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HALLIBURTON'S
HAND-BOOK OF PHYSIOLOGY




KIRKES' HAND-BOOK OF PHYSIOLOGY



HAND-BOOK



OF



PHYSIOLOGY



BY W. D. HALLIBURTON, M.D., F.E.S.

PROFESSOR OF PHYSIOLOGY, KING'S COLLEGE, LONDON.



TWENTIETH EDITION

WITH NEARLY SEVEN HUNDRED ILLUSTRATIONS

INCLUDING SOME COLOURED PLATES.



PHILADELPHIA :

P. BLAKISTON'S SON, & CO.
1012 WALNUT STREET

1905

[Printed in Grec^t B.-:tj.i/i.~[ , - ;,



QP3 A-



I



AUTHOR'S PEEFACE TO THE SIXTH
EDITION



I HAVE taken advantage of the alteration in the size of the page and
of the type which Mr Murray has thought wise to adopt, to make
considerable changes in the present edition. I have throughout,
however, endeavoured to remember that the main object of the work
is to supply students with a complete but elementary text-book.
Sections which treat of what may be termed " advanced work " have
therefore been made as brief as possible, and have been inserted in
small print. The student on reading the book for the first time will
find it best to omit these passages. When he has mastered the
continuous story told in the large text, he will then be able to study
what is given in small type.

During the past few years two important advanced text-books
have made their appearance ; one published in this country, under the
editorship of Professor Schafer, F.E.S., and the other in America,
under the editorship of Professor Howell. I am much indebted to
both of these for assistance in bringing this book up to date. I have
also to thank Professor Schafer for allowing me to copy several of
the instructive new diagrams which have appeared in his Essentials
of Histology.

The parts of this book which have undergone most revision are
those relating to the nervous system and to the circulation of the
blood. Under the latter head I have devoted some space to those
elementary principles of physics which underlie what is often called
hsemodynamics. Experience in teaching has shown me that although
students may have previously received instruction in physics and



vii



6839



Vlll PREFACE

chemistry, they are not as a rule capable of applying their knowledge
to the elucidation of physiological problems. Hence my present
attempt to supply them with the necessary aid. My friend Professor
T. G-regor Brodie has made a special study of the subject of hsemo-
dynamics, and I owe him my sincerest thanks for the assistance he
has given me in revising the part of the present edition which deals
with the circulation.

W. D. HALLIBUBTOK

KING'S COLLEGE, LONDON,
1904.



AUTHOR'S PREFACE TO THE PRESENT
EDITION

IN the year that has elapsed since the publication of the last
edition, I have again subjected the book to a thorough revision. The
only parts, however, which have been materially altered are the
chapters relating to the special senses, and to the generative organs
and development. I am much indebted to Dr C. S. Myers, Lecturer
on Experimental Psychology at this College, for his valued help in
revising the account given of the special senses; and to Professor
Arthur Kobinson, now of Birmingham University, for great assistance
in rewriting the sections relating to generation and development. In
former editions the chick has been largely taken as the type of a
developing vertebrate animal ; now the main descriptions relate to the
mammalian embryo. This has involved the disappearance of numer-
ous old illustrations, and the introduction of as many as thirty-six
new ones.

W. D. HALLIBUBTON.

KING'S COLLEGE, LONDON,
1905.



