GIFT OF

MICHAEL REESE

ELEMENTS

OF THE

COMPARATIVE ANATOMY

OF

VERTEBRATES.

ELEMENTS

OF THE

COMPAEATIVE ANATOMY

OF

VERTEBBATES.

ADAFPED FROM THE CTERMAN OF

EGBERT WIEDEESHEIM,

PROFESSOR OF ANATOMY, AND DIRECTOR OF THE INSTITUTE OF HUMAN AND COMPARATIVE ANATOMY
IN THE UNIVERSITY OF FREIBURG-IN-BADEN,

BY

W. NEWTON PARKER,

PROFESSOR OF BIOLOGY IN THE UNIVERSITY COLLEGE OF SOUTH WALES AND MONMOUTHSHIRE.

WITH ADDITIONS BY

OR AND TRANSLATOR

TWO HUNDRED AND SEVENTY WOODCUTS.

MACMILLAN AND CO.

AND NEW YORK.

1886.

The Right of Translation and Reproduction is Reserved.

w

RICHARD CLAY & SONS,

BREAD STREET HILL, LONDON',

Bvngay, Suffolk.

PEEFACE.

PROFESSOR WIEDERSHEIM'S Grundriss der vergleichenden
Anatomic der WirleUhiere, published at Jena in 1884, was
written to supply a need which had been felt for some time
past for a short text-book on Vertebrate Anatomy embodying
some of the more recent views on the subject. The present
book is a modified translation of the Grundriss, and it is
hoped that it will serve to render Professor Wiedersheim's work
more widely known amongst English students.

The plan of the original has been retained throughout, though
numerous additions and modifications have been made to the
work ; for many of these I have to thank Professor Wiedersheim,
— for others I am myself responsible. I must also express my
indebtedness to Professor Wiedersheim for revising the whole
translation with me last summer, and for much help while the
work was in progress.

Within the limits of a short text-book like the present, much
of the matter is of necessity greatly condensed : more detailed
accounts of the various parts and organs will be found in the
new edition of Professor Wiedersheim's Lehrlucli der vergl.
Anatomie der Wirbelthiere, which is to appear shortly, and on
the first edition of which the Grundriss was founded.

vi PREFACE.

The brevity of the descriptions is, however, to some extent
made up for by the number of woodcuts. Most of these are
taken from the German edition, but several new figures have
been added.

The arrangement of the book according to organs, and not
according to groups of animals, is likely to render it more difficult
for a beginner, and a general knowledge of zoology will be of
great assistance. The pages on which the different groups are
described are, however, collected together in the index, so that the
sections relating to any one group can be easily referred to. The
present arrangement seems to be the only possible one if the book
is to be founded on a scientific basis, for it is most important that
the student should grasp the fact that there has been an evolution
of organs, as well as of animals.

The more theoretical and detailed matter is printed in small
type, and in the form of notes : the student should in most cases
pass this over when reading the book for the first time. A black
and a spaced type have been used to render prominent important
words or sentences.

A bibliography is appended at the end of each chapter. This
in no case presumes to be anything like a complete list of the
literature of the subject : our object has been more particularly to
mention the recent and the more important works, though many of
these have doubtless been omitted. References to other researches
can be found by consulting the works mentioned.

At Professor Wiedersheim's suggestion, I have not inserted a
translation of the preface to the original, as it seemed unnecessary
so to do. I may, however, mention that the book was written
for students of Medicine, as well as for those of Comparative
Anatomy: the intimate connection of the two subjects renders it

PREFACE. vii

most important that medical students should have a general
scientific basis for their special anatomical knowledge.

My sincerest thanks are due to my friends Professors F.
Jeffrey Bell and G. B. Howes, who have kindly read through
the proof-sheets. To them I am indebted for numerous valuable
suggestions, as well as for correcting many faults of style and
expression which had escaped my notice. I must also express
my thanks to my father, Professor W. K. Parker, and to Dr.
Gadow, for many special details in connection with the skeleton,
as well as to Mr. E. Radford for help in making the index.

W. N. PARKER.

UNIVERSITY COLLEGE, CARDIFF,
May 1886.

ERRATA AND ADDENDA.

Page 46, 9th line, for "centra" read "centre."
,, 83, 5th line, for "when" read "where."
,, 136, in Fig. 109, SR should indicate the space below DM, and the line from A

should point to the layer indicated by SR.
,, 249, Fig. 203, for "Siluris" read "Silurus."

Insert under Bibliography : —
On p. 160 :—

GRAAF, HENRI "W. DE. — Zur Anatomic und Entwickelung der Epiphyse bei
Amphibien und Reptilien. Zool. Anzeiger, IX. Jahrgang, No. 219, 1886.

SPENCER, W. BALDWIN. — The Parietal Eye of Hatteria. Nature, May 13th,
1886. (De Graaf shows that the epiphysis really corresponds to an un-
paired sense-organ, and Mr. Spencer has found a .well-developed median
eye in Hatteria and other Lizards. Compare p. 133 of this book.)

JOHNSON, ALICE, and SHELDON, LILIAN. — On the Development of the Cranial
Nerves in the Newt. Proc. Roy. Soc. 1886.

On p. 294 : —

PARKER, T. JEFFERY.— On the Blood- Vessels of Mustelus antarcticus. A Con-
tribution to the Morphology of the Vascular System in the Vertebrata.
Abstract in Proc. Roy. Soc. 1886. (Not yet published in full.)

CONTENTS.

PAOB

Preface v

Errata and Addenda . viii

List of "Woodcuts xv

List of General Works on Comparative Anatomy and Embryology ...... xxv

INTRODUCTION 1

I. On the Nature and Meaning of Comparative Anatomy 1

II. Development and Structural Plan of the Vertebrate Body 2

III. Classified List of the Principal Vertebrate Groups 13

IV. Table showing the Gradual Distribution of the Vertebrata in Time ... 15

SPECIAL PART.

A. INTEGUMENT .16

of Fishes : 17

of Amphibians 18

of Reptiles 20

of Birds 20

of Mammals 23

Mammary Glands 27

Bibliography . . .' , . . . . 28

B. SKELETON 30

I. DERMAL SKELETON ... 30

II. EXDOSKELETON . . 33

I. VERTEBRAL COLUMN 33

of Fishes 34

of Amphibians 39

of Reptiles 42

of Birds 44

of Mammals . 46

Bibliography 47

II. RIBS 48

of Fishes and Dipuoaiib 48

of Amphibians ' 48

of Reptiles, Birds, and Mammals 4M

CONTENTS.

PAOE

III. STERNUM 51

Bibliography 54

IV. THE SKULL 54

Theory of tho Segmentation of the Skull 54

a. Brain-case (cranium) 57

&. The Visceral Skeleton (general description and develop-
ment 59

c. The Bones of the Skull (general description and develop-
ment) 61

Anatomy of the Skull (special part) 63

A. The Skull of Fishes 63

B. ,, of Amphibians 70

c. ,, of Reptiles 74

D. ,, of Birds 78

E. ,, of Mammals ^ 80

Bibliography 84

v. LIMBS 84

a. Unpaired Limbs 85

b. Paired Limbs 86

Pectoral Arch 87

of Fishes 87

of Amphibians and Reptiles 87

of Birds 90

of Mammals 91

Pelvic Arch 92

of Fishes 92

of Amphibians 92

of Reptiles and Birds 94

Mammals 98

Free Limbs 99

of Fishes and Dipnoans 99

General Considerations on the Limbs of the higher Vertebrata . 101

Free Limbs of Amphibians 104

of Reptiles 105

of Birds 106

of Mammals ! 108

Bibliography Ill

C. MUSCULAR SYSTEM ... n2

DERMAL MUSCULATURE .... 113

MUSCULATURE OF THE SKELETON . . 113

MUSCLES OF THE TJIUNK H3

of Fishes and Amphibians 1 1 3

of Reptiles .... 116

of Birds 117

of Mammals 117

CONTENTS. xi

PAGE

MUSCLES OF THE VISCERAL SKELETON AND HEAD . 118

of Fishes 118

of Amphibia 119

of Amriiota 120

MUSCLES OF THE APPENDAGES 121

DIAPHRAGM 122

Bibliography 122

D. ELECTRIC ORGANS 124

Bibliography 128

E. NERVOUS SYSTEM 129

I. THE CENTRAL NERVOUS SYSTEM 129

1. THE SPINAL CORD 129

2. THE BRAIN (general description and development) 131

MEMBRANES OF THE BRAIN AND SPINAL CORD 135

The Brain of Cyclostomi 136

of Elasmobranchii 137

of Teleostei 139

of Ganoidei, Dipnoi, and Amphibia 142

of Reptiles 144

of Birds . . . . . - 147

of Mammals 148

II. PERIPHERAL NERVOUS SYSTEM 151

SPINAL NERVES 152

CRANIAL NERVES 153

SYMPATHETIC • . . 160

SUPRARENAL BODIES 161

Bibliography 162

III. SENSORY ORGANS (general description) 162

SENSE-ORGANS OF THE INTEGUMENT 163

Rod-shaped Organs of Fishes, Dipnoi, and Amphibia, — Segmental

Sense-Organs 163

End-Bulbs 166

Terminal Ganglion-Cells 167

Bibliography 169

OLFACTORY ORGAN (general description and development) 170

of Fishes 171

of Dipnoi and Amphibia 172

of Reptiles 174

of Birds . . 175

of Mammals 176

JACOBSON'S ORGAN 177

THE SPOUTING APPARATUS OF GYMNOPHIONA 179

Bibliography 180

xii CONTENTS.

PAGE

EYE (general description and development) 181

of Fishes 184

of Amphibians 186

of Reptiles and Birds 186

of Mammals 188

Retina 189

Accessory Organs in Connection with the Eye 191

a. Eye-Muscles 191

b. Eyelids 191

c. Glands 192

Bibliography 194

AUDITORY ORGAN (general description and development) 194

of Fishes 198

of Amphibians 198

of Reptiles and Birds 199

of Mammals 201

Histology of the Mammalian Cochlea 206

Relation of the Auditory Organ to the Air-Bladder in Fishes . . 207

Bibliography 207

F, ORGANS OF NUTRITION 208

ALIMENTARY CANAL AND ITS APPENDAGES (general description) 208

I. MOUTH 212

Teeth (general description) 212

of Fishes and Amphibians 214

of Reptiles . ... 215

of Mammals , 216

Glands of the Mouth 220

of Amphibians 221

of Reptiles 221

of Birds '. 222

of Mammals - 222

Tongue -222

Thyroid Gland .225

Thymus Gland 226

II. (ESOPHAGUS, STOMACH, AND INTESTINE .

of Fishes and Amphibians ....

of Reptiles -231

of Birds

of Mammals -234

Histology of the Mucous Membrane of the Alimentary Canal . . 237

APPENDAGES OF THE ALIMENTARY CANAL ... • 240

Liver -240

Pancreas -243

Bibliography

CONTENTS. xiii

I'ACiE

G. ORGANS OF RESPIRATION .... 245

I. GILLS 245

of Amphioxus 246

of Cyclostomi . . - 246

of Fishes 248

of Dipnoi 250

of Amphibia 250

II. AIR-BLADDER AND LUNGS 251

1. THE AIR-BLADDER 251

2. THE LUNGS 252

Air-Passages 253

of Amphibians 253

of Reptiles 254

of Birds 255

of Mammals 255

The Lungs in a more Restricted Sense 257

The Lungs of Dipnoi 257

of Amphibia 257

of Reptiles 258

The Lungs and Air-Sacs of Birds 259

The Lungs of Mammals 263

ABDOMINAL PORES 265

Bibliography 267

H. ORGANS OF CIRCULATION 268

(VASCULAR SYSTEM) 268

Development of the Heart and Blood- Vessels 268

The Foetal Circulation 270

The Heart and its Vessels . ' 277

of Fishes 277

of Dipnoi 280

of Amphibia 281

of Reptiles . 284

of Birds and Mammals 286

Arterial System 288

Venous System 291

Retia Mirabilia 292

Lymphatic System 292

Bibliography 294

I. TTRINOGENITAL ORGANS 296

Development 296

URINARY ORGANS . 302

of Fishes and Dipnoans 302

of Amphibians 304

of Reptiles and Birds 307

of Mammals .... 308

xiv CONTENTS.

PAGE

GENERATIVE ORGANS 310

of Amphioxus 310

of Fishes 310

of Amphibians 314

of Reptiles and Birds 317

of Mammals 320

COPULATORY ORGANS 327

Bibliography 330

INDEX . 335

LIST OF WOODCUTS.

FIG. PACK

1. Diagram of the UD impregnated Ovum 3

2. Diagrams of the Segmentation of the Ovum 4

3. Diagram of a Segmented Meroblastic Ovum 5

4. Blastosphere - .... 5

5. Gastrula 6

6. Early Stage in the Differentiation of the Embryo 7

7. A and B. Diagrammatic Transverse Sections through a Developing Verte-

brate Embryo 8

8. Diagrammatic Transverse Section through the Body of an Adult Vertebrate 9

9. A, B, and C. Diagrams illustrating the Formation of the Amnion, Allantois,

and Yolk-Sac. A and B, in Longitudinal Section ; C, in Transverse

Section 11

10. Diagrammatic Transverse Section illustrating the Structure of the Skin in

Fishes 17

HA. Skin of Larva of Salamander (Salamandra maculosa) 19

HB. Section through the Skin of Adult Salamander (S. maculosa) 19

12. Six Stages in the Development of the Feather. (Mainly after Th. Studer.) 21

13. Six Stages in the Development of Hair ' . . . . 23

14. Longitudinal Section through a Hair. (Diagrammatic.) 25

15. Section through the Human Skin 26

16. A, True (Secondary) Teat ; and B, Pseudo- (Primary) Teat 27

17. a, Dermal Armature of Hypostoma commune (a Siluroid) ; b, Denticles from

the Skin of the Abdomen of Callichthys ; c, Plates from the Tail-fin of

Hypostoma. (After 0. Hertwig.) 30

18. Dermal Denticles of Protopterus 31

19. Dermal Armature of Callichthys 31

20. Transverse Section of the Vertebral Column of Ammoccetes 34

21. Portion of the Vertebral Column of Spatularia. (Side view.) 35

22. Transverse Section of the Vertebral Column of Acipenser ruthenus (in the

anterior part of the body) 35

23. Portion of the Vertebral Column of Protopterus. (Side view.) 35

24. Portion of the Vertebral Column of Polypttru-* 36

xvi LIST OF WOODCUTS.

FIG- I'AUK

25. Diagram showing the Inter vertebral Remains of the Notochord 36

26. Portion of the Vertebral Column of Lepidostnis. (After Balfour and

Parker.) 37

27. Portion of the Vertebral Column of a Young Dogfish (Scylliun canicula).

( After Cartier. ) 38

28. Portion of the Vertebral Column of Scymnus 38

29. Tail of Protopterus 39

30. Tail of Lepidosteus 39

31. Longitudinal Section through the Vertebral Column of Various Urodeles.

A, Ranodon sibericus ; B, Amblystoma tigrinum ; C, Gyrinophilus por-

phyriticus ; D, Salamandrina perspicillata 40

32. Vertebral Column of Discoglosms pictus 41

33. Tail of Archceopteryx 44

34. Pelvis of Owl (Strix bubo). (Ventral view.) 45

35. Third Cervical Vertebra of Woodpecker (Picns viridis). (Viewed

anteriorly.) 46

36. Skeleton of the Trunk of a Falcon 49

37. Costal Arch of Man 50

38. Ventral Portion of the Pectoral Arch of Rana escidenta 52

39. Pectoral Arch and Sternum of a Gecko (Hemidactylus verrucosus) .... 52

40. A, Sternum of Fox ; B, of Walrus ; and C, of Man 53

41. Diagram showing the Primitive Metameric Condition of the Head .... 55

42. First Cartilaginous Rudiments of the Skull 57

43. Diagrammatic Transverse Sections, of the Head in Embryo — (A) Sturgeons,

Elasmobranchs, Anura, and Mammals ; (B) Urodeles and Snakes ; and

(C) Certain Teleosteans, Lizards, Crocodiles, Chelonians, and Birds . . 58

44. Second Stage in the Development of the Primordial Skull 58

45. Diagrammatic Transverse Section of the Third Stage in the Development

of the Primordial Skull 59

46. Diagram showing the Relations of the Embryonic Visceral Skeleton ... 60

47. Semidiagrammatic Figure of an Elasmobranch Skull, showing the Relations

of the Segmental Cranial Nerves 60

48. Semidiagrammatic Figures of the Suspensorial Apparatus in Various Verte-

brates. (Mainly after Gegenbaur.) A, Notidanus ; B, Other Elasmo-
branchs ; C, Torpedo ; D, Teleosteans ; E Amphibians, Reptiles, and

Birds ; F, Mammals 61

49. Skull and Branchial Basket of Petromyzon planen 64

50. Skull of Heptanchus 64

51. Cranial Skeleton of Raja oxyrhi/ncha 65

52. Cranial Skeleton of Sturgeon (Acipenser] after Removal of the Exoskeletal

Parts 66

53. Skull of Polypterus bichir from the Dorsal Side 67

54. Cranial Skeleton, Pectoral Arch, and Anterior Extremity of Protopterus . . 68

55. Cranial Skeleton of Trout 69

56. Skull of Young Axolotl. (Ventral view.) 71

57. Skull of Salamandra atra (Adult). (Dorsal view.) 71

LIST OF WOODCUTS. xvii

H(i. PAGE

58. Skull of Salamandra atra (Adult). (Ventral view. ) 71

59. Skull of Rana esculenta. (Ventral view.) (After Ecker.) 72

60. Hyobranchial Apparatus of Urodeles. A, Axolotl (Siredon pisciformis) ;

B, Salamandra maculata ; C, Triton cristatus ; D, Spelerpes fuseus . . 73

61. Skull of Lizard (Lacerta agilis). (Dorsal view.) 76

62. A and B. Skull of Snake (Tropidonotus natrix) 76

63. Skull of Young Water-Tortoise (Emys europced). (Side view.) 77

64. Skull of a Young Crocodile. (Ventral view. ) 77

65. Skull of a Wild Duck (Anas boschas}. (A, from above ; B, from below ;

C, from the side.) 79

66A. Longitudinal Vertical Sections through the Skulls of— A, Salamandra

maailosa, B, Testudo grceca, and C, Corvus corone, to show the Relations

between the Cranial and Visceral Portions 81

66B. Longitudinal Vertical Sections through the Skulls of— A, Deer, B, Baboon,
and C, Man, to show the Relations between the Cranial and Visceral

Portions 82

67. Skull of Embryo of Armadillo (Tatusia hybrida). (Modified from a drawing

by W.K.Parker.) . . . .' 83

68. Diagram showing (A) the Undifferentiated Condition of the Paired and

Unpaired Fins in the Embryo, and (B) the Manner in which the Perma-
nent Fins are formed from the Continuous Folds 85

69. Pectoral Arch and Fin of Heptanchus 88

70. Left Pectoral Arch and Fin of the Trout. (From the outer side.) .... 88

71. Diagram of the Ground-Type of Pectoral Arch met with in all Vertebrata,

from the Amphibia up to Mammalia 89

72. Semidiagrammatic Figure of the Pe«toral Arch and Sternum of Urodela . . 89

73. Pectoral Arch of a Chelonian. (Ventral view. ) 89

74. Pectoral Arch and Sternum of Bombinator igneus 90

75. Pectoral Arch of Ornilhorhynchus paradoxus 91

76. Pelvis of Protopterus. (From the ventral side.) 92

77. Pelvis of Salamander (Salamandra maculosa). (Ventral view.) ..... 93

78. Pelvic Arch of Frog (Rana esculenta}. (A, from below ; B, from the

side.) 94

79. Pelvis of Lacerta muralis. (Ventral view.) 94

80. Pelvis of a Young Alligator Indies. (A, ventral, and B, side view.) ... 95

81. Pelvis of Iguanodon bernissartensis. (After Dollo. ) 96

82. Pelvis of Apteryx australis. (Lateral view.) (After Marsh.) 96

83A. Pelvis of a Six-Days' Chick. (After A. Johnson.) 97

'83B. Diagram showing the Relations of the Pelvic Bones to the Acetabulum . 97

84. Pelvis of Echidna. (From the left side. ) ( After Gegenbaur. ) 97

85. Pectoral Fin of Ceratodus fosteri 100

86. Diagram of the Predominant Uniserial Type of the Anterior Extremity of

Elasmobranchs 100

87. Diagrammatic Figures to show the Relations of the Free Extremity to the

Trunk in Fishes (A), and the Higher Vertebrates (B) 102

88. Posterior Extremity of Ranodon sibericus 103

• I

xviii LIST OF WOODCUTS.

FIG. PAGE

89. Eight Fore-Arm, Carpus, and Hand of Salamandra maculosa. (From

above.) 103

90. Right Tarsus of Discoylossus pictus. (From above.) 104

91. Eight Carpus of a Young A lligator lucius. (From above. ) 105

92. Anterior Extremity of A rchceoptcryx. (After C. Vogt.) 106

93. Anterior Extremity of Blackbird ( Turdus merula) 107

94. Posterior Extremity of Blackbird (Turdus merula) 108

95. Fore-Foot of Ancestral Forms of the Horse. 1. Orohippus (Eocene).

2. Mesohippus (Upper Eocene). 3. Miohippus (Miocene). 4. Protohippus

(Upper Pliocene). 5. Pliohippus (Uppermost Pliocene). 6. Eqims . . 110

96. Skeleton of the Left Fore-Limb of A, Pig ; B, Hyomoschus ; C, Tragulus ;

D, Roebuck ; E, Sheep ;- F, Camel. (From Bell, after Garrod.) .... 110

97. Lateral Muscles of A mphioxus 114

98. The Entire Musculature of Siredon pisciformis 114

99. The Musculature of Siredon pisciformis. (From the ventral side.) ... 115

100. Torpedo marmorata, with the Electric Organ (E) exposed 124

101. A and B. The Electric Organ of Grymnotus electricus. (B, from a prepara-

tion by A. Ecker.) 125

102. Electric Prisms of Torpedo marmorafa. (Semidiagrammatic. ) 126

103. Section through the Electric Chambers. (Greatly enlarged and semidia-

grammatic.) 127

104. The Entire Nervous System of the Frog. (After A. Ecker.) (From the

ventral side. ) 130

105. Diagram of the Embryonic Condition of the Central Nervous System . . 132

106. Longitudinal Section through the Skull and Brain of an (ideal) Vertebrate

Embryo. (In part after Huxley.) 133

107. Diagram of the Ventricles of the Vertebrate Brain 134

108. Cerebral Flexure of a Mammal 135

109. Brain-Membranes of Man. (After Schwalbe.) 136

110. Brain of Ammoccttes. (Dorsal view.) 137

111. Brain of Galeus canis, in situ. (Dorsal view.) (After Roh on.) 138

112. Brain of Myliobatis aquila, in situ. (From the ventral side.) (After

Rohon.) 139

113A. Longitudinal Vertical Section through the Anterior Part of the Teleostean

Brain. (Founded on a figure of the Trout's brain by Rabl-Riickhard.) . 140

113B. Transverse Section through the Fore- Part of the Teleostean Brain . . . 141

114. Brain of Perch (Perca schraetser). (Side view.) 141

115. Brain of Perca schraetser. (Dorsal view. ) 141

116. Brain of Polypterus bichir. (Side view.) , . 142

117. Brain of Salamandra maculosa. (A, dorsal, B, ventral view) 143

118. Brain of liana escidenta. (From the dorsal side.) 143

119. Brain of Blindworm (Anguis fragilis). (A, from the dorsal, B, from the

ventral side.) 144

120. Brain of Emys europxa. (A, side, B, ventral view.) 145

121. Brain of Alligator. (From the dorsal side.) (After Rabl-Riickhard.) . . 146

122. Brain of Pigeon. (A, from above ; B, from the side.) 147

LIST OF WOODCUTS. xix

FIG. PAGE

123. Human Braiu. (Median longitudinal vertical section.) (Mainly after

Reichert.) 148

124. Convolutions of the Human Brain. (After A. Ecker.) 149

125. Diagrammatic Figure of the Principal Bands of Nerve-Fibres of the Mam-

malian Brain. (From a drawing by A. Ecker.) 149

126. Casts of the Brain-Cases of Eocene Mammals. (After Marsh.) 150

127. Cranial Nerves and Brachial Plexus of Salamandra atra 153

128. Chiasma of the Optic Nerves. (Semidiagrammatic.) A, chiasma as seen

in the greater number of Teleostei ; B, in Herring ; C, in Lacerta agilis ;

D, in an Agama ; E, in a higher Mammal 156

129. Cranial Nerves of Anguisfragilis *. . 157

130. Cranial Nerves and Brachial Plexus of Scyllium canicula 159

131. A, peripheral nerve-ending, as seen in all the higher sensory nerves ; B,

rod-shaped end-cell of a sensory organ of the integument of a Fish or
Amphibian, or a taste-cell ; C, free, and D, ganglionated nerve-ending

of the integumentary sensory organs of terrestrial Vertebrates 163

132. Transverse Section of a Freely-Projecting Segmental Sense-Orgau .... 164

133. Distribution of the Lateral Sense-Organs in a Salamander Larva .... 164

134. Diagram showing the Distribution of the Sensory Organs of the Lateral

Line in Fishes 165

135. Organ of the Lateral Line of a Urodele. (Semidiagrammatic.) 165

136. A Tactile Spot from the Skin of the Frog. (Modified from Merkel.) ... 167

137. Tactile Corpuscle from the Tongue of a Bird 167

138. A Tactile Corpuscle (End-Bulb) from the Conjunctiva of a Mammal ... 168

139. A Pacinian Corpuscle from the Beak of the Duck. (After J. Carriere.) . 168

140. Epithelium of the Olfactory Mucous Membrane. A, of Pctromyzon planeri ;

B, of Salamandra atra 170

141. Anterior Portion of Head of Acipenser sturio 171

142. Anterior Portion of the Head of Polypterus 172

143. Olfactory Organ of Menobranchus lateralis. (From the dorsal side.) . . . 173

144. Transverse Section through the Olfactory Cavities of Plethodon glutinosus

(Myctodera) , 173

145. Diagram of the Olfactory Organ of a Lizard. (Longitudinal vertical sec-

tion.) 174

146. Transverse Section through the Right Nasal Cavity of a Shrike (Lanius

'minor) 175

147. Transverse Vertical Section through the Nasal Cavity of Man 177

148. Dissection of the Head of Epicrium glutinosum. (Dorsal view.) .... 178

149. The Left So-called "Tentacle" of Ccecilia oxyura. (Opened from the

dorsal side.) 179

150. Diagrams showing the Mode of Formation of the Eye in Invertebrates

(A) and Vertebrates (B) • .* 181

151 A. Diagram showing the Mode of Formation of the Primary Optic Vesicles

(ABl] 182

15lB. Semidiagrammatic Figure of the Secondary Optic Vesicle, and of the

Lens becoming separated off from the Epiblast 182

xx LIST OF WOODCUTS.

FIG. PAGE

152. Diagram of a Horizontal Section through the Right Human Eye. (Seen

from above.) 183

153. Eye of a Teleostean 185

154. Eye of Lacerta muralis, showing the Ring of Bony Sclerotic Plates . . . 187

155. Eye of an Owl 187

156. Retina. (After Merkel. ) .' 189

157. Harderian Gland (H, Hl) and Lacrymal Gland (Th) of Anguis frag His . . 193

158. Diagrammatic Transverse Vertical Section through the Eye of a Mammal . 193

159. Diagram of the Lacrymal Apparatus of Man 194

160. Head and Anterior Portion of Body of a Chick, (In part after Molden-

hauer.) 195

161. Semidiagrammatic Figure of the Auditory Organ of a Teleostean. (Modi-

fied from a figure of that of Murcena anguilla by Hasse.) 196

162. Longitudinal Section of an Ampulla of Gobius. (After Hensen. ) . . . 197

163. Membranous Labyrinth of Lacerta. (From the outer side.) (After C. Hasse.) 200

164. Membranous Labyrinth of the Crocodile. (From the outer side.) (After

C. Hasse.) 200

165. Membranous Labyrinth of the Pigeon. (After C. Hasse.) 201

166. Membranous Labyrinth of the Ox. (After C. Hasse.) 201

167. Diagram of the Entire Auditory Organ of Man 203

168. Bony Cochlea of Man. (After A. Ecker.) 204

169. Diagrammatic Transverse Section of the Cochlea of a Mammal 204

170. The Organ of Corti. (After Lavdowsky.) 206

171. Diagrams of the Oral Cavities of a Fish (A), Amphibian (B), Reptile or

Bird (C), and Man (D) 210

172. Diagram of the Entire Alimentary Tract of Man 211

173. Semidiagrammatic Figure of a Longitudinal Section through a Tooth . . 213

174. Skull of Batrachoseps attenuates. (From the ventral side, showing the

teeth on the parasphenoid. ) 214

175. A, Diagrams of Transverse Sections through the Jaws of Reptiles, showing

Pleurodont (a), Acrodont (6), and Thecodont (c) Dentitions. B, a, Lower

3a,wofZootocavivipara;b,ofAnguisfragilis. (After Leydig.) . . . 215

176. Figures of the Poison-Fangs of a Viperine Snake 216

177. Dentition of the Hedgehog (Erinaceus europceus). (The teeth of both

jaws from the side, and those of the upper jaw from below.) , 217

178. Dentition of the Dog (Canis familiar is) 218

179. Dentition of the Porcupine (Hystrix hirsutirostris) . 218

180. Dentition of Sheep (Ovis aries) 219

181. Dentition of a Catarrhine Monkey (Nasalis larvatus) 219

182. The Poison- Apparatus of the Rattlesnake 221

183. Head of Spelerpes fuscus, with the Tongue extended 223

184. A, Tongue, Hyoid Apparatus, and Bronchi of a Gecko (Phyllodadylus

europceus) ; B, Tongue of Lacerta ; C, of Monitor indicus ; D, of Emys

europcea 224

185. Thymus and Thyroid of a Young Stork 227

186. Intestinal Tract of a Shark . . 229

LIST OF WOODCUTS. xxi

FIG. PAGE

187. Alimentary Viscera and Air- Bladder of Lepidosteus, in situ. (After Balfour

and Parker.) 230

188. Intestinal Tract of Perch 231

189. Intestinal Tract of Siren lacertina 232

190. Intestinal Tract of Rana esculenta , 232

191. Diagram of the (Esophagus and Stomach of a Bird 233

192. Different Forms of Mammalian Stomachs 235

193. Diagram of the Structure of a Ccelenterate 237

194. Semidiagrarnmatic Transverse Section of a Portion of the Wall of the

Intestine. (Combined from the condition seen in both Lower and Higher
Vertebrates.) 238

195. Semidiagrarnmatic Figures of the Mucous Membrane of the Intestine of

Fishes, showing Intermediate Forms between Longitudinal Folds and

Bound Crypts 240

196. Liver of Rana esculenta. (From the ventral side. ) 241

197. Pancreas and Liver of Frog, to show the Arrangement of their Ducts . . 241

198. Viscera of Lacerta agilis, in situ 242

199. Amphioxus lanceolatus, x 2J. (From Gegenbaur, after Quatrefages.) . . 247

200. Diagram of a Longitudinal Section through the Head of Ammoccetes (A)

and Petromyzon (B) 248

201. Longitudinal Section through the Head of Ammoccetes 248

202. Horizontal Section through the Ventral Side of the Head of a Selachian.

(Semidiagrammatic.) The floor of the mouth is exposed 249

203. Horizontal Section through the Ventral Side of- the Head ofSilurus glanis.

(Semidiagrammatic.) 249

204. A, B, C, Diagrams showing the Mode of Development of the Lungs . . . 252

205. Diagram Illustrating the Phylogenetic Development of the Lungs .... 253

206. Cartilaginous Skeleton of the Laryngo-Tracheal Chamber of Rana escu-

lenta. (A, from above ; B, from the side.) 254

207. Larynx of Phyllodactylus europccus. (A, skeleton, and B, musculature of

larynx.) 254

208. Larynges of Various Mammals 256

209. Abdominal Viscera and Air-Sacs of a Duck after the Eemoval of the

Ventral Body -Wall. (From an original drawing by H. Strasser.) . . . 260

210. Left Lung of the Duck, in situ. (From an original drawing by H.

Strasser.) 261

211. Diagram of the Arrangement of the Bronchi in Mammals. (From the

ventral side.) 264

212. Diagram of the Pleural and Pericardial Cavities of Mammals, founded on

the relations of these parts in Man. (A, horizontal section ; B, transverse
section.) 265

213. Abdominal Pores of Various Vertebrates. (A, Cyclostome ; B, Elasmo-

branch ; C, Protopterus ; D, Spatularia.) 266

214. Diagram showing the Primitive Relations of the Different Chambers of

the Heart 269

215. Diagram of the Embryonic Vascular System 271

xxii LIST OF WOODCUTS.

KIG- . PAGE

216. Diagram of the Circulation of the Yolk-Sac at the eiid of the Third Day

of Incubation in the Chick. (After Balfour. ) . . .• 272

217. Diagram of t;he Venous Circulation in the Chick at the Commencement of

the Fifth Day. (After Balfour.) 273

218. Diagram of the Venous Circulation in the Chick during the Later Days of

Incubation. (After Balfour.) 274

219. A, B, C. Diagrams of the Development of the Paired Venous System of

Mammals (Man). (From Gegenbaur.) 275

219. D. Diagram of the Chief Venous Trunks of Man. (From Gegenbaur.) . . 275

220. Diagrammatic Section through the Human Gravid Uterus 276

221. Diagram showing the Transformations of the Aortic Arches — A, in a

Lizard ; B, in a Snake ; C, in a Bird ; and D, in a Mammal. (After
Rathke.) (Seen from below. ) 277

222. Hearts of Various Fishes —A, of the Hammer-headed Shark (Zygoma

malleus}; B, of Silurus glanis ; C, of a Selachian, cut open 278

223. Diagram of the Arterial System of Fishes 279

224. Diagram of the Heart and Branchial Vessels of Ceratodus. (Mainly after

J. E. V. Boas.) 280

225. Diagram of the Branchial Circulation of Protopterus 281

226. Diagram showing the Course of the Blood through the Heart in Urodela

(A) and Anura (B) 282

227. The Arterial Arches of the Larva of a Salamander. (Slightly diagram-

matic.) (After J. E. V. Boas.) 283

228. Arterial Arches of an Adult Salamandra maculosa, shown spread out.

(After J. E. V. Boas.) • 284

229. A, Heart of Lacerta muralis, and B, of a large Varanus, shown cut open ;

C, Diagram of the Eeptilian Heart 285

230A. Heart of the Swan, with the Right Ventricle cut open 286

230s. Transverse Section through the Eight (Vd) and Left (Vg) Ventricle of

Grus cincrea 286

231. Five Different Modes of Origin of the Great Vessels from the Arch of the

Aorta in Mammals 287

232. The Arterial System of Salamandra maculosa 289

233. The Arterial System of Emys europcea , 290

234A. Diagram of the (Secondary) Connection of the Mesonephric Tubules with

the Segmental Duct (Sg) 297

234s. Horizontal Section through an Embryo of Lacerta agilis. (After M.

Braun.) 297

234c. The Entire Excretory System of the Embryo of Eylodes martinicensis

(3 millimetres long). (After E. Selenka.) . 298

235. Diagrammatic Transverse Sections of the Body of a Lower Vertebrate, to

show the Relations of the Segmental Organs. (After Hensen.) The
right side of the figure represents a later stage than the left 299

236. Diagram Exhibiting the Relations of the Female (A) and of the Male (C)

Reproductive Organs to the General Plan (B) of these Organs in the
Higher Vertebrata 301

LIST OF WOODCUTS. xxiii

PAGE

237. Diagram of the Primitive Condition of the Kidney in an Elasmobranch

Embryo. (After Balfour.) 303

238 A. Diagram of the Arrangement of the Urinogenital Organs in an Adult

Female Elasmobranch. (After Balfour.) 303

238B. Diagram of the Arrangement of the Urinogenital Organs in an Adult

Male Elasmobranch. (After Balfour.) 303

239. The Entire Viscera of Siplwnops annulattis ( 9 ), in situ 305

240. Diagram of the Urinogenital System of a Male (A) and Female (B) Uro-

dele ; founded on a preparation of Triton tceniatus. (After J. "W.
Spengel.) 306

241. Excretory Apparatus of Monitor indicus 308

242. Male Urinogenital Apparatus of Heron (Ardea cincrea) 309

243. Diagrammatic Longitudinal Section through the Kidney of a Mammal . . 310

244. Male Urinogenital Apparatus -of the Sturgeon 312

245. Female Urinogenital Apparatus of Protopterus annectens. (From the

ventral side, natural size. ) (After H. Ay ers.) 313

246. Diagram of a Portion of the Male Generative Apparatus of the Gymno-

phiona 315

247. Testis and Anterior End of Kidney of Hana esculenta. (Semidiagram-

inatic.) 315

248. Urinogenital Organs of a Female Rana esculenta = 317

249. Female Urinogenital Apparatus of Lacerta muralis 318

250. Male Urinogenital Organs of Anguisfragilis. (After F. Ley dig. ) . . . 319

251. A, Male Urinogenital Organs of Orntihorhynch'ics paradoxus ; B, Female

Urinogenital Organs of Echidna hystrix 321

252. Female Generative Apparatus of Certain Marsupials. A, Didelphys

dorsigera (juv.) ; B, Phalangista vulpina ; C, Phascolomys wombat.
(After A. Brass.) 323

253. Various Forms of Uteri. A, B, C, D, diagrams showing the different

stages in the fusion of the Mullerian ducts : A, uterus bicornis ; B, uterus
eimplex ; C, uterus duplex ; D, uterus bipartitus. E, female Urinogenital
apparatus of Hustelina, with embryos in the uterus ; F, ditto of
Hedgehog (Erinaceus) 324

254. Section through a Portion of the Ovary of a Mammal, showing the Mode

of Development of the Graafian Follicles 325

255. Diagrammatic Section of the Testis of a Mammal 325

256. Male Urinogenital Apparatus of the Hedgehog (Erinaceus) 326

257. Semidiagrammatic Figure of the Human Penis. (A, transverse section ;

B, side view ; C, from below. ) 329

LIST OF GENERAL WORKS ON COMPARATIVE
ANATOMY AND EMBRYOLOGY.

BALFOUR, F. M. — A Treatise on Comparative Embryology. 2 vols. London, 1881-
1882. A Monograph, on the Development of Elasmobranch Fishes. London, 1878.

BELL, F. JEFFKEY. — Comparative Anatomy and Physiology. London, 1885.

BRONN, H. G. — Die Classen und Ordnungen des Thierrciches. Leipzig and Heidel-
berg, 1873 and onwards. (The volumes on Amphibia, Eeptilia, and Aves
have appeared up to the present time. )

CHAUVEATJ, A.—L'Anatomie comparee des Animaux domestiques. Paris, 1883.
Earlier Edition translated by Fleming. London, 1873.

CUVIER, G. — Lecons d' Anatomic comparee. 5 vols. Paris, 1799-1805. 2nd Ed.
1835-1845.

ECKEK, A. — Icones Physiologicce. Leipzig, 1852-1859.

Encyclopedia Britannica. Edinburgh, 1875 and onwards. (20 vols. have appeared
up to the present time.) (See articles on Amphibia, Anatomy, Ape, Biology,
Birds, Embryology, Evolution, Histology, Ichthyology, Mammalia, Physiology,
Reproduction, Reptiles, &c.)

FOL, H. — Lehrbuch der vergleichenden microscopischen Anatomie mit Einschluss der
vergleichenden Histologie und Histogenie. Leipzig, 1884. Now appearing in
parts.

FOSTER, M., and BALFOUR, F. M. — The Elements of Embryology. 2nd Ed. edited
by Sedgwick and Heape. London, 1883.

GEGENBAUR, C. — Elements of Comparative Anatomy (translated by F. Jeffrey Bell).
London, 1878.

GOTTE, A. — Entwickelungsgeschichte der Urike. Leipzig, 1875.

HAECKEL, E. — Generelle Morphologic der Organismen. 2 Bde. Berlin, 1866.

HERTWIG, 0. — Die Coelomtheorie. Jena, 1881.

HOWES, G. B. — Atlas of Practical Elementary Biology. London, 1885. (Contains
numerous figures of the anatomy, histology, and development of the Frog.)

HUXLEY, T. H. — Anatomy of Vertebrated Animals. London, 1871. Reprinted

1882.

KOLLIKER, A. — Grundriss der Entwickelungsgeschichte des Menschen und der hoheren
Thiere. Leipzig, 1880.

LEYDIG, F. — Lehrbuch der Histologie des Menschen und der Thiere. Frankfort,

1857.

MAC A LISTER, A. — Introduction to Animal Morphology. 2nd Vol. (Vertebrates.)
London, 1878.

MARTIN, H. K, and MOALE, W. A.— Handbook of Vertebrate Dissection. (Three parts
have appeared up to the present time : these contain descriptions of the Turtle,
Pigeon, and Rat.) Macmillan & Co., 1884.

xxvi LIST OF GENERAL WORKS.

MECKEL, J. F. — System der vergl. Anatomic. 6 Bde. Halle, 1821-1833.

MILNE-EDWARDS, H. — Lcgons sur la Physiologic et I' Anatomic compared, de VHomme
ct des Animaux. 20 Tom. Paris, 1857-1880.

MONRO, A. — The Structure and Physiology of Fishes. Edinburgh, 1785.
MULLER, E. — Vergl. Anatomic der Myxinoiden. Berlin, 1834-1845.
OWEN, E. — Anatomy of Vertebrates. London, 1866-1868.

PARKER, T. J. — A Course of Instruction in Zootomy (Vertebrates). London, 1834.
KOLLESTON, G. — Forms of Animal Life. Oxford, 1870.
SCHMIDT, 0. — Handbuch der vergl. Anatomic, 8 Aufl. Jena, 1882.
STANNIUS, H. — Handbuch der Anatomic (Zootomie] der Wirbelthiere. Berlin, 1854.
(Contains detailed descriptions of Fishes, Amphibians, and Reptiles.)

VOGT, C., and JUNG, E. — Lehrbuch der pract. vergl. Anatomic. Braunschweig,
1885. Traite d' Anatomic comparee pratique. Paris, 1884 and onwards.
(Only parts relating to Invertebrates have yet appeared.)

WEBER, MAX. — Studien uber Saiigethiere. Ein Beitrag zur Frage nach dem Ursprung
der Cetaceen. Jena, 1886.

WIEDERSHEIM, E. — Lehrbuch der vergl. Anatomic, auf Grundlage der EntwicTcelungs •
geschichte. 2nd Ed. Jena,

WILDER, BURT G., and GAGE, S. H. — Anatomical Technology. New York and
Chicago, 1882.

ELEMENTS

OF THE

COMPARATIVE ANATOMY

VERTEBRATES.

COMPAEATIVE ANATOMY.

INTRODUCTION.

I. ON THE NATURE AND MEANING OF COMPARATIVE
ANATOMY.

A THOROUGH knowledge of the animal body cannot be gained
by Comparative Anatomy alone, and it is therefore necessary to
call in the aid of other branches of science also, viz. : —

1. Ontogeny; 2. Palaeontology; 3. Histology; and 4.
Physiology.

The first of these treats of the developmental history of the
individual, while the second has to do with the development of
the races of animals in time (Phylogeny). As the different
phases of development of the race are usually repeated to a
greater or less extent in those of the individual, these two subjects
help to complete one another. Thus the object of both alike is
to enable us to ascertain the past by observing the present.

The third-mentioned branch. Histology, teaches us about the
structural elements — the building-stones of the organism. It
shows how these elements are combined to form tissues, and how
organs are constructed out of the latter. The organs again
combine to form systems of organs.

The structural elements consist primarily of cells and second-
arily of cells and fibres, and the different tissues may be divided
into four principal groups : —

1. Epithelium, and its derivative, glandular tissue.

2. Supporting-tissue (connective-tissue, cartilage, bone).

3. Muscular tissue.

4. Nervous tissue.

In accordance with their physiological character, epithelium
and supporting-tissue maybe described as passive, and muscular
and nervous tissue as active.
r

2 COMPARATIVE ANATOMY.

By an organ we understand an apparatus constructed to perform
a definite physiological function, as, for instance, — the liver, which
secretes bile, the gills and lungs, in which an exchange of gases
is effected with the surrounding medium, and the heart, which
pumps blood through the body.

The organ -systems, which will be treated of in order in this
book, ,aye: as: follows : — 1. The outer covering of the body, or
integument; 2. The skeleton; 3. The muscles, together with
electric organs; 4. The nervous system and sense-organs;
5. The organs of nutrition, respiration, circulation,
excretion, and reproduction.

The closely-allied branches of science defined above are united
together as Morphology, as opposed to Physiology, which
concerns the functions of organs, apart from their morphological
relations.

Morphology alone leads us to a satisfactory explanation of the
structural phenomena of the animal body, for it not only reveals to
us the law of heredity and the consequent relationship of
animals to one another, but it also helps to explain certain de-
graded and rudimentary forms, which, considered as isolated
adult animals, would always remain absolutely incomprehensible.
Further, it shows us on the one hand how the animal organism
is acted upon by the influence of its surroundings, and how it
is apt to change gradually and more or less continuously ; and on
the other hand how the capacity of adaptation resulting from
these changes varies inversely with the persistence of inherited
qualities. These two important opposing factors, adaptation
and heredity, constitute the formative principle of the
animal body.

II. DEVELOPMENT AND STRUCTURAL PLAN OF THE
VERTEBRATE BODY.

The structural elements described in the preceding section as
the building- stones of the organism, i.e. the cells, all arise from a
single primitive cell, the egg-cell or ovum. This forms the
starting-point for the entire animal-body, and a somewhat full
account of its structure and relations must therefore be given here.
The stages in its development will only be described in a very
general manner.

The unimpregnated ovum consists of a rounded vesicle (Fig. 1),
in the interior of which the following parts can be distinguished : —
The vitellus (D), the germinal vesicle (KB\ and one or more
germinal spots (KF}. Of these, the germinal vesicle is the most
important physiologically (comp. p. 3). The outer-covering of the
ovum is spoken of as the vitelline membrane.

INTRODUCTION.

Since the ovum in its primitive form as above described
represents a single cell, we may speak of the vitellus 1 as the
protoplasm of the egg-cell, the germinal vesicle as its nucleus,
and the germinal spot as its nucleolus. An outer limiting
membrane, corresponding to the vitelline membrane, is not an

.AT

FIG. 1. — DIAGRAM OF THE UNIMPBEGNATSD OVUM.
I), vitellus ; KB, germinal vesicle ; KFt germinal spot.

integral part of the cell, but may be developed as a hardening of
the peripheral protoplasm, consequent on a process of differen-
tiation.

In all Vertebrates, the contact of the generative products of the
male, the sperm-cells (spermatozoa) is an absolute necessity for
the development of the ova. A spermatozoon makes its way into
the interior of the ovum, and a portion of it finally unites in a
definite manner with the modified germinal vesicle to form a single
body — the first segmentation nucleus.

This modification of the germinal vesicle takes place as follows. Before
fertilisation occurs, two polar cells2 are constricted off from the ovum, part
of the germinal vesicle passing into each, and the remainder being spoken of
as the female pronucleus. The polar bodies are given off at different
times in different animals : they may be formed while the ovum is still within
the ovary, or, on the other hand, they may arise at the time of fertilisation.

1 The vitellus consists of two different substances — protoplasm and deutero-
plasm (yolk) — in varying proportions in different animals.

2 The two most important views as to the meaning of the polar cells are those
of (1) Balfourand van Beneden, and (2) Weismann. The first-named authors suppose
that the egg, being a product of both sexes, is primitively hermaphrodite. By the
extrusion of the polar bodies, the male portion of the egg is thrown out, and the
remainder thus becomes unisexual (female), and ready for the entrance of the
spermatozoon. This process would thus be a contrivance for the prevention of
parthenogenesis.

Weismann distinguishes in every animal body two kinds of cells, somatic and
generative cells. As all the cells arise as products of the segmentation of the
ovum, they are originally quite similar morphologically, and each would thus consist
of a "somatic" and of a "generative" portion. In order that certain of them
should give rise to definite generative cells, it is necessary that the somatic portion
should be got rid of, and this is effected by the extrusion of the polar bodies.

The first hypothesis -presupposes that in parthenogenesis no polar bodies are
formed. Weismann has lately, however, proved their existence in the partheno-
genetic summer eggs of Daphnidse, and this view is consequently rendered impro-
bable. In the development of the male generative cells, a certain portion of each
primitive seminal cell also remains passive, not giving rise to spermatozoa.

B 2

4 COMPARATIVE ANATOMY.

The head of the spermatozoon, on entering the ovum, is transformed into the
male pronucleus,1 which fuses with the female pronucleus to form the first
segmentation nucleus.

Impregnation then consists in a material fusion of the
generative products of both sexes, and hence in the new
individual we naturally find inherited qualities. The essential
cause of inheritance consists in the molecular structure of the
nuclei of both male and female germinal cells. This structure
(idioplasm) is the morphological expression of the characters of
the species and individual.

C D

FIG. 2. — DIAGRAMS OF THE SEGMENTATION OF THE OVUM.

A, first stage (two segments) : RK, polar bodies. B, second stage (four segments).
C, further stage. D, morula stage.

The next stages (Fig. 2) are as follows. The first segmentation
nucleus divides into two equal parts, each of which forms a new
centre for the division of the ovum into two halves. This division,
the beginning of the process of segmentation, takes place by
the formation of a farrow round the egg, which becomes deeper
and deeper, until the division is complete.

The first stage in the process of segmentation is thus completed :
the second takes place in exactly the same way, and results in a

1 If the egg is to be normally developed, not more than one spermatozoon must
enter. The latter may either pass through a definite opening (micropyle), or else
bore its way through the vitelline membrane.

INTRODUCTI

division of the ovum into four parts, and by a similar process are
formed eight, then sixteen, then thirty-two spheres, and so on, the
spheres becoming smaller and smaller, and each being provided
with a nucleus. In short, out of the original ovum, corresponding

SU

FIG. 3. — DIAGRAM OF A SEGMENTED MEROBLASTIG OVUM.
Bla, blastoderm ; Do, yolk.

to a single cell, a mass of cells is formed, which represents the
building-material of the animal body, and which, from its likeness
in appearance to a mulberry, is spoken of as a morula.1

FIG. 4. — BLASTOSPHERE.
BD, blastoderm ; Fff, segmentation cavity.

In the interior of this morula a cavity (segmentation cavity)
filled with fluid is formed, and the morula is now spoken of as the
blastosphere or blastula. The peripheral cells enclosing this
cavity form the germinal membrane or blastoderm (Fig. 4,

1 A segmentation of the entire ovum occurs in Amphioxus, Cyclostomes, Stur-
geon, Lepidosteus, Amphibians, and Mammals (with the exception of Monotremes). In
Elasmobranchs, Teleosteans, Reptiles, Birds, and Monotremes, in which a very large
amount of food-yolk (deutero plasm) is present in the protoplasm, the egg under-
goes only a partial segmentation, the main mass of the yolk remaining undivided
and serving merely as nutritive material for the developing embryo. The former
are spoken of as holoblastic (Figs. 1 and 2), the latter as meroblastic (Fig. 3).
The eggs of the Sturgeon, Lepidosteus, and in a less degree those of Cyclostomes and
Amphibians, though holoblastic, approach the meroblastic type.

6 COMPARATIVE ANATOMY.

BD).1 Consisting at first of a single layer of cells, the blastoderm
later on becomes two- and then three -layered. From the relative
positions of these, they are spoken of respectively as the outer,
middle, and inner germinal layers, or as epiblast, mesoblast,
and hypoblast.2

The question as to the origin of the germinal layers, on account of its im-
portant signification, is one of the most burning problems in Morphology, and
as yet we cannot arrive at any full and satisfactory conclusion on the subject.
It may, however, be affirmed with certainty that in all Vertebrates the blasto-
sphere passes — or did so in earlier times — into a stage called the Gastrula.3
One must imagine this form as being derived primitively from the blastula
by supposing that the wall of the latter (Fig. 4, BD) became pushed in, or

Wet-

FIG. 5. — GASTRULA.
Ekt, epiblast ; Ent, hypoblast ; £lp, blastopore ; U, archenteron.

invaginated, at one part, thus giving rise to a double-walled sac (Fig. 5). The
outer wall then represents the epiblast (Ekt\ which functions as an organ of
protection and sensation, while the inner, or hypoblast (Ent\ encloses a central
space, the primitive intestinal cavity (archenteron), and represents the
assimilating and digestive primary alimentary canal. The opening of the
latter to the exterior, where the two germinal layers are continuous, represents
the primitive mouth, and is called the blastopore (Fig. 5, Blp).

Out of the epiblast arise later the epidermis and its derivatives,
as well as the entire nervous system. The latter is formed as an

1 In meroblastic Vertebrate ova the blastoderm only extends part of the way
round the periphery of the ovum (Fig. 3).

2 The terms ectoderm, Iliesoderm, an(l endoderm are applied to the corre-
sponding layers in an adult animal.

3 The process of the formation of the gastrula may be actually observed at the
present day in the holoblastic ova of certain Fishes (see note, p. 5) and of Amphibia,
and the same process essentially occurs in the meroblastic ova of other Fishes, though
it is here more difficult to recognise. In the case of the Amniota the difficulty is
still greater, but although we have no direct proof of the existence of a gastrula
stage, the intimate connection of the developmental processes throughout the animal
kingdom renders it a priori certain that the gastrula is represented in them.

INTRODUCTION. 7

involution of the thickened dorsal region of the embryo (medul-
lary plate), which soon becomes constricted off from the epiblast
in the form of a hollow tube — the medullary cord or tube
(comp. Figs. 6 and 7), from which the brain and spinal cord are
formed. The hypoblast gives rise eventually to the epithelium of
the alimentary canal (Fig. 7, A and B, Enf) with its glands, as well
as to the epithelial parts of the lungs, thyroid and thymus glands,
liver, and pancreas.

Though we can look upon the epiblast and hypoblast, — that is, both the
primary germinal layers,1— as arising in the manner above described, the
problem as to the origin of the mesoblast is as yet by no means settled. All
that can be said at present is briefly as follows :— The mesoblast is a secondary

FIG. 6. — EARLY STAGE IN THE DIFFERENTIATION OF THE EMBRYO.

IW, blastoderm ; KS, germinal disk ; KA, body-walls ; R, medullary cord, light
and left of which are seen the mesoblastic somites ; G, brain.

formation, and phylogenetically younger than the other two germinal layers.
Reminding one in many points of the "mesenchyma" of Invertebrates, it
always arises at first from the point where epiblast and hypoblast pass into one
another, that is, from the region of the blastopore, or, what comes to the same
thing in the higher Vertebrates, from the primitive streak. Originating
between the other two layers, its first and most important function is the
formation of blood-corpuscles — first of white cells (leucocytes, lymph-cor-
puscles); later it gives rise to the heart, blood-vessels, supporting and
connective substances (connective- tissue, adipose tissue, cartilage, and bone),
serous membranes (peritoneum, pleura, pericardium, arachnoid), excretory and
reproductive apparatus, and muscles.

1 It must be observed that this important difference in the histological differentia-
tion of the individual germinal layers cannot be so definitely stated as regards the
whole animal kingdom : in certain types of Invertebrates it is not so strongly
marked.

COMPAEATIYE ANATOMY.

Fro. 7, A AND B. — DIAGRAMMATIC TRANSVERSE SECTIONS THROUGH A DEVELOPING
VERTEBRATE EMBRYO.

D, alimentary canal ; Ent, hypoblast, showing in Fig. A the thickening which will
form the notochord ; Chl (Fig. B), the note-chord now constricted off from the
hypoblast ; UW, mesoblastic somite ; UGf, primary urinary duct (segmental
duct) ; A, aorta ; SpP, splanchnic and SoP, somatic mesoblast ; Co, God, ccelome ;
Hj remains of the upper part of the ccelome in the interior of the mesoblastic
somites ; EJct, epiblast ; Med, medullary cord : — in Fig. A it is shown still con-
nected with the epiblast, from which it has become constricted off in Fig. B.

A large cleft appearing in the mesoblastic tissue divides it into
a parietal layer (Fig. 7, A and B, SoP), lying along the inner

INTRODUCTION. 9

side of the epiblast, and into a visceral layer (SpP), which
becomes attached to the hypoblast. The former, together with
the epiblast to which it is united, constitutes the somatopleure,
and the latter, together with the hypoblast, the splanchnopleure.
The cavity separating these is the body-cavity, or ccelome.1
The upper part of the mesoblast, or that which lies on either side
of the medullary cord and notochord (see p. 10), becomes separated
from the lower, and segmented to form a series of mesoblastic

KW

FIG. 8. —DIAGRAMMATIC TRANSVERSE SECTION THROUGH THE BODY OF AN ADULT

VERTEBRATE.

Med, spinal cord ; NR, neural tube ; KW, body- wall ; Co, derma ; Ep, epidermis ;
VR, visceral tube ; Ao, aorta ; Ms, mesentery ; Per, parietal layer of the peri-
toneum ; Per1, visceral layer of the petittfneum ; Msc, musculature of intes-
tine ; Subm, connective-tissue coat of the intestine ; Ep, epithelium of intestine ;
DH, lumen of intestine ; W, vertebral column.

somites, or protovertebrae (Fig. 7, B, UW, H), which lose
their cavities, and eventually give rise to the vertebral column
and longitudinal lateral muscles.

The Vertebrate body is formed on a bilaterally symmetrical
plan, and it may be described as consisting in the adult of two
tubes, a dorsal and a ventral (Fig. 8, NR, VR). The former, or
cerebro-spinal cavity, encloses the central nervous system (brain
and spinal cord), and may therefore be spoken of as the neural

1 The coelome may arise as a segmentally arranged series of outgrowths from the
archenteron, as, e.g., in Amphioxus (enterocceles), or it may be formed secondarily
by a splitting (delamination) of the mesoblastic tissue (schizoccele). The first of
these must be considered as the most primitive.

10 COMPARATIVE ANATOMY.

tube; the latter surpasses the former considerably in size, and,
as it encloses the viscera, it may be called the visceral tube.

A cellular, cartilage-like rod — the notochord (chorda dor-
sal is), arising primitively as an axial thickening of the hypoblast
(Fig. 7, A and B, Ch, Chl), forms the basis of the vertebral column,
that is, the segmented axial skeleton which characterises the
Vertebrate body.1 This segmentation of the axis, as well as of
other organs and systems of organs (musculature, ribs, roots of
spinal nerves, sympathetic cord, pro- and meson ephros), indicates
that the Vertebrata must have arisen from an invertebrate and
segmented ancestral form.

The anterior ends of the enlarged medullary cord and alimentary
tract enter into a close relation with the outer world, the former
giving rise to the brain and to certain parts of those sense-organs
with which the higher cerebral functions are connected, while from
the latter are developed the mechanisms for the taking in of
nutriment and for respiration.

The anterior section of the embryo, or head, passes behind
into the trunk, in the hinder part of which the anal and urinogenital
apertures are situated. These parts are classed together as the
body-axis, as distinguished from the limbs, or appendicular
organs, which arise from the trunk.

In Reptiles, Birds, and Mammals, a delicate investment, the
arnnion, is early formed round the embryo; it arises as a fold of
the somatopleure (Fig. 9, AF, A). A sac-like out-growth from
the hinder part of the primitive intestine (i.e. from the splanchno-
pleure) gives rise to the allantois (At) which becomes highly
vascular, and in Reptiles and Birds extends round the embryo
close under the egg-shell; it here serves as an embryonic respiratory
organ. In all Mammals, except Monotremes and Marsupials, the
allantois becomes attached to a definite region of the uterine wall,
and from it vascular processes or villi grow out into crypts of the
latter, which is also plentifully supplied with blood-vessels. Thus
a placenta is formed, in which interchanges can take place both
as regards nutritive materials and aeration between the blood of
the mother and that of the foetus.

Considerable differences are observable in the form of the placenta in dif-
ferent Mammals. The most primitive arrangement is most probably one in
which the allantois becomes attached along a discoidal region of the wall of
the uterus, and the various modifications seen in the different groups may be
looked upon as having arisen in order to increase the absorptive surface. This
may be effected either by the area of that part of the allantois which is covered
by placental villi becoming extended, or by the increase in complexity of the
villi and crypts. In the latter case, the interlocking between foetal and maternal
parts is so close that the mucous membrane of the uterus is torn away with
the foetal part of the placenta at birth, and the latter is then said to be
deciduate. In the former case, the discoidal placenta may -extend so as to

1 In the lowest Vertebrates, the segmentation of the body is indicated mainly by
somites.

INTRODUCTION.
1?

11

AF—/-

Al

FlG. 9, A, B, AND C. — DIAGRAMS ILLUSTRATING THE FORMATION OF THE AMNION,

ALLANTOIS, AND YOLK-SAC. A AND B, IN LONGITUDINAL SECTION ; C, IN
TRANSVERSE SECTION.

E, embryo ; Dh, alimentary tract ; Do, yolk-sac ; t, vitello-intestinal duct ; PP,
body-cavity; Ah, amniotic cavity; AF, ainniotic fold; A, amuiou ; Al,
allantois ; a, somatopleure ; b, splanclinopleure ; M, medullary cord ; C, notochord.

12 COMPARATIVE ANATOMY.

occupy a zonary area, or even so as to completely surround the foetus, when
it is spoken of as diffused. By the concentration of the villi of the diffused
placenta into definite patches, or cotyledons, a polycotyledonary form is
produced.

Rodents, Insectivores, and Bats possess a discoidal deciduate placenta,
and that of Sloths, Armadilloes, and Myrmecophaga approaches the same type.
In Carnivores, Elephants, Hyrax, and Orycteropus, it is deciduate and zonary.
In all other Mammals the placenta is non-deciduate, the maternal and foetal
parts simply separating from one another at birth. Amongst these the poly-
cotyledonary form is found in the Ruminants proper, and the diffused form
in Suidae, Hippopotamus, Perissodactyla, Tylopocla, Tragulidao, Manis,
Lemuridae, Sirenia, and Cetacea. The mode of development of the meta-
discoidal placenta of Primates shows that it has been derived from a diffused
placenta, the villi becoming restricted in the course of development to a
disk-shaped area, and their complexity increasing at the same time (Balfour).

In the course of development the embryo becomes more and more folded
off from the yolk-sac (umbilical vesicle) (Fig. 9, Do), the stalk of which latter,
and that of the allantois, enveloped by the base of the amnion, together form
the umbilical cord. At birth, the foetal membranes are shed, the intra-
abdominal portion of the allantois persisting as the urinary bladder and the
urachus (cp. the chapter on the vascular system).

Amongst Elasmobranchs, Mustelus Isevis and Carcharias possess a kind of
placenta formed by the yolk-sac, which becomes raised into folds fitting into
the vascular walls of the oviduct. Indications of such anumbilicalplacenta
are also seen in the early stages of Insectivora, Cheiroptera, and Rodentia.

Further investigations on the umbilical sac and allantois in Marsupials and
Moiiotremes are necessary. Indications of an umbilical placenta have been
observed in the former group.

INTRODUCTION.

13

SYSTEMATIC ZOOLOGY.

On the ground of their relationship to one another, animals
have been classified into certain divisions and subdivisions, which
are designated as Classes, Orders, Suborders, Families,
Genera, and Species.

A general classification of the principal existing Vertebrate
groups is given in the following table.

A. Acrania,

Amphioxus.

B. Craniata.

I. CYCLOSTOMATA (Suctorial Fishes).

1. Petrcmyzontidee.

2. Myxinoidse.

II. GNATHOSTOMATA (Animals provided with jaws),
(a.) ANAMNIA (without amnion).

1. PISCES (True Fishes). ^

a. Elasmobranchii (Sharks and Rays).

/3. Holocephali (Chimaera and Callorhynchus).

y. Ganoidei.

1. Selachoidei (Cartilaginous Ganoids — Aci-

penser, Polyodon, &c.).

2. Teleostoidei (Bony Ganoids — Poly pterus,

Lepidosteus, Amia, &c.).
<5. Teleostei.

1. Physostomi (with open pneumatic duct between

the air-bladder and pharynx, e.g. Cyprinus,
Salmo, Silurus, Mormyrus).

2. Physoclisti (air-bladder, when present, with

closed pneumatic duct, e.g. Perca, Gadus,
IchthyopsidaJ Lophius).

2. DIPNOI.

1. Monopneumones (Ceratodus).

2. Dipneumones (Protopterus).

3. AMPHIBIA.

a. Urodela.

1. Perennibranchiata (Proteus, Siren, Minobran-

chus).

2. Caducibranchiata.

Derotremata (Amphiuma, Menopoma).

Myctodera (Salamandra, Triton, Ambly-

stoma).

/3. An ura (Frogs and Toads).
y. Gymnophiona (Footless Coecilians).

14

COMPARATIVE ANATOMY.

(b.) AMNIOTA (Vertebrates wliicli develop an amnion during
foetal life).

1. KEPTILIA.

a. Crocodilia (Crocodiles and Alligators).
/3. Lacertilia (Lizards).
y. Chelonia (Turtles and Tortoises).
Sauropsida. \ 8. Ophidia (Snakes).

2. AVES.

a. Batitae (Cursorial Birds— Ostrich, Bhea, Emeu, &c.).
|8. Carinatse (Birds of flight).

3. MAMMALIA.

a. Prot'otheria or Ornithodelphia (Monotremata

— Ornithorhynchus and Echidna).
/3. Metatheria or Didelphia (Marsupialia— Kan-

garoos, Phalangers, Opossums, &c.).
y. Eutheria or Monodelphia (Placentalia).

Edentata. - s\otv

Sirenia.-

Cetacea. - -

Ungulata. ' r^

Hyracoidea. - ^

Proboscidea. ~ *

Rodentia. «-

Cheiroptera. -

Insect! vora. -

Carnivora. -
^- Lemuroidea. r-

Primates.

-

INTRODUCTION

SPECIAL PART.

A. INTEGUMENT.

THE skin consists of a superficial ectodermal and a deeper
mesodermal layer. The former is called the epidermis (scarf-
skin) and the latter the derma (corium, cut is). The subcu-
taneous connective tissue is usually not sharply marked off
from the derma, but either passes gradually into the other. The
epidermis always consists of cells only, while the derma is made
up principally of connective-tissue fibres, as well as those of an
elastic and contractile nature. Nerves, glands, pigment-cells
(chromatophores), bony-structures, and vessels occur principally
in the derma, the last two being found in this part of the
integument only.

Thedermaisalso permeated throughout by leucocytes (white blood-corpuscles),
and this is especially the case in Fishes. These leucocytes penetrate to the
superficial layer of the epidermis, and carry pigment granules.1 Here they
take on amoeboid movements, and break up into numerous small pigment-
containing particles, which are taken up by the epithelial cells.

From what has been said above, it is clear that the skin pre-
sents much variety both in form and function, and this cannot
be wondered at when one considers how accessible its outer surface
is to external modifying influences.

In the epidermis two layers may always be distinguished : — an outer,
composed of horny cells (stratum corneum, horny layer), and a deeper,
made up of soft protoplasmic cells (stratum Malpighii, mucous layer).
The latter serves as a matrix for the regeneration of the horny layer,
the superficial part of which is continually scaling off. From the epidermis
all the glands of the skin, and all other parts spoken of as epidermic
structures take their origin. Such are hairs, bristles, feathers, nails, claws,
hoofs, &c. The peripheral sensory end-organs of the skin are to be con-
sidered as arising by a differentiation of epidermic cells : the definite relation
which many of these organs have with the derma must be looked upon as a
secondary acquirement.

1 Pigment is never formed in the epithelial cells themselves, but always originates
in the derma.

INTEGUMENT.

17

Animals living in the water mostly possess a thinner horny layer which is
more capable of imbibition than that of land animals, which latter are usually
exposed to more mechanical clangers than the former. It may also be
mentioned that the connective-tissue bundles in the derma of Fishes,
Amphibians, and Reptiles show a typical arrangement in alternating
horizontal and vertical strands. Their disposition in Birds and Mammals is
irregular, that is, the fibres are more thoroughly felted.

Fishes. — In Amphioxus, the surface of the epidermis is
covered with cilia in the larval (gastrula) condition, and this

J?

FIG. 10. — DIAGRAMMATIC TRANSVERSE SECTION ILLUSTRATING THE STRUCTURE
OF THE SKIN IN FISHES.

Ep, epidermis ; Co, derma ; F, subcutaneous fat ; CS, cuticular margin ; B, B, slime-
cells (goblet-cells which open on the surface) ; Ko, goblet-cells ; A'o, granular
cells present in Petromyzou ; G, vessels which pass upwards in the vertical con-
nective-tissue bundles of the derma ; W, horizontal connective-tissue bundles.

must undoubtedly be considered as inherited from Invertebrate
ancestors. The striated cuticular border of the outer epidermic
layer in many fishes (e.g. Cyclostomes, Teleostei, and Dipnoi),
and, as will be mentioned presently, in Amphibian larvae, is perhaps
to be explained in the same way, the striation possibly corresponding
to coalesced cilia (Figs. 10 and 11 A, CS}.

InAmphioxus and the Cyclostomes long cylindrical cells provided with
-•stiff bristle-like processes appear amongst the ordinary epithelial cells. These,

18 COMPARATIVE ANATOMY.

as well as similar elements connected with complicated pieces of apparatus
will be treated of later in connection with the sense organs.

The meaning of the "granular cells" present in the integument of
Petromyzon, as well as of the so-called "club-" and "goblet-cells" in the
many-layered epidermis of osseous Fishes, is as yet by no means clear ; it
is, however, not improbable that the latter of these have to do with the pre-
paration of a secretion which protects the outer skin from the action of
the water.

The scales of Fishes lie in connective-tissue pouches of the
derma, and are formed as ossifications of the latter. In Teleostei
they are covered by the epidermis throughout life ; in Ganoids and
Eiasmobranchs this is only the case in the larva. (For further
details compare pp. 31 and 32.)

Pigment-cells (cp. p. 16), which are under the influence of
the nervous system and are able to cause a change of colour, are
present sometimes in both layers of the integument, sometimes in
the epidermis only. Muscles and glands, such as are found
in the skin of other Vertebrates, are not usually present
in Fishes.1

Phosphorescent organs are present in the integument of some Fishes.

Amphibia. — The structure of the integument in larval Am-
phibians somewhat resembles that of Fishes, while in adults it
more nearly approaches that seen in Reptiles.

The epidermis of those larva? which live in the water consists
of two sharply differentiated layers. The outer layer is made up
of flat cells with a striated border (Fig. 11 A, CS) on their free edge,
like that already described in Fishes : the inner layer is composed
of more cylindrical or cubical cells (£). The former corresponds to
the stratum corneum, the latter to the stratum Malpighii.

Later, with advancing development, the layers of the epidermis
become more numerous, and involutions towards the derma take
place in all parts, giving rise to a great number of globular and
tube-shaped glands, which are particularly abundant in certain
regions — more especially in the head and flanks.

Their secretion serves to keep the skin moist, but, as experi-
ments have shown, it also forms an important weapon of defence,
on account of its poisonous properties.2

1 There are, however, several exceptions to this rule. In male Eiasmobranchs
there is a large glandula pterygopodii (gland of the clasper) at the base of each
pelvic fin. It arises as a tube-like invagination of the skin, and is in relation with
the copulatory organs (cp. the chapter on these organs). In the "Weever (Trachinus)
there is a series of poison-glands lying on either side of the bnses of the spines of the
dorsal fin ; they are situated at the bottom of integumentary sacs, and their ducts
open close to the bases of the spines. In Thalassophryne the operculum is pro-
vided with a hollow spine, at the base of which a poison-sac is situated, and in
Synanceia there are also a series of "poison-bags" at the bases of the grooved
dorsal spines (Gu'nther). Poison organs appear to be present in certain other Fishes
(many Siltiroids, Aetobatis) but the existence of actual glands is not certainly known.

'2 The poison has no effect on other individuals of the same species ; but it acts
very powerfully on closely allied forms, as well as on the higher animals.

INTEGUMENT.

19

This richness in glands is a characteristic of the
skin of Amphibia, and to it they owe their moist and slippery
nature. Frequently, as for instance in Toads, the skin is not

smooth, but has a rough, warty appearance, caused by local
^ proliferations of the epidermis.

FIG. 11 A. — SKIN OF LARVA OF SALAMANDER (Salamandra maculosa).

Ep, epidermis ; Co, derma ; a, stratum corneum ; b, stratum Malpighii ; LZ>
Leydig's cells ; OS, striated border.

The pigment, accumulated principally in the derma — partly
diffused, partly enclosed within the cells — is under the control of the
nervous system, and thus renders a change of colour possible;
and as the colour becomes modified according to the surroundings of
the animal, it may serve as a protection.

FIG. HB.— SECTION THROUGH THE SKIN OF ADULT SALAMANDER (S. maculosa},

Ep, epidermis ; Co, derma, in the richly pigmented (Pi) connective-tissue stroma of
which the varioTis sized integumentary glands (A, C', D, D, E] lie embedded ; Ml,
the muscular layer of the glands, lying within the membrana propria (Pr) ; M,
the same, seen from the surface ; E, epithelium of glands ; S, secretion of
glands ; Mm, subcutaneous layer of muscles, through which vessels (G) extend
towards the derma.

Calcifications may also occur in the derma, or, as in Cera-
tophrys dorsata, definite bones may be formed. Such bony plates

c 2

20 COMPARATIVE ANATOMY.

were much more abundant in Amphibians of former times ; those
of the Carboniferous and Trias (Stegocephala, Labyrinthodonta)
were richly provided with them.

For a further reference to the ring-like scutes and scales found in Csecilians,
the reader is referred to the chapter on the dermal skeleton, and it is only
necessary to state here that the rings accurately correspond in number with
the vertebrae, — a very rare occurrence. The same is true of certain Amphis-
bsenians (e.g. Blanus cinereus).

Reptilia. — In contrast to the skin of Amphibians, that
of Reptiles is very deficient in glands. In Lizards, a series
of "femoral glands" occur along the ventral side of the thigh,
the secretion from which, as it passes out of the apertures, hardens
so as to form a series of papilloe or warts, which appear to serve as
clasping organs during copulation. In Amphisbsenians integu-
mentary glands are also present : they lie anteriorly to the cloaca,
and open into the " pre-anal pores."

The characteristic peculiarity of the skin of Reptiles
is its capacity of producing scales, warts, prickles, shields
(e.g. the " tortoiseshell" of Chelonians), claws, rattles (of Rattle-
snake), and suchlike structures.

All these integumentary organs, as already mentioned (p. 16),
are to be included in the same category as the feathers of
Birds and the hairs of Mammals : that is, all arise at first by a
proliferation of the epidermic cells, a portion of the dermal tissue
taking part in their formation later on. Certain special differences
are, however, always to be observed in the development of these
different structures, as will be mentioned presently.

As in Amphibia, calcifications or ossifications may occur in
the derma. The horny layer of the epidermis may be either
periodically cast off entire (Snakes), or in shreds from time to
time : it is renewed from the Malpighian layer. Pigment-cells
also occur, rendering possible in many cases a change of colour
(e.g. Chameleon).

Birds. — Birds possess a thinner derma than do any other
Vertebrates, and it is not very plentifully supplied with- blood-
vessels, although, as will be mentioned in another chapter, sensory
organs (tactile corpuscles) are abundant. In the deeper layers
there is a strongly developed network of muscle-fibres, which are
inserted into the feather-sacs, and serve to erect the feathers.

The feathers, arranged in so-called " feather- tracts " (pterylse)
separated by naked regions (apteria), form the most marked
peculiarity of the body of Birds, and their development is very
instructive. In the region where a feather is to be formed, the
dermal tissue becomes raised up towards the ectoderm (Fig. 12, A,
Cu, Sc, SM1^), and thus gives rise to a papilla (Pap). As this papilla
grows out to form an elongated cone with a pointed apex, the
feather-germ (Fig. 12, B, FK), its base sinks gradually deeper and

INTEGUMENT.

•21

deeper into the derma, and thus becomes surrounded by a sort of
pocket— the feather-follicle (F9 F1).

The horny, as well as the Malpighian layer of the epidermis
(8c, SM) extends into the base of the follicle, and thence into the
feather-germ (Sc\ AM1), the interior of which is throughout filled
by cells of the derma, which give rise to the pulp (P). As the
feather-germ keeps on growing, the cells of the Malpighian layer

FIG. 12.— Six STAGES IN THE DEVELOPMENT OF THE FEATHER.
(Mainly after Th. Studer.)

rv, derma ; SM, stratum Malpighii ; Se, stratum eorneum ; SM1, Sc1, extensions of
these tissues iota the feather-papilla, Pap ; FK, feather-germ ; F, F\ feather-
follicle ; P, pulp ; Fed (SM1), folds of the Malpighian layer extending into the
feather-germ, and enclosed externally by the horny layer, HS (Sc'1} : both layers
are seen in the transverse section (C) ; FSp, quill of feather, which breaks up
above into a tuft of rays or barbs (HSt) ; sec, sec, secondary rays (barbules)
arising from the latter ; £, rachis ; V, vexillum.

For further details as to the different stages A-F, compare text.

If one supposes that in Fig. A the papilla grows and becomes flattened and
bent downwards posteriorly, and that ossification occurs in the dermal portion,
the essential mode of development of a scale would be arrived at.

begin to proliferate rapidly, giving rise to a series of radial folds
arranged §,long a central axis, which extend inwards towards the
pulp, and are immediately bounded by the horny layer (Fig. 12, C,
Fal (SM1) and HS (Sc1). These folds then become cornified and
separated from the surrounding cells, and, by a gradual drying of the

22 COMPARATIVE ANATOMY.

central pulp-substance, give rise to a tuft of horny rays, which are,
however, at first bound together by the enclosing stratum cor-
neum. Most Birds are hatched when the feathers are in this
stage of development, and they thus appear as if covered with
pencil-like hairs.

By the shedding of the surrounding horny layer the rays or
barbs become free (Fig. 12, D, ffSt), and if they are all similar to
one another, an embryonic down-feather (pluma) is formed.
The whole feather-germ, however, does not become divided up into
barbs in this manner : its lower portion, embedded in the skin,
retains a more uniform character and forms the quill (calamus)
(FSP).

The embryonic down-feathers (Fig. 12, E), on the individual
barbs of which smaller secondary rays or barbules become
developed (sec, sec), may retain their character as such throughout
life or may be replaced by definitive feathers. In this case a second
follicle early arises from the base of the follicle of the down-feather,
with which it is connected by a cellular cord, and which it closely
resembles in, structure (Fig. 12, D, F1). The papilla developing
within the interior of this new follicle grows rapidly, gradually
pushes the base of the down-feather out of its follicle, and comes
to the surface. Each definitive feather at first closely resembles a
down-feather in structure, and consists of a tuft of similar rays or
barbs provided with barbules. In the course of further growth,
however, one of the rays becomes rapidly thickened, and forms a
main axis or stem (sea pus), to which the barbs are attached on
each side. ' The proximal or basal portion of the scapus which bears
no barbs is called the quill (calamus), and the distal part, to
which the barbs are attached, the shaft (rachis). The barbs
together constitute the vane (vexillum) (Fig. 12, F, E, HSt, sec).
The secondary rays or barbules are so arranged on each barb (HSt)
as to make the latter resemble an entire feather in appearance.

In many Birds, each quill of the ordinary feathers of the body bears two
vexilla, the second being spoken of as the af tershaft (hyporachis).

In this manner the contour feathers (pennse) are formed,
such, for instance, as those on the wings and tail. The individual
portions of the vexillum usually become very closely united
together, so that an extremely strong and resistant though pliant
structure is formed.

A periodic casting of feathers, or "moulting," takes place
in all Birds, and corresponds to the similar process of the casting of
the outer skin in Reptiles ; in Mammals there is a continual scaling
off of the epidermic cells of a similar nature.

The feather-covering of Birds must have been acquired in very early
geological periods, for Archseopteryx, found in the Jurassic strata of
Bavaria, possessed well-formed feathers with a very delicate shaft and vane.

Palseohtological researches have not yet brought to light any definite inter-
mediate stages between scales and feathers, but that they must once have
existed is shown by the development of these structures (camp. Fig. 12 and
description).

There is no trace of proper dermal bones (dermostoses) l or
calcifications in the skin of Birds, and the glands are reduced to
a single mass — the uropygial gland: this is situated at the
base of the tail (uropygium), arid its secretion serves to oil the
feathers.

Epidermic structures, such as claws, spurs, foot-scales,
and beak-sheaths, are strongly developed.

FKJ. 13.— Six STAGES IN THE DEVELOPMENT OF HAIR.

Sc, stratum corneum ; SM, stratum Malpighii ; C, derma ; F, follicle ; Dr, sebaceous
fland ; CZ, central, and PZ, peripheral zone of the hair-germ ; HX, hair-knob ;
P, beginning of the formation of the hair-papilla ; P\ the same in a later stage
of development, when it has become vascular.

Mammals. — The hair-like structures possessed by certain
Reptiles and Birds are historically quite distinct from the
true hairs of Mammals. The possession of hairs characterises
Mammals quite as much as feathers distinguish Birds from all
other animals.

1 See note on p. 62.

24 COMPARATIVE ANATOMY.

Each hair arises first as a proliferation of the epidermic cells in
the region of the Malpighian layer, which projects inwards towards
the derma (Fig. 13, A and B, Sc, SM, 0). In this manner the
hair-germ is formed. The thickening of the epidermis then
grows out into the form of a papilla, and becomes surrounded
by the cells of the derma, so that, as in the case of the feather,
it comes to lie within a kind of pocket, the hair- follicle (Fig. 13,
C, D, F). The originally uniform mass of cells of the hair-gerrn
later becomes differentiated into a peripheral and a central
portion (Fig. 13, E, F, PZ, GZ}. The latter consists of more elong-
ated cells, and gives rise later to the hair-s-haft with its medulla
or pith, and to the cortex, as well as to the cuticle of the shaft,
and to the so-called inner root-sheath ; the former gives rise to
the outer .root-sheath (comp. Fig. 14, which represents the
fully-formed hair). The base of the hair-shaft which fills up the
bottom of the follicle is broadened out to form the hair- knob
(Fig. 13,E,F,^AT), and the richly vascular hair-papilla (Fig. 13, E,
F, P, P1), which arises comparatively late, extends into it from
below. At Dr, in Fig. 13, the sebaceous glands are seen
arising by a proliferation of the Malpighian cells. The hair usually
breaks through the skin in an oblique direction. The character of
the medulla varies so much that upon it principally depend the
differences observable in the hair of Mammals.1 The colour of the
hair is due to three causes : — Firstly, to the greater or less accumula-
tion of pigment in the cells of the cortical layer ; secondly, to the
air contained in the intercellular spaces of the medulla; and lastly,
to the nature of the surface of the hair, i.e. whether it is rough
or smooth.

The hair-sheaths consist of an outer, or dermic coat (Fig.
14, F, F1), and of an inner or epidermic coat, the so-called
root- sheath (WS, WS1). The first of. these is formed entirely
from the connective-tissue of the derma — that is, from the above-
mentioned hair-sac or follicle, — while the latter is a product of
the Malpighian layer, with which it remains in connection.
Between the two hair-sheaths lies a homogeneous and thin
hyaline-layer (GIT).

The follicular tissue, which is richly provided with blood-
vessels, extends into the knob -like base or root of the hair-shaft
(bulb us), and gives rise to the hair -papilla (Fig. 14, HP). The
latter is the real matrix of the hair, and from this region a new
hair-shaft may develop when the hair is shed, periodically or
non-periodically as the case may be, by the formation of a new
papilla. Whether new hairs arise in the same manner as in
the embryo ("primary hair-formation") cannot be stated with
certainty.

Smooth muscles (arrectores pili) and nerves, as well as seba-

1 In Bats, tho hairs arc usually distinguished by scale-like projections of their
surface.

INTEGUMENT. 25

ceous glands (Fig. 14, HBD], are. in connection with each hair-sac,
the latter serving to oil the hair.1

Sch

FIG. 14.— LONGITUDINAL SECTION THROUGH A HAIR. (Diagrammatic.)

tie, stratum cornenm ; SM, stratum Malpighii ; Co, derma ; Ap, arrectores pili ;
Ft, adipose tissue ; F, outer longitudinal layer, and F1, inner transverse layer
of dermic coat (both composed of connective-tissue) ; Sck, hair-shaft ; M,
medulla ; E, cortex ; 0, cuticle of shaft ; WS, WS1, external and internal
root-sheath, — the latter reaches above only as far as the point of entrance of
the ducts of the sebaceous glands (HBD] • HP, hair-papilla, containing vessels ;
GH, hyaline layer, which lies between the inner and outer hair-sheaths, ie.,
between the root-sheath and the follicle (dermic coat).

As feathers are arranged, in definite tracts, so also hairs are
disposed more abundantly on some parts of the body than on others.

1 The arrectores pili have also the function of compressing the glands, though the
latter are provided with muscles of their own, the development of which is in inverse
proportion to that of the proper hair-muscles.

i>f> COMPARATIVE ANATOMY.

A richer hairy covering (1 ami go) is often met with in the embryonic
condition — as, for instance, in the human fetus — than occurs later ; and this
fact, together with the occasional appearance of abnormally hairy individuals,
indicates that at one time man was distinguished by a far more abundant
clothing of hair than at the present day. In the normal condition the Ainos
and the Australians are the most hairy races.

When pigment is present, it is always situated in cells of the
Malpighian layer ; particular parts, as, lor instance, the external
genitals (labia majora and scrotum), the arms, the teats, and the skin
of the axillae in Man, are especially well provided with it.

The outer layer of the derma, as may be seen by a glance at
Fig. 15, may be divided into an outer papillary and an inner
reticular portion. The former contains both nerves and blood- and

sxf

If w

M( ^^i^^ii^L^l^-'
^ I > ^v

FIG. 15. — SECTION THROUGH THE HUMAN SKIX.

8c, stratum corneum ; SM, stratum Malpighii ; Co, derma ; F, F, subcutaneous
fat ; NP, nerve-papillse ; GP, vascular papillae ; N and G, nerves and vessels of
la ; SD, SI), sweat-glands, with their ducts (SD1, &D1) ; H,

the derma
sebaceous glands (D).

hair witli

lymph-capillaries ; the latter, on the other hand, becomes lost
without any sharp boundary line in the sub-dermal connective-
tissue, and in the more or less strongly-developed fatty layer
(panniculus adiposus). Smooth muscle elements are distributed
throughout the derma ; they are particularly abundant in the
scrotum (dartos) and in the teats.

The integumentary glands are tube-shaped or else berry-
shaped or globular. The former kind, which we must consider as
the most simple and primitive, include the sweat-glands and their
modifications (e.g. ceruminous glands); while the latter, which are
more highly developed histologically, are spoken of as sebaceous
glands. To the latter group belong the already-mentioned glands

INTKOUMKNT. -27

of the hair-sacs, the glands of the prepuce, the perineal glands
present in many Mammals, the glandular dorsal grooves of the
neck of the Chamois, the Meibomian glands of the eyelids, and
many others.

Epidermic structures play a very important part in Mammals : such
are — claws, nails, hairs, bristles, and spines (Hedgehog, Porcupine) ; the so-called
whale-bone (baleen) of the Mystaceti, the nasal horns of the Rhinoceros, the
scales of Manis, and the palatal plates of Sirenia, belong to the same category.

MAMMARY GLANDS. — The mammary glands, which stand in
such a close relation to reproduction, are entirely confined to the
Mammalia, which owe their name to the possession of these organs.
Their phylogenetic relations are by no means clear; they must,
however, be considered as modified integumentary glands
(sebaceous glands).

The so-called mammary pouch of Echidna 1 may be taken as a
point of origin of the different forms of teats. It consists of a
pocket-like in-sinking of the skin of the abdomen, which is
possibly only formed periodically ; in it the eggs or unripe young
appear to be protected. How the latter, in the absence of true
teats, take in the milk is not at present known.

A ™ B

FIG. 16. — A, TRUE (SECONDARY) TEAT ; AND B, PSEUDO- (PRIMARY) TEAT.

This pouched condition repeats itself ontogenetically in every
Mammal by the epidermis extending inwards towards the derma,
and cylindrical more or less branched processes arising from the
base of the pouch thus formed. These processes only are the
proper glands, the mammary pouch being simply a part of the
outer surface of the skin which has sunk inwards, and thus it may
give rise to hairs and other integumentary structures.

The teats may become developed in one of two ways. In
the first of these, the skin surrounding the pouch becomes
raised up, and so forms a teat perforated by a canal, into the
base of which the proper ducts of the gland open (Fig. 16, B).
In the second case, the gland surface itself becomes elevated into
a papilla, while the surrounding skin remains almost on a level
with the rest of the integument (Fig. 16, A). In the latter

1 Whether the absence of a pouch in Oinithnrhynchus is the more primitive or
secondary condition is not certain : possibly one is formed periodically during "heat."

28 COMPARATIVE ANATOMY.

case the teat may be described as true or secondary (Mar-
supials, Rodents (? all), Lemurs, Monkeys, and Man), and in
the former as a pseudo- or primary teat (Garni vora, Pigs,
Horses, and Ruminants). The lattej condition is already
sketched out in certain Marsupials (Phalangista vulpina).

As a rule the number of teats corresponds to the number of
young born at a time. They are often situated in two nearly
parallel rows along the ventral side of the thorax and abdomen,
which gradually converge towards the inguinal region : in other
cases they may be restricted either to the inguinal (Ungulates and
Cetaceans) or to the thoracic region (Elephants, Sirenia, many
Lemurs, Cheiroptera, and Primates).

In the human male, the mammary apparatus becomes aborted, though
usually at birth and puberty true milk, the so-called " witches' milk "
(Hexenmilch), is produced. Male goats and castrated sheep have also been
known to give milk. The occasional existence in men of supernumerary teats,
and in women of supernumerary mammae and teats (poly mast ism and poly-
thelism) is very remarkable. They are usually situated in the thoracic region,
and must be considered as an atavism to a characteristic primitive form which
possessed numerous teats, and which produced a number of young at a time.
Such a transition from polymastism to bimastism may be seen plainly at the
present day in the Lemurs : in them the inguinal and abdominal teats are under-
going a retrogressive metamorphosis, while a single pair of thoracic teats remain
well developed.1 This accords with the fact that most Lemurs bear only a pair
of young ones at a time, which they carry with them at the breast.

The glands, which are at first solid, become secondarily hollowed
out and differentiated into acini, mammary ducts, milk
sinuses, and excretory ducts. The whole intermediate tissue
during lactation is filled with white blood-corpuscles (leucocytes),
and possibly the well-known structural elements of milk, known as
colostrums and milk-spheres, owe their origin to these corpuscles,
which have passed through the walls of the acini.

BlBLIOGKAPHY.

