Fore-brain or Prosencephalon

The fore-brain or prosencephalon consists of: (1)
the diencephalon, corresponding in a large measure to the third
ventricle and the structures which bound it; and (2) the telencephalon,
comprising the largest part of the brain, viz., the cerebral hemispheres;
these hemispheres are intimately connected with each other across
the middle line, and each contains a large cavity, named the lateral
ventricle. The lateral ventricles communicate through the interventricular
foramen with the third ventricle, but are separated from each other
by a medial septum, the septum pellucidum; this contains a
slit-like cavity, which does not communicate with the ventricles.


FIG. 90715– Mesal
aspect of a brain sectioned in the median sagittal plane.
 The Diencephalon.—The diencephalon is connected above
and in front with the cerebral hemispheres; behind with the mid-brain.
Its upper surface is concealed by the corpus callosum, and is covered
by a fold of pia mater, named the tela chorioidea of the third ventricle;
inferiorly it reaches to the base of the brain.		   The diencephalon comprises: (1) the thalamencephalon;
(2) the pars mamillaris

hypothalami; and (3) the posterior part of the third ventricle.
For descriptive purposes, however, it is more convenient to consider
the whole of the third ventricle and its boundaries together; this
necessitates the inclusion, under this heading, of the pars optica
hypothalami and the corresponding part of the third ventricle—structures
which properly belong to the telencephalon.		 The Thalamencephalon.—The thalamencephalon comprises:
(1) the thalamus; (2) the metathalamus or corpora
geniculata; and (3) the epithalamus, consisting of the
trigonum habenulæ, the pineal body, and the posterior commissure.


FIG. 90716– Dissection
showing the ventricles of the brain.
   The Thalami (optic thalamus) (Figs.
90716, 90717) are two large ovoid masses,
situated one on either side of the third ventricle and reaching for
some distance behind that cavity. Each measures about 4 cm. in length,
and presents two extremities, an anterior and a posterior, and four
surfaces, superior, inferior, medial, and lateral.		   The anterior extremity is narrow; it lies close
to the middle line and forms the posterior boundary of the interventricular
foramen.		   The posterior extremity is expanded, directed
backward and lateralward, and overlaps the superior colliculus. Medially
it presents an angular prominence, the pulvinar, which is continued
laterally into an oval swelling, the lateral geniculate body,
while beneath the pulvinar, but separated from it by the superior
brachium, is a second oval swelling, the medial geniculate body.		   The superior surface is free, slightly convex,
and covered by a layer of white substance, termed the stratum zonale.
It is separated laterally from the caudate nucleus by a white band,
the stria terminalis, and by the terminal vein. It is divided
into a medial and a lateral portion by an oblique shallow furrow which
runs from behind forward and medialward and corresponds with the lateral
margin of the fornix; the lateral part forms a portion of the floor
of the lateral ventricle, and is

covered by the epithelial lining of this cavity; the medial part is
covered by the tela chorioidea of the third ventricle, and is destitute
of an epithelial covering. In front, the superior is separated from
the medial surface by a salient margin, the tænia thalami,
along which the epithelial lining of the third ventricle is reflected
on to the under surface of the tela chorioidea. Behind, it is limited
medially by a groove, the sulcus habenulæ, which intervenes
between it and a small triangular area, termed the trigonum habenulæ.		   The inferior surface rests upon and is continuous
with the upward prolongation of the tegmentum (subthalamic tegmental
region), in front of which it is related to the substantia
innominata of Meynert.


FIG. 90717– Coronal
section of brain immediately in front of pons.
   The medial surface constitutes the upper part
of the lateral wall of the third ventricle, and is connected to the
corresponding surface of the opposite thalamus by a flattened gray
band, the massa intermedia (middle or gray commissure).
This mass averages about 1 cm. in its antero-posterior diameter: it
sometimes consists of two parts and occasionally is absent. It contains
nerve cells and nerve fibers; a few of the latter may cross the middle
line, but most of them pass toward the middle line and then curve
lateralward on the same side.		   The lateral surface is in contact with a thick
band of white substance which forms the occipital part of the internal
capsule and separates the thalamus from the lentiform nucleus of the
corpus striatum.		 Structure.—The thalamus consists chiefly of gray substance,
but its upper surface is covered by a layer of white substance, named
the stratum zonale, and its lateral surface by a similar layer
termed the lateral medullary lamina. Its gray substance is
incompletely subdivided into three parts—anterior, medial, and
lateral—by a white layer, the medial medullary lamina.
The anterior part comprises the anterior tubercle, the medial part
lies next the lateral wall of the third ventricle while the lateral
and largest part is interposed between the medullary laminæ and
includes the pulvinar. The lateral part is traversed by numerous fibers
which radiate from the thalamus into the internal capsule, and pass
through the latter to the cerebral cortex. These three parts are built
up of numerous nuclei, the connections of many of which are imperfectly
known.


