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LESSONS

ELEMENTARY PHYSIOLOGY

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THE MACMILLAN COMPANY

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MACMILLAN & CO., Limited

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LESSONS

IN ELEMENTARY

PHYSIOLOGY

THOMAS H. HUXLEY, LL.D., F.R.S.

BY

ENLARGED AND REVISED EDITION

THE MACMILLAN COMPANY
1917

A II rights reser^ied

iQTlo

First Edition printed 1866. Second^ 1868. Reprinted 1869, 1870, 1871,
March and May, 1872. Third, October. 1872. Reprinted i&fi, 1874, '875,
1876, 1878, 1879, 1881, 1883, January, Fehruary, May, September {twice),
and November, 1884, Fourth, 1885. Reprinted 1886, i888, 1890, 1892,
January and September, 1893, 1896. Fifth, 1900.
Reptrinted 1902, 1905, 1908, 1911.

Si.vth Edition 1915. Reprinted September, 1915; Aug. .it, 1916;
December, 1916; May, I9'i7.

\^\^

PREFACE

In approaching the revision of " Huxley's Physi-
ology," my feelings haA^e been similar to those of an
architect to whom is entrusted the restoration of a
historic building designed by a master hand.

Written by Huxley, the book was revised, and in
fact almost rewritten, by Foster. The former was
as great a writer as any scientist of his time, the
latter may almost be said to have created English
Physiology.

To " restore " the work of these men from the
dilapidations made by two decades of scientific
progress is the task now entrusted to me. The
sense of responsibility with which I approach it is,
if possible, heightened by the affection which I have
for the memory of Foster, who was my master.

I have faithfully left untouched any portion of
the fabric in which there was not an actual flaw;

3^2 3^

PREFACE

but where the structuie needed repair, it seemed to
me due not only to the readers of the book but to
the memory of the author, that the repair should
be thorough, substantial, and simple. Such have
been the principles on which I have tried to carry
out my work. I have been greatly helped by
Mrs. Thacker, Fellow of Newnham College, who
has read through my proofs, and made valuable
suggestions.

JOSEPH BARCROFT.

King's College, Cambridge,
November 24, 1914.

CONTENTS

LESSON I

A GENERAL VIEW OF THE STRUCTURE AND
FUNCTIONS OF THE HUMAN BODY. Pp. 3-29

§ 1. The Work of the Body as a JJ'hole.

2. The General Build of the Body.

3. The Tissues generally.

4. The Skeleton.

5. The Erect Position.

6. Sensory Organs.

7. Alimentary Organs.

8. Girculatory Organs.

9. Excretory Organs.

10. Respiratory Organs.

11. Coordinating Action of the Nervous System,

12. Life and Death.

13. Modes of Death.

LESSON n

THE ORGANS OF CIRCULATION. Pp. 30—92

Part I. — The Blood Vascular System and the
Circulation
§ 1. The Capillaries.

2. The Arteries and Veins.

3. The General Arrangement of Blood-vessels in the

Body.

CONTENTS

§ 4. 27te Heart.

5. The Valves of the Heart.

6. The Beat of the Heart.

7. The Action of the Valves.

8. The Working of the Arteries.

9. The Cardiac Impulse.

10. The Sounds of the Heart.

11. Blood-pressure.

12. The Pulse.

13. The Rate of Blood Flow.

14. The Nerooiis Control of the Arteries. Vaso-motor

Nerves.

15. The Vaso-motor Centre.

16. Vaso-dilator Nerves.

17. Nervous Control of the Heart. Cardiac Nerves.

18. The Proofs of the Circulation.

19. The Capillary Circulation.

20. Inflammation.

Part II. — The Lymphatic System and the Circulation
OF Lymph

§ 1. The General Arrangement of the Lymphatics.

2. The Origin and Structure of Lymphatics.

3. The Structure and Function of Lymphatic Glands.

4. Causes which lead to the Movements of Lymph.

LESSON III
THE BLOOD AND THE LYMPH. Pp. 93—122

1. Microscopic Examination of Blood.

2. The Red Corpuscles.

3. The White Corpuscles.

4. Blood Platelets.

5. The Origin and Fate of the Corpuscles.

CONTENTS

§ 6. rhe Physical Qualilies of Blood.

7. The General Composition of Blood.

8. The Proteins of Plasma.

9. The Clotting of Blood.

10. The Qjiantity and Distribution of Blood in the Body.

11. The Functions of the Blood.

12. Lymph: its Character and Composition.

13. The Mode of Fornudion of Lymph.

LESSON IV
EESPIRATION. Pp. 123—169

§ 1. The Gases of Arterial and Venous Blood.

2. The Nature and Essence of Respiration.

3. The Organs of Resjnratioii.

4. The Thorax caid Lungs.

5. The Movements of Besjnration.

6. The Amount of Aii' Respired.

7. The Changes of Air in Respiration. '

8. The Atruxint of Waste which leaves the Lungs.

9. The Natiire of the Respiratory Changes in the Lungs

and Tissues.

10. The Nervous Mechanism of Respiration.

11. Influence of Blood-Supply on the Respirato)-y Centre.

Dyspncea and Asphyxia.

12. llie Influence of Respiration on the Circulation.

13. Ventilation.

LESSON V

THE SOURCES OF LOSS AND OF GAIN TO THE BLOOD.
Pp. 170—223.

§ 1. Genercd Revieiv of the Gain and Loss.

2. The Kidneys.

3. The Structure of a Kidney.

CONTENTS

§ 4. The Composition of Urine and Chemistry of Urea.

5. The Secretion of Urine.

6. Th^ History of Urea.

7. The Structure of the Skin. Nails and Hairs.

8. The Composition and Quantity of Stveat.

9. A Comparison of the Lutigs, Kidneys, and Skin.

10. The Secretion of Siveat and its Nervous Control.

11. Animal Heat : its Production and Distribution.

12. Regulation of Body-temperature by Alterei Loss of

Heat.

13. Regulation of Body-temperature by Altered PrO'

duction of Heat.

14. The Temperature of Fever.

15. The Liver.

16. The Work of the Liver. Its Glycogenic Function.

17. The Thyroid Body or Gland.

18. The Supi-arenal Bodies.

19. The Thymus Gland.

20. The Spleen.

21. The Pituitary Body.

LESSON VI

THE FUNCTION OF ALIMENTATION. Pp. 284-284

Part I. —Digestion and Absorption

1. Waste made good by Food.

2. Food and Food-stuffs.

3. The Purpose and Means of Digestion.

4. Mastication and Sicallowing.

5. The Salivary Glands.

6. The Nervous Control and Nature of Salivary Se-

cretion.

7. The Composition and Action of Saliva.

8. Soluble Ferrnents or Enzymes.

9. The Structure of the Stomach.

CONTENTS

§ 10. The Nature and Action of Gastric Juice.

11. The General Arrangement and Structure of the In-

testines.

12. The Structure of the Villi.

13. The Structure of the Pancreas and its Changes during

Secretion.

14. The Nature and Action of Pancreatic Juice.

15. The Nature and Action of Bile.

16. The Changes Food Undergoes in the Intestines.

17. Absorption froiri the Intestines.

Part II.— Food assd Nutrition

§ 1. Some Aspects of Nutrition.

2. Some Statistics of Nutrition.

3. The Economy of a Mixed Diet.

4. The Effects of the several Food-stuffs.

5. The Erroneous Division of Food-stuffs into Heat-

producers and Tissue-fonners.

6. The Income and Expenditure of Energy.

LESSON vn

MOTION AND LOCOMOTION. Pp. 285-339

§ 1. The Source of Active Forcer and the Organs of

Motion. J! m-

2. Ciliated Epitnelium and Actwn oj ^^vx.

3. The Structure of Unstriated Muscle.

4. The Str^tctnre 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.

e". The General and Minute Structure of Bone.

xli CONTENTS

10. The Mechanics of Motion. The System of Levers.

11. The Joints of the Body.

12. TTie Various Movements of the Body.

13. The Mechanics <f Locomotion.

14. The Mechanism of the Larynx.

15. The Voice.

16. Speech.

LESSON VIII
SENSATIONS AND SENSORY ORGANS. Pp. 340-397

§ 1. MoxKment the Result of Rcjiex Action.

2. Sensations and Consciousness.

3. The Special Senses.

4. The General Plan of a Sense-organ.

5. The Skin as a Sense-Orgun.

6. The Mnscidcir Sense.

7. T/ie Sense of Taste.

8. The Sense of Smell.

9. V'/ie Ear and the Sense of Hearing.

10. Sensations governing co-ordination and equilibrium,.

LESSON IX
THE ORGAN OF SIGHT. Pp. 398-436

1. The General Structwre of the Eye.

2. The Eye as a Water-Camera.

3. The Mechanism of Accommodation.

4. The Limits of Accommodation, Use of Spectacles.

5. The Muscles of the Eyebcdl.

6. The Protective Appendages of the Eye.

7. The Structure of the Retina.

8. The Sensation of Light.

CONTENTS

§ 9. !Z7i€ '' Blind Spot."

10. The Duration of a Luminous Impression.

11. Sensations of Light Produced vnthout the Action of

Light.

12. The Functions of the Rods and Cones.

13. Sensations of Colour and Colour-blindness.

LESSON X

THE COALESCENCE OF SENSATIONS WITH ONE

ANOTHER AND WITH OTHER STATES OF

CONSCIOUSNESS. Pp. 437-461

§ 1. Sensations may be Simple or Composite.

2. Judgments are Delusive, not Sensations.

3. Subjective Sensations.

4. Delusions of Judgment.

5. The Incersion of the Visucd Image.

6. Single Objects give rise to Single Images.

7. The Judgment of Distance and jSi'te by the Brightness

and >ire of Visual Images.

8. Judgment of Form by Shadows.

9. The Judgment of Changes of Form.

10. Single Vision with Tiro Eyes. Corresponding Points.

11. The Judgment of Solidity.

LESSON XI

THE NEEVOirS SYSTEM AND INNERVATION.
Pp. 452—534

§ 1. The General Arrangement of the Nervous System,.

2. The Investing Membranes of the Cerebro- Spinal

System.

3. The Arrangement and General Structure of the Spinal

Cord and the Roots of the Spinal Nerves.

LIST OF ILLUSTRATIONS

t"IG. PAGE

1. Diagrammatic Sections of the Human Body . . . 11

2. The Vertebral Column 15

3. Side View of the Skull 16

4. The Pelvis . . 17

5. The Bones of the Limbs. Front View. Left

Limbs . . 18

6. Diagram illustrating the Attachments of some of

the most important Muscles which keep tlie
Body in the Erect Posture 19

7. Capillaries 30

8. Transverse Section of part of the Wall of a

medium-sized Artery, magnified 75 diameters.
(Schafer) 33

9. Transverse Section of an Artery and of a corre-

sponding Vein 35

10. The Valves of Veins 36

11. Diagram of the Heart and Vessels, with the

Course of the Circulation 37

12. Heart of Sheep, as seen after Removal from the

Body, lying upon the two Lungs 39

13. Transverse Section of the Chest, with the Heart

and Lungs in Place 41

14. The Heart, Great Vessels, and Lungs. (Front

View) 43

15. Cardiac Fibre Cells . 44

16. Right Side of the Heart of a Sheep ...... 46

17. Left Side of the Heart of a Sheep (laid open) . . 48

18. View of the Orifices of the Heart from below . . 49

b

LIST OF ILLUSTRATIONS

KIO. PAGE

19. The Orifices of the Heart seen from above 50

20. Transverse Section through the middle of the

Ventricles of a Dog's Heart in Diastole and in
Systole. (After Hesse) 52

21. Diagram to illustrate the Action of the Heart . 54

22. Diagi-am to illustrate the Position of the Vaso-

Motor Centre 71

23. Diagram to illustrate the Position of the Cardio-

Inhibitory Centre 76

24. Portion of the Web of a Frog's Foot seen under a

low Magnifying Power 78

25. Very small Portion of Fig. 24 very highly

Magnified 81

26. The Lymphatics of the Front of the Right Arm 85

27. The Thoracic Duct 87

28. Connective Tissue Fibres 88

29. Epithelioid (Jells Lining the Lymphatics .... 89

30. Diagrammatic Representation of a Lymphatic

(iland seen in Section. (After Sliarpey) ... 90

31. Red and White Corpuscles of the Blood

Magnified ... 95

32. Crystals of Haemoglobin. (After Funke) . .99

33. Successive Forms assumed by Colourless Cor-

puscles of Human Blood 100

34. Colourless Corpuscles of Blood. (Hardy) 101

35. Network of Filaments left after washing away

the Colouring Matter from a thin, flat Clot of
Blood. (Ranvier) 109

36. To illustrate a Simple E.xperiment on Diffusion . 121

37. A Section of the Moutli and Nose taken vertically . 130

38. Back View of the Neck and Thorax of a Human

Subject 131

39. Magnified View of the Lungs . . 132

40. Ciliated Epithelium Cells from the Trachea of the

Rabbit, highly magnified. (Schiifer) ... 134

41. Diagram of the Thorax sliowing the Position of

the Heart and Lungs 136

42. Transverse Section of the Chest, with the Heart

and Lungs in place 137

LIST OF ILLUSTRATIONS

FIG. PAGE

43. Sternum viewed from the Front 138

44. The Bony Walls of the Thorax 139

45. View of Four Ribs of the Dog with the Inter-

costal Muscles 140

46. Diagram of Models illustrating the Action of the

External and Internal Intercostal Muscles . . 141

47. The Diaphragm of a Dog viewed from the Lower

or Abdominal Side 144

48. Diagrammatic Sections of the Body ..... 147

49. Diagram to illustrate the Position of the Respira-

tory Centre 160

50. The Kidneys, Ureters, &c 176

51. Longitudinal Section of the Human Kidney . . 178

52. A Malpighian Capsule (highly magnified) .... 179

53. Circulation in tlie Kidney 180

54. Diagrammatic View of the Course of the Tubules

in the Kidney 181

55. Tjpes of the two chief kinds of Cells in the

Tubules of the Kidney 182

56. Blood Vessels of Kidney. (Cadiat) 183

57. Diagram to show the Structure of the Skin . . . 191

58. Coiled End of a Sweat-Gland 193

69. Longitudinal and Transverse Sections of a Nail 194

60. A Hair in its HairSac . 195

61. Part of the Shaft of a Hair inclosed Mithin its

Root-Sheaths 197

62. Section of the Skin showing the Roots of the Hairs

and the Sebaceous Glands 198

63. The Liver turned up and viewed from Below . 208

64. Section of Partially Injected Liver magnified 210

65. A Section of Part of the Liver .... .211

66. Termination of Bile Duct at Edge of Lobule . . 213

67. The Spleen with the Splenic Artery 221

68. A Diagram to illustrate the Structure of Glands 228

69. A Section of the Mouth and Nose taken vertical!}' . 230

70. Vertical and Horizontal Section of a Tooth . . . 232

71. Sectional Views of a Tooth 234

72. A Dissection of the Right Side of the Face . . . 237

73. Sections of the Submaxillary Gland 238

LIST OF ILLUSTRATIONS

no. PAGE

74. Sections of the Parotid Gland 239

75. Changes in the Parotid (Jland during Secr-eting

Activity 240

76. The Stoniacli Laid Open 245

77. One of the Glands which Secretes Gastric Juice . 246

78. The Stomach at Various Stages in Digestion . . 249

79. The Viscera of a Rabbit 250

80. The Alimentary Canal in the Abdomen .... 252

81. Diagram to show how tiie Wall of the Abdomen

is made up ...... • 253

82. Diagram of Two Villi and an adjacent Gland of

Lieberkiihn. (Hardy) 257

83. A Portion of the Pancreas of a Rabbit . . . 259

84. Columnar Ciliated Epithelium Cells from the

Human Nasal Membrane 286

85. A Fibre-cell from the Plain, Non-striated Muscu-

lar Coat of the Intestine 288

86. Fasciculi of Striated Muscle cut across . ... 290

87. Capillaries of Striated Muscle 291

88. To illustrate the Structure of a Striated Muscular

Fibre . • 292

89. A Muscular Fibre (of Frog) ending in Tendon . . 294

90. A Muscle Nerve Preparation 300

91. Longitudinal Section of the Shaft of a Human

Femur or Thigli-bone . . 304

92. The Bones of the Upper Extremity with the

Biceps Muscle 306

93. The Course of the Digastric Muscle 307

94. Transverse Section of Compact Bone 311

95. Transverse Section of Bone, liiglily magnified (300

diameters) 312

96. Representation of the three kinds of Levers . . . 315

97. The Right Knee-joint 318

98. A Section of the Hip-joint 321

99. Longitudinal and Vertical Section through the

Elbow- joint 322

100. The Atlas and Axis 323

101. The Bones of the Right Forearm in Supination

and Pronation 324

LIST OF ILLUSTRATIONS

KIG. PAGE

102. TheVertebral Column in the upper part of the Neck 327

103. Diagram of the Larynx 330

104. Vertical and Transverse Section through the

Larynx 331

105. The Parts surrounding the Glottis 332

106. View of the Human Larynx from above as

actually seen bj' the aid of the instrument called

the Laryngoscope 334

107. Diagram of a model illustrating the action of the

Levers and Muscles of the Larynx 336

108 Diagram to illustrate the Paths of Reflex Action. 342

109. Tactile Corpuscle within a Papilla of the Skin of

the Hand. (Ranvien 349

110. End-bulb from the Human Conjunctiva. (Long-

worth) 350

111. A Pacinian Corpuscle from a Cat's Mesentery.

(Ranvier) . .... 351

112. Outlines of Heat Spots and Cold Spots (After

Goldscheider) ..... . ■ 354

113. The Mouth widely opened to show tiie Tongue and

Palate 359

114. Diagram of a Circumvallate Papilla, and of Taste-

Buds 360

115. Vertical Longitudinal Sections of the Nasal Cavity 363

116. A Transverse and Vertical Section of the Osseous

Walls of the Nasal Cavity ..... 364

117. Cells of Olfactory Epithelium. (Max Schultze); . 366

118. Transverse Section through the Side Walls of the

Skull to show the Parts of Ear 368

119. A Diagram illustrative of the Relative Positions

of the Various Parts of the Ear 369

120. A Section through the Axis of the Cochlea, magni-

fied three diameters 375

121. Section of Coil of Cochlea 377

122. The Membrane of the Drum of the 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 Tem-
poral Bone ... 378

123. The Membranous Labyrinth, twice the natural size 391

LIST OF ILLUSTRATIONS

FIO. PAOE

124. Diagram to illustrate the endings of tVie Auditory

Nerve in the Membranous Labyrinth and
Cochlea 392

125. Longitudinal Section of Ampulla, cutting the Crest

Cro.sswise 394

126. Horizontal Section of the Eyeball 400

127. Pigment Cells from the Choroid Coat 401

128. View of Front Half of the Eyeball seen from

Behind . 403

129. The Formation of an Image on the Retina . 408

130. Diagram of the Images of a Candle-Flame seen by

Reflection from the Surface of the Cornea and

the two Surfaces of the Lens 410

131. The Changes in the Lens in Accommodation ... 411

132. The Muscles of the Right and Left Eyeball ... 414

133. Front View of the Right Eye 416

134. A Front View of the Left Eye 417

135. The Eyeball divided Transversely in the Middle

Line and viewed from the Front 418

136. Diagrammatic Views of the Nervous and Con-

nective Elements of the Retina 420

137. Diagram in illustration of the Nervous Structure

of the Retina 421

138. A Diagrammatic Section of the Macula Lutea, or

Yellow Spot . 423

1.39. Pigmented Epithelium of the Human Retina.

(Max Sehultze) 424

140. Diagram of a Rod and of a Cone 425

141. Diagram of Images on the Retina 428

142 Diagram, as seen from the Front, of the Spinal

Cord and the Spinal Nerves .... ... 455

143. Transverse Section of one-half of the Spinal Cord 457

144. The Spinal Cord .458

145. Transverse Section of a Medium-sized MeduUated

Nerve 459

146. To illustrate the Structure of Nerve Fi])res . 462

147. A Neuroglia-Cell from the White Matter of the

Spinal Cord. (Schiifer^ 466

148. A Large Nerve Cell from the Anterior Horn of

the Spinal Cord 467

LIST OF ILLUSTRATIONS

FIQ. PAGE

149. Diagram of a Typical Cell from the Grey Matter

of the Spinal Cord. (Sherrington) 468

150. Transverse Sections of half the Spinal Cord . . . 470

151. A Nerve Cell from the Ganglion on the Posterior

Root of a Spinal Nerve 471

152. Diagram to illustrate Experin)ents in proof of the

Functions of the Spinal Nerve-Roots and of the
Ganglion on the Posterior Root 473

153. Arrangement of a Nerve and Galvanometer for

Experiments on the Electrical Properties of a
Nerve 479

154. Arrangement of Nerve, Muscle and Lever for

determining the Velocity of a Nervous Impulse 481

155. Diagram to show the Position of Tracts of Ascend-

ing Degeneration in the White Matter of the
Spinal Cord at the Level of the Fifth Cervical
Nerve 491

156. Diagram to show the Position of Tracts of

Descending Degeneration in the White Matter
of the Spinal Cord at the same Level as in Fig.
155 .... 492

157. Diagram to illustrate the Distribution of the

Spinal Nerves and their Relationship to the
Ganglia of the S^'mpathetic Sj'stem .... 496

158. Pale Non-meduUated Fibres from the Pneumo-

gastric Nerve. (Ranvier) 497

159. Side View of the Brain and Upper Part of the

Spinal Cord 499

160. The Base or Under-Surface of the Brain .... 501

161. View of the Right Half of a Human Brain . . . 503

162. Diagram of the Ventricles of the Brain 505

163. Diagram of a Horizontal Section of the Brain.

(After Hirschfeld and Leveille) ... 508

164. Diagram to illustrate the Structure of the Super-

ficial Grey Matter of the Cerebellum . . . 512

165. Diagrammatic Figure to illustrate the Structure of

a Typical Section of the Cerebral Cortex . , 514

166. A Diagram illustrating the Superficial Origin of the

Cranial Nerves 518

LIST OF ILLUSTRATIONS

FIO. PAGE

167. Diagram to illustrate the Decussation of Fibres in

the Optic Chiasma 519

168. Diagram of Outer Surface of the Right Cerebral

Hemisphere 529

169. Diagram of the Inner Surface of the Right Hemi-

sphere 529

170. Diagram of Outer Surface of Right Cerebral

Hemisphere 531

171. Diagram of Inner Surface of the Right Cerebral

Hemisphere 531

172. Diagram of the Course of the Crossed Pyramidal

Tract from the (Motor) Cerebral Cortex to the
Spinal Cord 533

173. Diagram of the Ovum 637

174. The Successive Division of the Mammalian Ovum

into Blastomeres . 538

175. Two Epithelial Scales from the Interior of the

Mouth . . 542

176. Hyaline Cartilage. A thin Section highly magni-

fied 548

177. A Small Portion of a Section of Articular Cartilage

(Frog) very highly magnified 549

178. Section of White Fibro-Cartilage. (Hardy) . . . 551

179. Section of Yellow P^lastic Cartilage. (Hardy) 552

180. Connective Tissue Fibres 554

181. A Small Bundle of Connective Tissue 555

182. Elastic Fibres of Connective Tissue, forming a

loose Network 556

183. Two Connective Tissue Corpuscles 557

184. Elastic Fibres teased out and magnified. (Sharpey) 559

185. Adipose Tissue 560

LESSONS

IN

ELEMENTARY PHYSIOLOGY

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LESSONS

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 renund 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

I£ B 2

4 ELEMENTARY PHYSIOLOGY [less.

out of the body is hotter and damper tlian 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 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 delicacies of the art of experiment.

The sum of the facts and generalizations 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 ai-e brought about, con-
stitutes the science of Human Physiology. An elementary
outline of this science, and of so much anatomy as is inci-
dentally 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 ascer-
tained if the bodies of men were as easily procured, exa-
mined, and subjected to experiment, as those of animals.

Suppose a chamber with walls of ice, through which a
current of pure ice-cold air passes ; the walls of the cham-
ber 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 doing 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 requii-ed to lift his weight
as high and as often as he has raised himself at every step.
But, in addition, a certain (juantity of the ice will be
melted, or converted into water ; showing that the man
has given off heat in abundance. Furthermore, if the air
which enters the chamber be made to pass through lime-
water, it will cause no cloudy white precipitate of car-

WORK AND WASTE

bonate of lime, because the quantity of carbonic acid ^ in
ordinary air is so small as to be inappreciable 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 tlie heat, is given ofi' by the
man.

Again, even if tlie air be quite dry as it enters the cham-
ber (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,
therefore, is derived from the body.

After the expiration of the hour during which the ex-
periment has lasted, let the man be released and weighed
once more. fTe 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
unlimited 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 sensations called hunger and thirst. To still
these cravings, to restore the weight of the body to its
former amount, to enable 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 three things, and with three only.
These are, firstly, fresh air ; secondly, drink — consisting
of water in some shape or other, however much it may be
adulterated ; thirdly, food. That compound known to

• By "carbonic acid" we mean "carbonic acid gas." This should in
strictness be called carbon dioxide (CO2), carbonic acid being the com-
pound of this with water, H2CO3. But for simplicity's sake, and
because the expression " carbonic acid " is in general use and is
generally understood to stand for carbon dioxide, we shall use it
throughout this book.

6 ELEMENTARY PHYSIOLO(JY less.

chemists as Protein matter (Losson IIT.), and which con-
tains carbon, hydrogen, oxygen, nitrogen, and sulphur,
must form a part of this food, if it is to sustain life in-
definitely ; and fatty, starchy, or saccharine, i.e. carbo-
hydrate 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 excrement itions matter, having simply passed
through the alimentary canal without undergoing much
change, and without ever being incorporated into the
actual substance of the body. But, under healthy con-
ditions, and when only so much food as is necessary is
taken, no important proportion of either protein 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 liave determined that these products which are
tlirown out of the body and are called excretions, con-
taui if taken altt)gether, 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.i And care-
ful investigation of the air which leaves the chamber in
the imaginary experiment described above would show,
not only that it has gamed 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

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 car-
bonic acid, and a variable quantity of watery vapour. The constitvient
of the atmosphere, argon, present in small quantities, is here reckoned
in with the nitrogen. (See Lesson IV.)

WORK AND WASTE

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
gaining 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 ascend in the intervals, oscillating to
equal distances on each side of the average position,
which would never be maintained for longer than a few
minutes. There is, therefore, no such thing as a sta-
tionary condition of the weight of the body, and what we
call such is simply a condition of variation within narrow
limits — a condition in which the gains and losses of the
numerous daily transactions of the economy balance one
another.

Suppose this diurnally-balanced physiological state to
be reached, it can be maintained only so long as the quan-
tity 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 in-
creased by a definite amount, which cannot be made good
unless a proportionate amount of extra food be supplied
to him. Let 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
compounds containing comparatively little oxygen ; as
constantly, the elements of the food (whether before or

8 ELEMENTARY PHYSIOLOGY [less.

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 propor-
tioned 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 lieat 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 per-
formance 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 leg's ; and legs and arms again are subdivided
by theii- jt»ints into parts which obviously exhibit a rough
correspondence — thigh and upper arm, leg and fore-
arm, ankle and wrist, fingers and toes, plainly
answering to one another. And the two last, in fact, are
so similar that they receive the .same name of digits ;
while the several joints of tlie 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
lengthways 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 thti trunk, the cut faces of

I THE BUILD OF THE BODY 9

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 sepai-ate 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 communication between the
dorsal canal and the ventral cavity.

The sjjinal 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 dia-
phragm (Fig. 1, D), which is concave towards the abdo-
men, and convex towards the thorax. The alimentary
canal (Fig. 1, Al.) traverses these cavities from one end
to the other, piercing the diaphragm. So does a long
double sei'ies of distinct masses of nervous substance,
which are called ganglia, are connected together by
nervous cords, and constitute the so-called sympathetic
system (Fig. 1, Sy.). 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 abdomen ; the liver,
the pancreas or "sweetbread" and the spleen. The
thorax incloses, besides its segment of the alimentary'
canal and of the sympathetic, 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,

10 ELEMENTARY PHYSIOLOGY. less.

or cavity of the skull, opens into the spinal canal. It
contains a mass of nervous matter called the brain, which
is continuous with the spinal cord, the brain and the
spinal cord together constituting what is termed the
cerebro-spinal system (Fig. 1, G.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
CBSOphagUS) opens.

Thus, the study of a longitudinal section shows us
that the human body is a double tube, the two tubes being
completely separated by tlie 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 cere-
bro-spinal axis ; the ventral tube contains the alimentary
canal, the sympathetic nervous system, the heart, and the
lungs, Ijesides other organs.

Transverse sections, taken perpendicularly to the axis
of the vertebral coluu\n, or to that of the skull, show still
more clearly that this is the fundamental structure of the
human body, and that the great apparent difference be-
tween the head and the trunk is due to tlie different size
of the dorsal cavity relatively to the ventral. In the head
the former cavity is very large in proportion to tlie size of
the latter (Fig. 1, B) ; in the thorax, or abdomen it is
very small (Fig. 1, C).

The limbs contain no such chambers as are found in
the body and the head ; but with the exception of certain
brandling tubes filled witli fluid, which are called blood-
vessels and lymphatics, are solid or semi-solid,
throughout.

3. The Tissues generally. — Such being the general
character and arrangement of tlie jiarts of the human
body, it will next be well to consider into what constitu-
ents 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

THE TISSUES

11

Fio. 1.

A. A diagrammatic section of the human body taken vertically
through the median plane. C.S. the cerebni-spinal nervous system;
N, the cavity of the nose ; ^f, that of the mouth ; Al. Al. the alimentary
canal represented as a simple straight tube ; H, the heart ; O, the dia-
phragm ; 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.

12 ELEMENTARY PHYSIOLOGY less.

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 homy matter, and is called the
epidermis ; and the deeper part, the dermis, which
is dense and fibrous (Lesson V.). The epidermis, if
wounded, neither gives rise to pain nor bleeds. The
dermis, under like circumstances, is very tender, and
bleeds freely. A practical distinction is drawn between
the two in shaving, in the course of whicli operation the
razor ought to cut only epidermic 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 extei-ior, but at the margins of the apertures of the
body it seems to stop, and to be replaced by a layer
which is much redder, more sensitive, bleeds more readily,
and which keeps itself continually moist by giving out a
more or less tenacious fiuid, called mucus. Hence, at
these apertures, the skin is said to stop, and to be re-
placed by mucous membrane, which lines all those
interior cavities, such as tlie alimentary canal, into which
the apertures open. But, in truth, the skin does not
really come to an end at these points, but is dii'ectly con-
tinued into the mucous membrane, which last is simply
an integument of greater delicacy, but consisting funda-
mentidly of the same two layers — a deep, fibious layer,
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 continuation,
which lines the alimentary canal.

The dermis, and the deep, vascular layer, which answers
to it in the mucous membranes, are chiefly made up of

THE TISSUES 13

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, ^
because it is the great connecting mediuni by which the
different parts of the body are held together. Thus it
passes from the dermis between all the other organs, en-
sheathing 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 com-
plete 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
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
presence 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 pro-
perty of changing its dimensions — shortening and be-
coming thick in proportion to its decrease in length — when

1 Every such constituent of the body, as epidermis, cartilage or
muscle, is called a •' tissue." CSee Lesson XII.)

14 ELEMENTARY PHYSIOLOGY

influenced by the will as well as by some other causes,
called artificial stimuli, and of returning to its original
form when let alone. This temporary change in the di-
mensions 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 whicli su[)port 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 substance, oi'dinarily known as "gristle."
The harder are the bones, which are masses of tissue,
hardened by being impregnated with phosphate and
carbonate of lime. They are animal 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.

More than 200 separate bones are ordinarily reckoned
in the human body, though the actual ninnber of distinct
bones varies at different periods of life, many bones which
are separate in youth becoming united I-' 'gether in old age.
Thus there are originally, as we have seen, thirty-three
separate bodies of vertebrae in the spinal column, and the
upper twenty-four of these commonly remain distinct
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-
inif vertebne often run into one bony mass called the
coccyx.

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

Fio. 2. — ^The Vertebral Colt-mn

A, side view, left side ; B, back view ; C 1-7, cervical vertebrae ; D 1-12,
dorsal (thoracic) vertebrte ; L 1-5, lunibar vertobr?B ; S, sacrum ; 0,
coccyx ; sp, spinous processes ; tr, ti'ansverse processes.

16

ELEMENTARY PHYSIOLOGY

the breast-bone or sternum (see Lesson IV.). In the
girdle which supports the shoulder, two bones are always
distinguishable as the scapula and the clavicle. The
pelvis, to which the legs are attached, consists of two
separate bones called the ossa innominata in the adult ;
but each os innoininatum is separal)le into three (called
pubis, ischium, and ilium^ in the young.

Fin. 3.— SinR View of the SKfix.

/, frontal bone ; -p, parietal ; o, occipital ; (i, wing of sphenoid ; «, flat
part of temporal ; c, m, st, other jiarts of temporal ; (in, opening of ear
or external auditory canal ; z, process of temporal passing to j, the cheek
bone ; mx, the upper jaw bone ; n, nasal bone ; f, lacrymal ; pt, part of
jphenoid. The lower jaw bone is drawn downwards ; r//, its process
which articulates with the temporal ; cr, its process to which muscles of
mastication are attached ; lli, tij, hyoid bone.

There are thirty bones in each of the arms, and the
same number in each of the legs, counting the patella,
or knee-cap.

THE SKELETON

17

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 contact. The cartilages which thus form part
of a joint are called articular cartilages, and their free
surfaces, by which tliey rub against each other, are lined
by a delicate synovial membrane, which secretes a lubri-
cating Huid — the synovia.

Fifi. 4.— The Pelvis.

Sac. sacrum ; Cocc. coccyx; il, <s>, pu, ilium, ischium, pubis, three parts
of the innominate or hip bone ; acct, acetabuhini cup for head of femur ;
5 L. V, &th lumbar vertebra.

5. The Erect Position. — Though tlie bones of the skele-
ton are all strongly enough connected togetherby ligaments
and cartilages, the joints play so freely, and the centre of
gravity of the body, when erect, is so high up, that it is im-
possible to make a skeleton or a dead body support itself
in the upright position. That position, easyasitseems, is the
result of the contraction of a multitude of muscles which

C

^1 — %^^^^^^

hum

.rod:

rnetat

Fio.

-The Bones of the Limhs. Front View. Left Limbs.

A, the innominate and bones of the leg ; ?))(), innominate ; fern, femur;
pat, patella or knee-cap ; </';, tibia ; fih, fibula ; tdv, (seven) tarsal bones ;
mctat, (five) metatarsal bones ; phi, (fourteen) phalanges ; B, the seapula,
clavicle, and bones of the arm ; cl, clavicle or collar bone ; scap, scapula
or shouiclvr bone ; /ou/i, humerus ; rdi/, radius ; ii/h, ulna; <:•«)•, (eight)
?arpal bones ; inctac, (five) metacarpal bones ; phi, (fourteen) phalanges.

THE ERECT POSITION

19

oppose and balance one another. Thus, the foot affording
the surface of support, the muscles of the calf (Fig. 6, I)

Pig. 6. — A DiAORAM FLLUSTRATINQ THE ATTACHMENTS OF SOME OF THE

most important muscles whioh keep the bodt 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.

1. 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, b. Those of the front
of the neck. The.se tend to keep the body from falling backwards. The
arrows indicate the direction of action of the muscles, the foot being
fixed.

c 2

20 ELEMENTARY PHYSIOLOGY less.

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 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 accurately proi)ortioned action of a vast number of
muscles. What is it that makes tliem 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 re-
laxed and powerless. What has ha[)pened 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 unconsciousness, 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 (jf all to the
muscles, but an influence has been exerted upon a some-
thing 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 eff"ect as a blow.

These obser\ations might lead to the conclusion that it
is the mind which directly governs the muscles, but a
little further in(iuiry will show that such is not the case.
For people have been so stabbed, or shot in the back, as
to cut the sjiinal cord, without any considerable injury
to other jjarts : and then they have lost the power of stand-
ing upright as much as before, thougli their minds may
have remained perfectly clear. And not only have they

THE ORGANS 21

lost the power of standing upright under these circum-
stances, but they no longer retain any power of either feel-
ing 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
remains in the body. For if the soles of the disabled feet
be 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 move-
ments even more powerful than those which the will
could produce in an uninjured 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 pro-
foundly disorganise it altogether, all these phenomena
cease ; tickling the soles, or sending galvanic shocks along
the spine, will produce no eftect 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 contraction, 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 »ery different char-
acters. 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 circumstances, to produce these sensations. But
only very few and comparatively small portions of the

22 ELEMENTARY PHYSIOLOGY less.

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 par-
ticular kinds of vibi-ations, are able so to affect those
regions. These very limited parts of the body, which put
us in relation with particular kinds of substances, or forms
of force, are what are termed sensory organs. There
are two such organs for sight, t^vo for hearing, two for
smell, and one, or more strictly speaking two, for taste.

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 with-
out, we must consider the means by which all this Avonder-
ful 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,
implies consumption either of their own substance, or of
something else. And as the organism can make nothing,
it muse 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 gro.ss, 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 lirought about. The organs wdiich
convert food into nutriment are the organs of alimenta-
tion ; those which distribute nutriment all over the
body are organs of circulation ; those which get rid of
the waste products are organs of excretion.

7. Alimentary Organs. — The organs of alimentation
are the mouth, pharynx, gullet, stomach, and intestines,
with their appendages. What they do is, first to receive
and grind the food. They then act upon it with chemical
agents, of which they ])o.ssess 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 containing nutritious

THE ORGANS 23

matters in solution or suspension, and innutritious dregs
or faeces.

8. 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 sub-
stance 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 chey are connected, are all full of
li(juid. 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
rhythmically, or at regular intervals. By means of these
contractions the blood Avhich 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 nutritive
matters which are the result of the process of digestion,
traverses the very thin layer of soft and permeable
tissue which -separates tlie 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 torrent of the circulation, the blood, thus
charged ^\-ith nutritive 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 heart, loaded with usele-ss and
injurious excretions, which sooner or later take the form
of water, carbonic acid, and urea.

9. Excretory Organs. — These excretionaiy matters

24 ELEMENTARY PHYSIOLOGY less,

are separated fi-om 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
elimination 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 oft' a good deal of water. The
duty of the kidneys is to exci'ete urea (together with other
substances, chiefly salts), but at the same time they pass
away a large quantity of water and a trifling amount of
carbonic acid ; while thi; skin gives off' nuich water, some
amount of carbonic acid, and a certain (juantity of .saline
matter, among which a trace of urea may be, sometimes,
though very d(»ul)tfully, ])resent.

10. Respiratory Organs.- Finally the lungs play a
douljle part, l)eing not merely eliminators of waste, or
excretionary products, but importers into the economy of
a substance which is not exactly either food or drink, but
something as important as either, — to wit, oxygen.

As the carbtniic acid (and watei') is passing from the
blood through the lungs into the external air, oxygen is
passing from the air through the lungs into the blood,
and is carried, as Ave shall see, by tluj blood to all parts of
the body. We have seen (p. 6) that the waste which

I OXIDATION AND HEAT PRODUCTION 25

leaves the body contains more oxygen tlian 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 continually burning, some mure
rapidly and fiercely than others. And this burning,
though it is carried on in a peculiar manner, so as never
to give rise to a tiame, yet nevertheless produces an
amount of heat which is as efficient as a fire to raise the
blood to a temperature of about 37' C. (98'6' F.) ; and this
hot fluid, incessantlj' renewed in all parts of the economy
by the torrent of the circulation, warms the body, as a
house is warmed by a 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 perhajjs 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.

11. Coordinating Action of the Nervous System. —
These alimentary, circulatory or distril>utive, excretory,
and respiratory (oxidational) processes would however be
worse than useless if they were not kept in strict propor-
tion one to another. If the state of physiological balance
is to be maintained, not only must the ijuantity of aliment
taken be at least equivalent to the quantity of matter
excreted ; but that aliment nmst be distributed with due

26 ELEMENTARY PHYSIOLOGY less.

rapidity to the seat of each local waste. The circulatory
system is the commissariat of the physiological army.

Again, if tlie body is to* be maintained at a tolerably
even temperature, while tliat of the air is constantly vary-
ing, the condition of the hot-water ai)paratus 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 nutritious from innutritions matters, and
to exert the nuiscular actions needful for seizing, killing,
and cooking ; guides the hand to the mouth, governs all
the movements of tlie jaws and of the alimentary canal,
and determines the due supply of the juices necessary for
digestion. By it, the working of the heart can be
properly adjusted and the calibres of the distributing
pipes can be regulated, so as indirectly to govern the
excretory and oxidational processes, which are also
additionally and more directly affected by other actions
of the nervous system.

The nervous system has often been compared to a
telephone system with its exchange (the brain and spinal
cord) and its wires (the tibres) which go to and fro in
communication with the various instruments in which
the messages are given or heard (sensory or motor nerve
endings). In addition to this system there is another
coordinating system which may be compared to a wireless
system of communication. The messages do not run
along fibres but are chemical substances which are
produced* in one organ of the body and carried by the
blood to another organ, perhaps in some very remote
part. They throw the particular organ for which they
are destined into activity without aflfecting other parts of
the body. Such messengers are called hormones.

12. Life and Death. — The various functions which

I LIFE AND DEATH 27

have been thus briefly indicated constitute the greater
part of what are called the vital action'^ of the human
body, and so long 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) Iiocal 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 inces-
santly dying and being cast off, to be replaced by others
which are, as constantly, 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, occa-
sionally, local death occurs on a larger .scale, as the re-
.sult 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 hy their
regeneration. Not only all the forms of epidermis and
epithelium, 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 bo'iy as
a whole, and death of the tissues. By the former term is
implied 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 occurring until after an interval,
which is sometimes considerable.

28 ELEMENTARY PHYSIOLOGY less.

Hence it is that, for a while after what is ordinarily
called death, the muscles of an executed criminal may
be made to contract by the application of proper stimuli,
and the heart may even be excised and made to beat for
a considerable time. The muscles are not dead, though
the man is.

13. 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
medlllla oblongata (see Lesson XI.) 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 tlie secondary action of
the injury to that organ upon the lungs or the heart.
The functions of the brain cease, when either respiration
or circulation is at an end. But if circulation and respira-
tion are kept up artificially, the brain may be removed
without causing death. On the other hand, if the blood
be not aerated, its circulation by the heart cannot pre-
serve 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.

With the cessation of life, the everyday forces of the
inorganic world no longer remain the servants of the

I CHANGES OF MATTER 29

bodily frame, as they were during life, but become its
masters. Oxygen, the slave of the living 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 otfer 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 tlie 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
Ceesar may now enter into the composition of Csesar the
negro in Alabama, and of Caesar the house-dog in an
English homestead.

And thus there is sober truth in the words which
Shakespeare puts into the mouth of Hamlet —

" Imperial Caesar, dead and turned to clay.
Might stop a hole to keep the wind awuy ;
Oh that tiiat earth, which kept the world in awe,
Should patch a wall, to expel the winter's flaw ! "

LESSON n

THE ORGANS OF CIRCULATION

Part I.-

-The Blood Vascular System and the
Circulation

1. The Capillaries. —Almost all parts of the body are

vuscvlar ; that is to say, they are traversed by minute and
very close-set canals, which open into one another so as to
constitute a small-meshed network, and confer upon these
parts a spongy texture. The canals, or rather tubes, are
provided with distinct but very delicate walls, composed of
what at first sight appears to be a structureless membrane,

d—

Fio. 7. — Capillaries.

A, surface view ; B, cut lengthwise ; C, cut across ; e c, enduthelial cells ;
n, nuclei ; d, the lumen or bore.

BLOOD CAPILLARIES 31

but is in reality formed of a number of thin scales, called
"cells," cemented together at their edges (Fig. 7, A, e.c) ;
in each of these cells lies a small oval body (Fig. 7, n),
termed a nucleus.

These tubes are the blood capillaries. They varv in
diameter from 14 ^ to 16 /x (^J^q to j^^^ of an iiich)i ;
they are sometimes disposed in loops, sometimes in long,
sometimes in wide, sometimes in narrow meshes ; and the
diameters of these meshes, or, in other words, the inter-
spaces between the capillaries, are sometimes hardly wider
than the diameter of a capillary, sometimes many times as
wide (see Figs. 25, 39, 58, and 64). These interspaces are
occupied by the substance of the tissue which the capillaries
permeate, so that the ultimate anatomical components of
every part of the body are, strictly speaking, outside the
vessels, or extm-vasadar.

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 trans-
parent 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-va.scular, 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 ex-
ceedingly thin that their fluid contents readily exude
through the delicate membrane of which they are com-
posed, 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

1 The ordinarily used unit of histological nieas\irement is fn\,g of a
millimeter, and is usually represented by the Greek letter fi, which
sta:ids for micro-millimeter. Since one millimeter is very nearly ^ of
an inch iJi = ^,s„ of an inch.

32 ELEMENTARY PHYSIOLOGY

tubes, with thicker walla, 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 com-
municate 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 capillai-ies constitutes an important
difference between the capillaries and the small arteries
and veins ; for the walls of the latter are thus rendered
far less permealile to fluids, and that thorough irrigation
of the tissues, 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 thickei", but also more complex,
being composed of several coats, one, at least, of which
is muscular. 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 cells
which form the entire thickness of the wall of the capillaries.
Outside this comes the nuiscular coat, consisting of a thin
layer of muscle-fibres of the kind called plain or non-
striated (see Lesson VII.), made uj) 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
(see Lesson XII.). The walls of the smallest arteries
are thus seen to be made up essentially of three layers ;
the inner cellular, the middle muscular, the outer of connec-
tive tissue. The larger arteries are similarly composed of
three layers or coats, which are. however, thicker and

n STRUCTURE OF ARTERIES 33

more complex in structm-e. The inner coat now consists
of thin flattened cells lying on a distinct and special layer
of elastic tissue of variable thickness. The middle coat,
to which the thickness of the arterial wall is chiefly due,
consists of alternating layers of plain muscle fibres,
lying transversely to the axis of the vessel, and of elastic
fibres which as a rule run lengthwise. The outer coat,
also of considerable thickness, is made up of fibi'ous con-
nective tissue, mixed with fibres of elastic tissue.

h-a.-s

Pig. 8. — Transverse Section of part of the Wall of a mepifm-sized
Artery, magnified V5 diameters. (Schaj-er.)

a, epithelial (endothelial) layer of inner coat ; 6, elastic layer (fenes-
trated membrane) of inner coat, appearing in section as a bright line ;
c, muscular layers (middle coat) ; </, outer coat, consisting of connective
tissue bundles, interspersed with connective tissue nuclei, and, es-
pecially near the muscular coat, with elastic fibres cut across.

From the above description of the structure of an artery
we see at once that arteries are strong, muscular and
elastic. The largest arteries are, as a rule, characteris-
tically more elastic than the smaller, while in the latter
the muscular tissue is present in large amount relatively
to the elastic tissue. The significance of this difference
will become apparent later on (see pp. 62 and 65).

The plain muscular fibres in the arterial wall possess
that same power of contraction, or shortening in the long,
and broadening in the narrow, dii-ections, which, as was
stated in the preceding Lesson, is the special property of
muscular tissue. And when they exercise this power, they,

34 ELEMENTARY PHYSIOLOGY less.

of course, narrow the calibre of the vessel, just as squeezing
it with tlie hand or in any other way would do ; and this
contraction may go so far as, in some cases, to reduce the
cavity of the vessel almost to nothing, and to render it
practically impervious.

The state of contraction of those muscles of the small
arteries is regulated, like tliat Of other muscles, by
their nerves ; or, in other words, the nerves supplied
to the vessels determine wlietlier the passage through
these tubes should be wide and free, or narrow and ob-
structed. Thus while the small arteries lose the function,
which the capillaries possess, of directly irrigatnig the
tissues by transudation, they gain that of regulating
the supply of fluid to the irrigators or capillaries
themselves. The contraction, or dilation, of the arteries
which supply a set of capillai'ies, comes to the same result
as lowering or raising the sluice-gates of a system of
irrigation -canals. Thus the one great and all-important
use of the mu.scular tissue of the smaller arteries is
to deteniiine and cantrol the supply of blood to each part of
the body, according to the varying needs of that j)art.

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 princi^jal arterial
and venous trunks with the heart.

(ii) The Structure of a Vein. — The wall of a vein
is structurally similar to that of an artery in so far that it
consists essentially of the same three layers or coats,
but the distinction 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.
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.

THE VALVES OF THE VEINS

35

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.

FiQ. 9.

-Transverse Section of an Artery and of a corrk-
spoNDiNO Vein.

A, artery ; V, vein ; cc, endothelial cells ; m, muscular (middle) coat ;
c, connective tissue (outer) coat ; n, nuclei of endothelial cells.

These valves 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,
while they do not interfere with fluid passing in the oppo-
site direction (Fig. 10). 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

D 2

36 ELEMENTARY PHYSIOLOGY

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.

But if a linger be pressed upon the up{)er part of one
of these veins, and then passed downwai'ds along it, so as
to drive the blood which it contains backwards, sundry
swellings, like little knots, will suddenly make their ap-
pearance at several points in the length of the vein, where
nothing of the kind was visible before. These swellings
are simi)ly 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

Pio. 10. — The Valvks ok Vkins.

C, H, C, H, diagrammatic sections of veins with valves. In the upper
figure the blood is supposed to lie flowing in the direction of the arrow,
towards the heart ; in the lower, back towards the capillaries ; C, cai)illary
side ; H, heart side. A, a vein laid open to show a pair of pouch-shaped
valves.

valve, swinging back towards the wall of the vein, affords
no obstacle to its progress, and the distention caused by
its pressure tlisappears (Fig. 10).

These valves play an important part in determining the
flow of l)lood 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 veins from their exterior. Such pressure tends to
squeeze the blood out of that pare of the vein on which it
is brought to ber.r ; but since the valves only open
towards the heart the blood is thereby driven on in the

-IhJX

Fig. 11. — Di.^r.R\M of the Heart and Vkssels, with the Course or the

ClRCrL.\TIOX, VIEWED FROM BEHIND, SO THAT THE PROPER LEFT OF

THE Observer corresponds with the left side of the Heakt in
THE Diagram.

L.A. left auricle; L.V. left ventricle; Ao. aorta; ^1. arteries to the
upper part of the body ; A'-, arteries to the lower part of the body ; ff.A.
hepatic artery, which supplies the liver with part of its blood ; I'l. veins
of the upper part of the body ; V-. veins of the lower part of the body ;
F".?. vena port;* : 77. r. hepatic vein : V.C.I, inferior vena cava ; V.C.S.
superior vena cava : R.A. right auricle : /?. V. right ventricle; P. A. pul-
monary artery ; Li;. lung ; P. V. pulmonary vein ; Let. lacteals ; Lv. lym-
phatics ; Th.T>. thoracic duct: AL alimentary canal; Lr. liver. The
arrows indicate the course of the blood, IjTnph, and chyle. The vessels
which contain arterial blood have dark contours, while those which
carry venous blood have light contours.

38 ELExMENTARY THYSIOLOGY less. ;i

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 considerocl with that organ.

3. The General Arrangement of Blood-vessels
in the Body. — It will now be desirable ta take a general
view f)f the arrangement of all these different vessels, and
of their relations to the great central organ of the vascular
system — the heart (Fig. 11).

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. 11, I'.C.S. V.C.I.)
termed the superior and the inferior vena cava,
which debouch 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. 11, Ao.), 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. 11, F.A ), springing from
the lower division of the right side of the heart. The
veins of the lungs, on the contrary, open by four ti'unks
int:; the upper part of the left side of the heart (Fig. 11,
P. V. ), by tlie pulmonary veins.

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

Fig. 12. -Heart of Sheef, as seen after Resioval from the Body

LVIXO UPOK THE T^VO LlNOS. ThE PER.CARDirM HAS BEEN CUT A^IV,
BUT NO OTHER DISSECTION MADE. a wax,

RA auricular appendage of right auricle; L.A. auricular appendage
of left auricle ; R.f . ngUt ventricle ; I. V. left ventricle ; & F.C. simerifr
vena cava ; /. V C. mfenor vena cava ; P. A. pulmonary artery ; ^o aorta
^o, innominate branch from aorta dividing into subclavian knd cSd
artenes : L, lung; Tr. trachea. 1, solid cord often present? the remnant
of a once open communication between the puhnoiiarv artery and aorta
2, masses of fat at the bases of the ventricle hiding from view the g^eate^
part of the auncles. 3, line of fat marking thi division be^eS. th^
two ventricles. 4, mass of fat covering end oi trachea between the

40 ELEMENTARY PHYSIOLOGY less.

the great veins of the body, are connected with opposite
sides of the heart ; and the great arteiy of the hnigs and
the great veins of the lungs also with opposite sides of
that organ. (Jn the other hand, the veins of the body
open into the same side of tlie heart as the artery of the
lungs, and the veins of the lungs (jpen into the same side
of the heart as the artery of the body.

The arteries which open into the capillaries of the sub-
stance of the heart are called coronary arteries, and
arise, like the other arteries, from the aoita, 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 ven{« cavte, but pours the
blood which it contains directly into the division of the
heart into which these ven;c cavie open — that is to say,
into the right upper division (Fig. 19, h).

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. 11, V.P.), but this trunk does not open into
the inferior vena cava. On the contrarj', 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. 11, H.A.), branches. From this comnion capillary
mesli-work veins arise, and iniite, at length, into a single
trunk, the hepatic vein (Fig. 11, 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 continuous with the capillaries
of an organ, like an artery. But certain small veins in
the kidney are similarly arranged. (Lesson V.)

THE HEART

41

The shorted 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
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 aliment-
ary canal, spleen or pancreas. It then enteres Istly, the
capillaries of one of these organs ; 2ndiy, the capillaries

Fig. 13.— Transverse Section of the Chest, with the 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 ; 8. C. superior vena cava ;
P. A. pulmonary artery, divided into a branch for each lung ; L.P. R.P.
left and right pulmonary veins; 5)-. bronchi ; R.L. L.L. right and left
lungs ; (E. the gullet or wsophagus ; p. outer bag of pericardium ; pi. the
two layers of pleura ; v. azygos vein.

of the liver ; and, Srdly, after passing tlu-ough the right
side of the heart, the capillaries of the lungs, from which
it returns to the left side and eventually to the aorta.

4. The Heart.— The heart (Figs. 12 and 14), to which
all the vessels in tlie budy have now been directly or

42 ELEMENTARY PHYSIOLOGY

indirectly traced, is an organ, the size of which is usually
roughly estimated as e([ual 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 : and a pointed end which is called
its apex, tinned downwards and forwards, and to the
left side, so as to lie opposite 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 enclosed in a sort of
double bag— the pericardium (Fig. 13, p.). 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 continuous with the otlier half, which l(H)sely
envelopes botli tlie heart and the adiierent lialf of the
bag. Between the two layers of the pericardium, con-
sequently, there is a completely closed, narrow cavity,
lined by an epithelium, and containing in its interior a
small quantity of clear fluid, the pericardial fluid.'

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 altogether upon
the diaphragm for support, inasmuch as the great vessels
whicli issue from or enter it — and for the most part pass
upwards from its base — help to suspend and keejj it in
place.

Tims 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, completely
separated by a fixed partition which extends frt)m the
base to the apex of the heart ; and consequently, having

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 tlie walls of these sacs are fre-
quently termed sr-rnus uteiitOi-anes. The fluid is, however, in reality a
form of lymph. (See Lesson III.)

THE HEART

43

no direct communication with one another. Each <>{
these two great cavities is further subdivided, nut
longitudinally but transversely, by a movable partition.
The cavity above the transverse partition on each side is
called the aiiricle; the cavity below, the ventricle — rit;ht
or left as the case may be.

Each of the four cavities has the same capacity, and is
capable of containing from 4 to 6 cubic inches of water

J5^ ^■^^.■v, ! 'FV.

T^r.

Fig. 14. — The Heart, 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. r. pulnionary veins;
R.L. right lung; L.L. left lung; f..S^ vena cava superior; .^'.C. sub-
clavian vessels ; C. carotids ; 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.

(70 to 100 cubic centimeters). The walls of the auricles
are much thinner than those of the ventricles. The wall
of the left ventricle is much thicker than that of the
right ventricle ; but no such difference is perceptible
between the two auricles (Figs. 16 and 17, 1 and 3).

44 ELEMENTARY PHYSIOLOGY

In fact, as we shall see, the ventricles have more work
to do than the auricles, and the left ventricle more to do
than the right. Hence the ventricles have more muscular
substance than the auricles, and tlie left ventricle than the
right ; and it is this excess of muscular substance which
gives rise to the excess of thickness observed in the left
ventricle.

The muscular fibres of the heart are of a peculiar
nature, resembling those of the chief muscles of the body
in being transversely striated, but diflering from them in
many other respects.

Pio. 15.— Cardiac Fibre Cells.

Two cells isolated from the heart. . n, nucleus ; I, line of junctioa
between the two cells ; p, process joining a similar process of anotliei"
cell. (Magnified 400 diameters.) '■

Cardiac Muscular Tissue. — The muscular tissue of
the heart is intermediate in character between striated and
non-striated muscle. Like the non-striated muscle, it is
composed of cells, each containing a single nucleus, and
possessing no sarcolennna. But the cells (Fig. 15) are
generally short and broad, frequently branched or irregular
in shape, and their substance is more or less distinctly

THE VALVES OF THE HEART 45

striated, like the substance of a striated fibre. A number
of such cells are j-ined by cement substance mto sets of
anastomosing fibres, which are built up in a complex mter-
woven manner into the walls <.f the ventricles and auricles.
The cavities of the heart are lined by a smooth, shmy
membrane called the endocardiTim, which consists of a
layer of connective tissue covered with thin flattened cells
continuous ^^ ith and simUar to those which form the waU
of the capillaries and which line the arteries and veins.
At the junction between the auricles and ventricles,
the apertures of c<jmmunication between theii" cavities,
caUed the auriculo-ventricular apertures, are
strengthened bv fibrous rings of connective tissue. To
these" rings the movable partitions, or valves, between
the auricfes and ventricles, the arrangement of which
must next be c«>nsidered. are attached.

5. The Valves of the Heart.— There are three of
these partitions attached to the circumference of the right
auriculo-ventricular aperture, and two to that of the left
(Figs. 16, 17, 18, 19, t r, m t). Each is a broad, thm, but
very tough and strong triangular fold of connective tissue
(see Lesson XII.) covered by endocardium, attached by
its base, which joins on to its fellow, to the auriculo-
ventricular fibrous ring, and hanging with its point
domiwards into the ventricular cavity. On the right
side there are, therefore, three of these broad, pointed
membranes, whence the whole apparatus is called the
tricuspid valve. On the left .side, there are but two,
which, when detached from aU their connexions but the
auriculo-ventricular ring, l(»ok 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 tendineae, con-
nect them with some column-like elevations of the fleshy
substance of the walls of the ventricle, wliich are termed

46

ELEMENTARY PHYSIOLOGY

papillary muscles (Figs. 16 and 17, pp) ; similar
column-like elevations of the walls of the ventricles, but

RioHT Side of the Heart of a Sheep.

R.A. cavity of right auricle ; S. V.C. superior vena cava ; /. V.C. inferior
vena cava ; (a style has been passed through each of these ;) a, a style
passed from the auricle to the ventriulo through the auriculo-ventricular
orifice ; 6, a style passed into the eonmary vein.

R. F. cavity of right ventricle ; ti\ tv, two flaps of the tricuspid valve ;
tlie third is dimly seen behind thoni, the style <i passing between the
three. Between the two flaps, and attached to them by chordce tendinfce,
is seen a papillary muscle, pp, 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 fiumel in the pulmonary
artery, P. A. One of the pockets of the semiliuiar 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 pulmonary artery.

THE VALVES OF THE HEART 47

having no chordfe tendinefe attached to them, are
called columnae carnese.

It follows, from this arrangement, that the valves
oppose no obstacle to the passage of fluid from the
auricles to the ventricles ; but if any should be forced
the other way, it will at once get between the valve and
the waU of the 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 temlineoi 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 be-
tween the ventricle and the auricle, through which no
fluid can pass. .

Where the aorta opens into the left ventricle, and
where the pulmonary artery opens into the right ventricle
another valvular apparatus is placed, consisting m each
case of three pouch-like valves called the semilunar
valves (Fig. 16, s.v. ; Figs. 18 and 19, Ao. P. A.), which
are similar to those of the veins. Since they are placed on
the same 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
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 aper-
ture. The tricuspid and mitral valves then usually
become closed 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

48

ELEMENTARY PHYSIOLOGY

Fio. 17. — liEFT Side of the Heart of a Shf.ep (f.aid opf.n).

P. y. pulmonary veins opening into the left auricle by four opcniiiprs,
as shown by the stj'les ; a, ;i style p;issed from auricle into vcntriclo
throuyh the auriculo-ventricular orifice ; b, a style passed into the
coronary vein, which, though it has no connexion with the left auricle,
is, from its position, necessarily cut across in thus laying open the
auricle.

in.v. the two flaps of the mitral valve (drawn somewhat diat,''"am-
matically); pp, papillary muscles, belonging as Ijcfore to the part of the
ventricle cut away ; r, a style passed from ventricle in Ao. aox-ta ; Aoi^.
branch of aorta {see l'"ig. 12, Ao) ; P. A. pulmonary artery ; H. V.C. superior
yena cava.

1, wall of ventricle cut across ; 2, wall of auricle cut away around
auriculo-ventricular orifice ; 3, other portions of auiicular wall cut
across ; 4, mass of fat around base of ventricle (see Fig. 12, 2).

THE VALVES OF THE HEART

49

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 : tliat 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 con-

FiG. 18.— View of the Orifices of the Heart from below, the whole
OF THE Ventricles having been cut away.

R.A. V. riglit auriculo-ventricuhr orifice surrouuded bv the three flaps
t.v. 1 t.i-. 2, t.r. 3, of the tricuspid valve ; these are stretched bv weiehts
attached to the chorda tuadinfiK. '

I. A. r. left auriculo-ventricular orifice surrounded in same wav bv the
two flaps, m.v. 1, m.v. 2, of mitral valve ; P.A. the orifice of pulmonarv
artery, the semilunar valves having: met and closed together • Ao the
orifice of the aorta with its semilunar vahes. The shaded portion
leading from K.A. V. to P. A., represents the funnel seen in Fig 16

50

ELEMENTARY THYSIOLOGY

tents of the ventricles pass easily into the aorta and
pulmonary artery, while none can be made to tiavel the
other way from the arterial trunks to the ventricles.

6. The Beat of the Heart. — Like all other muscular
tissues, the substance of the heart is contractile ; but, un-
like most muscles, the heart contains within itself a some-

Fio. 10. — The Orifices OF the Hkart sken from above the Auricles
AND Great Ve.ssels being cut awav

P. A. pulmonary artery, with its semilvin.ar valves ; Ao. anrta do.

R.A. r. right auriculo-ventricular orifice with the three flaps (t. v. 1, 2, 3)
.of tricuspid valve.

L.A. V. left auriculo-ventricular orifice, with m.v. 1 .and 2, flaps of
mitral valve ; h, style puiscd into coronary vein. On the left part of
L.A. v., the section (if the auricle is carried through the auricular aji-
pondage ; hence the toothed appearaiiee due to the portions in relief
cut across. '

thing which causes its different parts to contract in a
definite succession and at regular intervals. The whole
of the heart is not alike in its faculty for contracting
spontaneously. Indeed this faculty of initiating the con-
traction is normally confined to a quite small jrortion.
Starting here the contraction spreads over the auricle by
a species of conduction, from the auricle it spreads on

THE BEAT OF THE HEART 51

over a delicate bridge of tissue, which is the functional
connection between the auricle and the ventricle, and
finally the whole ventricle becomes involved.

If the heart of a living animal be removed from the body,
it will, if its substance is fed with a suitable nutrient fluid,
go on beating much as it did while in the body. ^ And care-
ful 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 contract again in the same order as before,
and their contractions are followed by the same pause as
before.

The state of contraction of the ventricle or auricle is
called its syatole ; the state of relaxation, during which it
undergoes dilation, its diastole.

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 — A'V"" — A"V
— &c. Thus the contraction of the heart is rhyth-
mical, 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 movements taking place in the heart during one
complete beat and the pause are 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
minute. From this it follows that the ordinary duration
of each beat is /„ of a second. Of this period the
contraction of the auricles occupies ^V ^"^^ ^^^^^ °^ ^^^
ventricles fg, the remaining /„ bemg taken up by the
pause of the heart as a whole. During each cycle or beat
the heart undergoes certain changes of shape and position,
as to the details of whicli there is some uncertainty, but
which are, on the whole, as follows. During each systole

E 2

52

ELEMENTARY PHYSIOLOGY

LESS.

the width of the heart fx'oin side to side becomes less ; pro-
bably also the depth from back to front is at the same
time slit^htly increased. The result of tliis is that whereas
during diastole tlie shape of a section of the ))ase <}f the
ventricles is elliptical, during systole it becomes much
more nearly circular.

Fio. 20.— Tkansvlusk Skction ■iiiu<>i(iii the .middli-: of the Ventricles
OF A Doo's Heart in Diastole and in SvsTt)LE. (After Uekse.)

R. V. right ventricle ; Z. V. left ventricle.

Tlie length of the heart is very slightly lessened, if at
all, during systole, but the heart as a whole is twisted to
a certain extent on its long axis, from the left and beliind
towards the front and riglit. 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
impoi'tance in connection vvitli what we shall describe
presently as the "cardiac impulse" (see p. 57).

7. The Action of the Valves. — Having now acejuired
a notion of tlie arrangement of the difierent pipes and
reservoirs 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 l)lood, the first step in the
pulsation of the heart occurs and the auricles contract.

By this action each auricle tends to scjueeze 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, cis a whole, will take,

n THE ACTION OF THE VALVES 53

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 resist-
ance 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 bj' the semilunar valves,
which are necessarily closed. The return of blood into
the veins is further checked by a contraction of the great
veijis at their point of junction with the heart which im-
mediately precedes the ^stole 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 so will pass into and distend the
ventricles. As the ventricles fill and begin to re.sist
further distension, the blood, getting behind the auriculo-
ventricular valves, will j)ush 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
ventricular systole begins. The walls of each ventricle
contract vigorously', and the first effect of that contraction
is to complete the closure of the auriculo-ventricular
valves and so to stop all egress towards the auricle. The
pressure upon the valves becomes very considerable, and
they might even be driven upwards, if it were not for the
chordce tendineft: which hold down their edges.

As the contraction continues and the capacities of the
ventricles become diminished, the points of the wall of
the heart to which the chorda tendinece are attached ap-
proach the edges of the valves ; and thus there is a ten-
dency to allow of a slackening of these cords, which, if
it really took place, might permit the edges of the ^■alves

54

ELEMENTARY PHYSIOLOGY

to fl,ap back and so destroy their utility. This tendency,
however, is counteracted by the rhurdce tendincce being
connected, not directly to the walls of the heart, but to
those muscular pillars, the papillary mnsdcs, which stand
out from its substance. These muscular pillars shorten
at the same time as the substance of the heart contracts ;
and thus, just so far as the contraction of the walls of the
ventricles brings the papillary muscles nearer the valves,
do they, by their own contraction, pull the choroUe ten-
diiiece as tight as before.

m-

FiG. 21.— Diagram to ilhstrate the Action of the Heart.

aur. .■wiride ; rent, ventricle; VV. veins; a, aorta; m, mitral valve;
», semilunar valve.

In A the aviricle is contracting, ventricle dilated, mitr.al valve open,
semilunar valves closed. In B the auricle is dilated, ventricle contracting,
mitral valve elosed, semilunar valves open.

By the means which have now been described, the fluid
in the ventricle is debarred from passing back into the
auricle ; the whole foi-ce of the contraction of the ventri-
cular walls therefore is exjiended in overcoming the resist-
ance presented by the semilunar valves. This resistance
is partly the result of the mere weight of the vei-tical
column of blood which the valves support ; but is chiefly
due to the reaction of the distended elastic walls of the

n THE ACTION OF THE VALVES 55

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
needlessness 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 ai'e already full ; and if there were no auriculo-
ventricular 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, firstly, of the
auriculo-ventricular valves ; and, secondly, of the thick-
ness and strength of 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 folhjws 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
refilled by the pressure of the fluid in the great veins,
which is amply sufticient to overcome the passive resist-
ance of the relaxed auricular walls. And, at every systole
of the ventricles, 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.

66 ELEMENTARY PHYSIOLOGY less.

8. The Working of the Arteries.— We must now
consider what happens in the arteries wlien the contents
of the ventricles are suddenly forced into these tubes
(which, it must be recollected, are already full).

If the vessels were tul)es of a rigid material, like gas-
pipes, the forcible discliarge of the contents of the left
ventricle into the beginning of the aorta would send a
shock, travelling with great rapidity, right along the
whole system of tuljes, 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 tlie 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 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 tliem 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

THE CARDIAC IMPULSE 57

close the semilunar valves, and su prevent the return of
blood to the heart, and, on the other hand, to distend the
next portion of the a6rta, di-i\ing an extra quantity of
blood into it. And this second portion, 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 ah.ng
the arteries distending them as it goes, and ultimately
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.

9. 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 move-
ment is perceptible, which is known as the "cardiac
impulse." It is the result of the heart-beat making it-
self felt through 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 becomes suddenly hard and tense, as do all
muscles when they contract. At the same time the apex of
the heart, as the result of the peculiar movements already
described (p. 52), 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 contact with the wall of the chest at the moment
when the hardening takes place. It is nut due, as is so
frequently stated, to the heart " striking " or " tapping "
against the chest-wall.

10. 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
between every two beats. First comes a longish dull

58 ELEMENTARY PHYSIOLOGY less.

booming sound ; then a short sharp sound, then a pause,
then the long, then the sharp sound, then another
pause ; and so on. Tliese sounds are usually likened to
the pronunciation of the syllables "lubb," "dup." There
are many different opinions as to the cause of the tirst
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 vil)ration
of the auriculo-ventricular valves, when they become
suddenly tense or stretched as the ventricles begin to
contract. In reality tlie first sound has probably a douljle
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
obscured in cases 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 [ire-
vented.

The second sound is without doubt caused by the
membranes of tlie 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 that point on the chest-
wall under which the semilunar valves lie ; that it is
modified and obscured l)y disease of these valves ; and that
it may be made to cease by experimentally hooking
back the semilunar valves in a living animal.

11. Blood-pressure.— When an artery is cut, the
outflow of blood is nt)t uniform and smooth, but takes
place in jerks which correspond to each beat of the heart.
Moreover the blood spurts out iCUh considerable force,
which although it is greater at each jerk is still persistent
and large betwctni the jerks. The obvious conclusion to
be drawn from the above observation is that the blood in

n BLOOD-PRESSURE 59

the artery is always under considerable, though variable,
pressure. This pressure is called blood-pressure. We
have already explained how this pressure comes' to be
estabUshed ; but its impoi-tance is so great as a factor in
the circulation 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
resistance." 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 between it and the next beat, pass on into the
veins. The remainder is lodged in the arteries whose
walls, being distensible, are pxi on the stretch by the
pressure of the blood thrust into thein at each stroke of the
heart, and this pressure of the blood on the arterial waU
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 fx-esh 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 tlie 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

60 ELEMENTARY PHYSIOLOGY

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 l)lood-pre.ssure is still less than in the
capillaries, and diminishes gradually along their course
towards the heart.

These differences of pre.s.sure in the several parts of the
vascular system determine the How of blood along the
vessels ; the blood is always flowing from a higher to a
lower pressure ; the main work of the heart is to estjiblish
the large blood-pressure existing in the larger arteritis.

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.
Further the flow is in this case continuous and not jerky
as it is from a cut artery, in cfnrespondence with the
fact that there is no "pulse" in the veins as there is in
the arteries. But this stiitement recjuires that we should
next consider the nature and causes of the pulse.

12. 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 wri.st, what is known as the pulse will be
felt as a slight throbbing jtressure on the finger, coming
and going at regular intervals which correspond to the
successive ])eats of the heart. What is felt is in reality
the intermittent risu and fall of that piece of the arterial
wall which lies innnediately 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 arterial 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 corre-

THE PULSE / 61

sponds 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 distance of the artery from the heart. For example,
the pulse 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 deter-
mined and is found to be about 30 feet per second. This
rate of progression of the pulse-wa\e must be carefully
distinguished from the rate at which the blood is flowing
along the artery. Even in the aorta, where the blood
flows most rapidly (p. 64), the velocity is not more than
about 15 inches per second. In fact "the pulse-wave
travels over the nioving blood somewhat 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
Lesson IV.) This actual loss, or rather transformation of
the pulse is eflected hij means of the elasticity of the arterial
ivalls, called into i)la[i by the peripheral 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 ofter a veiy 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 tliat of the aorta, the blood ha.s a difliculty in
getting through the capillaries into the veins as fast as it

62 ELEMENTARY PHYSIOLOGY

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 these, an elastic tube has the power, if long enough and
elastic enough, to change a jerked impulse into a continu-
ous 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 difl"erently, according as
the opening at the far end is wide ou narrow. If it is
wide, the water flows out in jerks, nearly as distinct as
tliose from the glass tube. Tliere is little resistance to
the outflow, little distension of the india-rubber tul)e, little
elasticity 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 any way 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 l)ut in a stream, which is mox'e
and more comi)letely continuous the longer and more
elastic 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 arceries and capil-
laries, for the india-rubber tube the whole arterial system,
and you have exactly the .same result in the living body.
Througii the action of the elastic arterial walls the separate

THE PULSE 63

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 ; a part only is thus spent, the rest goes to
distend the elastic arteries. But during the interval
between that beat and the next the distended arteries are
narrowing again, by virtue of their elasticity, and so are
pressing the blood on into the capillaries with as much
force as they were themselves distended by the heart.
Then comes another beat, and the same process is re-
peated. 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 t lie blow to the capillaries
during the interval between the strokes.

The larger the amount of elastic arterial wall thus
brought into i)lay, 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 rhe arterial system, so to speak, being sheltered
by the lengths between it and the heart.

Every inch of the arterial system may, in fact, be con-
sidered 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 catlses above
mentioned, 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 arteries 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.

The phenomena so far described are the direct outcome

64 ELEMENTARY PHYSIOLOGY

of the mechanical conditions of the organs of the circula-
tion 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 hack 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.

13. The Rate of Blood Flow. —It has been found, by
experiment, that in tlio horse it takes about half a minute
for any substance, as for instance a chemical body, whose
presence in the blood can easily be recognized, to com-
plete the circuit, ex. gr. 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.

By far the greater portion of this half minute is taken
up by the passage through the small vessels, where the
blood moves, it is estimated, at the rate only of about one
and a half inches in a minute, wlioreas through tlie camtid
artery of a dog it flies along at the rate of about ten 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
(putting the other comparatively slight helps on one
side) the beat of the heart and tliat alone, however much
it may be modified, as we have seen, in character, it is
obvious that the velocity with which the blood moves
must be greatest in the aorta and diminish towards the
capillaries.

For with each l)ranching of the arteries the total area
of the arterial system is increased, the total width of the
capillary tubes if they were all put together side by side
being very much gi-eater than that of the aorta. Hence
the blood, or a corpuscle, for instance, of the blood Itoing

n VASOMOTOR NERVES 65

driven by the same force, viz. the heart's beat, over the
whole body, must pass much more rapidly through the
aorta than through tlie 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 undth of the ''bed," and this alone, which determines
the differences 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.

14. The Nervous Control of the Arteries. Vaso-
motor Nerves. — The arteries, as we have seen, are char-
acterised structurally by being elastic and muscular. In
the large 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 dis-
appearance of the pulse. It has also been pointed out

ELEMENTARY PHYSIOLOGY

(p. 34) that the small arteries may be directly affected
by the nervous system, whicli controls the state of con-
traction 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. Tiie control of the ner-
vous 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
familiar, either as experienced on themselves 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 ; there-
upon a hot flush is felt, the skin grows reil, and according
to the intensity of the emotion 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 I'eddens and grows hot, because
its vessels contain an increased cjuantity of this red and
hot fluid ; and its vessels contain more, because the small
arteries suddenly dilate, the natural moderate contraction
of their muscles being superseded l)y 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 moderate contraction is, for the time, sus-
pended.

On the other hand, in many peo])le, 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
conseipience 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 con-

VASOMOTOR NERVES 67

traction of the muscular coats of the arteries is brought
about by the increased action of the nervous system. '■

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 sym-
pathetic nerve (Fig. 22, C. 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 i-eason 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 pre-
viously slightly contracted, become altogether relaxed.

And now 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, whicli di-
minishes their calibre so much that the blood can haidly
make its way thiough them. Consequently, the ear
becomes pale and cold.

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 ai-teries ; and inasmuch as this con-
trol 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

1 Sudden paleness is perhaps most frequently due to a failure or
stoppage 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.

F 2

68 ELEMENTARY PHYSIOLOGY less.

of the arteries with which it is connected are always
contracted and the vessels themselves constricted, the
sympathetic is more precisely characterised 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 oitt along this nerve,
whereby the arteries are kept continually in a condition 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
supply 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 tliey become active they require
an increased amount of blood which could not be supplied,
at least by a vaso-constrictor mechanism, but for the exist-
ence 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 aftect the supply to
every part of the body at the same time, and what is
really wanted is a localised variation in supply to meet tlie
varying needs of each part or organ. Thus the vaso-
constrictor nerves act by carrying more.of less of the same
kind of impulse, leading to increase or loss of tone and
hence lessened or increased l)lood-supply ; they do not
act, as is so frequently and erroneously imagined, by
carrying one set or kind of impulses to produce constric-
tion, and another set or kind to produce dilation.

We have quoted blushing as being a characteristic and
familiar instance of the action of vaso-motor (vaso-con-
strictor) nerves. But other examples of exactly similar
action are met with througliout the whole body. Thus
when a muscle contracts, or when a salivary gland secretes

THE VASOMOTOR CENTRE 69

saliva, or when the stomacli 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 blush-
ing may, like the ordinary blushing described above,
be brought a})out by vaso-motor nerves. It may also be
brought about by chemical bodies produced in the organs,
which, acting on the muscular walls of their arteries,
cause them to relax. 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 nen-ous
system. Indeed everywhere all over the body, the ner-
vous system by its vaso-motor nerves is continually super-
vising 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 con-
gestion or inflammation, are due to, or at least associated
with, a disorder or failure of this vaso-motor activity.

15. 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 part
of the central nervpus 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
(see Lesson XI.) 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 ca.se the interesting question
arises as to where these impulses are generated before
their exit from the cord. Experiment shoAvs 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 originate in a very limited
portion of the central nervous system, located in that part'

70 ELEMENTARY PHYSIOLOGY less.

of it which we shall describe in a later Lesson (XI.) 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 arteines of the body as a whole, this
definitely localisefj part of the bulb has received the
name of the vaso-motor centre. (Fig. 22, V.M.C.).