CARRIERE, J. — Die postembryonale Entwickelung der Epidermis des Siredon pisci-

formis. Archivf. mikros. Anat. Bd. XXIV. 1884.
DOMBROWSKI, R. VON. — Geweihe u. Gehorne. Nalurwiss. Studic. Wien, 1885. (With

40 plates.)
ECKEH, A., und WIEDERSHEIM, R. — Die Anatomic dcs Frosehcs. Braunschweig,

1864—82.
GEGENBAUR, C. — Zur Morphol. dcs Nagels. MorpJi. Jahrbuch, Bd. X. 1885. Zur

genaueren Kenntniss der Zitzen der Sdugethiere. Morphol. Jahrb. Bd. I. 1876.
HUXLEY, T. H. — Tegumentary Organs. Todd's Cyclopaedia of Anat. and Physiol.
KERBERT, C. — Ueber die Haut der Reptilian und andrer Wirbelthivre. Arch. f.

miTcr. Anatomie, Bd. XIII.
KLAATSCH, H.— Zur Morphologic der Sdugethierzitzen. Morphol. Jahrb. Bd. IX.

1883.

LEICHTENSTERN, Ueber uberzdhlige Brustc. Arch. f. pathol. Anat. 1878.
LEYDIG, F. — Ueber die allgem. Bedeckungen der Amphibien. Arch. f. mikr.

Anatomie, Bd. XII. 1876.
LIST, J. — Ueber Wanderzellen in Epithcl. Archiv /. mikros. Anat. Bd. XV. 1885,

mid Zool. Anzeiger, No. 198, VII. Jalirgang, 1885.

1 In Hapalemur griseus the single pair of teats is situated on the arm (Becldard)-.

INTEGUMENT. 29

PAULICKI.— Ueber die Haut dcs Axolotls. Archiv f. mikros. Anat. Bd. XXIV.

1884.

PFITZNER, \V. — Die Epidermis der Amphibien. Morphol. Jahrb. Bd. VI. 1880.
RAUBER, A. — Ueber den, Ursprung der Milch und die Erndhrung der Frudit im

Allgemeinen. Leipzig, 1879.
REIX, G. — Untersuch. uber die embr. Entw.-Geschichte der Milchdrilse. Arch. /.

mikr. Anat. Bd. XX. uud XXI. 1882.

SCHULZE, F. E. — Epithel. und Driisenz Men. Arch. /. mikr. Anat. Bd. III.
STUDEB, TH. — Die Entwicklung der Federn. Inaug.-Diss. Bern, 1873. Beitrdge

zur EntwicTclungsgeschichte der Feder. Zeitschr. f. wiss. Zool. Bd. XXX.
UXXA, P. — Beitr. zur Histologie und Entw.-Geschichte der menschl. Oberhaut und

ihrer Anhangsgebilde. Arch. f. mikr. Anat. Bd. XII. 1876.
WALDEYER, AV. — Atlas der menschl. und thier. ffaare, <bc. Lahr, 1884.
"WIEDERSHEIM, R. — Die Kopfdrusen der geschwdnzten Amphibien, <tc. Zeitschr. f.

wisscnsch. Zoologie, Bd. XXVII.
ZANDER, R. — Die fruhcsten Stadien der Nagelentwiclclung und ihre Beziehungen zu

den Digitalnerven. Arch. f. Anat. u. Entw.-Gesch. 1884.

B. SKELETON.

I. DERMAL SKELETON.

THE dermal skeleton, as phylogenetically the older, is best
considered before the endoskeleton. Its relative age is shown

FIG. 17 (after 0. Hertwig). — a, DERMAL ARMATURE OF Hypostoma commune (a
Siluroid) ; b, DENTICLES FROM THE SKIN OF THE ABDOMEN OF Callichthys ; c,
PLATES FROM THE TAIL-FIN OF Hypostoma.

Z, dermal denticles, shown broken off from their bases at Z^ ; BP, basal plate.

not only by Palaeontology,1 but also by Ontogeny, inasmuch as
calcifications and ossifications in the derma or perichondrium

1 As examples of ancient forms which were protected by well-developed dermal
skeletons, may be mentioned the armoured Fishes of the Devonian and Silurian
strata, and the armoured Amphibians of the Carboniferous, Trias, and Jurassic.

DERMAL SKELETON.

31

appear much earlier in the developing animal than do definite
ossifications within particular parts of the cartilaginous skeleton.
The condition which obtains in Fishes and Amphibia well illustrates

FIG. 18.— DERMAL DENTICLES OF Protoptcms.

D, the apical portion of the denticle ; 8, S, the base of the denticle, the cavity of
which is seen in optical section through the transparent apical portion at S.I ami S.2.

this. To take a single example : the young File-fish (Balistes) is
provided with a complete dermal armour at the time when the
ossification of the primordial cranium has hardly begun.

FIG. 19.— DERMAL ARMATURE OF Callichthys.

B, barbules ; BrF, pectoral fin ; BF, pelvic fin ; RF, dorsal fin ; DS and VS, dorsal
and ventral bony shields ; t> lateral line.

The exoskeleton originates by the formation of small denticles
(Figs. 17 and 18) attached to basal plates, which lie scattered over

32 COMPARATIVE ANATOMY.

the whole skin, and which exhibit exactly the same structure as
the teeth proper, which will be described later.

Such dermal denticles are found in the skin ofElasmobranchs,
Ganoids, Siluroids, and Dipnoans; the large shields, which, in
the armoured Ganoids and Siluroids (Fig. 19), Lophobranchii and
Plectognathi, become united to form a strong bony cuirass, may be
derived from the gradual fusion of the above-mentioned basal-
plates to form bands and networks. One may even extend this
still further, and derive phylogenetically allthe scales of Fishes,
as well as the investing bones of the pectoral arch (e.g.
Teleostei) and of the primordial skull in the same manner (cp.
Fig. 53).1

Fossil genera of Amphibia have bequeathed but slight traces
of this strong dermal armour to the existing forms of the group :
as examples may be mentioned the bony plates in the skin of the
back of certain Anura (Ceratophrys dorsata and Ephippifer auran-
tiacus), as well as the scales lying between the ring-like scutes of
the footless Amphibia (Gymnophiona) (comp. p. 20). The latter
may be derived from such a scaly covering as that of the ancient
Salamander (Discosaurus) of the Carboniferous formation.

The dermal skeleton of fossil Reptilian genera, as, for instance,
of many Ornithoscelida (Stegosaurus), was still more highly
developed. In these, enormous bony plates and spines, sometimes
as much as 63 centimetres long, were present in the dorsal region.
Teleosaurus also, as well as the Triassic Aetosaurus ferratus,
possessed a strong exoskeleton. Amongst existing Reptiles, the
Crocodiles, many Lizards (Anguis, Cyclodus, Scincus), and more
especially the Chelonia, exhibit a well- developed dermal skeleton.
In the latter group a dorsal and ventral shield (carapace and
plastron) consisting of numerous pieces and completely enclosing
the body must be noticed. Both arise independently of the endo-
skeleton, which is preformed in cartilage, that is to say, they are true
exoskeletal membrane bones (cp. note on p. 62) ; the exoskeleton,
however, comes into the closest relation with the endoskeleton, and
may supplant it here and there : thus, in Testudo, for instance, the
thoracic and lumbar vertebrae and ribs become quite rudimentary.

Birds have no dermal skeleton, as already mentioned in the
chapter on the integument.

It is uncertain whether the dermal skeleton present in Arma-
dillos (Loricata)2 only among Mammals is to be derived directly

1 The dermal denticles of Elasmobranchs are often spoken of as placoid, and
the firmly-jointed scales of Lepidosteus and Polypterus as ganoid scales : both are
covered by a layer of enamel (probably) developed from the ectoderm, and thus both
epidermis and derma take part in their formation. Some Fishes (e.g. Electric Ray,
Spatularia, some Eels) are scaleless.

2 In Armadillos the dermal skeleton consists of a series of transverse bony
scutes, which are movable on one another, while in Glyptodon, a fossil member of
this group, the dermal plates were firmly united together to form a large shield,
which covered the whole body.

ENDOSKELETOK 33

from that of Reptiles, or whether it is to be considered as formed
independently, that is, as a new acquisition or " neomorph "
(Gadow).

Thus it will be seen that the exoskeleton tends
gradually to disappear as we ascend in the scale of
the Animal Kingdom, while, on the other hand, the
endoskeleton becomes of greater and greater importance.

BlBLIOGEAPHT.

CREDNEE, H. — Die Stegocephalen (Labyrinthodonten) aus dem Rothlicgenden dcs
Plauen'schen Grundes bei Dresden. Zeitschr. der deutsch. geolog. Gesellschaft,
1881, 1882, 1883.

FRITSCH, A. — Fauna der GasJcohle und der Kalksteine der Permformation Bohmens.
Prag. (not yet completed).

HERTWIG, 0. — Ueber Bau und Entwickelung der Placoidschuppen und der Zdhne der
Selachier. Jenaische Zeitschr. Bd. VIII. N.F. I. Ueber das Hautskelet der Fischc
(3 Essays). Morphol. Jahrb. Bd. II. 1876 ; Bd. V. 1879 ; Bd. VII. 1881.

MARSH, 0. C. — Numerous papers in the American Journal of Science and Arts.

WIEDERSHEIM, E. — Die Anatomie der Gymnophionen. Jena, 1879. Zur Histologie
der Dipnoerschuppen. Arch. f. mikr. Anatomie, Bd. XVIII. 1880.

II. ENDOSKELETON.

I. VERTEBRAL COLUMN.

An elastic rod, the notochord (chorda dorsalis), lying in the
long axis of the embryo, between the neural and visceral tubes (see
Fig. 7., B), is to be considered as the foundation of the axial skeleton.
Consisting of a meshwork of cells, which are early vacuolated,
the outer protoplasmic part of the notochord becomes differentiated
into a structureless cuticular sheath (the sheath of the notochord,
or elastica limitans in terna), which, however, disappears almost
entirely after the notochord has ceased to grow (Fig. 20, Cs).

Outside this inner sheath, a skeletogendus layer is formed
round the notochord from that part of the mesoblast which is
distinguished as the mesoblastic somites or protovertebrse.
Round the outer periphery of this layer another sheath is formed,
the outer sheath of the notochord, or elastica limitans
externa (Fig. 20, Ee).

The skeletogenous layer, consisting of fibrous tissue, now
extends dorsally over the spinal cord on each side, and thus gives
rise to a continuous membranous tube, JVhich is only broken
through at the points of exit of the spiral nerves. No proper
segmentation, — like that seen in the muscular system, — is to
be noticed in this membranous stage. The first indication of
segmentation is the formation of cartilaginous areas in -the mem-
brano-fibrous mass of the skeletogenous tissue, in the immediate
neighbourhood of the notochord : these show a segments! ar-
rangement (formation of metameres), and represent the first

D

34 COMPARATIVE ANATOMY.

traces of the vertebral bodies and arches. This is the
second or cartilaginous stage of the vertebral column, and
now ossification may occur (bony stage). Those parts of the
fibrous tissue which do not become consolidated in this manner
give rise to certain ligaments of the vertebral column.

During these differentiations of the skeletogenous tissue, the
noto chord suffers a very different fate in the various Vertebrate

FIG. 20. — TRANSVERSE SECTION OF THE VERTEBRAL COLUMN OF Ammoccetes.

C, uotochord ; Cs, inner sheath, and Ee, outer sheath, of the notochord ; SS, skele-
togenous layer ; Ob, upper arch ; Ub, lower arch ; F, fatty tissue ; M, spinal
cord ; P, pia mater.

groups ; it may increase in size and persist as a regular cylindrical
rod, or it may become constricted at definite intervals by the for-
mation of vertebral bodies, or even entirely disappear.

During the cartilaginous and bony stages the various processes (spinous,
transverse, articular processes, &c.) are formed : and the individual vertebrae
may sometimes become fused together, as for instance, in the regions of the
neck, sacrum, and coccyx.

All these ontogenetic stages find their exact parallel in the
phylogenetic development of Vertebrates, as the following pages
will show.

Fishes. — The vertebral column of all Fishes is distinguished by
a very uniform character of its elements, so that one can only
distinguish between trunk and caudal vertebrae.

The most embryonic type of notochord is seen in Amphioxus,
Ammoccetes, and Myxinoids, in which it is entirely unsegmented.
In the metamorphosed Petromyzon, cartilaginous elements already
make their appearance in the form of rudimentary arches and
spines, which do not meet above the spinal cord (comp. Fig. 49).

VERTEBRAL COLUMN.

35

To the condition found in Petromyzon, that seen in the
Cartilaginous Ganoids, ChimaBrse, and Dipnoi, is directly
connected, inasmuch as the metameric character is mainly indicated
by upper or dorsal arches.

Ub

FIG. 21. — PORTION OF THE VERTEBRAL COLUMN OF Spatularia. (Side view.)

FIG. 22. — TRANSVERSE SECTION OF THE VERTEBRAL COLUMN OF Acipenser
ruthenus (in the anterior part of the body).

Pa, spinous process ; EL, longitudinal elastic band ; SS, skeletogenous layer ; Ob,
upper arch ; Mt spinal cord ; P, pia mater ; Ic, intercalary pieces ; Cs, noto-
chordal sheath ; C, notochord ; Ee, elastica externa ; Ub, lower arch ; Ao, aorta ;
Fo, median parts of the lower arches, which enclose the aorta ventrally ; Z, basal
processes of the lower arches.

The strong, concentrically-layered notochordal sheath (Fig. 22,
Cs) here plays the part of vertebraa, and is surrounded by a fibrous
skeletogenous layer (Fig. 22, $#), in which paired dorsal and
ventral cartilages become developed. The former give rise to
the above-mentioned upper or neural arches, the latter to the
lower or haemal arches (Figs. 21,22, Ob, Ub]. In the caudal region

FIG. 23.— PORTION OF THE VERTEBRAL COLUMN OF Protopterus. (Side view.)
C, notochord ; DF, spinous process ; FT, interspinous bone ; FS, fin-ray.

the latter enclose the aorta and caudal vein ; further forwards the
cartilages do not meet in the middle line below, and consequently
the lower arches end on either side in a laterally-directed cartila-
ginous projection, or " basal process," which may develop an

D 2

36 COMPARATIVE ANATOMY.

articulation at its base, and thus give rise to a rib (cp. p. 48).
The relations of these parts in Elasmobranchs and Teleosteans is
similar to that above described. For the further strengthening of the
vertebral column so-called "intercalary pieces" (Figs. 21,22,
Ic) appear between the upper and lower arches in Cartilaginous
Ganoids and Elasmobranchs. The vertebral column of Bony
Ganoids reaches a much higher stage of development. Paired
dorsal and ventral cartilages arise above and below the notochord,

JOT

FIG. 24. — PORTION OF THE VERTEBRAL COLUMN OF Polypierus.
WK, centra ; BF, basal processes ; Ob, upper arches ; Ps, spinous processes.

but in the course of development so extend at the base as to com-
pletely surround it. From the dorsal cartilages the upper arches
take their origin, and from the ventral the lower, while the car-
tilage surrounding the notochord gives rise to the vertebral
bodies or centra. The whole vertebral column also becomes
strongly ossified (Fig. 24). The notochord is now no longer equal in
diameter throughout, but becomes constricted or actually divided

FIG. 25.— DIAGRAM SHOWING THE INTERVERTEBRAL REMAINS OF THE
NOTOCHORD.

(7, (71, expanded and constricted portions of notochord ; WK, centra ; Li, inter-
vertebral ligaments.

in the centrum of each vertebra (that is, vertebrally), while
intervertebrally it remains expanded, and so serves as a sort of
connecting- or packing- substance between two contiguous vertebra
(Figs. 25 and 27, 0, Cl). The same thing takes place in other
Fishes — Elasinobranchii and Teleostei — and thus deeply
biconcave (amphiccelous) centra are formed.

One of the Bony Ganoids, Lepidosteus, forms a marked exception to other
Fishes as regards its vertebral column, inasmuch as definite articulations are
formed between the vertebrae. A concavity is formed at the hinder end of

VERTEBRAL COLUMN.

37

each centrum (Fig. 26, J., en1), which articulates with a convexity (en) on the
next vertebra behind (opisthocoelous vertebra). The notochord (except in
the caudal region) entirely disappears in the adult ; in the larva it is seen to
be expanded vertebrally, and constricted intervertebrally, a condition of
things which appears again in the higher types, as, for instance, in Reptiles. In
a still earlier larval stage, however, the constrictions are vertebral, as in other
Fishes (see Fig. 27).

The primitive character of the vertebral column of Fishes is
shown by the fact that the arches only meet dorsally in rare in-
stances. As a rule, the closing in of the arch is effected by special

FIG. 26. — PORTION OF THE VERTEBRAL COLUMN OF Lepidosteus.
(After Balfour and Parker.)

A, a vertebra from anterior surface ; B, two vertebrae from the side ; en, anterior
convex face, and cnl, (posterior concave face of centrum ; h. a, basal process ;
n. a, upper arch; i.c, intercalary cartilages; ZJ, longitudinal ligament; i.s,
interspinous bone.

pieces of cartilage and by an elastic longitudinal band (Figs. 22,
26, i.c, LI) which is always present. This description of the upper
arches applies also to the lower ones present in the caudal region.
Elasmobranchs and Ganoids possess a greater number of vertebrae 1
than Teleosteans, in which we seldom meet with more than 70 :
the Eel, however, possesses more than 200. In Rays and ChimseraB
only amongst Fishes are definite articulations formed between the
skull and vertebral column.

1 In Alopecias vulpes there are 365 ; in Carcharias glaucus, 240 ; in
Lamna, 150; in Pristiurus, 140; in Scyllium, about 124; inSquatina, 117.

38

COMPARATIVE ANATOMY.

The caudal region of the vertebral column deserves particular
attention in Fishes, and the primitive condition of this region in
Amphioxus, Cyclostomi, and Dipnoi, may be taken as a starting-
point. In these, the notochord extends straight backwards to the
hinder end of the body, and is surrounded quite symmetrically

FK

FIG. 27. — PORTION OF THE VERTEBRAL COLUMN OF A YOUNG DOGFISH (Scyllium
caniculd). (After Car tier.)

G) notochord ; Kn> outer, and JTw1, inner, zone of cartilage ; FK, the fibre-carti-
laginous mass lying between these zones, which is undergoing calcification ; Li,
intervertebral ligament.

by the tail-fin, which is therefore spoken of as protocercal or
diphy cereal (see Fig. 29). This condition is met with also in
many Wishes of the Devonian strata, as well as in young stages of
Teleostei. In the latter, however, the ventral half of the tail-fin
with its supporting skeleton becomes, as a result of unequal growth,

FIG. 28. — PORTION OF THE VERTEBRAL COLUMN OF Scymnus.

WK, centra ; Ob, upper arches ; Ic, intercalary pieces. The apertures for the spinal

nerves are seen in the arches and intercalary pieces.

more strongly developed than the dorsal, and thus the vertebral
column is bent up dorsally, giving rise to a heterocercal tail.
This form of tail may be recognised externally, as in many Elasmo-
branchs, Ganoids, and numerous fossil Fishes, or may be masked by
a more or less symmetrical tail-fin, when it is only visible internally
(Lepidosteus (Fig. 30) and Amia to some extent, but more particu-
larly in most Teleosteans,1 e.g. Salmo, Esox).

1 The term homocercal is sometimes used to describe the masked heterocercal
condition of the tail in Teleostei.

VERTEBRAL COLUMN.

39

Amphibia. — The vertebral column of Urodeles may be
divided into cervical, thoraco-lumbar, sacral, and caudal
regions, and these regions can be recognised, except in certain
modified forms such as Snakes, from Urodeles up to the Mam-
malia. On account of the absence of extremities in Csecilians, the
vertebral column can only be divided into three regions, cervical,

FIG. 29. — TAIL OF Protopterus.

thoracic, and a very short caudal. In Anura, no special lumbar
region can be recognised, and the caudal portion is modified to
form a " urostyle " (see p. 42).

The notochord of Urodele larvae, like that of most Fishes,
undergoes vertebral constrictions, while intervertebrally it grows
thicker, and accordingly appears expanded. Thus the vertebras here

FIG. 30.— TAIL or Lcpidostcus.

also are amphicoelous. Later, intervertebral masses of cartilage
become developed, which, together with the bone which is formed
at the same time in the surrounding connective-tissue, extend in-
wards towards the centre, gradually constricting the notochord so
that eventually it may become entirely obliterated. Finally a dif-
ferentiation, as well as a resorption, extending inwards from the

40 COMPARATIVE ANATOMY.

periphery occurs in these cartilaginous parts : in the interior of
each an articular cavity is formed, so that in the vertebrae of the
higher Urodeles (Salamandrina perspicillata and certain Tritons)
an anterior convexity and a posterior concavity may be distin-
guished, both covered with cartilage ; they are, therefore,
opisthoccelous. A glance at Fig. 31, A to D, will make this clear.

D

FIG. 31. — LONGITUDINAL SECTION THROUGH THE VERTEBRAL COLUMN OF VARIOUS
URODELES. A, Eanodon sibericus ; B, Ainblystoma, tigrinum ; C, Gyrinophilus
porphyriticus (the three most anterior vertebrae, /, 21, III) ; D, Salamandrina
perspicillata.

Ch, notochord ; Jvk, intervertebral cartilage : CK, vertebral cartilage and fat-cells ;
K, peripheral bony covering of centrum ; R, ribs and transverse processes ; S,
vertebral constriction of notochord in Amblystoma tigrinum, without cartilage
and fat-cells ; **, intervertebral cartilaginous tracts ; Mh, Mh, marrow cavities ;
Gp, GJc, articular socket and head ; JJ'igt, intervertebral ligaments.

In the development of the vertebral column then of Urodeles
we can distinguish three stages : — (1) A connection of the indi-
vidual vertebras by means of the intervertebrally expanded
notochord ; (2) a connection by means of intervertebral masses of
cartilage; and finally (3) an articular connection. These three

VERTEBRAL COLUMN.

41

different stages of development find a complete parallel in the
phylogeny of tailed Amphibians, inasmuch as all fossil forms, e.g.
the Stegocephala of the Carboniferous Period and the Labyrintho-
donts, as well as the Perennibranchiata, Derotremata, and many
Salamanders, possess simple biconcave vertebrae, without differen-
tiation into definite articulations.

The bony parts of the vertebrae of Urodeles are not formed from
the cartilaginous sheath of the notochord, but in the surrounding
connective-tissue, there being only an intervertebral cartilaginous

Off

FIG. 32. — VEKTEBEAL COLUMN OF Diseoglossus pidus.

Pa, articular processes ; Ps, spinous processes ; Pt, transverse processes of trunk
vertebrae ; Ptc, transverse processes of caudal vertebrae (urostyle, Oc) ; S W,
sacral vertebra ; Ob, upper arch of first vertebra ; Sg, its lateral articular
surfaces ; Po, its anterior process ; R, ribs.

zone, extending into the ends of the centra. In the Anura, on the
other hand, as well as in Elasmobranchs, bony Ganoids, and the
higher Vertebrata, the vertebra are preformed in cartilage.
In the Anura true articulations are always formed between the
vertebrae, and, as a rule, the convexity is posterior, and the concavity
anterior (proccelous). A further difference is seen in the rela-
tions of the notochord, which persists verteb rally longer than
intervertebrally, a condition which leads towards the Reptiles.
The configuration of the caudal region of the vertebral column must

42 COMPARATIVE ANATOMY.

also be remarked upon, as it differs in tailed and tailless Amphi-
bians. The long caudal portion of the tadpole's vertebral column,
which is very similar to that of Urodeles, undergoes during
metamorphosis a gradual retrogressive change, and the vertebrae
of its proximal end become fused together and ossified to form a
Jong unsegmented dagger-like bone, the urostyle (Fig. 32, Oc).

The upper as well as the lower arches of the vertebras are in direct
connection with the centra. Lower arches are present only in the caudal
region of Urodeles, and evidently correspond to the already-mentioned basal
processes of the vertebrae of Ganoids. The most anterior in some cases
function as supports for the ribs, and this circumstance is sufficient to render
untenable the earlier view that the lower arches are modified transverse
processes 1 or fused ribs, as is the case in some Fishes.

The neural spines, as well as the transverse processes, which are as
a rule bifurcated at the base and are present from the second vertebra onwards,
show the greatest variety as regards shape and size, differing in the several
regions of the body. The transverse processes of the single sacral vertebra,
which give attachment to the pelvis, are particularly strongly developed,
especially in the Anura.

Articular processes (zygapophyses) which are usually
present in Fishes that possess a bony vertebral column, are well
developed in all Vertebrates from the Urodela onwards, and
consist of two pairs of projections arising respectively from the
anterior and posterior edges of the base of the neural arch. Their
surfaces are covered with cartilage, and overlap one another from
vertebra to vertebra like tiles on a roof: not unfrequently the
neural spines also articulate with one another, and thus a well-
articulated and mobile chain-like vertebral column results.

From the Amphibia onwards the first vertebra, or so-called
atlas (and this is the only cervical vertebra of Amphibia), becomes
differentiated from the others. In Amphibians it consists of a
simple ring which articulates with the two condyles and the basis
cranii. As numerous researches have shown, however, the first ver-
tebra of Amphibians does not correspond to that (i.e. the atlas) of the
higher Vertebrates, but is much more nearly homologous with the
second cervical vertebra of the latter — the axis (epistropheus).
This is demonstrated by a study of its development, which shows
that the real atlas loses its individuality as a separate mass, and
becomes united with the occipital region of the skull.

Reptilia. — In contrast to the numerous fossil forms, only a
few existing Reptiles, viz., Hatteria and the Geckos (Ascala-
bota) retain throughout life the primitive biconcave character of
their vertebrae, with the notochord expanded intervertebrally.

In all the others, the notochord remains expanded longer in the
vertebral regions than intervertebrally, but in the adult it becomes
entirely aborted and replaced by bony tissue. This stronger and
more solid ossification of the whole skeleton forms a characteristic

1 Traces of the transverse processes are present nearly to the end of the tail.

VERTEBRAL COLUMN. 43

difference between the skeleton of Ichthyopsida on the one hand
and Amniota on the other. As a rule the vertebrae of Reptiles
become definitely articulated with one another, and are of the
proccelous type: the above-named genera, with intervertebral
remains of the notochord, form an exception to this rule, as do
also Crocodiles and Birds, in which intervertebral disks or menisci
exist ; in the latter, however, they are not present in the cervical
region.1

What has been said as to the classification of the vertebrae into
different regions in Amphibia, as well as to the presence of processes,
applies here also, though there are always several cervical vertebrae
instead of a single one : there are also always at least two sacral
vertebrae. An atlas, usually consisting of three pieces, and an
axis, with an odontoid bone, are always well developed.2

The spinous processes of the upper arches vary in size, and
transverse processes arise from the centra themselves or close to
them. Lower arches (chevron bones) are present in the tail in
Lizards, Chelonians, and Crocodiles; and besides these, median or
paired inferior processes of the centra themselves are seen in
many of the vertebrae of Lizards and Snakes, as well as in Birds,
and to some extent in the lumbar region of certain Mammals.

In consequence of the absence of a pectoral arch, the vertebral column
of Snakes and AmphisbEenians, like that of Caecilians, can only be divided into
trunk and caudal vertebrae. The vertebral column of Chelonians deserves
particular notice as a large portion of it becomes anchylosed with the dermal
bones of the carapace, and it is thus rendered immovable in a certain region.

In Snakes and some Lizards (Iguana) extra articular processes (zygosphenes
and zygantra) are developed on the vertebrae. In Lizards small separate ossifi-
cations or subvertebral wedge-bones are often present on the ventral
side of the vertebral column between the centra ; and in the caudal region, an
unossified septum remains in the middle of each centrum, so that the tail
easily breaks off at these points. When this happens the tail grows again, but
proper vertebrae are not formed.

In fossil Eeptiles, which, both as regards size and number of species,
usually surpassed the existing representatives of the group, the sacrum,
which gives attachment to the pelvis, often consists of more than two
vertebrae, the number being four or five (Ornithoscelida).

The following facts will give some idea of the monstrous proportions
of these old genera of Keptiles : — Atlantosaurus immanis, a North
American Dinosaur, reached a length of about 80 feet, and its femur was
8 feet long and 25 inches thick at its proximal end. The transverse dia-
meter of the individual vertebrae amounted to 16 inches, and Apatosaurus
laticollis, found in the same strata, possessed cervical vertebrae which
reached a diameter of 3^ feet.

A knowledge of fossil genera of Reptiles is of the greatest
interest, as we can see, in many groups, important points of con-

1 In Crocodiles the vertebrae are mostly proccelous, an exception being seen in the
two sacrals and first caudal ; and in Cheloniaus there is great variation in the form of
the individual centra of the cervical vertebrae, while the thoracic and lumbar have
flattened faces, and are firmly united together by cartilage.

2 The os odontoideum corresponds morphologically to a part of a centrum of the
atlas.

44 COMPARATIVE ANATOMY.

nection with Birds. At the present day it cannot appear doubtful
to any morphologist that the latter are descended from Reptilian
ancestors.

Birds. — In Archseopteryx, found in the Solenhofen slates of
the Bavarian Jurassic, and already mentioned on p. 22, many of the
special peculiarities of Reptiles and Birds are united. The hinder
extremities are distinctly Reptilian, as is also the tail, which, like
that of a Lizard, is composed of numerous elongated free vertebrae.
A covering of true feathers, on the other hand, characterises it as
a Bird : the biserial arrangement of the tail-feathers is seen in
Fig. 33.

FIG. 33. — TAIL OF Archceoptcryx.

The vertebral column of Birds corresponds with that of Reptiles
not only in its phylogenetic relations, but also ontogenetically. In
both groups the notochord eventually disappears entirely, and the
whole skeleton becomes strongly ossified.1 The pelvis of Bird-
embryos, like that of existing adult Reptiles, is attached to the
vertebral column by two vertebrae only ; during further develop-
ment, however, a number of other vertebras (thoracic, lumbar, and
caudal) become fused with the sacrum (Fig. 34).

A further difference between the vertebral column of Reptiles
and Birds is seen in the character of the caudal region in the latter
group, which always remains apparently rudimentary. In this
peculiarity existing Birds stand in sharp contrast to their Jurassic
ancestors (see above).

It must, however, be well understood that the pygostyle of
Birds may be made up of six or more fused caudal vertebrae, and in

1 Ichthyornis (from the American Cretaceous), as well as Archaeopteryx, pos-
sessed biconcave vertebrae. The same type of vertebra is to be met with in many
fossil (e.g. in the Enaliosauria), and in some existing Reptiles (Ascalabota and
Hatteria), as well as in most of the free caudal vertebras in existing Birds.

VERTEBRAL COLUMN. 45

the sacrum even a greater number may be included (cp. the chapter
on the pelvis, p. 96) : thus in the common Duck (Anas boschas),
seven become united with the pelvis, eight remain free, and the
pygostyle is composed of ten separately ossified and fused segments,
making in all twenty-five vertebras originally present in the caudal
region of this Bird. In Archaeopteryx the pelvis was much shorter
than in existing Birds, and much fewer vertebrae were united with
it. Moreover, in embryos of an Australian parrot (Psittacus undu-
latus) more vertebrae are formed in the embryo than are seen in the
adult. The original type is well preserved to the present day in

FIG. 34. — PELVIS OF OWL (Strixbubo). (Ventral view.)

JV, position of the primary sacral vertebrae : between 2i and //, and behind W, are
seen the secondary sacral vertebrae, fused with the primary ; II, ilium ; Is,
ischium ; P, pubis ; t, foramen between ilium and pubis ; It, last pair
of ribs.

the Ratitae, in which the posterior caudal vertebrae remain free,
instead of uniting to form a pygostyle, and the secondary sacral
vertebrae remain longer distinct. Thus the chasm between
Archseopteryx and existing Birds is in this respect essentially
lessened.

The arches always become united into a single mass with the
corresponding centra, and are no longer separated from them for life
by sutures, as seen e.g. in Crocodiles, and exceptionally in Chelo-
nians. The same may be said of the atlas and axis, in which also no
sutures persist between the different parts. In the cervical region,
where by means of saddle-shaped articulations the vertebrae are
able to move easily on one another, the bifurcated transverse

46 COMPARATIVE ANATOMY.

processes may unite with the corresponding ribs (Fig. 35). In
the thoracic region, more or fewer of the vertebrae usually become
immovably united together.

'J

-» Pt

FIG. 35. — THIRD CERVICAL VERTEBRA OF WOODPECKER (Picus viridis). (Viewed

anteriorly.)

Sa, articular surface of centrum ; Ob, upper arch ; Pa, articular process ; Pt, Pt, the
two bars of the transverse process, shown on one side anchylosed with the cervical
rib (E) ; Ft, transverse (vertebrarterial) foramen ; Psi, haemal spine.

Mammalia. — No direct connection exists between the
vertebraB of Reptiles and Birds and those of Mammals. The
notochord persists longer intervertebrally than vertebrally, but it
disappears entirely by the time the adult condition is reached. A
jelly-like pulpy mass, the nucleus pulposus, persists, however,
throughout life in the centrSf of the fibro-cartilaginous menisci
which are developed between the vertebrae. Articulations between
the centra are never formed, but as in Amphibians, Reptiles, and
Birds, well-developed articular processes are present, arising from
the neural arches.1 The cervical region is usually the most
moveable, and the centra may be so much hollowed out in this
region as to give them an opisthoccelous character (e.g. Ungulata).
In some cases, on the other hand, the cervical vertebras may become
firmly fused together (Cetacea). The centra are provided with epi-
physes, except in all but those of the caudal region of Monotremes
and in Sirenia (?).

The atlas and axis essentially resemble those of Birds, though
the differentiation of the vertebral column into regions, characterised
by difference of form, is much more sharply marked than in any
other Vertebrates.

In long-necked Ungulates (Horse, Camel, Ox) the neural spines of the
anterior thoracic vertebrae are greatly developed, and corresponding with this,
a strong cervical ligament (ligamentum nuchse) is particularly well developed
to support the weight of the head. This is also true of antler-bearing animals
and of the Gorilla. There are as a rule 7 cervical vertebrae ; Bradypus, how-
ever, possesses 8 — 9, and Tamandua bivittata 8, while in Manatus and Choloepus
there are only 6.

The transverse processes of the cervical vertebra) usually unite with the
rudimentary ribs, as in Birds.

1 In certain Edentata (e.g. Myrmecophaga) extra articular processes are present
besides the ordinary zygapophyses on the posterior thoracic and lumbar vertebra.

tt,c
A ^

( UN3

VERTEBRAL COLUMN. , 47

X^l

In Mammals, as in Reptiles and Birds, two primary sacral
vertebra are present, but as a rule a few caudal become later
included in the sacrum, and are usually more or less closely united
with it. In Anthropoids, as in Man, the first sacral vertebra is
plainly marked off from the last lumbar by the appearance of the
so-called promontory.

The caudal vertebrae vary extremely in their development, and
excepting in most long-tailed Mammals — more particularly the
Sirenia and Cetacea — no longer develop lower arches. When
present these " chevron bones " are intervertebral in position.

In the higher Primates the tail forms a stump-like appendage,
the coccyx consisting of few (3 — 6) vertebrae. In the embryo,
however, the .notochord extends beyond the point corresponding
to the apex of the coccyx, and thus a longer caudal region must
formerly have been present.

The greatest number of caudal vertebrae is found in Microgale
longicauda (48), Manis macrura (46 — 49), Paradoxurus (about 36),
and in certain Monkeys (Semnopithecus, Ateles, 32 — 33).

In human embryos of 9 — 10 millimetres long (5th week) 38 vertebrse are
present, and these all consist of a cartilage-like tissue with the exception of
the two posterior caudals. In embryos 12mm. long (6th week) the three
posterior caudal vertebrae (36th, 37th, and 38th) fuse together, and the 35th
also loses its sharp contour. In embryos of 19mm. there are only 34 ver-
tebrse, the number present in the adult. In the stage with. 38 vertebrse, the
spinal cord and notochord extend to the extreme apex of the tail, almost
reaching to the skin, but a reduction of these parts takes place later.

BIBLIOGRAPHY.

AGASSIZ, L. — Beck, sur Us poissons fossiles. Neuchatel, 1833 to 1843.

C ARTIER, 0. — Beitr. zur Entw.-Geschichte der Wirbelsdule. Zeitschr. f. wiss. Zool.

Bd. XXV. Suppl. 1875.
DAMES, W., and KAISER, E. — Palceontol. Abhandlungen, Bd. II. Heft 3. Berlin,

1884. (Monograph on Archceopteryx.)

FLOWER, W. H. — Osteology of the Mammalia. London, 1885.
FOL, H. — Sur la queue de Vembryonhumain. Comptes rendus, 1885.
GEGENBAUR, C. — Unters. z. vergl. Anatomie der Wirbelsdule der Amphibien und

Beptilien. Leipzig, 1862. Beitr. zur Kcnntniss des Beckens der Vogel, &c.

Jenaische Zeitschr. Bd. VI.
GERLACH, L. — Ein Fall von SchwanzUldung lei einem menschl. Embryo. Morphol.

Jahrb. Bd. VI.
GOTTE, A. — Beitr. zur vergl. Morphologic des Skeletsystems der Wirbelthiere. Arch.

f. mikr. Anatomie, Bd. XV. 1878.
GUKTHER, A. — Description of Ceratodus Forsteri. Phil. Trans, of the Royal Society.

London, 1871.

HASSE, C.—Das naturl. System, der Elasmobranchier, &c. Jena, 1879-82.
HOFFMANN, C. K. — Beitr. z. vergl. Anatomie der Wirbelthiere. Niederl. Arch. /.

Zool. Bd. IV.
MARSH, 0. C. — Odontornithcs, a Monograph on the Extinct Toothed Birds of North

America. Washington, 1880.
MAYER, P. — Die unpaaren Flossen der Selachier. Mittheilungen der Zoolog. Station

zu Neapel. Bd. VI. 1885.
MIVART. ST. G. — On Various Axial Skeletons. Proc. and Trans. Zool. Soc. 1865,

1869, 1873.
EOSEXBERG, C.—Ueber die Occipital-region des Selachier -schddels. Dorpat, 1884.

Uebcr die Entwicklung der Wirbelsdule und das Centrale carpi des Menschen.

Morphol. Jahrb. Bd. I. 1876.

48 COMPARATIVE ANATOMY.

SAGEMEHL, M. — Beitrage zur Anat. der Fische. Morphol. Jahrb. IX., 1884.

WIEDERSHEIM, R. — Salamandrina perspicillata. Versuch. einervergl. Anatomie der
Salamandrinen. Genua, 1875. (Annali del Museo civico. Vol. VII.) Die
Anatomi& der Gymnophionen. Jena, 1879. Das STcelet und Nervensystem von
Lepidosiren annectens. Morphol. Studien, Heft I. Jena, 1880.

II. RIBS.

The ribs, standing in the closest connection with the nryocom-
mata (myotomes) of the great lateral muscles of the body, are
arranged segmentally, and onto- as well as phylogenetically, pass
through a membranous, a cartilaginous, and a bony stage. Their
development, which as a rule takes place first in the anterior part
of the body and then extends gradually backwards, is usually
entirely independent of the vertebral column, their connection with
it being a secondary one.1 **

Fishes and Dipnoi. — The cartilaginous or bony ribs are
attached to the basal processes already described, and extend
ventro-laterally from the corresponding vertebrae. The
ribs of Fishes show a very primitive condition, usually extending
along the whole length of the vertebral column (Lophobranchii,
Spatularia). In rare cases they are absent, though many Fishes
only possess rudimentary ribs (many bony Fishes, Elasmobranchs).

In others, as in numerous Teleosteans and Ganoids, they are
very well developed, and encircle the body-<avity like the hoops of
a cask ; but they never unite together in tte mid-ventral line.

The relations of the anterior portion of the vertebral column to the auditory
organ in certain Teleosteans will be described later (see p. 207).

Amphibia. — In the ribs of Amphibia there are evident
signs of degeneration ; as a rule they are confined to the region of
the trunk, or at most they extend in certain Urodeles in a very
rudimentary form as far as the first two caudal vertebrae ; in other
cases, as in the tailless Batrachia, they are so remarkably short
that they can no longer be said to encircle the body-cavity. In
many Anura the ribs are not distinctly articulated, as they become
fused with the broad transverse processes (Fig. 32, jR).

The ribs of Urodeles are forked at their proximal ends, and articulate with
the bifurcated transverse processes of the vertebrae. The ventral limb, only of
the transverse process corresponds to the basal process of Ganoids ; the dorsal
one is to be looked upon as a neoinorph. The bifurcated ends of the ribs in
Reptiles and Birds as well as the double articular facets of the ribs of Mammals
are to be explained in the same manner.

1 The ribs of Ganoids, and possibly also those of Dipnoi, seem to follow an
entirely different plan of development, in that they become segmented off from the
lower arches, as mentioned in the chapter on the vertebral column.

RIBS.

49

Excepting the first, all the trunk vertebrae are usually provided
with ribs in Urodeles : ribless (lumbar) vertebrae are met with very
rarely (Spelerpes).

Reptiles, Birds, and Mammals.— In these, well-developed
ribs are always present, and more or fewer of them almost always
unite ventrally with a breast-bone or sternum. Those which
unite with the sternum are sometimes spoken of as "true," the
others as " false " ribs.

FIG. 36. — SKELETON OF THE TRUNK or A FALCON.

S, scapula ; G, glenoid cavity for humerus ; Ca, coracoid, which articulates with the
sternum (St) at t ; Fu(Cl), furcula (clavicles) ; Cr, keel of sternum ; V, vertebral,
and Sp, sternal, portion of rib ; Un, uncinate process.

The ribs of Snakes show the least amount of differentiation
for, without giving rise to a sternum, they extend along the whole
trunk from the third cervical vertebra to the anus, and retain
throughout a similar form and size. In Lizards, where a dorsal
bony and a ventral cartilaginous portion can be distinguished, three

50 COMPARATIVE ANATOMY.

or four ribs reach to the sternum,1 and are not always completely
segmented off from it.

The ribs of Birds exhibit a much more marked segmentation
into vertebral and sternal portions, and this evidently stands in
relation to respiration; they moreover develop so-called unci-
nate processes (Fig. 36, Uri). In this latter, as in many other
points, they show a relation to certain Reptiles (viz. Hatteria and
Crocodiles).

The ribs of Archseopteryx are of special interest, as they are more simi-
lar to those of Reptiles than to those of Birds, though they do not closely
resemble the former. Their structure is delicate, their ends are pointed,
and no uncinate processes have been observed : in transverse section they are
ellipsoidal, and not flattened like those of Birds. Whether a connection with
a breast-bone existed is not certainly proved, as nothing is known of a sternum
or of sternal ribs. The breast-bone must at any rate have been very small, as
the " abdominal ribs " extend far forwards ; it was probably provided with a
keel, for the quills of the wing are well developed.

FIG. 37. — COSTAL ARCH OF MAN.

WK, centrum of vertebra ; Pt, transverse process ; Ps, neural spine ; Cp, body of
rib ; Ca, capitulum ; Co, neck ; Tb, tuberculum ; Kn, cartilaginous (sternal)
rib ; St, sternum.

It has already been mentioned that the cervical ribs and transverse
processes may become united together in representatives of all the Amniota,
and the fusion between the ribs and dermal plates in Chelonians may be
here noted.

In the true ribs of Mammals, and especially in those of Man,
a capitulum, a neck, a tuberculum, and a body may be distinguished
(Fig. 37). The capitulum articulates with the centrum, and the
tuberculum with the transverse process of the vertebra. v The
number of ribs which reach the sternum varies considerably.

1 In Crocodiles, eight to nine ribs reach the breast-bone ; in Birds, three to
eight. An ossification of the inscriptiones tendineae of the rectus abdominis takes
place in Crocodiles and Hatteria, and similar structures (so-called "abdominal
ribs") occur in numerous fossil Reptiles (Nothosaurus of the Trias, Enaliosaurus of
the Jurassic, Pterodactylus, &c.). Archseopteryx also possessed twelve to thirteen
veil-developed "abdominal ribs." These must not be confounded with the remains
of true abdominal ribs, which persist without the corresponding vertebral portion
in the Chameleon and certain Birds (W. K. Parker).

STERNUM. 51

Development teaches us that in the cervical, lumbar, and sacral
regions, where no ribs are apparent in the adult, they are present in the
embryo, even in Man, and this points back to primitive conditions.
The rudimentary character and variety in size of the eleventh and
twelfth ribs of Man shows that they are gradually disappearing
(cp. p. 53) : a gradual shortening of the thoracic portion of the
vertebral column and a corresponding lengthening of the cervical
and lumbar regions is also taking place in Mammals generally, and
thus the following general rule may be stated: — The reduction
in the number of ribs is correlated with a higher stage
in development of the Vertebrate body.

It has already been mentioned that sacral ribs are developed,
and it is only necessary to add that this statement holds good for
all Vertebrates. In other words: the pelvis is always sup-
ported by sacral ribs, whether these remain differentiated
throughout life (Urodeles), or whether they fuse with the corre-
sponding transverse processes of the sacral vertebrae (Amniota).

111. STEENUM.

Never present in Fishes, the sternum appears for the first time
in Amphibians in the form of a small variously -shaped plate of
cartilage situated in the middle line of the chest (Figs. 38 and 74, St).
It originates from two cartilaginous rods lying in the inscriptiones
tendineaB of the thoracic region, with which the coracoids, or rather
the epicoracoid plates of the pectoral arch come into more or less
close connection (Fig. 38, St, Co1). In-many tailless Batrachians (e.g.
Rana), the ventral portion of the pectoral arch is continued forwards
in the middle line as a slender bone tipped with cartilage — the
" omo-sternum " (Parker) (Fig. 38, .#/?). The phylogenesis of the
Amphibian sternum is still entirely unknown, and it is doubtful
whether it ought to be placed in the same category with the simi-
larly named structure in the Amniota. In the latter, the sternum
has a costal origin, and is due to a number of ribs on either side of
the middle line running together to form a continuous cartilaginous
tract. An unpaired cartilaginous sternal plate is formed by the
tract of either side becoming more or less completely fused with its
fellow, and from this plate the ribs become secondarily segmented
off by the formation of true articulations. Later it may become
calcified (Reptiles), or converted into true bone (Birds, Mammals).
In Reptiles, Birds, and Monotremes the coracoids (Fig. 75, Go, Co1}
always come into direct connection with the upper or the lateral
edges of the sternum (comp. Fig. 36, St, and Ca, and Figs. 38 and
39, St, Co).

The sternum is greatly developed in Birds, and consists of a
broad plate, usually (" Carinate Birds") provided with a projecting
keel (crista sterui), which forms a point of origin for the wing-

E 2

52

COMPARATIVE ANATOMY.

FIG. 38. — VENTRAL PORTION OF THE PECTORAL ARCH OF Rana esculcnta.

Sty bony, and Kn, cartilaginous sternum (xiphisternum) ; S, scapula ; KC, carti-
laginous portion between the latter and the clavicle (Cl) ; Co, coracoid ; Co1,
epicoracoid ; m, line of junction between the two "epicoracoids ; G, glenoid
cavity for the humerus; Fe, fenestra between the coracoid and clavicle; Ep,
omosternum.

FIG. 39. — PECTORAL ARCH AND STERNUM OF A GECKO
(Hemidadylus verrucosus}.

St, sternum ; R, ribs ; Si, cartilaginous cornua to which the last pair of ribs is
attached ; SS, suprascapula ; S, scapula ; Co, coracoid ; Co1, cartilaginous
epicoracoid ; Ep, interclavicle ; a, b, c, membranous fenestrse in the coracoid ;
Cl, clavicle ; G, glenoid cavity for the humerus.

STERNUM.

53

muscles. In contrast to these, the Ratitas (" Cursorial Birds ") are
characterised by a broad, slightly-arched, shield-like sternum without
a keel.

In both cases the sternum arises in two bauds connected with ribs, a greater
number often taking part in its formation than are present in the adult.
According to Lindsay, the two parts of this costal sternum, corresponding
to the breast-bone of Reptiles and Mammals, become connected by a portion,
the metasternum, which gives rise to the median and posterior portion of
the sternum of the adult. In the Ratitae, the metasternum, which probably
arises from the fused edges of the costal portion, remains partly cartilaginous
in the adult. In the Carinatae, it gives rise to a median ventral outgrowth,
the keel. This generally arises at the time of the fusion of the two halves,
but in some cases there appears to be a tendency for it to become differentiated
from the rest of the sternum. It is either- ossified by the fusion of a pair of
lateral bony centres, or else by means of a separate endosteal (see p. 63) centre.

Thus the keel of the Bird's sternum is probably of late phylogenetic
development, arising in correlation with the large development of the
pectoral muscles, and having no relationship to the interclavicle of Reptiles,
as is often asserted.

A far greater number of ribs are as a rule concerned in the
formation of the breast-bone of Mammals than is the case in

FIG. 40.— A, STERNUM OF Fox ; B, OF WALKUS ; AND C, OF MAN.
Mb, manubrium ; C, body ; Pe, xiphoid process ; R, ribs.

Reptiles and Birds. Consisting at first of a simple cartilaginous
plate, it later becomes segmented into definite bony regions, the
number of which originally corresponds to the affixed ribs (Fig. 40,
A, B). But in other cases, as, for instance, in Primates, the
individual bony segments usually run together to form a long plate
(corpus sterni), of which the proximal end becomes differen-
tiated into the so-called manubrium, and the distal end into the
xiphoid process (processus ensiformis). The latter (Fig. 40,
C, Pe] owes its origin in the embryo to the ventral fusion of a true
pair of ribs, arising independently or as a direct continuation of
the primary sternal tracts, from which it later becomes segmented

54 COMPARATIVE ANATOMY.

off, and, like the manubrium, ossified from a special centre. Thus,
in Man, an embryonic stage exists in which the eighth pair of ribs
are connected with the xiphoid process.

/ .

BIBLIOGRAPHY.

GOTTE, A. — Beitrdge zur vergl. Morphologic dcs Skeletsystcms der Wirbelthierc.
Arch.f. mikr. Anat. Bd. XIV.

HAS-SE, C., and BORN, G. — Bemerkungen ub. d. Morphologic d. Rippen. Zool. Anz.
1879.

HOFFMANN, C. K. — Beitrdge zur vergl. Anatomic der Wirbelthiere. Niederl. Arch,
f. Zoologie, Bd. IV., V.

LINDSAY, BEATRICE. — On the Avian Sternum. Proc. Zool. Soc. 1885, p. 684.

PARKER, W. K. — A Monograph on the Structure and Development of the Shoulder-
Girdle and the Sternum. Ray Soc. 1867.

RUGE, G. — Untcrsuch. ub. Entwick. am Brustbeine d. Menschen. Morphol. Jahrbuch,
Vol. VI. 1880.

IV. THE SKULL.

Theory of the Segmentation of the Skull.

In the skull, as in the vertebral column, three stages may
be distinguished ontogenetically as well as phylogenetically,
viz., a membranous, a cartilaginous, and a bony stage.
There is thus an important correspondence between these two
parts of the vertebral axis, which is considerably increased by
the following facts.

The notochord always extends for a certain distance into the
base of the skull, so that the latter has a similar origin to, and is
developed as a direct continuation of, the vertebral axis. Still
more important is the fact that a series of mesoblastic somites
(protovertebras) give origin to the greater part of the head as
well as to the main dorsal section of the trunk in the embryo,
so that both show a metameric mode of origin. Out of these
somites, each of which encloses a cavity originating from the
ccelome, are formed the muscles of this region as well as the
foundation of the proper cranial capsule. As development
advances, the original segmented arrangement gradually disappears,
and thus the cranium, especially in the lowest Vertebrates, as, for
instance, in Cartilaginous Fishes, forms a continuous structure.

A cartilaginous system of arches, which often later become
ossified, arises in serial order on the ventral side of the brain-case ;
these encircle the anterior part of the alimentary tract like hoops,
and are distinguished from the cranial region as the visceral
skeleton. The latter stands in important relation to gill-breathing,
inasmuch as each consecutive pair of arches enclose a passage
(gill-slit) communicating between the pharynx and the exterior;
this is lined by endoderm, and through it the water passes.
The foremost visceral arch bounds the aperture of the mouth, and

THE SKULL.

55

forming thus a firm support for it, gives rise to the skeleton of
the jaws, as well as, in higher types, to the main part of the facial
skeleton. The arches lying posterior to this function primarily as
gill-supports.

FIG. 41. — DIAGRAM SHOWING THE PRIMITIVE METAMERIC CONDITION
OF THE HEAD.

E, E, epiblast, which at N is invaginated to form the primitive olfactory pit, the
epithelium of which is supplied by the olfactory nerve (Olf) ; M, oral involu-
tion ; /, first somite, from which arise the superior, internal, and inferior
rectus, and inferior oblique muscles ; II, second somite, from which the
superior oblique muscle originates ; HI, third somite, which gives rise to the
external rectus ; IV, V, VI, fourth, fifth, and sixth somites : only the sixth
gives rise to muscle-rudiments ; VII, VIII, IX, seventh, eighth, and ninth
somites, from which the muscles extending from the skull to the pectoral arch
arise : the anterior part of the sterno-hyoid is also formed in this region ; a and
b indicate the first somites of the trunk ; ///, oculomotor, IV, trochlear,
VI, abducent, and XII1 to XIIs, hypoglossal nerves. All the above-named
nerves correspond to ventral roots of the nerves belonging to the head-somites,
/, II, III, VII, VIII, and IX. The ventral nerves belonging to somites
IV, V, and VI are not known : they probably lie in the territory of the tri-
geminal. RpV, ramus ophthalmicus profundus of the trigeminal, the dorsal
nerve of the first somite; V, the rest of the trigeminal, the dorsal nerve of the
second somite, supplying the maxillary and mandibular regions ; VII, VIII,
the acustico-facialis, the dorsal nerve of the third and fourth somites, supplying
the first primitive gill-cleft (spiracle) (1) ; IX, glossopharyngeal, the dorsal
nerve of the fifth somite, supplying the second gill-cleft (2) ; X1 to X4, vagus, the
dorsal nerves of the sixth to the ninth somites, supplying the third to the sixth
gill-clefts (3 to 6) ; £yj, Sv\ ventral, and Sdl, ScP, dorsal, roots of the two first
spinal nerves; m, first (mandibular) visceral arch; h, second (hyoiu) arch;
6l to b5, the five branchial arches ; E1, JK'2, first and second ribs.

On viewing the serial arrangement of the visceral arches, one
might be tempted to explain them as being homodynamous with
ribs, and to consider this, as well as the corresponding distribu-
tion of the branchial nerves as a further support for supposing

56 COMPARATIVE ANATOMY.

for the head a metameric origin of the same nature as that of
the body. This, however, is not admissible, inasmuch as the above-
described segmentation of the visceral section of the skull by
the formation of gill-slits does not correspond to a segmentation
of the same nature as that seen in the body, but arises quite
independently. To express it briefly — Metamerism does not
correspond to branchiomerism. It follows that a direct
parallelism of the branchial nerves to the intercostal nerves — which
correspond with trunk-metameres— does not exist, and the attempt
to solve the problem of the Vertebrate skull by indirect methods,
i.e. those of Comparative Anatomy, must lead to crude theories and
false conclusions.

The result of the above considerations may be shortly expressed
as follows : —

1. The Vertebrate skull is not a structure sui generis, but has
been derived by a metamorphosis of the most anterior section of
the skeleton of the body.

2. The proof of this lies in the common origin of both the
cranial and vertebral skeleton out of the protovertebra3 (somites,
metameres).

3. The skull is divided into two main sections, a dorsal and a
ventral. The former encloses the brain, and is spoken of as the
cranium, while the latter lies in the region of the fore-part of the
alimentary tube, has primitively to do with branchial respiration,
and is called the visceral skeleton.

4. The cranial section alone is to be looked upon as made up of
a series of mesoblastic somites : the segmentation of the visceral
skeleton must be regarded as a secondary acquisition, for the gill-
arches are developed as secondary supports for the hypoblastic
gill-clefts.

5. The attempt to explain the adult skull as a series ot
vertebrae1 fails completely; it is a question of protovertebrse
(somites) only, and thus is one that can only be solved along the
lines of Embryology, and not those of Comparative Anatomy.

6. The number of mesoblastic somites concerned in the
formation of the skull maybe fixed at nine,2 according to researches
up to the present time on Cyclostomes, Elasmobranchs, and Am-
phibians. In no case are there fewer, — in many instances possibly
more.

1 Rosenberg has, however, shown that in Carcharias glaueus, but apparently
not in other Selachians, the portion of the cranium lying between the exit of the
vagus and the vertebral .column is clearly composed of three vertebrae, which gra-
dually fuse with, and constitute a part of, the occipital region of the skull (Gadow
finds four vertebrae in embryos of Carcharias which thus become modified). It follows
that the cartilaginous cranium is not completely homologous throughout the Verte-
brata ; the skull of Carcharias corrresponds with that of Scyllium, for instance, plus
certain of the anterior vertebrae.

Sagemehl has found a somewhat similar modification in Ganoids.

2 Beard, in a recent paper, increases the number of segments in the head in
Sharks to eleven.

THE SKULL.

a. Brain-Case (Cranium).

The first cartilaginous rudiments appear in the primitively
membranous skull tube in the form of a pair of rods, the
trabeculge cranii. These lie along the base of the brain, their
posterior part embracing th'e notochord, and the}7 thus are divisible
into prochordal (anterior) and parachordal (posterior) regions
(Fig. 42, Tr). The parachordal tract may extend further along the
notochord as a direct backward growth of the trabecula?, or as one
or two separate cartilaginous tracts (Fig. 42, PE). The para-
chordals soon unite to form a basilar plate, which grows round
the notochord dorsally and ventrally, and thus early forms a solid
support for the brain. The slender trabeculas project forwards
and enclose a space, which may be spoken of as the primitive
pituitary space (Fig. 42, PE). (':Q

Fia. 42. — FIRST CARTILAGINOUS RUDIMENTS or THE SKULL.

C, notocho^d ; PE, separate paracliordal elements ; Tr, trabeculse cranii ; PR,
pituitary space ; N, A, 0, the three sense-capsules (olfactory, optic, and
auditory).

These structures may become further developed in many
different ways in the various Vertebrate groups : either the trabeculse
become completely united with one another in the median line
(Fig. 43, A, Tr), or the connective- tissue of the oral mucous mem-
brane becomes ossified to form a parasphenoid (Fig. 43, B, Ps).
In other cases, the trabeculse may become compressed and partly
aborted owing to the great development of the eyes ; this obtains
in certain Teleosteans and Reptiles and in all Birds, where a fibro-
cartilaginous interorbital septum appears in their place (Fig. 43,
C, Tr, />Sf).

COMPAPxATIVE ANATOMY.

In most cases a median cartilaginous bar (intertrabecula) is formed
between the trabeculse in front, fusing with them, and forming the ethmo-nasal
septum. It occasionally projects forwards to form a rostrum.

We must now follow further the processes of growth, taking as
a foundation the primary condition of things described above, in
which the trabeculse have united together in the middle line. The

FIG. 43. — DIAGRAMMATIC TRANSVERSE SECTIONS, OF THE HEAD IN EMBRYO —
(A) STURGEONS, ELASMOBRANCHS, ANURA, AND MAMMALS ; (B) URODELES AND
SNAKES ; AND (C) CERTAIN TELEOSTEANS, LIZARDS, CROCODILES, CHELONIANS,
AND BIRDS.

Tr, trabeculse cranii ; G, brain ; A, eyes ; Ps, parasphenoid ; IS, interorbital septum ;
F, frontal ; Olf, olfactory nerve.

cartilaginous basal plate now comes into relations with the olfactory,
optic, and auditory organs by the formation of processes which
serve — particularly in the case of the olfactory and auditory

FIG. 44. — SECOND STAGE IN THE DEVELOPMENT or THE PRIMORDIAL SKULL.

C, notochord ; B, basilar plate ; Tr, trabecula, which has united with its fellow in
front of the pituitary space to form the ethmonasal septum (S) ; Ct, cornu
trabeculse, and AF, antorbital process, which supports the olfactory organ (NK)
in front and behind ; 01, foramina for exit of the olfactory nerves from the
cranium ; PF, postorbital process of trabecula ; NK, nasal capsule ; A, eye ;
0, auditory capsule.

apparatus — to connect the skull with the cartilaginous sense-cap-
sules, and thus to act as supports for them. Thus an olfactory,
an orbital, and an auditory region are early differentiated.

THE SKULL. 5g

While the first and the last of these are gradually surrounded
by cartilage, and, especially in higher types, more and more drawn
in to the skull proper, the lateral walls of the basal plate become
raised up, and begin to grow round the brain on both sides,
eventually extending even to the dorsal region. Thus a con-
tinuous cartilaginous capsule is formed, such as persists through-
out life in Elasmobranchs for example. But in by far the greater
number of Vertebrates, the cartilage does not play so great a part,
and is, as a rule, confined to the base and lower parts of the sides
of the skull and to the sense-capsules, except in the occipital
region, where it always extends over the brain. The rest of the
skull, more particularly the roof, becomes directly converted from
membrane into bone. Thus it may be stated generally, that the
higher the systematic position of the animal, the less
extensive are the cartilaginous constituents and the
more important the bony.

I. The Visceral Skeleton.

The skeletal parts of the visceral arches, always formed in
hyaline cartilage, encircle the anterior section of the alimentary
canal, lying embedded in the inner part of the walls of the throat

FIG. 45. — DIAGRAMMATIC TRANSVERSE SECTION OF THE THIRD STAGE IN THE
DEVELOPMENT OF THE PRIMORDIAL SKULL.

C, notochord ; Tr, trabeculre, which enclose the brain (C) ventrally and laterally ;
0, auditory capsule ; RH, the cavity of the pharynx, enclosed by the visceral
skeleton ; 1 to 4, the individual elements composing each visceral arch, which
latter is united with its fellow by a basal piece (Cp).

(Figs. 45 and 46, B, B). Always present in a greater number (up to
as many as nine) in gill-breathing animals than in higher types, they
gradually become reduced, so that in the Amniota the remains of
the three or four anterior at most are seen : they further undergo
a change of function, for all but the first of these take on definite
relations to the auditory organ and larynx.

The most anterior arch, serving as a support for the walls of
the mouth and receiving its nerve supply from the trigeminal,

60

COMPARATIVE ANATOMY.

arises first, and is distinguished from the other or post-oral arches
as the mandibular arch. The post-oral arches only, function as gill-
bearers in the adult fish : even the first of these, the hyoid, which

FIG. 46. — DIAGRAM SHOWING THE RELATIONS OF THE EMBRYONIC VISCERAL

SKELETON.

N, nasal capsule ; A, eye ; 0, auditory capsule ; Tr, trabeculee, which, from being

and basi-branchials.

is ^ supplied by the facial nerve, becomes modified from those
lying < behind it: the latter, or branchial arches proper, are
supplied by the glossopharyngeal and vagus. Nevertheless,

FIG. 47. — SEMI-DIAGRAMMATIC FIGURE OF AN ELASMOBRA-NCH SKULL, SHOWING
THE RELATIONS OF THE SEGMENTAL CRANIAL NERVES.

N, nasal capsule ; A, eye ; 0,. auditory capsule ; Tr, trabecula; Q and PQ, quadrate
and palatopterygoid, which arc bound to the trabecula by ligament at t ; M,
Meckel's cartilage ; L, Ll, labial cartilages ; H, hyomandibular ; K, hyoid arch ;
a to e, branchial arches, between which the gill-clefts (7 to V] are seen ; S,
spiracle ; 0, notochord ; W, W, vertebrae ; V, trigeminal nerve, and 1, 2, 3, its
three main divisions : Rpl, its palatine branch ; VII, facial nerve ; Ep, its
palatine branch ; IX, glossopharyngeal ; X, vagus.

everything goes to prove that formerly a time existed in which all
the visceral arches must have borne gills, and in the embryos of
Elasmobranchs they even still do so.

Originally unsegmented in most cases, the individual arches
may become broken up into different (as many as four) pieces, of

THE SKULL.

61

which the uppermost becomes inserted under the base of the
skull, while the lowermost comes to lie ventrally, and is connected
with its fellow by a median piece, or basi-branchial (Fig. 45, 1
to 4, Cp).

The two anterior visceral arches also undergo a segmentation.
Thus the first becomes divided into a short proximal piece, the
quadrate, and into a long distal Meckel's cartilage (Fig. 46,
Qu, M). The quadrate grows out anteriorly into a process, the
palatoquadrate or palatopterygoid (Figs. 47 and 48, A to C,
PQ\ which becomes fixed to the base of the skull, and thus forms a
sort of primary upper jaw.

The quadrate, which serves as a support (suspensorium) for the
lower jaw, either remains separated from the skull by an articu-
lation, that is, is only united to it by connective-tissue, or it forms
one mass with it.

The hyoid arch, — which always stands in close relations to the
mandibular, and may also take part in its suspensorial apparatus,1

FIG. 48. — SEMI-DIAGRAMMATIC FIGURES OF THE SUSPENSORIAL APPARATUS IN
VARIOUS VERTEBRATES. (Mainly after Gegenbaur.) A, NOTIDANUS ; B, OTHER
ELASMOBRANCHS ; C, TORPEDO ; D, TELEOSTBANS ; E, AMPHIBIANS, REPTILES,
AND BIRDS ; F, MAMMALS.

M, Meckel'a cartilage ; PQ, palate-quadrate ; Hm, hyomandibular ; hy, hyoid
arch ; Sy, symplectic ; Q (in DandE), quadrate ; Q (in F), articular (malleus),
and Q\ quadrate (incus), both of which lie in the tympanic cavity (P) '; hl,
styloid process, connected with the anterior (lesser) corner of the hyoid (h) by
the stylohyoid ligament, indicated by the dotted lines ; b, the posterior (greater)
cornu, and c, the body of the hyoid in Mammals.

— becomes divided, as do the true branchial arches, into a great
number of pieces (Fishes), which are distinguished from above
downwards as hyomandibular, symplectic, and hyoid in a
narrower sense (Fig. 48, A to D, Em, Syt hy). In the mid-ventral
line there is a basi-hyal connecting the arch of each side, and this
becomes ossified, and is embedded in the tongue as the entoglossal
or glossohyal.

c. The Bones of the Skull.

Two kinds of bone, genetically distinct, may be distinguished,
one arising within cartilage, the other in connective-tissue, in those

^ l According to Dohrn, Meckel's cartilage and the palatopterygoid are separate in
origin, as are also the hyomandibular and hyoid proper, and thus the so-called
mandibular and hyoid arches each represents two.

62 COMPARATIVE ANATOMY.

regions of the skull which are only membranous.1 Again in other
cases, true bones are not formed at all, there being only a calcareous
incrustation of the cartilage (calcined cartilage).

The bones arising in the membranous regions of the skull come
under the category of the dermal skeleton and, as already men-
tioned with regard to the latter, are to be looked upon as originat-
ing genetically (Amphibia, Fishes) or phylogenetically (Amniota)
in connection with tooth-structures. In this manner, the bones
of the mouth-cavity of Fishes and Amphibians, for instance, still
arise, and this will not surprise us when we remember that the
epithelium of the oral cavity is formed by invagination of the
outer skin.

This mode of origin of the first skull-bones appears
to be the oldest or most primitive, and it is mos«t apparent
in the lower Vertebrates (Fishes). This hoMs good also for
those cases in which bones are formed merely by deposition of cal-
careous matter directly in the connective-tissue layer, without
giving rise to tooth-structures (e.g. in all investing bones, — those,
for instance, of the roof of the skull of all Vertebrates from the
Amphibia to the Mammalia) : this may be looked upon as an
abbreviated development.

The phylogenetically younger endochondral bones appear first in the
Anura and onwards, though in Urodeles only the perichondral mode of. origin
is seen, and even in Anura this mode occurs largely. Not unfrequently,
endochondral bones and investing bones come into apposition, and fuse together.
Thus it may happen that in the course of generations an investing bone may
take the place of a cartilage bone, and the formation of cartilage be entirely
suppressed, and not repeated again ontogenetically.

The following lists give a summary of the most important
bones according to their different relations to the skull.

I. Bones of the Mouth Cavity (partly lying within it,
partly bounding it on the outer side).

1. Parasphenoid.

2. Vonaer.

3. Premaxilla.

4. Maxilla.

INVESTING
BONES.

5. Jugal.

6. Quadratojugal (in part).

7. Dentary.

8. Splenial.

9. Angular.

10. Supra-angular.

11. Coronoid.

12. Palatine.

13. Pterygoid.

l The different varieties of ossification may be conveniently classified as follows : —
I- Membrane Bones. (&) Dermostoses — ossifications of the derma; (b)
Paro«toses — ossifications of the looser subcutaneous tissue; (c) Ectostoses — ossifi-
cations of the inner layer of the fibrous investment (perichondrium) of a tract of
cartilage : these may extend into the latter, replacing it, and thus give rise to

II. Cartilage Bones, which may, however, also be formed independently, a
bony deposit taking place within the cartilage itself (endostosis).

THE SKULL. 63

II. Bones of the Outer Surface (enumerated from before
backwards).

1. Premaxilla.

2. Maxilla.

3. Nasal.

4. Lacrymal.

5. Frontal.

INVESTING (
BONES.

6. Prelrontal (of Reptiles).

7. Postfrontal or postorbital.

8. Supraorbital.

9. Parietal.

10. Temporal or squamosal.

11. Supraoccipital (in part).

III. Cartilage Bones.

2 Baloid lPreSe^ °& in Amniota (formlng the base Of the

3. Presphenoid J sku11'"

4. Exoccipital (supraoccipital, in part).

5. Pro-, epi-, and opisthotic, also sphenotic and pterotic (in Teleostei),

(forming the bony auditory capsule).

„' . r. l °" | sphenoid, developed in the trabecular region.

8. Ethmoid, together with the rest of the cartilaginous skeleton of the

nose (septum, turbinals, &c.).

9. Quadrate.

10. Articular.

11. Visceral skeleton (in part).

ANATOMY OF THE SKULL.
SPECIAL PART.

A. Fishes.

The skulls of Fishes vary so greatly in their details that only a
general outline can be given here.

In the suctorial Fishes, or Cycles tomes, the skull is deve-
loped essentially in the manner described already for all Verte-
brates. Later, however, the form of the skull shows so many
peculiarities, probably in consequence of the suctorial (Petromyzon)
or parasitic (Myxine) mode of life of these animals, that it becomes
quite abnormal. The most important peculiarity is the absence of
proper jaws such as those of other Vertebrates ; for this reason
these Fishes are called Cyclostomata to distinguish them from
the other Vertebrates or Gnathostomata. Their visceral skeleton,
consisting of a delicate cartilaginous basketwork, also shows many
peculiarities (Fig. 49), such as, for instance, its very superficial
position ; we may accordingly speak of these cartilages as " extra-
branchials."

64

COMPARATIVE ANATOMY.

The skull ofElasmobranchs presents the simplest conditions
and most easily comprehensible relations, so that it may be taken
as the starting-point for the study of the skull of all other
Vertebrates. It consists of a simple cartilaginous and fibrous

jr.

FIG. 49. — SKULL AND BRANCHIAL- BASKET OF Petromyzon planeri.

Lb, labial cartilage ; R, cartilaginous ring-shaped support of the suctorial mouth :
A, B, C, three other supporting plates of the suctorial mouth; ZB, lingual
cartilage ; Na, external nostril ; N, nasal sac ; Tr, trabeculae ; PQ, palato-
quadrate ; SS, fibrous cranial tube, which is cut through behind at MC (medul-
lary canal) ; OB, auditory capsule ; Hy, hyoid ; Ko, gill-openings ; t, posterior
(pcricardial) cartilage of the branchial basket ; *,*, transverse bars of the branchial
basket ; C, notochord.

capsule either immovably united with the vertebral column
(Squalidae) or connected with it by articulations (Rays and
Chimserse).

FIG. 50.— SKULL OF Hcptanchus.

WS; vertebral column ; GK, auditory capsule ; PF, AF, postorbital and ant-
orbital processes ; Orb, orbit ; ^, rostrum ; NK, nasal capsule ; t, region of
articulation of the palatoquadrate (PQ) with the skull ; G, articulation of lower
jaw ; Md, mandible ; Z, teeth.

True bones are never developed, the cartilage bemg merely
calcified ; the palatoquadrate and the lower jaw are nevertheless
richly provided with teeth (Fig. 50, Z).

THE SKULL.

65

The olfactory sacs lie in the ventro-lateral parts of the nasal
region, which is often elongated to form a long cut-water or rostrum
(intertrabecula). Behind this are seen the deep orbital hollows
(Figs. 50 and 51), which are bounded posteriorly by the strongly
projecting auditory regions (GK).

The palatoquadrate is usually only united to the basis cranii by ligaments,
but in the Chimaerae it becomes immovably fused with it, whence their name
of Holocephali. In some forms, the palatoquadrate is not directly united to
the skull, but is suspended from it by the upper segment of the hyoid arch or
hyomandibular (Fig. 51, Hm). In this case the skull may be described as
hyostylic, to distinguish it from -autostylic skulls, in which the hyoid
takes no part in the suspensorium. A cleft, the spiracle, lies on the
anterior border of the hyomandibular, and leads into the cavity of the mouth,
and on its walls may be found remnants of the embryonic spiracular
(mandibular) gill.

The branchial skeleton is always richly developed, owing to
secondary segmentation and fusion of its parts, and exhibits
characteristic modifications. On the outer circumference of each
branchial arch radially-arranged cartilaginous rays are developed,

FIG. 51.— CRANIAL SKELETON OF Raja oxyrhyncha.

GK, auditory capsule; Orb, orbit; NX, nasal capsule;'.??, rostrum; LK, LK\
labial cartilages ; ^Sp, spiracular cartilage ; 8Pl, spiracle ; PQ, palatoquadrate ;
Md, mandible; Hm, hyomandibular ; hy, hyoid ;'/ to V, first to fifth branchial
arches ; a, b, c, d, the individual segments of the branchial arches, viz. the pha-
ryngo-, epi-, cerato-, and hypohranchials ; t, point of union of the fourth and
fifth branchial arches ; Cp, basibranchials.

which serve as supports for the gill-sacs. They are present also
on the hyomandibular and hyoid, and rudiments of mandibular
rays are present in Sharks.

While in Elasmobranchs the gill-slits open freely on to the sur-
face of the body, in Chimserge a fold of skin arising from the
hinder border of the hyomandibular, lies over them. This is the
first indication of a gill-cover or operculurn, such as we shall
meet with again in Teleosteans and Ganoids.

66

COMPARATIVE ANATOMY.

Amongst the Ganoids, the lowest condition is met with in
those forms in which the hyaline primordial skull, immovably
fixed to the vertebral column, is still retained. These forms are
spoken of as Cartilaginous Ganoids. The appearance of
definite bones, however, divides them sharply off from the Elas-
mobranchs, and proves their skull to be at a much higher stage of
development. These bones have the form of richly sculptured
plates and shields, and are developed partly from the mucous
membrane of the mouth (as for instance the parasphenoid, which
lies along the base of the skull), and partly from the outer skin
(compare the chapter on the dermal skeleton, p. 32).

Cop

FIG. 52.— CRANIAL SKELETON OF STURGEON (Acipenscr) AFTER REMOVAL OF THE

EXOSKELETAL PARTS.

WS, vertebral column ; SpN, apertures for spinal nerves ; Psp, spinous processes ;
Ob, neural arches ; G, notochord ; GK, auditory capsule ; PF, AF, postorbital
and antorbital processes ; Orb, orbit ; //, optic foramen ; x, vagus foramen ;
Na, nasal cavity ;^R, rostrum ; *, proininent ridge on the basis oranii ; Ps, TV,
Ps", parasphenoid; PQ, palatoquadrate ; Qu, quadrate; Md, mandible; De,
dentary; Ar, articular; Hm, hyomandibular ; tiy, symplectic ; Ih, interhyal ;
hy, hyoid ; / to V, first to fifth branchial arches, with their segments — the
double pharyngo-branchial (a), the epibranchial (b)', the cento-branchial (c), and
the hypobranchial (d) ; Cop, basal elements of the visceral skeleton ; Hi, ribs.

This dermal skeleton attains to a much more considerable
development in a second group of these Fishes — the bony
Ganoids — and gives rise to a strong armour composed of numerous
pieces lying on the roof of the skull (Fig. 53). The ossifications are
not restricted to the outer surface, but extend into all parts of the
skull, as, for instance, the trabecular regions and the lower jaw ;
the cartilage thus becomes greatly reduced.

The branchial skeleton in Ganoids consists of four or five more
or less strongly ossified gill-arches, decreasing in size antero-
posteriorly. In bony Ganoids the surface which looks towards the
throat is beset with teeth. A gill-cover, often supported by several
bony pieces, is always present.

In the form of their skull, the Dipnoi show many points of con-
nection with Eiasmobranchs, Ganoids, Teleosteans, and Urodeles.

TLIK SKULL.

67

In other points, however, they differ considerably from all these ;
and it is clear that the last-named group cannot have been directly
derived from them. The suspensorium, as well as the very massive
palatoquadrate bar, fuses with the skull, and, as in Amia calva (a

FIG. 53. — SKULL OF Polypterus bichir FROM THE DORSAL SIDE.

Pmx, premaxilla ; Na, external nostril ; N, nasal ; Sb, Sbl, anterior and posterior
suborbital; Orb, orbit ; M, maxilla; Sp, spiraeular bones; PO, preoperculum (?) ;
SO, suboperculum ; Op, operculum ; F, frontal ; P, parietal ; a, b, c, d, supra-
occipital shields. The two arrows pointing downwards under the spiraeular
shields show the position of the openings of the spiracles on to the outer surface
of the skull.

bony Ganoid), even some of the anterior vertebrae with distinct
neural arches and transverse processes are united with the occipital
region of the cranium (Fig. 54, Wt Wl). (Cp. note on p. 56.)

Posterior in addition to anterior nasal apertures
appear in the Dipnoi for the first time: this is an indication of
air-breathing.

Cranial bones are not nearly so numerous as in Ganoids, and

F 2

6.8 COMPARATIVE ANATOMY.

the underlying hyaline primordial skull persists entirely (Cera-
todus), or to a large extent. Gill-covers and branchiostegal rays

S!E

FIG. 54. — CRANIAL SKELETON, PECTORAL ARCH, AND ANTERIOR EXTREMITY OF

Pi'otopterus. ~y^ v v» Y\ t>

ir, Wl, the vertebrae which are fused with the skull, with their spinous processes
(Psp, Psp1) ; Occ, supraoccipital, with the liypoglossal foramina ; Ob, auditory
capsule ; Tr, trabecula, with the foramina for the trigemmal- and facial nerves ;
FP, fronto-parietal ; Ht, membranous fontanelle, perforated by the optic foramen
(II) ; SK, supra-orbital ; SJE, supra- ethmoid ; NX, cartilaginous nasal capsule ;
AF, antorbital process (the labial cartilage, which has a similar position and direc-
tion, is not indicated) ; PQ, palatoquadrate, which converges towards its fellow
of the other side at PQ1 j 8q, squamosal, covering the quadrate ; A, A1, articular
joined to the hyoid ( ffy) by a fibrous band (£) ; L>, external dentary ; ft, Meckel's
cartilage, which is freely exposed, and grows out into prominences ; SL, ena-
melled ridge ; a, b. teeth ; Op, Opr, rudimentary opercular bones ; / to VI, the
six branchial arches ; KR, cranial ribs ; LK, MK, lateral and median bony lamella?,
which ensheathe the cartilage of the pectoral arch (Kn, Kn1} ; co'fibrous band,
which binds the upper end of the pectoral arch with the skull ; x, articular
head of the pectoral arch, with which the basal segment (b) of the free extremity
articulates ; *,*, rudimentary lateral rays of the extremity (biserial type) ; 1, 2, 3,
the three next segments of the free extremity.

are present, though greatly reduced, and even the five or six car-
tilaginous gill-arches are in a very rudimentary condition. The
sharp, blade-like teeth, covered with enamel, deserve notice.

Teleostei. — In this group, the skull presents a large amount
of variation ; its ground-plan, however, may always be derived from
that of the bony Ganoids, and more particularly from Amia
calva. On the other hand, no relations with the Amphibia are
observable, and we must consider the whole group of the bony
Fishes as a side branch of the piscine phylum.

THE SKULL.

r \\
UK

-..

Much of the cartilaginous primordial skull persists in most
Teleostei ; the cranial cavity, in all skulls described up to the
present time, may either reach between the eyes as far as the
ethmoid al region, or it may become narrowed and arrested in
the orbital region (Fig. 43, C).

The palatoquadrate bar becomes differentiated into a perfect
row of bony plates, which are described as quadrate, meso- and
metapterygoid, pterygoid, and pa]atine. In the occipital and
auditory regions, as well as on the dorsal surface of the skull,
numerous groups of bone are developed, which cannot be further
described here. A canal, lying in the axis of the base of the skull of
many Tel eosteans, must be mentioned : it encloses the eye-muscles,
and opens on each side into the orbits.

' '

FIG. 55. — CKANIAL SKELETON OF TROUT.

Ep, opiotic ; Ptc, pterotic ; Sph, sphenotic ; Os, supraoccipital ; P, parietal ; F,
frontal ; Sp.cth, supra-ethmoid ; Can, aperture of the canal for the olfactory
nerve ; Nl, nasal ; Pmx, prem axilla ; AI, Ml, maxilla ; 7<y, jugaj ; Ms, ineso-
pterygoid ; Mtp, metapterygoid ; ooo, orbital ring ; Hm, Trycmiandiniilar ; s,
symplectic ; Qu, quadrate ; Pr, preoperculum ; lop, interopkrculum ; Sop, sub-
operculum ; Op, operculum ; Bs&, branchiostegal rays ; Ar, articular ; De,
dentary ; A, eye.

All the bones bounding the oral cavity, viz., the vomer, the
parasphenoid, the premaxilla, and the maxilla, may bear teeth.
The maxilla, however, is usually edentulous, and both it and the
premaxilla vary much as to their development: the latter may
even be absent.

Besides the above-mentioned palatoquadrate bar, the proper
cranial capsule of Teleosteans is surrounded by other outworks
consisting of plates and bars. These arise as dermal bones in the
region of the eyes (orbital ring) (Fig. 55, oco), and in the gill-
covers (opercular bones) (Fig. 55, Pr, Op, Sop, lop). A large
number of branchiostegal rays are developed in the ventral parts
of the opercular fold, or branchiostegal membrane (Fig. 5

70 COMPARATIVE ANATOMY.

Anteriorly, the opercular apparatus lies against a bony chain
consisting of three pieces — the hyomandibular, sympiectic, and
quadrate — which serves as a suspen serial apparatus for the lower
jaw (Fig. 55, Hm, S, Qu, and Fig. 48, D). The latter consists of
Meckel's cartilage and of several bony elements, the largest of which
is called the dentary (Fig. 55, Be). The others are the articular
(Fig. 55, Ar), angular, and coronoid. The last, however, is as a
rule absent, and the angular may also be wanting.

A curious asymmetry is seen in the head of adult Pletironec.tidae. When
hatched, these Fishes are quite symmetrical, but later on the eye of one
side becomes rotated, so that eventually both eyes come to lie on the same side ;
in consequence of this, the skull also becomes asymmetrical.

B. Amphibia.

Urodela. — The skull of tailed Amphibians is distinguished
from that of Fishes principally by negative characters, — on the one
hand by the presence of less cartilage in the adult, and on the
other by a reduction in the number of bones. In short, it presents
altogether a much simpler plan, reminding us of that of Ganoids
and Elasmobranchs. This is seen, for instance, in the larval con-
dition (Fig. 56), in which the chondrocranium still plays a great
part, its auditory, nasal, arid orbital regions having the relations
described in the introduction to this chapter. The auditory cap-
sules (Figs. 56 to 58, OB], — which are bound together by cartila-
ginous basi- and supraoccipital tracts,1 and generally become strongly
ossified later, — show a new and important arrangement as compared
with those of Fishes in the presence of an aperture, the fenestra
ovalis, which looks outwards and downwards (Figs. 56 and 58,
Fov). This fenestra is closed by a cartilaginous or bony plug, the
stapes, and will be spoken of again in connection with the anatomy
of the auditory organ.

In all Amphibians two condyles for articulation with the first
vertebra are developed on the ventral periphery of the foramen
magnum (Figs. 56 to 58, Cocc).

The large nasal capsules, consisting throughout life of consider-
able cartilaginous tracts (Fig. 57, Na\ are connected with the
\) auditory capsules by means of the trabeculse,2 which form the
side walls of the skull, and enclose a large cavity. This cranial
cavity becomes closed dorsally by the frontals and parietals (Fig.
57, F} P), and ventrally by the parasphenoid (Figs. 56 and 58,
Ps), which is sometimes provided with teeth similar to those
of many Teleostei. In front of it lie the jgMMps (Figs. 56 and

58, Vo), which bound the posterior nostriM^K in adults each

1 There are never more than rudiments of a supra- and basioccipital in Anura,
and not even rudiments of thes,> bones in Urodeles.

2 The trabecula1 become more or less entirely ossified as the sphencthmoid and
prootics..

THE SKULL.

Fio. 56. — SKULL OF A YOUNG

FIG. 57.— SKULL OF Salamandra atra
(ADULT.) (Dorsal view.)

-Can

FIG. 58.— SKULL OF

atm (ADULT). (Ventral view.)
Tr, trabeeula ; OJ3, auditory capsule ; Fov, fenestra ovalis, closed on one side by
the stapes (8t) ; Lyt, ligament between the stapes and suspensorium ; Cocc, occi-
pital condyles ; £p, cartilaginous basilar plate between the auditory capsules ;
Osp, dorsal tract of the occipital cartilage ; AV, internasal plate, which extends
laterally to form processes ( TF and AF] bounding the posterior nostrils (Ch) ;
NK, nasal capsule ; Can, nasal cavity ; Na, external nostrils ; 11, foramen for the
olfactory nerve ; Z, tongue-like outgrowth (intertrabecula) of the internasal
plate, which forms a roof for the internasal cavity (Fig. 57) ; Qu, quadrate ; Ptc,
cartilaginous ptorygoid : Put. ->, pvdj pedicle, and Pa, ascending

Process, of the' quadr_afe"; Px. parasphenoid ; Pt, bony p^terygoid ; lro, vomer ;
V, palatine; Pp, palatine process of maxilla; i'op, v.omero-palatiue ; Pmx,
prcmaxilla ; M, maxilla; Os, sphenethm'oid ; As, prootic ; ^V, nasal; Pf, pre-
frontal, perforated at D for the lacrymal duct; F, frontal; P, parietal; Squ,
squamosal ; //, optic, V, trigeininal, and VII, facial foramen ; R*, point of
entrance of the nasal branch of the fifth nerve into the nasal capsule.

72 COMPARATIVE ANATOMY. ^

becomes fused with the corresponding palatine (Fig. 58, VJj^*
which forms a delicate bar lying on the ventral surface of the
parasphenoid. These relations are secondary, for in^tfie larval
condition a typical palatoquadrate or pterygo-palatine bar is present
(Fig. 56, Pt, Pic, Pi).

On the outer side of the vomer lies the maxilla (Figs. 56 to 58,
M)y and in front of this a premaxilla (Pmx) which usually encloses,
or at least bounds a cavity. The latter bone extends on to the
dorsal surfa.ee of the skull, and abuts against the nasal, behind
which usually follows a prefrontal (Fig. 57, N, Pf).

The suspensorium is much more simple than that of Fishes,
as a glance at the diagrammatic Fig. 48, E, will show. It con-
sists of the quadrate only, which secondarily becomes fused with
the skull, and on the outer surface of which an investing bone, the
squamosal, becomes developed (Figs. 56 to 58, Squ).

The skull of the Gy inn op h ion a, which is characterised "by its extreme,
strength and solidity, reminds us of that of the fossil genera of Amphibia of
the Carboniferous period. It shows in many points a certain relationship to
the skull of Anura, and is of great interest on account of the very complicated
structure of the nasal capsule (compare the chapter on the olfactory organ).

FIG. 59. — SKULL OF Rana esculents. (Ventral view.) (After Ecker.)

The investing bones are removed on the right side.

Cocc, occipital condyles ; Olat, exoccipital ; GK, auditory capsule ; Qu, quadrate ;
Qj'g, quadrat ojugal ; Pro, prootic ;• P.v, parasphenoid ; As, alisphenoid region ;
Pt, bony ptcrygbid ; PP, palatopterygojd ; FP, frontoparietal ; E, sphcnethmoid
(girdle-bone) ; Pal, palatine ; Vo, vomer ; M, maxilla ; Pmx, premaxilla ;
N, Nl, cartilaginous supports of the nose ; //, V, VI, foramina for optic, tri-
geminalf and abducent nerves.

Anura. — The skull of the tailless Batrachia is at first sight very
similar to that of Urodeles. It undergoes, however, an essentially
different and much more complicated development, reminding

THE SKULL.

one in certain points of that of Petromyzon, and it cannot there-
fore be derived in any way directly from that of Urodeles.

A suctorial mouth, provided with labial cartilages and horny
teeth, is present in the larva. Far more important, however, is the

Tlttrl

KIT

JCeJl

FIG. 60. — HYOBEANCHIAL APPARATUS OF URODELES. A, Axolotl (Siredon pisci-
form-is); B, Salamandra maculata; C, Triton cristatus ; D, Spelerpcs fuscus.

Bbr I, II, first and second basibranchial ; KcH, ceratolij-al ; JffpH, hypohyal ;
Xebr I, II, first and second ceratcbranchial ; Epbr /to IV, first to fourth cpi-
branchial ; KU, KH1, small anterior and posterior pairs of cornua ; O.th, thyroid

bone ; G.th, thyroid^land.

r

formation of membranous and cartilaginous walls to the tympanic
cavity, which is closed externally by a tympanic membrane,
while internally it opens into the mouth by a Eustachian aperture.
The palatoquadrate bar unites anteriorly with the lateral part of

74 COMPARATIVE ANATOMY.

the cartilaginous nasal capsule (compare the chapter on the auditory
organ, p. 199).

With the exception of certain small regions on the dorsal side,
the skull of Anura consists of a complete cartilaginous box : in the
adult the bones are not so numerous as in Urodeles, and the frontal
and parietal of either side as a rule fuse together, thus giving rise
to a fron to-parietal. The maxillary bar grows backwards much
further than in Urodeles, and becomes connected with the suspen-
sorium by means of a small intermediate bone, the quadrat oju gal
(Fig. 59, Qjy). For the relations of the bones bounding the mouth-
cavity, compare Fig. 59.

