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THE NERVOUS SYSTEM 
OF VERTEBRATES 



BY 



J. B. JOHNSTON, Ph. D. 

PROFESSOR OF ZOOLOGY IN WEST VIRGINIA UNIVERSITY 



With One Hundred and Eighty Illustrations 



PHILADELPHIA 

P. BLAKISTON'S SON & CO. 

1012 WALNUT STREET 
1906 



225918 

Copyright, 1906, By P. BLAKISTON'S SON & CO. 



• ••••• ••• 

• ••• •••*• 



Printed by 

THE MAPLE PRESS CO., 

^'ork. Pa. 



PREFACE. 

The attempt has been made in the following pages to give an 
account of the nervous system as a whole, to trace its phylogenetic 
history and to show the factors which have determined the course 
of evolution. This has been made possible by recent studies 
directed toward the analysis of the nervous system on the basis of 
function. The functional point of view, which is the chief char- 
acteristic of the present book, brings the treatment of the nerv^ous 
system into close relation with the work of recent years on the 
beha\ior of animals. The study of behavior aims to give an 
account of the actions of animals in relation to the en\dronment. 
The study of the nervous system aims to describe the mechanism 
by which actions are directed and adapted to the conditions of life. 

A text-book of comparative neurology at the present time must 
meet the needs of workers of all grades, students, investigators 
and instructors. Its descriptions should be intelligible to students 
who have had one year of work in zoology or medicine, including 
the anatomy and embryology of some vertebrate. On the other 
hand there should be included all facts which are important for 
the functional and phylogenetic mode of treatment. How far 
these difi5cult conditions have been met only the use of the book 
can show. Little space is given to mere descriptive anatomy and 
some descriptive matter which is well presented in the text-books of 
zoology, anatomy and histology in common use, is omitted. This 
accounts for the brief treatment of the eye, ear and other sense 
organs, the distribution of the spinal nerves, etc. On the other 
hand, every effort has been made to bring out clearly the functional 
significance and relationships of the structures described, and to 
interest and train the student in the interpretation of structure in 
terms of function, adaptation and evolution. 

In the preparation of the text considerable time has been given 
to the re\dew of the author's earlier work and that of other writers, 
and to the study of many unsettled questions. In this way much 

iii 



IV PREFACE. 

material has been collected which is published here for the first 
time. The new observations, which are found in nearly every 
chapter, have to do chiefly with the brains of selachians, ganoids 
and amphibians, and with the origin of the organs of the sense of 
taste. The whole discussion of the phytogeny of the forebrain 
in chapter XVIII may be regarded as a new contribution to the 
subject, since the account given differs in essential respects from 
the earlier views of the author and from those of other writers. 
The proof that taste buds have their origin in the entodermal 
lining of the phar3mx establishes an exception to the statement 
that all nervous structures are derived from. ectoderm. It sug- 
gests the further possibility that some of the peripheral plexuses 
of the sympathetic system may form a second exception. 

The nomenclature of the nervous system is in a very imsatis- 
factory state for want of a guiding principle. It is believed that 
a guiding principle is furnished by the mode of treatment here 
attempted. The usual description of the nervous system based 
upon mere structural relations is quite lacking in life and meaning, 
and no set of terms has any commanding force. Significance 
and essential fitness of terms can come only with the study of the 
nervous system from the point of view of fimctional relationships. 
It is not yet time for a general revision of nomenclature on this 
basis. Accordingly the BNA terms which are now the most 
generally familiar, have been employed as far as they are adequate. 
The few new terms that have been introduced are necessary for 
the description of the functional divisions of the nervous system. 
In describing brain tracts compound names have been employed 
which indicate the origin and ending of the constituent fibers. 
Care has been taken to use as the first part of the compoimd name 
the name of the nucleus of origin of the fibers. For the sake of 
consistency and clearness it is necessary to put the names of most 
nerve centers and tracts into the Latin form. In many places, 
however, English equivalents have been used interchangeably 
with the Latin terms. The object has been to deviate as little as 
may be from common language in a subject in which technical 
terms must be so largely used. 

The majority of the illustrations are made from original draw- 



PREFACE. V 

ings. The drawings have been so constructed that in the case of 
a transverse section the reader looks at the caudal surface of the 
section, the dorsal border is uppermost and the right and left sides 
correspond to those of the reader. In the case of lateral views 
and sagittal sections the anterior end is placed at the reader's left. 
For the sake of the true illustration of the results of other workers 
and because of the peculiar value of good figures, a large number 
of illustrations have been copied. The author wishes to express 
his hearty thanks to the, writers who have given permission for 
the use of their figures. 

At the end of each chapter is a literature list giving the sources 
for the bulk of the facts contained in the chapter and the author- 
ities for the facts which support at critical points the general views 
presented. At the end of each chapter there are given also sug- 
gestions for laboratory work. These do not constitute a systematic 
laboratory course, although they ofifer the material from which 
each instructor can arrange a course suited to his conditions. The 
primary intention of these suggestions is to indicate the best or 
most easily obtainable material for the illustration of the facts 
in the chapter. It is hoped that in the near future there may be 
sufficient demand for courses in comparative neurology to warrant 
the preparation of a systematic laboratory guide. 

The first chapter is intended as a help to prospective investi- 
gators in the choice of material and methods for their studies. 
The beginning student may best pass over all but the last two 
paragraphs of the chapter. 

The author wishes to acknowledge his indebtedness to the larger 
books on the nervous system of man and mammals, of which those 
of Barker and Cajal have been the most useful. The most cordial 
thanks are due to Professor C. Judson Herrick for many helps, 
including the loan of numerous books and papers and the engrav- 
ings for Figures 4, 42, 43, 88, 89 and 90, and most of all for reading 
and criticizing the greater part of the manuscript. 

The Author. 



CONTENTS. 



CHAPTER I. 

PAGE 

The Study of the Nervous System i 

Point of view; anatomical, experimental and physiological 

methods. 
Morphological terms; planes of sectioning; neurological terms. 

CHAPTER II. 
General Morphology of the Nervous System 12 

CHAPTER III. 

Development of the Nervous System 34 

Neural tube; neural crest; neuromeres; secondary brain seg- 
ments; auditory and olfactor\' pits and optic vesicle; histo- 
genesis of central nervous system; longitudinal zones. 
Development of peripheral nerves and sense organs; part 

played by the neural crest and by ectodermal placodes. 
Morphology of the head. 

CHAPTER IV. 

Nerve Elements and their Functions 76 

Functions of nerv^e elements; stimuli and impulses; polar dif- 
ferentiation; parts of a nerve cell and course of impulses. 
Receptive and excitatory cells and reflex actions. 
The Neurone Theory. 

CHAPTER V. 

The Functional Divisions of the Nervous System .... 95 
The work of the nervous system in relation to soma and 
viscera ; four kinds of nervous activity and the correspond- 
ing divisions of the nervous system. 

vii 



VUl CONTENTS. 

CHAPTER VI. 

PAGE 

Somatic Afferent Division. General Cxttaneous Subdi- 
vision los 

General cutaneous endings and general cutaneous components 
in the spinal and cranial nerves; general cutaneous centers 
in spinal cord and brain; general survey. 

CHAPTER VII. 

Somatic Afferent Division. Special Cutaneous Subdi- 
vision 124 

Sense organs, morpholog\' and physiolog}-; special cutaneous 
components; special cutaneous centers; differentiation of 
cutaneous and auditory centers and cerebellum. 

CHAPTER VIII. 

Somatic Afferent Division. The Visual . Apparatus . . . 143 
Organism sensitive to light; development of special light per- 
cipient organ, the retina; optic tracts; structure of tectum 
opticum; origin and morphological position of the eyes. 
Pineal eyes. 

CHAPTER IX. 

The Visceral Afferent Division 155 

General Visceral Subdivision. 

Visceral afferent components; visceral sensor)' centers. 
Special Visceral or Gustatory System. 
Organs of the sense of taste, origin, structure and distribu- 
tion; innervation; gustatory centers; change of function 
in higher vertebrates. 

CHAPTER X. 

The Olfactory Apparatus 176 

Sense organ; nerve; olfactory centers. Relations to other 
systems. 

CHAPTER XI. 

The Somatic Motor Division 190 

Ventral nerves; roots wanting in cranial region; origin, 
arrangement and distribution of components; plexuses; 
"hypoglossus"; eye muscle nerves; relations of the somatic 
motor column in the brain. 



CONTENTS. IX 

CHAPTER XII. 

PAGB 

The Visceral Efferent Division 199 

Origin, arrangement and distribution of the components; 
fibers to the sympathetic; nerves of mastication; of expres- 
sion; vagus and spinal accessory complex; relations of the 
visceral efferent column. 

CHAPTER XIII. 

The Sympathetic System 206 

Development of ganglia and nerves; structure of the system; 
general constitution and relations. 

CHAPTER XIV. 

Centers of Correlation 218 

General constitution of central system; four chief zones; sub- 
stantia reticularis; functions and organization of; centers of 
correlation; their relation to complex activities; materials 
for study of these centers. Region of the myelencephalon. 

CHAPTER XV. 

The Cerebellum 226 

Phylogenetic history. 

Mammalian cerebellum; hemispheres and vermis; subdivisions 

and fissures; structure; review of vertebrate cerebellum. 

Structure of cerebellar cortex; fiber tracts of cortex. Deep 

gray masses and their fiber connections. 
Evolution of structure and fimction, general survey; fimction 

in mammals. 

CHAPTER XVI. 

Centers of Correlation. The Mesencephalon and Dien- 
cephalon 253 

1. The Cutaneous Apparatus. 

The lemniscus system; the differentiation of centers in 
the tectum mesencephali. 

2. The Optic Apparatus. 

Tectum opticum; development of optic centers in the 
thalamus; division of function between centers. 

3. Centers related to the posterior commissure. 



X CONTEXTS. 

CHAPTER XVII. 

PAGE 

Correlating Centers in the Diencephalon (Continued) . . 269 

4. The Olfactory and Gustaton* Apparatus. 

The hypothalamus; its limits, relations, structure and 
fiber tracts. The nucleus habenulae; structiu-e, rela- 
tions and fiber tracts. The hj'pothalamus and nu- 
cleus habenulae as derivatives of the visceral sensory 
zone of the brain. 

5. The nucleus of the tractus strio-thalamicus and the sub- 

stantia reticularis thalami. 

6. The Saccus Apparatus. 

CHAPTER X\TII. 

The Evolution of the Cerebral Hemispheres 292 

Survey of the forebrain of lower vertebrates. 
The archipallium and neopallium of mammals. 

CHAPTER XIX. 

The Neopallium 338 

Structure of the cortex. 
Cerebral localization. 
Evolution of the neopallium. 



LIST OF ILLUSTRATIONS. 



FIG. PAGE 

1. Outlines of the spinal cord with the dorsal and ventral roots in 

Petromyzon and in a mammal 14 

2. The brain of Heptanchus 15 

3. The medulla oblongata of the lake sturgeon (Acipenser rubi- 

cundus), to show the longitudinal zones 16 

4. Two views of the brain of the buffalo fish, Carpiodes velifer 

(Raf.); (i) from above; (2) from the right side. From C. 
Judson Herrick after C. L. Herrick 17 

5. Simple diagrams of the branchial nerves of lower vertebrates 

as seen from the left side 19 

6. A diagram of the lateral line canals and pit organs together 

with the nerves which supply them, in a ganoid fish (Amia 
calva). After E. Phelps Allis 21 

7. A sketch of the brain of Chimaera monstrosa from the left side 

to show especially the position of the nerve roots 23 

8. The outline of the brain ventricles as seen from above; A, of 

a cyclostome fish, Lampetra Wilderi; B, of a selachian, 
Mustelus canis; C, of a young specimen of a bony fish, Co- 
regonus albus; D, of a tailed amphibian, Necturus macu- 
latus 25 

9. A diagram of one side of the forebrain in Mustelus to show 

what is believed to be the primitive relations of the wall and 
ventricle 26 

10. The outline of the ventricles in man 26 

1 1 . The mesial surface of the right half of the brain of a selachian, 

Squalus acanthias ' ' 29 

12. A sketch of the brain of a cyclostome fish, Lampetra Wilderi, 

as seen from the left side 30 

13. Sections of the neural plate and folds in amphibia, Amblystoma 

tigrinum and A. punctatum 35 

14. Transverse section of the neural lube of Amblystoma puncta- 

tum just after closing 36 

xi 



XU LIST OF ILLUSTRATIONS. 

FIG. PAGE 

15. Transverse section through the neural tube, neural crest and 

ectoderm of Amblystoma at a later stage than that shown in 

Fig. 14 36 

16. Same as Fig. 15, later stage 37 

17. A part of the spinal cord of an 1 8-day Catostomus embryo, 

showing the giant ganglion cells 38 

18. Figures of the brain of a selachian embryo (Squalus acanthias) 

to show the history of the neuromeres. After Locy. ... 39 

19. Figures to show the history of the neuromeres in a bony fish 

and in the chick. After Hill 40 

20. The brain of a pig embryo of 12 mm. from the right side. 

From Minot as drawn and revised by Dr. F. T. Lewis. . . 41 

21. Transverse sections through the region of the optic vesicle in 

embryos of: A, Torpedo ocellata; B, Callus domesticus; 

C, Cavia cobaya. After Froriep 42 

22. The development of the optic vesicles in Amblystoma punc- 

tatum 43 

23. A part of a transverse section of the inferior lobe of the stur- 

geon, stained by the Golgi method 47 

24. A horizontal section of the nucleus praeopticus of the sturgeon, 

by the Golgi method 48 

25. A part of a transverse section of the optic lobe of the sturgeon, 

by the Golgi method 49 

26. The ganglion of the IX nerve in Amblystoma punctatum at 

the time of formation of the central processes 51 

27. A few cells of the trigeminal ganglion of Amblystoma puncta- 

tum with the fibers of the ramus mandibularis growing out 
from them 51 

28. A, a diagram of the head of Petromyzon at a stage when the 

neural crest is segmented into the anlages of the cranial 
ganglia. B, a similar diagram at a later stage when all the 
cranial nerves are to be recognized. After KoltzofiF. ... 52 

29. Three diagrams of the head of Squalus acanthias to show the 

difiFerentiation of the neural crest into the cranial ganglia. 
After Neal 53 

30. A reconstruction of the peripheral nerves in a four weeks 

human embr>'o, 6.9 mm. long. From Streeter 54 

31. Reconstruction of peripheral nerves in four weeks human 

embr\'o, 7.0 mm. long. From Streeter 55 



LIST OF ILLUSTRATIONS. XIU 

FIG. PAGE 

32. Reconstruction of peripheral nerves in six weeks human em- 

bryo, 17.5 mm. long. FromStreeter 56 

33. Three stages in the development of the acustico-lateral system 

in the sea bass. From H. V. Wilson 57 

34. A transverse section through the nasal sac of an embryo of a 

bony fish at about the time of hatching, to show the origin 

of the fibers of the olfactory nerve 6i 

35. Two figures representing the formation of unipolar cells in the 

spinal ganglion of the dog embryo. After Van Gehuchten . 62 

36. A median sagittal section of the head of an embryo Ambly- 

stoma punctatum 67 

37. Diagrams representing the development of the buccal cavity, 

hypophysis and nasal pit in Amphioxus and Petromyzon. 
After Legros 68 

38. A diagram representing the segmentation in a generalized 

vertebrate head 71 

39. Several types of nerve cells from the central and peripheral 

nervous system of vertebrates 78 

40. A unipolar nerve cell from the brain of Carcinas to illustrate 

Bethels experiment 80 

41. Diagrams intended to show several forms of reflex chains in 

the nervous system of vertebrates 83 

42. A portion of the subepithelial nervous network in the palate 

of the frog. From Prentiss 86 

43. A portion of the nervous network about the walls of a small 

vessel in the palate of the frog. From Prentiss 87 

44. Two ganglion cells of the nervous network in the intestinal 

wall of the leech Pontobdella, showing neurofibrillae pass- 
ing through the cells. From Bethe after Apathy .... 88 

45. A diagram of the component elements in the spinal cord and 

the nerve roots of a trunk segment, to illustrate the rela- 
tions of the four functional divisions of the nervous 
system 97 

46. Diagrams to represent the extent and arrangement of the 

functional divisions in the brain of a selachian 100 

47. A diagram to show the arrangement of the two afferent divis- 

ions in the brain of man loi 

48. General cutaneous endings in the skin of a cyclostome, Lam- 

petra Wilderi 105 



XIV LIST OF ILLUSTRATIONS. 

FIG. PAGE 

49. A diagrammatic representation of the general cutaneous com- 

ponents of a trunk segment 106 

50. A simple diagram of the general cutaneous components in the 

cranial and spinal nerves of a fish 107 

51. A reconstruction of the cranial nerves of Petromyzon dorsatus 

to show the arrangement and distribution of the several sys- 
tems of nerve components 108 

52.. The principal sensory collaterals in the spinal cord of the 

new-bom rat. From Cajal 109 

53. Some cells of the dorsal horn in the chick embryo of five 

days. From Cajal in 

54. Transverse section of the substance of Rolando of the cervical 

cord in the new-born cat. From Cajal 112 

55. Cells in the dorsal horn of the cord in a chick embryo of 

nineteen days incubation. From Cajal 113 

56. Transverse section through the spinal V tract and the sub- 

stance of Rolando in a new-born rabbit. From Cajal . . 114 

57. A, transverse section of the spinal cord of Lampetra to show 

the ceils of the dorsal horn 115 

58. A, transverse section of the tuberculum acusticum of Lam- 

petra; B, a sagittal section of the cerebellum of the same 
animal 116 

59. A diagram representing the centers and fiber tracts related to 

the general cutaneous components in fishes 118 

60. A diagram representing the general cutaneous centers and 

fiber tracts in the human brain 120 

61. A large and a small neuromast from a sucker embryo at about 

the time of hatching 124 

62. A diagram of the lateral line canals and nerves in Amia. 

After E. Phelps Allis 125 

63. A reconstruction of the chief rami of the cranial nerves in a 

bony fish, Menidia, to show the arrangement of the several 
systems of components. After C. Judson Herrick . . . . 129 

64. Transverse section of the brain of the sturgeon at the level of 

the VII and VIII nerves 131 

65. Transverse section of the brain of the sturgeon at the level of 

the V nerve 132 

66. Transverse section of the brain of Scyllium to show the fold- 

ing of the cerebellar crest and tuberculum acusticum ... 133 



LIST OF ILLUSTRATIONS. XV 

FIG. PAGE 

67. Transverse section of the acusticum of the sturgeon to show 

acusticum cells and a Purkinje cell 134 

68. A section through the same region as in Fig. 67, to show cell 

forms intermediate between the acusticum and Purkinje cells. 136 

69. A diagram to show the central endings of the special cutaneous 

components in fishes 138 

70. A diagram to show the central ending of the vestibular and 

cochlear nerves and of the optic tract in man and the chief 
secondary tracts related to them 140 

71. A, a section through the retina of a mouse embryo of 15 mm.; 

B, a section through the retina of a chick embryo of fourteen 
days incubation; C, a section of the retina of a dog. From 
Cajal 144 

72. Cells from the retina of the chicken. From Cajal 145 

73. The tectum opticum of the sturgeon 147 

74. A section of the optic lobe of a bird. From Cajal .... 149 

75. A series of diagrams intended to illustrate the origin and mode 

of formation of the optic vesicle in vertebrates 151 

76. A sketch showing the relations of the two epiphyses in verte- 

brates 152 

77. A diagrammatic representation of the general visceral sensory 

components in a trunk segment 155 

78. A transverse section through the region of Clarke's column of 

the thoracic cord of a new-born dog. From Cajal .... 156 

79. A reconstruction of the cranial nerve components in a tailed 

amphibian, Amblystoma. After Coghill 158 

80. A simple diagram of the visceral nerves of the head in fishes. 159 

81. Four transverse sections through the medulla oblongata of the 

frog to show the position and ending of the fasciculus com- 
munis and nucleus commissuralis 160 

82. Transverse section through the medulla oblongata of a mouse 

at the level of the nucleus commissuralis. From Cajal . . 161 

83. Transverse section through the medulla oblongata of a mouse 

four days old. From Cajal 162 

84. A, a transverse section through the medulla oblongata of the 

sturgeon at the level of the X ner\e; B, at the level of the 

IX nerve 163 

85. Sense organs of bony fishes. A, a taste bud from the oesopha- 

gus of Catostomus at the time of hatching; B, a taste bud 



XVI LIST OF ILLUSTRATIONS. 

FIG- PAGE 

from the pharynx of the same embryo; C, two neuromasts 

from the skin of the same embryo 165 

[86. A taste organ from the pharynx of the ammocoetes of Petro- 

myzon dorsatus 166 

87. A taste bud from the skin of an adult Lampetra 167 

88. A projection of the cutaneous branches of the communis root 

of the right facial nerve in a bony fish, Ameiurus. From 

C. Judson Herrick 168 

89. A parasagittal section through the brain of the spotted sucker, 

Minytrema melanops, to show the gustatory centers and 
tracts. From C. Judson Herrick 170 

90. A diagram of the gustatory paths in the brain of the carp as 

seen from the left side. From C. Judson Herrick .... 172 

91. Part of a sagittal section of the brain of a newly hatched 

bony fish 173 

92. A diagram representing the centers and tracts related to the 

visceral sensory components in fishes 174 

93. A transverse section through the olfactory epithelium of a 

bony fish at the time of hatching 177 

94. An oblique section through the forebrain of Lampetra . . . 178 

95. A horizontal section through the olfactory bulb of the 

sturgeon 179 

96. A spindle cell and two granules from the olfactory bulb of 

the sturgeon 180 

97. An olfactory glomerulus from the brain of the sturgeon . . 181 

98. Cells with short neurites in the olfactory bulb. FromCajal. 182 

99. A transverse section of the forebrain of the sturgeon at the 

level of the anterior commissure 184 

100. An outline of the median sagittal section of the forebrain of 

Lampetra 185 

loi. A diagram of the olfactory conduction paths in the sturgeon. 187 

102. A diagrammatic representation of the somatic motor compo- 

nents of a trunk segment 191 

103. An early stage in the formation of the pectoral fin and 

brachial plexus in a selachian, Spinax. After Braus ... 192 

104. The formation of the cervical plexus in a selachian, Hexan- 

chus. After Furbringer 193 

105. A transverse section through the nucleus of origin of the III 

nerve in Lampetra 195 



LIST OF ILLUSTRATIONS. XVll 

IG. PAGE 

06. A diagrammatic representation of the visceral efferent com- 

ponents in a trunk segment 199 

07. Diagram to show the central relations of the IX, X and XI 

nerves in mammals according to Onuf and Collins . . . 201 

08. A diagram of the sympathetic system and the arrangement 

of its neurones in a mammal. Chiefly after Huber . . . 209 

09. Diagram illustrating the spinal representation of the sympa- 

thetic nerve in a mammal. From Onuf and Collins ... 210 
IG. A diagram to illustrate Langley's "axone reflex" . . . . 213 

11. Tract cells in the spinal cord of the trout. After Van Ge- 

huchten 219 

12. The relations of the cerebellum, brachium conjunctivum and 

gustatory tracts in selachians (Scyllium) 226 

13. Transverse section through the cerebellum of the sturgeon . 227 

14. Transverse section through the cerebellum and secondary 

gustator>' nucleus of the sturgeon 228 

15. Transverse section of the brain of the sturgeon at the junc- 

tion of cerebellum and midbrain 229 

16. Transverse section through the midbrain of the sturgeon . . 230 

17. The relations of the cerebellum, brachium conjunctivum 

and gustatory tracts in a ganoid fish (Acipenser) .... 231 

18. A, the left lateral aspect of the brain of a pouch specimen 

of Dasyurus viverrinus; B, median sagittal section through 

the cerebellum of the same brain. After G. Elliot Smith . 232 

19. A diagram representing the fundamental and more constant 

secondary- fissures of the mammalian cerebellum spread out 

in one plane. After G. Elliot Smith 234 

20. The mesial surface of the right half of the brain of Squalus 

acanthias 236 

21. A diagrammatic transverse section of one fold of the cere- 

bellum. From Koelliker 238 

22. Net-like fiber ending in the human cerebellum. After Cajal. 241 

23. Two schemes to show the course of impulses in the cerebellar 

cortex. From Cajal 242 

24. Two transverse sections through the cerebellum of Scyllium. 244 

25. A transverse section through the cerebellum of Necturus . . 246 

26. A transverse section through the deep gray nuclei of the 

cerebellum of man 247 

127. Outline transverse sections through the mesencephalon of a 



XVUl LIST OF ILLUSTRATIONS. 

FIG. PAGE 

cyclostome, a selachian, a ganoid, a bony fish, an amphibian 
and a mammal 255 

128. Transverse section of the diencephalon of the rat at the 

level of the body of Luys. From Cajal 257 

1 29. Transverse section of the dorso-mesial portion of the posterior 

corpus quadrigeminum of the new-born dog. From Cajal. 259 

130. Transverse section of the anterior corpus quadrigeminum of 

the rabbit of eight days. From Cajal 260 

131. Lower part of the corpus geniculatum laterale of a new-bom 

cat. From Cajal 262 

132. Nucleus of posterior commissure in Lampetra 266 

133. Transverse section through the corpora mammillaria of the 

sturgeon 270 

134. Transverse section through the inferior lobes of the sturgeon. 272 

135. Transverse section through the posterior commissure of the 

sturgeon 273 

136. Scheme of the connections of the mammillary tracts, nucleus 

habenulae and the nucleus dorsalis thalami. From Cajal. 274 

137. Sagittal section of the tuber cinereum of a new-born rat. 

From Cajal 275 

138. Sagittal section of the tuber cinereum of the rat of eight 

days. From Cajal • 276 

139. A scheme to show the embryologicjal relations of the nucleus 

habenulae and the inferior lobes in fishes 277 

140. Nucleus habenulae of the sturgeon 278 

141. Transverse sections through the nucleus habenulae and the 

facial lobe of a young Amia 279 

142. Transverse section of the habenular nuclei in the dog. 

From Cajal 280 

143. The efferent tract to the saccus vasculosus in the sturgeon. 283 

144. A general scheme of the saccus apparatus 284 

145. A diagram of the fiber tracts in the forebrain of a cyclostome 

(Lampetra) 294 

146. An oblique section through the inferior lobes and forebrain 

of Lampetra 296 

147. A diagram of the fiber tracts in the forebrain of a selachian 

(chiefly after Kappers) 299 

148. A diagram of the fiber tracts in the forebrain of a bony fish 

(chiefly after Goldstein) 302 



LIST OF ILLUSTRATIONS. XIX 

FIG. PAGE 

149. A transverse section of the brain of the sturgeon behind the 

anterior commissure 303 

150. A diagram of the fiber tracts in the forebrain of a tailed 

amphibian (Necturus) 306 

151. Simple diagrams to show the histor>' of the epistriatum in 

fishes and amphibia 308 

152. Sketches of transverse sections through (A) the caudal part 

of the epistriatum in amphibia and (B) the corresponding 
structure in Ornithorhynchus. B after G. Elliot Smith . 310 

153. A sagittal section of the forebrain and interbrain of a chick 

embryo of 7.0 days. After Minot 312 

154. Part of a transverse section of the cortex of a chameleon. 

After Cajal 313 

157;. Transverse section through the forebrain of Lacerta at the 

level of the anterior commissure. After Cajal 314 

156. A transverse section through the right lateral lobe of the 

forebrain of Lacerta. After Cajal 315 

157. A diagram of the mesial surface of the hemisphere of a reptile 

to show the extent of the hippocampus and related structures. 316 

158. A mesial sagittal section of the brain of an embryo of Sphe- 

nodon punctatum. From G. Elliot Smith 317 

159. The mesial surface of the right half of the brain of Ornitho- 

rhynchus. From G. Elliot Smith 318 

160. The mesial surface of the cerebral hemisphere of a marsupial 

(Phascolarctos). From G. Elliot Smith 319 

161. Portion of a transverse section through the brain of a Moni- 

tor (Hydrosaurus). From G. Elliot Smith 320 

162. Transverse section of the cerebral hemispheres of Ornitho- 

rhynchus. From G. Elhot Smith 321 

163. Plan of cerebral hemispheres, lamina terminalis and optic 

thalami in horizontal section. From G. Elliot Smith . . 322 

164. Sagittal section of the commissural and precommissural 

regions of the hemisphere of Ornithorhynchus. From G. 
Elliot Smith 323 

165. Ventral surface of the brain of Ornithorhynchus. After G. 

Elliot Smith 324 

166. Transverse section of the brain of a rat of four days through 

the anterior commissure. From Cajal 325 

167. Diagram of the mesial surface of (A) the hemisphere of a 



XX LIST or ILLUSTRATIONS. 

FIG. PAGE 

monotreme and (B) that of a mammal to show the extent 
and relations of the hippocampus 326 

168. Schemes to explain the expansion of the corpus callosum 

and the formation of the septum pellucidum. After figures 

by G. Elliot Smith 328 

169. Scheme of cerebral commissures and the margin of the cor- 

tex of the human brain. From G. Elliot Smith .... 330 

170. Scheme of the structure and connections of the hippocampus. 

From Cajal 332 

171. Structure of the cerebral cortex 339 

172. Diagram showing the probable course of impulses in the cells 

of the cerebral cortex. After Cajal 340 

173. Scheme of long association tracts in the cerebral cortex. 

After Cajal 342 

174. Scheme of commissural and projection fibers of the cortex. 

After Cajal 343 

175. The primordial areas in the cerebral hemispheres, lateral 

surface. From Flechsig 346 

176. The primordial areas in the hemispheres, mesial surface. 

From Flechsig 347 

177. The primordial areas and border zones of the association 

fields, lateral surface. From Flechsig 350 

178. The primordial areas and border zones of the association 

fields, mesial surface. From Flechsig 350 

179. Diagram of lateral surface of hemisphere showing localiza- 

tion of functions 351 

180. Diagram of mesial surface of hemisphere showing localiza- 

tion of functions 351 



NERVOUS SYSTEM OF VERTEBRATES.' 



CHAPTER I. 
THE STUDY OF THE NERVOUS SYSTEM. 

The nervous system is a complex system of organs which stands 
in close relations >^ith all the other organs of the body. The study 
of the nenous system requires on the one hand an examination 
of the structure and mode of functioning of its component parts 
and on the other a consideration of its manifold relations to other 
organs. There must be considered (a) structural connections 
which provide for the reception and discharge of nerve impulses 
which play an essential part in the functioning of organs and in 
the correlation of their .activities; and (b) the more general 
relations of place, the arrangement of ner\^ous and other organs 
in the body. In both these respects the actual relations of the 
ner\'ous system in the organism have been determined by the 
conditions and course of the evolution of the vertebrate organism, 
in which the adaptation of structure to the mode of life is the 
controlling principle; and by the conditions of embryonic develop- 
ment, in which special structures adapted to the embryonic mode 
of life and inherited structures adapted to ancestral modes of 
life play important parts. The student who wishes to gain a 
knowledge of the ner\^ous system is of necessity thrown into this 
maze of complicated relationships, but fortunately he finds 
methods of investigation adapted to the problems to be solved. 

While the student may direct his attention especially to the phase 
of structure or the phase of function, he should understand from 
the outset that great progress and permanent results arc to be 
attained only when the study of structure and the study of function 
go hand in hand. While in general the detailed study of structure 
must precede the detailed study of function, the interpretation 



2 NERVOt'S SYSTEM OF VERTEBRATES. . 

of nervous structares, the statement of problems and the planning 
of investigations for their solution gain most from more general 
conjurations of function, namely, considerations of the conditions 
ajfd'mbde of life of the organism. 
**:•. Anatomical methods.— The method by which the study of the 
• * nervous system was first undertaken is also the first to be used by 
the student of today, namely, dissection. The purpose of dissec- 
tion is to show the general place relations of the parts of the nervous 
system to other organs and the apparent structural connections 
between the two. Even to the investigator of original problems 
the method is still useful, and as it must be practiced in preparation 
for physiological experimentation and operation for the study of 
degeneration phenomena, the importance of thorough training 
in dissection can scarcely be over-estimated. The refinement of 
dissecting microscopes and mechanical appliances in recent years 
has made dissection applicable to more and more minute objects 
and at the same time has extended its usefulness to problems 
which could be approached before only by more indirect and 
tedious methods. The great advantage of handling one's object 
and of seeing its parts in their three dimensions makes the method 
of dissection indispensable and warrants every effort to refine 
and extend it. 

When the dissection of the nervous system had reached the 
height of its efficiency by the appliances known to the earlier 
workers, as in the studies of the ner\'es of fishes by Stannius, the 
introduction of the microtome and the method of studying sections 
opened the way for great advances especially in the study of the 
central nervous system and of development. When the sections 
were stained by carmine or ha^matoxylin, more or less complete 
pictures were obtained of the structures in each section. By 
carefully keeping all the sections into which an object might be 
cut and mounting them in their serial order, it became possible 
to find the limits and form of structures too small to be dissected 
and to trace bundles of fibers from section to section throughout 
their course. Although for special investigations this method is 
largely replaced by more exact methods, it is still useful for the 
study of the general morpholog}- of the nervous system and the 



THE STUDY OF THE NERVOUS SYSTEM. 3 

topographical relations of nerve centers and fiber tracts, and for 
the purpose of reconstructions by which these relations can be 
shown on an enlarged scale in wax models and the like. Series 
of sections prepared with such a stain as Delafield's hsematoxylin, 
well differentiated, remain one of the best means of studying 
general anatomical relations, especially in the central ner\^ous 
system. 

Scarcely had this general method been developed when special 
methods were devised to gain more exact pictures of nerve struc- 
tures, especially by taking advantage of the selective activity of 
the chemicals to be employed. First among such methods was 
the impregnation of nerve elements by metalUc salts. Results 
obtained by this method were published by Golgi in 1875, ^^^ ^^ 
method did not come into general use for a number of years. The 
reasons for this were that the procedure followed by Golgi was 
slow, requiring several months for its completion, and the impreg- 
nation was more or less uncertain, while the method of Weigert 
which was soon introduced promised results more quickly and 
with less labor. About 1890 new procedures which were both 
more rapid and more certain in their action were brought forward 
by Cajal, Cox and others, and since that time the method of 
metallic impregnation has been more and more widely used. 
This technique has given some of the most valuable contributions 
to our knowledge of the nervous system and is still among the 
most important methods for research. The advantage of the 
method lies in the fact that it gives incomplete pictures. Few 
elements are stained, often in great detail; and these are not 
obscured by the richness and confusion seen in complete pictures. 
The coloring of the tissue is not a true stain but an impregnation 
with a metallic salt. Although neuroglia, capillaries and other 
elements may at times be impregnated, the procedure peculiarly 
affects the nerve elements. Of these only a few are impregnated, 
the selection apparently being due in some way to the physiological 
state of certain elements. The elements which are thus isolated 
may be studied with great certainty and completeness, but the 
full study of all the elements in a given region requires a suffi- 
ciently large number of preparations to enable the observer to 



4 NERVOUS SYSTEM OF VERTEBRATES. 

make up a composite picture approaching completeness. The 
method when used alone always has the disadvantage that the 
observer can never know when he has obtained an impregnation 
of all the elements present in the organ which he \vdshes to study. 
The peculiar value of the method lies in the fact that the nerve 
elements themselves are brought to view. . Not only can the form 
of the cells and their dendrites be seen but the neurites can be 
traced directly from their origins to their endings. The polarity 
of form of the nerve elements is shown and the direction of flow of 
impulses can be inferred with little danger of error. The method 
is especially adapted to the study of the central nervous system 
and gives facilities for determining the functional relations of 
centers and tracts scarcely afforded by any other anatomical 
method. It is equally trustworthy for the peripheral nervous 
system wherever clear pictures can be secured, but the technical 
difficulties are greater on account of the conditions of sectioning 
and the formation of precipitates between the tissues. 

The method of staining myelin sheaths commonly known as 
the method of Weigert, who introduced it in 1881, differs widely 
from the method of Golgi in that it stains non-nerv^ous elements 
only. The picture given by this procedure is complete so far 
as the myelin sheaths are concerned. Since the gray matter 
and the non-myelinated tracts are unstained, the myelinated 
tracts can be traced with relative ease. The method is especially 
adapted to the central nerv^ous system where it gives quickly and 
easily the course and general topographical relations of the fiber 
tracts. The method is limited in its usefulness because it does 
not give the origin or ending of neurites or the course of non- 
myelinated tracts, fibers or collaterals. Since neurites may run a 
longer or shorter distance before receiving their myelin sheaths and 
since the terminal branches of neurites run an unknown distance 
after passing the end of their sheaths, the study of the sheaths may 
lead to erroneous conclusions as to the groups of cells from which 
given fibers arise or in which they end. Since in recent years 
it has become possible lo apply the method to material fixed in 
formalin or in Flemming's fluid, even better preparations are 
obtained than formerlv and with jjrrcatcr ease. When the usual 



THE STUDY OF THE NERVOUS SYSTEM. 5 

Weigert preparations are treated with a sharp secondarj' proto- 
plasmic Stain, such as acid fuchsin, the method gives a much 
larger number of facts and more reliable pictures. In particular, 
cell bodies are stained and the origin and endings of neurites are 
sometimes shown sufficiently well to enable the observer to deter- 
mine the direction of impulses carried by a given tract. Even 
when this result is not obtained, the double-stained preparations 
are preferable to ordinary haematoxylin sections for the study of 
general topographical relations. As a special method, however, 
the Weigert procedure if used alone and on adult brains, can be 
relied upon for little in the way of functional relationships. The 
method is applicable to the study of the course of peripheral 
nerves by means of sections, and is especially valuable for the 
analysis of nerve trunks into their components. In addition to 
the usual Weigert technique, it may be mentioned that with small 
animals or small brains excellent Weigert effects may be obtained 
by simple fixation with vom Rath's picro-aceto-platino-osmic 
mixture, with or without after-staining with acid fuchsin. 

A third special method which has proved extremely valuable is 
the intra vitam staining with methylene blue introduced by Ehrlich 
in 1886. This differs from both the preceding in that it is an 
actual stain of nerve elements. The preparations are in general 
comparable to those obtained by the Golgi technique, since the 
pictures are incomplete pictures of the same character. The 
method gives in addition the internal structure of the nerve 
elements and is extremely useful for cytological study. It gives 
also more extensive and reliable information than does the Golgi 
method as to the details of fiber endings and the structural relations 
between nerve elements. The method is especially adapted to 
the study of the peripheral nerv^ous system and to small isolated 
portions of the central nervous system. It has not yet been 
extensively used for the study of centers and fiber tracts. It reaches 
the height of its efficiency where the tissues stained are suitable 
for study in the living condition. 

Of the great variety of cytological methods which may be applied 
to the nervous system two require mention as special nerve methods. 
The first of these is known as the Nissl method and is a selective 



6 NERVOUS SYSTEM OF VERTEBRATES. 

Stain of certain plastic materials within nerve elements which 
are doubtless either nutritive or excretory in their nature, or both. 
These materials constitute the so-called Nissl bodies whose form 
and volume change with changing ph)rsiological states of the nerve 
elements. The study of these elements is therefore of great 
importance to the physiologist and the pathologist. 

The other method includes a considerable number of proced- 
ures which may be spoken of collectively as neuro-fibrillar staining 
methods. These stains affect the more stable colloidal substances 
in nerve elements and so are complementary to the Nissl method. 
Besides the methylene blue stain mentioned above a number of 
methods have been used for this purpose, of which the latest are 
certain modifications of the photographic procedure introduced 
by Cajal, Bielschowsky and others. 

The methods thus far mentioned are anatomical methods 
chiefly applicable to the adult nervous system. When the nervous 
system is studied with especial reference to its development or 
by making use of any peculiar characters which it possesses 
during development, it is proper to speak of embryological methods. 
These include the study of the development of the nervous system 
by any of the procedures applicable to embryos and the study of 
histogenesis by various staining techniques, including those of 
Golgi and of Ehrlich mentioned above. A special embryological 
method of great importance, the method of Flechsig, consists 
in the study of the course and order of myelination of nerve tracts 
by means of the Weigcrt technique. This gives especially favorable 
opportunities for studying the course of fiber tracts because 
specific tracts can be studied at the time of appearance of their 
myelin sheaths, certain tracts frequently presenting themselves 
in entire isolation from others. 

Experimental methods. — The anatomical study of the nervous 
system may be greatly extended by means of experiments which 
produce artificial differences betw^een ner\'e tracts or nerve centers, 
which may then be brought to view by suitable staining. Most 
prominent among these methods is the study of fiber tracts which 
have been caused to degenerate in accordance with Waller's 
law. Negative pictures of such tracts are obtained when the 



THE STUDY OF THE NERVOUS SYSTEM. ^ 

sections are treated by the usual Weigcrt stain, or positive 
pictures are obtained by means of the technique of Marchi. The 
method is applicable wherever localized and known centers can 
be destroyed or kno^^Ti tracts can be severed from their cells of 
origin. The fibers then degenerate in a direction away from their 
cells of origin, and the course of the fibers is brought to view by 
selective staining of the degeneration products of their myelin 
sheaths. Random operations may reveal the position of centers 
or tracts not before suspected. Many operations and prepara- 
tions are needed in order to gain complete results. The time 
necessary for degeneration after the operation must be determined 
by extended trial and if not accurately determined, either no 
result or misleading results may follow. The staining technique 
is not always reliable. A source of serious error is found in the 
fact that fat globules resulting from the degenerative processes 
may be carried by blood or lymph currents to situations far removed 
from their place of origin, and when stained in those situations 
they give very misleading pictures. The method is extremely 
valuable as giving the gross facts regarding the course and func- 
tional relationship of myelinated fiber tracts. With regard to 
the place of ending of fiber tracts the method has the same limi- 
tations as the method of Weigert, but in even greater degree 
because the fibers may not have degenerated throughout their 
entire length or may have passed the proper condition for staining. 
The method is more reUable for the origin of the fibers studied, 
or rather for the direction of the impulses which they carry, pro- 
vided due care is taken to distinguish between Waller's degener- 
ation and Gudden's degeneration (see p. 91) which may also 
occur. The method requires great understanding and caution 
in its use but promises to be one of the most important of anatom- 
ical methods, and one which will greatly increase in usefulness. 

Allied to the last is the study of secondary degenerations due 
to lesions, operative or accidental, in related centers. Nature 
performs many experiments of the greatest value to the neurol- 
ogist, lesions in the central nervous system often offering oppor- 
tunity through the study of primary and secondary degenerations 
to determine pathways of impulses othen\dse difficult to follow. 



8 NERVOUS SYSTEM OF VERTEBRATES. 

Closely related to the embryological methods already men- 
tioned is the study of the processes at work in the degeneration 
and regeneration of cut nerve fibers. This may be prosecuted 
on either embryos or adults and throws especial light upon the 
character of neurones, their histogenesis, their relations to accessory 
structures and the mode and meaning of histological differenti- 
ation within the ner\'ous system. 

Recent work upon the rise and histogenesis of nervous elements 
and organs has shown that the embryological methods may be 
extended by experimental procedures to a degree little dreamed 
of a few years ago. Many problems of development and differ- 
entiation in the nervous system are capable of re-examination 
by experimental embryological methods, which will especially 
throw light on the causes at work in these processes. 

Physiological methods. — ^These may be either experimental 
or observational. In the one case nerv^ous actions are studied 
under conditions determined by operative or other artificial means. 
In the other case actions are studied under natural conditions 
believed to be sufficiently known. Among the simpler forms 
of experiments are those to determine the course of impulses. 
By the help of operations, or otherwise, suitable stimuli are applied 
to definite sensory areas, to nerve trunks or roots, to certain 
nerve centers, etc., and the actions called forth are observed and 
studied. The method is widely applicable and has been used 
independently for the investigation of the pathways of impulses 
and hence of functional connections in the nervous system. For 
its intelligent use it presupposes thorough anatomical knowledge 
and the most reliable results are obtained by combining one or 
more anatomical methods with it. Conduction paths may be 
determined by this method independently and some indications 
obtained as to the number and arrangement of their several steps. 
These data may then be tested and confirmed anatomically. 

The study of the rate, direction and mode of conduction of 
impulses in neurones and neurone-chains has been carried on by 
stimulation under normal and experimental conditions. Oppor- 
tunity is offered for the extension of this field in the direction of 
experimentation with chemical conditions combined with cyto- 



THE STUDY OF THE NERVOUS SYSTEM. 9 

logical Study to determine the character and mode of action of 
nerve protoplasm. 

Numerous operative experiments have been directed toward 
the study of the function of various parts of the brain and of other 
nervous organs by stimulation, extirpation, poisoning, etc. Ex- 
amples of such work are found in the numerous researches on 
the function of the cerebellum and upon the localization of fimc- 
tion in the cerebral cortex. Other studies take account of the 
modifications of normal action following upon the destruction 
of any conduction path or center anatomically known. Still others 
regard the modifications of responses to stimuli after the appli- 
cation of selective poisons such as curare and nicotin, which affect 
respectively the motor end plates and certain elements of the 
sympathetic system. 

The functions of sense organs have been studied by experiments 
to determine the conditions of their functioning, the character 
of stimuli proper to each kind of sense organ, the range of stimu- 
lation, etc. Examples are seen in recently renewed studies upon 
the ear and lateral line organs and upon the gustatory organs 
of fishes. 

The methods by observation may or may not be experimental 
in the sense that natural conditions are controlled or simplified 
by the observer without physical interference with the organism 
or its nervous mechanisms. This form of study is especially 
developed in the field of animal behavior and physiological 
psychology. 

The main object in the study of the ner\^ous system is to dis- 
cover what functional relationships are provided for by each of its 
parts. The words "functional rclationsliips" imply actions and 
organs which act. The nervous system is in relation both structur- 
ally and functionally with all parts of the organism. All actinties 
are directly or indirectly dependent upon the proper functioning 
of the nervous system. The study of the ncn^ous system impUes 
a knowledge of the whole structure and the whole life of the organ- 
ism. This breadth of view and attention to the functional side 
are two factors essential to the right attitude in the study of the 
nervous system which have been scarcely well enough appreciated. 



lO NERVOUS SYSTEM OF VERTEBRATES. 

Although the present book, as an introduction to the study of the 
nervous system, will deal chiefly >^ith structure it is hoped that 
the student will be led into full sympathy with the functional point 
of view in dealing with anatomical facts. 

One of the most serious questions before the student of the 
nervous system at present is, what names and terms will serve best 
for clear description. Although there is an unfortunate variance 
in the usage of different writers certain terms are in more or less 
common use and it is assumed that the reader is familiar enough 
with the anatomy of vertebrate animals to understand the meaning 
of these terms. For example, to indicate the two ends of a verte- 
brate animal the terms front and hind, anterior and posterior, 
cephalic or rostral and caudal ends are used, and for the relation 
of direction toward the head or toward the tail the convenient 
terms cephalad and caudad are frequently used. So the terms 
right and left, dextral and sinistral sides and dextrad and sinistrad 
express corresponding relations, while the term median means in 
the middle vertical plane which divides the body into right and 
left halves. The terms mesial and lateral, mesad and laterad 
are similar in meaning to those already mentioned. For the 
upper and lower surfaces of vertebrate animals the terms dorsal 
and ventral are regularly used and these together with the other 
terms mentioned will be appUed in the following pages to man in 
the same sense as to other vertebrates. 

Since most of the description of the nervous system is taken 
from microscopic sections it is necessary to understand the terms 
used to designate sections cut in different planes. Sections made 
at right angles to the longitudinal axis of the body are called trans- 
verse sections. In many cases, as in embryos with the head bent 
ventrad, sections transverse to the trunk will not be transverse 
in the head region. Unless otherwise stated transverse sections 
are understood to be at right angles to the longitudinal axis in the 
region under consideration. Sections taken at right angles to 
transverse sections and passing symmetrically through correspond- 
ing organs of the right and left side, such as the limbs, would 
be parallel with the horizontal plane in animals whose position 
is prone. In man such sections would be approximately parallel 



THE STUDY OF THE NERVOUS SYSTEM. II 

with the vertical face of the frontal bone. The terms horizontal 
and frontal may be used interchangeably to indicate such sections. 
Sections taken at right angles to both of these and parallel with 
the median vertical plane of the body are called sagittal sections, 
since in man such sections are parallel with the sagittal suture of 
the skull. 



12 NERVOUS SYSTEM OF VERTEBRATES. 



CHAPTER II. 
GENERAL MORPHOLOGY OF THE NERVOUS SYSTEM. 

In any vertebrate animal two chief parts of the nen-ons system 
are distinguished, the central system consisting of the brain and 
spinal cord and the peripheral system. The peripheral S)rstem 
includes the nerv^es which connect the central system with various 
parts of the body, the ganglia of those nerves, sense organs, and 
the sympathetic system. The central nervous system is situated 
dorsal to the alimentary canal and is surrounded by more or less 
strong skeletal structures which constitute the greater part of the 
skull and spinal column. 

In the trunk region the central ner\'ous system consists of a 
rounded cord which is enclosed within the neural arches of the 
vertebrae. In fishes this spinal cord extends the whole length of 
the spinal column. In higher forms the caudal portion is less 
developed, the cord as a whole grows less rapidly than the trunk 
and remains in the adult shorter than the spinal column, ending 
in the lumbar region. Beyond this a slender thread continuing 
into the tail represents the caudal portion of the spinal cord of 
fishes. In all vertebrates having well developed limbs the two 
regions of the spinal cord with which the nerxes of the limbs are 
connected are somewhat thicker than the rest of the cord. These 
thickened portions are known as the thoracic and lumbar enlarge- 
ments. The cord is usually more or less flattened dorso-ventrally 
and upon the dorsal and ventral surfaces in the middle line are to 
be seen longitudinal grooves, the dorsal and ventral fissures. The 
ventral fissure is usually a deep furrow. If the cord be cut across 
and the cut surface examined with a hand lens, the greater depth 
of the ventral fissure will be evident and in the median plane there 
will be seen a narrow opening. This is the central canal which 
extends throughout the length of the cord. The material surround- 
ing the central canal and filling up the inner portion of the cord 



GENERAL MORPHOLOGY OF THE NERVOUS SYSTEM. 13 

is very soft and somewhat gray in color, while the outer part is 
more firm and white. The gray matter is composed chiefly of 
cells, the white matter chiefly of fibers. The whiteness is due to 
the sheaths covering the fibers. The distinction between cellular 
and fibrous parts is not apparent in the cord of such an animal as 
Petromyzon whose nerve fibers are not provdded with such 
sheaths. The gray matter has in cross section somewhat the form 
of the letter H, the central canal being found in the crossbar of the 
H. In either half of the cord a part of the gray matter extends 
dorsal to the central canal and a part extends ventral to it. These 
parts are called respectively the dorsal and ventral horns of the 
gray matter. In mammals and man a smaller projection of the 
gray matter laterally is spoken of as the lateral horn. 

Each lateral half of the cord has connected with it the roots 
of the peripheral nerves. In all vertebrates there is in each 
typical segment of the body a dorsal root connected with the dorsal 
surface of the cord and a ventral root connected with the ventral 
surface. In the lowest vertebrates (Amphioxus and Cyclostomes) 
the dorsal and ventral roots, of the same side alternate with one 
another along the cord. This is due to the fact that the dorsal 
nerve is destined to go in larger part to the skin while the ventral 
nerves go to the muscles of the trunk. Since the muscles are 
arranged in simple transverse segments following one another, 
the dorsal nerve is placed in the interval between two segments 
where it passes in the intermuscular septum of connective tissue 
to the skin. The ventral nen-e, on the other hand, is situated 
opposite the middle of the muscle segment and is distributed 
directly to the muscle. Since part of the muscle lies above the spinal 
cord the ventral root divides into a dorsal and a ventral ramus. 
The dorsal root, after emerging from the spinal column becomes 
thickened by a collection of ganglion cells. This thickening is 
called the spinal ganglion. Beyond the ganglion the ner\'e divides 
into dorsal and ventral rami, which go to the skin. 

In true fishes and in all higher vertebrates there has come about 
a shifting of parts such that the dorsal and ventral roots of a 
given segment come to lie nearly in the same transverse plane of 
the body and the two roots unite at or just beyond the ganglion 



M 



NERVOUS SYSTEM OF VERTEBRATES. 



of the dorsal root (Fig. i, B). The dorsal rami of the two roots 
unite to form a common dorsal ramus and the ventral rami unite 
to form a conunon ventral ramus. From the ganglion or from the 
conunon ventral ramus just beyond the ganglion, there arises a 
branch which enters a ganglion of the sympathetic sj'stem, by way of 
which the \iscera are brought into connection with the central 
ner\'ous s\-stem. The branch to the sympathetic system is known 
as the ramus communkans. 

In the head region in all true vertebrates the central nervous 
sj-stem becomes considerably enlarged to form the brain. The 




Ku;. I. Outlines of the spinal cord with the dorsal and ventral nerve roots; A, 
in Pfin*myson: H. in a mammal, d. /., dorsal fissure; d. h., dorsal horn; d. «,, 
dorsiil nenv: /. h., lateral horn; v. /., ventral fissure; v. h., ventral horn; v. «., 
vrntRil nor\T. 

nor\-os connivtal Nv-ith the brain are called cranial nerves. There 
is no sharp limit between spinal cord and brain or between spinal 
and cranial nrrvcs. as then* is none between trunk and head. 
Tin* brain prrsrnls in ihc adult five constant and well marked 
lH)rlions whirh will srrvr as a ^uide in the description of the 
nervous systrm of \\\v hrad. These are kno^^^l as secondary 
si^KUU^nts of the brain and an* nanu*<l from behind forward: my- 
oleneephalon, inetencephalon. nuseneephalon, diencephalon and 
teleneephalon (Im^f.- ^^^ 

Tin* m\rlnhrftlhiloii, or mtulullit ohlon^iitUa, appears as a gradual 
enlargement e\tt*ndinj'. Icnwaitl fn)in the sj^nal ix>rd. As the 
spinal eohl tneim** into the biain the dorsal |H>rtions of the cord 
spn\\d apart atnl thr dopuil \'vi*<\\\v seems to widen out into a 



i6 



NERVOUS SYSTEM OF VERTEBRATES. 



as well as from without* certain ridges and grooves will be seen 
which indicate a division of the myelcncephalon into longitudinal 
2ones (Fig, 3). The ventral portion of the myelencephalon 
appears to be a continuation of the ventral part of the spinal cord 
wnth sUght modification. In the floor of the fourth ventricle is 

_^ Cerebellum 



NJ.LVIl 



Lobus Hncae 
lateralis 



Tubcrculum 
acusticum 



Fasc. long. med. 
Lobus visceral is 



IX 



Fig. 3.— The medulla oblongata and cerebellum of the lake sturgeon (Aciprt^str 
rubicundus), to show the lon^iudinal zones. A, dorsal view with the chr>roid 
plexus removed. B, C and D, sketches of sections at the Icveb indicated by the 
reference lines. The dark area with light circles is the continuation of the ventral 
hqrn of the cord. The dark area with reclangidar spaces is the continuation of 
the lateral horn. The area with oblique lines is the visceral scnsorj- column (lobus 
visccralis). The area with vertical lines is the somatic sensory column. 



to be seen a deep median groove bounded by two narrow but 
usually high ridges. The fiber bundles which make up these 
ridges bear important relations to the motor nerve roots. The 



GENERAL MORPHOLOGY OF THE NERVOUS SYSTEM. 



17 



ventral horns of the gray matter of the cord continue into the 
myelencephalon and are marked by two grooves which bound 
laterally the two ridges just mentioned. In the lateral zones 
there is much greater change from the. cord to the brain. The 
greater size of the myelencephalon is due chiefly to the greater 
volume of these lateral parts and to their bulging laterally. From 
the internal surface of the lateral wall there projects into the 
cavity a ridge constituted chiefly of a thickening of the gray 
matter. It extends from the caudal end of the myelencephalon 
to near the cephalic end, stopping abruptly opposite the seventh 
or facialis nerve. This ridge has been known as the lobus vagi 




Fig. 4. — Two views of the brain of the buffalo fish, Carpiodes velijer (Raf .) ; 
(i) from above, (2) from the right side. Twice the natural size. From C. Judson 
Herrick after C. L. Herrick. 

The vagal lobes (L. vg.) are very large and, with the overhanging cerebellum, 
completely conceal the facial lobe. In the upper figure the cerebellum appears as 
a nearly rectangular body in front of the vagal lobes and in front of this is the 
roof of the mesencephalon. The optic lobes are pushed wide apart by the enormous 
valvula cerebelli within and the shaded area in the figure represents a membranous 
portion of the roof connecting the optic lobes. In front of this is the basal ganglion 
of the forebrain, the membranous roof of which has been cut away. The olfactory 
bulbs are cut away. In the lower figure the large inferior lobes are seen below the 
optic lobes and behind the latter the cerebellum is produced ventrally as the superior 
secondary gustator>' nucleus. Immediately behind this and above the VIII nerve 
is the lobus lineae lateralis. The nerves of the trigemino-facial root complex are 
marked Fi. d, Fi. v, V2 and VI I j the auditory nerve VIII ^ and the glossopharyngcus 
and vagus IX and X. 



l8 NERVOUS SYSTEM OF VERTEBRATES. 

or the lobus fadalis, ovnng to the relation which it bears to the 
vagus and facialis nen-es. The ridge in cyclostome fishes is small, 
in selachians of moderate size, in ganoids (Fig. 3) relatively large 
and in some bony fishes enormous (Fig. 4, L. vg.). In some 
bony fishes, as in Carpiodes whose brain is shown in Figure 4, 
it is the caudal part of this ridge which is greatly enlarged. In 
other cases the cephalic portion is so large that it overtops all 
other parts of the medulla oblongata, and the ridges of the two 
sides may fuse in the median plane into a single mass, the lobus 
impar. As will be seen later (p. 1 59) this ridge is the place of ending 
of the sensory fibers of the visceral surfaces in the head and of 
the organs of the sense of taste. The ridge would therefore 
be appropriately named the lohus visceralis. In amphibians, 
reptiles, birds and mammals the part of the brain corresponding 
lo this lobe, although recognizable microscopically, is much 
smaller than in fishes and is not to be seen as a projecting ridge. 
The name visceral sensory column will be used for this whole 
structure. When it forms a projecting ridge the term lobus vis- 
ceralis may be used for the whole, or lobus vagi and lobus facialis 
for its two parts. 

The upper part of the lateral wall of the myelencephalon is 
formed by a thick ridge of gray matter which is continuous caudad 
with the dorsal horn of the spinal cord and cephalad with the 
cerebellum. This ridge is more or less prominent in all lower 
vertebrates (see Figs. 2, 3) and its equivalent in mammals is to 
be found in the acustic nuclei and the restiform bodies. 
The ridge has been known as the tuberculum acusticum on 
account of its relation to the eighth cranial or auditory nen^e. 
Since the ridge is the place of ending of all the cutaneous nerves 
of the head and since the term tuberculum acusticum is applied 
to a restricted portion of this region in mammals, it would be better 
to call this the samatk sensory column. In selachians and ganoids 
and to a less extent in cyclostome fishes a short part of this lobe 
projects prominently dorsad just behind the cerebellum. Since 
this is related wholly to ner\'es which supply the so-called lateral 
line organs, it is called the lobus lineae lateralis. 

Most of the cranial nenes are connected with the myelen- 



GENERAL MORPHOLOGY OF THE NERVOUS SYSTEM. 



19 



cephalon. In this region the dorsal and ventral nerves remain 
separate from one another throughout life. Of the dorsal nerves 
the most caudal which can be recognized in all vertebrates corre- 
sponds to the tenth cranial or vagus nerve of human anatomy. 
This arises by numerous small roots some of which are sensory, 
others motor. The motor are situated slightly ventral to the 
sensory. The roots all unite into one large trunk which descends 




Velu 



Nvil N.X ^ - branch. 1 



R. palat.VSI 

R, hyom 

R. pratnVll 



R. lingJX 
R. palat. iX 

B 



,R. IntesL 




R, pharyng. 
'R. posttr. 
R. preetr. 



Fig. 5. — Simple diagrams of the branchial nerves of lower vertebrates as seen 
from the left side. A, in a cyclostome; B, in a true fish. In B the trigeminus 
nerve is not shown. 



to the dorsal border of the second gill slit in fishes and there bears 
a ganglion (Fig. 5). From this ganglion a main trunk continues 
caudally and bears a ganglion over each gill slit. From each 
ganglion two rami arise. The first runs ventrally in front of the 
slit and is called the ramus praetrematicus. It gives the large 



20 NERVOUS SYSTEM OF VERTEBRATES. 

ramus pharyngeus to the mucosa of the roof of the pharynx, and 
then supplies the gill filaments and the mucosa of the lateral 
and ventral walls of the pharynx. The other is the ramus post- 
ireinaticus which runs ventrally behind the gill slit and supplies 
the muscles of the branchial arch, gill filaments, the mucosa and 
taste buds, and in cyclostomes the overlying skin. Beyond the 
last gill slit the vagus trunk is continued caudad as the ramus 
intestinalis. In many forms especially among bony fishes the 
position of the roots and ganglia of the vagus with reference to 
the gills becomes modified and is not so simple as indicated in 
the diagram. Figures 51, 63, 79 will show the arrangement in a 
cyclostome, a bony fish and an amphibian. 

The more caudal motor roots of the vagus series supply certain 
muscles connected with the shoulder girdle (trapezius muscula- 
ture). Owing to the disappearance of the gills in higher verte- 
brates and the consequent reduction of the more cephalic motor 
roots, these more caudal roots become more prominent higher in 
the scale of vertebrates. They have been set apart as an inde- 
pendent ner\^e under the name of the eleventh cranial or spinal 
accessory ner\'e. 

A short distance cephalad from the vagus appears the ninth 
cranial or glossopharyngeus nerve. It arises by a sensory and a 
motor root, bears a ganglion over the first gill slit, and gives rise 
to pharyngeal, pretrematic and posttrematic rami as in the case 
of each of the vagus ganglia. The phar}^ngeal ramus extends 
into the palate and is known as the ramus palatinus IX s and the 
posttrematic ramus is known as the ramus lingualis IX because 
it continues into the tongue. 

Above the glossophar}Tigeus, sometimes in front of and sometimes 
behind it, arises the nervus lincae lateralis. It is an independent 
sensory root which usually joins the trunk of the vagus and runs 
for some distance with it, then continues separately beneath the 
skin as the ncr\T of the special sense organs of the lateral line. 
This nerve enters the dorsal somatic sensor)^ lobe and cephalad 
from it two or three other roots enter the same lobe. The most 
caudal of these is the eighth cranial or auditory nerve. In front 
of this there are in cyclostomes, selachians and ganoids, two 



GENERAL MORPHOLOGY OF THE NERVOUS SYSTEM. 



21 



roots, the more dorsal connected with the lobus lineae lateralis 
and the more ventral connected with the tuberculum acusticum, 
which together supply the lateral line organs of the head. In bony 
fishes and aquatic amphibia only the more ventral root is present. 
In terrestrial vertebrates the roots which supply lateral line organs 
on both the trunk and head disappear because the sense organs 
are useful only in aquatic Ufe. Of this group of nerves only the 
auditory remains in higher vertebrates. The nerves which 
supply the lateral line organs of the head enter into close relations 



R. ophthal. superfic 



Supraorbital 
canal 




Hyomand 
canal 



Infraorbital canal 



Lateral line 
canal 



R. hyomandibularis 



Fig. 6. — A diagram of the lateral line canals and pit organs together with the 
nerves which supply them in a ganoid fish {Amia calva). After E. Phelps Allis. 
The canals are shaded with cross lines and the canal organs are shown as black 
<lLscs in the course of the canals. The pit organs are shown as rows of black dots. 
Only the i>eripheral nerve trunks are shown, the ganglia and roots being omitted. 



with the facialis or trigeminus ner\'e and their rami have been 
variously named as rami of the latter nerves. Three chief rami 
are formed, a supraorbital, an infraorbital and a mandibular, 
which will be more fuUv described in a later chapter (see Chap. 
VII). 

Ventrad or ventro-cephalad from the auditor^' root are the two 
roots, a sensory and a motor, which constitute the seventh cranial 
or facialis ntrvt. The two roots enter a ganglion from which 



22 NERVOUS SYSTEM OF VERTEBRATES. 

arise the ramus palalinus to the mucosa of the roof of the mouth 
and a ramus hyoideus which is joined for some distance with the 
mandibular ramus of the lateral line ner\'es in the common ramus 
hyomandibularis, and eventually supplies the hyoid muscles and 
the mucosa of the floor of the mouth. The ramus hyoideus is a 
posttrematic ramus, since it runs behind the spiracular cleft when 
that is present, and there is often a pretrematic ramus running 
in front of the cleft and arising in common with the ramus palatinus. 

The distribution of the organs of the sense of taste innervated 
by the X, IX and VII nerves is of great importance. In fish-like 
vertebrates the taste buds are found in the mouth and branchial 
ca\ities. They are also distributed more or less widely on the 
outside of the head and in extreme cases, as in some bony fishes, 
on the fins and over almost the entire body. They have usually 
no regular arrangement in rows and difi'er from the lateral line 
organs in that they usually project above the surface and are 
never depressed in pits or canals. In terrestrial forms the taste 
organs are confined to the mouth cavity. 

From the cephalic end of the somatic sensory column of the 
myelencephalon the fifth cranial or trigeminus nerve takes its 
origin by a more dorsal sensory and a more ventral motor root. 
The roots enter a ganglion which is partly divided into two portions, 
a dorso-cephalic and a ventro-caudal portion. From the dorso- 
cephalic ganglion a large ner\^e runs forward through the dorsal 
part of the orbit and suppUes the skin of the snout and dorsal 
surface of the head. This is the nervus ophthalmicus profundus 
and its ganglion may be knowTi as the profundus ganglion. The 
ventro-caudal ganglion is properly known as the trigeminal gang- 
lion. From it arise two large rami: the ramus maxillaris which 
supplies the skin beneath the eye and the lining of the front part 
of the roof of the mouth; and the ramus mandibularis which 
supplies the skin over the lower jaw and the muscles which move 
the lower jaw. These two rami are apparently comparable to 
the pretrematic and posttrematic rami of the branchial nen^es, 
the mouth taking the place of a branchial cleft. The nerve 
complex consisting of the ophthalmicus profundus and trigem- 
inus proper is fairly constant in its relations and size throughout 



GENERAL MORPHOLOGY OF THE NERVOUS SYSTEM. 



23 



the vertebrate series because of the constancy of the sensory area 
supplied by it, the skin of the anterior part of the head. 

Two ventral nerves are connected with the myelencephalon, 
the so-called hypoglossus and the abducens. The hypoglossus 
or twelfth cranial nerve arises by a variable number of roots in 
cephalo-caudal succession in about the region of junction of brain 
and spinal cord. These roots unite into a common trunk or 
plexus which supplies the muscles of the tongue. In lower ver- 
tebrates it is evident that the roots are the equivalent of several 
ventral segmental nerves and form a simple continuation forward 
of the series of ventral nerves of the tnmk. The number of such 
nerves present is greater in the more primitive forms. While 
in higher vertebrates an interval representing several segments 



Tectum mesencephali 

N M vn 

i;)|fiia<jry sac 

Olfactory bultj 



N lint^ JaterBlJs 




S0CCU& vascuktsus 



Fig. 7. — A sketch of the brain of Chimaera monstrosa from the left side to show 
especially the fX)sition of the nerve roots. The roots of the nerves were blackened 
by osmic acid and in this way one or more roots of the abducens and hypoglossus, 
not to be seen in ordinary dissections, were brought to view. The nerve roots are 
indicated by the usual Roman numerals. The hypK)glossal and spinal roots are 
numbered w, jc, y, 2, a, 6, c, 4, 5, 6, after Fiirbringer's scheme. 

intervenes between the roots of the hypoglossus and abducens, 
in such forms as Chimaera (Fig. 7), Heptanchus and related forms 
(Fig. 2), and in Petromyzon dorsatus (Fig. 51) only one or two 
ventral nerves are wanting in the series between the abducens 
and the ventral spinal ner\'es. In Bdellostoma the series of 
ventral nerves is quite complete except the eye-muscle nerves, 
w^hich are wanting. The abducens or sixth cranial nerve also 
arises by several rootlets which may extend from the level of the 
VII nerve root back nearly to the level of the IX nerve. The 



24 NERVOUS SYSTEM OF VERTEBRATES. 

nerve formed by these rootlets supplies the rectus extemus muscle 
of the eye-ball. 

The metencephalon is a short segment of the brain which ven- 
trally appears to be merely a continuation forward of the myelen- 
cephalon, but dorsally is sharply distinguished by the possession 
of a massive roof instead of a choroid plexus. This massive roof 
is the cerebellum. It is large in all true fishes and in some selachians 
it is the most prominent part of the whole brain. In bony fishes 
it projects inward also, encroaching upon the fourth ventricle 
and largely filling the ca\dty of the mesencephalon. In cyclo- 
stomes, dipnoans and amphibians the cerebellum is very small, but 
in reptiles, birds and mammals it becomes progressively larger 
and more important. Its size is evidently correlated with the 
activity of the animal and with the number and importance of the 
cutaneous sense organs. In all vertebrates the cerebellum consists 
fundamentally of an arch of gray matter covered externally by a 
fiber layer which forms a commissure dorsally. The two pillars 
of the arch are continuous with the somatic sensory column of 
the medulla oblongata. In mammals and man the ventral wall 
of the metencephalon is greatly thickened and forms a ventral 
protuberance known as the pons Varolii, 

That portion of the dorsal wall of the brain which qonnects 
the cerebellum with the mesencephalon is thin in most vertebrates 
and is known as the velum medullare anterius. This velum 
undergoes various modifications which are discussed in a later 
chapter (see p. 1 7 1). The fourth cranial or irochlearis nerve, which 
crosses with its fellow in the velum and emerges from the brain 
between the cerebellum and mesencephalon, is reckoned with the 
cerebellar segment. It supplies the superior oblique muscle of 
the eye-ball. 

The mesencephalon or midbrain is perhaps the most constant 
portion of the brain in vertebrates. Its ventral and lateral walls 
are always massive, its cavity a narrow canal, the aqueduct of 
Sylvius. Its dorsal wall is less thick and is divided by a longi- 
tudinal furrow into lateral portions which are kno^n as the optic 
lobesy because they serve as the place of ending of the fibers of the 
optic tract coming from the retina. In mammals the lateral 



GENERAL MORPHOLOGY OF THE NERVOUS SYSTEM. 



lobes become di\ided by transverse furro\\'s into anterior and 
posterior parts and the four bodies thus formed receive the name 
of corpora quadrigemina. The optic lobes van* in size in different 
classes of vertebrates, being noticeably larger in those animals 
in which the eyes are especially large and important (bony fishes, 
some selachians, birds, etc.). In mammals, however, the size of 
the eyes does not greatly affect the size of the coqjora quadri- 



Rec.neur. 



,W^^ 



Sw^^ 



p 



vent- 



-Aq- 



.F.of 



. jnf . 

\ F nf M' 



_ IV vent. 



vIV vent. 



Figs. 8» 9 and 10* — The outline of the brain and brain ventricles of several ver- 
tebrates a? seen from above. The relative size of the brains is ignored in the figures 
but the form of the brain and ventricles is accurately drawn from dissections or 
microscopic sections. 

Fig, 8. — A, the brain of a cyclostome fish, iMmpctra Wilder i. 

B, the brain of a selachian, Mustdus cams. The outline of the ventri- 
cle in the optic lobes and cerelx41tim is drawn in dotted tinrs. 

C, the brain of a young specimen of a bony fish, Coftgonus alhus. On 
the left side is shown in dotted line the form of the oplic ventriclej on the right side 
the outline and cavity of the inferior lobe of the diencephalun, 

D, the brain of a tailed amphibian, Nee turns maculatus. 

gemina (compare Chapter XVI). The cephalic border of the 
roof of the niid-brain is marked by the poskrwr cammissitre. 
The lateral and ventral walls of the mid-brain are in general 
comparable to the same portions of the medulla oblongata. From 
tie ventral surface of the mesencephalon arises the third cranial 
oculomotor nerve which supplies four of the eye muscles, rectus 
superior, rectus inferior, rectus inlernus and obliquus inferior. 
The diencephalon or inkrbrain^ although the smallest of the 



26 



NERVOUS SYSTEM OF VERTEBRATES. 



secoDdar)^ segments of the brain, is one of the most interesting 
and important and presents many points of morphological sig- 
nificance and many variations in different classes of vertebrates. 
WTien seen from the side (Figs. 2, 7), it appears as a wedge-shaped 
segment, the edge of the wedge being upward. The short dorsal 
wall is mostly membranous, but is thickened at one point by 
the so-called superior cammissure which connects the two small 
knob-like thickenings of the dorsal border of the lateral wall, 
the nu4:lei hahenidae. These two bodies which are constantly 



F.of M. 



Fig. 9* — A diagram of one side of the forcbrain of Musieltts cams to show what 
is believed to be the primitive relations of the wall and ventricle, 
FlG» :o.— The outline of the ventricles in man. 



present and of great significance in the vertebrate brain, are usually 
of unequal size on the right and left sides. Just behind the supe- 
rior commissure there arises from the dorsal surface of the dien* 
cephalon a small sac or tube which in cyclostomes, many fishes 
and reptiles extends through the cranium to end beneath the 
skin on the dorsal surface of the head. Tliis sac in the forms 
mentioned bears some resemblance in structure to an eye and in 
several cases is probably functional as a light-percipient organ. 



GENERAL MORPHOLOGY OF THE NERVOUS SYSTEM. 2^ 

It is hence called the pineal or parietal eye. It will be seen later 
(Chapter VIII) that there are m vertebrates two of these rudi- 
mentary eyes one behind the other. In amphibia, birds and 
mammals the parietal eye is quite rudimentary and in man is 
known as the pineal gland or body. The anterior border of the roof 
of the diencephalon is marked in lower vertebrates and m the 
embryos of all vertebrates by a deep transverse fold called the 
velum iransversutn (Fig. ii). The velum forms the cephalic wall 
of a larger or smaller median sac of the choroid roof of the dien- 
cephalon, which may be called simply the dorsal sac. In mammals 
a median sac occupying nearly the same topographical position 
bears the name of paraphysis. It is ner\'ous in character and its 
significance will be treated later (compare Chapter XVIII). 

The lateral walls of the diencephalon are known as the optic 
ihalami. They are thick and in lower vertebrates are traversed 
by the optic tracts on their way to the optic lobes, while in higher 
forms a large part of these tracts end in the optic thalami them- 
selves. The ventral wall of the diencephalon in lower vertebrates 
is expanded and is divided by a median ventral furrow into lateral 
halves, known as the inferior lobes. These lobes are relatively 
small in cyclostbme fishes and become progressively larger in 
selachians, ganoids and bony fishes. In vertebrates above the 
fishes the region corresponding to the inferior lobes is less expanded 
and in mammals it forms a funnel-shaped body with apex ventrad, 
which is called the infundibulum. (Compare Figs. 2, 7, 11.) 

An evagination of the caudal wall of the inferior lobes or infun- 
dibulum in all vertebrates forms a pair of bodies projecting some- 
what laterally and caudally which bear the name of mammillary 
bodies (corpora mammillaria. Figs. 2, 11). Between and ventral 
to these the floor is thin and is produced caudally and ventrally 
into a thin-walled sac which is supplied richly with blood spaces. 
It is hence called the saccus vasculosus. It is present in all verte- 
brates but is much larger in the true fishes than elsewhere. Con- 
nected with the ventro-cephalic surface of the saccus is a glandular 
body properly known as the hypophysis (see p. 66). The two 
together constitute the pituitary body. The so-called optic or 
second cranial nerve is connected with the ventral wall of the 



28 NERVOUS SYSTEM OF VERTEBRATES. 

diencephalon and marks the cephalic border of the inferior lobes. 
These are not true nenes but central brain tracts, and will here- 
after be called the optic tracts. In all vertebrates except bony 
fishes the two tracts as they enter the diencephalon form a deeus- 
sation which is more or less completely hidden in the wall of the 
brain. This is known as the optic chidsnia. In bony fishes the 
chiasma is carried out from the brain wall and in many cases 
the two tracts cross one another at a considerable distance from 
the brain wall, on their way to the eyes. 

The telencephalon or jorebrain in the different classes of verte- 
brates presents great differences in both size and structure. The 
forebrain of some primitive selachians (e.g. Heptanchus, Fig. 2) 
consists clearly of paired lateral lobes which are somewhat elong- 
ated. At its anterior end each lobe is produced forward and 
laterally as a slender cylinder which becomes enlarged as the 
olfactory bulb. This lies in contact with the inner surface of the 
olfactory sac. From the epithelium of the olfactory sac, nerve 
fibers pass through the wall and directly into the olfactory bulb. 
These fibers constitute the olfactory or first cranial nerve. The 
slender portion connecting the bulb with the forebrain proper is 
the olfactory tract, Dorsally a membranous roof (tela chorioidea) 
connects and covers the lateral lobes, enclosing the forebrain 
ventricle. The ventricle in its caudal part is common to both 
lobes and in its cephalic part divides in Y-shape and continues 
through the olfaclor}- tracts to the olfactorj' bulbs. The common 
cavity together with that of the diencephalon is usually known 
as the third ventricle of the brain. The lateral or olfactory por- 
tions are to be compared broadly with the lateral ventricles of 
the mammalian brain (see (Chapter X\T^I) and the point of their 
separation from the third ventricle is the foramen of Monro, 
It will apj)car later that the third ventricle extends a short distance 
forward beyond the opening into the literal ventricles and these 
openings are therefore true lateral structures. It is necessary, 
therefore, to speak of ])aired foramina of Monro. The caudal 
border of the roof of the forebrain is marked by the velum trans- 
versum. In front of this the roof is produced dorsally into a tube 
or sac uhich \'aries in size and is more or less complexly branched 



30 NERVOUS SYSTEM OF VERTEBRATES. 

or folded in different vertebrates. This is properly called the 
fiaraphysis and is not homologous with the structure of the same 
name in mammals mentioned above (Figs, ii, 36, 150, 152, 158). 
The ktero-ventral walls of the forebrain are thick and are loosely 
spoken of as the corpora striata. The thin portion connecting these 
in the mid-ventral line is known as the lamina terminalis and is 
thickened at one place by the fibers of the anterior commissure. 
In many other selachians (e.g. Squalus, Fig. 11, Scyllium, Raja) 
the forebrain is shorter and more compact and massive. The 
olfactory bulbs are larger and the olfactory tracts usually shorter. 
The lateral lobes are shorter, thicker, and more roimded. The 

V ^ — ^[^^js,^ Epiphysis Tectum mesencephaH Cefebdkim 



L. Inferior x-^^^^. 



Fig. la. A sketch of the brain of a cyclostome fish, Lampetra Wilderi, as seen 
from the left side. d. i, first dorsal spinal nerve; N. I. /., roots of the lateral line 
nerves; Vm., two motor roots of the trigeminus, the smaller of which innervates 
an cye-musrle; Tj., sensory r<M)t of the trigeminus. 

brain is wider between the olfactory tracts and the massive nervous 
structure extends up on the dorsal surface farther, so that the 
chonnd mof is shorter than in lleptanchus. So too, the median 
ventricle is shorter and the lateral ventricle is relatively more 
imix^rtant. (Compare Fi^s. 2, 11.) 