The cause of the phenomenon of arterial " tone " now
becomes quite clear. The vaso-motor centre continually
generates and sends out impulses to every part, or rather
to very many parts, of the body, which sufKce to keep
the muscle fibres of the arteries sui)plying those parts in
a condition of slight contraction. When the impulses are
increased to any part, 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 uj)on it in such a way as to determine those
variations in its activity which are essential to its adapt-
ing itself to the varying needs of either the body as a
whole or any small \rdrt of the body. These impulses
whicli govern the vaso-motor centre pass into it either
down from the brain above, 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 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 efi'ects of heat and cold ap))lied to the
body, as determining those variations of blood-sup] )ly to
the skin by wliich the temperature of the body is
so largely regulated (see Lesson V. ). Here the impulses
are started in the skin and, travelling along certain
sensory nerves, enter the spinal cord, pass up to the
vaso-motor centre, and as Ijefore lead to the necessary
changes in its activity.

VASODILATOR NERVES

71

16. Vaso-dilator Nerves,— Our consiaeration of vaso-
motor nerves has so far led us to the view that the dilation
or wideaing of an artery which leads to increase blood-

a.f. a.f.

■/-- V.M.C.

•Sp.C.

c.f,
•c.f.
c.f.

-Sp.C.

Fig. 22.— Diagram to illfstrate the Position of the Vaso-Motob
Centre, the Paths of Vaso-Constrictor Impulses from the Centre
along the Cervical .Sympathetic Nerve and (part of) the Ab-
dominal Splanchnic, and the Course of Impul-ses to the Centre
from the Brain and from an outlying Part of the Body.

Sp.C, Sp.C. spinal cord ; V.M.C. va-so-motor centre ; Art. artery of ear;
C.Sy. cervical sympathetic; S.C.G. superior cervical ganglion; I.C.G
inferior cervical ganglion; S.Ait. subclavian artery; A.V. annulus of
Vieussens ; St.G. stellate ganglion ; 4, 5, 6, 7, 8, fourth, fifth, sixth,
seventh and eighth thoracic ganglia; A.S. upper roots and part of ab-
dominal splanchnic nerve.' The dotted lines a.f., a.f. indicate paths of
conduction for impulses to the vaso-motor centre from the brain. The
dotted lines c.f., <•./., c.f. indicate paths for the passage of impulses to tlie
vaso-motor centre from some outlying part of the body such as tlip skin.
The .irrows show the directions in wliich the impulses travel along
each path.

72 ELEMENTARY PHYSIOLOGY less.

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 witli in tlio
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 7th cranial nerve (see
Lesson XL), which runs to the submaxillary salivary
glands. When this nerve is simply severed, no obvious
effect is produced on the blood-vessels of the gland.
But if now the cut end connected icilh the gland be sthau-
lated, the small arteries at once dihite powerfully, the
blood-supply is enormously increased, and the gland
becomes 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 iin])ulses passing along it give rise not to
constriction but to dilation. Hence it is spoken of as a
vaso-dilator nerve. Other instances of the occurrence
of similar vasodilator nerves are met with, but as our
knowledge of them is at present uncertain and incomplete
we must be content with having simply drawn attention
to their existence, and to one striking instance of their
action. It will be obsei-ved that vaso-constrictor nerves
only lead to dilation through interference with the vaso-
motor centre and tonic inij)ulses ; vaso-dilator nerves
bring about dilatiuu ilircctly.

17. Nervous Control of the Heart. Cardiac Nerves.
— The heart, as we all know, is not under the
direct influence 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 something which arrests
the heart as dead as you stop a stop-watch with a

n THE NERVES OF THE HEART 73

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 nervous system, and we must now consider the
more important details of this control.

The heart is well supplied with nerves. There are
many small ganrilia, or masses of ner\e cells, lodged in
the substance 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. 23). Of these the
most important 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 coui-se, the vagus (see Lesson XI.). Other
nerves reaching th,e heart seem to come from the sym-
pathetic, but may be traced back thi'ough the sjTupathetic
to the spinal cord, and, for reasons which will presently
become apparent, are called accelerator nerves.

The heart, as already exjjlained (p. 51), contracts
rh}i;hmically, but the regular i-hythmical succession of the
ordinary 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 continue even after the heart is removed from
the body. Hence we must conclude, and experiment
bears out the conclusion, that the muscle substance of ivhich
the heart is made is itself endowed xmth the power of con-
tracting and relaxing at regular intervals. On the other
hand the influences which alter the hearts action, as in
fainting or palpitation, 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.

74 ELEMENTARY PHYSIOLOGY less.

If a frog be pithed, or its brain be otherwise destroyed,
so as to obliterate 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 slowly, and at the same time each beat is rather
more feeble, as shown by the diminislied distance over
which the end of the lever moves. But if the stimulation
is strong, the lever almost innnediately stops dead, and
the heart will be found quiescent, with relaxed and dis-
tended walls. After a little time the influence of the
vagus passes off, the heart recommences its work as
vigorously as before, and the lever vibrates 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 eftbct is produced which is
exactly the opi)osite 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 tliat each beat is stronger.

No clearer proof could be desired than is afforded by
the above experiments, that the heart of the frog is con-
trolled by two antagonistic nerves of which one, the
vagus, carries impulses which slow and finally stop its

11 THE CARDIO-IXHIBITORY CENTRE 75

beat, while the other, the accelerator, conveys impulses
which make it beat faster. Since experiments have showTi
that the mechanism just described exists equally in the
mammalian heart, we may at once apply these striking
results t^ 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 it mechanically cause a stoppage of his
own heart-beat.

The heart, then, is controlled by two kinds of antagon-
istic influences, analogous to those previously described
as controlling the muscular walls of the arteries. More-
over 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 in-
fluences they convey to the heart must, as in the case of
the vaso-raotor nerves, originate in the central nervous
system. We saw, however (p. 69), that the impulses
carried by the vaso-motor nerves are generated in a very
specially localised part of the spinal bulb, and the inter-
esting question at o:ice arises : Is there a similarly local-
ised centre in which the impulses which modify the beat
of the heart take their origin ? The answer to this ques-
tion is in the affirmative, for experiment shows tliat the
impulses which, travelling along the vagus, can stop or,
as the physiologist savs "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 there-
fore known as the cardio-inhibitory centre. 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

ELEMENTARY PHYSIOLOGY

centre, be itself influenced by impulses which reach it
either from the brain above or the spinal cord below. In

c.b Vg

FlO. 23. — DiAORAM TO II.I-rsTRATE THR POSITION OF THE CaRPIO-

Inhibitory Centre, the Paths of Inhibitory and Accelerator

IlUTLSES 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, Sp.C. spinal cord; C.f.C. cardio-inhibitory centre; X. vagus
nerve at its origin ; J'.G. ganglion of the vagus, through which run the
fibres coming from the centre along A7, the spinal accessory nerve (see
Lesson Xl.>. ('(/. main trunk of the vagus ; c.b. Vg. cardiac brifnches of
vagus, supplying the heart; C.Hii. sympathetic nerve in the neck;
S.C.O. superior cervical ganglion ; I.C.G. inferior cervical ganglion ; S.Art.
subclavian artery; A.V. annulus of Vieussens ; St.G. stellate ganglion ;
4, 0, (5, fourth, fifth and sixth thoracic ganglia ; c.b.Sji. cardiac branches
of the sympathetic su]>plying the heart. The dotted lines a./., a.f.
indicate paths of condiiction fur impulses to the cardio-inhibitory centre
from the bi-ain. The dotted lines »»../., m.f. indicate paths for the passage
of impulses 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.

n THE CARDIO-INHIBITORY CENTRE 77

this way the heart is indirectly connected with all parts
of the body, so that by nervous agencies its beat may be
made to vary. For instance when a person faints from a
sudden emotion, an influence is started in the brain,
passes down to the centre in the spinal bulb, increases
its action and stops for a time the beating of the heart.
Or again, fainting may result from a blow on the
stomach ; in this case the influence starts at the part
struck, and passing up the spinal cord to the cai'dio-
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
cardio-inhibitory centre is quite clear and definite. It is
possible that a cardio-augmentor (-accelerator) centre may
also exist, but at present we have no exact knowledge of
its existence ; hence in the accompanying figure (Fig. 23)
the accelerator nerves are shown as originating in the
central nervous system, but not arising from any de-
finitely localised centre.

18. Tlie Proofs of the Circulation. — The evidence
that the Ijlood circulates in man, although perfectly con-
clusive, 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 arrange-
ment of the various valves, will not, as was shown by
Harvey, permit the blood to flow in any other direction

78

ELEMENTARY PHYSIOLOGY

LESS.

than in the one described above. Moreover, with a
syringe, we can easily inject a Uuid from the vena cava,

Fio. 24. — Portion of the Web o»- a Froo's Foot seen under a low
Maonifyino Power, the Blood-Vkssels only BEiNn rei'REsknted,

EXCEIT IN THE CORNER OF THE FlELP, WHERE IN THE PORTION MARKED

OFF TtiF. Pigment Spots are also prawn.
o, small arteries ; v, small veins ; the minute tubes joining the arteries
of the veins are the capillaries. The arrow.* denote the direction of the
circulation. The lartrer artery runnintr straight up in the middle line
breaks up into capillaries at jwints higher up than can be shown in the
drawing.

THE PROOFS OF THE CIRCULATION 79

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 the capillaries, because
when an artery is tied, in a living body, it swells up and
pulsates on the side of the ligature nearest the heart,
whereas on the other side it becomes empty, and the
tissues supplied by the artery become pale 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., &c., 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 circu-
lation can be most conveniently observed is the frog.
The web between its toes is very transparent, and the
corpuscles suspended 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. 24).

19. The Capillary Circulation.— The essential charac-
teristics of blood flow through the capillaries may be
easily studied by observation under the microscope of the
web of a frog's foot. In the smallest capillaries the cor-
puscles pass along singly, sometimes following each other
in close file, at other times leaving quite considerable

ELEMENTARY PHYSIOLOGY less, u

gaps in their succession. Frequently one or more cor-
puscles may remain stationary for a moment and tlien
pass on again. The red C()r[)uscles, whicli in the froy are
oval and comparatively large, glide along with tlieir 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. 25, G and H). In
the larger capillaries, more especially in mammals whose
corpuscles are smaller than in the frog, the corpuscles
often pass along two or three abreast. Further, in tliese
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. 50) 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 current.
Moreover, they frecjuently stoj) for an appreciable time,
as if sticking to the wall of the capillary, and then roll on
again ; probably because tliey are more adhesive than the
red corpuscles as a result of their power of executing
anueboid iiioveiuents (see p. 100).

20. Inflammation. - Everybody is moi-e or less familiar
with a i)eculiar and unusual condition whicli may arise in
almost any part of the body, and which they describe by
speaking of the part as "inflamed." To ordinary obser-
vation 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

Pio. 25.— Very small Portion of Fig. 24 verv highly magnified.

A, walls of capillaries ; B, tfssue of web lying between the capillaries ;
C, cells of ejjidermis covering web (these are only shown in the right-
hand and lower part of the field ; in the other part.s of the field the focus
of the microscope lies below the epidermis) ; D, nuclei of these epidermic
cells ; E. pigment cells contracted, not partially expanded as in Fig. 24 ;
F, red blood-corpuscle (oval in the frog) passing along capillary— nucleus
not visible : G. another corpuscle squeezing its way through a capillary,
the canal of which is smaller than its own transverse diameter; H,
another bending as it slides round a comer ; A', corpuscle in capillary
seen through the epidermis ; /, white blood-corpuscle.

6

82 ELEMENTARY THYSIOLOGY less.

merely the external evidences of changes taking place at
the same time in the minute blood-vessels and circulation
of tlie part affected, and since these changes throw an
interesting light on the relations ordinarily existing
between the walls of the blood-vessels and the neighbour-
ing blood, they are wortliy of a short consideration.

If when the web of a frog's foot, or other suitably
transparent part of an animal, is adjusted for observation
under the microscope, some irritant be applied to it such
as a trace of mustard,' 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 waniitli 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 adlicre more firmly than usual to the walls of
these vessels. Further, blood "platelets" (see p. 103),
not previously visible, begin to collect also with and
among the white corpuscles. Following 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 corpuscles
make their way by amoeboid movements thi'ough 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 con-
siderable (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 cliaracteristic
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 becomes slower and slower,

1 Used similarly as an irritaut in the form of the ordinary domestic
mustard poultice.

THE LYMPHATIC SYSTEM 83

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 multiplying
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 imlls of the capillaries (and of the
smallest veins and arteries) plays a veiy important part
in determining the characteristics of the normal circulation
through these passages. And since in an inflamed area
the flow of blood becomes slower and slower, and ulti-
mately 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 variations 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. 84) for the nutrition of the neigh-
bouring tissues.

Part IT.- The Lymphatic System and the Circulation
OF Lymph

1. The General Arrangement of the Lymphatics.—

Food, as we have .already pointed out (p. 23), after
digestion 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. 31) that the ultimate

G 2

84 ELEMENTARY PHYSIOLOGY less.

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 everywhere 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
producing directly into the blood for its removal as a
preliminary to its excretion. Both these difficulties are
however got over by the fact that « ■portion of the fluid
part of the blood is continuall]/ exiiding through the icalls of
the capillaries into tlie neiglibouring tissues, taking with
it the nutriment necessary 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,'
and may be regarded as a sort of " middleman " between
the blood on the one hand and the tissue on the other.
But if now this lymj)!! 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 Ij'mph, after having served the needs of the tissues,
is gathered up again and returned to the blood for further
use. Now this is exactly wliat does take place, and tlie
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 cajiillaries also
contain another set of what are termed lymph-capil-
laries, 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

1 The mode of formation, composition and properties of lymph are
dealt "with in Lesson III.

n

THE LYMPHATIC SYSTEM

85

connected with larger vessels of only one kind.

That is to say they open only into trunks which 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-capillaries,

but obstruct the flow of any liquid in

the opposite direction. But the Ij'm-

phatic vessels difi'er from the veins in

that they do not rapidly unite into

larger and larger trunks which present

a continually increasing calibre and

allow a flow without interruption 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 new lymphatic

vessels (Fig. 26, </.).

Sooner or later the great majority of
the smaller lymphatic vessels pour their
contents into a tube which is about as
large as a goose-quill, lies in front of
the backbone, and is called the thor-
acic duct. This opens at the root
of the neck into the conjoined trunks
of the great veins (jugular and sub-
clavian) which bring back the blood from the left side
of the head and the left ai-m. (Fig. 27, f.g.)

The remaining lymphatics, chiefly those of the right

Fig. 26.— The Lym-
phatics OK THE
Front of the
Right Arm.

g, lymphatic glands,
on the course of
the lymphatics.

86 ELEMENTARY PHYSIOLOGY less.

side of the head and neck, tlie right arm and right lung,
are connected by a common canal with the correspond-
ing vein of the right side.

The lower part of the tlioracic duct is dilated, and is
called the receptacle of the chyle (Fig. 27, a.). This
part receives more particularly the lymphatics from the
intestines, which, though they differ in no essential re-
spect from other lymphatics, are called lacteals, because,
after a meal containing much fatty matter, they are filled
with a milky fluid termed chyle. Tlie lacteals, or lym-
phatics of the small intestine, not only form networks in
its walls, but send blind prolongations into the little pro-
cesses termed villi, with which the mucous membrane of
that intestine is beset. (See Lesson VL)

Where the two principal trunks of tlie lymphatic system
open into the veins, valves are placed which allow of the
passage of fluid in one direction only, namely from the
lymphatic 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 re-
turned into the blood.

2. The Origin and Structure of Lymphatics.— The
tissues of the body' are built up of cells wliich, though
lying closely applied to each other, ai-e often separated
by extremely minute spaces. These spaces are particularly
plentiful in that form of tissue of wliich we have already
spoken as connective tissue, as a result of its structural
arrangement, for 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 (see Fig. 28).
Some of the spaces in this tissue are comparatively
large and are called areohe, whence this particular kind
of connective tissue is sometimes called areolar tissue
(see Lesson XII. i. This areolar tissue is, as we have said
(p. 13), present in every part of the body, and of course
supports tlie blood-capillaries, which are thus, in reality,
merely minute tubes lying imbedded in connective tissue.

THE THORACIC DUCT

87

Fig. -JT.— The Thoracic DrcT.

The Thoracic Duct occupies the middle of the figure. It lies UDon the
spinal coh..„n. at the sides of which are seen portlr.ns of the ribs m

u, the receptacle of the chyle; 6. the trunk of the homcic duct
opening at o into the junction of the left jugular ( /) and subdavian r„i
veins as they unite into the left innominate veii/wh^-h has been cut
across to show the thoracic duct running behind it d, lyn^ hltic llands
placed m the lumbar regions ; 7,, the superior vena cava firmed bv thi
junction of the right and left innominate vems lormea by the

88 ELEMENTARY PHYSIOLOGY

The chinks and spaces of the tissues are filled with that
fluid exudation from the blood-vessels and 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 lym))h-spaces thelympli passes into tlie lymph
capillaries. These are also essentially spaces in the mesh-

\-

N \ /^, , ,:f,,.^

Fig. 28.— Connective Tissue Fibres.

a, small bundles of white fibrous tissue ; b, larger bundles ; c, single
elastic fibres.

work of connective tissue, but they are now lined by a
single layer of extremelj' thin, flat, nucleated, epithelioid
cells, very similar to those composing the wall of a blood
capillary, but cliaiacterised by their edges being very
sinuous or indented. These cells are joined to each other
by their edges, tlie sinuosities of adjacent cells fitting into
one another, so tliat tliey form a system of minute tubes,
larger than blood-capillaries and wandering more irregu-
larly.

LYMPHATIC GLANDS 89

The lymphatic vessels into which the lymph-
capillaries poux" theii" contents on its way towards the
thoracic duct possess a structure essentially similar to
that of a vein (p. 34) ; but tliey difler 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, whose structure, however, is the
same as in the veins.

Fig. 29. — Epithelioid Cells Lining, and Chak.acteristic of, the
Lyjiphatics.
The outline of the cells has been brought into view by means of
nitrate of silver, which does not stain the nuclei ; the latter therefore
are not shown.

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 afferent lymphatic vessels enter the glands
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 ve.ssels.

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
projections, called trabeculae, inwards and towards the
centre of the gland which divide it up into compartments
or alveoli, the compartments being very regularly ar-
ranged at the outer portion or cortex of the gland and

90

ELEMENTARY PHYSIOLOGY

LESS.

irregularly in the more central parts or medulla (see
Fig. 30). Each alveolus is filled witli a network of con-
nective tissue, whose meshes are small and closely set in
the central part of the alveolus, wider or more open when
in contact with the tral)ecula?. The central small meshed
network is known as adenoid tissue, ^ is densely packed

\

Fio. 30. — Diagrammatic REPREsinTATiON of a Lymphatic Gland seen in
Section. (After Shari'ey.)

Cap. capeule ; Tr. trabccula? ; G.S. glandular substance; L.S. lymph
sinus. In the alveolus marked 1. 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. af-
ferent lymphatic; E.L. efferent lymph.atic. The arrows show the
direction in which the lymph enters and leaves the gland.

with lymph-corpuscles or leucocytes closely resembling
the colourless corpu.scles of blood (jj.IOO), and constitutes
what is usually spoken of as the glandxilar substance.
The more open-me.shed network which surrounds the
glandular substance and separates it from the trabeculse
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

1 Also often called retiform or lymphoid connective tissue
Lesson XII.)

(See

11 LYMPHATIC GLANDS 91

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 arteries
supplying blood to the gland pass along the trabeculae.
cross the lymph-sinus, enter the glandular substance, and
break up into a network of capillaries from which the
blood is collected and carried away by small veins.

The leucoc}i:es which crowd the glandular substance
present under the microscope appearances which leave no
doul)t 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 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 leucocj'tes 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.

The lymphatic glands have another very important
function. Bacteria which reach tliem iu the lymph from
some infected region are retained and destroyed in them.
Otherwise these bacteria would be poured with the lymph
into the blood and would infect the whole body.

4. Causes which lead to the Movements of Lymph.
— Throughout the preceding description of the lymphatic
system 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.

92 ELEMENTARY PHYSIOLOGY less, ii

The cause of this flow is not so immediately apparent as it
is in the case of the blood, for the lymphatic system pos-
sesses no central pump, such as the heart, to keep the
lymph in motion.' 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 some force, whose
nature will be considered in the next Lesson, is at work
to determine the initial exit of fluid from the blood-vessels
into the lymph-spaces. This force must obviously tend
to drive out the lymph already in those spaces, into and
along the channels leading from them. Further, as we
have seen, the blood-jiressure in the large veins near the
heart is very small and is certainly nnich 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
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 inspira-
tion 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. But the one
great and potent cause of lymph-flow is the presence, all
along the course of the lymphatics, of valves whose action
is, however, only brought into play by the movements of
the body. As in the veins (p. 36) so in the lymphatic
vessels ; when any pressure is applied to their outside the
lymph is driven out of the squeezed part, and since the
valves only open towards the junction of the thoracic duct
with the venous system, the lymph is thereby driven
along in the desired direction.

1 The frog possesses four lymph hearts, placed in two pairs at the
upper and lower end of the backbone Their strnctural arrangement is
very similar to that of the blood-honrt, and, V)eing rhythmically con-
tractile, they pump the lymph into the venous system.

LESSON m
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 Avith a microscope
magnifying at least three or four hundred diameters.
Provided with this instrument, a hand lens, and some
slips of thick and thin glass,' 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. ^A'hen
in this condition, if it 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 slips of thick glass, and covered
lightly and gently with a piece of the thin glass, 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 di-op be
dealt with in the same way, a few granules of common
salt being first added to the drop.

1 Slides and coverslips, as they are called by microscopists.

94 ELEMENTARY PHYSIOLOGY

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 disapj)ear, 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 the}^ 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,
indeed, the slide may be inverted without the disturbance
of the drop, wliich has become solidified, and may be
removed, with the point of a penknife, as a gelatinous
mass. The niass is quite soft and moist, so that this
setting, the clotting or coagulation, of a drop of
blood is sometliing 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 s<ilt therefore has
prevented the coagulation of the blood. Thus this very
simple investigation teaches that blood is comp(jsed of a
nearly colourless plasma, in which many coloured corpuscles
are susjjended ; that it has a remarkal)le power of
clotting ; and that tliis clotting may be prevented by
artificial means, such as the addition of salt.

If, instead of using tlie hand lens, the drop of blood 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

THE CORPUSCLES OF BLOOD

95

or colourless corpuscles. The former are much
more numerous than the latter, and have a J■ello^vish-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

Fig. 31.— Red and White CoRprsci.ES of the Blood Magnified.

A Moderately magnified. The red corpuscles are seen lying lu
rouleaux • at a and <i are seen two white corpuscles. ^ j.^.^.

B Red'corpuscles much more highly magnified seen m f ace : C ditto
seen in profile; D. ditto, in rouleaux, rather more highly magnified , £. a
red corpuscle swollen into a sphere by imbibition of water.

F A white corpuscle magnified same as B.

n. Red corpuscles puckurcd or crenate aU over.

/. Ditto, at the edge only.

several are seen lying one on the other, the redness
becomes obvious. The white, somewhat larger than the
red corpuscles, are, as theii' name implies, pale and devoid
of colouration.

96 ELEMENTARY PHYSIOLOGY

The corpuscles differ also in other and more important
respects.

2. The Red Corpuscles.— The red corpuscles (Fig. 31)
are flattened circular di.scs, on an average 7/x to 8/x (:^._,'oo
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 incji square,
and that the volume of each corpuscle does not exceed
i^oooo\)00oooth of a cubic inch.

The broad faces of tlie discs are not flat, but somewhat
concave, as if tliey were jjushed in towards (me another.
Hence the coriniscle is thinner in the middle than at the
edges, and when viewed under the micro.scope, by trans-
mitted light, looks clear in the middle and darker at the
edges, or dark in the middle and clear at the edges,
according 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 appear-
ance 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 di-awn, the
red corpuscles float about and roll, or slide, over each
other quite freely. After a short time (the length of
which varies in diflferent 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 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. 31, .4.).

The corpuscles remain thus coherent for a certain
length of time, but eventually separate and float freely

Ill THE RED CORPUSCLES 97

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 appear-
ance which is due to the concentrating by evaporation of
the fluid in which they are floating (Fig. 31, H, H.).

The red corpuscles ai'e very soft, flexible, and ela.stic
bodies, so that they readily squeeze through apertures and
passages narrower than their own diameters, and imme-
diately resume their proper shapes (Fig. 25, G, H.).
Examined under even a high power the red corpuscle
presents no very obvious structiu-e ; 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 consists of a sort of spongy colour-
less framework, the stroma, composed of the kind of
material known as protein 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 hsemoglobin,
and may by proper chemical treatment be resolved into a

98 ELEMENTARY PHYSIOLOGY less.

reddish-brown substance containing iron, called hae-
matin, and a colourless protein 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 sj)ongy 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 tlie other
hand, if the plasma be diluted with water, the latter foi'ces
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 discoidal.

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
corpuscles may therefore be regarded as simply so many
tiny 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
characteristic of acting as the carriers of oxygen from the
lungs to the tissues of all parts of the body. We have
already pointed out (p. 25) the general importance of this
transference of oxygen to the tissues ; the details con-
nected with the relationshi[) of hjiemoglobin to this trans-
ference may be more appropriately dealt with when we
study respiration in the next Lesson.

The colouring matter of the corpuscles is further

HEMOGLOBIN CRYSTALS

99

characterised by its property of crystallising more or less
readily. If a little rat's or dog's blood, from which the
fibrin has been removed (see p. Ill) 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 haemoglobin from the stroma into the neighbouring
fluid, in which it is now in solution. If the vessel con-

FiG. 32. — Crystals of H.emoglobin. (After Funke.)
a, squiiTel ; b, gfuinea-pig ; c, cat or dog ; d, mau ; e, hamster.

taining the laky blood be allowed to stand on ice for some
hours, a sediment usually forms at the bottom, and will
be found in a successful experiment, when examined with
the microscope, to consist chiefly of blood-crystals.
The crystals differ in shape according to the animal from
whose blood they were obtained ; in man they have the

H 2

v^

•^ I

100 ELEMENTARY PHYSIOLOGY less.

shape of prisms. The h<i;nioglobin of human blood
crystallises with ditHciilty, but that of the guinea-pig, rat,
or dog, much more readily.

3. The White Corpuscles.— The colourless corpuscles
(Fig. 'M, a a, F.) are larger than the red corpuscles, their
average diameter being 10/x (05^5 of an inch). They are
further seen, at a glance, to ditfer from the red corpuscles
by tlie irregularity of their form, and by their greater
stickiness or adhesiveness, shown by theii* tendency to
attach themselves to the glass slide, while the red
corpuscles float about and tumble freely over one another.

A feature of many of the colourless corpuscles, even
more remarkable th in tlie irregularity of their form is the
unceasing variation of shape which they exhibit so long as

Fid. 33. — Successive Forms assi'mkd by Colourless Corpuscles of
Human Blood. (Magnified about 600 diameters )

The intervals between the forms a, b, c, d, was a minute ; between d
and e two minutes ; so that the whole series of changes from a to e took
five minutes.

they are alive. The form of a red corpuscle is changed
only by influences from without, such as pressure, or the
like ; that of most colourless corpuscles is undergoing
constant alteration, as the result of changes taking place
in their own substance. To see these changes well, a
microscope with a magnifying power of live or six hvindred
diameters is requisite, and some arrangement for keeping
the preparation gently warmed (to 40' 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 coi'puscle represented in Fig. 33, in which

THE WHITE CORPUSCLES 101

a represents the form of the corpuscle when first observed ;
6, its form a minute afterwards ; c, that at the end of the
second ; d, that at the end of the third ; and e, that at
the end of the fifth minute.

Careful watching of such a colourless corpuscle, in fact,
shows that every part of its surface is constantly changing
— undergoing active contraction or being passively dilated
by the contraction of other parts. It exhibits con-
tractility in its lowest and most primitive form.

While they are thus living and active, a complete
knowledge of the structure of the colourless corpuscles
cannot be arrived at. Each corpuscle seems to be formed
simply of a mass of the coarsely or finely granular
substance called protoplasm in which no distinction of

Fin. 34.— Colourless Corpuscles of Blood. (H.ardy.)

A spherical and coai-sely granular, the non-granular part being oc-
cupied by the nucleus ; B" .spherical and finely granular ; C, showing
nucleus after action of acetic acid ; D, flattened, as the corpuscle moves,
and showing the nucleus.

parts can be seen (Fig. 34, A and B). This is especially the
case when the corpuscle is at rest and assumes a spheroidal
shape. Sometimes, liowever, the corpuscle, in the course
of the movements just described, spreads itself out
into a very thin fiat film (Fig. 34, D) ; and when that
is the case there may be seen in its interior a body,
differing in appearance from the rest of the corpuscle.
Again when a di'op of blood is diluted with water,
still better with very dilute acetic acid, the spongy proto-
plasm of the white corpuscles swells up and becomes
transparent, many of the granules becoming dissolved, and
in this case the same body becomes visible. This internal
body, which differs in nature from the rest of the

102 ELEMENTARY PHYSIOLOGY lbss.

substance of the corpuscle, is called the nucleus (Fig.
34, C) ; and when the blood is treated under the micro-
scope, with various staining fluids, such as solutions of
carmine or logwood, the nucleus generally stains more
deeply than the rest of the corpuscle.

Such a colourless corpuscle as has been described, with
its nucleus, is what is called a nucleated cell. It will
be observed that it lives in a free state in the plasma of
the blood, and that it exhil)its an independent contracti-
lity. In fact, except that it is dependent for the conditions
of its existence upon the plasma, it might be compared to
one of th'se simple organisms which are met with in
stagnant water, and are called Aincalxp, whence the name
' amoeboid ' given to the movements of the colourless cor-
puscles 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 embryos •
even of manmials the red corpuscles are at first nucleated.

Whilst all the colourless corpuscles of the blood are
nucleated only about four- fifths of them have the power
of amoeboid movement. Those which have not this power
usually, but not always, possess a cell-body which is quite
clear and transparent. They are known as lymphocytes.
The cell-body of the majority 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 essen-
tial to the structure of the corpuscle. What the real
sti'ucture of the living, contractile ground-substance or pro-
toplasm may be is still a matter of conjecture and dispute.
1 All embryo is the rudiuiLUtury unborn young of any creature.

ra THE WHITE CORPUSCLES 103

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 this solid part is made up
largely of proteins or substances closely allied to
proteins. 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 Lesson V.)

The parts played by the colourless corpuscles in the
animal economy are probably varied and numerous, but
our knowledge of them is very imperfect. We have seen
(p. 82) that under special circumstances these corpuscles
may, by means of their amoeboid 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 the 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 majority of colourless corpuscles
can flow round small solid particles and absorb them into
their cell-body ; in other words they can feed on sub-
stances 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 introduction of foreign matter such as bacteria, &c.
Moreover it is extremely probable that the colourless cor-
puscles may act on the blood and on any foreign matter it
may at times contain by means other than their amoeboid
movements ; namely chemically by the discharge into the
blood of substances formed within themselves. Finally
there are reasons for supposing that when blood is shed,
these corpuscles 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

104 ELEMENTARY PHYSIOLOGY less.

corpuscles, 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
corpuscle, 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. But since, on the whole,
the average numlier of each kind of corpuscle is main-
tained during healthy life, it is evident that new cor-
puscles must be continually forming to take the place of
those which have disappeared.

Our knowledge of the origin of the red corpuscles is
somewhat indefinite ; there is, however, no doubt that
in the Jidult the chief .seat of their formation lies in that
marrow found in the cavities of bones which, from being
very plentifully su{)plied with blood-vessels, is known as
red marrO"W. The majority of colourless corpuscles,
those which exhibit amu3l)oid movement and granular
cell-bodies also have their origin in the red marrow of the
bones. There is some doubt as to whether the cells
which give rise to red corpuscles in the marrow are similar
to ordinary white corpuscles, or are a particular kind of
cell ; and the question has not as yet been definitely de-
cided as to how the mammalian red corpuscle comes to
have no nucleus, although formed in or from cells which
are themselves nucleated. The lymphocytes originate
in the lymphatic glands and other similar structures,
are then passed along the lymphatic vessels into the
blood.

Apart from what is known as to the disappearance of
white corpuscles from the blocjd by migration through the

m PHYSICAL QUALITIES OF BLOOD 105

walls of the vessels, we cannot point with certainty to any
other fate which befalls them. There is no reason for sup-
posing that they are used up in giving rise to red corpuscles.

When we deal with the liver we shall see that the fluid
(bile) which it forms or "secretes" is highly coloured,
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
decomposition of haemoglobin, known as hsematin. If
haemoglobin 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.

It seems probable that some destruction both of red
and of colourless corpuscles takes place in the spleen.

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 (eighty 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 gi'avity being about 1'055. In 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 taken, and diminishing in the intervals between
meals. Average blood may be regarded as consisting of |
plasma and ^ corpuscles.

The blood is hot, its tempei-ature being about 37'^ C.
(98-6" F.).

7. The General Composition of Blood. — Considered

106 ELEMENTARY PHYSIOLOGY less.

chemically, the blood is a faintly alkaline fluid, consisting
of water, of solid and of gaseous matters.

The proportions of these several constituents vary ac-
cording 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 21 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 cjuarters water. Of the 21 parts
of dry solids, 12 ( = 4ths) l)elong to the corpuscles. The
remaining J> are about two-thirds (6"7 parts = 'jths) jjroteins
(substances like white of egg, coagulating by heat), and
one-third (== ith of the whole solid matter) a mixture of
saline, fatty, and carbohydrate matter.s 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. (or 100 cubic inches) of
blood will contain about 60 c.c. (or 60 cubic inches) of
gases. These gaseous matters are carbonic acid, oxygen,
and nitrogen ; 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 average composition of the l)lood gases is about
two-thirds or more carljonic acid, and one-third or less
oxygen, the quantity 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 l)e 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 oxidation. The connection between the
oxygen and the I'ed corpuscles is of a peculiar nature,
beinu a sort of loose chemical combination with one of

in THE PROTEINS OF PLASMA 107

their constituents, and that constituent is, as we have said
previousl)', the hgemoglobin ; for appropriate solutions of
hsemoglobin 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 tem-
perature and pressure. But unlike the oxygen, the
carbonic acid thus held by blood is not peculiarly associated
with the h;emogloljin 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
contaming 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 chlorme 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 Ij-mphatic, constitution.

Animal diet tends to increase the quantity of the red
corpuscles ; 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 thereby in a much greater proportion than
that of the other solid constituents of the blood.

8. The Proteins of Plasma.— By cooling or the
addition 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, of solids in
solution. The solids consist chiefly of that kind of
material which we have so frequently spoken of as protein.
Since these proteins are typical of their class, and since
proteins 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 characteristics of a protein.

108 ELEMENTARY PHYSIOLOGY less.

Pix>teins are, in the first place, extremely complex
substances, 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, those most frequently found in
the body are altered or coagulated, as in the well-known
change wliicli the white of an egg, itself a typical protein,
undergoes when boiled.

In the next place proteins are composed of the four
elements, carbon, hydrogen, oxygen and nitrogen, with
frequently a small amount of sulphur ; 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 except it is presented in the form of a protein.
The general percentage composition of proteins is, roughly
speaking, the same for all of them, and varies but slightly
on either side of the following numbers : — carbon 53
parts, oxygen 22, hydrogen 7, nitrogen 16, and sulphur
1-2. In the absence of any formula this percentage
composition becomes a most important characteristic.

All proteins give the three following reactions,
(i) When boiled with nitric acid they turn yellow, and
this yellow turns to orange on tlie addition of ammonia,
(ii) Boiled with Millon'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 protein in
solution or as a solid.

The solids in the plasma of blood are chiefly protein.s,
of which there are three. The first is known as
fibrinogen and is piecipitated by the addition to plasma
of 15 per cent, of sodium chloride (ordinai'y salt). This

in

THE PROTEINS OF PLASMA

109

result is readily attained by adding to the plasma an
equal volume of a saturated solution of sodium chloride
which contains about 30 per cent, of salt. The fibrinogen
separates out from solution as a fine, flocculent, viscid
precipitate. Fibrinogen is characterised by the fact
that it "sets" or coagulates when heated in solu-
tion to 56' C. (132' F.). The second is called para-
globulin and is similarly precipitated when the

Fig. 35. — Network of FiLAsrENTS left after washing away the
Colouring Matter from a thin, flat Clot of Blood. (Ranvier.)

plasma from whicli 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
filniiiugen, namely 75=^ C. (167" 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 familiar in the white of an egg, and while it

no ELEMENTARY PHYSIOLOOY less.

coagulates when heated to 75" C. (167" F.) as does para-
globulin, it differs from paraglobulin and also from
fibrinogen by not being precipitated when its solution is
saturated with sodium chloride.

Whilst these three i)roteins are characteristic of the
plasma of drawn blood, it would seem that during its
circulation in the body there is but one more complex
protein from which these are derived.

The different precipitability of fibrinogen and para-
globulin in presence of varying amounts of sodium
chloride or other "neutral " salt, such as the sulphates of
sodium and magnesium, has been the starting point of all
investigations on the processes concerned in the clotting
of blood ; we may therefore now proceed very appro-
priately to deal with the nature and causes of this
striking phenomenon.

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. 94. When this solid is
carefnlhj washed, by sti'eaming water over it very gently,
the colouring matter is removed and a coarse network of
extremely delicate fibres or filaments remains. (Fig. 35).

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 become a semi -solid mass. The
filaments are composed of a suV)stance called fibrin ;
hence it is this formatioji 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, by the ordinary proce.ss of opening a vein with
a lancet, a quantity of blood is collected into a basin,
it is at first perfectly fluid : but in a very few minutes it

in THE CLOTTING OF BLOOD 111

becomes, through clotting, 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 in-
crease in number, and run together, 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 cor-
puscles 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 form-
ing 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 thei-efore, when the blood is drawn, they
tend to sink very slowly towards the bottom, but as a
rule clotting 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 i)lasma becomes quite free
from red corpuscles before the fibrin forms in it ; and,
consequently, the uppermost layer of the clot is nearly
white : it then receives the name of the buffy coat. This
is frequently well seen in the blood of the horse, which often
clots slowly, and wliose corpuscle? are unusually heavy.

If the blood is " whipped " with a bunch of twigs as soon
as it is di'awn from the body, fibrin is formed as before.

112 ELExVlEXTARY PHYSIOLOGY less.

but in this case the clot is broken up as fast as it tends to
be formed. Under these circumstances the fibrin collects
upon the twigs, and a red fluid is left behind, consisting
of the serum j)lus 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 proteins, and is in fact itself a protein,
although somewhat impure.

The clotting of the blood is hastened, retarded, or
temporarily prevented by many circumstances.

(a) Temperature. — A temperature up to or slightly
above 40' C. (104' 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 esjiecially sulphate of sodium or mag-
nesium and sodium chloride (connnon 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 ivith living or not living matter. — Contao#
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 introduced into a living vein will
become coated with fibrin, although perfectly fluid blood
surrounds it.

In its normal surroundings, for instance in a portion of
a vein which is tied at each end, blood remains fluid for a
very long time. The lieart 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

Ill THE CLOTTTNCx OF BLOOD 113

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.

Even more remarkable is the immunity from clotting
which is conferred upon blood by the salivary secretion
of the leech. Blood which has been contauiinated with
even minute traces of this substance will remain fluid
indefinitely in an open vessel.

The clotting of blood being thus due to the appearance
in it of fibrin we may now consider how and why it is
formed when blood is .shed

The exact natui-e of the changes involved in the
formation of fibrin have not even yet been thoroughly
worked out ; but the following facts throw a good deal of
light on them : — The pericardium and other serous cavities
in the body, contain a clear fluid, which may be briefly
described as consisting of the elements of the blood
without the red blood-corpuscles. This fluid sometimes
clots spontaneously, as the blood plasma would do, but
very often shows no disposition to clot spontaneously,
especially if it is not removed from the body until some
hours after death. When the latter is the case, the fluid
may nevertheless be made to clot and yield true fibrin, by
adding to it a few drops of whipped blood, i.e. of blood
which has clotted, or a little serum of blood. Now if a
specimen of pericardial fluid, which has been thus observed
not to clot spontaneously, but to clot readily on the
addition of blood or serum, be treated with salt in the
same way as described above for blood plasma, a substance
will be thrown down which, at first siglft, looks exactly
like that thrown down from blood plasma under similar
circumstances. But there is a great diflference, for the
substance thus obtained from pericardial fluid when
dissolved in water will not clot spontaneously, though its
solutions may be made to clot at any time by the addition
of a little serum, or whipped blood. It is clearly an

I

114 ELEMENTARY PHYSIOLOGY less.

antecedent of fibrin, and indeed it was on this account
that it first received the name of fibrinogen, or " fibrin
maker," having been obtained from serous fluids before it
was shown to be present in plasma. It is undoubtedly
present in the substance thrown down by salt from blood
plasma, but then it is mixed with other bodies ; and the
presence of some or other of these bodies seems to be the
reason why in this case it is always converted into fibrin,
and So gives a clot. Conversely the absence of this body
or these bodies from pericardial fluid is the reason wliy
pericardial fluid, or fibrinogen prepared from pericardial
fluid, does not clot spontaneously.

We have previously descril)ed how this fibrinogen may
be separated from plasma. If, now, the yellow fluid or
serum which exudes from a blood-clot be treated with
salt in a similar way (see p. 109) no fibrinogen can ever
be obtained from it, while on the other hand it is found
to contain both paraglobulin and serum albumin. A
comparison of plasma and serum thus shows that during
clotting, i.e. during the formation of fibrin, one constituent
of the plasma, namely, fibrinogen, disappears, the other
two proteins being left to appear in the serum.

Putting these facts together there can be no doubt that
when blood clots the fibrin is formed out of fibrinogen.
But there must also be some substance in blood after it is
shed which leads to the ccmversion of fibrinogen into
fibrin ; for pericardial and other serous fluids contain
fibrinogen, but do not usually clot, and pui'ified solutions
of fibrinogen never clot spontaneously. What is this
substance ?

If serum be* precipitated with an excess of strong
alcohol and after some weeks the precipitate is collected
and extracted with distilled water, this watery extract
contains very little solid matter, but is found to be active
in causing the 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,

Ill THE CLOTTING OF BLOOD 115

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 fer-
ments are characterized by their power, even when
present in sniall quantities, of producing great changes in
other bodies without themselves entering into the changes.
Thus the particular ferment of which we are speaking,
and which has been called "fibrin ferment," or
"thrombin," produces fibrin, and yet does not itself
become part of the fibrin so produced, or at all events
only does so to a very slight extent.

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 some
kind of white corpuscle, or it may be of the blood-
platelets.

This breakdown yields a suljstance, ^^prothrombin,"
which appears to be united to salts of lime present in the
plasma, the resulting substance is thrombin. Yet one
other factor is concerned in the process of coagulation,
for the prothrombin and the salts of lime do not unite
spontaneously. A third substance, a special kind of
ferment known as a kinase, effects the union of the lime
salts with the prothrombin. This kinase is to be found
especially in injured animal tissues ; therefore, while as
we have already seen, blood clots very slowly in uninjured
blood-vessels, it clots with great readiness when in
contact with cut or bruised tissues. Clearly, in this very
complicated chemical reaction, or series of reactions, we
have a mechanism which is of great service to the body as
it provides for the ready clotting of the blood issuing
from a wound. The kinase is provided from the lips of
the wound. It unites the prothrombin from the shed
corpuscles with the lime salts of the plasma forming
thrombin. The thrombin in turn forms fibrin out of

I 2

116 ELEMENTARY PHYSIOLOOxY less.

the fibrinogen of the platsma and the fibrin fills up the
wound and assuages the bleeding.

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 precise ascertainment
of its amount is very dithcult. It may probably be
estimated, on the average, at not less than one-twentieth
or about 5 per cent, of the weight of the body.

Its distribution may be stated in round numbers as
follows : —

One quarter, in the heart, lungs and large blood
vessels.

One quarter, in the liver.

One quarter, in tlie skeletal muscles.

One ([uarter, in the other organs of the body.

11. The Functions of the Blood.— The function of
the blood is to supply nouiislnnent to, and tiike 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 absolutely e.ssential 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
tluough the walls of the vessels in which the blood is
contained. And this vivifying infiuence depends upon
the corpuscles of the blood. The proof of these state-
ments 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 tlie limb, without affect-
ing 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 tlie ligatures be removed before the death
stiffening has become thoroughly established and the

in LYMPH 117

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 whip{)ing. but containing its corpuscles, Ije arti-
ficially passed through the vessels, it will be found nearly
as efiectual 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.

12. Lsmaph : its Character and Composition.—
Lymph, as previously explained, is the fluid whicli 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 accompanied l)y a migration of some of the
colourless corpuscles of the blood. Hence it is at once
evident that lymph may, broadly speaking, be regarded
as so much blood minus its red corpuscles.

Lymph is most easily and plentifully obtained for
examination fiom 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 of collecting the lymph from a
fasting animal in order 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 strikinuly,
in one respect, as we shall see later on, from that which
comes from it in the alisence of food. Taking lymph,
then, from the thoracic duct of a fasting animal, in which
however, the lymph from the intestine still joins the
lymph formed in the rest of the body, it is found

118 ELEMENTARY PHYSIOLOGY less.

to be a transpai-ent, faintly yellow fluid. When
examined under the niicri)sci>i)e it is seen to contain
a luunber ' of corpuscles, the lymph-corpuscles or
leucocytes, very similar to the colourless corpuscles of
l)lood, though perhaps on the whole rather smaller, and
like the latter showing amoeboid movements, especially
if kept warm. These leucocytes 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. 91).

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," and this is due to a deficiency of
proteins. But the proteins present in lymph are the
same in kind as the three already described as found in
plasma, viz., fibrinogen, paraglobulin and serum albumin.
Hence lymph clots when left to itself and yields fibrin
identical with that obtained 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
ap{)reciable ; the bulk of the gas is carbonic acid and a
very small amount of nitrogen.

Average lymph is therefore very similar to plasma
somewhat diluted with water ; but it is important to
notice from what has been said above that the dilution
does not afi'ect the salts and the proteins to the same
extent. Neither is the dilution the same in lymph col-
lected from different regions of the body. Thus lymph
from the arm or leg contains less, and lymph from the
liver more solid matter than is present in average mixed
lymph from the thoracic duct. The significance of

1 Equal on the average to the number of white corpuscles present in
blood, so that in a drop of lymph very few would be seen and often none
at all.

2 Only a1x)ut 5 per cent, of its weight as compared with 8 to 10 per
cent, in plasma.

Ill LYMPH 119

these facts will become apparent when we speak of the
probable mode of formation of lymph. While lymph thus
differs in composition when derived from various parts of
the body, it also differs when collected from the same part
at different times. Usually the 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
essentially 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 "mUky." 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 mUky, 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 fall-
ing upon them and hence the fluid appears white. Some
of the fat in chyle exists in the form of minute g'.ol)ules,
similar to those present in mUk, but the larger part is so
finely divided that it can only be spoken of as " gran-
ules " 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. The word '' exudation " was used purposely, as not
imjjlying more than that some of the liquid part of the
blood passes out into the tissues, and certainlj^ as not ex-
pressing any view as to the nature of the processes con-
cerned in that passage. But now that we have dealt with
the composition of lymph, and especially since we have
seen that the composition varies according to the part of
the body from which it flows, we may profitably con-

120 ELEMENTARY PHYSIOLOGY less.

sider what are the causes which in the first place lead
to the presence of lymph in the lymph spaces of the
tissues.

Three processes suggest themselves at once as possible
causes ; these are filtration, diffusion, and Osmosis.
With the first of these every one is more or less familiar,
and we need say no more than that it consists in the
passage of fluid and of substances in solution through a
porous membrane as the result of a difference of pressure
on the two sides of the membrane. Diffusion, on the
other hand, is, broadly s})eaking, independent of the
difference of pressure necessary to cause filtration. A
simple experiment shows at once the essential feature of
diffusion. If a small dish of parchment, similar to that in
which " ices" are frequently served, be floated on water,
and into the dish a solution of common table salt be
placed, by chemical analysis the salt will soon be found to
have passed into the water outside through the substance of
the dish, even though this contains no visible holes. There
will never be any appreciable difference of level between
the surface of the fluid within and without the dish.

The difference between diffusion and osmosis will be
understood by comparing this experiment with the
following. 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 solution of gum acacia and fix the tube
vertically, as in Fig. 36, 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. The water passes
gradually through the paper into the tube so that the
liquid rises in the narrow part of the tube and may ulti-
mately stand an inch or more above the surface of that
which is in the beaker. The essential difference between
the two experiments lies in the fact that in the former both
the water and the salt pass (diffuse) freely through the
membrane, in the second the membrane is permeable to the

THE FORMATION OF LYMPH

121

water but not to the gum. The gum attracts water to its
side of the membrane, but does not itself traverse the
membrane.

Substituting the wall of the capillaries for the paper used
in the preceding ex[)eriment we have the conditions
necessary for a possibly diffusive
interchange between the blood on
the one side of that wall and the
fluid in the tissues on the other.
But here we may say at once that
diffusion will not account at all
completely for the formation of
lymph. In support of this state-
ment it may suffice to point out
that lymph contains a considerable
amount of proteins, and these are
characteristically non-dift'usible.^

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 for-
mation of lymph as the result of
filtration ? Here again we may at
once say that the passage of fluid
through the walls of the capillaries
under the influence 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 supply.
But we must not conclude therefore that the process is
entirely due to filtration. The process of osmosis also

Fig. 36. — To Illustrate a
Simple Experiment on
Diffusion.

t.t. thistle tube ; p.p.
paichiuent paptjr ; s. solu-
tion of gum ; b. beaker ;
w. water in beaker.

1 Substances such as the proteins of blood, also gelatin, which will not
diffuse, are known as colloids, in couti-.idistiuction to crystalline sub-
stances or crystalloids, which diffuse readily.

122 ELEMENTARY PHYSIOLOGY less, ni

enters largely into the cause of the flow of lymph!
Speaking broadly lymph flows much more freely from
organs which are in activity than from tliose which are at
rest. The immediate result of this activity is the pro-
duction of carbonic acid and more com[)licated substances.
The carbonic acid no doubt diffuses rapidly into the blood
but the more complicated bodies accunmlate in tlie lymph
outside the capillaries and attract water from the V)lood by
the process of osmosis, thus increasing the volume of the
lymph at the expense of that of the blood.

Lastly the capillary wall itself differs in character in
different places and is more permeable in some places
to particular substances than in others.

The great majority of the known facts about the cause
of the flow of lymph can be explained by the physical
factors which have been enumerated.

LESSON IV
RESPIEATION

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
complex product, not of any one organ or constituent
of the body, but of all. Many of its features are doubt-
less 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 aflected by the soil over which it
flows, by the water-weeds which cover its banks, and
by affluents from distant regions ; by irrigation works
whicli are supplied from it, and by drain-pipes which flow
into it.

One of the most remarkable and important of the

124 ELEMENTARY PHYSIOLOGY less.

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 (Lesson II.).

Thus, if l)lood 1)0 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 dark j)urplish hue. And as this con-
trast is met witli 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 mo.st 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
subjected to chemical examination, the differences pre-
sented 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 i)roportion
which the carbonic acid gas bears to the oxygen ; there
being a smaller quantity of oxygen and a greater quantity
of carbonic acid, in venous than in arterial blood.

Every 100 volumes of blood contain about 60 volumes
of gases. These may be extracted by boiling 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 ra[)id exit of the gases into the vacuum ;
they can now be collected and measured and their respec-
tive volumes determined. The composition of the blood-
is thus found to be the following : —

IV THE GASES OF BLOOD 125

Arterial Blood. Venous Blood.

Oxygen 20 vols 8-12 vols.

Carbonic acid ... 40 ,, 46 ,,

Nitrogen 1-2 , 1-2 ,,

This difference in their gaseous contents is the most
obvious 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 com4jlete 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 sei-ious im-
pediment to the passage of either gas.

Practically we may say that the most important differ-
ence between venous and arterial blood is not so much
the relative 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
of 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. Under
the former circumstances they may, not improbably,

126 ELEMENTARY PHYSIOLOGY

redect the light more strongly, so as to give a more
distinct colourtition 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-crystjils
(Lesson III.), even when perfectly free from actual blood-
corpuscles, change in colour from scarlet to purple, ac-
cording as they gain or lose oxygen. It has already been
stated (p. 98), that oxygen exists in the blood in loose
combination with ha'iuoglobin. And further, a solution
of hajmoglobin, when thus loosely combined with oxygen,
has a scarlet colour, while a solution of haemoglobin
deprived of oxygen has a purplish hue. Hence arterial
blood, in which tlie lu^nioglobin is richly provided
with oxygen, is naturally scarlet, wliile venous blood,
which not only contains an excess of carbonic acid, but
whose hiiemoglobin 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 have to discuss later on, namely horn 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 a{)i)arently in a state of simple
solution, as though the blood were so nmch water. A
very snaall part of the i>xygen is similarly simj^ly dissolved
in the blood, but practically almost the whole of it is in a
state of loose chemical combination loith the haviofilobin of
the red coi-jjuscles. The facts which prove this are simple
and conclusive. In the absence of red corpuscles plasma
and serum only absorb as much oxygen as does an equal
quantity of water, namely about one volume per cent. ;
but blood, where the red corpuscles are present, may con-
tain as much as 20 volumes per cent, of oxygen. Again,

IV THE ESSENCE OF RESPIRATION 127

solutions of hfemoglobin absorb oxygen as readily and
largely as blood does. Finally, the oxygen is known to
be loosely combined with the lipenioglobin, because when
blood is subjected to a gradually increasing vacuum, the
oxygen does not come off uniformly and progressively,
as the vacuum is made greater, in the way it would if
it were in mere solution ; on the contrary it escapes
icith a sxuiden rush after the 2Jressiire has been considerably
rediiced.

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
somewhat incomplete. It is known, however, that the
carbonic acid is combined chiefly in some constituents of
the plasma rather than with the corpuscles, and most
authorities consider that the larger part is present in
plasma united with sodium in the form of sodium bicar-
bonate, NaHCO,.

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 other waste products). The demand for oxygen
is met by a supply fi'om 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 whii-led away into the veins. The blood there-
fore leaves the tissue poorer in oxygen 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

128 ELEMENTARY PHYSIOLOGY less.

tissues is frefiviently spoken of as the respiration 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, 1st, that the blood
remains dark in the right auricle, the right ventricle, and
the pulmonai'y artery ; 2nd, that it is scarlet not only in
the aorta, but in the left ventricle, the left auricle, and
the pulmonary veins.

Obviously, then, the cliange from venous to arterial
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 sulistance of an extremely thin mem-
brane. This membrane is in contact with the air, so that
the blood in each cajjillary 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 between the V)lood and the air ; the latter
gaining moisture and carbonic acid, and losing oxygen.'

This is the essential step in respiration. That it really
takes place may be demonstrated very readily, by the
experiment described in the first Lesson (p. 5), in which
air expired was proved to differ from air inspired, by con-
taining more heat, more water, more carbonic acid, and
less oxygen ; or, on the other hand, by putting a ligature

1 The student mu.st guard himself against the idea that arterial blood
contains no carbonic acid, and venous blood no oxygen. In passing
through the hings 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 carbonic acid as there is of oxygen. See the table on
p. 126.

IV THE ORGANS OF RESPIRATION 129

on the windpipe of a living animal so as to prevent air
from passing into, or out of, the lungs, and then examin-
ing 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 venae cav?e 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 absolutely 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 extensive and complicated accessory
machinery — the arrangement and working of which must
next be carefully studied.

3. The Organs of Respiration.— The back of the
mouth or pliar3nix communicates by two channels with
the external air (see Fig. 37, gf./.e.). One of these is
formed by the nasal passages, which cannot be closed by
any muscular apparatus of their oa\ti ; 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.
38, Gl) — capable of being closed by a sort of lid — the
epiglottis (Fig. 37, e.) — or by the shutting together of
its side boundaries, formed by the so-called vocalcords.
The glottis opens into a chamber with cartilaginous walls
— the larynx ; and leading from the lai-jTix do-\\-nwards
along the front part of the throat, where it may be very
readily felt, is the trachea, or windpipe (Fig. 37, c, Fig.
38, Tr.).

If the trachea be handled through the skin, it will be
found to be firm and resisting. Its waUs are, in fact,

E

130 ELEMENTARY PHYSIOLOGY i.ess.

strengthened by a series of cartilaginous hoops, which
hoops are incomplete behind, their ends being united

Fio. 87.— A Section of the Mouth and Nose taken vertically, a

LITTLE TO THE LErf OF THE MlDDLE LlNE.

a, the vertebral column ; 6, the gullet ; c, the wind-pipe ; d, the
thyroid cartilage of the larynx ; ■', tho epiglottis; /, the uvula; g, the
opening of the left Eustachian tube ; h, the opening of the left lachrymal
duct; !, the hyoid bone; k, the tongue; I, the hard palate; m, n, the
base of the skull ; o, p, q, the superior, middle, and inferior turbinal
bones. The lettei*s g, f, e, are placed in the pharynx.

only by muscle and membrane, where the trachea comes
into contact with the cesophagus, or gullet. The trachea
passes into the thorax, and there divides into two branches,

THE ORGANS OF RESPIRATION

131

a right and a left, which are termed the bronchi (Fig.
38, Br.). Each bronchus enters the lung of its own side,
and then breaks up into a great number of smaller
branches, which are called the bronchioles or bron-
chial tubes. As these diminish in size, the cartilages,
which are continued all through the bronchi and their

Fig. 38. — Back View of the Neck akd 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 ; Tr. trachea ; L.L. left lung ; R.L. right lung;
£r. bronchus ; P. ^. pulmonary artery ; P. K. pulmonary veins ; ^o. aorta;
D. diaphragm ; H. heart ; V.C.I. vena cava inferior.

large ramifications, become smaller and more scattered
and eventually disappear, so that the walls of the smallest
bronchial tubes are entirely muscular or membranous.
Thus while the trachea and bronchi ai-e kept permanently
open and pervious to au* by their cartilages, the smaller

K 2

132

ELEMENTARY PHYSIOLOGY

bronchial tubes may be almost closed by the contraction
of their muscular walls.

Fig. 39.

A. Two infundibiila (h) with the viltimate bronchial tube (a) which
opens into them. (Magnified 'JO diameters.)

B. Diagrammatic view of an air-cell of A seen in action ; a, epi-
thelium ; b, partition 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 ; b, small branches of arteries and
veins.

D. Poition still more highly magnified.

Each 5ner bronchiole ends at length in an elongated dila-
tation about ^ of an inch in diameter on the average and
known as an infundibulum (Fig. 39, A. b). The wall

THE STRUCTURE OF THE LUNGS 133

of an infundibulum sends flattened projections into its
interior and thus forms a series of thin partitions by
which the cavity of the infundibukim is divided up into a
large number of little sacs or chambers. These sacs are
the alveoli or air-cells.

The very thin walls (Fig. 39, B h) which separate these
alveoh are supported by much delicate and highly elastic
hssue, and carry the wide and close-set capillaries into
which the ultimate ramifications of the pulmonary artery
pour its blood (Fig. 39, C, D). Thus, the blood contained
in these capillaries is exposed on both sides to the air-
being separated from the alveolus on either hand only by
the very delicate pellicle which forms the wall of the
capillary, and the lining of the alveolus. The partitions
between the alveoli are covered with extremely thin
flattened 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.

The infundibula are bound together in groups by con-
nective tissue to form larger masses termed lobules.
These lobules are similarly-bound together in groups to
form lobes and the several lobes are united to form a
lung. The blood-vessels, nerves and lymphatics of each
lung are carried by the connective tissue which binds the
whole together.

The trachea is essentially a tube whose waU is strenc^th-
ened and whose bore is kept open by C-shaped hoops or
rmgs of cartilage. These hoops lie imbedded in fibrous
connective tissue in the outer part of the waU, in which
also there is a certain amount of unstriated muscular
tissue, running chiefly across the space between the ends
of each cartilaginous hoop. The inner surface of the tube
IS lined with a mucous membrane, i This consists of

f»lT^'^ ""^"^ is appUed generaUy to the membranes lining those in-
ternal passages of the body which communicate with its surface such
.^s the respiratory passages, the alimentary canal, and the bladder
Mucous membranes become continuous with the skin at the edge of the
openmg on the surface. They derive theu- name from the fact that they

134

ELEMENTARY PHYSIOLOGY

an epithelium of ciliated cells, interspersed with
mucous cells ; these lie on a distinct basement mem-
brane and below this is a small amount of lymphoid and
elastic tissue. (See Lesson XII.) Between the mucous
membrane and the outer layer which carries the hoops of
cartilage, there is a certain amount of areolar connective

Fio. 40. — Ciliated EpiTHKi.iUMCKt.i.s from the Trachea of the Rabbit,

UlnHLY MAGNIFIED. (ScHAFER.)

ml, to2j «t3, mucus-secreting cells lying between the ciliated cells and
seen in various stages of mucin-formation.

tissue (p. 86), in which some small mucous glands are
imbedded ; this constitutes the submucous layer. The
ciliated cells are elongated columnar 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. (See also Lesson VII.)
The mucous and ciliated cells extend from the trachea into
the smallest branches of the bronchi.

are covered with a viscid secretion called mucus, whose characteristic
constituent, mucin, is secreted by speciiil mucous cells or by small
mucous glands, imbedded in or lying beneath the membrane.

THE THORAX AND LXJNGS 135

No conditions could be more favourable to a ready
exchange between the gaseous contents of the blood and
those of the air in the alveoli than the arrangements
which obtain in the pulmonary capillaries ; and, thus far,
the structure of the lung fully enables us to understand
how it is that the large quantity of blood poured through
the pulmonary circulation becomes exposed in very thin
streams, over a large surface, to the air. But the only
result of this arrangement would be, that the pulmonary
air would very speedily lose all its oxygen, and become
completely saturated with carbonic acid, if special luo-
vision were not made for its being incessantly 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 Lungs.— The lungs (and heart)
are enclosed in what is practically an air-tight box, whose
walls are movable. This box is the thorax. 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. 38).

The two lungs occupy almost all the cavity of this box
which is not taken up by the heart (Fig. 41). Each
is enclosed 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, quite 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 ' (Fig. 42, pi.). So long
as the walls of the thorax are entire, the cavity of each

1 There is a small amount of fluid between the two surface* of the
pleura, to facilitate their rubbing easily ajrainst one another. This
serous fluid is m reality, as is pericardial fluid, a form of lymph

136

ELEMENTARY PHYSIOLOGY

pleura is practically oblitei-ated, that layer of the pleura
which covers the lung being in close contact with that
whiqh lines the wall of the chest ; but if a small opening

Fio. 41. — Diagram of the Thorax showing the Position of the Heart

AND Lungs.

1-12, ribs ; 11-12, floating ribs ; .t, sternum ; r, rib ; c.c, costal cartilages ;

c, clavicle ; I, lungs ; a, apex of heart ; peric. pericardium, cut edge.

be made into the pleura, the lung at once shrinks to a com-
paratively small size, and thus develops a great cavity
between the two layers of the pleura. If a pipe be now
fitted into the bronchus, and air blown through it, the

rv THE THORAX AXD LUNGS 137

lung is very readily distended to its full size ; but, on
being left to itself, it collapses, the air being driven out
again with 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.

Fig. 42. — Trans\'erse Section of the Chest, wtth the Heart and
Lungs in pl.\ce. (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 cesophagus ; p. outer bag of pericardium ; pi. the two
layers of pleura ; v. azygos vein.

The lungs are kept distended in the dead subject, so
long as the walls of the chest are entire, b}- the pressure
of the atmosphere acting down the trachea, bronchi and
bronchioles 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

138

ELEMENTARY PHYSIOLOGY

LESS.

the chest, it cannot produce such a separation without
developing a vacuum between these two layers. To effect
this, the elastic tissilfe 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

Fio. 43.— Sternum viewed from the Front.

1-T, points of attachment of first seven ribs ; cl. points of attachments of
clavicles (collar-bones) ; x, lower projecting end of sternum.

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 atmosj)heric pressure inside the lung is
equalised by that outside it, and the elastic tissue, freed

THE RIBS

139

from its opponent, exerts its full power on the lung and
it collapses.

5. The Movements of Respiration. — The hinder ends
of the ribs are attached to the vertebral column so as to

Fig. 44.— The Boty "Walls of the Thor.\x.

a, b, vertebral column ; 1-12, ribs ; c, sternum ; d costal cartilages ;
e, united cartilages of lower true ribs.

be freely movable upon it. The front ends of the first
ten pairs of ribs are connected either directly (first seven
ribs), or indirectly (next three ribs), by the costal
cartilages to the sternum, the connection being therefore
flexible (Figs. 41, 44, 45). When left to themselves the ribs
take a position which is inclined obliquely do^Tiwards and
forwards.

140

ELEMENTARY PHYSIOLOGY

Two sets of muscles, called intercostals, pass between
the successive pairs of ribs on each side. The outer set,
called external intercostals (Fii,'. 45, A), run from
the rib above, obli([uely downwards and forwards, to the

'N, ^.

Fio. 45. — View of Four Ribs of the Dog with the Intercostal

Ml'SCLES.

a, the bony rib ; 6, the cartilage ; c, the junction of bone and cartilage ;
d, uno8sified, (, ossified, portion.s of the stcruum. A, external intercostal
muscle ; B, internal intercostal muscle. In the middle interspace, the
external intercostal has been removed to show the internal intercostal
beneath it.

rib below. The other set, internal intercostals (Fig.
45, 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 that when a

THE INTERCOSTAL MUSCLES

1.41

muscle contracts, it tends to shorten the distance between its
tico ends be borne in mind. Let a and h in Fig. 46, 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 xtoy will be inclined
downwards and forwards, and one from w to z will be
directed downwards and backwards. Now it is obvious
from the figure that the distance between x and y is

Pig. 46. — Diagram of Models illustrating the Action of th4
External and Internal Intercostal Muscles.

B, inspiratory elevation ; C, expiratory depression.

shorter in B than in A and much shorter than in C ;
hence when x y \s 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.

If the simple apparatus just described be made of wood,
hooks being placed at the points x ;/, 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

142 ELEMENTARY PHYSIOLOGY less.

the band being so short as to be put on the stretch when
hooked on to either x y, or w z, with the bars in the
horizontal position, A, the elasticity of the band, when
hooked on to x and y, 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
virtue 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.

Such a model, however, does not accurately represent
tlie ribs, with their numerous and peculiar curves, and
hence, while all are agreed that the external 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 the levatores costarum and perhaps
certain other muscles. The levatores costarum are
attached by their upper ends to the transverse pro-
cesses of the last cervical and first eleven dorsal ver-
terbraj, and each muscle is fastened by its lower end to
the rib next below the vertebra from whicli the muscle
itself springs. These muscles must also by their con-
traction 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 for-
wards, and must therefore thrust the sternum slightly
outwards, or aAvay from the vertebral column. By this
movement the size of the thorax is of course increased
from back to front, an increase wliich may be easily felt by
placing one hand on the back and one on the chest of a

THE DIAPHRAGM 143

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 toidth of the thorax from side to side, an increase
which may, as before, be i-eadily 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. 1, D). Frcflii its middle, which is
tendinous, muscular fibres extend downwards and outwards
to the ribs, and two, especially strong masses, which ai-e
called the pillars of the diaphragm, to the spinal column
(Fig. 47) . When 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. 48, A), or in other words when the diaphragm is
flattened, the size of the thorax is increased from above
downwards.

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 we have said (p. 137), 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

144

ELEMENTARY PHYSIOLOGY

elastic lungs to the required extent, and to prevent the
formation of any vacuum between the two pleurte. This
drawing of air into the lungs constitutes an inspiration.

Fio. 47.— The Diaphragm of a Doo viewed from the Lower or
Abdominal Side.

V.C.I, the vena cava inferior; 0, the oesophagus; Ao. the aorta ; the
broad white teiidinois middle (B.B.H) is easily distinguished from the
radiating muscular fibres (.4..i.ji) which pass down to the ribs and into
the pillars (C*. /)) in front of the vcrtcbrse.

At the end of each inspiration the diaphragm ceases to
contract and the external intercostal muscles relax. So
much of the elasticity of the lungs as was called into
play by the contraction of the diaphragm and the

IV LABOURED RESPIRATION 145

raising of the ribs now comes into action ; air is driven
out of the lungs and the diaphragm rises to its former
position (Fig. 48, B), being partly also 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 had been put on the stretch during inspiration,
and possibly also by the contraction of the internal inter-
costal muscles. By these means air is driven out of
the lungs, the forcing out of the air constituting an
expiration, which taken together with an inspiration
makes up respiration.

Thus it appears that we may have either diaphragmatic
resjnration, or costal resviration. As a general rule, how-
ever, the two forms of respiration coincide and aid one
another, the contraction of the diaphragm taking place at
the same time with that of the external intercostals, and
its relaxation with their relaxation.

In ordinary quiet respiration, inspiration is an active
process depending on the contraction of muscles ; ex-
piration, 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 stei'num ; 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 con-
traction of certain muscles which connect the ribs and
breast-bone with the pelvis, and form the front and side
walls of the abdomen, the abdominal muscles. They
assist expiration in two ways : first, directly, by pulling
down the ribs ; and next, indirectly, by pressing the

L

146 ELEMENTARY PHYSIOLOGY less.

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
flattened 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
viscera being driven against them by the descent of the
diaphragm — the spine is straightened, the head thrown
back, and the shoulders outwards, so as to afford the
greatest mechanical advantiige to all the muscles which
can elevate the ribs.

It is a remarkaljle circumstance that the mechanism of
respiration 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.

Si(jhi)ig is a deep and prolonged inspiration. " Sniffing "
is a more rapid inspiratory act, in which the mouth is kept
shut, and the air made to pass through the nose.

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 character-
istic 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 ex^jiratory 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 approxima-
tion of the soft palate and the base of the tongue, the air
•is forced through the nasal passages.

THE THORAX AS A BELLOWS

147

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 respiratory naovements is just the same as that of

Fig. 48. — Diagrammatic Sections of the Body in

A, inspiration ; £, expiration ; Tr, trachea ; St, sternum ; D, diaphragm ;
Ab, abdominal walls. The shading roughly indicates the stationary air.

the approximation and separation of the handles of the
bellows, which drive out and draw in the aii' through the
pipe. There is, however, one difference between the
bellows and the respiratory apparatus, of great importance
in the theory of respiration, though frequently overlooked ;
and that is, that the sides of the bellows can be brought

L 2

148 ELEMENTARY PHYSIOLOGY

close together so as to force out all, or nearly all, the air
wliich they contjiin ; while tlie walls of the chest, when
approximated as much as possible, still inclose a very con-
siderahle cavity (Fig. 48, 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 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.
Fir.st, the breath is drawn in or inspired, immediately
afterwards it is driven out or expired ; and these suc-
cessive acts are followed by a brief pause. Thus, just as
in the rhythm of the heart, the auricular systole, the ven-
tricular systole and then a pause follow in regular order ;
so in the chest, the inspiration, the expiration, and then a
pause succeed one another. But in the chest, unlike the
case of the heart, the pause is generally very short com-
pared with the active movement ; indeed, sometimes it
hardly exists at all, a new inspiration following imme-
diately on the close of expiration.

6. The Amount of Air Respired. — At each inspiration
of an adult well-grown man about 500 c.c. (20 to 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 con-
veniently 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. (from 75 to 100 cubic inches).

About as much more in addition to this remains in the
chest after an ordinary expiration, ahd is called Supple-
mental air.

IV THE AMOUNT OF AIR RESPIRED 149

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 reapiratory
or intal capacity. It varies according to a person's height,
weight, and age.

It results from these data that the lungs, after an
ordinary 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 inspiration. Apart from the circumstance, then,
that the fresh air inspired has to fill the cavities of the
hinder part of the mouth, and 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 exjiiration. But as
the bronchi branch out into a prodigious mnnber of
bronchial tubes, the inspired air can only peneti'ate 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 aij comprehended under these names merely
shifts its outer limit in the bronchial tubes, as the chest
ddates and contracts, without leaving the lungs, and is
hence called stationary air ; the tklal air, alone, being
that which leaves tlie 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 pai-t of a middleman between the two parties — the

150 ELEMENTARY PHYSIOLOGY less.

blood and the fresh tidal air — who desire to exchange
their commodities, 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
complete contact and continuity, and hence the exchange
between them must take place according to the ordinary
laws of gaseous diffusion.

Thus, the stationary air in the air-cells, or, as it is '
frequently called, Alveolar Air, gives up oxygen to the
blood, and takes carbonic acid from it, though the exact
mode in which the change is effected is not thoroughly
understood. By this process it becomes loaded with
carbonic acid, and deficient in oxygen. There is very
much greater excess of tlie one, and deficiency of the
other, than is exhibited by inspired air, seeing that the
latter acquires its composition by diffusion in the short
space of time (four or five seconds) during which it is in
contact with the alveolar air.

Dry alveolar air contains in each 100 volumes —

Oxygen Nitrogen Carbonic .-cid

14-5 80 rro

7. TherUhanges 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 37' C. (98*6° F.).

(ii) However dry the external air may be, that expired
is nearly, or quite, saturated with watery vj^our. This
vapour is not derived from the stationary air, but from
the walls of the outer aii' passages, so that the inspired
air is practically saturated with aqueous vapour before it
reaches the bronchi.

(iii) While ordinary inspired air contains in 100
volumes —

Oxygen Niti-ogeu Carbonic Acid

20-96 7900 04

IV CHANGES OF AIR IN RESPIRATION 151

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
decomposable kind. Nothing is known of their nature,
but they are probably the chief cause, why air which has
been breathed once is extremely unwholesome if breathed
a second time ; hence they are of great importance in con-
nection with ventilation (see p. 168).

(v) The volume of the tidal air is but little altered by
being breathed, because the two parts of oxygen in the
carbonic acid (CO.,) occupy the same volume as the car-
bonic acid itself, or in other words the volume of the
carbonic acid is equal to that of the oxygen contained in
it. But as a matter of fact very close analysis of the
expired air shows firstly that the volume of oxygen
which disappears is slightly greater than the volume
of carbonic acid which takes its place. This is because
all the oxygen taken in does not go to form car-
bonic acid ; some of it unites with hydrogen to form
water and some with other elements such as sulphur.
Hence the volume of the expired air is slightly (^q) less
than that of the inspired air. In the second place careful
analysis shows that the nitrogen in expired air may vary
very slightly : sometimes it is a little in excess of, some-
times slightly less than, that inspired, and sometimes it
remains unaltered.

8. The Amount ofWaste 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

152 ELEMENTARY PHYSIOLOGY less.

no exercise, in the course of twenty-four hours, and are
charged with carbonic acid, and deprived of oxygen, to
the extent of about four per cent. This amounts to about
450 litres (16 cubic feet) of the one gas taken in,
and of tlie other given out. Thus, if a man be shut uj) in
a close room, having 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 re{)resented by a piece of pure char-
coal weighing 225 grammes (eight ounces).

The quantity of water given oft" from the lungs in the
twenty-four hours varies very much, but may be taken on
the average as rather less than 250 c.c. (half a pint, or
about nine ounces). It may fall below tliis amount, or
increase to double or treble the (juantity.

The air expired during the first half of an expiration
contains less carbonic acid than that expired during the
second half. Further, when the frequency of respiration
is increased without altering the volume of each in-
spiration, though the percentage of carbonic acid in each
expiration 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 the second case remains more than half
of what it was in the first case, and hence the total
evolution is greater.

This does not imply that there is a greater formation of
carbonic acid in the tissues, but only that the carbonic
acid in tlie blood passes more rapidly into tlie alveolar air
and is in turn replaced by that in the tissues. Thus the

IV THE GASEOUS INTERCHANGE 153

quantity of carbonic acid in the body is reduced, and
indeed the whole condition is one which cannot be
maintained for more than a few minutes.

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 expelled : exercise and the taking of food
have a corresponding eflfect.

In proportion to the weight of the body, the activity of
the respiratory process is far greatest in children, and
diminishes gradually with age.

The excretion of carbonic acid is greatest during the
day, and gradually sinks at night, attaining its minimum
about midnight, or a little after.

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 vege-
table matters.

9. The Nature of the Respiratory Changes in the
Lungs and Tissues. — The essential difference between
venous and arterial blood is, as we have previously ex-
plained, entirely dependent upon the relative amounts of
the two gases, oxygen and carbonic acid, which they re-
spectively contain. We have also pointed out where the
changes from arterial to venous blood and vice versa, take
place, and have indicated the general causes of the con-
version as being an interchange between the blood and the
tissues on the one hand, and between the blood and
the stationary air in the lungs on the other. But we
have not so far dealt with the nature of the processes
involved in effecting the interchange, and to these we
must now turn our attention.

A clear understanding of certain facts and principles

154 ELEMENTARY PHYSIOLOGY LEsa

as to the behaviour of gases towards each other and
towards liquids with which they may be in contact is
essential as a preliminary.

When a gas is enclosed in a vessel, it exerts a
pressure on its walls which is measured by the height
of the column of mercury which it can support in a
vertical tube connected with the vessel. If two gases
are mixed in the vessel, each gas exerts its own
pressure just as if the other gas were not present ; the
total pressure of the two gases is therefore eqrial to the
swn of their separate pressures. The pressure due to
each gas in the mixture is called the partial pressure
of that gas, and is proportional to the qnantitij of the gas.
Hence if the toUil 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 oxj'gen is
1% ^ 7^0 ~ 159 6 mm. (6 '3 inches of mercury), and
that of the nitrogen is ^^q x 700 = 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 <«"o gases of eijual solubility
are in contact with the some liquid tliey 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 pressures of the gases in the liquid
are the same as the partial pressures of the gases in the
space. If the partial pressure of one of the gases be made
less in the space over the liquid, then some of tliat gas will
make its exit from the licjuid ; 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

rv THE GASEOUS INTERCHANGE 155

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 wlien the surface
of the fluid is separated from the neighbouring gases by
a thin, moist, porous film.

The air in the alveoli of the lungs is a mixture of gases
separated by the thin, moist, filmy wall of the capillaries
from the venous ))lood brought to them by the pulmonary
artery ; and what we have now to consider is whether,
in the absence of any other obvious cause, the differences
of partial pressure between the gases in that air and the
same gases in that blood are sufficient to account for
the interchange by which the venous blood becomes
arterial.