With the exception of the lower jaw, the visceral skeleton
of Urodeles undergoes various modifications in the different types.
We may consider the ground-form, as present in the larva, to
consist of five pairs of bars. The anterior pair, or hyoid, consists
of two pieces (Fig. 60, A, Hpff, KeH), as do also the two first
branchial arches (Kebr I, If, Mphr I, II). The third and fourth
branchial arches are much smaller, and each is composed of a
single segment (JSpbr III, IV). All the above-named arches are
connected with their fellows the other side by means of a single
'or double basal piece (Fig. 60, Bbr I and Bbr II). At the close of
larval life, when the lungs come into use, the two hinder pairs of
arches disappear entirely, while the two anterior pairs undergo
changes as regards form and position, and become more or less
strongly ossified (Fig. 60, B, C). In the genus Spelerpes, which
possesses a sling-like tongue, the lateral (dorsal) segment of the
first true gill-arch (epibranchial /) grows out into a long cartila-
ginous filament, which extends far back under the skin of the back
(Fig. 60, D).

In the Anura there is a much greater reduction of the
branchial skeleton at the close of larval life than in Urodeles. In
the larva, the main skeletal part consists of superficial branchial
cartilages (extra-branchials), which form a continuous structure
comparable to the branchial basket-work of the Lamprey. A hyoid
and small rudiments of the four proper internal branchial arches are,
however, present behind the mandible.

c. Reptilia.

The relationship between the skulls of Reptiles and Birds is very
close, while both are widely separated from those of Amphibians
and Mammals.

Excepting in the ethmoidal region, the whole chondrocranium
becomes almost obliterated by an extensive process of ossification.

In Snakes, Amphisbsenians, and Crocodiles, the cranial cavity
extends forwards between the orbits as far as the ethmoidal region,
while in Lacertilia and Chelonia — in which a fibre-cartilaginous

THE SKULL. 75

interorbital septum perforated by the olfactory nerve is present, — it
is arrested in the orbital region (compare p. 57).

The parasphenoid, which plays so important a part as an
investing bone of the roof of the mouth in Fishes and Amphibians,
begins to disappear ; amongst Reptiles it only attains to any im-
portant development in Snakes, where the anterior part remains,
and forms the base of the interorbital region. Its place is taken
by two cartilage bones, the basioccipital and basisphenoid, situated
along the basis cranii. In contradistinction to the Amphibia, only
a single condyle connects the skull with the vertebral column;
this, on close examination, is seen to be formed of three parts
(basioccipital and exoccipitals).

The roofing bones of the skull are well developed, as in
Teleostei, while the trabecular region (ali- and orbitosphenoids)
becomes of secondary importance, its place being taken by processes
growing downwards from the frontal and parietal, especially in
Snakes. The parietals are usually confluent in the adult, and in
Lacertilia are perforated by an aperture (parietal foramen).

For the topographical relations of the several bones to one
another compare Figs. 61 to 64. It will be seen in them that the
ground-plan of the Urodele skull, already somewhat fully explained,
is here fundamentally retained.

A new element, the transverse bone (Figs. 61, 62. and 64, Ts),
extending from the maxilla to the pterygoid, appears, except in
Chelonia and Typhlopidae. An imperfect circum orbital ring of
bones present in Lizards is also worthy of mention. The dentition
is stronger than in the forms as yet described, and may be borne,
as in Amphibians, on the palatines and pterygoids as well as on
the proper jaw-bones (Fig. 62, PI, Ft). Rasp-like sphenoidal
teeth are not present in Reptiles, and the Chelonia are altogether
toothless, the free edge of the jaws being covered by sharp horny
sheaths.

The skull of Crocodiles is of particular interest, owing to the
fact that the palatine processes of the maxillas (Fig. 64, Jf),,as well
as the palatines and pterygoids further behind (PI and Pi), meet I
together in the middle line, and thus form a secondary roof to^ the \
mouth-cavity, separate from the proper '(sphenoidal) base of {he \
skull. The cavity thus formed closes in the posterior prolongation
of the nasal chambers, in consequence of which the latter become
sharply differentiated from the mouth, and open far back into the
pharynx (Fig, 64, Oh). Thus the skull reaches a higher stage of
development, which,, only indicated in Chelonia, is characteristic

Mammals. In__all Reptiles the suspensorium consists mainly

the quadrate, which may be loosely attached to the skull
'Snakes,1 Lacertilia), or firmly fixed to it (Hatteria, Chelonia,

1 In Snakes i^Fig. 62, Qu) (except Tortrix), the quadrate is only indirectly con-
nected with the skull by means of the sqiiamosal .(Squ}, which extends backwards,
and thus throws the articulation of the lower jaw far backwards, giving rise to a very

76

COMPARATIVE ANATOMY

Cocc

FIG. 61.— SKULL 'OF LIZARD (Lacerta acjilis'}. (Dorsal view.)

e. , b*

A B

FIG. 612, A and B. — SKULL OF SNAKE (Tropidonotus natrix).

/occipital condyle ; Os, and Osp, supraoccipital ; 01, exoocipital ; f_
rifltalis ; Pe, periotic ; P, parietal ; Fp, parietal foramen ; F, frontal jjF1, post*
frontal ; P£, ^iueftQntal ; Eth, ethmoid ; N, nasal ; Na, external nostril ; Pmx,
])remaxilla ; M, maxilla; 0, 0, orbital ring; Bp, basioccipital ; Bs, • basi-
sphenoid ; Ch, posterior nostrils ; Vo, vonier ; PI, palatine ; Ft, pterygoid ; Ts,
transverse bone ; Qt/, quadrate ; Squ, squaiwosal ; Mp, supratemporaTT luff,
jugai \'^3rtt articular ; Ag, angular ; SA, supra-angular ; l)t, dentary ; II, optic
foramen.

THE SKULL.

-Cocc

FIG. 63. — SKULL OF YOUNG WATEII-TOUTOISE (Emys europcea}. (Side view.)

Osp, supraoccipital, which gives rise to a crest ; Pf, prefrontal, which forms a
great part of the anterior boundary of the orbit ; -/T^point of entrance of the
olfactory nerve into the nasal capsule ; Si, interorbital septum ; HK, horny
sheaths of jaws; Qjg, quadratojugal1; Mt, tympanic membrane; BP, carti-
laginous interval between basioccipital and basisphenoid ; Md, mandible. Other
letters as before. . •

Coec

FIG. 64. — SKULL OF A YOUNG CROCODILE. (Ventral view. )

Coco, occipital condyles ; Ob, basioccipital ; Ch, internal nostrils ; Pt, pterygoid
Orb, orbit ; PI, palatine ; .)/, palatine process of maxilla ; Pmx, premaxilla
2T6', transverse bofio ; J<t, jugal ; Qj, quadratojugal ; Qu, quadrate.

78 ( X KMPAIIATIVE ANATOMY.

Charneleontidse, Crocodilia). In most Lizards there is in addition
a rod-like bone, the epipterygoid, which extends from the hind part
of the pterygoid to the parietals.

A number of bones arise in connection with the lower jaw,
viz. a dentary, angular, supra- angular, splenial, coronoid, and
articular (Fig. 62, Dt, Ag, SA, Art).

In correspondence .with the absence of branchial respiration
during development, the branchial apparatus plays no great part in
Reptiles, and often only the slightest traces of it are seen : thus
in Snakes, for instance, only the hyoid remains, and this not
always. In Chelonians a basal piece (basi-hyo-branchial) as well
as the first branchial arch persist in addition.

D. Birds.

The skull of Birds is formed on the same plan as that of
Reptiles, and more particularly of Lizards, although it exhibits a
greater proportional development of the brain-case (Fig. 65). The
skull of Archa30pteryx was essentially similar to that of existing
Birds, and the bones were firmly united together. Teeth were,
however, present in both upper and lower jaws, and the fact that
the premaxillaB were toothed probably indicates that no horny beak
was present (compare the chapter on teeth).

All the bones have a tendency to run together by the obliter-
ation of the sutures originally present between them, and they thus
give rise to a united mass largely formed of endochondral bones.
It is only in the region of the nose that the cartilage persists
throughout life to any extent, and even here not always. In contrast
to all the Vertebrata as yet considered, the unpaired occipital condyle
no longer lies at the posterior boundary of the skull, but becomes
moved downwards and forwards along the base of the skull, so that
the axis of the latter lies at an angle with that of the vertebral
column. The basis cranii is formed by a basioccipital and a
basisphenoid, from which latter a bony rostrum, the remains of
the anterior part of the parasphenoid, extends forwards. The
posterior part of the parasphenoid persists as a large plate, the
basitemporal, which underlies the basisphenoid and part of
the basioccipital. Above the rostrum a small presphenoid is present
in the embryo.

All the bones are delicate and spongy, and thus contrast greatly
with those of Reptiles, in which they are often of an ivory-like
structure. (With regard to the pneumaticity of the bones,
already referred to in Crocodiles and certain fossil Reptiles, consult

wide gape. In most Snakes, and particularly in the Viperine forms (cp. Fig. 176, A),
the facial bones are capable of movement upon one another, but in Typhlops they are
immovably connected with the skull. The two rami of the mandible are connected
by a more or less elastic ligament.

THE SKULL.

7/7

ar

FIG. 65.— SKTJLL OF A WILD DUCK (Anas boschas). A, from above ; B, from below ;

C, from the side.

a.l.s, alisphenoid ; ag, angular ; ar, articular ; a.p.f, anterior palatine foramen ; b.t,
basitemporal ; b.o, basioccipital ; b.pg, basipterygoid ; b.s, basisphenoid ; d,
dentary ; e.n, external nostrils ; eth, ethmoid ; e.o, exoccipital ; e.u, Eustachian
aperture ; fr, frontal ; f.m, foramen magnum ; i.c, foramen for internal carotid
artery ; j, jugal ; Ic, lacrymal ; mx.p, maxillopalatine process ; mx, maxilla ;
n, nasal ; n.px, nasal process of the premaxilla ; px, premaxilla ; p, parietal ; ps,
presphenoid ; pg, pterygoid ; pi, palatine ; p.n, internal nostrils ; q, quadrate ;
q.j, quadratojugal ; sq, squamosal ; s.o, supraoccipital ; ty, tympanic cavity ;
v, vomer ; II, foramen for optic nerve ; V, for trigeminal ; IX, X, for glosso-
pharyngeal and vagus ; XII, for hypoglossal.

SO COMPARATI VE AN ATOM Y.

the- chapter on the respiratory system.) A complete bony palate
like that of Crocodiles is never present, so that the Bird's skull
here manifests a decidedly lower stage of development than that
of the higher Reptiles. The quadrate is nearly always movable
upon the skull, as is also the whole maxillopalatine apparatus, the
palatopterygoid bar sliding on the rostrum of the basisphenoid,
and so allowing the beak to be raised or lowered to a greater
or less extent. This mobility of the upper jaw is most marked
in Parrots, in which the frontonasal joint forms a regular hinge.
Teeth are no longer developed in existing Birds, their place being
taken functionally by horny sheaths covering the bones of the
jaw, and forming a beak. As in Reptiles, a fenestra ovalis and
tenestra rotunda are present, as well as a tympanic cavity opening
into the mouth.

The visceral skeleton is greatly reduced, though the first
branchial arch not only persists, but may, as in the Woodpecker,
grow out into a pair of very long jointed rods extending far
over the skull.

(For other details, compare Fig. 65, A, B, C.)

E. Mammals.

Tn Mammals there is a much closer connection between the
cranial and visceral regions of the skull than is the case in the
Vertebrates already described (comp. Figs. 66A and B). In the
fully-developed skull both maxillary and palatopterygoid regions
aj*e united to the cranium, though even in Man a facial arid a
nial region can be distinguished. The higher we pass in the
Mammalian series, the more doc-s the former come to lie below
tter ; so that, in the highest types, their mutual relations to
:ier can no longer be so well expressed as being before
uehxuu, aa by under and above. Thus in Man the facial
leton is proportionately small when contrasted with the great
portion, and the reduction of the. angle between the basi-
md vertebra] axes is carried still further than in Birds.
The base of the skull is preformed in cartilage, as in Reptiles
and Birds : the parasphenoid has disappeared almost entirely, the
anterior part of the basis cranii being formed by the ossification
of the basal cartilage, which may be distinct, as a presphenoid
(Marsupials, Rodents, and some Insectivores), or result from a
union, of the basal parts of the two orbito-sphenoids : a basi-
sphenoid and basioccipital are always present. Most of the bones
of the roof of the skull are developed directly in the subcutaneous
fibrous membrane.

The cranial cavity is closed in anteriorly by the cribriform
plate of .the ethmoid, which is perforated by the olfactory nerves.

THE SKULL.

81

Turbinals are present in the nasal cavity, but are never more than
rudimentary in Oetacea. For further details as to the olfactory
and auditory capsules and their mode of ossification, as well as the
formation of the auditory ossicles, consult the chapters on the

FIG. 66 A. — LONGITUDINAL VERTICAL SECTIONS THROUGH THE SKULLS OF —
A, Salamandra maculosa, B, Testudo grceca, AND C, Corvus corone, TO SHOW
THE RELATIONS BETWEEN THE CRANIAL AND VISCERAL PORTIONS.

olfactory and auditory organs. Remains of the primitive cartilage
are seen in the nasal region in adult Mammalian skulls.

It has already been stated that, as regards the hard palate,
Mammals essentially agree with Crocodiles, but the pterygoids

82

COMPARATIVE ANATOMY.

(except in Anteaters and some Cetacea) do not take part in its
formation.

FIG. 66B. — LONGITUDINAL VERTICAL SECTIONS THROUGH THE SKULLS OF — •
A, DEER, B, BABOON, AND C, MAN, TO SHOW THE RELATIONS BETWEEN
THE CRANIAL AND VISCERAL PORTIONS.

According to recent researches the premaxilla appears to
originate primitively by a double ossification on each side. In the
middle line it bounds a canal which forms a communication between

THE SKULL.

83

the nasal and oral cavities (incisive or naso-palatine canal). In the
lateral parts of the face of most Mammals, the jugal or malar
connects the maxilla with a process of the squamosal instead of
with the quadrate, as in Amphibia and Sauropsida: in Ungulata

e.n

FIG. 67. — SKULL OF EMBRYO OF ARMADILLO (Tahtsia hybrida). (Modified from a
drawing by W. K. Parker.)

"./.", tympanic annulus ; au, auditory capsule; b. Tiy, basihyal ;_ c.ky, ceratohyal ;
cr, cricoid ; d, dentary ; e.hy, epihyal ; e.n, external nostril ; co, excccipital ;
/, frontal ; hJiy, hypohyal ; i, jugal ; in, incus; Ic, lacrymalj ink, Meckel's
cartilage ; ml, malleus ; mx, maxilla ; n, nasal ; oc.c, occipital condyle ; p,
parietal ; pa, palatine ; px, premaxilla ; so, supraoccipital ; st, stapes ; s. t,
superior turbinal ; st.m, stapedius muscle ; sq, squamosal ; th, thyroid ; tr,
trachea ; If1, foramen for first, and V2 for second, division of the trigeminal ;
//, optic foramen.

A

and Primates, wheOne jugal is also connected with a process of
the frontal,1 the orbit becomes almost completely separated from
the temporal fossa.

1 Most of the true Ruminants are provided with horns projecting from the frontal
bones: these are of three kinds: — 1, hollow horns (in the Cavicomia) ; 2, solid
horns (antlers of Cervidse) ; and 3, horns of the Giraffe.

In the Cavicomia (Bovinse, Antelopinse, Caprime, Ovinte) bony processes are
developed from the frontals, which become enveloped by horn formed from the epi-
dermis. In the Cervidre, a membrane bone becomes developed in the derma round
each process of the frontal, with which it fuses. This grows out to form the antler, and
after attaining its full development, the skin covering it dries up owing to the develop-
ment of the "burr" at its base ; this constricts the vessels, and the antler, being
deprived of nutriment, falls off. This occurs periodically at the close of the breeding
season. In the young animal, the antlers are simple, but year by year they become
more complicated and branched. Giraffes possess persistent antlers covered by hair
without any process from the frontal, which do not become anchyloscd to the latter
bone.

The differentiation into " horn-" and "antler-bearers" first began in the Miocene
epoch.

G 2

84 COMPARATIVE ANATOMY.

The lower jaw, each ramus of which is composed of a single
piece, corresponds to the anterior portion only (dentary, splenial,
and coronoid) of the mandible of Sauropsida, and it articulates
secondarily with the squamosal. Concerning the primary relations
of these parts, compare the chapter on the auditory organ and
Fig. 67. The hyoid arch is often reduced to a fibrous band, the
stylo-hyoid ligament, and is connected proximally with the base of
the auditory capsule and distally with the third visceral (that is,
the first true branchial) arch. The latter forms the proper body of
the hyoid with its greater cornua. Remains of the fourth visceral
(second branchial) arch are present in some cases, as in the Porpoise
(Phocsena), for example. For the air-sinuses of the skull compare
the chapter on the air-sacs of Birds.

BIBLIOGRAPHY.

AHLBORN, F. — Ueber die Segmentation des Wirbelthierkorpers. Zeitschr. f. wiss.

Zool. Bd. XL. 1884.

CALLENDER, G. W. — Formation, tfcc., Human Face. Phil. Trans. 1869.
DAMES, W. — Veb. den Ban des Kopfes von Archceopteryx. Sitz. d. k. preuss. Akad.

d. Wiss. 37 and 38, 1882.
DOHRN, A. — Studien zur Urgeschiehte des Wirbelthierkorpers. Mittheilungen der

Zoolog. Station zu Neapel. Bd. VI. 1885.
DURSY, E. — Entw.-Gesch. des Kopfes des MenscTien und der hoheren Wirbelthiere.

Tubingen, 1869.
GEGENBATJR, C. — Unters. z. vergl. Anatomic der Wirbelthiere, H. III. Das

Kopfskelet der Selachier. Leipzig, 1872.
HERTWIO, 0. — Ueber das Zahnsystem der Amphibien und seine Bedeutung fur die

Genese des Skelets der Mundhohle. Arch. f. mikr. Anat. Vol. XI. Suppl. H.

1874.
HOWES, G. B. — On some Points in the Anatomy of the Porpoise. Journ. of Anat. and

Physiol. Vol. XIX. 1879.
MARSH, 0. C. — The Dinocerata, a Monograph of an Extinct Order of Gigantic

Mammals. Washington, 1884.
MOLDENHATJER, — Die Entwick. d. mittlcren u. des ausseren Ohr. Morphol. Jahrb.

Bd. III. 1878.

MtiLLER, J. — Vergl. Anatomie der Myxinoiden. Berlin, 1834-1845.
PARKER, W. K. — Numerous Papers on the Development of the Vertebrate Skull

in the Transactions of the lioyal, Zoological, and, Linnean Societies for the last

twenty years.
PARKER, W. K., and BETTANY, G. T. — The Morphology of the Skull. London,

1877.
STOHR, P. — Zur Entwick. des Kopfskeletes der Teleostei. Festschrift z. Feier des 300

Jdhrigen Bestetens der Jul. Max. Univ. zu Wurzburg. Leipzig, J882. Zur

Entwick. des Urodelen- und Anurenschddels. Zeitschr. fur Wiss. Zool. Bd. 33

und 36.
WIEDERSHEIM, R. — Salamandrina pirspicillata, &c. Versucheiner vergl. Anatomie

der Salamandrinen. Genua, 1875. Das Kopfskelet der Urodelen. Morphol.

Jahrb. Bd. III. 1877. Die Anatomie der Gymnophionen. Jena, 1879.
WYHE, J. W. VAN. — Ueber die Mesodermsegmente und die Entwicklung der Nerven des

Selachierkopfes. Amsterdam, 1882.

V. LIMBS. .

The limbs or extremities, which are, as appendicular organs,
distinguished from the axial organs (head, neck, and body),
serve mainly for locomotion, and may be divided into two groups,

LIMBS.

85

the paired and the unpaired limbs. Both arise in Fishes, as
linear proliferations of the epiblast, which form four folds or ridges
— a dorsal and a ventral, extending from the head backwards
to the tail, and two lateral (Fig. 68, A, D, S, /S1).1 Mesoblastic
elements extend into them later.

a. UNPAIRED LIMBS.

The unpaired limbs, which are characteristic of Fishes, are
developed from the dorsal and ventral ridges. They either remain
continuous in their further development, as in some Fishes and
tailed Amphibians, or else certain parts undergo degeneration, so that

Bri'

FlG. 68. — DlAGEAM SHOWING (A) THE UNDIFFERENTIATED CONDITION OF THE

PAIRED AND UNPAIRED FINS IN THE EMBB.YO, AND (B) THE MANNER IN
WHICH THE PERMANENT FINS ARE FORMED FROM THE CONTINUOUS FOLDS.

D, dorsal fin-fold ; S, S, lateral folds, which unite together at £] to form the ventral
fold ; RF, FF, dorsal fins ; SF, tail-fin ; AF, anal fin ; BrF, pectoral fin ;
BF, pelvic fin ; An, anus.

they only persist in certain regions, which are spoken of as dorsal,
caudal, and anal fins (Fig. 68, A, B) : in these regions muscles
and skeletal parts (fin-rays) become formed.

After the formation of the epiblastic fin-folds, the next parts of the unpaired
fins which appear are the muscles, and then follow the supporting cartilages ;
these latter arise, like the skeletal parts of the paired fins, entirely independently
of the axial skeleton. The connection between the latter and the unpaired fins
is only secondary. This holds good also for the caudal fin, in which the
relation between the axial skeleton and that of the fin is a very close one.

The caudal fin is the principal organ of locomotion in most Fishes, and it
acts in a horizontal direction : the paired fins play only a secondary part, and
are principally concerned in rising and sinking in the water.2

1 See also p. 86.

2 The curious suctorial disk on the dorsal side of the head of the Remora
(Echeneis), by means of which it attaches itself to foreign objects, arises in the
embryo from the anterior portion of the dorsal unpaired fin, and this is indicated
throughout life by the arrangement of the blood-vessels, nerves, and skeletal parts.

86 COMPARATIVE ANATOMY.

In many tailed Amphibians and Amphibian larvae (including
those of Csecilians), the unpaired fins are represented by a fold of
the skin extending along the dorsal and ventral sides of the tail.
In some cases this fold extends along the back right up to the
head, but it never gives rise to bony or cartilaginous supporting
elements. In the male Triton it becomes much enlarged during the
breeding season.

b. PAIRED LIMBS.

No other morphological problem has given rise during the last
twenty years to such extensive researches and to such varied
solutions as the question of the origin of the paired limbs. Two
very opposite views exist. According to one of these (Gegenbaur's
view), the proximal parts of the extremities, that is, the pectoral
and pelvic arches, are regarded as being derived from branchial
arches, and the distal or free portions as metamorphosed
fin -rays. According to this theory, the pelvis is to be looked
upon as a visceral arch which has changed its position so as to
lie far back along the body.

According to the other view (that of Dohrn), the origin of the
paired limbs has nothing to do with the visceral skeleton, but, like
the latter, they are to be looked upon as the localised remains in
definite regions of the body (thoracic and pelvic regions) of a series
of cartilaginous bars originally extending along the whole trunk,
and having a metameric arrangement. In other words, -just as
each body-segment of an Annulate may be looked upon as being
provided with a pair of limbs, so also was each primitive segment
of the Vertebrate body : recent researches seem to support this.

These researches were made on Elasmobranch embryos, and in these each
somite gives rise to a fin-element, each of which, consists of two dorsal and two
ventral bundles of muscle, two rods of cartilage, and a corresponding spinal
nerve. Both pectoral and pelvic fins are made up of a considerable number
of these fin-elements. It is interesting to note that these outgrowths from
the somites are present along the entire length of the lateral epiblastic
folds, that is, they exist at first between the pectoral and pelvic fins, as well
as behind the latter, but in these regions they soon become aborted. The
lateral epiblastic folds do not run parallel to one another, as was supposed by
Thatcher, Mivart, and Balfour, but converge towards the anus (Fig. 68, A) : the
presence of outgrowths behind the anus, however, points to the possibility of the
ventral impaired fin having been originally paired. This probably was the case
when the postanal gut of the embryo, as well as the coelome, extended through
the whole caudal region. After the formation of the definitive anus and the
disappearance of the postanal gut, the two lateral halves of each primordial
fin-fold fused together to form a median fin. Possibly the dorsal fin was also
originally paired, for it arises by paired outgrowths from the dorsal part of
the my o tomes, so that in this case it was not situated in the middle line, but
along each side of it.1 On the closure of the laminae dorsales in the formation

1 Dohrn has lately attempted to prove that the unpaired fins of Petromyzon arise
in a paired manner, and that this Fish must formerly have possessed paired pectoral
and pelvic fins, which have gradually become lost.

PECTORAL ARCH. 87

of the cerebro-spinal axis, these two folds fused together in the median dorsal
line. Thus all the four longitudinal epiblastic folds (of Thatcher, Mivart,
and Balfour) are possibly to be considered as arising originally from separate
metamefic outgrowths of the body-segments, that is, from parapodia, which
have thus become gradually transformed into the fins existing at the present
time.

Paul Meyer supports this hypothesis Jby finding parapodia-like outgrowths
arranged in four rows along the caudal region in embryos of Pristiurus and
Scyllium.

Later investigators in this subject 110 longer even accept the
hoinodynamy (i.e. the serial homology) of the pectoral and pelvic
arches and limbs, but suppose that even ontogenetically the
two arches can in no way be compared with one another, for,
arising in an entirely different manner, they can only be
regarded as ''apparently similar" structures. Which of these
attempts at an explanation of the problem comes nearer to the
truth cannot yet be definitely stated, and the relative merits of the
views just put forward cannot be discussed here.

^ Pectoral Arch.

Fishes. — Owing to the absence of paired fins in Amphioxus
and Cyclostomi, pectoral and pelvic arches are also wanting. In
Elasmobranchs the pectoral arch consists of an extremely simple
cartilaginous bar, the two halves of which are united ventrally
by cartilage or fibrous tissue (Fig, 69, $Z?) ; and it has at first a
similar structure in embryos of Ganoids and Teleostei.

Later, however, in both the last-named groups, a row of bony
structures arising in the perichondrium becomes developed in this
region ; so that one can distinguish between a secondary or bony
pectoral arch, and a primary or cartilaginous one. •

The free extremity, or fin, is connected with the hinder and
outer circumference of the (primary) arch, and its point of
attachment may be taken as separating an upper dorsal and a
lower ventral section. The former, which is often connected with
the vertebral column, corresponds to a scapula, and the latter to a
coracoid plus precoracoid (clavicle) of the higher Vertebrata.1

In Teleosteans and bony Ganoids, the bony (secondary) arch
forms the principal support of the fin in the adult, the main
element being a large clavicle (Fig. 70, D). The primitive rela-
tions are thus much altered. The arch becomes secondarily
connected with the skull ((7m). For further details, compare
Fig. 70.

Amphibia and Reptilia. — In these types, the secondary bony
apparatus is of less importance, for the primary arch is more

1 The pectoral arch of Dipnoi is intermediate in character between that of Elasmo-
branchs and Ganoids. It shows so many special peculiarities as regards form and
position, that it cannot be fully described here.

88

COMPARATIYE ANATOMY.

FIG. 69. — PECTORAL ARCH AND FIN OF Heptanchus.

SB, SB1, pectoral arch, with a nerve-aperture at NL ; Pr, Ms, Mt, the three
basal elements of the fin — pro-, meso-, and metapterygium ; JRa, cartilaginous
fin-rays ; a, b, the main fin-ray, lying in the axis of the metapterygium ;
t, single ray on the other side of the axis (indication of a bisenal type) ; FS,
horny rays, cut through.

Cm

Co(Cl)

FIG. 70.— LEFT PECTORAL ARCH AND FIN OF THE TROUT. (From the outer side.)
D, J}1, Z>2, chain of secondary bones of the pectoral arch (clavicle and supraclavicle),
which is connected with the skull by means of the post-temporal (Cm) ; S and
Co(Cl), bony scapula and coracoid, which have become developed in the cartilage
(Kri) ', L, foramen in scapula ; M\ metapterygium ; 2ia, lla, the second and
third, and 4, the fourth basal element of the fin ; Ea1, the second cartilaginous
row of radii ; HS, bony ray on the border of the fin, which is connected with the
fourth basal element ; F, S, bony fin-rays, shown cut away from their attachments.

PECTORAL ARCH. 89

extensively developed. In all the higher Vertebrates it is formed
on the same plan as in Amphibia, and becomes more or less
completely ossified.

It always consists on each side of a cartilaginous or bony
dorsal plate (scapula), which curves round the side of the body,
and becomes continuous ventrally with two processes — an anterior

FlG. 71. — DlAGHAM OF THE GROUND-TYPE OF PECTORAL ARCH MET WITH IN
ALL VERTEBRATA, FROM THE AMPHIBIA UP TO MAMMALIA.

S, scapula ; Co, coracoid ; Cl, precoracoid (clavicle) ; Ht humerus.

FIG. 72. — SEMIDIAGRAMMATIC FIGURE OF THE PECTORAL ARCH AND STERNUM

OF Urodela.

St, sternum ; a, point where the two coracoids overlap ; Cl, precoracoid (clavicle) ;
SS, suprascapula, shown extended transversely outwards on the left side ; t, bony
scapula ; H, humerus.

FIG. 73. — PECTORAL ARCH OF A CHELONIAN. (Ventral view.)
S, scapula ; Go, coracoid ; Co1, epicoracoid ; Cl, precoracoid (clavicle) ; B, fibrous
band between these two elements ; Fe, feiiestra between them ; G, glenoid
cavity.

(precoracoid, or clavicle), and a posterior (coracoid) (Fig. 71,
S, Cl, Co). It always becomes connected ventrally with the
sternal apparatus (compare Figs. 38, 39, 74).

The two coracoid plates either overlap one another in the mid-
ventral line, or else their free edges come into apposition and fuse
together (Figs. 74 and 38, Co, Co1}.

90 COMPARATIVE ANATOMY.

As in the rest of the skeleton, cartilage plays the most im-
portant part in the pectoral arch of Amphibia (see Fig. 74), while
in all the higher Vertebrates the cartilage is almost entirely
replaced by bone (scapula, coracoid, and clavicle). Unossified
spaces are often left in the coracoid, giving rise. to fenestras closed
over by fibrous membrane (Lizards) (Fig. 39, a, b, c).

In Lizards, a cross or T-shaped parosteal bony plate, the inter-
clavicle, lies in a groove on the under side of the sternum in the
middle line. In Crocodiles a slender rod-like interclavicle is also
present, and has the same relations, though clavicles are wanting.1

FIG. 74. — PECTORAL ARCH AND STERNUM OF Bomlinator igneus.

St, sternum, with its two processes, a, a1 : S, scapula ; SS, suprascapula, in situ on
the left side, spread out horizontally on the right ; Co, coracoid ; Co1, epicoracoid,
which on both sides overlaps the anterior part of the sternum ; Cl, precoraeoid ;
Cll, bony clavicle ; Fe, fenestra between clavicle and coracoid ; G, glenoid
cavity.

The presence of a pectoral arch in numerous footless Reptiles
(some Skinks, Amphisbaenians) indicates that they formerly pos-
sessed extremities ; rudiments of the latter may even be seen in the
embryo, though they disappear entirely later on (Anguis fragilis).

Birds. — In Birds, the scapula consists of a thin and narrow
plate of bone, often extending far backwards, the strong coracoid
being bent at a sharp angle with it in all Carinate Birds (Fig. 36,
S and Co). The lower end of the latter is firmly articulated in
a groove on the anterior edge of the sternum.

In almost all Flying Birds the clavicle is well developed, and
becomes united with its fellow to form a furcula2 (comp. Fig. 36,

1 It is doubtful whether the three anterior elements of the plastron of Chelonians
correspond to clavicles and interclavicle, as supposed by Parker and Huxley.

2 The median plate often present at the point of junction of the clavicle is some-
times described as an interclavicle, but its late appearance in the embryo seems to
negative this view.

PECTORAL ARCH. 91

Fu(Cl), for its relations to the rest of the pectoral arch and to
the sternum).

Amongst the Cursorial Birds, the Emeu (Dromaeus) and Casso-
wary (Casuarius) possess rudimentary clavicles : in the others
they are wanting.

In Archseopteryx, the scapular region only of the pectoral
arch has been satisfactorily made out, and this resembles that of
existing Birds.

Mammals. — In Monotremes only amongst Mammals does the
coracoid extend ventrally to reach the sternum ; in all the others
it becomes reduced, and simply forms a prominent process on the
scapula (coracoid process), which becomes ossified from a

FIG. 75. — PECTORAL ARCH OF Ornithorhynchus paradoxus.

8t, sternum ; Ep, interclavicle ; Co, coracoid ; Co1, epicoracoid ; S, scapula ; C7,
clavicle ; G, glenoid cavity.

separate centre. Thus the scapula alone serves to support the
extremity ; it becomes at the same time greatly broadened out,
and gives rise on its outer side — in connection with the highly
differentiated muscles of the limb — to a strong ridge (spina
scapulae), which extends downwards to form the so-called
acromion.

The distal end of the clavicle usually becomes connected with
the acromion, its proximal end articulating with the anterior edge
of the sternum.

In those Mammals in which the fore-limbs are capable of very varied and
free movements, the clavicles are very strongly developed. In others, such
as the Carnivora and Ungulata, they may be entirely wanting or only
rudimentary, and in the latter case their relations to the scapula become
altered.

92 COMPAKATIYE ANATOMY.

Pelvic Arch.

Fishes. — The Dipnoi have probably retained the most primi-
tive form of pelvis amongst existing animals. It consists of a
cartilaginous plate lying in the mid-ventral line, from which two
pairs of processes, an anterior and posterior, arise. The hinder
extremities articulate with the posterior (Fig. 76, &), while the
anterior pair are to be interpreted as iliac processes (Fig. 76, a).
The latter vary much both in form and size, and, as in young
specimens of Protopterus, may extend widely outwards, embedded
in the intermuscular septa (Ml\ A delicate rod-like process

FIG. 76. — PELVIS OF Protopterus. (From the ventral side.)

a, iliac process, which may become forked at its distal end ; b, process to which the
hinder extremity (HE} is attached ; Gr, sharp ridge, for attachment of muscles ;
c, median process ; M, M, myotomes ; M1, M1, intermuscular septa.

arises between them from the anterior border of the plate and
extends far forwards in the middle line (Fig. 76, c).

The pelvis of Elasinobranchs may be derived from that of
Dipnoi, although it must be looked upon as a degenerate form.
It consists of an unpaired or paired plate of cartilage, on which
processes corresponding to those described in Dipnoi may be seen,
though these are often very rudimentary. InChimsera an iliac
tract is much more plainly marked.

Amongst all other Fishes, the existence of a pelvic rudiment
has been proved with certainty only in the case of Polypterus.

Amphibia. — In Amphibia, as in all the remaining higher
Vertebrata, a dorsal and two ventral pelvic elements may be
distinguished. The former, or ilium, is connected with the sacral

PELVIC ARCH. 93

vertebrae, and corresponds to the iliac process mentioned above
as being present in some Fishes ; of the latter, the anterior ventral
element is spoken of as the pubis, the posterior as the ischium.
The articular cavity for the thigh-bone (acetabulum) is situated at
the point of junction of the pelvic elements. Thus we meet here
with a ground-form essentially similar to that of the pectoral arch
(Fig. 71).

In the pelvis of Urodela and Anura a single ventral plate is
seen on each side, which comes into contact with its fellow to form

FIG. 77. — PELVIS OF SALAMANDER (Salamandra maculosa). (Ventral view. )

II, ilium ; Is, ischium ; P, pubis (?) (pars acetabularis ?) ; Fo, obturator foramen ; Sy,
ischio-pubic symphysis ; t, two protuberances, present in many Urodeles ; Ep,
epipubic cartilage, with its forked ends (a, b) ; G, acetabulum.

a symphysis (Fig. 77, Sy), and the anterior part of which usually
remains cartilaginous throughout life. It cannot, at present, be
stated with certainty whether this part corresponds to the pubis,
or to a fourth element, the pars acetabularis (Fig. 77, P, and
78, Kn), which is present in many Amniota, inserted between the
pubis and the acetabulum (comp. Fig. 83A). The posterior part,
which always becomes ossified, is without doubt an ischium (Is).
In some Urodeles a delicate rod of cartilage arises from the middle
line of the anterior border of the pelvis, and becomes bifurcated
distally (Fig. 77, Ep, a, 5). Amongst the Anura, this e pi pubis
("ypsiloid cartilage") is only present in Dactylethra capensis,
where its form is somewhat different, reminding one of the delicate
median process of the pelvis of Dipnoi (Fig. 76, c). The so-called
" marsupial bones " of Monotremes and Marsupials have probably
been developed from the representative of the epipubis.

In correspondence with the mode of progression in Anura,
the ilium of each side becomes extended so as to form a. long rod
(Fig. 78, //), and the two ventral plates, which in Urodeles lie

COMPARATIVE ANATOMY.

in the plane of the abdominal walls, become closely pressed
together in the middle line, and so give rise to a well-marked
keel (Fig. 78).

II-

FIG. 78. — PELVIC ARCH OF FROG (Rana esculenta}. (A, from below ; B, from the

side.)

11, P, ilium ; Is, ischium ; Kn, pubis (?) (pars acetabularis ?) ; Cr, the median
ventral ischio-pubic crest ; G, acetabulum ; Oc, urostyle ; Ft, transverse process
of sacral vertebra.

Fia. 79. — PELVIS OF Lacerta muralis. (Ventral view.)

11, ilium ; Is, ischium ; Fo, foramen in the pubis for the obturator nerve ; Kn, Kn1,
cartilaginous elements lying at the symphysis of the ischium (Sy) and pubis
respectively ; B, fibrous band connecting these cartilages ; Fc, foramen cordi-
forine ; t, tubercle of ilium ; G, acetabulum.

Reptiles and Birds. — In these, the well-ossified elements
of the pelvis are sharply differentiated, and, as the pubis in

PELVIC ARCH.

95

Reptiles usually extends forwards for some distance in the middle
line, a large space (foramen cordiforme) is present between it and
the ischiuin : in Lizards, Crocodiles, and Turtles, this space
is divided into two halves by a median nbro-cartilaginous band
(Fig. 79, Kn, Knl, B). In land and fresh-water Tortoises, in
place of this band, the median ends of the pubis and ischium
extend towards one another and meet in the mid-ventral line, and
thus the space (obturator foramen) between them becomes entirely
surrounded by bone.

FIG. 80. — PELVIS OF A YOUNG Alligator Indus. (A, ventral, and B, side view.)

//, ilium ; Is, ischium ; P, pubis ; Sy, symphysis of ischium ; F, foramen cordiforme +
obturatum ; B, fibrous band between the symphyses pubis and ischii ; f, carti-
laginous apophysis of the ventral acetabular process of the ischium, which is
interposed between the process a, of the ilium and the pubis ; b, foramen in
the acetabulum, bounded posteriorly by the two processes, a and b, of the ilium
and ischium respectively, which here meet one another ; *, indication of a, for-
ward growth of the ilium, such as is met with in Dinosaurians and Birds ; Cf,
acetabulum ; /, //, first and second sacral vertebrae ; M, fibrous membrane ex-
tending between the anterior margin of the pubis and the last pair of "abdominal
ribB"(jRfi).

In Crocodiles we meet for the first time with a considerable
extension of the ilium (Fig. 80, B, //), so that now a longer
posterior, and a shorter anterior process may be distinguished. In

06

COMPARATIVE ANATOMY.

Dinosaurians the latter (Fig. 81,*) is more strongly developed,
and plainly leads towards the form of pelvis seen in Birds.

FIG. 81. — PELVIS OF Iguanodonberniszartcnsis. (After Dollo.)

*, preacetabular, and Jl, postacetabular process of the ilium ; a, acetabulum (per-
forated) ; P, pars acetabularis (pectineal process of pubis) ; P1, pubis ; Jst
ischium.

,'7

FIG. 82.— PELVIS OF Apteryx australis. (Lateral view.) (After Marsh.)
il, ilium ; is, ischium ; p, pectineal process of pubis ; p1, pubis ; a, acetabulum.

In Birds (Fig. 82), both pre- and postacetabular portions of
the ilium are largely developed, but vary in their relative propor-
tions (comp. p. 44). In Crocodiles the pubis is to a great extent

PELVIC ARCH.

97

J>

FIG. 83A. — PELVIS OF A SIX-DAYS' CHICK. (After A. Johnson.)
Jl, ilium ; Js, ischium j pb, pubis ; pp, pectineal process of pubis.

FIG. 83B.— DIAGRAM SHOWING THE RELATIONS OF THE PELVIC BONES TO THE

ACETABULTTM.

.7, ilium ; Js, ischium ; P, pubis ; A, acetabular bone ; Ac, acetabuluna.

FIG. 84.— PELVIS OF Echidna. (From the left side.) (After Gegenbaur.)
11, ilium ; Is, ischium ; P, pubis ; Om, marsupial bones ; Fo, obturator foramen.

98 COMPARATIVE ANATOMY.

shut out from the acetabulum by the cartilaginous pars acetabularis
(Fig. 80, B, t> -P), and in them, as well as in Birds, the acetabulum
is perforated (Fig. 80, B, &, and Figs. 81 and 82, a).

What most distinguishes the pelvis of Dinosauria and Birds
from that of Reptiles is the position of the pubis (.Figs. 81 and
82, P1). It has the form of a delicate bar of bone directed back-
wards, and running parallel and even fusing (Birds of flight) with
the ischium, which also extends far backwards. It is very difficult
to explain the homology of a strong bone, arising from the
antero-ventral border of the acetabulum of Dinosaurians, indi-
cated by the letter P in Fig. 81. It is, however, probably to be
looked upon as a special outgrowth from the pars acetabularis of
each side. Rudiments of this bone, which may be called the
pectineal process of the pubis, are also present in Birds (Figs.
82, # 8'3, pp).

In Birds the elements of the pelvis usually become anchylosed
to one another, while in Reptiles and Dinosaurians they remain
distinct. The pubis of either side meets its fellow only in Struthio,
and the ischium in Rhea.

The pelvis of Arcliseopteryx possessed a broad pre-acetabular and a long
and slender post-acetabular portion of the ilium. The ischia appear to have
been united by symphysis : nothing is known of the pubis.

Mammals. — The four elements of the pelvis here remain
separated for a long time by cartilage, but later they become fused
together. The angle between the axes of the ilium and sacrum
is smaller in Monotremes than in the higher Mammals.

The original type with both pubic and ischiatic symphyses is
seen in Monotremes, Marsupials, many Rodents, Insectivores,
Ungulates, and Carnivores. In many other Insectivora and
Carnivora, and more particularly in the highest forms, the Pri-
mates, the ischia no longer meet below. The greatest amount of
variety in the form of the pelvis in any one order is seen in
Insectivores, in some of which (e.g. Mole), as well as in most Bats,
the symphysis pubis is not closed.

There is no pars acetabularis (see Fig. 83B, A ) in Monotremes and Bats, nor
in numerous representatives of the other principal Mammalian groups. It
always lies anterior to the " incisura acetabuli," and is most strongly developed
relatively in the Mole (Talpa), where it shuts the ilium as well as the pubis
out of the acetabulum ; in by far the greater number of Mammals the pubis
only is thus excluded. The ischium always forms part of the acetabulum.
In older individuals the acetabular bone may become united with either of the
other three pelvic bones : thus in Man, some Rodents, and Marsupials, it
fuses with the pubis ; more commonly, however, it fuses with the ischium or
ilium. In the Pinnipedia all four elements take part in the formation of
the acetabulum.

The pars acetabularis always becomes ossified much later than the other
pelvic elements, and the pubis ossifies later than the ilium and ischium.

In Monotremes and Marsupials of both sexes, two strong
bones ("marsupial bones") (Fig. 84, Om) arise from the anterior

/?*>--; v

LIMBS. 99

>^£^!.

border of the pubes, right and left of the middle line, and extend
forwards in a straight or oblique direction. At present no satis-
factory morphological explanation has been given of them, and
we can only compare them with the epipubic cartilage of Dipnoi
and Amphibia, which has the same relations to the pyramidalis
muscle (comp. p. 117). Fibrous rudiments of them are to be seen
among Dogs.

Before leaving the pelvic arch it may be pointed out that,
like the pectoral, it is not restricted to any particular body-
segment, but that both present much variety as to position,
phylogenetically as well as ontogenetically.

FREE LIMBS.

Fishes and Dipnoi. — In the Dipnoi, taking Cera tod us more
particularly into consideration, both pectoral and pelvic fins are
supported by a cartilaginous axis, made up of a great number of
small segments which are movable upon one another.

Numerous small jointed cartilaginous rods or radii are disposed
serially along the dorsal and ventral sides of this axis, and these
gradually decrease in size towards its distal end. Towards the
] eriphery of the fin the place of these cartilages is taken by fine
horny rays, which are covered by fibrous tissue as well as by the
^kin ; thus a broad paddle-like fin is formed. Both pectoral and
pelvic fins have a similar form and structure, and in their natural
position a lateral (external) and a medial (internal) surface can
be distinguished. The dorsal radii are much more numerous
than the ventral, which have undergone reduction. Thus the
biserial type of fin is already modified in Ceratodus, and this
modification is carried still further in Elasmobranchs (comp.
Fig. 86), until eventually only a single series of radii (Fig. 69, J-ia)
remains. This series corresponds to the dorsal row of Ceratodus,
but in consequence of its position in the adult fin, where it divides
a dorsal from a ventral surface, it is spoken of as lateral.1
The radii are much jointed, the segments being arranged in a
mosaic, and closely bound together by fibrous tissue. They are
covered over by the shagreen-like skin, and are continued out-
wards towards the periphery of the fin by a large number of horny
rays (Fig. 69, FS), so that the size of the fin is thus considerably in-
creased. Three larger cartilaginous basal elements lie proximally
to the small radii, and are spoken of as pro-, meso-, and meta-
pterygium respectively (Fig. 69, TV, J/s, Alt}. They are connected
with the pectoral arch (SB), and the metapterygium (Alt) together
with the distal elements lying along the same axis (a, I) represents

1 Fig. 54 stows how much reduction the skeleton of the fins has undergone in
Protopterus, ^the lateral rays having almost entirely disappeared. The whole ex-
tremity consists simply of a long segmented cartilaginous filament, which no longer
serves as an organ of locomotion.

H 2

100

COMPARATIVE ANATOMY.

FIG. 85. — PECTORAL FIN OF Ceratodus fosteri.

a, b, the two first segments of the main axial ray ; f, t, lateral rays, or radii ; F8t
horny rays, shown only on one side.

HS

FIG. 86. — DIAGRAM OF THE PREDOMINANT UNISEHIAL TYPE OF THE ANTERIOR
EXTREMITY OF ELASMOBRANCHS.

HS, the axis passing through the main ray ; ***, mimerous lateral rays of one side ;
ttt, few lateral rays of the other side.

the main ray of the fin (basipterygium). This serves as the
chief support for all the other rays, and is to be considered as
the homologue of the axial ray of Ceratodus. The above

LIMBS. 101

description holds good for the pectoral fin only of Elasmo-
branchs; the pelvic fin, however, is formed on a similar plan, but
remains in a lower stage of development, which is mainly expressed
by a limitation in the number of its basal segments. Thus a meso-
pterygium is not developed, and the propte^gium is more or less
rudimentary, the metapterygium being in this case also the most
important element. In male Elasmobranchs, a cartilaginous ap-
paratus— the skeleton of the " claspers " — is connected with the
metapterygium (comp. p. 327).

In Ganoids, and still more in Teleosteans, the essential
plan of the cartilaginous portions of the fins may be derived from
that of Elasmobranchs ; the primary skeleton of the fins, however,
undergoes a considerable reduction, and, in consequence of the
appearance of membrane-bones in connection with it, aprimary
and a secondary skeleton may be distinguished.

The skeleton of the fins of Siluroids, Cyprinoids, and Gymno-
tidas, amongst the Teleostei, comes nearest to that of Ganoidei,
that of the Gymnotidse being the most primitive of .the three.

GENERAL CONSIDERATIONS ON THE LIMBS OF THE
HIGHER VERTEBRATA.

Though it is easy to derive the skeleton of the fin of all the
orders of Fishes from a single ground-type, it is far more difficult
to trace the connection of the latter with the extremities of
Amphibia. Between these two types of limb there appears to
be a wide gap, in consequence of the different conditions of life
existing between Fishes and Amphibians ; and the question thus
arises — In what manner has the limb of an air-breathing Verte-
brate, adapted for progression upon land, become derived from the
fin, only fitted for use in the water ?

Paleontology furnishes no answer to this question ; we know of
no fossil intermediate forms of limb, and it is at present, therefore,
only possible to suggest a hypothesis on the subject. We may
suppose that when the primitive Amphibian first began to take on
a terrestrial mode of life, its fin, which we may look upon as a
single-jointed lever, and which amply sufficed for the movement of
the body in a fluid medium, became gradually transformed into a
many-jointed system of levers.

In other words, as the function of the limb was no longer
simply to propel the body forwards, but also at the same time
to lift it up from the ground, the firmly connected elements
of the skeleton of the fin gradually became loosened from, and
placed at an angle to, one another (knee, elbow), definite articu-
lations being formed between them in a proximo-distal direction.
Moreover, the extremity must have changed its position with regard
to the body, so that, instead of projecting horizontally outwards, it

102 COMPARATIVE ANATOMY.

became bent downwards, and thus the angle between it and the
median plane of the trunk was gradually reduced, until in Mammals
eventually, the longitudinal axis of the limb, when at rest, came to
lie parallel with the median plane of the body. In k the higher
types this is more particularly the case as regards tie posterior
extremities, the anterior undergoing the most varied adaptations
and modifications, and giving rise to tactile, prehensile, or flying
organs, or, as in aquatic Mammals, becoming once more converted
into rowing organs.

Thus we may also reduce the limbs of all the higher Vertebrata
to a single ground-type, and we may further connect the latter
with the fin of Fishes by taking the ground-plan of the fin of
Ceratodus and Elasinobranchs, consisting of a main axis and
lateral rays, as a starting-point. Figs. 87 and 88 will render this
statement clear. In Fig. 88, a thick line (US) is seen beginning
at //, and passing through Ft i, c, c, 2, to //. This is the main

Pa
FIG. 87. — DIAGRAMMATIC FIGURES TO SHOW THE RELATIONS OF THE FREE

EXTREMITY TO THE TRUNK IN FISHES (A), AND THE HIGHER VETEBRATES (B).

/S', pectoral arch ; Mt, metapterygium, which corresponds to the main ulnar ray

( Ui) ; lid, secondary radial ray.

axis, and from its proximal end (at II) a lateral ray passes off
through T, t, to /. A second series of lateral rays arises from
the other side of the axis. Thus we have here also the primitive
biserial form, with a marked preponderance of one row of radii.
At the same time it must be borne in mind that this arrange-
ment of radii on an axis is less plainly seen in the embryonic con-
dition than in the adult limb, and we must therefore speak of
the relations of these parts as similar rather than as strictly
homologous.

The fore- and hind-limbs show a great similarity as regards the
form and position of their various parts. A division into four princi-
pal sections can always be recognised : in the case of the fore-liinb
these are spoken of as upper arm (brachium), fore-arm (anti-
brachium), wrist (carpus), and hand (manus); and in the
hind-limb as thigh (femur), shank (crus), ankle (tarsus),
and foot (pes). While the bone of the upper arm (humerus)
and of the thigh (femur), corresponding probably to the met a-

LIMBS.

103

IIS-

K

FIG. 88. — POSTERIOR EXTREMITY OF Ranodon sibericus.

II, humerus ; ITS, axial ray ; F, fibula ; T, tibia ; i, intermedium ; t, tibiale ; f,
fibulare ; c, c, the two centralia ; 1 to 6, distal tarsalia ; t, traces of a sixth ray
within the proximal row of the tarsus ; / to V, the five metatarsals.

FIG. 89. — RIGHT FORE-ARM, CARPUS, AND HAND OF Salamandra maculosa.

(From above.)

.#, radius ; V, ulna ; r, radiale ; i, u, intermedio-ulnare ; c, centrale ; 1 to 4, first io
fourth carpalia ; Me, Me, metacarpals ; Ph, phalanges ; / to IV, first to fourth fingers.

104

COMPARATIVE ANATOMY.

pterygium, is always unpaired, two bones are present in the
fore- arm and shank. The former are called radius and ulna,
and the latter tibia and fibula. The hand and foot may also be
respectively divided into two sections, a proximal metacarpus
and metatarsus, and a distal series of phalanges, which form
the skeleton of the fingers and toes (digits).

Both manus and pes are made up of several series of cylin-
drical bones. There are never more than five complete series,
which, except as regards number, present essentially similar primary
relations throughout the higher Vertebrates. The skeleton of the
carpus and tarsus, each of which always consists of a series of small
cartilages or bones, shows much variation, but the following may
be taken as a ground-type. Round a centrale, which may be
double, is arranged a series of other elements, in which three
proximal, and a varying number (four to six) of distal, may be dis-
tinguished. The proximal, in correspondence with their relations
to the bones of the fore-arm and shank respectively, are spoken of as
radiale or tibiale, ulnare or fibulare, and intermedium;
while the distal are called carpalia or tarsalia / to VI (in the
narrower sense). They are counted beginning from the radial or
tibial (pre-axial, Huxley) side (Figs. 88 and 89).

FIG. 90. — EIGHT TARSUS OF Discoglossiis pictus. (From above.)

Aa, astragalus ; Ca, calcaneum ; 1 to 4, the four separate (cartilaginous) tarsalia on
the tibial side, in relation with the rudimentary extra finger (7) and digits 1 to 3
(77. to TV) ; 5 + 6, fibrous band representing the tarsals of digits Fand VI ;
7, rudimentary extra digit on tibial side ; 77 to VI, metatarsals of digits 1 to 5 ;
t, single phalanx of the extra tibial digit.

Amphibia. — Whilst the anterior and posterior extremities of
Urodeles are formed essentially on the ground -plan described above
(Figs. 87 to 89), 1 in the case of the Anura the radius and ulna be-

1 Numerous secondary fusions of the individual elements of both carpus and
tarsus may, however, occur ; this applies also to the Anura. As a rule the anterior
extremity 'is only provided with four fingers, though there are reasons for supposing
that it at ore time, like the posterior, possessed five complete fingers. The number of
phalanges varies in different Amphibians.

LIMBS. 105

come fused together, and the intermedium is wanting. The proximal
row of the tarsus, moreover, consists of only two cylindrical bones,
which are usually united together by an envelope of cartilage.
One of these corresponds to a tibiale plus intermedium, and is
called the astragalus; the other or calcaneum answers to a
fibulare (Fig. 90, Aa, Co).

In the distal row there are as a rule four separate elements.
Rudiments of a fifth carpal, as well as of an extra digit on the radial
side, are usually present, and traces of an extra toe are also seen on
the tibial side of the tarsus.

In Anura, the metatarsals and phalanges, between which the
web of the foot is stretched, are very long and slender. The femur,
as well as the bones of the shank, which are fused together, are
also exceedingly long, in correspondence with the mode of
progression of these animals.

The skeleton of the extremities is more strongly ossified in
Anura than in Urodeles, in which many of the elements remain
cartilaginous.

Reptiles. — Chelonians and Lizards closely resemble Urodeles
in the structure of the carpus, and here also traces of the

ft if

FIG. 91. — RIGHT CARPUS OF A YOUNG Alligator Indus. (From above.)

7v', radius ; U, ulna ; r, radiale ; u, ulnare ; C, centrale ; 1 to 5, the five carpalia, as
yet unossified, of which 1 and 2, as well as 3, 4, and 5, have become fused
together ; f, pisiform ; / to V, the five metacarpals.

former possession of an extra finger on the radial side are to be
seen. The tibia and fibula always remain separate.1

In Crocodiles, which possess no trace of an intermedium, the
proximal row of the carpus consists of two hour-glass-shaped bones
— a larger radiale, and a smaller ulnare (Fig. 91). A rudiment
of a sixth ray is present on the outer side of the latter, and this

1 In Hatteria alone, amongst existing Reptiles, a double centrale is present in the
young animal.

106 COMPARATIVE ANATOMY.

corresponds to the so-called pisiform bone of Mammals. The
distal row of carpals is much less developed than the proximal.

In the fossil Flying Eeptiles (Pterodactylus, Rhauiphorhynchus) the fourth
finger was produced into a long jointed rod, which supported a wing-like
expansion of the integument.

In all Reptiles, the tarsus undergoes considerable fusion,
especially in its proximal portion, and leads gradually on to the
type seen in Birds. Thus in Chelonians and Lizards the proximal
tarsals all run together into a single mass, which corresponds to
tibiale, intermedium, fibulare, and centrale. Traces of an extra
radial ray are also present here.

In the distal row five separate tarsals are developed, but these
may unite partly with one another (Cheloniaus), and partly with the
corresponding metatarsals (Lizards), and thus there is an increas-
ing tendency for the movement of the foot to take place by means
of anintertarsal articulation, as in Birds.

In Crocodiles, there are two bones in the proximal row of
the tarsus, one of which corresponds to a tibiale, intermedium, and
centrale, the other to a fibulare. The former is spoken of as the
astragalus, the latter as the calcaneum, and on it a definite heel
(calcaneal process) is seen for the first time in the animal series.
The distal row consists originally of four small cartilages, but these
later undergo a partial reduction.

Birds. — While the skeleton of the manus of Archasopteryx (Fig.
92) agrees in many points with that of Reptiles, that of existing

FIG. 92. — ANTERIOR EXTREMITY OF Archceopteryx. (After C. Vogt. )

Birds has become considerably modified by adaptation for flight,
and correspondingly reduced. Of the five carpals of the embryo,
the three distal become fused with the corresponding meta-
carpals (Fig. 93, Me, Me), while the two proximal remain separate
as a radiale and an ulnare. The metacarpals themselves become
in part united together, and only bear a very limited number of
phalanges at their free ends.

The small size of the head of the humerus, as well as the absence of a
ridge for the insertion of the pectoralis major, and the probable small size of
the sternum, prove that Archseopteryx could not have been a good flier. In

LIMBS. 107

other points the humerus, radius, and ulna correspond closely with those of
existing Birds. There was only one carpal (radiale), and the manus consisted
of three free metacarpals and digits, of which the first possessed two, the
second three, and the third four phalanges : all the digits were provided
with claws (Fig. 92).

Ten families of existing Carinate Birds possess the same number (two) of
phalanges on the first finger as Archeeopteryx, the distal one bearing a claw.
Four families of Carinatse also possess three phalanges on the second finger,
like Archaeopteryx, but in only two of these families is there a claw on this
digit. The third finger in all existing Carinates has only one phalanx,1 as
compared with four in Archaeopteryx, and this never bears a claw. Amongst
the Ratitee, Apteryx and Casuarius possess only a single digit (the second),
and it, like the first finger of Struthio and Rhea, is provided with a claw.

The strongly-developed and pneumatic 2 bones of the arm and
fore-arm stand out in sharp contrast with the greatly reduced
skeleton of the manus ; and the anterior extremities in most Birds
of flight, as the principal organs of locomotion, greatly exceed the
posterior in size (Fig. 93, H, R, U).

FIG. 93. — ANTERIOR EXTREMITY OF BLACKBIRD (Turdus merula).

H, humerus ; E, radius ; U, ulna ; r, radiale ; u, ulnare ; Me, Me, the three meta-
carpals, with which the distal row of carpals has united ; / to ///, the three
digits.

The tarsus of Birds consists in the embryo of three elements,
two small proximal and a broader distal. The former (tibiale
and fibulare) unite later with the distal end of the tibia, thus
forming a tibio-tarsus, while the latter, which corresponds to
tarsalia 1 to V, becomes included in the base of the metatarsus.
Thus the foot of adult Birds no longer possesses any distinct tarsal
elements, though, as in Chelonians and Lizards, the foot really
moves by an intertarsal articulation. Of the original five meta-
tarsals, the fifth soon disappears, while the second, third, and fourth
become united with one another and with the distal element of
the tarsus to form a single bone, the tarso-metatarsus (Fig.
94, Mt}. The first metatarsal remains to a greater or less extent
independent.

1 In embryos of the Duck a cartilaginous rudiment of a second phalanx is pre-
sent in the third finger of the manus. The Ostrich alone amongst existing Birds
possesses a second phalanx on the third finger.

2 See p. 262, concerning the pneumatic character of the bones.

108

COMPARATIVE ANATOMY.

The number of toes varies between two and four ; that of the phalanges
is normally 2, 3, 4, 5, reckoning from the first to the fourth digit. The tibia,
even from the first, greatly exceeds the fibula in size.

Palaeontological discoveries prove clearly that the form of the Bird's tarsus
has been gradually evolved from that of Dinosaurian-like forms.

The foot of Archseopteryx was very similar to that of existing
Birds, though the primitive separation of the tarsal elements is

FIG. 94. — POSTERIOR EXTREMITY or BLACKBIRD (Tur dun mcrula}.

Fe, femur ; T, tibia, united with the fibula (F) ; t,*, apophyses of the tibia and meta-
tarsus, corresponding respectively with the proximal and distal tarsal-plate : the
original division of the metatarsus (Mt) into separate bones is indicated at its
distal end at t ; / to 17, first to fourth digits.

much clearer, and marked by deeper furrows. The first metatarsal
was turned slightly outwards, and the toe itself backwards, like
that of most existing Birds.

Mammals. — In Mammals the anterior extremity either
remains in the condition of a simple organ of locomotion, or it
gives rise to a prehensile organ.1 In the latter case the radius and
ulna, instead of being firmly connected together, remain separate,
and articulate with one another. The movements of rotation
which are thus rendered possible are spoken of as pronation
and supination.

1 In Bats, by the elongation of the fingers, between which a wing-membrane is
stretched, it serves for flight.

LIMBS. 109

The carpus and tarsus correspond essentially with those of
Urodeles and Chelonians, and, as in them, certain of the elements
may become fused together. Thus the intermedium and tibiale
as a rule unite to form an astragalus,1 while the fourth and
fifth carpals become fused to form the so-called^ u n cifojr m b ma p.t
and the corresponding tarsals give rise to the cuboid. A
centraJe is always present at an early stage in all five-fingered
Mammals, but as a rule it becomes fused later with the radiale,
as in the case of the Gorilla, the Chimpanzee, and Man, though it
may persist (in 4 cases per cent.) in the human subject throughout
life. In the tarsus the centrale (navicular) remains distinct, and
usually lies on the inner border of the foot. (Compare p. 106 and
small type below for mention of the pisiform bone.)

A considerable modification of the homologies of the carpal and tarsal
elements described above must be expected shortly. The results of recent
researches (Bardeleben, Baur, Albrecht) on this subject are briefly as
follows : —

The astragalus corresponds to an intermedium as well as to another element
which remains independent in Marsupials, but in other forms (e.g. Man) exists
only in the embryo, and unites later with the intermedium. This second element
is either to be looked upon as a first centrale, or perhaps as a second inter-
medium, and is represented in the carpus by the cuneiform (ulnare). The
navicular corresponds to the scaphoid of tlie Mammalian carpus, that is,
to a navicular proper plus a second centrale. The pisiform corresponds to
the whole calcaneum. In human embryos of the second month a distinct
cartilage is present on the tibial side of the tarsus, and this probably answers
to a small bone on the tibial border of the foot of Monotremes, American
Marsupials, Edentates, Carnivores, Rodents, Insectivores, and Monkeys. This
most likely corresponds to an extra (first) toe (" prehallux," Bardeleben). In
the animals mentioned above, with the exception of Monotremes and the
addition of Cheiroptera, a " prepollex " is also present in the manus, consist-
ing of a carpal and a rudimentary metacarpal. The distal rows of the carpus
and tarsus correspond as regards their individual elements. That the unciform
and cuboid originally each represent two elements2 (Bardeleben) is shown
by the fact that two digits are attached distally to each, and that in Marsupials,
Kodents, and Hyperoodon, an indication of a division into two parts persists
throughout life. Centetes madagascariensis alone has a double centrale in the
carpus.*

It is interesting to note the reduction which has taken place in the feet
of Ungulates in the course of time. In Fig. 95 sketches of the stages in the
phylogenetic development of the fore-foot of the Horse are given, showing
how it has been gradually derived from a tetra- or pentadactyle ancestor.
While in this case the third digit becomes greatly enlarged relatively (per -
issodactyleform), and eventually is the only one remaining, in cloven-footed
Ungulates the third and fourth digits are both functional, and equally strongly
developed (artiodactyle form), and may be united together to form a

1 In Marsupials only does the intermedium remain as an independent bone. In
the human embryo it exists as an independent cartilage, but later almost always
becomes fused with the tibiale.

2 According to Baur, the cuboid and unciform arise each as a single mass,
their double condition being secondary.

3 Baur states that a double centrale is never present in any Mammalian
embrya

no

COMPARATIVE ANATOMY.

" cannon-bone," while the others are gradually reduced. A similar reduction
takes place in the hind-foot, and is here as a rule more rapid.

As far back as the Eocene period Ungulates were separated into Perisso-
and Artiodactyles ; a long series of ancestors is hereby indicated.

If T

FIG. 95. — FORE-FOOT OF ANCESTRAL FORMS OF THE HORSE. 1. OEOHIPPFS
(Eocene). 2. MESOHIPPUS (Upper Eocene). 3. MIOHIPPUS (Miocene). 4. PROTO-
HIPPUS (Upper Pliocene). 5. PLIOHIPPUS (Uppermost Pliocene). 6. EQUUS.

FIG. 96.— SKELETON OF THE LEFT FORE-LIMB OF A, PIG ; B, HYOMOSCHUS ; C,
TRAGULTJS ; D, ROEBUCK ; E, SHEEP ; F, CAMEL. (From Bell, after Garrod.)

The tibia is the most important bone of the shank, just as the
radius is of the fore-arm, and the thigh is usually shorter than the
shank. A sesamoid bone developed in the distal tendons of the
great extensor muscles of the shank is known as the knee-cap or
patella. This is already present in Lizards and Birds.

LIMBS. Ill

The number of phalanges is similar in both hand and foot : in
the first digit there are only two, while in the others there are
three. An exception to this rule is seen in Cetacea, in which the
phalanges are numerous.

BIBLIOGRAPHY.

BARDELEBEN, K. — Das Os intermedium tarsi der Saiigethiere. Zool. Anz. VI.
Jahrgaug, 1883. Beitrage z. Morphol. des Hand- u. Fussskeletes. Sitz. Ber. d.
Jen. Gesellsch. f. Medic, u. Naturwiss. 1885.

BAUR, G. — Zur Morphologic des Carpus und Tarsus der Wirbelthiere. Zool. Anz.
YII1 Jahrgang, 1885. Zur Morph. d. Carpus und Tarsus der Eeptilien. Zool.
Anz. VI11 Jahrgang, 1885. Ueb. das Archipterygium und die Entwick. des
Cheiropterygium aus dem Ichthyopterygium. Zool. Anz. VIII Jahrgang, 1885.
On the Centrale Carpi of Mammals. American Naturalist, Vol. XIX. p. 195, and
Morphol. Jahrb. Bd. X. Heft 3. On the Morphology of the Tarsus in Mammals.
American Naturalist, Vol. XIX. p. 86, and Morphol.' Jahrb. Bd. X. Heft 3. The
Trapezium of the Camelidce. American Naturalist, Vol. XIX. p. 196, and
Morphol. Jahrb. Bd. XI. Heft 1. Bemerk. ub. d. "Astragalus" u. d. "Inter-
medium tarsi" der Saiigethiere. Morphol. Jahrb. Bd. XI. Heft 3.

BORN.— Die sechste Zche der Anuren. Morphol. Jahrb. Bd. I. 1875.

DAVIDOFF, M. v. — Beitrage z. vergl. Anat. d. hinteren Gleichmasscn d. Fische, I.
Morphol. Jahrb. Vol. V. 1879.

DOHRN, A. — Studien zur Urgeschichte des Wirbelthierkorpcrs. VI. Die paarigen
und unpaarigen Flossen der Selachier. Mittheil. aus der zoolog. Station zu
Neapel, Bd. V. Heft. 1, 1884.

GEGENBAUR, G. — Unters. zur vergl. Anatomie der Wirbelthiere: Schultergurtel der
Wirbelthiere. Carpus und Tarsus und Brustflosse der Fische. Leipzig, 1864,
1865. Ueber das Archipterygium. Jen. Zeitschr. Bd. VII.

GO'TTE, A. — Beitr. zur vergl. Morphologic des Skeletsystems der Wirbelthiere : Brust-
beinund Schultergurtel. Arch.f. mikr. Anat. Bd. XIV. 1877.

HOFFMANN, C. K. — Beitr. z. Kenntniss des Beckens der Amphibien und Eeptilien.
Niederl. Archiv fur Zool. Bd. 111.

HUXLEY, T. H. — The Characters of the Pelvis in the Mammalia, &c. Proc. Roy. Soc.
Vol. XXVIII. 1879. On Ceratodus fosteri, with some Observations on the Classi-
fication of Fishes. Proc. Zool. Soc. 1876.

JOHNSON, ALICE. — On the Development of the Pelvic Girdle and Hind Limbs in the
Chick. Quart. Journ. of Micros. Science, 1883, and Studies from the Morphol.
Lab. of the University of Cambridge, Vol. II. Part I. 1884.

LEBOUCQ, H. — Resume d'un mlmoire sur la morphologic du carpechez les mammi/dres.
Bull, de Tacad. r. de medecine de Belgique. 3 Ser. T. XVIII. Nro. 2; and
Archiv de Biologic, T. V. 1884.

LECIIE, "W. — Zur Anatomie der Beckenregion bei Insectivoren, <tc. K. Schwedischc
Academic d. Wisscnschaften, Bd. XX. 1882. Das Vorkommen und die Morphologic
Bedeutung des Pfannenknochens (Os acetabuli). Internationale Monatschrift
fur Anatomie und Embryologie, Bd. I.

MARSH, 0. C. — The Limbs of Sauranodon. Amer. Journ. of Science and Arts,
Vol. XIX. 1880.

MAYER, P. — Die unpaaren Flossen der Selachier. Mittheil. aus d. zool. Station zu
Neapel, Bd. VI. 1885.

MIVART, ST. GEORGE. — On the Fins of Elasmobranchii. Trans. Zool. Soc. Vol. X.

PARKER, W. K. — Structure and Development of the Shoulder-Girdle and the Sternum.
Ray Society, 1868.

ROSENBERG, E. — Uebcr die Entwicklung der Wirbelsdule und das Centrale Carpi des
Menschen. Morphol. Jahrb. Bd. 1. 1876.

THACHER, J. TH. — Median and Paired Fins, &c. Transact, of the Connecticut
Academy, III. 1877.

"WIEDERSHEIM, R. — Salamandrina perspic. d'C. Versuch einer vergl. Anat. der
Salamandrinen. Genua, 1875.

C. MUSCULAR SYSTEM.

THE muscles, commonly spoken of as flesh, may be divided
into two groups, according to their histological character, namely,
into those with smooth, and those with transversely-striated fibres.
The former are phylogenetically the older, and are to be looked
upon as the precursors of the latter.

The smooth involuntary muscle-fibres preponderate in the
viscera, derma, and vessels, and are not under the control of the
will ; the striated or voluntary muscles occur principally in the
body-walls and organs of locomotion, and are almost without
exception under the control of the will. The following general
statements refer exclusively to the latter kind of muscles.

In its simplest form, an origin, a belly, and an insertion,
may be distinguished in each muscle. The muscles of the trunk
are as a rule flat, while those of the extremities have usually an
elongated, cylindrical, or prismatic form. In some case*, however,
they assume the most various shapes ; for instance, there may be
more than one origin (bicipital, tricipital, or quadricipital forms),
the belly may be double (biventral or digastric form), or the muscle
maybe saw-shaped, or have its fibres arranged in a single or double
series like a feather.

All the muscles are surrounded by fibrous sheaths, or fasciae,
by means of which they are more or less firmly connected with
one another (intermuscular septa) and with the integument and
skeleton.

A muscle may undergo very considerable modification both in form
and position by a change of origin and insertion, by division (intermediate
tendons dividing it into two or more parts), or by splitting into layers, and it
may thus give rise to one or more new independent muscles. If the action of a
muscle becomes unnecessary, it either disappears partly or entirely, or what
remains of it contributes to the strengthening of a neighbouring muscle.

In the embryo, the individual groups of muscles (e.g. the flexor subiimis
and profundus, and the masticatory muscles) are not at first separate from one
another, but consist of a uniform (mesoblastic) blastema, the splitting of which
takes place later by the ingrowth of fibrous septa. In this manner the indi-
vidual muscles are differentiated, and they gradually become more distinct by

MUSCULAR SYSTEM. 113

use, that is, in post-embryonic time. Certain muscles may disappear in the
course of development, and changes of position may also take place.

Wherever a marked friction occurs, ossifications (sesamoids) may become
developed in the course of a muscle or tendon. The muscle thus gains an
extra point of attachment, and a single-jointed lever is converted into a
double-jointed one.

The higher one passes in the animal scale, the more numerous
do the differentiations of the muscular system become, and the
more varied its relations to the skeleton. One portion, the dermal
musculature, which is sometimes not largely developed, always
shows an independence as regards the rest of the muscular sys-
tem, though this independence is as a rule acquired secondarily.
Developed to a very slight extent in Fishes and Amphibia, the
dermal musculature is of great importance in Reptiles and Birds on
account of its relations to the scutes, scales, and feather?. It reaches
its greatest development amongst Mammals, where it may extend
over the back, head, neck, and flanks (Echidna, Dasypus, Pinni-
pedia, Erinaceus, &c.). In Man, only a slight rudiment is found of
it in the shape of the platysma myoides, which extends over
the neck and part of the breast and face.

MUSCULATURE OF THE SKELETON.

MUSCLES OF THE TRUNK.

Under this head are included all the muscles of the body
which remain after the removal of those connected with the limbs.
They arise from the muscle-plates of the embryo, — that is, from the
outer parts of the mesoblastic somites,1 and, particularly in higher
types, may be separated into various groups, viz., a cranial and
visceral, a dorsal and ventral.

Fishes and Amphibia. — In Fishes and the lower Amphibia
the dorsal and ventral groups of muscles form a uniform mass,
which is spoken of as the lateral body -muscle (M. lateralis).
On each side of the body this consists of two portions, a dorsal
and a ventral, which meet together laterally, as well as in the
mid-dorsal and ventral lines (Fig. 97, D, F), and which are made up
of a great number of metamerically arranged portions (myotomes
or myocommata), separated from one another by connective-
tissue septa (Fig. 97, M, M). Along the latter, ribs may be
developed, and thus a much greater degree of firmness is attained.

This metameric arrangement of the musculature of the trunk,
which has such an important relation to the spinal nerves as well

1 The development of the sub- vertebral, cutaneous, and diaphragmatic muscles,
requires further investigation.

I

114

COMPARATIVE ANATOMY.

as to the segmentation of the axial skeleton, forms a character-
istic feature in Vertebrates, and distinct indications of it may be
traced in all representatives of the group up to Man.

JIM v OB

FIG. 97. — LATERAL MUSCLES OF AmpMoxus.

D, dorsal, and V, ventral portions ; M, M, the individual myotomes ; QB, transverse
muscles of abdominal region ; C, cirrlii ; F, F1, tail-fin.

RM

FIG. 98. — THE ENTIRE MUSCULATURE OF Siredon pisciformis.

LI, lateral line. ; D, dorsal, and .F, ventral portion of caudal muscles; RM, dorsal
portion of lateral muscles of the trunk ; 0, 0, outermost layer of the external
oblique muscle, arising from the lateral line, and extending to the fascia, F ; at *
a piece of this layer is removed, exposing the second layer of the muscle (Ob) ;
at Re the oblique fibres of the, latter pass into longitudinal fibres, indicating the
beginning of the differentiation of a rectus abdominis ; at Re1 the rectus-system
is seen passing to the visceral skeleton ; Me, fibrous partitions between the myo-
tomes of the dorsal portion of the lateral muscles ; T, temporal ; Ma, masseter ;
Dg, digastric ; Mhl, mylohyoid (posterior portion) ; Go, external ceratohyoid
muscle ; Lv, levator arcuum braiichialium ; ftt, levator branchiarum ; Cph,
cervical origin of the constrictor of the pharynx ; Th, thymus gland ; Lt,
latissimus dorsi ; Ds, dorsalis scapulas ; Cu, cucullaris ; SS, suprascapula ; Ph,
procoraco-humeralis.

The cranio-visceral musculature is to be looked upon as having
been derived out of the lateral muscles in consequence of the
development of the visceral skeleton.

It may be asserted as a general rule that the dorsal portion of
the lateral muscles of the trunk, — except in the caudal region,

MUSCULAR SYSTEM.

115

where the uniform character persists botli dorsally and ventrally,
— retains throughout more primitive relations than does the ventral,

FIG. 99. — THE MUSCULATURE OF Siredon piscifarmis. (From the ventral side.)

0, outermost layer of the external oblique, passing into the fascia, which is shown cut
through at F ; Oh, second layer of the same muscle ; Re, rectus abdominis,
passing into the visceral musculature (sternohyoid) at Re1, and into the pector-
alis major at P ; Mh, J\Ihl, anterior and posterior portions of the mylohyoid,
which is cut through in the middle line, and removed on the left side, so as to show
the proper visceral musculature ; Ce, Ci, Ci1, external and internal ceratohyoid :
the former is inserted on to the hyoid (Hy) ; Add, adductor arcuum branchialium ;
C, constrictor arcuum branchialium ; Oph, portion of the constrictor of the
pharynx arising from the posterior branchial arch ; Dp, depressores branchinrum ;
Gh, genio-hyoid : P/>, procoraco-humeralis ; Spc, supracoracoideus ; Cbb, coraco-
brachialis brevis ; Clo, cloaca ; La, linea alba.

the latter becoming greatly modified in order to form the walls of
the body-cavity.

I 2

116 COMPARATIVE ANATOMY.

Thus, even in many Fishes, differentiations occur on the ventral
side which bring about the formation of straight and oblique
abdominal muscles (rectus et obliqui abdominis).

This differentiation is carried further in certain Dipnoi, and
is still more marked in tailed Amphibians. In the latter the
ventral muscles of the trunk become split into four layers, and in
the higher types, — such as the sexually-mature Salamander and
Triton, — a rectus abdominis lying right and left of the median
line is plainly differentiated (Fig. 99, He, He).

The outermost layer of the lateral muscles of the abdomen
does not appear to be retained in the higher types ; the other three
layers however remain, and are distinguished from without in-
wards, according to the direction of their fibres, as external and
internal oblique, and transversalis (Figs. 98, 99, 0, Ob).

The external and internal obliques extend from the visceral skeleton,
that is, from the floor of the mouth, to the pelvic arch, the former even
being directly continuous with the musculature of the tail (Fig. 98) ; the
transversalis ceases in the region of the heart, and stands in the closest
relation with the fascia transversalis and the peritoneum, on the outer side of
which it lies. A similar arrangement is seen in all Vertebrates from the
Urodeles onwards.

The muscular system of the trunk of Anura shows a negative condition
as compared with that of Urodeles as above described : the lateral muscles of
the abdomen consist of two layers only, and their metameric arrangement seen
in the larva becomes later more and more obliterated. The rectus abdominis
is always well differentiated, and possesses a varying number of myocommata.

Reptiles. — In Reptiles, the lateral muscles of the trunk attain
a much higher grade of development. This is to be accounted
for by the more perfect form of the skeleton, more especially of
the ribs and pectoral arch. The ribs and intercostal muscles now
play an important part in respiration, and changes, necessitated by
the more important development of the lungs, are thus brought
about.

The distinction between thoracic and abdominal regions becomes
gradually more plainly marked, and distinct external and inter-
nal intercostal muscles are now differentiated. In the lumbar
region the ribs become gradually withdrawn from the muscles lying
between them ; the muscles thus lose their intercostal charac-
ter, and form connected sheets, extending between the last pair
of ribs and the pelvic arch (e.g. the quadratus lumborurn,
which lies close against the vertebral column, and the obliqui).

The rectus abdominis, which is always well developed, be-
comes divided into three portions, — a ventral, an internal (a
subdivision of the latter), and a lateral.

While no important differentiation is noticeable in the dorsal
portion of the lateral body-muscles in Urodeles, a great sub-
division of these muscles is seen in Reptiles. In them may be
distinguished a longissimus, an ileocostalis, interspinales,
semispinales, multifidi, splenii, and levatores costarum,

MUSCULAR SYSTEM. 117

together with the scaleni, which belong to the last-mentioned
group.

The muscles of the main part of the tail retain primitive rela-
tions similar to those seen in Fishes : at the root of the tail,
however, new muscles become differentiated.

Birds. — In Birds the primitive character of the trunk-muscles
has disappeared far more than in Reptiles.

This is mainly to be accounted for by the excessive develop-
ment of the muscles of the anterior extremity, — the pectoralis
major more particularly, — and the corresponding backward
extension of the breast-bone.

External and internal oblique muscles are present, but only
slightly developed : this is more particularly true of the inter-
nal, which appears to be undergoing degeneration. No trace of
a trans versalis can be distinguished, but on the other hand, a
paired, unsegmented rectus is present.

External and internal intercostals are well developed, and a
triangularis sterni appears for the first time on the inner surface
of the sternal ends of the ribs.

The dorsal portion of the trunk musculature is only slightly
developed in the region of the body, though very strongly marked
in the neck.

All these modifications in Birds seem to be accounted for by
the great specialisation of the mechanisms for flight and respira-
tion, to assist which the greatest possible number of muscles are
brought into play, and which thus greatly influence the whole
organism : an essential difference is thus brought about between
Birds and Reptiles.

Mammals. — Three lateral abdominal muscles are present in
Mammals, an external and internal oblique and a trans-
versalis. Except in a single instance (Tupaia), they are entirely
unsegmented, and consist of broad uniform plates of muscle. To-
wards the middle line, they pass into strong aponeuroses, which en-
sheath the rectus abdominis. The latter consists of a single band
on each side, and possesses a varying number of myocommata ; it
is no longer connected with the axial muscles of the neck belong-
ing to the same system (sternohvoid, sternothyroid, &c.), as is the
case in Urodeles, for the sternum is always interposed between
them, as it is in Saufopsida.

In Monotremes and Marsupials, a strong pyramidalis mus-
cle lies on the ventral side of the rectus abdomnis. It arises from
the inner border of the "marsupial bones" (epipubes), stands in
important relation to the pouch (marsupium), and may extend
forwards as far as the sternum. In the higher Mammals, where
the epipubes are absent, the pyramidalis becomes greatly reduced
or entirely lost. Traces of it are, however, commonly to be met

118 COMPARATIVE ANATOMY.

with as far as the Primates, always arising from the anterior border
of the pubis, right and left of the middle line.

The external and internal oblique muscles are to be met with
in the thoracic region in Mammals, as in Sauropsida, in the form
of external and internal intercostals.

What has been said above as to the differentiation of the
dorsal portion of the trunk-muscles in Reptiles, applies also in
its essentials for Mammals.

MUSCLES OF THE VISCERAL SKELETON AND HEAD.

The common origin of the muscles of the visceral skeleton and
the ventral trunk musculature may be much more easily proved than
that of the cranial muscles with the latter system. The muscles
of the head may be divided into two sections, viz. facial muscles
and muscles of the jaws.

Fishes. — Leaving out of consideration the Cyclostomes, which
show a remarkable transformation of the cranio-visceral muscula-
ture in correspondence with their peculiar cranial skeleton (sucto-
rial apparatus) and branchial basket, these muscles in Elasmo-
branchs may all be regarded from the same standpoint. They
may be divided into the following four groups or systems :—

1. Superficial circular muscles.

2. Upper adductors of the arches.

3. Middle flexors of the arches.

4. Ventral longitudinal muscles.

The latter group occupies a more independent position than
the other three, which are more closely connected together.

The superficial circular muscle, receiving its nerve-supply from
the vagus, glossopharyngeal, facial, and trigeminal (third division)
acts essentially as a constrictor; it narrows the entire oral and
pharyngeal cavities, closes the gill-clefts, and elevates the whole
visceral skeleton, together with the floor of the mouth and
pharynx.

The main mass of the upper and middle flexors is supplied by.
the vagus and glossopharyngeal, and their muscles act essentially
as adductors of the branchial arches, bringing them nearer
to one another.

The ventral longitudinal muscles, supplied by the first
and second spinal nerves, are to be looked upon as a direct con-
tinuation of the ventral portion of the trunk -muscles, that is,
of the rectus- system, which is to a certain extent undifferentiated
in Fishes. Like the rectus, these muscles possess tendinous inter-
sections, and they extend from the anterior border of the pectoral
arch forwards to the lower jaw, or only to the hyoid arch (coraco-
mandibular and coraco-hyoid muscles).

MUSCULAR SYSTEM. 119

The structure of the cranio-visceral musculature of Teleostei
differs considerably from that roughly sketched out above, so that
the different groups of muscles must be arranged in an entirely
different manner. Thus the following divisions may be distin-
guished : — (1) Muscles of the jaws ; (2) muscles of the dorsal,
and (3) muscles of the ventral ends of the visceral arches.

Each of these groups may again be subdivided, but further
details about their arrangement, which is often very complicated,
cannot be given here.

Amphibia. — It is to be expected, a. priori, that the muscula-
ture of the visceral skeleton should be more highly developed in
gill-breathing than in lung-breathing Amphibians ; we thus find
that in the former, more primitive relations are met with, connecting
them with lower forms, while in the latter a greater modification,
or rather reduction, of these muscles takes place.

Between the two rami of the lower jaw there lies a muscle
with transverse fibres (the mylohyoid), supplied by the third
division of the trigeminal and the facial ; this represents the last
remnants of the constrictor muscle of Fishes. As the elevator of
the floor of the mouth, it stands in important relation to respira-
tion and deglutition, and is retained throughout the rest of the
Vertebrata up to Man (Fig. 98, 99, Mkt Mhl).

A continuation of the trunk-musculature (the omo-, sterno-, and
genio-hyoid) provided with tendinous intersections, lies above the
mylohyoid (Fig. 99, fie1, GJi). These muscles, which serve to pull
the visceral skeleton forwards and backwards, are supplied by the
first and second spinal nerves.

In contrast to Fishes, there is in Amphibia a definite differen-
tiation into muscles of the tongue, that is, into a hyoglossus
and a genioglossus, but these also must be considered as having
been derived from the anterior end of the ventral muscles of the
trunk ; they are present in all Vertebrates, from the Amphibia
onwards, and are supplied by the hypoglossal (the first spinal nerve
of Amphibians).

In the Perennibranchiata and in Salamander larvas the muscles
of the hyoid and of the visceral arches may, by analogy
with Fishes, be divided into a ventral and a dorsal group : the
latter disappears in adult Salamanders and Anura, only the ventral
persisting. Their function is to raise and depress the branchial
arches, as well as to draw them forwards and backwards. To these
may be added constrictors of the pharynx as well as (in gill-
breathing animals) levators, depressors, and adductors of the
external gill filaments (Figs. 98 and 99). They are innervated
by the vagus and glossopharyngeal.

The jaw-muscles may be divided into a depressor (digastric,
or biventer mandibulaB, Fig. 98, Dg), supplied by the facial, and
into elevators of the lower jaw (masseter, temporal, and pterygoid

120 COMPARATIVE ANATOMY.

muscles, Fig. 98, Ma, T), supplied by the third division of the
trigeminal. All these muscles, which may be derived from the
adductor of the mandible of Elasmobranchs and Ganoids, arise
from the auditory region of the skull.

Amniota. — With the simplification of the visceral skeleton in
Amniota, there is a considerable reduction of the musculature
belonging to it. All muscles connected with branchial respiration
are of course wanting, and the ventral trunk-muscles, as men-
tioned above, are always interrupted in their forward extension
by the sternum and pectoral arch. At the same time, the
muscles along the neck and on the floor of the mouth met with in
Amphibia are present here also; they are the mylo-, sterno-,
omo-, and genio-hyoid, as well as the hyoglossus and genio-
glossus. To these may be also added a sterno-thyroid, and a
thyro-hyoid, continued forwards as a prolongation of the former.

The stylo-hyoid, stylo-glossus, and stylo-pharyngeus of Mammals,
arising from the styloid process and stylo-hyoid ligament, and undergoing
numerous variations, are neomorphs. They are supplied partly by the facial,
partly by the glossopharyngeal, and act as retractors of the tongue and
levators of the pharynx and hyoid.

The muscles of the jaws resemble those of Amphibia,
although, especially in the case of the pterygoids, they are much
more sharply differentiated, and are throughout more strongly
developed. (A secondary subdivision of the muscles may occur
in Birds and Reptiles, as for instance in the case of the temporal
muscle.)

The facial muscles, forming a marked feature for the first time
in Mammals, arise originally in the neighbourhood of the eyes,
mouth, nose, and ears, around which they are principally grouped
throughout life ; they are thus connected with the most important
organs of sense. They are supplied by the facial nerve, and attain
their greatest development in Primates.

Following in the lines laid down by Gegenbaur in his Lehrbuch der
Anatomic des Menschen on the origin of the facial muscles, G. Huge has made
detailed researches on the facial musculature of Lemurs, from which he arrives
at the following results.

The fact that all the muscles supplied by the facial nerve belong to
the same series indicates that those related to the visceral skeleton, and
having originally nothing to do with the face, which are supplied by the same
nerve, must have shifted upwards from the region of the lower jaw and neck,
so as to come into close relation with the soft "parts surrounding the apertures
of the ear and mouth, that is, to the secondarily-formed lips and external
ear. From these points they extended further, taking on new relations to the
eye, nasal aperture, and frontal and temporal regions. The muscu-
lature further extended to the parietal region, the parts of it in front of the
aperture of the ear arising from the frontal and temporal regions, those behind
it from the occipital region. The upward change of position of the musculature
thus took place along two lines, — in front of, and behind the ear, as is
proved by its iiiuervation, to be described directly.

MUSCULAR SYSTEM. 121

Into all these regions the facial nerve extended, forming divergent branches
and plexuses : a posterior (occipital) and four anterior (temporal, maxil-
lary, niandibular, and a branch to the posterior belly of the biventer muscle)
main branches may be distinguished.

The complexity of the branching of the facial nerve is thus proportionate
to that of the facial muscles, and i.^ most marked in Primates, in which the
musculature gradually takes on new origins corresponding to the more and
more highly differentiated cranial skeleton.

The platysma myoides thus forms the matrix for the facial
muscles, and it represents the remnant of a musculature continued
forwards to the head, which has retained (e.g. in Man) an undifferentiated
form in the neck (Gegenbaur).

Besides the formation of new independent muscles, modifications of certain
of the facial muscles also took place, which resulted in their more or less
complete degeneration. Thus they have become replaced by tendinous
aponeurotic regions (viz. the fascia temporalis, parotideo-masseterica, and the
galea aponeurotica of Man), or even entirely obliterated.

Besides the platysma myoides there is a second deeper dermal system of
muscles of the neck, the sphincter coll i. This, like the platysma, also takes
on secondary relations to the head, and gives origin to the levator labii superi-
oris proprius, levator anguli oris, sphincter oris, depressor tarsi, buccinatorius,
and the proper muscles of the nose. The facial muscles not mentioned here
arise from the system of the platysma.