CONTENTS



CHAPTER I

PAGE

INTRODUCTORY ......... 1

Definition of the Science of Physiology ..... 1

Physiological Methods ....... 3

The Organs, Tissues, and Cells of the Body .... 4

Animal and Vegetable Cells ....... 5



CHAPTER II

THE ANIMAL CELL ........ 8

Protoplasm ......... 8

Nucleus . . . . . . . . .10

Attraction Sphere . . . . . . . .12

Protoplasmic Movement .... 12

Cell division ......... 16

The Ovum . . 20



CHAPTER III

EPITHELIUM ......... 22

Classification of Epithelium ....... 22

Pavement Epithelium ....... 22

Cubical, Spheroidal, and Columnar Epithelium . . . .25

Ciliated Epithelium . . . . . . . .27

Ciliary Motion ........ 29

Transitional Epithelium ....... 30

Stratified Epithelium . . . . . . . .31

Nutrition of Epithelium ....... 33

Chemistry of Epithelium ....... 33



X CONTENTS

CHAPTER IV

PAGE

THE CONNECTIVE TISSUES ........ 35

Areolar Tissue . . . . . . . .36

Fibrous Tissue . . . . . . . .41

Elastic Tissue ......... 43

Adipose Tissue . . . . . . . .43

Retiform Tissue ........ 46

Adenoid or Lymphoid Tissue ...... 47

Basement Membranes ....... 47

Jelly-like Connective Tissue ....... 48

CHAPTER V

THE CONNECTIVE TISSUES continued . . . . . .49

Cartilage ......... 49

Bone .......... 54

Ossification ......... 59

Teeth .......... 64

The Blood 76

CHAPTER VI

MUSCULAR TISSUE ......... 78

Voluntary Muscle . . . . . . . .79

Red Muscles ......... 87

Cardiac Muscle . . . . . . . .87

Plain Muscle ......... 87

Development of Muscular Fibres . . . . . .88

CHAPTER VII

NERVE .......... 90

Structure of . . . . . . . . .90

Terminations of ........ 95

Development of . . . . . .96

CHAPTER VIII
IRRITABILITY AND CONTRACTILITY . . . . . .99

CHAPTER IX

CONTRACTION OF MUSCLE Summary . . . . . .105



CONTENTS Xi

CHAPTER X

PAGE

CHANGE IN FORM IN A MUSCLE WHEN IT CONTRACTS . . . .107

Instruments used ........ 107

Simple Muscle Curve . . . . . . . .116

The Muscle-Wave 118

Effect of two Successive Stimuli . . . .119

Effect of more than two Stimuli* ...... 120

Tetanus ......... 121

Voluntary Tetanus ........ 121

CHAPTER XI

EXTENSIBILITY, ELASTICITY, AND WORK OF MUSCLE .... 125

CHAPTER XII
THE ELECTRICAL PHENOMENA OF MUSCLE ..... 133

CHAPTER XIII

THERMAL AND CHEMICAL CHANGES IN MUSCLE . . . . .147

Fatigue ......... 150

Rigor Mortis ......... 153

Chemical Composition of Muscle ...... 154

CHAPTER XIV

COMPARISON OF VOLUNTARY AND INVOLUNTARY MUSCLE . . . 158

CHAPTER XV

PHYSIOLOGY OF NATIVE . . . . . . . .161

Classification of Nerves . . . . . . .161

Investigation of Nerve Functions . . . . . .164

Degeneration of Nerve . . . . . . .164

Roots of the Spinal Nerves . . . . . . .168

Changes in Nerve during Activity . . . . . .171

Nerve Impulses ........ 172

Crossing of Nerves . . . . . . . .173

Chemistry of Nerve . . . . . . . .175

CHAPTER XVI

ELECTROTONUS . . . . . . . . .179



Xll CONTENTS

CHAPTER XVII

PAGE

NEUVE CENTRES ......... 190

Structure of Nerve-Cells . . . . . . .192

The Significance of Nissl's Granules ..... 200

Classification of Nerve-Cells ....... 203

Law of Axipetal Conduction ..... 204

CHAPTER XVIII

THE CIRCULATORY SYSTEM . ...... 206

