillaries, which opposes
a resistance to the flow of blood, and is hence spoken of as
the peripheral resistance, is so great that the blood cannot
pass through them into the veins as quickly as it escapes
from the ventricle into the aorta. Hence the contents of
the ventricle, driven by the force of the systole past the
semilunar valves, are at first lodged in the first part of the
aorta, the walls of which are stretched and distended by
the extra quantity of blood thus driven into it. But, as soon
as the ventricle has emptied itself and no more blood is
driven out of it to stretch the aorta, the elastic walls of this
vessel come into play ; they strive to go back again and
make the tube as narrow as it was before ; thus they return
back to the blood the pressure which they received from
the ventricle. The effect of this elastic recoil of the arterial
walls is, on the one hand, to close the semilunar valves, and
so prevent the return of blood to the heart, and, on the
other hand, to distend the next portion of the aorta, driving
an extra quantity of blood into it. And this second por-
tion, in a similar way, distends the next, and this again the
next, and so on, right through the whole arterial system.
Thus the impulse given by the ventricle travels like a wave
along the arteries, distending them as it goes, and ulti-
mately forcing the blood through the capillaries into the
veins, and so on to the heart again.
Several of the practical results of the working of the heart
and arteries just described now become intelligible.
10. The Cardiac Impulse. — If a finger be placed on
the chest over the space between the fifth and sixth ribs
on the left side, about one inch below the left nipple, and
slightly towards the sternum, a certain throbbing movement
is perceptible, which is known as the " cardiac impulse."
It is the result of the heart-beat making itself felt through
G
82 ELEMENTARY PHYSIOLOGY less.
the wall of the chest at this point, at the moment of the
systole of the ventricles. Even when the heart is at rest
the apex, in a standing position, lies close under and in
contact with this part of the chest- wall. When the systole
takes place the muscular substance of the ventricles be-
comes suddenly hard and tense, as do all muscles when
they contract. At tne same time the apex of the heart, as
the result of the peculiar movements already described
(p. 76), is brought into still firmer contact with the chest-
wall. The cardiac impulse is the outcome of this sudden
hardening of the ventricular walls, aided by their closer con-
tact with the wall of the chest at the moment when the hard •
ening takes place. It is ?wt due, as is so frequently stated,
to the heart striking or tapping against the chest-wall.
11. The Sounds of the Heart. — If the ear be applied
over the heart, certain sounds are heard, which recur with
great regularity, at intervals corresponding with those be-
tween every two beats. First comes a longish dull booming
sound ; then a short sharp sound, then a pause, then the
long, then the sharp sound, then another pause : and so on.
These sounds are usually likened to the pronunciation of
the syllables " lubb, dup." There are many different opin-
ions as to the cause of the first sound ; some physiologists
regard it as a muscular sound caused by the contraction of
the muscular fibres of the ventricle, while others believe it
to be due to the vibration of the auriculo-ventricular valves,
when they become suddenly tense or stretched as the
ventricles begin to contract. In reality the first sound has
probably a double origin, being partly muscular and partly
valvular, and this view is borne out by the following facts.
The sound is given out during the ventricular systole, and is
most plainly heard at the spot where the cardiac impulse
is most readily felt. It is greatly altered in character and
in BLOOD-PRESSURE 83
obscured in case of disease or experimental injury of the
auriculo-ventricular valves; but on the other hand it may
be heard, although modified, in a beating heart through
whose cavities the passage of blood is temporarily pre-
vented.
The second sound is without doubt caused by the mem-
branes of the semilunar valves becoming tense, and thus
thrown into vibrations, on their sudden closure at the end
of the ventricular systole. This^ is proved by the facts that
the sound is loudest at a point on the chest- wall near which
the semilunar valves lie ; that it is modified and obscured
by disease of these valves ; and that it may be made to
cease by experimentally hooking back the semilunar valves
in a living animal.
12. Blood-pressure. — When an artery is cut, the out-
flow of blood is not uniform and smooth, but takes place
in jerks which correspond to each beat of the heart. More-
over, the blood spurts out with conside7-able force, which,
although it is greater at each jerk, is still persistent and
large between the jerks. The obvious conclusion to be
drawn from the above observation is that the blood in the
artery is always under considerable, though variable, press-
ure. This pressure is called biood-pressure. We have
already explained how this pressure comes to be established ;
but its importance is so great as a factor in the circula-
tion that we may with advantage refer to this point once
more.
The smallest arteries and capillaries offer a considerable
frictional resistance to the flow of blood through them into
the veins, called, as we have already said, " peripheral resist-
ance." Owing to this resistance, of the total amount of
blood forced into the arteries at each beat of the heart, only
a portion can during the actual beat, apart from the pause
84 ELEMENTARY PHYSIOLOGY less.
between it and the next beat, pass on into the veins. The
remainder is lodged in the arteries, whose walls, being dis-
tensible, axe put on the stretch by the pressure of the blood
thrust into them at each stroke of the heart, and this press-
ure of the blood on the arterial wall is what we mean
by " blood-pressure." As soon as the arterial walls are
stretched their elastic properties come into play ; they
recoil and press on the blood with a force equal to that
which puts them on the stretch. This elastic recoil squeezes
the blood on in the intervals between the successive beats
of the heart, and thus renders the circulation continuous.
In short, the whole arterial system is always in a state of
distension ; the work of the heart consists in keeping up
this distended condition by thrusting fresh blood into the
arteries under pressure ; and the pressure thus established
forces the blood through the capillaries, on through the
veins, and so back to the heart.
Blood-pressure is greatest in the large arteries near the
heart and diminishes gradually along the arterial system
until we come to the smallest arteries and capillaries ; here
the pressure falls suddenly. The sudden fall of pressure is
due to the existence of what we have already referred to as
"peripheral resistance." This resistance must be overcome
in order to drive the blood on into the veins ; to overcome
a resistance work must be done, and to do work, force must
be employed and energy expended. Now blood-pressure is
the force available for overcoming the resistance, and if it
be thus used up there is less of it left, or, in other words,
the pressure falls. In the veins the blood-pressure is still
less than in the capillaries, and diminishes gradually along
their course towards the heart.
These differences of pressure in the several parts of
the vascular system determine the flow of blood along the
in THE PULSE 85
vessels ; the blood is always flowing from a higher to a
lower pressure ; the main, immediate work of the heart is
to establish the large blood-pressure existing in the larger
arteries.
When a vein is cut, the blood does not spurt out as it
does from a cut artery, but oozes or trickles out gently, the
reason being that the pressure in the veins is small. Fur-
ther, the flow is in this case continuous and not jerky as it
is from a cut artery, in correspondence with the fact that
there is no pulse in the veins as there is in the arteries.
But this statement requires that we should next consider
the nature and causes of the pulse.
13. The Pulse. — If the finger be placed on an artery
which lies near the surface of the body, such as the radial
artery at the wrist, what is known as the pulse will be felt
as a slight throbbing pressure on the finger, coming and
going at regular intervals which correspond to the succes-
sive beats of the heart. What is felt is in reality the in-
termittent rise and fall of that piece of the arterial wall
which lies immediately under the finger. This fact may
be easily proved by placing a light lever so as to rest over
the artery, whereupon its end may be seen to rise and fall
at the same regular intervals. This movement of the arte-
rial wall is due to that distension of the arteries of which
we have already spoken, which is started at each beat of
the heart by the extra quantity of blood driven into them
by the ventricle, then travels in the form of a wave from
the larger to the smaller arteries, and corresponds to the
jerky outflow of blood from a cut artery.
The pulse which is felt by the finger does not correspond
in time precisely with the beat of the heart, but takes place
a little after it, and the delay is longer, the greater the dis-
tance of the artery from the heart. For example, the pulse
86 ELEMENTARY PHYSIOLOGY less.
in the tibial artery on the inner side of the ankle is a little
later than the pulse in the temporal artery in the temple.
By suitable instruments the rate at which the pulse travels
along the arteries may be readily determined and is found
to be nearly 30 feet per second. This rate of progression
of the pulse-wave must be carefully distinguished from the
rate at which the blood is flowing along in the artery. Even
in the aorta, where the blood flows most rapidly (p. 89), its
velocity is not more than about 15 inches per second. In
fact, " the pulse-wave travels over the moving blood some-
what as a rapidly-moving natural wave travels along a
sluggishly-flowing river."
Under ordinary circumstances, the pulse is no longer to
be detected in the capillaries or in the veins. Sometimes
a backward pulse from the heart along the great venous
trunks may be observed ; but this is quite another matter,
and is the result of the movements of breathing. (See
p. 190.) The actual loss, or rather transformation, of
the pulse in the small vessels, is effected by means of the
elasticity of the arterial walls, called into play by the periphe-
ral resistance, in the following manner.
In the first place it must be borne in mind that, owing
to the minute size of the small arteries and capillaries, the
amount of friction taking place in their channels when the
blood is passing through them is very great ; in other words,
they offer a very great resistance to the passage of the blood.
The consequence of this is that, in spite of the fact that the
total area of the capillaries is so much greater than that of
the aorta, the blood has a difficulty in getting through the
capillaries into the veins as fast as it is thrown into the
arteries by the heart. The whole arterial system, therefore,
becomes over-distended with blood.
Now we know by experiment that, under such conditions as
HI THE PULSE 87
these, an elastic tube has the power, if long enough and elastic
enough, to change a jerked impulse into a continuous flow.
If an ordinary syringe or other convenient form of pump
be fastened to one end of a long glass tube, and water be
forced through the tube, it will flow from the far end in
jerks, corresponding to the jerks of the syringe. This will
be the case whether the tube be quite open at the far end,
or drawn out to a fine point so as to offer great resistance
to the outflow of the water. The glass tube is a rigid tube,
and there is no elasticity to be brought into play.
If now a long india-rubber tube be substituted for the
glass tube, it will be found to act differently, according as
the opening at the far end is wide or narrow. If it is wide,
the water flows out in jerks, nearly as distinct as those
from the glass tube. There is little resistance to the out-
flow, little distension of the india-rubber tube, little elas-
ticity brought into play. If, however, the opening be
narrowed, as by fastening to it a glass tube drawn out to
a fine point, or if a piece of sponge be thrust into the end
of the tube — if, in fact, in anyway resistance be offered
to the outflow of the water, the tube becomes distended,
its elasticity is brought into play, and the water flows out
from the end, not in jerks but in a stream, which is more
and more completely continuous the longer and more elas-
tic the tube, and the greater the resistance at its open end.
Substitute for the syringe the heart, for the finely-drawn
glass tube or sponge the small arteries and capillaries, for
the india-rubber tube the whole arterial system, and you
have exactly the same result in the living body. Through
the action of the elastic arterial walls, the separate jets
from the heart are blended into one continuous stream.
The whole force of each blow of the heart is not at once
spent in driving a quantity of blood through the capillaries ;
88 ELEMENTARY PHYSIOLOGY less
a part only is thus spent, the rest goes to distend the elas-
tic arteries. But during the interval between that beat and
the next, the distended arteries are narrowing again, by vir-
tue of their elasticity, and so are pressing the blood on into
the capillaries with a force equal to that by which they were
themselves distended by the heart. Then comes another
beat, and the same process is repeated. At each stroke
the elastic arteries shelter the capillaries from part of the
sudden blow, and then quietly and steadily pass on that part
of the blow to the capillaries during the interval between
the strokes.
The larger the amount of elastic arterial wall thus brought
into play, i.e. the greater the distance from the heart, the
greater is the fraction of each heart's stroke which is thus
converted into a steady elastic pressure between the beats.
Thus the pulse becomes less and less marked the farther
you go from the heart ; any given length of the arterial sys-
tem, so to speak, being sheltered by the lengths between it
and the heart.
Every inch of the arterial system may, in fact, be consid-
ered as converting a small fraction of the heart's jerk into a
steady pressure, and when all these fractions are summed up
together in the total length of the arterial system no trace of
the jerk is left.
As the immediate, sudden effect of each systole becomes
diminished in the smaller vessels by the causes above men-
tioned, the influence of this constant pressure becomes more
obvious, and gives rise to a steady passage of the fluid from
the arteries towards the veins. In this way, in fact, the arte-
ries perform the same functions as the air-reservoir of a fire-
engine, which converts the jerking impulse given by the
pumps into the steady flow from the nozzle of the delivery
hose.
Ill THE RATE OF BLOOD FLOW 89
The phenomena so far described are the direct outcome
of the mechanical conditions of the organs of the circulation
combined with the rhythmical activity of the heart. This
activity drives the fluid contained in these organs out of the
heart into the arteries, thence to the capillaries, and from
them through the veins back to the heart. And in the
course of these operations it gives rise, incidentally, to the
cardiac impulse, the sounds of the heart, blood-pressure, and
the pulse.
14. The Rate of Blood Flow. — It has been found, by
experiment, that in the horse it takes about half a minute
for any substance, as, for instance, a chemical body, whose
presence in the blood can easily be recognised, to complete
the circuit, e.g. to pass from the jugular vein down through
the right side of the heart, the lungs, the left side of the
heart, up through the arteries of the head and neck, and so
back to the jugular vein.
The greater portion of this half minute is taken up by the
passage through the capillaries, where the blood moves, it is
estimated, at the rate only of about one and a half inches in
a minute, whereas through the the carotid artery of a dog it
flies along at the rate of about twelve inches in a second.
Of course, to complete the circuit of the circulation, a blood-
corpuscle need not have to go through so much as half of an
inch of capillaries in either the lungs or any of the tissues of
the body.
Inasmuch as the force which drives the blood on is (put-
ting the other comparatively slight helps on one side) the
beat of the heart and that alone, however much it may be
modified, as we have seen, in character, it is obvious that
die velocity with which the blood moves must be greatest in
the aorta and must diminish towards the capillaries.
For with each branching of the arteries the total area of
go ELEMENTARY PHYSIOLOGY less.
the arterial system is increased, the total width of the capil-
lary tubes if they were all put together side by side being
very much greater than that of the aorta. Hence the blood,
or a corpuscle, for instance, of the blood, being driven by
the same force, viz. the heart's beat, over the whole body,
must pass much more rapidly through the aorta than through
the capillary system or any part of that system.
It is not that the greater friction in any capillary causes
the blood to flow more slowly there and there only. The
resistance caused by the friction in the capillaries is thrown
back upon the aorta, which indeed feels the resistance of
the whole vascular system; and it is this total resistance
which has to be overcome by the heart before the blood
can move on at all.
The blood driven everywhere by the same force simply
moves more and more slowly as it passes into wider and
wider channels. When it is in the capillaries it is slowest ;
after escaping from the capillaries, as the veins unite into
larger and larger trunks, and hence as the total venous area
is getting less and less, the blood moves again faster and
faster for just the same reason that in the arteries it moved
slower and slower. It is, in fact, the differences in the width
of the "bed" and these alone, which determine the differ-
ences in the rate of flow at the various points of the vascular
system.
A very similar case is that of a river widening out in a
plain into a lake and then contracting into a narrow stream
again. The water is driven by one force throughout (that
of gravity). The current is much slower in the lake than
in the narrower river either before or behind.
15. The Nervous Control of the Arteries. Vaso-motor
Nerves. — The arteries, as we have seen, are characterised
structurally by being elastic and muscular. In the large
in VASOMOTOR NERVES 91
arteries the elastic properties are more marked than the
muscular, whereas in the smaller arteries the muscular tissue
is present in large amount relatively to the elastic elements;
and we have dealt in detail with the significance of arterial
elasticity and its use in connection with the establishment of
blood-pressure and the disappearance of the pulse. It has
also been pointed out (p. 58) that the small arteries may be
directly affected by the nervous system, which controls the
state of contraction of their walls, and regulates their calibre,
and thus governs the supply of blood to each part of the
body according to its varying needs. The control of the.
nervous system over the circulation in particular spots is of
such paramount importance that we must now deal with this
also in some detail.
A phenomenon with which every one is more or less famil-
iar, either as experienced on himself or observed on other
persons, is that known as blushing. Now blushing is a
purely local modification of the circulation, and it will be
instructive to consider how a blush is brought about. An
emotion, sometimes pleasurable, sometimes painful, takes
possession of the mind ; thereupon a hot flush is felt, the
skin grows red, and according to the intensity of the emo-
tion these changes are confined to the cheeks only, or extend
to the " roots of the hair," or " all over."
What is the cause of these changes ? The blood is a red
and a hot fluid ; the skin reddens and grows hot, because
its vessels contain an increased quantity of this red and hot
fluid : and its vessels contain more, because the small arte-
ries suddenly dilate, the natural moderate contraction of
their muscles being superseded by a state of relaxation ; and
this relaxation comes on because the action of the nervous
system which previously kept the muscles in a state of mod-
erate contraction is, for the time, suspended.
92 ELEMENTARY PHYSIOLOGY LESi
On the other hand, in many people, extreme terror or
rage causes the skin to grow cold, and the face to appear
pale and pinched. Under these circumstances, in fact, the
supply of blood to the skin is greatly diminished, in conse-
quence of an increased contraction of the muscles of the
small arteries whereby these become unduly narrowed or
constricted, and thus allow only a small quantity of blood to
pass through them ; and this increased contraction of the
muscular coats of the arteries is brought about by the
increased action of the nervous system.1
That this is the real state of the case may be proved
experimentally upon rabbits. These animals may be made
to blush artificially. If, in a rabbit, the sympathetic nerve
(Fig. 33, Sy.), which sends branches to the vessels of the
head, is cut, the ear of the rabbit, which is covered by so
delicate an integument that the changes in its vessels can
be readily perceived, at once blushes. That is to say, the
vessels dilate, fill with blood, and the ear becomes red and hot.
The reason of this is that, when the sympathetic is cut, the
nervous impulse which is ordinarily sent along its branches
is interrupted, and the muscles of the small vessels, which were
previously slightly contracted, become altogether relaxed.
And it is quite possible to produce pallor and cold in the
rabbit's ear. To do this it is only necessary to irritate the
cut end of the sympathetic which remains connected with
the vessels. The nerve then becomes excited, so that the
muscular fibres of the vessels are thrown into a violent state
of contraction, which diminishes their calibre so much that
the blood can hardly make its way through them. Conse-
quently, the ear becomes pale and cold.
1 Sudden paleness is perhaps most frequently due to a failure or stop-
page of the heart's beat, as in fainting. But it may also be observed when
there is no change in the beat of the heart.
in VASO-MOTOR NERVES 93
This experiment on the blood-vessels of the rabbit's ear
is of fundamental importance as proof of the existence of
nerves which control locally the muscular elements of the
walls of the smaller arteries ; and, inasmuch as this control
consists in causing movements of the walls of the vessels, by
means of which their calibre is regulated, the nerves which
exert the control receive the general name of vaso-motor
nerves. But from the fact that, when the cut end of the
sympathetic nerve is irritated, or, as the physiologist says,
is " stimulated," the muscular walls of the arteries with which
it is connected are always contracted and the vessels them-
selves constricted, the sympathetic is more precisely charac-
terised as a vaso-constrictor nerve. Further, since merely
cutting the sympathetic leads to a dilation of the blood-
vessels of the ear, we are justified in assuming that vaso-
constrictor impulses are continually being sent out along this
nerve, whereby the arteries are kept continually in a condi-
tion of slight or medium constriction. To this condition
the name is given of arterial " tone." Now this " tone " is
of great importance, for by its existence it at once becomes
possible to increase the blood-supply to any part of the
body, as well as to diminish it. Did the arteries possess no
" tone " they would, under ordinary resting conditions, be
dilated to their full extent, and the part or organ they sup-
ply with blood would be receiving a maximum supply when
at rest. But the organs of the body are never at rest for
long, and when they become active they require an increased
amount of blood, which could not be supplied, at least by a
vaso-constrictor mechanism, but for the existence of this
arterial tone. It would of course be possible to increase the
blood-supply by means of an increased activity of the heart ;
but this would affect the supply to every part of the body at
the same time, and what is really wanted is a localised vari'
94 ELEMENTARY PHYSIOLOGY less.
ation in supply to meet the varying needs of each part or
organ. Thus the vaso-constrictor nerves act by carrying
more or less of the same kind of impulse, leading to increase
or decrease of tone and hence lessened or increased blood-
supply.
We have quoted blushing as being a characteristic and
familiar instance of the action of vaso-motor (vaso-constric-
tor) nerves. But other examples of exactly similar action
are met with throughout the whole body. Thus, when a
muscle contracts, or when a salivary gland secretes saliva, or
when the stomach is preparing to digest food, in each case
the small arteries of the muscle, salivary gland, or stomach,
dilate and so flush the part with blood. The organ in fact
blushes ; and this inner unseen blushing is, like the ordinary
blushing described above, brought about by vaso-motor
nerves. We shall see later on that the temperature of the
body is largely regulated by the supply of blood sent to the
skin to be cooled, and this supply is in turn regulated by
the vaso-motor nervous system. Indeed, everywhere, all
over the body, the nervous system by its vaso-motor nerves
is continually supervising and regulating the supply of blood,
sending now more, now less blood, to this or that part ; and
many diseases, such as those when exposure to cold causes
congestion or inflammation, are due to, or at least associ-
ated with, a disorder or failure of this vaso-motor activity.
16. The Vaso-motor Centre. — The vaso-constrictor
nerves, which, by causing the varying contraction in the
muscular walls of the arteries, thus control the supply of
blood to each region of the body, can all be traced back
to the spinal cord. They make their exit from this pari
of the central nervous system by the anterior roots of the
spinal nerves of the middle part of the cord, and after
passing through the ganglia of the sympathetic system
in THE VASOMOTOR CENTRE 95
(p. 516) are distributed to their various destinations. The
impulses which these nerves convey to the blood-vessels are
of course received by them from the spinal cord. This
being the case, the interesting question arises as to where
these impulses are generated before their exit from the
cord. Experiment shows that under ordinary circumstances
they come down the cord from a point higher up, i.e. nearer
the brain, than that at which the nerves themselves pass off
from the cord. In fact it has been shown that they origi-
nate in a very limited portion of the central nervous system,
located in that part of it which we shall describe in a later
Lesson (XII.) as the spinal bulb or medulla oblongata.
Here, then, the vaso-constrictor impulses are generated, and
since they are the chief agents in determining the state of
contraction or relaxation of the arteries of the body as a
whole, this definitely localised part of the bulb has received
the name of the vaso-motor centre. (Fig. 33, V.M.C.)
The cause of the phenomenon of arterial " tone " now
becomes quite clear. The vaso-motor centre continually
generates and sends out to every part, or rather to very
many parts, of the body, impulses which suffice to keep
the muscle fibres of the arteries supplying those parts in a
condition of slight contraction. When the impulses to any
part are increased, the supply of blood to that part is
lessened; when the impulses are lessened, the supply is
increased.
But if the vaso-motor centre is to be of use, it must itself
be under the influence of impulses which can be made to
play upon it in such a way as to determine those variations
in its activity which are essential to its adapting itself to the
varying needs of either the body as a whole or any small
part of the body. These impulses which govern the vaso-
motor centre pass into it either down from the brain above,
96 ELEMENTARY PHYSIOLOGY less.
or up from the spinal cord below. As an instance of the
former case we may refer once again to " blushing." Here
the emotion which leads to the blush starts impulses in the
brain (Fig. 33, a.f.), which then pass down to the vaso-
motor centre and modify its activity so as to lessen the
intensity of the impulses it sends to the blood-vessels of the
cheeks. As an instance of the second case we may refer to
the effects of heat and cold applied to the body, as deter-
mining those variations of blood-supply to the skin by which
the temperature of the body is so largely regulated (p. 229).
Here the impulses are started in the skin (Fig. 33, c.f.) and,
travelling along certain sensory nerves, enter the spinal cord,
pass up to the vaso-motor centre, and as before lead to the
necessary changes in its activity.
17. Vaso-dilator Nerves. — Our consideration of vaso-
motor nerves has so far led us to the view that the dilation
or widening of an artery which leads to increased blood-
supply is usually the result of cutting off or lessening con-
strictor impulses which were previously passing along the
nerves to the arteries. But instances are met with in the
body where the dilation is produced in an entirely different
way. Thus there is a certain nerve called the chorda
tympani, a branch of the facial or seventh cranial nerve
(p. 537), which runs to the submaxillary salivary glands.
When this nerve is simply severed, no obvious effect is pro-
duced on the blood-vessels of the gland. But if now the
cut end connected with the gland be stimulated, the small
arteries at once dilate powerfully, the blood-supply is enor-
mously increased, and the gland becomes bright red and
flushed. In this case we have to deal with a vaso-motor
nerve whose typical behaviour when stimulated is, speaking
broadly, the exact opposite to that of the vaso-constrictor
nerves. It is, in fact, a vaso-motor nerve such that impulses
VASO-DILATOR NERVES
97
passing along it give rise not to constriction but to dilation.
Hence it is spoken of as a vaso-dilator nerve. Other
Art
S.Art.--
V.M.C.
Sp.C.
A.S
Fig. 33. — Diagram to illustrate the Position of the Vaso-Motor Centre,
the Paths of Vaso-Constrictor Impulses from the Centre along the
Cervical Sympathetic Nerve and (part of) the Abdominal Splanchnic,
and the Course of Impulses to the Centre from the Brain and from
an outlying Part of the Body.
Sp.C. spinal cord; V.M.C. vaso-motor centre in spinal bulb; Art. artery of ear;
S.Art. subclavian artery; Sy. sympathetic nervous system, the cervical part with its
two ganglia above the subclavian artery, the thoracic part with several ganglia below
the artery; A.S. upper roots and part of abdominal splanchnic nerve, which carries
vaso-constrictor fibres to the abdominal organs. The dotted lines a.f. indicate paths
of conduction for impulses to the vaso-motor centre from the brain. The dotted lines
c.f. indicate paths for the passage of impulses to the vaso-motor centre from some
outlying part of the body such as the skin. The arrows show the directions in which
the impulses travel along each path.
instances of the occurrence of similar vaso-dilator nerves are
met with, but, as our knowledge of them is at present uncer
98 ELEMENTARY PHYSIOLOGY less.
tain and incomplete, we must be content with having sim-
ply drawn attention to their existence, and to one striking
instance of their action. It will be observed that vaso-
constrictor nerves lead to dilation only through interference
with the vaso-motor centre and tonic impulses ; vaso-dilator
nerves bring about dilation directly.
18. The Nervous Control of the Heart. Cardiac Nerves.
— The heart, as we all know, is not under the direct influ-
ence of the will, but every one is no less familiar with the
fact that the actions of the heart are wonderfully affected
by all forms of emotion. Men and women often faint, and
have sometimes been killed by sudden and violent joy or
sorrow ; and when they faint or die in this way, they do so
because the perturbation of the brain gives rise to a some-
thing which arrests the heart as dead as you stop a stop-
watch with a spring. On the other hand, other emotions
cause that extreme rapidity and violence of action which we
call palpitation. These facts suggest at once that the heart,
like the arteries, is subject to control by the central ner-
vous system, and we must now consider the more important
details of this control.
The heart is well supplied with nerves. There are many
small ganglia, or masses of nerve cells, lodged in the sub-
stance of the heart, more especially in the auricles, and
nerves spread from these ganglia over the walls, both of the
auricles and ventricles. Moreover, several nerves reach the
heart from the outside (Fig. 34). Of these the most im-
portant are branches of a remarkable nerve which starts
from the spinal bulb, and supplies not only the heart, but
the lungs, alimentary canal, and other parts, and which is
called the pneumogastric, or, from its wandering course, the
vagus (p. 538). Other nerves reaching the heart seem to
come from the sympathetic system, but may be traced back
in THE NERVES OF THE HEART 99
through this system to the spinal cord, and, for reasons
which will presently become apparent, are called accelera-
tor nerves.
The heart, as already explained (p. 75), contracts rhyth-
mically, but the regular rhythmical succession of the ordi-
nary contractions is not primarily dependent upon the ganglia
lodged in its substance, as was at one time supposed to be
the case. Neither does it depend on the action of the
nerves connected with the heart, since the movements con-
tinue even after the heart is removed from the body.
Hence we must conclude, and experiment bears out the
conclusion, that the muscular substance of which the heart
is made is itself endowed ivith the power of contracting and
relaxing at regular intervals. On the other hand, the influ-
ences which alter the heart's action, as in fainting or palpi-
tation, do as a rule come to the heart from without, and are
carried to the heart along the vagus and accelerator nerves.
This may be demonstrated on animals, such as frogs, with
great ease.
If a frog be pithed, or its brain destroyed, so as to oblit-
erate all sensibility, the animal will continue to live, and its
circulation will go on perfectly well for a prolonged period.
The body may be laid open without causing pain or other
disturbance, and then the heart will be observed beating
with great regularity. It is possible to make the heart
move a long lever backwards and forwards ; and if frog and
lever are covered with a glass shade, the air under which is
kept moist, the lever may vibrate with great steadiness for
a couple of days.
It is easy to adjust to the frog thus prepared a contri-
vance by which electrical shocks may be sent through the
vagus nerves, so as to stimulate them. If the stimulation is
only gentle or weak, the heart will be seen to beat more
ioo ELEMENTARY PHYSIOLOGY less.
slowly, and at the same time each beat is rather more feeble,
as shown by the diminished distance over which the end of
the lever moves. But if the stimulation is strong, the lever
almost immediately stops dead, and the heart will be found
quiescent, with relaxed and distended walls. After a little
time the influence of the vagus passes off, the heart recom-
mences its work as vigorously as before, and the lever vi-
brates through the same arc as formerly. With careful
management, this experiment may be repeated Very many
times ; and after every arrest by the stimulation of the
vagus, the heart resumes its work.
If, on the other hand, the stimulation be applied to the
sympathetic nerves, then an effect is produced which is
exactly the opposite to that which results from stimulating
the vagus. The lever moves more rapidly and over a
greater distance, showing quite clearly that the heart is now
beating faster and that each beat is stronger.
No clearer proof could be desired than is afforded by the
above experiments, that the heart of the frog is controlled
by two antagonistic nerves, of which one, the vagus, carries
impulses which slow and finally stop its beat, while the
other, the accelerator, conveys impulses which make it beat
faster. Since there is no reason for supposing that the
working mechanisms of a frog's heart differ in any essential
way from those of the mammalian heart, we may at once
apply these striking results to the human heart. It is, in
fact, recorded of a certain well-known physiologist, that,
having a small hard tumour in his neck, in close proximity
to the vagus nerve, he could press the vagus against this
tumour and by thus stimulating the nerve mechanically
cause a stoppage of his own heart-beat.
The heart, then, is controlled by two kinds of antagonistic
influences, analogous to those previously described as con-
in THE CARDIO-INHIBITOKY CENTRE 101
trolling the muscular walls of the arteries. Moreover, both
the cardiac nerves are connected with the central nervous
system, the one coming from the spinal bulb, the other from
the spinal cord, so that the influences they convey to the
heart must, as in the case of the vaso-motor nerves, origi-
nate in the central nervous system (Fig. 34). We saw, how-
ever (p. 94), that the impulses carried by' the vaso-motor
nerves are generated in a very specially localised part of the
spinal bulb, and the interesting question at once arises : Is
there a similarly localised centre in which the impulses
which modify the beat of the heart take their origin? The
answer to this question is in the affirmative, for experiment
shows that the impulses which, travelling along the vagus,
can stop or, as the physiologist says, "inhibit" the heart's
beat, are generated in a limited part of the spinal bulb, in
close proximity to the vaso-motor centre. This part is
therefore known as the cardio-inhibitory centre (Fig. 34,
C.I.C.). There are reasons for supposing that this centre,
like the vaso-motor centre, is continually at work sending
out impulses to the heart along the vagus, which check its
activity, so that in many animals the heart beats more
quickly after the vagus nerves are cut.
The cardio-inhibitory centre may, like the vaso-motor
centre, be itself influenced by impulses which reach it either
from the brain above or the spinal cord below. In this way
the heart is indirectly connected with all parts of the body,
so that by nervous agencies its beat may be made to van-
according to the varying needs of the body as a whole or of
its several parts. For instance, when taking exercise, the
restraining influence of the centre is lessened and the heart
beats faster, thus providing for an increased rapidity of the
circulation to meet the demands of the more actively con-
tracting muscles. It is, of course, possible that the faster
ELEMENTARY PHYSIOLOGY
beat of the heart may also be due to impulses along the
accelerator nerves. Again, when a person faints from a
sudden emotion, an influence is started in the brain, passes
a.f.
^ k
feb.Vg
-C.I.C.
--Sp.C.
■**-m.f.
Fig. 34. — Diagram to illustrate the Position of the Cardio-Inhibitorv
Centre, the Paths of Inhibitory and Accelerator Impulses from the
Central Nervous Svstem to the Heart, and the Course of Impulses
to the Centre from the Brain and from an outlying Part of the Body.
Sp.C. spinal cord; C.I.C. cardie-inhibitory centre; V.G. ganglion of the vagus;
Vg. main trunk of the vagus; c.b-.Vg. cardiac branches of vagus, supplying the
heart; S.Art. subclavian artery; Sy. sympathetic nervous system, the cervical part
with its two ganglia above the subclavian artery, the thoracic part with several gan-
glia below the artery, c.b.Sy. cardiac branches of the sympathetic supplying the heart.
The dotted lines a.f. indicate paths of conduction for impulses to the cardio-inhibitory
centre from the brain. The dotted lines m.f. indicate paths for the passage of im-
pulses to the cardio-inhibitory centre from some outlying part of the body such as
the stomach or intestines. The arrows show the directions in which the impulses
travel along each path. •
down to the centre in the spinal bulb (Fig. 34, a.f.), in-
creases its action and stops for a lime the beating of the
heart. Or again, fainting may result from a blow on the
in THE CARDIO-INHIBITORY CENTRE 103
stomach ; in this case, the influence starts at the part struck
(Fig. 34, m.f.), and, passing up the spinal cord to the
cardio-inhibitory centre, increases its activity and leads as
before to stoppage of the heart. The rapid and violent
beating of the heart which we speak of as " palpitation "
may, on the other hand, be often due to some emotion
which in this case lessens the activity of the centre and
hence diminishes the restraint which it ordinarily exerts
over the heart. But of course palpitation may also, at
times, be due to impulses reaching the heart along those
nerves which we have described above as the accelerators.
Our knowledge of the existence and position of the car-
dio-inhibitory centre is quite clear and definite. It is possi-
ble that a cardio-augmentor (-accelerator) centre may also
exist, but at present we have no exact knowledge of its exist-
ence ; hence in the accompanying figure the accelerator
nerves are shown, as originating in the central nervous sys-
tem, but not arising from any definitely localised centre.
19. The Proofs of the Circulation. — The evidence that
the blood circulates in man, although perfectly conclusive,
is almost all indirect. The most important points in the
evidence are as follows : —
In the first place, the disposition and structure of the
organs of circulation, and more especially the arrangement
of the various valves, will not, as was shown by Harvey,
the discoverer of the circulation (1628), permit the blood
to flow in any other direction than in the one described
above. Moreover, we can easily with a syringe inject a
fluid from the " vena cava, for instance, through the right
side of the heart, the lungs, the left side of the heart, the
arteries and capillaries, back to the vena cava ; but not the
other way. In the second place, we know that in the living
body the blood is continually flowing in the arteries towards
104
ELEMENTARY PHYSIOLOGY
the capillaries, because when an artery is tied, in a living
body, it swells up and pulsates on the side of the ligature
Fig. 35. — Portion of the Web of a Frog's Foot seen under a low Magni-
fying Power, the Blood-vessels only being represented, except in
the Corner of the Field, where in the Portion marked off the Pig-
ment Spots are also drawn.
a, small arteries; 7', small veins; the minute tubes joining the arteries and the
veins are the capillaries. The arrows denote the direction of the circulation. The
larger artery running straight up in the middle line breaks up into capillaries at
points higher up than can be shown in the drawing.
nearest the heart, whereas on the other side it becomes
empty, and the tissues supplied by the artery become pale
?H THE PROOFS OF THE CIRCULATION 105
from the want of a supply of blood to their capillaries. And
when we cut an artery the blood is pumped out in jerks
from the cut end nearest the heart, whereas little or no
blood comes from the other end. When, however, we tie a
vein the state of things is reversed, the swelling taking place
on the side farthest from the heart, etc. etc., showing that
in the veins the blood flows from the capillaries to the
heart.
But certain of the lower animals, the whole, or parts, of
the body of which are transparent, readily afford direct
proof of the circulation ; in these the blood may be seen
rushing from the arteries into the capillaries, and from the
capillaries into the veins, so long as the animal is alive and
its heart is at work. The animal in which the circulation
can be most conveniently observed is the frog. The web
between its toes is very transparent, and the corpuscles sus-
pended in its blood are so large that they can be readily seen
as they slip swiftly along with the stream of blood, when the
toes are fastened out, and the intervening web is examined
under a microscope (Fig. 35).
20. The Capillary Circulation. — The essential charac-
teristics of blood-flow through the capillaries may also be
easily studied in such a preparation. In the smallest capil-
laries the corpuscles pass along singly, sometimes following
each other in close file, at other times leaving quite con-
siderable gaps in their succession. Frequently one or more
corpuscles may remain stationary for a moment and then
pass on again. The red corpuscles, which in the frog are
oval and comparatively large, glide along with their long
axis parallel to the direction of the stream, and may often
be observed to be squeezed out of shape by pressure against
the wall of the capillary (Fig. 36, G and H). In the larger
capillaries, more especially in mammals whose corpuscles are
io6
ELEMENTARY PHYSIOLOGY
Fig. 36. — Very small Portion of Fig. 35 very highly magnified.
A, walls of capillaries; B, tissue of web lying between the capillaries; C, cells oi
epidermis covering web (these are shown only in the right hand and lower part of the
field; in the other parts of the field the focus of the microscope lies below the epider-
mis); D, nuclei of these epidermal cells; E, pigment cells contracted, not partially
expanded as in Fig. 3s: /■', red blood-corpuscle (oval in the frog) passing along
capillary nucleus not visible; (?, another corpuscle squeezing its way through a
capillary, the 1 anal "f which is smaller than its own transverse diameter; //, another
corpuscle bending as it slides round a corner; A", corpuscle in capillary seen througli
the epidermis; /, white blood-corpuscle.
in INFLAMMATION 10}
smaller than in the frog, the corpuscles often pass along two
or three abreast. Further, in these larger capillaries it may
be seen that the red corpuscles tend to keep to the centre
of the stream, leaving a clear layer of fluid along the sides
of the blood-vessels. This is due to the fact that the fluid
friction (already referred to on p. 81) is greater close to the
walls of the capillaries than in the middle of the stream, and
the corpuscles pass along where the resisting friction is least.
The colourless or " white " corpuscles usually move more
slowly and irregularly than the red, and may, as a rule, be
seen to lie in the clearer layer of fluid at the side of the cur-
rent. Moreover, they frequently stop for an appreciable
time, as if sticking to the wall of the capillary, and then roll
on again ; probably because they are more adhesive than
the red corpuscles, in harmony with their power of executing
amoeboid movements (see p. 126).
21. Inflammation. — All persons are more or less familiar
with a peculiar and unusual condition which may arise in
almost any part of the body, and which they describe by
speaking of the part as "inflamed." To ordinary observa-
tion the characteristics of the condition are that the inflamed
region becomes flushed and red, that it feels warmer than
usual, that it becomes swelled and painful, and, finally, if the
inflammation is severe, that a thick yellowish fluid is formed
which is commonly known as " matter," or more correctly
as pus. Such a series of changes may be observed during
the formation and breaking of a boil. But the several stages
just named are merely the external evidences of changes
taking place at the same time in the minute blood-vessels
and circulation of the part affected, and, since these changes
throw an interesting light on the relations ordinarily existing
between the walls of the blood-vessels and the adjacent
blood, they are worthy of a short consideration.
io8 ELEMENTARY PHYSIOLOGY less
If, when the web of a frog's foot, or other suitably trans-
parent part of an animal, is adjusted for observation under
the microscope, some irritant be applied to it such as a trace
of mustard,1 the following events may be readily observed.
The minute arteries dilate, the blood flows faster, and the
increased quantity of blood forced through the capillaries
distends them so that they, as well as the smallest veins,
appear to be similarly dilated. This accounts for the initial
greater redness and warmth of an inflamed part. Very
soon the colourless corpuscles are seen to be collecting in
large numbers in the clear layer of fluid next to the walls of
the capillaries and veinlets, and seem to adhere more firmly
than usual to the walls of these vessels. Further, blood
"platelets" (see p. 130), not previously visible, begin to
collect also with and among the white corpuscles. Follow-
ing upon this the stream of blood begins to flow more slowly
although the blood-vessels are still widely dilated. And now
a very striking phenomenon takes place. The white cor-
puscles make their way by amoeboid movements through the
thin walls of the capillaries and collect outside them in the
spaces in the neighbouring tissue. At the same time that
the corpuscles are in this way " migrating," a considerable
quantity of the fluid part of the blood also passes out through
the walls of the blood-vessels into the adjacent tissue. This
accounts for the characteristic swelling of an inflamed part.
If the action of the irritant is continued, more and more
white corpuscles collect in the vessels, the blood-flow be-
comes slower and slower, red corpuscles are arrested in large
numbers among the white, and finally the circulation stops
altogether. At this stage red corpuscles pass through the
walls of the vessels as well as the white, and the latter, multi-
1 Used similarly as an irritant in the form of the ordinaiy domestic mus-
tard poultice.
Ill THE LYMPHATIC SYSTEM 109
plying rapidly in the spaces of the tissue outside the blood-
vessels, and undergoing certain other slight changes, are
converted into pus corpuscles.
The appearances just described seem to indicate that the
condition of the walls of the capillaries (and of the smallest
veins and arteries) plays a very important but as yet obscure
part in determining the characteristics of the normal circula-
tion through these passages. And since in an inflamed area
the flow of blood becomes slower and slower, and ultimately
ceases, even while the blood-vessels are more widely dilated
than usual, the condition of the walls of these vessels may
evidently play a very important part in determining varia-
tions in that " peripheral resistance " which, as we have
previously explained, is of paramount importance to the
working of the circulation throughout every part of the
whole body. Moreover, it is evident that the condition of
the walls of the capillaries may also at any moment modify
the amount of the fluid part of the blood which is continually
passing out through those walls as lymph (p. no) for the
nutrition of the neighbouring tissues.
Part II. — The Lymphatic System and the Circulation
of Lymph
1. The General Arrangement of the Lymphatics. —
Food, as we have already pointed out (p. 22), after diges-
tion in the alimentary canal, is absorbed into the blood-vessels
and lacteals of that canal and whirled away in the current
of the circulation for distribution as nutritive material to all
parts of the body. But we have also drawn attention to
the fact (p. 56) that the ultimate anatomical components,
the cells and tissues, of every part of the body lie outside the
blood-vessels. It is therefore clear that the tissues are every-
no ELEMENTARY PHYSIOLOGY less.
where separated from the blood by at least the thickness of
the walls of the vessels, and in any case cannot draw the
nutriment they require directly from the blood, since they
are nowhere in direct contact with it. Neither can they, for
the same reason, discharge the waste they are always produc-
ing directly into the blood for its removal as a preliminary
to its excretion. Both these difficulties are however got over
by the fact that a portion of the fluid part of the blood is
continually exuding through the walls of the capillaries into
the neighbouring tissues, taking with it the nutriment neces-
sary for each tissue and providing a fluid connection between
the tissue and the blood across which the waste from the
tissues can be returned into the blood. The fluid which thus
exudes is called lymph,1 and may be regarded as a sort of
" middleman " between the blood on the one hand and the
tissue on the other. But if this lymph is to be thoroughly
efficient as a nutriment for the tissues it should, presumably,
contain more food material than the tissues actually require
as an average, and it must, therefore, be an economy to the
body if the lymph, after having served the needs of the tis-
sues, is gathered up again and returned to the blood for
further use. Now this is exactly what does take place, and
the means for ensuring the return of the lymph to the blood-
vessels are as follows.
Besides the capillary network and the trunks connected
with it which constitute the blood-vascular system, all parts
of the body which possess blood capillaries also contain
another set of what are termed lymph- capillaries, mixed up
with those of the blood-vascular system, but not directly
communicating with them, and, in addition, differing from
the blood-capillaries in being connected with larger vessels
1 The mode of formation, composition, and properties of lymph are dealt
with in Lesson IV.
Ill
THE LYMPHATIC SYSTEM
of only one kind. That is to say, they open only into trunks
that carry fluid away from them and thus bear the same
relationship to the lymph-capillaries that
the veins do to blood-capillaries. These
trunks are known as the lymphatic vessels,
and further resemble the small veins in the
general structure of their walls and in being
abundantly provided with valves, similar to
those in the veins, which freely allow of the
passage of lymph from the lymph-capilla-
ries, but obstruct the flow of any liquid in
the opposite direction. But the lymphatic
vessels differ from the veins in that they
do not rapidly unite into larger and larger
trunks which present a continually increas-
ing calibre and allow a flow without inter-
ruption to the heart. On the contrary,
remaining nearly of the same size, they at
intervals become connected with small,
rounded or often bean-shaped bodies called
lymphatic glands, entering the glands at one
side and emerging at the opposite side as FlG- 37-— The Lym-
0 . PHATICS OF THE
new lymphatic vessels (Fig. %7,g). Front of the
J i \ 5 OltSJ Right Arm.
Sooner or later the great majority of the , , ... . ,
0 J J g, lymphatic glands,
smaller lymphatic vessels pour their con- °" ,the course of
J l x the lymphatics.
tents into a tube which is about as large
as a goose-quill, lies in front of the backbone, and is
called the thoracic duct. This opens at the root of the
neck into the conjoined trunks of the great veins (jugular
and subclavian) which bring back the blood from the left
side of the head and the left arm. (Fig. 38,/, g.)
The remaining lymphatics, chiefly those of the right side
of the head and neck, the right arm and right lung, are con-
ii2 ELEMENTARY PHYSIOLOGY less,
nected by a common canal with the corresponding vein of
the right side.
The lower part of the thoracic duct is dilated, and is
called the receptacle of the chyle (Fig. 38, a). This part
receives more particularly the lymphatics from the intestines,
which, though they differ in no essential respect from other
lymphatics, are called lacteals, because, after a meal con-
taining much fatty matter, they are filled with a milky fluid
termed chyle. The lacteals, or lymphatics of the small
intestine, not only form networks in its walls, but send blind
prolongations into the little processes termed villi, with
which the mucous membrane of that intestine is beset.
(P. 280.)
Where the two principal trunks of the lymphatic system
open into the veins, valves are placed, which allow of the
passage of fluid in one direction only, namely from the lym-
phatic to the veins, the blood in the veins being unable to
get into the lymphatics, and in this way the lymph from
every part of the body is collected and returned into the
blood.
2. The Origin and Structure of Lymphatics. — The
cells of which the tissues of the body are built up, though
lying closely applied to each other, are often separated by
extremely minute spaces. These spaces are particularly
plentiful in that form of connective tissue called "areolar."
As has been seen (p. 49), it is made up of bundles of fine
threads or fibres which cross one another in all directions
and thus form a sort of feltwork of interlacing fibres. Some
of the spaces in this tissue are comparatively large and are
called areolce, whence the name areolar tissue. This tissue
is, as we have said (p. 11), present in every part of the
body, and of course supports the blood-capillaries, which
are thus, in reality, merely minute tubes lying imbedded in
HI
THE THORACIC DUCT
"3
connective tissue. The chinks and spaces of the tissues are
filled with that fluid exudation from the blood-vessels and
Fig. 38. —The Thoracic Duct.
The thoracic duct occupies the middle of the figure. It lies upon the spinal
column, at the sides of which are seen portions of the ribs (/).
a, the receptacle of the chyle; i, the trunk of the thoracic duct, opening at c into
the junction of the left jugular (_/) and subclavian {g) veins as they unite into the
left innominate vein, which has been cut across to show the thoracic duct running
behind it; if, lymphatic glands placed in the lumbar regions; h, the superior vena
cava, formed by the junction of the right and left innominate veins.
114 ELEMENTARY PHYSIOLOGY less.
tissue elements already spoken of as lymph, and hence are
themselves often called lymph-spaces. In these lymph-spaces
we see the origin or beginning of the lymphatic system.
From the lymph-spaces the lymph passes directly into
the lymph- capillaries (Fig. 39). These are also essentially
spaces in the mesh work of connective tissue, but they are
now lined by a single layer of extremely thin, flat, nucleated,
epithelial cells, very similar to those composing the wall of
Fig. 39. — Origin of Lymphatics. (After Landois.)
S, lymph-spaces opening directly into lymphatic capillary; A , lymph-spaces unil-
ing to form a lymphatic capillary: E, epithelial cells forming walls of capillnry.
a blood-capillary. These cells are joined to each other by
their edges so that they form a system of minute tubes,
larger than blood-capillaries and wandering more irregularly.
The lymphatic vessels, into which the lymph-capillaries
pour their contents on the way towards the thoracic duct,
possess a structure essentially similar to that of a vein
LSI LYMPHATIC GLANDS 115
(P- 59) > but they differ from a vein in that their walls are
thinner, so thin -as to be very transparent, are relatively more
muscular, and are more plentifully supplied with valves. The
structure of the latter is the same as in the veins.
3. The Structure and Function of Lymphatic Glands. —
Lymphatic glands occur at more or less frequent intervals
along the course of the lymphatic vessels. They are of very
variable size, being somewhat rounded when small, and
when large having more or less the shape of a bean. The
V
-A.L
Fig 40. — Diagrammatic Representation of a Lymphatic Gland seen in
Section. (After Sharpey.)
Cap. capsule; 7V. trabecules: G.S. glandular substance; L.S. lymph-sinus.
In the alveolus marked / all the leucocytes are supposed to have been washed out;
in the rest of the gland they are shown in the glandular substance, but washed out
of the lymph-sinuses. A.L. afferent lymphatic; E.L. efferent lymphatic. The
arrows show the direction in which the lymph enters and leaves the gland.
afferent lymphatic vessels enter the gland by several
branches on its more convex side, and emerge in diminished
numbers as efferent vessels from the opposite side. Blood-
vessels enter and leave the glands side by side with the
efferent lymphatic vessels.
nb ELEMENTARY PHYSIOLOGY less.
Each gland is covered externally by a capsule or coat of
connective tissue, with which some unstriated muscle fibres
are not infrequently mixed. This capsule sends partitions,
called trabeculae, inwards and towards the centre of the
gland, which divide it into compartments or alveoli, the
compartments being very regularly arranged at the outer
portion or cortex of the gland and irregularly in the more
central parts or medulla (see Fig. 40). Each alveolus is
filled with a network of connective tissue, whose meshes are
small and closely set in the central part of the alveolus,
wider or more open where in contact with the trabeculae.
The central small meshed network is known as adenoid tis-
sue (p. 53), is densely packed with lymph-corpuscles or
leucocytes closely resembling the colourless corpuscles of
blood (p. 126), and constitutes what is usually spoken of as
the glandular substance. The more open-meshed network
which surrounds the glandular substance and separates it
from the trabeculae is known as the lymph-sinus or lymph-
channel. The meshes of the lymph-sinus, like those of the
glandular substance, are crowded with leucocytes, but these
are not very firmly fixed in this network, as they are in that
of the glandular substance, and may be readily washed out
by shaking a thin slice of the gland in water.
The lymphatic vessels which bring lymph to the gland
open directly into the channel of the lymph-sinus, and those
vessels which gather up the lymph to carry it away from the
gland open out of the lymph-sinuses.
The leucocytes which crowd the glandular substance
present under the microscope appearances of cell division,
which leave no doubt that they are undergoing rapid and
probably large increase in numbers. But, since the size of
each gland is ordinarily constant, a continual removal of the
newly formed leucocytes must be taking place. This view
in LYMPHATIC GLANDS 117
is borne out by the observation that leucocytes are more
numerous in the lymph coming from a gland than in that
which flows to it. The removal takes place by a discharge
of leucocytes from the glandular substance into the meshes
of the neighbouring lymph-sinus, whence they are then
washed away in the current of lymph, which is always slowly
flowing through the sinuses. In this way the lymphatic
glands provide a constant supply of leucocytes, which are
passed ultimately into the blood and become those white or
colourless corpuscles with which we shall have to deal in the
next Lesson.
4. Causes which lead to the Movements of Lymph. —
Throughout the preceding description of the lymphatic sys-
tem we have spoken of the lymph as flowing along a series
of passages, from their origin in the tissues to the point
where they become connected with the blood-vessels. The
cause of this flow is not so immediately apparent as it is in
the case of the blood, for the lymphatic system possesses no
central pump, such as the heart, to keep the lymph in mo-
tion.1 In the absence, then, of any obviously propulsive
mechanism, to what may we attribute this continual passage
of lymph along the lymphatics?
The flow is in reality brought about by several causes.
We may point out, in the first place, that the processes
of filtration and diffusion, whose nature will be considered
in the next Lesson, are at work to determine the initial
exit of fluid- from the blood-vessels into the lymph-spaces.
These processes must obviously tend to drive out the lymph
already in those spaces into and along the channels leading
1 The frog possesses four lymph-hearts, placed in two pairs at the upper
and lower end of the backbone. Their structural arrangement is similar to
but simpler than that of the blood-heart, and, being rhythmically contractile,
they pump the lymph into the venous system.
n8 ELEMENTARY PHYSIOLOGY less, in
from them. Further, as we have seen, the blood-pressure
in the large veins near the heart is very small and is cer-
tainly much less than it is in the capillaries ; and since the
lymphatics originate at the capillaries and discharge their
contents into the great veins, this difference of pressure at
the two ends of the lymphatic system must tend to cause
an onward flow of lymph. Here also the movements of
respiration play a part, for, as will be seen when dealing
with respiration, the pressure in the great veins is suddenly
diminished at each inspiration, and lymph is thus sucked
out of the thoracic duct, no reversal of this action being
possible at expiration because of the valves guarding the
end of the duct. Finally, one great cause of lymph-flow
is the contraction of the muscles throughout the body and
the resulting pressure upon the lymphatics. As in the
veins (p. 61), so in the lymphatic vessels; when any press-
ure is applied to their outside, the lymph is driven out of
the squeezed part, and since the valves open only towards
the junction of the thoracic duct with the venous system, the
lymph is thereby driven along in the desired direction.
LESSON IV
THE BLOOD AND THE LYMPH
1. Microscopic Examination of Blood. — In order to
become properly acquainted with the characters of the
blood, it is necessary to examine it with a microscope
magnifying at least three or four hundred diameters. Pro-
vided with this instrument, a hand lens, and some glass
slides and coverslips, the student will be enabled to follow
the present lesson.
The most convenient mode of obtaining small quantities
of blood for examination is to twist a piece of string, pretty
tightly, round the middle of the last joint of the middle, or
ring finger, of the left hand. The end of the finger will
immediately swell a little, and become darker coloured, in
consequence of the obstruction to the return of the blood
in the veins caused by the ligature. When in this condi-
tion, if the finger be slightly pricked with a sharp, clean
needle (an operation which causes hardly any pain), a
good-sized drop of blood will at once exude. Let it be
deposited on one of the glass slides, and covered lightly
and gently with a coverslip, so as to spread it out evenly
into a thin layer. Let a second slide receive another drop,
and, to keep it from drying, let it be put under an inverted
watch-glass or wine-glass, with a bit of wet blotting-paper
inside. Let a third drop be dealt with in the same way, a
few granules of common salt being first added to the drop.
119
120 ELEMENTARY PHYSIOLOGY less
To the naked eye the layer of blood upon the first slide
will appear of a pale reddish colour, and quite clear and
homogeneous. But on viewing it with even a pocket lens,
its apparent homogeneity will disappear, and it will look
like a mixture of excessively fine yellowish-red particles,
like sand, or dust, with a watery, almost colourless, fluid.
Immediately after the blood is drawn, the particles will
appear to be scattered very evenly through the fluid, but
by degrees they aggregate into minute patches, and the
layer of blood becomes more or less spotty.
The " particles " are what are termed the corpuscles of
the blood ; the nearly colourless fluid in which they are
suspended is the plasma.
The second slide may now be examined. The drop of
blood will be unaltered in form, and may perhaps seem to
have undergone no change. But if the slide be inclined,
it will be found that the drop no longer flows ; and, in-
deed, the slide may be inverted without the disturbance
of the drop, which has become solidified, and may be re-
moved, with the point of a penknife, as a gelatinous mass.
The mass is quite soft and moist, so that this setting, the
clotting or coagulation, of a drop of blood is something
very different from its drying.
On the third slide, this process of clotting will be found
not to have taken place, the blood remaining as fluid as
it was when it left the body. The salt, therefore, has
prevented the coagulation of the blood. Thus this very
simple investigation teaches that blood is composed of a
nearly colourless plasma, in which many coloured corpuscles
are suspended ; that it has a remarkable power of clotting ;
and that this clotting may be prevented by artificial means,
such as the addition of salt.
If, instead of using the hand lens, the drop of blood
iv THE CORPUSCLES OF THE BLOOD 121
on the first slide be placed under the microscope, the
particles, or corpuscles, of the blood will be found to be
bodies with very definite characters, and of two kinds,
called respectively the red corpuscles and the white or
colourless corpuscles. The former are much more numer-
Fig. 41. — Red and White Corpuscles of the Blood, Magnified.
A. Moderately magnified. The red corpuscles are seen lying in rouleaux; at a
and a are seen two white corpuscles
B. Red corpuscles much more highly magnified, seen in face; C. ditto, seen in
profile; D. ditto, in rouleaux, rather more highly magnified; E. a red corpuscle
swollen into a sphere by imbibition of water.
F. A white corpuscle magnified the same as B.
H. Red corpuscles puckered or crenate all over.
/. Ditto, at the edge only.
ous than the latter, and have a yellowish-red tinge ; when
one of these corpuscles is seen, under a high power of the
microscope, lying by itself, it seems to be hardly more than
faintly yellow in colour, but when several are seen lying
one on the other, the redness becomes obvious. The
122 ELEMENTARY PHYSIOLOGY less.
white, somewhat larger than the red corpuscles, are, as
their name implies, pale and devoid of coloration.
The corpuscles differ also in other and more important
respects.
2. The Red Corpuscles. — The red corpuscles (Fig. 41)
are flattened circular discs, on an average 7/x. to 8/x (-g-gV^
of an inch) in diameter, and having about one-fourth of that
thickness. It follows that rather more than 10,000,000 of
them will lie on a space one inch square, and that the vol-
ume of each corpuscle does not exceed tto.o"ooit~o o7o"7 °f a
cubic inch.
The broad faces of the discs are not flat, but somewhat
concave, as if they were pushed in towards one another.
Hence the corpuscle is thinner in the middle than at the
edges, and when viewed under the microscope, by trans-
mitted light, looks clear in the middle and darker at the
edges, or dark in the middle and clear at the edges, accord-
ing as it is or is not in focus. When, on the other hand,
the discs roll over and present their edges to the eye, they
look like rods. All these varieties of appearance may be
made intelligible by taking a small, round, flat disc of clay
or putty and squeezing the central part of the two flat sides
between the thumb and finger, so as to make the centre
thinner than the edges ; the disc is now more or less similar
in shape to the red corpuscles, and may be turned into
various positions before the eye.
In a drop of blood immediately after it is drawn, the
red corpuscles float about, and roll or slide over each
other quite freely. After a short time (the length of which
varies in different persons, but usually amounts to two or
three minutes), they seem, as it were, to become sticky,
and tend to cohere ; and this tendency increases until, at
length, the great majority of them become applied face to
iv THE RED CORPUSCLES 123
face, so as to form long series, like rolls of coin. The end
of one roll cohering with the sides of another, a network of
various degrees of closeness is produced (Fig. 41, A).
The corpuscles remain thus coherent for a certain length
of time, but eventually separate and float freely again. The
addition of a little water, or dilute acids or saline solutions,
will at once cause the rolls to break up.
It is from this running together of the corpuscles into
patches of network that the change noted above in the
appearances of the layer of blood, viewed with a lens,
arises. So long as the corpuscles are separate, the sandy
appearance lasts; but when they run together, the layer
appears patchy or spotted.
The red corpuscles, rarely, if ever, all run together into
rolls, some always remaining free in the meshes of the net.
In contact with air, or if subjected to pressure, many of the
red corpuscles become covered with little knobs, so as to
look like minute mulberries — an appearance which is due
to the concentrating, by evaporation, of the fluid in which
they are floating (Fig. 41, H, H).
The red corpuscles are very soft, flexible, and elastic
bodies, so that they readily squeeze through apertures and
passages narrower than their own diameters, and immedi-
ately resume their proper shapes (Fig. 36, G, H). Exam-
ined under even a high power the red corpuscle presents
no very obvious structure; when, however, blood is frozen
and thawed one or more times, or when it is treated in
certain other ways, as, for instance, by the addition of water,
the colouring matter which gave each corpuscle its yellow
or yellowish-red tinge is dissolved out and passes into the
surrounding fluid, and all that is left of the corpuscle is a
colourless framework appearing often under the microscope
as a pale, hardly visible, ring. Each corpuscle in fact con-
i24 ELEMENTARY PHYSIOLOGY less,
sistsof a sort of spongy colourless framework, the stroma,
composed of the kind of material known as proteid and of
a peculiar colouring matter, which, in the natural condition,
is intimately connected with this framework, but may by
appropriate means be removed from it. This colouring
matter, which is of a highly complex nature, is called
haemoglobin, and may by proper chemical treatment be
resolved into a reddish-brown substance containing iron,
called hsematin, and a colourless proteid substance.
Each corpuscle therefore is not to be considered as a
bag or sack with a definite skin or envelope containing
fluid, but rather as a sort of spongy semi-solid or semi-fluid
mass, like a disc of soft jelly ; and as such is capable of
imbibing water and swelling up, or giving out water and
shrinking, according to the density of the fluid in which it
may be placed. Thus, if the plasma of blood be made
denser by dissolving saline substances, or sugar, in it, water
is drawn from the substance of the corpuscle to the dense
plasma, and the corpuscle becomes still more flattened and
very often much wrinkled. On the other hand, if the
plasma be diluted with water, the latter forces itself into
and dilutes the substance of the corpuscle, causing the
latter to swell out, and even become spherical ; and, by
adding dense and weak solutions alternately, the corpuscles
may be made to become successively spheroidal and dis-
coidal. Exposure to carbonic acid gas seems to cause
the corpuscles to swell out ; oxygen gas, on the contrary,
appears to flatten them.
The stroma or framework constitutes but a very small
part, 10 per cent., of the solid matter of which the red
corpuscles are composed, the remaining 90 per cent, con-
sisting of the colouring matter or haemoglobin. The cor-
puscles may, therefore, be regarded simply as so many tiny
IV
H.EMOGLOBIN CRYSTALS
125
masses of haemoglobin. Now haemoglobin, we may say at
once, possesses the remarkable property of uniting in a
peculiar way with considerable quantities of oxygen, and
thus confers on the red corpuscles their one great charac-
teristic of acting as the carriers of oxygen from the lungs
to the tissues of all parts of the body.
The colouring matter of the corpuscles is further charac-
terised by its property of crystallising more or less readily.
Fig. 42. — Crystals of Haemoglobin. (After Funke.)
a, squirrel; b, guinea-pig; c, cat or dog; d, man; e, hamster, a European rodent.
If a small quantity of rat's or dog's blood, from which the
fibrin has been removed (see p. 136), be shaken up with a
small quantity of ether, it loses its opacity and becomes
quite transparent in thin layers, or as it is often called "laky."
The transparency results from the discharge of the haemo-
globin from the stroma into the neighbouring fluid, in which
it is now in solution. If the vessel containing the laky blood
126 ELEMENTARY PHYSIOLOGY less.
be allowed to .stand on ice for some hours, a sediment usu-
ally forms at the bottom, and will be found in a successful
experiment, when examined with the microscope, to consist
chiefly of blood-crystals (Fig. 42). The crystals differ in
shape according to the animal from whose blood they were
obtained ; in man they have the shape of prisms. The haemo-
globin of human blood crystallises with difficulty, but that of
the guinea-pig, rat, or dog, much more readily.
3. The White Corpuscles. — -The colourless corpuscles
(Fig. 41, a, a, F) are larger than the red corpuscles, their
average diameter being 10/x ( 2 ^ 0 of an inch). They are
further seen, at a glance, to differ from the red corpuscles
by the irregularity of their form, and by their greater sticki-
ness or adhesiveness, shown by their tendency to attach
themselves to the glass slide, while the red corpuscles float
about and tumble freely over one another.
A still more remarkable feature of the colourless corpuscles
than the irregularity of their form is the unceasing variation
of shape which they exhibit so long as they are alive. The
form of a red corpuscle is changed only by influences from
without, such as pressure, or the like ; that of the colourless
corpuscle is undergoing constant alteration, as the result of
changes taking place in its own substance. To see these
changes well, a microscope with a magnifying power of five
or six hundred diameters is requisite, and some arrangement
for keeping the preparation gently warmed (to 400 C), since
heat makes the movements more active ; and, even then,
they are so gradual that the best way to ascertain their exist-
ence is to make a drawing of a given colourless corpuscle
at intervals of a minute or two. This is what has been done
with the corpuscle represented in Fig. 43, in which a repre-
sents the form of the corpuscle when first observed ; b, its
form one minute afterwards ; c, that at the end of the second
iv THE WHITE CORPUSCLES 12;
minute ; d, that at the end of the third; and e, that at the
end of the fifth minute.
Careful watching of a colourless corpuscle, in fact, shows
that every part of its surface is constantly changing — under-
going active contraction or being passively dilated by the
contraction of other parts. It exhibits contractility in its
lowest and most primitive form.
Fig. 43. — Successive Forms assumed by Colourless Corpuscles of Human
Blood. (Magnified about 600 diameters.)
The intervals between the forms a, b, c, d, were one minute each; between d and
t two minutes; so that the whole series of changes from a to e took five minutes.
While they are thus living and active, a complete know-
ledge of the structure of the colourless corpuscles cannot be
arrived at. Each corpuscle seems to consist simply of a
mass of coarsely or finely granular protoplasm (p. 32), in
which no distinction of parts can be seen (Fig. 41, F}.
This is especially the case when the corpuscle is at rest and
assumes a spheroidal shape. Sometimes, however, the cor-
puscle, in the course of the movements just described, spreads
itself out into a very thin flat film ; and when that is the
case there may be seen in its interior a rounded body, dif-
fering in appearance from the rest of the body of the cor-
puscle. Again, when a drop of blood is diluted with water,
still better with very dilute acetic acid, the spongy pro-
toplasm of the white corpuscles swells up and becomes
transparent, many of the granules becoming dissolved, and
in this case the same rounded body becomes visible. This
internal rounded body, which differs in nature from the rest
of the substance of the corpuscles, is the nucleus \ and when
128 ELEMENTARY PHYSIOLOGY : less,
the blood is treated under the microscope with various
staining fluids, such as solutions of carmine or logwood,
the nucleus generally stains more deeply than the rest of
the corpuscle.
The colourless corpuscle, with its nucleus, is a typical
nucleated cell (p. 31). It will be observed that it
lives in a free state in the plasma of the blood, and that it
exhibits an independent contractility. In fact, except that
it is dependent for the conditions of its existence upon the
plasma, it might be compared to one of those simple organ-
isms which are met with in stagnant water, and are called
Amoeba, whence the name " amoeboid " given to the move-
ments of the colourless corpuscles of blood.
While the colourless corpuscles are thus nucleated cells,
the red corpuscles have no such nucleus ; and this is true
not only of human blood but of the blood of all mammals,
i.e. of all those animals which suckle their young; in all
these the red corpuscle has no nucleus. In the case of
birds, reptiles, and fishes, however, the red corpuscles as
well as the colourless are nucleated ; and in the em-
bryos l even of mammals the red corpuscles are at first
nucleated.
The body of the colourless corpuscle may sometimes be
quite clear and transparent, though it more usually appears
to be granular from the presence in it of minute particles
which, varying in size, are spoken of as " fine " or " coarse."
We may regard these particles as simply imbedded in the
ground-substance of which the cell-body is made up, and,
since they are variable in size and numbers, as not essential
to the structure of the corpuscle. What the real structure
of the living, contractile ground- substance or protoplasm
may be is still a matter of conjecture and dispute.
1 An embryo is the: rudimentary unborn young of any creature.
iv THE WHITE CORPUSCLES 129
When the colourless corpuscles are examined chemically
they are found to consist chiefly of water, and only 10-12
per cent, of solid matter. As in the case of the stroma of
the red corpuscles, so here also the solid part is made up
largely of proteids or substances closely allied to proteids.
But frequently also some small amount of fat is found to be
present, as also of a representative of that class of substances
known as carbohydrates or starchy bodies, called glycogen,
which will be dealt with later on when treating of the liver.
(See p. 242.)
The parts played by the colourless corpuscles in the
animal economy are probably varied and numerous, but
our knowledge of them is very imperfect. ^Ye have seen
(p. 108) that under special circumstances these corpuscles
may, by means of their amceboid movements, migrate in
large numbers through the walls of the blood-vessels into
the tissues, and it is possible that here they may in some
way assist in the removal of the causes which are giving
rise to a disturbance. Quite probably a similar migration
is taking place on a smaller scale at all times, for some as
yet obscure but possibly similar purpose. Again, by their
amoeboid movements the colourless corpuscles can flow
round small solid particles and absorb them into their
cell-body ; in other words, they can feed on substances in
the blood and thus be continually busied in keeping this
fluid in a normal condition, more particularly when, as in
disease, the composition of the blood is altered by the in-
troduction of foreign matter such as bacteria, etc. More-
over, it is extremely probable that the colourless corpuscles
may act on the blood and on any foreign matter it may at
times contain by means other than their amceboid move-
ments ; namely, chemically, by the discharge into the blood
of substances formed within themselves. Finally, there are
i3o ELEMENTARY PHYSIOLOGY less.
•easons for supposing that when blood is shed, these cor-
puscles have something to do with starting that striking
change, to which we have already alluded, known as the
clotting or coagulation of blood.
4. Blood Platelets. — In addition to the red and white
corpuscles, a third kind of rounded, colourless particles
may, but with difficulty, be made out as existing in blood.
These are known as " blood platelets." They are extremely
minute, not much wider than the thickness of a red cor-
puscle, and usually disappear as soon as blood is removed
from the body. But so little is known about them that we
must not do more than simply draw attention to their
existence.
5. The Origin and Fate of the Corpuscles. — The exact
number of both red and colourless corpuscles present in the
blood varies a good deal from time to time ; and there is
reason to think that both kinds of corpuscles are continually
being destroyed or made use of. But since, on the whole,
the average number of each kind of corpuscle is maintained
during healthy life, it is evident that new corpuscles must
be continually forming to take the place of those which have
disappeared.
The colourless corpuscles are, as already described (p.
117), chiefly formed out of leucocytes which, originating in
the lymphatic glands and other similar structures, are then
passed along the lymphatic vessels into the blood.
Our knowledge of the origin of the red corpuscles is
somewhat less definite ; there Is, however, no doubt that
in the adult the chief seat of their formation lies in that
marrow found in the cavities of bones, which, from being
very plentifully supplied with blood-vessels, is known as
red marrow. It seems wholly probable that the cells which
give rise to red corpuscles in the marrow are a particular
iv THE PHYSICAL QUALITIES OF BLOOD 131
kind of coloured, nucleated cell ; but the question has not
as yet been definitely decided as to how the mammalian red
corpuscle comes to have no nucleus.
Apart from what is known as to the disappearance of
white corpuscles from the blood by migration through the
walls of the vessels, we cannot point with certainty to any
other fate which befalls them.
When we deal with the liver, however, we shall see that,
the fluid (bile) which it forms or " secretes " is highly col-
oured, though not red. Observation and experiment both
show that the substance to which the colour of bile is due is
probably derived from that coloured product of the decom-
position of haemoglobin known as haematin. If haemoglo-
bin is thus the parent substance of the colouring matter
of the bile, then, since bile is formed by the liver each day
in large quantities, a correspondingly large daily destruction
of red corpuscles must also be taking place.
6. The Physical Qualities of Blood. — The proverb that
" blood is thicker than water " is literally true, as the blood
is not only " thickened" by the corpuscles, of which it has
been calculated that no fewer than 70,000,000,000 (nearly
fifty times, the number of the human population of the
globe) are contained in a cubic inch, but is rendered
slightly viscid by the solid matters dissolved in the plasma.
The blood is thus rendered heavier than water, its specific
gravity being about 1.055. ^n other words, twenty cubic
inches of blood have about the same weight as twenty-one
cubic inches of water.
The corpuscles are heavier than the plasma, and their
volume is usually somewhat less than that of the plasma.
Of colourless corpuscles there are usually not more than
three or four for every thousand of red corpuscles ; but the
proportion varies very much, increasing shortly after food is
132 ELEMENTARY PHYSIOLOGY less
taken, and diminishing in the intervals between meals.
Average blood may be regarded as consisting of two-thirds
plasma and one-third corpuscles.
The blood is hot, its temperature being about 380 C.
(100.40 F.).
7. The General Composition of Blood. — Considered
chemically, the blood is a faintly alkaline fluid, consisting
of water, of solid and of gaseous matters.
The proportions of these several constituents vary accord-
ing to age, sex, and condition, but the following statement
holds good on the average : —
In every 100 parts of the blood there are 79 parts of
water and 2 1 parts of dry solids ; in other words, the water
and the solids of the blood stand to one another in about
the same proportion as the nitrogen and the oxygen of the
air. Roughly speaking, one-quarter of the blood is dry,
solid matter ; three-quarters water. Of the 2 1 parts of dry
solids, 12 (=7) belong to the corpuscles. The remain-
ing 9 are about two-thirds (6.7 parts = -|) proteids (sub-
stances like white of egg, coagulating by heat), and one-third
( = \ of the whole solid matter) a mixture of saline, fatty,
and carbohydrate matters and sundry products of the waste
of the body, such as urea.
The total quantity of gaseous matter contained in the blood
is equal to rather more than half the volume of the blood ; that
is to say, 100 c.c. of blood will contain about 60 c.c. of gases.
These gaseous matters are carbonic acid, oxygen, and nitro-
gen ; or, in other words, the same gases as those which
exist in the atmosphere, but in totally different proportions ;
for whereas air contains nearly three-fourths nitrogen, one-
fourth oxygen, and a mere trace of carbonic acid, the aver-
age composition of the blood gases is about two-thirds or
more carbonic acid, and one-third or less oxygen, the quan-
:v THE GENERAL COMPOSITION OF BLOOD 133
tity of nitrogen being exceedingly small, only 1-2 c.c. in
100 c.c. of blood.
It is important to observe that blood contains much more
oxygen gas than could be held in solution by pure water at
the same temperature and pressure. This power of holding
oxygen depends upon the red corpuscles, the oxygen, thus
held by them being readily given up for purposes of oxida-
tion. The connection between the oxygen and the red cor-
puscles is of a peculiar nature, being a sort of loose chemical
combination with one of their constituents, and that con-
stituent is, as we have said previously, the haemoglobin ; for
solutions of haemoglobin behave towards oxygen almost
exactly as blood does. Similarly, the blood contains more
carbonic acid than could be held in solution by pure
water at the same temperature and pressure. But unlike
the oxygen, the carbonic acid thus held by blood is not
associated with the haemoglobin of the red corpuscles ; in
fact, it seems to be chiefly retained by some constituents of
the plasma.
The corpuscles differ chemically from the plasma in con-
taining a large proportion of the fats and phosphates, all the
iron, and almost all the potassium, of the blood; while the
plasma, on the other hand, contains by far the greater part
of the chlorine and the sodium.
The blood of adults contains a larger proportion of solid
constituents than that of children, and that of men more
than that of women ; but the difference of sex is hardly at
all exhibited by persons of flabby, or what is called lym-
phatic, constitution.
Animal diet tends to increase the quantity of the red cor-
puscles ; a vegetable diet and abstinence to diminish them.
Bleeding exercises the same influence in a still more marked
degree, the quantity of red corpuscles being diminished
134 ELEMENTARY PHYSIOLOGY less.
thereby in a much greater proportion than that of the other
solid constituents of the blood.
8. The Proteids of Plasma. — By cooling or the addi-
tion of certain neutral salts the clotting of blood is retarded
or even entirely prevented. The corpuscles may now be
removed and the plasma obtained as a clear, faintly yellow
and slightly alkaline liquid composed of about 90 per cent,
water and 10 per cent, solids in solution. The solids
consist chiefly of that kind of material which we have so
frequently spoken of as proteids. Since these proteids are
typical of their class, and since proteids are without doubt
the most important substances met with in the body, it will
be as well to state at once what are the essential character-
istics of a proteid.
Proteids are, in the first place, extremely complex sub-
stances, so much so that chemists have not as yet been able
to determine their constitution or assign any formula to
them. Some are soluble in water, others only soluble in
solutions of a neutral salt, such as sodium chloride, while
others are insoluble in either of the preceding solvents.
When heated, with but few exceptions they are altered or
coagulated, as in the well-known change which the white of
an egg, itself a typical proteid, undergoes when boiled.
In the next place, proteids are composed of the four ele-
ments, carbon, hydrogen, oxygen, and nitrogen, with a small
amount of sulphur and frequently of phosphorus ; of these
the nitrogen stands out as having a supreme importance.
All the tissues of the body contain nitrogen and are continu-
ally undergoing a nitrogenous waste, and the body is quite
unable to make use of nitrogen for the repair of this waste
unless it is presented in the form of a proteid. The general
percentage composition of proteids is, roughly speaking, the
same for all of them, and varies but slightly on either side
THE PROTEIDS OF PLASMA
1 3b
of the following numbers: carbon 53 parts, oxygen 22,
hydrogen 7, nitrogen 16, and sulphur 1-2.
All proteids give the three following reactions, (i) When
boiled with nitric acid they turn yellow, and this yellow
turns to orange on the addition of ammonia, (ii) Boiled
with Million's reagent (a mixture of the nitrates of mercury)
they give a pink colour, (iii) When mixed with caustic
soda and a small amount of a solution of sulphate of copper
they give a violet colour. These reactions suffice for the
detection of any proteid in solution or as a solid.
Fig. 44. — Network of Filaments of Fibrin left after washing away the
Colouring Matter from a thin, flat Clot of Blood. (Ranvier.)
The solids in the plasma of blood are chiefly proteids and
are three in number. The first is known as fibrinogen, and
is precipitated by the addition to plasma of 15 per cent, of
sodium chloride (ordinary salt). This result is readily at-
tained by adding to the plasma an equal volume of a satu-
rated solution of sodium chloride, which contains about 30
per cent, of salt. The fibrinogen separates out from solu-
136 ELEMENTARY PHYSIOLOGY less.
tion as a fine, flocculent, viscid precipitate. Fibrinogen is
characterised by the fact that it "sets" or coagulates when
heated in solution to 560 C. (1320 F.). The second is
called serum-globulin and is similarly precipitated when the
plasma from which the fibrinogen has been removed is
subsequently saturated by the addition of as much sodium
chloride as it will dissolve. It coagulates, when heated in
solution, at a temperature much higher than does fibrino-
gen, namely 750 C. (1670 F.). The third is known as
serum-albumin. It may, roughly speaking, be regarded as
very like that kind of albumin with which every one is famil-
iar in the white of an egg. It coagulates when heated to
840 C. (1830 F.) ; it differs from serum-globulin and also
from fibrinogen by not being precipitated when its solution
is saturated with sodium chloride.
9. The Clotting of Blood. — If a drop of blood be spread
out in a thin layer on a slide and kept from drying it soon
becomes solid and gelatinous, as in the second experiment
described on p. 120. When this solid is carefully washed,
by streaming water over it very gently, the colouring matter
is removed and a coarse network of extremely delicate
fibres or filaments remains. (Fig. 44.)
These filaments are formed in the blood and, traversing
it in all directions, uniting with one another and binding the
corpuscles together, are the cause of the blood having be-
come a semi-solid mass. The filaments are composed of a
substance called fibrin ; hence it is this formation of fibrin
which is the cause of the solidification or clotting of the
blood ; but the phenomena of clotting, which are of very
great importance, cannot be properly understood until the
behaviour of the blood when drawn in much larger quantity
than a drop has been studied.
When a quantity of blood is drawn directly from the
iv THE CLOTTING OF BLOOD 137
blood-vessels of an animal into a basin, it is at first perfectly
fluid ; but in a very few minutes it becomes, through clot-
ting, a jelly-like mass, so solid that the basin may be turned
upside down without any of the blood being spilt. At first
the clot is a uniform red jelly, but very soon drops of a
clear yellowish watery-looking fluid make their appearance
on the surface of the clot, and between it and the sides of
the basin. These drops increase in number, and run to-
gether, and after a while it has become apparent that the
originally uniform jelly has separated into two very different
constituents — the one a clear, yellowish liquid ; the other a
red, semi-solid, slightly shrunken mass, which lies in the
liquid. The liquid exudes from the coloured mass because
the latter shrinks and so squeezes it out.
The liquid is called the serum ; the semi-solid mass the
clot. Now the clot obviously contains the corpuscles of the
"blood, bound together by some other substance ; and this
last, if a small part of the clot be examined microscopically,
will be found to be that fibrous-looking matter, fibrin, which
has been seen forming in the drop of blood. Thus the clot
is made up of the corpuscles plus the fibrin of the plasma,
while the serum is the plasma minus the fibrinous elements
which it contained.
The corpuscles of the blood are slightly heavier than the
plasma, and therefore, when the blood is drawn, they tend
to sink very slowly towards the bottom, but as a rule clot-
ting is complete before the corpuscles have had time to
sink appreciably. When, on the other hand, the blood
clots slowly, the corpuscles have so much time to sink
that the upper stratum of plasma becomes quite free from
red corpuscles before the fibrin forms in it ; and, conse-
quently, the uppermost layer of the clot is nearly white ;
it then receives the name of the buffy coat. This is well
138 ELEMENTARY PHYSIOLOGY less.
seen in the blood of the horse, which clots with remarkable
slowness.
If the blood is " whipped " with a bunch of twigs as soon
as it is drawn from the body, clotting takes place as before,
but in this case the clot is broken up as fast as it is formed.
Under these circumstances the fibrin collects upon the twigs,
and a red fluid is left behind, consisting of the serum plus
the red corpuscles and many of the colourless ones. The
fibrin adhering to the twigs may readily be washed in a
stream of water, and as thus obtained is a white, stringy,
elastic and very insoluble substance. It gives, when tested,
all the reactions characteristic of proteids, and is, in fact,
itself a proteid, although somewhat impure.
The clotting of the blood is hastened, retarded, or tempo-
rarily prevented by many circumstances.
(a) Temperature. — A temperature up to or slightly
above 400 C. (1040 F.) accelerates the clotting of the
blood ; a low one retards it very greatly ; so much so that
blood kept at a temperature close to freezing point may
remain fluid for a very long time indeed.