The eyelostonie forebrain (Ki^. 12), ah hough outwardly bearing 
a resemblanee to the massive selachian type, in reaUty owes its form 
to the pressure whit h is cxcrlrd upon it by the great buccal funnel 
in fnnit. The f«»rrbrain (»f ^^anoids and bony tishes resembles 
the elonj^'ilrd lyp<' of selat hian l)rain, hut is more slender and is 
simphM* hei aiise tlir olfai lory struclnres are less higlily developed. 
The tila (hoiioiiha i* more rxtcnsive and the lateral lobes are 
smaUer and moir jompail. *V\\r olfaelory tract varies in length. 
The hnnina teiminnUu v\ ncaily horizontal (.Figs. 148, 139). 



GENERAL MORPHOLOGY OF THE NERVOUS SYSTEM. 3 1 

The forebrain of dipnoans and amphibians differs from the 
compact type in selachians in two ways. First, the olfactory tract 
is absent as an external feature and the bulb is connected directly 
with the lateral lobe. Second, the lateral lobes are separated 
from one another in front (Compare Fig. 8, B, D, and Fig. 9) the 
median ventricle is still shorter and the lateral ventricles are 
relatively still more important. The lateral lobes have complete 
nervous walls and the tela chorioidea covers only the median 
ventricle. In reptiles, birds and mammals the lateral lobes are 
fundamentally of the amphibian type but become larger and greatly 
modified in connection with the development of the cerebral hem- 
ispheres. (See Chapter XVII.) 

In most vertebrates only one pair of ner\'es is connected with 
the forebrain. These are the olfactory nerves which were men- 
tioned above. The olfactory nerve always arises from the sense 
cells of the nasal epithelium, has no ganglion in its course and 
enters the olfactory bulb. In addition to the olfactory there is 
found in many selachian and in some ganoid and dipnoan fishes 
another pair of nerves connected with the forebrain, whose presence 
is of the greatest importance in the study of the morphology of 
the nervous system. This new nerve arises from the upper 
or lower surface of the forebrain near the median line, bears 
a ganglion which is often lodged in the angle between the 
olfactory tract and the forebrain, passes forward along the olfac- 
tory tract and over the olfactory bulb, divides into branches and 
is distributed to the epithelium of the nasal sac (Fig. 2). This 
nerve differs from the olfactory nerve in that its fibers are mye- 
linated, that it has a ganglion and that it enters the forebrain 
proper and not the olfactory bulb. Because of its attachment to the 
front end of the brain, this ner\^e has been called the nervus term- 
inalis. 

The figures in this chapter are intended rather to illustrate 
the typical form of the vertebrate brain and to bring out features 
of special morphological significance than to show the varying 
forms of the brain in different classes. For this the reader should 
refer to the figures in the larger text-books of zoology and com- 
parative anatomy. 



32 NERVOUS SYSTEM OF VERTEBRATES. 

DEMONSTRATION OR LABORATORY WORK. 

1. Dissect the nen-ous system of the dogfish or skate, the frog, and 
of a mammal (rabbit, cat, dog or man). 

2. Compare the brains of representatives of all classes of verte- 
brates (Petromyzon; Squalus acanthias or Raja; Ameiurus, Catostomus 
or other bony fish; frog; lizard or turtle; fowl; mammal). Especial 
attention should be given to the relative size of the different parts and 
the correlation of these with the activities (habits) of the animals and 
the number and importance of the sense organs. The brains should 
be dissected with care in such ways as to expose the ventricles and all 
of the points mentioned in the text. 

LITERATURE. 

AUis, E. P., Jr.: The Cranial Muscles and Cranial and First Spinal Nerves 
of Amia calva. Jour, of Morphol, Vol. 2, 1889. 

Dejerine, J.: Anatomic des centres nerveux Paris, 1895. 

Edinger, L. : Vorlesungen ueber den Bau der nervosen Zentralorgane des 
Menschen und der Thiere yte Aufl. Leipzig, 1904. 

Fischer, J. G.: Anatomische Abhandlungen Uber die Perennibranchiaten 
und Derotremen. Hamburg, 1864. 

Fiirbringer, Max.: Ueber die spino-occipitalen Nerven der Selachier und 
Holocephalcn und ihre vergleichende Morphologic. Gcgcnbaur's Festschrift, 
Bd. 3, Leipzig, 1896. 

Gaupp, E.: Anatomic des Frosches. Braunschweig, 1897. 

Gaupp, E. . Zirbel, Parictalorgan und Paraphysis. Mcrkel u. Bonnet's 
Ergebnisse, 1897. 

Gcgcnbaur, C: Grundriss der vergleichende Anatomic. 

vanGchuchtcn, A. : Anatomic du systcmc nerveux derhommc. Louvain, 
1897. 

His, W.: Zur allgcmeinen Morphologic des Gchims. Arch. f. Anat. u. 
Physiol., Anat. Abth., 1892. 

Mcrkel, Fr.: Ueber die Endigungen dcr scnsiblcn Nerven in der Haut der 
Wirbclthicrc. Rostock, 1880. 

Museum of the Royal College of Surgeons of England. Catalogue of 
Physiological Series. Vol. H. (Description of brains chiefly by G. Elliot 
Smith.) 

Quain: Text -book of Human Anatomy. 

Rabl-Ruckhard, H.: Zur Deutung und Entwickclung des Gehims der 
Knochenfisch. Arch. f. Anat. u.*Physiol., Anat. Abtheil., 1882. 

Retzius, G.: Das Menschenhim. 

Spalteholtz-Barker: Atlas of Human Anatomy. 1905. 

Stannius, H.: Das peripherische Ner\Tnsystem der Fische, anatomisch 
und physiologisch untersucht. Rostock, 1849. 



GENERAL MORPHOLOGY OF THE NERVOUS SYSTEM. 33 

Stannius, H.: Handbuch der Anatomie der Wirbelthiere. 2te. Aufl. 
Berlin, 1854. 

Wiedersheim, Robert: Vergleichende Anatomie der Wirbelthiere. 6te. 
Aufl. Leipzig, iqo6. 

Ziehen und Zander: Nervensystem. In Handbuch der Anatomie des 
Menschen. Herausg. von Bardeleben, 1 899-1903. 



34 NERVOUS SYSTEM OF VERTEBRATES. 



CHAPTER III. 
DEVELOPMENT OF THE NERVOUS SYSTEM. 

It is hoped in this chapter to bring to the attention of the student 
those features and processes in the development of the nervous 
system which are of the greatest morphological importance^ and 
especially to bring forward certain facts which have not received full 
treatment in text-books of embryology. There will not be space 
for such a full and orderly treatment of the development as an 
elementary student in embryology would require. For this reason 
it will be assumed that the student is familiar with the more 
general facts in the development of some vertebrate such as the 
frog or chick. 

It is commonly believed that all nervous functions are performed 
by structures derived from the ectoderm. In the early vertebrate 
embryo a certain area on the dorsal surface is to be recognized 
as the anlage of the greater part of the nervous system. This 
area is known as the neural plate. It becomes visible first as an 
area boimded on either side by slightly elevated ridges or folds, 
the neural jolds. These folds are continuous in front and behind 
and enclose an area which is broader in front, in the region corre- 
sponding to the future brain (Fig. i8 A). This somewhat banjo- 
shaped neural plate and the folds which bound it give rise to the 
brain and spinal cord, the greater part of the sensory nerves with 
their ganglia, the motor ner\'es and the retina. The olfactory 
epithelium, the sensory epithelium of the ear and lateral line 
organs, and the ganglia and nen^es of these organs are derived 
from parts of the ectoderm closely related to the neural plate. 
The organs of taste appear first in the embryo in the entodermal 
lining of the branchial cavities. In fishes they appear later in 
the outer skin also and their histor)- requires further investigation. 

In all vertebrates the neural plate becomes converted into a 
tube lying below the surface of the body. In all except cyclostomes 



DEVELOPMENT OF THE NERVOUS SYSTEM* 



35 



and bony fishes this is accomplished by the neural plate rolling 
or folding up so that the neural folds meet above. The folds begin 
to fuse together at a point which later falls in the region of the 
mesencephalon or farther caudally, and the fusion continues 
fon\''ard and backward until a complete tube, the neural iube^ is 
formed. The tube remains open for some time at the anterior 
end; the opening is called the neurapore. At the caudal end the 
tube remains for some time in connection with the archenteron 
by way of the neurenteric canal. The extent and relations of the 
neuropore will be important in later connections; the neurenteric 
canal need not claim further attention. In the mean time the 
general ectoderm fuses over the dorsal surface of the neural tube 



F1G.313.— Sectioos of the neural plate and folds in amphibia. A, the cephalic 
portion of the neurd plate in Ambly stoma tigrinum. The neural foldj arc more 
OArkiy shaded. It U evident that they are actual folds of the ectoderm. B, one 
side of the neural plate of Amhlystoma punctatum at a slightly more advanced 
stage of development andjat a higher magnification* 

and when the neuropore closes the tube lies wholly beneath the 
surface of the body. In cyclostomes and bony fishes the same 
results are reached by the neural plate thickening and sinking 
do^na as a solid cord of cells which comes to he beneath the ecto- 
derm- Within the cord of cells appear clefts which unite into a 
continuous canal. 

Roughly speaking the neural tube goes to form the spinal cord, 
brain and motor nerves. During its development, the material 



3^ 



NERVOUS SYSTEM OF VERTEBRATES. 



from which the sensorj' nerves are derived is taking on definite 
form in dose relation with the neural tube. The accompanying 
figures illustrate the development of the neural tube and the 
ganglia of the senson* nenes in amphibia. As seen in Figure 13 



'♦♦.^ 



Fig. 14. — Tninsverse section of ihc neural tube of A mbly stoma ^puncta turn jusi 
alter closing. The cells which will form the neural crest arc morc^darkJy shaded. 
Mitotic figures are seen at all levels in the wall of the tube. 

B, the neural plate consists of columnar ceUs, while the neural 
folds which bound it consist of two or more layers of somewhat 
cubical cells irregularly arranged. As the edges of the neural 
plate close up to form the neural tube (Fig. 14), the neural folds 



^. 



'.^^: 



Fig. 15, — Transvcr n through neural tube* neural crest and ectoderm of 

AmMyshma ^unciaium at a later stage than that shown in Fig- 14, 

form a sort of bridge connecting the tube with the ectoderm. Now 
the cells which originally formed the folds separate from the 
ectoderm and extend laterally on the surface of the neural tube, 
between it and the ectoderm (Fig. 15). Some of these cells remain | 




DE\^LOPMEKT OF THE NERVOUS SYSTEM, 



37 



connected with, and some actually enclosed ^Nithin, the dorsal 
wall of the tube adjacent to the seam of closure. There results 
a pair of continuous flaps or ridges of cells connected with the 
neural tube at its mid-dorsal line and spreading laterally between 
the tube and the ectoderm (Fig. 15). These are known as the 
neural ridges or crests and contain the cells which enter into the 
formation of the spinal ganglia* This condition of the gang- 
lionic material may be taken as t)pical for the embrj'os of verte- 
brates. In fishes a re!ati\ely large number of neural crest cells 
remain \\'ithin the neural tube and there give rise to sensor)^ fibers 



m'' 



Fig. 16, — Same as Fig. 15, later stas;*-. Tht' rrlls of the neural cresl have mi- 
atcd ventriiUy to form a spinal ganglion. Processes arc seen upon ncuroblasls 
irithin the tube and upon one sptinal ganglion cell. 

which run to the skin. These are the so-called giant ganglion 
cells of the spinal cord of fishes (Fig. 17). These facts suggest 
that the cells which give rise to sensory nen'e fibers originally 
lay within the neural tube and have migrated to fonn the spinal 
ganglia. Already in Amphioxus, however, a large part of the 
pinal ganglion cells are situated in the roots of the ner\'es outside 
"of the spinal cord. In the brain region the neural crest is fonned 
in essentially the same manner as in the trunk, but two ver\^ note- 
worthy facts are to be ]>ointed out. The first is that in the region 
opposite the ear the neural crest is entirely absent for a short dis- 



38 NERVOUS SYSTEM OF VERTEBRATES. 

tance. The second is that m front of the mesencephalon the crest 
is either very small or so modified that it can not be readily com- 
pared with that of the trunk. 

While these changes are taking place two special sense organs 
are making their appearance, the olfactory and auditory organs. 
Both are formed from parts of the ectoderm inmiediately adjacent 
to the neural plate. The olfactory organ appears as a pair of thick- 
ened patches of ectoderm at the cephalic border of the neural plate 
at either side of the neuropore. These patches eventually become 
depressed and form deep olfactory pits. The auditory organs 
arise on the dorso-lateral surface of the head opposite the region 
of the future myelencephalon. These are also at first thickened 
patches which sink in and form deep pits, which eventually separate 




r 1 T n r^"' ^"^ ^ 



Fig. 17. — A part of the spinal cord of an 18-day Ca toj/(>fiif^ embryo showing 
two giant ganglion cells. Golgi method. 

from the ectoderm as closed sacs and may press against the neural 
tube on either side. These auditory sacs exercise a great influence 
on the form and course of development of other organs. 

If now the neural tube be examined either in frontal sections 
or in embryos so dissected as to lay the tube bare, it will be seen 
that the tube is divided by slight transverse constrictions into 
successive segments approximately equal in length. These seg- 
ments are called neuromeres. The neural crest is divided into 
corresponding segments and, as shown in Fig. 18, the segmentation 
of the crest is present in the neural plate stage in selachian embryos. 
While these neuromeres have been clearly seen and described 
in the region of the hindbrain and spinal cord in all classes of 
vertebrates, in the anterior portions of the brain they are of shorter 
duration and are more difficult to study. In selachians, bony 



DEVELOPMENT OF THE NERVOUS SYSTEM. 



39 



fishes and the chick, however, they have been studied and described 
and some of the important stages in their history are shown in the 
accompanying series of figures. (Figs. i8, 19.) 

In the tnmk region where the number of neuromeres corre- 
sponds to the number of muscle segments and the neuromeres 
alternate in positon with the muscle segments, the embryonic 
condition is essentially continued into adult life. In the head 




Fig. 18. — Four stages in the development of the selachian brain to show the 
history of the neuromeres. After Locy. In A, B and C the whole head is shown, 
in D the brain alone. The Arabic numerals indicate the neuromeres. 

region, where great changes take place during embr}'onic develop- 
ment, the neuromeres share in these changes and in the adult 
brain the embryonic neuromeres are lost from new. The causes 
for this are manifold, but perhaps the chief ones are the bending 
downward of the head, the crowding of the brain produced by its 
growing more rapidly than surrounding organs, and above all 



40 



NERVOUS SYSTEM OF VERTEBRATES. 



the rapid growth of those parts of the brain which are destined to 
become large and important in the adult. One result of these 
forces is the appearance early in embryonic Ufe of certain bends 

5 




Fig. 19. — The history of the neuromercs in the bony fish and the chick. A, B, C, 
three stages in the development of the bony fish, as seen from the right side. D, E, 
the brain of the chick. After Hill. The letters c. e. g. mark corresponding fur- 
Tovrs in the chick brain; ep. i, ep. 2, anterior and posterior epiphyses; inf. inferior 
lobe; p. com., posterior commissure. The neuromeres are indicated by Arabic 
numerals. 

or flexures. These are illustrated in the figures which show the 
neuromeres (Figs. 18, ig) and also in Figure 20. In the neck 
region, owing to the bending of the head, the brain is bent down- 




Fig. 20. — Reconstruction of thf brain and cerebral nerves in a la mm. pig 
abno, (From Minol's *' Laboraiory Text-Book of Embryolog)*," Drawn and 
irised by F* T. Lewis,) 

Nerves* 3, oculomotor- 4, trochlear* 5» trigemmal, with its semilunar ganglion, 
W, and iKrce brantbes, — oph., ophthalmic; mx., maxillar>'; md.^ mandibular. The 
motor portion, w*hich* goes with the mandibular branch, is concealed in this view 
hy the sensory portion. 6. abducens. 7, geniculate ganglion of the intermedins. 
Fibers from this ganglion mix with the motor fibers of the facial ner\'e, so that both 
nsor)' and motor fibers are found in the three branches, — /, s. p,, large superficial 
itrosaJ; ck. /y., chorda tympani; /a. » facial, 8, acoustic, showing an upper vestib* 



4» 



NERVOUS SYSTEM OF \'ERTEBRATES. 



ular portion^ and a lower cochlear portion. 9, glossophar)'iigeal \dlh its supmcxr 
ganglion, J, above; its petrosal ganglion, p, below; and its three branches,— Ij^., 
tympanic; /.r., Lingual ramus; ^/^. r.^ pharyngeal ramus. 10, vagus, with its jugular 
ganglion, /\ extending posteriorly as a ganglionic commissure, com.; and below, its 
ganglion nodosum, n. Its branches form the laryngeal plejfus^ beyond which is 
the recurrent nerve, rec. Just below the jugular ganglion is the auricular branch 
of the vagus. lU accessor)', which joins the vagus; ex., its ramijs extemus. 
12, hypoglossal. F.^ Froriep*s hypoglossal ganglion. C l» C 2, C 3, cervical 
nerves. 

Brain and Sense Organs, Tdiff?., telencephalon. Z? i>n> » diencephalon. Mestn,^ 
mesencephalon. Meten., metencephalon. Myden, ^ myelencephalon. H., hem- 
isphere. Vcn. IV., roof of the fourth ventricle. Op„ optic cup. L, lens. Ni., nasai 
pit. Ot, ototyst. 

ward in relation to the spinal cord. Farther forward a sharper 
flexure in the opposite direction is found, called the pantial flexwre, 
because the pons of higher vertebrates appears at this point. 
Then the front end of the brain is curved down again in such a 
way as to bring the ventral surface of the brain just behind the 

Eye 



^Eye 



B 



Eye 



wv 



Fig. 21. — Transverse sections through the region of the opiic vesicles in selach* 
fan, avian and mammalian embrj'os : A, Torpedo occllata : B, Gallus domesticus ; 
C, Cavia cobaya. After Froriep. These and the following %ure show that ihe 
optic vesicles arise from the boraers of the neural plate. 

closed neuropore nearly into contact with the pontial flexure* 
The last bend, from its position in the parietal region of the head, 
is called the parietal flexure. It corresponds in position to the 
future mesencephalon. These flexures are in themselves of no 



DEVELOPMENT OF THE NERVOUS SYSTEM. 43 

fundamental importance, except as the adult form of certain parts 
of the brain and the position of some of its nuclei are doubtless 
determined by the influence of the flexures in these early stages. 
As the head of the embr)^o straightens out and as the growth of 
the other organs relaxes the pressure upon the brain, the flexures 
tend to disappear and they are to a large extent obliterated in the 
brains of lower vertebrates (compare Figs. 2, 11, 12, 158). 






:-•> i5 



B 

Fig, 22. — The development of the optic vesicles in AmUysloma punciatum^ 
Fig. 22 Ag should be compared mth a similar figure by Eyclcshymcr. In B the 
germinal cells in process of division are shown by small black figures. 

In the meantime the rapid growth of certain parts of the brain 
while the flexures are still present, results in deepening the con- 
strictions between certain of the neuromeres and the obliteration of 
others, and produces the prominent secondary segments which have 
been described in the previous chapter. Only a brief summary 
of these changes can be given here, since a full description would 



44 NERVOUS SYSTEM OF VERTEBRATES. 

require much more space than can be devoted to it. The forebrain 
is formed by the relatively enormous growth of the first neuro- 
mere, especially of its dorsal part. The optic vesicle, which will 
be treated more fully later (Chapter VIII), is formed from the 
dorsal part of the second neuromere. In the ventral portion of 
the second neuromere there is a great expansion which results 
in the formation of the inferior lobes and mammillary bodies. The 
floor of the second neuromere is depressed just behind the optic 
stalk and expanded backward beneath the third neuromere. The 
extreme ventro-caudal portion of the inferior lobes thus formed 
becomes thin-walled and is supplied with a great profusion of 
blood spaces. This is the saccus vasculosus which becomes 
closely related with the hypophysis in the pituitary body. The 
slight constrictions between neuromeres i and ii, and ii and ill 
quite disappear, while that between neuromeres ill and iv remains 
and is deepened. The formation of the parietal flexure is largely 
due to the great growth of the dorsal part of neuromeres iv and 
V, where the optic lobes are to be formed. The whole of neuro- 
meres iv and v fuses into the mesencephalon and the constriction 
between neuromeres v and vi is deepened more than any other. 
This marks the boundar}' between the midbrain and cerebellum and 
is known in the adult as the isthmus, Neuromere vi forms the 
metencephalon and the dorsal part of it becomes greatly enlarge 
in many vertebrates as the cerebellum. The neuromeres from 
number vii caudally enter into the medulla oblongata and spinal 
cord, the exact number belonging to the medulla being uncertain, 
since no definite limit can be fixed between brain and spinal cord. 
It is possible that this limit varies in different vertebrates. As the 
result of these changes, the brain loses its primitive segementation 
and comes to be divided into five secondary' segments of imequal 
size. 

The optic vesicle has been mentioned as formed from the dorsal 
part of the second neuromere, the first of the two which constitute 
the dienccphalon. At a \ery early stage while the nervous system 
is still in the form of a flat neural plate, the area which is destined 
to enter into the retina may sometimes be recognized in the broad 
cephalic part. The two retinal areas lie near the lateral borders 



DEVELOPMENT OF THE NERVOUS SYSTEM. 4$ 

of the neural plate. When the neural tube is formed these areas 
at once bulge laterally. They seem upon casual observation, 
for example in the chick, to involve nearly the whole lateral wall 
of the first two or three neuromeres. Careful examination shows 
clearly, however, that in fishes, amphibia, birds and mammals, 
these pouches belong strictly to the dorsal part of the brain, and 
they probably belong to a single neuromere, the second. The 
pouches grow out farther, retaining a portion of the brain ventricle, 
and finally come to be constricted next to the brain so that they 
remain connected with the brain wall only by a narrow stalk. 
The pouch is now known as the optic vesicle and the stalk as the 
optic stalk. Figures 1 8 to 20 show the optic vesicle in various 
stages of growth and Figures 21 and 22 represent it in transverse 
section. As the vesicle is formed it is crowded between the brain 
and ectoderm so that, it is flattened. The flattening continues 
until the cavity becomes a narrow cleft and the vesicle becomes 
somewhat saucer shaped. The constriction from the brain takes 
place from above downward so that the stalk comes to be attached 
near the ventral border of the vesicle. As a result of the same 
process the attachment of the stalk to the brain has shifted ventrad 
to reach the position afterward occupied by the optic chiasma. 
In the meantime the two walls of the optic vesicle begin to be 
differentiated, the outer wall growing thicker to form the retina 
and the inner wall remaining thin. Finally the portion of the 
brain ventricle which was carried out into the optic vesicle is 
obliterated but in many lower vertebrates a vestige of the ca\ity 
is retained in the stalk. This lumen connects with the preoptic 
recess^ a depression of the floor of the third ventricle in front of 
the optic chiasma. 

Histogenesis. — ^The development of the microscopic structure, 
the histogenesis^ of the brain, nerves and sense organs is going 
on at the same time with the development of the form of the brain. 
The neural tube consists at first of two incomplete layers of cells 
irregularly arranged (Figs. 13, 14), among which are some columnar 
cells which extend through the whole thickness of the wall. A 
little later the columnar cells become radially arranged around the 
lumen of the tube (Figs. 14, 15), and the shorter cells lie between 



46 



NERVOUS SYSTEM OF VERTEBRATES- 



them bordering upon the lumen or upon the extemaJ surface. 
The columnar cells retain their position throughout life, extending 
from the central canal to the surface. Since they give rise to a 
sort of meshwork or treUis which serves largely to support the 
nervous elements, these cells are called spongioblasts. The inner 
ends of neighboring spongioblasts fuse to form an internal limiting 
membrane bounding the central canal and the outer ends may 
form a similar external limiting membrane. The nuclei of the 
spongioblasts are situated in the middle and internal portions 
of the wall, the outer portion remaining relatively free from nuclei. 
Those spongioblasts whose nuclei occupy the internal ends of 
the cells become the ependyma cells of the adult brain. 

Among the sjx>ngioblasts are cubical or rounded cells which are 
multiplying rapidly, so that many of them contain mitotic figures. 
They are known as germinal cells. While a few of these cells lie 
in the thickness of the wall or even upon the external surface, 
the great majority border on the central canal (Figs. 13 B, 14, 15, 
16). The germinal cells are belie\ed to give rise to all the ner\'ous 
elements in the brain and spinal cord and to the stellate cells 
which with the spongioblasts make up the neuroglia. In general 
the histor)^ of both nen^e cells and gha cells is the same. The 
cells produced by the di\ision of the germinal cells migrate or 
are pushed toward the periphery of the cord or brain and then, 
supported by the meshwork of the spongioblasts, undergo develop* 
ment into specific nen^e or glia ceUs. 

Those cells which are desdned to form nen^e cells are called 
neuroblasts. These continue to multiply rapidly by mitosis but 
the number of mitodc figures seen in the ner\^ous system becomes 
rapidly less in later stages of development and in the adult the 
division of nerve cells is comparatively rare. The first step in 
the formation of a functional ner\*e cell from a neuroblast is the 
production of processes somewhat similar to the pseudopodia 
of an amoeba. First the neuroblast becomes drawn out at one 
end into a slender filament which is recognized as a nerve fiber 
(Fig. 16). The presence of this cell fiber is the only mark by which 
the neuroblast can be distinguished with certainty from spongi- 
oblasts or glia cells. The nerve fibers may collect into bundles 



DEVELOPMENT OF THE NERVOUS SYSTEM. 47 

as they are formed and in general the fibers run to the outer 
surface of the cord or brain, forming a peripheral fiber layer. 
Some time after the formation of the nerve fiber the neuroblast 
gives off other processes which branch more or less richly in the 
substance of the brain wall. The nerve fiber first formed remains 
always a relatively slender fiber of nearly uniform diameter. 
It is called the neurite. The processes later formed become more 
or less bush-like and are known as dendrites. 

In most parts of the central nervous system the greater number 
of nerve cells remain near the central canal and there form the 
central gray matter. But many cells migrate toward the surface and 
in some parts of the brain very rich layers of nerve cells are foimd 




Tr lobo4)ulbaris 



Fig. 23. — A part of a transverse section of the inferior lobe of the sturgeon, 
stained by the Golgi method. The cells all retain their primitive position adjacent 
10 the cavity. 

at the outer surface. In fishes it is observed that in any given 
part of the brain those cells which are farthest removed from the 
central canal have imdergone the greatest modifications of form 
as compared with the simple structure of the neuroblasts. 
The dendrites may become more richly branched, more nu- 
merous and more widely extended; they may be disposed par- 
allel with the surface or otherwise modified to increase their 



48 



NERVOUS SYSTEM OF VERTEBRATES. 



functional cfficiencj'. It is worthy of notice that in the cen- 
tral gray matter in lower vertebrates a large part of the cells 
have central processes which reach, and perhaps take part in, 
the internal limiting membrane. These cells seem to show that 
the bodies of cells formed from neuroblasts migrate toward 
the periphery, but their inner ends remain connected with the 
internal limiting membrane and become drawn out into long 
central processes. Other cells situated at the extreme periphery 
in the fish brain are probably derived from peripheral cells of 




Fig. 24. — A horizontal section of the nucleus praeopticus of the sturgeon. Golgi 
method. The cells have migrated toward the periphery but retain their connec- 
tion with the internal limiting membrane by means of central processes. 

the neural tube such as are seen in Figures 14 and 15. Figures 
23, 24, and 25, taken from the brain of the sturgeon, illustrate 
these relations. 

While the histogenesis is in progress certain changes of form 
in the neural tube are taking place. The most important of these 
is the appearance of longitudinal zones. In the region of the spinal 
cord it is noticeable that the walls of the neural tube do not increase 
in thickness equally. The lateral walls thicken, while along the 
mid-dorsal and mid-ventral lines the tube remains thin. These 
thin bands have been named by His roof-plaic and floor-fdate^ 



DEVELOPMENT OF THE NERVOUS SYSTEM. 



49 



respectively. They remain relatively thin throughout life. As 
the spinal cord grows it expands laterally and ventrally at the sides 
of the floor-plate, so that the floor-plate comes to form the bottom 
of a deep groove which was mentioned in the last chapter as the 
ventral fissure. At the mid-dorsal line such an open fissure does 
not appear but it is represented by a septum of connective tissue. 
Two other zones which probably have much greater significance 
are marked by a groove which appears on the inner face of each 
lateral half of the cord. This pair of grooves causes a widening 
of the central canal and a thinning of the wall of the tube, so that 
each half of the cord is divided into a dorso-lateral and a ventro- 
lateral zone or column. These zones were first brought to notice 
by His, who described them in the human embryo. There is 



r. opticus 




Fig. 25. — A part of a transverse section of the optic lobe of the sturgeon. The 
cells farther removed from the ventricle are more highly developed. 



reason to suppose that the dorsolateral zone includes the two 
sensory columns to be described in later chapters and that the 
ventro-lateral zone includes the two motor columns. 

Obser\'ations on these zones in embryos of other vertebrates 
are much to be desired. 

The development or the peripheral ner\'es, — ^The develop- 
ment of the sensory nerves is a very complex matter, especially 
in the case of the cranial nerves, and an account of its main out- 
lines only can be given. It was said above that the neural crest 
4 



50 NERVOUS SYSTEM OF VERTEBRATES, 

shared in the segmentation of the neural tube. The cells of the 
crest multiply by mitosis .and in the trunk region each segment 
moves latero-vcntrad at the sides of the spinal cord, between it 
and the adjacent ectoderm and mesoderm. At the same time 
the segments separate more and more from one another mitil 
there are formed a series of flaps attached to the dorso-lateral 
surface of the cord and hanging down on its lateral surface opposite 
the intenal between each two mesodermic somites. Each of these 
segments of the neural crest may now be spoken of as the anlage 
of a spinal ganglion. Each anlage moves further ventrad and 
may wholly lose its connection us-ith the cord (see Figs. 15, 16). 
The cells are now elongated, spindle-shaped and it is soon noticed 
that each cell elongates further as if each end were being drawn 
out into a slender thread. In fact a process analogpus to a pseu- 
dopxxiium grows out from each end of the cell. The process 
from the upper end of the cell, or central process, appears first 
and is from the first distinctly more slender than the opposite, 
or pheripheral process. The central process is therefore to be 
considered as the neurite, the peripheral process as the dendrite. 
Figure 26 is from part of a transverse section of an embryo of 
AniNystoma punctatum at the level of the glosso-pharyngeus 
ganglion. The central processes of the ganglion cells are growing 
upward, the peripheral processes are not yet formed. Figure 
27 shows a few ganglion cells of the trigeminus ner\'e. The per- 
ipheral processes of the cells are seen but the central processes 
bend backward so that they do not fall in the same section. 

The central processes soon reach the dorsolateral surface of 
the spinal cord and grow into the cord where they form a bimdle 
of afferent or sensor}- ner\e fibers. The peripheral fiber grows 
also and with its fellows forms a sensor\- nene or the sensory 
ponion of a peripheral nene trunk. The peripheral process 
sometimes divides, its branches entering two nen-e rami. This 
is analogous to the branching of the dendrites of central nen-e 
cells. The fibers continue their growth, following the path of 
least resistance between the muscles and other organs, until they 
R^ich the area of their ultimate distribution, for example in the 
skin, where ihev divide into terminal branches. The cells 



DEVELOPMENT OF THE NERVOUS SYSTEM. 



SI 



from which the fibers have grown out remain in a mass at the 
side of the spinal cord and form the spinal gangUon, 

It is ver\' important to notice that not all the cells of the neural 
crest give rise to nerve fiberSj but as in the central nen'ous system, 
a part of the cells become supporting elements and a part go to 
form the sheaths of the nerve fibers. It has recently been experi- 
mentally proved that cells derived from the neural crest migrate 
out along the bundles of the peripheral processes and form the 



^ 



Fig. 26.— 1 he ganglion of the IX nerve in Amblystama pMncUxium at the lime 
of formation of the central processes* 

Fic. 27. — A few cells of the tngeminal ganglion in AmUy stoma pundaium with 
the fibers of the ramus mandibulans growing out from them. 

neurilemma or cellular sheath of Schwann of the nen^e fibers. 
The fibers of the ventral nerv^e derive their sheath cells partly in 
this way and in part from the central ner\'ous system, the cells 
wandering out directly along the ventral roots. The origin of 
the cells of this sheath shows their genetic relation \\ith the neu- 
roglia cells, and consistent with this is the fact that in the cen- 
tral nervous system the sheath of Schwann is absent, the myelin- 
ated fibers being sumoimded by gUa cells. 





Fio, 29.— Three diagrams of the head of Squulus acarUhias to show the differen- 
tiation of the neural crcsl Into the cranial ganglia- After Neal. A, i, 7, 3, somites, 

manent cutaneous ncrv^es, while in all other vertebrates a variable 
number of these nerves fail to de\elop. (Compare Chapter VL) 
In front of this the neural crest is larger and is di\ided by a slight 




54 



NERVOUS SYSTEM OF VERTEBRATES. 



constriction into a longer caudal segment for the vagus nerve 
and a shorter cephalic segment for the glossopharyngeus. The 
development of the ganglia and fibers of these nerves proceeds 
in essentially the same manner as in the case of the spinal nerv^es, 
except that the ganglia migrate somewhat farther from the brain 



Opthal div^ 
Sup.max.div 
N.maStic^Torm 



tx-jX'Xt 9ffn9 ere it. 




Fig. 30. — Reconstruction of the peripheral nerves in a four weeks human 
embno, 6. 9 mm. long. Enlarged 16.7 diameters. From Streeter. Ot. v., audi 
tory vesicle; i, 2, 3, visceral arches; C. i, first cervical nerve; D. i, first thoradc 
nerve; L. i, first lumbar nerve; S. i, first sacral nerve. 

wall and come to lie outside the muscle somites and against the 
ectoderm above the gill slits. The ganglion of the glossopharyn- 
geus lies over or nearly over the second gill slit. The vagus is 
connected with a series of ganglia equal in number to the number 
of gill slits from the third onward. In cyclostomes (Figs. 28, A 
and B) the ganglion of the vagus is formed first over the third 



DEVELOPMENT OF THE NERVOUS SYSTEM. 



55 



slit and then grows backwards until a series of ganglia are formed, 
one over each gill slit. In bony fishes, amphibia and higher 
forms these ganglia tend to become consolidated and are not 
symmetrically placed over the gill slits. Where the ganglia 
reach the ectoderm over the gill slits the ectoderm becomes thick- 
ened by multiplication of its cells and at least in some lower 
vertebrates, cells proliferate from the ectoderm and join the 
ganglia. The thickening of ectoderm is called the epibranchial 
placode. Each ganglion is now composed of two parts, a median 
part derived from the neural crest and a lateral part derived from 
the epibranchial placode. The ganglion may now be called 
the epibranchial ganglion. In man the ganglion petrosum on the 
IX nerve and the ganglion nodosum on the X nerve seem to repre- 




AjL i^ryq^ Sup 

Fig. 31. — Reconstruction of the peripheral nerves in a four weeks human 
embryo, 7. o mm. long. Enlarged 16.7 diameters. From Streeter. 

sent the epibranchial ganglia of fishes, and ectodermal placodes 
are described in connection with these ganglia in man (Fig. 31), 
but no migration of cells from the ectoderm to join the gangUa 
has been made out. From each ganglion so formed the ramus 
j)OSttrematicus described in the previous chapter grows downward 
behind the gill slit and the ramus pharyngeus grows forward and 
inward to the roof of the pharynx. From the ramus pharyngeus 
in forms above the cyclostomes the ramus praetrematicus goes 



S6 



NERVOUS SYSTEM OF VERTEBRATES. 



downward in front of the gill slit. It has been suggested that 
the epibranchial placodes represent ancestral sense organs whose 

Vagus root gang. 

Accessory root gang. 



IX root gang, 



Gang petros. 
N tymp 



Br to 
carotid pfexus 




Froriep 



Sympotheitc. 



Fig. 32. — Reconstruction of the peripheral nerves in a six weeks human embryo, 
17.5 mm. long. From Streeter. 



DEVELOPMENT OF THE NERVOUS SYSTEM* 



0/ 



sense ceUs have come to join the ganglia. In fact very little is 
certainly known as to the nature of these ectodermal thickenings. 
The presence of the placodes in man and the separation between 
the root ganglia and those formed in connection with the placodes 
suggest that the placodes probably have the same history and 
function in all vertebrates* It is of the highest importance for 
the morphologj' of the nen^ous system that the exact Mstorj- and 
fate of the placodes should be traced in one or more classes of 





Fic, 33. — ^Thrce stages in the development of the acustico-lalcral system in the 
sea bass. From H. V. Wilson, ^1. s., auditory sac, a. s.t.^ anterior sensor)' tract; 
B. s. o.^ preauditon' pit; g. i., gill slk; L L, lateral line anlage; In., lens; m, con., 
furrow between midbrain and hindbrain; med., medulla oblongata; n, i., nasal 
sat; op. n., optic tract; op. 5., optic vesicle; 5, /., common senson- furrow. 

vertebrates and that the distribution, central connections and 
functions of the fibers formed from ectodermal cells, if any, should 
be ascertained. This can doubtless be done by extirpation 
experiments on embryos. 

In front of the anlage for the glossopharj^ngeus ner\*e there is a 
segment of the neural crest which gi\*es rise to the sensor}^ root 
of the VII nen'e. This is deeply constricted from the segment 
for the IX nerve and is wholly separated from the portion belong- 
ing to the trigeminus group. While the ganglion of the IX nerve 
grows down wholly behind the auditor)^ pit, that of the VII ner\^e 
comes into contact with the pit, in lower vertebrates in contact 
with its caudal surface, in higher vertebrates with its mesial walL 




58 NERVOUS SYSTEM OF VERTEBRATES. 

The ganglion extends ventrally beyond the auditory pit and comes 
into contact with the ectoderm over the first gill slit (spiracle). 
An ectodermal thickening fuses with the ganglion and from the 
fused mass the rami praetrematicus and palatinus of the VII nerve 
and the visceral sensory portion of the ramus hyomandibularis 
take their origin. This mass is comparable to the epibranchial 
ganglion of the IX nerve, but the significance of the ectodermal 
constituent is equally unknown. 

The auditory pit is itself a thickening of ectoderm and it plays 
a part in the development of certain cranial ganglia which gives 
it the name of a dorso-lateral placode. Where the neural crest 
anlage of the VII ganglion comes into contact with the auditory 
pit, cells undoubtedly wander from the walls of the pit and form the 
gangUon of the VIII nerve, no part of which is formed from the 
neural crest. In some selachians and bony fishes (Fig. 33) an 
ectodermal thickening continues for some distance forward and 
backward from the auditory pit, so that we may speak of a much 
elongated dorso-lateral placode. The cells which proliferate 
from the caudal portion of this placode continue to multiply by 
mitosis and form a large mass which grows and pushes backward 
along the inner surface of the ectoderm over the roots of the IX 
and X nerves. This mass grows caudally as a narrow band 
along the side of the body, ploughing through the deeper layers of 
the ectoderm. The cells derived from the placode behind the 
auditory pit form a ganglion lying over the ganglion of the IX 
or the first part of the X ganglion and send fibers toward the 
brain, forming the root of the lateral line nerve which is situated 
near that of the IX ner\T as described in the last chapter. The 
growing placode continues backward along the line of di\dsion 
between the dorsal and the lateral body muscles and at intervals 
gives rise to sense organs at the surface of the ectoderm. These 
organs consist of high columnar supporting cells which form the 
thickness of the epidermis, and shorter pear-shaped sense cells 
which do not extend to the full depth of the epidermis but do 
send hair-like processes out beyond the surface. The organs later 
sink down beneath the surface so that they come to lie at the 
bottom of the ectodermal pits (pit organs). The pits then be- 



DEVELOPMENT OF THE NERVOUS SYSTEM. 



59 



come enclosed as tunnel like canals and finally the successive 
canals unite to fonn a continuous canal, the canal of the lateral 
line. Meanwhile the cells of the ganglion send processes caudally 
beneath or imbedded in the growing placode, which come into 
relation with the sense organs as they are formed and constitute 
the nerve of the lateral line. 

The cephalic part of the placode, in front of the auditory pit, 
grows forward and divides into two branches which continue for- 
ward and give rise to sense organs in the same way as does the 
lateral line placode. One branch extends for^vanl above the eye 
and forms the supraorbital row of organs, the other goes ventral 
to the eye and forms the infraorbital line. Ganglion cells which 
proliferate from the placode in front of the auditory pit give rise 
10 the rami ophthalamicus superfacialis and buccalis which supply 
these lines and form the one or two roots of these rami which 
have been described above (p. 20, 21), 

Beneath and in front of the auditory pit a ventral projection 
of the common placode extends downward and forw^ard on the 
mandibular arch. This develops in the characteristic manner 
the hyomandibiilar line of sense organs. The nerve fibers which 
supply this row of organs are also derived from the placode cells 
and form a part of the ramus hyomandibularis. 

In this way the acustico-latcral system of sense organs and nerv^es 
in most fishes is derived from an extensive thickening of ectoderm 
on the side of the head, the dorso-lateral placode. Certain impor- 
tant ditTercnces in other forms must now be mentioned. The anterior 
part of the placode is not always directly continuoiis with the part 
which forms the auditor}' pit. Sometimes this part becomes 
invaginated to form another similar pit lying in front of the auditory 
pit and in front of the spiracukr cleft. This pit may even become 
an enclosed sac and contain well formed sense cells (Wilson) 
similar to those of the lateral line organs. In embryos of certain 
amphibia (Gymnophiona) a deep pit is formed in front of the 
auditory pit and independently of it. This gives rise to the nen^e 
which supplies the supraorbital row of sense organs. In Amblys- 
toma punctatum this pit appears earlier than the auditory pit. In 
cyclostomes (Fig. 28) two separate ectodermal placodes are formed 



6o NERVOUS SYSTEM OF VERTEBRATES. 

in front of the auditory pit, one in front of the other, which give 
rise to the supraorbital and infraorbital lines of sense organs and 
whose ganglia are closely related to the trigeminus and profundus 
ganglia. It is evident that in the early history of vertebrates the 
acustico-lateral system occupied three segments and that pits 
were formed to contain the sense organs. It is probable that 
at first the portions of the system in the three segments were 
independent of one another. 

In typical fishes the main rows of sense organs are enclosed in 
canals similar to the lateral line canal. In addition to these chief 
rows there are often auxiliary rows or groups of pit organs and 
in many cases the typical canals are incompletely formed, the 
organs remaining as pit organs. In cyclostomes and amphibia 
canals are not formed. In selachians there appear along the course 
of the main rows of organs depressions which sink in as pits and 
then become deep, narrow tubes and eventually extend beneath 
the surface to end in one of several masses of jelly-like tissue 
lodged in concavities of the skull. The deep ends of the tubes 
expand and develop sense organs which are related in structure 
to the lateral line organs. These are the ampullae of Lorenzini. 
The mode of development of the vesicles of Sa\i and of the 
nen^e sacs of ganoids, both of which belong to the acustico- 
lateral system of organs, is not known. 

The complete hiatus in the neural crest in front of the anlage 
for the VII nerve is connected in some way with the development 
of the acustico-lateral system opposite this point. Probably 
this system actually uses up the material of this part of the neural 
crest. Forward from this, overlying the region of the cerebellum 
and midbrain, is a large segment of the neural crest which gives 
rise to the trigeminus group of ner\xs. It is necessary to say group 
of ner\'es, because the adult trigeminus is formed from two nerves 
which arise independently in vertebrate embryos. 

From the crest covering neuromeres v and vi in selachians grow 
dowTi two ganglionic anlagcs which remain connected peripherally 
so that they form an U-shaped loop, having two connections 
with the brain. The caudal of these two anlagcs forms the gang- 
lion of the X. trigeminus proper and gives rise to the sensory por- 



DEVELOPMENT OF THE NERVOUS SYSTEM. 6l 

tion of the rami maxillaris and mandibularis trigemini. The 
anterior ajilage loses its connection with the brain and the fibers 
which grow centrally from its ganglion cells follow caudally over 
the trigeminal ganglion and enter the brain along with the root 
of the trigeminus. The peripheral fibers from this ganglion run 
cephalad through the dorsal part of the orbit and form the N. 
ophthalamicus profundus. Figure 29 shows the early stages in 
the differentiation of these two ganglia. While the above descrip- 
tion applies to selachians it may be taken in a general way as typical 
for vertebrates. In cyclostomes the two nerves are more indepen- 
dent in their origin and adult relations, and in all vertebrate embryos 
the two ganglia can be recognized. 
In certain selachians an anlage of a sensory nerve is formed 



Fig. 34. — A transverse section through the nasal sac of an embryo of a bony fish 
at about the time of hatching, to show the origin of the fibers of the olfactory nerve. 

from the neural crest between the mesencephalon and diencephalon. 
It is known as the nervus thalamicus (Fig. 29). The anlage is 
small, comes to be closely related to the profundus trunk and may 
furnish material for the ciliar>' ganglion. In the chick the ciliary 
ganglion is formed in part of cells which migrate out from the 
neural tube along the fibers of the oculomotor nen^e and in part 
of cells which come from the ophthalamicus profundus ganglion. 

In front of this point the neural crest does not give rise to any 
nerve which is found constantly in vertebrates. In selachians, 
however, the N. terminalis (see p. 31) possesses a ganglion and in 
Squalus acatUhias develops in much the same way as do other 
sensory nerves. In the chick ganglion cells are found in connec- 
tion with the olfactory nerve during its development which may 
bear some relation to this nene in fishes. 

The olfactory ner\'e differs from all other nerves in vertebrates 



62 



NERVOUS SYSTEM OF VERTEBRATES. 



in that its cells of origin arise and remain in the ectoderm. These 
are the sensory cells of the nasal epithelium. From the inner end 
of each of these cells arises a fiber (Fig. 34) which grows toward 
the forebrain and enters the bulbus olfactorius. These con- 
stitute the olfactory ner\'e. The fibers are never myelinated. 
Although so little is certainly known of the details of the part 
played by ectodermal placodes in the formation of the sensory 
nen-es and ganglia, it is clear that the ner\'ous ectoderm is not 
limited to the neural plate. It is probable that the limits of the 
ectoderm which give rise to the nervous system are indefinite and 
variable. All that can be said is that the neural plate gives rise 
to the spinal cord and brain with the neural crest, while the olfac- 
tory organ and ner\'e and the ganglia and nen^es of the acustico- 
lateral system are derived from ectoderm adjoining the neural 
plate. These ganglia and nen-es, including the olfactory, are to 
be compared with those derived from the neural crest. All are 




Fig. 35. — Two figures representing the formation of unipolar cells in the spinal 
ganglion of the dog embr\'o. After Van Gehuchten. r., root; p.^ peripheral nerve. 

derived from ectoderm which in the early embryo is closely adjacent 
to the lateral border of that which forms the central nervous system, 
and the neural crest is wanting in those segments in which the 
olfactory and acustico-lateral systems develop. The acustico- 
lateral ganglion cells remain in the general ectoderm for a time, 
the olfactory permanently so. It is probable that the epibranchial 
l^lacodcs, which are much farther removed from the neural plate, 
give rise to a part of the visceral sensorj- nerves, but whether 
general visceral or gustatory fibers are so formed is imknown. 
In Amphioxus and cyclostomes the sensory ganglion cells 
Remain throughout life bipolar, spindle-shaped cells such as have 
bei*n clescribixl. During the course of development in true fishes 



DEVELOPMENT OF THE NERVOUS SYSTEM. 63 

a part of the cells become unipolar and in higher vertebrates 
most of them do so. This change is brought about by a sort of 
bending of the cell body by which the fibers at the two ends are 
brought together at one side (Fig. 35). Then by the growth of 
that part of the cell to which the fibers are attached both fibers 
come to be borne on a single process. 

The development of the motor nerves takes place somewhat 
later than that of the sensory ner\ es. In the ventral part of the 
spinal cord and brain, where motor nerves are to be formed, 
certain large neuroblasts send their neurites out on the ventral 
surface. At this period numerous cells wander out of the cord or 
brain, following the neurites in their growth, migrate and multiply 
and eventually form the sheath of Schwann (cf. p. 51 above). 
The neurites grow until they reach the muscles which they are to 
innervate, when they branch or expand on the surface of the 
muscle fibers in the form of special motor end organs. These 
ventral motor nerves innervate only those muscles which are 
derived from the mesodermic somites. It has recently been 
shown that in amphibia fibers of a single nerve are distributed 
to muscle fibers of two adjacent muscle segments. This fact 
is wholly inconsistent with the view held by some anatomists 
that from the earliest embryonic stages processes of nerve cells 
are in connection with their muscles by means of strands of proto- 
plasm which become elongated to form motor nerve fibers. 

The nerve fibers which innervate the muscles derived from the 
lateral mesoderm are developed in the same way from neuro- 
blasts which lie in the lateral parts of the central gray. In lower 
vertebrates these fibers pass out of the cord or brain together with 
or close beneath the dorsal sensory nerve roots and pass through 
the sensory ganglia. In higher vertebrates a part of these fibers 
pass out with the ventral loots and in the cat, dog and monkey 
all of them are said to have this course. 

Morphology of the head. — ^In order to understand the 
brain and cranial nerves it is necessary to take into account the 
morphology of the organs of the head. For this reason a sum- 
mary of the more important conclusions of comparative embryology 
and anatomy will be given here. 



64 NERVOUS SYSTEM OF VERTEBRATES. 

At an early stage in the embn'o the dorsal border of the sheets of 
mesoderm becomes divided into a series of segments, which soon 
separate from the undivided lateral mesoderm and lie as indepen- 
dent blocks or sacs at the sides of the neural tube. These are known 
as mesodermic somites. The first somites to be formed lie in 
the n?gion which will form the neck and the segmentation continues 
from that pxjint forward and backward. In front of this point 
there are formed in selachians and cyclostomes a definite number 
of somites which were first described by vanW'ijhe in 1882. These 
head segments bear constant relations to other organs and may 
be designated by numbers indicating their order from before 
backwaal. Somite i lies behind and below the optic vesicle 
and givt^ rise to those eye-muscles which are inner\'ated by the 
III ner\e mim. rectus superior, rectus inferior, rectus intemus, 
obliquus inferior). In front of somite i there is found in selachians 
a pair of lateral masses of mesoderm known as anterior head 
caxiiies. These are believai 10 repn^sent a still more anterior 
pair of somites which dis;ippear in vertebrates \iithout forming 
any nvognizaMe structure. Somite 2 is larger and is directly 
connci'ial wiil\ the mandibular arch. It gives rise to the muscle 
inner\atal by il\e l\' nor\e yXn, obliquus superior). Somite 
^^ is voiuuvtixl \\i\\\ the hyoid arch and gives rise to the muscle 
inniiNatiil by thr \1 nerve .m. rectus extemus). Somite 4 
liv's nusial to vm* >lii:ht!y behind ihe auditor}* pit and is known 
,\s \\\v \\\>\ poNiaudiUMv MMiuie. It dves rise to muscle on the 
vioi^^.d Mu t,uc of ihc h^Mvi in v\vrlos:omi^; in all other vertebrates 
iMvaks ilown .uul vii^ap;HW>. Somite 5 in cyclostomes gives 
!\s,' 10 .\ iu\>vK' Kins: nc\: Inhit^l :ha: of somite 4; in all other 
\v^'.:vIm,\Iv-- vli-^.i)^|H\i:s. l;> V \ v'!o<:o:vas all :he following somites 
to:!n ju'.ir.anrnt inxv^ivMUv'^. anvl :v,\o:o:v.c :c lOiiet her with several 
iol!v>\\n\i\ i', ^vMul nu'.Nvlr b\:viN vivnxr. Vt:v!r.v: ar.vi forward beneath 
;b.v^ iMauxlwal ,i|»p,u,i;\is ;o !v^'.n\ ::\o >;;b branchial muscles corre- 
--'.s^n^hnv: to \\w ioi\;'.\ir i\\;>v\;la:i:'. v" v^t hichcr forms. 

In mI.\v1u,\\v. a NanaMr r.\:'.\\l\"v ot '.vsMv.v'.iion^' somites may 

'.'n; loun ".\M\\v' iwii'^vK' ;'l\i-» .;;\i ,;::v vAarv'.s break down and 

ov.',\ vvMw.iinur iv^ ti\r to\n\,\Mon v^: •/.^v"^v:^■"*v:r.c. The first somite 

u^ tvvtw pvMn^aiuMi nu\.vlv-. i-. n\;',v.»\^* " v^* S in n'^v^st selachians. 



DEVELOPMENT OF THE NERVOUS SYSTEM. 65 

Somite 8 or 9 and several following it send muscle buds ventrad 
and forward beneath the branchial apparatus to form the sub- 
branchial muscles as in cydostomes. In higher vertebrates 
the abortion of certain postauditory somites is more complete 
and the number which disappear may be greater. In various 
classes of vertebrates more or fewer vertebral segments may be 
secondarily fused with the primordial cranium, entering into the 
occipital region of the skull. In manmials and perhaps man four 
such vertebrae are added to the cranium. In all vertebrates 
the sub-branchial or tongue musculature is formed from muscle 
buds derived from post-branchial somites. 

In vertebrates above cydostomes the breaking down of somites 
behind the ear produces a shifting of the relative position of the 
somites and nervous organs. The auditory sac is known to shift 
backward during development through the length of from one to 
three hindbrain neuromeres (Fig. 18). Also the permanent somites 
shift forward somewhat so that at every stage the existing somites 
lie dose behind the auditory sac. The shifting forward of 
somites at the same time affects their position relative to the 
branchial apparatus. Also, the disappearance of somites leads 
to the disappearance of the ventral motor nerves which should 
inner\'ate them, and apparendy the shortening process in the 
region behind the ear leads to a decrease of the ectodermal area 
and the reduction or disappearance of one or more nerves of the 
skin. Finally, the development of the hypoglossal nerve is deter- 
mined by the migration into the sub-branchial region of the 
musde buds which it is destined to inner\'ate. 

The lateral mesoderm in the meantime becomes segmented 
in a passive manner by the formation of the gill slits. In most 
vertebrates these slits seem to have no definite position reladve 
to the somites. There is e\idence, howev'er, that the segmentation 
of the lateral mesoderm and pharynx in primitive vertebrates 
corresponded to that of the dorsal mesoderm. The mandibular 
and hyoid arches are connected with the second and third somites 
respectively. The third branchial arch is connected with the 
fourth somite, and in Petromyzon the total number of gill arches 
(d^t) corresponds to the number of somites (somites 2 to 9) 
5 




Flo- 57. — DiAgrams representing thedevetoptnent of the buccal cavity, hypophy- 
fii und naiAl pit In AmpkhxHs and PiW^^mywm. After Lcgros. 

in existing vrrtcb rates: in cvxlostomes^ 7 to 35; in selachians, 
5 to 7 ; in ampliibia and higher forms, 5 to 4. It is probable that 
the number in primitive vertebrates was large. The number of 





DEVELOPMENT OF THE NERVOUS SYSTEM* 



f-7 



anterior head cavities mentioned above. This most anterior 
mass of entoderm is present in amphibia also (Fig* 36), although 
the anterior head cavities are not separately developed* In 
this b seen the vestige of a communication between the hypoph* 
)^s and the archenteron. In many vertebrates an outgrowth 
of entoderm from this region, knowTi as SeeseFs sac, enters with 
the ectodermal pit into the formation of the adult hypophysis* 
In cyclostoraes the ectodermal pit becomes deep and large, and in 
Bdellostoma (Kupffer) has an open communication mth the arch- 
enteron at an early stage. This communication is afterward oblit- 



-Epiphysis 
I- Velum 



\ 



Opttc chtasnm 



Mouth 



Seescl's sac. 



iphysis 



Fig. 36. — A meilian sagittal section of the head of an embrvo of Amblystoma 
pmutatum, to show the relations of epiphysis, velum, paraptiysiSj hypophysis. 
Srescrs sac, etc. 

crated but the hypophysis remains throughout life as a large sac 
open to the exterior and extendingbeneath the brain. The communi- 
cation with the archenteron has been described also for the sturgeon* 
Also in Amphioxus» where the permanent mouth is formed from 
a gill slit, a canal which opens externally farther forward communi- 
cates with the archenteron at an early stage of development (Leg- 
ros). The relations of mouth and hypophysis in AmpMoxus and 
Petromyzon are represented diagrammatically in the accompany- 
ing 5gure from Legros (Fig. 37). The ancient vertebrate mouth, 
or paleostoma, is to be thought of as l>^g ver}^ near the anterior 
end of the animal, just beneath the region of the olfactor}' organs 
which alone extend farther forward. In the sides of the canal 



70 NERVOUS SYSTEM OF VERTEBRATES. 

The ophthalamicus profundus nene arises from the neural crest 
o\'er the mesencephalon; its root has shifted back to join that of 
the trigeminus. The trigeminus arises over the metencephalon 
and in cyclostomes remains j)ermanently attached to that segment. 
In all other vertebrates it moves back one segment to the first 
neuromerc of the myelencephalon. The facialis has shifted 
back not only one segment, but two, leaving the neuromere between 
it and the trigeminus \\ithout any nen'e root. The shifting of 
the VII is probably due to the shifting of the auditory sac which 
has in part pushed it back and in part moved back past it. By 
this movement the IX and X nencs are also affected. There 
is reason to believe that the IX has moved back one segment in 
cyclostomes, and that the IX and X have moved back two neuro- 
meres in fishes and three in reptiles, birds and mamniab. 

A word should be said regarding the complex character of the 
X ncr\'e. There is reason for thinking that each one of the epi- 
branchial ganglia of the vagus represents an independent branchial 
nene, and that these nerves one after another beginning at the 
caudal end of the gills have joined the ner\'e next anterior until 
all have united into a single root. In the myxinoids this process 
has gone one step farther than in other vertebrates, and the vagus 
itself is joined to the glossophar>Tigeus. The actual shifting for- 
ward and fusion of these nerves does not now take place during 
the ontogeny, so that it is to be regarded as a very primitive feature 
of vertebrate development. 

It is impossible in this book to follow further the evolution of 
the vertebrate head. The accompanying tables and figures are 
intended to make more clear the brief statements that have been 
made. Figure 38 is meant to show the relations of the various 
structures which enter into the primitive head segments. The 
two tables give further facts which should be taken into account 
in connection with this diagram. Tabic A is abbreviated from 
the tables of Hoffmann showing the shifting of organs which 
takes place during the development of the selachian head. Table 
B is meant to show ihe elements present in each segment of the 
head and the more important reductions and changes of position 
which have taken place. 



72 



NERVOUS SYSTEM OF VERTEBRATES. 



terrainalis, the nen'us thalamicus, the relalioQ of the lateral Hues of the head tolhtj 
auditory vesicle, and ihc prat^oral entoderm are taken from selachians* The ^ensoix] 
nerve roots are repre^sentcd as retaining their attachment to the dorsal surfaced 1 
the neural tube where they were fonned from the neural crest The segmeotAl 
p>ositJon of these nyols is about that which they have in the embryo of Petrornvzon, 
except the root of N. X, which has been shifted back a littk farther than it is in 1 
Petromyzon. The general cutaneous nerve shown in dotted outline over somiljelij 
is the Vagusanhang of Hatschek in Ammocoetes and the nerve which tutiCfii] 
with the vagus root in the embryo of selachians. This is the second dorsal spimlj 
nerve in Petromyzon dorsatus and other primitive cyclostomes* The poiiitionc 
the viscero-motor nuclei somewhat cauda>I to the several roots is indicated, 
viscero-motor nucleus of the vagus and accessorius is shown as a single 
nucleus extending through two segments. It might mure properly have been < 
tinued caudally until it came into connection with the i.'iscero-motor nucleus of 1 
trunk nerve. The accessorius ner\'e is not shown. The somatic motor nudettf | 
and root are shown for all the somites except somite 4^ where they arc shown 11 
dotted outline. The nerve for somite 4 is absent in Petromyzon, but is presenll 
Bdcllostoma. It is possible that this root has joined with N. VI in gnatko 

The pTOcess of cephaJization in vertebrates has consisted cl 
in (i) the development of special sense organs; (2) the conseque 
enlargement of the brain; (3) the formation of a rigid craniun 
to protect sense organs and brain; (4) the disappearance of ceria 
muscle segments and the change of posidon and function of otl 
(eye- muscles); (5) the reduction in number of gills, the formatio 
of a new mouth and the expansion of the persistence gills; (6) th^ 
disappearance of \ arious nert^es owing to the reduction of somite 
and gills; (7) the shifting of posidon of various organs and nc 
roots due to these changes; and (8) the great development 
higher vertebrates of the so-called higher brain centers. At ever 
step in the evoludon of vertebrates these changes ha\^e left ihei^ 
impression on the ner\^ous system so that the nervous system ' 
a greater extent than any or all other organs preserv^es a recor 
of the course of phylogenctic development. For tliis reason not' 
only does a knowledge of the nervous system throw light upon 
the morphology of the head, but a knowledge of the evolution o^H 
the head is necessary for a true understanding of the nen'ous^l 
system. 

TABLE A. 
Shifting of organs during deveJopment of selachian embryo> 

Length of embryo. .12^-13 mm* 40 mm. 

Duct of Cuvier shifts from trunk segment ..... 2 to tniok segment 10 
Omphalomesenteric artery " .... 4- 5 " 12-15 

Sixth branchial artery from occipital segment . 3 ** 

Ostium of Muller^s duct, from truok segment . 5-6 " 



g-r.. 

















13 


14 


15 


16 


n 


- 


xiii 
9 


xlv 


XV 


xvl 


xvli 


lO 


II 


12 


13 


h( 


br. 6 


a larger number of branchial arches in lower 
cyclostomes and primitive vertebrates 


4 sp. Cycl. 

)stomes. 
[eptanchus 


5 sp. Cycl. 6 sp. Cycl. 
permanent roots in Acanthias. 


7 sp. CycL 
pennanent 
root in 
Gnath. 


8sp.Cycl. 
2sp. ? 
Gnath. 


.ciistici. 








a 












6 BdeU. 

X 

nd ventral 


7 BdeU. 

y 

motor column. 


8 BdeU. 

z 


9 BdeU. 
I sp. Furb. 


lo BdeU. 
a sp. 





comp*t 
col'ct'd 
into 

N. X. 

f Cajal an( 


comp't 
in r. 
br.-int. X 

i visceral sensory 


comp't 
in r. 
br.-int. X 

column of spinal 


comp*t 
in r. 
br.-int. X 

cord. 


comp*t 
in r. 
br.-int. X 


illy distinguished 








i 










n 


mot. N. 
corct'd 
into N.X 

lotor colun 


N. access, 
an of spinal cord. 


N. access. 


N. access. 


N. access. 



74 NERVOUS SYSTEM OF VERTEBRATES. 

Dohrn, Anton: Die Entstehung der Hypophysis bd Petromyzon planeri. 
Mitth. Zool. Sta. NeapeL 

Eycleshymer, A. C. : The Development of the Optic Vesicles in Amphibia. 
Jour. Morph., Vol. 8. 1890. 

Froriep, A.: Ueber ein Ganglion des Hypoglossiis und Wirbelanlagen in 
der Occipitalregion. Arch. f. Anat. u. Physiol., Anat. Abth., 1882. 

Froriep, A.: Ueber Anlagen von Sinnesurganen am Facialis. Glos- 
sopharyngeus und Vagus, ueber die genetische Stellung des Vagus zv.m 
Hypoglossus und ueber die Herkunft der Zungenmuskulatur. Arch. f. Anat. 
u. Physiol., Anal. Abth., 1885. 

Froriep, A. : Die Entwickelung des Auges der Wirbclthiere. Hertwig's 
Handbuch der Entwickelungslehre. 1905. 

Fuchs, H.: Bemerkungen ueber die Herkimft und Entwickelung der 
Gehorkndchelchen bei Kaninchen-Embryonen u. s. w. Arch. f. Anat. u. 
Entwick. 1905. Suppl.-Bd. 

FUrbringer, Max: Ueber die spino-occipitalen Nerven der Selachier und 
Holocephalen und ihre vergleichende Morphologie. Gegenbaur's Festschrift. 
Bibliography. 

Gage, Susanna Phelps: A Three Weeks Human Embryo, with especial 
Reference to the Brain and Nephric System. Amer. Jour. Anat.. Vol. 4. 1905. 

Gaupp, E.: Zirbel, Parietalorgan und Paraphysis. Merkel u. Bonnet's 
Ergebnisse, Bd. 7. 1897. 

Harrison, R. G.: Ueber die Histogenese des peripheren Nervensystems 
bei Salmo salar. Arch. f. mik. Anat. u. Entw., Bd. 57. 1901. 

Hill, Charles: Developmental History of the Primary Segments of the 
Vertebrate Head. Zool. Jahrb., Abth. f. Anat. u. Ontog., Bd. 13. 1899. 

His. W.: Zur allgemeine Morphologie des Oehims. Arch. f. Anat. u. 
Physiol., Anat. Abth., 1892. 

His, W.: Ueber das frontale Ende des Gehimrohres. Arch. f. Anat. u. 
Enlw., Anat. Abth., 1893. 

His, W.: Anatomic menschlichen Embr\'onen. 

His, W.: Die Entwickelung des menschlichen Gehirns wahrend der ersten 
Monate. Leipzig, 1904. 

HoflFmann, C. K.: Ueber die Metameric des Nachhims und Hinlerhims 
und ueber Beziehungen zuden segmentalen Kopfner\'en bei Reptilien-Em- 
br\'onen. Zool. Anz., Bd. 12. i88q. 

Hoffmann, C. K.i Beitrage zur Entwickelung der Selachii. Morpb. 
Jahrb., Bde. 24, 25, 27. 1896-1899. 

Johnslon, J. B.: The Morpholog)' of the Vertebrate Head from the 
Viewpoint of the Functional Divisions of the Nervous System. Jour. Com. 
Xeur. and Psych. Vol. 15. IQ05. Bibliography. 

Koltzoff. \. K.: Entwickelungsge5.chichte des Kopfes von Petromyzon 
planeri. Bull, de la Soc. Imper. d. Natural, de Moscou, Annee 1901, 
No. 3.-4. igo2. 



DEVELOPMENT OF THE NERVOUS SYSTEM. 75 

von Kupffer, C: Studien zur vergleichenden Entwickelungsgeschichte 
des Kopfes der Rranioten. MUnchen u. Leipzig. 1894-1900. 

von Kupffer, C: l)ie Morphogenie des Centralnervensystems. Hertwig's 
Handbuch der Entwickelungslehre. 1905. 

Legros, Robert: Developpment de la cavite buccale de TAmphioxus 
lanceolatus. Archive Anat. Micros., Tome i, 1898. 

Locy, W. A.: A Contribution to the Structure and Development of the 
Vertebrate Head. Jour. Morph., Vol. 11. 1895. 

Minot, C.S.: A laboratory Text-book of Embryology. Philadelphia. 1903. 

Neal, H. V.: The Segmentation of the Nervous System in Squalus 
acanthias. Bull. Mus. Com. Zool. Harvard College, 31. 1898. 

Neal, H. v.: The Development of the ventral nerves in Selachii. I. The 
Spinal Nerves. Mark Anniversary Volume. 1903. 

Piatt, Julia B.: A Contribution to the Morphology of the Vertebrate Head 
based on a Study of Acanthias vulgaris. Jour. Morp., Vol. 5. 1891. 

Price, G. C: Some Points in the Development of a Myxinoid (Bdellostoma 
Stouti, L.) Verhdl. Anat. Ges. 10. Vers. Berlin. 1896. 

van Wijhe, J. W.: Ueber die Mesodermsegmente und die Entwickelung 
der Nerven des Selachierkopfes. Amsterdam. 1882. 

Wilson, H. v.: The Embryology of the Sea Bass (Serranus atrarius). 
Bull. U. S. Fish Com., Vol. 9. 1891. 

Zimmermann: Ueber die Metamerie des Wirbelthierkopfes. Verhdl. d. 
Anat. Ges. 5 Versam. Mtinchen. i8qi. 



76 NERVOUS SYSTEM OF VERTEBRATES. 



CHAPTER IV. 
^ NERVE ELEMENTS AND THEIR FUNCTIONS. 

The chief functions of nerve elements are" the origination 
and transmission of nenx impukes. A nerve impuke originates 
as the result of a. stimulus. Although the stimulus may be one 
of several kinds, nffechanical, chemical, thermal, photic, etc., it 
always consists in some change in the environment of the nen-e 
tell. Whatever the character of the stimulus, it produces in the 
protoplasm of the nerve cell chemical and physical changes of 
such a charatter that tBey may be propagated or transmitted 
through the protoplasm from one part of the nerve cell to another. 
These changes within a ner\-e cell constitute a nerve impulse. 
Although it is called^ forth by a stimulus, the impulse is not that 
stimulus caught up and passed on, but is a new thing consisting in 
the activity of the nerve cell itself. Besides the origination and 
transmission of impulses it is believed that other functions may be 
performed by ner#e elements. They may reinforce or strengthen 
the impulses during their transmission; they may store up weak 
impulses so as to discharge a stronger one after an interval (sum- 
mation) ; such discharge may be repeated at more or less regular 
intervals (rhythmical nerve action) ; or the impulses may be more 
or less completely blocked or impeded in their transmission 
(inhibition). The consideration of these processes within nenx 
elements belongs to the field of general nenx ph)rsiology with 
which the present book does not deal. 

Although the chemical and physical nature of a nerve impuke 
may and probably does differ in relation to the internal structure 
of the nerve cell and the character of the stimulus, the importance 
of the nerve impulse to the organism consists in the eCFect produced 
when the nen^e impuke is delivered by the nerv^e element to some 
other tissue element. What tissue element shall receive the 
impulse; after how long a time it shall be delivered ; whether it shall 



NERVE ELEMENTS AND THEIR FUNCTIONS. ^^ 

be delivered in full force, strengthened or inhibited, — ^these things 
which are all-important in the life of the organism depend upon 
the form of the nerve elements and the manner in which they 
are arranged. While the work done by the nen^ous system is 
our primary interest, wfe can understand this only by first under- 
standing the construction bf the mechanism which does the work. 
On the other hand the work done must be held in min4 in order 
that we may truly interpret the constituent parts of the mechanism.* 
The study of the minute structure and physiology of the nervous 
S)rstems of various classes of animals has shown that nerve elements 
usually transmit impulses in a given direction. This is equiva- 
lent to saying that the elements have a specific form, as otherwise 
all directions would be alike. In vertebrites as' a rule nerve 
elements present structurally and fimctionally a polar differen- 
tiation; the two ends of the element iJifSer in both form and 
function. The nerve element consists of a mas^ of protoplasm 
containing a nucleus, it is a cell. Usually the nucleus is iirimedi- 
ately surrounded by a larger mass of protoplasm which is called 
the cell-body. From this cell-body there 'Extend more or less 
slender strands of protoplasm called processes. In nearly all 
cases the processes are seen to be of two forms. Some are rela- 
tively thick, irregular and have numerous branches; one is rela- 
tively slender, imiform in diameter, gives off c^ateral branches 
and is profusely branched at the end of its course. The former 
bear a general resemblance to a bush or branch of a tree and are 
hence called dendrites or dendrons; the latter is more thread like 
and since it forms the axial and essential portion of a common 
nerve fiber it is called a neurite or axone. A nen-e cell commonly 
possesses two or more dendrites but usually has only one neurite. 
The volume of all the dendritic processes may exceed by many 
times the volume of the cell-body, and even the volume of the 
neurite may be considerable when it is very long or has many 
end branches. In certain cases the cell-body is elongated and the 
dendrites arise from one end and the neurite from the other. Most 
nerve cells, however, do not so clearly illustrate polar differentia- 
tion in their form. The dendrites may arise irregularly from 
various parts of the cell-body and the neurite may take its origin 



78 



NERVOUS SYSTEM OF VERTEBRATES. 



from one of the dendrites at some distance from the cell-body. 
Figure 39- shows several forms of nerve cells with reference to 
polar diCFerentiation. 

With respect to the form and disposition of their neurites, 
two chief types of nerve cells are distinguished. In the first and 
more common type (type I) the neurite is long and serves to make 
a connection with some distant nerve cell, muscle cell, gland cell, 
etc. In the second type (type II) the neurite is short and divides 




Fig. 39. — ^Several types of nerve cells from the central and peripheral nervous 
system of vertebrates. A, spinal ganglion cells; B, a Purkinje cell from the brain 
of the sturgeon; C, a cell from the nucleus praeopticus; D, a granule from the 
cerebellum; E, a cell of type II from the tectum opticum. 

into terminal branches in the near vicinity of its cell, serving to 
make connection with several other cells of the same nerve center 
(Fig. 39, E). 

The structural differentiation of the cell is correlated with a 
functional polarity. For, the nerv-e impulse within a given cell, 
whatever its source, begins in the dendrites or cell-body and is 
transmitted toward and along the neurite until it is passed on to 
some other cell through the branches of the neurite. In the case 



NERVE ELEMENTS AND THEIR FUNCTIONS. 79 

of the spinal ganglion cell there are but two processes from 
the cell and both these are slender, like neurites. As shown 
in the last chapter (p. 50) the process which receives the impulse, 
the peripheral fiber, corresponds in form and time of development 
to the dendrite of a cell of the usual form (Fig. 39, A and B). 
In the retina and in the olfactory epithelium the sensory cells 
seem to have no dendrites but instead only rod-like or hair-like 
peripheral processes. 

In the celk which are usually regarded as typical ner\'e cells 
in the central nervous system of vertebrates the impulse starting 
in the dendrites passes through the cell-body and along the 
neurite. Probably in most cells in the central nervous system, 
however, and in the unipolar spinal ganglion cells the neurite 
arises from a dendrite at a longer or shorter distance from the 
cell-body. In these cases the most direct route for the ner\'e 
impulse is to pass from the dendrites to the neurite without entering 
the cell-body. Although it has long been supposed that such is 
probably the course of the impulse, it has not been possible actually 
to demonstrate it in vertebrates. In the crab Carcinas, Bethe 
has found a ganglion in which it was possible to secure exper- 
imental proof of this hypothesis. In the ganglion which supplies 
a sensory nerve to the second antenna the sensory cells have a 
form analogous to that of the imipolar spinal gangUon cells of 
higher vertebrates. Each cell body gives off a single process 
which is very much elongated and divides into a dendrite which 
serves as a sensory fiber to the antenna and a neurite which makes 
connection with motor cells in another part of the ganglion (Fig. 
40). Without explaining the arrangement in detail, it may be 
said that the unipolar sensor)^ cells are so placed that by verj- 
careful manipulation with fine instruments it was possible to 
remove the cell-bodies without disturbing the point of division 
of the single process into dendrite and neurite or the course and 
terminations of these processes. By artificial stimulation of the 
antenna, now, it was found that the same muscular reflexes 
followed as in the normal animal. Evidently the nerve elements 
perform their functions in the usual way in spite of the cell-bodies 
being cut away. The phenomenon of summation was also wit- 



8o NERVOUS SYSTEM OF VERTEBRATES. 

nessed. It was further observed, however, that after a few da)rs 
the functional responses ceased, presumably because of the death 
of the nerve fibers whose cell-bodies had been cut away. 

It was supposed by the older anatomists and physiologists 
that the dendrites of a nerve cell were merely protoplasmic expan- 
sions which served to absorb the nutrient materials necessary for 
the cell, and they were called " nutritive processes. " It was thought 
that the cell-body and neurite carried on all the specific nervous 
functions. Since it has been shown that the dendrites play apart 




Fig. 40. — A cell from one of Bethe's figures to illustrate the experiment described 
in the text. The arrow shows where the single process was cut. 

in the nerve activities not less important than that of the cell- 
body and neurite, they have not on that account lost their impor- 
tance for nutrition. Since they oflFer by far the greatest surface 
for absorption of nutriment, together with a sufficient bulk of 
protoplasm for the purpose, it is probable that absorption does 
take place chiefly through the dendrites. But the final result 
of Bethe's experiment shows that the cell-body, or more especially 
the nucleus which it contains, is necessary for the nutritive activi- 
ties. In nerv^e cells as in other cells the nucleus plays an essential 
part in the metabolic processes. Deprived of its nucleus the nerve 
cell soon dies, although it may carry on its normal functions so 
long as the nutrient materials contained in the protoplasm sufl&ce 
for the necessary metabolic activities. 



NERVE ELEMENTS AND THEIR FUNCTIONS. 8l 

As each nerve cell has two poles, an in-coming and an out- 
going, so the nervous system as a whole receives stimuli from or 
through other tissues and gives out stimuli to other tissues. The 
nerve elements are so arranged in the nervous S) stem that certain 
elements serve to receive all the stimuli, while others give out 
impulses to the rest of the body. The former cells may be called 
receptive cells; the impulses which they carry into the central 
nervous system, afiereni impulses. The cells which give out 
impulses may be called excitatory cells; and their impulses efferent 
or out-going impulses. The afferent impulses do not always 
give rise to sensations. When they do, the impulses may properly 
be called sensory impulses and the fibers which carry them, sensory 
fibers. The efferent impulses go to glands as well as to muscles. 
Those which go to muscles may properly be called motor or excito- 
motor impulses; those which go to glands may be called excito- 
glandular impulses; and the fibers concerned may be given corre- 
sponding names. 

The elements of the nervous system are so arranged that the 
dendrites of the receptive cells are directed toward the periphery 
and constitute what are commonly known as sensory nerve fibers. 
The neurites of the excitatory cells likewise extend to the periphery 
and constitute excito-motor and excito-glandular fibers. These 
two sets of fibers, together with numerous sense organs, constitute 
the peripheral nervous system, within the limits of which the 
cell-bodies of the receptive cells may also be included. A great 
number of other cells, which make up the central nervous sys- 
tem, are engaged in transmitting impulses from the receptive to 
the excitatory cells, and in distributing and coordinating the im- 
pulses in such ways as to produce through the excitatory cells 
definite responses to the stimuli received. The only collective 
term for all the various categories of cells performing such func- 
tions is the term central cells. The relations of the sympathetic 
system to the peripheral and central portions of the nervous sys- 
tem will be taken up in a special section (Chapter XIII). 

