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Full text of "The Anatomy of the nervous system from the standpoint of development and function"

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THE ANATOMY 

of the 

NERVOUS SYSTEM 

FROM THE STANDPOINT OF DEVELOPMENT AND FUNCTION 



By 

STEPHEN WALTER RANSON, M. D., Ph. D. 

Professor of Anatomy in Northwestern University Medical School, Chicago 



WITH 260 ILLUSTRATIONS 
SOME OF THEM IN COLORS 



PHILADELPHIA AND LONDON 

W. B. SAUNDERS COMPANY 

1921 



Copyright, 1920, by W. B. Saunders Company 



Reprinted April, 1921 



PRINTED IN AMERICA 

PRESS OF 

W. B. SAUNDERS COMPANY 
PHILADELPHIA 



PREFACE 



IN the pages which follow the anatomy of the nervous system has been pre- 
sented from the dynamic rather than the static point of view; that is to say, 
emphasis has been laid on the developmental and functional significance of struc- 
ture. The student is led at the very beginning of his neurologic studies to think 
of the nervous system in its relation to the rest of the living organism. Struc- 
tural details, which when considered by themselves are dull and tiresome, become 
interesting when their functional significance is made obvious. This method of 
presentation makes more easy the correlation of the various neurologic courses 
in the medical curriculum. For physiologic and clinical neurology a knowledge 
of conduction pathways and functional localization is essential, and this informa- 
tion can best be acquired in connection with the course in anatomic neurology. 
In selecting the material to be included in this book the needs of the medical 
student have been kept constantly in mind, and emphasis has been placed on 
those phases of the subject which the student is most likely to find of value to 
him in his subsequent work. 

In many laboratories the head of the shark and the brain of the sheep have 
been used to supplement human material. The book has been so arranged as to 
facilitate such comparative studies without making it any the less well adapted 
to courses where only human material is used. 

During the past twenty years very considerable additions have been made to 
the science of neurology, and the more important of these have been included in 
the text. While a detailed presentation of the evidence concerning new or dis- 
puted points would be out of place in a book of this kind, whenever the state- 
ments made here differ from those found in other texts the authority has always 
been cited, the author's name and the date of his contribution being given in 
parentheses. A full list of these references to the literature has been included in 
a Bibliography at the end of the volume. 

The terminology adopted is that of the B. N. A., which has been used, for 
the most part, in its English form. But in the case of the fiber tracts the Basle 



12 PREFACE 

terms are often misleading, and wherever this is the case, other names have been 
substituted. 

An outline for a laboratory course in neuro-anatomy has been included, and 
this has been so arranged as to be easily adapted by the instructor to his par- 
ticular needs. 

Free use has been made of material gathered and arranged by others in the 
various handbooks, texts, and atlases that deal with the nervous system. The 
classification of the afferent paths and centers adopted here is based on the 
work of Sherrington. The terms which he introduced and which are now coming 
into general use have been employed. In the analysis of the cranial nerves the 
American conception of nerve components, so ably presented by Herrick, has 
been utilized. 

Illustrations have been borrowed from many sources, in each case duly 
accredited, and our indebtedness for permission to use them is gladly acknowl- 
edged. The majority of the figures have been made from drawings prepared for 
this purpose by Miss M. E. Bakehouse. The large number of illustrations and 
the excellent manner in which they have been reproduced is to be credited to the 
generous policy of the publishers, W. B. Saunders Co. My thanks are due to 
Dr. Olaf Larsell for reading the manuscript and for many valuable suggestions, 
and to Mr. Michael Mason for assistance in reading the proof. 

S. W. RANSON. 

CHICAGO, ILL. 



CONTENTS 



CHAPTER I PAGE 

ORIGIN AND FUNCTION OF THE NERVOUS SYSTEM 17 

The Diffuse Nervous System of Ccelenterates 19 

The Central Nervous System 20 

CHAPTER II 

THE NEURAL TUBE AND ITS DERIVATIVES 24 

The Brain of the Dogfish 26 

Development of the Neural Tube in the Human Embryo 31 

CHAPTER III 

HlSTOGENESIS OF THE NERVOUS SYSTEM 37 

Development of the Neuron 37 

Development of the Spinal Nerves 40 

Differentiation of the Spinal Cord 42 

CHAPTER IV 

NEURONS AND NEURON-CHAINS 43 

Form and Structure of Neurons 43 

Interrelation of Neurons 49 

The Neuron as a Trophic Unit 51 

The Neuron Concept 52 

Neuron Chains 53 

CHAPTER V 

THE SPINAL NERVES 56 

Metamerism 58 

Functional Classification of Nerve-fibers : 60 

The Spinal Ganglia 62 

Somatic Sensory Fibers and Nerve Endings 66 

CHAPTER VI 

THE SPINAL CORD 73 

External Form and Topography 73 

The Spinal Cord in Section 78 

Microscopic Anatomy 85 

The Spinal Reflex Mechanism 91 

CHAPTER VII 

FIBER TRACTS OF THE SPINAL CORD 95 

Intramedullary Course of the Dorsal Root Fibers 95 

Afferent Paths in the Spinal Cord 98 

Ascending and Descending Degeneration in the Spinal Cord 105 

Long Descending Tracts of the Spinal Cord 108 

CHAPTER VIII 

GENERAL TOPOGRAPHY OF THE BRAIN 113 

Anatomy of the Medulla Oblongata 118 

Anatomy of the Pons 123 

The Fourth Ventricle 125 

The Mesencephalon 129 

13 



14 CONTENTS 

CHAPTER IX PAGE 

THE STRUCTURE OF THE MEDULLA OBLONGATA 132 

The Rearrangement Within the Medulla Oblongata of the Structures Continued Upward 

from the Spinal Cord 133 

Decussation of the Pyramids 136 

Nucleus Gracilis, Nucleus Cuneatus, and Medial Lemniscus 137 

Olivary Nuclei 141 

Restiform Body 143 

Formatio Reticularis 144 

CHAPTER X 

INTERNAL STRUCTURE OF THE PONS 147 

Basilar Part of the POPS 147 

Dorsal Part of the Pons 149 

CHAPTER XI 

INTERNAL STRUCTURE OF THE MESENCEPHALON 158 

Tegmentum 158 

Basis Pedunculi 164 

Corpora Quadrigemina 165 

CHAPTER XII 

THE CRANIAL NERVES AND THEIR NUCLEI 168 

Somatic Efferent Column of Nuclei 170 

Special Visceral Efferent Column of Nuclei 174 

General Visceral Efferent Column of Nuclei 177 

Visceral Afferent Column 180 

General Somatic Afferent Nuclei 182 

Special Somatic Afferent Nuclei 185 

Summary of the Origin and Composition of the Cranial Nerves 190 

CHAPTER XIII 

THE CEREBELLUM 195 

Development 195 

Anatomy 196 

Morphology 199 

Nuclei of the Cerebellum 203 

Cerebellar Peduncles 204 

Histology of the Cerebellar Cortex 206 

Efferent Cerebellar Tracts 211 

CHAPTER XIV 

THE DlENCEPHALON AND OPTIC NERVE 213 

Thalamus 213 

Epithalamus and Metathalamus 220 

Hypothalamus 222 

Third Ventricle 223 

Visual Apparatus 225 

CHAPTER XV 

EXTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES 229 

Development of the Cerebral Hemispheres 229 

The Dorsolateral Surface 232 

The Medial and Basal Surfaces . . . 238 



CONTENTS 15 

CHAPTER XVI PAGE 

INTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES 243 

Corpus Callosum 243 

Lateral Ventricles 246 

Basal Ganglia of the Telencephalon : 252 

Internal Capsule 257 

Connections of the Corpus Striatum and Thalamus 262 

CHAPTER XVII 

THE RHINENCEPHALON 265 

Parts Seen on the Basal Surface of the Brain 265 

Hippocampus 269 

Fornix 270 

Anterior Commissure 273 

Structure and Connections of the Several Parts of the Rhinencephalon 274 

Olfactory Pathways 280 

CHAPTER XVIII 

THE CORTEX AND MEDULLARY CENTER OF THE CEREBRAL HEMISPHERE 283 

Structure of the Cerebral Cortex 283 

Cortical Areas 287 

Localization of Cortical Functions 290 

The Medullary Center of the Cerebral Hemisphere 296 

CHAPTER XIX 

THE GREAT AFFERENT SYSTEMS 302 

Exteroceptive Pathways to the Cerebral Cortex 302 

Spinal Path for Touch and Pressure 303 

Spinal Path for Pain and Temperature Sensations 306 

Secondary Trigeminal Paths 307 

Neural Mechanism for Hearing 309 

Neural Mechanism for Sight 310 

Proprioceptive Pathways 311 

Spinal Proprioceptive Paths (Muscle Sense) 311 

Cerebellar Connections of Vestibular Nerve 314 

CHAPTER XX 

EFFERENT PATHS AND REFLEX ARCS 316 

The Great Motor Path 317 

The Cortico-ponto-cerebellar Path 325 

The Cerebello-rubro-spinal Path 326 

Important Reflex Arcs 327 

CHAPTER XXI 

THE SYMPATHETIC NERVOUS SYSTEM 334 

Fundamental Facts Concerning Visceral Innervation 335 

Structure of the Sympathetic Ganglia 341 

Composition of Sympathetic Nerves and Plexuses 345 

Architecture of the Sympathetic Nervous System 346 

Important Conduction Paths Belonging to the Autonomic Nervous System 352 

A LABORATORY OUTLINE OF NEURO-ANATOMY 355 

BIBLIOGRAPHY . . 375 



INDEX . . 383 



THE ANATOMY OF THE NERVOUS SYSTEM 
FROM THE STANDPOINT OF DEVELOP- 
MENT AND FUNCTION 



CHAPTER I 

THE ORIGIN AND FUNCTION OF THE NERVOUS SYSTEM 

IRRITABILITY and conductivity, which, as every biological student knows, 
are two of the fundamental properties of protoplasm, reach their maximum 
development in the highly differentiated tissue of the nervous system. Indeed, 
it is in response to the need for increased sensitiveness to stimuli and for better 
transmission of the impulses aroused by them that the nervous system has 
developed and been perfected in the long process of evolution which has cul- 
minated in man. 

When an ameba is touched with a pointed glass rod it moves away from 
the source of stimulation. Changes are initiated in the superficial protoplasm 
which are transmitted through the unicellular organism, resulting in a flowing 
out of pseudopodia on the opposite side. Through a continuation of this stream- 
ing motion the entire organism moves forward. Thus the relatively undif- 
ferentiated living substance of which it is composed receives the stimulus, 
transmits the resulting disturbance, and carries out the appropriate response. 

When in the place of unicellular organisms we study simple metazoa, the 
sea-anemones for example, we find that considerable differentiation has occurred 
among the component cells. A cuticle has formed, designed to protect the 
subjacent parts from the action of the surrounding objects, while other cells 
have differentiated in the direction of contractile elements or muscle cells. 
Because the general body surface has been adapted to cope with the environ- 
ment it becomes necessary to have certain cells at the surface which are sensi- 
tive to environmental changes. These sensory elements are able to transmit 
the waves of activation developed in them directly to the subjacent muscle 
cells. But in higher animals, because of the large size of the body and the 

2 17 



i8 



THE NERVOUS SYSTEM 



complicated reactions required, long lines of communication have been estab- 
lished between peripheral sense organs and muscle-fibers in widely separated 
parts of the body. 

The sensory elements and the lines of communication constitute the nervous 
system and, together with the musculature, the neuromuscular mechanism. 
It is well to keep in mind the fact that the nervous system was developed for the 
purpose of enabling the musculature to react to changes in the environment of 
the organism. But in all higher animals the nervous system responds not only 
to stimuli from without but also to stimuli from within the body, and helps to 





Itl 



Fig. 1. Stages in the differentiation of the neuromuscular mechanism: A to C, Hypothetic 
early stages: A, epithelial stage; B, muscle cell at the stage of the sponge; C, partially differen- 
tiated nerve-cell in proximity to fully differentiated muscle-cell; D, nerve- and muscle-cell of 
coelenterate stage; E, a type of receptor-effector system found in many parts of sea-anemones, in- 
cluding not only receptors, r, with their nerve-nets, and of muscle cells, w, but also of ganglion 
cells, g, in the nerve-net; F, section at right angles to the sphincter of the bell of a jellyfish (Rhizos- 
toma): e, epithelium of the subumbrellar surface; n, nervous layer; w, muscle layer. (Parker.) 

bring about an internal adjustment of part with part. Here again it acts as a 
sensitive mechanism for receiving stimuli and conducting them to the appro- 
priate organs of response. These organs through which the nervous system 
produces its effects are known as effectors. While muscles and glands are by 
far the most important effectors, we must also include certain pigmented cells 
(or chromatophores) and electric and phosporescent organs under this heading. 
Except for the reactions produced through such effectors the nervous system 
would be meaningless. 

We can best understand the significance of the nervous system if we trace 
its early history. This, as it has been interpreted by Parker (1919), makes an 



THE ORIGIN AND FUNCTION OF THE NERVOUS SYSTEM 



interesting story. According to this author contractile tissue develops before 
any trace of the nervous system appears. In sponges, which are devoid of 
nervous elements, the oscula open and close in response to appropriate stimuli. 
These movements are brought about by a contractile tissue not unlike smooth 
muscle. The active element or effector is thus the first to make its appearance, 
and at this stage is brought into action by direct stimulation. Next in the order 
of development is the sensory cell, derived from the epithelium in the neigh- 
borhood of an effector, and specially differentiated to receive stimuli and trans- 
mit them to the underlying muscle (Fig. 1, D). This stage of development is 
reached by such ccelenterates as the sea-anemones. The advantage which 
these forms derive from the specialized sensory cells or receptors is seen in the 
character of their responses, which are more rapid than those of sponges. Such 



Cerebral ganglion- 

Esophageai connective --- 
Pharynx 

Ventral nerve cord --^ 





.... Cerebral ganglion 
-- Pharynx 
Esophageai connective 

- Ventral nerve cord 



A B 

Fig. 2. Anterior portion of the nervous system of the earthworm: A, Lateral view; B, dorsal view. 

a sensory cell may be compared to a percussion cap through which a charge of 
powder is ignited. 

But ccelenterates usually present a more complex arrangement of receptor 
and effector elements than that indicated in Fig. 1, D. Fine branches from the 
sensory cells anastomose with each other and form a nervous net within which 
are scattered nerve-cells. Such a nerve net is seen in many parts of sea-ane- 
mones (Fig. 1, E) and is well developed in the jellyfish (Fig. 1, F). It seems 
capable of conveying nerve impulses coming from the sensory cells in all direc- 
tions through the bell-shaped body of the jellyfish and to muscle-fibers far dis- 
tant from the receptors involved. The conduction of nerve impulses from 
receptors to effectors seems to occur diffusely through the net not in stated 
directions nor along fixed paths. In this respect the diffuse nervous system of 
the ccelenterates is in contrast with the more centralized system in the worms. 



20 



THE NERVOUS SYSTEM 



The sensory cells are not so directly connected with muscle-fibers hi the 
worms as in the sea-anemones, for between receptor and effector there is here 
interposed a central nemous system. This system, as it appears in the earth- 
worm, is illustrated in Fig. 2. It consists of a cerebral ganglion dorsal to the 
buccal cavity and a row of ventrally placed ganglia bound together by a ventral 
nerve cord. The most anterior of the ventral series of ganglia is connected to 
the dorsal one by nerve strands on either side of the esophagus. The ganglia 
of the ventral cord are placed so that one occurs in each body segment, and 
from each three pairs of nerves run to the skin and muscles of that segment. 
The arrangement of the constituent elements can best be studied in transverse 
sections (Fig. 3). The sensory cells are located in the skin, and from each of 
them a fiber runs along one of the nerves into the ganglion, within which it 
branches, helping to form a network known as the neuropil. Within each 




Fig. 3. Transverse section of the ventral chain and surrounding structures of an earthworm: 
cm, Circular muscles; ep, epidermis; Im, longitudinal muscles; me, motor cell-body; mf, motor 
nerve-fiber; sc, sensory cell-body; sf, sensory nerve-fiber; vg, ventral ganglion. (Parker.) 

ganglion are found large nerve-cells from which fibers run through the nerves 
to the segmental musculature. Here we have the necessary parts for the sim- 
plest reflex arc. Stimulation of the sensory cell causes nerve impulses to travel 
through its fiber to the neuropil, thence to a motor cell, and finally along a proc- 
ess of the latter to the muscle. In other words, we have a receptor, conductor, 
center, another conductor, and finally an effector; and all this is for the purpose 
of bringing the muscle-fiber under the influence of such environmental changes 
as are able to stimulate the sensitive receptor. 

In addition to the primary sensory and motor elements just enumerated the 
ganglia contain nerve-cells the fibers of which run from one ganglion to another 
and serve to associate these in co-ordinated activity. These internuncial ele- 
ments serve to establish functional connections among the different parts of 
the ganglionated nerve cord that constitutes the central nervous apparatus; 



THE ORIGIN AND FUNCTION OF THE NERVOUS SYSTEM 21 

and they lie entirely within this central organ. The slow waves of contraction 
that pass from head to tail as the worm creeps forward may be advanced from 
segment to segment by such internuncial or association elements. 

The nervous system of the earthworm differs from that of the ccelenterate 
in many ways, but the fundamental difference is one of centralization. In the 
former the greater part of it has separated from the skin and become con- 
centrated in a series of interconnected ganglia which serve as a central nervvts 
system. These ganglia receive nerve-fibers, coming from the sense organs, and 
give off others, going to the muscles; and the fibers are brought together and 
grouped into nerves for convenience of passage. The neuropil within a ganglion 
offers a variety of pathways to each incoming impulse which may accordingly 
find its \vay out along one or more of several motor fibers. The spreading of 
nerve impulses through the chain of ganglia is facilitated by the presence of the 
association fibers already mentioned. Nevertheless, conduction is not diffuse 
as in the nerve net of the medusa, but occurs along definite and more or less 
restricted lines. This is well illustrated by the experiment cited by Parker: 
"If an earthworm that is creeping forward over a smooth surface is suddenly 
cut in two near the middle, the anterior portion will move onward without much 
disturbance, whereas the posterior part will wriggle as though in convulsions. 
This reaction, which can be repeatedly obtained on even fragments of worms, 
shows that a single cut involves a stimulation which in a posterior direction 
gives rise to a wholly different form of response to what it does anteriorly; in 
other words, transmission in the nerve cord of the worm is specialized as com- 
pared with transmission in the nervous net of the ccelenterate." In the gan- 
glionated cord of the earthworm, as here described, we find many of the features 
characteristic of the central nervous system of higher forms. 

The vertebrate nervous system has much in common with that of the earth- 
worm. The central nervous system of the annelid is split off from the ectoderm 
by a process of delamination, as will be seen by comparing the ventral nervous 
cord of the marine worm, Sigalion, with that of the earthworm (Figs. 3, 4). 
Through a comparable process of infolding of the ectoderm to form a neural 
tube there is developed the central nervous system of the vertebrate (Fig. 6). 
The dorsal position of the neural tube in vertebrates as compared with the 
ventral position of the solid nerve cord of the annelid offers some difficulty and 
has led to ingenious theories in explanation of their phylogenetic relationship, 
theories which we need not consider here (Gaskell, 1908). In primitive chor- 
dates, such as the amphioxus, we already have a simple, dorsally placed, neural 



22 



THE NERVOUS SYSTEM 



tube associated with segmental nerves. In true vertebrates the anterior end of 
the neural tube becomes irregularly enlarged to form the brain, while the pos- 
terior end remains less highly but more uniformly developed and forms the 
spinal cord. 

The primary motor nerve-cells of vertebrates resemble very closely those of 
invertebrates in being located within the central nervous system and in send- 
ing motor nerve-fibers to the muscles (Fig. 31). The primary sensory cells lie 
outside the central system, as in invertebrates. Those for smell are located in 
the olfactory epithelium. But all others have migrated centrally along the 
sensory fibers, and now send one process toward the periphery and another into 



bra 




Fig. 4. Transverse section of the ventral nervous cord of Sigalion: bm, Basement mem- 
brane; c, cuticula; e, epidermis; gc, ganglion-cells; n, nerve-fibers and neuropil; s, space occupied 
by vacuolated supporting tissue. (Parker, Hatschek.) 

the central system. The relative positions of these cells in the annelid, mollusc, 
and vertebrate are illustrated in Fig. 5. In the latter the sensory cells are aggre- 
gated into masses known as the cerebrospinal ganglia, which are associated 
with peripheral nerves and are usually placed near the point of origin of these 
nerves from the brain or spinal cord. A comparison of Figs. 3 and 31 will show 
a striking similarity between the simple reflex arc in the earthworm and in man. 
If space permitted we might trace the development of the central nervous sys- 
tem in some detail, but perhaps enough has been given to suggest that the 
nervous system of man represents the culmination of a long process of evolu- 
tion which began with a simple sensory mechanism like that of the sea-anemones. 
We shall be concerned with a study of the vertebrate nervous system, almost 



THE ORIGIN AND FUNCTION OF THE NERVOUS SYSTEM 



2 3 



exclusively with that of the mammal, and more particularly with that of man. 
In man we are so accustomed to think of the nervous system as the organ and 
agent of the mind that its true physiologic position is often forgotten. In this 
introductory chapter we have attempted to show that the primary function of 
the nervous system is to receive stimuli arising from changes in the environment 
or within the organism, and to transmit these to effectors which bring about 
the adjustments necessary for life. Biologically speaking, the nervous system 
is not to be regarded as an intelligence bureau, which gathers information for 




CO- 



1 

FZ 


1 

> 


F 

h- 
X' 





Fig. 5. Peripheral sensory neurons of various animals: A, Oligochaetic worms (Lumbricus); 
B, polychaetic worms (Nereis); C, molluscs (Limax); D, vertebrates. The figure illustrates the 
gradual change in the position of the sensory cells in the phylogenetic series: e, Epithelial cells of 
sensory surface; c, cuticula; sz, cell-body of peripheral sensory neuron; rm, rete Malpighii of epi- 
dermis; sn, axon; co, central nervous system. (Barker, Retzius.) 

a sovereign mind, enthroned within the brain, nor yet as a chief executive officer 
to carry out that sovereign's decrees. Sensory impulses from many sources 
reach the brain, where they pass back and forth through a multitude of asso- 
ciation paths, augmenting or inhibiting each other before they finally break 
through into motor paths. Previous experience of the individual, having left 
its trace in the organization of the central nervous system, alters the character 
of the present reactions. It is in connection with the neural activity involved 
in these complex associational processes that consciousness appears shall I 
say as a by-product? at least as a parallel phenomenon. 



CHAPTER II 



THE NEURAL TUBE AND ITS DERIVATIVES 

Infolding of the Neural Tube. The vertebrate nervous system develops 
from a thickened plate of ectoderm along the middorsal line of the embryo. 
By the infolding of this neural plate there is formed the neural groove, which 
becomes transformed into the neural tube (Fig. 6). The neural tube detaches 
itself from the superficial ectoderm and gives rise through a thickening of its 
walls to the brain and spinal cord. The latter is formed by a process of uniform 



Neural groove Neural plate 



Neural groove Neural plate 




Ectoderm Neural groove 




Neural tube 




Neural tube 



D 



Neural cavity 



Fig. 6. Development of the neural tube in human embryos (Prentiss-Arey): A, An early embryo 
(Keibel) ; B, at 2 mm. (Graf Spec) ; C, at 2 mm. (Mall) ; D, at 2.7 mm. (Kollmann). 

thickening in the walls of the caudal portion of the tube. The derivatives of 
the rostral part are well illustrated in the accompanying diagram (Fig. 7). 

Brain Vesicles. At an early stage in the development of any vertebrate 
embryo the rostral portion of the neural tube is distinguished from the caudal 
part by the more rapid development of the former, its walls bulging outward 
to form three bulb-like swellings or vesicles, which together represent the brain, 
and are named from before backward, the prosencephalon, mesencephalon, and 
24 



THE NEURAL TUBE AND ITS DERIVATIVES 25 

rhombencephalon (Fig. 7). The more rostral vesicle becomes subdivided by a 
constriction into the telencephalon and diencephalon (Fig. 7, B, C). The rhom- 
bencephalon is less sharply subdivided into a rostral part, which includes the 
cerebellum, and is known as the metencephalon, and a more caudal portion, the 
myelencephalon. The optic nerves and retinae, not illustrated in the figure, 
develop as paired evaginations from the prosencephalon. 

The Cerebral Hemispheres. The telencephalon includes a thickened portion 
of the ventrolateral wall loosely designated as the corpus striatum or, since there 




Fig. 7. Diagrams illustrating the development of the vertebrate brain: A, First stage, side 
view, the cavity indicated by dotted line; B, second stage; C, third stage, side view of a brain with- 
out cerebral hemispheres; D, the same in sagittal section; E, fourth stage, side view of a brain with 
cerebral hemispheres; F, the same in sagittal section; G, dorsal view of the same with the cavities 
exposed on the right side. Rhin., rhinocoele; Lot. Vent., lateral ventricle; Int. For., interventricu- 
lar foramen; Vent. Ill, third ventricle; Vent. IV, fourth ventricle. /, Prosencephalon; / a, Telen- 
cephalon; I a-r, Rhinencephalon ; I a-p, Pallium; / a-lt, Lamina terminalis; / a-ch, Cerebral 
hemisphere; i a-cs, Corpus striatum; i b, Diencephalon; / b-t, Thalamus. 2, Mesencephalon ; 2c, 
Optic lobes; 2 d, Crura cerebri. j, Rhombencephalon; j a, Metencephalon; 3 a-c, Cerebellum; 
3 b, Myelencephalon. 

is one of these on either side, the corpora striata (Fig. 7, D). Another part of 
the wall is relatively thin and is known as the pallium, while the part directly 
associated with the olfactory nerve belongs to the rhinencephalon. The most 
important factor in the evolution of the vertebrate brain is the progressive evag- 
ination of the lateral walls of the telencephalon to form paired masses, the 
cerebral hemispheres. In primitive forms like the cyclostomes only a part of the 
rhinencephalon has been evaginated, and in them the hemisphere consists only 
of an olfactory bulb and olfactory lobe. This stage of development is roughly 



2 6 THE NERVOUS SYSTEM 

indicated in Fig. 7, C, D. In the selachians, as illustrated in Figs. 8, 9, 10, 
and 11, the evagination has progressed further than in cyclostomes. Still further 
progress in this direction has been made by the. amphibians, the cerebral hemi- 
spheres of which have reached about the stage of development indicated in Fig. 
7, E, F, G. Here the entire lateral wall, including the pallium and corpus 
striatum, has been evaginated in the formation of the cerebral hemisphere. 

The Brain Ventricles. The portions of the original cavity of the neural tube 
which are contained within the evaginated cerebral hemispheres are known as 
the lateral ventricles (Fig. 7, G). These paired ventricles communicate with the 
median prosencephalic cavity by openings known as the interventricular foram- 
ina. This median cavity, called the third ventricle, represents for the most 
part the cavity of the diencephalon, but its rostral part, bounded by the lamina 
terminalis, belongs to the telencephalon. It will be seen by a study of the 
accompanying diagrams that this lamina also belongs to the telencephalon and 
represents in a certain sense the rostral end of the brain. Its position should 
be carefully noted in each of the diagrams. The cavity of the rhombencephalon 
is known as the fourth ventricle and that of the mesencephalon as the cerebral 
aqueduct. The latter connects the third and fourth ventricles. It will help us 
to understand the morphology of the vertebrate brain if we now consider the 
shape and arrangement of the various parts of a simple brain like that of the 

dogfish. 

THE BRAIN OF THE DOGFISH SQUALUS ACANTHIAS 

The telencephalon of the selachian brain is evaginated to form a pair of 
laterally placed masses, the cerebral hemispheres, and in this respect is at a stage 
of development not far removed from that represented in diagrams E, F, and G 
of Fig. 7. The long axis of the brain is almost straight; and this freedom from 
ventrodorsal curvatures makes it especially easy to recognize the various funda- 
mental divisions already enumerated and to understand their relationship. 

The medulla oblongata, which together with the cerebellum forms the rhom- 
bencephalon, is continuous at the caudal extremity with the cylindric spinal 
cord, and within it the central canal of the spinal cord opens out into the fourth 
ventricle (Fig. 8). The medulla, which has somewhat the shape of a trun- 
cated cone, is considerably larger than the cord, but decreases in size as it is 
traced backward toward their point of junction. In the mammal a conspicuous 
transverse bundle of fibers, associated with the cerebellum, is found on the 
ventral and lateral aspects of that part of the medulla which belongs to the 
metencephalon and is known as the pons. But in the fish it is customary to 



THE NEURAL TUBE AND ITS DERIVATIVES 



consider the medulla oblongata as extending from the spinal cord to the mesen- 
cephalon. It forms the ventral and lateral walls of the fourth ventricle; and 
when the roof of this cavity has been removed these walls are seen to surround 
a long and rather broad depression the fossa rhomboidea or floor of the fourth 
ventricle which tapers caudally like the point of a pen (Fig. 9). 

The cerebellum forms an elongated mass the rostral end of which overhangs 
the optic lobes, while the caudal extremity projects over the medulla oblongata 




/'Nasal capsule 



---O'factory bulb 

/ -Nervus terminalis 

..Olfactory tract 



/ Olfactory nerve N. I 
' / Rhinoccele 



, Lateral ventricle 



! ' 



Cerebral hemisphere. 

Interventricular for 
Epiphysis 
-Optic nerve N. II 

Thalamus- 

Optic lobes- 

-Trochlear nerve N. Ill 




-Cerebellum 

Lobus linecR lateralis 

Facial nerve N. VIL 

Acoustic nerve N. VIII. 

Tuberculum acusticunt 

Medulla oblongata- 

Glossopharyngeal nerve N. IX 

Medial longitudinal fasc 

Visceral lobe.__ 



-Vagtts nerve N. X. 



-Spinal cord.- 




&? Telencephalon 



-- - - -;- Third ventricle 
) Diencephalon 

< 
---- '-Mesoccele 

> Mesencephalon 



f Metencephalon 
-j- Cerebellum 



(caudal part) 

i^ -I- Rhomboid fossa 
V Myelencephalon 



Fig. 8. The brain of the dogfish, 
Squalus acanthias, dorsal view. 



Fig. 9. The brain of the dogfish, 
Squalus acanthias, with the ventricles 
opened, dorsal view. 



(Fig. 8). Its dorsal surface is grooved by a pair of sulci arranged in the form 
of a cross. It contains a cavity, a part of the original rhombencephalic vesicle, 
which communicates with the fourth ventricle proper through a rather wide 
opening (Fig. 11). Behind the cerebellum the fourth ventricle possesses a thin 
membranous roof which was torn away in the preparation from which Fig. 8 
was drawn. 



28 



THE NERVOUS SYSTEM 



Mesencephalon. The optic lobes on the dorsal aspect of the mesencephalon 
are a pair of rounded masses separated by a median sagittal sulcus. They 
represent the bulging roof of the mesencephalic cavity and are accordingly 

Cerebellum. ... 
Optic lobe 



Thalamus 

Cerebral hemisphere 




Olfactory bulb 



v 
Vagus nerve N. X / 

Glossopharyngeal nerve N. IX ' 

Acoustic nerve N. VIII / 
Abducens nerve N. VI 



Olfactory tract 



> ! Optic nerve N. II 

\ Inferior lobe 
Oculomotor nerve N. Ill 
Saccus vasculosus 



Trigeminal and facial nerves Nn. V, VII TroMear nene N _ IV 

Fig. 10. The brain of the dogfish, Squalus acanthias, lateral view. 



spoken of as the tectum mesencephali. Within this roof end the fibers which 
come from the retinae through the optic nerves. The floor of the cavity is formed 
by the ventral part of the mesencephalon. This appears like a direct continua- 
tion of the medulla oblongata, and in the mammal bears the designation crura 



Paraphysis 

Cerebral hemisphere j | 
Olfactory tract 
Olfactory bulb 



Optic lobe 

Epiphysis . Mesoccde 

Cerebellum 



Metacode 



Tuberculum acusticum 
. Tela chorioidea 

Fourth ventricle 
Visceral lobe 




Telencephalon ( ! 

Preoptic recess 
Velum transversum 



\ Metencephalon Myelencephalon 
Saccus vasculosus 



Mesencephalon 
Optic chiasma Third ventric i e 

Fig. 11. The brain of the dogfish, Squalus acanthias, medial sagittal section. 

cerebri. Emerging from the roof of the mesencephalon between the cerebellum 
and optic lobe is the fourth or trochlear nerve, and from the ventral aspect of 
this division of the brain arises the third or oculomotor nerve. 

The Diencephalon. The thin roof of the diencephalon, which can easily 



THE NEURAL TUBE AND ITS DERIVATIVES 



2 9 



be torn away so as to expose the third ventricle (Figs. 8, 9), is attached by its 
caudal margin to a ridge containing a pair of knob-like thickenings, the habe- 
nular nuclei and a commissure connecting the two (Fig. 11). From a point 
just caudal to the middle of this commissure there projects forward over the 
membranous roof of the ventricle a slender tube, the epiphysis cerebri or pineal 
body, which comes in contact with the roof of the skull and ends in a slightly 
dilated extremity. The epiphysis and habenular nuclei belong to the epithala- 
mus. The thalamus forms the thick lateral wall of the third ventricle and is 
traversed by the optic tracts on their way to the optic lobes. The hypothalamus 



Nasal sac 



Epiphysis 
Superior oblique 

Trochlear nerve 
Medial rectus 
Superior rectus 
Lateral rectus 
Vestibule 

Spiracle- 

Semicircular canal 
Glossopharyngeal nerve 

Vagus - 
Branchial cleft i 




Superficial ophthalmic V, VII 
Olfactory capsule 

Inferior oblique 

Maxillary V 
Mandibular V 
Palatine VII 

Spiracle 

Hyomandibular VII 
Glossopharyngeal 

i. Branchial cleft 
Vagus 



Spinal cord Lateral line branch of vagus 

Fig. 12. Dissection of the brain and cranial nerves of the dogfish, Scyllium catulus. The 
eye is shown on the left side, but has been removed on the right. (Marshall and Hurst, Parker 
and Haswell.) 

is relatively large in the shark and presents, in addition to a pair of laterally 
placed oval masses, or inferior lobes, a thin walled vascular outgrowth, the saccus 
vasculosus. Closely related to the ventral aspect of the hypothalamus is a gland- 
ular mass, derived by a process of evagination from the oral epithelium, and 
known as the hypophysis. For a picture of this structure in the adult dogfish 
reference should be made to a paper on the subject by Baumgartner (1915). 
On the ventral surface of the hypothalamus the optic nerves meet and cross in 
the optic chiasma. 

The telencephalon includes all of the brain in front of the velum transfer sum, 



3 



THE NERVOUS SYSTEM 



a transverse fold projecting into the third ventricle from the membranous roof 
(Fig. 11), and consists of a median unpaired portion, and of the two cerebral 
hemispheres with their olfactory bulbs. The hemispheres are the evaginated 
portions of the telencephalon and are partially separated from each other by a 



Olfactory bulb 

Olfactory nerve 

(n.I) 
Somatic area 




r. ophthal. superfic. V 
r. ophthal. superfic. VII 

n. terminalis 

r. ophthal. profundus V 

Optic nerve (n. II) 



r. maxillaris V 
r. mandib. V 

Supra-orbital trunk 

Infra-orbital trunk 
Ganglion V 
r. palatinus VII 
Gang, geniculi VII 
Gang, later. VII 
r. prespirac. VII 

Spiracle 

r. hyomandib. VII 

n. IX 
n. X 

r. lateralis X 

r. branchialis X 
r. intestinalis X 



Fig. 13. Diagram of the brain and sensory nerves of the smooth dogfish, Mustelus canis, 
from above. Natural size. The Roman numerals refer to the cranial nerves The olfactory 
part of the brain is dotted, the visual centers are shaded with oblique cross-hatching, the acoustico- 
lateral centers with horizontal lines, the visceral sensory area with vertical lines, and the general 
cutaneous area is left unshaded. On the right side the lateral line nerves are drawn in black, the 
other nerves are unshaded. (From Herrick's Introduction to Neurology.) 

median sagittal fissure, which has been to a large extent obliterated by the 
fusion of their median walls. The shape of the lateral ventricle and the position 
of the interoentricular foramina are shown in Fig. 9. From the lateral side of 
the rostral end of the hemisphere there projects forward the long and slender 
olfactory tract with a terminal enlargement, the olfactory bulb. This lies in 



THE NEURAL TUBE AND ITS DERIVATIVES 3 1 

contact with the nasal sac to which it gives off a number of fine nerve bundles, 
which together constitute the olfactory or first cranial nerve. At the rostral end 
of the brain an additional nerve makes its exit from the hemisphere. It is 
known as the nervus terminalis and can be followed forward over the olfactory 
tract and bulb to the nasal sac (Fig. 8). 

The roof of the selachian forebrain presents a number ^structures of great morphologic 
interest, two of which have already been mentioned, namely, the epiphysis and velum 
transversum. The former is an outpocketing of the roof of the diencephalon; the latter 
is an infolding and marks the line of separation between the two divisions of the prosenceph- 
alon. Rostral to the velum the roof of the telencephalon is evaginated to form a thin-walled 
sac, the paraphysis. The velum and paraphysis are readily identified in the mammalian 
embryo, but become obscured in the course of later development. The morphology of this 
region has recently been studied in great detail by a number of American investigators: 
Minot (1901), Johnston (1909), Terry (1910), Warren (1911, 1917), and Bailey (1916). 

A good idea of the shape and connections of the various brain ventricles and 
of the relation of the various parts of the brain to each other can be obtained 
from a study of Figs. 9 and 11. In Fig. 13 there is indicated the location of the 
principal sensory areas of the brain of the smooth dogfish, and the relation of 
these areas to the corresponding peripheral nerves is apparent. The lateral 
line components of the seventh and tenth cranial nerves are indicated in black. 

DEVELOPMENT OF THE NEURAL TUBE IN THE HUMAN EMBRYO 

In its embryonic development the nervous system of man presents some- 
thing like a synopsis of the early chapters of its phyletic history. The neural 
groove is the most conspicuous part of an embryo of 2.4 mm. (Fig. 14). Near 
the middle of the body it has closed to form the neural tube, and from this 
region the closure proceeds in both directions. The last points to close are 
situated at either end and are known as the neuropores. The rostral end of the 
groove shows enlargements which upon clpsure will form the brain vesicles. 
The longer portion, caudal to these enlargements, represents the future spinal 
cord. Except that it is flexed on itself, the brain of the human embryo of Jive 
weeks (Fig. 15) shows a marked resemblance to the diagram of a vertebrate 
brain without cerebral hemispheres (Fig. 7, C, D). The prosencephalic vesicle 
is divided by a constriction into the telencephalon and diencephalon with freely 
intercommunicating cavities. The mesencephalon is well denned and presents 
a sharp bend, the cephalic flexure. The rhombencephalon shows signs of sepa- 
ration into the metencephalon and myelencephalon and is slightly bent dorsally 
at the pontine flexure. Another curvature which develops at the junction of 



THE NERVOUS SYSTEM 






the brain and spinal cord is known as the cervical flexure (Fig. 16). From 
the walls of the prosencephalon there develop outpocketings on either side, 
which form the optic cups and which are connected to the brain by the optic 
stalks. From the cup develops the retina and through the stalk grow the 
fibers of the optic nerve. These structures are, therefore, genetically parts of 
the brain. 

The Telencephalon of the Human Embryo. By the time the embryo has 
reached a length of 13 mm. the brain has passed into the stage represented by 



Mesencephalon 

Rhombenccphalon 
Myelencephalon 

Amnion (cut) 




Mesodermal segment 14 
Open neural groove 

Body stalk 
Fig. 14. Human embryo of 2.4 mm. showing the neural tube partially closed. (Kollmann.) 

diagrams E, F, G of Fig. 7. The lateral wall of the telencephalon, with the 
corpus striatum and olfactory brain or rhinencephalon, has been evaginated on 
either side to form paired structures, the cerebral hemispheres (Fig. 16). Ex- 
cept for the corpus striatum and rhinencephalon the evaginated wall is relatively 
thin, develops into the cerebral cortex, and is known as the pallium. The 
lateral ventricles within the hemispheres represent portions of the original telen- 
cephalic cavity and communicate with the third ventricle through the inter- 



THE NEURAL TUBE AND ITS DERIVATIVES 



33 



ventricular foramina, which at this stage are relatively large. The lamina 
terminalis, connecting the two hemispheres in front of the third ventricle, repre- 



Diencephalon 



Pallium 



Mesencephalon 



Cephalic 
flexure 



D 

Thalamus 




Pallium 



\Iesencephalon 



Optii 
cup 
Ponline flexure j 



Myelencephalon ? 



Meten- 
cephalon 
Corpus striatum. 
Optic recess 

Hypoihalamus 




Medulla oblongata 



Fig. 15. Reconstructions of the brain of a 7 mm. embryo: A, Lateral view; B, in median sagittal 

section. (His, Prentiss-Arey.) 

sents in a certain sense the rostral end of the brain. Immediately behind this 
lamina is a portion of the telencephalic cavity which forms the anterior part of 



Cerebral peduncle 
Hypothalamus .^ 
Epithalamus \ ^aJL 
Thalamnf; 
Diencephalon- r 



Pallium^ 
Telencephalon- -' 




Cerebral aqueduct 
[.^Mesencephalon 

^^,RhombencepJialic isthmus 



Cerebellum 
Metencephalon 
Rhomboid fossa 



^Myelencephalon 



1234 > 
Rhinencephalon | Corpus striatum Pans 
Lamina terminalis 




Spinal cord 



Fig. 16. A median section of the brain of a 13.6 mm. human embryo: 1, Optic recess; 2, ridge 
formed by optic chiasma; 3, optic chiasma; 4, infundibular recess. (His, Sobotta.) 

the third ventricle. The further development of these structures is readily 
traced in Fig. 17, which represents the brain of a human fetus of the third 

3 



34 



THE NERVOUS SYSTEM 



month. The most striking feature of the brain at this stage is the great size 
attained by the cerebral hemispheres. 

The Diencephalon. The three principal divisions of the diencephalon 
the thalamus, epithalamus, and hypothalamus faintly indicated in an embryo 



Diencephalon, 
Chorioid plexus 

Corpus striatum 
Telencephalon / 



Thalamus 

I Pineal body (epithalamus) 
Cerebral peduncle 
Cerebral aqueduct 
" Mesencephalon 




'-Isthmus 
'*- Cerebellum 
~ Metencephalon 
Rhomboid fossa 
Myelencephalon 



: Optic Hypo- 
j chiasma, physis Medulla 

Lamina terminalis / "Hypothalamus blon z ata 
Rhinencephalon 



' Spinal cord 
Central canal 



Fig. 17. The brain of a fetus of the third month in median sagittal section. (His, Sobotta.) 

of 13.6 mm., are well denned by the third month (Fig. 17). In transverse 
sections this division of the embryonic brain is seen to be composed of a pair of 
plates on either side, which with a roof and floor form the walls of the ventricle 

Roof plate (with chorioid plexus) 

Alar plate or Thalamus 

Sulcus limitans 
Basal plate or Hypothalamus 

"Mammillary recess 

Fig. 18. Transverse section through the diencephalon of a 13.8 mm. embryo. (His, Prentiss- 

Arey.) 

(Fig. 18). The dorsal lamina is known as the alar plate, the ventral as the basal 
plate. On either side these meet at an angle, forming the sulcus limitans. These 
laminae and the sulcus limitans between them can be traced back through the 




THE NEURAL TUBE AND ITS DERIVATIVES 



35 



mesencephalon and rhombencephalon into the spinal cord. The thalamus is 
produced by a thickening in the alar lamina and is separated from the hypo- 
thalamus by the sulcus limitans, which can be traced as far as the optic recess 
rostral to the ridge produced by the optic chiasma. 

The hypothalamus 1 represents the basal lamina and gives rise to the tuber 
cinereum, posterior lobe of the hypophysis, and the mammillary bodies. From the 
dorsal edge of the alar lamina, where this is attached to the thin roof plate, there 
is developed a thickened ridge, the epithalamus, which is transformed into the 
habenula and the pineal body. The roof plate of the diencephalon remains 
thin and forms the epithelial lining of the tela chorioidea or roof of the third 
ventricle. 

The Mesencephalon. The basal plate of the mesencephalon thickens to 
form the cerebral peduncles (Fig. 17), the alar plate forms the lamina quad- 
rigemina in which are differentiated the quadrigeminal bodies; the cavity be- 
comes the cerebral aqueduct. 

TABLE SHOWING SUBDIVISIONS OF THE NEURAL TUBE AND THEIR DERIVATIVES (Modified from a 
Table in Keibel and Mall, Human Embryology). 





Primary vehicles. 


Subdivisions. 


Derivatives. 


Lumen. 






( 

Telencephalon. . . { 


Cerebral cortex, 
Corpora striata, 
Rhinencephalon, 


Lateral ventricles. 
Rostral portion of 
the third ventricle 






( 


Pars-optica hypo- 
thalami. 






Prosencephalon. 


Diencephalon ^ 


Epithalamus, 
Thalamus, 
Hypothalamus, 
Hypophysis, 


The greater part of 
the third ventricle. 


Brain 






Tuber cinereum, 
Mammillary bodies, 
Metathalamus. 






Mesencephalon 


Mesencephalon . . . . < 


Corpora quadri- 
gemina, 
Crura cerebri. 


Cerebral aqueduct. 




Rhombencephalon < 


Metencephalon < 
Myelencephalon 


Cerebellum, 
Pons, 
Medulla oblongata.j 


Fourth ventricle. 


Spinal cord 






Spinal cord. 


Central canal. 



1 The pars optica hypothalami, including the optic chiasm, is, properly speaking, not a part 
of the hypothalamus at all, but belongs to the telencephalon (Johnston, 1909, Jour. Comp. Neur , 
vol. 19, and 1912, Jour. Comp. Neur., vol. 22). 



36 THE NERVOUS SYSTEM 

The Rhombencephalon. The ventral part of the rhombencephalon, includ- 
ing both alar and basal plates, thickens to form the pans and medulla oblongata 
(Fig. 17). Most of the roof of this division remains thin and forms the epithelial 
lining of the tela chorioidea of the fourth ventricle. But in the caudal portion 
of the myelencephalon the lumen of the neural tube becomes completely sur- 
rounded by thickened walls, forming the central canal of the closed portion of 
the medulla. The posterior edge of the alar plate in the metencephalon becomes 
greatly thickened and, fusing across the median line with the similar structure 
of the opposite side, forms the anlage of the cerebellum (Figs. 17, 137). Later 
we shall see that, in general, motor structures develop from the basal, and sen- 
sory parts from the alar, plate. 

The table on page 35 gives in brief the principal derivatives of the 
neural tube. 



CHAPTER III 

HISTOGENESIS OF THE NERVOUS SYSTEM 

Early Stages in the Differentiation of the Neural Tube. Hardesty (1904) 
has given a good account of the early development of the spinal cord in the pig. 
At first the neural plate consists of a single layer of ectodermal cells (Fig. 19, A). 
These proliferate and lose their cell boundaries. When the neural tube has 
closed its wall is formed of several layers of fused cells a syncytium bounded 
by an external and an internal limiting membrane (Fig. 19, B, C). The syn- 
cytium now becomes more open and sponge-like in structure. The nuclei are 
so arranged that three layers may be differentiated: (1) an ependymal layer, 
(2) a mantle layer, with many nuclei, and (3) a marginal or non-nuclear layer. 
The ependymal layer is represented by a row of elongated nuclei, among which 
are found the large mitotic nuclei of the germinal cells. 

These germinal cells divide and give rise to ependymal cells, and to the indif- 
ferent cells of the mantle layer. Through division of the latter spongioblasts 
and neuroUasts are formed. From the former comes the neuroglia or supporting 
tissue of the nervous system, while from the latter are derived the nerve-cells 
and fibers. 

The Development of the Neuron. A neuron may be defined as a nerve- 
cell with all its processes; and each is derived from a single neuroblast. From 
the pear-shaped neuroblast a single primary process grows out, and this be- 
comes the axis-cylinder of a nerve-fiber (Fig. 20). Other processes which de- 
velop later become the dendrites. The primary process, or axon, grows into the 
marginal layer, within which it may turn and run parallel to the long axis of the 
neural tube as an association fiber; or it may run out of the neural tube in a ven- 
trolateral direction as a motor axon. In this way the motor fibers of the cere- 
brospinal nerves are laid down. The axis-cylinder of each represents a process 
which has grown out from a neuroblast in the basal plate of the neural tube. 

Development of Afferent Neurons. The sensory or afferent fibers of the 
spinal nerves take origin from neuroblasts which are from the beginning out- 
side the neural tube. These neuroblasts are derived from the neural crest, a 
longitudinal ridge of ectodermal cells at the margin of the neural groove, where 
this becomes continuous with the superficial ectoderm. At first in contact with 

37 



THE NERVOUS SYSTEM 



the dorsal surface of the neural tube, the neural crest soon separates from it 
and comes to lie in the angle between it and the myotomes. In this position 
the neural crest gives rise to a series of sensory ganglia. From neuroblasts 
located in these ganglia arise the sensory fibers of the cerebrospinal nerves. 



Marginal layer Mantle layer Ependymal layer 

' I 




.Germinal 
cell 



Marginal layer Ependymal layer 
Mesoderm Marginal layer 




\Germinal 
cell 

S^p 

Internal limiting membrane 

Ependymal layer 

9 




^Germinal 



1 >' J cell 



External limiting membrane Mantle layer Internal limiting membrane 



External limiting membrane 

ST 1 




Germinal cell Internal limiting membrane 



Mesoderm Marginal layer 



Mantle layer 



Ependymal layer 



Fig. 19. Early stages in the differentiation of the neural tube: A, From a rabbit embryo 
before closure of neural tube; B, from a 5 mm. pig embryo after closure of tube; C, from a 7 mm. 
pig embryo; D, from a 10 mm. pig embryo. *, Boundary between nuclear and marginal layers. 
(Hardesty, Prentiss-Arey.) 

This last statement requires some qualification. The fibers of the olfactory nerve 
arise from cells in the olfactory mucous membrane. The fibers of the mesencephalic root 
of the trigeminal nerve, which in all probability are sensory, arise from cells located within 
the mesencephalon. The optic nerve is also an exception, but this is morphologically a 
fiber tract of the brain and not a true nerve. An ingenious theory, advanced by Schulte and 
Tilney (1915), attempts to bring this mesencephalic root and the optic nerve into more ob- 



HISTOGENESIS OF THE NERVOUS SYSTEM 



39 



vious relation with the other sensory nerves. They assume that the part of the neural crest, 
which lies rostral to the anlage of the semilunar ganglion, fails to separate from the neural 
tube. From this part of the neural crest, retained within the brain, they would derive the 
mesencephalic nucleus of the trigeminal nerve and the optic vesicles. 

On the other hand, there are observations which tend to show that some of the cranial 
sensory ganglia are derived at least in part from other sources than the neural crest. This 
is especially true of the acoustic ganglion (Streeter, 1912). According to Landacre (1910) 
many of the sensory ganglion cells of the seventh, ninth, and tenth nerves are derived from 





Fig. 20. A, Transverse section through the spinal cord of a chick embryo of the third day 
showing neuraxons (F) developing from neuroblasts of the neural tube and from the bipolar 
ganglion cells, d. B, Neuroblasts from the spinal cord of a seventy-two-hour chick. The three to 
the right show neurofibrils; C, incremental cone. (Cajal, Prentiss-Arey.) 

thickened patches of the superficial ectoderm, known as placodes, with which the ganglia of 
these nerves come in contact at an early stage in their embryonic development. The 
acoustic ganglion of the eighth nerve seems also to have a similar origin, i. e., from the cells 
of the otic vesicle which is formed by a process of invagination from the superficial ectoderm. 

The neuroblasts of these ganglia become bipolar through the development 
of a primary process at either end (Fig. 21). Originally bipolar, a majority of 
these sensory neurons in the mammal become unipolar through the fusion of 
the two primary processes for some distance into a single main stem. Beyond 
the point of fusion this divides like a T into two primary branches, one of which 



THE NERVOUS SYSTEM 



is directed centrally, the other peripherally. The centrally directed branch 
grows into the neural tube as a sensory root fiber (Fig. 20, A, d) ; the other grows 
peripherally as an afferent fiber of a cerebrospinal nerve. This general state- 
ment requires some qualification. It may be that some bipolar neuroblasts 
become unipolar by the absorption of one of the primary processes, while the 
remaining one divides dichotomously into central and peripheral branches 
(Streeter, 1912). It should also be noted that the cells of the sensory ganglia 

of the acoustic nerve remain bipolar 
throughout life. 

Development of the Spinal Nerves. 
We have traced the development 
of the chief elements entering into 
the formation of the cerebrospinal 
nerves, and will now see how these 
are combined in a typical spinal nerve. 
The spinal ganglion, derived from 
the neural crest, contains bipolar 
neuroblasts, which are transformed 
into unipolar neurons. The axon of 
such a nerve-cell divides into a cen- 
tral branch, running through the 
dorsal root into the spinal cord, and 
a peripheral branch, running distally 
through the nerve to reach the skin 
or other sensitive portion of the body. 
Mingled with these afferent fibers in 
the spinal nerves are efferent axons which have grown out from neuroblasts in 
the basal plate of the spinal cord, througiiJJie ventral root, and are distributed 
by way of the spinal nerve to muscles. 

So far we have dealt only with the origin of the axis-cylinders of the nerve- 
fibers. But these soon become surrounded by protective sheaths which are also 
ectodermal in origin. In the path of the outgrowing axons there are seen nu- 
merous spindle-shaped ectodermal cells, which have migrated from the anlage 
of the spinal ganglia (Harrison, 1906), and perhaps also from the neural tube 
along the ventral roots (Held, 1909). These cells form such a prominent feat- 
ure in a developing nerve that some workers have thought the axons differen- 
tiate in situ from them. This theory, which has been known as the cell-chain 




Fig. 21. A section of a spinal ganglion from 
a 44 mm. fetus, showing stages in the trans- 
formation of bipolar neurons, A, into unipolar 
neurons, B. Golgi method. (Cajal.) 



HISTOGENESIS OF THE NERVOUS SYSTEM 41 

hypothesis, and gives to each axon a multicellular origin, has been supported by 
Schwann, Balfour, Dohrn, and Bethe, and in modified forms by other workers. 
There are good reasons, however, for believing that each axon arises as an out- 
growth from a single cell or neuroblast. This idea, which is in keeping with 
what is known of the structure and function of the neuron and which forms an 
integral part of the now generally accepted neuron theory, was first developed in 
the embryologic publications of His. Convincing experimental evidence has 
been furnished by Harrison (1906). Using amphibian larvae, this author showed 
that if the neural crest and tube are removed no peripheral nerves develop. 
He further showed that isolated nerve-cells cultivated in clotted lymph will 



Roof plate 



Dorsal column 

Dorsal root 

Mantle layer 

Ventral column 

Ependymal layer 




Dorsal Juniculus 



Neural cavity 



Marginal layer 



Floor plate Ventral median fissure 

Fig. 22. Transverse section of the spinal cord of a 20 mm. human embryo. (Prentiss-Arey.) 

give rise to long axons in the course of a few hours. But the ectodermal cells, 
mentioned above, which migrate outward along the course of the developing 
nerve, take an important part in the differentiation of the fibers. From them 
is derived the nucleated sheath or neurilemma of the peripheral nerve-fiber. 
The myelin sheath is composed of a fatty substance of uncertain origin. It 
may be a product of the axon, of the neurilemma, or of both. 

The sympathetic ganglia consist of cells derived like those of the spinal 
ganglia from the neural crest, and, according to Kuntz (1910), also from the 
neural tube by migration along the course of the cerebrospinal nerves. These 
cells become aggregated in the ganglia of the sympathetic system and are asso- 
ciated with the innervation of smooth muscle and glands. 



42 THE NERVOUS SYSTEM 

The spinal cord of a 20 mm. human embryo presents well-defined ependymal, 
marginal, and mantle layers. Figure 22 should be compared with the appear- 
ance presented by a cross-section of the spinal cord in the adult (Fig. 55). The 
mantle layer with its many nuclei differentiates into the gray matter of the spinal 
cord, which contains the nerve-cells and their dendritic processes. The mar- 
ginal layer develops into the white substance as a result of the growth into it 
of the axons from neuroblasts located within the mantle layer. These form 
association fibers which ascend or descend through the marginal layer and serve 
to connect one level of the neural tube with another. It is not until these 
longitudinally coursing axons develop myelin sheaths that the white substance 
acquires its characteristic coloration. 

The cavity of the neural tube is relatively large, and at the point marked 
"neural cavity" in Fig. 22 a groove is visible. This is the sulcus limitans. It 
separates the dorsal or alar plate from the ventral or basal plate. The mantle 
layer of the alar plate develops into the dorsal gray column which, like the other 
parts developed from this plate, is afferent in function. The afferent fibers, 
growing into the spinal cord from the spinal ganglia, either terminate in this 
dorsal column or ascend in the posterior part of the marginal zone to nuclei 
derived from the alar plate in the myelencephalon. Most of the association 
fibers which run in the marginal layer have grown out from neuroblasts located 
in the dorsal column. The mantle layer of the basal plate gives rise to the 
ventral gray column. From the neuroblasts in this region grow out the motor 
fibers of the ventral roots and spinal nerves. 

From what has been said it will be clear that the entire nervous system is 
ectodermal in origin. The nervous element proper or neurons are derived from 
the neuroblasts; the supporting tissue of the brain and spinal cord, the neuroglia, 
is derived from spongioblasts ; while the neurilemma of the peripheral nerves is 
the product of sheath cells which have migrated out from the spinal ganglia 
and possibly also from the neural tube. 



CHAPTER IV 

NEURONS AND NEURON-CHAINS 

THE nervous system is composed of highly irritable cellular units, or neurons, 
linked together to form conduction pathways. In the preceding chapter we 
have seen that each neuron is the product of a single embryonic cell or neuro- 
blast, and that, therefore, the nerve-cell with all its processes constitutes a gen- 
etic unit. In the present chapter, as we examine the form and internal struc- 
ture of the neurons and their relation to each other, we shall learn that they are 
also the structural and functional units of the nervous system. 

Form. There is the widest possible variation in the shape of nerve-cells, 
but all present some features in common. About the nucleus there is an accumu- 
lation of cytoplasm which together with the nucleus forms what is often called 
the cell body. A convenient term by which to designate the circumnuclear 
cytoplasmic mass is perikaryon. From the perikaryon cytoplasmic processes 
are given off, some of which may be of great length. The external form of 
the neuron depends on the shape of the perikaryon and on the number, shape, 
and ramification of these processes. Since the variety of forms is almost with- 
out limit, we will content ourselves with studying a few typical examples. 

The pyramidal cells of the cerebral cortex are good examples (Fig. 23). The 
perikaryon is triangular in form. One angle, that directed toward the surface 
of the cortex, is prolonged in the form of a long thick branching process, the 
apical dendrite. From the sides and other angles of the perikaryon arise shorter 
branching dendrites, while from the base or from one of the basal dendrites 
arises a long slender process, the axon. The characteristic features of the den- 
drites are as follows: they branch repeatedly, rapidly decrease in size, and 
terminate not far from the cell body. Their contour is irregular and they are 
studded with short side branches, or gemmules, which give them a spiny appear- 
ance. Each neuron usually possesses several dendrites, but in some types of 
nerve-cells they are absent altogether. The axon, on the other hand, is char- 
acterized by its uniform smooth contour, relatively small diameter, and in most 
instances by its great length and relative freedom from side branches. It may 
give off fine side branches, or collaterals, near its origin; and these arise at right 

43 



44 



THE NERVOUS SYSTEM 



angles to the parent stem. The axon 
terminates in a multitude of fine branches 
usually at a considerable distance and 
sometimes as much as a meter from its 
origin. The origin of the axon from the 
perikaryon is marked by an expansion 
known as the cone of origin or im- 
plantation cone. This cone, like the 
axon, differs somewhat in structure from 
the perikaryon. Such long axons as 
have just been described are character- 
istic of the cells of Golgi's Type I. 

That not all axons are long and 
relatively unbranched is seen from Fig. 
24, which illustrates a cell of Golgi's 
Type II. The axons of these cells are 
short, branch repeatedly, and end in the 
neighborhood of the cell body. 

Another good example is furnished 
by the primary motor neurons. Figure 
25 illustrates such a cell from the anterior 
gray column of the spinal cord. This 
is a large nerve-cell with many rather 
long branching dendrites and an axon, 
which forms the axis-cylinder of a motor 
nerve-fiber and terminates by forming 
a motor ending in a muscle. As illus- 
trated in this figure, long axons tend to 
acquire myelin sheaths, and those 
which run in the cerebrospinal nerves 
are also covered by a nucleated mem- 
branous sheath the neurilemma. 

Nerve-cells with many processes, 
such as have just been described, are 
called multipolar. Examples of unipo- 
lar and bipolar cells are furnished by the 
cerebrospinal ganglia (Fig. 40). These cells, which will be described in more 




Fig. 23. A pyramidal cell from the cere- 
bral cortex of a mouse : a, Dendrites from the 
base of the cell; b, white substance of the 
hemisphere into which the axon, e, can be 
traced; c, collaterals from the first part of the 
axon; /, apical dendrite; p, its terminal 
branches near the surface of the cortex. 
Golgi method. (Cajal.) 



NEURONS AND NEURON-CHAINS 



45 



detail in another chapter, are devoid of dendrites. The axon of such a unipolar 
cell divides dichotomously into a central and a peripheral branch, each possess- 
ing the characteristics of an axon. 

It is not uncommon to regard the peripheral branch of a sensory neuron as a dendrite, 
because like the dendrites it conducts nerve impulses toward the cell body. But, since it 
possesses all the morphologic characteristics of an axon, and since any axon is able to con- 
duct nerve impulses throughout its length in either direction, and since these peripheral 
branches of the sensory neurons actually convey impulses distally in the phenomenon of 




Fig. 24. Neurons with short axons (Type II of Golgi) from the cerebral cortex of a child: a, 

Axon. Golgi method. (Cajal.) 

antidromic conduction (Bayliss, General Physiology, p. 474), it seems best to consider both 
central and peripheral branches as divisions of a common axonic stem. (See Barker, The 
Nervous System, p. 361.) 

From what has been said it will be apparent that a neuron usually possesses 
several dendrites and a single axon, but some have only one process, which is 
then an axon. It may be added that some neurons have more than one axon. 

Nerve-fibers are axons naked or insheathed. Two myelinated peripheral 
nerve-fibers are shown in Fig. 26. The axon or axis-cylinder is composed of 



THE NERVOUS SYSTEM 



delicate neurofibrils embedded in a semifluid neuroplasm. It is surrounded by 
a relatively thick myelin sheath and a nucleated membranous neurilemma sheath. 




Fig. 25. Primary motor neuron (diagram- 
matic): ah, Implantation cone of axon; ax, 
axon; c, cytoplasm; d, dendrites; m, myelin 
sheath; m', striated muscle; n, nucleus; ri, 
nucleolus; nR, node of Ranvier; sf, collateral; 
si, neurilemma ; tel, motor end-plate. (Barker, 
Bailey.) 



Fig. 26. Portions of two nerve-fibers 
stained with osmic acid (from a young rabbit). 
Diagrammatic. 425 diameters: RR, Nodes of 
Ranvier, with axis-cylinder passing through; a, 
neurilemma; c, opposite the middle of the seg- 
ment, indicates the nucleus and protoplasm ly- 
ing between the neurilemma and the medullary 
sheath. In A the nodes are wider, and the in- 
tersegmental substance more apparent than in 
B. (Schafer, in Quain's Anatomy.) 



The myelin sheath consists of a fatty substance, myelin, supported by a retic- 
ulum of neurokeratin. The latter, not seen in the living fiber, may be a coag- 
ulation product produced during fixation. The highly refractive myelin gives 



NEURONS AND NEURON-CHAINS 



47 



to the myelinated fibers a whitish color. This sheath is interrupted at regular 
intervals by constrictions in the nerve-fiber known as the nodes of Ranvier. 
The constrictions are produced by a dipping in of the neurilemma sheath toward 
the axon, which runs without interruption through the node. The part of a 
fiber between each node is an internodal segment, and each such segment pos- 
sesses a nucleus which is surrounded by a small amount of cytoplasm and lies 
just beneath the neurilemma. The latter is a thin membranous outer covering 
for the fiber. Each segment of the neurilemma sheath, together with the cell 
which lies beneath, is the product of a single sheath cell of ectodermal origin. 
Fibers such as have just been described are found in the cerebrospinal nerves, 
and give these their white glistening appearance. 

The myelinated filers of the brain and spinal cord are of somewhat different 
structure. There is no evidence of segmentation in the myelin sheath and 
neither the neurilemma nor its cells are present. This fact is of much im- 
portance in the phenomena of regeneration, as will be explained later. These 
are the fibers which give the characteristic color to the white matter of the 
brain and spinal cord. 

Unmyelinated fibers are of two kinds, namely, Remak's fibers and naked , 
axons. The former possess nuclei which may be regarded as belonging to a 
thin neurilemma. They are found in great numbers in the sympathetic nervous 
system, and many of the fine afferent fibers of the cerebrospinal nerves also 
belong to this class (Ranson. 1911). Naked axons are especially numerous in 
the gray matter of the brain and spinal cord, and it may be added that every 
axon at its beginning from the nerve-cell, as well as at its terminal arborization, 
is devoid of covering. 

By way of summary we may enumerate four kinds of nerve-fibers: (1) myelin- 
ated fibers with a neurilemma, found in the peripheral nervous system, especially 
in the cerebrospinal nerves; (2) myelinated fibers without a neurilemma, found 
in the central nervous system; (3) unmyelinated fibers with nuclei (Remak's 
fibers), especially numerous in the sympathetic system, and (4) naked axons, 
abundant in the gray matter of the brain and spinal cord. 

Neuroglia cells and fibers will be considered in connection with the structure 
of the spinal cord. 

Structure of Neurons. Like other cells, a neuron consists of a nucleus sur- 
rounded by cytoplasm, and these possess the fundamental characteristics which 
belong to nucleus and cytoplasm everywhere, but each presents certain features 
more or less characteristic of the nerve-cell. The nucleus is large and spheric; 



48 THE NERVOUS SYSTEM 

and, because it contains little chromatin, it stains lightly with the basic dyes 
(Fig. 27, A). It contains a large spheric nucleolus. The cytoplasm, enclosed 
in a cell membrane, is characterized by the presence of basophil granules and 
a fibrillar reticulum. The granules, which apparently are a product of the 
nucleus, are composed of nucleoprotein. They are grouped in dense clumps, 
known as Nissl bodies or tigroid masses, and stain deeply with methylene-blue. 
The size, shape, and arrangement of the Nissl bodies differ with the type of 
nerve-cell studied. They are much larger in motor than in sensory neurons 
(Malone, 1913). While they are found in the larger dendrites, the axon and 
its cone of origin are free from them. They are intimately concerned in the 




Axon 



Fig. 27. Nerve-cells stained with toluidin blue: A, From anterior horn of spinal cord of the 
monkey, shows Nissl bodies in cytoplasm; B, from the facial nucleus of a dog, shows a partial 
disappearance of the Nissl bodies (chromatolysis) resulting from section of the facial nerve. 
(Schafer.) 

metabolic activity of the cell, increasing during rest and decreasing as a result 
of fatigue. They also undergo solution as a result of injury to the axon even 
at a great distance from the cell, the so-called axon-reaction or chromatolysis 
(Fig. 27, B). 

The neurofibrils were first brought forcefully to the attention of neurologists 
by Bethe (1903). These are delicate threads which run through the cytoplasm 
in every direction and extend into the axon and dendrites (Fig. 28). The 
appearance of the fibrillae differs according to the technic employed in preparing 
the tissue for microscopic examination. While in the preparations by Bethe's 
method the fibrils do not appear to branch or anastomose with each other, those 
seen in Caial preparations divide, and by anastomosing with each other form 



NEURONS AND NEURON-CHAINS 



49 



a true network. The fibrillge can be traced to the terminations of the dendrites 
and axons. They have been looked upon by many as the chief elements in- 
volved in the conduction of the nerve impulse. 

Other elements such as pigment granules may be present. Mitochondria 
have been described in nerve-cells by Cowdry (1914) and Rasmussen (1919). 

Interrelation of Neurons. In the 
ccelenterates, as we have learned, a single 
nerve-cell may receive the stimulus and 
transmit it to the underlying muscle. 
But in vertebrates the transmission of a 
nerve impulse to an effector requires 
a chain of at least two neurons, the im- 
pulse passing from one neuron to the next 
along the chain. One of the most im- 
portant problems in neurology, there- 
fore, is this: How are the neurons re- 
lated to each other so that the impulse 
may be propagated from one to the 
other? The place where two such units 
come into such functional relation is 
known as a synapse. In a synapse the 
axon of one neuron terminates on the 
cell body or dendrites of another. Func- 
tional connections are never established 
between the dendrites of one neuron 
and the cell body or dendrites of an- 
other. In Fig. 29 the axon of a basket 
cell of the cerebellum is seen giving off 
collaterals which terminate about and 
form synapses with the Purkinje cells. 
Another type of synapse is illustrated in 
Fig. 70. 

The processes of one nerve-cell are not directly fused with those of others, 
but, on the contrary, each neuron appears to be a distinct anatomic unit. At 
least the most detailed study of Golgi and Cajal preparations, in which the 
finest ramifications of dendrites and axons are stained, has failed to demon- 
strate a structural continuity between neurons. In especially favorable material 




Fig. 28. Neurofibrils in a cell from the 
anterior gray column of the human spinal 
cord: ax, Axon; lii, interfibrillar spaces; n, 
nucleus; x, neurofibrils passing from one 
dendrite to another; y, neurofibrils passing 
through the body of the cell. (Bethe, Hei- 
denhain.) 



50 THE NERVOUS SYSTEM 

Bartelmez (1915) has shown that an axon and dendrite, entering into the forma- 
tion of a synapse, are each surrounded by a distinct plasma membrane and 
that there is no direct protoplasmic continuity. It has been maintained by 
Bethe and others that at such points of contact the neurofibrils pass without 
interruption from one neuron to another, but this has been denied by Cajal. 
The relation between two neurons at a synapse appears to be one of contact, 
but not of continuity of substance. 

Nerve impulses pass across the synapse in one direction only, i. e., from the 
axon to the adjacent cell body or dendrite. As a corollary of this it is obvious 
that impulses must travel within the neuron from dendrites to perikaryon and 
then out along the axon, as indicated by the arrow in Fig. 30. This is known 




Fig. 29. Basket cell from the cerebellar cortex of the white rat. The Purkinje cells are indicated 

in stipple. Golgi method. (Cajal.) 

as the law of dynamic polarity. The polarity is, however, not dependent upon 
anything within the neuron itself, but upon something in the nature of the 
synaptic interval which permits the impulses to travel across it in one direc- 
tion only. There are many lines of evidence which indicate that when once 
activated a nerve-fiber conducts equally well in either direction. When a motor 
fiber bifurcates, sending a branch to each of two separate muscles, stimulation 
of one branch will cause an impulse to ascend to the point of bifurcation, and 
then descend along the other branch to its motor ending (Fig. 30). This can 
often be demonstrated in regenerated nerves (Feiss, 1912). The phenomena 
of antidromic conduction and the axon reflex (Bayliss, 1915) are also explained 
by the assumption that impulses are able to travel along a nerve-fiber in either 
direction. 



NEURONS AND NEURON-CHAINS 



The Neuron as a Trophic Unit. All parts of a cell are interdependent, and a 
continuous interaction between the nucleus and cytoplasm is a necessary con- 
dition for life. Any part which is detached from the portion containing the 
nucleus will disintegrate. In this respect the nerve-cell is no exception. When 
an axon is divided, that part which is separated from its cell of origin and 
therefore from its nucleus dies, while the part still connected with the cell 
usually survives. The degeneration of the distal fragment of the axon extends 
to its finest ramifications, but does not pass the synapse nor involve the next 
neuron. 

It must not be supposed, however, that the part of the neuron containing the 
nucleus remains intact, for as a result of the division of an axon important 




Motor neuron 



Synapse 




Sensory neuron 



Fig. 30. Diagram of a reflex arc to illustrate the law of dynamic polarity. The arrows indicate 

the direction of conduction. 

changes occur in the cell body. The Nissl bodies undergo solution, the cell 
becomes swollen, and the nucleus eccentric. This phenomenon is known as 
chromatolysis, or the axon reaction, and is illustrated in Fig. 27, B. If the 
changes have been very profound the entire neuron may completely disin- 
tegrate; but, as a rule, it is restored to normal again by reparative processes. 
The nucleus becomes more central, the Nissl bodies reform and usually become 
more abundant than before, while from the cut end of the axon new sprouts 
grow out to replace the part of the axon which has degenerated. From what 
has been said it will be apparent that the nucleus presides over the nutrition of 
the entire neuron, that the latter responds as a whole to an injury of even a 
distant part of its axon, that the changes produced by such a lesion are limited 
to the neuron directly involved, and that nerve-fibers are unable to maintain 



52 THE NERVOUS SYSTEM 

a separate existence or to regenerate when their continuity with the cell body 
has been lost. This is what is meant by the statement that the neuron is the 
trophic unit of the nervous system. 

Degeneration and Regeneration of Nerve-fibers. As has already been stated, 
that portion of a divided fiber which has been separated from its cell of origin 
degenerates. The axon breaks up into granular fragments, the myelin under- 
goes chemical change and forms irregular fatty globules. Later the degenerated 
axon and myelin are entirely absorbed. The neurilemma cells of a degener- 
ated peripheral nerve-fiber increase in number, their cytoplasm increases in 
quantity, and they become united end to end to form nucleated protoplasmic 
bands or band-fibers. These changes in the nerve-fiber are known as Wallerian 
degeneration. 

In regeneration new axons grow out from the old ones in the central unde- 
generated portion of the nerve. These grow into the distal degenerated stump 
and find their way along the nucleated protoplasmic bands, mentioned above, 
to the terminals of the degenerated nerve. These band-fibers serve as conduits 
for the growing axons and from them the new neurilemma sheaths are differ- 
entiated. Thus, while the neurilemma cells and the band-fibers derived from 
them appear to be incapable of developing new nerve-fibers by themselves in 
the peripheral stump, they play an important part in nerve regeneration in 
co-operation with the new axons from the central stump (Cajal, 1908; Ranson, 
1912). It is important to note that the nerve-fibers of the brain and spinal 
cord, which, as has been stated before, are devoid of neurilemma sheaths, are 
incapable of regeneration. 

The neuron concept, which is based on such facts as have been presented 
in the preceding paragraphs, was first clearly formulated by Waldeyer in 1891, 
who was also the first to use the name neuron for the elements under considera- 
tion. The neuron doctrine may be summarized as follows: 

1. The neuron is the genetic unit of the nervous system each being derived 
from a single embryonic cell, the neuroblast. 

2. The neuron is the structural unit of the nervous system, a nerve-cell with 
all its processes. These cellular units remain anatomically separate, i. e., while 
they come into contact with each other at the synapses there is no continuity 
of their substance. 

3. The neurons are the functional units of the nervous system. They are 
conduction units and the conduction pathways are formed of chains of such 
units. 



NEURONS AND NEURON-CHAINS 



53 



4. The neuron is also a trophic unit, as is seen (a) in the degeneration of a 
portion of an axon severed from its cell of origin, (6) in the phenomenon of 
chromatolysis or axon reaction, and (c) in the regeneration of the degenerated 
portion of the axon by an outgrowth from that part of the axon still in con- 
tact with its cell of origin. 

5. Neurons are the only elements concerned in the conduction of nerve 
impulses. The nervous system is composed of untold numbers of such units 
linked together in conduction systems. 

While a majority of neurologists now accept the neuron doctrine as pre- 
sented here, there are dissenters (Marui, 1918). In his very interesting book, 




Fig. 31. Diagrammatic section through the spinal cord and a spinal nerve to illustrate a 
simple reflex arc: a, b, c, and d, Branches of sensory fibers of the dorsal roots; e, association neuron; 
/, commissural neuron. 

"Allgemeine Anatomic und Physiologic des Nervensystems," Bethe has vigor- 
ously controverted every one of the five cardinal points just presented. 

We will next examine some of the simpler chains of neurons to see how they 
enter into the formation of the conduction pathways. 

Neuron-chains. The simplest functional combination of neurons is seen in 
the reflex arc, and this again in its simplest form is illustrated in Fig. 31. Such 
an arc may consist of but two neurons, one of which is afferent and conducts 
toward the spinal cord; the other is efferent and conducts the impulses to the 
organ of response. The arc consists of the following parts: (1) the receptor, 
the ramification of the sensory fiber in the skin or other sensory end organ; 
(2) the first conductor, which includes both branches of the axon of the spinal 
ganglion cell; (3) a center including the synapse; (4) the second conductor, which 



54 



THE NERVOUS SYSTEM 



includes the entire motor neuron, with its cell body in the anterior gray column 
and its motor ending on the muscle, and (5) the effector or organ of response, 
which in this case is a muscle-fiber. A wave of activation, known as the nerve 
impulse, is developed in the sensitive receptor, travels over this arc, and on 
reaching the muscle causes it to contract. So simple a reflex is rare, but prob- 
ably the knee-jerk is an example (Jolly, 1911). A more common form of reflex 
arc involves a third, and purely central neuron, as illustrated on the right side 
of Fig. 31. Such central elements may be spoken of as association and com- 
missural neurons. Many of them serve to connect distant parts of the central 




Fig. 32. Diagram representing some of the conduction paths through the mammalian central 
nervous system. An elaborate system of central or association neurons furnishes a number of 
alternative paths between the primary sensory and motor neurons. (Redrawn from Bayliss.) 

nervous system with each other (Fig. 68). It is to the multiplication of these 
central neurons that we owe the complicated pathways within the mammalian 
brain and spinal cord. 

Pathways Through Higher Centers. A good idea of how the neurons of some 
of the centers in the brain are related to the primary motor and sensory spinal 
neurons is given by Fig. 32. It will be seen that many paths are open to an 
impulse entering the spinal cord by way of a dorsal root fiber: (1) It may pass 
by way of a collateral to a primary motor neuron in a two-neuron reflex arc. 
It may travel over an association neuron, belonging (2) to the same level of the 



NEURONS AND NEURON-CHAINS 55 

spinal cord, or (3) to other levels, in reflex arcs of three or more neurons each; 
or (4) it may ascend to the brain along an ascending branch of a dorsal root 
fiber. Here it may travel over one or more of a number of paths, each con- 
sisting of several neurons, and be finally returned to the spinal cord and make 
its exit by way of a primary motor neuron. The figure illustrates but a few of 
the possible paths, many of which we shall have occasion to consider in the 
subsequent chapters. 

For an incoming impulse a variety of paths are open, one or more of which 
may be taken according to the momentary resistance of each. There is reason 
to believe that the resistance to conduction across a synapse may vary from 
moment to moment, according to the physiologic state of the neurons involved. 
It is therefore not necessary that every impulse entering by a given fiber shall 
travel the same path within the central nervous system nor produce the same 
result. The pathways themselves are, however, more or less fixed, and depend 
upon the structural relations established among the neurons. Many of these 
synaptic connections are formed before birth, follow an hereditary pattern, 
and are approximately the same for each individual of the species. In the child 
these are illustrated by the nervous mechanisms involved in breathing and 
swallowing, which are perfect at birth. The newly hatched chick is able to run 
about and pick up food, acts which are dependent on nervous connections al- 
ready established according to hereditary pattern. In man and to a less extent 
in other mammals the nervous system continues to develop long after birth. 
This postnatal development is influenced by the experience of the individual 
and is more or less individual in pattern. It is probable "that in certain parts 
of the nervous mechanism new connections can always be established through 
education" (Edinger, 1911). 

The neurons which make up the nervous system of an adult man are there- 
fore arranged in a system the larger outlines of which follow an hereditary pat- 
tern, but many of the details of which have been shaped by the experiences of 
the individual. 



CHAPTER V 

THE SPINAL NERVES 

WE have had a glance at the earliest beginnings of a nervous system in the 
animal series and learned something of its biologic significance. We have 
traced briefly its development in the mammalian embryo, and become familiar 
with its chief subdivisions. We have studied the microscopic units of which it 
is composed, learning something of their development, structure, and function. 
With this information we are prepared to take up a more detailed study of the 
various subdivisions of the system. 

Subdivisions of the Nervous System. The most convenient and logical 
classification of the parts of the nervous system is that which emphasizes the 
distinction between the central organs and those peripheral portions which are 
concerned chiefly in conducting impulses to and from the central organs, as 
follows : 

The central nervous system: 
Brain, 
Spinal cord. 

The peripheral nervous system : 
Cerebrospinal nerves: 
Cranial nerves, 
Spinal nerves. 

The sympathetic nervous system. 

The anatomic relationships of these subdivisions in man are illustrated in 
Figs. 33 and 34. The brain lies within and nearly fills the cranial cavity. It is 
continuous through the foramen magnum with the spinal cord, which occupies 
but does not fill the vertebral canal. From the brain arises a series of nerves 
usually enumerated as twelve pairs and known as cranial or cerebral nerves; 
while thirty-one pairs of segmentally arranged spinal nerves take origin from the 
spinal cord. 

Branches of the cerebrospinal nerves reach most parts of the body. They 

are composed of afferent fibers, which receive and carry to the central nervous 

system sensory impulses produced by external or internal stimuli, and of efferent 

fibers, which convey outgoing impulses to the organs of response. It is through 

56 



THE SPINAL NERVES 



57 



the central nervous system that the incoming impulses find their way into the 
proper outgoing paths. To bring about this shunting of incoming impulses 
into the appropriate efferent paths requires the presence of untold numbers 

Ciliary ganglion Maxillary nerve 
Sphenopalaline ganglion , 
Superior cervical ganglion of sympathetic \ \ 




Cervical 
plexus 



Brachial f 
plexus | 



Greater 
splanchnic nerve 

Lesser 
splanchnic nerve 



Sacral 
plexus 




Pharyngeal plexus 

Middle cervical ganglion of sympathetic 
Inferior cervical gang, of sympathetic 
Recurrent nerve 
Bronchial plexus 

Cardiac plexus 



Esophageal plexus 
Coronary plexus 



Left vagus nerve 

Gastric plexus 
Celiac plexus 

Superior mesenteric plexus 



Aortic plexus 

Inferior mesenteric plexus 

Hypogastric plexus 

Pelvic plexus 

Bladder 
Vesical plexus 



Fig. 33. Fig- 34- 

Fig. 33. General view of the central nervous system, showing the brain and spinal cord in situ. 

(Bourgery, Schwalbe, van Gehuchten.) 

Fig. 34. Diagram of the sympathetic nervous system and its connections with the cerebrospmal 

nerves. (Schwalbe, Herrick.) 

of central or association neurons, and it is of these that the central organs- 
brain and spinal cord are chiefly composed. 

Many authors employ a classification which emphasizes the distinction be- 



5 THE NERVOUS SYSTEM 

tween the cerebrospinal nervous system, composed of the brain and spinal cord 
with their associated nerves, and the sympathetic nervous system. But this usage 
has the disadvantage that it is likely to engender an entirely false notion of the 
independence of the sympathetic system. 

The spinal nerves take origin from the spinal cord within the vertebral canal 
and make their exit from this canal through the corresponding intervertebral 
foramina. As component parts of such a nerve there may be recognized a 
ventral and a dorsal ramus, a ventral and a dorsal root, and associated with 
the latter a spinal ganglion. The fibers of the ventral root have their cells of 
origin within the spinal cord and are distributed through both ventral and 
dorsal rami. Since they conduct impulses from the spinal cord they are known 
as efferent or motor fibers. The sensory or afferent fibers of the dorsal roots 
and spinal nerves arise from cells located in the spinal ganglia. These fibers 
are also distributed through both ventral and dorsal rami (Fig. 37). 

Metamerism. That the spinal nerves are segmentally arranged, a pair for 
each metamere, is readily appreciated in the case of the typical body segments 
of the thoracic region. Here it is obvious that a nerve supplies the correspond- 
ing dermatome and myotome, or in the adult the skin and musculature of its 
own segment. While the thoracic nerves retain this primitive arrangement in 
the adult, the distribution of fibers from the other spinal nerves is complicated 
by the development of the limb buds and by the shifting of myotomes and 
dermatomes during the development of the embryo. 

Opposite the attachment of the limb buds the ventral rami of the correspond- 
ing nerves unite to form flattened plates, and from these plates the brachial and 
lumbosacral plexuses are developed. Within these plexuses the fibers derived 
from a number of ventral rami are intermingled in what appears at first to be 
hopeless confusion. Each nerve which extends from these plexuses into the 
limbs carries with it fibers from more than one spinal nerve. To determine 
the exact distribution of the fibers from each segmental nerve has been a very 
difficult problem, in the elucidation of which the work of clinical neurologists 
has been of the first importance. A study of the paralyses and areas of anes- 
thesia, resulting from lesions involving one or more nerve roots within the ver- 
tebral canal, has contributed much toward its solution. 

Sherrington (1894) attacked the problem of the distribution of the sensory 
fibers by experimental methods on cats and monkeys. He found that section of 
a single dorsal root did not cause complete anesthesia anywhere, and attributed 
this result to an overlapping of the areas of distribution of adjacent spinal nerves. 



THE SPINAL NERVES 



59 



Next, selecting a particular dorsal root for study, he cut two or three roots 
both above and below it. The zone in which sensation still existed and which 
was surrounded by an area of anesthesia represented the cutaneous field of that 
particular root. He found that each "sensory root field" overlapped those of 
adjacent roots (Fig. 35). In the thoracic region each such field has the shape of 
a horizontal band wrapping half-way around the body from the middorsal to 
the midventral lines (Fig. 36). 

Sherrington also found that, although in the plexuses associated with the 
innervation of the extremities each segmental nerve contributes sensory fibers 
to two or more peripheral nerves, the cutaneous distribution of these fibers is 
not composed of disjointed patches, but forms a continuous field running approxi- 
mately parallel to the long axis of the limb. The general arrangement of these 
sensory root fields in man is indicated on the right side of Fig. 36. On the 



Uth 

thoracic 

sensory < 

spinal 

^ kin field. 



///////////////////////////////I//// 

//////////// luiin iniini a 1 1 n 



\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ 
J\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ 



I 



\ 



3d 
thoracic. 



5th 
thoracic. 



Fig. 35. Diagram of the position of the nipple in the sensory skin fields of the fourth, third, 
and fifth thoracic spinal roots. The overlapping of the cutaneous areas is represented. (Sher- 
rington.) 

opposite side is indicated the distribution of the cutaneous nerves. It will be 
seen that in the extremities there is no correspondence between the areas sup- 
plied by these peripheral nerves and those supplied by the individual dorsal 
roots. It will also be evident that the fibers of a given dorsal root reach the 
corresponding sensory root field by way of more than one cutaneous nerve. 
A knowledge of the cutaneous distribution of the various nerve roots is of great 
importance in enabling the clinician' to determine the level of a lesion of the 
spinal cord or nerve roots within the vertebral canal. 

In the same way the shifting of muscles during embryonic development has 
been accompanied by corresponding changes in the spacial distribution of the 
motor fibers. A familiar example is furnished by the diaphragm, the musculature 
of which is derived from the cervical myotomes and which in its descent carries 
with it the phrenic nerve. This explains the origin of the phrenic from the 
third, fourth, and fifth cervical nerves. 



6o 



THE NERVOUS SYSTEM 



If, as seems probable, the musculature of the extremities has not developed 
along metameric lines, there can be no true metamerism of the motor nerves to 
the limbs (Streeter, 1912). Yet the fibers from each ventral root are distributed 
in a very orderly manner. As is indicated in the table on page 77, almost every 
long muscle receives fibers from two or more ventral roots. It will be apparent 
that the muscles of the trunk are innervated from the roots belonging to the 



Great auricular 
Cutaneous nerve of the neck 
Supraclavicular nerves 
Axillary 

Intercostobrachial 
Medial cutaneous of arm 
Posterior cutaneous of arm 
Medial cutaneous of forearm 
Musculocutaneous 

Radial 
Median 
Ulnar 

Genitofemoral ' 

Lateral cutaneous of the thigh' 
Intermediate cutaneous rami' 
Medial cutaneous rami 
Infrapatellar ramus 
Lateral sural 
Saphenous 
Superficial peroneal 
Sural 
Deep peroneal 

Fig. 36. Sensory root fields on the right, contrasted with the areas of distribution of cutaneous 

nerves on the left. 

several metameres from the myotomes of which these muscles * developed. The 
table shows in a general way the distribution of the fibers of the several ventral 
roots. 

Functional Classification of Nerve-fibers. Many years ago Sir Charles 
Bell (1811, 1844) showed that the dorsal roots are sensory in function and the 
ventral roots motor; and this has been known since then as Bell's law. He 
recognized that sensory and motor fibers are distributed to the viscera as well as 




THE SPINAL NERVES 



6l 



to the rest of the body. But GaskeU (1886) was the first to make a detailed 
study of the nerve-fibers supplying the visceral and vascular systems. We 
now recognize in the spinal nerves elements belonging to four functionally 
distinct varieties, namely, visceral afferent, visceral efferent, somatic afferent, and 
somatic efferent fibers (Fig. 37). 

Visceral Components. The fibers which innervate the visceral and vascular 
systems, including all involuntary muscle and glandular tissue, possess, as 
Gaskell (1886) pointed out many years ago, certain distinguishing character- 
istics. They are all fine myelinated fibers and end in sympathetic ganglia 

Somatic afferent fiber \ Dorsal root 
Visceral afferent fiber) 

Spinal ganglion 
Dorsal ramus 

f Ventral ramus 

| 

" Ramus communicans 

Sympathetic ganglion 

/*. Visceral efferent fiber], , . , 
S Somatic efferent fiber Central root 

Postganglionk fiber 

,Viscus 

Fig. 37. Diagrammatic section through a spinal nerve and the spinal cord in the thoracic region 
to illustrate the chief functional types of peripheral nerve-fibers. 

from which the impulses are relayed to involuntary muscles and glands by a 
second set of neurons (Fig. 37). They are usually designated as visceral efferent 
fibers, and they run by way of the white rami to the sympathetic ganglia. It 
is usually stated that they are found only hi the second thoracic to the second 
lumbar nerves inclusive, but Langley (1892) has shown that in the cat, dog, 
and rabbit they are present in all the thoracic and the first four lumbar nerves, 
and Miiller (1909) found white rami associated with the third and fourth lumbar 
nerves in man. 

There are also visceral afferent fibers distributed to the thoracic and ab- 
dominal viscera by way of the white rami from the thoracic and upper lumbar 




62 



THE NERVOUS SYSTEM 



nerves. These have their cells of origin in the spinal ganglia and are continued 
through the dorsal roots into the spinal cord (Fig. 37). We shall have much 
more to say about the visceral components of the spinal nerves in the chapter 
on the Sympathetic Nervous System. In the remaining pages of this chapter 
we will confine our attention to the somatic components, i. e., to those fibers which 
innervate the various parts of the body exclusive of the visceral and vascular 
systems. 

Somatic Efferent Components. The skeletal muscles are innervated by 
myelinated fibers, which are, for the most part, of large caliber. The axis- 
cylinders of these fibers are the axons of cells located in the ventral part of the 
gray matter of the spinal cord, and they end on the muscle-fibers in special 





Fig. 38. Nerve-ending in muscular fiber of a lizard (Lacerta viridis). Highly magnified: a, 
End-organ seen in profile; b, from the surface; s, s, sarcolemma; p, p, expansion of axis-cylinder. 
Beneath this is granular protoplasm containing a number of large clear nuclei and constituting 
the "bed" or "sole" of the end-organ. In b the expansion of the axis-cylinder appears as a clear 
network, branching from the divisions of the medullated fiber. (Kiihne in Quain's Anatomy.) 

motor end-plates. Such a primary motor neuron is illustrated in Fig. 25. A 
motor fiber undergoes repeated division as it approaches its termination, but 
each branch retains its myelin sheath until in contact with the muscle-fiber. 
At this point this sheath terminates abruptly, and the neurilemma becomes 
continuous with the sarcolemma (Fig. 38). The terminal branches of the 
axon are short, thick, and irregular. They lie immediately under the sarcolemma 
in a bed of specialized sarcoplasm containing a number of large clear nuclei. 
The wave of activation, which travels down an axon as a nerve impulse, is 
transmitted through these motor nerve endings to the muscle and initiates a 
contraction. 

The Spinal Ganglia. Since the afferent fibers in the spinal nerves take their 



THE SPINAL NERVES 63 

origin from the ganglia on the dorsal roots we will do well to interrupt for a 
moment our functional analyses of the spinal nerves and consider the struc- 
ture of these ganglia. 

The spinal ganglia are rather simple structures so far as their fundamental 
plan is concerned, but in recent years, chiefly through the studies of Cajal 
(1906) and Dogiel (1908), we have learned to recognize in them many complex 
histologic details, the significance of which is not yet understood. It has long 
been known that the typical cells of the mammalian spinal ganglion are uni- 
polar. The cell body is irregularly spheric. The axon, 1 which is attached to 
the perikaryon by an implantation cone, is coiled on itself in the neighborhood 
of the cell, forming what is known as a glomerulus (Fig. 39, /). It then runs 
into one of the central fiber bundles of the ganglion and divides in the form 
of a T or Y into two branches, of which one is directed toward the spinal cord 
in the dorsal root. The other and somewhat larger branch is directed distally 
in the spinal nerve. The cells vary greatly in size and the diameter of the axon 
varies with that of the cell from which it springs. An axon arising from a 
large cell usually forms a very pronounced glomerulus and soon becomes en- 
sheathed with myelin, and this myelin sheath is continued along both branches 
into which it divides. The branching occurs at a node of Ranvier. 

As was originally pointed out by Cajal (1906) and Dogiel (1908) and 
recently emphasized by Ranson (1911) the small cells of these ganglia give rise 
to fine unmyelinated fibers. These coil but little near the cell, or the glomerulus 
may be entirely lacking (Fig. 39, a). They divide dichotomously, just as do 
the myelinated fibers, into finer central and coarser peripheral branches. At 
the point of bifurcation there is a triangular expansion in place of the constric- 
tion so characteristic of a dividing myelinated fiber. It has been shown by 
Hatai (1902) and Warrington and Griffith (1904) that the small cells are con- 
siderably more numerous than the large cells, though because of their small 
size they constitute a less conspicuous element. 

A few cells retain the bipolar form characteristic of all the spinal ganglion 
cells at an early stage of development (Figs. 21, 40, d). 

The spinal ganglion cells are each surrounded by a capsule or membranous 
sheath with nuclei on its inner surface (Fig. 39, d, /) which is continuous with 
the neurilemma sheath of the associated nerve-fiber. The cells forming the 
capsule are of ectodermal origin, being derived like the spinal ganglion cells 
themselves from the neural crest. 

1 See fine print, page 45. 



04 THE NERVOUS SYSTEM 

In good methylene-blue preparations and in sections stained by the newer 
silver methods it is possible to make out many additional details of structure. 
The axon may split into many branches, which subdivide and anastomose, 
forming a true network in the neighborhood of the cell (Fig. 39, b). From this 
network the axon is again assembled and passes on to a typical bifurcation. 
Or the axon may be assembled out of a similar plexus which, however, is con- 

&. 



a 










Fig. 39. Neurons from the spinal ganglion of a dog: a, Small cells with unmyelinated axons; 
6, c, d, e, and /, large cells with myelinated axons; /, typical large spinal ganglion cell showing 
glomerulus and capsule. The arrow points toward the spinal cord. Pyridin-silver method. 

nected with the cell by several roots (Fig. 39, c}. Some of the fibers give off 
collaterals terminating in spheric or pear-shaped end-bulbs. Such an end bulb 
may rest upon the surface of its own perikaryon (Fig. 39, d] or elsewhere in the 
ganglion. From the body of some cells short club-shaped dendrites arise, which, 
however, terminate beneath the capsules which surround the cells. 

Based on such details as these Dogiel (1908) has arranged the spinal ganglion cells in 
groups and recognizes eleven different types. Two of his eleven types are of special interest. 
The cells of Type VIII resemble the typical spinal ganglion cell in all respects except that 



THE SPINAL NERVES 65 

the peripheral branch of the axon breaks up within the ganglion into numerous myelinated 
fibers, which after losing their sheaths terminate in what are apparently sensory endings. 
The central branch runs apparently without division to the spinal cord. The cells of Type 
XI possess, in addition to an axon, that apparently runs without division through the dorsal 
root to the spinal cord, several processes that resemble dendrites, in that they divide re- 
peatedly within the ganglion, but resemble axons in their appearance and in possessing 
myelin sheaths (Fig. 40, b). These processes after repeated divisions become unmyelinated 
and end within the ganglion and dorsal root in what appear to be sensory endings. It would 
lead us too far afield if we should attempt to summarise Dogiel's work. It should be pointed 
out, however, that he no longer believes in the existence of the cells which he formerly de- 
scribed under the head of spinal ganglion cells of Type II and which find a conspicuous place 
in most text-books. He believes that what he formerly described as the branching fibers 
of these cells are, in reality, the dendrite-like branches of the cells of Type XI. 