(b) The addition of neutral salts to the blood. — Many
salts, and more especially sulphate of sodium or magnesium
and sodium chloride (common salt), dissolved in the blood
in sufficient quantity, prevent its clotting ; but clotting sets
in when water is added so as to dilute the saline mixture.
(c) Contact with living or not living matter. — Contact
with not living matter promotes the clotting of the blood.
Thus, blood drawn into a basin begins to clot first where it
is in contact with the sides of the basin ; and a wire intro-
duced into a living vein will become coated with fibrin,
although perfectly fluid blood surrounds it.
On the other hand, direct contact with living matter
retards, or altogether prevents, the clotting of the blood.
iv THE CLOTTING OF BLOOD 139
Thus, blood remains fluid for a very long time in a portion
of a vein which is tied at each end. The heart of a turtle
remains alive for a lengthened period (many hours or even
days) after it is extracted from the body ; and, so long as it
remains alive, the blood contained in it will not clot, though,
if a portion of the same blood be removed from the heart, it
will clot in a few minutes. Blood taken from the body of
the turtle, and kept from clotting by cold for some time,
may be poured into the separated, but still living, heart, and
then will not clot.
The clotting of blood being thus due to the appearance
in it of fibrin, we may now consider how and why the latter
is formed when blood is shed.
Clotting is an altogether physico-chemical process, depen-
dent upon the properties of certain of the constituents of the
plasma.
A comparison of plasma and serum shows that during
clotting, i.e. during the formation of fibrin, one constituent
of the plasma, namely, fibrinogen, disappears, the other
two proteids, serum-globulin and serum-albumin, being left
to appear in the serum. Many facts show beyond doubt
that the fibrin is formed out of the fibrinogen. It was
on this account that the latter first received the name
of fibrinogen, or " fibrin-maker." But there must also be
some substance in blood after it is shed which leads to
the conversion of fibrinogen into fibrin ; for pericardial and
other serous fluids contain fibrinogen, but do not usually
clot, and purified solutions of fibrinogen never clot spon-
taneously. What is this substance?
If serum be precipitated with an excess of strong alcohol
and after some weeks the precipitate is collected and ex-
tracted with distilled water, this watery extract contains very
little solid matter, but is found to be active in causing the
140 ELEMENTARY PHYSIOLOGY less.
conversion of fibrinogen into fibrin. We do not as yet
know exactly what the substance is in this extract which
brings about the change of the fibrinogen, but for reasons
into which we cannot now enter, it is classed with the
"ferments," of which we shall have to speak when we
come to consider digestion. These ferments are charac-
terised by their power, even when present in small quan-
tities, of producing great changes in other bodies without
themselves entering into the changes. Thus, the particu-
lar ferment of which we are speaking, and which has been
called " fibrin ferment," produces fibrin, and yet does not
itself become part of the fibrin so produced.
This ferment is apparently not present in healthy blood
as it circulates in the living blood-vessels, but makes its
appearance when the blood is shed. We do not know
exactly from what source it comes, but there are reasons
for thinking that it arises from a breaking down of the
white corpuscles, or it may be of the blood platelets.
Finally, then, although the process of clotting is not yet
understood in full, we may say that fibrin as such does not
exist in the blood at the moment of its being shed, but makes
its appearance afterwards on account of the action of fibrin
ferment on fibrinogen. It is possible that other bodies are
concerned in the matter.
10. The Quantity and Distribution of Blood in the
Body. — The total quantity of blood contained in the body-
varies at different times, and the ascertainment of its pre-
cise amount is very difficult. It may probably be estimated,
on the average, at not less than one-thirteenth or about 7.5
per cent, of the weight of the body.
Its distribution at any moment may be stated in round
numbers as follows : —
One-quarter, in the heart, the vessels of the lungs, and
the large blood-vessels.
iv THE FUNCTIONS OF THE BLOOD 141
One-quarter, in the vessels of the liver.
One-quarter, in the vessels of the skeletal muscles.
One-quarter, in the vessels of the other organs of the
body.
11. The Functions of the Blood. — The function of the
blood is to supply nourishment to, and take away waste
matters from, all parts of the body. All the various tissues
may be said to live on the blood. From it they obtain all
the matters they need, and to it they return all the waste
material for which they have no longer any use. It is abso-
lutely essential to the life of every part of the body that it
should be in such relation with a current of blood that
matters can pass freely from the blood to it, and from it
to the blood, by transudation through the walls of the ves-
sels in which the blood is contained. And this vivifying
influence depends upon the corpuscles of the blood. The
proof of these statements lies in the following experiments :
If the vessels of a limb of a living animal be tied in such
a manner as to cut off the supply of blood from the limb,
without affecting it in any other way, all the symptoms of
death will set in. The limb will grow pale and cold, it
will lose its sensibility, and volition will no longer have
power over it; it will stiffen, and eventually mortify and
decompose.
But, if the ligatures be removed before the death stiffen-
ing has become thoroughly established, and the blood be
allowed to flow into the limb, the stiffening speedily ceases,
the temperature of the part rises, the sensibility of the skin
returns, the will regains power over the muscles, and, in
short, the part returns to its normal condition.
If, instead of simply allowing the blood of the animal
operated upon to flow again, such blood, deprived of its
fibrin by whipping, but containing its- corpuscles, be arti-
r42 ELEMENTARY PHYSIOLOGY less.
ficially passed through the vessels, it will be found nearly
as effectual a restorative as entire blood ; while, on the
other hand, the serum (which is equivalent to whipped
blood without its corpuscles) has no such effect.
It is not necessary that the blood thus artificially injected
should be that of the subject of the experiment. Men, or
dogs, bled to apparent death, may be at once and effectu-
ally revived by filling their veins with blood taken from
another man, or dog ; an operation which is known by
the name of transfusion.
Nor is it absolutely necessary for the success of this
operation that the blood used in transfusion should belong
to an animal of the same species. The b\)od of a horse
will permanently revive an ass, and, speaking generally,
the blood of one animal may be replaced without injurious
effects by that of another closely-allied species ; while that
of a very different animal will be more or less injurious,
and may even cause immediate death.
12. Lymph : its Character and Composition. — Lymph,
as previously explained, is the fluid which fills the lymphatic
vessels, and at the place where it is first formed is a mere
overflow of fluid from the blood through the walls of the
capillaries. This exudation of fluid may also be accom-
panied by a migration of some of the colourless corpuscles
of the blood. Hence it i.s at once evident that, broadly
speaking, lymph may be regarded as so much blood minus
its red corpuscles.
Lymph is most easily and plentifully obtained for exami-
nation from the thoracic duct. As procured from this vessel
it has the advantage of being representative of an average
specimen of lymph, since it is a mixture of fluid collected
from nearly all parts of the body. But the precaution must
be taken to collect the lymph from a fasting animal in order
tv LYMPH 143
to avoid the complication due to admixture of the lymph
from the body generally with certain special substances
which are taken up by the lymphatics of the intestine after
a meal. After a meal, the lymph from the alimentary canal
differs strikingly, in one respect, as we shall see later on,
from that which comes from it in the absence of food.
Lymph taken, then, from the thoracic duct of a fasting ani-
mal, is found to be a transparent, faintly yellow fluid. When
examined under the microscope it is seen to contain a num-
ber x of corpuscles, the lymph-corpuscles or leucocytes, very
similar to the colourless corpuscles of blood, though perhaps
on the whole rather smaller, and like the latter showing
amoeboid movements, especially if kept warm. These leuco-
cytes may represent some of the white blood-corpuscles which
migrated from the vessels, but by far the larger number are
formed in the lymphatic glands (see p. 116).
When examined chemically lymph is found to contain the
same salts as are present in plasma and in about the same
amount : the total solids are, however, considerably less than
in plasma,2 and this is due to a deficiency of proteids. But
the proteids present in lymph are the same in kind as the
three already described as found in plasma, viz., fibrinogen,
serum-globulin, and serum-albumin. Hence lymph clots
when left to itself and yields fibrin identical with that ob-
tained from blood, only in smaller quantities, so that the clot
is less firm than from blood. Some gas may also be extracted
from it, but in the absence of red corpuscles the amount of
oxygen it yields is scarcely appreciable ; the bulk of the gas
is carbonic acid.
1 Equal on the average to the number present in blood, so that in a drop
of lymph very few would be seen and often none at all. -
2 Only about 5 per cent, of its weight as compared with 8 to 10 per cent,
in plasma,
144 ELEMENTARY PHYSIOLOGY less
Average lymph is therefore very similar to plasma some-
what diluted with water ; but the dilution is not the same in
lymph collected from different parts of the body. Lymph
differs also in composition when collected from the same
part at different times. Usually this difference is slight, but
in the case of one source it is marked and important. In a
fasting animal the lymph coming from the intestines is essen-
tially the same as average lymph ; but after food has been
taken, and especially if the food contains much fat, and food
always contains some fat, this lymph appears to be quite
white or " milky." Owing to the" thinness of the walls of
the lymphatics the contents are visible from their exterior,
so that the vessels also appear white or milky, and hence
this particular set of lymphatics is known as the lacteals, and
the contents are called chyle. The only difference between
chyle and the lymph ordinarily present in the lacteals is that
chyle holds in suspension a large amount of fat (from 5 to
15 per cent.) in a state of extremely fine division. These
minute particles of fat reflect a great deal of the light falling
upon them and hence the fluid appears white. Some of the
fat in chyle exists in the form of minute globules, similar to
those present in milk, but the larger part is so finely divided
that it can only be spoken of as " granules " and in this form
is known as the molecular basis of chyle.
13. The Mode of Formation of Lymph. — In all which
we have so far said respecting lymph we have spoken of it
merely as an exudation of fluid from the walls of the capil-
laries. We may now consider what are the causes which
lead to the presence of lymph in the lymph-spaces of the
tissues.
Two physical processes suggest themselves at once as
possible causes ; these are filtration and diffusion. Filtra-
tion consists in the passage of fluid and of substances in
THE MODE OF FORMATION OF LYMPH
'45
-&
solution tlirough a porous membrane as the result of a differ-
ence of pressure on the two sides of the membrane. Diffu-
sion, on the other hand, is, broadly speaking, independent
of such a difference of pressure. A simple experiment
shows at once the essential feature of diffusion. Tie a piece
of parchment paper tightly over the wide end of an ordinary
" thistle tube " as used by chemists. Then fill the bulb and
about one inch of the tube with a
strong (20 per cent.) solution of
sugar or common table-salt and fix
the tube vertically, as in Fig. 45, in
a beaker of water, so that the surface
of the solution in the tube is at the
same level as that of the water in
the beaker. In a short time the
sugar or salt begins to pass out
through the paper and may be de-
tected in the water in the beaker.
At the same time water passes
through the paper in the opposite
direction into the tube and in con-
siderable quantity, so that the liquid
rises in the narrow part of the tube
and may ultimately stand several
inches above the surface of that
which is in the beaker.
Substituting the wall of the capillaries for the paper used
in the preceding experiment we have the conditions neces-
sary for a possibly diffusive interchange between the blood
on the one side of that wall and the fluid in the tissues on
the other. We may say at once that diffusion by itself will
not account for the formation of lymph. In support of this
statement it may suffice to point out that lymph contains a
L
Fig. 45. — To Illustrate a
Simple Experiment on
Diffusion.
1 1. thistle tube: p p, parch-
ment paper; s. sugar or salt
solution; b. beaker; w. water
in beaker.
t$6 ELEMENTARY PHYSIOLOGY less
considerable amount of proteids, and these are characteristi-
cally non-diffusible.1
On the other hand, the blood-pressure in the capillaries,
though much less than in the arteries, is not inconsiderable,
and is exerted against the walls of these vessels. Can we
then account for the formation of lymph as the result of fil-
tration ? Here again we may at once say that the passage
of fluid through the walls of the capillaries under the influ-
ence of pressure has a great deal to do with the formation
of lymph. We are justified in this view by the fact that, as
a general rule, increase of blood-pressure in the capillaries
leads to an increased flow of lymph from the parts they sup-
ply. But we must not conclude, therefore, that the process
is entirely due to filtration. Experiments may be made in
which while we know that the blood-pressure in the capilla-
ries is much greater than usual, no increased formation of
lymph takes place. Again, it is possible by certain means
to obtain a greatly increased flow of lymph from parts in
whose capillaries there is no obvious increase of blood-press-
ure. Neither of these results would hold good in the case
of any ordinary filter. But in the case of lymph, as a matter
of fact, it is not an ordinary filter with which we have to
deal. The wall of a capillary is made of cells which are
alive and are thus able to change their condition from time to
time. By this means the capillary wall is, as it were, the
master of the current of fluid passing across it under varying
filtrational pressure, and can determine by means at present
unknown to us not only how much fluid shall pass, but in
what relative proportions its several constituents shall make
their exit. When once this idea is clearly grasped many
1 Substances such as the proteids of blood, also gelatin, which will diffuse
either not at all or only with difficulty, are known as colloids, in contradis-
tinction to crystalline substances or crystalloids ', which diffuse readily.
iv THE FUNCTIONS OF THE LYMPH 14*
difficulties disappear. We can understand more easily why
the lymph differs in composition as formed in various parts
of the body. We see why arterial dilation is less potent to
increase lymph formation than is venous obstruction, for,
although they both increase the blood-pressure in the capil-
laries, venous obstruction is necessarily accompanied by a
stagnation of blood which presumably alters the condition
of the capillary wall. We can also now more easily appreci-
ate many details of inflammation as previously described
(p. 108).
The formation of lymph may thus be regarded as the
result of the passage of certain constituents of the blood-
plasma through the walls of the capillaries, the two processes
of diffusion and filtration probably s/iaring in the proceeding,
but the passage being made peculiar by the influence of the
living walls of the vessels through which it is taking place.
14. The Functions of the Lymph. — The lymph has
already been spoken of (p. no) as a " middleman " between
the blood on the one hand and the tissue on the other. With
the single exception of the lining epithelial membrane of the
blood-vessels, no tissue deals directly with the blood itself.
The lymph, before it is gathered into vessels for return to
the blood, surrounds and bathes the living cells. All sup-
plies of food and of oxygen are conveyed from the blood
to the cells by the lymph, and all waste matters in going from
the cells to the blood for ultimate excretion are carried by
the same medium. The presence of the lymph and its inti-
mate relation to the living substance thus make effective the
vivifying influence of the blood. The former is as essential
to life as is the latter.
LESSON V
RESPIRATION
1. The Gases of Arterial and Venous Blood. — The
blood, the general nature and properties of which have
been described in the preceding Lesson, is the highly com-
plex product, not of any one organ or constituent of the
body, but of all. Many of its features are doubtless given
to it by its intrinsic and proper structural elements, the
corpuscles ; but the general character of the blood is also
profoundly affected by the circumstance that every other
part of the body takes something from the blood and pours
something into it. The blood may be compared to a river,
the nature of the contents of which is largely determined
by that of the head waters, and by that of the animals
which swim in it ; but which is also very much affected by
the soil over which it flows, by the water-weeds which cover
its banks, by affluents from distant regions, by irrigation
works which are supplied from it, and by drain-pipes which
flow into it.
One of the most remarkable and important of the changes
effected in the blood is that which results, in most parts of
the body, from its simply passing through capillaries, or, in
other words, through vessels the walls of which are thin
enough to permit a free exchange between the blood and
the fluids which permeate the adjacent tissues (p. 56).
148
less, v THE GASES OF BLOOD 149
Thus, if blood be taken from the artery which supplies
a limb, it will be found to have a bright scarlet colour ;
while blood drawn, at the same time, from the vein of the
limb, will be of a purplish hue. And as this contrast is met
with in the contents of the arteries and veins in general
(except the pulmonary artery and veins), the scarlet blood
is commonly known as arterial and the dark blood as
venous.
This conversion of arterial into venous blood takes place
in most parts of the body while life persists. Thus, if
a limb be cut off and scarlet blood be forced into its
arteries by a syringe, it will issue from the veins as dark
blood.
When specimens of venous and of arterial blood are sub-
jected to chemical examination, the differences presented
by their solid and fluid constituents are found to be very
small and inconstant. But the gaseous contents of the
two kinds of blood differ widely in the proportion which
the carbonic acid gas bears to the oxygen ; there being a
smaller quantity of oxygen and a greater quantity of car-
bonic acid, in venous than in arterial blood.
Every 100 volumes of blood contain about 60 volumes
of gases. These may be extracted by placing the blood in
a vessel connected with the vacuum of a mercurial pump.
The reduction 'of pressure on the surface of the blood leads
to a rapid exit of the gases into the vacuum ; they can then
be collected and measured and their respective volumes
determined. The composition of the blood-gases is thus
found to be the following : —
Arterial Blood. Venous Blood.
Oxygen 20 vols 8-1 2 vols.
Carbonic acid ... 40 " 46
Nitrogen 1-2 " 1-2 "
ISO ELEMENTARY PHYSIOLOGY less.
This difference in their gaseous contents is the only
essential difference between venous and arterial blood, as
may be demonstrated experimentally. For, if venous blood
be shaken up with oxygen, or even with air, it gains oxygen,
loses carbonic acid, and takes on the colour and properties
of arterial blood. Similarly, if arterial blood be treated
with carbonic acid so as to be thoroughly saturated with
that gas, it gains carbonic acid, loses oxygen, and acquires
the true properties of venous blood ; though, for a reason
to be mentioned below, the change does not take place
so readily nor is it so complete in this case as in the
former. The same result is attained, though more slowly,
if the blood, in either case, be received into a bladder, and
then placed in the oxygen, or carbonic acid ; the thin moist
animal membrane allowing the change to be effected with
perfect ease, and offering no serious impediment to the
passage of either gas.
Venous blood is characterised not only by the large
amount of carbonic acid which it contains, but also by the
fact that the red corpuscles have given up a good deal of
their oxygen for the purposes of oxidation, or, as the
chemists would say, have become reduced. This is the
reason why arterial blood is not so easily converted into
venous blood by exposure to carbonic acid as is venous
blood into arterial by exposure to oxygen. There is, in
the former case, a want of some oxidisable substance to
carry off the oxygen from and so to reduce the red cor-
puscles. When such an oxidisable substance is added (as,
for instance, either ammonium sulphide or Stokes's re-
agent1), the blood at once and immediately becomes com-
pletely venous.
1 This is made by mixing some tartaric acid with a solution of ferrous
sulphate and then adding ammonia until the' mixture is alkaline.
v THE GASES OF BLOOD 151
Practically we may say that the most important difference
between venous and arterial blood is not so much the rela-
tive quantities of carbonic acid as that the red corpuscles
of venous blood have lost a good deal of oxygen, are
reduced, and ready at once to take up any oxygen offered
to them.
Similarly, the loss of oxygen by the red corpuscles is the
chief reason why the scarlet arterial blood turns to a more
purple or claret colour in becoming venous. It has indeed
been urged that the red corpuscles are rendered somewhat
flatter by oxygen gas, while they are distended by the action
of carbonic acid (p. 124). Under the former circumstances
they may, not improbably, reflect the light more strongly, so
as to give a more distinct coloration to the blood; while,
under the latter, they may reflect less light, and, in that way,
allow the blood to appear darker and duller.
This, however, can only be a small part of the whole
matter ; for solutions of haemoglobin or of blood-crystals
(see p. 125), even when perfectly free from actual blood-
corpuscles, change in colour from scarlet to purple, accord-
ing as they gain or lose oxygen. It has already been stated
(p. 125) that oxygen most probably exists in the blood in
loose combination with haemoglobin. And further, a solu-
tion of haemoglobin, when thus loosely combined with
oxygen, has a scarlet colour, while a solution of haemo-
globin deprived of oxygen has a purplish hue. Hence
arterial blood, in which the haemoglobin is richly provided
with oxygen, is naturally scarlet, while venous blood, which
not only contains an excess of carbonic acid, but whose
haemoglobin also has lost a great deal of its oxygen, is
purple.
The conditions under which the gases exist in blood are
peculiar and important in connection with a point we shall
152 ELEMENTARY PHYSIOLOGY less
have to discuss later on, namely, how venous blood becomes
arterial in the lungs and how arterial blood becomes venous
in the tissues. As to the nitrogen, we may say at once that
it is apparently in a state of simple solution, as though the
blood were so much water. A very small part of the oxy-
gen is similarly simply dissolved in the blood, but practically
almost the whole of it is in a state of loose chemical combi-
nation with the hemoglobin of the red corpuscles. The facts
which prove this are simple and conclusive. When blood
is subjected to a gradually increasing vacuum, the oxygen
does not come off uniformly and progressively, as the
vacuum is made greater, as it would if it were in mere
solution ; on the contrary, it escapes with a sudden rush
after the pressure has been considerably reduced. In the
absence of red corpuscles plasma or serum absorbs only as
much oxygen as does an equal quantity of water, namely,
about one volume per cent. ; but blood, where the red cor-
puscles are present, may contain as much as 20 volumes per
cent, of oxygen. Finally, solutions of haemoglobin absorb
oxygen as readily and largely as blood does.
The conditions under which carbonic acid exists in the
blood may also be shown to be those of a loose chemical
combination ; but beyond this fact our knowledge is some-
what incomplete. It is known, however, that the carbonic
acid is combined chiefly in some constituents of the plasma
and not with the corpuscles, and most authorities consider
that the larger part is present in plasma united with sodium
in the form of sodium bicarbonate, NaHCO;1.
2. The Nature and Essence of Respiration. — All the
tissues, as we have seen, are continually using up oxygen.
Their life, in fact, is dependent on a continual succession of
oxidations. Hence they are greedy of oxygen, while at the
same time they are continually producing carbonic acid (and
v NATURE AND ESSENCE OF RESPIRATION 153
other waste products). The demand for oxygen is met by
a supply from the red corpuscles, and the oxygen they give
up passes through the walls of the capillaries, across the
lymph, and so to the cells of which the tissue is composed.
At the same time the carbonic acid passes across the lymph
in the opposite direction, through the capillary walls and
into the blood, by which it is at once whirled away into the
veins. The blood therefore leaves the tissue poorer in oxy-
gen and richer in carbonic acid than when it came to it ;
and this change is the change from the arterial to the
venous condition. This gaseous interchange between the
blood and the tissues is frequently spoken of as the respira-
tion of the tissues or internal respiration.
On the other hand, if we seek for the explanation of the
conversion of the dark blood in the veins into the scarlet
blood of the arteries, we find, first, that the blood remains
dark in the right auricle, the right ventricle, and the pul-
monary artery ; secondly, that it is scarlet not only in the
aorta, but in the left ventricle, the left auricle, and the pul-
monary veins.
Obviously, then, the change from venous to arterial blood
takes place in the capillaries of the lungs, for these are the
sole channels of communication between the pulmonary
arteries and the pulmonary veins.
But what are the physical conditions to which the blood
is exposed in the pulmonary capillaries ?
These vessels are very wide, thin walled, and closely set,
so as to form a network with very small meshes, which is
contained in the substance of an extremely thin membrane.
This membrane is in contact with the air, so that the blood
in each capillary of the lung is separated from the air by
only a delicate pellicle formed by its own wall and the lung
membrane. Hence an exchange very readily takes place
154 ELEMENTARY PHYSIOLOGY less.
between the blood and the air ; the latter gaining moisture
and carbonic acid, and losing oxygen.1
This is the essential step in respiration. That it really
takes place may be demonstrated very readily by the ex-
periment described in the first Lesson (p. 3), in which air
expired was proved to differ from air inspired, by containing
more heat, more water, more carbonic acid, and less oxy-
gen ; or, on the other hand, by putting a ligature on the
windpipe of a living animal so as to prevent air from passing
into, or out of, the lungs, and then examining the contents
of the heart and great vessels. The blood on both sides of
the heart, and in the pulmonary veins and aorta, will then
be found to be as completely venous as in the vena? cavae
and pulmonary artery.
But though the passage of carbonic acid (and hot watery
vapour) out of the blood and of oxygen into it is the essence
of the respiratory process — and thus a membrane with
blood on one side, and air on the other, is all that is abso-
lutely necessary to effect the purification of the blood —
yet the accumulation of carbonic acid is so rapid, and the
need for oxygen so incessant, in all parts of the human body,
that the former could not be cleared away, nor the latter
supplied, with adequate rapidity, without the aid of exten-
sive and complicated accessory machinery — the arrange-
ment and working of which must next be carefully studied.
3. The Organs of Respiration. — The back of the mouth
or pharynx communicates by two channels with the external
1 The student must guard himself against the idea that arterial blood
contains no carbonic acid, and venous blood no oxygen. In passing
through the lungs venous blood loses only a part of its carbonic acid; and
arterial blood, in passing through the tissues, loses only a part of its oxygen.
In blood, however venous, there is in health always some oxygen ; and in
even the brightest arterial blood there is actually about twice as much car
bonic acid as there is of oxygen. See the table on p. 149.
V THE ORGANS OF RESPIRATION 155
air (see Fig. 46, g,f, e). One of these is formed by the
nasal passages, which cannot be closed by any muscular
apparatus of their own ; the other is presented by the mouth,
which can be shut or opened at will.
Immediately behind the tongue, at the lower and front
part of the pharynx, is an aperture — the glottis (Fig. 47,
Gl) — capable of being closed by a sort of lid — the epi-
glottis (Fig. 46, , the oesophagus or gullet; c, the trachea or windpipe;
d, the thyroid cartilage of the larynx ; e, the epiglottis ; _/, the uvula ; g; the opening of
the left Eustachian tube; //, the opening of the left lachrymal duct; i, the hyoid bone;
k, the tongue; /, the hard palate; m, n, the base of the skull; 0, p, q, the superior,
middle and inferior turbinal bones. The letters g,f, e, are placed in the pharynx.
incomplete behind, their ends being united by unstriated
muscle, where the trachea comes into contact with the oesopha-
gus, or gullet. The trachea is lined by a mucous membrane,
which consists of an epithelium of ciliated cells (Fig. 49), in-
terspersed with mucous cells ; these lie on a distinct so-called
THE ORGANS OF RESPIRATION
157
basement membrane and below this is a small amount of
lymphoid and elastic tissue. Between the mucous membrane
and the outer layer which carries the hoops of cartilage, there
is a certain amount of areolar connective tissue, in which some
small mucous glands are imbedded ; this constitutes the
submucous layer. The ciliated cells are elongated columnar
Fig. 47. — Back View of the Neck and Thorax of a Human Subject from
which the Vertebral Column and whole Posterior Wall of the Chest
are supposed to be removed.
M. mouth; Gl. glottis; TV. trachea; L.L. left lung; R.L. right lung; Br. bron-
chus; P. A. pulmonary artery; P.V. pulmonary veins; Ao. aorta; D. diaphragm;
H. heart; V.C.I, vena cava inferior.
cells with a large and distinct nucleus. During life the cilia
vibrate incessantly backwards and forwards, but work on the
whole in such a way as to sweep both liquid (mucus) and
solid particles outwards or towards the mouth.
The walls of the bronchi and bronchial tubes have a struc-
ture in general similar to that of the trachea. But, as the
<58
ELEMENTARY PHYSIOLOGY
tubes diminish in size, the cartilages become smaller and
more scattered and eventually disappear. At the same
time the muscular tissue increases in quantity and comes
to form a complete layer outside the mucous membrane.
D
^Y'
Fig. 48.
A. Two infundibula (5), with the ultimate bronchial tube (a) which opens into
them. (Magnified 20 diameters.)
B. Diagrammatic view of an air-cell of A seen in action: a, epithelium; b, parti-
tion between two adjacent cells, in the thickness of which the capillaries run ; c, fibres
of elastic tissue.
C. Portion of injected lung magnified: a, the capillaries spread over the walls
of two adjacent air-cells; /■>, small branches of arteries and veins.
D. Portion still more highly magnified.
Thus, while the trachea and bronchi are kept permanently
open and pervious to air by their cartilages, the smaller
bronchial tubes may perhaps be almost closed by the con-
traction of their muscular walls. Eventually the muscular
THE ORGANS OF RESPIRATION
15".
tissue largely disappears, and the character of the tissue
between the alveoli is quite different from that of the walls
of the bronchial tubes.
The very thin partitions (Fig. 48, B, b) which separate
these alveoli are supported by much delicate and highly
elastic tissue, and carry the wide and close-set capillaries
into which the ultimate ramifications of the pulmonary
artery pour its blood (Fig. 48, C, D). The partitions are
Fig. 49. — Cilivted Epithelium Cells from the Trachea of the Rabbit,
HIGHLY MAGNIFIED. (ScHAFER.)
>«', m", ms, mucus-secreting cells lying between the ciliated cells and seen in various
stages of mucin formation.
covered with extremely thin, flattened, non-ciliated cells,
which may be easily seen in the lung of a young animal, but
are reduced to almost nothing in the lung of an adult (Fig.
48, B, a). Thus, the blood contained in the capillaries is
exposed on both sides to the air — being separated from
the cavity of the alveolus on either hand only by the very
delicate pellicle which forms the wall of the capillary and
the lining epithelium of the alveolus.
i Go ELEMENTARY PHYSIOLOGY less
No conditions could be more favourable to a ready ex-
change between the gaseous contents of the blood and
those of the air in the alveoli than the arrangements which
obtain in the pulmonary tissue. It will readily be per-
ceived, however, that with the continual pulmonary circu-
lation the pulmonary air would very speedily lose all its
oxygen, and become completely saturated with carbonic
acid, if special provision were not made for its being inces-
santly renewed. The renewal is brought about by the
working of certain structural and mechanical arrangements
which must now be described in detail.
4. The Thorax and Pleura. — The lungs (and heart)
are inclosed in what is practically an air-tight box, whose
walls are movable. This box is the thorax or chest. In
shape it is conical, with the small end turned upwards, the
back of the box being formed by the spinal column, the
sides by the ribs, the front by the sternum or breast-bone,
the bottom by the diaphragm, and the top by the root of
the neck (Fig. 47).
The two lungs occupy almost all the cavity of this box
which is not taken up by the heart (Fig. 50). Each is in-
closed in its serous membrane, the pleura, a double bag
(very similar to the pericardium, the chief difference being
that the outer bag of each pleura is, over the greater part of
its extent, firmly adherent to the walls of the chest and the
diaphragm, while the outer bag of the pericardium is for the
most part loose), the inner bag closely covering the lung
and the outer forming a lining to the cavity of the chest 1
(Fig. 25, pi). So long as the walls of the thorax are entire,
the cavity of each pleura is practically obliterated, that layer
1 There is a small amount of fluid between the two surfaces of the pleura,
to facilitate their rubbing easily against one another. This " serous" fluid
is in reality, as is pericardial fluid, a form of lymph.
THE THORAX AND PLEURA
161
of the pleura which covers the lung being in close contact
with that which lines the wall of the chest ; but, if an open-
ing be made into the pleura, the lung at once shrinks to a
comparatively small size, and thus develops a great cavity
Fig. 50. — Diagram of the Thorax, showing the Position of the Heart and
Lungs.
1-12, ribs; 11-12, floating ribs; s, sternum; r, rib; c.c, costal cartilages; c, clavicle;
/, lungs; a, apex of heart; peric, pericardium, cut edge.
between the two layers of the pleura. If a pipe be now
fitted into the bronchus, and air blown through it, the lung
is very readily distended to its full size ; but, on being left
to itself, it collapses, the air being driven out again with
M
[62 ELEMENTARY PHYSIOLOGY less.
some force. The abundant elastic tissues of the walls of
the air-cells are, in fact, so disposed as to be greatly
stretched when the lungs are full ; and when the cause of
the distension is removed, this elasticity comes into play and
drives the greater part of the air out again.
The lungs are kept distended in the dead subject, so long
as the walls of the chest are entire, by the pressure of the
atmosphere acting down the trachea, bronchi, and bronchi-
oles upon the inner surfaces of the walls of the alveoli. For
though the elastic tissue is all the while pulling, as it were,
at the layer of pleura which covers the lung, and attempting
to separate it from that which lines the chest, it cannot
produce such a separation without developing a vacuum
between these two layers. To effect this, the elastic tissue
must pull with a force greater than that of the external air
(or fifteen pounds to the square inch), an effort far beyond
its powers, which do not equal one-fourth of a pound on the
square inch. But the moment a hole is made in the pleura,
the air enters into its cavity, the atmospheric pressure inside
the lung is equalised by that outside it, and the elastic
tissue, freed from its opponent, exerts its full power on the
lung and the latter collapses.
5. The Movements of Respiration. — The hinder ends
of the ribs are attached to the vertebral column so as to
be freely movable upon it. The front ends of the first
ten pairs of ribs are connected by the costal cartilages to
the sternum, the connection being therefore flexible (Figs.
50, 51, 52). When left to themselves, the ribs take a posi-
tion which is inclined obliquely downwards and forwards.
Two sets of muscles, called intercostals, pass between
the successive pairs of ribs on each side. The outer set,
called external intercostals (Fig. 52, A), run from the rib
above, obliquely downwards and forwards, to the rib below.
THE MOVEMENTS OF RESPIRATION
163
The other sel, internal intercostals (Fig. 52, B), cross
these in direction, passing from the rib above, downwards
and backwards, to the rib below.
The action of these muscles is somewhat puzzling at first,
but is readily understood if the fact be borne in mind that
when a muscle contracts, it tends to shorten the distance
The Bony Walls of the Thorax.
a, I, vertebral column; 1-12, ribs; c, sternum; d, costal cartilages; e, united car
tilages of lower true ribs.
betiveen its tivo ends. Let a and b in Fig. 53, A, be two
parallel bars, representing two consecutive ribs, movable by
their ends upon the upright c, which may be regarded as
the vertebral column at the back of the apparatus ; then
a line directed from x to y will be inclined downwards and
forwards, and one from w to z will be directed downwards
164
ELEMENTARY PHYSIOLOGY
and backwards. Now it is obvious from the figure that
the distance between x and y is shorter in B than in A
and much shorter than in C ; hence when x y is shortened
the bars will be pulled up from the position C or A to
or towards the position B. Conversely, the shortening of
w z will tend to pull the bars down from the position B
or the position A to or towards the position C.
Fig. 52. — View of Four Ribs of the Dot;, with the Intercostal Muscles.
a, the bony rib; b, the cartilage; c, the junction of bone and cartilage; d, unossi-
fied, e, ossified, portions of the sternum. A , external intercostal muscle; B, internal
intercostal muscle. In the middle interspace, the external intercostal has been re-
moved to show the internal intercostal beneath it.
If the simple apparatus just described be made of wood,
hooks being placed at the points x y, and w z, and an
elastic band be provided with eyes which can be readily
put on to or taken off these hooks, it will be found that,
the band being so short as to be put on the stretch when
THE MOVEMENTS OF RESPIRATION
165
hooked on to either x y, or w z, with the bars in the hori-
zontal position, A, the elasticity of the band, when hooked
on to x and v, will bring them up as shown in B ; while,
if hooked on to w and z, it will bring them down as shown
in C.
Substitute the contractility of the external and internal
intercostal muscles for the shortening of the band, in vir-
tue of its elasticity, and the model will exemplify the action
of these muscles; the external intercostals in shortening will
tend to raise, and the internal intercostals to depress, the
bony ribs.
Fig 53. — Diagram of Models illustrating the Action of the External and
Internal Intercostal Muscles.
B, inspiratory elevation; C, expiratory depression.
Such a model, however, does not accurately represent
the ribs, with their numerous and peculiar curves, and
hence, while most physiologists are agreed that the exter-
nal intercostals raise the ribs, the action of the internal
intercostals is not by any means so certain.
The raising of the ribs which results from the action of
the external intercostal muscles is further assisted by the
contraction of certain other muscles, the scaleni and leva-
tores costarum. The former are stretched between the
;66 ELEMENTARY PHYSIOLOGY less.
cervical vertebrae and the first two ribs, and serve to raise
and fix these ribs. The latter are attached by their upper
ends to the transverse processes of the last cervical and
first eleven dorsal vertebrae, and each muscle is fastened
by its lower end to the rib next below the vertebra from
which the muscle itself springs. These muscles must also
raise the ribs.
By means of these several muscles, the ribs can be
raised from their naturally downward-slanting position into
one more nearly horizontal. When this takes place, the
front ends of the ribs must move not only upwards but
forwards, and must therefore thrust the sternum slightly
outwards, or away from the vertebral column. By this
movement the size of the thorax is of course increased
from back to front, an increase which may be easily felt
by placing one hand on the back and one on the chest
of a person who is breathing. Again, when the ribs are
raised, each rib must evidently, by its upward motion,
tend to occupy the position previously held by the rib
next above it ; but the arched curve of each rib increases
in size from the first to the seventh pair of ribs, so that
this upward movement makes a rib with a larger arch take
the place of one with a smaller curve. This must clearly
result in an increase in width of the thorax from side to side,
an increase which may, as before, be readily felt by placing
the hands on the opposite sides of the chest.
The floor of the thorax is formed by the diaphragm, a great
partition situated between the thorax and the abdomen, and
always concave to the latter and convex to the former (Fig.
i, D). From its middle, which is tendinous, muscular
fibres extend in a sheet downwards and outwards to the
ribs, and two especially strong masses, which are called the
pillars of the diaphragm, to the spinal column (Fig. 54).
THE MOVEMENTS OF RESPIRATION
165
vVhen these muscular fibres contract, therefore, they tend
to make the diaphragm flatter, and to increase the capacity
of the thorax at the expense of that of the abdomen, by
pulling down the bottom of the thoracic box (Fig. 55, A),
or, in other words, when the diaphragm is flattened, the
size of the thorax is increased from above downwards.
Fig. 54. — The Diaphragm of a Dog, viewed from the Lower or Abdominal
Side.
V.C.I, the vena cava inferior; O. the oesophagus; Ao. the aorta; the broad white
tendinous middle (B.B.B.) is easily distinguished from the radiating muscular fibres
{A. A. A.) which pass down to the ribs and into the pillars (C, D) in front of the
vertebrae.
By means then of the movements of the ribs and of the
diaphragm the size of the thorax may be increased in all
its dimensions. Let us now consider what must happen to
the lungs when the thorax becomes larger. The lungs, as
168 ELEMENTARY PHYSIOLOGY less.
we have said (p. 162), are kept distended by the pressure
of the atmosphere acting down the trachea and keeping the
outer walls of each lung firmly pressed against the inner
wall of the chest. This being so, if the wall of the thorax
tends to move away from the wall of the lung, as it must do
when the thorax is enlarged, then the wall of the lung must
follow the wall of the thorax, air rushing in through the
trachea to increase the distension of the elastic lungs to
the required extent, and to prevent the formation of any
vacuum between the two pleurae. This drawing of air into
the lungs constitutes an inspiration.
At the end of each inspiration the diaphragm and the
external intercostal muscles relax. The diaphragm rises to
its former position (Fig. 55, B), being partly pushed up by
the abdominal viscera which were pushed down when the
diaphragm contracted. At the same time gravity acting on
the ribs tends to lower them, and this is assisted by the
elastic recoil of the lungs and of the tissues of the chest
wall which has been put on the stretch during inspiration,
and possibly also by the contraction of the internal inter-
costal muscles. So much of the elasticity of the lungs as
was called into play by the contraction of the diaphragm
and the raising of the ribs now comes into action. By these
means the thorax is diminished in size and air is driven out
of the lungs, the forcing out of the air constituting an
expiration. An expiration and an inspiration together con-
stitute a respiration.
Thus it appears that we may have diaphragmatic respi-
ration and costal respiration. As a general rule, the two
forms of respiration coincide and aid one another, the con-
traction of the diaphragm taking place at the same time
with that of the external intercostals, and its relaxation with
their relaxation. It is a remarkable circumstance that the
v THE MOVEMENTS OF RESPIRATION 169
relative importance of the two forms is somewhat different in
the two sexes. In men, the diaphragm takes the larger share
in the process, the upper ribs moving comparatively little ;
in women, the reverse is the case, the respiratory act being
more largely the result of the movement of the ribs.
In ordinary quiet respiration, inspiration, as has been seen,
is an active process depending on the contraction of
muscles ; expiration, on the other hand, is rather due to a
passive recoil of elastic structures which had been previously
put on the stretch. But at times, as when taking violent
exercise, the respiration becomes more forcible or, as it is
called, "laboured." In this case many accessory muscles
come into play to assist during inspiration in raising the
ribs and sternum; being chiefly muscles stretched between
the ribs and parts of the vertebral column — above them at
the back, and between the neck and the sternum in front.
At the same time expiration, from being passive, now also
becomes an active process, chiefly by the contraction of
certain muscles, the abdominal muscles, which connect the
ribs and breast-bone with the pelvis, and form the front and
side walls of the abdomen. They assist expiration in two
ways : first, directly, by pulling down the ribs ; and next,
indirectly, by pressing the viscera of the abdomen upwards
against the under surface of the diaphragm, and so driving
the floor of the thorax upwards.
It is for this reason that, whenever a violent expiratory
effort is made, the walls of the abdomen are obviously flat-
tened and driven towards the spine, the body being at the
same time bent forwards.
In taking a deep inspiration, on the other hand, the walls
of the abdomen are relaxed and become convex, the vis-
cera being driven against them by the descent of the dia-
phragm — the spine is straightened, the head thrown back.
170 ELEMENTARY PHYSIOLOGY less.
and the shoulders outwards, so as to afford the greatest
mechanical advantage to all the muscles which can elevate
the ribs.
Sighing is a deep and prolonged inspiration followed by a
long expiration. " Sniffing" is a more rapid inspiratory act,
in which the mouth is kept shut, and the air made to pass
through the nose.
A hiccough is the result of a sudden inspiration, due to
a contraction of the diaphragm, during which the glottis is
suddenly closed and the column of air, striking on the
closed glottis, gives rise to the well-known and characteristic
sound.
Coughing is a violent expiratory act. A deep inspiration
being first taken, the glottis is closed and then burst' open
by the violent compression of the air contained in the lungs
by the contraction of the expiratory muscles, the diaphragm
being relaxed and the air driven through the mouth. In
sneezing, on the contrary, the cavity of the mouth being shut
off from the pharynx by the approximation of the soft palate
and the base of the tongue, the air is forced through the
nasal passages.
It thus appears that the thorax, the lungs, and the trachea
constitute a sort of bellows without a valve, in which the
thorax and the lungs represent the body of the bellows,
while the trachea is the pipe ; and the effect of the respira-
tory movements is just the same as that of the approxima-
tion and separation of the handles of the bellows, which
drive out and draw in the air through the pipe. There is,
however, one difference between the bellows and the respira-
tory apparatus, of great importance in the theory of respi-
ration, though frequently overlooked ; and that is, that the
sides of the bellows can be brought close together so as to
force out all, or nearly all, the air which they contain ; while
THE MOVEMENTS OE RESPIRATION
i7i
the walls of the chest, when approximated as much as possi-
ble, still inclose a very considerable cavity (Fig. 55, B) ; so
that, even after the most violent expiratory effort, a very
large quantity of air is left in the lungs.
If an adult man, breathing calmly in the sitting position, be
watched, the respiratory act will be observed to be repeated
Fig. 55. — Diagrammatic Sections of the Body in
A, inspiration, B, expiration. Tr, trachea; St, sternum: D, diaphragm: Ab, abdomi-
nal walls. The shading roughly indicates the stationary air.
on an average about fifteen to seventeen times every minute ;
but the frequency of repetition is very variable. Each act
consists of certain components which succeed one another
in a regular rhythmical order. First, the breath is drawn
in or inspired ; immediately afterwards, it is driven out or
172 ELEMENTARY PHYSIOLOGY less.
expired ; and these successive acts are followed by a brief
pause. Thus, just as in the rhythm of the heart, the auricu-
lar systole, the ventricular systole, and then a pause follow
in regular order ; so in the chest, the inspiration, the expira-
tion, and then a pause succeed one another. But in the
chest, unlike the case of the heart, the pause is generally
very short compared with the active movement ; indeed,
sometimes it hardly exists at all, a new inspiration following
immediately on the close of expiration.
6. The Amount of Air Respired. — At each inspiration
of an adult well-grown man about 500 c.c. (30 cubic inches) of
air are inspired ; and at each expiration the same, or a slightly
smaller, volume (allowing for the increase of temperature of
the air so expired) is given out of the body. To this the
name of tidal air has been conveniently given.
The amount of air which, as already pointed out, cannot
be got rid of by even the most violent expiratory effort and
is called residual air, is, on the average, about 1,500 c.c.
(100 cubic inches).
About as much more in addition to this remains in the
chest after an ordinary expiration, and is called supple-
mental air.
Thus it follows that, after an ordinary inspiration,
1,500 + 1,500 + 500 = 3,500 c.c. (100+100 + 30 = 230
cubic inches) may be contained in the lungs. By taking the
deepest possible inspiration, another 1,500 c.c. (100 cubic
inches), called complemental air, may be added.
The sum of the supplemental, tidal, and complemental air
amounts to about 3,500 to 4,000 c.c. (230 to 250 cubic
inches), and is a measure of what is known as the respiratory
or vital capacity. It varies according to a person's height,
weight, and age.
It results from these data that the lungs, after an ordinary
V ' THE AMOUNT OF AIR RESHRED 173
inspiration, contain about 3,500 c.c. (230 cubic inches) of
air, and that only about one-seventh to one-eighth of this
amount is breathed out and taken in again at the next inspi-
ration. Apart from the circumstance, then, that the fresh
air inspired has to fill the cavities of the hinder part of the
mouth, the trachea, and the bronchi, if the lungs were
mere bags fixed to the end of the bronchi, the inspired air
would descend so far only as to occupy that one-fourteenth
to one-sixteenth part of each bag which was nearest to the
bronchi, whence it would be driven out again at the next
expiration. But as the bronchi branch out into a prodigious
number of bronchial tubes, the inspired air can only pene-
trate for a certain distance along these, and can never reach
the air-cells at all.
Thus the residual and supplemental air taken together
are, under ordinary circumstances, stationary — that is to
say, the air comprehended under these names merely shifts
its outer limit in the bronchial tubes, as the chest dilates and
contracts, without leaving the lungs, and is hence called sta-
tionary air; the tidal six, alone, being that which leaves the
lungs and is renewed in ordinary respiration.
It is obvious, therefore, that the business of respiration is
essentially transacted by the stationary air, which plays the
part of a middleman between the two parties — the blood
and the fresh tidal air — who desire to exchange their com-
modities : carbonic acid for oxygen, and oxygen for carbonic
acid.
Now there is nothing interposed between the fresh tidal
air and the stationary air ; they are gaseous fluids, in com-
plete contact and continuity, and hence the exchange be-
tween them must take place according to the ordinary laws
of gaseous diffusion.
Thus, the stationary air in the air-cells gives up oxygen
174 ELEMENTARY PHYSIOLOGY less.
to the blood, and takes carbonic acid from it, though the
exact mode in which the change is effected is not thor-
oughly understood. By this process it becomes loaded with
carbonic acid, and deficient in oxygen, though to what pre-
cise extent is not known. But there must be a very much
greater excess of the one, and deficiency of the other, than is
exhibited by expired air, seeing that the latter has acquired
its composition by diffusion in the short space of time (four
or five seconds) during which it has been in contact with the
stationary air.
7. The Changes of Air in Respiration. — Expired air
differs from the air inspired in the following particulars : —
(i) Speaking generally, whatever be the temperature of
the external air, that expired tends to be nearly as hot as
the blood, or has a temperature of about 370 C. (98. 6° F.).
(ii) However dry the external air may be, that expired is
nearly, or quite, saturated with watery vapour.
(iii) While ordinary inspired air contains in 100 vol-
umes —
Oxygen. Nitrogen. Carbonic Acid.
20.96 79.OO .04
the composition of expired air is on the average in 100
volumes —
Oxygen. Nitrogen. Carbonic Acid
16.50 79.50 4.00
Thus, speaking roughly, air which has been breathed
once has gained 4 per cent, of carbonic acid and lost rather
more than 4 per cent, of oxygen, the quantity of nitrogen
being practically unchanged.
(iv) Expired air contains, in addition, small quantities
of "animal matter" or organic impurities of a highly de-
composable kind. Nothing is known of their nature, but
they are probably the chief cause why air which has been
v WASTE WHICH LEAVES THE LUNGS 175
breathed once is extremely unwholesome if breathed a
second time ; hence they are of great importance in con-
nection with ventilation (see p. 191).
(v) The volume of the expired air is slightly (about J~0)
less than that of the inspired air. This is due to the fact
that the volume of oxygen which disappears is always
slightly greater- than the volume of carbonic acid which
takes its place ; for all the oxygen taken in does not go to
form carbonic acid ; some of it unites with hydrogen to
form water and some with other elements such as sulphur.
Furthermore, careful analysis shows that the nitrogen in
expired air may vary very slightly : sometimes it is a little
in excess of, sometimes slightly less than, that inspired, and
sometimes it remains unaltered.
8. The Amount of Waste which, leaves the Lungs. —
About 10,000 litres (from 350 to 400 cubic feet) of air are
passed through the lungs of an adult man taking little or no
exercise, in the course of twenty-four hours, and are charged
with carbonic acid, and deprived of oxygen, to the extent
of about 4 per cent. This amounts to about 450 litres
(16 cubic feet) of the one gas taken in, and of the other
given out. Thus, if a man be shut up in a close room hav-
ing the form of a cube seven feet in the side, every particle
of air in that room will have passed through his lungs in
twenty-four hours, and a fifth of the oxygen it contained
will be replaced by carbonic acid.
The quantity of carbon eliminated in the twenty-four
hours is pretty nearly represented by a piece of pure char-
coal weighing 225 grammes (eight ounces).
The quantity of water given off from the lungs in the
twenty-four hours varies very much, but may be taken on
the average as about 500 c.c. (one pint, or about sixteen
ounces). It may fall below this amount, or increase to
double or treble the quantity.
176 ELEMENTARY PHYSIOLOGY less.
The air expired during the first half of an expiration con-
tains less carbonic acid than that expired during the sec-
ond half. Further, when the frequency of respiration is
increased without altering the volume of each inspiration,
though the percentage of carbonic acid in each inspiration
is diminished, it is not diminished in the same ratio as that
in which the number of inspirations increases; and hence
more carbonic acid is got rid of in a given time.
Thus, if the number of inspirations per minute is in-
creased from fifteen to thirty, the percentage of carbonic
acid evolved in each expiration in the second case remains
more than half of what it was in the first case, and hence
the total evolution is greater.
The activity of the respiratory process is greatly modified
by the circumstances in which the body is placed. Thus,
cold greatly increases the quantity of air which is breathed,
the quantity of oxygen absorbed, and of carbonic acid ex-
pelled ; exercise and the taking of food have a correspond-
ing effect.
In proportion to the weight of the body, the activity of
the respiratory process is far greatest in children, and dimin-
ishes gradually with age.
The excretion of carbonic acid is greatest during the day,
and gradually sinks at night, attaining its minimum at about
9 p.m. and remaining there for six or seven hours.
Indeed, it would appear that the rule that the quantity of
oxygen taken in by respiration is, approximately, equal to
that given out by expiration, only holds good for the total
result of twenty-four hours' respiration. More oxygen ap-
pears to be given out during the daytime (in combination
with carbon as carbonic acid) than is absorbed ; while, at
night, more oxygen is absorbed than is excreted as carbonic
acid during the same period. And it is very probable that
v CHANGES IN THE LUNGS AND TISSUES 177
the deficiency of oxygen towards the end of the waking
hours, which is thus produced, is one cause of the sense of
fatigue which comes on at that time. This difference be-
tween day and night is, however, not constant, and appears
to depend a good deal on the time when food is taken.
The quantity of oxygen which disappears in proportion
to the carbonic "acid given out, is greatest in carnivorous,
least in herbivorous animals — greater in a man living on a
flesh diet, than when the same man is feeding on vegetable
matters.
9. The Nature of the Respiratory Changes in the Lungs
and Tissues. — When a gas is inclosed in a vessel, it exerts
a pressure on its walls. If two gases are mixed, each gas
exerts its own pressure just as if the other gas were not
present ; the total pressure of the mixture is equal to the sum
of the separate pressures. The pressure due to each gas in
the mixture is called the partial pressure of that gas, and is
proportional to the quantity of the gas. Hence if the total
pressure of the mixture is measured and its composition is
determined by analysis, the partial pressure of each gas is at
once known. Take, for instance, ordinary air when the
barometer stands at 760 mm. (30 inches of mercury). The
partial pressure of the oxygen is -^fa x 760= 159.6 mm.
(6.3 inches of mercury), and that of the nitrogen is T7^ x
760 = 600.4 mm. (23.7 inches of mercury).
When a gas is in contact with a liquid some of the gas is
absorbed by the liquid, the amount being dependent on the
pressure of the gas. If two gases are in contact with the
same liquid, they will be absorbed in quantities proportional
to their respective partial pressures in the space over the
liquid, and when the absorption is complete the partial press-
ures of the gases in the liquid are the same as the partial
pressures of the gases in the space. If the partial pressure
N
78 ELEMENTARY PHYSIOLOGY less.
of one of the gases be made less in the space over the liquid,
then some of that gas will make its exit from the liquid ; and
if its partial pressure be, on the other hand, increased, then
more of that gas will enter the liquid. Thus we see that
changes in the partial pressures of the gases in contact with
the liquid determine the exit and entry of those gases from
and into the liquid. Further, since gases diffuse readily
through thin porous films, the statements we have just made
will, broadly speaking, hold equally good in the case when
the surface of the fluid is separated from the neighbouring
gases by a thin, moist, porous film. In these facts we find
the causes of the conversion of venous to arterial blood in
the lungs and the reverse change in the tissues.
The air in the alveoli of the lungs is a mixture of gases
separated from the venous blood by the thin, moist, filmy
wall of the alveoli and capillaries. The partial pressures of
the gases of the blood are known. The composition of
alveolar air has not been determined as yet because it has
not been found possible to collect air direct from the alveoli.
But from the composition of expired air we can at once
determine the partial pressures of the oxygen and carbonic
acid in it, and although the partial pressure of the oxygen
in alveolar air must be less and of carbonic acid greater than
in expired air, there are reasons for supposing that the dif-
ference is not great. By applying the data thus obtained we
find that venous blood in contact with oxygen at the partial
pressure it probably has in alveolar air readily takes up oxy-
gen and becomes arterialised. The entry of the oxygen is
further assisted by the fact that the gas passes into loose
chemical combination in the red corpuscles. Similarly, we
may say that the exit of carbonic acid is due to the differ-
ence between the (lower) partial pressure of carbonic acid
in the alveolar air and the (higher) partial pressure it has in
v THE NERVOUS MECHANISM OF RESPIRATION 17$
the venous blood ; but the case is not quite so clear as it is
with regard to oxygen, for the partial pressure of carbonic
acid in alveolar air is not inconsiderable, and its exit from
the blood is opposed by the fact that it is in loose combina-
tion with some constituent of the plasma.
The blood thus fully arterialised is whirled away to the
tissues. Here the causes of the change are much more
easily understood, for the living tissues are greedy of oxygen,
which they stow away in compounds so stable that they
give up no oxygen to the vacuum of even the most powerful
pump ; the partial pressure of oxygen in the tissues is in
fact zero. Hence oxygen readily passes over from the
arterial blood. On the other hand, the living tissues are
always producing carbonic acid in greater or less amount
according as they are more or less active ; the partial press-
ure of carbonic acid here is therefore high and quite suffi-
cient to account for the passage of this gas from the tissues
into the neighbouring arterial blood. The blood therefore
becomes venous. The amount of oxygen left in the blood
is dependent on the varying activity of the tissues, and this
is the reason why the volume of this gas was given (p. 149)
as varying from eight to twelve volumes in each hundred
volumes of venous blood.
10. The Nervous Mechanism of Respiration. — Of the
various mechanical aids to the respiratory process, the nature
and workings of which have now been described, one, the
elasticity of the lungs, is of the nature of a dead, constant
force. The action of the rest of the apparatus is under
the control of the nervous system, and varies from time
to time.
As the nasal passages cannot be closed by their own
action, air has always free access to the pharynx ; but the
glottis, or entrance to the windpipe, is completely under the
180 ELEMENTARY PHYSIOLOGY less.
control of the nervous system — the smallest irritation about
the mucous membrane in its neighbourhood being conveyed,
by its nerves, to that part of the cerebro-spinal axis which is
called the spinal bulb or medulla oblongata (see Lesson
XII.). The spinal bulb thus stimulated gives rise, by a
process which will be explained hereafter, termed reflex
action, to the contraction of the muscles which close the
glottis, and commonly, at the same time, to a violent con-
traction of the expiratory muscles, producing a cough (see
p. 170). The muscular fibres of the smaller bronchial
tubes are similarly under the control of the bulb, sometimes
contracting so as to narrow and sometimes relaxing so as to
permit the widening of the bronchial passages.
These, however, are mere incidental actions. The whole
respiratory machinery is worked by a nervous apparatus.
From what has been said, it is obvious that there are many
analogies between the circulatory and the respiratory appa-
ratus. Each consists, essentially, of a kind of pump which
distributes a fluid (liquid in the one case, gaseous in the
other) through a series of ramified distributing tubes to a
system of cavities (capillaries or air-cells), the volume of
the contents of which is greater than that of the tubes.
While the heart, however, is a force-pump, the respiratory
machinery represents a suction-pump.
In each the pump is the cause of the motion of the fluid,
though that motion may be regulated, locally, by the con-
traction or relaxation of the muscular fibres contained in
the walls of the distributing tubes. But, while the rhythmic
movement of the heart chiefly depends upon an apparatus
placed within itself, which is then controlled by the central
nervous system, that of the respiratory apparatus results
mainly from the operation of a nervous centre lodged in the
spinal bulb, which has been called the respiratory centre.
v THE NERVOUS MECHANISM OF RESPIRATION 181
This centre is situated (see Fig. 56, R. C.) close to the
two previously described as the vaso- motor and cardio-
inhibitory centres (Figs. 33 and 34, pp. 97 and 102). Im-
pulses arise in this centre, pass down the spinal cord, and
leaving the cord along certain nerves, reach the various
muscles by whose contractions the movements of respira-
tion are produced. The respiratory muscles contract only
when they receive these impulses, and therefore all the
movements of respiration depend upon the activity of this
centre, and cease at once on injury of this part of the spinal
bulb.
The action of the centre is primarily automatic ; in other
words, the impulses it sends out appear to be the result of
changes started within itself, in the same way that the beat
of the heart is automatic as the outcome of changes started
in the muscle-tissue of which it is made up. This primary
automatism of the respiratory centre is subject, however, to
control, in a way to be described presently, by impulses
reaching it from outlying parts of the body, and more par-
ticularly by changes in the condition or quality of the blood
which circulates in the capillaries of the centre itself.
The intercostal muscles are supplied by intercostal nerves
coming from the spinal cord in the region of the back (Fig.
56, I.C.N.), and the muscular fibres of the diaphragm are
supplied by two nerves, one on each side, called the phrenic
nerves (Fig. 56, Phr.), which, starting from certain of the
spinal nerves in the neck, dip into the thorax at the root of
the neck, and find their way through the thorax by the side
of the lungs to the diaphragm, over which they are dis-
tributed. From the respiratory centre in the spinal bulb
impulses at repeated intervals descend along the upper part
of the spinal cord, and, passing out by the phrenic and in-
tercostal nerves respectively, reach the diaphragm and the
1S2 ELEMENTARY PHYSIOLOGY less.
intercostal muscles. These immediately contract, and thus
an inspiration takes place. Thereupon the impulses cease,
and are replaced by other impulses, which, though starting
from the same centre, pass, not to the diaphragm and exter-
nal intercostal muscles, but to other, expiratory, muscles,
which they throw into contraction, and thus expiration is
brought out. As a general rule, the inspiratory impulses
are much stronger than the expiratory ; indeed, in ordinary
quiet breathing expiration is chiefly brought about, as we
have seen, by the elastic recoil of the lungs and chest walls ;
these need no nervous impulses to set them at work ; as
soon as the inspiratory impulses cease and the diaphragm
and other inspiratory muscles leave off contracting, they
come of themselves into action. But, in laboured breath-
ing, very powerful expiratory impulses may leave the res-
piratory centre and pass to the various muscles whose
contractions help to drive the air out of the chest.
Everyday experience shows that no function of the body
is more obviously subject to sudden and marked changes
than is the respiration. It is quickened by exercise, quick-
ened or slowed by emotions; hurried by stimulation of the
skin, as by a dash of cold water, or brought to a standstill by
stimulating the mucous membrane of the nose by a pungent
vapour such as strong ammonia. The changes involved in
sneezing, laughing, coughing, etc., are profound and pecul-
iar. Finally, we can control our respiration by an effort of
the will within very wide limits and in almost any desired
way. The mechanism involved in the production of all
these changes is correspondingly complicated ; but certain
broad farts are fairly simple, and to these we may now turn.
The main trunk of the vagus nerve, which, as we shall
see, contains nerve fibres coming from ''the lungs (p. 538),
gives off a branch to the larynx as it passes down the neck
v THE NERVOUS MECHANISM OF RESPIRATION 18}
(Fig. 56, S.Lr.). If the vagus be cut below the poini of exit
of this nerve (as at x, Fig. 56), and the upper (central) end
(y, Fig. 56) connected with the spinal bulb and containing
i ,'a.f.
•R.C.
I.C.N.
— Sp.C.
Fig. 56. — Diagram to illustrate the Position of the Respiratory Centre,
the Connections of this Centre with the Intercostal Muscles and
Diaphragm, and the Paths by which Impulses pass to the Centre from
Outlying Parts of the Body and from the Brain.
Sp. C spinal cord; R.C respiratory centre in the bulb: I.C.N, three intercostal
nerves: Phr. one phrenic nerve passing to the diaphragm D. ; Vg. vagus nerve; V.G.
ganglion of vagus nerve; S.Lr. superior laryngeal nerve. The dotted lines, c.f., indi-
cate paths of conduction for impulses to the respiratory centre from some part of the
body such as the skin; the dotted lines, a.f., indicate similar paths from the brain to
the centre. The arrows show the direction in which impulses travel along each nerve
or path.
the pulmonary fibres be gently stimulated, the respiration
often becomes hurried. Thus, we have in the vagus a nerve
such that impulses passing up it may quicken the respiration
by their action on the respiratory centre.
184 ELEMENTARY PHYSIOLOGY less
If on the other hand the branch of the vagus supplying
the larynx, the superior laryngeal nerve, be cut, and its
central end be stimulated, the result is that the respiration
may be sloived, even to a complete cessation of all respiratory
movements.
In the case of the vagus, impulses seem to be ordinarily
always passing up it from the lungs to the respiratory centre,
for if the vagus nerves be simply cut, the respiration be-
comes at once extremely slow, and remains so.
These two nerves without doubt act in life as they act
upon artificial stimulation and may be taken as typical of
their kind, the one quickening, the other slowing the
respiration. But similar nerves run to the respiratory
centre from all parts of the body, notably from the skin,
also from the brain, and by their varied action largely
determine the action of the centre, and thus the manifold
changes which the respiratory movements from time to time
undergo.
11. Influence of Blood-supply on the Respiratory Cen-
tre. Dyspnoea and Asphyxia. — The function of respiration
has for its one great object the conversion of venous into
arterial blood. Hence we might expect that the mechanism
which controls it should be adjusted so as to be extremely
sensitive to the varying condition of the blood. This expec-
tation is justified by facts, for, although the respiratory centre
is keenly responsive to impulses brought to bear upon it
along various nerves, it is even more so to the influence
exerted by the varying quality of the blood circulating in the
capillaries of the spinal bulb. Thus, when by any means the
blood becomes less arterialized than it should be, the res-
piratory centre feels this change, and is at once stimulated
to greater activity in the endeavour, by an increased force
and frequency of the respiratory movements, to restore the
v INFLUENCE OF BLOOD-SUPPLY 185
blood to its proper condition. In other words, venous
blood makes the respiratory centre work faster and more
vigorously.
The blood becomes more venous whenever the free access
of air to the lungs is interfered with ; as, for instance, when
a man is strangled, drowned, or choked by food or other
obstacle in the trachea. But the blood may become unusu-
ally venous by means less violent than the above. Since
the rapidity of diffusion between two gaseous mixtures de-
pends on the difference of the proportions in which their
constituents are mixed, it follows that the more nearly the
composition of the tidal air approaches that of the stationary
air, the slower will be the diffusion of oxygen inwards, and
of carbonic acid outwards, and the more deficient in oxygen
and overcharged with carbonic acid will the air in the alveoli
become. Thus, by breathing in a confined space, the oxy-
gen in the tidal air is gradually diminished and the carbonic
acid gradually increased until at length a point is reached
when the change effected in the stationary air is too slight
to enable it to supply the pulmonary blood with oxygen,
and to relieve it of carbonic acid to the extent required for
its proper arterialisation.
"When from any of the above causes the blood sent to the
respiratory centre is more venous than usual, the centre is
stimulated and the respiratory movements become quicker
nnd more forcible. This condition is usually spoken of as
dyspnoea, or laboured breathing. It is characterised by the
increased force and frequency with which both the inspi-
ratory and expiratory muscles contract. If the offending
cause of dyspnoea be not removed, the blood becomes more
and more venous. By this means the respiratory centre
is spurred on to still greater activity. Not only do the
ordinary muscles of respiration contract more vigorously,
1 86 ELEMENTARY PHYSIOLOGY less.
but the accessory muscles (p. 169) come into more promi-
nent play, and chiefly those which assist expiration. Still
later, nearly all the muscles of the body are thrown into a
state of violent contracting activity, and with the onset of
these convulsions dyspnoea passes over into asphyxia. The
violence of the convulsive movements speedily leads to
exhaustion, and the convulsions cease. After this stage is
reached, a long-drawn inspiration takes place at intervals ;
but the intervals become longer and longer and the inspira-
tory movements more and more feeble until the last breath
is taken and breathing ends with an expiratory gasp.1
Venous blood is distinguished from arterial by two fea-
tures, by having less oxygen and more carbonic acid. Hence,
in asphyxia, two influences of a distinct nature are cooper-
ating ; one is the deprivation of oxygen, the other is the
excessive accumulation of carbonic acid in the blood. Oxy-
gen starvation and carbonic acid poisoning, each of which is
injurious in itself, are at work together; but of these, the
lack of oxygen is the real cause of asphyxia.
The effects of oxygen starvation may be studied sepa-
rately, by placing a small animal under the receiver of an
air-pump and exhausting the air ; or by replacing the air by
a stream of hydrogen or nitrogen gas. In these cases no
accumulation of carbonic acid is permitted, but, on the
other hand, the supply of oxygen soon becomes insufficient,
and the animal quickly dies with all the symptoms of as-
phyxia. And if the experiment be made in another way, by
placing a small mammal, or bird, in air from which the car-
bonic acid is removed as soon as it is formed, the animal will
nevertheless die asphyxiated as soon as the amount of oxy-
gen is reduced to 10 per cent, or thereabouts.
1 The term asphyxia is sometimes used to include all the stages, from
the onset of dyspnoea until death ensues.
v INFLUENCE OF BLOOD-SUPPLY 185
The directly poisonous effect of carbonic acid, on the
other hand, has been very much exaggerated. A very large
quantity of pure carbonic acid (10 to 15 or 20 per cent.)
may be contained in air, without producing any very serious
immediate effect, if the quantity of oxygen be simultaneously
increased.
Moreover, such symptoms as do occur when the carbonic
acid in the air breathed is increased without any corre-
sponding decrease in the oxygen, are not exactly those of
asphyxia but are said to resemble rather those of narcotic
poisoning. So that the chief cause of asphyxia in strangling,
drowning, or choking, or however produced, is the diminu-
tion of the oxygen in the air of the lungs and consequently
a diminution of the oxygen in the blood.
And that it is the lack of oxygen which is the important
thing is further shown by the asphyxiating effects of certain
poisonous gases. Thus sulphuretted hydrogen, so well known
by its offensive smell, has long had the repute of being a
positive poison. But its evil effects appear to arise chiefly,
if not wholly, from the circumstance that its hydrogen com-
bines with the oxygen carried by the blood-corpuscles, and
thus gives rise, indirectly, to a form of oxygen starvation.
Carbonic oxide gas (carbon monoxide, CO) has a much
more serious effect, as it turns out the oxygen from the
blood-corpuscles, and forms a very stable combination of
its own with the haemoglobin. The compound thus formed
is only very gradually decomposed by fresh oxygen, so that,
if any large proportion of the blood-corpuscles be thus ren-
dered useless, the animal dies before restoration can be
effected. Badly made common coal gas sometimes con-
tains 20 to 30 per cent, of carbon monoxide ; and, under
these circumstances, a leakage of the pipes in a house may
be extremely perilous to life.
1 88 ELEMENTARY PHYSIOLOGY less.
12. The Influence of Respiration on the Circulation. —
Just as there are certain secondary phenomena which
accompany, and are explained by, the action of the heart,
so there are secondary phenomena which are similarly
related to the working of the respiratory apparatus. Of
these the chief is the effect of the inspiratory and expira-
tory movements upon the circulation.
In consequence of the elasticity of the lungs, a certain
force must be expended in distending them, and this force
is found experimentally to become greater and greater the
more the lung is distended ; just as, in stretching a piece
of india-rubber, more force is required to stretch it a good
deal than is needed to stretch it only a little. Hence, when
inspiration takes place, and the lungs are distended with
air, the heart and the great vessels in the chest are sub-
jected to a less pressure tKan are the blood-vessels of the
rest of the body.
For the pressure of the air contained in the lungs is
exactly the same as that exerted by the atmosphere upon
the surface of the body ; that is to say, fifteen pounds on
the square inch. But a certain amount of this pressure
exerted by the air in the lungs is counterbalanced by the
elasticity of the distended lungs. Say that in a given con-
dition of inspiration a pound l pressure on the square inch
is needed to overcome this elasticity, then there will be
only fourteen pounds pressure on every square inch of the
heart and great vessels. And hence the pressure on the
blood in these vessels will be one pound per square inch
less than that on the veins and arteries of the rest of the
body, which lie outside the thorax. If there were no
aortic, or pulmonary, valves, and if the structure of the
1 A "pound" is stated here for simplicity's sake. As a matter of fact
the pressure required is much less than this, not more than 2 or 3 ounces,
v THE INFLUENCE OF RESPIRATION 1S9
vessels, and the pressure upon the blood in them, were
everywhere the same, the result of this excess of pressure
on the surface would be to drive all the blood from the
arteries and veins and the rest of the body into the heart
and great vessels contained in the thorax. And thus the
diminution of the pressure upon the thoracic blood-cavities
produced by inspiration would, practically, suck the blood
from all parts of the body towards the thorax. But the
suction thus exerted, while it hastened the flow of blood
to the heart in the veins, would equally oppose the flow
from the heart to the arteries, and the two effects might
balance one another.
As a matter of fact, however, we know —
(1) That the blood in the great arteries is constantly
under a very considerable pressure, exerted by their elastic
walls ; while that of the veins is under little pressure.
(2) That the walls of the arteries are strong and resist-
ing, while those of the veins are weak and flabby.
(3) That the veins have valves opening towards the
heart ; and that, during the diastole, there is no resistance
of any moment to the free passage of blood into the heart ;
while, on the other hand, the cavity of the arteries is shut
off from that of the ventricle, during the diastole, by the
closure of the semilunar valves.
Hence it follows that equal pressures applied to the
surface of the veins and to that of the arteries must pro-
duce very different effects. In the veins the pressure is
something which did not exist before ; and partly from the
presence of valves, partly from the absence of resistance in
che heart, partly from the presence of resistance in the
capillaries, it all tends to accelerate the flow of blood
towards the heart. In the arteries, on the other hand, the
pressure is only a fractional addition to that which existed
190 ELEMENTARY PHYSIOLOGY less.
before ; so that, during the systole, it only makes a com-
paratively small addition to the resistance which has to
be overcome by the ventricle ; and during the diastole, it
superadds itself to the elasticity of the arterial walls in
driving the blood onwards towards the capillaries, inas-
much as all progress in the opposite direction is stopped
by the semilunar valves.
It is, therefore, clear, that the inspiratory movement, on
the whole, helps the heart, inasmuch as its general result
is to drive the blood the way that the heart propels it.
In expiration, the difference between the pressure of the
atmosphere on the surface, and that which it exerts on the
contents of the thorax through the lungs, becomes less and
less in proportion to the completeness of the expiration.
Whenever, by the ascent of the diaphragm and the descent
of the ribs, the cavity of the thorax is so far diminished that
pressure is exerted on the great vessels, the veins, owing to
the thinness of their walls, are especially affected, and a
check is given to the flow of blood in them, which may
become visible as a venous pulse in the great vessels of the
neck. In its effect' on the arterial trunks, expiration, like
inspiration, is, on the whole, favourable to the circulation ;
the increased resistance to the opening of the valves during
the ventricular systole being more than balanced by the
advantage gained in the addition of the expiratory press-
ure to the elastic reaction of the arterial walls during the
diastole.
When the skull of a living animal is laid open and the
brain exposed, the cerebral substance is seen to rise and
fall synchronously with the respiratory movements ; the rise
corresponding with expiration, and being caused by the
obstruction thereby offered to the flow of the blood in the
veins of the head and neck.
v VENTILATION 19 1
The effects of the respiratory movements are the same
[or the thoracic duct. At inspiration the reduction of
pressure on the outside of the duct draws lymph up into it
from the abdominal lymphatic vessels. At expiration, the
lymph cannot pass down again, owing to the valves in
the duct, and is therefore sent on towards the junction of
the latter with the venous system. Hence the respiratory
movements are a not unimportant aid to the onward flow
of lymph (see p. 118).
13. Ventilation. — In the case of breathing the same air
over and over again, the deprivation of oxygen, and the accu-
mulation of carbonic acid, cause injury, long before any
signs of even dyspnoea are observed. Under these circum-
stances uneasiness and headache arise when less than 1 per
cent, of the oxygen of the air is replaced by other matters ;
the symptoms in this case, however, are due not so much to
the diminution of oxygen or the increase of carbonic acid,
as to the poisonous effects of the various organic matters
present in expired air which, though existing in minute quan-
tities, have a powerfully deleterious action. It need hardly
be added that the persistent breathing of such air tends to
lower all kinds of vital energy, and predisposes to disease.
Hence the necessity of sufficient air and of ventilation for
every human being.
The object of ventilation is to prevent the accumulation
of these organic impurities (p. 174) and any deficiency of
oxygen, such as may arise from burning gas in a room for
purposes of illumination. Since the organic matter does
not admit of direct estimation, the percentage of carbonic
acid in the air is usually taken as an indirect measure of its
amount, and this is at the same time a measure of the defi-
ciency of oxygen. Air which has been fouled by breathing
is injurious if it contains more than .05 per cent, of carbonic
i92 ELEMENTARY PHYSIOLOGY less, v
acid. If the percentage of carbonic acid is to be kept down
to this limit, a man should live in a room whose capacity is
not less than 28,000 litres (1,000 cubic feet) and into which
at least 60,000 litres (2,000 cubic feet) of fresh air are
admitted each hour.1
] A cubical room ten feet high3 wide, and long contains one thousand
cubic feet of air.
LESSON VI
THE SOURCES OF LOSS AND OF GAIN TO THE
BLOOD
1. General Review of the Gain and Loss. — The blood
which has been aerated, or arterialised, by the process de-
scribed in the preceding Lesson, is carried from the lungs
by the pulmonary veins to the left auricle, and is then forced
by the auricle into the ventricle, and by the ventricle into
the aorta. As that great vessel traverses the thorax, it gives
off several large arteries, by means of which blood is distrib-
uted to the head, the arms, and the walls of the body.
Passing through the diaphragm (Fig. 47, Ad), the aortic
trunk enters the cavity of the abdomen, and becomes what
is called the abdominal aorta, from which vessels are given
off to the viscera of the abdomen. Finally, the main stream
of blood flows into the iliac arteries, whence the viscera of
the pelvis and the legs are supplied.
Having in the various parts of the body traversed the
ultimate ramifications of the arteries, the blood, as we have
seen, enters the capillaries. Here the products of the waste
of the tissues constantly pour into it ; and, as the blood is
everywhere full of corpuscles, which, like all other living
things, decay and die, the products of their decomposition
also tend to accumulate in it, but these are insignificant
compared to those coming from the great mass of the
o 193
i94 ELEMENTARY PHYSIOLOGY less.
tissues. It follows that, if the blood is to be kept pure, the
waste matters thus incessantly poured into or generated in
it must be as constantly got rid of, or excreted.
Three distinct sets of organs are especially charged with
this office of continually removing or " excreting " waste
matters from the blood. They are the lungs, the kidneys,
and the skin. These three great organs may therefore be
regarded as so many drains from the blood — as so many
channels by which it is constantly losing substance.
On the other hand, the blood, as it passes through the
capillaries, is constantly giving up material by exudation
through the capillary walls into the surrounding tissues, in
order to supply them with nourishment, and thus in this
way also is constantly losing matter.
The material which the blood loses by giving it up to the
tissues consists of complex organic bodies, such as proteids,
fats, carbohydrates, and various substances manufactured out
of these, of certain salts, of a large quantity of water, and
lastly of oxygen.
The material which the blood loses by giving it up to the
skin, lungs, and kidneys, passes away from these organs as
water, as carbonic acid, as peculiar organic substances, of
which one, called urea, is much more abundant than the
others, and as certain inorganic salts. Speaking generally,
we may say that these organs together excrete from the
blood, water, carbonic acid, urea, and salts.
Another kind of loss takes place from the surface of the
body generally, and from the interior of the air-passages.
Heat is constantly being given off from the former by radia-
tion, evaporation, and conduction : from the latter, chiefly
by evaporation ; and the loss of heat in each case is borne
by the blood passing through the skin and air-passages re-
spectively. Besides this a certain quantity of heat is lost by
vi REVIEW OF THE GAIN AND LOSS 195
the urine and faeces, which are always warm when they leave
the body.
On the side of gain we have, in the first place, the various
substances which are the products of the activity of the sev-
eral tissues, muscles, brain, glands, etc., and which pass from
the tissues into the blood. We may speak of these as waste
products, and one of them; which is produced by all the tis-
sues, namely, carbonic acid, is emphatically a waste product
and is got rid of as soon as possible. But some of the sub-
stances which are returned to the blood from the tissues are
not wholly useless matters to be thrown off as rapidly as
possible ; they are capable of being used up again by some
tissue or other. Thus, as we shall see, the liver, at certain
times at all events, returns to the blood a certain quantity
of sugar, which is made use of in other parts of the body,
and similarly the spleen, while it takes up certain substances
from the blood, gives back to the blood certain other sub-
stances, which we can hardly speak of as waste matters in
the sense of being useless material fit only to be at once
thrown away.
In the second place, the blood is continually receiving
from the alimentary canal the materials arising from the
food which has been digested there. As we shall see,
some of this material passes directly from the cavity of
the alimentary canal into the blood, but some of it goes
in a more roundabout way through the lacteals or lym-
phatics. On its way to the blood, this latter is joined by
material which, escaping from the blood and not used by
the tissues, or passing from the tissues directly into the
lymphatics, is carried back to the blood by the thoracic
duct (see p. in).
In the third place, the blood is continually gaining oxy-
gen from the air, through the lungs.
i96 ELEMENTARY PHYSIOLOGY less.
Then again the blood, while it loses heat by the skin and
lungs, gains heat from the tissues. As we have already seen
(p. 24), oxidation is continually going on in various parts
of the body, and by this oxidation heat is continually being
set free. Some of this oxidation may take place in the
blood itself ; we do not know exactly how much, but prob-
ably very little. The greater part of the heat is generated
in the tissues, in the muscles, and elsewhere, and is given
up by the tissues to the blood. So that we may say that
the blood gains heat from the tissues.
These several gains and losses are for the most part
going on constantly, but are greater at one time than at
another. Thus the gain to the blood from the alimentary
canal is much greater some time after a meal than just
before the next meal, though, unless the meals be very far
apart indeed, the whole of the material of one meal has
not passed into the blood before the next meal is begun.
Again, though the muscles, even when completely at rest,
are taking up oxygen and nutritive material, and giving out
carbonic acid and other waste products, they give out and
take in much more when they are at work. So also cer-
tain " secreting glands," as they are called, which we shall
study presently, such as the salivary glands, have periods
of repose ; it is at certain times only, as when food has
been taken, that they pour out any appreciable quantity of
fluid. Hence, though they are probably taking up material
from the blood and storing it up in their substance even
when they appear at rest, they take up much more and
so become much more distinctly means of loss to the
blood when they are actively pouring out their secretions.
In the case of the liver, the loss to the blood is more
constant, since the secretion of bile, as we shall see, is con-
tinually going on, though greater at certain times than at
vi SOURCES OF LOSS AND GAIN 197
others ; and the materials for the bile have to be pro-
vided by the blood. Some of the constituents of the bile,
however, pass back from the intestines into the blood ; and
so far the loss to the blood by the liver is temporary only.
Of all the gains to the blood, perhaps the most constant
is that of oxygen, and of all the losses, perhaps the most
constant is that of carbonic acid ; but even these vary a
good deal at different times or under different circum-
stances.
Broadly speaking, then, the blood gains oxygen from the
lungs, complex organic food materials from the alimentary
canal, and various substances, which we may speak of as
waste substances, from the several tissues ; and it loses,
on the one hand, material, which we may speak of as con-
structive material, to the several tissues ; and, on the other
hand, material which passes away by the skin, lungs, and
kidneys, as water, carbonic acid, urea, and saline bodies.
And while it is continually receiving heat from the sev-
eral tissues, it is also continually losing heat by the skin,
lungs, and other free surfaces of the body.
The sources of loss and gain to the blood may be
conveniently arranged in the following tabular form : —
Sources of Loss and Gain to the Blood1
A. Sources of Loss : —
I. Loss of Matter.
1. The lungs: carbonic acid and water (fairly
constant).
1 The learner must be careful not to confound the losses and gains
of the blood with the losses and gains of the body as a whole. The two
differ in much the same way as the internal commerce of a country differs
from its export and import trade.
198 ELEMENTARY PHYSIOLOGY less,
2. The kidneys : urea, water, salines (fairly con-
stant) .
3. The skin: water, salines (fairly constant).
4. The tissues : constructive material (variable, es-
pecially in the case of those tissues whose
activity is intermittent, such as the muscles,
many secreting glands, etc.).
II. Loss of Heat.
1. The skin.
2. The lungs.
3. The excretions by the kidney and the alimen-
tary canal.
B. Sources of Gain : —
I. Gain of Matter.
1. The lungs: oxygen (fairly constant).
2. The alimentary canal : food (variable).
3. The tissues : products of their activity, waste
matters (always going on but varying
according to the activity of the several
tissues).
4. The lymphatics : lymph (always going on but
varying according to the activity of the
several tissues).1
II. Gain of Heat.
1. The tissues generally, especially the more ac-
tive ones, such as the muscles.