In order to understand the functional relations of the several 
kinds of nerve cells to one another and to the rest of the organism, 
a certain type of relatively simple actions which are performed 
6 



82 NERVOUS SYSTEM OF VERTEBRATES. 

by all animals may be examined. If a frog be touched or pinched 
it will usually make a single prompt movement which may result 
in drawing the part concerned out of danger. If a flash of light 
suddenly falls on the eye of a fish, or if a swiftly moving shadow 
crosses the water, the fish may make a quick dart due to a single 
stroke or a few strokes of the tail. Further movements of either 
frog or fish may be indirectly connected with the stimulus men- 
tioned, but it is the initial, relatively simple movement which 
concerns us here. We ourselves make similar movements 
under certain conditions. In our sleep we may move a hand 
to brush away a fly without being conscious of the act. We 
commonly toss about more or less in our sleep, and quite uncon- 
sciously. Certain waking movements are also of the same simple 
class, as when the eye-lid is suddenly closed to shut out a flying 
insect which is not consciously seen, or when sudden coughing 
is caused by some object entering the trachea. Probably many 
simpler mechanical operations, such as walking, fall at least at 
times under the same category' of simple actions. 

If these simple actions are studied experimentally under proper 
conditions we can determine what ner\'e elements are engaged and 
how they act. First, if in a frog or other lower animal the brain 
be entirely destroyed, the sort of actions mentioned are still per- 
formed with normal efficiency. Only the spinal cord and the 
peripheral elements connected with it are necessary. In the 
simplest case a single receptive cell whose body is situated in the 
spinal ganglion, receives the stimulus through the terminal branches 
of its dendrite in the skin and transmits an impulse along its cen- 
trally directed neurite. Within the spinal cord lateral branches 
are given off from the neurite and one of these collaterals carries 
the imjuilse to one or more excitatorv' cells lying in the ventral 
horn of the cord. These cells send impulses out along their neu- 
rites in the ventral root of a spinal nerve to certain muscles whose 
contraction produces the observed movement. Thus only recep- 
tive and excitatory cells are engaged in the whole action from the 
reception of the stimulus to the movement in response. This is 
illustrated in the left half of Fig. 41, A. The movement is called 
a reflex m(rcement, the whole act including the nervous processes 



NERVE ELEMENTS AND THEIR FUNCTIONS. 



83 



is a reftex act and the chain or series of nerve cells concerned is 
known as the reflex chain or reflex arc. In this simplest case 
the reflex chain has but two links. Probably only the simplest 
movements, such as a jerk or twitch, are produced by so simple 
a chain. 

Usually the aflFerent impulse instead of being handed over 
directly to an excitatory cell, is spread more widely through the 
spinal cord by means of a larger number of branhcesof the incoming 
neurite and by means of central cells (Fig. 41, B, D). The arrange- 
ment of these cells will be treated later (Chapter XIV) but it may 




tract cdl 



Fig. 41. — Diagrams intended to show several forms of reflex chains in the nervous 
system of vertebrates. A, somatic sensory and motor reflex; B, reflex by way of 
tract cells; C, visceral sensory and motor reflex; D, a diagram of the spinal cord 
and nerve roots from the side, showing the same elements as in \ B and C. d. c. 
/., direct cerebellar tract. 

be said that at least one set of cells usually interv'enes between 
the receptive and the excitatory cells. The function of these 
cells is to spread the impulse so that it will affect a 
larger number of excitatory cells and so provoke a more 
ample or more intense movement in response. The extent 
to which the muscles of the body may be called into movement 
by such a reflex mechanism depends chiefly upon the strength 
and the duration of the stimulus. A larger number of links may 
be introduced into the reflex chain and the excitator}' impulses 



84 NERVOUS SYSTEM OF VERTEBRATES. 

may pass out over several nerves and give rise to coordinated 
contractions of many muscles. In this case we may speak of a 
complex reflex. A large number of the ordinary actions of the 
body are carried out in this way without the intervention of the brain. 
A large part of the brain also, even in man, may take part in such 
reflexes without voluntary eflPort or effect in consciousness. When 
the brain is involved, however, even in the frog or other lower animal, 
the actions are likely to become so long continued, so complex 
and varied that they are diflBcult to analyze into their simple 
constituents and to describe as reflexes. Nevertheless in all these 
cases essentially the same events are taking place. Receptive 
cells when stimulated transmit impulses which, after passing 
through a larger or smaller number of central cells, are sent out 
by excitatory cells to pro^'oke contraction of muscles. If we 
speak of the cells through which an impulse is transmitted as the 
path of the impulse, the path in this case instead of being simple 
and direct is more complex, and the movements resulting may 
be much more complex and the means by which the end is attained 
more indirect than when the spinal cord alone is involved. 

It is evident that there is no limit to the extent to which this 
conception of the reflex acti\ity may be carried. So far as the 
nervous activities are concerned they are always of the same 
general tyipe as that represented in the reflex. The term reflex 
is properly used only when a stimulus, through the mediiun of 
the nervous system, provokes a responsive action in the organism. 
But, given a ner\^e impulse of any kind, aroused in any way, 
the series of events in the ner\'ous system is similar to that of a 
reflex act. The impulse may pass from cell to cell, the motor 
response may be inhibited, and the impulse traveling into the 
sensor}' areas of the cerebral cortex, a sensation may result. This 
may mark the beginning of a new series of events within the 
cerebral cortex, associations, memory, thought processes. In 
all cases we are dealing with the origination of nerve impulses 
and their transmission over definite paths which may be anatom- 
ically studied. It is these anatomical pathways, themselves 
determined by experience, hereditary and individual, which 
determine the course taken by ner\'e impulses and the responses 



NERVE ELEMENTS AND THEIR FUNCTIONS. 85 

of the organism to stimuli. Our object therefore is to trace out 
these pathways of impulses, to arrange them into systems of 
nerve elements with reference to their appropriate stimuli and 
responses, and to give an orderly account of these systems. 

THE NEURONE THEORY. 

The cell theory of Schleiden and Schwann (1838-39) was a state- 
ment of the general conclusions of anatomists and embryologists 
up to that time regarding the mode of construction of the animal 
and plant organism. In brief, it was to the effect that every 
Ugher animal or plant is made up of many individual organisms 
known as cells. The cell was regarded as the unit of structure. 
Each cell carried on its own processes and had its own life history, 
and at the same time joined with others to produce the structure 
and actions of an organism of higher order. The cells of each 
tissue were like one another and the diflferences between tissues 
depended upon the characters of the cells constituting each. As 
the knowledge of various animal and plant tissue was increased, this 
theory was extended and amplified and remained for fifty years 
the best expression of our knowledge. In 1891 the cell theory was 
stated in a special form as it applied to the nervous system. This 
statement of the cell theory of the nervous system has since been 
known as the neurone theory of Waldeyer, who formulated it. The 
theory may be stated as follows. The nervous system consists 
of cells each of which (i) arises from a single embryonic cell, 
processes of which grow out to form the neurite and dendrites; (2) 
remains as an independent cell in adult life, making connections 
with other cells only by contact of its processes with the processes 
or cell-bodies of other cells. The constituent elements of the 
nervous system were called by Waldeyer neurones. As the result 
of later discussion of this theory two further points have been 
added to it; (3) the structural and functional polarity of the neurone; 
and (4) that all parts of the neurone constitute a trophic unit for 
whose continued metabolic activity the presence of the nucleus 
is necessary. When the neurone is cut in two in any way only 
that part which retains the nucleus is capable of long continued 
functional existence. 



86 



NERVOUS SYSTEM OF VERTEBRATES. 



Even before this theorj' had been expressed the cell theory 
itself was being severely criticized and it may be said that since 
the year 1890 it has undergone important modifications. Although 
we may still speak of many-celled animals and may regard cells 
in a general way as units of structure, we can no longer consider 




Fig. 42. — A fMirtion of the subepithelial nervous plexus in the palate of the frog. 
From Prentiss. 

cells as unit organisms which join in the formation of an organism 
of a higher order. Cells are not always completely bounded by 
cell walls and in many cases adjacent cells are directly continuous 
with one another by means of strands of protoplasm. Moreover, 



NERVE ELEMENTS AND THEIR FUNCTIONS. 87 

in the course of embryonic development the number, form, posi- 
tion and multiplication of cells do not determine the course of 
differentiation. The cells are not controlling organisms which 
possess the iniative and directive power in these processes, but 
are perhaps only mass divisions made necessary by the metabolic 
relations of the nucleus and cytoplasm. The course of develop- 
mental differentiation is determined by a more fundamental 
organization which exists in the whole protoplasm of the organism 
before its division into cells, and by the interactions between the 
organism and its environment during development. The cells 
are rather the plastic material of differentiation. As the processes 
of differentiation complete themselves, however, cells and groups 
of cells become the means or organs for the performance of certain 




Fig. 43. — A portion of the network about the walls of a small vessel in the 
palate of the frog. From Prentiss. 

functions. Organs are cell-complexes and the function performed 
by an organ is the mass result of the functioning of its constituent 
cells. Thus it may be stated as a general rule that each mass of 
protoplasm containing a nucleus is essentially a structural and 
fimctional unit of the organ. It is the nucleated mass of protoplasm 
which maintains itself as a structural entity although it may be 
connected by plasm strands with its neighbors, and which per- 
forms its specific part of the function of the organ. 

These considerations apply to the nerve cell as well. It has 
been clearly shown that in many cases ner\'e cells are in continuity 
with one another. In the case of some peripheral plexuses (Figs. 



88 NERVOUS SYSTEM OF VERTEBRATES. 

42, 43), dendrites are fused with dendrites into an intricate net- 
work. In other cases neurites are fused with the protoplasm 
of other nerve cells instead of merely ending in contact with 
them. That part of the neurone theory which states that nerve 
cells make connections with one another only by contact is defi- 




Fig. 44. — Two ganglion cells of the nervous ncti*x»rk in the iotesdna] wall of the 
leech, Pontobd^Uii, showing neurofibrillae passing through the cdU. From Bethe 
after Apathy. 

nitely disproved. Other criticisms which ha\*e been brou^t 
against the neurone theory are rather in the nature of extensions 
and corrections of the conception of neurones. 

The relationships of the neurofibrillae have been represented 
as antagonistic to the neurone theory, .\lthough the existence 



NERVE ELEMENTS AND THEIR FUNCTIONS. 89 

of fibrillae in the protoplasm of nerve cells has been known for 
nearly fifty years, it is only in the last few years that they have 
been extensively studied and described. They have been found 
in many classes of animals and in many kinds of cells in the 
nervous system of invertebrates and vertebrates, so that their 
existence as structures characteristic of nervous tissue is quite 
certain. Further, it has been clearly shown that when two nerve 
cells are connected by strands of protoplasm the neurofibrillae 
may extend from one cell into the other. Neurofibrillae have been 
described as running through two, three or more cells without 
interruption. With regard to the origin, structural character and 
function of these fibrillae, further study is needed. It is said that 
they are formed outside of nerve cells and grow into them, that 
they may extend beyond the Umits of the protoplasm of nerve cells 
as in nerve-muscle endings, and that they are the medium of 
conduction of nerve impulses, while the nerve cells perform only 
the incidental function of nutrition. Indeed the neurofibrillae 
are regarded by some as quite new structures in the nervous 
system in addition to nerve cells; — structures which are more 
intimately and essentially concerned in specific nervous functions 
than arc the nerve cells themselves. 

Certainly in the present state of knowledge this view is extreme. 
It seems more reasonable to regard the neurofibrillae as a dense 
portion of the colloid substances in the cytoplasm of nerve cells, 
whose definite form and arrangement are conditioned upon the 
intimate structure of the protoplasm as a whole. Thus if the 
protoplasm has the structure of a foam, then the colloid substances 
forming the walls of the vesicles and filling the solid angles, in 
elongated strands such as dendrites and neurites would certainly 
appear as threads. In more rounded cell-bodies the colloid sub- 
stance would be more irregularly arranged and there would be 
the appearance of branching and crossing of threads, as is actually 
the case with the neurofibrillae. When the dendrites of two cells 
fuse together the neurofibrillae would of course continue from 
one cell into the other. In the most minute end branches of 
neurites the plasm may be so slight in amount that in prepa- 
rations in which the colloid substance alone is sharply stained, 



90 NERVOUS SYSTEM OF VERTEBRATES. 

the slender branches would appear to be cmnposed of a neuro- 
fibrilla alone. FinaUr, in ner\'e-mi]scle endings it is conceivable 
that the neuroplasm and sarcc^dasm should be fused tog^er 
and that the neurofibrillae should continue into the sarcoplasm. 
This is at present the most piobaUe int^iNnetation oi the appear- 
ances presented by the neurofibrillae. WTiether the fibrallae con- 
stitute a special conducting substance in the nerve cell is doubtful 
It is scarcdy conceivable that any one substance in the nerve ceD 
can originate and conduct a nen*e impulse without interacticHi 
with other substances in the ceU. However, if the nerve impulse 
and its transmission are phenomena in the pioductioii of which 
the \-arious substances in the cdl cooperate, then the neurofibrillae 
by reascm of their density and their staining properties may 
reasonably claim our anentk» as indicates of the course taken 
by impulses. 

The process of regenerati<xi of injured nerves^ altfaou^ it may 
not ccandde with the processes of normal growth of nerve dements, 
is instructix-e as to the nature of the neurone. The course of 
m^eneraiion of a cut nerv*e seems to be as follows. The part 
distal to the cut, i.e. away from the cell-body, degenerates in agree- 
ment with the ohserv-adoos of Waller. The fibers proximal to 
the cut remain in a healthy oxiditioQ and grow along die line of 
the des^?ne^i:ed distal rviit. Whether the growth of die fibers 
is suScien: to replace the whole kiwith of the pmit cut away has 
zsx been po«sid>i'rfy demecscratevi bu: » presumed in the absence 
of e%-3Crj::>.-r :o the cvTcnary. WSm the pctoimal ends are not 
pr^v^nrec froirr. srcwtsi: down jilcca: the de y n e i ateJ nerve the 
-ikt-i e\:ic<:oc> of :hc jl\£s s::.^±Ki<r^ nray be traced tor a con- 
>:ctr^'rli ii>:jLn:x\ Vh^ i;:^'r.vri::oc of :be cSssal pan of the 
i.xi5 r-liniirv^ IsLiis :o ciuiii^ i^: the sheadBSw Tbe myelin is 
rri'^^rSi'i j^i th-: >r.<i:h oeC>. . t jc:-< .^: :Sff::t. K»cin strazKis of 
T.i^r: -r^.r i.:\ :; r< -x^cir-.'.iv: j.> :~ •>-: r^rire oc embryonic 
— "^^^ '•^"'" --^ vr:\ir.Ml ;ro r: *>: -:— - i> tied to one side 
1 " ~ ^— ' '. ^r. . , „. 1 ,.-*^ :-j -^^-.^ ^s^^ ^- j^ said 

r-i:::^ :;-^: :-: i^'r-^•;-■: >:-vr/> ;.: .i-:.- T^^urocbrtHae and 
reo:-t irL: :: :':cm>:: — v,:>o^ :- . • . : ^^i-r^ils the strands 
:■: "•:: i:^ _:::-, :-_> \%.-.x- .l-*o - - .^ o;r;-oc:5oc: TKith the 



NERVE ELEMENTS AND THEIR FUNCTIONS. 9 1 

central end of the nerve is necessary; without it the strand formed 
from sheath cells degenerates. The conditions of the experiments 
of Bethe are not such as to show what takes place in the course of 
normal regeneration. It is not shown that the strands derived 
from sheath cells form part of the nerve when the proximal stump 
is imdisturbed. The later researches of Cajal are opposed to this 
"autogenetic regeneration" of Bethe and give positive evidence 
that the nerve is regenerated by outgrowth of the proximal stumps 
of the cut fibers. 

For the continued performance of its normal functions all 
parts of the neurone are necessary. The fact that a neurone may 
continue to function for some days after the cell-body containing 
the nucleus has been cut away shows that impulses follow the short- 
est path from dendrites to neurite. The further fact that the 
dendrites and neurite die after a few days proves the trophic unity 
of the neurone. The Wallerian degeneration is evidence of the 
same. It appears also that injury to one of the processes of the 
neurone may start destructive changes which the neurone is unable 
to combat and the whole neurone may degenerate as the result 
of the cutting of its neurite (Gudden's degeneration). It is quite 
unnecessary to think that all parts of the neurone must enter at 
once into the chemical and physical changes which constitute 
any single act of the neurone. The notion that the cell-body 
acts as a ganglionic center with reference to its dendrites and 
neurite is no part of the neurone theory. Facts opposed to such 
a conception were known long before that theory was formulated, 
namely the unipolar form of spinal ganglion cells and the origin 
of neurites from the dendrites of many cells in the brain. 

The neurone theory as an expression of our general ideas 
regarding the structure and functioning of the nervous system 
may be tentatively re-stated as follows. 

1. The nervous system is formed of cells each of which is 
derived from a single cell in the course of development. From 
each cell grow out one or more processes which are comparable 
in a general way to the pseudopodia of unicellular animals. Such 
cells may be called neurones. 

2. Although in their primitive condition, as commonly in 



94 NERVOUS SYSTEM OF VERTEBRATES. 

clinical study of the degeneration and regeneration in peripheral nerve fibers 
after severance of their connection with the nerve centers. Jour, of Physiol., 
Vol. 13, 14, 1892, 1893. 

Langley, J. N.: Note on the experimental junction of the Vagus nerve 
with the cells of the superior cer\'ical ganglion. Proc, Roy. Soc., Vol. 62. 1897. 

I^angley, J. N., and Anderson, H. K.: Observations on the Regeneration 
of Nerve Fibers. Jour, of Physiol., Vol. 29. 1902. 

Lenhossek, M.- Frage nach der Entwickelung der peripberischcn Nerv- 
enfasem. Anat. Auz., Bd. 28, 1906. 

Neal H. V.: The Development of the Ventral Nerves in Selachii. Mark 
Anniversary Volume. 1903. 

Nissl, Fr.: Ueber die Sogenannten Granula der Nervenzellen. Neurol. 
Centralb., Bd. 13. 1894. 

Nissl, Fr.: Die Neuronlehre und ihre Anh^ger. Jena, i^^ 

Perroncito, A.: La rigenerazione della fibre nervosc. Bollet. de Soc. 
Medico-Chirurg. di Pa via. 1905. 

Prentiss, C. W. : The Ner\'ous Structures in the Palate of the Frog; the 
Peripheral Networks and the Nature of their Cells and Fibers. Jour. Comp. 
Neur. and Psych.. Vol. 14. 1904. 

Sedgwick, A.: On the Inadequacy of the Cellular Theory of Develop- 
ment, and on the early Development of Nerves, etc. Quart. Jour. Micr. Sci., 
Vol. 37. 1895. 

Verwom, M.: Das Neuron in Anatomic und Physiologie, Jena. 1900. 

Waldeyer, W.: Ueber einige neuere Forschungen im Gebiete der 
Anatomic des Centralnen-ensystems. Deutch. med. Wochenschr., 1891. 

Waller, A.: Sur la reproduction des nerfs et les fonctions des ganglions 
spinaux. Miiller's Archiv, 1852. Also articles in Comptes Rendus, Tome 
34. 1852. 

Whitman, C. O.: The Inadequacy of the Cell Theory of Development. 
Jour. Morph., Vol. 8. 1803. 



FUNCTIONAL DIVISIONS OF THE NERVOUS SYSTEM. 95 



CHAPTER V. 

THE FUNCTIONAL DIVISIONS OF THE NERVOUS 

SYSTEM. 

The nen-ous system can be understood only by studying it 
from the standpoint of the work which it does. It must be insisted 
that a knowledge of mere structure is of little value and may be 
misleading without a knowledge of function. For this reason 
in speaking of the nervous system the term mechanism is preferable 
to architecture; for mechanism implies work as well as structure. 
It implies structure in action. The work of the ner\-ous system is 
-to adapt the activities of the animal to the conditions of its life 
and of the perpetuation of the species. As the necessary adap- 
tations and correlations are successfully carried out the organ- 
ization of the nervous system is perfected through experience. In 
all vertebrates the same general plan of organization is seen; 
the degree of organization is a measure of adaptation and is 
correlated with the sur\dval of the best adapted. The higher 
animals are adapted to more complex and changing conditions 
of life and have more highly organized nervous systems. 

We may distinguish two main groups of activities in the verte- 
brate organism which have determined the general plan of organ- 
ization of the nervous system: actions in relation to the external 
world, and internal activities having to do with the processes of 
nutrition and reproduction. The actions toward the external 
world consist of the finding and capturing of food, fighting 
with other animals, preparing nests or homes for protection 
against the physical elements, and many minor reactions to 
changes in heat, light, moisture, etc. The internal actinties include 
all the processes related to metabolism, the distribution of nutritive 
material to various parts of the organism, and the various pro- 
cesses connected with the formation of the reproductive elements 
and the nutrition of the embrvo. 



96 NERVOUS SYSTEM OF VERTEBRATES. 

With regard to each of these groups of activities the work of 
the ncn'ous system is two-fold. On the one- hand it receives 
stimuli from the external world or the internal organs; on the 
other hand it directs the responses to those stimuli. All actions are 
in response to stimuli; directly or indirecdy. The stimuli usually 
aflFect the organs only through the nervous system. It is noticed 
that certain characteristic responses habituallyjollow upon certain 
stimuli, either as the result of heredity or of individual experience. 
For example, a frog will jump to catch an object which moves in 
a manner characteristic of its usual food. So a hellbender will 
pay no attention to a living earthworm so long as it is quiet, even 
although the worm be Ijdng on the animal's nose. The moment 
the worm wriggles vigorously it is snapped up. This- is because 
these animals habitually judge their food by its movements. 
In other animals the reactions necessary for capturing food are 
called forth by stimuli received through the organs of smell or- 
taste. In any case the reactions toward the external world are 
in the nature of reflex acts, simple or complex. As far as the 
ner\'Ous processes are concerned the internal reactions are of the 
same character, although their reflex nature is not so readily seen 
as in the case of external activities. 

\Yith regard to the stimuli, then, those which aflFect the bodily 
N\*elfaro of the animal in its surroundings must be distinguished 
fn>m those which affect the internal actinties. A morphological 
distinction has long boon made between the soma and the z^iscera. 
The >oma comprises those org-ans skin, muscle, skeleton, etc.) 
by which the animal deals with its en\-ironment. The \-iscera 
cv^nipriso those orvrans alimentar\" canal and appendages, cir- 
culator}, oxcr<^tor>* and r<".^!\v?;.ctivo apparatus^ concerned with 
the vrwi^<t^ of nutaly^Hsrri by which the organism is built up 
and the r\:^r^vtuvt:vo c*c:v.cr.:> arv :orr.ievi. A pari of the soma, 
the >kir., vvr.u-^ tr.to vvntavt with the external world and offers 

h! rvach the nervxms system. The 
les ar.o >kele:or. n^sponds to these 
r-xr.ts ur.:;T the direction of the 
".vT-a. r.i— . . rSr> and the central 
L^ w:th >f>v,.:i anecixi: the welfare 



tr.e r.uv.:\:r.: :n- wntvh 


>ti:v. 


iTr.Mir.vter of the s*.vv.a 


:tt,:> 


stttt^uli : \ at vroi'tate 


tr.vn 


ne^^^^,^ >%>t;:!^, Th. 


tvty 


ntiv:h,ir:i>r:':> which ha\ 


.- :^x 



FUNCTIONAL DIVISIONS OF THE NERVOUS SYSTEM. 



97 



of the animal in its surroundings are arranged on a common plan 
in all segments of the body in all vertebrates, and constitute a 
distinct portion of the nervous system both structurally and func- 
tionally. These structures are best called collectively the somatic 
afferent division of the nervous system. 

The stimuli having reference to food and the visceral activities 
include those arising in the viscera and those produced by chem- 
ical changes in the surrounding medium. Chemical changes 
in the surroundings do not stimulate the somatic sense organs. 
The special organs of the sense of taste, which in fishes lie in 



R, dors 




R. ven 



Sympathetic 



Fig. 45. — ^A diagram of the component elements in the spinal cord and the nerve 
roots in a trunk segment, to illustrate the four functional divisions of the nervous 
83rstem. s, j., somatic sensory; v. s., visceral sensory; v. m , visceral motor; s, m., 
somatic motor. 

the skin as well as in the branchial and mouth cavities, and the 
olfactory organs are affected by chemical stimuli and are used 
in finding food. All the nervous structures concerned with 
impulses arising in the viscera, in the taste organs and in the 
olfactory organ are closely related and constitute the visceral 
afferent division of the nervous system. 

The movements of the soma are aroused chiefly by somatic 
afferent impulses and have to do with the relation of the animal 
to its sunoimdings. Thus, all the usual movements of locomotion, 
of offense and defense, and so forth, are directed ordinarily in 
response to stimuli from without. Somatic movements are also 
7 



98 



NERVOUS SYSTEM OF VERTEBRATES. 




oc tbf functions 

fsrskulatum. 
>>^. -^.-s tT»i i:xTcSal< Air ex- 



FUNCTIONAL DIVISIONS OF THE NERVOUS SYSTEM. 



99 



performed in respoQse to gustatory and olfactory stimuli and have 
for their object the capturing of food. It is shown by experi- 




FiG. 47. — A diagram to show the arrangement of the two afferent divisions in 
the brain of man. Compare with Fig. 46. A cutaneous mesencephalic root of 
V is hypothetical in man. 

ments on fishes and by clinical observations on man that vcn- 
strong gustatory stimuli can be localized without the help of 



lOO NERVOUS SYSTEM OF VERTEBRATES. 

tactile stimuli, but the responses given by fishes to such stimuli 
are less constant and precise than the reactions to combined gusta- 
tory and tactile stimulation. Usually the localization of objects 
detected by the gustatory or olfactory organs requires concurrent 
somatic stimuli. This is easily illustrated in our own experience. 
We can not tell with certainty the location of an object which we 
smell. If a wind brings the odor to us we can tell the direction 
of the object, but not its distance. The direction is known only 
by the pressure (somatic) stimuli due to the wind. If we are 
blindfolded in a room and wish to find the position of a boquet 
of flowers which we detect by its odor we must go about and sniff 
until we find the place where the odor is strongest. In general, 
while somatic movements may be called forth by visceral stimuli 
they are more tj-pically called forth by somatic stimuli and are more 
precise when they are directed in response to somatic stimuli. The 
nene centers and peripheral ner\-es which direct somatic move- 
ments constitute a distinct pordon of the nen-ous system called 
the soma:k e^erenf ^^motor) dmsion. 

The visceral aciiuties consist of contractions of visceral muscles, 
secnMon* pavescscs, \-aso-motor regubtion, etc. These all con- 
tribute dirvctlv or indirectly to the processes of nutrition in the 
wioos: sense, v>r c^f nfpn>ducdan. These activities are aroused 
ohicSy by xisoerJ Affensaa: Lr.pulses* including gustatory and 
oifAvTon in^pxUscs. How far they nny be called forth by somatic 
s;:r.v.,l: is no: tully vj:iv:er>:cv\i. 1: is a taa of common experience 
:h,i: :ho vvr.vxv: v^f :\v\: irv^usod by scii: or tactile perception 
r.\i\ v\;;.x' N.;l:\An s<vrc:k\r.. Vjio-rx^ror n?^e:ulaxiQQ is in large 
;.v:': :>.v t\>;,I; o: h;M: Ar>.i v\\v: s:™u^ rn ±!e siin. Pospiration 
:> :.;::s\: :. -:h Vx rise in :or: :vri:urc ^irun ie body. The nerve 
A. ,. >v -V. ,v-- Nx:^:.^ .vr.:rA .:>orrju icdviaes constitute 

, * .", ;, .*: ,v.M -•..,"'-, ," A'^.ir.^jLl :;-_? iir>is of aerrous actin- 

.^ . V .,-.\v / ^. v.\/ vr ,*: >vr.-.*5v sdai-Ji: ^,2) the 

i \\ A^ ,^ V.-. , v V N X : :s^ :--TQ« of \isceral 

^ ^" "* ^ "^^^ '-** ^* >v\ '. .'.-; :::i^ Corresponding 

'^ ' ^^^ ' V, X ,- . , .^ ^. \ ^ V -/ , - j:,r>i.r^>~wnrk distinct 

,^- ^v.N .-. N^ ., ,^ V x»^, -^ >^^^v ^- i.-\>.r;:r:: aad ccei^nt, 



FUNCTIONAL DIVISIONS OF THE NERVOUS SYSTEM. I OX 

visceral afferent and efferent divisions (Fig. 45). With exceptions 
to be noted in their proper places, the four kinds of activities ai*e 
called for in all segments of the body, and consequently each of 
the functional divisions is represented in each segment of the 
body and all the segments of a given division are serially homolo- 
gous with one another. These longitudinal divisions of the nervous 
system are therefore the most fimdamental and important divisions 
both structurally and functionally. The segmentation of the nervous 
system is to be regarded as a segmentation of each of the functional 
divisions. It is probable that the functional divisions of the 
nervous system are more fundamental than the metamerism of 
the body. 

One point of contrast should be noted between the two somatic 
divisions on the one hand and the two visceral divisions on the 
other. Although somatic afferent impulses may produce somatic 
reflexes directly without sensation, very commonly sensations 
are produced. When present the sensations are definitely local- 
ized and the responses may be consciously directed. Visceral 
afferent impulses, on the other hand, usually produce reflexes 
without sensation. When present the sensations are vague, 
general, poorly localized, and consciously directed visceral activities 
are very exceptional if not abnormal. The somatic activities 
are par excellence related to the conscious life. 

There is given here for reference a table of the four functional 
divisions with the structures included in each. In addition to 
the structures included in this outline there are certain brain 
centers which with their fiber tracts serve functions of correlation 
between the four primary divisions. These will be treated in 
later chapters (Chap. XIV and following). 

A. Somatic sensory division. 

I. Genend cutaneous subdivision. Consists of: 
free nerve endings in the skin, 
general cutaneous system of components, 

dorsal tracts of the cord, spinal V tract in the medulla oblongata, 

together with their accompanying nuclei : the dorsal horn, nucleus 

funiculi, nucleus spinalis trigemini, acusticum and cerebellum, 

secondary tracts and centers: internal and external arcuate fibers 



I02 ; \ : • NERVOUS SYSTEM OF VERTEBRATES. 

» • • 

, *••. forming the tractus spino- and bulbo-tectalis (lemniscus), tec- 

,\ • turn mesencephab\ and other nuclei, 

tertiary tracts to motor nuclei and coordinating centers. 

2. Special cutaneous subdivision. Consists of: 

neuromasts (acustico-lateral sense organs), 
neuromast components, 

spinal Vni tract and nucleus, nucleus funiculi, acusticum, cere- 
bellum, secondary tracts and centers and tertiary tracts as in i, 
the cochlea, its nerve and centers in higher vertebrates. 

3. Special sense organs belonging to the somatic sensory division. 

Lateral eyes. Consist of: 

retina, which includes the equivalent of sensory ganglion, nerve 

component, and primary brain center, 
optic tract and tectum opticum corresponding to the secondary 

tracts and centers of the cutaneous subdivision. 
Pineal eyes. (Compare Chap. VIII below.) 

B. Visceral sensory division. 

1. General visceral subdivision. Consists of: 

free nerve endings in the mucosae, 

fasciculus communis system of components, 

Clarke's column or its equivalent, nucleus commissuralis Cajal. 

lobus vagi, and lobus facialis, 
secondary visceral tract and its continuation in the cord,==the 

direct cerebellar tract in higher forms, 
secondary \'isceral nucleus,=:end nucleus of direct cerebellar tract 

in the vermis of higher forms, 
tertiary tracts to the hypothalamus in lower vertebrates. 

2. Special visceral subdivision. Consists of: 

end buds (taste buds) 

components and central nuclei and tracts not yet distinj;;uished 
from those of the general visceral subdivision. 

3. Special sense organ belonging to the v-isceral sensory division. 

Consists of: olfactory epithelium and nerve, bulbus olfactorius, 
tractus olfactorius, area olfactoria, and tertiary tracts to coord- 
inating centers in the diencephalon. 

4. Sympathetic system, afferent portion. An outgrowth or offshoot 

from visceral sensorj' ganglia which reaches a high specializa 
tion in the vertebrate series. Consists of simple visceral 
sensory component fibers and of ganglion cells which together 
with the efferent ix)rtion control contraction of smooth muscles 
and glandular secretion. 

C. Somatic motor division. Consists of: 

ventral horn of the cord, nuclei of origion of Nn. XII, VI, R", III, 
and nucleus of somatic motor fasciculus. 



FUNCTIONAL DIVISIONS OF THE NERVOUS SYSTEM. IO3 

motor components in ventral roots supplying musculature derived 
from the somites. 
D Visceral motor division. Consists of: 

1. Motor nuclei in lateral horn or intermediate zone of the cord and 

corresponding i:egion of the medulla oblongata, 
motor component in dorsal and ventral roots supplying muscula- 
ture derived from lateral mesoderm. 

2. S3rmpathetic system, efferent portion. Consists of ganglion cells 

and fibers concerned with glandular secretion, and the con- 
tractions of smooth muscle. They receive impulses from 
visceral efferent fibers. 

To illustrate the positions of the four divisions in the brain of 
vertebrates two diagrams are given, Figures 46 and 47. These 
show by conventional symbols the areas occupied by the functional 
divisions in the brain of a fish and of man. In these figures as 
in others the shading by right lines, either vertical or horizontal, 
indicates somatic sensory areas; the obUque lines indicate visceral 
sensory areas. The figures should be compared with Figs. 2, 
3, II, and with figures of the human brain in a text-book of 
anatomy. 

DEMONSTRATION OR LABORATORY WORK. 

1. Dissect the cranial nerves of a fish, a frog and a mammal with 
especial reference to the cutaneous and visceral rami. 

2. Dissect the brain of a large dogfish or skate and of a large 
Ameiurus, or other bony fish which has large vagal or facial lobes. 
Examine the form relations of the functional divisions in the medulla 
oblongata, and of the cerebellum, tectum mesencephali, inferior lobes 
and olfactory lobes and bulbs by means of hemisections and dissections 
of the brain and by sections under low power of the microscope. 

3. Compare the brain of some mammal with those of the fishes 
with regard to each of these points. 

The importance of dissections of the brain cannot be over-estimated. 
Very satisfactory results in demonstrations can be obtained by no more 
elaborate means than dissections and well prepared sections stained by 
Delafield's haematoxylin. 

LITERATURE. 

Allis, E. P., jr.: The Anatomy and Development of the Lateral Line 
System in Amia calva. Jour. Morph., Vol. 2. 1889. 

Cole, F. J.: On the Cranial Nerves of Chimaera monstrosa Linn., etc. 
Trans. Roy. Soc. Edinb., Vol. 38. 1896. 



104 NERVOUS SYSTEM OF VERTEBRATES. 

Ewart, J. C: On the Cranial Nerves of Elasmobranch Fishes, Proc. 
Roy. Soc., Vol. 45. 1889. 

Ewart, J. C: The Lateral Sense Organs of Elasmobranchs. I. The 
Sensory Canals of Laemargus. Trans. Roy. Soc. Edinb., Vol. 37. 1893. 

Gaskell, W. H.: On the Structure, Distribution and Function of the 
Ner\*es which innervate the Viscera and Vascular Systems. Jour, of Physiol, 
Vol. 7. 1886. 

Herrick, C. J.: The Cranial and First Spinal Nerves of Menidia; a Con- 
tribution, etc., Jour. Comp. Neur., Vol. 9. 1899. 

Herrick, C. J.: The Doctrine of Nerve Components and some of its 
Applications. Jour. Comp. Neur., Vol. 14, 1904. 

Johnston, J. B.: An Attempt to DdBne the Primitive Functional 
Divisions of the Central Nervous System. Jour. Comp. Neur., Vol. 12. 1902. 

Johnstiui, J. B.: Das Gehim und die Craniahierven der Ananmier. 
Mcrkel u. Bonnet*s Ergebnisse. Bd. 11. 1902. Bibliography. 

Ji^nston, J, B.: The Morphology of the Veitebrate Head horn the View- 
point of the Functional Divisions of the Xenrous System. Jo«ir. Comp. Neur. 
and Psxvh. Vol. 15. 1005. 

l.>$K\m, H. F.: A Contributico to the Internal Stracture of the Amphib- 
ian Hntin, Jour Morph,. Vol. 2. iSSS. 

$li\>ng, O. ;^: The Cranial Nerves of Amphibia. A Contzibiition. etc 
Jour Mor|>h.. Vol. i<x 1S05. 



SOMATIC AFFERENT DIVISION. lOS 



CHAPTER VI. 

SOMATIC AFFERENT DIVISION. GENERAL 
CUTANEOUS SUBDIVISION. 

In the skin of all vertebrates certain fibers belonging to the 
dorsal nerves end by free branches between the cells of the epider- 
mis (Fig. 48). These are the fibers of the sense of touch. In 
the trunk these fibers form the largest component of the dorsal 
spinal nerves and are distributed by way of both dorsal and 
ventral rami.. The rami reach the skin in the myosepta and may 
be distributed forward or backward from the myoseptum in which 




T^ 



Fig. 48. — General cutaneous endings in the cyclostome Lampetra Wilderi, 6 is 
taken from the border of a neuromast pit. 

they run, or both forward and backward, i.e. to the two adjacent 
segments. A diagram to show the central relations of the general 
cutaneous components is given in Fig. 49. 

Passing forward into the head we find a region between the 
permanent first spinal nerve and the vagus in which general 
cutaneous nerves are apparently absent. In cyclostomes, however, 
the full number of cutaneous nerves are present, and the same is 
apparently true in the primitive selachian Heptanchus, as shown 
in Figure 2. In the embryo of other selachians there is one 
vestigeal ganglion for each segment, which probably represents 
a cutaneous nerve. Even in mammalian embryos, as in the pig 
(Fig. 20) and in man (Fig. 32), rudimentary ganglia which may 



io6 



NERVOUS SYSTEM OF VERTEBRATES. 



represent the cutaneous roots of this region accompany the spinal 
accessory roots. These remain in the adult as a few scattered 
clumps of ganglion cells among the vagus roots. The reason 
for the failure of these nerves to develop has been suggested in a 
previous chapter (p. 65). Since a certain number of somites in 
this region have disappeared and the trunk somites have shifted 
forward, it is probable that the cutaneous area has been lessened 
and one or more nerves have disappeared in most vertebrates. 
The arrangement of the cutaneous components in the cranial 
nenes varies in different vertebrates. In cyclostomes such fibers 
form large components of the X, IX and VII roots. In selachians, 
ganoids, bony fishes and amphibia general cutaneous components 
are found in the X and IX roots, but rarely (KingsburjO in the 



R. dor: 




arcuate fibers 



F'v.. 40 - -A v::a»:rar.ir.ia::c n'prv>or.:dt:on v^f the general aitaneous components 

\'ll n.x^:. In mammals and man a general cutaneous component 
in :ho \ none has i:s cells of origia in iho jugular ganglion and 
foTT-.^s the ra:r.v.> aurioiilaris. A r.iviinicntan- root ganglion in the 
IX r.crvo vrv^baMy ro:^r<^on:> :h:> oor.::x^ncn: Figs, 2cx 30, 31, ^2). 
Ir. .•.!! vtr:ibra:c^:hc >er.>^>r) :rl5:vr.v.r.u> and the ophthalmicus 
rrv^:::r..:;:> r.c7\c> .-tx' forr^AV. vhicr.y or cxclusi\-ely of general 
c;::,ir.cov.> :V:-i:^. F:r.,il.>. :>..i: >:r..il'. ncr.e which is foimd con- 
r.c\:c\i 'A::h :h;^ :orvbr,i:r. :r. :r.ar.y <c*,i/r.:ar.>. Amia and Pro- 
:o::c-:>, :hi t:-::./ ,/".:':.:;./. i< vTvr^V'y general cutaneous 



r.v\ ;:> .vr.v.xvr.cn:5 is rela- 



GENERAL CUTANEOUS SUBDIVISION. 



107 



tively simple. The axrangement is shown in a generalized scheme 
in Figure 50. In cyclostomes (Fig. 51) the dorsal spinal nerves 
in the branchial region send dorsal rami to the skin of the back 
and their ventral rami join the epibranchial trunk. From this 
the cutaneous fibers are distributed by the posttrematic rami to 
the skin of the lateral and ventral surface of the gill region. The 
cutaneous component in the vagus root goes to the first post- 
trematic ramus of the vagus. In true fishes and all higher forms 
the cutaneous component of the X root is distributed by dorsal 
rami to the dorsal surface of the head. In fishes provided with 
an operculum general cutaneous fibers are not found in the bran- 
chial rami, but a ramus from X helps to supply the skin on the 
operculum. The cutaneous components in the branchial nerves 



N. ophth 
N. terminali 




Eye 



opercularis 
R mand. 



N. spin- dors. 



Fig. 50. — A simple diagram of the general cutaneous components in the cranial 
and spinal nerves of a fish. The dotted lines (except R. opercularis) represent 
components present in cyclostomes but not yet found in other fishes. 



of selachians have not been described. The cutaneous component 
in the IX nerve in cyclostomes goes by way of the ramus post- 
trematicus to the skin of the region of the first branchial arch. 
In other vertebrates only a dorsal ramus is present. 

The absence of a. general cutaneous component from the VII 
nerve in most vertebrates requires to be explained. In forms 
provided with an operculum, the operculum is innervated by rami 
from the V and X nerves and rami from the V nen e supply the ven- 
tral surface in the gill region. In cyclostomes, where there is no 
operculum, a simpler and more primitive arrangement is found. 




Fig. $1. — A reconstruction of the cranial ncnes of a c>'clostome fish^ Peiromyt&n 
dorsaius, Xo show the arrangement and distribution of the several systems of nerve 
components. Sym., sympathetic trunk; a g. t-, secondjgill cleft- 



MO NERVOUS SYSTEM OF VERTEBRATES. 

by rami from the V nerve. From this it would appear that the 
hyoid segment had originally its own cutaneous innervation and 
that when the operculum was formed the component in VII 
(lisapi)eared and the V nen'^e supplied the parts of the hyoid 
segment remaining exposed. 

The trigeminus gives small branches to the dorsal surface of 
the head behind the eye, and in fishes it gives a larger ramusophthal- 
micus superfacialis trigemini to the skin above the eye. It then 
forms the rami maxillaris and mandibularis which supply the 
skin of the upper and lower jaws and the lining of the stomodaeum. 

The ophthalmicus profundus nerve supplies the area in front 
of the eye to the tip of the snout. In tailed amphibians the 
niiixilhirj* ner\'e is greatly reduced and the profundus takes on 
the innervation of the territor}- of the maxillaris. 

Thk ckntral apparatus for cutaneous impulses. — ^This 
a>nsisls of the dorsal horn of the gray matter in the spinal cord 
and si\x>ndar}- tracts and centers connected with it, and of corre- 
s|H>nding structures in the brain. The cutaneous fibers have their 
gatiglion vvlls in the spinal ganglia. The central processes of 
those iranglion colls enter the dorsal part of the cord and there 
hiturvato in T or Y form. The twx) branches run one cephalad 
and \M\o oaudad* forming the dorsal tracts of the cord. In man 
it is known that the ivph;ilic bnuich is the Icmger and this seems 
t\^ Iv tho vaso in low or wncbratos as well. In man and TnAmmaJf; 
tho \\ :>haUo branchi^ of r.bcrs in iho nwrt^ caudal roots are pushed 
tvn\,rA? ;ho mi\li,\n :\L\no by :ho inv\>ming fibers of the successive 
wviN t\;,;V.ov ton\;\rvi. :\^ th^: :ho knig cx^phalic branches come to 
tov,v. ,; x*v;,r,i:o Vur.vilo :hc wtryi^* -ir««i-ii/iur » meaal to the 
;i'v,v o; ;>.x -vn^ xv:^h,/.iv' rA>:s :hc ^^^cJ fmrnicMims), Each 
^^;'.^^ ot .; xV.tArtvV;^ r.K- as :: rjz:> forrnid and back^-ard 
" ^ ' ?;'^'>^ x^' 'Tjc Ox<U:;'^',l b^i^^^h<^ Tliese are distrib- 

V. 'N' / \i ;.- :":. vV .:v>i^r >oif, :.> the voitial horn, 
V ' ^ o, . "^x' svv. i- o.- .7: ir. Fi4??w -ti, 5^. The col- 
\v ^^ ^0 ^\ -, ' ^^T s^r*-; f.v u>f 5i3i?le?t rejJexes. 
,\\ , .X V v-. "'. -"^v-i^v^ -wiSdr through the 
<\'^ V ,^ N, >v. ^^ ^. *-jv: Tvtsjrrie iDore ample 
"' ^ ^ "^ " , .wV:.*^ rriTJcbe^ ead r^allv 



;;: ; 


..\ n\" \: 


, -.A- 


;>, 


^^'^' 


/o >v:n 


%\ \'' 


A*x <SV 


■ '^ 


s ^. 



GENERAL CUTANEOUS SNBDIVISION. 



Ill 



in the dorsal horn. While the collaterals make connections 
which serve for relatively simple and direct reflexes, the connections 
by way of the dorsal horn serve for complex reactions and for 
sensation. 

The structure of the dorsal horn and its secondary connections 
are best understood in the region at the junction of the spinal 
cord and brain where the dorsal tracts of the cord have their 
ending and where a part of the cutaneous fibers of the head also 




Fig. 53. — Some cells in the dorsal horn of the chick embr>'o of five days. From 
Cajal (Textura, etc.). A, dorsal root; B, transverse cell of the substance of Rolando; 
C, cell of the substance of Rolando; D, cell of the posterior horn whose neuritc 
goes to the dorsal commissure; £, interstitial cell whose neurite goes to the dorsal 
commissure; F, dorsal commissure. 

end. In this region the dorsal horn is considerably enlarged in 
all vertebrates and in man it forms the two large nuclei of the dorsal 
funiculi. In lower vertebrates there is a single large nucleus of 
the dorsal funiculus. This nucleus consists of large and small 
nerve cells imbedded in a \ery rich and intricate interlacing of 
their own dendritic branches and of the end-branches of cutaneous 
ner\'e fibers. The small cells have relatively short and simple 
dendrites and send their neurites among the cells of the dorsal 



112 



NERVOUS SYSTEM OF VERTEBRATES. 



horn itself either at the same level or farther forward or backward. 
Such cells are seen in Figs. 53, 56. The function of these cells 
is to spread the incoming impulses more completely to all parts 
of the nucleus. A part of the neurites of these cells pass across 
the median plane dorsal to the canal, forming a part of the dorsal 
commissure of the cord, and end in the dorsal horn of the opposite 
side (Fig. 53). 




Vv. 5^ -- '>A'Vx\v:^isr ^;^^:.. r. .^' :b<^ juhsci^.-^f .-c RcCxsdo in the oerrical cord of 
:>:>- :^»*« :vv. .,;: Vr,r.*. v^jljlI JTev.urA. <^:c .-{. oflls oc the rertcx of the dorsal 
b>orr.. :" v^ x\ > .: :S,- <u^>:Ar.'r .:' Xv-ZJL:^i-: £. ioK» cohljitCTxIs; F, endings of 

r>.c *.i-vv .vU> ^d^^* .Arprr vie^.:n:r5 Ao:h long bxanches which 
>irvM,; :>.rv^;:j:h :>.o v^ors.^ her:: ir.i :he dorsal tracts (Figs. 
5^, 55. 5*' I ";* r.v,.:*.:.^ Cx" \rr.:rj.lly r<yoi:>d Ae doi^ horn, 
r.:>.- .'^. :^;* cr,;> .^: :>.^ v^v:.^ :^^^::;rr: ;\r.i. crc^Siing the median 
*.v.^.;' ^v^::^,^.". :.^ :".^ vvr: v.* vW:.*. r.^rr.-: tv'^i.: ^rrejirs in transverse 



GENERAL CUTANEOUS SUBDIVISION. 



"3 



section as a loop. They are therefore spoken of as arcuate fibers. 
The greater number of them run deeply imbedded in the wall of 
the cord (or brain) and are known as internal arcuate fibers to 
distinguish them from the remainder which have a part of their 
course on the surface and are known as external arcuate fibers. 
The internal arcuate fibers after reaching the opposite side of the 
cord or brain bend forward and help to form the ventrolateral 




Fig. 55. — Cells in the dorsal horn of the cord of a chick embryo of nineteen 
days incubation. From Cajal (Textura, etc.). A, large marginal cell; B, giant cell 
of the center of the horn; C, cell of the interstitial nucleus; a, b, c, neurites. 



fiber tracts. Continuing forward these fibers collect mto a bundle 
which is more definitely limited in higher vertebrates than in fishes. 
In man this bundle is known as the ascending lemniscus. In fishes 
the fibers go to the nuclei in the tectum mesencephali and form 
what is known as the tractus hulho-tectalis. Its relations in 
mammals are more complex and will be treated later (Chap. XVI). 
8 



GENERAL CUTANEOUS SUBDIVISION. 



IIS 



Spinalis trigemini, A small part of the fibers of the trigeminus, 
however, run directly to the cerebellum. The tractus spinalis 
trigemini is accompanied by an end-nucleus which is directly 
continuous with the gelatinous substance of the dorsal horn of 
the cord. This substantia gelatinosa is made up of small and 
large cells. The neurites of the small cells are either short, ending 
in the immediate vicinity, or are directed forward or backward 
within the substantia gelatinosa itself. The neurites of the 
large cells go as internal arcuate fibers to the opposite ascending 
lemniscus or to a part of the lemniscus system on the same side. 
The structure of the substantia gelatinosa of the rabbit is shown 
in Figure 56. 
The primitive condition of the general cutaneous centers as 




Fig. 57. — A transverse section of the spinal cord of a cyclostome fish, Lampetra 
Wilder i to show the cells of the dorsal horn (cf. h.). 



seen in lower vertebrates differs considerably from this. In fishes 
the trigeminus and ophthalmicus profimdus fibers may or may 
not bifurcate on entering the medulla oblongata. When they 
bifurcate one branch goes to the cerebellum and the other either 
into the tractus spinalis trigemini or into the deeper part of the 
tuberculum acusticum. When they do not bifurcate the fibers 
may take any one of these courses. Further, in selachians, 



ii6 



NERVOUS SYSTEM OF VERTEBRATES. 



ganoids and amphibians a bundle of very coarse fibers enters the 
brain yriih the sensory trigeminal root and ends in the tectum 
mesencephali. This may be called the mesencephalic root of 
the trigeminus and corresponds in position to* the bundle which 
in man is usually described as a motor root. Thus in lower verte- 
brates the cutaneous fibers are widely distributed through all 
that most dorsal* column of gray matter which connects the dorsal 
horns of the cord with the cerebellum and the dorsal part of the 
mesencephalon. Indeed, in cydostomes there is no marked 
distinction in structure between the different parts of this column- 




Arcuate fibers 



Arcuate fibers 

Fig. 58. — A, transverse section of the tuberculum acusticum of Lampetra; B, a 
sagittal section of the cerebellum of the same animal, ac.y acusticum. 

The structure and secondar)' connections of these centers in 
cydostomes may now be described as showing the most primitive 
known condition of the general cutaneous centers in the brain 
of vertebrates. The nudeus funiculi, although simple, has 
essentially the structure above described. The tuberculum 
acusticum differs from it in being somewhat more voluminous 
and in possessing a larger number of cells. The neurites of the 



GENERAL CUTANEOUS SUBDIVISION. II 7 

-small cells form a thin layer of fine fibers covering the outer surface 
of the acusticum and cerebelliun. This represents the molecular 
layer and the cell layer corresponds to the granular layer, which 
are such conspicuous features of the cerebellum of higher forms. 
The large cells are larger and more conspicuous and are arranged 
with more regularity, their bodies being near the fourth ventricle 
and their dendrites spreading in the fiber layer. Then neurites 
of these cells go as internal arcuate fibers to join those from the 
nucleus funiculi and nm forward. The large cells of the acusticum 
and of the cerebellum are shown in Fig. 58. The cerebellum 
IS very small and consists of a continuation of the acusticimi for- 
ward, upward and mesially so that the two halves of the cerebellum 
meet over the fourth ventricle. Here there occurs a commissure 
formed by the neurites of the small cells. By means of these 
fibers the cutaneous impulses are carried from the centers of one 
dde to those of the other. In histology also the cerebellum 
differs little from the acusticum. Its large cells are not special- 
ized as are the Purkinje cells in the cerebellum of higher forms, 
although by comparing them T^ith the Purkinje cells in other 
fishes one can see that they are the forerunners of such cells. 
The neurites of the large cells go as internal arcuate fibers to 
the opposite side of the brain, but whether they join those from the 
nucleus funiculi and acusticum is uncertain. The latter fibers 
pass forward and upward to enter the roof of the mesencephalon, 
forming the tractus htUbo-tectalis. 

The roof of the mesencephalon is thus a secondary center for 
general cutaneous impulses. Its structure and the differentia- 
tion of centers in it will be described in the chapters on the visual 
apparatus and on the higher correlation centers (Chaps. VIII 
and XVI). Here it is necessary only to state the course of tracts 
arising in the tectum mesencephali which are of importance for 
the cutaneous apparatus. The first is a tract of fibers which 
arise from the cells of the tectum and nm in a direction opposite 
to, but parallel with that of the tractus bulbo-tectalis and have 
their endings in the medulla oblongata or spinal cord. The tract 
is called the tractus tecto-hulharis et spinalis. It descends from 
the tectum over the outer surface of the mesencephalon and bends 



ii8 



NERVOUS SYSTEM OF VERTEBRATES. 



backward along the ventro-lateral surface of the myelencephalon. 



> 


4^ 




CO 


u 




n 


* 


0^ 


c 


w> 


;.:^ 


(D 




E 


H 


-o 




CO 






Iii;. so. \ tliagram ro|>rt'>i*ntinp ihc centers and fiber tracts related to the 
^encial i\it;uuH»us ioinjH>ncnts in fishes. 



GENERAL CUTANEOUS SUBDIVISION. 



119 



Here the fibers give collaterals inward and themselves turn inward 
to come into relation directly or indirectly with the cells which 
give origin to the motor nerves. A part of the tract, when it descends 
over the lateral face of the mesencephalon, instead of going back- 
ward along the same side of the brain crosses to the opposite side 
through the ventral wall of the mesencephalon, helping to form 
the large ventral commissure of this region of the brain. The 
fibers then join the tract of the opposite side and continue with it 
to similar endings. The tractus tecto-bulbaris et spinalis thus 
consists of a crossed portion and of an uncrossed or direct portion. 
Another tract from the tectum descends over the side of the mesen- 
cephalon and bends forward to end in the inferior lobe, the tractus 
tecto-loharis. By means of tracts which run from the inferior 




Fig. 60. — A diagram representing the general cutaneous centers and fiber tracts 
in the human brain. 

lobes to the forebrain and to the hindbrain and spinal cord, probably 
wider connections are set up and more complex correlations 
provided for. The several central tracts and nuclei belonging 
to the cutaneous apparatus in fishes are shown in the accompany- 
ing diagram (Fig. 59). The outline of a selachian brain is drawm 
schematically and the various tracts are shown as seen from the 
left side and as if projected upon the median plane. The cutaneous 
components m the roots of the V, VII, IX, and X nerv^es are 
shown, but the ganglia of those nerves are omitted. In Fig. 60, 
the cutaneous fibers and the ascending lemniscus in man are shown. 



I20 NERVOUS SYSTEM OF VERTEBRATES. 

In higher vertebrates the acusticum and the cerebdlum become 
more and more higher specialized in their histological structure 
and secondary connections. Since this specialization is largely 
due to the fact that these are the centers for the special cutaneous 
(acustico-lateral) system of nerves, the special structure and the 
course of differentiation will be described in the next chapter. 
At the same time that the specialization is going on the general 
cutaneous fibers for the most part cease to enter the acusticum 
and cerebellum and come to be concentrated in the spinal V tract, 
so that in higher vertebrates by far the greatest part of the cutaneous 
fibers end in the nuclei of the spinal V tract and of the dorsal 
funiculi. While this concentration of the primary tracts and 
nuclei is going on, a change takes place in the secondary connections 
as well. The secondary tract from the spinal cord and the nucleus 
funiculi, the lemniscus, now ends only in part in the roof of the 
mesencephalon. A larger part of its fibers end in nuclei situated 
in the lateral walls of the diencephalon. (See Chapter XVI.) 

We should expect the general cutaneous sjrstem of nerves 
to be the most primitive and widespread part of the ner\'ous 
sj-stem, unless the general >-isceral nerves be excepted. And so 
it is, but there is reason to think that even in the lowest vertebrates 
the s}'stem has imdergone considerable modification. A pair 
of cutaneous nenes and corresponding caiters is to be expected 
in each segment, but in c}*clostomes the first fi\'e segments of the 
bmin arx^ without cutaneous roots and the corresponding area 
of the skin is supi>lied by nerv-es whose roots enter the brain in 
the si\;h neuivmere. The fact that one of these, the ophthal- 
micus prv^tv;nvius, arisen in :he embryo from the fifth neuromere 
is taken ,is cxivicncv :iu: i: 01:0^ held dia: position in the adult 
K> -on x\ :he :in>: four s<r^.^cr.:5 of :he head are without cutaneous 
ner>'v^ ot :hcir o\>n An.: Are surr^ei by cutaneous rami coming 
tVn\ nv:\^ ca;^:aI >>xn:er.^>. In selichiins^ however, the N. 
:r,nun,i..x ,vr.n.v:A: 'A{:h :hc* nr^c neurosaere is probably the 
cuunsv,^ no'N\^ s^t :>a: >ocr:<r.:. KrAscsis wiQ be g^ven in a 
^Atvi v>/,. :v* v"x^\ :v: \ V/, :V- :>.i.ni:n^ tii: ^se catancoxis nerves 
04 ,>-,^ >AVi^* :>.'.'/ /,:v.^ v,.r\ ^v-vi-^^rs hix^ been modified 
u\;o x',N,^** o '</."': N. N ^ ,.- :^>: »" . >.:'^j:^n\:c,;:v rcvtuzadus has come 



GENERAL CUTANEOUS SUBDIVISION. 121 

to supply the corresponding area of skin. In higher forms the 
difiPerentiation of the gills and the development of an operculum 
have led to changes in the general cutaneous components, such 
that the VII and in some cases the IX nerve are without cutaneous 
fibers. Then in the occipito-spinal region a variable number of 
cutaneous nerves have been gathered into the single vagus root 
and others have disappeared on account of the shifting of the 
mesodermal organs and shortening of the cutaneous area. Within 
the brain correlated changes have taken place. A general cuta- 
neous center probably persists in the forebrain of selachians but 
little is known of its structure or relations. In other vertebrates 
the most cephalic cutaneous center is the tectum mesencephali. 
The ophthalmicus profundus nerve once arose from this segment 
of the brain and the trigeminus from the cerebellar segment. 
Both of these regions still receive general cutaneous fibers, at least 
in lower vertebrates, but both these and the tuberculum acusticum 
have lost most of the fibers of this component which once entered 
them. 

All these facts may be expressed or implied in a word by saying 
that there has been a process of concentration of the tactile appa- 
ratus of the head toward the caudal part of the cranial region. 
This has beeh due in part to the usurpation of the cephalic part 
of the brain by highly specialized somatic sensory organs, the 
eyes and acustico-lateral system, and in part to some undefined 
advantage that is probably gained by the concentration of a 
system of nerves and centers instead of their being equally distrib- 
uted segmentally. Such is the general cutaneous system; the 
most primitive and the least specialized system of nerves and 
centers, yet progressively more and more modified in its arrange- 
ment, chiefly through the influence of more highly specialized 
organs. 

DEMONSTRATION OR LABORATORY WORK. 

1. Review dissections of the dorsal spinal nerves and the trigeminus. 

2. Trace the position and relations of the dorsal tracts, acusticum, 
and cerebellum in the brain of a large fish, bullfrog and a mammal, by 
dissection. 

3. Trace the spinal V tract in Delafield haematoxylin or Weigert 



122 NERVOUS SYSTEM OF VERTEBRATES. 

sections of a fish, frog or mammalian brain. Note its relations with 
the dorsal tracts of the cord, and the relations of the substantia 
gelatinosa of medulla oblongata and cord. 

4. In Weigert sections of the brain of a selachian or frog, look for 
the sensory fibers of the trigeminus running to the tectiun mesencephaii 
and to the cerebellum. 

5. Study the ner\e elements in the dorsal horn, acusticum, sub- 
stantia gelatinosa and cerebellum in Golgi sections of the fish brain. 

6. Trace the ascending lemniscus in fish, frog or mammal in trans- 
verse or sagittal sections by the method of Weigert. 

LITERATURE. 

Cajal S. R. : Beitrage zum Studium der Medulla Oblongata. Leipzig. 1896. 

Cajal, S. R.: Textura del sistema nervioso del Hombre y de los Verte- 
brados. Madrid. 1904. 

Coghill, G. E.: The Cranial Nerves of Amblystoma tigrinum. Jour. 
Comp. Neur., Vol. 12. 1902. 

Cole, F. J.: On the Cranial Nerves of Chimaera monstrosa Linn., etc. 
Trans. Roy. Soc. Edinb., Vol. 38. 1896. 

Cole, F. J.: Observations on the Structure and Morphology of the 
Cranial Nerves and Lateral Sense Organs of Fishes, with especial reference 
to the (jenus Gadus. Trans. Linn. Soc. London, Ser. 2, Zool. 7. 1898. 

Ilcrrick, C. Judson: The Cranial and First Spinal Nerves of Menidia. 
Jour. Comp. Neur., Vol. 9. 1899. 

Ilcrrick, C. Tudson: A Contribution upon the Cranial Nerves of the 
Codfish. Jour. Comp. Neur., Vol. 10. 1900. 

Herrick, C. Judson: The Cranial Nerves and Cutaneous Sense Organs 
of North American Siluroid Fishes. Jour. Comp. Neur., Vol. 11. 1901. 

Johnston. J. B.: The Brain of Acij)enser. Zool. Jahrb., Abth. f. Anat. u- 
Ontoji;., Bd. 15. 1901. 

Johnston, J. B.: The Brain of Petromyzon. Jour. Comp. Neur., Vol. 12. 
1902. 

Johnston, J. B.: The Cranial Nerve Components of Petromyzon. Morph. 
Jiihrb., Bd. 34. IQ05. 

Johnston, J. B.: The Cranial and Spinal Ganglia and the Viscero-motor 
l<(M>ts in Am|)hio.xus. Biol. Bull.. Vol. 9. 1905. 

Kappors, C. U. A.: The Structure of the Teleostean and Selachian 
Bruin. Jtuir. Comj>. Xour. and Psych., Vol. 16. 1906. 

Kinjip*l>\iry. B. K.: The Structure and Morphology of the Oblongata in 
Mshcs. Jonr. Comp. Nour., Vol. 7. 1897. 

I.ory. \V. A.: ("^n a nowly nxrognized Nerve connected with the Fore- 
l>iain of Sohuhinns. .\nat. Anz.. Bd. 26. 1905. 



GENERAL CUTANEOUS SUBDIVISION. 1 23 

Pinkus, F.: Die Himnerven des Protoptems annectens. Morph. Arbeit, 
Bd. 4. 1894. 

Stannius, H.: Das j)eripherische Nervensystem der Fische, anatomisch 
und physiologisch untersucht. Rostock. 1849. 

Strong, O. S.: The Cranial Nerves of Amphibia. Jour. Morph., Vol. 10. 
1895- 



124 



NERVOUS SYSTEM OF V'ERTEBRATES, 



CHAPTER Vn. 

SOMATIC AFFERENT DIVISION. SPECIAL 
CUTANEOUS SUBDIVISION. 

The typical sense organs of this system are the pit and canal 
organs which are found in rows on the head and along the lateral 
line of cyclostODies, fishes and aquatic amphibia. In Figure 6i 
are shown a large and a small organ of this type from a sucker 
embryo at about the time of hatching. The organ consists of 
high columnar supporting cells which form the w^hole thickness 
of the epidermis within the area of the organ, and of shorter 
thicker pear-shaped cells which do not reach the whole depth 
of the epidermis. The latter ceUs bear at their outer ends cuticular 




\K 



Fig. 6i. — A large and a small neuromast from a sucker (Catostcmus) exDbiyo at 
about the time of hatching- 
hairs or brisdes which project beyond the surface. These are 
the sense cells. Beneath the organ a few nerve fibers come up 
from a deeper l>ing ner\'e, lose their medullary sheaths as they 
reach the organ and penetrate between the cells. Here the fibers 
divide in a very complicated manner (Bunker) and end by very 
fine branches on the surface of the cells. In the tv-pical pit organs 
the surrounding epidermis is much thicker and rises up as a wall 
on all sides, the bottom of the pit being formed by the sense organ. 
Organs of this type are found in fishes in a line along the lateral 
surface of the body, and in a supraorbital, an infraorbital and a 
hyomandibular row (Fig. 62). The organs of these typical rows 
in fishes are usually enclosed in canals in the manner described 
in the chapter on embryology, but in cyclostomes and amphibia 





SPECIAL CUTANEOUS SUBDIVISION. 



"S 



they remain in the form of pit organs. In many cases accessory 
lines of pit organs, or even canals, are present and in selachians 
the total number of organs is very large. The number of organs 
found in the typical rows of the head varies greatly, as is indicated 
by the following examples: Amia, 40; Menidia, 37; Gadus, 28; 
Chimaera, 93. It has recently been shown by experiment that 
the function of the pit and canal organs is to take account of 
vibrations in the water of a frequency too low for the production 
of sound G^etween 6 and 100 per second). 



R. ophthal. superfic 



Supraorbital 
canal 




Hyomand. 
canal 



Infraorbital canal 



Lateral line 
canal 



R. hyomandibularis 



Fig. 62. — A diagram of the lateral line canals and nerves in the ganoid fish, 
Amia calva. After E. Phelps Allis. 



In selachians there are found two other forms of sense organs 
which are histologically similar to the pit and canal organs and 
are also related to them in their ner\-e supply. These are the organs 
known as the ampullae of Lorenzini and the vesicles of Savi. The 
ampullae are foimd in groups including in some cases a large 
nimiber of organs, imbedded in a soft gelatinous connective 
tissue in cavities about the skull or between other organs. The 
bodies which are imbedded in these groups, however, are only 
the enlarged ends of slender canals which after a longer or shorter 
course open on the surface. The development of the organs shows 



126 NERVOUS SYSTEM OF VERTEBRATES. 

that the point at which the canal opens in the adult is the place 
from which the organ develops. Starting as pits at these points 
the tubes grow inward until they reach their final location. At 
the inner end of the tube there is developed a sense organ of the 
same type as the pit organ, but more complex in structure. The 
organs are supplied by branches of the nerves which supply the 
adjacent canals and are regarded as modified pit organs. The 
vesicles of Savi are small isolated sacs lying beneath the epidermis 
which contain pear-shaped hair cells and are also inner\'ated by 
the same system of nen^es as the canal organs. In ganoids also 
are similar organs called nerve sacs. Both are probably degen- 
erated or at least modified organs of the same system with the 
canal and pit organs. All this system of organs has been com- 
monly given the name of the lateral line system, because the lateral 
line canal is its most conspicuous part. Some years agp the term 
neuromasts was proposed by Wright for the organs whose sense 
cells are pear-shaped hair cells, as distinguished from the rod- 
shaped cells found in the organs which are now known to be taste 
organs. This is a much more satisfactory name than the term 
lateral line organs. 

It was shown in the chapter on embryology that the neuromast 
system develops in close connection with the ear. An examina- 
tion of the ear in all its relations has led to the conclusion that it 
is fundamentally a part of the same system of organs as the 
neuromasts. The endence of this in part will be given in detail 
in the present chapter but may be summarized here as follows. 
(i) The sense cells in the ear of all vertebrates are hair cells 
similar to those of the canal organs. (2) The sense cells are 
enclosed in canals which during development sink in from the 
surface as do the sensory canals and remain open to the surface 
for some time by way of the ductus endolymphaticus. (3) The 
hair cells of both neuromasts and ear respond to vibrations in 
the fluid which fills the canals. The cells of the ear of many fishes 
and of higher vertebrates respond to vibrations of a more rapid 
rate (more than 100 per second) than those to which the canal 
organs respond. (4) The sense cells of the ear are supplied 
by ner\'e fibers which in their development and their central endings 



SPECIAL CUTANEOUS SUBDIVISION. 1 27 

correspond very closely to those of the canal organs. (5) The 
centers in which the nerves of the canal organs and ear end form 
one continuous structure whose histology and secondary connections 
in the different classes of vertebrates show that it has been derived 
from the general cutaneous centers. 

The sense organs thus far spoken of lie in the semicircular 
canals and in the sacculus and utriculus and are the only sense 
organs of the ear in fishes. In addition to these there is developed 
in higher vertebrates the spiral-shaped cochlea with its compli- 
cated organ of Corti. The sense cells of this organ are also hair 
cells which respond to vibrations in fluid, of a still higher rate. 
Even this organ with its nerves and centers is so closely related 
to the rest of the ear that it must be regarded as a more highly 
specialized part of the same system of sense organs. 

The function of the system as a whole seems to have been at 
first to aid an aquatic animal in directing its movements. Vibra- 
tions in the water caused by surface waves, by the movements 
of other animals, by its own movements, — ^all these were capable 
of stimulating these organs, and reactions to such stimuli con- 
stituted the means by which the animal directed many of its move- 
ments. Some of these organs becoming deeply imbedded in 
canals were not directly influenced by large slow waves, but only 
by the shorter quicker waves which could enter the canals. Some 
of the canals becoming completely enclosed and being filled by a 
denser fluid which responded more readily to waves of less amplitude 
and higher rate, enabled the animal to be influenced by a wider 
range of vibrations, including those of high enough rate to produce 
sound. In this way the open pits, the canal organs, and the organs 
of the ear came to be differentiated and to serve for a wide range 
of stimuli. Finally a still higher and higher rate of stimuli were 
provided for by the development of the cochlea, and as verte- 
brates ceased to live an aquatic life the pit and canal organs which 
responded to slow waves in water ceased to be of use and were 
lost. 

The phenomenon of equilibration is a constant phase in the 
control of movements. Indeed, keeping the equiUbrium is a 
necessary condition for all directed movement. It is probable 



I30 



NERVOUS SYSTEM OF VERTEBRATES. 



of various nerve trunks* This \\iU be cleax from an examination 
of Figs. 62 and 63. In the former the canals of the head are 
shown with the nerves which iniier\'ate them but without the 
accompanying components of the other kinds. In Figure 63 the 
various components which constitute the cranial nenes of Menidia 
are sho\\Ti as they are combined in nerve trunks. The neuro- 
mast components are shown in outline but the canals Uiemselves 
are omitted on account of the complexity of the figure. The root 
and ganglion of the lateral line ner\^e is closely related to those 
of the vagus, as is nearly always the case. In many fishes also 
fibers from this root go into the IX ner\^e in which they run for 
some distance to reach their sense organs. In Menidia the fibers 
to the supraorbital canal, forming the ramus ophthalmicus super- 
facialis VII, run with the general cutaneous fibers which form the 
ramus ophthalmicus superfadalis V, the two constituting a supra- 
orbital trunk. This is the most common arrangement in other 
fishes also. The fibers for the infraorbital canal in Menidia run 
with general cutaneous and general \isceral fibers in the maxillary 
trunk. The fibers to the infraorbital row of sense oi^ans are 
known as the ramus buccaMs VII and in most fishes are independent 
of other comjxinents for the greater part of their course. The 
fibers destined to the hyomandibular canal form a component of 
the ramus hyomandibularis, which contains also general cutaneous, 
\isceral sensor)^ and visceral motor fibers. It will be seen imme- 
diately that all the neuromast fibers, no matter in w*hat rami they 
run, have the same central connections as well as the same type 
of peripheral organs, and are therefore justly considered as a 
single system of ner\^e components. The distribution of th^e 
components in the cyclostomes (Fig. 51) and in amphibia (Fig. 
79) shoiJd be compared with that in Menidia (Fig. 63). 

Special cutaneous centers, — In the brain of cyclostomes 
the neuromast fibers are intimately associated with the general 
cutaneous fibers and end in the same slightly specialized centere* 
(See previous chapter.) In the brain of a selachian or ganoid 
fish although the general and special cutaneous fibers are still 
almost as intimately associated, a higher development of the 
centers has taken place and the special cutaneous fibers end chiefly 



SPECIAL CUTANEOUS SUBDIVISION. 



t3i 



in the more highly developed portions. This can be Ulustrated 
best by describing the cutaneous centers with their roots and 
secondary connections in some detail. As stated abo\'e (p. 115) 
the general cutaneous fibers on entering the brain go in part by 
the spinal V tract to the nucleus of the dorsal funiculus and in 
part spread widely through the acusticum and cerebellum. The 
neuromast components also spread widely through the acusticum 
and cerebellum but only a small part of them go to the nucleus 
funiculi or to a small nucleus adjoining it. Thus the greater 
part of the nucleus funiculi, and, especially in higher vertebrates^ 



k^^ife^'' 



Cf rebcllum 

L. lineae lateralis 



-X. Im. lat. VII 
. K. VII sensory 



— N'» VII motor 

- Secondary gustatory 

Iract 



Spinal V tract 

Fig. 64.^ — A transverse section of the brain of the sturgef>n at the level of ihc 
VII and Vm nerves. 

cells accompanying the spinal V tract are related to general cu- 
taneous components alone, while in the acusticum and cerebellum 
both general and special cutaneous fibers intermingle and pre- 
sumably end in relation with the sanae cells. The intermingling 
of these components wiU be seen by reference to Figs. 64 and 65. 
In the acusticum and cerebellum there is to be noticed a great 
increase in size in true fishes as compared with cyclostomes and 
the presence of much more prominent granular and molecular 
layers. The outer portion of the cerebellum is composed of a 
dense layer of very fine fiber5, interspersed with few celk, which 
continues caudally over the dorsal or lateral surface of the acus- 




132 



NERVOUS SYSTEM OF VERTEBRATES. 



ticum and forms what is known as the cerebellar crest. This cor- 
responds to the molecular layer of the cerebellum of higher forms. 
The inner portion of both cerebellum and acusticum is composed 
of large and small cells and corresponds to the combined granular 
and Purkinje cell layer of higher forms. 

The small cells are vastly more numerous in the cerebellum 
than in the acusticum and the small bodies of the cells closely 
packed together suggest the names granule cells and granular 
layer. The great majority of these cells are very small, have from 
one to three short dendrites with small claw-Uke branches and 
give rise to extremely fine neurites. These are the granule cells 



Molcculiir 
layer 



Granular 

Uyer 

_Tr. tecto- 
cerebelUzis 



Tr. biilbo-tectalis " 



. Tubcrculum 

acusticum 

_ Spinal V tract 

__ Secondary gustatory 

tract 
• Tr. tecto-bulbam 



Fig. 65* — A transverse section of the brain of the sturgeon at the level of the V 
ner\'e. 

in a strict sense. Their neurites turn toward the outer surface 
of the cerebellum or acusticum, as the case may be, and run 
parallel uith the surface, forming the molecular layer. In most 
forms these fibers bifurcate on entering the molecular layer, 
one branch running forward and one backward. A considerable 
part of the neurites of granule cells in the cerebellum pass to the 
opposite side through the rcxjf, forming the superior commissure 
of the cerebellum. The larger part of the molecular layer fibers 
end within the cerebelhmi; the smaller part, consisting of both 
crossed and uncrossed fibers, pass caudally in the cerebellar crest 



SPECIAL CUTANEOUS SUBDIVISION. 



133 



which grows gradually smaller and dwmdles away toward the 
caudal end of the acusticum. The smaller number of small 
cells in the cerebellum and acusticum are of the form known as 
cells of type II, whose neurites divide into terminal branches in 
the near vicinity of the cell. A single granule cell is shown in 
Fig. 39, and in Figs. 64 and 65 are shown the relations of the 
granular and molecular layers in the brain of the sturgeon. In 
cyclostomes the molecular layer extends along the lateral surface 




Fig. 66. — A transverse section of the medulla oblongata of Scyllium to show the 
folding of the cerebellar crest and tuberculum acusticum. ac, tuberculum acus- 
ticum; c.c, cerebellar crest; L././., lobus lineae lateralis; L.v., lobus vagi. 

of the acusticum. In selachians there is a folding of the acusticum 
in such a way that the molecular layer forms the floor of a 
longitudinal gnx)ve on the lateral surface (Fig. 66) and in the 
sturgeon this folding has gone so far that the groove has been 
dosed up by the fusion of tl^e opposed surfaces of the molecular 
layer. That part of the acusticum which lies above the fold is 
the lobiis lineae lateralis (Fig. 3) and in the sturgeon it is almost 
separated from the rest of the acusticum by the cerebellar crest. 
The large cells in the acusticum and cerebellum may be described 



134 



NERVOUS SYSTEM OF VERTEBRATES. 



as of two main types with intermediate forms. The first type 
consists of cells with large bodies and rather coarse dendrites 
whose many branches spread widely through the granular layer. 
These cells show no special arrangement and no great peculiari- 
ties; they are most like the large cells in the dorsal horn of the 
cord or in the nucleus funiculi. Their neurites go as internal 
arcuate fibers to join the tractus bulbo-tectalis of the opposite 




Fig. 67. — Transverse section of the acusticum of the sturgeon to show acusticum 
cells and a Purkinje cell, c.c, cerebellar crest; L.v., lobus vagi. 

side of the brain. These cells will be referred to as acusticum cells. 
Such cells are shown in Fig. 58 A, from the acusticum of a cyclos- 
tome and in Fig. 67 from the sturgeon. 

The cells of the second type are also large but they differ from 
the first in having both a special arrangement and a special form 
dilTcrcntiation. The cell-bodies stand in the granular layer 
next to the molecular layer and are somewhat elongated vertically 



SPECIAL CUTANEOUS SUBDIVISION. 