MUSCLES OF THE APPENDAGES.

The following important factors must be taken into consi-
deration with regard to the muscles of the appendages : (1) the
homologies of the parts of the skeleton ; (2) the relative positions
of the neighbouring soft parts ; and (3) the nerve-supply.

The most primitive condition of the muscles of the extremities
is met with in Dipnoi, more particularly in Ceratodus. In this
case, the musculature of each surface of the fin forms a uniform
mass, there being hardly any indication of a separation into definite
layers and groups. E very th ing goes to prove that allthemuscles
of the appendages are to be looked upon as derivatives
of the lateral muscles of the trunk.

Two principal groups of appendicular muscles may always
be distinguished; one lying in the region of the pectoral and
pelvic arches, the other in the free extremity.

In the fins of Fishes, very simple conditions of the muscles
are met with ; in Amphibia, on the other hand, in correspondence
with the more highly-differentiated organs of locomotion, con-
siderable complication is seen, and there is a much more marked
separation into individual muscles, corresponding with the different
sections of the extremity. In Fishes, only simple elevators,
depressors, and adductors, for moving the extremity as a
whole, are present, while from Amphibia onwards there are added
rotators, flexors, extensors, and adductors of the upper arm
and thigh, of the fore-arm and shank, and of the hand and foot.
The digits are also moved by a highly- differentiated musculature.

122 COMPARATIVE ANATOMY.

In cases where, as in Primates, the anterior extremity is con-
verted into a prehensile organ, new groups of muscles appear,
known as 'pronators and supinators. The former are derived
from flexors, the latter from extensors.

On account of the relatively small amount of movement of
the pelvic arch as compared with the pectoral arch, one would
naturally not expect similar groups of muscles connected with
these two regions : entirely different relations are here frequently
to be met with.

DIAPHEAGM.

The first traces of a muscular partition-wall between the
thoracic and abdominal cavities are to be met with in Urodeles.
In them we find circular and semicircular fibres of the transversalis
muscle passing inwards between the pericardium and the liver. In
Chelonians, and more particularly in Crocodiles and Birds, where
the muscular fibres concerned in the formation of the partition
arise from the ribs,1 the indications of a diaphragm are much
plainer, but there is not a complete separation into thoracic and
abdominal cavities. A complete dome-shaped diaphragm, arising
from the vertebral column, ribs, and sternum, appears first in
Mammals, and is of great importance in respiration, as it allows
of a lengthening of the thoracic cavity in a longitudinal direc-
tion. It is supplied by the phrenic nerve, which arises from one
(4th to 6th) of the cervical nerves, and usually consists of two
parts, a pericardial and a pleural, arising independently of one
another. The former is fibrous, and forms the central tendon,
while the latter is muscular. In some cases (e.g. Echidna, Pho-
csena) the diaphragm is entirely muscular. The Mammalian dia-
phragm is probably not the homologue of the so-called diaphragm
( of other Vertebrates.

BIBLIOGRAPHY.

BAIIDELEBEN, C. — Muskeln und Fascie. Jenaische Zeitschr. Bd. XV. N.F. VIII.

CUNNINGHAM, D. J. — The Relation of Nerve-Supply to Muscle- Homology. Journ. of
Anat. and Physiol. Vol. XVI. 1882. Report on some Points in the Anatomy of
the Thylacine, &c., with an Account of the Comparative Anatomy of the Intrinsic
Muscles of the Mammalian Pes. " Challenger " Reports, Vol. V. The Structure
and Development of the Suspensory Ligament of the Fetlock in the Horse. American
Naturalist, Vol. XIX. p. 127.

DOBSON, G. E. — On the Digastric Muscle. Trans. Linn. Soc. Vol. II. Part 5.
Monograph on the Insectivora, 1882. Homologies of the Long Flexor Muscles.
Journ. of Anat. and Physiol. Vol. XVJI. 1883. On the Rectus Abdominis and
Sternalis. Ibid. The Comparative Variability of Bones and Muscles. Ibid. 1885.

DUGES, A. — Rech. sur V osteologie et la myologie des batracicns a leurs differents ages.
Paris, 1834.

1 In Birds, two entirely different structures have been described as a diaphragm.
Comp. the chapter on the air-sacs of Birds. )

MUSCULAR SYSTEM. 123

CKEK, A., and WIEDERSHEIM, R. — Die Anatomic des Froschcs. Braunschweig,

1864-1882.
.FURBRINGER, M. — DicKnochen und Muskeln dor Extremitdten bciden schlangcndhnl.

Sauriern. Leipzig, 1870. Zur vergl. Anat. d. Schultermuskeln. 1st and 2nd

parts in Jenaische Zcitschr. Bd. VII. und V1IL, 3rd part in Morphol. Jahrb.

Bd. I. 1876.
GADOW, H. — Observations on Comparative My ology. Jo-urn, of Anat. and Physiol.

Vol. XVI See also Papers on the Muscles of Reptiles. Alorphol. Jahrb. Bd. VII.
GOTTE. — Entwicklung der Unke. Leipzig, 1875.
HENLE, J. — Handbuch der systcmat. Anatomic des Menschcn. Braunschweig,

1868.
HUMPHRY, G.M. — Niimerous Papers in the Journal of Anatomy and Physiology,

Vols. III. and VI.

HUXLEY, T. H. — On the Respiratory Organs of Apteryx. Proc. Zool. Soc. 1882.
LECHE, "W. — Zur Anat. der Bcckenregion bei Insectivora, die. K. Schwcd. Acad, der

Wisscnsch. Bd. XX. No. 4, 1882.
MARSHALL, A. M. — On the Head Cavities and Associated 'Nerves in Elasmobranchs.

Quart. Journ. Micros. Science, Vol. XXI. 1881.
MULLER, J. — Vcrgl. Anat. d. Myxinoiden. Berlin, 1834-1845.
RUGE, G. — Ueber die Gcsichtsmuskulatur der Halbaffen. Morphol. Jahrb. Bd. XI.

Heft 2.
SCHNEIDER, A. — Beitr. z. vergl. Anat. u. Entw.-Gcsch. der Wirbelthiere. Berlin,

1879.
SMALTAN, C. — Beitrage zur Anatomic der Amphisbaenen. Zeitschr. fur wiss. Zool.

BandXLII.
TESTUT, 'L. — Les anmnalies musculaires chez Vhomme expliquees par V anatomic com-

paree : leur importance en Anthropologie. Paris, 1884.
VETTER, B. — Unters. z. vergl. Anat. der Kiemen- u. Kiefermuskulatur der Fische.

Jcnaische Zcitechr. Bd. VIII. und XII. N.F. I. Bd.

D. ELECTRIC ORGANS.

ELECTRIC organs are present in certain Fishes, and are most
strongly developed in aKay (Torpedo mar mo rat a), found in the
southern seas, in a South American Eel (Gymnotus electricus)
and in an African Siluroid (Malapterurus electricus).
Gymnotus, the electric Eel, possesses by far the strongest electric

FIG 100. — Torpedo mar morata, WITH THE ELECTRIC ORGAN (E] EXPOSED.
S, skull ; Sp, spiracle ; KK, gills ; Au, eye.

power, next to it comes Malapterurus, and then Torpedo. The
electric batteries of these three Fishes are situated in different,
parts of the body ; in Torpedo they have the form of a broad
mass, extending throughout the substance of that part of the
body lying between the gill-sacs and the propterygium on each

ELECTRIC ORGANS.

125

side of the head (Fig. 100, E) ; in Gymnotus they lie in the
ventral portions of the enormously long tail (Fig. 101, M) that is,
in the position usually occupied by the ventral portions of the
great lateral muscles; and finally, in Malapterurus, the electric
organ extends between the skin and muscles round almost the
entire circumference of the body, thus enclosing the Fish like a
mantle : it is especially strongly developed along the sides.

FIG. 101, A and B. — THE ELECTRIC ORGAN OF Gymnotus cledricus. (B, from a
preparation by A. Ecker. )

H, skin ; Fl, fin ; DM, DM1, dorsal portions of the great lateral muscles, seen partly
in transverse section, partly in longitudinal ; VM, FM1, ventral portions of
ditto ; E, the electric organ, seen in transverse section at E (B), and from the
side at E1 ; WS, vertebral column, from the side, showing the spinal nerves,
and WS1, in transverse section ; LH, posterior end of body-cavity ; Sep,
median longitudinal fibrous septum, which divides the electric organ and the
lateral trunk-muscles into two equal halves ; A, anus.

The electric power of those Fishes which were formerly known
as " pseudo-electric " has now been fully demonstrated, though it
is much feebler than in the forms described above. To this category
belong all the Rays, with the exception of Torpedo, the various
species of Mormyrus, and Gymnarchus (both belonging to the
Teleostei). In all these, the electric organs lie on either side

126 COMPARATIVE ANATOMY. *

of the end of the tail, and have a metameric arrangement like
that of the caudal muscles ; in the MormyridaB, for example, there
is on each side an upper and an under row of electric organs.

The electric apparatus in all the above-named Fishes is to be
regarded from the same point of view both as concerns its mode
of development and anatomical relations: all electric organs are
to be looked upon as metamorphosed muscular tracts,
and the nerve-endings belonging to them as homologues
of the motor end-plates which are ordinarily found on
muscles.

As regards the minute structure of the electric organs, the same
essential arrangements are met with in all. The framework is
formed of fibrous tissue, which, running partly longitudinally,
partly transversely through the organ, gives rise to numerous
polygonal or more or less rounded chambers or compartments.
These latter are arranged in rows, either along the longitudinal
axis of the body (Gymnotus, Malapterurus) or in a dorso-ventral
direction (Torpedo), forming definite prismatic columns (Fig. 102).

FIG. 102. — ELECTRIC PRISMS OF Torpedo marmorata. (Semidiagrammatic. )

Numerous vessels and nerves ramify in the connective-tissue
lying between these compartments, the nerves being enclosed in
very thick sheaths, and having a great variety of origin accord-
ing to the species of Fish under consideration. In Torpedo, they
arise from the "electric lobe" of the medulla oblongata, a
single branch coming also from the trigeminal ; in all pseudo-
electric Fishes, as well as in Gymnotus, in which over two hundred
nerves pass to the electric organ, they arise from the spinal cord,
and most probably are in the closest relation with the ventral
cornua of the latter, which are particularly well developed in
the last-named Fish. It is very remarkable that the electric
nerves of Malapterurus arise on each side from a single enormous
nerve-cell, which, lying in the neighbourhood of the second
spinal nerve, is continued into a very large primitive-fibre, which
passes towards the end of the tail, dividing as it goes. The
latter is invested by a thick sheath.

On continuing our examination into the more minute his-
tological structure of the peripheral nerve-endings, it becomes
necessary to give a definition of those structures which are usually

ELECTRIC ORG

127

called "electric-" or "end-plates." These can be summarily
described, as they are essentially the same in all electric Fishes.

After the nerve, running in the septum between the compart-
ments, has by degrees lost its thick sheath and thus has almost
ceased to show a double contour, it suddenly gives rise to a club-
shaped swelling, and then divides up into a number of primitive
fibres, which branch out in a tree-like manner, without, however,
giving rise to definite meshes, so that we cannot speak of a proper
nervous network. In Torpedo the nerve spreads out on the ventral
side of the structure known as an electric plate (Fig. 103, J£P\
while in Gynmotus it passes to the posterior surface, that is to the
one turned towards the tail. Finally in Malapterurus, the nerve,
as in Gynmotus, passes on to the posterior surface of the electric
plate ; it does not stop here, however, but perforates the plate, so

>

FIG. 103 —SECTION THROUGH THE ELECTRIC CHAMBERS. (Greatly enlarged, and

semidiagrammatic. )

B'T, framework of connective-tissue, forming walls of compartments ; EP, electric
plates ; Nt nerves entering into the septa between the compartments ; NN,
terminal fibres of the nerve, passing to the posterior (Gymnotus) or under (Torpedo)
surface of each compartment ; G, gelatinous tissue ; the arrow points towards
the head (Gymnotus), or towards the dorsal side of the animal (Torpedo).

as to spread out on the anterior surface, turned towards the head.1
This difference must be borne in mind on account of the direction
taken by the electric current, to be described later on.

Each electric plate consists of a homogeneous disk, transparent
in the fresh condition, and surrounded by a special membrane,
within which star-like cells with long processes are present. Both
surfaces of the plate exhibit irregular protuberances, separated
from one another by shallower or deeper notches, and giving the
whole an irregular appearance.

1 Babuchin, who had the opportunity of examining Malapterurus in the fresh
condition, disputes strongly the perforation of the plate by the nerve ; whether
rightly or not, new researches must show.

128 COMPARATIVE ANATOMY.

As this disk, which, as already mentioned, is to be looked upon as
metamorphosed muscle-substance, becomes inseparably fused with
the nerve- plate lying close to it, it follows that the electric plate is
not a uniform structure, as was formerly believed, but is to be
regarded as having arisen out of two tissue-elements. The com-
partments are not entirely filled by the electric plates ; a space
tilled by gelatinous tissue (Fig. 103, 6r), or sometimes only
by a fluid, always remains in the upper (Torpedo) or anterior
(Gymnotus, Malapterurus) side of each compartment along
the wall separating it from the next. The side of the electric
plate on which the nerve branches out is negative at the
moment of discharge, while the opposite side is positive, and thus
the different arrangements of the parts in Gymnotus and
Malapterurus render it clear that the electric shock must pass
in different directions in these Fishes : thus in Malapterurus it
passes from the head to the tail, but in the contrary direction in
Gymnotus. In Torpedo the discharge passes from below upwards.

Experiments have shown that all electric Fishes are proof
against the electric current, with the limitation that muscles
and nerves — even the electric nerves themselves — separated
out from the body, are capable of being excited by the current.
" The last and most important question with regard to the electric
Fishes is naturally concerning the mechanism whereby the electric
plates become temporarily charged with electricity. The reply to
this question, although probably not so difficult a one as that
relating to the mechanism of muscular contraction, is still far from
being answered " (Du Bois-Reymond). The only thing that can
be stated with certainty is, that the electromotive force is under
the influence of the will.

BIBLIOGRAPHY.

BABUCHIX. — Uebers.de?- neueren Untersuchungen iibcr EntwicTilung, d-c., der elektri-
schen und pseudoelektrischen Organs. Arch. f. Anat. und Physiol. 1876.

ECKER, A. — Entwickl. der Nerven des elektr. Organs von Torpedo Galv. Zeitschr. filr
wiss. Zool. Bd. I. 1848. Unters. zur Ichthyologie. Freiburg, 1857.

UEYMOND, E Du Bois. — Gcsammelte Abhandlungcn zur allg. Muskel- und Nerven-
Bd. II.

E. NERVOUS SYSTEM.

THE following elements, which are all derived from the epiblast,
may be distinguished in nervous tissues: — (1) Ganglion cells,
provided with processes, and supported by a connective-tissue
framework, the neuroglia; and (2) Fibres, entering into or aris-
ing from the former, and serving as conductors of sensory or
motor impulses. Each fibre maybe invested by a delicate cover-
ing or sheath, the neurilemma (primitive sheath, or sheath of
Schwann).

The nervous system may be divided into three main parts, the
central (brain and spinal cord), peripheral, and sympathetic
systems. The central part is the first to arise, and is formed as
a direct product of the epiblast; the other two become established
later.

I. THE CENTRAL NERVOUS SYSTEM.

1. THE SPINAL CORD.

The first indication of the central nervous system is a furrow
(medullary groove) which appears on the dorsal side of the
embryo, and which gradually becomes converted into a tube by the
meeting of its edges : this tube then becomes separated from the
epiblast and gives rise to the hollow medullary cord (cp. p. 7),
the walls of which are at first comparatively thin ; it consists of
a more expanded anterior, and a longer and more slender posterior
section. From the former arises the brain, from the latter the
spinal cord.

In an early stage of development the lumen of the medullary
cord is as a rule continuous posteriorly with that of the primitive
intestine (neurenteric canal). This connection, however, soon dis-
appears, and the spinal cord then consists of a cylindrical or more
or less flattened hollow cord, by no means filling the neural canal :
its walls become gradually more and more thickened, until finally
only a very narrow lumen, the central canal, remains; this is
lined by ciliated epithelium.

K

130

COMPARATIVE ANATOMY.

FIG. 104. — THE ENTIRE NERVOUS SYSTEM OF THE FROG. (After A. Eck«r.)

(From the ventral side. )

He, cerebral hemispheres (prosencephalon) ; Lop, optic lobes (mesencephalon) ; J)/,
spinal cord ; Ml to M10, spinal nerves, which are connected at 8M by branches
(rami communicantes) with the ganglia (SI to 810) of the sympathetic (8) ; No,
iemoral'nerve ; Ni, sciatic nerve ; / to X, first to tenth cranial nerves (for their
names, see text) ; G, ganglia of the vagus ; Vg, Gasserian ganglion ; o, eye ; N,
nasal sac ; Va to Vct the different branches or the trigemiual ; F, facial nerve ;
Vs, connection of the sympathetic with the Gasserian ganglion ; XI to X4, the
different branches of the vagus. Some of the fibres of the sympathetic should
be shown accompanying the vagus peripherally.

THE SPINAL COED. 131

The cord is at first of a uniform diameter throughout, but
later, in cases where the extremities are well developed and a
richer nerve-supply becomes needed, it exhibits in these regions
definite swellings— the brachial and lumbo -sacral enlarge-
ments. The spinal cord originally extends along the whole length
of the neural canal, but its growth is usually less rapid than that
of the vertebral axis, so that it comes eventually to be considerably
shorter than the latter. In such cases (Primates, Cheiroptera,
Insectivora, Aves, Anura, &c.) it passes at its posterior end into a
brush-like mass of nerves, the so-called cauda equina; these lie
mostly within the neural canal, and the nerves of the sacral plexus
are given off from them. An axial prolongation of the spinal
cord nevertheless extends far back, but is reduced to a thin
thread-like appendage, the filum terminale.

The bilaterally-symmetrical form of the spinal cord is pronounced
by the presence of longitudinal fissures running along it dorsally
and ventrally ; and if one imagines the points of exit of the dorsal
(sensory) and ventral (motor) nerve-roots to be respectively con-
nected together by a longitudinal line, each half of the spinal cord
would thus be divided into three columns, — a ventral, lateral, and
dorsal (anterior, lateral, and posterior columns of human
anatomy).

The external form of the spinal cord in certain Fishes (Orthagoriscus,
Trigla), and in embryos of Salamandra atra, as well as its histological struc-
ture in the higher Vertebrata, seems to indicate that the unsegmented spinal
cord of Vertebrates was primitively segmented and paired, and that it has
passed in its phylogenesis through a stage which was closely related to the
abdominal chain of ganglia of many Invertebrates (e.g. Annelids). A definite
segmentation of the mid-brain, cerebellum, and medulla oblongata is also seen
in the embryos of all the chief Vertebrate types (Kupffer).

As regards its minute structure, two parts can always be dis-
tinguished in the spinal cord, — a white substance, consisting of
medullated nerve-fibres, and a gray substance, composed
principally of multipolar nerve-cells and non-medullated fibres.
Their relative positions to one another vary greatly in the different
animal groups, as well as in the different regions of the cord ; the
white substance, however, has usually a more peripheral, the gray
a more central position.

The membranes of the spinal cord will be treated of later.

2. THE BEAIN.

At a very early stage three swellings may be seen on the anterior
enlarged part of 'the medullary tube, which are spoken of as the
primary anterior, middle, and posterior cerebral-vesicles
(fore-, mid-, and hind-brain). (Fig. 105, /, //, III.}

The cavities of the vesicles, corresponding with the ventricles
of the fully-formed brain, are in direct connection with the central

K 2

132 COMPARATIVE ANATOMY.

canal of the spinal cord. The primary fore-brain and hind-brain
each become differentiated into two parts, and thus five divisions
of the brain may be distinguished. Counted from before backwards
these are Prosencephalon (secondary fore-brain), Thalamen-
cephalon (primary fore-brain), Mesencephalon (mid-brain),
Metencephalon (secondary hind-brain), and Myelencephalon
(primary hind-brain). The prosencephalon is also spoken of later

FIG. 105. — DIAGRAM OF THE EMBRYONIC CONDITION OF THE CENTRAL NERVOUS

SYSTEM.

Gr, brain, with its three primary vesicles, 7, //, ///;. H, spinal cord.

as the cerebrum, the mid-brain as the optic lobes, or corpora
bigemina,1 the metencephalon as the cerebellum, and the
myelencephalon as the medulla oblongata.

The olfactory lobes arise from the secondary fore-brain, which
becomes divided into two cerebral hemispheres by a longi-
tudinal fold, the basal portion of the vesicle becoming thickened
to form a great mass of nerve-centres; this may be distinguished
from the remaining peripheral part of the vesicle, or pallium,
as the central portion (Fig 106, Cs).

Throughout the animal kingdom the prosencephalon plays a
most important part, for the intellectual condition of the animal
depends upon the extent of its development. It consequently
attains the greatest perfection in Mammals, and above all, in
Man. While in the lower Vertebrates the central portion of the
fore-brain is provided with only three small commissures, con-
necting its two halves, in Mammals the two hemispheres become
fused together along one portion of their inner surfaces, and thus
give rise to the great commissures spoken of as the corpus
c all o sum and the for nix.2 While the outer surface of the
hemispheres in all Vertebrates below the Mammalia is more or less
smooth, in the latter fissures (sulci) and convolutions (gyri)
may be present. These consist of folds of the gray cortical sub-
stance, which cause a greater or less increase of the superficial
area.

The following structures arise from the thalamencephalon : —
the optic thalami, formed as thickenings of its lateral ^alls ;
the primary optic vesicles, arising as paired basal and lateral
outgrowths, from which the optic nerves and retina are derived
later (Fig. 106, Tho, Opt} ; the pineal gland or epiphysis (Z),
developed as a tube-like outgrowth of the roof ; and finally, the

1 In Mammals, each optic lobe becomes divided into two parts, and indications of
a similar division are seen in some Lizards (see Fig. 119, A).

2 Traces of a fornix are seen in certain Reptiles (e.g. Psammosaums).

THE BRAIN. 133

infundibulum (/), formed as a funnel-like extension of the floor,
together with a part (the posterior lobe) of the pituitary body
(hypophysis) (//).* The other part (anterior lobe) of the pituitary
body arises by a gradual pinching offof the epithelium of the primary
oral involution, and gives rise later to a gland-like structure.

The cerebellum, in the higher types, becomes differentiated into
two lateral portions (lateral lobes), which may again be sub-
divided, and a median unpaired portion (superior vermis), which

Olf

FIG. 106.— LONGITUDINAL SECTION THROUGH THE SKULL AND BRAIN OF AN
(IDEAL) VERTEBRATE EMBRYO. (lu part after Huxley.)

L'c, basis cranii ; Oh, iiotocliord ; SD, roof of skull ; NH1, nasal cavity; VH,
secondary fore-brain (prosencephalon), showing the corpus striatum (Gs) at the
base, and the olfactory lobe (Olf) anteriorly ; ZH, thai,* rnencepha Ion (primary
fore-brain), which has given rise dorsally to the pineal gland (epiphysis) ' (Z\
and ventrally to the infundibulum (/), to which the pituitary body (hypophysis)
(77) is attached : anterior to this is seen the optic nerve (Opt}, arising from the
optic, thalanms (T/io) ; HO, posterior commissure; J777, mid-brain (mesen-
cephalon) ; 7777, cerebellum (meteucephalon, secondary hind-brain) ; Nff,
primary hind-brain (myelencephalon) ; Oe, Central canal of spinal cord.

connects these two. The other two portions of the brain (mid-brain
and medulla oblongata) do not become so greatly modified as
the fore-brain. It is therefore only necessary to mention that the
medulla oblongata, the roof of which undergoes a retrogressive
metamorphosis, gives origin to the greater number of the cranial
nerves, so that its physiological importance is very great.

The following important changes take place in the further
development of the brain.

The walls of the cerebral vesicles become more and more
thickened, so that their cavities, transformed into the ventricles
of the brain, undergo a gradual constriction.

1 Opinions are. much divided as to the meaning of the epiphysis and hypophysis.
vSome observers consider the epiphysis to be the homologue of the anterior neuropore
of embryos of Amphioxus and Tunica t-a, lhat is, as the remains of a last connection
of the brain with the epiblast : this opinion, however, is not tenable according to
numerous later researches, all of which agree that it arises secondarily from the
roof of the thalamencephalon. Others imagine it to be the last rudiment of a
sense-organ, and from the fact that it arises in a similar manner to the optic
vesicles it has been explained as an unpaired eye.

The hypophysis, as it seems to arise as a p aired structure, is thought by
Dohrn to correspond to a pre-oral gill-cleft : this is the latest of the many hypotheses
which have been put forward on the subject.

134

COMPARATIVE ANATOMY.

A series of unpaired ventricles, lying in the longitudinal axis
of the brain, as well as paired ventricles can always be distinguished.
The principal paired cavities lie within the cerebral hemispheres,
and are known as the lateral ventricles (Ventviculus 1 and 2)
(Fig. 107, SV) ; each of these communicates with the unpaired sys-
tem by means of an opening, the foramen of Monro (Fig. 107,
'FAT). In Teleostei, certain Amphibia, and Sauropsida, each optic
lobe also contains an optic ventricle, communicating with the
unpaired system of ventricles. The latter consists of a third
(within the thalamencephalon) and a fourth ventricle (in the
medulla oblongata), as well as of the aqueduct of Sylvius,

-11

FIG. 107. — DIAGRAM OF THE VENTRICLES OF THE VERTEBRATE BRAIN.

/ClCUlCli llCllllOUllCl COj l/UUVOUJJUJJE 1;1J.C J.CI U^i. C* J. V O J.A til lUi V^O \ / J f J ? ffJLJ.) Ulltl-AO,]

jphalon, with the third ventricle (///) ; in Mammals the paired septum lucidum,
ring anteriorly to the thalamencephalon, encloses the "fifth ventricle " ; each

VHy cerebral hemispheres, containing the lateral ventricles (SV) ; ZR, thalamen-
cej
lying

lateral ventricle communicates with the third ventricle by a small aperture, the
foramen of Monro (FM) ; MH, mid-brain, which encloses the aqueduct of Sylvius
(Aq), communicating between the third and fourth ventricles ; HH, cerebellum ;
NH, medulla oblongata, enclosing the fourth ventricle (IV} ; Cc, central canal
of the spinal cord ( J2).

which passes through the mid-brain and connects these two. For
further details, such as the relations of the different ventricles to
particular parts of the brain, compare Figs. 106 and 107. A so-
called fifth ventricle, lying between corpus callosum and fornix,
is found in Mammals, but morphologically it is quite different
from the others.

All five cerebral vesicles lie at first in the same horizontal
plane, but in the course of development the axis of the vesicles be-
comes bent downwards, so that at a certnin stage the mesencephalon
forms the apparent apex of the brain (Fig. 108, SB). In Mammals,
the parts of the brain become still further folded on one another, so

THE BRAIN. 135

that a parietal (Fig. 108, $Z?),a Varolian (#/?), and a cervical
(NE) bend may be distinguished.

While in Fishes and Amphibia the cerebral flexure later be-
comes practically obliterated, it persists more or less markedly in
the higher types, more particularly in Mammals. In the latter,
moreover, the original relation of the parts becomes still further
complicated by the large development of the cerebral hemispheres,
which grow backwards, and thus gradually come to overlie all the
other parts of the brain. This condition of things attains its
greatest perfection in Man. Thus from the primitive relations of
the various sections of the brain one behind another, they come

Vtt Ztt

FIG. 108. — CEREBKAL FLEXURE OF A MAMMAL.

Vff, prosencephalon ; Zff, thalamencephalon, with the pituitary body (H] at its
base ; MH, mesencephalon, which at SB forms the most projecting portion of
the brain, representing the so-called "parietal bend"; HH, metencephalon ;
NIT, myeleneephalon. forming the " cervical bend " (NB) : the " Varolian bend "
(BB) arises on the ventral circumference, at the junction between HH and
NH ; .ft, spinal cord.

to lie eventually more upon one another, the thalamencephalon,
mid-brain, cerebellum, and medulla oblongata, becoming covered
over by the hemispheres.

MEMBRANES OF THE BRAIN AND SPINAL CORD.

The enveloping membranes of the brain and spinal cord arise
by a differentiation of a connective-tissue layer lying between the
central organs of the nervous system and the surrounding skeletal
parts. In Fishes, only two membranes are distinguishable, one,
the dura mater, lining the inner surface of the cerebro-spinal
canal, and the other, pia mater, investing the brain and spinal
cord. The latter represents also the arachnoid of higher Verte-
brates, which is not here differentiated as a separate membrane.
The dura mater conveys vessels to the walls of the cerebro-spinal
canal, that is, to the perichondrium or periosteum, while the pia
mater, which is much richer in blood-vessels, has to do with the
nutrition of the cerebro-spinal axis. The dura mater consists of two
lamellae, which, however, only remain distinct along the whole

136 COMPARATIVE ANATOMY.

central nervous system in the lower Vertebrata. In higher Verte-
brates, its double nature persists only in the region of the vertebral
column, the two layers becoming fused in the cranial portion. As
the brain of Fishes by no means fills up the cranial cavity, a large
lymph-space' lies between the dura and pia mater ; this cor-
responds to the so-called sub-dural space of the higher
Vertebrata.

A differentiation of the primary vascular membrane of the
brain and spinal cord into pi a mater and arachnoid takes
place in the higher Vertebrates, and these two layers become
separated in those places where there are deep depressions be-
tween the individual parts of the brain ; the deeper of these (pia)

FIG. 109. — BRAIN- MEMBRANES OF MAN. (After Schwalbc.)

DM} dura mater ; Sfi, sub-dural cavity ; A, arachnoid ; PM, pia mater ; GR, gray
cortical substance of the brain.

adheres closely to the brain, and also penetrates into the ventricles
in the form of telse choroidese and plexus choroidei,
while the superficial one (arachnoid) simply bridges over the
depressions.

No certain explanation can as yet be given of the morphological
meaning of the hollow anterior end of the spinal cord in Amphi-
oxus, nor of the diverticulum connected with it which opens
freely to the exterior on the dorsal surface.

Fishes.

The Cyclostomi show a very low condition of the brain,
which in many points remains in an embryonic condition. This is
particularly the case in the larval condition (Ammoccetes, Fig.
110), in which the brain possesses a narrow and elongated form.
The individual vesicles lie in an almost horizontal direction
one behind the other, and it is of great importance to note that

THE BRAIN.

137

the part described on page 132 as the peripheral region
(pallium) of the prosencephalon consists of a single layer of
epithelial cells. This is covered on its dorsal surface by the pin
mater, and thus here, as is also the case with the secondary
fore-brain of Teleostei (see Fig. 113), there is a persistence of
that low stage of development in which the prosencephalon is re-
presented by a thin- walled and dorsally unpaired vesicle ; that is,
there is no separation into two hemispheres by a cleft in the
peripheral region. In Fig. 110 the peripheral region is not in-
dicated, and thus the central portion of the prosencephalon is seen,
the floor of the latter, or corpora striata, being exposed. The
olfactory lobes (Lol) are connected with the corpora striata anteriorly.

Lol

FIG. 110. — BHAIN OK Ammocwtes. (Dorsal view.)

Lol, olfactory lobes and nerves (/) ; VH, basal portion of the prosencephalon ; Zll,
thalamencephalon, with the pineal gland (Z)\ MI?, inesencephalon ; HH, meten-
cephalon, on each side of which is seen the origin of the trigeminal ( V) ; NH,
myeleucephalon ; Fr, fourth ventricle ; VII, VIII, the points of origin of the
facial and auditory nerves ; Med, spinal cord.

The meten- and myelencephalon of Ammoccetes are remark-
ably long, while in Petromyzon and Myxine, the individual
portions of the brain are broader and more closely approximated.
The epiphysis never breaks through the roof of the skull in
Cyclostomes, as it does in many Fishes.

Elasmobranchii. — The brain of these Fishes, like that of
Cyclostomes, is of a specialised form, characteristic of, and confined
to, the group, though the particular regions are much more highly
developed than in the Cyclostomi. According to their external form
two main types of Elasmobranch brains can be distinguished. One
of these, seen inSpinax, Scymnus, and Notidanus, is character-
ised by its very narrow and elongated form, while in the rest of the
Elasmobranchii the individual parts are more closely compressed

138 COMPARATIVE ANATOMY.

and approximated together. In almost all Sharks the prosen-
cephalon is relatively much longer than any of the other regions.
The olfactory tracts, the length of which varies much, are con-
nected with the anterior end of the prosencephalon, and pass
forwards into the large olfactory lobes, from which the olfactory
nerves arise (Fig. Ill, Tro, I/ol).

The thalamencephalon, appearing like a small commissure
wedged in between the prosencephalon and mid-brain, grows out
on its dorsal surface to form a chimney- or tube-like epiphysis ;

FIG. 111. — BRAIN OF Galeus canis, in situ. (Dorsal view..) (After Rohon.

Lol, olfactory lobe ; Tro, olfactory tract ; VH, prosencephalon, showing at fn a
foramen for blood-vessels ; ZH, thalamencephalon ; Afff, mesencephalon ; 7777,
metencephalon ; Nil, myelencephalon ; R, spinal cord ; 77, optic nerve ; 777,
oculomotor; IV, trochlear nerve; V, trigeminal ; L,Trig, trigeminal lobe;
G,rest, restiform body ; IX, glossopharyngeal ; A', vagus ; E.t, eminentiae
teretes, between which is the calamus scriptorius.

this may reach to such a length as to extend beyond the anter-
ior end of the brain for a considerable distance, and pass distally
into the roof of the skull.

Two pairs of small folds, spoken of as lobi inferiores and sacci
vasculosi or processus infundibuli (Fig. 112, UL), are present on the
floor of the thalamencephalon. They probably arise in connection with
the infundibulum, or perhaps with the hypophysis also.

The cerebellum is always very large, overlapping the medulla
oblongata to a greater or less extent : it is divided up into seve-
ral folds lying one behind the other (Fig. Ill, HIT). In Sharks
the medulla oblongata is an elongated cylindrical body (Fig.
Ill, NH\ while in Bays it is more compressed and triangu-
lar. In electric Rays a pair of lobi electrici arise from the gray
matter of the floor of the fourth ventricle, and these enclose a
mass of giant nerve-cells. For further details concerning, e.g., the
restiform bodies and trigeminal lobes, compare Figs. Ill and 112.

THE BRAIN.

139

In the Angler (Lophius piscatorius, a Teleostean) there is also a superficial
layer of enormous nerve-cells (about 209 in numbed) behind the calamus scrip-
torius of the sinus rhomboidalis, filling up the dorsal fissure of the spinal cord for
a certain distance : their discoverer, G. Fritsch, calls them"lobi nervi lateralis."
The axis fibres arising from these cells accompany the trigeminal and vagus,
but do not go to electric organs, which are entirely wanting in Lophius, but to
the integumentary sense-organs, which are enormously developed in this Fish,
and also to the "lure.'"' The similarity in position of these nerve centres
to the electric lobes of Torpedo, however, deserves notice.

FIG. 112.— BRAIN OF Myliobatis aquila, in situ.
(After Rohon.)

(From the ventral side. )

/, olfactory, //, optic, 777, oculomotor, IT, trochlear, T, trigeminal, VI, abducent,
VII, facial, VIII, auditory, IX, glossopharyngeal, and X, vagus nerves ; VH,
prosencephalon ; 77, HS, hypophysis and infundibulum ; UL, lobi inferiores ;
>Sv, saccus vasculosus ; Ctr, transverse commissure ; Geh, auditory capsule ; W,
vertebral column ; R, spinal cord.

Teleostei. — The type of brain found in Teleosteans is also
specialised, and restricted to the members of this order.

As is the case in nearly all Fishes, the brain by no means fills
the cranial cavity, as already mentioned in the description of the
brain-membranes, and as a rule it is separated from the roof of the
skull by a greater or less amount of a lymph-like fluid.

The form of the brain in Teleostei varies greatly, more by far
than in any other Vertebrate group. It is therefore difficult to
give a general description of it, and only the following essential
points can be mentioned here.

It never attains to so large a relative size as does that of Elas-
mobranchs. The peripheral region, as already ir entioned (p. 137),

140

COMPARATIVE ANATOMY.

remains in an embryonic condition, and can hardly be said to have a
physiological fu notion ; the brain of those Fishes in which this con-
dition is retained probably acts mainly as a reflex machine,
and there is little doubt that the mental processes

Mol

FIG 113A. — LONGITUDINAL VERTICAL SECTION THROUGH THE ANTERIOR PART OF
THE TELEOSTEAN BRAIN. (Founded on a figure of the Trout's brain by Rabl-
Kiickhard.)

Tco, roof of the optic lobes ; Tl, torus longitudinalis ; Cp, posterior commissure ;
Gp, pineal gland, with a cavity (Gfp1) in its inteiior ; Ep, Ep, the ependyma,
which lines the walls of the ventricles ; t, point at which the epithelial roof of
the secondary fore-brain (pallium. Pa) becomes continuous with the lining of the
anterior wall of the pineal tube: the former is folded inwards at /; V.cm,
common ventricle of the secondary fore -brain ; V.t, third ventricle ; B.ol, N.ol,
olfactory bulb and nerve ; C.st, corpus striatum, which was formerly taken to
represent the whole of the prosencephalou, and which lies on either side of the
middle line; Ch.n.opt, optic chiasma ; Oi, inferior commissure (Gadden) ; Ch,
horizontal commissure (Fritsch) ; J, infundibulum ; ff, Hl, hypophysis ; Sv,
saccus vasculosus ; Li, lobi inferiores ; Aq, aqueduct of Sylvius ; ftr, trochlear
nerve ; Val, valvula cerebelli.

which take place in the cortical gray substance of the
brains of higher Vertebrates do not obtain in them.

The mid-brain and cerebellum are by far the largest
portions of the brain (Figs. 114 and 115, Mil, HH\ while the
thalamencephalon is depressed between the prosencephalon and

THE BRAIN.

HI

FIG. 113B. — TRANSVERSE SECTION THROUGH THE FORE-PART OF THE TELEOSTEAN

BRAIN.

fr, frontal bone, underneath which the pineal tube, Gp, is visible in transverse sec-
tion, and below this the pia mater, Pm ; Pa, the pallium, or roof of the secondary
fore-brain, formed of a simple epithelial layer; Kern, common ventricle ; Ep,
ependyma ; Ty T, olfactory tracts at the base of the corpora striata (C.st).

FIG. 114. — BRAIN OF PERCH (Perm schraetser}. (Side view.)

Jt

FIG. 115. — BRAIN OF Perea schraetser. (Dorsal view.)

Lol, olfactory lobe ; Vff, basal part of prosencephalon ; ZH, thalamencephalon ; Afff,
mesencephalon ; HIT, metencephalon ; NH, myelencephalon ; R, spinal cord ;
/, olfactory nerve ; //, optic chiasma ; V, trigeminal ; UL, lobi inferiores ; Se,
central sulcus at the bottom of the sinus rhomboidalis ; Lp, lateral elevations
of the cerebellum (lobi posteriores).

142 COMPARATIVE ANATOMY.

mid-brain, and thus may be easily overlooked (Figs. 114 and
115, ZH).

The prolongation of the cerebellum into the ventricle of the mid-brain,
seen only exceptionally in Elasniobranchs, is present as a rule in Teleosteans,
but the extent' of its development varies much. The pineal gland does not
differ essentially from that of Elasmobranchs and Ganoids, though it never
extends into the roof of the skull, and usually remains within the brain-
membranes.

As in Elasmobranchs, lobi inferiores and a saccus vasculosus are
present in connection with the infundibulum. The saccus vasculosus is both
glandular and vascular in structure, and its duct passes into the infundibulum ;
hence it is sometimes called the " infundibular gland."

Ganoidei, Dipnoi,, and Amphibia. — Even apart from the
brains of Lep.idosteus and Amia, which are formed on the
Teleostean type, a common ground-plan cannot be laid down for the
brains of other Ganoids, Dipnoans, a«nd Urodeles; in a certain mea-
sure they may be said to form one group, but in many points they
resemble the brain of Petromyzon. They are all distinguished by

I T,ol VKS 1H NJL Jfff

JL P#fcCs Inf M

FIG. 116. — BRAIN OF Polypterus bichir. (Side view.)

/, olfactory nerve ; //, optic nerve ; Lol, olfactory lobe ; VH, prosencephalon, with
a lateral cleft at S, and the cerebral peduncles (Pedc) at its base, which radiate
into the hemispheres at Cs ; ZH, thalamencephalon, at the base of which is the
infundibulnm (Inf) with the pituitary body (H) • ME, mid-brain ; HH, cere-
bellum ; NH, medulla oblongata ; R, spinal cord.

a marked development of the cerebrum, while the cerebellum is
only represented by a small transverse fold of nervous matter
on the anterior end of the medulla oblongata (Figs. 116 and
117, HH).

The mid-brain is always paired ; it encloses the narrow'
aqueduct of Sylvius, and extends laterally outwards into the
optic tract. The extremity of the epiphysis sometimes extends
into the roof of the skull (Acipenser, Ceratodus), and the thalam-
encephalon is not visible to any great extent on the dorsal side,
though much more of it is exposed in Urodela than in Gym-
nophiona and Anura, in which the individual regions, especially
the largely developed hemispheres and the broadened mid-brain
(Fig. 118, VH, MH\ are much more closely approximated than in
Urodeles and Ganoids.1

1 As in Elasmobranchs, the epiphysis of Anura is produced into a long tube, the
distal end of which not only passes into the roof of the skull, but (in the embryo)
extends even to the skin. It becomes reduced later on.

THE

143

FIG. 117. — BEAIN OF Salamandra maculosa. (A, dorsal, B, ventral view.)
VHt cerebral hemispheres, marked off by a furrow (F) from the olfactory lobes (Lol) ;
ZHj thalamencephalon, with the* pineal gland (Z) and the ingrowth of the
choroid plexus on the dorsal side, the infundibnlnm (Inf), and the pituitary
body (H) ; ME, optic lobes ; HH, cerebellum ; NH, medulla oblohgata ; Frh,
fourth ventricle ; E, spinal cord ; /, olfactory nerve ; //, optic nerve, with its
chiasma ; ///, oculomotor ; V\ V'2, F3, first, second, and third divisions of the
trigeminal, which arise from the Gasserian ganglion (G) ; Co, commissure
between the roots of the trigeminal and facial nerves ; VI, abducent nerve ;
VII and VIII, facial and auditory nerves, arising from a common root ; Oh,
auditory capsule ; XI, X, glossopharyngeal and vagus grcup ; Gl, ganglion of
vagus ; I Sp, II Sp, first (hypoglossa.1) and second spinal nerves.

VJ£

FIG. 118. — BRAIN OF Puma eacuLenta. (From the dorsal side.)
VH, cerebral hemispheres marked off from the olfactory lobes (L,ol) by a furrow (/) ; I,
olfactory nerve ; ZH, thalamencephalon, with the pineal gland (Z] ; MH, rnid-
HH, cerebellum ; NH, medulla oblongata ; Frh, fourth ventricle.

brain

144

COMPARATIVE ANATOMY.

The brain of Anura, and still more that of Gymnophiona,
reaches a much higher stage than that of Urodeles, which
retains to a greater extent a resemblance to the brain of Fishes.
In Rana, moreover, the fore-parts of the hemispheres in the
region of the olfactory lobes are fused together in the middle line ;
in Urodeles and Protopterus they remain distinct throughout. In
Ceratodus the hemispheres are fused together dorsally, and in
Ganoids ventrally.

Olfactory lobes may be largely (Amphibia, Polypterus,
Ceratodus), only moderately (Sturgeons), or not at all (Protopterus)
developed.

Reptiles.

The brain of Reptiles reaches a considerably higher stage of
development than that of the forms already described, and this is

FIG. 119. — BRAIN OF BLINBWORM (Anguis fragttis).
B, from the ventral side.)

(A, from the dorsal,

Vff, cerebral hemispheres, narrowing anteriorly to pass into the olfactory tracts (Tro)
and the olfactory lobes (Lol) ; ZH, thalamencephalon; with the hypophysis (H) ;
MH, optic lobes, encircled behind by the roots of the optic tracts ( Trop) ; HH,
cerebellum ; NH, medulla oblongata ; Frh, fourth ventricle ; R, spinal cord ;
/, olfactory nerve ; II, optic nerve, with chiasma (Chi) ; III, oculomotor ; IV,
trochlear nerve ; V1, first division of the trigeminal, with its special ganglion (G1) ;
V2, V3, second and third divisions of the trigeminal, arising from a common root
( V) and ganglion (£2) ; VI, abducent ; VII, VIII, facial and auditory nerves,
arising from a common root ; IX, X, XI, glossopharyngeal, vagus, and spinal
accessory ; I Sp, II Sp, first and second spinal nerves ; BK, Varolian bend.

most pronounced by the individual parts coming to overlie one
another to a greater extent, and by the larger development
of both the peripheral and basal portions of the hemispheres.

THE BRAIN.

145

The former character is seen most plainly in the Agamse
and Ascalabotse (Geckos), the latter in Snakes, Chelonians,
and Crocodiles. A knowledge of the anatomy of the skull will
help us as regards the external form of the brain of Reptiles, and

FIG. 120. — BRAIN OF Emys europcea. (A, side, B, ventral view.)

VH, cerebral hemispheres ; Lol, olfactory lobe ; T, temporal lobe ; Inf, infundi-
bulurn ; If, hypophysis ; MH, optic lobes ; HH, cerebellum ; Nff, medulla
oblongata ; R, spinal cord ; 7, olfactory, and 77, optic nerves ; Tro, optic tract ;
Chi, optic chiasma ; 777, oculomotor; IV, trochlear ; V1 — Vz, first, second,
and third divisions of the trigeminal, all of which arise from the Gasserian gang-
lion, GG ; VI, abducent, VII, facial, VIII, auditory, IX, glossopharyngeal, X,
vagus, and XI, spinal accessory nerves; 7 Sp, II Sp, first and second spinal

the reader is referred to that section of the introduction to the
chapter on the skull in which the interorbital narrowing of the cranial
cavity is described (p. 57).

The brain of Lizards and Blindworms (Anguis) exhibits a far

L

146

COMPARATIVE ANATOMY.

lower organisation than that of other Reptiles. The hemispheres
are small and pyriform, and all the different parts are narrower
and more extended longitudinally ; the brain thus bears a closer
resemblance to that of Urodeles (compare Figs. 119, 120, A and B,
and 121). 'Olfactory lobes seem to be wanting in Crocodiles only.
An olfactory ventricle is usually present in each lobe.

FIG. 121. — BRAIN OF ALLIGATOR. (From the dorsal side. )
(After Rabl-Btickhard.)

VH, cerebral hemispheres ; Z, pineal gland ; MH, optic lobes ; HH and

median and lateral portions of the cerebellum ; Frh, sinus rhomboidalis,
bounded by the eminentiae acusticse (Eac), the tseniae medullares (T\ the obex
(Ob), and the clava (Cl) ; /, olfactory nerve ; //, optic nerve ; IV, trochlear nerve;
V, trigeminal, VIII, auditory, IX, glossopharyngeal, X, vagus, XI, spinal
accessory, and I Sp, II Sp, first and second spinal nerves.

The thalamencephalon is always depressed, and is hardly,
or not at all, visible from the dorsal side. It gives rise to a
distinct infundibulum as well as to an epiphysis, which in the
embryos of Lizards, as in those of Anura, extends into the roof of
the skull, but which becomes narrowed and reduced later.

The mid-brain always consists of a well-marked paired
portion, and from it the optic tracts pass downwards and forwards
to the chiasma. the fibres of the optic nerve taking on a secondary

THE BRAIN, 147

connection with the mid-brain. The cerebellum usually consists
of a thicker median, and two fold- or wing-like lateral portions.
It generally overlies the sinus rhomboidalis for some distance,
and& attains its greatest development in the Crocodilia (Fig. 121,
HE).

Judging from the casts of the cranial cavity, the brain of Dinosaurians
must have been very lowly organised, and much more nearly related to that
of Lizards than to that of Birds. The genus Stegosaurus possessed the
smallest brain of any terrestrial Vertebrate relatively to its siz;e.

Birds.

In Birds, the hemispheres are so largely developed that they
overlie the anterior part of the mid-brain, bending back the
pineal gland, and only leaving the cerebellum uncovered (Fig. 122,
A and B, VHt HH}.

A B

FIG. 122. — BRAIN OF PIGEON. (A, from above ; B, from the side.)

VH, cerebral hemispheres ; Z, pineal gland ; MR, optic lobes ; HH, HH1, cerebel-
lum (vermis and flocculi) ; NH, medulla oblongata ; R, spinal cord ; H, pituitary
body ; /, olfactory nerve ; Lol, olfactory lobe.

The cerebellum consists of a well-developed and folded median
lobe, and of two lateral portions (flocculi), which vary much both in
form and size. Posteriorly it completely covers the fourth ventricle.
The two optic lobes are separated from one another and pressed
downwards, so as to lie on the sides of the brain in the angle between
the hemispheres, cerebellum, and medulla oblongata (Fig. 122, MH\
and they are connected by a broad commissure. Olfactory lobes
are always present, but only slightly developed. The corpora striata
(basal portion of the cerebrum) lying within the hemispheres are
so largely developed that they form by far the greater part of this
region of the brain.

The toothed Birds of the Cretaceons period, with Hesperornis at their head,
possessed a very lowly organised Reptilian-like brain, with small hemispheres
and large olfactory lobes. The brain of Archseopteryx was highly developed,
nearly resembling that of existing Birds.

L 2

]48

COMPARATIVE ANATOMY.

Mammals.

While in many cases (e.g. Marsupials, Kodents, and Insecti-
vores) the mid-brain lies more or less freely exposed, in the series
of the Primates the hemispheres gradually come to cover all the
other parts of the brain. The presence of large commissures be-
tween the hemispheres — the corpus callosum1 and for nix —
is very characteristic of Mammals : the hemispheres are also differ-
entiated into lobes, which are usually more or less convoluted,
giving rise to gyri separated by sulci, which serve to increase

fMH

—-JW

FIG. 123. — HUMAN BRAIN. (Median longitudinal vertical section.)
(Mainly after Reichert.)<;

VH, cerebrum ; To, optic thalamus (thalamencephalon), with the middle commissure
(Cm) • Z, pineal gland ; T, infundibulum ; H, pituitary body ; Mff, corpora
bigemina, with the aqueduct of Sylvius (Aq), anterior to which is seen the
posterior commissure (Cp) ; HH, cerebellum ; NH, medulla oblongata, with the
pons Varolii (P) ; H, spinal cord ; B, corpus callosum ; 6r, fornix, which extends
antero-ventrally to the lamina terminalis (Col), in the upper part of which is seen
the anterior commissure (Co), and between the latter and the optic thalami (To)
the foramen of Monro (FM) ; Teh, tela choroidea ; /, olfactory nerve ; II,
optic nerve.

the superficial area. The amount of convolution varies much
in the different orders: thus in the brain of Primates frontal,
parietal, occipital, temporal, and central lobes may be dis-
tinguished.2 The central lobes correspond to the region described
above as the basal portion of the prosencephalon.

1 The corpus callosum is very small in Monotremes and Marsupials, only the part
corresponding to the anterior genu of higher types being developed, and this is the
part which is the first to appear in the embryo of the latter. The relative size of the
anterior commissure is in inverse proportion to that of the corpus callosum.

2 Corresponding with this division into definite lobes there is also a marked
differentiation of the lateral ventricles, so that an anterior, a posterior, and an
inferior .corn u can be distinguished in each.

THE BRAIN.

149

The division of the cerebellum into a median and two lateral
portions, already indicated in Reptiles, but much more plainly
marked in Birds, is carried to a still further extent in Mammals.

FIG. 124. — CONVOLUTIONS OF THE HUMAN BRAIN. (After A. Ecker.)

Lf, frontal lobe ; Lp, parietal lobe ; Lo, occipital .lobe ; T, temporal lobe ; «, b, c,
superior, middle, and inferior frontal gyri ; X, 0, anterior and posterior
central convolutions, separated from one another by the fissure of Eolando (ft) ;
em, the calloso-marginal sulcus, on the dorsal surface ; P, Pl, superior and
inferior parietal gyri, separated from one another by the interparietal fissure
(I) ; Po, parieto-occipital fissure ; FS, Sylvian fissure ; 1 to 3, superior, middle,
and inferior temporal convolutions ; HH, cerebellum ; NH, medulla oblongata ;
R, spinal cord.

The median portion gives rise to the so-called superior vermis,
while the lateral parts form the lateral lobes and flocculi.

jnf

.r Cacb

FIG. 125. — DIAGRAMMATIC FIGURE OF THE PRINCIPAL BANDS OF NERVE-FIBRES
OF THE MAMMALIAN BRAIN. (From a drawing by A. Ecker. )

Cacb, crura medullfe ad cerebellum ; Cap, crura cerebelli ad pontem ; Cac, crura
cerebelli ad corpora bigemina ; C.C., crura cerebri ; HM, hemispheres ; Cs, corpus
striatum ; Th, optic thalamus ; L, lemniscus ; P, pons Varolii ; HH, cere-
bellum.

The two lateral lobes are connected by a large commissure, the
pons Varolii (Fig. 123, P) : this extends round the medulla
oblongata ventrally, and is more largely developed the higher we
pass in the Mammalian series.

150

COMPARATIVE ANATOMY".

Other bands of nerve-fibres connecting parts of the brail
are spoken of as peduncles of the cerebellum (crura medullse ad
cerebellum, crura cerebelli ad pontem, and crura cerebelli ad
corpora bigemina) and cerebrum (crura cerebri) (Fig. 125).

In Mammals the mid-brain is of smaller relative size than in
other Vertebrates (Fig. 123, MIT). A transverse furrow across its
dorsal surface divides it into four lobes (corpora bigemina).

FIG. 126. — CASTS OF THE B^AIN-CASES OF EOCENE MAMMALS. (After Marsh.)

Skull, with brain indicated, of A, Tillotherium fodiens ; B, Brontotherium ingens ;
C, Coryphodon hamatus ; D, Dinoceras rairabile. E and F, ventral and lateral
views of casts of the brain of Dinoceras mirabile.

The pineal gland (Fig. 123, Z) is displaced by the enlarged
hemispheres, and rests upon the anterior lobes.

The reader is referred to Fig. 123 for details as to the relations
of the corpus callosum, the fornix, the thalamencephalon, the
three commissures of the central portion of the brain, &c. These
parts will be better understood by a comparison with the descrip-
tion of their development given in the introduction to this chapter.

PERIPHERAL NERVOUS SYSTEM. 151

aii] The brains of Tertiary Mammals were in a very low stage of
aq development (comp. Fig. 126, A to F). Quite apart from the
arj relatively diminutive size of the brain, and more particularly of the
i hemispheres, its structure reminds us in many points of the Reptilian
\i brain, though these animals were probably related to the Ungulata
si arid Proboscidea.

II. PERIPHERAL NERVOUS SYSTEM.

By means of the peripheral nervous system a physiological
connection is established between the periphery of the body and the
central nervous system in both centripetal and centrifugal directions.
All parts of it, whether formed of nerve-cells or fibres, appear to
arise as bands or outgrowths of the central nervous system, and
are consequently derivatives of the epi blast. Thus the whole
nervous system represents a single organ morphologically as well as
physiologically, and the connection of the nerves with their periphe-
ral end-organs is to be looked upon, at least as far as we know at
present, as a secondary one.

Two principal groups of peripheral nerves may be distinguished,
viz. spinal and cranial, that is, those which arise from the spinal
cord and brain respectively. The first are the more primitive and
simple structures, and they all show a similar arrangement along both
dorsal and ventral sides of the spinal cord, so that each segment of the
trunk possesses a dorsal and a ventral pair. The former consists of
sensory, the latter of motor fibres.

This regular arrangement can no longer be plainly recognised
in all the cerebral nerves. Their condition in the early embryo,
however, shows that they have a similar origin to the spinal nerves :
both groups arise from two continuous longitudinal ridges of cells
lying along the dorso-lateral regions of the medullary cord, which
become later differentiated into segmentally-arranged ganglia, the
intersegmental portions undergoing no further development.

Nerve-fibres (processes of the multipolar nerve-cells, consisting
of axis-fibres) now grow out from the dorsal region of the spinal
cord into this chain of ganglia, pass through them, and appear
again on the other side. The dorsal nerve-roots thus arise
secondarily, that is, after the conversion of the neural ridge
into a chain of ganglia, while the ventral roots are developed
independently from the medullary cord, and appear to be formed
later than the dorsal.

We must thus bear in mind that each dorsal or sensory nerve,
whether it belongs to the brain or to the spinal cord, has originally a
ganglionin connection with it, while in the ventral nerves a
ganglion is wanting.

On the distal side of each ganglion, both nerve-roots become
bound up in a common sheath, though many facts seem to indicate

152 COMPARATIVE ANATOMY.

that in the ancestors of existing Vertebrates the dorsal and ventral
roots remained distinct.

The common trunk formed by the junction of the two roots
divides up again into a dorsal, a ventral, and an intestinal
branch. The first of these goes to the muscles and skin of the
back, the second supplies the lateral and ventral portions of the
body-wall, while the intestinal branch comes into connection with
the sympathetic (see p. 160).

1. SPINAL NERVES.

As a general rule, each corresponding pair of dorsal and ventral
roots lies in the same transverse plane : an exception to this is seen
however, in Amphioxus, Cyclostomes, and Elasmobranchs. In
Amphioxus the mesoblastic somites of the right and left side
are arranged alternately, and thus the points of exit of the
nerves also alternate, -while in the two last-named groups of Fishes
each ventral pair alternates with a dorsal pair. In Ganoids also
lateral displacements of the nerve-roots are to be met with.

While in Fishes the greatest variations are seen as regards the
mode of exit of the nerves (which pass through the intercalary
pieces of the vertebral column, through the arches, or between them),
from the Amphibia onwards the nerves always make their exit on
each side between the arches, through the intervertebral foramina.
In their primitive undifferentiated condition the spinal nerves have
a strictly metameric arrangement, and are equally developed in all
regions of the body. As already pointed out in the chapter on the
spinal cord, this condition becomes modified by the development of
the appendages, so that a number of spinal nerves unite together to
form plexuses,1 which according to their position are spoken of as
cervical, brachial, lumbar, sacral, &c. The number and size
of the nerves composing them is usually directly proportional to
the development of the appendages : a special description of them,
however, cannot be given here, and only the following points will
be touched upon.

In contrast to Fishes, the. great variation in the plexuses of which
renders it impossible to reduce them to a common plan, we find
from the Amphibia onwards a typical grouping of the branches
of the brachial plexus. The following branches may be distin-
guished : — (1) Anterior thoracic nerves (the dorsalis scapulae
and thoracicus posterior s. lateralis of human anatomy) ; (2)
anterior brachial nerves, the homologues of the subscapulares,
cutaneus brachii internus minor (with limitations), axillaris, and
radialis; (3) posterior brachial and thoracic nerves (tho-
racici s. pectorales anteriores, cutaneus internus major s. medius
musculo-cutaneus, median, and ulnar nerves (with limitations).

1 For a description of their composition, see Wiedersheim's Lchrbuch der vergl.
Anatomic.

CRANIAL NERVES.

153

The lumbo-sacral plexus shows in general, and more
particularly in Mammals, much greater variations than does the

E7#

FIG. 127. — CEREBRAL NERVES AND BRACHIAL PLEXUS or Salamandra atm>

Va, ophthalmic branch of the trigeminal ; V*, its maxillary branch ; Vc, its
mandibular branch ; t \, entrance of the ophthalmic branch into the nasal capsule
and Va, its extension forwards to the snout ; VII, facial nerve ; VII*, its hyo-
mandibular branch ; VIP, its palatine branch, which enters the nasal capsule
at * ; Co, commissure between the facial and glossopharyngeal (IX ) ; IXa, branch
of the glossopharyngeal to tongue ; IX b, its pharyngeal branch ; X, vagus ; -2f/,
spinal accessory ; XII 1, hypoglossal (first spinal nerve, which becomes connected
peripherally with the second spinal nerve, XII '2} ; 1 to 6, the first five spinal
nerves ; Pl.brcwh., brachial plexus ; Sy, sympathetic cord, showing a connection
with the spinal nerves at Sy1 ; Or, orbit ; M, maxilla.

brachial plexus. The nerves arising from it are spoken of as
obturator, crural, and sciatic. The latter divides up in the
hind-limb into a tibial and a fibular nerve.

2. CRANIAL NERVES.

As already mentioned, the cranial nerves become so much
modified in the course of development that their primary rela-
tions, as a rule, can be no longer recognised.;,: Nevertheless, it is
important to understand these primary relations thoroughly before
pursuing our inquiries further. It must therefore be borne in mind
that the head is primitively composed of a series of metameres, and
that the brain and skull are correlated genetically.1

We must now ascertain, as far as is possible in the present
state of our knowledge, to which individual metameres the different
cranial nerves belong (see Fig. 41). The latest researches on this
subject have reference mainly to Elasmobranch embryos, though the
results obtained have been confirmed in other Fishes (Cyclostomi,
Teleostei), and to a certain extent in Mammals also.

1 For the appearance of dorsal ganglia of the cranial nerves in the embryo, compare
the chapter on the sensory organs of the integument, p. 166.

154

COMPARATIVE ANATOMY.

The following general summary gives a scheme of the primitive
relations of the head segments. It is to be noticed that the first
and second cranial nerves — the olfactory and the optic — are not
mentioned in the list, for reasons to be explained later.

TABLE SHOWING THE SEGMENT AL ARRANGEMENT OF THE CRANIAL NERVES,
WITH THEIR RELATION TO THE METAMERES OF THE HEAD.

1st Metamere (superior,
inferior, and internal rec-
tus, and inferior oblique
muscle).

Ventral branch.

Dorsal branch.

Oculomotor (///).

2nd Metamere
oblique).

(superior i Trochlear (IV).

3rd Metamere (external
rectus).

4th Metamere (muscles
which are early aborted).

5th Metamere (muscles
which are early aborted).

6th Metamere (very rudi-
mentary muscles).

7th to 9th Metame.res
(muscles extending from
the skull to the pectoral
arch, including the an-
terior portion of the
sterno-hyoid).

Abducent ( VI).

Wanting.

Wanting.

Appears to be want-
ing.

Hypoglossal (XII).

Ramus ophthalmicus pro-
fundus of the trigemin d

Trigeminal (with the excep-
tion of its ramus ophthal
micus profundus).

Facial (VII), and audi-
tory ( VIII).

Glossopharyngeal (IX).

Vagus (X).

The cranial nerves may be divided into four main groups,1 quite
apart from their metameric signification. The first consists of the
olfactory, or first, and the optic, or second cranial nerve; the
second of the nerves of the eye-muscles, i.e. the oculomotor,
trochlear, and abducent nerves, the third of the trigeminal
with the auditory and facial, and the fourth of the glossopharyn-
geal and vagus. The eleventh cranial nerve, or spinal accessory,
as well as the twelfth, or hypoglossal, although they occasionally
(in Mammals for instance) are included within the cranial cavity,
come under the category of spinal nerves.

Olfactory Nerve. — The olfactory, when compared with the
other cranial nerves, possesses many peculiarities, which seem to
give it an isolated position. It grows out secondarily from the

1 No satisfactory morphological explanation has yet been given of the two pairs of,
nerves branching out to supply the anterior end of the body in Amphioxus.

CRANIAL NERVES. 155

olfactory lobe or from a part of the brain developed from it, and
does not at first consist, like the ordinary cranial and spinal
nerves, of a series of non-nucleated axis fibres, but has the form of
nucleated bundles, which arise from a network of star-shaped
cells of the olfactory lobe. These nuclei are therefore identical
with the nuclei of nerve-cells.1

These observations were made on the human subject, and it is very desirable
that similar researches should be extended to the lower Vertebrates. It might
then be possible to explain the fact that the roots of the olfactory nerve are
usually double. The roots either eventually unite on either side to form a
common trunk, or else (less commonly) remain entirely separate, so that two
olfactory nerves perforate the ethmoid on each side (Gymnophiona).

A definite cribriform plate is not always present ; far more
commonly the whole undivided trunk of the olfactory nerve ex-
tends into the nasal cavity, and only then begins to break up.
This holds good for by far the greater number of Vertebrates below
Mammals, as well as for Monotremes.

It appears very probable that the olfactory nerve, in spite of its
peculiarities in the human subject, arises primitively, that is,
phylogenetically, in a similar manner to the ordinary cranial nerves.
The important circumstance that in Fishes it arises from the primi-
tive ridge of the medullary tube, from which all the other nerves
take their origin, is in favour of this supposition.

Optic Nerve. — As already mentioned, the optic nerve arises
from the stalk of that outgrowth of the primary fore-brain which is
spoken of as the primary optic vesicle. Inasmuch, therefore,
as it represents apart of the brain, it cannot be compared with,
any of the other nerves. It remains for further researches to show
whether it is possible, on phylogenetic grounds, to consider it as
originally a segmental nerve.

Three more or less sharply differentiated portions may in most
cases be distinguished in the optic nerve ; these are spoken of from
the proximal to the distal end respectively as the optic tract)
chiasma, and nerve.

The chiasma, that is, the crossing of the two optic nerves, is
always present, though not always freely exposed, for it may re-
tain a primitive position deeply embedded in the base of the brain,
as in Cyclostomes.

While in most Teleosteans the optic nerves simply overlie one
another (Fig. 128, A), in some of these Fishes (Harengus, Engraulis,

1 The olfactory lobes, which are closely united with the hemispheres, are originally
directly applied to the olfactory mucous membrane, which is perforated by numerous
fine fibres from them. This condition persists throughout life in Cyclostomes : in
other Fishes, in correspondence with the separation of the olfactory organs and brain
by the secondary growth of the head, each olfactory lobe becomes drawn out into an
olfactory tract, the main mass of the lobe remaining in connection with the
olfactory organ, and thus being widely separated from the brain. In Elasmobranchs
the olfactory lobes are usually short and thick, while in Teleosteans they are long and
slender.

156 COMPARATIVE ANATOMY.

Fig. 128, B), one nerve passes through a slit in the other, and this
condition of things is gradually carried still further in Reptiles, until
finally the fibres of the two nerves intercross in a very complicated
manner (Fig. 128, C, D), giving rise to a sort of basket-work ; this is
finest and most delicate in Mammals, where its structure can only
be analysed by comparing a series of sections.

FIG. 128. — CHIASMA OF THE OPTIC NERVES. (Sernidiagrammatic. ) A, chiasma as
seen in the greater number of Teleostei ; B, in Herring ; C, in Lacerta agilis ; D,
in an Agama ; E, in a higher Mammal.

CM, chiasma of the bundle of nerves lying centrally ; Cc, Ce\ S, S\ lateral fibres ;

Co, commissure.

Nerves of the Eye-muscles. — The nerves of the eye-muscles,
that is, the oculomotor, trochlear, and abducent, supply the
muscles which move the bulb of the eye, as already mentioned
in the table showing the metameric distribution of the cranial
nerves (see p. 154).

The so-called ciliary ganglion belongs to the ramus ciliaris,
or, what comes to the same thing, to the ramus profundus of the
trigeminal, and it thus represents the most anterior ganglion of the
head. Its relations to the oculomotor are secondary.

Trigeminal. — This is one of the largest of the cerebral nerves.
As its name implies, it divides up on each side into three main
branches, — an ophthalmic (1st division) consisting of a super-

CRANIAL NERVES. 157

ficial and a deep1 branch, a maxillary (2nd division), and a
mandibular (3rd division).

The first of these arises separately, like a dorsal root of a spinal
nerve, while the other two represent primitively a single branch,
corresponding to the mandibular, from which the maxillary grows
out secondarily.

FIG. 129. — CRANIAL NERVES OF Anguis fragilis.

G, Gasserian ganglion, from which proceed the three branches of the trigeminal, Va,
Vb, and Vc : behind it is seen a sling-like commissure of the sympathetic (Sy
and Co), which connects the trigeminal with the vagus-group (IX, X ) : from
this commissure arises a sympathetic ganglion (Gg], as well as a long cord (Sym)
passing to the second sympathetic ganglion, Gfy1 ; VIIa, VIIb, the facial nerve
appearing through two separate apertures ; f> connection between the palatine
branch of the facial and the maxillary division of the trigeminal ; *t, points of
entrance of the maxillary and ophthalmic divisions respectively of the trigeminal
into the nasal capsule ; Mm, Mm, branches of the mandibular branch to the
masticatory muscles ; GX, ganglion of the vagus ; Li, inferior laryngeal nerve ;
p, superior laryngeal ; Ri, intestinal branch of the vagus ; XII, hypoglossal (the
two first spinal nerves) ; 3 to 6, third to sixth spinal nerves ; 0, auditory capsule ;
Seap, scapula ; A, eye ; D, Dl. lacrymal and posterior part of Harderian gland.

The fact that in many Vertebrates the trigeminal arises by two
separate roots indicates its double nature, as does also the fact
that, contrary to the general rule, all three divisions do not unite in
a single ganglion (the Gasserian), but each main branch may be
provided with an independent ganglion.

In all Vertebrates, the first division of the trigeminal,
with its deep (naso-ciliary) and superficial branch, supplies the
integument of the forehead and snout, as well as the integu-
mentary coverings of the orbit and certain parts of the eye-
ball. It is entirely sensory.

The second division of the trigeminal, which is also a
sensory nerve, is connected with the facial, and extends first
along the floor of the orbit, then passes to the upper jaw, supplying
the teeth, and finally, as the infraorbital branch, perforates the

1 In Fishes and Amphibia the deep branch forms an independent twig ; in higher
forms it is bound up with the superficial branch as the naso-ciliary nerve.

158 COMPARATIVE ANATOMY.

skull to reach the integument in the region of the upper jaw, snout,
and upper lip.

The third division of the trigeminal is of a mixed nature ;
it supplies on the one hand the masticatory muscles, and also gives
rise to the great sensory nerve of the tongue (lingual or gustatory
nerve), while another branch, passing through the inferior dental
canal, supplies the teeth of the lower jaw, and then gives off
one or more branches to the integument of the latter and of
the lower lip. It is usually connected with the chorda tympani
branch of the facial.

Facial and Auditory Nerves. — Both arise from a common
ganglion, the former, as we have seen, coming into close relation
with the trigeminal.

The facial, which is originally a mixed nerve, divides into three
branches, a hyomandibular, a palatine, and abuccal. The first,
which is connected with the glossopharyngeal by means of the
so-called anastomosis of Jacobson, is distributed, as its name im-
plies, mainly to the region of the first and second visceral arches ;
thus in Fishes it goes to the parts around the spiracle and to the
muscles of the operculum and branchiostegal membrane. A small
remnant of this branch in the higher Vertebrates supplies the stylo-
hyoid muscle and the posterior belly of the digastric.

In Mammals the facial is a purely motor nerve, supplying
mainly the facial muscles, as well as the platysma myoides, which
has the closest relations to them (comp. p. 121).

The auditory is always a very large nerve, and soon after its
origin from the brain it divides into a cochlear and a vestibular
branch. The former passes to the cochlea, while the latter supplies
the rest of the auditory labyrinth.

Glossopharyngeal and Vagus. — These, which are of a mixed
nature, have not, like the other cranial nerves, their distribution
limited to the head.

In Fishes and gill-breathing Amphibians the vagus branches
out to the region of the visceral and branchial apparatus, as well
as to the muscles of the shoulder and anterior extremity
(Protopterus). It then extends backwards along the sides of
the body under the skin to the tail as one or more lateral
nerves, supplying sensory organs.1

Further, in all Vertebrates it is distributed to the anterior
part of the alimentary canal, giving rise to a pharyngeal, an
oesophageal, and a gastric plexus, besides giving off branches to the
heart and to the whole respiratory system, from the larynx to the
lungs (air-bladder).

Thus cephalic, cervical, thoracic, and abdominal por-
tions of the vagus can be distinguished in the higher Vertebrates.

1 Compare the chapter on sensory organs, p. 165.

CRANIAL NERVES.

159

Both vagus and glossopharyngeal are always closely connected with
the sympathetic system by anastomoses : in Fishes the glosso-
pharyngeal supplies the region of the first (hyobranchial) cleft, while
in the higher Vertebrates it passes to the tongue as the nerve of
taste, and, like the vagus, gives rise to a pharyngeal plexus.

JRlat

FIG. 130. — CRANIAL NERVES AND BRACHIAL PLEXUS OF Scyllium canicula.

II, optic nerve ; 777, oculomotor ; IV, trochlear ; Va (upper), superficial branch, and
Va (lower), deep branch of the first division of the trigeminal (the two branches
anastomose at * within the nasal capsule) ; Vbc, maxillo-mandibular branch ;
V>, maxillary branch ; Vc, mandibular branch ; VI, abducent ; F77, facial ;
VIIa, its hyomandibular branch ; VIIb, its palatine branch ; IX, glossopharyn-
geal ; X, vagus ; Elat, its lateral branch ; ttt, gill-clefts ; 1 to 14, the first
fourteen spinal nerves, forming the brachial plexus (Pl.bradi) ; 0, auditory
capsule ; Sp, spiracle ; Or, orbit ; MS, cleft of mouth.

Spinal Accessory. — This nerve arises some distance back
along the cervical portion of the spinal cord, in the region from
which the fourth to fifth cervical nerves come off; from this
point it passes forwards, taking up fibres from the cervical nerves
as it goes. It extends along the side of the medulla oblongata
into the cranial cavity, there becomes associated with the root of
the vagus, and leaves the skull through the same foramen as the
latter. It appears plainly for the first time in Chelonia, and
supplies certain of the muscles related to the pectoral arch, e.g.
the stern ocleidomastoid and the trapezius.

Hypoglossal. — This purely motor nerve closely resembles a
spinal nerve, and is distributed (having here and there anastomoses
with the cervical plexus) to certain muscles lying on the floor of the
mouthr and toothers extending between the pectoral arch (sternum)
and hyoid (which morphologically are trunk-muscles and not vis-
ceral muscles), as well as to the muscles of the tongue proper,
which are differentiated from the latter. (Compare p. 119). In
the Ichthyopsida it is not included within the skull, and is there

160 COMPARATIVE ANATOMY.

represented by the first and sometimes also the second spinal nerve.
Both, to a greater or less degree, take part in forming the brachial
plexus.1

SYMPATHETIC.

The sympathetic nervous system is derived from the ganglia of
the cranial and spinal nerves, and, as already mentioned, is
distributed mainly to the intestinal tract (in the widest sense),
the vascular system, and the glandular organs of the body.

From each spinal ganglion of the embryo, a nerve grows out,
which has been already referred to as an "intestinal" nerve (p. 152).
After extending a short distance, dorsal to the cardinal vein, each
nerve passes into a small, irregularly-shaped mass of nerve-cells,
and these ganglia, like those of the spinal nerves, show originally
a segmental arrangement. As they become united together by
longitudinal commissures, a chain-like paired sympathetic cord
is formed (Fig. 104, S, SI to 810). From its ganglia nerves pass
off to the above-mentioned systems of organs, and its original
connection with the central nervous system persists.

The sympathetic extends not only along the vertebral column,
but passes anteriorly into the skull, where it comes into relations
with a series of the cranial nerves similiar to those which it forms
further back with the spinal nerves. In the Frog, where it becomes
connected with the ganglion of the vagus, a considerable number of
its fibres pass together with the latter nerve to the heart, and thus
the nerve which is generally known as the vagus stem is in reality
vagus plus sympathetic.

The original segmental character usually disappears later on,
and this is especially the case in those regions where marked
modifications of the earlier metameric arrangement of the body
have taken place, e.g. in the neck.

In Crocodiles and Birds a part of the sympathetic cord runs
within the vertebrarterial canal alongside the vertebral artery, while
in all other Vertebrates the whole cord lies along the ventral side
of the vertebral column : it is generally situated close to the latter,
and overlies the vertebral ends of the ribs.

BIBLIOGRAPHY.

A.HLBORN, F. — Unters. uberd. Gehirn der Petromyzonten. Zeitsch.f. iviss. Zool. Bd

XXXIX. 1883.
DOHRN, A. — Stud. z. Urgesch. d. Wirbelthierk&rp&rs. Mitthl. d. zool. Stat. z. Neapel.

Bd. III. 1881 und Bd. IV. 1882.
ECKER, A. — Zur Entw. Gesch. der Furchenund Windungen dor Grosshirnhemispharen

im Fotus des Menschen. Arch. f. Anthropologic, Bd. III. Die Hirnwindungen

des Menschen. Braunschweig, 1869 (1883).
EHLERS, E. — Die Epiphyse am Gehirn der Plagiostomcn. Zeitsch.f. wiss. Zool. Bd.

XXX, -

1 Traces of dorsal roots of the hypoglossal have been found in Mammalian embryos.

SUPRARENAL BODIES. 161

GASKELL, W. H., and GADOW, H. — On the Anatomy of the Cardiac Nerves in certain

Cold-blooded Vertebrates. Journ. of Physiol. Vol. V. No. 4.
KUPFER, C. — Ueber primdre Metamcrie des Neuralrohres der Vertebraten. Sitz-

ungsb. d. K. Baierische Academie der Wissenschaften, Dec. 1885.
LEXHOSSEK, M. v. — Untersuch. ub. die Spinalganglien des Frosches. Archiv f.

mikr. Anat. Bd. XXVI. 1886.
MARSHALL, A. MILNES. — Various Papers on the Development of the Nerves in the

Journ. of Anat. and Physiol. Vol. XI. and XVI., and in the Quart. Journ. of

Micros. "Science, Vols. XVIII., XIX., and XXI.
MIHALKOVICS, V. v. — Entw.-Gesch. des Gehirns. Leipzig, 1877.
OSBORX, H. F. — Preliminary Notes on the Brain of Menopoma and JRana. Proc. Acad.

Nat. Sci. Philad. 1884.
RABL-RiJCKHARD, H.— Die gegenstit, Verhaltnisse der Chorda, Hypophysis, &c., lei

Haifischembryonen. Morph. Jahrb. Bd. VI. 1880. (See also the further works

of this author in the Zeitsch. f. wiss. Zool. Bd. XXX., and Archiv f. Anat.

und Physiol. 1882 and 1883, as well as in the Biolog. Centralb. 1883, No. 1.)
REICHERT, C.B. — Der Bau des menschl. Gehirns. Leipzig, 1859 und 1861.
SCHWALBE, G. — Lehrb. d. Neurologic. Erlangen, 1880.
STIEDA, L.— Cp. the works of this author in the Zeitsch. f. Zool. Bd. XVIII., XIX.,

XXIII., und XXV.
WIEDERSHEIM, R. — Skdet und Nervensystem von Lepidosiren annectens. Morph.

Studien, Heft 1. Jena, 1880.
WILDER, BURT G. — Encephalic Nomenclature. N. Y. Med. Journ., March 21 and

28, 1885.
WYHE, J. W. VAN. — Ueber das VisceralsJcelet und die Nerven des Kopfes der Ganoiden

und von Ceratodus. Niederl. Arch. f. Zool. Bd. V. 3. Ueber die Mesoderm-

segmente und die Entwicklung der Nerven des Selachierkopfes. Verhdl. der

R. Acad. der W. zu Amsterdam, 1882.

SUPRARENAL BODIES.

These bodies, which owe their name to the position which they
occupy in Mammals in front of the kidneys, orginate from the
mesoblastic tissue lying between the mesonephros and the aorta as
well as from the sympathetic.

In Elasmobranchs they are represented by a double row of
bodies lying right and left of the vertebral column (that is, arranged
segmentally) ; in these, a mesoblastic portion, consisting of richly
nucleated lobules, and a part arising from the sympathetic may be
recognised. In Teleostei the suprarenals are often wanting, but
when present they sometimes represent the metamorphosed anterior
(lymphoid) part of the kidney. In other cases, they are closely
united with the kidneys. It is probable that in all Vertebrates
they arise in connection with the pro- or meso-nephros. In
Amphibia, they either lie on the ventral side (Anura) or on the
inner border (Urodela) of the kidneys, receiving their blood-supply
both in Amphibians and Reptiles from the renal-portal vein.
In the latter group, as well as in Birds, they are of a bright yellow
colour, of an elongated or lobulated form, and lie in close contact
with the genital glands.

In Arnniota, and especially in Mammals, the suprarenal of
each side forms a definite and uniform mass, lying close to the
corresponding kidney, and in it an ectoderm al (i.e. sympathetic)
medullary, and a mesodermal cortical substance can always be
recognised, the two elements here being closely united together.

M

162 COMPARATIVE ANATOMY.

Their extraordinary richness in blood-vessels, which is seen
throughout life, points to the important function of these organs ;
but it is impossible to say at present what this function is.

BIBLIOGRAPHY.

BALFOUR, F. M. — Elasmobranch Fishes. London, 1878.

BRAUN, M. — Ueber Bau und Entwickl. d. Nebennieren bei Reptilien. Arb. d. zooL

Inst. zu Wjirzburg, Bd. V.
GOTTSCHAU, M. — Ueber Nebennieren der Sdugethiere spec, iiber die des Menschen.

Wilrzb. phys. med. Gesellsch. 1882. Structur und embr. Entwicklung der

Nebennieren bei Sdugethieren. Arch, f.' Anat. und Physiol. 1883.
JANOSIK. — BemerTcungcn ub. die Entwick. der Nebennieren. Archiv f. mikr. Anat.

1883.
MITSUKURI. — On the Development of the Suprarenal Bodies in Mammalia. Quart.

Journ. of Micros. Science, 1882.
ONODI, A. D.- — Ueber die Entwick. des sympatischen Nervensy stems. Archiv f. mikr.

Anat. Bd. XXVI. Heft 1.
WELDON, W. F. R. — On the Head-Kidney of Bdellostoma, with a Suggestion as to the

Origin of the Suprarenal Bodies. Quart. Journ. of Micros. Science, 1884. Cn

the Suprarenal Bodies of Vertebrata. Quart. Journ. of Micros. Science, _1885.

III. SENSORY ORGANS.

The specific sensory end-organs originate, like the nervous
system in general, from the epiblast. The peripheral terminations
of the sensory nerves are thus always to be found in cells of
epithelial origin, while mesoblastic elements (as investments, for
instance) are secondarily added to them.

The individual sense-organs, e.g. those of sight, smell, taste, and
hearing, are to be regarded as secondary differentiations
of a diffused sense, as will be mentioned later on. This is
not only indicated in many Invertebrates, but numerous facts in
the development of Fishes and Amphibia and in the structure of
Amphioxus point to the same conclusion.

From the Cyclostomi onwards this differentiation has already
taken place, and we find in them and in all Vertebrates above
them, that the olfactory, optic, and auditory organs are always
closely connected with the head. They are enclosed within
certain cavities and hollows of the skull (sense -capsules), and
thus differ somewhat from the second main group of sensory organs,
which are the means whereby the sensations of taste and temper-
ature, as well as other sensory impressions, are appreciated. These,
for the greater part, extend over the whole surface of
the body, and thus have a diffused distribution; more-
over, they remain throughout life near the ectoderm,
from which they originate.

In the higher organs of sense two kinds of cells are to be
distinguished, although they are genetically identical with one
another. The first of these are the proper rod-shaped sensory

SENSE-ORGANS OF THE INTEGUMENT.

163

cells, connected by nerves with the central nervous system, and
the second are the supporting cells, which lie between the
former and serve as a connecting and isolating material.

In all the higher sensory organs the medium surrounding
the end-organ is always moist, and as this is also the case with
the sensory organs of the integument of Fishes, we find
to a certain extent similar nerve-endings in both. Thus in both
cases, we meet with rod-shaped sensory cells, but in the latter, the
nerves coming from them do not pass through ganglion cells, as
they always do in the higher sensory organs. This indicates a
lower stage of development.

D

CJSf—

FIG. 131. — A, peripheral nerve-ending, as seen in all the higher sensory nerves; .Z?,
rod-shaped end-cell of a sensory organ of the integument of a Fish or Amphi-
bian, or a taste-cell ; C, free, and D, ganglionated nerve-ending of the integu-
mentary sensory organs of terrestrial Vertebrates.

.2V1, first, and N, second portion of the nerve-fibre in connection with the epithelial
end-cell, Gl ; G, ganglion cell interposed between these portions ; CS, cuticular
process of end-cell.
All the figures are diagrammatic, and are based upon a figure by Merkel.

In those animals which in the course of development give up
an aquatic life and come on land, the end-organs of the nerves
pass further inwards from the surface, undergoing at the
same time changes of form.

The rod-shaped end-cell now once for all disappears, and
two kinds of nerve-endings are seen in the skin — terminal
ganglion-cells and free nerve-endings.

SENSE-ORGANS OF THE INTEGUMENT.

I. ROD-SHAPED ORGANS OF FISHES, DIPNOI, AND AMPHIBIA.

a. Segmental Sense-Organs.

These organs show considerable similarity to certain structures
in Chsetopods and marine Rhipidoglossa (e.g. Fissurella).

They always consist of a central mass of cells, arranged in
the form of a rounded and depressed pyramid, and of a peripheral

M 2

164

COMPARATIVE ANATOMY.

mass grouped around the former. The central cells are in connection
with nerve-fibres ; each of them bears at its free end a stiff cuticular
hair, and they are to be looked upon as the proper sensory cells
(Fig. 132, CZ). The others (HZ, MZ1) function only as a supporting
mass (Fig. 135, a, b, c)

MZ

FIG. 132.— TRANSVERSE SECTION OF A FREELY PROJECTING SEGMENTAL SENSE-

OllGAN.

The cuticular tube and the surrounding epidermic cells are removed. CZ, central
(sensory) cells ; MZ, MZ1, peripheral cells.

FIG. 133. — DISTRIBUTION OF THE LATERAL SENSE-ORGANS IN A SALAMANDER

LARVA,

In cases where these organs project freely from the epidermis
— and this is always the case in the embryo — a delicate protective
hyaline tube arises from the summit; into it the terminal
hairs of the sensory cells project, and the tube opens distally into
the surrounding water (Fig. 135, K).

While in aquatic Amphibia these organs retain throughout
life their peripheral free position, on a level with the epidermis,1
in Fishes they may in post-embryonic time become enclosed in
depressions or complete canals, which are formed either by the
epidermis only, or, as is more usually the case, by the scales, and
bones of the head, and which open externally. The organs are
thus protected, and the hyaline tube is no longer seen.

Those sensory organs are distributed over the whole body, but as
a general rule they are most abundant along certain tracts, the
position of which is very constant. Thus in the bead, their course
is usually similar to that shown in Fig. 134. From this point

1 At the time when an Amphibian undergoes metamorphosis, and gives up its
aquatic life, these sensory organs sink downwards into the deeper layer of the skin,
and, as the epidermis grows together over them, they apparently become shut off' from
the exterior and reduced, and may finally disappear. According to other authors,
however, they persist, and remain open, being connected with the outer surface of ths
skin by a tube.

SENSE-ORGANS OF THE INTEGUMENT.

165

backwards the organs are arranged in metameric order, and,
always connected by longitudinal nerves, extend along the sides
of the body to the caudal fin in one or more "lateral lines"
(Fig. 133); they are thus often spoken of as organs of the

FIG. 134. — DIAGRAM SHOWING THE DISTRIBUTION OF THE SENSORY ORGANS OF
THE LATERAL LINE IN FISHES.

a, supra-orbital,' and b, infra-orbital, portion ; c, mandibular, d, occipital, and e
lateral portions, the latter extending backwards along the sides of the trunk and
tail.

FIG. 135.— ORGAN OF THE LATERAL LINE OF A URODELE. (Semidiagrammatic.

a, a, cells of the epidermis, through which the neuro-epithelium, b, b, can be seen ;
c, the terminal hairs of the latter (the peripheral Cecils are not represented) ; JR, the
hyaline tube ; N, the nerve-fibres passing to the* sensory cells.

lateral line. The portions lying in the region of the head are
innervated by the trigeminal, while the lateral line is supplied by
(Compare p. 158.)

the vagus.1

1 The development of the lateral branch of the vagus has not yet been
satisfactorily made out : it either grows backwards from the vagus ganglion, or, as
most authors maintain, it arises by a proliferation or differentiation of the deeper
layer of the epidermis in situ.

In Anguilla, Gymnotus, and Ceratodus, the lateral nerve is represented by a
branch of the facial.

166 COMPARATIVE ANATOMY.

The sensory sacs of Ganoids,1 which are confined to the head, and the
sensory tubes" of Elasmobranchs represent peculiar modifications of the
sensory organs in question. The former are sac-like invaginations of the
epidermis, while the latter have the form of delicate tubes, which give rise at
their base to one or more swellings or "ampullae." Both are lined by a
sensory epithelium of the same structure as that described on p. 164.

As regards the function of these sensory organs, it can
only be affirmed that they must have to do with the percep-
tion of mechanical irritations from the surrounding water :
in what manner the impulses give rise to perception cannot be
definitely stated. In many cases they are thought to be con-
cerned with the perception of sound, and we shall see that this
is not improbable when we come to consider the auditory organ.

The following is known with regard to the development of the lateral segmen-
tal organs. The dorsal roots of the cranial nerves ( F, F/7, F/Z7, IX, and X)
are during a certain embryonic period connected with the cells of the epiblast,
with which they become completely fused. Each of these masses then grows
and proliferates very rapidly, and the epiblastic thickenings thus resulting
represent the rudiments of the ganglia of the dorsal roots of the cranial nerves
as well as the first indications of the segmental sense-organs. Later, each
ganglion becomes separated from the skin, though it remains connected with
the corresponding sense-organ by means of a delicate nerve-fibre. A similar
mode of development also obtains in the segmental sense-organs of the trunk.

It is a very interesting fact that in embryos of the sheep and cow, 8 to
10 mm. long, the ganglia of the facial, glossopharyngeal, and vagus are fused
with the epiblast, and thus indications of segmental sense-organs are still
present, though they disappear or become modified later. Beard has also
found rudiments of these organs in chicks of the third day.2

/>. End-Bulbs.

In the organs described above (Fig. 132) great differences in
size and form between the central and peripheral cells may be
recognised : similar organs, however, exist near them in which both
kinds of cells are quite similar to one another in these respects.
These are the so-called end-bulbs.

In all Fishes they are scattered irregularly over the whole body,
but especially over the head ; from the Amphibia onwards, they
are present in the mouth-cavity only, and are no longer seen
outside it.3 In Amphibians they occur on the palate and on
the fungiform papillae of the tongue, and in Lizards and Blind-
worms they are also present on the inner sides of the upper and
lower jaws. In Mammals they are found on the soft palate, on the
walls of the pharynx, and on the epiglottis, and even extend into
the larynx ; but here also they are most constant and numerous on
the tongue, where they occur on the circumvallate and fungiform
papillae, as well as on the papilla foliata.

1 Similar organs are present in Amiurus cattus.

2 Beard proposes the name of branchial sense-organs for these structures, as
he considers them to be primitively the special sensory organs of the gill-clefts.