The Heart ......... 206

Course of the Circulation . . . . . .213

Arteries ......... 214

Veins ......... 216

Capillaries ........ 219

Lymphatic Vessels ....... 221

CHAPTER XIX

THE CIRCULATION OF THE BLOOD ...... 226

CHAPTER XX

PHYSIOLOGY or THE HEART . . . . . . .231

The Cardiac Cycle . . . . . . . .231

Action of the Valves of the Heart ... ... 233

Sounds of the Heart ........ 234

Coronary Arteries ........ 237

Cardiographs ......... 237

Intracardiac Pressure . . . . . . .240

Frequency of the Heart's Action ...... 244

Work of the Heart ..... .244

Innervation of the Heart . . . . . . 247

The Excised Heart . . . . . . . .252

CHAPTER XXI

THE CIRCULATION IN THE BLOOD-VESSELS ..... 259

Use of the Elasticity of the Vessels .261

Blood-pressure ........ 263

Velocity of the Blood-Flow . . . . . . .278

The Time of a Complete Circulation . . . . .285

The Pulse 287

The Capillary Flow . 293

The Venous Flow ........ 296

The Vaso-motor Nervous System . . . . . .297

Plethysmography ........ 307

Pathological Conditions ....... 310

Local Peculiarities of the Circulation . . . . 311



CONTENTS Xlll



CHAPTER XXII

PAGK

LYMPH AND LYMPHATIC GLANDS ...... 314

Composition of Lymph ....... 314

Lymphatic Glands ... .... 315

Lymph Flow ......... 317

Relation of Lymph and Blood . ' . . . . .318

Formation of Lymph . . . . . . . .318

Osmotic Phenomena . .321



CHAPTER XXIII

THE DUCTLESS GLANDS ........ 328

Spleen ......... 329

Haemolymph Glands ........ 333

Thymus ......... 334

Thyroid 335

Parathyroids ......... 337

Supra-renal Capsules ........ 338

Pituitary Body ........ 341

Pineal Gland ......... 341

Coccygeal and Carotid Glands ...... 342



CHAPTER XXIV

RESPIRATION ......... 343

Respiratory Apparatus ....... 343

Respiratory Mechanism . . . . . . .351

Nervous Mechanism of Respiration . . . . . . 360

Special Respiratory Acts ....... 364

Effect of Respiration on the Circulation ..... 366

Asphyxia ......... 370

Effects of Breathing Gases other than the Atmosphere . . . 373

Alterations in the Atmospheric Pressure ..... 374

Chemistry of Respiration ....... 374



CHAPTER XXV

THE CHEMICAL COMPOSITION OF THE BODY ..... 386

Carbohydrates ....... 386

Fats 393

Proteids ......... 395

The Polarimeter ........ 404

Ferments 405



XW CONTENTS

CHAPTER XXVI

PAGE

THE BLOOD . . . . . . . . , . 409

Coagulation of the Blood ....... 412

Plasma and Serum ........ 414

Blood-Corpuscles ...... .418

Blood Platelets ........ . 423

Development of the Blood-Corpuscles . . . . .425

Chemistry of the Blood-Corpuscles ...... 428

Haemoglobin ......... 429

Immunity ......... 439

CHAPTER XXVII
THE ALIMENTARY CANAL ....... 445

CHAPTER XXVIII

FOOD .......... 459

Milk . . . . . . . . .461

Eggs .465

Meat .......... 466

Flour 467

Bread .......... 468

Cooking of Food 468

Accessories to Food ........ 469

CHAPTER XXIX

SECRETING GLANDS *....... 470

Electrical Variations in Glands ...... 473

CHAPTER XXX

SALIVA .......... 474

The Salivary Glands . . . 474

Secretory Nerves of Salivary Glands . . . . .476

The Saliva ......... 479

CHAPTER XXXI

THE GASTRIC JUICE ........ 481

Composition ......... 483