2. The blood itself, probably to a very small
extent.
1 The gain from (hose lymphatics which are called lacteals, since it
:omes from the lalimeniary canal, varies much more.
in SECRETION IN GENERAL 195
2. Secretion in General. — Secreting glands have been
spoken of as sources of loss and gain to the blood. A brief
and general survey of their structure and mode of action
may profitably be made here. In principle, they are nar-
row pouches of mucous membrane, or of the integument of
the body, lined by a continuation of the epithelium, or
the epidermis (Fig. 57). According as the pouch has the
form of a tube or is dilated, the gland is said to be tubular ( 1 )
or saccular (3). Forms intermediate between these two
are not uncommon. When a single pouch exists, the gland
is called simple ; when divided into two or more pouches,
it is compound. Compound saccular glands are usually
termed racemose (6), from their fancied resemblance to a
bunch of grapes. The neck by which the gland communi-
cates with the free surface of the mucous membrane or skin
is called its duct (//). The epithelium lining the gland con-
stitutes the secreting portion. It is composed of conspicu-
ous, characteristic cells, bathed over their attached surfaces
by lymph and surrounded closely by a rich network of capil-
laries, Frequently a thin, inconspicuous membrane of fiat
cells, the basement membrane (fi), lies immediately outside
the secreting cells. The manifest function of the secreting
cells is to receive from the blood, through the lymph, water,
salts, and other substances, to manufacture from these raw-
materials certain specific chemical substances, and finally
to pass out through the duct to the free surface the result-
ing mixture of water, salts, and specific substances, as the
secretion.1
1 The word " secretion " is used by physiologists in three senses. Pri-
marily it is used to denote the sum total of the processes by which a gland
or organ forms the fluid which it gives out ; thus we say that the salivary
glands " secrete " saliva. Further, it often signifies merely the process of
extrusion of the fluid from the gland in which it is formed. Lastly, the
fluid is itself spoken of as "a secretion." The word "excretion" is usu
ELEMENTARY PHYSIOLOGY
A
Fig. 57. — A Diagram to illustrate the Structure of Glands.
A. typical structure of a mucous membrane; a, the layer of epithelium cells;
6, the basement membrane; c, the dermis, with e, a blood-vessel, andy, connective
tissue corpuscles.
1. A simple tubular gland; letters the same as in A.
2. A tubular gland divided at its base. In this and succeeding figures the blood
vessels are omitted.
3. A simple saccular gland.
4. A divided saccular gland, with a duct, d.
5. A similar gland still more divided.
6. A racemose gland, part only being drawn.
vi THE URINARY ORGANS aoi
A less obvious but not less important function of manv
glands is that of giving to the blood material which is thus
passed on to other glands for excretion or can be made use
of by other parts of the body. This property of internal
secretion, which has become well recognised only and is not
yet fully elucidated, belongs prominently to the liver and
certain so-called " ductless glands," such as the thyroid body
and the suprarenal bodies.
In the preceding Lesson we have described the operation
by which the lungs withdraw from the blood much carbonic
acid and water, and supply oxygen to the blood. In this
and the succeeding Lesson some other of the chief sources
of loss and of gain to the blood will be discussed in detail.
3. The Urinary Organs. — We now proceed to the sec-
ond source of continual loss, the Kidneys.
Of these organs there are two, placed at the back of the
abdominal cavity, one on each side of the lumbar region of
the spine. Each, though somewhat larger than the kidney
of a sheep, has a similar shape. The depressed, or concave,
side of the kidney is turned inwards, or towards the spine ;
and its convex side is directed outwards (Fig. 58). From
the middle of the concave side (called the hilus) of each
kidney, a long tube with a small bore, the ureter ( U) , pro-
ceeds to the bladder ( Vu).
The latter, situated in the pelvis, is an oval bag, the walls
of which contain abundant unstriped muscular fibre, while
it is lined, internally, by mucous membrane, and coated
externally by a layer of the peritoneum, or double bag of
serous membrane, which has exactly the same relations to
ally applied to any fluid which after its formation is useless and requires
to be at once got rid of. Tims, we say that urine is an excretion which
is secreted {i.e. formed) by the kidneys; ami we speak of those secretory
structures which get rid of waste as excretory organs.
ELEMENTARY PHYSIOLOGY
the cavity of the abdomen and the viscera contained in
them as the pleurae have to the thoracic cavity and the
lungs. The ureters open side by side, but at some little
distance from one another, on the posterior and inferior
wall of the bladder. Each ureter is lined by an epithelium
consisting of several layers of cells. Outside of these is a
muscular coat made up of unstriated muscle-fibres, arranged
Fig. 58. — The Urinary Organs seen from behind. (From Moore's Elemen-
tary Physiology.)
R, right kidney; U, ureter; Vu, bladder; Ua, commencement of urethra; A,
aorta; Ar, right renal artery; Ve, inferior vena cava; Vr, right renal vein.
in three layers and surrounded externally by some fibrous
connective tissue. In front of the ureters is a single aper-
ture which leads into the canal called the urethra (Fig. 58,
U, medullary portion,/ reaching to the summit of a pyramid.
/, Malpighian capsule; //, /', convoluted tubules; !I[, descending limb, and
IV, ascending limb, of the loop of Henle; VI, VII, VIII, collecting tubules;
IX, discharging tubule.
experiments show that they are such. They are surrounded
by a rich capillary network (Fig. 61). In the collecting
and discharging tubules the cells are cubical or columnar
(Fig. 63, b), quite free from granules, do not stain readily,
and apparently are not secretory. These portions of the
208
ELEMENTARY PHYSIOLOGY
tubules are probably purely conducting in function. So
far as the formation of the urine is concerned, the impor-
tant cells seem to be the capsular and the secreting cells.
The artery which supplies the kidney enters at the hilus
and divides into branches which "pass around the pelvis and
proceed outwards between the pyramids. At the junction
of the medulla and cortex these branches spread out
sideways and form arches (Fig. 64). From these arches
branches run (i) straight out to the surface of the kidney,
B
/•:
Fig. 63. — Types of Cells in the Tubules of the Kidney.
A, lubules cut lengthwise; B, tubules cut across.
a, type of (secreting) cell lining the convoluted, spiral, and irregu.ar tubules
b, type of cells lining the collecting and discharging tubules; n, nuclei; c, in B,
capillaries seen in section.
giving off smaller lateral branches, of which some pass to
the capsules while others supply the capillary network round
the tubules : (ii) down towards the pyramids, in whose sub-
stance they break up into capillaries. The veins also form
arches at the junction of the cortex and medulla, into which
the blood flows from the capillaries, and leave the kidney
by a course parallel to that of the entering arteries.
5. The Urine. — The renal secretion is a clear yellowish
fluid, whose specific gravity is not very different from that
vi THE URINE 209
of blood-serum, being 1.020. In health it has a slightly
acid reaction, due to the presence of acid sodium phos-
phate. It is composed chiefly of water, holding in solution :
(i) Organic substances, of which the chief is urea, with a
very much smaller amount of uric acid, (ii) Inorganic
'I
Fig. 64. — Blood-vessels of Kidnev. (Cadiat.)
a, part of arterial arch; b, interlobular artery; c, glomerulus; d, efferent vessel;
e, capillaries of cortex; f, straight arteries of medulla; g, venous arch; h, straight
veins of medulla ; i, interlobular vein.
salts, chiefly sodium chloride and sulphates and phosphates
of sodium, potassium, calcium, and magnesium, (iii) Col-
ouring matters, of which but little is known, (iv) Gases,
chiefly carbonic acid, with a very small amount of nitro-
gen and still less oxygen.
210 ELEMENTARY PHYSIOLOGY less.
An average healthy man excretes about 1,500 c.c. (3
pints) of urine each day. In this are dissolved 33 grammes
(ij oz. or about 2 per cent.) of urea and not more than
.5 gramme (8 grains) of uric acid. The amount of salts
is nearly equal to that of the urea, and the larger part
consists of sodium chloride.
The quantity and composition of the urine vary greatly
according to the time of day, the temperature and mois-
ture of the air, the fasting or replete condition of the ali-
mentary canal, the nature of the food, and the amount of
fluid consumed.
The quantity depends on the temperature and the mois-
ture of the air because, as we shall see (p. 230), these
determine the greater or less loss of water by the skin,
and thus leave less or more to be excreted by the kidneys.
The relationship of fluid consumed to the amount of urine
excreted is obvious. The composition varies with the kind
and amount of food, chiefly in respect of the amount of
urea excreted, for the nitrogen in urea represents nearly
all the nitrogen introduced into the body in the proteids.
This relationship of the nitrogen in food to the nitrogen
of urea confers upon urea its supreme importance as a
constituent of urine ; for the body cannot make good its
nitrogenous waste from any source other than the nitrogen
introduced into it in the form of proteids. Hence varia-
tions in the quantity of urea excreted thus become the
measure of the amount of nitrogen turned over or " metabo-
lised " in the body from time to time.
Urea is a white crystalline solid, very soluble in water, and
composed of carbon, oxygen, hydrogen, and nitrogen. Its
chemical formula is (NH.,)2CO, from which it is seen to
contain rather more than 46 per cent, of nitrogen.
Historically, urea is interesting as being the first organic
vi THE SECRETION OF URINE 211
animal product prepared (synthetically) from inorganic
sources (by Wohler in 1S28).
6. The Secretion of Urine. — Many of the constituents
of urine are present in blood. These appear in the urine
dissolved in a large quantity of water, whereas many other
substances also present in the blood do not, in a state of
health, make their way into the urine. This suggests the
idea that the kidney is a peculiar and delicate kind of filter,
which allows certain substances together with a large quan-
tity of water to pass through it, but refuses to allow other
substances to pass through. And when we come to studv
the minute structure of the kidney, we find much to support
this idea. Thus, we saw that the surface of the glomerulus
is, practically, in direct communication with the exterior by
means of the cavity of the tubule ; and, further, that in each
vessel of the glomerulus a thin stream of blood constantly
flows, separated from the cavity of the tubule only by the
capillary wall and the very delicate epithelial membrane
covering the glomerulus. The Malpighian capsule may, in
fact, be regarded as a funnel, and the membranous walls
of the glomerulus as a piece of very delicate but peculiar
filtering-paper, into which the blood is poured.
And indeed, though there are some objections to this
view, we have reason to think that a great deal of the water
of urine, together with certain of the constituents (the inor-
ganic salts), is thus, as it were, filtered off by the Malpighian
capsules. But it must be remembered that the process is
after all very different from actual filtering through paper ;
for filter-paper will let everything pass through that is really
dissolved, whereas the glomerulus, while letting some things
through, refuses to admit others, even though completely
dissolved. Filtration in the kidney acquires its peculiarities
from the fact that, as in the case of lymph-formation (p. 146),
2t2 ELEMENTARY PHYSIOLOGY less.
the filtration takes place through the substance of living
cells.
Speaking of the process, with this caution, as one of filtra-
tion, it is obvious that the more full the glomerulus is of
blood the more rapid will be the escape of urine. Hence
we find that when blood flows freely to the kidney the urine
is secreted freely, but that when the blood-supply to the
kidney is scanty the urine also is scanty. When the renal
nerves going to the kidney are cut, the branches of the
renal artery dilate, much blood goes into the kidney, the
blood-pressure is raised in the glomeruli, and the flow of
urine is copious. If the same nerves be stimulated, the
arterial tubes are narrowed or constricted, less blood goes
to the kidney, blood-pressure is reduced, and the flow of
urine is scanty or may be stopped altogether.
We can now explain, in part at all events, how it is that
the activity of the kidney is influenced by the state of the
skin. The quantity of blood in the body being about the
same at all times, if a large quantity goes to the skin, as in
warm weather and especially when the skin is active and
perspiring, less will go to the kidney and the secretion of
urine will be small. On the other hand, if the blood be
largely cut off from the skin, as in cold weather, more blood
will be thrown upon the kidney and more urine will be
secreted. Thus the skin and the kidneys play into each
other's hands in their efforts to get rid of the superfluous
water of the body.
But the whole of the urine is not thus excreted, through a
sort of filtering process, by the Malpighian capsules. The
circulation in the kidney is peculiar, inasmuch as the blood
coming from the glomeruli is not sent at once into a vein,
but is carried into a second capillary network, wrapped
round the tubules. The tubules are lined, as has been
vi THE HISTORY OF UREA 213
stated, by epithelium cells, and these cells, in certain parts
of the tubule, especially where these are coiled, are secreting
cells. That is to say, they have the power, by some means
which we do not at present fully understand, to take up
from the blood, which is flowing in the capillaries wound
round the tubules, or rather from the plasma which exudes
from those capillaries and bathes the bases of the cells, cer-
tain substances, and to pour these substances into the cavity
of the tubule.
And we have evidence that many of the most important
constituents of the urine, such as urea, uric acid, and others,
are thus secreted by the epithelium cells of the tubules, and
not simply filtered off by the Malpighian capsules.
The formation of urine is therefore a double process. A
great deal of the water, with probably some of the more
soluble inorganic salts, passes by the glomeruli, but the
urea, the colouring matters, and a great many other of the
constituents, are thrown into the cavities of the tubules by
a peculiar action of the epithelium cells.
7. The History of Urea. — Nitrogen enters the body as
proteid food and, practically, all of it leaves the body again
as urea. Somewhere or other, and by some means or other,
the nitrogen while in transit is turned over from the proteids
into urea. This change involves the whole nitrogenous me-
tabolism1 of the body and from its importance merits a short
statement of the chief facts which throw some light on the
question of where and how urea is formed.
In the first place the urea excreted in the urine is not
?nade in the kidney out of some other (antecedent) sub-
stance. The activity of the kidney consists in picking out
1 The word " metabolism " (MeTa/3oA>j = change) is conveniently used to
denote the sum total of those chemical changes which take place in living
matter, and in virtue of which we speak of it as " living."
214 ELEMENTARY PHYSIOLOGY less.
ready-made urea from the blood which passes through it
and discharging this urea into the channels of the tubules.
Hence urea must be made in tissues other than the kidney
and finds its way from these into the blood.
Nearly half the weight of the body is made up of muscular
tissue, the muscles. Even when at rest these muscles are
the seat of active oxidation, and this activity is enormously
increased at times when they are contracting. There must
therefore always be a considerable wear and tear going on in
them, and we must suppose that this leads to the formation
of waste ; of this some should contain nitrogen, since the
muscles are chiefly built up of nitrogenous material. But
this waste does not come out of the muscles as ready-made
urea, neither do we know as yet exactly in what form it does
leave them. In fact, all we know is that the muscles give
off nitrogenous waste, that this waste is presumably turned
into urea in some other part of the body, and the urea
picked out and excreted by the kidneys.
The liver (p. 233) is the seat of many activities with which
we shall deal later on, and among these there is no doubt
that the making of urea out of other substances brought to it
in the blood is not the least important of them. We know
to a certain extent what one of these " other substances " is.
When we study digestion we shall see that one of the
products of digestion of proteids is a nitrogenous, crystalline
substance known as leucin. This is absorbed through the
walls of the intestines, carried to the liver in the blood of
the portal vein, and apparently converted into urea by the
liver. Possibly the liver similarly converts other nitrogenous
products, which it receives from the tissues, into urea. But
one thing is certain, a considerable portion of the urea which
is excreted by the kidneys is made in the liver. Beyond this
fact our knowledge of anything definite as to the mode
vi THE STRUCTURE OF THE SKIN ■ 215
of origin of urea in the body is very imperfect and incom-
plete.
8. The Structure of the Skin. Nails and Hairs. — That
the skin is a source of continual loss to the blood may be
proved in various ways. If the whole body of a man, or one
of his limbs, be inclosed in a rubber bag, full of air, it will
be found that this air undergoes changes which are similar in
kind to those which take place in the air which is inspired
into the lungs. That is to say, the air loses oxygen and
gains carbonic acid ; it also receives a great quantity of
watery vapour, which condenses upon the sides of the bag,
and may be drawn off by a properly disposed pipe. Further,
there is a continual loss of heat taking place from the sur-
face of the body. Of these the loss of watery vapour and
of heat are of immense importance, for it is chiefly by means
of variations in their amount from time to time that the
temperature of the body is kept nearly constant. But be-
fore dealing with these activities of the skin we must under-
stand the main facts as to its structure.
The skin (Fig. 65) consists of two parts, an outer layer or
epidermis, resting on a deeper layer, the dermis. The skin
as a whole is connected with the tissues it covers by a layer
of loose fibrous connective tissue (see Fig. 14), called sub-
cutaneous tissue. This often contains fat, and is the part
which is cut through when an animal is skinned.
The dermis is made up of a dense feltwork of ordinary
connective tissue fibres mixed with many elastic fibres and
some connective tissue corpuscles. The surface of the der-
mis is raised up into little hillocks or elevations known as
the papillae. Arteries enter the dermis and break up into
capillaries, which are very close set at its surface and in the
papilla? ; thus the dermis is extremely vascular. Nerves also
run into the dermis, and passing outwards, form a network
2l6
ELEMENTARY PHYSIOLOGY
of fibres at its junction with the epidermis, and from this
network extremely fine nerve fibrils pass out and between
Fig. 65. — Diagram to show the Structure of the Skin.
E.c, homy layer of epidermis; E.m, Malpighian layer of epidermis; D.c, con-
nective tissue of dermis; /, papilla; gl, sweat gland, the coils of the tube cut across
or lengthwise; d, its duct;_/, fat; v, blood-vessels; n, nerve; t.c, tactile corpuscle.
VI THE STRUCTURE OF THE SKIN 217
the lower cells of the epidermis. In some parts of the body,
some of the branches of the nerves run up into the papillae,
where they are connected with special nervous structures,
such as tactile corpuscles and end-bulbs. But since these
are of importance solely in connection with the functions of
the skin as a sense-organ, they will be described later on
(seep. 373).
The epidermis lies on the dermis and dips down into all
Its depressions. It is composed entirely of cells and has
no blood-vessels.
The cells may be divided into two layers. Of these the
innermost or Malpighian layer (Fig. 65, -E.m) is made
up of nucleated cells, which are tall and columnar where
they rest on the dermis, become more rounded and wrinkled
as they pass outwards, and then flattened and granular.
The outer layer of the epidermis, or horny layer (Fig. 65,
-E.c), is made up of cells which, losing their nuclei, become
converted into flattened, thin scales, consisting of horny
material. These are the cells which become so strongly
developed on parts of the body subject to friction, such as
the hands and soles of the feet. They are always being
shed from the surface of the skin, and their place is taken
by new cells, pressed out from the deeper layers of the
epidermis (see also pp. 37, 38).
All over the body the skin presents minute apertures,
the ends of channels excavated in the epidermis, and each
continuing the direction of a minute tube, usually about
So/a (^0 of an inch) in diameter, and a quarter of an inch
long, the end of which is imbedded in the dermis. Each
tube is lined with an epithelium continuous with the epi-
dermis (Fig. 65, d). The tube sometimes divides, but,
whether single or branched, its inner end or ends are blind,
and coiled up into a sort of knot, interlaced with a mesh-
work of capillaries (Fig. 65,^/, and Fig. 66).
ELEMENTARY PHYSIOLOGY
LESS.
This coiled-up portion is called a sweat-gland, and the
tube leading from it to the surface of the skin is its duct.
The cells lining the duct are small and rounded, those in
the tube of the gland are larger and more columnar, and
may be readily stained.
The blood in the capillaries of the gland is separated
from the cavity of the sweat-gland only by the thin walls
Fig. 66. — A Sweat-gland (Fig. 65,^/), Epithelium not shown.
a, the gland; b, the duct; c, network of capillaries, inside which the gland lies.
of the capillaries and the glandular epithelium, which
together constitute but a very thin pellicle. This arrange-
ment, though different in detail from, is similar in principle
to, that which obtains in the kidney. In the latter, the
vessel makes a coil within the Malpighian capsule, which
ends a uriniferous tubule. Here the perspiratory tubule
coils about and among the vessels. In both cases the same
result is arrived at — namely, the exposure of the blood to a
vi THE STRUCTURE OF THE SKIN 219
large, relatively free surface, upon which certain of its con-
tents transude. In the sweat-gland, however, there is no
filtering apparatus like the Malpighian corpuscle of the
kidney, and the whole of the sweat appears to be secreted
into the interior of the tube by the action of the epithelium
cells which line it.
The number of these glands varies in different parts of the
body. They are fewest in the back and neck, where their
number is not much more than 400 to a square inch.
They are more numerous on the skin of the palm and
sole, where their apertures follow the ridges visible on the
skin, and amount to between two and three thousand on
the square inch. At a rough estimate, the whole integu-
ment probably possesses not fewer than from two millions
and a quarter to two millions and a half of these tubules,
which therefore must possess a very great aggregate secret-
ing power.
In certain regions of the skin the horny cells of the
epidermis are not at once thrown off in flakes, but are at
first built up in definite structures known as nails and hairs,
which grow by constant addition to the surfaces by which
they adhere to the epidermis. In the case of the nails the
process of growth has no limit, and the nail is kept of one
size simply by the wearing or cutting away of its oldest
or free end. In the case of the hairs, on the contrary,
the growth of each hair is limited, and when its term is
reached the hair falls out and is replaced by a new hair.
Underneath each nail the deep or dermal layer of the
integument is peculiarly modified to form the bed of the
nail. It is very vascular, and raised up into numerous
parallel ridges, like elongated papillae (Fig. 67, B, C). The
surfaces of all these are covered with growing epidermic
cells, which, as they flatten and become converted into
22C ELEMENTARY PHYSIOLOGY less.
horn, form a solid continuous plate, the nail. At the hinder
part of the bed of the nail the integument forms a deep
fold, from the bottom of which, in like manner, new epider-
FlG. 67.
A, a longitudinal and vertical section of a nail; a, the fold at the base of the
nail: b, the nail; c, the bed of the nail. The figure B is a transverse section of the
same — a, a small lateral fold of the integument; i, nail; c, bed of the nail, with its
ridges. The figure C is a highly-magnified view of a part of the foregoing — c, the
ridges; d, the deep layers of epidermis; e, the horny scales coalesced into nail sub-
stance. (Figs. A and B magnified about 4 diameters; Fig. C magnified about 200
diameters.)
mal cells are added to the base of the nail, which is thus
constrained to move forward.
The nail thus constantly receiving additions from below
THE STRUCTURE OF THE SKIN
and from behind, slides forwards over its bed, and projects
beyond the end of the finger, where it is worn away or
cut off.
Fig. 68. — A Hair in its Hair-sac.
a, shaft of hair above the skin; b, cortical substance of the shaft, the medulla not
being visible; c, newest portion of hair growing on the papilla (/) ; d, cuticle of hair;
f, cavity of hair-sac; f, epidermis (and root-sheaths) of the hair-sac, corresponding to
the Malpighian layer of the epidermis of the integument («/): g> division between
dermis and epidermis; //, dermis of hair-sac corresponding to dermis of integument (/);
k, mouths of sebaceous glands; n, horny layer of epidermis of integument.
A hair, like a nail, is composed of horny cells ; but
instead of being only partially sunk in a fold of the integu-
222 ELEMENTARY PHYSIOLOGY less.
ment it is at first wholly inclosed in a kind of bag, the hair-
sac or follicle, from the bottom of which a papilla (Fig.
68, i), which answers to a single ridge of the nail, arises.
The hair is developed by the conversion into horn, and
coalescence into a shaft, of the superficial epidermal cells
coating the papilla. These coalesced and cornified cells
being continually replaced by new growths from below,
which undergo the same metamorphosis, the shaft of the
hair is thrust out until it attains the full length natural to it.
Its base then ceases to grow, and the old papilla and sac
die away, but not before a new sac and papilla have been
dc £
Fig. 69. — Part of the Shaft of a Hair inclosed within its Root-sheaths
and treated with Caustic Soda, which has caused the Shaft to be-
come distorted.
a, medulla; b, cortical substance; c, cuticle of the shaft; from d to f, the root-
sheaths, in section. (Magnified about 200 diameters.)
formed by budding from the sides of the old one. These
give rise to a new hair. The shaft of a hair of the head
consists of a central pith or medullary matter (Fig. 69, a),
of a loose and open texture, which sometimes contains air
and is often wanting altogether ; of a cortical or fibrous
substance (Fig. 69, l>), surrounding this, made up of
coalesced elongated horny cells and containing pigment ;
and of an outer cuticle (Fig. 69, c) composed of flat horny
plates, arranged transversely round the shaft, so as to over-
lap one another by their outer edges, like tiles on the
\ri THE COMPOSITION AND QUANTITY OF SWEAT 223
roof of a house. The superficial epidermal cells of the
hair-sac also coalesce by their edges, and become converted
into root-sheaths (Fig. 69, d, £,f), which embrace the root
of the hair, and usually come away with it when it is
plucked out.
Fig. 70. — Section of the Skin, showing the Roots of the Hairs and
the Sebaceous Glands.
a, epidermis; b, muscle of c the hair-sheath, on the left hand; d, dermis; e, twoseba-
ceous glands attached to each hair-sac.
The sebaceous glands (Fig. 70) are small glands whose
duct opens into the follicle of a hair. They form a fatty
secretion which lubricates the hairs.
9. The Composition and Quantity of Sweat. — The
sweat-glands have the function of forming a fluid, the
sweat, which is passed out upon the surface of the body.
This fluid is composed chiefly of water containing a small
amount (1-2 per cent.) of solid matter in solution, of which
sodium chloride is a prominent constituent. In health,
sweat contains no appreciable amount of urea.
In its normal state the sweat, as poured out from the
proper sweat-glands, is alkaline ; but ordinarily, as it col-
lects upon the skin, it is mixed with the fatty secretion of
the sebaceous glands, and then is frequently acid. In addi-
tion it contains scales of the external layers of the epidermis,
which are constantly being shed.
Under ordinary conditions the sweat is evaporated from
the surface of the skin as fast as it is secreted \ in this case it
224 ELEMENTARY PHYSIOLOGY less.
is frequently spoken of as insensible perspiration. But when
violent exercise is taken, or when under some kind of mental
emotion, or when the body is exposed to a hot and moist
atmosphere, the sweat is secreted faster than it evaporates :
the perspiration then becomes sensible, that is, it appears in
the form of scattered drops on the surface of the body.
The quantity of sweat, or sensible perspiration, and also
the total amount of both sensible and insensible perspiration,
vary immensely, according to the temperature and other con-
ditions of the air, and according to the state of the blood
and of the nervous system. It is estimated that, as a gen-
eral rule, the quantity of water excreted by the skin is con-
siderably more than that given out by the lungs in the same
time.
The amount of matter which may be lost by perspiration
under certain circumstances, is very remarkable. Heat and
severe labour, combined, may reduce the weight of a man
two or three pounds in an hour, by means of the cutaneous
perspiration alone ; and, as there is some reason to believe
that the quantity of solid matter carried off from the blood
does not diminish with the increase of the amount of the
perspiration, the total amount of solids which are eliminated
by profuse sweating may be considerable.
10. The Secretion of Sweat and its Nervous Control. —
In analysing the process by which the perspiration is elimi-
nated from the body, it must be recollected, in the first
place, that the skin, even if there were no glandular struc-
tures connected with it, would be in the position of a mod-
erately thick, permeable membrane, interposed between a hot
fluid, the blood, and the atmosphere. Even in hot climates
the air is, usually, far from being completely saturated with
watery vapour, and in temperate climates it ceases to be so
saturated the moment it comes into contact with the skin,
vi THE SECRETION OF SWEAT 225
the temperature of which is, ordinarily, twenty or thirty
degrees above its own.
A bladder exhibits no sensible pores ; but if a bladder be
filled with water and suspended in the air, the water will
gradually ooze through the walls of the bladder, and disap-
pear by evaporation. Now, in its relation to the blood, the
skin is such a bladder full of hot fluid.
Thus, perspiration to a certain amount must always be
going on through the substance of the integument, but prob-
ably not to any great extent ; though what the amount of
this perspiration may be cannot be accurately ascertained,
because it is entirely masked by the secretion from the
sweat-glands.
When from any ordinary cause an increased formation of
sweat takes place, two things usually happen. The small
arteries which supply the capillary network surrounding the
coiled tube of the sweat-gland dilate and there is an increased
flow of blood through these capillaries. At the same time
the cells of the glands begin to pour out an increased quan-
tity of fluid, in other words they begin to secrete. The
first of the above two results is brought about by a lessening
of the vaso-constrictor impulses which had previously been
keeping the arteries constricted (see p. 94). But what, on
the other hand, is the cause of the simultaneously increased
activity of the sweat-glands? Do they simply secrete faster
because of the increased supply of blood brought to them,
as is the case with the Malpighian capsules of the kidney? Or
is it because their cells are urged on to greater activity by
special nervous impulses sent to them ? The latter is the
real explanation of the increased activity of the sweat-cells,
as is shown by the following facts.
It is possible to obtain an increased secretion of sweat by
the stimulation of nerves in parts of an animal's body from
9
226 ELEMENTARY PHYSIOLOGY less.
which the blood-supply has been previously cut off. Again,
certain drugs may lead to sweating without at the same time
producing any vascular changes, and the same effect is often
observed in sweating which results from mental emotions
and in the " cold sweats " of a disease such as phthisis. The
nerves which can thus make the cells of the sweat-glands
become more active may be called secretory nerves. They
appear to be connected with a centre or centres in the cen-
tral nervous system, the number and exact location of which
are not fully known, and by this means sweating may be
brought about reflexly, as when placing mustard in the mouth
causes the face to sweat. The possibility of such reflex
stimulation of the sweat-glands acquires an extraordinary
importance, as we shall see when we come to consider the
means by which the temperature of the body is regulated
(p. 231).
The ideas we have thus arrived at as to the process of
sweat secretion hold good for all secreting glands ; and we
shall have to consider them again later on, when dealing
with certain of the salivary glands, in which this indepen-
dence of secretion and blood-supply is much more strikingly
shown (see Lesson VII.).
11. A Comparison of the Lungs, Kidneys, and Skin. —
It will now be instructive to compare together in more detail
than has been done in the first Lesson (p. 23) the three
great organs — lungs, kidneys, and skin — which have been
described.
In ultimate anatomical analysis, each of these organs con-
sists of a moist animal membrane separating the blood from
the atmosphere.
Water, carbonic acid, and solid matter pass out from the
blood through the animal membrane in each organ, and con-
stitute its secretion or excretion ; but the three organs differ
vi ANIMAL HEAT 22?
in the absolute and relative amounts of the constituents the
escape of which they permit.
Taken by weight, water is the predominant excretion in
all three ; most solid matter is given off by the kidneys ;
most gaseous matter by the lungs.
The skin partakes of the nature of both' lungs and kidneys,
seeing that it absorbs oxygen and exhales carbonic acid and
water, like the former, while it excretes organic and saline
matter in solution, like the latter ; but the skin is more
closely related to the kidneys than to the lungs. Hence,
as has been already said, when the free action of the skin is
interrupted, its work is usually thrown upon the kidneys, and
vice versa. In hot weather, when the excretion by the skin
increases, that of the kidneys diminishes, and the reverse is
observed in cold weather.
This power of mutual substitution, however, only goes a
little way ; for if the kidneys be extirpated, or their func-
tions much interfered with, death ensues, however active the
skin may be. And, on the other hand, if the skin be cov-
ered with an impenetrable varnish, the temperature of the
body rapidly falls, and from this cause death takes place,
though the lungs and kidneys remain active.
12. Animal Heat : its Production and Distribution. — It
has been seen that heat is being constantly given off from
the skin and from the air-passages ; and everything that
passes from the body carries away with it, in like manner, a
certain quantity of heat. Furthermore, the surface of the
body is much more exposed to cold than its interior. Nev-
ertheless, the temperature of the body is in health main-
tained very evenly, at all times and in all parts, within the
range of two degrees or even less on either side of 370 C.
(98.60 F.).
This is the result of three conditions : the first, that
228 ELEMENTARY PHYSIOLOGY less.
heat is constantly being generated in the body ; the second,
that it is as constantly being distributed through the body ;
the third, that it is subject to incessant regulation as regards
both loss and production.
Heat is generated whenever oxidation takes place. As
we have seen, the tissues all over the body, muscles, brain-
substance, gland cells, and the like, are continually under-
going oxidation. The living substance of the tissue, built
up out of the complex proteids, fats, and carbohydrates, and
thus even still more complex than these, is, by means of
the oxygen brought by the arterial blood, oxidised, and
broken down into simpler, more oxidised bodies, which are
eventually reduced to urea, carbonic acid, and water. Wher-
ever life is being manifested these oxidative changes are
going on, more energetically in some places, in some tis-
sues, and in some organs, than in others. Hence every
capillary vessel and every extra-vascular islet of tissue is
really a small fireplace in which heat is being evolved, in
proportion to the activity of the chemical changes which
are going on.
The chief seat of this heat production is undoubtedly in
the muscles ; for, as already pointed out, they make up
about half the body-weight, and are carrying on an active
oxidation even while at rest. This gives rise to heat, and
when a muscle enters into a state of contracting activity,
the heat production becomes so rapid as to produce an
actual measurable rise of its temperature. After the mus-
cles we may regard the liver as the next great heat-produc-
ing organ of the body.
But as the vital activities of different parts of the body,
and of the whole body, at different times, are very different I
and as some parts of the body are so situated as to lose their
heat by radiation and conduction much more easily than
vi REGULATION OF BODY-TEMPERATURE 229
others, the temperature of the body would be very unequal
in its different parts, and at different times, were it not for
the arrangement by which the heat is distributed and regu-
lated.
Whatever oxidation occurs in any part, raises the tempera-
ture of the blood which is in that part at the time, to a pro-
portional extent. But this blood is swiftly hurried away into
other regions of the body, and rapidly gives up its excess
heat to them. On the other hand, the blood which, by
being carried to the vessels in the skin on the surface of the
body, begins to have its temperature lowered by evaporation,
radiation, and conduction, is hurried away, before it has
time to get thoroughly cooled, into the deeper organs ; and
in them it becomes warm by contact, as well as by the oxi-
dating processes there going on. Thus the blood-vessels
and their contents may be compared to a system of hot-
water pipes, through which the warm water is kept con-
stantly circulating by a pump ; while it is heated, not by a
great central boiler as usual, but by a multitude of minute
gas jets, disposed beneath the pipes, not evenly, but more
here and fewer there. It is obvious that, however much
greater might be the heat applied to one part of the system
of pipes than to another, the general temperature of the
water would be even throughout, if it were kept moving
with sufficient quickness by the pump. In this way, then,
the temperature of the body is kept uniform in its several
parts.
13. Regulation of Body-temperature by Altered Loss of
Heat. — If a system such as we have just imagined were
entirely composed of closed pipes, the temperature of the
water might be raised to any extent by the gas jets. On the
other hand, it might be kept down to any required degree
by causing a larger, or smaller, portion of the pipes to be
23o ELEMENTARY PHYSIOLOGY less.
wetted with water, which should be able to evaporate freely
— as, for example, by wrapping them in wet cloths. And
the greater the quantity of water thus* evaporated, the lower
would be the temperature of the whole apparatus.
Now, the regulation of the temperature of the human body
is chiefly effected on this principle. The vessels are closed
pipes, but a great number of them are inclosed in the skin
and in the mucous membrane of the air-passages, which are,
in a physical sense, wet cloths freely exposed to the air. It
is the evaporation from these which exercises a more im-
portant influence than any other condition upon the regula-
tion of the temperature of the blood, and, consequently, of
the body.
But, as a further nicety of adjustment, the wetness of the
regulator is itself determined, through the aid of the nervous
system, by the temperature of the body. The sweat-glands,
as we have seen, may be made to secrete by impulses reach-
ing them along certain nerves coming from a centre, or
centres, in the central nervous system. This centre is itself
connected by other nerves with the skin, and the ends of
these cutaneous nerves are so constituted that they are
stimulated by heat applied to the skin. When the body is
exposed to a high temperature (and the same occurs when
a part only of the body is heated), these cutaneous nerves
convey impulses to the central nervous system, from which
other impulses are then sent out along the secretory nerves
to the sweat-glands and cause them to pour forth a copious
secretion on to the skin ; and when the temperature falls,
the glands cease to act. Moreover, in this work of secret-
ing sweat, the sweat-glands are assisted by corresponding
changes in the blood-vessels of the skin. It has been stated
(see p. 91) that the small arteries of the body may be some-
times narrowed or constricted, and sometimes widened or
vi REGULATION OF BODY-TEMPERATURE 231
dilated. Now the condition of the small arteries, whether
they are constricted or dilated, depends, as we have also
seen, upon the action of certain nerves (vaso-motor nerves).
And it appears that when the body is exposed to a high
temperature these nerves are so affected as to lead to a
dilation of small arteries of the skin ; but when these are
dilated the capillaries and small veins in which they end
become much fuller of blood, and from these filled and
swollen capillaries much more nutritive matter passes through
the capillary walls to the sweat-glands, so that these have
more abundant material from which to manufacture sweat.
On the other hand, when the body is lowered in tempera-
ture the vaso-motor nerves are so affected that the small
arteries of the skin are constricted ; hence less blood enters
the capillaries of the skin, and less material is brought to
the sweat-glands.
Thus when the temperature is raised two things happen,
both brought about by the nervous system. In the first
place, the arteries of the skin are widened so that a much
larger proportion of the total blood of the body is carried
to the surface of the skin and there becomes cooled ; and,
secondly, this cooling process is greatly helped by the in-
creased evaporation resulting from the increased action of
the sweat-glands, whose activity is further favoured by the
presence in the skin of so much blood. Conversely, when
the temperature is lowered, less of the blood is brought to
the skin, and more of the blood circulates through the
deeper, hotter parts of the body, and the sweat-glands cease
their work (this quiescence of theirs being in turn favoured
by the lessened blood-supply) ; hence the evaporation is
largely diminished, and thus the blood is much less cooled.
Hence it is that, so long as the surface of the body per-
spires freely, and the air-passages are abundantly moist, a
232 ELEMENTARY PHYSIOLOGY less.
man may remain with impunity, for a considerable time,
in an oven in which meat is being cooked. The heat of
the air is expended in converting this superabundant per-
spiration into vapour, and the temperature of the man's
blood is hardly raised.
14. Regulation of Body-temperature by Altered Pro-
duction of Heat. — The temperature of the body is kept
constant by that carefully adjusted variation in loss of heat
from its surface which has been described in the preceding
section. But now we may point out that there is another
way by which this constancy might be attained, namely, by
altering the production of heat taking place in the body, in
correspondence to the changes of the surrounding tempera-
ture ; just as the temperature of a room may be regulated
by putting out or increasing the fire as well as by opening
or closing its windows. The question thus raised is very
interesting, but it is also very abstruse, and we must not do
more than just touch upon it.
All oxidation in the body involves the consumption of
oxygen, the production of carbonic acid and the genera-
tion of an exactly corresponding quantity of heat. We may,
therefore, take the difference in the amount of oxygen used
up (and of carbonic acid produced) at different times as
a measure of the amount of heat produced in the body
during the same periods. Working in this way it is found
that when a warm-blooded animal is exposed to cold, as
when it is put into a chamber which is cooled, it uses up
more oxygen and gives off more carbonic acid than when
put into a warm chamber. But this can only mean that in
the cooler surroundings the animal makes more heat than
when the surroundings are warm. Again we may point out,
as tending to the same conclusions, that our desire for food
is greater, on the whole, in the cooler winter time than in
vi THE STRUCTURE OF THE LIVER 233
the warmer summer ; and all food is oxidised in the body,
and during this oxidation gives rise to heat. Thus, there
are reasons for supposing that within certain limits altered
production of heat may play some part in keeping the
temperature of the body constant.
All the functions of the body which we have so far studied
have been seen to be under the guidance of the nervous
system. We may, therefore, suppose that the production of
heat will be no exception to the rule, and, indeed, there are
reasons, based largely on experiment and partly on the phe-
nomena of certain diseases, which justify this view. But the
nervous mechanism of this function is not yet fully known.
15. The Temperature of Fever. — The condition to
which the name of fever is given is characterised essentially
by the temperature of the body being higher than is usual
in health. Thus it may rise to as much as 410 C. (105 .8° F.),
or occasionally even above this point, and there has been
much dispute as to how this high temperature arises. A
common cause is a disturbance of the mechanism by which
heat is lost to the body, some diminution in loss of heat
leading naturally to a rise of temperature. On the other
hand, direct measurement shows that a fevered person often
gives off more heat than usual and at the same time uses up
more oxygen and produces more carbonic acid and urea
than usual. In such cases there is no doubt that the
abnormally high temperature is largely due to an over-
production of heat.
16. The Structure of the Liver. — The liver is a con-
stant source both of. loss, and, in a sense, of gain, to the
blood which passes through it. It gives rise to loss, because
it secretes a peculiar fluid, the bile, from the blood, and
throws that fluid into the intestine. It is also in another
way a source of loss because it elaborates from the blood
234 ELEMENTARY PHYSIOLOGY less.
passing through it a substance called glycogen, which is
stored up sometimes in large, sometimes in small, quantities
in the cells of the liver. This latter loss, however, is only
temporary, and may be sooner or later converted into a
gain, for this glycogen very readily passes into sugar, and
either in that form or in some other way is carried off by
the blood. In this respect, therefore, there is a gain to
the blood of kind or quality, though not of quantity of
material
Fig. 71. — The Liver of a Young Subject sketched from below and
behind. (From Moore's Elementary Physiology.}
R.L. right lobe; L.L. left lobe; g.bl. gall-bladder; v.c.i. inferior vena cava;
/. portal vein; on its right the bile-duct, on its left the hepatic artery.
The liver is the largest glandular organ in the body, ordi-
narily weighing about 1,400-1,700 grammes (fifty or sixty
ounces). It is a broad, dark, red-coloured organ, which
lies on the right side of the body, immediately below the
diaphragm, with which its upper surface is in contact,
while its lower surface touches the intestines and the
right kidney.
The liver is invested by a coat of peritoneum, which
vi THE STRUCTURE OF THE LIVER 235
keeps it in place. It is flattened from above downwards
and convex and smooth above, where it fits into the con-
cavity of the lower surface of the diaphragm (Fig. 71). It
is concave and irregular below, where it is in contact with
the stomach, the intestine, and the right kidney, irregular
behind, and ends in a thin edge in front.
Viewed from behind and below, as in Fig. 71, the inferior
vena cava, v.c.i., is seen to traverse a notch in the hinder
edge of the liver as it passes from the abdomen to the
thorax. At/ the trunk of the portal vein is observed enter-
ing into the substance of the organ. At its left the hepatic
artery, coming almost directly from the aorta, similarly
enters the liver, and ramifies through it. At the right of
the portal vein is the single trunk of the duct called the
hepatic duct, which conveys away to the intestine the bile
brought to it by its right and left branches from the liver.
Opening into the hepatic duct is seen the duct of a large
oval sac, g.bl., the gall-bladder.
The liver consists of two chief lobes, of which the right
is much larger than the left. Externally the lobes are
covered with a layer of connective tissue forming its capsule,
and a quantity of connective tissue forms a thick sheath
for the portal vein, the hepatic artery, and the bile-duct
as these plunge into the liver. This sheath accompanies
the vessels as they ramify in the liver, and finally forms a
number of partitions, continuous with the capsule on the
outside, which divide each lobe into a very large number
of small divisions called lobules (Figs. 72, A, and 73, L).
These partitions are much thicker and more conspicuous in
some animals, such as the pig, than they are in others, such
as the rabbit ; in the former it is very easy to see on the
outside of the liver the outlines of the lobules; in the latter
it is not so easy. The lobules are polyhedral in shape and
236
ELEMENTARY PHYSIOLOGY
LESS,
about y1^- of an inch in diameter, being thus visible to the
naked eye. Each lobule is seated on the branch of the
hepatic vein, the large vein which carries the blood away
from the liver, and is made up of a mass of cells, the hepatic
(From Quain's Anatomy.)
A. Two lobules of the liver (diagrammatic) (Schafer). /, interlobular branches
o. the portal vein, giving off capillaries into the lobules: //, intralobular veins, shown
in cross-section in the left-hand lobule, in longitudinal section in the right-hand
lobule; s, sublobular branch nf the hepatic vein: the arrows indicate the direction o(
the course of the blood. The liver cells are represented in a portion only of each
lobule.
B. Portion of lobule very highly magnified, a, liver cell with n, nucleus (two
are often present): b, capillaries cut across; c, minute biliary passages between the
celfs, injected with colouring matter.
THE STRUCTURE OF THE LIVER
237
cells, which lie in the meshes of a close-set network of
blood capillaries. These capillaries unite in a small blood-
vessel which runs down the centre of each lobule towards its
base ; this central blood-vessel is called the intralobular
vein (Fig. 72, A, h), and, passing out of the lobule at its
base, runs into a branch of the hepatic vein (Figs. 72 A, s,
and 73, H.V.).
Fig. 73. — A Piece of the Liver cut so as to show
//. V. a branch of the hepatic vein, L, the lobules of the liver, seated upon its walls,
and sending their intralobular veins into it.
If the branches of the hepatic artery, the portal vein,
and the bile-duct be traced into the substance of the liver,
they will be found to accompany one another, and to
branch out and subdivide, becoming smaller and smaller.
At length the ultimate branches of the portal vein (Fig. 72,
238 ELEMENTARY PHYSIOLOGY less.
A, p) reach the outer surfaces of the lobules, and passing
round and between them are known as the interlobular
veins. These veins pour their blood into the network of
capillaries which permeates each lobule. The branches
of the hepatic artery follow a course parallel to that of the
portal vein and finally, reaching the surface of a lobule, also
pour the blood they carry into the lobular capillaries.
Thus, the venous blood of the portal vein and the arterial
blood of the hepatic artery reach the surfaces of the lobules
by the ultimate branches of that vein and artery, become
mixed in the capillaries of each lobule, and are carried off
by its intralobular veinlet, which pours its contents into one
of the branches of the hepatic vein. These branches, join-
ing together, form larger and larger trunks, which at length
reach the hinder margin of the liver, and finally open into the
vena cava inferior, where it passes upwards in contact with
that part of the organ.
Thus the blood with which the liver is supplied is a
mixture of arterial and venous blood : the former brought
by the hepatic artery directly from the aorta, the latter by
the portal vein from the capillaries of the stomach, intes-
tines, pancreas, and spleen.
In the lobules themselves all the meshes of the blood-
vessels are occupied, as has been said, by the hepatic cells
or liver cells. These are many-sided, minute bodies, each
about 20/x (y^u- of an inch) in diameter, possessing a
nucleus in its interior, and frequently having larger and
smaller granules of fatty matter distributed through its
substance (Fig. 72, A, and B, a). It is in the liver cells
that the active powers of the liver reside.
The smaller branches of the hepatic duct, lined by an
epithelium which is continuous with that of the main
duct, and thence with that of the intestines, into which
vi THE WORK OF THE LIVER 239
ihc main duct opens, may be traced to the very surface of
the lobules, where they seem to end abruptly (Fig. 74).
But, upon closer examination, it is found that they com-
municate with a network of minute passages passing between
the hepatic cells, and traversing the lobule in the intervals
left by the capillaries (Fig. 72, B, c). These minute
passages are the bile canaliculi. The bile manufactured by
the hepatic cells finds its way first into these minute pas-
sages, from them into the ducts, and finally either into the
gall-bladder or the intestines.
17. The Work of the Liver. — The work of the liver,
and this, as has been said, is carried out by the hepatic
cells, may be considered as consisting of two kinds.
Fig. 74. — Termination of Bile Duct at Edge of Lobule.
(Somewhat diagrammatic.)
b, small bile duct, becoming still smaller at b' , the low, flat epithelium at last sud-
denly changing into the hepatic cells, /, the channel of the bile duct being continued
as small passages between the latter; c, capillary blood-vessels cut across.
On the one hand, the hepatic cells are continually en-
gaged in the manufacture of a complex fluid called bile,
which they pour into the minute passages spoken of above,
and theme into the branches of the hepatic duct, whence
240 ELEMENTARY PHYSIOLOGY less.
it flows through the duct itself into the intestines, or, when
digestion is not going on and the opening of the duct into
the intestine is closed, back to the gall-bladder. The
materials for this bile are supplied to the hepatic cells by
the blood ; hence the secretion of the bile constitutes a loss
to the blood.
The total quantity of bile secreted in the twenty-four
hours varies, but probably amounts to about 700 cubic
centimetres (1 pint). It is a golden yellow, slightly alka-
line fluid, of extremely bitter taste, consisting of water with
from 15 per cent, to half that quantity of solid matter in
solution. The solids consist of the so-called bile-pigments
and bile-salts, a remarkable crystalline substance called
cholesterin ; a small quantity of fat ; and some inorganic
salts.
The colour of bile is due to the pigment called bili-
rubin. By oxidation this may easily be converted into
a green pigment called biliverdin, and the differences in
colour of the bile of different animals depend on the rela-
tive amounts of these two pigments which they contain.
The bile-salts are sodium salts of two organic acids, one
called glycocholic, the other taurocholic acid. The former
consists of carbon, oxygen, hydrogen, and nitrogen, while
the latter contains additionally a considerable quantity of
sulphur.
Bile, as it is secreted by the liver, is a thin fluid, but after
its sojourn in the gall-bladder, where it is stored in the
intervals between its discharge into the intestines, it contains
a considerable amount of mucin, secreted into it by. the
cells which line the gall-bladder, and it is then viscid and
slimy.
Of these constituents of the bile the essential substances,
the bile acids and the colouring matter, are not discoverable
vi THE WORK OF THE LIVER 241
in blood which enters the liver ; they must therefore be
formed in the hepatic cells. How they are exactly formed
we do not at present clearly know. The material of which
they are composed is brought to the hepatic cells by the
blood, but the exact condition of that material — whether,
for instance, the blood brings something very like the bile
acids, and only needing a slight change to be converted into
bile acids ; or whether the hepatic cells manufacture the
bile acids from the beginning, as it were, out of the com-
mon material which the blood brings to the liver as to all
other tissues and organs — is not as yet quite determined.
There is, however, but little doubt that the pigment of bile is
in some way made out of the haemoglobin of the red blood-
corpuscles (see p. 131). The saline matters and choles-
terin, on the other hand, appear to be present in the blood
of the portal vein, and may therefore, like the water, be
simply taken up by the cells from the blood, and passed on
to the bile ducts.
Thus the bile is a continual loss to the blood. But,
besides forming bile, the hepatic cells are concerned in
other labours, the result of which can hardly be considered
either as a loss or as a gain, since these labours simply con-
sist in manufacturing from the blood and storing up in the
hepatic cells substances which, sooner or later, are returned,
generally in a changed condition, back into the blood.
As we shall presently see, the portal blood is, after a meal,
heavily laden with substances, the result of the digestive
changes in the alimentary canal. When these substances,
carried along in the portal blood, reach the hepatic cells,
in the meshes of the lobules, some of them appear to be
taken up by those cells and to be stored up in them in a
changed condition. In fact, the products of digestion pass--
ing along the portal veins suffer (in the liver) a further
R
242 ELEMENTARY PHYSIOLOGY less.
change, which has been called a secondary digestion.
Thus the liver produces a powerful effect on the quality
of the blood passing through it, so that the blood in the
hepatic vein is very different, especially after a meal, from
the blood in the portal vein.
The changes thus effected by the hepatic cells are prob-
ably very numerous, but they have not been fully worked
out, except in one particular case, which is very interesting
and deserves special attention.
It is found that the liver of an animal which has been
well and regularly fed, when examined immediately after
death, contains a considerable quantity of a substance which
is very closely allied to starch, consisting of carbon, hydro-
gen, and oxygen in proportions the same as in starch. This
substance, which may by proper methods be extracted and
preserved as a white powder, is in fact an animal starch,
and is called glycogen. As we shall see, common starch
is readily changed by certain agents into a grape-sugar, or
dextrose, as it should be called ; and this glycogen is simi-
larly converted with ease into dextrose. Indeed, if the
liver of such an animal as the above, instead of being ex-
amined immediately after death, be left in the body, or be
placed on one side after removal from the body for some
hours before it is examined, a great deal of the glycogen
will have disappeared, a quantity of dextrose having taken
its place. There seems to be present in the liver some
agent capable of converting the glycogen into dextrose, and
this change is particularly apt to take place if the liver is
kept at blood-heat or near that temperature.
Now if, instead of the liver of a well-fed animal, the liver
of an animal which has fasted for several days be examined
in the same way, very little glycogen indeed will be found
in it, and when this liver is left exposed to warmth for some
vi THE WORK OF THE LIVER 243
time very little dextrose is found. That is to say, the liver
has, in the first case, formed the glycogen and stored it up
in itself, out of the food brought to it by the portal blood :
in the second case, no food has been brought to the liver
from the alimentary canal, no glycogen has been formed,
and none stored up. If the liver in the first case be ex-
amined microscopically with certain precautions, the glyco-
gen may be seen stored up in the hepatic cells ; in the
second case little or none can be seen.
The kind of food which best promotes the storing up
of glycogen in the liver is one containing starch or sugar ;
but some glycogen will make its appearance even when an
animal is fed on an exclusively proteid diet, though not
nearly so much as when starch or sugar is given.
It would appear, then, that the hepatic cells can manu-
facture and store up in themselves the substance glycogen,
being able to make it out of even proteid matter, but more
easily making it out of sugar ; for, as we shall see, all the
starch which is eaten as food is converted into sugar in the
alimentary canal, and reaches the liver as sugar.
There are reasons for thinking that the glycogen, thus
deposited and stored up in the liver, is converted into sugar
little by little as it is wanted, poured into the hepatic vein,
and thus distributed over the body. So that we may re-
gard this remarkable formation of glycogen in the liver as
an act by which the blood, when it is over-rich in sugar,
as after a meal, stores it up or deposits it in the liver as
glycogen ; and then, in the intervals between meals, the
liver deals out the stored-up material as sugar back again
in driblets to the blood. The loss to the blood, therefore,
is temporary — no more a real loss than when a man de-
posits at his banker's some money which he has received,
until he has need to spend it.
^44 ELEMENTARY PHYSIOLOGY less.
This story of glycogen, important in itself, is also useful
as indicating other possible effects of a similar nature which
the hepatic cells may bring about in the blood, as it is
passing in the meshes of the lobules of the liver from the
veinlets of the portal to the veinlets of the hepatic vein.
The formation of urea by the hepatic cells has already been
discussed (p. 214). Glycogen and urea may rightly be
spoken of as internal secretions of the liver (see p. 201).
18. The Spleen. — The spleen, one of the so-called duct-
less glands, lies in the abdominal cavity, slightly below and
towards the left side of the stomach and immediately to the
left of the tail of the pancreas (Fig. 75, SpL). It is an
elongated, flattened, red body, abundantly supplied with
blood by an artery called the splenic artery, which proceeds
almost directly from the aorta. The blood which has trav-
ersed the spleen is collected by the splenic vein, and is
carried by it to the portal vein, and so to the liver. The
spleen is covered by a capsular sheath of connective tissue
mixed with a good deal of elastic tissue and some unstriated
muscle fibres. Somewhat in the same way as in a lymphatic
gland (p. 115) this capsule sends branching projections or
trabeculae inwards, which divide the organ up into a number
of irregular spaces, and these spaces are filled with a mass
of spongy tissue called the spleen-pulp. The pulp is
traversed by a network, the meshes of which are occupied
by red blood-corpuscles, by colourless corpuscles closely
similar to those of lymph, and by other kinds of cells pecul-
iar to the spleen. The latter, or spleen cells, resemble the
colourless corpuscles of blood, in that they can perforin
amoeboid movements, but they are larger and contain in
their substance red corpuscles in various stages of disinte-
gration.
A section of the spleen shows a dark red spongy mass
THE SPLEEN
245
dotted over with minute whitish spots. Each of these last
is the section of one of the spheroidal bodies called cor-
puscles of the spleen, or Malpighian corpuscles, which are
scattered through its substance. These corpuscles consist
of little masses of lymphoid or adenoid tissue, very similar
to that found in the lymphatic glands (p. 116) which sur-
round the smaller branches of the arteries. They are
crowded with leucocytes, and hence they stand out as
white specks against the dark red pulp of the spleen.
Vrrv.
Fig.
75-
The spleen (Spl.) with the splenic artery {Sp. A.). Below this is seen the
splenic vein running to help to form the portal vein ( V. P.). Ao, the aorta; D. a
pillar of the diaphragm; P.D. the pancreatic duct exposed by dissection in the sub-
stance of the pancreas; Dm. the duodenum; B.D. the biliary duct uniting with the
pancreatic duct into the common duct, x; y, the intestinal vessels.
The smallest branches of the arteries which carry blood
into the spleen open into the network of the spleen-pulp,
so that the blood flows into and through this network ; it is
then gathered up again into the ends of tiny veins, which
similarly open into the spleen-pulp, and carry the blood
away into the splenic vein.
246 ELEMENTARY PHYSIOLOGY less.
We are still very much in the dark as to the functions of
the spleen; they are without doubt of some importance ; but,
on the other hand, the spleen may be permanently removed
from the body without producing any obvious derangement
of its working.
The elasticity of the splenic tissue allows the organ to be
readily distended with blood, and enables it to return to its
former size after distension. It appears to change its dimen-
sions with the state of the abdominal viscera, attaining its
largest size about five hours after a full meal, and gradually
returning to its minimum bulk.
The blood of the splenic vein is found to contain more
colourless corpuscles than that of the splenic artery ; and it
has been supposed that the spleen is one of those parts of
the economy in which colourless corpuscles of the blood are
produced. It is also thought that red corpuscles there die
and are broken up.
19. The Thymus Gland. — This ductless gland lies over
the trachea, in the lower part of the neck and behind the
sternum at the base of the heart. It is conspicuous at birth,
but soon begins to waste away, and in the adult is replaced
by a small amount of connective tissue and fat. In structure
it somewhat resembles a lymphatic gland.
Nothing definite is known of the function or use of this
gland.
20. The Thyroid Body or Gland. — This organ consists
of two lobes, one lying on each side of the trachea, just be-
low the larynx and the two being joined across the trachea
by a connecting strip of thyroid tissue. Each lobe is cov-
ered with a capsule of connective tissue, from which branches
pass inwards and divide the interior into rounded spaces or
alveoli. Each alveolus is lined by a layer of cubical cells
so as to leave a large central closed space, which is filled
vi THE SUPRARENAL BODIES 247
with a clear, viscid, often semi-solid fluid. The body pos-
sesses no duct.
The thyroid gland seems to have much to do with the
nutrition of the body. When diseased in man, it often leads
to nutritive disorders, strikingly manifest in a puffed, swollen
appearance of the skin, but involving various organs and
tissues, especially the nervous system, and thus leading to
nervous troubles. Occasionally the degenerations of the
tissues take on the form of a change into a mucin-like
substance. These troubles may be largely mitigated by
taking doses of an extract of the fresh gland or by eating
the fresh gland-substance. Goitre is an enlargement of the
thyroid, and cretinism, a peculiar form of idiocy common
in some places, is associated with its diseased condition.
Recent experimental work seems to show beyond a doubt
that the thyroid secretes material which passes into the cir-
culation and is of use to the organism. The nature of this
internal secretion is not definitely known. Especially sig-
nificant, however, is the presence in the gland of iodine,
apparently in an organic compound, which has been called
iodo-thyrin. This compound is thought to be the active
constituent of the internal secretion. Disease of the gland
doubtless interferes with its production and thus causes the
characteristic disorders.
21. The Suprarenal Bodies. — The suprarenal bodies are
two in number, and are placed one on the upper edge of each
kidney. They are enveloped in an outer coat, or capsule.
of connective tissue, from which partitions pass into theii
interior, dividing it up into compartments. The spaces in
the cortical part are filled with groups of angular cells ;
those of the medullary part by cells larger and more irregu-
lar in shape.
The functions of the suprarenal bodies are important
248 ELEMENTARY PHYSIOLOGY less, vi
although as yet but little understood. When they are
both removed from an animal, death speedily ensues, ac-
companied chiefly by great muscular weakness. When dis-
eased in man, similar weakness is observed, together with a
characteristic "bronzing" or coloration of the skin. Extract
of the suprarenals, when injected into the body, has a power-
fully stimulating effect upon the muscular system, especially
the muscles of the heart and the arteries, and thus causes a
great increase of arterial pressure. Such extract probably
will be found, as in the case of the thyroid gland, to mitigate
the symptoms which result from the bodies being diseased.
As in the case of the thyroid also, the facts known regarding
the suprarenals are interpreted as indicating that these bodies
constantly secrete into the blood in minute quantities a sub-
stance or substances that are beneficial to the body, espe-
cially to the muscular system. In fact, an organic compound
has been obtained from the bodies which, when injected
into a living animal, has an effect upon the blood-vessels
similar to that of extracts of the bodies themselves. To
this compound the name epinephrin has been given. But
much remains to be discovered regarding not only the func-
tions of these and other ductless glands, but the general
physiology of internal secretion itself.
LESSON VII
THE SOURCES OF LOSS AND GAIN TO THE BLOOD
(continued) : THE FUNCTION OF ALIMENTATION
Part I. — Digestion and Absorption
1. "Waste made Good by Food. — We explained in the
first Lesson that a living active man is always expending
energy in the form of the mechanical (muscular) work he
performs and of the heat he gives off by his skin and lungs.
Further, we pointed out that the source from which the
energy is derived lies in that constant oxidational breaking
down of the tissues which results from their being supplied
with oxygen, introduced into the body by the lungs. And,
further, it was shown that the above processes result in a
waste of substance corresponding exactly to the amount of
energy expended. If the man's activity is to continue from
day to day, this continual waste of substance must be made
good. Now the only channel, except the lungs, by which
altogether new material is introduced into the body, is the
alimentary canal, and we may use the word alimentation to
denote the sum total of its operations in this connection.
These fall naturally under three heads, viz. the introduction
of food as new material ; the reduction of this food by
digestion to a condition such that it can pass through the
delicate structures which form the walls of the vessels
of the alimentary canal; and absorption, or the processes
249
250 ELEMENTARY THYSIOLOGY less.
by which the digested material is passed from the cavity
of the canal into the blood-vessels and lymphatics, by
which it is then distributed over the body. We may there-
fore most suitably begin by learning something of the
nature and composition of that "new material" which we
introduce into the body as food.
2. Food and Food- stuffs. — Every one is familiar with
the meaning of the term food, as exemplified by bread,
meat, potatoes, milk, etc. None of these substances,
however, is made up of one kind of material ; but when
analysed it is found that they all consist of varying amounts
of a few substances, and to these the name of food-stuffs is
given.
Food-stuffs are classified under four heads, (1) Proteids,
(2) Fats, (3) Carbohydrates, (4) Salts (mineral matter)
and Water. They may further be divided into two distinct
groups : — the nitrogenous and the non-nitrogenous. The
proteids alone contain nitrogen and thus form one group by
themselves ; the other food-stuffs are all non-nitrogenous.
Further, the first three classes, as being compounds of
carbon, are known as organic compounds, while the salts
and water are inorganic. They may therefore be tabulated
as follows : —
Organic Inorganic
(Nitrogenous) (Non-nitrogenous) (Non-nitrogenous)
II I
Proteids Fats Salts
I I
Carbohydrates Water
A. Nitrogenous Food-stuffs.
Proteids. — These are composed of the four elements
carbon, oxygen, hydrogen, and nitrogen, united with small
vii NON-NITROGENOUS FOOD-STUFFS 25:
amounts of sulphur and frequently of phosphorus (see
p. 134). Under this head come the albumin of the white
of egg and of blood-serum ; the casein of milk and cheese ;
the gluten of flour and other cereals ; the myosin of lean
meat (muscle) ; the globulins of blood and of the yolk of
an egg ; and the fibrin of blood.
Gelatin, the basis of connective tissue fibres, is composed
of the same elements as a proteid and in somewhat similar
proportions, and may be regarded as an outlying member
of this group. But gelatin is not a true proteid and cannot
entirely replace it in food.
B. Non-nitrogenous Food-stuffs.
(i) Fats. — These are composed of carbon, oxygen, and
hydrogen only, and contain less oxygen than would form
water if united to the hydrogen they contain. Butter
and all animal and vegetable oils come under this head.
(ii) Carbohydrates. — These are substances which also
consist of carbon, oxygen, and hydrogen only, but in them
the oxygen is present in an amount which would just suffice
to form water if it were united to their hydrogen. This
group includes starch, as in flour and potatoes ; ordinary
cane-sugar or beet-sugar, and other sugars such as dextrose
and milk-sugar ; also cellulose from all vegetable tissues.
(iii) Salts and Water. — Water is present in all foods, and
salts in most of them, such as meat, eggs, milk, and cheese.
The salts are chiefly the phosphates, chlorides, and carbon-
ates of sodium, potassium, and calcium, and some salts
of iron.
All food is made up of these food-stuffs, but the amount
of each present in different foods varies greatly. Thus
lean meat is chiefly proteid, but ordinarily contains a good
deal of fat ; bread contains a great deal of carbohvdrates,
252 ELEMENTARY PHYSIOLOGY less.
but also some proteid and a little fat. Only the fats and
oils may be regarded as composed of nearly pure material.
The composition of the chief foods is important and has
been carefully determined ; and to this we shall return
when we come to study their respective influences on the
body as a whole.
3. The Purpose and Means of Digestion. — All food-
stuffs being thus proteids, fats, carbohydrates, or mineral
matters, pure or mixed up with other substances, the whole
purpose of the alimentary apparatus is in the first place to
separate these proteids, etc., from the innutritious residue,
if there be any, and to reduce them into a condition either
of solution or of excessively fine subdivision, in order that
they may make their way through the delicate structures
which form the walls of the vessels of the alimentary canal.
In the next place this mechanical and physical change must
be accompanied by chemical changes whereby the food-
stuffs are brought into such a condition that when they
reach the tissues the latter can take them up or assimilate
them.
To these ends food is taken into the mouth and masti-
cated, is mixed with saliva, is swallowed, undergoes gas-
tric digestion, passes into the intestine, and is subjected to
the action of the secretions of the liver and pancreas with
which it there becomes mixed ; and, finally, after the more
or less complete extraction of the nutritive constituents, the
residue, mixed up with certain secretions of the intestines,
leaves the body as the faeces.
The actual digestive changes of food are brought about
chiefly by the action of fluids secreted by glands whose
ducts pour their secretions into the cavity of the alimentary
canal.
4. The Mouth and the Teeth. — The cavity of the mouth
vii THE MOUTH AND THE TEETH 253
is a chamber with a fixed roof, formed by the hard palate
(Fig. 76, /), and with a movable floor, constituted by the
lower jaw, and the tongue' (k), which fills up the space be-
tween the two branches of the jaw. Arching round the
margins of the upper and the lower jaws are thirty-two
Fig. 76. — A Section of the Mouth and Nose taken vertically, a little
to the left of the Middle Line.
a, the vertebral column; 6, the oesophagus or gullet; c, the trachea or windpipe,
d, the thyroid cartilage of the larynx; e, the epiglottis: /*, the uvula; g, the opening
of the left Eustachian tube; h, the opening of the left lachrymal duct; i, the hyoid
bone; k, the tongue; /, the hard palate; m, «, the base of the skull; o, p, g, the
superior, middle and inferior turbinal bones. The letters g,f, e, are placed in the
pharynx.
teeth, sixteen above and sixteen below, and external to
these, the closure of the cavity of the mouth is completed
by the cheeks at the sides, and by the lips in front.
254 ELEMENTARY PHYSIOLOGY less.
AYhen the mouth is shut the back of the tongue comes
into close contact with the palate ; and, where the hard
palate ends, the communication between the mouth and
the back of the throat is still further impeded by a sort of
fleshy curtain — the soft palate or velum — the middle of
which is produced into a prolongation, the uvula (/), while
its sides, skirting the sides of the passage, or fauces, form
double muscular pillars, which are termed the pillars of the
fauces. Between these the tonsils are situated, one on each
side.
The velum with its uvula comes into contact below with
the upper part of the back of the tongue, and with a sort
of gristly, lid-like process connected with its base, the
epiglottis (e).
Behind the partition thus formed lies the cavity of the
pharynx, which may be described as a funnel-shaped bag
with muscular walls, the upper margins of the slanting, wide
end of which are attached to the base of the skull, while the
lateral margins are continuous with the sides, and the lower
with the floor, of the mouth. The narrow end of the pharyn-
geal bag passes into the gullet or oesophagus (<£),a muscular
tube which affords a passage into the stomach.
There are no fewer than six distinct openings into the
front part of the pharynx — four in pairs, and two single
ones in the middle line. The two pairs are, in front, the
hinder openings of the nasal cavities ; and at the sides,
close to these, the apertures of the Eustachian tubes (g),
which connect the pharynx with the middle ears. The two
single apertures are, the hinder opening of the mouth be-
tween the soft palate and the epiglottis ; and, behind the
epiglottis, the upper aperture of the respiratory passage,
or the glottis.
Each tooth presents a crown, which is visible in the cavity
vii THE MOUTH AND THE TEETH 255
of the mouth, where it becomes worn by attrition with the
tooth opposite to it and with the food ; and one or more
fangs, which are buried in a socket or alveolus, furnished by
the jaw-bone and the dermis of the dense mucous membrane
of the mouth. This covering of the jaw-bone constitutes the
gum. The line of junction between the crown and the fang
is the neck of the tooth.
The eight teeth on opposite sides of the same jaw are con-
structed upon exactly similar patterns, while the eight teeth
which are opposite to one another and bite against one an-
other above and below, though similar in kind, differ some-
what in the details of their patterns.
The two teeth in each eight which are nearest the middle
line in the front of the jaw, have wide, but sharp and chisel-
like, edges. Hence they are called incisors, or cutting teeth.
The tooth which comes next is a tooth with a more conical
and pointed crown. It answers to the great tearing and
holding tooth of the dog, and is called the canine or eye-
tooth. The next two teeth have broader crowns, with two
cusps, or points, on each crown, one on the inside and one
on the outside, whence they are termed bicuspid teeth, and
sometimes false grinders. All these teeth have usually one
fang each, except the bicuspid, the fangs of which may be
more or less completely divided into two. The remaining
teeth have two or three fangs each, and their crowns are
much broader. Since they crush and grind the matters which
pass between them they are called molars, or true grinders.
In the interior of the tooth is a cavity communicating
with the exterior by canals, which traverse the fangs and
open at their points. This cavity is the pulp cavity (Fig.
77, l>). It is occupied and completely filled by a highly
vascular tissue richly supplied with nerves, the dental pulp,
which is continuous below, through the opening of the fangs,
250
ELEMENTARY PHYSIOLOGY
with the vascular dermis of the gum which lies between the
fangs and the alveolar walls, and plays the part of periosteum
to both.
The tissue which forms the chief constituent of a tooth is
termed dentine (Fig. 77, d). It is a dense and calcified
substance containing less animal matter than bone, perme-
ated by innumerable, minute, parallel, wavy tubules (Fig.
78, a), which give off lateral branches. The wider inner
ends of these tubules measure on the average 5^1 (45^0
inch) in diameter ■ they open into the pulp cavity, while
Fig. 77.
A, vertical, B, horizontal section of a tooth, a, enamel of the crown; b, pulp
cavity; c, cement of the fangs ; d, dentine. (Magnified about three diameters.)
the narrower outer terminations ramify at the surface of the
dentine, and may even extend into the enamel or cement
(Fig. 78).
The greater part of the crown and almost the whole of the
fangs consist of dentine. But the summit of the crown is
invested by a thick layer of a much denser tissue, which con-
tains only 2 per cent, of animal matter, and is the hardest
vii THE DEVELOPMENT OF THE TEETH 257
substance in the body ; so hard that it will strike fire with
steel. This is called enamel (Fig. 77, a). It becomes
thinner on the sides of the crown and gradually dies out on
the neck. Examined microscopically, the enamel is seen to
consist of six-sided prismatic fibres (Fig. 78, A, B) set
closely side by side, nearly at right angles to the surface of
the dentine. These fibres measure about 5 /a (-g-gVg- inch)
in transverse diameter and present transverse striations.
The third tissue found in teeth is a thin layer of true bone,
generally devoid of Haversian canals, which invests the
outer surface of the fangs and thins out on the neck. This
is termed cement (Fig. 77, A, c ; and Fig. 78, C, e).
The dental pulp is chiefly composed of delicate connec-
tive tissue. It is abundantly supplied with vessels and nerves,
which enter it through the small opening at the extremity
of the fang. The nerves are mainly sensory branches de-
rived from the fifth pair of cranial nerves (Lesson XII).
The superficial part of the pulp, which is everywhere in
immediate contact with the inner surface of the dentine,
consists of a layer of nucleated cells so close set that they
almost resemble an ■ epithelium. They are, however, in
reality connective tissue cells (and the layer is merely a
slightly modified condition of the stratum of undifferen-
tiated connective tissue which lies at the surface of every
dermal structure), and from them long filamentous processes
can be traced into the dentinal tubules.
5. The Development of the Teeth. — The teeth begin to
be developed long before birth, and while the jaw-bones are
in a very rudimentary condition. The epithelium covering
the gums thickens into a ridge and grows down into the un-
derlying dermis, which at the same time grows up at the
sides of the ridge. In this way a semicircular groove, the
dental groove, is developed in the dermis of the gum of
258 ELEMENTARY PHYSIOLOGY less.
each jaw. The epithelium of the gum, however, completely
fills the groove and passes from side to side smoothly over
it. Next, each groove becomes subdivided into ten pouches,
five on each side of the middle line, and behind the fifth on
each side there remains a residue of the groove, which may
be called a residual pouch.
Each of the first-mentioned pouches becomes gradually
more and more distinct from its neighbours, until at length
its walls unite and shut off the epithelium which it contains
from the cavity of the mouth. The result is a closed bag
full of epithelium, which is a milk tooth sac. At the same
time the dermis of the bottom of the sac has grown up as a
conical process into its interior; and this dental papilla is
the rudiment of the future tooth.
While the milk-tooth sac is thus shaping itself, its epithe-
lium grows out on one side into a small process, which gradu-
ally increases in size and takes on the characters of a second
tooth sac. This is the sac of the permanent tooth, which
answers to and will replace each milk tooth.
A similar change takes place in the residual pouches, each
of which gradually becomes divided into three sacs for the
three hindmost permanent teeth in each jaw.
The sacs of the milk teeth rapidly increase in size and
become separated from one another by partitions of bone
developed from the jaw with which they are in relation, and
which grow up round them. They thus become lodged in
alveoli.
The proper tooth substance first makes its appearance as
a very thin hollow cap of glassy calcareous deposit at the
summit of the papilla. This cap gradually extends and in-
creases in thickness, the increase of the tooth being accom-
panied by decrease of the papilla, which eventually remains
in the cavity of the finished tooth as the pulp.
THE DEVELOPMENT OF THE TEETH
259
The fully formed milk teeth press upon the upper walls
of the sacs in which they are inclosed, and, causing a more
Fig. 78.
A. Enamel fibres viewed in transverse section.
B. Enamel fibres separated and viewed laterally.
C. A section of a tooth at the junction of the dentine (a) with the cement (e);
i, c, irregular cavities in which the tubules of the dentine end; d, fine tubules con-
tinued from them; f, g, lacuna; and canaliculi of the cement. (Magnified about 40c
diameters.)
or less complete absorption of these walls, force their way
through. The teeth are then, as it is called, cut.
26o ELEMENTARY PHYSIOLOGY less.
The cutting of this first set of teeth, called deciduous, or
milk teeth, commences at about six months, and ends with
the second year. They are altogether twenty in number —
eight being cutting teeth, or incisors ; four, eye teeth, or
canines ; and eight, grinders, or molars.
It has been seen that each dental sac of the milk teeth, as
it is formed, gives off a little prolongation ; this becomes
lodged in the jaw below the milk tooth, enlarges, and de-
velops a papilla from which a new tooth is formed. As the
latter increases in size, it presses upon the root of the milk
tooth which preceded it, and thereby causes the absorption
of the root and the final falling out, or shedding, of the milk
tooth, whose place it takes. Thus every milk tooth is re-
placed by a tooth of what is termed the permanent dentition.
The permanent incisors and canines are larger than the milk
teeth of the same name, but otherwise differ little from them.
The permanent teeth, which replace the milk molars, are the
bicuspids.
We have thus accounted for twenty of the teeth of the
adult. The permanent back grinders, or molars, are de-
veloped in the sacs which are formed out of the residual
pouches above mentioned. The first of these teeth, the
anterior molar of each side, is the earliest cut of all the
permanent set, and appears at six years of age. The last,
or hindermost, molar is the last of all to be cut, usually not
appearing till twenty-one or twenty-two years of age. Hence
it goes by the name of the " wisdom tooth."
6. Mastication. — The muscles of the parts which have
been described have such a disposition that the lower jaw
can be depressed, so as to open the mouth and separate the
teeth ; or be raised, in such a manner as to bring the teeth
together \ or move obliquely from side to side, so as to cause
the face of the grinding teeth and the edges of the cutting
vii THE CESOPHAGUS AND SWALLOWING 261
teeth to slide over one another. And the muscles which
perform the elevating and sliding movements are of great
strength, and confer a corresponding force upon the grind-
ing and cutting actions of the teeth.
When solid food is taken into the mouth, it is cut and
ground by the teeth, the fragments which ooze out upon the
outer side of their crowns being pushed beneath them again
by the muscular contraction of the cheeks and the lips ; while
those which escape on the inner side are thrust back by the
tongue, until the whole is thoroughly rubbed down.
While mastication is proceeding, the salivary glands pour
out their secretion in great abundance, and the saliva mixes
with the food, which thus becomes interpenetrated not only
with the salivary fluid, but with the air which is entangled in
the bubbles of the saliva.
7. The (Esophagus and Swallowing. — When the food
is sufficiently ground it is collected, enveloped in saliva, into
a mass or bolus, which rests upon the back of the tongue,
and is carried backwards to the aperture which leads into
the pharynx. Through this it is thrust, the soft palate being
lifted and its pillars being brought together, while the back-
ward movement of the tongue at once propels the mass and
causes the epiglottis to incline backwards and downwards
over the glottis and so to form a bridge, by which the bolus
can travel over the opening of the air-passage without any
risk of tumbling into it. While the epiglottis directs the
course of the mass of food below, and prevents it from pass-
ing into the trachea, the soft palate guides it above, keeps
it out of the nasal chamber, and directs it downwards and
backwards towards the lower part of the muscular pharyngeal
funnel. By this the bolus is immediately seized and tightly
held, and the muscular fibres contracting above it, while
they are comparatively lax below, it is rapidly thrust into
and down the oesophagus.
262 ELEMENTARY PHYSIOLOGY less,
The oesophagus is lined with mucous membrane. This
rests on some fibrous tissue, outside of which is a thick coat
of muscular tissue, striated in the upper third of the tube,
unstriated lower down next to the stomach. This is arranged
in two layers, an outer layer in which the fibres run parallel
to the long axis of the tube ; an inner layer in which the
fibres are wrapped round the tube.
After food has been thrust into the oesophagus by the
action of the pharynx, a wave-like contraction, called peri-
staltic action, of the muscular wall of the oesophagus follows
the bolus and finally thrusts it into the stomach.
Drink is taken in exactly the same way as food. It does
not fall down the pharynx and gullet, but each gulp is
grasped and passed down. Hence it is that jugglers are
able to drink standing upon their heads, and that a horse,
or ox, drinks with its throat lower than its stomach, feats
which would be impossible if fluid simply fell down the
gullet into the gastric cavity.
During these processes of mastication, insalivation, and
deglutition, what happens to the food is, first, that it is
reduced to a coarser or finer pulp ; secondly, that any
matters it carries in solution are still more diluted by the
water of the saliva ; thirdly, that any starch it may con-
tain begins to be changed into sugar by the saliva, whose
formation and action we must next consider.
8. The Salivary Glands. — The mucous membrane
which lines the mouth and the pharynx is beset with
minute glands, the buccal glands ; but the great glands
from which the cavity of the mouth receives its chief
secretion are the three pairs which are called the parotid,
submaxillary, sublingual (Fig. 79).
Each parotid gland is placed just in front of the ear, and
its duct passes forwards along the cheek, until it opens in
THE SALIVARY GLANDS
263
the interior of the mouth, opposite the second upper grind-
ing tooth.
The submaxillary and sublingual glands lie between the
lower jaw and the floor of the mouth, the submaxillary
being situated farther back than the sublingual. Their
ducts open in the floor of the mouth below the tip of the
tongue. The secretion of these salivary glands, mixed with
that of the small glands of the mouth, constitutes the saliva.
Fig.
79-
A dissection of the right side of the face, showing a, the sublingual, b, the sub-
maxillary glands, with their ducts opening beneath the tongue in the floor of the
mouth at d; c, the parotid gland and its duct, which opens on the side of the cheek
at e .
The salivary glands are of the type shown in Fig. 57, 6.
Their essential part consists of the secreting cells which
line the dilated ends, or alveoli, of the finest branches of
their ducts. In a gland which is resting, that is, has not
been secreting for some time, the cells are large and nearly
fill the alveoli (Figs. So and 81, A). Each cell has a
nucleus placed either near its outer end (many of the sub-
maxillary alveoli), or in the middle of the cell (parotid).
The protoplasm of the body of the cell is more or less
264
ELEMENTARY PHYSIOLOGY
completely filled up with granules, which are better seen in
pieces of the fresh gland than in preserved specimens.
After the glands have been secreting for some ti?ne, as the
result either of taking food or of stimulating the nerves
supplied to them, the appearance of their cells is greatly
changed (Figs. 80, B, and 81, C). The cells are now
smaller ; the nucleus has become more distinct and in the
submaxillary cells has moved nearer the centre of the cell ;
the granules are fewer and now lie near the inner or alveo-
lar end of the cells;- and the protoplasm, being freed from
a
Fig. 80. — Sections of the Submaxillary Gland hardened and stained.
A, after rest; B, after secretory activity; a, a, so-called marginal cells.
granules, is now much more distinct. Between these two
extremes there is an intermediate stage shown in Fig. 81, B.
The differences in the size and appearance of the cells
after rest and after activity seem to show quite clearly that,
while at rest, the cells build up material which is stored in
their substance, and hence they are large. In the submax-
illary and the sublingual glands this substance is largely
mucinous, in the parotid albuminous, and it is deposited as
SALIVA AND ITS SECRETION
265
separate distinct granules in the body of the cell. Further, it
appears that during their activity both glands discharge their
store of material into the duct leading from them, and hence
the cells become smaller and more obviously protoplasmic.
Fig. 81. — Changes in the Parotid Gland during Secreting Activity (fresh).
(Slightly diagrammatic.)
A, after rest; B, after slight activity; C, after greater activity.
9. Saliva and its Secretion. — The mixed saliva from the
several glands consists chiefly of water, holding in solution a
small amount of proteid matter, some inorganic salts, to
which its faintly alkaline reaction is due, a small amount of
mucin, which gives to saliva its well-known sliminess, and
a small quantity of a peculiar substance called ptyaliii, to
which the digestive power of the liquid is due.