FIG. 90718– Coronal
section of brain through intermediate mass of third ventricle.
 Connections.—The thalamus may be regarded as a large
ganglionic mass in which the ascending tracts of the tegmentum and
a considerable proportion of the fibers of the optic tract end, and
from the cells of which numerous fibers (thalamocortical) take origin,
and radiate to almost every part of the cerebral cortex. The lemniscus,
together with the other longitudinal strands of the tegmentum, enters
its ventral part: the thalamomammillary fasciculus (bundle
of Vicq d’Azyr), from the corpus mammillare, enters in its
anterior tubercle, while many of the fibers of the optic tract terminate
in its posterior end. The thalamus also receives numerous fibers (corticothalamic)
from the cells of the cerebral cortex. The fibers that arise from

the cells of the thalamus form four principal groups or stalks: (a)
those of the anterior stalk pass through the frontal part of
the internal capsule to the frontal lobe; (b) the fibers of
the posterior stalk (optic radiations) arise in the
pulvinar and are conveyed through the occipital part of the internal
capsule to the occipital lobe; (c) the fibers of the inferior
stalk leave the under and medial surfaces of the thalamus, and
pass beneath the lentiform nucleus to the temporal lobe and insula;
(d) those of the parietal stalk pass from the lateral
nucleus of the thalamus to the parietal lobe. Fibers also extend from
the thalamus into the corpus striatum—those destined for the
caudate nucleus leave the lateral surface, and those for the lentiform
nucleus, the inferior surface of the thalamus.


FIG. 90719– Hind-
and mid-brains; postero-lateral view.
   The Metathalamus (Fig. 90719) comprises the geniculate bodies, which are two in
number—a medial and a lateral—on each side.		   The medial geniculate body (corpus geniculatum
mediale; internal geniculate body; postgeniculatum) lies under
cover of the pulvinar of the thalamus and on the lateral aspect of
the corpora quadrigemina. Oval in shape, with its long axis directed
forward and lateralward, it is lighter in color and smaller in size
than the lateral. The inferior brachium from the inferior colliculus
disappears under cover of it while from its lateral extremity a strand
of fibers passes to join the optic tract. Entering it are many acoustic
fibers from the lateral lemniscus. The medial geniculate bodies are
connected with one another by the commissure of Gudden, which passes
through the posterior part of the optic chiasma.		   The lateral geniculate body (corpus geniculatum
laterale; external geniculate body; pregeniculatum) is an oval
elevation on the lateral part of the posterior end of the thalamus,
and is connected with the superior colliculus by the superior brachium.
It is of a dark color, and presents a laminated arrangement consisting
of alternate layers of gray and white substance. It receives numerous
fibers from the optic tract, while other fibers of this tract pass
over or through it into the pulvinar. Its cells are large and pigmented;
their axons pass to the visual area in the occipital part of the cerebral
cortex.		   The superior colliculus, the pulvinar, and the lateral
geniculate body receive many fibers from the optic tracts, and are
therefore intimately connected with sight, constituting what are termed
the lower visual centers. Extirpation of the eyes in a newly
born animal entails an arrest of the development of these centers,
but has no effect on the medial geniculate bodies or on the inferior
colliculi. Moreover, the latter are well-developed in the mole, an
animal in which the superior colliculi are rudimentary.		   The Epithalamus comprises the trigonum habenulæ,
the pineal body, and the posterior commissure.		   The trigonum habenulæ is a small depressed
triangular area situated in front of the superior colliculus and on
the lateral aspect of the posterior part of the tænia thalami.
It contains a group of nerve cells termed the ganglion habenulæ.
Fibers enter it from the stalk of the pineal body, and others, forming
what is termed the habenular commissure, pass across the middle
line to the corresponding ganglion of the opposite side. Most of its
fibers are, however, directed downward and form a bundle, the fasciculus
retroflexus of Meynert, which passes medial to the red nucleus,
and, after decussating with the corresponding fasciculus of the opposite
side, ends in the interpeduncular ganglion.		   The pineal body (corpus pineale; epiphysis)
is a small, conical, reddish-gray body which lies in the depression
between the superior colliculi. It is placed beneath the splenium
of the corpus callosum, but is separated from this by the tela chorioidea
of the third ventricle, the lower layer of which envelops it. It measures
about 8 mm. in length, and its base, directed forward, is attached
by a stalk or peduncle of white substance. The stalk of the
pineal body divides anteriorly into two laminæ, a dorsal and
a ventral, separated from one another by the pineal recess of the
third ventricle. The ventral lamina is continuous with the posterior
commissure; the dorsal lamina is continuous with the habenular commissure
and divides into two strands the medullary striæ, which run forward,
one on either side, along the junction of the medial and upper surfaces
of the thalamus to blend in front with the columns of the fornix.		   The posterior commissure is a rounded band of
white fibers crossing the middle line on the dorsal aspect of the
upper end of the cerebral aqueduct. Its fibers acquire their medullary
sheaths early, but their connections have not been definitely determined.
Most of them have their origin in a nucleus, the nucleus of the
posterior commissure (nucleus of Darkschewitsch), which
lies in the central gray substance of the upper end of the cerebral
aqueduct, in front of the nucleus of the oculomotor nerve. Some are
probably derived from the posterior part of the thalamus and from
the superior colliculus, while others are believed to be continued
downward into the medial longitudinal fasciculus.		   The Hypothalamus (Fig. 90720) includes the subthalamic tegmental region and the
structures forming the greater part of the floor of the third ventricle,
viz., the corpora mammillaria, tuber cinereum, infundibulum, hypophysis,
and optic chiasma.		   The subthalamic tegmental region consists of
the upward continuation of the tegmentum; it lies on the ventro-lateral
aspect of the thalamus and separates it from the fibers of the internal
capsule. The red nucleus and the substantia nigra are prolonged into
its lower part; in front it is continuous with the substantia innominata
of Meynert, medially with the gray substance of the floor of the third
ventricle.		   It consists from above downward of three strata: (1)
stratum dorsale, directly applied to the under surface of the
thalamus and consisting of fine longitudinal fibers; (2) zona incerta,
a continuation forward of the formatio reticularis of the tegmentum;
and (3) the corpus subthalamicum (nucleus of Luys),
a brownish mass presenting a lenticular shape on transverse section,
and situated on the dorsal aspect of the fibers of the base of the
cerebral peduncle; it is encapsuled by a lamina