Dorsal root /, 



Dorsal ramus 




Ventral root 



Ramus communicans 



Ventral ramus 



Fig. 40. Diagrammatic longitudinal section of a spinal ganglion and a spinal nerve (cervical 
or sacral) : a, Small cells with unmyelinated axons; b, cell of Dogiel's type XI ; c, large cell possessing 
a myelinated axon and surrounded by a pericellular plexus; d, bipolar cell. 

According to Dogiel every spinal ganglion cell is surrounded by a network of 
fine branching and anastomosing fibers; and he believes that these are formed 
by the ramifications of fine myelinated and unmyelinated fibers that have 
entered the spinal ganglion from the sympathetic nervous system through the 
rami communicantes. While the origin of these fibers is open to question, there 
can be no doubt that such pericellular networks exist on at least a considerable 
proportion of the cells and constitute an important element in the structure of 
the ganglion (Fig. 40, c}. 

The fiber bundles of the ganglia are composed of both myelinated and un- 

5 



66 THE NERVOUS SYSTEM 

myelinated fibers representing the branches of the axons of the spinal ganglion 
cells. Both types of fibers can be followed through the dorsal roots into the 
spinal cord, as well as distally into the nerves. In the latter they mingle with 
the large myelinated fibers coming from the ventral roots (Fig. 40). When 
traced distally in the peripheral nerve the unmyelinated fibers are found to go 
in large part to the skin, though a few run in the muscular branches (Ranson, 
1911 and 1915). 

Classification of the Somatic Afferent Fibers According to Function. 
Sherrington (1906) in an instructive book on "The Integra tive Action of the 
Nervous System" has furnished us with a useful classification of the elements 
belonging to the afferent side of the nervous system. He designates those 
carrying impulses from the viscera as interoceptive, and subdivides the somatic 
afferent elements into exteroceptive and proprioceptive groups. The extero- 
ceptive fibers carry impulses from the surface of the body and from such sense 
organs, as the eye and ear, that are designed to receive stimuli from without. 
These fibers, therefore, are activated almost exclusively by external stimuli. 
The proprioceptive fibers, on the other hand, respond to stimuli arising within 
the body itself and convey impulses from the muscles, joints, tendons, and the 
semicircular canals of the ear. Each group has receptors or sensory endings 
designed to respond to its appropriate set of stimuli, and for each there are 
special connections within the brain and spinal cord. 

Exteroceptive fibers and sensory endings are activated by changes in the 
environment, that is to say, they are stimulated by objects outside the body. 
The impulses, produced in this way and carried by these fibers to the spinal 
cord, call forth for the most part reactions of the body to its environment; 
and, when relayed to the cerebral cortex, they may be accompanied by sensa- 
tions of touch, heat, cold, or pain. The receptors are, for the most part, located 
in the skin; yet it is convenient to include in the exteroceptive group the pressure 
receptors which are closely allied to those for touch, but which lie below the 
surface of the body. At this point it should be noted that sensibility to those 
forms of contact which include some slight pressure, such as the placing of a 
finger on the skin, is not abolished by the section of all of the cutaneous nerves 
going to the area in question, since the deeper nerves carry fibers capable of 
responding to such contacts (Head, 1905). This deep contact sensibility, which 
for lack of a better name we may call "pressure- touch," must not be overlooked 
in the analysis of cutaneous sensations. 

The balance of evidence is in favor of the assumption that each of the vari- 



THE SPINAL NERVES 67 

eties of cutaneous sensation is mediated by a separate set of nerve-fibers. But 
little progress has as yet been made toward identifying these various func- 
tional groups. We know that both myelinated and unmyelinated fibers are 
present in the cutaneous nerves (Ranson, 1915), but are not able to say with 
certainty which subserve each of the varieties of cutaneous sensation. There 
are many good reasons, however, for the belief that painful afferent impulses 
and possibly also those of temperature are carried by the unmyelinated fibers, 
and that those of the touch and pressure group are mediated by the myelinated 
fibers. The evidence on which this statement is based has been briefly sum- 
marized on pages 102-104. 




Fig. 41. Free nerve endings in the epidermis of a cat's paw: A, Stratum corneum; B, stratum 
germinativum Malpighii, and C, its deepest portion; a, large nerve trunk; b, collateral fibers; c, 
terminal branches; d, terminations among the epithelial cells. Golgi method. (Cajal.) 

All sensory nerve endings in the skin belong to the exteroceptive group, 
but it is not so easy to say which ones are responsible for each of the several 
varieties of cutaneous sensation, namely, touch, pain, heat, and cold. On 
structural grounds we may recognize three principal groups: (1) endings in hair- 
follicles, (2) encapsulated nerve endings, and (3) free terminations in the epi- 
dermis. 

Free Nerve Endings. Some of the myelinated fibers as they approach 
their terminations divide repeatedly. At first the branches retain their sheaths, 
but after many divisions the myelin sheaths and finally the neurilemma are lost 
and only the naked axis-cylinders remain. These enter the epidermis, where, 



68 



THE NERVOUS SYSTEM 



after further divisions, they end among the epithelial cells (Fig. 41). This type 
of nerve ending is found in the skin, mucous membranes, and cornea. Similar 
endings are also found in the serous membranes and intermuscular connective 
tissue. 

We do not know what form the endings of the afferent unmyelinated fibers 
may take, but it is not unlikely that they also ramify in the epidermis like the 
terminal branches of the myelinated fibers just described. It seems certain 
that at least a part of the free nerve endings in the epidermis are pain receptors. 
In the central part of the cornea, the tympanic membrane, and the dentine 
and pulp of the teeth, such free nerve endings alone are present, and pain is the 
only sensation that can be appreciated. 

Some of the nerve-fibers which enter the epidermis end in disk-like expansions 
in contact with specialized epithelial cells (Fig. 42). These have been known 




Fig. 42. Merkel's corpuscles or tactile disks from the skin of the pig's snout. The nerve- 
fiber, n, branches and each division ends in an expanded disk, m, which is attached to a modified 
cell of the epidermis, a; c, an unmodified epithelial cell. (Ranvier, Herrick.) 

as Merkel's touch-cells on the supposition that the endings in question are tactile 
receptors. 

Encapsulated Nerve Endings. Among the encapsulated nerve endings are 
the corpuscles of Meissner. These have quite generally been regarded as tactile 
end organs and are located in the corium or subepidermal connective tissue of 
the hands and feet, forearm, lips, and certain other regions. They are of large 
size, oval, possess a thin connective-tissue capsule, and within each terminate 
one or more medullated fibers (Fig. 43). Within the capsule the fibers lose their 
myelin sheaths, make a variable number of spiral turns, and finally break up 
into many varicose branches, which form a complex network. To another 
type of encapsulated end organ belong those known as the end bulbs of Krause. 
One of these is illustrated in Fig. 44. They are found in the conjunctiva, edge* 
of the cornea, lips, and some other localities. 



THE SPINAL NERVES 



6 9 





Fig. 43. Meissner's tactile corpuscle. 
Methylene-blue stain. (Dogiel, Bohm-David- 
off-Huber.) 



Fig. 44. End-bulb of Krause from con- 
junctiva of man. Methylene-blue stain. 
(Dogiel, Bohm-Davidoff-Huber.) 





Fig. 45. Pacinian corpuscles from mesorectum of kitten: A, Showing the fine branches of 
the central fiber; B, the network of fine nerve-fibers about the central fiber. Methylene-blue stain. 
(Sala, Bohm-Davidoff-Huber.) 

The Pacinian corpuscles, two of which are illustrated in Fig. 45, have a very 
wide distribution in the deeper parts of the dermis of the hands and feet, in the 



7 

tendons, intermuscular septa, periosteum, peritoneum, pleura, and pericardium. 
They are also numerous in the neighborhood of the joints. According to Her- 
rick (1918) it is probable that "by these end organs relatively coarse pressure 
may be discriminated and localized (exteroceptive function), and movements 
of muscles and joints can be recognized (proprioceptive function)." They are 



1 hst 




Tt is 



Fig. 46. Nerves and nerve endings in the skin and hair-follicles: hst, Stratum corneum; rm, 
stratum germinativum Malpighii; c, most superficial nerve-fiber plexus in the cutis; n, cutaneous 
nerve; is, inner root sheath of hair; as, outer root sheath; h, the hair itself; dr, glandulae sebaceae. 
(Retzius, Barker.) 

large oval corpuscles, made up in great part of concentric lamellae of connective 
tissue. The axis of the corpuscle is occupied by a core of semifluid substance 
containing the termination of a nerve-fiber. The fiber loses its myelin sheath 
as it enters the core, through which it passes from end to end. Its terminal 
branches end in irregular disks. Side branches are also given off within the core. 



THE SPINAL NERVES 



Nerve Endings in the Hair-follicles. It has long been known that the hairs 
are delicate tactile organs. The hair-clad parts lose much of their responsive- 




Fig. 47. Neuromuscular nerve end-organ from a dog. The figure shows the intrafusal 
muscle-fibers, the nerve-fibers and their terminations, but not the capsule nor the sheath of Henle. 
Methylene-blue stain. (Huber and De Witt.) 

ness to touch when the hair is removed. As would be expected on these grounds, 
the hair-follicles are richly supplied with nerve endings. Just below the open- 
ing of the sebaceous gland into the follicle myelinated nerve-fibers enter it, los- 



72 THE NERVOUS SYSTEM 

ing their myelin sheaths as they enter. They give off horizontal branches, 
which encircle the root of the hair, and from these ascending branches arise 
(Fig. 46). Some of these are connected with leaf-like expansions, associated 
with cells resembling Merkel's touch-cells. 

Practically nothing is known concerning the receptors for sensations of heat 
and cold. 

Proprioceptive Fibers and Sensory Nerve Endings. To this group belong 
the afferent elements which receive and convey the impulses arising in the 
muscles, joints, and tendons. Changes in tension of muscles and tendons and 
movements of the joints are adequate stimuli for the receptors of this class and 
excite nerve impulses which, on reaching the central nervous system, give in- 
formation concerning tension of the muscles and the relative position of the 
various parts of the body. For the most part, however, these impulses do not 
rise into consciousness, but serve for the subconscious control of muscular 
activity. The unsteady gait of a tabetic patient illustrates the lack of mus- 
cular control that results when these impulses are prevented from reaching the 
central nervous system. 

The proprioceptive fibers are myelinated and are associated with motor 
fibers in the nerves to the muscles. Some follow along the muscles to reach 
the tendons. Three types of end organs belong to this group, Pacinian cor- 
puscles, muscle spindles, and neurotendinous end organs. Many Pacinian 
corpuscles are found in the neighborhood of the joints. They have been de- 
scribed in a preceding paragraph. 

Neuromuscular End Organs. The afferent fibers to the muscles end on 
small, spindle-shaped bundles of specialized muscle-fibers (Fig. 47). These 
muscle spindles are invested by connective- tissue capsules; and within each 
of them one or more large myelinated nerve-fibers terminate. Within the 
spindle the myelin sheath is lost and the branches of the axis-cylinders wind 
spirally about the specialized muscle-fibers, or they may end in irregular disks. 
Somewhat analogous structures are the neurotendinous end organs or tendon 
spindles where myelinated nerve-fibers end in relation to specialized tendon 
fasciculi. 



CHAPTER VI 



THE SPINAL CORD 

THE spinal cord, or medulla spinalis, is a cylindric mass of nervous tissue 
occupying the vertebral canal. It is 40 to 45 cm. in length, reaching from the 
foramen magnum, where it is continuous with the medulla oblongata, to the 
level of the first or second lumbar vertebra. Even above this level the vertebral 
canal is by no means fully occupied by the cord (Fig. 48), which, as shown in 
Fig' 49, is surrounded by protective membranes, while between these and the 
wall of the canal is a rather thick cushion of adipose tissue containing a plexus 



Extradural fat and venous plexus 



Subarachnoid space 
Spinal nerve roots 



Spinal cord 



Dura mater 

Ligamentum denticulatum 




Fig. 48. Diagram showing the relation of the spinal cord to the vertebral column. 

of veins. Immediately surrounding the cord and adherent to it is the delicate, 
highly vascular pia mater. This is separated from the thick, fibrous dura mater 
by a membrane having the tenuity of a spider web, the arachnoid, which sur- 
rounds the subarachnoid space. This space is broken up by subarachnoid 
trabeculae and filled with cerebrospinal fluid. 

External Form. The spinal cord is not a perfect cylinder, but is somewhat 
flattened ventrodorsally, especially in the cervical region. Its diameter is not 
uniform throughout, being less in the thoracic than in the cervical and lumbar 
portions. That is to say, the cord presents two swellings (Fig. 51). The cer- 
vical enlargement (intumescentia cervicalis) comprises all that portion of the cord 

73 



74 



THE NERVOUS SYSTEM 



from which the nerves of the brachial plexus arise, that is, the fourth cervical 
to the second thoracic segments inclusive. The lumbar enlargement (intumes- 
centia lumbalis) is not quite so extensive and corresponds less accurately to the 
origin of the nerves innervating the lower extremity. At an early stage in the 
embryonic development of the spinal cord these enlargements are not present. 
In the time of their first appearance and in their subsequent growth they are 
directly related to the development of the limbs. 

Below the lumbar enlargement the spinal cord rapidly decreases in size 
and has a cone-shaped termination, the conus medullaris, from the end of which 
a slender filament, the filum terminale, is prolonged to the posterior surface of 
the coccyx (Figs. 50, 51). This terminal filament descends in the middle line, 
surrounded by the roots of the lumbar and sacral nerves, to the caudal end of 

Septum poslicum 

/Posterior spinal artery 

_,,,... , , ^^>_- Ligamentum denticulatum 

Subarachnoid trabecuke ---.-. 



Pia mater - 



Epidural trabeculce *=;2?5 
Anterior spinal artery' 




--Dura mater 
*- Subdural space 
Arachnoid 
'-Nerve root 



Subarachnoid cavity 
Linca splendens 
Fig. 49. Diagram of the spinal cord and meninges. 

the dural sac at the level of the second sacral vertebra. Here it perforates the 
dura mater, from which it receives an investment, and then continues to the 
posterior surface of the coccyx. The last portion of the filament with its dural 
investment is often called the filum of the spinal dura mater (filum durae matris 
spinalis). The filum terminale is composed chiefly of pia mater; but in its 
rostral part it contains a prolongation of the central canal of the cord. 

The spinal cord shows an obscure segmentation, in that It gives origin to 
thirty-one pairs of metameric nerves. These segments may be somewhat 
arbitrarily marked off from each other by passing imaginary planes through the 
highest root filaments of each successive spinal nerve (Donaldson and Davis, 
1903). The highest of these planes, being just above the origin of the first cer- 
vical nerve, marks the separation of the spinal cord from the medulla oblongata. 



THE SPINAL CORD 



75 



This is again an arbitrary line of separation, since both as to external form 
and internal structure the cord passes over into the medulla oblongata by in- 



Medulla oblongala 



f- N. cenicalis VIII 

' Ventral root of N. 
T.I II 

Dorsal root of N. 

T.IV 

Lateral funiculus 
Spinal dura mater 



- N. thoricalis XII 



\- Cauda equina 



I- N. lumbalis V 



Filum of spinal dura 
mater 



.Medulla oblongata. _ 



I Anterior median fissure 



A nterolateral sulcus 
-Cervical enlargement 
-A nterior funiculus 



-Thoracic portion of- - 
spinal cord 




I Lumbar enlargement 



Conus medullaris 



Filum terminale 



Rhomboid fossa 



Posterior median 
sulcus 

Posterior funic- 
ulus 

Posterior inter- 
mediate sulcus 

Dorsal root 



- Spinal nerve 



. 



^-Cauda equina 



Fig. 50. Fig. 51. Fig. 52. 

Figs. 50-52. Three views of the spinal cord and rhombencephalon : Fig. 50, Lateral view 
with spinal nerves attached; Fig. 51, ventral view with spinal nerves removed; Fig. 52, dorsal 
view with spinal nerves attached. (Modified from Spalteholz.) 

sensible gradations. According to this method of subdivision there are in the 
cervical portion of the cord eight segments, in the thoracic twelve, in the lumbar 
five, and in the sacral five, while there is but one coccygeal segment. 



76 THE NERVOUS SYSTEM 

Several longitudinal furrows are seen upon the surface of the cord (Figs. 51, 
52). Along the middle line of the ventral surface is the deep anterior median 
fissure (fissura mediana anterior). This extends into the cord to a depth 
amounting to nearly one-third of its anteroposterior diameter and contains a 
fold of pia mater. Along the middle line of the dorsal surface there is a shallow 
groove, the posterior median sulcus (sulcus medianus posterior). As may be 
seen in cross-sections of the spinal cord, it is divided into approximately sym- 
metric lateral halves by the two furrows just described and by the posterior 
median septum (Figs. 55, 56, 57). On either side, corresponding to the line of 
origin of the ventral roots, is a broad, shallow, almost invisible groove, the 
anterolateral sulcus (sulcus lateralis anterior). And again on either side, cor- 
responding to the line of origin of the dorsal roots, is the narrower but deeper 
posterolateral sulcus (sulcus lateralis posterior). These six furrows extend the 
entire length of the spinal cord. In the cervical region an additional longi- 
tudinal groove may be seen on the dorsal surface between the posterior median 
and posterolateral sulci, but somewhat nearer the former. It is known as the 
posterior intermediate sulcus and extends into the thoracic cord, where it grad- 
ually disappears. 

Funiculi. By means of these furrows and the subjacent gray matter each 
lateral half of the cord is subdivided into columns of longitudinally coursing 
nerve-fibers known as the anterior, lateral, and posterior funiculi (funiculus 
anterior, funiculus lateralis et funiculus posterior). In the cervical and upper 
thoracic regions the posterior intermediate sulcus divides the posterior funiculus 
into a medial portion, the fasciculus gracilis, and a lateral portion, the fasciculus 
cuneatus. 

Nerve Roots. From the lateral funiculus in the upper four to six cervical 
segments there emerge, a little in front of the dorsal roots of the spinal nerves, 
a series of root filaments which unite to form the spinal root of the accessory 
nerve (Fig. 125). This small nerve trunk ascends along the side of the cord, 
enters the cranial cavity through the foramen magnum, and carries to the 
accessory nerve the fibers for the innervation of the sternocleidomastoid and 
trapezius muscles. 

From the posterolateral sulcus throughout the entire length of the spinal 
cord emerge an almost uninterrupted series of root filaments (fila radicularia). 
Those from a given segment of the cord unite to form the. dorsal root of the cor- 
responding spinal nerve. The filaments of the ventral roots emerge from the 
broad, indistinct anterolateral sulcus in groups, several appearing side by side, 



THE SPINAL CORD 



77 



rather than in the accurate linear order characteristic of the dorsal roots. Those 
from a given segment unite with each other to form a ventral root; and that in 
turn joins with the corresponding dorsal root just beyond the spinal ganglion to 
form the mixed nerve (Fig. 50). 

Relation of the Spinal Cord and Nerve Roots to the Vertebral Column. 
At an early fetal stage the spinal cord occupies the entire length of the vertebral 



Infrahyoid muscles 



Diaphragm 



Muscles of shoulder, arm, 
and hand 




Cervical segments of spinal cord 



Thoracic segments of spinal cord 



Lumbar segments of spinal cord 

Sacral and coccygeal segments of 
spinal cord 



Abdominal musdes 



Flexors of hip / 

Extensors of the kneel 

and adductors of hip[ 

Other muscles of thigh, 
leg, and foot 

Perinea! and anal mus- 
cles 



Fig. 53. Diagram showing the level of the various segments of the spinal cord with reference to 
the vertebra?, with a table showing the distribution of the fibers of the several ventral roots. 

canal and the spinal nerves pass horizontally lateralward to their exit through 
the intervertebral foramina. As development progresses the vertebral column 
increases in length more rapidly than the spinal cord, which, being firmly an- 
chored above by its attachment to the brain, is drawn upward along the canal, 
until in the adult it ends at about the lower border of the first lumbar vertebra. 



78 THE NERVOUS SYSTEM 

At the same time the roots of the lumbar and sacral nerves become greatly 
elongated. They run in a caudal direction from their origin to the same inter- 
vertebral foramina through which they made their exit before the cord shifted 
its position. Since the thoracic portion of the cord has changed its relative 
position but little, and the cervical part even less, the cervical roots run almost 
directly lateralward, while those of the thoracic nerves incline but little in a 
caudal direction. 

Since the spinal cord ends opposite the first or second lumbar vertebra, the 
roots of the lumbar, sacral, and coccygeal nerves, in order to reach their proper 
intervertebral foramina, descend vertically in the canal around the conus medul- 
laris and filum terminale. In this way there is formed a large bundle, which is 
composed of the roots of all the spinal nerves below the first lumbar and has 
been given the very descriptive name cauda equina. 

The amount of relative shortening of the various segments of the cord differs 
in different individuals. In Fig. 53, where the quadrilateral areas represent the 
bodies of the vertebrae, we have indicated the average position of each segment 
of the spinal cord. This figure is based on data published by Reid (1889). It is 
obvious that the segments are longer in the thoracic than in the cervical and 
lumbar portions of the cord, while the sacral segments are even shorter (see 
also Fig. 59). 

We have been at some pains to explain the development of the cauda equina 
and the vertebral level of the various segments of the spinal cord because these 
are matters of much practical importance. In spinal puncture the needle is 
made to enter the subdural space caudal to the termination of the cord. In 
locating lesions of the spinal cord it is necessary to know the position of its 
various segments with reference to the vertebras. It is particularly important 
to be able to distinguish between an injury to the lower part of the spinal cord 
and one which involves only the nerve roots in the cauda equina, since, although 
the symptoms in the two cases may be nearly identical, damage to the spinal 
cord is irreparable, while the nerve roots will regenerate. 

The Spinal Cord in Section. When a section is made through any part of 
the brain or spinal cord one sees at once that they are composed of two kinds 
of tissue the one whitish in color, the other gray, tinged with pink. The white 
substance consists chiefly of myelinated fibers, the gray is made up of nerve- 
cells, dendrites, unmyelinated and myelinated fibers, and many blood-vessels. 
Both have a supporting framework of neuroglia. 

The gray substance (substantia grisea) of the spinal cord is centrally placed 



THE SPINAL CORD 



79 



and forms a continuous fluted column, which is everywhere enclosed by the 
white matter (Fig. 54). In cross-section it has the form of a letter H (Fig. 55). 
There is a comma-shaped gray field in each lateral 
half of the cord, and these are united across the 
middle line by a transverse gray bar. The enlarged 
anterior end of the comma has been known as the ven- 
tral horn, the tapering posterior end as the dorsal 
horn, and the transverse bar as the gray commissure. 
But, when it is remembered that the gray substance 
forms a continuous mass throughout the length of the 
spinal cord, it will be seen that the term "column" 
is more appropriate than "horn." The long gray mass 
in either lateral half of the cord is convex medially and concave laterally. It 
projects in a dorsolateral direction as the posterior column (columna posterior). 

As seen in a cross-section of the cervical cord, the posterior column is rela- 
tively long and narrow and nearly reaches the dorsolateral sulcus (Fig. 55). 




Fig. 54. Diagram of gray 
columns of spinal cord. 



Posterior intermediate sulcus and septum 

Collaterals from cuneate fasc. 

Substantia gelaiinosa 

Posterolateral sulcus 
\ 

Ceraix . 



Posterior column{ ^ if?" ~ \ 



Posterior median sulcus and septum 
Fasciculus gracilis \ Posterior 
\ Fasckulus cuneatus j fnniculus 

Dorsal root 

Dorsolateral fasciculus 
(Lissauer) 
lateral funiculus 



Reticular formation 




Posterior ...| 

com. 

Anterior ._S 
gray com. 

Anterior - 
white com. 
Anterior' 
column 

Anterolateral sulcus ' .7'^^ _ -Anterior funiculus 

A nterior median fissure 

Fig. 55. Section through seventh cervical segment of the spinal cord of a child. Pal-Weigert 

method. 

It presents a constricted portion known as the cervix, a pointed dorsal extrem- 
ity or apex, and between the two an expanded part sometimes called the caput. 
The apex consists largely of a special variety of gray substance, gelatinous in 



80 THE NERVOUS SYSTEM 

appearance in the fresh condition and very difficult to stain by neurologic meth- 
ods, which in sections has a A -shaped outline. It is known as the substantia 
gelatinosa Rolandi. In the thoracic portion the posterior column, which is here 
very slender, does not come so close to the surface; and in the lumbosacral seg- 
ments it is much thicker (Figs. 56, 57). 

The anterior column is relatively short and thick and projects toward the 
anterolateral sulcus. It contains the cells of origin of the fibers of the ventral 
root. From its lateral aspect nearly opposite the gray commissure there pro- 
jects a triangular mass, known as the lateral column (columna lateralis). This 
is prominent in the thoracic and upper cervical segments; but it blends with 
the expanded anterior column in the cervical and lumbar enlargements (Fig. 56). 

Posterior median sulcus and septum Posterior funiculus 

Substantia gelatinosa \ Dorsolateral fasciculus (Lissauer) 

Posterolateral sukus ^^^^^ Dorsal root 

BiBbU; Lateral funiculus 
Apex oj posterior column 

Nucleus dorsalis 
lateral column 




'! ^---^-- ^Central canal 
Anterior white commissure ' ^iwKSB^H ^f^^r .... 

. . , .--' ----. y-^ Anterior funicmus 

Anterior column' ;~^--^_ . ,. ,. 

" Anterior median jissure 

Fig. 56. Section through the seventh thoracic segment of the spinal cord of a child. Pal-Weigert 

method. 

The reticular formation (formatio reticularis) , situated just lateral to the cer- 
vix of the posterior column in the cervical region, is a mixture of gray and 
white matter (Fig. 55). Here a network of gray matter extends into the white 
substance, breaking it up into fine bundles of longitudinal fibers. The reticular 
formation is most evident in the cervical region, but traces of it appear at other 
levels. 

The gray commissure contains the central canal, and by it is divided into the 
posterior commissure (commissura posterior) and the anterior gray commissure 
(commissura anterior grisea). Ventral to the latter many medullated fibers 
cross the midline, constituting the anterior white commissure. 

The cavity of the neural tube persists as the central canal, which lies in the 
gray commissure throughout the entire length of the cord. The canal is so 
small as to be barely visible to the naked eye. It is lined with ependymal 



THE SPINAL CORD 



8l 



epithelium and the lumen is often blocked with epithelial debris. The canal, 
which is narrowest in the thoracic region, expands within the lower part of the 
conus medullaris to form a fusiform dilatation, the ventriculus terminates. 

Posterior median sulcus and septum 
Collaterals from fasciculus cuneat-us \ Posterior fun i en I us 



Dorsal root 
I 



Dorsolateral fascicuhis (Lissauer} 
! Poster olateral sulcus 



Substantia gelati 



{4 "bex - 
r 
Lervix 




Posterior commissure 

Anterior gray - 
commissure 

Anterior white com...- 
Anterior column.*- 



Fig. 57. Section through the fifth lumbar segment of the spinal cord of a child. Pal-Weigert 

method. 

Dorsal roots of lumbar and sacral nerves 

Posterior fun iculus 

., 

: ''-'- Substantia gelatinosa 

Dorsolateral fasciculus 
&5HB^. Posterior column 



^- Lateral funiculus 




'Anterior column 

*~ Ventral roots oj "lumbar and 
sacral nerves 

Fig. 58. Section of the third sacral segment of the human spinal cord and the lumbosacral nerve 
roots of the cauda equina. Pal-Weigert method. 

The White Substance. The long myelinated fibers of the cord, arranged in 
parallel longitudinal bundles, constitute the white substance which forms a 



82 



THE NERVOUS SYSTEM 



thick mantle surrounding the gray columns. In each lateral half of the cord it 
is divided into the three great strands or funiculi, which have been described 



White matter. 



Grey matter. 



-Ertrire secMon. 



1OO 

o 

eo 

40 
20 




I II lfl IY 



WIYIO 1 H HI W Y H MI H1I IX X XI XII 1 II IfllVYI 



Fig. 59. Curves showing the variations in sectional area of the gray matter, the white matter, and 
the entire cord in the various segments of the human spinal cord. (Donaldson and Davis.) 

on the surface of the cord. The anterior funiculus (funiculus anterior) is bounded 
by the anterior median fissure, the anterior column, and the emergent fibers 
of the ventral roots. The lateral funiculus (funiculus lateralis) lies lateral to 




V//C- VII I C 



vine -ID 



II D 



VII D 




XII D 



III S 



IV 5 C 

Fig. 60. Outline drawings of sections through representative segments of the human spinal cord. 

the gray substance between the anterolateral and posterolateral sulci, i. e., 
between the lines of exit of the ventral and dorsal roots. The posterior funiculus 
(funiculus posterior) is bounded by the posterolateral sulcus and posterior col- 



THE SPINAL CORD 83 

CHARACTERISTIC FEATURES OF TRANSVERSE SECTIONS AT VARIOUS LEVELS OF THE SPINAL CORD 



Level. 


Cervical. 


Thoracic. 


Lumbar. 


Sacral. 


Outline 


Oval, greatest di- 
ameter transverse 


Oval to circular 


Nearly circular 


Circular to 
quadrilateral 


Volume of gray 
matter 


Large 


Small 


Large 


Relatively 
large 


Anterior gray 
column 


Massive 


Slender 


Massive 


Massive 


Posterior gray 
column 


Relatively slender, 
but extends far 
posteriorly 


Slender 


Massive 


Massive 


Lateral gray 
column 


Absorbed in the 
anterior except in 
the upper three 
cervical segments 


\Vell marked 


Absorbed in the 
anterior column 


Present 


Processus 
reticularis 


Well developed 


Poorly developed 


Absent 


Absent 


White matter 


In large amount 


Less than in the 
cervical region, 
but relatively a 
large amount in 
comparison to the 
gray matter 


Slightly less than in 
the thoracic re- 
gion; very little 
in comparison to 
the large volume 
of the gray 


Very little 


Sulcus interme- 
dius posterior 


Present throughout 


Present in upper 
seven thoracic 
segments 


Absent 


Absent 



umn on the one side, and the posterior median septum on the other. The sep- 
tum, just mentioned, completely separates the two posterior funiculi from each 
other. Incomplete septa project into the white substance from the enveloping 
pia mater. One of these, more regular than the others, enters along the line of 
the posterior intermediate sulcus. It is restricted to the cervical and upper 
thoracic segments, is known as the posterior intermediate septum, and divides 
the posterior funiculus into two bundles, the more medial of which is known 
as the fasciculus gracilis, while the other is called the fasciculus cuneatus. 

Characteristics of the Several Regions of the Spinal Cord. It will be ap- 
parent from Figs. 55-58 that the size and shape of the spinal cord, as seen in 
transverse section, varies greatly at the different levels and that the relative 
proportion of gray and white matter is equally variable. Two factors are 



84 THE NERVOUS SYSTEM 

primarily responsible for these differences. One of these is the variation in the 
size of the nerve roots at the different levels; for where great numbers of nerve- 
fibers enter, they cause an increase in the size of the cord and particularly in 
the volume of the gray matter. It has already been pointed out that the cer- 
vical and lumbar enlargements are directly related to the large nerves supply- 
ing the extremities. The second factor is this: Since all levels of the cord are 
associated with the brain by bundles of long fibers, it is obvious that such long 
fibers must increase in number and the white matter increase in volume as we 
follow the cord from its caudal end toward the brain. All this is well illus- 
trated in a diagram by Donaldson and Davis reproduced in Fig. 59. 

The outline of a section of the spinal cord at the fourth sacral segment is some- 
what quadrilateral. The total area is small and the greater part is occupied 
by the thick gray columns (Fig. 60). The size of the cord is much greater at 
the level of the first sacral and fifth lumbar segments, as might be expected from 
the large size of the associated nerves (Figs. 57, 60). There is both an absolute 
and a relative increase in the white substance, which here contains the long 
paths connecting the sacral portions of the spinal cord with the brain. Both 
the anterior and posterior columns are massive, and the anterior presents a 
prominent lateral angle. The large nerve-cells in the lateral part of the an- 
terior column give rise to the fibers which run to the muscles of the leg. At the 
level of the seventh thoracic segment (Figs. 56, 60) the cross-sectional area is less 
than in the lumbar enlargement. Corresponding to the small size of the tho- 
racic nerves the gray matter in this region is much reduced, both anterior and 
posterior columns being very slender. The apex of the latter is some distance 
from the surface and its cervix is thickened by a column of cells known as the 
nucleus dorsalis. The columna lateralis is prominent. The white matter is 
somewhat more abundant than in the lumbar region, and increases slightly in 
amount as we follow the cord rostrally through the thoracic region (Fig. 59). 

A transverse section at the level of the seventh cervical segment is elliptic in 
outline and has an area greater than that of any other level of the cord (Figs. 
55, 60). The white matter is voluminous and contains the long fiber tracts 
connecting the brain with the more caudal portions of the cord. The gray 
matter is also abundant, as we might expect from the large size of the seventh 
cervical nerve. The ventral column is especially thick and presents a prominent 
lateral angle. The large laterally placed nerve-cells of the anterior column are 
associated with the innervation of the musculature of the arm. The posterior 
column is relatively slender, but reaches nearly to the dorsolateral sulcus. 



THE SPINAL CORD 85 

MICROSCOPIC ANATOMY 

Neuroglia. Occupying the interstices among the true nervous elements of 
the central nervous system is a peculiar supporting tissue, the neuroglia, which 
is of ectodermal origin. In the chapter on Histogenesis we learned that from 
the original epithelium of the neural tube there are differentiated spongioblasts 
and neuroblasts, as well as a special epithelial lining for the tube, the ependyma. 




Fig. 61. Ependyma and neuroglia in the region of the central canal of a child's spinal cord: 
A, Ependymal cells; B and D, spider cells in the white and gray matter, respectively; C, mossy 
cells. Golgi method. (Cajal.) 

The latter consists of long nucleated columnar cells which line the central canal 
of the spinal cord as well as the ventricles of the brain (Fig. 61). In fetal life 
their free ends bear cilia, which project into the lumen of the tube, and fine 
processes from the outer ends extend to the periphery of the cord. In the adult 
there are no cilia and the peripheral processes reach the surface only along the 
posterior median septum and in the anterior median fissure. 



86 THE NERVOUS SYSTEM 

The neuroglia cells are differentiated from the spongioblasts. These, when 
stained by the Golgi method, appear as small cells with many processes. Some 
have long slender processes, the spider cells or long rayed astrocytes; others have 
short thick varicose processes, the mossy cells or short rayed astrocytes (Fig. 
61). Special neuroglia stains, like that of Weigert, show that an astrocyte is 
composed of a glia cell associated with many glia fibers. Some authors main- 
tain that the fibers run through the cytoplasm, while others assert that they 
merely pass along the surface of the cell. In any case the fibers are to be re- 
garded as products of these cells. Neuroglia cells and fibers are found every- 
where throughout the gray and white matter of the spinal cord, forming a sup- 
porting framework for the nervous elements. A special condensation of neu- 




Unmyelinated fibers 



inated fibers 



Fig. 62. From a cross-section through the spinal cord of a rabbit showing the structure of the white 
matter as revealed by the Cajal method. (Cajal.) 

roglia surrounds the central canal and is known as the substantia gelatinosa 
centralis. In addition to the neuroglia this contains some nerve-fibers and 
cells. Beneath the pia mater and closely investing the spinal cord externally 
is a thin stratum of neuroglia, the glial sheath, which dips into the cord along 
with the pial septa. The posterior median septum is composed of neuroglia 
and greatly elongated ependymal elements, and is in no part formed by the 
pia mater. 

White Substance. The white matter of the spinal cord consists of longi- 
tudinally coursing bundles of nerve-fibers, bound together by a feltwork of 
neuroglia fibers in which are scattered neuroglia cells. A majority of the neu- 
roglia fibers run in a direction transverse to the long axis of the nerve-fibers. 
Blood-vessels enter the cord from the pia mater and are accompanied by con- 



t THE SPINAL CORD 87 

nective tissue from the pia and by the subpial neuroglia. It has been generally 
supposed that the white fascicles of the cord were composed almost exclusively 
of myelinated fibers; and it is true that these, partly because of their size, are 
the most conspicuous elements. In cross-sections stained by the Weigert 
method the myelin sheaths alone are stained; and since the fibers are cut at 
right angles to their long axes, they appear as rings. Cajal (1909) has shown 
that there are also great numbers of unmyelinated fibers in the longitudinal 
fascicles of the cord (Fig. 62). The different fascicles differ not only in the size 
of their myelinated fibers but also in the proportion of unmyelinated fibers 
which they contain. The fasciculus dorsolateralis or tract of Lissauer (Fig. 63) 
contains fine myelinated fibers with great numbers of unmyelinated axons. 




Fig. 63. From a cross-section of the spinal cord of the cat; a narrow strip extending across 
the apex of the posterior gray column: a, Fasciculus cuneatus; b, fasciculus dorsolateralis (Lis- 
sauer) ; c, dorsal spinocerebellar tract. The unmyelinated fibers appear as black dots. Pyridin- 
silver method. 

Close to it lies the dorsal spinocerebellar tract which is composed almost ex- 
clusively of large myelinated fibers. 

Gray Substance. The gray matter is composed of nerve-cells, including 
their dendrites, and of unmyelinated axons and smaller numbers of myelinated 
fibers all supported by a neuroglia framework and richly supplied with capil- 
lary blood-vessels. The axons of the cells of Golgi's Type I are very long and 
run out into the white substance or into the ventral roots. Those of the cells 
of his Type II are short and end within the gray matter. In addition, great 
numbers of collaterals from the dorsal root fibers and from the longitudinal 
fibers of the cord, as well as terminal branches of these fibers, enter the gray 
substance and ramify extensively within it, entering into synaptic relations 
with the neurons which it contains. The branches of the myelinated fibers 
soon lose their sheaths, and it is this relative scarcity of myelin which gives to 



88 



THE NERVOUS SYSTEM 



this substance its gray appearance. The ramification of dendrites and unmy- 
elinated fibers forms a very intricate feltwork throughout the gray substance 
(Fig: 64). 

The nerve-cells of the spinal cord vary greatly in size. The largest are 
situated in the anterior column and may measure more than 100 micra. They 
are all multipolar, possess each a single axon, and may be classified in four groups: 
(1) Some of the cells, found in the posterior horn and particularly in the sub- 
stantia gelatinosa Rolandi, belong to Golgi's Type II, with short axons confined 
to the gray substance. These, however, are present in relatively small numbers 
in the spinal cord. (2) The motor cells, situated in the anterior column and 




Fig. 64. From a section through the spinal cord of a monkey; showing part of the an- 
terior gray column including a multipolar nerve-cell and the surrounding neuropil. Pyridin- 
silver method. 

most numerous in the cervical and lumbar enlargements, are of large size and 
possess axons which leave the cord in the ventral roots. (3) Smaller cells are 
present in the lateral column in the thoracic region and give rise to the visceral 
efferent fibers of the ventral roots (Fig. 37). (4) Other cells of small or medium 
size, found chiefly in the posterior column, possess axons which pass into the 
white matter, where they bend sharply to become ascending or descending 
fibers, or divide dichotomously into ascending and descending branches (Fig. 
68). Some of the ascending fibers reach the brain; the others merely connect 
the different levels of the spinal cord. The fibers of the latter group constitute 
the fasciculi proprii and vary greatly in length, some connecting adjacent, 



THE SPINAL CORD 89 

others, more remote, segments. Their collateral and terminal branches re- 
enter and ramify within the gray substance. Those which remain throughout 
in the same lateral half of the cord are called association fibers; while others, 
known as commissural fibers, cross the median plane chiefly in the white com- 
missure (Fig. 68). Some of the commissural fibers are short and confined to a 
single level of the cord (Fig. 66). 

Cell-columns. The nerve-cells are not uniformly distributed throughout 
the gray matter, for many of them are arranged in longitudinal cell-columns. 
In transverse sections each of these columns appears as a distinct group of 
cells, somewhat separated from other similar groups within the gray matter 
(Fig. 65). The large motor cells of the anterior column, which give origin to 
the ventral root fibers, form several subgroups. One of these, known as the 
anteromedian cell-column, occupies the medial part of the anterior column through- 
out almost its entire length, being absent only in the fifth lumbar and first 
sacral segments. Behind it is the posteromedian cell-column, which is, however, 
present only in the thoracic and first lumbar segments and for a short stretch 
in the cervical region. The axons from these two medial groups of cells prob- 
ably supply the musculature of the trunk. In the cervical and lumbar enlarge- 
ments there are laterally placed groups of cells the axons of which supply the 
muscles of the limbs. These are: (1) the anterolateral cell-column, present in 
the fourth to the eighth cervical and in the second lumbar to the second 
sacral segments; (2) the posterolateral cell-column in the last five cervical, 
last four lumbar, and first three sacral segments; (3) the retro posterolateral 
cell-column in the eighth cervical, first thoracic, and first three sacral seg- 
ments, and (4) the central cell-column in the second lumbar to the second sacral 
segments. 

The intermediolateral cell-column is found in the lateral column in the tho- 
racic region of the cord and is prolonged downward into the upper lumbar seg- 
ments. It is composed of small cells, the axons of which run through the ven- 
tral roots, spinal nerves, and white rami communicantes into the sympathetic 
nervous system (Fig. 37). They have to do with the innervation of smooth 
and cardiac muscle and glandular tissue. The longitudinal extent of this 
column corresponds quite accurately to that of the spinal origin of the white 
rami. A group of cells, having a similar function, is also found in the third 
and fourth sacral segments. 

The cells of the posterior gray column are smaller, as a rule, than those of the 
ventral column: and except for the nucleus dorsalis they are not arranged in 



9 o 



THE NERVOUS SYSTEM 



definite groups. They are concerned with the reception and distribution of 
the impulses entering along the fibers of the dorsal roots. 




Si 



S4- 



Fig. 65. Outline sketches of ventral horn of left side of cord at different levels, showing the 
relative number and position of the chief cell-groups: C\, C\, T 6 , etc., indicate the segments e. g., 
first cervical, fourth cervical, sixth thoracic; C (b), lower part of eighth cervical. The following 
letters designate the cell-groups: v-m, Anteromedian; d-nt, posteromedian ; v-l, anterolateral ; 
d-l, posterolateral ; p. d-l, retroposterolateral; v in LV, L*, ventral; c in L%, L 4 , S\, central; /. c. in 
T 6 , Ti2, intermediolateral ; ace. in C\, C 4 , accessorius; phr. in C 4 , phrenic; Cl.c. in T 6 , Tu, nucleus 
dorsalis. (Bruce, Quain's Anatomy.) 

The nucleus dorsalis, or column of Clarke, is a group of large cells in the 
medial part of the base of the posterior column. It extends from the last cer- 



THE SPINAL CORD g r 

vical or first thoracic to the second or third lumbar segments. It is a prom- 
inent feature in cross-sections of the thoracic cord, appearing as a well-defined 
oval area richly supplied with collaterals from the dorsal roots. The cells have 
an oval or pyriform shape; each has several dendritic processes and an axon 
which enters the lateral funiculus, within which it runs toward the cerebellum 
in the dorsal spinocerebellar tract. 

The Spinal Reflex Mechanism. In the next chapter we will consider at 
length the long ascending and descending paths in the white substance of the 
cord by which afferent impulses from the spinal nerves reach the brain, and 
those through which the motor centers of the brain exert in return a controlling 
inf uence over the spinal motor apparatus. But fully as important as these are 
the purely intraspinal connections the spinal reflex mechanism. 




Fig. 66. Diagrammatic section through the spinal cord and a spinal nerve to illustrate a 
simple reflex arc: a, b, c, and d, Branches of sensory fibers of the dorsal roots; e, association neuron; 
/, commissural neuron. 

A reflex arc in its simplest form may be made up of only two neurons, the 
primary sensory and motor neurons with a synapse in the gray matter of the 
anterior column (Fig. 66). It consists of the following parts: (1) a receptor, 
the peripheral sensory endings; (2) a conductor, the afferent nerve-fiber; (3) a 
center, including the synapse in the anterior column; (4) a second conductor, 
the efferent nerve-fiber, and (5) an effector, the muscle-fiber. Usually, how- 
ever, there are interposed between the primary sensory and motor elements 
one or more intermediate neurons. These, when restricted to one side of the 
cord, are known as association neurons; when their axons cross the median 
plane, as many of them do through the anterior white commissure, they are 
called commissural neurons. When the circuit is complete within a single neural 



92 THE NERVOUS SYSTEM 

segment it may be said to be intrasegmental (Fig. 66) ; if it extends through two 
or more such segments it is an intersegmental reflex arc. 

Intersegmental Reflex Arcs. Impulses entering the spinal cord through a 
given dorsal root may be transmitted to the primary motor neurons of another 
segment in one of two ways: (1) by way of the ascending and descending branches 
of the dorsal root fibers, and (2) along the fibers of the fasciculi proprii (Fig. 67) . 
A full account of these two pathways will be presented in the next chapter, 
but a word of explanation is required here. The fibers of the dorsal root divide, 




Fig. 67. Diagram of the spinal cord, showing the elements concerned in a diffuse unilat- 
eral reflex: a, Spinal ganglion cell; b, motor cell in anterior column; c, association neuron. 
(Cajal.) 

soon after their entrance into the cord, into long ascending and shorter descend- 
ing branches, which together form the greater part of the posterior funiculus 
and give off many collaterals to the gray matter of the successive levels of the 
cord (Fig. 67). Many of the ascending branches reach the brain; but the others 
terminate, as do the descending branches and all the collaterals, in the gray 
matter of the cord (Fig. 68). The fasciculi proprii immediately surround the 
gray columns (Fig. 68) and consist of ascending and descending fibers, which 
arise and terminate within the gray substance of the cord. Most of these 
fibers remain on the same side as association fibers concerned in unilateral re- 



THE SPINAL CORD 



93 



flexes. Others cross in the anterior white commissure and are commissural 
fibers concerned in crossed reflexes. Afferent impulses may be transmitted 
along the cord in either direction by the branches of the dorsal root fibers; or by 
means of synapses in the gray matter they may be transferred to the long asso- 
ciation and commissural fibers and conveyed to the primary motor neurons of 
the same or opposite side in more or less distant segments. The course of a 
nerve impulse in a unilateral intersegmental reflex is indicated on the left side 



Dorsal root 



Ventral root 



Ascending branch of dorsal root fiber 

Association fibers -'-'' 
Descending branch of dorsal foot fiber 



Dorsal root 




Commissural fibers 

I 



Ventral root 



Fig. 68. Diagram of the spinal cord, showing the elements concerned in intersegmental reflexes. 

of Fig. 68, while on the right side of the same figure are shown the elements 
concerned in crossed reflexes. 

The observations of Coghill (1913 and 1914) and of Herrick and Coghill (1915) tend to 
show that the simple form of reflex arc illustrated in Fig. 66 is not the primitive type. In 
larval Amblystoma the first arcs to become functionally mature are composed of chains 
of many neurons, so arranged that every cutaneous stimulus elicits the same complex response 
of the entire somatic musculature, i. e., the swimming movement. It is of particular interest 
to note that in this primitive reflex mechanism the sensory fibers arise from giant cells located 
within the spinal cord and that the ventral root fibers are collaterals from the central motor 
tract. In adult Amblystoma these sensory and motor elements are replaced by the usual 
type of primary sensory and motor neurons. 



94 



THE NERVOUS SYSTEM 



We may mention as an example of a reflex arc involving many segments of 
the cord the ''scratch-reflex" of the dog, which has been very carefully investi- 
gated by Sherrington (1906). If, some time after transection of the spinal cord 
in the low cervical region, the skin covering the dorsal aspect of the thorax be 
stimulated by pulling lightly on a hair, the hind limb of the corresponding side 
begins a series of rhythmic scratching movements. By degeneration experi- 
ments it was shown that this reflex arc probably includes the following elements: 
(1) a primary sensory neuron from the skin to the spinal gray matter of the 
corresponding neural segment; (2) a long descending association neuron from the 




Fig. 69. Diagram of the spinal arcs involved in the scratch-reflex: Ra and Rp, Receptive 
paths from hairs in the dorsal skin of left side; Pa and P/3, association neurons; FC, motor fibers of 
ventral root. (Sherrington.) 

shoulder to the leg segments, and (3) a primary motor neuron to a flexor muscle 
of the leg (Fig. 69). 

A primary motor neuron seldom, if ever, belongs exclusively to one arc, but 
serves as the final channel to which many streams converge. Its perikaryon 
gives off wide-spread dendritic processes, through which it comes into relation 
with the ramifications of axons from many different sources. In this way 
impulses reach it from the dorsal roots, and from the fasciculi proprii of the 
spinal cord, as well as from a number of tracts which descend into the spinal 
cord from centers in the brain (the corticospinal, rubrospinal, tectospinal, and 
vestibulospinal tracts). The primary motor neuron is, as Sherrington has said, 
"the final common path." 



CHAPTER VII 

FIBER TRACTS OF THE SPINAL CORD 

THE fibers composing the white substance of the spinal cord are not scat- 
tered and intermingled at random, but, on the contrary, those of a given func- 
tion are grouped together in more or less definite bundles. A bundle of fibers 
all of which have the same origin, termination, and function is known as a fiber 
tract. The funiculi of the spinal cord are composed of many such tracts of 
longitudinal fibers, which, while occupying fairly definite areas, blend more or 
less with each other, in the sense that there is considerable intermingling of the 
fibers of adjacent tracts. It is convenient to have a name for certain obvious 
subdivisions of the funiculi which contain fibers belonging to more than one tract. 
Such a mixed bundle is properly called a. fasciculus. 

THE INTRAMEDULLARY COURSE OF THE DORSAL ROOT FIBERS 

The central end of a dorsal root breaks up into many rootlets or filaments 
(fila radicularia) , which enter the spinal cord in linear order along the line of 
the posterior lateral sulcus. As it enters the cord each filament can be seen to 
separate into a larger medial and a much smaller lateral division. The fibers of 
the medial division are of relatively large caliber and run over the tip of the 
posterior column into the posterior funiculus (Fig. 72). Those of the lateral 
division are fine and enter a small fascicle which lies along the apex of the pos- 
terior column, the fasciculus dorsolateralis or tract of Lissauer. Very soon 
after their entrance into the cord each dorsal root fiber divides in the manner of 
a Y into a longer ascending and a shorter descending branch (Fig. 70). 

The ascending branches of the fibers of the medial division of the dorsal root 
run for considerable but varying distances in the posterior funiculus; some from 
each root reach the medulla oblongata, others terminate at different levels in the 
gray matter of the spinal cord. At the level of their entry into the cord these 
fibers occupy the lateral portion of the fasiculus cuneatus; but in their course 
cephalad, as each successive root adds its quota, those from the more caudal 
roots are displaced medianward. In this way the longer fibers come to occupy 
the medial portion of the posterior funiculus (Fig. 71). In the cervical regior 

95 



96 THE NERVOUS SYSTEM 

the long ascending fibers from the sacral, lumbar, and lower thoracic roots 
constitute a well-defined medially placed bundle, the fasciculus gracilis, sepa- 
rated from the rest of the posterior funiculus by the posterior intermediate 
septum. Those of the long ascending fibers, which finally reach the brain, 
terminate in gray masses in the posterior funiculi of the medulla oblongata 




Fig. 70. Bifurcation of the dorsal root fibers within the spinal cord into ascending and 
descending branches, which in turn give off collaterals; the termination of some of these col- 
laterals in synaptic relation to cells of the posterior gray column. (Cajal, Edinger.) 

(nucleus of the funiculus gracilis and nucleus of the funiculus cuneatus). Since 
the number of these long ascending branches must increase from below upward 
it is easy to understand the progressive increase in size of the posterior funiculus 
from the sacral to the cervical region (Fig. 60). 

The fasciculus gracilis and fasciculus cuneatus are composed for the most 



FIBER TRACTS OF THE SPINAL CORD 



97 



Fasc. gracilis 
\ Fasc. cuneatus 



part of these ascending branches of the dorsal root fibers, the former contain- 
ing those which have the longest intramedullary course. 

The descending branches of the fibers of the medial division of the dorsal 
root are all relatively short. The shortest terminate at once in the gray matter 
of the posterior column. Others descend in the fasciculus interfascicularis, or 
comma tract of Schultze, which is situated near the center of the posterior fu- 
niculus; and still others run near the posterior median septum in the septomar- 
ginal fasciculus (Fig. 76). In both of these fas- 
cicles they are intermingled with descending fibers, 
arising from cells within the gray matter of the spinal 
cord. 

Collaterals. At intervals along both ascending 
and descending branches collaterals are given off which 
run ventrally to end in the gray matter (Fig. 70). 
They are much finer than the fibers from which they 
arise, and the total number arising from a given fiber 
is rather large. Some of them end in the ventral 
gray column; others, in the posterior gray column, 
including the substantia gelatinosa and the nucleus 
dorsalis; still others run through the dorsal com- 
missure to the opposite side of the cord, where they 
appear to end in the posterior columns (Fig. 72). In 
Fig. 70 there are illustrated the arborizations formed 
by some of these collaterals about cells of the posterior 
column. 

The terminals of the descending branches and of 
those ascending branches, which do not reach the brain, 
end as do the collaterals within the gray matter of the 
spinal cord. 

The fibers of the lateral division of the dorsal root are all very fine. The 
majority are unmyelinated and can be recognized only in preparations in which 
the axons are stained. A good account of their appearance in Golgi prepara- 
tions has been given by Barker (1899, pp. 466-468). In Weigert preparations 
we must look carefully to find the few myelinated fibers contained in this divi- 
sion. But in pyridin-silver preparations great numbers of fine unmyelinated 
fibers, accompanied by a few which are myelinated, can be seen to turn lateral- 
ward as the root filament enters the cord. These constitute the lateral division 




Fig. 71. Diagram to 
illustrate the arrangement 
of the ascending branches 
of the dorsal root fibers 
within the posterior funic- 
ulus of the spinal cord. 



THE NERVOUS SYSTEM 



of the root and enter the dorsolateral fasciculus or tract of Lissauer (Fig. 72).' 
The medial division, on the other hand, consists exclusively or almost exclu- 
sively of myelinated fibers. The fibers of the lateral division of the root divide 
into ascending and descending branches, both of which, however, are very 
short. The ascending branch, which is the longer of the two, does not extend 
at most more than the length of one or two segments in the long axis of the 
cord (Ranson, 1913, 1914). 

The dorsolateral fasciculus, or tract of Lissauer, lies between the apex of 
the posterior column and the periphery of the cord, and varies greatly in shape 
and size in the different levels of the cord (Figs. 55-58). It is composed of 



Medial division of dorsal root 



~^X. Fasciculus cuneatus 

Dorsolateral 
fasciculus 



Lateral division of dorsal 
root 




_ Dorsal spino- 
cerebellar tract 



Dorsal spinocerebellar tract 

\< (' ^S=fe- ^^ J 

__. Ventral spino- 
cerebellar tract 
Lateral spino- 
thalamic and 
spinotectal tracts 
Ventral spinothalamic 
tract 

Fig. 72. Diagram of the spinal cord and dorsal root, showing the divisions of the dorsal root, 
the collaterals of the dorsal root fibers, and some of the connections which are established by 
them. 

unmyelinated and fine myelinated fibers, which are derived in part from the 
lateral division of the dorsal root and in part arise from cells in the neighboring 
gray matter (Fig. 63). 

AFFERENT PATHS IN THE SPINAL CORD 

We have been at some pains to make clear the course and distribution of 
the dorsal root fibers within the spinal cord because all afferent impulses which 
reach the cord are carried by them. Interoceptive fibers from the viscera, 
proprioceptive fibers from the muscles, tendons, and joints, as well as extero- 
ceptive fibers from the skin are included in these roots; and among the latter 
group are probably several subvarieties, mediating the afferent impulses out 



FIBER TRACTS OP THE SPINAL CORD 99 

of which the sensations of touch, heat, cold, and pain are elaborated. An 
important problem which in great measure awaits solution is this: How are the 
fibers of the different functional varieties distributed in the spinal cord and 
along what paths are these various types of afferent impulses carried toward 
the brain? 

The proprioceptive fibers, which terminate at the periphery in neuromus- 
cular and neuro tendinous spindles and in Pacinian corpuscles, are known to 
be myelinated. They must, therefore, pass through the well myelinated medial 
division of the dorsal root into the posterior funiculus. As shown by Brown- 
Sequard in 1847 by a study of patients with unilateral lesions of the spinal 
cord, sensations from the muscles, joints, and tendons reach the brain without 
undergoing a crossing in the spinal cord. This and other evidence points un- 
mistakably to the long ascending branches of the dorsal root fibers, which are 
continued uncrossed in the posterior funiculus to the medulla oblongata, as the 
conductors of this type of sensation. When these fibers are destroyed by a 
tumor or other lesion confined to the posterior funiculus, muscular sensibility 
and the recognition of posture are abolished, while touch, pain, and tempera- 
ture sensations remain intact (Dejerine, 1914). 

No better exposition of the proprioceptive functions could be furnished than 
by describing the sensory deficiencies found in cases of tabes dorsalis or loco- 
motor ataxia, a disease in which there is degeneration of the posterior funiculi. 
Lying in bed, with eyes closed, a tabetic may not be able to say in what posi- 
tion his foot has been placed by an attendant because afferent impulses from 
the muscles, joints, and tendons fail to reach the cerebral cortex and arouse 
sensations of posture. Not only are the sensations of this variety lacking, but 
the unconscious reflex motor adjustments initiated by proprioceptive afferent 
impulses are also impaired. Standing with feet together and eyes closed, the 
patient loses his balance and sways from side to side. In walking his gait is 
uncertain and the movements of his limbs poorly coordinated. All of this 
motor incoordination is explained by a loss of the controlling afferent impulses 
from the muscles, joints, and tendons. 

The long ascending fibers of the posterior funiculus, which reach the brain 
and end in the nucleus gracilis and cuneatus, are for the most part proprio- 
ceptive in function (Fig. 235). The connections which they make there can 
best be considered in another chapter. Collaterals and many terminal branches 
end in the gray matter of the cord, entering into synaptic relations with the neu- 
rons of the spinocerebettar paths and with neurons belonging to spinal reflex arcs. 



100 THE NERVOUS SYSTEM 

Proprioceptive Paths to the Cerebellum. According to the researches of 
Marburg (1904) and Bing (1906) the spinocerebellar tracts are concerned with 
the transmission to the cerebellum of afferent impulses from the muscles, joints, 
and tendons, which remain, however, at a subconscious level (Dejerine, 1914). 
We may, therefore, appropriately consider these paths at this time. 

The dorsal spinocerebellar tract (fasciculus spinocerebellaris dorsalis, direct 
cerebellar tract of Flechsig, fasciculus cerebellospinalis) is a well-defined bundle 
at the surface of the lateral funiculus just ventral to the posterior lateral sul- 
cus (Figs. 72, 78). In cross-section it has the form of a flattened band, situated 
between the periphery of the cord and the lateral corticospinal tract. It begins 
in the upper lumbar segments and is prominent in the thoracic and cervical 
portions of the cord. It consists of uniformly large fibers, which take origin 
from the cells of the nucleus dorsalis of the same side. This nucleus forms a 
prominent feature of the sections through the thoracic portion of the cord, but 
is not found above the seventh cervical nor below the second lumbar seg- 
ments. A conspicuous bundle of myelinated collaterals from fibers of the 
fasciculus cuneatus run to this nucleus (Fig. 56) where their arborizations form 
baskets about the individual cells of the nucleus. The fibers arising from the 
cells of the nucleus dorsalis run laterally to the periphery of the lateral funiculus 
of the same side, where they turn rostrally and form the dorsal spinocerebellar tract. 
We will follow this tract into the brain in a latter chapter. Here we need only 
say that it reaches the cerebellum by way of the restiform body (Fig. 235). 

The ventral spinocerebellar tract constitutes the more superficial portion of 
a large ascending bundle of fibers, known as the fasciculus anterolateralis super- 
ficialis or Gower's tract, which also includes the spinotectal and lateral spino- 
thalmic tracts (Fig. 72). It is situated at the periphery of the lateral funiculus 
ventral to the tract we have just considered. It is said to consist of fibers which 
arise from the cells of the posterior gray column and intermediate gray matter of the 
same and the opposite side (Page May, 1906; Dejerine, 1914). In a subsequent 
chapter we will trace these fibers by the way of the medulla, pons, and an- 
terior medullary velum to the cerebellum (Fig. 235). 

From what has been presented above it will be apparent that collaterals 
and terminal branches of dorsal root fibers, doubtless of the proprioceptive 
group, enter into synaptic relations with certain intraspinal neurons, the axons 
of which run to the cerebellum by way of the ventral and dorsal spinocerebellar 
tracts. The entire path from periphery to cerebellum therefore consists of two 
neurons with a synaptic interruption in the gray matter. 



FIBER TRACTS OF THE SPINAL CORD IOI 

Interoceptive fibers are present in the thoracic and upper lumbar dorsal 
roots, but are either absent or very few in number in the others. We know 
practically nothing about their intraspinal course in mammals. They will be 
considered in the chapter on the Sympathetic Nervous System. 

Exteroceptive fibers carry cutaneous afferent impulses, and probably are 
subdivided into several varieties. Most authors agree that there are separate 
fibers for the impulses aroused by tactile and thermal stimuli; and Sherrington 
(1906) has presented evidence for the existence of a separate group of fibers, 
whose end organs are responsive only to agents capable of inflicting injury, 
that is, to noxious or painful stimuli. 

Conduction of Tactile Impulses in the Spinal Cord. The phenomena of sen- 
sory dissociation, characteristic of syringomyelia, show that the intraspinal 
path for the sensations of touch is rather widely separated from that for pain 
and temperature sensation (Fig. 73). In that disease a cavity is developed 
within the gray matter of the spinal cord; and sensations of pain and tem- 
perature may be abolished over a given cutaneous area which is still sensitive 
to touch. The separation of these two lines of conduction occurs at the place 
where the dorsal root fibers enter the cord. The fibers, mediating pain and 
temperature sensations, end almost at once in the gray matter, while those for 
touch ascend for some distance in the posterior funiculus of the same side (Head 
and Thompson, 1906; Dejerine, 1914). As these fibers ascend in the posterior 
funiculus they give off collaterals to the gray matter of the successive levels of 
the spinal cord through which they pass. The tactile impulses from a given 
root, therefore, do not enter the gray matter all at once, but filter forward through 
the collaterals and terminals of these dorsal root fibers to reach the posterior 
gray column in a considerable number of segments above that at which the 
root enters the cord. Within the posterior gray column at these successive 
levels the terminals and collaterals of the tactile fibers establish synaptic con- 
nections with neurons of the second order. The axons of these neurons form the 
ventral spinothalamic tract of the opposite side (Fig. 73). 

The ventral spinothalamic tract is an ascending bundle of fibers found in the 
anterior funiculus. It consists of fibers which take origin from cells in the pos- 
terior column of the opposite side, cross the median plane in the anterior white 
commissure, and ascend in the ventral funiculus to end within the thalamus (Fig. 
73). It is possible that many of the fibers do not reach the thalamus directly, 
but terminate in the gray matter of the cord and medulla oblongata in rela- 
tion to other neurons, whose axons continue the course to the thalamus. If 



IO2 



NERVOUS SYSTEM 



this be so the path consists in part of relays of shorter neurons (D6jerine, 

1914). 

The uncrossed path in the posterior funiculus for tactile impulses entering 
the cord through any given dorsal root overlaps by many segments the crossed 
path in the ventral funiculus (Fig. 230). Some of the uncrossed fibers even 
reach the nuclei of the funiculus gracilis and funiculus cuneatus in the medulla 
oblongata. This extensive overlapping of the uncrossed by the crossed paths 
accounts for the fact that lateral hemisection of the human spinal cord rarely 
causes marked disturbance of tactile sensibility below the lesion (Petren, 1902; 

Head and Thompson, 1906). 

I 



Ascending branch of dorsal root fiber - 
Myelinated fiber of dorsal root^ 
Spinal ganglion 

Unmyelinated fiber of dorsal root' 




~ Lateral spinothalamic tract 
(pain and temperature) 

Ventral spinothalamic tract 
(touch) 



Fig. 73. Exteroceptive pathways in the spinal cord. 

Since it seems clear that the dorsal root fibers subserving tactile sensibility ascend for 
some distance in the posterior funiculus, they must be included among the myelinated fibers 
of the medial division of the dorsal root, because only myelinated fibers ascend in that 
funiculus. This conclusion is in keeping with the facts already mentioned concerning the 
termination of myelinated fibers in the supposedly tactile end organs, such as Meissner's 
corpuscles and Pacinian corpuscles. It is also in. keeping with facts to be mentioned in 
a following paragraph concerning the structure of the median nerve. 

The Lateral Spinothalamic Tract. It seems to be well established that the 
dorsal root fibers, which serve as pain conductors, terminate in the gray matter 
almost at once after entering the cord, and come into synaptic relations with 
neurons of the second order, whose axons run in the lateral spinothalamic tract. 
From cells in the posterior column fibers arise, which in man cross to the opposite 
side of the cord in the anterior white commissure and ascend in the lateral spino- 
thalamic tract to end in the thalamus (Figs. 73, 231). This is a tract of ascending 



FIBER TRACTS OF THE SPINAL CORD 103 

fibers situated in the lateral funiculus under cover of the ventral spinocerebellar 
tract. Together with the spinotectal and ventral spinocerebellar tracts it forms 
the fasciculus anterolateralis superficialis (of Gowers). It mediates pain and 
temperature sensations. 

Conduction of Painful Afferent Impulses in the Spinal Cord. Not all of the fibers of 
the lateral spinothalamic tract reach the thalamus. According to May (1906), "Some of 
these fibers certainly pass directly to the thalamus, while others terminate in the inter- 
mediate gray matter, and thus, by means of a series of short chains, afford secondary paths 
to the same end station, which may supplement the direct path, or be made available after 
interruption of the direct path." It has been shown in many cases in man and animals that, 
after a complete hemisection of the spinal cord, the loss of sensibility to pain on the op- 
posite side of the body below the lesion was only temporary. In time there may occur a 
more or less perfect restoration of pain conduction, showing that the homolateral side of 
the cord is able to supplement or replace the heterolateral path. According to the researches 
of Karplus and Kreidl (1914) and Ranson and Billingsley (1916) these short chains, which are 
of secondary importance in man, are much better developed in the cat. In this animal 
pain conduction through the spinal cord is bilateral and is effected to a large extent through 
a series of short relays. 

According to Head and Thompson (1906) the path for pain in the spinal cord is the same 
whether the impulses arise in the skin or in the deeper parts, such as the muscles and joints. 
But Dejerine (1914) is of the opinion that painful impulses from the muscles may be trans- 
mitted in the posterior funiculus and remain uncrossed as far as the medulla oblongata. 

Until recently we possessed no information as to which dorsal root fibers served as pain 
conductors. But in the last few years evidence has been presented which points toward the 
unmyelinated fibers of the spinal nerves and dorsal roots as the pain fibers (Ranson, 1915). 
Space does not permit a detailed presentation of the evidence here. It should be noted, 
however, that the unmyelinated fibers of the lateral division of the dorsal root terminate in 
the gray matter almost immediately after their entrance into the spinal cord, and in this 
respect correspond to the known course of the fibers carrying painful impulses. The un- 
myelinated fibers are chiefly distributed in the cutaneous nerves, although a few run in the 
muscular branches. This coincides with the much greater sensitiveness to pain of the 
skin than of the deeper tissues. Furthermore, the median nerve at the wrist, a large nerve 
supplying a relatively small area of skin richly endowed with the sense of touch, contains 
relatively few unmyelinated fibers. On the other hand, nerves like the lateral cutaneous 
of the thigh and the medial cutaneous of the forearm, which supply relatively large cutaneous 
areas of low tactile sensibility, but not inferior to the fingers in sensitiveness to pain, are com- 
posed in large part of unmyelinated fibers. This difference between the composition of the 
median nerve and the medial cutaneous nerve of the forearm is just what should be expected 
if the touch fibers are myelinated and the pain fibers unmyelinated. Head and his co-workers 
(1905, 1906, 1908) have regarded the group of sensations (protopathic), to which according 
to their classification cutaneous pain belongs, as primitive in character and the first to appear 
in the phylogenetic series. It is well known that nerve-fibers in their earliest phylogenesis 
are unmyelinated. If our conception is correct, a great many of the afferent fibers of mam- 
mals remain in this primitive undifferentiated state and mediate a relatively primitive 
form of sensation. In this connection it is interesting to note that Dejerine (1914) believes 
that pain is conducted by the "sympathetic" fibers contained in the cutaneous and muscular 
nerves. He does not state the evidence on which this belief is based; but if by "sympathetic" 
he means to designate the unmyelinated fibers his view agrees perfectly with that presented 
in the preceding paragraphs. 



104 



THE NERVOUS SYSTEM 



The problem can be approached from the. experimental standpoint. The seventh lum- 
bar dorsal root of the cat is especially adapted for such a test. This root as it approaches 
the cord breaks up into a number of filaments which spread out in a longitudinal direction 
and enter the cord along the posterolateral sulcus. Within each root filament, as it ap- 
proaches this sulcus, the unmyelinated separate out from among the myelinated fibers and 
take up a position around the circumference of the filament and along septa that divide it 
into smaller bundles. As the root enters the cord, these unmyelinated fibers turn laterally 
into the dorsolateral fasciculus, constituting together with a few fine myelinated fibers the 
lateral division of the root (Fig. 74). Almost all of the myelinated fibers run through the 
medial division of the root into the cuneate fasciculus. A slight cut in the direction of the 



Exterior fin\lculus. 



Inmuel 'mated [tiers. 

Lissauers tract 
.Dorsal Toot 





qela/tinosa. 



Lateral 
funiculus 







Fig. 74. From a section of the seventh lumbar segment of the spinal cord of the cat, showing the 
unmyelinated fibers of the dorsal root entering the tract of Lissauer. 



arrow, which as shown by subsequent microscopic examination divided the lateral without 
injury to the medial division of the root, at once eliminated the pain reflexes obtainable 
from this root in the anesthetized cat, such as struggling, acceleration of respiration, and 
rise of blood-pressure. On the other hand, a long deep cut in the plane indicated by B, 
Fig. 74, which severed the medial division of the root as it entered the cord, had little or no 
effect on the pain reflexes. This series of experiments, the details of which are given else- 
where (Ranson and Billingsley, 1916), furnishes strong evidence that painful afferent im- 
pulses are carried by the unmyelinated fibers of the lateral division of the dorsal root. 

These fibers probably terminate in the substantia gelatinosa Rolandi, and, if so, it is 
not unlikely that intermediate neurons are intercalated between them and the neurons 
whose axons run in the ventral spinothalamic tra.ct. 



FIBER TRACTS OF THE SPINAL CORD 105 

The Conduction of Sensations of Pain, of Heat, and of Cold. It is well estab- 
lished on the basis of clinical observations that the paths for sensations of heat 
and cold follow closely those for pain. They pass through the gray matter im- 
mediately after entering the cord, cross to the opposite side, and ascend in the 
lateral spinothalamic tract. 

According to May (1906) "it is clear that there are distinct and separate 
paths for the impulses of pain, of heat, or of cold in the spinal cord, and that 
these different and specific qualities of sensation may be dissociated in an affec- 
tion of the spinal cord." That is, one of these forms of sensibility may be lost, 
although the other two are retained. "But as these paths are anatomically 
very closely associated from origin to termination these three forms of sensa- 
tion are usually affected to a like degree." 

From what has been said above it will be apparent that the paths, mediating 
pain and temperature sensibility, cross promptly to the opposite side of the 
cord and ascend in the lateral spinothalamic tract. The path for touch crosses 
more gradually, but finally comes to lie in the ventral spinothalamic tract of 
the opposite side; while the sensory impulses from the muscles, joints, and 
tendons, as well as some elements of tactile sensibility, are carried upward on 
the same side of the cord by the long ascending branches of the dorsal root fibers, 
which terminate in the nuclei of the funiculus gracilis and the funiculus cuneatus. 
The connections established within the brain by the fibers of these various paths 
cannot profitably be discussed at this point, but will be considered in Chapter XIX. 

Other afferent paths besides those already mentioned exist in the spinal 
cord. These include the spino-olivary and spinotectal tracts (Fig. 78). The 
former consists of fibers which arise from cells in the posterior gray column, 
cross to the opposite side of the cord, and ascend in the ventral funiculus, to 
end in the inferior olivary nucleus of the medulla oblongata. The spinotectal 
tract consists of fibers which arise from cells in the posterior gray column and 
which, after crossing, ascend in the lateral funiculus in company with those of 
the lateral spinothalamic path to end in the roof (tectum) of the mesencephalon, 
i. e., in the corpora quadrigemina. 

ASCENDING AND DESCENDING DEGENERATION OF THE SPINAL CORD 

When as a result of an injury a nerve-fiber is divided, that part which is 

severed from its cell of origin degenerates, while the part still connected with 

that cell usually remains intact. This is known as Wallerian degeneration, and, 

as will be readily understood, gives valuable information concerning the course 



io6 



THE NERVOUS SYSTEM 



of the fiber tracts. In case of a complete transection of the spinal cord all the 
ascending fibers whose cells are located below the cut will degenerate in the 
segments above; while those descending fibers whose cells of origin are located 
above will degenerate below the lesion (Fig. 75). Injury to the dorsal roots 
proximal to the spinal ganglia causes a degeneration of the dorsal root fibers 

Dorsal spinocerebellar tract 
f .fCorticospinal tract 
i 

"Ascending branches of dorsal root fibers 




Fasciculus proprius 

Descending branch of dorsal root fiber 



Fig. 75. Diagram of the spinal cord to illustrate the principle of Wallerian degeneration. 
The broken lines represent the degeneration resulting from 1, section of the ventral root; 2, 
section of the spinal nerve distal to the spinal ganglion; 3, section of the dorsal root proximal to 
the spinal ganglion, and 4, a lesion in the lateral funiculus. 

throughout their length in the spinal cord. Brain injuries may, according to 
their location, result in the degeneration of one or more of the tracts which 
descend into the spinal cord from above. 

By the study of a great many cases of injury to the central nervous system 
in man and of experimentally produced lesions in animals a very considerable 



FIBER TRACTS OF THE SPINAL CORD 



107 



amount of information has been obtained concerning the fiber tracts of the 
spinal cord (Collier and Buzzard, 1901, 1903; Stewart, 1901; Thiele and Horsley, 
1901 ; Batten and Holmes, 1913). This is summarized in the accompanying table 
and in Fig. 78. 

TABLE SHOWING THE LOCATION OF THE CHIEF FIBER TRACTS OF THE SPINAL CORD AND THE 
DIRECTION IN WHICH THEY DEGENERATE 





Ascending degeneration. 


Descending degeneration. 


Anterior funiculus 


Ventral spinothalamic tract 


Ventral corticospinal tract, 
Vestibulospinal tract, 
Tectospinal tract 


Lateral funiculus 


Dorsal spinocerebellar tract, 
Ventral spinocerebellar tract, 
Lateral spinothalamic tract, 
Spinotectal tract 


Lateral corticospinal tract, 
Rubrospinal tract, 
Bulbospinal tract, 
Tectospinal tract 


Posterior funiculus 


Ascending branches of the 
dorsal root fibers 


Fasciculus interfascicularis, 
Septomarginal tract 



The fasciculi proprii or ground bundles are composed of short ascending 
and descending fibers, which arise and terminate within the gray matter of the 
spinal cord and link together the various segments of the cord. These fascicles, 
one of which is present in each of the three funiculi, immediately surround 
the gray columns. After a transection of the spinal cord the fasciculi proprii 
undergo an incomplete degeneration for some distance both above and below 
the lesion (Figs. 75, 76). In cross-section the ground bundle of the posterior 
funiculus has the form of a narrow band upon the surface of the posterior column 
and posterior commissure, and was once called the cornu-commissural bundle 
(Fig. 78). In addition to this fascicle there are in the posterior funiculus two 
other tracts which in part belong to the same system the septomarginal tract 
and the fasciculus interfascicularis, or comma tract of Schultze. These are 
both composed of descending fibers, in part of intraspinal origin and in part 
representing the descending branches of the dorsal root fibers. The septomar- 
ginal tract is situated along the dorsal periphery of the posterior funiculus in 
the thoracic region; it takes up a position along the septum in the lumbar segments 
(oval area of Flechsig) ; and in the sacral region it forms a triangular field at the 
dorsomedial angle of the posterior funiculus (triangle of Gombault and Philippe) 
(Fig. 76). The fasciculus interfascicularis is best developed in the thoracic 
segments, where it occupies a position near the center of the posterior funiculus. 



io8 



THE NERVOUS SYSTEM 



In the anterior funiculus, in addition to the fasciculus proprius which imme- 
diately surrounds the gray matter, there is a thin layer of similar fibers spread 
out along the border of the anterior fissure and known as the sulcomarginal 
fasciculus. This tract also contains the fibers which descend into the cord from 
the medial longitudinal bundle of the medulla oblongata. 

As a general rule the short fibers of the fasciculus proprius lie nearer the 
gray substance than the fibers of greater length; and the long tracts, which 



Fasciculus gracilis .., 



Spinocerebellar, spinotectal, and lateral^ 
spinothalnmic tracts 






Fasciculus inter fascicular is 



Septomarginal fasciculus- 
Lateral corticospinal tract* 

Septomarginal fasciculus, oval area of Flechsig , 
Lateral corticospinal tract* 






Cervical enlargement 
ascending degeneration 



Upper thoracic 
ascending degeneration 



Middle thoracic 
site of compression 



Lower thoracic 
descending degeneration 



Upper lumbar 
descending degeneration 



Lower lumbar 
descending degeneration 



Fig. 76. Ascending and descending degeneration resulting from a compression of the thoracic 
spinal cord in man. Marchi method. (Hoche.) 

connect the spinal cord with the brain, occupy the most peripheral position. 
But the fact must not be overlooked that many fibers of the fasciculus proprius 
are intermingled with those of the long tracts. 

LONG DESCENDING TRACTS OF THE SPINAL CORD 

Fibers which arise from cells in various parts of the brain descend into the 
spinal cord, where they form several well-defined tracts. The most important 



FIBER TRACTS OF THE SPINAL CORD 109 

and most conspicuous of these are the cerebrospinal fasciculi, which are more 
properly called the corticospinal tracts. There are two in each lateral half of 
the cord, the lateral and the ventral corticospinal tracts. Their constituent 
fibers take origin from the large pyramidal cells of the precentral gyrus or motor 
region of the cerebral cortex and pass through the subjacent levels of the brain 
to reach the spinal cord (Fig. 77). Just before they enter the spinal cord they 
undergo an incomplete decussation in the medulla oblongata, giving rise to a 
ventral and a lateral corticospinal tract. 

The Lateral Corticospinal Tract (Crossed Pyramidal Tract, Fasciculus 
Cerebrospinalis Lateralis). The majority of the pyramidal fibers, after cross- 
ing the median plane in the decussation of the pyramids, enter the lateral fu- 



Cerebral hemisphere 




Spinal 
cord 



Fig. 77. Diagram of the corticospinal tracts. 

niculus of the spinal cord as the lateral corticospinal tract, which occupies a posi- 
tion between the dorsal spinocerebellar tract and the lateral fasciculus proprius 
(Fig. 78). In the lumbar and sacral regions, below the origin ot the dorsal 
spinocerebellar tract, the lateral corticospinal tract is more superficial. It can 
be traced as a distinct strand as far as the fourth sacral segment; and as it 
descends in the spinal cord it gradually decreases in size. Throughout its 
course in the spinal cord it gives off collateral and terminal fibers which end in 
the gray matter. 

The ventral corticospinal tract (fasciculus cerebrospinalis anterior or direct 
pyramidal tract) is formed by the smaller part of the corticospinal fibers, which 
do not cross in the medulla, but pass directly into the ventral funiculus of the 



no 



THE NERVOUS SYSTEM 



same side of the cord. They form a tract of small size, which lies near the 
anterior median fissure and which can be traced as a distinct strand as far as the 
middle of the thoracic region of the spinal cord. Just before terminating these 
fibers cross in the anterior white commissure. They end like those of the lateral 
corticospinal tract, either directly or perhaps through an intercalated neuron, 
in relation to the motor cells in the anterior column. The crossing of these 
fibers is only delayed, and it will be apparent that all of the corticospinal fibers 
arising in the right cerebral hemisphere terminate in the anterior column of the 
left side of the cord, and conversely, those from the left hemisphere end on the 
right side. It is along these fibers that impulses from the motor portion of the 
cerebral cortex reach the cord and bring the spinal motor apparatus under 
voluntary control. 



Fasciculus septomarginalis 

Fasciculus inlerfascicularis 

Fasciculus proprius 

Sensory fibers of the 
second order ~ 
Lateral corticospinal _ 
tract 

Rubrospinal tract I- 
Tectospinal tract - - 
Fasciculus proprius NT 

Bulbospinal tract 

Vestibules pinal tract--, 



Fasciculus gracilis 




.,-- Fasciculus cuneatus 

- - - Dorsolateral fasciculus 

^_ Dorsal spinocerebellar 
tract 

~~ Fasciculus proprius 

Ventral spinocere- 
bellar tract 
jf-.. Lateral s pinotltalamic 

tract 
Spinotectal tract 

- Ventral root 



~~ Ventral spinolhalamic tract 

* T . " Sulcomareinal fasciculus 

Ventral cortjcosptnal tract 

Fig. 78. Diagram showing the location of the principal fiber tracts in the spinal cord of man. 
Ascending tracts on the right side, descending tracts on the left. 

It is stated by some authors, although on the basis of rather unsatisfactory evidence, 
that the fibers of the lateral corticospinal tract ramify in the formatio reticularis (Mona- 
kow, 1895) and the nucleus dorsalis (Schafer, 1899). The corticospinal path is from the 
standpoint of phylogenesis a relatively new system and varies a great deal in different 
mammals. It is found in the ventral funiculus in the mole, while in the rat it occupies the 
posterior funiculus. In the mole it is almost completely unmyelinated, in the rat largely so. 
It contains many unmyelinated fibers in the cat, fewer in the monkey (Linowiecki, 1914). 
In man it does not become fully myelinated before the second year. An uncrossed ventral 
corticospinal tract seems to be present only in man and the anthropoid apes, and this tract 
varies greatly in size in different individuals. 

The rubrospinal tract (tract of Monakow) is situated near the center of the 
lateral funiculus just ventral to the lateral corticospinal tract (Fig. 78). Its 
fibers come from the red nucleus of the mesencephalon, cross the median plane, 



FIBER TRACTS OF THE SPINAL CORD 



III 



and descend into the spinal cord, within which some of them can be traced to 
the sacral region. Their collateral and terminal branches end within the an- 
terior column in relation to the primary motor neurons. 

Other Descending Tracts. The bulbospinal tract (olivospinal tract, tract of 
Helweg) is a small bundle of fibers found in the cervical region near the surface 
of the lateral funiculus opposite the anterior column. The fibers arise from 
cells in the medulla oblongata, possibly in the inferior olivary nucleus, and end 
somewhere in the gray matter of the spinal cord. The exact origin and ter- 



Fasciculus cuneatus 
\ 



Fasciculus gracilis 



Lateral corticospinal tract 

Fasciculi proprii 

Ventral corticospinal tract 




- Dorsal spinocerebellar tract 



Oval area of Flechsig 





D. Ill 



L. IV 



Fig. 80. 

Figs. 79 and 80. Diagrams of the sixth cervical, third thoracic, and fourth lumbar segments 
of the spinal cord, showing the location of the different tracts as outlined by Flechsig on the basis 
of differences in time of myelination. (van Gehuchten.) 

mination of the tract is unknown. The tectospinal tract, located in the ventral 
funiculus, is composed of fibers which take origin in the roof (tectum) of the 
mesencephalon, cross the median plane and descend into the anterior funiculus 
of the spinal cord, and end in the gray matter of the anterior column. The tract 
is concerned chiefly with optic reflexes. The vestibulospinal tract, also located 
in the anterior funiculus, arises from the lateral nucleus of the vestibular nerve 



112 THE NERVOUS SYSTEM 

in the medulla oblongata and conveys impulses concerned in the maintenance 
of equilibrium. Some of its fibers can be traced as far as the lower lumbar 
segments. They end in the gray matter of the anterior column. 

Hemisection of the spinal cord in man produces a characteristic symptom 
complex known as the Brown-Sequard's syndrome which the student is now in 
position to understand. Below the level of the lesion and on the same side 
there is found a paralysis of the muscles with a loss of sensation from the mus- 
cles, joints, and tendons; while on the opposite side of the body, beginning two 
or three segments below the level of the lesion, there is loss of sensations of 
pain and temperature. Tactile sensibility is everywhere retained (Dejerine, 
1914). 

Order of Myelination. The fiber tracts of the spinal cord do not all become 
myelinated at the same time. By a study of the fetal spinal cord at various 
developmental stages Flechsig was able to identify and trace many of these 
tracts because of the difference in the tune of myelination. His results agree 
in general with those derived frorh a study of spinal cords showing ascending 
and descending degeneration (Figs. 79, 80). Myelination begins during the fifth 
month of intra-uterine life. The order in which the fibers of the spinal cord 
acquire their myelin sheaths is as follows: (1) afferent and efferent root fibers, 
(2) those of the fasciculi proprii, (3) the fasciculus cuneatus, (4) the fasciculus 
gracilis, (5) the dorsal spinocerebellar tract, (6) the ventral spinocerebellar fas- 
ciculus, (7) the corticospinal tracts. 



CHAPTER VIII 



THE GENERAL TOPOGRAPHY OF THE BRAIN. THE EXTERNAL 
FORM OF THE MEDULLA OBLONGATA, PONS, AND MESEN- 
CEPHALON 

The General Topography of the Brain. The brain rests upon the floor of 
the cranial cavity, which presents three well-marked fossae. In the posterior 
cranial fossa are lodged the medulla oblongata, pons, and cerebellum, which 
together constitute the rhombencephalon (Fig. 81). This fossa is roofed over 
by a partition of dura mater, called the tentorium cerebelli, that separates the 
cerebellum from the cerebral hemispheres. Through the notch in the ventral 



Calvaria 



Prosen-( Telencephalon 
cephalon\Diencephalon 

Frontal lobe of cerebral 

hemisphere in anterior 

cranial fossa 
Temporal lobe of cerebral 

hemisphere in middle 

cranialfossa 




Parietal lobe of cerebral 
hemisphere 



Mesencephalon 



Occipital lobe of cerebral 

hemisphere 
Tentorium cerebelli 
Posterior cranialfossa 

Cerebellum 

Pons 

Medulla oblongata 

Spinal cord 



Fig. 81. Median sagittal section of the head showing the relation of the brain to the cra- 
nium. The sphenoid bone is shown in transparency, and through it the temporal lobe may be 
seen. 

border of the tentorium projects the mesencephalon, connecting the rhomben- 
cephalon below with the prosencephalon above that partition. The cerebral 
hemispheres form the largest part of the prosencephalon, occupy the anterior 
and middle cranial fossae, and extend to the occiput on the upper surface of the 
tentorium. 

The dorsal aspect of the human brain presents an ovoid figure, the large 
cerebral hemispheres, covering the other parts from view. In the sheep's brain the 

8 113 



THE NERVOUS SYSTEM 



hemispheres are smaller and fail to hide the cerebellum and medulla oblongata 
(Fig. 82). The cerebral hemispheres, which are separated by a deep cleft called 
the longitudinal fissure of the cerebrum, together present a broad convex surface 
which lies in close relation to the internal aspect of the calvaria. From the 
latter it is separated only by the investing membranes or meninges of the brain. 
The thin convoluted layer of gray matter upon the surface of the hemispheres is 
known as the cerebral cortex. 

The ventral aspect or base of the brain presents an irregular surface adapted 
to the uneven floor of the cranial cavity (Figs. 83, 86). The medulla oblongata, 



Face and tongue 

Head and eyes 

Fore limb 

Hind limb 

Gyrus sylviactis (arcuatus) 

Cyrus lateralis 
Gvri mediates 



Gyrus internus I / /- 



Vermis cerebelli 
Hemisptuerium cerebelli 

Medulla oblongata 
Medulla spinalis 




Gyrus frontalis medialis 
Gyrus frontalis superior 
Sulcus coronal is 
Sulcus splenialis 
Fissura ansata (cruciata) 
Fissura lateralis (Sylvii) 

Fissura suprasylvia 
Fissura longitudinalis 
Sulcus lateralis 
Sulcus intermedius 
Sulcus medialis 



Flocculus 



Neruus accessorius 
Nervus spinalis I 



Fig. 82. Dorsal view of the sheep's brain. The motor cortex is shaded on the left side. (Herrick 

and Crosby.) 

which is continuous through the foramen magnum with the spinal cord, lies on 
the ventral aspect of the cerebellum in the vallecula between the two cere- 
bellar hemispheres. Rostral to the medulla oblongata and separated from it 
only by a transverse groove is a broad elevated band of fibers, which plunges 
into the cerebellum on either side and is known as the pons. The cerebellum 
can be seen occupying a position dorsal to the pons and medulla oblongata, and 
can easily be recognized by its grayish color and many parallel fissures. A 
pair of large rope-like strands are seen to emerge from the rostral border of 
the pons and to diverge from each other as they run toward the under surface 



THE GENERAL TOPOGRAPHY OF THE BRAIN 



of the cerebral hemispheres. These are the cerebral peduncles and they form 
the ventral part of the mesencephalon. At its rostral extremity each peduncle 
is partially encircled by a flattened band, known as the optic tract, which is con- 
tinuous through the optic chiasma with the optic nerves. A lozenge-shaped 
depression, known as the inter peduncular fossa, is outlined by the diverging 
cerebral peduncles and by the optic chiasma and tracts. Within the area thus 
outlined and beginning at its caudal angle may be distinguished the following 
parts: the inter peduncular nucleus, which is very large in the sheep and occu- 



Longitudinal fissure of cerebrum^ 

Optic nerve^ 
Optic chiasma 
Rhinal fissure 

Insula- 
Lateral fissure 

Optic tract . 

Infundibulum -~ 
Mammittary body - 

Cerebral peduncle 
Inter peduncular fossa and 
nucleus 

Trigeminal nerve 

Abducens nerve--- 

Acoustk( Vestibular n ~ 
nene (Cochlearn. 
Glossopharyngeal nerve ~-' 
Vagus nerve 
Hypoglossal nerve '' 
Anterior median fissure'' 




' Olfactory bulb 

' Medial olfactory gyrus 

Anterior perforated substance 
- Lateral olfactory stria 
Lateral olfactory gyrus 
-Diagonal band 
..- Amygdaloid nucleus 

Pyriform area 
; Hippocampal gyrus 
L-- Trochlear neroe 



m,,.--Pons 
Jn. .-'A bducens nerve 

~*?^,_-- Facial nerve 

Trapezoid body 

Cerebellum 
'---Olive 

^Chorioid plexus 
" Accessory nerve 
^Tractus later alis minor 



Fig. 83. Ventral view of the sheep's brain. 



pies an area designated in man as the substantia perforata posterior; the corpus 
mammillare, which in man is divided by a longitudinal groove into two mam- 
millary bodies; and also the tuber cinereum, infundibulum, and hypophysis. 
Rostral to the optic tract there is on either side a triangular field of gray matter, 
studded with minute pit-like depressions and known as the anterior perforated 
substance. 

The Rhinencephalon. The olfactory bulb is situated near the rostral end 
of the hemisphere, to the ventral surface of which it is attached by the olfactory 



n6 



THE NERVOUS SYSTEM 



peduncle (and in man by the long olfactory tract). In the sheep's brain there 
diverge from the olfactory peduncle two well-defined gray bands, the medial 
and lateral olfactory gyri, which are less evident in man; and furthermore, the 
lateral olfactory gyrus is obviously continuous with the hippocampal gyrus, 
forming the pyriform area (Fig. 83). All of these structures are closely asso- 
ciated in function and belong to the rhmencephalon, or olfactory part of the 
brain, which, because of the greater importance of the sense of smell in the 
sheep, is better developed in that animal than in man. A prominent longi- 
tudinal fissure separates this part of the brain from the rest of the hemisphere. 

Inter-ventricular foramen Body of corpus callosum 



Anterior commissure 
Septum pellucidum^ 

Rostral lamina 
Rostrum of corpus callosum. \ 
Genu of corpus callosum { \ ' 



Body of fornix 

\ Hippocampal com. Roofs of third ventricle or tela choriotdea 

Stria med. /Haben. com. 



Splenium 
fineal 
body 



Suprapineal recess 
', Superior colliculus 
' -Primary fissure 

White center of vermis 




Olfactory bulb 
Medial olfactory gyrus , 
Anterior perf. substance'; 
Lamina terminalis 
Diagonal band 



>'/ ! / / ! Infundib. \ 
' / / ,' Third vent. 
! ' Massa intermedia 
i Optic chiasma 
Preoptic recess 



\ ' \ \ 'Pons 
\ \ 'Aqueduct 
* \Lamina quad. 
\ 'Posterior com. 
\ * Hypophysis 
Mammillary body 



Central canal 
\ Medulla 

\ Medial aperture of 
\ \ fourth ventricle 
\ \Tela chorioidea 
\ * Fourth ventricle 
''Anterior medullary 
velum 



Fig. 84. Medial sagittal section of the sheep's brain. 

This is known as the rhinal fissure; and all that portion of the cerebral cortex 
which lies dorsal to it is the new or non-olfactory cortex, the neopattium. In 
contrast to the older olfactory cortex or archipallium, which includes the pyri- 
form area, the neopallium is of recent phyletic development. It first forms a 
prominent part of the brain in mammals and is by far the most highly developed 
in man. 

Interrelation of the Various Parts of the Brain. An examination of a medial 
sagittal section of the brain will make clear the relation which the various parts 
bear to each other (Fig. 84). The medulla oblongata, pans, and cerebellum are 
seen surrounding the fourth ventricle, and are intimately connected with one 



THE GENERAL TOPOGRAPHY OF THE BRAIN 117 

another. The medulla oblongata is directly continuous with the pons, and on 
either side a large bundle of fibers from the dorsal aspect of the former runs into 
the cerebellum. These two strands, which are known as the restiform bodies 
or inferior cerebellar peduncles, constitute the chief avenues of communication 
between the spinal cord and medulla oblongata on the one hand and the cere- 
bellum on the other. The ventral prominence of the pons is produced in large 
part by transverse bundles of fibers, which when traced lateralward are seen to 
form a large strand, the brachium pontis or middle cerebellar peduncle, that 
enters the corresponding cerebellar hemisphere (Figs. 83, 86). The brachium 
conjunctivum or superior cerebellar peduncle can be traced rostrally from the 
cerebellum to the mesencephalon. The three peduncles are paired structures, 
symmetrically placed on the two sides of the brain (Figs. 87, 88). 

The Cerebrum. The mesencephalon surrounds the cerebral aqueduct and 
consists of the ventrally placed cerebral peduncles, and a dorsal plate with four 
rounded elevations, the lamina and corpora quadrigemina (superior and inferior 
colliculi). The cerebral hemispheres form the most prominent part of the 
cerebrum and are separated from each other by the longitudinal fissure (Fig. 
82), at the bottom of which is a broad commissural band, the corpus callosum, 
which joins the two hemispheres together (Fig. 85). Under cover of the cere- 
bral hemispheres and concealed by them, except on the ventral aspect of the 
brain, is the diencephalon. This includes most of the parts which help to form 
the walls of the third ventricle. These are from above downward, the epithal- 
amus, including the habenular trigone and pineal body near the roof of the 
ventricle; the thalamus, which forms most of the lateral wall of the ventricle, 
and is united with its fellow across the cavity by a short bar of gray substance, 
the massa intermedia; and the hypothalamus, including the mammillary bodies, 
infundibulum, and part of the hypophysis (Figs. 84, 85). 

The Brain Ventricles. The central canal of the spinal cord is prolonged 
through the caudal portion of the medulla oblongata and finally opens out into 
the broad rhomboidal fourth ventricle of the rhombencephalon. At its pointed 
rostral extremity this ventricle is continuous with the cerebral aqueduct, the 
elongated slender cavity of the mesencephalon. This, in turn, opens into the 
third ventricle, which is a narrow vertical cleft between the two laterally sym- 
metric halves of the diencephalon. It is bridged by the massa intermedia. 
Near the dorsal part of the rostral border of the ventricle is a small opening in 
each lateral wall, the inter-ventricular foramen or foramen of Monro. This 
leads into the lateral ventricle, the cavity of the cerebral hemisphere. 



THE NERVOUS SYSTEM 
THE ANATOMY OF THE MEDULLA OBLONGATA 

At its rostral end the spinal cord increases in size and goes over without 
sharp line of demarcation into the medulla oblongata, or myelencephalon, which, 
as we learned in Chapter II, is derived from the posterior part of the third brain 
vesicle. The medulla oblongata may be said to begin just rostral to the high- 
est rootlet of the first cervical nerve at about the level of the foramen magnum ; 



Marginal part of sukus cinguli 

Sulcus of corpus callosum \ 

Splenium of corpus callosum \ ;' 

Precuneux 
Sub parietal sulcus \ 
Parieto-occipital fissure\ 

Lamina quadrigemina 

Cuneus 
Superior vermis^ 




Calcarinefissure f\ 

Occipital pole f 

Lingual gyrus 
Transverse fissure 

Cerebellar hem. 
Medullary substance 
of vermis 

Inferior vermis'' 
Calamus scriptorius'' 
Central canal \ 

Spinal cord , , 
Tela chorioidea of fourth ventricle f 

Fourth ventricle ; 
Medulla oblongata 
Anterior medullary velum 

Cerebral aqueduct ! 