Ordinarily saliva is secreted in increased quantity as soon
as food is introduced into the mouth. This result is brought
about reflexly. The food stimulates the ends of certain
nerves (Vth and, IXth cranial, see p. 537) which supply the
walls of the inside of the mouth. Impulses pass up these
nerves to the brain, and from this organ other impulses
pass down to the glands and make their cells secrete.
Some of the experimental evidence that the salivary
glands are under nervous control is as follows : —
The submaxillary gland is supplied by a nerve which is a
branch of the Vllth cranial nerve (see p. 537), and which,
266 ELEMENTARY PHYSIOLOGY less.
since it crosses the tympanic cavity or drum of the ear
(see p. 406), is called the chorda tympani nerve. When this
nerve is stimulated three things happen : the arteries which
supply the gland with blood dilate, and there is a very
largely increased flow of blood through the gland ; the gland
begins to pour out its secretion ; and the cells of the gland
slowly change their size and appearance as already described.
These changes show that a good deal of the material with
which the cells were loaded during rest has been discharged.
The granules in the cells are the immediate forerunners of
the organic constituents of the saliva, the proteids, the
mucin, and the ptyalin, and undergo final chemical trans-
formation into these constituents at the time of discharge.
But at the same time the cells have discharged a large
quantity of water and some salts, and the water and salts
can have come only from the blood. The question at once
arises : has the increased supply of blood simply led to an
increased flow of water and salts through the cells, which
has carried away with it the accumulated materials of the
cell-substance, the whole process being largely filtrational ;
or has the stimulation of the nerve not only made the cells
discharge some of their substance, but also made them take up
water and salts from the blood and pass these as well through
the cells? The evidence in support of the latter mode of
action seems conclusive. For, first, an increased tempo-
rary secretion may be observed on stimulating the nerve even
after the blood-supply to the gland has been cut off; and,
secondly, if certain drugs, such as atropine, be injected into
the animal, then, although the arteries dilate to the full
extent when the nerve is stimulated, no increased secretion
takes place. Evidently, when the gland secretes it is
because the impulses which ?-each it along the nerve exert
a direct influence on its cells. These impulses make the
vil SOLUBLE FERMENTS OR ENZYMES 267
cells take up water and salts and discharge them, together
with the stored cell-substance, as saliva into the ducts.
The increased blood- supply, while not causing the secre-
tion, is necessary if the cells are to continue to secrete, for
it is from the blood alone that they can obtain all that they
require for the manufacture of the saliva.
10. The Action of Saliva. — Saliva does not act on
proteids or fats, but, if a little of it be mixed with ordinary
starch- paste and warmed to the temperature of the body, by
means of its ptyalin it turns that starch into sugar. This
sugar is identical with that obtained from malt in brewing,
and is hence known as maltose. Although this chemical
change is, without doubt, of some use to the body, its
importance must not be over-estimated. For in many
animals the action of their saliva on starch is very slight,
and, moreover (see p. 287), the larger part of the starch we
eat is digested, that is, changed into a sugar, while the food is
in the intestine and under the action of the pancreatic juice.
The chief use of the saliva is mechanical rather than chem-
ical, inasmuch as it moistens the food and thereby assists
mastication and makes the swallowing of the food easy.
11. Soluble Ferments or Enzymes. — The peculiar sub-
stance, ptyalin, to which the chemical action of saliva on
starch is due, belongs to a class of substances known as
soluble ferments or enzymes. The word ferment was origi-
nally applied to a living organism such as yeast, which, as
in brewing, while converting sugar into alcohol, causes at
the same time, on account of the simultaneous production
of carbonic acid gas, a boiling up or frothing of the liquor ;
hence the name "ferment " {Jervere = to boil up).
But it is known now that such organised ferments can
be made to yield extracts which may be filtered so as to
be quite free from organisms and still be able to produce
268 ELEMENTARY PHYSIOLOGY less.
the same changes as did the cells from which they are
prepared. Hence the name of soluble ferment or enzyme
(£vix-q = yeast) was given to the substance in solution which
can bring about the same changes as the parent cell.
Very little is known of the chemical nature of enzymes,
but they are strongly characterised by certain facts as to
the conditions under which their action takes place. Thus :
(i) Very minute quantities will effect a change in a mass
of the substance on which they are working, which is enor-
mously large compared with the minute mass of the enzyme.
(ii) Their action depends closely on temperature. At
o° C. (320 F.) they cease to act; as the temperature rises
they become increasingly active, and are most active at about
400 C. (1040 F.). At higher temperatures they become less
active and lose their powers permanently if once heated to
ioo° C. (2 1 20 F.), as by boiling: they are then said to be
" killed." (hi) Their action in many cases depends on the
reaction, whether acid or alkaline or neutral, of the solution
in which they are at work.1 (iv) Their action stops in pres-
ence of an excess of the special products of their activity.
And (v) it has not so far been conclusively proved that the
enzymes are themselves used up during the changes which
they produce on other substances.
Nearly all the chemical changes which the food under-
goes in the alimentary canal are brought about by the action
of these soluble ferments or enzymes.
12. The Structure of the Stomach. — The stomach, like
the gullet, consists of a tube with muscular walls lined by
mucous membrane and covered by peritoneum ; but it
differs from the gullet in several circumstances. In the first
place, its cavity is much larger, and its left end is produced
1 Thus, the pepsin of gastric juice acts best in presence of hydrochloric
acid and the trypsin of pancreatic juice in presence of sodium carbonate.
THE STRUCTURE OF THE STOMACH
269
into an enlargement, which, because it is on the heart side
of the body, is called the cardiac part (Fig. 82, b). The
opening of the gullet into the stomach, termed the cardiac
aperture, is consequently nearly in the middle of the whole
length of the organ, which presents a long, convex, greater
curvature, along its front or under edge, and a short, con-
cave, lesser curvature, on its back or upper contour.
Towards its right extremity the stomach narrows, and,
Fig. S2. — The Stomach laid open.
a, the oesophagus; b, the cardiac dilatation ; c, the lesser curvature; d, the pylorus;
e, the biliary duct;_/", the gall-bladder; g, the pancreatic duct opening in common
with the cystic duct opposite h / h, i, the duodenum.
where it passes into the intestine, the muscular fibres are
so disposed as to form a sort of sphincter around the aper-
ture of communication. This constriction is called the
pylorus (Fig. 82, d).
The muscular coat of the stomach, consisting of unstriated
muscular tissue, is made up of two chief layers, an outer
longitudinal and an inner circular, together with an incom-
270 ELEMENTARY PHYSIOLOGY . less.
plete layer of muscle fibres which are continuous with the
circular fibres of the oesophagus, and which, running ob-
liquely, merge into the internal circular layer of the stomach.
The mucous membrane which lines the stomach is loosely
attached to the muscular coat by a layer of areolar connec-
tive tissue. This is called the submucous coat, and it is in
this layer that the nerves, blood-vessels, and lymphatics run
for the supply of the mucous membrane.
The mucous membrane lining the wall of the stomach
contains, or rather is made up of, a multitude of small
glands, the gastric glands, packed closely side by side with
delicate adenoid tissue between them, and opening upon
the inner surface of the stomach. These are on the whole
simple in nature, being long, tubular glands, but they vary
in character, their blind ends being more divided and
twisted at one part of the stomach than another.
Each gland is lined by cells which at the mouth of the
gland are columnar and secrete mucin ; but deeper down
in the tubes they are cubical and granular. These are the
central cells (Fig. 83, c) . A second kind of cell may also
be seen lying scattered irregularly between the basement
membrane of the gland and its central cells : these are the
parietal cells (Fig. 83,/). Oval in shape, they have a well-
defined outline and their cell-substance is usually very finely
granular. The glands near the pyloric end of the stomach
differ from those of the rest of the mucous membrane, chiefly
and essentially by not containing any of these parietal cells.
13. Gastric Juice and its Secretion. — The liquid secreted
by the glands of the stomach is called gastric juice. Pure
gastric juice is a clear, acid fluid and consists of little more
than water containing a few saline matters in solution ; its
acidity is due to the presence of free hydrochloric acid to
the extent of .2 per cent. It possesses, however, in addi-
THE ACTION OF GASTRIC JUICE
271
tion a small quantity of a peculiar substance called pepsin,
a soluble ferment or enzyme in many respects similar to,
though very different in its effects from, ptyalin, and also
a similar ferment called reixnin.
When the stomach is empty, its mucous membrane is
pale and hardly more than moist. Its small arteries are
then in a state of constriction, and com-
paratively little blood is sent through it.
On the entrance of food a vaso- motor
action is set up, which causes these small
arteries to dilate ; the mucous membrane
consequently receives a much larger
quantity of blood, and it becomes very
red. At the same time the cells of the
glands begin to form their secretion.
The whole process is exactly similar in
principle to that already described in
the case of the secretory activity of the
submaxillary gland (p. 265). The gran-
ules of the central cells of the glands
gradually disappear and are believed to
be transformed into pepsin and perhaps
rennin. It has been thought, but it is
not definitely established, that the parietal
cells produce the hydrochloric acid of
the juice. The water and salts come
directly from the blood.
14. The Action of Gastric Juice. —
It is easy to ascertain the properties of
gastric juice experimentally, by putting a small portion of
the mucous membrane of a stomach into water made acid
by the addition of .2-5 per cent, of hydrochloric acid and
containing small pieces of meat, hard-boiled egg, or other
Fig. 83. — One of the
Glands which se-
crete Gastric Juice.
D, the duct or mouth
of the gland", m, mucous
cells lining the mouth of
the gland and covering
the inner surface of the
mucous membrane: c,
central cells; p, parietal
272 ELEMENTARY PHYSIOLOGY less.
proteids, and keeping the mixture at a temperature of abou
400 C. (1040 F.). After a few hours it will be found that
the white of egg, if not in too great quantity, has become
dissolved : while all that remains of the meat is a pulp, con-
sisting chiefly of the connective tissue and fatty matters
which it contained. This is artificial digestion, and it has
been proved by experiment that precisely the same opera-
tion takes place when food undergoes natural digestion
within the stomach of a living animal.
The solvent power of gastric juice over proteids is due
to the pepsin ; gastric juice which has been boiled, in which
case all the ferment it contains is " killed" (see p. 268), is
quite inactive although it contains the usual amount of acid.
The characteristic proteid which is formed during the
solvent action of the juice is called peptone, and has pretty
much the same characters whatever the nature of the pro-
teid which has been digested.
Peptone differs from all other proteids in its extreme
solubility, and characteristically in the fact that it is highly
diffusible, and hence in the readiness with which it passes
through animal membranes. Many proteids, as fibrin, are
naturally insoluble in water, and others, such as white of
egg, though apparently soluble, are not completely so, and
can be rendered quite solid or coagulated by being simply
heated, as when an egg is boiled. A solution of peptone,
however, is perfectly fluid, does not become solid, and is
not at all coagulated by boiling. Again, if a quantity of
albumin, such as white of egg or serum of blood, be tied
up in a bladder, and the bladder immersed in water, very
little if any of the proteid will pass through the bladder into
the water, provided that there are no holes.1 If, however,
l This experiment may be readily made with the apparatus shown in
Fig. 45, p. 145.
vii THE ACTION OF GASTRIC JUICE 27^
peptone be used instead of albumin, a very large quantity
will speedily pass through into the water, and a quantity of
water will pass from the outside into the bladder, causing it
to swell up. This diffusive passage of a substance through
a membrane is called osmosis, and is evidently of great
importance in the economy ; and the purpose of the con-
version of the various proteids by digestion into peptone
seems to be, in part at least, to enable this class of food-
stuff to pass readily into the blood through the thin partition
formed by the walls of the mucous membrane of the intes-
tine and the coats of the capillaries. Similarly, starch, even
when boiled, and so partially dissolved, is not diffusible and
will not pass through membranes, whereas sugar does so
with the greatest ease. Hence the reason of the conversion
of starch, by digestion, into sugar.
The rennin of gastric juice causes the casein in milk to
clot in a way very similar to that in which fibrin-ferment
gives rise to a clot of fibrin by its action on fibrinogen
(p. 140). This action of rennin is the basis of cheese-
making, and the "rennet" used for obtaining the curd in
the latter process is really an extract of the mucous mem-
brane of the stomach of a calf, in which the ferment is
peculiarly plentiful.
As far as we know, gastric juice has no direct action on
fats ; by breaking up, however, the proteid framework of the
cells in which animal and vegetable fats are imbedded, it
sets these free, and so helps their digestion by exposing
them to the action of other agents. It appears too, that
gastric juice has no direct action on carbohydrates ; on the
contrary the conversion of the starch into sugar begun in
the mouth appears to be wholly or partially arrested by
the acidity of the contents of the stomach, ptyalin being
active only in an alkaline or neutral mixture.
T
274 ELEMENTARY PHYSIOLOGY less,
By continual rolling about, with constant additions of
gastric juice, the food becomes reduced to the consistence
of pea-soup, and is called chyme. In this state, the larger
part is allowed to escape through the pylorus and to enter
the duodenum ; but a very small portion of the fluid (con-
sisting of peptone together with any sugar resulting from the
partial conversion of starch, or otherwise) may be at once
absorbed, making its way, by imbibition, through the walls
of the delicate and numerous vessels of the stomach into
the current of the blood, which is rushing through the gastric
veins to the portal vein.
15. The General Arrangement and Structure of the
Intestines. — The intestines (Figs. 84 and 86) form one
long tube, with mucous and muscular coats, like the stom-
ach ; and, like it, they are enveloped in peritoneum. They
are divided into two portions — the small intestine and the
large intestine ; the latter, though shorter, having a much
greater diameter than the former. The name of duodenum
is given to that part of the small intestine, about ten inches
in length, which immediately succeeds the stomach. It is
bent upon itself and fastened by the peritoneum against the
back wall of the abdomen, in the loop shown in Fig. 82, h, i.
It is in this loop that the head of the pancreas lies (Fig. 75).
The rest of the small intestine, of which the part next
the duodenum is called the jejunum and the rest the ileum, is
no wider than the duodenum, so that the transition from the
small intestine to the large (Figs. 85, a, k, and &6,Il,ccec) is
quite sudden. The opening of the small intestine into
the large is provided with prominent lips which project
into the cavity of the latter, and oppose the passage of
matters from it into the small intestine, while they readily
allow of a passage the other way. This is the ileo-ceecal
valve (Fig. 85, d).
THE INTESTINES
Fig. 84. — The Viscera of a Rabbit as seen upon simply opening the Cavi-
ties of the Thorax and Abdomen without any further Dissection.
A, cavity of the thorax, pleural cavity on either side; B, diaphragm; C, ven-
tricles of the heart; D, auricles; E, pulmonary artery; F, aorta; G, lungs collapsed,
and occupying only the back part of chest; H, lateral portions of pleural membranes;
/, cartilage at the end of sternum (ensiform cartilage) ; A", portion of the wall of body
left between thorax and abdomen; a, cut ends of the ribs; L, the liver, in this case
lying more to the left than the right of the body; M, the stomach, a large part of the
greater curvature being shown; iV, duodenum ; O, other portions of the small intes-
tine; P, the caecum, so largely developed in this and other herbivorous animals; Q.
the large intestine.
276 ELEMENTARY PHYSIOLOGY less.
The large intestine forms a blind dilatation beyond the
ileo-csecal valve, which is called the caecum (Figs. 85, k,
and 86, ccec) ; and from this an elongated blind process
is given off, which, from its shape, is called the vermiform
appendix of the caecum (Figs. 85, b, and 86, verm).
Fig. 85.
The junction of the ileum, a, with the caecum, k, and the continuation of the latter
into the colon, e; d, the ileo-cascal valve; c, the opening of the vermiform appendix
(3) into the caecum.
The caecum lies in the lower part of the right side of the
abdominal cavity. The colon (Fig. 86), or first part of the
large intestine, passes upwards from it as the ascending
colon ; then making a sudden turn at a right angle, it
passes across to the left side of the body, being called the
transverse colon in this part of its course ; and next,
suddenly bending backwards along the left side of the
abdomen, it becomes the descending colon. This reaches
the middle line and becomes the rectum, which is that part
of the large intestine which opens externally. The external
opening is called the anus.
The intestines are slung from the middle line, along
the vertebral column, of the abdominal cavity by a thin
membrane known as the mesentery (Fig. 87). This is a
THE INTESTINES
277
continuation of the peritoneum, the serous membrane that
lines the whole cavity of the abdomen. The mesentery
consists really of two layers, between which the nerves,
blood-vessels,' and lymphatics lie which supply the intestines.
Asol
verm
Fig. 86. — The Alimentary Canal in the Abdomen.
R_, right: L, left; tz, oesophagus; st, stomach; py, pylorus; duo, duodenum;
JeJ< jejunum; //, ileum; ccec, caecum; A.col, ascending colon; T.col, transverse
colon; D.col, descending colon; R, rectum; zierm, vermiform appendix.
The latter thus lie in a fold of the peritoneum, somewhat as
a man lies when slung in a hammock.
Other folds of the peritoneum similarly support the
Other organs in the abdomen. The peritoneum is thus a
i78
ELEMENTARY PHYSIOLOGY
double bag whose relation to the wall of the abdomen and
to the organs in it is similar to that of the pleurae to the
walls of the thorax and the lungs.
The intestines receive their blood almost directly from
the aorta. Their veins carry the blood which has traversed
the intestinal capillaries to the portal vein.
The intestines, like the stomach, are made up of four
coats : the external peritoneum, then a muscular coat con-
nected by a submucous layer with the inner or mucous coat.
IV Tt
d.m.
j>cn't ___
b.v.
rn.es :
incest
perit
Fig. 87.
■Diagram to show how the Wall of the Abdomen is made up,
and how the mesentery supports the intestine.
The body is supposed to be cut across, and the intestine is represented as the
section of a straight tube. In reality the space between the intestine and the body
wall is filled by the coils of the intestine and by other organs.
vert, vertebra; d.m, muscles of back; sk, skin; m1, m~, nfi, the three muscle
layers; perit, peritoneum; no, mesentery; latest, intestine; b.v, blood-vessels.
The muscular coat of the small intestine is made up of
two layers ; an outer longitudinal, an . inner circular. As in
the oesophagus, the circular fibres of any part are able to
contract successively, in such a manner that the upper
fibres, or those nearer the stomach, contract before the
vii THE INTESTINES 279
lower ones, or those nearer the large intestine. It follows
from this peristaltic contraction, that the contents of the
intestines are constantly being propelled, by successive and
progressive narrowing of their calibre, from their upper
towards their lower parts. And the same peristaltic move-
ment goes on in the large intestine from the ileo-csecal
valve to the anus.
The submucous layer is composed of loose (areolar)
connective tissue, and carries the blood-vessels, nerves,
and lymphatics.
The tube of mucous membrane which forms 'the inner
coat of the small intestine is longer than the muscular tube
which surrounds it ; hence, to get this greater length of
the former stowed away into the shorter length of muscular
tubing, the mucous membrane is thrown into folds, which
must evidently lie at right angles to its long axis. These
folds serve to increase the surface of the mucous membrane
and are called valvulae comiiventes.
The large intestine presents noteworthy peculiarities in
the arrangement of the longitudinal muscular fibres of the
colon into three bands, which are shorter than the walls of
the intestine itself, so that the latter is thrown into puckers
and pouches (Fig. 84, Q) ; these are known as the sacculi,
and serve for the same purpose as the valvules conniventes
of the small intestine. Moreover, the muscular fibres around
the anus are arranged so as to form a ring-like sphincter
muscle, which keeps the aperture firmly closed, except
when defalcation takes place.
The mucous membrane of both small and large intestine
consists largely of simple tubular glands packed side by side ;
they are known as the glands of Lieberkuhn (Fig. 88, G.L.).
Each gland is lined by a layer of columnar cells (Fig. 88, C),
among which occur a certain number of mucous cells. The
28o ELEMENTARY PHYSIOLOGY less.
glands are separated from one another by adenoid tissue
(p. 1 1 6). In the small intestine the tissue between the
mouths of the glands projects into the cavity of the intes-
tine as minute club-shaped processes, the villi, which are set
side by side over the surface of the mucous membrane like
the pile on velvet. These villi are absent in the large intestine.
At irregular intervals along the mucous membrane the
lymphoid tissue between the glands forms small rounded
masses crowded with leucocytes like lymphatic glands, and
called solitary glands. In parts of the small intestine
groups of these follicles are found packed closely together;
they are then known as Peyer's patches.
At the commencement of the duodenum are certain
small racemose glands, called the glands of Brunner, whose
ducts open into the intestine. Their function seems to be
quite unimportant.
16. The Structure of the Villi. — The average length of
a villus (Fig. 88, A) is about .5 -.7 of a millimetre (-gL __L
of an inch). Running up its centre or axis is a relatively
large lymphatic vessel, which ends blindly at the summit of
the villus, but at its base opens into the lymphatics in the
submucous tissue. This central lymphatic is called a lacteal.
Lying around the lacteal, and parallel to it, are a few small
fibres of unstriated muscle derived from the muscularis
mucosae, which is a thin layer of unstriated muscle in the
mucous membrane, lying next to the submucous coat (m.m.) ;
outside these again, close under the epithelium of the villus,
is a network of capillaries {c), which receive blood from an
artery in the submucous layer and return it by a small vein
to the veins of the same layer.
All space left between the several structures so far de-
scribed in the body of the villus is filled up with adenoid
(lymphoid) tissue, which is continuous with that between
THE STRUCTURE OF THE VILLI
281
the glands of Lieberkiihn, and whose meshes are more or
less crowded with leucocytes.
sf — m.m.
Fig. 88. — Diagram of Two Villi and an adjacent Gland of Lieberki'hn
(Hardy).
A, two villi with a gland of Lieberkiihn, G. L, between their bases; m.tn, mus-
cularis mucosas; /, central lacteal; c, blood-capillaries.
B, portion of epithelium of villus more highly magnified to show one " goblet "
cell (above) and two of the other epithelial cells; C, two of the cells which line the
tube of the gland of Lieberkiihn, more highly magnified.
The epithelium covering a villus is continuous with that
lining the glands of Lieberkiihn and is made up of cells of
two kinds (Fig. 88, B). Of these the large majority are
2S2 ELEMENTARY PHYSIOLOGY less.
tall, columnar, and granular, with an oval nucleus. The
outer end of each cell (on the surface of the villus) shows a
narrow, strongly striated border. These cells are concerned
in the absorption of digested food. Lying between these
are cells which, from their shape, are often called " goblet "
cells, but which in structure are practically the same as the
mucous cells of the submaxillary gland already described
(p. 263). These cells secrete the mucus which covers
the inside of the intestine.
17. Succus Entericus. — The glands of Lieberkiihn are
supposed to form a secretion known as succus entericus,
or intestinal juice, which they then discharge into the intes-
tine. The precise functions of this secretion are not wholly
known : it seems to be able to convert starch and various
kinds of sugar into that variety of sugar known as dextrose.
But, on the whole, it probably possesses comparatively little
importance as a digestive agent.
18. The Structure of the Pancreas and its Changes
during Secretion. — The pancreas is a racemose gland, but
the alveoli in which the ducts end are somewhat elongated
as compared with their more rounded shape in the salivary
glands. The cells in each alveolus are not unlike those of
the parotid gland (p. 263). When the gland has been at
rest for some time the cells are large, their outlines indistinct,
and they are thickly loaded except at their outer ends with
very obvious granules (Fig. 89, A.). After the gland has
been secreting for some time, the cells are smaller, their
outline distinct, and the granules have largely disappeared.
Those granules which remain are now placed at the inner
ends of the cells next to the lumen of the alveolus (Fig.
89, B.). These differences in the appearance of the cells in
the two conditions of rest and activity show quite clearly
that while at rest these cells build up material, which is
vii NATURE AND ACTION OF PANCREATIC JUICE 2S3
lodged in their substance as obvious granules, and discharge
this material as part of the secretion as soon as they become
active. Thus, the changes taking place in the cells of the
pancreas during secretion are essentially the same as those
previously described in the case of the salivary glands, and
have the same significance in explanation of the phenomena
of secretion.
19. The Nature and Action of Pancreatic Juice. —
Pancreatic juice is a somewhat viscid fluid, alkaline from
the presence of sodium carbonate and containing a fairly
B.
Fig. 89. — A Portion of the Pancreas of a Rabbit.
A. after rest; B. after activity.
a, granular central zone of the cells; b, clear outer zone; c, lumen of alveolus;
d, junction of two neighbouring cells.
large amount of proteid in solution. It contains further, as
its most important constituents, three soluble ferments. Of
these one, which is called trypsin, is so far like pepsin that
it converts proteids into peptones,1 but it differs from pepsin
in several respects. In the first place trypsin is most active
in an alkaline solution, such as of 1 per cent, sodium car-
1 An artificial pancreatic digestion of proteids may be carried on in the
way already described for pepsin (p. 271), using as a digestive fluid a 1 per
cent, solution of sodium carbonate to which some of the extract of pancreas
sold as " Liquor pancreaticus " has been added.
284 ELEMENTARY PHYSIOLOGY less.
bonate, while pepsin will act only in the presence of an'
acid. In the next place, the change which proteids undergo
by the action of trypsin does not end with the formation of
peptones, as it does in the case of pepsin, but proceeds
further, and some of the peptone is broken down into
the crystalline substances known as lencin and iyrosin.
Of these the leucin is peculiarly interesting, inasmuch as
after it is absorbed it is carried to the liver in the blood
of the portal vein and apparently is converted by the liver
into urea (see p. 214).
The second ferment in pancreatic juice, called amylopsin,
resembles the ptyalin of saliva in so far as it converts starch
into sugar, but it acts more energetically.
The third ferment, called steapsin, has no action on
either proteids or carbohydrates, but it acts on the ordinary
fats in such a way as to split them into glycerine and a fatty
acid. The latter uniting with the alkali of the pancreatic
juice forms soaps, and this process is known as saponifica-
tion. The soaps so formed are important, for they help
greatly in reducing the rest of the fats to that state of fine
subdivision, known as an emulsion, which is an important
aid to further saponification and ultimate absorption.
Pancreatic juice, as containing these three ferments, acts,
therefore, on all three classes of food-stuffs, peptonising the
proteids, saponifying and emulsifying the fats, and convert-
ing starch into sugar.
Although the most obvious function of the pancreas is to
secrete a digestive juice, there are reasons for supposing that
it has other important uses. If it be removed from an animal,
a large quantity of sugar speedily appears in the urine and
the animal wastes away. Such a condition is not infrequently
observed in man, where it is known as diabetes ; and in some
cases of diabetes the pancreas is found *o be diseased.
vii THE FUNCTION OF BILE 285
The pancreas thus seems to exert some control over the
nutrition of the body, probably by means of an internal
secretion, in a way somewhat similar to (though differing in
its results from) the influence exerted by the thyroid gland
and the suprarenal bodies (see p. 247).
20. The Function of Bile. — Bile has of itself no direct
chemical action on food-stuffs, but as an alkaline fluid,
poured into the intestine in large quantity, it serves to neu-
tralise the acidity of the chyme as the latter leaves the
stomach, and thus prepares it for the action of pancreatic
juice. Further, by means of the bile-salts bile plays an
important part, when mixed with pancreatic juice, in lead-
ing to the emulsification of fats, and also facilitates their
subsequent absorption. The bile-pigments and cholesterin
are excretions.
21. The Changes undergone by Food in the Intestines.
— The only secretions, besides those of the proper intestinal
glands, which enter the intestine, are those of the liver and
the pancreas — the bile and the pancreatic juice. The ducts
of these organs have a common opening in the middle of
the bend of the duodenum ; and, since the common duct
passes obliquely through the coats of the intestine, its walls
serve as a kind of valve, obstructing the flow of the con-
tents of the duodenum into the duct, but readily permitting
the passage of bile and pancreatic juice into the duodenum
(Figs. 75 and 82).
After gastric digestion has been going on some time, and
the semi-digested food begins to pass on into the duodenum,
the pancreas comes into activity, its blood-vessels dilate, it
becomes red and full of blood, its cells secrete rapidly, and
a copious flow of pancreatic juice takes place along its duct
into the intestine.
The secretion of bile by the liver is much more continu-
2S6 ELEMENTARY PHYSIOLOGY less.
ous than that of the pancreas, and is not so markedly in-
creased by the presence of food in the stomach. There is,
however, a store of bile laid up in the gall-bladder ; and as
the acid chyme passes into the duodenum, and flows over
the common aperture of the bile and pancreatic ducts, a
quantity of bile from this reservoir in the gall-bladder is
ejected into the intestine. The bile and pancreatic juice-
together here mix with the chyme and produce remarkable
changes in it.
In the first place, the alkali of these juices neutralises the
acid of the chyme ; in the second place, as has been seen,
both the bile and the pancreatic juice exercise an emulsify-
ing influence over the fatty matters contained in the chyme,
and this action is specially well marked when bile and pan-
creatic juice are mixed. The fat, as it passes from the
stomach, is very imperfectly mixed with the other constitu-
ents of the chyme ; and the drops of fat or oil (for all the
fat of the food is melted by the heat of the stomach) readily
run together into larger masses. By the combined action,
however, of the bile and pancreatic juice the large drops of
fat which pass into the intestine from the stomach are broken
up into exceedingly minute particles, and thoroughly mixed
with the rest of the contents ; they are brought in fact to
very much the same condition as that in which fat {i.e. but-
ter) exists in milk. When this emulsifying has taken place
the contents of the small intestine no longer appear grey,
like the chyme in the stomach, but white and milky ; in fact,
it and milk are white for the same reason, viz., on account
of the multitude of minute suspended fatty particles reflect-
ing a great amount of light.
The contents of the small intestine, thus white and milky,
are sometimes called chyle ; but it is best to reserve this
name for the contents of the lacteals, of which we shall have
to speak directly.
<7I CHANGES UNDERGONE BY FOOD 287
The emulsifying of the fats is not, however, the only
change going on in the small intestine. For this is simply
preliminary to their being split up, by the pancreatic juice,
into glycerine and fatty acid, and the subsequent formation
of soaps. It seems probable that much, if not all, of the
fat thus goes to form soaps, which are soluble and diffusible.
Moreover the pancreatic juice has an action on starch simi-
lar to that of saliva, but much more powerful. During the
short stay in the mouth very little starch has had time to be
converted into sugar, and in the stomach, as we have seen,
the action of the saliva is arrested. In the small intestine,
however, the pancreatic juice takes up the work again ; and,
indeed, by far the greater part of the starch which we eat is
digested, that is, changed into sugar, by the action of this
juice.
Nor is this all, for,-in addition to the above, the alkaline
pancreatic juice has a powerful effect on proteids very simi-
lar to that exerted by the acid gastric juice : it converts
them into peptones, and the peptones so produced do not
differ materially from the peptones resulting from gastric
digestion. At the same time a variable amount of leucin
and tyrosin make their appearance as the result of the
further action of pancreatic juice on the peptones.
Hence it appears that, while in the mouth carbohydrates
only, and in the stomach proteids only, are digested, in the
intestine all three kinds of food-stuffs, proteids, fats, and
carbohydrates, are either completely dissolved and made
diffusible, or minutely subdivided, and so prepared for their
passage into the vessels.
As the food is thrust along the small intestine by the
grasping action of the peristaltic contractions, the digested
matter which it contains is absorbed, that is, passes away
from the interior of the intestine into the blood-vessels and
288 ELEMENTARY PHYSIOLOGY less.
lacteals lying in the intestinal walls. So that, by the time
the contents of the intestine have reached the ileo-caecal
valve, a great deal of the nutritious matter has been re-
moved. Still, even in the large intestine, some nutritious
matter has still to be acted upon ; and we find that, in the
caecum and commencement of the large intestine, changes
are taking place, apparently somewhat of the nature of fer-
mentation, whereby the contents become acid. These
changes are largely the result of the activity of certain
minute organisms or organised ferments (bacteria, etc.).
One marked feature of the changes undergone in the
large intestine is the rapid absorption of water. Whereas,
in the small intestine, the amount of fluid secreted into the
canal about equals that which is removed by absorption, so
that the contents at the ileo-caecal valve are about as fluid
as they are in the duodenum ; in the large intestine, on the
contrary, especially in its later portions, the contents become
less and less fluid. At the same time a characteristic odour
and colour are developed, and the remains of the food, now
consisting either of undigestible material, or of material which
has escaped the action of the several digestive juices, or with-
stood their influence, gradually assume the characters of
faeces.
22. Absorption from the Intestines. — A great deal of
the absorption takes place in the small intestine (though the
process is continued in the large intestine) , and there can
be no doubt that it is largely effected by means of the villi.
Each villus, as we have seen (p. 280), is covered by a layer
of epithelium, and contains in the centre a lacteal radicle,
between which and the epithelium lies a network of capil-
lary blood-vessels imbedded in a delicate tissue. Now after
a meal containing fat the epithelium cells covering the villi
are loaded with minute droplets of fat. It has long been
vil ABSORPTION FROM THE INTESTINES 289
supposed that, in some way or other not thoroughly under-
stood, the majority of the minute particles of the finely
divided, emulsified fat in the intestine passed into the cells
of the epithelium directly. It now seems more probable
that the fat is absorbed in the form of dissolved soap, in
the same manner as the peptone and sugar to be dis-
cussed below, and after entering the cells is changed back
into fat droplets. However this may be, and the manner
of the absorption is not wholly clear, the droplets later leave
the epithelial cells, and go past the capillary blood-vessels,
into the central lacteal radicle ; so that, after a fatty meal,
these lacteal radicles of the villi become filled with fat.
The lacteal radicle is continuous with the interior of the
lymphatic vessels which ramify in the walls of the intestine,
and which pass into the larger lymphatic vessels running
along the mesentery towards the thoracic duct. Into these
vessels the finely divided fat passes from the lacteal radicle
of the villus, and, mixing with the ordinary lymph contained
in these vessels, gives their contents a white, milky appear-
ance. Lymph thus white and milky from the admixture of
a large quantity of finely divided fat is called chyle ; and
this white chyle may after a meal be traced along the lym-
phatics of the mesentery to the thoracic duct, and along the
whole course of that vessel to its junction with the venous
system. After a meal, in fact, this vessel is continually pour-
ing into the blood a large quantity of chyle, i.e. of lymph
made white and milky by the admixture of fats drawn from
the villi of the small intestine.
In the case of the proteids and carbohydrates, the result
of digestion has been to produce a solution of peptones and
sugars, which are extremely soluble and highly diffusible.
Now we know that if such a solution is separated by a thin
membrane from a solution of ordinary non- diffusible pro-
it
29o ELEMENTARY PHYSIOLOGY less,
teids, there will be a rapid transmission of the diffusible
substances through and across, the membrane. The con-
ditions necessary for such a process are evidently present
in the intestines, where the solution in its interior is sepa-
rated by what is practically a thin membrane from the
(albuminous) blood in the capillaries just below the epithe-
lial cells. It is thus very tempting to suppose that the ab-
sorption of peptones and sugars (also of salts) is the result
of their diffusibility and of the conditions to which they are
exposed. And indeed within certain limits this view is cor-
rect. But it does not by any means explain the whole pro-
cess. For if substances of differing diffusibilities be placed
in the intestines it is not found that the most diffusible sub-
stance is necessarily absorbed the fastest. In fact we find
that the details of the absorption are in many ways so pecul-
iar that we must look to the living epithelial cells of the
villi as determining and completely controlling the process,
which is thus partly physical but chiefly due to the special
activity of cells.
The fats pass, as already stated, into the lacteals and
thence through the lymphatic vessels and thoracic duct into
the blood. Peptones and sugar, on the other hand, appear
to be taken up by the capillary blood-vessels of the villus,
so that very little if any of them gets to the lacteal radicle.
From the capillaries of the villi the peptones and sugar are
then carried along the portal vein to the liver, where they
probably undergo some further change. So that while the
fat, though it gets for the most part into the general blood
current by a roundabout way, viz., by the lymphatics,
reaches the blood, as far as we know, very little changed ;
the peptones and sugars on the other hand, though -also
taking a roundabout course, viz., by the liver, are probably
altered before they are thrown into the general blood
vii SOME ASPECTS OF NUTRITION 291
stream ; for the portal blood in which they are carried is
acted upon by the liver before it flows through the hepatic
vein into the general venous system. But concerning both
the process of absorption itself and the changes undergone
by the absorbed products before they reach the heart, ready
to be distributed all over the body, we have probably much
yet to learn.
Part II. — Food and Nutrition
1. Some Aspects of Nutrition. — The digestive changes
which mixed food undergoes in the alimentary canal pre-
pares the food-stuffs, of which food is composed, for distri-
bution to the various tissues of the body. Entering the
tissues, the food-stuffs provide, by the oxidational changes
which they undergo, the energy which the body expends as
heat and mechanical work, and at the same time they make
good the waste of substance which results from previous
oxidation. While being in this way metabolised,1 or worked
through the tissues, the food-stuffs may give rise incidentally
to changes in the composition of any individual tissue or of
a group of tissues, and hence of the body as a whole. These
latter changes depend partly upon the total amount of mixed
food supplied to the body, but more particularly upon the
relative amounts of each kind of food-stuff which is present
in and goes to make up that variable total amount of food.
Thus, we have the phenomena of starvation as an extreme
instance of the effect of variation in the amount of food
given; and the special storage of glycogen (p. 243) and of
fat are obvious instances of the effects of individual food-
stuffs. The consideration of the possibilities thus indicated
provides some of the problems of nutrition. But nutrition
1 See note on p. 213.
292 ELEMENTARY PHYSIOLOGY less.
has also to deal with the quantitative relationships between
the amount of food supplied and the amount of waste ex-
creted ; to strike a balance between the two ; and to draw
conclusions from the balance-sheet as to how the business
of the body is being carried on. Further, since food not
only repairs waste but also provides energy, the balance-
sheet must take into account how much total energy is sup-
plied in the food and how this available income is expended
as heat and work.
2. Some Statistics of Nutrition. — The average weight
of a healthy full-grown man may be taken as 70 kilogrammes
(154 pounds). Such a body is made up, in round numbers,
as follows : —
Muscles and Tendons 41 per cent.
Skeleton 16 "
Skin 7 "
Fat 18
Brain 2 "
Thoracic viscera 2 "
Abdominal viscera 7 "
Blood1 7 "
100 "
The waste of water and other matter which this body
excretes daily and their distribution among the chief excre-
tory organs are shown in the table on the following page.
The " other matter " from the lungs is chiefly carbonic
acid, in which the larger part of the carbon is excreted,
bringing with it nearly all the oxygen originally taken in by
the lungs. From the kidneys the " other matter " includes
urea, which contains nearly the whole of the nitrogen
1 The total amount of blood in the body is about 5 litres, or more than a
gallon, and may be taken as being usually about fa or fc of the weight of the
body.
SOME STATISTICS OF NUTRITION
293
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294 ELEMENTARY PHYSIOLOGY less.
excreted, together with some 25 grammes (nearly 1 oz.) of
inorganic salts. From the skin the " other matter " is a
small amount of salts and some carbonic acid, and in the
faeces it includes some 5 grammes of salts. The total out-
put of salts from the body is thus about 30 grammes (or
rather more than 1 oz.).
This daily loss has to be made good by the new food
supplied. But in calculating the amount of material neces-
sary to replace the waste, we need only turn our attention
to the nitrogen and the carbon ; for the water lost represents
almost entirely water taken as drink or in the food, the oxy-
gen is that which is derived from the air, and the salts are
largely, though not entirely, introduced as salts with the food.
The daily waste of nitrogen and carbon may be taken in
round numbers as about 20 grammes (300 grains) of the
former and 270 grammes (or about g\ oz.) of the latter.
The nitrogen necessary to make good this loss can be
obtained only from proteids. The carbon may come from
proteids, carbohydrates, or fats, but most advantageously, as
we shall see, from a mixture of all three.
The necessity of constantly renewing the supply of proteid
matter arises from the circumstance that the body is unable
to employ for the renewal of its proteids nitrogen in any
other form than proteid itself. If proteid matter be not
supplied, the body must needs waste, because then there is
nothing in the food competent to make good the nitro-
genous loss. On the other hand, if proteid matter be sup-
plied, there can be no absolute necessity for any other but
the mineral food-stuffs, because proteid matter contains
carbon and hydrogen in abundance, and hence is competent
to make good not only the breaking down which is indicated
by the nitrogenous loss, but also that which is indicated by
the other great products of waste, carbonic acid and water.
vn SOME STATISTICS OF NUTRITION 295
It has been found advantageous, however, to balance
the total waste in some such way as is shown in the following
table of daily income.
TABLE SHOWING THE AVERAGE DAILY INCOME OF THE HUMAN
BODY
Nitrogen. Carbon.
Proteids 130 grammes (45 oz.) contain 20 grammes (J oz.) 70 grammes (25 oz.)
Fats
Carbo-
50
rammes
(2 oz.)
(14 oz.)
(1 oz.)
(5 pints)
-
40 (i|oz.)
160 " (55 oz.)
/400
640
3,750 g
20
grammes
270 grammes (9! oz.)
Salts
Water
Oxygen
Total
3. Diet. — Foods, as previously explained (p. 250), never
consist, except perhaps in the case of fats and oils, of one
kind of food-stuff only ; each article of food contains at most
an excess of some one kind of food-stuff, and no two foods
are exactly alike. Hence the selection of such foods as will
supply the amount of proteids, fats, and carbohydrates re-
quired by the above statement opens up the possibility of an
almost indefinite choice.
Suppose that we select lean meat, bread, potatoes, milk,
and fat, such as butter or dripping. From the amounts of
each food shown in the table on the following page, we may
obtain all that we require.
The amounts of the several foods shown in this table
suffice to cover the waste shown in the table on p. 293 and
constitute what is ordinarily known as a diet. But the data
thus given are to be taken rather as an illustration of how
the balance-sheet between food and waste is drawn up, than
as an example of exactly what a man ought to eat in the
way of food. As already pointed out, foods are many, and
vary in the relative amounts of the several food-stuffs they
contain, so that it is possible to draw up many such tables,
296
ELEMENTARY PHYSIOLOGY
8
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vii THE ECONOMY OF A MIXED DIET 297
all satisfying the condition of covering the daily loss of 20
grammes of nitrogen and about 270 or 300 grammes of
carbon.
In drawing up such a table of diet the question of cost
must also not be forgotten. Thus, for instance, it costs
more to obtain the requisite amount of carbon from fat
than from sugar or starch. Moreover, the value of a diet
depends also on the ease with which its constituents can be
digested and utilised. Mere chemical analysis is by itself
a very insufficient guide as to the usefulness and nutritive
value of an article of food. A substance to be nutritious
must not only contain some or other of the various food-
stuffs, but contain them in an available, that is a digestible,
form. A piece of beef-steak is far more nourishing than a
quantity of pease pudding containing even a larger propor-
tion of proteid material, because the former is far more
digestible than the latter; and a small piece of dry hard
cheese, though of high nutritive value as judged by mere
chemical analysis, will not satisfy the more subtle criticism
of the stomach.
4. The Economy of a Mixed Diet. — The body, as we
have pointed out, cannot obtain the nitrogen it requires
from any source other than the ready-made proteids.
Hence in the absence of these from the food of an animal
it must sooner or later die from what is known as nitrogen
starvation.
In this case, and still more in that of an animal deprived
of food altogether, the organism, so long as it continues to
live, feeds upon itself. In the former case, all the processes
involving a loss of nitrogen, in the latter, all the processes
leading to the appearance of all the several waste products,
are necessarily carried on at the expense of its own body ;
whence it has been rightly enough observed that a starving
298 ELEMENTARY PHYSIOLOGY less.
sheep is as much a carnivore as a lion. Protcid is thus the
essential element of all food, and since it contains carbon as
well as nitrogen it may suffice, under certain circumstances,
to maintain the body ; but it is a very disadvantageous and
uneconomical food-stuff when taken by itself.
Albumin, which may be taken as a type of the proteids,
contains about 53 parts of carbon and 15 of nitrogen in 100
parts. If a man were to be fed on white of egg, therefore,
he would take in, speaking roughly, 31 parts of carbon for
every part of nitrogen.
But we have seen that a healthy, full-grown man, keeping
up his weight and heat, and taking a fair amount of exer-
cise, eliminates per diem 270 to 300 grammes of carbon to
only 20 grammes of nitrogen, or, roughly, only needs one-
thirteenth to one-fifteenth as much nitrogen as carbon. If
he is to get his 270 grammes of carbon out of albumin, he
must eat 500 grammes of that substance. But 500 grammes
of albumin contain 75 grammes of nitrogen, or nearly four
times as much as he wants.
To put the case in another way, it takes about four
pounds (1,800 grammes) of fatless meat (which generally
contains about one-fourth its weight of dry solid proteids) to
yield the necessary amount of carbon, whereas one pound
(453 grammes) will furnish all the nitrogen that is required.
Thus, a man confined to a purely proteid diet must eat
an excessive quantity of it in order to obtain the amount of
carbon he requires. This not only involves a great amount
of physiological labour in comminuting the food, and a
great expenditure of power and time in dissolving and
absorbing it, but throws a great quantity of wholly profitless
labour upon those excretory organs that have to get rid
of the nitrogenous matter, three- fourths of which, as we have
seen, is superfluous.
vii THE ECONOMY OF A MIXED DIET 299
Unproductive labour is as much to be avoided in physio-
logical as in political economy ; and it is quite possible that
an animal fed with perfectly nutritious proteid matter should
die of starvation ; the loss of power in the various operations
required for its assimilation overbalancing the gain ; or the
time occupied in their performance being too great to per-
mit waste to be repaired with sufficient rapidity. The body,
under these circumstances, falls into the condition of a
merchant who has abundant assets, but who cannot get in
his debts in time to meet his creditors.
These considerations lead us to the physiological justifi-
cation of the universal practice of mankind in adopting a
mixed diet, in which proteids are mixed either with fats or
with carbohydrates, or with both.
Fats may be taken to contain about 80 per cent, of
carbon, and carbohydrates about 40 per cent. Now it has
been seen that there is enough nitrogen to supply the
waste of that substance per diem, in a healthy man, in
453 grammes (a pound) of fatless meat, which also contains
67 grammes (1,000 grains) of carbon, leaving a deficit of
200 grammes (3,000 grains) of carbon ; 250 grammes (say
half a pound) of fat, or 500 grammes (rather more than a
pound) of sugar, will supply this quantity of carbon.
Several apparently simple articles of food constitute a
mixed diet in themselves. Thus, butcher's meat commonly
contains from 30 to 50 per cent, of fat. Bread, on the other
hand, contains the proteid gluten, and the carbohydrates,
starch and sugar, with minute quantities of fat. But, from
the proportion in which these proteid and other constituents
exist in these substances, they are neither, taken alone,
such physiologically economical foods as they are when com-
bined in the proportion of about 200 to 75, or two pounds
of bread to three-quarters of a pound of meat per diem.
300 ELEMENTARY PHYSIOLOG\ less.
There is one largely consumed article of food which is
not merely composed of all the various food-stuffs requisite
to provide a mixed diet, but contains these substances in the
relative amounts very suitable for affording an economical
diet as regards the proportion of the nitrogen to the carbon.
This food is milk. Milk consists chiefly of water (86 p. c.)
in which proteids, casein, and some albumin are dissolved, as
also a carbohydrate, milk-sugar or lactose, and inorganic salts,
such as chlorides and phosphates of sodium, potassium, and
calcium. The fat present in milk is emulsified or suspended
in the water in the form of extremely minute globules, and
the white appearance presented by milk is due to the great
amount of light reflected from these minute particles of fat.
5. The Effects of the Several Food-stuffs. — When
proteid food is given to an animal, such as a dog, which
has been fasting, the larger part of the nitrogen given in
the proteid is not retained in the body but is excreted
almost immediately. If another larger meal of proteid be
given, the amount of nitrogen excreted is still further in-
creased, less and less being retained in the body. By pro-
ceeding in this way it is possible to increase the excretion
of nitrogen to such an extent that it ultimately becomes
equal to the amount administered in the food : the animal
is then said to be in " nitrogenous equilibrium." Such are
the facts, and their meaning is obvious. The proteid food
stirs up the nitrogenous metabolism of the body and stimu-
lates it to an increased activity. But this effect is not con-
fined to the nitrogenous metabolism alone, for if the output
of carbonic acid and the corresponding intake of oxygen
be measured during the above experiment, they are both
found to be considerably above the average. Hence pro-
teid food also stimulates the non-nitrogenous metabolism of
the body and thus leads to an increased waste of all kinds.
vii THE EFFECTS OF THE SEVERAL FOOD-STUFFS 301
This fact is, indeed, made use of for the treatment of obes-
ity by dieting, as in the Banting " cure," in which the carbo-
hydrates and fat in the food are reduced as far as possible
and large amounts of proteid are given.
Because of their lack of nitrogen the effects of carbohy-
drates and fats as foods cannot be studied by feeding an
animal with these alone, as is possible with proteids. But
this difficulty may be got over by administering a small,
fixed quantity of proteid with a variable amount of either
carbohydrate or fat. In this case it is found that an in-
crease of the carbohydrate in food very soon leads to the
laying on of fat ; and this corresponds to the everyday
experience which is frequently embodied in the expression
" sugar is fattening." At the same time analysis of the liver
shows that a large amount of glycogen is stored up in it, as
previously explained (p. 243).
If fats be given in increasing quantity they also finally
lead to a laying on of fat, but by no means so readily as
does an increase of carbohydrates. At the same time, no
storage of glycogen is observed in the liver. Fats are there-
fore not as fattening as might at first sight have been ex-
pected.
The salts which leave the body are largely the salts which
were introduced in the food. It might therefore at first
sight appear that they are merely unavoidable constituents
of food which are largely passed without change through the
body. But this is not the case. In some way or other the
salts of food play an essential part in directing the metab-
olism taking place in the tissues. Thus animals fed with
an abundance of food, which has, however, been freed as
far as possible from salts, soon die with symptoms of de-
fective nutrition, accompanied by paralysis and convulsions.
When an animal is deprived of all food whatsoever, it
^02 ELEMENTARY PHYSIOLOGY less.
begins to feed on its own tissues. Thus up to the day of
its death from starvation there is an output of urea and
of carbonic acid, though in amounts less than when food
is being taken. The loss of tissue-substance thus pro-
duced affects the several tissues to different extents; but
without entering into details we may simply point out that
the master-tissues suffer least, in the obvious effort to pro-
long life to the utmost. Thus the brain and spinal cord
are almost unaltered at death, and the blood and the mus-
cular tissue of the heart also lose but little as compared
with the fat and the skeletal muscles.
6. The Erroneous Division of Food-stuffs into Heat-
producers and Tissue-formers. — Food-stuffs have been
divided into heat-producers and tissue-formers — the car-
bohydrates and fats constituting the former division, the
proteids the latter. But this is a very misleading, and
indeed erroneous classification, inasmuch as it implies, on
the one hand, that the oxidation of the proteids does not
develop heat ; and, on the other, that the carbohydrates and
fats, in being oxidised, subserve only the production of heat.
Undoubtedly proteids are tissue-formers, inasmuch as no
tissue can be produced without them ; for all the tissues are
nitrogenous, some containing a large and others a small
quantity of nitrogen, and proteids are the only nitrogenous
food-stuffs ; they alone can supply the nitrogenous elements
of the tissues. But there is reason to think that the fats
and carbohydrates taken as food may also be directly
built up into the tissues.
Moreover, if the proteids alone were the tissue-formers,
then the energy set free during the contracting activity of
the preeminently nitrogenous muscles ought to come from
the metabolism of their proteids. But under most circum-
stances this is probably not the case, for muscular exercise
vii THE INCOME AND EXPENDITURE OF ENERGY 303
does not lead to any increased output of nitrogenous waste
which is in the least proportionate to the work being done.
On the other hand, exercise at once, and largely, increases
the excretion of carbonic acid, to an extent which may be
five times as great as during rest ; that is to say, the non-
nitrogenous part of the tissue seems to be used up more
quickly than the nitrogenous part ; and the consumption
of this particular constituent of the muscular substance may
be made good by non-nitrogenous food, by fats or carbo-
hydrates.
On the other hand, proteids must be regarded as heat-
producers also. For though in some tissues, as in muscles,
the non-nitrogenous part seems to be most rapidly changed,
yet the nitrogenous part, supplied by the proteids, is sooner
or later oxidised, and in being oxidised must give rise to
heat.
As soon as the elements of the food, in fact, get into the
tissues, the distinction between the two classes is lost ; both
form tissues, and both supply heat.
If it is worth while to make a special classification of the
food-stuffs at all, it appears desirable to distinguish the essential
food-stuffs, or proteids, from the accessory food-stuffs, or fats
and carbohydrates — the former alone being, in the nature
of things, necessary to life, while the latter, however impor-
tant, are not absolutely necessary.
7. The Income and Expenditure of Energy. — It is
quite certain that nine-tenths of the dry, solid food which
is taken into the body sooner or later leaves it in the shape
of carbonic acid, water, and urea ; and it is also certain not
only that the compounds which leave the body are more
highly oxidised than those which enter it, but that all the
oxygen taken into the blood by the lungs is carried away
out of the body in the various waste products.
J04 ELEMENTARY PHYSIOLOGY less.
The intermediate stages of this conversion are, however,
by no means so clear. It is highly probable that practically
all the food-stuffs which pass from the alimentary canal into
the blood, be they proteids, or fats, or carbohydrates, are
absorbed by some tissue or other (muscle, nervous tissue,
glandular tissue, and the like), before they are oxidised;
that, indeed, it is within some tissue or other that they
suffer oxidation, and that the amount of oxidation going on
in the blood is very small.
In the course of its oxidation, the food not only supplies
the energy which the body expends in doing work, but also
the energy which, as we have seen, the body loses as heat.
The oxidation of the elements of the food is indeed the
ultimate source of the heat of our bodies, all other causes
being of little moment. About this there can be no doubt,
and it is a further fact that the oxidation which thus gives
rise to heat is not the oxidation of the elements of the food
as they are carried about in the blood, but the oxidation of
the tissues, more especially the muscles, into which the
food-stuffs have been built up, and of which they have
become an integral part. The same may be said regard-
ing the source of the energy expended in muscular
work.
The amount of mechanical work a man does may be
determined with no great difficulty, whether we calculate it
as work done in walking or in turning some machine or in
some other effort which results in overcoming a resistance.
This work is measured in terms of the resistance overcome,
or weight lifted, multiplied by the height through which it is
raised. Thus, we speak of the work done in lifting one
pound through the height of one foot as a foot-pound ; or,
using the metric system and taking a kilogramme (2.2 lbs.)
and a metre (39.37 inches) as the units of weight and
yn THE INCOME AND EXPENDITURE OF ENERGY 305
distance, we call the unit of work a kilogramme-metre,
equal to 7.23 foot-pounds. Using the latter unit we may
say that a good day's work is about 150,000 kilogramme-
metres.
The unit of heat is the amount of heat required to raise
the temperature of one pound of water through i° F. Now,
as is seen in all ordinary engines, heat can do work ; and it
is found that one unit of heat can do 778 foot-pounds of
work. This is called the " mechanical equivalent of heat."
In the metric system the unit becomes the amount of heat
required to raise the temperature of one gramme (15.4
grains) of water through i° C. This is called a calorie, and
the mechanical equivalent of heat is 427 gramme-metres.
Using these data we can readily convert heat into its equiva-
lent of work, or vice versa.
The measurement of the amount of heat given off by the
body is by no means easy, and the sources of error are con-
siderable. But, allowing for these, a rough determination
may be made ; the heat thus measured may be calculated
as work by using the mechanical equivalent of heat, and
the result may be added to the actual work done as work.
The outcome of this calculation shows that of the total
energy expended by the body about one-sixth is put out as
work and five-sixths as heat. Finally, we find that the aver-
age total output of energy as work and heat (calculated as
work) may be taken as about 1,000,000 kilogramme-metres
daily.
We may now consider how far this expenditure is met by
the income of energy in food. When a substance is com-
pletely burnt, i.e. oxidised, to water and carbonic acid,
a certain amount of heat is produced, which can be meas-
ured. Thus, it is possible to determine how much heat is
produced by the complete combustion of one gramme of
306 ELEMENTARY PHYSIOLOGY less, vii
each of the food-stuffs, proteids, fats, and carbohydrates.
The result obtained is as follows : —
i gramme of proteid gives 5,700 calories.
I " " fat " 9,500 "
I " " carbohydrate " 4,000 "
Now this must also be the amount of heat produced by the
same quantity of each of these food-stuffs during their com-
plete oxidation in the animal body. In the case of the pro-
teid some deduction has to be made because the proteids
are not completely oxidised ; the nitrogen they contain
leaves the body as urea, which is still capable of under-
going further oxidation to water, nitrogen and carbonic
acid. One gramme of proteid gives rise to about ^ gramme
of urea, and the complete combustion of this amount of
urea gives rise to 844 calories. Hence, deducting these
from the 5,700 gives us about 4,800 calories, which we may
take as being the physiologically available heat of combus-
tion of one gramme of proteid. If we apply these values
to the diet given on p. 296 we find that : —
130 grammes of proteid give 624,000 calories.
50 " " fats " 475,000 "
400 " " carbohydrate " 1,600,000 "
2,699,000
If now we take the mechanical equivalent of this heat we
find it works out as 1,152,473 kilogramme-metres. Hence
the energy available by the oxidation in the body of this
particular diet is more than sufficient to balance the total
amount which we saw was expended.
LESSON VIII
MOTION AND LOCOMOTION
1. The Source of Active Power and the Organs of
Motion. — In the preceding Lessons the manner in which
the incomings of the human body are converted into its
outgoings has been explained. It has been seen that new
matter, in the form of organic and mineral food, is con-
stantly appropriated by the body, to make up for the loss
of old matter, which is as constantly going on in the shape,
chiefly, of carbonic acid, urea, and water, the formation
of this waste being the outcome of oxidation accompanied
by a liberation of energy.
The organic foods are derived directly, or indirectly, from
the vegetable world : and the products of waste either are
such compounds as abound in the mineral world, or im-
mediately decompose into them. Consequently, the human
body is the centre of a stream of matter which sets inces-
santly from the vegetable and mineral worlds into the min-
eral world again. It may be compared to an eddy in a
river, which may retain its shape for an indefinite length
of time, though no one particle of the water of the stream
remains in it for more than a brief period.
But there is this peculiarity about the human eddy, that
a large portion of the particles of matter which flow into
it have a much more complex composition than the parti-
cles which flow out of it. To speak in what is not altogether
3°7
3o8
ELEMENTARY PHYSIOLOGY
LESS.
a metaphor, the atoms enter the body, for the most part,
piled up into large heaps, and tumble down into small heaps
before they leave it. The energy which they set free in
this tumbling down, is the source of the active powers of
the organism.
These active powers are chiefly manifested in the form
of motion — movement, that is, either of part of the body,
or of the body as a whole, which last is termed locomotion.
The organs which produce total or partial movements
of the human body are of three kinds : cells exhibiting
amoeboid movejnents, cilia, and
muscles.
The amoeboid movements of
the white corpuscles of the blood
have been already described (p.
126), and it is probable that
similar movements are performed
by many other simple cells of the
body in various regions.
The amount of movement to
which each cell is thus capable
of giving rise may appear per-
fectly insignificant ; nevertheless, there are reasons for think-
ing that these amoeboid movements are of great importance
to the economy, and may under certain circumstances be
followed by very notable consequences.
2. Ciliated Epithelium and the Action of Cilia. — Cilia
are filaments of extremely small size, attached by their
bases to, and indeed growing out from, the free surfaces
of certain epithelial cells ; there being in most instances
very many (thirty for instance), but, in some cases, only
a few cilia on each cell (Figs. 49, 90). In some of the
lower animals, cells may be found possessing only a single
Fig. 90. — Columnar Ciliated
Epithelium Cells from the
Human Nasal Membrane.
Magnified 300 diameters.
(Sharpey.)
vin CILIATED EPITHELIUM AND ACTION OF CILIA 309
cilium. Cilia are in incessant waving motion, so long as
life persists in them. Their most common form of move-
ment is that "in which each cilium is suddenly bent upon
itself, becomes sickle-shaped instead of straight, and then
more slowly straightens again, both movements, however,
being extremely rapid and repeated about ten times or more
every second. These two movements are of course an-
tagonistic ; the bending drives the water or fluid in which
the cilium is placed in one direction, while the straightening
drives it back again. Inasmuch, however, as the bending
is much more rapid than the straightening, the force ex-
pended on the water in the former movement is greater
than in the latter. The total effect of the double move-
ment, therefore, is to drive the fluid in the direction towards
which the cilium is bent : that is, of course, if the cell on
which the cilia are placed is fixed. If the cell be floating
free, the effect is to drive or row the cell backwards ; for
the cilia may continue their movements even for some
time after the epithelial cell, with which they are connected,
is detached from the body. And not only do the move-
ments of the cilia thus go on independently of the rest of
the body, but they appear not to be controlled by the
action of the nervous system. Each cilium is comparable
to one of the mobile processes of a white corpuscle. A
ciliated cell differs from an amoeboid cell in that its con-
tractile processes are permanent, have a definite shape, and
are localised in a particular part of the cell, and that the
movements of the processes are performed rhythmically
and always in the same way. But the exact manner in
which the movement of a cilium is brought about is not
as yet thoroughly understood.
Although no other part of the body has any control over
the cilia, and though, so far as we know, they have no
310 ELEMENTARY PHYSIOLOGY, less.
direct communication with one another, yet their action
is directed towards a common end — the cilia, which cover
extensive surfaces, all working in such a manner as to sweep
whatever lies upon that surface in one and the same direc-
tion. Thus, the cilia which are developed upon the epithelial
cells which line the greater part of the nasal cavities and
the trachea, with its ramifications, tend to drive the mucus
in which they work, outwards.
In addition to the air-passages, cilia are found, in the
human body, in a few other localities ; but the part which
they play in man is insignificant in comparison with their
function in the lower animals, among many of which they
become the chief organs of locomotion.
3. The Structure of Unstriated Muscle. — It is custom-
ary to distinguish three varieties of muscle, unstriated, cardiac,
and striated, which differ from one another in structure and
in some respects in mode of action. Cardiac muscle, which
occurs in the heart only, has been already described (p. 74).
Fig. 91. — A Fibre-cell from the Plain, Non-striated Muscular Coat of
the Intestine.
f, fibre; u, nucleus; /, granular protoplasm around the nucleus.
Unstriated (also called " plain " or " smooth ") muscle
occurs in the walls of the alimentary canal, the blood-vessels,
the bladder, and other organs. It is composed of bundles
of fibres, which are bound together by connective tissue
carrying nerves and blood-vessels. The fibres are in reality
elongated, spindle-shaped cells whose length is about 100/x
(yto mcn) an(l width 6/x (-40V0- inch). Somewhere towards
the middle of each cell there is an elongated oval or some-
vin THE STRUCTURE OF STRIATED MUSCLE 311
times rod-shaped nucleus, surrounded by a small amount
of granular protoplasm.
The substance of the cell is clear and shows no transverse
striations, although it often shows signs of a very fine longi-
tudinal fibrillation. Each cell is said to be surrounded by
an extremely delicate sheath, but, as to this, opinions differ.
A number of such fibre-cells are united together by a mi-
nute quantity of cement, or intercellular substance, into a
thin fiat band, and a number of such bands are bound
together by connective tissue into larger bands or bundles.
Each fibre is capable of contracting, that is, of shortening
and becoming at the same time thicker.
4. The Structure of Striated Muscle. — Striated muscle
is also made up of fibres, though the fibres are very differ-
ent from the fibres or fibre-cells of unstriated muscle, and
these fibres are again similarly bound up together in various
ways by connective tissue, which carries the blood-vessels and
nerves, so as to form muscles of various shapes and sizes.1
Each muscle is thus made up of (i) an external wrapping
or perimysium; this is a sheath of connective tissue from
the inner face of which partitions proceed and divide the
space which it incloses into a great number of longitudi-
nally disposed compartments ; (ii) the muscular fibres which
occupy these compartments ; (iii) the vessels which lie in
the sheath and in the partitions between the compartments,
and thus surround the muscular fibres without entering them ;
(iv) the nerves which also at first lie in the sheath and in
the partitions between the compartments, but which eventu-
ally enter into the muscular fibres.
1 It is necessary to distinguish "muscle" as an organ from "muscle"
as a tissue. The biceps muscle (p. 326), for example, is an organ of a
complicated character, of which muscular tissue forms only the chief con-
stituent.
312 ELEMENTARY PHYSIOLOGY less
The perimysium forms a complete envelope around the
muscle, which, when it is sufficiently strong to be dissected
off, is known as a fascia ; at each end it usually terminates
in dense connective tissue (tendon), which becomes con-
tinuous with the bone or cartilage to which the tendon is
attached. The partitions given off from the inner surface
of the perimysium form at first coarse compartments, inclos-
ing large bundles of fasciculi (Fig. 92), each consisting of
a very great number of fibres. These large bundles are
again divided by somewhat finer connective-tissue parti-
tions into smaller bundles, and these again into still smaller
Fig. 92. — Fasciculi of Striated Muscle cut across.
Several fasciculi, f, bound together into large fasciculi to make up the muscle.
ones, and so on, the smallest bundles of all being composed
of a number of individual muscular fibres. In this way the
partitions become thinner and more delicate, until those
which separate the chambers in which the individual mus-
cular fibres are contained are reduced to little more than
as much connective tissue as will hold the small nerves,
arteries, veins, and capillary network together. As the
perimysium consists of connective tissue, it may be destroyed
by prolonged boiling in water. In fact, in " meat boiled to
rags " we have muscles which have been thus treated : the
THE STRUCTURE OF STRIATED MUSCLE
3*3
perimysial case is broken up, and the muscular fibres, but
little attacked by boiling water, are readily separated from
one another.
If a piece of muscle of a rabbit which has been thus
boiled for many hours is placed in a watch-glass with a
little water, the muscular fibres may be easily teased out
with needles and isolated. Such a fibre will be found to
have a thickness of somewhere about 6o/a (4-^0 inch) (they
Fig. 93. — Capillaries of Striated Muscle.
A. Seen longitudinally. The width of the meshes corresponds to that of a muscle
fibre, a, small artery; i, small vein.
B. Transverse section of striated muscle, a, the cut ends of the fibres; 5, capil-
laries filled with injection material; c, parts where the capillaries are absent or not
filled.
vary, however, a great deal), with a length of 30 or 40 milli-
metres, i.e. about 1^- inch. It is a cylindroidal or polygonal
solid rod, which either tapers or is bevelled off at each end.
By these it adheres to those on each side of it ; or, if it lies
at the end of a series, to the tendon.
The structure and properties of striated muscular tissue
314
ELEMENTARY PHYSIOLOGY
in the histological sense mean the structure and properties
of these fibres.
As we have already had occasion to remark, all tissues
undergo considerable alteration in passing from the living to
the dead state, but, in the case of muscle, the changes which
the tissue undergoes in dying are of such a marked charac-
ter that the structure of the dead tissue gives a false notion
of that of the living tissue.
«-
B
i am 4£V«5£E
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;,— ■-■■ U (,■. ,; •
Fig. 94. — To illustrate the Structure of a Striated Muscle Fibre.
A. Part of a muscle fibre (of a frog) seen in a natural condition, d, dim bands;
b, bright bands, with the granular line seen in many of them ; «, nuclei and the granu-
lar protoplasm belonging to them, very dimly seen.
B. Portion of prepared mammalian muscle fibre teased out, showing longitudi-
nal portions of variable (1, 2, 3, 4) thickness; 4 represents the finest portion (fibrilla)
which could be obtained; d, dim bancis; b, bright bands, in the midst of each of
which is seen the granular line g.
A living striated muscle fibre of a frog or a mammal is a
pale transparent rod composed of a soft, flexible, elastic sub-
stance, the lateral contours of which, when the fibre is viewed
out of the body, appear sharply defined, like those of a glass
rod of the same size ; but when the fibre is observed in the liv-
ing body, bathed in the lymph which surrounds it, the out-
vin THE STRUCTURE OF STRIATED MUSCLE 315
lines are not so sharply defined. In neither case can any
distinct line of demarcation between a superficial layer and
a deeper substance be recognised. The fibre appears trans-
versely striped, as if the clear glassy substance were, at
regular intervals (Fig. 94, A, d), converted into ground
glass, thus appearing dimmer. Each of these " dim bands "
is about 2fx wide, and the clear space or " bright band "
which separates every two dim bands is of about the same
size, or under ordinary circumstances somewhat narrower.
With a high power a very thin dark granular line equidistant
from each dim band is discernible in each bright band,
dividing the bright band into two. As these appearances
remain when the objective is focussed through the whole
thickness of the fibre, it follows that the dim bands, the
granular lines, and the clear spaces on each side of each
granular line, represent the edges of segments of different
optical characters, which regularly alternate through the
whole length of the fibre. Let the excessively thin seg-
ments, of which the thin granular lines represent the edges,
be called g, the thicker, pellucid segments, of which the
bright bands on each side of a granular line represent the
edges, b ; and the thickest, slightly opaque segments, of
which the ground-glass-like dim bands are the edges, d.
Then the structure of the fibre may be represented by d. b.
g. b. d. b. g. b., indefinitely repeated, and one inch of length
of fibre will contain about 30,000 such segments, or alterna-
tions of structure.
In a perfectly unaltered living fibre the striated substance
presents hardly any sign of longitudinal striation ; but near
to the surface of the fibre in mammalian muscle, though at
various points in the depth of the fibre in the muscles of
the frog, faint indications are to be observed of the existence
of nuclej, each surrounded by a small amount of granular
protoplasm (Fig. 94, A, ;/).
316
ELEMENTARY PHYSIOLOGY
If the muscle fibre be preserved and studied by the ac-
cepted histological methods, the nuclei can be made much
more conspicuous ; and, moreover, parallel, longitudinal
striae appear in greater or less numbers, until sometimes
the striated substance seems broken up into fine delicate
m
fibrils, each of which presents the same segmentation as the
whole fibre (Fig. 94, B, 4). Transverse sections of mus-
cular fibre present minute close-set circu-
lar dots, which appear to represent the
transverse sections of naturally existing
longitudinal fibrils. The most reasonable
interpretation of these facts is that the
fibre is really made up of fibrils, and that
these are invisible in the living muscle on
account of their having the. same refrac-
tive power as the interfibrillar substance.
By proper treatment of the fibre there
may be demonstrated a thin membrane
of glassy transparency, the sarcolemma,
which ensheathes the striated and fibril-
lated substance (Fig. 95, s).
These are the most important struc-
tural appearances presented by ordinary
striated muscle. But it may be noticed
further that the dim bands exert a power-
ful influence on polarised light. Hence when a piece of
muscle is placed in the field of a polarising microscope and
the prisms are crossed so that the field is dark, these bands
appear bright. The granular lines have a similar but very
much less marked effect.
In the embryo the place of the adult tissue is occupied
by a mass of closely applied, undifferentiated, nucleated
cells. As development proceeds, some of these cells are
Fig. 95. —A Mus-
cular Fibre (of
Frog) ending in
Tendon.
The striated mus-
cular substance, m,
has shrunk from the
sarcolemma, s, the
fibrils of the tendon,
/, being attached to
the latter.
vin THE STRUCTURE OF STRIATED MUSCLE 317
converted into the tissues of the perimysium, but others,
increasing largely in size, gradually elongate and take on the
form of more or less spindle-shaped rods or fibres. Mean-
while the nucleus of each cell repeatedly divides, and thus
each rod becomes provided with many nuclei, so that each
fibre is really a multi-nucleate cell. Along with these
changes the protoplasmic substance of the original cell
becomes, for the most part, converted into the character-
istically striated muscle substance, only a little remaining
unaltered around each nucleus.
The many-nucleated cell thus changed into a muscle
fibre is nourished by the fluid exuded from the adjacent
capillaries, and it may be said to respire, inasmuch as its
substance undergoes slow oxidation at the expense of the
oxygen contained in that fluid, and gives off carbonic acid.
It is, in fact, like the other elements of the tissues, an
organism of a peculiar kind, having its life in itself, but
dependent for the permanent maintenance of that life upon
the condition of being associated with other such elementary
organisms, through the intermediation of which its tempera-
ture and its supply of nourishment are maintained.
The special property of a living muscle fibre, that which
gives it its physiological importance, is its peculiar contrac-
tility. The body of a colourless blood corpuscle, as we
have seen, is eminently contractile, inasmuch as it under-
goes incessant changes of form. But these changes take
place at all points of its surface, and have no definite rela-
tion to the diameter of the corpuscle, while the contractility
of the muscular fibre is manifested by a diminution in the
length and a corresponding increase in the thickness of the
fibre. Moreover, under ordinary circumstances, the change
of form is effected very rapidly, and only in consequence of
the application of a stimulus.
318 ELEMENTARY PHYSIOLOGY less.
When a contracting striated fibre is observed under the
microscope, all the bands become broader (across the
fibre) and shorter (along the fibre) and thus more closely
approximated. Some observers think that the clear bands
are diminished in total bulk relatively to the dim bands ;
but this is disputed by others. When the fibre relaxes
again the bands return to their previous condition.
5. The Chemistry of Muscle. — If a muscle taken per-
fectly fresh from the body be cooled down with ice in order
to keep it from undergoing change (just as was previously
done with blood, p. 134) and subjected to considerable
pressure it yields a fluid called muscle-plasma. This re-
mains fluid so long as it is kept adequately cooled, but clots
spontaneously at ordinary temperatures. The clotting takes
place in a way very similar to that already described for
blood-plasma, and results in the formation of a semi-solid
gelatinous substance, called myosin, and a small amount of
fluid, or muscle-serum. Myosin is a proteid and belongs to
the same class of proteids as do the fibrinogen and serum-
globulin of blood, namely the globulins. During the for-
mation of myosin, the fluid, which when first squeezed out
was faintly alkaline, becomes distinctly acid owing to the
formation of an organic acid called sarcolactic acid. At a
longer or shorter time after death this clotting takes place
in the body within the muscles themselves. They become
more or less opaque, and, losing their previous elasticity,
set into hard, rigid masses, which retain the form which
they possess when the clotting commences. Hence the
limbs become fixed in the position in which death found them,
and the body passes into the condition of what is termed
the '-death-stiffening," or rigor mortis. This stiffening is
also accompanied by a change in the chemical reaction of
the muscle, for, while living muscle when tested with litmus
vin THE CHEMISTRY OF MUSCLE 319
is faintly alkaline or neutral, at least when at rest, it becomes
distinctly acid as rigor mortis sets in. And it may be added
that a similar but slighter acidity is developed even in a
living muscle, when it contracts.
After the lapse of a certain time the coagulated matter
liquefies, and the muscles pass into a loose and flabby
condition, which marks the commencement of putrefaction.
It has been observed that the sooner rigor mortis sets
in, the sooner it is over ; and the later it commences, the
longer it lasts. The greater the amount of muscular exer-
tion and consequent exhaustion before death, the sooner
rigor mortis sets in.
Rigor mortis evidently presents some analogies with the
clotting of the blood. Moreover, the substance which is
formed within the fibre (myosin) is in many respects not
unlike fibrin, and is thought to come from a substance called
myosinogen, which is believed to exist in the living muscle.
Besides myosin, muscle contains other varieties of pro-
teid material, about which we at present know little ; a
variable quantity of fat ; certain inorganic saline matters,
phosphates and potash being, as is the case in the red
blood-corpuscles, in excess ; and a large number of sub-
stances existing in small quantities, and often classed
together as " extractives." Some of these extractives con-
tain nitrogen ; the most important of this class is creatin,
a crystalline body which is supposed to be the chief form in
which nitrogenous waste matter leaves the muscle on its way
to become urea.
The other class of extractives contains bodies free from
nitrogen, perhaps the most important of which are sarcolac-
tic acid and glycogen.
Most muscles are of a deep, red colour ; this is due in
part to the blood remaining in their vessels ; but only in
320 ELEMENTARY PHYSIOLOGY less
part, for each fibre' (into which no capillary enters) has
a reddish colour of its own, like a blood-corpuscle, but
fainter. And this colour is due to the fibre possessing a
small quantity of that same haemoglobin in which the blood-
corpuscles are so rich.
6. The Phenomena of Muscular Contraction. — Every
fibre in a muscle has the property, under certain conditions,
of shortening in length, while it increases correspondingly
in width, so that the volume of the fibre remains unchanged.
This property is called muscular contractility, and when-
ever, in virtue of this property, a muscle fibre contracts it
tends to bring its two ends closer together. Since a muscle
is made up of a collection of these fibres, when the fibres
contract the muscle as a whole also contracts ; it becomes
shorter and thicker, and brings its two ends closer together,
along with whatever may be fastened to those ends. By
this action the muscles lead to the motion of the parts to
which they are attached and by these motions give rise to
locomotion or other activities.
The condition which ordinarily determines the contraction
of a muscle fibre is the passage along the nerve fibre, which
is in close anatomical connection with the muscle fibre, of a
nervous impulse, i.e. of a particular change in the substance
of the nerve, which is propagated from particle to particle
along the fibre. The nerve fibre is called a motor fibre,
because by its influence on a muscle it becomes the indirect
means of producing motion (see Lesson XII.).
The phenomena of muscular contraction may be con-
veniently studied in the large muscle from the calf of a frog's
leg, which, since the frog is a " cold-blooded " animal,
retains its power of contracting for some time after it is
removed from the body. This muscle is called the
gastrocnemius, and may be dissected out so as to be
nil PHENOMENA OF MUSCULAR CONTRACTION 321
still attached to a piece of the femur near the knee, and to
the nerve, the sciatic, which supplies it. The preparation as
thus taken out of the body is known as a muscle-nerve prep-
aration (Fig. 96).
The muscle may be suspended by the femur, and a weight
be hung on the tendon at its lower end, and then the muscle
Fig. 96. — A Muscle-nerve Preparation.
m, the muscle, gastrocnemius of frog; Sp.c, lower end of spinal column; n, the
sciatic nerve, all the branches being cut away excepting that supplying the muscle;
f, the femur; cl. a clamp to hold the femur; t.a. tendon of Achilles.
may be made to contract by stimulating the sciatic nerve
(see also Lesson XII.).
When the nerve is excited by a very brief stimulus, as, for
instance, by the momentary electric current often called an
X
322 ELEMENTARY PHYSIOLOGY less.
induction shock, the following changes take place in the
muscle : —
(i) It becomes shorter and thicker, lifts the weight
attached to it and then relaxes, allowing the weight to fall
again. The shortening and relaxing take place very rapidly,
the whole process occupying rather more than j1^ of a second.
(ii) The muscle may be inclosed in a small chamber
and made to contract several times. If now we examine
the air in the chamber in which this excised muscle has been
contracting, we cannot obtain satisfactory evidence of any
escape of carbonic acid from the muscle during its con-
traction; if carbonic acid is produced it must be retained
within the muscle, presumably in the form of some simple
chemical compound. That, however, the muscle within the
body does give off carbonic acid in some form during its
contraction is wholly probable. The substance of the
muscle may at the same time have become faintly acid, as
tested by litmus paper. The acidity is due to sarcolactic
acid.
(iii) The muscle becomes slightly warmer ; this can
only be due to the fact that heat is formed during the
contraction.
(iv) The muscle undergoes certain electrical changes.
At the moment of commencing contraction the muscle
becomes like a small battery cell, and generates a current
of electricity, which can be readily detected.1
We have already more than once insisted on the fact
that all the tissues of the body are continually taking up
oxygen which they stow away in the form of some com-,
pound, since no oxygen can be extracted from them by
l See also p. 502, where the electrical changes of an active nerve, which
are essentially the same as those of a contracting muscle, are described
in greater detail.
nil THE TETANIC CONTRACTION OF MUSCLES 32:,
an air pump. Tn muscle this storage of oxygen leads to
an instability of the contractile substance of which it is
composed, so that when the appropriate stimulus is given
to it, this unstable substance undergoes a sudden decom-
position, almost explosive in its nature ; and the energy set
free during the decomposition makes itself known partly as
the work which the muscle can do in overcoming a resist-
ance and partly as heat. This decomposition is accom-
panied by an electrical disturbance and the appearance of
the products of decomposition.
7. The Tetanic Contraction of Muscles. — When experi-
menting with a muscle-nerve preparation, as in the preced-
ing section, it is easy to stimulate the nerve twice in such
rapid succession that the second stimulus is given while the
muscle is in a state of contraction resulting from the first.
In this case the muscle responds to the second stimulus as
well as to the first ; in other words, while already contract-
ing, it contracts still more. The second contraction is rather
less in amount than the first, and is added to the first. If
a rapidly successive series of stimuli be applied to the nerve,
the muscle responds by an equally rapid series of contrac-
tions, each of which takes place before the preceding one
is over ; the contractions are thus added together, and the
muscle remains in a state of continued contraction as long
as the stimuli are continued, until exhaustion sets in. A
prolonged contraction made up of such a series of single
contractions superadded to one another is called a tetanic
contraction. The acidity and heat which are developed
at a "single contraction become much more obvious during
a tetanic contraction.
The voluntary contractions by which we execute the
various movements of our body are in reality, in at all
events nearly all cases, tetanic contractions, however short
354 ELEMENTARY PHYSIOLOGY l£ss
they may appear to be. Thus, when we contract one of
our muscles by an effort of the will it appears that a series
of impulses is sent out in rapid succession from the spinal
cord, perhaps at the rate of twelve or more in a second,
to throw the muscle into prolonged contraction. By this
means our control of the resulting movement is far greater
than it would be if we were only able to execute single,
short, and sudden contractions, such as result from sending
a single impulse along the nerve going to the muscle.
8. The Various Kinds of Muscles. — Muscles may be
conveniently divided into two groups, according to the
manner in which the ends of their fibres are fastened ;
into muscles not attached to solid levers, and muscles
attached to solid levers.
Muscles not attached to Solid Levers. — Under this head
come the muscles which are appropriately called hollow
muscles, inasmuch as they inclose a cavity or surround a
space ; and their contraction lessens the capacity of that
cavity, or the extent of that space.
The muscular fibres of the heart, of the blood-vessels,
of the lymphatic vessels, of the alimentary canal, of the
urinary bladder, of the ducts of the glands, of the iris of
the eye, are so arranged as to form hollow muscles.
In the heart the muscular fibres, which, though peculiar,
are striated, are arranged in an exceedingly complex manner
round the several cavities, and they contract, as we have
seen, in a definite order.