of nerve fibers and contains numerous medium-sized nerve cells, the
connections of which are as yet not fully determined.		   The corpora mammillaria (corpus albicantia)
are two round white masses, each about the size of a small pea, placed
side by side below the gray substance of the floor of the third ventricle
in front of the posterior perforated substance. They consist of white
substance externally and of gray substance internally, the cells of
the latter forming two nuclei, a medial of smaller and a lateral
of larger cells. The white substance is mainly formed by the fibers
of the columns of the fornix, which descend to the base of the brain
and end partly in the corpora mammillaria. From the cells of the gray
substance of each mammillary body two fasciculi arise: one, the thalamomammillary
fasciculus (bundle of Vicq d’Azyr), passes upward
into the anterior nucleus of the thalamus; the other is directed downward
into the tegmentum. Afferent fibers are believed to reach the corpus
mammillare from the medial lemniscus and from the tegmentum.


FIG. 90720– Median
sagittal section of brain. The relations of the pia mater are indicated
by the red color.
   The tuber cinereum is a hollow eminence of gray
substance situated between the corpora mammillaria behind, and the
optic chiasma in front. Laterally it is continuous with the anterior
perforated substances and anteriorly with a thin lamina, the lamina
terminalis. From the under surface of the tuber cinereum a hollow
conical process, the infundibulum, projects downward and forward
and is attached to the posterior lobe of the hypophysis.		   In the lateral part of the tuber cinereum is a nucleus
of nerve cells, the basal optic nucleus of Meynert, while close
to the cavity of the third ventricle are three additional nuclei.
Between the tuber cinereum and the corpora mammillaria a small elevation,
with a corresponding depression in the third ventricle, is sometimes
seen. Retzius has named it the eminentia saccularis, and regards
it as a representative of the saccus vasculosus found in this situation
in some of the lower vertebrates.		   The hypophysis (pituitary body) (Fig. 90721) is a reddish-gray, somewhat oval mass, measuring about 12.5
mm. in its transverse, and about 8 mm. in its antero-posterior diameter.
It is attached to the end of the infundibulum, and is situated in
the fossa hypophyseos of the sphenoidal bone, where it is retained
by a circular fold of dura mater, the diaphragma sella; this
fold almost completely roofs in the fossa, leaving only a small central
aperture through which the infundibulum passes.


FIG. 90721– The
hypophysis cerebri, in position. Shown in sagittal section.
 Optic Chiasma (chiasma opticum; optic commissure).—The
optic chiasma is a flattened, somewhat quadrilateral band of fibers,
situated at the junction of the floor and anterior wall of the third
ventricle. Most of its fibers have their origins in the retina, and
reach the chiasma through the optic nerves, which are continuous with
its antero-lateral angles. In the chiasma, they undergo a partial
decussation (Fig. 90722); the fibers from
the nasal half of the retina decussate and enter the optic tract of
the opposite side, while the fibers from the temporal half of the
retina do not undergo decussation, but pass back into the optic tract
of the same side. Occupying the posterior part of the commissure,
however, is a strand of fibers, the commissure of Gudden, which
is not derived from the optic nerves; it forms a connecting link between
the medial geniculate bodies.		 Optic Tracts.—The optic tracts are continued backward
and lateralward from the postero-lateral angles of the optic chiasma.
Each passes between the anterior perforated substance and the tuber
cinereum, and, winding around the ventrolateral aspect of the cerebral
peduncle, divides into a medial and a lateral root. The former comprises
the fibers of Gudden’s commissure. The lateral root consists
mainly of afferent fibers which arise in the retina and undergo partial
decussation in the optic chiasma, as described; but it also contains
a few fine efferent fibers which have their origins in the brain and
their terminations in the retina. When traced backward, the afferent
fibers of the lateral root are found to end in the lateral geniculate
body and pulvinar of the thalamus, and in the superior colliculus;
and these three structures constitute the lower visual centers.
Fibers arise from the nerve cells in these centers and pass through
the occipital part of the internal capsule, under the name of the
optic radiations, to the cortex of the occipital lobe of the
cerebrum, where the higher or cortical visual center
is situated. Some of the fibers of the optic radiations take an opposite
course, arising from the cells of the occipital cortex and passing
to the lower visual centers. Some fibers are detached from the optic
tract, and pass through the cerebral peduncle to the nucleus of the
oculomotor nerve. These may be regarded as the afferent branches for
the