3 They have also been found in the mouth and pharynx of Dipnoi.

SENSE-ORGANS OF THE INTEGUMENT. 167

These structures function from the Amphibia onwards as organs
of taste, while in Fishes they probably serve as tactile organs.
This latter function is naturally impossible in those cases where
they become somewhat withdrawn inwards from the free surface of
the epithelium, as is the case with those situated on the tongue, where
they can only be reached by fluid passing in to them.

II. TERMINAL GANGLION CELLS.

These structures are not directly connected with the surface of
the epidermis, and supporting cells are wanting.

"Tactile spots," consisting of groups of "tactile cells," are
met with for the first time in tailless Amphibians, where,
situated mainly on small elevations, they are distributed over the
skin of the whole body (Fig. 136, a, a). In Reptiles they are

FIG 136. — A TACTILE SPOT FROM THE SKIN OF THE FROG.
(Modified from Merkel.) "

A7", nerve, which loses its medullary sheath at N1 ; a, a, neuro-epithelium ;
b, epidermis.

-KIT

FIG. 137. — TACTILE CORPTJSCLE FROM THE TONGUE OF A BIRD.
N, nerve ; H, outer investment, with nuclei (KH] ; S, S, septa.

found chiefly in the region of the head, on the lips and sides of
the face, and on the snout, but in some cases (as in Blind worms
and Geckos), they extend over the whole body.1 In Snakes and
Birds the tactile cells are confined to the mouth-cavity (tongue)

1 Similar structures are also present in Crocodiles, and in the skin of the back of
Trionyx cellular bodies exist, which most probably are to be regarded as tactile
organs.

1G8

COMPARATIVE ANATOMY.

and to the beak (cere), and lie much more closely together, form-
ing definite masses, or "tactile corpuscles." Each of these
is surrounded by a nucleated connective-tissue investment, from
which septa extend into the interior, partially separating the
individual tactile cells from one another.

FIG. 138. — A TACTILE CORPUSCLE (END-BULB) FROM THE CONJUNCTIVA OF A

MAMMAL.

N, nerve (the neurilemma of which at t becomes continuous with the investment of
the tactile corpuscle ; K, K, nuclei in the investment ; N, the coiled termination
of the nerve (axis-fibre) passing to the tactile cells ( T, T).

t

Nl

FIG. 139. — A PACINIAN CORPUSCLE FROM THE BEAK OF THE DUCK.
(After J. Carriere.)

ZZ, cells of the nerve-sheath ; L, longitudinal, and Q, circular layers of the in-
vesting lamellae ; JX, central knob, with the two pillars of cells ; A, axis-fibre,
with protoplasmic investment, entering the corpuscle at A1 ; MS, medullary
sheath ; Nl, neurilemma, which becomes continuous with the investment of the
corpuscle at f, f.

In Mammals the tactile cells are either isolated, as, for instance,
on the hairless portions of the body, or they give rise to oval corpus-
cles, each consisting of a many-layered and nucleated investment,
into which a nerve passes, becomes twisted up, and ends in one or
more terminal ganglion cells (Fig. 138).

SENSE-ORGANS OF THE INTEGUMENT. 169

From the Reptilia onwards, so-called Pacinian corpuscles
are present in addition to the above-described tactile organs. They
undergo very numerous modifications, but each consists essentially
of numerous concentric lamella? (Fig. 139, Q, L\ enclosing the
termination of the axis-fibre with its protoplasmic sheath, which
dilates at the distal end into a sort of knob surrounded by a double
row of cells (Fig. 139, JK). Thus the axis-fibre receives the
external impressions indirectly, that is, by means of the cells in-
vesting the knob-like end-organ. Organs of this kind are uni-
versally present in the skin of Mammals, and differ from the
tactile spots and tactile corpuscles in position: they
are found in the deeper layer of the derma, as well as in
the panniculus adiposus, the interstitial connective-tissue, and in
the various organs of the abdominal cavity (e.g. the mesentery,
mesocolon, pancreas, and portal fissure of the liver of the Cat),
the fasciae, tendons, vas deferens, periosteum, pericardium, pleura,
corpora cavernosuin and spongiosum, the wing-membrane of
Bats, &c.

These organs are not entirely wanting in any part of the skin
of Birds, but are particularly abundant on the beak, and at the
bases of the contour- wing- and tail-feathers. So-called corpuscles
of Grandry are also present in the beak.

In all the tactile cells and tactile and Pacinian corpuscles we
have to do with organs of touch, or, expressed generally, with
means for the appreciation of sensations in the skin.

It is impossible to say definitely which nerve-endings have to
do with the perception of temperature ; it is not improbable
that the tactile cells, as well as the nerve-fibres with knob-like
swellings ending freely in the epidermis, are concerned in this
(Fig. 131, O).

BlBLIOGEAPHY.

BEARD, J. — On the Segmental Sense-organs, and on the Morphology of the Vertebrate
Auditory Organ. Zool. Anz. Nos. 161 and 162, 1884. On the Cranial Ganglia
and Segmental Sense-organs. Zool. Anz. No. 192, 1885. The System of Bran-
chial Sense-organs, and their Associated Ganglia in Ichthyopsida. Quart. Journ.
Micros. Science, 1885.

GOLDSCHEIDER, A. — Neue Thatsazhen ub. die Haut-sinnesnerven. Arch. f. Anat. u.
Physiol. 1885.

KOLLIKER, A. — Stiftchenzellen in der Epidermis von Froschlarven. Zool. Anz. 1885.

LEYDIG, F. — Ueber die SMeimeanale der Knochenfische. Arch. f. Anat. u. Physiol.
1850. Ueber Organe eines sechsten Sinnes. Nova acta acad. Caes. Leopold. Carol.
Germ. not. curios. Bd. XXXIV. 1868. Stiftchenzellen in der Oberhaut -von
Batrachierlarven. Zool. Anz. No. 212, 1885.

MALBRANC, M. — Sinnesorgane der Seitenlinie bei Amphibien. Zeitsch. f. iciss. Zool.
Bd. XXVI. 1875.

MERKEL, FR. — Ueber die Endigungen der sensibUn Nerven in der Haut der Wirbel-
thiere. Eostock, 1880.

SCHULTZE, F. E. — Ueber die becherform. Organe der Fische. Zeitsch. f. wiss. Zool.
Bd. XII. 1863. Ueber die Sinnesorgane der Seitenlinie bei Fischen und
Amphibien. Arch.f. mikr. Anat. Bd. VI. 1870.

SCHWALBE, G. — Lehrb. der Anatomie der Sinnesorgane. Erlangen, 1883.

SOLGER, B. — Seitenorgane der Fische. Arch. f. mikr. Anat. Bd. XVII. und
XVIII.

170

COMPARATIVE ANATOMY.

OLFACTORY ORGAN.

The olfactory organ, in its simplest form, consists of a paired,
pit-like depression of the integument above the cleft of the mouth.
A nerve passes to the base of this pit, and becoming enlarged
in the form of a ganglion sends off nerves to the sensory cells
(olfactory cells). The latter must be regarded as phylo-
genetic derivatives of the end-bulbs of that part of the integu-
ment which becomes pushed in to form the primitive olfactory
pit ("olfactory bulbs"), and therefore come under the same
morphological category as the taste-bulbs. At first (that is in
Fishes and Urodeles), they are only separated from one another by
interstitial epithelial tissue, but from the Anura onwards this tissue
gradually disappears in order to allow of an increased surface for
the olfactory epithelium. The ciliated cells lying amongst the
sensory cells serve to continually renew the outer medium — whether
that be water or air — by which the odoriferous particles are conveyed
(Fig.

FIG. 140. — EPITHELIUM OF THE OLFACTORY Mucous MEMBRANE.
Petromyzon planeri ; B, of Salamandra atra.

R, olfactory cells ; E, interstitial epithelial cells.

A, of

While the olfactory organs of all the true Fishes exhibit the
above-described simple sac-like form, from the Dipnoi onwards
they come to communicate with the cavity of the mouth as well
as with the exterior. In consequence of this, anterior or external,
and posterior or internal nostrils (choanse) can be distin-
guished, and as a free passage is thus formed through which the
external medium can pass, the olfactory organ takes on an important
relation to the respiratory apparatus.

OLFACTORY ORGAN. 171

These facts in the structure and development of the olfactory organ and
nerve have caused an attempt to be made to draw a parallel between the
olfactory pit and a primitive preoral gill- cleft,1 and this is further supported
by the general structure and histological relations of the olfactory mucous
membrane, which corresponds with that of the gills of Fishes in the possession
of end-bulbs. In a recent paper, however, Beard has put forward the view-
that "the nose is really a branchial (segmental) sense-organ, i.e. the sense-
organ of a non-existent gill- cleft, and not a gill-cleft itself."

Fishes. — In Petromyzon (Fig. 49, N, NCL) and Myxin-
oids the olfactory organ consists of a sac, unpaired externally,
lying close in front of the cranial cavity, and opening on the
dorsal surface of the anterior part of the head by a longer or
shorter chimney-like tube. Its mode of development and internal
structure, however, as well as the double olfactory nerve, seem to
prove that the olfactory organ of Cyclostomes must also have been
primitively a paired structure.2

The position of the olfactory organ in Elasmobranchs differs
from that of Cyclostomes in lying on the under instead of the

I

FIG. 141. — ANTERIOR PORTION OF HEAD OF Acipenser sturio. '

a, anterior, b, posterior opening of external nostrils ; o, isolated rosette of olfactory

folds.

upper surface of the snout. From these Fishes onwards throughout
all Vertebrates the organ is always paired, and is more or less
completely enclosed by a cartilaginous or bony investment, which
forms an outwork of the skull.

From the Ganoids onwards it always has a similar position with
regard to the skull, being situated between the eye and the end of the
snout, either laterally or more or less dorsally. In the course of
development each external nostril of Ganoids and Teleostei becomes
divided into two portions, an anterior and a posterior (Fig. 141, a, &,
and Fig. 142, AN, ANl\ by a fold of skin. The anterior often lies
at the summit of a longer or shorter tube, lined with ciliated cells,
and the distance between it and the posterior aperture varies

1 According to this view, the condition which is seen in Myxinoids and Dipnoans is
to be looked upon as the more primitive, and that of all other Fishes as secondary.

2 It is improbable that the naso-palatine duct, which opens into the oral
cavity in Myxinoids, but ends blindly in Petromyzon, is directly comparable to the
posterior nares of higher Vertebrates.

172 COMPARATIVE ANATOMY.

greatly, according to the width of the fold of skin which separates
them.

The olfactory organ of Polypterus is more highly developed than that of
any other Fish. It is not a 'simple sac-like involution, but consists of six
radially arranged compartments l separated from one another by complicated
septa, and lying round a central spindle. A transverse section has somewhat
the appearance of a cut orange. A short and distinct oval sac lies against
the olfactory organ towards the middle line, and is entirely shut off from the
rest of the apparatus ; it receives a special branch of the olfactory nerve.

-^r*/

t'

FIG. 142. — ANTERIOR PORTION OF THE HEAD OF Polypterus.

A, eye ; AN, AN}, anterior and posterior openings of the external nostril : t, t, t,
apertures of the sensory tubes.

The mucous membrane of the nasal organ of Fishes is always
raised up into a more or less complicated system of folds, which
may have a transverse, radial, rosette-like, or longitudinal (in re-
spect to the cranial axis) arrangement. The branches of the olfac-
tory nerve are distributed on them, and they serve to increase
the olfactory surface.

Dipnoi and Amphibia. — The olfactory organ of Dipnoi
and Perennibranchiata is always enclosed within a complete
or perforated cartilaginous capsule lying without the cranium
proper (Figs. 54, NK, 143, and N), and its mucous membrane
is raised into folds like those of Fishes. In all the other Am-
phibia it becomes included within the cranial skeleton, and
lies directly in the longitudinal axis of the skull in front of the
cranial cavity.

In Amphibia, turbinals appear for the first time (Fig. 144,
C, S, E) : they are processes of the cranial skeleton projecting into
the nasal cavity, and thus giving rise to an extension of the olfactory
surface. These structures, slight traces only of which are present
in tailed Amphibians, attain to a very considerable development in
Anura and Gymnophiona, especially in the latter, where the nasal
chamber is converted into a complicated system of spaces and
cavities. A main and an accessory cavity can in all cases
be distinguished, but more especially in the Derotremata and
Myctodera ; the accessory cavity, as it lies in the maxillary bone,
may be described as the maxillary cavity. In certain Gymno-
phiona this becomes entirely shut off from the main cavity, and

1 Each compartment resembles in structure the entire olfactory sac of other Fishes.

OLFACTORY ORGAN.

173

receives a special branch of the olfactory nerve, so that in these
cases two separate nasal cavities may be distinguished. This will
be referred to again later on (p. 178).

A farther neomorph are the internal nostrils (choanae) already
mentioned, as well as the glands lying under the olfactory
mucous membrane ; these are either diffused, or united to form

FIG. 143. — OLFACTORY ORGAN OF Menobranclius laMralis. (From the dorsal side. )

JV, olfactory sac ; 01, olfactory nerve ; Pmz, premaxilla ; F, frontal ; P, process of
the parietal ; PP, palato-pterygoid ; AF, antorbital process.

FIG. 144. TRANSVERSE SECTION THROUGH THE OLFACTORY CAVITIES OF

Plethodon glutinosus (Myctodera).

S S olfactory mucous membrane ; N, main nasal cavity ; K, maxillary cavity ; C,
cartilaginous, and S\ fibrous portion of the turbinal, which causes the olfactory
epithelium (E) to project far into the nasal cavity ; ID, intermaxillary gland,
shut off from the cavity of the mouth by the oral mucous" membrane (MS) :
F, frontal ; Pf, prefrontal ; M, maxilla ; Fop, vomero-palatine ; Sp, nasal

definite organs. They either open directly into the nasal cavity,
their secretion serving for the necessary moistening of the mucous
membrane (which is effected in Fishes and Dipnoi by the exter-
nal medium), or they pour their secretion into the pharynx or
posterior nostrils.

The latter always lie tolerably far forwards on the palate, and
are for the most part enclosed by the vomer, and sometimes by the
palatine also.

174 COMPARATIVE ANATOMY.

Finally, the naso-lacrymal duct of Amphibia must be
mentioned : it passes out from the anterior angle of the orbit, goes
through the lateral wall of the nose, and opens into the nasal
cavity on the side of the upper jaw. It conducts the lacrymal
secretion from the conjunctival sac of the eye into the nasal
cavity, and arises in all Vertebrates, from the My c tod era onwards,
as an epithelial cord which is separated off from the epidermis, and,
growing down into the derma, becomes secondarily hollowed.

Reptilia. — The Lacertilia and Ophidia possess the sim-
plest olfactory organs amongst Reptiles. The nasal cavity of the
former group is divided into two portions, a smaller outer
(anterior), and a larger inner (posterior), or olfactory chamber
proper. The latter only is provided with sensory cells, the for-
mer being lined by ordinary stratified epithelium continuous with
the epidermis, and glands being entirely absent in it.

A large turbinal, slightly rolled on itself, arises from the outer
wall of the inner nasal chamber, and extends far into its lumen ;
this is also well developed in Ophidia, in which a distinct outer

FIG. 145. — DIAGRAM OF THE OLFACTORY ORGAN OF A LIZARD. (Longitudinal

vertical section.)

AN, IN, outer and inner nasal chambers ; t, tube-like connection between them ;
Ch, internal nostrils ; P, papilla of Jacobson's organ ; Ca, aperture of com-
munication of the latter with the mouth ; MS, oral mucous membrane.

nasal chamber is wanting ; it may be derived from that of the
Amphibia.

A large gland which opens in the boundary between the
inner and outer nasal cavities lies within the turbinal. Below the
latter is the aperture of the lacrymal duct, though this in some
cases opens on the roof of the pharynx (Ascalabota), and in others
into the internal nostrils (Ophidia).

The structure of the nose in Cheloniaiis is very complicated and varied.
In marine Chelonians it is divided into two passages, one of which lies
above the other, but they are connected by means of a perforation of the
septum. The comparative paucity of glands in the olfactory organ of Lizards
and Snakes forms a marked contrast to the condition seen in Chelonians, the
nasal organ of which is characterised by a great abundance of them.

From the Crocodilia onwards the olfactory organ, which
up to this point lies entirely in front of the brain, gradually

OLFACTORY ORGAN. 175

extends further and further backwards, in correspondence with
the growth forwards of the facial region and the formation of
the palate ; its posterior part thus comes to lie below the brain
and base of the skull.

In Crocodiles, as in other Reptiles, there is only a single true
turbinal, but externally to it lies a second prominence, which
is spoken ofasapseudo-turbinal.1 Each nasal chamber of the
Crocodile is divided posteriorly into two superimposed cavities, the
upper of which represents the proper olfactory chamber, and is
lined by sensory epithelium, while the lower functions as a
respiratory portion only. Certain accessory chambers are con-
nected with the nasal cavity, which, however, serve only as air-
chambers. A large gland, similar to that of Lizards and Snakes,
lies in the cavity of the upper jaw, and opens into the nasal
cavity.

Birds. — In all Birds, as in Lizards, there is an outer chamber,
lined by stratified epithelium, and a proper olfactory chamber,
which lies above the former. Birds also possess only a single

FIG. 146. — TRANSVERSE SECTION THROUGH THE RIGHT NASAL CAVITY OF A
SHRIKE (Lanius minor).

OM, MM, superior (pseudo) and middle (true) turbinal ; a, upper, and b, lower nasal
passage ; LR, air-chamber, which extends into a hollow of the superior turbinal.

true turbinal, if by this term is understood a free independent
projection into the nasal cavity supported by skeletal parts. Two
other prominences (pseudo-turbinals) are, however, present, one of
which2 lies like the true turbinal in the proper olfactory chamber,
while the other, like the pseudo-turbinal of the Crocodile, is situated
in the outer portion : these are simply incurved portions of the
whole nasal wall (Fig. 146, MM, OM).

The form of the true turbinal, which is usually supported
by cartilage, more rarely by bone, varies greatly. It is either
represented by a moderate-sized prominence, or else it becomes
more or less rolled on itself (forming as many as three turns).
The lacrymal duct opens below and anteriorly to it. There

1 The meaning of the pseudo-turbirial will be discussed in the description of the
olfactory organ of Birds.

2 Its cavity communicates with an air-sinus lying in the anterior orbital space.

176 COMPARATIVE ANATOMY.

is no doubt that this turbinal is comparable to that of Urodeles
and "Reptiles.

The so-called external nasal gland of Birds does not lie in the
region of the upper jaw, but on the frontal or nasal bones.

Mammals. — Corresponding to the much more marked de-
velopment of the facial portion of the skull, the nasal cavity of
Mammals is proportionately much larger than in the forms yet
described, and consequently there is much more room for the
extension of the turbinal s : these give rise to a spongy labyrinth,
the cell-like compartments of which are lined by mucous membrane,
and thus variously shaped projections, supported partly by car-
tilage and partly by bone, are seen extending into the nasal
.-cavity. The normal number of these "olfactory scrolls" is
^ five. In Marsupials they have a very typical arrangement ; they
are situated obliquely, slanting from the postero-dorsal to the
antero-ventral side : the inferior is no longer covered by olfactory
epithelium, and it becomes connected with the maxilla.

The four other typical (ethmoidal) scrolls may persist as
such, or the two upper and two lower become united together,
in which case they are called respectively the superior and
middle turbinal s. Usually, however, the two upper primary
turbinals remain separate throughout life, so that in this case there
are two upper turbinals. The middle turbinal may also remain
partially or entirely separated into its two primitive component
parts.

While the superior and middle turbinals of Man, that is the
four primitive upper olfactory scrolls of Mammals, are to be regarded
as neomorphs, the inferior turbinal, below which the lacrymal
duct always opens, must be looked upon as derived genetically
from that of the lower Vertebrates. It corresponds to the single
true turbinal of Urodeles, Reptiles, and Birds, and in Mammals is
represented by an independent bone lying at the entrance of the
antrum maxillare s. Highmori (Fig. 147, J).1

In Man each nasal cavity is divided by the three turbinals
into three superimposed passages ; of these the two upper alone
(Fig. 147, b, c) conduct the air containing the odoriferous particles
to the ethmoidal labyrinth, that is to the proper olfactory
region of the nose, while the lower passage serves only as a
respiratory tract (Fig. 147, a).

The nasal chamber of Mammals not only communicates with
the maxillary sinus as in the lower Vertebrates, but also with the
neighbouring cavities, such as, in Man for instance, the frontal,
ethmoidal, and sphenoidal sinuses. These cavities arise in part
after birth, and often do not attain their maximum development
till the body is fully grown.2

1 In Cetacea the turbinals are never more than rudiinentary.

2 Compare the chapter on air-sacs of Birds, p. 259.

OLFACTORY ORGAN.

177

Their lining of mucous membrane is in direct connection with that of the
nasal cavity ; this is also the case with the glandular organs of the nose, which
are divided into two sets, — numerous small diffuse Bowman's glands, and a
large gland of Stenson. The early appearance of the latter in the embryo
indicates that it is an ancient structure (comp. nasal glands of Amphibia, Reptiles
and Birds, pp. 173-175). It lies inthelateral walls of the nasal cavity (Carnivora

FIG. 147. — TRANSVERSE VERTICAL SECTION THROUGH THE NASAL CAVITY OF

MAN.

/, II, III, inferior, middle, and superior turbinal ; a, b, c, inferior, middle, and

cavity ; M, maxilla ; Or, Avail of orbit.

Rodentia, &c), and in cases where a maxillary sinus is well developed (e.g. Man),
it extends into the latter, and lies in its inner wall, close to the aperture into
the nasal cavity. The duct opens at the anterior end of the middle turbinal.

The appearance of an external nose is very characteristic of
the olfactory organ of Mammals ; we must regard it as a derivative
of the outer chamber of the nose of Reptiles and Birds. It is
formed by an extension outwards of the nasal bones, and by a
special cartilaginous support which arises from the ethmoid. The
outer nose undergoes the most varied functional adaptations ; it may
give rise to a trunk-like organ, or even grow out to form a definite
trunk, and, by means of its abundant nerve-supply, serve as a
delicate organ of touch, and even as a prehensile apparatus. It
is always provided with muscles, which are sometimes very largely
developed.

JACOBSON'S ORGAN.

By Jacobson's organ is understood a paired accessory nasal
cavity, which in an early embryonic stage becomes entirely separated
off from the nasal chamber, and which is supplied by the olfactory
and trigeminal nerves; it communicates with the mouth by a
special aperture. 5

N

178

COMPARATIVE ANATOMY.

This definition applies accurately to the accessory nasal cham-
ber of Caecilians already mentioned, which is enclosed within
the maxillary cavity, and there can be no doubt that the latter
is homologous with the maxillary sinus of all Vertebrates. In no
other Vertebrates, however, does it retain the character of a kind
of separate nasal chamber, but on the contrary, the higher we pass
in the Vertebrate series, the more does the maxillary cavity become

FIG. 148. — DISSECTION OF THE HEAD OF Epicrium glutinosum. (Dorsal view.)
VH, cerebral hemispheres, separated by a furrow (F) from the olfactory lobes (Lol) J
Z, pineal gland ; MH, mid-brain ; HH, cerebellum ; NH, medulla oblongata >
R, spinal cord ; Se, septum nasi ; Id, dorsal pair of olfactory nerves ; Va> Vb, Vc,
first, second, and third division of the trigeminal ; Val and Fa2, lateral branches
of the first division, one of which goes to the olfactory mucous membrane, and
the other to the sheath of the " tentacle " (TtS] : the constrictor muscle of the
tentacular gland (Cg) is supplied from a lateral branch of the second division of
the trigeminal ; X, vagus ; ca, duct of the tentacular gland, which is surrounded
by the sheath of the tentacle ; Boc, globe of the eye ; mass, mass1, the two
portions of the masseter muscles ; Oh, auditory capsule.

separated physiologically from the olfactory organ; it loses its
olfactory epithelium, and finally degenerates into a simple air-
sinus.

In Lizards and Snakes an apparatus exists which is quite
unconnected with the Jacobson's organ of Gymnophiona, but which
nevertheless comes under the above definition. This (see Fig.
145, P) lies between the floor of the nasal cavity and the roof of
the mouth, and rnay be described as a small paired cavity lined by

JACOBSON'S ORGfAN. 179

olfactory epithelium, from the floor of which a papilla arises :
it communicates with the mouth by a special aperture in front of
the internal nostrils.

Jacobson's organs are not known in Crocodiles, Chelonians,
or Birds, but are very general in Mammals, being especially
well marked in Rodents, Ruminants, and Perissodactyles. They
here consist of two tubes lying at the base of the septum nasi,
enclosed by definite cartilages (ploughshare cartilages of Huschke) ;
they end blindly posteriorly, but open anteriorly into the mouth

FIG. 149. — THE LEFT SO-CALLED "TENTACLE" OF Ccecilia oxyura. (Opened from

the dorsal side.)

Me, constrictor muscle ; Re, retractor muscle, which becomes tendinous at Re1, and is
inserted into the ridge or papilla, Pa ; D, the large gland, surrounded by the
constrictor, with its two ducts (a, b), which further forwards unite to form a
common duct (c) ; IS, AS, inner and outer fibrous tubes ; A, eye ; S, S, skull,
with the tentacular gland (TD) lying in the nasal cavity, showing through. At
t the ducts of the tentacular gland (Co) pass out from the skull, and after extend-
ing a short distance, pass into the tentacular sheath.

by means of the incisive or naso-palatine canals (ducts of Stenson),
which extend through the palate behind the premaxillse.

The structures present in Man which have usually been described as rudi-
ments of Jacobson's organ probably correspond with the remnants of a gland
in connection with the nasal septum (Gegenbaur).

The physiological function of Jacobson's organ may consist in
bringing the food taken into the mouth under the direct control of
the olfactory nerve.

THE SPOUTING APPARATUS (SO-CALLED "TENTACLE") OF GTMNOPHIONA.

A very remarkable organ exists in Csecilians, which is closely related as
regards position both to the nasal cavity and to the orbit.

It consists of a fibrous capsule lying in the orbit, and surrounded by strong
muscle (Figs. 148, Cg, 149, Me), which extends forwards in the form of a

N 2

180 COMPARATIVE ANATOMY.

tube along a canal in the upper jaw, opening on the lateral surface of the head
near the snout. This prolongation consists of two fibrous tubes, one lying
within the other (Fig. 149, IS, AS).1

A longitudinal muscle, acting as a retractor (Fig. 149, Ee, Re1), extends
inside the organ along its whole length, and is inserted into a papilla (Pa) lying
in the aperture on the side of the head.

A large gland (orbital gland) 3 (D) is grouped around this muscle within
the broad vesicular portion of the organ, and empties its secretion into the
lumen of the tube -like portion (Fig. 149, a, &, c). The duct of a second large
gland (tentacular gland) which is embedded within the maxillary cavity
(Fig. 149, TD\ perforates the lateral wall of the maxilla, and opens into the
tube-shaped section of the organ, near its distal end, close to the above-
mentioned papilla.

The physiological function of this apparatus, which occurs quite isolated
in the Animal Kingdom, cannot at present be explained with certainty. It
probably acts as a spouting apparatus, and (if the secretion of the glands be
poisonous) as a weapon of offence ; and thus, together with the remarkably
developed olfactory organ, it would serve in some measure to make up for the
non-functional, or partly non-functional eyes and auditory organ. It is
improbable that it serves as a " tentacle," or organ of touch, as was formerly
supposed, us the necessary nerves and sensory epithelium are not known to
be present.

BIBLIOGRAPHY.

BLAUE, J. — Untersuch. ub. d. Bau der Nasenschleimhaut bei Fischen und Amphibien,

namentl. ub. Endknospen als Endapparate des Nerv. olfactorius. Arch.f. Anat.

u. Physiol. 1884.
BORN, G. — Numerous Papers on the structure of the nasal cavity of Amphibia, and

on the naso-lacrymal duct of all the chief Vertebrate groups, in the Morphol.

Jahrb. Bd. II., V., VIII.
GEGENBAUR, C. — Ueber die Nasenmuscheln der Vogel. Jen. Zeitschr. Bd. VII.

1873. Ueber das Rudiment einer septalen Nasendriise beim Menschen. Morphol.

Jahrb. Bd.*"XI. Heft 3, 1885.
KANGRO, C. — Ueber Entwick. u. Bau der Steno'schen Nasendriise der Sdugethiere.

Inaug. Dissert. Dorpat, 1884.
KLEIN, E. — Contributions to the Minute Anatomy of the Nasal Mucous Membrane,

and of Jacobson's Organ in the Guinea-pig. Quart. Journ. Micros. Science,

Vol. XXI. 1881.
KOLLIKER, A. — Ueber die Jakolson'schen Organe des Menschen. Grat.-Schrift der

Wurzburger med. Facultdt fur Rinecker, 1877. Zur Entwickl. d. Auges und

Geruchsorganes menschl. Embryonen. Grat.-Schrift fur die Zurichcr Uni-

versitat, 1883.

LEYDIG, F. — Die in Deutschland lebenden Arten der Saurier. Tubingen, 1872.
SCHWALBE, G. — Ueber die Nasenmii^cheln der Sdugethiere und des Menschen. Sitz.-

Ber. der physic. -cecon. Gesellsch. zu Konigsberg, XXIII. 1882. Lehrb. der

Anatomie der Sinnesorgane. Erlangen, 1883.
WIEDERSHEIM, R — Das Kopfskelet der Urodelen. Morphol. Jahrb. Bd. III. 1877.

Die Anatomie der Gymnophwnen. Jena, 1879.

1 In the embryo the " tentacle " appears to be wanting, but the eye is more distinct
than later.

2 This probably corresponds to a metamorphosed Harderian gland.

EYE.

181

EYE.

In contrast to the eyes of Invertebrates, which arise by a
differentiation of the cells of the epiblast (hypodermis),1 the
sensitive elements of the Vertebrate eye are formed from a paired
outgrowth of the primary vesicle of the fore-brain (Figs. 150, 151A,
ABl), as already mentioned on p. 132.

This outgrowth is spoken of as the primary optic vesicle,
and as it grows outwards towards the outer skin of the embryo,

FIG. 150. — DIAGRAMS SHOWING THE MODE OF FORMATION OF THE EYE IN
INVERTEBRATES (A) AND VERTEBRATES (B).

Ep, Ep, epiblast, which is invaginated in A to form the retina (It), and in B to form
the medullary groove, its cavity (V) representing the third ventricle of the
brain ; in both figures ft indicate the point where the external~epiblast becomes
continuous with the invaginated portion ; the arrow in Fig. A shows the direc-
tion in which the rays of light pass, and one can imagine the rays entering in
the same manner in Fig. B, and passing in the direction of the curved arrows :
they would thus fall upon the shaded cells BSP on the walls of the ventricle,
that is of the primary optic vesicle, which give rise later to the retina ; L, the
cells of epiblast, out of which the lens is formed later ; Jnf, infundibulum ;
N, optic nerve.

the portion which connects it with the brain becomes constricted,
and by degrees loses its cavity, giving rise to a solid cord, from
which the optic nerve is formed.

At the point where the vesicle touches the epiblast, the latter
becomes thickened, and gives rise to a mass of cells which is at
first hollow, but which becomes compact later. This thickened
portion of the epiblast then becomes driven inwards, carrying before

1 On closer consideration, this distinction does not appear to be an essential one,
for the outgrowths of the primary 'wall of the brain arise in the embryo at a time when
the medullary groove is still open to the exterior, and when the cells comprising it con-
sist simply of a thickened portion of the epiblast. When the primary optic vesicle
comes into apposition with the epiblast, and the lens and secondary optic vesicle are
formed, the relations of the parts are different, the sensitive (neuro-epithelial)
elements of the retina being turned away from' the light. (Comp. Figs. 150 and
151A and B, and p. 190.)

182 COMPARATIVE ANATOMY.

it the outer wall of the vesicle so as to form a double-walled cup,
the secondary optic vesicle (Fig. 15lB).

FIG. 15lA. — DIAGRAM SHOWING THE MODE OF FORMATION OF THE PRIMARY
OPTIC VESICLES (ABl}.

VH, fore-brain ; V, V, ventricular cavity of the brain, which, communicates freely
with the cavities of the primary optic vesicles at tt.

Fia. 151s. — SEMIDIAGRAMMATIC FIGURE OF THE SECONDARY OPTIC VESICLE, AND

OF THE LENS BECOMING SEPARATED OFF FROM THE EPIBLAST.

IB, inner layer of the secondary optic vesicle, from which the retina arises ; t, point
at which the latter is continuous with the outer layer (AE), from which the pig-
ment epithelium arises ; H, remains of the cavity of the primary optic vesicle ;
L, lens which arises as a cup-shaped involution of the epiblast (E) ; *, point of
involution of epiblast to form the lens ; MM, mesoblastic tissue, which at Ml, Ml
grows in between the epidermis and the lens as the latter becomes separated off,
and which gives rise to the cornea as well as to the iris ; C, vitreous chamber of
the eye, between the lens and retina, which becomes later filled by the vitreous
humour.

The inner and outer walls of the cup then become fused
together, and from the former the sensory epithelium of the

-KYE. 183

retina is formed, while from the latter the pigment epithelium

arises.

In the course of further development, the epiblastic thicken-
ing mentioned above becomes separated from the epiblast, sinks
more and more into the interior of the optic vesicle, and is
differentiated to form the crystalline lens (Fig. 151B, Z).
The remaining space within the optic vesicle becomes filled by
mesoblastic tissue, which grows in from the ventral side of
the secondary optic vesicle through the so-called choroidal
fissure, and which gives rise to the vitreous humour (Fig.
15 IB, C), the bulk of which, as compared with the lens, gradually
increases. Certain vessels (vasa centralia nervi optici, arteria
hyaloidea, tunica vasculosa lentis) also extend into the vesicle in
the same manner, and are of the greatest importance for the
nutrition of the embryonic eye.

FIG. 152.— DIAGRAM OF A HORIZONTAL SECTION THROUGH THE RIGHT HUMAN
EYE. (Seen from above. )

Op, optic nerve ; OS, sheath of optic nerve ; MF, blind spot ; Fo, yellow spot (fovea
centralis) ; lit, retina ; PE, pigment epithelium of the latter ; Ch, ehoroid, with
its lamina fusca (Lf) and vascular layer (GS) ; Sc, sclerotic ; Co, cornea ; Cj,
conjunctiva ; MD, membrane of Descemet ; CS, canal of Schlemm (the dotted
line should extend further through the sclerotic to the small oval aperture) ; Ir,
iris ; Lc, ciliary ligament ; C, ciliary process ; VK9 UK, anterior and posterior
chamber of the eye ; L, lens ; H, hyaloid membrane ; 2, zone of Zinn ; CP,
canal of Petit ; Cv, vitreous humour.

The secondary optic vesicle is now plentifully supplied with
blood-vessels in its interior, and others arise in its periphery, where
a definite vascular membrane, the ehoroid,1 is formed from the
surrounding mesoblast (Fig. 152, Ch).

Internally to the lens, the ehoroid gives rise to the ciliary
folds, while more towards the exterior it passes in front of the
lens to form the iris (Fig. 152, Ir), which retains in the centre
a circular or slit-like aperture, the pupil, through which the rays

1 I.e. the chorio capillaris and lamina fusca ; the pigment epithelium, as already
stated, is formed from the outer wall of the secondary optic vesicle.

184 COMPARATIVE ANATOMY.

of light pass. The amount of light admitted is regulated by the
dilator and constrictor (sphincter) muscles of the iris, which are
able to increase or lessen the size of the pupil ; the iris thus serves
as a screen to regulate the amount of light which enters the eye.

Not only is the size of the pupil inconstant, but the lens is also
capable of undergoing considerable change in form, becoming more
flattened, or more convex, as the case may be. The former con-
dition occurs when distant, the latter when near objects are looked
at. This delicate accommodating apparatus is regulated by a
muscle (the ciliary, or tensor choroidese), which arises in a
circle all round the eye from the point of junction of the iris and
sclerotic, and is inserted along the peripheral border of the iris
(Fig. 152, Zc).

External to the vascular layer of the choroid lies a lymph-sinus
(perichoroideal sinus) the walls of which are known under the
name of lamina fusca (Fig. 152, Lf), and finally, externally
to this is a firm, fibrous, partly' cartilaginous or even ossified
layer, the sclerotic (Fig. 152, Sc). While the latter passes inter-
nally into the sheath of the optic nerve (OS), and from thence
into the dura mater, it becomes continuous externally with the
cornea, the outer surface of which is covered over by an epithe-
lial layer continuous with the epidermis, the conjunctiva (Fig.
152, Co, Cf). The sclerotic and cornea together form a firm outer
support for the eye, and thus, together with the gelatinous mass
of the vitreous humour, guarantee the rigidity necessary for
the physiological activity of the nerve end -apparatus. Between
the cornea and iris there is a large lymph-space, the so-called
anterior chamber of the eye (Fig. 152, VK), its contained fluid
being called the aqueous humour.

The deep orbit, formed by the skull, serves as a further pro-
tection for the eye, as do also certain accessory structures, which
may be divided into three categories, viz. : —

1. Eyelids (Palpebrae).

2. Glandular organs.

3. Muscles (apparatus for moving the eye-ball).

The eyeball then is formed of a series of concentric layers
which are called from within outwards retina, choroid (with the
iris) (vascular layer), and sclerotic (with cornea) (skeletal layer).
The first corresponds with the nervous substance of the brain, the
oecond with the pia mater, and the third with the dura mater.
The interior of the eye contains refractive media, the lens and
vitreous humour. To these, certain accessory structures are
added.

Fishes.1 — The eyo of Cyclostomes remains at a very low
stage of development : this is seen not only in the structure of
the retina, but also (that is, in Myxinoids) in the absence of the

1 In Aniphioxus the presence of a visual organ has not been certainly proved.

EYE. 185

lens, iris, and of a differentiated sclerotic and cornea. Moreover,
the eye of Myxinoids and of Ammocoetes lies beneath the skin
and sub-dermal connective-tissue. In Petromyzon the skin covering
the eye becomes thinned out, and thus the animal, which was
blind in the larval state, can see on reaching the adult condition :
at the same time the eyeball increases in size, and becomes more
highly organised.

The eyes of all the true Fishes are, with few exceptions (e.g.
Siluroids and Eels), of considerable size. They have but little
power of movement, and as the cornea is very flat, and the lens
lies almost directly against it, the eyeball always possesses a
hemispherical or ellipsoidal form, and the anterior chamber is very
small. In other points, the eye is formed on the same plan as that
described in the introductory portion of this chapter, but a few
other details concerning it must now be considered.

Jr

FIG. 153. — EYE OF A TELEOSTEAN.

Op, optic nerve ; OS, sheath of optic nerve ; Et, retina ; PE, pigment epithelium ;
Tp, tapetum ; Lv, lamina vasculosa ; Ag, argentea ; Ls, lamina suprachoroidea ;
Sc, sclerotic, enclosing cartilage or bone (t) ; Co, cornea ; Ir, iris ; Lc, ciliary
ligament ; VK, anterior chamber ; L, lens ; Cv, vitreous humour ; Pr, processus
falciformis, and Cp, campanula Halle ri, here shown as if continuous with one
another.

The lens of Fishes, as in all aquatic animals, is globular, and
possesses therefore a high refractive index. It fills up the greater
part of the eyeball, so that not much space is left for the vitreous
humour. It forms an exception to that of other Vertebrates in the
fact that, in the condition of rest, it is accommodated for
seeing near objects. In place of a ciliary muscle, there is
only a fibrous ciliary ligament.

In the eye of Teleostei, a fold, the processus falciformis,
arises from the choroid and extends into the vitreous humour
towards the lens. The so-called campanula Halleri, which is
inserted round the periphery of the lens, and which is usually
described as a trumpet-shaped expansion of the processus falci-
formis, is, according to Virchow, entirely independent of the latter

186 COMPARATIVE ANATOMY.

(Fig. 153, Pr, Cp). The processus falciformis is never large in
Elasmobranchs and Ganoids.

In the interior of this structure lie nerves, vessels, and
smooth muscle- fibres, and the latter possibly exert an
influence on the lens, and thus serve as an apparatus for ac-
commodation.

External to the choroid proper, that is, between it and the
lamina fusca s. suprachoroidea, lies a silvery or greenish-gold
iridescent membrane, the so-called argentea. It extends either
over the whole interior of the eye (Teleostei), or is limited to the
iris (Elasmobranchs).

A second layer with a metallic lustre, the tapetum
cellulosum s. lucidum, lies internally to the iridescent portion,
and within this again there is the chorio-capillaris of the choroid.
No tapetum appears to be present in Teleostei or Petromyzon.

The so-called choroid gland, present only in Teleostei and
Amia, consists of a rete mirabile (comp. p. 292), composed of
arteries and veins, which has the form of a cushion, lying near
the entrance of the optic nerve, between the argentea and pigment
epithelium of the retina ; thus it has nothing to do with a " gland " :
it corresponds in position to the choroid.

The sclerotic is usually extensively chondrified (Elasmobranchs, Sturgeons),
and not unfrequently becomes calcined or ossified towards its junction with the
cornea : this also holds good for Teleosteans.

The eyeball is almost always surrounded by a gelatinous tissue, pene-
trated by simple and elastic connective-tissue fibres, and in Elasmobranchs
it is curiously articulated on its inner circumference with a rod of cartilage
arising from the lateral wall of the skull.

The eyes are reduced or abortive in Amblyopsis spelaeus, a Fish living in
the caves of Kentucky : a similar abortion of the eyes takes place in many
Invertebrates which live in caverns or in the deep sea (Vermes, Mollusca,
Crustacea, Insecta).

Amphibia. — The eyes of Amphibians are proportionately
smaller, and their form rounder than those of Fishes, but there are
many points of close correspondence between them. This is true,
for instance, of the more or less strongly chondrified sclerotic, the
slight convexity of the cornea, and the globular lens. In other
important points, however, the Amphibian eye is simpler than that
of Fishes ; thus, it is wanting in an argentea, a tapetum, a choroid
gland, and a processus falciformis and campanula Halleri. A proper
ciliary muscle is present in the whole series of animals from
this point onwards.

The eyes of Proteus and of the Gymnophiona always lie more or less
deeply beneath the skin ; they are very small, and are much degenerated
(Figs. 148, Boc, and 149, A). In Proteus the crystalline lens and vitreous
humour are both wanting.

Reptiles and Birds. — In these also, the sclerotic is in great
part cartilaginous, and in Lizards it is provided with a ring of
delicate bony plates around the external portion. Very many

EYE.

187

fossil Reptiles and Amphibians possessed similar plates, as do
also existing Birds (Figs. 154, 155, f) ; in Birds horse-shoe- or
ring-shaped bony structures are also usually present close to the
entrance of the optic nerve.

FIG. 154. — EYE OF Lac&rtn muralis, SHOWING THE RING OF BONY SCLEROTIC

PLATES.

While the eyeball of Reptiles has a globular form (Fig. 154),
that of Birds, and especially nocturnal Birds of prey (Owls), is
more elongated and tubular, an external larger segment being
sharply marked off from an internal smaller one (Fig. 155). The

FIG. 155. — EYE OF AN OWL.

fit, retina ; Ch, choroid ; Sc, sclerotic, with its bony ring at t ; CM, ciliary muscle ;
Co, cornea ; VN, point of junction between sclerotic and cornea ; Ir, iris ; VK,
anterior chamber ; L, lens ; Cv, vitreous humour ; P, pecten ; Op, OS, optic
nerve and sheath. The dotted line passing across the broadest portion of the
circumference of the eye divides the latter into an inner and an outer segment.

former is bounded externally by the very convex cornea (Co), and
encloses a large anterior chamber ( VK\ as well as a complicated
ciliary muscle (Crampton's muscle) composed of striped fibres.
This muscle is also transversely striated in Reptiles, and in them

188 COMPARATIVE ANATOMY.

is always well developed, though not to such an extreme degree as
in Birds.

In Reptiles (Lizards, for instance) a tapetum may be developed,
but an argentea and choroid gland are never present ; all these
structures are wanting in Birds. A structure which is homologous
with the processus falciformis of the eye of Fishes is, however,
present in Reptiles and Birds. Absent in Hatteria and Chelonia,
this so-called pecten is largely developed in other Reptiles and
in Birds (Fig. 155, P). In the latter it may extend from the point
of entrance of the optic nerve to the capsule of the lens, but as a
rule it does not reach so far. In Birds it is always more or less
folded, and consists mainly of a closely-felted network of capillaries,
and appears in both Reptiles and Birds to have an important
f relation to the nutrition of the contents of the eyeball and of the
\ retina. It has nothing to do with accommodation.

The iris, which is regulated by striated muscle, by means of which it is
able to respond very quickly to visual impressions, is often brightly coloured,
and this colour is due to the presence not only of pigment, but also of
coloured fat globules.

The pupil is as a rule rounded, but it may have the form of a vertical slit, as
" in many Reptiles and in Owls. In certain Fishes (Coregonus) and Amphibians
(Bombinator) it is angular.

Mammals. — In Mammals the eyeball is always more
completely enclosed within the bony orbit than in most other
Vertebrates, and this may partially account for the fact that the
sclerotic no longer shows traces of cartilage or bone, but is
entirely of a fibrous character.

With the exception of aquatic Mammals, in which it is some-
what flattened, the cornea is moderately convex, and the whole
eyeball is of a more or less rounded form.

A tapetum (tapetum cellulosum vel fibrosum), consisting either of
cells or fibres, exists in the choroid of numerous Mammals, and gives rise by
interference to a glistening appearance when seen in the dark (Carnivora,
Sirenia, Ruminants, Perissodactyla, &c.).

Certain structures homologous to the processus falciformis and pecten are
present in Mammals in the embryo only, but details of these structures cannot
be described here.

The ciliary muscle consists of smooth elements only, and serves to
accommodate the eye for seeing near objects (compare the eye of Fishes). The
lens of Mammals in its position of rest is accommodated for distance.

The external surface of the lens is less convex than the internal, which
latter lies in the so-called fossa patellaris of the vitreous humour.

The pupil is not always round, but may be transversely oval (Ungulates,
Kangaroos, Cetacea), or have the form of a vertical slit (e.g. Cat).

EETINA.

18(J

Retina.

The fibres of the optic nerve, which pass into the eyeball
at a right or acute angle, cross one another at the point of
entrance,1 and are then distributed to the sensitive elements of
the retina.

The latter is thus thickest at the point of entrance of the
nerve, which is known as the " blind-" or " Marriott's spot," and

_ 0

•jfiiiuiiijilifa ' I'J >B \iJS*B!l[!i!lii!.L —

FIG. 156. — RETINA. (After Merkel.) (The nervous portion is shown black, and
the supporting substance of a lighter tint.)

Ot layer of nerve-fibres ; GgL, layer of nerve-cells ; in.gr, inner granular layer ;
in.K, inner nuclear layer; du.gr, outer granular layer; auK, outer nuclear
layer ; Le, limitans externa ; St, layer of rods and cones.

gradually decreases in thickness towards the ciliary processes,
until, at the point of origin of the iris, it consists of a simple layer
of cells.

The retina, which is bounded both on its inner and outer
periphery2 by a structureless hyaline membrane (limitans

1 This has not been satisfactorily made out in Mammals.

2 The limitans externa encloses the entire retina externally in the embryo, but
later the rods and cones come to project through it (see Fig. 156).

190 COMPARATIVE ANATOMY.

interna and externa), is quite transparent in the fresh con-
dition, and consists of two structures which are histologically
and physiologically quite different: the}' are, a supporting part
and a nervous part. The former, or so-called fulcrum, which
is stretched as on a frame between the limitans interna and externa,
is shown in Fig. 156 as a light filigree-like tissue, the nervous
portion being indicated by a darker and more granular shade. The
latter consists of seven concentric layers, viz. : —

1. Layer of nerve -fibres.
9 pplh

— * ?? yy 9? v i i i>.

3. „ ,. inner granular or molecular layer.

4. „ „ inner nuclei.

5. „ „ intermediate nuclei or outer molecular layer.

6. „ „ outer nuclei.

7. „ „ rods and cones with the pigment epithelium.

Only the two last-mentioned layers (6 and 7) correspond to
the proper neuro-epithelium.

These layers are so arranged that the nerve-fibres lie next
to the vitreous humour, that is, internally, while the rods and
cones are situated towards the choroid, or are the most external.
Thus the terminal members of the neuro-epithelium are turned
away from the rays of light falling upon the retina, and the rays
must therefore pass through all the other layers before they reach
the rods and cones. ^

Fishes possess the longest, Amphibians the thickest rods, so that in the
latter there are only about 30,000 to a square millimetre, while in Man there
are from 250,000 to 1,000,000.

In Fishes the rods far exceed the cones in number, while in Reptiles and
Birds the reverse is the case. The cones of many Reptiles and all Birds are
distinguished by the presence of brightly colon red oil-globules, which are
also present in those of Marsupials.

In the retina of ajj. Vertebrates there is a specially modified region of most
acute vision. This is called the yellow-spot (fovea central is or macula
lutea), and lies in the inner portion of the eye. It is due to the thinning-out
of all the layers except that of the rods and cones, and even the rods disappear,
only the cones persisting (Fig. 152, Fo). The cones of the macula lutea are
distinguished in Fishes and Reptiles by a very elongated and narrow form,
while in Amphibia and Mammals they are quite similar to those of the rest of
the retina. The physiological function of the pigment-epithelium is to give
rise to a colouring material, the so-called visual purple or visual red.
This becomes dissipated by the light falling upon the retina, and we may thus
compare the latter to a photographic plate, or rather to a whole photographic
apparatus, in which the photographer, represented by the pigment- epithelium,
by continually laying on new sensitive material (" visual substance ") (purple),
renews the plate, removing the old picture. Thus the act of seeing may possibly
be looked upon as a photo-chemical process. The rods only possess the visual
purple, the cones being without it ; thus in animals the retina of which possesses
no rods (e.g. many Reptiles), as well as in the yellow-spot of others, it is wanting.
The fact that the visual purple is not present in these Reptiles as well as in
certain nocturnal animals (e.g. Caprimulgus, Vespertilio serotinue), and in
Fowls and Pigeons, proves, however, that the physiology of vision is not yet
thoroughly understood.

;--

EYE-MCSCLEl™ 191

X^J

In a recent paper, Engelmann has shown that the cones lengthen under the
influence of light and shorten in darkness. This contraction of the cones and
pigment- epithelium is directly connected with the nervous system, as is proved
by the fact that it may occur in eyes which are entirely shut off from the light,
as well as in those of decapitated animals -when the brain is intact. The optic
nerve must thus be looked upon as being made up of both centripetal sensory
and centrifugal motor fibres. A reflex excitation of the cones may also be
produced in Frogs by allowing light to fall on any one part of the body only,
and the same occurs in strychnine-tetanus, quite independently of light.

Accessory Organs in Connection with the Eye.

(a) EYE-MUSCLES.

The movement of the eyeball is always (except in Myxinoids)
effected by six muscles, which may be divided, according to the
direction they take, into four straight (rectus superior, in-
ferior, externus, and internus) and two oblique muscles
(obliquus superior and inferior). The former, which arise
from the inner portion of the orbit, usually from the dural sheath
of the optic nerve, together circumscribe a pyramidal cavity, the
apex of which lies against the inner portion of the orbit, while the
base surrounds the equator of the eyeball, that is, the region in
which the muscles are inserted into the sclerotic.

Both the oblique muscles usually take their origin, in close
proximity to one another, from a point on the anterior or nasal
side of the orbit, and as they respectively pass from this point
dorsally and ventrally in an equatorial direction round the eyeball,
they constitute a sort of incomplete muscular ring.

A deviation from this arrangement is seen in Mammals, in which the
superior oblique arises far down in the inner part of the orbit, and then
passes forwards in the long axis of the latter towards its anterior (internal)
angle, where it becomes tendinous, and passes through a fibre-cartilaginous
pulley (trochlea) attached to the upper border of the orbit, on the frontal
bone. Hence it is sometimes called the trochlear muscle. From this point it
changes its direction, and becomes reflected obliquely outwards and backwards
to the globe of the eye.

Besides these six muscles, others are usually present which are
known as the retractor bulbi (which is most developed in
Ungulates), the quadratus (bursalis), and the pyramidalis.
The last two are in connection with the nictitating membrane, and
are present in Reptiles and Birds. All three are supplied by the
abducent nerve. For an account of the innervation of the straight
and oblique muscles, the reader is referred to the chapter on the
cranial nerves (p. 154).

(6) EYELIDS (PALPEBR.E).

In Fishes the upper and lower eyelids are very rudimentary,
having simply the form of stiff folds of the skin ; and in all other
Vertebrates below the Mammalia they never reach a very high stage

192 COMPARATIVE ANATOMY.

of development. They are lined on the surface looking towards
the eyeball by the conjunctiva, and in the Ichthyopsida and
Sauropsida are usually not sharply marked off from the rest of the
skin, being capable of no, or of only very slight movement.1

The case is quite different in Mammals, in which the eyelids,
more particularly the upper one, are extremely moveable, and
are provided with hairs (eyelashes) on their free margin. They
are closed by a circular muscle which surrounds the whole slit
between the lids ; a levator is also present in the upper eyelid.
In Sauropsida and many Mammalia (e.g. Ungulates) there is also
a depressor of the lower lid.

The want of or slight development of upper and lower eyelids
in all Vertebrates below the Mammalia is compensated for, at
any rate to a certain extent, by the presence of a nictitating
membrane. This "third eyelid" differs from the others in
having nothing to do with the true outer skin, consisting simply
of a reduplicature of the conjunctiva, and being regulated by
special muscles (p. 191).

The nictitating membrane, which is present in rudiment in many
Elasmobranchs, and which encloses a cartilage, is situated beneath
the lower eyelid, or it may lie more towards the anterior angle of
the eye. The former condition is seen in Anura and Reptilia, for
instance, in which a third eyelid is so largely developed as to be
capable of covering the whole freely exposed portion of the eyeball.
In Birds and Mammals it always lies in the anterior angle of the
eye ; in Primates it becomes reduced to a small half-moon-shaped
fold (plica semilunaris), and so comes into the category of rudi-
mentary organs.2*

(c) GLANDS.

The glands in connection with the eye may be divided into
three sections : (1) the lacrymal, (2) the Harderian, or gland
of the nictitating membrane, and (3) the Meibomian glands.

The secretions of all these three serve to keep the free surface
of the eyeball moist, and to wash away foreign bodies.

In Fishes, the outer medium appears to suffice for this purpose,
but the first attempt of a Vertebrate to exchange an aquatic for an
aerial existence necessitates the development of a secretory apparatus
in connection with the eye.

Thus in Urodeles a glandular organ is developed from the
conjunctival epithelium along the whole length of the lower eyelid ;

1 In many Reptiles and Birds the upper eyelid is supported by a membrane-bone
or fibre-cartilage, and large lymphatic and cavernous spaces are developed within the
tissue of the lid. In Geckos and Snakes the two eyelids grow together to form a
transparent membrane overlying the eye, and this comes away with the rest of the
outer skin when the latter is shed.

2 In the Caucasian race the plica semilunaris is only 1| to 2 millimetres broad,
while in the Malayan Orang-Sakai race it reaches a breadth of 5 to 5J millimetres.

GLANDS OF THE EYE.

193

in Reptiles this becomes more developed in the region of the
anterior and posterior angles of the eye, and the original connecting
bridge gradually disappears : thus two glands are developed from
the primitively single one, each of which becomes further differ-
entiated both histologically and physiologically. From one is formed
the Harder ian gland, which always lies at the anterior angle of
the eye, surrounding to a greater or less extent the an tero- ventral
portion of the eyeball, while the other gives rise to the lacrymal
gland (Fig. 157, H, H1, Th). The latter retains throughout life

FIG. 157.— HARDERIAN GLAND (H, H1) AND LACRYMAL GLAND (Th) OF Anguis

fragilis.
M, muscle of jaw ; B, eyeball.

FIG. 158.— DIAGRAMMATIC TRANSVERSE VERTICAL SECTION THROUGH THE EYE OF

A MAMMAL.

Op, optic nerve ; B, eyeball ; Fo, Fo, fornix conjunctive : LH, LH, outer skin
of the eyelids, which at the free edges of the latter at t becomes continuous
with the conjunctiva ; T, the so-called tarsal fibre-cartilages, in which the
Meibomian glands (MD) lie embedded, the latter opening at * ; ff, H, eyelashes.

its primitive position at the posterior angle of the eye, and in
Reptiles and Birds even lies in the region of the lower eyelid,
bsing supplied by the second division of the trigeminal. In
Mammals it becomes gradually further subdivided, and extends
into the region of the upper eyelid, so that its ducts open above
the eye into the upper conjunctival sac (Fig. 159,**). Nevertheless,
even in the Primates, more or fewer ducts are present which open
into the lower conjunctival sac, and thus the primitive position of
the lacrymal gland is indicate:!.

194

COMPARATIVE ANATOMY.

A well-differentiated Harderian gland is present in the whole
series of animals from the tailless Amphibia to the Mammalia.
Amongst the latter it is wanting in the Cetacea and Primates only.

Tit

FIG. 159. — DIAGRAM OF THE LACRYMAL APPARATUS or MAN.

TJ), lacrymal gland, divided up into several portions ; **, ducts of the lacrymal
gland ; ft, puncta lacrymalia ; TR, TR1, upper and lower lacrymal canals ;
>y, lacrymal sac ; D, naso-lacrymal duct.

The Meibornian glands, belonging to the group of sebaceous
glands, are confined to the Mammalia, and lie embedded in the
substance of the eyelids in the form of branched tree-like tubes
or clustered masses. They open on the free edge of the lid, and
produce a fatty secretion.

BIBLIOGRAPHY.

BERGER, E. — Beitrdge zur Anatomie des Sehorgancs der Fische. Morphol. Jahrb. Bd.

Y1JL 1882.
BOLL, F. — Zur Anatomie und Physiologic, der Retina. Arch. f. Anat. u. Physiol.

1877, Physiol. Abthl.

CARRIERE, J. — Die Sehorgane der Thiere. Miinchen und Leipzig, 1885.
HEINEMANN, C. — Beitrdge zur Anatomie der Retina. Areh.f. mikr. Anat. Bd. XIV.

1877.

KESSLER, L. — Zur Entwicklung des Auges. Leipzig, 1877.
LEUCKART, K. — Organologie des Auges. In A. Grae/e und Th. Saemisch, Handbuch

der gesammten Augenheilkunde. Bd. I. Anatomie und Physiologic.
MANZ, W. — Entw.-Gesch. des menschl. Auges. Ibid.
MULLER, H. — Gesammelte und hinterlassene Schriften zur Anatomie und Physiologic

des Auges. Herausgegeb. von 0. Becker. Leipzig, 1872.
MULLER, W. — Ueb. die Stammcscntwick. des Schoryanes der Wirbelthiere. Festgabe

von C. Ludwig. Leipzig, 1874.
SARDEMANN, E. — 'Die Thracnendrilse. Preisschr. Frieburg i/B, 1884. Abstract in the

Zool. Anz. 1884.
SCHULZE, M. — Die Retina. Strieker's Handbuch der Lehre von den Geweben.

Leipzig, 1871. See alsD Archiv fur mikr. Anatomic, Bd. II., 111., lAr., \.,

VI., & VII.
SCHWALBE, G. — Lchrb. d. Anatomie der Sinnesorganc. Erlangen, 1883.

AUDITORY ORGAN.

It is very probable that the auditory organ, like those of
smell and taste, has been derived primitively from a modified
integumentary sense-organ (organ of the lateral line). The

AUDITORY ORGAN. 195

original form of both auditory and lateral line organs is a vesicle
derived from the epiblast. from which it later becomes separated
off; it is lined by elongated cells of sensory epithelium provided
with hair-like processes (auditory hairs), and by supporting cells.
Moreover, both structures are supplied by cranial nerves (VIII,
JC) which correspond to dorsal roots.

Like the other higher sense-organs, the paired auditory organ
of Vertebrates is situated in the region of the head, and it always lies
between the origins of the trigeminal and vagus nerves. The first
traces of it in the embryo are seen to the right and left of the hind
brain (Fig. 160, LB\ and after the vesicle of each side has be-
come separated off from the epiblast and connected with the auditory
nerve which grows out towards it from the brain, it sinks deeper
and deeper into the mesoblastic tissue of the skull : it then loses its
original pyriform or rounded shape, and becomes divided into two

FIG. 160. — HEAD AND ANTERIOR PORTION OF BODY OF A CHICK. (Iii part after

Moldenhauer. )

EG, olfactory pit ; A, eye ; / to IV, first to fourth visceral arches ; t, point at which
the external auditory passage begins to be formed ; LB, primitive auditory
vesicle seen through the wall of the head.

parts, called respectively the utriculus and sacculus (Fig. 161, Utt
S). From the former the semicircular canals become differen-
tiated, while from the latter the tube-like recessus vestibuli
(aqujjeductus vestibuli s. ductus endolymphaticus) and the cochlea
are formed (Fig. 161, S.B., F.B., H.B., D.e, O).

This whole, very complicated, apparatus constitutes the mem-
branous auditory organ or membranous labyrinth. It
becomes surrounded secondarily by mesoblastic tissue, which is at
first in close contact with it ; later, however, a process of absorption
takes place in the innermost layers of the mesoblast, thus giving
rise to a space, which closely repeats the form of the membranous
labyrinth, as does also the mesoblast which encloses this space, and
which later becomes chondrified, and often also ossified. We thus

o 2

196 COMPARATIVE ANATOMY.

get a membranous and a bony labyrinth, and between them
a cavity (cavum perilymphaticum) filled with a lymph-like fluid
(perilymph). The cavity within the membranous labyrinth,
which also contains a fluid (endo lymph), is spoken of as the
cavum endolymphaticum.

With the exception of the Cyclostomi, three semicircular canals
are always present, and these lie in planes at right angles to one
another. They are distinguished as the anterior vertical, the
posterior vertical, and the horizontal (external) canals.
The first and last-named (Fig. 161, S.B., H.JB.) arise from the
portion of the utriculus known as recessus utriculi (Re.ut.), and
each has a vesicle-like swelling or ampulla (S.A., If. A.) at its

11 '•''"'

&£

// 1 •' ^N.

F.B.

J^^Rkx,

MB:

FIG. 161. — SEMIDIAGRAMMATIC FIGURE OF THE AUDITORY ORGAN OF- A
TELEOSTEAN. (Modified from a figure of that of Murcena anguilla by Hasse.)

Ut., utriculus ; Re.ui. , recessus utriculi ; V.R., connecting- tube of the posterior ampulla
(F,A.); **, wide connecting-duct between the pars superior and pars inferior ;
>$', sacculus ; (7, cochlea; S.B., F.B,, H.B., anterior and posterior vertical, and
horizontal canals; Co., canal commissure, with its apex; S.A., II. A., P. A.,
ampullae of the anterior, horizontal, and posterior canals ; D.e, ductus endo-
lyrnphatieus, which arises from the point where the two tubes of the pars
superior of the labyrinth and the opening of the horizontal canal ( x ) join one
another.

origin. The posterior canal (F.JB.) also arises with an ampulla
(F.A?) from a prolongation of the utriculus, described in Fig. 161
as the " connecting-tube " ( V.E.).

The other end of the horizontal canal opens by a funnel-shaped
enlargement (Fig. 161, x) into the utriculus, while those of the
posterior and anterior canals fuse together to form a common
tube, the so-called canal commissure (sinus superior) (Co.),
which also opens into the utriculus.

The distribution of the branches of the auditory nerve and the
position of the sensory epithelium in the following parts of
the membranous labyrinth 'must now -be considered : * (1) the

1 Concretions composed mainly of carbonate of lime are present in the regions
of the various nerve end-plates of tbe auditory organ in all Vertebrates, as well as
in many Invertebrates. These "otoliths" present the greatest variety both iu

AUDITORY ORGAN.

197

three ampullae of the canals, in each of which the auditory cells
are situated on a ridge (crista acustica) projecting into the
lumen ; (2) the utriculus, in which a large " macula acustica " is
present ; this is continued into the recessus utriculi as well as into
the sacculus and the rudiment of the cochlea (the recessus cochleaa),
which arises from the sacculus. The several portions of the sensory
plate or macula acustica, which are originally continuous, become
later disconnected from one another, and from the Teleostei onwards
are seen as separate macula3 acusticse.1

FIG. 162. — LONGITUDINAL SECTION OF AN AMPULLA OF GOBIUS. (The exact form
of the epithelium of the crista is not indicated.) (After Henscn.)

??, the nerve passing into the connective-tissue of the crista ; a, base of semicircular
canal ; b, point of opening of the ampulla into the alveus communis ; c, the
somewhat cylindrical epithelium on the free wall of the ampulla ; d, the
auditory hairs.

The higher we pass in the Vertebrate series, the greater share
does the mesoblast take in the formation of the auditory organ.
At first, that is, in Fishes, the ear lies close under the roof of the
skull, and is thus easily accessible to the waves of sound, which are
conducted partly through the operculum (when present), and partly
through the gill-slits or spiracle : 2 as we pass to the higher animals,

form and size. The largest and most massive ones are seen in Teleosteans. They
either consist of a single mass, or are arranged in grbups in different regions of the
labyrinth. Nothing certain is known as to their physiological function.

1 Besides the above-mentioned areas of distribution of the auditory nerve, there
is still another independent one : it lies in Fishes on the floor of the utriculus, close
to the ductus sacculo-utricularis, to be described presently, and is called the macula
rieglecta. It is present from Fishes up to Birds, lying in Amphibia on the inner
side of the sacculus, and in Reptiles and Birds in the utriculus, as in Fishes : in
Mammals it undergoes a gradual reduction, and finally becomes obliterated. In all
Vertebrates except Mammals, all the auditory nerve-endings are characterised by only
two forms of cells (auditory and supporting cells) : in the Mammalian cochlea the
cells of the sensory epithelium possess a peculiar form.

2 Howes has shown that in the Skate the structure known as the "fenestra
vestibuli cartilaginei " corresponds physiologically to a kind of tympanum.

108 COMPARATIVE ANATOMY.

however, the auditory organ gradually sinks further and further
inwards from the surface. Thus a new method for conducting the
sound-waves is necessitated, and the following structures become
developed : — a canal passing inwards from the surface, the external
auditory passage or meatus ; this opens into a spacious cham-
ber, the tympanic cavity, in which are situated the auditory
ossicles, and which is connected by the Eustachian tube with
the pharynx. The whole of this canal, which is divided into an
outer and an inner portion at the junction of the external auditory
passage and tympanic cavity by a vibratory membrane, the tym-
panic membrane, lies in the position of the first embryonic
visceral (hyomandibular) cleft, or, what comes to the same thing,
in the position of the spiracle present in many Fishes. From Rep-
tiles and Birds onwards the first indications of a pinna (that is, the
part of the external ear which projects from the head) are seen,
though it only reaches a full development in Mammals.

The pinna arises from a series of swellings which surround the external
aperture of the hyomandibular cleft. These appear at an early stage in the
region of the mandible and hyoid, and soon fuse together to form a sort of
ring, from which are "formed later those characteristic protuberances of the
pinna which are known as tragus, antitragus, antihelix, &c.

Fishes. — Apart from Cyclostomes, the peculiarities of whose
auditory organ it is difficult to explain, that of all other Fishes
follows the general plan given above, and the same may be said for
all the higher Vertebrates. Everywhere we meet with a division
into a pars superior, — represented by the utriculus and
semicircular canals, — which remains essentially much in the
condition already described, and a pars inferior, — constituted
by the sacculus and cochlea, — which gradually becomes more
differentiated, and attains to a higher and higher degree of develop-
ment and functional perfection (Fig. 161). In Fishes the cochlea
consists simply of a small knob-like appendage ("lagena") of the
sacculus, which opens freely into the main cavity of the latter
by means of the sacculo-cochlear canal (Fig. 161, C}. The utriculus
and sacculus also communicate with one another by the sacculo-
utricular canal.

Amphibia. — Here all the parts remain much as described
above, with the exception of the cochlea, which, especially in the
Anura, points to a higher stage of development, in that it shows
an indication of a pars basilaris with another patch of nerve-
endings, the papilla acustica basilaris: it becomes further
constricted off from the lumen of the sacculus, with which it is
connected only by a very minute canal.

A further advance in structure as compared with Fishes is the
appearance of a cartilaginous plate which fits into the feriestra
ovalis of the auditory capsule, and corresponds to the base of the
stirrup-bone (stapes) of the higher Vertebrata (Figs. 56 and 58,

AUDITORY ORGAK - 199

St, Fov). In several Urodeles (e.g. Cryptobranchus, Menopoma,
Spelerpes) the stapedial plate becomes elongated by the addition
of a distal element, and thus forms a " columella." A tympanic
cavity, with a tympanic membrane lying on a level with the skin,
and a Eustachian tube opening into the pharynx, are met with first
in the Anura, in which also the columella is more perfect, con-
sisting oTlTbony and cartilaginous chain, expanded distally to fit
against the tympanic membrane. The whole columella probably
corresponds to the upper element of the hyoid arch (pharyngo-
hyal) : the fact that it is sometimes continuous with the wall of
the auditory capsule at an early stage may probably be explained
as a secondary modification.

Reptiles and Birds. — In the Chelonia, the auditory organ
shows many points of resemblance to that of Urodeles, and in all
Reptiles and Birds, the chief modifications are confined to the
cochlea, which shows gradually a higher condition of development
as we pass from Chelonians and Snakes to Lizards and Crocodiles.
In the Chelonia, where, as already mentioned, the auditory organ
remains in a lower stage of development, the cochlea grows out in
the form of a short canal (ductus cochlearis,* lagena) ; in Croco-
diles and Birds this canal is considerably longer, and at the
same time it becomes slightly coiled (Figs. 163 — 165). A more
marked differentiation also gradually takes place in the mem-
brana basilaris and the papilla acustica basilaris. Both
become more and more elongated, and, at the same time, distinct
indications of a sea la tympani and vestibuli are seen. (Compare
the description of these parts on p. 204.)

In the Lacertilia the most varied types of auditory organ are met with ;
in many (Phrynosoma, Pseudopus, Anguis), the membrana basilaris is hardly
more highly developed than in Ophidia. In Iguana, an advance towards
Lacerta and the other higher Lizards is to be noticed ; the membrana
basilaris is longer, though the lagena with its papilla is not so much marked.
In Acantias and Platydactyfus this state of things is carried still further,
and finally the more highly developed auditory organ of Plestiodon and
Egerina leads up to that of Crocodilia. Thus there i-s a continuous
and unbroken series from the lower forms to the higher.

The structure of the auditory organ of Hatter i a shows many striking
peculiarities : it thus, like that of Chamseleo, occupies an isolated position.

Whilst the cochlea gradually becomes more independent of the
sacculus, the latter shows the greatest variety both as to form
and size in the different types (Figs. 163, 164, #). Thus, for
instance, it is usually very small in Birds, while in Lizards (Lacerta)
it is of considerable size.

The aperture of communication between the utriculus and
sacculus persists, though it gradually becomes narrowed, as does
also that between the sacculus and cochlea. The connection
between the latter may be drawn out to form a canal (canal is
reunions), and this is particularly the case in Birds (Fig. 165) ;

200

COMPARATIVE ANATOMY.

in Crocodiles an intermediate condition between Birds and
Lizards is seen. The membranous labyrinth of Birds, however, is
always specially characterised by the peculiar arrangement of the

Fio. 163. — MEMBRANOUS LABYRINTH OF Lacerta.
Co

SB

FIG. 164. — MEMBRANOUS LABYRINTH OF THE CROCODILE. (Both from the outer

side.) (After C. Hasse.)

S, sacculus ; ut, utriculus ; Re.ut, recessus utrbuli ; VR, connecting-duct of the
posterior ampulla ; SB, FB, HB, anterior and posterior vertical, and horizontal
canals, with their ampulke (SA, FA, and HA) ; Co, commissure of the vertical
canals ; pb, pars basilaris cochleae : pi, macula acustica neglecta ; lag, Ingena ;
N, auditory nerve.

anterior and posterior canals, which are greatly arched, and the
position of the openings of which into the sinus superior (canal
commissure) is reversed.

In lower types (Swimming Birds) this peculiarity is less marked than in the
higher forms, and it would be very interesting to ascertain the condition of
these parts in the Struthionidee, as one would expect to find in them important
points of connection with Reptiles.

In spite of this higher stage of development of the auditory
organ in Crocodiles and Birds, we cannot speak of the presence

AUDITORY ORGAN.

201

in them of an organ of Corti in the cochlea. A tympanic mem-
brane is present in most Reptiles (with the exception of Ophidia,
Hatteria, and Chamseleo) and in all Birds. The osseo-cartilaginous
columella is well developed, and varies much in the different
forms.

FIG. 165. — MEMBRANOUS LABYRINTH OF THE PIGEOX. (After C. Hasse.)

,S', sacculus ; p&, pars basilaris ; mr, membrane of Reissner ; lag, lagena ; SB, FB,
HB, anterior and posterior vertical, and horizontal canals; SA, FA, HA,
ampullae of the anterior and posterior vertical, and horizontal canals.

FIG. 166. — MEMBRANOUS LABYRINTH OF Ox. (After C. Hasse.)

S, sacculus ; Re.ut, recessus utriculi ; VB, blind sac at the origin of the cochlea ; Cr,
canalis reuniens ; pb, pars basilaris ; lag, lagena ; SB, FB, JIB, anterior and
posterior vertical, and horizontal canals ; SA, FA, HA, the ampulla of these
canals ; Co, canal commissure ; N, auditory nerve.

Mammals, — The Mono tr ernes appear to form a connecting
link between the Reptilia, or, more correctly perhaps, the post-

202 COMPARATIVE ANATOMY.

Reptilia and other Mammals ; and their auditory organ is similar
in many points to that of Crocodiles. At the same time nothing is
yet certainly known of the phylogeny of the Mammalian auditory
organ, concerning which further and more extended researches are
necessary. The cochlea here reaches its highest development, for
it grows into a long tube which becomes spirally coiled on itself :
in Man it forms nearly three coils, and in other Mammals from
one and a half (Cetacea) up to as many as four or more.1 In this
spiral form of the cochlea, as well as in its more highly specialised
histological structure, lies the characteristic peculiarity of the
auditory organ of Mammals. The auditory nerve forms the axis
of the spiral (Fig. 166).

In consequence of the large development of the cochlea, the
papilla acustica, or, as it is called in Mammals, the organ of Corti,
is drawn out to a considerable length, and the part of the wall of
the cochlea on which this lies is called the basilar membrane,
while the opposite wall is spoken of as the membrane of
Reissner (Figs. 169 and 170, B, E). These parts will be referred
to again later on.

The aperture of communication between the pars superior and
pars inferior of the membranous labyrinth, that is, between the
sacculus and utriculus, is entirely obliterated in Mammals, the two
parts being only indirectly connected with one another by means of
the ductus endolymphaticus ; this bifurcates at its point
of insertion into the membranous labyrinth, one limb being con-
nected with the utriculus and the other with the sacculus (Fig.
167, at 2).

The tympanic membrane is situated deep down in the external
auditory meatus, and separates the latter from the tympanic cavity.
In place of the single bony columella of the Sauropsida there is in
Mammals a chain of three auditory ossicles, articulated with
one another, and extending between the tympanic membrane and
the fenestra ovalis. These are, the malleus, the incus, with its
orbicular apophysis, and the stapes, besides which there is often
a bony (interhyal) rudiment in the tendon of the stapedius muscle.
The malleus corresponds to the articular element of the mandible
of lower Vertebrates,2 and the incus to the quadrate, the former
arising by a constriction of the proximal end of Meckel's cartilage,
which extends through the so-called Glaserian fissure into the
tympanic cavity. As in the Sauropsida, the stapes corresponds to
the upper element of the hyoid arch (pharyngohyal or hyo-
mandibular of Fishes). The fact that in some cases the stapes

1 In the Rabbit there are two and a half, in the Ox three and a half, in the Pig
almost four, and in the Cat three coils in the cochlea. In other types the cochlea, as
well as the sacculus and all parts of the pars superior of the membranous labyrinth,
vary considerably both in form and arrangement.

2 Cp. the chapter on the skull, and Fig. 67, in which the mode of development of
these parts is shown.

AUDITORY ORGAN.

203

appears to be cut out of the substance of the periotic capsule, is
taken by some observers (Kolliker, Moldenhauer) to prove that it
has not a visceral origin, but this fusion is probably a secondary
condition (cp. columella of Amphibia, p. 198). In Monotremes,
several Marsupials, and some Edentates, the stapes is imperforate
and columelliform ; in all other Mammals it is stirrup-shaped, and
encloses the stapedial artery.

/

FIG. 167. — DIAGRAM OF THE ENTIRE AUDITORY ORGAN OF MAX.

External Ear. — M, M, pinna ; Mae, external auditory ineatus ; 0, wall of latter ;
Mt, tympanic membrane.

Middle Ear. — Ct, Ct, tympanic cavity; Ol, wall of same; SAp, sound-conducting
apparatus, drawn in the form of a rod, representing the auditory ossicles ;
the point t corresponds to the stapes which closes up the fenestra ovalis ; M,
fenestra rotunda ; Tb, Eustachian tube ; Tbl, its opening into the pharynx ; 0",
its wall.

Internal Ear, with the greater part of the bony labyrinth (KL, KL1} removed.—
S, sacculus ; a, b, the two vertical canals, one of which (b) is shown cut through ;
c, Cffj commissure of the • canals of the membranous and bony labyrinths re-
spectively ; S.e, D.e, saccus and ductus endolymphaticus ; ^ the latter bifur-
cates at 2 ; Cp, cavum perilymphaticum ; Cr, canalis reuniens ; Con, mem-
branous cochlea, which gives rise to a blind sac at + ; Con1, bony cochlea ; Sv
and St, scala vestibuli and scala tympani, which at * pass into one another
at the cupula terminalis (Ct) ; D.p, ductus perilymphaticus, which arises from
the scala tympani at d, and opens at D.p1. The horizontal canal is seen
between 2 and S.

To understand the auditory organ, and more particularly the
membranous cochlea, of Mammals, it is necessary to examine the
bony cochlea. The axis of the latter lessens in size from base
to apex (Fig. 168, A) and round it a bony lamella (lamina spiralis
ossea) winds in a spiral manner ; this extends into the cavity of
the coils of the cochlea without quite reaching as far as the

204 COMPARATIVE ANATOMY.

opposite wall (Figs. 168, Lso, Lso1, and 169-, Lo, Zo1). This is
continued outwards by two laterally-diverging lamellae (Fig. 169, i?,
J?), mentioned above as the membrana basilaris and mem-
brana Reissue ri; these lie at an angle to one another and
correspond to the inner walls of the membranous cochlea. The outer
wall of the latter is completed by a portion of the peripheral part
of the bony cochlea (the region between Ls and the peripheral end
of R in Fig. 169). The membranous cochlea, which thus appears

FIG. 168. — BONY COCHLEA OF MAN. (After A. Ecker.)

A, axis ; Lso, Lso1, lamina spiralis ossea, the free edge of which, perforated by the
fibres of the auditory nerve, is visible at t ; H, hamulus.

FIG. 169. — DIAGRAMMATIC TRANSVERSE SECTION OF THE COCHLEA OF A

MAMMAL.

KS, bony cochlea ; Lo, Lol, the two layers of the lamina spiralis ossea, between which
at N the auditory nerve (together with the ganglion, left of L) is seen ; L,
limbus laminae spiralis ; B, membrana basilaris, on which the neuro-epithelium
lies ; R, Reissner's membrane ; Sv, scala vestibula ; St, scala tympani j Sm,
scala media (membranous cochlea) ; C, membrane of Corti ; Ls, ligamentura
spirals.

approximately triangular in transverse section, is called the duct us
cochlearis or scalamedia. It is apparent that the scala media
does not by any means fill up the lumen of the bony cochlea, but
th,at a cavity is left on either side of it, corresponding to those we
have already met with in the auditory organ of Birds, and known
as the scala vestibuli and scala tympani (Fig. 169, Sv, St).

Both of these are continuous with the cavum perilymphaticum,
and, following the direction of the scala media, open into one another

AUDITOKY ORGAN. 205

at the blind end of the latter, that is, at the so-called cupula ter-
minalis (Fig. 167,*). The scala vestibuli is shut off from the
tympanic cavity (Ctt Ct) by the membrane of the fenestra ovalis,
to which the stapes is applied externally (Fig. 167, SAp, t) ; the
scala tympani is closed by the membrane of the fenestra rotunda
(Fig. 167, M}.

On the floor of the bony cochlea, not far from the fenestra
rotunda, there is an opening (Fig. 167, d) into a narrow canal, the
aqueductus cochleae, or ductus perilymphaticus (P#t
D.p1), which serves as a communication between the perilymph-
atic cavity and the peripheral lymphatic trunks of the head.1

The already-mentioned ductus endolymphaticus s.
aqueductus vestibuli is in relation with the endolymph
enclosed within the membranous labyrinth (Figs. 161 and 167,
D.e). It is an archaic structure, and is present from the lowest
Fishes (Myxinoids) up to Mammals, undergoing numerous varia-
tions and modifications in the Vertebrate series. In its primitive
form, the endolymphatic duct consists of a tube arising from the
inner wall of the sacculus, with the lumen of which it communi-
cates. Its upper end perforates the inner wall of the cartilaginous
or bony auditory capsule, passes into the cranial cavity, and
terminates by an expanded extremity (saccus endolymphaticus) in
the dura mater. Osmosis can thus occur between the lymph
contained in the endolymphatic and epicerebral lymph-spaces
respectively.

In Elasmobranch s the ductus endolymphaticus opens on the posterior part
of the roof of the skull, and is thus in free communication with the sea- water.
In many Reptiles its free end comes to lie close under the roof of the skull
beneath the parieto-occipital suture, and in the Ascalabota the duct even
leaves the cranial capsule, passes back between the muscles of the neck, and in
the region of the pectoral arch becomes swollen to form a large folded sac,
from which finger-shaped processes extend right to the ventral surface of the
vertebral column and to the sub-mucous tissue of the pharynx. These pro-
cesses may also branch out in a labyrinthic manner into the orbit, and they
are always filled with a white semi-solid mass of otolithic substance, con-
sisting of very minute crystals of carbonate of lime : otolithic matter is also
present in the ductus endolymphaticus of all Vertebrates, at any rate in the
embryo.

In Amphibians, and also in certain Teleosteans, the duct of either side,
by means of large sac-like enlargements, may come to lie close to its fellow,
either on the dorsal surface only, or on both dorsal and ventral sides of the
brain. The latter is the case in Anura, for instance.

In Birds and Mammals the ductus endolymphaticus never passes out of
the cranial cavity, and in its general relations corresponds with the tube-like
ground-form described above.

1 A ductus perilymphaticus can be plainly made out from Eeptiles onwards. It
arises in the cavum perilymphaticum on the outer side of the sacculus, then passes
along a deep furrow to the median wall of the cochlea, extends over the membrana
basilaris (scala tympani), passes through the foramen rotmidum, and comes into
connection with the epicerebral lymph-sinus.

206

COMPAKATIVE ANATOMY.

HISTOLOGY OF THE MAMMALIAN COCHLEA.

The fibres of the auditory nerve running along the axis of the bony cochlea
extend in their course laterally outwards, and come to lie between the two
plates of the lamina spiralis ossea (Fig. 168, Lso, Lso1, Figs. 169, 170, JV,
between Lo and Lo1}. On the free border of the latter, these pass out, and
break up into terminal fibrillae on the inner surface of the basilar membrane
(Fig. 170, N,.N\ N*}.

Lo

FIG. 170. — THE ORGAN OF Conn. (After Lavdovvsky.)

Lo, Lo1, the two plates of the lamina spiralis ossea ; N, auditory nerve with ganglion ;
JV1, N'2, the nerve branching up into fibrillae and passing to the auditory cells
(Gf, G] ; Ba, Ba, bacilli, or supporting cells ; Mz, membrana reticularis ; C, mem-
brane of Oorti ; Ls, ligamentum spirale, passing into the basilar membrane ;
Sm, scala media ; JR, membrane of Reissner ; B, B, basilar membrane.

The fibrillse extend to the sensory or auditory cells (G, G), and these are
stretched as in a frame between the firm supporting and isolating cells or
bacilli (Ba, Ba). From the surface of the bacilli a resistant net-like mem-
brane (membrana reticularis) extends laterally, and through the meshes of the
latter the hairs of the auditory cells project (Fig. 170, Mz}. The number of
the outer hair-cells may be estimated at about 12,000. The auditory cells
are covered by a thick and firm membrane — the membrana tectoria s. Corti
(Fig. 170, (7) — which probably acts as a damper, and which arises from the
labium vestibulare of the lamina spiralis ossea. The whole extent of the basilar
membrane consists of clear thread-like and very elastic fibres, of which about
16,000 to 20,000 can be made out in Man. These are capable of vibrating
freely, and, as their length varies definitely in different regions of the cochlea,
they might be looked upon (were it not that they are absent from Birds) as
forming a sort of keyboard or harp, that is, as a definite apparatus of strings
capable of analysing the different waves of sound, the vibrations of which are
communicated to the auditory cells, and thence by means of the nerves to
the brain.

AUDITORY/ ORGAN. 207

RELATION OF THE AUDITORY ORGAN TO THE AIR-BLADDER IN FISHES.

A relation between the auditory organ and air-bladder is observable in four
families of Teleosteans (Siluroidei, Cyprinoidei, Characini, and Gymnoti). The
apparatus is formed on the same plan in all these Fishes.1

A chain of bones extends between the anterior end of the air-bladder and
the auditory organ, by means of which the relative fulness of the air-bladder
can be appreciated by the Fish. This chain arises by the metamorphosis of
certain parts of the four anterior vertebra} (upper arches and spines and
transverse processes), and four segments may be distinguished. In many cases
processes of the air-bladder are produced outwards to the side-walls of the body,
where the skin becomes very thin, forming a sort of tympanic membrane.

(For further details, such as the relation of the whole apparatus to the
saccus endolymphaticus, the reader is referred to Wiedersheim's Lehrbuch der
vergl. Anatoriiie.}

BIBLIOGRAPHY.

HADDON, A. C. — On the Stridulating Apparatus of Callomystax gagata. J&urn. of

Atiat.. and Physiol. Vol. XV.
HASSE, C. — The numerous Papers of this Author, which refer to all the chief

Vertebrate groups, are published partly in the Zeitsch. f. wiss. Zool. (Bd. XVII.

und XVIII.), and partly in " Anatomischen Studien." Leipzig, 1870-1873.
HENSEN, V. — Physiologic des Gehors. In the Handbuch der Physiologic by L.

Hermann. Abthl. Sinncsorgane 2. Leipzig, 1880.
His, W. — Anatomic mensch. Embryonen. Leipzig, 1880-1885.
HOWES, G. B. — On the Presence of a Tympanum in the Genus Raia. Journ. of Anat.

and Physiol Vol. XVII., 1883.
KUHN. — See the works of this Author on the auditory organ of Fishes, Amphibia,

and Reptiles in the Arch.f. miJcr. Anat. Bd. XIV., XVIL, XX.
MOLDENHAUER, "\V. — Die. Entwicldung dcs mittlcren und dusscrcn Ohres. Morph.

Jahrb. Bd. 1JL, 1878.
PARKER, T. J. — On the Connection of the Air-Bladder and the Auditory Organ in the

Red Cod (Lotella bacchus}. Trans. N. Z. Institute, 1883.
RETZIUS, G. — Das Gehororgan der Wirbelthiere. I. Das Gehororgan der Fische und

Amphibien. Stockholm, 1881. II. Das Gehororgan der Reptilien, der Vogel,

und der Saugethiere. Stockholm, 1884.

WEBER, E. H. — De aure et auditu hominis et animaUum. Lipsiae, 1820.
AViEDERSHEiM, R.- — Zur Anatomic und Physiologic des Phyllodactylus europmis> &c.

Morph. Jahrb. Bd. I., 1876.

1 T. J. Parker has also described a connection between the auditory organ, air-
bladder, and skin in the Red Cod (Lotella bacchus).

F. ORGANS OP NUTRITION.

ALIMENTARY CANAL AND ITS APPENDAGES.

THE alimentary canal (tractus intestinalis) consists of a tube
which begins at the aperture of the mouth, passes through the
body cavity (ccelome), and ends at the anus. Its walls consist
essentially of three layers; an inner epithelial, a middle con-
nective-tissue, and an outer muscular layer. The first,
which corresponds to the hypoblast of the embryo, forms the lining
of the canal (Fig. 8, Ep)t and gives rise to numerous glandular
structures which have a secretory as well as a resorptive nature ;
the second (Siibm), consisting of connective and adenoid tissue,
serves chiefly to conduct the blood and lymph vessels ; while the
third (Msc) which, together with the second, corresponds to the
splanchnic layer of mesoblast of the embryo, is, as a rule, divided
into two layers, and consists of smooth muscular elements, the inner
being constituted by circular fibres, and the outer by longitudinal
ones. These serve for the contraction of the wall of the gut, and
thus fulfil the double function of bringing its nutritive contents
into the closest possible relation with the whole epithelial surface,
and at the same time of removing from the body the substances
which have not been absorbed. A fourth accessory serous coat,
enclosing the gut externally in the region of the coelome, must be
added to these three layers. It is covered on its free surface by
pavement epithelium, and is reflected round the entire body-cavity,
converting the latter into a large lymph-sinus. Its abdominal
portion is spoken of as the peritoneum, and its thoracic portion
as the pleura, the heart being invested by a special serous mem-
brane, the pericardium. In the cranial and cervical portions of
the alimentary tract the serosa is not developed.

A parietal layer, lining the body-cavity, and a visceral
layer reflected over the viscera, can then be distinguished in
the peritoneum (Fig. 8, Per, Per1). The region where one passes
into the other, which is thus primitively double, is called
the mesentery (Ms), and this serves not only to support the
alimentary canal from the dorsal body-wall, but also to conduct

ALIMENTARY CANAL. 209

the vessels and nerves passing from the region of the vertebral
column to the viscera. By far the greater number of the nerves
arise from the sympathetic system ; cerebral and spinal elements
are present only in the most anterior and posterior sections of the
alimentary canal, both of which regions contain striped muscular
fibres and are under the influence of the will. The mesentery gives
rise to a large system of folds arising from the inner dorsal surface
of the body-wall, in which the viscera are enveloped.

The most anterior section of the primitive alimentary tract
functions as a respiratory cavity as well as a food-passage, and
possesses for this purpose a row of apertures, lying one behind
the other :. round these, certain vessels are developed, by means of
which a continual interchange of gases can take place between
the blood and the water passing through the apertures. In short,
gills are developed (Fig. 171, A). Although these latter are only
functional in Fishes, Dipnoans, and aquatic (or larval) Amphibians,
even in the higher Vertebrates, the larger portion of the cavities
of the mouth and pharynx lying behind the internal nostrils serves
as a common air- and food-passage until a proper palate is formed
(Fig. 171, C, D).