Pans ! ; 

Posterior perforated substance , 
Oculomotor nerve 



Central sulcus in paracentral lobule 
Pineal body 
I Pineal recess 
! ', Posterior commissure 
I I 1 Tela chorioidea of third ventricle 
Massa intermedia 
. Gyrus cinguli 
Thalamus 

Body of corpus callosum 
Body offornix 
Septum pellucidum 
' Sulcus cinguli 
Interventric. foramen 
Column offornix 
Anterior commis- 



;> Superior frontal 
gyrus 



'-Frontal pole 
Cenu of cor pus callosum 
Nostrum of cor p. callosum 
-- Parolfactory area and sulci 
, X X X X X \ % \ \ s- Subcallosal gyrus 
X X X X X \ \ \ Hypothalamic sulcus 
X X X X \ \ X ^Lamina terminal is 
X X X X \ X Optic recess 
X X X X \ "Optic nerve 
X X X \ Optic chiasma 
X X X Infundibulum 
X \ "Anterior lobe\ , 

Posterior lobe 
"Mammillary body 






Fig. 85. Medial sagittal section of the human brain. (Sobotta-McMurrich.) 

and at the opposite extremity it is separated from the pons by a horizontal groove 
(Figs. 81, 85). Its ventral surface rests upon the basilar portion of the occipital 
bone; while its dorsal surface is in large part covered by the cerebellum. The 
shape of the medulla oblongata is roughly that of a truncated cone, the smaller 
end of which is directed caudally and is continuous with the spinal cord. In 
man it measures about 3 cm., or a little more than 1 inch, in length (Fig. 86). 
Like the spinal cord, the medulla oblongata presents a number of more or 



ANATOMY OF THE MEDULLA OBLONGATA 119 

less parallel longitudinal grooves. These are the anterior and posterior median 
fissures, and a pair each of anterior lateral and posterior lateral sulci (Figs. 86, 
89). By means of the fissures it is divided symmetrically into right and left 
halves; while these, in turn, are marked off by the sulci into ventral, lateral, and 
dorsal areas, which as seen from the surface appear to be the direct upward con- 
tinuation of the anterior, lateral, and posterior funiculi of the spinal cord. 
But, as we shall see in the following chapter, this continuity is not as perfect 
as it appears from the surface; because the tracts of the cord undergo a rear- 
rangement as they enter the medulla oblongata. The posterior median fissure 
does not extend beyond the middle of the oblongata, at which point its lips 
separate to form the lateral boundaries of the caudal portion of the fourth ven- 
tricle. The caudal half of the medulla oblongata contains a canal, the direct 
continuation of the central canal of the spinal cord, and is known as the closed 
portion of the medulla oblongata (Fig. 85). This canal opens out into the fourth 
ventricle in the rostral half, which helps to form the ventricular floor, and which 
is often spoken of as the open part of the medulla oblongata.* 

Fissures and Sulci. The posterior median fissure represents the continua- 
tion of the posterior median sulcus of the spinal cord and, as noted above, ends 
near the middle of the medulla oblongata. The anterior median fissure is con- 
tinued from the spinal cord to the border of the pons, where it ends abruptly 
in a pit known as the for amen ccecum. Near the caudal extremity of the medulla 
oblongata this fissure is interrupted by interdigitating bundles of fibers which 
pass obliquely across the median plane. These are the fibers of the lateral 
corticospinal tract, which undergo a decussation on passing from the medulla 
oblongata into the spinal cord, known as the decussation of the pyramids. The 
anterior lateral sulcus also extends throughout the length of the medulla ob- 
longata and represents the upward continuation of a much more indefinite groove 
bearing the same name in the spinal cord. From it emerge the root filaments 
of the'hypoglossal nerve. From the posterior lateral sulcus emerge the rootlets 
of the glossopharyngeal, vagus, and accessory nerves (Figs. 86, 88, 89). 

The ventral area of the medulla oblongata is included between the anterior 
median fissure and the anterior lateral sulcus, and has the false appearance of 
being a direct continuation of the anterior funiculus of the spinal cord. On 
either side of the anterior median fissure there is an elongated eminence, taper- 
ing toward the spinal cord, and known as the pyramid (pyramis Fig. 86). It 
is formed by the fibers of the corticospinal or pyramidal tract. Just before the 
fibers of this tract enter the spinal cord they undergo a more or less complete 



I2O 



THE NERVOUS SYSTEM 



decussation, crossing the median plane in large obliquely interdigitating bundles, 
which fill up and almost obliterate the anterior median fissure in the caudal 
part of the medulla oblongata. This is known as the decussation of the pyra- 
mids (decussatio pyramidum). In the sheep these fibers pass into the opposite 
posterior funiculus of the spinal cord. In man the crossing is incomplete, a 



Infundibidum 
Orbital sulci of frontal 
Orbital gyri of frontal lobe 

Hypophysis 
Temporal pole 

Anterior perfor, substance 

Oculomotor nerve ^ 
Uncus --, 
Mammillary body 

Cerebral peduncle - 
Pans - 

Trigeminal nerve - 

Temporal lobe 

Facial nerve 



Frontal pole olfadory sukus 

,. Olfactory bulb 

Olfactory tract 
Optic nerve 



Nervus intermedius - 



Acoustic nerve." 



Flocculus of cerebellum^' 
Cerebellum ' 



Chorioid plexus of ventricle IV 
Glossopharyngeal nerve 




Vagus nerve' 
Hypoglossal nerve 

Accessory nerve ' 
Root filaments of cervical nerve I 

Decussation of pyramids 



.-Optic chiasma 
- Lateral olfactory stria 
Tuber cinereum 
Maxillary nerve 

Ophthalmic nerve 
Portia minor of trigem. 
nerve 
Mandibular nerve 

Semilunar ganglion 
Trochlear nerve 



Inter peduncular fossa 



Abducens nerve 
'Olive 
Pyramid 

Medulla oblongata 
Tonsil of cerebellum 
* Occipital pole 
Spinal cord 
Vermis of cerebellum 

Fig. 86. Ventral view of the human brain. (Sobotta-McMurrich.) 



majority of the fibers descending into the lateral funiculus of the opposite side, 
a minority into the anterior funiculus of the same side (Fig. 77). We are al- 
ready acquainted with these bundles in the spinal cord as the ventral and lateral 
corticospinal tracts (direct and crossed pyramidal tracts). In addition to the 
pyramid the ventral area of the medulla also contains a bundle of fibers, the 



ANATOMY OF THE MEDULLA OBLONGATA 121 

medial longitudinal fasciculus, which is continuous with the anterior fasciculus 
proprius of the spinal cord. 

The lateral area of the medulla oblongata, included between the antero- 
lateral and posterolateral sulci, appears as a direct continuation of the lateral 
funiculus of the spinal cord; but, as a matter of fact, many of the fibers of that 
funiculus find their way into the anterior area (as, for example, the lateral cor- 
ticospinal tract) or into the posterior area (dorsal spinocerebellar tract). In 
the rostral part of the lateral area, between the root filaments of the glosso- 
pharyngeal and vagus nerves, on the one hand, and those of the hypoglossal, 
on the other, is an oval eminence, the olive (oliva, olivary body), which is pro- 
duced by a large irregular mass of gray substance, the inferior olivary nucleus, 
located just beneath the surface (Figs. 87, 88). By a careful inspection of the 
surface of the medulla oblongata it is possible to distinguish numerous fine 
bundles of fibers, which emerge from the anterior median fissure or from the 
groove between the pyramid and the olive and run dorsally upon the surface 
of the medulla to enter the restiform bodies. These are the ventral external 
arcuate fibers and are most conspicuous on the surface of the olive (Fig. 88) . 

In the sheep there are two superficial bands of fibers not seen in the human 
brain. Placed transversely near the caudal border of the pons is a belt-like 
elevation, known as the trapezoid body, through which emerge the roots of the 
abducens and facial nerves (Figs. 83, 87). In man the much larger pons covers 
this band from view and the sixth and seventh nerves emerge from under the 
caudal border of the pons. Another bundle, beginning on the ventral sur- 
face of the trapezoid body near the seventh nerve, describes a graceful curve 
around the ventral border of the olive and becomes lost in the lateral area of 
the medulla oblongata. This has been called the fasciculus lateralis minor. 

The dorsal area of the medulla oblongata is bounded ventrally by the pos- 
terolateral sulcus and emergent root filaments of the glossopharyngeal, vagus, 
and accessory nerves. In the closed part of the medulla oblongata it extends 
to the posterior median fissure, while in the open part its dorsal boundary is 
formed by the lateral margin of the floor of the fourth ventricle. The caudal 
portion of this area is, in reality, as it appears, the direct continuation of the 
posterior funiculus of the spinal cord. On the dorsal aspect of the medulla 
oblongata the fasciculus cuneatus and fasciculus gracilis of the cord are con- 
tinued as the funiculus cuneatus and funiculus gracilis, which soon enlarge into 
elongated .eminences, known respectively as the cuneate tubercle and the clava 
(Figs. 89, 91). These enlargements are produced by gray masses, the nucleus 



122 



THE NERVOUS SYSTEM 



gracilis and nucleus cuneatus, within which end the fibers of the corresponding 
fasciculi of the spinal cord. The clava and cuneate tubercle are displaced lat- 
erally by the caudal angle of the fourth ventricle. Somewhat rostral to the mid- 
dle of the medulla oblongata they gradually give place to the restiform body. 

More laterally, between the cuneate funiculus and tubercle on the one hand 
and the roots of the glossopharyngeal, vagus, and accessory nerves on the other, 
is a third longitudinal club-shaped elevation called the tuber culum cinereum. 
It is produced by a tract of descending fibers, derived from the sensory root of 
the trigeminal nerve, and by an elongated mass of substantia gelatinosa which 



Corona 



Lentiform nucleus 
Lateral geniculale body v 
Medial geniculale body 
Optic radiation \ 
Corona radiata , 
Pulvinar\ 

Inferior quadrigeminal brachium^* 

Superior colliculus - v 

Trochlear nerve O 

Inferior colliculus-^ 

Brachium pontis'*- 

Brachium conjunctivum'' 

Restiform body 



Acoustic nerve 

\ Cochtear n _ 

Dorsal cochlear nucleus 

Glossopharyngeal nerve - 

Vagus nerve and restiform body 

Accessory nerve " - 

Clava'"" . 

Cuneate tubercle'"' 




Anterior perforated substance 

Optic tract 

Optic nerve 

Infundibulum 

Mammillary body 

Hypophysis 

'Oculomotor nerve 

" Transverse peduncular tract 

Cerebral peduncle 

Pans 

Abducens nerve 

Trigeminal nerve 

Facial nerve 

Trapezoid body 

'* Olive 

" Tractus later alis minor 

Hypoglossal neme 



Fig. 87. Lateral view of brain stem of the sheep. 

forms one of the nuclei of this nerve (Fig. 111). This bundle of fibers and the 
associated mass of gray matter are known as the spinal tract and nucleus of the 
spinal tract of the trigeminal nerve. 

The restiform body (corpus restiforme or inferior cerebellar peduncle) lies 
between the lateral border of the fourth ventricle and the roots of the vagus 
and glossopharyngeal nerves in the rostral part of the medulla oblongata (Figs. 
87-89). There is no sharp line of demarcation between it and the more cau- 
dally placed clava and cuneate tubercle. It is produced by a large strand of 
nerve-fibers, which run along the lateral border of the fourth ventricle and then 
turn dorsally into the cerebellum. These fibers serve to connect the medulla 



ANATOMY OF THE PONS 123 

oblongata and spinal cord on the one hand with the cerebellum on the other. 
By a careful inspection of the surface of the medulla it is possible to recognize 
the source of some of the fibers entering into the composition of the restiform 
body. The ventral external arcuate fibers can be seen entering it after crossing 
over the surface of the lateral area; and the dorsal spinocerebellar tract can also 
be traced into it from a position dorsal to the caudal extremity of the olive. 

At the point where the restiform body begins to turn dorsally toward the 
cerebellum, it is partly encircled by an elongated transversely placed elevation 
formed by the ventral and dorsal cochlear nuclei (Figs. 87, 88). This ridge is 
continuous on the one hand with the cochlear nerve, and on the other with 
several bundles of fibers which run medialward over the floor of the fourth 
ventricle and are known as the stria medullares acusticce (Fig. 89). The cochlear 
nuclei are more prominent in the sheep, while the medullary striae are best seen 
in the human brain. Just caudal to this ridge there is sometimes seen another, 
running more obliquely across the restiform body, which is an outlying portion 
of the pons and has been described by Essick (1907) under the name corpus 
pontobulbare. 

Nerve Roots. From the surface of the medulla oblongata there emerge in 
linear order along the posterior lateral sulcus a series of root filaments, which 
continues the line of the dorsal roots of the spinal nerves. These are the root- 
lets of the glossopharyngealj vagus and accessory nerves. But unlike the dorsal 
roots, which are made up of afferent fibers, the spinal accessory nerve contains 
efferent fibers, while the vagus and glossopharyngeal are mixed nerves. The 
line of the ventral or motor roots of the spinal nerves is continued in the medulla 
oblongata by the root filaments of the hypoglossal neroe, which is also composed 
of motor fibers. The abducens, facial, and acoustic nerves make their exit along 
the caudal border of the pons in the order named from within outward. The 
abducens emerges between the pons and the pyramid, the acoustic far lateral- 
ward in line with the restiform body, and the facial with its sensory root, the 
nervus intermedius , near the acoustic nerve (Figs. 86-88). 

THE ANATOMY OF THE PONS 

The pons, which is differentiated from the ventral part of the metencephalon, 
is interposed between the medulla oblongata and the cerebral peduncles and 
lies ventral to the cerebellum. As seen from the ventral surface, it is formed 
by a broad transverse band of nerve-fibers, which on either side become aggre- 
gated into a large rounded strand, the brachium pontis or middle cerebellar 



124 



THE NERVOUS SYSTEM 



peduncle, and finally enter the corresponding hemisphere of the cerebellum 
(Figs. 83, 86). This transverse band of fibers, which gives the bridge-like 
form from which this part derives its name, belongs to the basilar portion of 
the pons and is superimposed upon a deeper dorsal portion that may be regarded 
as a direct upward continuation of the medulla oblongata. The transverse 
fibers form a part of the pathway connecting the cerebral hemispheres with the 
opposite cerebellar hemispheres; and the size of the pons, therefore, varies with 



Anterior limb of 
internal capsule 



Head of the can- ' 
date nucleus 



Anterior commissure'' 
A nterior perforated . - 
substance 

Optic nerve ' 
Basis pedunculi' 




Pons 

Nervus jportio minor 

trigeminus \portio major 

Acoustic nerve 

Facial nerve 

Glossopharyngeal and vagus nerves 
Olive 

Hypoglossal nerve 

Ventral external arcuate fibers 

Pyramid 

Ventral root N. cero. I 

Anterior lateral sulcns 

Ventral root N. cerv. II ~ 



rr~^^^---s Corona radiata 



Tail of the caudate nucleus 
Lenticulotha- 

lamic part Posterior 
Retrolenticular limb of 

part internal 

Sublenticular | capsule 

part J 

Thalamus 

Medial geniculate body 
-- Superior colliculus 

" ^Inferior quadrigeminal brachium 
"" Inferior colliculus 
" - Trochlear nerve 
- - Lateral lemniscus 

- - Brachium conjunctivum 
^~* Fila later alia pontis 
,> Dentate nucleus 
Restiform body 
Brachium pontis 

~~ ~~ Dorsal cochlear nuc. 
~~ Corpus pontobulbare 
" Restiform body 
~ Tuberculum cinereum 
* Accessory nerve 

--'"Dorsal root N. cerv. II 



Fig. 88. Lateral view of human brain stem. 

the size of these other structures. It is instructive to compare the brains of 
the shark, sheep, and man with this point in mind (Figs. 11, 84, 85). 

The ventral surface of the pons is convex from above downward and from 
side to side and rests upon the basilar portion of the occipital bone and upon 
the dorsum sellae (Fig. 81). A groove along the median line, the basilar sulcus, 
lodges the basilar artery (Fig. 86). 

The trigeminal nerve emerges from the ventral surface of the pons far lateral- 
ward at the point where its constituent transverse fibers are converging to form 



THE FOURTH VENTRICLE 125 

the brachium pontis. In fact, it is customary to take the exit of this nerve as 
marking the point of junction of the pons with its brachium. The nerve has two 
roots which lie close together: the larger is the sensory root, or portio major; 
the smaller is the motor root, or portio minor (Fig. 88). 

The posterior surface of the pons forms the rostral part of the floor of the 
fourth ventricle, along the lateral borders of which there are two prominent 
and rather large strands of nerve-fibers, the brachia conjunctiva (Figs. 88, 89). 

The brachia conjunctiva or superior cerebellar peduncles lie under cover of 
the cerebellum. As they emerge from the white centers of the cerebellar hemi- 
spheres they curve rostrally and take up a position along the lateral border of 
the fourth ventricle. They converge as they ascend and disappear from view 
by sinking into the substance of the mesencephalon under cover of the inferior 
quadrigeminal bodies. Each consists of fibers which connect the cerebellum 
with the red nucleus, a large gray mass situated within the midbrain ventral to 
the superior colliculus of the corpora quadrigemina. The interval between the 
two brachia conjunctiva, where these form the lateral boundaries of the fourth 
ventricle, is occupied by a thin lamina of white matter, the anterior medullary 
velum (Fig. 85). This is stretched between the free dorsomedial borders of the 
two brachia and forms the roof of the rostral portion of the ventricle. Caudally 
it is continuous with the white center of the cerebellum. The fibers of the 
trochlear nerves decussate in the anterior medullary velum and emerge from its 
dorsal surface (Fig. 89). As they run through the velum they produce a raised 
white line which extends transversely from one brachium to the other. 

THE FOURTH VENTRICLE 

The lozenge-shaped cavity of the rhombencephalon is known as the fourth 
ventricle. It lies between the pons and medulla oblongata, ventrally, and the 
cerebellum dorsally, and is continuous with the central canal of the closed por- 
tion of the medulla, on the one hand, and with the cerebral aqueduct on the 
other (Fig. 84). On each side a narrow curved prolongation of the cavity ex- 
tends laterally on the dorsal surface of the restiform body. This is known as 
the lateral recess (Figs. 89, 90). It opens into the subarachnoid space near the 
flocculus of the cerebellum; and through this lateral aperture of the fourth ven- 
tricle (foramen of Luschka) protrudes a small portion of the chorioid plexus 
(Fig. 90). There is also a median aperture (foramen of Magendie) through the 
roof of the ventricle near the caudal extremity. By means of these three open- 
ings, one medial and two lateral, the cavity of the ventricle is in communica- 



126 



THE NERVOUS SYSTEM 



tion with the subarachnoid space, and cerebrospinal fluid may escape from the 
former into the latter. 

The floor of the fourth ventricle is known as the rhomboid fossa and is formed 
by the dorsal surfaces of the pons and open part of the medulla oblongata, which 
are continuous with each other without any line of demarcation and are irreg- 
ularly concave from side to side (Figs. 89, 91). The fossa is widest opposite the 
points where the restiform bodies turn dorsally into the cerebellum; and it 
gradually narrows toward its rostral and caudal angles. The lateral boundaries 

Pineal body 



~ Superior colliculus 

Inferior colliculus 
Cerebral peduncle 
Trochlear nerve 
Median sulcus 
Locus caruleus 
Facial colliculus 
Medial eminence 
Sulcus limitans 

Lateral recess 

Stri(B medtillares 

Tcenia 

Trigonum hypoglossi 

Cuneate tubercle 

Tuber culum cinereum 

Clava 

Posterior median fissure 

Posterior intermediate 

sulcus 
'Posterior lateral sulcus 



Medial geniculate body--*- 

Inferior quadrigeminal T~ 

brachium 

Frenulum veli 

A nterior medullary velum 

Brachium conjunctivum 

Brachium pontis*--^ 
Restiform body- 

t 

Superior fovea 

Area acustica-<-=:~ 

Inferior fovea 

Restiform body 

Ala cinerea """ 

Funiculus separans"'" 

Area postrema-"'" 

Obex-''"' 

Funiculus gracilis -~~~ 
Funiculus cuneatus~ " 





Fig. 89. Dorsal view of human brain stem. 



of the fossa, which are raised some distance above the level of the floor, are 
formed by the following structures: the brachia conjunctiva, restiform bodies, 
cuneate tubercles, and clava. Of the four angles to the rhomboid fossa, two 
are laterally placed and correspond to the lateral recesses. At its caudal angle 
the ventricle is continuous with the central canal of the closed part of the me- 
dulla oblongata, and at its rostral angle with the cerebral aqueduct. Joining 
the two last named angles there is a median sulcus which divides the fossa into 
two symmetric lateral halves. 

The rhomboid fossa is arbitrarily divided into three parts. The superior 



THE FOURTH VENTRICLE 



127 



part is triangular, with its apex directed rostrally and its base along an imagin- 
ary line through the superior foveae. The inferior part is also triangular, but 
with its apex directed caudally and its base at the level of the horizontal por- 
tions of the taeniae of the ventricle. Between these two triangular portions is 
the intermediate part of the fossa, which is prolonged outward into the lateral 
recesses. The floor is covered with a thin lamina of gray matter continuous 
with that which lines the central canal and cerebral aqueduct. Crossing the 
fossa transversely in its intermediate portion are several strands of fibers known 
as the stria medullares acustica. These are subject to considerable variation in 
different specimens. Springing from the dorsal cochlear nuclei they wind 
around the restiform body in the lateral recess and run transversely across the 
fossa to disappear in the median sulcus. 

The inferior portion of the fossa bears some resemblance to the point of a 
pen and has been called the calamus scriptorius. It belongs to the medulla 
oblongata. In this part of the fossa there is on either side a small depression, 
the inferior fovea, shaped like an arrow-head, the point of which is directed toward 
the striae medullares. From the basal angles of this triangle run diverging sulci: 
a medial groove toward the opening of the central canal and a lateral groove 
more nearly parallel to the median sulcus. By these sulci the inferior portion 
of the fossa is divided into three triangular areas. Of these the most medial 
is called the trigone of the hypoglossal nerve or trigonum nervi hypoglossi. Be- 
neath the medial part of this slightly elevated area is located the nucleus of the 
hypoglossal nerve. The area between the two sulci, which diverge from the 
fovea inferior, is the ala cinerea or triangle of the vagus nerve. Both names 
are appropriate, the one, because of its gray color, and the other, because a 
nucleus of the vagus nerve lies subjacent to it. The third triangular field, 
placed more laterally, forms a part of the area acustica. 

The area acustica is, however, not restricted to the inferior portion of the 
fossa, but extends into the intermediate part as well. Here it forms a prominent 
elevation over which the striae medullares run. Subjacent to this area lie the 
nuclei of the vestibular nerve. A part of the acoustic area and all of the ven- 
tricular floor rostral to it belong to the pons. 

Rostral to the striae medullares there may be seen a shallow depression, 
the fovea superior, medial to which there is a rounded elevation, the facial 
colliculus. Under cover of this eminence the fibers of the facial nerve bend 
around the abducens nucleus. Extending from the fovea superior to the 
cerebral aqueduct is a shallow groove, usually faint blue in color, the locus 



128 



THE NERVOUS SYSTEM 



caruleus, beneath which lies the substantia ferruginea, composed of pigmented 
nerve-cells. 

Beginning at the cerebral aqueduct and extending through both the superior 
and inferior foveae is a very important groove, the sulcus limitans, which repre- 
sents the line of separation between the parts derived from the alar plate and 
those which originate from the basal plate of the embryonic rhombencephalon. 
Lateral to this sulcus lie the sensory areas of the ventricular floor, including the 
area acustica, all of which are derived from the alar plate. Medial to this 
sulcus there is a prominent longitudinal elevation, known as the medial eminence, 
which includes two structures already described, namely, the facial colliculus 
and the trigone of the hypoglossal nerve. Beneath the medial part of this 




Tel a chorioidea 

Choriold plexus 

Median aperture of 
fourth ventricle 

Fig. 90. Dorsal view of human rhombencephalon showing tela chorioidea and chorioid plexus of 

the fourth ventricle. 

trigone lies the nucleus of the hypoglossal nerve and beneath the lateral part is a 
group of cells designated as the nucleus intercalatus. 

One or two features remain to be mentioned. At the caudal end of the ala 
cinerea is a narrow translucent obliquely placed ridge of thickened ependyma, 
known as the funiculus separans. Between this ridge and the clava is a small 
strip of the ventricular floor, called the area postrema, which on microscopic 
examination is found to be rich in blood-vessels and neurogliar tissue. 

The roof of the fourth ventricle is formed by the anterior medullary velum, 
a small part of the white substance of the cerebellum, and by the tela chorioidea 
lined internally by ependymal epithelium (Fig. 85). Caudal to the cerebellum 
the true roof of the cavity is very thin and consists only of a layer of ependymal 
epithelium, which is continuous with that lining the other walls of the ventricle. 



ANATOMY OF THE MESENCEPHALON I2Q 

This is supported on its outer surface by a layer of pia mater, the tela chorioidea, 
rich in blood-vessels. From this layer vascular tufts, covered by epithelium, 
are invaginated into the cavity and form the chorioid plexus of the fourth ven- 
tricle (Fig. 90). The plexus is invaginated along two vertical lines close to the 
median plane and along two horizontal lines, which diverge at right angles from 
the vertical ones and run toward the lateral recesses. These right and left 
halves are joined together at the angles so that the entire plexus has the shape 
of the letter T, the vertical limb of which, however, is double. 

After the tela chorioidea with its epithelial lining has been torn away to 
expose the floor of the ventricle, there remains attached to the lateral bound- 
aries of the caudal part of the cavity the torn edges of this portion of the roof. 
These appear as lines, the teenies of the fourth ventricle, which meet over the 
caudal angle of the cavity in a thin triangular lamina, the obex (Fig. 89). Ros- 
trally each taenia turns lateralward over the restiform body and forms the caudal 
boundary of the corresponding lateral recess. 

THE MESENCEPHALON 

The midbrain or mesencephalon occupies the notch in the tentorium and 
connects the rhombencephalon, on the one side of that shelf-like process of 
dura, with the prosencephalon on the other (Fig. 81). It consists of a. dorsal 
part, the corpora quadrigemina, and a larger ventral portion, the cerebral pe- 
duncles. It is tunneled by a canal of relatively small caliber, called the cerebral 
aqueduct, which connects the third and fourth ventricles and is placed nearer 
the dorsal than the ventral aspect of the midbrain (Fig. 84). 

The cerebral peduncles (pedunculi cerebri, crura cerebri), as seen on the 
ventral aspect of the brain, diverge like a pair of legs from the rostral border of 
the pons (Fig. 83). Just before they disappear from view by entering the ven- 
tral surface of the prosencephalon they enclose between them parts of the hypo- 
thalamus, and are encircled by the optic tracts. On section, each peduncle is 
seen to be composed of a dorsal part, the tegmentum, and a ventral part, the 
basis pedunculi. Between the basis pedunculi and the tegmentum there inter- 
venes a strip of darker color, the substantia nigra (Fig. 113). By dissection it is 
easy to show that the basis pedunculi is composed of longitudinally coursing 
fibers which can be traced rostrally to the internal capsule (Fig. 88). In the 
other direction some of these fibers can be followed into the corresponding pyra- 
mid of the medulla oblongata. On the surface two longitudinal sulci mark the 
plane of separation between the tegmentum and the basis pedunculi. The 



130 



THE NERVOUS SYSTEM 



groove on the medial aspect of the peduncle, through which emerge the fibers 
of the third nerve, is known as the sulcus of the oculomotor nerve, while that on 
the lateral aspect is called the lateral sulcus of the mesencephalon. Dorsal to 
this latter groove the tegmentum comes to the surface and is faintly marked by 
fine bundles of fibers which curve dorsally toward the inferior colliculus of the 
corpora quadrigemina (Fig. 88). These fibers belong to the lateral lemniscus, 
the central tract associated with the cochlear nerve. 

The corpora quadrigemina form the dorsal portion of the mesencephalon, 
and consist of four rounded eminences, the quadrigeminal bodies or colliculi, 



Anterior limb of internal capsule.^ 
Stria terminalis^ / 

Habenular commissure 
Habenular trigone. \ 

Pineal body^ /\R 

Posterior limb of internal capsule^. )\l" 
Superior colliculus X"<- 

Optic radiation % ?\ v ^ 
Attachment anterior'^! 
medullary velum -J 
Inferior colliculus 

Superior fovea . ^ 
Brachium conjunctivum ^v\ 
O 

Brachium pontis 
Restiform body 
Dorsal cochlear nucleus. 
Acoustic area 
Inferior fovea and restiform body ~-~ 

Tcenia of fourth ventricle 
Clava 

Cuneale tubercle 
Posterior lateral sulcus 




Corona radiata 
-Head of cattdate nucleus 
.Stria medullaris of thalamus 
_ .- Third ventricle 

, Thalamus 
'\ ,, Tail of caudate nucleus 



\, Median sulcus 
W . Trochlear nerve 

/''/Facial colliculus 
' ,/,' Trigeminal nerve 

'/Sulcus limitans 
^'Medial eminence 
Ala cinerea 

.. Lateral recess of fourth ventricle 
Trigone of hypoglossal nerve 
Obex 

'--Posterior median fissure 

Posterior intermediate sulcus 

Funiculus gracilis 

Funiculus cuneatus 



Fig. 91. Dorsal view of brain stem of sheep. 

which arise from the dorsal aspect of a plate of mingled gray and white matter 
known as the quadrigeminal lamina (Figs. 89, 91). The superior colliculi are 
larger than the inferior, the disproportion being greater in the sheep than in 
man. A median longitudinal groove separates the colliculi on either side. In 
the rostral end of this groove rests the pineal body, while attached to its caudal 
end is a band which runs to the anterior medullary velum, and is known as the 
frenulum veli. A transverse groove runs between the superior and inferior collic- 
uli and extends on to the lateral aspect of the mesencephalon, where it inter- 
venes between the superior colliculus and the inferior quadrigeminal brachium 
(Figs. 87, 89). 



ANATOMY OF THE MESENCEPHALON 131 

The Brachia of the Corpora Quadrigemina. From each colliculus there runs 
ventrally and rostrally on the lateral aspect of the mesencephalon an arm or 
brachium (Figs. 87, 88). The inferior quadrigeminal brachium is the more con- 
spicuous and is the only one that can be readily identified in the sheep. It 
runs from the inferior colliculus to the medial geniculate body. This is an oval 
eminence, belonging to the diencephalon, which has been displaced caudally so 
as to lie on the lateral aspect of the mesencephalon. The superior quadrigeminal 
brachium runs from the superior colliculus toward the lateral geniculate body, 
passing between the pulvinar of the thalamus and the medial geniculate body. 
Some of the fibers can be traced beyond the lateral geniculate body into the 
optic tract. 



CHAPTER IX 

THE STRUCTURE OF THE MEDULLA OBLONGATA 

THE medulla oblongata contains the nerve-cells and fiber tracts associated 
with certain of the cranial nerves. These include the central mechanisms which 
control the reflex activities of the tongue, pharynx, and larynx, and in part those 
of the thoracic and abdominal viscera also. At the same time the ascending 
and descending fiber tracts, which unite the spinal cord with higher nerve 
centers, pass through the medulla oblongata. 

The central connections of the cranial nerves, except those of the first two 
pairs, are located in the medulla oblongata and in the tegmental portions of the 
pons and mesencephalon. In many respects they resemble the connections of 
the spinal nerves within the spinal cord. The following general statements on 
this topic, most of which are illustrated in Fig. 92, will help to elucidate the 
structure of the brain stem. 

1. The cells of origin of the sensory fibers of the cranial nerves (Fig. 92, 1) 
are found in ganglia which lie outside the cerebrospinal axis and are homologous 
with the spinal ganglia. These are the semilunar ganglion of the trigeminal, 
the geniculate ganglion of the facial, the superior and petrous ganglia of the 
glossopharyngeal, the jugular and nodose ganglia of the vagus, the spiral gang- 
lion of the cochlear, and the vestibular ganglion of the vestibular nerve. 

2. All of these sensory ganglia except the last two, the cells of which are 
bipolar, are formed by unipolar cells, the axons of which divide dichotomously 
into peripheral and central branches. The latter (or in the case of the acoustic 
nerve the central processes of the bipolar cells) form the sensory nerve roots, 
enter the brain stem and divide, each into a short ascending and a long descending 
branch. These branches give off numerous collaterals, which with the terminal 
branches end in gray masses known as sensory nuclei or nuclei of termination. 
It is the descending branches of the sensory fibers of the trigeminal neroe which 
form the spinal tract of that nerve illustrated in Figs. 92, 98, 99, 101. 

3. The ascending branch may be entirely wanting, as in the case of the sen- 
sory fibers of the seventh, ninth, and tenth nerves, all of which bend caudally and 
form a descending tract in the medulla oblongata, known as the tractus soli- 
tarius (Figs. 92, 101, 103). 

132 



THE STRUCTURE OF THE MEDULLA OBLONGATA 



133 



4. The sensory nuclei (Fig. 92, 4), within which the afferent fibers terminate, 
contain the cells of origin of the sensory fibers of the second order (Fig. 92, 2). 
Some of these are short; others are long, and these may be either direct or 
crossed. Many of them divide into ascending and descending branches. They 
run in the reticular formation and some of the ascending fibers reach the thal- 
amus. 

5. These sensory fibers of the second order give off collaterals to the motor 
nuclei. Direct collaterals from the sensory fibers of the cranial nerves to the 
motor nuclei are few in number or entirely wanting. 

6. The motor nuclei (Fig. 92, 5) are aggregations of multipolar cells which 
give origin to the motor fibers of the cranial nerves (Fig. 92, 3). 



Main sensory nucleus 
of trige minal nerve 

Afferent fiber of 
second order 



Tractus solilarius 

N2(deus of 

hypoglossal nerve 
Afferent fiber of 

second order 
Spinal tract of 

trigeminal nerve 

and Us nucleus 



Fig. 92. Diagram of the tongue and rhombencephalon to illustrate the central connections 
and functional relationships of certain of the cranial nerves: 1, Sensory neurons of the first order 
of the trigeminal and glossopharyngeal nerves; 2, sensory neurons of the second order; 3, motor 
fibers of the hypoglossal nerve; 4, sensory nuclei; 5, motor nucleus of hypoglossal nerve. (Cajal.) 

The Rearrangement Within the Medulla Oblongata of the Structures Con- 
tinued Upward from the Spinal Cord. At the level of the rostral border of the 
first cervical nerve the spinal cord goes over without a sharp line of demarcation 
into the medulla oblongata. The transition is gradual both as to external form 
and internal structure; but in the caudal part of the medulla there occurs a 
gradual rearrangement of the fiber tracts and alterations in the shape of the 
gray matter, until at the level of the olive, a section of the medulla bears no 
resemblance to one through the spinal cord. 

The realignment of the corticospinal tracts and the termination of the fibers 
of the posterior funiculi of the spinal cord are two of the most important factors 




134 



THE NERVOUS SYSTEM 



responsible for this gradual transformation. Traced rostrally from the spinal 
cord, the ventral corticospinal tracts are seen to enter the pyramids within the 
ventral area of the medulla oblongata, that is to say, they enter the medulla 
without realignment. But the fibers of the lateral corticospinal tracts on enter- 
ing the medulla swing ventromedially in coarse bundles, which run through 
the anterior gray columns and cut them off from the gray matter surrounding 
the central canal (Figs. 93, 95). After crossing the median plane in the decussa- 
tion of the pyramids these fibers join those of the opposite ventral corticospinal 
tracts and form the pyramids (Fig. 96). Thus fibers from the lateral funiculus 
come to lie ventral to the central canal and displace this dorsally; and at the same 
time a start is made toward breaking up the H -shaped gray figure characteristic 
of the spinal cord. 

Cerebral hemisphere 




Spinal 
cord 



Fig. 93. Diagram of the corticospinal tracts. 

Shortly after entering the medulla oblongata the fibers of the posterior funiculi 
end in nuclear masses which invade the funiculus gracilis and funiculus cuneatus 
as expansions from the posterior gray columns and central mass of gray sub- 
stance (Figs. 95, 96). These are known as the nucleus gracilis and nucleus cu- 
neatus. They cause a considerable increase in the size of the posterior funiculi 
and a corresponding ventrolateral displacement of the posterior columns of 
gray matter. The fibers of the posterior funiculi end in these nuclei about cells, 
the axons of which run ventromedially as the axis-cylinders of internal arcuate 
fibers. These sweep in broad curves through the gray substance, and decus- 
sate ventral to the central canal in what is known as the decussation of the medial 
lemniscus. After crossing the median plane they turn rostrally between the 



THE STRUCTURE OF THE MEDULLA OBLONGATA 



135 



pyramids and the central gray matter to form on either side of the median 
plane a broad band of fibers known as the medial lemniscus (Figs. 96, 97). 



Fasciculus gracilis - 

Fasciculus cuneatus 

Dorsolateralfasc. (Lissauer) 

Substantial gelatinosa 

Dorsal column 

Lateral cortices pinal tract 

Central canal " 
Ventral column 
Ventral corticospinal tract " 





Funiculus gracilis 
Nucleus gracilis 
Funicnlus cuneatus 
Spinal tract of trigem. nerve 
Nucleus of spinal tract of 
Dorsal column [N. V 

Lateral corticospinal tract 
Central canal 

Decussation of the pyramids 
Ventral column 



Fig. 94. 



Fig. 95. 




Funiculus gracilis 
Nucleus gracilis 
Funiculus cuneatus 
Nucleus cuneatus 
Spinal tract of trigeminal nerve 
Nucleus of spinal tract of N. V 
^Central gray matter 
Internal arcuate fibers 
Central canal 
Reticular substance 
Medial lemniscus 
Decussation of medial lemniscus 
Decussation of the pyramids 
Pyramid, corticospinal tract 



Fig. 96. 



- Fourth ventricle 
..-Dorsal motor nucleus of vagus 
Nucleus of hypoglossal nerve 
Tractus solitarius 
Nucleus of spinal tract of N. V 
~t Spinal tract of trigeminal nerve 
"Fibers of hypoglossal nerve 
" Reticular substance 
'Dorsal accessory olivary nucleus 
"Medial lemniscus 
'Inferior olivary nucleus 
'Medial accessory olivary nucleus 

Pyramid, ccrticospinal tract 
nganr 

Fig. 97. 

Figs. 9497. Diagrammatic cross-sections to show the relation of the structures in the 
medulla oblongata to those in the spinal cord: Fig. 94, First cervical segment of spinal cord; 
Fig. 95, medulla oblongata, level of decussation of pyramids; Fig. 96, medulla oblongata, level 
of decussation of medial lemniscus; Fig. 97, medulla oblongata, level of olive. 

At the level of the middle of the olive most of the fibers of the funiculus cune- 
atus and funiculus gracilis have terminated in their respective nuclei; and the 
nuclei also disappear a short distance farther rostrally (Fig. 97). With the 




136 THE NERVOUS SYSTEM 

disappearance of these fibers and nuclei there ceases to be any nervous sub- 
stance dorsal to the central canal, and this, which has been displaced dorsally 
by the accumulation of the corticospinal fibers and those of the lemniscus ven- 
tral to it, opens out as the floor of the fourth ventricle (Fig. 97). 

The outline of the gray matter in the most caudal portions of the medulla 
oblongata closely resembles that of the spinal cord. The anterior columns are 
first cut off by the decussation of the pyramids (Fig. 95). Then the posterior 
columns are displaced ventrolaterally due to the increased size of the posterior 
funiculi and the disappearance of the lateral corticospinal tracts from their 
ventral aspects. This rotation of the posterior column causes the apex of 
that column with its spinal tract and nucleus of the trigeminal nerve, which are 
continuous with the fasciculus dorsolateralis and substantia gelatinosa of the 
spinal cord (Fig. 94), to lie almost directly lateral ward from the central canal 
(Fig. 96). The shape of the gray figure is still further altered by the develop- 
ment of special nuclear masses, many of which are very conspicuous. These 
include the nucleus gracilis, nucleus cuneatus, inferior olivary nucleus, and the 
nuclei of the cranial nerves. The greater part of the gray substance now becomes 
broken up by nerve-fibers crossing in every direction, but especially by the 
internal arcuate fibers. This mixture of gray and white matter is known as the 
reticular substance. The central gray matter is pushed dorsad first by the pyra- 
mids and later by the medial lemniscus until it finally spreads out to form a thin 
gray covering for the floor of the fourth ventricle. 

The Pyramids and Their Decussation. We have had occasion repeatedly 
to refer to the crossing of the lateral corticospinal tracts in this and preceding 
chapters, but there remain some details to be presented. The pyramids are 
large, somewhat rounded fascicles of longitudinal fibers, which lie on either side 
of the anterior median fissure 01 the medulla oblongata (Fig. 86). The constit- 
uent fibers take origin from the large pyramidal cells of the anterior central 
gyrus or motor cerebral cortex. The decussation of the pyramids or motor 
decussation occurs near the caudal extremity of the medulla oblongata (Fig. 
93). Approximately the medial three-fourths of the corticospinal tract passes 
through the decussation into the lateral funiculus of the opposite side of the 
spinal cord, as the lateral corticospinal tract (fasciculus cerebrospinalis lateralis 
or lateral pyramidal tract); while the lateral one-fourth is continued without 
crossing into the ventral funiculus of the same side as the ventral corticospinal 
tract (fasciculus cerebrospinalis anterior or anterior pyramidal tract Figs. 
94, 95, 96, 98). The decussating fibers are grouped into relatively large bundles 



THE STRUCTURE OF THE MEDULLA OBLONGATA 



137 



as they cross the median plane, the bundles from one side alternating with 
similar bundles from the other, and largely obliterating the anterior median fis- 
sure at this level. There is great individual variation as to the relative size of 
the ventral and lateral corticospinal tracts; and there may even be marked 
asymmetry due to a difference in the proportion of the decussating fibers on the 
two sides. 

The nucleus gracilis and nucleus cuneatus (nucleus funiculi gracilis and 
nucleus funiculi cuneati) are large masses of gray matter located in the pos- 
terior funiculi of the caudal portion of the medulla oblongata. They are sur- 
rounded by the fibers of these funiculi except on their ventral aspects, where they 
are continuous with the remainder of the gray substance (Fig. 99). The fibers 



Funiculus gracilis 

Nucleus gracilis 

Spinal tract of trigeminal 
nerve 

Nucleus of spinal tract of 
N. V 

Central canal 

Decussation of the pyramids 
Anterior column 




Posterior median fissure 
Funiculus cuneatus 



Nucleus cuneatus 
Dorsal spinocerebellar tract 
Ventral spinocerebellar tract 
Ventral fasciculus proprius 
Bulbospinal tract 

Anterior median fissure 

Fig. 98. Section through the medulla oblongata of a child at the level of the decussation of the 
pyramids. Pal-Weigert method. (X6.) 

of the gracile and cuneate fasciculi terminate in the corresponding nuclei; and 
their terminal arborizations are synaptically related to the neurons, whose cell 
bodies and dendrites are located there (Fig. 100). Accordingly, in sections 
through successive levels we see the fibers decreasing in number as the nuclei 
grow larger (Figs. 98, 99, 101). It is due to the presence of these nuclei that the 
funiculi become swollen to form the club-shaped prominences with which we are 
already familiar under the names clava and cuneate tubercle. At the level of the 
pyramidal decussation the gracile nucleus has the form of a rather thin and 
ill-defined plate, while the cuneate nucleus is represented by a slight projection 
from the dorsal surface of the posterior gray column (Fig. 98). At the level of 
the decussation of the lemniscus both have enlarged and the gracile nucleus has 
become sharply outlined (Fig. 99). As the central canal opens out into the 



138 



THE NERVOUS SYSTEM 



fourth ventricle the nuclei are displaced laterally and gradually come to an end 
as the restiform body becomes clearly defined (Fig. 101). 

As one would expect from the fact that there is no sharp line of separation between the 
spinal cord and medulla oblongata, some of the fibers of the cuneate fasciculus end in the 
substantia gelatinosa (here known as the nucleus of the spinal tract of the trigeminal nerve) 
and in the remnant of the head of the posterior gray column (Fig. 100). There are three 
smaller gray masses within the funiculus cuneatus: (1) the external round nucleus, an iso- 
lated portion of the substantia gelatinosa, near which it is situated; (2) the internal round 
nucleus, more variable in position; and (3) the accessory or lateral cuneate nucleus superficial 
to the main nuclear mass. 



Funiculus gracilis 

Nucleus gracilis 

Spinal tract of trigeminal 
nerve 

Nucleus of spinal tract 
of N. V 

Dorsal motor nucleus of. 
vagus 

Nucleus of hypoglossal 
nerve 

Decussation of medial 
lemniscus 

Lateral reticular nucleus 

Medial accessory olivary 
nucleus 

Ventral external arcuate 
fibers 




Funiculus cuneatus 
Nucleus cuneatus 
Central canal 

Internal arcuate fibers 
Reticular substance 

Dorsal spinocerebellar 
tract 

Ventral spinocerebellar 
tract 

Ventral fasciculus 
proprius 

Hypoglossal nerve 

Pyramid, corticospinal 
tract 



Fig. 99. Section through the medulla oblongata of a child at the level of the decussation of the 
medial lemniscus. (Pal-Weigert method.) (X 6.) 

The Medial Lemniscus and its Decussation. The great majority of fibers 
which arise from the cells in the nucleus gracilis and nucleus cuneatus sweep 
ventromedially in broad concentric curves around the central gray substance 
toward the median raphe (Fig. 99). As has been stated on a preceding page, 
these are known as internal arcuate fibers, and as they cross those from the 
opposite side in the raphe they form the decussation of the lemniscus (decussatio 
lemniscorum, sensory decussation). After crossing the median plane they turn 
rostrally in the medial lemniscus (fillet), and end in the thalamus (Fig. 235). 
These longitudinal fibers constitute a broad band which lies close to the median 
raphe, medial to the inferior olivary nucleus, and dorsal to the pyramids (Figs. 
96, 97). By the accession of additional internal arcuate fibers this band in- 
creases in size and spreads out dorsally until at the level of the middle of the 
olive it is separated from the gray matter of the ventricular floor only by the 



THE STRUCTURE OF THE MEDULLA OBLONGATA 



139 



fibers of the fasciculus longitudinalis medialis and the tectospinal tract (Fig. 
101). The decussation of the lemniscus begins at the upper border of the 
decussation of the pyramids, where the sensory fibers are grouped into coarse 
bundles arching around the central gray matter (Fig. 99), and extends as far as 
do the gracile and cuneate nuclei, that is, to about the middle of the olive. In 
sections through the lower half of the olive the internal arcuate fibers describe 
broad curves through the reticular formation and their decussation occupies a 
considerable ventrodorsal extent of the raphe (Fig. 101). 



Nerve cell in the nucleus cuneatus 



Ramification of fibers from the fasciculus cuneatus 



Nucleus cuneatus 



Fasciculus 
cuneatus 




Subslantia 
gelatinosa 



Fig. 100. From a transverse section through the medulla oblongata of a kitten, to illustrate 
the termination of the fibers of the fasciculus cuneatus, and at a the beginning of the internal 
arcuate fibers. (Combined from drawings by Cajal.) 

The arcuate fibers of the medulla oblongata may be separated into two 
groups: those which run through the reticular formation constitute the inter- 
nal arcuate fibers; and those which run over the surface of the medulla, the 
external arcuate fibers. The internal arcuate fibers are of at least three kinds: 
(1) those described in the preceding paragraph, which arise in the gracile and 
cuneate nuclei and form the medial lemniscus; (2) sensory fibers of the second 
order, arising in the sensory nuclei of the cranial nerves; and (3) olivocerebellar. 
fibers, which will be considered in another paragraph. Our knowledge of the 
external arcuate fibers is less satisfactory. From the nuclei of the posterior funic- 



140 



THE NERVOUS SYSTEM 



uli and perhaps also from these funiculi themselves a group of dorsal external 
arcuate fibers make their way to the restiform body along the dorsal surface of 
the medulla (Fig. 101). According to Cajal these fibers are well developed in 
man, but absent in the cat and rabbit. The ventral external arcuate fibers are 
said to include a certain number which arise in the lateral reticular and arcuate 
nuclei and run dorsolaterally over the surface of the medulla to reach the 
cerebellum by way of the restifrom body (Fig. 104). The arcuate nuclei are 
small irregular patches of gray matter situated on the ventromedial aspect of 
the pyramid and continuous rostrally with the nuclei pontis, with which they 



Spinal vestibular 
nucleus 

Dorsal external 
arcuate fibers 

Tractus solitarius 
and nucleus 

Nucleus of 
hypoglossal nerve 

Internal arcuate 
fibers 

Dorsal spinocere- 
bellar tract 

Medial longitudinal 
fasciculus 

Ventral spinocere- 
bellar tract 

Tectospinal tract 
Medial lemniscus 

Inferior olivary 
nucleus 

Hilus of olivary 
nucleus 

Pyramid, cor ti co- 
spinal tract 

Fig. 101. Section through the medulla oblongata of a child at 

method. (X 6.) 




Dorsal motor 
nucleus of vagus 

Nucleus cuneatus 
Restiform body 



Spinal tract and 
nucleus N. V 

Nucleus ambiguus 

Reticular substance 

Lateral reticular 
nucleus 

edial accessory 
olivary nucleus 
nferior olivary 
nucleus 

Hypoglossal nerve 

Ventral external 
arcuate fibers 

the level of the olive. Pal-Weigert 



seem to be homologous (Figs. 101, 103). They probably receive fibers from the 
cerebral cortex by way of the pyramidal tracts; and, if so, the external arcuate 
fibers which arise from them are homologous with the transverse fibers of the 
pons. 

Although the facts stated above are pretty well established, only a small part of the 
ventral external arcuate fibers are thus accounted for. The origin and course of the majority 
of these fibers is still obscure. According to Cajal (1909) they arise from the nuclei of the 
posterior funiculus, curve ventrally and medially over the surface of the medulla oblongata, 
penetrate the pyramids or the anterior median fissure, cross in the median raphe, and join 
the medial lemniscus of the opposite side. On the other hand, Edinger (1911) gives to 



THE STRUCTURE OF THE MEDULLA OBLONGATA 



141 



them the name "tractus cerebello-tegmentalis bulbi," and believes that they descend from 
the cerebellum by way of the restiform body, then arch ventrally over the surface of the 
medulla, penetrate the pyramid or the anterior median fissure, and end in the reticular 
formation of the opposite side (Fig. 153). According to Van Gehuchten (1904) some of the 
ventral external arcuate fibers arise from cells in the reticular formation of the same and the 
opposite side, and run through the restiform body to the cerebellum. 

Olivary Nuclei. The oval prominence in the lateral area of the medulla, 
known as the olive, is produced by the presence just beneath the surface of a 
large gray mass, the inferior olivary nucleus, with which there are associated 




Fig. 102. Diagram to illustrate the structure of the inferior olivary nucleus. (Cajal, Edinger.) 

two accessory olivary nuclei. The inferior olivary nucleus is very conspicuous 
in the sections of this part of the medulla (Fig. 101). It appears as a broad, 
irregularly folded band of gray matter, curved in such a way as to enclose a 
white core, which extends into the nucleus from the medial side through an 
opening, known as the hilus. Considered as a whole this nucleus resembles a 
crumpled leather purse, with an opening, the hilus, directed medially. Sec- 
tions at either end of the nucleus do not include this Opening, and at these 
points the central core of white matter is completely surrounded by the gray 
lamina. The fibers which stream in and out of the hilus constitute the olivary 



142 



THE NERVOUS SYSTEM 



peduncle. The two accessory olives are plates of gray substance, which in trans- 
verse section appear as rods. The medial accessory olivary nucleus is placed be- 
tween the hilus of the inferior olive and the medial lemniscus, while the dorsal 
accessory olivary nucleus is located close to the dorsal aspect of the chief nuclear 
mass. 

Structure and Connections. The gray lamina of the inferior olivary nucleus 
consists of neuroglia and many rounded nerve-cells beset with numerous short, 
frequently branching dendrites, the axons of which run through the white core 
of the nucleus and out at the hilus as olivocerebellar fibers (Fig. 102) . About 
these cells there ramify the end branches of several varieties of afferent fibers, 
the origin of which is not well understood. Some come from a tract, designated 



Fourth ventricle 
Principal vestibular nucleus 
Spinal vestibular nucleus 
Nucleus intercalate 

Restiform body 
Spinal tract and 
nucleus N. V 
Ponlobulbar body 

Glossopharyngeal nerve 

Nucleus ambiguus 

Ventral spinocerebellar tract 
Dorsal accessory olivary 
nucleus 

Hilus of olivary nucleus 
Inferior olivary nucleus 

Medial accessory olivary nucleus 

Ventral external arcuate fibers 



Tcenia of fourth ventricle 




Nucleus of hypoglossal nerve 

Dorsal motor nucleus of vagus 
Tractus solitarius and 
nucleus 
Medial longitudinal 

fasciculus 
Reticular substance 
Olivocerebellar fibers 
Vagus nerve 

Lateral reticular nucleus 
Thalamo-olivary tract 
Inferior olivary nucleus 
Medial lemniscus 
Hypoglossal nerve 
Pyramid, corticospinal tract 
Arcuate nucleus 



Fig. 103. Section through the medulla oblongata of a child at the level of the restiform body. 

Pal-Weigert method. (X4.) 

as the thalamo-olivary fasciculus; but it is not certain that they have their 
origin in the thalamus; quite possibly they come from some other gray mass 
in that neighborhood. Another group of fibers, consisting chiefly of collaterals, 
comes from the ventral funiculus of the spinal cord and may be regarded as 
ascending sensory fibers (Cajal, 1909). These belong to the so-called spino- 
olivary fasciculus. 

Olivocerebellar Fibers. The axons from the cells of the inferior olivary 
nucleus stream out of the hilus, cross the median plane, and either pass through 
or around the opposite nucleus. Here they are joined by some uncrossed fibers 
from the olivary nucleus of the same side. Thence they curve dorsally toward 
the restiform body, passing through the spinal tract of the trigeminal nerve 



THE STRUCTURE OF THE MEDULLA OBLONGATA 



which becomes split up into several bundles (Fig. 103). They form an im- 
portant group of internal arcuate fibers, which run through the restiform body 
to the cerebellum and constitute the olivocerebellar tract (Fig. 104). 

The restiform body or inferior cerebellar peduncle is a large and prominent 
strand of fibers which gradually accumulate along the lateral border of the 
caudal part of the fourth ventricle. It forms the floor of the lateral recess of 
that cavity and then turns dorsally into the cerebellum (Figs. 88, 89, 103). It 
is composed for the most part of two large and important fascicles: (1) the 



-'Restiform body 
- -Olivocerebellar tract 
'Lateral reticular nucleus 



Medulla oblongata < 



Spinal cord 




""Arcuate fibers from arcuate 
nucleus 



Dorsal external arcuate fibers 



Dorsal spinocerebellar tract 



Fig. 104. Diagram showing the fiber tracts which enter the restiform body from the medulla 

oblongata. 

olivocerebellar fibers, both direct and crossed, but chiefly from the inferior olivary 
nucleus of the opposite side; and (2) the dorsal spinocerebellar tract, from the 
nucleus dorsalis of the same side of the spinal cord (Fig. 104). In addition, 
there are fibers in smaller number from other sources: (3) the dorsal external 
arcuate fibers from the gracile and cuneate nuclei of the same side; and fibers 
(4) from the arcuate nucleus, (5) from the lateral reticular nucleus, and possibly 
also from other cells scattered through the reticular formation (Van Gehuchten, 
1904). 



144 



THE NERVOUS SYSTEM 



The dorsal spinocerebellar tract can readily be traced in serial sections of 
the medulla because the large, heavily myelinated fibers of which it is composed 
cause it to be deeply stained by the Weigert technic. It can be followed from 
the spinal cord along the periphery of the medulla oblongata near the posterior 
lateral sulcus. At first it lies ventral to the spinal tract of the trigeminal nerve 
(Figs. 98, 99). But at the level of the lower part of the olive it inclines dorsally, 
passing over the surface of the spinal tract of this nerve to reach the restiform 
body (Fig. 101). Between this tract and the olive we find the ventral spino- 
cerebellar tract also in a superficial position. 

The spinal tract of the trigeminal nerve is formed by the descending branches 
of the sensory fibers of that nerve. They give off collateral and terminal 
branches to a column of gray matter, resembling the substantia gelatinosa 



Tractus solitarius and nucleus 

Dorsal motor nucleus of vagus 

Nucleus of hypoglossal nerve 

Nucleus amblguus 

Medial longitudinal fasciculus 

Tectospinal tract 

Dorsal accessory olivary nucleus 

Medial lemniscus 

Medial accessory olivary nucleus 



Corticospinal tract 



Fig. 105. Diagram showing the location of the nuclei 

at the level of the 




' Vestibular nudeus 
.--Nucleus cuneatus 



of the spinal tract N. V 

'" Dorsal spinocerebellar tract 
-- Spinal tract N. V 
Vagus nerve 



spinocerebellar tract 
" Spinothalamic tract 
Thalamo-olivary tract 
-Inferior olivary nucleus 

Hypoglossal nerve 



and fiber tracts of the medulla oblongata 
olive. 



Rolandi, with which it is directly continuous, and designated as the nucleus 
of the spinal tract of the trigeminal nerve (Figs. 92, 98, 99, 101, 103). The tract 
lies along the lateral side of the nucleus and is superficial except in so far as it 
is covered by the external arcuate fibers, the dorsal spinocerebellar tract, and the 
restiform body. It forms an elongated elevation, the tuberculum cinereum on 
the surface of the medulla oblongata (Fig. 88). 

The formatio reticularis fills the interspaces among the larger fiber tracts 
and nuclei. It is composed of small islands of gray matter, separated by' fine 
bundles of nerve-fibers which run in every direction, but which are for the 
most part either longitudinal or transverse. It is subdivided into two parts. 
The formatio reticularis alba is located dorsal to the pyramid and medial to the 
root filaments of the hypoglossal nerve and is composed in large part of longi- 



THE STRUCTURE OF THE MEDULLA OBLONGATA 145 

tudinal nerve-fibers belonging to the medial lemniscus, tectospinal tract, and the 
medial longitudinal fasciculus (Fig. 105). The latter is closely associated with 
the vestibular nerve and can best be described with the central connections of 
that nerve. Theformatio reticularis grisea is found dorsal to the olive and lateral 
to the hypoglossal nerve. In it the nerve-cells predominate and the trans- 
versely coursing internal arcuate fibers form a conspicuous feature. Its longi- 
tudinal fibers, though less prominent, are of great importance. The descend- 
ing fibers include those of the rubrospinal tract, which can be followed into the 
lateral funiculus of the spinal cord, and the thalamo-olivary fasciculus, which 
ends in the olive. Among the ascending filers are those of the ventral and 
dorsal spinocerebellor, the spinothalamic , and spinotectal tracts. 

The neme-cells of the reticular formation are scattered through the mesh of 
interlacing fibers. In certain localities they are more closely grouped and form 
fairly well-defined nuclei. Among these we may select two for special atten- 
tion. The lateral reticular nucleus or nucleus of the lateral funiculus is a long 
column of cells found along the deep surface of the ventral spinocerebellar tract, 
from which it is said by Andre Thomas to receive afferent fibers. At any rate, 
it receives fibers from the lateral funiculus of the spinal cord (Cajal, 1909) and 
sends its axons to the cerebellum by way of the restiform body (Van Gehuchten, 
1904; Yagita, 1906). It seems, therefore, to be a way station on a sensory 
path from the spinal cord to the cerebellum. Some large cells in the gray part 
of the reticular formation may be grouped together and called the motor nucleus 
of the tegmentum (nucleus magnocellularis of Cajal). Their axons become as- 
cending or descending fibers or may bifurcate into ascending and descending 
branches within the reticular formation. Kohnstamm has traced such fibers 
by means of the degeneration method, and has shown that they run for the 
most part in a caudal direction and that some of them reach the cervical por- 
tion of the spinal cord (tractus reticulospinalis Fig. 115). 

The nuclei of the cranial nerves can best be considered in a separate chapter. 
At this point it will only be necessary to enumerate and locate the nuclei of those 
nerves which take origin from the medulla oblongata. 

The nucleus of the hypoglossal nerve contains the cells of origin of the 
motor fibers which compose that nerve. It forms a long column of nerve-cells 
on either side of the median plane in the ventral part of the gray matter sur- 
rounding the central canal and in the floor of the fourth ventricle (Figs. 99, 101, 
103). In the latter region it lies immediately beneath that part of the floor 
which was described in the preceding chapter under the name of the trigonum 



146 THE NERVOUS SYSTEM 

hypoglossi (Fig. 89). In reality, it corresponds only to the medial part of this 
eminence, for on its lateral side there is found another group of cells known as 
the nucleus intercalatus (Fig. 103). From their cells of origin the fibers of 
the hypoglossal nerve stream forward through the reticular formation to emerge 
at the lateral border of the pyramid. 

The nucleus ambiguus is a long column of nerve-cells which give origin to 
the motor fibers that run through the glossopharyngeal, vagus, and accessory 
nerves to supply the striated musculature of the pharynx and larynx. It is 
located in the reticular formation of both the open and the closed portions 
of the medulla, ventromedial to the nucleus of the spinal tract of the trigeminal 
nerve (Figs. 101, 103). 

The dorsal motor nucleus of the vagus lies along the lateral side of the 
nucleus of the hypoglossal. It occupies the ala cinerea of the rhomboid fossa 
and extends into the closed part of the medulla oblongata along the lateral 
side of the central canal (Figs. 89, 99, 101, 103). From the cells of this nucleus 
arise the efferent fibers of the vagus nerve which innervate smooth muscle 
and glandular tissue. The afferent fibers of the vagus and glossopharyngeal 
nerves bend caudally and run within the tractus solitarius. 

The nucleus of the tractus solitarius is the nucleus of reception of the affer- 
ent fibers of the facial, glossopharyngeal, and vagus nerves, i. e., it contains 
the cells about which these afferent fibers terminate. The tractus solitarius 
can be traced throughout almost the entire length of the medulla. It decreases 
in size as the descending fibers terminate in the gray matter which surrounds 
it (Figs. 92, 101, 103). 



CHAPTER X 

INTERNAL STRUCTURE OF THE PONS 

THE pons consists of two portions which differ greatly in structure and sig- 
nificance. The dorsal or tegmental part resembles the medulla oblongata, of 
which it is the direct continuation. The ventral or basilar portion contains 
the longitudinal fibers which go to form the pyramids; but except for these it is 
composed of structures which are peculiar to this level. It is a recent phyletic 
development and forms a prominent feature of the brain only in those mam- 
mals which have relatively large cerebral and cerebellar hemispheres, as might 
be expected from the fact that it forms part of a conduction path uniting these 
structures. 

THE BASILAR PART OF THE PONS 

The basilar portion of the pons is the larger of the two divisions. It is 
made up of fascicles of longitudinal and transverse fibers and of irregular masses 
of gray substance, which occupy the spaces left among the bundles of nerve- 
fibers and which are known as the nuclei pontis. 

The longitudinal fasciculi of the pons consist of two kinds of fibers: (1) those 
of the corticospinal tract, which are continued through the pons into the pyra- 
mids of the medulla oblongata; and (2) those which end in the nuclei of the pons 
and are known as corticopontine fibers (Fig. 106). As they pass through the pons 
the corticospinal fibers give off collaterals which also end in these nuclei. The 
longitudinal fibers enter the pons at its rostral border from the basis pedunculi. 
At first they form on either side a single compact bundle; but this soon becomes 
broken up into many smaller fascicles, which are separated from each other 
by the transverse fibers and nuclei of the pons (Fig. 108). At the caudal border 
these bundles again become assembled into a compact strand, which is con- 
tinued as the pyramid of the medulla oblongata (Fig. 107). It is evident, how- 
ever, that the volume of the bundles is much greater at the rostral than at the 
caudal border. This is to be explained by the fact that the corticopontine 
fibers have left these bundles during their passage through the pons and have 
come to an end by arborization within the nuclei pontis. 

The transverse fibers are designated as fibres pontis and are divisable into a 
superficial and a deep group (fibrae pontis superficiales and fibrae pontis pro- 

147 



148 



THE NERVOUS SYSTEM 



funda). Those of the superficial group lie ventral to the longitudinal fasciculi; 
while the deep transverse bundles interlace with the longitudinal ones or lie 
dorsal to them. The majority of the fibrae pontis cross the median plane. These 
are joined by some uncrossed fibers and gathered together on either side of the 
pons to form a compact and massive strand, known as the brachium pontis or 
middle cerebellar peduncle, which curves dorsally to enter the white center of 
the cerebellum (Figs. 88, 108). 

^X * >v 

V Cerebral cortex, 

-y Corticobulbar tract 
- * Corticospinal tract 

Temporopontine tract 
Frontopontine tract 

- Pons 

- Cerebellum 

"' Nuclei pontis 

x Brachium pontis 



Lateral corticospinal tract 
Ventral corticospinal tract 



Fig. 106. Diagram of the cortico-ponto-cerebellar pathway and the corticospinal and cortico- 

bulbar tracts. 

Along the rostral border of the pons and brachium pontis one or two fiber bundles are 
sometimes found which run an isolated course to the cerebellum. These are known as the 
fila later alia pontis or Icenia pontis (Fig. 88). According to Horsley (1906) the constituent 
fibers arise from a ganglion situated caudal to the interpeduncular ganglion, decussate at once, 
and end in the cerebellum in the neighborhood of the dentate nucleus. Perhaps they rep- 
resent slightly displaced fibrae pontis. Some of the transverse fibers on reaching the median 
plane bend at right angles and run as fibrse rectae toward the pars dorsalis pontis (Fig. 108). 
According to Edinger (1911) these belong in part at least to the tractus cerebellotegmentalis 
pontis, which arises in the nuclei of the cerebellum and runs through the brachium pontis 
to end in the reticular formation of the opposite side (Fig. 153). Cajal (1909) is doubtful 
about the existence of such efferent fibers from the cerebellum in the brachium pontis. 

The nuclei pontis, which are continuous with the arcuate nuclei of the 
medulla oblongata, contain stellate nerve-cells of varying size, the axons of 




INTERNAL STRUCTURE OF THE PONS 149 

which are continuous with the fibrse pontis. There are also some small nerve- 
cells of Golgi's Type II, the short axons of which end in the adjacent gray mat- 
ter. Within these nuclei terminate the fibers of the corticopontine tracts and 
some collaterals from the corticospinal fibers. Collaterals from the medial 
lemniscus are also found arborizing in those nuclei of the pons which lie im- 
mediately ventral to that bundle. This gray matter, therefore, represents an 
important association apparatus within which there terminate fibers from 
several different sources. 

From what has been said it will be apparent that the pons serves to estab- 
lish an important and for the most part crossed connection between the cere- 
bral hemispheres and the cerebellum, a cortico-ponto-cerebellar path. The cor- 
ticopontine fibers take origin from pyramidal cells in the frontal and temporal 
lobes and end in the nuclei pontis. Arising from the cells in these nuclei, most 
of the transverse fibers cross the median plane and reach the opposite cerebellar 
hemisphere through the brachium pontis (Fig. 106). 

THE DORSAL OR TEGMENTAL PART OF THE PONS 

The dorsal or tegmental part of the pons (pars dorsalis pontis) resembles in 
structure the medulla oblongata (Fig. 108). On its dorsal surface there is a 
thick layer of gray matter which lines the rhomboid fossa. Between this layer 
and the basilar portion of the pons is the reticular formation divided by the 
median raphe into two symmetric halves. This has essentially the same struc- 
ture here as in the medulla oblongata, and contains the continuation of many 
longitudinal tracts with which we are already familiar. The restiform body at first 
occupies a position similar to that which it has in the medulla, along the lateral 
border of the rhomboid fossa; but it soon bends dorsally into the cerebellum. 

The Cochlear Nuclei. At the point of transition between the medulla and 
pons the restiform body is partly encircled on its lateral aspect by a mass of 
gray matter formed by the terminal nuclei of the cochlear division of the acoustic 
nerve (Fig. 107). There may be distinguished a dorsal and a -ventral cochlear 
nucleus at the dorsal and ventral borders of the restiform body. Within these 
nuclei the fibers of the cochlear nerve end; while those of the vestibular nerve 
plunge into the substance of the pons ventromedially to the restiform body to 
reach the floor of the fourth ventricle (Fig. 134). Fibers from the dorsal cochlear 
nucleus run medially upon the floor of the fourth ventricle in the striae medullares 
(Fig. 89), and sinking into the tegmentum join the fibers from the ventral coch- 
lear nucleus in the trapezoid body. 



THE NERVOUS SYSTEM 



The trapezoid body (corpus trapezoideum) , which in most mammals appears 
on the surface of the medulla near the border of the pons (Fig. 83), is covered 
in man by the enlarged pars basalis pontis. In sections through the more caudal 
portions of the pons the trapezoid body forms a conspicuous bundle of trans- 
verse fibers in the ventral portion of the reticular formation (Fig. 108). The 
fibers are associated with the terminal nuclei of the cochlear nerve, especially 
the ventral one, and with the superior olivary nucleus, around the ventral border 
of which they swing in such a way as to form a bay for its reception. Farther 
medialward they pass through the medial lemniscus at right angles to its con- 



Fourth ventricle 
Stria medullares 



Dorsal cochlear nucleus 




Vent, spinocerebellar tract 

Vent, external arcuate fibers 
Medial lemniscus 



Nucleus of eminentia teres 

Principal vestibular nucleus 

Lateral vestibular 
nucleus 

Nucleus of tract us 
solitarius 
Glossopharyngcal 

nerve 
Dorsal cochlear 

nucleus 

Restiform body 
Ventral cochlear 
nucleus 

Spinal tract and 
nucleus N. V 

Trapezoid body 
Pontobulbar body 
Medial longitudinal fasciculus 
Thalamo-olivary tract 

Inferior olivary nucleus 
Pyramid, corticospinal tract 
Arcuate nucleus 



Foramen cacum Pons 

Fig. 107. Section through caudal border of the pons and the cochlear nuclei of a child. 

Weigert method. ( X 4.) 



Pal- 



stituent fibers and decussate in the median raphe. The trapezoid body de- 
scribes a curve with convexity directed rostrally as well as ventrally, and as a 
result its lateral portions are seen best in sections through the lower border 
of the pons (Fig. 107), while the rest of it is in evidence in sections at a higher 
level (Fig. 108). Arising from the ventral nucleus of the cochlear nerve (Fig. 
107) these fibers pass, with or without interruption in the superior olivary 
nucleus, across the median plane (Fig. 108) ; and, on reaching the lateral border 
of the opposite superior olivary nucleus, they turn rostrally to form a longi- 
tudinal band of fibers known as the lateral lemniscus (Fig. 110). This is a 



INTERNAL STRUCTURE OF THE PONS 



part of the central auditory pathway, the connections of which are represented 
diagrammatically in Fig. 134. 

The superior olivary nucleus is a small mass of gray matter located in the 
ventrolateral portion of the reticular formation of the pons in close relation to 
the trapezoid body and not far from the rostral pole of the inferior olivary nucleus 
(Figs. 108, 110). It consists of two or three separate but closely associated 
nuclear masses composed of small fusiform nerve-cells, among which there 
ramify collaterals from the fibers of the trapezoid body. From the dorsal aspect 



Superior vestibular nucleus 



Abducens nerve 

Genu of facial N. / 

Medial longitudinal 
fasciculus 



Fourth ventricle 




Restiform body 
Brachium pontis 

Nucleus of abducens N. 
Facial nerve 

Spinal tract and nu- 
cleus N. V 

Nucleus of facial N. 
Thalamo-olivary tract 
Superior olivary nucleus 

Trapezoid body and 
medial lemniscus 

Deep stratum of pons 

Corticospinal and cortico- 
pontine tracts 

Nuclei pontis 

'ficial stratum of pons 



Fig. 108. Section through the pons of a child at the level of the facial colliculus. Pal-Weigert 

method. (X 4.) 

of this nucleus a bundle of fibers, known as the peduncle of the superior olive, 
makes its way toward the nucleus of the abducens nerve, and it may be that 
some of these fibers enter the medial longitudinal bundle (Fig. 124). 

The nuclei of the vestibular nerve lie in the floor of the fourth ventricle, 
where they occupy a field with which we are already familiar, namely, the area 
acustica (Fig. 89). The vestibular fibers on approaching the rhomboid fossa 
divide into ascending and descending branches, and terminate in four nuclear 
masses: (1) the medial (dorsal or principal) vestibular nucleus (Figs. 103, 107), 
(2) the lateral vestibular nucleus of Deiters (Fig. 107) , (3) the superior vestibular 



152 



THE NERVOUS SYSTEM 



nucleus of Bechterew (Fig. 108), (4) the spinal or descending vestibular nucleus 
(Fig. 103). These are represented diagrammatically in Fig. 136. 

The medial longitudinal fasciculus is an important bundle which extends 
from near the floor of the third ventricle to the spinal cord, and is especially 
concerned with the reflex control of the movements of the head and eyes. A 
large proportion of its fibers are derived from the lateral vestibular nucleus. 




~M. rectus medialis 
'i'\M. rectus lateralis 

Nucleus of med. long. fasc. 

Nucleus of oculomotor nerve 
Nucleus of trochlear nerve 

Nucleus of abducens nerve 
Medial longitudinal fasciculus 

r Lateral vestibular nucleus 

Vestibular nerve 



Fig. 109. Diagram showing the connections of the medial longitudinal fasciculus. (Modified 

from Villiger.) 

From this origin the fibers pass horizontally through the reticular formation to 
the median longitudinal fasciculus of the same or the opposite side, and there 
divide into ascending and descending branches (Fig. 109) . The former terminate 
in the nuclei of the oculomotor, trochlear, and abducens nerve, the latter in 
the nucleus of the spinal accessory nerve and in the columna anterior of the 
cervical portion of the spinal cord. In this way there is established a path for 



INTERNAL STRUCTURE OF THE PONS 153 

the reflex control of the movement of the head, neck, and eyes in response to 
stimulation of the nerve endings in the semicircular canals of the ears. Another 
important group of fibers within this fasciculus takes origin from a collection 
of cells situated in the hypothajamus just rostral to the red nucleus, which 
Cajal (1911) has called the interstitial nucleus, 1 but which might properly be 
designated as the nucleus of the medial longitudinal fasciculus. According to 
Cajal the fascicle also contains ascending fibers from the ventral fasciculus 
proprius of the spinal cord. Still other fibers serve to connect the nuclei of the 
oculomotor and abducens nerves. 

The medial longitudinal fasciculus is continued into the ventral fasciculus 
proprius of the spinal cord. These fibers are displaced dorsolaterally by the 
decussation of the pyramids (Fig. 98) and then still farther dorsally by the 
decussation of the lemniscus (Fig. 99) until they come to lie in the most dorsal 
part of the substantia reticularis alba (Fig. 101), which position they occupy 
throughout the remainder of their course. The fasciculus is found ventral to 
the nucleus of the hypoglossal nerve (Fig. 103) and in close apposition to the 
nuclei of the three motor nerves of the eye (Figs. 108, 114, 116). 

The medial lemniscus can also be traced within the reticular formation from 
the medulla into and through the pons. But this broad band of longitudinal 
fibers, which was spread out along the median raphe in the medulla, shifts 
ventrally in the pons, assuming first a somewhat triangular outline and a ven- 
tromedian position (Fig. 107); then by shifting farther lateralward it takes 
again the form of a flat band (Figs. 108, 110). But now it is compressed ven- 
trodorsally and occupies the ventral part of the reticular formation, its fibers 
crossing those of the trapezoid body at right angles. It must not be forgotten 
that the medial lemniscus is composed of longitudinal fibers, and it is by the 
gradual shifting of these that the bundle as a whole changes shape and posi- 
tion. As it is displaced ventrally it separates from the medial longitudinal 
bundle, which retains its dorsal position. 

The motor nucleus of the facial nerve occupies a position in the reticular 
formation dorsal to the superior olive (Fig. 108). It is an oval mass of gray 
matter, which extends from the lower border of the pons to the level of the 
facial colliculus, and contains the cells of origin of the fibers which innervate 

1 The interstitial nucleus of Cajal must not be confused with the nucleus of the posterior 
commissure of Darkschewitsch whicli lies in the mesencephalon just rostral to the oculomotor 
nucleus and which, according to Cajal, may or may not send fibers into the medial longitudinal 
bundle. 



154 



THE NERVOUS SYSTEM 



the platysma and muscles of the face. These fibers emerge from the dorsal 
surface of the nucleus and run dorsomedially toward the floor of the fourth 
ventricle. Somewhat widely separated at first, they become united on the 
medial side of the abducens nerve into a compact strand, which as the genu of 
the facial nerve partly encircles this nucleus, and which then runs ventrolateraUy 
between the spinal tract of the trigeminal nerve and its own nucleus toward 
its exit from the brain (Figs. 108, 124). 



Anterior medullary velum 



Medial longitudinal fasckulus 
Ventral spinocerebellar tract 

Trapezoid body: 
Superior olive, 

Lateral lemniscus 
Brachium pontis 



Fourth ventricle 

Brachium conjunctiwim 

Mesencephalic root of trigem- 

inal nerve 
Motor nucleus of trigeminal 

nerve 

Sensory nucleus of trigem- 
inal nerve 




Medial lemniscus 
Superficial stratum of pons 



Trigeminal nerve 
Corticospinal and corlico- 
pontine tracts 

Nuclei pontis 



Fig. 110. Section through the pons of a child at the level of the motor nucleus of the trigeminal 

nerve. Pal-Weigert method. (X 4.) 

The nucleus of the abducens nerve along with the genu of the facial pro- 
duces a rounded elevation in the rhomboid fossa, known as the facial colliculus 
(Figs. 89, 108). It is a spheric mass of gray matter containing the cells of origin 
of the fibers which innervate the lateral rectus. These emerge from the dorsal 
and medial surfaces of the nucleus and run ventrally more or less parallel to the 
median raphe toward their exit at the lower border of the pons. 

The Nuclei of the Trigeminal Nerve. In transverse section through approxi- 



INTERNAL STRUCTURE OF THE PONS 155 

mately the middle of the pons we encounter the fibers of the trigeminal nerve 
and two associated masses of gray matter, the motor and main sensory nuclei 
of that nerve (Fig. 110). These are located close together in the dorsolateral 
part of the reticular formation near the groove between the middle and supe- 
rior cerebellar peduncles. Of the two, the sensory nucleus is the more superficial. 
It is, in reality, not a new structure, but rather the enlarged rostral extremity 
of the column of gray matter which we have followed upward from the sub- 
stantia gelatinosa Rolandi of the spinal cord and have designated as the nucleus 
of the spinal tract of the trigeminal nerve (Figs. 98, 101). On its medial side is 
found the motor nucleus, a large oval mass of gray matter from the cells of which 
arise the motor fibers for the muscles of mastication. Some of the fibers of the 
trigeminal nerve, passing between these two nuclei, are continued as the mesen- 
cephalic root of the trigeminal nerve (Figs. 110, 111). Reaching the gray matter 
in the lateral wall of the rostral part of the fourth ventricle, this bundle of fibers 
turns rostrally along the medial side of the brachium conjunctivum (Fig. 112). 
It extends into the mesencephalon in the lateral part of the gray matter which 
surrounds the cerebral aqueduct (Fig. 114). The fibers of this root take origin 
from unipolar cells scattered along its course and known as the mesencephalic 
nucleus of the trigeminal nerve. 

It will be apparent from this description that there are four nuclear masses 
associated with the trigeminal nerve, namely, the nucleus of the spinal tract, 
and the main sensory, motor, and mesencephalic nuclei. The relations which 
each of these groups of cells bear to the fibers of the trigeminal nerve are illus- 
trated in Fig. 111. Note that those fibers which arise from cells in the semi- 
lunar ganglion divide into short ascending and long descending branches. The 
former end in the main sensory nucleus; while the latter run in the spinal tract 
of the trigeminal nerve and end in the nucleus which accompanies it. 

The brachium conjunctivum or superior cerebellar peduncle (Fig. 89) is seen 
in sections through the rostral half of the pons, where it enters into the lateral 
boundary of the fourth ventricle. It is a large strand of fibers which runs from 
the dentate nucleus of the cerebellum to the red nucleus of the mesencephalon 
(Fig. 115). As it emerges from the white center of the cerebellum this brachium 
is superficially placed, with its ventral border resting on the tegmental portion 
of the pons (Fig. 110). To its dorsal border is attached a thin plate of white 
matter, the anterior medullary velum, which roofs in the rostral part of the 
fourth ventricle. As the brachium ascends toward the mesencephalon it sinks 
deeper and deeper into the dorsal part of the pons until it is entirely submerged 



156 



THE NERVOUS SYSTEM 



(Fig. 112). Near the rostral border of the pons it assumes a crescentic outline 
and lies in the lateral part of the reticular formation. From its ventral border 




Fig. 111. Diagram of the nuclei and central connections of the trigeminal nerve: A, Semi- 
lunar ganglion; B, mesencephalic nucleus, N. V.; C, motor nucleus, N. V.; D, motor nucleus, N. 
VII; E, motor nucleus, N. XII; F, nucleus of the spinal tract of N. V; G, sensory fibers of the sec- 
ond order of the trigeminal path ; a, ascending and b, descending branches of the sensory fibers, 
N. V; c, ophthalmic nerve; d, maxillary nerve; e, mandibular nerve. (Cajal.) 

fibers stream across the median plane, decussating with similar fibers from the 
opposite side. This is the most caudal portion of the decussation of the brachium 



INTERNAL STRUCTURE OF THE PONS 



conjunctivum, which increases in volume as it is followed rostrally, reaching its 
maximum in the mesencephalon at the level of the inferior colliculi. In this 
decussation the fibers of the brachium undergo a complete crossing. 

The ventral spinocerebellar tract, which has made its way through the retic- 
ular formation of the pons, turns dorsolaterally near the rostral end of the 
pons, winds around the brachium conjunctivum, and enters the anterior medul- 
lary velum, in which it descends to the vermis of the cerebellum (Figs. 110, 
149). 

Fourth ventricle 
Dorsal longitudinal fasciculus 
Medial longitudinal fasciculus 

Thalamo-olwary 
Brachium conjunctivum 



tract 



Decussation of brachium 
conjunctivum 




Trochlear nerve 

Mesencephalic root of trigeminal 
nerve 

Lateral lemniscus and nucleus 
Dorsal nucleus oflegmentum 
Ventral nucleus of 

tegmentum 

Nucleus centralis superior 
Medial lemniscus 
Pons 



Fig. 112. Dorsal half of a section through the rostral part of the human pons. The index 
line to the mesencephalic root of the trigeminal nerve does not quite reach that structure. Pal- 
Weigert method. 

The lateral lemniscus is an important tract of fibers which we have already 
traced from the cochlear nuclei by way of the trapezoid body and striae medul- 
lares acusticae. It first takes definite shape about the middle of the pons, where 
it is situated lateral to the medial lemniscus (Fig. 110). As it ascends it becomes 
displaced dorsolaterally until it occupies a position on the lateral aspect of the 
brachium conjunctivum (Fig. 112). In this position there is developed in con- 
nection with it a collection of nerve-cells, the nucleus of the lateral lemniscus, 
to which its fibers give off collaterals. 



CHAPTER XI 



Lamina quadrigemina 

Cerebral aqueduct / 

Central gray stratum-^., .(^ \J 

Tegmentum 




THE INTERNAL STRUCTURE OF THE MESENCEPHALON 

A DIAGRAM of a transverse section through the rostral part of the mesen- 
cephalon will make clear the relation of the various parts of the midbrain to 
each other (Fig. 113). The cerebral aqueduct is surrounded by a thick lamina 
of gray matter, the central gray stratum (stratum griseum centrale). Dorsal to 
this lies the lamina quadrigemina, a plate of mingled gray and white matter 
which bears four rounded elevations, the corpora quadrigemina. The ventral 

part of the midbrain is formed by 
the cerebral peduncles, each of which 
is separated into two parts by a 
lamina of pigmented gray substance, 
known as the substantia nigra. 
Dorsal to this the peduncle consists 
of reticular formation continuous 
with that of the pons and known as 
the tegmentum. Ventral to the sub- 
stantia nigra is a thick plate of longitu- 
dinal fibers, called the basis pedunculi, 
composed of fibers which are continuous with the longitudinal fasciculi of the 
pons. 

The Tegmentum. The dorsal portion of the pons is directly continuous 
with the tegmentum of the mesencephalon. Both are composed of reticular 
formation, consisting of interlacing longitudinal and transverse fibers grouped 
in fine bundles and separated by minute masses of gray substance, in which are 
embedded important nuclei and fiber tracts. In the caudal part of the mid- 
brain and the rostral part of the pons are four cellular masses the locations of 
which are indicated in Fig. 112. They are the dorsal nucleus of the raphe, the 
superior central nucleus, the ventral tegmental nucleus, and the dorsal tegmental 
nucleus. The latter is a collection of small cells in the central gray substance, sep- 
arated from the ventral tegmental nucleus by the medial longitudinal bundle. 
Both the ventral and dorsal tegmental nuclei receive fibers from the mammillary 
body (tractus mamillotegmentalis) , and within the dorsal one there also ter- 



Basis pedunculi'" 

Substantia nigra--'' 
Fig. 113. Diagrammatic cross-section through 
the human mesencephalon. 



THE INTERNAL STRUCTURE OF THE MESENCEPHALON 



159 



minate fibers from the interpeduncular ganglion (Fig. 211). The tegmentum 
contains many longitudinal fiber tracts which are continued into it from the dor- 
sal part of the pons. The most conspicuous of these is the brachium conjunc- 
tivum. 

The Decussation of the Brachia Conjunctiva. In the sections of the pons we 
saw that, as the brachia conjunctiva ascend toward the mesencephalon, they 
sink deeper and deeper into the pars dorsalis pontis (Fig. 112). When they 
reach the level of the inferior colliculi of the corpora quadrigemina they are 



Aqueduct of cerebrum 

Mesencephalic root of N. V 

Medial longitudinal 
fasciculus 

Decussation of brachium 
conjunctivum 

Interpeduncular fossa 
Szibstantia nigra 




Commissure of inferior colliculi 
Inferior quadrigeminal brachium 
Nucleus of inferior colliculus 
Lateral lemniscus 
Trochlear nerve 
Thalamo-olivary tract 
Nucleus of trochlear nerve 



Medial lemniscus 
Basis pedunculi 
Posterior perforated substance 
Pons 

Fig. 114. Section through the mesencephalon of a child at the level of the inferior colliculus. 

Pal-Weigert method. (X 4.) 

deeply placed in the tegmentum; and here they cross the median plane in the 
decussation of the brachium conjunctivum (Fig. 114). After crossing, each brach- 
ium turns rostrally and forms a rounded bundle of ascending fibers, which al- 
most at once comes into relation with the red nucleus (Fig. 116). Many of the 
fibers enter this nucleus directly, while others are prolonged over its surface to 
form a capsule that is best developed on its medial surface. While the majority 
of these fibers ultimately end in the red nucleus, some reach and end within the 
ventral part of the thalamus (Fig. 115). By way of summary we may repeat 
that the fibers of the brachium conjunctivum, or at least the greater part of them, 



i6o 



THE NERVOUS SYSTEM 



arise in the dentate nucleus of the cerebellum; they cross the median plane 
in the tegmentum at the level of the inferior colliculi and end either in the red 
nucleus or in the thalamus. 

According to Cajal (1911) the fibers of the brachium conjunctivum give off two sets of 
descending branches, which he has seen in Golgi preparations of the mouse, rabbit, and cat. 
The first group are collaterals given off as the brachium enters the dorsal part of the pons 
and before its decussation (Fig. 115). They descend into the pons and medulla oblongata 
and constitute a direct descending tract from the dentate nucleus of the cerebellum to the 
reticular formation of the pons and medulla oblongata. The second group of descending 



Rubrospinal tract x 
Rubroreticular tract v 




From frontal lobe and corpus strialum 
Thalamus 

Red nucleus 
Brachium conjunctivum 
Dentate nucleus 

\ 



Pons 
Rubrospinal tract 

Medulla oblongata 

Reticulospinal tract 

Spinal cord 

Fig. 115. Diagram showing the connections of the red nucleus: A, Ventral tegmental 
decussation; B, decussation of the brachium conjunctivum; Cand D, descending fibers from bra- 
chium conjunctivum, before and after its decussation respectively. 

branches is formed by the bifurcation of the fibers of the brachium conjunctivum just beyond 
the decussation, and constitute a crossed descending tract from the dentate nucleus, which 
can be followed by degeneration methods through the reticular formation of the brain stem 
and probably into the anterior and lateral funiculi of the spinal cord (Fig. 115). 

The red nucleus (nucleus ruber) is a very large oval mass of gray matter, 
which in the fresh brain has a pink color. It is located on the path of the brach- 
ium conjunctivum in the rostral part of the tegmentum (Fig. 116). In trans- 
verse sections it presents a circular outline and can be followed from the level 
of the inferior border of the superior colliculus into the hypothalamus. In its 
caudal portion it contains great numbers of fibers derived from the brachium 



THE INTERNAL STRUCTURE OF THE MESENCEPHALON l6l 

conjunctivum, and stains deeply in Weigert preparations, but farther rostrally 
these fibers are less numerous and the nucleus takes on more and more the ap- 
pearance of gray substance. 

Afferent fibers reach the red nucleus chiefly through the brachium con- 
junctivum, but it also receives fibers from the cerebral cortex of the frontal 
lobe and others from the corpus striatum (Fig. 115). These descending fibers 
help to form the capsule of the nucleus and are most abundant along its medial 
surface. 

Efferent Fibers. From the cells of the red nucleus arise the fibers of the 
rubrospinal tract, which after crossing the median plane descend into the spinal 
cord. Other cells give origin to fibers, which decussate along with those of the 
rubrospinal tract and terminate in the nuclei of the reticular formation and in 
the nucleus of the lateral lemniscus. These form the tractus rubroreticularis 
(Fig. 115). Other fibers from the red nucleus reach the thalamus. 

The nerve-cells which are found in the red nucleus vary greatly in size. The smaller 
ones have the character of the cells of the reticular formation and send their axons into the 
tegmentum of the same and the opposite side. Another group of very large cells furnishes 
the axons that constitute the rubrospinal tract. This collection of large cells is phylogenetic- 
ally the older and forms the chief part of the red nucleus in the lower mammals. But in 
man, where the two parts are rather sharply differentiated, the chief mass is composed of 
the smaller cells. 

The red nucleus may be regarded as an especially highly developed portion of the motor 
nuclei of the tegmentum. In the lower mammals it serves as a center through which the 
cerebellum can influence the motor functions of the spinal cord and medulla oblongata. 
In man it has the same function, but is also more closely linked with the reticular formation 
of the pons by way of the rubroreticular tract. It is a significant fact that in man where 
the rubrospinal tract is relatively small the rubroreticular tract is especially well developed. 
This suggests the possibility that impulses from the red nucleus may be relayed through the 
reticular nuclei of the pons to the spinal cord (Fig. 115). 

The Tegmental Decussations. At the level of the superior colliculus and 
between the two red nuclei the median raphe presents an unusual number of 
crossing fibers (Fig. 116). Among these are included the dorsal tegmental de- 
cussation (fountain decussation of Meynert) and the ventral tegmental decussa- 
tion (fountain decussation of Forel). The latter is composed of fibers from the 
red nucleus, which, after crossing the median plane, descend through the brain 
stem into the lateral funiculus of the spinal cord as the rubrospinal tract (Fig. 
115). The dorsal tegmental decussation is composed of fibers which arise in the 
superior colliculi of the corpora quadrigemina, sweep in broad curves around the 
central gray stratum, and after crossing the median plane in the dorsal part of 
the raphe, go to form the tectobulbar and tectospinal tracts. 



162 



THE NERVOUS SYSTEM 



The median longitudinal fasciculus is more conspicuous in the mesencephalon 
than in other parts of the brain stem, but it occupies the same relative position, 
that is, near the median plane close to the central gray matter. At the level of 
the superior colliculus it forms a rather broad obliquely placed lamina, extending 
dorsolaterally from the median raphe, which together with the corresponding 
lamina of the opposite side produces in transverse sections a V-shaped figure 
(Fig. 116). The apex of this V is directed ventrally; and included between its 
two limbs are the oculomotor nuclei. At the level of the inferior colliculi the 



Stratum zonale 
Stratum griseum 
Stratum opticum 

Stratum Iemnisci'~r7 l 
Stratum profundum-^ 
Aqueduct of cere 

brum 
Medial lemnis- 



Superior colliculus 

Nucleus of oculomotor nerve 
Medial longitudinal fasciculus 
Thalamo-olivary tract 

Inf. quadrigeminal brack. 
Med. gen. body 




Dorsal tegmental decussation 

Ventral tegmental decussation 



Red nucleus 
Oculomotor nerve 



gf Basis pedunciili 
Substantia nigra 



Fig. 116. Section through the mesencephalon of a child at the level of the superior colliculus. 

Pal-Weigert method. (X 4.) 

medial longitudinal fasciculus lies immediately ventral to the nucleus of the 
trochlear nerve (Fig. 114). In the pons the nucleus of the abducens nerve is 
placed on its dorsolateral border. The close relation of this fascicle to the nuclei 
for the motor nerves of the eye is of considerable significance, since according 
to the law of neurobiotaxis (p. 179) it is an expression of the fact that the majority 
of the afferent fibers to these nuclei come from this fascicle. This bundle of 
fibers, the composition of which is discussed on pages 152 and 329, is a chief 
factor in the reflex control of the movements of the eyes, and especially in the 
coordination of these movements with those of the head and neck. 



THE INTERNAL STRUCTURE OF THE MESENCEPHALON 163 

The Lemnisci. In sections through the rostral border of the pons the two 
lemnisci form a broad curved band in the ventral and lateral portions of the 
tegmentum. The fibers of the lateral lemniscus are cut obliquely, indicating 
that they have begun to turn dorsally toward the inferior colliculus (Fig. 112). 
On entering the midbrain this lateral portion of the fillet separates from the 
medial lemniscus and runs toward the corpora quadrigemina, where it forms a 
capsule for the nucleus of the inferior colliculus (Fig. 114). Some of these 
fibers are prolonged beyond the nucleus and decussate with similar fibers from 
the opposite side. A large proportion of the fibers of the lateral lemniscus end 
in the inferior colliculus, but others form the inferior quadrigeminal brachium 
(Fig. 114), through which they reach the medial geniculate body (Figs. 116, 134). 
In the mesencephalon the lateral lemniscus, which, it will be remembered, is the 
central auditory tract from the cochear nuclei, is joined by the fibers of the 
spinotectal tract; and these run with it to the corpora quadrigemina. 

The medial lemniscus, or bulbothalamic tract from the gracile and cuneate 
nuclei of the opposite side, is continued through the tegmentum of the mesen- 
cephalon to end in the lateral nucleus of the thalamus (Fig. 235) . Incorporated 
with it in this upper part of its course are the fibers of the spinothalamic tract 
and a portion of the central sensory tract of the trigeminal nerve (Figs. 132, 234). 
In the caudal part of the mesencephalon this broad band of longitudinal fibers 
occupies the ventrolateral portion of the tegmentum (Fig. 114); but at the level 
of the superior colliculus it has been displaced dorsolaterally by the red nucleus. 
Here it lies not far from the medial geniculate body and inferior quadrigeminal 
brachium (Fig. 116). 

The Central Gray Stratum. The cerebral aqueduct is lined by ependymal 
epithelium and surrounded by a thick layer of gray matter, the central gray 
stratum, which, because of its paucity in myelinated fibers, is nearly colorless in 
Weigert preparations. This layer is continuous with the gray matter surround- 
ing the third ventricle, on the one hand, and with that covering the rhomboid 
fossa on the other. Numerous nerve-cells of various size and shape are scat- 
tered through this central gray substance; and, in addition, there are three 
compact groups of cells, which are the nuclei of the oculomotor and trochlear 
nerves and of the mesencephalic root of the trigeminus. 

The nucleus of the trochlear nerve contains the cells of origin of the motor 
fibers for the superior oblique muscle of the eye. It is a small oval mass situated 
in the ventral part of the central gray stratum at the level of the inferiof collic- 
ulus (Fig. 1 14) . The fibers of the trochlear nerve emerge from the dorsolateral 



164 THE NERVOUS SYSTEM 

aspect of this nucleus, curve dorsally around the central gray matter, and decus- 
sate in the anterior medullary velum (Fig. 112). 

The nucleus of the oculomotor nerve is composed of the cells of origin of 
the motor fibers for all of the ocular muscles except the superior oblique and 
lateral rectus. It lies in the ventral part of the central gray substance beneath 
the superior colliculus (Fig. 116). This nucleus, a part of which occupies a 
median position and supplies fibers to the nerves of both sides, is 6 or 7 mm. 
long and extends from a little beyond the rostral limit of the mesencephalon to 
the nucleus of the trochlear nerve, from which it is not sharply separated. From 
the nucleus the fibers of the oculomotor nerve stream forward through the 
tegmentum and red nucleus. They emerge through the oculomotor sulcus along 
the ventromedial surface of the basis pedunculi. 