Sphincter pupillæ and Ciliaris muscles. Other fibers have been
described as reaching the cerebellum through the superior peduncle;
while others, again, are lost in the pons.		 The Third Ventricle (ventriculus tertius) (Figs.
90716, 90720).—The third ventricle
is a median cleft between the two thalami. Behind, it communicates
with the fourth ventricle through the cerebral aqueduct, and in front
with the lateral ventricles through the interventricular foramen.
Somewhat triangular in shape, with the apex directed backward, it
has a roof, a floor, an anterior and a posterior
boundary and a pair of lateral walls.


FIG. 90722– Scheme
showing central connections of the optic nerves and optic tracts.
   The roof (Fig. 90723)
is formed by a layer of epithelium, which stretches between the upper
edges of the lateral walls of the cavity and is continuous with the
epithelial lining of the ventricle. It is covered by and adherent
to a fold of pia mater, named the tela chorioidea of the third ventricle,
from the under surface of which a pair of vascular fringed processes,
the choroid plexuses of the third ventricle, project downward,
one on either side of the middle line, and invaginate the epithelial
roof into the ventricular cavity.		   The floor slopes downward and forward and is
formed mainly by the structures which constitute the hypothalamus:
from before backward these are: the optic

chiasma, the tuber cinereum and infundibulum, and the corpora mammillaria.
Behind the last, the floor is formed by the interpeduncular fossa
and the tegmenta of the cerebral peduncles. The ventricle is prolonged
downward as a funnel-shaped recess, the recessus infundibuli,
into the infundibulum, and to the apex of the latter the hypophysis
is attached.		   The anterior boundary is constituted below by
the lamina terminalis, a thin layer of gray substance stretching
from the upper surface of the optic chiasma to the rostrum of the
corpus callosum; above by the columns of the fornix and the anterior
commissure. At the junction of the floor and anterior wall, immediately
above the optic chiasma, the ventricle presents a small angular recess
or diverticulum, the optic recess. Between the columns of the
fornix, and above the anterior commissure, is a second recess termed
the vulva. At the junction of the roof and anterior wall of
the ventricle, and situated between the thalami behind and the columns
of the fornix in front, is the interventricular foramen (foramen
of Monro) through which the third communicates with the lateral
ventricles.


FIG. 90723– Coronal
section of lateral and third ventricles. (Diagrammatic.)
   The posterior boundary is constituted by the
pineal body, the posterior commissure and the cerebral aqueduct. A
small recess, the recessus pinealis, projects into the stalk
of the pineal body, while in front of and above the pineal body is
a second recess, the recessus suprapinealis, consisting of
a diverticulum of the epithelium which forms the ventricular roof.		   Each lateral wall consists of an upper portion
formed by the medial surface of the anterior two-thirds of the thalamus,
and a lower consisting of an upward continuation of the gray substance
of the ventricular floor. These two parts correspond to the alar and
basal laminæ respectively of the lateral wall of the fore-brain
vesicle and are separated from each other by a furrow, the sulcus
of Monro, which extends from the interventricular foramen to the
cerebral aqueduct (pages 741 and 742). The lateral wall is limited
above by the tænia thalami. The columns of the fornix curve downward
in front of the interventricular foramen, and then run in the lateral
walls of the ventricle, where, at first, they form distinct prominences,
but subsequently are lost to sight. The lateral walls are joined to
each other across the cavity of the ventricle by a band of gray matter,
the massa intermedia (page 809).		 Interpeduncular Fossa (Fig. 90724).—This
is a somewhat lozenge-shaped area of the base of the brain, limited
in front by the optic chiasma, behind by the antero-superior surface
of the pons, antero-laterally by the converging optic tracts,

and postero-laterally by the diverging cerebral peduncles. The structures
contained in it have already been described; from behind forward,
they are the posterior perforated substance, corpora mamillaria, tuber
cinereum, infundibulum, and hypophysis.