The iris of the eye is like a curtain, in the middle of
which is a circular hole. The muscular fibres are of the
smooth or unstriated kind (see p. 310), and they are dis-
posed in two sets : one set radiating from the edges of the
hole to the circumference of the curtain ; and the other
set arranged in circles, concentrically with the aperture.
vin THE VARIOUS KINDS OF MUSCLES 325
The muscular fibres of each set contract suddenly and
together, the radiating fibres necessarily enlarging the hole,
the circular fibres diminishing it.
In the alimentary canal the muscular fibres are also of
the unstriated kind, and they are disposed in two layers,
one set of fibres being arranged parallel with the length of
the intestines, while the others are disposed circularly, or
rather at right angles to the former.
As has been stated above (p. 278), the contraction of
these muscular fibres is successive ; that is to say, all the
muscular fibres, in a given length of the intestines, do not
contract at once, but those at one end contract first, and
the others follow them until the whole series have con-
tracted. As the order of contraction is, naturally, always
the same, from the upper towards the lower end, the effect
of this peristaltic contraction is, as we have seen, to force
any matter contained in the alimentary canal from its
upper towards its lower extremity. The muscles of the
walls of the ducts of the glands have a substantially similar
arrangement. In these cases the contraction of each fibre
is less sudden and lasts longer than in the case of the heart.
Muscles attached to Definite Levers. — The great majority
of the muscles in the body are attached to distinct levers,
formed by the bones. In such bones as are ordinarily
employed as levers, the osseous tissue is arranged in the
form *of a shaft (Fig. 97, d), formed of a very dense and
compact osseous matter, but often containing a great central
cavity (t>), which is filled with a very delicate vascular and
fibrous tissue loaded with fat called marrow. Towards the
two ends of the bone, the compact matter of the shaft thins
out, and is replaced by a much thicker but looser sponge-
work of bony plates and fibres, which is termed the can-
cellous or spongy tissue of the bone. The surface even
ELEMENTARY PHYSIOLOGY
in
Fig. 97. — Longitudinal
Section of the Shaft
of a Human Femur
or Thigh-bone.
a, the head, which ar-
ticulates with the hip-
bone; b, the medullary
cavity, and d, the dense
bony substance of the
shaft; c, the part which
enters into the knee-joint,
articulating with the shin-
bone, or tibia
of this part, however, is still formed by
a thin sheet of denser bone.
At least one end of each of these
bony levers is fashioned into a smooth,
articular surface, covered with cartilage,
which enables the relatively fixed end of
the bone to play upon the corresponding
surface of some other bone, with which
it is said to be articulated (see p. 345),
or, contrariwise, allows the other bone
to move upon it.
It is one or other of these extremities
which plays the part of fulcrum when the
bone is in use as a lever.
Thus, in the accompanying figure (Fig.
98) of the bones of the upper extremity,
with the attachments of the biceps mus-
cle to the shoulder-blade and to one of
the two bones of the fore-arm called the
radius, P indicates the point of action
of the power (the contracting muscle)
upon the radius.
It usually happens that the bone to
which one end of a muscle is attached
is absolutely or relatively stationary ;
while that to which the other is fixed is
movable. In this case, the attachment
to the stationary bone is termed the ori-
gin, that to the movable bone the inser-
tion, of the muscle.
The fibres of muscles are sometimes
fixed directly into the parts which serve
as their origins and insertions ; but
nn THE VARIOUS KINDS OF MUSCLES 327
more commonly strong cords or bands of fibrous tissue,
called tendons, are interposed between the muscle proper
and its place of origin or insertion. When the tendons play
over hard surfaces, it is usual for them to be separated from
these surfaces by sacs containing fluid, which are called
bursce ; or even to be invested by synovial sheaths, i.e. quite
covered for some distance by a bag forming a double sheath,
very much in the same way that the bag of the pleura covers
the lunar and the chest-wall.
Fig. 98. — The Bones of the Upper Extremity, with the Biceps Muscle.
The two tendons by which this muscle is attached to the scapula are seen at a
P indicates the attachment of the muscle to the radius, and hence the point of action
of the power; F, the fulcrum, the lower end of the humerus, on which the upper end
of the radius (together with the ulna) moves; \V, the weight (of the hand).
Usually, the direction of the axis of a muscle is that of a
straight line joining its origin and its insertion. But in some
muscles, as the superior oblique muscle of the eye, the
tendon passes over a pulley formed by ligament, and com-
pletely changes its direction before reaching its insertion.
(See p. 440.)
Again, there are muscles which are fleshy at each end,
and have a tendon in the middle. Such muscles are called
328
ELEMENTARY PHYSIOLOGY
digastric, or two-bellied. In the curious muscle which pulls
down the lower jaw, and especially receives this name of
digastric, the middle tendon runs through a pulley connected
with the hyoid bone ; and the muscle, which passes down-
wards and forwards from the skull to this pulley, after trav-
ersing it, runs upwards and forwards to the lower jaw
(Fig- 99)-
9. The Structure of Bone. — A fresh long bone, such as
the femur and humerus of a rabbit, from which the attached
muscles, tendons, and ligaments have been carefully cleaned
Fig. 99. — The Course of the Digastric Muscle.
D, its posterior belly; D' , its anterior belly; between the two is the tendon passing
through its pulley connected with Hy, the hyoid bone.
away, but the surface of which has not been scraped or
otherwise injured, is an excellent subject for the study of
bone. It is a hard, tough body, which is flexible and highly
elastic within narrow limits, but readily breaks, with a clean
fracture, if it is pressed too far. The two articular ends are
coated by a layer of cartilage, which is thickest in the middle.
Where the margins of the cartilage thin out, a layer of vas-
cular connective tissue commences, and, extending over the
whole shaft, to the surface of which it is closely adherent,
constitutes the periosteum. If the bone is macerated for
some time in water, the periosteum may be stripped off in
vin THE STRUCTURE OF BONE 329
shreds with the forceps. Filaments pass from its inner sur-
face into the interior of the bone. If the shaft is broken
across it will be found to contain a spacious medullary cavity
(Fig. 97, b) filled by a reddish, highly vascular mass of con-
nective tissue, abounding in fat cells, called the medulla or
marrow ; and a longitudinal section shows that this medul-
lary cavity extends through the shaft, but in the articular
ends becomes subdivided by bony partitions and breaks up
into smaller cavities, like the areola; of connective tissue.
These cavities are termed cancelli, and the ends of the bone
are said to have a cancellated structure.- The wails of the
medullary cavity in the shaft are very dense and exhibit no
cancel] i, and appear at first to be solid throughout. But on
examining them carefully with a magnifying glass it will be
seen that they are traversed by a meshwork of narrow canals,
varying in diameter from 20^ to ioo/x. or more. The long
dimensions of the meshes lie parallel with the axis of the
shaft. These are the Haversian canals. This system of
Haversian canals opens by short communicating branches
on the one hand upon the periosteal and on the other upon
the medullary surface of the wall of the shaft ; and in a fresh
bone, minute vascular prolongations of the periosteum and
of the medulla, respectively, may be seen to pass into the
communicating canals and become continuous with the
likewise vascular contents of the Haversian canals. More-
over, at one part of the shaft there is a larger canal, through
which the vessels which supply the medulla pass. This is
the so-called nutritive foramen of the bone. At the two
ends of the bone the cavities of the Haversian canals open
into those of the cancelli ; and the vascular substance which
fills the latter thus further connects the vascular contents of
the Haversian canals with the medulla.
Thus the bone may be regarded as composed of (i) an
330 ELEMENTARY PHYSIOLOGY less.
interna], thick cylinder of vascular medulla ; (ii) an external,
hollow, thin, cylindrical sheath of vascular periosteum, com-
pleted at each end by a plate of articular cartilage ; (hi) of
a fine, regular, long-meshed vascular network, which connects
the sides of the medullary cylinder with the periosteal sheath
of the shaft ; (iv) of a coarse, irregular, vascular meshwork
occupying at each end the space between the medullary
cylinder and the plate of articular cartilage, and connected
with the periosteum of the lateral parts of the articular end ;
and (v) of the hard, perfect, osseous tissue which fills the
meshes of these two networks. Such is the general structure
of all long bones with cartilaginous ends, though some, as
the ribs, possess no wide medullary cavity, but are simply
cancellated in the interior. In some very small bones even
the cancelli are wanting. And there are many bones which
have no connection with cartilage at all.
If a bone is exposed to a red heat for some time in a
closed vessel nothing remains but a mass of white " bone-
earth," which has the general form of the bone, but is very
brittle and easily reduced to powder. It consists almost
entirely of calcium phosphate and carbonate. On the
other hand, if the bone is digested in dilute hydrochloric
acid for some time the calcareous salts are dissolved out,
and a soft, flexible substance is left, which has the exact
form of the bone, but is much lighter. If this is boiled for
a long time it will yield much gelatine, and only a small
residue will be left. Osseous tissue therefore consists essen-
tially of an animal matter impregnated with calcium salts,
the animal matter being collagenous, like connective tissue.
A sufficiently thin longitudinal section, made by grinding
down part of the wall of the medullary cavity of a bone —
which has been well macerated in water and then thoroughly
dried — if viewed as a transparent object with a magnify-
viii THE STRUCTURE OF BONE 331
ing glass, shows a series of lines, with dark enlargements at
intervals, running parallel with the Haversian canals. If
the section, instead of being longitudinal, were made
transversely to the shaft, and therefore cutting through the
majority of the Haversian canals at right angles to their
length, similar lines and dark spots would be seen to form
concentric circles at regular intervals round each Haversian
canal (Fig. 100). The hard bony tissue appears therefore
to be composed of lamellae, which are disposed concentri-
cally around the Haversian canals ; and a Haversian canal
with the concentric lamellae belonging to it forms what is
called a Haversian system. The soft substance from which
the bone-earth has been extracted is similarly lamellated,
and here and there presents fibres which may be traced into
the fibrous substance of the periosteum.
If a thin section of dry bone is examined with the micro-
scope (Fig. 101), by transmitted light, each dark spot is
seen to be a black body (of an average diameter of about
15/x) with an irregular jagged outline, and proceeding from
it are numerous fine dark lines which ramify in the sur-
rounding matrix and unite with similar branched lines from
adjacent black bodies. The matrix itself has a somewhat
granular aspect. In a transverse section these black bodies
are rounded or oval in form, but in a longitudinal section
they appear almost spindle-shaped ; that is to say, they are
lenticular or lens-shaped ; but flattened as it were between
the adjacent layers of the matrix. Examined by reflected
light the same bodies look white and glistening ; and if the
section, instead of being examined dry, be boiled in water
or soaked in strong alcohol, and brought under the micro-
scope while still wet, the black bodies with their branching
lines will be found to have almost disappeared, only faint
outlines of them being left. At the same time minute
332 ELEMENTARY PHYSIOLOGY less
bubbles of air will have escaped from the section. The
black bodies seen in the dry bone are in fact " lacunae,"
a
W^ptZJ--
i
5 $Wr*JNi*W* ^*F*
«|T4 W& ' WAS
Fig. ioo. — Transverse Section of Compact Bone
a, lamellae concentric with the external surface; />, lamella; concentric with the
.medullary surface; c, section of Haversian canals; <:', section of a Haversian canal
just dividing into two; d, intersystemic lamella;. Low magnifying power.
i.e. gaps, or holes in the solid matrix, appearing black by
transmitted light and white by reflected light, because they
THE STRUCTURE OE BONE
333
are filled with air; and the dark branched lines are
similarly minute canals, " canaliculi," also filled with air-
bubbles, drawn out, so to speak, into lines, also hollowed
out of the solid matrix, and placing one lacuna in communi-
cation with another. In each Haversian system the cana-
liculi and the lacunce of the innermost layer, or that nearest
Fig. ioi. — Transverse Section of Bone, highly magnified (300 diameters)
H, Haversian canals; /, lacunae with canaliculi.
the Haversian canal, communicate with it, while the cana-
liculi and the lacuna of the outermost layer communicate
only with those of the next inner layer. Hence the lactone
and canaliculi compose a meshwork of canals, which is
peculiar to each Haversian system, and by which the nutri-
tive plasma exuded from the vessels in the canal of that
334 ELEMENTARY PHYSIOLOGY less.
system irrigates all the layers of bone which belong to the
system.
A very thin section of perfectly fresh bone exhibits no
dark bodies, inasmuch as the lacunae and canaliculi con-
tain no air, but are permeated with the nutritive fluid.
Each lacuna, moreover, at all events in young bone, con-
tains a nucleated cell, which is altogether similar in essen-
tial character to a connective-tissue or cartilage corpuscle,
and if the term were not already misused might be called
a bone corpuscle. In fact, in ultimate analysis the essen-
tial character of bone shows itself to be this : that it is
a tissue analogous to cartilage and connective tissue in so
far as it consists of cells separated by much intercellular
substance ; and that it differs from them mainly in the fact
that calcareous matter is deposited in and associated with
the intercellular substance in such a way as to leave minute
uncalcified passages (the canaliculi), which open into the
larger uncalcified intervals (the lacuna) in the neighbour-
hood of the cells.
The function of these passages is doubtless to allow of a
more thorough permeation of the calcified tissue by the nutri-
tive fluids than could take place if the calcareous deposit
were continuous, and it is probable that, in an ordinary bone,
there is no particle i/n square which is not thus brought
within reach of a minute streamlet of nutritive plasma.
This circumstance enables us to understand that which
one would hardly suspect from the appearance of a bone,
namely, that, throughout life, or, at all events, in early life,
its tissue is the seat of an extremely active vital process.
The permanence and apparent passivity of the bone are
merely the algebraical summation of the contrary processes
of destruction and reproduction which are going on in it.
If a young pig is fed with madder, its bones will be found
viii THE DEVELOPMENT OF BONE 335
after a time to be dyed red. The madder dye, in fact, get-
ting into the blood, permanently dyes the tissue with which
it meets in its course through the bones. But if the pig is
fed for a time with madder, and is then deprived of it, the
amount of colour to be found in the bones depends on the
time which elapses before the pig is killed. And it is not
that the colouring matter is merely, as it were, washed out ;
the dye is permanent, but the bones nevertheless become
parti-coloured. In the shaft of a long bone, for instance, a
certain time after feeding with madder, a deep red layer
of bone in the middle of the thickness of its wall will be
found to have colourless bone on its medullary and on its
periosteal face. And the longer the time which has elapsed
since the feeding with madder, the more completely will the
deep red bone be replaced and covered up by colourless bone.
10. The Development of Bone. — Careful inspection of a
transverse section of the wall of the shaft of a long bone is by
itself sufficient to show that bone is constantly being formed
and as constantly being removed. Such a section exhibits,
as has been said, a number of Haversian canals surrounded
by circular zones formed of concentric layers of bone. But
interspersed between these there lie larger and smaller seg-
ments of zones formed of similar concentrically curved par-
allel lamellae, the so-called intersystemic lamellae (Fig. 100, d),
which have evidently at one time formed parts of complete
Haversian systems, but which have been partially destroyed
and replaced by new systems. In fact the formation of new
bone is constantly taking place : (i) at the surface in con-
tact with the periosteum; (ii) at the surface in contact
with cartilage; and (iii) at the surface in contact with the
medulla and its prolongations in the cancelli and the Haver-
sian canals ; and the bone thus formed is after a time de-
stroyed and replaced by new growths.
336 ELEMENTARY PHYSIOLOGY less.
To understand this we must study the origin of osseous
tissue. At a certain period of embryonic life there is no bone
in any part of the body. Nevertheless, the greater number
of the " bones," for example the vertebras, the ribs, the limb
bones, and some of the cranial and facial bones, exist in a
morphological sense, inasmuch that cartilages having the
general form of such bones exist in the places of the future
bones. In the place of the humerus and the femur, for
example, there are rods of pure cartilage, which are, so to
speak, small, rough models of the humerus and femur of
the adult. When the process of bone formation com-
mences, slight opaque spots, termed centres of ossification,
make their appearance in the substance of the cartilage,
the opacity being due to the deposit of calcareous salts
at these points.
Microscopic examination shows that the calcareous salts
are deposited in the intercellular substance, which, therefore,
is converted into a sort of bone, in which the lacunae are
represented by the cavities of the cartilage corpuscles.
These calcareous salts must reach the centres of ossification
dissolved in the plasma which is exuded from the perichon-
drial vessels and permeates the intercellular substance.
In the cartilaginous rudiment of a long bone three such
centres of ossification usually make their appearance, one
in the centre of the shaft and one in each end. Supposing
these centres to be formed at the same time (which may
not, however, be the case), what we have to start from is a
rudiment or model in cartilage of the future bone, converted
at three points into calcified cartilage ; that is to say there
are a central nodule (diaphysis) and two terminal nodules
(epiphyses) . If the deposit were to spread from the three
centres until the three nodules united, the result would be
a calcified cartilage in place of the formative cartilage.
viii THE DEVELOPMENT OF BONE 337
As a matter of fact, the deposit does spread through the
rudiment from each centre outwards so long as the bone is
growing. But the cartilage between the diaphysis and epi-
physes and beyond the ends, of the epiphyses also grows
and increases with the general growth of the bone. That
beyond the epiphysial ossification remains throughout life as
articular cartilage, while that between the epiphysial and
diaphysial ossifications is gradually encroached upon by
these and finally obliterated.
If this were all, the adult bone would consist of calcified
cartilage (Fig. 102, c) tipped at the ends with cartilage which
remained uncalcified. But this is not all ; such a mass of
calcified cartilage is not a true bone.
Very soon after the ossific centres have made their appear-
ance, there grow into them vascular processes of the peri-
chondrium, or membrane of connective tissue containing
blood-vessels that surrounds the cartilage and later is called
the periosteum. These processes make room for themselves
by, in some way, causing the destruction and absorption of
the calcified cartilage, thus giving rise to large irregular
spaces or areolae, which they occupy. The processes con-
sist of blood-vessels surrounded by a peculiar form of
connective tissue, characterised by the presence of large
nucleated cells called osteoblasts.
No sooner have these processes hollowed out the areola?
in the calcified cartilage than they begin to line them with
layers of true bone (c.d), the matrix of the connective tis-
sue of the processes being calcified in such a way as to leave
spaces, in which some of the cells or osteoblasts remain im-
bedded, fine branching canals being left in the matrix, or
being subsequently formed in it. In other words, layers of
true bone, with lacunas containing nucleated cells and with
branched canaliculi, are thus constructed as a lining to the
z
33S
ELEMENTARY PHYSIOLOGY
spaces hollowed out of the calcified cartilage. None of the
spaces, however, are completely filled up, and there are no
signs of regular Haversian systems with canals and concen-
tric laminae. The calcified cartilage
is simply replaced by a loose open
network of spongy bone, in the thick-
ness of the bars of which may be
seen the remains of the calcified
cartilage, and the cavities of which
are filled with blood-vessels and deli-
cate connective tissue, that is, with
marrow.
Meanwhile the perichondrium or
periosteum, in addition to sending
in these processes, which thus con-
vert the calcified cartilage into
spongy but true bone, also deposits
layers of somewhat denser but still
spongy bone on the outside of the
changed and changing ossific cen-
tre, in the form of a cylinder (p.b),
which grows in thickness by the
Fig. 102. — Longitudinal Sec- it.- r 1 •.. r
■noNOF Ossifying Humerus addition of new layers On its Surface,
...... immediately under the periosteum,
c, the original primitive carti- J 1
lage, calcined in its deeper por- ancl in length by the extension of
tion; c.b, spongy bone arising ° ■*
from ossification of calcified these cylindrical layers upwards and
cartilage; this has already been J
absorbed and replaced by me- downwards. The " periosteal " bone,
as this is called, is also true bone,
the deposition of calcic salts tak-
ing place in the matrix around the
osteoblasts in such a way as to leave lacunae and canaliculi.
Very soon after this sheath of periosteal bone has made
its appearance, the spongy bone first formed in the interior
dullaat »i: p.b. bone formed by
the periosteum; it is seen ex-
tending as a thin sheet upwards
and downwards outside the carti-
lage. (Magnified 7 diameters.)
vni THE DEVELOPMENT OF BONE 339
is itself absorbed by the same vascular processes which
formed it, so that soon what was at first the centre of ossifi-
cation, after passing from simple cartilage to calcified carti-
lage, and so to spongy bone, is resolved into marrow or
medulla (m), that is, into vascular connective tissue richly
loaded with fat.
The cartilage at each end of the medulla continues to
grow in length and thickness, and to be successively con-
verted, first, into calcified cartilage, and then into spongy
bone at its end nearest the medulla. The medulla also
increases rapidly in length, encroaching more and more
upon the spongy bone.
The whole is surrounded by the ring or cylinder of perios-
teal bone just described, which also grows in thickness and
length and assumes the form of a long, narrow dice-box,
with narrow but thicker walls in the middle, and with wider
but thinner walls at each end. The middle of the cylinder
is occupied by medulla alone, but each end is, as it were,
plugged by a disc of cartilage undergoing conversion into cal-
cified cartilage (c), then into spongy bone (c.b), and finally
into medulla (m).
As the developing bone grows, the discs get farther and
farther apart, and the medulla grows longer until the two
ends of the diaphysis meet the epiphyses, and unite with
them. The whole disc thus becomes at last spongy bone,
continuous with the similar spongy bone into which the
epiphysis is converted, and forms the spongy bone existing
at the ends of the long bones ; all that remains of the calci-
fied cartilage is an exceedingly thin layer just below the
articular cartilage at either end of the bone.
Thus, though the primitive cartilage serves as the model
of the future bone, a great deal of the bone, namely, the
dense, compact bone which forms the shaft and is con-
340 ELEMENTARY PHYSIOLOGY less
tinued as a shell over the two ends, does not come from the
cartilage at all but is deposited by the periosteum ; the
spongy bone at each end is the only part that is formed in
the cartilage, and even in that, as we have seen, there are
no remains of the cartilage itself.
Moreover, the bone even thus formed is subject to inces-
sant change. The periosteal bone is at first spongy and
slight in texture, and exhibits no true Haversian systems.
Little by little, spaces are scooped out in it by vascular pro-
cesses of the periosteum on the outside and of the medulla
on the inside, like those which formed it ; and such a space
when formed is in turn filled up in a solid fashion by layers
of bone deposited in a regular way -as concentric lamellae
round the blood-vessels of the process, which in the end
remains as the blood-vessel of the Haversian canal, in the
centre of the Haversian system thus deposited. And indeed
similar processes of absorption and fresh formation go on
certainly while the bone is increasing in size, and probably
also for some time afterwards.
A good many bones, such as the frontal and parietal bones
of the skull, have no cartilaginous precursors. The roof of
the skull of an embryo is formed of a membrane of con-
nective tissue, and in this each of the bones commences as
a calcification of that part of the connective tissue which
occupies the place of the centre of the future bone. The
calcification radiates from this centre outwards, so that it
soon has the form of a thin plate, the margins of which are,
as it were, frayed out in filaments. The vascular connective
tissue which incloses the plate becomes its periosteum, and
plays the same part in relation to the growing bone as the
periosteum of cartilage bone does to it. As the plate grows
thicker, medullary processes burrow into it and give rise to
canr.elU and Haversian systems.
vui THE MECHANICS OK MOTION. LEVERS 54.
11. The Mechanics of Motion. Levers. — To understand
the action of the bones, as levers, properly, it is necessary to
possess a knowledge of the different kinds of levers and be
able to refer the various combinations of the bones to their
appropriate lever-classes.
A lever is a rigid bar, one part of which is absolutely or
relatively fixed, while the rest is free to move. Some one
point of the movable part of the lever is set in motion
by a force, in order to communicate more or less of that
motion to another point of the movable part, which pre-
sents a resistance to motion in the shape of a weight or
other obstacle.
Three kinds of levers are enumerated by mechanicians,
the definition of each kind depending upo 1 the relative posi-
tions of the point of support, or fulcrum; of the point
which bears the resistance, weight, or other obstacle to be
overcome by the force ; and of the point to which the force,
or power employed to overcome the obstacle, is applied.
If the fulcrum be placed between the power and the
weight, so that, when the power sets the lever in motion,
the weight and the power describe arcs, the concavities of
which are turned towards one another, the lever is said to be
of the first class. (Fig. 103, I.)
If the fulcrum be at one end, and the weight be between
it and the power, so that weight and power describe concen-
tric arcs, the weight moving through the less space when the
lever moves, the lever is said to be of the second class.
(Fig. 103, II.)
And if, the fulcrum being still at one end, the power be
between the weight and it, so that, as in the former case,
the power and weight describe concentric arcs, but the
power moves through the less space, the lever is of the
third class. (Fig. 103, III.)
342
ELEMENTARY PHYSIOLOGY
LESS
In the human body the following parts present examples
of levers of the first class.
(a) The skull in its movements upon the atlas, as
fulcrum.
(b) The pelvis in its movements upon the heads of the
thigh-bones, as fulcrum.
(c) The foot, when, it is raised, and the toe tapped on
the ground, the ankle-joint being fulcrum. (Fig. 103, I.)
ill
Fig. 103.
The upper three figures represent the three kinds of levers; the lower, the foot
when it takes the character of each kind. — W, weight or resistance; F, fulcrum; P,
power.
The positions of the weight and of the power are not given
in either of these cases, because they are reversed accord-
ing to circumstances. Thus, when the face is being de-
pressed, the power is applied in front, and the weight to
the back part, of the skull ; but when the face is being
raised, the power is behind and the weight in front. The
like is true of the pelvis, according as the body is bent for-
ward, or backward, upon the legs. Finally, when the toes,
in the action of tapping, strike the ground, the power is at
the heel, and the resistance in the front of the foot. But
when, the toes are raised to repeat the act, the power is in
vin THE MECHANICS OF MOTION. LEVERS 343
front, and the weight, or resistance, is at the heel, being, in
fact, the inertia and elasticity of the muscles and other parts
of the back of the leg.
But in all these cases, the lever remains one of the first
class, because the fulcrum, or fixed point on which the
lever turns, remains between the power and the weight, or
resistance.
The following are three examples of levers of the second
class : —
(a) The thigh-bone of the leg which is bent up towards
the body and not used, in the action of hopping.
For, in this case, the fulcrum is at the hip-joint. The
power (which may be assumed to be furnished by the thick
muscle J of the front of the thigh) acts upon the knee-cap ;
and the position of the weight is represented by that of the
centre of gravity of the thigh and leg, which will lie some-
where between the end of the knee and the hip.
{b) A rib when depressed by the rectus muscle 2 of the
abdomen, in expiration.
Here the fulcrum lies where the rib is articulated with
the spine; the power is at the sternum — virtually the
opposite end of the rib ; and the resistance to be overcome
lies between the two.
(c) The raising of the body upon the toes, in standing
on tiptoe, and in the first stage of making a step forward.
(Fig. 103, II.)
Here the fulcrum is the ground on which the toes rest ;
the power is applied by the muscles of the calf to the heel
1 This muscle, called rectus, is attached above to the hip-bone and below
to the knee-cap (Fig. 6, 2, p. 18) . The latter bone is connected by a strong
ligament with the tibia.
2 This muscle lies in the front abdominal wall on each side of the middle
line. It is attached to the sternum above and to the front of the pelvis
below. (Fig. 6, 3.)
344 ELEMENTARY PHYSIOLOGY less.
(Fig. 6, I.) ; the resistance is so much of the weight of the
body as is borne by the ankle-joint of the foot, which of
course lies between the heel and the toes.
Three examples of levers of the third class are —
(a) The spine, head and pelvis, considered as a rigid bar,
which has to be kept erect upon the hip-joints. (Fig. 6.)
Here the fulcrum lies in the hip-joints, the weight is high
above the fulcrum, at the centre of gravity of the head and
trunk ; the power is supplied by the extensor muscles (Fig.
6, 2) in the front of, or the flexor muscles (Fig. 6, II.) at
the back of, the thigh, and acts upon points comparatively
close to the fulcrum.
(£) Flexion of the forearm upon the arm by the biceps
muscle, when a weight is held in the hand.
In this case, the weight being in the hand and the ful-
crum at the elbow-joint, the power is applied at the point of
attachment of the tendon of the biceps, close to the latter.
(Fig. 98.)
(c) Extension of the leg on the thigh at the knee-joint.
Here the fulcrum is the knee-joint ; the weight is at the
centre of gravity of the leg and foot, somewhere between
the knee and the foot ; the power is applied by the muscles
in front of the thigh (Fig. 6, 2 and Fig. 104), through the
ligament of the knee-cap, or patella, to the tibia, close to
the knee-joint.
In studying the mechanism of the body, it is very impor-
tant to recollect that one and the same part of the body
may represent each of the three kinds of levers, according
to circumstances. Thus, it has been seen that the foot may,
under some circumstances, represent a lever of the first, in
others, of the second class. But it may become a lever of
the third class, as when one dances a weight resting upon
the toes up and down, by moving only the foot. • In this
THE JOINTS OF THE BODY
345
case, the fulcrum is at the ankle-joint, the weight is at the
toes, and the power is furnished by the extensor muscles at
the front of the leg (Fig. 6, i), which are inserted between
the fulcrum and the weight. (Fig. 103, III.)
Fig. 104. — The Right Knee-joint. The Outer Half of the Femur and
Patella sawn away.
fern, femur; pat, patella; til, tibia; fib, fibula; caps, capsule of joint; /, crucial
ligaments; c, semilunar fibro-cartilages; e, tendon of extensor muscle.
12. The Joints of the Body. — It is very important that
the levers of the body should not slip, or work unevenly,
when their movements are extensive, and to this end they
are connected together in such a manner as to form strong
and definitely arranged joints or articulations.
Joints may be classified into imperfect and perfect.
(a) Imperfect joints are those in which the conjoined
levers (bones or cartilages) present no smooth surfaces
346 ELEMENTARY PHYSIOLOGY less
capable of rotatory motion, to one another, but are connected
by continuous cartilages or ligaments, and have only so
much mobility as is permitted by the flexibility of the join-
ing substance.
Examples of such joints as these are to be met with in
the vertebral column — the flat surfaces of the bodies of the
vertebras being connected together by thick plates of very
flexible fibro-cartilage, which confer upon the whole column
considerable play and springiness, and yet prevent any great
amount of motion between the several vertebrae. In the
pelvis (see Fig. 4), the pubic bones are united to each
other in front, and the iliac bones to the sacrum behind, by
fibrous or cartilaginous tissue, which allows of only a slight
play, and so gives the pelvis a little more elasticity than it
would have if it were all one bone.
(3) In all perfect joints, the opposed bony surfaces
which move upon one another are covered with cartilage,
and between them is placed a sort of sac, which lines these
cartilages, and, to a certain extent, forms the side walls of
the joint ; and which, secreting a small quantity of viscid,
lubricating fluid — the synovial fluid — is called a synovial
membrane.
The opposed surfaces of these articular cartilages, as
they are called, may be spheroidal, cylindrical, or pulley-
shaped ; and the convexities of the one answer, more or less
completely, to the concavities of the other.
Sometimes, the two articular cartilages do not come
directly into contact, but are separated by independent
plates of cartilage, which are termed inter- articular. The
opposite faces of these inter-articular cartilages are fitted to
receive the faces of the proper articular cartilages.
While these co-adapted surfaces and synovial membranes
provide for the free mobility of the bones entering into a
THE JOINTS OF THE BODY
347
joint, the nature and extent of their motion is defined, partly
by the forms of the articular surfaces, and partly by the
disposition of the ligaments, or firm, fibrous cords which
pass from one bone to the other.
As respects the nature of the articular surfaces, joints
may be what are called ball-and-socket joints, when the
spheroidal surface furnished by one bone plays in a cup
Fig. 105. — A Section of the Hip- joint taken through the Acetabulum or
Articular Cup of the Pelvis and the Middle of the Head and Neck
of the Thigh-bone.
L.T, Ligamentum teres, or round ligament. The spaces marked with an inter-
rupted line ( ) represent the articular cartilages. The cavity of the synovial
membrane is indicated by the dark line between these, and, as is shown, extends along
the neck of the femur beyond the limits of the cartilage. The peculiar shape of the
pelvis causes the section t3 have the remarkable outline shown in the cut. This will
be intelligible if compared with Fig. 4.
furnished by another. In this case the motion of the
former bone may take place in any direction, but the extent
348 ELEMENTARY PHYSIOLOGY less
of the motion depends upon the shape of the cup — being
very great when the cup is shallow, and small in proportion
as it is deep. The shoulder is an example of a ball-and-
socket joint with a shallow cup (Fig. 5, B) ; the hip of such
a joint with a deep cup (Fig. 5, A and Fig. 105).
Hinge-joints are single or double. In the former case,
the nearly cylindrical head of one bone fits into a corre-
sponding socket of the other. In this form of hinge-joint
the only motion possible is in the direction of a plane
perpendicular to the axis of the cylinder, just as a door can
only be made to move round an axis passing through its
hinges. The elbow is the best example of this joint in the
human body, but the movement here is limited, because the
olecranon, or part of the ulna which rises up behind
the humerus, prevents the arm being carried back behind
the straight line ; the arm can thus be bent to, or straight-
ened,.but not bent back (Fig. 106). The knee (Fig. 104)
and ankle present less perfect specimens of a single hinge-
joint.
A double hinge-joint is one in which the articular surface
of each bone is concave in one direction, and convex in
another, at right angles to the former. A man seated in a
saddle is " articulated " with the saddle by such a joint.
For the saddle is concave from before backwards, and con-
vex from side to side, while the man presents to it the
concavity of his legs astride, from side to side, and the con-
vexity of his seat from before backwards.
The metacarpal bone of the thumb is articulated with the
bone of the wrist, called trapezium, by a double hinge-joint.
A pivot-joint is one in which one bone furnishes an axis,
or pivot, on which another turns ; or itself turns on its own
axis, resting on another bone. A remarkable example of
the former arrangement is afforded by the atlas and axis,
THE JOINTS OF THE BODY
349
or two uppermost vertebras of the neck (Fig. 107). The
axis possesses a vertical peg, the so-called odontoid process
{b), and at the base of the peg are two obliquely placed,
articular surfaces («). The atlas is a ring-like bone, with a
massive thickening on each side. The inner side of the
front of the ring plays round the neck of the odontoid peg,
Fig. 106. — Longitudinal and Vertical Section through the Elbow-joint.
H, humerus; £//,ulna; 7V, the triceps muscle, which extends the arm; Bi, the
biceps muscle, which flexes it.
and the under surfaces of the lateral masses glide over the
articular faces on each side of the base of the peg. A
strong ligament passes between the inner sides of the two
lateral masses of the atlas, and keeps the hinder side of
the neck of the odontoid peg in its place (Fig. 107, A).
By this arrangement, the atlas is enabled to rotate through
35°
ELEMENTARY PHYSIOLOGY
a considerable angle either way upon the axis, without any
danger of falling forwards or backwards — accidents which
would immediately destroy life by crushing the spinal cord.
The lateral masses of the atlas have, on their upper faces,
concavities (Fig. 107, A, a), into which the two convex,
occipital condyles of the skull fit, and in which they play
upwards and downwards. Thus, the nodding of the head is
effected by the movement of the skull upon the atlas ; while,
in turning the head from side to side, the skull does not
move upon the atlas, but the atlas slides round the odontoid
peg of the axis vertebra.
Fig. 107.
A. The atlas viewed from above : a a, upper articular surfaces of its lateral masses
for the condyles of the skull ; b, the opening for the peg of the axis vertebra.
B. Side view of the axis vertebra; a, articular surface for the lateral mass of the
atlas; b, peg or odontoid process.
The second kind of pivot-joint is seen in the fore-arm.
If the elbow and fore-arm, as far as the wrist, are made to
rest upon a table, and the elbow is kept firmly fixed, the
hand can nevertheless be freely rotated so that either the
palm, or the back, is turned directly upwards. When
the palm is -turned upwards, the attitude is called supination
(Fig. 108, A) ; when the back, pronation (Fig. 108, B).
The fore-arm is composed of two bones ; one, the ulna,
which articulates with the humerus at the elbow by the
hinge-joint already described, in such a manner that it can
THE JOINTS OF THE BODY
351
move only in flexion and extension (see p. 348), and has
no power of rotation. Hence, when the elbow and wrist
are rested on a table, this bone remains unmoved.
But the other bone of the fore-arm, the radius, has its
small upper end shaped like a very shallow cup with thick
edges. The hollow of the cup articulates with a spheroidal
surface furnished by the humerus : the lip of the cup, with
a concave depression on the side of the ulna.
Fig. 108.
The bones of the right fore-arm in supination (A) and pronation (B). H, humerus;
R, radius; U, ulna.
The large lower end of the radius bears the hand, and
has, on the side next the ulna, a concave surface, which
articulates with the convex side of the small lower end of
that bone.
352 ELEMENTARY PHYSIOLOGY less
Thus, the upper end of the radius turns on the double
surface furnished t© it by the pivot-like ball of the humerus
and the partial cup of the ulna ; while the lower end of the
radius can rotate round the surface furnished to it by the
lower end of the ulna.
In supination, the radius lies parallel with the ulna, with
its lower end on the outer side of the ulna (Fig. 108, A).
In pronation, it is made to turn on its own axis above, and
round the ulna below, until its lower half crosses the ulna,
and its lower end lies on the inner side of the ulna (Fig.
108, B).
The ligaments which keep the mobile surfaces of bones
together are, in the case of ball-and-socket joints, strong,
fibrous capsules, which surround the joint on all sides. In
hinge-joints, on the other hand, the ligamentous tissue is
chiefly accumulated, in the form of lateral ligaments, at the
sides of the joints. In some cases ligaments are placed
within the joints, as in the knee, where the bundles of fibres
which cross obliquely between the femur and the tibia are
called crucial ligaments (Fig. 104, /) ; or, as in the hip,
where the round ligament passes from the bottom of the
socket, or acetabulum of the pelvis, to the ball furnished by
the head of the femur (Fig. 105, LT).
Again, two ligaments pass from the apex of the odontoid
peg to both sides of the margin of the occipital foramen,
i.e. the large hole in the base of the skull, through which
the spinal cord passes to join the brain; these, from their
function in helping to stop excessive rotation of the skull,
are called check ligaments (Fig. 109, a).
In one joint of the body, the hip, the socket or aceta-
bulum (Fig. 105) fits so closely to the head of the femur,
and the capsular ligament so completely closes its cavity
on all sides, that the pressure of the air must be reckoned
via THE VARIOUS MOVEMENTS OF THE BODY 353
among the causes which prevent dislocation. This has
been proved experimentally by boring a hole through the
floor of the acetabulum, so as to admit air into its cavity,
when the thigh-bone at once falls as far as the round and
capsular ligaments will permit it to do, showing that it was
previously pushed close up by the pressure of the external air.
13. The Various Movements of the Body. — The differ-
ent kinds of movement which the levers, thus connected,
are capable of performing are called flexion and extension ;
abduction and adduction; rotation and circumduction.
A limb is flexed, when it is bent ; extended, when it is
straightened out. It is abducted, when it is drawn away
from the middle line ; adducted, when it is brought toward
the middle line. It is rotated, when it is made to turn on its
own axis 5 ch'cumducted, when it is made to describe a coni-
cal surface by rotation round an imaginary axis.
No part of the body is capable of perfect rotation like a
wheel, for the simple reason that such motion would neces-
sarily tear all the vessels, nerves, muscles, etc., which unite
it with other parts.
Any two bones united by a joint may be moved one upon
another in, at fewest, two different directions. In the case
of a pure hinge-joint, these directions must be opposite and
in the same plane ; but, in all other joints, the movements
may be in several directions and in various planes.
In the case of a pure hinge-joint, the two practicable
movements — viz., flexion and extension — may be effected
by means of two muscles, one for each movement, and
running from one bone to the other, but on opposite sides
of the joint. When either of these muscles contracts, it
will pull its attached ends together, and bend or straighten,
as the case may be, the joint towards the side on which it
is placed. Thus, the biceps muscle is attached, at one end,
2 A
354
ELEMENTARY PHYSIOLOGY
to the shoulder-blade, while, at the other end, its tendon
passes in front of the elbow-joint to the radius (Figs. 98
and 106, Bi) : when this muscle contracts, therefore, it
bends, or flexes, the fore-arm on the arm. At the back of
the joint there is the triceps (Fig. 106, Tr) : when this
contracts, it straightens, or extends, the fore-arm on the
arm.
6-e
Fig. iog.
The vertebral column in the upper part of the neck seen from behind and laid open
to show, a, the check ligaments of the axis; b, b' , the broad ligament which extends
from the front margin of the occipital foramen along the hinder faces of the bodies
of the vertebrae; it is cut through, and the cut ends turned back to show, c, the special
ligament which connects the point of the odontoid peg with the front margin of the
occipital foramen; c is placed on the occipital bone; /, the atlas; //, the axis.
In the other extreme form of articulation — the ball-and-
socket joint — movement in any number of planes may be
effected, by attaching muscles in corresponding number
and direction, on the one hand, to the bone which affords
the socket, and on the other to that which furnishes the
head. Circumduction will be effected by the combined
and successive contraction of these muscles.
14. The Mechanics of Locomotion. — We may now pass
from the consideration of the mechanism of mere motion to
that of locomotion.
vin THE MECHANICS OF LOCOMOTION 355
When a man who is standing erect on both feet proceeds
to walk, beginning with the right leg, the body is inclined,
so as to throw the centre of gravity forward ; and, the right
foot being raised, the right leg is advanced for the length of
a step, and the foot is put down again. In the meanwhile,
the left heel is raised, but the toes of the left foot have not
left the ground when the right foot has reached it, so that
there is no moment at which both feet are off the ground.
For an instant, the legs form two sides of an equilateral
triangle, and the centre of the body is consequently lower
than it was when the legs were parallel and close together.
The left foot, however, has not been merely dragged
away from its first position, but the muscles of the calf,
having come into play, act upon the foot as a lever of the
second order, and thrust the body, the weight of which
rests largely on the left astragalus, upwards, forwards, and
to the right side. The momentum thus communicated to
the body causes it, with the whole right leg, to describe an
arc over the right astragalus, on which that leg rests below.
The centre of the body consequently rises to its former
height as the right leg becomes vertical, and descends again
as the right leg, in its turn, inclines forward.
When the left foot has left the ground, the body is
supported on the right leg, and is well in advance of the
left foot ; so that, without any further muscular exertion,
the left foot swings forward like a pendulum, and is carried
by its own momentum beyond the right foot, to the position
in which it completes the second step.
When the intervals of the steps are so timed that each
swinging leg comes forward into position for a new step
without any exertion on the part of the walker, walking
is effected with the greatest possible economy of force.
And, as the swinging leg is a true pendulum — the time of
356 ELEMENTARY PHYSIOLOGY less.
vibration of which depends, other things being alike, upon
its length (short pendulums vibrating more quickly than
long ones), — it follows that, on the average, the natural
step of short-legged people is quicker than that of long-
legged people.
In running, there is a period when both feet are off the
ground. The legs are advanced by muscular contraction,
and the lever action of each foot is swift and violent.
Indeed, the action of each leg resembles, in violent running,
that which, when both legs act together, constitutes a jump,
the sudden extension of the legs adding to the impetus,
which, in slow walking, is given only by the feet.
15. The Mechanism of the Larynx. — Perhaps the most
singular motor apparatus in the body is the larynx, by the
agency of which the voice is produced.
The essential conditions of the production of the human
voice are : —
(a) The existence of the so-called vocal cords.
(b) The parallelism of the edges of these cords, without
which they will not vibrate in such a manner as to give, out
sound.
((f) A certain degree of tightness of the vocal cords,
without which they will not vibrate quickly enough to
produce sound.
(d) The passage of a current of air between the parallel
edges of the vocal cords of sufficient power to set the cords
vibrating.
The larynx (Fig. no) is a short tubular box opening
above into the bottom of the pharynx and below into the
top of the trachea. Its framework is supplied by certain
cartilages more or less movable on each other, and these
are connected together by joints, membranes, and muscles.
Across the middle of the larynx is a transverse partition,
inn
MECHANISM OF THE LARYNX
357
formed by two folds of the lining mucous membrane,
stretching from either side, but not quite meeting in the
middle line (Fig. in). They thus leave, in the middle
line, a chink or slit, running from the front to the back,
called the glottis. The two edges of this slit are not round
and flabby, but sharp and, so to speak, clean cut ; they are
also strengthened by a quantity of
elastic tissue, the fibres of which are
disposed lengthwise in them. These
sharp free edges of the glottis are
the so-called vocal cords, or vocal
ligaments.
The thyroid cartilage (Fig. no,
Tli) is a broad plate of gristle bent
upon itself into a V-shape, and so
disposed that the point of the V is
turned forwards, and constitutes what
is commonly called " Adam's apple."
Above, the thyroid cartilage is at-
tached by ligament and membrane
to the hyoid bone (Fig. no, Nv) .
Below and behind, its broad sides are
produced into little elongations or
horns, which are articulated by liga-
ments with the outside of a great ring
of cartilage, the cricoid (Fig. no,
Cr), which forms, as it were, the top
of the windpipe.
The cricoid ring is much higher behind than in front,
and a gap, filled up by membrane only, is left between its
upper edge and the lower edge of the front part of the
thyroid, when the latter is horizontal. Consequently, the
thyroid cartilage, turning upon the articulations of its horns
Fig. ho.
Diagram of the larynx
seen from the right side, the
thyroid cartilage (77/) being
supposed to be transparent,
and allowing the right ary-
tenoid cartilage (Ar), vocal
cords (/"), and thyroaryte-
noid muscle (ThA), the
upper part of the cricoid car-
tilage (CV) , and the attach-
ment of the epiglottis (Ep) to
be seen. C.th, the right crico-
thyroid muscle: 7V, the
trachea; Hy, the hyoid bone;
ThA is placed just below
the "Adam's apple."
35§
ELEMENTARY PHYSIOLOGY
with the hinder part of the cricoid, as upon hinges, can be
moved up and down through the space occupied by this
membrane ; or, if the thyroid cartilage is fixed, the cricoid
cartilage moves in the same way upon its articulations with
the thyroid. When the thyroid moves downwards or the
cricoid upwards, the distance be-
tween the front part of the thyroid
cartilage and the back of the cri-
coid is necessarily increased ; and
when the reverse movement takes
place the distance is diminished.
There is, on each side, a large
muscle, the crico-thyroid, which
passes from the outer side of the cri-
coid cartilage obliquely upwards
and backwards to the thyroid, and
pulls the latter down ; or, if the
thyroid is fixed, pulls the cricoid
up (Fig. no, ah.).
Perched side by side upon the
upper edge of the back part of
the cricoid cartilage are two small,
irregularly - shaped but, roughly
speaking, pyramidal cartilages, the
arytenoid cartilages (Figs, no and
1 1 2, Ary.'). Each of these is artic-
ulated by its base with the cricoid
cartilage by means of a shallow joint, which permits of very
varied movements, and especially allows the front portions
of the two arytenoid cartilages to approach, or to recede
from, each other.
It is to the forepart of one of these arytenoid cartilages
that the hinder end of each of the two vocal cords is fas-
Fig. in. — Vertical and
Transverse Section
through the larynx,
the Hinder Half of which
is removed.
Ep, Epiglottis; Th, thy-
roid cartilage; a, cavities called
the ventricles of the larynx
above the vocal cords ( V) ; x the
right thyro-arytenoid muscle cut
across; Cr, the cricoid carti-
lage.
THE MECHANISM OF THE LARYNX
359
tened ; and they stretch from these points horizontally for-
ward across the cavity of the larynx, to be attached, close
together, in the re-entering angle of the thyroid cartilage
rather lower than half-way between its top and bottom.
Now when the arytenoid cartilages diverge, as they do
when the larynx is in a state of rest, it is evident that the
aperture of the glottis will be V-shaped, the point of the V
being forward, and the base behind (Figs. 112, 113).
Fig. 112. — The Parts surrounding the Glottis partially dissected and
viewed from above.
7V;., the thyroid cartilage: Cr., the cricoid cartilage; /', the edges of the vocal
cords bounding the glottis; Ary , the arytenoid cartilages; T/i.A., thyro-arytenoid;
C.a./., lateral crico-arytenoid; C.a.p., posterior crico-arytenoid; Ar.p., posterior
arytenoid muscles.
For, in front, or in the angle of the thyroid, the two vocal
cords are fastened permanently close together, whereas,
behind, their extremities will be separated as far as the
arytenoids, to which they are attached, are separated from
each other (Fig. 113, I, B). Under these circumstances
a current of air passing through the glottis produces no
sound, the parallelism of the vocal cords being wanting ;
360 ELEMENTARY PHYSIOLOGY less.
whence it is that, ordinarily, expiration and inspiration take
place quietly. Passing from one arytenoid cartilage to the
other, at their posterior surfaces are certain muscles called
the posterior arytenoid (Fig. 112, Ar.p.). There are also
two sets of muscles connecting each arytenoid with the cri-
coid, and called from their positions respectively the poste-
rior and lateral crico-arytenoid (Fig. 112, C.a.p., C.aJ.).
By the more or less separate or combined action of these
muscles, the arytenoid cartilages, and especially the front
part of these cartilages and, consequently, the hinder ends
of the vocal cords attached to them, may be made to
approach or recede from each other, and thus the vocal
cords rendered parallel (Fig. 113, I, A) or the reverse.
We have seen that the crico-thyroid muscle pulls the
thyroid cartilage down, or the cricoid cartilage up, and thus
increases the distance between the front of the thyroid and
the back of the cricoid, on which the arytenoids are seated.
This movement, the arytenoids being fixed, must tend to
pull out the vocal cords lengthwise, or, in other words, to
tighten them (Fig. 114).
Running from the re-entering angle in the front part of
the thyroid, backwards, to the arytenoids, alongside the
vocal cords (and indeed imbedded in the transverse folds,
of which the cords are the free edges), are two strong mus-
cles, one on each side (Fig. 112, T/i.A.), called thyro-
arytenoid. The effect of the contraction of these muscles
is to pull up the thyroid cartilage after it has been depressed
by the crico-thyroid muscles (or to pull down the cricoid
after it has been raised), and consequently to slacken the
vocal cords (Fig. 114).
Thus, the parallelism {I?) of the vocal cords is determined
chiefly by the relative distance from each other of the ary-
tenoid cartilages ; the tension (c) of the vocal cords is
THE VOICE
?6i
determined chiefly by the upward or downward movement
of the thyroid or cricoid cartilage ; and both these con-
ditions are dependent on the action of certain muscles.
The current of air {d) whose passage sets the cords
vibrating is supplied by the movements of expiration, which,
when the cords are sufficiently parallel and tense, produce
that musical note which constitutes the voice, but otherwise
give rise to no audible sound at all.
I A
Fig. 113.
I. View of the human larynx from above as actually seen by the aid of the instru-
ment called the laryngoscope; A, in the condition when voice is being produced; B,
at rest, when no voice is produced.
ee' , epiglottis (foreshortened).
c.v, the vocal cords.
c.v.s, the so-called false vocal cords, folds of mucous membrane lying above
the real vocal cords.
a, elevation caused by the arytenoid cartilages.
s, w, elevations caused by small cartilages connected with the arytenoids.
/, root of the tongue.
II. Diagram of the same.
16. The Voice. — Voice consists simply of the sound, or
musical note, which results from the vibration of the vocal
cords. Other things being alike, the musical note will be
362 ELEMENTARY PHYSIOLOGY less.
low or high, according as the vocal cords are relaxed or
tightened : and this again depends upon the relative pre-
dominance of the contraction of the thyro-arytenoid and
crico-thyroid muscles. For, when the thyro-arytenoid mus-
cles are fully contracted, the thyroid cartilage will be raised,
relatively to the cricoid, as far as it can go, and the vocal
cords will be rendered relatively lax ; while, when the crico-
thyroid muscles are fully contracted, the thyroid cartilage
will be depressed, relatively to the cricoid, as much as pos-
sible, and the vocal cords will be made more tense.
If, while a low note is being sounded, the tip of the
finger be placed on the crico-thyroid space (which can be
felt, through the skin, beneath the lower edge of the thy-
roid cartilage), and a high note be then suddenly produced,
the crico-thyroid space will be found to be narrowed by the
approximation of the front edges of the cricoid and thyroid
cartilages. At the same time, however, the whole larynx is,
to a slight extent, moved bodily upwards and thrown for-
ward, and the cricoid has a particularly distinct upward
movement; this movement of the whole larynx must be
carefully distinguished from the motion of the thyroid rela-
tively to the cricoid.
The range of any voice depends upon the difference of
tension which can be given to the vocal cords, in these two
positions of the thyroid cartilage. Accuracy of singing
depends upon the precision with which the singer can vol-
untarily adjust the contractions of the thyro-arytenoid and
crico-thyroid muscles — so as to give his vocal cords the
exact tension at which their vibration will yield the notes
required.
The quality of a voice — treble, bass, tenor, etc. — on
the other hand, depends upon the make of the particular
larynx, the primitive length of its vocal cords, their elasticity,
SPEECH
363
the amount of resonance of the surrounding parts, and so
on.
Thus, men have deeper notes than boys and women,
because their larynxes are larger and their vocal cords
longer — whence, though equally elastic, they vibrate less
swiftly.
17. Speech. — Speech is voice modulated by the throat,
tongue, and lips. Thus, voice may exist without speech ;
and it is commonly said that speech may exist without
Fig. 114.
Diagram of a model illustrating the action of the levers and muscles of the larynx.
The stand and vertical pillar represent the cricoid and arytenoid cartilages, while the
rod (b e), moving on a pivot at e. takes the place of the thyroid cartilage; a b is an
elastic band representing the vocal cord. Parallel with this runs a cord fastened at
one end to the rod b c, and, at the other, passing over a pulley to the weight B. This
represents the thyro-arytenoid muscle. A cord attached to the middle of be, and
passing over a second pulley to the weight A, represents the crico-thyroid muscle.
It is obvious that when the bar {be) is pulled down to the position ed, the elastic
band {ab) is put on the stretch.
voice, as in whispering. This is true, however, only if the
title of voice be restricted to the sound produced by the
vibration of the vocal cords ; for, in whispering, there is a
sort of voice produced by the vibration of the muscular
walls of the lips, which thus replace the vocal cords. A
whisper is, in fact, a very low whistle.
The modulation of the voice into speech is effected by
364 ELEMENTARY PHYSIOLOGY less.
changing the form of the cavity of the mouth and nose, by
the action of the muscles which move the walls of those
parts.
Thus, if the pure vowel sounds —
E (as in he), A (as in hay), A' (as in ah),
O (as in or), O' (as in oh), 00 (as in cool),
are pronounced successively, it will be found that they all
may be formed out of the sound produced by a continuous
expiration, the mouth being kept open, but the form of its
aperture, and the extent to which the lips are thrust out or
drawn in so as to lengthen or shorten the distance of the
orifice from the larynx, being changed for each vowel. It
will be narrowest, with the lips most drawn back, in E, wid-
est in A', and roundest, with the lips most protruded, in 00.
Certain consonants also may be pronounced without in-
terrupting the current of expired air, by modification of the
form of the throat and mouth.
Thus the aspirate, H, is the result of a little extra expira-
tory force — a sort of incipient cough. £ and Z, Sh andy
(as in jugular = G soft, as in gentry), Th, L, R, E, V, may
likewise all be produced by continuous currents of air forced
through the mouth, the shape of the cavity of which is pecul-
iarly modified by the tongue and lips.
All the vocal sounds hitherto noted resemble one another
so far, that their production does not involve the stoppage
of the current of air which traverses either of the modulat-
ing passages.
But the sounds of J/ and TV can be formed only by block-
ing the current of air which passes through the mouth, while
free passage is left through the nose. For M, the mouth is
shut by the lips; for N, by the application of the tongue to
the palate.
VIII SPEECH 365
The other consonantal sounds of the English language are
produced by shutting the passage through both nose and
mouth ; and, as it were, forcing the expiratory vocal current
through the obstacle furnished by the latter, the character
of which obstacle gives each consonant its peculiarity. Thus,
in producing the consonants B and P, the mouth is shut by
the lips, which are then forced open in this explosive man-
ner. In T and D, the mouth passage is suddenly barred
by the application of the point of the tongue to the teeth,
or to the front part of the palate ; while in K and G (hard,
as in go) the middle and back of the tongue are similarly
forced against the back part of the palate.
An artificial larynx may be constructed by properly
adjusting elastic bands, which take the place of the vocal
cords ; and, when a current of air is forced through these,
due regulation of the tension of the bands will give rise to
all the notes of the human voice. As each vowel and con-
sonantal sound is produced by the modification of the length
and form of the cavities which lie over the natural larynx,
so, by placing over the artificial larynx chambers to which
any requisite shape can be given, the various letters may be
sounded. It is by attending to these facts and principles
that various speaking machines have been constructed.
Although the tongue is credited with the responsibility of
speech, as the " unruly member," and undoubtedly takes a
very important share in its production, it is not absolutely
indispensable. Hence, the apparently fabulous stories of
people who have been enabled to speak after their tongues
had been cut out by the cruelty of a tyrant, or persecutor,
may be quite true.
Some years ago I had the opportunity of examining a
person, whom I will call Mr. R., whose tongue had been
removed as completely as a skilful surgeon could perform
366 ELEMENTARY PHYSIOLOGY less, viii
the operation. When the mouth was widely opened, the
truncated face of the stump of the tongue, apparently cov-
ered with new mucous membrane, was to be seen, occupying
a position as far back as the level of the anterior pillars of
the fauces. The dorsum of the tongue was visible with diffi-
culty ; but I believe I could discern some of the circumval-
late papillae upon it. None of these were visible upon the
amputated part of the tongue, which had been preserved in
spirit ; and which, so far as I could judge, was about 2\
inches long.
When his mouth was open, Mr. R. could advance his
tongue no further than the position in which I saw it ; but
he informed me that when his mouth was shut the stump
of the tongue could be brought much more forward.
Mr. R.'s conversation was perfectly intelligible ; and such
words as think, the, cow, kill, were well and clearly pro-
nounced. But tin became fin; tack, fack or pack ; toll,
pool; dog, thog; dine, vine; dew, thew ; cat, cat/; mad,
mad/; goose, gooth ; big, pig, bich, pick, with a guttural ch.
In fact, only the pronunciation of those letters the forma-
tion of which requires the use of the tongue was affected ;
and, of these, only the two which involve the employment
of its tip were absolutely beyond Mr. R.'s power. He con-
verted all t's and d's into/V, p's, v's, or th's. Th was fairly
given in all cases ; s and sh, I and r, with more or less of a
lisp. Initial g*s and k's were good ; but final g's were all
more or less guttural. In the former case, the imperfect
stoppage of the current of air by the root of the tongue was
of no moment, as the sound ran on into that of the follow-
ing vowel ; while, when the letter was terminal, the defect
at once became apparent.
LESSON IX
SENSATIONS AND SENSORY ORGANS
1. Movement the Result of Reflex Action. — The agent
by which all the motor organs (except the cilia) described
in the preceding Lesson are set at work, is muscular fibre.
But, in the living body, muscular fibre is, as a rule, made to
contract by a change which takes place in the motor or effe-
rent nerve which is distributed to it. This change again is
generally effected by the activity of the central nervous sys-
tem, with which the motor nerve is connected. The central
organ is thrown into activity, directly or indirectly, by the
influence of changes which take place in nerves, called sen-
sory or afferent,1 which are connected, on the one hand, with
the central organ, and, on the other hand, with some other
part, usually on the surface, of the body. Finally, the altera-
tion of the afferent nerve is itself produced by changes in
the condition of the part of the body with which it is con-
nected ; which changes usually result from external impres-
sions brought to bear on that part.
Sometimes the central organ enters into a state of activity
without our being able to trace that activity to any direct
influence of changes in afferent nerves ; the activity seems
to take origin in the central organ, and the movements to
which it gives rise are called " spontaneous," or " voluntary."
Putting these cases on one side, it may be stated that a
1 It should be mentioned that not all efferent nerves are motor, nor all
afferent nerves sensory. Compare p. 500.
367
3b8 ELEMENTARY THYSIOLOGY less.
movement of the body, or of a part of it, is to be regarded
as the effect of an influence (technically termed a stimulus)
applied directly, or indirectly, to the ends of afferent netves,
and giving rise to a modification of the condition of the
particles or molecules which form the substance of the nerve
fibres, i.e., to a molecular change called a nervous impulse,
which is propagated from molecule to molecule along the
fibres to the central nervous system with which these are
connected. The molecular activity of the afferent nerve
sets up changes of a like order in the fibres and cells of the
centra] organ ; from these the disturbance is transmitted
along the motor nerves, which pass from the central organ
to certain muscles. And, when the disturbance in the molec-
ular condition of the efferent nerves reaches the endings of
those nerves in muscular fibres, a similar disturbance is com-
municated to the substance of the muscular fibres, whereby,
in addition to the production of certain other phenomena, to
which reference has already been made (p. 320), the parti-
cles of the muscular substance are made to take up a new
position, so that each fibre shortens and becomes thicker,
and a movement ensues. Thus, for instance, if we uninten-
tionally prick one of our fingers or touch some very hot
object the hand is jerked away almost before we are aware
of what has happened.
Such a series of molecular changes as that just described is
called a reflex action : the disturbance in the afferent nerves
caused by the irritation being as it were reflected back, along
the efferent nerves, to the muscles. But the name is not a
good one, since it seems to imply that the molecular changes
in the afferent nerve, the central organ, and the efferent nerve
are all alike, and differ only in direction ; whereas there is
reason to think that they differ in many ways.
The several structures necessary for the carrying out of a
SENSATIONS AND CONSCIOUSNESS
369
muscular contraction, resulting in movement, in the way we
have described, may be made clear by the following diagram
(Fig. 115).
The stimulus is applied to a sensory surface (S) ; the
change thus set up is propagated as a nervous impulse along
the sensory (afferent) nerve a.f. to c, a part of the central
nervous system (the spinal cord). The changes which
then take place in c. result in the setting up of a nervous
Fig. 115. — Diagram to illustrate the Paths of Reflex Action.
Sp.C. spinal cord. S, some sensory surface; a.f. afferent or sensory nerve; c.
central connection in nervous system; e.f., e.f. efferent or motor nerves; M1, M2,
muscles. The arrows show the directions in which the impulses travel.
impulse in the motor (efferent) nerve e.f., which is conveyed
outwards along that nerve to the muscle M1, usually on the
same side of the body. Sometimes the impulse is sent out
along a motor nerve to some muscle, M2, on the opposite
side of the body.
2. Sensations and Consciousness. — A reflex action may
take place without our knowing anything about it, and hun-
dreds of such actions are continually going on in our bodies
without our being aware of them. But it very frequently
happens that we learn that something is going on, when a
2B
370 ELEMENTARY PHYSIOLOGY less.
stimulus affects our afferent nerves, by having what we call
a feeling or sensation. We class sensations along with emo-
tions and volitions and thoughts, under the common head
of states of consciousness. But what consciousness is we
know not ; and how it is that anything so remarkable as a
state of consciousness comes about as the result of irritating
nervous tissue is just as unaccountable as any other ultimate
fact of nature.
Sensations are of very various degrees of definiteness.
Some arise within ourselves, we know not how or where,
and remain vague and undefinable. Such are the sensations
of uncomfortableness, of faintness, of fatigue, or of restless-
ness. We cannot assign any particular place to these sensa-
tions, which are very probably the result of affections of the
afferent nerves in general, brought about by the state of the
blood, or that of the tissues in which they are distributed.
However real these sensations may be, and however largely
they enter into the sum of our pleasures and pains, they tell
us absolutely nothing of the external world. They are not
only diffuse, but they are also subjective sensations.
3. The Special Senses. — In the case of other sensations,
each feeling arises out of changes taking place in a definite
part of the body, is produced by a stimulus applied to that
part of the body, and cannot be produced by stimuli applied
to other parts of the body. Thus, the sensations of taste
and smell are confined to certain regions of the mucous
membrane of the mouth and nasal cavities ; those of sight
and hearing to the particular, parts of the body called the
eye and the ear ; and those of touch, though arising over a
much wider area than the others, are nevertheless restricted
to the skin and to some portions of the membranes lining the
internal cavities of the body. Any portion of the body to
which a sensation is thus restricted is called a sense-organ.
ex THE GENERAL PLAN OF A SENSE-ORGAN 371
It may be here remarked that, in the case of the sensation
of touch, the simple feeling of contact is accompanied by
information, not only as to what sense-organ, but also as to
what part of that sense-organ, is being affected. When we
touch a hot or a rough body with the tip of a finger, we
are aware not only that we are dealing with a hot or a
rough body, but also that the hot or rough body is in con-
tact with the tip of the finger ; we " refer," as is said, the
sensation to that part of the tip of the finger which is being
acted upon by the body in question. With the other sensa-
tions the case is different. When we smell a bad smell,
though we know that we smell by the nose, we do not con-
sider that the smell arises in the nose ; we conclude that
there is some object outside ourselves which is causing the
bad smell. We refer the origin of the sensation to some
external cause, and that even when the sensation is after all
due to changes taking place in the nose itself independently
of external objects, as in the unpleasant odours which accom-
pany certain diseases of the nose. Similarly, all our sensa-
tions of sight and of hearing are referred to external objects ;
and even in the case of taste, when a lump of sugar is taken
into the mouth, we are simply aware of a sensation of sweet-
ness and do not associate that sensation of sweetness with
any particular part of the mouth, though, by the sense of
touch, which the inside of the mouth also possesses, we can
tell pretty exactly whereabouts in the mouth the melting lump
is lying.
4. The General Plan of a Sense-organ. — In these sensa-
tions, thus arising in special sense-organs, and hence often
spoken of as "special" sensations, each sensation or feeling
results from the application of a particular kind of stimulus
to its appropriate sense-organ ; and, in each case, the struc-
ture of the sense-organ is arranged in such a manner as to
372 ELEMENTARY PHYSIOLOGY less.
render that organ peculiarly sensitive to its appropriate
stimulus.
Thus, the sensations of sight are brought about by the
action of the vibrations of the luminiferous ether ; and the
eye, or sense-organ of sight, is constructed in such a way
that rays of light, which falling on any other part of the
body produce no appreciable effect, give rise to vivid sensa-
tions when they fall upon it.
Further, we may, with more or less completeness, distin-
guish in each sense-organ two parts : an essential part,
through which the agent producing the sensation (be it
light, a series of sonorous vibrations, a sapid or odorous
chemical substance, a change in temperature, or a variation
in pressure) produces changes in certain structures which
are peculiarly associated with the delicate terminations of
the nerve distributed to the sense-organ ; and an accessory-
part, not absolutely necessary to the sense, but of great use-
fulness inasmuch as it assists in bringing the agent to bear,
in the most efficient way, upon the essential part. In the
case of the eye and ear this accessory part is extremely
complicated, and, indeed, seems to form the greater part of
the whole sense-organ ; in the case of the other senses 4 is
much more simple.
The essential part of each sense-organ is in turn composed
of minute organs, which upon examination appear to be in
reality modified epithelial cells ; and the delicate termina-
tions of the nerve filaments distributed to the sense-organ
may, with more or less distinctness, be traced to the immedi-
ate vicinity of these modified cells. These minute organs,
these modified epithelial cells, may be spoken of as sense-
organules ; they serve as intermediators in each case between
the physical agent of the sensation and the sensory nerve.
The physical agent is by itself unable to produce in the
ix THE SKIN AS A SENSE-ORGAN 373
fibres of the sensory nerve those changes which, reaching
the brain as nervous impulses, give rise to the special sensa-
tions. Thus, as we shall presently see, rays of light falling
upon the optic nerve cannot give rise to a sensation of sight.
The physical agent must act first on the sense-organules, and
these in turn act upon the filaments of the nerve. Thus,
light, falling upon the sense-organules situated in that essen-
tial part of the eye called the retina, sets up changes in them,
these changes set up corresponding changes in the delicate
nerve filaments which with the sense-organules go to make
up the retina, and the changes in the nerve filaments propa-
gated along the optic nerve to the brain give rise, in the
latter, to sensations of sight.
Hence in the essential part of each sense-organ we have
to distinguish between the sense-organules, i.e. the modified
epithelium, and the terminal expansion of the sensory nerve ;
and further, in each sense-organ, there is added to this essen-
tial part a more or less complicated accessory part.
Lastly, in all these special sensations, there are certain
phenomena which arise out of the structure of the sense-
organ, and others which result from the operation of the
central apparatus of the nervous system upon the materials
supplied to it by the sense-organ.
5. The Skin as a Sense-Organ. — The sense of touch
(including the senses of pressure, temperature, and pain) is
possessed, more or less acutely, by all parts of the free sur-
face of the body, and by the walls of the mouth and nasal
passages.
Whatever part possesses this sense consists of a membrane
(integumentary or mucous) composed of a deep layer made
up of fibrous tissue containing a capillary network, and of a
superficial layer consisting of epidermal or epithelial cells,
among which are no vessels. (Sec p. 215.)
374
ELEMENTARY PHYSIOLOGY
Wherever the sense of touch is delicate, the deep layer is
not a mere flat expansion, but is raised up into multitudes
of small, close-set, conical elevations (see Fig. 65, p. 216),
which are called papillae. In the skin, the coat of epithelial
or epidermal cells does not follow the contour of these pa-
pillae, but dips down between them and forms a tolerably
even coat over them. Thus,
the points of the papillae are
much nearer the surface than
the general plane of the deep
layer whence these papillae
proceed. Loops of vessels en-
ter the papillae, and sensory
nerve-fibres are distributed to
them. In some cases the
nerve-fibre ends in a papilla
in a definite organ, in what is
called a tactile corpuscle, or
in a similar body called an
end-bulb. Each of these or-
gans consists essentially of an
oval or rounded swelling,
formed by a modification and
enlargement of the delicate
connective tissue ensheathing
the nerve-fibre ; in the mid-
dle of the swelling the nerve-fibre itself ends abruptly in a
peculiar manner. These bodies are especially found in the
papillae of those localities which are endowed with a very
delicate sense of touch, as in the tips of the fingers, the
point of the tongue, etc. ; and the papillae which contain
tactile corpuscles generally contain few or no blood-vessels.
Tactile corpuscles (Fig. 116) occur most numerously in
Fig. 116. — Tactile Corpuscle within
a Papilla of the Skin of the
Hand. (Ranvier.)
«, «, two nerve-fibres passing to the
corpuscle; a, a, varicose terminations
of the nerve-fibres inside the corpuscle.
END-BULBS
375
the papillae of the skin of the palmar surface of the hand,
especially of the finger tips ; they are also present, but much
less numerously, on the plantar surfaces of the skin of the
feet, and are commonest on parts of the skin where there is
no hair. Each corpuscle forms an elongated, bulbous swell-
ing about 75/* (3^0 inch) in length at the end of the nerve-
fibre to which it is attached, and lies with its long axis in the
long axis of the papilla (t.c, Fig. 65, p. 216). The corpuscle
consists of a sheath or capsule of connective tissue which
sends into the interior incomplete transverse partitions. The
nerve which supplies the cor-
puscle approaches it at its side,
winds once or twice around it,
then enters the body of the cor-
puscle, and divides into a number
of branches, which end in enlarge-
ments.
End-bulbs (Fig. 117) are found
in the papilla? of the skin of the Fig. 117 - End-bulb from the
,. t • 1 • • m. Human Conjunctiva.1 (Long-
lips and in other situations, ihey worth.)
are Spheroidal and Smaller (/tOu . <*, the nerve-fibre; b. capsule
with nuclei; c, c, portions of nerve-
in diameter) than the tactile COr- fibre inside the end-bulb; d, e, cells
of the core.
puscles. They are not all exactly
alike, but the commonest form consists of a thin outer
sheath or capsule, which is nucleated and incloses a mass of
polygonal cells. The nerve-fibre enters the capsule and
ends among the cells in its interior.
The great majority, however, of the nerve-fibres going to
the skin do not end in any such definite organs. They divide
in the dermis into exceedingly delicate minute filaments,
the course and ultimate terminations of which are traced
1 The conjunctiva is the mucous membrane which lines the eyelids and
covers the front of the eyeball.
376 ELEMENTARY PHYSIOLOGY less,
with the greatest difficulty. Some of the finest filaments,
however, pass into the epidermis and are there lost among, or
possibly connected with, some of the epidermal cells, espe-
cially those of the lower layers.
Another kind of highly specialised nerve-ending is found
on the branches of the nerves which supply the skin of the
hand and foot, as they pass through the subcutaneous tissue,
and in other places. These are known as Pacinian corpus-
cles, called after Pacini, an Italian anatomist born in 1812,
who first carefully described them. From their position they
are not, strictly speaking, sensory endings of nerves in the
skin ; but they possess undoubtedly some sensory functions,
although we do not know what these may be.
The Pacinian corpuscles (Fig. 118) are long, ovoid, bul-
bous structures of considerable size, averaging ^ of an inch
in length. They are thus easily visible to the naked eye.
Each corpuscle consists of an elaborate capsule containing
an elongated central core of homogeneous material in which
the axis of the nerve is imbedded and terminates. The
capsule consists of some 30 to 40 capsules, made of connec-
tive tissue, and placed one outside the other like the layers
of an ordinary onion.
It is obvious, from what has been said, that no direct con-
tact takes place between a body which is touched and the
sensory nerve, — a thicker or thinner layer of epithelium, or
epidermis, being situated between the two. In fact, if this
layer is removed, as when a surface of the skin has been
blistered, contact with the raw surface gives rise to a sense
of pain, not to one of touch properly so called. Thus, in
touch, the essential part of the sense-organ consists either
of certain epithelial or epidermal cells of the general in-
tegument or of certain structures contained in the tactile
corpuscles, end-bulbs, and other similar organs. These epi-
PACINIAN CORPUSCLES
377
thelial cells, very slightly modified apparently in the general
skin, but more so in the tactile corpuscles and end-bulbs,
are the sense-organules ; they serve as intermediators be-
tween the physical agent — pressure — and the terminal
filaments of the sensory nerves. The accessory part of the
Fig. 118. — A Pacinian Corpuscle from a Cat's Mesentery. (Ranvier.)
n, nerve-fibre, passing through the core, m., and terminating at a.
sense-organ of touch is very slightly developed, being chiefly
supplied by the variable number and form of the papillae
and the variable thickness and character of the layers of
epidermal cells.
378 ELEMENTARY PHYSIOLOGY less.
(i) The Sensation of Pressure. — Mere contact of a single
object with the skin exerts a pressure on it which results in
a stimulation by means of which we become aware that
something is touching us. The power of discriminating
pressure and its differences we may call the sense of press-
ure. The sensitiveness of the various regions of the skin
in responding to pressure varies, and the difference may be
measured for each part of the skin by determining either
what the least weight is which can be just felt when allowed
to rest on that part, or else by determining the least differ-
ence in weight which can be distinguished between two
weights laid in succession on the same spot. Experiment-
ing in this way it may be shown that the sense of pressure
is most acute on the skin of the forehead and of the back
of the hand. The sense is less acute in the skin of the
finger tips. Careful investigation seems to show, with but
little doubt, that some points on the skin of any part are
peculiarly sensitive to pressure. They are spoken of as
" pressure spots." They are believed to overlie the end-
ings of the nerves which mediate the sensations of pressure,
but what the end-organs of the sense are is not known.
(ii) The Sensations of Temperature. — The feeling of
warmth, or cold, is the result of an excitation of sensory
nerves distributed to the skin, which are possibly distinct
from those which give rise to the sense of pressure. And
it would appear that the heat must be transmitted through
the epidermal or epithelial layer to give rise to this sensa-
tion ; for, just as touching a naked nerve, or the trunk of a
nerve, gives rise only to pain, so heating or cooling an ex-
posed nerve, or the trunk of a nerve, gives rise not to a
sensation of heat or cold, but simply to pain. Thus, if the
elbow be dipped into a mixture of ice and salt, the cold first
affects the skin of the elbow, giving rise to a sensation of
THE SENSATIONS OF TEMPERATURE
379
cold at the elbow, but afterwards attacks the trunk of the
ulnar nerve, which at the elbow lies not very far below the
skin ; and this latter effect is felt as a sensation, not of cold
but of pain. The pain, moreover, thus caused is not felt
in the trunk of the nerve at the elbow, where the cold is
acting, but in the parts where the fibres of the nerve end,
more particularly in the little and ring fingers.
Fig. 119. — Outlines of Heat Spots and Cold Spots. (After Goldscheider.)
The heat spots are cross-hatched and dark, the cold spots are dotted and light. In
some places the heat spots and cold spots overlap each other.
Again, the sensation of heat, or cold, is relative rather
than absolute. Suppose three basins be prepared, one filled
with ice-cold water, one with water as hot as can be borne,
and the third with a mixture of the two. If the hand be
put into the hot-water basin, and then transferred to the
mixture, the latter will feel cold ; but if the hand be kept a
380 ELEMENTARY PHYSIOLOGY less.
while in the ice-cold water, and then transferred to the very
same mixture, this will feel warm.
Like the sense of pressure, the sense of warmth varies in
delicacy in different parts of the body. The cheeks are
very sensitive, more so than the lips ; the palms of the
hands are more sensitive to heat than their backs. Hence
a washerwoman holds her flat-iron to her cheek to test the
temperature, and one who is cold spreads the palms of his
hands to the fire.
The differences in the sensitiveness of the skin to heat
and cold at various points may be readily determined by
touching the several points with the blunt end of a wire
whose temperature can be kept constant at any desired
degree. In this way it is found that some points respond
to heat but not to cold, others to cold but not to heat, so
that we meet with "heat spots" and "cold spots." The
accompanying figure (Fig. 119) shows the distribution of
these spots in a small area of the skin of the thigh. Their
localisation is different from that of the " pressure spots."
They probably mark the position of the terminal organs of
heat and cold, but these, like the organs of pressure, are
unknown.
(iii) The Sensation of Pain. — Pain is often regarded as
the result of an -excessive stimulation of any of the nerve-
endings which are concerned in giving rise to sensations.
Pain also results from stimulating the trunks of the nerves
leading from those endings to the central nervous system.
In the latter case the pain is " referred " outwards to the
end of the nerve, as in the experiment of cooling the elbow
described above. The nerves of any part may thus give
rise to pain, from this it might appear that we can scarcely
speak of any distinct and separate " sense " of pain. But
there are certain facts which show that sensations of pain
ix LOCALISATION OF TACTILE SENSATIONS 381
arc probably distinct from, though ultimately mixed up
with, other sensations. Thus, in many diseases of the ner-
vous system, such as locomotor ataxy, the sensitiveness of
the skin to touch may be almost entirely wanting, while pain
is readily felt. Further, observation shows that the impulses
giving rise to pain, as also those resulting from heat and
cold, pass along the spinal cord on their way to the brain
by paths which are distinct from those which convey the
impulses resulting from mere touch or pressure.
(iv) The Localisation of Tactile Sensations. — Certain very
curious phenomena appertain to the sense of touch ; some
of these are probably in part due to varying anatomical
arrangements, to the varying thickness of the epidermis,
and to the abundance or scantiness of special end-organs.
Not only is tactile sensibility to a single impression much
duller in some parts than in others — a circumstance which
might in many cases be accounted for by the different thick-
ness of the epidermal layer — but the power of distinguish-
ing double simultaneous impressions is very different. Thus,
if the ends of a pair of compasses (which should be blunted
with pointed pieces of cork) are separated by only one-tenth
or one-twelfth of an inch, they will be distinctly felt as two, if
applied to the tips of the fingers ; whereas, if applied to the
back of the hand in the same way, only one impression will
be felt ; and, on the arm, they may be separated for a quarter
of an inch, and still only one impression will be perceived.
Accurate experiments have been made in different parts
of the body, and it has been found that two points can be
distinguished by the tongue, if only one twenty-fourth of an
inch apart ; by the tips of the fingers if one twelfth of an
inch distant ; while they may be one inch distant on the
cheek or forehead, and even three inches on the back, and
still give rise to only one sensation.
382 ELEMENTARY PHYSIOLOGY less.
6. The Muscular Sense. — What is termed the muscular
sense is less vaguely localised than the sensations referred
to above in Section 2 (p. 370), though its place is still
incapable of being very accurately defined. This muscular
sensation is largely the feeling of resistance which arises
when any kind of obstacle is opposed to the movement of
the body, or of any part of it ; and it is something quite
different from the feeling of contact or even of pressure.
Lay one hand fiat on its back upon a table, and rest a
disc of cardboard a couple of inches in diameter upon the
ends of the outstretched fingers ; the only result will be a
sensation of contact — the pressure of so light a body being
inappreciable. But put a two-pound weight upon the card-
board, and the sensation of contact will pass into what
appears to be a very different feeling, viz., that of pressure.
Up to this moment the fingers and arm have rested upon
the table ; but now let the hand be raised from the table,
and another new feeling will make its appearance — that of
resistance to effort. This feeling comes into existence with
the exertion of the muscles which raise the arm ; and it is
the consciousness of that exertion which goes by the name
of " the muscular sense."
Any one who raises or carries a weight knows well enough
that he has this sensation : but he may be greatly puzzled to
say where he has it. Nevertheless, the sense itself is very
delicate, and enables us to form tolerably accurate judg-
ments of the relative intensity of resistances. Persons who
deal in articles sold by weight are constantly enabled to form
very precise estimates of the weight of such articles by bal-
ancing them in their hands ; and in this case they depend
in a great measure upon the muscular sense.
But the muscular sense embraces more than the mere con-
sciousness of the resistance to effort involved in lifting a
ix THE SENSE OF TASTE 383
weight. Thus, it is a matter within everybody's experience
that, even when the eyes are closed, we are perfectly well
aware of the direction and extent of any movement of any
part of the body. Moreover we are equally conscious of the
position of any part of the body at any moment, whether the
position is the result of our own voluntary movement or
the result of the action of some other person, who has placed
the part in position. In all such cases the muscular sense
supplies the basis of our knowledge of the position or of the
movements of the parts of our body.
The muscular sense is thus essentially concerned with sen-
sations arising from movements, whether active or passive.
Now the parts affected by these movements are chiefly the
following four ; the skin, the muscles, the tendons, and the
ligaments. It has been supposed that the impulses which
give rise to the sensations may be largely due to the stimu-
lation of cutaneous nerves resulting from the varying extent
to which the skin is put on the stretch by the movements ;
but the arguments in favour of this view are not conclusive.
On the other hand, we know that the muscles themselves and
the ligaments at the joints possess nerve-fibres which are cer-
tainly afferent, i.e., sensory ; and similarly afferent fibres, con-
nected with extremely minute end-bulbs, are distributed to
the tendons. And there is but little doubt that we must look
to the impulses generated in these nerves as providing the
sensations which form the basis of the muscular sense.