Now the oxygen and carbonic acid in blood are not in
mere solution but largely in combination with certain
constituents of the corjiuscles and plasma. Hence the
pressures they exert are mucli less than they would,be if
they were in simple solution. But on the other hand the
compounds formed by these gases in the blood are very
unstable and easily dissociated or broken up, so that
a sufficient difference of partial jJressure on the surface of
the blood may still easily start the interchange between
the blood and the alveolar air and between the blood
and the tissues.

The partial pressures of oxygen and carbonic acid in
alveolar air are usually about 100 and 40 mm. of mercury
respectively. By applying these data we find that venous
blood in contact with oxygen at the partial pressure it has
in alveolar air becomes arterialised as regards its ox5'gen.
And the entry of the oxygen is further assisted by the
fact that it is stowed away in loose chemical combination
in the red corpuscles. Similarly we may say that the exit
of carbonic acid is due to the difference between the

156 ELEMENTARY PHYSIOLOGY less.

(lower) partial pressure of carbonic acid in the alveolar
air and the (higher) partial pressure it has in the venous
blood ; but the case is not quite so clear as it is in respect
of the entry of oxygen. For the partial pressure of car-
bonic acid in alveolar air is not inconsiderable, and its
exit from the blood is opposed by the fact that it is in
loose combination with some constituent of the plasma.

The blood thus fully arterialised is whirled away to the
tissues, where it becomes once more venous. In the
tissues 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 power-
ful pum[) ; the partial pressure of oxygen in tlie tissues
may even l)e zero. Hence oxygen readily passes over from
the arterial blood. Again, the living ti.ssues are always
producing carbonic acid in greater or less amount accord-
ing as they are more or less active ; the partial pressure
of cj^rbonic acid is therefore high in the tissues and quite
sufficient to account for the passage of this gas from them
into the neighbouring arterial blood.

The amount of oxygen left in venous blood is dependent
on the varying activity of the tissues and of the quantity
of blood which is flowing through them, and this is the
reason why the volume of this gas was given (p. 125) as
varying from eight to twelve volumes in each hundred
volumes of venous blood and indeed it may even vary
within wider limits.

10. The Nervous Mechanism of Respiration. — Of
the various mechanical aids to the respiratory process,
the nature and workings of which have now been de-
scribed, 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 nervoas system, and
varies fronx time to time.

IV THE NERVES OF RESPIRATION 157

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 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 ob-
longata (see Lesson XI. ). The spinal bulb thus stimu-
lated 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 contraction of the expiratory
muscles, producing a cough (see p. 146). 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
apparatus. 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 ^i suction-pump.

In each the pump is the cause of the motion of the
fluid, though that motion may be regulated, locally, by
the contraction 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 con-
trolled by the central nervous system, that of the respi-
ratory apparatus results mainly from the operation of a

158 ELEiMENTARY PHYSIOLOGY less

nervous centre lodged in the spinal bulb, which has been
called the respiratory centre.

This centre is situated (see Fig. 49, R.G.) close to the
two previously described (Figs. 22 and 23) as the vaso-
motor and cardio-inhibitoi-y centres (pp. 69 and 75). 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
respiration 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 j)art of
the spinal bulb.

The action of the centre is primarily antomatic ; in
other words the impulses it sends out appear to be the
result of changes started irithin 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 sul)ject, however, to control by impulses reaching it
from outlying parts of the body, and more particularly
by changes in the condition or quality of the blood which
circulates in the capillaries of the centre itself, iji a
way to be described presently.

The intercostal muscles are supplied by intercostal
nerves coming from the spinal cord in the region of the
back (Fig. 49, ION, ICN, ICN), and the muscular filjres of
the diaphragm are supplied by two nerves, cme on each side,
called the phrenic nerves (Fig. 49, Phr.), which starting
from certain of the spinal nerves in the neck, dij) 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 distributed. Now from the nervous
respiratory centre in the spinal bulb impulses at repeated
intervals descend along the ujiper part of the spinal cord
and, passing out by the phrenic and intercostal nerves

IV THE NERVES OF RESPIRATION 159

respectively, reach the diaphragm and the 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
external intercostal muscles, but to other, expiratory,
muscles, which they throw into contraction, and thus
expiration is brought out. As a general rule the inspira-
tory 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 imjjulses
to set them at work, as soon as the inspiratory impulses
cease and tlie diaphragm and other inspiratory muscles
leave off contracting, thej' come of themselves into action.
But, in laboured breathing, very powerful expiratory
impulses may leave the respiratory centre and pass to the
various muscles whose contractions help to drive the air
out of the chest.

Every day 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,
quickened 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 si^ezing, laughing, coughing, <S:c., are
profound and peculiar. 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 corre-
spondingly complicated ; but certain broad facts are fairly
simple, and to these we may now turn.

The main trunk of the vagllS nerve (see Lesson XI.)
gives off a branch to the larynx as it passes down the neck
(Fig. 49, S.Lr.). If the vagus be cut hdoio the point oj
exit of this nerve (as at x, Fig. 49) and the upper (central)

160

ELEMENTARY PHYSIOLOGY

end (y, Fig. 49), connected with the spinal bulb be stimu-
lated, the respiration becomes hurried. Thus we have in

Fig. 49. — Diagram to ilu'strate 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, Sp.C, spinal cord ; R.C, respiratory centre in the bulb ; JCN, ICN,
ICN, three intercostal nerves ; Phr. one phrenic nerve passing to the
diaphragm D; Vg. vagus nerve; V.G, ganglion of vagi.s nerve; S.Lr,
superior laryngeal nerve. The dotted lines, cf, cf, <•/, indicate paths of
conduction for impulses to the respiratory centre from some part of the
body such as the skin ; the dotted lines, af, a/, similar paths from the
brain to the centre. The arrows show the direction in which impulses
travel along each nerve or path.

the vagns a nerve such that impulses passing up it may
quicken the respiration by their action on the respiratory
centre.

IV DYSPXCEA AND ASPHYXIA 161

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 respirator}' movements.

In the case of the vagus impulses seem to be ordinarily
always passing up it to facilitate the action of the respira-
tory centre, for if the vagus nerves be simply cut, the
respiration becomes at once extremely slow.

These two nerves 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
manifold changes which the respiratory movements from
time to time undergo.

11. Influence of Blood-Supply on the Respiratory
Centre. 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 extremelj' sensitive to the varying condition or
quality of the fluid (blood), whose gaseous composition it
has to regulate. This expectation 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 capillai'ies of the
spinal bulb. Thus, when by any means the blood becomes
less arterialised than it should be, the respiratory 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
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

H

162 ELEMENTARY PHYSIOLOGY

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 unusually venous by means less violent than
the above. Since the rapidity of difl'usion between two
gaseous mixtures depends on the difference of the propor-
tions in which their constituents are mixed, it follows that
the more nearly the composition of the tidal air aj)proaches
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 oxygen in the tidal aii- is gradually
diminished and the carbonic acid grathialbj 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 be-
come quicker and 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 inspiratory and expiratory muscles con-
tract. If the offending cause of dyspnuia 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, but the accessory muscles (p. 145) come
into more prominent play, and chiefly those ivhich assist
expiration. StiU later, nearly all the muscles of the body
are thrown into a state of violent contracting activity, and
with the onset of these convulsions dyspnossi passes
over into asphyxia. The violence of the convulsive
movements speedily leads to exhaustion, and the con-

IV

ASPHYXIA 163

vulsions cease. After this stage is reached, a long-drawn
inspiration takes place at intervals ; but the intervals
become longer and longer and the inspiratory movements
more and more feeble until the last breath is taken and
breathing ends with an expiratory gasp.i

After death by asphyxia the blood throughout the
whole body is of course venous. The right side of the
heart, the great (systemic) veins and the pulmonary
' arteries are highly distended with blood, while the left
side of the heart is empty. This condition of the vascular
system is brought about largely by an obstruction to the
usually easy flow of blood through the lungs, which is due
to a constriction of their small arteries caused by the
stimulation resulting from the venosity of the blood. But
it is helped by the unusually forcible drawing of blood
into the great veins which results from the increased
force of the respiratory movements (see p. 166).

Venous blood is distinguished from arterial by two
features, by ha\-ing less oxygen and more carbonic acid.
Hence, in asphyxia, two influences of a distinct nature
are co-operating ; one is the deprivation of oxygen, the
other is the excessive accximulation of carbonic acid in the
blood. Oxygen starvation and carbonic acid poisoning,
each of which is injurious in itself, are at work to-
gether.

The respiratory centre is very sensitive to variations
in the normal quantity of carbonic acid in the blood.
We have already stated that the carbonic acid in the
alveolar air exerts a ])artial pressure of 40 mm. of
mercury, this corresponds very closely to the pressure of
carbonic acid in the blood leaving the lung and reaching
the respiratory centre. If this pressure of carbonic acid
be raised by even a few millimetres the respiration

1 The tertQ asphyxia is sometimes used to include aU the above three
stages, from the onset of dyspnoea until death ensues.

H 2

164 ELEMENTARY PHYSIOLOGY less.

becomes accelerated. Indeed, if the amount of acid,
of whatever kind it may be, in the arterial blood be
materially increased, the respiratory centre is stimulated
and respiration is quickened. Small changes in the
partial pressure of oxygen in the air breathed do not
affect the respiratory rhythm but great deprivation of
oxygen rapidly jjroduces the symptoms of aspliyxia.
The reason of this is that in the absence of a sufficient
supply of oxygen the combustion in the tissues is
incomplete, large quantities of acid substances are
formed as the result, these are thrust into the blood
where they accumulate, and stimulate the respiratory
centre to make the desperate efforts v, hich are character-
istic of asphyxia. The acids produced in the tissues, in
their stimulating action on the respiratory centre, act as
hormones (p. 27).

Tliat the lack of oxygen is an 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
hydnjgen combines 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
moi'e serious effect, as it turns out the oxygen from the
blood-corpuscles, and forms a very stable combination of
its own with the hjemoglobin. The compound thus
formed is only very gradually decomposed by fresh
oxygen, so that if any large proportion of the blood-
corpuscles be thus rendered useless, the animal dies
before restoration can be effected. Badly made common
coal gas sometimes contains 20 to 80 per cent, of carbon
monoxide ; and, under these circumstances, a leakage

IV EFFECTS ON THE CIRCULATION 165

of the pipes in a house may be extremely perilous to
life.

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 re-
lated to the working of the respiratory apparatus. Of
these the chief is the effect of the inspiratory and expi-
ratory 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 requii'ed 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 subjected to a less pressure than 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
condition of insjiiration a pound ^ 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 tliese 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 ttian tiiis, not more than 2 or 3
ounces.

166 ELEMENTARY PHYSIOLOGY

vessels, and the pressure upon the blood in them, were
everywhere the same, the result of this excess of pressure
on tho surface would be to drive all the blood from the
arteries and veins of 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
resisting, 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
produce 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 resist-
aiice in the heart, partly from the presence of resistance
in the capillaries, it all tends to accelerate the flow of blood
toivards the heart. In the arteries, on the other hand, the
[)ressure is only a fractional addition to that which existed
before ; so that, during the .systole, it only makes a com-
paratively small addition to the resistance which has to

EFFECTS ON THE CIRCULATION 167

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 ex-
piration. 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 especi-
ally affected, and a check is given to the flow of blood
in them, which may become visible as a renons 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 pressure 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 sjTichronously 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.

The eflfects of the respiratory movements on the flow of
blood towards the heart must be the same for any other
structure contained in the thorax and connected with
vessels lying outside the thorax. Now the thoracic duct

168 ELEMENTARY PHYSIOLOGY less.

is a large, thin-walled tube placed inside the thorax and
communicating with tlie lymphatic vessels which lie in
the abdomen, and, further, it is plentifully supplied with
valves. 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 duct
with the venous system. Hence the respiratory move-
ments on the whole are a not unimportjint aid to the
onward flow of lymph (see p. 92).

13. Ventilation. — In the case of breathing the same
air over and over again the deprivation of oxygen, and
the accumulation of carljonic acid, cause injury, long
before any .signs of even dyspncjea are observed. Under
these circumstances 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 (jf 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 quantities, have a powerfully
deleterious action. It need hai'dly be added that the
persistent breathing of such air tends to lower all kinds
of vital energy, and predisposes to di.sease. 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. 151). Since the organic
matter does not admit (jf direct estimation, the percentage
of carbonic acid in the air is usually taken as an in-
direct measure of its amount. Air which has been
fouled by breathing is injurious if it contains more than
■05 per cent, of carbonic acid. Knowing the amount of
air passed through the lungs in one hour and the amount
of carbonic acid it contains (p. 152), calculation easily showa

IV VENTILATION 16&

that if the percentage of carbonic acid is to be kept down
to the limit of -05 per cent, 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 niue feet liigh, wide aud long, contains only 729
cubic feet of air.

LESSON V

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 arteiialised, by the
process described 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 aortsi. As that great vessel traverses
the thorax, it gives off several large arteries, by means of
which blood is distributed to the head, the arms, and the
walls of the body. Passing through the diaphragm (Fig.
47, Ao.), 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 arttjries, 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 tissues. It follows that, if the blood is to be
kept pure, the waste matters thus incessantly poured into,

LESS. V LOSSES OF THE BLOOD 171

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 (see Lesson I.). 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 radiation, 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 respectively. Besides this a certain
quantity of heat is lost by the urine and fgeces which are
always warm when they leave the body.

On the side of gain we have, in the first place, the

172 ELEMENTARY PHYSIOLOGY less.

various substances which are the products of the activity
of the several tissues, muscles, brain, glands, &c., 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 tissues, namely carbonic acid, is
emphatically a waste product and is got rid of as soon as
possible. But some of the substances 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 u.se 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
substances 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 what are called the lacteals
or lymphatics. 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 Le.sson II., p. 87).

In the third place, the blood is continually gaining
oxygen from the air through the lungs.

Then again the blood, while it loses heat by the skin and
lungs, gains heat from the tissues. As we have already
seen (Lesson I., p. 25) 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

GAINS AN0" LOSSES OF THE BLOOD 173

much, but probably 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
certain "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 appreci-
able 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 continually going on, though greater at
certain times than at others ; and the materials for the
bile have to be provided by the blood. Some of the
constituents of the bile, however, pass back from the

1 The word '■ secretion " is used by physiologists in two 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. But the fluid thus formed is itself
spoken of as "a secretion." The word "excretion" is usually applied
to any fluid which after its formation is useless and requires to be at
once got rid of. Thus we say that urine is an excretion which is se-
creted {i.e. formed) by the kidneys; and we speak of those secretory
structures which get rid of waste as excretory organs.

174 ELEMENTARY PHYSIOLOGY

intestines into the blood ; and so far the loss to the blood
by the liver is temporary only.

Of ail the gains to the blood perhaps the most con-
stant 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 cir-
cumstances.

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 kidney, as water, carbonic acid, urea, and saline bodies.

And while it is continually receiving heat from the
several 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 con-
veniently arranged in the following tabular form : —

Sources of Loss and Gain to the Blood. ^
A. Sources of Loss : —
I. Loss of Matter.

1. The lungs : carbonic acid and water (fairly

constant).

2. The kidneys : urea, water, salines (fairly

constant).

3. The skin : water, salines (fairly constant).

4. The tissues : constructive material (variable

especially in the case of those tissues
whose activity is intermittent, such as the
muscles, many secreting glands, &c.),
water, &c., to form lymj)!!.

1 The learner must be careful not to confound tho Ifisses and gains of
the blood with the lo.sses and gains of the bod;/ as a wliole. The two
differ in much the same way as the internal commerce of a country
differs from its export ajid import trade.

THE KIDNEYS 175

n. Loss of Heat. •

1. The skin.

2. The lungs.

3. The excretions by the kidney and the alimen-

tary canal.

B. SoTJRCES 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).^

II. Gain of Heat.

1. The tissues generally, especially the more

active ones, such as the muscles.

2. The blood itself, probably to a very small

extent.

2. The Kidneys.— 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 : we now proceed to the second 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.
50). From the niiddle of the concave side (called the

1 The gain from those lymphatics which are caUed lacteals, since it
comes from the aUmentary canal, vanes much more.

176

ELEMENTARY PHYSIOLOGY

hilus) of each kidney, a long tube with a small bore, the
Ureter (Ur.), proceeds to the Bladder (BL).

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

Fig. 50.

The kidneys (K); ureters (Ur.y; with the aorta (^o.) ; and vena cava
inferior (r.f./.) ; and the renal arteries and veins. Bl. is the bladder,
the top of which is cut off so as to show the openings of the ureters (1, 1)
and that of the urethra (2).

bag of serous membrane which has exactly the same
relations to the cavity of the abdomen and the viscera con-
tained in them as the pleurse 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 (Fig. DO, 1, 1). Each

V THE URINARY APPARATUS 177

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 in two layers and
surrounded externally by some fibrous connective tissue.
In front of the ureters is a single aperture which leads
into the canal called the Urethra (Fig. 50, 2), by which
the cavity of the bladder is placed in communication with
the exterior of the body. The openings of the ureters
enter the walls of the bladder obliquely, so that it is much
more easy for the fluid to pass from the ureters into the
bladder than for it to get the other wa}', from the bladder
into the ureters.

Mechanically speaking, there is little obstacle to the
free flow of fluid from the ureters into the bladder, and
from the bladder into the urethra, and so outwards ; but
certain muscular fibres arranged circularly around the part
called the "neck" of the bladder, where it joins the
urethra, constitute Avhat is termed a sphincter, and are
usually, during life, in a state of contraction, so as to close
the exit of the bladder, while the other muscular fibres
of the organ are relaxed.

It is only at intervals that this state of matters is
reversed ; and the walls of the bladder contracting, while
its sphincter relaxes, its contents, the urine, are dis-
charged. But, though the expulsion of the secretion of
the kidneys from the body is thus intermittent, the
excretion itself is constant. The urinary fluid is pro-
pelled drop by drop along the ureters by rhythmic
contractions which pass along their walls in the direction
of the bladder where it accumulates, until its quantity is
sufficient to give rise to the uneasy sensations which
compel its expulsion.

3. The Structure of a Kidney. — When a longitudinal
section of a kidney is made (Fig. 51), the upper end of the
ureter {U) seems to widen out into a basin-like cavity (P),
which is called the pelvis of the kidney. Into this
sundry conical elevations, called the pyramids (Pi/)

N

178

ELEMENTARY PHYSIOLOGY

LESS.

project ; and their summits present multitudes of minute
openings — the final terminations of the tubules, of which
the thickness of the kidney is chiefly made up. If the
tubules be traced from their openings towards the outer
surface, they are found, at first, to lie parallel with one
another in bundles, which radiate towards the surface,
and subdivide as they go ; but at length they spread about
irregularly, and become coiled and interlaced. From this

-SA.

Fio. 51. — Longitudinal Section of the Human Kidney.

Ct, the cortical substance ; M, the medullary substance ; P, the pelvis
of the kidney ; U, the ureter ; R.A, the renal artery ; Py, the pyramids.

circumstance, the middle part, or medulla (medulla,
marrow) of the kidney looks different from the superficial
part or cortex {cortex, bark) ; but, in addition, the
cortical part is more abundantly supplied with vessels
than the medullary, and hence has a darker aspect. Each
tubule after a very devious course ultimately terminates
in a dilatation (Fig. 52) called a Malpighian capsule.

THE KIDNEY 179

Into the summit of each capsule, a small vessel (Figs. 52
and 53, v.a), one of the ultimate branches of the renal
artery, which reaches the kidney at the concave side,
with the ureters, and divides into branches which pass in
between the pyramids (Fig. 51, RA), enters (driving the
thin wall of the capsule before it), and immediately breaks
up into a bunch of looped capillaries, called a glomer-
ulus (Fig. 52 gl), which nearly fills the cavity of the
capsule. The blood is carried away from this glomerulus

Fig. 52.— a Malpighian Capsule (highi.y magnified).

v.a, small branch of renal artery entering the capsule, breaking up into
the glomerulus, r/.l, and finally joining again to form the vein, t:e.

c, the tubule ; n, the epitheUum over the glomerulus ; b, the epi-
thelium lining the ciipsule.

by a small vein or vessel (re), which does not, at once,
join with other veins into a larger venous trunk, but opens
into the network of capillaries (Fig. 53) which surrounds
the tubule, thus repeating the portal circulation on a
small scale.

The course of the tubules is devious and peculiar.
After leaving the capsule each tubule becomes twisted
and is spoken of as convoluted (Fig. 54, II.). Passing
towards the medulla, at first in a slightly spiral course,
it proceeds straight down into the pyramid, where it bends

N 2

180

ELEMENTARY PHYSIOLOGY

LBSS.

hack upon itself and runs up again into the cortex. The
loop thus formed is known as the loop of Henle,' and
tlic two parts of which it is formed are called the
descending limb and the ascending limb of the
loop (Fig. 54, IIL IV.).

Reaching the cortex once more the tubule becomes
zigzag and then again convoluted, after which it

Fig. 53. — Circulation in the Kidney.

ai, small branch of renal artory reiving off the br.mch va, which enters
glonicniluR, issues as vc, and tlion breaks \ip into capillaries, which after
aurrovnidiug the tubule find their way by v into ri, branch of the renal
vein ; )/i, capillaries around tubules in parts of the cortical substance
where there are no glomeruli.

passes into a straight part or collecting tubule,
which, leaving the cortex finally for the medulla and unit-
ing with other similar collecting tubules, forms the dis-
charging tubule which finally opens near the summit of
a pyramid (Fig. 54, V.-IX.).

Each tubule is lined tliroughout by cells, the epithelium,
and the cells differ in their characters in the several

1 Who first described it.

THE KIDNEY

181

parts of its course. The details of these differences
are numerous and complicated, but the following state-
ment includes all that is most essential. The cells in
the convoluted, spiral and zigzag portions are, on the
whole, large, very granular and striated, and both the

Fig. 54.— Diagrammatic View of the Course of the Tubules in
THE Kidney.

r, cortical portion answering to Ct in Fig. 51, I- being close to the
surface of the kidneys ; g, p, medullary portion, p reaching to the
summit of the pyramid.

IX, opening of tubule on the pyramid ; VIll, VII, VI, the straight
portion of the tubules ; V-II, the twisted portion of the tubules ; /, the
Malpighian capsule.

cells and their nuclei stain readily and deeply. In the
collecting tubules the cells are flattened, cubical, quite
free from granules and do not stain readily. In the
loop of Henle the cells of the descending limb resemble
those in the collecting tubules in being flattened and

182

ELEMENTARY PHYSIOLOGY

LESS.

free from granules ; the cells of the ascending limb,
though small, are somewhat granular and often striated
and thus present attinities to the cells of the convoluted
tubules. These two chief types of cells are shown in
Fig. 55.

The artery which supplies the kidney enters at the
hilus and divides into branches which pass round the
pelvis and proceed outwards between the pyramids. At

Fig. 55.— Types of the two chief kinds of Cells in the Tubules
OF the Kidney.

A, tubules cut lengthwise ; B, tubules cut across.
a, type of (secreting) cell lining the convoluted (spiral and zigzag)
tubules ; b, type of cells lining the conducting, collecting and dis-
charging tubules ; n, nuclei ; c, in B, capillaries seen in section.

the junction of the medulla and cortex the.se branches
spread out sideways and form arches. From these arches
branches run (i) straight out to the surface of the kidney
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 substance 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,

URINE

183

and leave the kidney by a course parallel to that of the
entering arteries.

4. The Composition of Urine and Chemistry of
Urea. — The renal secretion is a clear yellowish tluid,

Fig. 5f>. — BLfioD Vessels of Kidkey. (Cadiat.)

a, part of arterial arch ; 6, interlobular artery ; c, glomerulus ; d,
ferent vessel ; e, capillaries of cortex ; f, small arteries of raedulla ;
g, venous arch ; k, straight veins of medulla ; i, interlobular vein.

whose specific gravity is not very different from that 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 solu-
tion (i) organic substances, of which the chief is urea.

184 ELEMENTARY PHYSIOLOGY LESS.

with a very much smaller amount of uric acid, (ii) In-
organic salts, chiefly sodium chloride and suli)hates and
phosphates of sodium, potassium, calcium and magnesium,
(iii) Golonrimf matters, of which but little is known,
(iv) Gases, chiefly carbonic acid with a very small amount
of nitrogen and still less oxygen.

An average healthy man excretes about 1,500 c.c.
(50 ounces or 2i pints) of urine each day. In this are
dissolved 33 grammes (1| oz. or about 2 per cent.) of urea
and not more than '5 grammes (10 grains) of uric acid.
The amount of salts is about half that of the urea, and of
this the larger part consists of sodium chloride.

The quantity and composition of tiie urine vary greatly
according to the time of day ; the temperature and mois-
ture of the air ; the fasting or replete condition of the
alimentary canal ; the nature of the food ; and the amount
of fluid consumed.

The qnantit]! depends on temperature and moisture of
the air, because, as we shall see (p. 198), 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 as
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, and the nitro-
gen in this waste leaves the body again chiefly as urea, a
very small part reappearing in the form of uric acid.
Hence variations in the quantity of urea excreted thus
become the measure of the amount of nitrogen turned
over or " metabolised " in the body from time to time.

V UREA 185

Urea is a white crystalline solid, very "soluble in water,
and composed of carbon, oxygen, hydi-ogen and nitrogen.
Its chemical formula is (NH,,)oCO, from which it is seen
to contain rather more than 46 per cent, of nitrogen. It
forms characteristic crystalline compounds with nitric
acid and oxalic acid, which serve for its qualitative identi-
fication. When acted on by sodium hypobromite, urea is
decomposed in such a way that aU the carbon becomes
carbon dioxide (carbonic acid gas) and the nitrogen is
given off as a gas :

(XH,).,CO + SNaBrO = Ng + CO., + 2H,0 + 3XaBr.

This is an important reaction, since by measuring the
nitrogen evolved the urea may be estimated quantita-
tively ; a method now very generally employed. When
in solution, under the influence of a ferment sometimes
secreted by the mucous membrane of the bladder, or of
organisms from the air, urea takes up water and becomes
ammonium carbonate ;

(NH.,).,CO + 2H.0 = (NH^),C03.

This accounts for the ammoniacal odour of stale urine.

Historically urea is interesting as being the first
organic animal product prepared (synthetically) from
inorganic sources.

5. 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 quantity of water to pass through it, but
refuses to allow other substances to pass through. And
when we come to study the minute structure of the kidney
we find much to support this idea. Thus we saw that the
surface of the glomerulus is, practically, free, or in direct

186 ELEMENTARY PHYSIOLOGY less.

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, only
separated from the cavity of the tubule by the capillary
wall and the very delicate membrane covering the glomer-
ulus. 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 we have reason to think that a great deal
of the water of urine together with certain of the con-
stituents (the inorganic .salts) is thus as it were filtered off
by the Malpighian capsules. But it nmst 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 as well as bodies so
small as blood-corpuscles, whereas the glomerulus, while
letting some things through, refuses to admit others, e.g.
the proteins of the plasma, even though they are in a
state of solution.

Speaking of the process, with this caution, as one of
filtrati(m, it is obvious that tlie 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 tlie 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 kiJney, the I)lood-pressure is rai.sed in the glomeruli,
and the flow of urine is copious. If the same nerves
be stimulated, the arterial tubes are narrowed and con-
stricted, less blood goes to tlie 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

V THE SECRETION OF URINE 187

in warm weather and especially when the skin is active
and perspiring, less will go to the tidney, 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 secreted, 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 stated, by epithelium cells, and these cells, in
certain parts of the tubule, especially where these are
coiled, are what is called 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, certain
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 ofi" by the Malpighian
capsules.

We may give two striking facts in support of this view.
In some animals the glomeruli of the kidney receive their
blood-supply by an artery, which is quite distinct from
the vessel which takes blood to the tubules. When the
artery supplying the glomeruli is tightly tied, no blood
can go to the glomeruli, but urea is still passed out from
the kidney and must come from the tubules. Again, a
certain colouring matter when injected into the blood is
excreted in the urine ; this colouring matter can easily be

188 ELEMENTARY PHYSIOLOGY less.

tracked thi'ough the kidney and be seen to pass through
the cells of the convoluted tubules and not through the
walls of the glomerular blood-vessels.

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.

6. The History of Urea. — Nitrogen enters the body
as protein 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 proteins into urea. This change involves the whole
nitrogenous metabolism ' of the body and from its impor-
tance merits a short statement of the chief facts which
throws some light on the (juestion of where and how urea
is formed.

In the first place the urea excreted in the urine is not
made in the kidney out of some other (antecedent) sub-
stance. The activity of the kidney con.sists entirely in
picking out 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. These muscles are the seat
of active oxidation even when at rest, and this activity
is enormously increased at times when they are contract-
ing. 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 which some should
contain nitrogen, since the muscles are chiefly built up
of niti'Ogenous material. But this waste does not come
out of the muscle as ready-made urea, neither do we

1 The word roetabotism (/ii.eTai3oA7) = change) is conveniently usei 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."

THE HISTORY OF UREA 189

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.

But there is another organ in the body of great size
and importance, the liver (p. 207). This organ is the
seat of many activities with which we shall deal later on,
among which the making, of urea out of other substances
brought in the blood is not the least important. We
also know to a certain extent what these " other sub-
stances " are. "When we study digestion we shall see
that the products of digestion of proteins are nitro-
genous, crystalUne substances known as amino-acids.
These are absorbed through the walls of the intestines,
carried to the liver in the blood of the portal vein,
and in part converted into urea hy the liver. Moreover
there are instances of animals which have survived
the functional removal of the liver for several days.
In such cases the excretion of urea by the kidney is
suspended and at the same time there is an accumulation
of ammonia in the blood. As it is known that the liver
has the power of converting ammonia into urea it seems
clear that a considerable proportion of the urea excreted
has ammonia as an antecedent. Whatever its exact
antecedents we may regard the urea secreted as coming
from two main sources, the waste of the tissues and the
superfluous nitrogen of the food which is undergoing
digestion in the alimentary canal ; the urea derived from
tissues is called endogenous uvea,, that derived directly from
the food, the antecedents of which have never, therefore,
been built into the living substance of the body, is called
exogenous urea. In both cases the final stage in the form-
ation of urea may be regarded as taking place in the liver.
7. 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 enclosed in a caoutchouc bag,

190 ELEMENTARY PHYSIOLOGY less.

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 uj)on
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 surface of the body. Of these the
loss of watery vapour and of heat are of immense im-
portance, 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 before dealing with these
activities of the skin we must understand the main facts
as to its structure.

The skin consists of two parts, an outer layer or
epidermis, resting on a deeper layer, the dermis.
The skin as a whole is connected to the tissues it covers
by a layer of loose fil)rous connective tissue (see Fig. 28),
called subcutaneous 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 (see Lesson XII.).
The surface of the dermis is raised up into little hillocks
or elevations known as tlie papillae. Arteries enter the
dermis and break up into capillaries which are very close
set at its surface and in the papilliB ; thus the dermis
is extremely vascular. Nerves also run into the dermis,
and passing outwards, form a bi-anching layer of fibres at
its junction with the epidermis, and from this layer
extremely fine nerve fibrils pass out and between the lower
cells of the epidermis. In some parts of the body, some
of the branches of the nerves run up into the papillee,
where they are connected with sp)ecial nervous structures
such as tactile corpuscles and end bulbs. But since
these are of importance solely in connection with the

THE SKIN

191

Fig. 57.— Diagram to show the Stkuctuke of the Skin.

E.c, epidermis corneous part ; E.m, epidermis Malpighian part ; D.c
connective tissue of dermis ; p, 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.

192 ELEMENTARY PHYSIOLOGY less.

functions of the skin as a sense-organ, they Avill be
described later on (see Lesson VIII.).

The epidermis lies on the dermis and dips down into all
its depressions. It is composed entii-ely of cells and has
no blood-vessels.

The cells may be divided into two layers. Of these the
innermost or Malpighian layer (Fig. 57, Em.) 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 corneous layer
(Fig. 57, Ec.) is made up of cells which, losing their
nuclei become converted into flattened, thin scales, con-
sisting of, horny material. These are the cells which
become so strongly developed on parts of the body sub-
ject 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 take by new cells passed up from the
deeper layers of the epidermis (see also Lesson XII.).

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
80/i (j5j^(j of an inch) in diameter, and a quarter of an inch
long, which is imbedded in the dermis. Each tube is
lined with an epithelium continuous with the epidermis
(Fig. 57, 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. 57, (il and Fig. 58).

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 flat, those in the
tube of the gland are larger and more colunuiar, 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
of the capillaries, that of the glandular tube, and its

SWEAT-GLANDS

193

epithelium, which, taken together, constitute but a very
thin pellicle ; and the arrangement, though different in
detail, 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 tubule. Here the
perspiratory tubule coils about, and among, the vessels.

Pio. 58.— Coiled End of a Sweat-Gland (Fig. 57, gl.), Epithblium

NOT SHOWN.

a, the coil ; 6, the duct ; c, network of capillaries, inside which the
gland lies.

In both cases the same result is arrived at, — namely, the
exposure of the blood to a large, relatively free, surface,
on to which certain of its contents 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.

194

ELEMENTARY PHYSIOLOGY

The number of these glands varies in different parts of
the body. They are fewest in the back and neck, where

Pio. 59.

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 ; 6, 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 substance, (b'igs. A
and B magnified about 4 diameters ; Fig. C magnified about 200
diameters.)

their number is not much more than 400 to a square inch.
They are more numerous on the skin of the palm and

HAIRS AND NAILS

195

Pig. 60.— a Hair in its Hair-Sac.

a, shaft of hair above the skin ; 6, cortical siibstance of the shaft, the
medulla not being visible ; c, newest portion of hair growing on the
papilla (i) ; d, cuticle of hair ; e, cavity of hair-sac ; /, epidermis (and
root-sheaths) of the hair-sac corresponding to that of the integument (»i) ;
g, division between dermis and epidermis ; h, dermis of hair-sac corre-
sponding to dermis of integument {I) ; k, mouths of sebaceous glands ;
n, horny epidermis of integument.

sole, where their apertures follow the ridges visible on the
skin, and amount to between two and three thousand on

196 ELEMENTARY PHYSIOLOGY less.

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
secreting power.

In certain regions of the skin the corneous 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 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 dei-viic 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 papilla} (Fig. 59,
B, C). The surfaces of all these are covered with growing
epidermic cells, which, as they flatten and become con-
verted into 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 epidermic cells are added to the base of the
nail, which is thus constrained to move forward.

The nail, thus constiintly receiving additions from
below and from behind, slides forwards over its bed, and
projects beyond the end of the finger, where it is worn
away or cut off.

A hair, like a nail, is composed of horny cells ; but
instead of being only partially sunk in a fold of the integu-
ment it is atfirst wholly enclosed in a kind of bag, the hair-
sac or follicle, from the bottom of which a papilla
(Fig. 60, 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 epidermic

HAIRS

197

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 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, of a loose and open texture, which
sometimes contains air ; of a cortical substance sui'-

b <" ^^^

A

Fig. 61. — Part of the Sn ' 'N''~lo5ed withik its Root-

Sheaths AND TREATED WITH CaUSTIC SoDA, WHICH HAS CAUSED THE

Shaft to become distorted.

a, medulla ; 6, cortical substance ; c, cuticle of the shaft ; from dtof.
the root-sheaths, in section. (Magnified about 200 diameters.)

rounding this, made up of coalesced elongated homy cells ;
and of an outer cuticle composed of fiat horny plates,
arranged transversely round the shaft, so as to overlap
one another by their outer edges, like tiles on the roof of
a house. The superficial epidermic cells of the hair-sac
also coalesce by their edges, and become converted into
root-sheaths, which embrace the root of the hair, and
usually come away with it when it is plucked out.

The sebaceous glands are small glands whose duct opens
into the follicle of a hair. They form a fatty secretion
which lubricates the hairs.

198 ELEMENTARY PHYSIOLOGY ^ less.

8. The Composition and Quantity of Sweat.— The
sweat glands have the function of forming a fluid, the
sweat, which is passed out on to the surface of tlie body.
This fluid is composed chiefly of water containing a small
amount (1-2 per cent.) of solid matter in solution, chiefly
sodium chloride.

In its normal state the sweat, as poured out from the
proper sweat-glands, is alkaline ; but ordinarily, as it
collects upon the skin, it is mixed with the fatty secretion
of the sebaceous glands, and then is frequently acid. In
addition it contains scales of the external layers of the
epidermis, which are constantly being shed.

Pro. 62. — Section of the Skin showing the Roots or the Hairs and
THE Sebaceous Glands.

o, epidermis ; 6, muscle of c the liair sheath, on the left hand.

Under ordinary conditions the sweat is evaporated from
the surface of the skin as fast as it is .secreted ; in this
case it is frequently spoken of as inseiisible perspiration.
But when violent exercise is taken, or 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 evapor;ites : 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 temi)era-
ture and other conditions of the air, and according to the

PERSPIRATION 199

state of the blood and of the nervous system. It is estimated
that, as a geneKftl rule, the quantity of water excreted by
the skin is about double that given out by the lungs in
the same time.

The amount of matter which may be lost by perspiration
under certain circumstftnces, 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 there is some reason
to believe that the total amount of solids which
are eliminated by profuse sweating may be consider-
able.

9. 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. 24),
the three great organs — lungs, kidneys, and skin — which
have been described.

In ultimate anatomical analysis, each of these organs
consists 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 constitute its secretion or excretion ; but the three
organs differ 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 kid-
neys ; 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

200 ELEMENTARY PHYSIOLOGY less.

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
functions much interfei'ed with, death ensues, however
active the skin may be. And, on the other hand, if the
skin be covered with an impeneti-able varnish, the tempe-
rature of the body rapidly falls, and death takes place,
though the lungs and kidneys remain active.

10. The Secretion of Sweat and its Nervous
Control.— In analysing the process by which the per-
spiration is eliminated from the body, it must be
recollected, in the first place, that the skin, even if there
were no glandular structures connected with it, would be
in the position of a moderately thick, permeable mem-
brane, interjjosed 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 itcomes into contact with the skin, 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
disappear by evaporation. Now, in its relation to the
blood, the skin is such a bladder full of hot fluid.

Thus, persjjiration to a certain amount, must always be
going on through the substance of the integument, but
probably 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

THE SECRETION OF SWEAT 201

and there is an increased flow of blood through these
capillaries. At the same time the cells of the glands Ijegin to
pour out an increased quantity 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
imprdses which had previously been keeping the arteries
constricted (see p. 68). 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 ? 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 cells, as
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 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
efiect 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 in the central nervous system, 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 tem-
perature of the body is regulated (p. 203).

11. Animal Heat : its Production and Distribution.
—It has l^een seen that heat is being constantly given
off from the skin and from the air-passages ; and every-
thing that passes from the body carries away with it,
in like manner, a certain quantity of heat. Further-
more, the surface of the body is much more exposed to

202 ELEMENTARY PHYSIOLOGY less.

cold than its interior. Nevertheless, the temperature
of the body is in health maintained very evenly, at
all times and in all parts, within the range of two
degrees or even less on either side of 37^ C (98*6°
Fahrenheit).

This is the result of three conditions : — the first, that
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 vvlienever oxidation takes place. As
we have seen, the tissues all over the body, muscle, brain-
substance, gland-cells and the like, are continually under-
going oxidation. The living substance of the tissue, built
up out of the complex proteins, fats, and carbohydrates,
and thus even still mure 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, carbimic acid, and water.
Wiierever life is being manifested these oxidative changes
are going on, more energetically in some places, in some
tissues' and in some organs, than in others. Hence every
capillary vessel and every extra-vascular islet of tissue is
really a small fireplace in wliich 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-weiglit, and are carrying on an active
oxidation even while at rest. This gives rise to heat, and
wlicn 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
muscles we may regard the liver and the other secreting
glands as the next great heat-producing organs of the
body.

But as the vital activities of different parts of the

V TEMPERATURE OF THE BODY 203

body, and of the whole body, at different times, are very
different ; and as some parts of the body are so situated
as to lose their heat by radiation and conduction much
more easily than 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 regulated.

Whatever oxidation occurs in any part, raises the tem-
perature of the blood which is in that part at the time,
to a proportional 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 oxidating 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 constantly 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 quick-
ness by the pump. In this way, then, the temperature of
the body is kept uniform in its several parts.

12. 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 wetted with water,

204 ELEMENTARY PHYSIOLOGY less.

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 tenipei'atui-e 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 important influence than any otiier condition upon
the regulation of the temperature of the blood, and, con-
sequently, 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 reaching them along certain nerves coming from
a centre 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 secreting sweat, the sweat-
glands are assisted by corresponding changes in the blood-
vessels of the skin. It has been stated (see p. 68) that
the small arteries of the body may be sometimes narrowed
or constricted, and sometimes widened or 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).

V TEMPERATURE OF THE BODY 205

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 mucli fuller of blood, and from tliese 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 manu-
facture sweat. On the other hand, when the body is
lowered in temperature 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 nmch
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 evapo-
ration 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
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.

13. Regulation of Body-temperature by Altered

206 ELEMENTARY PHYSIOLOGY less.

Production 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 alteriny the production of heat taking place in
the body, in correspondence to the changes of the sur-
rounding temperature ; 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 being 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. Perhaps
the most evident instance of increased heat production,
in response to unusual heat loss, is that of shivering.
When, owing to lowering of the external temperature
or to insufficient clothing, the ordinary sources of heat
fail to maintain the temperature of the body, impulses
pass from the brain to the muscles which cause them to
contract rhythmically. In other words the subject shivers.
Muscular contraction, as we have already seen, involves
oxidation (see p. 203), and oxidation is accompanied by
heat production. Again we may point out, as tending to
the same conclusion, that our desire for food is greater,
on the whole, in the cooler winter time than in the

V FEVER 207

warmer summer ; and all food is, sooner or later, oxidised
in the body and during this oxidation gives rise to I; eat.
There are reasons for supposing that within certain limits
altered production of heat plays a part in keeping the tem-
perature 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 nervous
impulses. 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 phenomena of certain diseases, which justify this view.
More than this we must not say.

14. The Temperature of Fever.— The condition to
which the name of fever is given is characterised essen-
tially by the temperature of the body being higher than is
usual in health. Thus it may rise to as much as 41 C.
(105'8'F.) or occasionally even above this point, and
there has been much dispute as to how this high tempera-
ture arises. By many it is regarded simply as the outcome
of 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 ; and probably, this is the most
common cause of the rise of temperature. But on the
other hand direct measurement shows that a fevered
person often gives off nwre heat th<i7i usual and at the same
time uses up more oxygen and produces more carbonic
acid and urea than is usual. In such cases there is no
doubt that the abnormally high temperature is largely due
to an over-production of heat.

15. The Liver. — The liver is a constant 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 passing
through it a substance called glycogen, which is stored
up bometiuies in large, sometimes in small, quantities in

208

ELEMENTARY PHYSIOLOGY

the cells of the liver. This latter loss, however, is only
temporary, and may sooner or later be 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.

The liver is the largest glandular organ in the body,
ordinarily 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

Fio. 63.— The liver Tuknep Up and Viewed from Below.

a, vena cava ; b, vena portse ; c, bile duct ; d, hepatic artery ; I, gall-
bladder. The termination of the hepatic vein in the vena cava is not
seen, being covered by the piece of the vena cava.

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 keeps
it in place. It is flattened from above downwards and
convex and smooth above, where it fits into the concavity
of the lower surface of the diaphragm. Flat and irregular
below (Fig. 63), it is thick behind, but ends in a thin edge
in front.

Viewed from below, as in Fig. 63, the inferior vena

THE LIVER 209

cava, a, is seen to traverse a notch in the hinder edge of
the liver as it passes from the abdomen to the thorax.
At h the trunk of the vena portae is observed dividing
into the chief branches which enter into, and ramify
through, the substance of the organ. At d, the hepatic
artery, coming ahnost directly from the aorta, similarly
divides, enters the liver, and ramifies through it. At c is
the single trunk of the duct, called the hepatic duct,
which conveys away 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, I, the
gall-bladder. The duct is smaller than the artery, and
the artery than the portal vein.

The liver consists of two chief lobes of which the right
is much larger than tlie 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 vena portfe, hepatic artery and bile-duct,
as these plunge into the liver. This sheath accompanies
the vessels as they ramify into 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 smaller divisions called lobules. 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 about y\j of an inch in diameter
and are thus visible to the naked eye. Each lobule is
made up of a mass of cells, the hepatic cells, which lie
in the meshes of a close-set network of blood capillaries.
These capillaries radiate from 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. 64, A, H.V.), and, passing out of the lobule
at its l)ase, runs into a branch of that great vein, the
hepatic vein, wl^ich carries the blood away from the

p

A. Section of partially injected liver magnified. The artificial white
line is introduced to mark the limits of a lobule. V.P, branches of
portal vein breaking up into capillaries, which run towards the centre of
the lobule, and join //. V, the intralobular branch of the hepatic vein.
The outline of the liver cells are seen as a fine network of lines through-
out the whole lobule.

B. Portion of lol)ulo very highly magnified, a, liver cell with n,
nucleus (two arc often present) ; l>, capillaries cut across ; r, minute
biliary passages between the cells, injected with colouring matter.

LESS. V

THE LIVER

211

liver. In this way each lobule comes to be seated bj its
base on a branch of the hepatic vein (Fig. 65, H.V.).

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

Pio. 65.— A Section of Part of the Liver to show

H. V, a branch of the hepatic vein, with I, the lobules or acini of the
liver, seated upon its walls, and sending their intralobular veins into it.

branch out and subdivide, becoming smaller and smaller.
At length the ultimate branches of the portal vein (Fig. 64,
V.P.) reach the outer surfaces of the lobules, and passing
round and between them are known as the interloblllar
veins. These veins pour their blood into the network
of capillaries which permeate each lobule. The branches

p 2

212 ELEMENTARY PHYSIOLOGY les&

of the hepatic artery follow a course parallel to that of
the portal vein and finally, reaching the surface of a
lobule, 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 loljule,
and are carried off by its intralobular veinlet, which pours
its contents into one of the branches of the hepatic vein.
These branches, joining together, form lai'ger 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 tiie blood with which the liver is supplied is a
mixture of arterial and venous blood : the former brought
by the hepatic artery directly frt)m the aorta, the latter
by the portal vein from the capillaries of the stomach,
intestines, pancreas, and spleen.

In the lobules themselves all the meshes of the blood-
vessels are occupied by the liver cells, or hepatic cells.
These are many-sided minute bodies, each about 25/i(Yf5^gth
of an inch) in diameter, possessing a nucleus in its
interior, and frequently having larger and smaller gran-
ules of fatty matter distributed through its substance
(Fig. (54, B, a). Tt is in the liver cells that the active
powers of the liver reside.

The smaller branches of the hepatic duct, lined by an
epithelium, wliich is continuous with that of the main
duct, and thence with that of the intestines, into which
the main duct opens, may be traced to the very surface of
the lobules, where they seem to end abruptly (Fig. 66).
But, upon closer examination, it is found that they
communicate with a network of minute passages passing
between the hepatic cells, and traversing the lobule in
the intervals left by the capillaries (Fig. 64, B, c). These

BILE

213

minute passages are the bile canaliculi. The bile

manufactured by the hepatic cells finds its way first into
these minute passages, and from tliem into the ducts.

16. The Work of the Liver. Its Glycogenic Func-
tion.— 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.

On the one hand, the hepatic cells are continually en-

FiG. 66.— Termination of Bile Duct at Edge of Lobule (somewhat
diagrammatic).

6, small bile duct, becoming still smaller at b', the low, flat epithelium
at last suddenly changing into the hepatic cells, I, the channel of the bile
duct being continued as small passages between the latter, c. capillary
blood-vessel cut across.

gaged in the manufacture of a complex fluid called bile,
which they pour into the minute passages spoken of
above, and thence into the branches of the hepatic duct ;
whence 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

214 ELEMENTARY PHYSIOLOGY less.

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 probaljly amounts to not less than from
two to three pounds. It is a golden yellow, slightly
alkaline fluid, of extremely bitter taste, ccmsisting of
water with from 15 per cent, to half that quantity of
solid matter in solution. We shall deal with the com-
position of bile and tlie nature of its constituents when
we come to speak of it in connection with digestion.
For the present we may say that its colour is due to
bile-pigments ; that it contains certain compounds of
sodium with organic acids called bile-salts ; a remark-
able crystalline substance called cholesterin ; and
some inorganic salts.

Of these constituents of the bile the essential sub-
stances, the bile acids and the colouring matter, ai-e not
discoverable 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
hepatjp 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 be-
ginning, as it were, out of the common material which
the blood brings to the liver as to all other tissues and
organs — is not as yet quite determined. There is how-
ever but little doubt that the pigment of bile is in some
way made out of the hremoglobin of the red blood-cor-
puscles. The saline matters and cholesterin, on the
other hand, aj)pear 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,

GLYCOGEN 215

besides forming bile, the hepatic cells are concerned in
other labours, the result of which can hardly be con-
sidered either as a loss or as a gain, since these labours
simply consist 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. For instance we have already seen
that salts of ammonium carried by the blood to the liver
are there converted into urea and are returned as such to
the blood.

Again, as we shall presently see, the portal blood is,
after a meal, heavily laden with substances, the result
of the digestive changes in tlie 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 passing along tl,e portal veins
suffer (in the liver) a further 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
probably 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
hydrogen, 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

216 ELEMENTARY PHYSIOLOGY less.

agents into grape sugar, or dextrose, as it should be
called ; and this glycogen is similarly converted with ease
into dextrose. Indeed, if the liver of such an animal as
the above, instead of being examined 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 dis-
appeared, a quantity of dextrose having taken its place.
There seems to be present in the liver some agent capable
of converting the glycogen into a special sort of sugar
called 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 been starved for several days be ex-
amined in the same way, very little glycogen indeed will be
found in it, and when this liver is left exposed to warmth
for some 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 glyco-
gen has been formed, and none stored up. If the liver
in the first case be examined microscopically with certain
precautions, the glycogen 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 protein diet, though not
nearly so much as when starch or sugar is given.

It would appear, tlien, tliat the hepatic cells can manu-
facture and store up in themselves the substance glycogen,
being able to make it out of even protein matter, but more
easil}^ 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 caual, and reaches the liver as sugar.

THE THYROID BODY OR GLAND 217

There are reasons for thinking that the glycogen, thus
deposited and stored up in the liver, is converted into
su>,'ar little by little as it is wanted, poured into the
hepatic vein, and thus distributed over the body. So that
we may regard 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 deposits at his banker's some money which he has
received until he has need to spend it.

This story of glycogen, important in itself, is also use-
ful as indicating other possible effects of a similar nature
which the hepatic cells may bring about on 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 contrast between the two types of function
exercised by the liver, the secretion of bile down the
bile duct and the secretion of sugar into the blood, was
emphasised by Claude Bernard, the celebrated French
physiologist, who discovered glycogen. The secretion
of bile he called an external secretion because it passed
away along a duct, the secretion of sugar into the blood
was termed an internal secretion. We shall have to con-
sider other examples of internal secretions and indeed the
secretory functions of glands which have no ducts must
be contined to the manufacture of such.

17. The Thyroid Body or Gland —This organ consists
of two lobes, one lying each side of the trachea just below
the larynx and joined across the trachea by a connecting
strip of its own tissue. Each lobe is covered 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

218 ELEMENTARY PHYSIOLOGY less.

so as to leave a large central space ; this space in each
alveolus is tilled with a clear, viscid, often semi-solid
fluid. The viscidity of this fluid is due to the presence
in it of a substance which in some respects is like the
mucin of mucus. This material is known as the colloid
substance of the thyroid and is remarkable among the
various constituents of the body as containing the
chemical element iodine.

The thyroid gland contributes an internal secretion to
the blood the importance of which may be judged from
the following circumstances. Children in which the thy-
roid is deficient present a very painful picture. They
develop neither physically nor mentally, at fifteen or
twenty years of age they have not advanced further than
a normal child of five years old, their stature is as small,
their appearance somewhat deformed, and they lack
any mental development. They are known as "cretins."
If such are made to eat thyroid glands, or extracts of
these glands, they at once develop rapidly and in the
course of a few years they become normal persons, and
remain such so long as they persist with the diet.
Perhaps even more remarkable is the fact that, when
pieces of thyroid from normal persons are grafted under
the skin of cretins, if the grafts establish themselves, the
patients develop rapidly into normal individuals.

Disease of the thyroid glands often leads to disorders,
strikingly manifest in the skin, but also involving other
organs and tissues, especially perhaps 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 m'h.y
be largely mitigated by administering an extract of the
fresh gland or by eatiiig the fresh gland-substance. Goitre
is an enlargement of the thyroid ; at one time surgeons
removed the gland, but the removal of the gland was
found to be followed by the symptoms which we have
described.

V THE SUPRARENAL BODIES 219

18. The Suprarenal Bodies. — The suprarenal bodies
are two in number, and are plrtced on the upper edge of each
kidney. They are enveloped in an outer coat or capsule
of connective tissue from which partitions pass into their
interior. In this way each suprarenal is divided up into
compartments. In the outer or cortical part the compart-
ments are long and narrow, and placed with their long
axis at right angles to the surface of the organ. In the
centre or medullary portion the connective tissue forms a
somewhat coarse network. The elongated spaces in the
cortex are filled with large angular cells, placed in rounded
groups immediately next to the capsule, but arranged
more in columns towards the central parts of the com-
partments. The cells which fill the spaces of the medullary
network are of irregular shape and usually branched.
These cells contain some peculiar substance, of which but
little is known beyond the fact that it gives a dark blue or
green colour with ferric chloride, and a bi'ight red colour
by treatment with oxidising agents.

The functions of the suprarenal bodies are important,
although, as yet, but little understood. When they are
both removed from an animal, death speedily ensues, ac-
companied chiefly by a nutritional upset of the skeletal
muscles. When diseased in man, a similar defect is
observed in the muscles, together with nervous weakness
and a characteristic "bronzing" or coloration of the
skin. They too contribute an internal secretion to the
blood.

Extracts of the medulla when injected into the circula-
tion have an extremely marked action in causing a rise of
blood-pressure. This is brought about by an extensive
constriction of tlie minute blood-vessels (arterioles) of the
body, and is due to the presence in the extract of the
crystalline substance known as "adrenalin." For
instance, if the ear of the rabbit be carefully observed
when a dose of adrenalin is being injected into a vein
with a syringe the vessels of the ear wiU almost disappear

220 ELEMENTARY PHYSIOLOGY less.

from view and the ear will become pale and cold. This
result we have already seen to take place (p. 67) when
the cervical sympathetic nerve, which supplies the
muscular wrapping of these vessels, is stimulated.
Indeed, when we analyse the action of the adrenalin we
find that it does actually stimulate the branches of the
cervical sympathetic not along the course of the fibres,
but at their termination just where they penetrate the
muscular fibres. Nor is the action of adrenalin confined
to nerve endings of sympathetic fibres going to blood-
vessels, but it stimulates the nerve endings of sympathetic
fibres generally, whatever their function. It therefore
causes acceleration of the heart,, secretion of saliva and
many other changes.

The amount of adrenalin necessary to produce these
effects is very small. In a rabbit a rise of blood-pressure
may be obtained by the injection of one millionth of a
gramme. The amount of adrenalin injected, therefore, bears
about the same proportion to the rabbit as Avould a drop
of water dropped into an express locomotive engine, and
this suffices not only to construct every muscular fibre in
the vascular system, but to produce a multitude of other
effects.

19. The Thymus Gland. — This is an organ which lies
over the trachea, in the lower part of the neck and
behind the sternum at the base of the heart. It is con-
spicuous 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 ; thus it has an external capsule from
which- trabeculie pass inwards and divide it up into
regular compartments or follicles. These follicles are
filled with a network of lymphatic connective tissue which
is crowded with leucocytes.

Nothing very definite is known of the function or use
of this gland.

20. The Spleen. — The spleen lies in the abdominal

THE SPLEEN

221

cavity, slightly below and towards the left side of the
stomach and immediately to the left of the tail of the
pancreas (Fig. 67). 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 traversed 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

tyrn

Fia. 67.

The spleen (Sp?) with the splenic artery (Sp.vl.). Below this is seen the
splenic vein running to help to form the vena portae (V.P.'). Ao, the
aorta; D, a pillar of the diaijhragm ; P.D. the pancreatic duct exposed
by dissection in the substance 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.

by a capsular sheath of connective tissue mixed with
a good deal of elastic tissue and in some animals a great
deal of unstriated muscle fibres. Somewhat in the same
way as in a lymphatic gland (p. 89) this capsule sends
branching projections or trabeculse 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 of branching cells whose processes are somewhat

222 ELEMENTARY PHYSIOLOGY less.

flattened and join on to the processes of neighbouring
cells. The meshes of this network are occupied by red
blood-corpuscles, by colourless corpuscles closely similar
to those of lymph, and by other kinds of cells peculiar to
the spleen. Some of the latter resemble a colourless
corpuscle of blood, in that they can perform amoeboid
movements, but they are larger and contain in their sub-
stance red corpuscles in various stages of disintegration.

A section of the spleen shows a dark red spongy mass
dotted over with minute whitish spots. Each of these
last is a section of one of the spheroidal bodies called
corpuscles of the spleen, or Malpighian cor-
puscles, which are scattered through its substance.
These corpuscles consist of little masses of lymphoid
or adenoid tissue, very similar to that found in the
lymi)hatic glands (p. 90), which surround the smaller
branches of the arteries. They are crowded with leuco-
cytes, and hence they stand out as white specks against
the dark red pulp of the spleen.

The smallest branches of the arteries which cany blood
into the spleen, open into the network of the spleen-pulp,
so that the blood flows into and through this network ; ib
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.

We are still very much in the dark as to the functions
of the spleen ; they are without doubt of some import-
ance ; but on the other hand the spleen may be perman-
ently 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 distention. It ai)i)ears to change
its dimensions with the state of the abdominal viscera,
attaining its largest size about six hours after a full meal,
and falling to its minimum bulk six or seven hours later,
if no further supply of food be taken.

THE PITUITARY BODY 223

The blood of the splenic vein is found to contain pro-
portionally fewer red corpuscles, but more c(Jourles3
corpuscles, than in the splenic artery ; and it has been
supposed that the spleen is one of those parts of the
economy in which, on the one hand, colourless corpuscles
of the blood are produced, and, on the other, red
corpuscles die and are broken up.

21. The Pituitary Body is a small structure under-
neath the brain from which its posterior lobe is
developed ; the anterior lobe and a portion joining the
two lobes together is developed from the epithelium of
the mouth.

The functions of the pituitary body are obscure, but
so far as they are known they are striking enough.
Enlargement of the anterior portion is associated with a
diseased condition in which many of the bones grow to an
abnormal size. Injection of extracts of the posterior
part, on the other hand, produces results somewhat
similar to, but not quite the same as, injection of adrenalin.

LESSON Yi

THE FUNCTION OF ALIMENTATION

Part 1. — 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 oft' by his skin and
lungs. Further, we pointed out that the source from
which the energy is derived lies in that constant oxida-
tional 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 corre-
sponding exactly to the amount of energy expended. If
the man's activity is to continue from day to day, this
continual waste of sul)stance must be made good. Now
the only channel, except the lungs, \>y which altogether
new material is introduced into the body, is the alimen-
tary 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 introduc-
tion 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
by wiiich the digested material is jiassed from the cavity
of the canal into the blood-vessels and lymphatics by

LESS. VI FOOD AND FOOD-STUFFS 225

which it is then distributed over the body. We may
therefore 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 famUiar 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 varj-ing
amounts of a few substances, and to these the name of
food-stuflfe is given.

Food-stuffs are classified under four heads, (1) Pro-
teins, (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 proteins 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)

Proteins Fats Salts

i I'

Carbohydrates Water

A. Nitrogenous Food-Stuffs.

Proteins. — These are composed of the four elements
carbon, oxygen, hydrogen and nitrogen united with
small amounts of sulphur (see p. 108). Under this
head come the albumin of the white of egg, and
blood-serum ; the casein of milk and cheese ; the

226 ELEMENTARY PHYSIOLOGY less,

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 jQbrin of blood.

Gelatin, the basis of connective tissue fibres, contains,
like protein, carbon, hydrogen, nitrogen, and oxygen
in the proportions in which they occur in protein, and
may be regarded as an outlying member of this group.
But gelatin contains no sulphur and cannot entirely
replace protein 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 oxj^gen is present in an amount which would
just suffice to form water if it were united to their hydro-
gen. This group includes starch, as in flour or potatoes ;
ordinary cane-sugar or beet-sugar, and other sugars
such as dextrose and mUk-sugar ; also cellulose
from all vegetable tissues.

(iii) Salts and Virater. — 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
carbonates 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 meat is chiefly protein, but ordinarily contains a
good deal of fat ; bread contains a great deal of carbo-
hydrates, but also some protein 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 ; but to this we shall
return when we come to study their respecti ''^influence on
the body as a whole.

THE PURPOSE OF DIGESTION 227

3. The purpose and means of Digestion.— All food-
stuffs being thus proteins, 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 proteins, &c., from the in-
nutritious residue, if there be any, and to reduce them
to a condition of fine subdivision and ultimately to
one of solution, 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
masticated, is mixed with saliva, is swallowed, undergoes
gastric digestion, passes into the intestine, and is sub-
jected 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 ali-
mentary canal. These glands are essentially groups of
cells supplied with nerves and blood-vessels ; but the
arrangement of these cells may be simple or complicated,
as in the types shown in Fig. 68.

Thus the glands of the walls of the intestines are tubular
(1). Those in the walls of the stomach are also tubular
but divided into two or more parts at their inner end
(2). The salivary glands and the pancreas are more com-
plicated ; their ducts divide and subdivide into multitudes
of smaller tubes, each of which ends in a dilatation in
which the secreting cells lie (6). These dilatations, at-
tached to the branched ducts, somewhat resemble a

Q 2

228

ELEMENTARY PHYSIOLOGY

Fig. 68. — A Diagram to illustrate the Stuccture ut Glands,

A. Typical struct\ire of tlie iiiurnus membrane, a, an upper, and 6 a
lower, layer of epithelium cells ; c, the dermis with c a blood-vessel,
and /, connective tissue corpuscles.

VI THE PALATE AND PHARYNX 229

bunch of grapes, whence glands of this type are called
raceiyiose.

4. Mastication and Swallowing.— The cavity of the
mouth is a cliamljer with a fixed roof, formed by the
hard palate (Fig. 69, I), and with a movable floor,
constituted bj' the lower jaw, and the tongue (A), which
fills up the space between the two branches of the jaw.
Arching round the margins of the upper and the lower
jaws are the thirty-two 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.

WTien the mouth is shut the back of the ix)ngu3 comes
into close contact with the palate ; and, ^hers che hard
palate ends, the communication betweer the mouth and
the back of the throat is still further 'inpeded by a sort of
fleshy curtain— the soft palate c:., 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 witli muscular walls, the upper margins of the slanting,
wide end of which are attached to the base of the skull,

B. The same, with only one layer of cells, a, and 6, the so-called basement
membrane between the epithelium o, and dermis c.

1. A simple tubular gland.

2. A tubular gland bifid 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.

230

ELEMENTARY PHYSIOLOGY

while the lateral margins are continuous with tlie sides,
and the lower with the tloor, of the mouth. The narrow

Fic. 60. -A Section of tiik Mouth and Nose taken vertically, a

LITTLE TO THE LEFT OF THE MlDDl.E LiNE.

a, the vertebral, column ; b, the oesophagus or giiUet ; c, the wind-pipe ;
rf, the thyroid cartilage of the larynx ; e, the epiglottis ; .(', the tivula ; rf,
the opening of the left Eustachian tube ; /(, the opening of the left
lachrymal duct ; (, the hyoid bone ; k; the tongue ; /. the hard palate ;
m, 71, the base of the skull ; o, p, q, the superior, middle, and inferior
turbiual bones. The letters g, j, e, are placed in the pharynx.

end of the pharyngeal bag passes into the gullet or
oesophagus (/>), a nmscular tube, which affords a passage
into the stomach.

THE TEETH 231

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 EjUStachian tubes
(g). The two single apertures are, the hinder opening of
the mouth between the soft palate and the epiglottis ; and,
behind the epiglottis, the upper aperture of the respira-
tory passage, or the glottis.

Each of the thirty-two teeth which have been mentioned
consists of a crown which projects above the gum, and
of one or more fangs, which are embedded in sockets, or
what are called alveoli, in the jaws (see Fig. 3).

The eight teeth on opposite sides of the same jaw are
constructed upon exactly similar patterns, while the eight
teeth which are opposite to one another, and bite against
one another above and below, though similar in kind,
differ somewhat 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 com-
pletely divided into two. The remaining teeth have two
or three fangs each, and their crowns are much broader.
As they crush and grind the matters which pass between
them they are called molars, or true grinders. In the
upper jaw their crowns present four points at the four
corners, and a diagonal ridge connecting two of them.
In the lower jaw the complete pattern is five-pointed,

232

ELEMENTARY PHYSIOLOGY

there being two cusps on the inner side and three on the
outer. Each tooth presents a crown, which is visible in
the cavity 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
furnished by the jawbone and the derma of the dense
nmcous membrane of the mouth, which constitutes the
gum. The line of junction between the crown and the

Fig. 70.

A, vertical, B, horizontal section of a tooth. — a, enamel of the crown ;
6, pulp cavity ; c, cement of the fangs ; d, dentine. (Magnified about
three diameters.)

fang is the neck of the tonth. Tn 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. It is occupied and com-
pletely filled by a highly vascular tissue richly supplied
with nerves, the dental pulp, which is continuous
below, through the openings of the fangs, with the
vascular dermis of the gum which lies between the fangs

VI THE TEETH 233

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. 70, A, B, d). It is a dense
calcified substance containing less animal matter than
bone, and further differing from it in possessing no
lacunae, or proper canaliculi. Instead of these it presents
innumerable, minute, parallel, wavy tubules (Fig. 71, d),
which give off lateral branches. The wider inner ends of
these tubules may measure 4/x or 9/m (sy^jg inch) ; they
open into the pulp cavity, while the narrower outer
terminations ramify at the surface of the dentine, and
may even extend into the enamel or cement (Fig. 71).

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 contains only 2 per cent, of animal matter, and is
the hardest substance in the body ; so hard that it will
strike fire with steel. This is called enamel (Fig. 70,
A, B, a). It becomes thinner on the sides of the crown
and gradually dies out on the neck. Examined micro-
scopically, the enamel is seen to consist of six-sided
prismatic fibres (Fig. 71, A, B) set closely side by side,
nearly at right angles to the surface of the dentine.
These fibres measure not more than 3/i to 5/i (gg^oo ^
xoVo 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. 70, A, c ; and Fig. 71, C).

The dental pulp is chiefly composed of delicate con-
Uictive 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 derived from the fifth pair of cranial nerves.

The superficial part of the pulp, which is everywhere in

234

ELEMENTARY PHYSIOLOGY

.A^^^nm^

. Fin. n.

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 («) ; 6, c, irregular cavities in which the tubules of the dentine
end ; rf, fine tubules continued from them ; /, g, lacunse and canaliculi of
the cement. (Magnified about 400 diameters.)

immediate contact with the inner surface of the dentine,
consists of a layer of nucleated cells so close set that they

VI MASTICATION AND SWALLOWING 235

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 dermic structure, and from them long filamentous
processes can be traced into the dentinal tubules.

The muscles of the parts which have been described
have such a disijosition that the lower jaw can be de-
pressed, so as to open the mouth and separate the teeth ;
or raised, in such a manner as to bring the teeth together ;
or more obliquely from side to side, so as to cause the
face of the gi'inding teeth and the edges of the cutting
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
grinding 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 contractions of the cheeks and
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 mixed with the food, which thus becomes inter-
penetrated not only with the salivary fluid, but with the
air which is entangled in the bubbles of the saliva.

When the food is sutticiently 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. Thr. ugh this
it is thrust, the soft palate being lifted and its pillars being
brought together, while the backward movement of the
tongue at once propels the mass and causes the epiglottis
to incline backwards and downwards over the glottis,

236 ELEMENTARY PHYSIOLOGY less.

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 for>d below, and prevents it from pa.ssing
into the trachea, the soft palate guides it above, keeps it
out of the na.sal chamber, and directs it downwards and
backwards towards the lower part of the muscular pha-
ryngeal 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 the oesophagus.

The resophagus is lined with mucous membrane. This
rests on .some fibrous tissue, outside of which is a thick
coat of mu.scular 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.

When food has been thrust into the ce.sophagus by the
action of the pharynx, the muscular wall of the oeso-
phagus just above the bolus contracts and pushes it
down into the next lower part. Then the wall of this
part contracts and pu.shes the mass a little further down
and so on. In this way the food is finally thrust into the
stomach by a .series of contractions of each part of the
oesophagus in succession : this is spoken of as peristaltic
action.

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 impossiVjle 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

THE SALIVARY GLANDS

237

matters it carries in solution are still more diluted by
the water of the saliva ; thirdly, that any starch it may
contain begins to be changed into sugar by the saliva,
whose formation and action we must next consider.

5. 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

A dissection of the right side of the face, showing, a, the sublingual,
b, the submaxillary glands, with their ducts opening beside 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.

secretion are the three pairs which, as has been already
mentioned, are called parotid, submaxillary, sub-
lingual, and which secrete the principal part of the
saliva (Fig. 72).

Each parotid gland is placed just in front of the ear,
and its duct passes forwards along the cheek, until it
opens in the interior of the mouth, opposite the second
upper grinding tooth.

238

ELEMENTARY PHYSIOLOGY

LESS.

The submaxillary and sublingual glands lie between the
lower jaw and the floor of the mouth, the submaxilliry
being situated further 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.

The salivary glands are built up on the type shown in
Fig. 68, 6. The essential part of the structure lies in the

■-AU'li

a

Fio. 73. — Sections of the Submaxillary Gland.

A, at rest ; B, after secretory activity.

a, a, demilune cells.

cells which line the dilated ends, or alveoli, of the finest
branches of their ducts. In a section of a submaxillary
gland which is resting, that is, has not been secreting for
some time, the cells are large and nearly fill the alveoli.
Each cell has a nucleus j)laced near its outer end and
surrounded by a small amount of protoplasm which is
granular and stains readily. The (larger) rest of the cell
is quite clear an,d transpai'ent and stains with great diffi-
culty if at all (Fig. 73, A). Since the material compos-

THE SALIVARY GLANDS

239

ing this clear part of the cell is of the nature of mucin,
the submaxillary gland is spoken of as a mucous gland.

In some of the alveoli a second kind of cell may be
seen (Fig. 73, A and B, a) ; these lie close against the
outer wall of the alveolus, and from their shape are often
called demilune cells. They are granular and stain
deeply.

In similar sections of the parotid gland, also in the
resting condition, the cells are smaller than in the alveoli
of the submaxillary gland. Further, the nucleus lies near

A
Fig.

A.

74.— Sections of the Parotid Gland.
at rest ; B, after secretory activity.

the middle of each cell, and the whole cell is extremely
granular and stains fairly easily (Fig. 74, A). The body
of each cell is composed of albumin and is free from any
trace of anything like mucin ; hence the parotid is known
as an albuminous gland.

After these glands have been secreting for hours, as
the result of stimulating the nerves supplied to them,
the appearance of their cells is greatly changed. The
cells of the submaxillary gland are now smaller ; the
nucleus is nearer the centre of each cell and the whole

240

ELEMENTARY PHYSIOLOGY

cell stains readily (Fig. 73, B). In the parotid gland the
cells are simihirhj smaller, the nucleus more distinct,
and the cell-substance stains more readily than in the
condition of rest (Fig. 74, B).

If instead of taking sections of the hardened gland a
piece of the fresh parotid be examined in the two states of
rest and activity the differences shown in Fig. 75 may be
seen.

At rest the cells are full of granules throughout, as in
Fig. 75, A. After activity the granules are fewer and
now lie near the inner end of the cells as in G. Between

Fig. 75.— Changes in the PARorin Gland during Secreting
Activity. (Slightly ihaorammatic.)

these two extremes there is an intermediate stage shown
inB.
6. The Nervous Control and Nature of Salivary

Secretion. — We have described in ihv iireceding section
the differences which may be observed Vjetween the cells
of a resting gland and of the same gland after it has been
secreting. Let us now see what may be learnt from a
study of these differences and of the way in which they
may be brought about.

In the first place the differences in the size and appearance
of the cells in the two conditions seem to show quite
clearly that while at rest they build up material which is
stored in their substance and hence the cells are large.

VI THE SECRETION OF SALIVA 241

In the submaxillary gland this substance is chiefly
mucinous, in the parotid albuminous, and some of it is
deposited as separate distinct granules in the body of the
cell (Fig. 75, A). Further it appears that during their
activity both glands discharge their store of material into
the duct leading from tliem and hence become smaller.

But further, the submaxillary gland is supplied by a
nerve which is a branch of the Vllth cranial nerve (see
Lesson XI.) and which, since it crosses the tympanic
cavity or drum of the ear (.see Lesson VIII.) is called the
chorda tympani nerve. When this nerve is stimulated
tliree things happen ; the arteries which sujjply 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 they
were loaded during rest has been discharged. But at the
same time the cells have discharged a large quantity of
water, for saliva, like all other secretions, except the
secretion of the sebaceous glands (p. 197), is largely com-
posed of water, and this water can only have come from
the blood. We are thus at once face to face with the
question ; — has the increased supply of blood simply led
to an increased flow of water through the cells which has
carried away with it the accumulated materials of the cell-
substance, the whole process being largely tiltrational, or
has the stimulation of the nerve made the cells not only dis-
charge some of their substance, but also made them take up
water from the blood and jjass this as well through thein-
selves ? The answer to this question is simple and the
evidence in its support is conclusive.

An increased temporary secretion may be observed on
stimulating the nerve even after the blood-supply to the
gland has been cut off, and if some drug, such as atropine,
be injected into the animal, then although the arteries
dilate to the full extent when the nerve is stimulated, no

242 ELEMENTARY PHYSIOLOGY

increased secretion takes place. No clearer proof could
be desired to show that when the gland secretes it is
because the impulses tohich reach it along the nerve exert a
direct influence on its cells. These impulses make the
cells take up water and discharge it, together with some
of their stored up cell-subst«ince, as saliva which passes
as the secretion of the gland into the ducts. It is thus
not the increased blood-supply which causes the secretion,
although of course it is necessary if the cells are to con-
tinue to secrete, for it is from tlie blood alone that they
can obtain all that they require for the manufacture of
their secretion.

Saliva is ordinarily secreted in increased quantity as
soon as food is introduced into the mouth or even smelt.
This result is brought about refle.vly. The food stimulates
the ends of certain nerves (Vth and IXth cranial, see
Les.S()ri XI.), wliich supply the inside of tlie walls of the
mouth or the Olfactory nerve which supplies the nose.
Impulses pass up these nerves to the brain, and from this
other impulses pass out down to the submaxillary gland
and make its cells secrete.

7. The Composition and Action of Saliva.— 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 ptyalin, which has certain very
remarkable properties. It does not act on proteids or
fats, but if a little saliva, i.e. ptyalin, be mixed with
ordinary starch-paste and warmed to the temperature of
the body, it turns that starch into a sugar identical with
that obtained from malt in brewing and hence known as
maltose.

But although this chemical change is without doubt of
some use to the body, its imj)ortance must not be over-
estimated. For in many animals the action of their saliva
on starch is very slight, and moreover (see p. 264) the larger

FERMENTS 243

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 chemical, inasmuch
as it moistens the food and thereby assists mastication
and makes deglutition, or the swallowing of food, easy.

8. Soluble Ferments or Enzjrmes.— The peculiar
substance, 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
originally applied to a living organism such as yeast which,
as in the case of brewing, while converting the sugar in
the wort 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 (fervere = 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
the same changes as did the cells from which they are
prepared. Hence the name of soluble ferment or enzyme
(^ii/-tj; = j'east) 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 enormously large compared with the minute mass of
the enzyme, (ii) Their action depends closelj' on tempera-
ture. At 0' C. (32'- F.) they cease to act ; as the tempera-
ture rises they become increasingly active, and are most
active at about 40' C. (104' F.). At higher temperatures
they become less active and lose their powers permanently
if once heated to 100 C. (212' F.) as by boiling : they are
then said to be " killed." (iii) Their action in many cases
depends on the reaction; whether acid or alkaline or

B 2

244 ELEMENTARY PHYSIOLOGY less.

neutrul, of the solution in which they are at work.^ (iv)
Their action stops in presence 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.

9. The Structure of the Stomach. — The stomach,
like the gullet, consists of a tube with muscular walls
composed of smooth niuscular fibres, and lined by nmcous
membrane ; but it differs from the gullet in several cir-
cumstances. In the first place, its cavity is greatly larger,
and its left end is produced into an enlargement which,
because it is on the heart side of the body, is called the
cardiac dilatation (Fig. 76, h). 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,
concave, lesser curvature, on its back or upper con-
tour. Towards its right extremity the stomach narrows,
and, where it passes into the intestine, the muscular
fibres are so disposed as to form a sort of sphincter around
the aperture of connuunication. This is called the
pylorus (Fig. 76, d).

The nuiscular coat of the stomach, consisting of un-
striated muscular tissue, is made up of two layers, an
outer longitudinal and an inner circular, together with
a certain amount of muscle fibres which are continuous
with the circular fibres of the cjesophagus and which,
running obliquely, merge into the internal circular
layer of the stomach. The mucous membrane which

1 Thus the pepsin of gastric juice acts best in presence of hydro-
chloric acid and the trypsin of pancreatic juice in presence of sodium
carbonate.

THE STOMACH

245

lines the stomach is loosely attached to the muscular coat
by a layer of fibrous connective 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 smaU

Fig. 76.— The Stomach Laid Open.

a, the cesophagus ; b, the cardiac dilatation ; c, the lesser currature ;
d, the pylorus ; ,:, the biliary duct ; /, the gall-bladder ; g, the pancreatic
duct, opening in common with the cystic duct opposite h ; h, i, the
duodenum.

glands, packed closely side by side, which open upon its
surface. 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

246

ELEMENTARY PHYSIOLOGY

the tubes they are cubical and slightly granular. These are
the central cells(Fig. 77, c). Asecondkind of cell inayalso
be seen lying irregidarly scattered between the outer wall
of the gland and its central cells : these are the parietal
or ovoid cells (Fig. 77, p).
Oval in shape, they have a well-
defined outline and their cell-
substance is usually very distinctly
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
ovoid cells.

When the stomach is empty, its
nnicous membrane is pale and
hardly more than moist. Its small
arteries are then in a state of con-
striction, and comparatively little
blood is sent through it. On the
entrance of food a vaso-motor action
is set u|), which causes these small
arteries to dilate ; the nmcous mem-
brane consequently receives a much
larger quantity of blood, and it
becomes very red. At the same
time the cells of the gland begin
to form their secretion, taking the
material they require for this pur-
pose out of the extra supply of
blood now coming to them. The
whole process is exactly similar in
principle to that already described
in the case of the secretory activity of the submaxillary
gland (p. 242).