FIG. 90724– Base
of brain.
 The Telencephalon.—The telencephalon includes: (1) the
cerebral hemispheres with their cavities, the lateral ventricles;
and (2) the pars optica hypothalami and the anterior portion
of the third ventricle (already described under the diencephalon).
As previously stated (see page 744), each cerebral hemisphere may
be divided into three fundamental parts, viz., the rhinencephalon,
the corpus striatum, and the neopallium. The rhinencephalon, associated
with the sense of smell, is the oldest part of the telencephalon,
and forms almost the whole of the hemisphere in some of the lower
animals, e. g., fishes, amphibians, and reptiles. In man it
is rudimentary, whereas the neopallium undergoes great development
and forms the chief part of the hemisphere.		 The Cerebral Hemispheres.—The cerebral hemispheres constitute
the largest part of the brain, and, when viewed together from above,
assume the form of an ovoid mass broader behind than in front, the
greatest transverse diameter corresponding with a line connecting
the two parietal eminences. The hemispheres are separated medially
by a deep cleft, named the longitudinal cerebral fissure, and
each possesses a central cavity, the lateral ventricle.		   The Longitudinal Cerebral Fissure (fissura
cerebri longitudinalis; great longitudinal fissure) contains a
sickle-shaped process of dura mater, the falx cerebri. It front
and behind, the fissure extends from the upper to the under surfaces
of the hemispheres and completely separates them, but its middle portion
separates them for only about one-half of their vertical extent; for
at this part they are connected across the middle line by a great
central white commissure, the corpus callosum.		   In a median sagittal section (Fig. 90720) the cut corpus callosum presents the appearance of a broad,
arched band. Its thick posterior end, termed the splenium,
overlaps the mid-brain, but is separated from it by the tela chorioidea
of the third ventricle and the pineal body. Its anterior curved end,
termed the genu, gradually tapers into a thinner portion, the
rostrum, which is continued downward and backward in front
of the anterior commissure to join the lamina terminalis. Arching
backward from immediately behind the anterior commissure to the under
surface of the splenium is a second white band named the fornix:
between this and the corpus callosum are the laminæ and cavity
of the septum pellucidum.		 Surfaces of the Cerebral Hemispheres.—Each hemisphere
presents three surfaces: lateral, medial, and inferior.		   The lateral surface is convex in adaptation to
the concavity of the corresponding half of the vault of the cranium.
The medial surface is flat and vertical, and is separated from
that of the opposite hemisphere by the great longitudinal fissure
and the falx cerebri. The inferior surface is of an irregular
form, and may be divided into three areas: anterior, middle, and posterior.
The anterior area, formed by the orbital surface of the frontal lobe,
is concave, and rests on the roof of the orbit and nose; the middle
area is convex, and consists of the under surface of the temporal
lobe: it is adapted to the corresponding half of the middle cranial
fossa. The posterior area is concave, directed medialward as well
as downward, and is named the tentorial surface, since it rests
upon the tentorium cerebelli, which intervenes between it and the
upper surface of the cerebellum.


FIG. 90725– Lateral
surface of left cerebral hemisphere, viewed from above.
   These three surfaces are separated from each other by
the following borders: (a) supero-medial, between
the lateral and medial surfaces; (b) infero-lateral,
between the lateral and inferior surfaces; the anterior part of this
border separating the lateral from the orbital surface, is known as
the superciliary border; (c) medial occipital,
separating the medial and tentorial surfaces; and (d) medial
orbital, separating the orbital from the medial surface. The anterior
end of the hemisphere is named the frontal pole; the posterior,
the occipital pole; and the anterior end of the temporal lobe,
the temporal pole. About 5 cm. in front of the occipital pole
on the infero-lateral border is an indentation or notch, named the
preoccipital notch.		   The surfaces of the hemispheres are moulded into a number
of irregular eminences,

named gyri or convolutions, and separated by furrows
termed fissures and sulci. The furrows are of two kinds,
complete and incomplete. The former appear early in
fetal life, are few in number, and are produced by infoldings of the
entire thickness of the brain wall, and give rise to corresponding
elevations in the interior of the ventricle. They comprise the hippocampal
fissure, and parts of the calcarine and collateral fissures. The incomplete
furrows are very numerous, and only indent the subjacent white substance,
without producing any corresponding elevations in the ventricular
cavity.		   The gyri and their intervening fissures and the sulci
are fairly constant in their arrangement; at the same time they vary
within certain limits, not only in different individuals, but on the
two hemispheres of the same brain. The convoluted condition of the
surface permits of a great increase of the gray matter without the
sacrifice of much additional space. The number and extent of the gyri,
as well as the depth of the intervening furrows, appear to bear a
direct relation to the intellectual powers of the individual.		   Certain of the fissures and sulci are utilized for the
purpose of dividing the hemisphere into lobes, and are therefore termed
interlobular; included under this category are the lateral
cerebral, parietoöccipital, calcarine, and collateral fissures,
the central and cingulate sulci, and the sulcus circularis.