With the formation of a definite palate,1 the primitive mouth-
cavity becomes divided into an upper respiratory, and a lower
nutritive portion, or into a nasal, and a secondary or de-
finitive mouth-cavity. The separation, however, is not a
complete one, the passage being common to both cavities for a
certain region (Fig. 171, at f). This region is called the pharynx,
and in Mammals it is partially separated from the mouth by a
fibrous and muscular fold, the velum palati, or free edge of the
soft palate.

The alimentary canal of all Vertebrates is divided into the
following principal sections: — Mouth (cavum or is), pharynx,
gullet (oesophagus), stornach(ventriculus) (not differentiated
in rare cases only), small intestine (duodenum, jejunum, and
ileum), and large intestine (colon and rectum). A caecum
is often present at the junction of the large and small intestine.
Between the stomach and duodenum as well as between the ileum
and large intestine there is, as a rule, a marked elevation of the
muscular coat serving as a sphincter (pyloric and ileo-colic
valves). There is also a sphincter muscle at the anus.

The small intestine is always the longest section of the ali-
mentary tract: the ducts of the liver and pancreas open into
its anterior portion (duodenum).

The course of the alimentary canal may be straight or more or
less coiled ; in the latter case it presents a greater absorptive
surface. As a general rule, it is longer in herbivorous than in
carnivorous animals.

A considerable increase of surface also commonly results from
1 Comp. pp. 75 and 81.

P

210

. COMPARATIVE ANATOMY.

—O

FIG. 173.— DIAGRAMS OF THE ORAL CAVITIES OF A FISH (A), AMPHIBIAN (B),
REPTILE OR BIRD (C), AND MAN (D).

'N, external nostrils ; Ch, internal nostrils ; D, alimentary canal ; K, gill-slits ; L,
lung ; T, trachea ; 0, oesophagus: the arrow marked^ indicates the respiratory
passage, that marked B the nutritive passage ; t, the point where the two
passages cross one another.

ALIMENTARY CANAL.

211

the elevation of the mucous membrane to form folds, villi, and
papillae.

A diagram of the human intestinal tract and its appendages
is given in Fig. 172. All the appendages of the canal arise in the

FIG. 172.— DIAGRAM OF THE ENTIRE ALIMENTARY TRACT OF MAN.

Gls, salivary glands ; Ph, pharynx ; Gl.th, thyroid gland ; Gl.thy, thymus gland ;
Lg, lung ; Oe, oesophagus ; Z, diaphragm ; Mg, stomach ; Lb, liver ; Pa, pancreas ;
Dd, small intestine ; Vic, ileo-colic valve ; Pv, vermiform process of caecum ;
Ca, ascending colon ; Ct, transverse colon ; Cd, descending colon ; jR, rectum ;
A, anus.

embryo as outgrowths from the hypoblast, and are thus of epithelial
origin : they either remain throughout life as glandular organs, or at

p 2

212 COMPARATIVE ANATOMY.

any rate they are formed on the same type as glands (lungs, thyroid,
thymus). Mesoblastic elements are added to them secondarily.

Beginning from the mouth the following appendicular organs
of the alimentary canal may be distinguished : —

(1) Salivary glands (Fig. 172, Gls).

(2) Mucous glands.

(3) The thyroid gland (Glth).

(4) The thymus gland (GLthy).

(5) The lungs (pulmones) (air-bladder) (Lg).

(6) The liver (Z6).

(7) The pancreas (Pa).

To these may be added the gastric and intestinal glands
(peptic glands, glands of Lieberkuhn, &c.), which are embedded in
the wall of the gut.

I. MOUTH.1

InAmphioxus the entrance to the mouth is provided with
cirrhi, and inCyclostomesitis surrounded by a ring of cartilage :
all other Vertebrates are provided with jaws.

Definite lips provided with muscles first appear in Mammals,2
and are most strongly developed in Monkeys, especially Anthropoids :
the space between them and the jaws is spoken of as the
vestibulum oris. This may become extended on either side
to form cheek -pouches, which serve as food reservoirs (many
Monkeys and Rodents).

The organs of the oral cavity may be divided into three
groups, viz. the teeth, the glands, and the tongue.

Teeth.

Both epiblast and mesoblast take part in the formation of the
teeth. The epithelium of the mouth grows inwards so as to give
rise to the so-called enamel-organ, which meets with dome-
shaped elevations of the underlying connective-tissue, the tooth-
germs or tooth-papillae. Both these tissues come into the
closest relations with one another, and, by very complicated pro-
cesses of differentiation, give rise to the different substances of
which the teeth are composed. These substances are, the enamel,
dentine (ivory), which is permeated by a fine system of canals,
and cement (Fig. 173, ^Sf, ZB, ZC).

1 The mouth of the Lamprey serves as a suctorial organ for attaching the animal
to foreign objects. The larvre of Lepidosteus and Anura are temporarily provided
with suctorial organs.

3 An exception is seen, however, in Dipnoi, the lips of which are provided with
well-developad muscles.

TEETH.

213

The root of the tooth embedded in the gums is provided at its
lower end with a small opening, and this leads into the pulp-
cavity (Fig. 173, PH1, PH). Into its interior, vessels and nerves
extend.

FIG. 173. — SEMIDIAGRAMMATIC FIGURE OF A LONGITUDINAL SECTION THROUGH A

TOOTH.

ZSy enamel ; ZB, dentine (ivory) ; ZC, cement ; PH1, aperture of the
pulp-cavity (PH).

While in Vertebrates below Mammals a succession of teeth
takes place throughout life, in the latter group this as a rule
occurs only once, that is, the first or so-called milk dentition is
only replaced once by a second stronger and more fully-developed
permanent dentition. In certain Mammals, such as the
Cetacea and Edentata (with the exception of Dasypus peba),
there is no succession, and they are therefore distinguished as
Monophyodonts from other Mammals, or Diphyodonts. The
teeth of Edentates are without enamel.

4

In Rhinolophus (Cheiroptera), some Rodents, and Sirenia, the milk-teeth
never cut the gum, and become entirely absorbed before birth. In Eodents
various conditions of the milk dentition are seen : in the Rabbit they cor-
respond in number with their successors,1 though the incisors disappear before
birth, the outer upper and the lower one being quite rudimentary. In others,
and in some Insectivores, no milk teeth at all are known : the Hedgehog has
a complete milk dentition, while no milk-teeth are known in the Shrew. In
Marsupials and Guinea-pigs only one milk molar is present. All these facts
indicate that a gradual reduction of the milk dentition is taking place.

In those cases where the teeth are similar in foim throughout,
as, for instance, in existing toothed Whales, we have a homodont
as opposed toaheterodont dentition. In the latter case the teeth

1 That is, with the incisors and premolars of the adult.

214 COMPARATIVE ANATOMY.

become differentiated into incisors, canines, and grinders
(premolars and molars).

Fishes and Amphibia. —The dermal denticles already de-
scribed in the chapter on the skeleton are structures homologous
with teeth, for both are developed in a similar manner. In
Teleostei all the bones bounding the mouth may bear teeth, as
may also the hyoid and branchial arches (pharyngeal bones). On
the latter, and also on the parasphenoid, they are arranged in a
single series, or in masses ; numerous teeth are also met with on
the parasphenoid in certain Urodeles (Fig. 174). In general,
however, there is in Amphibia a considerable diminution in the
number of teeth as compared with those of Fishes ; and at the
same time a much more uniform character is noticeable in their
form throughout.1

In Amphibia they are enlarged conically below, and rest on a
definite base, while above they become narrower, and slightly
curved, and end either in a double (Myctodera, Anura), or a single

FIG. 174. — SKULL OF Batrachoseps attenuatus. ~ (From the ventral side, showing the
teeth on the parasphenoid.)

apex (Perennibranchiata, Derotremata, Gymnophiona) ; the latter
is the more primitive condition. The teeth lie deeply embedded
in the mucous membrane, and are present, as a rule, on the pre-
maxilla, maxilla, and mandible, as well as on the vomer and palatine,
but rarely on the parasphenoid ; in the larvae of Salamanders and
in Proteus the splenial of the lower jaw is also toothed. Horny
jaws and horny teeth are present in larval Anura.2

1 Tn Fishes the teeth may be cylindrical, conical, or hooked ; or in some cases
(Scarus, and the Sarginse) they may be chisel-shaped, resembling the incisors of
Mammals, and working together like scissors ; in others they give rise to a definite
pavement, are rounded in form, and serve to crush the food. Again, they may be
delicate and bristle-shaped (Chaetodon), or sabre-shaped (Chauliodus).

2 The horny structures on the upper jaw in young stages of Polyp terus, in the
mouth of Cyclostonji, on the jaws of Siren lacertina, and on the lips of Dipnoi, belong
to the same category as these.

TEETH.

215

Reptilia. — Corresponding with the great firmness and solidity
of the skull in Reptiles, the dentition is usually strongly developed,
and occasionally at the same time it is more highly differentiated
than in Amphibians. The teeth are either situated upon a ledge
on the inner side of the lower jaw, with which they become fused
basally (pleurodont dentition, — Skinks, Amphisbsenians, and
others), or they lie on the free upper border of the jaw (acrodont
dentition), or finally, as in Crocodiles and numerous fossil Reptiles,
they are lodged in alveoli (thecodont dentition) (comp. Fig. 175,
A, a, b, c). Both upper and lower jaws, and occasionally the
palate also, are toothed ; the teeth have a single apex, except in
Lizards, in which the apex is double.1 In many Reptiles, however
(e.g. Hatteria, Uromastix spinipes, Agamse, and numerous fossil
forms, especially those of the Trias of South Africa), a heterodont
dentition, consisting of incisor- canine- and molar-like teeth, is
already seen.

FIG. 175. — A, DIAGRAMS OF TRANSVERSE SECTIONS THROUGH THE JAWS OF REPTILES,
SHOWING PLEURODONT (a), ACRODONT (&), AND THECODONT (c) DENTITIONS.
B, a, LOWER JAW OF Zootoca vivipara ; b, OF Anguis fragilis. (After Ley dig. )

The dentition of poisonous snakes deserves special
attention, for in them a varying number of maxillary tee'th are
differentiated to form poison- fangs. Thus in the common Viper
(Pelias berus and P. prester) there are on each side nine poison-
fangs arranged in transverse rows ; the stronger ones project freely,
while the lesser, reserve teeth, lie within the gum (Fig. 176, A) ;
only one of these teeth, however, is firmly fixed to the maxilla at a
time. Each fang is perforated by a poison-canal, which is in-
completely surrounded by the pulp-cavity, the latter having the
form of a half-ring in transverse section (Fig. 176, B, C, GC, PH) :
the duct of the poison-gland passes into an aperture at the base
of the tooth which leads into the poison-canal, and the latter opens

1 A peculiar tooth is present in the embryos of Lizards, Blindworms, and some
Snakes. It projects considerably beyond its neighbours, and lies in the median line
of the lower jaw extending vertically towards the snout, and serving the young as a
means of breaking through the egg-shell.

216

COMPARATIVE ANATOMY.

at a short distance from the apex of the tooth (see the course of
the arrow in Fig. 176, A).

Between the ordinary teeth of Snakes and the poison-fangs with closed
canals, there are numerous intermediate forms in which certain of the teeth are
simply grooved along their anterior side. A similar condition is also seen in
the teeth of the lower jaw of a poisonous Mexican Lizard (Heloderma).
(Comp. p. 222.)

Chelonians, like Birds, are provided with horny sheaths to the jaws in-
stead of teeth. The presence of teeth in embryos of Trionyx, however, proves
that this is only a secondary condition.

The teeth of the fossil Birds of America (Odontornithes) were either
si tuated in definite alveoli (I chthyornis), or simply in grooves (Hesperornis).
The premaxillee were toothless, and seem to have possessed a horny beak.
The single-pointed smooth teeth of ArcheEopteryx were 1 mm. long, and all of
similar size and form. Most probably they were situated in alveoli. All
existing Birds, as well as those of the Tertiary and Post-Tertiary strata, are
toothless.

FIG. 176. — FIGURES OF THE POISON-FANGS OF A VIPERINE SNAKE.

A, skull of Rattlesnake ; B, transverse section through about the middle of the
poison-fang of Vipera ammodytes ; C, transverse section through the poison-fang
of Vipera ammodytes near its distal end. (B and C after Leydig. )

Gz, poison-fang ; Ez, reserve fangs ; GO, poison-canal ; PIT, pulp-cavity.

Mammals. — The differentiatiofi-oi^the dentition here reaches
its extreme limit, corresponding with the manner in which the food
is taken in and masticated. As already mentioned, incisor,
canine, and grinding teeth (premolars and molars) can
as a general rule be distinguished. These are all embedded in

TEETH.

217

well-developed alveoli of the jaw-bones, and there are no longer any
teeth on the roof of the palate. The canine, which is most largely
developed in Carnivora, usually lies in a continuous series with
the incisors, which are situated in the anterior part of the jaws
(premaxilla in upper jaw). The premolars follow behind the canine,
the space usually present between them being called the d iastema,
and then come the molars, which lie mainly in the posterior part of
the jaw.1

The incisors are usually chisel-shaped, while the canines, in
those cases where they are most largely developed (Carnivora),

FIG, 177. — DENTITION OF THE HEDGEHOG (Erinaceus europceus}. (The teeth of
both jaws from the side, and those of the upper jaw from below. )

i, incisors ; pm, premolars ; m, molars.

possess a pointed, conical form, and are more or less curved. The
form of the premolar and molar teeth may be derived from that
of the incisors and canines ; originally two lateral and a median
cutting-edge can be distinguished in all, these edges having become
gradually metamorphosed phylogenetically for mastication in the
case of the grinding teeth. In the course of further development
the whole grinding surface becomes reduced to the same level,
and appears more or less flattened or tubercular.

In describing the teeth of a Mammal it is convenient to make use of a
dental formula in which their number and arrangement can be seen at a

1 The premolars are those teeth which correspond in position with the deciduous
milk molars, the molars proper having no predecessors.

218

COMPARATIVE ANATOMY.

IG. 178. — DENTITION OF THE DOG (Canis familiaris] .
i, incisors ; c, canines ; pm, premolars ; m, molars.

Fio. 179. — DENTITION OF THE PORCUPINE (Hystrix hirsutirostris).
References as before.

TEETH.

219

FIG. 180. — DENTITION OF SHEEP (Ovis aries).

References as before, but the teeth of the lower instead of the upper jaw are figured

from the surface.

FIG. 181. — DENTITION OF A CATARRHINE MONKEY (Nasalis larcatus).
References as before.

220 COMPARATIVE ANATOMY.

glance. Thus the dental formula of those animals the teeth of which are
represented in Figs. 177 to 181, would be —

Fig. 177. Hedgehog, /. f , c. §, pm. f , m. f .

„ N8. Dog, *. |, c. i, pm. £, m. f.

„ 179. Porcupine, i. }, c. §, m. f.

„ 180. Sheep, i. §, c. ^, ^m. §, m. f.

„ 181. Catarrhine Monkey, i. f , c. }, ^w. |, w. f.

The variations in the dentition of the different groups of Mammals are so
exceedingly numerous that it is impossible to describe them in detail here,
and only the following points will be briefly remarked upon.1

The essential arrangement of 'the teeth is such that there is an alternation
between those of the upper and lower jaw ; thus the teeth of one jaw do not
correspond in position with those of the other, but with the interspaces
between them.

A consideration of the rudimentary, functionless teeth which are commonly
present renders it probable that in the course of genealogical development the
teeth have undergone a decrease in number.2 An increase in number, on the
other hand, must be always considered as an atavism.

Finally, attention must be called to the commonly-existing sexual differ-
ences in dentition, as, for instance, in the Wild Boar, in the Narwhal
(Monodon), in the Dugong (Halicore), and in the Musk-deer. In the males
of these animals a modification of certain of the teeth (usually the canines) to
form tusks occurs, and these serve as fighting weapons. In the Elephant and
Walrus tusks are present in both sexes : in the former they correspond to
incisors, and in the latter to canines.

Glands of the Mouth.

The glands of the mouth, like those of the orbit and integument,
appear first in terrestrial animals, that is, from Amphibians on-
wards. They have the function of keeping moist the mucous
membrane which comes into contact with the outer air. From
being at first almost entirely unspecialised, and giving rise simply
to a slimy fluid, they become differentiated later into structures the
secretions of which take on a very important relation to digestion ;
they may also, as in the case of poisonous Snakes and Lizards,
constitute dangerous weapons of offence.

With their gradually increasing physiological importance a
greater morphological complication both in number and arrange-
ment takes place. Their histological character also becomes
changed in such a manner that the three ordinary forms of glands,
i.e. tubular, compound-tubular, and acinous, may be
recognised.

In the lower Vertebrata the two first forms preponderate, and

1 These variations not only consist in the various positions and forms of the
teeth, but also in the typical arrangement of the enamel on the crowns. Thus in the
non-Ruminant Bunodontia the molars are tubercular, while in the Ruminant
Selenodontia the surface of the grinders is made up of crescent-shaped elevations.

2 The last molar of Man,n>r so-called "wisdom-tooth," seems to be gradually dis-
appearing. It appears last and is lost first, and often does not reach the grinding
surface.

GLANDS OF THE MOUTH. 221

the glands are mostly arranged in groups; in the higher types, on
the contrary, the last- mentioned form, which is considerably higher
from a developmental point of view, is the more usual.

Amphibia. — With the exception of the Perennib'ranchiata,
Derotremata, and Gymnophiona, a tubular gland becomes
developed in all Amphibia from the anterior portion of the roof of
the mouth (comp. Fig. 144, ID], the main mass of which lies, in
Urodeles, in the cavity of the nasal septum or premaxilla
(intermaxillary or internasal gland).

In Anura its position is more anterior than in Urodeles, and
it is more largely developed ; but in both cases the ducts open on
to the anterior part of the palate. In Anura there is a second
gland (pharyngeal gland) present in the region of the internal
nostrils, the secretion of which, passes partly into the latter and
partly into the pharynx.

Numerous gland-tubes are also present in the tongue of
Amphibia.

Reptiles. — The mouth-glands in Reptilia show an advance on
those of Amphibia, inasmuch as they are separated into groups.
Thus not only is there a palatine gland, homologous with the in-
termaxillary gland, but lingual and sublingual, as well as upper
and lower labial glands are present. Chameleons and Snakes

Me j»- Kin

FIG. 182.— THE POISON-APPARATUS OF THE RATTLESNAKE.

S, the fibrous poison-sac, which is surrounded by the constrictor-muscle, Me ; at Me1
an extension of the latter towards the lower jaw can be seen ; Gfc, the duct
arising from the poison-gland, which passes into the poison-fang at t ; the latter
is embedded in a large sac of the mucous membrane, zf ; Km, masticatory muscles,
some of which are seen cut through at * ; posterior to this the cut edge of the
scaly integument is seen ; JV, external nostril ; A, eye displaced towards the
antero-dorsal aspect ; z, tongue ; za, aperture of the poison-fang.

are distinguished by a remarkable richness in glands, which become
most specialised into definite groups in the latter. In poisonous
Snakes the poison-gland (glandula venenata) becomes
differentiated from a portion of the labial gland in the upper
lip. It is enclosed in a strong fibrous sheath, and is acted upon

222 COMPARATIVE ANATOMY.

by powerful muscles, so that its secretion can be poured with
great force into the duct (Fig. 182, Gc), and thence into the
poison-fang (t).

The sublingual gland of a Mexican Lizard, Heloderrna, has a somewhat
similar poisonous nature. Its secretion passes out through four ducts, which
perforate the bones of the lower jaw in front of the grooved teeth.

In marine Chelonians and Crocodiles there are no large glands united
into groups connected with the mouth.

Birds. — In Birds, and more especially in climbing Birds
(Scansores), well-developed lingual glands are present, opening
on the floor of the mouth. There is no doubt that they are
homologous with those of Lizards, and they probably correspond
with the posterior upper labial gland which opens into the angle of
the mouth, that is, to the poison-gland of Snakes. The palatine
glands of Birds have also their homologues in Reptiles.

Mammals. — Three glands may be distinguished in connection
with the mouth in Mammals, which are called, according to their
position: (1) parotid, (2) submaxillary, and (3) sublingual
(Fig. 172, Gls). Each opens by means of a well-defined duct (ducts
of Steno, Wharton, and Bartholini respectively) into the
mouth. The former corresponds to the gland opening into the
angle of the mouth in Birds, arid consequently also to the poison-
gland of Snakes. As the last-mentioned is to be looked upon
as a differentiation of labial glands, the same origin must be
supposed for the parotid, — and this is confirmed by a study of its
development.

The fact that both the other salivary glands are homologous
with the sublinguals of lower Vertebrates needs no special proof,
and the numerous mucous glands lying at the sides of the tongue
and opening into the mouth come under the same category.

Concerning the tonsils of Mammals, which lie at the junction of the
mouth and pharynx, compare pp. 239 and 293.

Tongue.

In Fishes and Dipnoi the tongue is, as a rule, rudimentary,
usually consisting simply of a covering of mucous membrane ex-
tending over the basal part of the hyoid, which in all the higher
Vertebrates serves as the main point of origin for the tongue-
muscles. The tongue is not capable of movement apart from the
visceral skeleton in Fishes, and, except in Cyclostomes (where it has
to do with the suctorial apparatus), is wanting in a proper muscula-
ture ; it is covered with papillae, and serves only as a tactile organ,
or, when covered with teeth, as a prehensile organ also.

In the Perennibranchiata it remains in a similar condi-
tion to that seen in Fishes, but in all other Amphibia except
the Aglossa (Pipa and Dactylethra) it reaches a higher stage

TONGUE. 223

and becomes larger in size by the development of definite
muscles in connection with it. Its mobility varies greatly in the
different groups of Amphibia in accordance with the manner
in which it is fixed to the floor of the mouth. It is usually
attached only by the anterior end or by a portion of its ventral
surface ; in other cases it is free all round, and in Spelerpes
(Fig. 183) is capable of being extended far out of the mouth by
means of a complicated mechanism.

FIG. 183. — HEAD OF Spe lerpes fuscus, WITH THE TONGUE EXTENDED.

In most Reptiles and Birds the tongue is freely moveable,
but its form and relative size varies greatly in the different families
(see Fig. 184, A to D). It is least mobile in Chelonians and
Crocodiles : 1 in Chameleons, on the other hand, it is very long and
protrusible.

The tongue of Birds which is usually poorly provided with
muscles, may be looked upon as having been derived from a
similar form to that of Lizards, and its shape as a rule corresponds
more or less to the form of the beak. It possesses a horny cover-
ing, usually provided with papillae and pointed recurved processes ;
as in many Reptiles, it may be split up at its distal end, being
either bifurcated (Colibris) or having a brush-like form. In Wood-
peckers (the extraordinarily developed epibranchials of which have
already been mentioned in the chapter on the skull), the tongue
may be thrown far out from the mouth by means of a complicated
system of muscles, and it thus serves as a prehensile organ; in
this Bird and in the Duck it is richly provided with Pacinian
corpuscles. The tongue is largest in predatory Birds (Rapaces)
and Parrots, but its size is here not due so much to the special
development of muscles as to the presence of fat, vessels, and
glands.

The tongue reaches its most complete development in Mammals,
and, as elsewhere, undergoes the most various modifications as
regards size, mobility, and function, according to the method of
taking in food. It is as a rule flat, and rounded anteriorly, having
a band-like form, and being extensile. A fold, the so-called sub-
lingua (plica fimbriata and mediana), is present on its lower
surface. This represents a primitive organ which must be regarded
as the predecessor of the structure which we now speak of as the
tongue.

In spite of the various functional modifications of the tongue

1 The relative importance and degree of development of the tongue does not run
parallel with the systematic position of the animal.

224

COMPARATIVE ANATOMY.

FIG. 184. — A, TONGUE, HYOID APPARATUS, AND BRONCHI or A GECKO (Phyllo-
dactylus europwus) ; B, TONGUE OF Lacerta; C, OF Monitor indicus ; D, OF
Emys europcea.

Z, tongue ; ZK, body of hyoid ; VH and HH, anterior and posterior cornua of hyoid ;
K, larynx ; Th, thyroid gland ; T, trachea ; B, bronchi ; Lg, lung ; M, inan-
dible ; //, glottis ; ZS, sheath of tongue.

THYROID GLAND. 225

in Mammals, it is very uniform both as regards the extrinsic
muscles and their attachment to the hyoid and lower jaw, and the
arrangement of the intrinsic muscles. Only in such forms as
Myrmecophaga and Dasypus do the intrinsic muscles show a
peculiar character : in them a tendinous investment is present
lying under the rnucosa, and this serves as a point of attach-
ment for all the transverse and vertical muscles.

In many Mammals (e.g. Carnivora, Insectivora) a peculiar struc-
ture, the lytta ("worm"), is present, lying within the longitudinal
axis of the tongue. It is partly fibrous and partly muscular, and its
thread-like posterior end is connected with the body of the hyoid.
Its phylogenetic meaning is not yet clear, and it can only be said
that it functions as a point of origin and insertion for the intrinsic
muscles of the tongue.

In Man the tongue has a double origin, arising firstly from an unpaired
protuberance — the tuberculum impar (His) — lying on the floor of the
mouth of the embryo, and secondly from the swellings of the second and
third visceral arches, which meet together in the middle line. In this manner,
a part of the floor of the primary mouth becomes bridged over, forming a
narrow depression, covered by the root of the tongue. This depression
becomes further isolated by the approximation and union of the body of the
tongue — which arises from the above-mentioned tuberculum impar — to the
tongue-root, which is formed from the visceral swellings. From the cavity
thus shut off a double epithelial vesicle is formed, — the median thyroid
rudiment. This remains for a time in free communication with the cavity
of the mouth by a duct, the ductus thyreoglossus, which passes to the
surface of the tongue along the region where the body and root of the tongue
become later united. The foramen caecum, which can be occasionally traced
in the adult human subject into the substance of the tongue for 2£ centimetres
or more, is the last remnant of this ductus thyreoglossus. From the median
thyroid rudiment the middle lobe of the thyroid arises later, and this com-
monly extends forwards (upwards) as a " cornu medium " and sometimes also
as a hollow duct. Now and then this duct may become constricted into
definite vesicles (from two to four), which are known as bursa suprahyoidea,
bursa prehyoidea, &c. All these are remains of the ductus thyreoglossus. The
lateral thyroid rudiments of Man arise by the lower part of the
primary floor of the pharynx, lying near the glottis, becoming separated off
from the main cavity, and thus forming an independent structure lying
laterally to the larynx. The lateral and median rudiments of the thyroid
later become approximated.

Thyroid Gland.

The thryoid gland arises in all Vertebrates as one or more
diverticula of the ventral wall of the pharynx or floor of the
mouth. In Ammocoetes the single diverticulum remains in com-
munication with the pharynx, and a similar condition of things
is seen in Ascidians and Amphioxus ; it thus appears probable
that we have to do here with a very ancient glandular organ, the
secretory function of which in relation to the alimentary canal was
of great importance in the ancestors of existing Vertebrates.

In all the higher Vertebrates this organ always becomes

Q

226 COMPARATIVE ANATOMY.

constricted off from the pharynx in the course of development,
and the communication between them is once for all abolished.
(Comp. also small print on p. 225.) As a rule it shows a paired
arrangement, and lies right and left of the median line. Internally
it consists of closed glandular vesicles, surrounded by a capillary
network, or cylindrical branched tubes may be present (Mammals).
The whole is lobulated in structure, and is characterised,
particularly in Mammals, by a great wealth of blood-vessels. It
seems very probable that this organ, which is in many respects
rudimentary, has undergone a gradual change of function in the
course of phylogenetic development, but as yet it is impossible to
explain in what its function consists. As regards position, it either
remains throughout life in its locus nascendi on the floor of the
mouth, as in Fishes and Amphibia, or it extends a varying
distance backwards. In Birds, for instance, it lies on the origin
of the carotid artery (Fig. 185, Tr).1

The development of the thyroid has been studied most completely in the
Fowl and in Mammals. In the former, the organ arises as an unpaired hollow
vesicle in the median ventral line of the neck. Later, this becomes solid,
and then divided into two halves, which gradually separate from one another
until eventually they reach the position they occupy in the adult.

Amongst Mammals the development of this organ is best known in the Pig.
Here the so-called middle lobe or isthmus arises as an outgrowth of the
mucous membrane of the floor of the mouth on the level of the second
branchial arch. The lateral lobes arise in the region of the third branchial
cleft. The different lobes become united later on (comp. also small print at
end of section on the tongue).

Thymus Gland.

The thymus gland arises on either side from the mucous
membrane of the pharynx, as a proliferation of the epithelium
of the gill-clefts. It is impossible to state with certainty whe-
ther it was originally a glandular, that is, a secretory organ, or
whether it consists of material that was at one time designed
for the formation of gill-filaments. Certain discoveries in
Elasmobranch embryos seem to point to the latter view. In
them the organ has a segmental arrangement corresponding to the
number of gill-clefts, and in Gymnophiona indications of a
similar arrangement are to be seen.

According to His, the thymus of Man does not arise from the inner or
hypoblastic epithelium of the pharynx, that is, of the visceral clefts, but
from the epithelial (epiblastic) covering of the fourth, third, and partly also

1 According to A. Dohrn the hyoid arch corresponds genetically to two arches,
the first giving rise to the hyomandibular, and the second to the hyoid arch proper,
and the thyroid represents the remnant of a lost gill-cleft between the hyomandibular
and hyoid. Dohrn supports this by the relations of the thyroid to the vascular
system ; the thyroid artery arises like a true branchial vessel from the hyoid artery.

THYMUS GLAND.

227

of the second visceral clefts, which extends inwards from the integument, as well
as from the covering of the corresponding visceral swellings. All these parts

FIG. 185. — THYMUS AND THYEOID OF A YOUNG STORK.
T, trachea ; B, bronchi ; Oe, oesophagus ; H, heart ; Tm, thymus ; Tr, thyroid.

are gradually withdrawn inwards and separated from the surface at the
boundary between head and neck.1

1 His states that in human embryos the gill-clefts never become perforated, but
simply blind slits are formed from both epiblast and hypoblast. The same holds
good also for the tympano-Eustachian passage.

Q 2

228 COMPARATIVE ANATOMY.

In post-embryonic time the organ always shows a lymphoid
structure, and, on account of its richness in white blood-corpuscles,
certainly has important physiological relations to the organism as a
whole. This is probably especially the case in Mammals, as it
here attains a large development, extending in the embryo back-
wards from the region of the larynx above the sternum as far as
the diaphragm. Later it undergoes a retrogressive metamorphosis,
and finally becomes more or less completely obliterated, though it
persists for a considerable time in Man, often for many years after
birth. In all other Vertebrates it persists throughout life, and lies
as a lobulated or cord-like organ in the anterior or lateral region
of the neck; thus in bony Fishes, for instance, it is situated
behind the gills in the neighbourhood of the fibrous band which
connects the gill-cover with the pectoral arch, and in Amphibians
it lies behind and above the articulation of the lower jaw (comp.
also Fig. 185, Tm).

II. OESOPHAGUS, STOMACH, AND INTESTINE.

Fishes and Amphibia. — While in Amphioxus a widened
section of the alimentary canal is probably to be looked upon as a
sort of stomach, in Cyclostomi, Dipnoi, Chimserse, certain
Teleosteans, and many branchiate Amphibians, a stomach is
not plainly marked off from the rest of the gut, which usually has
a more or less straight course. In this case the only externally
visible boundary between the stomach and intestine is, as already
mentioned, the point where the bile-duct (ductus choledochus)
perforates the wall of the latter. In other Fishes, as, for instance,
in Squalidae, all Ganoidei, numerous Teleostei, as well as in
theDerotremata, Myctodera, and Anur a, the stomach appears
more or less dilated and sac-like ; it may also be curved on itself,
so that one can distinguish between a part running backwards
(descending portion) (Fig. 186, M), and another extending forwards
(ascending portion), the two lying parallel to one another (PJK). In
general, it becomes adapted to the form of the body : — thus Rays and
Anurans possess a far wider stomach than do most other Fishes and
Amphibians (cp. Figs. 189 and 190) — and this rule holds good also for
Reptiles. The stomach of Teleosteans varies considerably in form.

The oesophagus is short, and usually not distinctly marked
off from the stomach, though exceptions to this often occur, as,
for instance, in many Teleostei, and in Siren lacertina amongst
the Amphibia (Fig. 189, Oe).

A longitudinal fold extending into the lumen of the intestine,
the first traces of which are seen in Ammocoetes, is to be looked
upon as a structure designed for increasing the digestive surface :
this is also present inElasmobranchs, Dipnoj., and Ganoidei,

(ESOPHAGUS, STOMACH, AND INTESTINE.

229'

in which it has a spiral form, and is therefore called the spiral
valve. In the last-named Fishes, it begins to undergo degeneration
(Fig. 187, sp.v), and is no longer met with in other Vertebrates.1

The pyloric caeca (appendices pyloricse), which are charac-
teristic of the intestine of many Fishes, belong to the same physio-
logical category as the above. They are met with in Ganoids
and numerous Teleosteans, and consist of longer or shorter finger-
shaped processes of the small intestine, which are situated posteriorly

FIG. 186. — INTESTINAL TRACT OF A SHARK.

H, heart ; PC, pericardium cut through ; Sv, sinus venosus ; L, L, the two lobes of the
liver, separated from one another so that the stomach (M ), with its pyloric tube
(PR), and the region of the pylorus (P) are visible ; MD, small intestine, in'which
the spiral valve lies; ED, large intestine; Gsp, rectal gland; AT, cloacal
pockets ; Pa, Pa, abdominal pores ; Pn, pancreas.

to the pylorus in the region of the bile-duct (Figs. 187, c, and 188,
Ap). Their number varies from 1 (Polypterus and Ammodytes)
to 191 (Scomber scombrus). The pyloric caeca and the spiral valve
seem to be developed in inverse proportion to one another, for,

1 Amongst the Teleostei a spiral valve is present in Cheirocentr us, and pro-
bably also in B u t i r i n u s. An intestinal valve exists also amongst the S a 1 m o n i d a?.

230

COMPARATIVE ANATOMY.

to a certain extent, the more one is developed, the less important
is the other.

.-—St

FIG. 187. — ALIMENTARY VISCERA AND AIR-BLADDER OF Lepidosteus, in situ.
(After Balfour and Parker.)

a, anus ; a.b, air-bladder ; a.b1, its aperture into the throat ; b.d1, aperture of bile-
duct into intestine ; c, pyloric ca3ca ; g.b, gall-bladder ; hp.d, hepatic duct ; Ir,
liver ; py, pyloric valve : s, spleen ; sp. v, spiral valve ; st, stomach.

In the narrow-bodied Gyrnnophiona, the intestine is only
slightly coiled, while in Antira it becomes considerably folded
on itself. Its form in Salamanders is about mid- way between
these two extremes.

SOPHAGUS, STOMACH, AND INTESTINE.

231

With the exception of the Cyclostomi, Holocephali, Ganoidei,and
Teleostei, in which there is a separate anus, the large intestine of
all other Fishes, and of Dipnoi and Amphibia, opens into a cloaca,
common to it and to the urinogenital ducts. The large intestine
takes a straight course, and in Amphibia, as well as to some extent
in certain Ganoids and Teleosteans, is plainly marked off from

FIG. 188. — INTESTINAL TRACT OF PERCH.

Oe, oesophagus ; M, stomach; t, caecal process of latter ; P, P, short pyloric region ;
-dp> pyloric caeca (appendices pyloricae) ; MD, small intestine ; ED, rectum ;
A, anus.

the small intestine : in some cases it is considerably swollen,
even exceeding the stomach in capacity (Fig. 190, N). An out-
growth of the ventral wall of the cloaca in Amphibia gives
rise to the urinary bladder, and represents the rudiment of
the allantois.

Reptiles. — Corresponding with the more definitely differen-
tiated neck, we find that Reptiles have a longer oesophagus than
the animals as yet considered, and this is always plainly marked
off from the much wider stomach, which is usually sac-like, or
bent upon itself, when it lies transversely (Chelonians).1 The
stomach of Crocodiles is more specialised than that of other
Reptiles, and approaches that of Birds in structure.

Snakes, Snake-like Lizards, and Amphisbaenians pos-
sess a narrow, spindle-shaped stomach which lies in the long axis
of the body ; in correspondence with the large size of the masses of

1 The O3sophagus of marine Chelonians, like that of many Birds, is lined by
horny papillae.

232

COMPARATIVE ANATOMY.

food, which are swallowed whole, it is capable of great distension.
In these forms the intestine is only slightly coiled ; in Lizards the

•Oe

V-EV

FIG. 189. FIG. 190.

FIG. 189. — INTESTINAL TRACT or Siren laccrtina.

Oe, oesophagus, marked off from the stomach (M ) by a constriction, t ; P, pyloric
region ; MD, small intestine ; ED, large intestine.

FIG. 190. — INTESTINAL TRACT OF Rana esculenta.

Oe, oesophagus ; M, stomach ; Py, pyloric region ; Du, duodenum ; D, ileum : t,
boundary between the latter and the large intestine (It) ; A, opening of the
rectum into the cloaca (Cl) ; Hb, urinary bladder ; Mz, spleen.

coils are more marked, and in the other forms, with broad bodies,
the folding is carried still further.

CESOPHAGUS, STOMACH, AND INTESTINE.

233

The large intestine has a straight course, is often consider-
ably swollen, and opens into a cloaca. An account of the allan-
tois of the Amniota will be found in a subsequent chapter
(p. 273).

From the Reptilia onwards a process (generally asymmetrical)
of the anterior portion of the large intestine is usually formed,
giving rise to a caecum or blind-gut.

The function of the bursae anales of Chelonia, which consist of paired
caecal outgrowths of the cloacal wall, is not understood.

Birds. — In correspondence with the kind of nutriment, the
mode of life, and the absence of teeth, a division of labour occurs
in the stomach of Birds, which, instead of remaining simple,
generally becomes divided into two portions, an anterior and a

Oe —

FIG. 191. — DIAGRAM OF THE (ESOPHAGUS AND STOMACH OF A BIKD.

Oe, Oe\ oesophagus ; Ig, crop ; DM, glandular stomach ; MM, muscular stomach ;

MD, duodenum.

posterior. The former (Fig. 191, DM), which, on account of its
richness in glands, is called the glandular stomach (proventri-
culus), alone takes part in dissolving the food; while the latter
(Fig. 191, MM\ which is lined by a horny layer consisting of a
hardened glandular secretion, has simply a mechanical function, in
correlation with which a very thick muscular wall provided with
two tendinous disks is developed.

The latter portion is for this reason spoken of as the muscular
stomach, or gizzard, and the degree of its development stands

234 COMPARATIVE ANATOMY.

in direct proportion to the consistency of the food. In gramini-
vorous Birds we find the strongest muscular layer and the thickest
horny lining, while in the series of insectivorous Birds, up to the
Birds of prey, this condition becomes gradually less marked, and
the division of labour is less noticeable. Thus in the series of
existing Birds we can see the line along which the differentiation
of the organ has taken place phylogenetically.

Finally the dilatation of the gullet of Birds, which is known
as the crop (ingluvies), must be mentioned (Fig. 191, Ig). A
false crop serving only as a food reservoir, and entirely wanting
in glands, and a true crop, rich in glands, which have a chemical
function, can be distinguished. The former is seen, e.g., in many
Ducks, in the Cassowary, Haliseus, and Otis, and the latter in
'Rasores, Columbidse, and others. The small intestine is
usually of a considerable length, and becomes folded on itself to
a greater or less degree ; it varies, however, both in form, length,
and diameter. At about the middle of its extent there is a small
caecum-like structure, the remains of the vitello-intestinal duct,
or diverticulum caecum.

The straight large intestine opens into a cloaca, and varies
as to, its relative diameter. The caecum is usually paired, and
may reach an enormous length (Lamellirostres, Rasores,
Ratitae). All kinds of intermediate stages between this and an
entire absence of a caecum are to be met with.

In those cases where the caecum is largely developed, it has
an important relation to digestion, as an increase of surface of the
mucous membrane is thus effected ; this increase may even be
carried further by each caecum being provided with a spiral fold
consisting of numerous turns, as in the Ostrich.

The so-called bursa Fabricii is a structure peculiar to Birds.
It arises as a small solid epithelial structure, which later becomes
excavated to form a vesicle, and lies freely in the pelvic cavity
between the vertebral column and the posterior portion of the
intestine ; it extends to the outer section of the cloaca, into
which it opens, posteriorly to the urinogenital ducts. It is
probably present in all Birds, but becomes atrophied more or
less completely in the adult; its physiological function is quite
unknown.

Mammals. — The oesophagus, like that of Birds, is sharply
marked off from the stomach, and is differentiated at its proximal
end to form a pharynx, which is regulated by strong muscles.

Under the influence of the food the stomach here undergoes
much more numerous modifications than are met with in any other
Vertebrate class. As a rule it takes a more or less transverse
position and has a sac-like form, the portion into which the oeso-
phagus opens being called the cardiac, and the part between
this and the duodenum the pyloric portion.

OESOPHAGUS, STOMACH, AND INTESTINE. 235

Oe

P

Oe

FIG. 192. — DIFFERENT FORMS OF MAMMALIAN STOMACHS.

A, Dog. B, Mus dccumanus. C, Mus musculus. D, "Weasel. E, diagram of the
Ruminantstomach, the arrow showing the direction which the food takes ; It, £,
rumen and reticulum ; 0, psalterium ; A, abomasum. F, human stomach
inverted and prepared to show the muscles, a, b, c, on the inner side. G, sto-
mach of Camel ; R, R, rumen and reticulum ; A, abomasum ; WZ, water cells.
H, stomach of Echidna hystrix ; Cmi, lesser curvature ; Cma, greater curvature.
I, stomach of Brady pus tridactylus ; ft, the portion corresponding to the rumen ;
t, that corresponding to the reticulum of the Ruminant's stomach ; the former is
produced to form a csecal process at MB : **, pouches of the duodenum. Oe,
resophagus ; P, pylorus ; Sc, cardiac sac ; Sp, pyloric sac ; Co, cardia.

(Fig. G after Gegenbaur.) . -

236 COMPARATIVE ANATOMY.

Herbivorous Mammals, as a rule, possess a larger and more
complicated stomach than do Carnivorous Mammals (comp. Fig.
192, A to G),1 and it may become divided into two or more chambers.
Thus in Ruminants (Fig. 192, E) there are four chambers, which
are called respectively, rumen (paunch), reticulum, psal-
terium,2 and abomasum. The two first simply serve as storage
cavities, the food returning from them into the mouth, once more
to undergo mastication. It then passes into the psalterium, and
finally into the abomasum, the latter alone being provided with
rennet (gastric) glands, and serving as the true digestive stomach.
(The dotted arrow in Fig. 192, E, shows the course which the food
takes.)

The small intestine is usually long, and varies more as to length
and diameter in domesticated than in wild forms. Commonly, as
in the human subject, the relative length of the small intestine
is less in the foetus than in the adult.

The large intestine, which is made up of a varying number of
coils, usually reaches a great length in Mammals, and its diameter
is much greater than that of the small intestine : these two por-
tions are thus sharply marked off from one another, and the
distinction between them is rendered still more marked by the
sacculations of the anterior part of the large intestine. Only the
posterior portion of the latter, or rectum, which passes into
the pelvic cavity, corresponds to the large intestine of lower
Vertebrates ; the remaining and far larger part, must be looked
upon as a neomorph, and is called the colon. In this, further
subdivisions may often be distinguished, e.g. in Man.

The caecum, which is almost always present, undergoes the
most various modifications both as to form and size, according
to the kind of nutriment. Thus in Carnivora, Odontoceti,
Insectivora, and Cheiroptera, it is very small or even entirely
wanting, while in Herbivora, it may exceed the whole body in
length. An inverse development in size is usually noticeable
between it and the rest of the large intestine. In many cases
(many Monkeys, Rodents, and Man) an arrest of a portion of
the caecum takes place in the course of individual development
(Fig. 172, Pv\ giving rise to a processus vermiformis. In
Lepus, the enormous caecum is provided with a spiral valve, and
in Hyrax, besides a large sacculated caecum at the junction of the
small and large intestines, there is a pair of large simple conical
caeca further back.

Monotremes only amongst Mammals possess a proper cloaca,

1 In Cetacea and Bradypus (Fig. 192, I) however, the stomach is divided into
several chambers, and in some Eodents and the Horse a distinct cardiac and a pyloric
division can be recognised : some Marsupials (e.g. Halmaturus) also possess a com-
plicated stomach. In Ungulates numerous forms between a simple and an exceedingly
complex stomach are to be met with.

8 The psalterium is the latest to be differentiated, both phylo- and ontogenetically.

HISTOLOGY OF THE MUCOUS MEMBRANE. 237

though in Marsupials and some Rodents, the anal and urinogenital
apertures are surrounded by a common sphincter. In all the
others a complete separation between these apertures takes place.

HISTOLOGY OF THE MUCOUS MEMBRANE OF THE ALIMENTARY

CANAL.

With the exception of the epithelium of the mouth and anus,
which is usually stratified, that of the alimentary canal of Ver-
tebrates consists primitively, that is, phylogenetically, of
ciliated cylindrical cells. In some cases this is also true
ontogenetically, and in Amphioxus and Petromyzon (Ammo-
ccetes),1 the ciliated epithelium persists throughout life.

In all other Vertebrates, especially in the higher forms, cilia
are only seen exceptionally after the embryonic period, so that,
as a rule, only ordinary cylindrical epithelium is present.

A striated margin is observable along the free border of the
epithelial cells turned towards the lumen of the canal ; this is pro-
bably to be looked upon as the expression of the earlier ciliated

FIG. 193. — DIAGRAM OF THE STRUCTURE OF A C(ELENTEUATE.

AK, ectoderm ; IK, endoderin ; NN, cells of the endoderm which have thrown out
amoeboid processes and ingested food-particles ; UD, primary alimentary cavity
(archenteron), containing food-particles (N) ; Um, the primary mouth (blasto-
. pore).

covering, and in the lower Vertebrates it is capable of an active
amoeboid movement (Fig. 194, B). In this active participation of
the cells in the process of absorption, we recognise an old inherit-
ance from Invertebrates : in this connection the reader is referred
to Fig. 193, which represents diagrammatically the structure of
a Ccelenterate, in which the endoderm-cells lining the primitive
alimentary cavity, or archenteron (UD\ are directly concerned
with the taking in of nutritive particles by means of pseudopodia
(N) [intracellular digestion]. These endoderm-cells may be com-

1 It persists throughout the gut in Ammoccetes, and only in certain parts in
Petromyzon.

238

COMPARATIVE ANATOMY.

pared with the epithelial cells, ElEl, of the intestine of a lower
Vertebrate, in Fig. 194, A, where they are shown putting out

BH M1 Z L S

a

FIG. 194. — A, SEMIDIAGRAMMATIC TRANSVERSE SECTION OF A PORTION OF THE
WALL OF THE INTESTINE. (Combined from the condition seen in both lower
and higher Vertebrates.)

The connective-tissue layer and epithelium are designedly drawn much too large
as compared with the outer coats. To the left of the figure would be the
body-cavity ; to the right, the alimentary cavity.

P, peritoneal investment of the gut ; M, longitudinal muscular layer ; Ml, circular
muscular layer ; Z, connective-tissue layer ; S, mucous membrane, which is
raised to form villi at Zo ; G, G, vessels, the larger trunks running between the
peritoneum and the muscular layer : the finer vessels branch out into the inter-
mediate layer ; these surround the masses of lymph-cells, LL, as well as the
glands, and send fine loops into the villi (at Cf1) ; DD, apertures of the glands ;
E, E, epithelial cells of the mucous membrane, with their striated border, from
which at E1 amreboid processes are extruded : in Fig. B, a, b, these cells are
drawn to a much larger scale (Sa, striated border) ; Ly, scattered lymph-cells
in the intermediate layer ; Z1, Z3, lymph-cells in the act of passing through
the mucous membrane ; at Z2, several have already passed into the alimentary
cavity, and are beginning to ingest the contained nutritive particles by means
of their amoeboid processes ; LL, masses of lymph-cells (solitary follicles) ;
Lym, lymph -vessels in the villi.

amoeboid processes, and in Fig. 194, B, where the cells a, I, are
shown drawn to a larger scale. Besides these absorptive epithelial

HISTOLOGY OF THE MUCOUS MEMBRANE. 239

cells, other cells take part in the active ingestion of food, and this
again is most marked in the lower Vertebrates, more especially in
Fishes and Dipnoans. These are the lymph-cells (leucocytes,
which on this account have received the suitable name of phago-
cytes), which are present in great numbers in the connective-tissue
of the mucous membrane (Fig. 194, A, Ly), often united into
definite masses (Z/). These cells, which are capable of the most
complete amoeboid movements, become migratory, force their way
through the epithelium of the gut (Fig. 194, A, Z1, Z3), and come
into contact with the food in the lumen of the intestine (Z2, NN).
Others of these phagocytes again, seem to take up the nutritious
particles only after the latter have penetrated through the epithelium
into the connective-tissue layer. Here they become incorporated
by the lymph-trunks (Lym), and finally, through metabolism, by
the whole organism.

The phagocytes possess the further peculiarity of taking up noxious
substances or portions of tissues which have undergone retrogressive meta-
morphosis, wherever they may occur in the body, thus rendering them
innocuous. They thus exercise a kind of superintendence over the body,
acting as a sort of police force. This function is seen most plainly in the
migration of leucocytes from the tonsils into the mouth and pharynx, and
from the epithelium of the conjunctiva into the conjunctival sac. It is not
improbable that this process of ingestion by the leucocytes takes place at every
point where the mucous membrane becomes continuous with the outer skin
(e.g. nose, urethra, anus).

Thus we arrive at the result that, in the lower Vertebrates, —
and, with certain limitations, in the higher types also, — active or
mechanical processes take place in digestion. These appear
to be of great importance in most of the Anamnia; thus in
all Fishes and Dipnoi, for instance, glands, provided with
specially differentiated epithelial cells, are not present, or at any
rate only the first traces of them can be recognised. With the
exception of the liver, Amphioxus, Cyclostomi. and
Dipnoi possess no trace of glands, and even in Amphibia,
a marked differentiation does not seem to occur. It cannot be
affirmed that no chemical processes take place in the process of
digestion in these animals, for every individual epithelial cell of
the gut may be looked upon as a small gland ; but, at all events,
the chemical processes in the higher types from the Reptilia
onwards, become of far greater importance than the merely mech-
anical absorptive processes, owing to the development of highly
differentiated glandular organs (peptic glands and glands of
Lieberklihn).

In conclusion, attention must be directed to the formation of
folds of the mucous membrane. In Cyclostomes these have only
a longitudinal direction (Fig. 195, A), while, from Elasmobranchs
onwards, they take on a transverse arrangement; and thus crypts
arise which possess a sac-like form, often passing deeply into the

240

COMPARATIVE ANATOMY.

wall of the gut (B to E). By a further development these crypts
become more tube-like and elongated, and give rise to the above-
mentioned gastric and intestinal glands (peptic glands and glands
of Lieberklihn).

D

E

FIG. 195. — SEMIDIAGRAMMATIC FIGURES OF THE Mucous MEMBRANE OF THE
INTESTINE OF FISHES, SHOWING INTERMEDIATE FORMS BETWEEN LONGITUDINAL
FOLDS AND KOUND CRYPTS.

A, Petromyzon, showing the distinct spiral fold ; B, an Elasinobranch ; C to E,

various Teleosteans.

The finger-like outgrowths of the mucosa (villi intestinales)
(Fig. 194, A, Zo, Zd) are to be looked at from the same physio-
logical point of view, that is, they have to do with increasing the
absorptive surface. They may be derived through numerous
intermediate forms from ordinary folds, and appear as distinct
papillae from the Sauropsida onwards, reaching their greatest
development in Mammals.

APPENDAGES OF THE ALIMENTARY CANAL.
Liver.

The liver, the form of which is always closely adapted to that
of the body, underlies to a greater or less extent the ventral
side of the intestinal tract, and is present in every Vertebrate
(Amphioxus ?). It is united to the body-wall by a fold of the
peritoneum, and varies considerably in the number of its lobes.
We may nevertheless fix upon a ground-form consisting
of two lobes (Myxinoids) as the precedessor of the organ in all
Vertebrates. It always arises from the commencement of the small
intestine, and develops into a large vascular and glandular organ,
(Figs. 196 to 198) which gives rise to bile, and remains in com-

241

FIG. 196. — LIVER OF Eana esculenta. (From the ventral side.)
L, U. L*, the different lobes of the liver ; M, stomach ; Du, duodenum ; H, heart.

FIG. 197.— PANCREAS AND LIVER OF FROG, TO SHOW THE ARRANGEMENT OF THEIR

DUCTS.

L, Ll to L3, the lobes of the liver turned forwards ; G, gall-bladder ; Dcy, cystic ducts,
which, together with the hepatic ducts (Dh), form a network from which thr
collecting ducts arise, and these unite to form the common bile-duct (Dc) : the
latter passes through the substance of the pancreas (P), receiving further hepatic
ducts (Dh1), and the pancreatic ducts (P1) ; at Dcl it becomes free from the
pancreas, and passes back to open into the duodenum (Du) at Dc2 ; Lhp,
duodeno-hepatic omentum ; M, stomach ; Py, pylorus.

242

COMPARATIVE ANATOMY,

munication with the intestine by means of one or more ducts
(ductus choledochus) 1 (Fig. 197, DC).

A gall-bladder (vesica fellea) may or may not be present, and
in the former case it is connected with the system of hepatic
ducts by means of a cystic duct (comp. Fig. 197, G, Dcy, Dh)

FIG. 198. — VISCERA OF Lacerta agilis, in situ.

Oe, oesophagus ; M, stomach ; MD, small intestine ; ED, large intestine ; L, liver ;
GB, gall-bladder ; Pn, pancreas ; SI, urinary bladder ; Lg, Lgl, the two lungs,
with their network of vessels ; H, heart ; Ci, postcaval ; Tr, trachea.

The liver of the Anamnia (e.g. Ganoids and Perennibranchi-
ates) is, as a rule, relatively larger than that of the Amniota.
Carnivorous (fat-eating) animals generally possess a larger liver
than Herbivores.

1 The single-lobed liver of the Lamprey undergoes a histological retrogression
(fatty metamorphosis) after transformation. The tubuli disappear, and the cells
become filled with fat, and the gall-bladder and bile-duct become atrophied.

PANCREAS. 243

In Mammals the liver may always be derived from a ground-
form consisting of two lobes, but in most cases it becomes further
subdivided, so that in some cases there may be as many as six or
seven lobes (e.g. Dog, Weasel). The right primary lobe is always
the longest, and in it the gall-bladder, when present, lies partially
embedded.

Pancreas.

As already mentioned, this organ also arises from the proximal
portion of the small intestine, and comes into close relation with
the liver. Its point of origin from the intestine corresponds to the
aperture of the pancreatic duct, which penetrates the entire
organ.

With the exception of certain Fishes (e.g. Cyclostomi and
many Teleostei) and Perennibranchiates (Siren and Proteus), a
pancreas is always present in Vertebrates. Varying much in form
and size, it early gives rise to a band-shaped or more or less
lobulated organ, usually lying in the fold of the duodenum. Its
duct frequently becomes united with that of the liver (Fig. 197, P1,
Dcl, Z)c2), or there may be several independent ducts opening into
the intestine (e.g. Birds, Crocodilia, Emydse, and some Mammalia).

BIBLIOGRAPHY.

AYERS, H. — Beitrage zur Anat. u. Physiol. der Dipnoer. Jenaische Zeitschr. f.

Naturwissenschaft. Bd. XVIII. (N.F. Bd. XL), 1885.
BATTME. — Odontologische Forschungen, I. Leipzig, 1882.
BORN, G. — Ueber die Derivate der embryonalen Schlundbogen, &c. Arch. f. inikr.

Anat. Bd, XXII. 1883.

CATTANEO, G. — Istologia e sviluppo dell' apparato gastrico degli uecelli. Milano, 1884.
DOBSON, G. E. — On Peyer's Patches. Journ. Anat. 1884.
DOHRN, A. — Studien zur Urgeschichte des Wirbelthierkorpers. MUM. aus d. zool.

Station zu Neapel. Bd. V. Heft 1. Bd. VI. Heft 1, 1884. (These include an

account of the thyroid gland of Elasmobranchs, Petromyzon, Amphioxus, and

Tunicata. )
EDINGER, L. — Ueber die Schleimhaut des Fischdarmes, &c. Arch. f. mikr, Anat.

Bd. XIII. 1877.
FISCHELIS, PH. — Beitrage zur Kenntniss der Glandula thyreoidca u. Glandula thymus.

Arch. f. mikr. Anat. Bd. XXV. 1885.
FLOWER, W. H. — Lectures on Odontology. Med. Times and Gazette, 1871. Lectures

on the Comparative Anatomy of the Organs of Digestion of the Mammalia. Ibid.

Feb. to Dec., 1872.

FORBES, W. A. — On theBursa Fdbricii in Birds. Proc. Zool. Soc. 1875.
GADOW, H. — Versuch einer vergl. Anatomie des Verdauungssystemes der Vogel.

Jenaische Zeitschr. Bd. XIII. N.F. VI.
HENSEL, R. — Ueber Homologieen und Varianten in den Zahnformen einiger Sauge-

thiere. Morphol. Jahrb. Bd. V. 1879.
HERTWIG, 0. — Ueber das Zahnsystem der Amphibien und seine Bedeutungfilr die

Genese des STcelcts der Mundhohle. ArchTf. mikr. Anat. Bd. XI. 1874.
His, W. — Anatomie menschlicher Embryonen. Leipzig, 1880-1885.
HUXLEY, T. H. — Tegumentary Organs. Encycl. of Anat. and Physiol. Vol. V. (See

also Quart. Journ. Micros. Science, 1853.)
LEYDIG, T. — Ueber die Kopfdrusen einheimischcr Ophidier. Arch. f. mikr. Anat.

Bd. IX. 1873.

B 2

244 COMPAKATIVE ANATOMY.

LUDWIG, FERDINAND, Konigl. Prinz v. Bayern. — Zur Anatomie der Zunge.

Miinclien, 1884. Ueber Endorgane der sensiblen Nerven in der Zunge der

Spechte. Sitzungb. d. k. bayr. Acad. d. Wiss. 1884.
MAURER, F. — Schilddruse und Thymus der Teleostier. Morphol. Jahrb. Bd. XI.

1885.
MINOT, CH. S. — Studies on the Tongue. Anniversary Memoirs of the Boston Society

of Natural History. Boston, 1880.
NUNN, EMILY A. — On the Development of the Enamel of the Teeth of Vertebrates.

Proc. Roy. Soc. 1882.

OWEN, K.— Odontography. (With Atlas.) 1840-45.
PARKER, T. JEFFERY.— On the Intestinal Spiral Valve in the Genus Raia. Trans.

Zool. Soc. Vol. XI. 1880.
EATHKE, H. — Zur Anatomie der Fische (two Papers). Arch. f. Anat. und Physiol.

1837.
KEICHEL, P. — Beitrag zur Morphologic der Mundhohlendriisen der Wirbelthierc .

Morphol. Jahrb. Bd. VIII. 1882.
SCHAFER, E. A. — On the Part played by Amceboid Cells in Intestinal Absorption.

Internal. Journ. of Anat. and Hist. Vol. II. Part I. 1885.
STIEDA, L. — Unters. uber die Glandula thymus, thyreoidea und carotica. Leipzig,

1881.
TOMES, CH. S. — Manual of Dental Anatomy, Human and Comparative. London,

2nd ed. 1882. On the Development of Teeth. Quart. Journ. Micros. Science,

Vol. XVI. 1876, and Phil. Trans. 1875-76.
WATNEY, H. — The Minute Anatomy of the Thymus. Phil. Trans. Roy. Soc. p. Ill,

1882.
WIEDERSHEIM, K. — Die feineren Structurverhdltnisse der Drusen im Muskelmagen

der Vogel. Arch. f. mikr. Anat. Bd. VIII. 1872. Ueber die mechan. Aufnahme

der Nahrungsmittel in der Darmschleimhaut. Freiburger Festschrift zur 56
Versammlung deutscher Naturforscher und Aerzte, 1883.
WOLFLER, A. — Ueber die Entioicklung der Schilddruse. Berlin, 1880.

G. ORGANS OF RESPIRATION.

THE respiratory organs are closely connected with the ali-
mentary canal, both in position and development, and are of
two kinds, gills and lungs. The former, as the phylogenetically
older organs, are adapted for aquatic respiration, and are con-
nected with the pharynx in the region of the visceral or branchial
arches : the latter always arise as sac-like outgrowths of the primi-
tive oesophagus, which grow backwards so as to lie within the
body-cavity.

Both gills and lungs may be developed in the same individual,
but with few exceptions (Dipnoi, and possibly Siren amongst
the Perennibranchiata) they are not functional at the same time.1
Which of these are to serve as respiratory organs depends upon
the condition of the circulation, for respiration can only take place
where venous trunks, — the blood in which is laden with car-
bonic acid, — come into close relation with the surrounding
medium; the carbonic acid is then given off, oxygen being
taken up in its place. The venous blood is thus converted into
arterial blood, which is again distributed over the body.

As long as these condition for the oxidation of the blood are
not fulfilled, we cannot speak of a respiratory organ. Thus the
air-bladder of Fishes, which arises in a very similar manner to a
lung, that is, as an outgrowth of the fore-part of the alimentary
tract, has in no period of life the arrangement of the blood-vessels
described above: it receives arterial blood only, from the aorta,
and venous blood passes from it*f it is therefore morpho-
logically, but not physiologically, a lung.

I. GILLS.

The gills arise as a series of laterally-arranged outgrowths of
the throat lying one behind the other, and, in the course of their
development, they become open to the exterior. A passage is thus
formed for the water entering by the month, and in order that

1 Comp. air-bladder of Lepidosteus, p. 257.

246 COMPARATIVE ANATOMY.

there may be every means for its contained oxygen to become
absorbed, leaf-like or thread-like vascular processes, or gills,
become developed in the region of each gill-cleft. These are
internal or external according to their position.1

While Fishes possess gills throughout life, this is only the
case in a small section of the Amphibia, viz. in the Perenni-
branchiata; all the others simply pass through a gilled stage,
and later come to breathe by means of lungs, so that the study of
this one group of animals furnishes us with an excellent representa-
tion of the course of phylogenetic development through which all
the higher Vertebrates must have passed.

The best proof of this, as well as of the important meaning of
the branchial apparatus of animals in general, lies in the appear-
ance of gill-clefts and gill-arches throughout the entire series
of the Amniota up to Man, that is, in forms in which they no
longer possess a respiratory function. They are thus repeated
ontogenetically, but have undergone a change of function,
coming into relation with the auditory organ and tongue, as
already described in connection with the skull and auditory organ
(see pp. 78, 80, 84, and 198).

Amphioxus. — The numerous (80, 100, or more) gill-clefts of
Amphioxus, which are supported by elastic rods, extend back-
wards nearly to the middle of the body. At first they open freely
to the exterior, but in a later period of development they become
covered by two lateral folds of the skin.

The water passing through the gill-slits is conducted backwards
by means of the peribranchial chamber thus formed, and passes
out through an aperture, the atrial pore, which lies somewhat
behind the middle of the body (Fig. 199, c).

This extension of the branchial apparatus over such a large
section of the body, which points back to a very primitive condition,
becomes considerably limited even in the Cyclostomi.

Cyclostomi. — In Ammoccetes the osso'phagus is continued
directly backwards from the branchial cavity (Fig. 200, A), and at
the entrance of the latter there is a muscular fold covered by the
mucous membrane, the veltfm (Fig. 201, V).

The seven2 gill-sacs provided with leaf-like folds of mucous
membrane which are present in Ammoccetes, persist in Petro~
myzon ; but, with the formation of a suctorial mouth, the portion
of the oesophagus into which they open (respiratory tube) be-
comes closed posteriorly, and the oesophagus apparently grows

1 External gills persist after hatching as functional respiratory organs only in
Protopterus and the Amphibia, and even in the latter they are often soon replaced
by internal branchiae (comp. pp. 250 and 251).

2 In Ammoccetes there are primitively eight gill-clefts ; but the first pair, which
give rise in most Elasmobranchs and many Ganoids to the spiracle, and in Amniota
to the tympano-Eustachian passage, does not perforate the skin. V

GILLS.

247

forwards above the latter, and joins the mouth-cavity at the velum.
The anterior part of the oesophagus of the adult is thus a neo-
morph : it is at first solid, but becomes hollowed secondarily.

eg

FIG. 199. — Amphioxiis lanceolcdus, x

(From Gegenbaur, after Quatrefages.)

a, aperture of month, surrounded by cirrhi ; b, anus ; c, atrial pore ; d, branchial sac ;
e, stomach-like section of the intestine ; /, csecal process (? liver) ; g, intestine ;
7i, general body-cavity ; i, notochord, and under it the aorta ; k, aortic
arches ; I, aortic heart ; m, swellings on the branchial arteries ; n, contractile
postcaval vein ; 0, contractile portal vein.

Thus two canals pass backwards from the mouth, a ventral
branchial or respiratory tube, and a dorsal oesophagus
(Fig. 200, B).

248

COMPARATIVE ANATOMY.

InPetromyzon the individual branchial sacs open freely to the
exterior; but in Myxine this original condition becomes modified

FIG. 200.— DIAGRAM OF A LONGITUDINAL SECTION THROUGH THE HEAD OF
Ammoccetes (A) AND Petromyzon (B).

Ch Ji

FIG. 201. — LONGITUDINAL SECTION THROUGH THE HEAD OF Ammoat'les.

V, velum ; P, papillae of mucous membrane ; K,K,K, three anterior gills ; Th, thyroid
gland (hypobranchial furrow) ; N, nasal sac ; *, communication between the
ventricle of the olfactory lobe and that of the prosencephalon ; JEp, epiphysis ;
Jnf, infundibulum ; HIT, metencephalon ; ML, medulla oblongata ; b, c,
ventricles of the mid- and hind-brain ; o, subdural cavity ; Ch, notochord ;
R, spinal cord.

by the external gill-passages- growing out into long tubes, which
unite to form a common duct on either side ; this opens far behind
the branchial apparatus on the ventral side of the body.

Fishes. — From the Elasmobranchii onwards the gills come
into close relation with the skeletal part of the visceral arches, and
in these Fishes they consist of closely-approximated transverse
lamina? (Fig. 202), which are firmly attached to both sides of the
septa which separate the individual gill-sacs from one another, so
that each septum bears gill-laminae on both its anterior and pos-
terior surface. The gill-sacs, of which there are usually five (six
or seven in Notidanus), usually open separately to the exterior.

GILLS.

249

In the Holocephali, however, an opercular membrane is present,
and traces of a similar structure are seen in Chlamydoselache.

HP

FIG. 202. — HORIZONTAL SECTION THROUGH THE VENTRAL SIDE OF THE HEAD OF
A SELACHIAN. (Semidiagrammatic. ) The floor of the mouth is exposed.

KM, muscles of jaw ; Z, tongue ; Hy, hyoid arch cut through, behind which are
seen the five branchial arches, also cut through ; BM, mucous membrane of the
mouth ; Oe, oesophagus ; S, S, pectoral arch, cut through ; LH, body-cavity ; KD,
body-walls ; the arrows indicate the external apertures of the five branchial sacs.

VIG. 203. — HORIZONTAL SECTION THROUGH THE VENTRAL SIDE OF THE HEAD OF
Siluris glanis. (Semidiagrammatic. )

T, T, tentacles ; Zp, Zpl, teeth of the lower jaw ; BM, mucous membrane of the
floor of the mouth ; Oe, oesophagus ; KM, muscles of jaw ; KD, gill-cover, behind
which (at the arrow) the common branchial cavity opens.

sacs.

In Ganoids and Teleosteans there are no longer chambered gill-
The septa on which the gill-laminae are borne become greatly

250 COMPARATIVE ANATOMY.

reduced, so that the apices of the latter extend freely outwards ; the
whole branchial region is moreover covered over by the operculum
and branchiostegal membrane (comp. pp. 66 and 69), and thus
the gill-slits open into a common branchial chamber, which com-
municates with the exterior by a single slit-like aperture on either
side (Fig. 203).

Asa rule, the Teleostei possess only four gill-bearing visceral arches,1
and this holds good for all Ganoids. A rudimentary gill orpseudobranch
is present on both mandibular and hyoid arches of most Elasmobranchs, on the
mondibular arch of all Cartilaginous Ganoids (spiracular gills), and on the
hyoid of Teleostei, and a more complete hyoidean gill is seen in Acipenser and
Lepidosteus : this indicates that all these Fishes formerly possessed a more
extensive branchial apparatus than at present.

In many Teleostei certain accessory structures are developed in the region
of the branchial chamber by a modification of the branchial arches or cavity.
These serve to retain the water, and thus the Fish is able to live for some time
out of the water (Anabas, Saccobranchus, Heterobranchus, Clarias, &c.).

External gills are met with in young stages of Elasmobranchii, Holo-
cephali, Polypterus, and Cobitis ; these possibly have a nutritive function
in connection with the absorption of the yolk.

Fishes breathe by taking in water through the mouth, and, by
the contraction of the latter, forcing it out again through the gill-
slits.2 In this process the gill-arches rise and fall, separating
from one another during inspiration, and approximating during
expiration.

Dipnoi. — These, as their name implies, breathe both by gills
and lungs.

The branchial apparatus deserves particular attention, inas-
much as, in Protopterus (Fig. 54, K\ besides the internal
gills situated on the branchial arches, there are also three ex-
( ternal gills. These are situated on the posterior and upper
border of the pectoral arch, to which they are fixed by connective-
tissue and vessels, which latter pass to them from the third and
fourth arterial arches.

As in Ganoids and Teleosteans there is only a single external
branchial opening on either side, covered by a somewhat rudimentary
operculum.

Amphibia. — In all Urodele larvae as well as in the Perenni-
branchiata there are three gill-tufts, lying one over the other, and
decreasing in size from the dorsal to the ventral side ; these extend
backwards, projecting freely to the exterior, and are composed of
connective-tissue, unsupported by cartilage. They either have
the form of tufts, or they may be delicately branched, showing the
most varied arrangements for increasing the respiratory surface.

1 They may be reduced to three, or to two, and even these may become more or
less rudimentary.

2 In Petromyzon, when holding on by its suctorial mouth, inspiration as well
as expiration takes place through the gill-apertures.

AIK-BL ADDER. 251

These external gills are acted on by a complicated system of
muscles, and are covered by ciliated epithelium, which serves to
keep up a continual current in the surrounding medium.

In the Axolotl and in the larvae of Salamanders there are four, and in
Menobranchus and Proteus only two gill-clefts perforating the pharynx. The
former thus show a more primitive condition, while in the latter these
structures have become reduced. There is always only a single external
opening coArered over by an opercular-like fold of skin.

In the Derotremata the gills disappear entirely, but the gill-aperture
between the third and fourth branchial arches persists.

The external gills present at first in Anura soon disappear,
and their place is taken by internal gills. By the growth of the
opercular folds, the external respiratory aperture of either side
becomes gradually reduced in size, and the two branchial chambers
come eventually to open by a single aperture, which is situated
either in the median ventral line, or laterally.

In the larvae of Notodelphys and of Caecilia compressicauda bran-
chial vesicles are developed, covered over by a vascular respiratory network ;
in the former these are bell-shaped, and in the latter they have an irregular
sac-like form. In the embryo of Epicrium glutinosum, a feather-like and
highly- vascular gill-tuft arises on either side ; these are of unequal length, and
they move continually backwards and forwards in the egg-albumen. In certain
Batrachia, the broad and richly vascular swimming-tail lying against the
egg-membrane may serve as a larval respiratory organ.

Thus we arrive at the result that the gills of Vertebrates may
be divided into four groups, which show no direct connection with
one another. The first kind is seen inAmphioxus, the second
in Cyclostomes, the third in the adults of other Fishes, and
the fourth in Amphibians.

II. AIR-BLADDER AISID LUNGS.

1. THE AIR-BLADDER.

As has already been mentioned, the lungs and air-bladder are
developed in a similar manner, and differ only from one another in
the fact that the lungs always arise from the ventral side of the
primary ossophagus, while this is an exceptional case as regards ) V
the air-bladder (Polypterus, Erythrina) ; that organ is usually
formed on the dorsal side. The exact point of origin of the
air-bladder from the oesophagus varies, and its duct (ductus
pneumaticus) may either remain open throughout life, as in
all Ganoids and some Teleosteans (Physostomi), or it may
later become reduced to a solid fibrous cord, or even entirely
obliterated, as in other Teleostei (Physoclisti). In the latter
case there is no communication between the air bladder and the

252

COMPARATIVE ANATOMY.

external air, and it is probable that the contained gas is given off
from the walls of the air-bladder itself.

The air-bladder always lies above the peritoneum on the dorsal
side of the body-cavity, between the vertebral column, aorta, and
kidneys on the one hand, and the alimentary canal on the other :
it is invested by the peritoneum on the ventral side only. It is
more or less sac-shaped in form, and is only exceptionally paired
(Polypterus) ; it usually extends along the whole length of the
body-cavity, and has walls composed of connective, elastic, and
muscular tissue. In some Teleostei it is transversely constricted
so as to form several successive divisions ; in other cases it may

five rise to a more or less numerous series of csecal processes,
ts internal surface may be either smooth or spongy, owing to
the formation of a meshwork of trabeculse, the structure of which
reminds one of the lungs of Dipnoi and Amphibia.

Attention has already been directed to the relations which often
exist between the air-bladder and 'the auditory organ (see p. 207).

2. THE LUNGS.

The further development of the primitive lung-sacs is essen-
tially similar to that of a branched gland. They gradually increase
in size, and the part which connects them with the oesophagus
becomes drawn out into a tube, the windpipe or trachea; this
bifurcates to form two bronchi, one of which goes to either lung

Pit

PD

PD

FIG. 204. — A, B, C, DIAGRAMS SHOWING THE MODE or DEVELOPMENT OF THE

LUNGS.

PD, primitive alimentary tube ; S, Sl, the lung-sacs, which are at first unpaired ;
t, trachea ; b, bronchus.

(Fig. 204, S, Sl, t, V). In their further growth, the bronchi branch
out into finer and finer tubes, and finally end iu small vesicles or
infundibula, which are made up of a number of alveoli, and
are surrounded by blood capillaries, through the thin walls of which
the interchange of respiratory gases takes place (comp. Fig. 205).

In the course of further development, annular cartilages become
developed in this system of tubes : the most anterior of these, that

AIR-PASSAGES. 253

is, those lying round the glottis or entrance to the trachea (which
are phylogenetically the oldest cartilages of the whole
apparatus), become differentiated to form a special apparatus, the
larynx ; this is regulated by muscles, and has to do with the
production of the voice.

FIG. 205.— DIAGRAM ILLUSTRATING THE PHYLOGENETIC DEVELOPMENT OF THE
LUNGS ; A GRADUAL INCREASE OF THE RESPIRATORY SURFACE is SEEN IN

PASSING FROM A TO D.

The trachea, bronchi, and larynx thus constitute a kind of
hollow skeleton for the whole respiratory apparatus, and, as they
are formed secondarily, we should naturally expect them to be most
developed in the higher types.

Air-Passages.

Amphibia. — The first traces of cartilaginous supports to the
glottis are seen inUrodeles, there being no skeletal elements in
this region in Dipnoans (Protopterus). At the same time, dilator
and constrictor muscles appear round the glottis.

In Dipnoi, Salamandridae, and An ur a, there is no proper
trachea, but only a short laryngo-tracheal chamber leading
from the larynx to the lungs ; in the two last-mentioned groups
this is supported by cartilages. A definite trachea is, however,
present in Siren, Amphiuma, and the Gymnophiona ; it
reaches a length of 4 to 5 or more centimetres, and its wall is
strengthened by a series of small irregular cartilages, which usually
tend to unite into bands : only in the Gymnophiona, however, do
these bands' begin to take on the form of half-rings, and to surround
the trachea more or less completely.

In Anura a considerable advance is noticeable, as a highly
differentiated larynx is present in them. This is regulated by a
well-developed series of muscles, and is provided with vocal cords,
the sound produced by which is often intensified by the presence
of vocal sacs developed from the floor of the mouth. The laryngo-
tracheal chamber of Rana esculenta lies between the posterior
cornua of the hyoid (thyro-hyals), and is supported by a thin car-
tilage on either side of the glottis (Fig. 206, Co), as well as by a
ring-shaped cartilage, from which delicate processes pass backwards
to the roots of the lungs (Fig. 206, Cll — C74). The former correspond

COMPARATIVE ANATOMY.

to the arytenoids, and the latter to the cricoid cartilage of
higher Vertebrates. These are all firmly united to one another by

ce

ex.1

-Up

FIG. 206. — CARTILAGINOUS SKELETON OF THE LARYNGO-TRACHEAL CHAMBER OF
Eana esculenta. (A, from above ; B, from the side.)

Ca, Ca, arytenoid cartilage ; C.I.1 to (7.Z.4, cricoid cartilage ; Sp, process of the
latter ; P, plate-like broadening out of the ventral part of the cricoid ; SR,
glottis ; ***, three tooth-like prominences of the arytenoids.

connective-tissue, the vocal cords being situated on the inner surfaces
of the arytenoids.

Reptiles. — The larynx of Reptiles is supported by cartilaginous
elements comparable to those of Anura, there being two sets of

D

FIG. 207. — LARYNX OF Phyllodactylus europceus. (A, skeleton, and B, musculature

of laiynx.)

Ar, arytenoids ; Cc, cricoid ; S, anterior median process of cricoid ; Slt sphincter ;
D, dilator ; T, trachea ; Oe, basi-hyal.

cartilages, a paired arytenoid, and a ring-shaped cricoid (Fig.
207, Ar, Cc).

AIR-PASSAGES. 255

No considerable advance in structure is, however, seen ; there is
even a reduction noticeable as regards the musculature, for as a
rule, only a single dilator and constrictor are present (Fig. 207,
D, $!), instead of several, as in the Frog.

One point, however, must be specially noticed, viz., the close
connection which obtains between the larynx and the hyoidean
apparatus — more particularly the dorsal surface of the basi-hyal.
In Crocodiles and Chelonians, for instance, the larynx is
firmly embedded in a shallow depression of the latter, and it seems
probable that the thyroid cartilage of Mammals has been
derived from a part of the body of the hyoid.

A well-developed trachea, always supported by enclosing
cartilages, is present in all Reptiles, but the cartilages are not in
all cases fused together to form complete rings. The walls of the
bronchi are also usually provided with cartilaginous supports.

Birds. — In Birds there are two larynges, an upper and a
lower. The former lies in the usual position behind the tongue on
the floor of the pharynx, and is plainly homologous with that of
other Vertebrates, though it is incapable of producing sound. \
This is owing to the fact that both its skeleton and muscles are/
obviously undergoing a retrogressive metamorphosis.

The lower larynx, or syrinx, is of much greater interest ; it is
usually situated at the junction of the trachea and bronchi, or more
seldom at the lower end of the trachea alone or on the bronchi
alone. It functions as the organ of voice, and appears first in,
and is restricted to, Birds, no traces of a syrinx, which might be
expected, being found in Reptiles. In the most usual form, or
broncho-tracheal syrinx, there is a moveable connection
between the most anterior bronchial rings, with which a compli-
cated system of muscles is connected ; these, by their contraction,
cause a stretching or relaxing of certain vibratory membranes
(membrana tympaniformis interna and externa). The
lower specially modified end of the trachea also plays an impor-
tant part as a " tympanum," which attains a relatively enormous
development in some Water-Birds (e.g., the male Duck), where it
gives rise to a bony vesicle which serves as a resonance cavity.

The length of the trachea in Birds varies greatly, and its complete cartila-
ginous rings show a great tendency to become ossified. In some cases (e.g.
the Swan and Crane) it extends into the hollow keel of the sternum, where it
becomes more or less coiled, and then again passes out close to its point of
entrance, and enters the body-cavity. In certain representatives of the famil}T
of Sturnidseit extends between the skin and the muscles of the thorax, and
there gives rise to numerous spiral coils.

Mammals. — The larynx of Mammals is distinguished from
that of all other Vertebrates by the following characters : there is
always a very marked differentiation of the muscles, the
constrictors always exceeding the dilators in number; and an
epiglottis and a thyroid cartilage are constantly present.

256

COMPARATIVE ANATOMY.
A

B

Tr

FIG. 208. — LARYNGES OF VARIOUS MAMMALS.

A, larynx of Deer, seen from the left side ; B, longitudinal section through the
larynx of the Fox ; C, larynx of the Howling Monkey (Mycctes ursinus), from
the left side ; D, larynx of Chimpanzee (Simia troglodytes), from the ventral
side.

Tr, trachea ; Ctr, cartilaginous rings of the trachea ; S, mucous membrane of the
trachea and tongue ; Or, ventral, and Or1, dorsal plate of the cricoid ; Ct, Ctl,
thyroid cartilage ; oh, uh, anterior and posterior cornua of the latter ; Ca, ary-
tenoid cartilage ; pm, processus muscularis of the latter ; Ep, epiglottis ; H,
body of hyoid ; h, lesser, hl, greater cornua of the hyoid ; Lt, crico-thyroid
ligament ; Mth, thyro-hyoid ligament ; M, laryngeal pouch, which shows an
enlargement at t ; 1, 2, 3, the three resonance-cavities of Simia troglodytes ; mu,
submucous tissue with muscles ; M.ge, genioglossus muscle ; Z, tongue.

LUNGS. 257

The epiglottis serves as a protection to the aperture of the
glottis, and its form varies much ; occasionally it may undergo
degeneration. The thyroid cartilage, which, as already mentioned
(see p. 255), is probably to be derived from the visceral skeleton,
is originally paired (Monotremes). In higher types, it forms a
cartilaginous capsule which encloses the cricoid and arytenoids x
on the ventral side. The thyroid serves as a point of origin and
insertion for important muscles which stretch the vocal cords.

The vocal cords extend between the thyroid and the ary-
tenoids, and the mucous membrane above them becomes in-
voluted to form the laryngeal pouches. In Anthropoids, and
certain other Monkeys, these may reach such a large size that they
serve as resonance cavities, and come to lie partially within
the body of the hyoid, which is swollen to form a large bony
chamber (Fig. 208, D, 1, 2, 3}?

The folds of mucous membrane bounding the pouches of the
larynx anteriorly are spoken of as false vocal cords ; these are not
present in all Mammals.

An interesting adaptation for the method of lactation is seen in the larynx
of Marsupial embryos, in which it becomes elongated so as to extend up-
wards into the internal nostrils, where it is firmly embraced by the soft palate.
Thus respiration can go on freely while the milk passes down the oesophagus on
either side of the larynx. In Cetacea (e.g. Phocrena) a similar arrangement is
observable, and is here adapted for the aquatic life of the animal. In many
other Mammals the epiglottis is embraced by the soft palate, so that feeding and
respiration can go on without interfering with one another.

The Lungs in a more Restricted Sense.

Dipnoi. — In Ceratodus, the lungs form a wide unpaired
sac, without any trace of a dividing septum, while in other Dipnoi
they are distinctly paired posteriorly, though single anteriorly.

They extend through the whole length of the body-cavity, and
are covered by the peritoneum only on the ventral surface : the
mucous membrane lining them forms bands and networks similar
to those seen in the air-bladders of many Fishes (e.g. Lepidosteus).3

Amphibia. — The lungs of Menobranchus and Proteus
remain at a lower stage of development than those of the Dipnoi,

1 The cricoid may be complete or incomplete ventrally, and its dorsal portion
usually becomes raised to form a broad plate which articulates with the arytenoids
(Fig. 208, Or, Crl, Co). Each of the latter often gives rise to an outgrowth an-
teriorly, which, becoming separated from it, forms a cartilage of Santorini.
A further independent cartilage (cartilage of Wrisberg) is sometimes present
in the aryepiglottidean fold.

2 In the Gorilla the resonance vesicle extends above the sterno-cleido-mastoideus,
and reaches backwards as far as the shoulder and pectoralis major.

3 It is worthy of remark that Lepidosteus, like many other Fishes, comes to
the surface and appears to swallow air, but it cannot be stated whether its air-bladder
has any important respiratory function until the relations of the blood-vessels are
known.

S

258 COMPARATIVE ANATOMY.

inasmuch as their internal surface is perfectly smooth, and has,
therefore, a much smaller superficial extent. They consist of two
delicate elongated sacs, of unequal length, and constricted in their
middle ; in Proteus they extend much further backwards than in
Menobranchus. A difference in length between the two lungs is
seen also in other Amphibia, such as Amphiuma and Siren
lacertina, in which the two cylindrical lungs lie near together,
close to the aorta. Their internal surface is raised into a net-
work, corresponding with the distribution of the blood-vessels, the
meshes of which are much finer in Amphiuma, and still more so in
Menopoma, than in Siren.

In Salamanders the lungs as a rule are equal in size, and have
the form of cylindrical tubes extending backwards as far as
the end of the stomach ; their internal surface is more or less
smooth. The lungs of Gymnophiona are similar to those of Sala-
manders, but the right alone is fully developed, and this shows in
its interior a complicated trabecular network : the left is only a few
millimetres long. A similar abortion of one lung is also seen in
Snakes, and in both cases has to do with the elongated form of the
body (cp. the note below). The wide, elliptical lungs of Anura
are quite symmetrical. Their internal surface, which is partly
lined by ciliated epithelium, is raised up into a rich respiratory
network of trabecula3, and numerous smooth muscular fibres are
present in their walls.

Reptiles. — In Reptiles, as in all other air-breathing Verte-
brates, the form of the lungs is to a great extent regulated by that
of the body. In the higher types, such as the Chelonia and
C roc od ilia, their structure is much more complicated than in
Amphibia ; this complication finds expression in a very con-
siderable enlargement of the respiratory surface. With the ex-
ception of the thin- walled lungs of Lizards, which retain a very
primitive condition, we no longer meet with a large central cavity,
but the organ becomes penetrated by a branched system of
bronchi, giving it the character of a tubular and sponge -like mesh-
work.1 Thus the phylo^enetic stages of development are here
again repeated ontogenetically.

The kings of the Chameleon present a very curious modification. The
anterior portion is much more compact and spongy than the posterior, which
grows out into numerous sac-like processes, some of which reach as far back
as the pelvic region ; their form is very variable, being spindle-shaped, club-
shaped, or lobulated, and their walls are very thin ; they extend in amongst the
viscera wherever there is room. If these processes have any respiratory function,
it is at most only a very slight one. An indication of a similar arrangement

1 The lung of Snakes exhibits an intermediate form, for, in spite of the finely-
meshed tissue arising from the periphery, it still retains a narrow central cavity. As
already mentioned, only the right lung is as a rule fully developed in Snakes and
Amphisbsenians, owing to the elongated form of the body, while the left remains
in a rudimentary condition, or even disappears entirely.

LUNGS. 259

is seen in the lungs of Testudo, in which a single thin-walled process extends
backwards to the" pel vie region. These processes seeih to foreshadow a con-
dition which reaches its highest development in Birds.

A uniform ground-plan is to be observed in the arrangement
of the mtra-pulmonary bronchial system through the whole series
of the Amniota, from Crocodiles onwards. A continuation of
the bronchus, which is almost straight, always passes through the
lung to its posterior end. This may be called the main bronchus;
from it a series of lateral bronchi arise. The important and
typical relations of the latter to the main trunk of the pulmonary
artery and vein in Mammals, will be described later (p. 263) : it is
not yet known whether a similar arrangement obtains in Chelonians,
Crocodiles, and Birds.

Birds. — The respiratory apparatus of Birds presents so many
remarkable peculiarities, both in the structure of the lungs and the
presence of air-sacs, that it must be considered in some detail.1

LUNGS AND AIR-SACS OF BIRDS.

When the ventral body-walls of a Bird are removed, the heart, stomach,
liver, and intestines, are seen pressed towards the mid-line, and on either side
of them a tightly-stretched fascia, the oblique septum, is observable, which
shuts them off from a paired lateral sub-pulmonary chamber (Fig. 209,
D.th.a). Other chambers are situated in the anterior thoracic region, ventral
to the lungs, which latter lie close against the vertebral column and the heads
of the ribs, by which they are impressed : others, again, are seen in the region
of the heart and in the posterior part of the abdominal cavity.

These chambers are occupied by the air-sacs, the development and
physiological function of which will be described later on.

The most posterior chamber on either side encloses an abdominal
(posterior) air-sac (Fig. 209, r.Abd.S., l.Abd.S). In Apteryx, this is
completely closed in by the oblique septum, but in other Birds it gives rise to
a large diverticulum which extends between the corresponding kidney and the
body-walls, and even into the latter, as well as between the pelvic muscles.
Its volume is naturally dependent upon the state of distension of the viscera
at the time.

In front of this there are two air-sacs lying above and externally to the
oblique septum, and constituting the main part of the sub-pulmonary chamber ;
these may be called the anterior and posterior intermediate sacs (Fig.
209, f, ft)- A-teaasverse dividing- wall (s1) lies between these two, at the
level of the cceliac artery, and a second septum (*) shuts off the anterior
intermediate sac from the one lying in front of it, to be described presently.
The posterior intermediate air-sac presents the simplest and most constant
relations, and never communicates with any of the neighbouring chambers, as
is often the case with the anterior intermediate.

A pair ofprebronchial air sacs lies on either side of the oesophagus
above the bronchus, anterior to the hilum of the lung (Fig. 209, (7, (7), and
below this a sub-bronchial sac is situated, which is separated behind from
the anterior intermediate sac by a septum (Fig. 209, *). This is usually

1 For Figs. 209 and 210, as well as for many of the details in the above
description, we are indebted to Professor H. Strasser of Freiburg in Baden, who has
kindly allowed us to make use of the manuscript of a paper which is not yet
published. The terms used are those of Professor Huxley.

s 2

260

COMPARATIVE ANATOMY.

T
0/-J

Ifod

r.Abd.S.

FIG. 209. — ABDOMINAL VISCERA AND AIR-SACS OF A DUCK AFTER THE REMOVAL,
OF THE VENTRAL BODY- WALL. (From an original drawing by H. Strasser.)

T, trachea ; H, heart, enclosed within the pericardium ; rL, IL, right and left lobes
of liver ; Ish, suspensory (falciform) ligament, and led, Ics, right and left coronary
ligament of the liver ; Z>, intestine ; P, pectoralis major ; pa, pv, pectoral artery
and vein ; S, subclavius muscle ; Cd, coracoid ; F, furcula ; Ifcd, coraco-furcular
ligament ; Lg, Lgl, lung ; r.Abd.S, l.Abd.S, right and left abdominal (posterior)
air-sac; D.th.a, the fibrous oblique septum ; ft, posterior intermediate air-sac ;
f, anterior intermediate air-sac ; si, s1, partition-walls between these sacs ;
s, s, partition-walls between the anterior intermediate air-sacs and the unpaired
sub-bronchial sac, lying in the anterior part of the body-cavity ; v, portion of
anterior wall of latter ; p, axillary sac lying between the coracoid, scapula, and
the anterior ribs, and communicating with the sub-bronchial air-sac ; (7, C,
prebronchial sacs ; *, point of entrance of the bronchi into the lung ; Ap,
pulmonary artery ; Aa and Va, innominate artery and vein with their branches.