The secretion thus formed is the gastric juice.

Fio. 77.— One of the
Glands which Se-
cretes Gastric Juice.
I), the duct or mouth
of the gland ; m, mucous
cells liniug the mouth of
the gland and covering
the inner surface of the
mucous membrane ; c,
central cells ; p, parietal
or ovoid cells.

GASTRIC JUICE 247

10. The Nature and Action of Gastric Juice.— Pure

gastric juice is a clear, acid fluid and consists of little
more than water, containing a few saline matters in solu-
tion, and its acidity is due to the presence of free
hydrochloric acid to the extent of "4 per cent. It
possesses, however, in addition a small quantity of a
peculiar substance called pepsin, a soluble ferment
'or enzyme in many respects similar to, though very
different in its efl'ects from ptyalin.

It is easy to ascertain the properties of gastric juice
experimentally, by putting a small portion of the mucous
memljrane of a stomach into water made acid by the
addition of "2 — "5 per cent, of hydrochloric acid and con-
ta.ining small pieces of meat, hard-boiled egg, or other
proteids, and keeping the mixture at a temperature of
about 40° C. (104' 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, consisting 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 operation takes place when food
undergoes natural digestion within the stomach of a
living animal.

It takes a very long time (some days) for the dilute acid
alone to dissolve proteid matters, and hence the solvent
power of gastric juice must be chiefly attributed to the
pepsin ; moreover gastric juice which has been boiled, in
which case all the ferment it contains is "killed" (see
p. 243), is quite mactive although it contains the usual
amount of acid.

Thus gastric juice dissolves pi-oteins, and the character-
istic protein 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 protein which
has been digested.

248 ELEMENTARY PHYSIOLOGY less.

Gastric juice turns milk into curds ; whether this
coagulating power is due to a particular ferment called
rennin is less certain than it was formerly supposed
to be. It is in all probability the first action of the
pepsin on the milk-protein. This is the basis of cheese-
making, and the " rennet " used for obtaining the curd
in this process is really an extract of the mucous mem-
brane of the stomach of a calf.

Peptone differs from most other proteins in its extreme
solubility. Many proteins, as fibrin, are naturally in-
soluble in water, and others, such as white of egg, though
apparently soluble, are not completely so, and can be
rendered (juite 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.

As far as we know gastric juice has no direct action on
fats, unless in a state of very fine division (emulsion,
p. 260) ; however, by breaking up the protein frame-
work 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. Gastric
juice has no direct action on carbohydrates. Carbo-
hydrate digestion does take place however in the
stomach, especially when large quantities of food are
swallowed. The food mixed with the alkaline .saliva is
only penetrated slowly by the acid secreted by the
stomach wall. Whilst this is taking place the saliva has
the opportunity of converting starch into sugar. At the
end of about twenty minutes it is estimated that carbo-
hydrate digestion in the stomach is brought to a standstill
by the hydrochloric acid present.

When food is swallowed it accumulates in the cardiac
end of the stomach which gradually distends to receive it.
The mass is gradually digested over its surface by the
gastric juice. A layer of fluid, of the consistency of pea

THE INTESTINES

249

soup and to which the name chyme is given forms between
this mass of food and the stomach wall. The object of
the movements of the stomach is in part to work this
fluid towards the pylorus. Constrictions form round the
stomach and each constriction moves towards the pylorus,
and ultimately reaches it. After a time the pylorus, which
has hitherto been closed, opens on the arrival of one of
these waves and some chyme is passed into the intestine.

Fig. 78.

a, the stomach, emptj' ; b, shortly after a meal, showing peristaltic
constrictions ; c, full.

The pylorus then closes and remains closed till the chyme
in the intestine loses its acid reaction and becomes
alkaline as we shall see later is the case (p. 263). In
this way the larger part of the chyme is allowed to
enter the duodenum ; but a portion of the fluid (con-
sisting of a little fat together with some sugar may
be at once absorbed, making its way, by imbibi-
tion, 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.
11. The General Arrangemeiit and Stmcture of the

250 ELEMENTARY PHYSIOLOGY

Pio. 79. — The Viscera of a Rabbit as seen ii'on simply opening the
Cavities of the Thorax and Abdomen without any further
Dissection.
A, cavity of the thorax, pleural cavity on either side ; S, diaphragm;

C, ventricles of the heart ; D, auricles ; E, pulmonary artery ; F aorta;

VI THE INTESTINES 251

Intestines, —The intestines form one long tube, with
mucous and muscular coats, like the stomach ; and, like
it, they are enveloped in peritoneum. They are divided
into two portions— the small intestines and the large
intestines; the latter, though shorter, having a much
greater diameter than the former. The name of duo-
denum is given to that part of the small intestine,
about ten inches in length, which immediately succeeds
the stomach, and is bent upon itself and fastened by
the peritoneum against the back wall of the abdomen, in
the loop shown in Fig 76, h, i. It is in this loop that
the head of the pancreas lies (Fig. 67).

The rest of the small intestines, of which the part next
to the duodenum is called the jejunum and the rest the
ileum, is no wider than the duodenum, so that the tran-
sition from the small intestine to the large (Fig. 80 II) 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-c^cal
valve.

The large intestine forms a blind dilatation beyond the
ileo-cjBcal valve, which is called the ceecum ; and from
this an elongated, blind process is given off. which, from
its shape, is called the vermiform appendix of the
cyecum (Fig. 80 verm).

The caecum lies in the lower part of the right side of the

G, lungs collapsed, and occupying only back part of chest ; ff, lateral
portions of pleural membranes ; /, cartilage at the end of sternum
(ensiform cartilage) ; K, portion of the wall of body left between thorax
and abdomen ; a, cut ends of the ribs ; £, the liver, in this case lying
more to the left than the right of the body ; M, the -toma'^h. a large" p^irt
of the gi-eater curvature being shown ; A', duodenum ; 0, small intestine ;
P, the cpecum, so largely developed in tliia and other herbivorous
animals ; Q, the large intestine.

252

ELEMENTARY PHYSIOLOGY

abdominal cavity. The colon, 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

Acol

^\R

Fio.

-TiiK Alimentary Canal in the Abdomen.

R, right ; L, left ; c, (esophagus ; si. stomach ; py. pylorus ; duo.
duodenum ; Jej. jejunum ; Jl. ileum ; rtic. ofecum ; A. col. ascending
colon; T.col. transverse colon; D.col. descending colon; /f, rectum?
verm, vermiform appendix.

passes across to the left side of the body, being called the
transverse colon in tliis part of its course ; and next

THE INTESTINES

253

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.

2>mC ^

^ncest..

Fig. si.— Diagram to show how the Wall of the Abdomen is made up,

AKD HOW THE MeSENTERV SUPPORTS THE INTESTINE.

The body is supposed to be cut across, and the intestine is represented
as the section of a strais-ht 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 ; J.m, muscles of back ; «t. skin ; m>, m^, nt?, the three
muscle layers ; perit. peritoneum ; mes. mesentery ; intest. intestine ; b.v,
blood-vessels.

The intestines are slung from the middle line, along
the vertebral column, of the abdominal cavity by a thin
membrane known as the mesentery. This consists
really of two layers between which the nerves, blood-
vessels and lymphatics lie which supply the intestines.
These layers are continued outwards on each side from

254 ELEMENTARY PHYSIOLOGY less.

the middle line as a lining, the peritoneum, for the
whole cavity of the abdomen, and also pass over and round
the intestines. The latter thus lie in a fold of the peri-
toneum, somewhat as a man lies when slung in a hammock
(Fig. 81).

Other folds of the peritoneum similarly support the
other organs in the abdomen. The peritoneum is thus a
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 tliorax 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 are m;ide up of four coats : an external
thin serotis covering of connective tissue, beneath
which is a muscMlar coat connected by a submncoits layer
with the inner or mucous coat.

The muscular coat of the small intestines is made up of
two layers ; an outer longitudinal, an inner circular. 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 lower ones, or those
nearer the large intestine. It follows from this so-called
peristaltic contraction, that the contents of the intestines
are constantly being propelled, by succe.ssive and ])ro-
gressive narrowing of their calibre, from their upper
towards their lower parts. And the same peristaltic
movement 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 intestines is larger than the muscular tube
which surrounds it ; hence to get this greater length of
the former stowed away into the shorter length of

VI THE INTESTINES 255

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 valvules conni-
ventes.

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. 79, Q) ; these are known
as the sacculi, and serve for the same purpose as the
vdlvulcp conniveiites of the small intestine. Moreover, the
muscular fibres around the termiiiation of the rectum are
arranged so as to form a ring-like sphincter muscle, which
keeps the aperture firmly closed, except when defeeca-
tion takes place.

The mucous membrane of both small and large intestine
consists mainly of a number of simple tubular glands
packed side by side ; they are known as the glands of
Lieberkiihri (Fig. 82, G.L). In the small intestine
the tissue between the mouths of the neighbouring
glands is at frequent intervals thrown out into the
cavity of the intestine as club-.shaped processes or
projections, the villi, which are thus set side by
side over the surface of the mucous membrane like the
pile on velvet. These villi are absent in the large intes-
tine. The glands of Lieberkuhn are separated from each
other by tissue which is practically the same as that
already described as lymphoid or adenoid tissue (p. 90) ;
it is therefore a network of connective tissue fibres.
Wherever a villus occurs this adenoid tissue is prolonged
up into and forms the body of the villus, and in it run the
finer branches of the blood-vessels and lymjihatics supj^lied
to each villus. Each gland of Lieberkiihn is lined by a
layer of cubical cells, among which occur a certain number
of mucous cells, and this layer is continued outwards
over each villus as its external covering ; but the cells

256 ELEMENTARY PHYSIOLOGY LEsa

covering the villi differ in shape from those lining the
glands (Fig. 82, B, C).

At irregular intervals along the mucous membrane the
lymphoid tissue between the glands is developed into
small rounded masses called "solitary glands" or
"solitary follicles." These closely reseml)le the
glandular substance of the alveoli at the cortex of a lym-
phatic gland (p. 90), and are similarly crowded with leuco-
cytes. In parts of the small intestine, more particularly
in the ileum, groups of these follicles are found packed
closely together ; they are then known as " agminnted
glands," or Peyer's patches; these do not occur in
the large intestine.

At the commencement of tlie duodenum are certain
small racemose glands, called the glands of Brunner,
whose ducts open into the intestine. Their function
seems to be quite unimportant.

12. The Structure of the Villi.— A villus, as already
explained, is a projection into the cavity of the intestine
of the tissue between the glands of Lieberkiihn, and is
covered by an epitlielium continuous with that which
lines the.se glands. The average length of a villus is about
•5 — "7 of a millimetre ( Jq - 5^5- of an inch). Running up the
centre or axis of the villus 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 (Fig. 82, /). 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 un-
striated muscle in the mucous membrane, lying next to
the submucous coat (Fig. 82, m.m) ; outside these
again, close under the epithelium of the villus, is a net-
work of capillaries (Fig. 82, 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 the space left in between the several structures so

VI

THE VILLI

257

Fig. 82.— Diagram of Two Villi and an adjacent Gland of

LlEBERKUHN (HaRDY).

A, two villi with a gland of Lieberkiihn, G.L, between their bases;
m.m, muscularis mucoste ; /, 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
oells which line the tube of the gland of Lieberkuhn, more highly
magnified.

S

258 ELEMENTARY PHYSIOLOGY LRsa.

far described in the body of the villus is filled up with
adenoid (lymphoid) tissue, whose meshes are more or less
crowded with leucocytes.

The epithelium covering a villus is made up of cells of
two kinds. Of these the large majority are tall, columnar,
and granular, with an oval nucleus towards their inner
end. The outer end of each cell (on the surface of the
villus) shows a narrow, strongly striated border (Fig. 82).
Lying between these are cells which, from their shape,
are often called " goblet " cells, but which in structure are
practically the same as the nuicous cells of the submaxillary
gland already described (p. 238). These cells secrete the
mucus which covers the inside of the intestine. The
columnar cells are concerned in the absorption of digested
food.

13, The Structure of the Pancreas and its Changes
dliring Secretion. — The pancreas is a raccMUose gland,
but tlie 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. 239). When the gland has
been at rest for some time the cells are large, their outlines
indistinct, and the central part of each is thickly loaded
with very obvious granules (Fig. 83, A). After the gland
has been secreting for some time, the cells are smaller,
their outline distinct, and the granules have largely dis-
appeared. Those granules which remain are now placed
at the inner ends of the cells next to the lumen of the
alveolus (Fig. 83, 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 lodged in their substance as ob-
vious 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 se-
cretion are essentially the same as those previously
described in the case of the salivary glands, and have

PANCREATIC JUICE

259

the same significance in explanation of the phenomena
of secretion. In one i-espect there is a remarkable
difference between the pancreas and the salivary glands.
The latter always secrete as the result of a stimulus
reaching them along their nerves. It is not certain that
the cells of the pancreas are supplied with nerves at all
and it is certain that they can be made to secrete by a
chemical substance which is carried to them by the blood.
This hormone (p. 26) is called secretin and is manu-

FiQ. 83.— A Portion of the Pancreas of a Rabbit.

A, at rest ; B, in a state of activity.

a, granular central zone of the cells ; 6, clear outer zone ; c, lumen of
alveolus ; d, junction of two neighbouring cells.

factured in the cells of the mucous membrane of the
duodenum. The arrival of acid food from the stomach
into the duodenum causes the secretin to be discharged
into the blood by which it is carried to the pancreas. By
this mechanism a flow of pancreatic juice is insured for
the digestion of the food which arrives in tlie duodenum.
14. 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
large amount of protein in solution. It contains further,
as its most important constituents, three soluble ferments.

s 2

'260 ELEMENTARY PHYSIOLOGY less.

Of these the chief is one which is called trypsin, and is so
far like pepsin that it converts proteins into peptones,'
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 carbonate, while pepsin will
only act in presence of an acid. In the next place, the
change which proteins 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 leucine, tyrosine, and similar bodies.
These amino-acids are peculiarly interesting, inasmuch as
after absorption they are carried to the liver in the blood
of the portal vein and are converted in part into urea by
the liver (see p. 189).

The second ferment in pancreatic juice is one which
resembles the ptyalin of saliva in so far as it converts
starch into sugar, but it acts more energetically. The
sugar which it produces is maltose (CjjHjoOn).

The third ferment has no action on either proteins
or carbohydrates, but it acts on the ordinary fats in
such a way as to split them up into glycerine and
a fatty acid. Tlie latter uniting with the alkali of
the pancreatic juice forms soaps, and this process is
known as saponification. The soaps so formed are
soluble substances and therefore can pass into the wall of
the intestine. In this form then the fat can be absorbed.
Moreover, they help greatly in reducing the remaining
fats to that state of fine subdivision, known as an
emulsion,^ which, as we shall see presently, is an
important preliminary to the digestion of fat in the
intestines.

1 An artificial pancreatic digestion of proteids may be carried on in
the way already described for pepsin (p. 247), using as a digestive fluid a
1 per cent, solution of sodium carbonate to which some of the extract of
pancreas sold as "liquor pancreaticns " has been added. ■

2 When a substance is sub-divided into extremely minute particles
suspended in a fl\iid under conditions such that these particles do not run
together on standing, the substance is said to be emulsified and the fluid
is called an emulsion. Thus milk is a typical emulsion.

VI BILE 261

Pancreatic juice, as containing these three ferments,
acts thei-efore on all three classes of foodstutFs, splitting
the proteins, saponifying and emulsifying the fats, and
converting 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
(dextrose) speedily appears in the urine and the animal
wastes away. Such a condition is not infrequently observed
in man, and is known as diabetes ; and in some cases
of diabetes the pancreas has been found to be diseased.
In this respect the pancreas seems to exert some control
over the nutrition of the body in a way somewhat
similar (though differing in its results) to the influence
exerted by the thyroid gland and the suprarenal bodies
(see pp. 217 and 219). This property of the pancreas is
associated with certain groups of cells known as the
Islets of Langerhans which diflfer in appearance from
the rest of tlie pancreas.

15. The Nature and Action of Bile.— Bile is the
fluid secreted by the liver (see p. 213). In the con-
dition in which ifc may most ordinarily be observed it
has a greenish colour, but as it flows fresh from the
liver it is bright yellow with a brownish tinge. This
colour is due to a pigment called bilirubin. By
oxidation this may be easily converted into a green
pigment called biliverdin, and the differences in the
colour of the bile of different animals depends on the
relative amounts of these two pigments which they contain.
These colouring matters are usually known as the bile-
pigments, and of these the bilirubin is probably, as
already stated (p. 105), formed from the hfemoglobin of
the red blood-corpuscles by some peculiar action of the
cell-* of the liver. In addition to the pigments bile, which
consists of about 85 per cent, of water, holds in solution
certain salts of sodium, the bile-salts. These are salts

262 ELEMENTARY PHYSIOLOGY less.

of two organic acids, one called glycocholic, the other
taurocholic. The former consists of carbon, oxygen,
hydrogen, and nitrogen, while the latter contains ad-
ditionally 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 is now
viscid and slimy.

Bile has by itself no direct chemical action on food-stuffs*.
But it serves to neutralise the acidity of the chyme as it
leaves the stomach and thus prepares it for the action
of pancreatic juice. Further it plaj^s an important part,
when mixed with pancreatic juice, in leading to the
emulsification of fats, facilitating their subsequent absorp-
tion and enabling the ferments of the pancreatic juice to
act more rapidly and eft'ectively. It also possesses the
property of keeping the bowels sweet. In its absence
undue putrefactive changes take place.

16. The Changes Food Undergoes in the Intestines.
— The only secretions, besides those of the proper intesti-
nal glands, which enter the intestine, are those of the
liver and tlie 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 obliciuely through the coats of the
intestine, its walls serve as a kind of valve, obstructing
the flow of the contents of the duodenum into the duct,
but readily permitting the passage of bile and pancreatic
juice into the duodenum (Figs. 67 and 76).

The glands of Lieberkuhn are supposed to form a cer-
tain amount of a secretion known as succus entericus,
or intestinal juice, which they then discharge into the
intestine.

After gastric digestion has been going on some time,
and the semi-digested food begins to pass on into the

VI FAT EMULSIFIED 263

duodenum, the pancreas conies 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 mu(?h more con-
tinuous than that of the pancreas, and is not so markedly
increased 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 pas.ses into the duodenum,
and flows over the common aperture of the bile and pan-
creatic 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, both the bile
and the pancreatic juice appear to exercise an influence
over the fatty matters contained in the chyme, which
facilitates the subdivision of these fats into very minute
separate particles, and this action is specially well-marked
when bile and pancreatic juice are mixed. The fat, as it
passes from the stomach, is very imperfectly mixed with
the other constituents 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 larger drops of fat which pass into the intestine
from the stomach are etntdsified. that is to say 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. butter) 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
susperded fatty particles reflecting a great amount of light.

264 ELEMENTARY PHYSIOLOGY less.

The contents of the small intestitie, thus white and
milky, are sometimes called chyle ; but it is best to reserve
this name for the contents of the lacteals, of Avhich we
shall have to speak directly.

The emulsification and saponification of the fats are not,
however, the only changes going on in the small intestine.
The pancreatic juice has an action on starch similar to
that of saliva, but much more powerful. During the
short stay in the mouth veiy little starch has had time to
be converted into sugar, and in tlie 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 maltose, by
the action of this juice. A ferment in the succus
entericus converts the maltose (Ci2H,20ii) into dextrose
(CgHj-jOg) in which form the sugar is absorbed.

Nor is this all, for, in addition to the above, the alkaline
pancreatic juice has a powerful effect on proteins very
similar to that exerted by the acid gastric juice ; it con-
verts 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
leucine, tyrosine, and other amino-acids make their
appearance as the result of the further action of pan-
creatic juice on the first-formed peptones. Here again
the succus entericus aids digestion. Peptone if absorbed
into the blood would act as a deadly poison. A ferment
erepsin also breaks up the peptone into a number of much
simpler bodies known as amino-acids. These are com-
plicated organic acids which however contain nitrogen in
a form closely related to the nitrogen in ammonia and
urea.

Hence it appears that, while by the saliva carbo-
hydrates only, and by the gastric juice i)roteins only, are
digested, in the intestine all three kinds of food-stuffs,
proteins, fats, and carbohydrates, are completely dis-

VI FAT EMULSIFIED 265

solved, and made diffusible and so prepared for their
passage into the vessels.

As the food is thrust along the small intestines 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 lacteals lying in the intestinal walls.

All the way down the small intestines, the proteins,
carbohydrates, and fats of a meal are being dissolved
or otherwise changed, and passing away into the lacte-xls
or blood-vessels. So that, by the time the contents of
the intestine have reached the ileo-csecal valve, a great
deal of the nutritious matter has been removed. 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
fermentation, whereby the contents become acid. But
these changes do not appear to be brought about by any
soluble ferments secreted by tlie walls of this intestine ;
on the contrary they are largely the result of the activity
of certain minute organisms or organised ferments
(bacteria, &c.).

In herbivora a considerable quantity of the cellulose
they eat does not reappear in the fseces and a small
amount may be digested in man. The digestion of
cellulose probably takes place in the large intestine and
is brought about by the action of micro-organisms.

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 absorp-
tion, so that the contents at the ileo-csecal 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

266 ELEMENTARY PHYSIOLOGY less.

the remains of the food, now consisting either of un-
(ligestible material, or of material which has escaped the
action of the several digestive juices, or withstood their
influence, gradually assume the characters of fteces.

17. Absorption from the Intestines.— A great deal
of the absorption takes place in the small intestine
(though the process is continued on 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. 257),
is covered by a layer of epithelium, and contains iti the
centre a lacteal radicle, between which and the epithelium
lies a network of capillary blood-vessels embedded in a
delicate tissue. The soap and fatty acids pass into the
cells of the epithelium where they reunite with glycerine
to form droplets of fat which travel 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 large 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 the vessels, gives their
contents a white, milky appearance. 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 ti'aced along the lymphatics 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 pouring 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 proteins and carbohydrates, the
result of digestion has been to produce a solution of
nitrogenous organic acids and sugars which are extremely

VI ABSORPTION FROM THE INTESTINES 267

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 proteins, there would be a rapid
transmission of the diffusible substances through and
across the membrane. The conditions necessary for such
a process are evidently present in the intestines where
the solution in its interior is separated by what is practi-
cally a thin membrane from the (albuminous) blood in the
capillaries just below tlie epithelial cells. It is thus very
tempting to suppose that the absorption of amino-acids
and sugars (also of salts) is the result of their diffusibility
and of the conditions to which they are exposed. And
indeed loithin certain limits this view is correct. But it
does not by any means explain the whole process. 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 peculiar that we must again, as in the case of the fats,
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. Amino-acids 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
amino-acids and sugar are then carried along the portal
vein to the liver, where they probably undergo some
further change. So that while the fat reaches the blood
very little changed, the cleavage products of the proteins
and the sugars though also taking a roundabout course,
viz., by the liver, are probably altered before they are
thrown into the general blood-stream ; for the portal
blood in which they are carried is acted upon by the

268

ELEMENTARY PHYSIOLOGY

liver before it flows through the hepatic vein into the
general venous system. But concerning both the pro-
cess 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 NuTRiTioisr.

1. Some Aspects of Nutrition.— Nutrition, on the

statistical side, has also to deal with the (juantitative
relationships between tlie amount of food supplied and
the amount of waste excreted ; 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 supplied in the food and
how this available income js expended as heat and
work.

2. Some Statistics of Nutrition.— The average
weiglit of a healthy full-grown man may be taken as 70
kilogrammes (154 pounds). Such a body is made up, in
round numl)ers, as follows : —

Muscles and Tendons

Skeleton

Skin

Fat

Brain

Thoracic viscera
Abdominal viscera ...
Blood

42

per cent

16

7

19

2

2

7

5

100

VI STATISTICS OF NUTRITION 269

The waste which the body excretes and its distribution
among the chief excretory organs is shown in the table on
the following page, in which the ".water and other
matters " represent the total waste.

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 it includes urea, which
contains nearly the whole of the nitrogen excx'eted,
"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 output 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
necessary 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, although a small amount comes from the oxida-
tion of the hydrogen of the food ; the oxygen 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 9| oz.) of the latter.
The niti'ogen necessary to make good this loss can only be
obtained from proteins.

The necessity of constantly renewing the supply of
protein matter arises from the circumstance that whether
the body is fed or not, a breaking down of protein mate-
rial is continually going on, giving rise to a constant
nitrogenous waste, which leaves the body in the form of
urea. Now, this nitrogenous waste, coming from the
breaking down of protein material, can only be met by

270

ELEMENTARY PHYSIOLOGY

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VI STATISTICS OF NUTRITION 271

fresh protein material being supplied. If protein matter
be not supplied, the body must needs waste, because
there is nothing in the food competent to make good the
nitrogenous loss.

On the other hand, if protein matter be supplied, there
is not the same necessity for any other but the mineral
food-stuffs, because protein 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.

In fact, the final results of the oxidation of protein
matters are carbonic acid, water, and urea ; and these,
as we have seen, are the final shapes of the waste
products of the human economy.

, Proteins contain in round numbers about 15 per cent.
of nitrogen, or a little more, and 53 per cent, of carbon, so
that the 20 grammes of nitrogen might be obtained from
130 grammes of protein, which would at the same time
introduce about 65 grammes of carbon. But the daily
waste of carbon is (about) 270 grammes, thus more than
200 grammes of carbon are still required to balance the
excess in the excreta. This additional amount of carbon
may be obtained from either fats or carbohydrates,
but preferably from a mixture of the two. Now
fats contain 80 per cent, of carbon, and carbohydrates
contain 40 per cent. ; thus the desired amount of
extra carbon may be obtained by adding to the
130 grammes of protein, about 50 grammes of fats
which contain 40 grammes of carbon and 400 grammes
of carbohydrates which contain 160 grammes of car-
bon. Adding to these 30 grammes of inorganic salts
and 2,300 grammes of water the total waste is about
balanced thus : —

272 ELEMENTARY PHYSIOLOGY less.

and 2,300 grammes of water the total waste is about
balanced thus : —

Nitrogen. Carbon

Proteins 130 grammes (4i oz.) contain 20 graninics (J oz.) TO granniies (2J oz.)

Fats 60

Carbo- \.^
hydrates/*""
Salts .... 30
Water... 2,300

(•-' oz.)

(14 oz.)

(1 oz.)
,, (4 pints)

jrammes

"

40 „ (U oz.)
160 „ (5i oz.)

2,910 i

20

grammes

270 grammes (9i oz.)

Foods as previously explained (p. 226) 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 carbo-
hydrates re<iuired by the above statement opens up the
possibility of an almost indefinite choice. But the
composition of the more commonly used articles of food
as regards the amounts of the several kinds of food-stuffs
they contain, has l)een very carefully determined, so
that we may now proceed to select a meal such as will
give us the desired quantities of proteins, fats, and
carbohydrates.

Suppose, for instance, that we select lean meat, bread,
potatoes, milk, and fat, such as butter or dripping, as our
food. We may obtain all that we require from the
amounts of each food shown in the table on the following
page.

The amounts of the several foods shown in the above
table suffice to cover the waste shown in the table on j). 270
and constitute what is ordinarily known as a diet. But tlie
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

STATISTICS OF NUTRITION

273

Pn

S

cS

Li

o

fii

o

1^

O*

• o

o : o

O ; p

: o ■ :

: ^

-fi ■ i;

-^ • ^

M-iS •

: ^

ti • Ph

i< ; Ph

•^ h '■

cS

c6

CS 13S

O OJ ,^

=" O OT

• -tj

. S -S

. . a>

: a

-^ ^ S

-w -S g

+j +j -1^

r^ r^ fT<

274 ELEMENTARY PHYSIOLOGY less.

food-stuffs they contain, so that it is possible to draw up
many such tables all satisfying the condition of covering
the daily loss of 20 grammes of nitrogen and about 270 or
300 granimes of carljon.

In drawing up such a table of diet the question i>f cost
must also not be forgotten, as in the case of fixing the
diet for soldiers or prisoners. 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 useful-
ness 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 lai'ger proportion 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.

3. The Economy of a Mixed Diet.— The body, as we
have repeatedly pointed out, cannot obtfiin 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 vital food altogether, the organism, so long as it con-
tinues 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 ob.served that a starving sheep is as much a car-
nivore as a lion. Protein is thus the essential element of

VI ECONOMY OF A MIXED DIET 275

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 proteins,
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, 3^ 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 exercise, 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. However, 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 proteins)
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 protein 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, which have
to get rid of the nitrogenous matter, three-fourths of
which, as we have seen, is superfluous.

Unproductive labour is as much to be avoided in phy-
siological as in political economy ; and it is quite possible
that an animal fed with perfectly nutritious protein matter

T 2

276 ELEMENTARY PHYSIOLOGY less.

should die of starvation ; the loss of power in various
operations required for its assimilation overbalancing the
gain ; or the time occupied in their performance being
too great to permit 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 justi-
fication of the universal practice of mankind in adopting a
mixed diet, in which proteins are mixed with fats and
carbohydrates.

Fats may be taken t(j 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 gx-ammes (ratlier more than
a pound) of sugar, will supply this quantity of carbon.

Several apparently simi)le articles of food constitute
a mixed diet in themselves. Thus butcher's meat com-
monly contains from 30 to 50 per cent, of fat. Bread,
on the other hand, contains the protein gluten, and the
carbohydrates, starch and sugar, with minute quantities
of fat. But from the proportion in which these protein
and other constituents exist in these substances, they are
neither, taken alone, such physiologically economical
foods as they are when combined in the pi'opOrtion of
about 200 to 75, or two pounds of bread to three-quarters
of a pound of meat per diem.

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 most suitable for affording an
economical diet as regards the proportion of the nitrogen
to the carbon. This food is milk. Milk consists chiefly

VI GELATIN AS A FOOD-STUFF 277

of water (86 p. c.) in which proteins, casein and some
albumin, are dissolved, as also a carbohydrate, milk-sugar
or lactose, and inoi-ganic 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. Milk,
however, is deficient in one essential constituent of diet,
that of iron. We have seen that iron is an important
constituent of haemoglobin (p. 97). A child is born with
sufficient iron in it3 body to provide for the formation of
red blood corpuscles for nine months to a year. After
this period it must get iron from its diet. Milk does not
furnish this.

4. The Effects of the several Food stuffs.— When
some protein food is given to an animal, such as a dog,
which has been previously starved, the larger part of the
nitrogen given in the protein is not retained in the body
but is excreted almost immediately. This is the nitrogen
in what we have termed the "exogenous " urea (p. 189).
The nitrogen excreted during the starvation was of cour.se
solely of "endogenous " origin. If another larger meal of
protein be given the amount of nitrogen excreted is still
further increased, less and less being retained in the body.
By proceeding 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."

The eflfects of carbohydrates and fats as foods cannot
be studied by feeding an animal with these alone, as
is possible with proteins. But this difficulty may be
got over by administering a small, fixed quantity of protein
with a variable amount of either carbohydrate or fat. In
this case it is found that an increase of the carbohydrate
in food very soon leads to the laying on of fat ; and this
corresponds to the everyday experience which is frequently

278 ELEMENTARY PHYSIOLOGY

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 ex-
plained (p. 215).

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
therefore not as fattening as might at first sight have been
expected.

We have already mentioned gelatin as a food-stuflf
which consists of the same four elements as are present
in proteins and in somewhat similar proportions. But
the nitrogen in gelatin cannot completely replace that
of a protein for the repair of nitrogenous waste. An
animal fed with gelatin dies ultimately of "nitrogen
starvation," though not as soon as it would without this
constituent of food. Thus gelatin reduces the waste of
the body and, so far, may be a useful article of food.

Although we have laid stress on the necessity of protein
in diet, we should point out the possibility supporting
nitrogenous equilibrium on a diet which does not actually
contain them. In discussing the digestion and absorption
of proteins we saw that they were ultimately absorbed as
amino-acids. Of these each protein breaks up into a great
many of different characters. If we could collect all these
acids and mix them in exactly the correct proportions we
could no doubt supply the needs of the body. At present
this can be accomplished by artificial digestion of protein
and the use of the products as a diet. If, however, such a
diet fell short by this amino-acid or that, it would become
inadequate. In this fact we find the key to the incom-
petence of gelatin to support life. When it is digested
there are lacking in this mixture of acids produced, two
important bodies which are yielded by protein.

Some of these bodies necessary ai'e only required in
very small quantities. Orientals can live on a diet which

VI GELATIN AS A FOOD-STUFF 279

consists almost entirely of rice, but if the rice be devoid
of its natural husks they incur the risk of suffering from a
condition of mal-nutrition known as beri-beri. The
particular all impjortant constituent of the husks has been
isolated and estimated though the amount necessary to
the body daily is almost inconceivably small.

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 metabolism 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 defective nutrition,
accompanied by paralysis and convulsions.

When an animal is deprived of all food whatsoever, it
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 prolong life to the utmost. Thus the brain and spinal
cord are almost unaltered at death, and the blood and
the muscular tissue of the heart also lose but little as
compared with the fat and the skeletal muscles on which
the body is now chiefly living.

5. The Erroneous Division of Food stuffs into
Heat-producers and Tissue-formers. —Food-stuffs have
been divided into heat- producers and tissue-formers— the
carbohydrates and fats constituting the former division,
the proteins 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 proteins does not

280 ELEMENTARY PHYSIOLOGY less.

develop heat ; and, on the other, that the carbohydrates
and fats in being oxidised, suliserve only the production
of heat.

Undoubtedly proteins are tissKe-furmers, 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 proteins aie 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 proteins alone are the tissue-formers
then the energy set free during the contracting activity of
the pre-eminently nitrogenous muscles ought to come
from the metabolism of their proteins. But this is not
found to be the case, for even the most violent muscular
exercise does not lead immediately 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 sjiy, the non-nitrogen(jus part of the
tissue seems to be used up more quickly than the nitro-
genous part ; and the consumption of this particular
constituent of the muscular substance may be made good
by non-nitrogenous food, by fats or carbohydrates.

On the other hand, proteins must be regarded as heat-
producers also. For if oxidation be, as has Ijeen sug-
gested, largely confined to the tissues, though in some
tissues, as in muscles, the non-nitrogenous part seems to
be most rapidly changed, yet the nitrogenous part, sup-
plied by the proteins, is sooner or later oxidised, and in
being oxidised must give rise to heat.

As soon as the eleuients 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

VI THE EXPENDITURE OF ENERGY 281

the vital food-stuffs at all, it appears desirable to dis-
tinguish the essential food-stuffs, or proteins, 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 important, are not abso-
lutely necessary.

6. 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 tlie blood by the lungs
is carried away out of the body in the various waste
products.

The intermediate stages of this conversion are, however,
by no means so clear. It is highly probable that all the
food-stuffs which pass from the alimentary canal into the
blood, be they derived from proteins, or fats, or carbohy-
drates, become part and parcel of some tissue or other
(muscle, nervous tissue, glandular tissue, and the like),
before they are oxidised ; that indeed it is as constituent
elements of 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 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 further probable 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 oxida-
tion of them in the tissues, more especially the muscles,
in the cells of which they may be pictured in close contact
with, and available for, the actual needs of the fabric.

282 ELEMENTARY PHYSIOLOGY less.

The oxidation of the tissues, which are themselves built
up out of food, and lience ultimately the oxidation of the
food itself, thus provides for the energy expended as
muscular work and as heat. Now the amount of me-
chanical work a man does may be detremined 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 di.stance, we call the unit of work a kilo-
gramme-metre, equal to 7 '24 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
1° F. Now, as is seen in all ortlinary engines, heat can
do work ; and it is found that one unit of heat can do
772 foot-pounds of work. This is called the " meclianical
equivalent of heat." In the metric system tlie unit
becomes the amount of heat required to raise the tem-
perature of one gramme (I5'4 grains) of water through
V C. This is called a calorie, and the mechanical
equivalent of heat is now 424 gramme metres. Using
tliese data we can readily convert heat into its equivalent
of Avork or vice versd.

The measurement of the amount of heat given off by
the body is by no means easy, and the sources of error
are considerable. But allowing for these a rough deter-
mination may be made, and the heat thus measured may
be calculated as work by ucing the mechanical ecjuivalent
of heat and the result added to the actual work done as
work. The outcome of this calculation shows that of the

THE EXPENDITURE OF ENERGY 283

total energy expended by the body about one-sixth is put
out as work and five-sixths as heat. Finally we find that
the average total output of energy as work and heat (cal-
culated as work) may be taken as about 1,000,000
kilogramme-metres.

We may now consider how far this expenditure is met
by the income of energy in food. AVhen a substance is
completely burnt, i.e. oxidised, to water and carbonic
acid, a certain amount of heat is produced which can be
measured. Thus it is possible to determine how much
heat is produced by the complete combustion of one
gramme of each of the food-stuffs, proteins, fats and
carbohydrates. The result obtained is as follows : —

1 gramme of protein gives 5,700 calories.

1 „ „ fat „ 9,500 „

1 ,, ,, carbohydrate ,, 4,000 ,,

Kow this must also be the amount of heat produced by
the same quantity of each of these food-stuffs during their
oxidation in the animal body. In the case of the protein
some deduction has to be made because the proteins are
not completely oxidised ; the nitrogen they contain leaves
the body as urea, which is still capable of undergoing
further oxidation to water, nitrogen, and carbonic acid.
One gramme of protein 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
combustion of one gramme of protein. If now we apply
these values to the diet given on p. 272 we find that : —

130 grammes of protein give 624,000 calories,

50 „ „ fats ,, 475,000

400 „ „ carbohydrate ,, 1,600,000 „

2,699,000

284 ELEMENTARY PHYSIOLOGY less, vi

If now we take the mechanical equivalent of this heat
we find it works out as 1,144,376 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 VII
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
master, in the form of vital and mineral food, is constantly
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 vital 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
immediately decompose into them. Consequently, the
human body is the centre of a stream of matter which
sets incessantly from the vegetable and mineral worlds
into the mineral 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
particles which flow out of it. To speak in what is not
altogether a metaphor, the atoms enter the body for the

286

ELEMENTARY PHYSIOLOGY

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 movements, cilia, and muscles.

The amoeboid movements of
the white corpuscles of the blood
have been already described, and
it is probable that similar move-
ments 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 ; neverthe-
less, there are reasons for think-
ing that these amoeboid move-
ments are of great importance
to the economy, and may under certain circumstances be
followed by very notable consequences.

2. Ciliated Epithelium and Action of Cilia.— Cilia
are filaments of extremely small size, attached by their
bases to, and indeed growing out fi'om, 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. 40, 84). In some of the
lower animals, cells may be found possessing only a
single ciliuni. They are in incessant waving motion, so
long as life persists in them. Their most common form of
movement is that each cilium is suddenly bent upon itself,

Fio. 84.— Columnar Cili-
ated Epithelii'm Cells
FROM THE Human Nasal
Membrane.

Magnified 300 diameters.
(Sharpey.)

vii CILIA 287

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
antagonistic ; 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 contractile
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 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 direction. Thus, the cilia which are developed
upon the epithelial cells, which line the greater part
of the nasal cavities and the trachea, with its rami-

288 ELEMENTARY PHYSIOLOGY les&

fications, 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. — 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 bands of
fibres which are bound together by connective tissue
carrying nerves and blood-vessels. The fibres are in
reality elongated, flattened, spindle-shaped cells whose

FlO. 85.— A FiBRE-CEI.I. FROM THE PLAIN, N0N-8TRtATED MuSCULAB

Coat of the Intestine.
/, fibre ; n, nucleus ; p, granular protoplasm around the nucleus.

length is about 50/i (.Jg inch) and width 6/x (^^^ inch).
Somewhere towards the middle of each cell there is an
elongated oval or sometimes rod-shaped nucleus, sur-
rounded by a small amount of granular protoplasm which
is pointed at the ends of the nucleus.

The substance of the cell is clear and shows no trans-
verse striations, although it often shows signs of a very
fine longitudinal fibrillation. A number of such fibre-
cells ave united together by a minute quantity of cement,
or intercellular substance, into a thin flat band, and a
number of such bands are bound together by connective
I issue into larger bands or bundles. Each fibre is capable
of contracting, that is, of shortening and becoming at the
same time thicker. In addition to the actual physical

VTi STRIATED MUSCLE 289

connection which the cement substance affords there is
clearly some fundamental connection by which the heat is
conducted from tibie to fibre.

4. The Structure of Striated Muscle,— Striated
mu^le is also made up of fibres, though the fibres are
very different 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.' Each muscle
is thus made up of (i) an external wrapping or peri-
mysium ; 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 longitudinally disposed compartments ; (ii) the mus-
cular fibres which occupy these compartments ; (iii)
the vessels which lie in the sheath and in the
partitions between the compartments, and thus sur-
round the muscular fibres without entering them ; (iv)
the motor nerves which also at first lie in the sheath
and in the partitions between the compartments, but
which eventually enter into the muscular fibres.

The penmysimn 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),
w^hich becomes continuous 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, inclosing large bundles, each con-
sisting of a very great number of fibres. These large
bundles are again divided by somewhat finer connective
tissue partitions into smaller bundles, and these again
into still smaller ones, and so on, the smallest bundles of

1 It is necessary to distinguish "muscle" as an organ from "muscle '
as a tissue. The biceps miiscle (p. 306), for example, is an organ of a
complicated character, of which muscular tissue forms only the chief
constituent.

U

290

ELEMENTARY PHYSIOLOGY

all being composed of a number of individual muscular
fibres (Fig. 86). In this way the partitions become
thinner and more delicate, until those which separate the
chambers in which the individual muscular fibres are
contained are reduced to little more than as much
connective tissue as will hold the small nerves, arteries
and veins and capillary networks together. As the
perimysium consists of connective tissue, it may be
destroyed by prolonged boiling in water. In fact, in
" meat boiled to rags " vvc have musples which have been

Fio. 8f\ — FAScict'Li OF Striated Muscle cut across.

Several fasciculi /, bound together into larger fasciculi to make up the
muscle.

thus treated : the perimysial case is broken up, and the
muscular fibres, but little attacked by Ijoiling 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 Vje found to
have a thickness of somewhere about 60/i (4 ^o inch) (they
vary, however, a great deal), with a length of 30 or 40
millimetres, i.e., about Ih inch. It is a cylindroidal or
polygonal solid rod, which either tapers or is bevelled oil"

STRIATED MUSCLE

291

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
in the histological sense means 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

Capillaries of Striated Muscle.

A. Seen longitudinally. The width of the meshes corresponds to that of
an ultimate fibre, a, small artery ; 6, small vein.

B. Transverse section of striated muscle, a, the cut ends of the ulti-
mate fibres ; 6, capillaries filled with injection material ; c, parts where
the capillaries are absent or not filled.

to the dead state, but, in the case of muscle, the changes
which the tissue undergoes in dying, are of such a marked
character that the structure of the dead tissue gives a false
notion of that of the living tissue.

A living striated muscular fibre of a frog or a mammal
is a pale transparent rod composed of a soft, flexible,
elastic substance, the lateral contours of which, when the

u 2

292 ELEMENTARY PHYSIOLOGY less.

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 living body, bathed in the lymph
which surrounds it, the outlines are not so sharply de-

FlO. 88. — To ILLUSTRATE TllK STRI'fTtlRE OF A STRIATED MuSCULAR

Fibre.

A. Part of a muscular fibre (of a frog) seen in a nat\iral condition, d,
dim bands ; 6, bright bands, witli the granular line seen in manj' of
them ; n, nuclei and the granular protoplasm belonging to them, very
dimly seen.

B. Portion of prepared mammalian muscular fibre teased out, showing
longitudinal portions of variable (1, 2, 3, 4) thickness ; 4 represents the
finest portion (fibrilla) which could be obtained ; d, dim bands ; 0, bright
bands, in the midst of each of which is seen the granular line g.

fined. In neither case can any distinct line of demarc-
ation between a superficial layer and a deeper substance
be recognised. The fibre appears transversely striped, as
if the clear glassy substance were, at regular intervals
(Fig. 88, A, d), converted into ground glass, thus appear-
ing dimmer. Each of these "dim bands" is about 2/x
wide, and the clear space or "bright band" which
separates every two dim bands is of about the same size,

STRIATED MUSCLE 29.']

or under ordinary circumstances somewhat narrower.
With a high power a very thin dark granular line equi-
distant from each dim band is discernible in each bright
band, dividing the bright band into two. As these
apjjearances remain when the object glass is focused
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 segments, of which the thin gi-anular
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 byZ). B. g.B.D.B. g.B. indefinitely
repeated, and one inch of length of fibre will contain
about 30,000 such segments, or alternations of structure.

In a perfectly unaltered living fibre the striated sub-
stance 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 cavities each filled by a nucleus,
surrounded by a small amount of protoplasm (Fig. 88,
A, n).

As the muscular fibre dies it undergoes a rapid altera-
tion : — (i) parallel longitudinal strije, often less than 2fi,
apart, appear, in greater or less numbers until sometimes
the striated substance appears broken up into a mass of
fine delicate fibres ; (ii) the dim bands become much more
opaque, and hence the transverse striation appears better
marked, until the dim bands may appear like sharply
defined discs ; (iii) the nuclei acquire sharp irregular
contours and become much more conspicuous, and (iv)
especially under certain circumstances and after particular

294

ELEMENTARY PHYSIOLOGY

treatment, a thin superficial layer becomes sharply
separated from tlie deeper substance of tlie fibre as a
membrane of glassy transparency, the sarcolemma,
which ensheathes the striated and fil)rillated substance.

The bright bands and the granular lines, on the other
hand, undergo little alteration.

Under very higli powers each granular line looks like a
number of minute granules lying side
by side as an extremely thin plate,
the margins of which are attached to
the sarcolemma ; it is often spoken
of as Kratise's memhrnne.

If the sarcolemma of a dead fibre be
torn with needles, the striated sub-
stance breaks up in different ways
according to the treatment to which
the fibi-e has been previously subjected.
It may break up into discs, each of
which contains a dim band. Or it
may break up into fibrils, each of
which pi'esents tlie same segmentaticm
as the whole fibre. These artificial
fibrils vary much in thickness accord-
ing to mode of preparation and the
skill of the operator ; they may some-
times be obtained of exceeding fine-
ness (Fig. 88, B). Transverse sections
of muscular fibre, which have been
frozen while perfectly fresh, present
minute close-set circular dots, which appear to repre-
sent the transverse sections of bundles of naturally existing
longitudinal fibrils. These dots are known as Cohn-
heim's areas. If the muscle substance is really in this
case unaltered the only possible interpretation of the fact
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 refractive power as. the interfibrillar sub-

Fio. 89.— A Muscu-
lar Fibre (of Froo)
ENDING IN Tendon.

Tlie striated muscu-
lar substance, m, has
shrunk from the sar-
colemma, s, the fi-
brils of the tendon,
t, being attached to
the latter.

STRIATED MUSCLE 295

stance. But whether the finest artificial fibrils into which
dead muscle may be broken up are identical with these
apparently natural fibrils, it is not at present certainly
determined. In some cases the artificial fibrils seem
smaller than the natural ones, as if the latter, like the
fibre itself, were capable of longitudinal cleavage.

These are the most important structural appearances
presented by ordinary striated muscle. But it may
further be noticed that the dim bands exert a powerful
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 efl'ect.

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
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.
Meanwhile 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 characteristically striated muscle substance, only a
little remaining unaltered around each nucleus as a muscle
corpuscle.

The many-nucleated cell tlius changed into a muscular
fibre is nourished by the fluid exuded from tlie adjacent
capillaries, and it may be said to i-espire, insomuch 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

296 ELEMENTARY PHYSIOLOGY less.

such elementary organisms, through the intermediation of
which its temperature is kept constant and its supply of
nourishment maintained.

The special property of a living muscular filjre, that
which gives it its physiological importance, is its peculiar
contractility. The body of a colourless blood corpuscle,
as we have seen, is eminently contractile, insomuch as it
undergoes incessant changes of form. But these changes
take place at all points of its surface, and have no definite
relation 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.
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. — Whilst much in the
chemistry of muscle is obscure the muscular contraction
may be compared to an explosion, the explosive material
for which is manufactured on the spot. The muscle may
therefore be regarded as consisting of (1) the stored
explosive material, which is relatively small in quantity,
(2) the machinery and fuel for its manufacture, and (3)
the fabric of the building which forms the factory. Of
these three the last is protein in nature, the other two
we may consider as non-nitrogenous.

We are quite uncertain as to the actual nature of the ex-
plosion, i.e., the physical and chemical changes which are
directly associated with the change of shape in the muscle.
Theyappear capableof taking place in the ahsenceof oxygen
and under such circumstances there would be little or no

vn THE CHEMISTRY OF MUSCLE 297

evolution of heat or of carbonic acid, but the muscle
would become acid in reaction due to the production of
lactic acid. The obvious chemical changes which usually
accompany-the contraction of muscle are those involved
in the manufacture of fresh explosive material. This
material is a store of energy and is rebuilt at the expense of
the active oxidation of other substances, of which probably
sugar is usually the most important. The evidence of
this oxidation consists of (1) the using up of sugar from
the blood, (2) the using up of oxygen from the blood,
(3) the discharge of carbonic acid into the blood, (4) the
evcjlution of heat. When the supply of oxygen and
nourishment to the muscle is sufficient, and the exercise
is not excessive as in the case of the heart beating with
its normal rhythm, there is no reason to suppose that any
considerable quantity of acid such as lactic acid leaves the
muscle.

In the absence of oxygen, or in the presence of a
deficient supply of oxygen the reaction of the muscle
becomes more acid, and this acidity has an effect on what
we have spoken of as the protein fabric of muscle. At
first this becomes less mobile, so that the muscle contracts
with greater difficulty and under tliese conditions we say
that the muscle is fatigued — a condition which must be
distinguished from nervous fatigue or partial loss of
" grip " over our nmscles. Ultimately, if the muscle were
persistently deprived of oxygen, the acid reaction would
become so pronounced as to cause actual precipitation of
the protein fabric ; the muscle would then become
opaque and lose its contractility much as it does when
boiled. On stopping the circulation and thus cutting ofi"
the supply of oxygen these changes take place rapidly if
the muscle is stimulated, slowly if it is not. In either
case they spell death to the muscle and are a constant
phase of the death of the individual. This coagulation of
the protein fabric of the muscle on death is known as
"rigor mortis."

298 ELEMENTARY PHYSIOLOGY less.

After the lapse of a certain time the coagulated matter
liquefies, and the muscles pass into a loose and flabby con-
dition, which marks the commencement of putrefaction.

If a muscle taken perfectly 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. 107)
and subjected to considerable pressure it yields a fluid
called muscle plasma. This remains fluid so long as
it is kept ade(iualely cooled, but dots spontaneously at
ordinary temjjeratures, and may be made to clot when cold
by the addition of small quantities of acid. This clotting
results in the formation of a semi-solid gelatinous
substance, called myosin, and a small amount of fluid,
or muscle serum. Myosin is a protein and belongs to
the same class of proteins as do the fibrinof^en and para-
globulin of blood, namely the globulins.

Besides myosin, muscle contains other varieties of
protein 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 substiinces existing in small quantities, and often
clas.sed together as "extractives." Some of these
extractives contain nitrogen ; the most abundant of this
class is creatine. It has been inferred that creatine is
converted into urea, the most abundant waste product of
the body, but of this there is no proof.

The otiier class of extractives contains bodies free from
nitrogen, perhaps the most important of which are
sarcolactic acid and glycogen.

Most muscles are of a deep, red colour ; this is due in
part to the blood remaining in tlieir vessels ; but only in
part, for each fibre (into which no capillary enters) has
a reddish colour of its own, like a blood-corpu.scle but
fainter. And this colour is probably due to tlie fibre
possessing a small quantity of tliat same haemoglobin ia
which the blood-corpuscles are so rich.

VII THE CONTRACTION OF MUSCLE 299

6. The Phenomena of Muscular Contraction, —Every
fibre in a muscle has the property, under certain con-
ditions of shortening in length, while it increases corre-
spondingly in width, so that the volume of tlie fibre
remains unchanged. This property is called muscular
contractility, and whenever, in virtue of this property,
a muscle fibre contracts it tends to hruuj its two ends
closer together. Since a muscle is m«ade 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.

The condition which ordinarily determines the con-
traction of a muscular fibre is, the passage along the
nerve fibre, which is in close anatomical connection with
the muscular 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 thence called a motor fibre, because, by
its influence on a muscle, it becomes the indirect means
of producing motion (see Lesson XI.).

The phenomena of muscular contraction may be con-
veniently studied in the muscles from the calf of a frog's
leg, which, since the frog is a "cold-blooded" animal,
retains its powers 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
still attached to a piece of t\\e femur near the knee and to
the nerve, the sciatic, wliich supplies it. This muscle
as thus taken out of the body is known as a muscle-nerve
preparation (Fig. 90).

The muscle may now be suspended by the femur and a
weight hung on to the temlon at its lower end, and then
made to contract by stimulating the sciatic nerve (see also
Lesson XL).

300

ELEMENTARY PHYSIOLOGY

LESS.

When the nerve is stiniuhited by a stimulus which
lasts as short a time as possible, as, for instance, by the
momentary electric current often called an induction
shock, the following changes take place in the muscle.

Fio. 90.— A Muscle Nerve Prkparatkin.

TO, the rmiscle, gastrocnemius of frog ; np.f, lower cud of spiual column ;
n, the sciatic nerve, all the br.anches being out away excepting that
supplying the muscle ; /, the femur ; cl. a clamp to hold the femur ; t.a,
Achilles tendon.

(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 takes place very
rapidly, the whole px'ocess occupying rather more than
^ of a second.

THE CONTRACTION OF MUSCLE 301

(ii) The muscle may be enclosed in a small cliamber
free from oxygen and made to contract several times.
If now we examine the gas 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 contraction. That, however, the
muscle within the body richlij supplied with oxygen does
give off carbonic acid during its contraction is clearly
shown by the fact that the venous blood coming from a
contracting muscle contains relatively more carbonic acid
than it does wlien tlie muscle is at rest.

(iii) The muscle becomes slightly warmer ; this can
only be due to the fact that heat is formed during the
coMtraction. The rise of temperature is slight, but may
always be observed. The evolution of heat, like the
oxidation processes with which it is associated, is pro-
bably associated with the formation, after the contraction,
of fresh contractile material.

(iv) The muscle undergoes certain electrical changes.
At the moment of commencing contraction the muscle
becomes like a small battery cell, and can send an electric
current through a wire whose ends are suitably applied to
the surface of the muscle.^

7. The Tetanic Contraction of Muscles. — When
experimenting with a muscle-nerve preparation, as in the
preceding section, it is easy to stimulate the nerve twice
in such rapid succession that tlie 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 it
conti'acts still more while already contracting. The
second contraction takes place on top of the first, is
rather less in amount than the first, and is added on to
the first. If now a rapidly successive series of stitnuli be

1 See also Lesson XI., where the electrical changes of an active nerve,
which are essentially the same as those of a contracting muscle, are
described iu greater detail.

302 ELEMENTARY PHYSIOLOGY

applied to the nerve, the muscle responds by an equally
rapid series of contractions, each of which takes place
before the preceding one is over ; the whole series is
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 supei'added to
each other is called a tetanic Contraction. The acidity
and heat which are developed at a single contraction
become much more obvious during a continued 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, how-
ever short 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 con- •
tractions suct> as result from sending a single impulse
along the nerve going to tlie 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, inasnmch 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

VII VARIOUS KINDS OF MUSCLES 303

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 u'is 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. 288), and they are
disposed in two sets : one set radiating from the edges of
the hole to the circumference of the cux'tain ; and the
other set arranged in circles, concentrically with the aper-
ture. 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. 254), 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
contracted. As the order of contraction is, naturally,
always the same, from the upper towards the lower end,
the efiect 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 tlie 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

304

ELEMENTARY PHYSIOLOGY

are ordinarily eniplo^'ecl as levers, the osseous tissue is
arranged in the form of a shaft (Fig. 91 d.), formed of a

vn STRUCTURE OF A BONE 305

very dense and compact osseous matter, but often contain-
ing a great central cavity (b) which is filled with a very
delicate vascular and fibrous tissue loaded with fat called
marrcw. 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-woi'k of bony plates and
fibres, which is termed the cancellous tissue of the
bone. The surface even 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. 319), or, contrari-
wise, allows that 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. 92) of the bones
of the upper extremity, with the attachments of the biceps
muscle 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 atta,chment to the stationary bone is termed
the origin, that to the movable bone the insertion, of
the muscle.

The fibres of muscles are sometimes fixed directly into
the parts which serve as their origins and insertions ; but,
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 bursse ; or even to be invested by

X

306

ELEMENTARY PHYSIOLOGY

synovial sheaths, i.e. quite covered for some distance by
a synovial bag forming a double sheath, very much in the
same way that the bag of the pleura covers the lung and
the chest- wall.

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 mtperior oblique muscle of the eye,
the tendon passes over a pulley formed hy ligament, and

FiQ. 92.

-The noNEs OF the Upper Extremity with the Bicbps

Muscle.

The two tendons by which this musole is attached to the scapula are
•een 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 ; W, the weight (of the hand).

completely changes its direction before reaching its inser-
tion. (See Lesson TX.)

Again, there are muscles which are fleshy at each end,
and have a tendon in the middle. Such muscles are
called dignairic, or two- bellied. In the curious muscle
which pulls down the lower jaw, and especially receives
this name of di'jastrir, the middle tendon runs through a
pulley connected with the hyoid bone ; and the muscle,

BONE

307

which passes downwards and fprwards from the skull to
this pulley, after traversing it, runs upwards and forwards
to the lower jaw (Fig. 93).

9. The General and Minute 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 away, but the
surface of which has not been scraped or otherwise in-
jured, 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

Fig. 93. — The Codrse 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.

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 vascular connective tissue commences, and ex-
tending 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 shreds with the forceps. Filaments
pass from its inner surface into the interior of the bone.
If the shaft is broken across it Avill be found to contain a

X 2

308 ELEMENTARY PHYSIOLOGY less.

spacious medullary cavity filled by a reddish, highly
vascular mass of connective tissue, abounding in fat cells,
called the medulla or marrow ; and a longitudinal
section shows that this medullary cavity extends through
the shaft, but in the articular ends becomes subdivided
by bony partitions and bi'eaks up into smaller cavities,
like the areohie of connective tissue. These cavities are
termed cancelli, and the ends of the bone are said to
have a cancellated structure. The walls of the medullary
cavity in the shaft are very dense, and exhibit no cancelli
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/i to lOOfi 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 communi-
cating 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 comnmnicating canals and become
continuous with the likewise vascular contents of the
Haversian canals. Moreover, 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
internal, thick, cylinder of vascular medulla ; (ii) an
external, hollow, thin, cylindrical sheath of vascular peri-
osteum, completed at each end by a plate of articular
cartilage ; (iii) of a fine, roopilar, long-meshed vascular
network which connects the sides of the medullary

vn BONE 309

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 ; (v) of the hard,
perfect osseous tissue which tills 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 hydro-
chloric 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 gelatin, and only
a small residue will be left. Osseous tissue therefore
consists essentially 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 magnifying glass, shows a series of lines, with dark
enlargements at intervals, running parallel with the
Haversian canals. If the section, instead of being longi-
tudinal, were made transversely to the shaft, and there-
fore 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 cu'cles at regular

310 ELEMENTARY PHYSIOLOGY less.

intervals round each Haversian canal (Fig. 94). The hard
bony tissue appears therefore to be composed of lamellae,
which are disposed concentrically around the Haversian
canals ; and a Haversian canal with the concentric
lamellae belonging to it form 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
microscope (Fig. 95), by transmitted light, each dark spot
is seen to be a black body (of an average diameter of
about lofj.) with an irregular jagged outline, and pro-
ceeding from it are numerous fine dark lines which
ramify in the surrounding 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 microscope 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 bubbles of air will
have escaped from the section. The black bodies seen in
the dry bone are in fact "lacunae," i.e. gaps, or holes in
the solid matrix, appearing black by transmitted light
and white by reflected light, because they 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 communication with another. In each Haversian

BONE

311

i*-

"0^,

r-

;::;=— -^ir-V- i--

u

d^

7^ --4

'z:^^s-

Fig. 94.— Transverse Section of Comp.\ct Boxe.

o, lamellae concentric with the external surface : h, lamellae concentric
with the medullary surface : <■, section of Haversian canals ; c', section
of a Haversian canal just dividing into two; c^ intersystemic lamellae.
Low magnifying power.

312

ELEMENTARY PHYSIOLOGY

system the <analicnli, and the lacunae of the innermost
layer or that nearest the Haversian canal communicate
with it, while the canaliadi and the lacunct of the outer-
most layer communicate only with those of the next inner
layer. Hence the lacume and canaliculi compose a

Fio. 95.— Transverse Section of Bonk, highly magnified (300
diameters).

H, Haversian canals ; I, lacunae with canaliculi.

meshwork of canals, which is peculiar to each Haversian
system, and by which the nutritive plasma exuded from
the vessels in the canal of that 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 lacunte and canaliculi con-

VII BONE 313

tain no air, but are permeated with the nutritive fluid.
Each lacuna moreovei", at all events in young bone,
contains a nucleated cell, which is altogether similar in
essential character to a connective tissue or cartUage
corpuscle, and if the term were not already misused
might be called a "bone corpuscle." In fact, in
ultimate analysis the essential character of bone shows it-
self 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 uncalcitied passages (the canali-
culi), which open into the larger uncalcified intervals (the
lacu)ice\ in the neighboui-hood of the cells.

The function of these passages is doubtless to allow of
a more thorough permeation of the calcified tissue by the
nutritive. fluids than could take place if the calcareous
deposit were continuous, and it is probable that, in an
ordinary bone, there is no particle Ifi 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 after a time to be dyed red. The madder dye, in
fact, getting 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

314 ELEMENTARY PHYSIOLOGY less.

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 thickness of its wall will be found
to have colourless bone on its medullary and on its perio-
steal 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 Mechanics of Motion. The System of
Levers. — To understand the action of the bones, .as levers,
properly, it is necessai'y 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 presents 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 upon the relative
positions 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 po'wer 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 order. (Fig. 96, I.)

If the fulcrum be at one end, and the weight be between

LEVERS

315

it and the power, so that weight and power describe con-
centric arcs, the weiglit moving through the less space
when tlie lever moves, the lever is said to be of the
second order. (Fig. 96, IT.)

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

-tT

4

ill

Fio. 96.

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 resist-
ance ; F, fulcrum ; P, power.

the power moves through the less space, the lever is of
the third order. (Fig. 96, III.)

In the human body the following parts present ex-
amples of levers of the first order.

(a) The skull in its movements upon the atlas, as
fidcnim.

(6) 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 heing fidcrum. (Fig. 96, I.)

The positions of the weight and of power are not
given in either of these cases, because they are reversed
according to circumstances. Thus, when the face is being

316 ELEMENTARY PHYSIOLOGY LEsa

depressed, 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 beliind anil the weight in front. The
like is true of the pelvis, according as the body is bent
forward, 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 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 order :—

(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^ of the fi'ont 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 somewhere between the end of the knee
and the hip.

(b) A rib when depressed by the rectus muscle ^ of the
abdomen, in expiration.

Here the fulcrum lies where the rib is articulated with
the spine ; the power is at the sternvnn — virtually the
oj)posite end of the rib ; and the resistance to be over-
come lies between the two.

1 This muscle, called rectus, is attached above to the haunch-bone and
below to the knee-cap (Fig. d, 2, p. 19). The latter bone is connected by
a strong ligament with the tihia.

'^ 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. ti, 3).

yn

EXAMPLES OF LEVERS 317

(c) The raising of the body upon the toes, m standing
on tiptoe, and in the first stage of making a step forwards.

(Fig. 96, 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 (Fig 6 1.); the resistance is so much of the weignt
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 order are-

(a) The spine, head, and pelvis, considered as a rigid

bar/which has to be kept erect upon the hip-jomts.

^^Hei\he fulcrum lies in the hip-joints, the weight is
hi-h 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.

(6) 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. 92.) ,.,,••.

(c) Extension of the leg on the thigh at the knee-jomt
Here the fulcrum is the knee-joint ; the weight is at
the centre of gravity of the leg and foot, somewhere be-
tween the knee and the foot ; the power is applied by the
muscles in front of the thigh (Fig. 6, 2 and Fig. 97),
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 im-
portant to recollect that one and the same part of the
bodv may represent each of the three kinds of levers,
according to circumstarices. Thus it has been seen that
the foot may, under some circumstances, represent a lever
of the first, in others, of the second, order. But it may

318

ELEMENTARY PHYSIOLOGY

become a lever of the third order, as when one dances a
weight resting upon the toes, up and down, by moving
only the foot. In this 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, 1),

cccps

fth-

Fio. 97. — The right Knek-joint. The Outer Half of the Feuttr
AND Patella sawn away.

fein. femur ; pat. patella ; lib. tibia ; ./(6. fibula ; caps, capsule of joint ;
I, crucial ligaments ; c, semilunar flbro-cartilages ; e, tendon of extensor
muscle.

which are inserted between the fulcrum and the weight.
(Fig. 96, III.)

11. 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

VII JOINTS OR ARTICULATIONS 319

are connected together in such a manner as to form strong
and definitely-arranged joints or articulations.

Joints may be classified into imjDerfect and perfect.

(a) Imperfect joints are those in which the conjoined
levers (bones or cartilages) present no smooth surfaces,
capable of rotatory motion, to one another, but are con-
nected by continuous cartilages, or ligaments, and have
only so much mobility as is permitted by the flexibility of
the joining substance.

Examples of such joints as these are to be met with in
the vertebral column — the flat surfaces of the bodies of
the vertebrse 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 Plate, Fig. VI. and 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.

(6) 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 synovia — is called a
synovial membrane.

The opposed surfaces of these ariicnlar 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.

320 ELEMENTARY PHYSIOLOGY

While these co-adapted surfaces and synovial mem-
branes provide for the free mobility of the bones entering
into a 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 wliich 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 furnished by another. In this case the motion of the
former bone may take place in any direction, but the
extent 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. 98).

Hinge-joints are single or double. In the former
case, the nearly cylindrical head of one bone fits into a
corresponding 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, be-
cause 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
straightened, but not bent back (Fig. 09). The knee
(Fig. 97) and ankle present less perfect specimens of a
single hinge-joint.

A double hinge-joint is one in which the articular sur-
face 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 convex from side to side, while the man presents to it

PERFECT JOINTS

321

the concavity of his legs astride, from side to side, and
the convexity of his seat, from before backwards.

The metacarpal bone of the thumb is articulated with

Fio. 98. — 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 interrupted 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 to
have the remarkable outline shown in the cut. This will be intelligible
if compared with Fig. VI. in the plate.

the bone of the wrist, called trapezium, by a double hinge-
joint. _

A pivot-joint is one in Avhich one bone furnishes an
axis, or pivot, on which another turns ; or itself turns on

322

ELEMENTARY PHYSIOLOGY

its own axis, resting on another bone. A remarkable
example of the former ari'angement is afforded by the
atlas and axis, or two uppermost vertebrjB of the neck
(Fig. 100). The axis possesses a vertical peg, the so-called
odontoid process (h), and at the base of the peg are two,

Bi- -1

flo. 90.- lonoitudinal and vertical .section through the
Elbow-joint.

ff, humerus ; Ul. ulna ; Tr. the triceps muscle, which extends the arm,
Bi, the biceps muscle, which flexes it.

obliquely placed, articular surfaces (a). 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, 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

PIVOT JOINTS 323

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. 100, A). By this arrangement, the atlas
is enabled to rotate through 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. 100, A, a) into which the two con-
vex, occipital condyles of the skull fit, and in which they

Fig. 100.

A. The atlas viewed from above ; a a, upper articular surfaces of its
lateral masses for the condyles of the skull ; 6, the peg of the axis
verti 0 ra.

B. Side viev7 of the axis vertebra ; a, articular surface for the lateral
mass of the atlas ; 6, peg or odontoid process.

play upward and dowmward. 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.

The second kind of pivot-joint is seen in the forearm.

If the elbow and forearm, 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

t2

S24

ELEMENTARY PHYSIOLOGY

supination (Fig. 101, A) ; when the back, pronation
(Fig. 101, B).

The forearm 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

Fig. 101.

The bones of the right forearm in supination (A) and pronation (B).
H, humerus ; R, radius : U, ulna.

move only in flexion and extension (see p. 320), 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 forearm, the radius, has its
small upper end shaped like a very shallow cup with thick

LIGAMENTS 325

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.

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.

Thus the upper end of the radius turns on the double
surface, furnished to it by the pivot-like ball of the humer-
us, 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 to the outer side of the ulna (Fig. 101,
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. 101, B).

The ligaments which keep the mobile surfaces of bones
together are, in the case of ball and socket joints, strong
fibrous ca])sulcs 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. 97, I) ; 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. 98).

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, thi'ough 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. 102, a).

In one joint of the body, the hip, the socket or aceta-

326 ELEMENTARY PHYSIOLOGY less.

bulum (Fig. 98) 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
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.

12. The Various Movements of the Body. — The
different kinds of movement which the levers thus con-
nected are capable of performing are called flexion and
extension ; abduction and adduction ; rotation
and circumduction.

A limb is flexed, when it is bent ; extended, Avhen it is
straightened out. It is abducted, when it is drawn away
from the middle line ; adducted, when it is brought to the
middle line. It is rotated, when it is made to turn on its
own axis ; circumducted, when it is made to describe a
conical 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
necessarily tear all the vessels, nerves, muscles, &c., 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,

KINDS OF MOVEMENTS

327

as the case may be, the joint towards the side on which
it is placed. Thus the biceps muscle is attached, at
one end, to the shoulder-blade, while, at the other end,
its tendon passes in front of the elbow-joint to the radius
(Figs. 92 and 99, Bi) : when this muscle contracts, there-
fore, it bends, or flexes, the forearm on the arm. At the
back of the joint there is the triceps {Tr, Fig. 99) : when

Fig. 102.

The vertebral column in the upper part of the neck laid open to show,
a, the check ligaments of the axis ; fc, 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 ; /, the
atlas ; //, the axis.

this contracts, it straightens, or extends, the forearm on
the arm.

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
ind successive contraction of these muscles.

328 ELEMENTARY PHYSIOLOGY less.

13. The Mechanics of Locomotion. — We may now

pass from the consideration of the mechanism of mere
motion to that of locomotion.

When a man who is standing erect on both feet pro-
ceeds to tvalk, 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 wliich 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.

^Vhen 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.

WALKING 329

And, as the swinging leg is a true pendulum — the time of
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 ones.

In running, there is a period when both legs are oflF 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, consti-
tutes a jicmj), the sudden extension of the legs adding
to the impetus, which, in slow walking, is given only by
the feet.

14. The Mechanism of the Larynx.— Perhaps the
most singular motor apparatus in the body is the larynx,
by the agency of which 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.

(c) 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 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 carti-
lages 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,
formed by two folds of the lining mucous membrane,
stretching from either side, but not quite meeting in the

330

ELEMENTARY PHYSIOLOGY

LESS.

middle line. They thus leave, in the middle line, a chink
or slit, running fx'om 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 length-
ways in them. These sharp
free edges of the glottis are the
so-called vocal cords, or vocal
ligaments.

The thyroid cartilage (Fig.
103, Th) 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 com-
monly called "Adam's apple.'
Above, the thyroid cartilage is
attached by ligament and mem-
brane to the hyoid bone (Fig.
103, Hy). 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. 103, 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 with the hinder part of the cricoid, as upon hinges,
can be moved up and down through the space occupied by

Fio. 103.

Diagram of the larynx, the
thyroid cartilag-e (Th) being-
supposed to be transparent,
and allowing the right ary-
tenoid cartilage (Ar), vocal
cords (V), and thyro-aryte-
noid muscle ( Th A ), the upper
part of the cricoid cartilage
(Cr), and the attachment of
the epiglottis (Kp) to be seen.
C.th, the right crico thyroid
muscle ; Tr, the trachea ;
Hy, the hyoid bone.

THE LARYNX

:m

this membrane ; or, if the thyroid cartilage is fixed, the
cricoid cartilage moves in the same way upon its articula-
tions with the thyroid. When the thyroid moves down-
wards or the cricoid upwards,
the distance between the front
part of the thyroid cartilage
and the back of the cricoid 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 cricoid cartilage
obliquely upwards and back
wai-ds to the thyroid, and pulls
the latter down ; or, if the
thyroid is fixed, pulls the cricoid
up (Fig. 103, G.th).

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 car-
tilages (Fig. 105, Ary). Each
of these is articulated by its
base with the cricoid cartilage
by means of a shallow joint
which permits of very varied
movements, and especially al-
lows 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 ligaments is
fastened ; and they stretch from these points horizontally
across the cavity of the larynx, to be attached, close to-

FiG. 104. — 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 o_f
larynx above the vocal liga-
ments (F); X the right
thyro-arytenoid muscle cut
across ; Cr. the cricoid car-
tilage.

332

ELEMENTARY PHYSIOLOGY

LESS.

gether, 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 forwards, and the base behind.

For, in front, or in the angle of the thyroid, the two

^r-./i.

Fio. 105. — The parts si'RROUNmNo the Glottis partially dissected

AND VIEWED FROM ABOVE.

Th. the thyroid cartilage ; Cr. the cricoid cartilage ; F, the edges of the
vocal ligaments bounding the glottis; Ary, the arytenoid cartilages;
Th.A, thyro-arytcnoid ; C.aA. lateral criro-arytenoid ; C.a.-p, posterior
crico-arytenoid ; Ar.ip, posterior arytenoid muscles.

vocal ligaments 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. Under these circumstances a current
of air passing through the glottis produces no sound, the
parallelism of the vocal cords being wanting ; whence it
is that, ordinarily, expiration and inspiration take place
quietly. Passing from one arytenoid cartilage to the

THE LARYNX 333

other, at their posterior surfaces are certain muscles
called the posterior arytenoid (Fig. 105, Ar.p.).
There are also two sets of muscles connecting each
arytenoid with the cricoid, and called from their positions
respectively the posterior and lateral crico-ary-
tenoid (Fig. 105, C.a.p. C.a.l.). 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 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
lengthways, or in other words to tighten them.

Running from the re-entering angle in the front part
of the thyroid, backward, 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
muscles, one on each side (Fig. 105, Th.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 conse-
quently to slacken the vocal cords.

Thus the parallelism (6) of the vocal cords is deter-
mined chiefly by the relative distance from each other of
the arytenoid cartilages ; the tension (c) of the vocal cords
is determined chiefly by the upward or downward move-
ment of the thyroid or cricoid cartilage ; and both these
conditions are dependent on the action of certain muscles.

The current of air (d) whose passage sets the cords
vibrating is supplied by the movemaats of expiiation,

334

ELEMENTARY PHYSIOLOGY

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.

15. The Voice. — Voice consists simply of the sound,
or musical note, which results from the vibration of the

Fio. lOG.

I. View of the human larynx from above as actually seen by the aid of
the instrument called the laryngoscope ; A, in the condition when voice
is being produced ; B, at rest when no voice is produced.

e. Ejiiglottis (foreshortened).

c.v. The vocal cords.

c.r.s. The so-called false vocal cords, folds of mucous membrane lying
above the real vocal cords.

a. Elevation caused by the arytenoid cartilages.

«.w. Elevations caused by small cartilages connected with the ary-
tenoids.

I. Root of the tongue.

II. Diagram of the same.

vocal cords. Other things being alike, the musical note
will be low or high, according as the vocal cords are
relaxed or tightened : and this again depends upon the
relative predominance of the contraction of the crico-

vn VOICE 335

thyroid and thyro-arytenoid muscles. For when the
thyro-arytenoid muscles 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 thjrroid cartilage will be depressed,
relatively to the cricoid, as much as possible, 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
thyroid cartilage), and a high note be then suddenly pro-
duced, 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 forwards, and the cricoid has a particularly dis-
tinct upward movement ; this movement of the whole
larynx must be carefully distinguished from the motion
of the thyroid relatively 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 voluntarily adjust the contractions of the thyro-
arytenoid and crico-thjToid muscles — so as to give his
vocal cords the exact tension at which theii" vibration
will yield the notes required.

The quality of a voice — treble, bass, tenor, &c. — on
the other hand, depends upon the make of the particular
larynx, the primitive length of its vocal cords, their
elasticity, 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.

336

ELEMENTARY PHYSIOLOGY

16. 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
voice, as in whispering. This is only true, however, 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 wliistle.

Fio. 107.
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 (be), moving on a pivot at c, takes the
place of the thyroid cai-tilage ; d 6 is an elastic l)and representing the
vocal ligament. Parallel witli this runs a cord fastened at one end to the
rod 6 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
6 c, anti passing over a second pulley to the weight A, represents the
crico-thyroid muscle. It is obvious that when the bar (b c) is pulled down
to the position c d, the elastic band (a 6) is put on the srtretch.

The modulation of the voice into speech is effected by
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),

0 (as in or), 0' (;is in oh), 00 (as in cool),

are pronounced successively, it will be found that they

VII SPEECH 337

may be all formed out of the sound produced by a con-
tinuous ex})iration, 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, widest in A', and roundest, with the
lips most protruded, in 00.

Certain consonants also may be pronounced without
interrupting 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
expiratory force — a sort of incipient cough. S and Z, Sh
and J (as in jugular = G soft, as in cjentrif), Th, L, R, F,
V, may likewise all be produced by continuous currents of
air forced through the mouth, the shape of the cavity of
which is peculiarly modified by the tongue and lips.

All the vocal sounds hitherto noted so far resemble one
another, that their production does not involve the
stoppage of the current of air which traverses either of the
modulating passages.

But the sounds of M and JV can only be formed by
blocking 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.

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 manner. In T and D, the mouth pas-
sage 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

z

338 ELEMENTARY PHYSIOLOGY

the tongue are similarly forced against the back part of
the palate.

An artificial larj'nx 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
consonantal 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, Avhom I will call Mr. R., whose tongue had been
removed as completely as a skilful surgeon could perform
the operation. When the mouth was widely opened, the
truncated face of the stump of the tongue, apparently
covered with new mucous membrane, was to be seen,
occupying a position as far back as the level of the an-
terior pillars of the fauces. The dorsum of the tongue
was visible with difficulty ; but I believe I could discern
some of the circumvallate papillae upon it. None of these
were visible upon the amputated part of the tongue,
which had been presei'ved in spirit ; and which, so far as
I could judge, was about 2h 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 rae that, when his mouth was shut,

VII SPEECH 339

the stump of the tongue could be brought inucli more
forward.