FIG. 90726– Lateral
surface of left cerebral hemisphere, viewed from the side.
   The Lateral Cerebral Fissure (fissura cerebri
lateralis [Sylvii]; fissure of Sylvius) (Fig. 90726) is a well-marked cleft on the inferior and lateral surfaces
of the hemisphere, and consists of a short stem which divides into
three rami. The stem is situated on the base of the brain,
and commences in a depression at the lateral angle of the anterior
perforated substance. From this point it extends between the anterior
part of the temporal lobe and the orbital surface of the frontal lobe,
and reaches the lateral surface of the hemisphere. Here it divides
into three rami: an anterior horizontal, an anterior ascending, and
a posterior. The anterior horizontal ramus passes foward for
about 2.5 cm. into the inferior frontal gyrus, while the anterior
ascending ramus extends upward into the same convolution for about
an equal distance. The posterior ramus is the longest; it runs
backward and slightly upward for about 7 cm., and ends by an upward
inflexion in the parietal lobe.		   The Central Sulcus (sulcus centralis [Rolandi];
fissure of Rolando; central fissure)

(Figs. 90725, 90726)
is situated about the middle of the lateral surface of the hemisphere,
and begins in or near the longitudinal cerebral fissure, a little
behind its mid-point. It runs sinuously downward and forward, and
ends a little above the posterior ramus of the lateral fissure, and
about 2.5 cm. behind the anterior ascending ramus of the same fissure.
It described two chief curves: a superior genu with its concavity
directed forward, and an inferior genu with its concavity directed
backward. The central sulcus forms an angle opening forward of about
70° with the median plane.		 The Parietoöccipital Fissure (fissura parietoöccipitalis).—Only
a small part of this fissure is seen on the lateral surface of the
hemisphere, its chief part being on the medial surface.		   The lateral part of the parietoöccipital
fissure (Fig. 90726) is situated about 5
cm. in front of the occipital pole of the hemisphere, and measures
about 1.25 cm. in length.


FIG. 90727– Medial
surface of left cerebral hemisphere.
   The medial part of the parietoöccipital
fissure (Fig. 90727) runs downward and forward
as a deep cleft on the medial surface of the hemisphere, and joins
the calcarine fissure below and behind the posterior end of the corpus
callosum. In most cases it contains a submerged gyrus.		   The Calcarine Fissure (fissura calcarina)
(Fig. 90727) is on the medial surface of
the hemisphere. It begins near the occipital pole in two converging
rami, and runs forward to a point a little below the splenium of the
corpus callosum, where it is joined at an acute angle by the medial
part of the parietoöccipital fissure. The anterior part of this
fissure gives rise to the prominence of the calcar avis in
the posterior cornu of the lateral ventricle.		   The Cingulate Sulcus (sulcus cinguli; callosomarginal
fissure) (Fig. 90727) is on the medial
surface of the hemisphere; it begins below the anterior end of the
corpus callosum and runs upward and forward nearly parallel to the
rostrum of this body and, curving in front of the genu, is continued
backward above the corpus callosum, and finally ascends to the supero-medial
border of the hemisphere a short distance behind the upper end of
the central sulcus. It separates the superior frontal from the cingulate
gyrus.		   The Collateral Fissure (fissura collateralis)
(Fig. 90727) is on the tentorial surface
of the hemisphere and extends from near the occipital pole to within
a short distance of the temporal pole. Behind, it lies below and lateral
to the calcarine fissure,

from which it is separated by the lingual gyrus; in front, it is situated
between the hippocampal gyrus and the anterior part of the fusiform
gyrus.		   The Sulcus Circularis (circuminsular fissure)
(Fig. 90731) is on the lower and lateral
surfaces of the hemisphere: it surrounds the insula and separates
it from the frontal, parietal, and temporal lobes.		 Lobes of the Hemispheres.—By means of these fissures
and sulci, assisted by certain arbitrary lines, each hemisphere is
divided into the following lobes: the frontal, the parietal,
the temporal, the occipital, the limbic, and
the insula.		 Frontal Lobe (lobus frontalis).—On the lateral
surface of the hemisphere this lobe extends from the frontal pole
to the central sulcus, the latter separating it from the parietal
lobe. Below, it is limited by the posterior ramus of the lateral fissure,
which intervenes between it and the central lobe. On the medial surface,
it is separated from the cingulate gyrus by the cingulate sulcus;
and on the inferior surface, it is bounded behind by the stem of the
lateral fissure.