Innervation of the Gastric Glands ...... 485

Action of Gastric Juice . . . 486



CONTENTS XV



CHAPTER XXXII

PAGE

DIGESTION IN THE INTESTINES ....... 490

The Pancreas ........ 490

Composition and Action of Pancreatic Juice .... 491

Secretory Nerves of the Pancreas ...... 493

The so-called Peripheral Reflex Secretion of the Pancreas . . 494

The Succus Entericus . . . . . .495

Bacterial Action . . . . . . .498

Leucine and Tyrosine ....... 499

Extirpation of the Pancreas ....... 500



CHAPTER XXXIII

THE LIVER ......... 502

Functions ......... 507

Bile 508

Glycogenic Function of the Liver . . . . . .514

Nerves of the Liver . 518



CHAPTER XXXIV

THE ABSORFFION OF FOOD . . . . . . .519

CHAPTER XXXV

THE MECHANICAL PROCESSES OF DIGESTION ..... 525

Mastication ......... 525

Deglutition ......... 526

Movements of the Stomach ....... 528

Vomiting ......... 530

Movements of the Intestines ....... 531

CHAPTER XXXVI

THE URINARY APPARATUS ....... 535

Nerves of the Kidney ....... 543

Activity of the Renal Epithelium ...... 546

Work done by the Kidney ....... 547

Extirpation of the Kidneys ....... 548

Passage of Urine into the Bladder ...... 548

Micturition 549



xvi CONTENTS

CHAPTER XXXV1J

PAGE

THE URINE ......... 551

Urea .......... 552

Ammonia ......... 559

Uric Acid ......... 560

Hippuric Acid ........ 562

Creatinine ......... 563

Inorganic Constituents of Urine . . . . . .564

Urinary Deposits ........ 567

Pathological Urine ........ 570

CHAPTER XXXVIII
THE SKIN . 574

CHAPTER XXXIX

GENERAL METABOLISM ........ 583

Discharge of Carbon ....... 586

Discharge of Nitrogen . . . . . . .587

Balance of Income and Discharge in Health . . . .587

Inanition or Starvation ....... 589

Exchange of Material in Diseases . . . . . .592

Luxus Consumption . . . . . . . .594

CHAPTER XL

ANIMAL HEAT ......... 598

Regulation of the Temperature of Warm-blooded Animals . . 603

CHAPTER XLI

THE CENTRAL NERVOUS SYSTEM ....... 606

CHAPTER XLII
STRUCTURE or THE SPINAL CORD ...... 608

CHAPTER XLIII
THE BRAIN ......... 622

CHAPTER XLIV

STRUCTURE OF THE BULB, PONS, AND MID-BRAIN .... 626

The Cranial Nerves . 640



CONTENTS XV11

CHAPTER XLV

PAGE

STRUCTURE OF THE CEREBELLUM ....... 648

CHAPTER XLVI

STRUCTURE OF THE CEREBRUM ....... 652

Histology of the Cortex . > . . . . . .656

The Convolutions ........ 662

CHAPTER XLVII

FUNCTIONS OF THE SPINAL CORD ....... 667

The Cord as an Organ of Conduction . . . . .667

Reflex Action of the Cord . 669

Reflex Action in Man ....... 671

Spinal Visceral Reflexes ... ... 676

CHAPTER XLVIII

FUNCTIONS OF THE CEREBRUM ....... 678

Removal of the Cerebrum ....... 678

Localisation of Cerebral Functions ...... 679

Function and Myelination ....... 692

Association Fibres and Centres ...... 693

Sleep . . 697

CHAPTER XLIX

FUNCTIONS OF THE CEREBELLUM ....... 702

The Semicircular Canals 706

CHAPTER L
COMPARATIVE PHYSIOLOGY OF THE BRAIN . . . . .710

CHAPTER LI

SENSATION . . . . . . . . . .714

CHAPTER LII

CUTANEOUS SENSATIONS . . . . . . . .719

Tactile End Organs . .719

Localisation of Tactile Sensations ...... 723

Varieties of Cutaneous Sensations ...... 725