Mr. R.'s conversation was perfectly intelligible ; and
such words as think, the, cote, kill, were well and clearly
pronounced. But tin became Jin; tack, fack or jmck ;
toll, pool; dog, thog ; dine, vine; dew, theto ; cat, cat/;
mad, madf; goose, gooth; big, pig, bich, pick, with a
guttural rh.

In fact, only the pronunciation of those letters the for-
mation of which requires the use of the tongue was affected ;
and, of these, only the two which involve the employment
of its tip were absolutelj^ beyond Mr. R.'s power. He
converted all t's, and d's into /'s, 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 gr'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 following voAvel ; while, when the letter was ter-
minal, the defect at once became apparent.

S2

LESSON VIII
SENSATIONS AND SENSORY ORGANS

1. Movement the Result of Reflex Action.— The

agent by wliich all the motor oi'gans (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 cliange which takes
place in the motor or eflferent nerve, wliich is dis-
tributed to it. This change again is generally effected by
the activity of the central nervous system, with which
the motor nerve is connected. The central organ is
thrown into activity, directly or indirectly, by the in-
fluence of changes which take place in nerves, called
sensory or afferent, which are ccmnected, 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 alteraticm of the afferent nerve is itself pro-
duced by changes in tlie condition of the part of the body
with which it is connected ; which changes usually result
from external impressions brought to bear cm that part.

Sometimes the central organ enters into a state of
activity without our being able to trace that activity to
any dii'ect 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." Putting these cases on one side, it
may be stated that a movement of the body or of a
part of it, is to be regarded as the efiect of an influence

REFLEX ACTION 341

(technically termed a stimillus) applied directly, or in-
directly, to the ends of afferent nerves, and giving. rise to a
modification of the condition of the particles or molecules
which form the suVjstance of the nerve fibres, i.e., to a
molecular change, 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 central organ ; from these the dis-
turbance is transmitted along the motor nerves, which pass
from the central organ to certain muscles. And, when
the disturbance in the molecular condition of the efferent
nerves reaches the endings of those nerves in nmscular
fibres, a similar disturbance is communicated to the sub-
stance of the muscular fibres, whereby, in addition to the
production of certain other phenomena to some of which
reference has already been made (Lesson VII.), 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 un-
intentionally 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 aft'erent
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 muscular contraction, resulting in movement, in the
way we have described may be made clear by the following
diagram.

The stimulus is applied to a sensory surface (S) j the

342

ELEMENTARY PHYSIOLOGY

LESS.

change thus set up is j)ropagated as a molecular disturb-
ance (or 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 molecular disturbance in the motor
(efferent nerve), e./. , which is conveyed outwards along
that nerve to the muscle iH^ usually on the same side of
the body. Sometimes the impulse is sent out along a
motor nerve to some muscle MP- on the opposite side of
the body.

-- Sp.C.

Fia. 108.— Diagram to illustrate the Paths of Reflex Action.

Sp.C, spinal cord. S, some sensory surface ; a./, afferent or sensory
nerve ; c, central connection in nervous system ; «./, e.f, efferent or motor
fibres ; 3/1, 3/-, muscles. Tlie arrows sliow the directions in which the
impulses travel.

2. Sensations and Consciousness. — A reflex action
may take place without our knowing anything about it,
and hundreds 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 stimulus affects our afferent nerves, by having
what we call a feeling or sensation. We class sensa-
tions along with emotions and volitions, and

vni THE SPECIAL SENSES 343

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 ulti-
mate 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 sensa-
tions of uncomfortahleness, of faintness, of fatigue, or of
restlessness. ' We cannot assign any particular place to
these sensations, 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. And 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 sen-
sations, 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 sen-
sation is thus restricted is called a sense-organ.

It may be here remarked that in the case of the sensa-
tion 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.

344 ELEMENTARY PHYSIOLOGY less.

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 contact 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 rn question. With
the other sensations the case is different. When we smell a
bad smell, though we know that we smell by the nose, we
do not consider that the smell arises in the nose ; we con-
clude that there is some object outside ourselves which is
causing the bad smell. We refer the origin of the sensa-
tion to some external cause, and that even wlien the sen-
sation is after all due to changes taking place in the nose
itself independently of external objects, as in the un-
pleasant odour.s which accompany certain diseases of the
nose. Similarly all our sensations 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 sweetness 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 ex-
actly whereabouts in tlie mouth the melting lump is lying.

4. The General Plan of a Sense-organ. — In these
sensations, 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 structui-e of the sense-organ is arranged in such a
manner as to render that organ peculiarly sensitive to its
appropriate stimulus.

Thus the sensations of siglit 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
sensations when they fall upon it.

SENSE-ORGANS 345

We may distinguish in each sense-organ an essential
and an accessory part.

The essential part of each sense-organ is composed
of minute organs, which upon examination appear to be
in reality modified epithelial cells ; and the delicate ter-
minations of the nerve filaments distributed to the sense-
or-an may, ^vith more or less distinctness, be traced to
these modified cells, in which indeed they seem to end.
These minute organs, these modified epithelial cells, may
be spoken of as sense-organules ; they serve as inter-
mediators 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 fibres of the sensory
nerve those changes which, reaching the brain as nervous
impulses, give rise to the special sensations. Thus, as we
shall presently see, rays of light falling upon the optic
nerve cannot give rise to a sensation of sight. The physical
a-ent must act first on the^ sense-organules, and these in
turn act upon the filaments of the nerve. Thus light
fallina upon the sense-organules, situated in that essential
part o'f the eye called the retina, sets up changes in them,
these changes set up corresponding changes m the delicate
nerve filaments which with the sense-organules go to
make up the retina, and the changes in the nerve filaments
propagated along the optic nerve to the brain give rise,
in the latter, to sensations of sight.

Hence in the essential part in each sense-organ we have
to distinguish between the sense organules, i.e., the modi-
fied epithelium, and the terminal expansion of the sensory
nerve ■ and further, in each sense-organ, there is added to
this essential 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.

346 . ELEMENTARY PHYSIOLOGY less.

5. The Skin as a Sense -Organ.— The sensations which
originate in the skin and tlie parts which lie under it are
those of pressure, heat, cold, and pain. These sensations
are difficult to analyse.

Take for instance the sensation of pressure. If my
hand is held out horizontally with the palm upwards and
another person makes a considerable but unsuccessful
effort to lower it by pressing the palm downwards, I can
appreciate that pressure and even make an estimate of
it. This I could do were there no skin on my hand. I am
really appreciating the strain which is put upon my
muscles and tendons in successfully resisting the pressure.
The information is sent up to my brain from sense-organs
situated in the tendons, muscles, and joints. In contrast
to that experiment the following may be performed. The
skin on the palm of my hand may be just touched lightly
with a blunt object, or some cotton wool, and I may be
asked if, without using my eyes, I can appreciate and
locate the touch. In this experiment I am also appreci-
ating sensations of pressure, but the sensations are quite
different in kind as well as in degree from those estimated
in the former experiment. The sense-organs which feel
the cotton wool are in the skin. At once then we can
divide our appreciation of pressure into two quite different
faculties which are termed respectively the deep and the
superficial sensibility accoi'ding as they are located in
the parts below the skin or in the skin itself.

Even in the skin however there seems to be more
than one mechanism for the appreciation of any par-
ticular type of sensation (tactile sensation, heat, cold, or
pain).

Just as in a microscope we have a "coarse" and "fine"
adjustment for focussing the slide, so in the skin we have
a coarse and a fine adjustment for the feeling of, say,
pain. With the coarse adjustment only we get what
we may speak of as a blurred image of pain, of heat or of

SENSE-ORGANS 347

cold. We cannot be certain of the detail, small
differences of temperature cannot be recognised, a prick
is felt but the place where the prick was given is not
known with certainty. We must not press the analogy
of the microscope too far however. We might by
accident obtain a clear vision of the slide without recourse
to the fine adjustment of the microscope. No accident
would produce for us detailed sensations without the
corresponding mechanism in our skin.

For the trustworthy information which we possess on
this subject we are indebted to the devotion of a
physician who allowed the sensory nerve from part of the
skin of his arm to be cut. The nerve grew again, and as
the various sensations returned at different times, it was
possible to distinguish between them. After about three
months the rough as opposed to the detailed sensations
come back, a rough appreciation of temperature and a
rough sensation of pain -a thing which was really hot
could be felt to be hot, a thing which was really cold
could be felt to be cold, but he found it impossible to tell
which was tlie hotter of two bodies which were nearly of
the same temperature. To these rough sensations the
name protopathic was given. The mechanism by
which they are felt is that to which we have compared the
coarse adjustment of the microscope. For a long time
the skin had these sensations and these only. Then at a
much later date, the delicate sensations returned— those
of detailed touch, the ability to distinguish the contact
of a single point from that of two points in close prox-
imity, the power of appreciating small differences of
temperature and the exact localisation of trifling pains,
these sensations were called epicritic. The sensory
end organs of these— both protopathic and epicritic -are
situated in the skin.

Protopathic sensations, specially those of pain, are
not contined to the skin but spread over a much greater

348 ELEMENTARY PHYSIOLOGY less.

area ; they are found, for instance, in the alimentary canaL
Already we have said that a protopathic sensation may
give the impression of coming from a different place from
that at which it actually originates. This is especially
true of sensations wliich come from the viscera. Such
are often felt to be in the skin, indeed it is a re-
markable thing that there are particular situations on the
skin corresponding to various organs of the body so that
a pain which really has its origin in the stomach will be
felt always in one skin-area, whilst a pain from the
kidney will be felt in another. This phenomenon is
known as that of "referred pain."

Let us turn to the structure of the skin to ascer-
tain if possible the nature of the sensory end organs
in it.

Whatever part possesses this sense consists of a mem-
brane (integumentary or mucous) composed of a deep
layer made up of fibrous tissue containing a capillary
network, and of a superficial layer consisting of epithelial
or epidermic cells, among which are no vessels.

Wherever the sense of touch is delicate, the deep layer
is not a mere flat expansion, but is raised up into multi-
tudes of small, close-set, conical elevations (see Fig. 57,
p. 191), which are called papillae. In the skin, the coat
of epithelial or epidermic cells does not follow the contour
of these papillae, but dips down between them and forms a
tolerably even coat over them. Thus, the points of the
papilliB are much nearer the surface than the general
plane of the deep layer whence these papillae proceed.
Loops of vessels enter 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 organs consists essentially of
an oval or rounded swelling formed by a modification and
enlargement of the delicate connective tissue ensheathing

TACTILE CORPUSCLES

349

the nerve-fibre ; in the middle of the swelling the nerve-
fibre itself ends abruptly in a peculiar manner. These
bodies are especially found in the papillte of those
localities wliich are endowed with a very delicate sense of
touch, as in the tips of
the fingers, the point of
the tongue, «S:c. ; and the
papillae which contain tac-
tile corpuscles generally
contain few or no blood-
vessels.

Tactile corpuscles
occur most numerously
in the papillse 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 toes, and are
commonest on parts of
the skin where there is
no hair. Each corpuscle
forms an elongated, bulb-
ous swelling about 75/x
(;5^y inch) in length at
the end of the nerve-fibre
to which it is attached,

and lies with its long axis pointing to the top of the
papilla. The corpuscle consists of a .sheath or capsule of
connective tissue inside of which are some nucleated cells
intermixed with connective tissue derived from the outer
sheath. The ners'e which supplies the corpuscle ap-
proaches ^it at its side, winds once or twice round it and
then enters the body of the corpuscle, where it divides

Fig. 109. — Tactile Corpuscle with-
in A Papilla of the Skin ok

THE BaKD (RaNVIKR),

n.n, two nerve fibres passing to
the corjiuscle ; ".«. varicose termina-
tions of the nerve fibres inside the
corpuscle.

350

ELEMENTARY PHYSIOLOGY

into a number of branches which end in a manner not as
yet exactly determined.

End bulbs are found in tlie papillae of the skin of the
lips and in other situations. They are spheroidal and
smaller (40/i in diameter) than the tactile corpuscles.
They are not all exactly alike, but the commonest form
consists of a thin outer sheath or 9apsule which is
nucleated and encloses a mass of polygonal cells. The

nerve-fibre enters the cap-
sule and ends among the
cells in its interior.

The great majority, how-
ever, of the nerve-fibres
going to the skin do not end
in any such definite organs.
They divide in the dermis in-
to exceeding delicate minute
filaments, the course and
ultimate terminations of
which are traced with the
greatest difficulty. Some of
the finest filaments, however,
pass into the epidermis and
are there lost among or pos-
sibly connected with some
of the epidermic cells, especially 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 sub-
cutaneous tissue. These are known as Pacinian cor-
puscles, called after Pacini, who first carefully described
them. From their position they are not, strictly .speaking,
sensory endings of nerves in the skin : but they possess

1 Tlie cf)iijunctiva is the mucous membrane which lines the eyelids and
front of the eyeball.

Fio. 110.— End-bulb from the
Human Conjunctiva! (Lono-
worth).

a, the nerve-fibre ; h, capsule
with nuclei ; c.c, portions of
nerve-fibre inside the end-bulb ;
d.e, cells of the core.

PACINIAN CORPUSCLES

351

undoubtedly some sensory functions, although we do not
know what these may be.

Pig. 111.— a Pacinian Corpuscle from a Cat's Mesentery.
(Ranvier.)

n nerve-fibre, passing through the core m, and terminating at a.

The Pacinian corpuscles are long, oval, bulbous
structures of considerable size, averaging ^ of an inch in
length. They are thus easily visible to the naked eye.

352 ELEiMENTARY PHYSIOLOGY less.

Each corpuscle consists of an elongated central core of
glassy-looking material in which the axis of the nerve is
embedded and terminates. The core is surrounded by
some 30 to 40 capsules made of connective tissue, and
placed one outside the other like the layers of an ordinary
onion.

(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 pressure. The sensitiveness of the various
regions of the skin in responding to pressure varies, and
the difference may be measui-ed for each {)art 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 difference in weight which can be
distinguished between two weights laid in succession on
the same spot. Experimenting 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 sensi-
tive to pressure. Hence we may perhaps speak of
" pressure spots " in the same way that we do of " heat
spots" and "cold spots."

(ii) The Sensations of Heat and Cold. — The feel-
ing 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 touch. And it would appear that the heat must be
transmitted through the epidermic or epithelial layer, to
give rise to this sensation ; for, just as touching a naked
nerve, or the trunk of a nerve, gives rise only to pain, so
heating or cooling an exposed nerve, or the trunk of a

VTii TOUCH 353

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 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.

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 while in the ice-cold water, and
then transferred to the very same mixture, this will feel
warm.

Like the sense of touch, 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 fii'e.

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 temperatm-e can be kept constant at any desired
(Jewree. 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 shows the distribution of these
spots in a small area of the skin of the thigh.

354

ELEMENTARY PHYSIOLOGY

(iii) The Sensation of Pain. — Pain may be 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

Pio. 112.— Outlines of Heat Spots and Cold Spots. (After

GOLD.SCHEIDEK.)

Tlie heat spots are cross-liatched and dark, the cold spots are dotted
and light. In some places the heat spots and cold spots overlap each
other.

nervous system. In the latter case the"[^)ain 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

viii TOUCH 355

separate "sense" of pain. But there are certain facts
which show that sensations of pain are distinct from other
sensations. In the first place there is a protopathic
sensation of pain, in many places in which there is no such
sensation of contact. Again there are certain situations
in the body which are incapable of appreciating touch,
but which readily feel pain, and in many diseases of the
nervous system, such as locomotor ataxy, the sensitive-
ness 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.

(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 im-
pression much duller in some parts than in others — a cir-
cumstance which might in many cases be accounted for by
the different thickness of the epidermic layer — but the
power of distinguishing 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 per-
ceived.

Accurate experiments have been made in different
parts of the body, and it has been found that two points

A A 2

356 ELEMENTARY PHYSIOLOGY less.

can be distinguished by the tongue, if only one-twenty-
fourth of an inch ai)ai-t ; by the tips of the fingers if
one-twelftli of an inch distant ; vvliile 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.

Lastly, can we find any correspondence between the
various forms of end organ which have been described and
the various sensations felt in the skin ? The epicritic
sense of pressure is felt in many parts of the skin by
tactile corpuscles (Fig. 109). This is especially the case
where there are no hairs as on the palms of the hands.
The nerve-endings attached to the small hairs serve the
same function elsewhere. End-bulbs probably acquaint
us with " cold." We are uncertain as to the corresponding
organs for " heat."

6. The Muscular Sense. — What is termed the
muscular sense is less vaguely localised than the
sensations referred to above in Section 2 (p. 343),
though its place is still incapable of being very accur-
ately 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 flat on its back upon a table, and rest a
disc of cardboai'd 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 cardboard, 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

THE MUSCULAR SENSE 357

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 krows 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 judgments 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 balancing them in their hands ; and in
this case, they depend in a great measure upon the mus-
cular sense.

But the muscular sense embraces more than the mere
consciousness of the resistance to effort involved m lifting
a weight Thus it is a matter within everybody s experi-
ence 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 con-
scious of the podtiov. 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 AMth
sensations arising from movements whether active or
passive Now the parts affected by these movements are
chiefly the following three ; the skin, the muscles and the
tendons or ligaments. It has been supposed that the
impulses which give rise to the sensations may be largely
due to the stimulation of cutaneous nerves resultmg 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

358 ELEMENTARY PHYSIOLOGY

that the muscles themselves possess nerve fibres which
are certainly afferent, i.e. sensory ; and similarly afferent
fibres, connected 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, more especially the nerves of the tendons, 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 membrane of the tongue is raised up
into papillae ; but these are large, separate, and have
separate coats of epithelium. Towards the tip of the
tongue they are for the most part elongated and pointed,
and are called filiform ; over the rest of the surface of
the tongue these are mixed with other large papilL-e, with
broad ends and narrow bases, called fungiform ; but
towards its root there are a number of larger papillae,
arranged in the figure of a V with its point backwards,
each of which is like a fungiform papilla surrounded by a
wall. These are the circumvallate papilhe (Fig. 113,
G.p.). The larger of these papilhe have subordinate small
ones upon their surfaces.

In both the fungiform and circumvallate papillae, the
cells which are specially concerned in giving rise to sensa-
tions of 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 imbedded in the layers of epithelium
which cover the sides of each papilla.

Each "bud" is flask-shaped and consists of an outer
wall, made up of elongated cells placed side by side like the
staves of a barrel, and leaving an opening at the end of
the bud where it comes to the surface of the papilla. The
inside of the bud is filled with the true gustatory cells,

VlII

TASTE

359

packed side by side into the cavity of the bud. 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

Fig. 113.-

-The Mouth widely opened to show the Tonotje and
Palate.

Uv the uvula ; Tn. the tonsil between the aiitenor and posterior pillars
of the fauces • C v, circuinvallate papillae ; F.p, fungiform papill*. The
minute fiUfoi-inpapilte cover the interspaces between these On the
right side the tongue is partially dissected to show the course of the fila-
ments of the glossopharyngeal nerve, VIII.

delicate process, like a stiff cilium (but not vibratile),
which projects through the open mouth of the bud.

The papillse are very vascular, and they receive nervous
filaments from two sources, the one the nerve called

360

ELEMENTARY PHYSIOLOGY

glossopharyngeal, the other the gustatory, which
is a branch of the fifth nerve (see Lesson XI.). The
latter chiefly supplies the front of the tongue, the former
its back and the adjacent part of the palate ; and there
is reason to believe that difl'erent taste sensations are
supplied by the two nerves.

The peculiar cells in the taste-buds are the sense-
organules of taste, and, with the delicate terminations of
the glossopharyngeal and gustiitory nerve which may be
traced to them, constitute the essetdial parts of the organ

Pio. 114. — Diagram of a Circumvallatk Papilla, and of Taste-Buds.

A, A circumvallate papilla cut across ; 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 cells with processes; m,
processes projecting at mouth of buds.

of taste. The tongue itself, which by its movements
brings the sapid sulistances into immediate contact with
these modified epithelium cells, may be regarded as the
accessory part.

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

vm TASTE 361

nose is held tightly pinched, it is very diflBcult to distinguish
the taste of various objects. An onion, for instance,
the eyes being shut, may then easily be confounded with
an 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 investigate 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 sensitive to sweet
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 medium temperatures,
such as 20' — 30 C. (68' — 95' F.), 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 — firstly, by the character of
its cells and by possessing no cUia ; secondly, by receiving
a large nervous supply from the olfactory, or first, pair of
cerebral nerves (see Lesson XI.), as well as a certain
number of filaments of the fifth pair, whereas the rest
of the mucous membrane is supplied from the fifth pair
alone.

Each nostril leads into a spacious nasal chamber,
separated, in the middle line, from its fellow of the other
side, by a paitition, or septum, formed partly by cartilage
and partly by bone, and continuous with that partition

•362 ELEMENTARY PHYSIOLOGY less.

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. 115 and 116) ;
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 ordinary 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 jjartition
are the two hinder openings of the nasal cavities (which
are called the posterior nares) separated by the
termination of the septum ; and through these wide
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 Ave 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 cribriform plate (Fig. 116, Or.). It is this
plate (with the membraneous structures which line its two
faces) alone which, in this region, separates the cavity of
the nose from that which contains the brain. The ul-
factory 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

SMELL

363

Fig. 115.— Vertical Lonoitudinal Sections of the Nasal Cavity.

The upper flexure represeutB the outer wall of the left nasal cavity ; the
lower figure the right side of the middle partition, or septum {Sp.) of the
nose which forms"the inner wall of the right nasal cavity. 1, the olfac-
tory nerve and its branches; V, branches of the fifth nerve ; Pa. the
palate, which separates the nasal cavity from that of the mouth ; *. i ,
the superior turbinal bone ; 71/. r, the middle turbmal ; /.7 themfenor
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.

364

ELEMENTARY PHYSIOLOGY

delicate filaments, the olfactory nerves, which are distri-
buted as follows (Fig. 115) :—

On each wall of the septum the mucous membrane
forms a fiat 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
turbinal, or spongy bones. These bones are called

S/i. FL

Fio. 116.— A Transverse and Vertical Section of thb Osseous Walls
OF THE Nasal Cavity taken nearly through the letter / in the

FOREGOING FIGURE.

Cr. the cribriform plate ; S.T, M.T, the chambered superior and middle
turbinal bones on which and on the septum (*.;).) the filaments of the
olfactory nerve are distributed ; I.T, the inferior turbinal bono ; I'/, the
palate; An. the an^niHi. or chamber which occupies the greater part of
the maxillary bone and opens into the nasal cavity.

spongy because the interior of each is occui)ied by air
cavities separated from each other by very delicate
partitions only, and coiumuuicating "with the nasal
cavities. Hence the bones, though massive-looking, are
really exceedingly light and delicate, and fully deserve
the appellation of spongy (Fig. 116).

Over these upper and middle turbinal bones, and on

viii SMELL 365

both sides of the septum opposite to them, the mucous
membrane is specially modified, and receives the name of
olfactory mucous ' membrane ; and it is to this
olfactory mucous membrane that the filaments of the
olfactory nerve passing through the cribriform 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 imperfectly separates the au* passages from the
proper olfactory chamber (Fig. 115). It is covered by
the ordinary ciliated mucous membrane of the nasal
passage, and receives no filaments from the olfactory
nerve.

In the non-olfactory part of the nasal mucous mem-
brane the epithelium cells are ordinary ciliated epithelium
cells (see p. 286) ; but in the olfactory part the cells not
only lose their cilia, but become peculiarly modified.
Many of them become very slender and rod-shaped, and
the delicate terminations of the olfactory nerve filaments
appear to end in these modified epithelial cells, which
indeed are the sense-organules of the organ of smell.
The olfactory mucous membrane, with the filaments of
the olfactory nerve ending in it, thus constitutes the
essential part of the organ.

The cells of the olfactory mucous membrane are of two
kinds, and somewhat similar to those composing a taste-
bud ; but their arrangement is different. Thus one kind
of cell is long, slender and rod-shaped, with a large
nucleus towards its inner end. The cells 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.
The cells of the first kind, which are the most numerous,
are supposed to be those which are specially concerned in
giving rise to the sensations of smell. The olfactory
mucous membrane is made up of a mass of these two kinds
of cell, placed side by' side and intermixed. (Fig, 117.)

366

ELEMENTARY PHYSIOLOGY

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 in-
spiratory and expiratory cur-
rents will flow along the
comparatively wide, direct
passages afforded by so much
of the nasal chamber as lies
below the middle turbinal ;
and that they will hardly
move the air enclosed in the
narrow interspace between
the septum and the upper
and middle spongy bones,
which is the proper olfactory
chamber.

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 par-
ticles, 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 odoriferous air. Hence
it is that, Avhen 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 tlie olfactory chamber
at the same time as, or even before, it affects that at the

Fio. 117.— Cells of Olfactory
Epithelium. (Max Scbultze.)

1, From a frog ; 2, from man.

a, columnar epithelial cell ;
ft, olfactory rod-cell ; c, outer
limb, d, inner limb of olfactory
cell, the former being proloiigod
at e into fine hairs, the latter
being continuous with a nerve
filament from the olfactory
nerve.

HEARING 367

nostrils ; and thus must tend to draw a little air out of
that chamber from behind. At the same time, or imme-
diately 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
experience shows that the sense is extremely delicate,
the most minute amount of odoriferous matter, such as
musk, serving to excite it.

9. The Ear and the Sense of Hearing.— The ear, or
organ of the sense of hearing, is very much more complex
than either of the sensory organs yet described ; and in it
both the essential and the accessory parts are much more
highly developed.

In our discussion of cutaneous sensation we saw that
the skin could not be regarded as a single sense organ
but that it could appreciate sensations of very diflerent
characters for which there were different types of sense
organs. The same is true of the ear. It has two quite
different functions, one of which is to hear sounds, the
other is to acquaint the mind with the position, or
alterations of the position, in which the head is placed.
These different functions are located in two different parts
of the ear, which, taken together, are called the inner ear.
The inner ear is situated in the skull at some distance
from the surface ; between it and the outer, or visible
portion of the ear, is the middle ear.

(i) The Esternal Ear. — The outer extremity of the

368

ELEMENTARY PHYSIOLOGY

LBSS.

external meatus is surrounded by the concha or external
ear {Co. Fig. 118), a broad, peculiarl3'-sliai)ed, and for
the most part cartilaginous plate, the general plane of
which is at right angles with that of the axis of the
auditory opening. The concha can be moved by most
animals and by some human beings in various directions

Co.

i:.M.

Fio. lis.— Transverse Section throuoh the Sidr Walls of thb
Skull to show the Parts of Ear.

Co. Concha or external ear; E.M, external auditory meatus; Ty.M,
tympanic membrane; Inc. Mall, incus and malleus; A.S.C, P.S.C,
E.S.C, anterior, posterior, and external somicircular canals ; Cnc. cochlea ;
Eu. Eustachian tube; !.M, internal auditory meatus, through which the
auditory nerve passes to the organ of hearing.

by means of mu.scles, which pass to it from the side of the
head.

(ii) 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

THE AUDITORY OSSICLES

369

tympanum ; secondly, the long external passage, or
m.eatus Fig. 118).

The drum of the ear and the external meatus, which
together constitute the middle ear, would form one cavity
were it not that a delicate membrane, the tympanic mem-
brane (Ty.M, Figs. 118 and 119), is tightly stretched in an

Pig. 119.— a Diagram illustrativb of the Relative Positions of
THE Various Parts of the Ear.

S.M, external auditory meatus ; Tv.M, tympanic membrane ; Ty. tym-
panum ; Mall, malleus; Inc. incns ; Stp. stapes; F.o, fenestra ovalis :
Fr fenestra rotunda; Eu. Eustachian tube; -V.i, membranous laby-
rinth, onlv one semicircular canal with its ampulla being represented :
Sea. r, Scd.T, Sca.M, the scalae of the cochlea, which is supposed to be
unrolled.

oblique direction acro.ss the passage, so as to divide the
comparatively small cavity of the drum from the meatus.
The membrane of the tympanum thus prevents any
communication, by means of the meatus, between the drum
and the, external air, but such a communication is pro-
vided, though in a roundabout way, by the Eustachian

370 ELEMENTARY PHYSIOLOGY less.

tube (Eu. Figs. 118 and 110), which leads directly from
the fore part of the drum inwards to the roof of the
pharynx, where it opens. (See also Fig. 69.)

Two other orifices exist in the bony wall of the middle
ear, they are called the fenestra omdu (Fig. 119, F.o.) and
the fenestra rotunda (F.r.) respectively. Through these
the middle and inner ears would communicate with one
another were not a membrane stretched across each.
(See p. 375.)

(iii) The Auditory Ossicles. — Three small bones, the
auditory ossicles, lie in the cavity of the tympanum. One
of these is the stapes, a small bone shaped like a stirrup.
The foot-plate of this bone is firmly fastened to the mem-
brane of the fenestra ovalis, while its hoop projects
outwards into the tympanic cavity (Figs. 119 and 122).

Another of these bones is the malleus {Mall. Figs.
118, 119, 122), or hammer-bone, a long process, the so-
called handle, oi which is fastened to the inner side of the
tympanic membrane (Fig. 122); while a very much smaller
process, the slender process, is fastened, as is also the body
of the malleus, to the bony wall f)f the tympanum by
ligaments. The rounded surface 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
horizontal, and rests u[)on 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 articulates ' with the stapes (Figs. 119
and 122).

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

1 A minute bone, the os orbiculare, intervenes between the end of the
process of the incus and the stapes, so that the stapes is in reality articu-
lated with the OS orbiculare, wliich in turn is f.istcned to the process of
the incus. For simplicity's sake, mention of this is omitted above.

vni THE AUDITORY OSSICLES 371

above, articulated together in such a manner that they
may practically be considered as forming one bone which
turns upon a horizontal 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 tym-
panum. Its general direction is represented by the line
a 6 in Fig. 122, or by a line perpendicular to the plane of
the paper, passing through the head of the malleus in
Fig. 119.

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 sliorter 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 vibra-
tions, 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 move-
ment of the handle of the malleus, the long process of the
incus also moves inward and outward, 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 membrane of the drum, produces by means of the
chain of ossicles a corresponding vibration of the mem-
brane of the fenestra ovalis to which the stapes is attached.

(iv) The Muscles of the Tympanuin. — The char-
acters 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 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

B B 2

372 ELEMENTARY PHYSIOLOGY less.

the front wall of the drum to the malleus. Each of the
muscles when it contracts 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.

(v) The Inner Ear consists, substantially, 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 petrosal), which forms part of the
temporal bone, and lies at the base of the skull. The sac,
however, does not completely fill the cavitj', so that a
Rpace is left between the bony walls and the contained
sac. This space, otherwise continuous all round tlie sac,
is interrupted at certain places only where the mem-
branous sac is attached in the bony walls, and contains a
fluid provided by the lymphatics 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 fi-om
the endolymph, the two fluids being separated by the
walls of the membranous 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.

So mi^ch is true of the sac as a whole, l)ut the different
portions of it have very different functions. The general
shape of the sac is shown diagrammatically in Fig. 124.

thp: cochlea 373

It is divided roughly into two compartments, the utricle
and the saccule ; with tlie former is connected a system
of tubes known as the semicircular canals, with the
latter is connected a single tube which is coiled in the
shape of a helix. This tube is the canalis cochlearis, and
with the bony cavity in which it is enclosed, forms the
cochlea, the essential organ of hearing.

(vi) The Cochlea. — Connected with the saccule by a
narrow canal is an extension of the original membranous
sac, in the form of a long tube closed at the end (Fig. 124,
Coch.). This cochlear tube is lined with epithelium,
contains endolymph, and is lodged in a bony cavity filled
with perilymph. The canalis cochlearis is much smaller in
section than the bony cavity in which it lies. The amount
of perilymph surrounding it is therefore considerable. In
the cochlea the contour of the cochlear tube is, along its
whole length, totally different from that of the containing
cavity ; for, in transverse section, while the contour of the
containing cavity is almost circular, that of the cochlear
tube itself is nearly triangular. The cochlear tube in fact is,
in shape, what is often called triangular (as when we speak
of a triangular file), but should be called trihedral ; that
is to say it has three sides or faces (and three edges) ;
one of the sides is however not flat but convex, i.e., bulges
somewhat outwards.

The cochlear tube, or canalis cochlearis, closely adheres
to the bony wall, along the whole length of the tube, in
two regions, namely, over the whole of that face of the
trihedral tube which has just been described as being
convex, and at the edge opposite. Take a round ruler,
make a paper case which just fits it, and close the case at
one end. Then pare down the ruler on two sides until
it has two flat faces meeting at an edge, and slide it into
the case, so that it does not quite reach the closed end.
The ruler, if it were hollow, would represent the cochlear
tube ; and it will be observed that it divides the cavity of
the case into two passages, which are quite distinct from

374 ELEMENTARY PHYSIOLOGY less.

each other, except at the end of the case to which the
ruler does not reach. In a similar way, the cochlear tube,
containing endolymph, divides the cavity containing
perilymph, in which it lies, into two passages, called
scalas, which are seen in section (Fig. 120) to be placed
one above and the other below the triangular cavity of
the cochlear tube itself, and which communicate with
each other at the far end of the cochlear tube, but not
elsewhere.

In one point, however, the comparison with the ruler
and its case is not exact. The cochlear tube is not nearly
so wide as the containing cavity ; and the sharp edge
opposite the convex adherent face would not be in direct
connexion with the bony walls, were it not for a bony ledge
which, projecting from the bony walls towards the thin
edge of the cochlear tube, is united to it by membrane
and thus forms a partition or septum, which separates
the two scalse in the region where the cochlear tube
itself would otherwise leave a communication between
them.

As has been already stated the cochlear tube is not
straight or even simply curved, but is twisted up on itself
into a spiral of two and a half turns. In these twists it
is accompanied by the cavities above and below it, and
also by the septum spoken of above, which thus takes a
spiral course, and is spoken of as the lamina spiralis
(Figs. 120, L.S, 121, l.s). The whole arrangement some-
what resembles the shell of a snail ; hence the name. All
along the spiral the edge of the cochlear tube attached to
the lamina spiralis is directed inwards and the convex
face outwards ; so that when a section is made through
the axis of the spiral a succession of rounded spaces are
cut through, each space exhibiting, above and below, the
somewhat half-moon-shaped section of a scala, the two
scalse being separated on tlie outer side, by the cochlear
tube, and, on the inner, by the lamina spiralis (Fig. 120).

The triangular cavity which, as we have seen, contains

THE COCHLEA

375

endolvmph. and is continuous with the saccule, is called
the canalis cochlearis. The upper of the two cavities
containing perilymph, when traced d.nvu to the bottom of
the spiral, is found to be continuous with the cavity
containing perilymph which surrounds the vestibule ij.e
the utricle and saccule) ; hence it is called the scala
vestibuli. The lower cavity, when similarly traced to
the bottom of the spiral, ends at the fenestra rotunda,
which is closed bv a membrane. Since this lower cavity
is only separated from the middle ear or tympanum by

Fio. 120.

-A S2CTI0V THROrnH THE AXIS OF THE COCHI.EA
THBEK DIAMETERS.

MAONIFIBD

r r canalis cochlearis, or cochlear tube : Sc. r, scala vestibuli ; ScT
sca^-f 'tympani ; i S lamina spiralis ; Md. bony axis, or modiolus, round
which the scalJe are wound ; C. .V, cochlear nerve.

the membrane covering the fenestra rotunda, it 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 lamina
spiralis except at the very tip of the spiral, where tbese
latter end ; here the two seal* are prolonged beyond the
cochlear tube and join together, forming a common space,
as seen at the top of Fig. 120.

The vibrations of sound are brought, as we shall see,
to the perilymph chamber of the vestibule, whence they
spread on *the other into the scala vestibuli. Passing
upwards, in the spiral along the scala vestibuli. they enter at

376 ELEMENTARY PHYSIOLOGY less.

the summit the scala tympani, along which they descend,
and are eventually lost at the fenestra rotunda in which
that scala ends.

(vii) The Organ of Corti. — But besides this peculiar
arrangement of the perilymph chamber, there are other
and still more important differences between the cochlea
and the labyrinth.

The auditory nerve is, as we have seen, distributed to
certain parts only of the membranous labyrinth, namely,
to the crests of the ampulliB 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 lamina spiralis (Fig. 121, A.N) run
to and end in the canalis cochlearis along its whole length,
from the bottom to the top of the spiral, Fig. 124, 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. 121), we see that the upper side of the
cochlear tube (that which separates it from the scala vesti-
buli) i.s formed by a thin membrane (called the membrane
of Reissner, Fig. 121, m.R) 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 into and end among the cells
of this epithelium, which is spoken of as the organ of
Corti. (Fig. 121, 0.0.)

The membrane which separates the cavity of the
cochlear tube from the scala tympani, and on which the
organ of Corti is placed, is of a peculiar character, speci-

vin

THE COCHLEA

377

PlO 121.— SfCTION Ol ( OIL OF COCHLEA.

Sc.V, scala vestibuli ; Sc.T, scala tympani ; C.C. canalis cochlearis, or
scala media; O.C, organ of Cortl ; m.R, membrane of Reissner, m.t,
membrana tectoria (a gelatinous membrane overlying the organ of Corti,
and supposed to act as a damper). A.N, fibres of the auditory nerve
running in l.i, the lamina spiralis, and ending in the organ of Corti ;
a, connective tissue cushion to which the basilar membrane is attached
on the outside ; 6, bony walls.

The figure has, for simplicity's sake, been made somewhat diagram-
matic. The lamina spiralis has been drawn too short ; the proportions
of the lamina spiralis and the scalse are more exactly rendered in Fig. 120.

378

ELEMENTARY PHYSIOLOGY

LESS.

ally adapted for being thrown into vibrations, and is
called the basilar membrane. The oryan 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) thousands
of rods. On tlie inside and on the outside of the rods are
very peculiar epithelial cells, also arranged into rows, each

Fio. 122. — The Membranr of hie Drum of the Ear with the small
Bones of thk Ear seen from the Inner Siuk ; and the Walls
OF the Tnmpanum, with thk Air-Cbll8 in thk Mastoid Part or
THE Temporal Bone.

The petrous part of the temporal bone containing the labyrinth is svip-
posed to be reiiioveti, the foot-plate of the stapes having been detached
from the fenestra ovalis.

M.C, mastoid cells; Mall, malleus; Inc. incuR ; St. stapes; a i, lines
drawn through the horizontal axis on which the malleus and incus turn.

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 ; and the auditory nerves
passing through the lamina s[jiralis, reach the cochlear
tube along the whole length of the spiral and end in
filaments which are lost in the organ of Corti, but are
probably connected with the hair-cells.

(viii) The Bony Ijabyrinth. — The essential part of
the organ of hearing, the canalis cochlearis, as well as the

vm

THE BONY LABYRINTH 379

other structures developed from the original sac. is lodged
in chambers of the petrous part of the temporal bone.

In the fresh state, this collection of chambers m the
petrous bone is perfectly closed ; but, in the dry skull,
there are (as we have seen, p. 370) two wide openings,
termed fenestrse, or windows, on its outer wall : i.e.,
on the side nearest the outside of the skull. Of these
fenestra, one, termed ovalis (the oval window), is
situated in the wall of the vestibular cavity ; the other,
rotunda (the round window), behind and below this,
is, as we have seen, the open end of the scala tymjxmi
at the base of the spiral of the cochlea. In the fresh
state, each of these windows or fenestrse is closed by a
fibrous membrane, 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, the foot of the stapes (p. 370).

(ix) The Transmission of Sound \Vaves 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 aflfection of which alone can
excite that sensation, must next be considered.

All bodies which produce sound are in a state of vibra-
tion, 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.

The aerial waves, produced by the vibrations of sono-
rous bodies, in part enter the external auditory passage,
and in part strike upon the concha 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 struc-
ture of the skuU to the organ of hearing ; but before they

380 ELEMENTARY PHYSIOLOGY less.

reach it they must, under ordinary circumstances, have
become so scanty and weak, that they may be left out of
considei'ation.

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
i'ibrations 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 w^all is formed by the
membrane 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 bodily, without
any vibrations of its particles among themselves. 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

vin THE FUNCTIOXS OF THE OSSICLES 381

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 auditor}' 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 obvious 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 sub-
stance, but, on the other hand, are extremely favourable
to their vibration en viasse. 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 connection 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 compared with the length
of the waves of audible sounds, and physical considera-
tions teach us that in a like thin rod, similarly capable
of swinging en masse, the minute molecular vibrations
would be inappreciable. Thirdly, direct experiments,
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.

382 ELEMENTARY PHYSIOLOGY LESa

Thus, there is reason to believe that when the tym-
panic 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. 371, the turning of the head of the malleus
involves the simultaneous 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 advantageous. The
long process of the incus, however, is so fixed to the
stapes, and the stapes so attached to the membrane of
tlie 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.
But every vibration, every pull and push, imparts a
corresponding set of shakes to the perilymph, which fills
the vestibule, the scala vestibuli and the scala tympani.
These shakes are communicated to the endolymph in tlie
canalis cochlearis, and, in some way, stimulate the delicate
endings of the cochlear division of the auditory nerve.
Probably the vibrations of the basilar membrane play an
important part.

(x) Tlie Conversion of Sonorous Vibrations into
Sensations of Sound. — We do not at present know
what kind of changes the vibrations of the endolymph
give rise to in the cochlea ; nor do we at present know the
exact way in which the changes thus set up are able to
excite the terminal filaments of the auditory nerve. But

mi AUDITORY SENSATIONS 383

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 it into stimulations
of nerve fibres, the molecular changes of which are
transmitted along the auditory nerve as nervous im-
pulses. Passing along the auditory nerve, these mole-
cular changes, these nervous impulses, reach certain parts
of the brain, situated in the cortex of the temporo-
sphenoidal lobe, below the fissure of Sylvius (see Lesson
XI.), 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 periphe-
ral end-organ (the apparatus of the cochlea and laby-
rinth . 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 feeling
which we call the special sensation. The central end-
organ of hearing is often spoken of as the auditory
sensorium.

Between the emission of sound from a body and its
appreciation by the hearer there is a series of events of
diti'erent kinds. There are the vibrations started Vjy the
sounding body, and passing through the air, the tym-
panum, the perilymph, and the endolymph : these are all
of one order. Then there are the changes in the peri-
pheral end-organ, in the apparatus of the cochlea and
labyrinth ; 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 clianges of nervous matter, are yet of still another
order, and probably comprise in themselves a whole .series

384 ELEMENTARY PHYSIOLOGY less.

of events, the consequence of the last of which is the
sensation of sound.

Every sound consists, as we have seen, of vibrations.
Sometimes the vibrations are repeated with great regu-
larity ; and sounds, in which the regular recurrence of
the same vibrations is conspicuous, are called " musical
sounds." Sometimes 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 tlie central end-organ a
particular kind of sensation which we call a tone ; and tlie
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
sensations 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 noise ; but the sensations have not the quali-
ties of tone-sensations, and do not give rise to feelings of
melody or harmony.

A pure musical sound consists of a series of vibra-
tions repeated with exact regularity, the number of
vibrations occurring in a given time, e.g. in a second,
determining what is called tlie pitch of the " note." But
ordinary musical sounds are, for the most part, not simple,
consisting of one set of vibrations, but compound, con-
sisting of several sets of vibrations occurring together ;
in these musicians distinguish one set, called the funda-
mental tone, and other sets, varying 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 rapidity of the vibration, by the number of vibrations
repeated, for instance, in a second ; hence every tuning-fork

vin FUNCTION OF COCHLEA 386

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 niusicHl note is sounded close to a number of
tuning-forks of different pitch, only that tuning-fork the
pitch of wliich is the same as tliat 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 vibrating ; 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, 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 would 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 degrees ; and thus a particular series of nervous
impulses, the counterpart as it were of the musical sound
with its fundamental tone and overtones, would be trans-
mitted along the nerve filaments to the brain.

And 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. 121), with its thousands of epithelial cells
and rods of Corti lying upon it, represents, as it were, an
assemblage of thousands of tuning-forks, of various rates
of vibration, with a separate nerve filament attached to
each. So that, when a number of vibrations of diflFerent

C C

386 ELEMENTARY PHYSIOLOGY less.

periods, such aa constitutes an ordinary musical sound, are
transmitted by tlie tympanum to the cochlea, these as
they sweep along the canalis cochlearis throw into sym-
pathetic movement those parts, and those parts only, of
the basilar membrane with their overlying epithelium,
whose periods of vibration correspond to tlieir own
vibrations, and thus excite certain nerve filaments, and
these only It is this excitement of a group of nerve
filaments, some more intensely than others, which, reaching
the brain, give rise to the sensation which we associate
with the particular musical sound.

It must be distinctly understood that the picture which
has just been drawn only illustrates one way in which the
cochlear mechanism miijlit translate waves in the perilymph
into nervous impulses. There is no suggestion that this
indicates what actually takes place or even that it is
nearer the truth than any of several other suggestions
which have been made. It is possible for instance that
each wave in the perilymph causes the whole basilar
membrane to vibrate like the plate of a telephone or
itself to fall into waves which traverse its whole length,
in either case we can conceive that the hair cells could
appreciate such a condition.

Little as we know of the cochlear mechanism, we know
still less about the nature of the auditory sensorium or
central end-organ of the auditory nerve ; but it may be
conceived that each filament of the cochlear nerve is
connected with a particular 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 disturb-
ance 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 pi'ovided with two end-organs : one, peripheral,
in the organ of Corti, capable of being set in motion by

vin FUNCTION OF COCHLEA 387

vibrations of one quality only ; the other, central, in the
brain, capable of producing a psychical effect d one
quality only. It does not follow, however, that we are
distinctly and separately conscious of the nervous disturb-
ance in each central end-organ, it does not follow that
we have as many distinct and separate kinds of conscious
sensation as there are peripheral and central end-organs,
though how many such distinct 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 separ-
ately, so, by a reverse operation, the central mechanism
probably pieces together the nervous disturbances of a
number of central end-oi'gans, and thus produces a
sensation whose characters are determined by a com-
bination 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
understand the exact mechanism by which each particular
vibration excites its corresponding nerve filament. The
nerve filaments appear to end in the epithelial cells bear-
ing short hairs, which lie on each side of the rods of Corti ;
and we may therefore conclude that these "hair-cells"
have some share in producing the effect. 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
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

c c 2

388 ELEMENTARY PHYSIOLOGY less.

in some way or other affected by the vibrations of the
fluids in the cochlea.

(xii) Localisation of Bound. — 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. The con-
clusions wc thus arrive at are usually more or less accurate.
But sounds may be coming to us in different directions
and fi'om different distances, and when we endeavour to
form some estimate of either tlie one or the other of these
possible differences we find our means of doing so are very
imperfect. As to our estimate of the distance fi'om which
a sound is coming, we are guided chiefly by its varying
intensity coupled with previous experience and a know-
ledge of the laws which connect varying intensity with
the different distances of the source. For the discrimina-
tion 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, according 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 Ave assume that the sound is coming
along a line directed straight into that ear. In animals
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

\^II EUSTACHIAN TUBE 389

source of a sound heard through a snow storm. Again,
in ventriloquism our judgment is upset, not only as
regards the nature nf the source of sound, but also of its
distance and direction, by carefully planned simulation
and suggestion.

(xiii) The Functions of the Tympanic Muscles and
Eustachian Tube. — It has already been explained that
the stapedius and tensor tijmpani muscles are competent
to tighten the membrane of the fenestra ovalis and
that of the tympanum, and it is probable that they come
into action when the sonorous impulses are too violent,
and would produce too extensive vibrations of these mem-
branes. They may therefore be of use in moderating the
eft'ect 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 the.se movements the
end of the Eustachian tube which opens into the pharynx
is opened and the pressure on the two sides of the
tympanum is equalised.

10. Sensations governing co-ordination and equili-
brium.— The sense of sight is associated with the eye and
with no other sense organ, that of hearing with the ear
and so forth ; in each case we are fully conscious of our
use of these faculties. But there is another sense of
which we are as a rule unconscious, this is the sense of
co-ordination ; when this sense fails us however we become

390 ELEMENTARY PHYSIOLOGY less.

amply conscious of the failure, which we signify by the
word giddiness. This word is not always used in quite
the same sense. It will be worth while to inquire more
particularly what we mean by it.

To take an instance, many persons, if they have to look
down from a considerable height, such as the edge of a
precipitous cliff, or the tower of a high building, or in some
cases even a scaffold, say tliey feel giddy. This feeling,
which it is difficult to describe, at all events involves the
inability to carry out or co-ordinate ordinary movements,
and would if strong enougli lead to complete collapse, in
which case the person would not only l)e unable to pro-
ceed but would actually fall. Again, if a person turns
round rapidly a few times and then stops he describes
himself as feeling giddy, but in this case he has a perfectly
definite sensation that the room is swinging round ; at the
same time if he tries to move his gait is unsteady, and if
he is giddy enough he falls down.

The power of co-ordination of the muscles, which
enables the body to maintain its equilibrium, is the result
of sensations which come from more than one sense organ.
In the fii'st of the two instances of giddiness which have
been given, the cause of giddiness was in really a failure
of the sense of sight to adapt itself to unaccustomed
conditions ; indeed, in some persons the giddiness ex-
perienced in lofty positions is overcome by the adequate
use of glasses. The sense of sight is one of those which
contribute to the power of co-ordination. Sensations
which come from the limbs are also factors. This is
evident in the condition of persons stricken with locomotor
ataxy. The gait of one suffering from this malady is
extremely clumsy and unsteady, and, indeed, is only
possible because he can use his vision instead of the
sensations which, owing to the diseased condition of his
spinal cord, fail to come from his legs. He walks by
seeing a spot in front of him at a suitable distance and
planting his foot upon it ; blindfolded he would at once

VIII THE MEMBRANOUS LABYRINTH 391

fall. The particular sensation from the legs, which in a
normal i)erson helj) in co-ordination, are to some slight
extent those from the skin of the feet, but more especially
are those from the joints and the muscles. These sen-
sations from the limbs are thus the second class of
sensation which contribute to tlie power of coordination.

But even more important than either the sensations from
the eyes or from the limbs are a third set which come
from a special organ which exists for the purpose of
appreciating alterations in the position of the body with
regard to the objects around it just as definitely as the

Fig. 123.— The Membranous Labyrinth, twice the natural size.

Ut. the (Jtrkie, or part of the vestibular sac, into which the semi-
circular canals open ; A. A. A, the ampulla; ; P. A, anterior vertical semi-
circular canal ; F. V, posterior vertical semicircular canal ; //, horizontal
semicircular canal. The saccule is not seen, as in the position in which
the labyrinth is drawn the saccule lies behind the utricle. The white
circles on the ampullae of the posterior vertical, and horizontal canals
indicate the cut ends of the branches of the auditory nerve ending in
those ampullse ; the branch to the ampulla; of the anterior vertical
canal is seen in the space embraced by the canal, as is also the branch
to the utricle.

eye sees or the ear hears. This organ is the membranous
labyrinth of the ear, the utricle and the semicircular
canals. The giddiness which is invoked by turning
rapidly and then stopping suddenly gives rise to a
definite sensation because this definite organ is stimulated
in just the way that it would be if the person and the
room were rotating relatively to one another. In order
to see that this is so we must understand the structure of
the labyrinth of the ear.

392

ELEMENTARY PHYSIOLOGY

LESS.

(i) The Membranous Labyrinth. — The membranous
bag, as we have said, is not simple but complicated ;
it consists of several parts. In the first place there
is a somewhat oval sac, called the utricle (Fig.
123, Ut.) into which open three hoop-like semicircular
canals. Of these two are placed vertically, one directed
anteriorly, the other posteriorly, and are hence called the

A.//

Fm. 124.

Diagram to ilhistrate the endings of the auditory nerve In the mem-
branous labyrinth and cochlea. N.B. The drawinii i.f diagramiuatic.

A.N, auditory nerve dividing into several branches, and ending: — at
A.a.C, in the ampulla of the anterior vertical semicircular canal : P.S.C,
do. posterior vertical: S.S.C, do. external horizontal: I/, in the utricle:
S, in the saccule. Cock, the ending all along the canalis cochlearis ;
A.V, canal uniting the interior of utricle with that of saccule. C,
canal joining the saccule to the canalis cochlearis.

anterior {P. A) and posterior (P. F) vertical semi-
circular canals. Tlie third is placed horizontally and
directed outwards, hence it is called the exterior
horizontal semicircular canal (Fig, 123, H). It
will be observed that the three canals thus lie in the three
directions of space. Each of these three hoops is

vra THE MEMBRANOUS LABYRINTH 393

dilated at one of its two ends, where it opens into
the utricle, into what is called an ampulla (Fig. 123,
A, A, A), the other end having no ampulla. Thus
there is one ampulla to each canal. Those ends of the
two vertical canals which are not dilated into ampullae
join together (Fig. 124), before they open into the
utricle.

On 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.

Attached to the utricule is a similar smaller sac (form-
ing another division of the primitive membranous bag)
called the saccule, on the walls of which is a similar
rounded patch of modified epithelium, or macula. 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. 124, A.V.).

The utricle and saccule with the three semicircular
canals receive the name of the membranous laby-
rinth. It will be remembered that this membranous
labyrinth, filled with endolymph, lies in an intricate caAdty
with bony walls called the OSSeous labyrinth, of which
the part which contains the saccule and utricle is known
as the vestibule, and that between the walls of the
bony and the membranous labyrinth, which correspond
largely but not wholly in form, is a space filled with
perilymph.

Branches of the auditory nei've pass to this mem-
branous labyrinth and send fibres (Fig. 124) to the three

394

ELEMENTARY PHYSIOLOGY

LBSS.

Fro. 125.

-LoNniTuniNAL Section ok Ampulla, outtinq the Orkst
CKuSSWISE (S0ME'.\'HAT diaorammatic).

c, one end of the ampulla forming 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.h,
auditory hairs ; c.t, connactive tissue support to the auditory epithe-

THE MEMBRANOUS LABYRINTH 395

crests of the three ampullae, to the patch ou the utricle,
and to the patch on the saccule. In each crest and each
patch the epithelium is thickened and modified, and
altliough the crests are slightly different in structure from
the patches, the general features are the same m all.
Whereas over the rest of the inside of the membranous
labyrinth the epithelium consists (Fig. 125, e) of a single
layer of low, rather flat cells, in the crests and patches
the cells lie several deep, and are of a peculiar form.
Some are conical or cylindrical, and some are spindle-
shaped, and either the one or the other, or, according to
some authors, both, bear stiff hair-like filaments (Fig. 125,
a.h, a.h.) projecting into the cavity of the labyrinth.
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 ampulliB than
in the patches of the utricle and saccule. The fibres
of the auditory nerve may be traced through the connec-
tive tissue wall of the crest or patch into the epithelium,
where they break up into a delicate network among the
cells (Fi" 125, A, B, b) ; but it is not as yet exactly de-
termined how the filaments of this network end, whether
they actually join the conical cells, or the spindle cells, or
merely lie in contact with them.

lium- n fibres of the auditoiH- nerve passing into the aiaditory epithe-
lium ' "'epithelium intermediate between the auditory epithelium and
the ordinary epithelium of the rest of the ampulla. j-i.„„

A and B Diafn-ams to illustrate the character of the cells of the and tory
epUhelium/aid the two views taken as to the relation of the auditory
E to the cells. In both A and B, I is the auditory epithelium, II the
connecive tissue on which it rests, and a, a fibre of the auditory nerve
parsing through 11, and dividing into fine branching fi aments m , at 6
• P?n T cc cylindrical cell bearing auditory hairs, a.h: e^.ch cell bears
a group of fine hairs which adhere together as a long narrow cone , sp.c,
spindle-shaped cells, not bearing hairs. ,„j„,i1p «harpd cells

lu B, c.c, cvlindrical cells not bearing hairs spc, ^P>"'^l';-«^'*l,f,'*^'^?['^
bearing the auditory hair a.U, and supposed to be connected with the
nprvc-filaments : f. other supporting cells. .

In both A and B, the fibre n. of the auditory nerve passes mto the
epithelium, ai.d cuds in hue brauchcs, 6.

396 ELEMENTARY PHYSIOLOGY less.

Let us now revert to the relation of the semicircular
canals to giddiness. If a glass containing water in which
some powder has been placed be rotated, the observer will
see, by looking at the powder, that whilst the glass rotates
the water does not. This is so at first at all events, but if
the rotation be maintained the water begins to rotate
also, and lastly, if the glass be suddenly stopped the
water will continue to rotate. Suppose further that there
was a projection from the side of the glass, pointing
towards the centre : when the rotation of the glass com-
menced, this projection would have to go through the
stationary water ; there would therefore be an increased
pressure of fluid on the side of the projection which was
breasting the water and a decreased pressure on the side
which was retreating from it ; tliis would be so until the
water acquired a rotation as rapid as that of the glass.
When the glass was stopped the water would then stream
on past the projection and would press on the opposite
side to that whicli had at first suffered the increased
pressure ; that is to say the side which would have
had to breast the stream if the glass had originally been
rotated in the opposite direction. If in our minds we
replace the glass of water by the horizontal semicircular
canal, and the projection by the crista in the ampulla, and
if we suppose further that the hair-cells on the crista can
appi'eciate the alterations in the pressure of fluid upon
them, we will see, firstly, that they would at once acfjuaint
us of any rotation of the body. Were the body to rotate
in the direction of the clock the endolymph would press
on one side of the crista ; were it to rotate in the reverse
direction the endolymph would press on the ojjposite
side ; were it to rotate clockwise long enough for the endo-
lymph to acquire the rotation of the body, and then sud-
denly to stop, the pressure of fluid on the hair-cells of the
crista would be similar to that which would be occasioned
by the commencement of a rotation against the hands of
the clock, and the person would be deceived into thinking

VIII THE MEMBRANOUS LABYRINTH 397

he was rotating in that direction. This then is the explan-
ation of the giddiness which we experience under such
circumstances. So far we have considered the horizontal
semicircular canals only, but quite similar sensations may
be obtained by suitable stimulation of the anterior and
posterior canals. These are placed in vertical planes at
right angles to one another and are also of course at
right angles to the horizontal canal. As any motion
in space can be resolved into motions in three planes at
right angles, the sensations from the three semicircular
canals, when put together in the brain, form an adequate
mechanism for the judgment of any motion.

So far we have spoken only of the sense-organs involved
in enabling us to co-oi'dinate our movemeftts. These
organs are all united to the central nervous system by
nerves. When we trace the vestibular branch of the
auditory nerve to the brain we find its course diverges
from that of the cochlear branch, or auditory portion
proper of the nerve. The vestibular branch, if followed,
takes us to that portion of the brain known as the cere-
bellum, which forms the central organ for the appreciation
not only of impulses from the labyrinth, but also for the
sensations of which we have spoken from the eyes and
from tlie limbs.

Indeed, mischief in the essential portion of the cere-
bellum leads to giddiness and inco- ordination of move-
ment as certainly as derangement of the sensations
which we have been discussing.

^.( AIX6

LESSON IX
THE ORGAN OF SIGHT

1. The General Structure of the Eye.— Every sense-
organ consists of two parts ; the essential part, consisting
of the structures in wliich the sensory nerve supplied to
the organ terminate, and in which the impulses which
pass up that nerve are generated, and the accessory part,
arranged so as to bring the agent, which affects the organ,
to bear upon the essential part. In the case of the
eye, the accessory structures are so ■ complicated and
their action so striking that they seem, at first sight,
to form the greater part of the whole sense-organ.
Hence 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 expan-
sion of the optic nerve, comprise three kinds of appar-
atus : (a) a " water- camera," the eyeball; (h) muscles
for moving the eyeball ; (c) organs for protecting the
eyeball, viz. the eyelids, 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 EYEBALL 399

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 membrane, varying in thickness from
g^jth 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 organ connected with
sensory nervous fibres which can be afiected, by any
agent, in such a manner as to give rise to the sensation
of light.

The eyeball is composed, in the first place, of a tough,
firm, spheroidal case consisting of fibrous or connecti'<-e
tissue, the greater part of which is white and opaque, and
is called the sclerotic (Fig. 126, 2). In front, however,
this fibrous capsule of the eye, though it does not change
its essential character, becomes transparent, and receives
the name of the cornea (Fig. 126, 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 con-
sequently more convex, spheroid.

The corneo-sclerotic case of the eye is kept in shape
by what are termed the hvjnonrs — watery or semi-fluid
substances, one of which, the aqueous humour (Fig.
126, 7')j 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. 126, 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. 126,12^

400

ELEMENTARY PHYSIOLOGY

LESS

denser, and capal)le of refracting light more strongly
than either of the humours. The crystalline lens is com-
posed of fibres having a somewhat complex arrangement,

Pio. 126.— Horizontal Section of the Eyeball.

1, cornea; 1', coiijunctiva ; 2, sclerotic; 2', sheath of optic nerve;
3, choroid ; 3", rods and cones of tlie retina ; 4, ciliary muscle ; 4', cir-
cular portion of ciliary muscle ; T), ciliary process ; G, posterior chamber
between 7, the iris and the suspensory ligament ; 7', anterior chamber ;
S, artery of retina in the centre of the optic nerve ; 8', centre of blind
spot ; 8", macula lutea ; 9, ora scrrata (this is of course not seen in a
sei'tion such as this, but-is introduced to show its position) ; 10, space
behind the suspensory ligament (canal of Petit); 12, crystalline lens ;
13, vitreous humour; 14, marks the position of the ciliary ligament;
a a, optic axis ; b h, line of equator of the eyeball.

and is highly elastic. It is more convex behind than in
front, and it is kept in place by a delicate, but at the

CHOROID COAT

401

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. 126, 5, and 128, c). Tn the ordinary condition
of the eye this ligament is kept tense, i.e. is stretched
pretty tight, and the front part of the lens is consequently
flattened.

The choroid coat is highly vascular and consists of
blood-vessels arranged in a very complex way, bound
together with a little connective tissue among which,
towards its inner side, are a number of branched connec-

Pio. 127. — Pigment Cells from the Choroid Coat.

tive tissue corpuscles whose cell-sub.stance is loaded with
granules of black pigment (Fig. 127).

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. 139). But these
cells really belong to the retina and will therefore be
described later on (p. 424). They are separated from the
vitreous humour by the retina only. 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 ; but when it reaches

D i>

402 ELEMENTARY PHYSIOLO(;Y less.

the front part of the sclei'otic, its inner surface becomes
raised up into a number of longitudinal ridges, with
intervening depressions, like the crimped frills of a lady's
dress, terminating within and in front by 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. 128, c, and 126, 5), are the
above-mentioned ciliary processes.

The iris itself (Figs. 126, 7, and 128, o, 6) is, as has
been already said (p. 303), a curtain with a round hole in
the middle, the pupil, provided with circular and radiating
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 nmscular fibres, is extremely rapid. The
edges of the iris are firmly connected with the capsule of
the eye, at the junction of the cornea and sclerotic, by
the connective tissue which enters into the composition
of what used to be called the ciliary ligament. The
hinder surface of the iris is covered with cells containing
a black pigment, similar to that of the choroid coat, and
the different colours of eyes depend partly on the varying
amount and distribution of pigment in these cells, but
chiefly on pigment cells imbedded in and scattered
throughout the substance of the iris. Unstriped muscular
fibres, having the same attachment in front, spread back-
wards on to the outer surface of the choroid, constituting
the ciliary muscle (Fig. 126, 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 termination of the choroid),
this pulling forward of the choroid brings about a
relaxation of the tension of that suspensory ligament,
which, as we have just said, is in a resting condition
stretched somewhat tight, keeping the front of the lens
flattened.

CILIARY REGION

403

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. 126 and 131).

The retina lines the interior of the eye, being placed
between the choroid and vitreous humour, its rods and
cones (see pages 415-419) being imbedded in the pigment

Fio. 128. — View of Front H \'_;

.EBALL SEEN FROM BEHIMD.

a, circular fibres ; b, radiating fibres of the iris ; c, ciliary processes ;
d, choroid. The crystalline lens has been removed.

epithelium lining the former, and its inner limiting
membrane touching the latter.

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. 126, 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 len&

D D 2

404 ELEMENTARY PHYSIOLOGY

2. The Eye as a Water-Camera.— The impact of the
ethereal vibrations upon the sensory expansion, or essential
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 woi-ld would affect all parts of it ecjually, and
therefore the feeling aroused would be that of a generally
diffused luminosity. There would be no separate feeling
of light for each separate radiating point, and hence no
correspondence between the vi.sual 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, tlie
essential condition is, that distinct luminous points in the
external Avorld shall be represented by distinct feelings of
light. And since, in order to jjroduce tliese distinct
feelings, vibrations 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 sun-ounded ; and which throw
upon the retina luminous points, which correspond in
number, and in position relatively to one another, with
those luminous points in tlie external world from which
ethereal vibrations proceed towards the eye. The lumin-
ous 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

IX CONDITIONS OF DISTINCT VISION 405

correspond with the disposition of the luminous parts of
the object represented, and with the qualities of the light
which proceeds 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 screen or wall in a dark room, and 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 con-
vex the lens the nearer its focus ; the less convex, the
further off its focus.

406 ELEMENTARY PHYSIOLOGY less.

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.

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
outside the watch-glass that an image of its flame shall
fall on the opposite wall of the box. If, under these cir-
cumstances, a doubly convex lens of glass were introduced
into the water in the path of the rays, it would act (though
less powerfully than if it were in air) in bringing 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 -ind forwards,
and thus throw on the screen at the back of the box dis-
tinct images of bodies at various distances oflf. 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 camei'a 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, the crystalline to the glass lens, the
introduction of which was imagined. The back of the
box corresponds 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 litrht
and cutting oft' the marginal rays which, owing to certain

rs EYE AS A WATER CAMERA 407

optical properties of spheroidal surfaces, give rise to
defects in the image formed at the focus.

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
contractions 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 radiating fibres makes it larger or dilates it. Further
conversely relaxation of the circular fibres causes or helps
to cause dilation, and relaxation of the radiating fibres
causes or helps to cause constriction. Contraction of the
circular fibres and so const rictio7i of the pujjil is brouglit
about by means of fibres of the oculo-motor ner^'B, and
contraction of the radiating fibres and so active dilation is
brought about by means of fibres of the sympathetic
system and may be induced by stimulation of the sym-
pathetic in the neck.

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 aflerent impulses to a centre in the
brain lying beneath the front end of the aqueduct of
Sylvius (see Lesson XI), and the third 'oculo-motor)
cranial nerve (see Lesson XI) provides the path for
efferent impulses from the centre to the cii'cular fibres of
the iris. The dilation of the pupil when light is with-
drawn from the retina is in the main at least due to the
cessation of previously acting constrictor impulses.

But the pupil, or aperture of the iris, is either con-
stricted or dilated under many circumstances, other than
the mere action of light and darkness on the retina.
Thus it is constricted when the eye is accommodated for

408 ELEMENTARY PHYSIOLOGY less.

near objects, during deep sleep, or after the administration
of morphia and several otlier poisons, and in the early
stages of the action of alcohol and cliloroform. On the
other hand the pupil is dilated when the eye is accommo-
dated for distant objects, during violent muscular activity,
during dyspnea, after the ailministi'ation of atropine
and some other poisons, and in the later stages of the
action of alcohol and cliloroform.

In the case of the action of many poisons the effect
produced is due, notably in respect of atropine, to a purely
local action on the circular (sphincter) fibres of the iris, or
on the endings of the nerves in these fibres.

Rays of light passing into the eye undergo a bending or
refraction (i) as they enter tlie eye, at the surface of the
cornea, (ii) as they pass tlirough the lens ; and as a

Fig. 129. — The Formation of an Image on the Retina.

result of this action of the cornea and lens an image of
any object in the external world is formed on the retina.

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 cori'esponds with the left of the object and vice versa.
Exactly the same thing takes place in the eye with the
image focussed on the retina. It too is inverted. This
fact often gives rise to the question, Why then do Ave see
objects in the external world in an erect position and not

ACCOMMODATION 409

also inverted ? The answer is simple, and is given in
Lesson X.

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 distance, those of either
still nearer, or more distant, bodies would be blurred and
indistinct. In the ordinarj' 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
or accommodation of the eye is to stick two stout
needles upright into a straight piece of wood, not exactly,
but nearly in the same straight line, so that, on applying
the eye to one end of the piece of wood, 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

410 ELEMENTARY PHYSIOLOGY less.

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 distinctly at the same
time.

Multitudes of explanations have been given of this
remarkable power of adjustment ; but the true solution of
the problem has been gained bj' the accurate determina-
tion of the nature of the changes in the eye which

Fio. 130. — Diagram of the Images of a Candle-Flame seen by Re-
flection FROM the surface OF THE CORNEA AND THE TWO SURFACES

OF THE Lens.

A, as seen when the ej'e is adjusted for a distant object ; B, as they
appear when the eye is fixed on a near object.

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 in-
verted. One upright 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

ACCOxMMODATION

411

from the posterior face of the lens, which, being convex
backwards, is, of course, concave forwards, and acts as a
concave mirror (Fig. 130, A).

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 or apparent position must change if either the cornea,
or the back of the lens, alter either their form or their

Fio. 131.— The Changes in the Lens in Accommodation.

A, adjusted for distant ; B, for near objects.

e, cornea ; con. conjunctiva ; scl. sclerotic ; cA. choroid ; c.p, ciliai-y

process ; c.vi, ciliary muscle ; s.l, snspensory ligament.

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.
130, 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. 131.

For purposes of accurate experiment it is better to
employ the images cast by ttvo 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

412 ELEMENTARY PHYSIOLOGY less.

that the tvx> images of the middle pair come nearer to-
gether when the eye is accommodated for a near object
than it is to observe the slight movement and diminution
in size of the single imarje of a candle flame.

These may be regarded as the facts of adjustment with
which all explanations of that process must accord. They
at once exclude the hypothesis (1) that adjustment is the
result of the compression of the ball of the eye by its
muscles, which would cause a change in the form of the
cornea ; (2) that adjustment results from a shifting of the
lens bodily, for its hinder face does not move ; (3) that it
results from the pressure of the iris upon the front face of
the lens, for under these circumstances the hinder face of
the lens would not remain stationary. This last hypo-
thesis is further negatived by the fact that adjustment
takes place equally well when the iris is absent.

But one other explanation remains, which is not only
exceedingly probable from the anatomical relations of the
parts, but is also supported by direct experimental evi-
dence. 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 consequently has a flatter
form than it would take if left to itself. 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.
— Adjustment can take place only within a certain
range ; this, however, admits of great individual varia-
tions.

People possessing ordinary, or as it is called ' ' normal "

IX USK OF SPECTACLES 413

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 bkuTed and
indistinct, because the " near limit " of adjustment 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 focus-
sing, 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 adjust-
ment is much closer. Such persons can see distinctly ob-
jects as near to the cornea as even one or two inches ;
but they cannot adjust their eyes to objects at any great
distance off. Thus many of these " near-sighted " people,
as they are 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 flat as possible, the arrangements of the
eye are such that the image of an object onlj^ 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 tlie 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.
K, 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 bom really "long-sighted," inasmuch
as they can see distinctly only such objects as are quite
distant ; 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

414

ELEMENTARY PHYSIOLOGY

use convex glasses. In such persons the bulb of the
eye is generally too short.

A kind of long-siglitednesa 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 the lens
has become less elastic with years ; and hence adjust-
ment for near objects becomes impossible, though distant

Ch. m

Fio. 132.

B

A, the muscles of the right eyel'all viewed from above, and B of the
left eyeball viewed from the outer side ; H.R, the superior rectus ; Inf. R,
the inferior rectus ; E.K, In.R, the external rectus ; S.Ob, the superior
oblique; Inf.Oh. the inferior oblique; Ch. the chiasma of the optic
nerves (ll.) ; ///, the third nerve which supplies all the muscles except
the superior oblique and the external rectus.

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."

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 nmscles, the
obliqui (Fig. 132). The straight muscles are attached to
the back of the bony orbit, round the edges of the hole
through which the optic uerve passes, and run straight

IX MUSCLES OF EYEBALL 415

forward to their insertions into the sclerotic — one the
superior rectus, in the middle line above ; one, the
inferior, opposite it below ; and one half-way on each
side, the external and internal recti. The eyeball 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 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 passing forwards and becoming tendinous
at the upper and inner corner of the orbit, it traverses a
pulley-like loop of ligament, and then turns downwards*
and outwards to its insertion. The action of the oblique
muscles is somewhat complicated, but their general ten-
dency is to roll the eyeball on its axis, and pull it a little
forward and inward.

By means of the contraction of these several muscles
the eyeballs may be moved into any desired j^osition and
their optic axes (Fig. 126, a. a.) directed straight towards
any object. This mobility is largely of use in diminisliing
the necessity for such frequent movements of the whole
head as would otherwise be neces.sary. But the move-
ments are also chiefly of extreme importance as ensuring
that any object is seen as single, although there is an
image of it on the retina of each of the two eyes.

6. The Protective Appendages of the Eye. — The
eyelids are folds of skin containing thin plates of carti-
lage. They are fringed at the edges with hairs, the
eyelashes, and contain a series of small glands called
Meibomian glands. Circularly disposed fibres of
striped muscle lie beneath the integvunents of the ej'elids,
and constitute the orbicularis muscle which shuts tliem.

416

ELEMENTARY PHYSIOLOGY

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
mucous membrane, the conjunctiva, which lines the in-
terior of the lids and the front of the eyeball, its epithelial
layer being even continued over the cornea. The nume-
rous small ducts of a

\1'":;aV>.\

S.Oi.

gland which is lodged in
the orbit, on the outer
side of the ball (Fig. 1.33,
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 inner side of the
eye is a reddish fold,
the carunciala lachry-
malis, a sort of rudi-
ment of that third eye-
lid which is to be found
in many animals. Above
and below, close to the

caruncula, the edge of each eyelid presents a minute
aperture (the pnndvm lachri/male), 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. 134) which passes down from the orbit
to the nose, opening below the inferior turbinal bone
(Fig. 69, h). It is through this system of canals that the
conjunctival mucous membrane is continuous with that
of the nose ; and it is by them that the secretion of the

rn/!06.

Fia. 133.

The front view of the right eye
dissected to show Orb. the orbicular
muscle of the eyelids ; the pulley
and insertion of the superior oblique,
S.Ob., and the inferior oblique
Inf. Ob. ; L.G, the lachrymal gland.

THE RETINA

417

lachrymal gland is ordinarily carried away as fast as it
forms.

But, under certain circumstances, as when the con-
junctiva 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 lachrj-mal duct,
and the fluid, accumulating between the lids, at length
overflows in the form of tears.

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 deli-
cacy, 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 circular depression of
a yellowish hue, the macula
lutea, or yellow spot (Fig.
135, m.l. ; Fig. 126, 8"),—
not easily seen, however,
unless the eye be perfectly
fresh, — and, at some distance from this, towards the inner,
or nasal, side of the ball, is a radiating appearance, pro-
duced by the entrance of the optic nerve and the spreading
out of its fibres into the retina.

A very thin vertical slice of the retina, in any region
except the yellow spot and the entrance of the optic
nerve, may be resolved into the structures represented
separately in Fig. 136. The one of these (A) occupies
the whole thickness of the section, and comprises its
essential, or nervous, elements. The outer ^ fourth, or

CD.

Fig. 134.
A front view of the left eye,
with the eyelids partially dis-
sected 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

K E

418

ELEMENTARY PHYSIOLOGY

rather less, of the thickness of these consists of a vast
multitude of minute, either rod-like, or conical bodies,
ranged side by side, perpendicularly to the plane of the
retina. This is the layer of rods and. cones (/> c).
From the front ends or bases of the rods and cones
very delicate fibres pass, and in each is developed a
granule-like or nucleus-like body (// c'), which forms a

Pio. 135.— The Eyeball divided transversely in the Middle Line

AND VIEWED FROM THE FRONT.

s, sclerotic ; ck, choroid, seen in section only.,

r, the cut edges of the retina ; r.r, vessels of tlie retina springing
from 0, the optic nerve or blind spot; m.l, the yellow spot, the darker
spot in its middle being the fovea centralis.

part of what has been termed the outer nuclear layer.
It is probable that these fibres next pass into and indeed
form the close meshwoi'k of very delicate nervous fibres,

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 sur-
faces ; 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.

THE RETINA 419

the outer molecular layer, which is seen at d d' (Fig.
136, A). From the inner surface of this meshwork other
fibres proceed, containing a second set of granules or
nuclei, which forms the inner nuclear layer (/ /').
Inside this laj^er is a stratum of convoluted fine nervous
fibres, the inner molecular layer (g g') — and inside
this again are numerous nerve-cells (h h') constituting
the layer of ganglionic cells. Processes of these
nerve-cells extend, on the one hand, into the layer of
convoluted nerve-fibres ; and on the other are continuous
with the stratum of fibres of the optic nerve (0-

These delicate nervous structures are supported by a
sort of framework of connective tissue of a peculiar kind
(B), which extends from an inner or anterior limiting
membrane (I), which bounds the retina and is in con-
tact with the vitreous humour, to an outer or posterior
limiting membrane (a), which lies at the inner ends,
or bases, of the rods and cones near the level of 6' c' in A.
Thus the fi'amework falls short of the nervous sub.stance
of the retina, and the rods and cones lie altogether
outside of it, wholly unsupported by any connective
tissue. They are, however, as we shall see, imbedded in
the layer of pigment oti which the retina rests (p. 424).

The fibres of the optic nerve spread out between the
inner limiting membrane (J.) and the nerve-cells {h'), and
the artery which enters along with the optic nerve
pierces the centre of the nerve (Fig. 135), and ramifies
between the two limiting membranes. Most of the
branches run between the inner limiting membrane and
the inner nuclear layer (//'). Thus, not only the nervous
fibres, but the vessels, are placed altogether in front of
the rods and cones.

The structural appearance of the nervous elements of
the retina and the seven " layers " into which it may be
divided as described above is such as can be made out in
any ordinarily stained section. Such a section tells us very
little, except in the case of the rods and cones, as to tlie

£ E 2

420

ELEMP:NTARY physiology less.

Fiu. 130. — Diagrammatic Views of the Nervous (A) and the Con-
nective (B) Elements of the Retina, supposed to be separated

FROM ONE another.

A, the nervous structures — 6, the rods ; c , the cones ; b'.c', the granules
or nuclei of the outer layer, with which these are connected ; d.W, inter-
woven very delicate nervous fibres, from which fine nervous filaments,
bearing the inner granules or nuclei, /./', proceed towards the inner sur-
face ; ff.f/', the continuation of these fine nerves, which become convo-
luted and interwoven with the processes of the nerve cells h.k' ; i.i, the
expansion of the fibres of the optic nerve. B, the connective tissue —
a.a, external limiting membrane ; e.c, radial fibres passing to the internal
limiting membrane ; e'.e', nuclei ; d.d, the intergranular layer ; y.g, the
molecular layer ; I, the inner limiting membrane.

(Magnified about 250 diameters.)