LUNGS.

261

FIG. 210. — LEFT LUNG OF THE DUCK, in situ. (From an original drawing by

H. Strasser.)

The main bronchus is cut open ; internally to it lies the pulmonary vein, and
externally the pulmonary artery.

Oe, oesophagus ; m.l.c, muse, longus colli ; BrWs, thoracic vertebrae; v, v, ends of
free vertebral ribs; stv, stv, sections of ribs which are connected with the
sternum ; N, kidney ; Tr, trachea ; /, first entobronchium, and c, its aperture
of communication (ostium) with the pre bronchial air-sac ; i, Q,, e, its internal,
anterior, and external branches ; Hi, He, internal and external branch of the
second entobronchium : the end of He opens' into the sub-bronchial sac at d ;
///, third entobronchium, with the aperture e for the anterior intermediate air-
sac ; IV, fourth entobronchium ; au, opening of the main bronchus into the
a'bdominal sac ; b, opening of the outer lateral branch of the mesobronchium
into the posterior intermediate sac ; b', second ostium of the latter, more
towards the middle line (present in Passeres). The boundary of the pulmonary
aponeurosis is seen along the outer edge of the lung, and the costo-pulmonary
muscles are shown extending to it from the ribs.

unpaired, the sac of either side fusing with its fellow to form an interclavi-
cular chamber, bounded by the furcula ; 1 it communicates with neighbouring

1 In some Birds (e.g. Rhea, Vulture, Adjutant) a median septum is present
separating the two sub-bronchial sacs.

262 COMPARATIVE ANATOMY.

air-cavities which lie between the pericardium and sternum, and in the axilla,
outside the body-cavity (axillary sac) (Fig. 209, p, p).

The lower surface of each lung is closely invested by a thin fibrous
membrane, the pulmonary aponeurosis,1 into which are inserted a variable
number of muscular bands (costo-pulmonary muscles). These arise from
the vertebral ribs, and are supplied by the intercostal nerves (Fig. 210).

The main bronchus (mesobronchium) runs close to the ventral
surface of the lung surrounded by the lung-parenchyma, and extends to its
posterior end, where, as a rule, it opens directly into the abdominal air-sac
(Fig. 210, au). From it a large lateral bronchus branches off, which opens
into the posterior intermediate sac by one or two (e.g. in Passeres) apertures (Fig.
210, &, &'). Besides this there are from four to six other lateral bronchi (Fig.
210, I to 2V), all of which become broadened out in a fan-like manner on the
ventral surface of the lung. These may be called entobronchia (bronchi
divergentes, Sappey) : they all arise from the anterior portion of the meso-
bronchium. The first of these radiates out anteriorly to the hilum of the lung,
and gives off internal, external, and anterior branches, one of which opens
into the prebronchial sac (Fig. 210, c). The other entobronchia give rise to
two series of branches, one of which extends inwards and backwards between
the factors of the pulmonary vein, and the other outwards between the
arterial branches. Almost without exception a large aperture or ostium is
present on the wall on the third entobronchium, communicating with the
anterior intermediate air-sac (Fig. 210, e). A branch of the second ento-
bronchium opens externally to the hilum of the lung into the sub-bronchial
sac (Fig. 210, lie, d).

The lateral bronchi considered as yet have to do with the ventral surface
of the lung only ; but besides these there are a variable number of ecto-
bronchia, arising from the dorsal side of the main bronchus posteriorly to
the entobronchia (see Fig. 210). These come off in a double longitudinal row,
those of the outer row being larger than those of the inner. They pass
dorsally to the costal face of the lung. Both ecto- and entobronchia give off
numerous bronchi of a third order, orparabronchia: the walls of these are
raised into numerous transverse net-like folds, into which the pulmonary
capillaries extend.

The following points must be noticed as regards the genesis
and function of the air-sacs.

Early in the embryonic period, delicate-walled hollow pro-
cesses, lined by pavement epithelium, arise from the pulmonary
vesicles : these grow rapidly, and soon exceed the lung proper in
size, so that they extend amongst the viscera. Their form and
extent depend entirely upon their surroundings, and they simply
consist of interstitial cavities lined by the membrane of the air-
sacs. Moreover, they are not confined to the body-cavity, but in
numerous places extend beyond it, passing in between the muscles,
beneath the skin, and even into most of the bones. The latter
are thus rendered pneumatic,2 and consequently the specific

ivity of the body is lessened, and the power of flight increased,
pneumaticity of the bones is not, however, an essential pecu-
liarity connected with flight, for in many Birds which are extremely

1 The pulmonary aponeurosis, as well as the oblique septum, is often spoken of
as a "diaphragm" (comp. p. 122).

2 This cannot of course take place until the marrow of the bones has performed
the greater part of its bone-forming function.

LUNGS. 263

good fliers (e.g. Larus, Sterna) the bones are not pneumatic.1 In
these cases, however, a compensation is effected by a more marked
development of the muscles, and the abdominal (posterior) air-sac,
which in no Birds appears to be entirely wanting, is here well
developed.

The air-sacs must be looked upon as integral parts of the respiratory
apparatus : a greater amount of air can, by their means, pass in and out during
inspiration and expiration, especially through the larger bronchi, and con-
sequently there is less necessity for the expansion of the lung-parenchyma.
The function of the prolongations of the air-sacs lying towards the outer surface
of the body consists in the giving off of watery vapour and in regulating the
heat of the body. Those which extend in between the muscles, and supplant
the connective and fatty tissue in these regions, have a further importance in
causing less power to be lost in friction.

But by far the greatest importance of the air-sacs lying towards the
periphery consists in the enlargement of the anterior thoracic region, princi-
pally that surrounded by the pectoral arch. A larger development of the
skeleton can thus take place, giving an increase of surface for muscular
attachment without any considerable increase in weight. Everything, in fact,
combines to establish an organ of flight with a large wing-surface and an
increased strength of the muscles.

Mammals. — As already mentioned, main and lateral
bronchi can be distinguished in Mammals. The pulmonary artery
crosses the main bronchus at its anterior end, and this point may
be taken as dividing the lateral bronchi into two systems — an
eparterial lying anterior (above) and a hyparterial lying
posterior to (below) the artery.

The hyparterial series is always well developed, and consists of
a double row 2 of lateral bronchi ; the eparterial system, on the
other hand, gradually becomes of much less importance, and in
certain cases is represented only by a single lateral bronchus on
either side (Fig. 211, c, b, a), and, as a rule, even the left of these
disappears, only the right remaining. This eparterial bronchus,
whether developed on one or on both sides, may change its position
on the main bronchus so as to arise from the trachea.3

In by far the greater number of Mammals then, the left epar-
terial bronchus has disappeared, while the right is retained ; this

1 The pneumaticity of the bones is not a special peculiarity of Birds : amongst
Mammals, frontal, maxillary, and sphenoidal sinuses are present in Anthropoids,
Elephants, and Marsupials for instance ; the skull of Crocodiles is also strongly
pneumatic. All these sinuses communicate with one another, and also with the tym-
panic cavity. They are in many cases developed in order to give a greater surface for
the attachment of muscles, and also to effect a saving of material and a lightening of
the skull.

2 The pulmonary artery passes backwards between the roots of the hyparterial
bronchi, while the corresponding vein runs along the ventral side of the main
bronchus (Fig. 211, A, V).

3 The left eparterial bronchus may also disappear (Hystrix), and thus the ter-
minal stage of a process begun in Birds is reached. It is difficult to find an explana-
tion for these facts ; they may possibly have something to do with the gradual
disappearance of the cervical and lumbar ribs, in connection with the shortening of
the thorax (comp. p. 51).

264

COMPARATIVE ANATOMY.

is the case, for instance, in Man. It leads to the following
conclusions.

As the anterior lobe of the right lung belongs to the eparterial
and that of the left lung to the first hyparterial bronchus, these lobes

FIG. 211. — DIAGRAM OF THE ARRANGEMENT OF THE BRONCHI IN MAMMALS.
(From the ventral side. ) .

a, a, eparterial bronchus of either side ; b, series of ventral, and c, of dorsal
hyparterial bronchi ; A and V, pulmonary artery and vein.

are evidently not homologous, the middle right lobe corresponding
much more nearly to the anterior lobe of the left side. Thus
there is a want of symmetry between the right and left
sides, the right lung retaining one element more than
the left.

The formation of lobes, which always begins at the anterior
end of the lung, is of less fundamental importance as regards the
structure of the lung than is the arrangement of the bronchial
system, as a single lateral bronchus is present to each lobe. Thus it
follows that what has hitherto been known in human anatomy as
the inferior (posterior) lobe does not correspond to a true lobe, but
represents the mainaxisof the lung, enclosing the main bronchus.

In the description of the peritoneum (p. 208), attention has
already been directed to the fact that the thoracic cavity is
lined by a serous membrane, the pleura. As in the case of the
peritoneum, a parietal and a visceral layer may be distinguished

HUNJVi $ j

ABDOMINAL PORES. 265

(Fig. 212, P, P1) : the latter is spoken of as the pulmonary
pleura, the former as the costal pleura. Towards the middle
line, the pulmonary pleura of either side is reflected so as to form
a septum between the right and left thoracic cavities. This septum

ffi

FIG. 212. — DIAGRAM OF THE PLEURAL AND PERICARDIAL CAVITIES OF MAMMALS,

FOUNDED ON THE RELATIONS OF THESE PARTS IN MAN. (A, horizontal

section ; B, transverse section. )

Tr, trachea ; Br, bronchi ; L, L, lungs ; E, heart ; W, vertebral column ; P, parietal,
and P1, visceral layer of the pleura ; t t, points at which these pass into one
another at the hilum pulmonalis (Hi) ; m, mediastinum ; PC, Psl, parietal and
visceral layers of the pericardium ; JR, ribs (wall of thorax) ; S, sternum.

is called the mediastinum (Fig. 212, m), and the space between
its two layers the mediastinal space: through this, the aorta,
oesophagus, and postcaval vein run, and in the region of the heart,
the mediastinum is reflected over the parietal layer of the peri-
cardium (see p. 268, and Fig. 212, PC).

There is a lymphatic fluid between the two layers of the pleura,
which renders the movements of the lungs smooth and easy.

ABDOMINAL PORES.

By the term abdominal pores is understood a perforation
— almost always paired — of the posterior end of the peritoneal
cavity, which puts the ccelome into direct communication with the
exterior.1 These pores are present in Cyclostomi, Elasmo-
branchii, certain Teleostei, Dipnoi, Chelonia, and Croco-
dilia. The perforations always take place through the ectoderm,
close to the apertures of the urinogenital organs and intestine ; each
is either situated on a papilla, or in the walls of the cloaca.

It can only be stated positively as regards the function of the
abdominal pores, that in Cyclostomes and a few other Fishes they
serve to conduct the generative products to the exterior. As in the
rest of the Vertebrata special ducts are present for this purpose,

1 Other connections of the coelome with the exterior (nephrostomes of Anamnia
and oviducts of all Vertebrata) will be mentioned later on (see pp. 297, 300, and 302).

266

COMPARATIVE ANATOMY.
B

—So.

ED

FIG. 213. — ABDOMINAL PORES or VARIOUS VERTEBRATES. (A, Cyclostome ;
B, Elasrnobranch ; C, Protopterus ; D, Spatularia.)

A, anus ; Pa, Ra, PC, abdominal pores ; Pp, papilla ; AT, cloacal pockets ; UG and
a, urinogenital apertures ; L, L, lip-like margin of cloacal aperture ; CE,
cloaca ; DED, longitudinal folds of the rectum, which end sharply at RF ;
t, point at which the rectal gland opens ; BF, pelvic fin ; ft, claspers. The
arrow in A points towards the hcnd, and all the figures are placed similarly to A.
In Fig. C, Cl indicates the blind sac of the cloaca, the dorsal wall of which is
visible at DW '; t, the unpaired apeiture of the generative ducts ; ED, ED\
the rectum, cut open. The arrow indicates the aperture of the ureters.

ABDOMINAL PORES. 267

the persistence of the abdominal pores is difficult to account for ;
they must have undergone a change of function. It is as difficult
to say what this function is as to explain the fact that they have
disappeared in the Amphibia, which are certainly a very ancient
group, while they are again met with amongst Reptiles.

In Protopterus the abdominal pores open in front of, and in
Ceratodus behind the cloaca. In Ceratodus their arrangement is
similar to that seen in Elasmobranchs, and they are always paired :
in Protopterus, on the contrary, they undergo numerous individual
variations ; as a rule only one is developed, and this lies on the
same side as the vent — sometimes to the right, sometimes to the
left of the middle line, and opens either within or without the
sphincter of the cloaca. If both pores are present, they always
open within the cloaca, on its dorsal wall, behind the aperture of
the rectum.

Gegenbaur considers that the abdominal pores are not homolo-
gous throughout the series of the Vertebrata, and that they must
be considered in relation with other structures — more particularly
the generative organs. Rathke and Huxley have pointed out
that in the series of the Salmonidas a gradual disappearance of
the oviducts is observable, their function being undertaken by
abdominal pores. This would seem to indicate that the abdominal
pores present in female SalmonidaB are not homologous with those
of other Fishes (e.g. Cyclostomi, Elasmobranchii, Dipnoi).

BIBLIOGRAPHY.

AEBY, CH. — Der Bronchialbaum der Saugethiere und des Menschen, Leipzig, 1880.
AYEES, H. — Untersuchungen iiber die Pori abdominales. Morphol. Jahrb. Bd. IX.

1884.
BRIDGE, T. W. — Pori Abdominales of Vertebrata. Journ. of Anat. and Physiol.

Vol. XIV.
FISCHER, J. G. — Anatom. Abhandlungen iiber die Perennibranchiaten und Derotremen.

Hamburg, 1864.
FURBRINGER, M. — Beitrdge zur Kenntniss der Kehlkopfmu^kulatur. Jena, 1875.

(Contains also a copious bibliography of the larynx in general.)
GEGENBAUR, C. — Bemerk. iib. die Pori abdominalea. Morphol. Jahrb. Bd. X. 1885.
HENLE, J. — Vergl. anatom. Beschreitning des Kehlkopfes. Leipzig, 1839.
HUXLEY, T. H. — On the Respiratory Organs of Apteryx. Proc. Zool. Soc. 1882. On

the Oviducts of Osmerus, with remarks on the relations of the Teleostean to the

Ganoid Fishes. Ibid. 1883.
KOLLIKER, A. — Zur Kenntniss des Baues der Lungen des Menschen. Verhandl. der

med. Gesellsch. z. Wurzbu,rg. N. F. Bd. XVI. (Compare also the text-books of

Anatomy of Quain, Aeby, Henle, Krause, &c.)
MULLER, J. — On Certain Variations in the Vocal Organs of Passer es. Konig. Akad.

d. Wiss. zu Berlin, 1846, 1848. Eng. trans. (Bell), Oxford, 1878.
PARKER, T. J. — On some Embryos of Callorhynchus antarcticus. New Zealand

Journ. of Science, 1883, and Nature, Vol. XXIX. p. 46.
KATHKE, H. — Zur Anatomie der Fische. Arch. f. Anat. und Physiol. 1838.
SAGEMEHL, M. — Beitrage zur vergl. Anat. der Fische. Morphol. Jahrb. Bd. X. 1884.

(This includes an account of the anatomy and physiology of the air-bladder.)
STRASSER, H. — Die Luftsdcke der Vogcl. Morphol. Jahrb. Bd. III. 1877.
"WlEDERSHEiM, R. — Das Respirations -sy 'stem der Chamceleoniden. Berichte der

Naturforsch. Gescllachaft zu Freiburg i/B. Bd. I. Heft III. 1886.

H. ORGANS OF CIRCULATION.

(VASCULAR SYSTEM.)

THE organs of circulation consist of a central organ, the heart,
peripheral organs, the vessels, and nutritive fluids, composed of
plasma and structural elements (cells),1 the blood and
lymph. The latter, which occurs partly within closed canals,
partly in various spaces and cavities of the body, and which
penetrates all the tissues, will be spoken of later, and the blood-
vascular system in its more restricted sense will be treated of
first. This consists of a series of completely closed tubes (vessels),
which, according as they contain oxygenated or impure blood, are
spoken of as arteries or veins. This, however, is not an abso-
lute rule, for setting aside the chemical condition of the blood, all
vessels which empty their contents into the heart are called veins,
while those which arise from the heart are spoken of as arteries.

The heart, which is enclosed within the pericardium,2 serves as
the central organ of the circulation, and acts both as a suction-
pump and a force-pump. It arises, like the entire vascular
system, from the mesoblast, either as a single or as a paired
tubular cavity ; it originates in the splanchnic layer along the ventral

1 The blood- and lymph-corpuscles are the last results of segmentation in the
mesoblast. The first to be formed are white corpuscles, which are nucleated and
amoeboid (these are the only kind present in Amphioxus) ; the red appear secondarily ;
whether they originate from the white corpuscles or independently is not known,
though the former mode of development seems the most probable. Both primitive
red and primitive white corpuscles possess a nucleus, which in the case of the
latter persists throughout life, though it is often only visible by means of reagents.
In the case of the red corpuscles the nucleus persists, and the whole cell is biconvex
in all Vertebrates below Mammals, and, even in these, nucleated red cells may be
seen in the marrow of the bones, in the blood of the spleen, and often in that of the
portal vein. In all other parts of the body of Mammals they lose their nuclei and
become biconcave. In all Mammals, except the Camelidse, the red corpuscles have
the form of circular disks ; in the last-mentioned group and in all other Vertebrates
except Cyclostomes they are oval. Siren possesses the largest red corpuscles, then
comes Proteus, and then Salamandra ; the smallest are found in the Tragulidse.

2 The pericardium consists of a parietal and a visceral layer: the former is
invested by the mediastinum (see p. 265), and the latter is closely applied to the
Iieart.

VASCULAK SYSTEM. ,269

region of the throat, close behind the gill-clefts.1 Thus it is formed
from the same blastema as the muscular coat of the alimentary
canal, and its wall becomes differentiated into three layers, an
outer serous (pericardial), a middle muscular, and an inner
epithelial. In this it essentially corresponds in structure
with the larger vessels, in the walls of which three layers
can also be distinguished.2 By a study of its development we
thus see that the heart represents essentially a strongly deve-
loped blood-vessel, which at first lies more or less in the longi-
tudinal axis of the body ; later, however, it becomes much more
complicated by the formation of various folds and swellings. In
this manner the folded tubular heart becomes divided into two
chambers, an atrium and a ventricle. Between these, valvular
structures arise, which only allow the blood to flow in a definite
direction on the contraction of the walls of the heart, viz. from the
atrium to the ventricle ; any backward flow is thus prevented.

-Sir

FIG. 214.— DIAGRAM SHOWING THE PRIMITIVE RELATIONS OF THE DIFFERENT

CHAMBERS OF THE HEART.

Sv, sinus venosus, into which the veins from the body open ; A, atrium ; V, ventricle ;
Ca, conus arteriosus ; Ba, bulbus arteriosus.

The valves are formed by a process of differentiation of the mus-
cular trabeculaB of the walls of the heart, as will be explained later
on. The atrium, into which the blood enters, represents primi-
tively the venous portion of the heart, while the ventricle, from
which the blood flows out, corresponds to the arterial portion. The
venous end further becomes differentiated to form another chamber,
the si ii us venosus (Fig. 214, Sv), and the arterial end gives rise
distally to a conus or truncus arteriosus; this is provided
with more or less numerous valves, and is continued forwards into
the arterial vessel, the enlarged base of which is spoken of as the
bulbus art eriosus. ((7a, Ba).

1 The primitive aortse arise independently of the heart : they are formed by
peripheral (segmentally arranged ?) vascular processes, which pass towards the middle
line, and there turn forwards and backwards and become confluent, so as to form
longitudinal trunks.

2 The walls of the smallest blood-vessels, the capillaries, consist of a single
cellular layer, which corresponds to the inner epithelial layer (intima) of the larger
vessels.

270 COMPARATIVE ANATOMY.

This condition of things persists throughout life in many
Fishes, and is at any rate passed through in the individual
development of all Vertebrates.

With the appearance of pulmonary respiration, important
changes take place in the primitively simple heart, which finally
result in the formation of a septum in both atrium and ventricle,
and thus lead to the presence of two atria or auricles, and two
ventricles : the conus arteriosus and sinus venosus become even-
tually incorporated in the ventricles and right auricle respectively.
Thus a right (venous) and a left (arterial) half can be distin-
guished, and a new vessel, the pulmonary artery, arises from
the right ventricle ; this conveys venous blood to the lungs, while
special vessels (pulmonary veins) return the oxygenated blood
from the lungs to the left side of the heart, from which it passes
into the general circulation of the body.

The gradually increasing complications which occur in passing from the
lower to the higher Vertebrates will be more easily understood after the
condition of the circulation in the embryo has been considered.

THE FCETAL CIRCULATION.

In an early stage, the bulbus arteriosus (Fig. 215, E) is
continued forwards towards the head by a long unpaired vessel
(branchial artery), which gives off right and left a series of
symmetrical transverse branches or arches (Ab\ each of which
runs between two consecutive gill-clefts (KL). After the first pair
has given off branches to the head (carotids), they all unite
above the clefts to form a longitudinal trunk on either side (S, Sl).
These branchial veins give rise further back to the right and
left roots of the aorta (RA, RA}.

In all Vertebrates the aorta (A) is throughout life the most
important artery of the body ; it extends backwards along the
ventral side of the vertebral axis as a large unpaired trunk, which
gives off numerous branches, and forms the caudal artery in the
tail (Actl).

The vitelline or omphalo-mesenteric arteries (Fig. 215,
Am, and Fig. 216, R.Of.A, L.Of.A), which are very important
up to a certain stage in development, arise from the aorta, and
carry blood to the surface of the yolk, whence it is returned by
means of the vitelline or omphalo-mesenteric veins (Fig. 216,
R.Of, L.Of). In embryos of Fishes these open into a subintes-
tinal vein, lying on the ventral side of the alimentary tract, and
opening into the heart : in the higher forms this vein is repre-
sented by the caudal vein and the ductus venosus (Figs. 217
and 218, DV) ; the latter passes through the liver on its way to the
heart, but disappears on the formation of the portal circulation.
Before passing into the sinus venosus the blood becomes mixed

THE FCETAL CIRCULATION.

271

with the venous blood of the ductus Cuvieri or precaval
sinus (Figs. 215 to 218, Si, S.V, DC, D).

-All

Acd

FIG. 215. — DIAGRAM OP THE EMBRYONIC VASCCJLAR SYSTEM.

A, A, dorsal aorta ; RA, RA, right and left roots of the aorta, which arise from the
branchial vessels, Ab, by means of the collecting trunks (branchial veins), S, Sl ;
c, c1, the carotids ; Sb, subclaviau artery ; KL, gill-clefts ; Si, sinus venosus ;
A, atrium ; V, ventricle ; B, bulbus arteriosus ; Vm, vitelline veins ; Am,
vitelline arteries ; Ic, Ic, common iliac arteries ; E, E, external iliac arteries ;
All, allantoic (hypogastric) arteries ; Acd, caudal artery ; VG, HC, anterior and
posterior cardinal veins ; Sb1, subclavian vein ; D, Cuvierian duct (precaval
veins), into which the anterior and posterior cardinals open.

The ductus Cuvieri, which runs transversely, is formed on either
side by the fusion of the anterior and posterior cardinal
veins — two large vessels which bring back the blood from th'e

272

COMPARATIVE ANATOMY.

head, mesonephros (see p. 296), and body-walls (Fig. 215, VCy HO
and Fig. 216,S.Ca.V, V.Ca).

The anterior cardinal veins, which return the blood from the head and
anterior portion of the body persist, and give rise to the large jugular veins
of the adult (comp. Figs. 217 to 219) ; the posterior cardinals soon become
largely obliterated, and are replaced by the azygos and vertebral veins,
and more especially by the postcaval (vena cava posterior s. inferior)
(see Figs. 217 to 219). For further details as to the venous circulation, e.g. the
portal system (which becomes established in Fishes by the subintestinal
vein), the reader is referred to Figs. 217 to 219.

SJT.

FIG.

216. — DIAGRAM OF THE CIRCULATION OF THE YOLK-SAC AT THE END OF
THE THIRD DAY OF INCUBATION IN THE CHICK. (After Balfour.)

heart: A A, the second, third, and fourth aortic arches: the first has become
obliterated in its median portion, but is continued at its proximal end as the
external carotid, and at its distal end as the internal carotid ; Ao, dorsal aorta ;
L.Of.A, left vitelline artery ; R.Of.A, right vitelline artery ; S. T, sinus termi-
nalis ; L. Of, left vitelline vein ; R. Of, right vitelline vein ; S. V, sinus venosus ;
D. (7, ductus Cuvieri ; S. Ca. V, anterior cardinal or jugular vein ; V. Ca, posterior
cardinal vein. The veins are marked in outline, and the arteries are made
black. The whole blastoderm has been removed from the egg, and is supposed
to be viewed from below. Hence the left is seen on the right, and vice versd.

To return once more to the arterial system : — We must consider
next the two branches of the dorsal aorta known as the allantoic
arteries (Fig. 215, All). As their name implies, these arteries
branch out over the allantois, which arises as an outgrowth from

THE FOETAL CIRCULATION.

the posterior part of the primitive intestine in all Vertebrates but
Fishes (comp. Fig. 9, p. 11). As the allantois grows further and
farther outwards, it comes, in the Sauropsida, to lie against the
internal surface of the egg-shell, and thanks to the porous nature
of the latter, which permits the air to pass through it, serves as an
important respiratory organ. In Mammals it serves in the embryo
both for respiration and nutrition (comp. pp. 10 and 274). The
further development of the embryonic vessels may take place in one
of three ways.

Fir:. 217. — DIAGRAM OF THE VENOUS CIRCULATION IN THE CHICK AT THE
COMMENCEMENT OF THE FIFTH DAY. (After Balfour.)

77, heart ; DC\ ductus Cuvieri : into the ductus Cuvieri of either side falls J, the
jugular vein ; Su. V, the anterior vertebral ; Wt the vein from the wing, and Ct
the posterior cardinal vein ; S. V, sinus venosus ; Ofi vitelline vein ; U, allantoic
vein, which at this stage gives off branches to the body-walls; V.C.I,
postcaval.

The embryo may either leave the egg, and take on an aquatic
existence (Anamnia), making use of its branchial vessels as a
gill-breather ; the entire allantois, in the case of the Amphi-
bia, giving rise to the bladder. In the case of terrestrial animals
(Amniota), which from the first breathe by means of lungs,
a modification and reduction of the branchial vessels and allantois
takes place, and the latter may even disappear entirely (certain
Reptiles and all Birds) (comp. p. 308) ^ In the third case the

1 Concerning the amnion, which encloses the embyros of Sauropsida and Mam-
malia, comp. p."lO and Figs. 9 and 220.

274

COMPARATIVE ANATOMY.

embryo undergoes a longer mtra-uterme existence, the allantois
coming into close connection with the walls of the uterus by means
of its villi : the allantoic vessels extend into the wall of the uterus
and come into more or less close relations with the maternal
vessels, thus serving for the respiration and nutrition of the foetus.

-svy.

FIG. 218. — DIAGRAM OF THE VENOUS CIRCULATION IN THE CHICK DURING THE
LATER DAYS OF INCUBATION. (After Balfour.)

//, heart; VS. R, right precaval ; VS.L, left precaval ; S. V, sinus venosus : the
two precavals are the original "ductus Cuvieri " : they open into the sinus
venosus ; /, jugular vein ; SU. V, anterior vertebral vein ; W, subclavian ;
V.C.I, postcaval ; HP, hepatic veins ; DV, ductus venosus ; PV, portal vein ;
M, mesenteric vein ; Of, vitelline vein ; 17, allantoic vein. The three last-
mentioned veins unite together to form the portal vein.

In this way arise a placenta and a placental circulation (comp.
pp. 10-12). The embryos of all Mammals except Monotremes
and Marsupials reach this high stage of specialisation.

The allantois then no longer has a simple sac-like form, but
becomes solid. The part outside the body of the foetus disappears

THE FCETAL CIRCULATION.

J B C

275

FIG. 219, A, B, C. — DIAGRAM OF THE DEVELOPMENT OF THE PAIRED VENOUS
SYSTEM OF MAMMALS (MAN). (From Gegenbaur.)

A. — Stage in which the cardinal veins have already disappeared. Their position is

indicated by dotted lines.
B. — Later stage, when the blood from the left jugular vein is carried into the right

to form the single precaval ; a remnant of the left precaval however still

remains.
C. — Stage after the left vertebral vein has disappeared, the right vertebral remaining

as the azygos vein. The coronary vein remains as the last remnant of the left

precaval.
/, jugular vein ; cs, precaval, s, subclavian veins ; c, posterior cardinal vein ; v,

vertebral vein ; az, azygos vein ; cor, coronary vein.

FIG. 219, D.— DIAGRAM OF THE CHIEF VENOUS TRUNKS OF MAN.
(From Gegenbaur. )

os, precaval ; s, subclavian vein ; ji, internal jugular ; je, external jugular ; az,
,-.» azygos vein; ha, hemiazygos vein; c, dotted line showing previous position

of'cardinal veins ; ci, postcaval ; r, renal veins ; il, iliac ; hy, hypogastric veins ;

h, hepatic veins. The dotted lines show the position of embryonic vessels

aborted in the adult.

entirely at birth, while the intra-abdominal remains of it give rise
in Dart to a solid fibrous cord, the urachus, and in part to the

T 2

276

COMPARATIVE ANATOMY.

definitive urinary bladder and urethra. Indications of the
point of exit of the allantois and vitello-intestinal duct (umbilical
cord) from the body-cavity can be seen in the adult at the navel,
or umbilicus, which represents the last point at which the
body-walls become united.

PC(Chf)

.Dv

FIG. 220.— DIAGRAMMATIC SECTION THROUGH THE HUMAN GRAVID UTERUS.

U, uterus ; Tb, Tb, Fallopian tubes ; Uff, uterine cavity ; Dv, decidua vera, which
at Pu passes into the uterine portion of the placenta ; Dr, decidua reflexa ; Pf,
foetal portion of the placenta (chorion frondosum, Chf) ; Chi, chorion laeve ; A, A,
the cavity of the amnion filled with fluid : in the interior of the amnion is seen
the embryo suspended by the twisted umbilical cord ; H, heart ; A, aorta ;
cs, precaval ; ci, postcaval ; p, portal vein ; Al, allantoic (umbilical) arteries ;
f, the liver, perforated by the umbilical -vein ; D, the remains of the yolk-sac
(umbilical vesicle).

The branchial vessels never become functional as such, in any
period of development either in Mammalia or Sauropsida, but those
which persist give rise to important vascular trunks of the neck,
head (carotids), upper extremity (subclavian), and lungs (pulmonary
artery), and also to the roots of the aorta, one or both of which may
remain (comp. Fig. 221, A to D).

THE HEART AND ITS VESSELS.

277

FIG. 221. — DIAGRAM SHOWING THE TRANSFORMATIONS OF THE AORTIC ARCHES —
A, IN A LIZARD ; B, IN A SNAKE ; C, IN A BIRD ; AND D, IN A MAMMAL.
(After Rathke.) (Seen from below.)

A . — a, internal, and b, external carotid ; c, common carotid ; d, ductus Botalli between
the third and fourth arches ; e, right aortic arch ; /, subclaviaii ; g, dorsal aorta ;
h, left aortic arch ; i, pulmonary artery ; k, rudiment of the ductus Botalli
between the pulmonary artery and the aorta.

B. — a, internal, and b, external carotid ; c, common carotid ; d, right aortic arch ;
e, vertebral artery ; /, left aortic arch ; h, pulmonary artery ; i, ductus Botalli
of the latter.

C. — a, internal, and b, external carotid ; c, common carotid ; d, base of the aorta ; e,
fourth arch of the right side (root of the aorta) ; /, right subclavian ; g, dorsal
aorta ; h, left subclavian (fourth arch of the left side) ; i, pulmonary artery ; k
and I, right and left ductus Botalli of the pulmonary arteries.

D. — a, internal, and 5, external carotid ; c, common carotid ; d, base of the aorta ; e,
fourth arch of the left side (aortic root) ; /, dorsal aorta ; g, left vertebral artery ;
h, left subclavian ; i, right subclavian (fourth arch of the right side) ; k, right
vertebral artery ; I, continuation of the left subclavian ; m, pulmonary artery ;
n, ductus Botalli of the latter.

THE HEART AND ITS VESSELS.

Fishes. — While the heart of Amphioxus is not specially
differentiated, as it is in the Vertebrata, that of Fishes is well
developed, and is situated in the anterior part of the body-cavity,
close behind the head. It is always formed on the same type
as that described on p. 269.1 In it may be distinguished a

There is no truncus arteriosus in Cyclostomi.

278

COMPARATIVE ANATOMY.

tr

Ba

FIG. 222. — HEARTS OF VARIOUS FISHES— A, OF THE HAMMER-HEADED SHARK
(Zygcena malleus) ; B, OF Sihirus glanis ; 0, OF A SELACHIAN, CUT OPEN.

In A and B, A, A, atria ; a, a (in A), auricular appendages ; V, ventricle ; Ba, bulbus

arteriosus ; tr, ventral aorta.
In C, a, a, indicate the atrio-ventricular valves, and J, the valves in the conus

arteriosus (Co) ; A, atrium ; V, ventricle.

ventricle (Fig. 222, V) and an atrium (A), the latter receiving
its blood from a sinus venosus, and being laterally expanded
to form the appendices auriculae (A, a, a).

THE HEART AND ITS VESSELS.

279

In correspondence with the different function which each
portion has to perform, the walls of the atrium are thin, while
those of the ventricle are much stronger, its muscles giving rise
in the interior to a network and also usually to a series of
large trabecula3; this holds good throughout the Vertebrata
(Fig. 222, C, A).

Between the ventricle and atrium, at the margins of the atrio-
ventricular aperture, membranous valves are present ; of these
there are usually two, but this number may be increased to as
many as six (Fig. 222, C, a, a). Numerous valves, arranged in rows,
are present in the muscular truncus or conus arteriosus
(Fig. 222, C, Ca, V) ; these are most numerous in Elasmobranchs and
Ganoids. There is a tendency however for the posterior ones, or
those which lie towards the ventricle, gradually to undergo reduc-
tion. The most anterior row always persists, and corresponds to
the single row of valves between the ventricle and bulbus in
Teleostei. Together with the reduction of these valves, the conus
arteriosus of Teleosteans also becomes reduced, so that the non-
contractile bulbus arteriosus usually • lies close against the
ventricle (Fig. 222, B, Ba).

ce

RA

FIG. 223. — DIAGRAM OF THE ARTERIAL SYSTEM OF FISHES.

ff, heart ; c, c1, anterior and posterior cardinal veins ; a, branchial arteries ; R,
capillaries of the branchial vessels ; b, branchial veins ; ce, circulus cephalicus ;
ca, carotids ; RA, root of the aorta ; A, dorsal aorta ; E, artery to viscera
(cceUaco-rnesenteric) ; N, renal arteries.

The heart of Fishes contains venous blood only, which it forces
through the branchial arteries (Fig. 223, a) into the capillaries

280 COMPARATIVE ANATOMY.

of the gills (R), where it becomes oxygenated, to pass thence
into the branchial veins (Fig. 223, &). The manner in which
the aortic roots become formed from the latter has already been
described.

Dipnoi. — In the Dipnoi, as in Fishes proper, the heart lies far
forwards, near the head. In correspondence with the double mode
of respiration, by lungs as well as by gills, it reaches a stage of
development mid-way between that seen in Fishes and in Amphi-
bians. The atrium becomes divided into two chambers by a septum,
as does also the ventricle to some extent. The conus arteriosus is

FIG. 224.— DIAGRAM OF THE HEAUT AND BRANCHIAL VESSELS OF CERATODUS.
(Mainly after J. E. V. Boas. )

V, ventricle ; A, A1, atria ; Co, conus arteriosus ; Co, and Cp, anterior and posterior
cardinal veins ; DC, ductus Cuvieri ; /to/F, branchial arteries ; 1 to 4, branchial
veins ; Ca, carotid ; Ap, pulmonary artery ; RE, capillaries of lung ; Vp, pul-
monary vein ; RA, roots of the aorta, beginning at t ; Ao, dorsal aorta.

twisted spirally on itself (Fig. 224, Co) : in Ceratodus it is pro-
vided with eight transverse rows of valves, and begins to be divided
into two chambers. InProtopterus this division is complete, so
that two currents of blood, an arterial and a venous, pass out
from the heart side by side (Fig. 225, a,&). The former comes
from the pulmonary vein, from which it passes into the left
atrium, thence into the left ventricle, and so to the two anterior

THE HEART AND ITS VESSELS. 281

branchial arteries (Fig. 225, /, II). The venous current, on the
other hand, passes from the right ventricle into the third and fourth
branchial arteries and thence to the corresponding gills, where it
becomes purified ; it reaches the aortic arches by means of the
branchial veins (///, IV, 3, 4, RA). The pulmonary artery
(Ap) arises from the fourth branchial vein, so that the blood is thus

RA

-Ao

FIG. 225. — DIAGRAM OF THE BRANCHIAL CIRCULATION OF PROTOPTERUS.

Co, conns arteriosns, which consists of two divisions, a and b : through b pure arterial
blood passes to the two anterior branchial arteries (7 and //) ; through a
venous blood passes to the two posterior branchial arteries (III and IV) ; 3 and
4 indicate the branchial veins and capillaries of the gills ; Ap, the pulmonary
artery, present only on the left side (?) ; RA, arch of the aorta ; Ao, aorta ;
C'«, carotid.

once more purified before it passes by means of the pulmonary
vein into the left ventricle. In Ceratodus the lung is supplied
with blood from the cceliac artery, and owing to the fact that the
longitudinal valve of the conus is incomplete, the blood passing to
the two anterior branchial arteries is of a mixed nature (comp.
Fig. 224, 7,77).

Amphibia. — With the exception of the Gymnophiona, in
which it is situated some distance back, the heart in all Amphi-
bians lies far forwards, below the anterior vertebraB. As in the
Dipnoi, there is a more or less complete septum atriorum (that
is, the septum is either entire or fenestrated). There are always
two fibrous pocket-like atrio-ventricular valves, which are con-
nected with the walls of the ventricle by cords.

The cavity of the ventricle is unpaired, and neither in Urodela
nor Anura shows any trace of a septum, so that the blood passing
out from it must have a mixed character (Fig. 226). The
ventricle is usually of a short and compressed form, but is more

282 COMPARATIVE ANATOMY.

elongated in Amphiuma, Proteus, and the Gymnophiona. It
is continued anteriorly into a conus arteriosus, as in Elasmo-
branchs, Ganoids, and Dipnoans; this has usually a slight
spiral twist, and possesses a transverse row of valves at either end,
as well as a spiral fold extending into its lumen.1 This holds good
for the Axolotl, Amblystoma, Salamandra, Amphiuma,
and Siren. In others (e.g. Menobranchus, Proteus, Gymno-
phiona), retrogression is seen in a lengthening of the conns, the
disappearance of the spiral fold, and the presence of only a single
row of valves.

In Anura, the fold lying within the conus extends so far
back that no undivided portion of the cavity is left. The conse-
quence of this is that the blood in one section of the branchial

ci ce,

FIG. 226. — DIAGRAM SHOWING THE COURSE OF THE BLOOD THROUGH THE HEART
IN Urodela (A) AND Anura (B).

A, right atrium ; A1, left atrium ; V, ventricle ; tr, conns arteriosns, divided in
Annra (B) into two portions, tr, tr1 : through tr venous blood passes into the
pulmonary arteries, Apl, Apl, while through tfr1 mixed blood goes to the carotids,
ci — ce, and to the roots of the aorta, RA ; Ir, Iv, pulmonary veins ; r, v,
pre- and postcavals (only one precaval is indicated) opening into the right
atrium.

vessels — that from which the pulmonary artery arises — is mainly
venous, while the others contain mixed blood (Fig. 226, B) ; for,
owing to the spongy nature of the ventricle, there is no time for
its contained blood to get thoroughly mixed before it is forced into
the conus.

As in the Dipnoi, four branchial arteries arise on either side
from the short conus in the Amphibia, which — taking as a good
type the larva of Salamandra — have the following relations.

The three anterior branchial arteries pass to numerous external
gill-tufts, where they break up into capillaries (Fig. 227, 1, £, &).
From the latter three branchial veins (/to III) arise, which pass to
:he dorsal side, and there unite on either side to form the aortic roots.
The fourth branchial artery, which is smaller than the others (4),
does not pass to a gill, but to the pulmonary artery, which arises

1 This spiral fold is to be looked upon as derived from a series of fused valves.

THE HEART AND ITS VESSELS. 283

from the third branchial vein (Fig. 227, 4, Ap}. The pulmonary
artery therefore contains far more arterial than venous blood, and
thus the lungs of the Salamander larva, like the air-bladder of
Fishes, can have 110 important respiratory function.

The internal carotid (ci) arises from the first branchial vein,
towards the middle line, the external carotid (ce) coming off
further outwards.

The latter, as it passes forwards, becomes connected with the first
branchial arch (1) by net-like anastomoses (t), and these give rise
later to the so-called carotid gland1 of the adult, which functions
as an accessory heart. Direct connections exist between the second
and third branchial arteries and the corresponding veins (see Fio-
227, a, a).

FIG. 227. — THE ARTERIAL ARCHES OF THE LARVA OF A SALAMANDER. (Slightly
diagrammatic.) (After J. E. V. Boas.)

tr, conns arteriosus ; 1 to 3, the three branchial arteries ; 7 to III, the corresponding
branchial veins ; 4, the fourth arterial arch, which. becomes connected with the
pulmonary artery (Ap) ; a, a, direct anastomoses between the second and third
branchial arteries and branchial veins ; ce, external carotid ; ci, internal carotid ;
t, net-like anastomoses between the external carotid and the first branchial
artery, which give rise later to the carotid gland ; RA, aortic roots ; Ao, dorsal
aorta. The arrows show the course which the blood takes.

Towards the end of the larval period, the second branchial vein
increases considerably in relative size, and the fourth arterial arch
also becomes larger. By a reduction of the anastomosis with the
third branchial vein, the fourth arterial arch furnishes the main
amount of blood for the pulmonary artery, and the latter thus
contains far more arterial than venous blood. When branchial
respiration ceases, the anastomoses between the branchial arteries
and veins no longer consist of capillaries, but a direct connection
between them becomes established (Fig. 228, #, 3, 4). Finally, the
connection between the first and second branchial arches disappears,

1 The "carotid gland" loses its character as a rete mirabile (comp. p. 292),
and in the adult consists simply of a muscular vesicle with septa in its interior.

284 COMPARATIVE ANATOMY.

the former giving rise to the carotids and the latter forming the
large aortic root (Fig. 228, cet ci, EA) ; an anastomosis remains
throughout life, however, between the fourth arch, which forms
the pulmonary artery, and the second and third (Fig. 228). This
is the ductus Botalli.

The third arch varies greatly in its development; it may be
present on one side only, or even may be entirely wanting.

ci

FIG. 228. — ARTERIAL ARCHES OF AN ADULT Salamandra maculosa, SHOWN SPREAD
OUT. (After J. E. V. Boas.)

co, tr, conns (truncus) arteriosus ; 1 to 4> the four arterial arches ; ce, external caro-
tid ; cd, carotid gland ; ci, internal carotid ; the fourth arterial arch, which
gives rise to the pulmonary artery (Ap), has increased considerably in size rela-
• tively, and is only connected by a delicate ductus Botalli (t) with the second and
third arches ; EA, root of the aorta ; ce, cesophageal vessels.

,-ln the larva? of Anura there are also four branchial arteries
present on either side, but these are connected with the correspond-
ing veins by capillaries only, there being no direct anastomoses
(compare Fig. 227, a, a). The consequence of this is that all the
blood becomes oxygenated.

In the adult Frog the third arterial arch becomes entirely
obliterated, and the first is completely separated from the second.
In other points the arrangement is similar to that seen in the
Salamander.

Reptiles. — As in all Amniota, the heart of Reptiles arises far
forwards in the neighbourhood of the gill-clefts, but on the forma-
tion of a neck it comes to lie much further back than is the case in
the Anamnia.1 The vagus nerve, which supplies the heart, is
thus correspondingly elongated, and the carotid arteries and jugular
veins also become increased in length.

The principal advance in structure as compared with the

1 It lies furthest forwards in most Lizards and in Chelonians, while in Amphis-
bsenians, Snakes, and Crocodiles it is situated much further back.

THE HEART AND ITS VESSELS.

285

Amphibian heart is seen in the appearance of a ventricular
septum, which may be incomplete, as in Lizards, Snakes, and
Chelonians, or complete, as in Crocodiles.1

A right and left aortic root or arch (Fig. 229, C, t and *)
are always present, and unite to form the dorsal aorta. Each

FIG. 229.— A, HEART OF Laccrta muralis, AND B,
OF A LARGE Varanus, SHOWN CUT OPEN ; C,
DIAGRAM OF THE REPTILIAN HEART.

V, F1, ventricles ; A, A1, atria ; tr, Trca, innomi-
nate trunk ; 1, 2, first and second arterial arch ;
Ap, Apl, pulmonary artery ; Vp, pulmonary
vein ; t and *, right and left aortic arch ; RA,
root of aorta ; Ao, dorsal aorta ; Ca, Cal, carotids ; Asc, As, subclavian arteries ; «/,
jugular vein ; Vs, subclavian vein ; Ci, postcaval. These three veins open into the
sinus venosus, which lies on the dorsal side of the heart, above the point indicated
by the letter S. In the diagram C the pre- and postcavals are indicated by Vc,
Ve, only one precaval being represented.

aortic root may be made up at its origin of two arches, anas-
tomosing with one another (Lacerta) (Fig. 229, A, 1, #), or of one
only (certain Lizards, Snakes, Chelonians, and Crocodiles)
(Fig. 229, B, RA, RA). The most posterior arterial arch gives
rise to the pulmonary artery (Ap) (comp. also Fig. 221, A, B).

1 A small aperture of communication between the ventricles, the foramen
Panizzae, exists in Crocodiles. Near it, between the origin of the left aorta and the
pulmonary artery, there is a small cartilage, as in Chelonians.

286 COMPARATIVE ANATOMY.

The blood from the right ventricle passes into the latter as well
as into the left aortic arch, and, according as the septum ventricu-
lorum is complete or incomplete, is either entirely venous (Croco-
diles) or mixed (other Reptiles, Fig. 229, C).

The valves of the heart have undergone a considerable
reduction in Reptiles : at the origin both of the aorta and of the
pulmonary artery there is only a single row ; this is also the case
in all other Amniota.

Birds and Mammals. — In these, the atrial and ventricular
septa are always complete, and there is no longer any mixture of
the arterial and venous blood. The ventricles are much larger than

FIG. 230A. — HEART OF THE SWAN, WITH THE EIGHT VENTRICLE CUT OPEN.

Vw, ventral wall of right ventricle turned on one side, thus putting the atrio- ven-
tricular valve — which arises by two muscular folds, a and b— on the stretch ;
f, point of insertion of these folds on the ventral wall of the ventricle : above c
is the atrio- ventricular aperture ; S, septum ventriculorum ; *, *, *, the three
semilunar valves of the pulmonary artery ; V, left ventricle.

FIG. 230B. — TRANSVERSE SECTION THROUGH THE RIGHT (Vd) AND LEFT (Vg)
VENTRICLE OF Grus cinerca.

S, septum ventriculorum.

the atria, and their muscular walls are strongly developed and very
compact. This is particularly the case in the left ventricle, on the
inner wall of which the papillary muscles are well developed :
the left ventricle is partially surrounded by the right, the cavity

THE HEART AND ITS VESSELS. 287

of the latter having a semilunar transverse section, and its walls
being much thinner than those of the other (Fig. 230B, Vd, Vg].

In both Mammals and Birds the blood from the head and
body passes by means of the precavals and postcaval into the right
atrium, as does also that from the walls of the heart through the
coronary vein: the right atrium is separated from the right
ventricle by means of a well-developed valve. In Birds, the latter
(Fig. 230A, a, b, c, t) is very large and entirely muscular, while in
most Mammals it consists of three membranous lappets (tricuspid
valve), to which are attached tendinous cords (chordae ten-
dinese), arising from muscular processes (musculi papillares)
of the walls of the heart.

In both Birds and Mammals the left atrio-ventricular aperture
is provided with a valve consisting of two membranous folds, called
the bicuspid or mitral valve: three semilunar pocket-like
valves are also present at the origins of both pulmonary artery
and aorta (Fig. 230A, **,*).

As regards the origin of the great vessels, Birds are distinguished
from Mammals by the fact that in them the right (fourth) arterial
arch persists, while in Mammals the left remains as the aortic
arch ; the corresponding arch of the other side in both cases gives
rise to part of the subclavian artery. Thus in both Birds
and Mammals there is only a single aortic arch. As in
Amphibians, the posterior arterial arch gives rise in both cases
to the pulmonary artery (comp. Fig. 221, C, D).

Amongst the more important points in the development of the
heart may be mentioned the fact that at first the two atria freely
communicate with one another by means of the foramen ovale,
through which the blood from the postcaval passes into the left
ventricle.

FIG. 231. — FIVE DIFFERENT MODES OF ORIGIN OF THE GREAT VESSELS FROM
THE ARCH OF THE AORTA IN MAMMALS.

Ao, aortic arch ; tb, brachiocephalic trunk ; c, carotids ; s, subclavians.

Great variations are seen in the mode of origin of the carotids
and subclavian s from the arch of the aorta in Mammals. Thus
there may be a brachiocephalic trunk on either side (Fig.
231, A), or an unpaired common brachiocephalic, from which
the carotid and subclavian of one or both sides arise (B, C, E), or,
finally, a common trunk of origin for the carotids, the subclavians
arising independently on either side of it (D).

288 COMPARATIVE ANATOMY.

ARTERIAL SYSTEM.

It has already been mentioned that in all Vertebrates there
is a large sub-vertebral vessel running in the longitudinal axis
of the body, called the aorta (Figs. 215 and 223, A), and that
this is formed by the union of the branchial vessels. From the
latter are also formed the carotids, which go to the head and
neck: of these, the internal carotid passes mainly into the
cranial cavity, and supplies the brain with blood, while the ex-
ternal carotid goes to the external parts of the head (face,
tongue, and muscles of mastication).

The origin of the subclavian artery, which supplies the
anterior extremity, is very inconstant, being sometimes symme-
trical, sometimes asymmetrical. It arises either in the region
of the branchial vessels, or from the roots or main trunk of the
aorta (Fig. 215, Sb, and Figs. 232 and 233, Sc). Extending
outwards towards the free extremity, the subclavian passes into
the axillary artery, and, on reaching the upper arm, becomes
the brachial artery. This finally divides into two branches
for the fore-arm — the radial and ulnar arteries; in the hand
these give rise respectively to the deep and superficial palmar
arches, as well as to the digital arteries.

In the dorsal aorta a thoracic and an abdominal portion
can be distinguished, and from them arise intercostal, lumbar,
and intestinal arteries, supplying the body- walls as well as
the thoracic and abdominal viscera. The intestinal arteries may
again be divided into two principal groups, namely, those which
supply the intestinal tract with its appendages (liver and pancreas)
and the spleen, and those which go to the urinogenital organs.
The branches of all these vary greatly both in number and
relative size ; thus, for instance, there is sometimes a single
cceliaco-mesenteric artery (Fig. 232, Cni), sometimes a
separate coeliac, and one or more mesenteric arteries. The renal
and genital arteries also vary in number and arrangement.

The abdominal aorta is continued posteriorly into the caudal
aorta, which usually lies within a canal formed by the ven-
tral arches of the vertebrse (Fig. 232, Aoc, and Fig. 233, 0) ;
the degree of its development naturally corresponds to the size of
the tail. In cases where the latter is rudimentary, as in Anthro-
poids and Man for instance, the caudal aorta is spoken of as the
median sacral artery, and the aorta here appears to be directly
continued, not by it, but by the common iliac arteries, which
pass outwards into the pelvic region (Fig. 232, lie).

Each common iliac artery becomes divided into an internal
iliac, or hypog^astric, supplying the viscera of the pelvis, which
is derived from the embryonic allantoic artery, and an exter-
nal iliac which is continued into the crural, and supplies the

ARTERIAL SYSTEM.

289

-Co,

FIG. 232. — THE ARTERIAL SYSTEM OF Salamandra maculosa.

II A, roots of the aorta ; Ao, Ao, dorsal aorta ; Sc, subclavian artery, from which the
cutaneous artery (Cu) arises : the latter anastomoses posteriorly with the epigastric
artery .Z?; Ov, ovarian arteries ; Cm, cceliaco-mesenteric ;JBT, hepatic artery ; /, /, /,
intestinal arteries passing to the small intestine ; M, M, rectal (hemorrhoids!)
arteries ; R, R, renal arteries ; lie, common iliac ; Or, crural artery ; Hy, hypo-
gastric artery ; A, A, vesical (allantoic) arteries ; Aoe, caudal aorta ; P, pharynx
and oesophagus ; m, stomach ; p, pancreas ; I, liver ; d, d, small intestine ; c,
rectum ; Bl, urinary bladder ; Cl, cloaca.

U

290

COMPARATIVE ANATOMY

V

FIG. 233. — THE ARTERIAL SYSTEM OF Emys europcca.

7V, trachea ; JBr, Br, the two bronchi ; m, stomach ; d, d, small intestine ; e, large
intestine ; Ap, pulmonary artery ; Cac, common carotids ; Tr, Oe, tracheal
and cesophageal branches ; Sc, subclavian artery ; Per, vertebral artery ; RA,
roots of the aorta ; Ao, dorsal aorta ; Co, Co1, and Me, cceliaco-mesenteric artery,
which here arises as a bundle of separate vessels ; UG, urinogenital arteries ; Cr,
crural artery ; E, epigastric artery ; H, hypogastric artery ; Is, sciatic artery ;
MB, rectal arteries ; C, caudal aorta.

(fu

VENOUS SYSTEM. 291

^3^

hinder extremity (Fig. 232, lie, Hy, Cr). In some cases the internal
and external iliacs come off separately from the aorta. The
function of the external iliac may be largely taken by a sciatic
artery arising separately from the aorta or iliac artery in the
pelvic region (Birds, and to a less extent in Reptiles) (comp
Fig. 233, Is).

The main vessels branch out in the limb in a manner
essentially similar to that already described for the anterior
extremity.

VENOUS SYSTEM

The numerous phases of development through which the
venous system passes are very instructive.

The cardinal veins,1 which open into the ductus Cuvierj, have
been already described, and it is only necessary to add that in
Fishes there is a renal-portal system interposed between the
posterior cardinals and the caudal vein, the latter dividing up into
capillaries in the kidney. The vessels returning the venous blood
from the alimentary canal, the pancreas, and the spleen, also
divide up into capillaries in the liver (hepatic portal system).
The blood passes out from the* latter by the hepatic veins into the
postcaval, whence it is conducted into the right auricle.2

From the Amphibia onwards the postcaval becomes of in-
creasing importance ; it receives the blood from the kidneys and
generative organs, as well as that from the posterior extremities,
pelvis, and body-walls. The posterior cardinals decrease propor-
tionately, and become to a certain extent lost.

The unpaired anterior abdominal (allantoic) vein plays
a great part in Reptiles and Amphibians : it arises mainly from
the crural vein of either side, and to a lesser degree also from
the veins of the urinary bladder ; it then passes along the ventral
body- walls to the liver, where it anastomoses with the portal
system, a small branch sometimes (e.g. Frog) going to the heart.
In Birds and Mammals the allantoic vein has an important function
in the embryo, but becomes obliterated on the atrophy of the
allantois at the close of foetal life : it seems, however, to be retained
throughout life in Echidna (Beddard).

From Birds onwards a renal-portal system no longer appears ;
but a hepatic portal system persists in all the higher Vertebrates.

1 It is interesting to note the almost lacunar condition of the veins in many
Elasmobranchs : the two posterior cardinals, for instance, unite in the middle of
the body-cavity to form a large sinus, with which others bringing back the blood
from the generative organs are connected.

2 In the Skate a large lateral vein lies along the outer side of the body-cavity
on either side, and receives the blood from the pectoral fins, abdominal walls, and
iliac region. A similar vein has been found in embryos of other Elasmobranchs
(e.g. Scymnus, Acanthias, Mustelus), and it seems highly probable that it corresponds
to the vein of the primitive lateral fin-folds (T. J. Parker) (ccmp. pp. 85 and 86).

u 2

292 COMPARATIVE ANATOMY.

The two anterior cardinal veins give rise to the right and
left precavals and to the jugular veins. In Monotremes and
Marsupials, as well as in many Rodents and Insectivores, both
precavals persist throughout life ; but in other Mammals the main
part of the left disappears, all the blood from the head and anterior
extremities passing into the right : this explains how it is that
the left azygos gives up its connection with the left precaval and
unites by means of transverse anastomoses with the right precaval.

Most of the veins are provided with valves, which are
adapted to prevent the reflux of the blood : they have the form
of semilunar folds of the internal coat, and each is usually made
up of two folds, placed opposite to one another.

RETIA MIRABILIA.

By this term is understood the sudden breaking-up of a venous
or arterial vessel into a cluster of fine branches, which, by anasto-
mosing with one another, give rise to a capillary network ; the
elements of this network may again unite to form a single vessel.
The earlier condition maybe described as a unipolar, the later as
a bipolar rete mirabile. If it is made up of arteries or of veins
only, it is called a rete mirabile simplex; if of a combination
of both kinds of vessels, it is a rete mirabile duplex.

The retia mirabilia serve to retard the flow of blood, and thus
cause a change in the conditions of diffusion. They are extremely
numerous throughout the Vertebrate series, and are found in the
most varied regions of the body, as, for instance, in the kidneys
(glomeruli), — where their above-mentioned function is most clearly
seen ; — on the ophthalmic branches of the internal carotid ; on
the pseudobranchia of Fishes ; along the intercostal arteries of
Cetacea ; on the mesentery of Man ; on the portal vein ; on the
vessels of the air-bladder of Fishes ; and along the caudal portion
of the vertebral column in Lizards and Blindworms. In the last-
mentioned case they are relatively very large, and probably have
to do with the power these animals have of reproducing the tail
when it is lost (comp. p. 43). A well-developed bipolar rete
mirabile may also be seen on the dorsal wall of the pharynx in
the Frog.

LYMPHATIC SYSTEM.

In the Anamnia and Reptilia the lymphatic vessels
occur mainly alongside the great blood-vessels, as well as on the
bulbus arteriosus and ventricle, and lie in the connective-tissue
surrounding these structures. Numerous independent lymphatic
vessels are also found in Fishes, arising from a capillary network

LYMPHATIC SYSTEM. 293

under the skin, and extending into the intermuscular septa
and the bases of the fins. The intestinal tract and the viscera
generally, of Sharks and Skates, are especially well provided with
lymphatic vessels. Elasmobranchs, moreover, possess a large
number of small lymph- hearts communicating with delicate
venous networks. Lymph-hearts are also to be met with in
Amphibians,1 Reptiles, and Birds, but are fewer in number in the
two last-mentioned groups than in the first ; they are either con-
fined to the posterior end of the body (pelvic region), or, as in
the Frog, are present also between the transverse processes of the
third and fourth vertebras. Their walls are capable of rhythmical
contraction, owing to the presence of muscular fibres. Similar
structures are not known to be present in Mammals.

Large lacunar lymph-sinuses are present under the skin of
tailless Amphibia, and the skin is thus only loosely attached to
the underlying muscles. These subcutaneous lymph-sinuses are
connected with those of the peritoneal cavity. Amongst the latter
the sub vertebral lymph- sinus is of great importance in
Fishes and Amphibia: it surrounds the aorta and is connected
with the (mesenteric) sinus lying amongst the viscera, into which
the lymphatic vessels of the intestine open. In Fishes there is also
a large longitudinal lymphatic trunk lying within the spinal canal.

The higher we get in the animal series the more commonly are
lymphatic trunks with independent walls to be met with;
thus from Birds onwards a large longitudinal subvertebral trunk
(the thoracic duct) is always present. In Mammals this arises
in the lumbar region, where it is usually dilated to form the
cisterna or receptaculum chyli; it receives the lymph from
the posterior extremities, the pelvis, and the urinogenital organs,
as well as the lacteals or lymphatics of the intestines. In
Mammals it opens anteriorly into the left, and in Sauropsida into
both left and right brachiocephalic veins. The lymphatics of the
head, neck, and anterior extremities open into the same veins.

The lymphatic vessels of Birds and Mammals are, like the
veins, provided with valves, the arrangement of which allows the
lymph- stream to pass in one direction only, and that a forward
one.

The lymph, like the blood, consists of two elements, a fluid
(plasma) and cells (lymph-corpuscles, leucocytes). The latter
have been already mentioned and their important physiological
function indicated in the chapter on the alimentary canal. We
have seen that they migrate from the solitary follicles and Peyer's
patches through the mucosa into the lumen of the gut ; and the
same thing occurs with the leucocytes of the so-called tonsils.
These appear to be present only in Mammals, and have the form

1 In Salamandra maculosa and Siredon piscifonnis, eight to twelve lymph-hearts
are present under the skin along the sides of the body and tail, at the junction of the
dorsal and ventral bodv-musclcs.

294 COMPARATIVE AN ATOM T.

of a paired organ lying on either side of the fauces, that is, in the
region where the mouth passes into the pharynx : they consist of
a retiform (adenoid) connective-tissue ground-substance enclosing
a number of lymph-corpuscles, which are arranged in so-called
follicles.

Lymphoid tissue plays a very important part in the body-cavity
of Fishes and Amphibia. Apart from the alimentary canal, it is
present in considerable quantity in the region of the urinary and
genital glands, which are often regularly embedded in it (e.g. Dipnoi).
The mass of lymphoid tissue on the heart of the Sturgeon, and
possibly also the so-called "fat-bodies" of Amphibia and Rep-
tilia, and the "hibernating gland" of certain Rodents, may be
placed in this category.

The agglomeration of a number of these follicles gives rise to
those structures which are spoken of as "lymphatic glands,"
These are always interposed along the course of a lymphatic
trunk so that an afferent and efferent vessel to each can be dis-
tinguished. They probably appear first in Birds, and are most
numerous in Mammals, where they are present in abundance in
various regions of the body ; they differ greatly in size.

The spleen, which is present in almost all Vertebrates, is
closely related to these structures. It usually lies near the
stomach, though it is occasionally met with in other regions of
the intestinal tract, as, for instance, at the commencement of the
rectum (Anura, Chelonia). In some cases (e.g. Sharks) it is broken
up into a number of smaller constituents.

Both the lymphatic glands and the spleen have to do with the
formation of lymph-cells, but their complete physiological function
is as yet by no means clear.

BIBLIOGRAPHY.

AYERS, H. — Beitr. zur Anatomie und Physiologic dcr Dipnocr. Jenasche Zeitschrift
fur Naturwissenschaften, Bd. XVIII. N. Folge, Bd. XI. 1885.

BEDDARD, F. E. — Note on the Presence of an Anterior Abdominal Vein in Echidna.
Proc. Zool. Soc. 1884. On the Heart of Apteryx. Ibid. 1885.

BOAS, E. V. — See numerous Papers On the Vascular System of Fishes and Amphibia
in the Morphol. Jahrb. Bd. VI. 1880, Bd. VII. 1881, and Bd. VIII. 1882.

GOMPERTZ, C. — Ueber Hers und Blutkreislauf bei nacktcn Amphibien. Arch. f.
Anat. und Physiologic (Physiologische Abtheilung), 1884. (This contains a
masterly description of the Anatomy and Physiology of the Heart of Anura. )

LANKESTER, E. RAY. — On the Valves in the Heart of Ornithorhynchus paradoxus
compared with those of Man and the Rabbit, with some Observations on the Fossa
Ovalis. Proc. Zool. Soc. 1882. On the Right Cardiac Valve of Echidna and
Ornithorhynchus. Ibid. 1883. On the Heart described by Professor Owen in
1841 as that of Apteryx. Ibid. 1885. On the Right Cardiac Valve of the Specimens
of Apteryx dissected by Sir Richard Owen in 1841. Ibid. 1885.

MASCAGNI. — Prodome d'un ouvrage sur lesystemedes vaisseaux lymphatiques. Sienne,
1784. Vasorum lymphaticorum corporis humani historia et iconographia.
Senis, 1787.

MULLER, J. — Ueber die Lymphherzen der Amphibien. Arch. f. Anat. u. Physiol.
1854.

MULLER, W. — Uebtr d?n feineren Bau der MHz. Leipzig, 1865.

VASCULAR SYSTEM. 295

PARKER, T. ,T. — On the Venous System of the Skate. Trans, of the N. Zealand

Institute, Vol. XIII. 1880.
RATHKE, H. — Uebcr die Entwicklung der Arterien, welche bei den Saugethieren von

den Bogen der Aorta ausgehen. Arch. f. Anat. u. Physiol. 1843.
RUSCONI. — Hist, not., devcloiipement et metamorphose de la Salamandre terrestre.

1854.
SABATIER, A. — Observations sur les transformations du syst&me aortique dans la

serie des Vertebres. Annal. d. Sc. Nat. 5 Ser. Tom. XIX. Etudes sur la c&ur

dans la serie des Vertebres. Montpellier et Paris, 1873.
SAPPEY, PH. C. — fitudes sur Vapparcille mucipare et sur le systeme lymphatique des

poissons. Paris, 1880.
STOHR, PH. — Conus arteriosiw der Selachier und Ganoiden. Morphol. Jahrb. Bd.

II. 1876. Ueber den Bau der Conjunctiva palpebrarum. Sitz. Ber. d. phys.-

medicin. Gesellsch. zu Wiirzburg, 1885.
WELIKY, W. — Ueber vielzdhlige Lymphherzcn bei Salamandfa maculosa und Siredon

pisciformis. Zool. Anzeiger, Jahrgang VII. Nr. 183, 1884.

(Further references may be found in the Text-Books on Anatomy mentioned at
the beginning of this book. )

I. URINOGENITAL ORGANS.

THE urinogenital organs of all Vertebrates arise in the region
of the dorsal body- wall, right and left of the middle line.

The first part to be developed isanunsegmented and paired
duct, which arises from the somatic mesoblast, and runs parallel
to the long axis of the body. This duct opens anteriorly into*
the body-cavity by means of one or more ciliated funnel-shaped
apertures, which communicate with it by means of convoluted
tubes. The latter constitute the pronephros or head-kidney;
they are formed as outgrowths of the tube itself, which opens
posteriorly into the cloaca, and which is known as the segmental
or pronephric duct.

The pronephros, as far as is known, is always present in types
with a larval development (i.e. all Fishes except Elasmobranchs, and
Amphibians), but it is usually only transitory. In other types
(Elasmobranchii and Amniota) it is practically absent, or at any rate
never has any physiological function.1 The segmental duct, how-
ever, persists, and serves to carry off the products of excretion from
a second series of glandular segmental tubules, which appear later,
and constitute the mesonephros or Wolffian body. This
also consists of a series of segmentally- arranged ciliated tubules
or nephridia, lying transversely to the longitudinal axis of the
body, which arise as buds from the peritoneal epithelium,

1 Mikalovics has lately shown that the primitive excretory organ in the embryos
of the Lizard, Duck, and Fowl consists of two parts, an anterior and a posterior. The
former consists of a number of vesicles, the cavities of which in a certain stage of
development communicate on one hand, by means of funnels, with the crelome, and
on the other with the cavities of the mesoblastic somites. The posterior portion of
the organ arises as a series of primitively solid structures in the mesoblastic tissue,
which do not communicate either with the ccelome or with the cavities of the somites.
The segmental duct arises on the outer and dorsal side of this apparatus, all the
constituent parts of which give rise later to hollow tubules, which come to open into
the segmental duct, and in each of which a glomerulus is formed. Mikalovics con-
siders that the anterior part of the organ corresponds to the pronephros of the
Anamnia, and the hinder part to the mesonephros. Sedgwick has also found traces
of a pronephros in chick embrycs. "We may hope for confirmation of these views in
further researches.

URINOGEN1TAL ORGANS.

297

and only 'secondarily communicate with the segmental
duct (Fig. 234A to c, and Fig. 235).

Each pronephric and niesonephric tubule is made up of the
following portions (see Fig. 234A) : — (1) a funnel-shaped ciliated
aperture, communicating with the body-cavity (peritoneal funnel,
nephrostome, Fig. 234A, ST) ; (2) a rounded mass of capillaries

FIG. 234A.— DIAGRAM OF THE (SECONDARY) CONNECTION OF THE MESONEPHRIC
TUBULES WITH THE SEGMENTAL DUCT (SG).

The two anterior tubules are already connected with the duct, while the two posterior

have not yet reached so far.
tiT, nephrostome ; M, glomerulus ; DS, coiled glandular tubule ; ES, terminal

portion of latter.

Utr

FIG. 234B.— HORIZONTAL SECTION THROUGH AN EMBRYO OF Lacerta agilis.
(After M. Braun.)

Pep, peritoneal epithelium ; Sg.bl, segmental vesicles ; Uwt mesoblastic somites.

(glomerulus) (M\ arising from the renal artery, which, lies within
an expanded portion of the tubule — the Malpighian capsule,
the outer wall of which is pushed in to receive it ; (3) a coiled
glandular tubule (DS) ; and (4) a terminal portion (US),
connecting the latter with the segmental duct (SG). (Comp.
also Fig. 235.)

298 COMPARATIVE ANATOMY.

Thus the primitive urinary system, besides its main function of
excreting waste products by means of the epithelial cells, serves
also to conduct the peritoneal fluid from the body.

This secondary urinary system, or mesonephros, is of greatest
importance in the Anamnia : in most Fishes it serves exclusively
as a urinary organ, but in others (most Elasmobranchs) it also
takes on certain relations to the generative apparatus, giving
rise to the parorchis, parovarium, and to other more or less

FIG. 234c.— THE ENTIRE EXCRETORY SYSTEM OF THE EMBRYO OF Hylodcs
martinicensis (3 millimetres long). (After E. Selenka. )

A, urinary bladder ; c, stalk of the latter which communicates with the intestine ;
Gl, glomerulus of the pronephros ; P, peritoneal epithelium ; S, S-shaped con-
volutions of the segmental duct ; Tg, segmental duct ; W, ciliated regions of
the peritoneal epithelium ; zz, urinogenital cord (formative region of the
mesonephric vesicles ; 1, 2, 3, the three csecal processes of the right and
left pronephros, with their branches ; a, a, apertures of the pronophrie
ducts into the bladder ; u, rudiments of the anterior urinary tubules, in the form
of solid ccrds ; ul to u*, urinary tubules ; r, r, apertures of the urinary tubules
into the segmental duct (Vgl, Vtf\ which thus becomes the duct of the
mesonephros.

rudimentary organs of secondary importance. Nevertheless, it
may remain as the permanent urinary organ (Elasmobranchs,
Amphibians1), or may entirely disappear as such (Amniota) ; in the
latter case, a third series of tubules are formed, giving rise to the

1 The glands formed by the posterior urinary tubules in Elasmobranclis and
Urodeles which give rise to the functional excretory organs, and which may be
provided with special ureters, are considered by Balfour to be the equivalents of the
kidneys proper (metanephros) of Amniota (comp. Fig. 238).

URINOGENITAL ORGANS. j'j'j

kidney proper (metanephros1), together with its special duct,
the ureter.2

The generative cells, that is, the ova and seminal cells,
have a similar origin throughout Vertebrates. They become dif-
ferentiated from the peritoneal epithelium.v This germinal
epithelium arises on the dorsal side of the body-cavity, on either

TO

FIG. 235. — DIAGRAMMATIC TRANSVERSE SECTIONS OF THE BODY OF A LOAVEI;
VERTEBRATE, TO SHOW THE RELATIONS OF THE SEGMENTAL ORGANS. (After
Hensen.) The right side of the figure represents a later stage than the left.

•»?, spinal cord ; Sp, ganglion of a spinal nerve ; MP, muscle-plate ; JD, wall of intes-
tine ; Ch, notochord ; WG, Wolffian duct ; A, aorta ; K, germinal epithelium ;
above K on the left side is seen the peritoneal aperture of a segmental tubule :
Gl, Malpighian capsule, shown on the left side arising as an expansion of a
urinary tubule, and on the right in a fully-formed condition, containing a
glomerulus, and communicating with the segmental duct by means of the convo-
luted tubule ; My Miillerian duct in process of formation ; X, cellular trabecuke
growing out from the Malpighian capsule into the generative gland (K).

side of the mesentery, and the adjacent mesoblastic stroma pene-
trates into it (comp. Fig. 235, K). The primitive germinal cells
are at first entirely undifTerentiated, so that it is impossible to

1 According to the researches of Mikalovics the mesonephros does not disappear,
as such, suddenly, but its greater part remains functional together with the
metanephros for some time : in Lizards, for instance, it only becomes reduced after
the first winter's sleep, that is, in the second year. Thus, to a certain extent, inter-
mediate stages exist between the condition of things seen in the Anamnia and
Amniota respectively. At one time Amniota must have existed in which the meso-
nephros served as the main excretory organ throughout life, but on the appearance
of the metanephros it became no longer needed, and was gradually reduced.

2 The metanephric duct arises in the form of a paired canal, which grows out frpm
the "Wolffian duct at the point where the latter opens into the cloaca. Its anterior
end then comes into relation with a series of segmental tubules, provided with
glomeruli, and the posterior end loses its connection with the Wolffian duct, and
come's to communicate with the allantois (urinary bladder).

300 COMPARATIVE ANATOMY.

say which will give rise to spermatozoa, and which to ova. In
the course of further development, this differentiation takes place,
and the germinal epithelium comes into relation with the meso-
nephros, as already mentioned. The final result is the formation
of a male or a female generative gland, i.e. a testis or an ovary.

The mode of development of the ova and spermatozoa is briefly as follows :—
Ova. — The cells of the germinal epithelium grow inwards amongst the
stroma of the ovary in the form of clustered masses : some of these increase
in size more than the others, and give rise to the primitive ova, while the
smaller cells form an investment or follicle round them, and serve as a
nutritive material. The investing cells multiply, and in Mammals a cavity
containing a fluid is formed in the middle of each follicle (Graafian follicle)
(see Fig. 254) : the main mass of the follicular' cells which enclose the ovum
project, as the discus proligerus (Fig. 254, Z)), into the cavity of the
follicle. When ripe, the ovum, surrounded by its vitelline membrane, comes
to the surface of the ovary and breaks through its walls into the abdominal
cavity ; it then passes into the Fallopian tube by means of the cilia on the
fimbriated aperture of the latter. A certain amount of blood is poured out
through the broken ends of the vessels in the stroma of the ovary into the
cavity of the Graafian follicle in which the ovum lay : this " wound " then
closes up, and its contained blood undergoes fatty degeneration, and gives rise
to a body of a yellow colour, known as the corpus luteum.

Spermatozoa. — As in the case of the female, primitive germinal cells
can be at first distinguished in the development of the male generative elements.
The nucleus of the larger of these gives rise to the main part of the so-called
"head" of the spermatozoon, while the surrounding protoplasm becomes
differentiated to form the motile " tail," which serves as an organ of propulsion.
Either each of the primitive germinal cells forms one spermatozoon only,
or a division of its nucleus into several portions takes place, and the cell
divides up into several spermatozoa. But there is no important difference
between these two modes of development, for in both cases we have to do with
a simple cell-division. Thus each spermatozoon is really the mor-
phological equivalent of an ovum, so that an absolutely similar
and equal portion from either parent is concerned in the
production of the embryo.1

In order to understand the general relations of the urinogenital
organs as a whole, we must now consider briefly the canals which
carry off the generative products.

In Cyclostomi and a very few other Fishes the generative
products are simply shed from the ovary or testis into the body-
cavity, whence they pass to the exterior by means of the abdo-
minal pores (see p. 265). This is probably to be looked upon
as the most primitive condition.

In all other Vertebrates except bony Fishes, a canal, called the
Mullerian duct, is formed in the female. This either becomes
split off from the segrnental duct (Elasmobranchs, Amphibia), or
arises independently as an involution of the peritoneum near the
latter (Amniota).2

1 It must be remembered that the process of fertilisation has to do essentially
with the fusion of the nuclei only of the male and female cells (Weismann and
Van Beneden). (Comp. pp. 3 and 4.)

3 According to Mikalovics, the Miillerian duct arises in the Lizard, Duck, and Chick
as a solid mesodermal rod, arid thus has originally no connection with the ccelome.

URINOGEN1TAL ORGANS.

301

In the first case the other product of the segmental duct, or
so-called secondary mesonephric duct, serves primarily in the
male as the urinogenital duct (Ley dig's duct), and in the
female simply as the urinary duct (Figs. 237, 238A and B,
w.d, vd, and 240, A, B, lgt Ig (Ur).

In the second case, the entire primary mesonephric duct gives
rise in the male to the seminal duct (Fig. 236, C, Vd)

A

B

FIG. 236. — DIAGRAM EXHIBITING THE RELATIONS OF THE FEMALE (A) AND OF THE
MALE (C) REPRODUCTIVE ORGANS TO THE GENERAL PLAN (B) OF THESE
ORGANS IN THE HIGHER VERTEBRATA.

MG, Milllerian duct ; Ut, uterus ; Ot, abdominal aperture of the Fallopian tube ;
Vg, vagina • Vm, uterus masculinus ; GM, hydatid of Morgagni ; WG, Wolfi&an
duct; Gg, Gartner's? duct ; Vd, vas ydeferens ; DC, ductus ejaculatorius ; Vs,
vesicula seminalis ; Urn, mesonephros ("Wolffian body) ; Po, parovarium ; Ep,
epididyrnis ; g, genital gland ; Ov', ovary ; Ho, testis ; N, kidney" ; Ur, ureter ;
B, urinary bladder ; Su, urinogenital 'sinus ; Cl, cloaca ; Ba, gland of Bartho-
lini ; GD, Cowper's gland ; Pr, Pr, prostate gland ; £, rectum ; Ggl, copulatory
organ. (In Fig. A the clitoris is indicated, but not lettered.)

(Wolffian duct

or

vas deferens), while in the female it

usually becomes rudimentary, and is then spoken of as Gartner' s
duct (Fig. 236, A, Gg). The metanephric duct or ureter
(Fig. 236, Ur) (which is possibly confined to the Amniota),2 serves
to carry off the products of urinaiy excretion in both sexes.

1 The term Wolffian duct is used to describe the duct of the mesonephros after
the formation of the Mlillerian duct, whether the latter is developed independently
or not. 2 Comp. note on p. 298.

302 COMPARATIVE ANATOMY.

The Miillerian duct (Fig. 236, B, MG) of the female gives rise
to the oviduct, and in Mammals becomes distinctly differentiated
into three portions, — a Fallopian tube, a uterus, and a vagina,
the latter of which opens to the exterior (Fig. 236, A, Ot, Ut, Vg).
The Fallopian end of the oviduct always opens into the abdominal
cavity by a ciliated funnel-shaped aperture. This abdominal aper-
ture represents the only connection between the body-cavity and
the exterior in the Amniota, where the nephrostomes no
longer appear in the course of development.1

In the male, the Miillerian duct is always developed, but
plays no important part, coming into the category of rudiment-
ary organs. It will be further treated of together with the
generative organs in a later chapter.

URINARY ORGANS.2

Fishes and Dipnoi. — The Myxinoids retain throughout
life a functional pronephros which is provided Avith numerous
peritoneal apertures and a limited number of glomeruli.

In Petromyzon slight rudiments of the pronephros of the
larva (Ammoccetes) alone persist, and the mesonephros with its
(segmental) duct becomes the functional urinary organ.

In the Teleostei the pronephros may possibly persist in
some cases, but further researches are necessary on this point. The
mesonephros constitutes the main, if not the entire, excretory
organ of the adult, and consists of a narrow band varying in size,
which lies on the dorsal side of the body-cavity, between the verte-
bral column and the air-bladder.3 Secondary fusions between the
organ of either side often occur, and this is also true of Ganoids.
The urinary duct in both groups probably represents the primary
segmental duct, and may lie more or less freely, or be embedded in
the substance of the kidney. Posteriorly the two ducts usually fuse
together and become expanded to form a kind ofurinarybladder,
which has evidently nothing to do with the similarly-named organ
(allantoic bladder) of Amphibia and Amniota (comp. p. 273). The
"bladder" usually opens behind the anus, — either independently,
or together with the genital ducts, — by a simple pore, or on the
summit of a urinogenital papilla.

The splitting of the segmental duct into a Wolffian and a
Miillerian duct is not known to occur in Teleostei; in Elasmo-
b ranch s this differentiation does take place, and at the same time a

1 For a different view see Mikalovics in the notes on pp. 296 and 300.

2 No urinary organ is at present known to exist in Amphioxus.

3 The delicate glistening threads from which the nest of the Sea-Stickleback
(Spinachia vulgaris) is made are formed as a secretion of the urinary tubules, which
undergo a change of function at the breeding- season. The secretion is mucin, which
becomes hard in water (Mobius).

URINARY ORGANS.

303

division of the mesonephros into an anterior and a posterior section
may be observed (comp. Figs. 237, pd, and 238, m.d, w.d, s.t). In

FlG. 237. — DlAGKAM OF THE PRIMITIVE CONDITION OF THE KlDNEY IN AN

ELASMOBRANCH EMBRYO. (After Balfour.)

pd, segmental duct : it opens at o into the body-cavity, and its other extremity com-
municates with the cloaca ; x, line along which the division appears which
separates the segmental duct into Leydig's (Wolffian) duct above, and the
Mullerian duct below ; s,t, segmental tubes : they open at one end in the body-
cavity, and at the other into the segmental duct.

FIG. 238A. — DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS IN
AN ADULT FEMALE ELASMOBRANCH. (After Balfour.)

m.d, Mullerian duct; w.d, Leydig's (Wolffian) duct; d, ureter; s.t, segmental
tubes : five of them are represented with openings into the body-cavity : the
posterior segmental tubes form the mesonephros ; ov, ovary.

FIG. 238B. — DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS IN
AN ADULT MALE ELASMOBRANCH. (After Balfour.)

m.d, rudiments of Miiilerian duct ; w.d, Leydig's (Wolffian) duct, marked vd in
front, and serving as vas deferens ; s. t, segmental tubes : two of them are
represented with openings into the body-cavity ; d, ureter ; t, testis ; nt, canal
at the base of the testis ; VE, vasa efferentia ; Ic, longitudinal canal of the
Wolffian body.

the male, the former comes into connection with the genital gland
(Fig. 238B, VE, nt, £), and its tubules open directly into the

304 COMPARATIVE ANATOMY.

Wolffian duct ; while the latter, which persists as the permanent
kidney (comp. note on p. 298), empties its secretion by means of
separate ducts into the Wolffian duct, which thus serves to conduct
both urinary and genital products. In the female (Fig. 238A)
the genital gland has no connection with the Wolffian duct, and
the ova pass to the exterior by means of the Miillerian duct.

A narrower anterior, and a broader middle and posterior portion can
usually be distinguished in the kidneys of Elasmobranchs. The outer border
is usually notched, and this, together with the arrangement of the nephro-
stomes in the embryo, points to the original segmental arrangement of the
organ. The segmental character, however, disappears later on ; in the adult
the nephrostomes are without exception much less numerous than the vertebra
of this region, but their number and size varies much in different genera and
even in individuals.

The kidneys of Sturgeons appear to show many points of
similarity to those of Elasmobranchs ; further investigations are,
however, necessary before their relations can be fully explained,
and the same may be said with regard to the kidneys of Dipnoans
and bony Ganoids. In the Dipnoi, the existence of nephrostomes,
although not proved, is very probable ; the urinary organ of these
animals corresponds to the mesonephros. In Ceratodus, the duct
lies freely in a peritoneal fold, while in Protopterus it is embedded
in the substance of the kidney. In both forms the kidney
is lobulated; it is relatively much smaller in Ceratodus than in
Protopterus. The lobes do not correspond to the segmentation
of the vertebral column.

A close examination of the organ which has usually been spoken of as the
kidney in Teleostei and Ganoids shows that a larger or smaller portion of it
— more particularly the anterior part — consists of an adenoid or lymphoicl
substance.

Amphibia. — In these, the most primitive condition is met
with in the Gymnophiona, in which the kidneys (Fig. 239,
Ni) consist of long narrow varicose bands, usually extending from
the heart to the anterior part of the cloaca, which latter is often
much elongated. In the embryo they consist of definite masses,
which are arranged segmentally (that is, correspond with the
segmentation of the vertebral column), and in each of them a
glomerulus, a nephrostome, and an excretory duct can be
distinguished (compare Fig. 234A).

This condition sometimes persists in the anterior portion of the
kidney, while, owing to secondary processes of growth, as many as
twenty nephrostomes are later on met with in a single body-
segment. The number of nephrostomes in the entire kidney may
amount to a thousand or more.

As regards the urinary duct and the relations of the entire
renal apparatus to the generative organs, the Gymnophiona in all
essential points resemble other Amphibians.

URINARY ORGANS.

305

The kidneys of U rode la and Anura lie in the usual position
on the dorsal side of the body-cavity ; in the former they are band-
like and more extended longitudinally than in the latter, in which

FIG. 239. — THE ENTIRE VISCERA OF Siphonops annulatus ( ? ) in situ. (The body-
wall is slit up along the median ventral line, and its two halves reflected.)

Intestinal tract. — Oes, oesophagus ; Mq, stomach; Dd, Ddl, small intestine;
Dda; large intestine ; Cl, cloaca ; Bl, Bl1, the anterior larger and posterior
smaller end of the urinary bladder ; Lcb, liver ; Bis, gall-bladder ; Pan, pan-
creas ; If, spleen ; Per, peritoneum (gastro-hepatic omentum).

Urinogenital organs. — Ov, Ov, ovaries; Afg, Mg, Miillerian ducts (oviducts);
Ni, Ni, kidneys ; Ur. ureter.

Eespiratory organs. — L, well-developed right lung ; Ll, rudimentary left lung ;
Tra, trachea.

Organs of circulation. — Ve, ventricle; At, atrium; Ba, conus arteriosus ; Ao,
aorta ascendens of left side : that of the right is not specially indicated ; Aod,
aorta descendens of left side ; Ap, Ap, pulmonary artery ; Vp, pulmonary vein ;
Vn, vein receiving blood from the urinogenital organs, the muscles of the back,
and the vertebral canal ; J, jugular vein ; Ci, postcaval ; DC, ductus Cuvieri ;
Vep, Vep, hepatic portal veins.

they are shorter and more compact, and confined to the middle
portion of the coelome.

In Urodeles they always consist of a narrow anterior, and a

x

306

COMPARATIVE ANATOMY.

broader and more compact posterior portion. The latter gives rise
to the functional kidney (Fig. 240, N), while the former becomes
connected in the male with the generative organs. Delicate ducts,

—711,

a

Ov.~-

FIG. 240. — DIAGRAM OP THE URINOGENITAL SYSTEM OF A MALE (A) AND FEMALE
(B) UEODELE ; FOUNDED ON A PREPARATION OF Triton tceniaius. (After
J. W. Spengel.)

Ho, testis ; Ve, Ve, vasa efferentia of testis, which fall into the longitudinal canal of
the Wolffian body, t ; a, collecting tubes of the niesonephros, which fall into
the Wolffian (urinogenital or Ley dig's) duct (Ig, Ig} ; the latter serves in the
female (Fig. B, Ig) simply as the urinary duct (Ur) ; the system of the vasa
efferentia (testicular network) is here rudimentary ; mg, mg1 (Od), Miillerian
duct ; Ot, peritoneal aperture of latter in the female ; Ov, ovary ; GN, anterior
sexual portion of kidney (parorchis of the male) ; N, posterior non-sexual portion
of kidney.

or vasa efferentia, pass out from the testis (Fig. 240, A, Ho, Ve,
Ve) into the substance of the anterior portion of the kidney, and there
open into the urinary tubules ; they may either enter the kidney

URINARY ORGANS. 307

direct, or else open first into a longitudinal collecting duct (t), from
which fine canals pass to the urinary tubules. Thus the seminal
fluid passes through the nephridia as well as through the Wolfnan
duct, which serves as a urinogenital duct (Fig. 240, A, Ig, a).

In Urodela and Anura of both sexes the Wolffian duct opens
separately on either side into the cloaca, receiving first, in Urodeles,
a number of ducts from the posterior part of the kidney.1

The urinary bladder, which is usually bilobed, opens on the
ventral wall of the cloaca opposite to the urinogenital apertures.
The morphological signification of the bladder has already been
explained in the chapters on the alimentary canal and vascular
system (pp. 231 and 273).

Slight indications of a segmental arrangement are found only
in the anterior sexual portion of the kidney of Urodeles ; in the
posterior part, and in the entire kidney of Anura, all traces of
segmentation have disappeared. In both cases, however, the
iiephrostomes remain throughout life in great numbers on the
ventral surface of the kidney, which is covered over by the
peritoneum.

The nephrostomes are connected with the urinary tubules in larval Anura,
but later on they become separated from them, and open into the renal-portal
vein. In consequence of this change of function, for such it must be con-
sidered, the body-cavity of adult Anura serves as a closed lymph-sinus,
as in the Amniota ; the peritoneal fluid, which in the larva was carried to the
exterior and lost, is in the adult poured into the general circulation, like the
rest of the lymph.

Reptiles and Birds. — In these, as in all other Amniota, the
mesonephros, so far as it is retained beyond the embryonic period,
is entirely separate from the functional excretory apparatus ; this
consists of a metanephros. entirely wanting in nephro-
stomes (compare p. 298).

The metanephros never extends so far along the body-cavity as
does the mesonephros ; as a rule it has the form of a small compact
or lobulated organ, usually lying within the posterior half of the
body-cavity, or even entirely confined to the pelvic region ; it has
the latter position, for instance, in most Reptiles (Fig. 241, JV) and
all Birds (Fig. 242, N). The posterior end of the kidney, which is
generally narrower than the rest, may even extend under the root
of the tail, as in Lacerta, in which region there is a fusion of the
organ of either side.