135 



I 



I 



I 



I 
I 

I 



to the surface. The dendrites arise from the end of the cell next 
to the molecular layer and spread in that layer. The dendrites are 
noticeably straight, have a stiff appearance, and aJl their branches 
in the molecular layer are provided with great numbers of litde 
spine-like projections. The possession of such dendrites brings 
these cells into the same category with the Purkinje cells of the 
human cerebellum. Since the dendrites are imbedded in the 
myriads of fine fibers of the molecular layer it is probable that the 
small spines serve for contact or perhaps a closer connection with 
the fine fibers. A single Purkinje cell is shown in Fig. 39 and one 
is drawn in the upper part of Fig. 67. From the inner end of the 
cell-body, which is frequently slender and pointed, arises the 
neiirite which may take one of three courses. Some of them go 
ventro-mesially close braeath the floor of the fourth ventricle 
and either make connections with the motor nuclei of the cranial 
nerves of the same region or enter the longitudinal fiber tracts 
closely related to those nuclei and go to nuclei of more distant 
nenes. The fibers from the cerebellum which have this desti- 
nation form two or more bundles which cune down over the 
inner face of the acusticum and reach the motor column in the 
region of the trochlearis and abducens nuclei. All these fibers 
may be referred to as the short motor connections of the acusticum 
and cerebellum. Other fibers from the Purkinje cells, especially 
in the acusticum, go as internal arcuate fibers to join the tractus 
bulbo-tectalis. Still other fibers from Purkinje cells in the acusti- 
cum go as arcuate fibers on the outer surface of the brain to desti- 
nations which are as yet little understood. Some may go to a 
nucleus comparable with the lower olive of human anatomy, 
others go to the cerebellum. The latter would correspond to the 
external arcuate fibers of man. The destiiation of the larger 
part of the neurites of Perkinje cells in the cerebellum of lower 
forms is not known. In mammals (p. 245) the neurites of Purkinje 
cells are very widely distributed to distant parts of the brain 
and spinal cord and it is an interesting problem to know how 
low in the scale of \erteb rates this condition makes its appearance. 
The cells in the acusticum intermediate between these two types 
are equally large but have not such definite form and arrange- 




136 



NERVOUS SYSTEM OF VESTEBRATES. 



ment as the Purkinje ceUs (Fig. 68). The difference lies chiefly 
in the form and position of the dendrites. A part of the dendrites 
ramify in the granular layer and a part in the molecular layer. 
Those which lie in the granular layer are like those of the first 
class of cells, but any dendrites which enter the molecular layer 




Fig. 68. — A section through the same region as in Fig. 67. to show cell fonns 
intermediate between acusticum and Purkinje cells. 

take on the characters of Purkinje cell dendrites. In these inter- 
mediate forms every gradation is found between the acusticum 
and Purkinje cells. The number of the acusticum and inter- 
mediate cells is much greater in the acusticum, that of the Purkinje 
cells much greater in the cerebellum. 



SPECIAL CUTANEOUS SUBDIVISION, 



137 



These facts lead to the conclusion that all these cells have been 
derived by modification from the simple large cells of the general 
cutaneous nuclei. The modification is due directly to the influence 
of the fine fibers of the molecular layer and in the cerebellum, 
where the granule cells arc collected in greatest numbers and the 
fine fibers are most numerous, the large cells are nearly all special* 
ized into Purkinje cells. The presence of great numbers of 
granule cells, of cells of type II and of highly developed Purkinje 
cells marks the cerebellum as the most highly specialized part 
of the special cutaneous nuclei. The specialization of both cere- 
bellum and aciisticum is to be attributed to the influence of 
the special cutaneous system of sense organs. In liigher verte- 
brates th^e centers no longer receive many general cutaneous 
fibers, but undergo a still higher speciallzadon, in part as the 
centers for the ear and in part as an apparatus for controlling 
bodily movements. 

It is necessary now to follow the tract us bulbo-tectalis which 
receives the greater part of the secondary fibers from the special 
cutaneous nuclei, and see its relations in the mesencephalon. 
The roof of the mesencephalon in the fishes begins to show a 
differentiation into two parts, a median somewhat dome-shaped, 
bi-lobed tectum opticum and a lateral thicker mass forming a 
semicircular border about the tectum on either side. These 
lateral masses are known as the collkuH or the lateral mesen- 
cephalic nuclei. The origin and significance of these parts are 
more fully treated in Chapter XVL A process of differentiation 
is seen in this region analogous to that which has been described 
in thtr cutaneous centers of the medulla oblongata. In the simplest 
condition in vertebrates the fibers which pass from the cutaneous 
center in the hindbrain to the midbrain (tractus bulbotectalis) 
end indiscriminately in all parts of the roof of the midbrain. 
WTien the colliculi are well developed it is noticed that the greater 
part of the tract ends in them^ not in the tectum opticum. From 
these nuclei in bony fishes tertiarj' tracts go to the tectum opticum, 
as well as to the inferior lobes and the motor centers of the medulla 
oblongata. From the tectum opticum in fishes an important 
tract goes to the cerebelhmi. The presence of this tract is one 




X38 



NERVOUS SYSTEM OF VERTEBRATES* 



of the first evidences of the development of coordinating functions 
on the part of the cerebellum, which was originally a simple 
general cutaneous center. 

The distribution of the root fibers of the acustico-lateral system 
of nerves to the special cutaneous nuclei in the medulla oblongata 
and cerebellum of fishes is shown in Fig. 69. This figure should 
be compared with that for the general cutaneous centers (Fig, 
59). The central tracts from the special cutaneous centers in 
fishes are identical with those from the general cutaneous centers. 

In aquatic amphibia and in the tadpoles of land forms the 



V^=^'- 



Hi 



L.Iineae lateralis 

Juberc. acust. 

Nuc, funic. 



N. LLV 



v;:> 



IX'Vit 



VI 
VIII 



X 



^ '4i 



N. lineae lateralis 



Fig. 69. — A diagram to show the ceniraJ endings of the special cutaneous com- * 
ponents in fishes. 

acustico- lateral system has essentially the same relations as in 
fishes. In terrestrial forms, however, the pit organs disappear 
because they are serviceable only in the water. Only the enclosed 
canals of the ear with their sense organs persist. In reptiles, 
birds and mammals also, the inner ear is the only representative 
of this system of sense organs which holds so prominent a place 
in fishes. In mammals the VIII nerve is divided into two parts, 
the N. vestibularis and the N. cochlearis, and for these nen^es 
two sets of nen'e centers have been developed from the acusticum 
of fishes. The vestibular ner\'e supplies the sense organs in 



SPECIAL CUTANEOUS SUBDIVISION. 



Z39 



the vestibule and semicirculax canals which represent nearly the 
whole of the ear of lower vertebrates, and its centers retain in 
general the form and position of the acusticum in fishes. For 
the detailed description of these nuclei the student must be referred 
to the larger text-books on the human brain, but the following 
general summary is in place here. The fibers upon entering the 
'brain bifurcate into smaU ascending and larger descending branches. 
The bundles of the large descending root are surrounded by and 
interspersed with cells which constitute its end-nucleus. MesiaUy 
this is continuous with a broad nucleus beneath the ventricle, 
called the mesial nucleus of the vestibular nerve. The two 
together correspond to the acusticum of fishes caudal to the VIII 
root* The anterior portion of the mesial nucleus is especially 
important and lateral to it is the lateral nucleus usually known 
as Deiter's nucleus, which is closely related to the bifurcation 
of the vestibular fibers and to the first portion of the descending 
branches. The ascending branches pass upward in a tortuous 

[course toward the cerebellum. Many of them end in the superior 
nudeus of the vestibular ner\'e, dorsal to the lateral nucleus, 

J and the rest enter the region of the nudeus tecti of the cerebellum. 

[ These ascending branches with their nuclei are the equivalent of 
the ascending VIII fibers and the continuous gray matter of the 
acusticum and cerebellum in fishes. 

From these several nuclei the following chief tracts arise 
(Fig. 70): (1) fibers from the nudeus of the descending root to the 
cerebellum; (2) fibers from the same nudeus which go as internal 
arcuate fibers to the opposite side and then either ascend in the 
medial lemniscus (p. 258) or descend into the spinal cord; (3) 
fibers from Deiter's nucleus, some of which go to the cerebdlum, 
but most of which go to the opposite side to run forward or back- 
ward in two or more bundles. The larger part of the fibers 
which cross to the opposite side run in the fasciculus longitudinalis 
medialis and may correspond to the '* short motor connections" 
of the acusticum in fishes. Although there is greater complexit}', 
the central apparatus of the vestibular ner\'e in mammals corre- 
sponds in a striking manner to that of the acustico-lateral system 
in fishes. 




I40 



NERVOUS SYSTEM OF VERTEBRATES, 



It is usually stated that the cochlear nen^e includes a branch 
to the macula acustica of the sacculus in addition to the fibers 
to the organ of Corti. Recent studies on the human embryo show^ 
however, that the branch in question belongs to the vestibular 
nen^e and is quite independent of the cochlear nen^e (Streetcr). 
The fibers of the cochlear nerve bifurcate on entering the medulla 
oblongata and end for the most part in two nuclei, the ventral 
and dorsal cochlear nuclei, which he lateral and dorsal to the 
vestibular nuclei. Two important things are to be noticed in 



Cgenic mes 



Pulvinar^ 



Lin^la 



Ceretvllum 



C.fienfc. 



^Tr spin V'U 



lo certhJ 



Lemniscus 
-t.*fnniscus 



l3t 



mfd 



Comm. -^ / 
anterior 



. Corp. mam. 



N. cochleans 
N. VEsiflHilaris 



Tr. opticus^ 



J Fig. 70. — A diagram to show the central endings of the vestibular and cocUeaf 
nerves and of the optic tract in man and the chief secondary tracts related to thcjiL 
Compare Fig. 60. 

regard to the centers for the cochlea* The first is that these 
nuclei are superficial with respect to the vestibular nuclei. In 
this they offer a clear illustration of the general law that the more 
highly speciaUzed structures in the brain, and hence those which 
have appeared later in the phylogeny, are placed toward the outer 
surface with respect to older structures to which they are related. 
It is probable that these nuclei have been developed from the 
acusticum of lower vertebrates and have taken up the superficial 
position as they developed. The second point is that no cochlear 
fibers go to the cerebellum. Apparently the development of the 
cochlea has come so late as compared with the evolution of the 
brain that the cerebellum had already assumed functions of 



I 





SPECIAL CUTANEOUS SUBDIVISION. 



141 



correlation inconsistent with its serving primary sensory nerv^es. 
On the other hand, some fibers of the cochlear ner\'e go beyond 
its primar)^ nuclei, by way of the corpus trapezoid eum and the 
secondary tracts, to higher centers. The fibers arising in the two 
cochlear nuclei go by a direct or indirect course chiefly to the 
lateral lemniscus of the same or opposite side (p. 258). The 
plain facts with regard to the central connections of the cochlear 
And vestibular ncrv^es are sho\\TL in the most diagrammatic form 
in Fig. 70, in order to faciUtate comparison with the arrangements 
in lower vertebrates (Fig. 69), The further consideration of the 
tentral relations will be more appropriate in the chapter on the 
correlating centers. 

I DEMONSTRATION OR LABORATORY WORK. 

I. Dissect the lateral line system and ear and their nerves in the 
Idogfish or skate. 

' 2. Study sections of lateral line organs from a selachian or from 
teleost embryos, stained with iron haematoxylin or prepared by a 
special nerve method, 

3. Review the dissection of the selachian and teleost brain (Chap- 
ter II, No. 2 and Chapter V, No. 2) with reference to the great 
deve-lopment of the acusticum and cerebellum in selachians, and the 
correlation with the large number of acustico-lateral organs. 

4. Study the roots of the acusttco- lateral system of nerves in the 
|brain of a selachian or ganoid fish in Weigert or Golgi sections. 

5. In Golgi sections of a fish brain study cells of acusticum and 
cerebellum to follow the evolution of the Purkinje cells. Notice the 
granules and cells of type II. 

6. In Weigert or Golgi sections of a fish brain note the internal 
arcuate fibers to the lemniscus system. 

LITERATURE. 

Allis, E. P., jr.: Lateral Line Sysiem in Amia. Jour. Morp., Vol. 2. iSSg. 

Aycrs, H.: Vertebrate Ceplialogenesis IL A Contribution to the Mor* 
phoJogy of the Vertebrate Ear. with a Reconsideration of its Functions. 
Jour. Morph., Vol. 6. 1892. 

Barker, L. F.: The Nervous System and its Constituent Neurones. i8g^. 

Beard. J.: On the Segmetital Sense Organs of die Lateral Line and the 
' Morphology of the V'ertebrate Auditory Organ. Zool Anz., Jahrg. 7. 1884. 

Braucr, A.: Bcilrage znr Kenntniss der Entwickelung unci Analomie der 
Gynmophionen. Zool. Jahrb., Suppl, 7. 1904. 




142 NERVOUS SYSTEM OF VERTEBRATES. 

Bunker, F. S.: On the Structure of the Sense Organs of the Lateral Line 
of Ameiurus nebulosus Le. S., Anat. Anz., Bd. 13. 1897. 

Coggi, A.: Sviluppo degli organ! di senso laterale, della ampoUe di 
Lorenzini e loro nervi rispettivi in Torpedo. Archivio 2>x>L, Vol. i. 1902. 

Coggi, A.: Su lo sviluppo e la morphologia della ampolle di Lorenzini e 
loro nervi. Archivio ZooL, Vol. 2. 1905. 

Cole, F. J.: Cranial Nerves of Chimaera monstrosa. Proc. Roy. Soc. 
Edinb.. Vol. 38. 1896. 

Cole, F. J.: Cranial Nerves of Gadus. Trans. Linn. Soc. London. Ser. 2, 
Zool. 7. 1898. 

Ewart, J. C: The Sensory Canals of Laemargus. Trans. Roy. Soc. 
Edinb., Vol. 37. 1893. 

Ewart, J. C. and Mitchell, J. C: The Sensory Canals of the Common 
Skate (Rata hatis). Trans. Roy. Soc. Edinb., Vol. 37. 1893. 

Herrick, C. Judson: The Cranial Nerves of Menidia. Jour. Comp. Neur. 
Vol. 9. 1899. 

Herrick, C. Judson: Cranial Nerves of Siluroid Fishes, Jour. Comp. 
Neur., Vol. 11. 1901. 

Johnston, J. B.: The Brain of Acipenser. ZooL Jahrb., Abth. f. Anat. u, 
Ontog., Bd. 15. 1901. 

Johnston, J. B.: The Brain of Petromyzon. Jour. Comp. Neur., Vol. 12 
1902. 

KoltzofiF, N. K.: Entwickelungsgeschichte des Kopfes von Petromyzon 
planeri. Bull. Soc. Imper. Natural de Moscou, Annee 1901, No. 3-4. 1902. 

Leydig, Franz: Ueber die SchleimcansUe der Knochenfische. Arch. f. 
Anat., Physiol, u. wiss. Med. 1850. 

Leydig, F. : Ueber Organe eines sechsten Sinnes. Dresden. 1868. 

Mayser, P.: Vergleichend-anatomische Studien tiber das Gehim der 
Knochenfische mit besonderer Beriichtigung der Cyprinoiden. Zeit. f. wiss. 
Zool, Bd. 36. 1881. 

Merkel, Fr.: Ueber die Endigungen der sensiblen Nerven in der Haut 
der Wirbelthiere. Rostock. 1880. 

Mitrophonow, Paul: Etude embryogenique sur les Selaciens. Arch, de 
2>x>l. Exper., Tome i. 1893. 

Parker, G. H.: Hearing and allied Senses in Fishes. Contr. Biol. Lab. 
U. S. F. C, Woods Hole, Mass. 1903. 

Parker, G. H.: The Functions of the Lateral Line Organs in Fishes. 
Bull. Bureau of Fisheries. Vol. 24. 1904. 

Streeter, G. L. : Concerning the Development of the Acoustic Ganglion 
in the Human Embryo. Verh. anat. Ges. 19. Vers. 1905. 

Wilson, H. v.: The Embryology of the Sea Bass. Bull. U. S. Fish Com., 
Vol. 9. 1891. 

Wright, R. Ramsay: On the Skin and Cutaneous Sense Organs of 
Ameiurus. Proc. Canadian Inst. Toronto, N. S. Vol. 2. 1884. 



ISE VISUAL APPARATUS. 



J43 



CHAPTER VIIL 

SOMATIC AFFERENT DIVISION. 
APPARATUS. 



THE VISUAL 



The general and special cutaneous systems serve for the recep- 
tion of stimuli from the external world due to mechanical coatact 
or pressure or to vibrations in fluids. Another set of stimuli of 
the greatest importance to the vertebrate animal, namely those 
pven by light, seem to common obsen^ation not to affect these 
cutaneous organs. As a matter of fact, it has been shown that 
. light stimuli do affect the endings of general cutaneous nerves 
in such a way as to produce characteristic reactions. If a light 
of suitable intensity be allowed to fall upon a frog whose eyes 
Aave been removed, the animal wiU turn its head toward the source 
of light and jump toward it. If the frog^s skin also be covered 
fium the light no such reaction takes place. Other amphibia, 
^ine Sshes and reptiles are influenced by light which falls on the 
^kin alone. In the case of ammocoetes, which lives buried in 
^he mud, the skin of the tail is more sensitive to light than any 
^ther part of the body, including the eyes. This condition is 
^ef i_ij to the animal, since it burrows head foremost and the sen- 
Siti\^^xiess of the tail ensures that it shall completely bury itself. 
v^'^liile light must be reckoned as one of the most important 
*aetors in the external world influencing the organism^ it is evident 
"*^^ the free nerve ending in the skin are not an adequate means 
*>"■ tilie perception of light. It is beyond the pro\iace of this 
woric to inquire how light has influenced the organism so as to 
p*x>clvi^^ an organ for its percepdon. It is, however, the business 
^^rnparative morphology to consider whether the organism 

IF^^nded to external influences by producing an entirely new 
^^^Ure or whether the organ produced was a modification of 
^^Tie structure already existing, and if the latter, what was its 
P^Oable mode or course of evolution. The attempt to answer 




ElG. 71- — Histogenesis and structure of t^c retina. From Cajal* (Die Retuia 
der Wiibelthiere.) 

A, a section throuffb tlie retina of a mouse embryo of 15 mm. afb,c, neuro* 
blasts; fi,«, epithelial (Miillerian) cells; /, nucleus of rod celL 

B, a section through the retina of a chick embryo of fourteen days, a A epi- 
thelial cells; Cj rod cell; d^ deep granule; e,n, cone cells; fw, bipolar cell; s^u, 
amacrine cells. 

C, a section of the retina of a dog. a, cones; b, rods; c and e, bipolar cells: 
related respectively to rods and cones; /, giant bipolar cell; h^ amacrine ccU?^ 
/, centrifugal Eber; n^ layer of ganglion cells. 






THE VISUAL APPARATUS. 



145 



"^ Jnese theoretical questions should add in- 
"fc^^rcst to the study of the development and 
^:s.erve connections of the visual apparatus. 
It was shown in an earlier chapter that 
Ttle lateral eyes are developed as a pair of 
^^esides from the dorso-lateral wall of the 
nkDrain in about the region of the second 
:xneuiomere (Figs. 18-22). Each vesicle 
^zximes to be attached to the ventral wall of 
^She brain by a hollow stalk. There now ap- 
3)ears in the ectoderm a thickening which 
^nks in to form a pit and finally separates 
rfrom the ectoderm as a closed sac. This 
Tthen develops into the lens of the eye. 
Meanwhile the optic vesicle becomes cup- 
shaped toward the outside, enclosing the 
lens, and the wall of the concave side 
thickens to form the retina, while the con- 
ytx wall remains thin. For the further de- 
velopment and adult relations of the eye-ball 
and accessory structures the reader must be 
referred to the larger text-books of embry- 
ology and anatomy. The histogenesis of 
(he retina proceeds in a manner similar to 
that of the spinal cord or brain. Supporting 
structures are formed (the so-called Miillerian 
fibers) which are to be compared with the 
spongioblasts of the brain, and among these 
^re foimd neuroblasts which give rise to the 
nerve cells of the retina (Fig. 71 A). On 
the inner surface of the retina, that is next 
to the portion of the brain ventricle which 
extends into the optic vesicle, there are formed 
spindle-shaped cells whose one process ex- 
tends to the ventricular surface and ends in 
a lod-shaped or cone-shaped structure, and 
whose other process runs deeper into the 

TO 



OQ^ 



:.-^'-> 






<Ll 



u 



Fig. 72. — Cells from 
the retina of the chicken. 
From Cajal. a, 6, centri- 
fugal fibers. 



i 



146 NERVOUS SYSTEM OF VERTEBRATES. 

retina and branches in relation with the dendrites of cells form- 
ing the inner nuclear layer (Figs. 71, 72). These cdls in turn 
set up connections with the elements of the gan^omc cdl layer. 
The fibers from the gan^onic cells form a fiber layer on the 
outer, concave siuf ace of the retina. The fibers collect toward a 
p)oint near the center of the retina, dip into its substance and 
pierce it to its inner surface. From this surface of the retina 
the fibers pass along the optic stalk to the brain. This arrange- 
ment is brought about during development by a diange of 
form of the retina and shifting of position of the stalk. At first 
the stalk is at the ventral border of the retina and the fibers 
run to the ventral border and thence to the brain in the stalk. 
Later the retina grows in such a way as to bring the attachment 
of the optic tract nearer its center. Finally, in all vertebrates 
except cyclostomes and some fishes and urodule amphibia, the 
hollow stalk degenerates and leaves the optic tract as the only 
connection of the retina with the brain. It should be noticed 
now that this optic tract, although conunonly called the optic 
ner\e, differs from all peripheral nerves in several ways, (i) 
The cells from which its fibers arise are derived from the wall of 
the brain. (2) They form part of a many-layered ner\t)us 
structure whose development and histology suggest comparison 
with the brain wall. (3) Although the fibers carry aflferent 
impulses they enter the ventral wall of the brain. Indeed, it has 
long been recognized that this is not a peripheral nerve but a 
central nerve tract and that the retina is a part of the brain wall. 
When the optic tracts arrive at the ventral wall of the dien- 
cephalon (or before in bony fishes) they decussate, forming the 
optic chiasma. As a general rule the fibers from one retina pass 
to the opposite side of the brain, but in manunals a complete 
decussation of the optic tracts is rare. In different manmials m~^ 
variable part of the fibers remain uncrossed and in man approxi — 
matcly one-third enter the same side of the brain as the eye froncm 
which they come. In vertebrates below the mammals most authoi s 
agree that the decussation is total. The beginnings of parti^^ 
decussation are seen, however, in fishes. Golgi preparations ^zdI 
the chiasma of the sturgeon show a considerable number of fibe tts 



THE \lSrAL APPARATUS. 



T47 



of the optic tracts giving off tliick coUaterab which enter the 
thalamus of the same side. The number of uncrossed fibers in 
any animal seems to depend upon the position of the eye. When 
the eyes are laterally placed as in most submammalian orders the 
uncrossed fibers are at a mimmum. Among mammals, those 
whose eyes are lateral, so that the fields of vision do not overlaj), 
have few xmcrossed fibers; while those (apes, man) whose fields 
of \iaon largely overlap have a large part of the optic tracts xm- 
crossed. The uncrossed fibers in man arise chiefly from the tern- 
poral part of the retina and run in a fairly well isolated bundle in 
the optic tracts and chiasma. Beyond the chiasma the optic tracts 



Fr. opticus 



Nuc. 



maija tecli 

Decussatio dorsali4 



Fio. 73. — Part of a section of the tectum opticurn of the sturgeon, schematic 



I 

I 



run up in the lateral part of the thalamus and end in the dorsal 
part of the thalamus and in the tectum opticurn. 

STRUCrmiE OF the XEcrrM opticum, — ^In the lower fishes the 
tectum contains a large number of cells of several forms, most 
of w^hich lie near the ventricle. The outer portion of the tectum 
is composed of fiber layers* Cells of both type I and type II are 
present. The cells of type II (Figs. 39 and 73) are slender 
spindle- shaped elements vertically placed near the ventricle, 
whose single thick dendrites rise toward the surface and branch 
profusely in the outer layers of the tectum* The neurite arises 
from some point of this dendrite far removed from the cell-body 




148 NERVOUS SYSTEM OF VERTEBRATES. 

and branches very richly in the middle layers of the tectum in the 
immediate vicinity of the cell. All the other cells of the tectum 
may be described as cells of t)rpe I whose dendrites spread more 
widely and do not reacK the outer surface of the tectum. Some 
of these cells have vertical spindle-shaped bodies and one or more 
branching dendrites, others have stellate bodies with several den- 
drites diverging widely, and still others are spindle-shaped but 
are placed tangentially and have two dendrites running parallel 
with the surface of the tectum. The neurites may arise from 
the cell-bodies or from any part of the dendrites, even from 
the tip of one of the tangential dendrites. The neurites arising 
from the cells of type I have various courses and destinations, 
(i) They cross to the opposite side, forming the dorsal decussation 
of the tectum. Whether any or all of these fibers leave the tectum 
after crossing is not known. (2) They enter the tractus tecto- 
bulbaris et spinalis and go either to the same or opposite side of 
the medulla oblongata and spinal cord. (3) They go down 
through the lateral wall of the mesencephalon or diencephalon 
to end in the inferior lobes, the tractus tecto-lobaris. Part of 
these fibers cross to the opposite side in the ansulate commissure in 
the ventral wall of the mesencephalon or in the postoptic decussa- 
tion, or in both. (4) They go along the lateral border of the tec- 
tum to enter the cerebellum, the tractus tecto-cerebellaris. (5) They 
go out m the optic tract to end in the retina (Fig. 72). The last 
are called centrifugal fibers. The elements in the optic lobe of 
birds are sho\Mi in Fig. 74. 

A speciid apparatus is described in connection with the tectum 
opticum of all classes of vertebrates which is supposed to serve 
at least in early life for direct and prompt reflexes in response 
to optic impulses. In the mesial part of the tectum is found a 
nucleus consisting of a variable number of cdls which are usually of 
extruordinar}- size. This is known as the roof nucleus or nucleus 
magmKelluIan's Urii, The neurites of these ceUs are said to enter 
the ventricle and form the structure known as Reissner's fiber, 
which makes connections with cells in the gray matter of the 
spinal cord. By mo;ms of this apparatus it is supposed that 
aquatic animals arc able to avoid obstacles and danger by move- 



THE VISUAL APPARATUS. 



ments more prompt than those which are directed by the more 
complex brain tracts. 

In cyclostomes all the optic tract fibers end in the tectum opticum 




•^^r^^i^r^^^^ 



Sll 



FiC. 74- — ^A section of the optic lobe of a bird* From Cajal (Tcxtura.eic). 
The lettera refer to extended descriptions in Cajat's text, 

but in selachians a part of the tract ends in the diencephaJon 
and in all higher forms an increasingly large part of the tract 
has this ending. The chief nucleus in which the fibers end is 



3SO 



NERVOUS SYSTEM OF VERTEBRATES* 



situated in the lateral wall of the thalamus and is known as the 
corpus genicuiatum laierale. The morphology of the thalamic 
and midbrain optic centers and their tertiar)^ connections in higher 
vertebrates will be discussed in a later chapter (Chap, XVI), 

The question of the origin of the eye and of its relation to other 
sensory systems may now be reviewed. Obviously a dose rela- 
tion would not be expected between the eye and the nerves or 
organs of \isceral sensation. Is the eye, then, related to cutaneous 
sense organs and has it been formed by modification of any pre- 
existing organ, or is it a wholly new structure? The facts show a 
remarkable similarity between the eye and the general cutaneous 
structures of other segments. The retina is derived from the 
dorso-lateral wall of the second neuromere. It therefore repre- 
sents a dorso-lateral nucleus and as such corresponds to the 
cutaneous nuclei in the hindbrain and spinal cord. The optic 
tract which arises in the retina is a secondare' brain tract for which 
the tectum opticum is the secondary nucleus. The secondary 
tract decussates in the ventral wall of the brain as do the secondary 
tracts from the cutaneous nuclei, and ends in the same center 
with those tracts, the tectum opticum. The tectum itself is a 
primary cutaneous center, since in several classes of vertebrates 
it receives a part of the sensory trigeminus nerve, and in the 
embryos of lower vertebrates the ophthalmicus profundus nen^e 
arises from the roof of the midbrain. In lower vertebrates the 
mode of formation of the optic vesicle bears a significant resem- 
blance to the mode of formation of the general cutaneous ganglia* 
In selachians, for example, both are formed as hollow outgrowths 
from the dorso-lateral wall of the neural tube. 

If to these facts there be added now the physiological facts 
with which this chapter was begun, a remarkably strong body of 
evidence is presented for a relationship between the eye and the 
general cutaneous sensor)^ s)rstem. The general cutaneous nerves 
are susceptible to light stimulation and the impulses carried to the 
brain produce reactions which correspond to those produced when 
the eyes are the vehicles of light perception. Whether the frog 
has the use of its eyes or only of its skin it turns its head toward 
the light and jumps toward it. Not only are the cutaneous nervesa 



THE VISUAL APPARATUS. 



151 



and retina both aflfected by light waves, but the nerve centers of 
the two are in part identical, so that the impulses arriving in the 
brain from either source produce the same reflexes. 

From these facts it must be supposed that the general ectoderm 
was originally sensitive to light and that in ancestral vertebrates 
the sensitiveness became greatest in an area favorably situated 
on the top of the head. The sensitive elements in the skin are 
the free endings of the dendrites of nerve cells. When the central 
nervous system sank below the surface the cells whose dendrites 
were distributed to the skin were in part enclosed within the neural 




Fig. 75. — A series of diagrams intended to ill'isrrate the origin and mode of for- 
mation of the optic vesicle in vertebrates. 



tube (p. 37). The greater part formed the neural crest. In 
connection with the front part of the brain no neural crest is 
formed and it must be supposed that in this region the whole of 
the nervous ectoderm was included in the neural tube. In this 
area then the cells which were especially sensitive to light became 
the rod and cone cells of the retina. Each of these is a bipolar 
cell whose two processes are comparable respectively with the 
dendrites and neurite of a typical ner\'e cell or with the peripheral 
and central processes of a spinal ganglion cell. With the growing 
thickness and opacity of the muscles and skeleton overl)dng the 



153 



NERVOUS SYSTEM OF VERTEBRATES. 



brain tube it became necessary for the retinal area to project 
toward the skin hi order to receive light stimuli. This area coa- 
tained both primarj^ receptive cells and brain centers* As it was 
carried out from the brain and the optic vesicle was formed, the 
stalk of the vesicle consisted chiefly of the fibers of the optic tract* 
Since these fibers were destined to decussate in the ventral wall 
of the brain, it was advantageous for the stalk to shift ventrad 
and so allow the tract to become shorten This is illustrated in 
an accompanying diagram (Fig, 75). Although this special visual 
organ has been developed, the general cutaneous endings retain 
their sensitiveness to fight in var)^ing degrees. 

In Amphioxus special light percipient cells are contained in 
the front end of the brain and throughout the spinal cord* These 
have been shown to be similar to the light percipient cells in the 



Epiphysis 2( 
epiphysis U 



flectum 



_ f Comrn. 
Paraphysis WW P^st. 

Velun 
Fig. 76, — A sketch showing the relations of the two epiphyses in vertebrate. 



Kuc. haben. 



ganglia of certain worms. Whether there is any relation b€ 
tween the light percipient cells of Amphioxus and the lateral eye 
of vertebrates is uncertain. 

In addition to the lateral eyes two other organs of light percep- 
tion are present in vertebrates, the degenerate or reduced pineal 
eyes. Although in most vertebrates only one of these structxires 
is present, at least in the adult, both are present in adult cydos- 
tomes, and in some reptiles one in front of the other. Ther 
position and innervation of the organ found in other dasse^^ 
of vertebrates show that sometimes it is the anterior ancft^ 
sometimes the posterior organ which is present. The anterio^^ 
organ sends its nerve fibers into the nudeus babenulae or adjacent: 
center. Those from the posterior organ go to the region of th^" 
posterior commissure and probably enter the tectum opticum- 




THE VISUAJ. APPARATUS. 



IS3 



These relations are illustrated by Figure 76- Although degen- 
erate in all vertebrates, yet one or the other of these organs is capa- 
ble of light perception, at least in cyclostomes and some reptiles, 
and perhaps in some fishes and amphibia. The structure of the 
organ is apparently much simpler than that of the retina, but in 
some forms the presence of rods or cones has been described^ 
There is evidence that the pineal eyes are not median structures. 
The organ is never quite median in its adult form and in the 
embryo paired organs begin to develop and only one persists. 
In selachians there have been described in early stages a series 
of ** accessory optic vesicles" following the true optic vesicles. 
The pineal eyes were probably originally paired organs serially 
homologous with the lateral eyes, and the three pairs represent 
the cutaneous sensory system in those segments of the head in 
which the cutaneous nerves and gangha are w^anting (p. 61), 

DEMONSTRATION OR LABORATORY WORK. 

1. Study the development of the optic vesicle, lens and optic tract 
in some vertebrate embryos. Compare the histogenesis of the retina 
with that of the brain waU, 

2. Histology of the retina in Golgi preparations. 

5. Trace the course of the optic tracts and compare the chiasma 
with the decussation of internal arcuate fibers in the medulla oblongata 

4, Study the structure of the tectum opticum in Golgi sections of 
the brain of a fish and the frog. 

5. In Golgi or Weigert sections study the tracts arising in the 
tectum and colliculus* 

LITERATURE. 

Barker^ L. F,: The Nervous System and its Constituent Neurones, New 
York. 1899. Chapter LIII especially. 

Bovcri, T.: Ueber die phylogenetischc Bedeutung der Sehorgan des 
Amphiojcus. Zool Jahrb., SuppL Bd. 7. 1904. 

Cajal, S. R»: Die Retina der Wirbelthiere. Wiesbaden. 1894. 

Eyclesh>Tner, A. C: The Development of the Optic Vesicles in Amphibia 
Jour. Morph., Vol 8. 1890. 

Froriep, A.: Die Entwickelung des Auges der Wirbelthiere. Hertwig's 
Handbuch der Entwickelungslehre. 1905. 

Hesse, R.: Die Sehorgane des Amphioxus. Zeit. f. wiss. Zool, Bd. 63. 
1898. 




154 NERVOUS SYSTEM OF VERTEBRATES. 

Johnston, J. B.: Das Gehim und die Cranialnerven der Anamnier. Merkel 
u. Bonnet's Ergebnisse, Bd. ii. 1902. 

Johnston, J. B.: The Morphology of the Vertebrate Head. Jour. Comp. 
Neur., Vol. 15. 1905. 

Johnston, J. B.: The Radix mesencephalica trigemini. Anat. Anz.. Bd. 
26. 1905. 

Kerr, J. Graham: The Development of Lepidosiren paradoxa. Part 3. 
The Development of the Skin and its Derivatives. Quart. Jour. Mic. Sci., 
Vol. 46. 1902. 

Locy, W. A.: Contribution to the Structure and Development of the 
Vertebrate Head. Jour. Morph., Vol. 11. 1895. 

Parker, G. H. : The Skin and Eyes as Receptive Origans in the Reactions 
of Frogs to Light. Amer. Jour, of Physiol., Vol. 10. 1903. 

Parker, G. H.: The Stimulation of the Integumentary Nerves of Fishes 
by Light. Amer. Jour, of Physiol., Vol. 14. 1905. 

Sargent, Porter E.: The Optic Reflex Apparatus of Vertebrates for 
Short-circuit Transmission of Motor Reflexes through Reissner's Fiber. 
Part I. Fish-like Vertebrates. Bull. Mus. Comp. Zool. Harvard Coll.. Vol. 45. 
1904. 



THE VISCERAL AFFERENT DIVISION. Z55 



CHAPTER IX. 

THB VISCERAL AFFERENT DIVISION. 
General Visceral Subdivision. 

'T^ln.c visceral aflferent fibers bring impulses from the viscera to 
ti^^ crcntral nervous system. They are distributed to the mucous 
^"*^^^^c:es in much the same way as the general cutaneous fibers 
^^^ tlrxe skin. In the absence of special knowledge as to their 
^"I^I>ic^CDpriate stimuli it may be supposed that the fiber endings 
^■^"^ ^^timulated by pressure as are the general cutaneous endings. 
'^^'-'^l^^iJugh it would be confusing to apply the term tactile to visceral 



R^doi 




i 



Fig. 77. — A diagrammatic representation of the general visceral sensory com- 
t^nents in a trunk segment. 

Impulses, it is probable that there is a close analogy between the 
two. The diflFerence between cutaneous and visceral sensory ap- 
paratus is not in the form of the endings or the mode of stimula- 
tion, but in the connections of the two kinds of fibers in the central 
nervous system. 
The visceral afferent fibers form a component of each of the 




Fio. 78, — ^A transverse section through the region d Clarke'* column of the 
thoracic cord of a new-born dog. FroniCajal (Textura, etc.). A, Clarke's column; 
B, ending of collaterals in it; C* collatfraJs ending in the intermediate nucleus; 
/?, reflex-motor collaterals; jE, ventral commissure i F, middle commissure; G, dor* 
sal commissure; II ^ cells of the dorsal commksure. 

of sympathetic nerves to some of the organs of the viscera. In 
the spinal cord these fibers have thdr central endings in a part 
of the gray matter l>ing at the base of the dorsal horn, kno\Mi 
as Clarke's columnj and perhaps in connection with other cells 



I 



THE VISCERAL AFFERENT DIVISION. IS? 

which lie near the median plane dorsal to the central canal (Fig. 
78)* This column of cells and its central relations have long been 
known in man and mammals but it is only in recent years that its 
function as the center for sensor>^ fibers of the sympathetic system 
has been proved. The neurites from the cells of Clarke's column 
go laterad to the surface of the cord and there tum cephalad to 
form a well defined tract known as the direct cerebellar iracty since 
it enters the cerebellum of the same side. As compared with the 
cutaneous sensory system the \isceral afferent system in the Xnmk 
is very small. 

In the head, on the other hand, the visceral sensory surface is 
greatly increased on account of the gills and there are present 
special organs of the sense of taste whose fibers run in the same 
nerves as the general \dsceral afferent fibers. The relation of 
these organs and their fibers to the general visceral fibers is so 
close that they may be spoken of as special visceral sensorj* struc- 
tures. Because of the extensive branchial surface and the great 
number of gustatory organs in the head, the combined visceral 
systems may be larger than the cutaneous. The visceral afferent 
fibers form usually the largest component of the X, IX and VII 
nerv^es. They are distributed to the giUs and the lining of the 
pharyTix and also to the mouth and the surface of the head and 
body wherever taste organs are found. In fishes the ganglion 
cells of this component in the vagus and glossophar\^ngeos are 
situated in the epibranchlal ganglia and the peripheral fibers reach 
their destination by way of the branchial and pharyngeal rami 
and the ramus intestinalis vagi. In the \TI ner\T the visceral 
ganglion cells form a ganglion which in different classes of verte- 
brates may be more or less closely in contact with the gangUon of 
the VIII> the lateral line or the V ner\T. The fibers enter the ramus 
palatinus and other rami of the facialis which reach the mucosa 
of the mouth and the gill of the spiracular cleft when that is present. 
In cyclostomes this component of the VII nerve is very small 
and its distribution has probably not been completely made out. 
The general arrangement of these components in the cranial 
nen-es is shown in Figs. 51, 63, 79, 80. 

In amphibia and all higher vertebrates owing to the disappear- 



THE VISCSRAL AFFERENT DIVISION. 



159 



superficial petrosal and main trunk of VII; pharyngeal and lin- 
gual rami of IX; and pharyngeal, superior laryngeal, pulmonar}% 
oesophageal, gastric and sympathetic branches of the X nerve. 
Possibly other branches also, such as the vidian nerve, carry fibers 
of this component. 

Visceral sensory centers. — The brain centers in which the 
visceral afferent fibers end in cyclostomes, ganoids, bony fishes, 
amphibia and mammals have been foimd to be directly continuous 
with the region of Clarke*s column, namely, a column occupying 
the mesial portion of the base of the dorsal hom. In all cases 
these columns become greatly enlarged at about the junction of 
the spinal cord and medulla oblongata, rise dorsaUy mesial to the 
dorsal horns and join to form a median dorsal nucleus, the nucleus 



N.IX 



N. branch. 1 



R^ intest. 




R. phary ng, 
R, posttr, 
R. prt^lr. 

Fio. 80. — A simple dmgrani of the visceral nerves of the head in fishes. 

commissurailsy first described by Cajal in the mouse. The cells 
of the nucleus commissuralis lie imbedded among the fibers of a 
commissure which has been known in fishes as the commissura 
infima. Just cephalad of this nucleus and commissure the lateral 
walls of the brain spread apart and are connected dorsaUy only 
by the choroid plexus of the fourth ventricle. The visceral 
columns now appear as more or less prominent ridges continuing 
forward in the lateral walls of the fourth ventricle, which have 
been described as includinj^ the vagal and facial lobes. 
As the visceral fibers in the X, IX and VII nen^es enter these 



i6o 



NERVOUS SYSTEM OF VERTEBRATES. 



lobes many of them divide into terminal branches inmiediately, 
and in many fishes the visceral lobe is strongly thickened opposite 
each nerve root so that it appears somewhat like a string of beads 
(Figs. 2, 3, ii). The bifurcation of the fibers and the formation 
of a longitudinal tract are not such prominent features in the 
visceral system as in the cutaneous. A part of the fibers, however, 
do turn caudad and in fishes form a diffuse bundle which extends 
to the nucleus commissuralis, where a part of the fibers cross 




Fig. 8i. — Four transverse sections through the meduUa oblongata of the frog to 
show the position and ending of the f asciouus communis and the nudeiis commis^ 
suralis. C. inj. H.^ commissura infima; nuc. of C, nucleus commissuralis; /. c, 
fasciculus communis; d. h., dorsal horn. 

to the opposite side in the commissura infima. In amphibia 
(Fig. 8i) these fibers form a well defined bundle which on account 
of its relation to the three nerves, VII, IX and X has been given 
the name fasciculus communis. In terrestrial vertebrates the 
loss of the gills has led to a great reduction in size of this system, 
and the centers no longer form conspicuous ridges projecting into 
the ventricle. Instead, there is in mammals only a small, sharply- 
defined longitudinal bundle of fibers comparable to the fasciculusi^- 
communis, which is known as the fcLSciculus solUarius^ and a*-— - 



THE VISCERAL AFFERENT DIVISION. 



i6i 



small column of cells accompaiiyiiig this bundle. In the mouse 
(Figs. 82, 83}, as in fishes and amphibia^ the descending fibers 
enter the nucleus commissuraHs and help to form the commissure. 
In all classes a part of the fibers continue caudad beyond the 
commissura infima in the visceral colimin of the cord. 

In fishes, where these centers are best known> their structure 
is relatively simple. The terminal branches of the afferent fibers 
are short but ven- profusely subdivided and the dendrites of the 
cells often have the same characteristics, so that sections of this 






AX^W 



•^^.t 



K^i 



' ^i^ 



M\ 



Fig- 82* — Transverse sedion through the medulla oblongata of the mouse at the 
* ^!^Ycl of thr nucleus commissuralis. From Cajal (Beltragc u, s. w.). A, nucleus 
mmissuralis; B, nucleus of hypoglossus; C» decussation of lemniscus j D, fasci- 
Vilus scilitarius; fe> c^ endings of libers of LX and X nerves. 



^:^nter prepared by the Golgi method show dense brushes of 

^:ftntricatcly inter^voven fibers which have as a whole a ver)' furry 

^•^ppearance (Fig, 84). Many of the cells are of type 11 whose 

fc^eurites terminate within the lobe. In fishes whose gustatory 

^B^pparatus is largely developed these intrinsic neurones are very 

mmetpus. Other cells send their neurites out of the sensory 




Fxc. ^^. — Transverse section through the medulia oblongata of a mouse four 
days' old. From Cajal (Bcilrjigc u. s. w.). .1, hypoglossal nucleus; B, nucleus 
cornmi&su rails; C, olive; D, spinal V tract; £, motor roots of IX and X, F^ 
nucleus ambiguus; G, caudal portion of ihe nucleus of the descending root of the 
nervus vestibularis; H, fasciculus soHtanus; d, pyramids; /, general cutaneous 
components in IX and X (?); A» collaterals of fasciculus solilanus ending in its 
ftccorapanying nucleus. 

cephalic and caudal branches or run forward or backward with- 
out dividing. The descending fibers form a diffuse bundle which, 
after giving part of its fibers to a nucleus adjacent to the nucleus^ 
comniissuraUs (bony fishes), grows smaller caudally and is k 



i. 





Fig, 84. — Transverse stsnions through ihemedulla oblongata of the sturgeon; A, 
^t the level of the X nen'e; B, at the level of the IX nerve. A is at a higher nmg- 
^lification than B. 



[<Fig5. 89, 92), This bundle has been called the secondary vagus 

h^ract in those fishes in which the vagal part of the visceral lobe 

[5s especially developed, but it is evidently a secondary \isceral 

l:ract and the nucleus in which it ends, a secondary' \isceral nucleus. 

There is reason to beheve that this tract and nucleus correspond 



i64 



KERVOUS SYSTEM OF VERTEBRATES. 



to the direct cerebellar tract and its nycleus in man, but it is vei 
desirable that the structures should be worked out in inter- 
mediate classes, especially reptiles and lower mammals. 

Since the \isceral centers are most highly developed in those 
forms in which the gustatory organs are most numerous, the further 
description of the secondar)^ \isceral nuclei will be given in the 
following section. 



Special Visceral or Gustatory System, 



1 



In fishes, as already stated (p, 22), the gustator)^ organs have 
a much wider distribution than in higher vertebrates. As far as 
is at present known the distribution of taste organs in the different 
classes of vertebrates is as follows. In cyclostome lan^al forms 
they have been found only in the pharynx, on the inner surface of 
the branchial arches. In adults they arc also irregularly distrib- 
uted over the surface of the head and branchial region of the 
body. Whether they occur in the trunk and tail region has not 
been ascertained. In ganoids they are present in the mouth 
and phar)iix and in considerable numbers over the surface of 
the head. In embr)'0s of bony fishes they are found in the pharjTix, 
oesophagus and mouth. In adults they reach their greatest develop- 
mcTit both as to number and distribution, being found also on 
the surface of the head and in many forms on the fins and practi- 
cally o\*er the whole body. In amphibia they are found on the 
tongue and mucosa of the mouth, being especially numerous on 
the papillae of the tongue. The so-called multicellular glands 
in the roof of the phar}^nx of tadpoles are probably taste organs. 
In man they arc foimd in small numbers on the general surface 
of the tongue, more numerous along the sides of the tongue, and 
most numerous on and around the drcumvallate papillae and on 
a region at the back of the tongue corresponding to the papillae 
foliatae of some mammals. They are also numerous on the 
anterior surface of the soft palate and on the posterior surface 
of the epiglottis. H 

The following facts show that the taste organs have their origin 
in the entoderm, (i) They arise in the lining of the pharynx in 
Ammocoetes, and are not found in the skin during larval life. 




THE VISCERAL AJFERENT DIVISION. 



X65 



(2) In ganoids and bony fishes (Amiaf Catosiomus) they arise in 
the entodermal Ening of the pharynx, oesophagus and mouth. 
After the time of hatching, the mucosa pushes out over the lips 
and taste organs appear in the spreading entoderm. (3) In am* 
phibians {Amblystama pumiatum^ Rana) the taste buds arise and 
remain throughout life in the entodermal area of the pharv'nx and 
mouth. Although the limits of ectoderm and entoderm have not 
been determined in reptiles, birds and mammals, the most rea- 



B 



Fto. 85. — ^Sense organs of bony fishes. A, a taste bud from the oesophagus of 
Cck>sU>mus at the time of haiching; B, a taste bud from the phannx of the same 
embn'o; C, two neuromasts from the skin of the same embryo. 

sonable inference from the position of the organs is that they lie 
in entodermal territory. 

The taste organs differ from the pit and canal organs: (i) in 
being surface organs not sunken in pits but sometimes projecting 
above the surface as hillocks; and (2) in that the sense ccUs are 
long, slender and rod-shaped and extend the full depth of the 




i66 



NERVOUS SYSTEM OF VERTEBRATES. 



epidermis. Each sense cell tenninates at the surface by a rod- 
like or hair-like pjiojection which is much shorter than the 
sense hairs of the neuromast organs. Since the taste organs are__ 
stimulated by chemical changes they do not need the long sensdB 
hairs which adapt the neuromasts to stimulation by vibrations 
in fluids. Beneath the organ nen^e fibers penetrate the dermis, 
lose their sheaths and end by fine branches in contact with the 
sense cells. A more intimate xmion between the cells and terminal 
branches of the fibers has not been seen. To facilitate comparison 
between neuromasts and taste organs there are shown in Fig. 85 
two taste organs and two neuromasts from the same embr^'o of a 
bony fish. The tall form of taste organ is taken from the lining 
of one of the^giirarches and represents the prevailing form in the 





Fig. 66,- 
dorsotus. 



-A taste organ from thejfphary'Tix of the ammoccetes of Peiromy 



pharynx, where the organs are best developed. The lower organ 
is taken from the oesophagus near the opening of the duct of th^H 
swim bladder and is of the same form as tliose in the mouth. 
The sharp contrast between the sense cells in these and in the 
neuromasts requires no comment. In Fig. 86 is draiMi a taste 
organ from the pharynx of the lar\'a of a cyclostome and in Fig. 
87 a taste organ from the skin of an adult of another species. The 
great difference in form of the two organs suggests a profound 
influence of the surrounding structures, but both organs have 
the same type of sense cells as are in the taste organs of all verte-| 
brates. 

The innen^ation of the taste organs is as follows. Those 
the pharynx are innervated by the \'isceral branches of the vagusJ 




THE VISCERAL AFFERENT DIVISION. 

and glossophar>^ngeus nerv^es. Those in the mouth are supplied 
by the palatine and hyomandibular rami of the facialis, by the 
phary^ngeal ramus of IX which extends into the roof of the mouth, 
and by the ramus lingualis of the IX nerve. Those on the surface 
of the head and body in ganoids and bony fishes are supplied 
by components of the ophthalmic and maxiilary rami and by a 
great system of superficial nerves which ha\'e been known imder 
the name of ramus lateralis accessorius (or nerve of Weber)* The 
name is unfortunate because it is likely to suggest some relation 
wiih the lateral line nerve, with which this nerve is to be sharply 
contrasted. The present nene (Figs. 63, SS) arises from the 
visceral afferent root of VII and sometimes in smaller part from 

m ■ 



m 



Fte. 87. — A taste organ from the skin of nn adult LampeirOm 



^he corresponding root of X, passes up through ihe cranium to 
the dorsal surface of the head and is distributed to the back, 

f the tail and the fins, wherever taste buds are found. It has been 
definitely shown that taste buds, where%x'r they are situated, 
are always inner\ated by fibers derived from the visceral afferent 
roots of the VII, IX and X nerv^es and ending centrally in the 
visceral lobe. Where the taste buds are very numerous in the 
skin they are always innervated from the root of the faciahs nervT 
and there is developed a special pars facialis of the visceral lobe. 
The innerv^ation of the taste buds in man is still somewhat 
In doubt. It was long supposed that fibers were supplied to 




i68 



NERVOUS SYSTEM OF \^RTEBR.\TES. 



taste buds by three nerves, the ramus Ungualis IX, the chorda 
t>Tiipani and the ramus lingualis V. In certain cases in which 
the Gasserian ganglion was removed and in which the clinical 
obser\^ations were continued for an unusually long lime, it was 
clearly sho\\Ti that taste was not at all impaired and that the 
trigeminus pro\'ided only for tactile sensation. This is in agree- 
ment with the conditions in lower vertebrates, but it is still possible 
that taste fibers nm in the trigeminus in exceptional cases. The 
homolog)^ of the chorda tympani has been in doubt because it 
was not certainly known whether the chorda of mammals is a 
pretrcmatic or posttrematic ramus. It seems dear now that 



Fig. 88.^A pmjeclion of tJie cutant'ous branches of the communis root of the 
right fadaJ nvrvc in a bony fish, Ameiurus. From C. Judson Herrick. All ihe 
branches drawn arc gustaton- in function* Thase wliich supply taste buds \\ithm 
the mouth are not shown. 

is a posttrematic ncr\T and that the ramus mandibularis VII 
as seen in the frog tadpole and in fishes is its homologue. The 
ramus lingualis IX is the homologue of the posttrematic ramus 
of the IX ner\'e in fishes and amphibia, which is prolonged into^ 
the tongue. 

The taste fibers enter the same centers with the general visceral 
fibers and no means has yet been discovered of distinguishing 
between the two. However it is probable that of the two sorts 
of secondary' fibers mentioned in the last section, one serves 
general visceral and the other chiefly gustator)^ functions. Those 
which make direct connections with the motor nuclei of the cranial 
nen^es carr)^ only impulses from the visceral surfaces analogous 




THE VISCERAL AFFERENT DIVISION* 



I 



I 
I 

I 

I 



to tactile impulses from the skin and give rise to reflex contractions 
of the branchial muscles controlling the respirator}^ movements. 
It is inherently probable that a part of the libers of the long tract 
to the metencephalic nucleus have general visceral functions, 
especially as such a tract is present in mammals coming forward 
from the trunk, where taste organs are out of the question. We 
must at present supjx)se that when taste buds were first developed 
in vertebrates they came to be innerv^ated by the general visceral 
fibers which already supplied the area in which the taste buds 
appeared. As the gustator}^ system came to be more important 
the long secondary tracts came especially into its service for the 
reason that these long tracts make possible more complex reflexes 
adapted to the capturing of food. 

It is in the bony fishes, where the gustatory^ apparatus has 
reached an enormous development, that the secondary and tertiary 
connections of the taste center have been most fully worked out 
(Figs. 89, 90). Here the secondar}^ visceral tract has the same 
general airangemenl as has been described above. As the gusta- 
tory elements greatly preponderate in it, we may call it the secondary 
gustatory tract. Caudally a part of this tract extends into the 
spinal cord and a part ends in an inferior secondary gustatory 
ntdceiis adjoining the nucleus commissuralis. Complex rela- 
tions between this and the adjacent nucleus funiculi (cutaneous 
center) perhaps enable the animal to correlate tactile with gusta- 
tory impulses in the control of movements for the capture of 
food. Cephalad the tract ends in the metencephalic nucleus, 
which Herrick calls the superior secondary gustatory nucleus. 
Part of the fibers end in the nucleus of the same side and part 
cross to the opposite side in a commissure which may be called 
the inferior cerebellar commissure. Tliis commissure contains 
also fibers arising from the cells of the secondar}^ nucleus. The 
nucleus is an enlargement of the gray matter bounding the fourth 
ventricle laterally and is very rich in cells and in terminal rami- 
fications of fibers. From this nucleus, in addition to the com- 
missural fibers, arises a tract which runs to the inferior lobes of 
fhe diencephalon. Other fibei^ from the secondary gustatory 
nucleus seem to go into the cerebellum and the tectum mesen- 



THE VISCERAL ATTERENT DIVISION. 



171 



» 



cephali. These may serve the more fully to bring about correlations 
with the somatic muscles in addition to the connections in the 
funicular region. From the inferior lobes two tracts already 
well known in lower vertebrates may forward the gustatory impulses. 
One of these is the tracius lobo-bulbaris which runs back through 
the myelcncephalon and makes connections with the motor nuclei 
of the cranial nen'es. The second tract is the tr actus lobo-epi- 
sfriaiicus which carries impulses to that large coordinating center 
of the forebrain (epistriatum) which is primarily a part of the 
olfactory apparatus and in amphibia, reptiles and mammals 
develops into the hippocampal lobe and adjacent parts of the 
true olfactor)^ cortex- (See Chapter XVIII,) It is possible that 
this tract is retained in higher vertebrates and serves as the path 
of gustatory sensations in the true sense. The chief central 
gustatory connections arc shown in Figs, 89 and 90. 

The arrangement of the secondar>^ gustatory tracts and nuclei 
in selachians throws light on certain important structures peculiar 
to the brain of ganoids and teleosts» In both these classes the 
superior secondary gustator)^ nucleus is large, lies in the lateral 
i^raU of the metencephalon and sends its commissure through a 

Ftc. S9. — A parasagittal section through the brain of the spotted sucker, Miny- 
Srema meianops, to show the gustatory centers and tracts. From C. Judson 
HcjTick. 

The sketch is designed to illustrate tiie course of the ascending secondary 
gustatory tract and the connections of its terminal nucleus. The plane of the 
section is slightly oblique so that the caudal end and the ventral side arc nearer the 
median line than are the cephalic and dorsal borders. The figure is a composite, 
made by outlining one section mth the camera lucida and filling in the details from 
this section and the three scciions of the same scries on each side immediately 
adjacent, omitting irrelevant detail. The features introduced are schematized as 
little as possible. The whole course of the ascending secondary gustatory tract 
from the fadal lobe is shovi-n. The origin of the tract from the vagal lobe hes 
fa^rther bteml 

A., tract between secondary gustatorj' nucleus and n. lateralis valvulae; com.h,^ 
commissura horixontalis, Fkitsch; com. r. VII ^ communis (gustatory) root of the 
facialis; descsec.X, descending secondary gustator>' tract from the vagal lobe; /J.wr,, 
fasciculus longitudinalis medialis; injjob.lat.y lateral lobule of inferior lobe 
Chypoaria, C. L. Herrick); tnj.hb.m,^ median lobule of inferior lobt^ (mammil- 
Ure, C. L. Herrick); nxori., nucleus corticalis, FniTScn; n.IX, motor nucleus of 
the glossopharyngeus; nMt., nucleus lateralis valvulae; n.roL^ nucleus rotunduSp 
Fritsch; n.st.^ nucleus subthakraicus, C. L. Hf.krick; n.V^ motor nucleus of the 
trigeminus; n.VIIj motor nucleus of the facialis ;r.-V.j., sensory root dbei^ of the 
vagus; sfcgHstJ., ascending secondary gustatory tract from the fadal and vagal 
lobes; tr.Lb., tracius lobo-bulbaris; trJ-c, tract us tecto-cerebcUaris; tr.t-lob., tract us 
tccto-lobaris, Johnston (commissura ven trails, C. L. Herrick). The area marked 
nJunkuJi contains also the inferior secondary* gustatory nucleus. 



172 



NERVOUS SYSTEM OF VERTEBRATES. 



part of the cerebellum which projects into and largely fills up the 
ventricle of the mesencephalon, — ihc valvula cereheUi, In selach- 
ians no valvula is present. The cerebellum corresponds to that 
part of the cerebellum of bony fishes which is folded upward and 
outward. This will be clear from a comparison of Fig* 91, which 
represents a sagittal section of the cerebellum of a newly hatched 
bony fish, with Fig. 11, representing a sagittal section of a selachian 
brain. The superior second an^ gustator}^ nuclei in selachians 
are situated higher in the dorso- lateral wall of the metencephalon 
than in bonv fishes and their commissure crosses in the roof ol* 



cetebeHumj 



UL 



Jobus I/// 
fobus (X 
Jobus 



fore 
brmn 



valvu/a 
cersbe/// 



JJ 



fd\ 



m 



'^n'or (ob^ 



.y//s 



JXsf 



.X.s 



Fig, 90.^ — A diagram of the gustator>' paths in the brain of the carp as seen from 
the left side. From C. Judson Herrick. 

n.VlLs., n.IX,s.f and n.X.s.^ represent the setisory root fibers of the facialis, 
glossopharyngeus and vagus respectively, or gustatory neurones of the first order 
(/)> The secondary tracts, l>oth ascending and descending, are marked //. The 
tertiary path to the inferior lobe is marked ///; the path to the cerebelluro and 
vaKiila, iiu The return path from the inferior lobe to the motor nuclei of the 
oblongata (traclus lobo-bulbaris) is marked IV, The commissures of the inferior 
and superior secondary nuclei are indicated by shaded areas (the latter marked c ). 
n,op., the optic ner\'e. The area marked n.jun. includes the funicular nucleus and 
the inferior secondary gustatorj* nucleus. 



the brain at the junction of the tectum opticum and cerebellum. 
This commissure has been described in selachians and amphibia 
as the decussadon of the velum medullare anterius. It is obvious 
that it is homologous with the inferior cerebellar commissure 
which passes through the valvula of bony fishes. Hence it must 




THE VISCERAL AFFERENT DIVISION. 



173 



I 



I 



be supfKJsed that the valvula has been formed by a growth and 
folding inward of the velum of selachians. The cause for the 
growth of this large structure is to be found in the great increase 
in the gustator)^ system in ganoids and bony fishes. 

In selachians a part of the secondary fibers from the visceral 
lobe cross to the opposite side as do the internal arcuate fibers 
from the cutaneous nuclei. The tertiar}^ tract from the superior 
secondar)^ gustatory nucleus goes as in bony fishes to the inferior 
lob^. Apparentiy in the more primitive brains the inferior lobes 
as a whole were related to both giistator)^ and olfactor)^ systems 
( cf. Chap. XA'^II). The relations in selachians are taken as the 
basis for the general diagram of visceral sensor)^ structures^ Fig- 
ure 92. Figures 112-117 in Chapter XV may also be consulted. 

The taste bitds are evidently unable to persist on the surface 




Secondary gU5tati>r>' tract 



Fig. 91* — Part of a sagittal section of the brain of a newly hatched bony fish, 
^QtostomuSf to show the relations of the secondary gustatory tract and the valvula 
<^^rebeUt 

■C>f the body in terrestrial animals, and in amphibia, reptiles, 
^irds and mammals they are confined to the mouth cavity* Besides 
the great modification in structure of the gustatory system which 
t^liis change entailed, a great change in function has been brought 
^bout. In primitive fishes the taste organs in mouth and 
t^harjnx detected indications of food brought by the respiratory 
"vs^ater current. In fishes where the taste buds are situated on the 
surface of the body they sen^e to detect the presence of food, and 



174 



NERVOUS SYSTEM OF VERTEBRATES. 



frequently special organs such as the barblets about the mouth, 
or the fins, are richly provided with taste buds and are dragged 
about on the bottom or otherwise used in the active search for food. 
In short, the fimction of the taste organs of fish-like vertebrates 
is to detect food and to discover its location. In terrestrial animals, 
on the other hand, the taste organs in the mouth can rarely be 
used in the search for food. Their only service is to test the 
food after it is taken into the mouth to discover if it is desirable 
to eat. Terrestrial vertebrates depend chiefly on the senses of 
smell and sight for finding food. This explains the relatively 



Tertiaiy gustatory Irai 
Tr- lobo-bulbi 



itcLisstUio veil 




vtsceralfs cerebelli 
Visceral sensory 
column 

Fasc communis 

Qomm. 
infima 
Nuc. 
funic 



Tr. lobcMepistr. 
Lohus inferior 



Corpus mammiiiaxe 111 



X Xii spvj 

i^usiatory tract 



Tr. mammillo-bulbaris 



Fig. 92. — A diagram representing the centers and tracts related to the visceral 
sensory comp>oncnts in fishes. 



slight importance of the sense of taste and the small size of its 
apparatus in higher vertebrates. At the same time, in higher 
vertebrates the general visceral system has been reduced in the 
head by the loss of the gills and more highly developed in the 
trunk in connection with the sympathetic system. 

A comparison of the diagram for the \dsceral and gustatory 
apparatus with those for the gen6ral and special cutaneous appa- 
ratus will show how widely these di\asions of the nervous S5rstem 
differ from one another. The student should now turn to the 
tabular definition of the two divisions (on p. loi) and review this in 
connection with the figures in this and previous chapters. 



THE VISCERAL AFFERENT DIVISION. 175 

DEMONSTRATION OR LABORATORY WORK. 

1. Review the dissections called for in Chapter V, Nos. i and 2 
'wnth reference to the visceral nerves and the visceral lobe in the brain. 

2. Study the taste buds in sections of young fry of bony fishes, in 
£irog tadpoles, and in the mammalian tongue. 

3. In haematoxylin or Weigert sections of the brain of a bony fish, 
s^elachian or frog, study the sensory roots of the X, IX and VII nerves 
SL,nd the formation and course of the fasciculus communis. 

4. Study the endings of visceral sensory fibers and the types of 
Cecils in the vagal and facial lobes in sections of the fish brain prepared 
"by the method of Golgi. 

5. The secondary' and tertiary gustatory' tracts and the secondary' 
nuclei are best studied in Golgi and Weigert sections in the brains of 
^eleosts, ganoids or selachians. 

LITERATURE. 

Cajal, S. R.: Beitrilge zum Studium der Medulla oblongata. Leipzig. 1896. 

Gushing: The Taste Buds and their Independence of the N. trigeminus. 
Johns Hopkins Hospital Bulletin, Vol. 14. 1903. 

Dohm, Anton: Studien zur Urgeschichte des Wirbelthierkorpers. No. 
- :xni. Mitth. Zool. Sta. zu Neapel. 1888. 

Gaskell, W. H.: On the Structure, Distribution and Functions of the 
l^erves which innervate the Viscera and Vascular Systems. Jour, of Physiol., 
Vol. 7. 1886. 

Herrick, C. Judson: The Cranial Nerves and Cutaneous Sense Organs 
of North American Siluroid Fishes. Jour. Comp. Neur., Vol. 11. 1901. 

Herrickr C. Judson: The Organ and Sense of Taste in Fishes. Bull. 
U. S. F. C. 1902. 

Herrick, C. Judson: On the Phylogeny and Morphological Position of 
the Terminal Buds of Fishes. Jour. Comp. Neur., Vol. 13. 1903. 

Herrick, C. Judson: The Central Gustatory Paths in the Brains of Bony 
Fishes. Jour. Comp. Neur. and Psych., Vol. 15. 1905. 

Johnston, J B.: The Brain of Petromyzon. Jour. Comp. Neur., Vol. 12. 
1902. 

Johnston, J. B.: The Cranial Nerve Components of Petromyzon. Morph. 
Jahib., Bd. 34. 1905. 

Johnston, J. B.: The Radix mesencephalica trigemini. Ganglion isthmi. 
Anat. Anz., Bd. 26. 1905. 

Onuf and Collins: Experimental Researches on the Central Localization 
of the Sjrmpathetic with a Critical Review of its Anatomy and Physiology. 
Arch. Neur. and Psychopath., Vol. 3. 1900. 

SchafFer, Jos.: Ucber das Epithel des Kiemendarms von Ammocoetes nebst 
Bemericungen ikber intraepitheliale DrUsen. Arch. f. mik . Anat. Bd. 45 . 1895. 



176 



NERVOUS SYSTEM OF VERTEBRATES. 



CHAPTER X. 
THE OLFACTORY APPARATUS. 

The olfactorj^ apparatus is treated here because of the physiolog- 
ical relationship between it and the gustatory apparatus. Whether 
a morphological relationship exists or not has yet to be determined. 

The peripheral organ of smell consists of a sensory epithelium 
situated at the extreme anterior end of the head. It arises in 
all vertebrates as a pair of thickenings of the ectoderm one at 
either side of the lower or anterior border of the neuropore. The 
thickenings are therefore formed from ectoderm which hes dose 
at the border of the neural plate before it rolls up into the neural 
tube. While the brain and optic vesicles are developing and the 
secondary di\isions of the brain are taking form, the two epithelial 
thickenings become depressed and sink into the angle between 
the cephalic surface of the optic vesicles and the brain, and even- 
tually form deep pits or sacs each with an external opening. At the 
same time that these nasal pits are fonidng there appears below 
and between them a single invagination which has been described 
as the hypophysis (p. 66). The hypophysis is the remnant of 
the ancient mouth and in some forms still has a connection with 
the archenteron for a short time during development. It appears 
that the olfactory organ was originally situated in the roof of the 
mouth or in front of it. In this position the organ must have been 
useful both in linding and selecting food. When the hypophysis 
ceased to serve as mouth and a new^ mouth was formed from a 
pair of gill slits the olfactor)^ organ w^as farther removed from the 
mouth but still in front of it. Finally, in air-breathing \x*rtebrate3 
the olfactory sac comes to have a connection with the pharynx 
and to serve as a passage for air in respiration. In this position 
the organ is favorably placed for detecting odors and so for finding 
food. 

In the waU of the nasal sac a part of the cells develop into sense 



4 



4 



THE OlFACTORY APPARATUS. 



177 



I 



cells, the remainder act as supporting cells. The sense cells 
differ from those in any other sense organs of vertebrates and 
resemble a common type of sense cells in invertebrates* Each is 
a slender spindle-shaped ccU from the inner end of which a nen^e 
fiber arises and nms toward the brain (Fig. 93). The fibers 
grow out from the olfactor}^ cells during development and enter 
the olfactor}^ bulbs of the forebrain, where they end in a manner 
to be described below. These fibers constitute the olfactory 
neRT. They^ never havt myelin sheaths. 

The olfactory centers. In fishes the greater part of the 
forebrain is related exclusively to olfactor\^ impulses. The primary 




Fig. 93. — A section of the olfactory epithelium of a bony fish al the time of 
hatching, to show the origin of the fibers of the olfactory uerv^e. 

center for olfactory fibers is the olfactor)^ bulb. In cyclostomes the 
bulb consists of several layers of cells surrounding the ventricle 
and of a superficial zone of fibers (Fig. 94). Nearly all the cells 

■ are either spindle-shaped and provided with a single dendrite 
vertically placed in the wall of the bulb or are stellate and have 
two or more dendrites which run obliquely toward the surface. 

P The final branches of the dendrites are intermingled with the 
end-branches of olfactory fibers near the surface of the bulb* 
The olfactory fibers run in bundles as they enter the bulb and as 
the individual fibers divide and subdivide the bundles form larger 

■ or smaller cylindrical masses of interwoven nerv^e fibrils. Into 
each of these masses the dendrites of numerous cells penetrate 
and so receive impulses from the olfactory fibers. The combined 
mass of fibers and dendrites is a glomerulus. There is noobscrvable 
difference in the relations of the various nerve cells of different 
sizes and shapes to these glomeruU. There is to be seen, how- 
ever, the beginning of a special t}^e of cells w^Wch come to pre- 




178 



NERVOUS SYSTEM OF VERTEBRATES. 



dominate in higher vertebrates. In the outer portion of the cell 
layer are seen a few cells of relatively large size whose dendrites 
are short and thick and more profusely branched than are those 
of other cells, and enter only one or two glomeruli. Although 




Fig. 94. — An oblique section of the forebrain of Lampetra^ passing through the 
optic chiasma and the olfactory commissure, a, 0., olfactory lobe; c. h,^ optic 
cniasma; c. 0., olfactory commissure; /. h. ven., fore brain ventricle; /. o. olfactory 
bulb; m, mitral cells; r. po., recessus postopticus; 5, striatum; //, optic tract. 

these cells are not highly specialized in cyclostomes they can be 
recognized as the forerunners of the so-called mitral cells. 

In true fishes and amphibia the structure of the bulb is much 
the same as in cyclostomes but there is a growing specialization 



i8o 



NERVOUS SYSTEM OF VERTEBRATES. 



that the glomerulus becomes an exceedingly complex mterladng 
of dendrites and olfactory fibers. The glomeruli also receive 
the dendrites of stellate and spindle cells (Figs. 96, 97) and some 
small glomeruli are formed by the dendrites of these cells alone. 
The transition from this to the condition found in mammals 
could probably be found in reptiles but has not yet been studied. 




Fig. 96. — A spindle cell and two granules from the olfactoi}' bulb of thesturg^^^ 
gt granules; gl., glomeruli; sp.^ spindle cell. 

In mammals the mitral cells are very highly developed and tb-^ 
are in addition to them numerous cells with short neurites. (Fig- ^* 
In the deeper parts of the bulb are a great number of small c^^ 
which seem to have no neurites, and whose dendrites rise to'^v^^ 
the glomerular zone without entering the glomeruli. These ^' 



THE OLFACTORY APPARATUS. 



l8l 



JfC-nown as granule cdls and probably represent the greater number 
^^l spindle and stellate cells of lower forms. It is supposed that 
3^^ the coinse of evolution the mitral cells became predominant 
-^^hile the smaller and less efficient cells lost their fimction and 
^:^ow exist only as vestigeal structures, the granule cells. By 
^>ome authors, however, the granule cells are described as of two 
J^^ds; one possessed of short neurites ending within the bulb, 
-i^lie other giving rise to excessively fine neurites which run in the 
^>lfactory tract to the forebrain nuclei. Mention must be made 

Olfactory fibers. 




M lira) cell dendrite. 
Bendfites of deep cells. 



^IG. 97. — An oUactorv glomerulus from the brain of the sturgeon to show the 
^*^*^ taken by the dendntes of deep'cells in forming the glomeruli. 



^5 cenirifugal fibers which come forward from the forebrain 
^ the olfactory tract and end in the olfactory bulb. It is probable 
^bat these are commissural fibers coming from the bulb of the other 
side. 

In all vertebrates the cells whose dendrites help to form the 
^omeruli send their neurites inward toward the ventricle and 
backward toward the forebrain, and there is no evidence that 
the neurites from the different types of cells have any different 
destinations or behavior. The whole number of fibers arising 



l82 



NERVOUS SYSTEM OF VERTEBRATES. 



in the olfactory bulb (or ending in it from behind) form the olfac- 
tory tract. In many fishes and some reptiles the olfactory bulbs 
are drawn forward some distance from the forebrain so that 
the olfactory tract is a long cylindrical structure which contains 
a slender continuation of the forebrain ventricle. In cyclostomes, 
amphibia and mammals the bulb is closely applied to the fore- 




FlG. 98. — Cells with short neurites in the olfactory bulb of mammals. Schematic 
figiire from Cajal. A , Golgi cell; B^ cell with peripheral neurite; C, horizontal fusiform 
cell of the internal plexiform layer; Z>, cell with horizontal neurite; E and F, 
periglrmerular corpuscles; a, collateral of mitral cell neurite; 6, collateral of a small 
brush cell. 



brain so that the olfactory tract does not run for any considerable 
distance free from surrounding gray matter. In all cases the fibers 
of the olfactory tract enter one or other of several groups of cells 
which collectively should be called the olfactory lobe or area. 



THE OLFACTORY APPARATUS. 



183 



The olfactory tract is the secondarv^ central tract of the olfactory 
apparatus; the nuclei of the olfactory' lobe are the secondary 
nuclei. 

These secondary nuclei form a larger or smaller part of the 
forebrain in diflferent vertebrates according to the importance 
of the olfactory organ. In cyclostomes the lateral lobes of the 
forebrain are divided by a vertical groove into anterior and pos- 
terior halves. The anterior half is the olfactor)^ bulb; the posterior 
half is mostly occupied by the lobe, which is not di\ided into nuclei 
but is a continuous mass. In selachians the bulb is distinct 
from the lobe and a longer or shorter olfactor}^ tract is present. 
The olfactory lobes occupy the anterior, the lateral and a larger 
or smaller part of the dorsal wall of the forebrain. The olfactory 
organ is very important in selachians and the large development 
of the olfactory centers accounts for the relatively great size of the 
forebrain in those fishes. In bony fishes the olfactory organ 
is much less important and the centers are correspondingly small, 
being confined to the lateral and ventral part of the forebrain. 
The caudo-lateral part becomes especially differentiated and is 
known as the nucleus thaeniae. In amphibia and reptiles the 
olfactory lobe has a similar extent. In mammals, owing to the 
great development of the cerebral hemispheres, the olfactory lobe 
is relatively very small It includes the tuberculum olfactorium 
and praecommissural body occupying a small area on the ventral 
and mesial aspect of the hemisphere in front of the commissures, 
and the pyriform lobe which extends along the ventral surface 
of the corpus striatum at the junction with the lateral cortex. 
The olfactory lobe may be divided into cephalic and lateral 
portions. The cephalic portion occupies the front wall of the 
forebrain, in front of the commissiires. In lower vertebrates 
It includes two or more collections of cells which have been called 
"t:he (median, lateral and dorsal) postolfactory nuclei. The 
lateral portion is an extensive nucleus covering the outer surface 
i the corpus striatum, a part of which is the nucleus thaeniac. 
The cells of all of these nuclei are irregular stellate or spindle 
lis which show no detmite orientation. The fibers which end 
these nuclei are all olfactor>^ tract fibers which come from the 



184 



NERVOUS SYSTEM OF VERTEBRATES. 



bulb of the same side or from that of the opposite side, crossing 
in the anterior commissure. The fibers which arise from the i 
nuclei form two main tracts, the tractm oljacto-hypothalamicus 
and the tr actus oljacto-kabenularis. The fibers of the former 
tract arise from the cells of both portions, run backward through 
the lateral and ventral parts of the corpus striatum and at the 
junction of the forebrain and thalamus appear in a medial and a 
lateral bundle. These enter the diencephalon and end in the 
hypothalamus. The tractus olfactohabenularis is smaller. It 
arises chiefly from the nucleus thaeniae and sometimes from the 
other olfactor)^ nuclei including the nucleus praeopticus in the 
walls of the preoptic recess, and runs upward and backward 
through the cpistriatum to the dorsal part of the diencephalon* 
There each tract wholly or partially crosses to the opposite side, 



Tr. olf act o- 
corticalis. 



Tr. olfacto-haben. 



Tr. strio-thalam, 




Fiber of olfac 
tory tract. 



Lateral olfdctory 
nucleus. 



Fro, gg, — A transverse section of the brain of the sturgeon at the level ol \ 
interior commissure. 



the decussation of the two tracts forming what is known as the 
superior or habenular commissure. The tracts end in the nuclei 
habenulae. These two tracts are tertiarv^ olfactory tracts. 

,%iother secondar)^ nucleus in the forebrain is to be described. 
The central gray matter surrounding the forebrain ventricle, 
bounded in front by the postolfactory nuclei and covered laterally 
and ventrally by the lateral olfactor)^ nucleus and the corpus 
striatum, is known as the epistriatum. The epistriatum is com* 
posed of p)Tamidal cells arranged in rows, the cell-bodies being 
near the ventricle and the dendrites directed toward the surface. 



f 




THE OtFACTORY APPARATUS. 



i8s 



A laxge number of fibers of the olfactor)^ tract which decussate 
in the anterior comniissure end in the epistriatum. As has been 
mentioned (p. i8i), the epistriatum receives also fibers from the 
hypothalamus, which probably in large part or wholly carry 
gustatory impulses. The epistriatum is thus related to both 
the sense organs which are concerned with the food relations of 
the organism. A large part of the epistriatum cells have short 
neurites which end in the iinderl}ing corpus striatum (Fig. 149) 
from which the tracius strio-ihalamicus goes backward. This 
constitutes a third path by which olfactorj^ impulses may reach 
the diencephalon. As the result of the distribution of these three 
tracts the greater part of the gray matter surrounding the third 
ventricle is brought into the ser\ice of the olfactor)' organ. 
In selachians, where the olfactory apparatus reaches an unus- 

Tr pinealis I 
Tr, pinealis II 

Tectum 

Post comm 
Tr. hab-pedunc. 
Commissura superior 




Comm. aot 



Fic. 100, — An outline of the median sag^ittal section of the forebrain of Lampetra. 

ually great development, the nuclei and commissures require some 
explanation^ In all lower vertebrates the olfactory portion of 
the anterior commissure is more or less distinct from the portion 
which is formed of fibers coming forward from the hypothalamus. 
In Pctromyzon the olfactory^ commissure is quite separate and 
is situated farther for^^ard and upward on the front end of the 
forebrain fFig. 100). In selachians the dorsal and lateral olfactory 
nuclei extend up from the front and side walls to form the thick 
nerv*ous tooI of the forebrain which has been mentioned. No 




1 86 NERVOUS SYSTEM OF VERTEBRATES. 

olfactory commissure has been seen in selachians but a large tract 
arises from the dorsal olfactory nucleus on either side and passes 
to the opposite side in the roof of the forebrain to end in the epi- 
striatum. The decussation of these tracts seems to take the place 
of the larger part of the anterior commissure of other fishes, since 
the anterior commissure proper in selachians is very small. It 
should be noticed that this is a tertiary olfactory tract and that 
it forms a decussation and not a true commissure. In some other 
fishes similar fibers are found which run from the olfactory nuclei 
on one side through the anterior commissure to the epistriatum 
of the other side. The peculiarities in selachians are the large 
size of the olfactory nuclei and of the tertiary tract and the conse- 
quent shifting forward of this tract (cf. further Chap. XVIII). 