The interpeduncular ganglion is a median collection of nerve-cells in the 
posterior perforated substance situated between the two cerebral peduncles near 
the border of the pons (Fig. 1 14) . It receives fibers from the habenular nucleus 
of the epithalamus by way of the fasciculus retroflexus of Meynert; and from 
it spring fibers that run to the dorsal nucleus of the tegmentum (Fig. 211). 

The substantia nigra is a broad thick plate of pigmented gray matter, which 
separates the basis pedunculi from the tegmentum and extends from the border 
of the pons throughout the length of the mesencephalon into the hypothalamus. 
In transverse section it presents a semilunar outline. Its medial border is super- 
ficial in the oculomotor sulcus and is thicker than the lateral border, which 
reaches the lateral sulcus of the mesencephalon. Its constituent nerve-cells, 
irregular in shape and deeply pigmented, send their axons into the tegmentum. 
But we are still ignorant as to the destination these may have; and the func- 
tion of the substantia nigra is equally obscure. It receives collaterals from the 
corticifugal fibers of the basis pedunculi. Furthermore, there terminates within 
it a bundle, consisting of both direct and crossed fibers from the corpus striatum, 
the strionigral tract (Fig. 117). 

The basis pedunculi is a broad compact strand, crescentic in transverse sec- 
tion, which consists of longitudinal fibers of cortical origin. These are con- 
tinued from the internal capsule into the longitudinal bundles of the pons 
through the basis pedunculi. It consists of four tracts. The medial and lat- 
eral fifths are occupied by fibers which terminate in the nuclei pontis. Those 
of the medial one-fifth arise from the cortex of the frontal lobe of the cerebral 
hemisphere and constitute the frontopontine tract. Other fibers, arising from 
the temporal lobe, form the temporopontine tract and occupy the lateral one- 



THE INTERNAL STRUCTURE OF THE MESENCEPHALON 



fifth of the basis pedunculi. The intermediate portion, approximately three- 
fifths, is formed by the corticospinal tract, the fibers of which after giving off 
collaterals to the nuclei pontis are continued into the pyramids of the medulla 
oblongata and thence into the spinal cord. Many of the fibers of the cortico- 
bulbar tract are intermingled with the more medially placed corticospinal fibers; 
but even at this level two large fascicles destined for the nuclei of the cranial 
nerves have separated from the main strand of motor fibers (Dejerine, 1914). 
These have been called the medial and lateral corticobulbar tracts (Figs. 106, 
117). 

The Corpora Quadrigemina. The rostral portion of the midbrain roof or 
tectum mesencephali is in all vertebrates an end-station for the optic tracts. In 
the lower vertebrates there are but two elevations in the roof, the optic lobes or 
corpora bigemina, and these, which correspond in a general way to the superior 




Temporopontine tract 
Tr. corticobulbaris lot. 

Strionigral tract 
Corticospinal tract 



Frontopontine tract Tr. corlicobulbaris med. 

Fig. 117. Diagram of the basis pedunculi. 

colliculi, are visual centers (Fig. 13). In mammals the development of a spir- 
ally wound cochlea is associated with the appearance of two additional eleva- 
tions, the inferior colliculi, within which many of the fibers of the central audi- 
tory path terminate. The entire tectum receives fibers from the spinal cord 
and medulla oblongata and sends other fibers back to them ; it also receives fibers 
from the cerebral cortex. It contains important reflex centers, those in the 
superior colliculus being dominated by visual, those in the inferior colliculus 
by auditory, impulses. 

The inferior colliculi or inferior quadrigeminal bodies each contain, in addi- 
tion to the laminated gray matter of the tectum, a large gray mass, oval in 
transverse section, and known as the nucleus of the inferior colliculus (Fig. 114). 



1 66 



THE NERVOUS SYSTEM 



The lateral lemniscus has been traced to this nucleus, and while some of the 
fibers plunge directly into it, others sweep around it to form a capsule, within 
which it is enclosed. The majority of these fibers ultimately end in this nu- 
cleus, but some pass beyond it, reach the median plane, and decussate with sim- 
ilar fibers from the opposite side (Fig. 118). The ramifications of fibers from the 
lateral lemniscus form an intricate interlacement within the nucleus, and 
throughout this network are scattered many nerve-cells of various shapes and 




Fig. 118. Semidiagrammatic section through the inferior colliculus of the mouse: A, Nucleus 
of inferior colliculus; B, gray matter of the lamina quadrigemina; C, inferior quadrigeminal bra- 
chium; D, central gray substance; K, decussation of the brachium conjunctivum; a, b, c, d, fibers 
of the lateral lemnisus. Golgi method. (Cajal.) 

sizes. On the medial side of this circumscribed nuclear mass we find some of 
the laminated gray matter of the tectum, within which are embedded large mul- 
tipolar cells with axons directed ventrally in the stratum profundum. These 
partially encircle the central gray matter and after undergoing a partial decus- 
sation enter the tectobulbar and tectospinal tracts. 

The inferior quadrigeminal brachium begins on the lateral side of the nucleus 
of the inferior colliculus and consists of fibers from the lateral lemniscus which 



THE INTERNAL STRUCTURE OF THE MESENCEPHALON 167 

run to and terminate within the medial geniculate body (Figs. 114, 116). The 
fibers of the lateral lemniscus carry auditory impulses from the terminal nuclei 
of the cochlear nerve. Some of these terminate in the inferior colliculus and 
are concerned with reflexes in response to sound. Other fibers, some of which 
are branches of those to the inferior colliculus, run to the medial geniculate 
body, from which the impulses that they carry are relayed to the cerebral cor- 
tex. The inferior quadrigeminal brachium also contains fibers of cortical origin, 
chiefly from the temporal lobe, which end within the inferior colliculus (Beevor 
and Horsley, 1902). 

The superior colliculi, or superior quadrigeminal bodies, are composed of 
laminated gray matter. Each consists of four superimposed, dor sally convex 
layers (Fig. 116). The most superficial of these is a thin lamina with many 
transversely coursing nerve-fibers, the stratum zonale. The second layer is much 
thicker, contains few myelinated fibers, and is known as the stratum griseum. 
The third and fourth layers, stratum opticum and stratum lemnisci, are rich in 
myelinated fibers. The majority of the afferent fibers of the superior colliculus 
come from the optic tract by way of the superior quadrigeminal brachium and 
enter the stratum opticum. Many of these end in the superimposed stratum 
griseum. The superior colliculus also receives fibers from the cerebral cortex 
and from the spinotectal tract. 

It has been generally supposed that the fibers of the stratum zonale come from the 
optic tract, but according to Cajal (1911) this cannot be the case, since they remain intact 
in animals which have been operated on in such a way as to produce degeneration of the optic 
fibers. According to him it is also probable that the fibers from the cerebral cortex, which 
reach the colliculus by way of the superior quadrigeminal brachium, end in the stratum 
lemnisci. The fibers of the spinotectal tract run with the lateral lemniscus in the upper part 
of its course and enter the superior colliculus by way of the stratum profundum. 

The tectobulbar and tectospinal tracts have their origin within the tectum of 
the mesencephalon, more of the fibers coming from the superior than from the 
inferior colliculi. These fibers, arising from cells in more superficial layers, are 
assembled in the stratum profundum and sweep ventrally in broad curves around 
the central gray substance (Figs. 116, 118). The majority of the fibers, after 
crossing the median plane in the dorsal tegmental decussation, run in a caudal 
direction just ventral to the medial longitudinal bundle in the tectospinal tract. 
They give off collaterals to the reticular formation and the red nucleus. But 
some of them, instead of taking part in this decussation, leave the mesencephalon 
by way of the lateral lemniscus of the same side, constituting the lateral tecto- 
bulbar and tectospinal tracts (Cajal, 1911; Edinger, 1911). 



CHAPTER XII 

THE CRANIAL NERVES AND THEIR NUCLEI 

THE cranial nerves contain, in addition to the general somatic and visceral 
components, which were encountered in the study of the spinal nerves, also 
other functional groups of fibers of more restricted distribution and specialized 
function. These special somatic and visceral components supply the organs of 
special sense and the visceral musculature, derived from the branchial arches, 
which differs from other visceral musculature in that it is striated. The fibers 
which supply this special musculature are designated as special visceral efferent 
fibers. The eye and ear, being special somatic sense organs, are supplied by 
special somatic afferent fibers. The olfactory mucous membrane and the taste 
buds are special visceral sense organs and are supplied by special visceral af- 
ferent fibers. 

From what has been said it will be evident that there are seven distinct 
functional components in the cranial nerves, namely: somatic efferent, general 
somatic afferent, special somatic afferent, general visceral efferent, special vis- 
ceral efferent, general visceral afferent, and special visceral afferent components 
(Figs. 119, 120). No single nerve contains all seven types of fibers and the 
individual cranial nerves vary greatly in their functional composition. On 
entering the brain a nerve breaks up into its several components, which separate 
from each other and pass to their respective nuclei, enumerated below. These 
nuclei may be widely separated in the brain stem. Fibers having the same func- 
tion tend to be associated together within the brain irrespective of the nerves 
to which they belong. For example, all the visceral afferent fibers of the facial, 
glossopharyngeal, and vagus nerves are grouped in the tractus solitarius (Fig. 
120, yellow). The nerve-cells, with which the fibers of the several functional 
varieties are associated within the brain stem, are arranged in longitudinal 
nuclear columns. The analysis of the cranial nerves into their functional com- 
ponents has involved a great amount of labor which has been carried through 
for the most part by American investigators. Among those who have made 
important contributions to this subject may be mentioned the following: Gas- 
kell (1886), Strong (1895), Herrick (1899), Johnston (1901), Coghill (1902), 
Norris (1908), and Willard (1915). 

1 68 



Special somatic afferent _ 

nucleus 
General somatic afferent 

nucleus 
Alar lamina' 
Visceral afferent nucleus- 
General visceral efferent__ 

nucleus 
Special visceral efferent 

nucleus 
Basal lamina" 

Somatic efferent nucleus *~ 




Somatic muscle 

Sympathetic ganglion 

Visceral mucous membrane 

Smooth muscle 



XN Sensory ganglion 
Branchial muscle 



Fig. 119. 



Sensor v nucleus N. V* 

V 

Nucleus of abducens nerve\ \ 

Facial nerve, \ 
Vestibular nuclei \ \ 

\ ' 



Vestibular ganglion 
and nerve 




DC 

Bulbar rootlet of accessory nerve 

Spinal root of accessory nerve'' 

Nucleus ambiguus-' 

Tractus solitarius 



Nucleus of Edinger-Westphal 
Nucleus of oculomotor nerve 

Nucleus of trochlear nerve 
M esencephalic nucleus N. V 

Trigeminal nerve 
and semilunar 
ganglion 

r . Spinal tract and 
nucleus N. V 

. Cochlear nuclei 



Spiral ganglion 
and cochlear 



.. Glossopharyn- 

geal nerre 
Vagus nerve 

. salivatorius superior 
'. salivatorius inferior 
Dorsal motor nucleus N. X 



Cervical spinal nerve 



Fig. 120. 

Figs. 119 and 120. Diagrams showing the origin, course, and termination of the functional 
components of the cranial nerves. Somatic afferent and efferent, red; visceral afferent, yellow; 
general visceral efferent, black; special visceral efferent, blue. Fig. 119 shows the locations of the 
several functional cell columns in a section through the medulla oblongata of a human embryo and 
the peripheral terminations of the several varieties of fibers. Fig. 120, dorsal view of the human 
brain stem, showing the location of the nuclei and the intramedullary course of the fibers of the 
cranial nerves. 169 



THE NERVOUS SYSTEM 

Longitudinal Nuclear Columns. In a previous chapter we learned that at 
an early stage in its development the lateral wall of the neural tube consists of a 
dorsal or alar and a ventral or basal plate, separated by a groove, the sulcus 
limitans (Fig. 119). The sensory nuclei of the cranial nerves develop within the 
alar plate and the motor nuclei within the basal plate. In the rhombencephalon 
both plates come to lie in the floor of the fourth ventricle, the alar occupying 
the more lateral position. And, in spite of the changes of position which occur 
during development, the sensory nuclei retain, on the whole, a lateral, and the 
motor nuclei a more medial, location. From the basal plate there differentiate 
a somatic and a visceral column of efferent nuclei, and from the alar plate a 
visceral and a somatic column of afferent nuclei. 

The somatic e/erent column includes the nuclei of those motor nerves which 
supply the striated musculature derived from the myo tomes, i.e., the extrinsic 
muscles of the eye and the musculature of the tongue (Figs. 119-121). 

The visceral efferent column undergoes subdivision into: (1) a ventrolateral 
column of nuclei, from which arise the special visceral efferent fibers to the striated 
visceral or branchial musculature, and which includes the nucleus ambiguus and 
the motor nuclei of the fifth and seventh nerves; and (2) a more dorsally placed 
group for the innervation of involuntary musculature and glandular tissue, of 
which the dorsal motor nucleus of the vagus is the chief example. The former 
may be called the special visceral efferent and the latter the general visceral ef- 
ferent column. 

The visceral afferent column is represented by the nucleus of the tractus 
solitarius, within which end the afferent fibers from the visceral mucous membrane 
and the taste buds, i. e., both the general and special visceral afferent fibers. 
The somatic afferent column splits into two: a general somatic afferent column, 
within which terminate the sensory fibers from the skin; and a special somatic 
group of nuclei for the reception of the fibers of the acoustic nerve and, in aquatic 
vertebrates, of the lateral line nerves also. 

THE SOMATIC EFFERENT COLUMN 

As can be seen by reference to Figs. 101, 108, 114, and 116 the nuclei of the 
hypoglossal, abducens, trochlear, and oculomotor nerves are arranged in linear 
order in the central gray matter near the median plane. They represent the 
continuation into the medulla oblongata of the large cells of the anterior column 
of the spinal cord. The cells of these nuclei are large and multipolar with 
well-developed Nissl bodies (Fig. 126). From them arise large myelinated 



THE CRANIAL NERVES AND THEIR NUCLEI 171 

fibers, which innervate the striated musculature derived from the myotomes. 
This group of nuclei is indicated in red in Fig. 120 and by small circles in Figs. 
121 and 122. 

The nucleus of the oculomotor nerve is an elongated mass of cells in the cen- 
tral gray matter ventral to the cerebral aqueduct at the level of the superior 
colliculus (Figs. 121, 122). Even a superficial examination shows that it is 
divided into a lateral paired and a medial unpaired portion (Fig. 116). The 



Nuc. Ill E-W. 
Nuc. Ill lat. 

Nuc. Ill med. 
Nuc. IV 



Velum medul- 
lare superiua 



Nuc. mot. V 

Nuc. VI 

Nuc. mot. VII 

Nuc. sal. sup. 

Nuc. sal. inf. 

Ala cinerea 

Nuc. dorsal. X 

Nuc. ambiguua 

Nuc. XII 



Colliculus sup. 
Corp. genicula- 
tum mediale 




Nuc. com. 
Cajal 



Fig. 121. Dorsal view of the human brain stem with the positions of the cranial nerve nuclei 
projected upon the surface. Sensory nuclei on the right side, motor nuclei on the left. Circles 
indicate somatic efferent nuclei; small dots, general visceral efferent nuclei; large dots, special 
visceral efferent nuclei; horizontal lines, general somatic sensory nuclei; cross-hatching, visceral 
sensory nuclei; stipple, special somatic sensory nuclei. (Herrick.) 

lateral groups of cells spreads out upon the surface of the medial longitudinal 
bundle, extends throughout the entire length of the nucleus, and may be divided 
into ventral and dorsal portions (Fig. 123). The medial group of cells is placed 
exactly in the median plane and is found only in the rostral half of the nucleus. 
Dorsolateral from this median group, and restricted to the most rostral part of 
the nucleus, is a collection of small cells which form the nucleus of Edinger- 
Westphal. This is a visceromotor nucleus and will be considered elsewhere. 



172 



THE NERVOUS SYSTEM 



The fibers from the medial nucleus enter both right and left nerves. Some 
from the caudal portion of the dorsal division of the lateral nucleus cross the 
median plane. The others remain uncrossed. After sweeping in broad curves 
through the tegmentum and red nucleus the fibers emerge through the oculo- 
motor sulcus. All of the extrinsic muscles of the eye except the lateral rectus 
and superior oblique are supplied by the medial and lateral groups of cells just 

described. 

t , Nucleus of Edinger-Westphal 

,. Nucleus of oculomotor nerve 

--Corpora quadrigemina 

Cerebral aqueduct 
-Nucleus of trochlear nerve 
Trochlear nerve 
'Anterior medullary velum 

'Motor nucleus N. V 

. - 'Nticleus of facial nerve 
, - Fourth ventricle 
Nucleus of abducens nerve 



Mesencephalon 
Oculomotor nerve 

Pans 
Portia minor N. V- 



Facial nerve 



Abducens nerve- - ' 



Medulla oblongata- 




- Nuc. salivatorius superior 

- Nuc. salivatorius inferior 

Nucleus of hypoglossal nerve 

- - Dorsal motor nucleus N. X 
.--Central canal 

Nucleus ambiguus 



Fig. 122. Motor nuclei of the cranial nerves projected on a median sagittal section of the 
human brain stem. Circles indicate somatic efferent nuclei; small dots, general visceral efferent 
nuclei; large dots, special visceral efferent nuclei. 

As one might expect from the fact that the oculomotor nerve supplies several distinct 
muscles, its nucleus seems to be made up of a number of more or less distinct groups of cells; 
but the efforts to locate subordinate nuclei have given rise to contradictory results. The 
most significant work in this field has been done by Bernheimer (1904), who extirpated in- 
dividual eye muscles in monkeys and studied the resultant changes in the cells of the oculo- 
motor nuclei. According to him, the various muscles are supplied by the lateral nucleus in 
the following order, beginning at the rostral end: levator palpebrae superioris, rectus supe- 
rior, rectus medialis, obliquus inferior, and rectus inferior. Bernheimer says that the fibers 
for the rectus inferior are entirely crossed, those for the obliquus inferior are in greater part 
crossed, those for the rectus medialis for the most part uncrossed, those for the rectus superior 
and levator palpebrse superioris entirely uncrossed. 



THE CRANIAL NERVES AND THEIR NUCLEI 



173 



V 



The nucleus of the trochlear nerve has already been located in the central 
gray matter ventral to the cerebral aqueduct at the level of the inferior collic- 
ulus, close to the caudal extremity of the oculomotor nucleus (Figs. 114, 121, 
122). The fibers of the trochlear nerve emerge from the dorsal and lateral 
aspects of this nucleus, and, encircling the central gray matter along an angular 
course which carries them also caudally, enter the anterior medullary velum, 
decussate within it, and make their exit from its dorsal surface (Fig. 112). They 
supply the superior oblique muscle. 

The nucleus of the abducens nerve was encountered in the dorsal portion 
of the pons as a spheric gray mass, which with the genu of the facial nerve forms 
the facial colliculus of the rhomboid fossa (Figs. 
108, 121, 122). The fibers of the abducens nerve 
leave the nucleus chiefly on its dorsal and medial 
surfaces and become assembled into several root 
bundles, which are directed ventrally toward their 
exit from the lower border of the pons near the 
pyramid of the medulla oblongata. It supplies 
the lateral rectus muscle. 

The axons, which ramify within the three nuclei 
for the motor nerves of the eye, are derived from 
many sources. The most important of these 
sources are the corticobulbar tract, the medial lon- 
gitudinal bundle, and the tectospinal tract. The 
nucleus of the abducens receives fibers also from 
the central auditory apparatus through the pe- 
duncle of the superior olive. These various fibers 
provide for voluntary movements of the eyes, and 

for reflex ocular movements in response to vestibular, visual, and auditory 
impulses. The nuclei probably also receive branches from the central sensory 
path of the fifth nerve. 

The nucleus of the hypoglossal nerve is a slender cylindric mass of gray 
matter nearly 2 cm. in length, extending from the level of the fovea inferior to 
that of the decussation of the pyramids. We have already identified it in both 
the open and the closed portions of the medulla oblongata (Figs. 99, 103). In the 
floor of the fourth ventricle it lies beneath the trigonum hypoglossi, while more 
caudally it lies ventral to the central canal (Figs. 121, 122). The root fibers 




Fig. 123. Diagram of the 
nuclei of the oculomotor nerve: 
M, Median nucleus; E.W., nu- 
cleus of Edinger-Westphal; V.L., 
D.L., ventral and dorsal portions 
of the lateral nucleus. (Ober- 
steiner.) 



174 THE NERVOUS SYSTEM 

are assembled into bundles which run ventrally toward their exit along the 
lateral border of the pyramid. 

A conspicuous plexus of myelinated fibers gives the hypoglossal nucleus a 
characteristic appearance in Weigert preparations. Fibers from many sources 
reach the nucleus and ramify within it. These include some from the cortico- 
bulbar tract and others from the sensory nuclei of the fifth nerve and from the 
nucleus of the tractus solitarius. The part which such fibers may play in reflex 
movements of the tongue is illustrated in Fig. 92. 

THE SPECIAL VISCERAL EFFERENT COLUMN 

The special visceral efferent column of nuclei contains the cells of origin of 
the motor fibers for the striated musculature derived from the branchial arches, 
as distinguished from the general skeletal musculature that develops from 
the myotomes. The branchial musculature includes the following groups of 
muscles: the muscles of mastication, derived from the mesoderm of the first 
branchial arch and innervated by the trigeminal nerve; the muscles of expression, 
derived from the second or hyoid arch and innervated by the facial nerve; the 
musculature of the pharnyx and larynx, derived from the third and fourth arches 
and innervated by the glossopharyngeal, vagus, and accessory nerves; and prob- 
ably also the sternocleidomastoid and trapezius muscles, innervated through the 
spinal root of the accessory nerve. Some authors prefer to call this column, 
which includes the motor nuclei of the fifth and seventh nerves and the nucleus 
ambiguus, the lateral somatic column, because the cells in these nuclei and the 
fibers which arise from them possess the characteristics of somatic motor cells 
and fibers (Malone, 1913). The nuclei are composed of large multipolar cells 
with well-developed Nissl bodies. These cells give origin to large myelinated 
fibers which run through the corresponding nerve and terminate in neuromus- 
cular endings in one or another of the muscles indicated above. 

The motor nuclei of the fifth and seventh nerves and the nucleus ambiguus 
of the ninth, tenth, and eleventh nerves form a broken column of gray matter, 
located in the ventrolateral part of the reticular formation of the pons and 
medulla oblongata some distance beneath the floor of the fourth ventricle (Figs. 
121, 122). The cells of this column and the special visceral efferent fibers which 
arise from them have been colored blue in Figs. 119 and 120. 

The motor nucleus of the trigeminal nerve lies on the medial side of the 
main sensory nucleus of that nerve, and is located at the level of the middle 
of the pons in the lateral part of the reticular formation some distance from the 



THE CRANIAL NERVES AND THEIR NUCLEI 



175 



ventricular floor (Figs. 110, 121, 122). The fibers, which take their origin here, 
are collected in the motor root or portio minor of the fifth nerve and run with its 
mandibular division to the muscles of mastication. Within the nucleus there 
terminate fibers from the corticobulbar tract and many fibers, chiefly collaterals, 
from the central sensory tract of the trigeminal nerve. It also receives collat- 
erals from the mesencephalic root of the trigeminal and from other sources 
(Fig. 131). 

The motor nucleus of the facial nerve is located in the ventrolateral part 
of the reticular formation of the pons near its caudal border (Figs. 108, 121, 
122). Its constituent cells are arranged so as to form a varying number of sub- 
groups which may possibly be concerned with the innervation of individual facial 
muscles. 



Root of facial nerve, first part 
Abducens nucleus 



Root of facial nerve, genu 



Root of facial nerve, second part 
Facial nucleus 




Nucleus of abducens nerve 

Root filaments of abducens nerve 
Stalk of superior olive 

Root of facial nerve, first part 
Spinal root and nucleus N. V 
Nucleus of facial nerve 
Root of facial n., sec. 
Superior olive [part 



Abducens nerve 



Fig. 124. Diagram of the root of the facial nerve, shown as if exposed by dissection in a thick 

section of the pons. 

From the dorsal aspect of this nucleus there emerge a large number of fine 
bundles of fibers, directed dorsomedially through the reticular formation. These 
rather widely separated bundles constitute the first part of the root of the facial 
nerve (Fig. 124). Beneath the floor of the fourth ventricle the fibers turn sharply 
rostrad and are assembled into a compact strand of longitudinal fibers, often 
called the ascending part of the facial nerve. This ascends along the medial side 
of the abducens nucleus dorsal to the medial longitudinal bundle for a consid- 
erable distance (5 mm.). The nerve then turns sharply lateralward over the 
dorsal surface of the nucleus of the abducens nerve, and helps to form the eleva- 
tion in the rhomboid fossa, known as the facial colliculus. This bend around 
the abducens nucleus, including the ascending part of the facial nerve, is known 



176 



THE NERVOUS SYSTEM 



as the genu. The second part of the root of the facial nerve is directed ventro- 
laterally and at the same time somewhat caudally, passing close to the lateral 
side of its own nucleus, to make its exit from the lateral part of the caudal 
border of the pons (Fig. 108). 

Fibers from many sources terminate in the facial nucleus in synaptic rela- 
tion with its constituent cells. Those from the corticobulbar tract place the 
facial muscles under voluntary control. Others are collaterals from the sec- 
ondary sensory paths in the reticular formation and are concerned with bulbar 
reflexes. Some of these collaterals are given off by fibers arising in the trapezoid 
body and carry auditory impulses. Others are collaterals of fibers arising in 
the nucleus of the spinal tract of the fifth nerve; and still others are given off by 
ascending sensory fibers from the spinal cord (Cajal, 1909). 



Bulbar rootlets of accessory nerve ^-^ 
Foramen magnum-\. 

Spinal root of accessory nerve 




-. Vagus nerve 

-'Jugular foramen 

Internal ramus \ * 
77 , ; } Accessory nerve 

External ramus / * 



1 

Fig. 125. Diagram of the roots of the vagus and accessory nerves. 



The nucleus ambiguus is a long slender column of nerve-cells, extending 
through the greater part of the length of the medulla oblongata in the ventro- 
lateral part of the reticular formation (Figs. 103, 121, 122). Its constituent 
cells give rise to the special visceral efferent fibers that run through the glosso- 
pharyngeal, vagus, and accessory nerves to supply the musculature of the 
pharynx and larynx. It reaches from the border of the pons to the motor de- 
cussation, but is most evident in transverse sections through the caudal part of 
the rhomboid fossa. Here it can be found in the reticular formation ventral to 
the nucleus of the spinal root of the trigeminal nerve. The fibers arising from 
its cells are at first directed dorsally; then curving laterally and ventrally they 
join the root bundles of the ninth, tenth, and eleventh nerves with which they 



THE CRANIAL NERVES AND THEIR NUCLEI 177 

emerge from the brain (Fig. 105). A few of the fibers cross the median plane 
and join the corresponding root bundles of the opposite side. 

The accessory nerve consists of a bulbar and a spinal portion. The fibers of the spinal 
root take origin from a linear group of cells in the lateral part of the anterior gray column 
in the upper cervical segments of the spinal cord. This root ascends along the side of the 
spinal cord, passes through the foramen magnum, and is joined by the bulbar rootlets of the 
accessory (Fig. 125). The nerve then divides into an internal and an external branch. In 
the latter run all the fibers of spinal origin and these are distributed to the trapezius and 
sternocleidomastoid muscles. If, as seems probable, these muscles are derived from the 
branchial arches (Lewis, 1910), the fibers which supply them may be regarded as special 
visceral efferent fibers; and the spinal nucleus of the accessory nerve may be considered as 
homologous to the nucleus ambiguus. The bulbar rootlets of the accessory nerve, which con- 
tain both general and special visceral efferent fibers, form a well-defined fascicle, readily 
distinguished from the spinal portion of the nerve, which, as the internal ramus, joins (he 
vagus nerve and is distributed through its branches (Fig. 120 Chase and Ranson, 1914). 

The sensory collaterals which arborize among the cells of the nucleus am- 
biguus are derived from the central tracts of the trigeminal, glossopharyngeal, 
and vagus nerves, from ascending sensory fibers of spinal origin, and from other 
longitudinal fibers in the reticular formation. Other fibers reach this nucleus 
from the corticobulbar tract. 

THE GENERAL VISCERAL EFFERENT COLUMN 

The general visceral efferent column of nuclei is composed of the cells from 
which arise the efferent fibers innervating cardiac and smooth muscle and glan- 



- ' O^S^A * iaS 
JP 





A B 

Fig. 126. Two types of motor nerve-cells from medulla oblongata of lemur: A, Cells of the 
somatic motor type from the hypoglossal nucleus; B, cells of the visceral efferent type from the 
rostral part of the dorsal motor nucleus of the vagus. Toluidin blue stain. (Malone.) 

dular tissue. The cells of these nuclei are of small or medium size and their 
Nissl bodies are not well developed (Fig. 126). They give rise to the general 



178 THE NERVOUS SYSTEM 

visceral efferent fibers of the cranial nerves. These are small myelinated fibers, 
which end in sympathetic ganglia, where they arborize about sympathetic 
cells, the axons of which terminate in smooth or cardiac muscle or in glandular 
tissue. The neurons of this series are, therefore, characterized by the fact that 
the impulses which they transmit must be relayed by neurons of a second order 
before reaching the innervated tissue (Fig. 119). This group of nuclei is indi- 
cated by black in Fig. 120 and by fine stipple in Figs. 121 and 122. 

The dorsal motor nucleus of the vagus (nucleus vagi dorsalis medialis) has 
been noted in the transverse sections through the medulla oblongata (Figs. 99, 
103). It lies along the dorsolateral side of the hypoglossal nucleus, subjacent 
to the ala cinerea of the rhomboid fossa, and along the side of the central canal 
in the closed part of the medulla oblongata. The general visceral efferent fibers, 
which arise from the cells in this nucleus, leave the medulla oblongata through 
the roots of the vagus and accessory nerves; but those entering the accessory 
nerve leave that nerve by its internal ramus and join the vagus (Fig. 120). 
Hence all of the fibers from this nucleus are distributed through the branches of 
the vagus to the vagal sympathetic plexuses of the thorax and abdomen for the 
innervation of the involuntary musculature of the heart, respiratory passages, 
esophagus, stomach, and small intestines (Van Gehuchten and Molhant, 1912), 
and for the innervation of the pancreas, liver, and other glands. 

There are relatively few sensory collaterals reaching the dorsal motor nucleus, 
and these come in large part from sensory fibers of the second order, arising in 
the receptive nuclei of the trigeminal, glossopharyngeal, and vagus nerves. 

The nucleus salivatorius is located in the reticular formation, some distance 
from the floor of the fourth ventricle at the junction of the pons and medulla 
oblongata near the caudal end of the facial nucleus and the rostral end of the 
nucleus ambiguus (Figs. 121, 122). The more caudal portion, or nucleus sal- 
ivatorius inferior, sends general visceral efferent fibers by way of the glosso- 
pharyngeal nerve to the otic ganglion for the innervation of the parotid gland. 
The rostral part, or nucleus salivatorius superior, lies dorsal to the large motor 
nucleus of the facial nerve, to which nerve it sends general visceral efferent 
fibers. These run from the facial nerve through the chorda tympani to the sub- 
maxillary ganglion for the innervation of the submaxillary and sublingual sal- 
ivary glands (Kohnstamm, 1902, 1903, 1907; Yagita, 1909; Feiling, 1913). 

The Edinger-Westphal nucleus is a group of small nerve-cells located in 
the rostral part of the nucleus of the oculomotor nerve. Here it is placed 
dorsolateral to the median unpaired portion of that nucleus (Figs. 121-123). 



THE CRANIAL NERVES AND THEIR NUCLEI 1 70 

This group of small cells gives origin to the general visceral efferent fibers of the 
oculomotor nerve which run to the ciliary ganglion for the innervation of the 
intrinsic muscle of the eye. 

Neurobiotaxis. The position of the motor nuclei of the brain stem varies greatly in 
different orders of vertebrates, and is determined by the source of the principal afferent 
impulses which reach them. The perikarya of the neurons migrate under the influence of an 
attraction, which has been called neurobiotaxis, in the direction of the chief fiber tracts 
from which they receive impulses (Ariens Kappers, 1914, 1917; Black, 1917). "When from 
different places stimuli proceed to a cell, its chief dendrite grows out and its cell body shifts in 
the direction whence the majority of the stimuli proceed," while the axon grows in the op- 
posite direction (Fig. 127). The nature of the attractive force is not altogether clear. Kap- 





Cell 

Axiscyh 



B 

Fig. 127. Diagram to illustrate the principle of neurobiotaxis. The axis-cylinder grows in 
the direction of the nervous current, indicated by the arrow, while the dendritic outgrowth and 
the final shifting of the cell body occur against the nervous current : A , Dendrites grown out to- 
ward the center of stimulation; B, the cell body has shifted toward the center of stimulation; the 
axis-cylinder is consequently elongated. (Kappers.) 

pers believes that it is a galvanotropic phenomenon, on the basis of the fact that the stimu- 
lation center is electrically negative, i. e., a cathode with reference to the surrounding tissue. 

Numerous instances might be cited of the action of this taxis, but two will suffice. It 
has already been noted that the eye-muscle nuclei receive most of their collaterals from the 
optic and vestibular reflex tracts; and these appear to be the most important factors in the 
determination of the positions occupied by those nuclei. The changes in position of the nuclei 
in the vertebrate series appear to run parallel to the changes in these tracts. The reader 
will now appreciate the significance of the close relation of these nuclei to the medial longi- 
tudinal and tectospinal fasciculi which convey to them impulses from the vestibular and 
optic centers. 

The position of the nucleus of the facial nerve and the curved course of its fibers within 
the pons may be explained in the same way. In a 10 mm. human embryo the nucleus of the 
facial nerve lies rostral to that of the abducens and the motor fibers pass directly lateralward 



i8o 



THE NERVOUS SYSTEM 



to their exit from the brain (Fig. 128). This nucleus, which supplies the muscles that sur- 
round the mouth, receives axons from the primary taste center in the medulla oblongata 
(the nucleus of the tractus solitarius) which is located at a more caudal level. Accordingly, 
the facial nucleus migrates caudally toward that center. It also receives fibers from the 
nucleus of the spinal tract of the trigeminal nerve and migrates ventrolaterally toward it. 
Thus is explained the adult position of the nucleus of the facial nerve, not far from the 
spinal tract of the trigeminal nerve and near the rostral end of the nucleus of the tractus 
solitarius. In the same way the curved course of the facial nerve within the pons may be 
explained. These examples are perhaps sufficient to illustrate the general principle of neuro- 
biotaxis. 

Nuclei of Origin and Terminal Nuclei. The efferent nuclei, which we have 
examined, all have this in common, that the axons, which take origin from their 
constituent cells, leave the brain through the efferent roots of the cranial nerves. 
Hence they may all be included under the term nuclei of origin. On the other 
hand, the afferent fibers of the cerebrospinal nerves have their cells of origin located 



Genu internum n. facialis 









Sulcus 



Sulcus 



Sulcus 



Fig. 128. Diagram illustrating three stages in the development of the genu of the facial 
nerve, the youngest, A, corresponding to the 10 mm. embryo, and the oldest, C, the newborn 
child. The relative position of the nucleus of the n. abducens is represented in outline. Sulcus, 
Sulcus medianus fossae rhomboideae. (Streeter, in Keibel and Mall's Embryology.) 

outside the central nervous system and, with the exception of the first two cranial 
nerves, in the cerebrospinal ganglia. These fibers enter the central nervous 
system and end by entering into synaptic relations with sensory neurons of the 
second order located in terminal nuclei. These are classified according to the 
function of the fibers which end in them as visceral afferent and somatic afferent 
nuclei. 

THE VISCERAL AFFERENT COLUMN 

All of the visceral afferent fibers of the cranial nerves, except those of the first 
pair, are contained in the facial, glossopharyngeal, and vagus nerves. These 
include: (1) the fibers from the taste buds, which since they mediate the special 
sense of taste, may be called special visceral afferent fibers; as well as (2) others 
from the posterior part of the tongue, and from the pharynx, larynx, trachea, 
esophagus, and thoracic and abdominal viscera, which are known as general 



THE CRANIAL NERVES AND THEIR NUCLEI 



181 



visceral afferent fibers. The majority of the taste fibers run through the seventh 
(via the chorda tympani and lingual) and ninth nerves (Gushing, 1903), but a 
few reach the epiglottis by way of the tenth (Wilson, 1905 Fig. 129). All 
cf these general and special visceral afferent fibers, whether contained in the 
seventh, ninth, or tenth nerves, enter the tractus solilarius, within which they 
descend for varying distances (Fig. 120, yellow). They terminate in a column 
of nerve-cells, which in part surround the tract and in part are scattered among 
its fibers. This is known as the nucleus of the tractus solitarius (Figs. 121, 130). 
It is a long slender nucleus, which extends throughout the entire length of the 
medulla oblongata and is best developed at the level of origin of the vagus nerve, 




Fig. 129. Diagram of the trigeminal, facial, and glossopharyngeal nerves showing the course 
of the taste fibers in solid black lines. The broken and dotted lines indicate the course which ac- 
cording to certain investigators some of the taste fibers are supposed to take: G. G., Gasserian 
ganglion; G. g., geniculate ganglion; G. sp., sphenopalatine ganglion; g.s.p., great superficial petro- 
sal nerve; N. Jac., the tympanic nerve of Jacobson; N. vid., vidian nerve; s.s.p., small superficial 
petrosal nerve. (Gushing.) 

where it lies ventrolateral to the dorsal motor nucleus of that nerve and some 
little distance below the floor of the fourth ventricle (Fig. 103). The fibers 
from the seventh and ninth nerves terminate in the rostral portion of the 
nucleus, which is therefore the part especially concerned with the sense of taste, 
while those from the vagus end in the caudal part. Some of these vagus fibers 
after undergoing a partial decussation terminate in a cell mass, the commissural 
nucleus, which lies dorsal to the central canal in the closed part of the medulla 
and unites the nucleus of the tractus solitarius on one side with the correspond- 
ing nucleus on the other side (Fig. 121). 

The secondary afferent paths from the nucleus of the tractus solitarius are 
not well denned. Since gustatory impulses arouse sensations of taste and the 



182 THE NERVOUS SYSTEM ^ 

afferent impulses from the viscera may be vaguely represented in conscious- 
ness, there must be a visceral afferent path to the thalamus; but concerning 
the character and location of this path we are entirely ignorant. 1 The fibers 
arising from the nucleus of the tractus solitarius enter the reticular formation, 
and it is probable that a majority of them are distributed to the visceral motor 
nuclei of the medulla oblongata, including the nucleus ambiguus and the dorsal 
motor nucleus of the vagus. In this way arcs are established for a large and 
important group of visceral reflexes. Some of these fibers descend to the spinal 
cord and may play an important part in the reflex control of respiration and 
in initiating reflex coughing and vomiting (Figs. 245, 246). 

THE GENERAL SOMATIC AFFERENT NUCLEI 

The general somatic afferent nuclei receive fibers from the skin and ecto- 
dermal mucous membrane of the head by way of the trigeminal nerve. These 
have their cells of origin in the semilunar ganglion, and within the pons the)' 
divide into short ascending and long descending branches (Fig. 131). The as- 
cending branches terminate in the main sensory nucleus; the descending branches 
run through the spinal tract and terminate in the nucleus of the spinal tract of 
the trigeminal nerve. Since these nuclei receive sensory fibers from the skin 
and ectodermal mucous membrane of the head, they are exteroceptive in function. 
The spinal tract and its nucleus also receives a few cutaneous afferent fibers 
through the glossopharyngeal and vagus nerves from the skin of the external ear 
(Fig. 120). 

The main sensory nucleus of the trigeminal nerve is located at the level of 
the middle of the pons in the lateral part of the reticular formation some dis- 
tance from the floor of the fourth ventricle (Figs. 110, 121, 130). The spinal 
nucleus, with which it is continuous, at first lies deeply under cover of the resti- 
form body; but when it is traced caudally it approaches the surface and, covered 
by the spinal tract, forms the tuberculum cinereum (Figs. 99, 103). It finally 
becomes continuous with the substantia gelatinosa Rolandi of the spinal cord. 
Thus we have a continuous column of gray matter extending from the sacral por- 
tion of the spinal cord into the brain stem and ending abruptly in an enlarge- 
ment, the main sensory nucleus of the trigeminal nerve. This entire column 
receives afferent fibers from the skin and belongs to the exteroceptive portion of 
the somatic afferent division of the nervous system. 

1 Kohnstamm and Hindelang (1910) and von Monakow (1913) have described a secondary 
visceral afferent path which arises from the gray matter in and around the tractus solitarius and 
terminates in the thalamus. 



THE CRANIAL NERVES AND THEIR NUCLEI 



183 



Secondary Afferent Paths. From the cells of the main sensory and spinal 
nuclei of the trigeminal nerve arise fibers which enter the reticular formation 
and are there grouped into longitudinal bundles from which collaterals are given 
off to the motor nuclei of the brain stem (Fig. 131). There are at least two such 
longitudinal bundles in each lateral half of the brain. The ventral secondary 
afferent path of the trigeminal nerve consists for the most part of crossed fibers 
and is located in the ventral part of the reticular formation, close to the spino- 
thalamic tract in the medulla, and dorsal to the medial lemniscus in the pons 



Mesencephalon \- 



Pons-\-- 




Ventral cochlear nucleus 



Medulla oblongata 



Cerebral aqueduct 

-/ Inferior colliculus 

Mesencephalic nucleus of N. V 

Sensory nucleus of N. V 
-/^-Fourth ventricle 

Vestibular nucleus 

Dorsal cochlear nucleus 

-Nucleus of tractus solitarius 

Nucleus of spinal tract N. V 



Central canal 



*p& M 

Fig. 130. Sensory nuclei projected upon a median sagittal section of the human brain stem. 
Horizontal lines, general somatic sensory nuclei; cross-hatching, visceral sensory nucleus; stipple, 
special somatic sensory nuclei. 

and mesencephalon (Fig. 132). It is composed in large part of long fibers which 
reach the thalamus. The dorsal secondary afferent path of the trigeminal nerve 
consists chiefly of uncrossed fibers and lies not far from the floor of the fourth 
ventricle and the central gray matter of the cerebral aqueduct. It consists in 
considerable part of short fibers (CajaL 1911; Wallenberg, 1905; Economo, 
1911; Dejerine, 1914). 

The proprioceptive nuclei of the cranial nerves are not well known. They 
have to do with afferent impulses arising in the muscles of mastication and in 



1 84 



THE NERVOUS SYSTEM 



the extrinsic muscles of the eye. Johnston (1909) has shown that the large 
unipolar cells of the mesencephalic nucleus of the fifth neroe which give rise 




Fig. 131. Diagram of the nuclei and central connections of the trigeminal nerve: A, Semi- 
lunar ganglion; B, mesencephalic nucleus, N. V.; C, motor nucleus, N. V.; D, motor nucleus, N. 
VII E, motor nucleus, N. XII ; F, nucleus of the spinal tract of N. V. ; G, sensory fibers of the sec- 
ond order of the trigeminal path: a, ascending and b, descending branches of the sensory fibers, 
N. V.; c, ophthalmic nerve; d, maxillary nerve; e, mandibular nerve. (Cajal.) 

to the fibers of the mesencephalic root of that nerve, are probably sensory in 
function. Willems (1911) and Allen (1919) believe that these are sensory fibers 



THE CRANIAL NERVES AND THEIR NUCLEI 



185 



to the muscles of mastication. If this interpretation is correct we are pre- 
sented with an exception to the rule that the afferent fibers of the cerebrospinal 
nerves take origin from cells located outside the cerebrospinal axis. This nucleus 
lies in the lateral wall of the rostral portion of the fourth ventricle and in the 
lateral part of the gray matter surrounding the cerebral aqueduct (Figs. 114, 
121, 130). The origin and termination of the afferent fibers for the extrinsic 




Fig. 132. Diagram to show the location of the secondary sensory tracts of the trigeminal 
nerve (solid black) in the tegmental portion of the rostral part of the pons: B.C., Brachium con- 
junctivum; D.T.T.N., dorsal secondary sensory tract of the trigeminal nerve; L.L., lateral lemnis- 
cus; M. L., medial lemniscus; M.L.F., medial longitudinal fasciculus; V.T.T.N., ventral secondary 
sensory tract of trigeminal nerve. 

muscles of the eye are unknown, although we know that such afferent fibers 
are present in the oculomotor, trochlear, and abducens nerves. 

SPECIAL SOMATIC AFFERENT NUCLEI 

The special somatic afferent nuclei are associated with the acoustic nerve, 
which is composed of two divisions. One part, the cochlear neroe, conveys im- 
pulses aroused by sound waves reaching the cochlea through the outer ear 
and tympanic cavity. Since it responds to stimuli from without, the cochlear 
apparatus subserves exteroceptive functions. The vestibular neroe, on the other 
hand, conveys impulses from the semicircular canals of the ear. These are im- 
portant proprioceptive sense organs and give information concerning the move- 
ments and posture of the head. 

The cochlear nuclei are the terminal nuclei of the cochlear nerve, the fibers 
of which take origin in the spiral ganglion of the cochlea. This is composed of 
bipolar cells, each having a short peripheral and a longer central process (Fig. 
133). The peripheral process terminates in the spiral organ of Corti. The 
central process is directed toward the brain in the cochlear nerve. These central 
fibers terminate in two masses of gray matter, located on the restiform body near 
the point where the latter turns dorsally into the cerebellum (Figs. 107, 121, 



i86 



THE NERVOUS SYSTEM 



130). One of these masses, the dorsal cochlear nucleus, is placed on the dorso- 
lateral aspect of the restiform body and produces a prominent elevation on the 
surface of the brain (Fig. 91). The other, known as the ventral cochlear nucleus, 
is in contact with the ventrolateral aspect of the restiform body. 

Secondary Auditory Path. From the cells of the ventral cochlear nucleus 
arise fibers which stream medialward in the ventral part of the pars dorsalis 
pontis and form the trapezoid body (Figs. 108, 134). The fibers cross the median 
plane and on reaching the lateral border of the opposite superior olivary nucleus 
turn rostrally as a compact bundle known as the lateral lemniscus (Figs. 110, 




Fig. 133. Section of the spiral ganglion and organ of Corti of the mouse: A, Bipolar cells of 
the spiral ganglion; B, outer hair cells; C, sustentacular cells; D, terminal arborization of the 
peripheral branch of a bipolar cell about an inner hair cell; T, tectorial membrane. Golgi method. 
(Cajal.) 

112, 114). Some of the fibers of the trapezoid body end in the superior olivary 
nuclei and in the nuclei of the trapezoid body, while others give off collaterals to 
these nuclear masses. Some of the fibers arising in these nuclei, especially in 
the nuclei of the trapezoid body, join in the formation of the lateral lemniscus; 
but according to Cajal (1909) a majority of the fibers from the superior olivary 
nucleus belong to short reflex pathways in the reticular formation connecting 
the cochlear nerve with the nuclei of the motor nerves of the head and neck. 
Fibers arising in the dorsal cochlear nucleus, and possibly also some from the 
ventral cochlear nucleus, sweep over the dorsal surface of the restiform body 
and the floor of the fourth ventricle as the s trice medullares acusticce. These may 



THE CRANIAL NERVES AND THEIR NUCLEI 



i8 7 



lie just beneath the ependyma or may be buried in the gray matter of the rhom- 
boid fossa. On reaching the median plane these fibers decussate, sink into the 
reticular formation, and join the trapezoid body or lateral lemniscus of the 
opposite side. Some probably fail to cross, since clinical experience and evi- 
dence based on animal experiments tend to show that a part of the fibers in the 
lateral lemniscus represent an uncrossed path from the cochlear nuclei of the 
same side (Kreidl, 1914). 



Transverse temporal gyrus 






Auditory radiation 

Medial geniculate body 
Inferior cotticulus 




-^"Lateral lemnisci 



Collaterals to nucleus of 
lateral lemniscus 



Rostral portion of the pons- 



Caudal portion of the pans 

Superior olive-'' 



/Stria medullares 



, Dorsal cochlear nucleus 

-Ventral cochlear micleus 
Cochlear nene 
> Vestibular nerve 



Trapezoid body ' 

Nucleus of the trapezoid body 
Fig. 134. Diagram of the auditory pathway. (Based on the researches of Cajal and Kreidl.) 

As the lateral lemniscus ascends in the reticular formation of the pons, there 
are scattered among its fibers many nerve-cells which together constitute the 
nucleus of the lateral lemniscus. To these cells it gives off collaterals and pos- 
sibly also terminal branches, and from them it is said to receive additional fibers. 
But according to Cajal the axons arising here do not ascend in the lateral lem- 
niscus, but are directed medially into the reticular formation. 

On reaching the mesencephalon the lateral lemniscus terminates in part in 
the inferior cotticulus, but also sends branches and direct fibers by way of the 
inferior quadrigeminal brachium to the medial geniculate body. While the me- 



i88 



THE NERVOUS SYSTEM 



dial geniculate body is a way-station on the auditory path to the cerebral cor- 
tex, the inferior colliculus serves as a center for reflexes in response to sound. 
The Vestibular Nuclei. The fibers of the vestibular nerve take origin from 
the bipolar cells of the vestibular ganglion located in the internal auditory meatus 
(Fig. 135). The cochlear and vestibular divisions of the acoustic nerve sepa- 
rate at the ventral border of the restiform body. Here the vestibular nerve 




Fig. 135. The vestibular ganglion and the termination of the peripheral branches of its bi- 
polar cells in a macula acustica: A, Hair cells and B, sustentacular cells of the macula; D, terminal 
arborization of the peripheral branches of the bipolar cells of the vestibular ganglion (G) about the 
hair cells of the macula; F, facial nerve; R, central branches of the bipolar cells directed toward the 
medulla oblongata T. Mouse. Golgi method. (Cajal.) 

penetrates into the brain, passing between the restiform body and the spinal 
tract of the trigeminal nerve toward the area acustica of the rhomboid fossa. 
Under cover of the area acustica the fibers divide into short ascending and 
longer descending branches (Figs. 134, 136). There may be enumerated five 
cellular masses within which these fibers terminate, namely: (1) the principal 
or medial nucleus, (2) the descending or spinal nucleus, (3) the superior nucleus 



THE CRANIAL NERVES AND THEIR NUCLEI 



189 



of Bechterew, (4) the lateral nucleus of Deiters, and (5) the cerebellum (Figs. 
130, 136). 

The principal, medial, or dorsal vestibular nucleus is very large. It lies sub- 
jacent to the major portion of the area acustica and belongs, therefore, to both 
the pons and the medulla oblongata (Figs. 89, 103, 107). The gray matter, 
associated with the descending branches from the vestibular nerve and lying on 
the medial side of the restiform body, constitutes the spinal or descending 
vestibular nucleus. Along with the descending fibers it can be followed in serial 



Nuc. of oculomotor _ 
neroe 



Nuc. of trocMear nerve -^- 



Brachium 



Nuc. of abducens nerve. 

Rhomboid fossa' 

Medulla oblongata- 




Superior colliculus 



Inferior colliculus 

Med. longitudinal 
- " ' fasciculus 

Superior vestibular 



Vestibulocerebellar 
tract 

4 Lateral vestibular nuc. 



" Vestibular nerve 



' Spinal vestibular nuc. 

Principal vestibular 
nucleus 



Fig. 136. Diagram of the nuclei and central connections of the vestibular nerve. (Based on 

figures by Herrick and Weed.) 

sections as far as the rostral extremity of the nucleus gracilis. The lateral vestib- 
ular nucleus of Deiters is situated close to the restiform body at the point where 
the fibers of the vestibular nerve begin to diverge (Fig. 107). It is composed of 
large multipolar cells like those found in motor nuclei. Directly continuous with 
the medial and lateral nuclei is a mass of medium-sized cells, the superior vestib- 
ular nucleus of Bechterew, located in the floor and lateral wall of the fourth 
ventricle lateral to the abducens nucleus, and the emergent fibers of the facial 
nerve (Fig. 108). It extends as far rostrad as the caudal border of the main 
sensory nucleus of the trigeminal nerve (Weed, 1914). 



190 THE NERVOUS SYSTEM 

Many of the ascending branches of the vestibular nerve, after giving off 
collaterals to the nuclei of Deiters and Bechterew, are prolonged in the tractus 
vestibulocerebellaris, to end in the cortex of the cerebellum (Cajal, 1909). These 
are joined by fibers arising in the superior and lateral vestibular nuclei which 
also run to the cerebellum (Fig. 136). From the standpoint of its embryologic 
development the cerebellum may properly be regarded as a highly specialized 
vestibular nucleus (p. 196). 

Secondary Vestibular Paths. In addition to the fibers to the cerebellum 
mentioned in the preceding paragraph two important tracts of fibers take origin 
in the superior and lateral vestibular nuclei. One of these was encountered in 
the study of the medial longitudinal bundle. Cells in the superior and lateral 
vestibular nuclei give rise to fibers which run to the medial longitudinal fascicle 
of the same and of the opposite side, and through it reach the motor nuclei of the 
ocular muscles (Fig. 136). In this way there is established an arc, which makes 
possible the reflex response of the eye muscles to afferent impulses arising in the 
vestibule and semicircular canals of the ear. The other bundle was considered 
in connection with the spinal cord as the vestibule spinal tract, the fibers of 
which take origin from the cells of the lateral nucleus and descend into the 
anterior funiculus of the same side of the cord. These fibers serve to place the 
primary motor neurons of the spinal cord under the reflex control of the vestib- 
ular apparatus. 

From the medial border of the principal vestibular nucleus many scattered 
fibers cross the raphe and enter the reticular formation of the opposite side, where 
they become longitudinal fibers. No tract to the thalamus is known, a fact 
which is in keeping with this other, that ordinarily the activities of the vestib- 
ular apparatus are not clearly represented in consciousness. 

SUMMARY OF THE ORIGIN, COMPOSITION, AND CONNECTIONS OF THE CRANIAL 

NERVES 

The olfactory and optic nerves and the nervus terminalis, which have not 
yet been considered in detail, have been included in this summary for the sake 
of completeness. 

The nervus terminalis is a recently discovered nerve which arises from the 
cerebral hemisphere in the region of the medial olfactory tract or stria. It is 
closely associated with the olfactory nerve and its fibers run to the nasal septum. 
The origin, termination, and function of its component fibers are not yet under- 
stood (McKibben, 1911; Huber and Guild, 1913; McCotter, 1913; Johnston, 



THE CRANIAL NERVES AND THEIR NUCLEI 191 

1914; Brookover, 1914, 1917; Larsell, 1918, 1919). Since it was unknown at 
the time the cranial nerves were first enumerated, it bears no numerical desig- 
nation. 

I. Olfactory Nerve. Superficial origin from the olfactory bulb in the form of 
a number of fine fila which separately pass through the openings in the cribri- 
form plate. It is composed of special visceral afferent fibers with cells of origin 
in the olfactory mucous membrane. The fibers terminate in the glomeruli of 
the olfactory bulb. 

II. Optic Nerve. Not a true nerve; but both from the standpoint of its 
structure and development a fiber tract of the brain. Superficial origin, from the 
optic chiasma, or after partial decussation, from the lateral geniculate body, 
pulvinar of the thalamus, and superior colliculus. Component fibers: special 
somatic afferent exteroceptive; origin, ganglion cells of the retina; terminations 
in the lateral geniculate body, pulvinar of the thalamus and superior colliculus. 
The fibers from the nasal half of each retina cross in the optic chiasma. 1 

III. Oculomotor Nerve. Superficial origin, from the oculomotor sulcus on 
the medial aspect of the cerebral peduncle. Composition: 

1. Somatic Efferent Fibers. Cells of origin, hi the oculomotor nucleus of the 
same and to a less extent of the opposite side (Fig. 120). Termination, in the 
extrinsic muscles of the eye except the superior oblique and the lateral rectus. 

2. General Visceral Efferent Fibers. Cells of origin in the Edinger-Westphal 
nucleus. Termination in the ciliary ganglion, from the cells of which post- 
ganglionic fibers run to the intrinsic nuscles of the eye. 2 

IV. Trochlear Nerve. Superficial origin, from the anterior medullary ve- 
lum. Composed of somatic efferent fibers; cells of origin in the trochlear nucleus; 
decussation in the anterior medullary velum; termination in the superior oblique 
muscle of the eye (Fig. 120). 

V. Trigeminal Nerve. Superficial origin, from the lateral aspect of the 
middle of the pons by two roots: the portio major or sensory root and the portio 
minor or motor root. Composition (Fig. 120): 

1. General Somatic A/erent Fibers. A, Exteroceptive Cells of origin in the 
semilunar ganglion (Gasserii), chiefly unipolar with T-shaped axons, peripheral 

1 It has been demonstrated by Arey that there are also efferent fibers in the optic nerves of 
fishes which control the movement of the retinal elements in response to light, Jour. Comp. Neur., 
vol. 26, p. 213. 

2 It is probable that the oculomotor, trochlear, and abducens nerves contain proprioceptive 
fibers for the extrinsic muscles of the eye, but the cells of origin and the central connections of 
these sensory components are unknown. 



IQ2 THE NERVOUS SYSTEM 

branches to skin and mucous membrane of the head, central branches by way 
of the portio major to the brain. Termination in the main sensory nucleus and 
nucleus of the spinal tract of the trigeminal nerve. 

2. General Somatic Afferent Fiber's. B, Proprioceptive Cells of origin prob- 
ably located in the mesencephalic nucleus of the fifth nerve. Fibers by way 
of the portio major, distributed as sensory fibers to the muscles of mastication. 

3. Special Visceral Efferent Fibers. Cells of origin in the motor nucleus of 
the fifth nerve. Fibers by way of the portio minor and the mandibular nerve 
to the muscles of mastication. 

VI. Abducens Nerve. Superficial origin, from the lower border of the 
pons just rostral to the pyramid. Composed of somatic efferent fibers; cells of 
origin in the abducens nucleus; termination in the lateral rectus muscle of the 
eye. 

VII. Facial Nerve and Nervus Intermedius. Superficial origin from the 
lateral part of the lower border of the pons separated from the flocculus by the 
eighth nerve. Composition (Fig. 120) : 

1. Special Visceral Afferent Fibers. Cells of origin in the ganglion geniculi, 
chiefly unipolar, with T-shaped axons. The peripheral branches run by way of 
the chorda tympani and lingual nerves to the taste buds of the anterior two- 
thirds of the tongue. The central branches run by way of the nervus intermedius 
to the tractus solitarius and end in the nucleus of that tract. It is probable that 
the taste fibers terminate in the rostral part of this nucleus. 1 

2. General Visceral Efferent Fibers. Cells of origin in the nucleus saliva torius 
superior. These fibers run by way of the- nervus intermedius, facial nerve, 
chorda tympani, and lingual nerve to the submaxillary ganglion for the in- 
nervation of the submaxillary and sublingual salivary glands. 

3. Special Visceral Efferent Fibers. Cells of origin in the motor nucleus of 
the facial nerve. These fibers run by way of the facial nerve to end in the super- 
ficial musculature of the face and scalp, and in the platysma, posterior belly of 
the digastric, and stylohyoid muscles. 

VIII. Acoustic Nerve. Superficial origin from the lateral part of the lower 
border of the pons near the flocculus. Consists of two separate parts known as 
the vestibular and cochlear nerves. 

1 Herrick (1918) describes general visceral afferent fibers in the facial nerve which he says 
mediate deep visceral sensibility and are probably found in all the branches of the facial. And 
Rhinehart (1918) has described a cutaneous branch of the facial in the mouse. This branch con- 
tains general somatic afferent fibers, which arise in the geniculate ganglion and terminate in the 
skin. 



THE CRANIAL NERVES AND THEIR NUCLEI 193 

The Vestibular Nerve. The component fibers belong to the special somatic 
afferent group and are proprioceptive. Cells of origin, in the vestibular ganglion, 
are bipolar. Their peripheral branches run to the semicircular canals, utricle and 
saccule. Their central branches terminate in the principal, lateral, superior, and 
spinal vestibular nuclei. Some of them run without interruption to the cerebellum. 

The Cochlear Nerve. The component fibers belong to the special somatic 
afferent group and are exteroceptive. Cells of origin, in the spiral ganglion of 
the cochlea, are bipolar. Their peripheral branches end in the spiral organ of 
Corti. Their central branches terminate in the ventral and dorsal cochlear nuclei. 

IX. The Glossopharyngeal Nerve. Superficial origin, from the rostral 
end of the posterior lateral sulcus of the medulla oblongata in line with the 
tenth and eleventh nerves. Composition (Fig. 120): 

1. General Visceral Afferent Fibers. Cells of origin in the ganglion petrosum, 
peripheral branches form the general sensory fibers to the pharynx and posterior 
third of the tongue, central branches run to the tractus solitarius and its nucleus. 

2. Special Visceral Afferent Fibers. Cells of origin in the ganglion petrosum, 
peripheral branches to the taste buds of the posterior third of the tongue, central 
branches, to the tractus solitarius and its nucleus. 

3. General Visceral Efferent Fibers. Cells of origin in the inferior salivatory 
nucleus; fibers run to the otic ganglion, from the cells of which postganglionic 
fibers carry the impulses to the parotid gland. 

4. Special Visceral Efferent Fibers. Cells of origin in the nucleus ambiguus. 
Termination in the stylopharyngeus muscle. 

X. Vagus Nerve. Superficial origin from the rostral part of the posterior 
lateral sulcus of the medulla oblongata in line with the ninth and eleventh and 
just caudal to the ninth. Composition (Fig. 120) : 

1. General Somatic Afferent Fibers. Cells of origin in the ganglion jugulare; 
peripheral branches to the skin of the external ear by way of the ramus auricularis; 
central branches to the spinal tract of the trigeminal nerve and its nucleus. 
According to Herrick, some of these fibers from the external ear run by way of 
the glossopharyngeal nerve also. 

2. General Visceral Afferent Fibers. Cells of origin in the ganglion nodosum; 
peripheral branches run as sensory fibers to the pharynx, larynx, trachea, esopha- 
gus, and the thoracic and abdominal viscera ; central branches run to the tractus 
solitarius and terminate in its nucleus. 1 

According to Wilson (1905) there are also special visceral afferent fibers in the vagus for 
the taste buds of the epiglottis. These also terminate in the nucleus of the tractus solitarius. 

13 



194 THE NERVOUS SYSTEM 

3. General Visceral Efferent Fibers. Cells of origin in the dorsal motor nucleus 
of the vagus. Fibers run to the sympathetic ganglia of the vagal plexuses for 
the innervation of the thoracic and abdominal viscera. 

4. Special Visceral Efferent Fibers. Cells of origin in the nucleus ambiguus. 
Termination in the striated musculature of the pharynx and larynx. 

XI. Accessory Nerve. Superficial origin from the posterior lateral sulcus 
of the medulla oblongata caudal to the ninth and tenth and from the lateral as- 
pect of the first five or six cervical segments of the spinal cord. Composition 
(Fig. 120): 

1. General Visceral Efferent Fibers. Cells of origin in the dorsal motor 
nucleus of the vagus. Fibers run in the bulbar rootlets and then by way of the 
internal ramus of the accessory to join the vagus, and end in the sympathetic 
plexuses, associated with the vagus nerve, for the innervation of thoracic and 
abdominal viscera. 

2. Special Visceral Efferent Fibers. These fall into two groups: A, fibers, 
whose cells of origin are located in the nucleus ambiguus, and which run by way of 
the internal ramus of the accessory to join the vagus and are distributed through 
it to the striated muscles of the pharynx and larynx; B, fibers, whose cells of 
origin lie in the lateral part of the anterior gray column of the first five or six 
cervical segments of the spinal cord, and which ascend in the spinal root of the 
accessory nerve and then run in its external ramus to end in the trapezius and 
the sternocleidomastoid muscles. 

XII. Hypoglossal Nerve. Superficial origin from the anterior lateral sulcus 
of the medulla between the pyramid and the olive. It is composed of somatic 
efferent fibers, whose cells of origin are located in the hypoglossal nucleus and 
whose termination is in the musculature of the tongue. 



CHAPTER XIII 



THE CEREBELLUM 



DEVELOPMENT OF THE CEREBELLUM 



THE dorsal border of the alar lamina occupies a lateral position in the rhom- 
bencephalon and, as a result of the development of the pontine flexure, acquires 
a V-shaped bend at the apex of which is the lateral recess of the fourth ventricle 
(Fig. 137, A). This dorsal border becomes everted and forms a prominent 



Mid-brain 



Cerebellum 



Lateral recess 

Rhombic lip 



Corpora quadrigemina 
Cerebrum 



A nlage of 
vermis 

Lateral lobe of 
cerebellum 



Rhombic lip 




Lateral lobe of cerebellum Lobules of vermis 



Obex 




Flocculus 



Uvula 



Nodulus 



Fig. 137. Dorsal view of four stages in the development of the cerebellum: A, of a 13.6 
mm. embryo (His); B, of a 24 mm. embryo; C, of a 110 mm. fetus; D, of a 150 mm. fetus. (Pren- 
tiss and Arey.) 

ridge known as the rhombic lip. From the portion of this ridge caudal to the 
lateral recess develop the taenia of the fourth ventricle and the obex. At the 
level of the recess the fibers of the acoustic nerve reach the dorsal edge of the 
alar lamina, which, accordingly, undergoes development at this point into 
vestibular and cochlear nuclei. More rostrally it undergoes an excessive devel- 

195 



196 THE NERVOUS SYSTEM 

opment, which is stimulated by the growth into it of afferent fibers from the 
vestibular nerve and of sensory fibers of the second order, bringing afferent 
impulses from other sources, chiefly from the somatic musculature. This 
part of the alar lamina, which may be regarded as an overgrown portion of the 
vestibular nucleus, develops into the cerebellum. As the paired cerebellar plates 
increase in thickness during the second month of embryonic development, they 
bulge inward toward the ventricle and take up a transverse position (Fig. 137, 
5). As they increase in size they invade the roof plate and unite in the median 
plane forming a transverse bar above the fourth ventricle. The lateral ex- 
tremities of this bar expand, and the entire structure assumes a dumb-bell 
shape, the lateral masses representing the future cerebellar hemispheres and the 
intermediate part the future vermis. 

At the close of the third month transverse sulci begin to appear in the vermis. 
The first of these, the fissura prima or sulcus primarius, extends into the lateral 
masses on either side and separates an anterior lobe from the remainder of the 
cerebellum. Other transverse fissures soon appear, due to the rapid expansion 
and resultant folding of the cortical layers. 

The cerebellum differs from the other parts of the nervous system, which we 
have thus far studied in detail, in that the relative position of the gray and white 
matter is reversed. The gray substance forms a thin superficial layer, the 
cerebellar cortex, which covers a central white medullary body (corpus medullare). 
Originally the cerebellar plate is formed, like other parts of the neural tube, of 
an ependymal, a nuclear or mantle, and a cell-free marginal zone. The neuro- 
blasts of the mantle zone take no part in the formation of the cortex, but become 
grouped in the internal nuclear masses of the cerebellum. The superficial or 
marginal zone is at first devoid of nuclei; the neuroblasts, from which the cere- 
bellar cortex is differentiated, migrate into this zone from the ependymal and 
perhaps also from the mantle layers of the rhombic lip. These developing neu- 
rons send their axons inward instead of outward as in the case of the spinal cord. 
These axons accumulate, along with others which enter the cerebellum from 
without, in the deep part of the marginal layer and form the central medullary 
body of the cerebellum, separating the developing cortex from the deep nuclear 
masses that are differentiating from the mantle layer. 

THE ANATOMY OF THE CEREBELLUM 

It is customary to consider the cerebellum as composed of three parts: a 
small unpaired median portion, called the vermis, because superficially it re- 



THE CEREBELLUM 



197 



sembles a worm bent on itself to form almost a complete circle; and two large 
lateral masses, the cerebellar hemispheres, which are connected with each other 
by the verrm's (Figs. 138, 139). Although morphologically incorrect, this sub- 
division has the advantage of convenience as well as of established usage. On 
the rostral aspect of the cerebellum the vermis forms a median ridge, not sharply 
marked off laterally from the hemispheres. This part has been called the superior 
vermis, and in contradistinction the remainder is known as the inferior vermis. 
The latter forms a prominent ridge, marked off from the hemisphere on either 
side by a well-defined sulcus. It lies in a deep groove between the hemispheres, 
known as the vallecula, within which the medulla oblongata is lodged. The 
hemispheres are also partially separated from each other by deep notches, the 



Anterior cerebellar notch 



Central lobule 
f Ala of central lobule 



Quadrangu-$ Ant, portion 
lar lobule \p os t. portion 

Cerebellar hemi- 
sphere superior 
surface 



Superior semi- . 
lunar lobule 



Cerebellar folia 
Inferior semilunar lobule 




Primary fissure 



Postclival sulcus 
Horizontal cerebellar sukus 



Folium of vermis 
Posterior cerebellar notch 

Fig. 138. Dorsal view of the human cerebellum. (Modified from Sobotta-McMurrich.) 

incisura cerebelli. The anterior cerebellar notch (semilunar notch) is broad and 
deep; and as seen from above it is occupied by the brachia conjunctiva and 
the inferior colliculi of the corpora quadrigemina. The posterior cerebellar 
notch (marsupial notch) is smaller, and within it is lodged a fold of the dura 
mater, the falx cerebelli. 

The superior vermis is divided by transverse fissures into the following 
lobules (Fig. 138): 

1. Lingula, closely applied to the anterior medullary velum between the two 
brachia conjunctiva. 

2. Central lobule, associated with the small alae lobuli centralis of the hemi- 
sphere. 



198 



THE NERVOUS SYSTEM 



3. Monticulus, which is further subdivided into the culmen and declive. The 
former goes over laterally without line of demarcation into the anterior portion 
of the quadrangular lobule, and the latter into the posterior portion of the same 
lobule in the hemisphere. 

4. Folium vermis at the posterior extremity of the superior vermis. 

The rostral or dorsal surface of the hemisphere is subdivided by curved 
transverse fissures, which are continued across the vermis, into the following 
parts : 

1. The anterior part of the quadrangular lobule, continuous with the culmen 
monticuli of the vermis. 

2. The posterior part of the quadrangular lobule, continuous with the declive 
monticuli. 

Nodule of vermis Flocculus 



Inferior vermis 



Cerebellar hemisphere 
inferior surface^--. 



Tonsil 




Bivenlral lobule 



^Inferior semi- 
lunar lobule 



.- Horizontal cere- 
bellar stdcus 

Superior semilunar 
lobule 



Uvula of vermis j p os i er - lor N N Tuber of vermis 
Pyramid of vermis ' cerebettar Folium of vermis 
notch 

Fig. 139. Ventral view of the human cerebellum. (Sobotta-McMurrich.) 

3. The superior semilunar lobule, occupying a large crescentic area along the 
dorsolateral border of the rostral surface. 

The inferior vermis (Fig. 139) is divided by transverse sulci into the follow- 
ing lobules: 

1. The tuber vermis, next to the folium. 

2. The pyramis. 

3. The uvula. 

4. The nodulus. 

The caudal surface of the hemisphere presents the following subdivisions: 
1. The inferior semilunar lobule, occupying a large part of this surface along 
its dorsolateral border. 



THE CEREBELLUM 



199 



2. The biventral lobule, occupying the ventrolateral part of the inferior surface. 

3. The tonsil, a small rounded lobule near the inferior vermis. 

4. The flocculus is the smallest of the lobules; and from it there runs toward 
the median plane a thin white band, the posterior medullary velum, and the 
peduncle of the flocculus. 

Structure of the Cerebellum. The cerebellum is composed of a thin super- 
ficial lamina of gray matter, spread over an irregular white center that con- 
tains several compact nuclear masses. This white medullary body forms a 
compact mass in the interior and is continuous from hemisphere to hemisphere 
through the vermis, within which, however, it is smaller than in the hemi- 
spheres (Figs. 140, 141). As is most readily seen in sagittal sections through the 
cerebellum, the medullary body gives off numerous thick laminae, which pro- 



Dentate nucleus 



Central lobule 
Lingula 




Fissura prima 

Declive 

Folium 



Nodule Uvula 



Fig. 140. Fig. 141. 

Figs. 140 and 141. Sagittal sections of the human cerebellum: Fig. 140 passes through the 
hemisphere and dentate nucleus; Fig. 141, through the vermis in the median plane. 

ject into the lobules of the cerebellum; and from these there are given off sec- 
ondary and tertiary laminse at various angles. Thus a very irregular white 
mass is formed, over the surface of which the much folded cortex is spread in 
a thin but even layer. Supported by the white laminae, the cortex forms long 
narrow folds, known as folia, which are separated by sulci and which are aggre- 
gated into lobules that, in turn, are separated by more or less deep fissures. 
Sections through the cerebellum at right angles to the long axis of the folia thus 
present an arborescent appearance to which the name arbor vita has been ap- 
plied. This is particularly evident in sections through the vermis (Fig. 141). 

MORPHOLOGY OF THE CEREBELLUM 

According to Elliott Smith (1903) and Bolk (1906), who have carried out extensive 
investigations on the morphology of the mammalian cerebellum, the fissura prima is an 



2OO 



THE NERVOUS SYSTEM 



important and constant fissure. It extends in a continuous curved line across the rostral 
aspect. of the vermis and both hemispheres. It has been found by Ingvar (1918) in reptiles 
and birds. All investigators who have given attention to this subject in recent years agree 
in designating the portion of the cerebellum which lies rostral to the fissura prima as the 
anterior lobe. The portion behind this fissure is composed of several individual lobules, each 
of which, though subject to considerable variation in form in the different genera, can be 
identified in every mammalian cerebellum. These lobules have been variously grouped into 
lobes by different investigators. Here we will follow the grouping employed by Ingvar, which 
is based on a comparison of the mammalian cerebellum with that of birds and reptiles (Fig. 
142). He recognizes three major divisions of the cerebellum, which he designates as the 
anterior, middle, and posterior lobes. The middle lobe contains those parts of the cerebellum 
which have been the last to appear during phyletic development, and it is here that the 
greatest variations are found in the different orders of mammals. 





1. 





Fig. 142. Schematic drawing of the cerebellum of 1, lizard; 2, crocodile; 3, bird, and 4, 
mammal. Vertical lines, anterior lobe; stipple, middle lobe; horizontal lines, posterior lobe; white, 
lobus ansoparamedianus. (Ingvar.) 

The anterior lobe includes all that part of the cerebellum that lies on the rostral side of 
the fissura prima (Figs. 143, 144, 146). In this lobe the folia have a transverse direction and 
extend without interruption across the vermis into both hemispheres. In the sheep the an- 
terior lobe is bounded laterally by the parafloccular fissure. It includes the three most 
rostral lobules of the superior vermis, which are designated in order from before backward, the 
lingula, lobulus centralis, and culmen monticuli. In man it also includes a large wing-shaped 
portion of each hemisphere (the pars anterior lobuli quadrangularis) ; and the entire lobe has 
the shape of a butterfly (Fig. 146). Morphologically, it is a median unpaired structure. 

The middle lobe is subdivided into four parts (Fig. 142). The most rostral of these 
is the lobulus simplex. It is separated from the anterior lobe by the fissura prima, and like 
that lobe it consists of transverse folia which extend across the superior vermis into both 



THE CEREBELLUM 



201 



hemispheres (Figs. 143, 144). In man the lobulus simplex forms a broad crescentic band 
across the rostral surface of the cerebellum, including what is ordinarily designated as the 
posterior part of the quadrangular lobule and the declive monticuli (Fig. 146). Like the 
anterior lobe, it is a median unpaired structure. The remainder of the middle lobe is sub- 
divided into median and lateral portions. The median part, known as the tuber vermis 



Fissura prima 



Lobulus ansiformis 



Lobulus paramedianus 




Lobits anterior 
^ Lobulus simplex 

' Paraflocculus 

-Fissura parafloccularis 



"^ Tuber vermis 



Fig. 143. Cerebellum of the sheep, dorsorostral view. 

(lobulus medius medianus of Ingvar and lobulus C 2 of Bolk), forms a conspicuous S-shaped 
lobule in the vermis of the sheep (Fig. 145) and may be readily identified at the occipital 
extremity of the inferior vermis in man (Figs. 139, 141). The paired lateral portions of the 
middle lobe each consist of two parts, called the lobulus ansiformis and lobulus paramedianus. 
The lobulus ansiformis, relatively small in most mammals (Fig. 144), is very large in man, 

Fissura prima 

i 

i 

, Lobus anterior 
/ f Lobuhis simplex 



Flocculus-" 



Paraflocculus' 



Lobulus paramedianus ' 




"Lobulus ansiformis 



Tuber vermis 



Lobulus medianus posterior 



Fig. 144. Cerebellum of the sheep, lateral view. 



and forms approximately the dorsolateral half of the hemisphere, occupying considerable 
parts of both the rostral and caudal surfaces. It corresponds to what has been known as 
the superior and inferior semilunar lobules and the biventral lobule (Figs. 146, 147). The 
lobulus paramedianus, or tonsilla of the B. N. A., is located on the lateral surface of the 
sheep's cerebellum, but is displaced on to the caudal surface in man by the great expani 
of the lobulus ansiformis. 



202 



THE NERVOUS SYSTEM 



The posterior lobe, as outlined by Ingvar, is composed of median and lateral portions. 
The median part, known as the posterior median lobule, comprises all of the inferior vermis 
except the tuber, from which it is separated by the prepyramidal sulcus. It is subdivided 
into three sublobules, known as the nodule, uvula, and pyramid (Figs. 139, 141, 145). The 
lateral part of the posterior lobe is formed on either side by two lobules, known as the flocculus 
and paraflocculus. These form the most lateral portion of the hemisphere in most mammals 
(Figs. 142, 144). In man the paraflocculus is rudimentary and the flocculus lies upon the 
caudal surface of the hemispheres (Fig. 147). It is connected with the nodule by a thin 
sheet of white matter, the posterior medullary velum. 

Functional Localization in the Cerebellum. We have described the cerebellum in 
terms of the subdivisions of Bolk and Ingvar, because these have morphologic and physio- 
logic significance, which is not true of the parts into which the cerebellum had previously 
been divided. By comparison of the size of these subdivisions with the degree of develop- 
ment and functional importance of the various groups of muscles in different animals Bolk 
endeavored to show that each of these parts was related to a particular group of muscles. 
On the basis of these comparative studies he concluded that the median unpaired portions 
of the cerebellum serve as coordination centers for the muscles which function in bilateral 



Tuber vermis 



Prepyramidal snlcus^ 



Paraflocculus -- 




-' Lobulus ansiformis 



Lobulus paramedianus 



~ Lobulus mcdianus posterior 



Fig. 145. Cerebellum of the sheep, caudal view. 



synergy. The muscles of expression and mastication, those of the eyes, pharynx, larynx 
and neck, and many of the trunk muscles are called into action simultaneously on both sides 
of the body, and should, according to this theory, have a median unpaired representation 
in the cerebellum. Bolk located the coordination center for the musculature of the head 
in the anterior lobe, that for the muscles of the neck in the lobulus simplex (Figs. 146, 147). 
A median center for those movements of the extremities which are strictly bilateral is found 
in the most dorsal sublobule of the vermis inferior, known as lobulus C 2 or tuber vermis. 
The remainder of the inferior vermis forms, according to this theory, a center for the bilateral 
movements of the trunk. In addition to a median center in the tuber vermis, the limbs are 
represented in the cerebellum by lateral centers for the coordination of unilateral move- 
ments. The lateral center for the arm is located in the rostral part or crus primum of the 
lobulus ansiformis (superior and inferior semilunar lobules) and that for the legs in the caudal 
part or crus secundum (biventral lobule), and perhaps also in the lobulus paramedianus 
(tonsil) . 

The conclusions concerning the localization of function in the cerebellum, reached by 
Bolk on the basis of morphologic studies, have been confirmed in so far as the centers for the 
neck and extremities are concerned by animal experimentation (Van Rynberk, 1908, 1912; 



THE CEREBELLUM 



203 



Andre Thomas and Durupt, 1914) and by clinical observations (Barany, 1912). There 
are, however, good reasons for skepticism regarding his localization of centers for the head 
and trunk. Ingvar (1918) presents evidence which indicates that the anterior and posterior 
lobes are probably concerned with the maintenance of the equilibrium of the body as a whole. 
The middle lobe, on the other hand, contains a number of separate centers, which correspond 
to those outlined by Bolk, for the control of the musculature of the neck and extremities. 

It has long been known that the degree of development of the cerebellar hemispheres in the 
different classes of vertebrates is closely correlated with that of the pons and cerebral cortex. 
This is particularly true of the lobulus ansiformis and lobulus paramedianus, which, like the 
neopallium, are recent phyletic developments. These belong to what Edinger (1911) calls 



B. N. A. 

Ala lobuli centralis 

Lobulus centralis 

Culmen monticuli 

Pars anterior lobuli 

quadrangular is 

Pars posterior lobuli 

quadrangularis 

Declive monticu'i 

Lobulus semilunaris 

superior 



Lobulus centralis 

Ala lobuli centralis 

Brachium pontis 

Flocculus 

Brachium conjunctivum 

Nodulus 

Uvula 

Tonsilla 

Lobulus biventer 

Pyramis 

Tuber 

Lob. semilun. inf. 

Sulcus horizontalis 

Lobulus semilunaris 

superior 




Fig. 146. 




BOLK 
Lobus anterior 



Sulcus primarius 
Lobulus simplex 
S. postdivalis 
Lobulus ansiformis 



Lobus anterior 

Cerebellar peduncles (cut) 
Flocculus 

Sulcus uvulo-nodularis 

Lobulus paramedianus 
Fissura secunda 

Lobulus ansiformis 



Fig. 147. 



Figs. 146 and 147. Outline drawings of the human cerebellum showing the localization of 
function according to the theory of Bolk. On the right side the parts are designated according 
to Bolk's terminology, on the left according to the B. N. A. Fig. 146, dorsal view. Fig. 147, 
ventral view. (Herrick.) 

the neocerebellum, receive the majority of the fibers from the brachium pontis, and may 
properly be regarded as cortical dependencies. They take an important part in the co- 
ordination of the voluntary movements of the extremities. 



THE NUCLEI OF THE CEREBELLUM 

The dentate nucleus is a crumpled, purse-like lamina of gray matter within 
the massive medullary body of each cerebellar hemisphere (Fig. 148). Like 
the inferior olivary nucleus, which it closely resembles, it has a white center 
and a medially placed hilus. In close relation to this hilus lies a plate of gray 
matter, the emboliform nucleus, and medial to this is the small globose nucleus. 



204 



THE NERVOUS SYSTEM 



Close to the median plane in the medullary body of the vermis, where this forms 
the tent-like covering of the fourth ventricle, is the nucleus of the roof or nucleus 
fastigii. 

The dentate nucleus is well developed only in those animals which possess 
large cerebellar hemispheres. It receives fibers from the cortex of the cere- 
bellar hemisphere, while the nuclei fastigii and globosi receive fibers chiefly 
from the vermis (Clark and Horsley, 1905; Edinger, 1911). It is probable that 



Decussation of brachia conjunctiva --, : 
Medial longitudinal fasciculus--^' 

\"' 
Brachium 



Molecular layer 
Granular layer 



Rhomboid fossa 
' 

^y A nterior medullary velum 
Lingula of cerebellum 
Fastigial nucleus 

Hilus of dentate nucleus 
Dentate nucleus 




Medullary lamina' 4 
Cerebellar folia-'-' 

Medullary substance of /' 
hemisphere 
Emboliform nucleus 

Globose nucleus 



Vermis 



Capsule of dentate nucleus 
Posterior cerebellar notch 



Fig. 148. Horizontal section through the cerebellum showing the location of the central nuclei. 

(Sobotta-McMurrich.) 

a functional localization similar to that in the cerebellar cortex will be found 
to exist in the central nuclei. In histologic structure the central nuclei closely 
resemble the inferior olive. 

THE CEREBELLAR PEDUNCLES 

The white core of the cerebellum is formed in large part of fibers which enter 
and leave the cerebellum through its three peduncles. 

The brachium pontis, or middle cerebellar peduncle, is formed by the trans- 
verse fibers of the pons and carries impulses which come from the cerebral cortex 
of the opposite side. It enters the cerebellum on the lateral side of the other 
two, and is distributed in two great bundles: one from the rostral part of the 
pons radiates to the caudal part of the cerebellar hemisphere; the other, from the 
caudal part of the pons, spreads out to the rostral portion of the hemisphere. 
In man, as might be expected from the large size of the pons and cerebellar 



THE CEREBELLUM 



205 



hemispheres, the brachium pontis is the largest of the three peduncles (Fig. 
89). But this is not true in most mammals, where, as in the sheep, the cere- 
bellum receives the majority of its afferent fibers from the spinal cord and medulla 
oblongata by way of the relatively large restiform bodies (Fig. 91). 

The restiform body ascends along the lateral border of the fourth ventricle; 
and at a point just rostral to the lateral recess it makes a sharp turn dorsally 
to enter the cerebellum between the other two peduncles (Figs. 87, 88). It 
consists of ascending fibers from the spinal cord and medulla oblongata and prob- 
ably also of descending fibers from the cerebellum to the reticular formation 
of the medulla (fastigiobulbar tract, p. 211). Among the ascending fibers are 
those of the following bundles: (1) dorsal spinocerebellar tract, which arises 




/ Tectocerebellar tract 



Cerebellum- 
Restiform body - 
Dorsal spinocerebellar tract * N 
Ventral spinocerebellar tract x^ 



f / 

' .; Corpora quadrigemina 

/ ''! 



Brachiumconjunctivum 



-Pons 
Fig. 149. Diagram of the spinocerebellar and tectocerebellar tracts. 

from the cells of the nucleus dorsalis of the same side of the spinal cord and 
ends in the cortex of the vermis; (2) the olivocerebellar tract, which consists of 
fibers from the opposite inferior olivary nucleus and to a less extent from that 
of the same side and which ends in the cortex of the vermis and of the hemi- 
sphere and in the central nuclei; (3) the dorsal external arcuate fibers, from the 
nuclei of the posterior funiculi of the same side; (4) ventral external arcuate 
fibers from the arcuate and lateral reticular nuclei (Fig. 104). 

The so-called medial part of the restiform body consists of bundles of fibers 
belonging to the tractus nucleocerebellaris, which course along the medial side of 
that peduncle as it turns dorsally into the cerebellum (Fig. 110). These come 
from the sensory nuclei of the cranial nerves. Most of them arise from the 
superior and lateral vestibular nuclei or represent the ascending branches of the 



206 THE NERVOUS SYSTEM 

fibers of the vestibular nerve and constitute the tractus vestibulocerebellaris . 
According to Cajal (1911) the fibers of this tract are distributed to the cortex 
of the cerebellum, the majority of them going to the vermis, a smaller proportion 
to hemisphere. In view of the newer ideas concerning the morphology of the 
cerebellum, the statements concerning the termination of all these cerebellar 
afferent fibers require re-examination. 

The brachium conjunctivum (Fig. 88) consists of efferent fibers from the 
dentate nucleus to the red nucleus and the thalamus of the opposite side. It is 
the smallest and most medial of the three peduncles. The ventral spinocere- 
bellar tract enters the cerebellum in company with the brachium conjunctivum. 
It ascends through the medulla oblongata and pons, curves over the brachium 
conjunctivum (Fig. 110), and enters the anterior medullary velum, within which 
it runs to the cerebellum (Fig. 149). Its fibers terminate in the rostral part of 
the vermis and in the nucleus fastigii (Horrax, 1915). According to Edinger, 
a bundle of fibers, the tectocerebellar tract, arises in the tectum of the mesencepha- 
lon and descends alongside of the brachium conjunctivum to the cerebellum, 
probably conveying impulses from visual centers. 

According to MacNalty and Horsley (1909) and Ingvar (1918) the fibers of the ventral 
spinocerebellar tract end in the lobulus centralis, culmen, and most rostral part of the declive. 
The fibers of the dorsal spinocerebellar tract have the same termination and, in addition, 
many of them go to the pyramis, and smaller numbers to the uvula and nodule. Practically 
all of the fibers which end in the cortex, therefore, go to the anterior and posterior lobes 
(Ingvar). The fact that the anterior lobe receives the majority of these fibers, which convey 
proprioceptive impulses from the trunk and extremities, is a strong argument against Bolk's 
conception of the anterior lobe as a co-ordination center for the musculature of the head. 

HISTOLOGY OF THE CEREBELLAR CORTEX 

The cerebellar cortex differs from that of the cerebral hemispheres in pos- 
sessing essentially the same structure in all the lobules. This would indicate 
that it functions in essentially the same way throughout, though as a result of 
different fiber connections the various lobules act on different muscle groups. 

A section through the cerebellum, taken at right angles to the long axis 
of the folia, shows each folium to be composed of a central white lamina, covered 
by a layer of gray cortex. Within the white lamina the nerve-fibers are arranged 
in parallel bundles extending from the medullary center of the cerebellum into 
the lobules and folia. A few at a time these bundles turn off obliquely into the 
gray matter, and there is no sharp demarcation between the cortex and the sub- 
jacent white lamina. The cortex presents for examination three well-defined 



THE CEREBELLUM 207 

zones: a superficial molecular layer, a layer of Purkinje cells, and a subjacent 
granular layer. 

The cells of Purkinje have large flask-shaped bodies and are arranged in an 
almost continuous sheet, consisting of a single layer of cells and separating 
the other two cortical zones (Fig. 150). They are more numerous at the summit 
than at the base of the folium. Each has a pyriform cell body. The part 
directed toward the surface of the cortex resembles the neck of a flask and from 




Fig. 150. Semidiagrammatic transverse section through a folium of the cerebellum. (Golgi 
method): A, Molecular layer; B, granular layer; C, white matter; a, Purkinje cell; b, basket cells; 
d, pericellular baskets, surrounding the Purkinje cells and formed by the arborizations of the 
axons of the basket cells; e, superficial stellate cells;/, cell of Golgi Type II; g, granules, whose 
axons enter the molecular layer and bifurcate at i; h, mossy fibers; j and m, neuroglia; n, climb- 
ing fibers. (Cajal.) 

it spring one or two stout dendrites. These run into the molecular layer and 
extend throughout its entire thickness, branching repeatedly. This branching 
occurs in a plane at right angles to the long axis of the folium; and it is only in 
sections, taken in this plane, that the full extent of the branching can be ob- 
served. In a plane corresponding to the long axis of the folium the dendrites 
occupy a more restricted area (Fig. 151). In this respect the dendritic ramifica- 
tions resemble the branches of a vine on a trellis. From the larger end of the 
cell, directed away from the surface of the cortex, there arises an axon which 



208 



THE NERVOUS SYSTEM 



almost at once becomes myelinated and runs through the granular layer into the 
white substance of the cerebellum. According to Clarke and Horsley (1905) and 
Cajal (1911) these axons end in the central cerebellar nuclei. Near their origin 
they give off collaterals, which run backward through the molecular layer to 
end in connection with neighboring Purkinje cells an arrangement designed 
to bring about the simultaneous discharge of a whole group of such neurons. 
The granular layer, situated immediately subjacent to that which we have 
just described, is characterized by the presence of great numbers of small neurons, 
the granule cells. Each of these contains a relatively large nucleus, surrounded 
by a small amount of cytoplasm; and from each there are given off from three 
to five short dendritic branches with claw-like endings. These are synaptically 
related with the terminal branches of the moss fibers, soon to be described, and 



Purkinje cell" 

Basket cell" 

Granule cell " 




"Purkinje cell 
' Granule cell 



Fig. 151. Diagrammatic representation of the structure of the cerebellar cortex as seen 
in a section along the axis of the folium (on the right), and in a section at right angles to the axis 
of the folium (on the left). 

form with them small glomeruli comparable to those of the olfactory bulb (Fig. 
208). Each granule cell gives origin to an unmyelinated axon, which extends 
toward the surface of the folium and enters the molecular layer. Here it divides 
in the manner of a T into two branches. These run parallel to the long axis of 
the folium through layer after layer of the dendritic expansions of the Purkinje 
cells, with which they doubtless establish synaptic relations (Fig. 151). Besides 
the granules just described, this layer contains some large cells of Golgi's Type 
II (Fig. 150, /). Most of these are placed near the line of Purkinje cells and 
send their dendrites into the molecular layer, while their short axons resolve 
themselves into plexuses of fine branches in the granular zone. 

The molecular layer contains few nerve-cells and has in transverse sections 
a finely punctate appearance. It is composed in large part of the dendritic 
ramifications of the Purkinje cells and the branches of axons from the granule 



THE CEREBELLUM 



209 



cells (Fig. 150). It contains a relatively small number of stellate neurons, the 
more superficial of which possess short axons and belong to Golgi's Type II. 
Those more deeply situated have a highly specialized form and are known as 
basket cells. From each of these there arises, in addition to several stout branch- 
ing dendrites, a single characteristic axon, which runs through the molecular 
layer in a plane at right angles to the long axis of the folium (Fig. 151). These 
axons are at first very fine, but soon become coarse and irregular, giving off 
numerous collaterals which are directed away from the surface of the cortex. 
These collaterals and the terminal branches of the axons run toward the Purkinje 
cells, about which their terminal arborizations form basket-like networks (Fig. 29) . 



Purkinje cell 
Dentate nucleus ^ 

Brachium conjunc- 
tivum 



Brachium pontis 



Restiform body 

Climbing fibers' 
Mossy fibers -' 




x Basket cell 
Granule cell 



Fig. 152. Diagram to illustrate the probable lines of conduction through the cerebellum. 

Nerve-fibers. The axons of the Purkinje cells form a considerable volume 
of fibers directed away from the cortex. There are also two kinds of afferent 
fibers which enter the cortex from the white center, and are known as climbing 
and mossy fibers respectively. The latter are very coarse and give off numerous 
branches ending within the granular layer. The terminal branches are provided 
with characteristic moss-like appendages. These mossy tufts are intimately 
related to the claw-like dendritic ramifications of the granule cells (Fig. 152). 
The climbing fibers, somewhat finer than those of the preceding group, pass 
through the molecular layer and become associated with the dendrites of the 
Purkinje cells in the manner of a climbing vine. Branching repeatedly, they 
14 



210 THE NERVOUS SYSTEM 

follow closely the dendritic ramifications of these neurons and terminate in free 
varicose endings. 

It would seem reasonable to suppose that the two kinds of afferent fibers, 
just described, have a separate origin and functional significance. According 
to Cajal (1911) it is probable that those entering the cerebellum through the 
brachium pontis are distributed as climbing fibers, and those from the restiform 
body as mossy fibers. The accompanying diagram represents the probable 
course of impulses through the cerebellum (Fig. 152). The mossy fibers, prob- 
ably derived from the restiform body, transfer their impulses to the granule 
cells; and these, in turn, relay them, either directly or through the basket neu- 
rons, to the Purkinje cells. The climbing fibers, which probably come from the 
brachium pontis, transfer their impulses directly to the dendrites of the Purk- 
inje cells. We do not known to which class the fibers of the vestibulocerebellar 
tract should be assigned. The efferent path may be said to begin with the 
Purkinje cells, whose axons terminate in the central cerebellar nuclei. From 
these nuclei, especially the dentate, arise the fibers of the brachium conjunc- 
tivum, the great efferent tract from the cerebellum. By means of the axons 
of the granule cells, basket cells, and neurons of Golgi's Type II, as well as by 
the collaterals from the axons of the Purkinje cells, an incoming impulse may be 
diffused through the cortex. 

The cerebellum probably receives fibers from all the somatic sensory centers, 
but especially from those of the proprioceptive group, through which afferent 
impulses are conveyed to it from the muscles, joints and tendons, and from 
the semicircular canals of the ear. Its connection with the vestibular appa- 
ratus is especially intimate. In fact, as already stated, it may be regarded from 
the standpoint of development as a very highly specialized portion of the ves- 
tibular nucleus. It is the great proprioceptive correlation center. Further- 
more, it sends efferent impulses to the various somatic motor centers and plays 
an important part in the coordination of muscular contraction and in the main- 
tenance of muscular tone. It is the chief center for equilibration, which depends 
upon the proper adjustment of the muscles in response, very largely, to the 
impulses from the semicircular canals. In man and mammals it also receives 
impulses from the cerebral cortex by way of the pons, which probably set the 
coordinating cerebellar mechanism into activity to bring about the proper 
adjustment of voluntary movements. For additional details concerning the 
functions of the cerebellum the reader should consult the recent paper by 
Holmes (1917). 



THE CEREBELLUM 



211 



THE EFFERENT CEREBELLAR TRACTS 

The efferent cerebellar tracts arise in the central nuclei. It is probable that 
no fibers of cortical origin leave the cerebellum except, perhaps, some to Deiter's 
nucleus (Clarke and Horsley, 1905). 

The brachium conjunctivum, or tractus cerebellotegmentalis mesencephali, 
arises for the most part at least in the dentate nucleus and terminates in the red 
nucleus and thalamus of the opposite side (Fig. 153). It constitutes the chief 
tract leading from the cerebellum and has been more fully described on page 
159. It undergoes a complete decussation beneath the inferior colliculus in 
the tegmentum of the mesencephalon. Both before and after this crossing its 



Brachium conjunctivum 




Thalamus 

-Red nucleus 
Nucleus fastigii 



TV Nucleus dentatus 



Fastigiobulbar tract 



- Tractus cerebellotegmentalis 

pontis 

Lateral vestibtilar nucleus 

Fastigiobulbar tract 



Fig. 153. Efferent tracts which arise in the central nuclei of the cerebellum. (Modified from 

Edinger.) 

fibers give off branches, which descend in the reticular formation of the pons 
and medulla. Some of the impulses reach the thalamus, but the others are 
relayed in the red nucleus along the rubrospinal and rubroreticular tracts to 
motor neurons in the brain stem and spinal cord (Fig. 115). 