7. The Sense of Taste. — The organ of the sense of taste
is the mucous membrane which covers the tongue, especially
its back part, and the hinder part of the palate. Like that
of the skin, the deep, or vascular, layer of the mucous mem-
brane of the tongue is raised up into papillae (Fig. 120) ;
but these are large, separate, and have separate coats of epi-
thelium. Towards the tip of the tongue they are for the
3»4
ELEMENTARY PHYSIOLOGY
most part elongated and pointed, and are called filiform ;
over the rest of the surface of the tongue these are mixed
with larger papillae, with broad ends and narrow bases, called
fungiform {F.p.) ; but towards its root there are a number
of still larger papillae, arranged in the figure of a V with its
point backwards, each of which is like a fungiform papilla
Fig. 120. — The Mouth widely opened to show the Tongue anp Palate.
Uv, the uvula; Th, the tonsil between the anterior and posterior pillars of the
fauces; C.p, circumvallate papillae; F.p, fungiform papilla;. The minute filiform
papillae cover the interspaces between these. On the right side the tissues are
partially dissected to show the course of the filaments of the trigeminal nerve, V,
and the glossopharyngeal nerve, VIII.
surrounded by a wall. These are the circumvallate papillae
(Fig. 1 20, C.p, and 121, A).
In both the fungiform and circumvallate papillae, the cells
which are specially concerned in giving rise to sensations of
TASTE-BUDS
J8S
taste are arranged in bulbous groups, somewhat like the
leaves in a bud, and hence these groups are known as taste-
buds. In the circumvallate papillae these taste-buds lie im-
bedded in the layers of epithelium which cover the sides of
each papilla.
Each " bud " (Fig. 121, B) is flask-shaped and consists of
an outer wall, made up of elongated cells placed side by side
like the staves of a barrel (vr) and leaving an opening at the
end of the bud where it comes to the surface of the papilla.
Fig. 121. — Diagram of a Circumvallate Papilla, and of Taste-buds.
A. A circumvallate papilla cut lengthwise; e, epidermis; d, dermis; t, taste-
buds; n, nerve-fibres.
B. Two taste-buds; e, epidermis; d, dermis; c, the outer or cover cells shown in
the lower bud; n, four inner or gustatory cells with processes; in, processes project-
ing at mouth of buds.
The inside of the bud is filled with the gustatory cells (;/),
packed side by side. Each of these cells is long and very
thin, with a large nucleus at its middle point, and each cell
has at its outer end a delicate process, like a stiff cilium (but
not vibratile), which projects through the open mouth of the
bud.
The papillae are very vascular, and they receive nervous
filaments from two sources, the one the nerve called glosso-
pharyngeal, the other the gustatory, which is a branch 0!
386 ELEMENTARY PHYSIOLOGY less.
the fifth nerve (p. 537). The latter chiefly supplies the
front and sides of the tongue, the former its back and the
adjacent part of the palate ; and there is reason to believe
that different taste sensations are supplied by the two
nerves.
The peculiar cells in the taste-buds are the sense-organ-
ules of taste, and constitute the essential part of the organ of
taste. The nerve-fibres enter the taste-buds and terminate
amongst the gustatory cello. The tongue itself, which by its
movements brings the sapid substances into immediate con-
tact with these modified epithelium cells, may be regarded
as the accessory part of the organ of taste.
The great majority of the sensations we call taste, how-
ever, are in reality complex sensations, into which smell, and
even touch, and the temperature sense, as in the sensation
of cold produced by peppermint, largely enter. When the
sense of smell is interfered with, as when the nose is held
tightly pinched, it is very difficult to distinguish the tastes of
various objects. A piece of onion, for instance, the eyes
being shut, may then easily be confounded with a bit of
apple. This explains the not uncommon device of pinching
the nose when taking nauseous medicine.
But the so-called " tastes," which are thus affected by the
absence of smell, ought rather to be spoken of as " flavours "
than as tastes. They are distinctly due to the odoriferous
particles the substances emit, and thus people are in the
habit of " sniffing " a glass of wine in order to appreciate
what they call its taste. True taste is independent of smell,
as in the case of sugar or quinine. When we come to investi-
gate the matter closely, we find that the various real tastes
may be arranged under four heads : these are — sweet, bitter,
sour or acid, and salt. These tastes are not excited equally
all over the surface of the tongue. Thus, the tip is most
IX THE SENSE OF SMELL 38?
sensitive to sweet and salt substances, and the back to bitter,
while the sides of the tongue most readily respond to
acids.
The sense of taste is most acute at the temperature of the
body, and substances to be tasted must be in solution.
8. The Sense of Smell. — The organ of the sense of smell
is the delicate mucous membrane which lines the upper part
of the nasal cavities. In this part the mucous membrane
is distinguished from the rest of the mucous membrane of
these cavities — first, by- the character of its cells and by
possessing no cilia ; secondly, by receiving a large nervous
supply from the olfactory, or first, pair of cerebral nerves
(p. 535), as well as a certain number of filaments of the
fifth pair, whereas the rest of the mucous membrane is sup-
plied from the fifth pair alone.
Each nostril leads into a spacious nasal chamber, sepa-
rated, in the middle line, from its fellow of the other side,
by a partition, or septum, formed partly by cartilage and
partly by bone, and continuous with that partition which
separates the two nostrils one from the other. Below, each
nasal chamber is separated from the cavity of the mouth
by a floor, the bony palate (Figs. 122 and 123) ; and when
this bony palate comes to an end, the partition is continued
down to the root of the tongue by a fleshy curtain, the soft
palate, which has been already described. The soft palate
and the root of the tongue together constitute, under ordi-
nary circumstances, a movable partition between the mouth
and the pharynx ; and it will be observed that the opening
of the larynx, the glottis, lies behind the partition : so that
when the root of the tongue is applied close to the soft
palate no passage of air can take place between the mouth
and the pharynx. But in the upper part of the pharynx
above the partition are the two hinder openings of the nasal
?88
ELEMENTARY PHYSIOLOGY
cavities (which are called the posterior nares) separated by
the termination of the septum ; and through these wide
Fig. 122. — Vertical Longitudinal Sections of the Nasal Cavity.
The upper figure represents the outer wall of the left nasal cavity; the lower
figure the right side of the middle partition, or septum (Sfi.) of the nose, which forms
the inner wall of the right nasal cavity. /, the olfactory nerve and its branches; I",
branches of the fifth nerve; Pa, the palate, which separates the nasal cavity from
that of the mouth; S.T, the superior turbinal bone; M. T, the middle turbinal; I.T,
the inferior turbinal. The letter /is placed in the cerebral cavity; and the partition
on which the olfactory lobe rests, and through which the filaments of the olfactory
nerves pass, is the cribriform plate. In the upper figure the branches of the olfac-
tory nerve are represented as coming somewhat too far down.
IX THE SENSE OF SMELL 389
openings the air passes, with great readiness, from the
nostrils along the lower part of each nasal chamber to the
glottis, or in the opposite direction. It is by means of
the passages thus freely open to the air that we breathe, as
we ordinarily do, with the mouth shut.
Each nasal chamber rises, as a high vault, far above the
level of the arch of the posterior nares — in fact, about as
high as the depression of the root of the nose. The upper-
most and front part of its roof, between the eyes, is formed
by a delicate horizontal plate of bone, perforated like a sieve
by a great many small holes, and thence called the cribri-
form plate (Fig. 123, Cr.). It is this plate alone (with the
membranous structures which line its two faces) which, in
this region, separates the cavity of the nose from that which
contains the brain. The olfactory lobes, which are directly
connected with, and form indeed a part of, the brain,
enlarge at their ends, and their broad extremities rest upon
the upper side of the cribriform plate, sending through it
immense numbers of delicate filaments, the olfactory nerves,
which are distributed as follows (Fig. 122) : —
On each wall of the septum the mucous membrane forms
a flat expansion, but on the side walls of each nasal cavity it
follows the elevations and depressions of the inner surfaces
of what are called the upper and middle turbiiial or spongy
bones. These bones are called spongy because the interior
of each is occupied by air cavities separated from each other
by very delicate partitions only, and communicating with
the nasal cavities. Hence the bones, though massive-look-
ing, are really exceedingly light and delicate, and fully
deserve the appellation of spongy (Fig. 123).
Over the upper turbiiial bones, and on both sides of the
septum opposite to them, the mucous membrane is specially
modified, and receives the name of olfactory mucous mem-
390
ELEMENTARY PHYSIOLOGY
brane ; and it is to this olfactory mucous membrane that the
filaments of the olfactory nerve passing through the cribri-
form plate are distributed.
There is a third light scroll-like bone distinct from these
two, and attached to the maxillary bone, which is called the
inferior turbinal, as it lies lower than the other two, and im-
perfectly separates the air passages from the proper olfactory
chamber (Figs. 122, 123). It is covered by the ordinary
ciliated mucous membrane of the nasal passage, and receives
no filaments from the olfactory nerve.
.An.
Fig. 123. — A Transverse and Vertical Section of the Osseous Walls of
the Nasal Cavity taken nearly through the letter / in the Fore-
going Figure.
Cr, the cribriform plate; S.T, M . T, the chambered superior and middle turbinal
bones on the former of which and on the septum (5/.) the filaments of the olfactory
nerve are distributed; /. T, the inferior turbinal bone; PL the palate; An. the antrum
or chamber which occupies the greater part of the maxillary bone and opens into the
nasal cavity.
In the non-olfactory part of the nasal mucous membrane
the epithelium cells are ordinary ciliated epithelium cells
(see p. 308), and many glands secreting mucus are present ;
but in the olfactory part the epithelium cells not only lose
their cilia, but become peculiarly modified.
IX
THE SENSE OF SMELL
391
They are of two kinds and somewhat similar to the cells
composing a taste-bud ; but their arrangement is different, the
two kinds being intermingled. One kind of cell is long,
slender and rod-shaped, with a large nucleus towards its inner
end (Fig. 124, b). Those of the second kind are also thin
and rod-like at their inner ends, but
beyond the nucleus the outer end
is wide and columnar (a). The
cells of the first kind, which are the
more numerous, are supposed to
be specially concerned in giving
rise to the sensations of smell.
The delicate olfactory nerve fila-
ments appear to end in these
modified epithelial cells, which, in-
deed, are the sense-organules of
the organ of smell. The olfactory
mucous membrane thus constitutes
the essential part of the organ.
The accessory part of the organ
of smell may be described as
follows : —
From the arrangements which
have been described, it is clear
that, under ordinary circumstances,
the gentle inspiratory and expira-
tory currents will flow along the
comparatively wide, direct passages
afforded by SO much of the nasal filament from the olfactory nerve.
chamber as lies below the middle turbinal ; and that they
will hardly move the air inclosed in the narrow interspace
between the septum and the upper and middle spongj
bones, which is the proper olfactory chamber.
Fig. 124. —Cells of Olfactory
Epithelium. (Max Schcltze.)
1, From a frog; 2, from man.
a, columnar epithelial cell;
b, olfactory rod-cell; c, outer
limb, ti, inner limb of olfactory
cell, the former being prolonged
at e into fine hairs, the latter
being continuous with a nerve
392 ELEMENTARY PHYSIOLOGY less
If the air currents are laden with particles of odorous
matter, these can only reach the olfactory membrane by
diffusing themselves into this narrow interspace ; and, if
there be but few of these particles, they will run the risk
of not reaching the olfactory mucous membrane at all, unless
the air in contact with it be exchanged for some of the odor-
iferous air. Hence it is that, when we wish to perceive a
faint odour more distinctly, we " sniff" or snuff up the air.
Each sniff is a sudden inspiration, the effect of which must
reach the air in the olfactory chamber at the same time as,
or even before, it affects that at the nostrils ; and thus must
tend to draw a little air out of that chamber from behind.
At the same time, or immediately afterwards, the air sucked
in at the nostrils entering with a sudden vertical rush, part
of it must tend to flow directly into the olfactory chamber,
and replace that thus drawn out.
The loss of smell which takes place in the course of a
severe cold may, in part, be due to the swollen state of the
mucous membrane which covers the inferior turbinal bones,
impeding the passage of odoriferous air to the olfactory
chamber.
Very little is known of the physiology of smell, and smells
have not so far been classified except as agreeable or the
reverse ; but recent observations seem to show that a much
more detailed classification is possible. Everyday experi-
ence shows that the sense is extremely delicate, the most
minute amount of odoriferous matter, such as musk, serving
to excite it. The sense is, however, much more highly de-
veloped in, and much more important in the daily lives of,
some of the lower animals, such as the dog, than in man.
9. The Ear and the Sense of Hearing in General. — The
ear, or organ of the sense of hearing, is very much more
complex than any of the sensory organs yet described ; and
rv THE EAR AND THE SENSE OF HEARING 393
in it the accessory parts especially are much more highly
developed.
The essential part, on each side of the head, lies in the
walls of a very peculiarly formed membranous bag. This
bag, when the ear first begins to be formed, is a simple
round sac, but it subsequently takes on a very complicated-
form, and becomes divided into several parts, which receive
special names. It is lodged in a cavity of correspondingly
intricate shape, hollowed out of a solid mass of bone (called
from its hardness petrous) , which forms part of the temporal
bone, and lies at the base of the skull. The sac, however,
does not completely fill the cavity, so that a space is left
between the bony walls and the contained sac. This space,
which is continuous all round the sac, being interrupted at
certain places only where the membranous sac is attached
to the bony walls, contains a fluid provided by the lym-
phatics of the neighbourhood, and called perilymph.
The membranous sac, the walls of which consist chiefly
of connective tissue, is lined by an epithelium, and contains
a fluid of its own called endolymph. The perilymph, it
will be understood, is quite distinct from the endolymph,
the two fluids being separated by the walls of the membra-
nous sac.
Over a great part of the interior of the membranous sac
the epithelium is simple in character, but at certain places
to be presently described it assumes special features, being
greatly thickened, and bearing hairlike processes, or being
otherwise modified, so as to be easily affected by even such
slight movements as the vibrations which produce sound.
Where these patches or tracts of modified or auditory epi-
thelium, as it is called, exist, the membranous sac is more
closely attached to the bony walls ; and branches of the
eighth, acoustic or auditory, nerve (see p. 537), passing
394 ELEMENTARY PHYSIOLOGY less.
through channels in the bony walls, through the tissue
attaching the membranous sac to the bony walls, and
through the wall of the membranous sac itself, come into
peculiar relation with, and end among, the cells of these
patches of auditory epithelium. It is only to the places
where the epithelium is thus modified that filaments of the
auditory nerve are distributed. The auditory epithelium
constitutes the essential part of the sense-organ of hearing.
The membranous sac is known as the membranous laby-
rinth, and the bony cavity in which it lies is similarly called
the osseous labyrinth ; together they constitute the internal
ear. Outside of this lies the middle ear, or drum, and still
further outward is the external passage opening upon the
side of the head, which with the pinna, or " ear " in popular
language, constitutes the external ear. All of these parts
except the auditory epithelium are accessory parts of the
organ of hearing.
What takes place in hearing may briefly be stated as fol-
lows. The vibrations set up by a sounding body are con-
ducted, by the accessory apparatus to be presently described,
to the perilymph, and from thence through the walls of the
membranous sac to the endolymph. As the vibrations trav-
elling along the endolymph reach those particular places
where the epithelium is modified, and where the filaments
of the auditory nerve end, they in some way or other affect
the epithelium cells. Through the intermediation of these
cells the delicate endings of the auditory nerve are stimu-
lated, so that molecular changes constituting a nervous
impulse are set up in the substance of the nerve, and trans-
mitted along the nerve from particle to particle, until they
reach that part of the brain the molecular disturbance of
which gives rise to sensations of sound.
Thus, until the auditory epithelium is reached, that which
THE MEMBRANOUS LABYRINTH
395
takes place in the ear when we hear a sound is simply a
transmission of vibrations of the same order as those which
are produced by the sounding body ; but the processes
which intervene between the epithelium and the brain are
not of the same kind ; here there is no transmission of such
vibrations, but what takes place is a series of changes of
nerve substance of the same order as, though perhaps not
exactly like, those which are set up by the action of a stimu-
lus on any other nerve.
10. The Membranous Labyrinth. — The membranous
bag, as we have said, is not simple but complicated : it
consists of several parts, namely, the utricle, the saccule,
the membranous semicircular canals, and the membranous
cochlea.
(i) The Utricle, the Saccule, and the Membranous Semicircu-
lar Canals. — The utricle is a somewhat ovoid sac (Fig. 125,
IT), into which open the three hooplike, semicircular canals.
Of these, two are placed vertically : one is situated high up
and directed anteriorly and outwards, the other is lower and
directed posteriorly and outwards; they are called the supe-
rior (A.S.C) and posterior (P.S.C) semicircular canals.
The third is placed horizontally and directed outwards,
hence it is called the external or horizontal semicircular
canal (Fig. 125, E.S.C). The three canals thus lie nearly
at right angles to one another in the three directions of
space ; this has nothing to do with judging the directions of
sound, but has a relation to other functions of the canals.
Each of these three hoops is dilated at one of its two ends,
where it opens into the utricle, into what is called an ampulla
(Fig. 125), the other end having no ampulla. Thus there
is one ampulla to each canal. Those ends of the two verti-
cal canals which are not dilated into ampullae join together
before they open into the utricle.
59b
ELEMENTARY PHYSIOLOGY
LESS,
In each ampulla is a ridge or crest, called crista acustica,
placed crosswise, and projecting into the cavity of the canal.
Each crest is formed partly by an infolding and thickening
of the connective tissue wall of the ampulla, and partly by a
thickening of the epithelium, which here has the peculiar
characters already referred to. A similar but oval patch of
thickened, modified, auditory epithelium, with a thickening
of the wall beneath it, is found in the utricle itself; this is
called a macula acustica.
E.S.C
AJF
BS.C
Coch
Fig. 125. — Diagram to illustrate the Membranous Labyrinth and the
endings of the auditory nerve.
U, utricle, containing a macula acustica; A.S.C, E S.C, P.S.C, superior, exter-
nal, and posterior semicircular canals; in each case the letters point to the ampulla of
the canal, which contains a crista acustica; S, saccule, containing a macula acustica ;
A. V., canal uniting the utricle with the saccule; Coch, cochlea, with the nerve fila-
ments supplying the organ of Corti; c, canal uniting the saccule with the cochlea;
A.N, auditory nerve dividing into several branches.
Attached to the utricle is a similar smaller sac (forming
another division of the primitive membranous bag) called
the saccule (Fig. 125, s), on the walls of which is a similar
rounded patch of modified epithelium, or macula acustica.
The cavity of the saccule is cut off from that of the utricle,
except for a curious roundabout connection by means of a
narrow canal (Fig- 125, A.V.).
ix THE MEMBRANOUS LABYRINTH 397
Branches of the auditory nerve pass to these parts of the
membranous labyrinth and send fibres to the three crests of
the three ampullae, to the patch on the utricle, and to the
patch on the saccule. In each crest and each patch the
epithelium is thickened and modified, and although the crests
are slightly different in structure from the patches, the general
features are the same in all. Whereas over the rest of the
inside of the membranous labyrinth the epithelium consists
(Fig. 1 26, A, e) of a single layer of low, rather fiat cells, in the
crests and patches the cells lie several deep, and are of a pecul-
iar form. Like the cells in the olfactory epithelium, they are
of two kinds. Some are columnar and bear each a stiff,
hairlike filament projecting into the cavity of the labyrinth
(Fig. 126, c.c, a.h). These filaments, often called auditory
hairs, appear at first sight to resemble cilia, but they are stiff,
and, unlike cilia, have no active movement of their own.
They are longer and more conspicuous in the crests of the
ampulla; than in the patches of the utricle and saccule.
The other cells of the epithelium of the crests and patches
are long and slender bodies with ?. bulging nucleus and no
hair, and are probably only supporting in function (sp. c).
The fibres of the auditory nerve may be traced through the
connective tissue wall of the crest or. patch into the epi-
thelium, where they break up into delicate filaments, which
appear to end, not in the cells, but among them (n, a, b).
It is very clear that movements in the endolymph may
set in motion these hairs, very much as waves of the wind
set in motion stalks of standing grain, and that the move-
ments of the hairs, by help of the cells to which the hairs
belong, may excite the delicate nervous filaments and so set
up disturbances or impulses which pass along the auditory
nerve to the brain. It is probable, as we shall learn more
fully later, that the utricle, saccule, and canals are not con-
ELEMENTARY PHYSTOLOHY
Fig. 126. — Diagrams to show the Structure of the Crista Acustica.
A, Longitudinal section of ampulla, the crest being cut crosswise.
c, one end of the ampulla opening into the semicircular canal; u, the other end
opening into the utricle; e, ordinary epithelium lining the greater part of the ampulla;
cr, the crest with a.e, auditory epithelium; a./i, auditory hairs; c.t, connective
tissue support to the auditory epithelium; n, fibres of the auditory nerve passing into
the auditory epithelium; i, epithelium intermediate between the auditory epithelium
and the ordinary epithelium of the rest of the ampulla.
B, Diagram to illustrate the character of the cells of the auditory epithelium and
the relation of the auditory hairs to the cells. I, the auditory epithelium; II, the
connective tissue on which it rests; c.c, cylindrical cells bearing auditory hair, a.h;
sp.c, supporting cells, not bearing hairs.
it, a fibre of the auditory nerve passing through II and dividing into fine branch-
ing filaments at b.
ix THE MEMBRANOUS COCHLEA 399
cerned specifically with the function of hearing, but have
other totally different functions.
In the utricle and saccule, where, as has been said, the
hairs are not so conspicuous, a mass of small calcareous
particles, called otoliths, imbedded in a soft substance, lies
in contact with the tips of the hairs. In some of the lower
animals these minute particles are replaced by one large
stone.
(ii) The Membranous Cochlea. — An important part of
the membranous labyrinth remains to be described, and
that is the cochlea, which, as we shall see, is the specifically
auditory part of the ear.
Connected with the saccule by a narrow canal is an exten-
sion of the original membranous sac, in the form of a long
tube, closed at the end (Fig. 125, Cocli). This cochlear
tube, like the parts of the sac already described, is lined
with epithelium, contains endolymph, and is lodged in a
bony cavity filled with perilymph. So far it resembles the
rest of the labyrinth, but in many other respects it is very
different.
In the first place, in the semicircular canals the mem-
branous walls follow, in general, the contour of the bony
walls, so that in a section the membranous canal presents a
flattened circular contour lying in the larger circular contour
of the bony canal. But in the cochlea, on the contrary, the
contour of the cochlear tube is, along its whole length, to-
tally different from that of the containing cavity ; for, in
transverse section, the contour of the containing cavity is
almost circular, a bony ledge, the spiral lamina, projecting
from the bony wall upon one side for a certain distance into
the cavity (Fig. 128, /.s) ; but the section of the cochlear
tube itself is nearly triangular (C.C). The cochlear tube
in fact is, in shape, what is often called triangular (as when
4oo ELEMENTARY PHYSIOLOGY less.
we speak of a triangular file), but should be called trihedral:
that is to say, it has three sides or faces (and three edges).
In the second place, in the utricle and saccule, the sac is
for the most part free from the bony walls, being attached
only at the places where the nerve fibres pass into it, and,
more loosely, at some few other points ; but in the cochlea,
on the contrary, the cochlear tube closely adheres to the
bony wall, along the whole length of the tube, in two regions,
namely, over one face and at the edge opposite. The one
face is attached firmly to one side of the bony wall, and the
opposite edge adheres to the projecting edge of the spiral
lamina. Thus the cochlear tube, containing endolymph,
together with the spiral lamina, divides the cavity contain-
ing perilymph, in which it lies, into two passages, called
scalse, which are seen in section (Fig. 127) to be placed
one above and the other below the triangular cavity of the
cochlear tube itself. The membranous tube is a trifle shorter
than the bony one, hence the two scalse communicate with
each other at the far end of the tube, but not elsewhere.
In the third place, the cochlear tube is not straight or
even simply curved, but is twisted upon itself, into a spiral
of two and a half turns. In these twists it is accompanied
by the scalae and also by the spiral lamina, whence the name
of the latter (Figs. 127, L.S, 128, l.s). The whole arrange-
ment somewhat resembles the shell of a snail ; hence the
name cochlea. All along the spiral the edge of the cochlear
tube attached to the lamina spiralis is directed inwards and
the attached face outwards ; so that when a section is made
through the axis of the spiral a succession of rounded spaces
is cut through, each space exhibiting, above and below, the
somewhat half-moon-shaped section of a scala, the two scalse
being separated, on the outer side, by the cochlear tube, and,
on the inner, by the spiral lamina (Fig. 127).
IX
THE MEMBRANOUS COCHLEA
401
The triangular membranous tube which, as we have seen,
contains endolytnph and is continuous with the saccule, is
called the canal of the cochlea, or scala media (because it
lies between the two other scala;). The upper of the two
cavities containing perilymph, when traced down to the
bottom of the spiral, is found to be continuous with the
cavity containing perilymph which surrounds the utricle
and saccule and is called the vestibule; hence the upper
scala is called the scala vestibuli. The lower cavity, when
So-4t j'0.p-
Fig. 127. — A Section through the Axis of the Cochlea, magnified Three
Diameters.
Sc.M, scala media; Sc.V, scala vestibuli; Sc.T, scala tympani; L.S, lamina
spiralis; Md, bony axis, or modiolus, round which the scalae are wound; C.N,
cochlear nerve.
similarly traced to the bottom of the spiral, ends against the
inner wall of the middle ear or tympanum by an opening,
called the fenestra rotunda, which is closed by a membrane.
Hence this lower cavity is called the scala tympani. Thus,
the scala vestibuli and scala tympani begin at different
points, and are separated along their whole course by the
cochlear tube and the spiral lamina, except at the very tip
of the spiral, where these latter end ; here the two scalae
are^ prolonged beyond the cochlear tube and join together,
forming a common space, as seen at the top of Fig. 127.
The vibrations of sound are brought, as we shall see, to
the perilymph chamber of the vestibule, whence they spread
}D2 ELEMENTARY PHYSIOLOGY less.
/nto the scala vestibuli. Passing upwards in the spiral along
the scala vestibuli, they enter at the summit the scala tym-
pani, along which they descend, and are eventually lost at
the fenestra rotunda in which that scala ends.
(iii) The Organ of Corti. — But besides this peculiar ar-
rangement of the chambers, there are other and still more
important differences between the cochlea and the rest of
the labyrinth.
The auditory nerve is, as we have seen, distributed to
certain parts only of the rest of the membranous labyrinth,
namely, to the crests of the ampulla? and to the patches on
the utricle and the saccule ; but, in the case of the cochlea,
fibres, running in canals excavated in the bony core of the
spiral, and in the spiral lamina (Fig. 128, AN), run to and
end in the canal of the cochlea along its whole length, from
the bottom to the top of the spiral (Fig. 125, Coch). And
the mode of ending of these nerves is very peculiar.
If we examine a section of one of the spirals of the cochlea
(Fig. 128), we see that the upper side of the cochlear tube
(that which separates it from the scala vestibuli) is formed
by a thin membrane (called the membrane of Reissner, Fig.
128, mR), lined internally by simple epithelium. The outer
convex side of the cochlear tube, that side by which it is
firmly attached to the bony wall, is also lined internally by
simple epithelium. Neither here nor in the membrane of
Reissner do any fibres of the auditory nerve end. But the
remaining side of the tube, that which looks towards the scala
tympani, possesses on its inner face, along the whole length
of the tube, from the bottom to the top of the spiral, a very
remarkable and strangely modified epithelium ; and, along
the whole length of the tube, fibres of the auditory nerve
pass to and end among the cells of this epithelium, which is
spoken of as the organ of Corti (Fig. 128, O.C).
THE MEMBRANOUS COCHLEA
403
The membrane which separates the cavity of the cochlear
tube from the scala tympani, and on which the organ of
Fig. 128. — Section of Coil of Cochlea.
Sc.V, scala vestibuli; Sc.T, scala tympani: C.C, canalis cochlearis, or scala
media; O.C, organ of Corti; >«R, membrane of Reissner: »it, membrana tectoria
(a gelatinous membrane overlying the organ of Corti, and supposed to act as a
damper). AN, fibres of the auditory nerve running in l.s, the lamina spiralis, and
ending in the organ of Corti: a, connective tissue cushion to which the basilar mem-
brane is attached on the outside: b, bony walls.
The figure has, for simplicity's sake, been made somewhat diagrammatic. The
spiral lamina has been drawn too short; the proportions of the spiral lamina and the
seals are more exactly rendered in Fig. 127.
404
ELEMENTARY PHYSIOLOGY
Corti is placed, is of a peculiar character, consisting of thou-
sands of delicate fibres placed side by side and extending
across the canal ; it is called the basilar membrane. The
organ of Corti itself consists of, in the first place, the so-
called rods of Corti, peculiarly shaped long bodies, which
are seen in section leaning, as it were, against each other.
There is an inner row of these and an outer row all along
the spiral, each row consisting of several (four to six) thou-
sands of rods. At the inner side and at the outer side of the
Fig. 129. — Transverse Section through the Side Walls of the Skull to
show the Parts of the Ear; the Left Ear seen from in front. (After
Arnold.) (From Quain's Anatomy.)
1, Pinna; 2 to 2', external auditory meatus; 2', tympanic membrane; 3, cavity of
the middle ear; above 3 the chain of small bones; 4, Eustachian tube; 5, internal
auditory meatus, containing the auditory (lower) and facial nerves coming from the
brain; 6, bony labyrinth of interna] ear; a, petrous part of temporal bone; c, e,f,
other parts of temporal bone; b, internal carotid artery; d, facial nerve.
rods are very peculiar epithelial cells, also arranged in rows,
each row consisting of several thousand cells. Each of these
cells bears short hairs on its free surface, hence they are called
hair-cells, inner and outer. The fibres of the auditory nerves
THE BONY LABYRINTH
405
passing through the spiral lamina reach the cochlear tube
along the whole length of the spiral, and branch into fila-
ments which go to the organ of Corti and terminate among,
but probably not in, the hair-cells.
11. The Bony Labyrinth. — It will be remembered
that the membranous labyrinth, filled with endolymph, lies
in an intricate cavity with bony walls called the osseous
labyrinth (Fig. 129, 6), which corresponds to the former
largely but not wholly in form. The bony vestibule contains
the membranous saccule and utricle ; the bony semicircular
Fig. 130. — The Membrane of the Drum of the Right Ear, with the Small
Bones of the Ear seen from the Inner Side; and the Walls of the
Tympanum, with the Air-cells in the Mastoid Part of the Temporal
Bone.
The petrous part of the temporal bone containing the labyrinth is supposed to be
removed, the foot-plate of the stapes having been detached from the fenestra ovalis.
M.C, mastoid cells; Mall, malleus; Inc, incus; St, stapes; a b, lines drawn
through the horizontal axis on which the malleus and incus turn.
canals contain the membranous semicircular canals ; the
bony cochlea, with its scala vestibuli and scala tympani, con-
tains the membranous canal of the cochlea, or scala media.
Between the membranous walls and the bony walls is a space
filled with perilymph. The cavities of the osseous labyrinth
are chambers in the petrous part of the temporal bone.
,In the living body, this collection of chambers in the
406 ELEMENTARY PHYSIOLOGY less.
petrous bone is perfectly closed ; but, in the dry skull, there
are two wide openings, termed fenestrae, or windows, in its
outer wall ; i.e. on the side nearest the outside of the skull
and between the internal and middle ears. Of these fenes-
trae, one, termed ovalis (the oval window) (Fig. 131, F.o.),
is situated in the wall of the vestibular cavity ; the other,
rotunda (the round window) F.r., behind and below this,
is, as we have seen, the open end of the scala tympani at the
base of the spiral of the cochlea. In the living body, each
of these windows or fenestrae is closed by a fibrous mem-
brane, continuous with the periosteum of the bone.
The fenestra rotunda is closed by membrane only ; but
fastened to the centre of the membrane of the fenestra ovalis,
so as to leave only a narrow margin, is an oval plate of bone,
part of one of the little bones to be described shortly.
12. The Middle Ear. — The outer wall of the internal
ear is still far away from the exterior of the skull. Between
it and the visible opening of the ear, in fact, are placed in
a straight line, first, the drum of the ear or tympanum ;
secondly, the long external passage, or meatus (Fig. 129).
The drum of the ear, which constitutes the middle ear,
and the external meatus would form one cavity, were it
not that a delicate membrane, the tympanic membrane
(Fig. 129, 2'), is tightly stretched in an oblique direction
across the passage, so as to divide the comparatively small
cavity of the drum from the meatus.
The membrane of the tympanum thus prevents any com-
munication, by means of the meatus, between the drum and
the external air, but such a communication is provided,
though in a roundabout way, by the Eustachian tube (Fig.
129, 4), which leads directly from the fore part of the drain
inwards to the roof of the pharynx, where it opens. (See
also Fig. 76, #)
THE AUDITORY OSSICLES
4C7
(i) The Auditory Ossicles. — Three small bones, the au-
ditory ossicles, lie in the cavity of the tympanum. One
of these is the stapes, a small bone shaped like a stirrup.
It is the foot-plate of this bone which, as already mentioned,
is firmly fastened to the membrane of the fenestra ovalis,
while its hoop projects outwards into the tympanic cavity
(Fig. 130, St., and Fig. 131, Stp.).
Fig. 131. — A Diagram illustrative of the Relative Positions of the Vari-
ous Parts of the Ear.
EM, external auditory meatus; Ty.M, tympanic membrane; Ty, tympanum;
Mall, malleus; Inc, incus; Stp, stapes; F.o, fenestra ovalis; F.r, fenestra rotunda;
Eu, Eustachian tube; M.L, membranous labyrinth, only one semicircular canal with
its ampulla being represented; Sca.V, Sca.T, Sea. ill, the scalse of the cochlea,
which is supposed to be unrolled.
Another of these bones is the malleus {Mall., Figs. 130,
131), or hammer- bone, a long process, the so-called handle
of which is fastened to the inner side of the tympanic mem-
brane ; while a very much smaller process, the s fender process,
is fastened, as is also the body of the malleus, to the bony
wall of the tympanum by ligaments. The rounded surface
4o8 ELEMENTARY PHYSIOLOGY les^
of the head of the malleus fits into a corresponding hollowed
surface in the end of a third bone, the incus, or anvil-bone,
thus forming a joint of a somewhat peculiar character. The
incus has two processes ; of these one. the shorter, is hori-
zontal, and rests upon a support afforded to it by the walls
of the tympanum ; while the other, the longer, is vertical,
descends almost parallel with the long process of the malleus,
and articulates1 with the stapes {Inc., Figs. 130 and 131).
The three bones thus form a movable chain between the
fenestra ovalis and the tympanic membrane. The malleus
and incus are, by the peculiar joint spoken of above, articu-
lated together in such a manner that they may practically
be considered as forming one bone which turns upon a hori-
zontal axis. This axis passes through the horizontal process
of the incus and the slender process of the malleus, and its
ends rest in the walls of the tympanum. Its general direc-
tion is represented by the line ab in Fig. 130, or by a line
perpendicular to the plane of the paper, passing through the
head of the malleus, in Fig. 131.
The two bones may be roughly compared to two spokes
of a wheel, of which the axle is represented by the axis just
described ; it should be added, however, that one spoke, the
incus, is shorter than the other, and that the movement of
the two spokes is limited to a very small arc of a circle.
When the membrane of the drum, thrown into vibration
by some sound, moves inwards and outwards in its vibrations,
it necessarily carries with it, in each inward and outward
movement, the handle of the malleus which is attached to it.
But with each inward and outward movement of the handle
1 A minute hone, the os orbiculare, intervenes between the end of the pro-
cess of the incus and the stapes, so that the stapes is in reality articulated
with the os orbiculare, which in turn is fastened to the process of the incus.
For simplicity's sake, mention of this is omitted above.
ix THE MUSCLES OF THE TYMPANUM 409
of the malleus, the long process of the incus also moves in-
wards and outwards, carrying with it the stapes which is
attached to its end. Hence each vibration, each inward
thrust, and each outward or backward return of the mem-
brane of the drum, produces by means of the chain of
ossicles a corresponding vibration of the membrane of the
fenestra ovalis to which the stapes is attached ; 1 but the
vibrations of this membrane are in turn communicated to
the perilymph of the labyrinth and cochlea. Thus, by means
of the chain of ossicles and the membranes to which these are
attached at each end, the aerial vibrations passing down the
meatus are transformed into corresponding vibrations of the
fluids of the inner ear. The vibrations of the perilymph
passing up the scala vestibuli, and down the scala tympani,
reach at last the membrane covering the fenestra rotunda
and throw this into vibration ; and as a matter of fact it has
been observed that when the membrane of the fenestra
ovalis moves inwards, that of the fenestra rotunda moves out-
wards, and vice versa.
The vibrations of the perilymph thus produced will affect
the endolymph, and thus the hairs, and so the auditory epi-
thelium of the labyrinth ; by which, finally, the auditory
nerves will be excited.
(ii) The Muscles of the Tympanum. — The characters of
the vibration of a membrane, and the readiness with which
it takes up or responds to aerial vibrations reaching it, are
largely modified by its degree of tension ; the membrane
acts differently when it is tightly stretched from what it
1 Owing to certain characters in the attachment of the stapes to the mem-
brane of the fenestra ovalis on the one hand, and to the os orbiculare on
the other, the movements of the foot of the stapes in the fenestra ovalis are
somewhat peculiar; but the details of these as well as the functions of the
peculiar articulation of the incus with the malleus have, for simplicity's sake,
been omitted.
4io ELEMENTARY PHYSIOLOGY less.
does when it is loose. Now, within the cavity of the
tympanum are two small, but relatively strong muscles.
One, called the stapedius, passes from the floor of the
tympanum to the foot of the stapes and the orbicular bone,
the other, the tensor tympani, from the front wall of the
drum to the malleus. Each of the muscles when it con-
tracts tightens the membrane to which it is thus indirectly
attached, the tensor tympani, the membrane of the drum,
and the stapedius, the membrane of the fenestra ovalis.
The effect of thus tightening the membrane is probably to
restrict the vibrations of the membrane, at least as far as
concerns grave, or low-pitched sounds ; but the complete
action of these muscles is too intricate to be dwelt on here.
13. The External Ear. — The outer extremity of thi
external meatus is surrounded by the pinna, the two together
constituting the external ear (Fig. 129, 1). The pinna is a
broad, peculiarly shaped, and for the most part cartilagi-
nous plate, the general plane of which is at right angles with
that of the axis of the auditory opening. The pinna can be
moved, by most animals and by some human beings, in
various directions by means of muscles, which pass to it
from the side of the head.
14. The Transmission of Sound Waves to the Inner
Ear. — The manner in which the complex apparatus now
described intermediates between the physical agent, which
is the primary cause of the sensation of sound, and the
nervous expansion, the affection of which alone can excite
that sensation, must next be considered.
All bodies which produce sound are in a state of vibration,
and they communicate the vibrations of their own substance
to the air with which they are in contact, and thus throw
that air into waves, just as a stick waved backwards and
forwards in water throws the water into waves.
ix TRANSMISSION OF SOUND WAVES 411
The aerial waves, produced by the vibrations of sonorous
bodies, in part enter the external auditory passage, and in
part strike upon the pinna of the external ear and the outer
surface of the head. It may be that some of the latter
impulses are transmitted through the solid structure of the
skull to the organ of hearing ; but before they reach it they
must, under ordinary circumstances, have become so scanty
and weak, that they may be left out of consideration.
The aerial waves which enter the meatus all impinge upon
the membrane of the drum and set it vibrating, stretched
membranes, especially such as have the form and characters
of the tympanic membrane, taking up vibrations from the
air with great readiness.
The vibrations thus set up in the membrane of the
tympanum are communicated, in part, to the air contained
in the drum of the ear, and, in part, to the malleus, and
thence to the other auditory ossicles.
The vibrations communicated to the air of the drum
impinge upon the inner wall of the tympanum, on the
greater part of which, from its density, they can produce
very little effect. Where this wall is formed by the mem-
brane of the fenestra rotunda the communication of motion
must necessarily be greater. All these vibrations, however,
may probably be neglected.
The vibrations which are communicated to the malleus
and the chain of ossicles may be of two kinds : vibrations
of the particles of the bones, and vibrations of the bones as
a whole. If a beam of wood, freely suspended, be very
gently scratched with a pin, its particles will be thrown into
a state of vibration, as will be evidenced by the sound given
out, but the beam itself will not be visibly moved. Again,
if a strong wind blow against the beam, it will swing bodilv,
without any vibrations of its particles among themselves.
412 ELEMENTARY PHYSIOLOGY less.
On the other hand, if the beam be sharply struck with a
hammer, it will not only give out a sound, showing that its
particles are vibrating, but it will also swing, from the
impulse given to its whole mass.
Under the last-mentioned circumstances, a blind man
standing near the beam would be conscious of nothing but
the sound, the product of molecular vibration, or invisible
oscillation of the particles of the beam ; while a deaf man in
the same position would be aware of nothing but the visible
oscillation of the beam as a whole.
Thus, to return to the chain of auditory ossicles, while it
may be supposed that, when the membrane of the drum
vibrates, these may be set vibrating both as a whole and in
their particles, the question arises whether it is the large
vibrations, or the minute ones, which make themselves obvi-
ous to the auditory nerve, which is in the position of our
deaf, or blind, man.
The evidence is distinctly in favour of the conclusion,
that it is the vibrations of the bones, as a whole, which are
the chief agents in transmitting the impulses of the aerial
waves.
For, in the first place, the disposition of the bones and
the mode of their articulation are very much against the
transmission of molecular vibrations through their substance,
but, on the other hand, are extremely favourable to their
vibration en masse. The long processes of the malleus and
incus swing, like a pendulum, upon the axis furnished by the
short processes of these bones ; while the mode of connec-
tion of the incus with the stapes, and of the latter with the
membrane of the fenestra ovalis, allows the foot-plate of that
bone free play, inwards and outwards. In the second place,
the total length of the chain of ossicles is very small com-
pared with the length of the waves of audible sounds, and
ix TRANSMISSION OF SOUND WAVES 413
physical considerations teach us that in a like thin rod,
similarly capable of swinging en masse, the minute molecular
vibrations would be inappreciable. Thirdly, direct experi-
ments, such as attaching to the stapes of a dissected ear a
light style, the movements of which are recorded on a
travelling smoked glass plate or in some other way, show
that the chain of ossicles does actually vibrate as a whole,
and at the same rate as the membrane of the drum, when
aerial vibrations strike upon the latter.
Thus, there is reason to believe that when the tympanic
membrane is set vibrating, it causes the process of the
malleus, which is fixed to it, to swing at the same rate ; the
head of the malleus consequently turns through a small arc
on its pivot, the slender process. But, as stated on p. 408,
the turning of the head of the malleus involves the simultane-
ous turning of the head of the incus upon its pivot, the short
process. In consequence the long process of the incus also
swings at the same rate. The length of the long process of
the incus, measured from the axis, on which the two bones
turn, is less than that of the handle of the malleus ; hence
the end of it moves through a smaller space. The arc
through which it moves has been estimated as being equal
to about two-thirds of that described by the handle of the
malleus. The extent of the push is thereby somewhat
diminished, but the force of the push is proportionately
increased ; in so confined a space this change is advantage-
ous. The long process of the incus, however, is so fixed to
the stapes, and the stapes so attached to the membrane of
the fenestra ovalis, that the incus cannot vibrate without
throwing into vibrations, to a corresponding extent and at
the same rate, the membrane of the fenestra ovalis.1 But
every vibration, every pull and push, imparts a correspond-
1 See foot-note, p. 408.
414 ELEMENTARY PHYSIOLOGY less.
ing set of shakes to the perilymph, which fills the bony
labyrinth external to the membranous labyrinth. These
shakes are communicated to the endolymph in the latter
chamber, and, by the help of the modified auditory epithe-
lium described above, stimulate the delicate endings of at
least the cochlear division of the auditory nerve.
15. The Conversion of Sonorous Vibrations into Sensa-
tions of Sound. — We do not at present know what kind of
changes the vibrations of the endolymph give rise to in the
epithelial cells of the organ of Corti ; nor do we at present
know the exact way in which the changes thus set up in
these epithelial cells are able to excite the terminal filaments
of the auditory nerve. But there can be no doubt of the
fact that the elaborate apparatus of the cochlea is able to
translate, so to speak, the sonorous vibrations which reach
them into stimulations of nerve-fibres, the molecular changes
of which are transmitted along the auditory nerve as audi-
tory nervous impulses. Passing along the auditory nerve,
these molecular changes, these nervous impulses, reach cer-
tain parts of the brain situated in the cortex of the temporo-
sphenoidal lobe, below the fissure of Sylvius (see p. 550),
and there in turn set up those molecular disturbances of
nervous matter which form the immediate cause of the states
of feeling called "sounds." Thus, the auditory nerve may
be said, and a similar statement may be made in the case
of the other nerves of special sensations, to be provided
with two "end-organs." There is the peripheral end-organ
(the apparatus of the cochlea) by which the physical agent
is enabled to excite the sensory nerve-fibres ; and there is
the central end-organ, in the brain, in which the nervous
impulses of the sensory nerve excite the special state of feel-
ing which we call the special sensation. The central end-
organ of hearing is often spoken of as the auditory sensorium.
ix ACTION OF THE AUDITORY END-ORGANS 415
Between the sounding body and the actual hearing of a
sound, there is a chain of events of different kinds. There
are the vibrations started by the sounding body, and pass-
ing through the air, the tympanum, the perilymph, and the
endolymph ; these are all of one order. Then there are
the changes in the peripheral end-organ, in the apparatus of
the cochlea ; these are of another order. Then follow the
molecular disturbances travelling along the auditory nerve ;
these are of still another order. Lastly, there are the
changes in the central end-organ, in the brain ; these, though
resembling the preceding in so far as they are changes of
nervous matter, are yet of still another order, and probably
comprise in themselves a whole series of events, the conse-
quence of the last of which is the sensation of sound.
16. The Mode of Action of the Auditory End-organs. —
Every sound consists, as we have seen, of vibrations. Some-
times the vibrations are repeated with great regularity ; and
sounds, in which the regular recurrence of the same vibra-
tions is conspicuous, are called "musical sounds." Some-
times no regular repetition of vibrations can be recognised ;
the sound consists of vibrations, few of which are like each
other, and which fall irregularly on the ear ; such sounds are
called " noises."
When we listen to musical sounds, each set of regularly
repeated vibrations generates in the central end-organ a
particular kind of sensation which we call a tone; and the
simultaneous or successive production of different tone-
sensations gives rise in us to the feelings which we speak of
as those of harmony or melody.
When we listen to a noise the vibrations generate sensa-
tions which are of a certain intensity, according to which
we call the noise slight or great, low or loud, and which also
have certain characters by which we recognise the kind of
4i6 ELEMENTARY PHYSIOLOGY less.
noise ; but the sensations have not the qualities of tone-sen
sations, and do not give rise to feelings of melody or har-
mony.
A pure musical sound consists of a series of vibrations
repeated with exact regularity, the number of vibrations
occurring in a given time, e.g. in a second, determining what
is called the pitch of the " note." But ordinary musical
sounds are, for the most part, not simple, consisting of one
set of vibrations, but compound, consisting of several sets of
vibrations occurring together; in these musicians distinguish
one set, called the fundamental tone, and other sets, vary-
ing in intensity or loudness, called overtones.
A tuning-fork, when set vibrating, vibrates with a given
rapidity ; and the note given out is'determined by the rapid-
ity of the vibration, by the number of vibrations repeated,
for instance, in a second ; hence every tuning-fork has its
own proper note. Now, a tuning-fork will be set vibrating
if its own particular note be sounded in its neighbourhood,
but not if other notes be sounded. Hence, when a pure
musical note is sounded close to a number of tuning-forks
of different pitch, only that tuning-fork the pitch of which is
the same as that of the note sounded is set vibrating;
the others remain motionless. When an ordinary musical
sound, such as a note sung by the human voice, is produced
among such a group of tuning-forks, several are set vibrat-
ing ; one of these corresponds to the fundamental tone, and
the others to the various overtones of the sound. Similarly,
if the top of a piano be lifted up or removed, and any one
sings into the wires with sufficient loudness a note, such as
the tenor c, a number of the wires will be set vibrating, one
corresponding to the fundamental tone, and the others to
the overtones.
If we were to imagine an immense number of tuning-forks,
ix ACTION OF THE AUDITORY END-ORGANS 417
each vibrating at different periods, so arranged that each
fork, when vibrating, in some way or other stimulated or
excited a minute delicate nerve filament attached to it, it is
obvious that a musical sound uttered near these tuning-
forks wouhd set a certain number of them into vibration,
some more forcibly than others, and that in consequence a
certain number, and a certain number only, of the delicate
nerve filaments would be excited, and that to various de-
grees ; and thus a particular series of nervous impulses, the
counterpart as it were of the musical sound with its funda-
mental tone and overtones, would be transmitted along the
nerve filaments to the brain.
It is suggested that the basilar membrane of the cochlea,
consisting as it does of thousands of fibres stretching across
from the inside to the outside (from left to right in Fig.
128), with its thousands of epithelial cells and rods of Corti
lying upon it, represents, as it were, an assemblage of thou-
sands of tuning-forks, of various rates of vibration, with a
separate nerve filament adapted to each. So that, when a
number of vibrations of different periods, such as consti-
tutes an ordinary musical sound, are transmitted by the
tympanum to the cochlea, these, as they sweep along the
canal of the cochlea, throw into sympathetic movement
those parts, and those parts only, of the basilar membrane
with their overlying epithelium and rods of Corti whose
periods of vibration correspond to the incoming vibrations,
and thus excite certain nerve filaments, and these only. It
is this excitement of a group of nerve filaments, some ex-
cited more intensely than others, which, reaching the brain,
gives rise to the sensation which we associate with a particu-
lar musical sound.
We know something in general about the position in the
brain of the auditory sensorium or central end-organ of the
418 ELEMENTARY PHYSIOLOGY less.
auditory nerve ; but we know very little about the nature of
this sensorium. It may be conceived, however, that each
filament of the cochlear nerve is connected with a particu-
lar portion of the nervous matter of the central end-organ,
in such a way that the molecular movements of one of these
particular portions of nervous matter, brought about by a
molecular disturbance reaching it through its appropriate
filament, produces a psychical effect of one kind only, more
or less intense it may be, but still always of one kind. If
this be so, each cochlear fibre or filament may be considered
as being provided with two end-organs : one, peripheral, in
the organ of Corti, capable of being set in motion by vibra-
tions of one quality only ; the other, central, in the brain,
capable of producing a psychical effect of one quality only.
It does not follow, however, that we are distinctly and sepa-
rately conscious of the nervous disturbance in each central
end-organ, it does not follow that we have as many distinct
and separate kinds of conscious sensation as there are periph-
eral and central end-organs, though how many such dis-
tinct kinds of sensation we may have we do not know. Just
as the peripheral mechanism sifts out the several vibrations
of which a musical sound is composed, and transmits them
separately, so, by a reverse operation, the central mechanism
probably pieces together the nervous disturbances of a num-
ber of central end-organs, and thus produces a sensation
whose characters are determined by a combination of the
nervous disturbances taking place in each end-organ.
Some such a view is indeed exceedingly probable ; but it
must be remembered that we do not at present at all under-
stand the exact mechanism by which each particular vibra-
tion excites its corresponding nerve filament. The nerve
filaments appear to end among the epithelial cells bearing
short hairs, which lie on each side of the rods of Corti ; and
ix LOCALISATION OF SOUND 419
we may, therefore, conclude that these " hair-cells " have
some share in producing the effect and constitute the essen-
tial part of the organ of hearing. But the whole matter is
at present very obscure ; the functions of the rods of Corti
are particularly difficult to understand ; for these do not
seem in any way connected with the nerve filaments, and
their movements can only affect the latter by influencing in
some way the hair-cells.
The fibres of the cochlear nerve, or their endings in the
brain itself, may be excited by internal causes, such as the
varying pressure of the blood and the like : and in some
persons such internal influences do give rise to sensations of
sounds and even to veritable musical spectra, sometimes of
a very intense character. But, for the appreciation of music
produced external to us, we depend upon the organ of Corti
being in some way or other affected by the vibrations of the
fluids in the cochlea.
It has been suggested that the utricle, saccule, and semi-
circular canals enable us to appreciate noises \ but such a
view presents great difficulties. Between noises and musi-
cal sounds no hard and fast line can, in fact, be drawn. It
seems probable that the cochlea deals with both kinds of
sonorous vibrations.
17. Localisation of Sound. — The apparatus of the ear
which we have described, provides us simply with auditory
sensations ; enables us to appreciate high notes and low
notes, to discriminate between musical sounds and noises.
Experience then enables us to base upon these sensations
certain conclusions as to the nature of the source which is
giving rise to each sound. But sounds may be coming to
us in different directions and from different distances, and
when we endeavour to form some estimate of either the one
or the other of these possible differences, we find that our means
420 ELEMENTARY PHYSIOLOGY less.
of doing so are very imperfect. As to our estimate of the
distance from which a sound is coming, we are guided
chiefly by its intensity coupled with previous experience.
For the discrimination of the direction from which a sound
is coming, we have to rely almost entirely on the different
effect the sound produces on each of our two ears, accord-
ing as it falls more directly into one of them than into the
other. Thus when we are endeavouring to localise a source
of sound, we usually turn the head into various positions,
until we find one position in which the sound is loudest as
it falls into one ear, and then we assume that the sound is
coming along a line directed straight into that ear. In ani-
mals with large and movable external ears, the movement of
the ear to a great extent takes the place of the movement
of the head ; this may be readily observed in an animal
such as the horse.
Anything which interferes with the ordinary laws of
transference of sound causes us to form a wrong judgment
as to the distance of the source, as in the case of listening
to speech through a telephone or in a phonograph.
Similarly, it is difficult to estimate the distance of the
source of a sound heard through a snow storm. Again,
in ventriloquism our judgment is upset, not only as
regards the nature of the source of sound, but also of its
distance and direction, by carefully planned simulation and
suggestion.
18. The Functions of the Tympanic Muscles and
E:\stacbian Tube. — It has already been explained that
the stapedius and tensor tympani muscles are competent
to tighten the membrane of the fenestra ovalis and that of
the tympanum respectively, and it is probable that they
come into action when the sonorous impulses are too vio-
lent, and would produce too extensive vibrations of the«;e
IX FUNCTIONS OF THE SEMICIRCULAR CANALS 421
membranes. They may therefore be of use in moderating
the effect of intense sound, in much the same way that, as
we shall find, the contraction of the circular fibres of the iris
tends to moderate the effect of intense light in the eye ;
they may, however, have other purposes.
The function of the Eustachian tube is, probably, to
keep the air in the tympanum, or on the inner side of the
tympanic membrane, of about the same tension as that on
the outer side, which could not always be the case if the
tympanum were a closed cavity. The unpleasant sensa-
tion often experienced, as of a " tightness " in the ear,
when diving under water, is due to the compression of
the air in the tympanic cavity under the increased external
pressure. It may be largely removed by merely performing
the movements of swallowing. By these movements the
end of the Eustachian tube which opens into the pharynx
is opened and the pressure on the two sides of the tym-
panum is equalised.
19. The Functions of the Semicircular Canals, the
Utricle, and the Saccule. — It is probable that the semi-
circular canals, the utricle, and the saccule have nothing to
do with hearing, and it is known that they have other very
definite functions, namely, that of enabling the body to
maintain its equilibrium.
We have seen that the semicircular canals lie in three
planes at right angles to one another (p. 395). When any
one of the canals is experimentally injured, the animal in
many cases executes a series of oscillatory movements of
the head, which are, broadly speaking, in the plane of the
canal. When all three canals are injured, the animal is
thrown into continuous movements of the most varied and
often extraordinary kind, and has lost all power of balancing
itself in a normal way. Not unfrequently in man these
422 ELEMENTARY PHYSIOLOGY less, ix
canals undergo injury as the result of disease, and in this
case the feelings experienced by the patient are those of
extreme giddiness, and an inability to balance the body,
while the symptoms exhibited to an onlooker are those of a
want of co-ordination in the execution of movements. Thus,
there is no doubt that the canals enable us to appreciate
the movements of the head in all planes in space, and thus
act as sense-organs for the guidance of our bodily move-
ments. A movement of the head causes a change of
pressure in the endolymph, and thus the hair-cells of the
crista; are stimulated. It is a suggestive fact that the
canals are relatively largest in animals, such as birds and
fishes, that live in a fluid medium rather than upon the
ground, and whose locomotor movements are often sudden
and delicate.
Some movements of the body also are apparently appre-
ciated by means of the utricle and saccule, but these parts
of the labyrinth seem, in addition, to give us notions of the
position of the resting body in space. Probably the con-
stant pressure of the otoliths on the hair-cells of the macula;
acts as a constant stimulus, the pressure being varied accord-
ing to the position in which the body rests, whether upright,
lying down, etc.
These various organs doubtless act together and enable
us to control all our bodily movements very perfectly and
thus to maintain our equilibrium under all circumstances.
The vestibular branch of the auditory nerve, which supplies
these organs, is distinct from the cochlear branch, and,
instead of ending with the latter in that part of the brain
that has to do with hearing, goes to the cerebellum, which,
as we shall see, has as its function the co-ordination of bodily
movements.
LESSON X
THE ORGAN OF SIGHT
1. The General Structure of the Eye. — In studying
the organ of the sense of sight, the eye, we may, perhaps
with advantage, consider the accessory parts first, and then
pass on to the essential structures.
The accessory organs, by means of which the physical
agent of vision, light, is enabled to act upon the expansion of
the optic nerve, comprise three kinds of apparatus : (a) a
"water camera," the eyeball; (b) muscles for moving the
eyeball ; (r) organs for protecting the eyeball, viz. the eye-
lids, with their lashes, glands, and muscles ; the conjunctiva ;
and the lachrymal gland and its ducts.
The ball, or globe, of the eye is a globular body, moving
freely in a chamber, the orbit, which is furnished to it by
the skull. The optic nerve, the root of which is in the brain,
leaves the skull by a hole at the back of the orbit, and enters
the back of the globe of the eye, not in the middle, but on
the inner, or nasal, side of the centre. Having pierced the
wall of the globe, it spreads out into a very delicate mem-
brane, varying in thickness from ^ of an inch to less than
half that amount, which lines the hinder two-thirds of the
globe, and is termed the retina. This retina is the only or-
gan connected with sensory nervous fibres which can be
affected, by any agent, in such a manner as to give rise to
the sensation of light. It contains the essential part of the
423
424 ELEMENTARY PHYSIOLOGY less
organ of vision, the rods and cones, and the one pre-eminent
function of the accessory structures is to bring the rays of
light entering the eye from external objects to a focus upon
the rods and cones.
The eyeball is composed, in the first place, of a tough,
firm, spheroidal case consisting of fibrous tissue, the greater
part of which is white and opaque, and is called the sclerotic
(Fig. 132, 2). In front, however, this fibrous capsule of the
eye, though it does not change its essential character, be-
comes transparent, and receives the name of the cornea
(Fig. 132, 1). The front surface of the cornea is covered
by an epithelium, in which the cells are very similar and
similarly arranged to those in the epidermis of the skin. The
corneal portion of the case of the eyeball is more convex than
the sclerotic portion, so that the whole form of the ball is
such as would be produced by cutting off a segment from
the front of a spheroid of the diameter of the sclerotic, and
replacing this by a segment cut from a smaller, and conse-
quently more convex, spheroid.
The corneo-sclerotic case of the eye is kept in shape by
what are termed the humours — watery or semi-fluid sub-
stances, one of which, the aqueous humour (Fig. 132, 7'),
which is hardly more than water holding a few organic and
saline substances in solution, distends the corneal chamber
of the eye, while the other, the vitreous humour (Fig. 132,
13), which is rather a delicate jelly than a regular fluid,
keeps the sclerotic chamber full.
The two humours are separated by the very beautiful,
transparent, doubly convex crystalline lens (Fig. 132, 12),
denser, and capable of refracting light more strongly than
either of the humours. The crystalline lens is composed of
fibres having a somewhat complex arrangement, and is highly
elastic. It is more convex behind than in front, and it is
GENERAL STRUCTURE OF THE EYE
425
kept in place by a delicate, but at the same time strong
membranous frame or suspensory ligament, which extends
from the edges of the lens to what are termed the ciliary-
processes of the choroid coat (Figs. 132, 5, and 134, c).
In the ordinary condition of the eye this ligament is kept
— C^ULiM JuUM
r..l bruua-
dudkflrldu
Fig. 132.
- Horizontal Section of the Eyeball.
1, cornea; i', conjunctiva; 2, sclerotic; 2', sheath of optic nerve; 3, choroid;
3", rods and cones of the retina; 4, ciliary muscle; 4', circular portion of ciliary
muscle; 5, ciliary process; 6, posterior chamber between 7, the iris, and 10, the sus-
pensory ligament: 7', anterior chamber; 8, artery of retina in the centre of the optic
nerve; 8', centre of blind spot; 8", macula lutea; 9, ora serrata (this is of course not
seen in a section such as this, but is introduced to show its position) ; 10, the sus-
pensory ligament; 12, crystalline lens; 13, vitreous humour; 14, space in tissue called
the canal of Schlemm; a a, optic axis; b b, line of equator of the eyeball.
tense, i.e. is stretched pretty tight, and the front part of the
lens is consequently flattened.
The choroid coat (Fig. 132, 3) is highly vascular and
consists of blood-vessels arranged in a very complex way,
426
ELEMENTARY PHYSIOLOGY
bound together with a little connective tissue among which,
towards its outer side, are a number of branched connective-
tissue corpuscles whose cell-substance is loaded with gran-
ules of black pigment (Fig. 133).
The choroid is in close contact with the sclerotic exter-
nally, and internally is in contact with a layer of very
peculiar cells, also full of pigment (Fig. 145) belonging to
the retina. The choroid lines every part of the sclerotic,
except just where the optic nerve enters it at a point below,
and to the inner side of the centre of the back of the eye ;
Fig. 133. — Pigment Cells from the Choroid Coat.
but, when it reaches the front part of the sclerotic, its inner
surface becomes raised up into a number of longitudinal
ridges, with intervening depressions, like a fluted ruffle, ter-
minating within and in front with rounded ends, but passing,
externally, into the iris. These ridges, which when viewed
from behind seem to radiate on all sides from the lens
(Figs. 134, c, and 132, 5), are the above-mentioned ciliary
processes.
The iris itself (Figs. 132, 7, and 134, a, &) is, as has
been already said (p. 324), a curtain with a round hole in
the middle, the piipil, provided with circular and radiating
X GENERAL STRUCTURE OF THE EYE 427
unstriped muscular fibres, and capable of having its central
aperture diminished or enlarged by the action of these
■fibres, the contraction of which, unlike that of other
unstriped muscular fibres, is extremely rapid. The hinder
surface of the iris is covered with cells containing a black
pigment, similar to that of the choroid coat, and the differ-
ent colours of eyes depend partly on the varying amount and
distribution of pigment in these cells, and partly on pigment
cells imbedded in and scattered throughout the substance
of the iris. The outer edges of the iris are continuous with
the choroid. Unstriped muscular fibres, originating in the
sclerotic at its junction with the cornea, spread backwards
on to the outer surface of the choroid and constitute the
ciliary muscle (Fig. 132, 4). If these fibres contract, it is
obvious that they will pull the choroid forwards ; and as the
frame or suspensory ligament of the lens is connected with
the ciliary processes (which simply form the anterior ter-
mination of the choroid), this pulling forward of the choroid
comes to the same thing as a relaxation of the tension of
that suspensory ligament, which, as we have just said, is in
an ordinary condition stretched somewhat tight, keeping
the front of the lens flattened.
The iris does not hang down perpendicularly into the
space between the front face of the crystalline lens and
the posterior surface of the cornea, which is filled by
the aqueous humour, but applies itself very closely to the
anterior face of the lens, so that hardly any interval is left
between the two (Figs. 132 and 137).
The retina, the structure of which will be considered later.
lines the interior of the eye, being placed between the choroid
and vitreous humour, its rods and cones being imbedded in
the pigment epithelium lying just within the former, and its
inner limiting membrane touching the latter (Fig. 132, 3").
428 ELEMENTARY PHYSIOLOGY less.
About a third of the distance back from the front of the
eye the retina seems to end in a wavy border called the
ora serrata (Fig. 132, 9), and in reality the nervous ele-
ments of the retina do end here, having become consider-
ably reduced before this line is reached. Some of the
connective-tissue elements, however, pass on as a delicate
kind of membrane at the back of the ciliary processes
towards the crystalline lens.
Fig. 134. — View of Front HalF'Of the Eyeball seen from Behind.
a, circular fibres; b, radiating fibres of the iris; c, ciliary processes; d, choroid.
The crystalline lens has been removed.
2. The Eye as a Water Camera. — The impact of the
vibrations of the ether upon the sensory expansion, or essen-
tial part of the visual apparatus, alone is sufficient to give
rise to all those feelings which we term sensations of light and
of colour, and, further, to that feeling of outness which
accompanies all visual sensation. But, if the retina had
a simple transparent covering, the vibrations radiating from
any number of distinct luminous points in the external
world would affect all parts of it equally, and therefore the
feeling aroused would be that of a generally diffused lumi-
nosity. There would be no separate feeling of light for
each separate radiating point, and hence no correspondence
x THE EYE AS A WATER CAMERA 429
between the visual sensations and the radiating points which
aroused them.
It is obvious that in order to produce this correspond-
ence, or, in other words, to have distinct vision, the essential
condition is, that distinct luminous points in the external
world shall be represented by distinct feelings of light.
And since, in order to produce these distinct feelings, vibra-
tions must fall on separate parts of the retina, it follows that,
for the production of distinct vision, some apparatus must be
interposed between the retina and the external world, by the
action of which distinct luminous points in the latter shall be
represented by corresponding points of light on the retina.
In the eye of man and of the higher animals, this acces-
sory apparatus of vision is represented by structures which,
taken together, act as a biconvex lens, composed of sub-
stances which have a much greater refractive power than
the air by which the eye is surrounded ; and which throw
upon the retina luminous points, which correspond in
number and in position, relatively to one another, with
those luminous points in the external world from which
ethereal vibrations proceed towards the eye. The luminous
points thus thrown upon the retina form a picture of the
external world — a picture being nothing but lights and
shadows, or colours, arranged in such a way as to correspond
with the disposition of the luminous parts of the object
represented, and with the qualities of the light which pro-
ceeds from them.
That a biconvex lens is competent to produce a picture
of the external world on a properly arranged screen is a
fact of which every one can assure himself by simple
experiments. An ordinary magnifying glass is a trans-
parent body denser than the air, and convex on both
sides. If this lens be held at a certain distance from a
+30 ELEMENTARY PHYSIOLOGY less.
screen or wall in a dark room ami a lighted candle be
placed on the opposite side of it, it will be easy to adjust
the distances of candle, lens and wall in such a manner
that an image of the flame of the candle, upside down, shall
be thrown upon the wall.
The spot on which the image is formed is called a focus.
If the candle be now brought nearer to the lens, the image
on the wall will enlarge, and grow blurred and dim, but
it may be restored to brightness and definition by moving
the lens further from the wall. But if, when the new
adjustment has taken place, the candle be moved away
from the lens, the image will again become confused, and,
to restore its clearness, the lens will have to be brought
nearer the wall.
Thus a convex lens forms a distinct picture of luminous
objects, but only at the focus on the side of the lens
opposite to the object ; and that focus is nearer when the
object is distant, and further off when it is near.
Suppose, however, that, leaving the candle unmoved,
a lens with more convex surfaces is substituted for the
first, the image will be blurred, and the lens will have to
be moved nearer the wall to give it definition. If, on
the other hand, a lens with less convex surfaces is sub-
stituted for the first, it must be moved further from the wall
to attain the same end.
In other words, other things being alike, the more convex
the lens, the nearer its focus ; the less convex, the further
off its focus.
If the lens were made of some extensible, elastic sub-
stance, like india-rubber, pulling it at the circumference
would render it flatter, and thereby lengthen its focus;
while, when let go again, it would become more convex,
and of shorter focus.
X THE EYE AS A WATER CAMERA 431
Any material more refractive than the medium in which
it is placed, if it have a convex surface, causes the rays of
light which pass through the less refractive medium to that
surface to converge towards a focus. If a watch-glass be
fitted into one side of a box, and the box be then filled
with water, a candle may be placed at such a distance out-
side the watch-glass that an image of its flame shall fall on
the opposite wall of the box. If, under these circumstances,
a doubly convex lens of glass were introduced into the water
in the path of the rays, it would (though less powerfully than
if it were in air) bring the rays more quickly to a focus,
because glass refracts light more strongly than water does.
A camera obscura is a box, into one side of which a lens
is fitted, so as to be able to slide backwards and forwards,
and thus throw on the screen at the back of the box dis-
tinct images of bodies at various distances. Hence the
arrangement just described might be termed a water
camera.
The eyeball, the most important constituents of which
have now been described, is, in principle, a camera of the
kind described above — a water camera. That is to say,
the sclerotic answers to the box, the cornea to the watch-
glass, the aqueous and vitreous humours to the water filling
the box, and the crystalline to the glass lens, the introduc-
tion of which was imagined. The back of the box corre-
sponds with the retina.
But, further, in an ordinary camera obscura it is found
desirable to have what is termed a diaphragm (that is, an
opaque plate with a hole in its centre) in the path of the
rays, for the purpose of moderating the light and cutting off
the marginal rays, which, owing to certain optical properties
of spheroidal surfaces, give rise to defects in the image
formed at the focus.
£J2 ELEMENTARY PHYSIOLOGY less,
In the eye, the place of this diaphragm is taken by the
iris, which has the peculiar advantage of being self-regu-
lating : contracting its aperture and admitting less light
when the illumination is strong ; but dilating its aperture
and admitting more light when the light is weak. It thus
acts like the various " stops " which a photographer uses
according to the varying light.
These changes in the pupil are brought about by the con-
tractions of the circular and radiating muscle-fibres of the
iris; contraction of the circular or sphincter fibres makes
the pupil smaller or constricts it, contraction of the radiat-
ing fibres makes it larger or dilates it. Further, conversely,
relaxation of the circular fibres causes or helps to cause dila-
tion, and relaxation of the radiating fibres causes or helps
to cause constriction. Contraction of the circular fibres
and so constriction of the pupil are brought about by means
of fibres of the oculo-moior nerve, and contraction of the
radiating fibres and so active dilation are brought about by
means of fibres of tne sympathetic system.
The constriction of the pupil observed when light falls
upon the retina is a reflex action in which the optic nerve
provides the path for afferent impulses to a centre in the
brain lying beneath the front end of the aqueduct of Sylvius
(p. 523), and the third (oculo-motor) cranial nerve (p. 536)
provides the path for efferent impulses from the centre to
the circular fibres of the iris. The dilation of the pupil
when light is withdrawn from the retina is, in the main at
least, due to the cessation of previously acting constrictor
impulses.
The pupil is also constricted when the eye is accommo-
dated for near objects, and during deep sleep ; and it is
dilated when the eye is accommodated for distant objects.
Rays of light coming from an object and passing into the
TIIK MECHANISM OF ACCOMMODATION
43J
eye undergo a bending or refraction (i) as they enter the
eye, at the surface of the cornea, (ii) as they pass through
the lens ; and as a result of this action of the cornea and
lens an image of the object is formed on the retina (Fig.
135)-
In the water camera the image brought to a focus on the
screen at the back is inverted ; the image of a tree, for
instance, is seen with the roots upwards and the leaves and
branches hanging downwards. The right of the image also
corresponds with the left of the object, and vice versa.
Exactly the same thing takes place in the eye with the
Fig. 135. — The Formation of an Image on the Retina.
image focussed on the retina. It too is inverted. This fact
often gives rise to the question, Why then do we see objects
in the external world in an erect position and not also
inverted? This matter is discussed in Lesson XI, p. 466.
3. The Mechanism of Accommodation. — In the water
camera, constructed according to the description given above,
there is the defect that no provision exists for adjusting the
focus to the varying distances of objects. If the box were
so made that its back, on which the image is supposed to be
thrown, received distinct images of very distant objects, all
near ones would be indistinct. And if, on the other hand,
it were fitted to receive the image of near objects, at a given
Z F
434 ELEMENTARY PHYSIOLOGY less.
distance, those of either still nearer, or more distant, bodies
would be blurred and indistinct. In the ordinary camera
this difficulty is overcome by sliding the lenses in and out, a
process which is not compatible with the construction of our
water camera. But there is clearly one way among many in
which this adjustment might be effected — namely, by
changing the glass lens ; putting in a less convex one when
more distant objects had to be pictured, and a more convex
one when the images of nearer objects were to be thrown
upon the back of the box.
But it would come to the same thing, and be much more
convenient, if, without changing the lens, one and the same
lens could be made to alter its convexity. This is what
actually is done in the adjustment of the eye to distances.
The simplest way of experimenting on the adjustment ox
accommodation of the eye is to stick two stout needles up-
right into a straight piece of wood, not exactly, but nearly
in a line parallel with the edge of the piece, so that, on
applying the eye to one end of the piece, one needle
(a) shall be seen about six inches off, and the other (b) just
on one side of it at twelve inches or more distance.
If the observer look at the needle b, he will find that he sees
it very distinctly, and without the least sense of effort ; but
the image of a is blurred and more or less double. Now let
him try to make this blurred image of the needle a distinct.
He will find he can do so readily enough, but that the act
is accompanied by a sense of effort somewhere in the eye.
And in proportion as a becomes distinct, b will become
blurred. Nor will any effort enable him to see a and b dis-
tinctly at the same time.
Multitudes of explanations have been given of this re-
markable power of adjustment ; but the true solution of the
problem has been gained by the accurate determination of
X THE MECHANISM OF ACCOMMODATION 435
the nature of the changes in the eye which accompany the
act. When the flame of a taper is held near, and a little on
one side of, a person's eye, any one looking into the eye
from a proper point of view will see three images of the
flame, two upright and one inverted (Fig. 136, A). One up-
right bright image is reflected from the front of the cornea,
which acts as a convex mirror. The second, less bright,
proceeds from the front of the crystalline lens, which has
the same effect ; while the inverted image, which is small
and indistinct, proceeds from the posterior face of the lens,
Fig. 136. — Diagram of the Images of a Candle-flame seen by Reflection
from the Surface 07 the Cornea and the Two Surfaces of the Lens.
A, as seen when the eye is adjusted for a distant object: B, as they appear when the
eye is fixed on a near object.
which, being convex backwards, is, of course, concave for-
wards, and acts as a concave mirror.
Suppose the eye to be steadily fixed on a distant object,
and then adjusted to a near one in the same line of vision,
the position of the eyeball remaining unchanged. Then the
upright image reflected from the surface of the cornea, and
the inverted image from the back of the lens, will remain
unchanged, though it is demonstrable that their size 01
436
ELEMENTARY PHYSIOLOGY
LESS
apparent position must change if either the cornea, or the
back of the lens, alters either its form or its position
But the second upright image, that reflected by the front
face of the lens, does change both its size and its position ;
it comes forward and grows smaller (Fig. 136, B), proving
that the front face of the lens has become more convex.
The change of form of the lens is, in fact, that represented
in Fig. 137.
A I b
Fig. 137. — The Changes in the Lens in Accommodation.
A, adjusted for distant; B, for near objects.
c, cornea; con, conjunctiva; scl, sclerotic; ch, choroid; c.fi, ciliary process; cm,
ciliary muscle; s.l, suspensory ligament.
For purposes of accurate experiment it is better to
employ the images cast by two small luminous points placed
one above the other. In this case three pairs of images are
seen by reflection ; and it is easier to observe that the two
images of the middle pair come nearer together when the
eye is accommodated for a near object than it is to observe
the slight movement and diminution in size of the single
image of a candle flame.
These may be regarded as the facts of adjustment with
which all explanations of that process must accord. The
following explanation, which was proposed by Helmholtz, is
the most generally accepted one. It seems probable from the
x USE OF SPECTACLES 437
anatomical relations of the parts, and it is supported by
direct experimental evidence. The lens, which is very
elastic, is kept habitually in a state of compression by the
pressure exerted on it by its suspensory ligament, and con-
sequently has a flatter form than it would take if left to it-
self. If the ciliary muscle contracts, it must, as has been
seen, relax that ligament, and thereby diminish its pressure
upon the lens. The lens, consequently, will become more
convex ; it will, however, since it is highly elastic, return to
its former shape when the ciliary muscle ceases to contract
and allows the choroid to return to its ordinary place.
Hence, probably, the sense of effort we feel when we
adjust for near distances arises from the contraction of the
ciliary muscle.
4. The Limits of Accommodation. Use of Spectacles.
— Accommodation can take place only within a certain
range ; this, however, admits of great individual variations.
People possessing ordinary, or, as it is called, "normal"
sight can adjust their eyes so as to see distinctly objects as
near to the eye as five or six inches ; but the image of an
object brought nearer than this becomes blurred and indis-
tinct, because the "near limit" of accommodation is then
passed. They can also adjust their eyes for objects at a very •
great distance, the indistinctness of the images of objects
very far off being due, not to want of proper focussing, but
to the details being lost through the minuteness of the
image.
Some people, however, are born with, or at least come to
possess, eyes in which the " near limit " of accommodation is
much closer. Such persons can see distinctly objects as
near to the cornea as even one or two inches ; but they can-
not adjust their eyes to objects at any great distance off.
Thus, many of these "near-sighted" people, as they are
438 ELEMENTARY PHYSIOLOGY less.
called, cannot see distinctly the features of a person only
a few feet off. Though their ciliary muscle remains quite
relaxed so that the suspensory ligament keeps the lens as
fiat as possible, the arrangements of the eye are such that
the image of an object only a few feet off is brought to a
focus in front of the retina, somewhere in the vitreous
humour. By wearing concave glasses these near-sighted
people are able to bring the image of distant objects on to
the retina and thus to see them distinctly.
The cause of near-sightedness is not always the same, but
in the majority of cases it appears to be due to the bulb of
the eye being unusually long from back to front. If, in the
water camera described above, when the lens and object
were so adjusted that the image of the object was distinctly
focussed on the screen, the. box were made longer, so that
the screen was moved backwards, the distinctness of the
image on it would be lost.
Some people are born really " long-sighted," inasmuch as
they can see distinctly only such objects as are quite dis-
tant ; and, indeed, have to contract their ciliary muscles,
and so make their lens more convex even to see these.
Near objects they cannot see distinctly at all unless they use
convex glasses. In such persons the bulb of the eye is
generally too short.
A kind of long-sightedness also comes on in old people ;
but this is different from the above, and is simply due, in
the majority of cases at all events, to a loss of power of
adjustment. The refractive power of the eye remains the
same, but the ciliary muscle fails to work; and hence
adjustment for near objects becomes impossible, though
distant objects are seen as before. For near objects such
persons have to use convex glasses. They should perhaps
be called " old-sighted " rather than " long-sighted."
TIIK MUSCLES OF THE EYEBALL
439
5. The Muscles of the Eyeball. — The muscles which
move the eyeball are altogether six in number — four straight
muscles, or recti, and two oblique muscles, the obliqui (Fig.
138). The straight muscles are attached to the back of the
bony orbit, round the edges of the hole through which the
optic nerve passes, and run straight forward to their inser-
tions into the sclerotic — one, the superior rectus, in the
middle line above ; one, the inferior, opposite it below ; and
one on each side, the external and internal recti. The eye-
Fig. 138.
A, the muscles of the right eyeball viewed from above, and B, of the left eyeball
viewed from the outer side; S.R., the superior rectus; ftif.R., the inferior rectus;
E.R., In.R., the external rectus; S.Ob., the superior oblique; Iti/.Ob., the inferior
oblique; C/i., the chiasmaof the optic nerves (//.) ; ///, the third nerve, which sup-
plies all the muscles except the superior oblique and the external rectus.
ball is completely imbedded in fat behind and laterally ; and
these muscles turn it as on a cushion ; the superior rectus
inclining the axis of the eye upwards, the inferior down-
wards, the external outwards, the internal inwards.
The two oblique muscles, upper and lower, are both
attached on the outer side of the ball, and rather behind its
centre; and they both pull in a direction from the point of
440 ELEMENTARY PHYSIOLOGY less.
attachment towards the inner side of the orbit — the lower,
because it arises here ; the upper, because, though it arises
along with the recti from the back of the orbit, yet, after pass-
ing forwards and becoming tendinous at the upper and inner
corner of the orbit, it traverses a pulley-like loop of liga-
ment, and then turns downwards and outwards to its inser-
tion. The action of the oblique muscles is somewhat
complicated, the upper rolling the eyeball downwards and
outwards, the lower rolling it upwards and outwards.
By means of the contraction of these several muscles in
various combinations the eyeballs may be moved into any
desired position and their optic axes (Fig. 132, ad) directed
straight towards any object. This mobility is largely of use
in diminishing the necessity for such frequent movements
of the whole head as would otherwise be necessary. But
the movements are also of extreme importance in that they
bring the two images of an object upon corresponding points
in the retinas of the two eyes (see p. 472) and thus insure
that the object is seen as single.
6. The Protective Appendages of the Eye. — The eyelids
are folds of skin containing thin plates of cartilage, and
fringed at the edges with hairs, the eyelashes, and with a
series of small glands called Meibomian, which secrete an
oily substance. Circularly disposed fibres of striped muscle
lie beneath the integuments of the eyelids, and constitute
the orbicularis muscle which shuts them (Fig. 139, Orb.).
The upper eyelid is raised by a special muscle, the levator
of the upper lid, which arises at the back of the orbit
and runs forwards to end in the lid. The lower lid has no
special depressor.
At the edge of the eyelids the integument becomes
continuous with a delicate, vascular, and highly nervous,
membrane, the conjunctiva (Fig. 132, 1'), which lines the
PROTECTIVE APPENDAGES OF THE EYE
441
■S.U6
interior of the lids and the front of the eyeball, its epithelial
layer being even continued over the cornea. The several
small ducts of a gland which is lodged in the orbit, on the
outer side of the ball (Fig. 139, L.G.), the lachrymal gland,
constantly pour its watery secretion into the interspace
between the conjunctiva lining the upper eyelid and that
covering the ball. On the nasal side of the eye is a reddish
elevation, between which and the eyeball is a narrow vertical
fold of conjunctiva, the semilunar fold ; the latter is a rudi-
ment of that third eyelid which is to be found in many
animals. Above and below, near this, the edge of each eye-
lid presents a minute aperture
(the punctum lacrimale), the
opening of a small canal. The
canals from above and below
converge, and open into the
lachrymal sac ; the upper blind
end of a duct (L.D., Fig. 140)
which passes down from the
orbit to the nose, opening be-
low the inferior turbinal bone
(Fig. 76, h). It is through this
cvatpm r,f r-onolc tVml- thp mil dissected to show Orb., the orbicular
system ot canals mat tne con- muscle of the eye]ids. the pulley and
innrtival mnrnnc; mpmhnnp is insertion of the superior oblique, .9.
juncuvai mucous memDrane ib oi. the inferjor obUque( Inf_ obx
continuous with that of the nose ; L-G- the lachr>'mal s^nd.
and it is by them that the secretion of the lachrymal gland
is ordinarily carried away as fast as it forms.