The olfactory apparatus of fish-like vertebrates may be sum- 
marized as follows. The fibers of the olfactory nerve arise from 
the sense cells and end in the glomeruli of the olfactory bulb. 
The fibers from the cells of the bulb constitute the (secondary) 
olfactory tract which is distributed to the olfactory lobe and the 
epistriatum. A part of the fibers of the olfactory tract, including 
probably all those to the epistriatum, cross to the opposite side 
of the brain in a special part of the anterior commissure. From 
the nuclei of the lobe tertiary tracts go to the epistriatmn through 
the anterior commissure, to the nuclei habenulae through the 
habenular commissure and to the hjrpothalamus for the most part 
without crossing. The epistriatum gives its fibers to the striatum 
which in turn sends a tract to the thalamus. From the thala- 
mus and hypothalamus tracts are sent to the medulla oblongata 
and cerebellum where they make direct and indirect connections 
with the motor nuclei. From the nuclei habenulae a tract goes 
to the corpus interpedunculare and adjacent nuclei in the base 
of the mesencephalon, from which further connections with 
motor nuclei are made. See Figure loi and the figures in Chapter 
XVIII. 

In amphibia, reptiles, birds and mammals the olfactory appa- 
ratus differs from that described for fishes chiefly in the greater 
development of the tertiary centers within the forebrain which 
constitute the olfactory cortex of the cerebral hemispheres. The 



i8g 



NERVOUS SYSTEM OF VERTEBRATES. 



4 



which the organs are constantly bathed* In both cases the nen*e 
impulse is probably aroused by the entrance into the sense cell 
of a small modicum of the stimulating substance. Since there 
is this similarity in the stimuli and in the purpose which the gusta- 
tory and olfactor)^ organs sen^e, the question whether there is 
any morphological relation between the two becomes of con- 
siderable interest. 

A direct comparison of the olfactory organ and the taste buds 
is impossible, for the reason that the sense ceUs of the olfactory 
organ themselves give rise to the fibers which carry their impulses 
to the central nervous system, while the fibers which carry taste 
impulses arise from ganglion cells situated in the cranial nerve 
ganglia. The olfactor)' sense cells are to be grouped together with 
the ganglion ceUs of general visceral and gustatory fibers as pri- 
marj' visceral receptive cells, while the rod and cone cells of the ^ 
retina and the ganglion cells of the general cutaneous and acustico- ^M 
lateral fibers are primarj^ somatic receptive cells. The cells of the ^^ 
twx) categories are broadly homologous, as primary receptive cells 
possessing neurites. The taste cells and neuromast cells are ac* 
cessory sense cells without neurities. Turning to the central 
nen ous S}'stem, a certain connection between the gustator>' and 
olfactorj' apparatus is at once evident since the chief tertiary tracts 
of both s)*stems enter the hypothalamus. When the structural 
relations of the correlating centers of the brain are examined in 
the foUo^inng chapters, evidence will appear that all of the gusta-^ 
lory and olfactory centers bdoog to a continuous zone of the^ 
brain of which the >Tsceral lobe in the medulla oblongata i^ 
a part. It b necessaiy thai organs which function as viscer^^^ 
soasotr organs sboukl be relattd to the same set of central nud 



DEMOlfSTRATlOF OR LABORATORY WORK. 

u Snachr the devdoiiaMal and UMlogr of tbe otfurtoiy organ. 
isl^ un py b fa in or ckM^ enbijros hf die aietbod cl GolgL 

a. SHidtjp Ae UMlDgv of tlie kbctDnr bolb in the bnun of a 
orfio|5. Go|pmtte»4. 

^ Sti4r ^ mmiinMac oi tike socoodary midei and tracts 
dfaaecixoB» of tke bniuft tf a scbcUuaad a 6^ aad m semi sectic»ns 
by tbe Wciiem Md G^ 



«i 




THE OLFACTORY APPARATUS. 



i«9 



I 



LITERATUBE, 

CajaJ, S, R.: Origen y terminacion de las fibras nerviosas olfactorias. 
Gac* san. dc Barcelona. 1890, 

Cajal, S. R.: Textufa del slstema nem'oso del Hombre y los Vertebrados. 
Madrid 1904. Tomo II, segunda parte. 

Catoia, E. H.: Recherchcs sur I'histologie et ranatomie microscopique dc 
reocephale chez les poissons. Bull. sci. de la France et de la Bclgique, Tome 
36, 1901. 

Disse. J.: Riechschieimhaut u. Riechnerv u. s. w. Merkel u. Bonnet*s 
Ergebnfsse, Bd. ir. 1901, 

Edinger, L.: Untersuchungen iiber die vergleichende Anatomine des 
Gehims. I. Das Vorderbim, Frankfurt a. M. 1888. rH, Neiie Studien 
iiber das V'orderhim der Replilien. 1S96, 

Van Gehucbten, A.: Contribution a Tetude de la muqueuse olf active chez 
Ics mammiferes. La Cellule, 1891. 

Goldstein, Kurt: Untersucbungi^n tiber das Vorderhim und Zwischenhim 
einigcr Knochenfische, nebst einigen Beitragen liber Mittelhim und KJeinbim 
deiselben. Arch. f. mik. Anat., Bd. 66. 1905, 

Johnston, J. B,: The Olfacton,^ Lobes. Forebrain and Habenular Tracts 
of Acipenser. Zool. Bull, Vol. i. 1898. 

Johnston. J. B.: The Brain of Petromyzon. Jour. Comp* Neur., Vol. 12, 
1903, 

Kappers^ C. U* A.r The Structure of the Teleostean and Selachian Brain. 
Jour. Comp. Neur. and Psych., Vol. 16, 1906. 

KoeUiker A.: Gewebelebre, 6te Au^., Bd. 2. 

Rubashkin, W.: Ueber die Beziehungen de5 Nervus Trigeminus zur 
Riechschieimhaut. Anat Anz., Bd, 22. 1903. 

Schultze^ M.: Untersuchungen uber den Bau der Nasenschleimhaut, 
namcollich die Struklur und Endigungsweise der Geruchsnerv^en beim Men* 
schcn imd den Wirbelthieren. AbhandL d. Naturf. Gesellsch, zu Halle» Bd. 
7. 1862. 



190 NERVOUS SYSTEM OF VERTEBRATES. 



CHAPTER XI. 

THE SOMATIC MOTOR DIVISION. 

This division of the nervous system directly controls the 
actions of the typical body muscles; namely, those derived 
from the dorsal mesoderm or somites. It would be expected 
that each segment of the body which has such muscles would 
have a pair of somatic motor nerves. Inasmuch as several somites 
in the occipital region degenerate in most vertebrates without 
producing muscle, these nerves are wanting in those segments. 
Somites i, 2 and 3 produce the eye muscles and these are inner- 
vated by the cranial nerves numbered III, IV and VI. Between 
these and the first somatic motor nerve of the spinal or trunk 
region is a gap owing to the absence of a variable number of 
postotic myotomes. In cyclostomes, where the postotic somites 
all develop into myotomes, one or more nerves are absent (Petro- 
myzon), apparently because one or two somites only partially 
develop. In one cyclostome (Bdellostoma) it is now known that 
a complete series of somatic motot nerves is present in this region, 
one nerve for each postotic myotome. In the adult pf this animal, 
however, the eye muscle nerves are wholly lacking. Finally, 
it is to be mentioned that one rudimentary somite is known in 
selachians anterior to those which produce the eye muscles. It 
is probable that the segment to which this somite belonged pos- 
sessed muscles in primitive or ancestral vertebrates. The somatic 
motor dinsion is to be thought of as incomplete owing to the 
loss, in various segments, of the muscles which it should innervate. 
Otherwise this division is the simplest and least modified of the 
four functional divisions of the nervous system. 

In the trunk region (Fig. 102) the somatic motor nerve fibers 
arise from the cells of the ventral horn of the spinal cord and make 
their exit from the ventral surface of the cord as the ventral roots 
of the spinal nerves. In cyclostomes these ventral nerves arise 



THE SOMATIC MOTOR DIVISION. 



191 



opposite the middle of the myotomes, pass through the membranous 
skeleton and divide into dorsal and ventral rami. The rami of 
each nerve nm upon the inner face of the myotome to which 
they belong and twigs from them penetrate the myotome to end 
in relation with the muscle fibers. These ventral nerves make 
no connection with the dorsal nerves, which lie in the spaces 
between each two myotomes. In all higher classes each ventral 
nerve unites with an adjacent dorsal nerve to form a composite 
structure called a spinal nerve. The union takes place at or near 
the distal end of the ganglion of the dorsal nerve and the composite 
nerve immediately divides into dorsal and ventral rami. At 




arcuate fibers 



Flo. 102. — A diagrammatic representation of the somatic motor components of 
a trunk segment. 



about the same point, or from the ventral ramus, the ramus com- 

municans is given off to the sympathetic ganglion. (See p. 200.) 

In urodeles (Bardeen) where the trunk muscles have the same 

simple segmental arrangement as in fishes, the spinal nerve hes 

m a myoseptum between two muscle segments. The sensory 

fibers are distributed to the skin both before and behind this 

septum, and the motor fibers enter both the adjacent muscle 

segments and innervate the muscle fibers at their ends. Each 

nerve therefore helps to innervate two muscle segments and any 

muscle fiber may be innervated from two spinal nerves. These 

facts seem to have an important bearing on the question of met- 

ajnerism in the vertebrate body and also" upon' the problems of 



192 



NERVOUS SYSTEM OF VERTEBRATES. 



histogenesis of motor nen^es (d, p, 63). In fcdgher vertebrates 
when the myotomes lose their simple segmental arrangement 
and become diiided into special muscles, the relations of the 
nen^es to the muscle segments become more obscure. 

The greatest modifications affecting the ventral nerves occur 
in connection with the innenation of the limbs. The limbs are 
innervated by rami going out from plexuses formed by the union 
of the ventral rami of several spinal nerves. Two such plexuses 
are formed , the brachial for the fore limb, the lumbar plexus for 
the hind limb. The presence of such plexuses is explained by 
the mode of origin and evolution of the vertebrate limbs. The 
limbs first arose as folds of skin and musde extending along the 



4 




Fig. 103. — An early stage in the formation of the pectoral fin and badial 
plexus in a selachian, Spinax, The musde buds are in dark shading, tbe i 
m black. 



side of the body. These long limb folds are formed in the embryos 
of lower vertebrates by outgrowths or buds from a number of 
myotomes together with mesenchjine (Fig. 103). The myotofflc 
buds form the muscles of the limb; the mesenchyme gives rise to 
the skeleton and the connective tissiie into which the blood vessds 
and nerves grow as the development of the limb proceeds. As many 
nen^es are involved in the inner\'ation of the limb muscles as there 
are myotomes involved in the limb fold* In higher vertebrates (afltl 
in later stages in the growth of the individual), as the limb grog's 
in length it comes to have a shorter base where it is attached to 
the trunk. At the same time the ner\^es in order to reach their 
muscles which have shifted out into the limb, must converge to 
enter the narrow base of the Umb. This together with various 
modifications of position due to the formation of the special 
muscles of the limbs have caused the ner\'es to unite into intricate 
plexuses. 



n 



THE SOMATIC MOTOR DIVISION. 



193 



|The most cephalic of the ventral spinal nerves form the 
led cervical plexus. This is closely related to the bracMal 
plexus^ some nerves usually contributing to both. The rami 
arising from the ccnical plexus go to innenale muscles extending 
between the pectoral girdle and the region of the tongue, commonly 
called the tongue musculature. This musculature is formed by 
buds from a number of myotomes in the cervical region, and^ in 
vertebrates which possess gills, these muscles are separated from 
the dorsal muscles and from the place of origin of the nerves by 
the expansion and shifting backward of the gill apparatus. In 
consequence, the nerves destined to the tongue muscles must 
run around behind and forward beneath the gills. As the more 




Rdt Rsbsp 



Fig. 104,— The constitution of the cemcal plexus in a selachmn, Hexanctms. 
M\ti Fiirbringer. vg.^ vagus; u\x,y,z^ spino-occipital nerve roots; i and 2, spinal 
nerves; the letters d and v indicate respectively the dorsal and ventral roots; Rdl, 
rami for the dorso-laterai trunk raiisdes; Rsbsp^ rami for the subspinal muscles; 
if 16, rami for the inlerbasal muscles. 



anterior nerves run back to get aroimd the gills they unite with 
the more caudal ones and so give rise to the cemcal plexus (Fig. 
104)- In higher vertebrates this plexus becomes somewhat 
modified. The anterior part of it, consisting of nenx roots arising 
in the occipital region and innervating the muscles of the tongue, 
becomes relatively independent of the plexus and is kno\^Ti as 
the hypoglossal ner\T. The remainder of the plexus gives rise 
to rami supplying various muscles of the neck and is even con- 
nected with the spinal accessory ner\' e which belongs to the visceral 
motor system. 




194 



NEKVOUS SYSTEM OF VERTEBRATES. 



The number and segmental position of the roots represented 
by the hypoglossus varies in different vertebrates, omng to the 
difference in the number of gills and to the extent caudally of the 
branchial apparatus. The tongue musculature arises from 
myotomes situated immediately behind the gills, and in fishes 
and higher \ertebrates possessing five or four gills it is probable 
that one or two more anterior myotomes may enter into the tongue^ 
muscles than in forms like the cyclostomes where there are sevenfl 
or more gills. In all higher vertebrates the hypoglossal roots 
include the first ventral roots following the eye muscle nerves. 
In cyclostomes, however, and to a less extent in selachians, a 
number of myotomes are presen^ed anterior to those which contrib- 
ute to the tongue muscles and to supply these muscles ventral 
nen^es are present anterior to the hypoglossal roots. In Petro- 
myzoix all the myotomes behind the ear form permanent muscles, 
while the first myotome to contribute to the tongue muscles is 
the seventh behind the ear (myotome lo). However, from one 
to three nerxes are absent in different species of Pctromyzon 
in spite of the presence of permanent muscles formed from their 
myotomes, and there remain from three to five nerv^es anterior 
to those which enter into the formation of the hypoglossus. The 
most anterior ventral root present in Petrmnyzon dorsaius belongs 
to the sanie segment as the vagus (Fig. 51). The segment of the] 
glossopharv^ngeus also has a somatic motor nen^e in Bdellostomat 
so that, as noted above, the series of somatic motor roots in this 
animal is complete from the segment of the glossopharyngeus 
backward. H 

The eye muscles are developed from the first, second and 
third somites. From the first somite are derived the rectus superior 
and inferior, the rectus intemus and the obliquus inferior. From 
the second somite comes the obliquus superior, and from the thiixi 
comes the rectus extemus. The ner%-e which innen'ates the rectus 
extemus muscle is the VI or abducens. It is the ventral motor 
nen^e of the third somite and is comparable in every way with a 
ventral spinal nen-e. In Petromyzon there is no VI nen^e. The 
muscle which is usually regarded as the homologue of the rectus 
extemus is innervated by a branch from the trigeminus ganglion 



I 




THE SOMATIC MOTOR DIVISION. 



I9S 



which probably arises from the trigemiQus motor nucleus. The 
source of the muscle — whether from dorsal or lateral mesoderm — 
is uncertain. The ner>^e mnerv^ating the obUquus superior muscle 
is the IV or trochlearisi which is the ventral nen^e of somite 2, 
This also is comparable with a ventral spinal nerve except that 
I its root starting from a ventral motor nucleus runs upward in 
the brain wall, decussates with its fellow in the roof of the brain 
and emerges from the dorsal or lateral surface between the cere- 
bellum and tectum mesencephali. No satisfactorv^ explanation 
has yet been found for the curious coun^e of thi^ nene. The 




FtG. 105, — ^A transverse section through the nucleus of ongirt of the III nerve in 
a cvdostome, fjimpeita. nJIlj nucleus of III nerve; b,M.^ bundle of Meynert 
(tractus habenalo-peduncularis) J d.b.\f,^ decussation of the sjtme; «,6.Af., end- 
Dudeiis of the same; g.M,^ one of the guint eelb of Mauthner. 



I 



nerve which innerv^ates the four muscles derived from the first 
somite is the III or oculomotorius, which arises from a ventral 
motor nudeus in the base of the mesencephalon. It is a noticeable 
peculiarity in the origin of this nerve that a large part of its fibers 
arise from the nucleus of one side and cross to enter the root of 
the opposite side (Fig. 105). The same arrangement is found in 
the roots of other ventral ner\^es but to a much less degree. 

All the somatic motor nerves arise from a portion of the gray 
matter which Ues latero-ventral to the central canal or ventricle. 
In the brain region it is usually marked by a pair of special grooves 
or furrows in the floor of the xentricle, one at either side of the 




196 



NERVOUS SYSTEM OF VERTEBRATES. 



mid-ventral furrow (Figs. 3, 46, etc.). The motor ceUs are large 
and have large dendrites whose branches spread widely through the 
latero- ventral part of the brain or cord. The neurites from the eel 
which lie in the immediate \'icinity of a ner\*e root may pass out 
that root, but most neurites must run forward or backward in order 
to reach their nerv^ roots. Indeed, it is probable that the neurites 
do not all enter the nen^e root which lies nearest to their cells of 
origin but that many neurites run through one or more complete 
segments before going out of the brain or cord as fibers of a ventral 
ner\^e. The neurites which run from segment to segment Be 
mesial to the motor nuclei, where they run in a definite bundle 
at either side of the mid- ventral groove of the ventricle- The 
large bundle of coarse fibers in this posiUon is known as the Jasci- 
cuius langifiidinalis medialis (dorsahs or posterior), and forms 
one of the most conspicuous landmarks in the brain of any verte-^ 
brate. This contains, however, other fibers in addidon to lh( 
here mentioned. That portion of it which is made up of motor 
fibers on their way to become fibers of ventral nerves may be 
called the samatic motor jascktdus. The motor ceUs together 
with this fasciculus make up the somatic motor column or zone 
of the spinal cord and brain. In those segments of the brain which 
have no ventral nerves the somatic motor cells are wanting and 
the column is represented only by the somatic motor fasciculus. 
This fasciculus continues fon\^ard beyond the oculomotor nerve 
and its fibers take their origin from the cells of a special nucleus 
cephalad from the nucleus of the III ner\ e in the ventral prt 
of the central gray of the thalamus. This nucleus is frequently 
considered to have some relation to the fibers of the po&lerior 
commissure (cf. p. 265 and Fig. 134, p. 272). 

Owing to the importance of bodily movement in neariy all the 
acti\dties of the animal, the connections of the somadc motor 
centers with the rest of the ner\'ous system are numerous and varied 
in character. Necessarily the motor neurones form a Unk iB 
every reflex chain which leads to a bodily movement, whatever 
the source and character of the exciting stimulus. Some of the 
chief classes of impulses and the tracts which bring them to th^ 
motor centers will be mentioned here. The simplest case is that 



be 



rte-^ 



4 
4 

^ 





THE SOMATIC MOTOR DI\1SI0N. 



197 



of tactile impulses wHch are brought into the spinal cord by the 
cutaneous fibers of the dorsal roots* Collateral branches of these 
jBbers carry the impulses directly to the motor cells. A somewhat 
inore complex course for similar impulses is illustrated in the 
relations of the acusticum and cerebellum to the motor centers. 
Impulses from the skin, the lateral line organs and the ear are 
given directly or indirectly to the large cells of the acusticum and 
cerebellum* The neurites from a part of these cells form bundles 
which have been described as going down close to the ventricle, 
to the somatic motor column (p. 135). Such bundles have been 
ieen in selachians going to the nucleus of each of the eye- muscle 
lerves. A part of the fibers enter the fasciculus longitiidinalis 
medialis and may go to the segments of the somatic motor column 
in the spinal cord which control the movements of the body and 
limbs* A third and more complex course for such impulses is 
that by way of the roof of the mesencephalon (cf. p. 117). 
The fibers descending from the tectum form the iracius iecto- 
kulbaris. The end branches of these fibers make direct or indirect 
connections with the motor centers in the medulla oblongata and 
spinal cord. Finally, when in a mammal or man such impulses 
are carried op to the cerebral cortex and give rise to a sensation, 
there may follow a voluntar}^ motor impulse which descends over 
the fibers of the pyramidal tract and reaches the motor centers 
of the spinal cord. The simpler courses for tactile impulses 
from the cutaneous to the somatic motor nenes are indicated in 
Figure 59, p. 1 18. The student should construct sindlar diagrams 
to illustrate the course of impulses from the ear and the eye. 

It is not yet certainly known whether visceral sensory impulses 
(from the general visceral surfaces, from taste organs or the 
olfactory organs) go directly to the somatic motor nuclei. Further 
study of the central gustatory apparatus may be expected to throw 
light on this question. Until such investigations are made it can 
only be said that in general the connections between the visceral 
sensory and somatic motor ner\'es are much more indirect than 
those between the cutaneous and somatic motor nerves. Some 
of the indirect paths of taste impulses leading to somatic motor 
centers are shown in Figure 92, p. 174. 





198 NERVOUS SYSTEM OF VERTEBRATES. 

DEMONSTRATION OR LABORATORY WORK. 

1. Review the dissections of spinal and cranial nerves already 
made, with especial reference to the position of the ventral roots and 
the relation of the rami to the muscle segments. Dissect the brachial 
and cervical plexuses of a selachian. 

2. Examine carefully the somatic motor cranial nerves, noting their 
segmental arrangement with reference to the dorsal roots and the num- 
ber and position of the "hypoglossal roots" present. Selachian brams 
are especially useful for this. Small roots which would ordinarily be 
overlooked can be brought sharply to view by painting the dissection 
with a one percent, solution of osmic acid which is well washed away 
with water as soon as the nerves are blackened. 

3. In sections by the Weigert or Golgi method study the somatic 
motor nuclei and the formation of the ventral roots. Selachian or 
frog. 

4. Upon the basis of the descriptions given in previous chapters 
construct diagrams representing the course of impulses — the centers 
and tracts involved — ^in somatic motor reflexes aroused by cutaneous, 
auditory, optic and olfactory stimuli. 

LITERATURE. 

Bardeen, C. R. : The Bimeric Distribution of the Spinal Nerves in EUs- 
mobranchii and Urodela. Amer. Jour, of Anat., Vol. 3. 1904. 

Fiirbringer, M.: Ueber die spino-occipitalen Nerven der Selachier u. s. w. 
Gegenbaur's Festschrift, Bd. 3. 1897. 

Johnston, J. B.: The Cranial Nerve Components of Petromyzon. Morph. 
Jahrb., Bd. 34. 1905. 

KoltzofiF, N. K.: Entwickelungsgeschichte des Kopfes von Petromyzon 
planeri. Bull. Soc. Imper. d. Natural, de Moscou, Annee 1901, No. 3-4. igf>^- 

Neal, H. V.: The Development of the Hypoglossus Musculature in 
Petromyzon and Squalus. Anat. Anz., Bd. 13. 1897. 

Neal, H. V.: The Development of the Ventral Nerves in Sclachii. i- 
The Spinal Nerves. Mark Anniversary Volume. 1903. 

van Wijhe, J. W.: Ueber die Mesodermsegmente und die Entwickelung 
der Nerven des Selachierkopfes. Amsterdam 1882. 

Worthington, Julia: The Descriptive Anatomy of the Brain and Cranial 
Nerves of Bdellostoma Dombeyi. Quart. Jour. Mic. Sci., Vol. 49. 1905. 



THE VISCERAL EFFERENT DIVISION. 1 99 



CHAPTER XII. 

THE VISCERAL EFFERENT DIVISION. 

The visceral efferent division controls the smooth muscles 
in the viscera and elsewhere in the body, the muscles of the heart 
and blood vessels, certain striated muscles derived from the 
lateral mesoderm, and the glands of the body. In higher verte- 
brates at least, the smooth muscle and glands do not receive impulses 
directly from the central nervous system, but the S3rmpathetic 
system offers intermediate neurones by which cerebro-spinal 



R. doi 




Fig. 106. — A diagrammatic representation of the visceral efferent components in 
a trunk segment. 



impulses are carried (see following chapter). In the lower verte- 
brates where the sympathetic system is poorly developed, it is 
probable that some of the functions performed by it in higher 
forms are performed by the visceral efferent nerves and centers. 
As it is not possible at present to distinguish the excito-glandular 
from the excito-motor cells in the central nervous system, the 



200 



NERVOUS SYSTEM OF VERTEBRATES. 



nuclei and nen^ components may be described under the general 
name of visceral efferent structures. 

The visceral efferent nuclei in the spinal cord occupy a position 
dorsal to the ventral honii between it and the \isceral afferent 
column (Fig; 106). The visceral efferent nuclei in the higher 
vertebrates and man form here a lateral projection of the gray 
matter known as the lateral horn. The neurites from the cells 
of this column pass out of the cord in the dorsal nerv^e roots in 
lower vertebrates. In higher vertebrates a part of these fibers^ 
and in some cases aU of them, pass out by way of the ventral 
roots. The fibers then go through the wliite rami communi- 
cantes into the ganglia of the sympathetic chain. Their further 
relations wiU be given in the next chapter. 

In lower vertebrates the \isceral efferent column of the cord is 
continued forward as a column in the brain constituting the 
nuclei of origin of the efferent roots of theX, IX, VII and V nerves. 
In fishes, where the voluminous musculature of the gill apparatus 
is to be inner\'ated, this is a large and important column of gray 
matter at either side of the ventricle as far fon^^ard as the cere- 
bellar segment. In higher vertebrates, where the gills have been 
lost, the column becomes less important and is di\ided into dorsal 
and ventral portions. The former is the dorsal vago-glosso- 
pharyngeal nucleus, and gives rise to efferent sympathetic fibers. 
The ventral portion consists of discontinuous masses known as the 
nucleus ambiguiis, and gives rise to fibers w^hich innen'ate striped 
muscles (Fig, 107 ) . The neurites from the cells of this column form 
the \isceral efferent component of the X, IX, VII and V ner\^es in 
all vertebrates. In the gill breathing forms (Figs, 51 ^ 63, 79) 
these components run in the posttrematic ramus of each of the 
branchial ner\^es. This ramus nms down along the anterior side 
of each branchial arch and hence behind the gill slit. The fibers 
in question are distributed to the muscles which control the gill 
arches in respiratory movements. The component in the trigem- 
inus supplies more specialized muscle in all vertebrates. In 
fishes this comf>onent runs in the mandibular ramus of the trigem- 
inus which holds the same relation to the mandibular arch and! 
mouth that the posttrematic rami of the branchial nerves hol(3 



* 



J 



THE VISCERAL EFFERENT DIVISION. 



201 



to the gill arches and slits. The motor fibers of the irigcminus 
supply the muscles which move the mandibular arch, i,e. the 
chief muscles of mastication. In all gnalhostome vertebrates 
the muscles of mastication are supplied in this way; with the 
exception of the posterior belly of the digastric, which is supplied 
by the corresponding component in the VII ner\T. In higher 
vertebrates the motor component of the facial nen'c also controls 
highly specialized muscles which move the skin of the face, of the 



N.homXI. 



N.xri 



.>tJ 



O. 






XII- 



I 



FkS, 107. — Diagram to show the central rcktioas of the IX, X and XI nen^es in 
miunnmls. From Onuf and CoDins* N'.X.di?rs.j dejrsal vagus or vago-glos* 
sopfaajyngeal nucleus,^ nucleus for the visceral or vegetative (sympathetic) e^erent 
fibers; N.amb., nucleus ambiguus,^ nucleus for the striped muscie innervated by 
the IX, X and XI nerves; soL, fasciculus solitarius; N JimywLCt.^ nucleus homolo- 
gous wHlh Clarke's column; N.XII, nucleus of hypoglossus; spin,ViR.^ spinal V 
tract. 



scalp and ears, the muscles of expression. This mode of inner- 
vation indicates that these muscles are derivatives of the branchial 
muscles of the hyoid segment which have spread forward to their 
present position, whither the motor branches of the facial nerv^e 
have followed them. This is one example of the way in which 




202 



NERVOUS SYSTEM OF \'TRTEBRATES. 



terms Ul-adapted to the comparative description of the nen-ous 
system of vertebrates have come into use. The nene is named 
the facial from the distribution of its larger brancJies in man, but 
the comparative anatomy shows that this distribution has been 
secondarily acquired and only in higher vertebrates. The fadal 
nen-e is really the nerve of the hyoid segment. 

As in the case of the somatic motor nuclei the course of tie 
neurites of the \isceral motor cells is not always simple. Not 
only do the fibers run longitudinally in the brain for longer or 
shorter distances before going out in their nerve roots, but there 
have occurred shiftings of segmental relations which are at first 
sight difficult to understand. In the case of the trigeminus, a 
part of the nucleus of origin of its motor component lies in all 
vertebrates caudal to the root, and the fibers from this part of the 
nucleus run forward lateral to, or in the lateral part of, the fasci- 
culus longitudinalis medialis to join the remainder of the root. 
In the case of the facialis the entire nucleus always lies caudal to 
the plane of exit of the root and the fibers nm fon^^ard as a dis- 
tinct bundle in the fasciculus longitudinalis medialis and turn 
laterad to form the root. In man this is knovvTi as the geniculated 
root of the facialis. The roots and nuclei of the IX and X nerves 
are arranged in much the same w^ay as those of the V nerve. This 
condition is more pronounced in the vagus and the fibers do not 
all unite into one root but the vagus has a number of motor as 
well as of sensory rootlets. This is explained by the fact that 
the vagus has gathered into it all the branchial nerves of the 
segments following that of the glossophar^'ngeus. The roots 
of the more caudal branchial ner\Ts have gradually shifted forward 
until they have united with that of the vagus, but the union is 
not complete in any class of vertebrates. In some fishes the number 
of rootlets approaches twenty and they are scattered for a con- 
siderablc distance along the side of the medulla oblongata (Figs. 
2, 3j 7, 12). The fibers of the most caudal of these motor rootlets 
in fishes go to supply certain muscles connected with the pectoral 
arch which are homologized with a part of the trapezius muscu- 
lature in mammals. In higher vertebrates the nuclei of these 
more caudal rootlets apparently extend farther back in the spinal 



1 

4 





THE VISCERAL EFFERENT DIVISION. 



203 



I 



cord and the roots lake their exit from the dorso-lateral surface of 
the cord between the dorsal and ventral roots of the spinal nerves, 
run forw^ard along the side of the medulla oblongata and join 
the trunk of the vagus (Figs, 20, 32}, These roots have the name 
of the spinal accessory or XI cranial ner\'e. In mammals the 
nucleus and roots of this nerve extend farther caudad than in 
other classes, are more variable in position and show a greater 
tendency to a segmental arrangement and a closer relation with 
the dorsal roots of the spinal nerves. The more caudal roots 
are smaller, more nearly segmental and are placed nearer the 
dorsal roots (I*ubosch), It seems that above the amphibia the 
increasing importance of the trapezius muscles have been corre- 
lated \Aith an increase in the extent of the accessorius nucleus. 
The fact that skeletcd muscles important in the movements 
of the fore limb are innervated by nerv^es arising from the visceral 
motor column requires a word of explanation. All other muscles 
involved in general bodily movements are derived from the dorsal 
mesoderm and are innenated by nerves from the ventral motor 
column. A somewhat similar anomaly is seen in the muscles 
of masdcation. Although these are derived from the lateral 
mesoderm and are innen'ated by visceral motor fibers they are 
voluntary muscles which move skeletal parts whose fimcdons are 
much more than merely visceral functions. Although primarily 
all muscles derived from the lateral mesoderm may have been 
related solely to the walls of the alimentary canal and have been 
involuntary in their acdon, it is evident that neither of these 
characters are retained by all such muscles. What is found to be 
Constant is that muscles derived from the lateral mesoderm are 
ilinen^ated by nerv^es arising from the visceral motor column. 
The chief question regarding the trapezius musculature, then, is 
llow it comes to be attached to the skeleton of the arm. The 
^^nly probable explanation is that the shoulder girdle or pectoral 
arch did not have its origin as a part of the skeleton of a limb, 
t»ut existed as a branchial arch before the limb w^as formed. It 
is believed that primitive vertebrates possessed a considerably 
larger number of gills than are now found in most vertebrates 
and it is supposed that the skeleton of one or more branchial 




204 



NEEVOUS SYSTEM OF V*ERTEBRATES, 



arches has been retained as a girdle for the attachment of the 
fore limb. The muscles which moved this branchial arch» or 
perhaps those of several arches, have in part been preserv^ed as 
muscles of the limb girdle. These muscles and their nerves were 
of course visceral muscles and nerves like those of other branchial 
arches, and they have secondarily acquired somatic functions. 
This interpretation is supported by the following facts: (i) The 
position of the brachial plexus in mammals sho\^'s that the shoulder 
girdle has shifted backward from its primitive position. (2) The 
most primitive vertebrates now possess a large number of gills, 
as many as thirty-five (Price), (3) There are in amphibian 
embryos signs of gill slits extending back into the trunk region 
caudal to the position of the fore limb (Piatt). 

As pointed out in the last chapter, the somatic and visceral 
efferent nuclei differ in the source of the impulses which come 
to them and in the tracts which bring thcra. Tracts from higher 
brain centers bring impulsei^ to both sets of motor nuclei, but 
much remains to be done in order to explain the mechanisms by 
which somatic and visceral activities are correlated* 

Collaterals from afferent -vdsceral fibers directly to the visceral 
efferent nuclei are probably present in mammals (see Figs. 55 
and 78). The short viscero-motor connections described in a previ- 
ous chapter (p. 162) form a two linked chain between the visceral 
sensory and the visceral motor apparatus. The tertiary connec- 
tions of the inferior secondary gustatory* center are not known. 
From the superior secondary gustatory center a large tract goes 
to the inferior lobes of the diencephalon» from which tracts go 
to the cerebelhmi and medulla oblongata. The greater part 
of the tract to the medulla oblongata ends in the region of the 
visceral motor nuclei and it is undoubtedly chiefly these nuclei 
which receive the impulses. Olfactory impulses also may come 
over the same tract to the visceral motor nuclei. It is probable 
that even in fishes tracts from other correlating centers, such as 
the cerebellum or the mesencephalic nuclei, bring impulses to the 
visceral motor nuclei for the control of some of the more complex 
movements, especially for the coordination of somatic and \isceral 
muscles in the act of seizing food. 



i 




THE VISCERAL EFFERENT DIVISION. 205 

DEMONSTRATION OR LABORATORY WORK. 

1. Review the dissections already made, with especial reference to 
the viscero-motor rami of the cranial nerves. 

2. Study the spinal accessory nerve in a mammal, either by dissec- 
tion or in embryos such as the pig. Compare the arrangement of the 
X and XI roots in a 12 mm. pig embryo with that in a selachian or 
the frog. 

3. In Golgi or Weigert sections of fish or frog brain study the origin 
of the viscero-motor roots of the cranial nerves. 

LITERATURE. 

Coghill, G. E.: The Cranial Nerves of Amblystoma. Jour. Comp. Neur., 
Vol. 12. 1902. 

Fiirbringer, M.: Spino-occipitalen Nerven der Selachier u. s. w. Gegen- 
baur's Festschrift, Bd. 3. 1897. 

Herrick, C. Judson: The Cranial and First Spinal Nerves of Menidia. 
Jour. Comp. Neur., Vol. 9. 1899. 

Johnston, J. B.: The Morphology of the Vertebrate Head, etc. Jour. 
Comp. Neur. and Psych., Vol. 15. 1905. 

Lubosch: Vergleichend-anatomische Untersuchungen Uber den Ursprung 
und die Phylogenese des N. Accessorius Willisii. Arch. f. mik. Anat., Bd. 54. 
1899. 

Osbom, H. F. : A Contribution to the Internal Structure of the Amphib- 
ian Brain. Jour. Morph., Vol. 2. 1888. 

Piatt, Julia B.: Ontogenetische DiflFerenzirung des Ektoderms in Necturus. 
Arch. f. mik. Anat., Bd. 43. 1896. 

Price, G. C: Some Points in the Development of a Myxinoid (Bdellos- 
toma Stouti L.) Verhdl. Anat. Ges. 10. Vers. Beriin. 1896. 

Stannius, H.: Das peripherische Nervensystem der Fische, u. s. w. 
Rostock. 1849. 

Streeter, G. L.: The Development of the Cranial and Spinal Nerves in 
the Occipital Region of the Human Embryo. Amer. Jour. Anat., Vol. 4. 1904. 

Strong, O. S.: The Cranial Nerves of Amphibia. Jour. Morph., Vol. 10. 

1895. 
See also the list at the close of the following chapter. 



2o6 NERVOUS SYSTEM OF VERTEBRATES. 



CHAPTER XIII. 
THE SYMPATHETIC SYSTEM. 

To understand the sympathetic system it is necessary to begin 
with the study of its development. At a time when the spinal 
ganglia and the dorsal and ventral nerve roots are formed there is 
noticed on the mesial side of the composite ventral ramus of each 
nerve a collection of ganglion cells which later forms the sympa- 
thetic ganglion. A stage a little earlier than this has been recog- 
nized in mammals. The beginning of the development of the 
s)rmpathetic is the outgrowth of fibers from the ventral root and also 
from the dorsal root ganglion, in the direction of the aorta. Then 
cells from the spinal ganglion are seen to migrate along these 
fibers. These constitute the group of cells first mentioned. As 
development proceeds the group of cells moves away from the 
spinal ganglion toward the aorta, but remains connected with 
the spinal ner\'e by the strand of fibers which grew out first. There 
are thus formed a pair of ganglia in each segment, lying below the 
notochoid and lateral to the aorta, and connected with the spinal 
nerves by the rami communicantes. The ganglia are those known 
as the ganglia of the chain but at this stage they are not yet con- 
nected longitudinally into a chain. 

The process of growth of fibers and the migration of cells along 
them continues beyond these chain ganglia and results on the one 
hand in joining the ganglia together by longitudinal cords, and on 
the other hand in the formation of additional ganglia. The cells 
which migrate from the chain ganglia form first certain median 
prevertebral ganglia or plexuses. These are in man the cardiac, 
solar and hj-pogastric plexuses. Further migration of cells 
carries them to or even among the tissues of se\-eral of the organs 
inner\-ated by the s\Tnpathetic, where peripheral ganglia are 
formed. Examples of these are the small ganglia in the heart, 
the plexuses of Auerbach and Meissner in the wall of the digestive 



THE SYMPATHETIC SYSTEM, 



207 



canal, etc. Wliiie the sympathetic elements are migrating in 
this way to their definitive positionsj some of the cells of the 
chain ganglia and perhaps of other ganglia send fibers back along 
the rami communicantes into the spinal gangha or the rami 
of the spinal nerves. The tibers which grow out from the spinal 
nen^es acquire myelin sheaths and so become white fibers, and 
the part of each ramus communicans formed by them is known as 
the white ramus communicans. The fibers which grow from the 
sympathetic cells back into the spinal ganglion or nerve form 
the gray ramus communicans. 

In selachians a pair of chain ganglia is formed in each segment 
in the trunk and for some distance into the tail. The anterior six 
pairs of trunk ganglia disappear during development. This Hoff- 
maim attributes to the shifting backward of the heart and other 
organs with reference to the spinal column and nen-es. In man 
the three cervical ganglia which are found in the adult are supposed 
to be formed by thefusionof a larger number of primary segmental 
ganglia of the chain. It is important to notice that there is not 
a complete segmental series of white rami communicantes in itiam- 
mals. In the cervical segments from which the spinal accessory 
nerve roots take their origin there are few or no myelinated fibers 
in the rami commimicantes, while in the segments immediately 
follo\\ang the last root of the accessory ner\T there appears suddenly 
a great increase in the number of such fibers. A second increase 
in these fibers begins at the caudal border of the brachial plexus 
and extends to the beginning of the lumbar plexus. The fibers of 
the white rami are small myelinated fibers which are excito- motor 
or excito-glandular in function and are in a broad way serially 
homologous with the efferent fibers of the vagus and spinal acces- 
sory ner\'es. The question of this homology will be taken up 
again a little later (p. 215). 

The development of the sympathetic system in the head has not 
been well studied. Some indicadons as to its source can be 
obtained by considering the character of the cranial nerves from 
which it is derived, and from its structure in the most primidve 
vertebrates. In selachians the development of the ciliary^ ganglion 
has been repeatedly described but it is still imperfectly understood. 





208 



NERVOUS SYSTEM OF VERTEBRATES* 



It seems to be formed in part from the ganglionic anlage known 
as the Nervus thalamicus, and early comes into relation with 
the ophthalmicus and oculomotorius ner\'es. In the chick it is 
derived in part from the neural tube and in part from the most 
anterior portion of the ganglion of the ophthalmicus profundus 
nerv^e. The development of the other sympathetic ganglia of the 
head has not been directly followed. Ganglia are present in bony 
fishes in connection ^\ith each of the dorsal cranial nerves, from 
which presumably they are derived during development- Asfl 
these nerves have no somatic motor component it is evident that 
this component does not enter into the head sympathetic. It is 
therefore probable that the fibers which grow out from the ventral 
spinal nerv^es to help form the sympathetic are not somatic motor 
fibers. Indeed, it is known from the adult structure in mammals 
that the fibers which go by the ventral spinal roots to the sym- 
pathetic take their origin from cells in the vbceral motor column 
of the cord* In cyclostomes the only connections of the sym- 
pathetic with the cranial ner\'es are vnih the visceral portions of the 
facialis and vagus. Indeed, from what we know of the develop- 
ment and structure of the s}Tnpathedc system in lower vertebrates, 
the general conclusion must be drawn that it is an outgrowth 
from the visceral nen^es, including etlcrent fibers and gangUon 
cells. The formadon of a relatively distinct sjrstem is due pri- 
marily to the migration of ganglion cells along the primitive ner\^es 
of the viscera toward the areas supplied by them. 

In describing the structure of the sympathetic system four 
types of nerve elements must be considered: (i) sensory fibers 
whose ganglion cells are in the spinal ganglia; (2) efferent fibers 
whose cells of origin are in the spinal cord or brain; (3) sympathetic 
excitatory cells; and (4) sympathetic sensory cells* The descrip- 
tion of these elements will be more clear if Fig* 108 is consulted 
in connection with the text. 

(i) Sensof}^ fibers whose ganglion cells are in the spinal ganglia. 
These fibere are the visceral sensory fibers already described in 
a preuous chapter (p, 1 56). They are the largest of the myelinated 
fibers running in the sympathetic nerves and may be seen to pass 
through one or more of the sympathetic ganglia without forming 





Scnootli 

musde Type II 



THE SYMPATHETIC SYSTEM. 

^--» Somatic motor 
-..„...,.„... General cutaneous 

Visceral sensory 

Visceral motor 
Sympathetic 

_ _ Dotsal root 



Dors, ramus. 



209 




Chain 
ganglidn 



_„ Periph. g. 
Bl. V. 



Sens, ending 
Pacinian 
' * corpiiscle 

Fig. 108. — A diagram of the sympathetic system and the arrangement of its neu- 
rones in a mammal. On the left are shown the typical elements of a trunk segment 
including the sympathetic system. On the right are shown only the somatic afferent 
and efferent neurones of the spinal nerve. Of the sympathetic system are shown 
the white and gray rami, three ganglia of the chain, one prevertebral ganglion and 
one peripheral ganglion. The symbols used are explained in the figure. In most 
respects the diagram follows one of Ruber's figures. 

14 




Fig, 109. — Diagram illuslratlng the spinal representation of the sympaihetic 
nerves in a mammals From Onuf and Collins. CL Clarke's column; inim. Z., 
intermediate zone; BcchL N., Bechterew's nucleus; lai,, lateral horn cell-group; 
parCf paracentral cell -group. 

syntpathetic system and that the sympathetic ganglia are placed 
along the course of these primitive visceral sensorj^ fibers. Cen- 
trally, these fibers enter the visceral sensory colunm of the cord 
or brain as already described. 

(2) Efferent fibers whose cells of origin are in the spinal cord^ 
or brain. The location of the cells of origin of these fibers has been 
determined with accuracy in the spinal cord of the cat. As indi- 



THE SYMPATHETIC SYSTEM* 



211 



in Fig. 109 they are located in a zone of the gray matter 

tween the dorsal and ventral horns and extending from the 
central canal to the lateral horn and the base of the dorsal horn. 
This is the portion of the gray matter which has pre\iously been 
tailed the visceral efferent column (p* 200). As already stated, 
the fibers from this column in lower vertebrates pass out through 
Ittie dorsal roots, but in the mammals wliich have been most used 
for the study of the sympathetic they pass out by way of the 
ventral roots* 

The fibers are small myelinated fibers, usually less than 4 ;i 
jb diameter, which 'enter the ganglia of the chain and find eadings 
in relation with the cells of these or other sympathetic gangEa. 
They may (a) end in the chain ganglion first entered, (b) run 
through it to end in another ganglion of the chaiUj (c) end in one 
©f the prevertebral ganglia, or (d) in one of the peripheral ganglia. 
During their course the fibers may give collaterals to one ganglion 
and pass on to end in another. The method of ending of these 
fibers is important. They pierce the capsule of the sympatheric 
cells and their branches interlace to form more or less complex 
plexuses or baskets immediately in contact with the sympathetic 
.cells. By means of these pericellular baskets the impulses sent 
ut from the central nen^ous system are transferred to the sympa- 

£tic excitatory cells. Such endings are found in thepreverte- 

ral and peripheral ganglia as well as in the ganglia of the chain, 

mid it is believed that the great majority if not aU of the excitatory 

cells of the sympathetic are thus brought under the direct influence 

Df the central ner\^ous system. 

(3) Sympathetic excitator)^ cells. The sympathetic cells have 
n general the same forms as cells in other parts of the ner\^ous 
System. They may have a single process which is a neurite, or a 
lieurite and one dendrite, or a neurite and several dendrites. 
The last is the rule for the great majority of cells, at least in mam- 
ED&ls. The cell- body is surrounded by a nucleated capsule 
irhich is pierced by the dendrites. Outside the capsule the 
3endrites divide and subdivide into ver\' delicate branches which 
interlace with those of other cells to form a rich plexus. In most 
:ases the dendrites end within the ganglion in which the cell lies, 





212 NERVOUS SYSTEM OF VERTEBRATES. 

but it has been shown that occasional dendrites pass along a 
sympathetic nerve and reach another ganglion, in which they 
break up into end-branches. It appears that the dendrites of 
sympathetic cells play a minor part in the reception of impulses 
and it is not clear that either the pericellular plexuses of dendrites 
or the passage of dendrites from one ganglion to another has any 
functional significance. The neurite of the sympathetic celjl 
arises either from the cell-body or from a dendrite and may or 
may not become myelinated. When it is myelinated it is so fine 
as still to be distinguished from the smallest fibers of cerebro- 
spinal origin and the myelin sheath may extend for a longer or 
shorter part of the course of the fiber. The neurites, after a longer 
or shorter course in the splanchnic nerves or by way of the gray 
rami communicantes and one of the peripheral rami of the spinal 
or cranial nerves, end in (a) involuntary muscle, (b) heart muscle, 
(c) glands, or (d) other sympathetic ganglia. All smooth muscle, 
whether in the wall of the alimentary canal, in the ducts of glands, 
in the urinogenital system, in blood vessels, the skin or the eye 
is innervated by neurites from s)rmpathetic cells. The ending is 
by means of simple branches often with small knobs or enlarge- 
ments. The heart muscle is innerv^ated by neurites from the 
cells in the intrinsic sympathetic ganglia of the heart. The 
endings may be more complex, somewhat like those in striated 
muscle. The secreting cells of glands are innervated by simple 
endings of s)rmpathetic neurites which enter the glands along 
the ducts or blood vessels or which come from neurones situated 
in the glands themselves. The ending of the neurites of S)rmpa- 
thetic cells in other sympathetic ganglia is still a matter of dispute. 
Histologists have described the endings of neurites upon the 
dendrites of sympathetic cells, but physiologists have obtained 
no functional evidence to corroborate the supposition that these 
are the endings of neurites arising from s)rmpathetic neurones. 
Inasmuch as the efferent cerebro-spinal fibers are universally 
believed to end in the pericellular baskets within the capsules of 
sympathetic cells, it is probable that the endings in connection with 
the dendrites come from sympathetic cells and that suitable forms 
of experiment for determining their fimctions have not yet been 



THE SYMPATHETIC SYSTEM. 



213 



t- 



de\ised. It is also uncertain as yet whether these fibers run from 
one of the chain ganglia to a more peripheral ganglion or from a 
more peripheral to a more proximal ganglion. The important 
question regarding these endings is whether the visceral excitatory 
chain consists of more than t\\*o links. The physiologists claim 
that only one sympathetic cell intervenes in any case between 
the efferent cerebrospinal fiber and the muscle or gland imier\^ated» 
The exbtence of sympathetic endings in sympathetic gangha, if 
clearly established, would seem to show that in some cases two 
such neurones enter into the excitatory chain, 

(4) Sympathetic sensor)^ cells* Certain ceEs in the peripheral 
ganglia, as in Auerbach*s plexus, have longer dendrites than 
those of ordinary sympathetic cells and these dendrites are sup- 
posed to be distributed to the mucosae and to serve as sensory 
fibers. The neurites of these cells pass through one or more 
ympathetic ganglia to which they give branches. These branches 



Symp. g 




Postg. fiber 



Fig. no. — A dia^am to illustrate Langley's *'axone reflex**. After Langley, 
-^r§g, fiber ^ pregatiglionic fiber; Fostg. fiber, postganglionic fibers; Inf. rms, gang., 
^ofetior mesentenc ganglion; h^ bladder. 



^^ter into the plexus of dendrites in the ganglion and may serve 
Xo arouse peripheral reflexes by stimuladng the exdtator}^ cells 
in the gangUon, It is stated as probable (Dogiel) that these 
:»ieurites nm on through the gray rami commnnicaEtes and form 
^he pericellular endings which are known to occur in the spinal 
ganglia of several classes of \'ertebrates. These are complex 
^^dings immediately aroimd the bodies of certain spinal ganglion 
«^d]s which are described as cells of type II. These second type 



214 



NERVOUS SYSTEM OF VERTEBRATES. 



ceUs have neurites which break up into branches within the spinal 
ganglion and form pericellular baskets about the bodies of 
ordinaf)^ spinal ganglion celk. The functions and the structural 
arrangement of the sensory sympathetic cells and their supposed 
connection with the spinal ganglion cells require further study. 

The statement is made (Onuf) that fibers wWch enter the 
visceral sensor>^ column of the spinal cord are caused to degenerate 
by cutting the rami communicantes of the sympathetic^ This 
would indicate that sensory cells situated in the sympathetic 
system send their neurites directly into the spinal cord. Suchj 
cells and fibers are not shown in Figure 108. 

Another form of peripheral reflex has been suggested in which 
the branches of a single neurone only w^ould be involved. It is 
supposed that an impulse may travel from a peripheral ganglion 
back along an efferent fiber and go out from it along a collateral 
to stimulate a sympathetic excitatory cell. This form of reflexj 
is illustrated in Figure no. It must be said that this hypothesisl 
seems v^ry improbable in \iew of what we know of the polarityl 
of neurones in other parts of the nervous system, and that there is] 
little direct evidence in its support* 

The essential feature of the sympathetic system is that in the! 
visceral reflexes governing smooth muscle^ heart muscle and glands, ' 
there is interpolated in the efferent limb of the reflex chain a per* 
ipheral neurone between the cerebro-spinal fiber and the organ 
inner\'ated. There may be two such neurones interpolated and^ 
the sympathetic may carry out peripheral reflexes without the 
aid of cerebro-spinal elements, but these things are still uncertain 
as arc also the sensory sympathetic neurones. The sympathetic^ 
system does not to any great extent carr}' on independent or 
automatic functions. The great majority of its actions are directly 
aroused by efferent impulses coming from the brain or spinal 
cord and in response to the stimulation of visceral sensorj^ fibers 
which run through, but have no connection with, the sympathetic 
ganglia. In a strict sense the sympathetic consists solely of the 
neurones whose cell-bodies lie in the various gangha, the excitatory 
and sensory sympathetic cells above described. The afferent 
and efferent neurones whose cell- bodies lie respectively in the 



THE SYMPATHETIC SYSTEM, 



2IS 



cranial or spinal ganglia and in the brain or cord belong properly 
to the cerebro-spinal system and not to the sympathetic. 

The efferent cerebro-spinal fibers which end in sympathetic 
gangUa were given in the last chapter (p. 200) as the \isceral 
efferent fibers of the spinal nen^es. Similar fibers occur in the 
cranial nerves in all vertebrates in which the sympathetic is devel- 
oped in the head region. Where the head sympathetic is not devel- 
oped the functions of the sympathetic, if performed at all, are prob- 
I ably performed by efferent \isceral fibers in the cranial nerv^es with- 
out the intervention of peripheral neurones. There isj of course, 
(the further alternative that there may exist peripheral ganglia 
which are not developed from the cranial ganglia in the typical 
manner. K in the lower fishes the head sympathetic is wanting 
there can scarcely be any sharp distinction drav^Ti between the 
fibers which innervate smooth musde and glands and those which 
innervate the striated muscles of the gill arches. WTierever the 
sympathetic is developed, however, such a distinction is very 
clear, since in the one case a peripheral neurone is interpolated 
in the reflex chain, in the other case not. The two sorts of fibers 
take their origin from the same zone in the brain and spinal cord 
but it happens that all the fibers w^hich innervate striated muscle 
are confined to the cranial nerves, including the spinal accessory, 
while the sympathetic system extends through both head and trunk. 
The nuclei of origin for fibers which innervate striated muscle 
have probably become distinct from the nuclei of origin of svTTipa- 
thetic efferent fibers (see Figure 107). An explanation of this 
condition may be offered as follows. In primitive vertebrates 
all visceral muscles were presumably non-striated and the visceral 
reflex chain consisted of simple afferent and efferent limbs without 
peripheral neurones. The more active branchial muscles became 
striated and retained their direct innervation. For the inner- 
vation of the non-striated muscle and glands neurones migrated 
from the cranial and spinal gangUa and came to be interpolated 
in the efferent pathway. As to the morphological status of these 
migrated neurones of the sympathetic ganglia no sufficient expla- 
nation can at present be given. As to the sensor}^ neurones, it 
would be not at ail surprising that ganglion cells should migrate 





2l6 



NERVOOS SYSTEM OF VERTEBRATES. 



toward their innervation territor)^, from the spinal ganglia into 
the viscera, but it is just these sensory cells whose existence and 
arrangement arc in dispute. The excitatory neurones in the 
sjTnpathetic system belong to a special category. They seem lo 
have had their origin from neural crest material, but whether as 
modified spinal ganglion cells or directly from indifferent ecto- 
dermal cells there is no evidence. The double origin of the cilian* 
ganglion in the chick is of interest in this connection. 

Regardless of these unsettled theoretical questions it should be 
held dearly in mind that the sympathetic system is an offshoot 
or subsidiary portion of the \isceral afferent and efferent divisions 
of the nen^ous system which has come to have a special structure 
and arrangement owing to the conditions of visceral activities. 

DEMONSTRATION OR LABORATORY WORK. 

1. Dissect the sympathetic system of a frog and a mammaU 

2, Study sections of sympathetic ganglia prepared by the Golgi or 
methylene blue method. 

LITERATURE. 

Balfour, F. M.: Monograph on the Development of Elasmobranch Fishes* 
1878. 

Balfour, F. M.: Comparative Erabr^'ology. Vol, 2. 1885. 

Cajal, S. R, : Tejctura del sistcma nervioso del Hombre y dc los VertdjnuJos. 
Tomo II, segunda parte. 

DogieL A. S.: Zur Frage iiber den feineren Bau des sympathischcn Nerven- 
systems bei den Saugethieren. Arch.Lmik.Anat., Bd. 46. 1895. 

GaskeU, W. H.: On the Structure. Distribution and Function of the Nerves 
which innervate the Visceral and Vascular Systems. Jour, of Physiol, Vol 
7, 1S86. 

VanGehuchten, A.: Les cellules nerveuses du sympathique. La Cellule, 
Tome 8. 1892. 

Herrick, C. Judson: The Cranial Nerves of Mcnidia. Jour. Comp. Neur.* 
Vol. g. i&gg. 

His, W., jr.: Die Entwickelung des Herznervensystems bei Wirbelthiereo. 
Abhdl. Math.'physischen Classe d. K5nigl Sachsischen Gesell.dWiss., Bd. 8. 
Leipzig. 1 89 1. 

Hoffmann, C, K.; Zur Eatwkkelungsgeschichle des Sympathicus. i. Die 
Entwickelung des Sympathicus bei den Selachiem (Acanthias vulgam). 
Verhandl. K. Acad. Wetensch. Amsterdam. 1900. 2. Die Entwickelungsge- 
schichtc des Sympathicus bei den Urodelen. Ibid. 1902, 



THE SYMPATHETIC SYSTEM. 217 

Hubcr, G. C: Lectures on the Sympathetic Nervous System. Jour. 
Comp. Neur., Vol. 7. 1897. 

Huber, G. C: A contribution on the Minute Anatomy of the Sympathetic 
Ganglia of the DifiFerent Classes of Vertebrates. Jour. Morph., Vol. 16. 1899. 

Johnston, J. B.: The Cranial Nerve Components of Petromyzon. Jour. 
Comp. Neur. and Psych., Vol. 15. 1905. 

Langley, J. N. : The Arrangement of the Sympathetic Nervous System based 
chiefly on Observations on Pilo-motor Nerves. Jour, of Physiol., Vol. 15. 1893. 

Langley, J. N.: A Short Account of the Sympathetic System. Pamphlet 
Physiol. Congress Berne. 1895. 

Langley, J. N.: The Sympathetic and other related systems of nerves. 
Text-book of Physiology. Edited by Schafer. Vol. 2. 1900. 

Lan^ey, J. N.: The Autonomic Nervous System. Brain. Vol. 26. 1903. 

Lan^ey, J. N.: Das Sympathische und verwandte nervose Systeme der 
Wirbdthiere (autonomes nervoses System). Asher u. Spiro's Ergebnisse. 
n. Jahrg., n. Abtheil. 1903. 

Onuf and Collins: Experimental Researches on the Central Localization 
of the Sympathetic with a Critical Review of its Anatomy and Physiology. 
Arch. Neur. and Psychopath., Vol. 3. 1900. 



2l8 NEKVOrS STSITM OF TEKIEBKAIES. 



CHAPTER XIV. 

CERTERS OF CORSELATIOH. 

The whole nennous system has bcea treated thus far as conasting 
of four main diviskxiSy each of whidi is ooimected with a spedal 
set of fuDctioiis. For a miew and summaiy oi these functicmal 
divisioiis the student should turn to the oudine ^ven in Chapter 
V. The central portions ol the functional divisions constitute 
four longitudinal zones ol the brain and ^xnal ooid. The four 
zones fie in the same^relative position throiig^hout die whole length 
of the central nennoiK system, except in certain segments of the 
brain where one or odier zmie is largdr or wholly wanting. It 
may be supposed that the spedafization ci these four zones has 
taken place within a rdativelT uixfifkxcntiated neural tube which 
was possessed by the remote ancestocsolyeitduates and that in all 
existing vertebrates the functional dfiiisions are quite distinct 
from one another. The process of spmafiratinn oi the functional 
divisions has left over, so to ^)eaky a certain amount of material 
which has come to sen-e the purpose at connecting the functional 
divisions with one another and the centers of aae segment with 
those of another. The elements which properly bdong to the 
primary fimctional di\ia(xis as described in previous chapters 
may be summarized as follows, (i) In die afferent or sensory 
columns: (a) recepti\-e neurones, which receive the end-branches 
of afferent nbers; (b) intrinsic neurones, whose neurites whether 
long or short are confined to the given column; and (c) extrinsic- 
neurones, whose neurites go beyrmd the given column to carry im- 
pulses to other centers. These other centers are primarily located 
in the same colunm from which the neurites spring but they fall 
under the categor\- of the centers of correladcNi which are here 
under consideration. The neurones {h) and (c) may both be at 
the same time receptive neurones. (2) In the efferent columns: 
motor or excito-glandular neurones whose neurites go to the 
periphery- as fibers of efferent nerves. 



CENTERS OF CORRELATION. 



219 



Neurones which do not properly belong to any one division 
are at first scattered through all four divisions and in the spinal 
cord this condition is maintained, so that the material in question 
appears as cells scattered throughout the whole gray area. These 
cells have come to be known as homolateral and heterolaieral 
traci cells (Fig. iii)» In the brain a large part of the corre- 
sponding material is retained in its embr>*onic position adjacent 
to the ventricle. The term central gray matter has been appUed 
to this circum-ventricular zone of cells. Since many of the cells 
of the primary sensor}^ and motor centers lie in this central gray, 



Fig. III. — Tract cells in the spinaJ cord of the trout. Combined from two 
figures by Van Gehucbten. 



especially in lower vertebrates, it is necessary to use some other 
term to designate the material which senses functions of correlation. 
Whether these cells are situated adjacent to the ventricle or are 
scattered through the wall of the brain, their dendrites intermingle 
with the fiber tracts which form a large part of the brain wall. 
These areas consisting of mingled fiber tracts, cells and dendrites 
may be called the substanUa retkularis. The portion in which 
the fiber tracts predominate is the substantia reticularis alba; that 
which is composed chiefly of cells is the substantia reticularis 
grisea. 

The tract cells in the spinal cord illustrate best the functions 
which this unspecialized material first served. Since these cells 




a3o 



NERVOUS SYSTEM OF VERTEBRATES* 



are widely scattered through the cord, some of them may receive 
impulses from one source, some from another. Their neurites 
enter the lateral tracts of the cord and run forward or backward 
for a longer or shorter distance, ending in relation with motor 
cells. There has been observed anatomically no order or system 
about these cells and their fibers. They seem to offer opportimities 
for the wide spread of all kinds of impulses from segment to 
segment of the cord. The heterolateral cells add the possibility 
of impulses reaching the opposite side of the cord. Whether 
any regularity in the relations of these neurones is constant in the 
species and is inherited from generation to generation is unknowTi. 
It seems more probable that these neurones oflfer a relatively 
indifferent material in the embr^'o, providing for the diffusion 
of impulses from segment to segment and from one side to the 
other, and that definite paths for impulses are set up chiefly as the 
result of the experience of the individual. If impulses traveling 
through certain cells and fibers sen^e for the performance of an 
act efficiently, the success attending the act will lead the young 
child to repeat the attempt. The repetition will render the 
impulse-pathway more easy for succeeding impulses to follow. 
Thus a habitual pathway is set up, while other possible pathways 
offered by the indifferent tissue of the embrj'onic nervous system 
become after a time unavailable through lack of use. So in the 
early life of the child it is probable that certain orderly sets of 
connections are established by way of these indifferent tract cells 
by means of which complex reflexes are carried out, and the actions 
of two divisions of the nen^ous system correlated. It is indeed 
just this development of orderiy connections in the central nerv^ous 
system which is going on during and as a result of the aimless 
movements of the infant in the first few months of its life* 

In the brain the same kind of processes have been at work but 
the indifferent material is proportionately greater in amount 
and in certain regions special brain centers have been formed. 
In Older to understand these it is necessary to look at them from 
the generic point of view, especially as regards their relations to 
the functional divisions. Strictly speaking, no fast line of division 
can be drawn between the elements of the functional divisions 



I 

I 

4 




CENTERS OF CORRELATION. 



321 



proper and these of the substantia reticularis. Not only are the 
neurones of the reticularis scattered throughout the four longitud- 

* inal zones, but in the brain region at least, the reticularis neurones 
[ are especially related to the functional di\'ision in which they lie. 

In all segments of the brain cephalad from the VII nervT one or 
I other of the primar)^ functional divisions is either greatly reduced or 
wholly wanting. Throughout this region of the brain special cen- 
I ters are formed which in most cases clearly lie within the bounds of 
one of the primary' longitudinal zones. Whether neurones belonging 
'primarily to the functional divisions have entered into the forma- 
tion of special centers, and to what extent this may have happened 
are questions which cannot now be decided. For the present the 

* following practical criterion may be applied; where the structure of 
I centers and the disposition of their tracts do not follow the typical 
I arrangement of centers and tracts in one of the functional divi- 
sions, the centers in question are treated as centers of correlation. 

I Since these centers are at least largely derived from the substantia 
I reticularis it may be stated that the most fundamental difference 
I between the brain and spinal cord is the presence in the former 
j of a relatively larger volume of this indifferent material for the 
formation of correlating centers. It will be the purpose of this 
and the following chapters to give in a broad and general way an 
' account of the phylogenetic histon^, morphology and functional 
I relations of these special brain centers. 

The reason for the use of the term centers of correlation is 
very simple. As has already been showTi (p. 8i), simple reflexes 
are mediated by direct connections between sensor)- and motor 
nuclei. Even large movements of a vague and ill-directed sort 
may be carried out in this way. But when complex movements 
invohing the action of many muscles directed to a definite end are 
called forth by a stimulus, the inten'ention of secondary* and 
tertiary centers with their fiber paths is neccssarj^ for the control 
iof the muscles as to the time, extent and force of contraction. 
The experiments with the brainless frog (p. 82) show that for 
movements of short duration up to a certain grade of complexity the 
tract cells of the spinal cord serve the purpose of correlating 
centers, but for the direction of movements for a longer period 




222 



NERVOUS SYSTEM OF VERTEBRATES. 



of time involving adjustment and readjustment of muscles with 
reference to some whole act destined to reach a given end, the 
better organized brain centers are necessa^^^ It is the function of 
relating several simple actions with reference to some common 
end, the co-relation of activities, which these brain centers serve. 

The degree of complexity of the acti\ities controlled by the 
centers of correlation in various animals is directly paralleled by 
the complexit}^ of the brain itself* The efforts at escape made by a 
normal frog when seized are much more complex and long con- 
tinued than those of a frog whose brain has been destroyed. To 
the human observer, however, the efforts of the normal frog are 
very simple. The frog has no cerebral hemispheres related to 
somatic sensation and somatic movement. The efforts of a 
mammal %\dth its cerebral cortex are enormously more complex 
and may involve keen observation, connected effort through 
relatively long periods, employment of indirect means, etc. 

In the 'lowest vertebrates, cyclostomes, a large part of the sub- 
stantia reticularis of the brain remains in its primitive indifferent 
condition; few special nuclei are developed and the activities of 
the animals are correspondingly simple. To any stimulus that 
may come the animal can respond only in a ver>^ Kmited number 
of ways. To two similar stimuU little or no difference in response 
is to be expected. In the whole hindbrain region no special nuclei 
in the substantia reticularis have been found. Even the secondary 
gustatOHr* nuclei are so little developed that they have not yet been 
seen. The cerebellum is w^holly unspecialized. The tectum 
optic um and the nucleus of the tractus habenulo-peduncularis 
in the mesencephalon, the nucleus habenulae, the nucleus of the 
posterior commissure and the inferior lobes in the diencephalon, 
and the striatum and epistriatum in the telencephalon probably 
comprise all the special nuclei at present know^n in the cyclostome 
brain. These wiU be considered in connection with the brain 
of true fishes below. 

In the different classes of vertebrates the process of differ* 
entiation of many of the special centers which are well marked 
in the mammals is seen in various stages of completeness. In 
some selachians and ganoids parts of the brain are little more 



4 



CENTERS OF CORRELATION. 



213 



highly specialized than in cyclostomes. Other parts of the brain^ 
however, as the olfactory centers in selachians and the gustatory 
centers in some bony fishes, are ver\^ largely developed and appear 
more complex than in other fishes. The cerebellum also is 
extremely large in many selachians and bony fishes. The study 
of these structures in those forms in which they are highly developed 
has led to some results which could not be so well attained from 
the study of any other forms. In bony fishes the development 
of special centers from the indifferent gray substance has gone 
farther than in other lower vertebrates. There has been a collect- 
ing and sorting of elements which are more diffusely placed in 
ganoids and selachians. Tliis fact makes the braio of the teleost 
an especially rich field for the study of the centers and fiber tracts 
constituting the apparatus for the performance of specific functions. 
Since the study of less speciaUzed brains has given the fundamental 
plan of structure of the vertebrate brain, the brain of the teleost 
should now^ be subjected to careful and detailed study in order to 
determine the early form of the central apparatus which directs 

i| known activities. Herrick has just done tliis for the gustatory 
apparatus, and it h much to be desired that the centers and fiber 
paths involved in other functions should be worked out in the same 
way. Generally speaking, any vertebrate in which any system 

I of organs is unusually highly developed presents special opportu- 
nities for the study of the central apparatus of that system and also 
of the process and method of brain differentiation in generaL 
Although much remains to be done in the way of rendering 
our knowledge of the special centers and their relation to the 
main functional di\isions complete and exact, the description 

, of these centers wall be given as far as possible in the form of an 
account of the functional system of neurones of which they form 
parts. 
First there are to be mentioned a number of neurones wliich 

;Se€m to be a vestige of invertebrate structures which are quite 
lost in higher A-ertcbrates. These are the Mullerian cells and 
fibers of cyclostomcs and the cells of Mauthner of fishes and 
amphibia. There are in the brain of Petromyzon over twenty 

I gigantic cells lying in the somatic and visceral motor columns 



224 



NERVOUS SYSTEM OF VERTEBRATES. 



in the region of the cranial nen^ roots* Their neuriles run back 
into the spinal cord. In other fishes a pair of such cells lies 
adjacent to the motor root of the VII nerve, whose neuriles cross 
and run back into the spinal cord. These elements remind one 
of the large cells and thick fibers characteristically found In the 
nervous system of invertebrates and the fact that they are found 
only in lower vertebrates and most numerous in the cyclostomes, 
suggests that they are ver)' archaic elements which are not to be 
counted among the typical elements of the vertebrate nervous 
system. 

In the region of the myclencephalon the substantia reticularis 
has on the whole the same relations as in the spinal cord. In 
lower vertebrates the formation of special nuclei from this material 
has not proceeded far, but it is probable that further study will 
show a tendency even in fishes for the segregation of the neumnes 
related to somatic centers from those related to visceral centers. 
Especially is this to be expected in any forms in which one system, 
such as the acustico-lateral in selachians or the gustator}' in bony 
fishes, is greatly developed. Indeed it seems dear that the inferior 
and superior secondar)' gustator}^ nuclei, which have already been 
described, are specially developed parts of the substantia reticularis 
which were primitively closely related to the visceral sensory 
column. 

At the caudal end of the myelencephalon in fishes a collection 
of substantia reticularis cells about the roots of the first ventral 
spinal ner\'es (hypoglossus) forms the inferior alive. This nucleus 
becomes of great size and complexity in mammals but its functional 
relations are not yet well understood in any vertebrate. 

In the ventral part of the cerebellar segment occurs a collec- 
tion of several nuclei which in mammals causes the large protu- 
berance kno^Mi as the pans. These nuclei are highly developed 
only in the mammals, where they are related to the pjTamidal 
tracts and the cerebellum. A part of them receive numemus 
collaterals and tenninal branches from the pyramidal tracts and 
are beUeved to send fibers to the motor centers of the spinal cord 
and to the cerebellum. These pontial nuclei are thus interpolated 
in the direct pathway between the cerebral hemisphere and the 



i 

I 

4 
4 





CENTERS OF CORRELATION. 225 

spinal cord and ako in the indirect path by way of the pons 
and the cerebellum. The mesial part of the pontial nuclei receives 
collaterals from the lemniscus and hence is related to the somatic 
sensory apparatus. 

DEMONSTRATION OR LABORATORY WORK. 

1. Study the tract cells of the spinal cord in Golgi sections of young 
fish fry, frog tadpoles or chick embryos of the fifth to eighth day of 
incubation. 

2. Study the lower olive and pontial nuclei in Golgi sections of the 
brain of the mouse or other small mammal. 

3. Study the tract cells of the spinal cord of a fish or amphibian 
and the cells and their relations to the four primary columns in the 
medulla oblongata. 

LITERATURE. 

Cajal, S. R.: Beitrage zum Studium der Medulla oblongata. Leipzig 1896. 

Cajal S. R.: Textura del sistema nen'ioso del Hombre y de los Vertebra- 
dos. Madrid 1904. 

Van Gehuchten, A.: La moelle epiniere de la truite (Trutta jario). La Cel- 
lule. Tome II. 1895. 



IS 



aa6 



NERVOUS SYSTEM OF VERTEBRATES. 



CHAPTER XV. 
THE CEREBELLUM. 

In the most primitive existing vertebrates the cerebellar segment 
shows little or no higher organization than any segment of the 
medulla oblongata. In man the cerebellum is, next to the cerebral 
hemispheres, the most complex part of the brain and is connected 
by a great number of tracts with nearly all parts of the nervous 



Decussatio veli^ 
Rad mesenc, V 
Tectum 



Secondary gustatory nuc. 



L. fineae lateralis 




Tertiary 
gustatory tract 



Fig. 112. — The relations of the cerebellum, brachium conjunctivum and gusta- 
toxy tracts in selachians (Scyllium). Projection upon the median sagittal plane. 



system. The chief facts in the evolution of this complex organ 
and its relationship will be dealt with in this chapter. 

In primitive vertebrates the cerebellar segment differs from 
those following it chiefly in the absence of visceral nerve roots 
and their primary sensor)- and motor nuclei. The dorsal part 



THE CEREBELLUM* 



227 



of the segmeDt belongs to the somatic sensory column (cf. p. 115) 
and receives general and special cutaneous nen^e fibers. The 
cutaneous centers arch over the ventricle and are connected 
dorsally by a commissure. In the lateral wall> at least in true 
fishes, the nsceral sensor}^ column is represented by the secondary 
gustatory nucleus. In the ventral wall is the somatic motor 
nucleus of the trocUearis nerve. The central gray and the super- 
ficial zone of longitudinal fiber tracts with a decussation in the 
central raphe complete the structure of the cerebellar segment. 

Tr. tecto-cerebellaris 



Brachium 
conjunct. 
fSladuc mcsenc. V. 
Sec gustatory tract ' 
Tr. bulbo-tcctalis ' 
Tr. lecto-bulbaris 



Tr. haben.- 
pedunc. 



Frc. 1 1 3, -^Transverse section through the cerebellum of the sturgeon. 



I 



In cyclostomes the central gray matter presents nothing of 
special interest and the visceral sensory structures are as yet 
not studied. The somatic sensory centers consist of large and 
small cells. The small celb give rise to neurites of line caliber 
which form a superficial fiber zone and a part of which cross to 
the opposite side, forming a dorsal commissure. The neurites 
of the large cells. take the characteristic couise for secondary 
cutaneous fibers, decussating in the ventral wall of the mesen- 
cephalon. Whether they run to the tectum mesencephali or to 
some other center is not certainly kno\\Tt, They may represent 





228 



NERVOUS SVSTEM OF \TRTEBRATES. 



the fibers in the brachium conjunctivimi of higher vertebrate 
which arise from the nucleus dentatus and run to the optic 
thai ami. 

In selachians the somatic sensorj' portion is greatly increased ^ 
in size and complexity. It forms a large, sometimes enormous^ij 
arched and folded roof to the metencephalon, in which histological ' 
specialization has gone much farther than in the cerebellum of 
cyclostomes. The increased volume is due to a much greater 
\'olume of small cells and to a larger aze and greater number of 
the large cells. The small cells constitute the granular layttJ 
and are known as granule cells. Their netmtes constitute the! 
greater part of the molecular layer and form the commissuiej 

Tr. tecto-ccrcbcllaris 



Sec. gusta^ 
tory tract 
BracbiuiTi! 
conjunct. 
Tr. tecto-btil bans' 

Tr. iobo-bulbaris 
Tn bill bo tec talis 




Radix ineitocc. V 

Tr, haben^'pedunc. 



Fig. 1 1 4. ^Transverse section thmugb the cerebcllmn and secondary gusUW 
nucleus of the sturgeon. 



of the cerebellum and the so-called cerebellar crest over the 
acusticum (cf. Chap. VII)* The large cells are definitely arrangpi 
on the border between the granular and molecular layers and their 
dendriteSj which expand in the molecular layer, have the character- ^g 
istic appearance of Purkinjc cell dendrites. This portion of the ^M 
cerebellum receives in addition to somatic sensory root fibers ^ 
secondar)^ tracts from somatic sensory centers, including on 
from the tectum opticum. This shov^^ that it senses both as 
primar)* somatic sensory center and as a center of correlation fc 




THE CEREBELLUM. 



229 



actinties called forth by impulses from different organs of somatic 
sensation. A well developed brachium conjuncdvum is present 
and probably consists in larger part of fibers arising in the cere- 
bellum (Fig. 124, A, B). 

I In the lateral walls of the metencephalon in selachians lie the 
secondary visceral sensor)^ nuclei (p, 172 above)* These nuclei 
receive the secondary visceral or gustatory tracts and give rise to a 

■ commissure which crosses dorsally at the junction of the cere- 
bellum and optic lobes, in the velum medullare anterius. This is 
£as the decussaiio veli and lies in close relation with the 
tion of the IV nene. The secondarv' gustatory- nucleus 



\'alvula 



Tectum mesciic. 



Tr* tecto.cercbel. 

Sec, ffust. tract 

Tertiary vust. tract 

Brachium conjunc. 

Tr. tectO'bulbaris 

Tr, habcD^-pctlunc. 




IV 



Radix mescnc. V^ 

Tf. bul bo- tec tali a 
Tr. lob(>butbari5 



Fto. 115* — Transverse section of the brain of the sturgeon at the junction of the 
cerebellum and midbrain. 



also sends a large tract foniN-ard and downward to the inferior 
lobe, the tertiary gustatory tract (p. 173). The arrangement 
of the brachium conjunctivum, and the tracts related to the 
secondary gustator)^ nucleus is shown in Fig. 112. 

In ganoids and bony fishes the somatic sensory portion of the 
metencephalon is not essentially different from that in selachians. 
It is not so large but the secondary tracts from other somatic 
sensory centers- — external arcuate fibers and tractus tecto-cere* 
bellaris — ^are more highly developed. The connections of the 
cerebellum with the secondary centers for the skin, ear and eye 




230 



NERVOUS SYSTEM OF VERTEBR.\TES. 



become more important relative to its primary connections than 
in lower fishes. The \isceral nuclei (Figs. 114, 115) are situated 
somewhat farther %*entrally and are both actually and relatively 
much larger than in selachians owing to the greater development 
of the gustatory organs. The position of the commissure between 
the gustator}^ nuclei is ver}^ dififerent from that in selachians* ■ 
Instead of lying in the roof it lies deeply imbedded in a massi\'e ^ 
median structure— the valvula cerebelli — which largely fills up 
the ventricle of the cerebellar and mesencephalic segments. The 
position of the commissure is sho\\Ti in Figure 91, which represents 



Valvula 



Inferior camtrriss. of cerebelluiB 



Tectum njes«nc. 