Other efferent tracts arise in the nucleus fastigii of the same and opposite 
side, and run, probably by way of all three cerebellar peduncles, to the retic- 
ular formation of the pons and medulla oblongata. One bundle of these fibers 
winds around the brachium conjunctivum before descending through the pons 
and medulla (Fig. 153). It is probable that other fibers descend by way of the 
restiform body, and are distributed in the reticular formation of the medulla 



212 THE NERVOUS SYSTEM 

oblongata on the same side, or are continued as ventral external arcuate fibers 
to end on the opposite side. The bundles which run from the nucleus fastigii 
to the medulla oblongata may be designated as the fastigiobulbar tracts (tractus 
cerebellotegmentales bulbi). These include fibers which terminate in the 
lateral vestibular nucleus. It is said that some fibers belonging to this system 
leave the cerebellum by way of the brachium pontis (tractus cerebellotegmentalis 
pontis). 

Since the dentate nucleus receives fibers from the cortex of the correspond- 
ing cerebellar hemisphere, and the nucleus fastigii receives similar fibers from 
the vermis, it may be inferred that the brachium conjunctivum is the chief 
efferent tract for the hemisphere and that the fastigiobulbar tracts serve the 
same purpose for the vermis (Strong, 1915). 



CHAPTER XIV 

THE DIENCEPHALON AND THE OPTIC NERVE 

Development. In an earlier chapter we traced briefly the development of 
the prosencephalon and showed that the cerebral hemispheres were developed 
through the evagination of the lateral walls of the telencephalon (Fig. 16). It 
is, however, only the alar lamina which is involved in this evagination. The 
basal lamina of the telencephalon retains its primitive position and forms the 
pars optica hypothalami. This part of the hypothalamus, along with the 
lamina terminalis and the most rostral part of the third ventricle, constitutes 
the telencephalon medium (Johnston, 1912). Through the excessive growth of 
the hemisphere the diencephalon becomes covered from view (Fig. 17), and 
appears to occupy a central position in the adult human brain. It is separated 
from the hemisphere by the transverse cerebral fissure, which is formed by the 
folding back of the hemisphere over the diencephalon. The differentiation of 
the alar lamina of the diencephalon into the thalamus, epithalamus, and meta- 
thalamus, and of its basal lamina into the hypothalamus was briefly traced on 
page 34. The sulcus limitans, which separates these two plates in the embryo, 
corresponds to the more caudal portion of the hypothalamic sulcus of the adult; 
but, since the latter can be followed to the interventricular foramen, while the 
former ends near the optic chiasma, the rostral ends of these two sulci are not 
related. The roof plate of the prosencephalon remains thin and constitutes 
the epithelial roof of the third ventricle, which along the median plane becomes 
invaginated into the ventricle as the covering of a vascular network to form 
the chorioid plexus. 

THE THALAMUS 

The thalamus is a large ovoid mass, consisting chiefly of gray matter, placed 
obliquely across the rostral end of the cerebral peduncle (Figs. 154, 155). Be- 
tween the two thalami a deep median cleft is formed by the third ventricle. 
The rostral end is small and lies close to the median plane. It projects slightly 
above the rest of the dorsal surface, forming the anterior tubercle of the thalamus, 
and helps to bound the interventricular foramen (Fig. 154). The caudal ex- 
tremity is larger and is separated from its fellow by a wide interval, in which the 

213 



214 



THE NERVOUS SYSTEM 



corpora quadrigemina appear. It forms a marked projection, the pulvinar, 
which overhangs the medial geniculate body and the brachia of the corpora 
quadrigemina (Figs. 88, 154). For purposes of description it is convenient to 
recognize four thalamic surfaces, namely, dorsal, ventral, medial, and lateral. 

The dorsal surface of the thalamus is free (Figs. 91, 154). It forms the 
floor of the transverse fissure of the cerebrum and is separated by this fissure 
from the parts of the cerebral hemisphere which overlie it, that is, from the 



Free portions of columns offornix. 
Head of caudate nucleus-^ 
Medullary strior v ""' 
Third ventricle x N 

Eabenular trigone ^ 
Pineal bodv - 



Superior colliculus J 



Tail of caudate nucleus- 



Super, quadrigeminal brack,- 

Infer, quadrigeminal brack. - ' 

Cerebral peduncle- ' 

Corpora quadrigemina-'' 

lateral filaments of pans--' 

Anterior medullary velum- - 



Lingula of cerebellum 



Tela chorioidea of fourth ventricle 




Corpus callosum 

/Lamina of septum pellucidum 
/' Columns offornix 

"^ y A nterior commissure 
^~ Optic recess of ventricle III 
- A nterior tubercle of thalamus 

, Terminal stria 
^ s Tania chorioidea 

^-- Habenular commissure 
x'Z,awzwa affixa 
^ .Superior quadrigeminal 

brachium 
^-' Pulvinar of thalamus 

.Lateral geniculate body 



~~ -Medial geniculate body 
~~ Inferior colliculus 
- Trochlear nerve 
Brachium conjuctivum 
, Lateral recess of fourth ventricle 
Brachium pontis 
.Peduncle of flocculus 



Flocculus of cerebellum 

Lateral aperture of- ventricle IV 
^Chorioid plexus of ventricle IV 
\Rhomboidfossa (intermediate portion) 
Medial aperture of ventricle IV 

Funiculus gracilis 



Medulla oblongata 
Fig. 154. Dorsal view of the human brain stem. (Sobotta-McMurrich.) 

fornix and corpus callosum. Laterally it is bounded by a groove, which separates 
it from the caudate nucleus and contains a strand of longitudinal fibers, the 
stria terminalis and a vein, the vena terminalis (Figs. 154, 155). The dorsal 
surface is separated from the medial by a sharp ridge, the tania thalami, which 
represents the torn edge of the ependymal roof of the third ventricle. The 
taeniae of the two sides meet in the stalk of the pineal body. The prominence 
of this torn edge of the roof is increased by a longitudinal bundle of fibers, 



THE DIENCEPHALON AND THE OPTIC NERVE 



215 



the stria medullaris thalami. This fascicle, together with the closely related 
habenular trigone and the pineal body, belong to the epithalamus and will be 
described later. 

The dorsal surface of the thalamus is slightly convex and is divided by a faint 
groove into two parts: a lateral area, covered by the lamina affixa and forming 
a part of the floor of the lateral ventricle; and a larger medial area, which forms 
the floor of the transverse fissure of the cerebrum. The oblique groove separat- 
ing these two areas corresponds to the lateral border of the fornix (Figs. 154, 155). 
The lamina affixa is part of the ependymal lining of the lateral ventricle superim- 



Fornix 
Stratum zonale 



Chorioid plexus of lateral ventricle 
Lamina affixa 



Internal medullary 
lamina 

Chorioid plexus of 
third ventricle 

Third ventricle 



Lenticular nucleus- 



Internal capsule' ? 



Hypothalamic 
nucleus 



^Transverse fissure of the cerebrum 
Stria medullaris 

-, Corpus callosum 
Lateral ventricle 




Caudate nucleus 

Stria terminalis 
and vena ter- 
minalis 

._ External medull- 
ary lamina 

Anterior nucleus 
of thalamus 

$--, ^Lateral njifleus 
of thalamus 



- Medial nucleus 
of thalamus 



Red nucleus"- 

Optic tract 

% Basis pedunculi 

Fig. 155. Diagrammatic frontal section through the human thalamus and the structures which 

immediately surround it. 



Substantia nigra ' 



posed upon this part of the thalamus. It is not present in the sheep, where the 
fornix is larger and the entire dorsal surface of the thalamus belongs to the floor 
of the transverse fissure. These features are well illustrated in Figs. 179 and 
180, as is also the position of the transverse fissure. This fissure intervenes be- 
tween the thalamus and the cerebral hemisphere, and contains a fold of pia 
mater, known as the tela chorioidea, of the third ventricle. 

The medial surface of the thalamus forms the lateral wall of the third ven- 
tricle (Figs. 158, 159). It is covered by the ependymal lining of that cavity. 
The medial surfaces of the two thalami are closely approximated, being separated 



2l6 THE NERVOUS SYSTEM 

from each other by the cleft-like space of the third ventricle, and are united across 
the median plane by a short bar of gray substance, the massa intermedia. 

The lateral surface is hidden from view. It lies against the broad band of 
fibers, known as the internal capsule, which connects the cerebral hemispheres 
with the lower levels of the central nervous system. This surface is best examined 
in sections through the entire cerebrum (Figs. 155-157). Many fibers stream 
out of the thalamus through its lateral surface and enter the internal capsule, 
through which they reach the cerebral cortex. To this important stream of 
fibers the name thalamic radiation is applied. 

The ventral surface of the thalamus is also covered from view and lies on the 
hypothalamus, by which it is separated from the tegmentum of the mesencepha- 
lon (Figs. 155, 157). Many fibers, representing such ascending tegmental paths 
as the medial lemniscus, spinothalamic tract, and brachium conjunctivum, enter 
the thalamus through this surface. 

Structure of the Thalamus. The thalamus consists chiefly of gray matter, 
within which there may be recognized a number of nuclear masses. Its dorsal 
surface is covered by a thin layer of white matter, called the stratum zonale, 
which in the region of the pulvinar consists in large part of fibers derived from 
the optic tract. On the lateral surface of the thalamus next the internal cap- 
sule there are many myelinated fibers, which constitute the external medullary 
lamina (Figs. 155, 156). The medial surface is covered by a layer of central 
gray matter, continuous with that which lines the cerebral aqueduct and forms 
the floor of the third ventricle. This central gray matter consists of neuroglia 
and of scattered nerve-fibers and cells (the nucleus paramedianus of Malone, 
1910). Some of these fibers are continued through the gray matter that lines 
the aqueduct and the floor of the fourth ventricle, as the dorsal longitudinal 
bundle of Schutz (Fig. 112). It is probable that this portion of the thalamus 
forms a center for vasomotor and visceral reflexes, since lesions in this region 
are often accompanied by disturbances in the nervous control of the blood- 
vessels and viscera (Edinger, 1911; Rogers, 1916). If this be true, it is probable 
that the dorsal longitudinal bundle of Schutz serves to bring this thalamic 
mechanism for visceral adjustments into connection with the visceral efferent 
nuclei of the brain. 

From the stratum zonale, which clothes its dorsal surface, there penetrates 
into the thalamus a vertical plate of white matter, called the internal medullary 
lamina. This subdivides the thalamus into three parts: the anterior, medial, 
and lateral nuclei. At the rostral extremity of its dorsal border the internal 



THE DIENCEPHALON AND THE OPTIC NERVE 



217 



medullary lamina bifurcates and includes between its two limbs the anterior 
nucleus. 

The anterior nucleus (or dorsal nucleus) of the thalamus is located in the 
dorsal part of the rostral extremity of the thalamus and penetrates like a wedge 
between the medial and lateral nuclei. It protrudes somewhat above the 
general level of the dorsal surface, forming the anterior tubercle of the thalamus. 
It receives a large bundle of fibers from the mammillary body, the mamillotha- 
lamic tract or bundle of Vicq d'Azyr (Figs. 156, 204, 205), and sends fibers to the 
caudate nucleus of the corpus striatum (Fig. 196). 



TatHia tecia Stria* Lancisii 




Subitaut'ui nigra 



catidatta 

Nticleta anterior tkalami 
KucUm lakralis tkalami 
Nuclrmt miJiafa tkalami 

lint thatamiaa 

nsa lenticularis 

Trattiu oftlcia 
Pa feduuaiU 



Fig. 156. Frontal section through the mammillary body, thalamus, and adjacent structures. 

Weigert method. (Villiger-Piersol.) 

The medial nucleus of the thalamus is situated between the central gray 
matter of the third ventricle and the internal medullary lamina, which separates 
it from the lateral nucleus except in the caudal part, where the line of separation 
between the two is not distinct. It is said to receive fibers from the olfactory 
centers and to send fibers to the caudate nucleus and the subthalamus. 

The lateral nucleus of the thalamus is by far the largest of the three. It 
extends farther caudad than the medial nucleus and includes all of the pulvinar. 
Through the external medullary lamina and the internal capsule it sends fibers 
to the cerebral cortex in the thalamic radiation and receives corticothalamic 



2l8 



THE NERVOUS SYSTEM 



fibers in return. Especially in its ventral subdivision it receives all of the as- 
cending sensory tracts from the tegmentum of the mesencephalon, as well as 
fibers from the brachium conjunct! vum and red nucleus. It is much more richly 
supplied throughout with myelinated fibers than are the other nuclei of the thala- 
mus. 

The lateral nucleus is subdivided into a dorsal portion, the lateral nucleus 
proper, and a ventral part, better known as the ventral nucleus of the thalamus. 
Within the latter are two well-defined nuclear masses. The more medial of 



Tunis umlclrcula 



Corfus gtniatlatitm lattrale 



Gyrus dentalus 




Fig. 157. Frontal section through the human pons, basis pedunculi, thalamus and adjacent 
structures. Weigert method. (Villiger-Piersol.) 

the two is known as the nucleus centralis (nucleus globosus or centrum media- 
num) and is surrounded by a well-defined capsule of myelinated fibers (Fig. 157). 
Ventrolateral to this is the well-defined nucleus arcuatus, which because of its 
shape is also called the nucleus semilunaris. The pulvinar is a very large mass 
which forms the most caudal part of the thalamus and is usually considered as 
a part of the lateral nucleus. 

Function. The medial and anterior thalamic nuclei are closely associated in 
function and from a phylogenetic point of view represent the older part of the 
thalamus. They serve as centers for the more primitive thalamic correlations 



THE DIENCEPHALON AND THE OPTIC NERVE 219 

such as occur in lower vertebrates that lack the cerebral cortex (Herrick, 1917) 
Both receive fibers from the olfactory centers and both send fibers to the corpus 
striatum, but none to the cerebral cortex (Sachs, 1909). There is some evidence 
of a clinical nature to show that the activity of these centers may be accompanied 
by a crude form of consciousness (Head and Holmes, 1911; Head, 1918). Pa- 
tients in whom the paths from the thalamus to the cortex have been interrupted 
are aware of many sensations, but cannot discriminate among them. The 
thalamus seems to be the chief center for the perception of pain and the affec- 
tive qualities of other sensations, and in this respect it plays an important 
role in consciousness independently of the cerebral cortex. 

The more lateral group of centers, which includes the lateral nucleus of the 
thalamus, the pulvinar, and the geniculate bodies, is of more recent origin and 
has been called the neothalamus. They serve as relay stations on the somatic 
sensory paths to the cerebral cortex. The medial lemniscus and spinothalamic 
tracts terminate in the ventral subdivision of the lateral nucleus. In the pul- 
vinar and lateral geniculate body terminate fibers from the optic tracts, while 
the lateral lemniscus ends in the medial geniculate body. From these nuclei 
sensory fibers of the third order run to the cerebral cortex. The lateral nucleus, 
exclusive of the pulvinar, is therefore a relay station on the paths of cutaneous 
and deep sensibility, and it is connected with the parietal and frontal cortex 
through the thalamic radiation. The pulvinar and lateral geniculate body are 
stations on the optic pathway, and the medial geniculate body on that for hearing. 

The thalamic radiation can best be considered in detail after we have ac- 
quired some familiarity with the structure of the cerebral hemisphere (p. 263). 

The fiber tract connections, established by the various nuclear masses composing the 
thalamus, among themselves and with other parts of the brain, are not as yet well known. 
This is particularly true of the descending tracts. It is known that from the region of the 
thalamus a large bundle, the thalamo-olivary tract, descends to the inferior olivary nucleus. 
Some authors also describe a thalamospinal tract which arises in the thalamus and is closely 
associated with the rubrospinal tract. 

It is fairly well established that each of the ascending sensory tracts of the tegmentum 
has its own particular field of distribution within the ventral nucleus of the thalamus; and it 
is, therefore, probable that there are corresponding functional differences in the various 
subdivisions of this nucleus. Beginning at the lateral side and passing medialward, the 
terminals of these various tracts are as follows: The spinothalamic tract ends in the most 
lateral part of the ventral nucleus. Next comes the field, within which terminate the fibers 
of the central tract of the trigeminal nerve, and which includes the nucleus arcuatus and 
nucleus centralis. The medial lemniscus ends in the most medial part of the inferior nucleus, 
including the nucleus centralis. This corresponds to the relative position which these tracts 
occupy in the tegmentum of the mesencephalon, where the spinothalamic tract is the most 
lateral of the three. 



22O THE NERVOUS SYSTEM 

THE METATHALAMUS 

The metathalamus is composed of two small protuberances, the geniculate 
bodies, which, having been displaced by the excessive development of the 
thalamus, are situated upon the dorsolateral surface of the rostral end of the 
mesencaphalon (Figs. 87-89, 154, 161). The lateral geniculate body is an oval 
swelling in the course of the optic tract. Its connections will be more fully 
considered in connection with the discussion of the course of the visual impulses. 
The medial geniculate body is overhung by the pulvinar, from which it is separated 
by a deep sulcus. It receives fibers by way of the inferior quadrigeminal bra- 
chium from the lateral lemniscus, which we have learned to know as the central 
auditory path from the cochlear nuclei. From it fibers run to the auditory 
area of the cerebral cortex (the thalamotemporal or acoustic radiation). 

THE EPITHALAMUS 

The epithalamus includes the pineal body, stria medullaris, and habenular 
trigone. The latter is a small triangular depressed area located on the dorso- 
medial aspect of the thalamus rostral to the pineal body (Fig. 158) . In the sheep, 
as in most other mammals, it is much larger than in man and bulges both dor- 
sally and medially beyond the surface of the thalamus (Figs. 91, 159). It marks 
the position of a nuclear mass, called the habenular ganglion, which receives fibers 
from the stria medullaris, a fascicle which runs along the border between the 
dorsal and medial surfaces of the thalamus subjacent to the taenia thalami 
(Figs. 154, 155). The stria medullaris takes origin from the anterior perforated 
substance and other olfactory centers on the basal surface of the cerebral hemi- 
sphere and, partially encircling the thalamus, reaches the habenular ganglion, 
in which it ends. (See p. 281.) Not all of the fibers terminate on the same 
side; some cross to the ganglion of the opposite side, forming a transverse bundle 
of myelinated fibers which joins the caudal end of the two ganglia together and 
is known as the habenular commissure. From the cells in this ganglion arises 
a bundle of fibers, known as the fasciculus retroflexus of Meynert or the tractus 
habenulopeduncularis. This bundle of fibers is directed ventralward and at 
the same time caudally along the medial side of the red nucleus toward the 
base of the brain, where it crosses to the opposite side and ends in the inter- 
peduncular ganglion (Fig. 189). The stria medullaris, habenular ganglion, 
and fasciculus retroflexus are all parts of an arc for olfactory reflexes, as indi- 
cated in Fig. 211. According to Edinger (1911) the cells, from which the stria 
medullaris arises, are intimately related to a bundle of ascending fibers from 



THE DIENCEPHALON AND THE OPTIC NERVE 



221 



the sensory nuclei of the trigeminal nerve. If this be true, the mechanism in 
question may receive afferent impulses from the nose, mouth, and tongue and 
be concerned with feeding reflexes. 

The pineal body is a small mass, shaped like a fir cone, which rests upon the 
mesencephalon in the interval between the two thalami. Its base is attached 
by a short stalk to the habenular and posterior commissures, and into the stalk 
there extends the small pineal recess of the third ventricle. The pineal body is 
a rudimentary structure and is not composed of nervous elements. In some 



Hypothalmic sulcus 

Habenula 
Habenular commissure^ \ 

Suprapineal recess i > 

\ \ \ 

Posterior commissure \ \ \ 

Pineal body \ \ \ 
Splenium of corpus callosum N \ \ \ *. ' 
Lamina quadrigemina \ \ \ 

\ N ^/ X v > 

Cerebral aqueduct N N ^ v x s A \ \ \^-^ 

s N v v'-^rr 

Anterior medullary velum, 

Fourth ventricle N 
5w/>. z;erw. of cerebellum x 

Fissura prima N_J 



Inferior vermis 
of cerebellum 



Epithelial roof and chori- 
oid plexus of fourth''' 
ventricle 




, Body of fornix 

Chorioid plexus of third ventricle 
Massa intermedia 

Epithelial roof of third ventricle 
I Lamina commissure hippocampi 
1 t Corpus collosum 





Genu of corpus 
' callosum 

Septum pelluci- 
J^'^ dum 

^"~~ Ros.ofcor. callosum 
~ - Lamina rostralis 
" Columna fornicis 
^-~~^ Interventr icular foramen 
^^.^ Anterior commissure 
*-^~ Lamina terminalis 
^-C^Optic recess 
^*- ^Optic chiasma 
~ -^ Infundibulum 

Hypophysis 
Mammillary body 
Oculomotor nerve 
\ *Subthalamus 

Tegmentum of mesencephalon 
Pans 
Medulla 

'Central canal 



Fig. 158. Median sagittal section through the human brain stem. 



vertebrates, certain lizards for example, it is more highly developed, resembles 
in structure an invertebrate eye, and lies close to the dorsal surface of the head. 
The posterior commissure is a large bundle of fibers which crosses the median 
plane dorsal to the point where the cerebral aqueduct opens into the third 
ventricle (Figs. 154, 156). The source and termination of the fibers which 
constitute the bundle are still obscure. 



222 THE NERVOUS SYSTEM 

THE HYPOTHALAMUS 

The hypothalamus consists of three parts: (1) the pars optica hypothalami, 
which belongs to the telencephalon, (2) the pars mamillaris hypothalami, and 
(3) the subthalamus. 

The pars mamillaris hypothalami includes the corpora mamillaria, tuber 
cinereum, infundibulum, and hypophysis. The mammillary bodies are a pair of 
small spheric masses of gray matter, situated close together in the interpedun- 
cular space rostral to the posterior perforated substance (Figs. 86, 158, 159). 
Each is enclosed in a white capsule and projects as a rounded white eminence 
at the base of the brain (Fig. 156). In the sheep's brain the two are fused to- 
gether into a single eminence (Fig. 83). Each mammillary body is composed 
of two nuclear masses: a large medial group of small cells and a smaller lateral 
collection of large cells. The white capsule is formed by fibers from the hippo- 
campus, which sweep in a broad curve around the thalamus, forming a bundle 
known as thefornix (Figs. 204, 205). This descends in front of the interventric- 
ular foramen and reaches the mammillary body, within, which a large part of 
these fibers end. From the dorsal aspect of the medial nucleus springs a stout 
fascicle, which runs dorsally, to end in the anterior nucleus of the thalamus, and 
is known as the mammillothalamic tract or bundle of Vicq d'Azyr (Figs. 156, 204, 
205). A short distance from the mammillary body there branches off from this 
tract another, the mammillotegmental tract of Gudden, which runs caudally in 
the tegmentum of the mesencephalon and probably ends in the dorsal tegmental 
ganglion. The lateral nuclear mass is also connected with the tegmentum by 
way of the peduncle of the mammillary body (Fig. 211). 

The tuber cinereum, as seen -from the ventral surface of the brain (Figs. 
83, 86), is a slightly elevated gray area rostral to the mammillary bodies. It is 
one of the olfactory centers. To it there is attached the funnel-shaped stalk 
of the hypophysis, known as the infundibulum. The hypophysis is a small 
gland of internal secretion, which is not composed of nervous tissue and which 
interests us here only because its posterior portion is developed as an outpock- 
eting of the ventral wall of the diencephalon, to which it remains attached by 
the infundibulum. A detailed account of this structure may be found in the 
papers by Tilney (1911 and 1913) listed in the Bibliography at the end of this 
volume. 

The subthalamus is situated between the thalamus and the tegmentum of 
the mesencephalon and forms a zone of transition between these two struc- 
tures (Figs. 156, 157). The long sensory tracts of the tegmentum run through 



THE DIENCEPHALON AND THE OPTIC NERVE 



223 



it on their way to the thalamus. The red nucleus and the substantia nigra 
project upward into it from the mesencephalon. An additional mass of gray 
matter is found in this region lateral to the red nucleus and ventral to the thala- 
mus. It is known as the hypothalamic nucleus and has the shape of a biconvex 
lens. Its function and fiber connections are not well understood; but it is prob- 
ably a motor coordination center receiving fibers from the thalamus, corpus 
striatum, and pyramidal tract, and sending fibers downward in the cerebral 
peduncle. 

THE THIRD VENTRICLE 

Since the third ventricle is chiefly surrounded by structures belonging to the 
diencephalon, it will be convenient to consider it at this point and to give at 



Interventricular foramen Body of corpus callosum 
Anterior commissure | ' \ Body of fornix 
Septum pellucidum^ \ \ \ 

Rostral lamina \ \ \ 
Rostrum of corpus callosum. > \ \ '< 
Genu of corpus callosum , ', \ ', \ < 



Hippocampal com. Roofs of third ventricle or tela chorioidea 



Stria med. 

Habenular 

Trigone 



/' Haben. com. 
,' Splenium 
',' Pineal 
! i body / 



Suprapineal recess 
.Superior colliculus 
/Primary fissure 

/White center of vermis 




Olfactory bulb , . 

Medial olfactory gyms' /, 

Anterior perf. substance' / 

Lamina terminalis / 

Diagonal band 



Central canal 
\ Medulla 

Medial aperture of 
\ fourth ventricle ' 
\ \Tela chorioidea 
' Fourth ventricle 
'Anterior medullary 
velum 



Fig. 159. Medial sagittal section of the sheep's brain. 

the same time an account of the parts of the telencephalon which help to form 
its walls. These include the lamina terminalis, anterior commissure, and the 
optic chiasma (Figs. 158, 159). The latter, formed by the decusssation of the 
fibers of the optic nerve, projects as a transverse ridge in the floor of the ven- 
tricle. The lamina terminalis is a thin plate joining the two hemispheres, which 
stretches from the optic chiasma in a dorsal direction to the anterior commis- 
sure. Here it becomes continuous with the thin edge of the rostrum of the 
corpus callosum, known as the rostral lamina. As indicated on page 26, the 



224 THE NERVOUS SYSTEM 

lamina terminalis is to be regarded as forming the rostral end of the brain; 
and the part of the third ventricle, which lies behind it and dorsal to the optic 
chiasma, belongs to the telencephalon. The anterior commissure is a bundle of 
fibers which crosses the median plane in the lamina terminalis and serves to 
connect certain parts of the two cerebral hemispheres, which are associated with 
the olfactory nerves. The anterior commissure and the lamina terminalis form 
the rostral boundary of the third ventricle, and between the latter and the optic 
chiasma is a diverticulum, known as the optic recess. 

The third ventricle is a narrow vertical cleft, the lateral walls of which are 
formed for the greater part by the medial surfaces of the two thalami. Ventral 
to the massa intermedia is seen a groove known as the hypothalamic sulcus, which 
if followed rostrally leads to the interventricular foramen, while in the other 
direction it can be traced to the cerebral aqueduct. Below this groove the 
lateral wall and floor of the ventricle are formed by the hypothalamus. 

In the floor of the ventricle there may be enumerated the following structures, 
beginning at the rostral end: the optic chiasma, infundibulum, tuber cinereum, 
mammillary bodies, and the subthalamus. 

The roof of the third ventricle is formed by the thin layer of ependyma, which 
is stretched between the striae medullares thalami of the two sides, and whose 
torn edge, in the dissected specimen, is represented by the tania thalami (Figs. 
85, 155, 159). Upon the outer surface of this ependymal roof is a fold of pia 
mater in the transverse fissure. This is known as the tela chorioidea; and from 
it delicate vascular folds are invaginated into the ventricle, carrying a layer of 
ependyma before them by which they are, in reality, excluded from the cavity. 
These folds are the chorioid plexuses. There are two of them extending side by 
side from the interventricular foramina to the caudal extremity of the roof. 
Here they extend into an evagination of the roof above the pineal body, known 
as the suprapineal recess. 

There are three openings into the third ventricle. The aqueduct of the cere- 
brum opens into it at the caudal end; while at the opposite extremity it com- 
municates with the lateral ventricles through the two interventricular foramina. 

THE VISUAL APPARATUS 

Development of the Retina and Optic Nerve. There is but one pair of 
nerves associated with the diencephalon, and these, the optic nerves, are not 
true nerves, but fiber tracts joining the retinae with the brain. It will be re- 
membered that the retina develops as an pagination of the lateral wall of the 
prosencephalon in the form of a vesicle whose cavity is continuous with that of 



THE DIENCEPHALON AND THE OPTIC NERVE 



225 



the forebrain. By a folding of its walls in the reverse direction, i. e., by invag- 
ination, the optic vesicle becomes transformed into the optic cup (Fig. 15) ; and 
the cavity of the vesicle becomes reduced to a mere slit between the two layers 
forming the wall of the cup. The inner of these two layers develops into the 
nervous portion of the retina; and nerve-fibers arising in it grow back to the brain 
along the course of the optic stalk, which still connects the optic cup with the 
forebrain. This mode of development serves to explain why the structure of 
the retina resembles that of the brain more than it does that of other sense 
organs, and why the optic nerve-fibers, like those of the fiber tracts of the cen- 
tral nervous system, are devoid of neurilemma sheaths. These fibers take origin 
from the ganglion cells of the retina, the structure of which must be briefly con- 
sidered at this point. 



Optic nerve 

Optic chiasma 



GanglioniciStratum oplicum ^- 

neurons \ Ganglionic layer " 

( Inner molecular layer 

Bipolar } Inner nuclear layer 
neurons I 

^ Outer molecular layer 

Rod and f uter nuckar layer 

cone < Ex. limiting membrane 

neurons \Layerofrodsandcones 




Optic tract 

Lateral geniculate body 
Medial geniculate body 
i * Pulvinar 

Superior colliculus 



Fig. 160. Schematic representation of the retina and the connections established by the optic 

nerve-fibers. 

The retina presents for consideration three layers of superimposed nervous 
elements: (1) the visual cells, (2) the bipolar cells, and (3) the ganglion cells 
(Fig. 160). These, with some horizontally arranged association neurons and 
supporting elements, form the nervous portion of the retina and are derived 
from the inner layer of the optic cup. The pigmented stratum of the retina is 
derived from the outer layer of the cup. 

The visual cells are bipolar elements, whose perikarya are located in the 
outer nuclear layer (Fig. 160). Each presents an external process in the form of 
a rod or cone, so differentiated as to respond to photic stimulation and thus to 
serve as a visual receptor. The other process terminates in the outer molecular 
layer in relation to processes from the bipolar cells. These latter elements have 
their perikarya in the inner nuclear layer and branches in the inner and outer 
molecular layers. The ganglion cells send their dendrites into the inner molec- 
ular layer, where they are related to the inner branches of the bipolar cells; 
15 



226 



THE NERVOUS SYSTEM 



while the axons form the innermost stratum of the retina, the stratum opticum, 
through which they enter the optic nerve. It will be apparent from Fig. 160 
that the visual cells are the receptors and neurons of the first order in the optic 
path. The impulses are transmitted through the bipolar cells to the ganglion 
cells, whose axons, in turn, carry them by way of the optic nerves to the supe- 
rior colliculus, lateral geniculate body, and pulvinar of the thalmus. In the same 
figure it may be seen that the nerve also contains some efferent fibers which 
terminate in the retina (Arey, 1916). 

The Optic Chiasma and Optic Tracts. The optic nerve emerges from the 
bulbus oculi at the nasal side of the posterior pole and, after entering the cranium 
through the optic foramen, unites with its fellow of the opposite side to form the 

Pulvinar of thalamus Aqueduct of cerebrum Red nucleus 
Medial geniculate body, 

' \ .^fr ' *'' *j5^L/* > \^ ubstantia ni & ra 

Lateral geniculate body ^F ^Mlr ""* ''' iBPL X^lfc '' 'Base of peduncle 

\ 









Cerebral peduncle '' 

Optic tract -'xj 

4Q 
Posterior perforated substance '' 




Mammillary body 



* Anterior perforated substance 



Tuber cinereum 

Optic nerve 



'Olfactory trigone 

\ 

| Ynfundibulum 

Optic chiasma 



Fig. 161. The connections and relations of the optic tracts. The mesencephalon has been cut 
across and the specimen is viewed from below. (Sobotta-McMurrich). 

optic chiasma, in which a partial decussation of the fibers takes place (Figs. 
161, 162). Beyond the decussation fibers from both retinae are continued in 
each of the optic tracts. In the chiasma the fibers from the two optic nerves 
are so distributed that each tract receives the fibers from the lateral half of the 
retina of its own side and those from the medial half of the opposite retina. 
The optic tracts partially encircle the ends of the cerebral peduncles. Each 
tract divides into a medial and a lateral root, of which the former goes to the 
medial geniculate body and does not consist of optic nerve-fibers. The lateral 
root is much larger and runs to the lateral geniculate body and pulvinar of the 
thalamus and to the superior colliculus of the corpora quadrigemina. In addi- 
tion to the optic fibers each tract contains a bundle of fibers, known as the com- 



THE DIENCEPHALON AND THE OPTIC NERVE 



227 



mis sure of Gudden, which crosses the median plane in the posterior part of the 
optic chiasma and, for the most part at least, connects the medial geniculate 
bodies of the two sides. These are the fibers which form the medial root of the 
optic tract. 

The Optic Radiation. The superior colliculus is a reflex center, and the fibers 
of the optic nerve, which terminate in it, subserve optic reflexes. On the other 
hand, the visual impulses, brought to the external geniculate body and the pul- 



Superior oblique muscle 
Retina 

Optic nerve 
Optic chiasma 

Commissure of Gudden 
Trochlear nerve 
O.ptic trad 
Thalamus 

Medial geniculate body 
Lateral genictilate body 




"~ -Superior colliculus 
~~- Inferior colliculus 
^^Nucleus oftrochlear nerve 
radiation 



Cuneus 



Occipital pole 



Fig. 162. Schematic representation of the optic pathways. The index line to the commissure of 

Gudden does not reach that structure. 

vinar of the thalamus, are relayed to the cerebral cortex and give rise to visual 
sensations. These two parts of the diencephalon are connected with the cere- 
bral cortex on both sides of the calcarine fissure by projection fibers, which 
form a conspicuous bundle that sweeps backward through the retrolenticular 
portion of the internal capsule into the occipital lobe. It is known as the optic 
radiation (Fig. 162). In addition to corticipetal fibers arising in the pulvinar 
and lateral geniculate body, the optic radiation contains corticifugal fibers 



228 THE NERVOUS SYSTEM 

arising in the cortex and terminating in the pulvinar, lateral geniculate body, 
and superior colliculus of the corpora quadrigemina. 

The significance of the partial decussation of the nerves is made clear by 
Figs. 162 and 163. The properties of the refracting media of the eyes are such 
that images of objects to the left of the axis of vision are produced on the nasal 
side of the left eye and the temporal side of the right eye. And, due to the man- 
ner of decussation of the optic nerve-fibers, impulses from both these sources 
reach the visual area of the right cortex. In the same way the visual cortex 
of the left side receives impressions from objects to the right of the axis of vision. 
That is to say, the sensory representation of the outer world in the cerebral 
cortex is contralateral in the case of sight just as it is in the case of cutaneous 




Fig. 163. Diagram to show why a destruction of one optic tract causes blindness in both eyes for 
the opposite lateral half of the field of vision. 

and auditory sensations. Furthermore, it will be evident that, while destruc- 
tion of one optic nerve causes total blindness in the corresponding eye, destruc- 
tion of one optic tract, its thalamic connections, their optic radiations, or the 
visual cortex in which these radiations terminate, will produce blindness in both 
eyes for the opposite lateral half of the field of vision. This condition is known 
as hemianopsia, and is produced by a lesion in the optic pathway anywhere be- 
hind the chiasma. 



CHAPTER XV 

THE EXTERNAL CONFIGURATION OF THE CEREBRAL 
HEMISPHERES 

Development. The cerebral hemispheres are formed by the evagination of 
the alar laminae of the telencephalon, the rest of which remains as the boundary 
of the rostral part of the third ventricle, and is known as the telencephalon 
medium. The cavities of the evaginated portions are known as the lateral ven- 
tricles and communicate with the third ventricle by way of the interventricu- 
lar foramina (Figs. 15-17). Each of the cerebral hemispheres consists of two 
ventrally placed portions, the rhinencephalon or olfactory lobe and corpus stria- 
turn, and a third part, more extensive than the others, the pallium or primitive 
cerebral cortex. The pallium expands more rapidly than the other parts, both 
rostrally and caudally, and comes to overlie the diencephalon, from which it is 
separated by the transverse fissure (Fig. 17). The fold of pia mater which is 
inclosed within this fissure is known as the tela chorioidea; and from it a vascular 
plexus grows into the lateral ventricle through the thin portion of the medial 
wall of the hemisphere, where this is attached to the diencephalon. This forms 
the chorioid plexus of the lateral ventricle and carries before it an epithelial cover- 
ing from the ependymal lining, by which it is, in reality, excluded from the 
ventricular cavity. This invagination of the medial wall of the hemisphere 
produces the chorioid fissure. Ventrally the thickened part of the hemisphere, 
known as the corpus striatum, remains in uninterrupted continuity with the 
thalamus. 

At first the cerebral hemisphere has a relatively large cavity and thin walls. 
As the pallium and ventricle enlarge they become bent around the thalamus 
and corpus striatum (Fig. 17). The hemisphere becomes bean shaped and 
the cavity curved. It expands rostrally to form the frontal lobe, caudally to 
form the occipital lobe, and ventrolaterally to form the temporal lobe (Fig. 164). 
Into each of these there is carried a prolongation of the lateral ventricle forming 
respectively the anterior, posterior, and inferior horns. Between the temporal 
and frontal lobes a deep fossa appears which is the forerunner of the lateral 
fissure. At the bottom of this fossa is the insula, a portion of the cortex which 

229 



230 



THE NERVOUS SYSTEM 



overlies the corpus striatum and develops more slowly than the surrounding areas 
(labelled lateral fissure. Fig. 164). Folds from the surrounding cortex close in 
over the insula, burying it from sight in the adult brain. These folds are known 
as the opercula, and the deep cleft which separates them as the lateral fissure. 

Development of the Cerebral Cortex. At first the pallium, like other parts 
of the neural tube, consists of three primitive zones: the ependymal, mantle, 
and marginal layers. But during the third month neuroblasts migrate outward 
from the ependymal and mantle layers into the marginal zone and there give 
rise to a superficial layer of gray matter the cerebral cortex. Nerve-fibers 
from these neuroblasts and others growing into the hemisphere from the thala- 



Suicus postcenlralls 



Sulcus centralis 



Lobus 
parietalis 
superior 
Supra- 




Occipital 
pole 



Inferior 

frontal 

sulcus 

Ascend- 
ing 
ramus 
Lateral 
fissure 
(Syhii) 



Temporal 
lobe 



Superior temporal gyms Middle temporal gyrus 
Fig. 164. Lateral view of the right cerebral hemisphere from a seven months' fetus. (Kollmann.) 

mus accumulate on the deep surface of the developing cortex and form the 
white medullary substance of the hemisphere. As the brain increases in size 
the area of the cortex expands out of proportion to the increase in volume of 
the white medullary layer upon which it rests, and is thrown into folds or gyri 
separated by fissures or sulci. All the larger mammalian brains present well- 
developed gyri, while the smaller brains are smooth; and it would thus appear 
that the size of the brain is an important factor in determining the* amount of 
folding that occurs in the cortex. 

As we shall learn, the cortex does not differentiate in exactly the same man- 
ner throughout, but may be subdivided into structurally and functionally dis- 



THE EXTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES 231 

tinct areas. The sulci develop in more or less definite relation to these areas, 
the great majority making their appearance along the boundary lines between 
them. These are known as terminal sulci, of which the rhinal fissure and central 
sulcus are examples. Sometimes the folding occurs entirely within such an 
area, i. e., along its axis, producing what is known as an axial sulcus. But 
there are still others in which the relation to these functional areas is not so evi- 
dent. The arrangement of the fissures and sulci in a seven month fetus is shown 
in Fig. 164. 

The Development of the Septum and Commissures. The two hemispheres 
are connected by the lamina terminalis, which serves as a bridge for fibers which 
cross from one hemisphere to the other. These form three important bundles: 





Fig. 165. Schematic representation of the development of the septum pellucidum and 
telencephalic commissures: A. C., Anterior commissure; C. C., corpus callosum; C. F., columna 
fornicus; C. S. P., cavum septi pellucidi; F., fornix; H. C., hippocampal commissure; /. F., 
interventricular foramen; Fis., chorioid fissure; L. T., lamina terminalis. (Based on drawings of 
models of the telencephalon of a four months' fetus (.4) and of a five months' fetus (B) by Streeter.) 

the anterior commissure, the hippocampal commissure, and the corpus callosum. 
The two former connect the olfactory portions of the hemispheres, while the 
latter is the great commissure of the non-olfactory cortex or neopallium. Every- 
one admits that the anterior commissure develops in the lamina terminalis 
(Fig. 165); and the corpus callosum and hippocampal commissures are said to 
form in its dorsal part (Streeter, 1912). According to this account the lamina 
terminalis becomes stretched by the great development of the corpus callosum 
and appropriates part of the paraterminal body. This is the portion of the 
rhinencephalon that lies immediately rostral to the lamina terminalis in the 
medial wall of each hemisphere. Eventually the lamina terminalis presents a 
large cut surface in the median sagittal section and includes the commissures 



232 THE NERVOUS SYSTEM 

as well as the septum pellucidum. The portion of the lamina terminalis which 
enters into the formation of the septum becomes hollow as a result of the stretch- 
ing to which it is subjected, and the resulting cavity is known as the cavum septi 
pellucidi. 

The cerebral hemispheres are incompletely separated from each other by 
the longitudinal fissure of the cerebrum, at the bottom of which lies a broad band 
of commissural fibers, the corpus callosum, which forms the chief bond of union 
between them. Each hemisphere has three surfaces: a convex dorsolateral 
surface (Fig. 166), a median surface flattened against the opposite hemisphere 
(Fig. 170), and a very irregular ventral or basal surface. A dorsal border sepa- 
rates the dorsolateral from the medial surface; and a lateral border marks the 
transition between the dorsolateral and basal surfaces. One may recognize 
also frontal, occipital, and temporal poles (Fig. 166). The long axis of the hemi- 
sphere extends between the frontal and occipital poles, and in man is placed 
almost at right angles to the long axis of the body (Fig. 33) ; while in other mam- 
mals it corresponds more nearly to the body axis. On this account it will be 
convenient in the description of the human cerebral hemisphere to take the 
occiput as a point of reference and use the term "posterior" in place of "caudal." 
Otherwise our directive terms remain the same rostral, dorsal, and ventral 
except that for the term "ventral" we shall often use the word "basal." 

The cerebral cortex is a layer of gray matter spread over the surface of the 
hemisphere; and its area is greatly increased by the occurrence of folds or gyri 
separated by deep sulci. That part of the cortex which belongs to the rhinen- 
cephalon and is phylogenetically the oldest is designated as the archipallium. 
It is separated from the newer and in mammals much larger neopallium or non- 
olfactory cortex by the rhinal fissure (Figs. 83, 171). 

The Neopallium. The development of the neopallium is so much greater 
in man than in the sheep, and the arrangement of the gyri and sulci is so dif- 
ferent in the two forms that but little can be learned by a cursory comparison of 
these structures in the two brains. We shall, accordingly, confine our atten- 
tion almost exclusively to the arrangement of the neopallium in man. 

THE DORSOLATERAL SURFACE OF THE HEMISPHERE 

By means of some of the more important sulci the cortex is marked off into 
well-defined areas, known as the frontal, parietal, temporal, and occipital lobes 
(Fig. 167). To these should be added a lobe buried at the bottom of the lateral 
fissure and known as the insula (Fig. 169). In the delimitation of these lobes 



THE EXTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES 



233 



the lateral fissure and the central sulcus play a prominent part. Some of the 
more important sulci are designated as fissures. This usage is regulated by 
custom, but it may be said that a number of the fissures are invaginations of 
the entire thickness of the wall of the hemisphere and produce corresponding 
elevations projecting into the lateral ventricle. 

The lateral cerebral fissure, or fissure of Sylvius, begins on the basal sur- 
face of the brain as a deep cleft lateral to the anterior perforated substance 
(Fig. 172). From this point it extends lateralward between the temporal and 
frontal lobes to the lateral aspect of the brain, where it divides into three branches 
(Figs. 166, 167). The anterior horizontal ramus of the lateral fissure runs ros- 



Operculum 



Opercular portion of inferior 
frontal gyr, 



Superior frontal 
gyrus 



Middle frontal 
gyrus 



Frontal pole 
Triangular portion ** 
of inf. front, gyrus ..* 

Lateral cerebral fissure^'' 

Temporal pole' 

Superior temporal gyrus-' 
Superior temporal sulcus'' 

Middle temporal gyrus 

Middle temporal sulcus 
Inferior temporal gyrus 



Precentral sulcus 

f Anterior central gyrus 
Central sulcus 

' Posterior central gyrus 
'' Inter parietal sulcus 




Supramarginal gyrus 
Interparietal sulcus 
Angular gyrus 

Superior parietal 
- lobule 
'-^-Inferior parietal 

lobule 

-'' Parieto-occipital 
fissure 

^Lateral occipital 
gyri 



Occipital pole 
Transverse occipital sulcus 
\ Superior temporal sulcus 
Posterior limb of lateral cerebral fissure 



Fig. 166. Lateral view of the human cerebral hemisphere. (Sobotta-McMurrich.) 

trally and the anterior ascending ramus dorsally into the frontal lobe. The 
posterior ramus of the lateral fissure is much longer, and runs obliquely toward the 
occiput and at the same time somewhat dorsally. The terminal part turns 
dorsally into the parietal lobe. This fissure is, in reality, a deep fossa, at the 
bottom of which lies the insula. It separates the frontal and parietal lobes 
which lie dorsal to it from the temporal lobe. 

The central sulcus or fissure of Rolando runs obliquely across the dorsolateral 
surface of the hemisphere, separating the frontal from the parietal lobe (Figs. 
166, 167). It begins on the medial surface of the hemisphere a little behind the 
middle of the dorsal border and extends in a sinuous course rostrally and toward 



234 



THE NERVOUS SYSTEM 



the base, nearly reaching the posterior ramus of the lateral fissure. It makes 
an angle of about 70 degrees with the dorsal border. It is customary to recog- 
nize two knee-like bends in this sulcus; one located at the junction of the dorsal 
and middle thirds with concavity forward, and the other at the junction of the 
middle and basal thirds with concavity backward. If the margins of the sulcus 
are pressed apart a deep annectant gyrus may often be seen extending across 
it, by which the continuity of the sulcus is to some extent interrupted. This is 
explained by the fact that the sulcus usually develops in two pieces, which be- 
come united as the depth of the sulcus increases. 

Lobes, The frontal lobe lies dorsal to the lateral cerebral fissure and rostral 
to the central sulcus (Fig. 167). The remainder of the dorsolateral surface is 
subdivided rather arbitrarily into the parietal, occipital, and temporal lobes. 




Frontal pole 

Lateral ( Ant. hor. ram. 
cerebral I Ant. ascend, ram. ' 
fissure { Post. ram. " 

Temporal pole' 



-Parietal lobe 
,'- Temporal lobe 

-- Parieto-occipital fissure 
r - Occipital lobe 

Preoccipital notch 
''Occipital pole 



Fig. 167. Diagram of the lobes on the lateral aspect of the human cerebral hemisphere. 

The rostral border of the occipital lobe is usually placed at a line joining the end 
of the parieto-occipital fissure with the preoccipital notch. The latter is a 
slight indentation on the lateral border of the hemisphere about 4 cm. rostral 
to the occipital pole; while the parieto-occipital fissure is a deep cleft on the 
median surface (Fig. 170), which cuts through the dorsal border about midway 
between the occipital pole and the central sulcus, but a little nearer the former. 
The parietal lobe is situated between the central sulcus and the imaginary line 
joining the parieto-occipital fissure with the preoccipital notch. It lies dorsal 
to the lateral fissure and an imaginary line connecting that fissure with the 
middle of the preceding line. The remainder of the dorsolateral surface belongs 
to the temporal lobe. 

The Frontal Lobe. The rostral part of the hemisphere is formed by the 



THE EXTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES 



235 



frontal lobe. Within it one may identify three chief sulci, which are, however, 
subject to considerable variation. The precentral sulcus is more or less parallel 
with the central sulcus and is often subdivided into two parts, the superior and 
inferior precentral sulci (Fig. 168). The superior frontal sulcus usually begins 
in the superior precentral sulcus and runs rostrally, following in a general way 
the curvature of the dorsal border of the hemisphere which it gradually ap- 
proaches. The inferior frontal sulcus usually begins in the inferior precentral 
sulcus and extends rostrally, arching at the same time toward the base of the 
hemisphere. 

Between the precentral and central sulci lies the anterior central gyrus in 
which is found the motor area of the cerebral cortex. The remainder of this 



Anterior central gyrus 

Superior precentral sulcus \ 

Superior frontal gyrus 

Superior frontal sulcus ----- .^^ 
Middle frontal gyrus-. 
Middle frontal sulcus -/^ 
Inferior frontal sulcus V_ ~,~* 

Inferior precentral sulcus I 

Inf. (Parsopercularis -1 ~_~~~ 

front. \ Pars triang. -4 

gynts\ Pars orbitalis-\ _ 



Lateral ( Ant. nor. ram. 
cerebral] Ant. ascend, ram.'' / 
fsstire ( Post, ram..''' 

Superior temporal siikus*'' 
Superior temporal gyrus 




, Posterior central gyrus 
Postccntral sulcus 



,,Supramarg. gyrusl 
^Angular gyrus 

''Superior parietal lobule 
Interparielal sulcus 



Trans, occipital sulcus 



Sulcus lunatus 
* \Infer tor temporal gyrus 
\ Middle temporal sulcus 
Middle temporal gyrus 



Fig. 168. Sulci and gyri on the lateral aspect of the human cerebral hemisphere. 

surface of the frontal lobe is composed of three convolutions, the superior, 
middle, and inferior frontal gyri, separated from each other by the superior and 
inferior frontal sulci. The inferior frontal gyrus, which in the left hemisphere 
is also known as Broca's convolution, is subdivided by the two anterior rami of 
the lateral sulcus into three parts, known as the orbital, triangular, and oper- 
cular portions. The orbital part of the inferior frontal gyrus lies rostral to the 
anterior horizontal ramus of the lateral sulcus; the triangular part is a wedge- 
shaped convolution between the two anterior rami of that fissure; while the 
opercular portion lies in the frontal operculum between the precentral sulcus 
and the anterior ascending ramus of the lateral fissure. 

The Temporal Lobe. Ventral to the lateral fissure is the long tongue-shaped 



236 THE NERVOUS SYSTEM 

temporal lobe which terminates rostrally in the temporal pole. The superior 
temporal sulcus is a very constant fissure, which begins near the temporal pole 
and runs nearly parallel with lateral cerebral fissure. Its terminal part turns 
dorsally into the parietal lobe. The middle temporal sulcus, ventral to the pre- 
ceding and in general parallel with it, is usually composed of two or more dis- 
connected parts. The inferior temporal sulcus is located for the most part on 
the basal surface of the temporal lobe. Dorsal to each of these fissures is a 
gyrus which bears a similar name: the superior temporal gyrus, between the 
lateral fissure and the superior temporal sulcus; the middle temporal gyrus, be- 
tween the superior and middle temporal sulci; and the inferior temporal gyrus, 
between the middle and inferior temporal sulci. The lateral fissure is very deep ; 
and the surface of the superior temporal gyrus that bounds it is broad and marked 
near its posterior extremity by horizontal convolutions, known as the transverse 
temporal gyri. One of these, more marked than the others, has been called the 
anterior transverse temporal gyrus or Heschl's convolution and represents the 
cortical center for hearing (Fig. 174). 

The Parietal Lobe. The postcentral sulcus runs nearly parallel with the 
central sulcus and consists of two parts, the superior and inferior postcentral 
sulci, which may unite with each other or with the inter parietal sulcus. Often 
all three are continuous, forming a complicated fissure, as shown in Fig. 168. 
The interparietal sulcus extends in an arched course toward the occiput and 
may end in the transverse occipital sulcus. These four sulci are often included 
under the term "interparietal sulcus." The interparietal sulcus proper is then 
designated as the horizontal ramus. 

The posterior central gyrus lies between the central and postcentral sulci. 
The interparietal sulcus separates the superior parietal lobule from the inferior 
parietal lobule. Within the latter we should take note of two convolutions: 
the supramarginal gyrus, which curves around the upturned end of the lateral 
fissure; and the angular gyrus, similarly related to the terminal ascending por- 
tion of the superior temporal fissure. 

The Occipital Lobe. Only a small part of the dorsolateral surface of the 
hemisphere is formed by the occipital lobe. This is a triangular area at the 
occipital extremity, bounded rostrally by a line joining the parieto-occipital 
fissure and the preoccipital notch (Fig. 167). The transverse occipital fissure 
may help to bound this area or may lie within it. Other inconstant sulci help 
to divide it into irregular convolutions. Sometimes the visual area which lies 
on the mesial aspect of this lobe is prolonged over the occipital pole to the lateral 



THE EXTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES 



2 37 



aspect. In this case a small semilunar furrow develops around it on the lateral 
surface and is known as the sulcus lunatus (Fig. 168). This sulcus, called by 
Riidinger the "Affenspalte," forms a conspicuous feature of the lateral surface of 
the cerebral hemisphere in the lower Old World apes (Ingalls, 1914). 

The Insula. The part of the cortex which overlies the corpus striatum lags 
behind in its development and becomes overlapped by the surrounding pallium. 
The cortex, which thus becomes hidden from view at the bottom of the lateral 
fissure, forms in the adult a somewhat conical mass called the insula or island of 
Reil (Fig. 169). Its base is surrounded by a limiting furrow, the circular sulcus, 
which is, however, more triangular than circular, and in which we may recognize 
three portions: superior, inferior, and anterior. The apex of this conical lobe 



Parietal lobe 



,/ Central sulcus of insula 
Circular sulcus 
, Frontal lobe 



Occipital lobe x 




V?- Short gyri of insula 



Temporal lobe i ong gyrus O f i nsu la 

Fig. 169. Lateral view of the human cerebral hemisphere with the insula exposed by removal of 

the opercula. (Sobotta-McMurrich.) 

is known as the limen insulce; and the remainder is subdivided by an oblique 
groove (sulcus centralis insulae) into the long gyrus of the insula and a more 
rostral portion, which is again subdivided into short gyri. 

The Operculum. As the adjacent portions of the pallium close over the 
insula (Fig. 164) they form by the approximation of their margins the three 
rami of the lateral fissure. These folds constitute the opercula of the insula. 
Each of the three surrounding lobes takes part in this process; and we may 
accordingly recognize a, frontal, a temporal, and a parietal operculum (Fig. 166). 

At this point it will be instructive to examine the lateral surface of the cerebral 
hemisphere of the sheep. It will be seen that the region which corresponds to 
the insula is on a level with the general surface of the hemisphere; no opercula 
have developed, and the lateral sulcus is only a shallow groove (Fig. 173). 



2 3 8 



THE NERVOUS SYSTEM 



THE MEDIAN AND BASAL SURFACES 

The occipital lobe comes more nearly being a structural and functional 
entity than any of the other lobes. It corresponds in a general way to the 
"regio occipitalis" as outlined by Brodman (Figs. 216, 217), and it is probably 
all concerned directly or indirectly with visual processes. We have seen that 
it forms a small convex area on the lateral surface near the occipital pole; 
and we now note that it is continued on to the medial surface of the hemi- 
sphere, where it forms a somewhat larger triangular field between the parieto- 
occipital and anterior portion of the calcarine fissure dorsorostrally and the 



Sulcus cinguli 
Sulcus of corpus callosum 




Sup. fronta 

gyrus'-- 

Frontal par. of 

stilcus cinguli 

Frontal pole 

Genu of corp. cat.' 

Septum pellucidum 
Rost. of corpus callosum 
Anterior parolfactory sulcus / / / ^ 
Par olfactory area < I 

Temporal pole jUncus 
Anterior commissure Fimbria 

Hippocampal gyms 



J3ody of corpus callosum 
' Paracentral lobule 
-' ' / Central sulcus 

Marginal portion of sulcus cinguli 
Precuneus 
'Column of fornix 
-'Subparietal sulcus 
'.Cms of fornix 

..- Parieto-occip. fis. 

Splen. of corp. cal. 
\.,-lsth. of gyms 
fornicatus 
j Cuneus 

Calcarine 
Ussure 



'Occipital pole 



' Lingual gyms 
Inferior temporal gyrus 
Inferior temporal sulcus 
Fusiform gyrus 

Collateral fissure 
Fasciola cinerea 



Fig. 170. Human cerebral hemisphere seen from the medial side. The brain has been 
divided in the median plane and part of the thalamus has been removed along with the mesen- 
cephalon and rhombencephalon. (Sobotta-McMurrich.) 

collateral fissure ventrally. On this aspect of the brain it includes two constant 
and well-defined convolutions: the cuneus and the lingual gyrus (Figs. 170, 
171). 

The calcarine fissure begins ventrally to the splenium of the corpus callosum 
and extends toward the occipital pole, arching at the same time somewhat 
dorsally. It consists of two portions. The rostral part, the calcarine fissure 
proper, is deeper, more constant in form and position, and phylogenetically 
much older than the rest, and produces the elevation on the wall of the lateral 
ventricle known as the calcar avis (Fig. 181). This part terminates at the point 



THE EXTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES 239 

where the calcarine is joined by the parieto-occipital fissure. The other portion, 
sometimes called the ''posterior calcarine sulcus," arches downward and back- 
ward from this junction toward the occipital pole, and occasionally cuts across 
the border of the hemisphere to its dorsolateral surface. The parieto-occipital 
fissure, which is really a deep fossa with much buried cortex at its depth, appears 
to be the direct continuation of the rostral part of the calcarine fissure. It cuts 
through the dorsal border of the hemisphere somewhat nearer to the occipital 
pole than to the central sulcus. These fissures form a Y-shaped figure whose 
stem is the calcarine fissure and whose two limbs are the parieto-occipital fissure 
and the "posterior calcarine sulcus." If the fissures are opened up the stem is 
seen to be marked off from the two limbs by buried annectant gyri. 

The cuneus is a triangular convolution with apex directed rostrally, which 
lies between the diverging parieto-occipital and calcarine fissures. The rest of 
the medial surface of the occipital lobe belongs to the lingual gyrus, which lies 
between the calcarine and collateral fissures. 

The remaining sulci and gyri on the median and basal surfaces may now be 
briefly described. 

The sulcus of the corpus callosum (sulcus corporis callosi) begins ventrally to 
the rostrum of the corpus callosum, encircles that great commissure on its con- 
vex aspect, and finally bends around the splenium to become continuous with 
the Mppocampal fissure (Fig. 171). The latter is a shallow groove, which runs 
from the region of the splenium of the corpus callosum toward the temporal 
pole near the dorsomedial border of the temporal lobe. It terminates in the 
bend between the hippocampal gyrus and the uncus. 

The sulcus cinguli (callosomarginal fissure) begins some distance ventral 
to the rostrum of the corpus callosum and follows the arched course of the 
sulcus of the corpus callosum, from which it is separated by the gyrus cinguli. 
It terminates by dividing into two branches. One of these, the sub parietal 
sulcus, continues in the direction of the sulcus cinguli and ends a short distance 
behind the splenium. The other, known as the marginal ramus, turns off at a 
right angle and is directed toward the dorsal margin of the hemisphere. A side 
branch, directed florsally, is usually given off from the main sulcus some dis- 
tance rostral to its bifurcation, and is known as the paracentral sulcus. 

The collateral fissure begins near the occipital pole and runs rostrally, sepa- 
rated from the calcarine and hippocampal fissures by the lingual and hippo- 
campal gyri. It is sometimes continuous with the rhinal fissure. The latter 
separates the terminal part of the hippocampal gyrus, which belongs to the archi- 



240 



THE NERVOUS SYSTEM 



pallium, from the rest of the temporal lobe, and is a very conspicuous fissure in 
most mammalian brains (Fig. 83) . 

Convolutions. Dorsal to the corpus callosum is the gyrus cinguli between 
the sulcus of the corpus callosum and the sulcus cinguli. The superior frontal 
gyrus is continued over the dorsal border of the hemisphere from the dorso- 
lateral surface and reaches the sulcus cinguli. Surrounding the end of the 
central sulcus is a quadrilateral convolution, known as the paracentral lobule. 
It is bounded by the sulcus cinguli, its marginal ramus and the paracentral 
sulcus. Another quadrilateral area, known as the precuneus, is bounded by 
the parieto-occipital fissure, the subparietal sulcus, and the marginal ramus of 
the sulcus cinguli. The hippocampal gyrus lies between the hippocampal fissure 



Superior frontal gyrus 

Sulcus of corpus callosum ,- x 
Gyrus cinguli -. 
Sulcus cinguli -~,,^ 
Corpus callosum-,,^ 
Gyrus fornicatus - 
Frontal lobe- 

Post. par olfactory sulcus -- 
Parolfactory area- 



Ant, par olfactory sulcus''' 

Temporal lobe ' 



S. centralis 
Paracentral sulcus 




Paracentral lobule 

- - Parietal lobe 
*- Marginal ramus 
__..-- Precuneus 

vV Subparietal sulcus 

^'\- - - Parieto-occipital fissure 

- Cuneus 

-- Calcarine fissure 

- Lingual gyrus 

, . Isthmus of gyrus 

fornicatus 
" Hippocampal fissure 



Rhinal fissure ' 

Uncus ' Hippocampal gyrus J 

Inf. temporal gyrus 



Collateral fissure 
Fusiform gyrus 



Inferior temporal sulcus 



Fig. 171. Diagram of the lobes, sulci, and gyri on the medial aspect of the human cerebral 

hemisphere. 

dorsally and the collateral and rhinal fissures ventrally. Its rostral extremity 
bends around the hippocampal fissure to form the uncus. It is connected with 
the gyrus cinguli by a narrow convolution, the isthmus of the gyrus fornicatus. 
Under the name gyrus fornicatus it has been customary to include the gyrus 
cinguli, isthmus, hippocampal gyrus, and uncus. Between the collateral fissure 
and the inferior temporal sulcus is the fusiform gyrus which lies on the basal 
surface of the temporal lobe in contact with the tentorium of the cerebellum 
(Figs. 170, 172). 

It has been customary to apportion parts of the medial and basal surfaces 
of the cerebral hemisphere to the frontal, parietal, occipital, and temporal 
lobes, as indicated in Fig. 171. According to this scheme the gyrus fornicatus 



THE EXTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES 



241 



stands by itself and is sometimes designated as the limbic lobe. This plan of 
subdivision, which was based on the erroneous belief that all portions of the 
gyrus fornicatus belonged to the rhinencephalon, should be abandoned. A 
simpler and more logical arrangement assigns the hippocampal gyrus and uncus 
to the temporal lobe and divides the gyrus cinguli between the frontal and 
parietal lobes. 



Longitudinal fissure of cerebrum 
/ Frontal pole 

rectus 
^Olfactory sulcus 

Orbital sulci 

Olfactory trigone 
Mammillary body 
.-Uncus 



L Middle temporal sulcus 
Base of cerebral peduncle 
Substantia nigra 



Optic chiasma 

Orbital gyri 
Anterior perforated substance^ 

Temporal pole._ 



Lateral cerebral 
(Sylvian) fissure 



Middle temporal sulcus-.. 



Tuber cinereum- 

I'VE* "*TWk TjiW ""* 3* !^^fc ^^^^B^^^T m j^s t ^H9T 

^Inferior temporal gyrus 

Fusiform gyrus 

Hippocampal gyrus 
Corpus quadrigeminum 
Isthmus of gyms fornicatus 
Lingual gyrus 
"Gyrus cinguli 

\ Splenium of corpus callosum 
\ Parieto-occipital fissure 
Occipital pole 

Fig. 172. Basal aspect of the human cerebral hemisphere. (Sobotta-McMurrich.) 

The basal surface of the hemisphere (Fig. 172) consists of two parts: (1) 
the ventral surface of the temporal lobe, whose sulci and gyri have been de- 
scribed in a preceding paragraph, and which rests upon the tentorium cerebelli 
and the floor of the middle cranial fossa; and (2) the orbital surface of the frontal 
lobe resting upon the floor of the anterior cranial fossa. The latter surface 
presents near its medial border the olfactory sulcus, a straight, deep furrow, 
directed rostrally and somewhat medially, that lodges the olfactory tract and 
bulb. To its medial side is found the gyrus rectus. The remainder of the 
orbital surface of the frontal lobe is subdivided by irregular orbital sulci into 
equally irregular orbital gyri. 

16 



Hippocampal fissure 



Collateral fissure' 
Inferior temporal sulcus' 




Cerebral aqueduct 

Collateral fissure 



CuneuS 



242 



THE NERVOUS SYSTEM 



From the foregoing account it will be apparent that almost the entire sur- 
face of the human cerebral hemisphere is formed by neopallium. Of the parts 
already described only the uncus and adjacent part of the hippocampal gyrus 
belong to the archipallium. Other superficial portions of the rhinencephalon, 
such as the olfactory bulb, tract and trigone, and the anterior perforated sub- 
stance, will be described in connection with the hidden parts of the rhinen- 
cephalon in Chapter XVII. 

Suprasylvian fissure 



Cerebral hemisphere ^ 
Cerebellum 
Postmedian lobule K, 
Ansiform lobule'r- 
Parafiocculus\~ 
Paramedian lobule*' - 
Flocculus 1 



; Lateral fissure 
Insula 



Chorioid plexus of 
fourth ventricle 




XII' 
XI' 



X // / / / 
1X''/ VIII; : V 'IV / 
Olive VII '; VI 
Trapezoid body 



Pan* 



Rhinal Opic 
fissure fissure 
I Mammillary body 
Hippocampal gyrus 
Cerebral peduncle 



\ Olfactory bulb 

Lateral olfactory gyrus 



Fig. 173. Lateral view of the sheep's brain. 

The surface form of the cerebral hemisphere of the sheep is illustrated in 
Figs. 83, 84, and 173. On these figures are indicated the names of the chief 
sulci and gyri. It will be of interest to note the position of the motor cortex 
in the sheep as given in Fig. 82. Since this corresponds to the precentral gyrus 
in man, it will be seen that there is little in the sheep's brain to correspond to the 
rostral part of the frontal lobe in man. 



CHAPTER XVI 

THE INTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES 

WHEN a horizontal section is made through the cerebral hemisphere at the 
level of the dorsal border of the corpus callosum the central white substance 
will be displayed in its maximum extent and will appear as a solid, semioval 
mass, known as the centrum semiovale (Figs. 174, 175). It will also be apparent 
that lamellae extend from this central white substance to form the medullary 
centers of the various convolutions, and that over this entire mass the cortex is 
spread in an uneven layer, thicker over the summit of a convolution than at 
the bottom of a sulcus. This medullary substance is composed of three kinds 
of fibers: (1) fibers from the corpus callosum and other commissures joining the 
cortex of one hemisphere with that of the other; (2) fibers from the internal cap- 
sule, uniting the cortex with the thalamus and lower lying centers; and (3) 
fibers running from one part of the cortex to another within the same hemi- 
sphere (p. 296). 

The Corpus Callosum. At the bottom of the longitudinal fissure of the 
cerebrum is a broad white band of commissural fibers, known as the corpus 
callosum, which connects the neopallium of the two hemispheres. While the 
medial portion of this commissure is exposed in the floor of the longitudinal 
fissure, its greater part is concealed in the white center of the hemisphere where 
its fibers radiate to all parts of the neopallium, forming the radiation of the 
corpus callosum. When examined in a median sagittal section of the brain the 
corpus callosum is seen to be arched dorsally and to be related on its ventral 
surface to the fornix and septum pellucidum (Figs. 84, 158, 170). The latter 
consists of two thin membranous plates, stretched between the corpus callosum 
and the fornix and separated by a narrow cleft-like space, the cavum septi 
pellucidi (Fig. 177). If the septum has been torn away it will be possible to 
look into the lateral ventricle and see that the corpus callosum forms the roof 
of a large part of that cavity. At its rostral extremity it curves abruptly toward 
the base of the brain, forming the genu, and then tapers rapidly to form the 
rostrum. The latter is triangular in cross-section, with its edge directed toward 
the anterior commissure to which it is connected by the rostral lamina. The 

243 



244 



THE NERVOUS SYSTEM 



body of the corpus callosum (truncus corporis callosi), arching somewhat dor- 
sally, extends toward the occiput and terminates in the splenium, a thickened 
rounded border situated dorsal to the pineal body and corpora quadrigemina. 
Related to the concave or ventral side of the corpus callosum are the fornix, 
septum pellucidum, lateral ventricles, tela chorioidea of the third ventricle, and 
the pineal body (Fig. 170). 



Genii of corpus callosum 



Cingulum (cut) ~^pF 



Corpus callo-... 
sum 

Centrum semi- 
ovale 

Medial longi-.. 
ludinal stria 



Cingulum (cut) J 



Splenium of _,, 
corp. callosum" 




Frontal part of 
' radiation of 
corp. callosum 

Intersection of 
fibers from cor- 
. pus callosum 
and corona 
radiata 

.Superior longi- 
tudinal fas- 
ciculus 



Radiation of 
corp. callosum 

^Transverse tem- 
poral gyri 



Optic radiation 



Occipital part of 
radiation of 
corp. callosum 



Fig. 174. Dissection of the human telencephalon to show the radiation of the corpus callosum. 

Dorsal view. 

Turning again to the dorsal aspect of the corpus callosum, a careful inspec- 
tion will show that at the bottom of the great longitudinal fissure it is covered 
by a very thin coating of gray matter, continuous with the cerebral cortex in 
the depths of the sulcus of the corpus callosum (Figs. 174, 175). This is a rudi- 
mentary portion of the hippocampus and is known as the supracallosal gyrus or 
indusium griseum. In this gray band there are embedded delicate longitudinal 



THE INTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES 



245 



strands of nerve-fibers. Two of these, placed close together on either side of 
the median plane, are known as the medial longitudinal stria. Further lateral- 
ward on either side, hidden within the sulcus of the corpus callosum, is a less 
well -developed band, the lateral longitudinal stria. 

The corpus callosum is transversely striated and is composed of fibers that 
pass from one hemisphere to the other. By dissection these may be followed 
into the centrum semiovale, where they constitute the radiation of the corpus 



Genii of corpus callosum 
Medial longitudinal stria 



- Hippocampal rudiment 
~;_-Body of corpus callosum 

Radiation of corpus callosum 

~ \--Corona radiata 

Intersection of corona ra- 

/A diata and radiation of 
A corpus callosum 




Lateral longitudinal stria 
Splenium of corpus callosum 

Fig. 175. Dissection of the telencephalon of the sheep to show the radiation of the corpus cal- 
losum. Dorsal view. 

callosum and intersect those from the internal capsule in the corona radiata 
(Figs. 174, 175). The fibers of the genu sweep forward into the frontal lobe, 
constituting the frontal part of the radiation. Fibers from the splenium bend 
backward toward the occipital pole, forming the occipital part of the radiation 
or forceps major. In the human brain fibers from the body and splenium 
of the corpus callosum sweep outward over the lateral ventricle, forming the 
roof and lateral wall of its posterior horn and the lateral wall of its inferior 
cornu. Here they constitute a very definite stratum called the tapetum. 



246 



THE NERVOUS SYSTEM 
THE LATERAL VENTRICLE 



When the corpus callosum and its radiation are cut away a cavity, known 
as the lateral ventricle, is uncovered. It is lined by ependyma, continuous with 
the ependymal lining of the third ventricle by way of the interventricular for- 
amen. This cavity, which contains cerebrospinal fluid, varies in size in differ- 
ent parts, and in some places is reduced to a mere cleft between closely apposed 
walls. The shape of the ventricle is highly irregular (Fig. 176). As constit- 
uent parts we recognize a central portion, anterior and inferior horns, and in 
man also a posterior horn. The latter part develops rather late in the human 
fetus as a diverticulum from the main cavity. 

Third ventricle 



Ant. horn 



[Lateral ventricle 
Inf. horn) Central P art 




y Interventricular for. 
' " Optic recess 
* ' Infundibulum 
\ > Third ventricle 
\^ Inf. horn 
\^ Suprapineal recess 
x Cerebral aqueduct 



Fourth ventricle 



Fourth ventricle 



Post, horn 

A B 

Fig. 176. Two views of the brain ventricles of man: A, Dorsal view; B, lateral view. 

The anterior horn, or cornu anterius, is the part which lies rostral to the 
interventricular foramen. Its roof and rostral boundary are formed by the 
corpus callosum. Its medial wall is vertical and is formed by the septum pellu- 
cidum, which is stretched between the corpus callosum and the fornix (Figs. 
177, 178). The sloping floor is at the same time the lateral wall, and is formed 
by the head of the caudate nucleus, which bulges into the ventricle from the 
ventrolateral side. In frontal section the cavity has a triangular outline; and 
in such a section its walls and the relation which they bear to the rest of the brain 
can be studied to advantage (Fig. 186). 

The central part or body of the lateral ventricle extends from the inter- 
ventricular foramen to the splenium of the corpus callosum, where in man the 
cavity bifurcates into posterior and inferior horns. The roof of the central 



THE INTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES 



247 



part is formed by the corpus callosum, and the medial wall by the septum pellu- 
cidum. The floor, which slants to meet the roof at the lateral angle, is com- 
posed from within outward of the following structures: the fornix, chorioid 
plexus, lateral part of the dorsal surface of the thalamus (in man, but not 
in the sheep), the stria terminalis, vena terminalis, and the caudate nucleus 
(Figs. 177-180, 188). The caudate nucleus tapers rapidly as it is followed 
from the anterior horn into the body of the ventricle (Fig. 177). The cavity 



Longitudinal fissure of cerebrum 
Lamina of septum pellucidum / 

Column of fornix 
Caudate nucleus 

Interventricular foramen 



Thalamus 



Body of fornix 



Chorioid plexus 



Transverse fissure of / 
cerebrum 




Rostrum of corpus callosum 
' ^Corpus callosum 

/ Cavity of septum pellucidum 
Anterior horn of lateral ventricle 
/Caudate nucleus 

Chorioid plexus of lateral 
ventricle 
, Terminal stria 

Central portion of 
lateral ventricle 



- Chcrioid glomus 



- Cms of fornix 

.-Inferior horn of 
lateral ventricle 



Splenium of corpus callosum 



* Posterior horn of lateral ventricle 
Calcarine fissure 
Cerebellum 



Fig. 177, Dissection of the human telencephalon. The corpus callosum has been partly removed, 
and the lateral ventricles have been exposed. Dorsal view. (Sobotta-McMurrich.) 

is lined throughout by an ependymal epithelium, indicated in red in Fig. 155. 
Between the caudate nucleus and the fornix this layer of ependyma constitutes 
the entire thickness of the wall of the hemisphere. In man, where the fornix 
and caudate nucleus are more widely separated than in the sheep, this epithelial 
membrane rests upon the thalamus and becomes adherent to it as the lamina 
affixa (Figs. 154, 155). At the margin of the fornix a vascular network from the 
tela chorioidea, i. e., from the pia mater in the transverse cerebral fissure, is 



248 



THE NERVOUS SYSTEM 



invaginated into the ventricle, pushing this epithelial layer before it and con- 
stituting the chorioid plexus. 

The posterior horn, or cornu posterius, extends into the occipital lobe of 
the human brain, tapering to a point, and describing a gentle curve with con- 
cavity directed medially (Figs. 177, 181). 

The tapetum of the corpus callosum forms a thin but distinct layer in the 
roof and lateral watt of the posterior horn, and is covered in turn by a thicker 
layer of fibers belonging to optic radiation or radiatio occipitothalamica (Fig. 
190). In the medial wall two longitudinal elevations may be seen. Of these, 



Corpus callosum 



Head of caudate 
nucleus " 



Body offornix--^ 



Fimbria of hippo- 
campus 



Hippocampus - 



Splenium of corpus 
callosum 




Anterior horn of 
lateral ventricle 

Thick portion of 
septum pellucidum 

~ Lateral fissure 



Interventricular 
foramen 

- Lateral ventricle 



Fig. 178. Dissection of the telencephalon of the sheep to show the lateral ventricle and the 
structures which form its floor. Dorsal view. 



the more dorsal one is known as the bulb of the posterior horn (bulbus cornu), 
and is formed by the occipital portion of the radiation of the corpus callosum 
or forceps major. The other elevation, known as the calcar avis, is larger and 
is produced by the rostral part of the calcarine fissure, which here causes a fold- 
ing of the entire thickness of the pallium (p. 238). 

The inferior horn, or cornu inferius, curves ventrally and then rostrally into 
the temporal lobe (Fig. 181). The angle between the diverging inferior and 
posterior horns is known as the collateral trigone. This horn lies in the medial 
part of the temporal lobe and does not quite reach the temporal pole. The roof 



THE INTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES 249 

is formed by the white substance of the hemisphere, and along its medial border 
are the stria terminalis and tail of the caudate nucleus. At the end of the latter 



Genii of corpus 



callosum 
Septum pellucidum 



Thick portion of sep- 
tum pellucidum 



Hippocampus -9f~~ 



Inferior horn of, 
lateral ventricle 




-Head of caudate 
nucleus 

i- I nterventricular 
foramen 

"Chorioid fissure 



ft -Fimbria of 

hippocampus 



Transverse fissure of cerebrum ! 
Tfialamus 



Fig. 179. 



\ Hippocampal commissure 
Pineal body 



Lateral ventricle -- 



Septum pellucidum 



Thick portion of 
septum pellucidum 



Column of fornix 




Thalamus I ', 
Third ventricle i 
Pineal body 



\ Thalamus 
\ Tania of thalamus 
'Habenular trigone 



'Genu of corpus 
callosum 



Head of caudate 
nucleus 



- I nterventricular 
foramen 

Fimbria of hippo- 
campus 

- Inferior horn of 
lateral ventricle 

" Hippocampus 



Fig. 180. 

Figs. 179 and 180. Dissections of the rostral part of the sheep's brain to show the relation 
of the lateral ventricles, fornix, fimbria, and hippocampus to the transverse fissure, thalamus, and 
third ventricle. Dorsal views. In Fig. 180 a triangular piece, including portions of the fornix, 
fimbria, and hippocampus, has been removed. 

the amygdaloid nucleus bulges into the terminal part of the inferior horn (Fig. 
185). The floor and medial wall of the inferior horn are formed in large part 



250 



THE NERVOUS SYSTEM 



by the following structures, named in their order from within outward: the 
fimbria, hippocampus, and (in man) the collateral eminence (Figs. 181, 182, 
189). Upon the fimbria and hippocampus there is superimposed the chorioid 
plexus (Fig. 183). The hippocampus is a long, prominent, curved elevation, 
with whose medial border there is associated a band of fibers, representing a 
continuation of the fornix and known as the fimbria. These parts will be de- 



Lamina of septum pellucidum 



Columns of fornix 
Anterior tubercle of thala- 



Uncus. 

Hippocampal 
digitations\ 



Hippocampal,. 
gyrus 

Collateral eminence 
Fimbria of hippo- 
campus . 
Collateral trigone 
Posterior commissure 

Hippocampus 

Calcar avis 
Posterior horn of lateral ventricle 




f Longitudinal fissure of cerebrum 
Corpus callosum 

, Cavity of septum pellucidum 
Interventricular foramen 
Anterior horn of lateral ventricle 
Head of caudate nucleus 

,Massa intermedia 
, Third ventricle 
, Habenular commissure 

,-Habenular trigone 

^Inferior horn of lateral 
ventricle 



Posterior horn of lat- 
eral ventricle 



Pineal body 



Corpora quadrigemina 
Vermis of cerebellum 



Fig. 181. Dissection of the human brain to show the posterior and inferior horns of the lateral 
ventricle. The body and splenium of the corpus callosum have been removed, as have also the body 
of the fornix and the tela chorioidea of the third ventricle. A sound has been passed through the 
interventricular foramina. Dorsal view. (Sobotta-McMurrich.) 

scribed in connection with the rhinencephalon. The collateral eminence is an 
elevation in the lateral part of the floor produced by the collateral fissure. 

The thin epithelial membrane, described above as joining the edge of the 
fornix with the caudate nucleus (Fig. 155), continues to unite these structures as 
they both curve downward, the former in the floor, the latter in the roof, of the 
inferior horn. A vascular plexus from the pia mater is invaginated into the 



THE INTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES 251 

lateral ventricle along this curved line, carrying before it an epithalial covering 
from this thin membrane. In this way there is formed the chorioid plexus of the 
lateral ventricle (Figs. 183, 184). The line along which this imagination occurred 
is the chorioid fissure; and when the plexus is torn away, the position of the 



Lateral ventricle 



Inter-ventricular foramen 




Hippocampus' 
Fimbria of hippocampus' / / 

Body offornix 

Optic tract / 
Internal capsule 



/ Olfactory bulb 
I \ Rhinoccele 

I Genu of corpus cattosum 

I Body of corpus callosum 
Septum pellucidum 



Fig. 182. Dissection of the cerebral hemisphere of the sheep to show the lateral ventricle. 

Lateral view. 

fissure is indicated by an artificial cleft extending into the ventricle, which be- 
gins at the interventricular foramen and follows the fornix and fimbria in an 
arched course into the temporal lobe (Fig. 205). 

Hippocampus Chorioid plexus of lateral ventricle 




Fig. 183. Outline drawing from Fig. 182, to show the location of the chorioid plexus of the lateral 

ventricle. 



The chorioid plexus of the lateral ventricle (Figs. 183, 184, 188) is continuous 
with that of the third ventricle at the interventricular foramen, from which 
point it can be followed backward through the central part into the inferior 
horn. It is coextensive with the chorioid fissure and is not found in the anterior 
or posterior horns. It consists of a vascular network derived from the pia 



252 



THE NERVOUS SYSTEM 



mater, and especially from that part of it enclosed in the transverse fissure and 
known as the tela chorioidea of the third ventricle. It is covered throughout 



Longitudinal fissure of cerebrum 



Anterior horn of lateral 
ventricle 

Corpus striatum-, : 
Interventricular for. 




Columns offornix 

Central portion of 

lateral ventricle 

Internal cerebral 

veins 

Chorioid vein 
Chorioid artery 



Inferior horn of 
lateral ventricle 



Collateral trigone 
Posterior horn 




Body of corpus callosum 
.Lamina of septum pellucidum 

-Cavity of sept, pellucidum 

L .Lamina: of septum 

pellucidum 

-Vein of septum 

pellucidum 

Terminal vein 
,-Thalamus 



-J, --Corpus striatum 
Lateral chorioid 
2<jL - ' plexus 
"^ / -Tel a chorioidea 
of third ventricle 

'Chorioid glomus 



Calcar a^jis ll^^^B 



Great cerebral vein Hippocampal Body of corpus Body offornix Crura offornix 
commissure callosum 

Fig. 184. Dissection of the human brain to show the tela chorioidea of. the third ventricle 
and the hippocampal commissure. The body of the corpus callosum and the fornix have been 
divided and reflected. Dorsal view, except that the ventral surfaces of the reflected corpus 
callosum and hippocampal commissure are seen. (Sobotta-McMurrich.) 

by a layer of epithelium of ependymal origin, which is adapted to every uneven- 
ness of its surface (Fig. 155). 

THE BASAL GANGLIA OF THE TELENCEPHALON 

There are four deeply placed masses of gray matter within the hemisphere, 
known as the caudate, lentiform and amygdaloid nuclei, and the claustrum. The 



THE INTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES 



253 



two former, together with the white fascicles of the internal capsule which 
separate them, constitute the corpus striatum (Fig. 185). 

The caudate nucleus (nucleus caudatus) is an elongated mass of gray matter 
bent on itself like a horseshoe, and is throughout its entire extent closely re- 




Caudate nucleus 
Thalamos 



Lenticular nucleus 
Amygdaloid nucleus 
Caudate nucleus 

Thalamus 

Tail of caudate nucleus 

Internal capsule 
Lenticular nucleus 

Caudate nucleus 
Thalamus 

Tail of caudate nucleus 
Internal capsule 



Fig. 185. Diagrams of lateral view and sections of the nuclei of the corpus striatum with the 
internal capsule omitted. A and B below represent horizontal sections along the lines A and B 
in the figure above. The figure also shows the relative position of the thalamus and the amygda- 
loid nucleus. (Jackson- Morris.) 

lated to the lateral ventricle (Figs. 91, 177, 178, 186, 187, 188, 191). Its swol- 
len rostral extremity or head is pear shaped and bulges into the anterior horn of 
the lateral ventricle. The remainder of the nucleus is drawn out into a long, 
slender, highly arched tail. In the floor of the central part of the ventricle the 
head gradually tapers off into the tail, which finally curves around into the roof 



254 



THE NERVOUS SYSTEM 



of the inferior horn and extends rostrally as far as the amygdaloid nucleus. 
Because of its arched form it will be cut twice in any horizontal section which 
passes through the main mass of the corpus striatum, and in any frontal section 
through that body behind the amygdaloid nucleus (Figs. 185, 189, 191). The 
head of the caudate nucleus is directly continuous with the anterior perforated 
substance; and ventral to the anterior limb of the internal capsule it is fused with 
the lentiform nucleus (Fig. 186). 

The lentiform or lenticular nucleus (nucleus lentiformis) is deeply placed 
in the white center of the hemisphere and intervenes between the insula, on the 



medialis. 



Stria 
longitu 

dinalis I lateralis 



Corpus ._ 
callosum 

Caput nuclei . j 
caudati * 

I 

Claustrum _j. 

Capsula 
externa 

Capsula 
interna 

Nucleus lentifor- 
mis (Putamen) 



Fibers from 
the tractus 
olfactorius 



Gyrus rectusr 



Fissura longi 
tudinalis 
cerebri 



Polus temporalis -' 




Fissura longitudi- 

nalis cerebri 
i_..Gyrus cinguli 

Sulcus corporis 
callosi 



Cornu anterius 
ventriculi 
lateralis 
..Vena septi 
pelluciiti 

_ Septum 
pellucidum 

~.Fissura cerebri 
lateral is(Sylvii) 



., Rostrum cor- 
"poris callosi 

""--- Gyrus sub- 
callosus 



.Area parolfac- 
toria (Brocae) 

Fissura cerebri 
lateralis (Sylvii) 



Fig. 186. Frontal section of the human brain through the rostral end of the corpus striatum and 
the rostrum of the corpus callosum. (Toldt.) 

one hand, and the caudate nucleus and thalamus on the other (Figs. 185, 191, 
194). In shape it bears some resemblance to a biconvex lens. Its lateral, 
moderately convex surface is nearly coextensive with the insula from which it 
is separated by the claustrum. Its ventral surface rests upon the anterior per- 
forated substance and the white matter forming the roof of the inferior horn of 
the lateral ventricle (Figs. 187-189). Its sloping medial surface is closely 
applied to the internal capsule. The lentiform nucleus is not a homogeneous 
mass, but is divided into three zones by internal and external medullary lamina. 
The most lateral zone is the largest and is known as the putamen. The two 
medial zones together form the globus pallidus. 



THE INTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES 



2 55 



Nucleus caudatus 
(Caput) 



Capsula interna^ 
(Pars frontalis 



_. Fissura longitudinalis 

cerebri 
f . Corpus callosum 

Cornu anterius 
ventriculi 
laterlais 
[_,. Plexus chorio- 
ideus ventriculi 
lateral!* 

^JC-^ Septum pellu- 
cidum 




Foramen inter-- 
ventriculare 

(Monroi) 
Substantia per- 
forata anterior' 

Uncus'' 



- Fissura cerebri 
lateralis(Sylvii) 

insulae 

ssus opti- 
cus ventriculi 

tertii 
"~ Tractus opticus 

Chiasma opti- 
-- cum (posterior 
part; 

^ Commissura inferior 
(Citddeni) 



Fig. 187. Frontal section of the human brain through the anterior commissure. (Toldt.) 



Ventriculus lateralis x 

(Pars centralis) 
Plexus chorioideus, 
ventriculi lateralis ~ 

Nucleus 

caudatus""^--^ 
Massa inter-, 
media 

Capsula interna 

Putamen~ 
Nucleu 
lenti- 
formis 

pallidus 
Capsula externa.. 

Claustrum.^^ 
Ansa peduncu- 

laris 

Tractus opticus- ~ 
Pedunculus. tha-^-^' 

lami inferior 

Cornu inferius ve: 

triculi lateralis 

Digitationes# 
hippocampi 

N. oculomotorius 



Ansa lenticularis 







j*if_. Nucleus hypo- 
5, thalamicus 
I (Corpus Luysi) 

Substantia 

nigra 
Basis pedunculi 

Corpus 
~~-~.mamillare 



Fossa inter- 
- peduncularis 



-Pens (Varoli) 



Fig. 188. Frontal section of the human brain through the mammillary bodies. (Toldt.) 



The putamen is larger than the globus pallidus and is encountered alone in 
frontal sections through either the rostral or caudal extremities of the corpus 
striatum (Fig. 189), and also in horizontal sections above the level of the globus 



256 



THE NERVOUS SYSTEM 



pallidus (Fig. 191). It is fused rostrally with the caudate nucleus, which it 
resembles in color and structure. 

The globus pallidus is lighter in color and is subdivided into two parts, of 
which the medial is the smaller. Both parts are traversed by many fine 
white fascicles from the medullary laminae. 

Especially in the anterior part of the internal capsule bands of gray sub- 
stance stretch across from the lentiform to the caudate nucleus, producing a 
striated appearance (Fig. 187). This appearance, which is accentuated by the 
medullary laminae and the finer fiber bundles in the lentiform nucleus, makes 



Tela chorioidea 
ventriculi tertii 



Capsula in 




Nucleus 
habenulae 
Cauda nuclei 

caudati 
Tractus opticus ~; 

Fimbria hippo- 
campi 

Fascia dentata 
hippocampi 

Pedunculus cerebri 



V cerebri interna. 



Plexus chorio- 
'ideus ventriculi 

tertii 

Commissura 

habenularum 

Commissura 

posterior 

Aditus ad aquae- 
ductum cerebri 

Fasciculus retro- 
flexus(Meynerti) 

Cornu inferius 
ventriculi 
lateralis 

...Nucleus rubcr 

Nucleus hypo- 

thalamicus 

(Corpus Luysi) 



- x - Substantia nigra 
Pons (Varoli) 



Recessus posterior fossae interpeduncularis/ 
Fig. 189. Frontal section of the human brain through the rostral part of the pons. (Toldt.) 



the term corpus striatum an appropriate name to apply to the two nuclei and 
the internal capsule, which separates them. 

The claustrum is a thin plate of gray substance, which, along with the white 
matter in which it is embedded, separates the putamen from the cortex of the 
insula. Its lateral surface is somewhat irregular, being adapted to the convolu- 
tions of the insula, with which it is coextensive (Figs. 188, 191). Its concave 
medial surface is separated from the putamen by a thin lamina of white matter, 
known as the external capsule. By some authorities the claustrum is thought 
to be a detached portion of the lentiform nucleus, while others believe that it 
has been split off from the insular cortex. It is probable that neither of these 
views is strictly correct. However, according to the recent work of Elliot 



THE INTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES 



257 



Smith (1919), the claustrum, putamen, amygdaloid nucleus, and the greater 
part of the caudate nucleus are pallial derivatives and are closely related mor- 
phologically to the neopallium; while the globus pallidus is the representative 
in the mammalian brain of the corpus striatum of lower forms, as seen in the 
shark (Fig. 9). 

The Amygdaloid Nucleus. In the roof of the terminal part of the inferior 
ventricular horn, at the point where the tail of the caudate nucleus ends, there 
is located a small mass of gray matter, known as the amygdaloid nucleus (Fig. 



Radiatio corporis N 

callosi 

Bui bus cornu-. 
posterioris 

Calcar 



Hippocampus -^ '' , 
* - - /,' s 

Corpora 

tjuadrigemina -- . 
Nucleus 
.colliculi 
inferioris 

Aquaeductus /_/. 
cerebri 

?.*, 

Nucleus n 
trochlearis ... .... x 

T^ I ''/' : 'j '' 

Fasciculus _. 

longitudinalis 
medial is 

Cerebellum--' 
Brachium pontis --' 

Flocculus 

Pyramis medullae oblongatae.-'" 




, Splenium cor- 
poris callosi 

Tela chorio- 

idea ven- 
triculi tertii 
T Corpus 
>'' pineale 

Cornu posle- 
,'' rius-ventri- 
culi lateralis 
^, Glomus 

chorioideum 
yA-r- J ':'" ^--^--- Tapetum 

_ ...v.t Radiatio occi- 

i \\ \\' pitothalamica 
Eminentia 
collateralis 

Fissura 
*. \;> collateralis 

Lemniscus 
' J ^ lateralis 
""J Brachium con- 
junct! vum 
-- Stratum griseum 

centrale 
~~- Lemniscus medialis 



N. vagus 



Fig. 190. Frontal section of the human brain through the splenium of the corpus callosum. View 
into the posterior horn of the lateral ventricle. (Toldt.) 

185). It is continuous with the cerebral cortex of the temporal lobe lateral to 
the anterior perforated substance (Fig. 198; Landau, 1919). 

The external capsule is a thin lamina of white matter separating the claus- 
trum from the putamen. Along with the internal capsule it encloses the lenti- 
form nucleus with a coating of white substance. 

THE INTERNAL CAPSULE 

The internal capsule is a broad band of white substance separating the 
lentiform nucleus on the lateral side from the caudate nucleus and thalamus on 
the medial side (Figs. 191, 192). In a horizontal section through the middle 

17 



2 5 8 



THE NERVOUS SYSTEM 



of the corpus striatum it has the shape of a wide open V. The angle, situated 
in the interval between the caudate nucleus and the thalamus, is known as the 



Truncus corporis callosi. 
Septum pellucid 
Corpus fornii 



Genu corporis callosi 




entriculi lateralis 
:lei caudati 
nna fornicis 

^apsula interna 

.Insula 

,Capsula externa 
Claustrum 



Putamen 

.Globus 
pallidus 



Nucleus 
lent!- 



Glomus chorio- 
ideum 



Radiatio occi- 
pitothalamica 
(Gratioleti) 



Splenium corporis ca 



Massa inter- 
media 
Ventriculus 

tertius 
L.Stria medullaris 

thalarni 
- Nucleus 
habenulae 

Habenula 

-Cauda nuclei 
- caudati 
Fimbria hippo- 
campi 

^Corpus pineale 
^Hippocampus 

Eminentia 
collaterals 

Calcar avis 



Cornu posterius 
'entriculi lateralis 



Fissura calcarina 



Fig. 191. Horizontal sections of the human brain through the internal capsule and corpus 
striatum. The section on the right side was made 1.5 cm. farther ventralward than that on the 
left. (Toldt.) 



genu. From this bend the frontal part or anterior limb of the internal capsule 
extends laterally and rostrally between the thalamus and the head of the caudate 
nucleus; while the occipital part or posterior limb of the internal capsule extends 



THE INTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES 



259 



laterally and toward the occiput between the lentiform nucleus and the thala- 
mus. 

The anterior limb of the internal capsule, intervening between the caudate 
and lentiform nuclei, is broken up by bands of gray matter connecting these 
two nuclei. It consists of corticipetal and corticifugal fibers. The former 
belong to the frontal stalk of the thalamus or anterior thalamic radiation from the 
lateral nucleus of the thalamus to the cortex of the frontal lobe. The cortici- 



Septum pellucidum ,^ 
Fornix^ 

Chorioid fissure 
Third ventricle ^ 

Thalamus^ s^ 

/ ~"> 

Habenular trigone -^K 

Habenular commis- ^U 
sure I 

Transverse fissure ^| 

Pineal body 

Inferior horn of _ 
lateral ventricle 

Superior colliculus 




':: 



-' Genu of corpus callosum 
.^Anterior horn of lateral ventricle 

, Anterior limb of internal cap- 
sule 

fe.- Head of caudate nucleus 
~~Insnla 

r- External capsule 

-^ Lentiform nucleus 

'Claitstrum 

\ N " % *. Genu of internal 

capsule 

\ 

\"y ^Posterior limb of 
internal capsule 

v/ Chorioid fissure 
\j& ; * ^Fimbria of hippocampus 
\) "Hippocampus 
^-^Cerebellum 



Medulla oblongata 



Fig. 192. Horizontal section through the sheep's brain, passing through the internal capsule and 

corpus striatum. 

fugal fibers form the frontopontine tract from the cortex of the frontal lobe to 
the nuclei pontis (Fig. 193). 