But under certain circumstances, as when the conjunctiva
is irritated by pungent vapours, or when painful emotions
arise in the mind, the secretion of the lachrymal gland
exceeds the drainage power of the lachrymal duct, and the
fluid, accumulating between the lids, at length overflows in
the form of tears.
Fig. 139.
The front view of the right eye
442
ELEMENTARY PHYSIOLOGY
7. The Structure of the Retina. — If the globe of the
eye be cut in two, transversely, so as to divide it into an
anterior and a posterior half, the retina will be seen lining
the whole of the concave wall of the posterior half as a
membrane of great delicacy, and, for the most part, of even
texture and smooth surface. But almost exactly opposite
the middle of the posterior wall, it presents a slight oval
depression of a yellowish hue, the macula lutea, or yellow
spot (Fig. 141, m. I; Fig. 132, 8"), — not easily seen, how-
ever, unless the eye be perfectly fresh, — and, at some dis-
tance from this, towards the inner or
nasal side of the ball, is a radiating
appearance, produced by the en-
trance of the optic nerve and the
spreading out of its fibres into the
retina.
A very thin slice of the retina
from its inner x to its outer surface,
in any region except the yellow
spot and the entrance of the optic
nerve, may be resolved into the
structures represented diagram matically in Figs. 142 and
143. These comprise eight layers, seven of which consist
largely of nerve-cells and their processes. By the application
of very special methods of staining microscopic sections the
true structure and relationships of the layers have only
LG
CD.
Fig. 140.
A front view of the left eye,
with the eyelids partially dissected
to show lachrymal gland, L.G,
and lachrymal duct, L.D.
1 In the following account of the retina, the parts are described in re-
lation to the eyeball. Thus, that surface of the retina which touches the
vitreous humour, and so is nearer the centre of the eyeball, is called the
inner surface ; and that surface which touches the choroid coat is called
the outer surface. And so with the structures between these two surfaces;
that which is called inner is nearer the vitreous humour, and that which is
called outer is nearer the choroid coat. Sometimes anterior, or front, is
used instead of inner, and posterior instead of outer.
x THE STRUCTURE OF THE RETINA 443
recently been discovered. Enumerated from the outer
surface (in contact with the choroid) to the inner surface
(next the vitreous humour), these layers are as follows : —
(i) The layer of pigment-cells,
(ii) The layer of rods and cones.
(hi) The outer nuclear layer.
(iv) The outer molecular layer.
(v) The inner nuclear layer,
(vi) The inner molecular layer.
(vii) The layer of nerve-cells,
(viii) The layer of nerve-fibres.
(i) When seen from the surface by which they are in
contact with the choroid, the pigment-cells present the
appearance of small black hexagons arranged in a sort of
mosaic (Fig. 145, a). They send long processes, loaded
with dark granules, among the rods and cones (3, c).
(ii) The rods and cones constitute the essential part of
the organ of sight, for it is they that receive the rays of light
and inaugurate the nervous impulse. They are processes
of modified epithelial cells, and they may also be called
nerve-cells, since they originate in the brain and grow out
along the optic nerve to the retina. They possess the
shape, relative size, and peculiar striated appearance shown
in Fig. 143, and are joined directly each with the nucleated
body of its own cell lying in
(iii) The outer nuclear layer. This layer receives its
name from these nuclei. The rod-cells are prolonged
through this layer each by a fine filament, which terminates
in the next inner layer in a small knob. Each of the cone-
cells sends a thick fibre through this layer to break up into
a brush of terminal filaments in
444
ELEM ENTA RY PH YSIO LOGY
(iv) The outer molecular layer. Here the end-knobs oi
the rod-cells and the terminal filaments of the cone-cells
are in contiguity, but not in actual continuity, with
branched processes from a second series of nerve-cells, the
rod and cone bipolar cells (Fig. 143, r.b.p., c.b.p). The
nucleated bodies of these bipolar cells lie in
Fig. 141.
The Eyeball divided transversely in the Middle Line and
viewed from the front.
s, sclerotic; ch, choroid, seen in section only.
r, the cut edges of the retina; r.v, vessels of the retina springing from o, the
optic nerve or blind spot- ;«./, the yellow spot, the darker spot in its middle being
the fovea centralis.
(v) The inner nuclear layer. Like the rod and cone-
cells of the outer nuclear layer each sends inwards a fibre
into the next layer,
(vi) The inner molecular layer. The cone bipolar cells
here terminate in expanded branches. Facing these, but
not in actual continuity with them, lie the processes from
the nerve-cells in the so-called layer of nerve-cells. The
fibre from each rod bipolar cell passes through this layer
and enters
THE STRUCTURE OF THE RETINA
445
(vii) The layer of nerve-cells, to end in branching pro-
cesses which surround the body of one of the nerve-cells.
These nerve-cells, while outwardly in relation with the rod
and cone bipolar cells, on the inner side are in direct con-
tinuity each with a fibre of the optic nerve. On their way
Outer surface.
(i) Layer of pigment-cells,
(ii) Layer of rods and cones.
(iii) Outer nuclear layer,
(iv) Outer molecular layer.
(v) Inner nuclear layer.
(vi) Inner molecular layer.
J)(vii) Layer of nerve-cells.
J: (viii) Layer of nerve-fibres.
Inner surface.
Fig. 142. — Diagrammatic Section of the Human Retina (Schiltze).
(From Quain's Anatomy.)
over the retina to the place of exit of the nerve these fibres
form
(viii) The layer of nerve-fibres.
In this complex way each rod and each cone is brought
446
ELEMENTARY PHYSIOLOGY
into relationship with a fibre of the optic nerve ; but, as will
be readily understood from the figure, the path of connec-
tion in each case shows two breaks in its structural continu-
ity, and the nervous impulses originating in the rods and
Fig. 143.
■Diagram in Illustration of the Nervous Structure of the
Retina.
ii-viii, the several " layers " of the retina
r.o, r.i, outer and inner limbs of a rod; r.f, rod fibre; r.n, rod nucleus; r.b.p, rod
bipolar cell; c.o, c.i, outer and inner limbs of a cone; c.f, cone fibre; c.n, cone
nucleus; c.b.p. cone bipolar cell: g.c, g.c, two cells of the nerve-cell layer; op.f, op.f,
fibres of optic nerve; s, h.c, cells of inner nuclear lr.yer, relationships of which are
not fully known.
THE STRUCTURE OE THE RETINA
44?
cones must necessarily pass across these breaks on the way
to the optic nerve.
These delicate nervous structures are supported by a
sort of framework of connective tissue of a peculiar kind,
which permeates all the layers
except the rods and cones and
the pigment-cells.
The artery supplying the
retina with blood enters in the
centre of the optic nerve, side
by side with the outgoing vein,
and then divides into several
branches (Fig. 141) ; the re-
sulting capillaries exist simply
in the four inner layers.
In addition to the bipolar
cells(Fig. 143, r.b.p and c.b.p),
which chiefly confer upon the
inner nuclear layer the char-
acteristic appearance from
which it derives its name,
other cells also occur in this
layer. These are shown in
h.c and s ; but their rela-
tionships to the other struc-
tural elements of the retina
are so uncertain that we must
content ourselves with merely
drawing attention to their ex-
istence.
At the entrance of the optic
nerve itself, the nervous fibres predominate, and the rods
and cones are absent. In the yellow spot, on the contrary,
44S
ELEMENTARY PHYSIOLOGY
the cones are abundant- and close set, becoming at the same
time longer and more slender, while rods are scanty, and
are found only towards its margin. In the centre of the
macula lutea (Fig. 144) the layer of fibres of the optic
nerve disappears, and all the other layers, except that of
the cones, become extremely thin.
lilP
Fig. 145. — Pigmented Epithelium of the Human Retina (Max Schultze, .
Highly Magnified.
a, cells seen from the outer (choroidal) surface; i, two cells seen sidewise, with
fine processes on their inner side; c, a cell still in connection with the layer of rods
of the retina.
8. The Sensation of Light. — The most notable property
of the retina is its power of converting the vibrations of ether,
which constitute the physical basis of light, into a stimulus
to the fibres of the optic nerve. The central ends of these
fibres are connected with certain parts of the brain which
constitute the visual sensorium, just as other parts, as we
have seen, constitute the auditory sensorium. The molecu-
lar disturbances set up in the fibres of the optic nerve are
transmitted to the substance of the visual sensorium, and
produce changes in the latter giving rise to the state of
feeling which we call a sensation of light.
The sensation of light, it must be understood, is the work
of the visual sensorium, not of the retina; for, if certain
parts of the brain be destroyed or affected, no sensation.
X THE " BLIND SPOT" 449
of light is possible even though the retina and indeed the
whole optic nerve be intact ; blindness is then the result,
because the visual sensorium cannot work.
Light, falling directly on the optic nerve, does not excite
it ; the fibres of the optic nerve, in themselves, are as blind
as any other part of the body. But just as the peculiar hair-
cells of the labyrinth, and the organ of Corti of the cochlea,
are contrivances for converting the delicate vibrations of
the endolymph into impulses which can excite the audi-
tory nerves, so the structures in the retina appear to be
adapted to convert the infinitely more delicate pulses of
the luminiferous ether into stimuli of the fibres of the optic
nerve.
9. The "Blind Spot." — The sensibility of the different
parts of the retina to light varies very greatly. The point
of entrance of the optic nerve is absolutely blind, as may
be proved by a very simple experiment. Close the left
eye, and look steadily with the right at the cross on the
page, held at ten or twelve inches distance from the eye.
*
The black dot will be seen quite plainly, as well as the
cross. Now, move the book slowly towards the eye, which
must be kept steadily fixed upon the cross ; at a certain
point the dot will disappear, but, as the book is brought
still closer, it will come into view again. It results from
optical principles that, in the first position of the book, the
image of the dot falls between that of the cross (which
throughout lies upon the yellow spot), and the entrance
of the optic nerve : while, in the second position, it falls on
2G
45°
ELEMENTARY PHYSIOLOGY
the point of entrance of the optic nerve itself; and, in the
third, it falls on the other side of that point. The three
positions of the dot and cross, and of the resulting images
of each on the retina, are shown in the accompanying
figure, 146.
So long as the image of the spot rests upon the entrance
of the optic nerve, it is not perceived, and hence this region
of the retina is called the blind spot.
The experiment proves that the vibra-
tions of the ether are not able to
excite the fibres of the optic nerve
itself.
10. The Duration of a Luminous
Impression. — The impression made
by light upon the retina not only
remains during the whole period of
the direct action of the light, but has
a certain duration of its own, how-
ever short the time during which the
light itself lasts. A flash of lightning
is, practically, instantaneous, but the
sensation of light produced by that
flash endures for an appreciable pe-
riod. It is found, in fact, that a
luminous impression lasts for about
one-eighth of a second ; whence it
follows, that if any two luminous im-
pressions are separated by a less
interval, they are not distinguished
from one another.
For this reason a " Catherine-wheel," or a lighted stick
turned round very rapidly by the hand, appears as a circle
of fire ; and the spokes of a coach wheel at speed are not
Fig. 146. — Diagram to il-
lustrate the Blind Ispot.
A SENSATIONS OF LIGHT 451
separately visible, but only appear as a sort of opacity, 01
film, within the tire of the wheel.
The same fact is made use of in the production of the
" animated photographs " which are now so perfectly shown
by the apparatus called the kinetoscope. A series of instan-
taneous photographs of some object in motion, taken at the
rate of many per second, is printed on a long transparent
film of celluloid. The film is then passed through a magic-
lantern at such a rate that not less than ten of the consecu-
tive photographs are projected upon the screen in each
second. At this rate, the impression produced by one
photograph has not had time to die out before the next
one produces its slightly different later effect. The result
is that the consecutive pictures on the screen blend in suc-
cession one into the other and so reproduce the appearance
of the original moving object.
11. Sensations of Light produced without the Action
of Light. — The sensation of light may be excited by other
causes than the impact of the vibrations of the luminiferous
ether upon the retina. Thus, an electric shock sent through
the eyeball may give rise to the appearance of a flash of
light : and pressure on any part of the retina produces a
luminous image, which lasts as long as the pressure, and is
called a phosphene. If the point of the finger be pressed
upon the outer side of the ball of the eye, the eyes being
shut, a luminous image — which, in most cases, is dark in
the centre, with a bright ring at the circumference (or, as
Newton described it, like the " eye " in a peacock's tail-
feather) — is seen ; and this image lasts as long as the
pressure is continued. Most persons have experienced
the remarkable display of subjective fireworks — have seen
" stars," following a heavy blow about the region of the
eyes.
4^2 ELEMENTARY PHYSIOLOGY less.
The sensation of light is, as already explained, the work
of those parts of the brain which, as the visual sensorium,
respond to the impulses reaching them through the optic
nerve. The retina is the usual means of supplying the
impulses to the sensorium and may be made to do so by
light ordinarily, but also by other kinds of stimulation.
But the visual sensorium itself may at times be affected by
influences other than those which reach it from the retina.
In this case also (subjective) luminous sensations of the
most vivid and startling kind may be experienced, which
give rise to delusive judgments of the most erroneous kind
(see p. 464).
12. The Functions of the Rods and Cones. — We have
seen that the fibres of the optic nerve ramify in the inner
fourth of the thickness of the retina, while the layer of rods
and cones forms its outer fourth. The light, therefore, must
fall first upon the fibres of the optic nerve, and only after
traversing them and the other layers of the retina can it
reach the rods and cones. Consequently, if the fibrillar of
the optic nerve themselves are capable of being affected by
light, the rods and cones can only be some sort of supple-
mentary optical apparatus. But, in fact, it is the rods and
cones which are affected by light, while the fibres of the
optic nerve are themselves insensible to it. The evidence
on which this statement rests is : —
(i) The blind spot is full of nerve-fibres, but has no cones
or rods.
(ii) The yellow spot, where the most acute vision is situ-
ated, is full of close-set cones, but has no nerve-fibres.
(iii) If one goes into a dark room with a single small
bright candle, and, looking towards a dark wall, moves the
light up and down, close to the outer side of one eye, so as
to allow the light to fall very obliquely into the eye, what
x SENSATIONS OF COLOUR 453
are called Furkinje's figures are seen. These are a series
of diverging, branched, dark, sometimes reddish, lines on an
illuminated field. The lines are the images of shadows
thrown by the retinal blood-vessels (Fig. 141). As the
candle is moved up and down, the lines shift their position,
as shadows do when the light which throws them changes
its place.
Now, as the light falls on the front face of the retina, and
the images of the vessels to which it gives rise shift their
position as it moves, whatever constitutes the end-organ,
through which light stimulates the fibres of the optic nerve,
must needs lie on the other side of the vessels. But the
fibres of the optic nerve lie among the vessels, and the only
nervous structures of the retina which lie outside them are
the rods and cones with their attached cell-bodies.
The image of the retinal blood-vessels may be also very
readily seen by looking at • a bright surface, such as the
frosted globe of a burning lamp or a white cloud on a sunny
day, through a pinhole in a card. When the card is moved
rapidly from side to side, but so as. to keep the pinhole
always within the limits of the width of the pupil, the retinal
blood-vessels are "seen" as a fine branched network of
black lines in the bright field of vision.
(iv) Just as, in the skin, there is a limit of distance within
which two points give only one impression, so there is a
minimum distance by which two points of light falling on
the retina must be separated in order to appear as two.
And this distance corresponds pretty well with the diameter
of a cone.
13. Sensations of Colour and Colour-blindness. — We
have spoken of the eye so far simply as the instrument by
which luminous sensations arise when the retina is stimu-
lated ; as an instrument which enables us to appreciate the
454 ELEMENTARY PHYSIOLOGY less.
position of a source of light, and differences in the intensity
of the light which it emits or reflects, and hence to perceive
objects in the world around us as regards their position,
shape, and size. But the objects we see are characterised
by something more than mere shape and size ; they differ
also in respect of what we call their colour.
When we look at a rainbow we are conscious of seven
broadly distinct kinds of colour-sensations ; these are red,
orange, yellow, green, blue, indigo-blue, and violet, and
when ordinary white light is passed through a prism and
then allowed to fall into the eye we experience the same
seven coloured sensations. The prism has, in fact, resolved
the light into its several coloured constituents, and these are
known as the " colours of the spectrum." Each colour
which we recognise as such is characterised, just as in the
case of sounds, by certain qualities; these are (i) hue, or
colour, as we ordinarily use the word to denote what we call
reds, greens, blues, and so on. This quality is dependent
on the wave-length of the ethereal vibrations which are giv-
ing rise to the sensation, and hence corresponds to the
" pitch " of a sound, (ii) intensity or brightness. This
depends on the amount of light which falls on the retina in
a given time and corresponds to the loudness of a sound,
(iii) saturation, or the amount of admixture with white
light. Thus, we speak of a colour as being " pale " if
mixed with much white, and as being "deep," "rich," or
"full," if highly saturated, i.e. unmixed with white.
The colours of objects depend on the power they possess
of absorbing some of the constituents of ordinary white light
and allowing others to pass or to be reflected. Thus, a
piece of transparent glass is red if it allows the red rays to
pass through to the eye and stops the others. Similarly, the
colour of an opaque red object is due to an absorption of the
X SENSATIONS OF COLOUR 455
spectral colours other than red, and the reflection of the
unabsorbed red rays.
When white light has been split up into its coloured con-
stituents by means of a prism, these may be gathered up
again by a second prism, suitably placed, and recombined
to make white light. In this experiment the several col-
ours of the spectrum are mixed once more after having been
sorted out or separated, and the mixing is a physical process.
But colours may also be mixed physiologically by taking ad-
vantage of that persistence of luminous impressions to which
we have already drawn attention (p. 450). Thus, if the
several colours of the spectrum are painted in sectors on a
circular disc and the disc is made to spin rapidly round its
centre, the sensations due to each colour are blended to-
gether and the disc appears white. The common instru-
ment used in this mode of mixing colours is called a " colour
lop."
By the use of a colour top it is at once possible to mix
not merely all the spectral colours but any two or three of
them. Experimenting in this way with pairs of colours we
find that there are several pairs which when mixed give rise
to the sensation of white : thus red and green, orange and
blue, yellow and indigo-blue, greenish-yellow and violet.
Colours which when mixed in this way in pairs give white
are known as complementary colours, and every colour has
some other colour which is complementary to it. If instead
of mixing the colours in pairs we mix them in threes, then
it becomes still more easy to produce a resultant white.
Thus, by mixing red, green, and blue, with due regard to
the relative amount and intensity of each, an excellent white
is readily obtained. But these three colours enable us to
do more than merely produce white. By properly adjust-
ing the proportions of each on the disc of the colour top we
456 ELEMENTARY PHYSIOLOGY less
can easily produce an orange and a yellow, as also a violet,
In other words, these three colours and their mixtures give
rise to all the several kinds of colour-sensation which we
derive from a spectrum. Further, by suitable mixture of
these colours, together with white or black, we can pro-
duce the other colours which we see in natural objects
around us but which are wanting in the spectrum. Thus,
purple is extremely common in the world and can be made
at once by mixing red and blue. Hence these three col-
ours have come to be regarded as primary colours, and we
may speak of the sensations to which they give rise as
primary sensations.
The foregoing considerations lead at once to the view
that all our sensations of colour maybe regarded as the out-
come of a very limited number (three) of simple or primary
sensations, corresponding to red, green, and blue. In
accordance with this fact a theory has been put forward1
that there are in the nervous apparatus of vision three kinds
of nervous structure, the nature and exact location of which
are wholly unknown, but of which each corresponds to one
of the primary colours and is most easily set in action by
one of these colours. Thus, the stimulation of one of them
gives rise to one of the primary sensations, the simultaneous
stimulation of all three to the same extent gives rise to the
sensation of white, and their simultaneous stimulation to
varying degrees gives rise to all the other sensations of col-
our of which we are at any time conscious.
This theory attempts to account for observed facts, and
goes a long way in doing so ; but it does not account com-
1 This theory was first propounded by an Englishman, Dr. Thomas
Young, the originator of the undulatory theory of light. In later times it
was adopted and amplified by Helmholtz, and is therefore known as the
Young-Helmholtz theory.
x COLOUR-BLINDNESS 457
pletely for all the facts of colour vision. Other theories,
likewise insufficient, have been proposed, but a discussion
of their respective merits, to be of value, must necessarily be
lengthy and detailed, and hence out of place in an element-
ary text-book.
The excitability of the retina is readily exhausted. Thus,
looking at a bright light rapidly renders the part of the
retina on which the light falls insensible ; and on looking
from the bright light towards a moderately lighted surface,
a dark spot, arising from a temporary blindness of the retina
in this part, appears in the field of view. If the bright light
be of one colour, the part of the retina on which it falls be-
comes insensible to the rays of that colour, but not to the
other rays of the spectrum. This is the explanation of the
appearance of what are called negative after-images. For
example, if, as in the form in which the experiment is most
commonly made, a bright red wafer be stuck upon a sheet
of white paper, and steadily looked at for some time with
one eye, when the eye is turned aside to the white paper a
greenish spot will appear, of about the size and shape of the
wafer. The red image has, in fact, fatigued the part of the
retina on which it fell for red light, but has left it sensitive
to the remaining coloured rays of which white light is com-
posed. But we know that, if from the variously coloured
rays which make up the spectrum of white light we take
away all the red rays, the remaining rays together make up
a sort of green. So that, when white light falls upon this
part, the red rays in the white light having no effect, the
result of the operation of the others is a greenish hue.
The colour of the negative after-image is thus of necessity
complementary to that of the object looked at. If the wafer
be green, the after-image is of course red.
Colour-blindness. — Most people agree very closely as to
458 ELEMENTARY PHYSIOLOGY less, x
differences between different colours and different parts of
the spectrum. But there are exceptions. Thus a certain
number of persons see very little difference between the
colour which most people call red, and that which most peo-
ple call green. Such colour-blind persons are perhaps unable
to distinguish between the leaves of a cherry-tree and its fruit
by the colour of the two ; they are only aware of a difference
of shape between the two. Cases of this "red-blindness"
or " red-green " blindness are not uncommon; but other
forms of colour-blindness are much more rare ; and extremely
rare, though of undoubted occurrence, are the cases of those
who are wholly colour-blind, i.e. to whom all colours are mere
shades of one tint.
This peculiarity of colour-blindness is simply unfortunate
for most people, but it may be dangerous if unknowingly
possessed by engine-drivers or sailors, particularly since red-
green colour-blindness is most common and red and green
are exactly the two colours ordinarily used for signals. It
probably arises either from a defect in the retina, which
renders that organ unable to respond to different kinds of
luminous vibrations, and consequently insensible to red, yel-
low, or other rays, as the case may be ; or the fault may lie
in the visual sensorium itself.
For ordinary purposes colour perception may be most
easily and successfully tested by asking the person under
examination to make matches between skeins of coloured
wool. In this way it is found that a red-green colour-blind
person matches a red with a green skein.
The phenomena of colour-blindness can, to a certain
extent at least, be explained according to the theory of col-
our-vision which has been given above. Thus, a red-green
colour-blind person is supposed to lack either the red-per-
ceiving or the green-perceiving structures normally present
either in the retina or the visual sensorium.
LESSON XI
THE COALESCENCE OF SENSATIONS WITH ONE
ANOTHER AND WITH OTHER STATES OF CON-
SCIOUSNESS
1. Sensations may be Simple or Composite. — In ex-
plaining the functions of the sensory organs, we have
hitherto confined ourselves to describing the means by
which the physical agent of a sensation is enabled to irritate
a given sensory nerve ; and to giving some account of the
simple sensations which are thus evolved.
Simple sensations of this kind are such as might be pro-
duced by the irritation of a single nerve-fibre, or of several
nerve-fibres by the same agent. Such are the sensations of
contact, of warmth, of sweetness, of an odour, of a musical
note, of whiteness, or redness.
But very few of our sensations are thus simple. Most of
even those which we are in the habit of regarding as simple,
are really compounds of different simultaneous sensations,
or of present sensations with past sensations, or with those
feelings of relation which form the basis of judgments. For
example, in the preceding cases it is very difficult to sepa-
rate the sensation of contact from the judgment that some-
thing is touching us ; of sweetness, from the idea that some-
thing is in the mouth ; of sound or light, from the judgment
that something outside us is shining or sounding.
The sensations of smell are those which are least compli
459
460 ELEMENTARY PHYSIOLOGY less.
cated by accessories of this sort. Thus, particles of musk
diffuse themselves with great rapidity through the nasal pas-
sages and give rise to the sensation of a powerful odour.
But beyond a broad notion that the odour is in the nose,
this sensation is unaccompanied by any ideas of locality and
direction. Still less does it give rise to any conception of
form, or size, or force, or of succession, or contemporaneity.
If a man had no other sense than that of smell, and musk
were the only odorous body, he could have no sense of
outness — no power of distinguishing between the external
world and himself.
Contrast this with what may seem to be the equally
simple sensation obtained by drawing the finger along the
table, the eyes being shut. This act gives one the sensation
of a flat, hard surface outside one's self, which sensation
appears to be just as simple as the odour of musk, but
is really a complex state of feeling compounded of —
(a) Pure sensations of contact.
(&) Pure muscular sensations of two kinds, — the one
arising from the resistance of the table, the other from the
actions of those muscles which draw the finger along.
(. 372) exist, the sensory fibres are connected with them
THE STRUCTURE OF NERVES
489
by means of their axis-cylinder, from which the neurilemma
and medulla have disappeared. If, as before, we follow the
sensory nerve-fibres back towards the spinal cord, we find
that they pass through the ganglion on one of the posterior
roots, and then enter the substance of the cord, passing
towards the posterior cornu. Like the motor nerve-fibres,
they lose their noded neurilemma as they enter the cord, so
that in this case also it is again the axis-cylinder which pro-
vides the actually continuous connection between the sense-
organ and the central nervous system.
The neurilemma, with its nucleus and the medulla, may
be regarded as a covering which provides for the protection
and nourishment of each successive length of the essentially
important neuraxis or axis-cylinder.
Fig. 153. — Pale non-Meduxlated Fibres from the Pneumogastric Nerve
(Ranvier).
«, nucleus; /, protoplasm belonging to the nucleus.
The fibres which make up the essential structure of the
nerves with which we have so far dealt are spoken of as
medullated, because except at their peripheral and central
terminations they possess the characteristic medulla (p. 485).
But scattered among these medullated fibres are a few which
are often spoken of as non-medullated, because they possess
no medulla. These non-medullated fibres are peculiarly
abundant in the nerves of the sympathetic system, so much
so that they are frequently called " sympathetic fibres."
49Q ELEMENTARY PHYSIOLOGY "less.
They appear under the microscope as pale flattened bands,
about as wide as small medullated fibres, often fibrillated
longitudinally, and frequently dividing (Fig. 153). They
appear, in fact, to be naked axis-cylinders, without medulla,
and apparently without a neurilemma, though they bear at
intervals on their surface nuclei, which may represent the
internodal nuclei of ordinary nerve-fibres.
6. The Minute Structure of the Spinal Cord and Spinal
Ganglia. — The white matter of the spinal cord consists
mainly of medullated nerve-fibres running for the most
part lengthwise. In a transverse section the fibres hence
show their cut ends, and the white matter appears to con-
sist of multitudes of minute rings (medulla), each contain-
ing a dot (axis-cylinder). The nerve-fibres are supported
by a fine felt-work of extremely delicate fibres which con-
stitutes what is known as the neuroglia (vevpov = nerve, and
yXta = glue), since it binds the nerve-fibres together. The
fibres of the neuroglia are, in reality, processes from number-
less minute cells, the neuroglia-cells (Fig. 154), in each of
which the body of the cell is extremely small, and the pro-
cesses unusually numerous.
At frequent intervals all over the surface of the cord and
in the fissures, the pia mater sends conspicuous longitudinal
partitions (Fig. 148), composed of connective tissue, into
the substance of the white matter. These partitions divide
and subdivide as they extend farther into the cord ; they
carry blood-vessels and lymphatics and provide for the inti-
mate distribution of these vessels throughout the nervous
tissue. The connective tissue and neuroglia are continued
on into the grey matter.
The most striking feature of the grey matter is the pres-
ence in its neuroglia of nerve-cells, many of which are very
large and conspicuous, while others are smaller, but still
xii MINUTE STRUCTURE OF THE SPINAL CORD 491
very evident ; these cells and their processes, together with
the comparative absence of medullated nerve-fibres, and the
presence of a closely interwoven network of non-medullated
nerve-fibres, form the chief contrast between the structure
of the grey and the white matter of the spinal cord.
The Cells of the Grey Matter. — These cells are not scat-
tered uniformly throughout the grey matter, but are arranged
in groups. The largest cells occur at the end of the anterior
horn (see Figs. 148 and 156), and since these are typical,
as regards the main features of their structure, of all the
cells of the grey matter, we may take one of them for
detailed description.
Fig. 154. — A Neuroglia-cell from the White Matter of the Spinal Core
(Schafer).
The body and processes of the cell appear black, since they were deeply stained in
order to bring out their details.
The body of each cell is large (varying in diameter from
50/A to 140/x; T^¥ to yJr- of an inch), and contains a very
conspicuous nucleus (Fig. 155). The cell-body is pro-
longed into a varying number of dendrites (usually many),
dividing and subdividing into branches, which may be traced
to some distance from the cell, becoming finer and finer, and
finally ending. Besides these branching processes the cell
bears one axis-cylinder process, which does not divide in
this way, passes straight away from the cell, and is soon cov-
ered by a layer of myelin or a medulla ; after its exit from
492
ELEMENTARY PHYSIOLOGY
the cord, it acquires additionally a neurilemma or primitive
sheath. In this way this process becomes the axis-cylinder
or neuraxis of a medullated nerve-fibre, and is continuous
to the organ, usually a muscle, to which it is distributed.
Fig. 155. — Diagram of a Typical Cell from the Grey Matter of the
Spinal Cord (Sherrington).
n, nucleus; d, d, d, branched processes (dendrites) from the cell-body; /, pig-
ment; c, part of cell-body which stains very readily (chromophilic substance); a,
axis-cylinder process, or neuraxon, which acquires first a medulla, m, and then (out-
side the cord) a neurilemma.
A, represents the processes (dendrites) from a neighbouring cell interlacing with,
but not joined on to, the processes of the cell figured.
The arrows indicate the direction in which the processes conduct nerve impulses.
The Differences in Structure of the Spinal Cord at Various
Levels. — These differences show themselves most conspicu-
ously with respect to (i) the shape of the column of grey mat-
ter at various levels, (ii) the position of the chief groups of
nerve-cells in the grey matter, and (iii) the amount of white
matter relatively to the grey matter at each level. The cord
is largest in the cervical region, smallest in the thoracic
(dorsal) region, and increases in size again in the lumbar
region ; that is, it is large in those parts which supply with
nerves not simply the portions of the trunk of the body that
lie at those levels, but the arms and legs in addition. The
MINUTE STRUCTURE OF THE SPINAL CORD 403
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494
ELEMENTARY PHYSIOLOGY
chief structural differences are very clearly indicated in Fig-
ure 156, which represents sections, drawn to scale, of (half)
the spinal cord at the level of A, the sixth, cervical, B,
the sixth, thoracic (dorsal), and C, the third lumbar spinal
nerves respectively.
The Structure of a Spinal Ganglion. — A spinal ganglion is,
as we have said (Fig. 149, G11.), an elongated swelling on
the posterior root of a spinal nerve. In a longitudinal sec-
tion it is seen to consist of an external sheath of connective
tissue which incloses groups of large nerve-cells, of which
the largest group lies at its outer side. The nerve-fibres
which enter the distal end of the ganglion on their way to
Fig. 157. — A Nerve-cell from the Ganglion on the Posterior Root of a
Spinal Nerve.
n.c, the body of the nerve-cell, with n, nucleus, ;;', nucleolus, /, protoplasmic
body; c, capsule of the nerve-cell; «", nuclei of the capsule; n.f, the nerve -fibre
which, at the node, d, divides into two. At a the neuraxis of the fibre is lost in the
substance of the cell; at b it acquires a medulla; at «'" nuclei are seen on the fibre.
At the division the neuraxis, d, is seen to divide, and besides the neurilemma, «./, the
fibre has an additional sheath, s, continuous with the capsule of the nerve-cell.
the spinal cord pass in bundles in between the groups of
nerve-cells, and a certain amount of connective tissue, with
accompanying blood-vessels and lymphatics, also passes in
amongst the nerve-cells and nerve-fibres. Each nerve-cell
(Fig. 157) consists, like a nerve-cell of the spinal cord, of
a large nucleus, with a nucleolus, and of a cell-body ; but
the cell-body is, in most cases at all events, prolonged into
xii FUNCTIONS OF ROOTS OF SPINAL NERVES 495
one process only, so that the whole cell is pear-shaped.
This process soon acquires a medulla and a neurilemma ; it
thus becomes an ordinary medullated nerve-fibre, which
then divides into two fibres, one of which may be traced
into the nerve trunk, and the other along the posterior root
to the spinal cord. Hence the nerve-cells of the ganglion
appear to be lateral appendages of the nerve-fibres, forming
a junction with them after the fashion of a T-piece. On the
central, or cord, side of the ganglion the fibres continue
their course into the substance of the spinal cord towards
the posterior horn. Like the motor nerves they lose their
neurilemma as they join the cord. Some of them pass on
at once into the grey matter of the posterior horn ; but the
majority turn aside as they enter the cord and run upwards
for some distance in the posterior column of the white sub-
stance before they enter the grey matter.
Structurally we may regard the nerve-fibres of the pos-
terior roots of the spinal cord as taking their origin from
one process of a cell in the spinal ganglion in the same
way that the fibres of the anterior root originate in one
process of a cell in the anterior horn of the grey matter.
This accounts for the peculiar way in which the fibres of
the posterior root make their connection with the cord, and
also for the most obvious function of the spinal ganglia, of
which we shall speak presently.
7. The Functions of the Roots of the Spinal Nerves.
— The physiological properties of the organs now described
are very remarkable.
If the trunk of a spinal nerve be irritated in any way
(at x in Fig. 158), as by pinching, cutting, galvanising, or
applying a hot body, two things happen : in the first place,
all the muscles to which filaments of this nerve are dis-
tributed contract ; in the second, pain is felt, and the
496 ELEMENTARY PHYSIOLOGY less.
pain is referred to that part of the skin to which fibres
of the nerve are distributed. In other words, the effect
of irritating the trunk of a nerve is the same as that of
irritating its 'component fibres at their terminations. •
The effects just described will follow upon irritation of
any -part of the branches of the nerve: except that when
a branch is irritated the only muscles directly affected, and
the only region of the skin to which pain is referred, will
be those to which that branch sends nerve-fibres. And
these effects will follow upon irritation of any part of a
nerve, from its smallest branches up to the point of its
trunk, at which the anterior and posterior root fibres unite.
If the fibres of the anterior root be irritated in the
same way (at_y, Fig. 158), only half the previous effects
are brought about. That is to say, all the muscles to
■ which the nerve is distributed contract, but no pain is
felt.
So, again, if the posterior, ganglionated root be irritated
(at z, Fig. 158), only half the effects of irritating the whole
trunk are produced. But it is the other half; that is to say,
none of the muscles to which the nerve is distributed con-
tract, but pain is referred to the whole area of skin to which
the fibres of the nerve are distributed.
It is clear enough, from these experiments, that all the
power of causing muscular contraction which a spinal nerve
possesses is lodged in the fibres which compose its anterior
roots ; and all the power of giving rise to sensation, in those
of its posterior roots. Hence the anterior roots are com-
monly called motor, and the posterior sensory.
The same truth may be illustrated in other ways. Thus,
if, in a living animal, the anterior roots of a spinal nerve be
cut, the animal loses all control over the muscles to which
that nerve is distributed, though the sensibility of the region
xii FUNCTIONS OF ROOTS OF SPINAL NERVES 497
of the skin supplied by the nerve is perfect. If the poste-
rior roots be cut, sensation is lost, and voluntary movement
remains. But if both roots be cut, neither voluntary move-
ment nor sensibility is any longer possessed by the part
supplied by the nerve. The muscles are said to be para-
lysed ; and the skin may be cut, or burnt, without any sen-
sation being excited.
If, when both roots are cut, that end of the motor root
which remains connected with the trunk of the nerve be
irritated, the muscles contract ; while, if the other end be
so treated, no apparent effect results. On the other hand,
■Ma^t /-4^°*
Fig. 158. — Diagram to illustrate Experiments in Proof of the Functions
of the Spinal Nerve Roots and of the Ganglion on the Posterior
Root.
AF, anterior fissure of spinal cord; PF, posterior fissure; AR, anterior root of
spinal nerve; PR, posterior root; T, trunk of spinal nerve; Gti, ganglion of pos-
terior root.
if the end of the sensory root connected with the trunk
of the nerve be irritated, no apparent effect is produced ;
while, if the end connected with the cord be irritated, pain
immediately follows.
When no apparent effect follows upon the irritation of
any nerve, it is not probable that the molecules of the
nerve remain unchanged. On the contrary, it would appear
that the same change occurs in all cases ; but a motor nerve
is connected with nothing that can make that change appar-
ent save a muscle, and a sensory nerve with nothing that can
show an effect but the central nervous system.
2K
498 ELEMENTARY PHYSIOLOGY less.
It is an interesting fact that the continued life of any
nerve-fibre is dependent upon the continuance of its
connection with the cell from which it arises. This de-
pendence is shown by the simple experiment of cutting a
nerve, and preventing the cut ends from reuniting. Thus,
if the anterior (motor) root of one of the spinal nerves be
cut at y (Fig. 158), all the fibres of that root beyond y
towards and along the trunk of the nerve T degenerate.
This degeneration shows itself by structural changes in the
nerve-fibres, which result ultimately in a total disappearance
of the axis-cylinders and medullary sheaths. While these
structural changes are taking place, and even before they
become obvious, the irritability of the nerve becomes gradu-
ally less, so that soon the nerve makes no response to any
stimulus which may be applied to it. But the changes we
have described do not occur in that (central) part of the
nerve which is still connected with the cells of the spinal
cord ; the portion of the root between y and the spinal cord
does not degenerate. Hence the " nutritional centre," as
it may be called, of the efferent fibres of the spinal nerves
lies in the nerve-cell bodies of the anterior cornu of the
spinal cord.
If we apply the same method of experiment to the
posterior root the following results are observed : when
the root is cut at w (Fig. 158), the fibres of that root
towards and along the trunk of the nerve T degenerate ; the
central parts connected with the ganglion do not. If, on
the other hand, the posterior root is cut at z, then the part
of the root which lies between z and the spinal cord degen-
erates, whereas the portion still connected with the ganglion
does not. Evidently the life of the fibres in the posterior
root is dependent upon their continued connection with the
cells of the ganglion, of which the fibres are processes.
xii PHYSIOLOGICAL PROPERTIES OF A NERVE 499
These facts lead to the inevitable conclusion that the func-
tion of the ganglion of the posterior root is to provide for
the nutrition of the afferent fibres of the spinal nerves.
This method of determining and localising the nutritional
centres from which nerve-fibres grow is known as the " de-
generation method," 1 and has proved to be most helpful in
determining the various " tracts," or paths in the spinal cord
and brain along which nervous impulses of various kinds
pass ; with these we shall have to deal later on (see p.
512).
8. The Physiological Properties of a Nerve. — It will
be observed that in all the experiments described in the
first part of the preceding section there is evidence that,
when a nerve is irritated, something which is spoken of as
a nervous impulse and consists, probably, of a change in
the arrangement of its molecules, is propagated along the
nerve-fibres. If a motor or a sensory nerve be irritated at
any point, contraction in the muscle, or sensation (or some
other corresponding event) in the central organ, immedi-
ately follows. But if the nerve be cut, or even tightly tied
at any point between the part irritated and the muscle or
central organ, the effect at once ceases, just as cutting a
telegraph wire stops the transmission of the electric current
or impulse. When a limb, as we say, " goes to sleep," it is
frequently because the nerves supplying it have been sub-
jected to pressure sufficient to interfere with the nervous
conductivity of the fibres, that is, their power to transmit
nervous impulses. We lose voluntary control over, and sen-
sation in, the limb, and these powers are only gradually
restored as that nervous conductivity returns.
Having arrived at this notion of an impulse travelling
1 Also as the " Wallerian method," after the name of the physiologist
who first employed it.
500 ELEMENTARY PHYSIOLOGY less.
along a nerve, we readily pass to the conception of a sen-
sory nerve as a nerve which, when active, brings an impulse
to the central organ, or is afferent : and of a motor nerve
as a nerve which carries away an impulse from the organ, or
is efferent. It is more convenient to use these terms to
denote the two great classes of nerves than the terms motor
and sensory ; for there are afferent nerves which are not
sensory in the sense of giving rise to a change of conscious-
ness, or sensation, while there are efferent nerves which are
not motor, in the sense of inducing muscular contraction.
The nerves, for example, by which the electrical fishes give
rise to discharges of electricity from peculiar organs to
which those nerves are distributed, are efferent, inasmuch as
they carry impulses to the electric organs, but are not motor,
inasmuch as they do not give rise to movements. The
pneumogastric when it stops the beat of the heart cannot be
called a motor nerve, and yet is then acting as an efferent
nerve. Similarly, the nerves which cause the cells of a
gland to commence secreting, such as those to the salivary
glands, sweat glands, pancreas, etc., are not motor nerves
but are strictly efferent as regards the direction in which
they convey their impulses.1 It will, of course, be under-
stood, as pointed out above, that the use of these words
does not imply that when a nerve is irritated in the middle
of its length the impulses set up by that irritation travel
only away from the central organ if the nerve be efferent,
and towards it if it be afferent. On the contrary, we have
evidence that in both cases the impulses travel both ways.
All that is meant is this, that the afferent nerve from the dis-
position of its two ends, in the skin, or other peripheral
1 In the human and higher vertebrate body it is, in fact, customary to
classify efferent nerves into *he three groups of motor, inhibitory, and
secretory;
xii PHYSIOLOGICAL PROPERTIES OF A NERVE 501
organs on the one hand, and in the cential organ on the
other, is of use only when impulses are travelling along it
towards the central organ, and, similarly, the efferent nerve
is of use only when impulses are travelling along it away
from the central organ.
There is no difference in structure, in chemical or in
physical character, between afferent and efferent nerves.
The impulse which travels along them requires a certain
time for its propagation, and is vastly slower than many
other movements — even slower than sound. (See p. 504.)
We know but little of the nature of a nervous impulse.
We know that it may be started in a nerve by various artifi-
cial means such as by pinching or knocking the nen e, or
by suddenly warming or cooling it, and, most readily, by
stimulating the nerve electrically. And we suppose that by
any of these means there is set up in that bit of nerve to
which any one of the above " stimuli " is applied a disturb-
ance, which is then propagated in succession from one
particle (or molecule) of the axis-cylinder to the next, so
that it ultimately reaches a point in the nerve remote from
that in which it was started. In this way we come to speak
of a nervous impulse as due to the propagation of a " molec-
ular disturbance " along a nerve. But this expression
serves rather to hide our ignorance than to explain what the
impulse really is.
If we may illustrate what is meant by this expression, by
likening the process of the transmission of a nervous impulse
to the transmission of any other condition with which most
people are familiar, we might compare it with the passage
of the explosion along a train of gunpowder when a spark is
applied to one end of it. In this case the spark merely sets
up a molecular change or disturbance in the grains of pow-
der to which it is applied ; the change thus set up leads to
502 ELEMENTARY PHYSIOLOGY less.
a similar change in the next neighbouring grains, and so on
along the whole train of powder, so that ultimately the result
of applying the spark at one end makes its appearance as a
similar result at the other end of the train. Similarly, in a
nerve we may regard the stimulus as setting up a change,
whose nature we do not as yet understand, at the point to
which it is applied ; this change sets up a similar change in
the next neighbouring particles of the nerve, and so on until
it finally appears at the furthest end of the nerve. But a
nerve, unlike the train of gunpowder, relays itself so long as
it is alive, as soon as the impulse has passed along it, whereas
the train of powder is " dead " after the passage of the explo-
sion, and must be artificially relaid for further use. It should
be borne clearly in mind that the simile that we have just
used is to be taken in its broadest outlines only, and that we
are quite ignorant of what is really going on in a nerve when
in action.
The Electrical Properties of a Nerve. — In the case of a mus-
cle we saw (p. 322) that its entry into a state of (contract-
ing) activity was accompanied by an easily recognised change
of shape, by chemical changes and by changes of tempera-
ture. In a nerve, when it is active, i.e. is conveying an
impulse, the first of these changes is of course entirely want-
ing and the others have not so far been shown to take place.
But we saw also that the contracting activity of a muscle is
accompanied by an electrical disturbance ; a similar dis-
turbance takes place in a nerve as the impulse sweeps along
it, and is,, indeed, the only evidence we possess of the pas-
sage of the impulse at any moment.
This electrical phenomenon consists in the fact that each
successive portion of the nerve becomes electrically negative
as the impulse passes it. This electrical change sweeps over
the nerve in the form of a wave. It may readily be demon-
xii RATE OF A NERVOUS IMPULSE 503
strated in an excised piece of nerve, as, for instance, the
sciatic nerve of a frog (see Fig. 96), by connecting two
points of the nerve with a sensitive galvanometer1 (Fig. 159)
and stimulating at some other point. The deflection of the
needle of the galvanometer indicates the moment when the
particular portion of the nerve connected with the galva-
nometer goes into activity, and the intensity of its activity.
Fig 159. — To show Arrangement of a Nerve and Galvanometer for Ex-
periments on the Electrical Properties of a Nerve.
AB, a piece of nerve; G, a galvanometer connected by wires and the electrodes
a, b, with the end B and the middle point C of the nerve.
The Rate of Transmission of a Nervous Impulse. — By means
of a complicated arrangement of apparatus it is possible to
determine very exactly the rate at which the electrical
change passes over the- nerve, and, by inference, the rate of
transmission of the nervous impulse. This is found to be
about 28 metres, or 90 feet per second, in the nerve of a frog.
The rate of transmission of the impulse may also be
determined in a much simpler way, by using a muscle-nerve
preparation such as is figured on page 321. The muscle is
suspended from a clamp, as shown in Fig. 160 ; a light hori-
zontal lever is attached by a hook to the tendon at the lower
end of the muscle, so that when the muscle is made to con-
tract the free end of the lever move's upwards and thus indi-
cates the moment at which the contraction of the muscle
commences. The sciatic ..erve is then arranged in such a
1 A galvanometer is an instrument used for the detection and measure-
ment of electric currents.
5G4 ELEMENTARY PHYSIOLOGY less
way that it may be stimulated either at a point x (Fig. 160,
as close as possible to its junction with the muscle, or at a
point j' as far away as possible from the muscle. By the use
of suitable apparatus it is easy to measure the interval of
time which elapses between the moment of applying the
stimulus at x and the moment at which the end of the lever
begins to move. This is found to be, in an ordinary experi-
ment, about y^-g- of a second. If now the nerve is stimu-
lated at y, it is found that the end of the lever begins to
move slightly later than it did when the stimulus was applied
at x ; that is to say, the muscle begins to contract rather
later when its nerve is stimulated at y than at x. This dif-
ference can only be due to the fact that when the impulse is
started at y it takes longer to reach the muscle than when it is
started at x. Since the length of the piece of nerve between
y and x is known by direct measurement, it becomes a sim-
ple matter to calculate the rate at which the impulse travels
from y to x. The result thus obtained agrees quite closely
with the one arrived at in the experiment previously de-
scribed in which a galvanometer was used, namely 28 metres
or 90 feet per second.
The rate at which an impulse travels along a nerve is
closely dependent on the temperature of the nerve, and
diminishes as the nerve is cooled ; thus, by cooling a frog's
nerve the rate may be reduced to as little as 1 metre (3 feet)
per second. Hence it is not surprising that, when experi-
ments are made on the nerves of a warm-blooded human
being, the rate of transmission is found to be somewhat
greater, viz. 33 metres (or rather over 100 feet) per second,
than in the cold-blooded frog.
^The idea is frequently expressed that a nervous impulse is
of the nature of an electric current similar to that which
passes along a wire as used for telegraphy. But this is by
XII
RATE OF A NERVOUS IMTULSE
5°5
no means the case, since, without going into any other more
abstruse reasons, we have shown that the rate at which an
impulse travels along a nerve is on an average about 33
metres, or 100 feet, per second, whereas we know that
electricity travels along a wire at a rate such that the trans-
mission of signals over the wires of an ordinary land-line is
practically instantaneous. Even in one of the cables across
the Atlantic Ocean (2,500 miles in length) only two-tenths
of a second elapse after contact is made with the battery at
»t>
B"
Fig. 160. — Arrangement of Nerve, Muscle, and Lever for determining the
Velocity of a Nervous Impulse.
J, femur: in, gastrocnemius muscle; t a, tendon; /, lever movable about the end
0; to, weight to keep the muscle stretched; «, the nerve; x and y, the two points at
which the nerve is stimulated.
one end before the effect can be first detected at the other
end. Now, if a nerve could be used for transmitting an
impulse from, say, London to Liverpool (200 miles), the
impulse would take nearly three hours (176 minutes) to
reach its destination, travelling as it does at the rate of 100
feet per second.
5o6 ELEMENTARY PHYSIOLOGY less.
9. The Functions of the Spinal Cord. — Up to this point
our experiments have been confined to the nerves. We may
now test the properties of the spinal cord in a similar way.
If the cord be cut across (say in the middle of the back),
the legs and all the parts supplied by nerves which come off
below the section will be insensible, and no effort of the will
can make them move ; while all the parts above the section
will retain their ordinary powers.
When a man hurts his back by an accident, the cord is
not unfrequently so damaged as to be virtually cut in two,
and then insensibility and paralysis of the lower part of the
body ensue.
If when the cord is cut across in an animal the cut
end of the portion below the division, or away from the
brain, be irritated, violent movements of all the muscles sup-
plied by nerves given off from the lower part of the cord
take place, but no sensation is felt by the brain. On the
other hand, if that part of the cord which is still connected
with the brain, or better, if any afferent nerve connected
with that part of the cord be irritated, sensations ensue, as is
shown by the movements of the animal ; but in these move-
ments the muscles supplied by the nerves coming from the
spinal cord below the cut take no part; they remain per-
fectly quiet.
Thus, it may be said that, in relation to the brain, the
cord is a great mixed motor and sensory nerve. But it is
also much more.
Reflex Action through the Spinal Cord. — If the trunk of a
spinal nerve be cut through, so as to sever its connection
with the cord, an irritation of the skin to which the sensory
fibres of that nerve are distributed produces neither motor
nor sensory effect. But if the cord be cut through any-
where so as to sever its connection with the brain, irrita-
xii THE FUNCTIONS OF THE SPINAL CORD 507
tiun applied to the skin of the parts supplied with sensory
nerves from the part of the cord below the section, though
it gives rise to no sensation, may produce violent motion of
the parts supplied with motor nerves from the same part
of the cord.
Thus, in the case supposed above, of a man whose legs
are paralysed and insensible from spinal injury, tickling
the soles of the feet will cause the legs to kick out con-
vulsively. And as a broad fact, it may be said that, so
long as both roots of the spinal nerves remain connected
with the cord, irritation of any afferent nerve is competent
to give rise to excitement of some, or the whole, of the
efferent nerves so connected.
If the cord be cut across a second time at any distance
below the first section, the efferent nerves below the second
cut will be affected no longer by irritation of the afferent
nerves above it, but only by irritation of those below the
second section. Or, in other words, in order that an affe-
rent impulse may be converted into an efferent one by the
spinal cord, the afferent nerve must be in uninterrupted
material communication with the efferent nerve by means
of the substance of the spinal cord.
This peculiar power of the cord, by which it is compe-
tent to convert afferent into efferent impulses, is that which
distinguishes it physiologically, as a central organ, from a
nerve, and is called reflex action. It is a power possessed
by the grey matter, and not by the white substance of the
cord.
The number of the efferent nerves which may be excited
by the reflex action of the cord is not regulated alone by
the number of the afferent nerves which are stimulated by
the irritation which gives rise to the reflex action. Nor
does a simple excitation of the afferent nerve by any means
508 ELEMENTARY PHYSIOLOGY less
necessarily imply a corresponding simplicity in the arrange-
ment and succession of the reflected motor impulses. Tick-
ling the sole of the foot is a very simple excitation of the
afferent fibres of its nerves ; but in order to produce the
muscular actions by which the legs are drawn up, a great
multitude of efferent fibres must act in regulated combina-
tion. In fact, in a multitude of cases a reflex action is to
be regarded rather as the result of a dormant activity of
the spinal cord awakened by the arrival of the afferent im-
pulse, as a sort of orderly explosion fired off by the afferent
impulse, than as a mere rebound of the afferent impulse
into the first efferent channels. open to it.
The various characters of these reflex actions may be
very conveniently studied in the frog. If a frog be decapi-
tated, or, better still, if the spinal cord be divided close to
the head, and the brain be destroyed by passing a blunt
wire into the cavity of the skull, the animal is thus de-
prived (by an operation which, being almost instantaneous,
can give rise to very little pain) of all consciousness and
volition, and yet the spinal cord is left intact. At first the
animal is quite flaccid and apparently dead, no movement
of any part of the body (except the beating of the heart)
being visible. This condition, however, being the result
merely of the so-called shock of the operation, very soon
passes off, and then the following facts may be observed :
So long as the animal is untouched, so long as no stimu-
lus is brought to bear upon it, no movement of any kind
takes place : volition is wholly absent.
If, however, one of the toes be gently pinched, the leg
is immediately drawn up close to the body.
If the skin between the thighs around the anus be
pinched, the legs are suddenly drawn up and thrust out
again violently.
Xu THE FUNCTIONS OF THE SPINAL CORD 509
If the flank be very gently stroked, there is simply a
twitching movement of the muscles underneath ; if it be
more roughly touched, or pinched, these twitching move-
ments become more general along the whole side of the
creature, and extend to the other side, to the hind legs,
and even to the front legs.
If the digits of the front limbs be touched, these will be
drawn close under the body as in the act of clasping.
If a drop of vinegar or any acid be placed on the top
of one thigh, rapid and active movements will take place
in the leg. The foot will be seen distinctly trying to rub
off the drop of acid from the thigh. And what is still
more striking, if the leg be held tight and so prevented
from moving, the other leg will begin to rub off the acid.
Sometimes, if the drop be too large or too strong, both
legs begin at once, and then frequently the movements
spread from the legs all over the body, and the whole
animal is thrown into convulsions.
Now all these various movements, even the feeblest and
simplest, require a certain combination of muscles, and some
of them, such as the act of rubbing off the acid, are in the
highest degree complex. In all of them, too, a certain pur-
pose or end is evident, which is generally either to remove
the body, or part of the body, from the stimulus, from the
cause of irritation, or to thrust away the offending object
from the body : in the more complex movements such a
purpose is strikingly apparent.
It seems, in fact, that in the frog's spinal cord there are
sets of nervous machinery destined to be used for a variety
of movements, and that a stimulus passing along a sensory
nerve to the cord sets one or the other of these pieces of
machinery at work.
Thus, one important function of the spinal cord is to
5i° ELEMENTARY PHYSIOLOGY less.
serve as an independent nervous centre, capable of origi-
nating combined movements upon the reception of the im-
pulse of an afferent nerve, or rather, perhaps, a group of
such independent nervous centres.
In all these reflex actions of the spinal cord, the struc-
tures necessary for their performance are, as already pointed
out (p. 369), a sensory surface, an afferent nerve, a portion
of the grey matter of the cord, an efferent nerve, and a
muscle or group of muscles (Fig. 150, B). In the case
of the headless frog, the actions are of course quite invol-
untary, and performed unconsciously, and the same remark
holds good in the case of a man whose spinal cord is so
injured as to be practically cut in two. But even in an
uninjured, healthy man, similar reflex actions, although now
under the control of the will, are strikingly manifest, and
play an important part in his everyday life. Thus, the act
of walking, though started by the will, is subsequently a
reflex action. When engaged in conversation or buried in
thought, a person walks with all his ordinary dexterity, but
in entire unconsciousness of the action. In this case the
afferent impulses are largely started from the stimulation of
the skin of the feet and legs which results from the varying
pressure and contact with the ground. Hence the stagger-
ing gait in cases where, as a result of disease, the chain
of structures requisite for the liberation of the reflexes is
broken, as for instance by disease of the posterior (affe-
rent) roots of the spinal cord. In such cases walking is fre-
quently possible only as the result of looking at the ground;
this accords with the fact that even in health afferent im-
pulses started in the sensory surface (retina) of the eye
play an important part in giving rise to the reflexes of
walking. But, on the other hand, blind persons walk with
no little dexterity, using other sensory impulses.
xii THE FUNCTIONS OF THE SPINAL CORD 511
Again, the actions of micturition and defalcation are really
reflex actions carried out by the spinal cord as soon as they
have been started by the will ; here the sensory surfaces are
the mucous membrane of the bladder or rectum, the neces-
sary stimulus being supplied as the result of their distension
by the accumulated urine or faeces.
Using the expression reflex action in a rather wider and
more general sense we may here again draw attention to
the importance of these actions to the working and welfare
of the body as regards the relationships of its internal
mechanisms. Thus, we have seen that certain parts of the
spinal bulb, or medulla, are connected with the heart (cardio-
inhibitory centre), blood-vessels (vaso-motor centre), and
respiratory muscles (respiratory centre), in such a way that
impulses arising in outlying parts of the body lead reflexly
to such modified activity of each of the above systems as may
from time to time be necessary (see pages 95, 101, 180).
Reflex action is a property of the central nervous system
which is not confined to the spinal cord alone, or to the
spinal bulb to which we have just extended it, but is also a
marked characteristic of the varied activities of the brain.
But to this point we shall return later on.
The Paths of Conduction of Impulses along the Spinal Cord.
— The spinal cord has a further most important function
beyond reflex action, namely that of transmitting nervous
impulses, as a great mixed motor and sensory nerve leading
from the brain, between the brain and the various organs,
such as the muscles and the skin, with which the spinal
nerves are connected. When we move a foot, certain
nervous impulses, starting in some part of the cerebral hemi-
spheres, pass down along the whole length of the spinal
cord as far as the roots of the spinal nerves going to the
legs, and issuing along the fibres of the anterior bundles of
512 ELEMENTARY PHYSIOLOGY less.
these roots find their way to the muscles which move the
foot. Similarly, when the sole of the foot is touched, affe-
rent impulses travel in the reverse way upwards along the
spinal cord to the brain. And the question arises, in what
manner do these efferent and afferent impulses travel along
the spinal cord?
This question is one very difficult to answer, and indeed
a complete and exact statement is not, at present, possible.
The method by which a large amount of our present infor-
mation on this matter has been obtained is the degeneration
method already described (p. 498). As in the case of a
nerve, so, if the spinal cord be cut across, degeneration
changes of the fibres of the white matter start from the
place of the cut, and advance upwards and downwards along
the cord. These changes affect certain definite areas,
which differ above and below the cut. Degeneration up-
wards is known as ascending degeneration : degeneration
downwards, as descending degeneration. The areas or tracts
thus affected represent paths of conduction along the cord.
In general, within the cord, fibres degenerate in the same
direction in which they conduct, hence from the direction
of degeneration the direction of conduction may be
inferred.
The chief tracts of ascending degeneration and conduc-
tion are shown in Fig. 161, and those of descending degen-
eration and conduction in Fig. 162. It will be seen that
the majority of the afferent (sensory) impulses on their way
to the brain pass up the cord in the posterior and lateral
columns, the postero-median tracts {p.m.) being the chief
path to the cortex of the cerebrum (see pp. 518, 529), the
two others that are figured going to the cerebellum (see
pp. 518, 530). The efferent (motor) impulses, however,
come down the cord from the brain mainly in the anterior
XII THE FUNCTIONS OF THE SPINAL CORD 513
and lateral portions, the crossed pyramidal tract ( Cr.p.)
being the most conspicuous column and conveying the im-
pulses directly down from the cortex of the cerebrum.
Besides these tracts for ascending and descending con-
duction, there is constant intercommunication going on
between the two sides of the cord. This is made possible
by fibres which cross from one side to the other in the
bridge connecting the two halves (see Fig. 148,4 and 5).
Cb.
ELEMENTARY PHYSIOLOGY
cp
Fig. 165. — The Base or Under-surface of the Brain.
A, frontal lobe; B, temporal lobe of the cerebral hemispheres; Cb, cerebellum;
/, the olfactory lobe; //, the optic nerve; ///, IV, VI, the nerves of the muscles of
the eye; V, the trigeminal nerve; VII, the facial nerve; VIII, the auditory nerve;
IX, the glossopharyngeal ; X, the pneumogastric; XI, the spinal accessory; XII,
the hypoglossal, or motor nerve of the tongue. The number J Yis placed upon the
pons Varolii. The crura cerebri are the broad bundle of fibres which lie between
the third and the fourth nerves on each side. The medulla oblongata (M) is seen to be
really a continuation of the spinal cord; on the lower end are seen the two crescents
of grey matter; the section, in fact, has been carried through the spinal cord, a little
below the proper medulla oblongata. From the sides of the medulla oblongata are
seen coming off the X, XI, and XII nerves; and just where the medulla is covered,
so to speak, by the transversely disposed pons Varolii, are seen coming off the VII
nerve, and more towards the middle line, the VI. Out of the substance of the pons
springs the V nerve. In front of that is seen the well-defined anterior border of the
pons; and coming forward in front of that line, between the IV and ///nerves on
either side, are seen the crura cerebri. The two round bodies in the angle between
the diverging crura are the so-called corpora albicantia, and in front of them is P,
the pituitary body. This rests on the chiasma, or junction, of the optic nerves; the
continuation of each nerve is seen sweeping round the crura cerebri on either side.
Immediately in front, between the separated frontal lobes of the cerebral hemispheres,
is seen the corpus callosurn, CC. The fissure of Sylvius, about on a level with /on
the left and // on the right side, marks the division between frontal and temporal lobes.
Xii THE ANATOMY OF THE BRAIN 521
spheres, separated by a median fissure whose sides are in
close contact. But if the sides of this fissure are carefully
pushed apart, the cerebral hemispheres may be seen to be
connected with each other by an elongated transverse and
horizontal mass of nerve-fibres known as the corpus callo-
sum (shown as CC in Fig. 165). If the hinder ends of
the cerebral hemispheres are raised, the whole upper surface
of the cerebellum comes into view, and if the cerebellum is
now lifted up, the posterior surface of the bulb is exposed.
Unlike the anterior surface, which is conspicuously convex
(see Fig. 165, M~), the posterior surface of the bulb is marked
by a shallow, elongated, diamond-shaped depression, forming
the cavity of the fourth ventricle. This cavity arises from
the gradual divergence of the posterior white columns of the
spinal cord, while the depth of the posterior fissure is at the
same time diminished, so that the central canal of the spinal
cord approaches the floor of the fourth ventricle, and act-
ually opens into the lower end of the cavity (Fig. 166) ;
this lower end of the ventricle is known as the calamus
scriptorius, from its fancied resemblance in shape to the
nib of a pen. The narrowed upper end of the fourth ven-
tricle is continued forwards under the cerebellum.
Having thus made out so much of the arrangement of
the brain as may be seen by mere external inspection, we
may now proceed to examine its internal structure. For
this purpose the most instructive method is to cut a verti-
cal, longitudinal section through the brain from front to
back, passing through the middle line, and thus dividing
it into two similar and symmetrical halves. When the cut
surface of the right half of the brain, as exposed by this
section, is examined, the following further structural details
may be made out, and are shown in Fig. 166.
The corpus cattosum is seen cut across at c.c, 'c.c, c.c. Above
522
ELEMENTARY PHYSIOLOGY
this, and extending forwards and backwards, is the flattened
exposed surface of the right cerebral hemisphere, which
forms one side of the median fissure between the hemi-
spheres. The upper end of the spinal cord, Sp.c, passes
into the bulb B, in front of which the transverse fibres of
the pons are seen in section at P, while the longitudinal
Fig. 166. — View of the Right Half of a Human Brain as shown b\ A
Longitudinal Section in the Median Line through the Longitudinal
Fissure. (After Sherrington.)
Sp.c, spinal cord; .5, bulb; P, pons; CR, cms cerebri; M, corpus albicans; CI.
cerebellum; c.c, central canal of spinal cord opening into 4, the fourth ventricle; V.V.
valve of Vieussens ; QP, QA , posterior and anterior corpora quadrigemina, beneath
which is the aqueduct of Sylvius leading from the fourth ventricle into 3, the cavity
of the third ventricle; P, pineal gland: F, fornix or roof of third ventricle; 07]
optic thalamus; H, pituitary body; OP, optic nerve cut across at the optic decus-
sation (see Figs. 165 and 172) ; SL, a part of the septum lucidum, of which th<
remainder has been cut away to reveal JVC, LV, the cavity of the lateral ventricle;
this communicates with the third ventricle by means of the foramen of Monro
whose position is marked by a small x at the front end of the third ventricle; c.c, c.c,
c.c, corpus callosum, above which is the mesial surface of the right cerebral hemi
sphere.
fibres of the bulb run forwards above the pons to emerge
in front as one of the (right) crura cerebri. Anteriorly
this crus disappears out of the section since it diverges
to the right (see Fig. 165) from the median line of the
brain to enter the corresponding cerebral hemisphere.
xii THE ANATOMY OF THE BRAIN 523
The cerebellum, Cb, is seen in section overhanging the
bulb, and between it and the bulb is the cavity, shaded
and marked with a 4, to which we have previously alluded
as the fourth ventricle. The central canal, c.c, of the
spinal cord is shown as an opening into the hinder end
of the cavity of the fourth ventricle, while the front end
of the cavity is prolonged into a narrow passage, the aque-
duct of Sylvius, which leads into a much larger cavity,
known as the third ventricle and marked by a J. Above
this aqueduct are four largely developed masses of tissue,
but of these two only are seen in the section at QA, QP,
since the four are arranged in two pairs, one pair being
placed each side of the middle line of the brain ; from
their number (four) these structures have received the
name of corpora quadrigemina. In front of the corpora
quadrigemina is a small structure, seen in section, the
pineal gland, P. The posterior corpus quadrigeminum is
continuous with a thin layer of nervous tissue which leads
back into the cerebellum ; this forms an overhanging roof
to the front end of the fourth ventricle, and is known as
the valve of Vieussens (Fig. 166, V.V). The floor of the
third ventricle is produced forwards and downwards into a
funnel-shaped space, to the tip of which is attached a body
of a glandular nature known as the pituitary body (Fig. 165, .
P, and Fig. 166, If). The roof of the third ventricle is
provided by a layer of pia mater, called the velum inter-
positum, and not shown in the figure ; this is covered by
a tract of fibres seen in section and known as the fornix
(Fig. 1 66, F) ; this is connected posteriorly with the
hinder end of the corpus callosum, and in front it curves
downwards and backwards into the lateral wall of the third
ventricle. The vertical space between the fornix and the
corpus callosum is filled in by a thin double layer of ner-
524 ELEMENTARY PHYSIOLOGY less.
vous tissue ; this is known as the septum lucidum. It lies
in the plane of the paper on which the figure is printed,
but only a small portion of it is shown at SL. The remain-
ing part has been cut away in order to reveal a feature of
which, so far, no mention has been made, viz. the darkly
shaded cavity JVC, LV, lying in the middle of the cere-
bral hemisphere, and known as the lateral ventricle. The
cavity of this ventricle communicates with that of the third
Fig. 167. — Diagram to show the Shape of the Cavity of the Left Lateral
Ventricle, its Connection with the Third Ventricle, and the Connec-
tion of the Latter with the Fourth Ventricle, and hence with the
Central Canal of the Spinal Cord.
Drawn from a cast of the ventricles. (After Welcker.)
c.c, canal of spinal cord; 4, fourth ventricle; A. S. aqueduct of Sylvius; 3, third
ventricle; F.M, foramen of Monro; LP', LV, LV, lateral ventricle with its anterior
cornu A.C, posterior cornu P. C, and inferior cornu I.C.
ventricle by a small opening at x, the foramen of Monro.
Since the septum lucidum consists of two layers, there is
a small flattened closed space between these layers in the
middle line of the brain ; this is spoken of as the fifth
ventricle, but it has no actual connection with the other
ventricles.1 (See Fig. 168, 5.) Each lateral ventricle is
a cavity of a very peculiar shape, one branch running for-
wards towards the front end of the hemisphere and one
1 The two lateral ventricles, one in each cerebral hemisphere, are some-
times reckoned as the first and second ventricles; hence the space between
the layers oHhe septum lucidum is known as the fifth ventricle.
xu THE ANATOMY OF THE BRAIN 525
backwards towards the hinder end, and from the latter a third
branch runs downwards and once more forwards (Fig. 167).
These correspond respectively to the chief lobes of which
each hemisphere is made up, namely, the frontal lobe, the
parietal and occipital lobes, and the temporal lobe. These
lobes are marked off on the surface of the hemispheres by
fissures, of which the most conspicuous are the fissure of
Sylvius, and the fissure of Rolando. (See Fig. 173.)
The cerebellum is firmly connected to the rest of the
brain by the transverse fibres which help to form the pons
Varolii (Fig. 165), and constitute the middle peduncle of
each half of the cerebellum. But each half has a further
attachment by means of two other bands of fibres. Of these
one coming out of the central part of the cerebellum on
each side runs upwards towards, and disappears under, the
corpora quadrigemina ; this forms the superior peduncle.
The other runs downwards towards the bulb and merges, as
the inferior peduncle, into that part of the bulb which is a
continuation upwards of the lateral columns of white matter
of the spinal cord.
We have seen that the spinal cord consists essentially of
grey matter containing nerve-cells, external to which is a
covering of white matter composed of nerve-fibres, the
arrangement of the grey and white matter being compara-
tively simple, and the nervous tissue surrounding a central
canal. Now from the description we have so far given of
the brain, it is evident that the brain may also be regarded
as being built up of structures which are placed round the
sides of a central canal, which is really continuous with the
canal of the spinal cord. But, unlike the latter, the canal
of the brain, consisting of the ventricles and aqueduct, is not
a simple straight tube, but has a very peculiar shape (Fig.
167)0 Moreover, although the brain is made up of grej
526 ELEMENTARY PHYSIOLOGY less.
and white matters, which by their greater or less develop-
ment form the structures of varying size which make up the
brain as a whole, the grey and white matters are not ar-
ranged in any simple way as they are in the spinal cord.
On the contrary, although in the brain a great deal of the
grey matter is placed externally to the white, the latter is
interspersed with localised deposits of grey matter, some
large, some small, which give to the whole an extraordinary
complexity. And this complexity is still further increased
by the existence of strands or bundles of nerve-fibres, which
serve to interconnect all these various deposits of grey
matter, so as to insure the possibility of co-ordinated action
between all the individual parts of which the brain as a
whole is built up. It would be neither possible nor desira-
ble to attempt to deal in any detail in this book with the
varied arrangements of the several deposits of grey matter
in the brain, and with their connections by strands of white
matter. But some of them stand out so conspicuously as
structures, and are so important in their functions, that we
must of necessity take them into consideration.
The Corpora Quadrigemina. — These have already been
described as four conspicuous masses of tissue lying in two
pairs above the aqueduct of Sylvius. They consist of depos-
its of grey matter in the otherwise thin wall of the roof of
the aqueduct. Each deposit is surrounded by white matter,
and from each bands of fibres run obliquely downwards and
forwards, those from the anterior pair of the corpora making
connection with structures connected with the optic nerve
(Fig. 165, II), while those from the posterior pair are be-
lieved to make similar connections with the nerves con-
cerned in hearing (Fig. 165, VIII).
The Optic Thalami. — The longitudinal fibres of the bulb,
passing between the transverse fibres of the pons, reappear,
THE ANATOMY OF THE BRAIN
527
as we have seen, in front of the pons as the crura cerebri.
These diverge from the middle line to enter the cerebral
hemispheres. As each crus passes into the base of the cor-
responding hemisphere, it receives on its upper surface a
large deposit of grey matter placed somewhat obliquely
across its course ; this mass of grey matter is the optic
Fig. 168. — Diagram of a Horizontal Section of the Brain above the Floor
of the Lateral Ventricles. (After Hirschfeld and Leveille.)
Sp.c, spinal cord; B, bulb; Cb, Cb, cerebellum; 4, fourth ventricle; QP, QA,
corpora quadrigemina; P, pineal gland; 3, third ventricle; 5, fifth ventricle; cc,
front part of corpus callosum; LI', LI', LI', lateral ventricle: OT, optic thalami;
CS, CS, corpus striatum; C, commissure of optic thalami. On the left side CS'
marks the corpus striatum, into which an incision has been made and a flap,_/", turned
back to show its internal striated appearance.
thalamus. Lying thus to one side of the third ventricle, and
under the lateral ventricle, it is easily seen how each optic
thalamus comes to form a projection in the outer side-wall
of the third ventricle, and on the floor of the lateral ventri-
cle. Thus the optic thalamus is shown at OT in Fig. 166
5 28 ELEMENTARY PHYSIOLOGY less
as part of the wall of the third ventricle, and in Fig. 168
as part of the floor of the lateral ventricle, the latter fig-
ure representing in diagram a horizontal section through the
hemispheres passing above the floor of the lateral ventricles.
The inner sides of the optic thalami are connected by a
small commissure (Fig. 168, C), which extends across the
third ventricle ; their outer sides are imbedded in the sub-
stance of the cerebral hemispheres with which they are con-
nected by nerve-fibres, and from their hinder end a bundle
of fibres sweeps forwards and downwards to pass into the
tract of the optic nerves.
The Corpora Striata. — Each corpus striatum may be re-
garded as a mass of grey matter deposited obliquely, as was
each opticf thalamus, on the course of the crura cerebri, but
lying somewhat in front of the optic thalami. Hence the
corpora striata are seen as a projection on the floor of the
lateral ventricles (Fig. 168, CS, OS'), and as part of
the side wall of the front end of this ventricle (Fig. 166,
JVC). The larger part of each corpus striatum is imbedded
in the neighbouring substance of the cerebral hemisphere
with which it is intimately connected by nerve-fibres. It is
also similarly connected with the fibres of the crus on which
it lies.
The Membranes of the Brain. — The brain is invested by
three membranes which are the same in name, and similarly
placed and related to each other as those which we have
previously described as covering the spinal cord (see p.
477). Of these the pia mater is highly vascular, and carries
blood-vessels down into the nervous matter, especially in the
sulci or grooves to which the convoluted appearance of the
surface of the brain is due. Moreover, it forms a roof to
the hinder part of the cavity of the fourth ventricle, and a
highly developed layer of the pia mater is tucked in under
xii THE MINUTE STRUCTURE OF THE BRAIN 529
the hinder end of the cerebral hemispheres to form the roof
of the third ventricle ; this is known as the velum inter -
positum. The edges of this velum as it lies beneath the
fornix project on each side into the cavities of the lateral
ventricles and are here known as the choroid plexuses, the
whole being arranged with a view to the nutrition of the
internal parts of the brain. The cavities of the cerebral
ventricles, and hence of the central canal of the spinal cord,
are placed in communication with the subarachnoid space
by a small opening in the pia mater covering the hinder end
of the fourth ventricle ; this opening is known as the
foramen of Magendie.
12. The Minute Structure of the Brain. — In the spinal
bulb the arrangement of the white and grey matter is sub-
stantially similar to that which obtains in the spinal cord,
that is to say, the white matter, composed of nerve-fibres,
is external and the grey internal ; but the grey matter,
containing, as in the spinal cord, nerve-cells, is more
abundant than in the spinal cord, and the arrangements of
white and grey matter become much more intricate and
complex.
Above the bulb there are internal deposits of grey matter,
containing nerve-cells at various places, more especially in
the pons Varolii, the crura cerebri, the corpora quadri-
gemina, optic thalami, and corpora striata. And there is a
remarkably shaped deposit of grey matter in the interior of
the cerebellum, on each side. But what especially charac-
terises the brain is the presence of grey matter of a special
nature on the surface of the cerebral hemispheres, contain-
ing peculiarly shaped nerve-cells, and known as the cortex,
and similarly a special grey matter forms the surface of the
cerebellum. This superficial grey matter covers the whole
surface of both these organs, dipping down into the fissures
2M
530 ELEMENTARY PHYSIOLOGY less.
(sulci) of the former, and following the peculiar plaits or
folds (convolutions) into which the latter is thrown.
The Cerebellum. — The surface of the cerebellum presents
a corrugated or laminated appearance. When a section is
made through one of its hemispheres it is seen that the
depressions which separate the laminse give off secondary
lateral depressions as they pass towards its centre, so that
the surface is really divided up into a very large number of
leaf-like foldings which are known as the lamellae. The
central part of the cerebellum consists of white matter which
is essentially the same as the white matter of the spinal cord,
that is to say, it is made up chiefly of medullated nerve-
fibres. Portions of this white matter extend outwards into
the primary foldings and secondary lamellae of the cerebellar
surface, and are covered by grey matter, the arrangement
thus presenting a very characteristic arborescent appearance
when seen in section.1
When a section of the external grey matter is cut at right
angles to the surface of a lamella, stained, and examined
under the microscope, it is found to consist of two layers.
The innermost, lying next to the central white matter, is
made up of a large number of small, closely packed cells
supported by neuroglia (see p. 490) and is known as the
nuclear layer (Fig. 169, N). The outer layer, immediately
under the pia mater, shows a few cells, but the chief appear-
ance it presents is that of a granular mass made up of closely
set dots. These dots are in reality the cut ends of fibres, of
which some belong to the supporting neuroglia, but of
which the majority are nerve fibrils. From its punctated
appearance (x) this layer, which is much broader than the
nuclear layer, is known as the molecular layer (M). Be-
tween these two layers lies a row of nerve-cells of very strik-
l This is somewhat imperfectly shown in Figs. 166 and 168.
THE CEREBELLUM
53^
ing and characteristic appearance, known as the cells of
Furkinje (i). These are pear-shaped, with a large and
Fig. 169. — Diagram to illustrate the Structure of the Superficial Grey
Matter of the Cerebellum as seen in a Transverse Section of a
Lamella.
M, molecular layer; N, nuclear layer; W, central white matter; 1, cell of Pur-
kinje; 2, spider cell ; 5, cell of Golgi: 3, basket-cell with one of its baskets, b\ 4, an-
other kind of cell in the molecular layer; t, tendril fibre; m, moss-fibre.
In the case of each cell a is the axon, d is a dendrite: _r, customary punctated
appearance of the molecular layer when seen in microscopic sections.
532 ELEMENTARY PHYSIOLOGY less.
conspicuous nucleus, the bulbous inner end resting on the
nuclear layer, while the outer end divides into a large num-
ber of processes which run out into the molecular layer as
finer and finer branches. The granular appearance of the
molecular layer is in part due to the close juxtaposition of
the cut ends of these branches or dendrites from the cells of
Purkinje\ The inner end of each cell bears a single process
which is usually cut through near the cell but. is really pro-
longed down into the central white matter as a medullated
nerve-fibre. Such are the details which can be made out
in an ordinarily stained section. But by employing special
methods of staining many further details come into view,
and putting all these together we are justified in construct-
ing the preceding diagrammatic Figure 169 to show the
nature and relationships of the cells of the cerebellar cortex
and of its two layers to the fibres of the central white matter.
In this figure the cells which call for special attention are
the following : The cell of Purkinje (1) with its central
axon {a) and peripheral dendrites (d). The basket-cell
(3) with its axon (a) and baskets (b) ; the baskets in
reality surround the bodies of cells of Purkinje, which for
the sake of clearness are not shown in the diagram. The
spider-cell (2) in the nuclear layer with its axon (a) running
into the molecular layer and dendrites (d). Also, in addi-
tion to the fibre derived- from the inner end of the cell of
Purkinje^ it is important to notice the moss-fibre (m) whose
outer end terminates by branching in the nuclear layer and
the tendril-fibre (7) which passes further outwards, but ends
similarly in the molecular layer. The direction in which
impulses are supposed to travel along these fibres is indicated
by arrows.
The Cerebral Cortex. — The structure of the superficial grey
matter of the cerebellum is practically the same in each part
THE CEREBRAL CORTEX
533
of the cerebellar cortex. In the cerebrum, on the other
hand, the details of structure vary not inconsiderably, ac-
Fig. 170. — Diagrammatic Figure to illustrate the Structure of a Typical
Section of the Cerebral Cortex.
I, Molecular layer. II, Layer of pyramidal cells. Ill, Layer of polymorphous
cells.
c and c'\ cells of the molecular layer; /*•/">/'"> pyramidal cells; P, cell of the
polymorphous layer: My, medullary ray of nerve fibrils from central white matter:
x, y, z, tangential bundles of nerve fibrils
534 ELEMENTARY PHYSIOLOGY less.
cording to the region of the cortex from which a section is
prepared. Into these differences we cannot enter, but must
content ourselves with a somewhat diagrammatic description
and figure in illustration of the general structural arrange-
ment of the cells and fibres of the cortex as a whole.
The grey matter is permeated throughout its whole thick-
ness by a neuroglia which is essentially the same as that of
the rest of the central nervous system. This forms the
supporting tissue in which the nerve-cells of the cortex are
imbedded, and through which the fibrils of nerves pass to
and from these cells from and to, the central white matter.
The latter is composed, as in the cerebellum, of medullated
nerve-fibres. The neuroglia is most marked in the outer-
most parts of the cortex, immediately below the pia mater,
and since in a section its wavy fibres are mostly seen as
sectional dots, this layer of the cortex is known as the
molecular layer (Fig. 170, I). Internally to this layer the
cortex is characterised by the presence of nerve-cells whose
shape is pyramidal with the apex of each cell pointed to-
wards the surface of the brain. This layer may therefore be
spoken of as the layer of pyramidal cells (Fig. 170, II).
These cells vary in size in the several parts of this layer, the
largest being found in the inner portion, the smallest next
to the molecular layer. That part of the cortex which lies
immediately external to the central white matter is character-
ised by the presence of nerve-cells of a somewhat irregular
form ; hence this layer is known as the layer of polymorphous
cells (Fig. 170, III).
In addition to the nerve-cells and their processes, which
characterise the several layers of the cortex, nerve fibrils
pass up into and through the cortex from the central white
matter. Of these some are arranged in bundles at right
angles to the surface of the cortex, medullary rays (Fig.
XII THE CRANIAL NERVES 535
170, M/-), while others lie parallel to the surface as tan-
gential rays (Fig. 170, x,y,z).
13. The Cranial Nerves. — Nerves are given off from
the brain in pairs, which succeed one another from before
backwards, to the number of twelve (Figs. 165 and 171).