Tr. tccto-ccrrbcL 
Tr. tecto-bu1bari« - 
TcTtiafy gtisL tr*- 
IV.- 
Tr. lobo-bulbarift- 
Tr. mamm,-bul-.^ 



baria 



r: 



Tr, lobo^etcbd. 
Rftdix mesencV 



Brachium conjunct. Tr, habcn.-pcdtiaculam 

Fig. 116. — Transverse section through the midbrain of the sttirgeon. 

a sagittal section of the brain of a teleost a few days after hatching 
(see also Figs. 115 and 116), The form of the adult cerebellum 
of teleosts is essentially the same as this. The place of crossing 
of the commissure and of the IV nen-e is homologous with the 
velum in selachians and it is clear that the valvula has been formed 
by an infolding of the roof in the region of the velum. Since the 
structure of the valvula is essentially that of the cerebellum it may 
be assumed that the t}T3icaI layers of the cerebellum have spread 
forward into the valvula. The infolding of the roof, however,] 
came about undoubtedly on account of the great size of the second- 





THE CEREBELLUM. 



^31 



ary gustatory nucleus and it is probable that the nucleus has 
invaded the valvula along tlie commissure. The dorsal limb of 
the valvula is thin and belongs to the tectum mesencephali. The 
arrangement of tracts related to the gustatory nucleus in a ganoid 
is shown in Figure 117. 

The cerebellum in the higher classes of vertebrates does not 
continue to increase in size and complexity as it does through 
the, several classes of fishes. On the contrar)', in amphibia and 
reptiles it is relatively ver>' small^ owing to the comparatively 



Valvula. 



Tectum, 



'VA^ 



Xomm. inf.cerebelli 



^Sec. gustatory nuc. 
.Rad. mesenc. V 



Brachium conjunct. 
L. inferior ) Tertiary gustatory tract 



■Tr. spin.V 

^ Sec. gust, tract 
Lemniscus sys. 



FtG. 117. — The relations of the cenebcllum, brathium conjunctivuro and gusta* 
lory tracts in a ganoid fish (Acipenser), Projection on the median plane* 



sluggish habits of these animals and the consequent reduction 
in the number of cutaneous and gustatory sense organs. The 
reduction of the cerebellum in amphibia as compared with selach- 
ians is chiefly a decrease in size only. The somatic sensor}^ portion 
constitiites lateral lobes which are simply arched over the ventricle, 
not folded, but in these the typical granular and molecular layers 
atid Purkinje cells are as clearly marked as in selachians. Simi- 
larly the secondary \isceral nuclei are present in the lateral walls 
but no val%^a is developed. The size of the cerebellum is closely 
related to the degree of activity of the animals. In active forms 




232 



NERVOUS SYSTEM OF \^ltTEBRATES. 



such as the frog and crocodile the somatic sensory lateral lot 
are greatly increased in size. This increased size is not due chiefly 
to a greater number of sense organs or to a richer cutaneous 
innervation, but to an increase in the secondary tracts from the 
cutaneous, auditor}^ and optic centers to the cerebellum, and to 
a greater number of Purkinje cells whose fibers go to motor nuclei. 
In other words, the cerebellum in the more highly organized 
amphibia and reptiles shows an increase in the correlating mechan- 
isms in comparison with the primary" sensor>^ apparatus. 

The further steps in the development of the correlating mechan- 
ism in birds and mammals are not well understood for lack of 
comparative studies. A careful study of the structure and fiber 



Julb^oll 
Fiss-rhia p^,^f^ 



jMesenceph. 

^Cerebellum 
^Fiss- postn. 

Rss. pra?c^ 



'lesenceph. 
Fiss, prima 



Tuberc. acust. 
Floe Vdum' 

A Plex. chorioid - 



^Fiss- 
sec- 

Rss- postn - 
B 



Fig. 118* — A, the left lateral aspect of the brain of a pouch specimen of Dasy^ i 
urus viverrinusi B, median sagittal section of the cerebellum of the same brain- 
After G. Elliot Smith. Fiss. praec, fissura praeculminis; Fiss, postn , fissure 
post nodularis; Fiss. sec, fissura secunda; Fhc, flocciUus; Farafloc.^ paraflocculu^- 



comiections of the cerebellum in a series of forms induding repre- 
sentatives of monotremes, marsupials, bats, rodents and carnivores 
is much needed in order to sho%v the course of evolution of the^ 
extremely complex cerebellum of man. No connected account; 
of this can be given at present, but it is evident that a larger num- 
ber of speciaEzed centers come into relation with the cer 
beUum in man than in fishes. At the same time the structur 
of the cerebellum itself has become more complex. This coux^ 
plexit}^ of structure and of anatomical connections indicates tK^ 



3^ 





THE CEREBELLUM. 233 

number and variety of functional relationships into which the 
cerebellum enters. The cerebellum has become in man a great 
center concerned with correlated movements in response to impulses 
from many sources. A brief review of the human and mammalian 
cerebellum will illustrate this. 

In order to understand the manunalian cerebellum it is necessary 
first to discard the cumbersome and meaningless description of 
lobes and surface divisions based upon the adult appearances, which 
is found in text-books of anatomy. The simpler method of dividing 
the cerebellum based upon the developmental history is much to 
be preferred, but even this gains its significance only when it is 
combined with a consideration of the centers and tracts within. 
First of all must be pointed out the fallacy of the common state- 
ment that the cerebellar hemispheres of manmials are new form- 
ations, not found in sub-mammalian classes. The hemispheres 
are formed first. The outline of the brain of a young marsupial 
given in Fig. ii8 shows that in these forms the cerebellar hemis- 
pheres correspond closely to those of reptiles, amphibia or even 
of fishes (compare Figs. 2, 3, 7). The tuberculum acusticum 
occupies the same position as in lower vertebrates, at the dorsal 
border of the medulla oblongata immediately behind the cere- 
bellima, and in front of it the border of the cerebellar hemisphere 
is formed of the flocculus and paraflocculus and these are con- 
tinued upward by the middle lobe as the arch of the cerebellum. 
In the early human embryo after the cerebellar arch has been 
formed by the massive walls fusing in the roof, the lateral lobes 
are more prominent than the middle portion and the whole cere- 
bdlimi has the same form as in lower vertebrates. The vermis is 
formed later than the hemispheres, not earlier. So in the phylo- 
genetic history: the lateral lobes are constant in the vertebrate 
series, the vermis is formed only in birds and manmials. What 
does happen in the higher mammals to produce a cerebellum 
which seems to differ greatly from that of lower vertebrates is: 
(i) a great growth of the median or keystone region of the arched 
cerebdlum to form the vermis; (2) a great growth of that part 
(rf the hemispheres which lies in front of the floccular lobe, to form 
what is conunonly known as the hemispheres; and (3) an arrest 



334 



NERVOUS SYSTEM OF VERTEBRATES. 



of growth of the flocculus and paraflocculus, together with the 
submersion of a part of the lateral lobes of lower vertebrates to 
form the nucleus denlatus. 

Those portions of the cerebellum which become large and impor- 
tant in mammals are subdiiHded by variously placed fissures which i 
are merely the result of mechanical conditions of growth and most 
of which are of no importance except as they may sen^e as land- 






\^ 






iitOJ/>-^t«rt»Jijirt.oJti»^^ 



■ iA^ 



l&taA-oi-^ 



<juJa'V^^-^aM' 



m 



tii'li 



-ty--" 






iwir^Kfr^" 



^iM. 



\:^)&^I^ 



£Q 



^jrAMhaZ 






o 



Fig. 119.— A dingram representing the more fundamental and constant fissures 
of the mamniaHan cerebellum spread out in one plane. After G EJliot Smith. 



marks in practical work. Those fissures which develop earlier in all 
mammals may be regarded as the more primitive and important 
and may be taken as the boundar>^ lines of the chief divisions of 
the cerebellum. Upon this basis the cerebellum may be divided 
into (A) the flocciilar lobe, including the flocculus and para- 
flocculus; and (B) the rest of the cerebeUum; which is subdivided 




TKE CEREBEIXtTM, 



235 



by two transverse fissures into (a) anterior, (b) middle and (c) 
posterior lobes. The trans%Trse fissures are named the fissura 
prima and fissura secunda. These divisions are sho\^ii in a 
median sagittal section of the marsupial cerebellum in Fig* 118 
and in a diagrammatic dorsal view of the higher mammalian 
brain in Fig. 119. The anterior and posterior lobes do not expand 
laterally but are formed from the region adjacent to the mid- 
dorsal line. They become divided by later fissures into subsidiary 
lobules which are of no importance in the present connection. The 
anterior lobe includes at its cephalic border the lingtda which is 
connected by means of the valve oj Vieussens with the corpora 
quadrigemina. This region therefore corresponds in position to 
the velum medullare anterius of the selachian brain and the valvula 
cerebelli of the brain of ganoids and teleosts. The posterior lobe 
includes the two transverse folds known as the uvula and mdtdus^ 
to the latter of which is attached the membranous roof of the fourth 
ventricle. The nodulus therefore corresponds to the meso-caudal 
border of the cerebellum to which the choroid plexus is attached 
in all vertebrates. (Compare Figs. 118 B and 120.) The middle 
lobe in the embryo extends laterally and connects with the extreme 
lateral portions which form the floccular lobe. The middle lobe 
is divided into a median portion or vermis and lateral lobes, each 
of which is further divided by subsidiary^ transverse fissures. 
The anterior lobe corresponds to the larger part of the superior 
vermis of anatomists, while the inferior vermis includes the |X)s- 
tenor lobe and most of the median portion of the middle lobe. 

This brief survey of the surface characters of the cerebellym 
gains in significance when the structure and arrangement of its 
deeper parts are considered. Throughout the whole cerebellum 
except the anterior lobe it consists of the following layers from 
without inward, (i) Molecular layer^ consisting of (a) cells, 
(b) non-myelinated fibers derived from the granule cells, and (c) 
the dendrites of Purkinje cells. (2) Layer of Purkinje cell- 
bodies. (3) Granular layer consisting chiefly of granule cells. (4) 
Layer of myelinated fibers. This layer is ver^^ voluminous and 
its subdivision to the various lobes gives rise to the well known 
arbor viiae. In the region of the anterior lobe the structure is 





THE CEREBELLUM. 



237 



the same as elsewhere except that the fourth or fiber layer is 
largely occupied by a number of gray masses or nuclei. These 
gray masses form the roof of the fourth ventricle al its anterior 
end* In this region are recognized near the median line the 
paired nuclei tecti or jasiigii; lateral to them on either side the 
smaller nucleus globosus and nucleus embolijormis; and farther 
laterally the large nucleus denialus which in man is convoluted 
like the lower olive but in lower mammals has a simple compact 
form. In the region of these nuclei is the commissure of the 
deep or white layer of the cerebellum. 

The general features of the vertebrate cerebellum may now be 
summarized. It consists in all classes of the dorso-lateral walls 
of the metencephalic segment which are arched over and connected 
above the ventricle, where there is a commissure of molecular 
layer fibers. In most vertebrates the lateral walls bulge outward 
and forward as the auricular lobes, the floccular lobes in man* 
In selachians the cerebellum contains a part of the fourth ventricle 
which extends up into all the folds (Fig. 120)* In mammals 
the great increase of the white layer has obliterated the ca\dty 
in the several cerebellar folds and its position is indicated only 
by the branches of the arbor \itae. In median sagittal secdon 
the roof of the fourth ventricle extends up into the cerebellum 
in the form of an inverted letter V. The apex of the A enters the 
base of the arbor vitae and separates the gray masses beneath 
the anterior lobe from the nodulus. The position of these deep 
gray masses and the commissure which passes through them 
corresponds fully to that of the velum medulla re anterius and 
neighboring parts in selachians. The nodulus, floccular lobe and 
the acustic nuclei bear the same relations in mammals as do the 
caudal border of the cerebellum, auricular lobe and tuberculum 
acusticum in selachians. The mammalian vermis is formed by a 
great thickening of what corresponds to the mid-dorsal region 
of the cerebellum of any lower vertebrate. The hemispheres 
correspond to the dorso-cephalic wall of the lateral lobes in lower 
vertebrates. At the base of the lateral walls in fishes lie the 
secondary gustator\^ nuclei whose commissure crosses through 
the velum. In mammals in the region corresponding to the velum 



238 



NERVOUS SYSTEM OF VERTEBRATES. 



there is a deep commissure accompanied by nuclei which will be ' 

described below. In all vertebrates the cerebellum has two 
brachia, a posterior and an anterior. The posterior bracliium 
contains primarily fibers belonging to both somatic and visceral 
sensory systems. As already described, primary root fibers of the 



1 r^^ 



*' 



V"' 



/ 



^ 



// 



Fig. 121.— A diagrammatic transverse section of one fold of the cerebeUum. 
From Koelliker. fy moss fibers; gl, glia cells; gr^ granules; gr\ neuritcs of gran- 
ules seen in the molecular layer; A, net-like fibers; k\ endings of the same; fp», 
small cells of the molecular layer; m\ basket cell; tk^ pericellular basket of the 
same; ft, short neurite of large granule- 



somatic sensory nerves and external arcuate (secondary sensory) 
fibers enter the cerebellum in aU vertebrates. In lower verte- 
brates the secondary gustatory tract runs ventral to and parallel 
with the somatic sensor}^ tracts and ends in the secondary gustatory 




THE CEREBELLUM. 239 

nucleus. In 'mammals the direct cerebellar tract, which is the 
secondary visceral sensory tract from the spinal cord, runs lateral 
to the somatic sensory tracts and both are included in the corpus 
resHjarme. The anterior brachium, hrachium conjunctivum, in 
lower vertebrates (Figs. 112, 117) runs from the lateral lobe partly 
through and partly beneath the secondary gustatory nucleus and 
passes diagonally downward and forward to the base of the mes- 
encephalon. In mammals (Fig. 126) it is enormously larger and 
has made room for itself by shifting other structures to one 
side. In all vertebrates the anterior and posterior brachia cross 
one another as they enter the cerebellum like the two limbs of a 
letter X, the anterior brachium passing mesially to the posterior. 
In mammals a third brachium is added, the brachium pontis, 
which goes vertically downward between the diverging anterior 
and posterior brachia. 

Structure of the cerebellar cortex. — ^The most con- 
spicuous elements in the cerebellum are the Purkinje cells. These 
differ from those in lower vertebrates chiefly in the mode and 
regularity of their arrangement. The cell-bodies are situated 
between the molecular and granular layers and the dendrites 
spread in the molecular layer. The arrangement is such that 
in any fold of the cerebellum the dendrites of each Purkinje cell 
spread like a fan across the fold (Fig. 121). The dendrites are 
provided with small lateral spines. From the deeper end of the 
cell-body arises the neurite which penetrates the white layer. In 
the first part of its course it gives off several collateral branches 
which rise toward the surface and end in the granular or molecular 
layer. The neurites then become myelinated and proceed as 
constituent fibers of the white fiber layer. 

The granular layer contains both granule cells and cells of 
Golgi's type II. The granule cells are very numerous and closely 
packed together and, like those of lower vertebrates, have short 
dendrites which end in claw-Uke tufts of small branches. The 
neurites are exceedingly fine, rise directly into the molecular layer 
and divide in T-form into two branches which run parallel with 
the surface of the folds of the cortex. Among these fine fibers 
of the molecular layer are imbedded the dendrites of the Purkinje 



240 



NERVOUS SYSTEM OF VERTEBRATES. 



cells, the small spines of which apparently serve iot cormectioiis 
with the fine fibers. The cells of the second type are much larger 
than the granule cells, their dendrites spread in both granular 
and molecular layers, and their neurites branch immediately 
and profusely in the granular layer. fl 

The molecular layer contains two chief t)T>es of cells. In the 
deeper part of the layer are numerous cells the behavior of whose 
neurites has given them the name of baskei cells. The neurites 
begin as slender fibers which grow thicker as they nm parallel 
with the Purkinje cell layer. At intervals they give off collateral 
branches which run down and branch to form basket-like net- 
works around the bodies of the Purkinje cells. Although such 
cells have been seen in rare cases in the cerebellum of fishes, they 
are not highly developed or numerous below the mammals. In 
the outer part of the layer are numerous small cortical cells 
whose neurites are short and either branch repeatedly close to the 
cell or have a longer or shorter horizontal course before ending. 

Fiber tracts of the cerebellum.— The fibers connecting 
the cerebellar cortex with other parts of the brain in part arise in 
the cerebellum, in part end in it. Since the structure of the 
cortex and the arrangement of the fiber endings are cveryw^here 
the same, it follows that all three peduncles of the cerebellum 
carry both in-coming and out-going fibers. The in-coming fibers 
are of two forms: moss fibers, which bear peculiar bundles of 
short branches in their course and at their ends, and fibers which 
end by complex net-like end-branches. The moss fibers occasion- 
ally bifurcate and give ofi^ frequent collaterals, so that the terminal 
branches are widely distributed. The peculiar small tufts of 
end-branches stand in relation with the similar branches of the 
dendrites of the granule cells. The second kind of fibers rise 
through the granular layer, apply themselves to the surface of the 
Purkinje cell dendrites and ramify upon the branches of these 
dendrites (Fig. 122). There is a remarkable uniformity in the 
character and arrangement of the ner\*e cells and in- coming fibers 
in all parts of the cerebellar cortex in mammals. The incoming 
fibers in mammals are ver\^ different from those in lower verte- 
brates. In lishes the great majority of in-coming fibers are somatic 





THE CEREBELLUM. 



241 



aisory fibers which end in the granular layer. In mammals 
>matic sensory root fibers are still received from the spinal 
erves by way of the dorsal funiculi and the corpus restiforme, 
"om the vestibular nerve and from the trigeminus, but the niunber 




Fig. 122. — Ending of a net-like fiber in the cerebellum of^man. After^Cajal. 



f such fibers is very small in comparison with the number of fibers 
Dming to the cerebellum from other brain centers. In spite 
f this great change in the character of the incoming fibers the 
lode of receiving impulses in the cerebellum has remained essen- 
16 




Fig* 125. — Two schemes to show the course of impulses in the cerebellar cc»*^-j^'. 
After Cajal In the upper figure; A, moss fibers; B, neuntes of Purkinje ^^^^^ 
a^ granules; b, fibers of molecular layer; c, cell of type 11; d, Purkinje odi* 
the lower figure: b, basket cell; c, Purkinje cell; d, nct*ltke fiber. 

their impiJses either to the granule cells or directly to ^^^ 
Purkinje cells. All impulses received by the granules ^^^ 
transferred to the Purkinje cells through the molecular la-Y* 





THE CEREBELIUli. 



243 



either directly or by means of basket cells or other cells with 
short neurites as indicated in the accompanying schemes (Fig. 

Recent experimental work on the cerebellum of the rabbit has 
led Van Gehuchten to the conclusion that the in-coming libers 
enter the cerebellum in the corpus restiforme and brachium 
pontis, while the out-going fibers go by way of the brachium 
conjuncdvnim. 

The fibers which enter the cerebellar cortex may be grouped 
under the following categories: 

a* Primar)* somatic sensory fibers from the roots of the spinal, 
vestibular and trigeminal nerves. 

b. Secondary somatic sensory fibers. These arise from the 
cells of the nuclei of the dorsal funiculi and from the superior and 
lateral nuclei of the vestibular ner\^e. The fibers from the nuclei 
of the fimiculi run in part in the corpus restiforme of the same 
side, in part as external arcuate fibers in the corpus restiforme 
of the opposite side* 

c* Fibers from the gray matter of the cord of the same and 
opposite side which run up in the fasciculus spino-cerebeUaris 
centralis (tract of Gowers) and enter the cerebeUum in part by 
way of the brachium conjunctivum and in part by the corpus 
restiforme^ to end in the superior vermis and hemispheres. 

d. Tractus olivo-cerebellaris, from the lower olive to the cere* 
bellar cortex. 

e. Fibers from the nuclei of the pons. Some of these constitute 
with Purkinje ceE neurites a two-linked commissure between the 
two hemispheres of the cerebellum. Others fonvard impulses 
which are brought to the pons from the cerebral hemispheres 
by way of the pyramids, 

f. Fibers from the nucleus ruber to the cerebellum by way of 
the brachium conjunctivum (Edinger's tractus tegmento-cerebel- 
laris)* Recent researches render the existence of such fibers very 
doubtful. 

I g. Fibers from the nucleus of the lateral lemniscus and the 
\ posterior colliculus of the corpora quadrigemina. These are 
secondary nuclei of the auditory paths* 



244 



NERVOUS SYSTEM OF VERTEBRATES.' 




IW, mesenc V 



Vilf 
Secondary gustatory tract 



Secondary gustatory nuc. 



Decussatio veli. 



^^^'\ .,* Tertiajy gustatory 
v^ /^0;Ar!.:::^i^;".Ai. tract 



Rad. mesencV 



\\ \ i,*^l\^^,^^^1^mniscus System 



^rachium conjunctivum 
B 



Fig. 124. — Two transverse seriions through the cerebellum of Scyllium. A, a 
section at the extreme caudal end of the secondan- gustaton- nucleus, the position 
of which is marked by the cross; B, through the secondan* gustatory nucleus, the 
decussatio veli and the origin of the tertian- gustatory* tract. The secondary gusta- 
tory nucleus lies beti^-een the brachium conjunctivum and the velum. 



THE CEREBELLtTM, 



24S 



h. Secondary visceral sensoiy fibers. These are the fibers 
of the direct cerebellar tract from darkens column in the cord. 
Most authors state that this tract ends in the deep gray nudei 
but it is sometimes described as ending in the cortex of the vermis. 

The fibers which go out from the cerebellum are all neurites 
of Purkinje cells. They go to the spinal cord, the lower olive, 
Deiter's nucleus, nuclei of the pons and perhaps to other centers* 
By way of these various paths the cerebellum may exercise control 
over both somatic and visceral motor nen-es. A part of the fibers 
which go through the middle peduncle to the pons go directly 
to the spinal cord to comiect wdth the somatic motor column. 
Other fibers end in the nuclei of the pons, from which fibers go 
to the opposite cerebellar cortex or to the cerebral cortex. Many 
Purkinje cell neurites may end in the nucleus dentatus, from 
which the large brachium conjunctivum goes to the optic thai- 
ami. By this path and the tractus thalamo-spinalis the cerebel- 
lum may gain a widespread connection with motor nen-es* 

There is no one fact more striking in the study of the human 
brain thaii the great complexity of structure and fiber connections 
of the cerebellar cortex and the great increase in the number of 
relationships of the cerebellum from lower to higher vertebrates. 
Much remains to be done, especially on the course of Purkinje 
ceU neurites, in order to gain an exact knowledge of these rela- 
tionships. 

Nucleus dentatus*— The nucleus dentatus is taken up sepa- 
rately from the other deep gray masses because of its apparent 
closer reladon with the cortex. The peculiar form of the nucleus 
which gives it its name in man is not seen in lower mammals. 
In these it is a simple gray body which lies nearer to the surface 
of the brain and nearer the junction of the cerebellum with the 
medulla oblongata. The fibers which enter the nucleus dentatus 
are not fully imderstood but they seem to include external arcuate 
fibers from the nuclei of the dorsal funiculi, fibers from some 
of the nuclei of the vestibular nen'e and fibers of Purkinje cells 
of the cortex. The neurites arising in the nucleus dentatus go 
mostly or wholly into the brachium conjunctivum which is com- 
posed almost exclusively of such fibers (Van Gehuchten). The 





246 



NERVOUS SYSTEM OF VERTEBRATES. 



brachium decussates in the ventral wall of the mesencephalon, 
passes through the nucleus ruber and ends m the optic thalami. 
The nucleus dentatus is concerned chiefly with somatic sensory 
impulses and is more closely related to the cortex than to the deep 
nuclei. This indicates a correspondence between this submerged 
and folded nucleus of the human cerebellum and the portion of 



Tectum 



Tr teao-buJbaris 



DecQssatJo veli 




vni 

Secondary 
gustatory tract 
Brachium conjunct 



Secondary 
gustatory nuc 

Rad mesenc. V 



1^ Hertiary gusiaiory ima 



Umniscus system 



Fig. 125. — A transverse section through the cerebellum of Necturus, The 
figure is somewhat diagrammatic in that the whole course of the decussatio veil as 
drawn does not fall in the same section with the other structures. The actual 
section from which the outline was drawn inclined forward somewhat, so that it 
passed through the tectum in front of the velum. The word Cerebellum at the l^t 
is placed in the part of the fourth ventricle which extends into the auricular lobe. 



the selachian cerebellum with which the brachium conjunctivimi 
is connected. In selachians the brachium conjimctivum enters 
the latero-ventral part of the cerebellum close to the junction 
with the tuberculum acusticum, a region which is wholly con- 
cerned with primar)' sensory and external arcuate fibers. It 
seems probable that in the evolution of the cerebellum this nu- 



THE CEREBELLUM. 



247 



deus has lost its superficial position and been overgrown by the 
greatly expanded hemispheres. The course and destination of the 
brachium in mammals and Petromyzon (p. 117) suggests com- 
parison with the ascending fibers of the lemniscus system (cf. p. 
258). 

Nuclei tecti, globosus and emboliformis. — ^The fiber con- 
nections of these nuclei are not well understood. The nucleus 
tecti is known to receive part of the fibers of the vestibular nerve 
and of the external arcuate fibers from the nuclei of the dorsal 
funiculi. A connection between the nucleus tecti and the superior 
olive of the pons has been described. The conflicting, descrip- 



Nuc. dentatus 

uc. emboliformis 
globosus 




'Lemniscus medians 
Brachium poniis 



Fig. 126. — A transverse section through the deep gray nuclei of the cerebellum 
of man. V, the radix mesancephalica V; VIII ^ root fibers of the vestibular nerve 
going to the cerebellum; By the position of BechtereVs nucleus; P, the position 
of Belter's nucleus. 



tions of the direct cerebellar tract do not allow us to decide whether 
it is confined to the deep gray or extends also to the overlying 
cortex of the anterior lobe. It seems certain, however, that the 
nucleus globosus and nucleus emboliformis are related to this 
tract and that the commissure which runs through these nuclei 
is formed either by the fibers of this tract or by neurites arising 
from these nuclei. It seems probable that these nuclei may 
represent the secondary gustatory nucleus of the cerebellum of 
lower vertebrates. Nothing is certainly known about gustatory 



248 



NERVOUS SYSTEM OF VERTEBRATES. 



paths in man but it is to be expected that the gustatory nudeus 
mil be found related to the nucleus of ending of the direct cerebellar 
tract (secondary \isceral sensor}^ tract). 

In Figures 124, 125 and 126 are drawn transverse sections 
through corresponding regions in the brain of a selachian, an 
amphibian and man. With these may be compared Figures 
112, 115, 116 and 117, In all these forms, although they differ 
widely in many respects, the relations of the brachium conjunctivuffi. 
mesencephaUc root of V, the primary and secondary somatic 
sensorv^ tracts and the cerebellum are essentially the same. J^t 
the direct cerebellar tract has its ending in one of the deep nud^^i 
of the cerebellum it would correspond to the gustatory^ tracts c^^f 
lower forms. It appears that the nuclei in the roof have bee— t::^ 
pushed up from the lateral wall on account of the great size c=r>4 
the brachium conjunctivum. This appears the more probab~Z!l^« 
if the brachium conjunctivum in mammals is related solely to H -m ^ 
nucleus dentatus as Van Gehuchten^s recent studies indicate. 

The evolution of the structure and fimction of the cerebelliuzzzam 
may be summarized as follows. In fishes the cerebellum consi^^^- '•s 
of a large dorsal somatic sensory nudeus and of a second 
visceral (gustatory) nucleus ventral to it. The gustatory nude 
has a dorsal comjnissure through the cephalic border of the cenr — '^^ 
bellum. The presence of this commissure leads to extensL ~^^^^ 
changes in the form of the cerebellum when the gustatory nude^ ^m^js 
is large. Nothing further can be said regarding the n ll^^ ~ ^I 
portion of the cerebellum until the center for the direct cerebeM^ ^^^ 
tract in mammals is more fully studied and the question of i^s 
homolog}' with the gustator\^ nudeus of fishes is settled. 

The somatic sensory portion is not only very large in tist:^^ 
but shows a higher specialization of structure than would be caM- 
for by its purely sensor)^ function. Its great size is direcr^t-^V 
accounted for by the great development of the acustico- lateral s.-;^'^'^- 
tern of sense organs. Its structure, however, dififers from tha't: ^ 
the tuberculum acusticum, which also serv^es as the center for X:^Mne 
same system, in the presence of a greater number of granule c^^^-^\ 
and the higher development of the Purkinje cells. In additio^^ 
the cerebellum receives a fiber tract from the tectum opticu-*^ 




THE CEREBELLUM. 249 

which serves to bring optic stimuli into relation with cutaneous. 
The cerebelliun is no longer a purely cutaneous center, but im- 
pulses may be sent out from it in response to retinal stimuli also. 
The development of special functions has not gone far in the 
selachians, for removal of the cerebellum alone in the dogfish does 
not produce visible effects on locomotion (Bethe). In combined 
operations on the cerebellum and other parts of the brain, how- 
ever, it is shown that the cerebellum plays some part in the 
coordination of movement. 

With the disappearance of lateral line organs in land amphibia 
the cerebelliun is greatly reduced in size. The reduction aflFects 
chiefly the primary sensory center, and from the amphibia onward 
the relations of the cerebellum with other brain centers increase. 
The fibers from other brain centers end chiefly in the dorso-median 
and cephalic regions of the cerebellum and in higher vertebrates 
the vermis and adjacent parts of the hemispheres are developed 
from these regions. Fibers come to these parts from the primary 
and secondary nuclei of the cutaneous, vestibular and cochlear 
nerves, and from the inferior olive, the nuclei of the pons and 
perhaps other sources. The fibers which go out from the cere- 
bellum make direct or indirect connections with nearly the whole 
range of motor nuclei, probably both somatic and \isceral. 

It is reasonable to suppose that the presence of the somatic 
pallium in mammals has influenced the evolution of function of 
the cerebelliun. In fishes and perhaps in amphibia and reptiles 
the cerebellum and the roof of the mesencephalon share between 
them the functions of higher centers to which somatic sensory 
impulses of the second and third orders are sent and from which 
impulses go out to control complex motor responses. In mammals 
these impulses are carried to the neopallium, which has taken on 
the direction of all voluntary movement. What then is the 
function of the cerebellar cortex? Disease of the cerebellum 
in man and its extirpation in animals always results in disturbances 
of voluntary muscular action. Animals from which one hemi- 
sphere or the whole cerebellum has been removed are unable to 
staod or walk until they have learned to make compensatory 
efforts. Does the cerebellum have the special function of main- 



250 



NERVOUS SYSTEli OF VERTEBRATES. 



lung 



taining the equilibrium, or is it necessary for the coordination of 
muscular contractions with reference to definite movements ? In 
the results of experimental investigations on mammals the function 
of the cerebellum which stands out most prominently is different 
from either of these. Dogs which have lost one cerebdlar 
hemisphere, although they are imable to stand or w^alk, can swim 
well in water (which supports their body weight), both coordinating 
their movements and maintaining their equihbrium. Such animiJs 
learn after a time to compensate for the loss of the cerebellum 
certain voluntar)^ modifications of their movements; e.g., cun 
the spine so as to bring the center of gravity over the sound 1 
spreading the feet wide apart, etc. They can then stand and 
walk. These and other facts show that the loss of the cerebellum 
does not involve loss of the power of equilibration nor of cutaneous 
or muscle sense on which the power of coordinated movements 
depends, but does result in weakness of muscular action on the 
injured side. It seems, therefore, that the cerebro-spinal mechan- 
isms are sufficient to carry out all voluntary movements without 
the aid of the cerebellum, but the movements are lacking in strength, 
precision and regularity. The cerebellum is not showTi to be a 
necessary link in the ner\^ous mechanisms which control muscular 
action but it seems to add something to the voluntary movement. 
According to Luciani the function of the cerebellum is to maintain 
the tone of muscles during rest, to increase the energy of contrac- 
tion when called forth by voluntary impulses and to determine 
the rhythm of motor impulses. In this way the imperfect actions 
of the dog deprived of its cerebellum would be perfected into 
normal movements. Whether the cerebellar cortex actually 
ser%'es other functions, such as the coordination of specific move- 
ments, remains for further investigation to decide. 
The fact that the cerebellum receives fibers from so many and 

various sources is important in this connection. It would seem 1 

that the maintenance of tone is not an abstract thing which 
unrelated to present acti\ity or sensory stimuli. If the fund 
of the cerebellum is to maintain the tone of muscles, it is e\iden"^i3 
that it sends out the necessary impulses in response to impul 
brought to it from related sensory areas or special sense 01 



THE CEREBELLUM. 



251 



I 
I 



Thus coordinated movements in response to sensory impulses are 
directed by the mesencephalic and cerebral apparatus to be 
described in future chapters, while the side circuit through the 
cerebellum furnishes the means of maintaining the requisite tone. 
It is evident that the cerebellum has changed its functions in 
the course of vertebrate historv^ but the indications are that the 
broad functional relationships of its main parts have been retained* 
The somatic sensory part has come in mammals to play a special 
role in the senso-motor functions. It is still concerned chiefly 
mth the reacdons of the organism toward the external world 
by means of its somatic muscles* It is to be hoped that future 
experimental studies will be directed toward discovering the 
structural and functional relationships of the deep nuclei of the 
anterior lobe. The suggestion that some of these are homologous 
^dth the gustatory nucleus of fishes will be justified if it stimulate 
investigation in this direction. 

DEMONSTRATION OR LABORATORY WORK. 

I. Review the laboratory work on the somatic sensory relations and 
the structure of the cerebellum in fishes given in Chapters VI and \1I. 

a. Study the structure of the cerebellar cortex in a mammal in 
Golgi sections. 

3. Examine by dissections and by sections in various planes the 
relations of the several parts of the cerebellum in fishes and mammals. 
Verify the comparisons made above, especially the constancy of the 
hemispheres in the vertebrate series. 

4. Examine in the same way the relations of the cerebellar peduncles 
and of the secondary gustatory nuclei and the velum. 



I 



LITERATURE. 



Archivio cli 



Bianchit Arturo: Sulle vie dl connessione del cervelletto. 
Anat. e di EmbrioL, Vol. 2. 1903, 

Barker, L. F.: The Nervous System, 

Bethe, A.: Die Locomotion des Haifisches u.s.w. Arch. f. d. ges, Physiol, 
Bd. 76, 1899. 

Cajal S. R,: Textura del sistema nervioso del Hombre, etc. Madrid. 1904. 

Catoist E. H,: Recherches sur Thistologie et ranatoraic microscopique 
dc l*encephale chez les poissons. Bull. Sci. dc la France, Tome 36. igoi, 

Edinger u. Wallenberg: Untersuchungen iiber den Gehim der Tauben. 
Anat, Anz., Bd. 15. 1899. 

Van Gehuchten, A.: Le corps restiforme et les connexions buibo-cerebel- 
euses. Le Nevraxe, Tome 6. 1904, 





252 NERVOUS SYSTEM OF VERTEBRATES. 

Van Gehuchten, A.: Les pedoncules cerebelleuses superieurs. Le Nevraxe, 
Tome 7. 1905. 

Johnston, J. B.: The Brain of Acipenser. Zool. Jahrb., Bd. 15. 1901. 

Johnston, J. B.: The Brain of Petromyzon. Jour. Comp. Nenr., Vol. 12. 
1903. 

Kappers, C. U. A.: The Structure of the Teleostean and Selachian Brain. 
Jour. Comp. Neur. and Psych., Vol. 16. 1906. 

Koelliker, A.: Gewebelehre. 6te. Aufl. Bd. 2. 1896. 

Luciani, L.: Das Kleinhim. Asher u. Spiro's Ergebnisse. III. Jahrg., H. 
Abth. 1904. 

Ramon, P.: Investigaciones micrograficas en ed encefalo de los batrados 
y reptiles. Zaragossa, 1894. 

Schaper, A.: Zur feineren Anatomie des Kleinhims- der Teleostier. Anat. 
Anz., Bd. 8. 1893. 

Schaper, A.: Die morphologische und histologische Entwickelung dcs 
Kleinhims der Teleostier. Morph. Jahrb., Bd. 21. 1894. 

Smith, G. Elliot: On the Morphology of the Brain in the Mammalia. 
Trans. Linn. Soc. London, Ser. 2, Zool. 8. 1903. 

Smith, G. Elliot: Further Observations on the Natural Mode of Sub- 
division of the Mammalian Cerebellum. Anat. Anz., Bd. 23. 1903. 

Stroud, B. B.: The Manunalian Cerebellxmi. The Development of the 
Cerebellum in Man and the Cat. Jour. Comp. Neur., Vol. 5. 1895. 

The literature of the mammalian cerebellum will be found in the general 
works cited above. 



CENTERS OF CORRELATION. 



253 



CHAPTER XVI. 



CENTERS OF CORRELATION (Continued). THE MESEW- 
■ CEPHALON AND DIENCEPHALON. 



I 



I 

I 



I. THE CUTANEOUS APP.\RATUS. 



In describing the centers and liber tracts connected with the 
general and special cutaneous nerves (Chapters VI and VII) it 
was shown that the largest secondar\^ tract from the primary 
centers goes in the form of internal arcuate fibers to the opposite 
side of the brain and passes forward to end in the roof of the 
mesencephidon* This tract is called in lower vertebrates the 
tracttis bulbo-tectaiis, in mammals the lemniscus. The tract 
has a wider distribution in the mesencephalon and diencephalon 
in mammals than in lower vertebrates. All the fibers which have 
this general course may be known as the lemniscus system. The 
evolution of this system and of its end nuclei will be considered 
here. 

The first important point to notice is that the lemniscus system 
in mammals is divided into two chief parts, the medial and the 
lateral lemniscus. Each of these parts is believed to contain 
descending fibers in addition to the ascending ones. It must be 
emphasized that the ascending fibers alone are of interest in the 
present connection. The medial lemniscus arises in small part 
from the dorsal horns of the cord and chiefly from the nuclei of 
the doi^al funicufi and from the nucleus of the spinal V tract. 
It is therefore a general cutaneous conduction path. Some ascend- 
ing fibers from the nuclei of the vestibular nen^e probably join 
the medial lemniscus and others probably run separately to 
similar destinations in the diencephalon. The lateral lemniscus 
arises from the various nuclei of the cochlear nerve and is therefore 
an auditor)^ conduction path. The chief place of ending of the 
lateral lemniscus is the posterior corpora quadrigemina, that of 





254 



NERVOUS SYSTEM OF VERTEBRATES. 



bj ects 
rntm 



the medial lemniscus is the anterior corpora quadrigemina and 
certain nuclei in the thalamus. The separation of the lemniscus 
system into two parts is the result of a differentiation in the mesen- 
cephalon and diencephaion the main features of which can be 
outlined, but the details of which offer most interesting su bjects 
for investigation. 

The mesencephalon in primitive vertebrates was a segment 
simple tubular form comprising two primary neuromeres similar 
to the cerebellar neuromere or even to a segment of the spinal 
cord. In typical fishes the roof of the mesencephalon is gready 
enlarged and is as highly specialized as the cerebellum. This is 
due to the entrance into the mesencephalic roof of secondary 
tracts (i) from the general and special cutaneous centers of the 
medulla oblongata and spinal cord, and (2) from the retina. 
These tracts (tractus bulbo-tectalis and tractus opticus) have 
been described and their morphology discussed in pre\dous 
chapters. In lowly fishes the centers for these tracts are not 
isolated but are more or less confused or intermingled in the brain 
roof. Indeed the differentiation of special centers within an indiffer- 
ent region is in progress in existing fishes. fl 

In typical fishes the midbrain roof is an arch whose bases rest 
on the ventrolateral walls of the brain and whose keystone is 
formed by a thin lamina (roof plate of His) traversed by the dorsal 
decussation. Enlargement of the centers in the roof is secured 
by the widening of the arch and expansion of the ventricle. The 
thickness of the walls varies much less in different fishes than the 
extent of the roof. In most fishes there is formed in this way a dor- 
sal expansion of the iter kno\>Ti as the optic ventricle or optocoele. 
In Fig. 127 are shown outline sections through the tectum of a 
cyclostome, a selachian, a bony fish, an amphibian and a mammal 
to illustrate the changes of form which this region undergoes. The 
first important fact is that the expansion of the optic ventricle takes 
place by bulging the side walls of the arch without separating its 
bases. From this it results that the ventrolateral part of the 
tectum remains beneath the side of the expanded optic ventricle 
and is overhung by the bulging dorso-lateral portion of the tectum. 
The portion of the original roof which now lies beneath the optic 




CENTERS OF CORRELATION. 



255 



aitricle is much thicker than the rest and is known as the cpllic- 
or region. The portion which roofs pver the optic ventricle 




Rad. 

esenc. 



otliculus^ 

Lemniscus^ 
system 

Arcuate fibers 

Decuss. and 

Nuc. tr^ hab. pedunc: 




Rad. mesenc. V 




Fiff. 137. — Outline transverse sections through the mesencephalon of various 
teDrates to illustrate the changes of form of the tectal region. A, a cydostome; 
a selachian; C, a ganoid; D, a bony fish; E, an amphibian; F, man (after 
lelliker). r, nucleus ruber; S. n., substantia nigra. 



known as the tectum opticum. (It must be remembered always 
lat the tectum opticum is only a part of the tectum mesencephalic 



2 $6 



NEUVQUS SYSTEM OF VERTEBRATES. 



Although differentiation between these centers is not complete 

in fishes it may be said in general that the optic tract predominates 
in the tectum opticum, while the coUicular region is especially 
related to the tractus bulbo-tectalis. That the speciaKzation 
of these centers is not complete is dear from the fact that in blind 
fishes and those from which the eye has been experimentally 
removed, the tcctvun opticum is not atrophied but only certain 
layers of its cells disappear (Ramsey). Evidently the secondar)^ 
cutaneous tracts enter the tectum optictmi as well as the colliculus, 
but the optic tract is more nearly coniined to the tectum opticum. 
The descending fibers from both the tectum opticum and the 
colliculus run in the same tracts so that they are all grouped under 
the name of tractus tecto-bulbaris* fl 

In the amphibia a marked change in the form of the mesen- 
cephalon is seen (Fig. 127 E). The form of the transverse section^ 
is more nearly that of a simple tube. The wall is relatively thicifl 
and there is only a slight indication of a special optic ventricle. 
In selachians, ganoids and bony lishes there is an increasing expan- 
sion of the tectum opticum and thickening of the massive coUicuIus. 
The size of the tectum opticum is directly affected by the size 
and importance of the eyes, while the great volume of the col- 
liculus is due to the great development of the acustico-lateraL^ 
system. In amphibia the reduction of the roof is due to dH 
decrease in both optic and cutaneous tracts which end in it. 
In most amphibia the eyes are less important than in most 
fishes and the number of optic fibers which enter the tectum is 
much farther reduced by the greater development of the optic 
centers in the thalamus. On the other hand, the cutaneous 
tracts are gready reduced by the complete disappearance of thc^ 
lateral line organs in the adult. The amphibia are beheved tdf 
have descended from fish-like ancestors whose affinities are wiih 
the lower orders of existing fishes, The amphibian brain shows 
resemblances to that of both selachians and cyclostomes. The 
reduction of the midbrain roof has left it in a primitive form. 
There is no clear distinction between tectum opticum and collie- 
iiluSj but the greater thickness of the wall in the caudal part of 
the roof indicates the beginning of formation of the posterior 





Fig. taS, — ^Transversc section of ihc diencephalon of the rat at the level of the 
■y of Luys. From CajaJ (Tcxtura. etc.). A^ optic tract; B diid C, nuclei of the 
corpus genirulatum iaterale: D, sensor)' nucleus (end nucleus of lemniscus); £, 
nucleus of Luys; F^ zona inccrta; /, posterior commissure; N, nucleus triangularis. 



to be fonned. In its caudal pari the roof is comparable lo the 
I colliculus of fishes rather than to the tectum opticum; in its cep- 
I haHc part the reverse is true. 

The form of the midbrain roof in reptiles and mammals is to 



258 NERVOUS SYSTEM OF VERTEBRATES. 

essentials similar to that in amphibia. The wall has thickened 
and the anterior and posterior corpora quadrigemina have differ- 
entiated. The anterior corpus quadrigeminum serves as the 
place of ending of optic and secondary cutaneous tracts and is 
comparable to the cephaUc part of both tectum opticum and 
coUiculus in fishes. The posterior corpus quadrigeminum serves 
as the chief place of ending of the secondary auditory paths and is 
roughly comparable to the caudal part of the colliculus in fishes. 
In birds a large tectum opticum has been developed on accoimt 
of the large size of the eyes and the mesencephalon of birds is 
more like that of bony fishes than that of other lower vertebrates. 

The lateral lenmiscus arises, as has been said, from the nuclei 
of the cochlear nerve. A large part of the fibers come as internal 
arcuate fibers from the nuclei of the opposite side, the remainder 
as direct fibers from the nuclei of the same side. The lateral 
lemniscus passes forward into the mesencephalon, bends upward 
and separates from the medial lemniscus, and the greater part 
of it enters the posterior corpus quadrigeminum. A part of the 
fibers pass forward to end in the anterior quadrigeminum and a 
part go on to end in the medial corpus geniculatum or adjacent 
nuclei in the thalamus. From the posterior corpus quadrigeminum 
fibers go to the medial corpus geniculatum and from this center 
auditory impulses are forwarded to the cerebral hemispheres. 
From the posterior quadrigeminum other fibers go down through 
the lateral lemniscus to end in the various cochlear nudd (recur- 
rent fibers). 

The medial lenmiscus consists of both crossed and imcrossed 
fibers from the centers for cutaneous nerves. The tract receives 
fibers also from many other sources not related to the cutaneous 
nen-cs. As the tract passes forward fibers go from it to end in 
the gray matter of the medulla oblongata, pons, isthmus, midbrain 
and hypothalamus. It is shown, however, that the great majority 
of the fibers which arise in the cutaneous nuclei pass forward to 
the thalamus and it is believed that the other fibers which enter 
and leave the tract do not belong to the lemniscus system proper. 
The secondar}' cutaneous fibers end in the so-called nucleus 
ventralis of the thalamus. It must be noticed that this nucleus 



/ 



CENTERS OF CORRELATION, 



259 



is by no means situated in the ventral part of the thalamus (as often 
stated) but lies mesial to the corpus geniculatum late rale in the 
dorsal half of the thalamus (Fig» 128). It would be better to call 
it as Cajal does the sensory milieus of the thalamus^ especially 
as the term nucleus ventralis is used for a different center in fishes. 
From this sensory nucleus impulses are forwarded to the cerebral 
cortex and descending fibers from the cortex also end in this 
, nucleus. In the cat (Tschermak) a part of the fibers of the me- 



I 



\\* 



Fig. 129. — ^Transverse section of the dorso-mesial portion of the posterior corpus 
^uadrigeminuzii of the newborn dog* From Cajal (Tcxtura, etc.). A, peripheral 
6ber layer; B, cells of the second layer; C, stellate and fusiform cells of the third 
layer; Z), fibro-cellidar layer; £, central gray matter; iS, commissure; F, plexm 
<sf second and third layers. 



^al lemniscus are fotmd by the degeneration method going 
through the thalamus to reach the cortex directly. There is some 
evidence that such bulbo-pallial fibers exist also in man. In view 
of the ending of secondary cutaneous fibers in the midbrain in 
lower vertebrate sit may be expected that some of the fibers of the 





Fig. 131.— Lower part of the corpus gen icula turn laterale of the newborn cat^ 
From Cajal (Bcitxage u.s,w.). X, B, C, D, endings of opticus fibeis. 



and some fibers end in the center of similar structure situated ir^ 
the pul\inar of the thalamus. The relations in mammals shot^^'* 
that with the increasing size of the corpus geniculatura there ha-*E 
gone hand in hand a process of differentiation of function betweiLJj — n 



CENTERS OF CORRELATION. 



263 



it and the tectum opticum. In man the anterior corpus quad- 
rigeminum is small and poorly developed as compared with that 
of lower animals. The descending tract from it is relatively 
small and represents only a part of the tractus tecto-bulbaris of 
lower vertebrates. The bimdles of the two sides descend over the 
central gray of the midbrain and form a decussation in its ventral 
wall called by Forel the *^ fountain- like" decussation. As the 
tracts pass on toward the medulla oblongata they give collaterals 
and terminals to the nuclei of the eye muscle ner\'es. This is 
the chief connection of the tract going out from the anterior quad- 
rigeminum and it shows that the chief funcrion of that nucleus 
is to direct eye musde reflexes to visual stimuli. The existence 
of fibers from the anterior quadrigemina to the cerebral cortex has 
not been clearly demonstrated and the complete loss of the ante- 
rior quadrigemina does not affect light or color \dsion. 

The corpus geniculatum, on the other hand, sends a large tract 
to the occipital region of the cortex and the geniculatum is the 
chief intermediate center in visual perception. The connection of 
the geniculatum and pulvinar with the occipital cortex is clearly 
shown by the secondary degeneration of these nuclei after destruc- 
tion of the visual cortical area. The tract has been followed 
dearly in animals by the method of Golgi (Cajal) and in the new- 
bom babe the tract from the geniculatum to the cortex is medul- 
lated while adjacent tracts are not, so that Weigert staining gives 
a dear picture of this conduction path (Flechsig). There is e\i- 
dence that in lower vertebrates fibers pass from the tectum 
opticum and the geniculatum to the forebrain and it is probable 
that this tract was one of the first to stimulate the development 
of the neopallium. Little is certainly known of the relations of 
the geniculatum in lower vertebrates, although connections with 
various parts of the brain have been described (Catois, Ediager). 

The existence of the corpus geniculatum laterale in selachians 
shows that it is far from being a new or phylogenetically young 
optic center. It is equally true, however, that it does not become 
important relative to the tectum opticum until the cerebral hemi- 
spheres are developed. Through the vertebrate series there has 
been a gradual shifting cephalad of the endings of optic tract 



264 NERVOUS SYSTEM OF VERTEBRATES. 

fibers. In fishes they are distributed throughout the whole length 
of the tectum mesencephali and a few end in the thalamus. In 
mammals the caudal half of the tectum no longer receives optic 
tract fibers and most of them (in man 80 per cent.) end in the 
thalamus. This shifting of optic tract fibers is part of the process 
of specialization of the mesencephalon described in the last section. 
The caudal part comes into the service of the secondary auditory 
tracts, the cephalic part becomes a visual reflex center for eye 
movements, while the conduction paths for general cutaneous 
and visual perception both leave the tectum mesencephali and 
end in thalamic nuclei. All these changes have resulted in greater 
compactness and directness in the arrangement of centers and 
tracts. Generally speaking, each conduction path in mammals 
is the shortest and most direct that could be evolved out of the 
unspecialized centers and tracts which already served the same 
set of functions in lower vertebrates. The caudal part of the 
tectum is claimed by the auditory paths which come from behind, 
the cephalic part by the optic path which comes from in front. 
The secondary general cutaneous path shifts from the tectum to 
the thalamic nuclei, which are nearer the cerebral cortex to which 
the impulses of general sensation are destined. Similarly the 
visual impulses destined for the occipital cortex are transferred 
to the corpus geniculatum instead of going to the more distant 
corpus quadrigeminum anterior. The shifting forward (prosen- 
cephalization) of visual centers is explained by the advantage 
of gaining shorter paths to the cerebral cortex, for the reason that 
visual impulses are significant in higher mammals chiefly for 
space and color perception of the surroundings. In fishes visual 
impulses arouse reflex movements of the body for the capture 
of food, avoidance of obstacles, etc. In mammals the reflexes 
which depend solely on visual impulses are few (lid reflex, etc.). 
Visual impulses are for the most part carried to the pallium where 
they are associated with impulses of other sorts (tactile, auditory) 
for the formation of a complex perception of the situation. Under 
the influence of this percept voluntary impulses arouse move- 
ments adapted to meet the situation. This may be taken as a 
characteristic illustration of the difference between the brain 



CENTERS OF CORRELATION. 



265 



of man and that of a hsh. Whereas the sensory impulses in a 
fish arc correlated by the relatively slightly organized centers of 
the mesencephalon and call forth simple, direct and often \try 
quick responses; the center for correlation in man is removed 
to the highly organized cerebral cortex and the responses lose 
something in quickness but gain vastly in precision and in the 
completeness with which they are adapted to the several factors 
of the situation. 

There should be added lo this account some mention of the 
centrifugal fibers in the opUc tracts which end in the retina. The 
presence of such fibers is demonstrated by the Golgi method 
which shows their origin, course and their endings in the internal 
molecular layer of the retina (Figs. 71, 72.). Their presence 
and course is also shown by the methods of primarj' and secondary 
degeneration. In fishes in which one eye has long been lost the 
optic tract of the opposite side degenerates with the exception of 
these fibers, w^hich persist and are stained by the Weigert method. 
In mammals, following section of the optic tract there occurs 
secondar)^ degeneration of cells in the anterior quadrigeminum, 
and in the dorsal part of the geniculatum laterale and pulvinar. 
These findings in mammals agree with those in fishc»s by the 
Golgi and degeneradon methods, where the centrifugal fibers 
arise from the tectum opticum and geniculatum (Catois). The 
significance of these fibers is not understood but their presence 
in all vertebrates seems to show that they have some constant 
function. It has been suggested that they are examples of fibers 
which pass forward from the cutaneous center of one segment 
to that of a center farther forward, and hence homologous with the 
fibers of the lemniscus system. 

3, CENTERS RELATED TO THE POSTERIOR COMMISSURE. 

The posterior commissure, although it has long been used as 
one of the most prominent and constant landmarks in the brain, 
is still verj^ imperfectly understood. In Petromyzon (Fig. 132) 
the fibers of the commissure arise from cells widely scattered 
through the dorso-caudal part of the thalamus and the cephalic 
part of the tectum opticum and the collicular region. The fibers 





266 



NERVOUS SYSTEM OF VERTEBRATES. 



are very fine until they approach the point of crossing, when they 
thicken into relatively coarse fibers. After crossing they bend 
ventro-caudally and spread widely through the wall of the mid- 
brain, mingling with the fiber tracts descending to the medulla 
oblongata. In mammals the commissure arises from a nucleus 
situated at the cephalic border of the anterior corpora quadri- 
gemina, between the tractus habenulo-peduncularis, the nucleus 
ruber and the root of the III nerve (Koelliker, p. 445). In other 
vertebrates the origin of the commissure has not been clearly 
distinguished. It is variously described as arising in, ending in or 
serving as a true commissure for a nucleus lying cephalad from the 
nucleus of origin of the III nerve. The same nucleus is described 




Fig. 132. — Nucleus of posterior commissure in Lampetra. vent., the aqueduct of 
Sylvius below, the optic ventricle above. 



as the nucleus of origin of the fasciculus longitudinalis medialis, 
and the fibers of the posterior commissure itself are said to form 
that fasciculus. These vague and conflicting statements are due 
to imperfect methods. Those who have worked by the Weigert 
technique have been misled by the fact that the fibers of the com- 
missure do not become myelinated until they approach the point 
of crossing. In Scyllium the fibers approach the median plane 
from the lateral direction just as in Petromyzon and presumably 
arise from a nucleus situated dorsally as in Petromyzon. The 
same is true in amphibia. The nucleus praetectalis of Edinger 



CENTERS OF CORRELATION. 267 

which has been described in fishes by various authors lies in the 
region of junction of thalamus and tectum mesencephali and 
probably is at least a part of the nucleus of the posterior commissure. 
After crossing, the limbs of the commissure bend downward and 
backward through the central gray more or less parallel with the 
tractus habenulo-peduncularis and are lost among the longitu- 
dinal and decussating tracts in the region of the III nucleus and 
the ansulate commissure. Most authors agree that the tract is 
related to the nucleus which gives rise to the fasciculus longi- 
tudinalis medialis and to the nuclei of the eye muscle nerves. 
The conmiissure contains many fibers which arise from or end in 
the tectum opticimi and its whole nucleus of origin in fishes is 
closely related to the tectum. All the evidence goes to show that 
the system of the posterior commissure is closely related to the 
optic centers. 

DEMONSTRATION OR LABORATORY WORK. 

1. Trace the lemniscus systems in Weigert sections of the brain of a 
bony fish, frog and a mammal. Note especially the relations of the 
thalamic centers. 

2. Study in Golgi sections the structure of the tectum mesencephali 
(corpora quadrigemina) and the corpus geniculatum laterale in the 
brain of a selachian, frog and the mouse or rat. 

3. In the Golgi and Weigert sections used above search carefully for 
the origin and destination of the fibers of the posterior commissure. 

LITERATURE. 

Baricer, L. F.: The Nervous System. Chapter LIII. 

Cajal, S. R.: Beitr&gezurStudium der Medulla oblongata. Leipzig. 1896. 

Cajal, S. R. : Textura del sistema nervioso del Hombre y de los vertebrados. 
Madrid. 1904. 

Catois, E. H.: Recherches sur Thistologie et Panatomie microscopique de 
I'encephale chez les poissons, Bull, de Sci. de la France, Tome 31. 1901. 

£dinger, L.: Voriesungen uber den Bau der nervosen Centralorgane- 
Leipzig. 1904. 

Johnston, J. B.: The Radix mesencephalica trigemini. Ganglion isthmi. 
Anat Anz., Bd. 26. 1905. 

Kappeis, C. U. A.: Teleostean and Selachian Brain. Jour. Comp. Neur. 
and Psych., Vol. 16. 1906. 

Koelliker, A: Gcwebelehre, 6 Aufl. Bd. 2. 



268 NERVOUS SYSTEM OF VERTEBRATES. 

Myers, B. D.: The Chiasma of the Toad and of some other Vertebrates 
Zeit. f. Morph. u. Anthrop. Bd. 3. 1901. 

Ramon. P.: Investigaciones sobre los centros 6pticos. etc. 2^ragoza. 1890. 

Ramon, P.: El enc^phalo de los reptiles. Zaragoza, 1891. 

Ramon, P.: El enc^phalo del camele6n Rev. trim, micrografica. tomo I. 
1896. 

Ramon, P.: Centros 6pticos de las aves. Rev. trim, micrografica. tomo I. 
X898. 

Ramsey, E.: The Optic Lobes and Optic Tracts of Amblyopsis spdeus 
de Kay. Jour. Comp. Neur., Vol. 11. 1901. 

Van Gehuchten, A.: La structure des lobes optiques chez Tembryon de 
poulet. La Cellule. 1892. 



CORRELATING CENTERS IN THE DIENCEPHALON. 269 



CHAPTER XVII. 

CORRELATING CENTERS IN THE DIENCEPHALON 

(Continued). 

4. THE OLFACTORY AND GUSTATORY APPARATUS. 

The centers considered in the last chapter have to do with 
somatic sensory impulses and are all differentiated from the 
dorsal part of the mesencephalon and diencephalon. The centers 
now to be considered have to do .with visceral sensory impulses 
and lie, with one exception, in the ventral part of the diencephalon. 
It was shown in Chapters IX and X that gustator}^ impulses are 
carried to the inferior lobes of the diencephalon by a tract from 
the superior gustatory nucleus and that olfactory impulses come 
to the same region in two or three isolated bundles of the tractus 
olfacto-hypothalamicus. 

It is important to understand the exact limits of the hypo- 
thalamus in fishes and to determine the corresponding structures 
in higher vertebrates. In all fishes a pair of rounded lobes, the 
inferior lobes, containing a wide extension of the third ventricle 
project ventro-caudally behind the optic chiasma. In cyclostomes 
and selachians these lobes are relatively small and have a simple 
and primitive structure, but in ganoids and bony fishes they are 
greatly expanded and show a somewhat higher histological develop- 
ment. Caudo-ventrally the wall of the lobe is produced into a 
median thin-walled and vascular sac known as the saccus vascu- 
losus. The caudal wall of the lobes above the opening of the 
saccus shows special characters and is known as the corpus mam- 
millare. The relations of these structures may be seen in sagittal 
sections of the brain (Figs. 2, 11, and Chap. XVIII). 

The whole area which shows the structural characteristics 
of the inferior lobes and is related to olfactor\' and gustatory tracts 
may extend somewhat beyond the lobes proper, and the Umits 



270 



NERVOUS SYSTEM OF VERTEBRATES. 



between this area and the thalamus have not been clearly appre- 
ciated. ThuSj the nucleus of origin of the fasciculus longitudinalis 
medialis has been spoken of as lying in the hypothalamus (Edinger). 
If a section through the inferior lobes of the sturgeon brain be 
compared mth one through the III nerve (Figs. 133, 116) it will 
be seen that the tectum mesencephali and lateral walls are the 
same in both, and also that the fasciculus longitudinalis medialis 



Rfldix mesenc.V_ 

Tr. tecto-cc rebel, ' 
Tf- lobo-bulbaris - 



Tr. iccto. I 

bam 



^Ir. hulbo-tectaiift 



Tr. habcn.-pedunc< "^^ 
Tr. lobo-bulbaris , 



Tr. ^acco tbal -^ 




Tr, thal,-saccii? - 
S ace us 
Fig. 133. — ^Transverse section through the corpora mammillaria of the sturgeon. 



and the fundamental bundles of the lateral tracts extend forward 
into the thalamus. In front of the nucleus of the fasciculus longi- 
tudinalis medialis the nucleus of the tractus strio-thalatnicus and 
the tract itself continue fonvard at about the same level. These 
several structures represent the brain base or stem. The greatly 
expanded inferior lobes ventral to them are anomalous structures. 
The characteristic structure of the inferior lobes extends up to 




CORRELATING CENTERS IN THE DIENCEPHALON. 



271 



the lateral border of the nucleus of the tractus strio-thalamictis, 
which belongs to the thalamus. The hypothalamic structure 
extends up lateral to this nucleus, however, upon the lateral surface 
of the thalamus and midbrain nearly to the ventral border of the 
tectum mesencephah (Figs. 133, 134, 135)* This area is quite 
free from m)'elinated fiber tracts but is traversed by the myriads 
of fine fibers of the tr actus iobo-bulbaris^ to which it contributes 
additional fibers. The dorsal border of this area is defined by a 
dense bundle of this tract. In bony fishes the typical hypothal- 
amic structure extends up in precisely the same way upon the 
lateral surface of the thalamus and midbrain and on account of 
the great size of the tectum mesencephali the base of the latter 
is crowded down into contact with this portion of the hypothal- 
amus. In both the sturgeon and the bony fish the hypothalamic 
structure is sharply limited dorsaUy by a groove and a septum of 
connective tissue extending deep into the brain substance. The 
limit between the thalamus and hypothalamus (Fig. 134) runs 
^irom a point near the base of the tectum mesencephali to the 
lateral border of the nucleus of the tractus strio-thalamicus. 
The structure of the hypothalamus is uniform throughout and is 
remarkably constant in the series of vertebrates. The fiber tracts 
connected with the hypothalamus are quite different from those 
connected virith the thalamus. To what extent the structure of 
the hjrpothalamus extends up on the lateral surface of the thaJa- 
mus in other classes than ganoids and teleosts is not known. 

Fortunately we have an account of the genetic relations of this 
h^Tpothalamic structure w*hich appeal^ so anomalous in the adult 
brain. In early teleost embr\'os the brain consists of a number 
of ring-shaped neuromeres which have been described in Chapter 
IIL As indicated in that chapter, the first ncuromere gives 
rise to the forebrain, the second gives rise to the retina and comes 
to have the optic chiasma at its ventro-cephalic border (Chap. 
VIII)* The third neuromere forms the thalamus, nucleus haben- 
ulae and epiphysis. The fourth and fifth neuromeres form the 
mesencephalon. One of the earliest differentiations of form to 
appear in the brain segments of bony fishes is an expansion of 
the ventral wall of the second neuromere^ which begins to crowd 






Tf . opticus 



Tr. urio-thalam 



Tr. thaL-4«ccu« Tr. lciiio-eptiitrtaticti» 

f*ig' '35- — ^Transvcrse section through the posterior commissure of the sturgeon. 

The gross relations of the hypothalamus being established 
it remains to discuss its functional relations as indicated by the 
fiber tracts connected with it. The tracts which bring impulses 
to the hypothalamus are primarily the gustator}^ and olfactor)' 
tracts of the thiixl orden Althoiiglx the gustatory tracts have only 
recently been demonstrated ihey are probably not less fundamental 





274 



NERVOUS SYSTEM OF VERTEBRATES. 



than the olfacton^ tracts which have long been known. The 
olfactory tracts in fishes come to the hyj^othalamus either isolated j 
or mingled with the tractus strio-thalamicus. On account of 
the ktter fact the view has been advocated by some that a partof 
the olfactor}^ impulses were delivered to nuclei in the thalamus 
proper. This is probably incorrect. WTien the medial olfacto- 



^m" 



iV'M- 



)\ 



x^.. 






Fig> 136, — Scheme of the coonections of the mammillarv tracts, the nudeti^^ 
habenuJae, and the nudeus dorsalis thalami. From Cajal (Tcxtura, etc.). A^ 
corpus mammillare; 5, nucleus dorsalis thalami; C, upper part of this nucleus^ 
D, nudeus habcnulae; E, corpus interpedunculare; F, dorsal audetis of ihc: 
tegmentum; /, optic chiasma; a, aqueduct of Sylvius; b, commissura habenulans j 
c, commissura posterior; d, tractus habcnulo-peduncuiaris; e, peduncle 
corpus mammillare; /, bundle of Vicq d' Azyr; g^ tractus mammi]Jo-pedun< 
/t J olfacto f)' tract of projection (tractus olf acto-hv'pothalamicus) ; *, stria ! 
(tractus olf acto-habenularis) ; w, ihalamo-corticai fibers; », cortico-th&lamic 6bcT$; 
o, p^ fibers of the stria thalami which cross in the habenular conunissure. 



hypothalamic tract is mingled with the tractus strio-thalamicm] 
there are always many libers going vcntrally from the latter tract! 
into the inferior lobes and it is whoUy probable that all olfactory] 
impulses enter the hypothalamus (except those to the nucleus] 
habenulae, see below). The tertiary gustatory tracts also enter 



CORRELATING CENTERS IN THE DIENCEPHAION. 



^75 



the inferior lobes and both gustatory and olfactory tracts are 
mdely distributed in the hypothalamus. In higher vertebrates 
nothing is knoiATi of the central gustatory tracts. The corpora 
mammillaria have long been known as the place of ending of the 
chief olfactor>^ conduction path, the fornix. From the mam- 
millary bodies the tracius mammiUo-pedumularis goes caudally 
and is comparable with the tractus mammillo-bulbaris in fishes. 
Collateral branches from the fibers of this tract near their origin 
form the tractus mammiUo-thalamkus to the nucleus dorsalis 
thalami (Fig* 136). The cephalic part of the hypothalamus, the 
tuber cinereum^ has never been well understood in mammals. 




» Fig. 137. — Sagittal section of the tuber cinereum of the newborn rat. From 
CajaJ (Textura, etc.). .1, anterior or chief nucleus; B, posterior nucleus; C, internal 
nucleus of the corpus mammillarc; D, optic chiastna; £, afferent tract from the 
forebrain (tractus olfacto-h>pothalantiicus); F, nucleus supraopticus; a and 6, 
branches of bifun:ation, and if , terminal branches of the afferent fibers. 



The first important contribution toward a clear understanding of 
its relations has been made recently by Cajal. This author shows 
that the two nuclei of the tuber cinereum receive bundles of non- 
myelinated and myelinated fibers which come from the septum 
pellucid um of the cerebrum, run close over the optic chiasma 
and end in these nuclei and in the mammillar}^ bodies. These 
tracts are illustrated in Figures 137, 138, taken from Cajal's text- 
book. As the discussion in the next chapter will show, these 
fibers are homologous \^ith the medial olfacto-hypothalamic tract 



J 



276 



NERVOUS SYSTEM OF VERTEBRATES. 



of fishes and other vertebrates. They are tertiary olfactar\^ fibers 
and it is evident that the whole hypothalamus receives such fibers 
in all vertebrates, although the caudal part, the corpora mam- 
miliaria, is especially developed as the end nucleus of olfactory 
fibers of the fourth order, the fornix. Considering the conditions 
in fishes it is to be expected that a tertiar)' gustatory tract in 
mammals will be found ending in the tuber cinereum. 

In fishes a tract from the tectum mesencephali enters the h\T)0- 
thalamus, the tractus tecto-lobaris. This tract would provide 



^"^^-^-Jt 



•?;mv 



^\^^ 



Fig. 138. — SagiUal section of the tulxT cincreum of the rat of eight da>*s, FMs^ 
Cajal (Tcxtura, ctt .). -4, anterior or chief miclcus; B^ posterior or acccssoiy nucleus; 
C, internal nucleus of corpus mammiUare; D, nen^ tract coming from the septum 
pellucidnni (tractus olfactO'h>'poth:iJ amicus medialis); £, effereni fibers fnom the 
tuber which disappear in the central gray matter; f, efferent tract from the corpus 
mammillarc; 6\ upper nucleus of the lubtT; A', optic chlasma. 



for the correlation of olfactory and visual or cutaneous impulses. 
The homologue of this tract in mammals has not been recognized, 
but it is known that fibers from other sources besides the olfactory 
systems describtn^l above end in the tuber cinereum. 

The tract going out from the corjiora mammillaria, as already 
mentioned, is the same in all vertebrates. Tracts going from the 



CORRELATING CENTERS IN THE DIENCEPHALON. 



277 



tuber cinereum have been followed only into the central gray 
of the thalamus (Fig. 137). This is the direction taken by the 
tr. lobo-bulbaris in lower vertebrates. Another tract from the 
hypothalamus which is not known in mammals and but poorly 
understood in lower vertebrates, is an ascending tract to the 
forebrain, the iractus lobo-epislriaiicus. 

A second olfactory conduction path, already described for 
fishes, is found in mammals; namely, that by way of the nucleus 
habenulae. The tract from the forebrain to the nucleus habenulae 
comes partly from the secondary olfactory nucleus, the nucleus 
thaeniae or nucleus amygdalae, and in part from the hippocampus. 
Those from the hippocampus are a part of the fornix system. 



tr o4^actD4uibefkulam 




KuctD^v^pothol 



Fig. 139. — A scheme to show the embryological relations of the nucleus habenulae 
and the inferior lobes in fishes. 1,2, neuromeres. 



The tract forms a part of the striae medullaris and enters the 
nucleus habenulae, where it contributes to the habenular commis- 
sure as in lower forms. From the nucleus habenulae the Iractus 
habentdo-peduncularis descends to the base of the mesencephalon, 
where the paired tracts form an intricate decussation and end in 
the corpus inter pedunclare. 

The fact that the two olfactory conduction paths diverge so 
widely seems at first sight difficult to understand. The accom- 
panying diagram will serve to show how this has come about. In 
Fig. 1 01 the olfactory paths are sketched into the outline of a 
sagittal section of a fish brain, and in Fig. 139 a reconstruction is 
given in which the several centers are dra^^^l in their embryonic 



278 NERVOUS SYSTEM OF VERTEBRATES. 

position. It will be seen that while in the adult the hypothalamus 
lies in an extreme ventral position, somewhat caudal to the nucleus 
habenulae, its embryonic and presumably early phylogenetic 
position was one neuromere cephalad from the nucleus habenulae 
and not so far ventrad as in the adult On the other hand, while 
the nucleus habenulae in the adult lies at the dorsal border of the 
brain wall it was overtopped in primitive vertebrates by the 
somatic sensory center. When it is considered that the large center 
for the tractus strio-thalamicus and the central gray have separated 
the h)rpothalamus and nucleus habenulae, that the hypothalamus 




vent. 
Tr. habenulo-peduncularis 

Fig. 140. — Transverse section of the nucleus habenulae of the sturgeon. 

has protruded ventrally as the result of its expansion, and that 
the nuclei habenulae have been drawn dorsally by the habenular 
commissure connecting them, it becomes altogether probable 
that these two tertiary olfactory centers have been developed 
from the same column of indifferent material, namely, the sub- 
stantia reticularis grisea of the second and third neuromeres. They 
are morphologically not dorsal and ventral structures, but represent 
the substantia reticularis of successive segments. Both receive 
identical tracts, including fornix fibers, which have been bifur- 
cated by the mechanical shifting apart of their end-nuclei. 



CORRELATING CENTERS IN THE DIENCEPHALON. 



279 



An additional evidence in favor of this view is seen in the rela- 
tion of the nucleus habenulae to the ventricle and the membranous 
roof. The nucleus habenulae in fishes (Fig. 140) is a lobe pro- 



Commsup. 

^Plex. chorioid. 



-orp. genic. 
lat. 




Jr. opt. 
med. bdl. 



Tr, opt Jat bdl. 




Tr. spin 



L. vise 



gust, tract 



Fig. 141. — Transverse sections through the brain of Amia at the level of the 
nucleus habenulae and of the facial lobe. Comm. sup. indicates the position in 
which the habenular commissure appears in the next adjacent section. The draw- 
ings^ were made from Golgi sections with the aid of the camera. 



jecting into the ventricle, its cells either border upon the ventricle 
or are arranged in rows parallel with the ventricular surface, and 
the membranous roof passes beyond the nucleus habenulae and is 



98o 



NERVOUS SYSTEM OF VERTEBRATES. 



attached to the dorsal border of the corpus geniculatura laterale. 
This shoi^^ clearly that the latter body forms the dorsal border 
of the brain wall. The nucleus habenulae has risen up mesial 
^o it and pushed it outward. This will become clearer on compar- 
isan with the relations of the \isceral and somatic sensory nuclei 




Fig. 1 4 2. —Transverse section of the habenular nuclei in the dog* From C 
(Tcxtura, etc.). .4, internal nucleus; B, external nucleus; C, stria thaiami; i>,~' 
tractus hal)enulo-]K'du ocularis. Only the intcimal nucleus corresponds to the 
nucleuis habenulae in the sturgeon (Fig, 140). 



in the medulla oblongata. In Figure 141 are drawTi two sections 
from ihe brain of a young freshwater dogfish (Amia), one through 
the medulla oblongata and one through the nucleus habenulae. 
It will be seen that in both the somatic sensor)^ centers, which 
are morphologically dorsal, are turned out to the side and are 




CORRELATING CENTERS IK THE DIENCEPHALON. 



281 



overtopped by the visceral lobe and the nucleus habenulae respect- 
ively. The position of the nucleus habenulae is in every way 
analogous to that of the vagal or facial lobe. In both cases a 
more median and \'entral body has been hypertrophicd and has 
pushed up imtil it has overtopped a more dorsal and lateral body. 
The nucleus habenulae has overtopped the geniculalum just as 
the vagal lobe or the facial lobe in bony fishes has overtopped 
the acuslicum. 

In mammals the nucleus habcnylae is composed of a mesial 
portion similar in structure to the nucleus in fishes, and of a 
lateral portion which has a %*er>" different structure (Fig, 142). 
Two similar portions are to be recognized in the nucleus habenulae 
of some bony fishes also. Several fiber tracts are said to be con- 
nected with the nucleus habenulae in addition to those mentioned 
above but it is not known how the two portions of the nucleus are 
related to these tracts. 



5. THE NUCLEUS OF THE TRACTUS STRIO-THALAMICUS AND THE 
SUBSTANTIA RETICULARIS THAL.\ML 

The large nucleus of the tractus strio-thalamicus, as almady 
stated* lies in the stem region of the thalamus. In bony fishes 
it is di\ided into the two large nutiei dorsalis and veniralis (nucleus 
rotund us Fritsch). From these nuclei arise the large tr actus 
ihalamo-huibares ei spifiaks which constitute an important part 
of the fundamental bundles in the medulla oblongata. It is to 
be noticed that the striatum is not a secondar)- olfactorv' nucleus, 
but is a correlation center for olfactory, gustatorj^ and perhaps 
other impulst^, and that the tractus strio-thalamicus is a descending 
tract or motor tract. The pathway is broken once in ihe thalamus 
and the fibers arising here probably make connection with widely 
separated motor nuclei in the brain and spinal con3. These 
functional relations emphasize the statement made above that these 
tracts reprtsent the stem or base of the interbrain and forebrain. 
Essentially the same motor conduction path is found in all verte- 
bratt^s, although its functional relations may be somewhat modi- 
fied in mammals on account of the cerebral cortex. 

In all vertebrates there remains surrounding the ventricle a 




282 NERVOUS SYSTEM OF VERTEBRATES. 

mass of central gray and in more highly specialized forms (teleosts, 
birds, mammals) special nuclei are present in addition to those 
mentioned. The functional relations of these are not yet suf- 
ficiently understood. It must be noted that some of these nuclei 
which constitute correlating centers of a higher order than those 
described above, by some authors have been assigned without 
reason to the hypothalamus. Thus the nucleus rotundus and 
even the corpus interpedunculare of bony fishes have been included 
in the hypothalamus. Equally unfounded is the assignment of 
the corpus Luysii, the field of Forel and adjacent centers in mam- 
mals to the hypothalamus. The term hypothalamus should 
be strictly limited to the infundibular and mammillary regions 
which have secondarily bulged ventrally and which servx as 
centers related to olfactory conduction paths. 

6. THE SACCUS APPARATUS. 

In all lower vertebrates the saccus vasculosus is a wide sac, 
sometimes much branched, with epithelial lining and with an ex- 
tremely rich blood supply in its walls. This outgrowth of the 
brain wall comes into very close relation with the vestige of the 
hypophysis, the branching tubes of the saccus often being inter- 
digitated with the epithelial sacs of the hypoph)rsis. This fact 
has led many authors to ascribe to the hypophysis nervous struc- 
tures which really belong to the saccus. 

Thd saccus is a part of the brain wall and is composed of elements 
characteristic of the brain wall. The vascular plexus is of course 
of mesodermal origin. Within this the saccus consists of a layer 
of nerve fibers and an epithelium bounding the cavity which is 
a part of the brain ventricle. The epitheUal lining is made up of 
supporting cells and nerve cells. The supporting cells form an 
internal limiting membrane as in the rest of the brain and extend 
through the fiber layer in which they form a supporting meshwork. 
These cells are therefore comparable with the ependyma. cells of 
the brain. The nerv^ cells have been described in several fishes 
both in embryonic and adult stages (Lundborg, Johnston). They 
are rather large spindle-shaped cells whose inner ends bear a tuft 
of cilia projecting into the ventricle. Such cells are already 



CORRELATING CENTERS IN THE DIENCEPHALON. 