The Kmsesthetic Sense . . 728



XV111 CONTENTS

CHAPTER LIII

PAGE

TASTE AND SMELL ......... 729

Taste .......... 729

Smell 734

CHAPTER LIV

HEARING .......... 738

Anatomy of the Ear ........ 738

Physiology of Hearing ....... 745

CHAPTER LV

VOICE AND SPEECH ........ 751

Anatomy of the Larynx . . . . . . .751

Movements of the Vocal Cords . . . . . .757

The Voice ......... 758

Speech ......... 760

Defects of Speech ........ 761

CHAPTER LVI

THE EYE AND VISION ........ 764

The Eyeball ......... 765

The Eye as an Optical Instrument . . . . . .776

Accommodation ........ 780

Defects in the Eye ........ 784

Functions of the Iris . . . . . . . . 788

Functions of the Retina ....... 789

The Ophthalmoscope ....... 792

The Perimeter ........ 795

Colour Sensations . . . . . . . .796

Changes in the Retina during Activity . . . . .801

Various Positions of the Eyeballs ...... 805

Nervous Paths in the Optic Nerves ...... 808

VisualJudgments ........ 809

CHAPTER LVII
TROPHIC NERVES . . ....... 813

CHAPTER LVIII

THE REPRODUCTIVE ORGANS . . . . . . .816

Male Organs 816

Female Organs . . . . . . . .821



CONTENTS
CHAPTER LIX



INDEX



PAGE



DEVELOPMENT ...... 827

The Ovum ......... 827

Maturation of the Ovum .... 828

Impregnation ..... 830

Segmentation ..... 831

The Decidua and Foetal Membranes .... 836

Development of the Foetal Appendages and Membranes . . .839

Development of the Framework of the Body . . 843
Development of the Vascular System . . . . .849

Development of the Nervous System ..... 858

Development of the Alimentary Canal .... 867

Development of the Respiratory Apparatus .... 870

Development of the Genito-urinary Apparatus . . 871



881



"FAHRENHEIT
and
CENTIGRADE
SCALES.


MBASUR

FRENCH IN'
LENGTH.
10 Setres 1 - 39*37 English

iSSBttS/fr'i^SW^


B MB NTS.

rO ENGLISH.

A grain equals about 1-16 gram.,
a Troy oz. about 31 grams.,
a Ib. Avoirdupois about Kilogrm.,
and 1 cwt. about 50 Kilogrms.


F.

500
401
392
383
374
356
347
338
329
S20
311
302
284
275
266
248
239
230
212
203
194
176
167
140
122
113
105
104
100


C.

260
205
200
195
190
180
175
170
165
160
155
150
140
135
130
120
115
110
100
95
90
80
75
60
50
45
40-54
40
37-8


CAPACITY.


1 decimetre ^ _ 3 . Q37 . h


1,000,000 cubic centimetres / metre.


iSdK/^^"^


1 cubic decimetre ^
or V = 1 litre.
1000 cubic centimetres J
OR,
ONE LITRE = 1 pt. 15 oz. 1 dr. 40.

(For simplicity, Litre is used to signify
1 cubic decimetre, a little less than 1
English quart.)
Decilitre (100 c.c.) = 3i oz.
Centilitre (10 c.c.) = 2| dr.
Millilitre (1 c.c.) = 17 m.
Decalitre = 2* gals.
Hectolitre = 22 gals.
Kilolitre (cubic metre) = 27J bushels.
A cubic inch = 16*38 c.c. ; a cubic foot
= 28-315 cubic dec., and a gallon =
4 54 litres.


1 centimetre \ = '3937 or about
10 millimetres / (nearly f inch).
1 millimetre = nearly ^ inch.

OR,

ONE METRE = 39-37079 inches.

(It is the ten-millionth part of a quarter
of the meridian of the earth.)
1 Decimetre = 4 in.
1 Centimetre = & in.
1 Millimetre = & in.
Decametre = 32-80 feet.
Hectometre = 109-36 yds.
Kilometre = 0-62 mile.


One foot = 3-047 Decimetres.
One yard = 0-91 of a Metre.
One mile = 1-60 Kilometre.