The posterior limb of the internal capsule intervenes between the thalamus 
and the lentiform nucleus, and bends around the posterior end of the latter 
on to its ventral surface (Fig. 194). It accordingly consists of three parts, 
designated as lenticulothalamic, retrolenticular, and sublenticular. The lentic- 
ulothalamic part consists of fibers belonging to the thalamic radiation intermingled 
with others representing the great efferent tracts which descend from the cere- 



260 



THE NERVOUS SYSTEM 



bral cortex (Fig. 193). Of these, the corticobulbar tract to the motor nuclei of 
the cranial nerves occupies the genu, and the cor tico spinal tract the adjacent 
portion of the posterior limb. The fibers of the corticospinal tract are so ar- 




Caudate nucleus 

Frontopontine tract 
Anterior thalamic radiation 

^Corticobulbar tract 
'Globus pallidus 
^-Corticorubral tract 

- Corticospinal tract (arm) 
-Corticospinal tract (leg) 
-Putamen 

- Thalamic radiation (sensory fibers) 

Auditory radiation 



Thalamus 



Optic radiation 
Fig. 193. Diagram of the internal capsule. 

ranged that those for the innervation of the arm are nearer the genu than those 
for the leg. Accompanying the corticospinal tract are descending fibers from 
the cortex of the frontal lobe to the red nucleus, the corticorubral tract. Those 

Coronal fibers from posterior limb of 
internal capsule 



Coronal fibers from anterior L^ 
limb of internal capsule 

Lentiform nucleus 




Coronal fibers from retro- 
lenticular part of inter- 
nal capsule 



Coronal fibers from sublenticular part 
of internal capsule 



Anterior commissure-' 
Ansa peduncularis'' 

Fig. 194. The lentiform nucleus and the corona radiata dissected free from the left human 

cerebral hemisphere. Lateral view. 



fibers of the thalamic radiation which run to the posterior central gyms and con- 
vey general sensory impulses from the lateral nucleus of the thalamus are sit- 
uated behind the corticospinal tract. The retrolenticular part of the internal 



THE INTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES 



26l 



capsule rests upon the lateral surface of the thalamus behind the lentiform 
nucleus and contains: (1) the optic radiation from the pulvinar and lateral 
geniculate body to the cortex in the region of the calcarine fissure, and (2) the 
acoustic radiation from the medial geniculate body to the transverse temporal 
gyrus. The sublenticular part of the internal capsule lies ventral to the pos- 
terior extremity of the lenticular nucleus and contains the temporopontine tract 
from the cortex of the temporal lobe to the nuclei pontis. 

Dissections of the Internal Capsule (Figs. 87, 88, 91, 194, 195). A large 
part of the fibers of the internal capsule, including the corticopontine, cortico- 
bulbar, and corticospinal tracts, are continued as a broad thick strand on the 
ventral surface of the cerebral peduncle, with which we are already familiar 



Radiation of corpus cal- 
losum forming roof 
of lateral ventricles 

A nterior limb of inter 
nal capsule (can- -^ d 
date impression) ".^EJ 

X 



Frontal pole - 



Posterior limb of internal capsule 
/(thalamic impression) 



/ Tapelum 




Genii internal cap- ,--^< 
side 

Anterior commissure 

Optic tract 
Temporal lobe - 

Basis pedunculi Optic radiation 

Fig. 195. Dissection of the human cerebral hemisphere, showing the internal capsule exposed 
from the medial side. The caudate nucleus and thalamus have been removed. 

under the name basis pedunculi. By removing the optic tract, temporal lobe, 
insula, and lentiform nucleus this strand can easily be traced into the internal 
capsule where it is joined by many fibers radiating from the thalamus and 
spreads out in a fan-shaped manner (Figs. 87, 88), forming a curved plate which 
partially encloses the lentiform nucleus. As seen from the lateral side, the line 
along which the fibers of the internal capsule emerge from behind the lentiform 
nucleus forms three-fourths of an ellipse (Fig. 194). Beyond the lentiform nu- 
cleus the diverging strands from the internal capsule, known as the corona 
radiata, join the central white substance of the hemisphere and intersect with 
those from the corpus callosum (Figs. 174, 238). 

An instructive view of the internal capsule may also be obtained by remov- 



262 



THE NERVOUS SYSTEM 



ing the thalamus and caudate nucleus from its medial surface. It is then seen 
to bear the imprint of both of these nuclei, and especially of the thalamus; and 
between the two impressions it presents a prominent curved ridge (Fig. 195). 
This ridge is responsible for the sharp bend known as the genu, which is evi- 
dent in horizontal sections at appropriate levels through the capsule. Many 
broken bundles of fibers, representing the thalamic radiation, are seen enter- 
ing the capsule upon its medial surface. 

THE CONNECTIONS OF THE CORPUS STRIATUM AND THALAMUS 

What is the function of the corpus striatum, and what connection does it 
have with other parts of the nervous system? These questions, to which no 



Caudate nucleus 




Parietal stalk of thalamus 
Corticospinal tract 

Insula 
Claustrum 

Putamen 

Globus pallidus 
Ansa peduncularis 
Red nucleus 

\ )\ ;^x>\ 

Ansa lenticularis 
Substantia nigra 
Hypothalamic nucleus 

Fig. 196. Diagram of the connections of the caudate and lenticular nuclei. 

final answer can as yet be given, have recently become of great importance, 
because of the frequency with which degeneration of the lentiform nucleus has 
been found at autopsy in patients who have shown serious disturbances of the 
motor mechanism (Wilson, 1912-1914). It seems probable that the corpus 
striatum exerts a steadying influence upon muscular activity, the abolition of 
which results in tremor during voluntary movement. The probable connec- 
tions of the corpus striatum are indicated in Fig. 196. Striopetal fibers reach 
the caudate nucleus from the anterior and medial nuclei of the thalamus (Sachs, 
1909). According to Cajal, the corpus striatum also receives collaterals from 
the corticospinal tract. Internuncial fibers join together various parts of the 
corpus striatum. The majority of these seem to run from the caudate nucleus 



THE INTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES 263 

to the putamen, on the one hand, and from the putamen to the globus pallidus 
on the other. The striofugal fibers arise, for the most part at least, in the globus 
pallidus. They are collected into a bundle of transversely directed fibers, known 
as the ansa lenticularis (Fig. 188), which is distributed to the thalamus, red 
nucleus, hypothalamic nucleus, and substantia nigra. Other fibers belonging 
to the same general system break through the ventral third of the internal 
capsule to reach the thalamus (Wilson, 1914). The importance of the connec- 
tion with the red nucleus is obvious, since by way of the rubrospinal and rubro- 
reticular tracts the corpus striatum is able to exert its influence upon the pri- 
mary motor neurons of the brain stem and spinal cord. The fibers to the sub- 
stantia nigra have already been mentioned under the name strionigral tract 
(p. 164). The impulses which travel along them are, in all probability, re- 
layed through the substantia nigra to lower lying motor centers, although the 
functions and connections of this large nuclear mass are still obscure. 

The Thalamic Radiation. We are now in position to understand the course 
and distribution of the fascicles, which unite the thalamus with the cerebral 
cortex and which consist of both thalamocortical and corticothalamic fibers. This 
thalamic radiation may be divided into four parts: the frontal, parietal, occip- 
ital, and ventral stalks of the thalamus, which will now be traced as fasciculi, 
without reference to the direction of conduction in the individual fibers. 

The ventral stalk, or inferior peduncle of the thalamus, streams out of the 
rostral portion of the ventral thalamic surface and is directed lateral ward under 
cover of the lentiform nucleus. Some of these fibers belong to the ansa lentic- 
ularis and run from the lentiform nucleus to the thalamus. The others, form- 
ing a bundle known as the ansa peduncularis , runs lateralward ventral to the 
lentiform nucleus and are distributed to the cortex of the temporal lobe and 
insula (Fig. 196). 

The frontal stalk, or peduncle of the thalamus, consists of fibers which run 
through- the anterior limb of the internal capsule from the lateral thalamic 
nucleus to the cortex of the frontal lobe (Fig. 193), and in small part to the cau- 
date nucleus also. 

The parietal stalk, or peduncle, emerges from the lateral surface of the 
thalamus, and runs through the posterior limb of the internal capsule in close 
association with the great motor tracts (Figs. 193, 196). It connects the lateral 
nucleus of the thalamus with the cortex of the parietal and posterior part of the 
frontal lobe. 

Many of these fibers, especially those terminating in the posterior central 



264 THE NERVOUS SYSTEM 

gyrus, are afferent fibers of the third order mediating sensations of touch, heat, 
cold, and perhaps also pain as well as sensations from the muscles, joints, and 
tendons (Head, 1918). These sensory fibers are located behind the corticospinal 
tract in the posterior limb of the internal capsule. According to Wilson (1914) 
the medullary laminae of the lentiform nucleus do not contain any thalamocor- 

tical fibers. 

The occipital stalk, or peduncle, is also known as the optic radiation and as 
the radiatio occipitothalamica. Its fibers stream out of the pulvinar and lateral 
geniculate body, pass through the retrolenticular part of the internal capsule, 
and run in a curved course toward the occiput, around the lateral side of the 
posterior horn of the lateral ventricle to the cortex of the occipital lobe, and es- 
pecially to the region of the calcarine fissure (Figs. 190, 191). It also contains 
some fibers arising in the occipital cortex and ending in the superior quadrigeminal 
body. We have learned that it forms an important part of the visual path 
(Fig. 162). 

Closely associated with the optic radiation in the retrolenticular part of the 
internal capsule is the acoustic radiation (radiatio thalamotemporalis) . This 
connects the medial geniculate body with the anterior transverse temporal gyrus 
and the adjacent part of the superior temporal gyrus, and mediates auditory 
sensations. It should be included as a part of the thalamic radiation. 



CHAPTER XVII 

THE RHINENCEPHALON 

THE olfactory portions of the cerebral hemisphere may all be grouped to- 
gether under the name rhinencephalon. Phylogenetically very old, this part of 
the brain varies greatly in relative importance in the different classes of verte- 
brates. The central connections of the olfactory nerves form all or almost all of 
the cerebral hemispheres in the selachian brain (Fig. 13); while in the mammal 
the non-olfactory cortex or neopallium has become the dominant part. Even 
among the mammals there is great variation in the importance and relative 
size of the olfactory apparatus. The rodents, for example, depend to a great 
extent on the sense of smell in their search for food, and possess a highly developed 
rhinencephalon. Such mammals are classed as macrosmatic. Man, on the 
other hand, belongs in this respect with the microsmatic mammals, because in 
his activities the sense of smell has ceased to play a very important part, and 
his olfactory centers have undergone retrogressive changes. The carnivora and 
ruminants are in an intermediate group. The sheep's brain furnishes a good 
illustration of this intermediate type, and displays much more clearly than the 
human brain the various parts of the rhinencephalon and their relation to each 
other. 

Parts Seen on the Basal Surface of the Brain. A comparison of the basal 
surface of the sheep's brain with that of the human fetus of the fifth month shows 
a remarkable similarity in the parts under consideration (Figs. 197, 198). The 
olfactory bulb, which is the olfactory center of the first order, is oval in shape and 
attached to the hemisphere rostral to the anterior perforated substance. It 
lies between the orbital surface of the cerebral hemisphere and the cribriform 
plate of the ethmoid bone. Through the openings in this plate numerous fine 
filaments, the olfactory nerves, reach the bulb from the olfactory mucous mem- 
brane. It contains a cavity, the rhinoccele, continuous with the lateral ventricle 
(Fig. 182). In the adult human brain the cavity is obliterated and the connec- 
tion between bulb and hemisphere is drawn out into the long olfactory tract. 
This is lodged in the olfactory sulcus on the orbital surface of the frontal lobe 
and in transverse section presents a triangular outline (Fig. 172). It contains 

265 



266 



THE NERVOUS SYSTEM 



olfactory fibers of the second order connecting the bulb with the secondary ol- 
factory centers in the hemisphere. At its point of insertion into the hemisphere 
the olfactory tract forms a triangular enlargement, the olfactory trigone. 

From the point of insertion of the olfactory bulb or tract a band of gray 
matter, the medial olfactory gyrus, can be seen extending toward the medial 
surf ace of the hemisphere (Figs. 159, 197, 198). A similar gray band, the lateral 
olfactory gyrus, runs caudalward on the basal surface of the sheep's brain. Along 



Longitudinal fissure of cerebrum^ 

Optic nerve 
Optic chiasma 
Rhinal fissure 

Insula 
Lateral fissure- 
Optic tract - 

Infundibulum - 
Mammillary body -. 

Cerebral peduncle 
Inter peduncular fossa and 
nucleus 

Trigeminal nerve 
A bducens nerve 

Acoustici Vestibular n - 
nerve \Cochlearn. - 
Glossopharyngeal nerve ~-~ 
Vagus nerve' 
Hypo glossal nerve---' 
Anterior median fissure 




^V' Olfactory bulb 

\ \' Medial olfactory gyrus 

^^t, V 

Anterior perforated substance 
'Lateral olfactory stria 

--'Lateral olfactory gyrus 

^-Diagonal band 

Amygdaloid nucleus 

i ~ Pyriform area 



-- Trochlear nerve 



-~ Abducens nerve 
~ Facial nerve 

---- Trapezoid body 
Cerebellum 
- -Olive 

^Chorioid plexus 
" Accessory nerve 
* Tractus later alls minor 



Fig. 197. Ventral view of the sheep's brain. 

its lateral border it is separated from the neopallium by the rhinal fissure; while 
its medial border contains a band of fibers, the stria olfactoria lateralis (Fig. 197). 
The same gyrus is seen in the brain of the human fetus, but here it is directed 
outward toward the insula (Fig. 198). In the adult human brain these olfactory 
convolutions are very inconspicuous, and with the fibers from the olfactory tract 
which accompany them are usually designated as the medial and lateral olfactory 
s tries. 



THE RHINENCEPHALON 



267 



The medial olfactory gyrus and stria require further investigation. It has been gen- 
erally supposed that the stria is formed by olfactory fibers of the second and third order 
running to the olfactory centers in the rostral part of the medial surface of the hemisphere. 
These are certainly few in number in the higher mammals, and Cajal (1911), who worked 
chiefly with rodents, has been unable to identify any such fibers in these animals. The sig- 
nificance of the medial olfactory gyrus is also obscure. According to Elliot Smith (1915), 
"the rudiment of the hippocampal formation that develops on the medial surface begins 
in front alongside the place where the stalk of the olfactory peduncle (which becomes the 
trigonum olfactorium) is inserted; it passes upward to the superior end of the lamina termi- 
nalis, from the rest of which it is separated by a triangular mass of gray matter called the 
corpus paraterminale" (Fig. 200). This description, as well as the figure which accompanies 
it, suggests a close relation between the rostral end of the hippocampal rudiment and what 
is ordinarily known as the medial olfactory gyrus. The subdivision of the olfactory lobe 
into anterior and posterior portions by the morphologically unimportant sulcus parol- 
factorius posterior, although adopted in the B. N. A., is without justification and leads only 
to confusion (Elliot Smith, 1907). 



Olfactory bulb 



Lateral olfactory gyrus (stria) 

Posterior parolfactory sulcus 

Amygdaloid nucleus 




Medial olfactory gyrus (stria) 
Olfactory tract 

Limen insula 

A nterior perforated substance 

Hippocampal gyrus 



Fig. 198. Brain of a human fetus of 22.5 cm. Ventral view. (Retzius, Jackson-Morris.) 

Between the olfactory trigone and the medial olfactory gyrus, on the one 
hand, and the optic tract on the other, is a depressed area of gray matter known 
as the anterior perforated substance, through the openings in which numerous 
small arteries reach the basal ganglia (Figs. 172, 197). The part immediately 
rostral to the optic tract forms a band of lighter color, known as the diagonal 
gyrus of the rhinencephalon or the diagonal band of Broca (Fig. 197). This 
can be followed on to the medial surface of the hemisphere, where it is continued 
as the paraterminal body or subcallosal gyrus (Fig. 200). Rostral to this gyrus 
the hippocampal rudiment, which corresponds in part to the parolfactory area 
of Broca, extends as a narrow band from the rostrum of the corpus callosum 
toward the medial olfactory gyrus. In those mammals which possess an espe- 
cially rich innervation of the nose and mouth, the region of the anterior per- 
forated space is marked by a swelling, sometimes of considerable size, called 



268 



THE NERVOUS SYSTEM 



the tuber culum olfactorium. According to Retzius, a small oval mass is present 
in the anterior perforated substance of man immediately adjacent to the ol- 
factory trigone, which represents this tubercle. 

Olfactory bulb 

Anterior commissure 
Anterior perforated substance 
-Amygdaloid nucleus 
Pyriform area 




Fig. 199. Ventral view of a sheep's brain, pyriform area shaded and anterior commissure 

exposed. 

The Pyriform Area. The lateral olfactory gyrus is continuous at its caudal 
extremity with the hippocampal gyrus (Figs. 197, 198), and the two together 
form the pyriform area or lobe (Fig. 199). In the adult human brain it is more 
difficult to demonstrate the continuity of these parts. As the temporal lobe is 



Hippocampal rudiment 

Corpus callosum -. 
Septum pellucidum 

Fornix . 
Anterior commissure. 




Paraterminal body ?^~ , -= 
Hippocampal rudiment -.. 

Olfactory trigone , *"., 
Olfactory tract ^^^^ 
Olfactory bulb^ 



Intermediate olfactory stria'' 

Lateral olfactory gyrus and stria' 

t 

Anterior perforated substance 
Limen insulce 



L Hippocampus (gyri 
Andrea Retzii) 

-Fascia dentata 
|T"-- Fimbria of hippocampus 

" Hippocampus (proper) 
Hippocampus 
** Hippocampal gyrus 
N - Cauda fascia dentata 
Uncus 



Diagonal band 
Fig. 200. Diagram of the rhinencephalon. 

thrust rostrally and the insula becomes depressed, the pyriform area is bent 
on itself like a V (Fig. 198). The knee-like bend forms the limen insulce, and 
with the rest of the insula becomes buried at the bottom of the lateral fissure. 
The continuity of the pyriform area is not interrupted in the adult, though part 



THE RHINENCEPHALON 



269 



of it is hidden from view. It includes the lateral olfactory stria and the cortex 
subjacent to it (or lateral olfactory gyrus), the limen insulce, the uncus, and at 
least a part of the hippocampal gyrus (Figs. 169, 172, 200). It is not easy to 
determine just how much of the human hippocampal gyrus should be included. 
Cajal (1911) apparently includes the entire gyrus, while Elliot Smith (1915) 
limits it to the part of the gyrus dorsal to the rhinal fissure. In Fig. 200 we 
have followed the outlines of the hippocampal region as given by Brodmann 
(1909). 

The Hippocampus. An olfactory center of still higher order is represented 
by the hippocampus, which was seen in connection with the study of the lateral 



Inferior horn of lateral ventricle 



Hippocampus 



Collateral eminence 



Tapelum 
Collateral trigone 



Posterior horn of lateral ventricle 




Hippocampal dictations 



,' Uncus 



Dentate fascia of hippocampus 
Hippocampal gyrus 
Hippocampal fissure 
Fimbria of hippocampus 



Bulb of posterior horn 
Calcarine fissure 



Calcar avis 



Fig. 201. Part of temporal lobe of human brain showing inferior horn of lateral ventricle and the 
hippocampus. Dorsal view. (Sobotta-McMurrich.) 

ventricle. If we turn again to the floor of the inferior horn of the lateral ven- 
tricle we shall see a long curved elevation projecting into the cavity (Figs. 181, 
201). This is the hippocampus and is formed by highly specialized cortex 
which has been rolled into the ventricle along the line of the hippocampal fissure 
(Figs. 204, 209). It is covered on its ventricular surface by a thin coating of 
white matter, called the alveus, which is continuous along its medial edge with 
a band of fibers known as the fimbria of the hippocampus. This, in turn, is 
continuous with the fornix (Fig. 201). In Figs. 201 and 204 there may be seen, 
along the border of the fimbria, a narrow serrated band of gray matter, the 
fascia dentata, which lies upon the medial side of the hippocampus. It is sepa- 
rated from the hippocampal gyrus by a shallow groove, called the hippocampal 



270 THE NERVOUS SYSTEM 

fissure, that marks the line along which the hippocampus has been rolled into 
the ventricle. 

The hippocampus and fascia dentata belong to the archipallium. In the 
marsupials and monotremes this extends dorsally on the medial surface of the 
hemisphere in a curve, which suggests that of the corpus callosum (Fig. 202). 
In the higher mammals the presence of a massive corpus callosum seems to 
inhibit the development of the adjacent part of the hippocampal formation, 
which remains as the vestigial indusium griseum, or supracallosal gyrus. This 
hippocampal rudiment is a thin layer of gray matter on the dorsal surface of the 
corpus callosum, within which are found delicate strands of longitudinal fibers. 
Two of these strands, placed close together on either side of the median plane, 

Cerebral cortex 

^. 

, Hippocampal fissure 

Hippocampus and fascia 
dentata 

Chorioid fissure 
- Thalamus 



Olfactory bulb . ^_^- , _, 

^ ^ " Pynform area 




Tuberculum olfactorium 



Rhinal fissure 



Fig. 202. Median view of the cerebral hemisphere of a monotreme Ornithorhynchus. (Elliot 

Smith.) 

are more conspicuous than the others, and are known as the medial longitudinal 
stria. On either side, where the supracallosal gyrus bounds the sulcus of the 
corpus callosum, there is a less distinct strand, the lateral longitudinal stria 
(Figs. 174, 175). The hippocampal rudiment can be traced upon the medial 
surface of the hemisphere from the region of the medial olfactory gyrus (or stria) 
toward the rostrum of the corpus callosum, then around the dorsal surface of 
that great commissure to the splenium, behind which' it becomes continuous 
with the hippocampus proper, where this comes to the surface in the angle 
between the fascia dentata and the hippocampal gyrus (Fig. 200 Elliot Smith, 
1915). 

The Fornix. Within the hippocampus fibers arise which run through the 
white coat on its ventricular surface, known as the alveus, into thefimbria. This 



THE RHINENCEPHALON 271 

is a thin band of fibers, running along the medial surface of the hippocampus 
and joining with the alveus to form the floor of the inferior horn of the lateral 
ventricle (Figs. 201, 204, 209). The fimbria increases in volume as it is traced 
toward the splenium of the corpus callosum, to the under surface of which it 
becomes applied, where, together with its fellow of the opposite side, it forms 
the fornix. 

The fornix, which is represented diagrammatically in Fig. 203, is an arched 
fiber tract, consisting of two symmetric lateral halves, which are separate at 
either extremity, but joined together beneath the corpus callosum. This 
medially placed portion is known as the body of the fornix. From its caudal 
extremity the fimbria diverge, and one of them runs along the medial aspect of 
each hippocampus. In man the hippocampus does not reach the under surface 

Column of fornix 

Body of fornix 



- Hippocampal commissure 

| 

Cms of fornix 

Fimbria of hippocampus 

Fig. 203. Diagram of the fornix. 

of the corpus callosum, and the part of the fimbria which joins the body of the 
fornix, being unaccompanied by hippocampus, is known as the cms fornicis. 
Rostrally the fornix is continued as two arched pillars, the columnce fornicis, 
to the mammillary bodies. 

The body of the fornix is triangular, with its apex directed rostrally. It con- 
sists in large part of two longitudinal bundles of fibers, representing the con- 
tinuation of the fimbriae, widely separated at the base of the triangle, but closely 
approximated at the apex, whence they are continued as the columnae fornicis. 
At the point where these longitudinal bundles diverge toward the base of the 
triangle they are united by transverse fibers which join together the two hippo- 
campi by way of the fimbriae. These fibers constitute the hippocampal com- 
missure. This part of the fornix, because of its resemblance to a harp, was 
formerly known as the psalterium (Fig. 184). The hippocampal commissure 




272 



THE NERVOUS SYSTEM 



is not very evident in the human brain, but can be easily dissected out in the 
sheep (Fig. 204). 

The columns fornicis are round fascicles which can be traced ventrally in 
an arched course to the mammillary bodies (Figs. 203-205). They are placed 
on either side of the median plane. Each consists of an initial free portion, 
which forms the rostral boundary of the interventricular foramen, and a cov- 
ered part, which runs through the gray matter in the lateral wall of the third 
ventricle to reach the mammillary body (Figs. 204, 205). 

The relations of the fornix are well shown in Figs. 155, 200, and 205. The 
body of the fornix intervenes between the corpus callosum, septum pellucidum, 



Body of corpus callosum 

Lateral ventricle 
Genu of corpus callosum 



Body of fornix 

Hippocampal commissure 
! Thalamus 




Splenium of corpus callosum 
. Lateral ventricle 

Chorioid fissure 
Hippocampus 



Anterior commissure 



- Fimbria of hippo- 
campus 
-W- Hippocampal 

fissure 

-/- Hippocampal gyms 
^^^r^ 

' Dentate fascia 

Mammillothalamic tract 
Mammillary body 
Infundibulum 

Fig. 204. Dissection of the cerebral hemisphere of the sheep to show the fornix and hippocampus. 

Median view. 



Lamina terminalis 

Optic chiasma \ 

Column of fornix. 



and cavity of the lateral ventricle on the one hand, and the transverse fissure of 
the cerebrum and the thalamus on the other. The fimbria and body of the for- 
nix form one boundary of the chorioid fissure. This fissure, which is shown but 
not labeled in Fig. 205, represents the line along which the chorioid plexus is 
invaginated into the lateral ventricle. When this plexus has been torn out, 
the fissure communicates with the interventricular foramen. 

The septum pellucidum is the thin wall which separates the two lateral ven- 
tricles and fills in the triangular interval between the fornix and the corpus 
callosum (Fig. 205). It consists of two thin vertical laminae separated by a 
cleft-like interval, the cavity of the septum pellucidum (Fig. 177). Each lamina 



THE RHINENCEPHALON 



273 



forms part of the medial wall of the corresponding hemisphere (Fig. 182); and 
the cavity, although sometimes called the fifth ventricle, develops as a cleft 
within the lamina terminalis and, therefore, bears no relation to the true brain 
ventricles, which are expansions of the original lumen of the neural tube (Fig. 
165). 

The anterior commissure, like the hippocampal commissure, belongs to the 
rhinencephalon. It is a rounded fascicle which crosses the median plane in the 
dorsal part of the lamina terminalis just rostral to the columnae fornicus (Fig. 
205). In a frontal section of the brain, like that represented in Fig. 187, it can 



Splenium of corpus callosum 
Sulcus cinguli 



Parieto-occipital fissure 



Cuneus 
Cakarine \ 
fissure 



,/ // Body of corpus callosum 
Body of form* / " Free potion of col. of fornix 

Septum pdlucidum 
Intervent. foramen 
Anterior commiss. 

Genu of 
corpus 
callosum 




Occipila 
lobe 

Crus offornix 

Thalamus 

Fimbria of hippocampus 
Dentate fascia of hippocampus 



Uncus 



Olfactory 

bulb 

Olfactory tract 
Rostrum offorpus col. 
, Rostral lamina 
Optic nerve 

' 'Covered portion of column of 
Mammillary body fornix 



Mammillothalamic tract 

Fig. 205. Dissection of the human cerebral hemisphere to show the fornix. Median view. 

(Sobotta-McMurrich.) 

be traced lateralward through the most ventral part of the lentiform nucleus. 
It consists of two parts (Fig. 206). Of these, the more rostral is shaped like a 
horseshoe and joins together the two olfactory bulbs. This part can be readily 
dissected out in the sheep's brain (Fig. 199), but is poorly developed in man. The 
remaining portion, and in man the chief component, joins the pyriform areas 
of the two hemispheres together (Cajal, 1911). 

We are now sufficiently acquainted with the anatomy of the rhinencephalon 
to undertake a study of the structure and connections of its various parts. 
Because of the wealth of detail which this subject offers we must confine our at- 

18 



274 



THE NERVOUS SYSTEM 



tention to the more important facts. Cajal (1911) has carried out extensive 
investigations concerning the structure and connections of the olfactory parts 
of the brain both in man and the smaller macrosmatic mammals, especially the 
mouse. His results, which differ in many respects from the ideas previously 
current, have been brought together in his "Histologie du Systeme Nerveux," 
Vol. II, pp. 646-823. The account which follows is largely based on his work. 




Fig. 206. Horizontal section of the rostral portion of the cerebral hemispheres of a mouse to 
show the anterior commissure. Golgi method. A, anterior and B, posterior portions of anterior 
commissure; G, anterior column of the fornix. (Cajal.) 

Structure and Connections of the Olfactory Bulb. In the olfactory portion 
of the nasal mucous membrane there are located bipolar sensory cells, each with a 
thick peripheral process, the ciliated extremity of which reaches the surface of 
the epithelium. These are the olfactory neurons of the first order, and their 
slender central processes are the unmyelinated axons which constitute the olfac- 
tory nerves. These fibers are gathered into numerous small bundles, the fila- 



THE RHINENCEPHALON 



275 



ments of the olfactory nerve, which pass through the cribriform plate of the eth- 
moid bone and immediately enter the olfactory bulb (Fig. 207). Here they 
form a feltwork of interlacing fibers over that surface of the bulb which is in 
contact with the cribriform plate. 

The olfactory bulb of man is solid, and the original cavity is represented by a 
central gray mass of neuroglia. This is surrounded by a deep layer of myelinated 




Fig. 207. Diagram showing the direction of conduction in the olfactory nerve bulb and tract: 
A, lateral olfactory stria; B, anterior portion of the anterior commissure; C, bipolar cells of the 
olfactory mucous membrane. (Cajal.) 

nerve-fibers passing to and from the olfactory tract. Superficial to this are several 
layers of gray matter of very characteristic structure, and this, in turn, is covered 
with the superficial layer of unmyelinated fibers from the olfactory nerve fila- 
ments. Within the gray matter of the bulb are found three types of neurons, 
the mitral, tufted, and granule cells. The large mitral cells are the most char- 



276 



THE NERVOUS SYSTEM 



acteristic element; and their perikarya are closely grouped together, forming 
a well-defined layer (Fig. 208, C). The tufted cells are smaller and more super- 
ficially placed (Fig. 208, B). The larger dendrites from both these types of' 
neurons are directed toward the superficial fiber layer. Each of these dendrites 




Fig. 208. Section of the olfactory bulb of a kitten. Golgi method. A, Layer of glomeruli; 
B, external plexiform layer; C, layer of mitral cells; D, internal plexiform layer; E, layer of granules 
and white substance; /, J, granule cells; a, b, glomeruli, showing the terminations of the olfactory 
nerve-fibers; c, glomerulus, showing the terminal arborization of a dendite of a mitral cell; d, 
tufted cells; e, mitral cell; h, recurrent collateral from an axon of a mitral cell. (Cajal.) 

breaks up into many branches, which form a compact rounded bushy terminal. 
The terminal ramifications of olfactory nerve-fibers interlace with these dendritic 
branches, and the two together form a circumscribed, more or less spheric ol- 
factory glomerulus (Fig. 208, A). These relations were demonstrated by Cajal 



THE RHINENCEPHALON 277 

in 1890, and possess considerable theoretic and historic interest. Since in these 
glomeruli the olfactory nerve-fibers come into contact with only the dendritic 
ramifications of the mitral and tufted cells, it is evident that these dendrites 
must take up and transmit the olfactory impulses. That is to say, these glomer- 
uli furnished positive proof that the dendrites are not, as had been thought by 
many investigators, merely root-like branches which serve for the nutrition of 
the cell. The mitral cells are larger than the tufted cells and their axons are 
thicker. These coarse axons are directed for the most part into the lateral ol- 
factory stria; while the finer axons of the tufted cells pass through the anterior 
commissure to the opposite olfactory bulb (Fig. 207). The axons of the deeply 
placed granule cells are relatively short and are directed toward the surface of 
the bulb. 

The olfactory tract consists of fibers passing to and from the olfactory bulb. 
Through it each bulb receives fibers from the other by way of the anterior com- 
missure as well as from the hippocampal cortex. The fibers leaving the olfac- 
tory bulb are the axons of the mitral and tufted cells. By far the greater number 
of the axons of the mitral cells are continued into the lateral olfactory stria. A 
much smaller number terminates in the olfactory trigone and in the tuberculum 
olfactorium within the anterior perforated substance. Other fibers are said to 
pass by way of the medial olfactory stria to the parolfactory area of Broca, to 
the subcallosal gyrus, and to the septum pellucidium, but this is open to ques- 
tion. The fibers of the lateral olfactory stria run upon the surface of the lateral 
olfactory gyrus, also known as the frontal olfactory cortex, to which they give 
off collaterals (Fig. 207). The terminal fibers reach the uncus and part of the 
hippocampal gyrus. The chief olfactory centers of the second order are, there- 
fore, found in the pyriform area. 

According to Cajal (1911), the hippocampal gyrus may be subdivided in man, as in the 
mammals, into five areas: (1) the external region near the rhinal fissure; (2) the principal 
olfactory region, the most salient part of the convolution; (3) the presubiculum, a transitional 
area between 2 and 4; (4) the subiculum, near the hippocampal fissure, and (5) the caudal 
olfactory region, including the caudal part of the hippocampal gyrus. Of these five regions, 
Cajal finds fibers from the lateral olfactory stria going to the second or principal olfactory 
region only. The presubiculum and subiculum and the caudal olfactory region represent 
olfactory association centers. The subiculum is characterized by the presence of a thick 
layer of myelinated fibers upon its surface. 

The hippocampus, which constitutes an olfactory center of a still higher 
order, is directly continuous with the portion of the hippocampal gyrus known 
as the subiculum (Fig. 209), and is formed by a primitive portion of the cortex 



278 



THE NERVOUS SYSTEM 



that has been rolled into the ventricle along the line of the hippocampal fissure. 
Upon its ventricular surface it is covered by a thin layer of white matter, known 
as the alveus, through which the fibers arising in the hippocampus reach the 
fimbria and the fornix. Beginning at the line of separation from the fascia 
dentata, we may enumerate the constituent layers of the hippocampus as fol- 
lows: the molecular layer, the layer of pyramidal cells, and the layer of poly- 
morphic cells (Figs. 209, 210). 

The molecular layer contains a superficial stratum of tangential fibers derived 
from the corresponding layer of the subiculum and from bundles of fibers that 




Fig. 209. Cross-section of the hippocampus and hippocampal gyrus of man. (Edinger.) 

perforate the cortex of the subiculum (Fig. 210). More deeply placed is another 
fiber layer, containing collaterals from the pyramidal cells as well as collateral and 
terminal fibers from the alveus, and known as the stratum lacunosum. The 
molecular stratum in the hippocampus resembles that in other parts of the cortex 
in containing the terminal branches of the apical dendrites from the pyramidal 
cells, and a few nerve-cells which for the most part belong to Golgi's Type II. 
The Layer of Pyramidal Cells. The pyramidal cells are all of medium size 
and their fusiform bodies are rather closely packed together, forming a well- 



THE RHINENCEPHALON 



279 



defined zone, the stratum lucidum. Their apical dendrites are directed toward 
the molecular layer and form the chief constituent of the stratum radiatum. 
The axons of these cells, after giving off collaterals, enter the alveus. 

The layer of polymorphic cells, also known as the stratum oriens, contains 
cells of Martinotti, that send their axons into the molecular layer, and still other 
cells the axons of which enter the alveus. 

The alveus is a thin white stratum which separates the preceding layer from 
the ventricle. It is continuous, on the one hand, with the white center of the 



Alveus 
Layer of polymorphic cells 

Layer of pyramidal 
cells 

Stratum lucidum\ 
Stratum radia- 
tum 



. Molecular layer 
I Stratum lacunosum 
Tangential fibers 



Lateral ventricle 



Fimbria 




Hippocampus / 

Fascia dentata 



Molecular layer 
Granule layer 
Layer of polymorphic cells 



Subictilum 



Fig. 210. Diagram of the structure and connections of the hippocampus. The arrows 
show the direction of conduction: A, molecular layer, and B, pyramidal cell layer of the subic- 
ulum; F, hippocampal fissure. (Cajal.) 

hippocampal gyrus, and on the other with the fimbria. Through it the efferent 
fibers of the hippocampus enter the fimbria and fornix. The fibers of the hippo- 
campal commissure are also carried in the fimbria and enter the hippocampus 
through the alveus. 

The fascia dentata also belongs to the archipallium and is closely related to 
the hippocampus, which it resembles somewhat in the structure of its three 
strata: the molecular layer, granule layer, and layer of polymorphic cells (Fig. 
210). The granules may be regarded as modified pyramidal cells of small size, 
ovoid or fusiform in shape. Each possesses instead of a single apical dendrite 
'two or three branching processes which extend into the molecular layer. The 



2 g THE NERVOUS SYSTEM 

axons are directed into the layer of pyramidal cells of the hippocampus. Orig- 
inally this layer of pyramidal cells was continuous with the granule layer of 
the fascia dentata, but in all the higher mammals a break in this cellular stratum 
has occurred at the point of transition between the two divisions of the archi- 

pallium. 

THE OLFACTORY PATHWAYS 

Impulses reach the glomeruli of the olfactory bulb along the fibers of the 
olfactory nerve and are here transferred to the dendrites of the mitral cells. 
Axons arising from these cells and running in the lateral olfactory stria transmit 
the impulses to the pyriform area (Fig. 207), whence they are conveyed to the 
hippocampus and fascia dentata by fibers entering the molecular layer in both 
of these parts of the hippocampal formation (Fig. 210). 

According to Cajal, the fibers of the lateral olfactory stria terminate in the principal 
olfactory region of the hippocampal gyrus, and there are present within the cortex of the 
pyriform area sagittal association fibers which unite the principal olfactory region with the 
caudal olfactory region of the hippocampal gyrus. From this latter region fibers reach the 
hippocampus and fascia dentata. These are relatively thick fibers which are found at first 
in the angle of the subiculum and can be traced through all the layers of that center into 
the molecular layer of the hippocampus and fascia dentata (Fig. 210, B). Within the molec- 
ular layer the impulses are transferred from these fibers to the dendrites of the pyramidal 
and granule cells. It was formerly supposed that fibers from the trigonum olfactorium, 
substantia perforata anterior, and septum pellucidum reached the hippocampus through 
the strize longitudinales and the fornix, and served as the chief conductors of afferent im- 
pulses toward the hippocampus. But according to Cajal, "The hippocampus does not receive 
olfactory impulses from the frontal region of the brain, nor through the intermediation of the 
septum pellucidum." 

The efferent fibers from the hippocampus represent the axons of the pyra- 
midal cells. These penetrate the stratum oriens and enter the alveus (Fig. 
210). Thence they are continued into the fimbria and fornix. They include 
both commissural and projection fibers. The commissural fibers serve to unite 
the two hippocampi and run through the hippocampal commissure as the trans- 
verse fibers of the psalterium. The projection fibers are continued rostrally; 
and in their course through the body of the fornix they form on either side of 
the median plane a longitudinal bundle, which is continued into the columna 
fornicis (Fig. 203). The latter bends caudally into the hypothalamic region, 
giving off fibers to the tuber cinereum and the mammillary body. The remaining 
fibers of the columna fornicis undergo a decussation just behind the mamillary 
body and are continued in the reticular formation of the brain stem as far, at 
least, as the pons. It will be obvious that the fornix is the efferent projection 



THE RHINENCEPHALON 



28l 



tract of the archipallium and serves to convey impulses from the hippocampus 
to the hypothalamus and reticular formation of the brain stem. Through the 
mammillary bodies olfactory impulses are relayed along the mammillothalamic 
tract to the anterior nucleus of the thalamus, and along the mammillotegmental 
bundle to the tegmentum of the pons and medulla oblongata (Fig. 21 1,/, g). 

The frontal olfactory projection tract takes origin from the gray matter of 
the olfactory peduncle or trigonum olfactorium and the gyrus olfactorius later- 




Fig. 211. Diagram of the afferent and efferent paths of the mammillary body, habenular 
ganglion, and interpeduncular ganglion: A, Medial nucleus of the mammillary body; B, C, 
anterior nucleus of the thalamus; D, habenular ganglion; E, interpeduncular ganglion; F, dorsal 
tegmental nucleus; J, optic chiasma; T, tuber cinereum; P, pons; a, cerebral aqueduct; b, habenular 
commissure; c, posterior commissure; d, fasciculus retroflexus of Meynert; e, peduncle of the mam- 
millary body;/, fasciculus mamillothalamicus; g, tegmental tract of Gudden; h, frontal olfactory 
projection tract; i, stria medullaris thalami. The arrows indicate the direction of conduction. 
(Cajal.) 

alis. It traverses the subthalamic region to reach the pons and medulla oblon- 
gata. A bundle of fibers, consisting in part of collaterals, is given off from it, 
to enter the stria medullaris thalami, which we have already traced to the habe- 
nular ganglion (Fig. 211, h,i). 

The stria terminalis is a delicate fascicle of nerve-fibers which lies in the sulcus between 
the thalamus and caudate nucleus (Figs. 155, 177), and accompanies the tail of the latter in 



282 THE NERVOUS SYSTEM 

the roof of the inferior horn of the lateral ventricle. According to Cajal (1911), it contains 
both commissural and projection fibers, the majority of which take origin from the olfactory 
cortex of the hippocampal gyms. A smaller number may arise in the amygdaloid nucleus. 
After following the curved course of the caudate nucleus, it bends ventrad toward the 
anterior commissure. Some of the fibers cross in the anterior commissure and end in the 
olfactory cortex of the opposite temporal lobe and in the septum pellucidum. The majority 
of the fibers, however, enter the mesencephalon and apparently end in the interstitial nucleus. 

The striae longitudinales, fornix longus, and the fiber tracts found in the 
subcallosal cortex and septum pellucidum have apparently been subject to 
much misinterpretation; but the subject is too extensive to be considered here. 
(See Cajal, Histologie du Systeme Nerveux, Vol. II, pp. 783-823.) 

The anterior perforated substance, or at least its more rostral part, which 
corresponds to the tuberculum olfactorium of macrosmatic mammals, receives 
besides fibers from the olfactory tract other afferent fibers which, according to 
Edinger (1911), come from the pons, perhaps from the sensory nucleus of the 
trigeminal nerve. It is probably "especially concerned with the feeding reflexes 
of the snout or muzzle, including smell, touch, taste, and muscular sensibility, 
a physiologic complex which Edinger has called collectively the 'oral sense' " 
(Herrick, 1918). 



CHAPTER XVIII 




THE CORTEX AND MEDULLARY CENTER OF THE CEREBRAL 

HEMISPHERE 

THE cerebral cortex forms a convoluted gray lamina, covering the cerebral 
hemisphere, and varies in thickness from 4 mm. in the anterior central gyrus 
to 1.25 mm. near the occipital pole. When sections through a fresh brain are 
examined macroscopically, the cortex is seen to be composed of alternating 
lighter and darker bands, the light stripes being produced by aggregations of 
myelinated nerve-fibers (Fig. 212). 

Nerve-fibers. In addition to a very thin superficial white layer of tangential 
fibers there are in most parts of the cerebral cortex two well-defined white bands, 
the inner and outer lines of Baillarger 
(Figs. 212, 215). These two bands con- 
tain large numbers of myelinated nerve- 
fibers running in planes parallel to the 
surface of the cortex. In the region of 
the calcarine fissure only the outer line 
is visible; but this is very conspicuous 
and is here known as the line of Gennari. 
Myelinated fibers enter the cortex from 
the white center in bundles that in 
general have a direction perpendicular 
to the surface of the cortex. These 
bundles radiate into each convolution from its central white core and separate 
the nerve-cells into columnar groups, thus giving the cortex a radial striation 
(Fig. 215). 

Many of the fibers in these radial bundles are corticifugal, representing the 
axons of the pyramidal and polymorphic cells of the cortex. Within the medul- 
lary center they run (1) as association fibers to other parts of the cortex of the 
same hemisphere, (2) as commissural fibers through the corpus callosum to the 
opposite hemisphere, or (3) as projection fibers to the thalamus and lower .lying 
centers. The others are corticipetal and are derived in part from the thalamic 
radiation; but an even greater number of them are the terminal portions of as- 

283 



Fig. 212. Schematic sections of cerebral 
gyri showing the alternate lighter and darker 
bands which compose the cerebral cortex: 1 
shows the layers as seen in most parts of the 
cerebral cortex; 2, the layers as seen in the 
region of the calcarine fissure. (Baillarger, 
Quain's Anatomy.) 



284 



THE NERVOUS SYSTEM 



sociation and commissural fibers from other parts of the cortex. Many of these 
fibers end in the most superficial stratum of the cortex, the plexiform layer, where 
the terminal branches of the apical dendrites of the pyramidal cells are widely 
expanded (Fig. 214). Others terminate as indicated in Fig. 213, where they 




Fig. 213. From the anterior central gyrus of 
the human cerebral cortex, showing the terminations 
of corticipetal fibers: a, b, Afferent fibers; B, dense 
network produced by the terminal branches of such 
fibers. Golgi method. (Cajal.) 




Fig. 214. Nerve-cells and neuroglia 
from the cerebral cortex: A, Neuroglia; B, 
horizontal cells of Cajal ; C, pyramidal cells; 
D, cell of Martinotti; E, stellate cell. 



are seen forming a close network of unmyelinated fibers. Enmeshed in the 
dense fiber plexus indicated at B, Fig. 213, are the pyramidal cells illustrated 
in Layer III of Fig. 215. 

The nerve-cells of the cortex are disposed in fairly definite layers as indicated 
in Fig. 215. We may enumerate five well-recognized varieties: (1) the pyra- 



THE CORTEX AND MEDULLARY CENTER OF THE CEREBRAL HEMISPHERE 285 

midal, (2) the stellate, and (3) the polymorphous cells, as well as (4) the hori- 
zontal cells of Cajal, and (5) the cells of Martinotti. 

The pyramidal cells are the most numerous and are classified as small, 
medium, large, and giant pyramidal cells (Fig. 215). From the base of a pyra- 
midal cell body an axon extends toward the subjacent white matter, giving 
off collaterals which ramify in the adjacent cortex (Figs. 23, 214, C). The den- 
drites are of two kinds: a large apical dendrite and numerous smaller ones at- 
tached to the base and sides of the pyramid. The apical dendrite appears as an 
extension of the cell body and is directed toward the surface of the cortex, near 
which it ends in spreading branches. Its length varies with the depth of the 
cell body from the surface. To an even greater extent than other dendrites it is 
provided with short thorny processes called "spines" or "gemmules." These 
are supposed by some to effect contact with neighboring axonic ramifications 
and to be retractile. Upon retraction of these gemmules, conduction across 
the synapse would be interrupted for the time being; and one might explain 
the varying sensory thresholds of an individual in sleep or during attention by 
the varying degree of expansion of the gemmules. But as yet no satisfactory 
evidence in support of the theory has been presented. 

The stellate cells are also known as granules. They are, for the most part, 
of small size, and their short axons branch repeatedly and terminate in the 
neighborhood of the cell of origin. That is to say, they are cells of Golgi's 
Type II. Although they occur in most layers of the cortex, they are especially 
numerous in the fourth stratum, which is accordingly designated as the layer 
of small stellate cells (Figs. 214, ; 215). 

The cells of Martinotti, which are also found in most of the cortical strata, 
have this as their distinguishing characteristic, that their axons are directed 
toward the surface of the cortex and ramify in the superficial layer (Fig. 214, D}. 

The horizontal cells of Cajal, which are present only in the superficial layer, 
are fusiform, with long branching dendrites directed horizontally. Their axons 
are long and form tangential myelinated fibers in the superficial layer (Fig. 214,5). 

Polymorphous cells, fusiform or angular in shape, are found in the deepest 
stratum of the cortex (Figs. 214, 215). Their axons enter the subjacent white 

matter. 

CELL AND FIBER LAMINATION 

The size and type of cells found in the cortex vary at different depths from 
the surface, that is to say, the cells are disposed in fairly definite layers. As 
already indicated, many of the myelinated fibers are arranged in bands parallel 



286 



THE NERVOUS SYSTEM 



to the surface. By means of this cell and fiber lamination Brodmann (1909) 
recognizes six layers in the cerebral cortex (Fig. 215). Other authors, notably 
Campbell (1905) and Cajal (1906), number these layers somewhat differently. 
Moreover, the arrangement varies in different parts of the cortex. In certain 



Via 



VIb 




> - >- 

4({\\$^ 




Fig. 215. Diagram of the structure of the cerebral cortex: 7, Molecular layer; II, layer of 
small pyramidal cells; ///, layer of medium-sized and large pyramidal cells; IV, layer of small 
stellate cells; V, deep layer of large pyramidal cells; VI, layer of polymorphic cells; ja 1 , band of 
Bechterew; 4, outer band of Baillarger; 56, inner band of Baillarger. (Brodmann.) 

regions one or more of the strata may be reduced, enlarged or subdivided, but 
the arrangement in most parts is substantially like that illustrated. The six 
layers are as follows: 

1. The molecular layer (plexiform layer, lamina zonalis) is the most super- 
ficial. It contains the superficial band of tangential myelinated fibers and many 



THE CORTEX AND MEDULLARY CENTER OF THE CEREBRAL HEMISPHERE 287 

neuroglia cells. The nerve-cells are of two kinds: (1) horizontal cells of Cajal, 
and (2) cells of Golgi's Type II. Within this layer ramify the terminal branches 
of the apical dendrites from the pyramidal cells of the deeper layers. 

2. The layer of small pyramidal cells (lamina granularis externa) contains a 
large number of small nerve-cells. Most of these are small pyramids with axons 
running to the white center of the hemisphere. Others belong to the short- 
axoned group (Golgi's Type II). 

3. The layer of medium-sized and large pyramidal cells (lamina pyramidalis) 
may be subdivided into two substrata, the more superficial stratum containing 
chiefly medium-sized pyramids and the deeper one chiefly large pyramids. There 
are also present cells of Golgi's Type II and cells of Martinotti. According to 
Cajal (1900-1906) and Campbell (1905), it is within this layer that the outer 
stripe of Baillarger is located, but Brodmann places this line in the next layer. 

4. The layer of small stellate cells (lamina granularis interna) is characterized 
by the presence of a large number of small multipolar cells with short axons 
(Golgi's Type II). Scattered among these are small pyramids. Brodmann 
places the outer line of Baillarger in this stratum. 

5. The deep layer of large pyramidal cells (lamina ganglionaris) contains the 
largest cells of the cortex. In the motor region these are known as the giant 
pyramidal cells of Betz and give origin to the fibers of the corticospinal tract. 
The apical dendrites of these cells are very long and, like those of the more super- 
ficial pyramidal cells, reach and ramify within the molecular layer. Smaller 
cells, both of the pyramidal and short-axoned type, are also present. The 
horizontal fibers of Baillarger 's internal line are found in this layer in most of 
the cortical areas. 

6. The layer of polymorphic cells (lamina multiformis) contains irregular 
fusiform and angular cells, the axons of which enter the subjacent white matter. 

Cortical Areas. The six layers of the cortex are arranged in most regions 
essentially as shown in Fig. 215. But each of more than forty areas presents its 
own characteristic variation in the structure, thickness, and arrangement of 
the cellular layers, in the thickness of the cortex as a whole, in the number of 
afferent and efferent myelinated fibers, and in the number, distinctness, and posi- 
tion of the white striae. On the basis of such differences the entire cortex has 
been subdivided into structurally distinct areas. Maps of such cortical areas 
have been furnished by Brodmann (1909), Campbell (1905), and Elliot Smith 
(1907) ; and while these vary in detail, they agree in their larger outlines. The 
existence and general boundaries of these regions are now well established; and 



288 



THE NERVOUS SYSTEM 



as a result of experimental and pathologic research it is known that specific 
differences in function are correlated with these differences in structure. 

The maps of the cortical areas furnished by Brodmann are reproduced in 
Figs. 216 and 217. He recognizes eleven general regions, and each of these may 




20 

Fig. 217. 

Figs. 216 and 217. Areas of the human cerebral cortex each of which possesses a distinctive 
structure: Fig. 216, lateral view; Fig. 217, medial view. (Brodmann.) 

be subdivided into smaller areas on the basis of characteristic differences in 
structure. Some of these differences are visible to the naked eye and have 
been represented in Fig. 218. 



THE CORTEX AND MEDULLARY CENTER OF THE CEREBRAL HEMISPHERE 289 

Myelination. The fibers in the various parts of the cortex acquire their 
myelin sheaths at different tunes. On this basis Flechsig (1896) identified 
thirty-six areas, which are numbered in Fig. 219 in the order of myelination. He 
recognizes three main groups: primary (Nos. 1 to 12), intermediate (Nos. 13 




Fig. 218. Diagram showing the differences in thickness and in the arrangement of the lighter 
and darker bands in the human cerebral cortex in different regions as seen with the naked eye: 
A, Motor cortex from anterior central gyrus; B, sensory cortex from the posterior central gyms; 
C, visual cortex from the region of the calcarine fissure; D, auditory cortex from the anterior 
transverse temporal gyrus. (Redrawn after Elliot Smith.) 

to 28), and late (Nos. 28 to 36). According to Flechsig, the primary areas, 
which are myelinated at birth, are projection centers and receive the sensory 
radiation from the thalamus; while the other parts of the cortex, not being pro- 
vided with projection fibers, serve only as association centers. He believed that 




Fig. 219. Lateral view of the human cerebral hemisphere, showing the cortical areas as 
outlined by Flechsig on the basis of differences in the time of myelination of their nerve-fibers. 
The primary areas (first to become well myelinated) are cross-hatched; the intermediate are 
indicated by vertical lines; the late areas are unshaded. (Lewandowsky.) 

myelination of nerve-fibers takes place in the order of conduction, that is, the 
sheaths are developed first on the afferent fibers, reaching the cortex from the 
thalamus, and later on the association fibers, linking the various areas together. 
According to this conception fibers of like function tend to become myelinated 
19 



2 QO THE NERVOUS SYSTEM 

at the same time. Much of Flechsig's work has failed to stand the test of rigid 
examination. It is now known that practically all regions of the cortex, in- 
cluding those designated by him as association centers, are connected with the 
thalamus or lower lying centers by afferent or efferent projection fibers. It 
has been shown that the more mature areas fade off gradually into those whose 
differentiation is less advanced, and that sharply outlined zones such as are 
indicated in his figures do not exist. Nevertheless, it is true that the regions 
designated by him as primary areas, though not sharply outlined by this method 
from the surrounding cortex, do mature first, and the myelination spreading 
from these areas reaches its completion last in those areas included in his late 
group (Brodmann, 1910). The primary areas include the region surrounding 
the central fissure, the region around the calcarine fissure, a portion of the 
superior temporal gyrus, and a part of the hippocampal gyrus. These areas 
are associated with especially important projection tracts and may properly 
be spoken of as projection centers. 

CORTICAL OR CEREBRAL LOCALIZATION 

In opposition to the crude conceptions of the localization of cerebral functions 
introduced by Gall (1825), which formed the basis for phrenology, the French 
physiologist Florens maintained the doctrine that all parts of the cerebrum are 
functionally equivalent. In 1861 Broca demonstrated that destruction of the 
left third frontal convolution may result in a loss of ability to speak; and nine 
years later Fritsch and Hitzig (1870) discovered that electric excitation of the 
cortex in the region of the central sulcus will elicit movements from muscles of 
the opposite side of the body. These observations, confirmed and extended 
by many observers, definitely proved that certain cortical areas possess spe- 
cialized functions. Physiologic and pathologic researches have served to out- 
line a number of these with considerable precision, and it is possible to identify 
them with regions of characteristic cell and fiber lamination. In this way evi- 
dence derived from histologic studies reinforces that drawn from physiology and 
pathology. 

The motor projection center is located in the anterior -wall of the central sulcus, 
in the adjacent part of the anterior central gyrus, and in that part of the para- 
central lobule which lies rostral to the continuation of the central sulcus on the 
medial surface of the hemisphere (Figs. 220, 221). It coincides fairly closely 
with Area 4 of Brodmann's charts (Figs. 216, 217). This is the center from which 
the impulses initiating voluntary movements on the opposite side of the body 






THE CORTEX AND MEDULLARY CENTER OF THE CEREBRAL HEMISPHERE 2QI 



descend to the motor nuclei of the cerebrospinal nerves. It is subdivided into 
areas, each of which controls the muscles moving a given part of the opposite 
half of the body; and these are arranged in inverted order, beginning with the 
center for movement of the toes near the dorsal border of the hemisphere, and 
ending with that for the face at the lower end of the anterior central gyrus (Fig. 
236). 

The structure of the motor cortex is characteristic. Here the gray matter 
reaches the maximum thickness, the lines of Baillarger are broad and diffused 
(Fig. 218). The fifth layer contains the giant pyramidal cells of Betz, from 
which arise the fibers of the corticospinal and corticobulbar tracts. These 
cells undergo chromatolysis when these motor tracts are cut; and when the motor 
cortex is destroyed the tracts degenerate (Holmes and May, 1909). 



Motor projection center 



Somesthetic area 

Auditory re- 
ceptive center 




Motor projection center 



Somesthetic area 



Visual receptive center 

Fig. 220. Diagram of the cortical pro- 
jection centers on the lateral aspect of the 
cerebral hemisphere. 




Visual re- 
Olfactory center ce P the center 

Fig. 221. Diagram of the cortical pro- 
jection centers on the medial aspect of the 
cerebral hemisphere. 



The motor cortex of the chimpanzee corresponds in its arrangement with 
that of man; and by the electric excitation of its different portions muscular 
contractions can be excited in the corresponding parts of the opposite side of 
the body (Griinbaum and Sherrington, 1903). In addition, there is an area 
farther forward in the frontal lobe the stimulation of which produces conjugate 
movements of the eyes. A similar center for the conjugate deviation of the 
head and eyes is situated in the posterior part of the middle frontal gyrus in 
man. It is probable, however, that this motor center is of a different kind 
from those found in the anterior central gyrus, from which all of the fibers of 
the pyramidal system are believed to take their origin (Fig. 236). 

The sensory projection centers are the areas within which terminate the 
sensory projection fibers. We have learned to locate such centers for vision, 



THE NERVOUS SYSTEM 



hearing, smell, and the general sensations from the surface of the body and the 
deeper tissues. The latter region, known as the common sensory or somesthetic 
area, is located in the posterior central gyrus (Areas 1, 2, and 3 of Brodmann). 
It receives fibers belonging to the thalamic radiation from the lateral nucleus of 
the thalamus and representing neurons of the third order in the afferent paths 
from the skin, muscles, joints, and tendons. 

The most conclusive evidence of the sensory function of the posterior central 
gyrus is furnished by Cushing's (1909) observations on the electric excitability 
of the human cerebral cortex. These tests were made on unanesthetized patients 
in the course of operations for brain tumors. Stimulation of the cortex within 
the posterior central gyrus caused the patients to experience cutaneous sensa- 
tions, which seemed to come from the skin of the hand, but did not elicit any 
motor responses; while in these same cases stimulation of the anterior central 



Calcarine fissure- 



Transition between striate 
and peristriate areas 

Cuneus - 




Tangential fibers 

^ - - -Stria of Gennari 

-White center 



Fig. 222. Section through the most rostral part of the cuneus. Pal-Weigert method. 



gyrus gave rise to no sensations, but did call forth muscular contractions. On 
the other hand, Head (1918), in a recent study of "Sensation and the Cerebral 
Cortex," would include in the somesthetic area the anterior as well as the posterior 
central convolution, and also the anterior part of the superior parietal lobule 
and the angular gyrus. This study shows, perhaps better than any other work, 
how intricate and difficult the problem of cortical localization really is and how 
far we are from an ultimate solution. 

The visual receptive center is located in the cortex forming the walls of the 
calcarine fissure and in the adjacent portions of the cuneus and the lingual 
gyrus (Figs. 217, 221). Rostral to the point where the calcarine is joined by the 
parieto-occipital fissure the visual cortex is located only along the ventral side 
of the former. Sometimes the center may extend around the occipital pole on 
to the lateral surface of the brain (Fig. 216, Area 17). The structural peculiar- 
ities of the visual cortex are very evident. It is not more than one-half as thick 



THE CORTEX AND MEDULLARY CENTER OF THE CEREBRAL HEMISPHERE 293 

as the motor cortex, and the outer line of Baillarger is greatly increased in thick- 
ness and known as the line of Gennari (Fig. 218, C). Because of the prominence 
of this line the region is known as the area striata. It is surrounded by cortex 
of quite different structure; and nowhere can the differences in adjacent cortical 
areas be better illustrated than at its border, where the prominent line of Gennari 
is seen to terminate abruptly (Fig. 222). The fibers of the optic radiation from 
the pulvinar and lateral geniculate body terminate in the visual projection center. 
These fibers carry impulses from the temporal side of the corresponding retina 
and the nasal side of the opposite one. The visual cortex of one hemisphere, 
therefore, receives impressions from the objects on the opposite side of the line 
of vision (Figs. 162, 163). 

The auditory receptive center is located in the anterior transverse temporal 
gyrus, which lies buried in the floor of the lateral sulcus. The area comes to 
the surface near the middle of the dorsal border of the superior temporal gyrus 
(Fig. 220). It receives the auditory radiation from the medial geniculate body. 
The olfactory receptive center is located in the uncus and adjacent portions 
of the hippocampal gyrus (principal olfactory area of Cajal). Within it ter- 
minate the fibers of the lateral olfactory stria. They form a rather thick layer 
of tangential fibers on its surface, which increases the thickness of the plexiform 
layer. 

Association Centers. It will be seen that the sensory and motor projection 
centers occupy only a small part of the entire area of the cortex. The remaining 
parts are connected with these centers by association fibers and are known as 
association centers. Each area of sensory projection is surrounded by a zone 
closely linked up with it by such fibers, and therefore probably under the dom- 
inating influence of the particular sensory impulses reaching that projection 
center. Their positions are indicated by lighter shading in Figs. 220 and 221. 
Campbell (1905) has applied to them the designations "audito-psychic" and 
"visuo-psychic fields" (Figs. 223, 224). The same author has designated 
the portion of the frontal cortex immediately rostral to the motor projection 
center the "intermediate precentral area," and is of the opinion it is especially 
concerned with the "execution of complex movements of an associated kind, 
of skilled movements, and of movements in which consciousness or volition takes 
an active part." There still remains more than half of the cortical area, in- 
dicated in white in Figs. 220 and 221, which is probably less intimately related 
to any particular projection center. The fact that the increased size of the 
human cerebral hemisphere over that of the higher apes is due to the much 



294 



THE NERVOUS SYSTEM 



greater development of the association centers in man, suggests that these are of 
especial significance for the higher intellectual functions. 




VisuQ-tCHSOry 



Fig. 223. 




Fig. 224. 

Figs. 223 and 224. Areas of the human cerebral cortex each of which possesses a distinctive 

structure. (Campbell.) 

In the present state of our knowledge of cortical activity and its relation to 
consciousness it is the part of wisdom to be very conservative in locating any 
mental faculty or fraction of our conscious experience in any particular part of 



THE CORTEX AND MEDULLARY CENTER OF THE CEREBRAL HEMISPHERE 295 

the cerebral cortex. We know upon which areas the auditory, visual, and olfac- 
tory impulses impinge, and less accurately that in which the thalamic radiation, 
mediating general bodily sensibility, terminates. Destruction of these areas 
causes impairment or loss of the corresponding sensations with reference to the 
opposite side of the body or the opposite half of the field of vision. Total loss 
of cutaneous sensibility even within circumscribed areas never results from cor- 
tical lesions; and it seems probable that the thalamic centers are in themselves 
sufficient for a certain low grade, non-discriminative consciousness or awareness 
of cutaneous stimulation. This is particularly true of painful sensations, which 
seem to be for the most part of thalamic origin (Head, 1918). Furthermore, 
the various parts of the cerebral cortex are so intimately linked together by as- 
sociation fibers that when afferent impulses reach a given projection center they 
must not only activate this center, but be propagated to other parts of the cortex 




Motor speech center 



Auditory speech center Visual s P eech center 
Fie. 225. The cortical areas especially concerned with language. 

as well. In view of these facts it is best to express the known facts of cortical 
localization in terms of the relation of particular areas to the known projection 
fiber systems. 

Aphasia. Some idea of the significance of the so-called association centers 
may be obtained from a study of the group of speech defects included under the 
term "aphasia." In right-handed individuals these result from lesions in the 
left hemisphere. Destruction of the triangular and opercular portions of the 
inferior frontal gyrus usually causes loss of ability to carry out the coordinated 
movements required in speaking, but does not impair the ability 'to move the 
tongue or lips (Fig. 225). This defect is known as motor aphasia. Broca's 
center, as this particular part of the cortex is sometimes called, is located in 
Campbell's intermediate precentral area; and motor aphasia serves as a good 
illustration of the importance of the entire intermediate precentral area for the 



296 THE NERVOUS SYSTEM 

execution of skilled volitional movements of an associated kind. In the same 
way, after a lesion in the posterior part of the left superior temporal gyrus, 
the patient may hear the spoken word, but no longer comprehend its meaning. 
This is sensory aphasia or word deafness. Word blindness, the inability to under- 
stand the printed or written language, although there is no impairment of vision, 
may result from lesions in the angular gyrus. These three areas are often spoken 
of as speech centers and are closely united together by association fibers. In 
fact, it is not altogether clear to what extent such defects as those mentioned 
above are dependent upon the destruction of these association tracts which lie 
subjacent to the speech centers. 

THE MEDULLARY CENTER OF THE CEREBRAL HEMISPHERE 

The medullary center of the cerebral hemisphere underlies the cortex and 
separates it from the lateral ventricle and corpus striatum. It varies greatly 
in thickness, from that of the thin lamina separating the insula and the claus- 
trum (Fig. 191) to that of the massive centrum semiovale (Fig. 174). The 
myelinated nerve-fibers of which it is composed are of three kinds, namely, as- 
sociation fibers, projection fibers, and commissural fibers. 

Commissural Fibers. As was stated in Chapter XV, there are three com- 
missures joining together the cerebral hemispheres. Of these, the corpus callo- 
sum is by far the largest and its radiation contributes largely to the bulk of the 
centrum semiovale (Fig. 174). The fibers which compose it arise in the various 
parts of the neopallium of each hemisphere; they are assembled into a broad 
compact plate as they cross the median plane, and then spread out again to 
terminate in the neopallium of the opposite side. As they spread through the 
centrum semiovale they form the radiation of the corpus callosum. Some cor- 
tical areas are better supplied with these fibers than others, few, if any, being 
associated with the visual cortex about the calcarine fissure (Van Valkenburg, 
1913). The majority of the callosal fibers do not connect together symmetric 
portions of the cortex; but, after crossing the median plane, the fibers from a 
given point in one hemisphere spread out to many parts of the opposite side. 
The anterior and hippocampal commissures connect portions of the rhinencephalon 
in one hemisphere, with similar parts on the opposite side. The anterior com- 
missure connects together by its rostral part the two olfactory bulbs and by its 
caudal part the two pyriform areas (Figs. 187, 194, 195). The hippocampal 
commissure is composed of fibers which join together the two hippocampi by 
way of the fimbriae and the psalterium. 



THE CORTEX AND MEDULLARY CENTER OF THE CEREBRAL HEMISPHERE 297 

Projection Fibers. Many of the fibers of the medullary white center connect 
the cerebral cortex with the thalamus and lower lying portions of the nervous 
system. These are known as projection fibers, and may be divided into two 
groups according as they convey impulses to or from the cerebral cortex. The 
corticipetal or afferent projection fibers include the following: (1) the optic radia- 
tion, which arises in the pulvinar of the thalamus and the lateral geniculate 
body and ends in the visual cortex about the calcarine fissure (Fig. 221); (2) the 
auditory radiation, which arises in the medial geniculate body and terminates in 
the auditory cortex of the anterior transverse temporal gyms; (3) the thalamic 
radiation which unites the lateral nucleus of the thalamus with various parts of 
the cerebral cortex, and which forms the ventral, frontal, and parietal stalks of 
the thalamus (Fig. 195). The fibers of the parietal stalk include the sensory 
fibers to the somesthetic cortex in the posterior central gyrus. The lateral ol- 
factory stria, which conveys impulses from the olfactory bulb to the pyriform 
area, is not a projection system in the strict sense of the word, since it begins 
and ends within the telencephalon. 

Efferent projection fibers convey impulses from the cerebral cortex to the 
thalamus, brain stem, and spinal cord. They represent the axons of pyramidal 
cells. The most important groups are those of the corticospinal and corticobulbar 
tracts, which together form the great motor or pyramidal system. These fibers 
begin in the motor cortex of the anterior central gyrus as axons of the giant cells 
of Betz. Entering the white medullary center of the hemisphere, they are as- 
sembled in the corona radiata (Fig. 194) and enter the internal capsule (Fig. 
88). Their course beyond this point has been traced in the preceding chapters. 
They convey impulses to the primary motor neurons of the opposite side of the 
brain stem and spinal cord. Another important group of corticifugal fibers is 
contained in the corticopontine tracts. Of these there are two main strands. 
The frontopontine tract consists of fibers which begin as axons of cells in the cortex 
of the frontal lobe, traverse the centrum semiovale, corona radiata, frontal part 
of the internal capsule and medial one-fifth of the basis pedunculi, and finally 
terminate in the nuclei pontis. The temporopontine tract has a similar origin 
from the cortical cells of the temporal lobe and possibly of the occipital lobe also, 
passes through the sublenticular part of the internal capsule and lateral one- 
fifth of the basis pedunculi, and finally terminates in the nuclei pontis (Figs. 
88, 106). The ascending thalamic radiation is paralleled by descending 
corticothalamic fibers, which should be included among the efferent projection 
systems, although their physiologic significance is not fully understood. Similar 



298 THE NERVOUS SYSTEM 

efferent fibers are contained in the optic radiation. They arise in the cortex 
about the calcarine fissure and terminate in the pulvinar, lateral geniculate 
body, and superior colliculus of the corpora quadrigemina (Fig. 162). A corti- 
corubral tract descends from the frontal lobe through the posterior limb of the 
internal capsule to end in the red nucleus of the mesencephalon. There do not 
appear to be any strictly corticostriate fibers, but, according to Cajal (1911), 
collaterals from the corticospinal fibers are given off to the corpus striatum. 
The efferent projection tracts which we have considered all have their origin in 
the neopallium. 

There are several projection tracts from the rhinencephalon, and of these the 
most important is the fornix. The fibers of this fascicle take origin in the hip- 



Cingulu 




Inferior longitudinal 
fasciculus 

Fig. 226. Some of the important association bundles projected upon the medial aspect of the 
cerebral hemisphere. (Sobotta-McMurrich.) 

pocampus, follow an arched course already described, and, entering the dien- 
cephalon, terminate in part in the mammillary body and in part in the teg- 
mentum of the brain stem (Fig. 205). 

The frontal olfactory projection tract arises from the gray matter of the ol- 
factory peduncle and the lateral olfactory gyrus. It enters the brain stem and 
terminates in the pons and the medulla oblongata (Fig. 211). 

Association Fibers. The various parts of the cortex within each hemisphere 
are bound together by associatiorrfibers of varying length. The short associa- 
tion fibers are of two kinds: (1) those which run in the deeper part of the cortex 
and are designated as intracortical, and (2) those just beneath the cortex, which 
are known as the subcortical fibers. The greater number of these subcortical 
association fibers unite adjacent gyri, curving in U-shaped loops beneath the 



THE CORTEX AND MEDULLARY CENTER OF THE CEREBRAL HEMISPHERE 



299 



intervening sulci, and are accordingly often designated as arcuate fibers (Fig. 
226). Others unite somewhat more widely separated gyri. The long association 
fibers form bundles of considerable size, deeply situated in the medullary center 
of the hemisphere, and unite widely separated cortical areas. There are five 
of these which may be readily displayed by dissection of the human cerebral 
hemisphere, namely, the uncinate, inferior occipitofrontal, inferior longitudinal, 
and superior longitudinal fasciculi, and the cingulum. Another, known as the 
fasciculus occipitofrontalis superior, is less easily displayed. 

The cingulum is an arched bundle which partly encircles the corpus callosum 
not far from the median plane (Figs. 174, 226). It begins ventral to the rostrum 
of the corpus callosum, curves around the genu and over the dorsal surface of 

Optk radiation External capsule and lentiform nucleus 

Corona radiata / Frontal lobe 




~--Fas. occipitofrontalis 

inferior 
~'Fas. uncinatus 

"-Temporal lobe 



Fig. 227. Lateral view of a dissection of a human cerebral hemisphere. The dorsal part 
of the hemisphere has been cut away. On the lateral side the insula, opercula, and adjacent parts 
have been removed. 

that commissure to the splenium, and then bends ventrally to terminate near the 
temporal pole. It is closely related to the gyrus cinguli and the hippocampal 
gyrus and is composed for the most part of short fibers, which connect the various 
parts of these convolutions. 

The uncinate fasciculus connects the orbital gyri of the frontal lobe with the 
rostral part of the temporal lobe. It is sharply bent on itself as it passes over 
the stem of the lateral fissure of the cerebrtffii (Figs. 227, 228). The inferior 
longitudinal fasciculus is a large bundle which runs through the entire length of 
the temporal and occipital lobes (Fig. 226). It connects the occipital pole, 
the cuneus, and other parts of the occipital lobe with the temporal cortex, ex- 
tending as far forward as the temporal pole. According to Curran (1909) the 



300 



THE NERVOUS SYSTEM 



uncinate and inferior longitudinal fascicles are formed by the shorter and more 
superficial fibers of a larger and longer tract, the inferior occipitof rental fasciculus, 



Superior longitudinal fasciculus 




Uncinate fasciculus '' 



Inferior occipitofrontal fasciculus 

Fig. 228. Some of the long association bundles projected upon the lateral aspect of the cerebral 

hemisphere. 

which unites the cortex of the frontal and occipital lobes (Figs. 227, 228). Along 
with the uncinate fasciculus it may easily be exposed by dissection, as it courses 
along the ventrolateral border of the lentiform nucleus. 




Cingulum 

Fas. occipitofrontalis sup. 

Corpus callosum 

Fas. longitudinalis sup. 

Caudate nucleus 

Internal capsule 

Lentiform nucleus 
Insula 

Fas. occipitofrontalis inf. 

Fas. uncinatus 

Amygdaloid nucleus 

Fig. 229. Frontal section of the cerebral hemisphere through the anterior commissure showing the 
location of the long association bundles. 

The superior longitudinal fasciculus (fasciculus arcuatus) is a bundle of as- 
sociation fibers which serves to connect many parts of the cortex on the lateral 



THE CORTEX AND MEDULLARY CENTER OF THE CEREBRAL HEMISPHERE 301 

surface of the hemisphere (Fig. 228). It sweeps over the insula, occupying the 
base of the frontal and parietal opercula, and then bends downward into the 
temporal lobe (Fig. 174). It is composed for the most part of bundles of rather 
short fibers which radiate from it to the frontal, parietal, occipital, and temporal 
cortex. 

The superior occipitofrontal fasciculus runs in an arched course close to the 
dorsal border of the caudate nucleus and just beneath the corpus callosum. It 
is separated from the superior longitudinal fasciculus by the corona radiata 
(Fig. 229). 

The weight of the brain varies with the sex, age, and size of the individual. 
The average weight of the brain in young adult men of medium stature is 
1360 grams. It is less in women and in persons of small size or advanced age. 
It is doubtful if there is any close correlation between the brain weight and 
intelligence or between the latter and the size and arrangement of the cerebral 
convolutions (Donaldson, 1898). 



CHAPTER XIX 

THE GREAT AFFERENT SYSTEMS 
EXTEROCEPTIVE PATHWAYS TO THE CEREBRAL CORTEX 

As has been intimated elsewhere, it is chiefly those nervous impulses, which 
are aroused by stimuli acting upon the body from without, that rise above the 
subconscious level and produce clear-cut sensations. The importance of these 
sensations in our conscious experience is no doubt correlated with the fact that 
it is through the reactions, called forth by such external stimuli, that the organism 
is enabled to respond appropriately to the various situations in its constantly 
changing environment. To meet these complex and variable situations cor- 
rectly requires the nicest correlation of sensory impulses from the various sources 
as well as their integration with vestiges of past experience, and it is in connec- 
tion with these higher correlations and adjustments that consciousness appears. 
The responses initiated by interoceptive and proprioceptive afferent impulses 
are more stereotyped and invariable in character ; and these reactions are for the 
most part carried out without the individual being aware either of the stimulus 
or the response. 

It is known that the cerebral cortex is the organ within which occur at least 
the majority of those complex and highly variable correlations and integrations 
which have consciousness as their counterpart. A single object may appeal 
to many sense organs, and our perception of that object involves a synthesis of 
a corresponding number of sensations and their comparison with past experience. 
For example, when I meet a friend and grasp his hand in greeting, my perception 
of him includes not only the image of his face but also the sound of his voice 
and the warm contact of his hand. Thus thermal, tactile, auditory, and visual 
sensations may be fused in the perception of a single object, and this involves an 
integration of the corresponding afferent impulses within the cerebral cortex. 
Accordingly, it becomes of special interest to trace the course of these afferent 
impulses from the various exteroceptive sense organs to their cortical receptive 
centers. 

As we shall see, the outer world has for the most part a crossed representation 
in the cerebral cortex. Cutaneous stimuli, received from objects touching the 
302 



THE GREAT AFFERENT SYSTEMS 

right side of the body, and optic stimuli produced by light waves coming from 
the right half of the field of vision, are propagated to the cortex of the left hemi- 
sphere. The crossed representation in the case of hearing is less complete, partly 
because every sound wave reaches both ears, but also because the crossing of 
the central auditory pathway seems to be incomplete. 

The grouping of the afferent fibers in the peripheral nerves differs from that 
in the spinal cord. In each of the spinal nerves several varieties of sensory fibers 
are freely mingled. In the cutaneous branches are found conductors of thermal, 
tactile, and painful sensibility; while the deeper nerves contain fibers for pain 
and sensations of pressure-touch as well as for muscle, joint, and tendon sensi- 
bility. Because of the intermingling of the various kinds of fibers a lesion of a 
spinal nerve results in a loss of all modalities of sensation in the area supplied 
exclusively by that nerve. 

But in the spinal cord a regrouping of the afferent impulse occurs, such that 
all of a given modality travel in a path by themselves. All those of touch and 
pressure, whether originally conveyed by the superficial or deep nerves, find 
their way into a common path in the cord. In the same way all painful impulses, 
whether arising in the skin or deeper parts, follow a special course through the 
cord. Another intramedullary path conveys impulses from the muscles, joints, 
and tendons. These various lines of conduction within the cord are so distinct 
from each other that a localized spinal lesion may interrupt one without affecting 
the others. A striking illustration of this is the loss of sensibility to pain and 
temperature over part of the body surface without any impairment of tactile 
sensibility as a result of a disease of the spinal cord, known as syringomyelia. 
While we shall here confine our attention to the afferent channels leading 
directly toward the cerebral cortex, it should not be forgotten that these are in 
communication with the reflex apparatus of all levels of the spinal cord and brain 
stem. 

The Spinal Path for Sensations of Touch and Pressure. Tactile impulses 
which reach the central nervous system by way of the spinal nerves are relayed 
to the cerebral cortex by a series of at least three units. 

Neuron I. The first neuron of this conduction system has its cell body, 
which typically is unipolar, located in the spinal ganglion; and its axon divides 
in the manner of a T or Y into a central and a peripheral branch. The per- 
ipheral branch runs through the corresponding spinal nerve to the skin, or in 
the case of those fibers subserving the tactile functions of deep sensibility, to the 
underlying tissues. The central branch from the stem process of the spinal 



THE NERVOUS SYSTEM 



ganglion cell enters the spinal cord by way of the dorsal roots. The touch fibers 
are probably myelinated and enter the cuneate fasciculus through the medial 
division of the dorsal root; and, like all other dorsal root fibers, they divide into 
ascending and descending branches. The ascending branches run for varying 
distances in the posterior funiculus, giving off collaterals before they terminate 

Internal capsule 



~ Thalamus 



Spinothalamic tract ~ 



Ascending branches of 
dorsal root fibers 



Ventral Spinothalamic tract 




Mesencephalon 



Medulla oblongata 



Medial lemniscus -|- 



Spinal cord 



Dorsal root and spinal ganglion 



Fig. 230. Diagram of the tactile path. 

in the gray matter of the spinal cord, some few at least even reaching the nucleus 
gracilis and cuneatus in the medulla oblongata. At varying levels they enter 
the gray substance of the columna posterior and form synapses with the neurons 
of the second order (Fig. 230). 



THE GREAT AFFERENT SYSTEMS 

Neuron II, with its cell body located in the posterior gray column, sends its 
axon across the median plane into the ventral spinothalamic tract in the opposite 
anterior funiculus. In this the fiber ascends through the spinal cord and brain 
stem to the thalamus. This tract gives off fibers, either collateral or terminal, 
to the reticular formation of the brain stem. Other neurons of the second order 
in the tactile path are located in the gracile and cuneate nuclei of the medulla 
oblongata, and their axons after crossing the median plane ascend in the median 
lemniscus of the opposite side to end in the thalamus. All of these secondary 
tactile fibers end within the ventral part of the lateral thalamic nucleus. 

The course of the ventral spinothalamic tract through the medulla oblongata and pons 
is not accurately known. It has generally been figured as joining the lateral spinothalamic 
tract dorsolateral to the olive (Fig. 230. See also Herrick, Fig. 81). But, since lesions in 
the lateral area of the medulla oblongata may cause a loss of pain and temperature sensation 
over the opposite half of the body without affecting tactile sensibility, it is not improbable 
that Dejerine (1914) is correct in supposing that it follows a median course, its fibers inter- 
mingled with those of the tectospinal tract which run, however, in the opposite direction 
(Fig. 234; Economo, 1911; Spilkr, 1915). 

There is reason to believe that the ventral as well as the lateral spinothalamic tract 
consists in part of short relays with synaptic interruptions in the gray matter of the spinal 
cord and brain stem, and the two tracts are sometimes designated as the spino-reticulo-thala- 
mic path. 

In the spinal cord there appear to be two tracts which convey tactile im- 
pulses toward the brain, an uncrossed one in the posterior funiculus and another 
that crosses into the opposite anterior funiculus. Since these overlap each 
other for many segments, this arrangement would account for the fact that con- 
tact sensibility is usually unaffected by a purely unilateral lesion (Head and 
Thompson, 1906; Rothmann, 1906; Petren, 1902). Among the fibers of contact 
sensibility, which ascend in the posterior funiculus to the cuneate and gracile 
nuclei of the same side, are those that subserve the function of tactile discrim- 
ination, or, in other words, the ability to recognize the duality of two closely 
juxtaposed points of contact, as when the two points of the compasses or dividers 
are applied simultaneously to the skin. Furthermore, those elements of tactile 
sensibility, which underlie the appreciation of the form of objects or stereognosis, 
ascend uncrossed in the posterior funiculus to the gracile and cuneate nuclei. 

Neuron III. The neurons located in the ventral portion of the lateral nucleus 
of the thalamus, with which the tactile fibers of the second order enter into syn- 
aptic relations, send their axons by way of the thalamic radiation through the 
posterior limb of the internal capsule and the corona radiata to the somesthetic 
area of the cerebral cortex in the posterior central gyrus (Fig. 220). 



THE NERVOUS SYSTEM 



THE SPINAL PATH FOR PAIN AND TEMPERATURE SENSATIONS 

Pain and temperature sensations are mediated by closely associated though 
not identical paths, and it is convenient to consider them at the same time. 

Neuron I. The first neuron of this system has its cell of origin located in 
the spinal ganglion. Its axon divides into a peripheral branch, directed through 

Internal capsule 



Thalamus 



X Mesencephalon 




Medulla oblongata 



- Lateral spinothalamic tract 
Spinal card 



Dorsal root and spinal ganglion 
Fig. 231. Diagram of the path for pain and temperature sensations. 

the peripheral nerve to the skin, or in the case of the pain fibers also to the deeper 
tissues, and a central branch, which enters the spinal cord through the dorsal 
root and almost at once terminates in the gray matter of the posterior gray column 
(Fig. 231). As was shown in Chapter VII, there is reason to believe that the 



THE GREAT AFFERENT SYSTEMS 



307 



fibers of painful sensibility, and possibly those of temperature sensations as 
well, are unmyelinated and enter the cord through the lateral division of the 
dorsal root to end in the substantia gelatinosa Rolandi. 

Neuron II. From these dorsal root fibers the impulses are transmitted 
(perhaps through the intermediation of one or more intercalated neurons) to the 
neurons of the second order. These have their cell bodies located in the pos- 
terior gray column, and their axons cross the median plane and ascend in the 
lateral spinothalamic tract to end in the ventral part of the lateral nucleus of 
the thalamus. In addition to this long uninterrupted path, there probably 
also exists a chain of short neurons with frequent interruptions in the gray 
matter of the spinal cord, which serves as an accessory path to the same end 
station. In the medulla oblongata the spinothalamic tract lies dorsolateral to 
the inferior olivary nucleus. In the pons it joins the medial lemniscus and 
runs in the lateral part of this fillet through the pons and mesencephalon to the 
thalamus (Figs. 231, 234). 

Neuron III. Fibers, arising from nerve-cells located in the lateral thalamic 
nucleus, convey thermal and possibly also painful impulses to the somesthetic 
area of the cerebral cortex in the posterior central gyrus by-way of the thalamic 
radiation, and the posterior limb of the interal capsule. It is important to 
note that it is not necessary for painful afferent impulses to reach the cerebral 
cortex before they make themselves felt in consciousness, the thalamus being 
in itself sufficient for the perception of pain (Head and Holmes, 1911 ; Head, 1918). 

The Exteroceptive Paths Associated with the Trigeminal Nerve. The tri- 
geminal nerve mediates tactile, thermal, and painful sensations from a large part 
of the cutaneous and mucous surfaces of the head. While there is reason to be- 
lieve that the tactile impulses mediated by this nerve follow a central course 
distinct from that of thermal and painful sensibility, we cannot as yet assign 
definite paths to either group, and shall consider the exteroceptive connections 
of this nerve as a unit. 

Neuron I. The axon of a unipolar cell in the semilunar ganglion divides 
into a peripheral branch, distributed to the skin or mucous membrane of the 
head, and a central branch, which runs through the sensory root (pars major) 
of the trigeminal nerve into the pons. Here it divides into a short ascending 
and a long descending branch. The former terminates in the main sensory 
nucleus, and the latter in the spinal nucleus of that nerve (Fig. 232). 

Neuron II. The fibers of the second order in the sensory paths of the tri- 
geminal nerve arise from cells located in the main sensory and the spinal nucleus 



38 



THE NERVOUS SYSTEM 



of that nerve; and after crossing the raphe they run in two tracts to the ventral 
part of the lateral nucleus of the thalamus. The ventral secondary afferent 
path is located in the ventral part of the reticular formation, close to the spino- 
thalamic tract in the medulla oblongata and dorsal to the medial lemniscus in 
the pons and mesencephalon (Figs. 132, 234). The dorsal tract lies not far 
from the floor of the fourth ventricle and the central gray matter of the cerebral 



'Medial lemniscus 
Mesencephalon 



o'o^ 1 *~o j- Medial lemniscus 





Pons 

Dorsal secondary tract N. V 
Ventral secondary tract, N. V 

^- Main sensory nucleus N. V 
Pons 

N. V 
-- Spinal tract N. V 

Spinal nucleus N. V 

Medulla oblongata 
Fig. 232. Diagram of the exteroceptive pathways associated with the trigeminal nerve. 

aqueduct. It consists in considerable part of uncrossed fibers and of fibers hav- 
ing a short course (Wallenberg, 1905; Economo, 1911; Dejerine, 1914). 

Neuron III. The afferent impulses are relayed from the thalamus to the 
cortex of the posterior central gyrus by fibers of the third order, which run through 
the posterior limb of the internal capsule. Their cells of origin are located in 
the lateral nucleus of the thalamus. 



THE GREAT AFFERENT SYSTEMS 



309 



The Neural Mechanism for Hearing. The spiral organ of Corti within the 
cochlea is connected with the auditory center in the cerebral cortex by a chain 
of three or more units. 

Neuron I. The bipolar cells of the spiral ganglion within the cochlea send 
each a peripheral process to end in the spiral organ of Corti. Each sends a central 
branch to ramify in the cochlear nuclei, where it forms sy nap tic connections 
with the auditory neurons of the second order (Fig. 233). 

Transverse temporal gyrus 

Auditory radiation 

Medial genicnlate body 
Inferior cotticulus 



v Lateral lemnisci 



Collaterals to nucleus of 
lateral lemniscus 



/Strife medullares 



,-Dorsal cochlear nucleus 

-Ventral cochlear nucleus 
Cochlear nerve 
r Vestibular nerve 




Rostral portion of the pons-/- ( 



!~\ 

J ! 
Caudal portion of the pons-\^ 



Superior olive '' 

Trapezoid body ' 

Nucleus of the trapezoid body 

Fig. 233. Diagram of the auditory pathway. (Based on the researches of Cajal and Kreidl.) 

Neuron II. The cells located in the ventral and dorsal cochlear nuclei give 
rise to fibers, which after crossing the median plane form the lateral lemniscus 
of the opposite side. Those from the ventral cochlear nucleus cross the pons in 
the trapezoid body, giving off collaterals to the superior olivary nuclei and the 
nuclei of the corpus trapezoideum, and may be joined by fibers taking origin in 
these nuclei. Lateral to the contralateral superior olivary nucleus they turn 
abruptly rostrad in the lateral lemniscus. The fibers from the dorsal cochlear 
nucleus run in the striae medullares of the fourth ventricle, and then, dipping 



310 THE NERVOUS SYSTEM 

into the reticular formation of the pons, cross the median raphe to join the trape- 
zoid body and enter the lateral lemniscus. While this tract is for the most part 
a crossed one, some fibers probably enter the lateral lemniscus from the cochlear 
nuclei of the same side. This accounts for the fact that it is very rare to have 
total deafness in either ear resulting from damage to the auditory pathway 
within the brain. The fibers of this fillet give off collaterals to the nucleus of 
the lateral lemniscus, from which some additional fibers may be contributed to 
the tract, which finally terminates in the medial geniculate body and the inferior 
colliculus of the corpora quadrigemina. The latter, however, serves only as a 
reflex center, while the medial geniculate body is the way station on the 
auditory path to the cerebral cortex. 

Neuron III. Through synapses in the medial geniculate body the auditory 
impulses are transferred to neurons of the third order, whose cell bodies are 
located in this nucleus and whose fibers run through the auditory radiation 
and the retrolenticular part of the internal capsule to the auditory receptive 
center in the cerebral cortex. It will be remembered that this center is situated 
in the anterior transverse temporal gyms, located upon the dorsal surface of 
the temporal lobe within the lateral cerebral fissure, and in the small portion 
of the superior temporal convolution with which that gyrus is directly continuous. 

The Neural Mechanism for Sight. The nervous impulses responsible for 
vision travel over a conduction system composed of at least four units. Since 
this mechanism has already been considered as a whole on pages 225-228 
it is only necessary for us to enumerate here the separate units of which it is 
composed (Figs. 160, 162). 

Neuron I. Visual cells of the retina including the rods and cones, which are 
differentiated as receptors for photic stimuli. 

Neuron II. Bipolar cells of the retina, forming synapses with the visual 
cells, on the one hand, and the ganglion cells on the other. 

Neuron III. Ganglion cells of the retina, whose axons enter the optic nerve, 
undergo a partial decussation in the optic chiasma, and end in the lateral genic- 
ulate body, pulvinar of the thalamus, and superior colliculus of the corpora 
quadrigemina. 

Neuron IV. From cells in the lateral geniculate body and the pulvinar of 
the thalamus axons run by way of the optic radiation through the retrolenticular 
part of the internal capsule to the visual receptive center in the cerebral hemi- 
sphere. This is located in the cortex on both sides of the calcarine fissure and 
occupies portions of the cuneus and the lingual gyrus. 



THE GREAT AFFERENT SYSTEMS 311 

PROPRIOCEPTIVE PATHWAYS 

We have traced the course of the afferent impulses from the skin and from 
the eye and ear to the cerebral cortex, and have learned that they play an es- 
pecially important part in conscious experience. The stimulation of these ex- 
teroceptive sense organs initiates both conscious and reflex adjustments of the 
body to its environment. But the resulting movements serve to excite the 
sensory nerve ending in the muscles, joints, and tendons; and any quick move- 
ment or change in position of the head will also excite the nerve terminals in 
the semicircular canals of the ear. From these sources afferent impulses pour 
back into the nervous system along special paths to centers which to a great 
extent are separate from those devoted to the exteroceptive functions and serve 
to regulate the movements already initiated. The necessity for such regulation 
is well illustrated by the ataxic gait of a tabetic in whom the afferent impulses 
from the muscles, joints, and tendons are more or less completely lost. In a 
sense the proprioceptive functions of the nervous system are secondary to the 
exteroceptive, since the purpose of both is the proper adjustment of the organism 
to its environment by means of reactions, called forth by external stimuli, 
but regulated and controlled through afferent impulses arising within the 
body. 

Since in the regulation of movement the proprioceptive subdivision of the 
nervous system has to deal with constant factors, inherent in the arrangement 
of the muscles, the resultant responses are more stereotyped and invariable in 
character and are, for the most part, subconsciously executed. These reactions 
belong more to the province of the cerebellum than to that of the cerebrum. 

Of the long ascending channels mediating afferent impulses from the muscles, 
joints, and tendons, only one extends to the cerebral cortex by way of the thala- 
mus; all the others end in the cerebellum. In fact, the cerebellum is the great 
correlation center for afferent impulses of the propriceptive group, whether they 
are conveyed by the vestibular nerve or the muscular branches of the spinal 
nerves. 

It will be understood that on the motor side these two subdivisions of the 
nervous system are not as distinct as on the afferent side. On the contrary, 
both tend to discharge into common efferent systems. This is particularly true 
of the primary somatic motor neuron, which serves as "the final common path" 
for both. 

The Spinal Proprioceptive Path to the Cerebral Cortex. The conduction 
system, along which those afferent impulses travel which underlie the rather 



312 



THE NERVOUS SYSTEM 





Fig. 234. Diagrams showing the location of the most important tracts of the brain stem 
based on figures by Dejerine. Solid red, aberrant bundles of the corticobulbar tract; red stipple, 
corticospinal tract; solid blue, secondary afferent paths of the trigeminal nerve; horizontal blue 
lines, the medial lemniscus (proprioceptive) ; blue stipple, ventral spinothalamic tract (or tactile 
path); blue circles, spinal root of the trigeminal nerve; solid black, lateral spinothalamic tract 
(pain and temperature); black triangles, ventral spinocerebellar tract; black circles, dorsal spino- 
cerebellar tract; black stipple, rubrospinal tract. A, Through the mesencephalon at the level of 
the inferior colliculus; B, through the rostral part of the pons; C, through the medulla at the level 
of the olive. 



THE GREAT AFFERENT SYSTEMS 313 

vague sensations of position and posture and of active and passive movements, 
consists of a chain of at least three units. 

Neuron I. The cell bodies of the neurons of the first order belonging to this 
system are located in the spinal ganglia. Their axons are myelinated and divide 
into peripheral branches, running to specialized end organs within the muscles, 
joints and tendons, and central branches directed through the medial division 
of the dorsal root into the posterior funiculus of the spinal cord. Here they 
divide; and their ascending branches run through the posterior funiculus to 
terminate in the gracile and cuneate nuclei of the medulla oblongata, where they 
enter into synaptic relations with neurons of the second order (Fig. 235). 

Neuron II. From cells located in the gracile and cuneate nuclei the axons 
run as internal arcuate fibers across the median raphe in the medulla oblongata 
and ascend by way of the medial lemniscus to end in the ventral part of the lateral 
nucleus of the thalamus, where they form synapses with neurons of the third order. 

Neuron III. From cells in the lateral nucleus of the thalamus fibers pass by 
way of the thalamic radiation through the posterior limb of the internal capsule 
to the posterior central gyrus or somesthetic area of the cerebral cortex. 

SPINAL PROPRIOCEPTIVE PATHS TO THE CEREBELLUM 

Impulses from the muscles, joints, and tendons may reach the cerebellum by 
three routes: 

A. By Way of the Dorsal External Arcuate Fibers : 

Neuron I of this chain is the same as in the path to the cerebral cortex just 
described, the fibers from the dorsal root reaching the gracile and cuneate nuclei. 

Neuron II. From cells located in these nuclei axons run as posterior external 
arcuate fibers to the restiform body of the same side, and thence through the 
white center of the cerebellum, to end in the cerebellar cortex (Fig. 235, red). 

B. By Way of the Ventral Spinocerebellar Tract: 

Neuron I. The first neuron in this chain is similar to the primary neuron in 
the two preceding paths. The impulses, however, travel over collateral and 
terminal branches of the dorsal root fibers to reach the posterior gray column 
and intermediate gray matter of the spinal cord. 

Neuron II. From cells located in the posterior gray column and intermediate 
gray matter fibers run in the ventral spinocerebellar tracts of the same or 
opposite side through the spinal cord, medulla oblongata and pons, bend around 
the brachium conjunctivum, and then course back along the anterior medullary 
velum to the cortex of the rostral part of the vermis (Fig. 235, blue). 



THE NERVOUS SYSTEM 



C. By Way of the Dorsal Spinocerebellar Tract: 

Neuron I. The first neuron of this chain is similar to the primary neuron 
in the. three preceding paths. The impulses, however, travel over those col- 
lateral and terminal branches of the dorsal root fibers which ramify about the 
cells of the nucleus dorsalis. 

Internal capsule 



Thalamus 



Ventral spinocerebellar tract 



Cerebellum 



Restiform body 
Medulla oblongata 




Ascending branches of 
dorsal rool fibers 



--Dorsal external arcuate fiber 



Ventral spinocerebellar tract 



Dorsal spinocerebellar tract 
Dorsal root and spinal ganglion 



Fig. 235. The proprioceptive paths. 