THE RETINA

421

real nature of the several structures which give to each
layer its characteristic appearance. Still less does it give
any clue to the nature of the connections of the successive

Fig. 137. — Diagram iJf Illustration of the Nervous Structure of the
Retina.

/ — VII, the sever.ll "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, ^.c, two cells of the
ganglionic cell layer ; op.f, op.t, fibres of optic nerve.

layers with each other by which the functional continuity
of the rods and cones with the fibres of the optic nerve
is brought about. But by the application of very special

422 ELEMENTARY PHYSIOLOGY

methods of staining many details have recently been
made out which seem to justify the construction of the
following diagrammatic figure (Fig. 137) in illustration of
the structure of the retina and of the relationships of its
several layers. In this figure the facts of chief and
immediate interest are those which refer to the mode of
connection between a rod on the one hand and a cone on
the other with the fibre of the optic nerve.

In the case of a rod the rod-fibre (r./.) is seen to end
in the outer molecular layer ( V) as an extremely minute
knob. This knob is surrounded by processes from the
outer end of a rod bipolar cell (r.b.p.) whose body lies in
the inner nuclear layer (/T-^.

The inner end of this cell ends in branching processes
which surrovnd the body of one of the cells in the
ganglionic-cell layer (JJ). which is itself in direct continuity
with a fibre of the optic nerve {op.f.).

In the case of a cone the cone-fibre {cf.) ends as a
flattened expansion, fi-oni which fine processes extend, in
the outer molecular layer ( V). Immediately below these
processes of the base of a cone-fibre, but not actually
continuous with them, lie the processes from the outer
end of a cone bipolar cell (c.b.p.) whose body is again
situated in the inner nuclear layer (TV). The inner end
of this cell terminates in expanded branches in the
inner molecular layer (///). Facing these, but not in
actual continuity with them, lie the processes from the
outer end of one of the cells in the ganglionic-cell layer
(II) which, as before, is itself directly connected with a
fibre of the optic nerve.

In this way each rod and cone is brought into relation-
ship with a fibre of the optic nerve ; but the path of
connection in each case shows tivo breaks in its structured
continuity. In the case of a rod these breaks lie in the
outer molecular layer (F) and ganglionic layer (11) ; in the
caoe of a cone they lie in the outer and inner molecular
layers respectively {V and ///).

IX

THE RETINA

42S

a ^' "^ <<^ o* '^"

Fio. 138.— A Di AORAMMATio SBOTroif or THE Macula Lctea, or

Yellow Spot.
a a the pigment of the choroid ; b.c, rods and cones ; d.d, outer
granular or nuclear layer ; f.f, inner gi-anular or nuclear layer ; y.cj, mole-
cvdar layer ; h.h, layer of nerve cells ; i.i, fibres of the optic nerve.
(Magnified about 60 diameters.)

In addition to the bipolar cells {r.h.p. and c.h.p.) which
chiefly confer upon the inner nuclear layer {IV) the

424

ELEMENTARY PHYSIOLOGY

LESS.

characteristic appearance from which it derives its name,
other cells also occur in this layer. These are shown at h. c.
and s. ; but their relationships to the other structural
elements of the retina are so uncertain that we must
content ourselves with merely drawing attention to their
existence.

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 contx'ary, the cones are abun-
dant and close set, becoming at the same time longer and
more slender, while rods are scanty, and are found only

Fio. 139.— Pigmented Ehithelium of the Hi'man Retina. (Max
ScHULTZE.) Highly magnified.

a, cells seen from the outer (choroidal) surface ; 6, two cells seen side-
ways, with fine processes on their inner side ; c, a cell still in connection
with the layer of rods of the retina.

towards its margin. In the centre of the macula lutea
(Fig. 138) the layer of fibres of the optic nerve
disappears, and all the other layers, except that of the
cones, become extremely thin.

The outer ends of the rods and cones lie buried among
certain fine processes of those pigment cells, adjacent to
the choroid coat, to whose existence we have previously
alluded (pp. 401, 419). When seen from the surface by
which they are in contact with the choroid, these cells
present the appearance of small black hexagons arranged
in a sort of mosaic (Fig. 139, a.).

IX

RODS AND CONES

425

e.l.m.

Seen sideways each cell is found to be provided with
long, fine, hair-like processes, which stretch in towards
the retina and envelop the outer ends of the rods and
cones (Fig. 139, b.c).

The rods and cones each consist of two parts, an outer

limb and an inner
limb. In the rods the
outer limb is quite cylin-
drical or rod-shaped, and
is transversely striated.
The inner limb is about
as long as the outer, but
bulges out slightly in its
middle part, so that it
forms an elongated cylin-
der tapering at each end.
The outer part of each
inner limb is longitudi-
nally striated. The inner
end of each inner limb
is prolonged into a fine
filament, which carries a
conspicuous nucleus, often
marked transverselj', and
may be easily traced into
the outer molecular layer
(Fig. 140, R.).

The outer limb of a
cone is much shorter than
the outer limb of a rod,
and instead of being rod-
shaped, is conical and
tapering to its outer point ; this outer limb is
transversely striated. The inner limb of each cone is
thicker than the inner limb of a rod, but of the same
general shape. It is striated" longitudinally in its outer
part and carries a large nucleus from which a thick fibre

o.m.l

R. C

FlQ.

140. — Diagram of

OF A CONF..

Rod and

R, rod ; C, cone ; o, outer limb ; ;',
inner limb ; e.l.m, external limiting
membrane of retina ; o ni.l, outer
molecular layer of retina.

426 ELEMENTARY PHYSIOLOGY

passes straight into the outer molecular layer of the
retina (Fig. 140, ('.).

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 molecular 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 of light is possible even though
the retina and indeed the whole optic nerve be intact ;
blindness is 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 cochlea, are contrivances for
converting the delicate vil)rations of the perilymph and
endolymph into impulses which can excite the auditoi-y
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 difterent
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 BLIND SPOT 427

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 : whUe, in the second position,
it falls on the point of entrance of the optic nerve itself ;
and, in the thii'd, 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, 141.

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 experi-
ment proves that the vibrations 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 o^vn, however short the
time during which the light itself lasts. A flash of light-
ning is practically instantaneous, but the sensation of
light produced by that flash endures for an appreciable
period. It is found, in fact, that a luminous impression
lasts for about one-eighth of a second ; whence it follows,

428

ELEMENTARY PHYSIOLOGY

LESS.

that if any two luininoiis impressions 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 vei-y rapidly by the
hand, appears as a circle of fire ;
and the spokes of a coach wheel at
speed are not separately visible,
but only appear as a sort of opacity,
or film, within the tyre of the
wheel.

The cinematograph is based
upon this fact. A series of in-
stantaneous 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 consecutive
photographs are projected on to
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 succession 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

FUNCTION OF ROBS AND CONES 429

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 nnage, Av^nch
Lts as long as the 'pressure, and is called a phos-
phene. 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 m the
centre, with a bright ring at the circumference (or as
Newton described it, like the "eye" in a peacocks taU-
feather)-is seen ; and this image lasts as long as the
pressure is continued. i • j iu

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 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 aflFected 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. 442).

12 The Functions of the Rods and Cones.- i he
last paragraph raises a distinction between the - hbres of
the optic nerve "and the " retina " which may not have
been anticipated, but which is of much importance.

We have seen that the fibres of the optic nerve raniify
in the inner fourth of the thickness of the retina, while
the layer of rods and cones forms its outer fourth, ihe
liaht therefore, must fall first upon the fibres of the optic
nerve, and, only after traversing them, can it reach the
rods and cones. Consequently, if the fibrillar of the optic
nerve themselves are capable of being aftected by hght,
the rods and cones can only be some sort of supple-
mentary optical apparatus. But, in fact, it is the rods and
con«s which are aflected by light, while the fibres of the

430 ELEMENTARY PHYSIOLOGY

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
situated, 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 oblicjuely into the eye,
one of what are called Purkinje's figures is seen.
This is a vision of a series of diverging, branched, dark,
sometimes reddish, lines on an illuminated field, and in
the interspace of two of these lines is a sort of cup-shaped
disc. The branched lines are the images of shadows
thrown by the retinal blood-vessels, and the disc is that
of the shadow thrown by the edge of the yellow spot.
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 inner 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, or outer, side of
the vessels. But the fibres of the optic nerve lie among
the vessels, and the only retinal structures which lie out-
side them are the nuclear layers and the rods and cones.

(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.

The image of the retinal blood-vessels may be also very
readily seen by looking at a bright surface, such as the

COLOUR- VISION 431

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 uidth of the piqiil,
the retinal blood vessels are " seen " as a fine branched
network of blaok lines in the bright field of vision.

13. Sensations of Colour and Colour-blindness. —
^^ e have spoken of the eye so far simply as the instru-
ment by which luminous sensations arise when the retina
is stimulated ; as an instrument which enables us to
appreciate the 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 giving rise to the sensation, and
hence corresponds to the "pitch" of a sound, (ii) In-
tensity 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

432 ELEMENTARY PHYSIOLOGY less.

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 ])ass or to be reflected. Thus
a piece of glass is red if it allows the red rays to pass to
the eye and stops the others. Similarly the colour of an
oparpie red object is due to an absorption of the spectral
colours other than red by the superficial layer of the
object and the reflection of the unabsorbed red rays from
its internal parts.

When white light has been split up into its coloured
constituents 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 colours 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 physio-
^^gficrtiii/ by taking advantage of that2)ersistence of luminous
impressions to which we have already drawn attention
(p. 423). Thus if the several colours of the sj)ectrum 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 together and the disc appears
white. The instrument used in this mode of mixing
colours is called a '•^colour top" and the principle it
embodies will be not unfamiliar to most people in the
form of a common toy.

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, in the right proportions, 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

COLOUR-VISION 433

every colour has some other colour which is comple-
mentary to it. If instead of mixing the colours in
pairs we mix them in threes, then it Ijucomes 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 adjusting the
proportions of each on the disc of the colour-top we 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 produce 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 colours 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 may be regarded as the
outcome 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 forward ^ that there are in the visual apparatus three
kinds of nervous structure 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

1 This theory was first propounded by an Englishman, Dr. Thomas
Young, the originator of the uniulatory theory of light. In later times
it was adopted and amplified hiy Helmholtz, and is therefore known
as the Young-Helmholtz theory.

FF

434 ELEMENTARY PHYSIOLOGY

sensations of colour of which we are at anj- time
conscious.

Another theory has been started in whose support very
much may be said.' It resembles the previous one
inasmuch as it also assumes that all our sensations
of colour result from a limited number of primary sen-
sations. But it differs in the number and selection of
the primary colours. Thus according to thi.s theory there
are three pairs of primary sensations arising from
three pairs of primary colours : these are red-green,
yellow-blue, white-black. Further, this theory
assumes the existence in the visual apparatus of three
kinds of visual substance of which each corre-
sponds to one of the above pairs of colours, and
it assumes that these substances may be made, some-
what in the same way as the sensitive salts on a photo-
graphic plate, to undergo a destnictive change by the action
of light in such a way as to give rise to the sensations of
red, yellow and white. Finally it supposes that the other
three antagonistic sensations arise from the subsequent
constructive building up again of their visual substances
which must take place as soon as the luminous cause of
their destructive change has been removed. Thus the
breaking down of the red-green visual substance gives rise
to the sensation of red, and its building up again to the
sensation of green, and similarly for the other two pairs.

Both these theories attempt to account for observed
facts, and each goes a long way in doing so ; but neither
of them accounts completely for all the facts of colour
vision. Neither may we enter into any discussion
of their respective merits, for such discussion would be
useless if it were not lengthy and detailed, and its
intricacy Avould be still further out of place in an elemen-
tary work which excludes as far as possible all debatable
matter.

The excitability of the retina is readily exhausted.
1 This is the theory of Bering.

COLOUR-VISION 435

Thus, looking at a bright light rapidly renders the part o!'
the retina on which the liyht fall^, 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 becomes 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 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 composed. 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 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 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 people call green. Such colour-blind persons
are 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-

F F 2

436 ELEMENTARY PHYSIOLOGY less, ix

green " blindness are not uncommon ; but another form
of colour-blindness in which blue and yellow cannot be
distinguished from each other is much more rare ; and
still rarer, though of undoubted occurrence, are the cases
of those who are n-holly colour-blind, i.e. to whom all
colours are mere shades of one tint.

This peculiarity of colour-blindness is simply un-
fortunate for most people, but it may be dangerous if
unknowingly possessed by engine-drivers or sailors, parti-
cularly 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, yellow, 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. A more
satisfactory test than the matching of wools is furnished
by the use of coloured lights, and for a detailed in-
vestigation of the sensations of the colour-blind more exact
observations by help of the spectrum are necessary.

The phenomena of colour-blindness can, to a certain
extent at least, be explained according to either of the
theories of colour-vision which have been given above.
Thus by the Ycjung-Helmholtz theory a red-green colour-
blind person lacks either the red-perceiving or the green-
perceiving structures normally present either in the
retina or the visual sensorium. According to the theory
of Hering they lack the red-green visual substance.

LESSON X

THE COALESCENCE OF SENSATIONS WITH ONE •

ANOTHER AND WITH OTHER STATES OF

CONSCIOUSNESS

1 Sensations may be Simple or Composite.-In

explaining 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
produced by the irritation of a single nerve-fibre, or ot
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 ot
iudcrments. For example, in the preceding cases it is very
diffi'cult to separate the sensation of contact from the
iud-ment that something is touching us ; of sweetness,
from the idea of something 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
complicated by accessories of this sort. Thus, particles

438 ELEMENTARY PHYSIOLOGY less.

of musk diffuse themselves with great rapidity through
the nasal passages 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 concejjtion 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.

(6) Pure muscular sensations of two kinds, — tie one
arising from the resistance of the table, the other from the
actions of those muscles which draw the finger along.

(c) Ideas of the order in which these pure sensations
succeed one another.

(d) Comparisons of these sensations and their order,
with the recollection of like sensations similarly arranged,
which have been obtained on previous occasions.

(e) Recollections of the impressions of extension, flat-
ness, &c., made on the organ of vision when these previous
tactile and muscular sensations were obtained.

Thus, in this case, the only pure sensations are those
of contact and muscular action. The greater part of what
we call the sensation is a complex mass of pi'esent and
recollected sensations and judgments.

Should any doubt remain that we do thus mix up our
sensations with our judgments into one indistinguishable
whole, shut the eyes as before, and, instead of touching

TACTILK JUDGMENTS 439

bhe table with the finger, take a round lead pencil
between the fingers, and draw that along the table.
The "sensation" of a flat hard surface will be just as
clear as before ; and yet all that we touch is the round
surface of the pencil, and the only pure sensations we
owe to the table are those afforded by the muscular sense.
In fact, in this case, our " sensation " of a flat hard surface
is entirely a judgment based upon what the muscular
sense tells us is going on in certain muscles.

A still more striking case of the tenacity with which
we adhere to complex judgments, which we conceive to
be pure sensations, and are unable to analyse otherwise
than by a process of abstract reasoning, is afforded by our
sense of roundness.

Any one taking a marble between two fingers will say
that he feels it to be a single round body ; and he will
probably be as much at a loss to answer the question how
he knows that it is round, as he would be if he were asked
how he knows that a scent is a scent.

Nevertheless, this notion of the roundness of the
marble is really a very complex judgment, and that it is
so may be shown by a simple experiment. If the index
and middle fingers be crossed, and the marble placed
between them, so as to be in contact with both, it is
utterly impossible to avoid the belief that there are two
marbles instead of one. Even looking at the marble,
and seeing that there is only one, does not weaken
the apparent proof derived from touch that there are
two.i

The fact is, that our notions of singleness and round-
ness are, really, highly complex judgments based upon
a few simple sensations ; and when the ordinar}' conditions
of those judgments are reversed, the judgment is also
reversed.

1 A ludicrous form of this experiment is to apply the crossed fingers to
the end of the nose, when it at once appears double; and in spite of
the absurdity of the conviction the mind cannot expel it so long as the
•eusations last.

440 ELEMENTARY PHYSIOLOGY

With the index and the middle fingers in their ordinary
position, it is of course impossible that the outer sides
of each should touch opposite surfaces of one spheroidal
body. If, in the natuial and usual position of the fingers,
their outer surfaces simultaneously give us the impression
of a spheroid (which itself is a complex judgment), it is
in the nature of things that there must be two spheroids.
But, when the fingers are ci-ossed over the marble, the
outer side of each finger is really in contact with a
spheroid ; and the mind, taking no cognizance of the
crossing, judges in accordance with its universal experience,
that two sphei'oids, and not one give rise to the sensations
which are perceived.

2. Judgments are Delusive, not Sensations. —
Phenomena of the kind described in tlie preceding
section are not uncommonly called dehisiuns of the senses ;
but there is no such thing as a fictitious, or delusive,
sensation. A sensation must exist to be a sensation, and
if it exists, it is real and not delusive. But the judgments
we form respecting the causes and conditions of the
sensations of which we are aware, ai-e very often erroneous
and delusive enough ; and such judgments may be
brought about in the domain of every sense, either by
artificial combinations of sensations, or by the influence
of unusual conditions of the body itself. The latter give
rise to what are called subjective setisutio7is.

Mankind would be sul)ject to fewer delusions than they
are, if they constantly bore in mind their liability to false
judgments due to unusual combinations, either artificial
or natural, of true sensations. Men say, "Ifelt," "I
heard," "I saw" such and .such a thing, when, in ninety-
nine cases out of a hundred, what they really mean is,
that they judge that certain .sensations of touch, liearing,
or sight, of which they were conscious, were caused by
such and such things.

3. Subjective Sensations. — Among subjective sensations
within the domain of touch, are the feelings of creeping

X SUBJECTIVE SENSATIONS 441

and the prickling of the skin, which may sometimes be
due to certain states of the cii'culation, but probably more
frequently to processes going on in the central nervous
system. The subjective evil smells and bad tastes which
accompany some diseases are, in a similar way, very
probably due to disturbances in the brain in the central
end-organs of the nerves of smell and taste.

Many persons are liable to what may be called anditory
spectra — music of various degrees of complexity sounding
in their ears, without any external cause, while they are
wide awake. I know not if other persons are similarly
troubled, but in reading books written by persons with
whom I am acquainted, I am sometimes tormented by
hearing the words pronounced in the exact way in which
these persons would utter them, any trick or peculiarity
of voice, or gesture, being, also, very accurately repro-
duced. And I suppose that everyone must have been
startled, at times, by the extreme distinctness with
which his thoughts have embodied themselves in apparent
voices.

The most wonderful exemplifications of subjective sen-
sation, however, are aflForded by the organ of sight.

Any one who has witnessed the suiferings of a man
labouring under delirium tremens (a disease produced by
excessive drinking), from the marvellous distinctness of
his visions, which sometimes take the forms of devils,
sometimes of creeping animals, but almost always of
something fearful or loathsome, will not doubt the inten-
sity of subjective sensations in the domain of vision.

But in order that illusive visions of great distinctness
should appear, it is not necessary for the nervous system
to be thus obviously deranged. People in the full
possession of their faculties, and of high intelligence, may
be subject to such appearances, for which no distinct
cause can be assigned. An excellent illustration of this
is the famous case of ]\Irs. A. given by Sir David Brewster,
in his Xatuial Magic. This lady was subject to un-

442 ELEMENTARY PHYSIOLOGY less.

usually vivid auditory and ocular spectra. Thus on one
occasion she saw her husband standing before her and
looking fixedly at her with a serious expression, though
at the time he was at another place. On another occa-
sion she heard him repeatedly call her, though at the
time he was not anywhere near. On another occasion
she saw a cat in the room lying on the rug ; and so vivid
was the illusion that she had gieat difficulty in satisfying
herself that really there was no cat there. The whole
account is well worthy of perusal.

It is obvious that nothing but the singular courage and
clear intellect of Mrs. A. prevented her from becoming a
mine of ghost stories of the most excellently authenti-
cated kind. And the particular value of her history lies
in its showing, that tlie clearest testimony of the most
unimpeachable witness may be quite inconclusive as to
the objective reality of something which the witness has
seen.

Mrs. A. undoubtedly saw what she said she saw. The
evidence of her eyes as to the existence of the apparitions,
and of her ears to those of the voices, was, in itself, as
perfectly trustworthy as their evidence would have been
had the objects really existed. For there can be no doubt
that exactly those parts of her retina which would have
been affected by the image of a cat, and those parts of
her auditory organ which would have been set vibrating
by her husband's voice, or rather the portions of the
sensorium with which those organs of sense are connected,
were thrown into a corresponding state of activity by
some internal cause.

What the senses testify is neither more nor less than the
fact of their own affection. As to the cause of that affec-
tion they really say nothing, but leave the mind to form
its own judgment on the matter. A hasty or superstitious
person in Mrs. A.'s place would have formed a wrong
judgment, and w'ovild have stood by it on the plea that
*' she must believe her senses."

DELUSIONS 443

4. Delusions of Judgment.— The delusions of the
judgment, produced not by al)norm;il conditions of the
body, but by unusual or artificial combinations of sensa-
tions, or by suggestions of ideas, are exceedingly numer-
ous, and, occasionally, are not a little remarkable.

Some of those which arise out of the sensation of touch
have already been noted. We do not know of any produced
through smell or taste, but hearing is a fertile source of
such errors.

What is called ventriloquism (speaking from the
belly), and is not uncommonly ascribed to a mysterious
power of producing voice somewhere else than in the
larynx, depends entirely upon the accuracy with which
the perft)rmer can simulate sounds of a particular charac-
ter, and upon the skill with which he can suggest a belief
in the existence of the causes of these sounds. Thus, if
the ventriloquist desire to create the belief that a voice
issues from the bowels of the earth, he imitates with great
accuracy the tones of such a half-stifled voice, and
suggests the existence of some one uttering it by directing
his answers and gestures towards the ground. These
gestures and tones are such as would be pi'oduced by a
given cause ; and no other cause being apparent, the
mind of the bystander insensibly judges the suggested
cause to exist.

The delusions of the judgment through tlie sense of
sight — optical delusions, as they are called — are more
numerous than any others, because such a great number
of what we think to be simple visual sensations are really
very complex aggregates of visual sensations, tactile sen-
sations, judgments, and recollections of former sensations
and judgments.

It will be instructive to analyse some of these judg-
ments into their princi^iles, and to explain the delusions
by the application of these principles.

5. The Inversion of the Visual Image. — Wlwn we
look at an external object, the 'ima(je of the object falls on

444 ELEMKNTARY PHYSIOLOGY less.

the retina at the end of the visual axis, i.e. a line joining
the object and the retina and traversing a particular region
of the centre of the eye. Conversely, 'when a p trt of the
retina is excited, by whatever means, the sensation is
referred by the mind to some cause outside the body in the
direction of the visual axis.

When we look at an external object which is felt by the
touch to be in a given place, the image of the object falls
upon a certain part of the retina. Conversely, when a
part of the retina is excited, by whatever means, the sensa-
tion is referred by the mind to some cause outside the body
occupying such a position that its image would fall on that
part.

It is for this reason that when a phosphene is created
by pressure, say on the outer and lower side of the eye-
ball, the luminous image appears to lie above, and to the
inner side of, the eye. Any external object which could
produce the sense of liglit in the part of the retina pressed
upon nmst, owing to the inversion of, the retinal images
(see p. 408), in fact occupy this position ; and hence the
mind refers the light seen to an object in that position.

The same kind of explanation is applicable to the
apparent paradox that, while all the pictures of external
objects are certainly inverted on the retina by the refract-
ing media of the eye, we nevertheless see them upright.
It is difficult to understand this, until one reflects that
the retina has, in itself, no means of indicating to the
mind which of its parts lies at the top, and which at the
bottom ; and that the mind learns to call an impression
on the retina high or low, right or left, simply on account
of the association of such an impression with certain
coincident tactile impressions. In other words, when one
part of the retina is affected, the object causing the affec-
tion is found to be near the right hand ; when anotbei,
the left ; when another, the hand has to be raised to
reach the object ; when yet another, it has to be depressed
to reach it. And thus the several impressions on the

VISUAL JUDGMENTS 445

retina are called right, left, upper, lower, quite irrespec-
tively of their real positions, of which the mind has, and
can have, no cognizance.

6. Single Objects give rise to Single Images.— TF/ien
on external body is ascertained by touch to be simple, it
forms but one image on the retina of a single eye ; and when
two or more images fall on the retina of a single eye, they
ordinarily proceed from a corresponding number of bodies
which are distinct to the touch.

Conversely, the sensation of tivo or more images is judged
by the mind to proceed from two or more objects.

If two pin-holes be made in a piece of cardboard at a
distance less than the diameter of the pupil, and a small
object like the head of a pin be held pretty close to the
eye, and viewed through these holes, two images of the
head of the pin will be seen. The reason of this is, that
the rays of light from the head of the pin are split by the
card into two minute pencils, which pass into the eye on
either side of its centre, and, on account of the nearness
of the pin to the eye, meet the retina before they can be
united again and brought to one focus. Hence they fall
on different parts of the retina, and each pencil of rays
being very small, makes a tolerably distinct image of its
own of the pin's head on the retina. Each of these
images is now referred outward (p. 444) and two pins are
apparently seen instead of one. A like explanation
applies to multiplying glasses and doubly refracting crystals,
both of which, in their own ways, split the pencils of
light proceeding from a single object into two or more
separate bundles. These give rise to as many images,
each of which is referred by the mind to a distinct
external object.

7. The Judgment of Distance and Size by the
Brightness and Size of Visual Images.— Ccrtoi?i visual
phenomma ordinarily accompany those products of tactile
sensation to which we give the name of size, distance, and
form. Thus, other things being alike, the space of the retina

446 ELEMENTARY PHYSIOLOGY less

covered by the image of a large object is larger than that
covered bij a sirudl object; while that covered by an object
when near is larger than that covered by the same object
when distant; and, other conditions being alike, a near
object is more brilliant than a distant one. Furthermore,
the shadoius of objects differ according to the forms of their
surfaces, as determined by touch.

Conversely, if these visual sensations can be produced,
they inevitably suggest a belief in the existence of objects
competent to produce the corresponding tactile sensations.

What is called perspective, whether solid or aerial in
drawing, or painting, depends on the application of these
principles. It is a kind of visual ventriloquism — the
painter putting upon his canvas all the conditions requisite
for the production of images on the retina, having the size,
relative form, and intensity of colour of those which would
actually be produced by the objects themselves in nature.
And the success of his picture, as an imitation, depends
upon the closeness of the resemblance between the images
it produces on the retina, and those which would be pro-
duced by the objects represented.

To most per.sons the image of a pin, at three or four
inches from the eye, ap[)ears blurred and indistinct —
the eye not being cai)able of adjustment to so short a
focus. If a small hole be made in a piece of card, the
circumferential raj's which cause the blur are cut off, and
the image becomes distinct. But at the same time it is
magnified, or looks bigger, because the image of the pin,
in spite of the loss of the circimiferential rays, occupies a
much larger extent of the retina when close than when
distant. All convex glasses produce the same effect —
while concave lenses diminish the apparent size of an
object, because they diminish the size of its image on the
retina.

Objects, as is well known, appear larger when seen in a
fog. In this case the actual size of the image on the
retina is the same as if there were no fog. But the indis-

X VISUAL JUU<;MENTS 447

tinctness with which the object is setsn leads to the wrong
conchision that it is situated at some considerable distance
from the observer. Hence the judgment is formed that
the object is large, because if it were not large it could
not, at the apparently greater distance, produce an miage
on the retina of the size it does.

The moon, or the sun, when near the horizon appears
very much larger than when it is high in the sky. This
is usually said to be due to the fact that when in the
latter position we have nothing to compare it with, and
the small extent of the retina which its image occupies
suggests small absolute size. But as it sets, we see it
passing behind great trees and buildings which we know
to be very large and very distant, and yet it occupies a
larger space on the retina than they do. Hence the
vague suggestion of its larger size. But this has really
very little to do with the delusion, for the appearance is
the same if the sun or moon is seen near the horizon
over the open sea, where no comparison with other
objects is possible. Probably one cause of the delusion
is that when low down the sun or moon is seen less dis-
tinctly, on account of mist. and vapour, and thus ''looks"
large for the same reason that a man seen in a fog appears
unduly big, or the delusion may be due to the fact that
to most people the distance from them to the horizon
appears greater than the distance straight above them
to the summit of the vault of the heavens (or the zenith).
Hence though the actual size of the image of the sun
or moon on the retina is the same whether they be low
down or high up, the idea that they are further off when
low down suggests that they are of greater size.

8. Judgment of Form by Shadows.— If a convex
surface be lighted from one side, the side towards the
light is bright — that turned from the light, dark, or in
shadow ; while a concavity is shaded on the side towards
the light, bright on the opposite side.

If a new half-crown, or a medal with a well-raised head

448 ELPLMENTARY PHYSIOLOOV

upon its face, be lighted sideways by a candle, we at once
know the head to be raised (or a cameo) by the disposition
of the light and shade ; and if an intaglio, or medal on
which the head is hollowed out, be lighted in the same
way, its nature is as readily judged by the eye.

But now, if either of the objects thus lighted be viewed
with a convex lens, which inverts its position, the light
and dark sides will be reversed. With the reversal the
judgment of the mind will change, so that the cameo will
be regarded as an intaglio, and the intaglio as a cameo ;
for the light still comes from where it did, but the cameo
appears to have the shadows of an intaglio, and vice versa.
So completely, however, is this interpretation of the facts
a matter of judgment, that if a pin be stuck beside the
medal so as to throw a shadow, the pin and its shadow,
being revei'sed by the lens, will suggest that the direction
of the light is also reversed, and the medals will seem to
be what they really are.

9. The Judgment of Changes of Torm..— Whenever
an external object is ivatched rapidly changing its form, a
continuous series of different pictures of the object is im-
pressed upon the same spot of the. retina.

Conversely, if a continuous series of different pictures of
one object is impressed upon one part of the retina, the mind
judges that they are due to a single external object, under-
going changes of form.

This is the principle of the curious toy called the thau-
matrope, or " zootrope," or " wheel of life," by the help
of which, on looking through a hole, one sees images of
jugglers throwing up and catching balls, or boys playing
at leapfrog over one another's backs. This is managed
by painting at intervals, on a disc of card, figures and
jugglers in the attitudes of throwing, waiting to catch, and
catching ; or boys " giving a back," leaping, and coming
into position after leaping. The disc is then made to
rotate before an opening, so that each image shall be
presented for an instant, and follow its predecessor before

SINGLE VISION WITH TWO EYES 449

the impression of the latter has died away. The result
is that the succession of different pictures irresistibly
suggests one or more objects undergoing successive
changes — the juggler seems to throw the balls, and the
boys appear to jump over one another's backs The same
explanation holds good for the cinematograph. (See
p. 428).

10. Single Vision with Two Eyes. Corresponding
Points. — When an external object is asceiiained by touch
to be single, the centres of its retinal images in the two eyes
fall upon the centres of the yellow spots of the tico eyes, when
both eyes are directed toicards it ; but if there be two external
objects, the centres of both their images cannot fall, at the
same time, upon the centres of the yelloic spots.

Conversely, when the centres of tioo images, formed
simidtaneously in the two eyes, fall upon the centres of the
yellow spots, the mind judges the images to be caused by a
single external object ; but if not, by two.

This seems to be the only adn>issible explanation of the
facts, that an object which appears single to the touch
and when viewed with one eye, also appears single when
it is viewed with both eyes, though two images of it are
necessarily formed ; and on the other hand, that when
the centres of the two images of one object do not fall on
the centres of the yellow spots, both images are seen
separately, and we have double vision. In squinting, the
axes of the two eyes do not converge equally towards the
object viewed. In consequence of this, when the centre
of the image formed by one eye falls on the centre of the
yellow spot, the corresponding part of that formed by the
other eye does not, and double vision is the result.

For simplicity's sake we have supposed the images to
fall on the centre of the yellow spot. But though vision
is distinct only in the yellow spot, it is not absolutely
limited to it ; and it is quite possible for an object to be
seen as a single object with two eyes, though its images
fall on the two retinas outside the yellow spots. AU that

G G

450 ELEMENTARY PHYSIOLOGY less.

is necessary is that the two spots of the retinas on which
the images fall should be similarly disposed towards the
centres of their respective yellow spots. Any two points of
the two retinas thus similarly disposed towards their re-
spective yellow spots (or more exactly to the points in which
the visual axes end), are spoken of as corresponding
points ; and any two images covering two corresponding
areas are conceived of as coming from a single object. It
is obvious that the inner (or nasal) side of one retina
corresponds to the outer (or cheek) side of the other.

11. The Judgment of Solidity. — ir/ien, a body of
moderate size, ascertained by touch to be solid, is viewed loith
both eyes, the images of it, formed by the two eyes, are
necessarily different (one shotciny more of its right side, the
other of its left side). Nevertheless, they coalesce into a
common image, which gives the impression of solidity.

Conversely, if the two images of the right and left
aspects of a solid body be viade to fall upon the retinas of
the two eyes in such a way as to coalesce into a common
imxtge, they are judged by the mind to proceed from the
single solid body xvhich alone, under ordinary circumstances,
is competent to produce them.

The stereoscope is constructed upon this princij^le.
Whatever its form, it is so contrived as to throw the images
of two pictures of a solid body, such as would be
obtained by the right and left eye of a spectator, on to
such parts of the retinas of the person who uses the
stereoscope as would receive these images, if they really
proceeded from one solid body. The mind nnmediately
judges them to arise from a single external solid body, and
sees such a solid body in place of the two pictures.

The operation of the mind upon the sensations pi'esented
to it by the two eyes is exactly comparable to that which
takes place when, on holding a marble between the finger
and thumb, we at once declare it to be a single sphere
(p. 439). That which is absolutely presented to the mind by
the sense of touch in this case is by no means the sensa-

JUDil.MKXT OF SOLIDITY 451

tion of one spheroidal body, but two distinct sensations of
two convex surfaces. That these two distinct convexities
belong to one sphere, is an act of judgment, or process of
unconscious reasoning, based upon many particulars of
past and present experience, of which we have, at the
moment, no distinct consciousness.

G G 2

LESSON XI
THE NERVOUS SYSTEM AND INNERVATION

1. The General Arrangement of the Nervous
System. — Tho sensory organs are, as we have seen, the
channels through which pai'ticular physical regents are
enabled to excite the sensory nerves with which these
organs are connected ; and the activity of these nerves is
evidenced by that of the central organ of the nervous
system, which activity becomes manifest as a state of
consciousness — the sensation.

We have also seen that the muscles are instruments by
which a motor nerve, excited by the central organ with
which it is connected, is able to produce motion.

The sensory nerves, the motor nerves, and the central
organ, constitute the greater part of the nervous system,
which, with its function of innervatinn, we nmst now study
somewhat more closely, and as a whole.

The nervous apparatus consists of two sets of nerves
and nerve-centres, which are intimately connected together
and yet may be conveniently studied apart. These are the
cerebro-spinal system and the sympathetic system.
The former, or central nervous system, consists of the
brain (see Fig. 1), including with this the spinal bulb
or medulla oblongata, and spinal cord and the
cranial and spinal nerves, which are connected with
this axis. The latter comprises the chain of sym-
pathetic ganglia, the nerves which they give ofl', and
the various cords by which they are connected with one

LESS. XI COVERINGS OF C. N SYSTEM 453

another and with the cerebro-spinal nerves. (See
Fig. 142.)

Nerves are made up entirely of nerve-flbres, the
structure of which is somewhat different in the cerebro-
spinal and in the sympathetic systems. (See p. 459 )
Nerve centres, on the other hand, are composed of
nerve-cells mingled with nerve-fibres. (See p. 466.)
Such nerve-cells are found in various parts of the brain and
spinal cord, in the sympathetic ganglia, and also in the
ganglia belonging to spinal nerves as well as in certain
sensory organs, such as the retina and the internal ear.

2, The Investing Membranes of the Cerebro-Spinal
System. — The brain and spinal cord lie in the cavity of
the skull and spinal column, the bony walls of which
cavity are lined by a very tough fibrous membrane,
serving as the periosteum of the component bones of this
region, and called the dura mater. This is composed of a
thick layer of densely interwoven fibres of connective
tissue, with which a small amount of elastic tissue is mixed.
The brain and spinal cord themselves are closely invested
by a very vascrdar membrane of fibrous connective tissue,
called pia mater. The numerous blood vessels supplying
these organs run for some distance in the pia mater, and
where they pass into the substance of the brain or cord,
the fibrous tissue of the pia mater accompanies them to a
greater or less depth.

Between the pia mater, and the dura mater, lies another
delicate membrane, called the arachnoid membrane.
These three membranes are connected with each other at
various points, and the arachnoid, which is not only very
delicate, but also less regular than the other two, divides
the space between the dura and pia mater into two spaces,
each containing fluid, and each more or less lined by a
delicate epithelium. The space between the dura mater
and the arachnoid, often called the subdural space, is
nowhere very large ; but the space between the arachnoid
and the pia mater, often called the subarachnoid

454 ELEMENTARY PHYSIOLOGY less, xi

space, though small and insignificant in the region of
the brain, becomes large in the region of the spinal cord,
and here contains a considerable quantity of fluid, called
cerebro-spinal fluid. This fluid has the aj)pearance
of ordinary lym])h, but there the resemblance ends ; for
cerebro-sprnal fluid contains only a minute amount of
proteids (globulins), does not clot as true lymph does, and
contains a peculiar reducing substance, which is, however,
not a sugar.

3. The Arrangement and General Structure of the
Spinal Cord and the Roots of the Spinal Nerves. —The
spinal cord (Fig. 142) is a column of greyish-white soft
substance, extending from the top of the spinal canal,
where it is continuous by means of the spinal bulb with
the brain, to about the second lumbar vertebra, where it
tapers off" into a filament. Starting at the level of the
junction of the atlas vertebra with the skull, the spinal
cord gives off laterally thirty-one pairs of spinal nerves
whose trunks pass out of the spinal canal by a])ertures
between the vertebra;, called the intervertebral
foramina, and then divide and subdivide, their ultimate
branches going for the most part to the muscles and to
the skin. Each nerve originates from the cord by two
roots, consequently there are twice as many roots as there
are spinal nerves (Fig. 144). After their exit from the
s[)inal canal the spinal nerves become connected with a
cliain of ganglia which lies parallel to the spinal cord and
constitutes the sympathetic nervous system (Fig. 142),
which will be described later on.

Transverse sections of the cord show thac a deep,
somewhat broad, fissure, the anterior fissure (Fig. 143,
1), divides it in the middle line in front, nearly down to
its centre : and a similar deeper but narrower cleft, the
posterior fissure (Fig. 143, 2), also extends nearly to
its centre in the middle line behind. The pia mater
extends more or less into each of these fissures, and
supports the vessels which supply the cord with blood.

S'lo. 142. — Diagram, as sef.x fp.om the Front, of the jSpinal Cord,
THE Spinal Nerves, and on one side of the Sympathetic Chain of
Ganglia. (After Allen Thomson.)

The numbers of the thirty-one spinal nerves are shown on the right

456 ELEMENTARY PHYSIOLOGY less.

In consequence of the presence of these fissures, only a
narrow bridge of the substance of the cord connects its
two halves, and this bridge is traversed throughout its
entire length by a minute canal, the central canal of
the cord (Fig. 143, 3).

The lines of attachment of the roots of the spinal nerves
divide the cord longitudinally into three parts, called
respectively the anterior, lateral and posterior columns
(Fig. 143, 8, 7, 6), those roots which arise along the line
which is nearer to the anterior surface of the cord being
known as the anterior roots ; those which arise along
the other line are the posterior roots (Figs. 143 and
144). A certain number of anterior and posterior roots,
on the same level on each side of the cord, converge and
form anterior and posterior bundles, and then the two
bundles, anterior and posterior, coalesce into the trunk
of a spinal nerve ; but before doing so, the posterior
bundle presents an enlargement — the ganglion of the
posterior root (Fig. 144, Gn.).

A transverse section of the sjjinal cord (Fig. 144, B,
and Fig. 143), shows further that each half consists
of two substances — a white matter on the outside,
and a greyish-red substance in the interior. A.nd
this grey matter, as it is called, is so disposed
that, in a transverse section, it looks, in each half,
something like a crescent, with one end bigger than
the other, and with the concave side turned outwards.
The two ends of each crescent are called its horns or

half of the figure. C, 1-8, cervical; D, 1-12, thoracic (dorsal); L, 1-5,
lumbar; S, 1-6, sacral; Br, brachial plexus; Sc, great sciatic nerve;
X, terminal filament of spinal cord.

a, superior, 6, middle, c. inferior cervical ganglion of the sympathetic
system ; of these c is fused with d the first thoracic (dorsal) ganglion.
In some animals (dog and cat) the ganglion corresponding tf> c, the
inferior cervical ganglion of man, is fused with the upper three thoracic
ganglia into a common ganglion called tlie " stellate" ganglion (see Figs.
22, 23, St. 6.) and the ganglion corresponding to h, the middle cervical
ganglion of man, is in this case known as the inferior cervical ganglion.

d', the eleventh thoracic sympathetic ganglion ; I, the first lumbal
ganglion. The ganglia below ss are the sacral ganglia.

STRUCTURE OF SPINAL CORD

4o:

cornua, the one directed forwards being the anterior
COrnu (Fig. 143, e e) ; the one turned' backwards the
posterior cornu (Fig. 143, a a). The convex sides of

Fio. 143. — Transverse Section of one-half of the Spinal Cord (in
THE Lumbar Region), magnified.

1, anterior fissure; 2. posterior fissure; 3, central canal; 4, and 5,
bridges connecting the two halves (posterior and anterior commissures) ;
G, posterior column ; 7, lateral column ; 8, anterior column ; 9, posterior
root ; 10, anterior root of nerve.

a a, posterior horn of grey matter ; c c e, anterior horn of grey matter.
Through the several columns I3, 7, and 8, each comjiosed of white matter,
are seen the prolongations of the pia mater, which carry blood-vessels
into the cord from the outside. The pia mater itself is seen over the
whole of the cord.

458

ELEMENTARY PHYSIOLOGY

the crescents of the grey matter approach one another, and
are joined by the bridge wliich contains the central canal.
The portion of this bridge which lies immediately behind
the central canal is known as the posterior grey
commissure ; that portion which lies in front of it as
the anterior grey commissure. The latter is
separated from the inner end of the anterior fissure by a
thin bridge of white matter known as the anterior
white commissure. (See Figs. 143 and 150. )

There is a fundamental difference in structure between

'L J?Je.

ClLj'^^^. ^ '"^''

FiQ. 144.— The Spinal Cord,

A. A front view of a portion of the cord. On the right side, the
anterior roots, A.R., are entire ; on the left side they are cut, to show the
posterior roots, P.li.

B. A transverse section of the cord. A, the anterior fissure; P, the
posterior fissure ; O, the central canal ; C, the grey matter ; W, the white
matter : A.R. the anterior root, P.K, the posterior root, Gn. the ganglion,
and 7', the trunk, of a spinal nerve.

the grey and the white matter. The white matter consists
almost entirely of nerve-fibres supported in a delicate
framework of connective tissue, and accompanied by
blood-vessels. Most of these fibres run lengthways in the
cord, and consequently, in a transverse section, the white
matter is really composed of a multitude of the cut ends
of these fibres.

The grey matter, on the other hand, contains in addi-
tion a number of nerve cells, some of them of considerable
size. These cells are wholly, or almost wholly, absent in
the white matter.

MEDULLATED NERVE-FIBRES

459

Many of the nerve-fibres of which the anterior roots
are composed may be tiaced into the anterior cornu, and,
indeed, into the nerve-cells lying in the cornu, while
those of the posterior roots, for the most part, pass into
this posterior column of white matter (Fig. 143, 6).

4. The Minute Structure of MeduUated Nerves.—
The white matter of the spinal cord consists chiefly of
nerve-fibres ; we may therefore, with advantage, consider

Fio. 145.— Transverse Section of a Medium-sized Medullated Nerve.

ep, ep, general connective tissue sheath or epineurium ; /, /, /, bundles
of nerve-fibres bound together by the perineurium per, per, per ; A,A,V,
blood-vessels ; L, lymphatic vessel.

the structure of these nerve-fibres before dealing in detail
with the minute structure of the cord itself.

If a small piece of a nerve, which may be easily obtained
from the leg of a freshly killed frog or rabbit, be teased
out with needles on a glass slide and examined under a
microscope it is seen to be made up chiefly of minute
fibres. When the nerve has been suitably hardened it
becomes possible to cut a transverse section of it ; if this
section be similarly examined the cut ends of the fibres

460 ELEMENTARY PHYSIOLOGY

may be readily seen as little circular dots arranged
in groups whicli compose the larger part of the
section. These fibres are bound together in bundles,
which are rounded as seen in section, by an external
sheath or case of connective tissue called the peri-
neurium, from whose imier surface verj' delicate layers
of connective tissue pass in between the fibres of which
each bundle is composed. The several bundles are
themselves bound together by connective tissue to form
the trunk of the nerve, and the whole nerve, thus built
up of bundles of nerve-fibres, is surrounded and held
together by an external layer of connective tissue.

The nerve-fibres, which are the essential elements of
the nerve, vary in diameter from 2/n to 12/x or more. In the
living state they are very soft cylindrical rods of a glassy,
rather strongly refracting aspect. No limiting membrane
is distinguishable from the rest of the substance of the
rod, but running through the centre of it a band of some-
what less transparency than the rest may be discerned.
At intervals, the length of which varies, but is always
many times greater than the thickness of the rod, the
nerve fil)re presents sharp constrictions, which are termed
nodes (Fig. 146, A, n ; B. n it). Somewhere in the inter-
space between every two nodes, very careful examination
will reveal the existence of a nucleus (Fig. 146, B. ?i, c),
invested by more or less protoplasmic substance and
lying in the substance of the rod, but close to the
surface.

As the fibre dies, and especially if it is treated with
certain re-agents, these appearances rapidly change.
1. The outermost layer of the fibre becomes recognisable
as a definite membrane, the neurilerama ^ (the so-called

1 This word was formerly used to denote the whole nerve-case, now
called perineurium ; but its similarity to the word sarcolemvia led to
great confusion in the minds of students. ' It is undoubtedly a whole-
some rule never to use an old word in a new sense ; but the striking
similarity between the two words "neurilemma" and " sarcolemma,"
and between the nerve-fibre sheath and the muscle-fibre sheath, seems an
adequate excuse for an exception to the rule.

XI MEDULLATED NERVE-FIBRES 461

"primitive sheath" or "sheath of Schwann"). 2. The
central band becomes more opaque, and sometimes ap-
pears marked with fine longitudinal striae as if it were
composed of extremely fine fibrillse ; it is the neuraxon
or axis-cylinder. 3. Where the axis-cylinder traverses
one of the nodes the neurilemma is seen to embrace
it closely, but in the intervals between the nodes a
curdy-looking matter, which looks white by reflected light,
occupies the space between the neurilemma and the
axis-cylinder. This is the medulla (the "white
substance of Schwann ") largely composed of a complex
fatty substance often spoken of as myelin. If the
neurilemma of a fresh fibre is torn, the myelin
flows out and forms irregular lumps as if it were
viscous. The medulla sometimes breaks, by oblique
lines (Fig. 146, C. m'), extending from the axis-cylinder
to the neurilemma, into segments, the faces of which
are obliquely truncated and fit closely against one
another. These may be seen even in quite fresh and
living nerve-fibres. 4. The internodal nucleus is more
sharply defined ; and it will be seen to be attached to the
inner surface of the neurilemma.

The essential part of each fibre, regarded as an
instrument for the transmission of that molecular dis-
turbance which is spoken of as a "nervous impulse," is
the axis-cylinder. This is suggested by the fact that
the axis-cylinder alone is apparently continuous through-
out each fibre, since it passes across each node ; but
all doubt is removed when we find that the axis-cylinder
alone provides the actual connection between the central
nervous system and the distant structures to or from
which the motor (efferent) or sensory (afferent) nerves
run. Thus, if we follow along the course of a motor
nerve, proceeding to its muscle, we find that it enters the
perimysium (with which the superficial layer of the peri-
neurium becomescontinuous), and divides in the perimysial
septa into smaller and smaller branches, each of which

462

ELEMENTARY PHYSIOLOOY

LESS.

FlO. 146.— To ILLUSTRATE THE STRUCTURE OF NeRVE-FIBRES.

A. A nerve-fibre seen without the use of reagents, showing the "double
contour" due to the medulla, andn, a node. Neither axis-cylinder nor
neurilemma can be distinctly seen. (Magnified about 300 diameters.)

B. A thin nerve-fibre treated with osniic acid, showing, nc, nucleus
with protoplasm, p, surrounding it, beneath the neurilemma ; n n, the
two nodes marking out the segment to which the nucleus belong.s,
(Magnified 400 diameters.)

C. Portion of fibre (thicker than B), treated with osmic acid to show
the node n; m, the densely stained medulla; at m' the medulla is seen
divided into segments. (Magnified 350 diameters.)

I). Portion of nerve-fibre treated to show the passage of the axis-
cy'inder, nx, through the node, n ; m, the medulla. At nx' the axis-
cylinder is swollen by the reagents employed and large and irregular.
(Magnified 300 diameters.)

E. Portion of nerve-fibre treated with osmic acid, showing the nucleus,
nc, embedded in the medulla; c, fine '.:erineurial sheath lying outside
the neurilemma ; the outline of the latter can only be recognised over
the nucleus nc ; the nucleus, nc', belongs to this perineurial sheath.
(Magnified 400 diameters.)

F. Portion of nerve-fibre deprived of its neurilemma and showing the
medulla broken up into separate fragments, m m, surrounding the axis-
cylinder, nx.

XI MOTOR NERVE-ENDINGS 463

contains the continuation of a certain number of the fibres
of the nerve trunk, bound up into a bundle by themselves.
In these larger ramifications of the nerve trunk there is
no branching of the nerve fibres themselves (at any rate
as a rule), but merely a separation of the fibres of
the compound nerve bundles. In the finer branches,
however, the nerve fibres themselves may divide ; the
division, which always takes place at a )>ode, is generally
dichotomous — that is, one fibre divides into two, each of
these again into two, and so on. An ultimate branch
consisting of one or two nerve fibres, or of one only, with
a very delicate connective tissue envelope (Fig. 146,
E c), passes to some single muscle fibre, and each nerve
fibre applies itself to the outer surface of the sarcolemma.
At this point, if it has not done so before, the medulla
disappears, the neurilemma becomes continuous with
the sarcolemma, and the axis-cylinder ending abruptly is
applied to a disc of protoplasmic substance containing
many nuclei, thus forming what is called a motor
end-organ or end-plate,' which is interposed be-
tween the striated nmscle substance and the sarco-
lemma at this point. Before ending the axis-cylinder
divides and its divisions anastomose freely, but the exact
relations of the various parts of the end-plate to the
muscle-substance have not yet been clearly made out.
The whole appears, however, to constitute an apparatus by
which the molecular disturbances of the substance of the
axis-cylinder (the essential part of the nerve) may be
efficiently propagated to the substance of the muscle.

If, instead of following the motor nerve to its distribu-
tion in the muscle, we trace it the other way, towards
the spinal cord, we shall find no alteration of any moment
until we arrive at the point at which the anterior root
enters the cord. From the finest branches of the motor
nerve (in which, as has been stated, the nerve-fibres

1 This is tlie arr.an£remei;t in most vertehnited animals. In the frog the
axifl-cylinder branches out without entering a distinct motor end-pla^e.

464 ELEMENTARY PHYSIOLOGY less.

themselves divide) to this point of entry each nerve-fibre ex-
tends ensheathed as 07ie continnons imdivided axis-ajlindcr
in a long succession of internodal segments. At the point
of entry into the cord the perineurium passes into the pia
mater and the general connective tissue framework of the
cord. The neurilemma and the nodes disappear. Often
the axis-cylinder can be traced towards the anterior horn of
the grey matter, invested only by a sheath of medidla
which gradually becomes thinner and thinner until at
length it disappears, and the fibre, thus reduced,
passes into one of the processes of one of the
large nerve cells, which lie in the anterior cornu of the
grey matter (see p. 467).

The axis-cylinder of a motor nerve-fibre, therefore, is in
fact an extremely fine and long process of a nerve cell,
which passes at its peripheral end into one or more muscle
fibres ; in other words, the nerve cell and the muscle cells
are the central and peripheral end-organs of the nerve-fibre.

With one or two exceptions, sensory (afferent) nerve-
fibres are not distinguishable by any structural character
from motor nerve-fibres. Wherever special-sense or-
ganules (p. 345) exist, the sensory fibres are connected
with them by means of their axis-cylinder from which the
neurilemma and medulla have di.sappeared. 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 po.sterior 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 provides 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 protec-
tion and nourishment of each successive length of the
essentially important axis-cylinder.

STRUCTURE OF SPINAL CORD 465

5. The Minute Structure of the Spinal Cord and
Spinal Ganglia. — The spinal cord consists, as already
described, of a central canal surrounded by grey matter
composed largely of nerve-cells, and arranged in two
crescent-shaped masses. The grey matter is surrounded
by white matter, consisting chiefly of medullated nerve-
fibres, and the whole is invested by the pia mater com-
posed of connective tissue.

The pia mater carries the blood-vessels and lymphatics
which supply the substance of the cord ; it dips down
into and completely fills the narrow so-called posterior
fissure, and similarly lines the wider cavity of the anterior
fissure. At frequent intervals, all over the surface of the
cord, and in the fissures, the pia mater sends conspicuous
prolongations (see Fig. 143) into the substance of the
white matter, forming partitions or septa which run on
the whole towards the grey matter, and thus carry the
blood-vessels into the cord. These larger primary septa
give off" fine secondary septa, wliich still further subdivide
the white matter, and provide for the more intimate dis-
tribution of minute blood-vessels throughout its substance.
The inner ends of the septa are continued on into the grey
matter for purposes similar to those which they subserve
in the white matter.

The spaces in the white matter between the septa
derived from the pia mater are filled by (i) medullated
nerve-fibres, whose structure has been previously described,
which run on the whole lengthwise or parallel to the long
axis of £he cord, and are supported by (ii) a fine felt-work
of extremely delicate fibres, which constitutes what is known
as the neuroglia, (i'fi}poi' = nerve, and 7Xta = glue) since
it binds the nerve-fibres together. The fibres of the
neuroglia are, in reality, processes from numberless
minute cells, in which the body of each cell is extremely
small, and the processes unusually numerous ; these cells
are known as neuroglia-cells (Fig. 147)-

The processes of the neuroglia-cells are wrapped closely

H H

466 ELEMENTARY PHYSIOLOGY

round the nerve-fibres, and since they thus form a support
and a covering for the fibres^ the latter no longer need
their natural external covering ; or, in other words, the
nerve-fibres of the white matter possess no neurilemma.

As in the white matter of the cord, so also in the grey
matter, neuroglia occupies the spaces between the septa
derived from the pia mater, and forms the supporting
basis for the nervous constituents of the grey matter.
The neuroglia is gathered into a specially well-marked
layer immediately surrounding the central canal of the
cord (Fig. 150, c.g.s.), and also, in a modified form, into a

Fio. 147. — A Neitroolia-Cell from the White Matter of the Spinal
Coed. (Schafer.)

The body and processes of the cell appear black, since they were
deeply stained in order to bring out their details.

conspicuous somewhat transparent mass at the outer end
of the posterior horn of the grey matter, where it is
knowai as the substantia gelatinosa of Rolando.
(See Fig. 150, sg.).

The most striking feature of the grey matter is the
presence in its neuroglia of nerve cells, many of which
are very large and conspicuous, while others are smaller,
but still very evident ; the presence of these cells and
of a closely interwoven network of non-medullated
nerve-fibres, together with the covipar/itive absence of
niedullated nerve-fibres, form the chief contrast between
the structure of the grey and the white matter of the
sj)inal cord.

NERVE-CELLS OF THE SPINAL CORD

467

The Cells of tbe Grey Matter. — These cells are not
scattered uniformly throutrhout the grej' matter, but
arranged rather in groups. The largest cells occur in the
outer end of the anterior horn (see Figs. 143 and 150),
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.

Fio. 148. — A Larok Nekve Cell from the Anterior Horn or the
Spin.^l Cord.

n. Nucleus : »', small body, called the nucleolus, inside the nucle\is ;
p, branched processes or dendrites ; n.p, unbranched axis-cylinder
procesa or axon continued into the neuraxis of a motor nerve fibre.

The body of each cell is large (varying in diameter from
50/x to lOOfx ; 5^0 to 2 50 of an inch), and contains a very
conspicuous nucleus (Fig. 148). The cell-body is pro-
longed into a varying number (usually many) of processes
called dendrites dividing and subdividing into branches,
which may b^ traced to some distance from the cell, be-
coming finer and finer, and finally ending abruptly.
Besides these branching processes the cell bears one
process, the axis-cylinder process, which does not divide
in this way, passes straight away from the cell and is
soon covered by a laj'er of myelin or a medulla ; after
its exit from the cord, it acquii'es additionally a neurilemma

H H 2

468

ELEMENTARY PHYSIOLOGY

or primitive slieath. In this way this process becomes
t!ie axis-cylinder of a medullated nerve-fibre, and is
continuous to the organ, usually a muscle, to which it is
distributed.

The fact is, the distinction between the so-called nerve-
cell and the nerve-fibre is a wholly artificial one ; these
two, together with the nerve-ending, or a portion of it,
being all constituent parts of one cell, which is the real
unit of nervous architecture. This unit is termed the
NEURON. Every nerve-fibie then, whether within or
without the central nervous system, whether medullated
or non-medullated, is part of a neuron.

Fia. 149. — Diagram of a Typical Cell from the Grev Matter of the
Spinal Cord. (Sherrinotok.)

n, nucleus ; d, d, d, branched processes (dendrites) from the cell-body ;
p, pigment; c, part of cell-body which stains very readily (chromatin);
a, axis-cj'linder process, whicli acquires first a medulla, m, and then
(outside tlic cord) a neurilemma.

A, represents the processes (dendrites) from a neighbouring cell inter-
lacing with, but not joined on to, the processes of the cell figured.

The other cells of the grey matter are generally similar
in structure to the one described, though smaller than the
cells of the anterior horn. Certain of them however ex-
hibit particular features, on which we need not dwell here.

The rest of the grey matter, apart from the neuroglia,
is made up of an interlacing network of naked (non-

XI SPINAL CORD AT VARIOUS LEVELS 469

m-xlullated) axis cylinders ; but intermixed with these
are a certain number of very fine meduUated nerve-fibres,
and a few large medullated fibres.

The Differences in Structure of the Spinal Cord at
Various Levels. — These differences show themselves most
conspicuously with respect to (i), the shape of the grey
matter 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 widest in the cervical region,
smallest in the thoracic (dorsal) region, and widens out
again in the lumbar region. The chief structural differ-
ences to which we have alluded are very clearly indicated
in Figure 150, which represents sections, drawn to scale,
of (half) the spinal cord at the level of A the sixth
thoracic (dorsal), B the sixth cervical, and C the third
lumbar spinal nerves respectively.

The Structure of a Spinal Ganglion. — A spinal
ganglion is, as we have said (Fig. 144, Gn.), an elongated
swelling on the posterior roots of the spinal nerves. In a
longitudinal section it is seen to consist of an external
sheath of connective tissue which encloses 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 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. 151, consists like a
nerve cell of the spinal cord, of a large nucleus, with a
nucleoius, and of a cell body ; but the cell body is, in most
cases at all events, prolonged into 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 meduUated nerve fibre, which
then divides into two fibres, one of which may be traced
into the nerve trunk, and the other along the posterior

470

ELEMENTARY PHYSIOLOGY

xt

FUNCTIONS OF NERVE ROOTS

471

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 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 enter the
cord. The majority of them turn aside as they enter the
cord and run upwards in the posterior column of the cord

Fio. 151.— A Nerve Cell from the Ganomov ok thk Postertor Root
OK A [Spinal Nerve.

The nerve cell, with n, nucleus, 71', nucleolus, p, protoplasmic body;
f, capsule of the nerve cell ; n", 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
n'" nuclei are seen on the fibre. At. the division the neuraxis J is seen
to divide, and besides the neurilemma, n.l., the fibre has an additional
sheath, s, continuous with the capsule tif the nerve cell.

to the brain. On their way thither the fibres give off
numerous branches called collaterals ; these are fine
medullated fibres, they pass into the grey matter of the
posterior horn and if their connections were traced ihey
would lead for the most part to cells of the anterior horn
or those of Clarke's column. They or their connections
end in contact with these cells (A Fig. 150). Some of the
posterior root fibres cross the cord and connect with
neurons which ascend the opposite side. Therefore an
impulse started in the skin would ultimately pass from
the sensory neuron in either one or more of three direc-
tions : (1) To the sensory portion of the brain, (2) To

472 ELEMENTARY PHYSIOLOGY less.

motor nerve, (3) to Clarke's column (Fig. 143), and thence
to the cerebellum along the direct cerebellar tract (p. 491).

Structurally we may regard the nerve-fibres of the
posterior 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 rot)t originate in
one process of a nerve-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.

6. The Functions of the Roots of the Spinal
Nerves.— If the trunk of a spinal nerve be irritated in
any way (at x in Fig. 152), 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 distributed contract ; in the second, pain is
felt, and the pain is referred to that part of the skin to
which fibres of the nerve are distributed. In other words,
the efi'ect of irritating the trunk of a nerve is the same as
that of irritating its component fibres at their terminations.

The eff"ects 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 aff'ected, and
the only region of the skin to which pain is referred, will
be those to which that branch sends nerve-fibres. And
theee effects will follow upon irritation of any part of a
nerve from its smallest branches up to the point of its
trunk, at wiiich the anterior and posterior bundles of root
fibres unite.

If the anterior bundle of root fibres be irritated in the
same way (at ;/, Fig. 152) 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 t\iQj)ufiterior, ganglionated bundle be irritated
(at 2, Fig. 152) only half the effects of irritating the
whole trunk is produced. But it is the other half ; that is
to say, none of the muscles to which the nerve is distributed

XI FUNCTIONS OF NERVE ROOTS 473

contract, 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 causinfj; 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 commonly called motor, and the pos-
terior 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 nmscles
to which that nerve is distributed, though the sensibility

3.jr

Fig. 152.— Diagram to illi'stratk Experiments ik proof of the Func-
tions OF THE Spinal Nkkve-Roots and of the Gangliok 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 ;

Gn, ganglion of posterior rout.

of the region of the skin supplied by the nerve is perfect.
If the posterior roots be cut, sensation is lost, and volun-
tary movement remains. But if both roots be cut, neither
voluntary movement nor sensibility is any longer possessed
by the part supplied by the nerve. The muscles are said
to be paralysed ; and the skin may be cut, or burnt, with-
out any sensation 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 tlie otlier end be
so treated, no ajjparent effect results. On the other hand,
if the end of the sensory root connected with the trunk of
the nerve be irritated, no apparent effect is produced,

474 ELEMENTARY PHYSIOLOGY

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
niirve 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 apparent save a muscle, and a sensory nerve
with nothing that can show an effect but the central
nervous system.

We have already explained (p. 468) that a fibre of the
anterior root of a spinal nerve is really part of a neuron
the cell of which is situated in the anterior horn of the
grey matter of the spinal cord. This being the case it is
not at all surprising to find that the continued life of any
of the efferent (motor) nerve-fibres is dependent upon the
continuance of their connection with the cells from which
they arise. That this dependence does really exist is
shown by the simple experiment of cutting an efferent
(motor) nerve, and preventing the cut ends from reuniting.
When this is done it is found that shortly after the
operation, those (peripheral) parts of the nerve beyond the
point of section, i.e., whose connection ivith the cells of the
spinal cord has been cut off, undergo what is called a
"degeneration." This degeneration shows itself by
structural changes in the nerve fibres. The medulla
breaks up into oily drops, the axis cylinder also breaks
into pieces and the neuclei of the neurilemma increase in
number, together with an increase in the amount of
granular protoplasm, which lies near them. The frag-
ments of the medulla are next largely absorbed and
disappear, and their place is taken by the protoplasm and
nuclei derived from the neurilemma. While these
structural changes are taking place, and even before
they become obvious, the irritability of the nerve becomes
gradually less, so that soon the nerve makes no response
to any stimulus which may be applied to it. But the
changes we have descx'ibed do not occur in that (central)

XI FUNCTIONS OF NERVE ROOTS 475

part of the nerve which is still connected with the cells of
the spinal cord ; this part does not degenerate in the
same way. Thus if the anterior eflFerent (motor) root
of one of the spinal nerves be cut at y (Fig. 152), all
the fibres of that root beyond y towards and along the
trunk of the nerve T degenerate, while the portion of the
root between y and the spinal c(n'd does not degenerate.

If, now, we apply the same method of experiment to a
posterior root the following results are observed. When
the root is cut at w (Fig. 152), 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 degenerates and the degeneration may be
traced as far as the cut axis-cylinders penetrate into the
central nervous system, whereas the portion still con-
nected with the ganglion does not. Evidently, then, the
life of the fibres in the posterior root is dependent upon
their continued connection with the ganglion of that root,
that is to say with the cells of that ganglion, of which the
fibres are processes, as we have previously explained.
These facts lead to the inevitable conclusion that the
ganglion on the posterior root is the structure upon which
the proper nutrition of the afferent filn-es depends, or, in
other words, the one clear and definitely ascertained
function of the ganglion is to provide for the nutrition of
these efi"ei-ent nerve fibres wliich originate from the
processes of the nerve cells in the ganglion.

This method of determining and localising the
nutritional centres from wliich nerve-fibres grow is known
as the " degeneratiim method," ^ 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. 489).

1 Also as the " Walleriau method," after the name of tlio physiologist
who first employed it.

476 ELEMENTARY PHYSIOLOGY Lilss.

7. 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, a something which is spoken of as
a nervous impulse and consists, probably, of a change
in the arrangement or condition of its molecules, is propa-
gated 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, immediately 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 subjected 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 sensation in, the
limb, and these powers are only gradually restored as
that nervous conductivity returns.

Having arrived at this notion of an impulse travelling
along a nerve, we readily pass to the conception of a
sensory 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 very convenient to
use these terms to denote the two great classes of nerves ;
for, as we shall find (p. 483), there are afferent nerves
which are not sensory in the sense of giving rise to a
change of consciousness, or sensation, while there are
efferent nerves which are not motor, in the sense of in-
ducing muscular contraction. The nerves, for example,
by which the electi-ical fishes give rise to discharges of
electricity from peculiar organs to which those nerves are
distributed, are eff"erent, inasmuch as they carry impulses
to the electric organs, but are not motor, inasmuch as

XI AFFERENT AND EFFERENT NERVES 477

they do not give rise to movements. The pneuinogastric
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
such as the salivary glands, sweat glands, &c., to
commence secreting are not motor nerves but are
strictly efferent as regards the direction in which they
convey their impulses. It will, of course, be understood,
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 afierent. 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 disposition of its two ends, in tlie skin, or other
peripheral organs on the one hand, and in the central
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. 481.)
We know but little of the nature of a iiervous impulse.
We know that it may be started in a nerve by various
artificial means such as by pinching or knocking the
nerve, or by suddenly warming or cooling it, and, most
readily, by stimulating the nerve electi-ically. 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 disturbance, which is then propagated in suc-
cession 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

478 ELEMENTARY PHYSIOLOGY less.

way we come to speak of a nervous impulse as due to
the propagation of a " molecular disturbance " along a
nerve. But this expression serves rather to hide our
ignorance than to explain wliat the impulse really is.

Electrical Properties of a Nerve. — In the case of a
muscle we saw (p. 300) that its entry into a state of (con-
tracting) activity was accompanied by an easily recognised
change of shape, by chemical changes and by changes of
temperature. Of these the first is of course entirely
wanting in a nerve when it Ijecomes active, i.e. is conveying
an impulse, and the other two kinds of change have not
so far been shown to take place in a nerve. But we saw
also that the contracting activity of a muscle is accom-
panied by an electrical disturbance ; a similar disturbance
takes place in a nerve as the impulse sweeps along it, and
is indeed the only evidence we possess of any change
which accompanies the transmission of that impulse.
Hence in a nerve this electrical phenomenon becomes of
extreme interest and merits a short consideration.

If a piece of nerve, as for instance the sciatic nerve
(see Fig. 90), from a freshly-decapitated frog, is removed
from the body and suitably examined, it is found that
each cat end of the nerve is electrically iiegative as compared
with any other jjoint of the nerve nearer to its middle or
equator. Hence if one end li (Fig. 153), of the nerve and
its middle point C are brought into contact with the
terminals a,b, of a sensitive galvanometer ^ G, the needle
of this instrument is at once deflected in a direction which
shows that an electric current is passing (through the
galvanometer) from the middle of the nerve to the cut
end. If, now, when the needle has come to rest under the
influence of this current, the end A of the nerve be
stimulated at x, the needle of the galvanometer is seen to
swing back towards the position it occupied before it icas
deflected by the current from the nerve. This means that as

1 A galvanometer is .aii instrument used for the detection and
■measurement of electric currents.

XI ELECTRIC PROPERTIES OF THE NERVE 479

the impulse which was started by the stimulus at x passes
under tlie terminal 6, that part of the nerve on which this
terminal rests, becomes momentarily less electrically
positive, that is to say becomes electrmdly negative as cmi-
pared ivith its condition before the passage of the impulse.
This statement holds equally good when the terminal b is
applied to any other point of the nerve, either towards A
or towards B, any difference in the result of stimulating
the nerve at x being merely one of degree (as regards the
extent to which the needle of the galvanometer moves),
and not of kind (as to the direction in which the needle
moves). Hence we may say without any possibility of

Fig. 153.— To show Akkangement ot a Nerve and Galvanometer for
Experiments on the Elei;trical 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.

doubt that when an impulse travels along a nerve each
point of the nerve becomes electrically negative as the
impulse reaches that point ; and conversely we may use
this electrical change in the nerve as unfailing evidence of
the passage of an imindse along it. Apart from this
electrical change we have no other means, such as exist in
the case of a muscle, of determining when a nerve enters
into a state of activity during the passage of an impulse.

The Rate of TranBinission of a Nervous Impulse. —
By means of a complicated arrangement of apparatus it is
possible to determine very exactly the interval of time
which elapses between the moment at which the stimulus
is applied to the nerve at x (Fig. 153) and the instant at

480 ELEMENTARY PHYSIOLOGY less.

which the needle of the galvanometer begins to move as
the result of the passage of the impulse, started at jc,
under the terminal b at the point C. If now we measure
the length of the piece of nerve between x and b we can
at once calculate the I'ate at which the impulse travels
along the nerve. Thus if the distance from x to b is 25
millimetres (1 inch), about -00089 of a second elapses
before the impulse started at x makes itself obvious as an
electrical disturbance at b. That is to say the impulse
travels at the rate of about 28 metres or DO feet per
second in the nerve of a frog.

The rate of transmission of an impulse along a (motor)
nerve may also be determined in the following way, using
a muscle nerve preparation such as is figured on page 298.
The muscle is suspended from a clamp, as shown in
Fig. 154 ; a light horizontal lever is attached by a hook to
the tendon at the lower end of the muscle, so that when
the muscle is made to contract the free end of the lever
moves upwards and thus indicates the moment at which
the contraction of the muscle commences. The sciatic
nerve is then arranged in such a way that it may be
stimulated either at a point .i; (Fig. 154) as close as possible
to its junction with the muscle, or at a point y 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 ajjplying 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 experiment,
about Y5o*^^^ ^^ ^ second. If now the nerve is stimulated
at y, it is found that the end of the lever begins to move
slightly later than it did wlien 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
difference can only be due to the fact that ivhen the
impulse is started at y it tnlies longer to reacli 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,

A NERVOUS IMPULSE

481

it becomes a simple matter to calculate the rate at which
the impulse travels from 1/ to x. The result thus
obtained agrees quite closely with the one arrived at in
the experiment previously described in which a gaWano-
meter 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

FiQ 154 -Arrangement of Nerve, Muscle and Lever for deter-
j.ia. -^^,^1^0 THE Velocity of a Nervous Impulse.

f femur ■ vi muscle ; t.a, tendon ; I, lever, movable about the end 6 ;
J. weight to keep the Muscle stretched ; n, the nerve ; x and y, the two
points at which the nerve is stimulated.

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
experiments are made on the nerves of a warm-blooded
human being, the rate of transmission is found to be
somewhat greater, viz., about 100 metres (or over 300
feet) per second, than in the cold-blooded frog.

The most efficient stimulus which can be artificially

I I

482 ELEMENTARY PHYSIOLOGY less.

applied to a nerve for starting an impulse along it, is, as
we have said (p. 477), an electrical stimulation. Further,
as we have seen, each point of the nerve undergoes an
electrical change, as the impulse reaches that point.
These two facts fre(}uently give rise to the entirely
erroneous idea that a nervous impulse is of the nature of an
electric current, similar to that wliich passes along a wire,
as used for telegraphy. But this is by no means the case,
since, without goinginto any othermore abstruse reasons, we
have shown that the rate at which an impulse travels along
a nerve is on an average about 100 metres, or 300 feet, per
second, whereas we know that electricity travels along a
wire at a rate such that the transmission of signals over
the wires of an ordinary hand-line is practically instan-
taneous. 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 one end
before the etfect can be first detected at the other end.
Now, if a nerve could be used for transmitting an impulse
as a signal frou), say. Land's End to John o'Groat's
(600 miles), the signal woidd take nearly three hours
(176 minutes) to reach its destination, travelling as it does
at the rate of 300 feet pev second. We have spoken of
a nervous impulse as a "molecular disturbance" pro-
pagated along a nerve. And 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 powder
to which it is applied ; the change thus set up leads to 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.

FUNCTIOXS OF SPINAL CORD

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 explosion, and must be artificially relaid for further
use.

8. The Properties or Functions of the Spinal Cord. —
Up to thi.s 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 oft" below the section, will
be insennble, 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 virtuallj' 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
supplied by nerves given ofi" 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 con-
nected with the brain, or better, if any afferent nerve
connected with that part of the cord be irritated, sensations
ensue, as is shown bj^ the movements of the animal ; but
in these niovements the mnscles supplied by the nerves
coming from the spinal cord below the cut take no part ;
they remain perfectly quiet.

Thus, it may be said that, in relation to the brain the

I I 2

484 ELEMENTARY PHYSIOLOGY less.

cord is a great mixed motor and sensory nerve. But
it is also much more.

Reflex ActioQ through the Spiaal 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 anywhere so as to sever its connection with
the brain, irritation applied to the skin of the parts sup-
plied with sensory nerves from the part of the cord below
the section, though it gives rise to no sensation, may pro-
duce 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 paralj'sed and insensible from spinal injui;y, tickling
the soles of the feet will cause the legs to kick out convul-
sively. 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.

It the cord be cut across a second time at any distaiice
below the first section, the efferent nerves below the second
cut will no longer be affected by irritation of the afferent
nerves above it — but only of those below the second
section. Or, in other words, in order that an afferent
impulse may be converted into an efferent one by the
spinal cord, the afferent nerve must be in definite
material communication with the efferent nerve, by means
of a neuron in the sjjinal cord. The nature of these
communications we have already seen (p. 472).

This peculiar power of the cord, by which it is com-
petent 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

REFLEX ACTION 485

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 necessarily impl}- a corresponding simplicity
in the arrangement and succession of the reflected motor
impulses. Tickling 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 combination. 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 affei'ent impulse, 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 deca-
pitated, 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 sl)ock 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
stimulus 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

486 ELEMENTARY PHYSIOLOGY less.

pinched, the legs are suddenly drawn up and thrust out
again violently.

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, the-se 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 tlie whole
animal is thrown into convulsions.

Now all these various movements, even the feeblest
and simplest, require a certain conibination of muscles,
and some of them, such as the act of rubbing off" the acid,
are in the highest degree complex. In all of tliem, too, a
certain purpose 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 tln-ust 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 coi'd 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
serve as an independent nervous centre, capable of origin-
ating combined movements upon the reception of the

XI REFLEX ACTION 487

impulse of an afferent nerve, or rather, perhaps, a group of
such independent nervous centres.

In all these reflex actions of the spinal cord, the
structures necessary for their performance are, as already
pointed out (p. 340), 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. In the case of the
headless frog, the actions are of course quite involuntary,
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 everj'day life. Thus the act of walk-
ing, 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 stimula-
tion of the skin of the feet and legs which results from
the varying pressure and contact with the ground. Hence
the staggering 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 (afferent) roots of the spinal nerves. In such cases
walking is frequently possible only as the result of looh'uKj
at the ground (see p. 390) ; this accords with the fact that
even in health afferent impulses 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.

Again, the actions of micturition and defalcation are
really reflex actions carried out by the spinal cord, as
soon as tliey have been started by tlie will ; here the
sensory surfaces are the mucous membrane of the bladder
or rectum, the necessary stimulus being supplied as the re-
sult of their distension by the accumulated urine or faeces.

488 ELEMENTARY PHYSIOLOGY less.

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 t(j the working and wel-
fare of the body as regards the relationships of its internal
mechanisms. Thus we have seen that certain parts of the
spinal bulb, or medulla, which for our present purpose
may be regarded as part of the spinal cord, are connected
with the heart (cardio-inhibitory centre), blood-vessels
(vaso-motor centre), and respiratory muscles (respiratory
centx'e) 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 7o, 70, 158).

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 maiked characteristic of the varied activities
of the brain. But to this point we shall return later
on.

Tbe Paths of Conduction of Afferent and Efferent
Impulses along the Spinal cord. The spinal cord has
a further most important function beyond reflex action,
namely that of ti-ansmitting 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 cerebi'al hemisphei'es,
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 these
roots find their way to the muscles which move the foot.
Similarly, when the sole of the foot is touched, afferent
impulses travel in the reverse way upward along the
spinal cord to the brain. And the question arises, in
what manner do these efierent and afferent impulses travel
along the spinal cord ?

XI THE SPINAL CORD AS A CONDUCTOR 489

We must now explain tlie method by which most of
our present information has been obtained, and point
out the chief and most definite facts which have been
arrived at.

We have seen previously (p. 475) that when a nerve is
cut, a structural change of its fibres starts at the cut.
This is spoken of as a "degeneration," and since it
indicates a breakdown in the proper nutrition of those
fibres which are seen to degenerate may, as in the case of
the roots of the .spinal nerves, be used to determine the
relationship of nerve fibres to the centres upon which
their nutrition depends. The white matter of the spinal
cord is composed of fibres which are in all essentials the
.same as those of an ordinary medullated nerve, and these
fibres of the white matter may similarly degenerate when
cut off from the centres on which their nutrition depends.
Hence, if the whole spinal cord be cut across transversely,
or if transverse cuts be made into any part, or the whole
of any one or more of the columns of white matter of
which the cord is so largely made up, degenerative changes
may start from the point of section, and by the course they
pursue up and down the cord, enable us to follow the
course of certain fibres or bundles of fibres in that
white matter. Now this is exactly what does happen when
the cord is cut, and this "degeneration method" has
provided the best means for answering the question as to
how and along what paths aiferent and efferent impulses
travel up and down the spinal cord.

