- - - Caudate nucleus
Frontopontine trad
' Anterior thalamic radiation
zCorticobulbar 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 anterio
limb of internal capsul
Lentiform nucleus
Coronal fibers from posterior limb of
Coronal fibers from retro-
lenticular part of inter-
nal capsule
Coronal fibers from sublenticular part
Ansa peduncularis"'' f internal capsule
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 gyrus 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
261
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
gyms. 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
Anterior limb of inter-
nal capsule (can- ^ ,
date impression) "jS.
V
Posterior limb of internal capsule
\/ (thalamic impression)
/T
Frontal pole i%
Genu internal cap- ,*<
side
Anterior commissure
Optic tract
Temporal lobe
Tapetum
Optic radiation
Basis pedunculi
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
Thalamus
Parietal stalk of thalamus
Corticospinal tract
Insula
Claustrum
Putamen
Globus pallidus
Ansa peduncularis
Red nucleus
''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 lateralward 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
2 6 5
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
surface 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 -
MammiUary body
Cerebral peduncle
Inter peduncular fossa and
nucleus
Trigeminal nerve
Abducens nerve-
Acoustici Vestibular n -
nen>e (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
L.~ Amygdaloid nucleus
A
W Pyriform area
\ \ p- Hippocampal gyrus
f,'- Trochlear nerve
J.l-~ 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 later alls (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
stria.
THE RHINENCEPHALON
267
The medial olfactory gyrus and stria require further investigation. It has been gen-
erally supposed that the stria is forme'd 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 parolfact&ry sulcus
Amygdaloid nucleus
Medial olfactory gyrus (stria)
Olfactory tract
'Limen insula
Anterior 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 inner vation 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 tuberculum 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 pellucidim
Fornix .
Anterior commissure
Paraterminal body
Hippocampal rudiment .,
Olfactory trigone ,
Olfactory tract
Olfactory bulb^
Intermediate olfactory stria'' /
Lateral olfactory gyrus and stria-' ;
Anterior perforated substance'
Limen insulce
~ Hippocampus (gyri
Andreas Retzii)
Fascia dentata
"~- Fimbria of hippocampus
" Hippocampus (proper)
Hippocampus
"^ Hippocampal gyrus
N - Caiida 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 insula, 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.
Tapetum
Collateral trigone
Posterior horn of lateral ventricle
Hippocampal digitations
,- 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
Pyriform area
Olfactory bulb
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 ihefimbria. 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
I
Crus 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 columns 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 columnar 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