CONVERSION SCALE.
To convert GRAMMES to OUNCES avoir-
dupois, multiply by 20 and divide by 567.
To convert KILOGRAMMES to POUNDS,
multiply by 1000 and divide by 454.
To convert LITRES to GALLONS, mul-
tiply by 22 and divide by 100.
To convert LITRES to PINTS, multiply
by 88 and divide by 50.
To convert MILLIMETRES to INCHES,
multiply by 10 and divide by 254.
To convert METRES to YARDS, multi-
ply by 70 and divide by 64.


98-5
95
86
77
68
50
41
32
23
14
+ 5
- 4
-13
-22
-40
-76


36-9
35
30
25
20
10
5

- 5
-10
-15
-20
-25
-30
-40
-60


WEIGHT.

(One gramme is the weight of a cubic
centimetre of water at 4 C. at Paris.)

1 gramme ~\
10 decigrammes 1 = 15*432349 grs.
100 centigrammes j r nearly 15).
1000 milligrammes J


1 decigramme ^ .,
10 centigrammes I TJ^ftES?
100 milligrammes \ than ^ grailu


SURFACE MEASUREMENT.
1 square metre = about 1550 sq. inches
(or 10,000 sq. centimetres, or 10*75 sq. ft.)
1 sq. inch = about 6-4 sq. centimetres.
1 sq. foot = ,, 930 ,,


1 centigramme ) = rather more
10 decigrammes f than & grain.


ENERGY MEASURE.
1 kilogrammetre = about 7*24 ft. pounds.
1 foot pound = -1381 kgm.
1 foot ton = ,, 310 kgms.


1 deg. F. = '54C.
1-8 = 1C.
3-6 - 2C.
4-5 = 2-5 C.
5-4 = 3C.


1 milligramme rather more
than 2 ;| n grain.
OR,

1 Decagramme = 2 dr. 34 gr.
1 Hectogrm. = 3J oz. (Avoir.)
1 Kilogrm. = 2 Ib. 3 oz. 2 dr. (Avoir.)


HEAT EQUIVALENT.
1 kilocalorie = 424 kilogrammetres.


To convert de-
grees F. into de-
grees C., subtract
32, and multiply
byf.


ENGLISH ]
Apothecaries Weight.
7000 grains = 1 Ib.
Or,
437-5 grains = 1 oz.


MEASURES.

Avoirdupois Weight.


To convert de-
grees C. into de-
grees F., multiply
by f , and add 32.


16 oz. = 1 Ib.
28 Ibs. = 1 quarter.
4 quarters = 1 cwt.
20 cwt. = 1 ton.



Measure of 1 decimetre, or 10 centimetres, or 100 millimetres.



1 .




1




1 1 1 1


1


1


1
1


' 1



10




Cranium.

7 Cervical Vertebrae.

Clavicle.
Scapula.

12 Dorsal Vertebra.
Humerus.

5 Lumbar Vertebrae.



Ilium.
Ulna.
Radius.
Pelvis.



Bones of the Carpus.

Bones of the Meta-
carpus.

Phalanges of Fingers.



Femur.



r Patella.



Tibia.
Fibula.



_ Bones of the Tarsus.

Bones of the Meta-
tarsus.
Phalanges of Toes.



THE SKELETON (AFTER HOLDEN).




Anterior Su-
perior Spine
of the Ilium



Symphysis Pubis.



DIAGRAM OF THORACIC AND ABDOMINAL REGIONS.



A . Aortic Valve.
M. Mitral Valve.