When the cord is cut across degenerative changes take
place in pai'ts of the white matter, both above and below
the point of section. These changes only affect limited
parts of the white matter, and the parts which are affected
abore the cut, that is up towards the brain, are different
in position from the parts which are affected below the
point of section. The changes which start from the
cut and take place uptvards along the spinal cord, towards
the brain, are spoken of as ascending degenerations ;
those which are observed to occur downivards from below

490 ELEMENTARY PHYSIOLOGY

the section, are known as descending degenerations, the
terms ascending and descending being thus used to denote
structural changes which start from the section and pass up
towards or down from the bi'ain respectively. Moreover,
since the parts which degenerate are limited as to their
transverse sectional area Avhile running for very consider-
able distances along the white matter of the spinal cord,
they ai'e usually spoken of as "tracts, "and since these
tracts serve very definitely for the transmission of
impulses up and down the cord, they denote very
definite paths of conduction along the spinal cord.

Having thus explained the method of experimenting,
we may now state the chief results obtained by its
application.

(A). Tracts of ascending degeneration,

(i). The Posterior Column comprising^ the Postero-
median and Postero - lateral Tracts. — These tracts
occupy the posterior white columns of the spinal cord,
adjacent to the posterior fissure, in each half of the cord.
(Fig. 155, p.m., p.l.).

The degenei-ation which marks out this tract follows not
only upon sections of the cord itself, but more especially
from cutting the posterior roots of the spinal nerves ; it is
therefore the result of a severance of the fibres in this
part of the cord from their nutritive centres in the cells of
the ganglion on the posterior root. Hence it marks the
course of fii)res passing up the cord from the posterior
roots, and denotes the path along which afferent (sensory)
impulses travel up from the spinal nerves. Since these
nerves are given oflf all along tlie cord, the tract is
necessarily found to exist throughout the whole extent of
the cord, and may be traced up into the spinal bulb,
where it ends on the side up which it passes. The
postero-median portion of the tract consists of fibres
coming from the lower part of the body, the postero-
lateral portion from the upper portion, hence the latter
only exists in the upper part of the cord .

(ii). The Direct Cerebellar Tract. — This tract lies in

THE SPINAL CORD AS A CONDUCTOR 491

the outer and hinder part of the lateral columns (Fig.
155, Ch., CO.). The degeneration which marks its course
results solely from sections of the cord itself, and not of
any of the spinal nerve roots. It begins in tlie lower end
of the cord at the level of the second lumbar nerve,
passes straight up to the spinal bulb and then into the
cerebellum by means of the inferior peduncle (see p. 506)

p m. .

. P'^-

asc.a.i.-'

asc. a.L.

AF

Fig. 155. — Diagram to show the Position of Tracts o? Ascwtdino
Degeseratios is thk White Matter of the Spisal Cord at the
Level of the Fifth Cer%-ical Nerve.

A.V, anterior fissure; P.F. posterior fissure; p.m., p7. the median
and lateral posterior tracts, or traut of fibres from the posterior roots of
the spinal nerves; Ch, Cb, the cerebellar tract; ate. a.l., asc. a.l. the
ascending antero-lateral tract.

The grey matter of the cord is shaded black.

ot«this part of the brain. The fibres in this tract originate
from processes of those cells in the grej' matter which
form a conspicuous group at the base of the dorsal horn,
and are known as Clarke's column. These cells are
seen most conspicuously in the thoracic, the upper cervical
and the sacral regions of the cord. Those from the sacral
region do not contribute to the Direct Cerebellar Tract.
(See Fig. 150, 3).

(lii). The Ascendine: Antero-lateral Tract. — This tract,
like the cerebellar, can only he made evident as the
result of injury to the cord itself. Its fibres originate in
the cells of Clarke's column in the lower portion of the
spinal cord. It lies in the outer and anterior part of the

492 ELEMENTARY PHYSIOLOGY

lateral column (Fig. 155, asc. a.l., asc. a.l.) and commences
rather lower down in the cord than does the cerebellar
tract, but like the latter runs up to the si)inal bulb, and
enters the cerebellum by mean of its superior peduncle.

B. Tracts of descending degeneration.

(i) The Crossed Pyramidal Tract. — This is a large
and conspicuous tract in the inner and hinder part of the
lateral column (Fig. 156, Cr.p., Cr.'p.').

It extends along the whole length of the cord, passing
down into the cord from the spinal bulb. The fibres of

PF.

D/py A.F.

FlO. 156. — DiAOKAM TO SHOW THE POSIIION OF TRAUTS OF DESCENDING

Deokneration in the Wfiite Matter of the Spinal Cord at the
SAME Level as in Fig. I'm.

Cr.p., Cr'.p'. crossed pyramidal tracts; D'.p'., D.p. direct pyramidal
tracts ; dexu. a.l ., desc. a.l. descending antero-lateral tract ; p.p., p.p.
prepyrainidal tract.

this tract are believed to communicate with those cells in
the anterior horns of the grey matter who.se process, as
previously described (p. 468), gives rise to the nerve fibres
which leave the cord as the efferent (motor) anterior roots
of the spinal nerves. The communication is made by a
small neuron interposed between the two. It may thus
be regarded as the path for efferent impulses coming
down the cord on their way to outlying parts of the body.
But this tract, unlike those we have so far described,
does not end, or rather we .sjiould now .say begin, in the
spinal bulb. On the contrary, it may be traced up through

XI " TRACTS IN SPINAL CORD 493

tlie bulb into the higher parts of the brain and is found
to start from a certain portion of wliat we shall describe
later on as the cortex of the cerebral hemispheres.
It is upon the cells of this i)art of the cortex that the
fibres of the pyramidal tracts depend for their nutrition ;
hence injury to this portion of the cortex leads to a
degeneration which extends right down to the lowest end of
the spinal cord. These facts still further confirm the idea
that this tract provides a path for efferent (motor) im-
pulses in the cord, since, as we shall see, that part of the
cerebral cortex of which we are now speaking, is
specially concerned in the development of etierent
(motor) impulses.

The fibres of this tract which enter the spinal bulb
from, say, the left side of the brain, cross over in the
bulb, in what is known as the decussation of the pyramids
(p. 520) just above the origin of the first cervical nerve, and
then pass down the right side of the spinal cord. For
this reason it receives the name of the "crossed"
pyramidal tract.

(ii) The Direct Pyramidal Tract. — This is a small tract
in the median part of the anterior white columns, adjacent
to the anterior fissure. (Fig- 156 D'.p'., D-P-) It really
consists of a small portion of those fibres which passed
into the bulb as the main pyramidal tract coming from
the cortex of the cerebral hemispheres, but which have
not yet crossed over in the bulb. Instead of crossing in
the brain they cross in the cord. Hence the tract grows
small as it descends and ultimately disappears. Thus in
Fig. 156 the direct tract D.j). comes from the same side
of the brain as the crossed pyramidal tract Ch:p., and a
similar remark applies to D'.p'. and Gr'.p'.

(iii) The Descending Antero-lateral Tract.— This
tract is not very clearly marked, in fact the fibres of the
ascending and descending antero-lateral tract intermingle
to some extent. It forms part of an alternative route
from the cerebral cortex to the anterior horn cells.
(Fig. 156, desc.a.l., desc.a.l.)

494 ELEiMENTARY PHYSIOLOGY less.

(iv) The Pre-pyramidal Tract. —Like the descending
tracts which we have already mentioned tlie pre-pyraniidal
tract ends in connection witli the anterior horn cells.
The impulses which its fibres carry come, however, not
from the cereljrum but from the cerebellum We have
seen (p. 471) that a sensory impulse reaching the central
nervous system by a fibre in the posterior root may
expend itself either in the cerebrum, the cerebellum or
the anterior horn ; we now see that an anterior horn
cell (motor neuron) may be played upon by impulses
coming either from the cerebrum, the cerebellum or the
sensory neurons of the posterior roots.

Such are the functions of the spinal cord, taken as a
whole. The spinal nerves arc, as we have said, chiefly
distributed co the muscles and to the skin. But other
nerves, such as those for instance belonging to the blond-
vessels, the so-called vatto-motor nerves (Lesson II. p. 6J)),
though many of them run for long distances in the sym-
pathetic system, may ultimately be traced to the spinal
cord. Along the spinal column the spinal nerves give off
branches which run into and join the sympathetic system.
And the vaso-motor fibres which run along in the sym-
pathetic nerves do really spring from the spinal cord,
finding their way into the symi)athetic system through
these communicating or commissural branches. Besides
which, some vaso-motor fibres run in spinal nerves along
their whole course.

Experiments, moreover, go to show that the nervous in-
fluences which, through these vaso-motor nerves, regulate
the blood-vessels, now forcibly constricting them, now
allowing them to dilate, and now keeping them in a state
of moderate or tonic constriction, proceed from the spinal
cord.

The cord is, therefore, spoken of as containing centres
for the vaso-motor nerves or, more shortly, vaso-motor
centres.

For example, the muscular walls of the blood-vessels

XI THE SYMPATHETIC SYSTEM 495

supplying the ear and the skin of the head generally, are
made to contract, as has been already mentioned, by
nervous fibres derived iinmediatel}' from the sympathetic
(Fig. 22, C.Sij.). These filires are non-medullated and
arise from culls situated in the superior cervical ganglion.
The ganglion in turn is connected with the spinal cord by
medullated fibres which do notarise from the sympathetic
ganglia, but simply pass through them on their way from
the upper dorsal region of the cord. Irritation of this
region of the cord produces the same effect as irritation of
the vaso-motor nerves themselves, and destruction of
this part of the cord paralyses them.

It has, however, been further shown that the nervous
influence does not originate here, but proceeds from
higher up, from the medulla oblongata in fact, and simply
passes down through this part of the spinal cord on its
way to join the sympathetic nerves.

9. The Ssmipathetic Nervous System.— The sym-
pathetic system consists chiefly of a double chain of
ganglia lying at the sides and in front of the spinal
column, and connected with one another, and with the
spinal nerves, b^' coumiissural cords (Fig. 142). From
these ganglia, nerves are given off which for the most
part follow the distribution of tlie blood-vessels, but
which, in the tliorax and abdomen, form great networks,
<u- plexiises, upon tlie heart and about the stomach and
other abdominal viscera. Every efl'erent impulse which
passes along the sympathetic system leaves the spinal cord
by a medullated fibre — a jjortion of a neuron the cell of
which is in the grey matter. This neuron ends in the
sympathetic system (either in a ganglion or elsewhere)
in connection with the cell of another neuron, non-
medullated, which carries the impulse to the end organ.
Some of these non-medullated fibres run back into the
spinal nerves for distribution to the blood-vessels of the
limbs.

By means of the sympathetic nerves the muscles of the
vessels generally, and those of the heart, of the intestines,

496

ELEMENTARY PHYSIOLOGY

LESS.

and of some other viscera may, as we have seen, be in-
fluenced ; and the influence thus conveyed, it may be
remarked, is generally difl"erent to, or even antagonistic to
that which is conveyed to the same organs by the fibres
running in the spinal or cranial nerves. Thus while
irritation of the (cranial) pneumogastric fibres stops the
heart, irritation of the sympathetic fibres going to the
heart increases the beat.

Fig. 157.— Diagram to illustrate the Distribution of the Spinal
Nerves and their Relationship to the Ganglia of the Sympa-
thetic System.

A.F, anterior fissure ; P.F. posterior fissure ; Gr, grey matter ; W, white
matter of spinal cord ; A, anterior root of spinal nerve ; Gn, ganglion on
the posterior root ; A, the trunk of a spinal nerve ; N', spinal nerve
proper, ending in a skeletal muscle M, in a sensory cell or surface 6' ;
F, a branch (white ramus communicans) of thu spinal nerve passing to
2 a ganglion of the sympathetic system, then passing on as V' to some
more distant ganglion <r, and then as V" to some peripheral ganglion o-',
and ending in a muscle Hi of the blood-vessels or viscera, in s an internal
(visceral) sensory cell or surface.

From 2 a nerve r.v. (grey ramus communicans) runs back and passes
partly towards the spinal cord and partly to the peripherj', as vaso-motor
fibres and other v.m., in connection with the spinal nerve JV', to m the
muscles of blood-vessels in certain parts e.g. of the limbs.

Sy, St/, the main chain of the sympath'jtic system which unites the
several ganglia 2 of that system. (See Fig. 142.)

But the influences which thus reach these organs
through the sympathetic nerves, do not originate in the
sympathetic system itself, but are derived from the spinal
cord or brain. We have seen (p. 69) this to be the case
in reference to vaso-motor nerves, and the same is true of
the sympathetic nerves going to the heart and other

SYMPATHETIC GANGLIA

497

viscera. Whatever may turn out to be the function of
the sympathetic ganglia, there is at present no adequate
evidence that they in any way act as nervous centres,
either of reflex action, or of any other form of nervous
activity. Hence the sympathetic is not to be regarded as
a separate nervous system, but as being in reality merely
an outlying part of the cerebro-spiual system, an outlying
chain of ganglia, through which the fibres of a part of
the trunk of each spinal nerve pass on their way to the
viscera. This relationship is made quite clear by the
accompanying diagram (Fig. 157).

We have spoken of the fibres which proceed from cells

Fio. 158. — Pale Non-medullated Fibres from the Pneumooastric
Nerve. (Ranvier.)

n, nucleus ; p, protoplasm belonging to the nucleus.

situated in the sympathetic system as non-medullated,
because they possess no medulla. They appear under the
microscope as pale flattened bands, about as wide as
small meduUated fibres, often fibrillated longitudinally,
and frequently dividing. They appear, in fact, to be
axis-cylinders, without medulla, and in cases perhaps
without a neurilemma, though they bear at intervals
on their surface nuclei which may represent the inter-
nodal nuclei of ordinary nerve fibres.

The ganglia of the sympathetic system are composed of
nerve cells bound together by a small amount of loose
connective tissue. The cells diff"er somewhat in ap-
pearance and arrangement according to the ganglion in

K K

498 ELEMENTARY PHYSIOLO(JY lkss.

which they are seen, but speaking broadly and generally
they may be said to resemble in their most obvious
features the motor cells of the spinal cord, whose struc-
ture we have previously desci'ibed (p. 407). Like the
latter, each nerve cell of a sympathetic ganglion contains
a large and conspicuous nucleus, and the cell body is pro-
longed into a varying but usually large number of
branching processes (dendrites). Moreover, eai^h cell
possesses one process which does not branch but passes
away from the body of the cell as a (usually) non-
medullated nerve fibre, to be distributed to the various
tissues (p. 495) which it influences, and is in this respect
similar to the axis-cylinder process of a cell of the spinal
cord. Each ganglion is also connected with nerve fibres
which come to it from the central nervous system (Fig.
157). These fibres mostly end in connections with the
nerve cells. Sometimes a fibre does not end in the first
ganglion it meets, but passes right through it into one
of the nerves going off from that ganglion, and so reaches
some other more distant ganglion. The number of
nerve fibres which thus pass into the ganglion from the
spinal cord is much less than the number of nerve cells
in the ganglion ; hence many more nerve fibres are found
coming, from the ganglion than entering it. By this
arrangement each ganglion provides, as it were, a sort of
junction by means of which any nervous impulses which
reach it along any one path may be the more readily and
widely distriljuted, along several paths, to the tissues.

10. The Structural Arrangements of the Brain and
Spinal Bulb (Medulla Oblongata).— The brain is a very
complex organ consisting of many parts. It occupies the
cavity of the skull and is thus placed at the upper end of
the spinal cord with which it becomes connected by means
of the spinal bulb ' ; this passes insensibly into, and in its
lower part has the same structure as, the sjjinal cord.
When viewed from the side, after the removal of the parts

1 Throughout this section we shall use the word "bulb" in shortness
for spinal bulb, and instead of the older name " medulla oblongata."

XI

THE BRAIN

499

which cover in the cerel)ro-spinal system, the brain pre-
sents the ap]jearance shown in the following figure. The
spinal cord (N) widens out into the bulb (M.Ob.) whose

Fig. 159.— Side View of the Brain and Upper Part of the Spinal

Cord in place— the Parts which cover the Cerebro-Spinal

Centres being removed.

C. C. the convoluted surface of the right cerebral hemisphere ; Cb. the

cerebellum ; M.Ob, the medulla oblongata ; B. the bodies of the cervical

vertebrae ; Up. their spines ; N. the spinal cord with the spinal nerves.

upper end passes on into the large convoluted structure
(G'.CC'.) which is called the (right) cerebral hemisphere.

500 ELEMENTARY PHYSIOLOGY less.

Lying beneath the hinder end of the hemisphere is a
large laminated mass which overhangs the posterior side
of the bulb and is known as the cerebellum (Cb.). When
the brain is removed from the skull and looked at from
its base or under surface many further details may be
at once made out. Thus it becomes evident that the
brain consists of two halves, corresponding to each half of
the spinal cord, lying symmetrically on each side of a line
joining GC. and M. in Fig. 160. The bulb (M) widens out
at its upper end and gives oft' from each side a number of
nerves (vii. — xii.) which are analogous to the spinal nerves
but, as originating from the brain, are called cranial
nerves. The other six pairs of cranial nerves (i. — vi.)
come off from parts of the brain in front of (above) the
bulb. The cerebellum is seen to send out from each side
towards the central line a large mass of transverse fibres
which sweep across the brain and meet, with a depression
in the middle line, thus forming a sort of bridge from one
half of the cerebellum to the other ; this bridge lies just in
front of (above) the bulb and is called the pons Varolii.
The number VI. is placed upon the pons in Fig. 160. The
longitudinal nerve fibres of the bulb pass forwards
(upwards in the figure), among and between the transverse
fibres of the j^oiik, and become visible again in front of it
as two broad diverging bundles called crura cerebri,
which plunge into the corresponding cerebral hemi-
sphere of each half of tlie brain.

When the brain is viewed from above nothing is visible
beyond the convoluted surfaces of the two cerebral hemi-
spheres, sepai'ated b}- a median fissure whose sides are in
close contact. But if the sides of this fissure are cai-efully
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
callosum (shown as CO., in Fig. 160). 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 BRAIN

501

Fio. 160. — The Base or Under-Surface of the Braik.

A, frontal lobe ; B, temporal lobe of the cerebral hemispheres ; Cb.
cerebellum : /, the olfactory nerve ; //, the optic nerve ; ///. />', VI. the
nerves of the muscles of "the eye ; (', the trigeminal nerve : VII, the
facial nerve ; VIII. the auditory nerve ; IX, the glossopharyngeal ;
A', the pneumogastric ; A7, the spinal accessory ; A7/, the hypoglossal,
or motor nerve of the tongue. The number VI is placed upon the pons
Varolii. The medulla oblongata (i1/) is seen to be really a continu-
ation of the siiinal cord ; on the lower end are seen the two cres-
cents of grey matter ; the section, in fact, has been carried thro\igh
the spinal cord, a little below the proper medulla oblongata. From
the siiles of tlie 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 dispose i pons Varolii, are seen coming off the
/■// nerve, and more towards the middle line the VI. Out of the
substance of the pons springs the V nerve. In fmnt of that is sen
the wcU-dcfiued anterior border of the ^)ons ; and coming forward

502 ELEMENTARY PHYSIOLOGY less.

the bulb is exposed. Unlike the anterior surface, which
is conspicuously convex (see Fig. 160, M) the posterior
surface 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 dimin-
ished, so that the central canal of the spinal cord
approaches the floor of the fourth ventricle, and actually
opens into the lower end of the cavity (Fig. 161) ; 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
ventricle 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 vertical,
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. 161.

The corpus callosum is seen cut across at cc. cc. cc. Above
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 hemis-
pheres. The upper end of the spinal cord, Sp.c, passes
into the bulb B, in front of which the transverse fibres

in front of that line, between the /f and /// nerves on either side, are
seen the crura cerebri. The two round bodies in the angle between tlie
diverging crura are the so-called corpora albicanlia, and in front of them
is P, the pituitary body. This rests on the chiasnia, or junction, of the
optic nerves ; the continuation of each nerve is seen swee])ing round the
crura cerebri on either side. Immediately in front, between the separ-
ated frontal lobes of the cerebral hemispheres, is seen the coipvs
callomm, CC. The fisstire of Sylvius, about on a level with 7 on the loft
and // on the right side, marks the division between frontal and
temporal lobes.

XI

THE BRAIN

603

of the pons are seen in section at P, while the longitudinal
fibres of the bulb run forward above the pons tf) emerge
in front as one of the (right) crura cerebri. Antericnly
this crus disappears out of the section since it diverges bo
the right (see Fig. 160) from the median line of the brain

Fio. 101.— View of the Right Half of a Human Brain as shown by

A LOS'CITUDINAL SECTION I.V THE MEDIAN LiNE THROUGH THE LONGI-
TUDINAL Fissure. (Aftek Sherringtos. )
Sp.c. spinal cord; B, bulb; F, pons; Cli, ovs cerebri ; M, corpus al-
bicans: CO, cerebellum; r, central canal of spinal cord opening into 4,
the fourth ventricle ; V.l', valve of Vieussens ; QP, QA, corpora quadri-
gemina, beneath which is the aijueduct or Si/lvitis leading from the
fourth ventricle into 3, the cavity of tlie third ventricle; P, pmeal
gland ; F, fornix or roof of third ventricle ; OT, optic thalamus :
H, pit'uitarv body; OP, optic nerve cut across at the optic decussation
(see Figs. 160 and 167); 6i. apart of the septum htcidvm, of which the
remainder has been cut away to reveal NC, I V, the cavity of the lateral
ventricle ; this cumniunicates with the third ventricle by means of the
foramtn of Monro, whose position is marked by a small x at the front
end of this thirl ventricle ; r.c, c.c, c.c, corpus calldsum, above which is the
mesial surface of the right cerebral hemisphere.

to enter the corresponding cerebral hemisphere. The
cerebellum Cb, is seen in section ovevhnnglng the bulb, and
between it and the bulb is the cavity, shided and marked
with a 4, to which we have i)reviously alluded as the/o»r/7i
ventricle. The central canal c.c. of the spinal cord is shown

604 ELEMENTARY PHYSIOLOGY less.

as an opening into the hinder end of the cavity of the
fourth ventricle, while the front end of the cavity ia
prolonged into a narrow passage, the aqueduct of
Sylvius, which leads into a much larger cavity
known as the third ventricle, and marked by a 3.
Above this aijueduct are four largely developed masses of
tissue, but of these tiL-a only are seen in the section at QA ,
QF, 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 structui'es 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 quadri-
geminum 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. 161, 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. 160, P, and
Fig. 161, //). The roof of the third ventricle is provided
by a layer of tissue seen in section and known as the
fornix (Fig. 161, F) ; this is connected posteriorly with
the hinder end of the corpus callosum, and in front it
curves downwai'ds and backwards into the lateral wall (A the
third ventricle towards the corpus albicans, M. The
vertical space between the fornix and the corpus callosum is
filled in by a thin donhle layer of nervous 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 remaining 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 AC,
LV., lying in the middle of the cerebral hemisphere, and
known as the (riglit) lateral ventricle. The cavity of
this ventricle communicates with that of the third ventricle
by a small opening at x, the foramen of Monro, Since

XI

THE BRAIN

505

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.'
(See Fig. 163, 5. ) Each lateral ventricle is a cavity of a very
peculiar shape, one branch running forwards towards the
front end of the hemisphere and one backwards towards
the hinder end, and from the latter a third branch runs
downwards and once more forwards. These correspond
respectively to the chief lobes of which each hemisphere
is made up, namely the frontal lobe, the parietal and

/?0

Fio. 162.— Diagram to show the Shape of the Cavity of the Left
Lateral Ventricle, its Connection with the Third Ventricle,
AND the Connection of the Latter with the Fourth Ventricle,

AND hence with THE CENTRAL CaNAL Of THE SFINAL 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 ; LV, L T, L V, lateral ventricle
vrith its anterior comu, A.C., posterior cornu, P.C., and inferior cornu, I.C.

occipital lobes, and the temporal lobe. These lobes are
marked off on the surface of the hemispheres by fissures,
of wliich the most conspicuous are the fissure of
Sylvius, and the fissure of Rolando. (See Fig. 168).
The cerebellum is firmly connected to the rest of the
brain by the transverse fibres which help to form the

1 The two lateral ventricles, one in each cerebral hemisphere, are
reckoned as the first and second ventricles : hence the space between the
layers of the septum lucidum ia known as the fifth ventricle.

506 ELEMENTARY PHYSIOLOGY less.

pons Vai-olii (Fig. 160), 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
(medullary) part of the cerebellum on each side, runs
forwards towards, and disaj^pears under, the corpora
quadrigemina ; this forms the superior peduncle.
The otlier runs backwards towards the bulb and merges,
as the inferior peduncle, into that part of the bulb
wliich is a continuation ujjwards of the lateral columns of
white matter of the spinal cord.

We have seen that the spinal cord consists essentially
of a central canal surrounded by grey matter containing
nerve cells, external to which is a covering of white
matter composed of nerve fibres ; and the arrangement of
the grey and white matter is comparatively simple. Now
from the description we have so far given of the bi'ain, it
is evident that the brain may also be regarded as being
built up of structures which arc 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 sliape.
Moreover, although tlie brain is made up of grey and
white mattei's, which by their greater or less development
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 de-
posits of grey matter, some large, some small, wliich give
to the whole an extraordinary complexity. And this
complexity is still fui-ther increased by the existence of
strands or bundles of nerve fibres, which serve to inter-
connect all these various deposits of grey m.itter, so as to
ensure the possibility of co-ordinated action between the

THE BRAIN 507

individual parts of which the brain as a whole is built up.
It would be neither possible nor desirable to attempt to
deal in any detail in this book with the varied arrange-
ments 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 Quadrigexnina. — These have already been
described as four conspicuous masses of tissue lying in
two pairs above the aqueduct of Sylvius. They consist
of deposits 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. 160, II.),
while those from the posterior pair are believed to make
similar connections with the nerves concerned in hearing
(Fig. 160, VIII.).

The Optic Thalami. — The longitudinal fibres of the
bulb, passing between the transverse fibres of the pons
reappear, as we have seen, in front of the pons as the
cruya cerebri. These diverge from the middle line to
enter the cerebral hemispheres. As each crus passes into
the base of the corresponding 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 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 pro-
jection in the outer side-wall of the third ventricle, and
on the floor of the lateral ventricle. Thus the optic
thalamus is shown at 0. T. in Fig. 161, as part of the wall
of the third ventricle, and as 0. T. in Fig. 163, which re-
l)resents in diagram a horizontal section through the
hemispheres passing above the floor of the lateral

508

ELEMENTARY PHYSIOLOGY

ventricles. The inner sides of the optic thalami are
connected by a small coniinissure (Fig. 163, C), which
extends across the third ventricle ; their outer sides are
imbedded in the substance of the cerebral hemispheres
with which they are connected by nerve fibres, and from

Pio. 163.— Diagram of a Hokizontal Section of the Brain above thk
Floor of the Lateral Ventricles. (After Hirschfeld and Leveill^.)

Sp.c. spinal cord ; B, bulb ; Cb, Cb, cerebellum ; 4, fourth ventricle ;
Q.P., Q.A, corpora quadrigemina ; P, pineal gland; 3, third ventricle;
5, fifth ventricle ; cc, front part of corpus callosum ; iK, IV, H\ lateral
ventricle ; OT., optic thalami ; CS, CS, corpus striatum ; C, com-
missure 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.

their hinder end a bundle of fibres sweeps forward to
pass into the tract of the optic nerves.

The Corpora Striata. — Each corpus striatum may be
regarded as a mass of grey matter deposited obliquely, as
was each optic thalamus, on the course of the crura

XI THE BRAIN 509

cerebi'i, 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. 163, G.S., C.S.), and as
part of the side wall of tlie front end of this ventricle
(Fig. ]61, NC). Each corpus consists of two parts, one
lying in front of the optic thalamus, the other further
back and by the side of the thalamus. Tlie 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.

A clear understanding of the position and relations of
the optic thalami and corpora striata is essential in con-
nection with the course of a tract of nerve fibres with
which we shall deal later, known as the internal
capsule.

The Membranes of the Brain. — The brain is invested
by three membranes whicli are the same in name, and
similarly placed and related to each other as tliose which
we have previously described as covering the spinal
cord (see p. 453). Of these the pia mater is highly
vascular, and carries blood-vessels down into the grey
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 the
hinder end of the cerebral hemispheres to form the roof
of the third ventricle ; this is known as the velum
interpositum. The edges of this velum as it lies
beneath the fornix project on each side into the cavities
of the lateral ventricles arid 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

510 ELEMENTARY PHYSIOLOGY less.

a small opening in the pia mater covering the hinder end
of the fourth ventricle ; this opening is known as the
foramen of Magendie.

11. The Minute Structure of the Brain.— In the
spinal bulb the arrangement of the wliite and grey matter
is substantially similar to that which obtains in the
spinal cord, that is to say, the white matter 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. The
structure of the white matter of the brain is essentially
the same as that of the spinal cord.

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, containing peculiarly sliaped nerve cells,
on the surface of the cerebral hemispheres 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 (sulci) of the former, and
following the peculiar plaits or folds into which the latter
is thrown.

The Cerebellum. — The surface of the cerebellum pre-
sents a corrugated or laminated appearance. When a
section is made through one of its hemispheres it is seen
that the depressions which separate the lamina} give ofi'
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

THE BRAIN 511

matter of the spinal cord, that is to say, it is made up
chiefly of niedullated 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.'

When a section of the external grey matter is cut at
right angles to the surface of a lamella, stained, and ex-
amined under the ijiicroscope, 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. 465) and is
known as the nuclear layer (Fig. 164, N). The outer
layer, immediately under the pia mater, shows a few cells,
but the chief appearance 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 (Fig.
164, X.) this layer, which is much broader than the
nuclear layer, is known as the molecular layer
(Fig. 164, M.). Between these two layers lies a row of
nerve cells of very striking and characteristic appearance,
known as the cells of Purkinj^ (Fig. 164, 1). These
are pear-shaped, with a large and conspicuous nucleus,
the bulbous inner end resting on the nuclear layer,
while the outer end divides into a large number of
processes which run out into the molecular layer as finer
and finer branches. The granular apjjearance of the
molecular layer is in part due to the close j uxtaposition 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 niedullated
nerve fibre. Such are the details which can be made out
1 This is somewhat imperfectly shown in Figs. 161 and 163.

512

ELEMENTARY PHYSIOLOGY

Fio. 164.— Diagram to illustrate the Structure of the SupERriciAL

Gkey Matter of the Cerebellum as seen in a Transverse Section

OF a Lamella.

it/, molecular layer ; N, nuclear layer ; W, central white matter ;

1, cell of Purkinje ; 2, spider-cell ; 5, cell of Golgi ; 3, basket-cell with

one of its baskets, 6 ; 4, another kind of cell in the molecular layer ;

t, tendril fibre ; m, moss-fibre.

In the case of each cell a is the axon or main undivided process, d is
a dendrite or divided process.

XI THE CEREBRAL CORTEX 513

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 con-
structing the preceding diagrammatic Figure 164 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 («) and baskets (h) ; the
baskets in reality surround the bodies of cells of Purkinje
which, for the sake of clearness are not showm 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 addition 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 (t) 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 of the cerebellar cortex. In the cerebrum, on
the other hand, the details of structure vary not incon-
siderably according 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 arrangement of the cells and fibres of
the cortex as a whole.

The grey matter is permeated throughout its whole
thickness 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

L L

514

ELEMENTARY PHYSIOLOGY less.

Fig. 165. — DlAGKA.MMATIC FlGLKE TO ILLUSTRATE THE STRUCTURE OF A

Typical Section ob- the Cerebral Cortex.

I. Molecular layer. II. Layer of pyramidal cells. ///. Layer of poly-
morphous cells.
c and c', cells of the molecular layer ; p', p", p'", pyramidal cells ;
P, cell of the polymorphous layer ; m.r. medullary ray of nerve fibrils
from central white matter ; x, y, 2, tangential bundles of nerve fibrils.

THE CRANIAL NERVES 515

which tlie fibrils of nerves pass to and from these
cells from the central white matter. The latter is
composed, as in the cerebellum, of medullated nerve
fibres. This neuroglia is most marked in the outermost
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. 165, I). Internally to this layer
the cortex is characterised by the presence of nerve cells
whose shape is pyramidal with the apex of each coll pointed
towards the surface of the brain. This layer may there-
fore be spoken of as the layer of pyramidal cells
(Fig. 165, 11). 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 characterised by the presence of nerve
cells of a somewhat irregular form, hence this layer is
known as the layer of polymorphous cells (Fig. 165,
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. 165, m.r.) while others lie parallel to the surface
as tangential rays (Fig. 165, x.y.z.).

12. The Cranial Nerves.— Nerves are given oflF from
the brain in pairs, which succeed one another from before
backwards, to the number of twelve (Figs. 160 and 166).
These are often called "cramai" 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.
But these two nerves require special notice. That which
is commonly called the olfactory "nerve " is really a lobe

L L 2

516 ELEMENTARY PHYSIOLOGY less.

of the brain and contains nerve cells. The proper olfac-
tory nerves are bundles of fibres which proceed from the
under surface of the above and traverse the cribriform
plate to be distributed to the olfactory mucous membrane.
And it is an extremely remarkable fact that these fibres
closely resemble the non-medulla,ted fibres of the sympa-
thetic nerves, in being hardly anything more than
neuraxes, bearing nuclei at intervals. A sheath, appar-
ently representing the neurilemma, is however present in
each fibre.

The optic "nerve " is also properly speaking a lobe of
the brain, and it retains its chai-acter as a part of the
centi'al nervous system in so far as its fibres have no
neurilemma and are nodeless, but it contains no nerve cells
along its course.

The third pair is called motor OCUli (mover of the
eye), because they are distributed to all the nmscles 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
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 sensoiy root. It
is the nerve which supplies the skin of the face and the
muscles of the jaws, and, having three chief divisions,
is often called trigeminal. One branch containing
sensory fibres supplies the fore-part of the mucous
membrane of the tongue, and is often spoken of as the
gustatory.

The seventh pair furnish with motor nerves the muscles
of the face, and some other muscles, and are called
facial.

XI THE CRANIAL NERVES 517

The eighth pair are the auditory nerves. The auditory
is, as we have seen (p. 392), divided into the cochlear
and vestibular nerve. (See later, p. 52.3).

The ninth pair, or glossopharyngeal nerves, 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.

The tenth pair is 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 pneumogastric supplies the larynx, the lungs, the
liver, and the stomach, and branches of it are connected
with the heart.

The eleventh pair again, called spinal accessory,
differ widely from all the rest, in arising, in part, from the
sides of the spinal cord, between the anterior and posterior
roots of the cervical 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
glossopharyngeal. They are purely motor nerves, supply-
ing certain muscles of the neck, while the pneumogastric
is mainly sensory, or at least afferent.

The ticelfthpair, or hypoglossal nerves, are the motor
nerves which supply the muscles of the tongue.

Of these nerves, the two foremost pairs do not properly
deserve that name, but are, as we have said, really
processes of the brain. The olfactory pair are prolonga-
tions of the cerebral hemispheres ; the optic pair, of the
walls of the third ventricle.

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 160) and are continued on backwards,

518

ELEMENTARY PHYSIOLOGY

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
corpora quadrigeuiina. 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 C70ss

Fio. 166.— A Diagram illustkating the Superficial Origin of the
Cranial Nervks.

H, the cerebral hemispheres ; C.S. corpus striatum ; Tk. optic thalamus ;
P. pineal body ; Ft. pituitary body ; C.Q. corpora quadrigemiiia , Ch. cere-
bellum ; M. medulla oblongata ; /. — XJI. the pairs of cerebral nerves ;
Sp. 1, Sp. 2, the first and second pairs of spinal nerves.

over to the opposite side of the brain ; the fibres from the
other half of each retina pass into the brain without
crossing. Hence the right optic tract contains the fibres
from tlie nasal half of the left retina and from the other
or temporal half of the right retina, and similarly the
left optic tract is made up of the fibres of the temporal

XI FUNCTIONS OF THE SPINAL BULB 519

half of the leffc 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 " corresponding points and single vision with two
eyes " (see p. 445). Going through the optic cliiasma,
there are also fibres joining one side of tlie brain to the
other via the optic tracts and probably fibres joining the
two eyes.

13. The Functions of tlie 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

Fig. 167. — Diaokam to illustrate the Decussation of Fibres in the
Optic Chiasma.

R, right eye; i, left eye; R.Op. f-ight 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.

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, so that injuries
to the bulb may arrest the respiratory process (p. 158).
Further it contains centres for the regulation of the
heart-beat (p. 77) and of the condition of the blood-vessels
over the whole body (p. 69). But beyond these the bulb
also contains centres for the nervous act of swallowing,
for the reflex secretion of saliva, and for many other
actions. Thus we find that simple puncture of, one side

520 ELEMENTARY PHYSIOLOGY LESa

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 kidne3's, and thus this slight injury to
the medulla produces a temporary disorder closely re-
sembling 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 their way through the bulb
from or to the spinal nerves. And a similar 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 crussed pyramidnl tract the
crossing of the fibres which compose the tract takes place
by means of what is called the decussation of the
pyramids in the anterior columns of the bulb (Fig. 172).
This point is indicated in Fig. 160 by a group of small
converging marks on the surface of the bulb just above
the cut end marked M. The fibres of the small direct
pyramidal tract cross below the bulb in the spinal cord
by means of its anterior white commissure (see Fig. 143).
Similarly the fibres concerned 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 un-
frequently happens, a blood-vessel gives way in the right
cerebral hemisphere, leading to a destruction of nervous
matter there, tlie result is that the left arm, and left leg,
and left side of the body generally are paralysed, tliat is,
the will has no longer any power to move the muscles of

FUNCTIONS OF THE CEREBELLUM 521

that side, and impulses started in the skin of that side
cannot awaken sensations 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 right cerebral hemisphere, the left
side of the man's face is paralysed as well as the left side
of his body, that is to say, impulses cannot pass to and
from his brain and the left facial and fifth nerves. The
impulses along these nerves also decussate, and reach the
right 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 right), above the decussation of the pyramids,
in such a way that the fifth and facial ner\'es 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 tlie left side of his body, is
paralysed on the right side of his face.

14. The Functions of the Cerebellum. — When
speaking of reflex actions we pointed out (p. 487) that the
complicated movements of walking when once started by
the will are essentiallj' reflex in their continued produc-
tion. Moreover we also drew attention to the fact that
the co-ordhiatioii of the efferent impulses which, although
distributed to many difi"erent 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 becomes 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. 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 pro-
duction depends largely on tactile and visual impulses.

522 ELEMENTARY PHYSIOLOGY less.

Now experiment and observation in cases of disease have
shown quite conclusively that the one (jreat function of
the cerebellum is to play a most important part in the co-
ordination of the actions, nervous and muscular, by ^vhich
the inovements of the body are carried on.

After the cerebullum has been completely removed, an
animal does not differ in any essential respect from its
normal condition as i-egards its intelligence or its special
senses, such as sight or hearing. But with regard to its
movements a great difference is observed in the absence
of the cerebellum ; all movements are now clumsily
executed — there is a want of orderliness or co-ordination.
This statement sums up our knowledge of the function of
the cerebellum.

We do not know hoio the cerebellum works in thus
keeping an orderly grip over the mechanisms of move-
ment ; but we see how easily it may do so when we
consider its connections with the spinal cord and with the
rest of the brain. We saw (p. 494) that the cerebellum
as well as the cerebrum sends fibres which connect with
the motor neurons in the anterior horn and (p. 491) that
two large tracts of afferent fibres from the spinal cord
pass into the cerebellum, viz. the cerebellar tract by the
inferior peduncle and the ascending antero-lateral tract by
the superior peduncle. Moreover the cerebellum is
connected with that part of the bulb in which the posterior
colum7i ends. Thus it may be a recipient of a vast
number of afferent sensory impulses, which are so
essential for co-ordinated movement. But each half of
the cerebellum is further connected with the cortex of
the cerebral hemisphere of the opposite side in two ways,
firstly, by the fibres of its middle peduncle across the
pons Varolii, and secondly, and more directly, by fibres
in its superior peduncle (see p. .o06). 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.

FUNCTIONS OF THE CEREBELLUxM 523

When describing the arrangements of the internal ear,
it was stated that the semicircular canals have functions
other than that of hearing. 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 vestibule, the utricle,
saccule, and semicircular canals. The.se two nerves, the
axons of which originate in the ear, terminate in connection
with groups of cells lying in the spinal bulb, and the
group of cells as.sociated with the vestibular nerve is
directly connected by a strand of fibres with the cerebellum.
Thus there is a path by which afferent (sensory) impulses
from the semicircular canals may directly reach the
cerebellum and there be turned to account in the co-
ordination of movements. Bearing this in mind, it is not
surprising to lind that the semicircular canals play a
very important part in the guidance of co-ordinated
movement.

The semicircular canals lie in three planes at right
angles to each other (p. 392). When any one of the canals
is experimentally injured, the animal executes a series of
oscillatory movements of the head, which are, broadly
speaking, in the plane of that canal. When all three
canals are injured, the animal is thrown into continuous
movements of tlie most varied and oft«n extraordinary
kind, and has lost all power of balancing itself in a
normal way. Not infrequently in man these canals
undergo injury as the result of disease, and in this case
the feelings experienced by the patient are those of ex-
treme 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 semicircular canals are
of very great importance as organs such that impulses
arising in them, enable us to maintain oir bodily
equilibrium, and there are reasons for thinking that the
utiicle and saccule also have a very similar function.

524 ELEMENTARY PHYSIOLOGY

15. The Functions of the Cerebral Hemispheres.—

The Hemispheres the Seat of Intelligence and W^ill. —

The functions of most of the parts of the brain wliich lie
in front of the spinal bulb are, at present, very ill under-
stood ; but it is certain that extensive injury, or removal,
of the cerebral hemispheres puts an end to intelligence
and voluntai-y 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 spontaneous movements ; tliey never
occur unless the animal be stimulated from without.
Removal of the cerebral hemispheres is alone sufficient to
deprive the frog of all spontaneous or voluntary move-
ments ; but the presence of the 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, resting 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 con-
tinues swimming until it finds some object on which it

XI FUNCTIONS OF CEREBRAL HEMISPHERES 525

can rest ; and when placed on its back immediately turns
over and resumes 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, wUl at
last reach the edge, upon which when the board becomes
vertical he will seat liimself with apparent great content.
Nevertheless, though his movements when they do occur,
are extremely weU combined and apparently identical
with those of a frog possessing the whole of his brain,
he never moves spontaneously, and never stirs unless
irritated.

Thus the parts of the brain below the cerebral hemi-
spheres constitute a complex nervous machinery for
carrying out intricate and orderly movements, in which
aflerent 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 phenomena which we term intelligence and will ;
and there is experimental and other evidence which seems
to indicate a connection between particular parts of the
surface' of the cerebral hemispheres, and particular acts.
Thus 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 is said to cause blindness. But
the exact way in Avhich these effects are brought about is
not yet thoroughly understood ; and even if it should be
ultimately proved beyond all doubt, that the central end-

526 ELEMENTARY PHYSIOLOGY

organ of vision (p. 421) 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. 386) consists of other nerve-cells lying else-
where on "the cerebral surface, it will still leave us com-
pletely in the dark as to what goes on in the cerebral
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 ante-
cedent to 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 difierent 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
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
completely 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 constitute
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
afferent 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

XI REFLEX ACTIONS OF THE BRAIN 527

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 in the production of speech. Per-
haps the reader is standing up and accompanying the
lecture with appropriate gestures. And yet every one of
these muscular acts may be performed with utter uncon-
sciousness, on his part, of anything 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
artificial reflex actions, that is to say, an action may
require all our attention and all our volition for its first,
or second, or third performance, but by frequent repeti-
tion 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 leai'n
his drill — for instance, to put himself into the attitude 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, omiot. There
is a story, which is credible enough, though it may not be
true, of a practical joker, who, seeing a discharged veteran
carrying home his dinner, suddenly called out " Atten-
tion ! " whereupon the man instantly brought his hands
down, and lost his mutton and potatoes in the gutter.

528 ELEMENTARY PHYSIOLOGY

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
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 whicli 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.

Iiocalisation of Function in tbe Cortex of the
Cerebral Hemispheres. — We have already alluded (p. 525)
to the fact that there is a connection between particular
parts of the surface of the cerebral hemispheres and
particular acts or special sensations. The possibility thus
indicated is of extraordinary impoi-tance and must now be
dealt with in some detail.

The cerebral hemispheres are separated along the
middle 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. 160 and 161). The
surface of each hemisphere is folded into a large luimber
of convolutions or gyri separated from each other
by sinuous depressions or sulci (see Fig. 159, C.C.).
Some of these depressions are deeper and more marked
than others, and are spoken of as fissures. Of these
the most conspicuous ai-e known as the fissure of
Sylvius, the fissure of Rolando, the parieto-
occipital fissure, and the calcarine fissure. The

THE CEREBRUM

529

position of these is shown in the accompanying diagrams.
These fissures may be taken as roughly dividing the
surface of the brain more or less distinctly into several

„^ F R 0 N Ta

P»rielo-occipit»l
Fissure

'f^'PORAL tC^'

Fio. 16S.— Diagram of ()i ter Surface of thb Right Cerebral
Hemisphere.

Fissure of Rolando

^^^PORAL LOBE

Fig. 169. — Diagram of the Inner (Mesial) Surface of the Right
Hemisphere to show the Parieto-Occipital akd Calcarine
Fissures.
The corpus callosum is seen shaded in section.

lobes, frontal, parietal, occipital, and temporal.
The extent of these is shown on Figs. 168 and 169.

When the surface of the hemisphere is stimulated

Al SI

530 ELEMENTARY PHYSIOLOGY less.

electrically close to the fissure of Rolando along its
anterior margin, very definite movements take place in
the limbs of the opposite side of the body. If care is
taken to localise the stimulation as far as possible witliin
the limits of a small area of the cortex tlie resulting move-
ments are found to l)e limited to a correspondingly small
group of muscles of the limb affected. Again, if that
piece of cortex whose stimulation gives rise to movements
be cut out or extirpated, tlie animal so operated on is
found to have lost the power of executing this particular
set of movements. The outcome of such experiments
makes it clear that the cerebral cortex along the course of
the fissure of Rolando is concerned in the develoj^ment of
muscular movements ; hence the name of " motor areas "
was given to these parts of the cortex. Our knowledge
of the existence and position of these areas as derived
from experiments on animals is moreover completely con-
firmed 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
similar to those obtainable by stimulation or extirpation
of cortical areas in animals.

Proceeding in a similar way it has been further found
that certain portions of the cortex are peculiarly connected
with the development of sensations, so that we come to
speak also of "sensory areas." 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 experi-
ment revealed the existence and position of the centres to
which we have so far referred it was noticed by a French
physician named Broca that patients who had exhibited a
curious inability to pronounce definite words or syllables

LOCALISATION IX CEREBRAL CORTEX 531

duriny life were found after death to have suifered from
disease or injury of the third frontal concolntion of the left

PISSI//>£ OF RoLANOO

Fr.L

OcL

Te.L
FiQ. 170. — Diagram of Outer Si-rface of Right Cerebhal Hemisphbbk

TO SHOW THE POSITION OF CORTICAL AREAS.

The areas for the leg, arm, and face are marked by vertical lines,
horizc^ntal lines, and dots respectively. The area for speech lies really in
a similar position on the left hemisphere.

Fr.L. frontal lobe ; Oc.L. occipital lobe ; Te.L. temporal lobe ;
Sy.P. fissure of Sylvius.

Fissure of Rolando

Par Oc F

OcL

TeL

Fig. 171. — Diagram of Inner (JIesjalj Surface of the Right Cerebral
Hemisphere to show the Position of Cortical Areas.
The cofpus callosum is seen shaded in section.

Fr.L. frontal lobe ; Oc.L. occipital lobe ; Te.L. temporal lobe ;
Par-Oc.F. parieto-occipital fissure.

side of the brain immediately above the Sylvian fissure ;
hence, this part of the cortex is known as Broca's con-

ii M 2

532 ELEMENTARY PHYSIOLOGY less.

volution (see Fig. 170). The disorder is, from its nature,
known as aphasia («, privative of, (fida-is, speech) and
may take several forms ranging from complete inability
to speak at all to an inability to utter certain words, and
hence to speak 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.

We need say no more in proof of the connection of
part of the right side of the brain in those who are left-
handed. Speech however is a very complicated operation
involving a number of mental processes such as the
thinking of the words to say, and the command of
muscular movements for their production. Broca's area
is probably involved in the latter and is by no means the
only portion of the brain involved in speech.

The portion of the brain concerned with tactile sen-
siition is close to the motor areas, being situated just
beliind, instead of just in fnnit of the fissure of Rolando.
The situations in which, so far as we know, afferent
impulses are converted into tliose altered states of con-
sciousness to which we give the name of sensations are
shown on Figs. 170 and 171.

The Internal Capsule. — The brain, as we have
previously said, may be regarded as built up round a very
peculiarly shaped central canal by means of thickenings
of the walls of that canal due to the development of
masses of nerve fibres and deposits of grey matter. We
have described the position of the most conspicuous of
these deposits and have incidentally referred to many of
the more important inter-connections of the chief parts of
which the brain as a whole consists. Moreover, we have
referred in some detail to the nature of the connection of
the spinal cord with the biain by means of very definite
" tracts " of fibres. Among these, particular stress was
laid on that tract which is known as the (crossed)
pyramidal tract, and it was pointed out (p. 492) that the

THE INTERNAL CAPSULE

533

fibres of this tract really start in, and depend for their
nutrition upon, the cells of the cortex of the cerebral
hemispheres. But this dependence is not extended to
the cells of the cortex generally ; on the contrary it is
limited to the cells of those parts of the cortex to which
we applied the expression " motor areas," in the preceding
section. Hence the pyramidal tract degenerates when

Fio. 172. — Diagram ot the Coirse of the Cros-sed Pyramidal Tract
FROM THE (Motor) Cerebral Cortex to thf. Spinal Cord.

C.C. corpus callosum : O.T. optic thalamus; C.S. corpus striatum;
Int. Cap. internal capsule ; Cb, cerebellum ; D. P. decussation i.f the
pyramids; Cr.p., Cr.p. crossed pyramidal tracts (see Fig. 156); S}}.C.
spinal cord.

the cells of the " motor " areas are destroyed, and thus we
come to regard tlie pyramidal tract as the path bj' which
the (motor) impulses developed in the cortex are dis-
tributed to the cells of the spinal cord as a preliminary to
tlieir exit from the cord along the anterior roots of the
spinal nerves. We traced the pyramidal tract into the
bulb, and now we may, in conclusion, follow its further
cour.sc until its fibres make connection with the cerebral
cortex.

534 ELEMENTARY PHYSIOLOGY

When this tract is traced forwards above the bulb it is
found to pass into the cms cerebri of its own side and to
pass along the lower or ventral part of the crus, which is
known as the pes. The optic thalami and corpora striata
are, as we have seen (p. 508), deposits on the course of
the crura as the latter sweep forward into the hemispheres.
Now when tlie fibres of the pyramidal tract reach the level
of the thalamus they rise up out of the cms, and, spread-
ing like a fan, pass posteriorly between the optic thalamus
and the hinder end of the corpus striatum, and anteriorly
between the two parts of wliich the front end of the corpus
striatum is composed. The fibres of the pyramidal tract
as they thus pass between the optic thalamus and corpus
striatum are known as the internal capsule, and the
bend the fibres make as they rise out of the crus is known
as its "knee." From this point onwards the fibres pass
directly to their connections with the cells in the grey
matter of the cortex along the region of the fissure of
Rolando. The preceding Figure 172 shows diagrammatic-
ally the course of the pyramidal tract from the cerebral
cortex to the spinal cord.

LESSON xn

HISTOLOGY ; OR, THE MINUTE STRUCTURE OF
THE TISSUES

1. The Body built up of Tissues and the Tissues

of Cells. — In the first chapter attention was directed fx)
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.

lu dissecting a limb there is no difficulty in distinguish-
ing tlie bones, the cartilages, the nmscles, 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 similar 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 homoiomera, " similar
parts." In modern times they have been termed tissxtes,
and the branch of biology which is concerned witli 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 physiohigy is, in

536 ELEMENTARY PHYSIOLOGY less.

ultimate analysis, the investigation of the vital properties
of the histological units of which the body is composed.
And even the elements of physiology cannot be thoroughly
comprehended 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 tlie 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 application of the microscope to the
elucidation of the minute structu.j of the tissues ; and
this has had the remarkable result of proving that these
tissues themselves are made up of extremely small
homoio^nera, or similar parts, which are primitively alike
in form in all the tissues.

Every tissue therefore is a compound structure : a
multiple of histological units, or an aggregation of his-
tological elements ; and the properties of the tissue are
the sum of the properties of its components. The dis-
tinctive character of every fully formed tissue depends on
the structure, mode of union, and vital properties of its
histological elements wlien tliey are fully formed.

2. The Primitive Tissues. — Each tissue can be traced
backtoayouu^ or embryonic condition, in which it has no
chanuteristic 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 nucleated cells. The colourless blood corpuscle
(Lesson III., p. 100) is a typical representative of such a cell.
And it is substantially correct to say (1) that the histolo-

THE TISSUES

537

jfejaV^X^-®

gical 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
aggregation of such cells.

3. The Body starts as a Single Cell, the Ovum,
which then divides into primitive cells.— The body of a
man or of any of the higlier animals commences as an ovum
or egg. This (Fig. 173) is a minute transparent spheroidal
sac 200/i, {j^Q of an inch) in diameter in man, which con-
tains a similarlj- spheroidal mass
of protoplasm, in which a single
large nucleus is imbedded.

The first step towards the
production of all the complex
organisation of a mammal out
of this simple body is the di-
vision of the nucleus into two
new nuclei which recede from
one another, while at the same
time the protoplasmic body be-
comes separated, by a narrow
cleft which runs between the
two nuclei, into two masses, or
blastomeres (Fig. 174, a), one for each nucleus. By the re-
petition of the px-ocess 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 nutriment 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 appearance, 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 aggregates of
undifferentiated cells.

Fig. 173.— Diaukam of the
Ovum.

a, granular protoplasm ;
6, nucleus, called "germinal
vesicle " ; c, nucleolus, called
" genninal spot.'

538

ELEMENTARY PHYSIOLOGY

LESS.

4. The Differentiation of the Primitive Cells.—
Gradually, these undifferentiated cells become changed
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

Fio. 174. — The Si'ccessivk Division of the Mammalian Ovum into
Blastomeres. Somewhat diagrammatic.

a, division into two, 6, into four, c, into eight, and d, into several
blastomeres. The clear ring seen in each case is the zonii pellucida, or
membrane investing the ovum.

organs, each organ generally consisting of more than one
tissue.

And this differentiation of structure 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 structure. But coincident with their
differentiation into tissues, a division of labour takes

THE TISSUES 539

place, so that in one tissue the cells manifest special pro-
perties and carry on a special work ; in another they have
other properties, and other work ; and so on.

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.

(i) The most important tis.sues are the musctllar and
nervous 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 of the
body : 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 of the
body.

(iii) The remaining principal tissues of the body,
namely the so-called connective tissue, cartila-
ginous 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 their 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 of the body (see Lesson
II.), 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 tegumentary tissue : a
similar combination of epithelium with other tissues

540 ELEMENTARY PHYSIOLOGY

constitutes the mucous membrane lining the alimentary
canal, and also occurs in the so-called "glandular" tissue.
The structure of these, as also of muscle and nerve and
bone, has been already described, so that we may confine
our attention here to the other principal tissues^
epithelial tissues, the connective tissues and cartilage.

6. The Epidermis. — A good example of this tissue is
to be found in the skin, which, as we have seen (Lesson
v.), consists of the superficial epidermis which is non-
vascular and epithelial in nature, and of the deep dermis,
which is vascular, and is indeed chiefly composed of
connective tissue carrying blood-vessels and nerves. And
in all the mucous membranes there is a similar superficial
epithelial layer, which is here simply called epitheliem,
and a deep layer, which similarly consists of connective
tissue carrying blood-vessels and nerves and may also be
spoken of as dermis.

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 de-
tached by maceration will be found to be tolerably dense
and coherent, 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
papillae, the deep or under surface of the epidermis
presents innumerable depressions into which the papillfe
fit, giving it an irregular appearance, somewhat like a net-
work. Hence it used not unfrequently to be called the
network of Malpighi (rete Malpighii), after a great
Italian anatomist of the seventeenth century, who first
properly described it. On the other hand, its soft and
gelatinous character 'led to its being called mucous layer
{stratum Tnucosum). Chemical analysis shows that the
firm outer layer of the epidermis differs irvm the deep
soft part by containing a great deal of horny matter.

THE SKIN 541

Hence this is distinguished as the horny layer (stratum
corneum).

In the living subject the superficial layers of the
epidermis 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
derma, and, accumulating between the upper and lower
layers of the epidermis, detaches the latter.

The epidermis is constantly growing upon the deep
or dermic side in such a manner that the horny layer
is continually being shed and replaced. The "scurf"
which collects between the haii's and on the whole surface
of the body, and is removed by our daily brushing and
washing, is nothing but shed ejjidermis. When a limb
has been 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 epidermic scurf is mixed with water
and examined under a power magnifying 300 or 400
diameters, it will seem to consist of nothing but irre-
gular 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
character of nucleated cells, in which the protoplasmic

542 ELEMENTARY PHYSIOLOGY less.

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
tu»"ned down, its surface veiy gently scraped with a blunt-

Pio. 175. — Two Epithelial Scales from the Interior of the Mouth.

A small nucleus n is seen in euoh, a.s well as fine granulations in the
body of the plate. The edges of the plates are irregular from pressure.
Magnified about 400 times.

edged knife, and the substance removed be spread out,
and 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 epidermic
scales, but more transparent, and each provided with a
nucleus in its centre (Fig. 175).

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 homy
external layer of the epidermis is composed of coherent

THE EPIDERMIS 543

comified 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 nucleated protoplasmic substance, that is,
into nucleated cells which in the innermost or deepest
part of the layer are columnar in form, being elongated
perpendicularly to the face of the dermis, on which they
rest, and which in the intermediate region present
transitions in form and other respects between the.se and
the shed scales.

A thin vertical section of epidermis (see Fig. 57, p. 191)
in undisturbed relation with the subjacent dermis, leaves
not the smallest doubt (a) that the epidermis consists of
nothing but nucleated cells, with perhaps an infinitesimal
amount of cementing substance between them ; (h) that
from the deep to the superficial part of the dermis, the
cells always present a succession from columnar or sub-
cylindrical protoplasmic forms to flattened completely
comified forms. And since we know that the latter are
constantly being thrown off, it follows (c) that these
gradations of form represent cells of the deep layer
which are continually passing to the surface, and being
thrown off" there.

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 ^
primitive embryo cell.

544 ELEMENTARY PHYSIOLOGY less.

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
multiplication for the whole epidermis ; they are in fact
the progenitors of the superficial cells which, as they are
thrust away by the intercalation of new cells between
the last formed and the progenitors, become meta-
morphosed 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 which follows its own laws of growth and multiplica-
tion, and is dependent upon nothing save the due supply
of nutriment fi'om 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 w^hich 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 further, the learner should try to form a definite
notion of their actual and relative dimensions by com-
parison with more familiar objects. A hair of the human
head of ordinary fineness has a diameter of about ^Joth
(say 0"003) of an inch, or 0"08 mm. (millimeter). 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 yg'tfoth of an inch, i.e. 0"001 inch,
or about 0"025 mm. ; while the fine point of such a hair
may be as little as o^^goth of an inch, about 0"001 mm.,
or even less in diameter.

In microscopic histological investigations the range of
the magnitudes with which we have to deal ordinarily lies
between 0"1 and O'OOl millimeter ; that is to say roughly
between one two hundred and fiftieth and one twenty-five
thousandth of an inch. It is therefore extremely con-
venient to adopt, as a unit of measurement, 0 001

XII MUCOUS MEMBRANE 545

millimeter, called a micro-millimeter, and indicated by the
symbol /ii, of which all greater magnitudes are multiples.^
Thus, if the extreme point of a rabbit's hair has a
diameter of 1m, the middle of the shaft will be 2o/i, and
the shaft of a human head hair 80/i.

Adopting this system, the deep cells of epidermis have
on an average a diameter of 12/x or more, the nuclei of
4/x to 5/i, while the superficial cells are plates of about 2bn,
the nuclei retaining about the same dimensions. The
diameter of a white corpuscle of the blood is about lOu,
that of a red corpuscle being 7fi to 8/x. Hence the deep
cells of the epidermis are rather larger than white blood
corpuscles, and the uppermost ones much larger, at least
in superficial area.

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 juxta-
position. But except in the region of the mouth, where,
as we have seen, the epithelium, like the epidermis, is
composed of many layers of cells, arranged as a soft
Malpighian layer and a hard corneous layer, and the
oesophagus where the structure is similar, the epithelium
of the alimentary canal and the continuations of that
epithelium into the ducts and alveoli of the various glands,
consists 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 inconspicuous small cells may be seen
below the latter. The cells constituting this single layer
vary in shape, being cylindrical or conical or, as especially
in the glands, cubical or spheroidal ; but they always are
delicate masses of protoplasm, each containing a nucleus.

1 Since 1 millimeter is very nearly equal to ^ of an inch, n = rdva ©^
an inch.

N N

546 ELEMENTARY PHYSIOLOGY less

The polygonal hepatic cells (see p. 212), are in ro;ility the
epithelium cells beloni^ing to the minute biliary ca,nals
passing between them.

In the trachea and bronchi, the epithelium of the
mucous membrane consists again of a single layer of
cells, which are cylimlrical in form and ciliated. In
the ureter and bladder, on the other hand, the epithelium
consists of several layers of cells which are frequently
irregular in form.

Lastly, the blood-vessels and lymphatic vessels and the
large serous cavities, such as the peritoneal and pleural
cavities, are lined by a peculiar epithelium, different in
origin from the epithelium of the skin and mucous mem-
branes. It consists of a single layer of flat, nucleated
plates cemented together at their edges. The form of
the plate or cell varies, being sometimes polygonal,
sometimes spindle-shaped, sometimes quite irregular (see
Fig. 29).

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 e{)ithelium, 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 material 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 themselves and called a
matrix. In epithelium, though the cells are sometimes
joined together by a cement material, this is never abun-
dant 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 breastbone. A
thin but tough layer of vascular connective tissue invests,

CARTILAGE 547

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 more-
over it is elastic, returning to its original shape when
the pressure is removed. It may be easilj' cut into very
thin slices, which are as transparant as glass, and to the
naked eye appear homogeneous. Dilute acids and akalies
have no effect upon it in the cold ; but if it is boiled in
water, it yields a substance similar to gelatin, but some-
what 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 perichondrium. The vascular perichondrium therefore
stands in the same relation to the non-vascular cartila-
ginous tissue as the vascular dermis does to the non-
vascular epidermis. But, since the cartilage is invested
on all sides by the perichondrium, 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 perichondrium, 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 employment of an ordinary hand magnifier is sufficient

N N 2

548 ELEMENTARY PHYSIOLOGY less.

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. 176) these jjoints prove to be nucleated
cells, the cartilage corpuscles, varying in shape, but
generally more or less spheroidal, sometimes 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 ceU. And sometimes globules

Fio. 176. — HvALiNB Cartilaoe. a thin Section iiiculv Magnified.

m, matrix ; a, group of two cartilage cells ; 6, a grovip of four cells ;
c, a cell ; n, nucleus.

of fat appear in the protoplasmic bodies of the cells, and
may completely fill them.

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-spheriodal cells in contact, and hemi-spheriodal cells
separated by a mere film of intercellular substance, and
widely separate spheroidal, ellipsoidal, or otherwise shaped

CARTILAGE

549

cells. In size, the cells vary very much, some being as
small as 10/ii, and others as large as 50/i, 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 contain them, and the appearance of the
tissue is greatly altered.

Fio. 177.— A Small Portion of a Section of Articular Cartilaqb
(Frog) very highly magnified (600 diam.).

s, matrix or intercellular substance ; p, the protoplasmic body of the
cartilage corpuscle ; n, its nucleus, with n\ 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 oi)lique line
indicates the earlier division into two.

No structure is discernible in the matrix or intercellular
substance under ordinary circumstances ; but it may be
split up into thin sheets or laminse. The portions of
matrix immediately surrounding the several cavities some-
times differ in appearance and nature from the rest of
the matrix, so as to constitute distinct capsules (Fig.

550 ELEMENTARY PHYSIOLOGY

177, c) for the 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
substance, until at length the transparent chondrigenous
material is replaced by the fibrous collagenous substance
of connective tissue (p. 554), and the cartilage cells take
on the form of "connective tissue corpuscles."

In a very young embryo we find in the place of a
sterno-cnstal cartilage nothing but a mass of closely-
applied, undifferentiated, nucleated cells, having the same
essential characters as colourless blood-corpuscles, or as
the deepest epidermic 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 quantity, between the central
cells of the mass, and a delicate sterno-costal cartilage is
thus formed. This is converted 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 ; (6) 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
increased is not certainly made 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
oflf fresh outer cornified layers, we should have an epi-
dermic structure which would resemble cartilage except
that the "intercellular substance" would be corneous
and not chondrigenous. And it is possible that the
intercellular aubstance of cartilage may be formed in this

xii FIBRO-CARTILAGE 551

way. But it is possible that the chondrigenous material
may be, as it were, secreted by and thrown out between
the cells, as the constituents of the bile are thrown out
between the hepatic cells, or at all events manufactured
in some way by the agency of the cells, without the
substance 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 know-
ledge 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.

Fia. 178. — Section of White Fibro-Cartilaqe. (Hardy.)

The embryonic cells, which give rise to cartilage, are
not distinguishable by any means we at present possess
in any respect of importance from those which give rise
to epidermis.

Nevertheless, the common form must disguise a different
molecular machinery, inasmuch as the two, when set
going by the conditions of temperature, supply of oxygen
and nutriment to which they are exposed in the living
economy, work out, as their ultimate products, tissues
which differ so widely as cartilage and epidermis.

The embryonic cartilage cells, like the embryonic epi-
dermic cells, are living organisms in which certain

552 ELEMENTARY PHYSIOLOGY less.

definitely limited possibilities of growth and metamor-
phosis are inherent, as they are in those equally simple
organisms, the spores of the conunon moulds, Penicilliuvi
and Mtccor. Given the proper external conditions, the
latter grow into moulds of two diffei'ent kinds, while the
former grow into cartilage and horny plates.

(ii) TVhite Fibro-Cartilage. — Since cartilage is a tissue
which serves chiefly for the purposes of supporting
and connecting other structures of the body, it requires,
in certain positions, to be somewhat more tough and re-
sistent, 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

Fia. 179. — Section of Yellow Elastic Cartilage. (HARDr.)

articular cartilage (see Fig. 97, c). Similar discs lie in
between and are attached to the bodies of the vertebrae.
They act not only as a sort of cushion to break the "jar "
arising from a sudden concussion of the vertebral column,
but also bind the vertebrte into a column which is re-
sistent but at the same time flexible. The additional
strength required by the cartilages of these discs is pro-
vided by the introduction into their matrix of bundles of
white fibrous connective tissue ; hence the name, white
tibro-cartilages (Fig. 178).

(iii) Yellow or Elastic Fibro-Cartilage. — In certain
other parts of the body cartilage is required to be pecu-

xii AREOLAR TISSUE 553

liarly elastic and flexible, as in the epiglottis and cartilage
of the external ear. In this case the requisite elasticity
is given to it by the introduction into the matrix of a
dense feltwork of fibres of yellow or elastic connective
tissue (Fig. 17'.)).
11. The Connective Tissues.

(i) Areolar Tissue. — If a specimen of the loose sub-
cutaneous tissue which binds the skin to the body or of
the similar tissue from between the nuiscles of a limb,
be examined, it is found to be a soft stringy substance,
which, if a small portion is cai-efuUy spread out in fluid on
a glass slide and examined without the aid of any micro-
scope, is seen to consist of semi-transparent whitish bands
and fibres, of very various thicknesses, interlaced so as to
form a network, the meshes of which are extremely
irregular. Hence the older anatomists termed this tissue
areolar or cellular.

Boiled in water, the connective tissue swells up and
yields gelatin, which sets into a jelly as the water cools.
After prolonged boiling, especially under pressure, it
almost entirely dissolves away into gelatin, only a small
filamentous solid residue remaining behind.

Dilute acids and dilute alkalies also cause connective
tissue to swell up and acquire a glassy transparency, but
they do not dissolve it. For if to a portion of the tissue
thus altered by either acid or alkali, alkali or acid is added
sufficient to neutralise the first, the tissue returns to its
normal condition.

If a specimen thus rendered transparent by dilute
acetic acid is examined with a magnifying glass, fine
dark lines and dots are seen to be scattered through
the apparently homogeneous substance. Placed under
the microscope, the lines are seen to be sharply defined
fibres of a strongly refracting substance. They are very
elastic and are unaffected by even strong acids or alkalies
or by prolonged boiling. Hence these elastic fibres
formed a considerable part of the residue above mentioned.

654

ELEMENTARY PHYSIOLOGY

The dots seen with the magnifying glass are shown by
the microscope to be small nucleated cells. They are
termed connective tissue corpuscles, just as car-
tilage cells are called cartiku/e corpuscles.

Thus, connective tissue resembles cartilage in so far as
it consists of cells separated by a large quantity of inter-
cellular substance ; but this intercellular substance is
soft, areolated fibrous, and, for the most part, either

a, small bundles of white fibrous tissue ; '), larger bundles ;
c, single elastic fibres.

collagenous or elastic, in contradistinction from that of
cartilage, which is hard, solid, laminated and chondri-
genous.

A specimen of fresh connective tissue prepared for the
microscope in its own liuid exhibits a very different
appearance. The field of view is occupied by strings or
threads of extremely various thicknesses which cross one
another in all directions and are often wavy. Some of
tJhe threads can be recognised as elastic by their strongly

CONNECTIVE TISSUE

555

refracting character, but the majority of them are pale
and not darkly contoured. All the thicker threads and
strings present a fine longitudinal striation as if they were
bundles of extremely fine fibrillte (Fig. 181, a). At intervals
.such bundles are often encircled by rings of a more re-
fractive sub.stance, and fibres of the like character may be
disposed spirally round the bundles.

When dilute acetic acid is added to the specimen, the
pale threads and longitudinally striated strings swell up
and the longitudinal striation disappears : hence it is that
the specimen becomes so transparent (Fig. 181, b). More-

a

Fig. 181.

A. A small bundle of connective tissue, showing longitudinal fibril-
lation, and at a and 6 encircling (annular, spiral) fibres. JIagnified 400
diameters.

B. A similar bundle swollen and rendered transparent by dilute acid
The encircling fibres are seen at a, a, a.

over it is these striated threads and strings which are
dissolved by boiling water, and yield gelatin. We may
therefore speak of them as collagenous or gelatin-
yielding fibres, by way of distinction from the fibres of
elastic substance, which do not yield gelatin on boiling,
and are of a diff'erent chemical nature.

By various modes of maceration the collagenous fibres
may be resolved into filaments which answer to the space
between the strise, and are of such extreme fineness
that they may measure less than l/i in diameter. It

556

ELEMENTARY PHYSIOLOGY

would appear therefore that the intercellular substance
of the connective tissue in question is composed of (a)
collagenous filaments, united by some cementing
substance into bundles, and of (6) elastic fibres. These
latter are generally united into long meshed networks
(Fig. 182).

With care, the cells or connective tissue corpijs-
cles may also be seen even in fresh, living connective
tissue (Fig. 183) ; but, as has been stated, they are most
distinctly visible when the tissue is treated with dilute
acetic acid. These cells, when seen in the fi-esh tissue,

Fio. 182.— Elastic Fibres of Connective Tissue, formino a loobk
Network.

Obtained by special preparation from subcutaneous tissue. Magnified 800
diameters.

care being taken to prevent the post-mortem changes
which they readily undergo, are found to be flattened
plates almost like epithelial scales, but with very irregular
contours. They closely adhere to, and are, as it were,
bent round the convex faces of the larger bundles of colla-
genous fibres.

Besides these ^xerf connective tissue corpuscles as they
are called, white blood corpuscles, or lymph corpuscles, or
bodies exceedingly like them, are found lying loose in the
fluid which occupies the meshes of the netwox'k of fibres,
and appear to wander or travel through the spaces of the

XII CONNECTIVE TISSUE 557

network by virtue of their power of amoeboid movement
(Lesson III.). Such cells are spoken of as wandering or
migratory cells.

Such are the characters of that which may be regarded
as a typical specimen of connective tissue. But in
different parts of the 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

Fig. 183.— Two Connective Tissue Corpuscles.

Each is seen to consist of a protoplasmic branched body, containing a
nucleus. Very highly magnified.

case in the superficial layer of the dermis and some other
places.

(6) The intercellular substance may be abundant, and
the collagenous elements, with fibrils strongly marked and
arranged in close-set parallel bundles, leaving mere clefts
in the place of the wide meshes of ordinary connective
tissue. This structure is seen in tendons and most
ligaments.

(c) The elastic element may predominate as in certain
(few) ligaments and the vocal cords.

(d) The fibrous or elastic elements may abound, but a
greater or less amount of chondrigenous substance is
developed around the corpuscles. These are respectively
the fibro-cartilages and elastic cartilages, which we

558 ELEMENTARY PHYSIOLOGY less.

have already described and which present every transition
between ordinary cartilage and ordinary connective tissue
(epiglottis, intervertebral ligaments). Where a tendon is
inserted into a cartilage, as in the case of the tendo Achillis,
(Fig. 90), 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
connective-tissue corpuscles.

(e) The intercellular substance may largely disappear
and the interlacing bundles of collagenous fibres may
actually join together at the points when they cross one
another. In this way a spongy network of branching
fibres may be formed whose meshes are filled with fluid,
as in the lymphatic glands.

(/) Finally, in many parts of the body fatty matter is
found within the protoplasmic substance of the connective
tissue corpuscles just as we have seen it to be formed in
cartilage corpuscles.

In this way we ai'rive at modifications of the funda-
mental type of connective (areolar) tissue which are so
characteristic as to merit separate description.

(ii.) W^hite Fibrous Tissue. This form is met with in
the dense and strong connective tissue of which tendons
and most ligaments are composed. In these structures
the collagenous fibres of areolar tissue are arranged in
dense, parallel bundles, among which are a certain number
of peculiarly flattened connective tissue corpuscles,
arranged in rows and now called tendon cells. The
structure of a tendon thus provides the qualities so
essential to it of great strength, complete flexibility, but
absolute want of all elasticity or extensibility.

(iii.) Yellow Elastic Tissue. — This form occurs
typically in the strong ligament (liiiamentuni nuchae) at the
back of the neck (see Fig. 102, b) which, while giving support
to the head, permits it at the same time to be bent
forward, since the ligament is extensible. This ligament

ADIPOSE TISSUE

559

is very highly developed in long-necked animals such as
the horse. The vocal cords of the larynx are also com-
posed of this tissue.

It consists of the elastic fibres of areolar tissue, now
arranged in dense, more or less parallel, bundles. The
fibres are sufficiently thick to show a well-marked outline.
They also frequently branch into finer fibres, and when
teased out the broken ends
of the fibres are character-
istically curled (Fig. 184).

(iv.) Adenoid, Retiform
or Lymphoid Tissue. — As
already described (p. 90) the
functionally essential parts
of a lymphatic gland are
composed of this tissue, and
it permeates the "pulp" of
the spleen, although here it
is in a somewhat modified
form.

Adenoid tissue is a simple
network of branching fibres
(see Fig. 30) whose substance
IS nearly identical with that
of the collagenous fibres of
areolar tissue, but has some
affinities to that of elastic
tissue.

(v.) Adipose Tissue. — This tissue is the ordinary
"fat" found in many parts of the body. It consists
simply of areolar connective tissue in which the connective
tissue corpuscles are present in vast numbers and contain
neutral fat, composed of a mixture of stearin, olein and
palmitin. These modified cells are held together by a
vascular framework furnished by the connective tissue to
which they belong.

The cells are at first indistinguishable from ordinary

Fio. 184.— Elastic Fibres teased
oot and magnified about 200
Diameters. (Sharpev.)

560

ELEMENTARY PHYSIOLOGY

connective tissue corpuscles, but by degrees minute
granules and droplets of fat appear in the cell-substance,
increase in numbers, distend the body of the cell, and

Fio. 185.— Adipose Tissue.

Five fat cells, held together by bundles of connective tissue /. m, the
membrane or envelope of the fat cell ; n, the nucleus, and p, the remains
of the protoplasm pushed aside by the large oil drop a. Magnified 200
diameters.

take the place of the cell-substance,
cell becomes a spheroidal sac full
nucleus pushed to one side (Fig. 185).

Thus finally each
of fat, with the

APPENDIX

ANATOMICAL AND PHYSIOLOGICAL
CONSTANTS

The weight of the body of a full-grown man may betaken
at 70 kilogrammes (154 lbs.).

I. General Statistics.

Such a body would be made up of —

Per cent lbs.

Muscles and tendons .... 42 . . 64 '7

Skeleton 16 . . 24-6

Skin 7 . . 10-8

Fat 19 . . 29-3

Brain 2 . . S'l

Thoracic viscera 2 . . 3*1

Abdominal viscera 7 • • 10"8

Bloodi 5 . . 7-7

100 1541

Or of—

Water 57 . • 88

Solid matters 43 . . 66

1 The total quantity of blood in the body is calculated at about ^ of
the body weight or rather more.

O O

582 ELEMENTARY PHYSIOLOGY append.

The solids would consist of the elements oxygen, hy-
drogen, carbon, nitrogen, phosphorus, sulphur, silicon,
chlorine, fluorine, potassium, sodium, calcium (lithium),
magnesium, iron (manganese, copper, lead), and may be
arranged under the heads of —

Proteins. Carbo-hydrates. Fats. Minerals.

Such a body would lose in 24 hours — of water, about
2,600 grammes (6 lbs. or 4j pints) ; of other matters, about
940 grammes (2 ll)s.), which would contain about 270-300
grammes (or rather more than i lb.) of carbon, 20 grammes
Q oz.) of nitrogen and 30 grammes (about 1 oz.) of
mineral matters (inorganic salts).

It could do about 150,000 kilogramme-metres (480 foot-
tons ^) of work, and gives ofTas 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 2,700
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 (3,180 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. 268.