Neuron II. From cells in the nucleus dorsalis fibers run to the dorsal spino- 
cerebellar tract of the same side and through the restiform body to the cortex 
of both the rostral and the caudal portions of the vermis (Fig. 235, red). 

Cerebellar Connections of the Vestibular Nerve. The vestibular nerve 



THE GREAT AFFERENT SYSTEMS 315 

conducts impulses from specialized sense organs in the semicircular canals, sac- 
cule and utricle, which are stimulated by movements and changes in posture 
of the head. 

Neuron I. From the bipolar cells of the vestibular ganglion (of Scarpa), 
located within the internal auditory meatus, peripheral processes run to the 
maculae of the utricle and saccule and to the cristae of the semicircular canals. 
The central processes are directed through the vestibular nerve toward the 
floor of the fourth ventricle and divide into ascending and descending branches. 
While the descending and many of the ascending branches terminate in the 
vestibular nuclei, many other ascending branches pass without interruption to 
end in the cerebellar cortex and particularly in that of the vermis (Fig. 136). 

Neuron II. Some of the cells situated in the vestibular nuclei send their 
axons, along with the ascending branches mentioned above in the vestibulo- 
cerebellar tract, to the cortex of the vermis, and to a less extent to the cortex 
of the cerebellar hemispheres also. 



CHAPTER XX 

EFFERENT PATHS AND REFLEX ARCS 

THE motor apparatus is a complex mechanism into which the pyramidal 
system enters as a single factor. The primary motor neurons of the brain stem 
and spinal cord are also under the influence of other motor centers than those 
found in the cerebral cortex. They receive impulses from the corpora quadri- 
gemina through the tectospinal tract, from the lateral vestibular nucleus by way 
of the vestibulospinal tract, from the large motor cells of the reticular formation 
through the reticulospinal path, from the cerebellum, and probably also from 
the corpus striatum by way of the red nucleus and the rubrospinal fasciculus. 
Perhaps, also, impulses descend from the thalamus or subthalamus by way of a 
thalamospinal tract. 

We must not think of the individual parts of this complex mechanism as 
functioning separately, since each of these motor centers contributes its share 
to the control of the primary motor neuron, upon which as the "final common 
path" all these efferent pathways converge. Only by keeping this fact con- 
stantly in mind can the motor functions be properly understood. The same 
idea has been well stated by Walshe (1919): 'Tn stimulation experiments on the 
motor cortex we see a complex motor mechanism at work under the influence 
of an abnormally induced, crude form of hyperactivity of the predominant partner 
in this mechanism. Conversely, after destructive lesions, we observe it at work 
liberated from the control of this predominant partner and deprived of its actual 
cooperation." 

On the other hand, the grave motor disturbances resulting from lesions in 
the basal ganglia and especially the corpus striatum with little or no involvement 
of the corticospinal tracts (paralysis agitans, Auer and McCough, 1916; bilateral 
athetosis, Cecile Vogt, 1911; and progressive lenticular degeneration, Wilson, 
1912-14) have recently called attention to the importance of the corpus striatum 
and the extrapyramidal motor path (see p. 324). In these diseases voluntary 
movements are impeded by tremor, rigidity, and athetosis; and in all probability 
these disturbances arise because the pyramidal system is deprived of the co- 
operation of one of the subordinate "partners" in the motor combine. 
316 



EFFERENT PATHS AND REFLEX ARCS 

Even after cerebral control has been entirely eliminated in the dog by de- 
cerebration, many reflex functions remain, which represent the unguided activity 
of the lower elements in the motor mechanism ; and we now know that a similar 
independent reflex activity may occur in the spinal cord of man after total trans- 
verse lesions (Riddoch, 1917). 

THE GREAT MOTOR PATH 

The great motor path from the cerebral cortex to the skeletal musculature, 
through which the bodily activities are placed directly under voluntary control, 
is in man and mammals the dominant factor in the motor mechanism. We 
have seen that afferent channels from the various exteroceptors reach the cere- 




Fig. 236. Cortical localization upon the lateral aspect of the human cerebral hemisphere. (Starr.) 

bral cortex; and that through the correlation of the olfactory, auditory, visual, 
tactile, thermal, and painful afferent impulses which pour into it, there is built 
up within the cortex a representation of the outer world and its constantly chang- 
ing conditions. The responses appropriate to meet the entire situation in which 
the individual finds himself from moment to moment are in large part at least 
initiated in the cerebral cortex and are executed through the motor mechanism. 
In these responses the great motor path is the dominant factor, although other 
parts of the mechanism are secondarily called into action, especially the pro- 
prioceptive reflex arcs, including the coordinating and tonic mechanism of the 
cerebellum. 

This great motor path consists of two-unit chains. The so-called upper 
motor neurons conduct impulses from the motor cortex to the motor nuclei of the 



THE NERVOUS SYSTEM 



cerebral nerves or to the anterior gray columns of the spinal cord; whence the 
lower motor neurons, also known as primary motor neurons, relay the impulses 
to the muscles. It is possible that another and much shorter element is inter- 
calated between the two chief units of this conduction system. 

The motor cortex occupies the rostral lip of the central sulcus and the ad- 
jacent portion of the anterior central gyrus, extending over the dorsal border of 

Motor cortex 




...Posterior limb of internal capsule 



. Genu of corpus cattosum 



Basis pedunculi of mesencephalon 



----^Longitudinal fascicles of pons 



Pyramid of medulla ablongata 

Lateral corticospinal tract 
Ventral corticospinal tract 



Fig. 237. The corticospinal path. 

the hemisphere into the paracentral lobule. Within this area the skeletal mus- 
culature is represented hi inverted order, that moving the toes near the dorsal 
border of the hemisphere. The area from which the corticobulbar tract arises 
is only a small part of the whole, and is situated near the lateral cerebral fissure 
(the region marked Eyelids, Cheeks, Jaws, Lips in Fig. 236). From all the rest 
of the motor cortex arise the fibers of the corticospinal tract. 



EFFERENT PATHS AND REFLEX ARCS 



319 



The motor path for the spinal nerves includes the corticospinal tract and the 
spinal primary motor neurons. 

Neuron I, or upper motor neuron. The giant pyramidal cells of the motor 
cortex give rise to the fibers of the corticospinal tract which is also known as 



Fissura longitudinalis cerebri 
Radiatio corporis callosi,. 



Septum pellucidum x 



Plexus chorio- 

ideus ventricul 

lateralis 



Corona radiata,. 



Columna, 
fornicis 
'Plexus chorio- 
ideus ventriculi ~"~V 

tertii 
Capsula interna 



Thalamus 

Ventriculus - 

tertius 

Fossa inter- 
peduncularis 

(Tarini) 

Cornu inferius 
ventriculi 
lateralis 



Peduncuh 
cerebri 

Brachium pontj 

Fasciculi longit 
nales(pyramida 
pontis 

Facies inferio 



, Gyrus frontalis superior 

f Truncus corporis callosi 

( Cornu anterius ventriculi 
lateralis 
Caput nuclei caudati 




Nn.facialisund 
acusticus 

Flocculus 
glossopharyngctis 



N. vagus 
Nucleus olivaris inferior 



Fibrae pontis super 

Pyramis medullae oblongatae / ^ Decussatio pyramidum 

Fig. 238. Section through the brain in the axis of the brain stem, showing the entire extent of 

the corticospinal tract. (Toldt.) 

the cerebrospinal fasciculus or pyramidal tract. These fibers traverse the rostral 
half of the posterior limb of the internal capsule, the intermediate three-fifths 
of the basis pedunculi, the basilar portion of the pons, and the pyramid of the 
medulla oblongata, and after undergoing a partial decussation are continued into 
the spinal cord (Figs. 237, 238). At the pyramidal decussation in the caudal 



320 



THE NERVOUS SYSTEM 



part of the medulla oblongata the greater part of the tract crosses to the opposite 
side of the spinal cord and is continued as the lateral corticospinal tract in the 
lateral funiculus. The smaller part is continued directly into the ventral fu- 
niculus of the same side, as the ventral corticospinal tract. The fibers of the 
ventral tract cross the median plane a few at a time and terminate, as do those of 
the lateral tract, directly or indirectly in synaptic relations with the primary 
motor neurons within the anterior gray column (Fig. 239). The ventral tract 
is not evident as a well-marked bundle below the level of the midthoracic region. 



Mesencephalon 

N.IV 

Pans 

Corticobulbar tract 




Medulla oblongata 

Ventral corticospinal tract 
Lateral corticospinal tract 

Spinal cord 



Ventral root 
Fig. 239. The corticobulbar and corticospinal tracts. 

It has long been known that in the higher mammals the lateral pyramidal tract, although 
consisting predominatingly of crossed fibers, contains a few homolateral fibers also (Simpson, 
1902), and according to the observations of Dejerine (1914) and other investigators this 
holds true for man also. Dejerine speaks of these uncrossed fibers in the lateral corticospinal 
tract as a third bundle arising out of the motor decussation, and calls it the "homolateral" 
corticospinal fasciculus. A good account of this tract and of the superficially placed bundle 
of uncrossed pyramidal fibers that is to be found in the ventral part of the lateral funiculus 
in the cervical portion of the spinal cord is given by Barnes (1901). 

Neuron II. To the lower or primary motor neurons belong the large multi- 
polar cells of the anterior gray column of the spinal cord. These give rise to the 
motor fibers that leave the spinal cord through the ventral roots and are dis- 
tributed through the spinal nerves to the skeletal musculature. 

The motor path for the cranial nerves is less well known. It includes the 
corticobulbar tract and those fibers of the cranial nerves which innervate striated 
musculature. 

Neuron I, or upper motor neuron. The corticobulbar fibers arise from the 



EFFERENT PATHS AND REFLEX ARCS 



321 



giant pyramidal cells of the part of the motor cortex near the lateral fissure. 
These fibers run through the genu of the internal capsule and the basis pedunculi 
to end, directly or indirectly, in synaptic relation to the primary motor neurons 
of the somatic motor and special visceral motor nuclei of the brain stem. Be- 
fore terminating, the majority cross the median plane, but some end in the motor 
nuclei of the same side (Fig. 239). 

Neuron II, lower or primary motor neuron. From the large multipolar 
cells of the somatic motor and special visceral motor nuclei arise fibers, which 
run through the cranial nerves to end in striated musculature. 



Tr. corticosp. 
Tr. corticobulb. 

F. A. Sth. (Ill) 

F. A.Pd. / Tr. cb. lot. 
(Ill, VI,XI)\Tr. cb.med. 

F. A.P. (V, X, XI, XII) 



F.A.B.P. (VII) 

Tr. corticosp. 

Tr. corticobulb. 

X I, C II-IV 

C 1 1 -IV 

Tr. corticosp. med. 

C II-IV 

XI, C II-IV 




Puhinar 

Med. lemniscus 
Nuc. N. Ill 
Corpora quad. 
Nuc. N. IV 
Nuc. N. V 

Fourth vent. 
Nuc. N. VI 

Nuc. N. VII 

Nuc. ambiguus Nn. IX and X 

Nuc. N. XII 

Nuc. gracilis 
Nuc. cuneatus 
Nuc. XI 

XI, XII, C II-IV 
Tr. corticosp. lot. 



Fig. 240. The course of the fibers of the corticobulbar tract. Redrawn from Dejerine. 
Corticobulbar tract, solid black; corticospinal tract, vertical lines; the medial lemniscus, horizontal 
lines. F. A. B. P., Bulbopontine aberrant fibers; F. A. P., aberrant fibers of the pons; F. A. Pd., 
aberrant fibers of the peduncle; F. A. Sth., subthalamic aberrant fibers; Tr. cb. lat., tractus cortico- 
bulbaris lateralis; Tr. cb med., tractus corticobulbaris medialis. The Roman numerals indicate 
the nuclei of the cranial and cervical nerves which are supplied by the various bundles. 

The Corticobulbar Tract. According to Dejerine (1914), who, because of the careful 
study which he and his associates have made of this efferent system, is most entitled to speak 
authoritatively on the subject, the corticobulbar fibers occupy chiefly the medial part of the 
basis pedunculi and its deeper layer. The fibers separate into two major groups. One 
part follows the course of the corticospinal tract and descends in the basilar portion of the 
pons and the pyramids of the medulla oblongata. Another part, which he designates as 
the system of aberrant pyramidal fibers, detaches itself from the preceding in small bundles 
at successive levels of the brain stem. These enter the reticular formation and descend 
within the region occupied by the medial lemniscus, giving off fibers to the motor nuclei of 
the cranial nerves (Fig. 240). The fibers undergo an incomplete decussation in the raphe 



322 THE NERVOUS SYSTEM 

and go chiefly to the nuclei of the opposite side. The decussating fibers are grouped in very 
small bundles, those for a given nucleus crossing at the level of that nucleus. There is great 
variation in the course of the bundles of aberrant pyramidal fibers in different brains. 

The chief aberrant bundles which can be traced dorsalward into the reticular formation 
(indicated in solid red in Fig. 234) are as follows: 

1. The aberrant fibers of the peduncle (Fig. 240, F. A. Pd.) form two bundles, which 
have been called by some authors the median and lateral corticobulbar tracts. These 
descend in the territory of the medial lemniscus (Figs. 234, 240) and give off fibers to the 
nuclei of the third, sixth, and eleventh cranial nerves. With these two bundles run some 
fibers destined for the upper cervical segments of the spinal cord. This group of aberrant 
fibers therefore controls the movements of the eyes and the associated movements of the head. 

2. The aberrant fibers of the pons (Fig. 240, F. A. P.) which join the preceding in the 
medial lemniscus run to the motor nuclei of the trigeminal and hypoglossal nerves and to the 
nucleus ambiguus. 

3. The bulbopontine aberrant fibers (Fig. 240, F. A. B. P.) leave the main trunk of the 
pyramidal system near the level of the sulcus between the pons and medulla. They reinforce 
the preceding groups, supply the motor nucleus of the facial nerve, and send fibers to the 
nucleus ambiguus and to that of the hypoglossal nerve. 

These facts are of the greatest importance for the clinical neurologist. Lesions re- 
stricted to the basilar portion of the pons are likely to destroy at the same time the cortico- 
spinal fibers and those of the corticobulbar tract which end in the facial nucleus. A lesion 
confined to the reticular formation and involving the medial lemniscus may, according to its 
level, sever the corticobulbar fibers for the motor nuclei of the eye-muscle nerves or those 
for the motor nuclei of the trigeminal, accessory, and hypoglossal nerves without involve- 
ment of the corticospinal tracts. Conjugate deviation of the head and eyes, not often seen 
as a result of damage to the basilar portion of the pons, may result from tegmental lesions 
involving the aberrant fibers of the peduncle. 



The physiologic and clinical significance of the course of the corticospinal and 
corticobulbar tracts is obvious. It is because of the decussation of these fibers 
that the muscular contractions produced by cortical stimulation occur chiefly 
on the opposite side of the body, and that the paralyses resulting from lesions 
in the pyramidal system above the decussation are contralateral. If the lower 
motor neuron is injured, the associated muscle atrophies and a flaccid paralysis 
results. Injury to the upper motor neuron, on the other hand, leads to a loss 
of function without atrophy, but rather with an increased tonicity of the affected 
muscle, i. e., to a spastic paralysis. By means of such differential characteristics 
as these it is possible to tell which of the two links in the motor chain has been 
broken. 

In order to understand the combination of symptoms, which result from 
damage to the motor path at different levels, it is necessary to have in mind the 
topography of its constituent parts. Some of these relations are indicated in 
Fig. 241. Since the motor cortex is spread out over a rather extensive area, 
it is usually not entirely destroyed by injury or disease. A restricted cortical 



EFFERENT PATHS AND REFLEX ARCS 



323 



lesion may cause a monoplegia, i. e., paralysis of a single part, such as the arm or 
leg (Fig. 241, A). But in the internal capsule the motor fibers are grouped 
within a small area and are frequently all destroyed together. This causes 
paralysis of the opposite half of the body or hemiplegia (Fig. 241, B). Damage 
to the pyramidal system in the cerebral peduncle, pons, or upper part of the 
medulla oblongata may also cause hemiplegia; but in such cases those cortico- 




To the arm 



To the leg 
Fig. 241. Diagram to illustrate the effects of lesions in various parts of the motor path. 

bulbar fibers, which leave the main strand of pyramidal fibers above the level 
of the lesion, may escape injury and the corresponding cranial nerves need not 
be involved (Fig. 241, C). Furthermore, in lesions of the brain stem the motor 
nucleus or emergent fibers of one of the cranial nerves may be destroyed along 
with the pyramidal fibers, in which case there would result a paralysis of the 
muscles supplied by that nerve as well as a paralysis of the opposite half of the 
body below that level a crossed paralysis (Fig. 241, C). While damage to the 



324 TTTF. NERVOUS SYSTEM 

spinal cord may affect only one lateral half and cause a homolateral paralysis 
below the lesion (Fig. 241, D), it is common for both lateral halves to be involved 
and for the resulting paralysis to be bilateral (Fig. 241, ). 

The Extrapyramidal Motor Paths. In recent years it has become increasingly evi- 
dent that the pyramidal system is not the only channel through which volitional impulses 
are able to reach the primary motor neurons of the brain stem and spinal cord. Rothmann 
(1907) found that, after section of the lateral corticospinal and the rubrospinal tracts in 
monkeys at the level of the third cervical nerve, voluntary movements were lost for a time, 
but soon reappeared; and he concluded that there must be an extrapyramidal volitional 
path in the ventral funiculus. Three years later Schafer (1910) showed that in monkeys 
the paralysis, which results from section of the pyramids of the medulla oblongata, is not 
complete and persistent; and he agreed with Rothmann that there must be some other path 
for volitional impulses. He believes that this alternative path is formed by descending 
fibers in the ventral funiculus and in the ventral part of the lateral funiculus, since section 
of these fibers produces as complete and persistent paralysis in monkeys as does section of 
the pyramids themselves. 

Sherrington and Graham Brown (1913) excised the arm area of the cerebral cortex 
in the chimpanzee, and found that function in the corresponding limb was completely re- 
stored in a few weeks. They were able to show that this was not attributable to the vicarious 
activity of the corresponding postcentral or the opposite precentral cortex. Horsley's (1909) 
patient, who recovered some degree of control over the arm after the removal of its cortical 
center in the precentral gyrus, shows that the observations of Sherrington and Brown are at 
least in part applicable to man. 

We know that the cerebral cortex is connected through efferent projection tracts with 
the thalamus and red nucleus and through collaterals from the corticospinal fibers with the 
corpus striatum (Cajal). But we do not know which, if any, of these systems of projection 
fibers constitutes a part of the extrapyramidal path for volitional impulses. 

A great deal of attention has recently been given by clinical neurologists to the dis- 
turbance of voluntary movement by tremor, rigidity, and athetosis, which results from lesions 
of the corpus striatum. This body seems to contain an important motor center, and ac- 
cording to Wilson (1912 and 1914) it exerts a steadying influence upon voluntary move- 
ments. The globus pallidus seems to be connected with the spinal primary motor neurons 
by way of the striorubral and rubrospinal tracts. It is also possible, especially in view 
of the important motor functions attributed to the ventrolateral descendnig tracts of the 
spinal cord by Rothmann and Schafer, that efferent impulses reach the spinal cord from the 
globus pallidus by way of the substantia nigra over the strionigral, the somewhat hypothetic 
nigroreticular, and the reticulospinal tracts. It is known that the axons arising in the sub- 
stantia nigra run into the reticular formation of the mesencephalon, beyond which they 
cannot be traced (Cajal, 1911). According to Collier and Buzzard (1901) the rubrospinal, 
vestibulospinal, tectospinal, and reticulospinal tracts probably represent the original paths 
for impulses from higher to lower parts of the nervous system; and the path from the 
cerebrum to the spinal cord, at first indirect, has been short-circuited in the mammal 
through the evolution of the pyramidal system. 

When it is remembered that the pyramidal system is a late development, present only 
in mammals, it does not seem unreasonable to think that some other and older path for 
volitional impulses may also exist. The globus pallidus, the representative of the primitive 
corpus striatum of the lower vertebrates, has been called the paleostriatum (Elliot Smith, 
1919). From this basal nucleus there arises in all vertebrates an important efferent bundle, 



EFFERENT PATHS AND REFLEX ARCS 



325 



"the basal forebrain bundle" of Edinger (1887), which is represented in mammals by the 
striofugal fibers of the ansa lenticularis. It is clear that this fascicle, which persists through- 
out the vertebrate series, must subserve important functions; and it is probable that it 
forms a part of the extrapyramidal motor path. 

THE CORTICO-PONTO-CEREBELLAR PATH 

The cortico-ponto-cerebellar path is an important descending conduction 
system which places the cerebellum under the influence of the cerebral cortex. 
Since a part of the corticopontine fibers are collaterals given off to the nuclei 
of the pons by the corticospinal fibers, and since in many mammals practically 

Red nucleus 



\Purkinje cell 
i Cerebellum 




Frontopontine tract 

Corticospinal tract 

Nuclei pontis 

Muscle 
Spinal cord 



- Dentate nucleus 

~ Brachium conjunctivum 

" * Brachium pontis 
Rubrospinal tract 

Corticospinal tract 



Fig. 242. The cortico-ponto-cerebellar and cerebello-rubro-spinal paths. (Modified from Cajal.) 

all of the corticopontine fibers are represented by such collaterals (Cajal, 1909), 
one can scarcely avoid the conclusion that through this system the coordinating 
and tonic mechanism of the cerebellum is brought into play for the regulation 
of movements initiated from the cerebral cortex. In this sense the idea of 
Cajal (1911) that there exists an indirect motor path to the spinal cord through 
the cerebellum is probably correct (Fig. 242). 

Neuron I. From pyramidal cells in the frontal lobe of the cerebral cortex 
fibers pass through the anterior limb of the internal capsule and the medial one- 



326 THE NERVOUS SYSTEM 

fifth of the basis pedunculi; and similar fibers from the temporal lobe descend 
through the sublenticular part of the internal capsule and the lateral one-fifth 
of the basis pedunculi. These fibers, together with the corticospinal tract, 
form the longitudinal fasciculi of the pons; and, along with collaterals from that 
tract, they end within the nuclei pontis in synaptic relations with the neurons 
of the second order (Figs. 106, 242). 

Neuron II. Arising from cells in the nuclei pontis, the transverse fibers of 
the pons cross the median plane and run by way of the brachium pontis and 
white substance of the cerebellum to the cerebellar cortex of the opposite side. 

THE CEREBELLO-RUBRO-SPINAL PATH 

The cerebello-rubro-spinal path is the conduction system through which the 
cerebellum contributes its important share to the control of the primary motor 
neurons of the spinal cord in the interest of muscular coordination, equilibration, 
and the maintenance of muscle tone. Other efferent connections of the cerebel- 
lum have been discussed on page 211. 

Neuron I. From the Purkinje cells of the cerebellar cortex fibers run to 
terminate in the central nuclei of the cerebellum, especially the dentate nucleus 
(Fig. 242). 

Neuron II. Arising chiefly, if not entirely, from the cells of the dentate 
nucleus, fibers run through the brachium conjunctivum, undergo decussation 
in the tegmentum of the midbrain ventral to the inferior colliculi, and end in the 
red nucleus and thalamus (Figs. 242, 243). 

Neuron III. From cells in the red nucleus arise the fibers of the rubrospinal 
tract, which cross the median plane in the ventral tegmental decussation, and 
descend through the reticular formation of the brain stem and the lateral funic- 
ulus of the spinal cord. Here this tract occupies a position just ventral to the 
lateral corticospinal tract, and its fibers end in the anterior gray column in 
relation to the primary motor neurons. 

We have learned that the cerebellum is the chief center of the proprioceptive 
system and is concerned with the maintenance of the proper tonicity of the 
muscles, the coordination of their contractions, and especially with those re- 
actions necessary to maintain or to re-establish that evenly balanced spacial 
orientation known as equilibrium. The cerebello-rubro-spinal path is the con- 
duction system primarily concerned in these reactions. 

What is perhaps the first direct experimental evidence of the function of this 
system has been given by Weed (1914). The extensor rigidity, so characteristic 



EFFERENT PATHS AND REFLEX ARCS 



327 



of decerebrated dogs, which Sherrington (1906) clearly showed to be a proprio- 
ceptive reflex that under normal conditions serves to keep the limbs from bend- 
ing under the weight of the body, is apparently dependent upon the integrity of 
the cerebello-rubro-spinal path. Weed showed that removal of the cerebellum, 
section of the superior cerebellar peduncles, or transection of the mesencephalon 
below the level of the red nucleus obliterated or greatly decreased this rigidity. 



Rubrospinal tract ^ 
Rubroreticular tract 




From frontal lobe and corpus striatum 
" Thalamus 



I Red nucleus 

Brachium conjunctivum 
' Dentate nucleus 



Pons 
Rubrospinal tract 

J Medulla oblongata 

u 

Reticulospinal tract 

Spinal cord 

Fig. 243. Diagram showing the connections of the red nucleus: A, Ventral tegmental 
decussation; B, decussation of the brachium conjunctivum; C and D, descending fibers from bra- 
chium conjunctivum, before and after its decussation respectively. 

On the other hand, stimulation of the area occupied by the red nucleus on the 
cut surface of the mesencephalon in decerebrated dogs increased the rigidity. 

IMPORTANT REFLEX ARCS 

We have considered the afferent paths leading to the cerebral cortex and to 
the cerebellum as well as the efferent channels which conduct impulses from these 
centers to the skeletal musculature. But there are many more direct paths 
by which impulses may travel from receptor to effector, and these are known as 
reflex arcs. It will be worth while to review briefly a few of the more important 
of these rather direct receptor to effector circuits. 



328 THE NERVOUS SYSTEM 

REFLEX ARCS OF THE SPINAL CORD 

Neuron I. Primary sensory neurons, with cell bodies in the spinal ganglia, 
convey impulses from the sensory endings to the spinal cord, then along the 
ascending and descending branches resulting from the bifurcation of the dorsal 
root fibers within the cord, and along the collaterals of these branches to the 
primary motor neurons, either directly or through an intercalated central unit 
(Figs. 66-68). 

Neuron II. The central neurons have their cell bodies in the posterior gray 
column and may belong to Golgi's Type II, having short axons restricted to the 
gray matter; or their axons may be long, running through the fasciculi proprii 
to the ventral horn cells at other levels of the cord. Some of these central axons 
cross the median plane in the anterior commissure. 

Neuron III. Primary motor neurons, with cell bodies in the anterior gray 
column, send their axons through the ventral roots and spinal nerves to the 
skeletal musculature. Or in the case of visceral reflexes, the motor neuron has 
its cell body located in the intermediolateral cell column, and its axon runs as a 
preganglionic fiber to a sympathetic ganglion, whence the impulses are relayed 
by a fourth or postganglionic neuron to involuntary muscle or glandular tissue. 

The reflex paths of the cranial nerves are similarly constituted, except that 
rarely if ever do the sensory fibers form synapses directly with the motor cells. 
The central neuron, which has its cell located in the sensory nucleus of a given 
nerve, sends its axon through the reticular formation to the motor nucleus of 
the same or of some other nerve (Figs. 92, 111). Two of the reflex circuits con- 
nected with the vestibular nerve require special attention. 

VESTIBTJLAR REFLEX ARC THROUGH THE MEDIAL LONGITUDINAL BUNDLE 

Neuron I. The bipolar cells of the vestibular ganglion in the external audi- 
tory meatus send peripheral processes to the cristae of the semicircular canals 
and maculae of the saccule and utricle. Their central processes run through 
the vestibular nerve to the vestibular nuclei (Figs. 135, 244). 

Neuron II. Cells in the lateral and superior vestibular nuclei send their axons 
to the medial longitudinal fasciculus of the same or the opposite side, where they 
divide into ascending and descending branches, which run in this bundle. From 
these branches twigs are given off to the nuclei of the oculomotor, trochlear, and 
abducens nerves and to the motor cells of the cervical portion of the spinal cord 
(Fig. 244). 

Neuron III. Primary motor neurons of the oculomotor, trochlear, abducens, 



EFFERENT PATHS AND REFLEX ARCS 



329 



accessory, and cervical spinal nerves send their axons to the muscles that move 
the head and eyes. 

This arc is concerned with the reflex regulation of the combined movements 
of the head and eyes in response to the vestibular stimulation which results from 
every movement and change of posture of the head. Strong stimulation of the 
semicircular canals, vestibular nerve, or Deiters' nucleus causes an oscillatory 
side to side movement of the eyes, known as nystagmus, a reflex response of an 
abnormal character mediated through this arc (Wilson and Pike, 1915). 

M. rectus medialis 



Oculomotor nerve 



Vestibular nerve ' 
Lateral vestibular nucleus - 

Vestibules pinal tract ' 

Median longitudinal --' 
fasciculus 



M. sternocleidomas- _ 
toideus 




M. rectus lateralis 



Nuc. of oculomotor nerve 
Abducens nerve 



Nuc. of abducens nerve 



Median longitudinal 
fasciculus 

Spinal root of accessory 
nerve 



t N. ceroicalis II 



Fig. 244. Vestibular reflex arcs. (Modified after Edinger.) 

A vestibules pinal reflex -arc is established between the vestibular sense organs 
and the skeletal musculature and consists of the following parts : the vestibular 
nerve; the vestibulospinal tract, which has its origin in the lateral vestibular 
nucleus, and descends in the ventral funiculus of the same side of the spinal 
cord; and the primary motor neurons of the spinal cord (Fig. 244). 

The afferent impulses reaching the medulla oblongata by way of the vagus 
give rise to a great variety of reflexes. While these are for the most part purely 
visceral, a few are executed by the somatic musculature and should receive 
attention at this point. 



33 



THE NERVOUS SYSTEM 



The Respiratory Reflex Mechanism. The maintenance of the normal res- 
piratory rhythm is dependent upon a respiratory center in the caudal part of 
the medulla oblongata, which is sensitive to changes in the carbon dioxid con- 
tent of the blood. But this rhythm is also influenced by afferent impulses coming 
from the lungs by way of the vagus nerve and the tractus solitarius. It is 
probable that these impulses are relayed through the nucleus of the tractus soli- 
tarius and descending fibers that arise in that nucleus (tractus solitariospinalis) 
to the primary motor neurons belonging to the phrenic and intercostal nerves 
(Fig. 245). There must also be a descending tract from the respiratory center 
to these neurons. Cajal (1909) believes that this center is, in fact, identical 
with the lower part of the nucleus of the tractus solitarius (the commissural 



Dorsal motor X nucleus 
Nucleus offascic. solitarius 

Fasciculus solitarius 

Vagus ganglion 

Vagus nerve 

Tr. solitario-spinalis 

Sympathetic ganglion 




Blood-vessel 
Respiratory center 



Intercostal nerve 

Intercostal muscle 

Phrenic nerve 



Diaphragm 
Fig. 245. Reflex mechanism of respiration. (Herrick, Cajal.) 

nucleus), and that this responds both to changes in the chemical composition of 
the blood and to the afferent impulses coming by way of the vagus nerve. If 
this be true, the fibers from the nucleus of the tractus solitarius would be the 
only descending tract needed to carry the respiratory impulses to the spinal 
cord. Although on its afferent side the respiratory reflex is visceral, it is ex- 
ecuted by somatic muscles which are under voluntary control; and hence breath- 
ing may be temporarily suspended or the rhythm altered at will. 

The reflex mechanism for vomiting and coughing is illustrated in Fig. 246. 
As the result of an irritation of the gastric mucous membrane a wave of excitation 
travels along the afferent fibers of the vagus nerve and the tractus solitarius. 
After passing through synapses in the nucleus of that tract, the impulses probably 



EFFERENT PATHS AND REFLEX ARCS 



331 



travel along the descending fibers, which arise in that nucleus, to the primary 
motor neurons of the spinal cord that give rise to the fibers innervating the dia- 
phragm and abdominal muscles. At the same time the musculature of the 
stomach is excited to contraction by that part of the wave of excitation which 
reaches the dorsal motor nucleus of the vagus. These impulses reach the mus- 
culature of the stomach over the visceral efferent fibers of the vagus and an 
intercalated postganglionic neuron. 

A similar neural circuit is probably responsible for reflex coughing. From 
the irritated respiratory mucous membrane, as, for example, of the larynx, the 



Vagus ganglion 




Intercostal muscle 



Diaphragm 



Stomach 



Dorsal motor vagus 
nucleus 

Nucleus of fasciculus 
solilarius 

Fasciculus solitarius 
Tr. solitariospinalis 



Phrenic nerve 
Intercostal nerve 

Nerve to abdominal 
muscles 

Sympathetic ganglion 
Postganglionic 



Fig. 246. Reflex mechanism of coughing and vomiting. (Herrick, Cajal.) 



disturbance is propagated along the afferent fibers of the vagus, through the 
nucleus of the tractus solitarius and the descending fibers arising in it to the 
spinal primary motor neurons, which innervate the diaphragm and the inter- 
costal and abdominal muscles. 

The corpora quadrigemina are important reflex centers. The path for re- 
flexes in response to sound begins in the spiral organ of Corti and follows the coch- 
lear nerve and its central connections, including the lateral lemniscus, to the 
inferior colliculus of the opposite side, and to a less extent of the same side also 



332 



THE NERVOUS SYSTEM 



(see p. 309). Thence the path follows the tectospinal and tectobulbar tracts 
to the primary motor neurons of the cerebrospinal nerves (see p. 167). The 
visual reflex arc begins in the retina, follows the optic nerve and optic tract with 
partial decussation in the chiasma, to the superior colliculus of the corpora 
quadrigemina (p. 226) ; thence it is continued by way of the tectospinal and tecto- 
bulbar paths to the primary motor neurons of the cerebrospinal nerves (Fig. 162). 
Pupillary Reactions. The iris is innervated by two sets of sympathetic 
nerve-fibers derived from the ciliary and the superior cervical sympathetic ganglia 
respectively. Impulses reaching the iris through the latter ganglion induce 
dilatation of the pupil; those through the ciliary ganglion cause constriction. 
The latter reaction always accompanies accommodation. When vision is fo- 



N.II 



Ciliary ganglion 



N. 



Sup. colliculus 
Sensory nuc. N. V 

Pons- 



Upper thoracic segments of < 
spinal cord 




N. V 
\ Carotid plexus 

Sup. cervical sympatltetic ganglion 
"- Cervical sympatlielic trunk 



Fig. 247. Pupillary reflex arcs. 



cused on a near object, contraction of the ciliary muscle results in accommoda- 
tion; and at the same time contraction of the two internal rectus muscles brings 
about a convergence of the visual axes. These two movements are always 
associated with a third, the contraction of the sphincter pupillae. In addition 
to this constriction of the pupil, which accompanies accommodation, two other 
pupillary reactions require attention (Fig. 247). 

The Pupillary Reflex (Light Reflex) When light impinges on the retinae 
there results a contraction of the sphincter pupillae and a corresponding constric- 
tion of the pupil. The reflex circuit, which is traversed by the impulses bringing 
about this reaction, begins in the retina and includes the following elements: 
the fibers of the optic nerve and tract, with a partial decussation in the optic 



EFFERENT PATHS AND REFLEX ARCS 333 

chiasma; synapses in the superior colliculus of the corpora quadrigemina ; fibers 
of the tectobulbar tract ending in the nucleus of Edinger-Westphal (visceral 
efferent portion of the oculomotor nucleus); the visceral efferent fibers of the 
oculomotor nerve, ending in the ciliary ganglion; and the postganglionic fibers 
extending from the ciliary ganglion to iris. 

The pupillary-skin reflex is a dilatation of the pupil following scratching of 
the skin of the cheek or chin. This is but one example of the fact that dilatation 
of the pupil can be induced by the stimulation of many sensory nerves and con- 
stantly occurs in severe pain. The path includes the following parts : the fibers 
of these sensory nerves and their central connections in the brain stem and spinal 
cord; preganglionic visceral efferent fibers, which arise from the cells of the inter- 
mediolateral column of the spinal cord and run through the upper white rami 
and the sympathetic trunk to the superior cervical sympathetic ganglion; and 
postganglionic fibers, which arise in that ganglion and run through the plexus on 
the internal carotid artery to end in the iris (Fig. 247). 

We have in the case of the pupillary reactions an illustration of the double 
and antagonistic innervation, which, as we shall see in the next chapter, is a 
rather characteristic feature of the autonomic nervous system. 



CHAPTER XXI 

THE SYMPATHETIC NERVOUS SYSTEM 

THE sympathetic nervous system is an aggregation of ganglia, nerves, and 
plexuses, through which the viscera, glands, heart, and blood-vessels, as well as 

Ciliary ganglion Maxillary nerve 
Sphenopalaline ganglion v 
Superior cervical ganglion of sympathetic \ \ 



Cervical plexus 



Brachial plexus 



Greater splanchnic nerve 
Lesser splanchnic nerve 



Lumbar plexus 



Sacral plexus 




Pharyngeal plexus 

Middle cervical ganglion of sympathetic 
Inferior cervical gang, of sympathetic 
Recurrent nerve 

Bronchial plexus 



Cardiac plexus 

Esophageal plexus 
^Coronary plexui 



Left vagus nerve 

Gastric plexus 
Celiac plexus 

Superior mesenteric plexus 



Aortic plexus 

Inferior mesenteric plexus 
I 

Hypogastric plexus 

Pelvic plexus 

Bladder 
Vesical plexus 



. ^ 
Fig. 248. The sympathetic nervous system. (Schwalbe, Herrick.) 



smooth muscle in other situations, receive their innervation. As illustrated in 
Fig. 248 it is widely distributed over the body, especially in the head and neck 

334 



THE SYMPATHETIC NERVOUS SYSTEM 



335 



and in the thoracic and abdominal cavities. It must not be too sharply de- 
limitated from the cerebrospinal nervous system, since it contains great numbers 
of fibers which run to and from the brain and spinal cord. For example, the 
vagus nerve contains many fibers which are distributed through the thoracic 
and abominal sympathetic plexuses for the innervation of the viscera. In the 
same way the spinal nerves are connected by communicating branches or rami 
communicates with the sympathetic trunks. 

The sympathetic trunks are two nerve cords which extend vertically through 
the neck, thorax, and abdomen, one on each side of the vertebral column (Fig. 
248). Each trunk is composed of a series of ganglia arranged in linear order 
and bound together by short nerve strands. Every spinal nerve is connected 
with the sympathetic trunk of its own side by one or more gray rami commu- 
nicantes through which it receives fibers from the sympathetic trunk. Fibers 
reach this trunk from the thoracic and upper lumbar nerves by way of the white 
rami communicantes (Fig. 257). The sympathetic trunk also gives off branches 
which enter into the formation of the nerve plexuses which are associated with 
the larger arteries. The largest of these plexuses is the celiac, which is associ- 
-ated with the upper portion of the abdominal aorta and its branches. In this 
plexus and located in close relation to the abdominal aorta are the celiac, 
mesenteric, and aorticorenal ganglia, all of which are in man grouped hi a pair 
of large irregular masses designated as the celiac ganglia and placed one on 
either side of the celiac artery (Fig. 257). The sympathetic ganglia may be 
grouped into three series as follows: (1) the ganglia of the sympathetic trunk, 
arranged in linear order along each side of the vertebral column and joined 
together by short nerve strands to form the two sympathetic trunks; (2) col- 
lateral ganglia, arranged about the aorta and including the celiac and mesenteric 
ganglia; and (3) terminal ganglia, located close to or within the structures 
which they innervate. As examples of the latter group there may be men- 
tioned the ciliary and cardiac ganglia and the small groups of nerve-cells in 
the myenteric and submucous plexuses (Fig. 257). 

FUNDAMENTAL FACTS CONCERNING VISCERAL INNERVATION 

General visceral afferent fibers are found in the ninth and tenth cranial 
nerves and in many of the spinal nerves, especially in those associated with the 
white rami (thoracic and upper lumbar nerves) and in the second, third, and 
fourth sacral nerves. These afferent fibers take origin from cells in the cerebro- 
spinal ganglia (Fig. 249). From these ganglia the fibers run through the corres- 



336 THE NERVOUS SYSTEM 

ponding cerebrospinal nerves to the sympathetic nervous system, through which 
they pass without interruption in any of its ganglia to end in the viscera. These 
fibers are of all sizes, including large and small myelinated fibers and many which 
are unmyelinated (Chase and Ranson, 1914; Ranson and Billingsley, 1918). 

The afferent impulses mediated by these fibers serve to initiate visceral re- 
flexes, and for the most part remain at a subconscious level. Such general vis- 
ceral sensations as we do experience are vague and poorly localized. Tactile 
sensibility is entirely lacking in the viscera and thermal sensibility almost so, 
although sensations of heat and cold may be experienced when very warm or 
cold substances enter the stomach or colon (Carlson and Braafladt, 1915). 
Pain cannot be produced by pinching or cutting the thoracic or abdominal 
viscera. Acute visceral pain may, however, be caused by disease, as in the pas- 
sage of a stone along the ureter. 

From the cerebrospinal ganglia the visceral afferent impulses are carried to the brain 
and spinal cord by the sensory nerve roots. The relations within the cerebrospinal ganglia 
are not entirely clear; but it seems probable that the visceral afferent impulses are conducted 
through the ganglion by way of the two branches of the typical unipolar sensory neuron 
(Fig. 249). Many authors believe that there are also sensory fibers which arise from cells 
in the sympathetic ganglia and terminate in the spinal ganglia in the form of pericellular 
plexuses (Fig. 40, C). Through these plexuses visceral sensory impulses are supposed to be 
transmitted to somatic sensory neurons and to be relayed by them to the spinal cord. Since 
it has not been clearly demonstrated that any sensory fibers arise from cells in the sym- 
pathetic ganglia, this interpretation of the pericellular plexuses of the spinal ganglia must be 
regarded as purely hypothetic. 

Langley (1903) has presented strong evidence that few if any sensory fibers arise in the 
sympathetic ganglia. Physiologic experiments show that the visceral afferent fibers run in 
the white rami, yet all or practically all of the fibers of a white ramus degenerate if the cor- 
responding spinal nerve is severed distal to the spinal ganglion. Huber (1913) states that 
"it has not been determined that the fine medullated fibers or the unmedullated fibers which 
appear to enter the spinal ganglia from without and end in pericellular plexuses are, in 
fact, the neuraxes of sympathetic neurones." The hypothesis that these pericellular plexuses 
represent the termination of visceral afferent fibers is, therefore, not well supported. This 
subject is treated in more detail in a series of papers on the sympathetic nervous system by 
Ranson and Billingsley (1918). 

Visceral Efferent Neurons. The general visceral efferent fibers of the 
cerebrospinal nerves take origin from cells located within the cerebrospinal axis. 
They do not run without interruption to the structures which they innervate; 
instead, they always terminate in sympathetic ganglia, whence the impulses, 
which they carry, are relayed to their destination by neurons of a second order 
(Fig. 249). This important information we owe to Langley (1900 and 1903), 
who showed that the injection of proper doses of nicotin into rabbits prevents 



THE SYMPATHETIC NERVOUS SYSTEM 



337 



the passage of impulses through the sympathetic ganglia, although an undi- 
minished reaction may be obtained by stimulation of the more peripheral sym- 
pathetic nerves By a long series of experiments Langley has shown that there 
are always two and probably never more than two neurons concerned in the 
conduction of an impulse from the central nervous system to smooth muscle 
or glandular tissue. The neurons of the first order in this series are designated as 
preganglionic, those of the second order as postganglionic, with reference to the 
relation which they bear to the ganglion containing their synapse. 

Preganglionic neurons have their cell bodies located in the visceral efferent 
column of the cerebrospinal axis. The cells of this series are smaller than those 



Spinal ganglion 
Dorsal ramus 




,' Ventral ramus 



Ramus communicans 



--- Sympathetic ganglion 



<y\ Visceral efferent fiber 
Somatic efferent fiber 

<^1 Postganglionic fiber 



root 



' ______ ,Viscus 



Fig. 249. Diagrammatic section through a spinal nerve and the spinal cord in the thoracic region 
to illustrate the chief functional types of peripheral nerve-fibers. 

of the somatic motor column and contain less massive Nissl granules. From 
these cells arise the fine myelinated visceral efferent fibers which run through 
the cerebrospinal nerves to the sympathetic nervous system and terminate in 
the sympathetic ganglia (Fig. 249) . 

Postganglionic neurons have their cell bodies located in the sympathetic 
ganglia. In fact, these cells with their dendritic ramifications and the terminal 
branches of the preganglionic fibers synaptically related to them are the es- 
sential elements in the sympathetic ganglia. Their axons for the most part 
remain unmyelinated and run as Remak fibers through the sympathetic nerves 



338 



THE NERVOUS SYSTEM 




s 



O J2 



C/5 '-5 



J3 



S J3 



THE SYMPATHETIC NERVOUS SYSTEM 339 

and plexuses, to end in relation with involuntary muscle or glandular tissue. 
A very few postganglionic fibers acquire delicate myelin sheaths. 

Three streams of preganglionic fibers leave the cerebrospinal axis (Fig. 250). 
The cranial stream includes the general visceral efferent fibers of the oculomotor, 
facial, glossopharyngeal, vagus, and accessory nerves. These fibers end in the 
terminal ganglia, already mentioned, which are located close to or within the 
organ which they innervate. In the cervical nerves there are no visceral ef- 
ferent fibers, the cranial stream being separated from the next by a rather wide 
gap. The thoracicolumbar stream includes the fibers which arise from the cells 
of the intermediolateral column of the spinal cord and make their exit through 
the thoracic and first four lumbar nerves (Langley, 1892; Miiller, 1909). After 
leaving the spinal nerves by way of the white rami they enter the sympathetic 
nervous system and terminate in the ganglia of the sympathetic trunk or in the 
celiac and associated collateral ganglia (Fig. 250). The sacral stream includes 
the visceral efferent fibers of the second, third, and fourth sacral nerves. These 
arise from cells in the lateral column of gray matter in the sacral portion of the 
spinal cord and run through the visceral branch of the third sacral and a similar 
branch from either the second or fourth sacral nerves. These fibers end in the 
ganglia of the pelvic sympathetic plexuses. 

The Autonomic Nervous System. For many reasons it is convenient to have 
a name which will designate the sum total of all general visceral efferent neurons, 
both preganglionic and postganglionic, whether associated with the cerebral 
or spinal nerves. For this purpose the term "autonomic nervous system" is 
in general use. It designates that functional division of the nervous system 1 
which supplies the glands, heart, and smooth musculature with their efferent in- I 
nervation (Fig. 250). It is important to bear in mind that this is a functional 
and not an anatomic division of the nervous system, that it includes only efferent 
elements, and that the preganglionic neurons lie in part within the cerebrospinal 
nervous system. The terminal portions of the preganglionic fibers and the 
postganglionic neurons are located in the sympathetic system. According to 
the origin of the preganglionic fibers, we may recognize the following three 
subdivisions of the autonomic system: (1) the cranial autonomic system, whose 
preganglionic fibers make their exit by way of the third, seventh, ninth, tenth, 
and eleventh cranial nerves; (2) the thoracicolumbar autonomic system, whose pre- 
ganglionic fibers make their exit by way of the thoracic and upper lumbar spinal 
nerves; and (3) the sacral autonomic system, whose preganglionic fibers run in 
the visceral rami of the second, third, and fourth sacral nerves (Fig. 250). 



040 THE NERVOUS SYSTEM 

The fibers of the thoracicolumbar stream run by way of the white rami to 
the sympathetic trunk, while the fibers of the cranial and sacral streams make 
no connection with that trunk, but run directly to the sympathetic plexuses. 
And while the thoracicolumbar preganglionic fibers terminate hi the ganglia of 
the trunk, those of cranial and sacral origin end in the terminal ganglia. In 
these two respects the cranial and sacral streams agree with each other and differ 
from the thoracicolumbar outflow. Also in their response to certain drugs, 
like atropin and adrenalin, the two former agree with each other and differ from 
the latter. It is, therefore, desirable to group the cranial and sacral systems 
together as the craniosacral autonomic system. This has been called by many 
physiologists the parasympathetic system. It stands in contrast to the thoracico- 
lumbar autonomic system to which many physiologists have unfortunately applied 
the name "sympathetic system." The importance of recognizing these two 
principal subdivisions is further emphasized by the fact that most of the struc- 
tures innervated by the autonomic system receive a double nerve supply and are 
supplied with fibers from both subdivisions. The thoracicolumbar fibers are 
accompanied in most peripheral plexuses by craniosacral fibers of opposite func- 
tion so that the analysis of these plexuses is greatly facilitated by subdividing 
the autonomic system in this way. 

Visceral Reflexes. In the gastro-intestinal tract and perhaps within other 
viscera there may be a mechanism for purely local reactions as indicated in 
the following paragraph. With this exception the evidence strongly indicates 
that all visceral reflex arcs pass through the cerebrospinal axis. In such an 
arc there are at least three neurons, namely, (1) visceral afferent, (2) pregang- 
lionic visceral efferent, and (3) postganglionic visceral efferent neurons (Fig. 249) . 

The purely local reactions which occur in the gut wall after section of all of 
the nerves leading to the intestine are known as myenteric reflexes and must de- 
pend upon a mechanism different from that of other visceral reflexes (Langley 
and Magnus, 1905; Cannon, 1912). Practically nothing is known of this mech- 
anism beyond the fact that it must be located in the enteric plexuses. Some 
authors have assumed that within these plexuses there is a diffuse nerve net 
similar to that found in the ccelenterates (Parker, 1919). While the evidence 
is far from satisfactory, it may be that such a net does exist in this situation and 
that it is responsible for these local reactions. 



THE SYMPATHETIC NERVOUS SYSTEM 



STRUCTURE OF THE SYMPATHETIC GANGLIA 



341 



The nerve-cells of the sympathetic ganglia are almost all multipolar, but there 
are also a few that are unipolar or bipolar. Each cell is surrounded by a nucleated 
membranous capsule. Some of the dendrites ramify beneath this capsule and 
are designated as intracapsular. Others pierce the capsule, run long distances 
through the ganglia, and are known as extracapsular dendrites. 




Fig. 251. Neurons from the human superior cervical sympathetic ganglion (pyridin-silver 
method): A, Three nerve cells and the intercellular plexus: a, unicellular glomerulus; b, neuron 
with extracapsular dendrites. B, Tricellular glomerulus. C, Neuron surrounded by subcapsular 
dendrites. 

Intracapsular dendrites are numerous in the sympathetic ganglia of man, 
but rare in those of mammals (Marinesco, 1906; Cajal, 1911; Michailow, 1911; 
Ranson and Billingsley, 1918). Beneath the capsule these dendrites may form 
an open network more or less uniformly distributed around the cell (Fig. 251, C), 
or they may be grouped on one side of the cell, causing a localized bulging in 
the capsule (Fig. 251, A, a). Such a localized mass of subcapsular dendrites 
with interlacing branches is known as a glomerulus. Following CajaPs classifi- 
cation we may distinguish four types of glomeruli according to the number of 



342 



THE NERVOUS SYSTEM 



neurons whose dendrites enter into their formation, namely, unicellular (Fig. 
25 1 , A , a) , bicellular , triceUular (Fig. 25 1 , B) , and multicellular glomeruli. Short 
intracapsular dendrites with swollen ends are sometimes present in the sym- 
pathetic ganglia of mammals (Fig. 252, A}. 




Fig. 252. Sympathetic ganglion cells showing various types of dendrites. Redrawn from 
Michailow. Methylene-blue stain. A, From superior mesenteric ganglion, horse; B, from celiac 
ganglion, horse; C, from stellate ganglion, horse; D, from superior cervical ganglion, dog; E, celiac 
ganglion, horse; F, superior cervical ganglion, dog. 

Extracapsular dentrites pierce the capsule, run for longer or shorter dis- 
tances among the cells, and help to form an intercellular plexus of dendritic and 
axonic ramifications (Fig. 251, -4). These dendrites may end in a variety of 
ways. Some of these types of endings may be enumerated as follows: (1) 
brush-like endings (Fig. 252, A); (2) plate-like or bulbous terminals applied 



THE SYMPATHETIC NERVOUS SYSTEM 



343 



against the outer surface of the capsule of another cell (Fig. 252, B, C) ; (3) inter- 
lacing branches, which form a plexus upon the outer surface of the capsule of 
an adjacent cell (Fig. 252, D}. 

Dogiel (1896) thought that the cells possessing the longest dendrites were sensory, but 
Cajal (1911) could find no evidence for this, and was unable to trace any of them from the 
ganglia and associated nerves to the viscera. Carpenter and Conel (1914), using the size 
and arrangement of the Nissl granules as a criterion,- were able to find only one cell type in 
the sympathetic ganglia, and concluded that these ganglia do not contain sensory nerve-cells. 




Fig. 253. Neurons and intercellular plexus from the superior cervical sympathetic ganglion of a 

dog (pyridin-silver method). 

The axons of sympathetic ganglion cells are usually unmyelinated, but a few 
of them acquire thin myelin sheaths. They are the postganglionic fibers which 
relay the visceral efferent impulses to the innervated tissue. According to 
Cajal (1911), who states that his anatomic studies are in accord with the physio- 
logic experiments of Langley, the axons of the cells in the ganglia of the sympa- 
thetic trunk dispose themselves in one of the three following ways: (1) Usually 
they run transversely to the long axis of the ganglion to enter a gray ramus. 



344 



THE NERVOUS SYSTEM 



(2) The axons may run through a connecting nerve trunk into another ganglion. 
He is not able to say whether these axons only run through the second ganglion 
or whether they make connections with its cells. In the chick embryo he at one 
time described collaterals coming from those longitudinal fibers of the ganglia, 
which take origin in neighboring ganglia. Now, however, he is inclined to doubt 
this observation, and thinks it likely that these collaterals all come from fibers 
that have entered the sympathetic trunk through white rami at other levels. 




Fig. 254. 




Fig. 255. 

Figs. 254 and 255. Preganglionic fibers and pericellular plexuses of the frog. Fig. 254, Pre- 
ganglionic fibers, the branches of which form pericellular plexuses; Fig. 255, a unipolar sympathetic 
ganglion cell in connection with which a preganglionic fiber is terminating. Methylene-blue. 
(Huber.) 

(3) In some cases the axons, arising from cells in the ganglia of the sympathetic 
trunk, run toward the neighboring arteries in the visceral nerves. 

There is no anatomic evidence worth mentioning in favor of the existence of association 
neurons, uniting one sympathetic ganglion with another or one group of cells with another 
within such a ganglion. But there is strong physiologic evidence against the existence of 
such association neurons (Langley, 1900 and 1904) ; and Johnson (1918) has shown that none 
are present in the sympathetic trunk of the frog. 

Termination of the Preganglionic Fibers. The spaces among the cells of a 
sympathetic ganglion are occupied by a rich intercellular plexus of dendritic 



THE SYMPATHETIC NERVOUS SYSTEM 345 

branches and fine axons (Figs. 251, A ; 253). The fine axons represent the rami- 
fications of preganglionic fibers and they degenerate when the connection 
between the ganglion and the central nervous system is severed (Ranson 
and Billingsley, 1918). Similar fibers pierce the capsules surrounding the 
cells and intertwine with the intracapsular dendrites. No doubt synaptic 
relations are established between the axonic and dendritic ramifications in 
these plexuses. 

Another and very characteristic type of synapse is established in the peri- 
cellular plexuses, formed by the terminal ramifications of preganglionic fibers upon 
the surface of the cell bodies of postganglionic neurons. Huber (1899) showed 
that fibers from the white rami branch repeatedly in the sympathetic ganglia 
and that the branches terminate in subcapsular pericellular plexuses (Figs. 254, 
255). 

In the sympathetic ganglia of the frog the pericellular plexus seems to be the only type 
of synapse and there is no intercellular plexus. In the mammalian sympathetic ganglion 
these pericellular plexuses are harder to demonstrate and are probably less numerous, while 
the intercellular plexus is much in evidence. It is well established that one preganglionic 
fiber may be synaptically related to several postganglionic neurons, probably in some in- 
stances to as many as thirty or more (Ranson and Billingsley, 1918). 

COMPOSITION OF SYMPATHETIC NERVES AND PLEXUSES 

Some of the sympathetic nerves are as well myelinated as the cerebrospinal 
nerves and present a white glistening appearance. This is true, for example, of 
the cervical portion of the sympathetic trunk, the white rami, and the splanch- 
nic nerves. Such white sympathetic nerves are composed at least in large part 
of fibers running to and from the central nervous system. Other nerves like 
the gray rami and branches to the blood-vessels are gray, because they are com- 
posed chiefly of unmyelinated postganglionic fibers. In preceding paragraphs 
we have shown that there are probably no association or sensory neurons in 
the sympathetic ganglia; and, if this be true, there are no axons, arising from such 
cells, in the sympathetic nerve trunks and plexuses. These nerves and plexuses 
are composed of the following three kinds of fibers (Fig. 256) : (1) Preganglionic 
visceral efferent fibers, which are of small size and myelinated, have their cells 
of origin in the cerebrospinal axis, and terminate in the sympathetic ganglia. 
(2) Postganglionic fibers, which are for the most part unmyelinated, have their 
cells of origin in the sympathetic ganglia and terminate in involuntary muscle or 
glandular tissue. (3) Visceral afferent fibers, which include myelinated fibers 
of all sizes as well as many that are unmyelinated, have their cells of origin in 



346 



THE NERVOUS SYSTEM 



the cerebrospinal ganglia and terminate in the viscera. The statements con- 
tained in this paragraph should not be applied without qualification to the ter- 



Spinal ganglion 
Dorsal root 



Pacinian corpuscle ' 

Motor ending on smooth 
muscle' 



Ventral root 
Splanchnic nerve 




Collateral ganglion 



Blood-vessel-s? 






^- Ganglion of sympathetic trunk 

Jfr^Tji Gray ramus 

"" White ramus 



Sympathetic trunk 

Dorsal ramus 

Ventral ramus 

f$ Gland 
'^'^^-^ Blood-vessel 

*~ White ramus 
_ x Gray ramus 

Ganglion of sympathetic trunk 

Sympathetic trunk 



Sensory ending 



Fig. 256. Diagram showing the composition of sympathetic nerves. Black lines, visceral 
afferent fibers; unbroken red lines, preganglionic visceral efferent fibers; dotted red lines, post- 
ganglionic visceral efferent fibers. 

minal ganglia and plexuses, since it is probable that these contain additional 
elements either in the nature of sensory neurons or of a nerve net. 



ARCHITECTURE OF THE SYMPATHETIC NERVOUS SYSTEM 

The sympathetic trunks are two ganglionated cords, each of which consists 
of a series of more or less segmentally arranged ganglia, bound together by as- 
cending and descending nerve-fibers and extending from the level of the second 
cervical vertebra to the coccyx (Figs. 248, 257). The two trunks are symmetrically 
placed along the anterolateral aspects of the bodies of the vertebrae. There are 
21 or 22 ganglia in each chain; and of these, 3 are associated with the cervical 
spinal nerves, 10 or 11 with the thoracic, 4 with the lumbar, and 4 with the sacral 
spinal nerves. The sympathetic trunks are connected with each of the spinal 
nerves by one or more delicate nerve strands, called rami communicantes (Figs. 



THE SYMPATHETIC NERVOUS SYSTEM 347 

248, 257). To each spinal nerve there runs a gray ramus from the sympathetic 
trunk. The white rami, on the other hand, are more limited in distribution and 
unite the thoracic and upper four lumbar nerves with the corresponding portion 
of the sympathetic trunk. 

The white rami consist of visceral afferent and preganglionic visceral efferent 
fibers directed from the central into the sympathetic nervous system. They 
contribute the great majority of the ascending and descending fibers of the 
sympathetic trunk (Fig. 257). While some of the fibers may terminate in the 
ganglion with which the white ramus is associated, and others run directly 
through the trunk into the splanchnic nerves, the majority of the fibers turn 
either upward or downward in the trunk and run for considerable distances within 
it (Fig. 250). The fibers from the upper white rami run upward, those from the 
lower white rami downward, while those from the intermediate rami may run 
either upward or downward. The cervical portion of the sympathetic trunk 
consists almost or quite exclusively of ascending fibers, the lumbar and sacral 
portions of the trunk largely of descending fibers from the white rami. The 
afferent fibers of the white rami merely pass through the trunk and its branches 
to the viscera. The preganglionic fibers, with the exception of those which run 
out through the splanchnic nerves, end in the gang^a of the trunk. Here they 
enter into synaptic relations with the postganglionic neurons. The majority 
of the postganglionic neurons, located in the ganglia of the sympathetic trunk, 
send their axons into the gray rami (Figs. 250, 256). 

The gray rami are composed of postganglionic fibers directed from the sym- 
pathetic trunk into the spinal nerves. These unmyelinated fibers, after joining j 
the spinal nerves, are distributed with them as vasomotor, secretory, and pilo- 
motor fibers to the blood-vessels, the sweat glands, and the smooth muscle of 
the hair-follicles. 

Especially in the cervical region there are other important branches from the 
sympathetic trunk, which resemble the gray rami in structure and which convey 
postganglionic fibers to certain of the cranial nerves and to the heart, pharynx, 
the internal and external carotid and thyroid arteries, and through the plexuses 
on these arteries to the thyroid gland, salivary glands, eye, and other structures 
(Figs. 248, 250, 257). 

The cranial portion of the sympathetic trunk consists of three ganglia bound 
together by ascending preganglionic fibers from the white rami. In the cat it has 
been shown to contain few if any sensory or postganglionic fibers. The superior 
cervical ganglion is the largest of the three ganglia and from it there are given off 



548 THE NERVOUS SYSTEM 

numerous gray nerve strands. These are all composed of postganglionic fibers 
which arise in this ganglion. They run to the neighboring cranial and spinal 
nerves, to which they carry vasomotor, pilomotor, and secretory fibers, and to the 
heart, pharynx, and the internal and external carotid arteries (Figs. 248, 250, 
257). The most important of these branches of the superior cervical ganglion 
are the three following: (1) The superior cervical cardiac nerve, which runs 
from the superior cervical ganglion to the cardiac plexus, carries accelerator 
fibers to the heart. (2) The internal carotid nerve runs vertically from the 
ganglion to the internal carotid artery, about which its fibers form a plexus, 
known as the internal carotid plexus (Fig. 257). It is by way of this nerve and 
plexus that the pupillary dilator fibers reach the eye (Fig. 247). (3) The branch 
of the superior cervical ganglion to the external carotid artery breaks up into a 
plexus on that artery. A continuation of this plexus extends along the external 
maxillary artery, and carries secretory fibers to the submaxillary salivary gland. 

The middle and inferior cervical sympathetic ganglia are smaller. Among 
the branches from these ganglia we may mention the gray rami to the adjacent 
spinal nerves and the middle and inferior cardiac nerves to the cardiac plexus 
(Figs. 248, 257). 

The thoracic portion of the sympathetic trunk is connected with the thoracic 
nerves by the gray and white rami. In addition to the rami communicantes 
and some small branches to the aortic and pulmonary plexuses, there are three 
important branches of the thoracic portion of the sympathetic trunk known as 
the splanchnic nerves. These run through the diaphragm for the innervation 
of abdominal viscera (Figs. 248, 257). The greater splanchnic nerve is usually 
formed by branches from the fifth to the ninth thoracic sympathetic ganglia 
and after piercing the diaphragm joins the celiac ganglion. The smaller splanch- 
nic nerve is usually formed by branches from the ninth and tenth thoracic 
sympathetic ganglia and terminates in the celiac plexus. The lowermost splanch- 
nic nerve arises from the last thoracic sympathetic ganglion and terminates in 
the renal plexus. These splanchnic nerves, although they appear to be branches 
of the thoracic sympathetic trunk, are at least in major part composed of fibers 
from the white rami, which merely pass through the trunk on their way to the 
ganglia of the celiac plexus (Figs. 250, 257; Langley, 1900; Ranson and Billings- 
ley, 1918). 

THE SYMPATHETIC PLEXUSES 

The Sympathetic Plexuses of the Thorax. In close association with the 
vagus nerve in the thorax are three important sympathetic plexuses. The 



THE SYMPATHETIC NERVOUS SYSTEM 349 

cardiac plexus lies in close relation to the arch of the aorta, and from it sub- 
ordinate plexuses are continued along the coronary arteries. It receives the 
three cardiac sympathetic nerves from the cervical portion of each sympathetic 
trunk, as well as branches from both vagus nerves (Figs. 248, 257). The pregan- 
glionic fibers of the vagus terminate in synaptic relation with the cells of the 
cardiac ganglia. They convey inhibitory impulses which are relayed through 
these ganglia to the cardiac musculature (Fig. 250). The cardiac sympathetic 
nerves contain postganglionic fibers which take origin in the cervical sympa- 
thetic ganglia; and they relay accelerator impulses, coming from the spinal cord 
by way of the upper white rami and sympathetic trunk to the heart (Fig. 250). 
The pulmonary and esophageal plexuses of the vagus are also to be regarded as 
parts of the sympathetic system (Fig. 257). 

The celiac plexus (solar plexus) is located in the abdomen in close relation 
to the celiac artery (Figs. 248, 257). It is continuous with the plexus which 
surrounds the aorta. Subordinate portions of the celiac plexus accompany 
the branches of the celiac artery and the branches from the upper part of the 
abdominal aorta. These are designated as the phrenic, suprarenal, renal, 
spermatic or ovarian, abdominal aortic, superior gastric, inferior gastric, he- 
patic, splenic, superior mesenteric, and inferior mesenteric plexuses. The celiac 
plexus contains a number of ganglia which in man are grouped into two large 
flat masses, placed one on either side of the celiac artery and known as the 
celiac ganglia. These ganglia are bound together by strands which cross the 
median plane above and below this artery. Somewhat detached portions of 
the celiac ganglion, which lie near the origin of the renal and superior mesenteric 
arteries, are known respectively as the aorticorenal and superior mesenteric 
ganglia. In addition, there is a small mass of nerve-cells in the inferior mesen- 
teric plexus close to the beginning of the inferior mesenteric artery. This is 
known as the inferior mesenteric ganglion. 

Preganglionic fibers reach the celiac plexus from two sources, namely, from 
the white rami by way of the sympathetic trunk and splanchnic nerves and from 
the vagus nerve (Fig. 257). Most if not all of the preganglionic fibers contained 
in the splanchnic nerves terminate in the ganglia of the celiac plexus. At the 
lower end of the esophageal plexus the fibers from the right vagus nerve become 
assembled into a trunk which passes to the posterior surface of the stomach and 
the celiac plexus. The fibers of the left vagus pass to the anterior surface of 
the stomach and to the hepatic plexus (Fig. 257). It is probable that the pre- 
ganglionic fibers of the vagus do not terminate in the ganglia of the celiac plexus, 



35 



THE NERVOUS SYSTEM 



N. VII 



Internal carotid plexus 

ToN.X 

ToN.IX 

To cervical N. I 



To sacral N. I 

N.II 
N.III 

IV 




Visceral branches 
sacral nerves 

N.IV 
N. V 
To coccygeal nerve 



Ciliary ganglion 

Splenopalatine ganglion 

N. IX 

Otic ganglion 

Superior cervical ganglion 

Pharyngeal plexus 

N, VII 

Submaxillary ganglion 

Middle cervical ganglion 

Superior cardiac N. 

Middle cardiac N. 

Inferior cardiac N. 

Cardiac branches of vagus 

Vagus and left pulmonary plexus 

Cardiac plexus 

Left coronary plexus 

Esophageal plexus 

Splanchnic nerves 



Hepatic plexus 
Left vagus nerve 

^Gastric plexus 

.Myenteric and sub- 
mucous plexuses 

Splenic plexus 

Celiac plexus 
perior mesenteric 
plexus 
^ * 
Inferior mesenteric plexus 



^Abdominal aortic plexus 

4 

Hypogastric plexus 



Fig. 257. Diagram of the sympathetic nervous system. The red lines indicate the branches 
of the cerebrospinal nerves which join the sympathetic system and those sympathetic nerves which 
are composed in major part of fibers from the cerebrospinal nerves. (Modified from Jackson- 
Morris.) 

but merely pass through that plexus to end in the terminal ganglia, such as the 
small groups of nerve-cells in the myenteric and submucous plexuses of the in- 
testine (Fig. 250). 



THE SYMPATHETIC NERVOUS SYSTEM 351 

The my enteric plexus (of Auerbach) and the submucous plexus (of Meissner), 
located within the walls of the stomach and intestines, receive filaments from 
the gastric and mesenteric divisions of the celiac plexus. They also receive 
fibers from the vagus either directly, as in the case of the stomach, or indirectly 
through the celiac plexus (Fig. 257). Unfortunately, very little is known con- 
cerning the synaptic relations established in the ganglia of these plexuses. Ac- 
cording to Langley, the postganglionic fibers from the celiac ganglia run through 
these plexuses without interruption and end in the muscular coats and glands 
of the gastro-intestinal tract. The preganglionic fibers from the vagus probably 
end in synaptic relation to cells in these small ganglia; and the axons of these 
cells serve as postganglionic fibers, relaying the impulses from the vagus to the 
glands and muscular tissue. As was indicated in a preceding paragraph, the 
enteric plexuses must also contain a mechanism for purely local reactions, since 
peristalsis can be set up by distention in an excised portion of the gut. But 
as yet we are entirely ignorant as to what that mechanism may be. 

The hypogastric plexus is formed by strands which run into the pelvis from 
the lower end of the aortic plexus and are joined by the visceral branches of the 
second, third, and fourth sacral nerves and by branches from the sympathetic 
trunk (Figs. 248, 257). As the hypogastric plexus enters the pelvis it splits into 
two parts, which lie on either side of the rectum and are sometimes called the 
pelvic plexuses. From these plexuses branches are supplied to the pelvic vis- 
cera and the external genitalia. 

The Cephalic Ganglionated Plexus. In close topographic relation to the 
branches of the fifth cranial nerve are four sympathetic ganglia, known as the 
ciliary, sphenopalatine, otic, and submaxillary ganglia. Each of these is con- 
nected with the superior cervical sympathetic ganglion by filaments derived 
from the plexuses on the internal and external carotid arteries and their branches 
(Fig. 257). These filaments are designated in descriptive anatomy as the sym- 
pathetic roots of the ganglia. Each ganglion receives preganglionic fibers from 
one of the cranial nerves by way of what is usually designated as its motor root 
(Fig. 257). Thus the ciliary ganglion receives fibers from the oculomotor nerve; 
the sphenopalatine ganglion receives fibers from the facial nerve by way of the 
great superficial petrosal nerve and the nerve of the pterygoid canal; the otic 
ganglion receives fibers from the glossopharyngeal nerve (Miiller and Dahl, 1910) ; 
and the submaxillary ganglion receives fibers from the facial nerve by way of 
the nervus intermedius and the lingual nerve. Postganglionic fibers arising 
in these ganglia are distributed to the structures of the head. From the ciliary 



352 THE NERVOUS SYSTEM 

ganglion fibers go to the intrinsic musculature of the eye. Some of the fibers 
arising in the sphenopalatine ganglion go to the blood-vessels in the mucous 
membrane of the nose. Fibers from the otic ganglion reach the parotid gland. 
And those arising in the submaxillary ganglion end in the submaxillary and 
sublingual salivary glands (Fig. 250). 

IMPORTANT CONDUCTION PATHS BELONGING TO THE AUTONOMIC NERVOUS 

SYSTEM 

Thanks to the work of Langley, we know that the conduction pathways in 
the sympathetic nervous system are at least as sharply defined as those in the 
brain and spinal cord. A great deal has already been done in the way of tracing 
these pathways; and some of the more important of these are given in the out- 
line which follows: 

1. Paths for the efferent innervation of the eye (Figs. 247, 250): 

(a) Ocular craniosacral pathway. 

Preganglionic neurons: Cells in the Edinger-Westphal nucleus, 
fibers by way of the third cranial nerve to end in the ciliary ganglion. 

Postganglionic neurons: Cells in the ciliary ganglion, fibers by 
way of the short ciliary nerves to the ciliary muscle and the circular 
fibers of the iris. 

Function: Accommodation and contraction of the pupil. 

(b) Ocular thoracicolumbar pathway. 

Preganglionic neurons: Cells in the intermediolateral column of 
the spinal cord, fibers by way of the upper white rami and sympathetic 
trunk to end in the superior cervical ganglion. 

Postganglionic neurons: Cells in the superior cervical ganglion, 
fibers by way of the internal carotid plexus to the ophthalmic division 
of the fifth nerve, the nasociliary and long ciliary nerves of the eyeball; 
other fibers pass from the internal carotid plexus through the ciliary 
ganglion, without interruption, into the short ciliary nerves and to 
the eyeball. 

Function: Dilatation of the pupil by the radial muscle-fibers of 
the iris. 

2. Paths for the efferent innervation of the submaxillary gland (Fig. 250) : 
(a) Submaxillary craniosacral pathway. 

Preganglionic neurons: Cells in the nucleus salivatorius superior, 
fibers by way of the seventh cranial nerve, chorda tympani, and 



THE SYMPATHETIC NERVOUS SYSTEM 353 

lingual nerve to end in the portion of the submaxillary ganglion 
located on the submaxillary duct. 

Postganglionic neurons: Cells in a number of groups along the 
chorda tympani fibers as they follow the submaxillary duct, fibers 
distributed in branches to the submaxillary gland. 

Function : Increases secretion. 
(&) Submaxillary thoracicolumbar pathway. 

Preganglionic neurons: Cells in the intermediolateral column of 
the spinal cord, fibers by way of the upper white rami, and the sym- 
pathetic trunk to end in the superior cervical ganglion. 

Postganglionic neurons: Cells in the superior cervical ganglion, 
fibers by way of the plexuses on the external carotid and external 
maxillary arteries to the submaxillary gland. 

Function: Increases secretion. 

3. Paths for the efferent inner vation of the heart: 
(a) Cardiac craniosacral pathway. 

Preganglionic neurons: Cells in the dorsal motor nucleus of the 
vagus, fibers through the vagus nerve to the intrinsic ganglia of the 
heart, in which they end. 

Postganglionic neurons: Cells in the intrinsic cardiac ganglia, 
fibers to the cardiac muscle. 

Function: Cardiac inhibition. 
(&) Cardiac thoracicolumbar pathway. 

Preganglionic neurons: Cells in the intermediolateral column of 
the spinal cord, fibers by way of the upper white rami and the sym- 
pathetic trunk to the superior, middle, and inferior cervical ganglia. 

Postganglionic neurons: Cells in the cervical ganglia of the sym- 
pathetic trunk, fibers by way of the corresponding cardiac nerves to 
the musculature of the heart. 

Function: Cardiac acceleration. 

4. Paths for the efferent innervation of the musculature of the stomach 

exclusive of the sphincters (Fig. 250) : 
(a) Gastric craniosacral pathway. 

Preganglionic neurons: Cells in the dorsal motor nucleus of the 
vagus, fibers by way of the vagus nerve, to end in the intrinsic ganglia 
of the stomach. 



THE NERVOUS SYSTEM 

Postganglionic neurons: Cells in the intrinsic gastric ganglia, fibers 
to end in the gastric musculature. 

Function: Excites peristalsis. 
(6) Gastric thoracicolumbar pathway. 

Preganglionic neurons: Cells in the intermediolateral column of the 
spinal cord, fibers by way of the white rami from the fifth or sixth to 
the twelfth thoracic nerves, through the sympathetic trunk without 
interruption, and along the splanchnic nerves to the celiac ganglion, 
where they end. 

Postganglionic neurons: Cells in the celiac ganglion, fibers by way 
of the celiac plexus and its offshoots to the stomach, to end in the 
musculature of the stomach. 

Function: Inhibits peristalsis. 
5. Paths for the efferent innervation of the musculature of the urinary 

bladder, 
(a) Vesical craniosacral pathway. 

Preganglionic neurons: Cells in the lateral part of the anterior 
gray column in the sacral portion of the spinal cord, fibers by way 
of the second and third sacral nerves and their visceral rami through 
the pelvic plexus to the plexus upon the wall of the bladder. 

Postganglionic neurons: Cells in the small ganglia of the vesical 
plexus, fibers to the vesical musculature. 

Function: Excites contraction of the vesical musculature exclusive 
of the internal sphincter (trigonal area), the contraction of which it 
inhibits and thus produces urination. 
(6) Vesical thoracicolumbar pathway. 

Preganglionic neurons : Cells in the caudal part of the intermedio- 
lateral cell column, fibers by way of the lower white rami to the infe- 
rior mesenteric ganglion. 

Postganglionic neurons: Cells in the inferior mesenteric ganglion, 
fibers through the inferior mesenteric plexus to the musculature of 
the bladder. 

Function: Excites contraction of the internal sphincter (trigonal 
area of the vesical musculature), causing retention of urine. 
It will be noted that the viscera receive a double autonomic innervation, and 
that the impulses transmitted along the craniosacral pathways are usually 
antagonistic to those transmitted along the thoracicolumbar paths. 



A LABORATORY OUTLINE OF NEURO-ANATOMY 

THE following directions for the study of the gross and microscopic anatomy of 
the nervous system are intended to aid the student in making the best use of his time 
and laboratory material. Free use is made of the sheep's brain because in most in- 
stitutions the number of human brains available is limited, and these are often poorly 
preserved and entirely unsuited for dissection. Even if an unlimited supply of well- 
preserved human brains were at hand, there would still be an advantage in the use of 
the sheep's brain because in it certain structures (such as the olfactory tracts and centers 
and the really significant subdivisions of the cerebellum) are more easily seen and more 
readily understood. 

The outline has been written in such a way that it can be readily adapted by the 
instructor to meet his own needs. It is assumed that each instructor will furnish his 
students with a schedule for the laboratory work, showing the number of laboratory 
periods available and the topics to be covered each period. This will help the student 
properly to apportion his time and enable the instructor to arrange the order of the 
laboratory work to his own liking. The paragraphs have been numbered serially in 
order that in such a schedule they may be referred to by number. It is not necessary 
that the topics be taken up in their numeric order. And in a course of one hundred 
hours some of the topics should be omitted altogether. How much should be omitted 
will depend largely on the amount of drawing required. It is assumed that the in- 
structor will indicate on the laboratory schedule the drawings which he wishes to have 
made. For this reason we have, for the most part, omitted specific directions for draw- 
ings. 

Since it will be necessary for the student in using the outline to make frequent 
references to figures in the text, it will be convenient to keep in the book several strips 
of thin paper to serve as bookmarks. 

METHODS OF BRAIN DISSECTION 

Much information concerning the gray masses and fiber tracts of the brain can be 
obtained by dissection. This should be carried out, for the most part, with blunt 
instruments. It is rarely necessary to make a cut with a knife. An orangewood mani- 
cure stick makes an excellent instrument. It should be rounded to a point at one end 
for teasing, while the larger end should be adapted for scraping away nuclear masses. 
A pair of blunt tissue forceps of medium size with smooth even edges and fine transverse 
interlocking ridges is also an essential instrument. This is useful in grasping and strip- 
ping away small bundles of fibers. In dissecting out a fiber tract it is necessary to have 
in mind a clear idea of the position and course of the tract, and the dissecting instru- 
ments should be carried in the direction of the fibers. Where it is necessary to remove 
nuclear material in order to display fiber bundles, it will be found very helpful to let a 
stream of water run over the specimen while the dissection is in progress. 

355 



356 THE NERVOUS SYSTEM 

DISSECTION OF THE HEAD OF THE DOGFISH 

1. The dogfish is the smallest of the sharks. Either the spiny dogfish (Squalus 
acanthias) or the smooth dogfish (Mustelus canis) may be used for dissection. 

2. The special sense organs include the olfactory organs, the eyes, the ears, and 
certain sense organs in the skin, known as the lateral line canals, and the ampullae of 
Lorenzini. 

3. Locate the position of the lateral line canal which produces a light colored ridge 
in the skin extending from head to tail along either side of the body. The line may be 
recognized by the presence of numerous small pores which open into the canal. It 
extends on to the head and there forms the supraorbital, infraorbital, and hyoman- 
dibular canals. The ampulla of Lorenzini are bulb-shaped bodies connected by long 
canals with pores in the skin. They are irregularly arranged and are most numerous 
on the snout. 

4. Locate the olfactory organs or nasal capsules which have their openings on the 
ventral surface of the snout in front of the mouth. 

5. Note the gills and spiracles (Fig. 12). Find two minute apertures near the 
midline between the spiracles. These are the openings of the endolymphatic ducts. 

6. The internal ear, a membranous labyrinth inclosed in a cartilaginous capsule, 
should be exposed on the left side. Shave off the cartilage in thin slices in the region 
between the spiracle and the median plane. The membranous labyrinth can be seen 
through the translucent cartilage, and care should be exercised to avoid injuring it while 
the cartilage is being removed. It consists of a spheric sac, the utriculosaccular chamber, 
to which there are attached three semicircular canals (Fig. 12). The endolymphatic 
duct is a small canal, which extends from this chamber through the roof of the skull to 
the small opening in the skin, which has previously been identified. Note the enlarge- 
ment at one end of each semicircular canal, known as the ampulla, and observe that 
each of these canals lies in a plane at right angles to the planes of the other two. 

7. The Brain and Cranial Nerves. Remove the remainder of the roof of the skull 
and expose the brain, eyes, and cranial nerves. 

8. Examine the brain as seen from the dorsal surface. Note the continuity of the 
medulla oblongata with the spinal cord. Identify the cerebellum, the thalamus, epiphysis, 
habenula, cerebral hemispheres, and olfactory bulbs (Fig. 8 and pp. 26-31). 

9. By dissection display on the left side the eye-muscles and the nerves which in- 
nervate them, as well as the optic nerve (Fig. 12). 

10. Find the nervus terminalis (Fig. 8). Now locate each of the cranial nerves 
from the second to the tenth inclusive, and trace them from the brain as far as possible 
toward their peripheral terminations (Figs. 12, 13). Note particularly that Nn. VII 
and X each have an extra root, indicated in black in Fig 13, which carries fibers from 
the lateral line organs to the acusticolateral area of the medulla. 

11. Attention should now be paid to the functional types of nerve-fibers which 
compose each of the cranial nerves (see pp. 168-170 and Figs. 119, 120). The ac- 
companying table shows in which of the cranial nerves of the dogfish each of the four 
principal functional groups of fibers are to be found (Herrick and Crosby, 1918). 



A LABORATORY OUTLINE OF NEURO-ANATOMY 
CRANIAL NERVE COMPONENTS OF THE DOGFISH 



357 



Somatic sensory. 



Somatic motor. 



Visceral sensory. 



Visceral motor. 



II. Optic 

III. Muscle sense 

IV. Muscle sense 

V. General cutaneous 
VI. Muscle sense 
VII. Lateral line fibers 

VIII. To the ear 
IX. Lateral line fibers 



X. Lateral line and 
general cutaneous 
fibers 



III. To eye-muscles 

IV. To eye-muscles 
VI. To eye-muscles 



I. Olfactory 



VII. General visceral 
and gustatory 

IX, X. General visceral 
and gustatory 



III. For intrinsic muscles 
of the eye 

V. To the jaw muscles 

VII. To hyoid muscula- 
ture 

IX, X. To branchial and 
general visceral mus- 
culature 



12. There are six pairs of cranial nerves associated with the medulla oblongata. The 
tenth cranial or vagus nerve is one of the largest and arises by two series of roots. One 
group of rootlets springs from the dorsolateral aspect of the medulla oblongata near its 
lower end, and contains fibers which are distributed through the branchial and gastro- 
intestinal rami of the vagus, while a large root, carrying fibers for the lateral line sense 
organs, runs farther cephalad and enters the acusticolateral area. The ninth or glosso- 
pharyngeal nerve, the nerve of the first branchial arch, arises from the medulla ob- 
longata just ventral to this root of the vagus. Since the gills, as well as the gastro- 
intestinal tract, are visceral organs, both the ninth and tenth nerves carry many visceral 
fibers. The eighth or acoustic nerve arises from the side of the medulla opposite the 
caudal part of the cerebellum in company with the fifth and seventh nerves, and ends 
in the membranous labyrinth of the ear. Like the vagus, the facial or seventh cranial 
nerve has, in addition to its main root, another, which runs further dorsally into the 
acusticolateral area. This root carries sensory fibers for the lateral line organs of the 
head. The sixth or abducens nerve arises more ventrally at the same level as the eighth^ 
The fifth, or trigeminal nerve, which sends many branches to the skin of the head, is 
represented by a large root emerging from the medulla oblongata in company with 
the seventh. Some idea of the peripheral distribution of these nerves can be gained 
from a study of Figs. 12 and 13. 

13. The floor of the fourth ventricle should now be exposed by carefully tearing away 
the membranous roof of that cavity. The floor presents for examination a series of 
longitudinal ridges and furrows which are of importance because they mark the position 
of longitudianl columns (Figs. 8, 13), to each of which a special functon can be assigned. 
A ridge on either side of the midline represents the position of the median longitudinal 
bundle, beneath which lie the nuclei of the third, fourth, and sixth cranial nerves. 
Since these nerves supply somatic musculature, the longitudinal elevation marks 
the position of the somatic motor column. Separated from this ridge by a broad furrow 



258 THE NERVOUS SYSTEM 

is a more prominent ridge with tooth-like secondary elevations. Within this second 
ridge terminate the fibers of visceral sensation and taste from the seventh, ninth, and 
tenth nerves. It is known as the visceral lobe or -visceral sensory column. Beneath the 
groove which separates these two ridges are located the motor nuclei of the fifth, 
seventh, ninth, and tenth cranial nerves. These nuclei supply visceral musculature 
and constitute the visceral motor column. The dorsal part of the lateral wall of the fossa 
forms another prominent ridge, which just caudal to the cerebellum is redundant and 
folded on itself to form an ear-shaped projection. This auricular fold, sometimes 
called the lobus linese lateralis, and the prominent margin just caudal to it belong to the 
acusticolateral area and contain the centers for the reception of impulses coming from 
the ear (N. VIII) and from the sense organs of the lateral line (Nn. VII and X). Ad- 
jacent to the acusticolateral area is a portion of the medulla oblongata which is concerned 
with the reception of sensory impulses from the skin which reach the medulla oblongata 
along the fifth and tenth nerves. The nuclei of the acusticolateral and general cutane- 
ous areas together constitute the somatic afferent column. 

14. Locate these functional columns on your specimen. Note the close relation of 
the olfactory bulb to the nasal sac. By comparison with Fig. 13 locate on your speci- 
men the olfactory portions of the brain. What part of the brain is especially associated 
with the eyes? 

15. Cut the nerve roots at some distance from the brain. Remove the brain, 
being careful not to injure the olfactory bulbs. Now study the lateral and ventral 
surfaces of the brain in order to locate more accurately the points of origin of the various 
cranial nerves (Fig. 10). 

16. Now study the parts of the brain which belong to the rhombencephalon. Which 
parts are they, and what is their relationship to each other? (Figs. 8, 10 and p. 26.) 

17. Study the parts of the brain which belong to the mesencephalon. Which 
are they, and what relationship do they bear to each other? (Figs. 8, 10 and 
p. 28.) 

18. In the same way study the parts belonging to the diencephalon (Figs. 8, 10 
and pp. 28, 29). Make a list of these parts. Tear away the membranous roof of the 
third ventricle and examine that cavity. 

19. Note the external form of the telencephalon and the parts which compose it 
(Figs. 8, 10). Students working at adjacent* tables should cooperate in the work 
which follows in order that two sharks' brains may be available. With a sharp razor 
blade divide one in the medial sagittal plane; and with a sharp scalpel open up the 
ventricles in the other as indicated in Fig. 9. Study the ventricles of the brain as they 
are displayed in these preparations and in Figs. 9 and 11. 

20. Find the velum transversum and the ridge produced by the optic chiasma. 
All that part of the brain which lies rostral to these structures belongs to the telen- 
cephalon. Study the telencephalon in detail (Figs. 8-11 and p. 30). Of what parts 
is it composed, and what are their relations, to each other? Pay special attention to 
the several parts of the telencephalic cavity. 

THE BRAIN OF THE FETAL PIG 

21. Using a pig embryo of about 35 mm., slice off the skin and a small amount of 
the underlying tissue on either side of the head with a sharp razor. Then at one careful 



A LABORATORY OUTLINE OF NEURO-ANATOMY 



359 



stroke split the specimen lengthwise in the median plane. This provides two prepara- 
tions for dissection, which should be used by two students. 



Pineal body 



Third ventricle 
Hypothalamus 

Thalamus 

Chorioid plexus of lateral 
ventricle 

Lateral ventricle 
Corpus striatum 

Lamina terminalis 

Rhinencephalon 

Hypophysis 



Tongue 



Fig. 258. Medial sagittal section of the head of a 35 mm. pig embryo. (Redrawn from Prentiss- 

Arey.) 

22. First study the medial section of the brain, noting the five divisions of the 
brain, the ventricles, and the relation of the cerebral hemispheres to other parts of the 



Cerebral aqueduct 

Lamina quadrigemina 
Cerebral peduncle 
Cerebellum 
Chorioid plexus of fourth ventricle 

Fourth ventricle 
Medulla oblongata 



Central canal of spinal 
cord 




Semilunar ganglion N. V 
Mesencephalon 



Cerebellum 



Hypothalamus 



Geniculate gang. N. VII 

Ganglion N. VIII 

Medulla oblongata 

Jugular gang. N. X 

Gang, of Froriep 

Gang. N. cerv. I 

Accessory nerve 

Hypoglossal nerve 

Ganglion nodosum N. X 

Gang. N. cerv. V 




Cerebral hemisphere 

N. V, ophthalmic N. 

Rhinencephalon 

N. opticus 

N. V, maxillary N. 

N. V, mandibular N. 



Chorda tympam 
Facial N. 



Fig. 259. Dissection of the head of a 35 mm. pig embryo. Lateral view. (Redrawn from 

Prentiss-Arey.) 



brain (Fig. 258. See also Figs. 16, 17 and pp. 32-36). Of what three parts is the 
cerebral hemisphere composed? Locate each of the subdivisions of the diencephalon. 



360 THE NERVOUS SYSTEM 

To which part does the pineal body belong? The hypophysis? Locate the quadri- 
geminal lamina, cerebral peduncle, cerebellum, and medulla oblongata. 

23. Now turn the specimen over and carefully dissect away what remains of the 
skin and mesodermal tissues so as to expose the brain and cranial nerves from the lateral 
side. Identify all the parts labeled in Fig. 259. 

GENERAL TOPOGRAPHY OF THE BRAIN 

24. The adult mammalian brain should now be compared with that of the shark 
and with that of the fetal pig. If two sheeps' brains are available, one should be divided 
into lateral halves by a cut made exactly 1 mm. to the left of the median sagittal plane. 
Use a long, thin brain knife and make the cut with a single sweep. Put away the right 
half for future study. On the left half and on the intact brain identify all of the chief 
divisions of the brain, determine their embryologic derivation, and compare them 
with similar parts in the brains of the shark and fetal pig. (See the table on p. 36, 
pp. 113-116, and Figs. 82-84.) 

25. By a study of the medial aspect of the left half of the brain ascertain what 
relations the various subdivisions bear to each other. (See Fig. 84 and pp. 116-118.) 
Note the difference in color between the cortex and the white center of the cerebellum. 
By tearing away the cerebellum a little at a time make a dissection of the cerebellar 
peduncles on this half of the brain (Figs. 87, 91). Scrape away the superficial gray 
matter from the rostral end of the left hemisphere and uncover the white substance 
beneath. The superficial gray matter is known as the cerebral cortex and this covers 
the white center of the cerebral hemisphere. 

NEUROLOGIC STAINS 

26. Some knowledge of how various stains act on the nervous tissues is essential 
for an understanding of the special preparations which are to be studied. The technic 
involved in preparing such material is described in books devoted to technical methods 
(Hardesty, 1902; Guyer, 1917). 

27. Osmic Acid. Small nerves may be fixed in osmic acid. This stains the myelin 
sheaths black. Why? Axons remain unstained. 

28. The Weigert or Pal-Weigert Method. When a portion of the brain or spinal 
cord has been treated for several \veeks with a solution containing potassium bichromate 
(Miiller's fluid) the myelin sheaths acquire a special affinity for hematoxylin, by 
virtue of which they become deep blue in color when stained by this method. Axons, 
nerve-cells, and all other tissue elements remain colorless unless the preparation has 
been counterstained. The method is adapted for the study of the development and 
extent of myelination and for tracing myelinated fiber tracts. This method may also 
be used for a study of degenerated fiber tracts, which remain colorless in preparations 
in which the normal fiber tracts are well stained. 

29. The Marchi method is a differential stain for degenerating fibers. These 
contain droplets of chemically altered myelin. The tissue is fixed in a solution contain- 
ing potassium bichromate (Muller's fluid). This treatment prevents the normal 
myelinated fibers from staining with osmic acid, but does not prevent the droplets of 
chemically altered myelin in the degenerated fiber from being stained black by this 



A LABORATORY OUTLINE OF NEURO-ANATOMY 361 

reagent. In a section prepared by this method the normal myelinated fibers are light 
yellow, while the degenerated fibers are represented by rows of black dots. 

30. The newer silver stains, including the Cajal method and the pyridin-silver technic, 
depend upon the special affinity for silver nitrate possessed by nerve-cells and their 
processes. After treatment with silver nitrate the tissue is transferred to a solution 
of pyrogallic acid or hydroquinon which reduces the silver in the neurons to a metallic 
state. Nerve-cells and their processes are stained yellow or brown by these methods. 
Myelin sheaths remain unstained. The axis-cylinders of the myelinated fibers are 
light yellow, the unmyelinated axons are dark brown or black. The neurofibrils are 
stained somewhat more darkly than other parts of the cytoplasm. 

31. The Golgi method furnishes preparations which demonstrate the external 
form of the neurons, and make it possible to trace individual axons and dendrites for 
considerable distances. The method also stains neuroglia. It is selective and rather 
uncertain in its results, since only a small proportion of the nerve-cells are impregnated 
in any preparation. The stain is due to the impregnation of the nerve-cells and their 
processes with silver. 

32. The best stains for demonstrating the tigroid masses or Nissl bodies are 
toluidin blue and Nissl' s methylene-blue. Both are basic dyes; and in properly fixed 
nervous tissue they color the tigroid masses as well as the nuclear chromatin of nerve- 
cells blue. 

THE PERIPHERAL NERVOUS SYSTEM 

33. The Spinal Ganglia. Study a longitudinal section through a spinal nerve and 
its roots, including the spinal ganglion, stained by the pyridin-silver method. How 
are myelinated and unmyelinated axons stained by this method? What kinds of cells 
do you find? Study their axons. (See Figs. 39, 40 and pp. 62-66.) Look for the 
bifurcation of the myelinated and unmyelinated fibers. Note the differences in 
composition of the ventral and dorsal roots. What becomes of the various kinds of 
fibers when traced peripherally? When traced toward the spinal cord? What is the 
origin of the unmyelinated fibers? 

34. Study the vagus nerve of the dog in osmic acid and pyridin-silver preparations. 
How are the various kinds of nerve-fibers stained in each? How does the structure of 
the vagus differ from that of a spinal nerve? 

35. Study the cervical portion of the sympathetic trunk, which in the dog lies in a 
common sheath with the vagus. Of what kind of fibers is it composed? What is the 
origin and termination of these fibers? (See pp. 345-347.) 

36. Study the pyridin-silver preparation from the superior cervical sympathetic 
ganglion. What is the source of the fine black fibers, and where do they end? Study 
the ganglion cells. What becomes of their axons? (See Figs. 251, 253 and pp. 341-344.) 

THE SPINAL CORD 

37. Review the development and gross anatomy of the spinal cord (p. 42 and pp. 
73-78). Examine the demonstration preparations of the vertebral column, showing 
the spinal cord exposed from the dorsal side. In these preparations study the meninges 
and ligamentum denticulatum, as well as the shape and size of the spinal cord. Note 



362 THE NERVOUS SYSTEM 

the level of the termination of the spinal cord, the level of the origin of the various 
nerve roots and of their exit from the vertebral canal, and the level of the various seg- 
ments of the cord with reference to the vertebrae. Note the filum terminale and the 
cauda equina. From your text-books of anatomy study the meninges and blood- 
supply of the cord. 

38. The Spinal Cord in Section. Examine the Pal-Weigert sections of the cervical, 
thoracic, lumbar, and sacral regions, and from them reconstruct a mental picture of the 
topography of the entire cord. How does it vary in shape and size at the different 
levels? Identify all the fissures, sulci, septa, funiculi, gray columns, commissures and 
nerve roots, the reticular formation, the substantia gelatinosa and the caput, cervix, 
and apex of the posterior gray column. (See pp. 78-84.) 

39. The Microscopic Anatomy of the Spinal Cor d Study all of the histologic 
preparations of the spinal cord which have been furnished you. (See pp. 85-90.) 
Study the neuroglia in Golgi preparations. Study the pia mater, septa, blood-vessels, 
and ependyma in hematoxylin and eosin preparations. Study the nerve-cells in Nissl, 
Golgi, and silver preparations. Study the myelinated fibers in Weigert preparations 
and both the myelinated and unmyelinated fibers in the silver preparations. Note 
the arrangement of each of these histologic elements and be sure that you understand 
the relations which they bear to each other. 

40. Draw in outline, ventral side down, each of four Pal-Weigert sections taken, 
respectively, through the cervical, thoracic, lumbar, and sacral regions of the human 
spinal cord. Make the outlines very accurate in shape and size, with an enlargement 
of 8 times. Put in the outline of the gray columns, the central canal, and the substantia 
gelatinosa Rolandi. Put each outline on a separate sheet and do not ink the drawings 
at present. 