FIG. 90728– Principal
fissures and lobes of the cerebrum viewed laterally.
   The lateral surface of the frontal lobe (Fig. 90726) is tranversed by three sulci which divide it into four gyri:
the sulci are named the precentral, and the superior and inferior
frontal; the gyri are the anterior central, and the superior, middle,
and inferior frontal. The precentral sulcus runs parallel to
the central sulcus, and is usually divided into an upper and a lower
part; between it and the central sulcus is the anterior central
gyrus. From the precentral sulcus, the superior and inferior
frontal sulci run forward and downward, and divide the remainder
of the lateral surface of the lobe into three parallel gyri, named,
respectively the superior, middle, and inferior frontal
gyri.		   The anterior central gyrus (gyrus centralis
anterior; ascending frontal convolution; precentral gyre) is bounded
in front by the precentral sulcus, behind by the central sulcus; it
extends from the supero-medial border of the hemisphere to the posterior
ramus of the lateral fissure.		   The superior frontal gyrus (gyrus frontalis
superior; superfrontal gyre) is situated above the superior frontal
sulcus and is continued on to the medial surface of the hemisphere.
The portion on the lateral surface of the hemisphere is usually more
or less completely subdivided into an upper and a lower part by an
antero-posterior

sulcus, the paramedial sulcus, which, however, is frequently
interrupted by bridging gyri.		   The middle frontal gyrus (gyrus frontalis
medius; medifrontal gyre), between the superior and inferior frontal
sulci, is continuous with the anterior orbital gyrus on the inferior
surface of the hemisphere; it is frequently subdivided into two by
a horizontal sulcus, the medial frontal sulcus of Eberstaller,
which ends anteriorly in a wide bifurcation.		   The inferior frontal gyrus (gyrus frontalis
inferior; subfrontal gyre) lies below the inferior frontal sulcus,
and extends forward from the lower part of the precentral sulcus;
it is continuous with the lateral and posterior orbital gyri on the
under surface of the lobe. It is subdivided by the anterior horizontal
and ascending rami of the lateral fissure into three parts, viz.,
(1) the orbital part, below the anterior horizontal ramus of
the fissure; (2) the triangular part (cap of Broca),
between the ascending and horizontal rami; and (3) the basilar
part, behind the anterior ascending ramus. The left inferior frontal
gyrus is, as a rule, more highly developed than the right, and is
named the gyrus of Broca, from the fact that Broca described
it as the center for articulate speech.		   The inferior or orbital surface of the
frontal lobe is concave, and rests on the orbital plate of the frontal
bone (Fig. 90729). It is divided into four
orbital gyri by a well-marked H-shaped orbital sulcus. These
are named, from their position, the medial, anterior, lateral,
and posterior orbital gyri. The medial orbital gyrus presents
a well-marked antero-posterior sulcus, the olfactory sulcus,
for the olfactory tract; the portion medial to this is named the straight
gyrus, and is continuous with the superior frontal gyrus on the
medial surface.		   The medial surface of the frontal lobe is occupied
by the medial part of the superior frontal gyrus (marginal gyrus)
(Fig. 90727). It lies between the cingulate
sulcus and the supero-medial margin of the hemisphere. The posterior
part of this gyrus is sometimes marked off by a vertical sulcus, and
is distinguished as the paracentral lobule, because it is continuous
with the anterior and posterior central gyri.		 Parietal Lobe (lobus parietalis).—The parietal
lobe is separated from the frontal lobe by the central sulcus, but
its boundaries below and behind are not so definite. Posteriorly,
it is limited by the parietoöccipital fissure, and by a line
carried across the hemisphere from the end of this fissure toward
the preoccipital notch. Below, it is separated from the temporal lobe
by the posterior ramus of the lateral fissure, and by a line carried
backward from it to meet the line passing downward to the preoccipital
notch.


FIG. 90729– Orbital
surface of left frontal lobe.
   The lateral surface of the parietal lobe (Fig. 90726) is cleft by a well-marked furrow, the intraparietal sulcus
of Turner, which consists of an oblique and a horizontal portion.
The oblique part is named the postcentral sulcus, and commences
below, about midway between the lower end of the central sulcus and
the upturned end of the lateral fissure. It runs upward and backward,
parallel to the central sulcus, and is sometimes divided into an upper
and a lower ramus. It forms the hinder limit of the posterior
central gyrus.		   From about the middle of the postcentral sulcus, or
from the upper end of its inferior ramus, the horizontal portion
of the intraparietal sulcus is carried backward and slightly upward
on the parietal lobe, and is prolonged, under the name of the