P. Pulmonary Valve.
T. Tricuspid Valve.



HANDBOOK OF PHYSIOLOGY



CHAPTER I

INTRODUCTORY

BIOLOGY is the science that treats of living things, and it is divided
into two main branches, which are called respectively Morphology
and Physiology. Morphology is the part of the science that deals
with the form or structure of living things, and with the problems
of their origin and distribution. Physiology, on the other hand,
treats of their functions, that is, the manner in which their individual
parts carry out the processes of life. To take an instance : the eye
and the liver are two familiar examples of what are called organs ;
the anatomist studies the structure of these organs, their shape, their
size, the tissues of which they are composed, their position in the
body, and the variations in their structure met with in different
parts of the animal kingdom. The physiologist studies their uses,
and seeks to explain how the eye fulfils the function of vision, and
how the liver forms bile, and ministers to the needs of the body in
other ways.

Each of these two great branches of biological science can be
further subdivided according as to whether it deals with the animal
or the vegetable kingdom; thus we get vegetable physiology and
animal physiology. Human Physiology is a large and important
branch of animal physiology, and to the student of medicine is
obviously the portion of the science that should interest him most.
In order to understand morbid or pathological processes it is neces-
sary that the normal or physiological functions should be learnt first.
Physiology is not a study which can be put aside and forgotten when
a certain examination has been passed; it has a most direct and
intimate bearing in its application to the scientific and successful
investigation of disease. It will be my endeavour throughout the
subsequent pages of this book to point out from time to time the
practical relationships between physiology and pathology.



2 INTRODUCTORY [CH. I.

Human physiology will be our chief theme, but it is not a portion
of the great science that can be studied independently of its other
portions. Thus, many of the experiments upon which our knowledge
of human physiology rests have been performed principally on certain
of the lower animals. In order to obtain a wide view of vital pro-
cesses it will be occasionally necessary to go still further afield, and
call the science of vegetable physiology to our assistance.

The study of physiology must go hand in hand with the study of
anatomy. It is impossible to understand how the body or any part
of the body acts unless we know accurately the structure of the
organs under consideration. This is especially true for that portion
of anatomy which is called Microscopic Anatomy or Histology.
Indeed, so close is the relationship between minute structure and
function that in this country it is usual for the teacher of
physiology to be also the teacher of histology. Another branch
of anatomy, namely, Embryology, or the process of growth from
the ovum, falls also to some extent within the province of the
physiologist.

But physiology is not only intimately related in this way to its
sister science anatomy, but the sciences of chemistry and physics
must also be considered. Indeed, physiology has been sometimes
defined as the application of the laws of chemistry and physics to
life. That is to say, the same laws that regulate the behaviour of
the mineral or inorganic world are also to be found operating in the
region of organic beings. If we wish for an example of this we may
again go to the eye ; the branch of physics called optics teaches us,
among other things, the manner in which images of objects are pro-
duced by lenses; these same laws regulate the formation of the
images of external objects upon the sensitive layer of the back of the
eye by the series of lenses in the front of that organ. An example
of the application of chemical laws to living processes is seen in
digestion ; the food contains certain chemical substances which are
acted on in a chemical way by the various digestive juices in order to
render them of service to the organism.

The question arises, however, is there anything else ? Are there
any other laws than those of physics and chemistry to be reckoned
with ? Is there, for instance, such a thing as " vital force " ? It
may be frankly admitted that physiologists at present are not able to
explain all vital phenomena by the laws of the physical world ; but
as knowledge increases it is more and more abundantly shown that
the supposition of any special or vital force is unnecessary ; and it
should be distinctly recognised that when, in future pages, it is
necessary to allude to vital action, it is not because we believe in any
specific vital energy, but merely because the phrase is a convenient
one for expressing something that we do not fully understand, some-



CH. I.] INTRODUCTORY 3

thing that cannot at present be brought into line with the physical
and chemical forces that operate in the inorganic world.

Physiology proper may be conveniently divided into three main
branches :

1. Chemical physiology ; or the application of chemistry to living

processes.

2. Physical physiology; oi> the application of physics to living

processes.

3. The physiology of the nervous system where the application of

such laws is at present extremely difficult.

But just as there is no hard-and-fast line between physiology and
its allies pathology, anatomy, physics, and chemistry, so also there is



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