283 



present in the floor of the brain ventricle in Amphioxus in a 
position corresponding to that of the saccus; a fact which indicates 
how ancient a structure the saccus is and that it functions in 
relation to the brain ventricle. In fishes the ciUated cells taper 
to a point at their outer ends and give rise to nerve fibers which 
help to form the fiber layer. These cells may be compared with 
typical primitive nerve cells elsewhere in the brain. The nerve 
fiber arises from the peripheral end of the cell as in the case of the 
neuroblasts in the embryo, and the cell-body retains the pos- 
ition in the epithelial Uning which is characteristic of the 



Lobus inferior 




Fig. 143. — The efferent tract to the saccus vasculosus in the sturgeon. The 
drawing represents a sagittal section a little to one side of the median plane through 
the front part of the saccus and a part of the ventral wall of the inferior lobe. 
The course and main branches of three fibers are shown, from among the many 
impregnated in the section. 



germinal cells and many neuroblasts. Well developed nerve cells 
which retain their epithelial position are very numerous in the 
brain of fishes (cf. p. 48). The saccus cells do not produce 
dendrites but instead bear cilia projecting into the ventricle. It 
must be supposed that these cells receive stimuU of some sort, 
whether vibratory or chemical, from the cerebro-spinal fluid. 



284 



NERVOUS SYSTEM OF VERTEBRATES. 



The impulses are carried by the neurites of the sense cells to the 
thalamus. The fibers form symmetrical tracts which in the 
sturgeon run up through the walls of the corpora mammillaria to 
end in a nucleus adjacent to the nucleus of the tractus strio- 
thalamicus in the extreme ventral part of the thalamus (Figs. 133, 
134). In bony fishes the nucleus is situated lower down and a 
part of the tract decussates in the caudal wall of the corpora 
mammillaria (Goldstein). The tract in teleosts also arises from 
the ciliated cells of the saccus (Johnston) and a similar tract is 
present in selachians. The tract does not arise in the dorsal part 
of the thalamus and end in the saccus as Edinger described it, 
but arises in the saccus. The secondarj*^ connections of the end- 
nucleus of this tract are not clear, but in teleosts a secondary- tract 
has been traced caudally over the ansulate commissure. 



Tr. opt.\ 



[( / L. inferior / \ 
Tr. thalamo-saccus 



^Corpus mamillare 
-Tr. sacco-thalani 



Hypophysis 



Saccus 



Fig. 144. — A general scheme of the saccus tracts as projected upon the mediin 
plane. 



In addition to the tractus sacco-thalamicus a tract goes out 
from the hypothalamus to end in the saccus. This tract comes 
in the sturgeon from the region just behind the optic chiasma (Fig- 
143) and in the salamander the origin of its fibers from ceils in 
this position has been demonstrated (Bochenek). The fibers 
of this tract go to all parts of the saccus vasculosus and ramify 
richly among the cells of the epitheUum. In the salamander 



CORRELATING CENTERS IN THE DIENCEPHALON. 285 

the ciliated cells are said to be absent. A general scheme of the 
saccus apparatus is given in Fig. 144. 

In mammals there is a sac with epithelial lining and dorsal to 
it a thick mass containing numerous cells of doubtful character 
and a rich plexus of nerve fibers (Berkeley, Cajal). From this 
plexus fibers pass into the epithelium to end freely among its cells 
(Cajal, Gemelli). The nerve plexus is connected with the brain 
by a large tract which runs along the raphe of the tuber cinereum. 
The tract takes origin from a nucleus situated directly over the 
optic chiasma. The tract and nucleus correspond in position to 
the efferent tract, and its nucleus in fishes. The epithelial sense 
cells and the tractus sacco-thalamicus have not been described in 
mammals. 

In all classes of vertebrates this outgrowth of the brain wall is 
present and is provided with nen^ous elements. Although the 
structure has been very incompletely studied, enough is known 
from fishes, amphibia and mammals to indicate that the relations 
of the saccus are fairly constant in the vertebrate series. The 
only suggestion regarding its function is that it serves as an organ 
for controlling the character of the cerebro-spinal fluid. Its plenti- 
ful blood supply and its thin wall adapt it for secreting fluid into 
the brain ventricle. The existence of a double nerv^e supply, both 
centripetal and centrifugal, indicates that it does more than simply 
secrete. The ciliated cells must be regarded as sense cells and it is 
conceivable that they may be stimulated by changes of cither 
pressure, density or chemical character in the cerebro-spinal fluid. 
In response to these stimuli the saccus may secrete some specific 
constituents of the ventricular fluid. The tract which ends in the 
saccus epithelium would arouse or control this secretive activity. 

7. Mention may be made here of an epithelial structure resem- 
bling in some respects the saccus epithelium, which forms the base 
of the epiphysis in the roof of the dicncephalon. This epithelium 
receives the free endings of ner\e fibers and gives rise to fibers 
which go in various directions. Part of the in-coming and out-going 
fibers form a commissure over the base of the epiphysis (Holt, 
Johnston). 

The fate of the four functional divisions in the midbrain and 



286 



NERVOUS SYSTEM OF VERTEBRATES. 



mterbrain may now be renewed in a few words. The motor 

columns are represented only by the nucleus of the III nerv^e and 
the thalamic nucleus of the fasciculus longitudinalis medialis, 
which belong to the somatic motor division* The presence of 
efiferent sympathetic fibers in the in nen^e in mammals indicates 
the presence in the midbrain of cells representing the visceral 
efferent column, but they have not yet been recognized as a definite 
column, unless the nucleus of origin of the mesencephalic root of 
the V nerve be that column. The somatic sensory division is 
represented by the tectum mesencephali, the corpus geniculatum 
laterale and mediale, and the nucleus of the medial lemniscus in the 
thalamus. With the exception of the mesencephaUc root of the 
trigeminus, only secondary or tertiary tracts end in these nuclei. 
The tectum mesencephah is clearly the continuation forward of 
the primary cutaneous centers of the cord and medulla oblongata. 
It has been modified into a center which is chiefly secondar\\ 
In the diencephalon the corpora geniculata and the nucleus of the 
lemniscus are situated in the dorsal region and hold essentially the 
same relation to sensory tracts as do the several nuclei in the 
tectal region of the mesencephalon. In both midbrain and inter- 
brain a relatively indifferent region has developed special nuclei 
for visual, cutaneous and auditory impulses. The fact is dear 
that the position of all the centers mentioned with relation to the 
axis of the brain and to other chief columns is the same as that 
of the somatic sensor}^ columns in other segments, and the con- 
duction paths in which these centers form stations are aU somatic 
sensory conduction paths. The recent description of the brach- 
ium conjunctiviim in mammals as a tract from the nucleus 
dentatus to the optic thalamus of the opposite side, brings the 
brachium into dose comparison with the lemniscus system and 
adds an important fact to the grounds upon which the above 
interpretation of the diencephalon is based. 

The visceral sensory division is not represented by any known 
special centers in the midbrain but in the intcrbrain it is largely 
developed and includes centers in gustatory pathways* The 
\isceral sensory column (or the substantia reticularis belonging 
to it) has been distorted by the expansion and shifting of the 




CORRELATING CENTERS IN THE DEENCEPHALON. 287 

centers lying in the two interbrain segments in such a way that 
a part of this column lies far dorsad (nucleus habenulae) and a 
part far ventrad (hypothalamus). This has been fully set forth 
above. 

Finally, a considerable part of the adult midbrain and interbrain 
consists of substantia reticularis or nuclei derived from it, whose 
morphological and genetic relations to the primary functional 
divisions are unknown. 

The COMMISSURES of the brain. — The facts regarding the 
chief brain commissures which have been scattered through the 
foregoing pages may be brought together here for convenience 
of reference. It should be noted at the start that the fiber- 
crossings in the lower vertebrates are for the most part mere 
decussations. The dorsal decussation of the spinal cord in higher 
vertebrates contains \dsceral sensory (sympathetic) fibers, collat- 
erals from cutaneous and visceral fibers, and secondary fibers 
from both somatic and visceral sensory columns. At the junc- 
tion of the spinal cord and brain, i.e. just behind the choroid 
plexus of the ventricle, in all vertebrates this decussation is greatly 
enlarged. This enlarged portion, known as the commissura infima 
(Figs. 81, 82, 83, 90, 92), is due chiefly to an increase of the vis- 
ceral sensory fibers from the roots of the VII, IX and X nerves 
and of secondary visceral fibers arising from the nuclei of those 
nerves. Other fibers in this commissure come from the cells of the 
nucleus funiculi. The dorsal decussation of the cord is therefore 
mixed somatic and visceral in character. These two components 
must be rigidly distinguished if the dorsal decussations of the brain 
are to be understood. 

The dorsal decussation of the medulla oblongata is not obliter- 
ated on account of the non-nervous roof, but its elements are 
crowded forward or backward. Behind the choroid plexus the 
conmiissura infima contains the visceral sensory elements proper 
to the segments of the VII, IX and X nerves. It is probable that 
the course of the root fibers of these nerves within the brain has 
been influenced by the crowding backward of their decussation 
and median nucleus by the choroid plexus. It is further prob- 
able that those fibers which take this caudal course are the more 



288 NERVOUS SYSTEM OF VERTEBRATES, 

primitive components of these nen'es, namely the general visceral 
fibers as distinguished from taste fibers. The point of special 
interest is that the concentration of the visceral decussation for the 
VII, IX and X nerves behind the choroid plexus precludes the 
expectation that the visceral elements of the first order will be 
found in the dorsal decussations farther forward. There are no 
visceral nen^es anterior to N. VII. 

The somatic sensory elements have behaved differently with 
reference to the IV ventricle. Instead of concentrating behind it 
they have concentrated in front of it. In those vertebrates in 
which the cerebellum is most primitive (Petromyzon, Protopterus, 
Urodeles) a decussation constitutes a prominent part of it. This 
decussation consists of axones of granule cells situated in the 
cerebellum destined to the somatic sensory nuclei of the medulla 
oblongata. This is therefore to be considered as the homologue 
of the somatic sensory portion of the dorsal decussation of the 
spinal cord. It is an important decussation in all lower verte- 
brates. 

A second prominent cerebellar decussation is found in fishes. 
This is situated in the velum meduUare anterius or in the enlarged 
equivalent of the velum, the valvula cerebelli of ganoids and bony 
fishes. Instead of connecting the dorsal portions or lateral lobes 
of the cerebellum, this commissure connects two nuclei which in 
fishes lie in the lateral walls distinctly ventral to the somatic 
sensor}^ centers, the superior secondary gustatory nuclei. The 
fibers of the secondary gustatory tract coming from the Ndsceral 
sensory column end in part in the secondary gustatory nucleus 
of the same side and in part cross to the opposite side. The 
remainder of the decussation is formed of the neurites of the cells 
of these nuclei. The destination of these fibers is not certainly 
known, so that it is uncertain whether a true commissure is present. 
It is evident, however, that the inferior cerebellar commissure 
belongs to the visceral sensor}^ division of the ner\-ous system. 

The dorsal decussation of the tectum mesencephali must be 
regarded as a somatic sensor\^ decussation comparable with the 
somatic sensor}' i)ortion of the dorsal decussation in the spinal cord. 

The posterior commissure has been discussed above (p. 265). 



CORRELATING CENTERS IN THE DIENCEPHALON. 289 

In the roof of the diencephalon two decussations are present, 
the well-known superior orhabenular commissure and a decussation 
closely related to the base of the epiphysis known only in a few 
forms. The habenular commissure contains decussating fibers 
from the olfactory nuclei of the forebrain (tractus olfacto-habenu- 
laris) and also probably true commissural fibers. It is to be 
compared with the inferior cerebellar commissure. Decussating 
fibers of the second and third order are present in each case and 
the nuclei in both cases are specialized parts of the substantia 
reticularis related to visceral functions. 

The post-epiphysial decussation has been described only in 
ganoids (Johnston), bony fishes (Holt) and the horse (Favoro). 
It is poorly understood but deserves further study. 

The anterior commissure, which will be described in the next 
chapter, is a dorsal decussation which, so far as at present known, 
is related in lower vertebrates to centers belonging to the visceral 
sensory system (olfactory and gustatory). In higher vertebrates 
two large commissures, one related to the olfactory cortex, the other 
to the somatic pallium, are developed from the anterior commissure. 

The dorsal decussations may be summarized by saying that 
the mixed dorsal decussation of the cord has been differentiated 
in the brain into separate somatic and visceral sensory decussations. 
The commissura infima, the inferior conamissure of the cerebellum, 
the habenular commissure and the anterior commissure represent 
the visceral portion. The superior commissure of the cerebellum, 
the dorsal decussation and posterior commissure in the mesen- 
cephalon (and the corpus callosum in the telencephalon) represent 
the somatic portion. 

The ventral decussation of the spinal cord consists of internal 
arcuate fibers and of the neurites of heterolateral tract cells. It is 
in smaller part a decussation of secondary sensory tracts, in larger 
part belongs to the substantia reticularis. In the medulla 
oblongata the secondary sensory elements (lemniscus system) are 
more numerous and in the base of the mesencephalon a large 
number of fiber tracts decussate in the region of the nuclei of the 
III and IV nerves. These tracts are m part tertiary sensory 
tracts, in part descending tracts from somatic and visceral correlat- 
19 



^90 NEKVOUS SYSTEM OF VERTEBRATES. 

ing centers going to make motor coimecdons in the medulla 
oblongata and spinal cord. The complexity of these decussations 
is very great and the region is by no means well understood. For 
attempts at the analysis of the midbrain decussations the student 
must be referred to the papers by Edinger, Johnston, and Goldstein. 
The great number of decussating fibers in this region is due to the 
projection \' en trad of the inferior lobes, which has crowded back 
the decussations from the segment of the diencephalon. 

At the cephalic border of the h}TDOthalamus another collection 
of decussating fibers is due in part to the same cause. These 
constitute the postoptic decussations. The optic chiasma has 
already been compared to the fibers of the lemniscus system in 
the ventral decussation of the medulla oblongata. The postoptic 
decussations arc to be compared in a broad way with the othcT 
elements of the ventral decussation. They belong chiefly to the 
substantia reticularis, either of the hypothalamus or of the nudeLJ 
in the colUcular region of the mesencephalon. These decxissa- 
tions have not yet been fully analyzed and the comparison of ' 
those in fishes and mammals is especially difficult. For the most 
recent treatment of these decussations the reader is referred to 
the papers of Myers, Kappers, and Goldstein. In Petromyzon 
the tractus lobo-epistriaticus, and in selachians the tractus oUacto- 
hypothalamicus lateralis decussate in the postoptic region. 



DEMONSTRATION OR LABORATORY WORK. 

1. Study the general relations of the thalamus, hypothalamus, saccus 
and nucleus habenulae in haematoxylin preparations of the brain of a 
ganoid, bony fish or selachian, in the frog and in a mammal. 

2. Study the cells and fibers in the hypothalamus and nucleus hab- 
enulae in a fish brain by the method of Golgi. Study tlie general 
course of the chief fiber tracts In Wefgert sections of the same brain, 

LITERATURE. 

Berkeley: The Nerve Elements of the Pittiitary Gland. Johns Hopkins 
Hospital Reports, Vol. 4. 1895. 

Bocheiick, A,: Ncue Beitrlige zum Ban der Hypophysis cenebri bei 
Amphibien. Bull, interna!. Akad. Sc. Cracovic. 1902. 

Boeke* J.: Die Bedeutung des Infundibulums in der Entwickelung dcr 
Knochenfische. Anat. Anz., Bd, 20, 1901. 



CORRELATING CENTERS IN THE DIENCEPHALON. 29 1 

Boeke, J.: Ueber das Homologon des Infundibularorganes bei Amphioxus 
lanceolatus. Anat. Adz., Bd. 21. 1902. 

Cajal, S. R. : Textura del sistema nervioso del Hombre y de los vertebrados. 

Edinger, L.: Utersuchiingen u.s.w. 2. Das Zwischenhim. Abhdl. d. Senken- 
berg. Naturf.-Gesell. 1888. 

Edinger, L.: Untersuchungen u.s.w. 4. Studien uber das Zwischenhim der 
Reptilien. Ibid. 1899. 

Gemelli: Nuove Richerche sull Anatomia e sull Embriologia dell Ipofisis. 
Boll, della Soc. med.-chimrg. de Pavia. 1903. 

Goldstein, K.: Vorderhim und Zwischenhim einiger Knochenfische, 
u.s.w. Arch. f. mik. Anat., Bd. 66. 1905. 

Herrick, C. Judson: The Central Gustator}' Paths in the Brains of Bony 
Fishes. Jour. Comp. Neur. and Psych., Vol. 15. 1905. 

Hill, Charles: Developmental History of the Primary Segments of the 
Vertebrate Head. Zool. Jahrb., Bd. 13. 1899. 

Holt, E. W. L.: Observations on the Development of the Teleostean Brain 
with especial reference to that of Clupea harengus. Zool. Jahrb., Bd. 4. 1890. 

Johnston, J. B.: The Brain of Acipenser. The Brain of Petromyzon. 

Kappers, C. U. A. : The Stmcture of the Teleostean and Selachian Brain. 
Jour. Comp. Neur. and Psych., Vol. 16. 1906. 

Koelliker, A.: Gewebelehre. 6te. Aufl. Bd. 2. 

Lundborg, H.: Die Entwickelung der Hypophysis und des Saccus vas- 
culosus bei Knochenfischen und Amphibien. Zool. Jahrb., Bd. 7. 1895. 



292 NERVOUS SYSTEM OF VERTEBRATES. 



CHAPTER XVIII. 
THE EVOLUTION OF THE CEREBRAL HEMISPHERES. 

The cerebral hemispheres of man are the largest and most 
complex part of the nervous system. They are also proportion- 
ately larger and more complex than in animals. The degree of 
development of the cerebral hemispheres is correlated with the 
mode of life of the animal. The greater size and the complexity 
of internal structure of the hemispheres in man are a measure of 
the degree of organization of his activities, the perfection of his 
adjustment to the manifold aspects of his environment, and the 
correlation of experience which makes such adjustment possible. 
The cerebral hemispheres constitute a mechanism whose structure 
is determined by all the experience of the race and of the individual, 
and in the working of whose minute parts is found the means of 
directing every one of the more complex activities of the man. 
In order to understand this highly organized mechanism every 
means of study must be employed, and nowhere has the compara- 
tive method proved more useful than here. The differences 
between the human cerebrum and that of lower a.nimaJs would 
seem at first sight to be so great as to make intelligent comparison 
impossible. In fact, as will appear below, the most fundamental 
parts of the cerebral hemispheres of man are present and have 
the same structure in all vertebrates, while for the study of the 
most highly specialized parts comparison with the various orders 
of mammals is very fruitful. 

The hemispheres of man are divisible into three widely diflferent 
parts. The part by which the hemispheres are directly connected 
with the rest of the brain is a thick bi-lobed mass consisting of 
collections of ner\^e cells pierced by numerous large bundles of 
fibers and is known as the basal ganglion or corpus striatum. It 
is by way of the corpus striatum that the chief fiber tracts connecting 
the hemispheres with the rest of the brain pass to or from the 



THE EVOLUTION OF THE CEREBRAL HEMISPHERES. 



293 



thalamus* Lying in front of and below the corpus striatum 
and forming part of the lower and mesial wall of the hemispheres 
at the anterior end arc the oljactory bulb and olfaciory lobe. 
On the ventral surface of the striatum is the nucleus amygdalae 
which is continuous caudally with the pyrijorm lobe and the hip- 
pocampus in the temporal region of the hemispherca These sev- 
eral structures, together with the jornix and hippocampal com- 
missure constitute the second main portion of the hemisphere, 
and may be spoken of collectively as the central olfactor}^ 
apparatus. All the rest— much the greater part— of the cer- 
ebrum is concerned with sensor}^ impulses from the external 
world which come from various parts of the body including 
the special sense organs of sight and hearing; with the cor- 
relation of these impulses with one another and with habitual 
tendencies produced by previous actions; with volimtar)^ impulses 
sent out to arouse, direct or inhibit actions in response to stimuli; 
with sensations; and with thought processes. THs portion of 
the cerebral hemispheres may be spoken of as the somaiic palliunL 
The phylogenctic history of these three portions of the forebrain 
has formed one of the most obscure chapters of comparative 
morpholog}% If it is possible to frame a connected account of 
the evolution of these structures, it will render the study of the 
human cerebrum simpler and its relations more intelligible. 

In order to gain a clear view of the vertebrate forebrain it will 
be necessary to begin with the lowest classes and give as concise 
an account as possible of the centers and fiber tracts and their 
functional relations in one class of vertebrates after another, 
endeavoring to fix with as great certainty as possible the homology 
of the most important structures and to harmonize those incon- 
sistencies which arise from differences of nomenclature or inter* 
pretation of known facts. In such a surv'ey the guiding principle 
must be functional relationship. When a ^ven center or fiber 
tract is clearly recognized anatomically, the questions must be 
asked, what is its function; with what other centers or fiber 
tracts is it related; with what kind of impulses is it concerned? 

The FOREBRAIN OF CYCLOSTOMES consists of paired lateral 
lobes and of a mesial portion connected with the diencephalon. 





NERVOUS SYSTEM OF \^RTEBRATES. 



The lateral lobes are rounded and are divided into anterior and 
posterior portions by a slight groove on the lateral surface, neariy 
vertical in position. If the brain be cut into halves in the median 
sagittal plane and one half looked at from the mesial surface 
(Fig. 145) the relation of all the parts can be better seen. In 
such a hemisection there is seen in the dorsal wall of the dien- 



Tr. bat) en. 'pcd line. 





rr. lobo- 
cpt»triatict]s 



Lobu<i inleriof 



Fig, 145 .' — A diagram of the fiber tracts in the forebraln of a cydostomev LamMr^ 
Wilderi. The mesial surface of the right half of the forcbrain and intcrbrain i» 
drawn. The dct'per shading indicates the wider parts of the ^tntride and the darlt' 
oval opening is the foramen of Monro. The fiber tracts and parts of them whic*^'" 
lie farthest latcrad arc drawn in the darkest shade. 



cephalon the thick nucleus habentilae and opposite it ventrall 
the optic chiasma and the decussations of other tracts. Behin. 
this is the depressed hypothalamus. The limit between the dien — 
cephalon and telencephalon is roughly indicated by a line drav^'M:^^ 
between the opdc chiasma and the nucleus habenulae. In froiiv* 





THE EVOLUTION OF THE CEREBRAL HEMISPHERES. 



295 



of this line the median or third ventricle extends fon\ard through 
the whole length of the telencephalon (cf. Fig. S). It is bounded 
laterally by thick walls but doi^ally and ventrally the w^alls are 
much thinner. Just in front of the optic cliiasraa the floor of the 
ventricle is suddenly depressed to fomi the preoptk recess. In 
front of this recess is the point corresponding to the lower edge 
of the neuropore in the embrj^o. From this point forward and 
upward the brain wall is formed by the closing of the neuropore 
in the jnedian line. As this is regarded as the end of a closed 
tube the front w^all of the median ventricle is called the lamina 
ierminalis. The lower portion of the lamina temiinalis is thick- 
ened by the afiierior commissure^ and the upper portion of it is 
also thickened by a transverse band of fibers knowTi as the olfac- 
tory decussalwn. Immediately above the olfactory decussation 
is seen a short triangular projection of the ventricle which marks 
the point at which the neural tube remained longest in connection 
with the ectoderm. This was probably at the upper border of the 
neuropore and the little sac is called the recessus neuroporkus. 
From the recessus neuroporicus back to the nuclei habenulae the 
roof of the median ventricle is membranous. This is a part of 
the true brain roof and does not belong to the lamina terminalis. 
There is no such thing as a pars supraneuroporica of the lamina 
terminalis. In the front part of the forebrain the median ventricle 
expands laterally into the ca\ities of the lateral lobes. These 
cavities are the lateral venirkks and each has an anterior and a 
posterior branch corresponding to the two parts of the lateral 
lobes. These must not be confused with the anterior and posterior 
horns of the lateral ventricles in man, since both together corre- 
spond more nearly to the anterior hom in man. The connection 
of the median with the lateral ventricle of each side is knowTi as 
the foramen oj Monro. 

Into the anterior half of each lateral lobe the olfactory nerve 
enters and its fibers are distributed to all parts of the anterior half 
of each lobe. This anterior part of the lateral lobe is therefore 
the olfactor}^ bulb. It consists of a great number of slightly 
differentiated cells (Fig. 94) from which arise the fibers of the 
idfactory tract. These fibers pass back into the posterior portion 




296 



NERVOUS SYSTEM OF VERTEBRATES. 



of the lateral lobes, to the whole extent of which they are distrib- 
uted. These are the olfactory lobes, including the equivalent of 
the nucleus thaeniae of higher forms. The vertical groove in the 




Fig. 146. — An oblique section through the inferior lobes and forebrain of 
peira. III^ median ventricle; a.o., olfactory lobe; d.p.^ p>ostoptic decussation; e^ 
striatum; ep.^ epiphysis; tr.e-s.f neurites of epistriatum cells to striatum; tr. 
tractus lobo-bufbaris; tr.o.f tractus opticus; tr.o-h.f tractus olfacto-habenuli 
tr.S'th.f tractus strio-thalamicus; tr.th-sac.f tractus thalamo-saccus; sac^ sa< 
▼asculosus. 




lateral wall separating the olfactory lobes from the bulbs is 
only external indication of the olfactory tract, which is us 
recognizable externally in other vertebrates. A part of the ol 




THE EVOLUTION OF THE CEREBRAL HEMISPHERES. 297 

tory tract fibers do not pass back on the same side but cross to the 
opposite side in the upper part of the lamina terminaUs, forming 
the olfactory decussation. These fibers then enter the dorsal part 
of the wall of the median ventricle and do not go to the lateral 
lobes. This lateral wall of the ventricle is directly continuous 
caudally with the lateral wall of the diencephalon and through the 
foramen of Monro with the caudal wall of the olfactory lobe. The 
nucleus which occupies this area is the epistriaium (Fig. 146). 
The olfactory tract, then, goes in greater part to the olfactory lobe 
of the same side and in lesser part through a decussation in the 
lamina terminalis to the epistriatum of the other side. 

From the olfactory lobe two tracts arise. One passes internal 
to the optic tracts and enters the inferior lobes of the diencephalon, 
the tractus oljacto-hypoihalamkus. The other goes upward and 
backward through the epistriatum to the nucleus habenulae, 
where the larger part of its fibers cross to the opposite nucleus, 
forming the commissura habenularis. This is the tractus oljacto- 
habenularis. In addition to fibers of the olfactory tract which 
end in the epistriatum an equally large tract comes to it from the 
hypothalamus by way of a decussation behind the chiasma. This 
is an important tract the equivalent of which is probably found in 
all vertebrates, usually in close relation with the tractus strio- 
thalamicus. It should be recognized as an independent tract 
under the name of the tractus lobo-epistriaticus. The epistriatum 
is characterized by pyramidal cells whose dendrites are studded 
with little knobbed spines. The cell-body Ues next the ventricle 
and the neurite arises from the basal part of a dendrite and pro- 
ceeds away from the ventricle. The neurites descend through the 
lateral wall of the median ventricle and end in the ventral half 
of the same wall, which constitutes the corpus striatum. The 
cells of the striatum are irregularly arranged bipolar or stellate 
cells whose neurites form the tractus strio-thalamicus, which 
ends in the central gray of the thalamus. Surrounding the 
preoptic recess is a layer of cells of very primitive character 
forming the nucleus praeopticus. The neurites from this nucleus 
in part nm dorsad to enter the tractus olfacto-habenularis and 
in part run back over the chiasma into the hypothalamus. The 



298 NERVOUS SYSTEM OF VERTEBRATES. 

fibers which constitute the anterior commissure have not been 
definitely traced. 

This is the most primitive type of forebrain that has been 
studied. It is concerned with olfactory impulses and possibly 
with gustatory impulses which may reach the epistriatum by way 
of the hypothalamus. It will be seen as the account proceeds 
that the centers and fiber tracts which have been described are 
preserved in higher vertebrates. 

The SELACHIAN FOREBRAIN. — ^The forebrain of the primitive 
selachians (e.g. Heptanchus, Fig. 2) and of the Holocephali (Chi- 
maera, Fig. 7) is rather slender and elongated. That of the 
more specialized forms (e.g. Squalus acanthias, Figs. 11 and 147, 
Scyllium, Raja, Torpedo, etc.) is much more massive and compact 
in form. The difference is chiefly a difference in size due to the 
greater development of certain parts of the brain in more specialized 
selachians. In both, the walls of the median ventricle are essen- 
tially as in cyclostomes except that the ventro-cephalic and the front 
part of the lateral wall are enormously thickened. The foramen 
of Monro leads laterally from the front end of the median ventricle 
into the lateral ventricle which traverses the thick lateral mass 
and continues forward through a more or less elongated olfactory 
tract into the olfactory bulb (Fig. 8). This is the typical form 
for the olfactory bulb in vertebrates. In cyclostomes the bulb 
has been pushed out to the side and backward by pressure from 
the large buccal funnel, and the side wall of the forebrain (olfactory 
lobe) has been folded outward and backward so as to form a sort 
of posterior horn to the lateral ventricle which is quite peculiar 
to the cyclostomes. 

The great mass at the front of the brain is composed of two parts 
the limits between which are readily seen in Figs. 11 and 147. 
Above and in front of the foramen of Monro the median ventricle 
is produced into a small pointed sac, the recessus neuroporicus. 
This meets a fibrous strand from the pia which descends through 
a narrow canal from the dorso-cephalic surface of the forebrain. 
This canal marks the dorsal border of the lamina terminalis and 
the mass lying in front of it is formed by a thickening of the lamina 
terminalis and of the adjacent wall, between the bases of the two 



THE EVOLUTION OF THE CEREBRAL HEMISPHERES. 



299 



olfactory tracts. As indicated in Figures 8 B and 9, the great 
growth of the front walls of the lateral lobes has resulted in their 
apposed mesial faces fusing together so as to give the appearance 
of an enormously thickened lamina temninalis. The preservation 
of the anterior branch of the lateral ventricle and of the canal 
from the dorsal surface serv^es to show the primary form of this 
region. At either side of the recessus neuroporicus in Figure 8 



I 



Laterail olfactory nucleus 

Hpistrjaturn I Tr. olf.habcnularis 

Tr* oHacto.corticaIis \ Tt. hahcn.-pedunc. 



' K. termrnalis 



Tr. olfactorius 
Medial olf. nuc. ^ 




V\ 



^y 



/ i 



^?^^ 



Striatum / 

Tr. oirhypolhat med. 



/ 



Tr- Dli.* 
hypothal. I at. 



\ \ L. inicrior 

Tr. ftirio.'thalam. 



I 



Fig. 147. — A diagram of the fiber tracts in the forebmin of a selachian- The 
tnesiftl surface of the right half of the brain of Squalus acanthias is drawn and the 
fiber tracts projected upon it. The course of the tracts is taken chiefly from the 
description by Kappers^ but the work of Catois, Edinger, Houser, Locy and othera 
has been conside: 



appe 
red. 



B is seen a ridge projecting into the lateral voitride, the re^o 
uncinaia. This represents the front wall of the brain immediately 
adjacent to the lamina terminalis. The great mass which has 
been formed by the expansion, thickening and bending upward 
of the front walls of the brain between the olfactory bulbs will 
be called the mesial olfactory nucleus. 

The lateral walls have also grown up, thickened enormously 
and fused together over the ventricle to form the massive roof 



300 NERVOUS SYSTEM OF VERTEBRATES. 

seen in Figure 147 behind the canal leading to the recessus neu- 
roporicus. The basal portion of the lateral wall is occupied by 
the corpus striatum. The outer layers of the lateral wall and of 
the thick roof constitute the lateral oljactory nucleus. The inner 
surface of both the striatum and the lateral olfactory nucleus is 
covered by a gray layer rich in small cells which bounds the ven- 
tricle both ventro-laterally and dorsally. This is the epistriatum. 
The course of the fiber tracts will show the functional signifi- 
cance and homology of the various parts of the selachian brain. 
The olfactory tract spreads through the whole anterior and lateral 
wall and ends in the mesial and lateral olfactory nudei. From 
each of these nuclei a tract runs to the hypothalamus. The first 
gathers from all parts of the mesial nucleus, curves downward and 
backward in the thick front wall, runs back near the mid-ventral 
line and joins the tractus strio-thalamicus. This tract should be 
called the tractus olfacto-hypothalamicus medialis. The second 
tract collects from the lateral olfactory nucleus, runs downward 
and backward through the lateral wall, passes over the optic 
chiasma, forms a decussation behind the chiasma and ends in the 
hypothalamus. This tract has usually been called the pallial 
tract because the nucleus from which it arises forms the roof of 
the forebrain. This nucleus, however, is the place of ending of 
olfactory tract fibers and is hence merely a part of the olfactory 
lobe. The tract in question should therefore be called the tractus 
oljacto-hypothalamicus lateralis. From the lateral part of the 
lateral olfactory nucleus another tract arises which runs along the 
dorsal border of the narrow lateral wall of the posterior part of 
the forebrain and enters the nucleus habenulae. Here the tract 
decussates with its fellow to form the habenular commissure and 
ends in the nucleus habenulae of the opposite side. This is the 
tractus olfacto-habenularis and corresponds to the tract of the 
same name in cyclostomes. That part of the lateral olfactory 
nucleus from which it arises may be called the nucleus thaeniae. 
The two tracts to the hypothalamus correspond to the one in 
cyclostomes, the existence of a separate medial tract in selachians 
being due to the extraordinary development of the olfactory 
apparatus. 



THE EVOLUTION OF THE CEREBRAL HEMISPHERES. 3OI 

The commissures in the forebrain of selachians differ in some 
respects from those in other vertebrates. In the floor (lamina 
terminalis) in front of the recessus praeopticus is the anterior 
commissure which contains two sets of fibers: one coming from the 
tractus strio-thalamicus and the other from the lateral olfactory 
nucleus and both ending in the deeper parts of the lateral wall, 
the epistriatum. In the roof is a large commissure which contains 
two kinds of fibers. The greater part of it consists of a decussation 
which is said to be composed of fibers from the dorsal part of the 
mesial olfactory nucleus and not of olfactory tract fibers. The 
fibers of this decussation end in the regio uncinata and in the 
epistriatum. In addition to this a direct tract arises in the mesial 
nucleus and ends in the epistriatum of the same side. These 
tracts in the selachian brain are new as compared with the brain 
of cyclostomes. It must be held clearly in mind that these fibers 
form the third link in a chain of which the olfactory nerve and 
the olfactory tract form the first two links. These fibers therefore 
constitute a tractus olfacto-corticalis. The nucleus which receives 
these fibers is a center of the same grade as the olfactory cortex of 
higher vertebrates. That part of the epistriatum which forms 
this nucleus must be called the olfactary cortex or archi- pallium. 
The fibers which go out from this center are not well understood, 
but it is known that the neurites of cells in the epistriatum connect 
it with the striatum (tr. cortico-medialis). From this the tractus 
strio-thalamicus carries impulses to the thalamus. In the roof 
also is another fiber crossing which is said to be composed of true 
commissural fibers between the lateral olfactory nuclei. This 
commissure has no counterpart in other vertebrates, but an 
analogous commissure is seen in the medulla oblongata of some 
fishes. In some bony fishes the facial lobes are so largely developed 
as to fuse together over the ventricle and in the lobus impar so 
formed are found commissural fibers. So here, the lateral olfac- 
tory nuclei have arched over the ventricle and a large commissure 
is formed where they have fused together. 

The scheme of the fiber tracts given in Figure 147 will make 
dear the general relations of the central olfactory' apparatus. 
It should be noticed particularly that the tractus olfacto- 




Fig. 14S. — A diagram of the fiber tracts in the forebrain of a bony fish. The 
figure follow's in the main a figure by Goldstein, and is drawn on the same plan as 
Figs. 146 and 147. 



the lamina terminalis is nearly horizontalj the lateral walls are 
thick but not high and the whole roof as far forward as the olfac- 
tory bulbs is membranous (Fig, 148). The median ventricle is 
large and spreads out wide under the membranous roof and at 
the cephalic end it bifurcates (foramina of Monro) into the ven- 
tricles of the olfactory bulbs. In the thick lateral walls arc found 
nuclei corresponding to those in the selachian brain but much less 
well developed. The corpus striatum occupies the chief part of 



4 



304 NERVOUS SYSTEM OF VERTEBRATES. 

fishes and from the whole olfactory lobe and nudeus praeopticus 
of ganoids arise the fibers of the tractus olfacto-habenularis which 
decussate in part in the habenular commissure and end in the 
nucleus habenularis. The epistriatum in the sturgeon receives 
a large tract of ascending fibers from the hypothalamus, the 
tractus lobo-epistriaticus, which decussates in the anterior com- 
missure. This tract may bring gustatory impulses to the epi- 
striatum. The neurites of the epistriatum cells in the sturgeon 
find endings in the striatum; in bony fishes they are said to go in 
the tractus strio-thalamicus to the thalamus. The anterior com- 
missure contains fibers from the lateral olfactory nudeus to the 
epistriatum of the other. side. From the nudeus praeopticus a 
tract goes back over the optic chiasma to the hypothalamus. 

The scheme of fiber tracts given in Figure 148 shows that in 
essentials they agree with those of selachians. The olfactory lobe 
is smaller and does not extend up into the roof of the ventride. 
The greater extent of the choroid plexus is due to the receding of 
the olfactory centers which in selachians help to form the toof. 
On this account there is no dorsal commissure but tertiary olfac- 
tory fibers cross in the anterior commissure and end in the epistri- 
atum as in selachians. The primitive crossing of olfactory tract 
fibers which has not been seen in selachians is present in ganoids 
and bony fishes. The tracts to the hypothalamus and nudeus 
habenularis are essentially aUke in all groups thus far described. 
The epistriatum of selachians is described as the place of ending 
of tertiary olfactory fibers only, while in ganoids it receives both 
secondary and tertiary olfactory tracts. Further investigation 
will probably show that the secondary olfactory fibers to the 
epistriatum are not absent in selachians. A prolonged discussion 
as to the nature of the paUium in bony fishes has been due espe- 
cially to the elevation of the so-called pallium of selachians to the 
dignity of a true cortical center. Now that it is known that the 
pallium of selachians is a part of the olfactory lobe and that there 
is a close correspondence between the fiber tracts in sdachians and 
bony fishes, it requires only the recognition of the slight devdop- 
ment of the olfactory apparatus in bony fishes to explain the 
condition of the pallium. A greater or less part of the proper 



THE EVOLUTION OF THE CEREBRAL HEMISPHERES. 305 

membranous roof has been displaced in selachians by the growing 
up, folding over and fusion of the lateral walls. In other fishes 
the corresponding part of the lateral wall is much smaller and the 
membranous roof is more extensive than in selachians. In 
some bony fishes (Fig. 151 C) there seems to be a sort of inversion 
of the lateral wall, so that the striatum and epistriatum bulge 
high up in the ventricle and the pallium passes over them and is 
attached to the lateral wall on the ventro-lateral aspect of the brain. 
A membranous roof over the median ventricle is universally 
present in the forebrain of vertebrates. In the more specialized 
selachians the greater part of it is displaced by the upgrowth from 
the lateral walls; in bony fishes it is extraordinarily broad because 
of the reduction and receding of the lateral walls. 

The nervus terminalis which is characteristic of selachian brains 
is foimd also in Amia, in Protopterus and in embryonic stages of 
Ceratodus, but nothing is known of its central connections. 

The forebrain of amphibia presents two external peculiarities: 
first, the relatively great size and independence of the lateral lobeSi 
and second, the f?ct that they extend forward far beyond the 
lamina terminalis. A glance at Figures 8 and 9 will show that the 
real difference between the amphibian and selachian brains in 
this regard is slight. In front of the lamina terminalis the lobes 
are separated by an open space knowTi as the sagittal fissure. In 
the frog this fissure is bridged across by the fusion of the two 
olfactory bulbs, but remains open behind the bulbs. The internal 
structure of the amphibian forebrain has been puzzling chiefly 
because its fiber tracts and commissures are difl&cult to, compare 
with those of other vertebrates. An understanding of the gross 
relations in one of the lower tailed forms, such as the hellbender 
(Cryptobranchus) or mud-puppy (Necturus), will help to make 
the finer structure intelligible. The median ventricle is relatively 
short (Fig. 150). Its floor in front of the optic chiasma is formed 
by the nucleus praeopticus and farther forward by the lamina 
terminalis, which is very greatly thickened by two commissures 
crossing one above the other in nearly the same plane. In front 
of the commissures the lamina terminalis is nearly vertical in posi- 
tion and becomes continuous with the membranous roof. This is 
20 



3o6 



NERVOUS SYSTEM OF VERTEBRATES, 



folded into the ventricle to form the choroid plexus. The recessus 
neuroporicus is recognizable in the frog a little above and in fmni 
of the commissure as in Fetromyzon, The caudal part of the 
roof is produced dorsally into a more or less branched sac, the 
paraphysis. The median ventricle is connected by mde foramina 
of Monro ^ith the lateral ventricles, which extend forward into 
the olf actor)' bulbs and backward into the rounded posterior 



Nuc. habcnulae 
Tr. olf.-habe&tilaris 
Hippocampus 



Tr. habcn.-pcilunc. 



Bulbus Qlt 



Tr. olL-eorticatU 



\ 



-*iC 



Tr. oir / > 

Tr. olf.'hypothal. med. / 

Upper commissure 
Lower commissure 



Nuc. praeo|iticiis | 

Tr> strio-thalam, 
Tr. Jobo'epistfiaticus 



L* inferior 



Fornix 



Fig. 150. — ^A diagram of the fiber tracts in the forcbrain of a tailed amphibian, 
Nedttrus. The representation of the commissure is based wholly upon Wcigcrt 
sections of the brain of Nectunis. In drawing the other tracts the descriptions of 
P. Ramon, Van Gchuchten and Bochenek for other amphibia have been consulted. 
The tractus ollacto-hypothal amicus lateralis is represented as joining the tracttis 
strio-thalamicus from above and going off from it to the inferior lobe* 



part of the lateral lobes. For the purpose of describing centers 
and hber tracts there may be distinguished in the lateral lobes a 
base or ventral wall, a lateral wall, a roof and a mesial wail. It 
will be convenient to anticipate the results of the following dis- 
cussion and divide the mesial wall into a portion in front of the 
foramen of Monro called the septum (precommissural or parater- 



THE EVOLUTION OF THE CEREBRAL HEMISPHERES. 307 

mihal body), and a portion above the foramen of Monro which 
together with the adjacent dorsal and caudal parts of the hemi- 
sphere forms the hippocampus. 

The fibers of the olfactory tract are distributed to the whole of 
the lateral wall and to the front part of the roof, base and septum 
and also to the nucleus praeopticus. A part of the olfactory tract 
decussates in the lower of the two commissures and goes to the 
lateral wall. As in selachians, then, the lateral and front walls 
of the hemispheres constitute the olfactory lobe. The base of the 
hemisphere is the corpus striatum. Fibers from the lateral wall 
pass back to the thalamus mingled with the tractus strio-thalami- 
cus. They constitute a tractus olfacto-hypothalamicus lateralis, 
homologous with that in fishes. From the septum and part of 
the roof and base at the front end arise fibers which curve for- 
ward and downward and form a tract which runs backward near 
the mid-ventral line and enters the hypothalamus (tractus cortico- 
medialis of P. Ramon in the frog). This corresponds to the tractus 
olfacto-hypothalamicus medialis in fishes. From the ventral and 
lateral part of the lateral wall, an area corresponding to the nucleus 
thaeniae in fishes, arises the tractus olfacto-habenularis which runs 
as in fishes. From the septum, which belongs to the olfactory 
lobe, fibers arise which run up and back and end in the caudal 
part of the mesial wall and roof, the tractus oljactorius septi of 
authors. This is a tertiary olfactory tract and corresponds to the 
tractus olfacto-corticalis in selachians, and should be given the 
same name. 

The commissures are difl&cult to analyze and compare with 
those of other vertebrates. The lower commissure contains fibers 
of the tractus olfacto-hypothalamicus from the cephaUc and lower 
half of the mesial wall (precommissural or paraterminal body, 
mesial olfactory nucleus), fibers of the tractus olfactorius which 
end in the recessus praeopticus, and fibers of the tractus strio- 
thalamicus. The lower commissure is placed in the base and 
lateral walls in very much the same position as the anterior com- 
missure in fishes and it is usually called the anterior commissure 
proper. The upper commissure is very peculiar in its position 
and composition. It crosses from side to side in the lamina 



3o8 



NERVOUS SYSTEM OF VERTEBRATES. 



terminalis, causing a high ridge across the floor of the ventride. 
At either side it rises up in the side walls just behind the foramen 
of Monro and bends forward over the foramen to be distributed 
to the upper half of the mesial wall and adjacent part of the roof. 
It also spreads back into the posterior part of the lateral lobe, 
into the part which is usually called the occipital pole. This 
name must not be taken to suggest any comparison with the 
occipital cortex of the human brain and would better be avoided. 




Fig. 151. — Simple diagrams to show the history of the epistriatum in fishes and 
amphibia. The epistriatum is represented by coarse dots adjoining the ventride, 
and the figures indicate how it is involved in the changes of form of the forebrain 
and how there are developed from it the hippocampal formation and the epistriatum 
proper of the amphibian brain. A, Petromyzon; B, Acipenser; C, Teleost; D, 
Chimaera; E, Squalus acanthias; F, Necturus. 



The upper commissure is usually described as a true commissure 
of the mesial and caudal regions just mentioned. It is, however, 
not so simple, for near its point of crossing it receives fibers from 
four other sources. First, fibers enter it from the lateral walls 
of the hemispheres; second, fibers from the medial olfactory 
nucleus; third, fibers from the tractus strio-thalamicus; and 
fourth, fibers which come directly from the hypothalamus and 
enter the commissure above the tractus strio-thalamicus. The 
commissure also has a third place of distribution in the dorsal 



THE EVOLUTION OF THE CEHEBRAL HEMISPHERES. 



309 



I 
I 

I 



I 



part of the brain, namely, in the lateral wall of the median ventricle 
anterior to the central gray of the thalamus. The composition 
and distribution of the commissure is shown in Figure 150. As 
it runs up behind the foramen of Monro it divides into two parts, 
the larger going into the medial wall of the hemisphere and the 
smaller turning into the lateral wall of the median ventricle. The 
latter bundle runs for a short distance \'entral to and distinct from 
the tractus olfacto-habenukris and spreads out in the central 
gray cephalo-ventral to the nucleus habenulae. This region of 
central gray is clearly seen in Cr)^ptobranchus, Necturus and in 
tadpoles of Amblystoma. It is a ver}^ compact body whose cells 
are arranged in rows next the ventricle and whose outer part is 
a fiber layer formed by the commissural bundle. In its structure, 
position and its relation to the tractus olfacto-habenularis, this 
body corresponds to the epistriatum of Petromyzon and to the 
caudal part of the epistriatum in Acipenser. As the cell layer 
is traced forw^ard it curves through the foramen of Monro, forming 
the dorsal border of the foramen, and becomes continuous with the 
ventricular cell layer of the dorsal half of the mesial wall. Inas- 
much as the commissure is distributed to both the lateral wall of 
the median ventricle and to the mesial wall of the lateral ventricle 
and as the ventricular cell layer is continuous throughout these 
regions, it is Just to conclude that they form a structural and 
functional unit. The composition of the commissure gives further 
ground for regarding this whole re^on as homologous with the 
epistriatum in fishes, with which a part of it agrees in position. 
The epistriatum in fishes receives an ascending tract from the 
hypothalamus which decussates in the anterior commissure. 
Such a tract is not certainly found in the lower commissure in 
amphibia but is present in the upper commissure. These fibers 
presumably constitute the tractus lobo-epistriaticus and the area 
in which they end is the epistriatum. In selachians it has been 
noticed that the region called the epistriatum receives a tertiary 
olfactory tract from the mesial olfactory nucleus, the tractus 
olfacto-corticalis. In selachians and ganoids a part of this tract 
crosses to the opposite side. In amphibia the tractus olfactorius 
septi and the bundle which enters the upper commissure from 





Fig. 152. — Sketches of transverse sections through (A) the caudal part of the] 
cpistriatum in amphibia and (B) the corresponding structure in Omithorhynckus. 
B from G. Elliot Smith. In A the tracts are drawn from the brain of AVflwrwij 
and the cells in the epistmium (£) are drawn schematically to show their disposition I 
next the veniide and the continuity of the cell layer through the foramen of Monro ] 
with that of the hippocampal formation (Hipp,), c, the tractus cortico-habenularis* j 
On the right side the section is supposed to pass just behind the foramen of Monro* I 

In B: f.d., fascia dentata; f,m,t foramen of Monro; hip., hippocampus; •#.♦ 
neopallium; para., paralerminal body; plxJ., choroid plexus of the lateral ventricle; 
rec.s., recessus superior. The rcccssus superior extends dorsad between two 
vertical shaded lateral walls which correifipond to the caudal part of the epistnatum 
in amphibia* This is erroneously called the paraphysis in mammals. 



THE EVOLUTION OF THE CEREBRAL HEMISPHERES. 



3" 



I 

I 



the mesial olfactory nucleus are homologous with the tractus 
olfacto-corticalis, and the region to which the upper commissure| 
is distributed must be considered as a part of the cpistriatum. 
The part of the epistriatum which receiver tertiaiy olfactory 
fibers is the olfactor)^ cortex or hippocampus. A part of the 
epistriatum which does not receive such tibers covers the inner 
surface of the striatum and in all higher vertebrates is regarded 
as an integral part of that body. 

The amphibian hemispheres differ in two chief ways from those 
of selachians. First, the secondar>^ olfactorv^ centers are less 
massive, the wall is thinner and the ventricle larger, and the 
epistriatum is relatively larger. Second, there is a more completed 
folding of the lateral wall than in selachians. In selachians the 
lateral walls rose up, arched over and fused. In amphibia the 
lateral walls rose up and folded over without fusing with one 
another. The result is to form on each side a lateral hem- 
isphere whose ventricle is a continuation of the primitive 
lateral or olfactory ventricle and whose dorso-mesial wall w^as 
primidvely the dorsal part of the lateral wall of the median 
ventricle. The lateral ventricle of the hemisphere is not 
completely surrounded by nen^ous walls as is the primitive 
olfactor)' ventricle. In amphibia the mesial wall of each 
hemisphere is connected with its fellow over the median 
ventricle by a membranous roof, wliich is homologous with 
that in fishes. The diagrams in Figure 151 will show how the 
amphibian hemispheres have been formed by folding over of 
the lateral walls, and also the history of the epistriatum in fishes 
and amphibia. The epistriatum is in reality no more or less than 
the central gray matter of the forebrain in fishes. The dorsal 
part of this central gray in selachians receives olfactory fibers of 
the third order and in consequence becomes differentiated as the 
olfactory" cortex. This region first becomes distinct in the amphib- 
ian brain as the result of the folding described. The caudal 
portion of the forebrain wall has not been folded into the lateral 
hemisphere and the epistriatum retains the position which it has 
in the caudal part of the forebrain in fishes. In higher forms 
this body becomes reduced but is stiO recognizable in mammals 




312 



NERVOUS SYSTEM OF VERTEBRATES. 



a short distance in front of the nucleus habenulae. Its nen^ous 
nature is evident in embryos but in the adult it has been confused 
with an entirely different structure under the name of paraphysis. 
In the accompanying Figure 152 the relations of the so-called 
paraphysis of a monotreme (Omithorh)mchus) are compared 
with those of the caudal part of the epistriatum in a tailed amphib- 




Itltor •*.-"-" 



JihHygJ 



SrV-'- 6X1. .-.---: 



Fig. 153. — A sagittal section of the forebrain and interbrain of a chick embryo 
of 7.0 days. From Minot. F.B.f forebrain; if.B., midbrain; chi., optic chiasma; 
ep,f epiphysis; Ay., hjrpophysis; Inf.g., infundibular gland (saccus vasculosus); 
P., paraphysial arch; Par., paraphysis; p.c.^ posterior commissure; V, velum. 



ian (Necturus). The structure which is properly known as the 
paraphysis is in lower vertebrates an outgrowth from the mem- 
branous roof of the forebrain in front of the velum transversum 
(Figs. 36, 147 and 153). 

The limits of the secondary and tertiary olfactory centers in 
amphibia are not clear but the dorso-medial and dorso-caudal 




^K- 154' — ^ATt of a transverse section of the cortex of a chameleon. After 
Cajal (Textura, etc*)- A, superficial plexiforra layer; B, layer of pyramids; C^deep 
plexiform layer; />, white substance; E, ependyma. 



and corresponds to the hippocampal formation in the majnmaliaii 
brain. The commissure of this region must therefore be compared 
functionally with the hippocampal commissure, x^natomically, 
however, the commisstire in amphibia is not the same as that in 
mammals. The upper commissure in amphibia lies beneath the 
median ventricle and behind the foramen of Monro and could not 




3^4 



NERVOUS SYSTEM OF VERTEBRATES* 



by any process of shifting be brought bodily into the position 
occupied by the Hppocampal commissure in mammals. More- 
over, the presence in this commissure of the ascending tract 
from the hypothalamus marks it as a part of the anterior commis- 
sure. In amphibia, then, the commissural fibers of the hippo- 
campal formation run by way of the anterior commissure, in which 
they constitute a large bundle nearly separate from the rest of the 
commissure. The presence of the tract from the hy^thalamus 



Epistriatum- 
Comm. hippJ 
Fornix- 



Comm, 
anterior 



Tr. strio-thalain. 

Fig* 155.— Transverse section ihrough the farebrain of Lacerta at Uic levd 
the anterior commissure. After Cajal (Tcxtura^ etc.). 



suggests that the olfactory cortex ser\xs also as a gustator}^ cortex. 
There is iinally to be mentioned a small tract from the dorso- 
caudal pole of the hemisphere which runs to the hj-pothalamus, 
taking a course over and behind the anterior commissure and 
separate from the tractus olfacto-hypothalamicus mcdiaUs, which 
runs beneath the anterior commissure. This small tract is to be 
compared with the jomix of the mammalian brain. 

No vestige of the nen-us terminalis or its center has been recpg-j 
nized in the amphibian brain. 

The reptilian brain differs from the amphibian chiefly in] 



THE EVOLUTION OF THE CEREBRAL HEMISPHERES. 



3IS 



having larger hemispheres with a somewhat more complex struc- 
ture in the olfactory cortex (Fig. 154), and in the position of the 
hippocampal commissure. The olfactory lobe occupies the 
anterior, lateral and ventral surfaces of the hemisphere, the base 
is the corpus striatum and the dorsal and medial walls constitute 
the hippocampus or olfactory cortex. The fiber tracts connect- 
ing the olfactory lobe with the cortex and connecting both 
with the hypothalamus and the nucleus habenulae seem to 
be identical with those in amphibia. The tract from the dorso- 
caudal pole to the hypothalamus is much larger than in amphibia. 
Overlying the striatum in the ventrolateral wall of the hemisphere 




Fig. 156. — A transverse section through the right lateral lobe of the forebrain of 
Lacerta. .Mxa Cajal (Textura, etc.). E, epistriatum; P, whivt substance of the 
hippocampal fofination; 5, striatum. 



is a body called the epistriatum which represents the ventral, 
unspedalized portion of the epistriatum in fishes. The anterior 
commissure (Fig. 155) includes olfactorv' tract fibers, fibers from 
the lateral dfactory area to the epistriatum, and fibers of the 
tractus strio-thalainicus. The commissure of the hippocampal 
region crosses in the upper part of the lamina terminalis and 
passes in frmt erf the foramen of Monro to reach the hippocampus. 
The positioD erf the commissure is the same as that of ie kip- 
pocampal commissure in monotremes and lower mammals. The 



3^^ 



NERVOUS SYSTEM OF VERTEBRATES, 



tractus olfacto-habenularis receives fibers from the dotso-caudal 
pole of the hemisphere. These fibers cxjnstitute a traclus cariice- 1 
habenularis. A commissure is found in Hzards and some other 
reptiles which connects the dorso-caudal poles of the hemispheres 
directly^ above the ventricle. This commissure runs in the velum 
transversum and does not correspond in position to any com- 
missure found in other vertebrates. It is therefore called the 
commissura aberrans (Fig. 158). The position and extent of the 
several parts of the forebrain are shoi^Ti in Figures 155, 156 and 
157, The last figure does not show the whole extent of the hip. 




Fig. 157. — A diagram of the mesial surface of ihe liemisphcre of a reptile 
show the extent of the hippocampus and related structures. The lamina terminalis 
is shaded with horizontal lines and in it are shown the anterior and 
commissures. The hippocampal area is shaded with oblique lines, 
olfactorius; p, para terminal or precommissural body (septum). 



pocampus but shows the position of its mesial part relative to the 
lamina terminalis, the foramen of Monro» the commissures anil 
the mesial olfactory nucleus (septum). 

The mammalian forebrain. — In monotremes and marsupiak 
the relations of the olfactor}^ centers are essentially as in amphibia 
and reptiles, although there is a higher development in internal 
structure. The olfactor}^ bulbs and lobes are relatively small 
{Figs. 159, 160) and the rest of the hemisphere very large in 
proportion. Indeed, in monotremes and marsupiak as in mam- 
mals, the lateral and doi^al walls of the hemispheres do not belong 
to the olfactorj^ apparatus as in reptiles and lower forms, but 





THE EVOLUTION OF THE CEREBRAL HEMISPHERES. 317 

constitute the s&malk pallium. The olfactory cortex is now 
confined to a part of the mesial wall of the hemisphere. Its 
extent is best seen in a new of the mesial surface of the brain 
(Fig. 160). The dorsal part of the mesial wall belongs with the 
dorsal wall to the general cortex or neopalluim whose functions 
are chiefly somatic. The region corresponding to the lateral 
olfactory nucleus of lower vertebrates is crowded dowTi upon the 
ventral surface of the hemisphere and forms the pyrijorm lobe 
(Fig, 159). This is separated from the general cortex by the 
fissura rhinalis. The mesial olfactorj^ nucleus is in the same 




fii^ cJt 



p,^ 



fp 



n.JK 



/r 



cU 



pLk£: 



cut. 



•v 



pLx.1^ 



>Sp 



Fig. 158. — A mesial sagittal section of the brain of an embn^o of Sphenodon 
punctatum. From G. Elliot Smith (Aberrant Commissure, etc.)* fl.5., aqueduct 
of Sylvius; b.o,, bulbus olfactorius; c.c, commissura aheirans; (r.rf., hippocampal 
commissure; c.h*, habenular commissure; c.p.^ posterior commissure; cak^ anterior 
commissure; cff., cerebral hemisphere; hyp.j hypophysis; /./,, lamina terminaJis; 
opt^ tractus opticus; par., parapbysis; p.o.^ olfactory peduncle; p.s.^ parietal 
stalk; plxJI.f lamina chorioidea; tttb,f tuber cinereum; vJIL^ third ventricle. 



position as in lower vertebrateSj forming the iuberculum oljaciorium 
and the lower portion of the mesial wall called the precommissural 
body (G. Elliot Smith). In the lamina terminalis is a very large 
anterior commissure and above it in the dorsal border of the 
lamina terminalis is a smaller hippocampal commissure. This 
commissure crosses in front of the median ventricle and enters 
the hippocampus above the foramen of Monro, and therefore 
corresponds in position to the hippocampal commissure of reptiles. 




THE EVOLL'TION OF THE CEREBRAL HEMISPHERES. 319 

150) and reptiles (Fig. i6i), and ends in the homologue of the 
recessus neuroporicus of fishes. This part of the median ventricle 
has been called the recessus superior and in both reptiles and 
monotremes its walls contain nervous elements and correspond to 
the caudal part of the epistriatum in amphibia as explained above 
(cf. Figs. 152 and i6i). A transverse section through the com- 
missures of a monotreme (Fig, 162) shows the general cortex 
forming the lateral and dorsal wall of the lateral ventricles, while 



L^-fr=<:'^ 



^/mM 



^LirM- 



:Vifv^ 



/'^. 



.^ 



^1 









-/■; 



'i-^.-e^ 



Hg* t6o. — ^The mesial surface of the right cerebral hemisphere of a marsupial 
(PkascolarclQs). Fmm G. Elliot Smkli (Relation of Fornix, etc*), a, extra ventricular 
alveus; d,d\ fascia dentata; /, fimbria; g, neopallium; I ft lamina tcrminalis;*?, olfac- 
tory bulb; it', olfactory peduncle; p, precommissural bodyj r^ pyriform lobcjt, 
tubcrculura olfactoriumj v^ ventral commissure (anterior commissure plus com- 
nu&sural fibers of the neopallium); uf, hippocampal commissure; jc, optic chiasma; 
% thalamus. 



the strongly infolded mesial wall is the hippocampus. The 
infolding is marked by a groove which appears on the mesial 
surface of the hemisphere as the hippocampal fissure (Fig. 159). 
The lower border of the hippocampus shows a higher special- 
ization of structure and is known as the fascia dentata. The 
lower wall of the brain is formed by the pyrifomi lobe and the 
corpus striatum through which runs the large anterior commissure. 





3^0 



NERVOUS SYSTEM OF VERTEBRATES, 



Where the commissure crosses in the lamina terminaUs the latter 
is thickened by gray matter which has invaded it from the adjacent 
mesial olfactor>^ nucleus or paratenninal body (Fig. 163)- In the 
upper part of the lamina terminahs the hippocampal commissure 
crosses in front of the foramen of Monro and covers the face of the 
hippocampus which bounds the lateral ventricle. Upon the 
upper surface of the hippocampal commissure in the middle line 
appears the small recessus superior. The hippocampal commis- 
sure is derived wholly from the hippocampal fold of the mesial 
wall of the hemisphere, while the dorsal and lateral wall or^gen- 
eral cortex contributes fibers to the anterior commissure. 



hip. 



ad- 



CVf- 



-recs. 



-rec.t 



y 




pStA* 



J. 



fJBCt^, 



C^, 



FiR- 1 61. ^Portion of a transverse section through the brain of a MoniiU 
(Rydrosaurus). From G, EJliot Smith. In the figure to the left the Hue jr^, I 
shows the plane of the section- oHk^ alveus; c*/., colnmna fomids; /asct fasdc 
ulus niarginalis; hip., hippocampus; para^^ paraterminaJ body; ucs.^ reccs&us 
superior; c.d,, hippocampal commissure; c.v,^ anterior commissure. 



In sagittal section near the medial plane (Fig. 164) are to be 
seen the follomng tracts belonging to the olfactor}^ apparatus. 
The tractus olfactorius (olfactory peduncle) enters the tuberculum^ 
olfactorium and the precommissural body and also sends a part 
of its fibers up into that part of the hippocampal formation known 
as the fascia denlata* The latter fibers are a vestige of the 
olfactory tract fibers %vhich run to the epistriatum in fishes. From 
the tuberculum olfactorium a large tracts not shown in the figure» 
goes up through the precommissural body to the hippocampus. 



THE EVOLUTION OF THE CEREBRAL HESdlSPHERES. JJI 

This corresponds to the tractus oUacto-corticatis of sdachians, 
amphibia and reptiles (tractus olfactorius sq>ti). A diffuse bundle 
of fine fibers arising in the precommissural body runs backwaid 
bdow the anterior commissure and is widely distributed in the 
hypothalamus (cf. p, 275). This tract is e\idcnily identical with 
the tractus olfacto-h>^thaIamicus medialis of lower vertebrates. 
Fibers ari^g from the whole arch of the hippocampus (Fig, 160) 
collect toward the commissural region in the lamina terminalis. 



,aM 



I 

I 



ay: p^m.. 

Fig. 162, — Transveree section through the cerebral hemispheres of Ornitharhyn' 
chys. From G- Elliot Smith. c.d,y hippocarapal commissure; c.t^, anterior commis- 
sure plus neopiilliaJ fibers; f.d., fascia dentata; hip., hippocampus; i,p,, lobus 
pyriformis; para.^ paratermina! body; rec^s.^ reccssus superior. 

Those from the caudal portion of the hippocampal arch run forward 
on its ventricular surface forming the fimbria. When the fimbria 
arrives at the lamina terminalis a part of its fibers go to form the 
hippocampal commissure (Fig. 164, w). The remainder of the 
fimbria fibers together with numerous fiber bimdles from the 
whole anterior portion of the hippocampal formation collect into 
a large bundle close upon the upper surface of the anterior com- 




322 



NERVOUS SYSTEM OF VERTEBRATES. 



missure and run backward through the hypothalamus to the 
corpus mammillare. This is the fornix (columna fomicis) and its 
course is the same as that of the bundle called fornix in amphibia 
and reptiles* Just above and behind the anterior comnussure 
the fornix column crosses the stria meduUaris tiialami which is 
coming up from the nucleus amygdalae in the caudal part of the 
ventral wall of the hemisphere* This region corresponds to the 
lateral olfactor)' nucleus of lower vertebrates and the stria i^ 



i^ 






jbo. 



fvZ 



iieMl 



.oC 



Fig, 163. — Plan of cerebral hemispheres, lamina lerminalis and optic thaUtoi 
in horizontal section. From G. Elliot Smith. b.o>, olfactory buJb; / J/.» foraJUfn 
of Monro; l.t., lamina terminalis; a.r, optic thalami; para*, pAratcrminA] bod^* 
V.J., lateral ventricle; vJIL^ third ventricle. 



identical with the tractus olfaclo-habenularis. The fibers which 
join this tract from the fornix columns constitute the tractus cor- 
tico-habenularis. Finally^ a large tract (Fig* 164, J) runs up 
from the region Just in front of the optic chiasma through the 
precommissural body and enters the fimbria to go to all parts of 
the luppocampus. These fibers probably come from the lateral 
olfactory area (pyriform lobe or nucleus amygdalae). They cor- 1 



THE EVOLUTION OF THE CEREBRAL HEMISPHERES. 323 

respond to the crossed tract us olfacto-corticalis of fishes and to 
the crossed sphenoidD-hip[x>campal tract in higher mammals. 

It was stated above that the region corresponding to the lateral 
olfactory nucleus of lower vertebrates is crowded down upon the 




Fig. 164. — Sagittal section of the commissural and precommissural regions of 
the right henaisphere of Ornithorhynchus. From G. Elliot Smith, aw., tractus cor- 
tico-habenularis;^,, nucleus habemilae; d, fascia dentata; g^ neopalliiitn ; o', olfactory 
peduncle; p., precommissural body; /., tuberculum olfactorium; i»., anterior com- 
missure plus neopallial libers; w., hipfsocampal commissure; 3, column of fornix; 
1', fibers of the same from the anterior end of the hippocampus passing beneath 



of the hippocampus passing 1 
Draniissuralis, probably comin 



the anterior commissure; 3, fasciculus praecoramissuralis, probably coming from 
the pyriform lobe; 4, tract us olfacto-h\^thal amicus j 5, fasciculus mareinalis, 
a jmrt of the olfactory tract which goes to the hippocampus; 6, stria medullaris 
thAlanti* 



I 



ventral surface of the hemisphere to form the pyriform lobe. This 
lobe appears on the ventral aspect of the brain as a ridge which 
extends the whole length of the lower wall of the hemisphere (Fig. 
165). As the olfactory tract comes from the bulb it divides into 




324 . NERVOUS SYSTEM OF VERTEBRATES. 

a mesial portion whose connections in the tuberculum olfactorium, 
precommissural body and hippocampus have just been described, 
and a lateral portion which enters the p)rriform lobe. This 
lateral portion, the external root or external olfactory radiation 
of authors, is distributed through the whole length of the pyriform 
lobe and to the caudal portion of the ventral wall of the hemisphere 
from which arises the stria medullaris thalami mentioned above. 
In higher manmials the anterior part of this region is kno^n 




Fig. 165. — Ventral surface of the brain of Ornithorhynchus to show the position 
of the pyriform lobe. Outline after a figure by G. Elliot Smith. 6., olfactory bulb; 
/>., pyriform lobe; /. tuberculum olfactorium; g.c.^ general cortex or neopallium. 



as the nucleus amygdalae, the posterior part as the pyriform lobe 
or sphenoidal cortex and from it arises the taenia semicircularis 
to the hypothalamus (Fig. 166). This is homologous with the 
tractus olfacto-hypothalamicus lateralis in fishes. As indicated 
above a tract from this same region goes to the hippocampus 
by way of the precommissural body and the fimbria. 

This general surv-ey shows that there is no essential difference 
in the arrangement of the central olfactory apparatus in mono- 
tremes and in amphibia and reptiles. The large size of the anterior 



THE EVOLmON OF THE C£R£BR.\L HEHISPHERES. 325 

commissure b due to the fact that it contains large numbers of 
fibers connecting the lateral walls of the hemispheres. These 
lateral walls together with the dorsal part of the mesial wall of the 
hemisphere correspond to the similarly placed general cortex or 
somatic pallium in man. This region has been called neopallium 
to distinguish it from the olfactory^ cortex (archipallium) which 






Fig. lUi. — i riiisvi r-t^ section through the brain of the rat of four days at the 
Icvrl of the anterior commissure. From Cajal (Textura^ etc). .4* columna 
fomicis; B, C and A' traclusolfacto-hypothalamicusC olfactory projection tract"); 
D, luhus pyriformis; E, nucleus lentiformis of corpus striatum; P, tractus opticus; 
H, anterior commissure; 7, cingulum; /?, nucleus caudatus of corpus striatum; 
7*, fasciculus longitudinalis superior. 



is traceable continuously from selachian fishes to mammals» 
The neopallium has developed in the lateral and dorsal wall of 
the hemispheres, crowding between the mesial and lateral parts 
of the olfactor}' lobe and pushing the lateral part down upon 
the ventral surface. The commissural fibers of this neopallium 





THE EVOLUTION OP THE CEREBRAL HEMISPHERES. 



327 



pallium from the anterior commissure to the hippocampal com- 
missure. The fibers appear first in the front part of the hippo- 
campal commissure, with the fibers of which they are mingled. 
As the neopallial fibers in this position increase in number the 
hippocampal fibers are displaced backward and the dorsal com- 
missure comes to be formed of two parts, an anterior corpus 
callasum and a posterior hippocampal commissure or psalterium. 

The further development of the mammalian forebrain consists 
chiefly in the enlargement and increasing complexity of the 
neopallium, the enlargement of the corpus callosum and the shift- 
ing of parts consequent on these changes. The enlargement of 
the neopallium and corpus callosum results in the degeneration 
of the hippocampus through the greater part of its length. The 
facts of chief importance are to be seen in the mesial aspect of 
the hemisphere of various mammals or in sagittal sections near 
the median plane. In the monotreme and marsupial (Fig. 167) 
the hippocampus begins just above and behind the olfactory 
peduncle, where it forms the boundary between the general cortex 
(neopallium) and the precommissural body, and extends back 
above the foramen of Monro and forms the mesial margin of the 
roof of the hemisphere. Some distance from its anterior end the 
hippocampus crosses the dorsal border of the lamina terminalis, 
to which it is attached by reason of the fact that its commissure 
crosses through the border of the lamina. Here also, just above 
and in front of the commissure the lamina terminalis joins the 
membranous roof of the median xentricle, the angle marking the 
position of the recessus neuroporicus in the embryo. In the 
marsupial and simplest mammalian brains the hippocampal 
commissure takes on the form of a compact lamina and the corpus 
callosimi extends for^vard from it, so that the two form an inverted 
V-shaped figure whose caudal descending limb is the hippocampal 
commissure. The two commissures cause a thickening of the 
lamina terminalis as indicated in the accompanying diagram (Fig. 
i68, C). 

The fibers of the corpus callosum in assuming this position 
have run transversely through the liippocampal formation and 
have mingled with the hippocampal ends of all the longitudinal 




328 



NERVOUS SYSTEM OF VERTEBRATES. 



fiber tracts which have been described above as forming the 
fornix system. It is very important that this intermingling of the 
transverse fibers of the corpus callosum with the longitudinal 
fibers which connect the hippocampus with the olfactory lobe, 
the base of the brain and the hypothalamus should be held in 
mind. The effect of the corpus callosum piercing the hippocampus 
is to cause it to degenerate in its middle part, adjacent to the 
lamina terminalis. In more highly developed (mammalian) brains 
the hippocampus from this point forward degenerates to a mere 




• Fig. 1 68, — Schemes to explain the expansion of the corpus callosum and the 
formation of the septum pellucidum. Alter G.Elliot Smith. A, reptile; B.mono- 
treme; C, marsupial; D, bat; E, higher mammal, c.a.^ anterior commissure; ex., 
corpus callosum; c,h., hippocampal commissure; /?., fimbria; hipp.j hippocampus; 
ind.j indusium; p., precommissural body. 



rudiment, the position of the original hippocampus being occupied 
by the enlarged corpus callosum. The rudiment of the hippo- 
campus remains on the dorsal surface of the corpus callosum and 
extends forward from it along the line of demarcation between 
the precommissural body and the neopallium. The fibers of the 



THE EVOLmOX OF THE CEREBRAL HEMISPHERES. 329 

several tracts of the fornix sj-stem which in monotremes entered 
the hippocampus anterior to the lamina terminalis are presented 
in small numbers and still enter the nidimentan* hippocampus, 
either running around the front edge (genu) of the corpus callosum 
or piercing it iperjoraiing fibers of authors, fornix Icngus of Forel). 
These perforating fibers maintain the same position and course 
which they have in all vertebrates, the corpus callosum is a new 
structure which has run transversely through and mingled with 
them, as explained above. 

The growth of the corpus callosum not only reduces the hippo- 
campus to a rudiment but also changes its position. As the neo- 
pallium enlarges and spreads backward the callosum spreads 
in the same direction, pushing back part of the hippocampus 
before it and stretching the rudimentar}- hippocampus which lies 
on its dorsal surface. The corpus callosum is formed in the 
lamiQa terminalis and its caudal border is mingled with the hip- 
pocampal commissure. As the callosum tends to grow broader 
antero-posteriorly its two borders are confined. In order to widen 
it must either stretch the lamina terminalis backward or it must 
bend and fold upon itself. It does both. In the simplest mammals 
(bats) no change is seen from the marsupial condition, but in such 
animals as the rabbit and hedgehog the corpus callosum has arched 
upward in the form of an inverted U, stretching the rudimentary 
hippocampus (Fig. 168). In higher mammals such as the cat, 
and in man the callosum has grown much larger and has pushed 
back over the hippocampal commissure. At the same time the 
U-shaped bend in the callosum has become more and more sharp 
until the two Umbs have met and in man it appears on superficial 
examination that the caudal border of the callosum is merely 
thickened. In fact, this portion, knowTi as the spleniuniy is formed 
by an actual folding of the callosum due to its growth in width 
while its borders remained fixed. The real border of the callosum 
is beneath its body, some distance in front of the splenial border, 
adjoining the psalterium (Fig. 169,/'')- Not only does the folded 
callosum extend back over the hippocampal commissure, but it 
stretches that part of the lamina terminalis containing that com- 
missure backward far from its original position. This stretching 



330 



^NERVOUS SYSTEM OF VERTEimATES. 



of the lanrdiia terminalis is e\ident upon comparing a sagittal 
section of the bat or rabbit brain with that of the cat or man. It 
is shown diagrammatically in Figure i68, A further effect is to 
stretch the mesial walls of the hemisphere at the same time with 
the lamina terminalis. As the anterior, hippocampal and callosal 
commissures form in the lamina tenninalis the lamina becomes 
thickened by the invasion of gray substance from the adjacent 
precommissural body. This gray substance forms a bed for the 



k' 



K'^l 



K^.;; 



Fig. i6g. — Scheme of cerebral commissures and margin of the concx in 
human brain. From G* Elliot Smith. a',a*. extra ventriculAral veus; c,c'» corpus 
callosum; d, fascia dcntatai IJ\r\ finibria; k*,h'\h"\ reduced hippocampus; 
ifl',l''f lamina terminalis; o,a\o'\ olfactory bulb and peduncle; p, precommissural 
body; p\ septum pelluciduin] r^ pyriform lobe; *, splenium of corpus caiiosum; 
V, anterior commissure; JC, optic duasma. 



commissural fibers. As the callosum expands and bulges and 
folds upward and backward it stretches the commissure-bed and 
also the closely related precommissural body, until the upper 
part of that body is drawn out into a tliin membrane. As this 
occurs in the mesial walls of both hemispheres two thin membranes 
are formed facing each other and fiUing in the somewhat triangular 
space between the two Embs of the callosum. These membranes 



THE EVOIITTION OF THE CEREBRAL HEMISPHERES, 

contain a few nerv^e cells and fibers of the fornix system and 
constitute the two leaves of the septum pellucidum. The space 
between the two leaves of the septum pellucidum is merely a part 
of the great sagittal fissure of the brain which by this process 
becomes roofed in by the corpus callosum and is called the ** fifth 
ventricle*'. WTiile these changes in the commissures are taking 
place the part of the hippocampus which retains its full development 
is pushed back until it no longer touches the lamina terminaHs 
in higher mammals (Fig. 169). 

While the expansion of the neopallium and its commissure has 
been so profoundly affecting the form of the hemisphere and the 
relations of the archipallium, the hippocampal formation itself 
has undergone changes of form and become more complex in 
structure. In monotremes and marsupials the hippocampus 
occupies the mesial wail of the hemisphere and is slightly infolded 
into the lateral ventricle, the line of infolding being marked by 
the hippocampal fissure. Along the lower border of the hippo- 
campus the cells multiply and proliferate from the ventricular 
layer to form the fascia dentata. As the hippocampus folds more 
strongly and the fissure deepens the thick fascia dentata which 
forms the lower limb of the fold retains its position on the surface 
and the upper limb, or hippocampus proper, is drawn deeper in 
and wrapped around the fascia dentata (Fig. 170) until the greater 
part of it is submerged within the hippocampal fissure. In most 
mammals the in-rolling goes so far that a larger or smaller part of 
the ventricular surface of the hippocampus is brought out upon 
the external surface of the brain where it is exposed except for a 
thin layer of fibers belonging to the fimbria. This inverted por- 
tion is the only part of the hippocampus, except the fascia dentata, 
which is exposed to view in the human brain. Of the two parts 
of the hippocampal formation the fascia dentata seems to serve 
wholly for the reception of olfactor}' impulses and to send its 
fibers into the hippocampus proper, while the hippocampus 
receives fibers from olfactory and other centers and sends out com- 
missural fibers and fibers of projection to the corpus mammillare 
and nucleus habenulae. 

This long and complex history of the evolution of the cerebral 




hemispheres mar be summarized by gi\ing a brief renew of the 
chief parts of the hemisphere of a higher mammal or man^ with an 
indication of their homcdogues in lower vertebrates. The base of 
the hemisphere is formed chiefly by the corpus striatum which 
includes the caudate and lentiform nuclei and is trax'er^ied by 
ascending and descending fiber tracts to the general cortex and 
hippocampus. In addidon to these, many fibers come up to the 
corpus striatimi from the lower parts of the brain and end in it, 
and a smaller number from the p}Tamidal cells of the general 
cortex also end here. Fibers arising from the cells of the striatum 
end in the centers of the thalamus, forming a tractus strio-thalami* 
cus. The nucleus caudatus, from its position next to the \'entricle 
and from the fact that a large part of its fibers end in the nucleus 
lentiformis, strongly reminds one of the unspecialized epistriatum 
of fishes. 