Thus, according to the position of the kidneys, the ureters
either do not extent! any distance along the body-cavity, or they
may have a longer or shorter free course. The latter is the case,
for instance, in Crocodiles, and more especially in Birds (Fig.
242, Ur) : in the last-named the kidneys lie closely embedded

1 In Anura the Wolffian ducts pass some distance independently along the body-
cavity, in correspondence with the position of the kidneys ; in the male each is often
provided with an enlargement, the vesicula seminalis.

x 2

308

COMPARATIVE ANATOMY.

within the pelvis, and their ventral flattened surface is usually
lobulated, and often penetrated by deep furrows and clefts in which
the veins lie embedded (Fig. 242, V, V} ; their posterior ends may
fuse together in the middle line, as in Lizards.

There is not always a perfect symmetry between the organ of
either side, and this is most marked in Snakes, in which the

FIG. 241. — EXCRETORY APPARATUS OF Monitor indicus.

The right kidney is shown in its natural position, while the left is turned on its
longitudinal axis, so that the ureter and the collecting tubes are visible. The
urinary bladder is not represented.

N, A", kidneys ; SG, collecting tubes which open into the ureter (Urt Ur1} ; Url
aperture of ureter into the cloaca.

greatly lobulated kidneys, like those of limbless Lizards, are
elongated, narrow, and band-like, in correspondence with the form
of the body.

A urinary bladder, arising from the ventral wall of the
cloaca, is present in Lizards and Chelonians ; it is usually bilobed,
as in Amphibia, and so points to a primitively paired condition.
A bfadder is wanting in Snakes, Crocodiles, and Birds.

Mammals. — The kidneys of Mammals are proportionately
small, and lie on the quadratus lumboruin muscle and ribs. They
usually possess a convex outer, and a concave inner border ; the
latter is called the hilum, and at this point the ureters arise and
the blood-vessels enter. The expanded proximal portion of the
ureter is divided up to form one or more calyces (Fig. 243, Co),
into which small papilliform processes of the pyramids (see

U BINARY ORGANS.

309

p. 310) project ; on the summits of these the urinary tubules open
in varying number (between Pr and Co). The calyces are continu-
ous with a large cavity in the widened portion of the ureter called
the pelvis (Pi), and from this the ureter (Ur) passes backwards to

An

FIG. 242. — MALE URINOGENITAL APPARATUS OF HEEON (Ardea cinerea).

N, kidneys ; Ur, ureter, opening into the cloaca (Cc) at Sr ; Ho, testis ; Ept epi-
didymis ; Vd, vas deferens, which opens at Vdl on a papilla in the cloaca ;
V, V, furrows on the ventral surface of the kidney in which veins lie embedded ;
Ao, aorta ; £F, bursa Fabricii, which opens into the cloaca at BF1.

open into the bladder on its dorsal side, sometimes nearer the apex,
sometimes towards the fundus.

The urethra, arising from the bladder, is always short in the
female, while in the male it is drawn out into a long tube which
extends through the penis, and is lined by erectile tissue (corpus
spongiosum) (comp. p. 329).

The definitive kidney (metanephros) is greatly lobulated in

310 COMPARATIVE ANATOMY.

the embryo ; this condition may remain throughout life (e.g.,
Cetacea, Pinnipedia, Ursus, Lutra, Bos), or the lobes may become
more or less completely united. In the latter case the original
division into lobes may still be recognised more or less plainly
internally. A section of the kidney shows an inner layer, the

Pi

FIG. 243. — DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE KIDNEY or A

MAMMAL.

E, R, cortical substance ; M, M, medullary substance arranged in pyramids (Pr) :
between the latter the cortical substance extends in the form of the columns of
Bertini (B, B} ; Co,, calyces ; Pe, pelvis ; Ur, ureter.

medullary substance, arranged in the form of wedges, —
the urinary pyramids, — and an outer layer, or cortical
substance, extending as the columns of Bertini between the
pyramids (Fig. 243, E, B}. The pyramids correspond roughly to
the embryonic lobes of the kidney, though several lobes may fuse
together in one pyramid.

The glomeruli as well as the coiled tubules, which are surrounded by a
network of blood-capillaries, lie in the cortical substance, while the so-called
straight tubules occur principally in the pyramids, where they gradually
anastomose to form larger collecting tubes.

Concerning the morphological relations of the urinary bladder of
Mammals, comp. p. 274.

GENERATIVE ORGANS.

In Amphioxus the generative " glands " or gonads remain for
a long time in an undifferentiated condition as regards sex. They
have a marked segmental arrangement, and each portion opens
separately into the peribranchial cavity, whence the generative
products pass to the exterior through the atrial pore.

Fishes. — Specially differentiated generative ducts are wanting
in Cyclostomes, as well as in Eels, female SalmonidaB,
and Laemargus borealis (an Elasmobranch). In these cases,
both sperm and ova are shed directly into the body-cavity, whence
they pass out through the abdominal pores (see p. 265.)

GENERATIVE ORGANS. 311

The generative gland of Cyclostomes consists of a long un-
paired organ suspended to the dorsal wall of the body-cavity by
a fold of peritoneum, the mesoarium or mesorchium, as the
case may be. In other Fishes, the gonads are only exceptionally
unpaired, and even then, this is only a secondary condition : in
all other Vertebrates they are also originally paired. There is
usually a want of symmetry observable between the organ of the
right and left sides, and that of one side may even entirely disappear,
as in Ammodytes tobianus, Cobitis barbatula, and others.
Secondary fusions may also take place.

The ovaries and testes of Teleostei closely correspond with
one another as regards position and the arrangement of their
ducts. The ovary usually forms an elongated sac, which is blind an-
teriorly, and on the inner walls of which the ova arise ; this sac is
continued backwards to form the oviduct. The latter, which is
generally short, as a rule fuses with its fellow to form an unpaired
canal ; this opens either on a level with the integument or on
a papilla, which may become elongated to form a tube or
" ovipositor." l

The testis of Teleosteans is elongated, and often lobulated in
form. Its duct, which is often intensely white, opens between the
rectum and the urinary aperture, after uniting with its fellow to
form an unpaired canal.

In cases where traces of copulatory organs or other accessory structures —
which are spoken of as " seminal vesicles," or " prostates," — are present, they,
like the so-called urinary bladder, have nothing whatever to do with the
similarly-named structures of higher Vertebrates.

In the above description the terms " Miillerian " and " Wolffian
ducts" have been purposely avoided, for it is very doubtful
whether the generative ducts of Teleostei are in any way homo-
logous with them, and further investigations are necessary to decide
the question (comp. note on p. 313).

In by far the greater number of Elasmobranchs the ovaries
are paired, and this is always the case as regards the oviducts,
which, unlike those of Teleosteans, are always separate from the
ovaries, and correspond to the Miillerian ducts. Their anterior
portion has a common opening into the body-cavity, and further
back each is provided with a so-called "oviducal gland." The
anterior part of the oviduct is always narrower and more delicate
than the posterior, which dilates to form a kind of uterus, in
which (when the shark is viviparous) the embryo undergoes

1 In the genus Girardinus, which, like many other Fishes (numerous Elasmo-
branchs for instance), brings forth its young alive, the body-cavity of the female
is much longer than that of the male, and the ovary serves also for a uterus.
Contrary to the general rule, the ova do not become free from their place
of origin before fertilisation, so that the spermatozoa must bore their way
through the germinal and follicular epithelium, and thus even pass into
the parenchyma of the ovary.

312

COMPAEATIYE ANATOMY.

i

FIG. 244. — MALE URINOGENITAL APPARATUS OF THE STURGEON.
N, Nl, N2, the different portions of the kidneys ; SG, SG1, SG2, the different parts
of the ducts of the kidneys ; Vd, Vdl, Vd2, vas deferens ; Ho* festis :
Ve, Te, network of the vasa efferentia ; \MH~ Miillerian duct, which unites with
the collecting duct of the kidneys at M Gfl ; t, point at which the two collecting
ducts of the kidneys unite,

development.1 Posteriorly, the oviducts open into the cloaca
somewhat behind the apertures of the ureters — either separately,
or by a common canal.

1 The greater number of Sharks are viviparous. (Concerning the umbilical
placenta found in Mustelus leevis and Carcharias, conip. p. 12.)

The embryos of Mustelus antarcticus are provided with membranes which bear

GENERATIVE ORGANS.

313

The test is of Elasmobranchs is always paired and sym-
metrical, and usually lies, supported by the mesorchium, towards

a

FIG. 245. — FEMALE URINOGESITAL APPARATUS OF Protoptcrus annccLcns. (From
the ventral side, natural size.) (After H. Ayers.)

a, vent ; h.d, rectum ; Hg, apertures of ureters into cloaca ; L, L, lymphoid tissue ;
Mg, Miillerian duct ; M.tr, abdominal aperture of latter ; N, kidney ; 0, ovary ;
p, lung ; V.c.a, postcaval vein ; V.u, urinary bladder.

the anterior portion of the body-cavity. The relations of the
vasa efferentia to the mesonephros have been already mentioned
(comp. pp. 298 and 303, and Fig. 238B), and a somewhat similar
arrangement is seen in Lepidosteus.

Amongst Ganoids, the female organs of Lepidosteus l are formed on the
same type as those of the Teleostei, while in cartilaginous Ganoids a more
or less incomplete splitting of the segmental duct into a Miillerian and a

a superficial resemblance to the amnion and chorion of Mammals, but these are
both formed from the maternal tissues (T. J. Parker).

1 The cavity within the ovary of Lepidosteus is formed by a fold of peritoneum
on either border of the gland growing towards its fellow, and meeting in the middle
line ; into this cavity the ova are shed, and are carried to the exterior by an oviduct
which is probably formed by continuations of these peritoneal folds behind the
ovary. A similar mode of development of the oviduct possibly also obtains in
Teleostei.

314 COMPARATIVE ANATOMY.

"Wolffian duct takes place (Fig. 244, MG). The latter probably serves in
the male as a urinogenital duct, and in the female as a urinary duct only.
Should more complete histological examination confirm these statements, the
mode of development of the generative organs of cartilaginous Ganoids will be
seen to resemble closely those of Elasmobranchs and Amphibians.

Hermaphrodite structures have been observed in certain Fishes : in
the different species of Serranus, for instance, they are constantly present.
Hermaphroditism also occasionally occurs in Sargus, Gadus morrhua, and
many others.

In the Dipnoi, the gonads and their ducts lie along the outer
border of the kidneys. During the breeding-season they become
greatly enlarged, and extend round the entire gut. The oviducts
are long and slightly coiled, reminding us in many points of those
of Amphibia : each communicates with the body-cavity by a funnel-
shaped aperture near the pericardium, and is provided with a well-
developed albumen gland. The ovaries undergo the greatest
variations according to age and the time of year. In the unripe
condition they have the form of long and narrow bands, which
extend along the whole body-cavity. In the young Ceratodus
they are distinctly lobulated, and in both Ceratodus and Proto-
pterus each ovary of the adult has the form of a thin- walled sac,
in the inner walls of which the ova are developed. The eggs are
shed into the body-cavity by the bursting of the walls of the sac,
and they pass thence into the oviducts.

In the male, the manner in which the sperm is conducted to
the exterior is not certainly known : it may possibly pass out
through the abdominal pores. The Miillerian ducts, although
less developed than in the female, are clearly present in the male.
The structure of the testis requires further investigation.

Amphibia. — The gonads of Amphibia are usually situated in
about the middle of the body-cavity : they are paired and symme-
trical, and lie right and left of the vertebral column ; their form is
usually modified by the shape of the body. Thus in the Gymno-
phiona (Fig. 239, Ov), the ovary has the form of a long and
narrow band, while the testis is made up of a long chain of small
bodies united together by a collecting duct (Fig. 246, Ho, Sg).
Each individual portion of the testis of Csecilians is made up of a
double row of rounded capsules (Fig. 246, K), in which the sperm
is formed, and from which it is passed into a collecting duct (8g\
which perforates each portion of the organ. A transverse canal
(Q) is given off from the free portion of the collecting duct lying
between every pair of testis lobes ; this passes towards the kidneys
(N, N), and opens into a longitudinal canal (Z). From the latter
the sperm passes through a second system of transverse canals
(Q\ C1) to the Malpighian capsules, and thence through the urinary
tubules into the urinogenital duct (JET&).

The male generative apparatus of all Urodela and certain
Anura (Bufonidse), corresponds in the main with the arrange-

CNJV
GENERATIVE ORGANS.

315

FIG. 246. — DIAGRAM OF A PORTION OF THE MALE GENERATIVE APPARATUS OF

THE GYMNOPHIONA.

Ho, Ho, testis ; Sg, collecting duct of testis ; K, K, testicular capsules ; Q, Q, transverse
canals connecting the collecting duct with the longitudinal canal (L, L) ; Q1, Q*,
second series of transverse canals ; M, M, Malpighian capsules ; N, N, kidney ;
ST, nephrostome ; S, convoluted portion of urinary tubule ; HS, urinogenital
duct.

FIG. 247. — TESTIS AND ANTERIOR END OF KIDNEY OF Rana csculenta.
(Semidiagrammatic. )

Ho, testis ; q, q, transverse canals of the testicular network, which give rise to blind
processes at 1 1 ; L, longitudinal canal of the testicular network, from which the
inter-renal network (C, G) arises ; N, kidney ; Ur, urinogenital duct.

316 COMPARATIVE ANATOMY.

ment which has already been described in the chapter on the
urinary organs (see p. 306). The external form of the testis,
however, varies greatly, and is either pointed at one or both ends
(Fig. 240, A, Ho), or more or less round or oval (Anura).

In Eana, Bombinator, and Alytes the vasa efferentia of the testis
become gradually separated from the kidney, that is, they either open directly
into the urinogenital duct, without becoming connected with the
urinary tubules (Rana) (Fig. 247, C, £7r), or the greater number of the
posterior canals end blindly, while only the anterior ones are directly connected
with the urinogenital duct (Bombinator). In Alytes, those vasa efferentia
at the anterior end of the kidney which possess a lumen open into the
Miillerian duct : this is a very special condition, and is not known to occur
in any other animal. The urinary duct, which comes off from the posterior
end of the kidney also opens into the Miillerian duct, the portion of which
anterior to this point serves as a vas deferens, while its posterior part functions
as a urinogenital duct.

In all other Amphibians, Miillerian ducts are always present, but in
the male they always remain in a more or less rudimentary condition, and lie
along the outer border of the kidneys in a similar position to those of the
female. They may or may not be provided with a lumen and apertures of
communication with the body-cavity and cloaca.

Hermaphroditism occasionally occurs amongst the Anura. In the males
of Rana temporaria " ova " are at times developed, embedded within the sub-
stance of the testis (Hermaphrodite gland, or ovotestis), and one testis
may even be replaced by a rudimentary ovary. In these cases, the Miillerian
duct may be as well developed as in the female. A body attached to the
anterior end of the testis in various species of Toads (" Bidder's organ ") also
apparently represents a rudimentary ovary.

The ovaries of Urodela are always formed on a common
plan. Each consists of an elongated closed tube, with a con-
tinuous lumen. In Anura, on the contrary, the ovarian sac
(Fig. 248, Ov) is divided up into a longitudinal row of (3 to 20)
separate pockets or chambers. In both cases a mesoarium is
always well developed, and there is no direct connection between
the ovaries and oviducts. The latter open far forwards into the
body-cavity by funnel-shaped apertures (Od, Ot\ and at a con-
siderable distance from the anterior ends of the kidneys: they
take a tolerably straight course along the outer borders of the
kidneys to the cloaca in young animals, but become greatly
coiled and convoluted in the adult (Fig. 248, Od). A short dis-
tance from their termination each oviduct becomes dilated to
form a thin-walled sac, and, after becoming again narrowed, usually
opens separately on a papilla on the dorsal wall of the cloaca
(Fig. 248, Ut, P). In the genera Bufo and Alytes alone, the
two oviducts fuse together into a posterior unpaired canal.

After receiving a gelatinous coating from the glandsin the wall of the middle
part of the oviduct, the eggs pass into the dilated portion of the duct, and
become united together into irregular masses (Frog) or chains (Toad).1

1 According to P. B. and C. F. Sarasin, Epicrium glutinosum (Gymnophiona)
is oviparous. The eggs are very similar to those of Sauropsida : they are exception-
ally large (9mm. long), of an oval shape, and possess a large yolk, which is light-

GENERATIVE ORGANS.

317

Finally, the so-called fat-bodies (corpora adiposa) must be mentioned :
these are present in all Amphibia in connection with the generative glands,
and are formed of adenoid substance, fat, and leucocytes, and contain numerous
blood-vessels. They are apparently formed by the degeneration of the anterior
part of the genital ridge, and " Bidder's organ " (see p. 316) in the Toad seems
to represent a part of the ridge which has not become degenerated so far.
The corpora adiposa probably have an important physiological (nutritive)
relation to the generative glands ; this gives an explanation of the fact that
Amphibians, after remaining for months, throughout their winter sleep, with-
out food, are able as scion as spring arrives to give rise to thousands of offspring.
The curious lymphoid organs of many Fishes and Reptiles have probably a
similar function (comp. pp. 304 and 320, and Fig. 245).

-~0t

FIG. 248. — URINOGEKITAL ORGANS OF A FEMALE liana esculcnta.
Ov, left ovary (that of the right side is removed) ; Od, oviduct ; Ot, abdominal
aperture of oviduct ; lit, the dilated posterior end of the oviduct ; P, opening of
latter into the cloaca ; N, kidneys ; S, Sl, apertures of ureters into the cloaca, sur-
rounded by longitudinal folds (*), which are separated by a deep depression (t).

Reptiles and Birds. — In these, as in other animals, the form
of the gonads becomes modified by that of the body. Thus in

yellow in colour, and consists of both yellow and white granules. They are coated
with a tough albumen in the oviduct, and this becomes drawn out at the poles into
chalazee, by means of which the eggs are connected together like the beads of a
necklace. Thev are laid in the earth, and the mother coils herself round them.

318

COMPARATIVE ANATOMY.

Chelonians they are broad, while in Snakes and snake-like Lizards
they are more elongated. In the latter cases, as well as in other
Lizards, they are asymmetrical, so that the organ of one side comes

Ot-

FIG. 249. — FEMALE URINOGENITAL APPARATUS OF Lacerta muralis.

N, kidneys ; Url, apertures of the ureters into the cloaca (01) ; B, urinary bladder ;
jB1, neck of the latter (cut open) ; R, rectum ; JB1, opening of rectum into the
cloaca ; Ov, ovaries ; t, remains of mesonephros ; Od, oviducts, which open into
the cloaca at Od1 ; Ot, abdominal openings of oviducts.

to lie in front of that of the other. More room is thus obtained
for the development of the ovaries, and, in cases where the eggs
are very large, the organ of one side tends to disappear ; in Birds,

GENERATIVE ORGANS.

319

for instance, the left ovary only is completely developed and
functional.

In Reptiles the ovaries lie near the vertebral column, and are
covered by peritoneum ; their lumen is penetrated by a highly vas-
cular network of trabeculse, enclosing the ova. In the lymph-
cavities thus formed the formation of ovarian follicles takes place.
The development of follicles occurs throughout life in Reptiles, as

FIG. 250, — MALE UKINOGENITAL ORGANS OF Anguis frcujilis.
F. Leydig.)

(After

Ho, testis ; f, the so-called "yellow body" (suprarenal); Ep, parorchis ; Vd, vas
deferens ; p, p, common aperture of the ureter (Ur, Url) and vas deferens on a
papilla on the dorsal wall of the cloaca (01) ; B, urinary bladder ; r, rectum ;
N, kidney ; mg, rudiment of the Mullerian duct.

in the Anamnia, while in other Amniota it takes place only in
the embryo, or at any rate for only a short time after birth.

The oviducts (Fig. 249, Od, Of) possess wide funnel-shaped
abdominal apertures, and are usually much folded transversely ;
their walls are provided with numerous muscular elements and
glands for the formation of the albumen and egg-shell. They
increase in size in the breed ing -season. In Birds they are con-
siderably coiled.

320 COMPARATIVE ANATOMY.

Only slight remnants of the mesonephros and Wolffian duct remain in the
female in Eep tiles, and these undergo fatty degeneration. They lie asym-
metrically, arranged in a single row on either side, between the oviduct and
vertebral column. The remains of the Wolffian duct are more marked in
female Snakes, Chelonians, and in Geckos than in other Lizards..

The testes of Sauropsida correspond in position with the
ovaries, and, like them, increase in size in the breeding-season.
They have an oval, round, or pyriform shape (Figs. 242 and 250,
Ho), and are made up of greatly convoluted seminal tubules, held
together by fibrous tissue. In Reptiles (Lacerta, Anguis), " yellow
bodies," which correspond to suprarenals, lie along the outer
border of the testes, and at this point transverse canals pass out
from the testis to the parorchis (Figs. 242 and 250, Ep).

The latter consists of greatly convoluted canals, and from
it arises the vas deferens (Wolffian duct), which either takes a
straight course, or is more or less coiled (Figs. 242 and 250, VcT).
In Birds it opens by an independent aperture (Fig. 242, Vdl) into
the cloaca, while in Lizards it fuses with the ureter shortly before
entering the latter.

Remains of the anterior portions of the Miillerian ducts are present in the
male, their position corresponding with those of the female. Their lumen is
not continuous throughout, but the abdominal aperture may remain open
(Emys europsea).

Lymph oid organs are present in many Rep tiles, and probably have a
physiological relation to the generative organs (comp. p. 317). In many
Lizards they are large and variously coloured, and lie within the pelvic
region ; in Snakes they extend along almost the entire body-cavity.

Mammals. — In Mammals the generative apparatus no longer
extends along the entire body-cavity, as in the lower groups of
Vertebrates, but is confined to the lumbar and pelvic regions.
Moreover, in correspondence with the close relations which take
place between mother and embryo, there is a much greater differ-
entiation of the generative organs than occurs in lower types. The
transition is not, however, a sudden one, for in the lowest Mammals,
viz. the Monotremes1 and Marsupials, these organs show many
points of resemblance with those of Reptiles and Birds (comp.
Figs. 251 and 252).

Thus in Monotremes the left ovary is more strongly
developed than the right, and each has the appear-
ance of a bunch of grapes; the cloaca persists, and the
Miillerian ducts (oviducts) remain distinct from one
another. A more or less complete separation of the oviducts is
also seen in Marsupials, and this point deserves special attention
on account of its important morphological significance.

In order to explain the gradual differentiation of these parts,
their condition in the Didelphidse (Opossums), — which come

1 It has been recently proved that both Ornithorhynchus and Echidna lay eggs
(Caldwell and Haacke) (comp. p. 5). Further details on these points are to be
expected shortly.

GENERATIVE ORGANS.

321

nearest to the Monotremes, — must now be described in greater
detail.

A dilated portion of each oviduct (Fig. 252, A, Od\ giving rise
to a uterus (Ut\ is plainly distinguishable from the rest, and its

J>

FIG. 251. — A, MALE URINOGENITAL ORGANS OF Ornithorhynchw paradoxiis ;

B, FEMALE URINOGENITAL ORGANS OF Echidna hystrix.

AT, kidneys ; Ur, ureter ; B, urinary bladder ; Sug, urinogenital sinus ; Ho, testis ;
Ve, vas efferens ; Ep, epididymis ; Vd, vasdeferens ; Od, oviduct ; r, rectum ;
Cl, cloaca, opening to the exterior at Cll; m to m3, muscles of the cloaca and pen is ;
Gp, glans penis, enclosed within its fibrous tube ; Pp, prepuce ; Cli, clitoris ;
*, *, aperture through which the copulatory organ extends into the cloaca.

narrowed posterior end comes into close contact with its fellow in
the middle line. At this point (f ) each uterus is connected with

322 COMPARATIVE ANATOMY.

the portion of the oviduct lying more posteriorly, or vagina (Vg\
by a distinct os uteri. The vagina then curves sharply outwards,
and, passing backwards, opens close to its fellow into the elongated
urinogenital sinus (Sug). The ureters, as in all other Marsupials
in which the vaginae have a similar arrangement, pass between the
curved portions of the vaginae to the bladder (B).

From the condition of the female generative organs in Didel-
phys that seen in other Marsupials can be easily explained. In
Phalangista vulpina and Phascolomys wombat (Fig. 252,
B and C) the anterior ends of the knee-shaped bends of the vagina
(comp. Fig. 252, A, f) come to lie closer and closer together, and
begin to extend backwards towards the urinogenital sinus, the
septum between them disappearing at the same time. A vaginal
caecum is thus formed (Fig. 252, B, C, VgB\ and this may become
more elongated, and finally extend backwards so as to meet the
upper (anterior) wall of the urinogenital sinus, into which it may
open by the formation of a so-called third vagina. This is
known to occur in seven species of Halmaturus, two of Petrogale
and Osphranter respectively, and in Onychogalea fraenata.

In all other Mammals the posterior portions of the Miillerian
ducts become fused together to form an unpaired vagina, and a
cloaca exists only in the embryo (comp. p. 236). A fusion may
also take place more anteriorly, and, according to its extent, the
most various forms of uteri result (uterus duplex, bicornis,
bipartitus, and simplex), as is shown in Fig. 253, A to D. The
Primates possess a simple uterus1 (Fig. 253, B), and in this case
the primitively paired condition of the Miillerian ducts is seen only
in the Fallopian tubes. The latter vary much in form, and their
abdominal apertures are usually provided with fringe-like append-
ages (fimbriae). The ureters, unlike those of Marsupials, always
pass to the outer sides of the genital passage, the vagina being
single.

The ovaries are usually small, and rounded or oval in shape,
their surface being either smooth, irregular, or furrowed. The
point at which the nerves and vessels enter is not covered by
peritoneum, and is called the hilum.

The reader is referred to p. 300 and Fig. 254 for further details
as to the more minute histological structure of the ovary and the
formation of the ova.

Remains of the mesonephros, known as the parovarium, are
present in the neighbourhood of the ovary, oviduct, and uterus.
These usually consist of small caecal tubes, forming a network,
which are connected together by a collecting duct. In cases
where the Wolrfian duct persists in the female, it passes from
the parovarium to the urinogenital sinus, and is spoken of as
Gartner's duct (Fig. 236, A, Gg\ as already mentioned oh p. 301.

1 The abnormalities which sometimes occur in the human uterus and vagina can
be often explained as atavisms.

0&

FIG. 252. — FEMALE GENERATIVE APPARATUS OF CERTAIN MARSUPIALS. A, Didel-
phys dorsigera (juv.) ; B, Phalangista vulpina ; C, Phascolomys wombat. (After
A. Brass.)

NN, kidneys ; Ur, ureters ; Ov, ovary ; Ot (Fim), abdominal opening of Fallopian
tube ; Od, oviduct ; Ut, uterus ; Utl, openings of uteri into the vaginal caecum,
VgB ; t, bend between uterus and vagina, Vg ; Vgl, apertures of vaginae into the
urinogenital sinus (Siig) ; B, urinary bladder ; r, rectum, which opens into
the cloacal region (01) at r1 ; g, clitoris ; *, t, rectal glands.

Y 2

324

COMPARATIVE ANATOMY.

A curious fold of the skin of the abdomen is present to a greater or less degree
in Marsupials and in Echidna. This pouch or marsupium serves to protect
the young, which are born in a very unripe condition, and thus renders possible

FIG. 253. — VARIOUS FORMS OF UTERI. A, B, C, D, diagrams showing the
different stages in the fusion of the Miillerian ducts : A, uterus bicornis ; B,
uterus simplex ; C, uterus duplex ; D, uterus bipartitus. E, female urinogenital
apparatus of Mustclina, with embryos (*, *) in the uterus. F, ditto of Hedgehog
(Erinaceus).

Od, Fallopian tube ; Ut, uterus ; Vg, vagina ; Ce, cervix uteri ; Ot, abdominal aper-
ture of Fallopian tube ; t, t, accessory sexual glands ; r, rectum ; Sug, urino-
genital sinus ; N, kidney ; Nn, adrenal ; Ur, ureter ; B, urinary bladder.

a longer connection between the mother and embryo during lactation. The aper-
ture of the marsupial pouch opens anteriorly or posteriorly, according to the
mode of life of the animal, and is provided with a muscle capable of closing it.

GENERATIVE ORGANS.

PS KE

325

FIG. 254.— SECTION THROUGH A PORTION OF THE OVARY OF A MAMMAL, SHOWING
THE MODE OF DEVELOPMENT OF THE GRAAFIAN FOLLICLES.

KE, germinal epithelium, ingrowths from which extend into the stroma of the ovary
to form the ovarian tubes (PS) : the stroma is penetrated by vessels (g, g) ;
Z7, U, primitive ova ; S, cavity between the follicular epithelium (tunica granu-
losa, Mg) and the primitive ova ; Lf, liquor folliculi ; D, discus proligerus ; Ei,
ripe ovum, with its germinal vesicle (K) and germinal spot ; Mp, zona pellucida,
showing radiated structure ; Tf, theca folliculi.

FIG. 255. — DIAGRAMMATIC SECTION OF THE TESTIS OF A MAMMAL.
Ho, testis ; NH, epididymis ; Vd, vas deferens ; A, albuginea of the testis, which
gives rise to the trabeculse (t, t) and the corpus Highmori (t) ; L, L, coils of the
seminal tubules ; Ve, vasa efferentia (rete Halleri) ; Cv, coni vasculosi, which
are connected together by the collecting duct, Vep ; Fa, vas aberrans.

As regards the male generative organs of Mammals, the
testes arise in the same position as do the ovaries of the female.

326

COMPARATIVE ANATOMY.

The ovaries, however, never move in the course of development
further backwards than the pelvis ; but each testis may pass out
of the abdomen through a space in its wall, the inguinal canal;
it thus comes to lie within a purse-like appendage of the hypo-
gastric region called the scrotalsac. The two scrotal sacs may

FIG. 256.— MALE URINOGENITAL APPARATUS OF THE HEDGEHOG (Erinaceus).

N, kidney ; Ur, ureters ; B, urinary bladder ; Pm, membranous portion of urino-
genital tube ; Cpc, corpus cavernosum ; Pp, prepuce ; Gp, glans penis ; PD,
preputial glands ; Cd, Cowper's glands ; Pr, Prl, the different lobes of the
prostate ; Sb, vesicula seminalis ; Ho, testis ; Ep, epididymis ; Vd, Vdl, vas
deferens.

remain separate, or unite to form a scrotum. In passing out,
the testes carry with them a portion of the peritoneum — the
tunica vaginalis: if the inguinal canals remain open, they
may be at times again withdrawn into the abdomen (e.g. in
Rodentia) ; this is effected by means of the cremaster muscle,

COPULATORY ORGANS. 327

a continuation of the fibres of the internal oblique and transver-
salis. When the inguinal canals become obliterated, the testes
remain throughout life in the scrotum.

In many Mammals, however, the testes remain permanently
within the abdomen. Their size is not always proportionate to
that of the body ; they are smooth, and somewhat oval in form,
and are covered by a fibrous investment (Fig. 255, A), from which
processes (trabeculse) usually extend inwards. In this way the
seminal tubes are separated into definite bundles (L, L\ and a sort
of lattice-work is also formed (corpus Highmori, f), by means of
which the elements of the rete Halleri (that is, the vasa
efferentia, Ve) pass to the epididymis (NH).

In the epididymis the seminal tubules become rounded off to
form the so-called coni vasculosi, and these are connected to-
gether by a collecting duct, the vas epididymis (Fig. 255, Cv,
Cv, Vep). The vas deferens ( Vd) arises from the last conus vas-
culosus, and gives rise towards its distal end, shortly before it
opens into the urinogenital sinus, to glandular outgrowths (vesi-
culse seminales), which may attain a relatively enormous size
in Rodents and Insectivores (Fig. 256, Sb). From this point to
its termination the seminal canal is spoken of as the duct us
ejaculatorius.

In many Mammals rudiments of the Miillerian ducts are pre-
sent, and open into the urinogenital sinus. In some (e.g. Man), only
the most posterior end of the latter remain, in the form of an un-
paired vesicle (uterus masculinus), which lies embedded within
an accessory genital gland, the prostate. This gland, which more
or less completely surrounds the urinogenital sinus, consists of
glandular tubules, connected together by means of fibrous and
muscular tissue : its secretion is poured into the urinogenital sinus
(compare Figs. 236, A to C, and 256).

COPULATORY ORGANS.

External organs of generation, such as are present in the higher
Vertebrates, are never found in Fishes, though in male Elasmo-
branchs a specially modified portion of the pelvic fin serves as
a copulatory organ ("clasper" or "pterygopodium"). It
consists of a series of cartilages which are moveable upon one
another, is covered by skin and muscles, and is provided with a
channel along the inner side. It must be looked upon as a deriva-
tive of the fin-rays.

These claspers are inserted, in a closed condition, into the
cloaca of the female, and thence into the oviducts; they are
then opened out by means of special muscles, and the seminal
fluid flows ulong their channels into the distended oviducts. In

328 COMPARATIVE ANATOMY.

connection with this apparatus,— which looks like a series of
surgical instruments, — there is a gland, surrounded by muscular
fibres, which is formed as an involution of the integument ; in
its histological character this calls to mind the uropygial gland
of Birds.

Amongst the Amphibia, male Gymnophiona alone possess
a true copulatory organ; this simply consists of the eversible
cloaca, which reaches a length of five centimetres, and is regu-
lated by a well-developed musculature. In Urodeles there
is merely a marked swelling of the lips of the cloaca and urino-
genital papilla during the breeding-season.

Two kinds of copulatory organs are found in Reptiles, the
one being seen in Lizards and Snakes, and the other in Chelonians
and Crocodiles.

In the former case, there are two erectile penes lying outside
the cloaca, under the skin at the root of the tail. By a compli-
cated muscular mechanism these can be drawn into the cloaca and
thence everted, the seminal fluid passing along a spiral furrow in
each. Similar organs are present — though much less developed —
in the female also.

Chelonians and Crocodiles possess a copulatory organ which is
united to the ventral wall of the cloaca : it consists of two fibrous
masses which are fused together in the middle line. Each half
encloses a large cavity, which contains a large number of blood-
vessels. The organ is regulated by well- developed muscles, and
possesses a groove along its free surface, which may divide up
into a series of channels at its apex. A representative of this
organ is also present in the female.

In most Rat it 86, as well as in some Carinatse (e.g. in Water-
Birds), a copulatory organ is present, and consists of an eversible
tube, strengthened by two fibrous bodies ; in a condition of rest it
lies coiled up in the left side of the cloaca. The everted organ is
retracted by means of an elastic band. The penis of the Ostrich
lies within a diverticulum of the cloaca, and somewhat resembles
that of Chelonians and Crocodiles,

The copulatory organs of Mammals may be divided into two
groups, — viz., those of Monotremes, and those of other Mammals :
the organs of Marsupials may be placed in a subdivision of the
latter. In all cases the female apparatus (clitoris), — although as
a rule less developed and usually not perforated by the urethral
canal, — is formed essentially on the same type as that of the male.

In Monotremes, the copulatory organ lies enclosed within a
sac situated on the boundary between the urinogenital sinus and
cloaca, and is fused with the ventral wall of the latter (Fig. 251,
Gp) : in all other Mammals the organ arises in the embryo from
the "genital prominence" on the ventral wall of the cloaca.
A channel passes along the side facing the cloaca to the opening of
the urinogenital sinus : this condition is usually retained through-

COPULATORY ORGANS. 329

out life in the case of the female, while in the male the groove
becomes closed to form a canal, and the urethra is considerably
lengthened. Three cylindrical bodies, composed of erectile tissue
(two lateral corpora cavernosa and a median corpus spon-
giosum), become developed in connection with the penis: these
are connected with one another by a network of fibrous tissue,
and are partially invested by muscles (Fig. 257, Ccp, Ccu\ Corpora

-Cc/i

nt'

FIG. 257. — SEMIDIAGRAMMATIC FIGURE OF THE HUMAN PENIS. (A, transverse
section ; B, side view ; C, from below. )

A, albuginea penis ; A1, albuginea urethrse, which gives rise to a septum at Sp ;
S, sulcus dorsalis penis ; Ccp, corpus cavernosum ; Ccu, corpus spongiosum,
which gives rise to the glans penis at Gp, and forms an oval enlargement (bulbus)
at B ; rd, rd1, crura of the corpora caVernosa.

cavernosa are also present in the clitoris, but a corpus spongiosum
is wanting. In Marsupials and the higher Mammals the corpora
cavernosa diverge at their proximal ends to form two crura (rd, rd1)
which are almost always attached to the corresponding ischia.

In many Mammals a membrane bone (os penis) becomes
developed in the septum between the corpora cavernosa (e.g. many
Monkeys, Rodents, Bats, Carnivores). In the Seal there is an os
clitoridis in the female also.

330 COMPARATIVE ANATOMY.

The apex of the organ is spoken of as the glans(glans penis
vel clitoridis) ; this varies much in form, and lies enclosed within
a fold of the skin, which in the male is called the foreskin or
prepuce. The glans is provided with a special kind of tactile
corpuscle.

Finally, the accessory organs, which are known in the male as
preputial and Cowper's glands, and the representatives of the
latter in the female (glands of Bartholini), must be mentioned
(Fig. 256, CcT). All these, as well as the prostate (PD), — which
belongs to the same category, — open at various points into the
urethra, and have always a paired arrangement.

BlBLIOGKAPHY.

BALBIANI, G. — Lemons sur la, generation des Vertebres. Paris, 1879.

BALFOUE, F. M. — On tlie Nature of the Organ in Adult Teleosteans and Ganoids,

which is usually regarded as the Head-Kidney or Pronephros. Quart. Journ.

Micros. Science, Vol. XXII. 1882.
BALFOUR, F. M., and PARKER, W. N. — On the Structure and Development of Lcpi-

dosteus. Phil. Trans. 1882.
BENEDEN, E. VAN. — Recherches sur la maturation de I'osuf, la fecondation et la

division cellulaire. Gand, Leipzig and Paris, 1883.
BOURNE, A. G. — On Certain Abnormalities in the Common Frog. Quart. Journ.

Micros. Science, Vol. XXIV. 1884.
BRASS, A. — Beitr. z. Kenntniss des weiblichen Urogenitalsystems der Marsupialier.

Inaug. Dissert. Leipzig, 1880.
BRAUN, M. — Das Urogenitalsystem der einheim. Eeptilien. Arb. a. d. zool. zootom.

Institut d. Universitdt Wurzburg, Bd. IV. 1877.
BROCK, J. — Beitr. zur Anatomie und Histologie der Geschlechtsorgane der Knochen-

fische. Morphol. Jahrb. Bd. IV. 1878.
EMERY, C. — Zur Morphologic der Kopfniere der Tcleostei. Zool. Anz. VIII. Jahr-

gang, 1885.
FURBRINGER, M. — Zur vergl. Anatomie und Entw.-Geschichte der Excretionsorgane

der Vertebraten. Morphol. Jahrb. Bd. IV. 1878. (Contains also a complete

bibliography. )
GROSGLIK, S. — Zur Morphologic der Kopfniere der Fische. Zool. Anz. VIII. Jahr-

gang, 1885. Zur Frage ub. die Persistenz der Kopfniere der Teleosteer. Ibid.

IX. Jahrgang, 1886.
HENSEN, V. — Physiologie der Zeugung. Handbuch der Physiologic von L. HERMANN.

Bd. VI. 2 Thl.

KNAPPE. — Das Bidder' sche Organ, &c. Morphol. Jahrb. Bd. XI. Heft 4, 1886.
LEREBOTJLLET. — Rech. sur I'anatomie des organes g€nitaux des animaux vertebres.

Nov. act. Acad. Leop.-Carol, &c. 1851.
LISTER, J. J., and FLETCHER, J. J. — On the Condition of the Median Portion of the.

Vaginal Apparatus in the Macropodidce. Proc. Zool. Soc. 1881. In reference to

same subject, see also J. J. FLETCHER in Proc. Linn. Soc. N.S. W. 1881, 1882,

and 1883.
MARSHALL, A. MILNES. — On Certain Abnormal Conditions of the Reproductive Organs

in the Frog. Journ. of Anat. and Physiol. Vol. XVIII. 1884.
MIHALKOVICS, V. v. — Entwick. d. Harn- u. Geschlechtsapparat der Amnioten. I.

Abhandl. Der Excretionsapparat. Internal. Monatschrift f. Anat. und Hist.

Bd. II. 1885.
MOBIUS, K. — Ueber die Eigenschaften und den Ursprung der Schleimfdden des

Seestichlingnestes. Archivf. Mikr. Anat. Bd. XXV. 1885.
PARKER, T. J. — On the Gravid Uterus of Mustelus antarcticus. Trans. N. Zealand

Inst. 1883.

PFLUGER, E. — Die Eierstocke der Sdugethiere und des Menschen. Leipzig, 1863.
SACK, ARNOLD. — Ueber die Verbindung der Crura penis mit dem Becken bei Beutel-

thieren. Zool. Anz. IX. Jahrgang, 1886.

URINOGENITAL ORGANS. 331

SARASIN, P. B. and C. F. — Ueber die Entwick. von Epicrium glutinosum. Arbeit.

zool. zootom. Inst. Wiirzburg, Bd. VII. 1885.
SEDGWICK, A. — Several Papers On the Development of the Excretory System in. the

Chick. Quart. Joum. Micros. Science, Vols. XX. and XXI. 1880 and 1881.
SEMPER, C. — Das Urogenitalsystem der Plagiostomen, <kc. -Arbeiten a. d. zool.

zootom. Inst, z. Wiirzburg, Bd. II. 1875.
SPENGEL, J. W. — Das Urogenitalsystem der Amphibien. Arb. a. d. zool. zootom.

Institut d. Universitdt Wiirzburg, Bd. III. 1876.
WALDEYER, W. — Eierstock und Ei. Leipzig, 1870.
WEISMANN, A. — Die Entstehung der Sexualzellen bei den Hydromedusen. With Atlas.

Jena, 1883.
WELDON, "W. F. R. — Note on the Early Development of Lac-erta muralis. Quart.

Journ. Micros. Science, 1883. On the Head-Kidney of Bdellostoma. Ibid.

1884.

(For further references compare the text-books on Human Anatomy by QUAIN and
HENLE, and also other works mentioned at the beginning of this book. )

INDEX

INDEX.

AMPHIOXTJS : — segmentation of ovum, 5 ;
development of body-cavity, 9 ; inte-
gument, 17 ; notochord, 34, 38 ; spinal
cord, 136 ; mouth, 212 ; thyroid, 225 ;
alimentary canal, 228, 287, 239 ; gills,
246 ; blood-corpuscles, 268 ; generative
organs, 310

CYCLOSTOMES : — segmentation of ovum, 5 ;
integument, 17 ; vertebral column; 34,
38 ; skull, 63 ; branchial basket, 64, 118 ;
brain, 136 ; cranial nerves, 153 ; olfactory
organ, 171 ; eye, 184 ; semicircular canals,
196 ; mouth, 212 ; horny teeth, 214 ;
thyroid, 225 ; alimentary canal, 228 ;
alimentary epithelium, 237, 239 ; liver,
240 ; pancreas, 243 ; gills, 246 ; ab-
dominal pores, 265 ; blood-corpuscles,
268 ; heart, 277 ; urinogenital organs,
300, 302, 310

FISHES : — integument, 16 ; dermal skeleton,
30 ; vertebral column, 34 ; ribs, 48 ;
skull, 63 ; unpaired fins, 85 ; paired fins,
86 ; pectoral arch, 87 ; pelvic arch, 92 ;
free limbs, 99 ; dermal muscles, 113 ;
muscles of the trunk, 113 ; muscles of
the visceral skeleton and head, 118 ;
muscles of the appendages, 121 ; electric
organs, 124 ; spinal cord, 131 ; brain,
134, 136 ; spinal nerves, 152 ; cranial
nerves, 153 ; suprarenal bodies, 161 ;
sense-organs of the integument, 163 ;
olfactory organ, 171 ; eye, 184 ; retina,
189 ; eye-muscles and eyelids, 191 ;
auditory organ, 198, 205 ; relations of
auditory organ with air-bladder, 207 ;
teeth, 214 ; tongue, 222 ; thyroid, 225 ;
thymus, 226 ; alimentary canal, 228 ;
liver, 242 ; pancreas, 243 ; gills, 246,
248 ; air-bladder, 251 ; abdominal pores,
265 ; blood-vessels, 270 ; heart and its
vessels, 277 ; arterial system, 288 ;
venous system, 291 ; retia mirabilia, 292 ;
lymphatic system, 292 ; spleen, 294 ;
urinary organs, 296, 302 ; generative
organs, 300, 310 ; claspers, 327

DIPNOANS : — integument, 17 ; dermal
skeleton, 30 ; vertebral column, 35, 38 ;

ribs, 48 ; skull, 66 ; pectoral arch, 87 ;
pelvic arch, 92 ; free limbs, 99 ; muscles
of the trunk, 116 ; muscles of the ap-
pendages, 121 ; brain, 142 ; cranial
nerves, 153 ; sense-organs of the integu-
ment, 163; olfactory organ, 172; lips,
212 ; tongue, 222 ; alimentary canal,
228 ; alimentary epithelium, 239 ; gills,
246, 250 ; laryngo-tracheal chamber,
253 ; lungs, 257 ; abdominal pores, 265 ;
heart and its vessels, 280 ; urinary
organs, 302 ; generative organs, 314

AMPHIBIANS : — segmentation of the ovum,
5 ; integument, 17, 18 ; dermal skeleton,
19, 32 ; vertebral column, 39 ; ribs, 48 ;
sternum, 51 ; skull, 70 ; pectoral arch,
87 ; pelvic arch, 92 ; free limbs, 104 ;
muscles of the trunk, 113 ; muscles of
the visceral skeleton and bread, 119 ;
muscles of the appendages, 121 ; "dia-
phragm," 122 ; spinal cord, 131 ; brain,
134, 142 ; spinal nerves, 152 ; cranial
nerves, 158 ; suprarenal bodies, 161 ;
sense-organs of the integument, 163 ;
tactile cells, 167 ; olfactory organ, 172 ;
spouting apparatus of Gymnophiona,
179 ; eye, 186 ; retina, 189 ; eye-muscles
and eyelids, 191 ; glands of the eye,
192 ; auditory organ, 198, 205 ; teeth,
214 ; glands of the mouth, 221 ; tongue,
222 ; thyroid, 225 ; thymus, 226 ; ali-
mentary canal, 228 ; liver, 241 ; pancreas,
243 ; gills, 246, 250 ; air-passages, 253 ;
lungs, 257 ; blood-corpuscles, 268 ; al-
lantois, 273 ; heart and its vessels, 281 ;
arterial system, 288 ; venous system,
291 ; lymphatic system, 292 ; spleen,
294 ; urinary organs, 296, 304 ; genera-
tive organs, 310 ; copulatory organs, 328

REPTILES : — integument, 20 ; dermal
skeleton, 32 ; vertebral column, 42 ;
ribs, 49 ; sternum, 51 ; skull, 74 ; pec-
toral arch, 87 ; pelvic arch, 94 ; free
limbs, 105 ; muscles of trunk, 116 ;
muscles of the visceral skeleton and
head, 120 ; muscles of the appendages,
121; "diaphragm," 122; brain, 132,
134, 144 ; spinal nerves, 152 ; cranial

336

INDEX.

nerves, 153 ; suprarenal bodies, 161 ;
end-bulbs, 166 ; tactile cells, 167 ; Pa-
cinian corpuscles, 169 ; olfactory organ,
174 ; Jacobson's organ, 178 ; retina, 189 ;
eye-muscles and eyelids, 191 ; glands of
eye, 192 ; auditory organ, 199 ; teeth,
215 ; glands of mouth, 221 ; tongue,
223 ; alimentary canal, 231 ; alimentary
epithelium, 239 ; pancreas, 243 ; air-
passages, 254 ; lungs, 258 ; abdominal
pores, 265 ; allantois, 273 ; heart and its
vessels, 284 ; arterial system, 288
venous system, 291; retia mirabilia, 292
spleen, 294 ; urinary organs, 296, 307
generative organs, 317 ; »copulatory
organs, 328

BIRDS : — segmentation of ovum, 5 ; in-
tegument, 20 ; vertebral column, 43,
44 ; ribs, 50, 53 ; sternum, 51 ; skull,
78 ; pectoral arch, 90 ; pelvic arch, 94 ;
limbs, 106 ; muscles of the trunk, 117 ;
muscles of the visceral skeleton and
head, 120 ; spinal cord, 131 ; brain, 134,
147 ; cranial nerves, 153 ; suprarenal
bodies, 161 ; tactile cells, 167; Pacinian
corpuscles, 169 ; olfactory organ, 175 ;
eye, 186 ; retina, 189 ; eye-muscles and
eyelids, 191 ; glands of the eye, 192 ;
auditory organ, 199, 205 ; glands of the
mouth, 222 ; tongue, 223 ; thyroid, 225 ;
thymus, 226 ; alimentary canal, 233 ;
pancreas, 243 ; air-passages, 255 ; lungs
and air-sacs, 259 ; circulation in embryo,
272 ; allantois, 273 ; heart and its
vessels, 286 ; arterial system, 288 ;
venous system, 291 ; lymphatic system,
293 ; lymphatic glands, 294 ; urinary
organs, 296, 307 ; generative organs,
300, 317 ; copulatory organs, 328

MAMMALS : — segmentation of ovum, 5 ;
integument, 23 ; mammary glands, 27 ;
dermal skeleton, 32 ; vertebral column,
43, 46 ; ribs, 50 ; sternum, 51 ; skull,
80 ; pectoral arch, 91 ; pelvic arch, 98 ;
limbs, 108 ; muscles of trunk, 117 ;
muscles of visceral skeleton and head,
120 ; muscles of appendages, 121 ; dia-
phragm, 122 ; spinal cord, 131 ; brain,
132, 134, 148 ; spinal nerves, 152 ;
cranial nerves, 153 ; suprarenals, 161 ;
end-bulbs, 166 ; tactile cells, 168 ; Pa-
cinian corpuscles, 169 ; olfactory organ,
176 ; Jacobson's organ, 179 ; eye, 188 ;
retina, 189 ; eye-muscles and eyelids,
191 ; glands of eye, 192 ; auditory organ,
201 ; histology of cochlea, 206 ; lips,
212 ; teeth, 213, 216 ; glands of mouth,
222 ; tongue, 223 ; thyroid, 225 ;
thymus, 226 ; alimentary canal, 211,
234 ; alimentary epithelium, 239 ; liver,
243 ; pancreas, 243 ; air-passages, 255 ;
lungs, 263 ; blood-corpuscles, 268 ;

allantois and placenta, 274 ; heart and
its vessels, 286 ; arterial system, 288 ;
venous system, 291; retia mirabilia, 292;
lymphatic system, 293 ; tonsils, 293 ;
lymphatic glands, 294 ; urinary organs,
296, 308 ; generative organs, 300, 320 ;
copulatory organs, 328

Abdominal pores, 265
Abomasum, 236
Acetabulum, 93—98
Acrodont dentition, 215
Acromion, 91
Adrenals, 161

Aetosaurus, exoskeleton, 32
Air-bladder, 245, 251
Air-passages, 253
Air-sacs, 259

Alimentary canal, 208 — 237
development of, 7
appendages of, 240 — 243
mucous membrane of, 237
Allantois, 10, 231, 273, 274
Alopecias, number of vertebrae, 37
Alveoli of lung, 252
Amblyopsis, abortion of eyes, 186
Amblystoma, development of vertebral

column, 40
Amia, fusion of anterior vertebrae with

skull, 67

Ainiurus, sensory sacs, 166
Ammocoetes : — notochord, 34 ; brain, 137 ;

eye, 185 ; thyroid, 225 ; alimentary

epithelium, 237 ; gills, 246
Ammodytes, pyloric caecum, 229
Amnion, 10, 276
Amphicoslous vertebrae, 36
Amphiuma, trachea, 253
Ampullae of semicircular canals, 196
Anabas, branchial chamber, 250
Anastomosis of Jacobson, 158
Anguilla, lateral nerve, 165
Anguis : — embryonic limbs, 90 ; brain, 144;

teeth, 215
Auteater, palate, 82
Antibrachium, 102
Antihelix, 198
Antitragus, 198
Antlers, 83

Antrum maxillare, 1 76
Apatosaurus, dimensions of vertebrae, 43
Aponeurosis, pulmonary, 262
Appendices auriculae, 278
Apteryx : pelvis, 96 ; manus, 107
Aqueduct of Sylvius, 134, 142
Aqueductus cochleae, 205
Aqueductus vestibuli, 205
Arachnoid, 135
Archenteron, 6, 237
Arches, neural and haemal, 85

INDEX.

337

Archieopteryx : — vertebra, 44 ; rihs, 50 ;
skull, 78 ; scapula, 91 ; pelvis, 45, 98 ;
wing, 106 ; foot, 108 ; brain, 147 ;
teeth, 216

Argentea of choroid, 186

Armadillo : — dermal skeleton, 32 ; skull, 83

Arteries, development of, 268, 270

Arteries .— allantoic, 272, 276, 289 ; aorta
(dorsal), 270, 277 — 290 ; aorta (ventral),
278 ; aortic arches or roots, 270, 276—
290 ; arterial arches, 270, 283—285 ;
axillary, 288 ; brachiocephalic, 287 ;
branchial, 270, 279—283 ; carotid, 270,
277, 288, 290; caudal, 270, 288—290;
cceliac, 288, 29?) ; cceliaco-mesenteric,
279, 288—290 ; crural, 288—290 ; cu-
taneous, 289 ; digital, 288 ; epigastric,
289, 290 ; genital, 288 ; hepatic, 289 ;
hyaloid, 183 ; hypogastrio, 288—290 ;
iliac, 288, 289, 291 ; intercostal, 288 ;
intestinal, 288, 289 ; lumbar, 288 ;
median sacral, 288 ; mesenteric, 288 ;
cesophageal, 284, 290 ; omphalo-mesen-
teric, 270 ; ovarian, 289 ; palmar arches,
288 ; pulmonary, 270, 277—287, 290 ;
radial, 288 ; rectal, 289, 290 ; renal,
279, 288, 289 ; sciatic, 290, 291 ;
stapedial, 203 ; subclavian, 276—288,
290 ; urinogenital, 290 ; ulnar, 288 ;
vertebral, 277, 290 ; vesical, 289 ; vitel-
line, 270

Artiodactyle foot, 109

Arytenoid cartilages, 254

Ascalabota, vertebrae, 42 ; ductus endolym-
phaticus, 205

Ascidians, thyroid, 225

Ateles, caudal vertebrae, 47

Atlantosaurus, dimensions, 43

Atlas : — of Amphibians, 42 ; of Reptiles,
43 ; of Birds, 45 ; of Mammals, 46

Atrial pore of Amphioxus, 246

Atrio-ventricular aperture, 279, 287

Atrium, 269

Auditory organ, 194 — 207 : — development
of, 195 ; relations with air-bladder, 207

Auditory ossicles, 198, 202 •

Auditory region of skull, 58

Autostylic skulls, 65

B,

Bacilli of basement membrane of cochlea,

206

Baleen, 27

Balistes, exoskeleton, 31
Basal processes of vertebrae, 35, 42
Basilar plate, 57
Basipterygium, 99
Bidder's organ, 316, 317
Bile-duct, 242
Blastoderm, 5
Blastopore, 6
Blastosphere, 5

Blustula, 5

Blood-corpuscles, development and form
of, 268

Boar, tusks of, 220

Body-cavity, 9

Bones, cartilage-, membrane-, and invest-
ing-, 62

Brachium, 102

Bradypus, cervical vertebrae, 46

Brain, 131—151 :— development of, 131 ;
membranes of, 135; cerebellum, 132,133;
cerebrum, 132; con volutions, 132; corpora
bigemina, 132 ; corpora restiformia, 138 ;
corpus callosum, 132, 148 ; crura cerebri
etcerebelli, 150; epiphysis, 133; foramen
of Monro, 134 ; fornix, 132 ; hypophysis,
133 ; infundibulum, 133 ; lobi electrici,
138 ; .lobi inferiores, 138, 142 ; lobi
nervi lateralis, 139 ; lobes, central, of
prosencephalon, 148 ; medulla oblongata,
132 ; mesencephalon, 132 ; meten-
cephalon, 132 ; inyelencephalcn, 132 ;
optic chiasma, 155 ; optic lobes, 132 ;
optic thalami, 132; optic vesicles
(primary), 132, (secondary), 182 ;
pallium", 132, 137 ; peripheral portion
of prosencephalon, 132 — 137 ; pineal
gland, 132, 142 ; pituitary body, 133 ;
processus infundibuli, 138, 142 ; pros-
encephalon, 132, 137—150 ; sacci
vasculosi, 138, 142 ; thalamencephalon,
132 ; ventricles, 131, 133, 134 ; vermis,
133

Brain-case, 57

Branchial arches, 60, 65—68, 74, 78, 80,
84, 248

Branchial clefts, 54, 159, 198, 245

Branchiomerism, 56

Branchiostegal membrane and rays, 69

Bronchi, 252 :— main and lateral, 259, 262,
263 ; eparterial and hyparterial, 263

Bulbus arteriosus, 269, 279

Bursa Fabricii, 234

Bursae an ales, 233

Butirinus, spiral valve, 229

0.

Cvmim, 209, 233, 236

Calamus, of feather, 22

Calcaneum, 105, 106, 109

Calyces, 308

Campanula Halleri, 185

Canalis reunions, 199

Canine teeth, 214, 216

Canis :— teeth, 218 ; stomach, 235

Cannon-bone, 110

Capillaries, 269

Carapace, 32

Carcharias : — placenta, 12 ; number of

vertebrae, 37

Cardiac portion of stomach, 231
Carina sterni, 51

338

INDEX.

Carpus, 102—109

Cartilages :— Meckel's, 61, 202 ; plough-
share, of Huschke, 179 ; of Wrisberg
and Santorini, 257

Cartilage-bones, 62

Cauda equina, 131

Cavicornia, horns of, 83

Cavum endolymphaticum, 196
maxillare, 172, 176, 178
perilymphaticum, 196

Cells, 1 : — granular, 18 ; goblet, 18 ;
ganglion, 129, 163 ; sensory and sup-
porting, 162

Cement of teeth, 212

Centra of vertebrae, 36

Centrale, 104, 105, 109

Ceratodus : — fins, 99 ; muscles of fins, ^'.
121 ; lateral nerve, 165 ; lungs, 257 ;
abdominal pores, 267

Ceratophrys, dermal skeleton, 32

Cere, tactile cells of, 167

Cerebral flexure, 135

Ceruminous glands, 26

Cetacea : — cervical vertebra, 46 ; phal-
anges, 111 ; dentition, 213

Chfetodon, teeth, 214

Chameleon : —tongue, 223 ; lungs, 258

Chauliodus, teeth, 214

Cheirocentrus, spiral valve, 229

Chevron bones, 43, 47

Chimpanzee, larynx, 256

Chlamydoselache, operculum, 249

Choanse, 170, 173

Choloepus, cervical vertebrae, 46

Choroid, 183

Choroid fissure, 183

Choroid "gland," 186

Chorion frondosum and laeve, 276

Ciliary folds, 183

Ciliary ligament, 185

Cirrhi of mouth of Amphioxus, 212

Circulation (fcetal), 270

Cisterna chyli, 293

Claspers, 101, 327

Classification of Vertebrates, 13

Clavicles, 87—91

Clitoris, 328

Cloaca, 231—234, 236

Cobitis, external gills, 250

Coccyx, 47

Cochlea, 195 ; bony, 203 ; histology of,
206

Coelenterate, diagram of, 237

Ccelome, 9

Colibris, tongue, 223

Colon, 209, 236

Colostrums, 28

Columella auris, 199

Columns of Bertini, 310

Cones of retina, 190, 191

Coni vasculosi, 327

Conjunctiva, 184, 192

Contour feathers, 22

Conus arteriosus, 269, 279—282

Copulatory organs, 327

Coracoid, '89—91

Corium, 16

Cornea, 184

Corpora adiposa, 317

Corpora cavernosa and corpus spongiosum,

329

Corpus Highmori, 327
Corpuscles : — of blood, 268 ; Paciuian,

169, 223 ; of Grandry, 169
Cortex of hair, 24
Cranial nerves, 153
Cranium, 57
Cribriform plate, 80 .
Crista acustica, 197
Crop, 234
Cms, 102

Crypts of intestine, 239
Cuboid, 109
Cuneiform, 109
Cupula terminalis, 205
Cuticle of hair, 24
Cutis, 16

D.

Dactylethra, epipubh, 93

Dasypus, milk dentition, 213

Ddcidua, vera and reflexa, 276

Dental formulae, 220

Denticles, dermal, 31

Dentine, 212

Dentition, milk, 213

Deer, larynx, 256

Derma, 16

Dermal skeleton, 30—33

D^rmostosis, 62

Deuteroplasm, 5

Development: — general, 2 — 12; feathers,
20 ; hairs, 24 ; teats, 27 ; dermal skele-
ton, 31 ; vertebral column, 33 ; tail of
Fishes, 38 ; coccyx, 47 ; ribs, 48 ;
sternum, 51, 53 ; skull, 54, 57, 59, 61 ;
horns, 83 ; limbs, 85, 86 ; muscles,
112, 120, 121 ; electric organs, 126 ;
central nervous system, 129 ; brain,
131 ; nerves, 151 ; sympathetic, 160 ;
suprarenals, 161 ; sensory end-organs,
162 ; olfactory organ, 170 ; eye, 181 ;
glands of eye, 192 ; auditory organ,
194 ; teeth, 212 ; tongue, 225 ; thyroid,
225 ; thymus, 226 ; intestinal crypts,
239 ; gills, 245 ; air-bladder, 251 ; lungs,
252 ; air-sacs, 262 ; heart, 269 ; placenta,
274 ; iirinogenital organs, 296 — 302

Diaphragm, 122

Diastema, 217

Didelpl^s, urinogenital organs, 320

Digestion, intracellular, 237

Digits, 104—110

Dinosauria :— dimensions, 43 ; pelvis,
96, 98

Diphyccrcal tail, 138

INDEX.

339

Diphyodouts, 213
Discosaurus, dermal skeleton, 32
Diverticulum cfecum, 234
Down-feathers, 22

Duck : — caudal vertebrae, 45 ; rudiment
of third finger, 107 ; lungs and air-sacs,
260, 261

Duct :— Leydig's, 301, 303 ; hepatic, 242;
naso-lacrymal, 174 ; naso-palatine of
Myxinoids, 171 ; pancreatic, 243 ; pneu-
matic, 251 ; of Steno, Wharton, and
Bartholini, 222
Ductus BotaUi, 284

coclilearis, 199, 204
Cuvieri, 271 <
eudolymphaticus, 202, 205
ejaculatorius, 327
perilymphaticus, 205
thyreoglossus, 225
sacculo-utricularis, 177, 19£
venosus, 270
Dugong, teeth, 220
Duodenum, 209
Dura mater, 135

E.

Ear :— internal, 195—206 ; external, 198

Echeneis, suctorial disk, 85

Echidna : — mammary pouch, 27 ; pelvis,
97 ; anterior abdominal vein, 291 ; eggs,
320 ; urinogenital organs, 321

Ectobronchia, 262

Ectoderm, 6

Ectostosis, 62

Edentata, teeth, 213

Egg-cell, 2

Elastica limitans externa and interna, 33

Electric organs, 124 — 128

Electric plate, 127

Emys, tongue, 224

Enaliosaurus, abdominal ribs, 50

Enamel, 32, 212

End-bulbs, 166

Endoderm, 6

Endolymph, 196

Endostosis, 62

Ensiform process, 53

Enterocceles, 9

Entobronchia, 262

Entoglossal, 61

Ephippifer, exoskeleton, 32

Epiblast, 6, 151

Epicrium : — external gills, 251 ; eggs, 316

Epidermis, 16

Epididymis, 327

Epiglottis, 255

Epiphyses of vertebra?, 46

Epiphysis cerebri, 132, 142

Epipubis, 93, 98, 117

Epithelium, 1 : — pigment of retina, 183 ; of
alimentary canal, 237 ; germinal, 299

Erinaceus, teeth, 217

Erythrina, air-bladder, 251

Ethmonasal septum, 58

Eustachian aperture, 73

Eustachian tube, 198

Exoskeleton, 30—33

Extra-branchials, 63, 74

Eye, 181 — 194 : — glands in connection

with, 192 ; muscles of, 191
Eyelids, 191

F.

Fallopian tube, 302, 322

Fat-bodies, 294, 317

Feathers, 20

Femur, 102

Fenestra :— rotunda, 80, 205 ; ovalis, 70,

80, 198, 205 ; vestibuli cartilaginei, 197
Fertilisation of ovum, 3
Fibres, 1 : — connective-tissue, 16 ; nerve,

129

Fibula, 104
Fibulare, 104
Filum terminale, 131
Fimbriae of Fallopian tube, 322
Fin-rays, 85, 99
Fins, 85
Food-yolk, 5
Foramen cordifonne, 95
Foramen ovale (of heart), 287
Foramen Panizzae, 285
Fulcrum of retina, 190
Furcula, 90

G.

Gall-bladder, 242

Ganglia of sensory nerves, 151

Ganglion, ciliary, 156

Gartner's duct, 301, 322

Gastrula, 6

Generative cells, development of, 299, 300

Generative organs, 310 — 327

Germinal epithelium, 299

Germinal membranes, 5

Germinal spot, 2

Germinal vesicle, 2

Gill-arches, 59—61, 63—66, 68, 74, 80, 84

Gill-clefts, 54, 159, 198, 245

Gill-cover, 65

Gills, 245, 251

Gills, external, 246, 251

Gills, spiracular, 250

Giraffe, horns of, 83

Gizzard, 233

Glands : — Bowman's, 177 ; of Bartholini,
330 ; ceruminous, 26 ; cervical, of Cha-
mois, 27 ; of claspers, 327 ; Cowper's, 330 ;
digestive, 239; femoral, 20; genital, 310 —
320, 325—327 ; Harderiau, 193 ; hibern-
ating, 294 ; integumentary, 18—20, 23 —
28 ; intermaxillary or internasal, 193 ;

340

INDEX.

labial, 221 ; lacrymal, 193 ; of Lieber-
kiikn, 239; lingual, 221, 222; lymphatic,
294 ; mammary, 27 ; Meibomian, 27,
194 ; nasal (external) of Birds, 176 ;
of olfactory mucous membrane, 173,
175 ; orbital, of Gymnophiona, 180 ;
ovidncal, 311 ; palatine, 221 ; parotid,
222 ; peptic, 239 ; perineal, 27 ; pharyn-
geal, 221 ; poison, 221 ; preputial, 27,
330 ; prostate, 327, 330 ; sebaceous, 24,

, 26 ; of Stenson, 177 ; sublingual, 222 ;
sweat, 26; "tentacular," of Gymno-
phiona, 180 ; uropygial, 23

Glaus penis and clitoridis, 330

Glaserian fissure, 202

Glomerulus, 297, 304

Gnathcstomata, 63

Gonads, 310—320, 325—327

Guinea-pig, milk dentition, 213

Gullet, 209, 228, 234

Gut, postanal, 86

Gymnarchus, electric organ, 125

Gymnophiona : — scutes, 20, 32 ; olfactory
nerves, 155 ; spouting apparatus, 179 ;
external gills, 251 ; eggs, 316

Gymnotus : — electric organs, 124 ; lateral
nerve, 165

Gyri, 132

Gyrinophilus, vertebral column, 40

H.

Hair, 24

Halrnaturus, third vagina, 322
Hatteria : — vertebrae, 42 ; uncinate pro-
cesses, 50 ; teeth, 215
Heart, 268—287
Hedgehog, teeth, 217, 220
Heloderma, teeth, 216
Heptanchus, skull, 64 ,
Heredity, 2

Hermaphrodite structures, 314, 316
Hesperornis :— brain, 147 ; teeth, 216
Heterocercal tail, 38
Heterodont dentition, 213
Hilum, of kidney, 308

of ovary, 322
Histology, 1
Holoblastic ova, 5
Homocercal tail, 38
Homodont dentition, 213
Homodynainy of limb-arches, 87
Horns, 83
Humour, vitreous, 183

aqueous, 184

Hyoid, 60, 78, 84, 199, 202, 222
Hyomandibular, 61, 70, 202, 22d
Hyostylic skulls, 65
Hypoblast, 6
Hypophysis cerebri, 132
Hyrax, caeca of, 236
Hystrix, teeth, 218

1.

Ichthyornis. vertebrae, 44

Idioplasm, 4

Iguanodon, pelvis, 96

Ileum, 2t)9

Iliac processes of Dipnoi, 92

Iliac tract of Chimaera, 92

Ilium, 92—98

Impregnation, 4

Incisor teeth, 214, 216

Incisura acetabuli, 98

Incus, 202

Infundibula of lung, 252

Inguinal canal, 326

Insectivora, milk dentition, 213

Integument, 16 — 28 ; sensory organs of, 163

Intercalary pieces of vertebrae, 36

Interclavicle, 90

Intermedium, 104, 109

Interorbital septum, 57, 75

Interspinous bones, 37

Intertrabecula, 58, 65

Intestine, small and large, 209

Iris, 183

Ischium, 93 — 98

J.

Jacobson, anastomosis of, 158 ; organ of,

177
Jejunum, 209

K.

Kidney, 296—310

Labium vestibulare, 206

Labyrinth : — membranous, 195 ; bony,

196

Lacerta : — internal ear, 200 ; tongue, 224
Lacteals, 293
Lagena, 198, 199
Lamina fusca, 184
Lamina spiralis ossea, 203
Lamna, number of vertebrae, 37
Lanugo, 26

Laryngeal pouches, 257
Laryngo-tracheal chamber, 253
Larynx, 253, 257

Lateral line, sensoiy organs of, 165, 194
Lemurs, teats of, 28
Lens, 183, 185

Lepidosteus : — vertebra, 37 ; tail, 38 ; de-
velopment of oviduct, 313
Leucocytes, 28, 239, 293
Ligaments, intervertebral, 34
Ligamentum nuchse, 46
Limbs : — unpaired, 85 ; paired, 86 — 111
Limitans externa and interna of retina, 189
Liver, 209

Lizards : — subvertebral wedge-bones, 43 ;
caudal vertebrae, 43

INDEX.

341

Lobes of lung, 264

Lophius, lure of, 139

Lophobranchii, ribs, 48

Lungs, 252—265

Lymph, 268, 293

Lymph-hearts, 293
sinuses, 293

Lymphatic glands, 294
system, 292
trunks, 205, 293

Lymphoid substance in relation with
urinogenital organs : — of Teleostei and
Ganoidei, 304 ; of Amphibia, 317 ; of
Reptilia, 320

Lytta, 225

M.

Macula lutea, 190

acustica, 197

neglecta, 197

Malapterurus, electric organs, 124
Manatus, cervical vertebrae, 46
Mandible, 64—84
Mandibular arch, 60
Manis : — scales, 27 ; number of caudal

vertebrae, 47
Manubrium sterni, 53
Manus, 102

Marsupial bones, 93, 98, 117
Marsupium, 324
Meatus, external auditory, 198
Meckel's cartilage, 61, 202
Mediastinum, 265
Medullary plate, 7

cord, 7, 129

groove, 129
Membrana basilaris, 199, 202, 204

Reissneri, 202, 204

reticularis, 206

tectorias. Corti, 206

tympani, 73, 198, 199

tympaniformis, 255
Membranous labyrinth, 195
Menisci of vertebrae, 43
Meroblastic ova, 5, 6
Mesentery, 208
Mesoarium, 311
Mesoblast, 6
Mesoblastic somites, 9
Mesobronchium, 262
Mesoderm, 6

Mesonephros, 296—299, 302—304, 322
Mesopterygium, 99
Mesorchium, 311
Metacarpus, 104

Metamerism of head and body, 54
Metanephric duct, 301, 307, 309
Metanephros, 299, 307—310
Metapterygium, 99
Metasternum, 53
Metatarsus, 104

Microgale, number of caudal vertebrae, 47
Micropyle, 4

Milk dentition, 213

Molars, 214, 216

Monitor, tongue, 224

Monophyodonts, 213

Morphology, 2

Morula, 5

Moulting, 22

Mouth, 212

Mlillerian duct, 300—304, 316, 320

Mus, stomach, 235

Muscles, 112—122 :— of appendages, 121 ;
arrectores pili, 24 ; of branchial arches,
118 ; buccinatorius, 121 ; bursalis, 191 ;
ciliary, 184 ; constrictor of iris, 184 ;
constrictor of glottis, 253, 255 ; coraco-
hyoid, 118; coraco-mandibular, 118;
costo-pulmonary, 262 ; Crampton's,
187 ; cranio-visceral, 118 ; cremaster,
326 ; depressor of eyelid, 192 ; de-
pressor tarsi, 121 ; dermal, 112, 113 ;
diaphragmatic, 122 ; digastric, 119,
158 ; dilator of glottis, 253, 255 ; dila-
tor of iris, 184 ; of eye (recti, obliqui,
and retractor bulbi), 191 ; facial, 120;
genio-hyoid, 119, 120; genioglossus,
119, 120 ; hyoglossus, 119, 120 ; ileo-
costalis, 116 ; intercostal, 116, 118 ;
interspinales, 116 ; lateral, 113 ; levator
anguli oris, 121 ; levatores costarum,
116 ; levator of eyelid, 192 ; levator
labii superioris propriup, 121 ; longissi-
mus, 116 ; masseter, 119 ; masticatory,
112 ; multifidi, 116 ; mylohyoid, 119,
120 ; obliquus abdominis, 116, 117 ;
omohyoid, 119, 120 ; pectoralis major,
117; platysma myoides, 113, 121 ; pro-
nators, 122 ; pterygoid, 120 ; pyramid-
alis abdominis, 99, 117 ; pyramidalis of
eye, 191 ; quadratus of eye, 191 ; quad-
ratus lumborum, 116; rectus abdominis,
116, 117; scaleni, 116; semispinales,

116 ; sphincter oris, 121 ; sphincter
collis, 121 ; splenii, 116 ; stapedius,
202 ; sternocleidomastoid, 159 ; sterno-
hyoid, 117, 119, 120; sternothyroid, 117,
120 ; stylohyoid, 120, 158 ; styloglossus,
120 ; stylopharyngeus, 120 ; supinators,
122; temporal, 119, 120; thyro-hyoid,
120 ; transversalis abdominis, 116, 117,
122 ; trapezius, 159 ; triangularis sterni,

117 ; of trunk, 113 ; of visceral skele-
ton and head, 118

Musk-deer, teeth, 220

Mustelus : — umbilical placenta, 12 ; foetal

membranes, 312
Mycetes : — larynx, 256
Myocommata or myotomes, 48, 113

N.

Kares, external and internal, 67, 170,

173
Xarwhal, teeth, 220

342

INDEX.

Nasalis, teeth, 219

Naso-palatine duct of Cyclostomes, 171

Naso-lacrymal duct, 174, 194

Navicular, 109

Neomorph, 33, 48, 236

Nephridia, 296

Nephrostomes, 297, 302, 304, 307

Nerve, lateral, 158, 165
phrenic, 122

Nerve-roots, 151, 152

Nerves: — cranial, 153—160; olfactory, 154;
optic, 132, 155 ; oculomotor, trochlear,
and abducent, 156; trigeminal, 118 — 120,
126, 156, 165, 193; facial, 60, 118—120,

158 ; auditory, 158 ; glossopharyngeal,
60, 118—120, 158 ; vagus, 60, 118, 119,
158, 160, 165, 284 ; spinal accessory,

159 ; hypoglossal, 119, 159
Nerves : — spinal, 152

Nervous system : — central, 129 — 151; peri-
pheral, 151— 160; sympathetic, 129, 152,
160

Neural tube, 9

Neurenteric canal, 129

Nenrilemma, 129

Neuroglia, 129

Neuropore, of Tunicata and Amphioxus,
133

Nictitating-membrane, 192

Nose, external, 177

Nothosaurus, abdominal ribs, 50

Notochord, 10, 33—47

Notodelphys, external gills, 251

Nucleus, 3

O.

Obturator foramen, 95

Odontoid bone, 43

Odontornithes, teeth, 216

(Esophagus, 209, 228, 23 i

Olfactory organ, 170 — 177

Olfactory region of skull, 58

Olfactory scrolls, 176

Omosternum, 51

Ontogeny, 1

Opercular bones, 69

Operculum, 65, 158, 249, 250

Orbicular apophysis, 202

Orbital region of skull, 58

Orbital ring, 69

Organ of Corti, 201, 202, 206

Organs, 1 : — electric, 128 ; sensory, 162 —
207 ; of nutrition, 208—243 ; of respira-
tion, 245 — 265 ; of circulation, 268 —
294 ; urinogenital, 296—330

Ornithorhynchus : — eggs, 5, 320 ; pectoral
arch, 91 ; uriuogenital organs, 321

Ornithoscelida, sacral vertebra}, 43

Orthagoriscus, spinal cord, 131

Os penis and clitoridis, 329

Ossification, 62

Otoliths, 196

Ovary, 311—320, 322, 325
Oviducal gland, 311
Oviduct, 302, 311—32
Ovipositor, 311
Ovotestis, 316
Ovum, 2

P.

Palaeontology, 1

Palate, 75, 81, 209

Palpebrae, 184

Pancreas, 243

Panniculus adiposus, 26

Papilla acustica, 198, 199, 202

Papillae, of tongue, 166, 222

Parabronchia, 262

Parachordal cartilages, 57

Paradoxurus, number of caudal vertebra1,

47

Parorchis, 298, 306, 320
Parostosis, 62
Parovarium, 298, 322
Parrot : — number of vertebra?, 45; fronto-

nasal joint,_80
Pars acetabularis, 93—98
Pars basilaris, 198
Pars superior and inferior of membranous

labyrinth, 198
Patella, 110
Pecten, 188

Pectineal process of pubis, 98
Pectoral arch, 86 — 91
Pelvic arch, 86, 92
Pelvis of kidney, 309
Penis, 328
Pennse, 22

Pericardium, 208, 265, 268
Perilymph, 196
Perissodactyle feet, 109
Peritoneal funnels, 297, 302
Peritoneum, 208
Pes, 102
Phalanges, 104

Phalangista, urinogenital organs, 322
Phagocytes, 239

Phascoiomys, urinogenital organs, 322
Pharynx, 209

Phyllodactylus : — tongue, 224; larynx, 254
Phylogeny, 1
Physiology, 2
Physoclisti, 251
Physostomi, 251
Pia mater, 135
Pigment of retina, 183
Pigment of skin, 19
Pineal gland, 132, 142
Pinna of ear, 198
Pisiform bone, 106, 109
Pituitary body, 133
Pituitary space, 57
Placenta, 10, 274
Placenta, umbilical, 12

INDEX.

343

Plastron, 32

Pleura, 208, 264 '

Pleurodont dentition, 215

Pleuronectidffi, asymmetry of head, 70

Plexus choroidei, 136

Plica semilunaris, 192

Pluma, 22

Pneumatic bones, 107, 262

Poison-fangs, 215

Polar cells, 3

Polymastism, 28 '

Polypterus :— olfactory organ, 172 ; horny
teeth, 214 ; external gills, 250 ; air-
bladder, 251, 252

Polythelism, 28

Precoracoid, 87, 89

Prehallux, 109

Premolars, 214, 216

Prepollex, 109

Prepuce, 330

Pristiurus : — number of vertebrae, 37 ; de-
velopment of fins, 87

Processus falciformis, 185

Processus vermiformis, 236

Prochordal cartilages, 57

Pronation, 108

Pronephric duct, 296, 303

Pronephros, 296, 298, 302

Pronucleus, male and female, 3,' 4

Proteus, eyes, 186

Protocercal tail, 38

Protoplasm, 3

Protopterus : — dermal denticles, 31 ; ver-
tebral column, 35 ; skull and pectoral
fin, 68 ; pelvis, 92 ; abdominal pores,
267 ; urinogenital organs, 313, 314

Proto vertebrae, 9, 56

Proventriculus, 233

Psalterium, 236

Pseudobranch, 250

Pseudo-turbinal, 175

Pterodactylus: — abdominal ribs, 50; maims,
106

Pterygopodium, 327

Pubis, 93—99

Pulp-cavity, 213

Pupil, 183, 188

Purple, visual, 190

Pyloric caeca, 229

Pylori c portion of stomach, 234

Pyramids of kidney, 308, 310

Quill, 22

Q.

U.

Radiale, 104

Radii, of fins, 99

Radius, 104

Rana :— alimentary canal, 232 ; liver and

pancreas, 241 j larvngo-tracheal cartilages,

254

Ranodon : — development of vertebral

column, 40

Rattlesnake, poison-apparatus, 221
Receptaculum chyli, 293
Rec3ssus cochleae, 197
utriculi, 196
vestibuli, 195
Rectum, 209, 228—236
Reproduction of tail in Lizards, 43, 292
Resonance-cavities, 257
Respiratory organs, 245 — 265
Respiratory tube, 246
Rete Halleri, 327
R«tia mirabilia, 292
Retina, 182, 189
Rhamphorhynchus, manus, 106
Rhinolophus, milk-teeth, 213
Ribs, 48—51
Ribs, abdominal, 50
Rodentia, milk-teeth, 213
Rods of retina, 190, 191
Root-sheaths of hair, 24
Rostrum of skull, 58, 65 ' "
Ruminant stomach, 235, 236

8.

Sacci vasculosi, 138, 142

Sacculo-cochlear canal, 198

Sacculus, 195

Saccus endolymphaticus, 205

Salamandrina, development of vertebral

column, 40
Salmonidie : — intestinal valve, 229 ; ab -

dominal pores, 267
Sarginse, teeth, 214
Scala vestibuli, tympani, and media, 193

204

Scales, 18, 32
Scapula, 87—94 -
Scapus, 22
Scarf skin, 16
Scarus, teeth, 214
Schizocceles, 9
Sclerotic, 184
Scrotal sacs, 326

Scyllium, number of vertebrae. 37
Segmental duct, 296, 303
Segmental sense-organs, 163
Segmentation of ovum, 4
Semicircular canals, 195
Seminal duct, 301
Seminal tubes, 327
Semnopithecus, number of caudal vertebrae,

47

Sense-capsules, 58
Sensory organs, 162—207

of integument, 163, 194
Sensory tubes and sacs, 166
Septum, interorbital, 57, 75
Septum, oblique, 259
Sesamoids, 110, 113
Shark, intestinal tract, 229

344

INDEX.

Sheath of Schwann, 129

Shrew, milk-teeth of, 213

Sinus : — frontal, cthmoidal, and sphe-
noidal, 176; maxillary, 172, 176, 178

Sinus superior of membranous labyrinth,
196

Sinus venosus, 269

Siphonops, viscera, 305

Siren : — horny teeth, 214 ; intestinal
tract, 232 ; trachea, 253

Sirenia : — palatal plates, 27 ; milk-teeth,
213

Skeletogenous layer of vertebral column,
33

Skeleton, 30—111

Skin, 16

Skinks, pectoral arch of, 90

Skull, 54—84

bones of, 61 : — alisphenoid, 75, 79 ;
basioccipital, 75, 76 ; basisphenoid,
75, 76, 79 ; basitemporal, 78, 79 ;
epipterygoid, 78 ; ethmoid, 80 ; ex-
occipital, 72 — 77 ; frontal, 70, 75 ;
frontoparietal, 74 ; jugal, 83 ;
maxilla, 69 — 79 ; mesopterygoid,
69 ; metapterygoid, 69 ; orbito-
sphenoid, 75 ; palatine, 69 — 79 ;
palatopterygoid, 61 — 72; palato-
quadrate, 61 — 72 ; parasphenoid.
57, 66—72, 80; parietal, 70—79;
premaxilla, 69 — 82 ; prefrontal, 76,
77 ; presphenoid, 78, 80 ; pterygoid,
69—81 ; quadrate, 61, 70—79, 202 ;
quadratojugal, 72, 74; squamosal,
71 — 83 ; supvaoccipital, 68, 69, 76,
77, 79 ; symplectic, 61, 66, 69, 70 ;
transverse, 75 — 77 ; turbinals, 81,
172, 175—177 ; vomer, 69—79
bones of mandible : — angular, 70, 76,
79 ; articular, 70, 76, 79, 202 ; coro-
noid, 70, 84, 87 ; dentary, 70, 76,
79, 84 ; splenial, 78, 84 ; supra-
angular, 78, 79

Somatopleure, 9

Somites, 9, 54

Spatularia : — vertebral column, 35 ; ribs,
48

Spelerpes, tongue, 74, 223

Spermatozoa, 3, 300

Sphincters of intestine, 209

Spina scapulae, 91

Spinachia, nest of, 302

Spinal cord, 129

Spinal nerves, 152

Spiracle, 65, 198, 246

Spiral valve of intestine, 229

Splanchnopleure, 9

Spleen, 294

Spots, blind and yellow, of retina, 189,
190

Spouting apparatus of (rymnophiona, 179

Squalid se, connection of skull with ver-
tebral column, 64

Squatina, number of vertebrae, 37

Stapes, 70, 198, 202

Stegosaurus : — dermal skeleton, 32 ; brain,

147

Stomach, 209, 228—236
Stork, thymus and thyroid, 227
Stylo-hyoid ligament, 84
Stratum corneum and Malpighii, 16
Structural elements, 1
Subdural space, 136
Sublingua, 223
Sub-pulmonary chamber, 259
Suctorial organs, 85, 212
Sulci, 132
Supination, 108
Suprarenal bodies, 161, 320
Suspensorium, 65, 70
Sympathetic, 160
Symphysis pubis, 98
Syrinx, 255

T.

Tactile cells, 167
corpuscles, 168

Tamandua, cervical vertebrae, 43

Tapetum, 186

Tarsus, 102, 105-109

Taste, organ of, 167

Teats, 27

Teeth, 212—220

Teeth, horny, 214

Telse choroidepe, 136

Temporal fossa, 83

"Tentacle," of Gymnophiona, 179

Testis, 311—326

Testudo : — dermal skeleton, 32

Thecodont dentition, 215

Thoracic duct, 293

Thymus gland, 226

Thyroid cartilage, 255 — 257

Thyroid gland, 225

Tibia, 104

Tibiale, 104

Tibio-tarsus, 107

Tissues, 1

Tongue, 222

muscles of, 119

Tonsils, 239, 293

Tooth-structures, 62

Torpedo, electric organ, 124

Trabeculae cranii, 57, 70

Trachea, 252

Tragus, 198

Trigla : — spinal cord, 131

Triton, dorsal tin, 8.5

Truncus arteriosus, 269, 279, 280, 282

Tuberculum impar, 225

Tubules, of kidney, 29o

Tunica vaginalis, 326

Tupaia, abdominal muscles, 117

Turbinals :— of Amphibians, 172; of Rep-
tiles, 174 ; of Birds, 175 ; of Mammals*,
81, 176

Tusks, 220

Tympanic membrane and cavity, 73, 198
Tympano-Eustachian passage, 227, 246
Typhlops, skull, 78

U.

Ulna, 104
Ulnare, 104
Umbilical cord, 276

vesicle, 12
Umbilicus, 276
Unciform bone, 109
Uncinate processes, 50
Ungulata : — cervical vertebrae, 46 ; orbit,

83 ; brain of Eocene forms, 1 51
Urachus, 275
Ureter, 299, 301, 322
Urethra, 276, 309

Urinary bladder, 231, 276, 307, 308 ; of
Fishes, 302

duct, 302, 314

organs, 302 — 310
Urinogenital duct, 301, 307, 314, 316

organs, 296 — 330

sinus, 322
Urostyle, 42
Uterus, 276, 302, 311, 321, 322

masculinus, 195

V.

Vagina, 302, 322

Valves, atrio-ventrieular, 279, 287
of conus arteriosus, 279
ileo-colic, 209
pyloric, 209
semilunar, 286, 287
of veins, 292
of lymphatics, 293
Vane, 22
Vas deferens, 301, 327

epididymis, 327
Vasa centralia nervi optici, 183

efferentia, 306, 316, 327
Vascular system, 268 — 294
Veins :— allantoic, 273, 274, 291 ; anterior
abdominal, 291 ; azygos, 272, 275 ;
branchial, 270, 279—283 ; cardinal,
anterior and posterior, 271, 272, 275,
280, 291 ; caudal, 35, 270, 291 ; coro-
nary, 275, 287 ; ductus Cuvieri, 271,
273, 280 ; ductus venosus, 270, 273,

INDEX. \\ , 345

"J)r-

274 ; hepatic^ 27-f, ^75, 291 ; hypo-
gastric, 275 ; iliac, 2/5 ; jugular, 272 —
275, 285, 292 ; lateral, 291 ; omphalo-
mesenteric, 270, 273, 274 ; portal
(hepatic), 272, 274, 291, (renal) 161,
291 ; postcaval, 272—275, 291 ; precaval,
274, 275, 282, 292 ; pulmonary, 270 ;
renal, 275 ; suhclavian, 274, 275, 285 ;
subintestinal, 270, 272 ; umbilical, 276 ;
vertebral, 272—275 ; vitelline, 270, 273,
274

Velum, 246

Velum palati, 209

Venous system, 291

Ventricles of heart, 269, 277, 285—287

Vertebral column, 33 — 47

Vertebrarterial canal, 46

Vesicles, cerebral, 131

Vesicula seminalis, 307, 327

Vestibulum oris, 212

Villi of intestine, 211, 240
of placenta, 10, 274

Viper, teeth, 216

Vitelline membrane, 2

Vitellus, 2

Vocal chords, 253, 257

Vocal sacs of Anura, 253

W.

Weasel, stomach, 235
Wolffian body, 296
Wolffian duct, 301, 303, 304, 322
Woodpecker: — cervical vertebrae 46;
tongue, 223

X.

Xiphoid process, 53

Y.

Yolk-sac, 12
Ypsiloid cartilage, 93

Z.

Zootoca, teeth, 215
Zygantra, 43
Zygosphenes, 43

A A

RICHARD CLAY & SONS,

1JREAD STREET HILL, LONDON,

Bungay, Suffolk.

THIS BOOK IS DUE ON THE LAST DATE
STAMPED BELOW

AN INITIAL FINE OF 25 CENTS

WILL BE ASSESSED FOR FAILURE TO RETURN
THIS BOOK ON THE DATE DUE. THE PENALTY
WILL INCREASE TO SO CENTS ON THE FOURTH
DAY AND TO $1.OO ON THE SEVENTH DAY
OVERDUE.

isu

Lai^Alni^

SfP 1»15^

OD ^ 11 AR

A

1 1 1035

rK 5 iJ*tu

JAN 1 6 1936

I //

APR 1 7 19^

/ /

AHA 1 1438

n. nCiAS

A'ttJ & lijoo

JVJN ^ ^

MAR 23 fqoq

JUN261948

•-> t • r?
i i 'f% -4 o injin

MAR *>c 1P47

p-CD > -, IM^fl

MAR 9 1940

5fP fi -

* '947

rro Ol IJLId

rto X5JI i«»^n

.•av 3\«A8

MAR 2 8 1949

HIM 1 Q 1Q£T

NW °

juii i y iy%)/

•

LD 21-100m-8,'34

U.C. BERKELEY LIBRARIES

CDEblbOETE

UNIVERSITY OF CALIFORNIA LIBRARY