41. Identify the various cell columns in the gray matter and note how they vary 
in the different levels of the cord (Nissl or counterstained Weigert preparations). 
(See pp. 89, 90 and Fig. 65.) Indicate these cell groups in their proper places in the 
four outline sketches of the spinal cord. What becomes of the axons arising from 
each group of cells? Why are the anterolateral and posterolateral cell groups seen 
only in the regions associated with the brachial and lumbosacral plexuses? The 
intermediolateral column only in the thoracic and highest lumbar segments? Why is 
the gray matter most abundant in the region of the intumescentiae and the white matter 
most abundant at the upper end of the spinal cord? 

42. What elements are concerned in spinal reflexes? (See pp. 91-94.) 

43. What connections do the fibers of the spinal nerves establish in the spinal cord? 
What is the origin and the peripheral termination of the somatic efferent fibers, of the 
visceral efferent fibers, of the somatic afferent fibers, and of the visceral afferent fibers 
of the spinal nerves? (See pp. 60-63 and Fig. 37.) What are the proprioceptive 
and exteroceptive fibers, and in what peripheral structures do they end? (See pp. 
66-72.) 

44. In a pyridin-silver preparation of the cervical spinal cord of a cat note that as 
the dorsal root enters the cord the unmyelinated fibers run through the lateral division 
of the root into the dorsolateral fasciculus (Fig. 72). The medial division of the root 
is formed of myelinated fibers which enter the posterior funiculus. Read about the 
intramedullay course of these fibers (pp. 95-98)., 



A LABORATORY OUTLINE OF NEURO-ANATOMY 363 

45. The fiber tracts, of which the white substance is composed, cannot be distin- 
guished in the normal adult cord. They can be recognized from differences in the degree 
of their myelination in fetal cords (p. 112 and Fig. 79) and in preparations showing 
degeneration resulting from disease or injury in various parts of the nervous system 
(p. 105; Figs. 75, 76). From such preparations as are available for this purpose and 
from your reading (pp. 95-112) form a clear conception of the origin, course, and ter- 
mination of each of the fiber tracts. 

46. Indicate the location of each of these tracts in the outline drawing of the 
cervical portion of the spinal cord, entering the ascending tracts and the ventral cortico- 
spinal tract on the right side, and all of the descending tracts except the ventral cortico- 
spinal tract on the left side. Why should the ventral and lateral corticospinal tracts 
be indicated on opposite sides of the cord? Wax crayons should be used to give the 
several tracts a differential coloring. Use the following color scheme: 

Somatic afferent tracts: 

Proprioceptive yellow. 

Exteroceptive blue. 
Somatic motor tracts: 

Corticospinal tracts red. 

Rubrospinal tract brown. 
All other tracts black. 

47. The fasciculus cuneatus and fasciculus gracilis should be colored yellow and 
then dotted over with blue to indicate that while the proprioceptive fibers predominate, 
there are also some exteroceptive fibers in these tracts. 

THE BRAIN STEM 

48. Now take the human brain and identify all of its principal divisions. Dissect 
out the arterial circle of Willis, and identify the branches of the internal carotid, ver- 
tebral, and basilar arteries. Read about the blood-supply and meninges of the brain 
in your text-book of anatomy. Identify all of the cranial nerves (Fig. 86). 

49. Examine again the cerebellar peduncles in the three specimens of the sheep's 
brain (Figs. 87, 91). Now remove the cerebellum from the previously intact sheep's 
brain. Cut through the peduncles on both sides of the brain as far as possible from 
the pons and medulla, sacrificing the cerebellum to some extent in order to leave as 
much of the peduncles as possible attached to the brain stem. Be careful not to damage 
the anterior medullary velum and the tela chorioidea which lie under cover of the 
cerebellum (Fig. 84). In the same way remove the cerebellum from the human brain. 

50. Study the roof of the fourth ventricle in both the human and the sheep's brain 
(pp. 128, 129 and Figs. 84, 90, 154). Examine the chorioid plexus of the fourth ven- 
tricle. Note the line of attachment of the tela chorioidea. Tear this membrane away. 
The torn edge which remains attached to the medulla is the taenia of the fourth ventricle 
(Figs. 89, 90). Study the attachments of the anterior medullary velum. The decus- 
sation of the trochlear nerve within the velum can easily be seen in the sheep. Remove 
this membrane. The floor of the fourth ventricle is now fully exposed. 

51. Remove the pia mater from the brain stem, carefully cutting around the roots 
of the cranial nerves with a sharp-pointed knife to prevent these nerves being torn 
away from the brain when this membrane is removed. 



364 



THE NERVOUS SYSTEM 



52. Carefully examine the medulla, pans, floor of the fourth ventricle, and the mesen- 
cephalon, observing all the details mentioned on pp. 118-131 and illustrated in Figs. 

84, 86-89, 91. 

53. Take selected transverse sections through the human brain stem and, by com- 
parison with the gross specimen, determine the level of each section. 

54. Draw in outline each of these transverse sections through the brain stem. 
Put each drawing on a separate page, ventral side down, with the transverse diameter 
corresponding to the longer dimension of the paper. Study each preparation in detail 
and identify all of the parts, indicating them lightly in pencil. Do not label the draw- 
ings at this time. Make sure that all proportions are correct. The sections through 
the medulla should be enlarged eight diameters, those through the pons and mesen- 
cephalon four diameters. 

55. Section Through the Decussation of the Pyramids. Keep in mind the tracts 
which extend into the brain from the spinal cord and note the changes in their form 
and position. Identify the decussation of the pyramids, the nucleus gracilis and nucleus 
cuneatus, the spinal root of the trigeminal nerve and its nucleus, the reticular formation. 
Note the change in the form of the gray substance (pp. 132-137; Figs. 94, 95, 98). 

56. Section Through the Decussation of the Lemniscus. Note the rapid change in 
the form of the gray matter. Identify the internal and external arcuate fibers, the 
decussation of the lemniscus and the beginning of the medial lemniscus, as well as the 
structures continued up from the preceding level (Figs. 96, 99; pp. 137-139). 

57. Section Through the Olive and the Hypoglossal Nucleus. At this level the central 
canal opens out into the fourth ventricle. The posterior funiculi and their nuclei are 
disappearing or have disappeared. The dorsal spinocerebellar tract lies lateral to the 
spinal tract of the trigeminal nerve and is directed obliquely backward toward the 
restiform body. Identify, in addition to those structures which are continued from 
the preceding level, the inferior olivary nucleus with the olivocerebellar fibers, the 
dorsal and medial accessory olivary nuclei, the external arcuate fibers, the nucleus and 
fibers of the hypoglossal nerve, the dorsal motor nucleus of the vagus, the tractus 
solitarius and its nucleus, the nucleus ambiguus and the lateral reticular nucleus (Figs. 
97,101; pp. 139-142). 

58. Section Through the Restiform Body. The restiform body and the spinal tract 
of the fifth nerve are conspicuous in the dorsolateral part of the section. In the floor 
of the fourth ventricle locate the nucleus of the hypoglossal nerve, the dorsal motor 
nucleus of the vagus, the medial and the spinal vestibular nuclei. The spinal tract of 
the fifth nerve and its nucleus are deeply situated ventral to the restiform body and 
broken up by the olivocerebellar fibers (Fig. 103; pp. 143-146). 

59. Section Through the Lower Margin of the Pons. Identify such portions of the 
pons, brachium pontis, and cerebellum as are contained in the section. Dorsolateral 
to the restiform body is the dorsal cochlear nucleus, and ventrolateral to it the ventral 
cochlear nucleus. Identify the striae medullares and the beginning of the trapezoid 
body, also the medial and lateral vestibular nuclei (Fig. 107; pp. 149-152). ' 

60. Section Through the Facial Colliculus. Differentiate between the ventral and 
the dorsal portions of the pons, and in the ventral portion identify the longitudinal 
fasciculi, transverse fibers, and the nuclei pontis (pp. 147-149). In the dorsal part 
identify the nuclei and root fibers of the sixth and seventh nerves including the genu 



A LABORATORY OUTLINE OF NEURO-ANATOMY 365 

of the seventh nerve. Locate the spinal tract of the fifth nerve and its nucleus, the 
trapezoid body, and superior olivary nucleus (Fig. 108; pp. 151-154). 

61. Section Through the Middle of the Pans Showing the Motor and Main Sensory 
Nuclei of the Fifth Nerve. In addition to these nuclei note the beginning of the mesen- 
cephalic root of the fifth nerve. The brachium conjunctivum makes its appearance 
in the dorsal part of the section (Fig. 110; pp. 154-157). 

62. Section Through the Inferior Colliculus. Identify the basis pedunculi, substantia 
nigra, medial and lateral lemnisci, cerebral aqueduct, central gray matter, mesence- 
phalic root of the fifth nerve, fasciculus longitudinalis medialis, nucleus of the trochlear 
nerve, and the decussation of the brachium conjunctivum (Figs. 113, 114; pp. 158, 165). 

63. Section Through the Superior Colliculus. Identify, in addition to the structures 
continued upward from lower levels, the red nucleus, the nucleus of the third nerve, 
and the root fibers of that nerve, the ventral and dorsal tegmental decussations, the 
inferior quadrigeminal brachium, and the medial geniculate body (Fig. 116; pp. 160, 
167). 

THE CEREBELLUM 

64. Compare the human cerebellum with that of the shark and the sheep. How 
is its size related to the size of the pons and to the extent of the cerebral cortex? 

65. On both the human and sheep's cerebellum identify the vermis, hemispheres, 
and divided peduncles (Figs. 138, 139, 143-145). In the medial sagittal section of 
the sheep's brain identify the white medullary body of the cerebellum, the arbor 
vitae, cerebellar cortex, folia, and sulci (Fig. 84; pp. 196-199). 

66. Study the morphology of the cerebellum in the sheep (Figs. 143-145). Lo- 
cate these same fundamental subdivisions in the human cerebellum (Figs. 146, 147). 
What functions have recently been assigned to each of these subdivisions? (See 
pp. 199-203.) 

67. Divide the human cerebellum in the median plane. Cut the right half into 
horizontal sections and the left into sagittal sections and study the medullary center 
and nuclei of the cerebellum (Figs. 140, 141, 148; pp. 199, 203). 

68. Study the histologic sections of the cerebellar cortex and master the details 
of its structure (Figs. 150, 151; pp. 206-210). 

FUNCTIONAL ANALYSIS OF THE BRAIN STEM 

69. Review the sections of the brain stem as directed in the following paragraphs, 
paying special attention to the functional significance of the various nuclei and fiber 
tracts as far as they can be followed in the series of sections. In general, the afferent 
tracts and nuclei should be entered in color on the right side of the drawings already 
made, and the efferent tracts and nuclei on the left side. But this order must be re- 
versed in certain cases to allow for the decussation of the tracts. Label the various 
tracts and nuclei. Use the following color scheme: 

Somatic afferent: 

Exteroceptive blue . 

Proprioceptive yellow. 
Visceral afferent orange. 
Visceral efferent purple. 



366 THE NERVOUS SYSTEM 

Somatic efferent red. 

All cerebellar connections not strictly proprioceptive brown. 

Other tracts black. 

PROPRIOCEPTIVE PATHS AND CENTERS (pp. 311-315) 

70. The cerebellum is the chief proprioceptive correlation center, and the restiform 
body consists for the most part of proprioceptive afferent paths (Fig. 235). Note its 
shape, position, and connections in all the gross specimens. In the left lateral half of 
the sheep's brain follow it caudally by dissection, separating it from the other peduncles. 
Cut and reflect the dorsal cochlear nucleus of the eighth nerve. Trace the restiform 
body backward and note the accession of external arcuate fibers. At the level of the 
inferior olive it receives the dorsal spinocerebellar tract. Trace this by dissection from 
the restiform body obliquely across the upper end of the tuberculum cinereum and 
then caudally along the ventral border of this elevation to the spinal cord. (See Figs. 
87, 88, 104; pp. 143, 205.) 

71. Now take the sections of the medulla, locate the dorsal spinocerebellar tract 
in each, and indicate its position in yellow on the right side of your outlines (p. 144). 
Locate the external ar cute fibers (p. 139). From where do they come and where do they 
go? Draw in yellow those belonging to the right peduncle. Locate in your sections 
the oliwcerebellar tract, and with brown indicate in your outline the fibers running into 
the right peduncle (Fig. 103). 

72. From your texts ascertain the course of the ventral spinocerebellar tract and 
indicate its position in yellow on the right side of the outlines (Fig. 149; p. 157). 

73. Proprioceptive Path to the Cerebral Cortex. Indicate in yellow the terminal 
portion of the right dorsal funiculi, and with yellow stipple the right nucleus gracilis 
and nucleus cuneatus (Figs. 98, 99). Study the internal arcuate fibers and the medial 
lemniscus, drawing the internal arcuate fibers from right to left and the medial lemniscus 
on the left side (yellow). Where do the fibers of the medial lemniscus terminate? 
What is the source and what the destination of the impulses which they carry? (See 
Figs. 101, 103, 107, 108, 110, 114, 116, 235 and pp. 138, 312.) 

74. Locate the vestibular nuclei and indicate them with yellow stipple on the right 
side of the outlines (Figs. 101, 103, 107, 108). Locate the vestibulocerebellar tract 
(pp. 151, 188; Fig. 136). 

EXTEROCEPTIVE PATHS AND CENTERS (pp. 302-310) 

75. The Cochlear Nerve and its Connections. On the sheep's brain note the two 
divisions of the acoustic nerve as well as the ventral and dorsal cochlear nuclei and the 
trapezoid body (Fig. 87). Examine the cochlear nuclei and the striae medullares in the 
human brain (Fig. 89). Locate the lateral lemniscus where it forms a flat band of 
fibers directed rostrally and dorsally upon the lateral surface of the mesencephalon. 
It occupies a triangular space dorsal to the basis pedunculi and rostral to the pons and 
is superficial to the brachium conjunctivum (Fig. 88). 

76. Now take the section through the lower border of the pons and study the 
cochlear nuclei, the stria medullares, and the beginning of the trapezoid body (Fig. 107). 
In the section through the facial colliculus study the trapezoid body and the superior 



A LABORATORY OUTLINE OF NEURO-ANATOMY 367 

olivary nuclei (Fig. 108). In the section through the middle of the pons identify the 
lateral lemniscus. Trace this tract to the inferior colliculu,s (Fig. 114) and through the 
inferior quadrigeminal brachium to the medial geniculate body (Figs. 114, 116). Color 
these central connections of the cochlear nerve blue, indicating the cochlear nuclei on 
the right side and the lateral lemniscus on the left (Fig. 134; pp. 149, 185). 

77. Dissection of the spinal tract of the fifth nerve. On the left half of the sheep's 
brain locate the fifth nerve and tear away the transverse fibers of the pons caudal to 
that nerve until the longitudinal fibers of its spinal tract are exposed. By carefully 
scraping away the structures superficial to this tract follow it to the lower end of the 
medulla. 

78. Locate the sensory nuclei of the fifth nerve in your sections and indicate them with 
colored stipple on the right side of your drawing (pp. 154, 182; Fig. 131): the mesen- 
cephalic nucleus, yellow (Fig. 114); the main sensory nucleus, blue (Fig. 110); the 
nucleus of the spinal tract, blue (Figs. 98, 99, 101, 103, 107, 108). At the same time 
color the spinal tract of the right side blue. What becomes of the fibers which arise 
from the cells of the main sensory and the spinal nuclei of the trigeminal nerve? (See 
pp. 183, 307; Fig. 232.) 

79. From the text ascertain the course of the spinothalamic tract and trace it up 
through the brain stem (Figs. 105, 230, 231, 234). Where do these fibers come from, 
and where do they end? What kind of sensations do they mediate? Enter it in blue 
on the right side of your drawings. (See pp. 101, 102, 145, 305.) 

VISCERAL AFFERENT PATHS AND CENTERS 

80. Identify the tractus solitarius and its nucleus (Figs. 101, 103, 120). What is 
the origin, termination, and function of the fibers constituting this tract? (See pp. 
180, 181.) Indicate the tract with orange and the nucleus with orange stipple on the 
right side of your drawing. 

VISCERAL MOTOR CENTERS 

81. In the sections of the brain stem identify the dorsal motor nucleus of the vagus 
(Figs. 101, 103) and the following special visceral motor nuclei: the nucleus ambiguus 
(Figs. 101, 103), the motor nucleus of the fifth (Fig. 110), and the motor nucleus of the 
seventh nerve (Fig. 108). Stipple these nuclei purple on the left side. How are visceral 
afferent and efferent elements connected to form visceral reflex arcs? (See pp. 174-178.) 

SOMATIC MOTOR TRACTS AND CENTERS 

82. The Corticospinal and Corticopontine Tracts. From the cerebral cortex the 
fibers of the pyramidal tract run through the internal capsule and brain stem to the 
somatic motor and special visceral motor nuclei of the cranial nerves and to the anterior 
gray column of the spinal cord. Along with these it will be convenient to study the 
cortico-ponto-cerebellar pathway. Take the left lateral half of the sheep's brain and, 
being careful not to injure the optic tract and optic radiation, follow the fibers of the 
basis pedunculi by dissection through the internal capsule to the cerebral cortex (Fig. 
260) . Now tear away the transverse fibers of the pons a few at a time and follow them 
by dissection into the brachium pontis. Observe that some of the fibers of the basis 
pedunculi end in the pons (corticopontine fibers) and that others (corticospinal fibers) 



368 THE NERVOUS SYSTEM 

can be traced through the pons into the pyramid of the medulla. Carrying the dis- 
section caudally, observe the decussation in the lower end of the medulla. 

83. Examine again the series of sections through the brain stem and color the 
corticospinal tract red on the right side of your drawings. Draw the fibers from right 
to left in the decussation (Fig. 237; pp. 136, 317). 

84. With red stipple indicate the somatic motor nuclei on the left side of your draw- 
ings. Which nuclei are they? (See pp. 170-173.) 

CEREBELLAR CONNECTIONS 

85. The inferior peduncle has already been studied and the cortico-ponto-cerebellar 
path has been dissected. Review this path in your sections. Color the corticopontine 
tracts of the left side brown ( Fig. 117). Indicate the nuclei pontis of the left side by 
brown stipple. Draw the transverse fibers of the pons from the left nuclei pontis to 
the right brachium pontis (Fig. 106; pp. 147-149). 

86. In the left lateral half of the sheep's brain follow the brachium conjunctivum 
by dissection into the tegmentum of the mesencephalon and note its decussation 
beneath the inferior colliculus. In your sections trace it rostrally, noting its decus- 
sation and termination (Figs. 110, 112, 114-116). Indicate it in brown on your 
drawings, beginning on the right side and tracing it through the decussation to the left 
red nucleus. Stipple both red nuclei with brown. (See pp. 159, 326.) 

87. The Rubrospinal Tract. Trace the rubrospinal tract from the red nucleus 
through the ventral tegmental decussation (Fig. 116) and the reticular formation of the 
brain stem. In the reticular formation it occupies a position ventromedial to the 
nucleus of the spinal root of the trigeminal nerve (Figs. 115, 234; pp. 161, 326). Color 
it brown on the left side of your drawings. 

THE RETICULAR FORMATION 

88. Study the reticular formation in the various sections. Of what is it composed? 
How many kinds of internal arcuate fibers can you find? What is the source of the 
longitudinal fibers of the reticular formation? Locate the tectospinal tract and in- 
dicate it in black on the left side of your drawings. (See pp. 144, 145). 

89. The Fasciculus Longitudinalis Medialis. Examine all nine sections, and enter 
this bundle in black on both sides of your drawings. What is the source of its fibers 
and what is its function? (See Fig. 109; pp. 152, 162). 

PROSENCEPHALON 

90. With a sharp brain knife divide the human brain exactly in the median sagittal 
plane, and then cut the left cerebral hemisphere into a series of frontal sections. The 
planes of the sections should pass through (1) the rostrum of the corpus callosum, 
(2) the anterior commissure, (3) the mammillary body, (4) the habenular nucleus, 
(5) the pineal body and the splenium of the corpus callosum (Figs. 186-190). 

91. Take the right half of the sheep's brain and make such dissections as may be 
necessary to secure a good preparation of the structures indicated in Fig. 84. Begin 
at the rostral angle of the fourth ventricle and follow the cerebral aqueduct, tearing 
away with tissue forceps any parts of the left lateral wall which have not been cut away. 



A LABORATORY OUTLINE OF NEURO-ANATOMY 369 

Follow the aqueduct into the third ventricle, removing from the latter the remains of 
its left lateral wall. Care is required in removing the rostral part of this wall in order 
that the lamina terminalis may be left intact. Now remove such portions of the left 
cerebral cortex as are still attached to the preparation. By this dissection a much more 
instructive preparation is obtained than when the original section is made exactly in 
the median plane. 

92. Take the left lateral hah of the sheep's brain and tear away what remains of the 
septum pellucidum and body of the fornix and locate the caudate nucleus. For the 
identification of these structures see Figs. 84 and 204. ^ Cut through the internal capsule, 
which has previously been exposed from the lateral side in this specimen, along a line 
extending horizontally toward the occipital pole from the highest part of the dorsal 
border of the caudate nucleus. Remove the portion of the cerebral hemisphere that 
lies dorsal to the plane of this section and thus expose the dorsal surface of the thalamus 
(Fig. 91). 

93. Diencephalon. Study the thalamus as it appears in all of these preparations 
(pp. 213-216). Examine the dorsal surface of the thalamus on the left half of the sheep's 
brain (Figs. 89, 91, 180). The lateral surface of the thalamus rests against the internal 
capsule, as can be readily understood from a study of this dissection. The medial 
surface forms a part of the wall of the third ventricle (Figs. 158, 159). 

94. Study the epithalamus in both the human and the sheep's brain. Of what 
parts is it composed? (See Figs. 91, 158, 159; pp. 220, 221.) 

95. Locate all the parts which belong to the hypothalamus in both the human and 
the sheep's brain (Figs. 84, 86, 158, 159; pp. 222, 223). 

96. Study the shape and boundaries of the third -ventricle (Figs. 158, 159; pp. 
223, 224). 

97. The Metathalamus. On the left half of the sheep's brain identify the medial 
geniculate body (Fig. 87). Immediately rostral to this body is a slight elevation in the 
optic tract produced by the subjacent lateral geniculate body. Identify both of these 
bodies on the human brain (Figs. 88, 89, 154). 

98. In the frontal sections of the left human cerebral hemisphere identify the various 
parts of the diencephalon (Figs. 188, 189). From these sections something can be 
learned concerning the internal structure of the thalamus, but more information can 
be obtained on this subject from sections stained by the Weigert method (Figs. 156, 
157; p. 216). In these sections trace the basis pedunculi into the internal capsule and 
the medial lemniscus into the thalamus. 

99. Dissection of the Optic Tract Take the left lateral half of the sheep's brain 
and, grasping the optic chiasma with the tissue forceps, pull the optic tract lateralward, 
separating it from the surface of the peduncle. It separates easily until the position 
of the lateral geniculate body is reached just rostral to the medial geniculate body. 
Stronger traction will cause it to tear away from the lateral geniculate body, which is 
now exposed as a prominent curved ridge of gray matter. This nucleus extends rostrally 
and dorsally from the medial geniculate body and is continuous with the pulvinar of 
the thalamus. Continued traction will cause the optic fibers to strip off from the sur- 
face of the pulvinar. Here they form a rather thick white lamina, the stratum zonale. 
Continue the dissection, raising the fibers of the optic tract as far as the groove rostral to 
the superior colliculus. Now cut the transverse peduncular tract, which lies in this 

24 



THE NERVOUS SYSTEM 



groove, by making a superficial incision across the groove along the lateral border of 
the optic fibers. Scrape away the superficial gray matter (about 1 mm.) of the superior 
colliculus and expose the stratum opticum (Fig. 116). Now continue the traction on 
the optic tract and a striking demonstration will be obtained of the fact that the stratum 
opticum is composed of fibers from this tract (Figs. 161, 162; pp. 226, 227). 

100. Dissection of the Optic Radiation. In the left half of the sheep's brain scrape 
away part of the gray matter of the pulvinar. Follow fibers from the pulvinar into the 
posterior limb of the internal capsule. These belong to the optic radiation, which may 
now be followed by dissection to the cortex near the occipital pole of the cerebral hemi- 
sphere (Fig. 260; pp. 227, 228). Now take the right half of the cerebral hemisphere 
and identify the visual area of the cerebral cortex (Fig. 221). 



Optic radiation ' ' /* 
Superior colliculus:' / 
Inferior colliculus ' \ 

Pulvinar '^| 
Medial geniculate body '" 
Cerebral peduncle 




Mammillary body 

Optic tract 
Posterior limb of internal capsule 

Optic nerve 



Intersection of corona radiata and 

radiation of corpus callosum 
Anterior limb of internal capsule 
Anterior perforated substance 



Fig. 260. Dissection of the cerebrum of a sheep showing the internal capsule and corona radiata. 
The lentiform nucleus has been removed. 

101. Surface Form of the Cerebral Hemispheres. Compare the basal surface of the 
human brain with that of the sheep. Note in each the parts belonging to the rhinen- 
cephalon and locate the rhinal fissure, which separates the neopallium and the archi- 
pallium. Nearly all of the surface of the human cerebral hemisphere is formed by the 
neopallium (Figs. 83, 86; pp. 115, 116). 

102. Examine the right cerebral hemisphere of the human brain and identify the 
poles, fissures, sulci, lobes, and gyri (Figs. 166-168, 170, 171; pp. 232-242). Draw 
the margins of the lateral fissure apart and locate the insula (Fig. 169). Study 
the insula in the frontal sections through the left cerebral hemisphere (Figs. 186-189; 
p. 237). 

103. Internal Configuration of the Cerebral Hemisphere. Take the sheep's brain 
from which the cerebellum has been removed and slice away successive thin layers from 
the dorsal aspect of both hemispheres. These thin sections should be cut in planes 
parallel to the dorsal surface of the corpus callosum and the last cut should be inch 
dorsal to that commissure. The direction and relative depth of the dorsal surface of 



A LABORATORY OUTLINE OF NEURO-ANATOMY 371 

the corpus callosum can be determied by examination of the medial aspect of the right 
half of the sheep's brain. As the sections are removed note the relation of the gray 
and white matter (Fig. 175). Gently press apart the two hemispheres and note the corpus 
callosum at the bottom of the longitudinal fissure. Now with a blunt instrument 
dissect away the gray and white matter from the dorsal surface of the corpus callosum 
(Fig. 175). Be careful not to injure a thin layer of gray matter, the indusium griseum, 
which covers this surface. Study the corpus callosum in this specimen and in the median 
sagittal sections of the sheep and human brains (Figs. 158, 159, 175; pp. 243-245). 
Examine the septum pellucidum in the median sagittal sections. 

104. The Lateral Ventricles (pp. 246-251). Cut through the corpus callosum of the 
sheep's brain as indicated in Fig. 178, leaving a median strip in position. Make a 
careful examination of all the parts thus exposed, including the septum pellucidum. 
On the right side of the specimen expose the entire extent of the inferior horn of the 
lateral ventricle by freely cutting away the lateral portion of the hemisphere as indicated 
in Fig. 182. Remove the caudate nucleus to demonstrate the entire extent of the ante- 
rior horn, and finally demonstrate the continuity of the lateral ventricle with the cavity 
of the olfactory bulb (Fig. 182). Now study the lateral ventricle and the structures 
which form its walls as these are illustrated on the two sides of this specimen. Note 
the chorioid plexus (Fig. 183) and chorioid fissure. 

105. Study the lateral ventricle as seen in the frontal sections of the left hemi- 
sphere of the human brain (Figs. 186-189). It has an additional part, the posterior 
horn, not seen in the sheep. Endeavor to reconstruct a mental picture of its shape 
(Fig. 176). 

106. The Corpus Striatum (pp. 253-257). Examine again the caudate nucleus as 
it bulges into the lateral ventricle (Fig. 178). Take the right lateral half of the sheep's 
brain and make a horizontal section through the cerebral hemisphere, passing through 
the lower border of the genu of the corpus callosum and the lower border of the habenular 
trigone. Locate the lentiform and caudate nuclei, the claustrum, and the internal 
and external capsules (Fig. 192). 

107. Dissection of the Lentiform Nucleus and the Internal Capsule. On the left 
side of the sheep's brain, in which the lateral ventricles have been exposed, remove the 
cortex and white matter superficial to the lentiform nucleus. Begin by grasping with 
tissue forceps the olfactory bulb close to its peduncle and tear it away, pulling in a 
lateral and caudal direction. There should come away with it the superficial part of 
the anterior perforated substance and part of the lateral olfactory gyrus (Fig. 83). 
This will expose the ventral part of the lentiform nucleus, and the structures lateral 
to that nucleus can now be removed. With a blunt dissecting instrument scrape away 
everything superficial to the lentiform nucleus and continue the dissection until the 
nucleus and the corona radiata are fully exposed (Fig. 87). Now scrape away the 
lentiform nucleus and expose the internal capsule (Fig. 260). In removing the nucleus 
you can obtain a clear idea of its shape and size. 

108. Dissection of the Internal Capsule. In the same specimen remove the optic 
tract and trace the basis pedunculi into the internal capsule and follow the fibers from 
the internal capsule into the corona radiata. Trace the optic radiation from the poste- 
rior extremity of the internal capsule to the cortex near the occipital pole (Fig. 260). 

109. Dissection of the Caudate Nucleus. On the left side of the same sheep's 



372 THE NERVOUS SYSTEM 

brain note that the tail of the caudate nucleus extends ventrally into the roof of the 
inferior horn of the lateral ventricle. With a blunt instrument scrape away the head 
and first part of the tail of the nucleus, exposing the medial surface of the internal cap- 
sule (Fig. 91). Note the shape and size of this nucleus as you are removing it. 

110. Study a horizontal section stained by the Weigert method through the internal 
capsule and basal ganglia. From this section and from the dissections endeavor to 
form a clear mental picture of the internal capsule and its relations (Figs. 191, 193; 
pp. 257-261). 

111. Now take the frontal sections of the left hemisphere of the human brain 
and identify the various parts of the corpus striatum and internal capsule (Figs. 186- 
190). 

112. Rhinencephalon. Study the olfactory portions of the brain to be seen on the 
ventral surface of the cerebral hemisphere in the human and sheep's brains (Figs. 172, 
197, 199 ; pp. 265-269) . Study the hippocampus, alveus, and fimbria as they lie exposed 
in the inferior horn of the lateral ventricle of the sheep's brain (Figs. 178, 182). Open 
up the inferior horn of the lateral ventricle on the left side of this specimen so as to 
expose the hippocampus and fimbria. Raise the hippocampus and fimbria on both sides 
at the same time, leaving them still attached to the fornix. This should be done without 
damaging the underlying tela chorioidea of the third ventricle, which occupies the great 
transverse fissure. Examine the under surface of the hippocampus, fimbria, and for- 
nix. Note that the two fimbriae unite to form the triangular body of the fornix. 
The transverse fibers in this triangle constitute the hippocampal commissure (lyra). 
Note the fascia dentata and hippocampal fissure. Figure 204 will help you to interpret 
the parts seen in this dissection. 

113. The chorioid plexuses of the prosencephalon are now fully exposed, and their 
relations to each other and the brain ventricles can be readily studied (pp. 224, 251). 

114. Remove the tela chorioidea of the third ventricle and again identify the parts 
of the thalamus and epithalamus which may be seen from above (Figs. 91, 180). 

115. Replace the fornix and hippocampus in position and divide the fornix and what 
remains of the cerebral hemispheres by a sagittal section \ millimeter to the right of 
the median plane. Take the left half of the preparation and, tearing away any por- 
tions of the right columna fornicis that may still be attached to the preparation, follow 
the left column of the fornix to the mammillary body. This can be accomplished by 
scraping away some of the medial surface of the thalamus (Fig. 204). At the same time 
expose the mamillothalamic tract. Remove the posterior part of the thalamus and the 
remainder of the brain stem by a cut made just caudal to the mamillothalamic tract, 
as indicated in Fig. 204. This gives a connected view of the entire fornix system. 
Find the cut surface of the hippocampal commissure and separate it for a few milli- 
meters from the rest of the fornix. Identify again the fimbria, fascia dentata, hippo- 
campal fissure and hippocampal gyrus, and study the fornix as a whole (Figs. 200, 
203; pp. 270-272). 

116. Study the septum pellucidum in the right half of the human brain (Fig. 158; 
p. 272). Also locate the anterior commissure. 

117. Dissect the anterior commissure in the right lateral half of the sheep's brain. 
Locate the commissure on the median surface and by blunt dissection follow it to the 
olfactory bulb (Fig. 199; p. 273). 



A LABORATORY OUTLINE OF NEURO-ANATOMY 373 

118. In the frontal sections of the left cerebral hemisphere of the human brain study 
the relations of the septum pellucidum, fornix, fimbria, hippocampus, and anterior 
commissure (Figs. 186-190). 

119. The Cerebral Cortex. On the right hemisphere of the human brain identify 
the motor, somesthetic, auditory, and visual centers (Figs. 220, 221; pp. 290-293). 
With a scalpel remove a cube of cortex and subjacent white matter from each of these 
areas. Each block should measure about 1 cm. in each dimension. With a sharp 
razor make section through each of these blocks at right angles to the surface of the cortex 
and perpendicular to the long axis of the gyrus from which the block was cut. Note the 
differences in thickness of the cortex in the various regions. Observe the white 
striations in the cortex, and note how these differ in the several specimens (Fig. 218). 
Study the stained and mounted sections of the cerebral cortex which are furnished 
you. What details of cell and fiber lamination do these preparations show, and how 
does this lamination differ in the several regions of the cortex? (See Fig. 215; pp. 
284-287.) 

120. Association Fibers (Figs. 226, 228; pp. 298-301). If the human brain is reason- 
ably well preserved the larger bundles of association fibers may be easily exposed by 
dissection. This can be done on the right hemisphere. But if the material is very 
soft this half of the brain can more profitably be laid into a series of horizontal sections 
and these used for a review of the form and relations of the component parts of the 
cerebral hemisphere. If the material is fairly well preserved, make the following 
review dissection and at the same time expose and study the various bundles of asso- 
ciation fibers. 

121. Review Dissection of the Human Brain. Take the right half of the 
human brain and scrape away the cerebral cortex from a portion of the dorsal 
surface of the frontal lobe. This will expose the short association or arcuate fibers 
(Fig. 226). 

122. Now make a horizontal section through the hemisphere parallel to the dorsal 
surface of the corpus callosum and J- inch dorsal to it. Note the centrum semiovale. 
Scrape away the cortex of the gyrus cinguli and the white matter immediately sub- 
jacent to it. In making this dissection carry the orangewood stick in an anteroposterior 
direction, removing the white matter a little at a time until a longitudinal bundle of 
fibers, the cingulum, is exposed (Fig. 174). The indusium griseum and striae longi- 
tudinales should now be uncovered. 

123. Remove the cingulum, scrape away the indusium griseum, and expose the 
radiation of the corpus callosum as indicated on the right side of Fig. 174, but do not 
cut the optic radiation or expose the tapetum at this time. 

124. Remove the parietal operculum a little at a time. This can be done with 
tissue forceps. Grasp small portions and t^ar them away by upward traction. Note 
the bundles of transverse fibers which enter this operculum from the corpus callosum 
and internal capsule. These intersect at right angles with the fibers of the superior 
longitudinal fasciculus which should come into view as the dissection progresses (Fig. 
174). The transverse bundles should be made to break off at the point where they pass 
through the superior longitudinal fasciculus. Complete the dissection of this fasciculus, 
carrying the dissecting instrument in the direction of its fibers. Now demonstrate the 
intersection of the corona radiata w r ith the radiation of the corpus callosum (Fig. 174). 



374 



THE NERVOUS SYSTEM 



By this dissection the insula and the dorsal surface of the temporal lobe have been ex- 
posed. Note in particular the transverse temporal gyri. 

125. Now dissect away the dorsal part of the temporal lobe and remove the insula. 
This will expose the uncinate and inferior occipitofrontal fasciculi as well as the external 
capsule (Fig. 227). These fiber bundles can best be displayed by carrying the dis- 
secting instrument in the direction of the fibers. Complete the dissection of the corona 
radiata and the optic radiation (Fig. 227). 

126. Now turn the specimen over and make a dissection of the column ofthefornix 
and the mamillothalamic tract as in Fig. 205, but do not cut away the brain stem as 
indicated in that figure. 

127. Dissection of the Internal Capsule from the Medial Side (Fig. 195). Tear 
away the fornix and septum pellucidum, opening up the lateral ventricle. With the 
brain knife cut away a slice from the medial surface of the hemisphere, varying in thick- 
ness from T inch at the frontal end to i inch at the occipital end, cutting through the 
corpus callosum and into the ventricle, but not into the basal ganglia. With a scalpel 
and tissue forceps remove what remains of the medial wall of the lateral ventricle, 
except in the inferior horn. Grasp with tissue forceps the stria terminalis in the rostral 
end of the sulcus terminalis and tear it away, carrying the forceps toward the occipital 
pole (p. 214). By blunt dissection remove the thalamus and subthalamus as well as the 
tegmentum and corpora quadrigemina of the mesencephalon. In scraping away these 
parts carry the dissecting instrument from the sulcus terminalis in a ventral direction. 
This will uncover the basis pedunculi and its continuation into the internal capsule. 
The fibers of the thalamic radiation will be broken off at the point where they enter the 
internal capsule (Fig. 195). Remove the ependymal lining of the posterior horn of the 
ventricle and uncover the tapetum. Scrape away the caudate nucleus, carrying the 
dissecting instrument in the direction of the fibers of the internal capsule (Fig. 195). 
Trace the anterior commissure to the point where it disappears under the anterior 
limb of the internal capsule. Study the internal capsule as seen from the medial sur- 
face, and note particularly the direction of the fibers, the anterior limb, the posterior 
limb, the optic radiation, and the curved ridge which represents the genu, 

128. Now turn again to the lateral side of the specimen (Fig. 227), and grasping 
with tissue forceps individual strands of the uncinate fasciculus in temporal lobe strip 
them forward into the frontal lobe. Remove the entire fasciculus in this manner. In 
the same way strip away the fibers of the inferior occipitofrontal fasciculus, beginning 
in the frontal lobe and tracing them toward the occiput. Strip off the fibers of the ex- 
ternal capsule and expose the lentiform nucleus and the corona radiata (Fig. 194). 
Pay special attention to the fibers of the corona radiata which come from the sublen- 
ticular part of the internal capsule and enter the temporal lobe. Follow the anterior 
commissure to the point where it disappears under the lentiform nucleus. 

129. Remove what remains of the temporal lobe and examine the hippocampus, 
fimbria, and inferior horn of the lateral ventricle from the dorsal surface (Fig. 201). 

130. Next scrape away the lentiform nucleus and trace the basis pedunculi into the 
internal capsule (Fig. 88). Study the corona radiata, internal capsule, and basis 
pedunculi from both sides of this preparation. The thalamus and the caudate and 
lentiform nuclei produce well-marked impressions on the internal capsule (Figs. 88, 195). 



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380 BIBLIOGRAPHY 

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INDEX 



NOTE. In cross references the key words are italicized. 
the pages on which the structures are illustrated. 



The numbers in Italics refer to 



ACCOMMODATION of vision, 332 

Acoustic area of cortex. See Center, auditory. 

Acousticolateral area, 358 

Affenspalte, 237 

Ala cinerea, 127 

lobuli centralis, 797 
Alveus, 270, 278, 279 
Ameba, 17 

Ammon's horn. See Hippocampus. 
Ampulla of semicircular canal, 358 
Amygdala. See Nucleus, amygdaloid. 
Ansa lenticularis, 263 

peduncularis, 263 
Aperture, lateral, of fourth ventricle, 125 

medial, of fourth ventricle, 125 
Apex columnse posterioris, 79 
Aphasia, 295 
Aqueductus cerebri (aqueduct of Sylvius), 26, 

158 

Arachnoid, 73 
Arbor vitae, 199 

Archipallium, 116, 242, 270, 277, 278, 279 
Area, acousticolateral, 358 
acustica, 127 

cortical, 287. (See also Center.) 
oval, of Flechsig, 107 
parolfactoria of Broca, 267 
postrema, 129 
pyriform, 116, 268, 277 
striata, 293 

Association bundles of cerebrum, 298 
arcuate, 298, 300 
cingulum, 299 
inferior longitudinal, 299 

occipitofrontal, 300 
superior longitudinal, 300 

occipitofrontal, 301 
uncinate, 299 
Ataxia, 99 

Auditory apparatus, 186, 309 
Auerbach's plexus, 351 
Autonomic system, 339 
cranial, 339 
craniosacral, 340, 354 
sacral, 339 

thoracicolumbar, 339, 354 
Axon (axis-cylinder), 37, 43, 45 

hillock. See Cone, implantation. 
Axonal reaction. See Chromatolysis. 

BAILLARGER, lines of, 283 
Band, diagonal, 267 
Basis cerebri, 115, 120 

pedunculi, 129, 158, 164 
Basket-cells, 209 
Bell's law, 60 



Betz, cells of, 290 

Bladder, innervation of, 354 

Body of cell, 43 

of fornix, 271 

geniculate, lateral, 131, 220 
medial, 131, 167, 220 

mammillary, 222, 280 

of Nissl, 48, 51 

paraterminal, 267 

pineal, 221 

pituitary. See Hypophysis. 

quadrigeminal, 130, 165 

restiform, 122, 143, 205 

striate. See Corpus striatum. 

tigroid. See Nissl body. 

trapezoid, 121, 150, 186 

Brachium (or brachia), conjunctivum, 125, 155, 
159, 160, 206, 211 

of corpora quadrigemina, 131 

pontis, 123, 204 

quadrigeminum inferius, 131, 163, 166 

superius, 131, 167 
Brain, 56, 113 

development, 25 

divisions of, 25 

end-. See Telencephalon. 

fore-. See Prosencephalon. 

hind-. See Metencephalon and Rhombenceph- 
alon. 

inter-. See Diencephalon. 

stem. See Medulla oblongata, Pans, Mesen- 
cephalon, and Ganglia, basal. 

vesicles, 24, 25 

weight, 301 

Broca's convolution, 235 
Brown-Sequard syndrome, 112 
Bulb, olfactory, 265, 274 

of posterior horn, 248 
Bundle. (See also Fasciculus and Tract.} 

association, of cerebrum, 298, 299, 300 

cornucommissural, 107 

ground. See Fasciculus proprius. 

of Gudden, tegmental. See Tract, mammillo- 
tegmental. 

marginal. See Fasciculus dorsolateralis. 

oval. See Area, oval. 

posterior longitudinal. See Fasciculus, medial 
longitudinal. 

of Turck. See Tract, ventral corticospinal. 

ventral longitudinal. See Tract, tectospinal. 
Burdach, column of. See Fasciculus cuneatus. 

nucleus of. See Nucleus cuneatus. 

CAJAL, commissural nucleus of, 330 

horizontal cells of, 285 
Calamus scriptorius, 127 

383 



384 



INDEX 



Calcar avis, 238, 248 
Canal, central (canalis centralis), 80, 136 
lateral line, 356 
semicircular, 315, 356 
spinal, 73 

Capsule, external, 257 
internal, 257, 259, 261 
nasal, 356 

of spinal ganglion cell, 63 
Cauda equina, 78 
Cavum septi pellucidi, 272 
Cell. (See also Neuron.) 
basket, 209 
of Betz, 290 
body, 43 

ependymal, 37, 85 
germinal, 37 
granule, of cerebellum, 208 

of cerebral cortex. See Neurons, stellate. 

of olfactory bulb, 276 
mitral, 275 
neuroglia, 85, 86 
of Purkinje, 207 
pyramidal, 285 

Cell-columns of Clarke. See Nucleus dorsalis. 
intermediolateral, 89 
of spinal cord, 89, po 
Center, cortical, 290 

association, 293 

auditory, 293 

motor, 290, 317, 318 

olfactory, 293 

optic, 292 

projection, 290 

somesthetic, 292 

of speech, 295 

visual, 292 
for pain, 219 
projection, 290 
respiratory, 330 

Central nervous system, 20, 21, 56, 57 
Centrum medianum thalami, 218 

semiovale, 243 
Cerebellum, 195 

in birds and reptiles, 200 

central white matter, 199 

cortex, 199, 206, 207, 208, 209 

development of, 195 

in the dogfish, 27, 28 

fiber tracts of, 204, 205, 206, 209, 210, 211 

folia, 199 

hemispheres of, 197, 198 

histpgenesis, 196 

laminae, 199 

lobes or lobules, 197, 198, 200, 201, 202 

in mammals, 200 

microscopic structure, 206 

morphology of, 199 

notches, 197 

nucleus dentatus, 203, 211 

emboliformis, 203 

fastigii or tecti, 204, 211 

globosus, 203 
peduncles, 204 

inferior, 122, 143, 205 

middle, 123, 204 

superior, 125, 155, 159, 160, 206, 211 
section, median, 199 

through hemisphere, 199 



Cerebellum in the sheep, 200, 201, 202 
vermis of, 196 
white matter, 199 

Cerebral aqueduct. See Aqueductus cerebri. 
cortex, 114, 232, 283 
area of, acoustic, 293 
association, 293 
audito-psychic, 293, 294 
audito-sensory, 294 
of Broca, 295 
motor, 290, 317, 318 
striata, 293 

visuo-psychic, 293, 294 
visuo-sensory, 294 
centers of, 290, 292, 295 
. development, 230 
electric excitability of, 291 
frontal olfactory, 277 
hippocampal, 278, 279 
histogenesis, 230 
layers of, 286, 287 
localization of function in, 290 
myelination of fibers, 289 
nerve-cells, 284, 285 
nerve-fibers, 283, 284 
neuroglia-cells, 284 
structure, 283, 284, 285, 286 
hemispheres, 113, 229, 232 
borders, 232 
commissural fibers, 296 
convolutions. See Gyri. 
corticifugal or efferent fibers, 283 
corticipetal or afferent fibers, 283 
development, 25, 32, 229 
in the dogfish, 27, 28, 30 
external conformation, 229 
fissures. See Fissure. 
gyri. See Cyrus. 
lobes. See Lobe. 
lobules. See Lobule. 
medullary center, 243, 296 
pallium, 25, 32, 33, 229 
poles, 232 
sulci. See Sulcus. 
surfaces, 232 

ventricles, lateral, 246 .'. 

peduncles. See Peduncles. 
vesicles, 24, 25 
Cerebrospinal fluid, 73, 126 

system, 58 
Cerebrum, 117 

Cervix, columnae posterioris, 79 
Chiasma, optic, 223, 226 
Chorda tympani, 192, 352 
Chorioid fissure, 229, 251 
plexuses. See Plexus. 
Chromatolysis, 51 

Chromophilic bodies. See Nissl bodies. 
Cingulum, 299 

Clarke, column of. See Nucleus dorsalis. 
Claustrum, 256 
Clava, 121, 137 
Climbing fibers, 209, 210 
Clivus monticuli. See Declive monticuli. 
Cochlea, 185 
Coelenterates, 19 
Cold, sensations of, 105, 306 
Collateral fibers, 43, 97 
Colliculus facialis, 127 



INDEX 



385 



Colliculus, inferior, 130, 165 

superior, 130, 165, 167 
Column, anterior, 80 

of Burdach. See Fasciculus cuneatus. 

of Clarke. See Nucleus dorsalis. 

dorsal (columna dorsalis grisea), 42 

of fornix, 272 

of Goll. See Fasciculus gracilis. 

gray, 79 

inter mediolateral, 89 

lateral, 80 

nuclear, of brain stem, 168, 170, 171, 174 

posterior, 79 

somatic afferent, 170, 182, 185 
efferent, 170 

ventral, 42, 80 

vesicular. See Nucleus dorsalis. 

visceral afferent, 170, 180 

efferent, 170, 174, 177 
Comma tract of Schultze. See Fasciculus inter- 

fascicularis. 

Commissura anterior alba, 80 
grisea, 80 

habenularum, 220 
Commissure or commissures, anterior cerebri, 

223, 231, 273, 296 
gray, 80 
white, 80 

great transverse. See Corpus callosum. 

of Gudden, 227 

habenular, 220 

hippocampal, 231, 271, 280, 296 

of inferior colliculi, 759 

middle. See Massa intermedia. 

optic. See Chiasma, optic. 

posterior, of cerebrum, 221 
of spinal cord, 80 

superior. See Commissure, habenular. 
Components of nerves, 61, 168. (See also 

Nerve-fibers.) 

Conduction of nerve impulses, 50 
Cone, implantation, 44 

of origin. See Cone, implantation. 
Cones of retina, 226 
Consciousness, 23, 302 
Conus medullaris, 74 
Convolution. See Cyrus. 
Coordination, 99, 210, 311 
Cornu ammonis. See Hippocampus. 
Cornucommissural bundle, 107 
Corona radiata, 261 
Corpus (or corpora) callosum, 243, 296 
development, 231 

fornicis, 271 

geniculatum laterale, 220 
mediale, 131, 167, 220 

mamillaria, 222, 230 

pineale, 221 

ponto-bulbare, 123 

quadrigemina, 130, 165 

restiforme, 122, 143, 205 

striatum, 25, 32, 33, 256, 262, 324 

subthalamicum (Luysi), 223 

trapezoideum, 121, 150, 186 
Cortex, cerebellar, 199, 206, 207, 208, 209 
localization of function in, 202 
neurons of, 207, 208, 209 

cerebral. See Cerebral cortex. 
Corti, ganglion of. See Ganglion, spiral. 

25 



Corti, organ of, 185, 186 

Cough, mechanism of, 331 

Crus (or crura) cerebri. See Peduncle, cerebral. 

fornicis, 271 

Crusta. See Basis pedunculi. 
Culmen monticuli, 198 
Cuneate tubercle, 121, 137 
Cuneus, 239, 292 
Cup, optic, 32, 33, 225 
Cytoplasm of nerve-cells, 42, 47, 48 

DECLIVE monticuli, 198 

Decussation (decussatio) of brachium conjunc- 
tivum, 156, 159 

dorsal tegmental, 161, 167 

of fillet. See Decussation of lemniscus. 

of Forel. See Decussation, ventral tegmental. 

fountain. See Decussation, dorsal tegmental. 

of lemniscus (lemniscorum), 134, 138 

of Meynert. See Decussation, dorsal tegmental. 

optic. See Chiasma, optic. 

of pyramids, 119, 120, 134, 136 

tegmental. See Decussations, ventral and 
dorsal tegmental. 

ventral tegmental, 161 
Degeneration of fiber tracts, 105, 106, 107 

of nerve-fibers, 51, 52 

Wallerian, 105, 106, 107 
Deiters, nucleus of, 151, 189 
Dendrites or dendrons, 43 
Dermatome, 58 

Development of the nervous system, 24, 31 
Diencephalon, 24, 25, 26, 28, 31, 33, 213 
Digitationes hippocampi, 269 
Dogfish, brain of, 26, 27, 28 
Dogiel's Type II cells, 65 
Dura mater, 73 
Dynamic polarity, law of, 50 

EARTHWORM, nervous system of, 19 

Edinger-Westphal nucleus, 178 

Effector, 18, 19, 54, 91 

Embryology of nervous system, 31, 37, 195, 213, 

229 
Eminentia cinerea. See Ala cinerea. 

collateralis, 250 

facialis. See Colliculus facialis. 

hypoglossi. See Trigonum hypoglossi. 

medialis, 129 

teres. See Eminentia medialis. 
Encephalon. See Brain. 
End-brain. See Telencephalon. 
End-plates, motor, 62 
Ependyma, 85 
Epiphysis, 29, 31 
Epithalamus, 29, 35, 220 

Exteroceptor, exteroceptive, 66, 182, 185, 304 
Eye, development, 225 

innervation, 225 

retina, 225 

FASCIA dentata, 269, 279 

Fasciculus, 95. (See also Tract and Bundle.) 

anterior proprius, 107 

anterolateralis superficialis, 100 

arcuatus, 300 

cerebellospinalis. See Tract, dorsal spinocere- 
bellar. 

cerebrospinalis. See Tract, corticospinal. 



3 86 



INDEX 



Fasciculus cerebrospinalis, anterior. See Tract, 

ventral corticospinal. 
lateralis. See Tract, lateral corticospinal. 
cuneatus, 76, 83, 95, 96, 121, 137 
dorsal longitudinal (Schutz), 216 
dorsolateralis (Lissauer), 79, 87, 98, 104 
gracilis, 76, 83, 96, 121, 137 
interfascicularis, 97, 107 
lateralis, minor, 121 

proprius, 107 
longitudinalis inferior, 299 

medialis, 145, 152, 162, 190, 328 
superior, 300 

medial longitudinal, 145, 152, 162, 190, 328 
of Meynert, 220 
occipitofrontalis, inferior, 300 

superior, 301 

peduncularis transversus, 369 
posterior longitudinal. See Fasciculus, medial 

longitudinal. 

proprius of spinal cord, 107 
pyramidal. See Tract, corticospinal. 
retroflexus, 220 
septomarginal, 97, 107 
solitarius, 132, 181, 330 
sulcomarginalis, 108 
superior longitudinal, 300 
thalamornamillaris. See Tract, mammillo- 

thalamic. 
uncinatus, 299 

Fibers, fibrae. (See also Nerve-fibers.) 
arcuate, of cerebrum, 299 
of medulla oblongata, 139 

external, 121, 123, 139, 140, 143 
internal, 134, 138, 139 
association, 92, 298 

cerebello-olivary. See Fibers, olivocerebellar. 
climbing, 209, 210 
commissural, 296 
mossy, 209, 210 

olivocerebellar, 139, 142, 143, 205 
pontis, 147 

postganglionic, 337, 343 
preganglionic, 337, 344 
projection, 297 

propriae. See Fibers, arcuate, of cerebrum, 
rectae, 148 

Fila lateralia pontis, 148 
Fillet. See Lemniscus. 
Filum durae matris spinalis, 74 
terminale, 74 
externum, 74 
internum, 74 

Fimbria hippocampi, 250, 269 
Final common path, 94, 311 
Fissure (or fissura), calcarine, 238, 292 
callosal. See Sulcus of corpus callosum. 
callosomarginal, 240 
central, of Rolando, 233 
cerebri lateralis, 233 
chorioid, 229, 251 
collateral, 239 

dentate. See Fissure, hippocampal. 
development, 230, 231 
great longitudinal, 232 

transverse. See Fissure, transverse cere- 
bral. 

hippocampal, 239, 269, 270 
lateral cerebral, 233 



Fissure, longitudinal cerebral, 114, 232 

mediana, anterior, of medulla oblongata, 119 

of spinal cord, 76, 82 
posterior, of medulla oblongata, 119 

parieto-occipital, 239 

prima, 196, 199 

rhinal, 116, 240 

of Rolando. See Sulcus, central. 

secunda, 203 

Sylvian. See Fissure, lateral cerebral. 

transverse cerebral, 213 
Flechsig, direct cerebellar tract of. See Tract, 

dorsal spinocerebellar. 
Flexure, cephalic, 31, 33 

cervical, 32, 33 

pontine, 31, 33 
Flocculus, 199 

Fluid, cerebrospinal, 73, 126 
Folium vermis, 198 
Foramen caecum, 119 

interventricular, 26, 118 

of Luschka. See Aperture, lateral, of fourth 
ventricle. 

of Majendie. See Aperture, medial, of fourth 
ventricle. 

of Monro. See Foramen, interventricular. 
Forceps, major, 245 

minor (frontal part of radiation of corpus cal- 
losum). 

Fore-brain. See Prosencephalon. 
Forel, fountain decussation of. See Decussa- 

tion, ventral tegmental. 
Formatio reticularis, 80, 136, 144 
Fornix, 270, 280 

body, 271 

columns, 271, 272 

commissure, 271, 280 

crura, 271 

fimbria, 270, 271 

longus, 282 
Fossa interpeduncularis, 115 

rhomboid, 126, 127 
Fountain decussations of Forel and of Meynert, 

161, 167 
Fovea, inferior, 127 

superior, 127 

Frenulum veli medullaris anterior, 130 
Frog, sympathetic ganglia of, 344, 345 
Funiculus, 95 

anterior, 76, 82 

cuneatus, 121, 137 

dorsal. See Funiculus, posterior. 

gracilis, 121, 137 

lateralis, 76, 82 

posterior, 76, 82 

separans, 129 

teres. See Eminentia medialis. 

ventral. See Funiculus, anterior. 

GANGLIATED cord. See Trunk, sympathetic. 
Ganglion or ganglia, autonomic. See Ganglia, 
sympathetic. 

basal, 252 

celiac, 349 

cerebrospinal (sensory ganglia on the cerebro- 
spinal nerves), 38 

cervical, inferior, 348 
middle, 348 
superior, 347 



Ganglion, ciliary, 351 

of Corti. See Ganglion, spiral. 

enteric, small ganglia of myenteric and sub- 
mucous plexuses, 351 

of facial nerve. See Ganglion, geniculate. 

Gasserian. See Ganglion, semilunar. 

geniculate, 192 

habenula?, 29, 220 

interpeduncular, 115, 164 

jugular, 193 

mesenteric, 349 

nodosum, 193 

otic, 351 

petrosal, 193 

of Scarpa. See Ganglion, vestibular. 

semilunar, 191 

sensory, 38 

sphenopalatine, 351 

spinal, 62 

development of, 38, 40 
structure of, 63, 64, 65 

spiral, 185, 186 

submaxillary, 351 

sympathetic, collateral, 335 
development of, 41, 335 
prevertebral. See Ganglia, collateral sym- 
pathetic, 
structure of, 341 
of sympathetic trunk, 335 
terminal, 335 

vertebral. See Ganglia of sympathetic 
trunk. 

of trigeminus. See Ganglion, semilunar. 

vestibular, 188 
Gemmules, 43 
Geniculate body. See Body. 

ganglion. See Ganglion. 
Gennari, line of, 283 
Genu of corpus callosum, 243 

of internal capsule, 258, 262 

internum of facial nerve, 175, 176, 180 
Glia-cells. See Cells, neuroglia. 
Glial sheath, 86 
Globus pallidus, 254, 256, 324 
Glomeruli, cerebellar, 208 

olfactory, 276 

of sensory axons, 63 

of sympathetic ganglia, 341 
Golgi cells of Type II, 44, 87 

method of, 361 
Goll, column or tract of. See Fasciculus gra- 

cilis. 
Gowers, bundle of. See Fasciculus anterolater- 

alis superficialis. 

Granular layer of cerebellum, 208 
Gudden, commissure of, 227 
Gustatory apparatus, 181 
Gyrus (or gyri), angular, 236 

annectent, 234 

anterior central, 235, 290 

ascending parietal. See Gyrus, posterior cen- 
tral. 

breves or short gyri of insula, 237 

callosal. See Gyrus cinguli. 

centralis, anterior, 235, 290 
posterior, 236, 292 

cinguli, 240 

dentatus. See Fascia dentata. 

diagonal, of rhinencephalon, 267 



INDEX 387 

Gyrus fornicatus, 240 

frontal, ascending. See Gyrus, anterior cen- 
tral, ^r 

inferior, 235 

middle, 235 

superior, 235, 240 
fusiform, 240 

hippocampal, 116, 240, 277 
insulae, 237 

limbic. See Lobe, limbic. 
lingual, 239, 292 
longus insulae, 237 

marginalis. See Gyrus, superior frontal, 
olfactory, lateral, 116, 266, 277 

medial, 116, 266 
orbital, 241 

postcentral. See Gyrus, posterior central, 
posterior central, 236, 292 
precentral. See Gyrus, anterior central, 
rectus, 241 

subcallosus (pedunculus corporis callosi), 267 
supracallosal, 244, 270 
supramarginal, 236 
temporal, inferior, 236 

middle, 236 

superior, 236 

transverse, 236, 293 
uncinatus. See Gyrus, hippocampal. 

HABENULA. See Nucleus habenulae. 

Hearing, organs of, 185, 186, 187, 309 

Heart, innervation of, 353 

Heat, sensations of, 105, 306 

Hemianopsia, 228 

Hemiplegia, 323 

Hemispheres, cerebellar, 197, 198 

cerebral. See Cerebral hemispheres. 
Hilus nuclei olivaris, 141 
Hind-brain. See Metencephalon and Rhomben- 

cephalon. 
Hippocampal gyrus, 116, 240, 277 

commissure, 231, 271, 280, 296 
Hippocampus, 250, 269, 277 
Histogenesis of cerebellar cortex, 196 

of cerebral cortex, 230 

of nervous system, 37 

of peripheral nervous system, 40 

of spinal cord, 38, 39, 42 

ganglia, 38, 40 

Horizontal cells of Cajal, 285 
Horn of lateral ventricle, 246. (See also 

Column.} 
Hypophysis, 222 

in the dogfish, 29 
Hypothalamus, 35, 222 

in the dogfish, 29 

pars mamillaris, 222 
optica, 35 

INCISURA. See Notch. 
Indusium griseum, 244, 270 
Infundibulum, 222 
Insula, 229, 237 

Inter-brain. See Diencephalon. 
Interoceptor, interoceptive, 66, 101 
Interpeduncular fossa (or space), 115 
Interventricular foramen, 26, 118 
Intumescentia cervicalis, 73, 84 
lumbalis, 74, 84 



388 INDEX 

Island of Reil. See Insula. 
Iter a tertio ad quartum ventriculum. See 
Aqueductus cerebri. 

JELLY-FISHES, 19 

Joints, sensory fibers of, 72 

KRAUSE, end-bulb of, 68 

LAMINA affixa, 215 

alar. See Plate, alar. 

basal. See Plate, basal. 

medullaris involuta. See Stratum lacunosum. 

quadrigemina, 130, 158 

rostralis, 223, 243 

septi pellucidi, 272 

terminalis, 25, 33, 223, 231 
Laminae medullares of lentiform nucleus, 254 

thalami, 216 
Lancisi, nerve of. See Stria longitudinalis me- 

dialis. 

Lateral line organs, 356 
Layers of cerebellar cortex, 208 

of cerebral cortex, 286, 287 

ependymal, 37 

mantle, 37, 42, 196 

marginal, 37, 42, 196 

of retina, 225 

Lemniscus, lateral, 130, 157, 163, 165, 166, 186, 
187, 309 

medial, 135, 138, 145, 153, 163, 219, 313 

spinal. See Tract, spinothalamic. 

trigeminal. See Path, secondary afferent, of 

trigeminal nerve. 
Ligamentum denticulatum, 74 
Limen insulae, 237, 268 
Line (or lines) of Baillarger, 283 

of.Gennari, 283 
Linea splendens, 74 
Lingula of cerebellum, 197 
Lissauer, tract of. See Fasciculus dorsolater- 

alis. 

Lobe (lobus or lobes) of cerebellum, 197, 198, 
200, 201, 202 

of cerebrum, 234 

frontal, 234 

inferior, 28 

insular. See Insula. 

limbic. See Cyrus fornicatus. 

lineae lateralis, 27 

occipital, 236, 238 

olfactory, 267 

optic, 27, 28, 165 

parietal, 236 

pyriform. See Area, pyriform. 

temporal, 235 

visceral, 27 
Lobule (or lobulus) ansiformis, 201 

bi venter, 199 

centralis, 197 

paracentral, 240, 290 

paramedianus, 201 

parietal, inferior, 236 
superior, 236 

postcentral. See Gyrus longus insulae. 

precentral. See Gyri breves insulae. 

quadrangularis, 198 

quadrate. See Precuneus. 

semilunaris, inferior, 198 



Lobule semilunaris, superior, 198 

simplex, 200 
Localization of function in cerebellum, 202 

in cerebral cortex, 290 

in thalamus, 219 
Locus caeruleus, 128 
Luschka, foramen of. See Aperture, lateral, of 

fourth ventricle. 

Luys, nucleus of. See Nucleus hypothalamicus. 
Lyra. See Commissure, hippocampal. 

MACROSMATIC mammals, 265 

Magendie, foramen of. See Aperture, medial, of 

fourth ventricle. 
Mammillary body, 222, 280 
Mantle. See Cerebral cortex. 

layer. See Layer. 

Marchi stain for degenerated nerves, 360 
Martinotti, cells of, 285 
Massa intermedia, 216 
Matter, central gray, 136, 158 
gray, 42, 79, 87 
white, 42, 79, 86 
Medulla oblongata, 114, 118 
closed portion of, 119 
development, 35. (See also Myelencepha- 

lon.) 

in the dogfish, 26, 27, 28 
fissure, anterior median, 119 

posterior median, 119 
form, 118, 119, 120, 121, 122 
gray matter, 136 
internal structure, 132 
length, 118 

motor nuclei, 170, 174 
open portion of, 119 
sensory nuclei, 180, 182 
sulci, 119 

spinalis. See Spinal cord. 
Meissner, corpuscles of, 68 

plexus of, 351 
Meninges, 73, 74 
Merkel, corpuscle of, 68 
Mesencephalon, 129, 158 
development, 24, 31, 35, 36 
in the dogfish, 27, 28 
form, 129 

internal structure, 158 
Metamerism, 58. (See also Segmentation.) 
Metathalamus, 220 
Metencephalon, 31, 33, 36 
Meynert, fasciculus retroflexus of, 220 

fountain decussation of. See Decussation, 

dorsal tegmental. 
Microsmatic mammals, 265 
Mid-brain. See Mesencephalon. 
Mitochondria, 49 
Molecular layer of cerebellum, 208 

of cerebral cortex, 286 

Monakow, bundle of. See Tract, rubrospinal. 
Monro, foramen of. See Foramen, interventric- 

ular. 

Monticulus, 198 

Moss-fibers of cerebellum, 209, 210 
Motor apparatus, 316 

area of cerebral cortex, 290, 317, 318 
end-plate, 62 
Muscle, branchial, 174 
cardiac, innervation of, 353 



INDEX 



389 



Muscle of eyeball, innervation of, 352 

of facial expression, innervation of, 192 

of larynx, innervation of, 194 

of mastication, innervation of, 192 

nerve endings in, 62, 72 

sense (proprioceptive), 72, 99, 100, 311 

skeletal. See Muscle, branchial and somatic. 

smooth or unstriated. See Muscle, visceral. 

somatic, innervation of, 62, 170 

striated. See Muscle, branchial and somatic. 

of tongue, innervation of, 194 

visceral, innervation of, 61, 174, 177 
Muscle-spindles, 72 
Myelencephalon, 31, 32, 33, 36 
Myelin, 46 

sheath. See Sheath. 
Myelination in cerebral cortex, 289 

in spinal cord, 112 
Myotome, 58, 170 

NEOPALLIUM, 116, 232, 242 

Neothalamus, 219 

Nerve (Nervus), abducens, 123, 154, 173, 192 

accessory, 123, 176, 177, 194 

acoustic, 123, 185, 192 

auditory. See Nerve, acoustic. 

cardiac, 348, 349 

cerebrospinal, 56 

chorda tympani, 192, 352 

ciliary, 352 

cochlear, 149, 185, 193 

components, 61. (See also Nerve-fibers.) 

cranial, 56, 132, 133, 168 

facial, 123, 153, 175, 192 

glossopharyngeal, 123, 193 

hypoglossal, 123, 173, 194 

intermedius, 123, 192 

of Lancisi. See Stria longitudinalis medialis. 

lingual, 192 

oculomotor, 130, 164, 171, 172, 191 

olfactory, 191, 265 

optic, 191, 225 

phrenic, 59 

pneumogastric. See Nerve, vagus. 

spinal, 56, 58, 65 
development of, 40 

splanchnic, 348 

sympathetic, 345 

terminalis, 27, 190 

thoracic, 58 

trigeminal, 124, 154, 174, 182, 191 

trochlear, 125, 163, 173, 191 

vagus, 123, 178, 193 

vestibular, 149, 185, 193, 314 

of Wrisberg. See Nervus intermedius. 
Nerve-cells, 43. (See also Neurons and Cells.) 

autonomic. See Neurons, sympathetic. 

motor, for involuntary muscles, 177 
for voluntary muscles, 777 

processes, 43 

shape, 43 

structure, 47 

types of, 43, 44 
Nerve-endings, encapsulated, 68 

free in epidermis, 67 

in free arborizations, 67 

in hair- follicles, 70, 71 

in Meissner's corpuscles, 68 

on Merkel's touch-cells, 68 



Nerve-endings in muscle-spindles, 71, 72 
in Pacinian corpuscles, 69 
peripheral, 66-72 

plexuses of sensory nerve-fibers, 67 
in synapses. See Synapse. 
in tactile corpuscles, 68 
in tendons, 72 
in voluntary muscles, 62 
Nerve-fibers, 45. (See also Fibers.) 

afferent, 58, 63. (See also Nerve-fibers, so- 
matic and visceral afferent.) 
autonomic. See Nerve-fibers, preganglionic 

and postganglionic. 
of cerebellar cortex, 209 
of cerebral cortex, 283 
classification of, 60 
collateral, 43, 97 

degeneration of, 52, 105, 106, 107 
development, 40, 41 
of dorsal root, 95 
efferent, 58 
exteroceptive, 66 

gray. See Nerve-fibers, postganglionic. 
interoceptive, 66 
to involuntary muscles, 61 
medullated. See Nerve-fibers, myelinated. 
motor, 59, 62, 94 

myelinated, 45, 46, 47, 63, 66, 67, 87 
non-medullated. See Nerve-fibers, unmyelin- 

ated. 

postganglionic, 337, 343 
preganglionic, 337, 344 
primary motor, 62, 90 
proprioceptive, 66, 72 
regeneration, 52 

of Remak. See Nerve-fibers, unmyelinated. 
somatic afferent, 61, 66 

general, 168, 182, 192, 193 
special, 168, 191, 193 
efferent, 61, 62, 168, 191, 192, 194 
sympathetic. See Nerve-fibers, postgangli- 
onic. 

unmyelinated, 47, 63, 66, 67, 87, 98, 104 
visceral afferent, 61 

general, 168, 181, 193, 335 
special, 168, 180, 192, 193 
efferent, 61 

general, 168, 178, 192, 193, 194, 336 
special, 168, 174, 192, 193, 194 
to voluntary muscles. See Nerve-fibers, so- 
matic efferent and special visceral efferent, 
of white rami, 61, 347 

substance of brain and cord, 47 
Nerve-root. See Root. 
Nervous system, autonomic, 339 
cranial, 339 
craniosacral, 340 
sacral, 339 

thoracicolumbar, 339, 340 
central, 20, 21, 56, 57 
cerebrospinal, 58 
development of, 24, 32, 36, 37 
diffuse, 18, 19, 340 
invertebrate, 19, 20, 21, 22 
peripheral, 56 
subdivisions of, 56 
sympathetic, 56, 57, 334 
vertebrate, 21, 22 
Net, nervous, 19, 340. (See also Plexus.) 



39 



INDEX 



Neural crest, 37 

groove, 24, 31 

tube, 24, 31, 36 
Neurilemma, 41, 46, 47 
Neurobiotaxis, 179 
Neuroblasts, 37, 39 
Neurofibrils, 48, 49, 50 
Neuroglia, 85, 86 
Neuromuscular end-organ, 72 

mechanism, 17 
Neuron or neurons, 43. (See also Nerve-cells.} 

basket cell, 50 

bipolar, 39, 44, 63 

chains, 43, 49, 53, 54 

concept, 52 

of cerebellar cortex, 207, 208, 209 

of cerebral cortex, 285 

development of, 37 

form of, 42 

horizontal, of Cajal, 285 

interrelation of, 49 

lower motor, 318 

of Martinotti, 285 

motor, 22, 44, 46, 177 

multipolar, 44 

of olfactory bulb, 275 

polarization of, 50 

postganglionic, 337 

preganglionic, 337, 339, 341 

of Purkinje, 207 

pyramidal, 43, 44, 285 

of retina, 225, 226 

sensory, 22, 23, 37, 63 

stellate, 285 

structure of, 47 

sympathetic, 341 

theory of. See Neuron concept. 

type I, 44, 87 

type II, 44, 45, 87, 88 

unipolar, 39, 44, 63 

upper motor, 317 
Neuropil, 20, 21 
Neuropore, 31 

Nissl bodies or granules, 48, 51 
Nodes of Ranvier, 47 
Nodule of vermis, 198 
Non-medullated fibers. See Nerve-fibers, unmye- 

linated. 
Notch, anterior cerebellar, 197 

posterior cerebellar, 197 

preoccipital, 234 

Nucleated sheath. See Neurilemma. 
Nucleus (or nuclei) of abducens N., 154, 173 

accessory cuneate, 138 

of accessory N., 194 

of acoustic N. See Nuclei, cochlear and 
vestibular. 

ambiguus, 146, 176 

amygdaloid, 249, 257 

anterior thalami, 217, 218 

arcuate, 140, 143 

arcuatus thalami, 218 

of Bechterew, 152, 189 

caudatus, 253 

centralis, superior, 157 
of thalamus, 218 

of cerebellum, 203, 204 

cochlear, 123, 149, 185 

commissural, 330 



Nucleus of corpus mamillare, 222 
cuneatus, 122, 134, 137, 139 
of Darkschewitsch, 153 
of Deiters, 151, 189 
dentatus, 203, 206, 211 
dorsalis, 90, 100 
of dorsal funiculus. See Nucleus gracilis and 

Nucleus cuneatus. 
dorsal motor, of vagus, 146, 178 

thalamic. See Nucleus, anterior thalami. 
of Edinger and Westphal, 178 
emboliformis, 203 
external round, 138 
of facial N., motor, 153, 175, 179 
of fasciculus cuneatus. See Nucleus cuneatus. 

gracilis. See Nucleus gracilis. 

solitarius. See Nucleus of tractus solitarius. 
fastigii, 204, 211 

of fifth nerve. See Nuclei oi trigeminal nerve, 
of fourth nerve. See Nucleus of trochlear 

nerve, 
funiculi cuneati. See Nucleus cuneatus. 

gracilis. See Nucleus gracilis. 
globosus of cerebellum, 203 

of thalamusr, 218 

of glossopharyngeal nerve. See Nucleus am- 
biguus and Nucleus of tractus solitarius. 
of Goll. See Nucleus gracilis. 
gracilis, 122, 134, 137 
habenulse, 29, 220 
of hypoglossal nerve, 145, 173 
hypothalamicus (Corpus Luysi), 223 
of inferior colliculus, 165 
internal round nucleus, 138 
interpeduncular, 115, 164 
interstitial, 153 
of lateral lemniscus, 157, 187 
lateral reticular, of medulla oblongata, 143, 

145 

lateral thalamic, 217, 219 
lemnisci lateralis, 157, 187 
lenticular, 254 
lentiform, 254 

of Luys. See Nucleus hypothalamicus. 
of medial longitudinal fasciculus, 153 
medial thalamic, 217, 218 
mesencephalic. See Nucleus of trigeminal N. 
motor, of tegmentum (motorius tegmenti), 

145, 161 

of nerve-cell, 47 
of oculomotor N., 164, 171 
olivary, 141, 142 

accessory, 142 
dorsal, 142 
medial, 142 

inferior, 141 

superior, 151, 186 
of origin, 180 
pontis, 148, 149 

radicis descendemtis N. tngemini. See Nu- 
cleus of tractus spinalis of N. V. 
red, 159, 160 

roof, of cerebellum. See Nucleus fastigii. 
ruber. See Nucleus, red. 
salivatory, 178 

of Schwalbe. See Nucleus, medial vestibular. 
semilunar, of thalamus, 218 
of sixth nerve, 154, 173 
somatic afferent, 182, 185 



INDEX 



391 



Nucleus, somatic efferent, 170 
of spinal tract X. V, 136, 144, 155, 182 
tecti. See Nucleus fastigii. 
tegmental, dorsal, 158 

ventral, 158 
terminal, 180 
thalamic, 217, 218 
of tractus solitarius, 146, 181, 330 

spinalis N. trigemini, 136, 144, 145, 182 
of trapezoid body, 186 
of trigeminal N., 154, 156 
main sensory, 155, 182 
mesencephalic, 155, 184 
motor, 155, 174 
spinal, 136, 144, 155, 182 
of trochlear N., 163, 173 
of vagus, motor. See Nucleus, dorsal motor, 

of vagus and Nucleus ambiguus. 
sensory. See Nucleus of tractus solitarius. 
ventral thalamic, 218 
vestibular, 151, 188 
visceral afferent, 180 
efferent, 174, 177 

OBEX, 129 

Olfactory apparatus, 274-282 

bulb, 265, 274 

cells of nasal mucous membrane, 274 

cortex, 277, 278, 279. (See also Archipallium.) 

glomeruli, 276 

gyri, 116, 266, 277 

lobe, 267 

nerve, 265, 275 

roots. See Gyri, olfactory. 

stria;, 266, 277 

tract, 265, 277 

trigone, 266 

tubercle, 268, 282 
Olive (oliva, olivary body), 121 

accessory, 142 

inferior, 141 

superior, 151, 186 
Opercula, 230, 237 
Optic apparatus, 225 

chiasma, 223, 226 

cup, 32, 33, 225 

lobes, 27, 28, 165 

nerve, 225, 226 

radiation, 227 

tectum. See Cotticulus, superior. 

tract, 226 

vesicle, 225 
Organ of Corti,"185, 186 

lateral line, 356 

spiral, 185, 186 

PACINIAN corpuscles, 69 

Pain, apparatus of, 68, 103, 105, 306 

Palaeothalamus. See Thalamus, old. 

Pallium, 25, 32, 33, 229 

Paraflocculus, 202 

Paralysis, 322, 323 

Paraphysis, 31 

Parasympathetic system. See Nervous system, 

craniosacral autonomic. 
Pars anterior lobuli quadrangularis, 198 

basilaris pontis, 124, 147 

dorsalis pontis, 124, 149 

frontalis capsulse internae, 258, 259 



Pars intermedia of Wrisberg. See Nervus in- 

termedius. 

mamillaris hypothalami, 222 
occipitalis capsulae internae, 258, 259 
optica hypothalami, 35 
posterior lobuli quadrangularis, 198 
Path (or pathway), afferent cerebellar, 313, 314 

spinal, 98, 303 
auditory, 186, 309 
cerebello-rubro-spinal, 326 
cortico-ponto-cerebellar, 149, 325 
craniosacral, 352, 353. 354 
efferent, 216 
for eye, 352 
for heart, 353 
for stomach, 353 
for submaxillary gland, 352 
for urinary bladder, 354 
exteroceptive, 66, 101, 102, 302 
extrapyramidal motor, 324 
final common, 94, 311 
motor, 109, 216 

for cranial nerves, 320 
for spinal nerves, 319 

for muscular sense. See Path, proprioceptive. 
olfactory, 280 
for pain, 103, 104, 105, 306 
proprioceptive, 72, 99, 100, 311 
secondary afferent, from tractus solitarius, 

181 

of trigeminal N., 163, 183, 185, 307 
vestibular, 190 

for thermal sensibility, 105, 306 
thoracicolumbar, 352, 353, 354 
for touch, 101, 102, 303 
vestibular, 190 
visual, 226, 227, 228, 310 
Peduncle (or peduncles), cerebellar, 204, 205, 

206, 211 

cerebral, 129, 158 

of corpus callosum. See Cyrus subcallosus. 
of mammillary body, 222 
olivary. See Stalk of superior olive, 
of pineal body. See Stalk of pineal body. 
Perforated space, anterior. See Substantia per- 

forata anterior. 
Perikaryon, 43 

Pes pedunculi. See Basis pedunculi. 
Pia mater, 73 
Pineal body. 130 

Pituitary body. See Hypophysis. 
Plate, alar, 34, 42, 194 
basal, 34, 42 
neural, 24 

roof, of prosencephalon, 213 
Plexus of Auerbach, 351 
brachial, 58 
cardiac, 349 
celiac, 349 
chorioid, lateral, 251 
of fourth ventricle, 128 
of third ventricle, 223 
esophageal, 349 
gastric, 349 
hypogastric, 351 

intercellular, of sympathetic ganglion, 344 
lumbosacral, 58 
Meissner's, 351 
mesenteric, 349 



392 



INDEX 



Plexus, myenteric, 351 

pelvic, 351 

pericellular, of spinal ganglion, 65 
of sympathetic ganglion, 345 

pulmonary, 349 

solar, 349 

submucous, 351 

sympathetic, 345, 348 

vesical, 354 

Polarity of the neuron, 50 
Poles of cerebral hemisphere, 232 
Pons (Varoli), 114, 123 

basilar or ventral part of, 124, 147 

dorsal or tegmental part of, 124, 149 

form, 123 

internal structure, 147 

longitudinal fasciculi, 147 

nuclei of, 148 

taenia of, 148 

transverse fibers of, 147 
Ponticulus. See T&nia of fourth ventricle. 
Portio major N. trigemini, 125 

minor N. trigemini, 125 
Postganglionic fibers, 337, 343 
Precuneus, 240 
Preganglionic fibers, 337, 344 
Pressure, apparatus of sensibility to, 66 
Presubiculum, 277 
Processus reticularis. See Reticular formation of 

spinal cord. 
Projection centers, 290 

fibers, 297 
Proprioceptor, proprioceptive, 72, 99, 100, 183, 

185, 311 

Prosencephalon, 24, 25, 31, 36, 113 
Protoplasm, 17 

Psalterium. See Commissure, hippocampal. 
Pulvinar, 214, 217, 227 
Purkinje, cells of, 207 
Putamen, 254, 255 
Pyramid (or pyramis) of cerebellum, 198 

of medulla oblongata, 119, 136 

of vermis, 198 
Pyriform lobe, 116, 268, 277 

RADIATION (or radiatio), auditory or acoustic, 
261 

of corpus callosum, 243, 245 

occipitothalamica. See Radiation, optic. 

optic, 227, 261, 264 

sensory, 264 

thalamic, 216, 217, 260, 263 

thalamotemporal, 264 

Radix descendens (mesencephalica) N. tri- 
gemini. See Root, mesencephalic N. V. 

N. facialis, 175 

Ramus communicans, 335, 346 
gray, 335, 347 
white, 335, 347 

dorsal, 58 

ventral, 58 

Ranvier, constrictions or nodes of, 47 
Receptor, 19, 53, 91 
Recess, lateral, of fourth ventricle, 125 

lateralis fossae rhomboideae, 125 

optic, 223 

pineal, 221 

suprapineal, 221 
Reflex act, 91 



Reflex arc, 20, 53, 91, 92, 93, 327 
auditory, 331 

of brain stem, 328, 329, 330, 331, 332 
for coughing and vomiting, 330 . 
of medulla oblongata, 328, 329, 330 
myenteric, 340 
optic, 332 
pupillary, 332, 333 
respiratory, 330 
scratch, 94 

of spinal cord, 91, 92, 93, 94, 328 
vestibular, 328, 329 
visceral, 340 

Regeneration of nerve-fibers, 52 
Reil, island of. See Insula. 
Respiratory apparatus, 330 
Restiform body, 122, 143, 205 

medial part of, 205 

Reticular formation (or substance), 80, 136, 144 
Retina, 225 

Rhinencephalon, 25, 32, 115, 265 
Rhombencephalon, 25, 31, 32, 35, 36, 113 
Rhombic lip, 195 
Rod and cone cells, 226 
Rolando, fissure of. See Sulcus centralis. 
substantia gelatinosa of, 80 
tubercle of. See Tuberculum cinereum. 
Root of abducens nerve, 123 
of accessory nerve, 76, 123 
of acoustic nerve, 123 
anterior spinal. See Root, ventral, 
dorsal, 58, 76, 95, 96, 97 
of facial nerve, 123 
field. See Sensory root field, 
of glossopharyngeal nerve, 123 
of hypoglossal nerve, 123 
mesencephalic, N. V. 155, 156 
of oculomotor nerve, 130 
posterior, spinal. See Root, dorsal, 
spinal, 78 

of trigeminal nerve, 124, 125 
of trochlear nerve, 191 
of vagus nerve, 123 
ventral, 58, 76 

Rostrum of corpus callosum, 243 
Rudiment of hippocampus, 244, 267, 270 

SACCULE, 193 

Saccus vasculosus, 28, 29 

Scarpa, ganglion of. See Ganglion, vestibular. 

Schultze, comma-tract of, 97, 107 

Schwalbe, vestibular nucleus of. See Nucleus, 

medial vestibular. 

Schwann, sheath of. See Neurilemma. 
Sea-anemones, 17, 19 
Segmentation of spinal cord, 74 
Semicircular canals, 193 
Septomarginal bundle or fasciculus, 97, 107 
Sensation (or sensibility) of cold, 105, 306 

of hearing, 185, 186, 187, 309 

of heat, 105, 306 

muscular, 72, 99, 100, 311 

of pain, 68, 103, 105, 306 

of pressure, 303 

of sight, 225, 228 

of smell, 265 

of taste, 181 

of touch, 66, 77, 101, 303 

visceral, 336 



INDEX 



393 



Sensory root field, 59, 60 
Septum pellucidum, 243, 272 
posterior intermediate, 83 

median, 83 
posticum, 74 
Shark. See Dogfish. 
Sheath, glial, 86 

medullary. See Sheath, myelin. 
myelin, 41, 46, 47 
of Schwann. See Neurilemma. 
Sight, organs of, 225-228 
Smell, organs of, 265-282 
Solitary bundle. See Tractus solitarius. 
Somesthetic area, 292 
Speech, apparatus of, 295, 296 
Spider-cells, 86 
Spinal cord, 56, 72, 75 

cervical enlargement, 73, 79, 84 
characters of different regions, 83 
columns of gray matter, 79 

of white matter. See Funiculus. 
of cells. See Cell-columns. 
commissures, 80 
coverings, 73 
cornua. See Columns. 
degenerations from brain lesions, 105, 106 
from cord lesions, 105, 106 
from section of dorsal roots, 106 
development, 41, 42 
in fetus and infant, 77 
fissure, anterior median, 76 
funiculi, 82 
glial sheath, 86 

gray matter or substance, 78, 79, 80, 81, 87 
cell-columns, 89, 90 
columns, 79 
horns. See Columns. 
microscopic structure, 87 
nuclei. See Cell-columns. 
relation to size of nerves, 84 
horn. See Column. 
internal structure, 85 
lumbar enlargement, 74, 81, 84 
microscopic structure, 85 
relation to vertebral canal, 77 
reflex mechanism of, 91, 92, 93 
sacral region, 74, 81, 84 
segmentation, 74 
sulcus, anterolateral, 76 
posterior, 76 

intermediate, 76 
posterolateral, 76 
thoracic region, 80, 84 
tracts, 95-112, 110 
white matter (or substance), 81, 86 
area in different regions, 82 
microscopic structure, 86, 87 
ganglion. See Ganglion. 
nerve. See Nerve. 
Spiracle, 356 
Splanchnic nerves, 348 
Splenium corporis callosi, 244 
Spongioblasts, 37 
Stalk, optic, 32, 225 
of pineal body, 221 
of superior olive, 151, 175 
Stomach, innervation of, 353 
Stratum griseum centrale, 163 
of superior colliculus, 167 



Stratum lacunosum, 278 
lemnisci, 167 
lucidum, 279 
opticum, 167 
oriens, 279 
profundum, 166, 167 
radiatum, 279 
zonale of superior colliculus, 167 

of thalamus, 216 
Stria (or striae) acustica. See Stria medullares 

acustica. 

of Baillarger, 283 
of Gennari, 283 
longitudinalis lateralis, 245, 270 

medialis, 245, 270 
medullaris acustica, 123, 127, 186 

thalami, 215, 220, 281 
olfactoria lateralis, 266, 277 

medialis, 266 

semicircularis. See Stria terminalis. 
terminalis, 214, 281 
Stripe of Baillarger, 283 

of Gennari, 283 
Subarachnoid space, 73 
Subiculum, 277, 280 
Substantia alba, 42, 79, 86 
ferruginea, 128 
grisea, 42, 79, 87 

centralis, 136, 158, 163 
gelatinosa, Rolandi, 80 
centralis, 86 

externa. See Sheath, glial. 
nigra, 129, 158, 164 
perforata, anterior, 267, 282 

posterior, 115 

reticularis. See Reticular formation, 
alba, 144 
grisea, 145 
Subthalamic tegmental region. See Subthala- 

mus. 

Subthalamus, 222 
Sulcus (or sulci), anterior lateral, 76, 119 

parol factory, 239 
basilar, 124 

callosal. See Sulcus of corpus callosum. 
central, of Rolandi, 233 
cerebellar, 199 
cerebral, 233, 235, 236, 239 
cinguli, 239 
circularis insulae, 237 
of corpus callosum, 239 
cruciate, 114 
frontal, inferior, 235 
middle, 235 
superior, 235 
horizontalis cerebelli, 197 
hypothalamicus, 223 
insulae, 237 

intermedius, posterior, 76, 127 
intra parietal, 236 
lateral, of mesencephalon, 130 
lateralis, anterior. See Sulcus, anterior lateral. 

posterior. See Sulcus, posterior lateral, 
limitans, 34, 42, 129 

insulae. See Sulcus circularis insulae. 
lunatus, 237 

median us posterior of spinal cord, 76 
of medulla oblongata, 119 
occipitalis transversus, 236 



394 



INDEX 



Sulcus of oculomotor nerve, 130 
olfactory, 241 
orbital, 241 
paracentral, 239 
parolfactorius, anterior, 239 

posterior, 267, 239 
postcentral, inferior, 236 

superior, 236 
postclivalis, 197 
posterior lateral, 76, 119 
parolfactory, 239, 267 
precentral, 235 
inferior, 235 
superior, 235 
prepyramidal, 202 
primarius. See Fissura prima. 
rhinalis. See Fissure, rhinal. 
of spinal cord, 76 
subparietal, 239 
temporal, inferior, 236 
middle, 236 
superior, 236 
uvulo-nodularis, 203 
Sylvius, aqueduct of, 26, 158 

fissure of, 233 
Sympathetic ganglia. See Ganglion. 

system, 50, 57, 334 
Synapse, 49, 50, 51, 55 
Syncytium, 38 
System. See Nervous system. 

TACTILE corpuscles, 68 
Taenia chorioidea, 214 

of fourth ventricle, 126 

pontis. See Fila lateralia pontis. 

tecti. See Stria longitudinalis lateralis. 

thalami, 214, 224 

ventriculi quarti, 126 
Tapetum, 245 
Taste, apparatus of, 181 
Tectum mesencephali, 28, 165 
Tegmentum, 129, 158 
Tela chorioidea of fourth ventricle, 128 

of third ventricle, 215, 224 
Telencephalon, 36 

development, 25, 31, 32, 33 

in the dogfish, 27, 28 

medium, 212, 229 

Temperature, apparatus of, 105, 306 
Tendon, nerve endings in, 72 
Tentorium cerebelli, 113 
Thalamencephalon. See Diencephalon. 
Thalamus, 213 

development, 35, 213 

in the dogfish, 29 

ending of sensory tracts in, 219 

lamina, external medullary, 216 
internal medullary, 216 

new, 219 

nuclei, 217 

old, 218 

pulvinar, 218 

radiation of, 216, 217, 260, 263 

stalks, 263 

stratum zonale, 216 

thalamocortical fibers, 263 

tubercle, anterior, 213 
Tigroid bodies. See Nissl bodies. 
Tonsil (tonsilla cerebelli), 199 



Touch, apparatus of, 66, 71, 101, 303 
Tract or tracts, 95. (See also Bundle and Fas- 
ciculus.) 

bulbospinal, 111 

of Burdach. See Fasciculus cuneatus. 
central sensory. See Path. 
cerebellobulbar. See Tract, fastigiobulbar. 
cerebellotegmental, 211, 212 
comma, 97, 107 
corticobulbar; 165, 260, 321 
corticopontine, 147, 164. (See also Tracts, 

frpntopontine and temporopontine.) 
corticorubral, 161, 260 
corticospinal, 109, 133, 136, 147, 165, 260, 320 

lateral, 109, 134, 136 

ventral, 134, 136 
corticothalamic, 263 
direct cerebellar. See Tract, dorsal spinocere- 

bellar. 

dorsal spinocerebellar, 110, 143, 144, 145, 205 
efferent, from cerebellum, 211 

from cerebral hemisphere, 297 

from mesencephalon. See Tracts, tecto- 

spinal, tectobulbar, and rubrospinal. 
fastigiobulbar, 212 

of Flechsig. See Tract, dorsal spinocerebellar. 
frontal olfactory projection, 281 
frontopontine, 164, 259 
of Goll. See Fasciculus gracilis. 
of Gowers. See Tract, ventral spinocerebellar. 
habenulo-peduncular. See Fasciculus retro- 

flexus. 

of Helweg. See Tract, bulbospinal. 
lateralis minor. See Fasciculus lateralis minor, 
of Lissauer. See Fasciculus dorsolateralis. 
mamillotegmental, 222, 281 
mamillothalamic, 217, 222 
mesencephalic, of N. V. See Root, mesen- 

cephalic, N. V. 

of Meynert. See Fasciculus retroflexus. 
of Monakow. See Tract, rubrospinal. 
nucleocerebellar, 205 
olfactory, 265, 277 

olivocerebellar. See Fibers, olivocerebellar. 
olivospinal. See Tract, bulbospinal. 
optic, 226 

pontocerebellar. See Brachium pontis. 
pontospinal. See Tract, reticulospinal. 
predorsal. See Tract, tectospinal. 
prepyramidal. See Tract, rubrospinal. 
projection, 297 
pyramidal, 109 

aberrant, 321 

direct, 109 

crossed, 109 

uncrossed lateral, 320 
reticulospinal, 160 
rubroreticular, 160, 161 
rubrospinal, of Monakow, 110, 145, 161 
of Schultze, 107 
septomarginal, 97, 107 
solitariospinalis, 330 
solitary (solitarius), 132, 181, 330 
spinal, of N. V., 132, 136, 144 
of spinal cord, 94-112, 110 
spinocerebellar, dorsal, 100, 314 

ventral, 100, 313 
spino-olivary, 105 
spinotectal, 105, 145 



INDEX 



395 



Tract, spinothalamic, 145, 163, 219, 307 
lateral, 102 
ventral, 101, 305 

strionigral, 164, 263 

sulcomarginal, 108 

tectobulbar, 161, 167 

tectocerebellar, 206 

tectospinal, 111, 145, 161, 167 

tegmentospinal. See Tract, reticulospinal. 

temporopontine, 164, 261 

thalamocortical, 263 

thalamo-olivary, 145, 219 

thalamospinal, 219 

transverse peduncular, 369 

trigeminothalamic, 183, 185 

ventral spinocerebellar, 100, 144, 145, 157, 206 

vestibulocerebellar, 190, 206 

vestibulospinal, 111, 190, 329 

of Vicq d'Azyr. See Tract, mamillothalamic. 
Trapezium. See Trapezoid body. 
Trapezoid body, 121, 150, 186 
Triangle of Gombault and Philippe, 107 
Trigone (or trigonum) acustici. See Area 
acustica. 

collateral, 248 

habenulae, 220 

hypoglossi, 127 

interpedunculare. See Fossa interpeduncula- 
ris. 

olfactory, 266 

vagi. See Ala cinerea. 
Trophic unity of neuron, 51 
Truncus corporis callosi, 244 
Trunk, sympathetic, 335, 346, 347, 348 
Tuber vermis, 198, 201 

Tubercle (or tuberculum) acusticum. See Nu- 
cleus, dorsal cochlear. 

anterior, of thalamus, 213 

cinereum, 122, 280 

cuneate, 121, 137 

olfactorium, 268, 282 

of Rolando. See Tuberculum cinereum. 
Tufted cells, 276 
Tiirck's bundle. See Tract, ventral cortico- 

spinal. 

UNCUS, 240, 269, 277 
Utricle, 193 
Uvula vermis, 198 



VALLECULA of cerebellum, 197 

Valve of Vieussens. See Velum, anterior med- 
ullary. 

Velum, anterior medullary, 125, 128, 155 
anticum. See Velum, anterior medullary, 
interpositum. See Tela chorioidea of third 

ventricle, 
medullare, anterius, 125 

inferius. See Velum medullare, poste- 

rius. 

posterius, 202 

superius. See Velum, anterior medullary, 
transversum, 29, 31 

Vena terminalis, 214 

Ventricle (or ventricles) of the brain, 25, 26, 27, 

117 

development, 26, 33, 34 
in the dogfish, 27, 28, 30, 31 
fourth, 26, 118, 125, 126, 127, 128 
lateral, 26, 118, 246 
third, 26, 118, 223 

Ventriculus lateralis, 26, 246 
terminalis, 81 
tertius. See Ventricle, third. 

Vermis, inferior, 197, 198 
superior, 197 

Vesicles, cerebral, primary, 24, 25 
optic, 225 

Vestibular apparatus, 188, 189, 190 

Vicq d'Azyr, bundle of. See Tract, mamillo- 
thalamic. 

Vieussens, valve of. See Velum, anterior med- 
ullary. 

Visceral innervation, 335 

Visual apparatus, 225 
receptive center, 292 

Visuo-psychic area, 293, 294 

Vomiting, mechanism of, 331 

WALLERIAN degeneration, 105, 106, 107 
Weight of brain, 301 

Worms, nervous system of, 19, 20, 21, 22 
Wrisberg, nerve of. See Nervus intermedius. 

ZONE, cortical. See Center, cortical, 
ependymal, 37 
mantle, 37, 42, 196 
marginal, 37, 42, 196