The ventral surface of the striatum is covered by the nucleus 
amygdalae which is continuous with the pyriform lobe or sphe- 
noidal cortex. These areas together correspond to the lateral 
olfactory area of fishes, amphibia and reptiles. From this region 
arise three tracts; first, the stria medullaris or taenia thalami, to 
the nucleus habenulae (tractus olfacto-habenularis) ; second, the 
thaenia semicircularis, to the hypothalamus (tractus olfacto-hj'po- 
thalamicus lateralis); third, a tract to the hippocampus by way of 
of the precommissural body (gyrus subcallosus) and the fimbria 
(tractus olfacto-corticalis). 

The inner surface of the corpus striatum forms the floor of the 
lateral ventricle, the anterior horn of which extends in front of 
the striatum into the frontal lobe of the hemisphere and reaches 
into the olfactory bulb, except where it becomes obliterated in the 
adult as in man. This is homologous with the primitive lateral 
or olfactory ventricle in lower vertebrates. The ventral and 
mesial wall of the anterior horn in front of the lamina terminalis 
is formed by the anterior and mesial part of the secondary olfactory 
center, homologous with the mesial olfactory nucleus in fishes. 
In man this region includes the tuberculum olfactorium and the 
small region in front of the lamina terminalis known as the gyrus 
subcallosus, better called the precommissural body. In the lamina 



334 



NERVOUS SYSTEM OF VERTEBRATES. 



terminalis in front of the median ventricle is a small anterior 
commissure connecting the corpora striata. This is largely made 
up of olfactory tract fibers coming from the olfactory bulb. The 
lamina terminalis is continued upward and greatly stretched 
backward to join the splenium of the corpus callosum. In this 
stretched portion of the lamina terminalis runs a thin band of 
transverse fibc^rs constituting a commissure of the hippocampi, 
the psalterium. The corpus callosum is a thick lamina of trans- 
verse fibers belonging to the neopallium, slightly arched. The 
mesial wall beneath it is formed of the stretched and thinned 
dorsal portion of the precommissural body and is known as the 
septum pellucid um. 

The floor of the temporal horn of the lateral ventricle is formed 
by the hippocampus which approaches the under side of the 
splenium of the callosum. The hippocampus does not end here 
but bends backward and cLir\'es over the splenial border of the 
callosum to run forward upon the dorsal surface of the latter. 
The portion of the Mppocampus in this position consists of paired 
rudiments of the hippocampal fold (indusium jalsum) in which 
runs a strand of fibers belonging to the fornix system, the stfiae 
Lancisii^ and a median film of gray matter connecting the hip- 
pocampal rudiments {hidusium vcrum). This rudimentary hippo- 
campus runs the whole length of the callosum in man, and in 
some mammals continues forward to the olfactory peduncles, 
forming the boundary line between the praecommissural area and 
the neopallium in the original position of the hippocampus in 
monotremes and marsupials- From the chief part of the hippo- 
campus in the temporal region fibers run forward over its surface 
forming the fimbria. When the hippocampus bends back to 
gain the upper surface of the callosum the fimbria continues 
forward in the lamina terminalis, and is usually termed the body 
of the fornix. The two bodies converge forward and the triangular 
space between them is bridged by the exchange of fibers which 
form the psalterium* This is a true commissure of the hippocam- 
pus and is homologous with the hippocampal commissure of rep- 
tiles and monotremes. In the lamina terminalis the bodies of 
the fornix bend downward in front of the foramina of Monro and 




THE EVOLUTION OF THE CEREBRAL HEMISPHERES, 



335 



then backward to become the pillars of the fornix. From the 
rudimentary hippocampus above the callosum similar fibers nm 
in the striae Lancisii and eventually break through the callosum 
or run around its anterior border and through the septum to join 
the pillars of the fornix. The latter then run into the thalamus 
and end in the corpus mamraillare. Fibers of the olfactory tracts 
and fibers from the tuberculum olfactorium and precommissural 
body enter the hippocampus by way of the septum and fimbria 
or as fibers perforating the callosum. Fibers running by the 
same routes come to the hippocampus from the pyriform lobe 
and nucleus amygdalae. The hippocampus also receives or sends 
out fibers through the corpus striatum. It is possible that these 
latter indude the equivalent of the tractus lobo-epistriaticus of 
fishes and that they bring up gustatory impulses from the h}T50thal- 
amus. It is especially interesting in this connection to note that 
gustatory sensation is thought by Flechsig to be localized in the 
hippocampus or the area immediately adjoining it. 

A remarkable Constance and similarity of structure is seen in 
the olfactory central apparatus throughout the classes of vertebrates. 
Speaking broadly, the primary (bulb) and secondary centers (lobe) 
and cortex with their respective tracts are already formed in 
selachians upon a plan which is retained in all higher vertebrates. 
A relative increase in the tertiaiy^ (cortical) centers and an increas- 
ing complexity of structure in these centers is the chief difference 
between higher and lower vertebrates* In fishes, amphibia and 
reptiles, as far as known, the whole forebrain with the exception 
of the center for the nerves terminalis in those forms which possess 
it, is devoted to the olfactorv'^ functions. The cortical center also 
serves the gustatory system. Apparently suddenly in monotremes 
appears a large lateral and dorsal cortex which is devoted to 
somatic functions. This is the neopallium whose structure and 
functions will be considered in the next chapter. Its position and 
the relations of its commissure show that it began its histor)^ in a 
dorsO'lateral position at the anterior end of the forebrain between 
the mesial and lateral olfactory nuclei, and that it spread back 
from this point, pushing the two olfactory areas do\\Ti upon the 
ventral surface and crowding the olfactory cortex to the extreme 




336 -- NERVOUS SYSTEM OF VERTEBRATES. 

dorso-mesial margin. The neopallial commissure at first ran 
in the primitive forebrain commissure (anterior commissure) 
and later followed the movements of the hippocampal commissure 
upward to the dorsal edge of the lamina terminalis. The long- 
itudinal tracts of the neopallium pass up and down through the 
corpus striatum lateral to and independent from the olfactorj' 
tracts and to quite different destinations (Fig. 166). It is impossi- 
ble to believe in the face of all that we know of the evolution of 
structure in the central nervous system that the neopallium has 
appeared spontaneously as a new formation in the brain of primi- 
tive mammals. It has arisen by the greater development of 
some structure previously existing in the vertebrate brain. That 
it should have arisen by modification of some part of the olfactory 
centers is inconsistent with all that has gone before in the present 
volume. The neopallium may be wholly absent from the brains 
of existing members of the classes below mammals but those 
brains have by no means been studied exhaustively enough to 
warrant such a statement. Indications of the existence of a 
homologue of the neopallium are to be looked for in the form of 
tracts connecting somatic sensory centers with centers in the 
forebrain isolated from the olfactory apparatus. A tract is 
certainly present in fishes nmning from the tectum opticum to the 
forebrain, but the center has not been isolated, perhaps because of 
its small size in all submammalian classes. The existence of a 
cutaneous nerve connected with the forebrain in many fishes 
(nervus terminalis) gives encouragement to the expectation that 
a forerunner of the neopallium may be found in lower vertebrates 
in the form of a segment of the somatic sensory column in the 
forebrain. It is to be" expected that further study of amphibia 
and reptiles from this point of view will reveal the rudiment of the 
neopallium, whose rapid development in primitive mammals 
led to the dominance of this class of vertebrates. 

DEMONSTRATION OR LABORATORY WORK. 

I. Upon careful dissections of the forebrain of a selachian; a bony 
fish, an amphibian and a mammal verify the general morphological 
relations described in this chapter. 



THE EVOLUTION OF THE CEREBRAL HEMISPHERES. 337 

2. Upon Golgi or Weigert sections study the forebrain tracts in the 
selachian and amphibian brain and if possible in the brain of a small 
rodent, bat or mole. 

3. If material is available, study with the aid of Smith's descriptions 
the history of the hippocampus and the fornix system by means of 
dissections and Weigert sections of the brain of the opossum, bat, 
mole, guinea pig, rat, rabbit, and cat. 

LITERATURE. 

Barker, L. F.: The Nervous System. 1899. 

Bochenek, A.: Die Nervenbahnen des Vorderhims von Salamandra 
maculosa. Bull. Intemat. Akad. Sci. Krakovie. 1899. 

Cajal, S. R.: Textura del sistema nervioso del Hombre. etc. 

Catois, E. H.: Recherches sur rhistologie et Tanatomie microscopique de 
Tencephale chez les poissons. Bull. Sci.d. France. Tome 36. 1901. 

Edinger, L.: Untersuchungen u.s.w. I. Das Vorderhim. 1888. 

Van Gehuchten^ A.: Contribution a I'^tude du syst^me nerveuse des T^l^o- 
st^ens. La Cellule, Tome 10. 1894. 

Goldstein, Kurt: Vorderhim und Zwischenhim einiger Knochenfische. 

Johnston, J. B.: The Brain of Acipenser. The Brain of Petromyzon. 

Kappers, C. U. A. : The Structure of the Teleostean and Selachian Brain. 

Ramon, P.: Investigaciones microgrdficas en el enc^fale de batracios y 
reptiles. Zaragoza. 1894. 

Ramon, P.: L'encefale des amphibiens. Bibliographie anatomique. 1896. 

Smith, G. Elliot: The Brain of a Foetal Omithorhynchus. Quart. Jour. 
Mic. Sci., Vol. 39. 1896. 

Smith, G. E.: The Morphology of the True Limbic Lobe, etc. Jour. 
Anat. and Physiol., Vol. 30. 1896. 

Smith, G. E.: The Fornix Superior. Jour. Anat. and Physiol., Vol. 31. 1897. 

Smith, G. E.: The Relation of the Fornix to the Margin of the Cerebral 
Cortex. Jour. Anat. and Physiol., Vol. 32. 1897. 

Smith, G. E.: The Origin of the Corpus Callosum, etc. Trans. Linn. Soc. 
London, Vol. 7. 1897. 

Smith, G. E.: Further Observations on the Fornix, with special reference 
to the Brain of Nyctophilus. Jour. Anat. and Physiol., Vol. 23. 1897. 

Smith, G. E.: Further Observations on the Brain of the Monotremata. 
Joiu*. Anat. and Physiol., Vol. ;^;^. 1898. 

Smith, G. E. : On the Morphology of the Cerebral Commissures in the 
Vertebrates, with special reference to an aberrant Commissure found in the 
Forebrain of certain Reptiles. Trans. Linn. Soc. London., Vol. 8. 1903. 

Unger. L. : Untersuchungen iiber die Morphologic und Faserung des Rep- 
tiliengehims. Sitzungsb. d. k. Akad. d. wiss. Wien, Math.-Naturw. Classe, 
Bd. 113. 1904. 
22 



338 NERVOUS SYSTEM OF VERTEBRATES. 



CHAPTER XIX. 
THE NEOPALLIUM. 

The evolution and general morphology of the cerebral hemi- 
spheres have been described in the last chapter and as full a 
description of the archipallium given as the limits of this book 
will permit. The neopallium, although it occupies the greater 
part of the manmialian hemisphere, is essentially dorso-lateral 
in position, lying between the p)niform lobe ventro-laterally and 
the hippocampal formation dorso-mesially. It is connected 
with the lower parts of the brain by large bimdles of fibers 
which run down through the corpus striatum, forming the cap- 
sula interna (Figs. 162 and 166). By means of the corpus 
callosum the neopallial areas of the two hemispheres are connected 
with one another. 

Structure of the cortex. — ^The neopallium ever)rwhere 
consists of a thick internal layer of fibers and of a superfical layer 
of cells, the cortex. This cortical layer varies in thickness from 
about 1.70 mm. to about 3.50 mm. Its total volume increases 
enormously from lower to higher mammals on account of the 
increasing size of the hemisphere as a whole, and still more on 
account of the superficial folding in the higher forms. The 
cortex consists of several zones of cells of diflFerent forms which 
are to be recognized with certain modifications in all parts of the 
pallium. The typical structure of the cortex as seen in Golgi 
sections is illustrated in the accompanying Figure 171, which is 
combined from numerous figures of Cajal representing sections 
of the frontal and parietal cortex of the young child. As indicated 
by arable numerals at the left of the figure, seven layers are dis- 
tinguished according to the size and form of their ceUs and the 
disposition of their neurites. 

I. Plexiform layer. This layer contains (a) small and medium 
sized cells with short neurites and ib) large horizontal cells whose 



THE XEOPALLITM. 53Q 

neurhes form tangential fibers <rf this layer. The la3rer also cod* 




Fig. 171.— Structure of the cerebral cortex. Explanation in the text. 

tains the terminal branches of the dendrites of pjTamids and of 



340 NERVOUS SYSTEM OF VERTEBRATES. 

Other cells situated in deeper layers, and the end branches of the 
neurites of various cells of deeper layers. 

2. Layer of small pyramids. This layer contains four types 
of cells: (c) small pyramids, each provided with a number of 
small basal dendrites and an apical dendrite which ascends to the 




Fig. 172. — Diagram showing the probable course of impulses in the cerebral 
cortex. From Cajal (Nouvelles idees, etc.). Ay small pyramid; B, large p>Tamid; 
C and D, polymorphic corpuscles; £, terminal fiber from other centers; F, collaterals 
from the white substance; G, fiber bifurcating in the white substance. 

plexiform layer, and ha\dng a long neurite which descends to the 
white substance; (d) large cells with short neurites; (e) small 
cells with short neurites which end in a very rich and dense arbor- 
ization; (/) fusiform, ovoid, stellate or triangular cells without 
radial dendrites whose neurites form the "fibers of Martinotti" 



THE NEOPALLIUM. 



341 



which ramify in the plexiform layer. Cells of the last type are 
found in all layers of the cortex and one is shown in the seventh 
layer in Figure 171. 

3. External layer of medium and large pyramids. In addition 
to {g) the p3n^midal cells, whose form is well shown in the figure, 
this layer contains several varieties of cells with short neurites, 
two of which are shown in the figure. One of these (A) is foimd 
in other layers as well. It has two sets of dendrites and a neurite 
with a great number of regular, smooth branches. The second 
(i) gives rise to a neurite which bears one or more pericellular 
baskets ending about the bodies of pyramidal cells. 

4. Layer of small stellate cells and of pyramids, usually called 
the layer of granules. This layer contains a few small, medium 
and large pyramids similar to those in the adjacent layers, but the 
most numerous and characteristic elements of the layer are the 
small stellate cells. By reason of their great number and the 
distribution of their neurites these cells are very important elements 
of the cortex. They are of several varieties of which there are 
shown in the figure: (j) cells whose neurites form long horizontal 
branches in the fourth layer; (k) cells whose neurites ramify 
in the third layer; (/) cells with ascending neurites with very 
numerous and exceedingly fine branches; and (w). cells whose 
neurites ascend to the first or second layer. 

5. Deep layer of large pyramids. In various regions of the 
cortex are found discontinuous groups or islands of giant pyramidal 
cells, two of which are shown in the figure (w). The layer contains 
also cells with short neurites distributed to the fifth and sixth 
layers, or ascending to the first layer. 

6. Deep layer of medium pyramids. This layer contains in 
addition to the pyramids (o) and triangular (p) and fusiform cells 
related to the pyramids, cells with short neurites of which several 
are shown in the figure (g,r). 

7. Layer of triangular and fusiform cells. Some of the elements 
of this layer are true pyramidal cells (5), others fusiform or tri- 
angular cells (/) related to pyramids. The remainder are cells 
witih short neurites («), including cells (/) which give rise to fibers 
of Martinotti. 



342 NERVOUS SYSTEM OF VERTEBRATES. 

Fibers of the cortex. — Four categories of fibers are connected 
with the neopallium, (i) AflFerent or exogenous fibers^ coming 
to the cortex from other parts of the brain. Such fibers come from 
the sensory nuclei in the thalamus and perhaps from other sensory 
centers, such as the corpus quadrigeminum anterior and the nuclei 
of the VIII nerve. Such fibers therefore bring sensory impulses 
to the cortex, in which they end by widely spread arborizations. 
In addition to these fibers, collateral fibers of unknown significance 
rise into the cortex from the white matter. (2) ItUracorticd 
fibers. This group includes a great variety of fibers which serve 
to spread impulses in the cortex or to bring into relation more or 
less distant areas. There may be mentioned: (a) short neurites 




Fig. 173. — Scheme of long association tracts in the hemisphere. From Cajal 
(Nouvelles idees, etc.). a,6,c, pyramidal cells; rf, terminal arborization; «, collat- 
erals of the fibers of association. 



connecting the superficial layers with the deeper; (6) short neurites 
which connect more or less distant elements in the same or 
adjacent layers; (c) neurites which connect distant parts of the 
plexiform zone; {d) homolateral fibers of association which con- 
nect various parts of the cortex in the same hemisphere. (3) Fibers 
oj the corpus callosum. These connect the cortex of one hemis- 
phere with that of another and are in part direct neurites of 
pyramidal cells and in part collaterals of fibers of association 
or of projection of one hemisphere which cross to the other. 
The place and manner of ending of the callosal fibers in man 
is not fully known, but it is thought that they are distributed 
to the motor and association areas. In some mammals they 



THE NEOPALLIUM. 



343 



have been traced to endings in the motor sphere of the cortex. 
(4) Fibers 0} projection. These arise from the pyramidal cells 
and descend through the white matter to form constituent fibers 
of the internal capsule of the corpus striatum and end in various 
lower centers of the brain or spinal cord. They are therefore 
eflferent fibers which carry impulses from the cortex and bring 
other parts of the nervous system under its influence. 

Functional areas of the neopallium.— The archipallium, 
as described in the last chapter, is devoted to olfactory and possibly 
gustatory functions. The neopallium is concerned with visual, 




Fiff. 174. — Scheme of commissural and projection fibers of the cortex. From 
Cajal (Nouvelles idees, etc.). ^4, corpus callosum; B, anterior commissure; C, 
pyramidal tract. 



auditory and general bodily sensations, with voluntary actions 
and with all the complex associational processes involved in our 
more highly organized activities, and in the formation and expres- 
sion of ideas. But the whole of this large cortical area is not de- 
voted indifferently to these several functions. It is shown by several 
independent methods of investigation that there exists in the 
neopallium a division of labor which is expressed by the term 
cerebral localization of functions. Thus, in case of any disease 
or injury which produces a lesion of a part of the cerebral cortex, 
one or more bodily functions may be interfered with and the study 



344 NERVOUS SYSTEM OF VERTEBRATES. 

of many cases has shown clearly that the functions affected depend 
upon the specific regions of the cortex injured. A sufficient 
number of facts of this sort have been collected from Hiniral 
observations, post-mortem examinations and surgical operations 
to render fairly certain and accurate the determination of the area 
of the cortex involved in case of a brain tumor, degeneration of 
cortical substance or other cause of disturbance. If the patient 
shows symptoms of disturbance in the functions of sight, hearing, 
bodily sensation, or voluntary movement including speech, certain 
specific areas of the cerebral cortex may be pointed out as the seat 
of the disease, and in the case of bodily sensation or movement 
certain subdivisions of the cortical area may be assigned to certain 
parts of the body. 

These clinical observations on man are supported by experi- 
mental investigations on animals in which either certain areas of the 
cortex in the living animal are directly stimulated and the effect 
noted, or the degenerations produced by the extirpation of certain 
cortical areas are studied by the method of Marchi. These 
investigations combined with the study of degenerations in human 
brains where the symptoms have been recorded during life, have 
led to the mapping out of functional areas on the cortex and to 
the description of the course of the tracts connected with those areas. 
Now the histological study and comparison of the areas thus 
marked out shows that in the normal brain certain differences exist 
between the different regions of the cortex and these differences 
are in some cases sufficiently marked to be of assistance in deter- 
mining the limits of the various areas. Thus the cortex in vari- 
ous regions differs in total thickness; in the relative thickness 
of its layers, in the presence, number and arrangement of giant 
pyramids and other cells; and above all, in the character of fibers 
connected with it. The skilled observer can distinguish between 
sections from various cortical areas. In the case of the cortical 
area for hearing, the thickness alone is sufficient guide and in the 
visual area the tangential fibers connected with it are so well 
marked that the limits of the area may be seen in dissections 
with the naked eye. 

The differences in the fiber tracts connected with the several 



THE NEOPALLIUM. 345 

areas give by far the most simple and direct and probably the 
most accurate means of mapping them out. The clinical and 
pathological data come into account here, but the most illuminat- 
ing studies are those based upon the order of myelinization of 
the fiber tracts (method of Flechsig). Flechsig has shown that 
in the developing human brain certain fiber tracts produce their 
myelin sheaths earlier, others later, and that the order of mye- 
linization is measurably regular and constant. Certain practical 
advantages in tracing fiber tracts were gained by this discovery. 
The tracts which are myelinated early may often be traced with 
great ease in early stages before adjacent tracts have received 
their myelin. When the tracts which are myelinated early are 
fully known in early stages of development the tracts which become 
myelinated later may be studied with greater certainty. The 
great advantage for the study of cerebral localization, however, 
lies in the fact that the majority of fibers connected with any nerve 
center or engaged in carrying impulses of the same category, 
become myelinated at the same time. If, then, the fibers which 
go up to the cortex from the centers for hearing, for example, 
become myelinated at the same time, the area of the cortex con- 
cerned with hearing will be indicated by the distribution of this tract 
when it is first myelinated and can be traced most easily. Further- 
more, the production of myelin seems to be the result of the begin- 
ning of functional activity of nerve fibers and the myelin appears 
first near the cells of origin and is progressively formed toward the 
end of the fiber and so indicates the direction in which the impulses 
travel. Thus, when a sufl&cient series of developing brains are 
studied this method should give three classes of facts: first, the 
course of the fibers engaged in a specific function and the cortical 
area to which they are related; second, the order in time when 
various nervous pathways begin to function; and third, the direc- 
tion in which impulses are carried by given fiber tracts. 

The senso-motor areas. — The afferent or sensory fibers 
which come to the neopallium from lower brain centers end in 
certain regions, and from these regions alone (Flechsig) arise the 
great majority of the efferent or motor fibers which carry impulses 
to the motor centers of the brain and spinal cord. Hence these 



346 



NERVOUS SYSTEM OF VERTEBRATES. 



regions arc given the name of the sensa-motor areas. Three 
sensory centers are recognized, the visual^ the auditory and the 
somaesthefic areas. The last is the area for general bodily sen- ] 
sations. Since motor tracts descending from the Wsual and 
auditor)^ fields are not well known, a general area corresponding 
roughly to the somacsthetic area is commonly known as the maiar 
area. The only way of describing the limits of these areas is 



». 



Fig. 175.— The primordial areas in the cerebral hemispheres, la tend surface. 
From Flechsig (EiniRe Bemerkungen u.s.w.). The numerals indicate the order 
of myelmkation of the several are^s. The areas 9-13, although myelinated early 
have no projection fibers. The areas 1-8 belong to the primar)* sensorj* areas 
(compare Figs. 179 and 180). 



by means of the superficial sulci and g>"ri of the brain. Theii 
position will be described here in a general way; for their 
boundaries the student must be referred to the original articles^ 
dealing with the subject. 

I. The somaesthetic area. This includes the central gyri, the 
lobulus paracentrahs and part of the adjacent frontal gyri and 
of the gyrus fomicatus; that is, a part of the lateral and mesial « 
surfaces of each hemisphere in the middle region. The extentj 



THE NEOPALLIUM- 



347 



of the area is indicated in the accompanying Figures 175 and 176. 
This area receives fibers from the chief sensor)' nucleus of the 
thalamus and perhaps from other centers of the general cutaneous 
apparatus (cf. p. 2^()). With respect to the time of myelinization 
these fibers are divided into three groups. The fibers of the first 
group recei%^e their myelin at the beginning of the ninth month of 
foetal life and are the first fibers entering the neopallium to become 
myehnated. Part of the fibers of the olfactorj^ tract are mye- 
Knated somewhat earlier. The fibers which are first myelinated 



2/ 



U 



ss 



Fig. 176. — The priniordial areas in' the cerebral hemispheres, mesial surface. 
From Flechsig. See Fig. 175. 



carrj^ impulses from the limbs and this accords with the evident 
importance in the early life of the infant of the tactile impressions 
received through the limbs. The fibers carrjing impulses from 
the trunk and head are myelinated later. 

2. The visual area. This occupies a small part of the lateral 
surface and a large part of the mesial surface in the occipital 
region. The fibers from the optic centers in the thalamus (p. 261) 
receive their myelin just later than the greater part of the fibers of 





348 NERVOUS SYSTEM OF VERTEBRATES. 

the olfactory pathways and earlier than the last group of somacs- 
thetic fibers. 

3. The auditory area. This area on the lateral surface of 
the temporal lobe close to the island of Reil is better defined than 
most other areas, since its cortex is much thicker thali that of the 
immediately adjacent regions. Its afferent fibers are myelinated 
soon after those of the visual field. 

4. The gustatory area. As already suggested in the previous 
chapter, the gustatory center is closely related to the olfactory. 
Injury to the cortex or fiber tracts close to the splenium of the 
corpus callosum in one hemisphere leads to loss of taste on the 
opposite half of the tongue. Whether the taste center is located 
in this area of the cortex (area 6 of Flechsig's figures) which is 
continuous with the subiculum comu Ammonis, or in the hippo- 
campus itself is uncertain. When the gustatory connections 
in lower vertebrates are taken into account it seems very probable 
that gustatory as well as olfactory sensation is provided for in 
the archipalliiun and that both come into relation with the mechan- 
ism for conscious and voluntary actions only through the asso- 
ciation centers. 

5. The motor area. This is practically co-extensive with the 
somaesthetic area. In this area the pyramidal cells reach a 
greater development than elsewhere and the large pyramids are 
especially numerous. In those areas in which arise fibers which 
run to the lower part of the spinal cord many of the pyramidal 
cells are of gigantic size (e.g. the giant cells of the lobulus para- 
centralis, the center for the lower limb). The motor fibers always 
develop myelin later than the sensory fibers of the corresponding 
area. From the whole motor area arise fibers which descend 
through the internal capsule and cerebral peduncle and continue 
through the pons and medulla oblongata into the spinal cord. 
In the internal capsule they make up the knee and two-thirds 
of the posterior Umb and they constitute about three-fifths of 
the cerebral peduncle. In the base of the midbrain and pons 
many of the fibers terminate in relation with the motor neurones 
of the III, IV, V, VI and VII nerves, most of them crossing to 
end in the nuclei of the opposite side. Many fibers end also 



THE NEOPALLIUM. 349 

in the nuclei of the pons. Those fibers which pass through the 
pons form in the medulla oblongata the well-known pyramidal 
tracts. From these, fibers are given off to the motor nuclei of the 
IX, X and XII nerves and at the posterior end of the medulla 
oblongata the two tracts enter into the pyramidal decussation. 
The decussation is not complete but the smaller part of the fibers 
continue into the cord on the same side, forming the ventral 
cerebro-spinal fasciculus. The crossed fibers together with a 
small number of the uncrossed fibers form the lateral cerebro- 
spinal fasciculus. Both of these bundles decrease in size as they 
descend the spinal cord, the ventral bundle being used up in the 
cervical and thoracic region and the lateral bundle gradually 
diminishing but extending even into the sacral cord. The fibers 
end in relation with the motor neurones of the spinal nenes. 
Part of the fibers of the ventral bundle cross to the opposite side 
and since the lateral bundles also contain uncrossed fibers it 
is probable that throughout the whole length of the spinal cord 
there are both direct and crossed connections of the cerebral 
cortex with the motor nerv^es. 

6. The areas of association. When the areas which have 

been described are myelinated there remains two-thirds of the 

cortical area without myelinated fibers. At the same time the 

projection tracts of the cerebral cortex seem to be fully formed. 

Although it is quite possible that a certain number of projection 

fibers may receive their myelin after this time and that some of 

Such fibers may be connected with parts of the cortex not included 

in the above areas, it is clear that these are the supreme projection 

areas of the cortex. Although the method of Golgi and the 

degeneration methods give evidence that projection fibers arise 

from the whole cortex, the method of Flechsig is the more reliable 

for this question and it must be regarded as clearly established 

that the number of projection fibers connected with other parts 

of the cortex is small compared with the number connected with 

the senso-motor areas above described. 

The remaining two-thirds of the cortex is intercalated between 
the several senso-motor areas so that each of them is separated by 
d considerable space from the others. This whole area constitutes, 



THE KEOPALLITM. 



.'^^ 



nI 



accoidmg to Fkririg, die asscKJJOioii centers and it may be divided 
into three main nelds, tiie cmliruir^ mMU and postrruw assiKi^ 
ation pdds iTig. 179^ That these fields are primarily related 




Fig. 179. — Diagram of lateral surface of hemi^bere showing localization of 
functions. 




Fig. 180. — Diagram of mesial surface of hemisphere sho\i'ing localization of 
functions. 



to the senso-motor areas is shown by the fact that the myelinization 
of the cortex spreads from the senso-motor areas to the cortex 



352 NERVOUS SYSTEM OF VERTEBRATES. 

bordering on them. A comparison of Figures 175 and 176 with 
Figures 177 and 178 shows that around the senso-motor areas 
a number of border zones have been myelinated while the central 
zones of the association fields remain without myelin. This 
seems to indicate that when the senso-motor areas become func- 
tional the association areas immediately adjoining them first come 
into relation with them, and the central parts of the association 
fields become functional last. A difference in fimction, then, is 
probably to be attributed to the border zones and the central 
zones of the association fields. 

By association fields must not be understood areas in which 
the functions of association are carried out without the aid of the 
senso-motor areas. This would be physically impossible and 
the term correlation centers would more truly express the function 
of the association fields. In other words, the function of the 
association centers is to correlate the actions of the senso- 
motor centers. The cells in the senso-motor areas are by no 
means all connected with projection fibers; only a few of them 
give rise to such fibers. The remainder give rise to association 
fibers of greater or less length. The shorter ones serve to connect 
the layers in the same gyrus or to carry impulses to adjacent gyri. 
The longer ones carry impulses to more distant parts of the cortex. 
It appears that these fibers do not go from one senso-motor area 
to another, but that the several senso-motor areas are brought 
into relation through the association, centers. The fibers reach 
first to the adjoining border zones of the association fields and 
later to the central zones and even to more distant association 
centers. The longer association fibers enter into the formation 
of certain long association tracts of which there may be mentioned : 
(i) the fasciculus longitudinalis superior or arcuatus, apparently 
connecting the occipital and frontal lobes; the fasciculus long- 
itudinalis inferior, connecting the occipital and temporal lobes; 
the cingulum, connecting the hippocampus and perhaps other 
olfactory nuclei with the lower gyri of the frontal lobe; the fas- 
ciculus uncinatus, connecting the temporal lobe with the same 
region of the frontal lobe; and the fasciculus occipito-frontalis 
(tapetum) which Dejerine believes consists of fibers arising in 



THE NEOPALLIUM. 353 

the frontal lobe and ending in the occipital lobe. In addition to 
these tracts which connect distant lobes, within each lobe are 
numerous association bundles, such as the calcarine, vertical, 
and transverse bundles in the occipital lobe. These various 
association bundles are very complex, consisting in most cases of 
fibers running in both directions and of fibers which enter and 
leave the bundles at various points of their course. Their grouping 
into bundles is due merely to mechanical conditions arising from 
the form of the brain. The greater part of the corpus callosum 
fibers go to the anterior and posterior association fields; the middle 
field receives few commissural fibers. 

In order to understand the functions of the association centers 
it is necessary to define more exactly the functions of the sensory 
areas. The phenomena of clear and sharp sense impressions is 
dependent upon the sense areas. Disease or injury to one of these 
centers interferes with the clearness and definiteness of the sen- 
sations with which that area is concerned. At the same time the 
memory of such sense impressions and percepts formed from 
several kinds of sense impressions may remain intact. Also, 
according to Flechsig, the perception of the spatial and temporal 
relations between sense perceptions depend upon the sense areas. 
Thus the clear recognition of the form of an object when felt by 
the hand depends upon the proper functioning of the hand and 
arm portion of the somaesthetic area, while the memory picture 
of the same object made up from previous sense impressions 
(tactile, visual, etc.) is a function of some of the association centers 
and may be preserved when the somaesthetic area is diseased. 

As the sense areas send fibers into the adjoining border zones of 
the association fields the participation of these zones will provide 
for the combination of simple sense impressions into perceptions 
of slight complexity. Thus general images of form based upon 
the examination of various objects by the hand may be localized 
in the border zone adjoining the sense area for the hand. In 
some such way the border zones stand in relation with the several 
sense areas and provide for relatively simple association of sense, 
impressions from nearly related regions of the body. More 
complex images of the form of objects dependent upon the com- 
23 



354 NERVOUS SYSTEM OF VERTEBRATES. 

bination of tactile and \dsual impressions, for example, require 
the cooperation of association centers which receive fibers from 
both somaesthetic and visual areas. The order of myelinization 
of association centers corresponds to the order of development 
of more and more complex actions and mental states in the child. 
The central zones of the association fields, then, serve for the 
higher psychic states and more complex processes of thought. 
The specific functions of the three association fields are deter- 
mined in large part at least by their position with reference to the 
senso-motor areas. The middle association field (island of Reil) 
situated as it is between the auditory area and that part of the 
somaesthetic area which receives sensation from the lips, tongue, 
throat, etc., is chiefly the association center for speech. The 
anterior and posterior fields have much more complex relations 
and functions. The posterior field, situated between the somaes- 
thetic, visual and auditory areas, receives from those areas 
impressions concerning the external world. The functions of this 
field are to construct external objects from the several kinds of 
sense impressions and to form ideas concerning the relations of 
objects and physical processes to one another and to the self. 
In a word, the objective relations of the individual and all those 
processes which we commonly call "intellectual" are the peculiar 
province of the great posterior association field. That this is at 
least in general the true interpretation of this field is evidenced by 
the mental symptoms in cases of disease affecting the posterior 
association field. The most common phenomena met with are 
various forms of loss of memory and of the power of association. 
If the sensory areas are not affected the perception of sense 
impressions by the touch, sight and hearing is not impaired, but 
these impressions can not be combined into objects which are 
recognized as previously experienced. The individual presents 
to the observer the phenomena of mind-blindness, mind-deafness 
and the like, while he himself loses the power of connecting his 
several impressions into orderly experience. Whatever set of 
associations are thus affected the corresponding set of sense 
impressions cease to interest and finally cease to attract the atten- 
tion of the individual and he appears listless with regard to things 



THE NEOPALLIUM. 



355 



i»l 



Iiterested him, or is unable to recognize objects, to give 

aes to things, to remember appointments, etc. When 

the posterior field become affected these symptoms 

general and the patient loses his interest in the 

and practical affairs. He may, however, retain his 

i own personal relations, his self-respect may be 

|erved, and so far as his powers of intelligent action 

te may be entirely true to his personal duties and 

The posterior association field deals then with 

ktion in the external world, with recognition, memory 

Ion, v^ith judgment and the weighing of processes 




r association field lies in proximity to the somaesthetic 

LOved from the other sensory areas. It would 

mon with the posterior area the impressions due to 

external objects and to the movements of the body, 

of speech, etc. Although the distant connections 

lobe are not well understood, it seems clear that 

e visual and auditory areas are of subordinate 

hile association bundles from the olfactory (and 

:ers are next in importance to those from the somaes- 

Id the light of comparative neurology it is more than 

sensation from the viscera reaches the archipallium 

play a part in conscious states through the asso- 

to the frontal lobes. If so, this would strengthen 

dy expressed by Flechsig that the impulses which 

ntal lobes have to do especially with experiences 

luiil and hence the anterior association field is 

li subjective states, and with the emotions, with 

h the ^vill. Here are to be sought the mechanisms 

(r> the bodily states which accompany or constitute 

md those connected with attention and apperception. 

conditions, such as muscular tension, are connected 

;ind active apperception, and the association centers 

li these are related to the somaesthetic and somatic 

[ere, also, cUnical observation supports the reasoning 

Ed data. In cases of disease of the frontal lobes 



356 NERVOUS SYSTEM OF VERTEBRATES. 

the symptoms which appear during life involve loss of appreciation 
of the indi\ddual's personality and of the value of things to him. 
Self-depreciation and lack of confidence or the opposite extremes, 
incapacity for moral and aesthetic judgment, uncertainty of 
action and lack of will, are common in such persons. Their 
self-control suffers and under the influence of excitement their 
conduct becomes immoral or criminal. The anterior association 
field has to do, then, with ideas of the individual's personality and 
with the appreciation of his personal relations. For conduct in 
the full sense, that is for moral conduct, there is required the normal 
functioning of both anterior and posterior association fields; since 
both objective and subjective relations must be considered, both 
judgment and will are involved. For moral conduct the individ- 
ual must respond to both the external or objective and the indind- 
ual or subjective factors in his situation, and the perfection of the 
response depends upon the grade of organization of the association 
centers and the balance between them. 

The evolution of the neopallium. — ^The general subject of 
the origin of the neopallium has been considered in the last chapter, 
but a certain interest attaches to the order and grade of develop- 
ment of its various parts. The direct data for the study of these 
subjects are very meager. The collection of such data would 
require the study of cerebral localization in the various orders of 
manmials together with the study of habits and the grade of organ- 
ization of intelligent action. Such studies are not yet complete 
enough for this purpose but the facts of localization in the human 
brain give some indications of the probable course of development. 
First, the facts that the senso-motor areas are everywhere separated 
by association areas and that myelinization proceeds from the senso- 
motor areas into the association areas warrant the inference that 
the association areas have been differentiated from the borders of 
already existing senso-motor areas. The senso-motor areas 
must have occupied the greater part or the whole of the neopallium 
in lower mammals and the greater size of the hemispheres in 
higher mammals is due chiefly to the expansion of the association 
areas. Second, the position of the several senso-motor areas 
indicates the order of development in the neopallium. The phy- 



THE NEOPALLIUM. 



357 



I 



tic and ontogenetic development of the several main regions 
of the forebrain show that the neopallium began its history at the 
front end of the forebrain and that it has expanded backward. 
Consequently the centers which were iirst developed must now be 
found in the posterior part of the hemisphere and those which 
were last developed, in the anterior part. The %isual centers, 
then, which occupy the extreme occipital pole, were first formed^ 
next the auditor)^ which are depressed into the temporal lobe by 
the development of the association centers, and last the somaesthetic 
and somadc motor area* It was pointed out in the last chapter 
that a forerunner of the neopallium is probably present in the fish 
brain in the form of a tract from the tectum opticum to the forebrain, 
and this accords well wdth the conclusion that the visual center 
was the first to be developed in the cortex. The order indicated 
by the position of the organs in the cerebral cortex of man accords 
also with the order of importance of the sense organs in phy-, 
logenetic history. While the cutaneous system is the oldest it is 
also the least specialized. The eye is the first special somatic 
sense organ in vertebrates and the auditory system followed it. 
It may easily be seen that these special sense organs which give 
knowledge of objects at a distance would be of greater value for 
the development of a higher correlating center such as the cortex 
than the cutaneous system. Although the cutaneous system 
was the last to reach the cortex its center in the neopallium has 
become larger than both visual and auditor)' combined. The 
relative areas are approximately proportional to the number of 
peripheral sensory fibers. The great importance of the somaes- 
thetic area in apes and man is connected with the development 
and mobility of the limbs; especially with the high organization 
of the hand as a grasping and tactile organ, and the use of tools. 
It may be supposed that while the senso-motor areas were being 
developed, association centers made their appearance between 
them, and that the posterior association center is therefore older 
than the anterior. The expansion of the association centers has 
helped to determine the position of the several areas in man. 
Indications of this are seen in the shifting of the visual area from 
a position about half on the mesial surface and half on the lateral 




358 



NERVOUS SYSTEM OF VERTEBRATES, 



surface in the apes to a position almost wholly on the mesial surface 
of the hemisphere in man, and also in the position of the auditor}^ 
area in the temporal lobe. Finally the anterior association field 
has developed from the front border of the somaesthetic area and 
is relatively small in the apes and probably absent in the lower 
mammals. The development of this field seems to be dirccdy 
proportional to the grade of se//- consciousness and in the several 
races of men, to the grade of civilization. So, the full appreciation 
of the self has probably been the last and highest factor in the 
development of individual and social conduct. 

The development of the several areas demands an increase in 
cortical surface. This is secured by surface foldings which form 
the g>^ri and sulci. Such foldmgs took place (i) within the sensory 
areas themselves, serving to increase their surface; (2) at the border 
of the sensor)^ areas and so forming the boundaries between sens- 
ory and association areas; and (3) within the association fields 
themselves. The deeper and more constant furrows belong to 
the first and second groups, since the furrows in the ELSSodation 
fields appear much later than the others. After the fundamental 
furrows were formed they were mechanically prolonged, branched 
and bent until it is difficult to compare the furrow patterns of the 
brains of different mammals. 

One other consideration concerning the association centers 
should be touched upon in dosing this book, namely their sig- 
nificance for education and morals. The object of human knowl* 
edge IS the world in its multitudinous forms and man's mani- 
fold relationships in it. The knowledge already acquired — ^when 
taken in comparison with the simpUdty of the child's brain and 
knowledge — has become enormously complex. It is so complex ' 
that when it is systematized and divided and subdivided many 
times, the average man can grasp only one small subdivision. 
How many of these shall the child entering school today learn to 
understand^ with how many can he become reasonably familiar^ 
in how many vviU he be able to add to human knowledge ? This 
will depend chiefly upon the number of related facts which corac 
to his notice during the plastic period of his brain. If he is early 
brought into contact with the world of matter and of life under 



THE XEOPALUm. 359 

many aspects, a corresponding number and \^riety of association 
tracts in his brain wiD be developed and fixed through use and will 
be ready for the condation of the experience of later yeani. Either 
in the schoolroom, on the playgroimd or in vacation time a richness 
of experience must supply the association centers ^-ith the condition 
for their high organization. Without this there can be in later 
years no richness of mental life and no great power of research, 
invention or constructive statesmanship. So on the moral side, 
the realities of human relationships must come into the experience 
of the youth during the period of plasticity of his brain if he is to 
rise to the higher planes of moral life. 

DEMONSTRATION OR LABORATORY WORK. 

1. Dissect the brain of a cat, dog, sheep or man, working out the 
relations of the neopallium to the archipallium and corpus striatum. 

* With the aid of figures in a larger text-book or atlas trace the limits 
of the neopallium in man and the chief g}Ti and sulci and the location 
of the senso-motor and association areas. 

2. As far as time and material permit, the structure of the cortex, 
the course of projection and commissural fibers and the arrangement 
of association bundles may be studied in Golgi or Weigert sections of 
a mammalian brain. 

LITERATURE. 

Beevor and Horsley: A record of the results obtained by electrical excitation 
of the so-called motor cortex and internal capsule in an Orang-Outang. Phil. 
Trans. 1890-91. 

Beevor and Horsley: An experimental investigation into the arrangement 
of the excitable fibers of the internal capsule of the bonnet monkey. Phil. 
Trans. 1890-91. 

Beevor and Horsley: A further minute analysis of the electric stimulation 
of the so-called motor region (facial area) of the cortex cerebri in the monkey. 
Phil. Trans., 1895. 

Cajal S. R. : Textura del sistema ner\'ioso del Hombre y de los Vertebrados. 

Cajal. S. R.: Les Nouvelles idees sur la structure du syst^me ner\'eux. 
Paris. 1895. 

Dejerine, J,: Anatomie des centres nerveuses. 1895--1904. 

Dejerine, J.: Sur les fibres de projection et d'association des hemispheres 
c^r^braux. C. R. Soc. Biol., Paris. 1897. 

Donaldson, H. H.: The Growth of the Brain. 1895. 



360 NERVOUS SYSTEM OF VERTEBRATES. 

Edinger, L.: Volesungen tiber den Bau der nervosen Centralorgane. 1904. 

Flechsig, P.: Gehim und Seele. Leipzig. 1896. 

Flechsig, P.: Die Localization der geistigen VorgS.nge insbesondere der 
Sinnesempfindungen. Leipzig. 1896. 

Flechsig, P.: Einige Bemerkungen Uber die Untersuchungsmethoden der 
Grosshimrinde, insbesondere des Menschen. Leipzig. 1904. 

Golgi. C: Sulla fina anatomia degli organi centrali del sistema nervoso. 
Riv. sper. di freniatr., Reggio -Emilia, Vol. 8, 1882; Vol. 9, 1883; Vol. 10. 1885. 

Koelliker, A.: Gewebelehre. 6te. Aufl. Bd. 2. 1896. 

von Monakow, C: Ueber den gegenwHrtigen Stand der Frage nach der 
Localization in Grosshim. Asher und Spiro's Ergebnisse, I. Jahrg., II. Abth. 
1902. 

Retzius, G.: Das Menschenhim. 1896. 

Smith. G. Elliot: Studies in the Morphology of the Human Brain with 
Special Reference to that of the Egyptians. No. i. The Occipital Region. 
Rec. Egypt. Gov. School Med., Vol. 2. 1904. 



INDEX. 



The numenls in ordinary type irfer to pasTS- iha» in hold^tao^ type 
refer to ngurcs. 



Abducent nerve, see N. abduccns 
Acustica-bteral system, ici.124 

nerves and centers. 124. 253. 51, 
62, 63, 70, 79 

pbcodes. 5& 33 

sense organs, t24. 51, 62 
Adaptation of acti\ities. 95 
Afferent impulses and nbers. 81. 96 
Amphibia 

acustico-lateral s\*5tem. ^9. 138. 

79 

brain. 31. 249, 256, 284, 305, 36, 
81, 125, 127, 150 

cranial ner\*cs. 79 

histogenesis. 14, 15, 16, 26, 27 

neural tube. 35, I3f I4t I5f 16 

optic vesicles. 43, 22 

placodes, 59 

segmental relations of ner\'es, 191 

sensitiveness to light, r43 
Ampullae of Lorenzini, 60, r25 
Anterior head cavities, 64, 38 
Aqueduct of Syh-ius, 24 
Ait:hipallium, 3or-336, 145-170 
Arcuate fibers 

external, 135, 229, 243. 245 

internal. 113. 137, 253, 49, 57, 
59.70 
Association centers, 349-356, 175- 
180 

apperception, 355 

attention, 355 

imagination, 354 

judgment, 354 

memory, recognition. 354 

personality. 355 

speech, 354 
Association tracts. 352 
Auditory apparatus, loi. 124, 253, 
70 

area of cortex, 348, 357 

centers, primar)-. 124. 64-70 
secondary, 124, 253, 127, 129 



Auditory apparatus 

cochlea. 127 

cochlear ner\-e. 140 

nerA-e. see N. acusticus 

sac. 58. 65 

semicircular canab. 12S 

sense organs. 126. 33, 61 

^'estibula^ nerve, 13S 
Birds. 39. 61. 25S. I9» 74 
Bony fishes 

brain. iS. 24. 27. 30, 161. 160. 
22s. 229. 271. 284. 302. 4, 8. 

IQ, 89, 90, 91. 127, 148 

cranial ner\*es. 106. 128, lOO. 63, 

88 
gxistatory paths. 169. 89, 90 
neuromeres. 38. 19 
taste organs. 164. 85 
Brachium conjuncti^•um. 220. 23S. 

243. 245. 112-117, 124. 125, 

126 
pontis. 239 
posterior. 238 
Brain 

general morphology. 14. 2, 3, 

4, 7-12, 20, 36, 159, 160, 

165 

flexures, 40. 20 
histogenesis. 45, 13-16 
longitudinal zones. 16, 48. loi, 

3, 11,46,47 
neuromeres, 38. 18, 19 
secondar}' segments, 14. 43. 2, 
20 
Branchiomeres, 66, 28, 29, 38 
Bulbus olfactorius. 28. 177. 293. 298, 
302, 305, 316, 2, 7, 8, 12, 94- 
98, loi, I45-I48> 150, 159. 
160 
Canal organs, 124. 51, 62 
Capsula interna, 338, 348, 162, 166 
Carcinas, 79, 40 
Cardiac plexus. 206, 212 



361 



362 



INDEX. 



Centers of correlation, 218, 226, 253, 
269 
central gray, 219 
substantia reticiilaris, 219 
tract cells, 219 
Cephalization in vertebrates, 69, 72 
Cerebellar crest, 132, 64-68 
Cerebellum, 24, 115, 131, 226, 2, 11, 
1 12-120 
basket cells, 240, 121 
brachiumconjunctivum, 229, 238, 
243,245, 112-117, 124, 125, 
126 
cortex of, 239, 121, 123 
deep gray nuclei, 237, 245, 126 
fiber tracts, 240 
function, 248-251- 
general cutaneous center, 114, 

59 

granular layer, 132, 239, 121, 

"3 

gustatory nucleus, 169, 247 
inferior commissure, 169, 288 
molecular layer, 240, 121, 123 
moss fibers, 240, 121 
motor connections, 245 
net-like endings, 240, 121, 122 
origin, 44 

phylogeny, 226, 248 
Purkinje cells, 239, 121 
special cutaneous center, 130, 

69 

superior commissure of, 227, 288 
valvula, 172, 91 

Cerebral hemispheres, 28, 186, 292, 
338. 145-180 
amphibians, 305, 36, 150-152 
bony fishes, 302, 148, 151 
cortex of, 338, 171- 180 
cyclostomes, 293, 94, 145, 146 
evolution of, 292 
ganoids, 302, loi, 149 
gyri and sulci, 358 
localization in, 343, 175-180 
mammals, 316, 159-180 
reptiles, 314, 154-158, 161 
selachians, 298, 11, 147, 151 

Chiasma opticorum, 28, 146, 2, 7, 11, 
36, 94. 100 

Chief sensory nucleus of thalamus, 
259, 60, 128 

Chorda t}Tnpani, 168 

darkens column, 156. 159, 78 



CoUiculus, 137, 255, 127, 129, 130 
Commissura aberrans, 316, 158 

ansulata, 119, 289 

anterior, 30, 289, 295, 301, 304, 
307. 315. 99-101, 145-150, 

162, 164, 166 

dorsalis of spinal cord, 112, 287, 

. 49,53,78 

inferior cerebelli, 169, 172, 288, 

91, 116, 117 
infima, 159, 287, 81, 82, 83, 92 
habenularis, 26, 184, 289, 10 1, 

145, 147, 148, 150, 158, 159 
hippocampi, 313, 315, 317-324. 

334, 150, 152, 155, 158-162, 

163, 167, 168 

posterior, 25, 265, 59, 132, 135, 

145 

superior, see C. habenularis 

Corpora albicantia, see C. mammil- 

laria 

mammillaria, 27, 44, 269, 322, 

47, loi, 133, 136, 148, 159 

quadrigemina, 25, 254-261, 47, 

60, 127, 159 
striata, 30. 185, 292-315, 333, 
145-150, 156 
Corpus callosum, 289, 327-331, ^^S, 
342, 166-169 
ectomanMnillare. 261 
geniculatum laterale, 150, 261. 
128, 131, 141 
mediale, 258 
interpedunculare, 277 
restiforme, 239, 243, 126 
Cortex cerebri, 338, 171- 174 
Cortex cerebelli, 239, 121- 123 
Cranial nerves, 18, 49, loi, 51, 63, 

79 

Cr3^tobranchus, 305, 309 
Cutaneous-motor reflex, 82, 41, 49 

52,78 
Cutaneous sensory apparatus, loi, 
105, 124, 253 
general cutaneous centers, no, 
52-60 
components, 105, 49, 50, 

5.1, 63, 79 
endings, 105, 48 
general cutaneous subdivision, 

loi, 104 
special cutaneous subdivision, 
loi, 124 



Ga>£X. 



x>^\> 



Cjdosiomcs 

amsdoo4atenl srsem. ^a 150. 

bnin. i&. 30. 116. 13CX 150. 177. 
1S3. 227, 265. 205. 12, 57, 58, 
94, 100, 105, 145. 146 

czanial noTcs. 23. 105. 100. i^ 

neunl tube. 55 

placxxks. 59. 28 

filial nennes. 13. i 
Decussatio dorsilis teed. 14& 254, 73 

olfactoria, 181. 185. 295. 304. 334. 
loi, 145. 148* 

vdL 229. 112, 124, 125 
Degeneration of nerve nbcrs. 7. 90, 91 
Dendrites. 47. 77. 80. 39 
Development, 34-73 

acustico-lateral system. 58. 33 

cranial ganglia. 49-63. 26-32 

neural tube. 34. 13-16 

olfactory epithelium. 61. 34 

peripheral ner\*es, 49. 13-16. 26 

-33 

Schwann's sheath. 51 

spinal ganglia. 49. 13-16 
Diencephalon, 2^. 44, 253. 269, 131- 

152 
DifiFerentiation of brain cells, 47. 23, 

24,25 
Dipnoi 

brain, 31 

N. terminalis. 31 
Direct cerebellar tract, 245, 247 
Dorso-lateral zone, 49 
Ear, 126 

see also auditor}' apparatus 
Epiphysis, 27, 102, 152, 11, 12, 19, 

36,76 
Epistriatum, 184, 297, 300, 304, 309, 

315. 333^ 145-152, 155, 156 

Equihbration, 127 

Ezdto-glandular impulses and fibers, 

81, 211 
Excito-motor impulses and fibers, 81, 

190, 199, 211 
Eye, 44» i43' 15O' 21, 22, 71, 72, 75 

see also visual apparatus 
Eye muscles 

origin, 64 

innen-ation, 194 

reflexes, 263 
Facial ner\'e, see N. facialis 



Faddi doiaita- 31a. 351. 159^ 160, 

162, 164, 169* 170 
Fascknhis communis. 160. 8x, 92 
Fascacuhis »licarius. i6a 82, 83 
Fasdcuhis longitudinalis medialis^ dor* 
silis or posterior. iq6. 3, 64, 65 
thalamic nudeus of. iq6« 270. 

134 

Fascknhis spino-cerebellaiis ventimlis. 

^3 
Fimbiia. 321, 334. 159, 16a, x67, 

168. 169* 170 
Fissures 

of spinal cord. 12. x 
of cerebellum. 234. x i8» i X9 
Fissuia hippocampi. 319, 331, X59, 

160, 162 
Fissura liiinalis. 317. 159 
Floor {^te. 48 
Food habits. 96 
Foramen of Monro. 28, 295-320, 8, 

145, 150 
Fornix. 314, 322, 334. 161, 164, 166 
Frog. 81 

sensitiveness to light, 143 
Funiculus dorsalis. no. 52-55, 59, 

60 
Functional divisions, 3, 45, 46, 47 
definition of, 95, loi 
fate in mesencephalon and dien- 
cephalon. 285 
longitudinal zones of brain, 16, 
48 
Ganglion cells 

bipolar and unipolar, 62, 35 
Ganglion habenulae, isthmi, etc., 
see Nucleus habenulae, etc. 
Ganglion dliare. 61, 216, 38 
Ganglion Gasseri, 22, 60, 5, 27, 28, 

29, 30, 51, 63, 79 

Ganglion of Ehrenritter, 106, 20, 

32 
Ganglion jugulare, 106. 20, 32 
Ganglion nodosum, 55, 20, 30, 31, 

32 
Ganglion pctrosum, 55, 20, 30, 31, 

Ganglion superior glossopharyngei, 

106, 20, 32 
Ganoids 

acustico-lateral system. 62 
brain, 130, 261, 270, 280, 302, 3, 
loi, 127 



364 



INDEX. 



Ganoids 

bulbus olfactorius, 178, 95, 96, 

97 

cerebellum, 171, 229, 64-68, 113 

""7 

fasdculus londtudinalis media- 
lis. 270, 3, 64, 134 

gustatory paths, 117 

inferior commissure of cerebel- 
lum, 172, 116, 117 

inferior lobes, 270, 23, 133, 134, 

135 

N. terminalis, 31, 106 
nucleus habenulae, 280, 140 
optic chiasma, 146 
saccus vasculosus, 282, 133, 143 
special cutaneous centers, 130, 

64-68 
taste buds, 164 
tectiun mesencephali, 147, 73, 

115, "6 
tuberculum acusticum, 130, 64- 

68 
valvida cerebelli, 171, 115, 116, 
117 
Greneral cutaneous subdivision, loi 
centers, no, 45, 46, 47, 49, 52- 

60 
peripheral components, 105, 50, 

51, 63, 79 
phylogenetic history, 120 
Germinal cells, 46, 13-16 
Giant cells, 37, 17 
Gill clefts, 65, 28, 29, 38 

relation of nerves to, 19, 54, 58 
Glossopharyngeal nerve, see N. glos- 

sopharyngeus 
Granular layer of cerebellum, 132, 

239, 121 
Granule cells 

of cerebellum, 132, 239, 121 
of olfactory bulb, 181, 95, 96 
Gustatory apparatus, 164, 85-92 
Acipenser, 65, 84, 11 3-1 17 
Amia, 165 
Amphibia, 165, 125 
area of cortex, 348, 176, 178, 

180 
bony fishes, 164-171, 85, 88, 

89 

brain centers and paths, 168, 275, 
89, 90, 92, 112-117 

diencephalic centers, 169, 273 



! Gustatoiy apparatus 

inferior gustatory nucleus, 169, 

I 81, 82, 83 

I origin of taste buds, 164 

peripheral fibers, 166, 51, 63, 
79, 88 

I Petromyzon, 164, 86, 87 

relation to olfactory, 187 
selachians, 92, 112, 124 
sense organs, 165, 85, 86, 87 
superior gustatory nucleus, 169, 

89, 92, 112-117 
tactUe correlations, 169 
tertiary tract, 169, 89, 92, 112- 

I 117 

Gyrus subcallosus, 333 
Heptanchus, 28, 2 
I Hippocampus, 307-335, 150-170 
I commissure of, 313-334, 150, 

152, 155, 158-102, 164, 
168 
i Histogenesis, 45, 13-16, 23-27 
I Hyomandibular line of neuromasts, 
59, 51, 62, 63 
Hyomandibular nerve, see R. hyom. 
Hypogastric plexus, 206 
I Hypoglossal nerve, see N. hypo^ossus 
' Hypophysis, 27, 66, 2, 7, 1 1, 36, 37 
Hypothalamus, 27, 269, 11, 133-139, 

145-150 
Indusium falsum, 334 
Indusium verum, 334 
Inferior lobes, 27, 44, 269, 2, 11, 12, 
133-139, 145-150 
j Infraorbital line of neuromasts, 59. 
' 51, 62, 63 

Infraorbital neuromast nerve, 59, 130 
Infundibulum, 27, 
i Isthmus, 44 

I Iter, see Aqueduct of Sylvius 
I Lamina terminalis, 30, 295-330, 145- 
150 
Lateral horn, 200 
I Lateral line nerves, 124, 253, 51, 62, 
63, 70, 79 
Lenmiscus system, 253, 59, 60, 127, 
128, 133, 134 
see also Tr. bulbo-tectalis 
Light, sensitiveness of skin to, 143 
Lobi inferiores, 27, 44, 169, 269, 2, 

11,12,133-139,145-150 
Lobus facialis, 17, 159, 3, 89, 141 
impar, 18 



IXDEX. 



5^5 



Lobus £adafis 

lineae btenlis. 18. 3, 64, 66 
oliactorius. 182. 296-307. 316. 12, 

99. 14S-150 
pyriionnis. 317. yt^ ^^. 159, 

160, 162, 165, 166, 167, 

169 
vagElis. 17. 159- 3t 4* ^ 
visceralis. 17, 159, 3« 4* "1 ^ 

Localization in cerebral conej[. 343. 

175-180 
Mammals 

auditOTT centers. 138. 70 
cerebeUum, 233. 118, 119, 121, 

122, 123. 126 
cerebrum, cortex. ^^I^ 171- 174 

localization in. 343. 175-180 

morphology of, 316. 145-170 
corpora quadrigemina. 253. 47, 

60, 127, 159 
general cutaneous centers, no. 
52, 54. 56, 60 

fibers. 106 
gustator)' fibers. 167 
h>'pothaiamus, 273. 136, 137, 

138 
olfactory centers, 180, 332, 98, 

169/170 
placodes, 55. 31 
>'isual centers, 261, 347, 128, 

130, 131, 179. 180 
Mauthner's fibers, 223 
Mesencephalon 

collicular region. 137, 253, 127, 

129, 130 
correlation centers. 253-261 
general morpholog>% 24, 44 
tectum mesencephali, 117, 137, 

254,2,7, 12, 127 
tectimi opticum, 147, 255. 261. 

73 

telencephalon 

general morphology, 24, 2, 7, 
II, 12 

origin of cerebellimi, 44 
Xiethods, general, 2 

of Flechsig, 344 
\iolecular layer of cerebellum, 240, 

121 
Morphology of the head, 63, 38 
Motor area of cortex, 345, 348 
Mouth. 66 



MuUerian cells. 223 
Musdes 

of eyebalL 64 

somatic. 64 

sub-branchial. 64 

trapezius. 203 

visceral. 66, 200 
MyekncephaloD 

gieneral morphology, 14. 2, 3» 4^ 
II, 12 

origin of cranial nerres. 18. 2, 3» 

7. " 
Xeopalliiun. 33S. 1 71- 180 

e^-olution of. 356 
Ncr\-e components 

general cutaneous. 105. 50^ 51, 

63. 79 

special cutaneous. 128, 51, 62, 

63,79 
general ^■isce^al. 155, 51, 63, 77, 

79 
gustator}-. 166. 51, 63, 79 
somatic motor. 190, 51, 63, 79, 

102 
visceral motor, 200, 51, 63, 79, 

106 
Xer\-e fibers 

origin and growth. 40, 91 
degeneration. 90 
regeneration, 00 
sheath of Schwann, 51 
Xer\'e impulses. 76 

afferent and efferent. 81 
Xer\*e sacs of ganoids, 126 
Ner\'es 

branchial, 19, 5, 51, 63, 79 
development of. 49, 26-34 
motor, 23-25, 81. 190. 199, 45, 

102, 104, 106 
segmental relations, 40, 70, 191. 

38 
sensor)'. i8, 81, 105, 128. 155, 

i76.'45. 49, 77 
shifting of, 70 
spino-occipital. 23, 52, 63, 105, 

190, 20, 30, 31, 32, 104 
Ner\-ous system 

development of, 34, 13-36 

functions of. 76. 95 

functional divisions of, 95, 3, 45, 

46, 47 
general plan of structure, 77. 

95 



366 



INDEX. 



Nervus abducens, 23, 194, 2, 7, 63 
acusticus, 20, 138, 63 
centers, 1 30-1 41, 64-70 
development, 58, 33 
cochlearis 

central connections, 138, 258, 
70 
facialis, 21, 2, 3, 7, 12 
components of, 108, 155, 166, 

200, 5 If 63, 79 
development, 57 
motor nucleus, 202 
relation to auditory sac, 57, 70 
gill cleft, 22, 58 
placodes, 58 
sensory centers, 108, 159, 64, 
90,92 
glossopharyngeus, 20, 2, 3, 7, 12 
components of, 106, 155, 167, 

200, 51, 63, 79 
development, 52-57, 26 
motor nucleus, 200, 107 
relation to gill cleft, 20, 55 
myotomes, 54 
placodes, 55, 31 
sensory centers, 106, 159, 89, 
92 
hypoglossus, 23, 193, 2, 7, 12, 

.104 
lineae lateralis, 20, 3, 7, 12, 51, 

63 

development, 58, 33 

relation to N. vagus, 20, 130 
oculomotorious, 25, 190, 195, 2, 

7, 12, 63, 105 
olfactorius, 28, 31, 177, 292, 2, 
7, 12, 95 

development, 61, 34 
ophthalmicus profundus, 22, 20, 

50,51,63,79 

development, 61 

distribution, no 
opticus, see Tr. opticus 
spinalis accessorius, 20, 203, 20, 
3<>-32 

relation to sympathetic, 207 
terminalis, 31, 61, 106, 302, 336, 

2, 147 

thalamicus, 61, 29 
trigeminus, 22, 2, 5, 7, 12, 50 
components of, 106, 200, 51, 

63,79 

development, 60, 27-30 



Nervus trigeminus 

distribution, 22 

motor nucleus, 202 

relation to mouth deft, 22 

sensory center, 114, 253, 56 
trochlearis, 24, 194, 2, 7, 12, 

63. 124 
vagus, 19, 2, 3. 7, 12 

components of, 106, 157, 166, 
200, 51, 63, 79 

compound of segmental nerves, 

70 
development, 52-57, 28-32 
motor nuclei, 200, 202 
origin, 19, 157, 200 
relation to gill clefts, 19, 54, 28 
myotomes, 54 
placodes, 55, 31 
sensory centers, no, 159, 167 
84, 89, 92 
vestibularis, 138, 70 
Neural crest, 37, 13, 14, 15 
Neural plate and folds, 34, 13 
Neural tube, 35, 14, 15, 16 
Neurilemma, origin of, 51 
Neurite, 47, 77, 91-92 
Neuroblasts, 46, 16 
Neurofibrillae, 88, 92, 44 
Neuroglia, 46 

Neuromasts, 20, 58, 124, 61 
Neuromeres, 38, 44, 18, 19 
Neurones 

arrangement of, 81 
functions of, 76 
fusions between, 87, 92, 44 
polarity of, 77. 91 
trophic relations, 91, 92 
types I and 11, 78, 39 
Neurone theory, 85-93 
Nucleus acustici spinalis, 139 
amygdalae, 322, 333, 
caudatus, ^S3^ 166 
commissuralis, 159, 81, 82, 83 
dentatus, 245, 126 
emboliformis, 247, 126 
funiculi, III 
globosus, 247, 126 
gustus inferior, 169, 81, 82, 83 
gustus superior, 169, 89, 92, 

112-117 
habenulae, 26, 184, 277, 11, loi, 
136, 140, 141, 142, 145- 
150 



INDEX. 



367 



Nucleus 

isthmi, 261 
lentiformis. $^^, 166 
magnocellulans tecti. 148, 73, 

135 

of posterior commissure, 265, 

of fasc. long, medialis, 196, 270, 

134 

olfactorius lateralis. 300, 99, 

145-150 
olfactorius medialis, 299. loi, 

147, 148, 150 
praeopticus. 297. 304. 145, 149, 

150 
praetectalis. 266 
rotundus. 281 
ruber, 243, 246 
tecti. 247. 126 
thaeniae. 183. 296, 300, 303 
ventralis thalami, 258, 128 
Oculomotor ner\*e, see N. oculomo- 

torius 
Olfactor}' apparatus 

bulb, 28. 117, 293, 298, 302, 305, 

316, 2, 7, 12, 94-98 
centers, 182, 292, 94-98, 145- 

170 
decussation, 181, 185, 295, 304, 

334, loi, 145, 148 
epithelium, 31, 61, 176, 34 
^omerulus, 177, 94-97 
granule cells, 181, 95, 96 
lobe, 182, 296-307, 316, 12, 99, 

145-150 
mitral cells, 178, 95 
ner\'e, see N. olfactorius 
relation to gustator)' apparatus, 

t88 
relation to mouth, 176, 28, 37 
Olive, 224 
Operculum, cutaneous ner\'e supply, 

107, 63 
Optic centers, 147, 261, 73, 74, 128, 
130, 131, 141 
prosencephalization, 264 
see also visual apparatus. 
Optic lobes, 24, 147, 254-261, 2, 7, 

12,73,74 
Optic ncr\'es, see Tr. opticus 
Optic thalami, 25, 44, 246, 258-265, \ 

128, 141 
Optic tract, see Tr. opticus I 



Optic ventricle, 254, 127, 134 
Optic vesicle, 44. 143. 150, 18-22, 75 
Pallial tract. seeTr. olfacto-h>potluLla- 

micus lateralis 
Pallium, see archipallium and neo 

pallium, 
of bony fishes, 304. 147, 151 
Paraph>-sis.' 30, 312, 11, 36, 150, 

152, 153 
Parietal eyes, 27, 102, 152, 11, 12. 

19,' 36, 76 
Paraterminal bodies. 306, 163 
Pathways of impulses, 84, 92, 220 
Peripheral nerves 

development of, 49, 13-16, 26- 

33 

Pineal eyes, 27, 102, 152, 11, 12, 19, 
36,76 

Pit organs, 58, 124 
Pituitar)' body, 27 
Placodes 

dorso-lateral, 58, 28, 33 

epibranchial, 55, 28, 31 
Plexuses 

in palate. 42, 43 

of Auerbach, 206 

of Meissner, 206 
Polarity 

of ner\*ous system, 81 

of neurones, 77, 39 
Pons, 24. 224, 243, 245 
Posterior commissure, 25, 265, 59, 

132, 135, 145 

Preauditory pit, 59 

Precommissural body, 317, 330, 157- 

169 
Protopterus, 106, 288 
Psalterium, 313, 315, 317-324. 334. 

150, 152, 155, 158-163, 

167, 168 
Pulvinar, 261 
Purkinje cells, i33-i37» 239. 67, 68, 

121 
P>Tamidal tract, 349 
Pyriform lobe, 317, 323, 3?^, 159, 

160, 162, 165, 106, 167, 

169 
Rami communicantes, 14, 156, 207, 

45, 77, 108 
relation to N. spinalis access., 207 
Ramus buccalis, 21, 59, 130, 51, 62, 

63,79 
hyoideus, 22, 58, 79 



368 INDEX. 

Ramus ' Selachians 

hyomandibularis, 22, 59, 130, . acustico-lateral system, 58, 125 



31, 62, 63 

lingualis IX, 20, 159, 63, 79 
lingualis V, 168 



brain, 18, 24, 28, 130. 133, 171, 
185, 228, 261, 266, 298, 2, 8, 
9, II, 18,66,92, 112, 124, 
mandibularis V, 22, 106, 51, 63, 127, 147 

79 I general cutaneous fibers, 105. 106 

maiallaris. 22, 106, no, 51, 63, | gustatory centers and tracts, 171- 

79 I 173^92 

ophthalmicus superficialis V, no. \ olfactory centers, 177. 185, 298, 

51, 63 I 147 

ophthalmicus superficialis VII. posterior commissure, 266 

130, 51, 63 I spino-occipital nerves, 53, 2 

palatinus IX, 20, 5 1 Sensations, 345, 353 

palatinus VII, 22, 157, 5, 63, ' somatic and visceral, loi, 354, 

79 355 

pharyngeus IX, 20, 55, 159 Sense organs 

pharyngeus X, 20, 55. 159 1 auditory, 126 

posttrematicus X, 20, 55, 5, 5i» gustatory, 164, 85, 86, 87 

63, 79 I olfactory, 31, 61, 176, 34 

posttrematicus IX, 20, 55, 5, 51, tactile, 105, 48 

63, 79 i visual, 143, 152, 71, 72 

posttrematicus VII, 22, 5, 51, Senso-motorareaof cortex, 345, 175- 

63, 79 I 180 

general cutaneous component. Sensory impulses and fibers. 81 

108, 51 ' Septum pellucidimi, 334, 168, 169 

praetrematicus X, 20, 55, 5, 63, ■ Sheath of Schwann, 51 

79 ( Shifting of organs, 69, 72 
praetrematicus IX, 20, 55, 5, 63, Shoulder girdle, 203 

79 Solar plexus, 206 

praetrematicus VII, 22, 5, 63 ' Soma, the, 96 

Recessus praeopticus, 45, 295, 11, , Somaesthetic area of cortex, 346, 175- 
145, 147, 148, 150 180 

neuroporicus, 295, 11, 145, 150 1 Somatic motor division, loi, 45, 46, 
superior, 319, 152, 159, 161, 47, 102 

162 column in brain and cord, 98, 190 

Reflexes, 81-85 ' impulses coming to. 196 

Reflex arc, 83, 41 j nerves, 190. 51, 63 

Regeneration of nerve fibers, 90 Somatic sensory division, see general 

Regio uncinata, 299, 8, 9 1 and special cutaneous sub- 

Reissner*s fiber, 148 divisions 

Relationships of nervous system, i, 9 ' Somatic sensor}' column, 18, 97, 45, 
Retina, 145, 71, 72 46, 47 

centrifugal fibers, 148, 265 fate in mesencephalon and dien- 

evolution of, 151 | cephalon, 286 

Roof-plate, 48 Somatic pallium, 293, 317, 325, 335, 

Root ganglion of glossopharyngeus, 1 ^^S, 171- 180 

106, 20, 32 Somites, mesodermic, 64, 38 

Saccus vasculosus, 27, 282, 7, 11, I Special cutaneous subdivision, 102 

143-148 centers, 130, 64-70 

Sections, planes of, 10 function, 127 

Segmentation, ^S, 63, 38 peripheral ner\'es. 20, 58,128, 51, 

relations of nerves, 49, 70, 191 ; 62, 63, 79 



INDEX. 



369 



Special cutaneous subdivision 

phylogenetic history, 127 

sense organs, 124, 61 
Spinal cord, 12, i 

central canal, 12 

dorsal tracts, no, 52-55, 59, 60 

longitudinal zones, 48 

neuromeres, 38 

white and gray matter, 13 
Spinal ner\'es, dorsal, 13, i 

components, loi, 105, 155, 200, 

45 

Spinal ner\'es, ventral, 13, 190, 200, 

1,45 

Spino-occipital nerves, 23, 52, 63, 105, 
190, 20, 30, 31, 32, 104 

Spongioblasts, 46 
Stimulus, 76 
Striae Lancisii, 334 
Stria medullaris, 322, ^^^ 

see Tr. olfacto-habenularis 
Striatum, see Corpus striatum 
Subiculum comu Ammonis, 348, 170 
Substantia gelatinosa, 115, 56 
Substantia reticularis, 219 
Supraorbital neuromasts and nerve, 

59, 130,51,62,63 
Sympathetic system, 206, 108, 109, 
no 

axone-reflex, 214, iio 

chain ganglia, 206 

development, 206 

efferent fibers, 210 

excitatory cells, 211 

peripheral ganglia, 206 

prevertebral ganglia, 206 

rami communicantes, 14, 156, 
207, 45, 77, 106, 108 

relation to visceral divisions, 214- 
216 

sensory cells, 213, 108 

sensory fibers, 208, 77, 108 
Tactile impulses, 105, 155 
Tacto-motor collaterals, 82, no, 197, 

41,52 
Taenia semicircularis, 324, 166 
Taenia thalami, 322, ^^^, loi, 145- 

150 
Taste buds, 164, 85, 86, 87 

distinct from neuromasts, 165 

distribution, 22, 164 

origin of, 164 

phylogenetic histor}% 173 

24 



Tectum opticum, 147, 255, 261, 73, 74 
Tela chonoidea, 28, 305, 2, 145-151 
Telencephalon, 28, 2, 7, 8, 11, 12, 

145-170 
evolution of, 292 
Tertiary gustatory tract, 169, 89, 92, 

112-117 
Tract of Gowers, 243 
Tractus bulbo-tectaUs, 117, 59, 112- 

117, 124, 125, 133, 134 
see also Lemniscus system 
Tractus cortico-habenularis, 323, 152, 

164 
Tractus gustus secimdus, 169, 89, 90, 

92, 112-117 
Tractus gustus tertius, 169, 89, 90, 

92, 112-117 
Tractus habenulo-peduncularis, 184, 

277,101,133,134,136,140, 

142, 145-150 
Tractus lobo-bulbaris, 171, 89, 92, 

loi, 116, 133 
Tractus lobo-epistriaticus, 297, 92, 

145, 150 
Tractus mammillo-bulbaris (pedun- 

cularis), 275, 92, loi, 136 
Tractus mammillo-thalamicus, 275, 

136 
Tractus olfactorius, 182, 295, 2, loi, 

145-150, 157 
Tractus olfactorius septi, 307, 150, 

155 

Tractus olfacto-corticalis, 301, 307, 

147, 150 
Tractus olfacto-habenularis, 184, 297 

ff., loi, 145-150, 164 
Tractus olfacto-hypothalamicus, 184, 
297 
lateralis, 300, 303, 307, 10 1, 

147, 150 
medialis, 275, 300, 303, 307. 
loi, 137, 145-150, 164 
Tractus olivo-cerebellaris, 243 
Tractus opticus, 28, 70, 128, 135, 
141 
centers of, 147, 261, 73, 74, 

128, 130, 131, 141 
centrifugal fibers, 148, 265, 72 
chiasma of, 28, 146 
Tractus saqco-thalamicus, 282, 134, 

Tractus spinalis trigemini, 114, 56, 
59, 60, 64 



370 



INDEX. 



Tractus spino-cerebellaris dorso-later- 

alis, 245, 247 
Tractus spino-cerebellaris ventralis, 

243 
Tractus strio-thalamicus, 185, 297 ff., 

loi, 134, 135, 145-150 

Tractus tecto-bulbaris et spinalis, 117, 

148. 59, 65, 133 
Tractus tecto-cerebellaris, 148, 228, 

229, 127, 133, 134, 135 
Tractus tecto-lobaris, 148, 134 
Tractus thalamo-spinalis, 245, 281, 

148 
Tractus thalamo-saccus, 282, 134, 

135, 143, 144 

Trapezius musculature, 203 
Trigeminal nerve, see N. trigeminus 
Trochlear nerve, see N. trochlearis 
Tuber cinereimi, 275, 137, 138 
Tuberculum acusticum, 18, 3, 11, 

58, 59, 64-69, 84, 118 
general cutaneous center, 115 ff. 
motor connections, 135 
special cutaneous center, 130 
Tuberculum olfactorium, 317, 333, 

159, 160, 164, 167 
Vagus nerve, see N. vagus 
Valvula cerebelli, 24, 172, 89, 91, 116 
Velum meduUare anterius, 24, 172, 

112, 118, 124 
Velum transversum, 316, 1 1, 36, 153, 

1S8 



Ventricles, 15, 24, 28, 8, 9, 10, 11 

lateral, 28, 295 ff. 
Ventro-lateral zone, 49 
Vesicles of Savi, 60, 125 
Viscera, the, 96 

Visceral motor division, 98, 199, 45, 
46, 106 
relations to sjrmpathetic, 208, 
210, 215 
Visceral sensory division, 97, 155, 45, 
46, 47, 77 
relation to S3rmpathetic, 208, 216 
Visceral sensory column, 97, 155, 3, 
11,45,46,77-84,89,92 
diencephalic representatives of, 

188, 269-281, 133-142 
intrinsic neurones, 161 
secondary tract, 163, 169, 89, 

92 

stimuli, nature of, 155 
Viscero-motor connections. 204 
Visual apparatus, 27, 102, 143, 261 

centers, 147, 261, 73, 74, 128, 
130, 131, 141 

eye, 44, 143, 150, 21, 22, 71, 72, 

75 

optic tract, 28, 146, 148, 265, 70, 

128, 135, 141 
perception, 263 
reflexes, 264 
visual area of cortex, 347, 175- 

180 



Stanford University Liiiranr 

Stanford, California 



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