The gains and losses of this body would be about as
follows : —

Creditor: — Solid dry food . 600 grammes (1|: lbs.)
Oxygen ... 640 „ (IJ „ )
Water . . . .2,300 „ (5^ „ )

3,540 gi-ammes (8 lbs.)

Debtor :— Water . . . . 2,600 grammes (6 lbs.)
Other matters . 940- ,, (2 ,, )

3,540 grammes (8 lbs.)

1 A foot-ton is the equivalent of the work required to lift one ton
one foot high.

CONSTANTS

563

n. NXTTRITION.

Such a body would require for daily food, carbon
270-300 grammes, nitrogen 20 grammes.

Now proteins contain, in round numbers, about 15 per
cent, nitrogen and 53 per cent, carbon, while carbo-
hydrates 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. 271) : —

Proteids . . . 130 gmis. containing 20 grms. nitrogen 70 grms. carbon

Carbo-hydrates 400 „ „ „ 160 „

Fats .... DO „ „ „ 40 „ „

Minerals . . 30 ,, „ „ — „ „

Water . . 2,300 „

2,910

20

270

This might in turn be obtained, for instance, from : —

Lean meat
Bread .
Potatoes
Milk. .
Fat . .
Water .

230 grammes (h lb.)
480 „ (^y 1 lb.)

. 660
. 500
. 30
.2,300

(U lb.)
(I pint)
(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 80 grammes (4 cubic inches or 3 ounces)
of blood.

The blood would probably move in the great arteries at
the rate of about 8 inches (200 millimetres) in a second ;

o o 2

564 ELEMENTARY PHYSIOLOGY append.

in the capillaries at the rate of 1-2 inches (25-50 milli-
metres) in a mimite. The shortest time taken up in per-
forming the complete circuit would probably be about ;:50
seconds.

The left ventricle would probably establish a blood-
pressure in the aorta equal to the pressure (per square
inch) of a column of blood about 6 feet (1*8 metres) in
height ; or of a column of mercury 5-6 inches (140 milli-
metres) in height.

Sending out 80 grammes of blood at each stroke
against this pressure the left ventricle does 80 x 1800
gramme millimetres or 144 gramme-metres of work at
each stroke : in 24 hours, at 72 strokes per minute, the
total work done is about 15,000 kilogramme-metres.
The work of the ricjlit 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 20,000 kilogramme-
metres, or 68 foot-tons.

IV. Respiration.

Such a body would breathe about 17 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. (20 to
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 culiic inches).

There would pass through the lungs, per diem, about
10,000 litres (350 to 400 cubic feet) of air.

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.) ;

CONSTANTS 565

there would be produced about the same volume (or rather
less) of carbonic acid, which would contain about 225
grammes (8 ounces) of carbon. During the same time
about 250 grammes (half a pint or 9 ounces) 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
atmosphere 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 surrounding 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
1000, 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 (11 stone) ought, therefore, to have
at least 1,000 cubic feet of well-ventilated space.

V. Cutaneous Excretion.

Such a body would throw off by the skin — of water
about 650 grammes (23 ounces or 1 pint) ; of solid matters
about 20 grammes (300 grains) ; of carbonic acid about
25 grammes (400 grains) in 24 hours.

VI. Renal Excretion.

Such a body would pass by the kidneys — of water about
1,500 grammes or cubic centimeters (53 ounces or 2h
pints) ; of urea about 33 grammes (500 grains or IJ oz.),
and about the same quantity of other solid matters in 24
hours.

566 ELEMENTARY PHYSIOLOGY

VIL Nervous Action.

A nervous impulse travelm along a nerve at the rate of
about 80 feet in a second in the frog, but much more
rapidly in man.

VIII. HiSTOLOGT.

The following are some of the most important histo-
logical measurements : —

Red blood-corpuscles, breadth jrg'iygth of an inch, or

7 /I* to 8 ju..

White blood-corpuscles, breadth ^s^yjjth of an inch, or
ID ^..

Striated muscular fibre (very variable), breadth jj^^th of
.11 inch, or 60 jn ; length Ih inch, or 30 to 40 millimetres.

Non-striated muscular fibre (variable), breadth ^-jj'oiT^h
of an inch, or 6 /n ; length ^^gth of an inch, or 50 /x.

Nerve fibre (very variable), breadth ^-^fyijth to aoVfj*^^
of an inch, or 2 /i to 12 fx.

Nerve cells (of spinal cord) excluding processes, breadth
gjfjth to 25T5*^^ o"* r"ore of an inch, 50 fj. to 100 /n or more.

Fibrils of connective tissue, breadth 2 s^orr^^ ^^ ^^ inch,
or 1 n-

Superficial cells of epidermis, breadth ys's^ff*'^ ^^ ^^
inch, or 25 ju.

Capillary blood-vessels (variable), with ^^gjjth to ofyViyth
of an inch, or 7 m to 12 /n.

Cilia, from the wind-pipe, length ^qVcj ^^ ^" i"'^^ ^^

8 ju..

Cones in the yellow spot of the retina, width g(j\jffth of
an inch, or 3 jm.

INDEX

Abdomen (abdo, I hide), 8

walls of, 253
Abdominal aorta, 170

viscera, weight of, 268
Abdiicens (o6, from ; duco, I lead) nerve, 516
Abduction, 326

Absorption (a6, from ; sorbeo, I suck up) from alimentary canal, 23, 172,

224
blood, 171
mtestines, 264, 265
stomach, 249
of oxygen, 24, 106, 128, 162,
174, 570
see Water
Accelerator nerves, 73
Accessory food-stuflfs, 281
Accommodation of the eye, mechanism of, 409

limits of, 412
Acetabulum (a vessel holdiiis; vinegar), construction of, 325, 326
Acid, acetic, appearance of blood treated with, 101

connective tissue treated with, 553
reaction of gastric juice, 246

urine, 183
carbonic, see Carbonic acid
hydrochloric, calcareous salts dissolved out of bone by, 309

in gastric juice, 247
taurocholic, 262
uric, 184
Acids of the bile, 214

on respiration, 164
Acts, particular, connected with particular parts of bram surface, 525
" Adam's Apple," 330
Adduction (ad, to ; duco, I lead), 326
Adenoid tissue, 90, 5,59
Adipose (adeps, fat) tissue. 559
Adjustment of the eye, how accomplished, 409
Aerial waves from sonorous bodies, 379
Afferent and efferent impulses, path of, along cord, 488
, in medulla oblongata, 520

nerves. 340, 476
Air, atmospheric, composition of, 6 .„,„„,„. .o,

changes in, effected by respiration, 4, 6, 128, 13d, 564

ELE. PHYS.

568 INDEX

Air, alveolar, 155

in lungs, residual, stationary, and tidal, 148-150, 564
odoriferous, 366, 367
waste, 151
Air cavities in turbinal bones, 364
Air cells in lungs, 133, 140, 150
Air tension in ear, regulation of, 389
Albumin (white of egg, alhinn, white) in blood, 109

as a food, 225, 274
Albuminous glands, 239
Alimentary canal, functions of, 227

muscular fibres of, 303
organs, 22
Alimentation (alo, I nourish), fmiction of, 224-284

organs of, 227
Alkaline reaction of bile, 263
blood, 106

living muscle, 299, 300
pancreatic juice, 25'.)
sweat, 108
Alveoli (a small hollow vessel) of lymphatic glands, 89
of infundibulum, 133
of salivary glands, 238
Alveolar air, 155

Amocbir (o>JOl^o^;. reciprocal), likeness of colourless corpuscles to, 102
Amtpboid movements of wliite corpuscles, 102, 286, 557
Ampulla' (am}iuUa, a tiask or bottle) of semicircular canals of ear, 393
Amputation (ambo, around ; p!<to, 1 cut) of tongue, effect of, 338
Amyloids (afivAoi'. starch) as food, 226
digested in mouth, 264
not acted on directly by gastric juice, 248
" Animal starch," 215
Animated photographs, 428

Anterior and posterior cornua (horns) of spinal cord, 457
Anterior nerve roots of cord motor in function, 473

connected with nerve cells of anterior
cornua, 492
Anterior pyramids of medulla oblongata, decussation of, 520
Antero-lateral tracts of spinal cord, 491, 493
Aorta (aeipuj, I take up, or carry), 38
amount of jiressure on, 564
abdominal, 170
valves of, 47
Apex of heart, felt in " beating " of the heart, 57

its position, 42
Appendix, vermiform, 251
Aqueduct of Sylvius, 5041, 505
Aqueous (aqua, water) humour of eye, 399
Arachnoid (a.paxvr\<;, a spider, or spider's web ; e'Sos, shape), its fluid

and membrane, 453
Areolar (areola, a little space) tissue, 86, 553
Arterial blood, 124, 126
Arterial tone. 68

Arteries (upT>)p. that by which anything is suspended),
bleeding in jets from, when cut, 58, 79

calibre of, regulation by vaso-motor system, 34, 66, 67, 205
elasticity of, 33, 56, 62
filling of, 56

INDEX 569

Arteries, nervous control of, 65
pressure, 59
pulsation of, 57
valves in primary. 38
structure of, 31-35
Arteries or artery —

aorta. .38. 47, 131

abdominal, 170
coronary, 40
hepatic, 40, 209
iliac, 170

pulmonary, 38, 131
radial, 60

renal, 179, 180, 182
splenic. 221
temporal, 61
tibial, 61
Articular (articulus. a joint) cartilages, 319, 549
Articulations of bones, 305, 318-326
Arj-tenoid (apxneivai, a pitcher, or ladle ; elSo?. shai)e

cartilages, 331, 332
Asphyxia (o. privative ; (r<i>\j^u>, I beat, of the pulse), modes of death

from. 162. 163
Assimilation of food, 227
Association, law of, 528
Astragalus (ao-rpayoAos, an ankle bone), 328
Atlas (a. euphonic : T^rjfiio. 1 bear) vertebra. 322
Atmospheric (ot^os. vapour ; <7-<ioipo, a sphere)
pressure, 154

how erjualLsed in ear. 389
an obstacle to dislocation of hip, 326
opposed by elasticity of lungs, 137
Atropine, effect of injection of, 241
Auditory (audio. I hear) hairs, 378, 387
nerve. 376. 393, 501, 517
ossicles, 370
sensorium. 386
spectra. 441
Auricles (auricula, a little ear), 43
Auricular appendage, 50
Auriculo-ventricular apertures, 45
Axis (aiuv. an axle) vertebra described, 322
Azygos (afvyos, unyoked) vein, 41

Bacteria, action of, in fermentation, 265
Balance, physiological, how maintained, 7, 25
Ball and socket joints, 320

capsular ligaments to, 325
Basilar (basis, a base) membrane of ear, 378
Beating of the heart, 57
Beet -sugar, 226
Biceps (bis, twice ; caput, head) piuscle, 13

its attachments, 305, 306
Bicuspid (bis. twice : cuspis, point of a weapon) teeth, 231
Bile, flow of, into duodenum. 263

nature and action of, 261 et seq.

-pigments. 261

570 INDEX

Bile-salts, 261

secretion of, 105, 213, 214
Bilirubin, 261
Biliverdin, 261

Bipolar ceils — see Rod and Cone
Bladder, 176

Biastomeres OAao-rb?, a bud ; M^po^. a division), 538
Blind spot of eye. 426, 427
Blister, how formed, 541
Blood, 93-129

arterial and venous, 123-129

in capillaries, 31, 170

chemical composition of, 105

circulation of, 23, 64

evidence of indirect, 77

coaKulation of, 04, 110

composition of, 105

corpuscles, 94-105, 223

under intlaniniation, 80, 82

longest possible course of. 41

crystals, 99, 126

flow, rat« of, 61, 64

functions of, 23, 116

gains and losses to, 170-175, 206

gases in, 106, 124

heat of. 25, 66, 105, 202

of hepatic vein, sugar in, 217

microscopic examination of, 93

oxygen carried by, 24, 106, 127

platelets, 82, 104

portal, 215

pressure, 59

solids in plasma of, 108

specific gravity of, 105

of splenic vein, paucity of red corpuscles in, 223

transfusion of, 117

quantity of, in body. 116, 561

supply, its influence on respiratory centre, 161

weight of, in the body, 267
Blood vessels 30 et seq.

peculiar epithelial lining of, 546
regulation of, by vaso-motor nerves, 67, 205, 496
of the retina, 431
Blushing, how effected, 66, 69
Body, human, general build of, 8, 561

diagraiiitiiati(' section of, 11

clemtMits present in, 562

e(|uilibrium of, 523

joints of. 318 et seq.

various movements of the, 326 et seq.

weights of various parts, 268, 561

work done by, 282
Bone, canaliouli of. 310

cancellated structure of, 308

structure of, 304, 307, 311
Bones, considered as levers, 305, 314

corpuscles, 313

number of, in body, 14

INDEX 571

Bones, astragalus, 327

atlas, 323

axis, 323

clavicle, 16

coccyx, 14

of ear, 370

femur, 304, 321

humerus, 324, 327

hyoid, 230

ilium, 16

incus, 378, 379

innominatum, 16

ischium, 16

of lower extremity, 14, 304, 316, 320

malleus, 378, 380

maxillary, 365

metacarpal, 321

orbiculare, 370

patella, 16, 317

pelvis, 319

pubis, 16, 319

radius, 306. 324

ribs, 14, 136, 139, 140, 145, 309

sacrum, 14

scapula, 16

of skull, 16, 230, 368

temporal, 378

turbinal, 230, 363, 365, 416

ulna, 320, 322

of upper extremities. 14, 305, 306, 324, 325
vertebrae, 9. 15, 135, 454
Brain, base of. illustrated, 501

component parts of, 498 et seq.
effect of destruction of, in frog, 74, 485, 524
respiration on, 168
venous blood on, 163
grey matter of. 506, 510
hemispheres of. .500
injury to, death caused indirectly by, 28

on one side affects opposite side of body, 52
lobes of, 505

localisation of powers in, 525
membranes of. 453, 509
minute structure of, 510 et seq.
olfactory lobes and nerve form part of, 362, 517
optic nerve forms part of, 517
pia mater of, 453
reflex action of. 526

sensation, mental action and will seated in, 21, 525
spinal cord continuous with. 10, 11
structural arranaements of, 498 et seq.
ventricles of, 505
weight of 268
Bread, as food. 225. 272-276, 563
Breathina. see Respiration

Brewster, Sir Da\-id. nuoted as to illusions, 441
Broca. M., his convolution, 530, 531
Bronchi (ppofxo^, the windpipe), 131, 137

572 INDEX

Bronohial tubRs. ciliated epithelium in, 134

Bronchioles, 131

" Bronzing " of skin, cause of, 219

Bninner, s'ands of, 256

ISuccal {bucca, the mouth) glands, 237

Buffy coat of blood, 111

Bursae (bursa, a pouch), 305

C.KCUM (i.e. intestinurn ccecum, the blind gut), 251
Calamus scriptorius, .502
Calcic salts in bone, 14, 307
Calcarine fissure, 529
Calorie, a, 282

Camera obscura (dark chamber) described, 406
Canal, alimentary, 224, 235 Pt seq.
central, of spinal cord, 456
semicircular, 392, 523
spinal, 9
Canaliculi of bones, 310

Canalis cochlearis (koxAo?, a spiral shell) of ear, 375
Canals, Haversian, 308

semicircular, of ear, 392, 523
Cancellous (cancdli, lattice-work) tissue of bone, 305, 308
Cane-sugar, 226

Canine (canis. a dog) teeth, 231

Capillaries (capillus, a hair), continuous with veins and arteries, 23, 31
dilatation of. under influence of heat. 205
exudation througli, 23, 84, 171, 187, 193
friction in, 56, 61
lymphatic, 84

microscopic examination of, 31. 79
pulmonary, oxidation of blood takes place in, 127

distribution of, 133
pulse lost in, 61

of stomach and intestines, 257, 264
structure of, 30
of the villi, 266
walls of, 30, 83
Capillary circulation, 79
Capsule, Malpiphian, 178, 181, 186
Capsules in cartilage, 549

internal, of brain, 532, 534

of corpora striata, 509
of liver, 209
of lymphatic gl.ands, 89
of spleen, 221
Carbohydrates as food, 225, 226, 277

given up by the blood to the tissues, 171
Carbon (carbo, a coal), in foods, 225, 226 '

amount of, eUmiiiated per diem, 269, 271, 272, 564, 565
Carbonic acid, excess of, in venous blood, 125, 127, 163

by lungs, 24, 154, 162, 269, 566
by muscle, 206
by skin, 190, 565
mode of poisoning by, 163, 164
a product of dissolution, 29
Carbonic oxide gas, effect of, on blood corpuscles, 164

INDEX 573

Cardiac (xapSia, the heart) dilatation of stomach, 244
impulse, 57
muscular tissue, 44
nerves, 72
Cardio-tnhibitorj- centre, 75

Cartilage (cartUaoo. gristle) on articulating surfaces, 305
corpuscles, 545, 549
growth and structure of, 546
in trachea and bronchi, 129, 131
Cartilages, 14 ^

articular, 17, 321
hyaline, 541
inter-articular, 321
sterno-costal, 541
thyroid, 330
Caruncula (dim. of caro. flesh) lachrymalis, 416
Casein (caseiis. cheese), 225

" Catherine wheel," continuous appearance of, on the retina, 428
Cell-bodv of white corpuscles, 102
Cells, ciliated, 134, 286, 359

columnar ciliated epitiielial, 286
cornification of, 192, 542
demilune, 238, 239
differentiation of. 538
epidermic, 542

epithelial, continuous with epidermic, 12, 545
modified for sense organs, 345

of hearing, 368
sight, 418
smell, 365
taste, 360
touch, 348
epithelioid, 89
fat, 560

incessant reproduction of, 27, 192
Uver or hepatic, 209. 212
as living organisms, 102, 295, 551
mucous, 134
nerve, 464
nucleated, 467

in capillaries, 30
of cartilage, 548
of connective tissue, 554
in embryonic tissues, 295, 536, 550
pigment, 401
primitive. 537

differentiation of, 538
of Pnrkinje, 511
of the retina. 417 et seq.
secreting, 187, 238
various forms of, 544, 545, 546
wandering, 557
Cellular tissue, see Areolar.
Cellulose, 226

digested by herbivora, 266
Cement of teeth, 233
Centres, cerebro-spinal, 21, 452
respiratory, 158

574 INDEX

Centres, vaso-motor, 494
Cerebellar tract of spinal cord, 490
Cerebellum, functions of, 521 et seg.

removal of, 522
Cerebral cortex, 493, 510, 513

localisation of function in, 528
hemispheres, functions of, 524 et seg.
stimulation of. 530
Cerebellum (dim. oi cerebrum), position of, 500

structure of, 510
Cerebral (cerebrum, the brain) hemispheres, functions of. 524
Cerebro-spinal system, 10, 452, 453
Cerehro-spinal Huid, 454
Check ligaments, 323
Cholesterin (xoKt), bile ; <rreap, fat), 214
Cliondrin (xoi'Spos, cartilage), 547
Chorda tympani, 72, 241
ChordtB tendineop, 45, 53

Choroid (^^opioi/. investint: membrane of foetus; elSos, form) coat, 401
pigment cells from, 401
plexuses of brain, 500
Chyle (xuAiW, juice), formation of. 266

in the intestines, 264
in lymph, 1 19
receptacle of, 86
Chyme (xv^tbs, pulpy juice), 219, 263
Cilia (citium, an eyelash) described, 284
on bronchial epithelium, 134
on p.irt of nasal mucous membrane, 366
from the windpipe, length of, 566
Ciliary ligament and muscle, 402
processes of choroiil, 401
Circular fibres of eye, 403, 407
Circulation of the blood, 23, 30
capillary, 79

control of, by the vaso-motor system, 34, 67, 494
constant of, 563
course of, 38, 41

effects of respiration on tlie, 165
evidence of the indirect, 77
in kidney. 179
resistance, see Peripheral
Circulation, portal. 46, 215
Circumduction (a leading around), 324

Circumvallate (circum, around ; vallum, a wall) papillce, 354, 356
Cistern of the chyle. 86
Clarke's colunui, 491
Clavicle (clancula a small key), 16
Clotting of blood, 94, 103, 110, 111

of muscle plasma, 298
Coagulation (cow, together ; aqo, I drive) of blood, 94, 110
Coal-gas. risk from breathing, 164
Coats of arteries and veins, 32
Coccyx (kokkv^, a cuckoo), 14
Cochlea (icoxAia?, a spiral shell) of ear described, 373-379

functions of, 385
Cochlear nerve, 387 , 523
Cohnheim's areas, 294

INDEX 575

Cold, respiration affected by, 159

sensations of. 352
Cold spots. 35-2, 353
CoUaglnous (xoAAa, alue : yt^vvato, I produce) fibres of connective

tissue, 555
Colloids, 1211

Colon (kioKov, a part or di\ision), 252, 253
Colour, sensations of, 426
Colour-blindness, 431 , 435
" Colour top," experiments with, 432
Colouring matters in urine. 184
Colourless corpuscles, see White.
Colours, complementary, 432

primary. 433
Columnae carnea> (fleshy columns). 47
Combination of muscular actions, 20. 486. 524, 523
Commissural {con. together ; witto, I send) cords, 494. 495
Complementary colours seen as result of retinal fatigue, 435
Concha (,K6y\o^. a sea shell) of ear described, 368
Concussion of brain, 20
Conduction of impulses, 488
Cone bipolar cell, 422

Cones (kCivos, a fir cone) of retina, 403, 418
abundant in yellow spot, 424
w-idth of. 566
Conjunctiva (con. together ; jungo, 1 join) 416
Connective (con, together ; necto, I fasten) tissue, 13, 553

corpuscles. 550, 554, 556
fibres of, 88, 553-5.56
fibrils of, 555, 566
lymphoid. 901
« perimysium formed of, 289

retiform. 90
varieties of, 557
Consciousness, states of, 343

Consonants (con, ■with ; .'iono, I sound), pronunciation of, 337
Contact (con, with ; tango. I touch), sense of, 352
Contractility (con, together ; traho, I draw) of bronchial tubes, 132
of colourless corpuscles, 100, 101, 296
of muscular fibre, 288. 296
Contraction of heart rhythmical. 50, 51
of hollow muscles, 302
of intercostal muscles, 140
of iris, 303, 402

of muscles, 13, 21, 33, 50. 288, 296, 340, 472
of muscular coat of arteries, 33, 66
of muscular fibre, 296
peristaltic, of gland ducts, 303

of intestines, 254, 303
of sphincter muscles, 177, 253
Convolutions of brain, 528

Broca's, 531, 532
of the cerebral hemispheres, 528
Co-ordination, 26, 521
Cornea (comeus. horny), 399
Corneous laver of skin, 199
Cornified cells, 196
Cornu (a horn) of spinal cord, anterior and posterior, 457

576 INDEX

Coronary (corona, a crowTi) arteries, 40
Corpora (|uaclrigeinina, 503, 507, 518
Corpora striata, 508
Corpus albicans, 5(»4

Corpus callosuin (tlie hard body), 500, 504

Corpuscles (corpusculum, dim of corpus, a body), of the bloo^
94, 95, 80

bone, 313

cartilage, 546, 548, 551

of connective tissue, 554

elfect of the spleen on, 223

Malpighian, 222

measurement of, 545, 566

migration of, 82, 103

of the spleen, 222, 559

pacinian 350, 351

tactile. 348, 349

under inllanunation, 80, 82
Cortex of lymphatic glands, 90

of kidney. 178
Corti, organ and rods of, 37G, 377, 378, 385 et aeq.
Coughing. 146, 159

Cranial nerves, arrangement of, 500, 515
Creatine (icpeas flesh), 298
Cretinism. 218

Cribriform (cribra, a sieve ; forma, shape) plate, 362, 364
Cricoid (kpiko?, a ring) cartilage, 330
Crico-aryteiioid muscle, 333
Crico-thyroid muscle, 331, 333
Crista acustica (acoustic crest), 393
Crossing over of nervous impulses in cord, 493, 520

in med^flla oblongata, 521
Crown of tooth, 231, 232
Crucial ligament, 325
Crura cerebri, .500
Crus cerebri, 534
Crystals in blood, 99

change of colour of, by oxygenation, 126
doubly refracting, 445
Crystalline lens, 399
Crystalloids. 12U

Cubic feet of air needed for respiration, 169, 566
Curvatures of stomach, 244
Cutaneous excretions, constant of, 565

Death from asphyxia, 163

of the blood corpuscles, 104

general and local, 27

immediate causes of, 28

of muscle, changes caused by, 293, 297

stiffening after. 297
Decomposition after death. 29
Decussation of the pyramids, 520

cranial nerves, 521
sensory, 520
Defsecation, 487
Degeneration of nerve-fibres, 474, 489

INDEX 577

Degeneration method, 475

tracts of ascending, 490
descending, 492
Delirium tremens (trembling delirium), 441
Delusions of the judgment, 440, 441, 443

optical. 442, 44.3
Demilune cells, 238. 239
Dendrites of cells of Purkinje, 511
Dental (dens, tooth) pulp, 232

tissues, 233
Dentine, 233

Dermis (Sip/Jia. skin), 12, 190,
Dextrose (dexter . right-handed, from the direction of light polarised

through it), 226
Diabetes, a form of, produced by injur.v to medulla oblongata, 520
Diaphragm (6ia, across ; i^pao-o-w, T separate bj' a fence), 9
action of, in respiration, 143, 144
connection of pericardium with, 42
of camera obscura, 406
Diastole (6i', apart ; o-reAAu, 1 place), 51, 52
Diet, mixed, economy of, 274
Differentiation of cells, 538, 551
Diffusion of lymph, 120

Digastric (6i for 61?, twice : yao-Trjp, the belly), muscles, 306, 307
Digestion, artificial, 247, 260'

fluid for, 2601
constant of, 563
purpose and means of, 224, 227
secondary, 215
Digits of hands and feet, 8
Dim bands of striated muscular fibre, 292
Direct pyramidal tract of spinal cord, 493, 515
Distance, judgment of, by the eye, 445
Division of labour in cells. 538

nucleus of epidermic cells, 343
mammalian ovum, 537
Double hinge joint, 320

vision, as result of squinting, 449
Drill, reflex nature of actions taught by, 527
Drinking, mechanism of, 236
Drum of the ear, 368
Duct, bile, 208, 213
hepatic, 209
lachrymal, 417
pancreatic, 221, 245
thoracic, 85
Ductless glands, 217
Duodenum (duodeni. twelve, from being twelve finger-breadths long, 251

secretions flowing into the, 262
Dura mater, 453
Dyspnoea (6us, bad ; nvew, 1 breathe), 161, 162

Eae described, 367 et seq.

experiment on blood supply to, 67, 495

" tightness " of, 389

transmi.ssion of sound-waves to the inner, 379

external, :i)68

ELE. PHYS. p "p

578 INDEX

Ear middle, 368

Efferent (ex, out of ; fero. I bear) impulses, course of, 488
nerves defined, 476

degeneration of, 474

muscular fibre contracts by means of, 340

Elasticity of artery walls, 32, 34, 56, 62

cartilage, 547

lungs, 137, 165

Elbow joint, 320, 322

Electrical fishes, efferent nerves of, 476

properties of a nerve, 478
Elements present in liumaii body, 562
Embryo, growth of muscle in, 295

red corpuscles nucleated In, 102
Embryonic form of all tissues, 536
Emotions, various, 342

eflfect of, on the heart, 73

on perspiration, 201
on the vaso-motor system, 66
painful, tears a consequence of, 417
Emulsification of fats, 263
Emulsion of fats, 260, 263
Enamel of teeth, 233, 234
End-bulb of nerve-fibre. 190, 348, 350
End-organs, central, of special sensations, 386, 526
Endocardium (ti/fioK, within ; xapSCa, the heart), 45
Endolymph ('ii'Sov, witliin ; li/mpha, water) contained in ear sac, 372

vibrations of, 382
Energy (cV, in ; (pyov, work) supplied by oxidation, 7, 25, 224

income and expenditure of, 281
Enzymes, 243

Epidermis (en-i, upon ; Sfpna. skin), 12, 190
breadth of superdcial cells of, 545
cells of, converted into horn, 192
continuous with epithelium, 12, 545
an execretory organ, 196
growth of, 542
non- vascular, 31

its relation to the derma, 543, 544
scales of, continually shed, 541
structure of, 540, 543
Epiglottis (eTT-i, upon ; yAoiTra, a tongue), 129, 227
Epithelial tissue, 539
Epithelioid cells, 89
Epithelium (eTrt, upon ; floAAw, I grow)
auditory, 378
cells of, mcessantly reproduced, 27

nucleated, 542, 545
ciliated, 286

in nasal mucous membrane, 365
epidermis, continued into, 12
modified in sense organs, 345
of mucous membrane, 545
non-vascular, 31, 544
olfactory, 365, 36C
of the retina, 424
of serous cavities, 546

INDEX 579

Epithelium, secreting, in sweat glands. 192

in tubules of kidney, 180, 182
Equilibriuni, bodily, maintenance of, 523
Erect position, how maintained, 17
Essential food-stutfs, 281

Ether, vibrations of, physical basis of light, 426
Eustachian tube, 2:U, 369

probable office of, 389
Evaporation from the lungs, 5

from the skin, 5, 204
Excretions {ex, from : cerno, I separate), a table of, 270
amount of oxygen contained in, 6
solid matter in, 565
Excretory organs, 23, 171
Expiration and inspiration (exspiro, I breathe out), 145, 167

usually performed silently, 332
Expired air, analysis of, 564
Extension of limbs, 326
Eye, the, 398 et seq.

accommodation of, 409
as a water-camera, 404
blind spot of, 427
muscles of, 306, 403, 414
nerve supply to, 510
yellow spot of, 418
Eyeball, component parts of, 398
Eyelids and eyelashes, 415

Face, cavity of, 10

Facial nerves. 516

Faeces (fcex. grounds), 23. 227. 265, 266, 270

Fainting effected by action of the pneumogastric, 72

Faintness. sense of, 343

Fangs of teeth. 231. 232

Fascia (a band) of a muscle, 289

Fat cells, 559

Fatigue, a cause of. 343

of retina, 435
Fats, absorbed by the lymphatics, 266

in blood corpuscles, 103

emulsified in duodenum, 260, 263

as food, 225, 226, 272, 569

given up from the blood to the tissues, 171, 271

acted on by gastric juice, 248

weight of, in body, 268
Fatty tissue, 559
Fauces 229

Femur (the thigh), structure of, 299, 304
Fenestra (a window or opening) ovalis, 370

rotunda, 370, 375
Ferments, action and composition of, 243
in blood, 115
in csecum, 265
Fever, temperature of, 207
Fibre cells, cardiac. 44
Fibres, circular or sphincter, of eye, 402, 403, 407

p p 2

580 INDEX

Fibres, of connective tissue. 88. 5r)3

mecliillated and uoii-meduUated, 495, 497, 498

motor, 299

muscular, 44, 289, 566

breadth of, 566
nervous, 289, 459, 566
of pyramidal tract, 492, 532
sympathetic, 495
Fibrils of connective tissue, 553

breadth of, 566
muscle, 292, 294
Fibrin, 111, 113

ferment, 115
Fibrinogen, 114
Fibro-cartilage, white, 551, 552

yellow, or elastic, 551, 552
Fibrous tissue, 12

arteries sheathed by, 32
Filtration of lymph, 120

of urine, 186
Filiform (filium, a thread ; forma, a shape) papillae of tongue, 358
Fishes, electrical, efferent nerves of, 476
Fissure of Sylvius, Wo, 528
Fissures of brain. 505
calcarine, 528

of cerebral hemispheres, 528
parieto-occipital. 528
of Rolando, 505. 528
of spinal cord, 454, 465
Flexion of limbs, 326
Fluid, cerebro spinal. 454

of labyrinth of ear, 372, 393
pericardial, 42, 113
Focus, the, in .sight, 405

Fog, effect of, on judgment of size, and distance 446, 447
Follicle, 196

solitary, 256
Food, average amount taken, 563

change undergone in the intestines, 262
necessary constituents of, 5, 225, 226
oxidation of, in the body, 7, 24, 224
talcen up by the blood, 172
waste made good by, 224
and nutrition, 268 et seq.
Food-stuffs classified, 225, 226

effects of several, 277
erroneous classification of, 279
essential and accessory, 281
Foot, the, 18, 19

as lever, 315
Foramen (a hole ; from foro, I pierce), nutritive, of bone, 308
of Magendie, 510
of Monro, 504
Foramina, intervertebral, 454
Form, visual judgment of, by shadows, 447

of changes of, 448
Fornix, 504, 509
Friction of blood in capillaries, 56, 61

INDEX- 581

rate of transmission of nervous impulse m, u66

fi;?cS?Jdan:^Stion of, m various levers, 314
Fungiform papiH'''' oi tongue, 358

GALL-BLADDER, 208, 209, 213

storage ot bile m the, 2bl o, .79

Galvanism, effect of, on spinal ^^d and nerves 2 1 472
Ganglia (7ai.v\.o^, a hard gathermg) of the heart, <,J

on sensory roots of fifth pair of nerves, ol6

sympathetic, 0, 452, 495 497

of the posterior root, 456

SSSm o?^^^^i^-rr gases and liquids, 154
in blood, 124, 126

poisonous, 164 ■ • • ai

proportions of, m atmospheric air, b'
in urine, 184 ,

Gastric (vao-Tijp, the stomach) glands, 24o

juice. 244, 246, 247
Gastrocnemius muscle, 299
Gelatin (!7«-?o. I freeze). 226, 2/8

obtained from connective tibsue, Id, oai
Germinal spot and vesicle, 527 , . .^

Glands (n^ans. an acorn), a source of loss to the blooa, 14U
structure of. 228
albuminous. 239
of Brunner, 256
buccal. 237
cutaneous. 192, 197
gastric. 245. 246
intestinal. 227, 255
lachrymal, 416
of Lieberkuhn, 25»
lymphatic, 89, 220
mesenteric, 253
mucous, 239
parotid, 237, 239
pineal. 504
racemose, 229
salivary, 173, 227, 237
sebaceous, 197, 198
solitary, 256
sublingual, 237
sub-maxillary, 237, 238
thymus, 220
thyroid, 217
Glandular substance, 90
Glasses, multipljing, 44o

gS'.]i.:i'(ciinf of glomus a clue of thread) of M,,ey, 179, 186
Glottis (vA<;<TTa, the tongue), described, 129, 330

position of. 231, 362
Glosso-pharyngeal nerve, ^6^ 5^^1, 5^17^^^ ^^^^^^^ .^ j^,„,tion, 517

582 INDEX

Gluten {gltio, I draw together), 226

Glycocholic acid, 262

Glycogen (yAuicu? sweet ; yewdui, I produce) in liver cells, 207, 215, 278

in blood-corpuscles, 103

conversion of, into grape-sugar, 217

non-nitrogenous, 208
Goitre (qidiur, tlie throat), 218
Granular layers of eye, 418
Grape-sugar formed from glycogen, 217
Grey commissure of spinal cord, 4.58
Grey matter of brain, special nature of, 506, 510
in medulla oblongata, 506
of spinal cord, 456, 465
Gristle, 14
Gullet, 230

passage of fluids in, 236
Gum, of month, 231, 232
Gustatory Uiusto. I taste) nerve, 360, 516
Gyri of cerebral hemispheres, 528

H.CMATix (aiVarii/o!, charged with blood), 98, 105
Haemoglobin (aVo, blood ; globus, a globe), 97, 98
acted on by carbonic oxide, 164
combination of, with oxygen, 107, 126
crystallisation of, 99
decomposition of, 105
Hair, non-vascular, 31

its growth limited, 196
Hair-like processes On auditory epithelium, 377, 378, 387
Hairs, growth of, 195-197

measurement of, 545
Haversian canals, 308, 310
Hearing, mechanism of, 343, 382 et aetj.
Heart, action of, helped by respiration, 167

increased by irritation of sympathetic, 74, 496
nervous control of, 74, 75

stopped by irritation of pneumogastric, 74, 496
beat of, 50
divisions of, 42
ganglia of, 73

inhibition of heart-beat, 75
muscular fibres of, 44, 303
position of, 136, 137
of sheep, 39, 46, 48

rhythmical contraction of, 23, 50, 51, 73
size of, 42
sounds of, 57
valves of, 45
work done by, 563
Heat, constant loss of, in the body, 5, 171, 201

measurement of amount given off by the body, 282
muscular, 202, 206

produced by oxidation, 25, 172, 202, 224
-producing food-stuffs, 279
regulation of, 203
sensation of, 352
spots, 352-354
Henle, loop of, 180

INDEX 583

Hepatic (vTrap. the liver) artery, 40, 209

cells, 2IJ9, 212

their action, 213

duct. 209

vein, 209
Herbi\orous animals, development of caecum in, 2511
Bering, Prof., history of sensations of colour, 434, 436
Hiccough. 146
Hilus of the kidney, 176
Hinge joints. 320
Hip-joint, section of, 321

Histology (icTTos, a tissue ; Aoyos, a discourse), defined, 535
Histological measurements, 544, 566
Hollow muscles, 302

Homoiomera (o/ioios, like ; tifpo<;, a di\'ision), 535
Hoops, cartilaginous, of trachea, 130
Hormone, 26, 164, 259
Horn, epidermic cells converted into, 196
Humerus (the shoulder) articulation of, 320
Humours of the eye, 399
Hyaline cartilage. 546

Hydrochloric (vSuip. water : x^iupo?- pale green) acid in gastric juice. 247
Hydrogen (viuip, water : yevfdM, 1 produce) in foods, 108, 226

sulphuretted, poisonous effects of, 164
Hyoid (v. the letter upsilon ; cTSos, shape) bone, 330
H>TX)glossal (i/iro, beneath ; yAuJTTa, the tongue) nerve, 501, 517

Ileo-C.ecax valves. 251

Ileum (eiAe'w, I roll). 251

Iliac (ilia, the flanks) arteries, 170

Ilium, 16

Illusions, spectral, 441

Imperfect joints, 319

Impression, retinal, corrected by sense of touch, 444

Impulse, cardiac. 57

nervous, rate of transmission of, 479
vaso-constrictor. effect of perspiration on, 203
Impulses, nervous conduction of, 68, 160, 299, 4s8
decussation of. 520

require time for propagation, 477, 566
Incisor {incido, I cut) teeth, 231
Incus (an anvil), 370
Inflammation, 80
Inhibition of heart-beat, 75
Innervation, 452

Innominatum (nameless) bone, 16
Inorganic compounds in foods, 225
Insertion of a muscle. 305
Inspiration (in, spiro. I breathe)

heart's action helped by, 167

mechanism of, 139, 156, 332

rate of, per minute, 564
Integument (in. upon ; tego. I cover), double. 12. 540
InteUigence. destroyed by removal of cerebral hemispheres, 524
Inter-articular cartilages, 319
Intercellular substance of cartilage, 546
Intercostal (inter, between ; conta, a rib) muscles, 140, 144

nerves, 158

584 INDEX

Interlobular vein, 211

Intestines, all food-stuffs dissolved in, 227, 264, 265
arrangement and structure of, 249
coats of, 254
glands of, 227, 255
small and laree, 251
Intralobular vein, 209, 212

Inverted position of retinal image, no obstacle to upright vision, 444
Iris (a rainbow) described, 402
muscular fibres of. 303, 40:5
Irritation of cut end of sympathetic, 67, 495
motor nerves. 472
pneumoKastric, 496
trunk of spinal nerve, 472
upper dorsal region of cord, 495
Ischium (iaxioi', the hip), 16

Jaw, lower and upper, 231

jejunum, 251
Jerks, blood issues from cut artery by, 58, 79

obviated by elasticity of tubes, 61
Joints, of the body, 318

ball and socket. 320
exemplifying lever action, 314
hinge, 320
right knee, 318
perfect and imperfect, 319
pivot, 321
Judgment combined with sensations, 438
delusions of the, 440, 445
visual images interpreted by the, 448, 450
Jugular vein, 43, 87
Juice, gastric, 246, 247, 248
intestinal, 262
pancreatic, 259
Jumping, 329

Kidneys, amount of excretion from, 269, 270, 565

dGScribcd ITo

excretory' functions of, 23, 177, 185, 187

minute structure of, 177

position of, 9

tubules of, 178

artery of, 182
Knee, riuht, joint of, 318
Krause's membrane, 294

Labyrinth (Aa^vpivSos, a maze) of ear, membranous, 391-397

osseous, 393
Lachrymal {lachryma, a tear) duct and sac, 416

gland, 416
Lacteal (lac, milk) radicles and vessels, 256

absorption of fat by, 266
Lacteals, 86, 119, 256
Lactic acid, 297

INDEX 585

Lactose in food, 277
Lacunae of bones, 310
LamellEB of cerebellum, 510
Lamina spiralis (spiral plate) of ear, 374
Langerhaus, Islets of, 261
Laryngeal nerve, superior. 161
Larynx (Aopuyf, throat), mechanism of, 129, 327
artificial, 338
voice produced by, 329
Lateral lieanients, 325
Layers of the retina. 418
Leather made from the dermis, 12
Lens (a lentil seed), adjustment of. 409

crystalline. 399, 405
Lenses, concave and convex, 440

Leucin and other amino acids, urea secreted from, 189, 264
Leucocytes in lymphatic glands, 90, 118, 256
Levator (lero, I raise). 416
Levatores costarum, 142

Levers {levo, 1 raise), bones considered as, 14, 303, 314-318
Lieberkiihn. glands of, 255
Life and Death, 26
Ligaments (lif/o. 1 bind), 320, 325
ciliary. 402
forming pulleys, 415
lateral. 325
round. 325

suspensory, of lens, 400, 401
vocal, 330
Ligamentum nuchse, 553
Light, sensation of, 426, 428, 429
Limbs, 8. 18

ascending and descending, of kidney, 180
outer and inner, of rods and cones. 425
Lime, salts of, in bone. 14, 309
Lime-water, how changed by breathing ttirough, 5
Liquids, behaviour of gases and, 134
Liver, blood supply to the, 209, 210
described, 207
formation of urea by, 189
glycogen stored in the, 213, 215
position of, 9

secretion of bile by the, 213, 261
vessels of the, 40
work of the, 213
Lobes of liver, 209
of lungs, 133
Lobules of the liver, 209
of the lungs, 133
Local death unceasing, 20
Locomotion (locux, a place ; vioveo, 1 move), how effected, 26

mechanics of, 328
Long sight. 413
Loop of Henle. 180
Losses of the blood. 170-175

body. 562
Luminous impression on eye, duration of, 427
Lungs, absorption of oxygen by, 24, 127, 162, 173, 564

586 INDEX

Lungs, elasticity of, 137, 165

as excretory organs, 2:3, 151, 170, 269
position of, 9, 136, 137, 142
structure of, 135
veins and arteries of, 38
Lympli (li/mpha, water), 83, 93, 117
-channel, 91
-hearts, 92
-corpuscles, 118
modes of formation of, 119
movement of, 91
-sinus, 91

-spaces, 88 '

Lymphatic glands, 88

origin and structure, 86
spaces, 88
system, 83, 84
vessels, 85, 89
, Lymphoid connective tissue, 90, 256, 559

Macula acustica (acoustic spot), 393

lutea (yellow) of retina, 423, 424
Madder, experiment with, as to growth of bone, 313
Magendie, foramen of, 510
Malleus (a hammer), 368 et seq.
Malpighian capsule, 178, 186
corpuscles, 212
layer, 192
Malpighii rete, 540
Maltose, 242, 260

Mammal, embryonic growth of, 535
Manufacture of bile-acids in liver, 214

of some constituents of urine in kidney, 177, 187
of glycogen by hepatic cells, 216
Marrow in bones, 305, 308

red, formation of blood corpuscles by, 104
Mastication, 229, 235
Matrix, 546
' Matter," 80
Matter, its changes, 28

solid, lost by perspiration, 565

passed from alimentary canal, 227
kidneys and skin, 565
Maxillary (maxilla, jaw-bone) bones, 365
Measurements, histological, 544
Meat as food, 226, 273, 276
" boiled to rags," 288
Meatus {meo, I pass) of ear, 369
Median posterior tract of spinal cord, 491
Medulla of lymphatic glands, 90

of kidney, 178
Medulla oblongata (oblong marrow), 452, 461

decussation of impulses in, 521

effect of venous blood on, 163

injury to, result of, 28, 519, 521

nervous centre for respiration in, 157, 519

for vaso-motor nerves, 69, 494, 519

INDEX 587.

Medulla oblongata, structure of, -498
Medullary cavity of bones, 308
matter of hairs. 197
rays, of cerebral cortex. 515
substance of the kidney, 178
Medullated nerve fibres. 459

nerves, the minute structure of, 459
Meibomian glands, 415
Membrane, arachnoid, 453
Krause's. 294
limiting, of eye, 419
mucous, 12, 133
of Reissner, 377
" serous," 42i
syno\-ial. 17, 319
vascular. 448
vibration of, 380
Membranes of the brain, 453, 509
Membranous labyrinth of ear, 376, 392
Mesentery (/xeVos, middle ; ivTfpov, intestine), 253
Metabolism. 188', 268 et seq.

Metacarpal (jieri, beyond ; Kapiri*;, the wrist) bone of thumb, 321
Micturition. 487

Migration of blood-corpuscles, 82, 103
Migratory cells, 557
Milk, as food. 272-277, 563

-sugar. 226. 277
Mind not the sole governor of muscle, 20
ilinerals as food. 225, 562
Molar (rnolo. I grind) teeth. 231

Molecular (nwlecula, dim. of moles, a mass) change in cerebral siibstanre.

526
in stimulated nerves,
341
layer of cerebellum, 511

of cerebral cort«x. 515
inner and outer, of the retina. 419
vibrations. 384
Monro, foramen of. 505

Mortification (mors, death : facio. I make), 27
Motion in li^"ing body incessant, 1, 285
Motor areas. 531

fibres. 289, 299
end-organ. 463
nerves," 289, 340, 473

composition of, 461
plates. 463
Motores oculi nerves, 516
Mouth, 10, 130. 229

epithelial scales from interior of, 542
Movements, amoeboid, 102, 286, 557
ciUary, 134, 286
of joints, 305
Mucin, 1341

in saliva. 242
in bile, 262
Mucor, 552
Mucous membrane, 12

.588 INDEX

Mucous membrane, epithelium of, 545

of alimentary canal, 545
olfactory, ifi')
of trachea, 133
glands, 239
Mucus, 12, 134
Murmurs, respiratory, SHt

Muscle (musrulus, a little mouse), contractility of, 13, 21, 33, 50, 73,
288-303, 341, 472
the chemistry of, 296 et seq.
-nerve preparation, 299, 300
-plasma, 298
-serum, 298
striated, 44, 289, 296
tetanic contractions of, 301
unstriated, 32, 288, 402
weight of. 268
Muscles, various kinds of, 302 et, seq.

attached to definite levers, 303
carbonic aci<l secreted by, 297
composition of, 171, 302
death of, 27, 2«3

changes caused by, 292
elasticity of, 301
hollow, 302

insertion and origin of, 305
oxidation of, 175
Muscles, arytenoid, 333 <

biceps, 13, 305
cihary, 400, 411
crico-arytenoid, 333
crico-t.hyroid, 331, 333
cricoid, 330
digastric, 306. 307
facial. 516

intiercostal. external and internal, 140, 144
oblifiue, of the eye, inferior, 414, 415

superior, 306, 414, 415, 516
orbicularis, 415
papillary, 46, 54
pharyngeal, 517
rectus, of abdomen. 316

of eye, external and internal, 414, 415. 516

superior and inferior, 414, 415, 516
of leg. 316'
stapedius, 371, .389
tensor tympani, 371, 389
superior obli(iue, 516
thyro-arytenoid, 333
triceps, 305
tympanic, 389
Muscular coat of arteries, 32

fibre, breadth of. 566
fibres of the heart, 44, 303

of striated muscle, 289
of iris, 303, 402
heat, 202
sense, the, 356

INDEX 589

Muscular tissue, development of, 295

Muscularis mucosa', 256

Musical sounds, how produced, 382

notes, varying with the tension of vocal chords, 334
Myelin, 461
Myosin, 226, 298

coagulation of, in rigor mortis, 297

Nails, growth of, 194, 196

non-vascular, 31
Nares (nostrils), anterior and posterior, 362
Nasal (nasus, nose) bones, 363

ca\-ities, ciliated cells in, 286
Natural Magic, quoted from, 441
" Near sight," 413
Neck of Tooth, 232
Nerves, accelerator, 73

afferent or sensory, 340, 461, 473

arterial, 34

auditory, 375 et seq., 501, 523

cardiac, 72

chorda tympani, 240

cochlear, 375 et seq., 523

cranial, 500, 515

effect of irritation on, 426, 472, 484

efferent or motor. 340, 461, 473

electrical properties of, 478

facial, 516

glosso-pharyngeal, 359, 501, 517

gustatory, 360, 516

of the heart, 72

hypoglossal, 501

intercostal, 158

laryngeal, 161

medullated, the minute structure of, 459

motor, 289, 340, 461, 473

motores oculi, 516

oculo-niotor, 407

olfactory, 363, 501, 515

optic, 419, 424, 515, 517

phrenic, 158

pneumogastric, or vagus, 73, 496, 501 517

posterior and anterior roots of, 456

renal, 186

sciatic, 299

sensory, 340, 461, 473

of special sensations, end-organs of, 383

spinal, 454, 469, 493, 495

spinal accessory, 501, 517

splanchnic, 71

sweat, 200, 201

sympathetic, 9, 67, 452, 495

trigeminal, 501, 516

vaso-constrictor, 68

vaso-dilator, 70

secretory, 202

590 INDEX

Nerves, vaso-motor, 65, 67, 204, 494

vestibular, 392, 523
Nerve-cells of cord, 467

breadth of, 566
in olfactory " nerve," 516
in grey matter, 456, 458
in nerve centres, 456
of sympathetic ganglia, 498
Nerve centre, spinal cord an independent, 486
Nerve centres, composition of, 450
function of, 21. 486
Nerve-Hbres, in blind spot of eve, 430
diameter of, 460, 566
in ear, 383
meduUated, 469
nodes of, 460
nucleated, 460
structure of, 453, 459
in tactile corpuscles, 349

white matter of cord and brain composed of, 458
Nerve roots, functions of, 472
Nervous apparatus, duplexity of, 452

impulse, rate of transmission of, 478

molecular change in nerve-fibres caused by, 383,

341, 426
transmitted from brain by spinal cord, 486
system, 452

as co-ordinating organ, 26
as controlling circulation, 33, 34, 66
evaporation, 203
glaiulnlar action, 193
muscular action, 472
respiration, 156 et seg.
Neuraxon, 461
Neurilemma (vevpof, a nerve ; Ae>/j.a, a peel or skin), 460, 467

continuous with sarcolemma, 463
Neuroglia, the, 465

Nitrogen (I'tVpor, potash ; yti-couj, I produce) not absorbed by lungs,
1.50, 152, 269
daily waste of, 270 et seq.
in proteid foods, 108. 188, 225
starvation from lack of, 274, 277
in urea, 184
in blood, 125
Nitrogenous foods, 225
Nodes of nerve-fibres, 460
Noises, 384

Non-meduUated nerve-fibres, 497
Non-nitrogenous foods, 225, 226
Non-vascular tissues, 31
Nose, 130, 362

Nuclear layer, of cerebellum, 511
Nuclear layers of retina, 418
Nucleated cells, in capillaries, 31
in cartilage, 548

of epidermis and epithelium, 542, 545
in lacunae of bone, 313
all tissues primitively composed of, 295, 537

INDEX 591

Nucleolus of nerve cell. 468

ovum, 537
Nucleus (a kernel) in white corpuscles, 95

division of, in growth of ovum, 537, 538
in cells of capillary walls, 31
in nerve-flbres, 460

in unstriped muscular fibre-cells, 44, 288
Nutrition effected by circulation of blood. 23

connection of thjToid gland with, 218
food and. 268 et seq.
some statistics of, 268
Nutritive foramen of bone, 308

Oblique muscles of the eye, 306, 414, 415, 516

Occipital lobe. 505. 529

Oculomotor nerve. 407

Ocular spectra, 441

( klontoid (66ous, iidin-os, a tooth ; ft5os. form) process, 322

•Esophagus (oio-w, obsolete = </)e'pu, I bear ; (Jayeif, to eat), 10, 130,

230, 236
Olecranon (<iAerT). the elbow ; Kp6.vo<;, a helmet), 310
Olfactory {olfacio. I smell) lobes, 362
epithelium, 365 et seq.
membrane. 365
nerves. 501. 515

prolongations of cerebral hemispheres, 515, 518
Optic chiasma. 517, 519
nerve, 417, 501. 515

not directly excited by light. 426
a prolongation of third ventricle of brain, 517
ramifications of, 419
thalami. 507
tracts. 518
Optical delusions. 441. 443
Ora serrata (serrated border). 403

Orbicular (orbiculus. a small round ball) bone, 370 note
Orbicularis muscle, 415
Orbit of the eye. 398
Organ oi Corti. 376 et seq.
Organic compounds in foods. 225
Organules of special sense, 343
Origin of a muscle, 305
Osmosis (IU0-M05. impulsion), 120
Ossa innominata. 16
Osseous labyrinth of ear. 393

tissues. 307
Ossicles (ossicula. a little bone), auditory. 370, 378
■' Outness," sense of. accompanying sense of smeU, 438
Oven, heated, conditions of safely remaining in, 206
Overtones, their nature. 385
Ovum, mammalian, described. 537
Oxidation, change to arterial blood caused by, 125, 126
of proteid matter. 225
in tissues, the source of energy, 7, 225, 282

of heat. 24. 172. 202. 225
Oxygen (ufOs, acid : yevvdai, I produce), absorption of, by the lungs,
24, 127, 152, 154, 162, 173, 564

592 . INDEX

Oxygen, amount of, consumed, 564

blood corpuscles apparently flattened by presence of, 100,

101, 126
colour of arterial blood caused by, 125, 126
combination of, with h;i'nioglobin, 107, 126
effect of privation of, IfiS
excess of, in arterial blood, 124
in excretions, 6
in proteids, 108

PAOmiAN corpuscles, 351
Pain, sensations of, 346 et seq.
Palate, hard, 229

soft, 229, 362
Palpitation caused by emotions, 73, 77
Paleness, cause of sudden. <i7i
Pancreas (n'lv, all ; Kp^as-, llesh), 9, 258

glands of, 227
Pancreatic juice, 244", 264
Papilla, circumvallate, 358

filiform, 358

fungiform, 358

of dermis. 190

of hair, 195-197 ♦

Papillae, tactile, 348

of tongue, 358
Papillary muscles, 46, 54
Paraglobulin, 100, 114

Paralysis (Ti-apa, beside ; Au'u, I loosen), injury to brain, 521
Parietal lobe, 505, 529
Parieto-occipital fissure, 528

Parotid (n-apa, beside : ou?, li-b?, the ear) gland, 237, 239
Patella (a dish or plate), 16, 317
Peduncle, inferior. 401. 506
middle, 506
superior, 492, 506
Pelvis (a basin), 16, 17, 321

of the kidney, 177
Penicillium, 552

Pepsin (iren-Tiu, I digest), 2441, 247
Peptone, 247

solubility of, 248
Perfect joints, 319
Pericardial fluid, 42
Pericardium (Tepl, about ; KapSia, the heart), 42

contents of, 113
Perichondrium (Trepl, about ; Yoi'Spos, cartUage), 547
Perilymph (fept, about ; lympha. water), ear-sac surrounded by, 372
Perimysium (fff pi, about ; /niis, a muscle), consists of connective tissue,

289
Perineurium (n-epi, about ; vevpov, a nerve), 460
Periosteum (n-epl, about ; oariov, a bone), 307
Peripheral resistance, 56, 59
Peristaltic action, 236. 254

Peritoneum (Trepl, about ; relvto, I stretch) described, 176
intestines and stomach enveloped in, 251, 253
liver surrounded by, 208

INDEX 593

Permeability of membrane, 192

Perspective, aerial and solid, 450 ff„„t„j k„ omntinn

Perspiratiou (per. through ; spiro, I breathe) affected by emotion,
201

amount of matter lost by. 199

sensible and insensible, 198
Pes of the crus cerebri, 528
Petrosal (jrerpa. a rock) bone, 372
Pever's patches, 256

Phalanges (<f>dXav^. a rank of soldiers), 18
Pharyngeal muscies. 517
Pharvnx (^ipi/.-f, the throat), 10. 129 235
Phosphene (*w?, Ught : <i>aCrtj. I display). 429
Phosphorus present in human body. •''62
Phrenic l't>prif. the diaphragm) nerves, 1.^8
Physiology, human, defined. 4

ultimate analysis of, 536
Pia mater, 453. 465 „ ^ ,. •, ,n^ .oj

Pigment {pigmentuyn. paint) cells of choroid. ■»01.„*''*

of web of frog, /8, 81
Pillars of the diaphragm, 143

of the fauces, 229
Pineal gland. 503, 504 .

Pituitary (pituita, phlegm or mucus) body, o02

Pivot joint, 321 ^ n, no ^r^y

Plasma (nXda-^a. workmanship) of the blood, 94-98, 107

solids in, 108, 114
Platelets, blood-. 82, 104
•• Playing at sight," 527
Pleura (TrAeupa. a rib or side), 13d

Plexuses of the sympathetic system. 49o „o,„„= 7q t;ni

Pneumogastric (r-n;^<o.'. lung : yao-r-ip, the stomach) nerves, /3, 501

heart's action arrested by means of, 496
Poisoning by carbonic acid, 163, 164 . -j^ i«,

by sulphuretted hydrogen and carbonic oxide, 164
Polymorphous layer of cerebral cortex, 515
Pons Varolii, 500, 501, 506
Portal (porta, a gat«), circulation, 40. 210. 211. 21/. 249

passage ot lood into the, 266
Position, erect, how maintained, 17
Posterior cornu, 457 .

nerve roots, sensory in function, 4<3

root, ganglion of the, 456
Postero-median'tract of spinal cord, 491
Potatoes, 563

as food, 273, 563

Pressure, atmospheric, 154 . . , , j ■ • • ..• i ^q

on heart, diminished durmg inspiration, lo9

partial, of gases. 154

sensations of, 352
" Pressure spots." 352
" Primitive sheath " of nerve-fibres, 461

Pronation (promts, face downwards) of hmbs, 325 „,„nroa+iP

Protein (^pi>ros, first ; eI5o9, shape) material, acted^on^by pancreatic

blood corpuscles formed

of, 97, 103
composition of, 108

Q Q

594 INDEX

Protein (ttpwtos, first; dSo;, shape) material, dissolved by gastric

juice, 24", 248
as food, 6, 225-227,

274
given up to the tissues
from the blood, 171
of plasma, 107
nitrogen in, 188, 225
reactions of, 108
Protopathic sensations, 347
Protoplasm, colourless corpuscles formed of, 100, 101, 536

of ovum, 537
Psychical (4'vxri, the spirit) phenomena, connection inconceivable

between molecular changes and, 526
Ptyalin (jttuw, I spit ; aKivos, salted), properties of, 242, 264
Puliis. 16

Pulleys, ligamentous, 415
Pulmonary {pulmo, lung) capillaries. 127

veins, 35, 38, 41
Pulp cavity of tooth, 232
Pulse, the, 60

lost in capillaries 61
venous, 167
waves, 61
Punctum lachrymale (lachrymal point), 416
Pupil, 402

constriction and dilation of, 407, 408
Purkinj6, cells of, 511
Purkinj6's figures, how produced, 430
Pus. 80

Pylorus (TTuAwpo?, a gate-keeper), 244
Pyramids, anterior, of medulla oblongata, 521

decussation of, 520
Pyramidal layer of cerebral cortex. 515

tract, its coiuicction with the cerebral cortex, 532
crossed, of spinal cord, 492, 520

QUADRIOEMINA, corpora, 518

Rabbit, experiment on ear of. 67

Racemose (racemus, a bunch of grapes) glands, 228

Radial artery, 60

Radicles, lacteal, 257

Radius (a ray or spoke of a wheel), 306

articulation of, 324
Recti (straight) muscles of the eye, 414. 415

nerve supply to, 515
Rectum (intestinum rectum), 253, 516
Rectus muscle of abdomen, 316

of leg, 3161
Receptacle of the chyle, 86
Red corpuscles, 94, 96

action of oxygen on, 100, 101, 126

possibly broken up in spleen, 222

of spleen, 222

size of, 95, 96, 566

structure of, 96, 98, 104

INDEX 595

Reflex action, paths of. 341, 342
of the brain, 526
of the cord, 484, 524
in conghing, 157
Reissner, membrane of, 376
Renal (ren, a kidney) artery, 179-183

constant of, 565 '

Rennet, 248
Rennin, 248

Reproduction of tissue, 27, 556
Residual air, 148, 564
Resistance to etfort, sense of, 352
Respiration, 123

constant of, 564
costal, 139

cubic feet of air needed for, 169, 565
diaphragmitic, 139
effect of, on circulation, 165
essential of, 123
internal, of the tissues, 128
mechanism of, 135, 141-148, 156, 159
nervous apparatus of, 156. 165
rat« of, per minute, 148, 152, 564
waste in, 151, 269
Respiratory centre in medulla oblongata. 156, 158
influence of blood supply on, 161
changes, 153
organs, 24
Restlessness, sensation of, 343
Rete (a net). Malpighii, 540
Retiform connecti^•e tissue, 90, 559
Retina (rete, a net) described, 399, 420
blood vessels of, 430
cells of, 419 et seg.

distinguished from fibres of the optic nerve, 42y
formation of the image on the, 408
inverted image, 444
its sensibility soon exhausted, 426
nervous structure of. 421
pigmented epithelium of, 424
Retinal impressions corrected by sense of touch. 444
Rhytlimical (pv9/ib5, measured motion) pulsation of heart, 23, 50,51,73
Ribs. 14. 139, 142, 307
Ftigor mortis (stiffness of death), 297
Rod bipolar cell, 422
Rod-shaped cells of olfactory ner\-es, 366
Rods and cones, layer of. 406, 420
functions of, 429
Rods of Corti. 378. 387
Rolando, fissure of, 505. 528
Roots of spinal nerves, anterior and posterior, 456
Rotation of joints, 322
Rouleaux, red corpuscles collect in, 95
Round ligments, 321
Running, how effected, 329

Sacrum, os (the sacred bone, because offered in sacrifice), 14
Saline matters, clotting retarded by, 1 12

596 INDEX

Saline matters, excretion of, 6, 23, 171, 184

in food, 225
Saliva, action of, 240, 261

nervous centre for secretion of, 240
secretion of, 237 et seq.
Salivary glantls. 173, 237
Salts excretion by the kidneys, 269, 273, 279
in food, 225, 226, 279
neutral, action on blood, 112, 114
in saliva, 242
Saponification by pancreatic juice, 260
Sarcolactic acid, 298

Sarcolemma (adp^, flesh ; Aeni(u.a, a bark or skin), 294
Saturation in light, 431

Scala, (a ladder) of the cochlea, 369. 375, 377
Scales of epidermis continually shed, 541
Scapula, 16
" Schwann, sheath of," 461

white substance of, 461
Sciatic nerve, 299
Sclerotic (o-xArjpo?, hard). 399
Scurf, nature of, 541
Sebaceous (sebum, suet) glands. 195-198
Secretin, 259
Secretory nerves. 201

Semicircular canals of ear, 368, 369, 390-397
Semilunar valves, 47, 58
Sensations, 339 et seq.

auditory, 382
composite, 438
diffuse, 346
of light, 426 et seq.
of pain, 354
simple. 437
subjective, 346
tactile, 355
Sense of hearing, 367
muscular, 356
of sight, 398
of smell, 361
of taste, 358
of touch, 347, 355
of warmth, 352
Sense-organs, 21, 343

essential and accessory parts of, 345, 398
Sense-organules described. 345, 464
of taste, 360
of touch, 349
Senses, the special, 343
Sensorium auditory, 382 et seq.

visual, 426
Sensory areas, 530

decussation, 520

or afferent nerves, 340. 472

collected into the posterior roots, 470, 471
Septum (a partition -, sepio, I fence in) of the nose, 364
Septum of the cochlea, 374
Septum lucidum, 504

INDEX 597

Serous cavities, peculiar epithelium lining, 546
fluid, 135'
membranes, 42
Serum-albumin, 1(>9, 114

(whey, hutterniilk). Ill, 114, 117
Sex, mechanism of respiration varies according to, 146

voice varies accordins to, 335
Shadows, judgment of form by, 447
Shaft of bones acting as levers, 304
" Sheath of Schwann," 461
Sheep, heart of, examined, 39, 46, 48
Shoulder joint, 318
Sighing, 146
Sight, long, near, and old, 413

sensation of, 343, 344, 398
Single vision with two eyes, 415, 449
Skeleton (o-iceAAcj, 1 am dried up), 14

weight of, 561
Skin, blood not rendered venous in the, 156
a double integument, 12, 535
an excretory organ, 23, 269, 565
kidneys affected by stat« of the, 187
a source of loss to the blood, 189
structure of, 191
weight of, 268, 561
SkuU, 8

number of bones of, 16
Smell, organ of, 343, 361
••Sniffing," 146, 361

air drawn into olfactory chamber by, 366
Sneezing, 146
Sodium In bile. 214
Solids of the body, 562
Solidity, ^^sual judgment of, 450
Solitary follicles. 256

glands, 256
Sound, conversion of sonorous vibrations into sensations of, 382
localisation of, 388
perception of, 367

waves, transmission of, to inner ear, 371, 379
Sounds, cardiac, 57

musical, 335, 384
Special senses, the, 343 et seq.
Spectacles, the use of, 412
Spectra, auditory. 441

ocular. 441
Speech, mechanism of, 336

Spiiincter (<T<i>i.yyio . I throttle or bind) muscle of bladder, 177
fibres of eye, 407, 408
of rectum, 164
Spinal accessory nerves, 501, 517

bulb (meiluUa oblongata), 28, 69, 497, 502

functions of. 519
cord, described, 9, 454, 465

acts as independent nervous centre, 21, 486

effect of galvanism on, 21, 472

fissures of, 454

grey matter of, 456, 458

598 INDEX

Spinal cord, paths of conduction of afferent and eflferent impulses, 488 ,
400
properties and functions of, 483
reflex action tlirousli, 484
result of injury to, 20, 473, 477
transmission of nervous impulses by, 486
white matter of, 456, 458
vaso-niotor centres in, 494
ganglia, 460

nerves, 455, 4;?6, 475, 493
Splanchnic nerve, abdominal, 71
Spleen, 9

its functions, 222
corpuscles of the, 222
-pulp, 221
Splenic artery and vein, 221
Spongy bones of nose, 364
Spot, blind, of eye, 427

germinal, of ovum, 537
yellow, of eye. 423
Squinting, double vision a result of, 449
Stapes (a stirrup), 369 et .wi/.

its attachments, 37o
Stapedius muscle, 371

possible use of, 389
Starch, action of ptyalin on, 242, 261
as food, 226

converted into sugar in alimentary canal, 216
by pancreatic juice, 261
by ptyalin, 242, 261
Starting at noise, a cerel)ral reflex action, 526
Stereoscope (o-Ttpeb?, solid : a-Koniui, 1 view), 450
Sterno-costal cartilages, 546

embryonic growth of, 550
Sternum (<TTepvov. the breast), 16, 135-138, 147, 316
Stiffening of muscle after death, 297
Stimulation of nerves, 72, 74, 241

of cerebral hemispheres, 530

of muscles, sec M uscles, contractility of

tetanic contraction of
Stimulus, 14, 341

Stoke's agent, action on blood, 1:^5
Stomach (a-Toixa, a mouth), 244

walls of, 227, 244
Stratum corneum and mucosum of epidermis, 540, 541
Striped muscular fibre, 289

in heart, 44, 303
Stroma, 97, 98
Sub-arachnoid space, 453
Snb-diind space, 453

Sublingual gland. 237 *

Submaxillary gland, 237, 238
Substantia gelatinosa, 466
Succus entericus, 262

Suction pump, respiratory machinery regarded as, 157
Sugar, action of, in intestines. 267

in blood increased by injury to the medulla oblongata, 520
conversion of glycogen into, 217

INDEX 590

Sugar, its fattening property, 278

as food. 226, 264

formed from action of ptyalin on starch, 242, 261

starch converted into, 242
Sulci of brain, 509, 528
Sulphur in proteids, 108

Sulphuretted hydrogen, mode of action at poison, 164
Supination (t^Kpinux, lying on the bacl<) of limbs, 324
Supplemental air. 148. 149, 564
Supra-renal bodies, 219
Swallowing, 229, 235, 236
Sweat, composition and quantity of, 198

glands, 192, 196

stimulated by warmth, 204

secretion of, and its nervous control, 200
" Sweet-bread," 9
Sylvius, aqueduct of, 503, 504

fissures of. 505. 528
Symmetry {crvy, together ; i^fTpov, a measure), bilateral, of body, 8
Sympathetic (aiiv. together ; Trdfc'os, feeling) nerve, blushing governed
bv, 66. 495
system. 9. 452. 495, 496
Syno\aa (ai/v, with ; wbr. an egg), and synovial membrane, 17, 319
Systole {crva-TiKKia, I draw togetlier, contract), 51, 52, 57

Tactile (tango, I touch) corpuscles. 190, 348. 349
impressions, education of the eye by, 444
sensations. 352
Tangential rays of cerebral cortex, 515
Taste, sense of. 343. 358 et seq.

organ of, 358
Taste-buds. 358, 360

Taurocholic (raOpo?, a bull ; \o\'ij, bile) acid. 262
Tears, secretion of, 417
Teeth, 31, 231

enamel of, 233
Temperature of blood, 25, 66, 105. 202

of body, due to oxidation, 24, 172, 202

regulated by blood supply to skin, 66, 202, 206
effect of, on coagulation of blood, 112

on vaso-niotor nerves, 205
of expired air, 135
of fever, 207

sense of, relative rather tiian absolute, 352
Temporal (tempora, the temples) artery, 61
bones, 378
lobe, 529. 531
Tendons (tendo. I stretch), 289, 305
structure of, 558
weight of, 268
Tensor tympani (stretcher of the drum) muscle, 371, 389
Teres ligamentum (tlie round ligament). :i21
Terror, its effect on the vaso-motor system, 66
Tetanic contraction of muscles, 3U1
Thigh, 8, 304
Thoracic duct, 85

viscera, weight of, 268

600 INDEX

Thorax (9<ipa|, the chest) described, 135, 139

organs within the, 8
Thoughts, 343
Thymus gland, 220
Thyroid (euptb?, a shield ; elSo?, shape) body or gland, 217

cartilage, 330
Thyroid-arytenoid muscle, 333
Tibia (a pipe or Hute), 318
Tibial artery, (11

Tickling, paralysed limbs not insensible to, 21, 484
Tidal air, 148, 149, 564

effect of change in, 162
" Tightness " of the ear, 389

Time required for propagation of nervous impulse, 481, 566
Tissue, adenoid, retiform, or lymphoid, 90, 255, 559

adipose, 559, 560

areolar, 86, 553

cancellous, 305

cartilaginous, 546

connective, conversion of food into, 228
examination of, 553

epithelial, 539

osseous, 307

muscular, 289'

retiform, 559

white fibrous, 558

yellow elastic^ 558
Tissue-forming food-stuffs, 278
Tissues of the body. 530, 539

combinations of, 539

minute structure of, 536

oxidation of, 280

primitive, 536

reproduction of, 27, 561

respiration of, 128, 153

various, 10, 539
Tone, arterial, 68
Tongue, 229

nerve supply to, 359, 360, 517
speech possible after amputation of, 338
Tonsils, position of, 229, 359
Touch, retinal impressions corrected by, 444

sense of, 343, 346

varying sensibility of different parts of the body to, 356
Trabeculae of lymphatic gland capsules, 89

of spleen, 221
Trachea (arteria trachea ; rpoxus, rough : the rough artery), 129-133

ciliated cells in the, 285
Transfusion, 117

Transudation through capillaries, 31, 34, 113
Trapezium (dim. of Tpdm^a, a table), 321
Triceps muscle, 322, 327

Tricuspid {tres, three ; cuspis, point of a weapon) valve, 45
Trigeminal nerve, 501, 516
•• Tripod of life," 28
Trochlear, 511

Trunk of spinal nerve, effect of irritation on, 472
Trypsin, reactions of, 260

INDEX 601

Tube, Eustacliian, 229, 370, 392
Tubules of kidney, 178 et seq.

collecting, 180

convoluted, 179, 180

spiral, 179

zigzag, 180
Tuning fork, vibrations of, 384
Turbinal (turbo, I whirl) bones, 230, 363, 364, 416
Tympanic muscle, 371, 389
Tympanum (TVfj.wavov, a drum) of ear, 368 et seq.
Tyrosine, 360

Ulna (uikev-q, the elbow), articulation of, 322
Uncomfortableness, sense of, 343
Unstriated muscular fibre, 288

in alimentary canal, 303
in bladder, 177
in coat of arteries, 32
in fibres of iris, 303, 402
structure of, 288
Urea (oupor, urine), excreted by kidneys, 6, 24,171, 269
chemistry of, 183 et seq.
history of, 185, 188
weight of, passed per diem, 565
Ureters, 9, 176
Urethra, 177
Uric acid, 184

Urine, average amount excreted daily, 184
composition of, 183

secretion of, influenced by state of skin, 187
Utricle (a small bag) of ear, 391 et seq.
Uvula (dim. of uva, a grape), 229

Vagus (wandering) or pneumogastric nerve, 73, 501, 516

stimulation of, 159
Valve of Vieussens, 504
Valves in arteries, 38, 85

course of circulation governed by, 78, 166
ileo-ca?cal, 251
of heart, 45 et seq.
in lymphatics, 85
in veins, 35, 36, 85, 166
Valvulse conniventes, 255
Varnish, result of covering the skin with, 200
Varolii, pons, connection of, with cerebellum, 500, 503, 506
Vascular membrane, 453

system, 30 et seq.
Vaso-constrictor nerve, 68
-dilator nerves, 70

impulses, effect of perspiration on, 201
-motor centres, in spinal cord. 69, 70, 494, 519
nerves, 65, 67, 205, 494

ultimately traceable to spinal cord, 494
Veins, 23 et seq.

coUapse when empty, 34

602 INDEX

Veins, no pulse in, 60, 61

valves in, 35, 36, 85, 166
Veins, azygos, 41
cerebral, 35
coronary, 40, 48
hepatic, 38, 40, 209, 210, 217
innominate, 87
interlobular, 211
intralobular, 209, 212
jugular, 43, 87

portal, 35, 40, 210, 211, 217, 247
pulmonary, 35, 38, 41
splenic, 221
subclavian, 87
Veinlet, intralobular, 212
Velum (a curtain), the, 229

interpositum, of brain, 509
Vena cava (the empty vein), inferior, 38-40, 208, 209, 212

superior, 38, 87
Vena porta, see Portal circulation
Venous blood, dark colour of, 124
causes of_, 161

effect of, on brain, 162 et seq.
pulse, 167
Ventilation, necessity of, 168, 564

Ventricles (ventriculus, a little belly) of the brain, 502 et seq.
of the heart, 43, 52

worii done by, 564
contraction of, 51
thickness of walls of, 55
Ventriloquism, effect of, due to suggestion, 443
Vermiform (vermis, a worm) appendix, 251
Vertebrae (.verto, I turn), bodies of, 9

coalescence of, in sacrum and coccyx, 14
of neck, 323
Vertebral column, 15

as example of imperfect joints, 316
Vesicle, germinal, 532
Vessels, muscular, 289
Vestibular nerve, 392 et seq., 523
Vestibule (or porch) of ear, 393
Vibrations, of cilia, 134

in endolymph, 383
of ether, physical basis of light, 426
molecular, 382

musical sounds, due to regularity of, 384
of the ossicles, 371
sensory organs affected by, 22
sonorous, 37), 379, 382 et seq.
of tympanic membrane, 371, 379
of vocal chords, 334
Vieussens, valve of, 504
Villi (villus, shaggy hair), structure of, 256
prolongation of the lacteals into, 86, 255
absorption by means of, 266, 267
Vision, conditions of, 398

explanation of the singleness of, 415, 449
probable seat of end-organ of, 526

INDEX 603

Visual seasorium. 426
Vital actions, 4, 26

foods derived from the vegetable world, 285
ultimate analysis of, 225
Vitreous (ritriim. glass) humour, 399
Vocal chords, 129. 329 et seg.

voice due to the presence of, 329
Voice, 334

production of, 329
quality of, 335
range of, 335
VoUtions, 342

absent, where brain is absent, 485
Voluntary muscular contraction, brain the source of, 21
Vowel sounds, how formed, 336

Walking, mechanism of, 328, 487

Wallerian method, 475'

Walls of vessels, differing structure of, 32

bony, of thorax, 139
Wandering cells, 556, 557
Warmth, sense of, 352
Wast«, in contraction of muscle, 149
made good by food, 224
nitrogenous, 146
in respiration, 151, 269
as result of work, 6, 7, 22
Wast€ matter in blood, excretion of. 171
Waste products of work ui tissues, not all useless, 171
Wat«r, absorption of, by the large intestine, 265
in food, 225, 226, 272, 563
excretion of, 6, 24, 171, 199, 272, 562
bv kidnevs, 177, 183, 565
by lungs. 24, 135. 564
by skin, 1 98, 565
proportion of, in bile, 214
in blood, 106
• Water-camera described, 406

eye-ball considered as, 404
" Watering " of the mouth, 242

Weight, proportional, of component parts of the body. 268, 561
White commissure of spinal cord, 4.58

White corpuscles of the blood, 94-96, 100, see Colourless corpuscles
amoeboid movements, 102, 222
changing form of, 100, 101, 286, 557
possibly produced in spleen, 223
relative number of, 105
size of, 566
of spleen, 223
typical nucleated cells, 536
corpuscles, cell-body of, 102
fibrous tissue, 558

matter of brain and meduUa oblongata, 510
of Schwann, 461

of spinal cord, 456-458, 465, 489
Will, seat of, 524
Winking, a cerebral reflex action, 526

604 INDEX

Work, physiological, 4

estimate of, in foot-pounds, 282, 562

waste, a result of, 22
Wrist, 8

Yellow elastic tissue, 558

spot of eye, 42:5, 424

wdth of cones in, 566
Young-Helmholtz theory of sensations of colour, 433, 436
Youth, bones afterwards united, are separated in, 14

Zona pelUioida of ovum. 538

Zootrope (^wof, an auiinal ; rponos, a turning), 448

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PRINTED IN U.S. A CAT. NO. 24 161 m

A 000 50o"558

QTIOU
H986L

1915
Huxley. Thomas H

Lessens in elementary physiology

MEDICAL SCIENCES LIBRARY

UNIVERSITY OF CALIFORNIA, IRVINE

IRVINE, CALIFORNIA 92664