These are often called "cranial" nerves, to distinguish
them from the spinal nerves.
The first pair, counting from before backwards, are the
olfactory nerves, and the second are the optic nerves. The
functions of these have already been described. The ol-
factory nerves are bundles of fibres which proceed from the
under-surface of the olfactory lobes of the cerebrum (Fig.
165,/) and traverse the cribriform plate to be distributed
to the olfactory mucous membrane. These fibres are non-
medullated and, with the olfactory lobes, are in a certain
sense prolongations of the cerebral hemispheres.
The optic " nerve " is also properly speaking a lobe of
the brain, being an outgrowth in the embryo from the walls
of the third ventricle. It retains its character as a part of
the central nervous system in so far as its fibres have no
neurilemma.
The optic nerve from each eye meets its fellow nerve
from the other eye at the base of the brain below the
third ventricle. Here they cross each other in what is
called the optic chiasma (covered by the pituitary body
P in Fig. 165) and are continued on backwards, to make
connection with the brain, as the optic tracts.
These are connected, as already stated, with the hinder
part of the optic thalami and with the anterior pair of
the corpora quadrigemina. At the chiasma the fibres of the
optic nerves undergo a remarkable partial decussation. The
fibres from each half of the retina nearest to the nose cross
over to the opposite side of the brain : the fibres from the
536
ELEMENTARY PHYSIOLOGY
other half of each retina pass into the brain without cross-
ing. Hence the right optic tract contains the fibres from
the nasal half of the left retina and from the other or tem-
poral half of the right retina, and, similarly, the left optic
tract is made up of the fibres of the temporal half of the
left retina and the nasal half of the right retina. This
arrangement is essential to the eye as a sense organ with
reference to what we have previously spoken of as " corre-
sponding points " and " single vision with two eyes " (see
p. 472).
Fig. 171.-
■ A Diagram illustrating the Superficial Origin of the Cranial
Nerves.
H. , the cerebral hemispheres; C.S., corpus striatum; 77;., optic thalamus; P,
pineal body; Pi, pituitary body; C.Q., corpora quadrigemina; Cb, cerebellum; M,
medulla oblongata; XII-I, the pairs of cerebral nerves; Sp 1, Sp 2, the first and
second pairs of spinal nerves.
The third pair are called motor oculi (mover of the
eye), because they are distributed to all the muscles of
the eye except two.
The nerves of the fourth pair, trochlear, and of the
sixth pair, abducens, supply, each, one of the muscles of
the eye, on each side ; the fourth going to the superior
XII - THE CRANIAL NERVES 537
oblique muscle, and the sixth to the external rectus.
Thus the muscles of the eye, small and close together as
they are, receive their nervous stimulus by three distinct
nerves.
Each nerve of the fifth pair is very large. It has two
roots, a motor and a sensory, and further resembles a
spinal nerve in having a ganglion on its sensory root. Its
sensory part supplies the skin of the face, and its motor
part the muscles of the jaws, and, having three chief divi-
sions, it is often called trigeminal. One branch containing
sensory fibres supplies the fore-part of the mucous mem-
brane of the tongue, and, from its supposed share in medi-
ating sensations of taste, is often spoken of as the gustatory.
Fig. 172. — Diagram to illustrate the Decussation of fibres in the Optic
Chiasma.
R. right eye; L, left eye; R.op , , right optic tract; L.op., left optic tract. The
decussation is shown by the distribution of the right (shaded) and the left (unshaded)
tract to the retinas of the two eyes.
The seventh pair furnish with motor nerves the muscles
of the face and some other muscles, and are called facial.
The eighth pair are the auditory nerves. The auditory
is divided into the cochlear and vestibular nerve. (See
later, p. 542.)
The ninth pair in order, the glossopharyngeal, are mixed
nerves ; each being, partly, a nerve of taste, and supplying
the hind-part of the mucous membrane of the tongue, and,
partly, a motor nerve for the pharyngeal muscles.
53S ELEMENTARY PHYSIOLOGY less.
The tenth pair are the two pneumogastric nerves, often
called the vagus. These very important nerves, and the
next pair, are the only cranial nerves which are distributed
to regions of the body remote from the head. The pneu-
mogastric has the widest distribution of any of the cranial
or spinal nerves. It contains both afferent and efferent
fibres, and supplies the larynx, the lungs, the liver, the
oesophagus, stomach, and intestines, and branches of it are
connected with the heart.
The eleventh pair, again, called spinal accessory, differ
widely from all the rest, in arising largely from the sides of
the spinal cord, between the anterior and posterior roots of
the dorsal nerves. They run up, gathering fibres as they
go, to the medulla oblongata, and then leave the skull by
the same aperture as the pneumogastric and glossopharyn-
geal. They are purely motor nerves, supplying certain
muscles of the neck.
The twelfth, and last, pair, the hypoglossal, are the motor
nerves which supply the muscles of the tongue.
14. The Functions of the Spinal Bulb or Medulla
Oblongata. ■ — The bulb plays so important a part in the
economy of the body that we may almost enumerate its
functions by recalling all the instances in which we have
made mention of its activities in the earlier lessons of this
book. Thus, we have seen that it contains a centre which
gives rise to the contractions of the respiratory muscles and
keeps the respiratory pump at work ; hence injuries to the
bulb may arrest the respiratory process (p. 180). Further,
it contains centres for the regulation of the heart-beat (p.
101) and of the condition of the blood-vessels over the
whole body (p. 95). But beyond these the bulb also con-
tains centres for the nervous act of swallowing, for the reflex
secretion of saliva, and for many other actions. Thus, we
xii THE FUNCTIONS OF THE SPINAL BULB 539
find that simple puncture of one side of the floor of the
fourth ventricle produces for a while an increase of the
quantity of sugar in the blood beyond that which can be
utilised by the organism. The sugar passes off by the kid-
neys, and thus this slight injury to the medulla produces a
temporary disorder closely resembling the disease called
diabetes. Hence we speak of a diabetic centre in the bulb.
Beyond this the bulb acts as a great conductor of impulses ;
for all impulses passing up and down between the higher
parts of the brain and the spinal cord must make then way
through the bulb from or to the spinal nerves. And a simi-
lar statement holds good for impulses along the cranial
nerves, with the exception of the olfactory, optic, and third
and fourth nerves.
The impulses which pass through the bulb cross, for the
most part, from one side to the other on their way along it.
In the case of the main efferent or crossed pyramidal tract
of the spinal cord, the crossing of the fibres which compose
the tract takes place by means of what is called the decus-
sation of the pyramids in the anterior columns of the bulb
(Fig. 177). This point is indicated in Fig. 165 by a group
of small converging marks on the surface of the bulb just
above the cut end marked M. Similarly, the fibres con-
cerned in the transmission of afferent impulses largely cross
in the bulb by paths which are varied, but of which one is
well marked as the sensory decussation. This general
decussation of efferent and afferent fibres leads to the result
that disease or injury of one side of the brain affects the
opposite side of the body. Thus, when, as not unfrequently
happens, a blood-vessel gives way in the left cerebral hemi-
sphere, leading to a destruction of nervous matter there, the
result is that the right arm, and right leg, and right side of
the body generally are paralysed, that is, the will has no
54© ELEMENTARY PHYSIOLOGY less.
longer any power to move the muscles of that side, and
impulses started in the skin of that side cannot awaken sen-
sations in the brain.
But there is also a decussation of impulses in the case of
the nerves arising from the medulla above the decussation
of the pyramids. Thus, in the case quoted above of a blood-
vessel bursting in the left cerebral hemisphere, the right side
of the man's face is paralysed as well as the right side of his
body, that is to say, impulses cannot pass to and from his
brain and the right facial and fifth nerves. The impulses
along these nerves also cross over, decussate, and reach the
left side of the brain.
It sometimes happens, however, that disease or injury
may affect the medulla oblongata itself, on one side only
{e.g. the left), above the decussation of the pyramids, in
such a way that the fifth and facial nerves are affected in
their course before they decussate, that is to say, on the
same side as the injury. The man then, while still paralysed
on the right side of his body, is paralysed on the left side of
his face.
15. The Functions of the Cerebellum. — When speaking
of reflex actions we pointed out (p. 510) that the compli-
cated movements of walking when once started by the will
are essentially reflex in their continued production. More-
over, we also drew attention to the fact that the co-ordination
of the efferent impulses which, although distributed to many
different muscles, give rise by their united action to the
orderly movements of walking, is dependent upon afferent
impulses from various parts of the body. Thus, walking be-
comes unsteady or even impossible in the absence of the
normal sensory impulses from the skin, or of visual impulses
from the eyes ; and to these we might have added afferent
impulses from the sensory nerves of the muscles themselves.
xii THE FUNCTIONS OF THE CEREBELLUM 541
When we take cases of movements which are less obviously
reflex, that is, more strictly voluntary, than are those of
walking, we find that here again their orderly or co-ordinated
production depends largely on tactile and visual impulses.
Now experiment and observation in cases of disease have
shown quite conclusively that the one great function of the
cerebellum is to play a most important part in the co-ordina-
tion of the actions, nervous and muscular, by which the
movements of the body are carried on.
After the cerebellum has been completely removed, an
animal does not differ in any essential respect from its
normal condition as regards its intelligence or its special
senses, such as sight or hearing. But with regard to its
movements a great difference is observed ; all movements
are now clumsily executed — there is a want of orderliness
or co-ordination. The above statement sums up our knowl-
edge of the function of the cerebellum.
We do not know how the cerebellum works in thus keep-
ing an orderly grip over the mechanisms of movement ; but
we see how easily it may do so when we consider its con-
nections with the spinal cord and with the rest of the brain.
We saw (p. 512) that two large tracts of afferent fibres from
the spinal cord pass into it. Moreover, it is connected with
that part of the bulb in which the postero-median tract ends.
Thus it may be a recipient of a vast number of afferent sen-
sory impulses, which are so essential for co-ordinated move-
ment. But each half of the cerebellum is further connected
with the cortex of the cerebral hemisphere of the opposite
side. And we shall see that it is exactly in the cortex of
the cerebral hemispheres that impulses chiefly arise for the
initiation of muscular movements.
When describing the arrangements of the internal ear, it
was stated that the semicircular canals, the utricle, and the
542 ELEMENTARY PHYSIOLOGY less.
saccule enable the body to maintain its equilibrium (p. 421).
Now the auditory nerve consists of two quite distinct parts,
the cochlear nerve, which is distributed to the cochlea, and
the vestibular nerve, which is distributed to the above-men-
tioned parts of the ear. These two nerves originate in
groups of cells lying in the spinal bulb, and the group of
cells which gives rise to the vestibular nerve is directly con-
nected by a strand of fibres with the cerebellum. Thus
there is a path by which afferent (sensory) impulses from
the vestibular organs and the canals may reach the cere-
bellum directly and there be turned to account in the co-
ordination of movements. Bearing this in mind, it is not
surprising to find that these organs play a very important
part in the guidance of co-ordinated movement.
16. The Functions of the Cerebral Hemispheres. —
The Hemispheres the Seat of Intelligence and Will. — The func-
tions of most of the parts of the brain which lie in front of
the spinal bulb are, at present, very ill understood ; but it is
certain that extensive injury, or removal, of the cerebral
hemispheres puts an end to intelligence and voluntary
movement, and leaves the animal in the condition of a
machine, working by the reflex action of the remainder
of the cerebro-spinal axis.
We have seen that in the frog the movements of the body
which the spinal cord alone, in the absence of the whole of
the brain, including the bulb, is capable of executing, are
of themselves strikingly complex and varied. But none of
these movements arise from changes originating within the
organism ; they are not what are called voluntary or sponta-
neous movements ; they never occur unless the animal be
stimulated from without. Removal of the cerebral hemi-
spheres is alone sufficient to deprive the frog of all sponta-
neous or voluntary movements ; but the presence of the
xii FUNCTIONS OF THE CEREBRAL HEMISPHERES 543
bulb and other parts of the brain (such as the corpora
quadrigemina, or what corresponds to them in the frog, and
the cerebellum) renders the animal master of movements
of a far higher nature than when the spinal cord only is left.
In the latter case the animal does not breathe when left to
itself, lies flat on the table with its fore-limbs beneath it in
an unnatural position ; when irritated kicks out its legs, and
may be thrown into actual convulsions, but never jumps
from place to place ; when thrown into a basin of water
falls to the bottom like a lump of lead, and when placed
on its back will remain so, without making any effort to turn
over. In the former case the animal sits on the table, rest-
ing on its front limbs, in the position natural to a frog ;
breathes quite naturally ; when pricked behind jumps away,
often getting over a considerable distance ; when thrown
into water begins at once to swim, and continues swim-
ming until it finds some object on which it can rest ; and
when placed on its back immediately turns over and re-
sumes its natural position. Not only so, but the following
very striking experiment may be performed with it : Placed
on a small board it remains perfectly motionless so long as
the board is horizontal ; if, however, the board be gradually
tilted up so as to raise the animal's head, directly the board
becomes inclined at such an angle as to throw the frog's
centre of gravity too much backwards, the creature begins
slowly to creep up the board, and, if the board continues to
be inclined, will at last reach the edge, upon which, when
the board becomes vertical, he will seat himself with appar-
ent great content. Nevertheless, though his movements
when they do occur are extremely well combined and appar-
ently identical with those of a frog possessing the whole of
his brain, he never moves spontaneously, and never stirs
unless irritated.
544 ELEMENTARY PHYSIOLOGY less.
Thus, the parts of the brain below the cerebral hemi-
spheres constitute a complex nervous machinery for carry-
ing out intricate and orderly movements, in which afferent
impulses play an important part, though they do not give
rise to clear or permanent affections of consciousness.
There can be no doubt that the cerebral hemispheres are
the seat of powers essential to the production of those phe-
nomena which we term intelligence and will ; and there is
experimental and other evidence which indicates a connec-
tion between particular parts of the surface of the cerebral
hemispheres and particular acts. Thus, as we shall see more
fully later, irritation of particular spots in the anterior part
of a dog's brain will give rise to particular movements of
this or that limb, or of this or that group of muscles ; and
the destruction of a certain part of the posterior lobes of the
cerebral hemispheres causes blindness. But the exact way
in which these effects are brought about is not yet thor-
oughly understood ; and although it seems to be proved
beyond doubt that the central end-organ of vision (p. 448)
consists of certain nerve-cells lying in a particular part of
the posterior surface of the cerebral hemisphere, and that
the central end-organ of hearing '(p. 414) consists of other
nerve-cells lying elsewhere on the cerebral surface, we are
still completely in the dark as to what goes on in the cere-
bral hemispheres when we think and when we will.
There is no doubt that a molecular change in some part
of the cerebral substance is an indispensable accompani-
ment of every phenomenon of consciousness. And it is
possible that the progress of investigation may enable us
to map out the brain according to the psychical relations
of its different parts. But supposing we get so far as to
be able to prove that the irritation of a particular frag-
ment of cerebral substance gives rise to a particular state
xn FUNCTIONS OF THE CEREBRAL HEMISPHERES 545
of consciousness, the reason of the connection between
the molecular disturbance and the psychical phenomenon
appears to be out of the reach, not only of our means of
investigation, but even of our powers of conception.
Reflex Actions of the Brain. Even while the cerebral
hemispheres are entire, and in full possession of their
powers, the brain gives rise to actions which are as com-
pletely reflex as those of the spinal cord.
When the eyelids wink at a flash of light or a threatened
blow, a reflex action takes place, in which the afferent
nerves are the optic, the efferent the facial. When a bad
smell causes a grimace, there is a reflex action through
the same motor nerve, while the olfactory nerves consti-
tute the afferent channels. In these cases, therefore, reflex
action must be effected through the brain, all the nerves
involved being cerebral.
When the whole body starts at a loud noise, the affe-
rent auditory nerve gives rise to an impulse which passes
to the medulla oblongata, and thence affects the great
majority of the motor nerves of the body.
It may be said that these are mere mechanical actions,
and have nothing to do with the operations which we
associate with intelligence. But let us consider what takes
place in such an act as reading aloud. In this case, the
whole attention of the mind is, or ought to be, bent upon
the subject-matter of the book, while a multitude of most
delicate muscular actions are going on of which the reader
is not in the slightest degree aware. Thus, the book is
held in the hand, at the right distance from the eyes ; the
eyes are moved from side to side, over the lines and up
and down the pages. Further, the most delicately adjusted
and rapid movements of the muscles of the lips, tongue, and
throat, of the laryngeal and respiratory muscles, are involved
2N
546 ELEMENTARY PHYSIOLOGY less.
in the production of speech. Perhaps the reader is stand-
ing up and accompanying the lecture with appropriate gest-
ures. And yet every one of these muscular acts may be
performed with utter unconsciousness, on his part, of any-
thing but the sense of the words in the book. In other
words, they are reflex acts.
Similar remarks apply to the act of " playing at sight "
a difficult piece of music. The reflex actions proper to
the spinal cord itself are natural, and are involved in the
structure of the cord and the properties of its constituents.
By the help of the brain we may acquire an infinity of arti-
ficial reflex actions ; that is to say, an action may require
all our attention and all our volition for its first, or sec-
ond, or third performance, but by frequent repetition it
becomes, in a manner, part of our organisation, and is
performed without volition, or even consciousness.
As every one knows, it takes a soldier a long time to
learn his drill — for instance, to put himself into the atti-
tude of " attention " at the instant the word of command
is heard. But, after a time, the sound of the word gives
rise to the act, whether the soldier be thinking of it or
not. There is a story, which is credible enough, though
it may not be true, of a practical joker, who, seeing a dis-
charged veteran carrying home his dinner, suddenly called
out " Attention ! " whereupon the man instantly brought his
hands down, and lost his mutton and potatoes in the gut-
ter. The drill had been thorough, and its effects had
become embodied in the man's nervous structure.
The possibility of all education (of which military drill
is only one particular form) is based upon the existence of
this power which the nervous system possesses, of organ-
ising conscious actions into more or less unconscious, or
reflex, operations. It may be laid down as a rule, which
xii FUNCTIONS OF THE CEREBRAL HEMISPHERES 547
is called the Law of Association, that if any two mental
states be called up together, or in succession, with due
frequency and vividness, the subsequent production of the
one of them will suffice to call up the other, and that
whether we desire it or not.
The object of intellectual education is to create such
indissoluble associations of our ideas of things, in the order
and relation in which they occur in nature ; that of a moral
education is to unite as fixedly the ideas of evil deeds with
those of pain and degradation, and of good actions with
those of pleasure and nobleness.
Localisation of Function in the Cortex of the Cerebral Hemi-
spheres.—We have already alluded (p. 544) to the fact that
there is a connection between particular parts of the sur-
face of the cerebral hemispheres and particular acts or
special sensations. The possibility thus indicated is of
extraordinary importance and must now be dealt with in
some detail.
The cerebral hemispheres are separated along the mid-
dle line of the brain by a narrow deep fissure, across which
the corpus callosum passes as a bridge from one hemisphere
to the other (see Figs. 165 and 166). The surface of each
hemisphere is folded into a large number of convolutions
or gyri separated from each other by sinuous depressions
or sulci (see Fig. 164, C, C). Some of these depressions
are deeper and more marked than others, and are spoken
of as fissures. Of these the most conspicuous are known as
the fissure of Sylvius, the fissure of Rolando, the parieto-
occipital fissure, and the calcarine fissure. The position
of these is shown in the accompanying diagrams (Figs. 173
and 174). These fissures may be taken as roughly dividing
the surface of the brain more or less distinctly into several
lobes, frontal, parietal, occipital, and temporal.
548
ELEMENTARY PHYSIOLOGY
When the surface of the hemisphere is stimulated elec-
trically close to the fissure of Rolando and on either side
of this fissure, very definite movements take place in the
limbs of the opposite side of the body. If care is taken to
Parieto-occipita.1
Fissure
Fissure of
Sylvius
Fig. 173. — Diagram of Outer Surface of the Right Cerebral Hemisphere
Fissure of Rolando
oil
^"CORU LOBE
Fig. 174. — Diagram of the Inner (Mesial) Surface of the Right Hemi-
sphere TO SHOW THE PaRIETO-OCCIPITAL AND CaLCARINE FlSSURES.
The corpus c alio sum is seen shaded in section.
localise the stimulation as far as possible within the limits
of a small area of the cortex, the resulting movements are
found to be limited to a correspondingly small group of
muscles of the limb affected. Again, if that piece of cor-
tex whose stimulation gives rise to movements be cut out
xn FUNCTIONS OF THE CEREBRAL HEMISPHERES 549
or extirpated, the animal so operated on is found to have
lost the power of executing this particular set of move-
ments. The outcome of such experiments makes it clear
that the cerebral cortex along the course of the fissure of
Rolando is concerned in the development of muscular
movements ; hence the name of " motor areas " was given
to these parts of the cortex (Figs. 175, 176). Our knowl-
edge of the existence and position of these areas as de-
rived from experiments on animals is, moreover, completely
confirmed by the observation of the results of Nature's own
experiments on man ; as, for instance, by an examination
after death of the brains of patients who during life had,
as the result of cerebral disease, exhibited symptoms simi-
lar to those obtainable by stimulation or extirpation of
cortical areas in animals.
By proceeding in a similar way it has been found further
that certain portions of the cortex are peculiarly connected
with the development of sensations, so that we come to
speak also of "sensory areas" (Figs. 175, 176). In this
case observations on man are specially instructive, since the
patient can give an account of his sensations, whereas
another animal cannot.
One of the earliest known and most interesting cases of
localisation of function in the cerebral cortex is that of the
centre for speech. Some long time before experiment re-
vealed the existence and position of the centres to which
we have so far referred, it was noticed by a French physi-
cian named Broca that patients who had exhibited a curious
inability to pronounce definite words or syllables during life
were found after death to have suffered from disease or
injury of the inferior frontal convolution of the left side of
the brain immediately above the Sylvian fissure ; hence,
this part of the cortex is known as Broca's convolution (see
55°
ELEMENTARY PHYSIOLOGY
Fig- x75)- The disorder is, from its nature, known as
aphasia (a, without, <£acns, speech) and may take one of
F/&SI/REOF ftOLAf/DO
Oc.L
Te.L
Fig. 175. — Diagram of Outer Surface of Right Cerebral Hemisphere to
show the Position of Cortical Areas.
The areas for the leg, arm, and face are marked by vertical lines, horizontal lines,
and dots, respectively. The area for speech lies really on the left hemisphere.
Fr.L, frontal lobe; Oc.L, occipital lobe; Te.L, temporal lobe; Sy.F, fissure of
Sylvius.
Fissure of Rolando
Fr.L
Par Oc F
OcL
TeL
Fig. 176. — Diagram of Inner (Mesial) Surface of the Right Cerebral
Hemisphere to show the Position of Cortical Areas.
The corpus callosum is seen shaded in section.
Fr.L, frontal lobe; Oc L, occipital lobe; Te L, temporal lobe; Par-Oc F. parieto-
occipital fissure.
several forms ranging from complete inability to speak at all
to an inability to utter certain words, and hence to speak
xii THE PATHS OF CONDUCTION OF IMPULSES 551
coherently. This centre for speech is, curiously, and unlike
most of the other centres, unilateral, being situated on the
left side of the brain in ordinary right-handed persons and
in the corresponding part of the right side of the brain in
those who are left-handed.
17. The Paths of Conduction of Impulses in the Brain.
— Corresponding to the greater complexity of the brain in
Int. Cap
Cr.p. Bu]b
Sp.C
Fig. 177. — Diagram of the Course of the Crossed Pyramidal Tract from
the (Motor) Cerebral Cortex to the Spinal Cord.
Cb.H., cerebral hemisphere; C.C., corpus callosum; O.T., optic thalamus; C.S.,
corpus striatum; Int. Cap, internal capsule; Cb, cerebellum; D.P., decussation of
the pyramids; Cr.p. , Cr.p., crossed pyramidal tracts (see Fig. 162); Sp.C, spinal cord.
general, as compared with the spinal cord, the paths of con-
duction in the former are much more numerous and intri-
cate than in the latter ; and one of the chief problems of
the neurology of the present day is to trace out these paths.
We have already referred incidentally to some of these.
Many of the sensory fibres of the spinal cord, after cross-
ing from one side to the other in the sensory decussation
(p. 539) in the spinal bulb, can be traced upwards into the
552
ELEMENTARY PHYSIOLOGY
UAiku**
MukUuu^
"hvdulU t-Uou^utt
n
Fig. 178. — Diagram showing the Probable Relation
Principal Cells and Fibres of the Cerebro-spinal
One Another (Schafer). (From Quain's Anatomy.')
s, :; sensory surface, such as the skin; 7, 8, afferent fibre belonging ft^^Lbody b
which lies in ganglion of posterior root of spinal nerve; 8, ascends spinal con
tero-median tract (Fig. 161, p.m.); 9, 10, 11, 12, 13, branches of 8 in spinal" 6
terminating about cell-bodies in posterior horn (9, 12,), anterior horn (10), Clarke'
column (11), and bulb (13); 14,14, relations of certain cells of posterior horn to those
of anterior horn; 17, neuron of bulb sending fibre to one of the small cells in cortex
of cerebrum; 18, relation of small cell to large pyramidal cell 1; 2, axis-cylinder
process of 1 giving off branch {call.) to corpus callosum and thence to cortex of
opposite hemisphere, branch (sir.) to corpus striatum, then proceeding downwards
through bulb and cord in pyramidal tract, and finally dividing into branches 3, 3, 3, 4,
which end about cells of anterior horn; 5, 5, 5, 5, efferent fibres from cell-bodies in
anterior horn going to peripheral organs, e.g. to muscle, in; 15, ascending, and 16,
descending fibres in relation with cells of Purkinjd in cerebellum. The simplest
reflex action would involve at least 7, 8, 10, and 5.
xii THE PATHS OF CONDUCTION OF IMPULSES 553
cerebral hemispheres and ultimately to the sensory areas of
the cerebral cortex. Conversely, many fibres from the
motor areas can be followed as a very definite tract, the
pyramidal tract, downwards through the crura cerebri to
Fig. X7Q. — Diagram showing Nervous Inter-relations of Sense-organs and
Motor organs. (From Landois & Sterling's Text-book 0/ Human Physiology).
s, s', s", a, paths of sensory impulses going to brain; m, m', paths of motor im-
pulses from brain to muscles of lips and hand: within brain are centres of sight (V),
hearing (A), for muscles of hand (W), and for speech (E). Arrows indicate the
direction of the nervous impulses.
the decussation of the pyramids in the bulb, and thence to
the descending columns of the cord. These sensory and
554 ELEMENTARY PHYSIOLOGY less, xn
motor fibres together converge in a fan-like manner from
their respective cortical terminations into a large and very
pronounced bundle between the optic thalamus and corpus
striatum on each side, which is known as the internal cap-
sule (Fig. 177).
We have already referred to the corpus callosum and the
pons Varolii as composed of commissural fibres connecting
the two halves of the cerebrum and cerebellum respectively.
These are the largest of several commissures, besides which
large numbers of commissural fibres are not collected into
definite bundles.
One of the most interesting of all the various pathways is
that of the so-called association fibres, which run between
different parts of the same hemisphere in both the cerebrum
and cerebellum. These constitute definite tracts, by which
the various sensory and motor areas are connected, and the
harmonious action of the parts is assured.
Figure 178 shows very diagrammatically the cellular rela-
tionships of some of the parts of the cerebro-spinal system to
one another. Figure 179 shows, also very diagrammatically,
how the centres of sight and of hearing may be associated
with each other, and with the motor areas concerned in
speech and writing.
APPENDIX
ANATOMICAL AND PHYSIOLOGICAL CONSTANTS
The weight of the body of a full-grown man may be taken
at 70 kilogrammes (154 lbs.).
I. General Statistics
Such a body would be made up of —
Per cent.
lbs.
. . . . 41 .
6.3
. . . . 16 .
25
IO.7
28
7
Fat
. . . . 18 .
Brain
2
3
3
10.7
2
. . . . 7 .
Blood1
10.7
x54
of—
100
89
.... 42 .
65
The solids would consist of the elements oxygen, hydro-
gen, carbon, nitrogen, phosphorus, sulphur, silicon, chlorine,
iodine, fluorine, potassium, sodium, calcium, lithium, mag-
1 The total quantity of blood in the body is calculated at about A to A
of the body weight.
555
556 APPENDIX
nesium, iron, manganese, copper, and lead, and may be
arranged under the heads of —
Proteids. Carbohydrates. Fats. Minerals.
Such a body would lose in 24 hours — of water, about
2,780 grammes (6 lbs. or 6 pints) ; of other matters, about
940 grammes (2 lbs.), which would contain about 270-300
grammes (or rather more than ^ lb.) of carbon, 20 grammes
(f oz.) of nitrogen and 30 grammes (about 1 oz.) of mineral
matters (inorganic salts).
It could do about 150,000 kilogramme-metres (540 foot-
tons 1) of work, and gives off as much heat (2,300 kilogramme
degree units) as would be able to do five times as much
work again, say 850,000 kilogramme-metres (or about 3,100
foot-tons). The total energy expended by the body as
heat and work (calculated entirely as work) is thus about
1,000,000 kilogramme-metres (3,640 foot-tons), of which
one-sixth is expended as work and five-sixths as heat.
The loss of substance would occur through various organs
and to the respective amounts shown in the table on
p. 293.
The gains and losses of this body would be about as
follows : —
Creditor: — Solid dry food . . 600 grammes (\\ lbs.)
Oxygen .... 640 " (i| " )
Water 2,500 " (5^ " )
3,740 grammes (8£ lbs.)
Debtor : — Water 2,800 grammes (6^ lbs.)
Other matters . . 940 " (2 " )
3,740 grammes (8£ lbs.)
1 A foot-ton is the equivalent of the work required to lift one ton one
fool high.
APPENDIX 557
II. Nutrition
Such a body would require for daily food, carbon 270-300
grammes, nitrogen 20 grammes.
Now proteids contain, in round numbers, about 15 per
cent, nitrogen and 53 per cent, carbon, while carbohydrates
and fats contain respectively 40 per cent, and 80 per cent,
carbon. Hence the necessary amounts of nitrogen and
carbon, together with the other necessary elements, might
be obtained as follows (see p. 295) : —
Proteids .... 130 grms. containing 20 grms. nitrogen 70 grms. carbon
Carbohydrates 400 " 160 "
Fats 50 " " 40 " "
Minerals ... 30 " ■
Water .... 2,500 "
This might in turn be obtained, for instance, from : —
Lean meat 230 grammes (| lb.)
Bread 480
Potatoes 660
Milk 500
Fat 30
Water 2,000
(1 lb.)
(ii lb.)
(t P^t)
(1 oz.)
(4 pints)
This table, however, must be understood as being intro-
duced for the sake of illustration only. Many other similar
tables may be constructed by the use of various kinds of
food.
III. Circulation
In such a body the heart would beat about 72 times in a
minute and probably drive out at each stroke from each
ventricle about 100 to 125 grammes (6 to 7 cubic inches or
3| oz.) of blood.
558 APPENDIX
The blood would probably move in the great arteries at
the rate of about 12 inches (300 millimetres) in a second:
in the capillaries at the rate of 1-2 inches (25-50 milli-
metres) in a minute. The time taken up in performing the
complete circuit would probably be a little less than 30
seconds.
The left ventricle would probably establish a blood-pres-
sure in the aorta equal to the pressure (per square inch) of
a column of blood about 7 or 8 feet (2 metres) in height;
or of a column of mercury 6-7 inches (150 millimetres) in
height.
Sending out 100 grammes of blood at each stroke against
this pressure the left ventricle does 100 x 2,000 gramme-
millimetres or 200 gramme-metres of work at each stroke ;
in 24 hours, at 72 strokes per minute, the total work done
is about 20,000 kilogramme-metres. The work of the right
ventricle is about one-quarter of that done by the left, since
it works against a smaller blood-pressure in the pulmonary
artery. The total work of both ventricles is therefore about
25,000 kilogramme-metres, or 90 foot-tons.
IV. Respiration
Such a body would breathe about 1 7 times a minute.
The lungs would contain of residual air about 1,500 c.c.
^100 cubic inches), of supplemental or reserve air about
1,500 c.c. (100 cubic inches), of tidal air 500 c.c. (30 cubic
inches), and of complemental air 500 c.c. (100 cubic inches).
The vital capacity of the chest — that is, the greatest
quantity of air which could be inspired or expired — would
be about 3,500 c.c. (230 cubic inches).
There would pass through the lungs, per diem, about
10,000 litres (350 to 400 cubic feet) of air.
APPENDIX 559
In passing through the lungs, the air would lose from 4 to
6 per cent, of its volume of oxygen, and gain 4 to 5 per
cent, of carbonic acid.
During 24 hours there would be consumed of oxygen
about 450 litres (16 cubic feet) or 640 grammes (1^- lb.) ;
there would be produced about the same volume (or rather
less) of carbonic acid, which would contain about 225
grammes (8 oz.) of carbon. During the same time about
500 grammes (1 pint or 20 oz.) of water would be given
off from the respiratory organs.
In 24 hours such a body would vitiate 1,750 cubic feet
(1 cubic foot = 28.3 litres) of pure air to the extent of 1 per
cent, or 17,500 cubic feet of pure air to the extent of 1 per
1,000. Taking the amount of carbonic acid in the atmos-
phere at 3 parts, and in expired air at 470 parts in 10,000,
such a body would require a supply per diem of more than
23,000 cubic feet of ordinary air, in order that the surround-
ing atmosphere might not contain more than 1 per 1,000 of
carbonic acid (when air is vitiated from animal sources with
carbonic acid to more than 1 per 1,000 the concomitant
impurities become appreciable to the nose). But for health,
the percentage of carbonic acid should be kept down to half
this amount or .5 per 1,000, so that the body should be
supplied with at least about 50,000 cubic feet of fresh air
each day. A man of the weight mentioned (154 lbs.) ought,
therefore, to have at least 1,000 cubic feet of well-ventilated
space.
V. Cutaneous Excretion
Owing to its excessive variation exact figures regarding
cutaneous excretion are of very little, if any, value. The
body mentioned might, however, throw off by the skin in 24
hours — of water 600 grammes (20 oz. or 1^ pint); of
560 APPENDIX
solid matters 12 grammes (185 grains) ; of carbonic acid
10 grammes (150 grains).
VI. Renal Excretion
Such a body would pass by the kidneys in 24 hours — of
water about 1,500 grammes or cubic centimetres (53 oz.
or 3 pints) ; of urea about 33 grammes (500 grains or 1^
oz.), and about the same quantity of other solid matters.
VII. Nervous Action
A nervous impulse travels along a nerve at the rate of
about 90 feet in a second in the frog, and cf about 100 feet
a second in man ; but the rate in man varies very much
according to circumstances.
VIII. Histology
The following are some of the most important histological
measurements : —
Red blood-corpuscles, breadth 3-2V0 of an inch, or 7/u. to
8/A.
White blood-corpuscles, breadth ^V^ °f an incn> or
I Oft.
Striated muscular fibre (very variable), breadth -^\-§ of
an inch, or 60/x ; length 1^- inch, or 30 to 40 millimetres.
Non-striated muscular fibre (variable), breadth 4-,^ of
an inch, or 6/u, ; length 2x0 °f an incn> or 100/4.
Nerve-fibre (very variable), breadth jyvws to TsVo °f
an inch, or 2/jl to 16/x.
Nerve-cells (of spinal cord) excluding processes, breadth
,')() to T7T or more °f an inch, 50/x to 140/x or more.
APPENDIX 561
White fibres of connective tissue, breadth 2TQo~5 °f an
inch, or i/a.
Superficial cells of epidermis, breadth j^Vo °f an mcn> or
Capillary blood-vessels (variable) width .i^Q0 to ^ ^ 0 of
an inch, or 7/x, to 12/t.
Cilia, from the wind-pipe, length goVo °f an mcn or
8/x.
Cones in the yellow spot of the retina, width ygVo °f
an inch, or 2/x.
SO
INDEX
Abdomen, 6.
Abduction of limb, 353.
Absorption of digested food, 249,
274, 288.
Accelerator nerves, 99.
Accommodation of the eye, 433 ;
diagram of, 435, 436; limits of,
437-
Acetabulum, 352.
Acid, carbonic, see Carbonic acid ;
glycocholic, 240; hydrochloric, in
gastric juice, 270; sarcolactic, 318,
319, 322; taurocholic, 240; uric,
209.
Action, reflex, see Reflex action ;
spontaneous, 367.
Adam's apple, 357.
Adduction of limb, 353.
Adenoid tissue, 53, 116.
Adipose tissue, 53 ; figure of, 54.
Afferent nerve, 367, 500.
After-images, negative, 457.
Air, amount of, respired, 172 ;
changes of, in respiration, 174;
complemental, 172 ; composition of
atmospheric, 4 ; composition of in-
spired and expired, 174; expired,
temperature of, 174; residual, 172;
stationary, 173 ; supplemental, 172 ;
tidal, 172; when injurious, 187,
191.
Air-cells in lungs, 155, 173.
Albumin as food, 251, 298.
Alimentary canal, 7,249; figure of,
277 ; muscle of, 325.
Alimentary organs, 21.
Alimentation, 249.
Alveolus, of gland, 263; of lung, 155,
173; of tooth, 255.
Amoeba, colourless corpuscle com-
pared to, 128.
Amoeboid movement, 128, 308.
Ampulla of semicircular canals, 395.
Aniylopsin, 284.
Anus, 276.
Aorta, 63; abdominal, 193.
Appendix, vermiform, 276.
Aqueous humour, 424.
Arachnoid membrane, 477.
Areolar tissue, 112.
Arm, figure of bones of, 17, 326.
Artery (or arteries), 22; and vein,
56 ; contractility of, 58 ; coronary,
65; elasticity of, 57, 81, 84, 86;
function of muscle of, 59 ; hepatic,
65, 235 ; iliac, 193 ; nervous con-
trol of, 90; peripheral resistance
in, 81, 83, 84, 109; pressure in, 83;
pulmonary, 63; renal, 204; struc-
ture of, 57 ; figure showing struc-
ture of, 58, 60 ; tone of, 93 ; working
of, 80.
Articular cartilages, 17, 346.
Articulations or joints, 345.
Arytenoid cartilages, 358.
Asphyxia, 186.
Aspirate sounds, 364.
Association, law of, 547.
Association nerve-fibres, 553.
Astragalus, 355.
Atlas, 348.
Auditory organs, 392.
Auditory epithelium, 393.
Auditory sensorium, 414.
563
564
INDEX
Auditory spectra, 463.
Auricle of heart, 68.
Auriculo-ventricular apertures, 69.
Automatism of the respiratory centre,
181.
Axis, 348.
Axis-cylinder, 485, 487; process, 481.
Axon, 481.
Ball-and-socket joints, 347, 352, 354.
Basilar membrane, 404; function of,
417.
Beat of heart, 75.
Bile, 239; colour of, 131; function
of, 286; secretion of, 285.
Bile duct, figure showing origin of,
239-
Bile-pigments, 240, 285.
Bile-salts, 240, 285.
Bilirubin, 240.
Biliverdin, 240.
Bladder, 8, 201.
Blastomeres, 33.
Blind spot, 449, 450.
Blister, how formed, 36.
Blood, 119; arterial and venous com-
pared, 150, 186; changes of, in
lungs, 178 ; circulation of, 22, 55 ;
clotting of, 120, 136; colour of
arterial and venous, 149 ; composi-
tion of, 132; distribution of, 140;
functions of, 141 ; gases of, 148 ;
microscopic examination of, 119;
physical qualities of, 131 ; quantity
of, 140; sources of loss and gain
to, 193, 249; specific gravity of,
131; temperature of, 132; trans-
fusion of, 142.
Blood-capillaries, 56.
Blood-corpuscles, 120; colourless, 32,
121, 126, 129; of embryos, 128;
figure of, 121, 127; migration of,
129; movement of, 107, 126; num-
ber of, 131 ; origin and fate of, 130 ;
properties of, 131 ; red, 121, 122.
Blood-crystals, 126; figure of, 125.
Blood-flow, rate of, 89.
Blood-plasma, 120, 134.
Blood-platelets, 130.
Blood-pressure, 83 ; in capillaries, 146.
Blood-strum, 137.
Blood-vessels, general arrangement
of, 61, 62.
Blushing, 91.
Body, chief tissues of, 34 ; composi-
tion of, 292, 555 ; daily income of,
295; daily outgo from, 293; de-
composition of, 28; diagrammatic
section of, 9 ; figure illustrating
erect posture of, 18 ; general build
of, 6; movements of, 353; oxida-
tion in, 24; table of gains and losses
of, 556; temperature of, 227; tem-
perature of, and blood supply, 94;
work of, 1, 305, 556.
Bone (or bones), 325; composition
of, 14 ; development of, 335 ; of
ear, 407, 412; ethmoid, 389; figure
showing development of, 338;
figure showing structure of, 326,
332, 333 ; hyoid, 357 ; marrow of,
329; number of, 14; orbicular,
408; periosteal, 338; petrous, 393;
spongy, 338 ; structure of, 328 ;
temporal, 393; turbinal, 389.
Bone corpuscles, 334.
Bony tissue, 330.
Brain, 8, 475 ; anatomy of, 517 ; basal
view of, 520; and consciousness,
544; effects of destruction of, in
frog, 99, 508 ; horizontal section of,
527 ; lobes of, 547 ; median view of,
522; membranes of, 528; minute
structure of, 529 ; paths of conduc-
tion in, 551; reflex action of, 545;
side view of, 518.
Bread as food, 299.
Breathing, see Respiration.
Broca's convolution, 549.
Bronchi, 155, 157, 158.
Bronchial tubes, 155, 157, 158.
Bronchioles, 155.
Brunner, glands of, 280.
Buccal glands, 262.
INDEX
565
Buffy-coat, 137.
Bulb, 517,519,521, 538.
Bursas, 327.
Caecum, 276; figure of, 276.
Calamus scriptorius, 521.
Calorie, 305.
Camera obscura, 431.
Canal, alimentary, 7, 249; alimentary,
figure of, 277; central, of spinal
cord, 478; Haversian, 329; semi-
circular, 395, 421, 541 ; spinal, 7.
Canaliculi in bone, 333.
Cancelli of articular end of bone, 329.
Cancellous or spongy tissue of bone,
325-
Capillaries, 22; condition of walls of,
109 ; figure showing distribution of,
104, 106; pressure in, 146; pulmo-
nary, 153 ; structure of, 55 ; figure
showing structure of, 55.
Capsule, of cartilage, 45 ; internal,
552; of joints, 352; Malpighian,
204, 206, 211, 212.
Carbohydrate, 4 ; absorption of, 289,
290; digestion of, 267, 282, 287 ; as
food, 250, 251, 299, 301 ; heat-equiv-
alent of, 306.
Carbon, amount eliminated by lungs,
175 ; daily waste of, 294.
Carbonic acid, 3; in air, 4, 174; in
blood, 132, 149, 152; effect of, on
blood-corpuscles, 151 ; effect of, on
organism, 187; in lymph, 143; in
urine, 209.
Carbonic oxide, effect of, 187.
Cardiac aperture, 269.
Cardiac cycle, 76.
Cardiac impulse, 81.
Cardiac muscle, 74.
Cardiac nerves, 98.
Cardio-inhibitory centre, 101, 538;
diagram of, 102.
Cartilage, articular, 17, 346; aryte-
noid, 358; calcified, 336; cricoid,
357; development of, 47 ; elastic or
yellow fibro-, 46 ; figure of elastic,
46; figure of hyaline, 44,45 ; figure
of white fibro-, 47 ; hyaline, 42; in-
ter-articular, 346 ; structure of, 42 ;
thyroid, 357 ; white fibro-, 46.
Cartilage corpuscles, 44.
Casein as food, 251.
Cells, 32; amoeboid, 308; central, of
gastric glands, 270 ; ciliated, 157 ;
differentiation of, 33 ; gustatory,
385; parietal, of gastric glands,
270; of Purkinje, 531; secreting,
199, 207, 219, 238, 263, 271, 282;
figure of secreting, 208, 264, 265,
271, 283 ; wandering, 51.
Cement of teeth, 257.
Centre, cardio-inhibitory, 101, 538 ;
of ossification, 336; respiratory,
180; vaso-motor, 94, 514, 538.
Cerebellum, 518, 523 ; figure illustrat-
ing structure of, 531 ; functions of,
540; peduncles of, 525; structure
of, 530.
Cerebral hemispheres, 518, 519, 522;
diagram of localization in, 550;
diagrams of surface of, 548 ; func-
tions of, 542; localization of func-
tion in, 547.
Cerebro-spinal nervous system, 8,
475 ; membranes of, 476.
Cerebrum, 518 ; diagrams of locali-
zation in, 550 ; diagrams of surface
of, 548 ; functions of, 542 ; localiza-
tion of function in, 547.
Chest, 160; figures of, 66 157, 161
163, 171.
Cholesterin, 240; 285.
Chondrin, 43.
Chorda tympani nerve, 96, 266.
Chordae tendineae, 69 ; action of, 78.
Choroid coat, 425.
Choroid plexuses, 529.
Chyle, 112, 286, 289; compared with
lymph, 144; receptacle of, 112.
Chyme, 274.
Cilia, 41, 308 ; action of, 157, 308.
Ciliary muscle, 427.
Ciliary processes of choroid, 425.
566
INDEX
Ciliated cells, 157; figure of, 159,
308.
Ciliated epithelium, 41.
Circulation of the blood, 22, 55 ; cap-
illary, 105 ; figure of capillary, 104,
106, 158 ; figure showing course
of, 62 ; influence of respiration on,
188 ; in the kidney, 205, 208 ; portal,
65 ; proofs of, 103 ; statistics of,
557-
Circulatory organs, 22, 55.
Circumduction of limb, 353.
Clavicle, 17.
Coagulation of blood, 120, 136.
Coccyx, 15.
Cochlea, 400; bony, 405; canal of,
401 ; diagram of, 401, 403 ; mem-
branous, 399.
Cold, sensations of, 378.
Colloids, 146.
Colon, 276.
Colour, qualities of, 454; sensations
of, 453-
Colours, complementary, 455 ; pri-
mary, 456.
Colour-blindness, 457.
Columnse carneae, 69.
Complemental air, 172.
Cones of retina, 443, 452.
Conjunctiva, 375, 440.
Connective tissue, 11, 49; adeno'd,
retiform, or lymphoid, 53, 116; are-
olar, 49, 112; development of, 53;
elastic, 52; fatty or adipose, 53;
fibrous, 52; structure of, 49.
Connective tissue corpuscles, 50;
figure of, 52.
Connective tissue fibres, figure of, 49,
50, 51-
Consciousness and brain, 544.
Consciousness and sensations, 369.
Consciousness, states of, 370.
Consonants, 364.
Contact, sense of, 382.
Contractility, of colourless corpuscle,
127 ; of muscle fibre, 317, 320.
Contraction, peristaltic, 262, 325.
Convolution, Broca's, 549; of cer-
ebrum, 547.
Coordinating action of nervous sys-
tem, 25.
Coordination of movements, 540.
Cornea, 424.
Cornua of spinal cord, 480.
Coronary arteries, 65.
Coronary vein, 65.
Corpora quadrigemina, 519, 523, 526.
Corpora striata, 528.
Corpus callosum, 521, 523.
Corpuscles, of blood, see BLood-cor-
puscles ; of bone, 334 ; of cartilage,
44; of connective tissue, 50 ; of the
spleen, 245; tactile, 217, 374.
Corresponding points, 440, 472.
Cortex, of cerebellum, 530; of cer-
ebrum, 529, 532; of cerebrum,
figure illustrating structure of, 531,
533; of cerebrum, localization of
function in, 547 ; of kidney, 204.
Corti, organ of, 402; organ of, dia-
gram of, 403 ; organ of, function
of, 417 ; rods of, 404.
Coughing, 170.
Cranial nerves, 475,519,535 ; diagram
of, 536.
Creatin, 319.
Cretinism, 247.
Cribriform plate, 389.
Cricoid cartilage, 357.
Crista acustica, 396 ; diagram of, 398.
Cruracerebri, 510, 522, 526.
Crystalline lens, 424, 437.
Crystalloids, 146.
Death, and life, 25 ; local and general,
26 ; modes of, 27.
Death-stiffening, 318.
Decomposition of the body, 28.
Decussation, of pyramids, 539; sen-
sory, 539.
Defsecation, 511.
Degeneration, ascending, 512; de-
scending, 512; in spinal cord, dia-
gram of tracts of, 513, 514.
INDEX
567
Degeneration-method, 499.
Delirium tremens, 463.
Delusions, of judgment, 461, 462,
465 ; optical, 466.
Dendrite, 481.
Dental groove, 257.
Dental papilla, 258.
Dental pulp, 255, 257.
Dentine, 256.
Dentition, milk, 260; permanent, 260.
Dermis, 10, 11, 35, 215.
Dextrose, 251.
Diabetes, 284, 539.
Diaphragm, 7, 166; action of, 167;
of camera, 431; figure of, 167;
pillars of, 166.
Diaphysis, 336.
Diastole of heart, 75.
Diet, 295, 557; economy of mixed,
297 ; effect of, on red corpuscles,
133-
Differentiation of primitive cells, 33.
Diffusion, 144 ; figure illustrating, 145.
Digestion, 249; intestinal, 285; pur-
pose and means of, 252.
Digits, 7.
Distance, judgment of, 468.
Drinking, mechanism of, 262.
Drum of ear, 406.
Duct, hepatic, 235; lachrymal, 441 ;
pancreatic, 285; thoracic, in.
Ductless glands, 201.
Duodenum, 274 ; figure of, 245.
Dura mater, 477.
Dyspnoea, 185.
Ear, 392 ; figure of, 404, 407 ; external,
394, 410; internal, 394; transmis-
sion of sound waves to, 410; mid-
dle, 406; middle, diagram of, 405.
Education and nervous system, 547.
Efferent nerve, 367, 500.
Egg, 32; diagram of, 32; segmenta-
tion of, 33 ; diagram of segmenta-
tion of, 34.
Electric shock, effect of, on muscle,
322.
Electrical fishes, 500.
Electricity, of contracting muscle,
322 ; of nerve, 502.
Embryo, 128.
Emulsification, 2860
Emulsion, 284.
Enamel, 257.
End-bulbs, 217, 375 ; figure of, 375.
Endocardium, 75.
Endolymph, 393.
End-organ, mode of action of audi-
tory, 415 ; motor, 487.
End-plate, 487.
Energy, daily output of, 305 ; income
and expenditure of, 303 : source
of vital, 304.
Enzymes, 267, 268.
Epidermis, 10, 215; growth of, 38;
structure of, 35, 217.
Epiglottis, 155, 254.
Epinephrin, 248.
Epiphyses, 336, 337.
Epithelium, 11; auditory, 393; o:
blood-vessels, etc., 42 ; ciliated, 41 ;
figure of cells of, 38 ; of mucous
membrane, 41 ; olfactory, 389 ;
secreting, 199.
Equilibrium, maintenance of bodily,
422.
Erect position, how maintained, 17,
18.
Eustachian tube, 254, 406; function
of, 421.
Excretion, 4 ; oxygen in, 4 ; statistics
of cutaneous, 559; statistics of
renal, 560.
Excretory organs, 23, 194,
Expiration, 168.
Expired air, composition of, 174.
Explosive sounds, 365.
Extension of limb, 353.
Eye, 423; accommodation of, 433;
movements of, 439 ; protective
appendages of, 440 ; structure of,
423 ; as water camera, 428.
Eyeball, 424; diagram of muscles oi,
439 ; section of, 425.
568
INDEX
Eyelashes, 440.
Eyelids, 440; muscles of, 441.
Face, cavity of the, 8 ; figure showing
section of, 156, 253.
Faeces, 22, 252.
Fainting, 92.
Fascia, 312.
Fasciculi of muscle, 312.
Fat, absorption of, 288, 290; in blood-
corpuscles, 129 ; in chyle, 144 ; di-
gestion of, 284, 286 ; as food, 250,
251,299,301 ; heat-equivalent of, 306.
Fat-cells, 53 ; figure of, 54.
Fatigue, one cause of, 177 ; of retina,
456-
Fauces, 254.
Femur, figure showing structure of,
326.
Fenestra ovalis, 406, 407.
Fenestra rotunda, 401, 406, 411
Ferments, 140, 267, 268.
Fever, temperature in, 233.
Fibres, association, 553; collagenous,
50 ; elastic, 50 ; white, 50.
Fibrin, 136 ; figure of, 135.
Fibrin ferment, 140.
Fibrinogen, 135, 139, 143.
Filtration, 144.
Fishes, electrical, 500.
Fissure, anterior, of spinal cord,
478 ; calcarine, 547 ; parieto-occipi-
tal, 547; posterior, of spinal cord,
478 ; of Rolando, 525, 547 ; of Syl-
vius, 525, 547.
Flexion of limb, 353.
Fluid, cerebro-spinal, 477.
Focus of lens, 430.
Food, 4, 250 ; changes of, in intestine,
285; proteid and carbohydrate, 4;
as source of energy, 304 ; waste
made good by, 249.
Food-stuffs, 250 ; accessory, 303 ;
effects of the several, 300 ; essen-
tial, 303; as heat-producers and
tissue-formers, 302 ; nitrogenous,
250; non-nitrogenous, 251.
Foot as lever, 342.
Foot-pound, 304.
Foramen, of Magendie, 529 ; of Mon-
ro, 524; occipital, 352.
Foramina, intervertebral, 478.
Forearm, figure of bones of, 351.
Fore-brain, 519.
Form, judgment of, 470, 471.
Fornix, 523.
Gall-bladder, 235 ; figure of, 269
Galvanometer, 503.
Ganglia, in heart, 98 ; spinal, 479 ,
spinal, structure of, 494; sympa-
thetic, 7, 475, 514.
Ganglion-cell, figure of, 494.
Gases, exchange of, in lungs, 178;
of inspired and expired air, 174;
partial pressure of, 177.
Gastric glands, 270; figure of, 271.
Gastric juice, 270; action of, 271;
secretion of, 271.
Gelatine, of bone, 11,330; as food,
251.
Gland (or glands) , in general, 199 ; of
Brunner, 280; buccal, 262; changes
in, in secretion, 263, 271, 281 ; duct-
less, 201; ducts of, 199; figure show-
ing structure of, 200 ; gastric, 270;
figure of, 271; lachrymal, 441; of
Lieberkuhn, 279; lymphatic, in;
lymphatic, figure of, 115; Meibo-
mian, 440; parotid, 262; pineal,
523; saccular, 199; salivary, 262;
sebaceous, 223 ; secreting, 199 ;
simple and compound, 199; sub-
maxillary, 262; sublingual, 262;
sweat, 217; thymus, 246; thyroid,
246; tubular, 199.
Glomerulus of kidney, 204 ; figure of,
205, 206, 209.
Glottis, 155, 254, 357; figure of, 359.
Gluten as food, 251.
Glycocholic acid, 240.
Glycogen, in blood-corpuscles, 129;
in liver, 234, 242; in muscle, 319.
Goitre, 247.
INDEX
569
irey matter, of brain, 526, 529; of
spinal cord, 480; of spinal cord,
cells of, 491 ; of spinal cord, figure
of cells of, 492.
Gristle, 12.
Gullet, 8.
Gum of mouth, 255.
Gustatory cells, 385.
Gustatory nerve, 385.
Gyri, 547.
Hcematin, 124, 131.
Haemoglobin, 124, 151 ; crystals of,
125 ; in muscle, 320.
Hair, 221 ; figure of, 221, 222, 223.
Hairs, auditory, 397.
Hair-cells, of cochlea, 404; of organ
of Corti, 418.
Hand, movements of, 351.
Harvey, discoverer of the circulation,
103.
Haversian canals, 329.
Haversian system, 331.
Head, movements of, 349, 350.
Hearing, sense of, 392.
Heart, 22, 66 ; beat of, 75 ; diagram
showing action of, 78 ; figures of,
64, 68, 70, 71, 72, 73 ; ganglia in,
98 ; nervous control of, 98 ; pal-
pitation of, 103 ; sounds of, 82 ;
structure of, 66, 74; valves of, 69;
valves of, action of, 76.
Heat, 227.
Heat, developed by contracting mus-
cle, 322, 323; loss of, 229; mechan-
ical equivalent of, 305 ; production
of, 232; unit of, 305.
Heat spots and cold spots, 380; dia-
gram of, 379.
Hiccough, 170.
Hilus of kidney, 201.
Hind-brain, 519.
Hinge-joints, 348, 352.
Hip joint, 352; figure of, 347.
Histology, 30; statistics of, 560.
Homoiomera, 30.
Humerus, 350.
Humours of the eye, 424.
Hyoid bone, 328, 357.
Hypoglossal nerve, 538.
Ileo-caecal valve, 274.
Ileum, 274.
Ilium, 17.
Incus, 408, 412.
Inflammation, 107.
Infundibulum of a bronchial tube,
155-
Innervation, 475.
Innominate bone, 17.
Insertion of muscle, 326.
Inspiration, 168.
Inspired air, composition of, 174.
Intelligence, seat of, in cerebrum, 542.
Intercellular substance, 32, 42, 44.
Intercostal muscles, 162 ; diagram of,
165 ; figure of, 164.
Intercostal nerves, 180.
Internal capsule, 552.
Internal secretion, 201 ; by liver, 244 ;
by thyroid body, 246 ; by supra-
renal bodies, 248.
Intestinal juice, 282.
Intestines, absorption from, 287, 288 ;
arrangement of, 274 ; changes of
food in, 285 ; structure of, 278.
Iodo-thyrin, 247.
Iris of the eye, 421, 426, 432 ; arrange-
ment of muscle in, 324 ; figure of,
428.
Ischium, 17.
Jejunum, 274.
Joints, 345-353 ; ball-and-socket, 347,
352> 354; hinge, 348; imperfect,
345; perfect, 346; pivot, 348.
Judgment, of changes of form, 471 ;
delusive, 461, 462, 465 ; of distance,
468; of form, 470, 471 ; of size, 468;
of solidity, 473.
Jumping, mechanics of, 356.
Kidneys, 8, 201; figure of, 202, 204;
figure showing cells of, 208; figure
57°
INDEX
showing circulation in, 206, 209 ;
function of, 23, 211 ; lungs and skin
compared, 226; and skin, 212; sta-
tistics of excretion by, 560 ; struc-
ture of, 203; types of cells in, 206.
Kilogramme-metre, 305.
Kinetoscope, 451, 472.
Knee-joint, diagram of, 345.
Labyrinth, bony, 394, 405; mem-
branous, 393, 395; membranous,
diagram of, 396.
Lachrymal gland, 441.
Lachrymal sac, 441.
Lacteals, 112, 144, 280.
Lacunae of bone, 332.
Lamellae, of cerebellum, 530; of bony
tissue, 331.
Lamina, spiral, 399.
Larynx, 155; mechanism of, 356;
figure of, 357, 358, 361 ; figure illus-
trating action of, 363.
Law of association, 547.
Leg, figure of bones of, 16.
Lens, crystalline, 424, 437.
Lenses, properties of, 429.
Leucin, 214, 284, 287.
Leucocytes, 116, 143.
Levatores costarum, 165.
Levers, bones as, 341-345 ; classes of,
341 ; figure of, 342.
Lieberkiihn, glands of, 279.
Life, and death, 25; tripod of, 27.
Ligaments, 17, 347; capsular, 352;
check, 352; crucial, 352; lateral,
352; round, 352; suspensory, 425.
Light, sensations of, 448, 451.
Liver, 8, 233 ; figure of, 234, 236, 237 ;
function of, 214, 239 ; structure of,
233-
Liver-cells, 236, 238; figure of, 236,
239-
Lobes, of brain, 525; of liver, 235.
Lobes and lobules of the lungs, 155.
Lobes, olfactory, 535.
Locomotion, mechanics of, 354.
Longsightedness, 438.
Lungs, 8, 155 ; amount of waste leav-
ing, 175 ; anatomy of, 155 ; elastic-
ity of, 162; figure of, 66, 68; figure
showing structure of, 158 ; function
of, 23 ; kidneys, and skin com-
pared, 226.
Lymph, no, 142 ; composition of,
143 ; functions of, 147 ; mode of
formation of, 144; movements of,
117.
Lymph-channel, 116.
Lymph-corpuscles, 143.
Lymph-sinus, 116.
Lymph-spaces, 114.
Lymphatic glands, in; figure of,
115; function of, 115; structure
of, 115.
Lymphatic system, 109.
Lymphatic vessels, 109, in, 114; gen-
eral arrangement of, 109 ; figure of
course of, 111; figure of origin of,
114; origin of, 112; structure of, 112.
Lymphoid tissue, 53, 116.
Macula acustica, 396.
Macula lutea, 442, 448; section of,
447-
Magendie, foramen of, 529.
Malleus, 407, 412.
Malpighi, network of, 36.
Malpighian capsule, 204, 206; figure
of, 205, 206; function of, 211, 212.
Malpighian layer of epidermis, 36,
217.
Maltose, 267.
Marrow of bone, 325, 329; red cor-
puscles formed in, 130.
Mastication, 260.
Matrix of cartilage, 42, 44.
Meat as food, 299.
Meatus of ear, 406.
Mechanical equivalent of heat, 305.
Medulla, of bone, 329; of kidney,
204; of lymphatic gland, 116; of
nerve-fibre, 485 ; oblongata, 517 ;
oblongata, functions of, 538.
Meibomian glands, 440.
INDEX
57i
Membrane, arachnoid, 477 ; base- [
ment, 199 ; basilar, 404, 417 ; mu- !
cous, 11, 41; of Reissner, 402;;
serous, 67 ; synovial, 17, 346.
Membranous labyrinth, 393, 395;
diagram of, 396.
Mesentery, 276 ; figure of, 278.
Metabolism, 213, note.
Micro-millimetre, 40.
Micturition, 511.
Mid-brain, 519.
Milk as food, 300.
Milk-teeth, 258, 260.
Mitral valve, 69.
Monro, foramen of, 524.
Mouth, 252; figure of section of, 156,
253-
Movements, amoeboid, 308 ; of the
body, 353 ; mechanics of, 341 ; co-
ordination of, 540.
Mucin, 240.
Mucous membrane, 11,41.
Mucus, 11.
Muscle (or muscles), attached to def-
inite levers, 325 ; not attached to
solid levers, 324 ; biceps, 326, 353 ;
biceps, figure of, 327 ; capillaries
of, figure of, 313 ; changes of a con-
tracting, 322; chemistry of, 318;
ciliary, 427 ; colour of, 319 ; con-
traction of, 12, 320, 323; crico-
thyroid, 358 ; death of, 318 ; devel-
opment of, 316; digastric, 328; of
eyeball, 439; external rectus, 537;
figure showing fasciculi of, 312;
gastrocnemius, 320 ; hollow, 324 ;
insertion of, 326; intercostal, 162;
intercostal, diagram of, 165 ; inter-
costal, figure of, 164 ; kinds of, 324 ;
lateral crico-arytenoid, 360; levator
of eyelid, 440; levatores costarum,
165 ; obliqui of eye, 439 ; orbicu-
laris, 440; as organ and as tissue,
311; origin of, 326; papillary,
69; plain, 310; posterior arytenoid.
360 ; posterior crico-arytenoid, 360 ;
recti of eye, 439 ; rectus abdominis,
343; rectus femoris, 343; scaleni,
165; smooth, 310; sphincter, 203;
stapedius, 410, 420; striated, 311;
superior oblique, 327, 537; tensor
tympani, 410, 420; tetanic contrac-
tion of, 323, thyro-arytenoid, 360;
triceps, 354 ; of tympanum, 413 ; of
tympanum, function of, 420; un-
striated, 310.
Muscle-fibre, cardiac, 74; contrac-
tility of, 317, 320; size of, 313 ; stri-
ated, 313; striated, figure of, 314,
316; unstriated, 310.
Muscle-nerve preparation, 321.
Muscle-plasma, 318.
Muscle-serum, 318.
Muscular sense, 383.
Muscularis mucosae, 280.
Musical sounds, 415.
Myelin, 485.
Myosin, 318; as food, 251.
Myosinogen, 319.
Nails, 219 ; figure showing structure
of, 220.
Nares, posterior, 388.
Nasal cavity, figure of, 388, 390.
Nearsightedness, 438.
Nerve (or nerves), 476; abducens,
536; accelerator, 99; afferent, 367,
500; auditory, 393, 402, 422, 537,
542; cardiac, 98 ; chorda tympani,
96, 266 ; cochlear, 422, 542 ; cranial,
475- 5IQ. 535 ; cranial, diagram of,
536; efferent, 367, 500; electrical
properties of, 502; facial, 537 ; glos-
sopharyngeal, 385, 537 ; gustatory,
385 ; hypoglossal, 538 ; inhibitory,
500; intercostal, 181; medullated,
484; motor, 367,500; motor oculi
or oculo-motor, 432, 536 ; olfactory,
535; optic, 423, 535 ; phrenic, 181;
physiological properties of, 499'
pneumogastric, 98, 182, 538 ; sciatic,
321; secretory, 226, 500; sensory,
367, 500; spinal, 475, 478; spinal,
roots of, 478, 495 ; spinal accessory,
572
INDEX
538 ; structure of, 483 ; superior
laryngeal, 184 ; sympathetic, 92,
515; trigeminal, 537; trochlear,
536 ; vagus, 98, 182, 538 ; vasocon-
strictor, 93 ; vaso-dilator, 96; vaso-
motor, 90, 93, 231 ; vestibular, 422,
542. •
Nerve-cell, 476, 481, 491, 516, 531 ;
figure of, 482, 492, 494; general
function of, 483 ; of retina, 444.
Nerve-centres, cardio-inhibitory, 101,
538 ; diabetic, 539 ; in medulla ob-
longata, 511, 538 ; motor,549; res-
piratory, 180, 538 ; secretory, 538 ;
sensory, 549 ; of speech, 549 ; in
spinal cord, 510; of swallowing,
538 ; vaso-motor, 94, 514, 538.
Nerve-fibres, 476, 484 ; degeneration
of, 482; medullated, 489; medul-
lated, figure of, 486; motor, 320;
motor, ending of, 487 ; non-rae-
dullated, 489, 517; origin 0^482;
of retina, 446 ; sensory, ending of,
489; sympathetic, 489.
Nerve-impulse, 320, 368, 499, 501 ;
rate of, 503, 560.
Nervous system, 475 ; cerebro-spinal,
475 ; coordinating action of, 25 ;
diagram of structure of, 552; and
education, 547; sympathetic, 514.
Nervous tissue, structural elements
of, 481.
Neuraxis, 481, 485, 487.
Neuraxon, 485.
Neurilemma, 485.
Neuroglia, 490, 534.
Neuroglia-cells, figure of, 491.
Neuron, 476, 481, 491, 516, 531;
figure of, 482, 492, 494; general
function of, 483; see also Nerve-
cell.
Nitrogen, in blood, 149; daily elimi-
nation of, 294.
Nitrogen starvation, 297.
Nodes of nerve-fibre, 485.
Noises, 415.
Nose, 387 ; figure of, 156, 253,388, 390.
Nucleus of cell, 32 ; function of, 482.
Nutrition, 291; statistics of, 292, 557.
Odontoid process, 349.
CEsophagus, 8, 254 ; function of, 261 ;
structure of, 262.
Olfactory epithelium, 389; figure of,
391-
Olfactory lobes, 535 ; nerves, 535.
Optic chiasma, 535 ; diagram of, 537.
Optic delusions, 466 ; nerves, 423,
535; thalami, 526; tracts, 535.
Ora serrata, 428.
Orbit of eye, 423.
Organ of Corti, 402; function of, 417.
Organs, in the abdomen, 8; alimen-
tary, 21 ; circulatory, 22; excretory,
23, 194; respiratory, 23, 154; sen-
sory, 20, 367; in the thorax, 8;
urinary, 201.
Origin of muscle, 326.
Os orbiculare, 408.
Osmosis, 273.
Ossa innominata, 17.
Ossicles, auditory, 407, 412.
Ossification, centres of, 336.
Osteoblasts, 337.
Otoliths, 399 ; function of, 422.
Overtones, 416.
Ovum, 32; segmentation of, 32.
Oxidation in the body, 24.
Oxygen, in blood, 132, 133, 149, 152;
effect of, on red corpuscles, 151 ;
in excretions, 4; in muscle, 323;
starvation, 186; taken in by the
lungs, 24.
Pacinian corpuscle, 376; figure of,
377-
Pain, sensation of, 380.
Palate, hard, 253; hard, figure of,
384 ; soft, 254.
Paleness, 92.
Palpitation of the heart, 103.
Pancreas, 8; figure of, 245 ; figure of
cells of, 283; secretion by, 282;
structure of, 282.
INDEX
573
Pancreatic juice, 283, 286.
Papillae, circumvallate, 384; dermal,
215, 374; filiform, 384; fungiform,
384-
Papillary muscles, 69; action of, 78.
Paralysis, 506, 539.
Parotid gland, 262 ; figure of cells of,
265.
Patella, 17, 344.
Pelvis, 17; figure of, 15; of kidney,
203.
Pepsin, 271.
Peptone, 272.
Pericardial fluid, 67.
Pericardium, 67.
Perichondrium, 42, 338.
Perilymph, 393.
Perimysium, 311, 312.
Perineurium, 484, 485.
Periosteum, 328, 338.
Peristaltic action, 262, 325.
Peritoneum, 201, 277.
Perspiration, sensible and insensible,
224.
Petrous bone, 393.
Peyer's patches, 280.
Phalanges, 7.
Pharynx, 8, 154.
Phosphene, 451.
Phrenic nerves, 181.
Physiology, scope of human, 2.
Pia mater, 477.
Pigment-cells, of retina, 443; figure
of, 426, 448.
Pillars, of diaphragm, 166 ; of fauces,
254-
Pineal gland, 523.
Pinna, 394, 410.
Pituitary body, 523.
Pivot joints, 348.
Plasma, 120; proteids of, 134 ; solids
in, I3S-
" Playing at sight," 546.
Pleura, 160.
Plexuses, choroid, 529; of sympa-
thetic nervous system, 515.
Pneumogastric nerve, 98, 182, 538.
Polarised light, affected by striated
muscle, 316.
Pons Vaiolii, 519, 522.
Portal vein, 65, 235.
Pressure, arterial, 83; sensations of,
378 ; spots, 378.
Primitive sheath, 485.
Pronation of arm, 350.
Proteids, 4, 134; absorption of, 289,
290; in colourless corpuscles, 129;
digestion of, 271, 283, 287 ; as food,
250, 298, 300; heat-equivalent of,
306 ; in lymph, 143 ; non-diffusive,
146 ; in plasma, 134 ; tests for, 135.
Protoplasm, 32; of colourless cor-
puscles, 127.
Protoplasmic processes of neuron,
481.
Ptyalin, 265.
Pubis, 17.
Pulmonary artery, 63.
Pulmonary capillaries, 153.
Pulmonary veins, 63.
Pulp cavity, 255.
Pulse, 85 ; velocity of, 86 ; venous,
190.
Punctum lacrimale, 441.
Pupil of the eye, 426, 432.
Purkinje, cells of, 531.
Purkinje's figures, 453.
Pylorus, 269.
Pyramids, decussation of, 539.
Racemose glands, 199.
Radius, 351.
Rage, 92.
Reading aloud, nervous mechanism
of. 545-
Receptacle of the chyle, 112.
Reflex action, 368, 507, 545 ; artificial,
546; of brain, 545; diagram of
paths of, 369; movement the re-
sult of, 367; natural, 546.
Reissner, membrane of, 402.
Rennin, 271, 273.
Residual air, 172.
Residual pouch, 258.
574
INDEX
Respiration, 148; abdominal, 169;
amount of air needed for, 192,
559; costal, 168; diaphragmatic,
168; effect of, on circulation, 188 ;
essential of, 154; internal, 153;
mechanism of, 162; movements
of, 162; nature of, 152; nervous
mechanism of, 179 ; organs of, 23,
154; rate of, 171, 558; statistics of,
558 ; of the tissues, 153.
Respiratory capacity, 172.
Respiratory centre, 180, 538; dia-
gram of, 183; influence of blood-
supply on, 184.
Respiratory organs, 23, 154.
Retiform tissue, 53, 116.
Retina, 423, 427, 441 ; diagram show-
ing circulation in, 444; diagram
showing formation of image on,
433; inversion of image on, 433,
466 ; section of, 445, 446 ; structure
of, 441.
Ribs, 15, 162.
Rigor mortis, 318.
Rods and cones of retina, 443, 452.
Rods of Corti, 404.
Rolando, fissure of, 525, 547.
Rotation of limb, 353.
Running, mechanics of, 356.
Saccule, 396; functions of, 421, 542.
Sacculi, 279.
Sacrum, 15.
Saliva, 263, 265 ; action of, 267 ; se-
cretion of, 265.
Salivary glands, 262.
Salts as food, 250, 251, 301.
Saponification, 284.
Sarcolactic acid, 318, 319, 322.
Sarcolemma, 316, 485.
Scalae of ear, 400, 401.
Scaleni muscles, 165.
Scapula, 17.
Schwann, sheath of, 485 ; white sub-
stance of, 485.
Sclerotic, 424.
Sebaceous glands, 223.
Secreting cells, 199, 207, 219, 238, 263,
271, 282; figure of, 208, 264, 265,
271, 283.
Secretion, in general, 199; internal,
201, 244, 246, 248; three senses of
word, 199.
Semicircular canals, 395; functions
of, 421, 541.
Semilunar fold of eye, 441.
Semilunar valves, 72.
Sensations, auditory, 414; coales-
cence of, 459 ; of colour, 453 ; and
consciousness, 369; of light, 448,
451; of movement, 383; muscular,
382 ; of pressure, 378 ; referred to
objects, 371 ; and sensory organs,
367; simple or composite, 459;
subjective, 370, 463 ; tactile, locali-
sation of, 381 ; of temperature, 378.
Sense, of contact, 382 ; of hearing, 392;
of movement, 383 ; muscular, 382 ;
of pain, 380; of pressure, 378 ; of
taste, 383 ; of temperature, 378 ; of
touch, 373 ; of smell, 387.
Senses, special, 370.
Sense-organs, 20, 370; accessory part
of, 372; essential part of, 372; gen-
eral plan of, 371 ; and motor or-
gans, diagram showing relation of,
553-
Sense-organules, 372.
Sensorium, auditory, 414 ; visual, 448,
452.
Septum lucidum, 524.
Septum, nasal, 387.
Serous membranes, 67.
Serum, of blood, 137 ; of muscle, 318.
Serum-albumin, 136, 143.
Serum-globulin, 136, 143.
Sexes, differences of respiration in,
169; differences of voice in, 363.
Sighing, 170.
Sight, organ of, 423.
Single vision with two eyes, 472.
Size, judgment of, 468.
Skeleton, 12.
Skin, diagram of structure of, 216;
INDEX
575
as excretory organ, 23, 223; excre-
tory products of, 23, 223, 559 ; func-
tions of, 23, 223, 373 ; and kidneys,
212; lungs, and kidneys, 226; as
sense-organ, 373; statistics of ex-
cretion by, 559 ; structure of, 10,
215-
Skull, 6; cavity of, 8; side view of,
14.
Smell, sense of, 387 ; and taste, 386.
Sneezing, 170.
Soaps, 284, 287.
Solidity, judgment of, 473.
Sounds, of heart, 82; localisation of
419; musical, 415.
Space, subarachnoid, 477, 529; sub-
dural, 477.
Spectacles, use of, 437.
Spectra, auditory, 463 ; ocular, 464.
Speech, 363 ; centre of, 549.
Sphincter muscle, 203.
Spinal bulb, 517, 519, 521 ; function
of, 538.
Spinal canal, 7.
Spinal column, figure of, 13.
Spinal cord, 7, 475 ; anatomy of, 477 ;
figure of, 476, 479, 480, 493 ; func-
tions of, 506 ; microscopic structure
of, 490; paths of conduction in,
511; reflex action through, 506;
structure of, at various levels, 492.
Spinal ganglia, 479 ; structure of, 494.
Spinal nerves, 478 ; diagram of dis-
tribution of, 516 ; diagram of roots
of, 497 ; function of roots of, 495.
Spiral lamina, 399.
Spleen, 8, 244; figure of, 245.
Spontaneous actions, 367.
Stapedius, 410,420.
Stapes, 407, 412.
Stationary air, 173.
Steapsin, 284.
Stereoscope, 473.
Sternum, 15.
Stimulus, 12, 368.
Stomach, figure of, 269; structure of,
368.
Stroma, 124.
Subarachnoid space, 477, 529.
Subdural space, 477.
Sublingual glands, 262.
Submaxillary glands, 262; figure of
cells of, 264.
Succus entericus, 282.
Sulci, 547.
Supination of arm, 350.
Supplemental air, 172.
Suprarenal bodies, 247.
Suspensory ligament, 425.
Swallowing, 261.
Sweat, composition of, 223 ; quantity
of, 224; secretion of, 224.
Sweat-glands, 217 ; and body tem-
perature, 230 ; figure of, 218.
Sweet-bread, 8.
Sylvius, aqueduct of, 523 ; fissure of,
525. 547-
Sympathetic ganglia, 7, 475, 514.
Sympathetic nerve, effect of cutting,
92.
Sympathetic nervous system, 7, 475,
5*4-
Synovia, 17.
Synovial fluid, 346.
Synovial membrane, 17, 346.
Synovial sheaths, 327.
Systole of heart, 75.
Tactile corpuscle, 217, 374; figure
of, 374-
Taste, sense of, 383 ; and smell, 386.
Taste-buds, 385.
Taurocholic acid, 240.
Tears, 441.
Teeth, 253, 254; alveolus of, 255;
bicuspid, 255 ; canine, 255 ; crown
of, 254 ; development of, 257 ; fangs
of, 255 ; figure showing structure of,
256, 259; incisor, 235; milk, 258,
260; molar, 255; permanent, 258 ;
pulp of, 255, 257; structure of,
255; wisdom, 260.
Temperature of body, 227 ; in fever,
233 ; regulation of, 94, 228, 229, 232,
576
INDEX
Temperature, effect of, on clotting of
blood, 138 ; sensations of, 378.
Tendon, 52, 312, 327.
Tensor typani, 410, 420.
Terror, 92.
Tetanus, 323.
Thalami, optic, 526.
Thaumatrope, 471.
Thoracic duct, 111 ; effect of respira-
tion on, 191 ; figure of, 113.
Thorax, 160 ; changes in size of, 166 ;
figure of, 157, 161, 163, 171.
Thymus gland, 246.
Thyroid body, 246.
Thyroid cartilage, 357.
Tidal air, 172.
Tissue, in general, 10, 34; adenoid,
116; adipose, 53; areolar, 112;
cancellous or spongy, 325 ; a com-
pound structure, 30; connective, 11,
49; elastic, in the lungs, 159, 162;
embryonic, 31; epithelial, 34; glan-
dular, 35 ; minute structure of, 30 ;
muscular, 34; nervous, 34; osse-
ous, 325, 330; renewal of, 21;
vascular, 35.
Tone, fundamental, 416; musical,
4i5-
Tongue, figure of, 384 ; in speech, 365.
Tonsils, 254.
Tooth sac, 258.
Touch, localisation of sensations of,
381 ; organs of, 373.
Trachea, 155; structure of, 156.
Tracts, crossed pyramidal, 513,539;
crossed pyramidal, diagram of,
551; optic, 535; postero-median,
512,541; pyramidal, 552; of spinal
cord, 512; of spinal cord, diagram
of, 513, 514.
Trapezium, 348.
Tricuspid valve, 69.
Trypsin, 283.
Tubules of kidney, 203, 205.
Tympanic membrane, 406.
Tympanic muscles, 409 ; functions
of, 420.
Tympanum, 401, 406.
Tyrosin, 284, 287.
Ulna, 350.
Unit used in histological measure
ment, 40.
Urea, 194, 209, 210; heat-equivalent
of, 306; history of, 213.
Ureter, 8, 201.
Urethra, 202.
Uric acid, 209.
Urinary organs, 201 ; figure of, 202.
Urine, 208; composition of, 209; dis-
charge of, 203,511 ; quantity of, 210;
secretion of, 211.
Uriniferous tubules, 203, 205; figure
showing course of, 207.
Utricle, 395; functions of, 421, 541.
Uvula, 254.
Vagus nerve, 98, 182, 538.
Valve (or valves), cardiac, 69; car-
diac, action of, 76; cardiac, figure
of, 70,71, 72, 73; ileo-cascal, 274;
ileo-ccecal, figure of, 276; semilu-
nar, 72; venous, 59; ofVieussens,
523-
Valvulae conniventes, 279.
Vaso-constrictor nerves, 93.
Vaso-dilator nerves, 96.
Vaso-motor centre, 94, 514, 538 ; dia-
gram of, 97.
Vaso-motor nerves, 90, 93, 231 ; course
of, 95-
Vein (or veins) , 22 ; and arteries, 56 ;
coronary, 65 ; hepatic, 65, 236 ; in-
terlobular, 238; intralobular, 237;
portal, 65, 235; pulmonary, 63;
structure of, 59; figure showing
structure of, 60; valves of, 59;
valves of, figure of, 61.
Velum interpositum, 523, 529.
Vena cava inferior, 63, 65.
Vena cava superior, 63.
Ventilation, 191.
Ventricles, of brain, first, 524; ot
brain, second, 524; of brain, third,
INDEX
577
523; of brain, fourth, 521 ; of brain,
fifth, 524; of brain, lateral, 524; of
brain, lateral, diagram of, 524; of
heart, 68 ; of larynx, 358.
Ventriloquism, 420, 465.
Vermiform appendix, 276.
Vertebrae, 7, 15.
v'ertebral column, figure of, 13,
354-
Villi, 112, 280; figure of, 281; struc-
ture of, 280.
Viscera, figure of, 275.
Visual image, inversion of, 433, 466;
referred to an object, 467.
Visual sensorium, 448, 452.
Vital actions, 25.
Vital capacity, 172.
Vitreous humour, 424.
Vocal cords, 155, 356, 357.
Voice, 361 ; accuracy of, 362 ; modu-
lation of, 363; production of, 356;
quality of, 362 ; range of, 362.
Vowels, 364.
Walking, mechanism of, 355, 510,
54°.
Wallerian method, 499.
Wandering cells, 51.
Waste, made good by food, 249; of
the tissues poured into the blood,
193; which leaves the lungs, 175.
Water, absorption of, 288 ; in food,
251 ; quantity given off by lungs,
175 ; camera, 431.
White matter, of brain, 480, 529; of
spinal cord, 480.
Will, 542.
Winking, 545.
Work, of body, 1, 305, 556; unit of,
304 ; and waste, 3.
Yellow spot of eye, 442, 448.
Young-Helmholtz theory of colour
vision, 456.
Zoetrope, 471.
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