occipital ramus, on to the occipital lobe, where it divides
into two parts, which form nearly a right angle with the main stem
and constitute the transverse occipital sulcus. The part of
the parietal lobe above the horizontal portion of the intraparietal
sulcus is named the superior parietal lobule; the part below,
the inferior parietal lobule.		   The posterior central gyrus (gyrus centralis
posterior; ascending parietal convolution; postcentral gyre) extends
from the longitudinal fissure above to the posterior ramus of the
lateral fissure below. It lies parallel with the anterior central
gyrus, with which it is connected below, and also, sometimes, above,
the central sulcus.		   The superior parietal lobule (lobulus parietalis
superior) is bounded in front by the upper part of the postcentral
sulcus, but is usually connected with the posterior central gyrus
above the end of the sulcus; behind it is the lateral part of the
parietoöccipital fissure, around the end of which it is joined
to the occipital lobe by a curved gyrus, the arcus parietoöccipitalis;
below, it is separated from the inferior parietal lobule by the horizontal
portion of the intraparietal sulcus.		   The inferior parietal lobule (lobulus parietalis
inferior; subparietal district or lobule) lies below the horizontal
portion of the intraparietal sulcus, and behind the lower part of
the postcentral sulcus. It is divided from before backward into two
gyri. One, the supramarginal, arches over the upturned end
of the lateral fissure; it is continuous in front with the postcentral
gyrus, and behind with the superior temporal gyrus. The second, the
angular, arches over the posterior end of the superior temporal
sulcus, behind which it is continuous with the middle temporal gyrus.		   The medial surface of the parietal lobe (Fig. 90727) is bounded behind by the medial part of the parietoöccipital
fissure; in front, by the posterior end of the cingulate sulcus; and
below, it is separated from the cingulate gyrus by the subparietal
sulcus. It is of small size, and consists of a square-shaped convolution,
which is termed the precuneus or quadrate lobe.		 Occipital Lobe (lobus occipitalis).—The occipital
lobe is small and pyramidal in shape; it presents three surfaces:
lateral, medial, and tentorial.		   The lateral surface is limited in front by the
lateral part of the parietoöccipital fissure, and by a line carried
from the end of this fissure to the preoccipital notch; it is traversed
by the transverse occipital and the lateral occipital sulci. The transverse
occipital sulcus is continuous with the posterior end of the occipital
ramus of the intraparietal sulcus, and runs across the upper part
of the lobe, a short distance behind the parietoöccipital fissure.
The lateral occipital sulcus extends from behind forward, and
divides the lateral surface of the occipital lobe into a superior
and an inferior gyrus, which are continuous in front with the
parietal and temporal lobes. 125		   The medial surface of the occipital lobe is bounded
in front by the medial part of the parietoöccipital fissure,
and is traversed by the calcarine fissure, which subdivides it into
the cuneus and the lingual gyrus. The cuneus is a wedge-shaped area
between the calcarine fissure and the medial part of the parietoöccipital
fissure. The lingual gyrus lies between the calcarine fissure
and the posterior part of the collateral fissure; behind, it reaches
the occipital pole; in front, it is continued on to the tentorial
surface of the temporal lobe, and joins the hippocampal gyrus.		   The tentorial surface of the occipital lobe is
limited in front by an imaginary transverse line through the preoccipital
notch, and consists of the posterior part of the fusiform gyrus
(occipitotemporal convolution) and the lower part of the lingual
gyrus, which are separated from each other by the posterior segment
of the collateral fissure.		 Temporal Lobe (lobus temporalis).—The temporal
lobe presents superior, lateral, and inferior surfaces.		   The superior surface forms the lower limit of
the lateral fissure and overlaps the insula. On opening out the lateral
fissure, three or four gyri will be seen springing from the depth
of the hinder end of the fissure, and running obliquely forward and
outward on the posterior part of the upper surface of the superior
temporal gyrus; these are named the transverse temporal gyri
(Heschl) (Fig. 90730).		   The lateral surface (Fig. 90726) is bounded above by the posterior ramus of the lateral fissure,
and by the imaginary line continued backward from it; below, it is
limited by the infero-lateral border of the hemisphere. It is divided
into superior, middle, and inferior gyri by the superior and middle
temporal sulci. The superior temporal sulcus runs from before
backward across the temporal lobe, some little distance below, but
parallel with, the posterior ramus of the lateral fissure; and hence
it is often termed the parallel sulcus. The middle temporal
sulcus takes the same direction as the superior, but is situated
at a lower level, and is usually subdivided into two or more parts.
The superior temporal gyrus lies between the posterior ramus
of the lateral fissure and the superior temporal sulcus, and is continuous
behind with the supramarginal and angular gyri. The middle temporal
gyrus is placed between the superior and middle temporal sulci,
and is joined posteriorly with the angular gyrus. The inferior
temporal gyrus is placed below the middle temporal sulcus, and
is connected behind with the inferior occipital gyrus; it also extends
around the infero-lateral border on to the inferior surface of the
temporal lobe, where it is limited by the inferior sulcus.


FIG. 90730– Section
of brain showing upper surface of temporal lobe.
   The inferior surface is concave, and is continuous
posteriorly with the tentorial surface of the occipital lobe. It is
traversed by the inferior temporal sulcus, which extends from
near the occipital pole behind, to within a short distance of the
temporal pole in front, but is frequently subdivided by bridging gyri.
Lateral to this fissure is the narrow tentorial part of the inferior
temporal gyrus, and medial to it the fusiform gyrus, which
extends from the occipital to the temporal pole; this gyrus is limited
medially by the collateral fissure, which separates it from the lingual
gyrus behind and from the hippocampal gyrus in front.		   The Insula (island of Reil; central lobe)
(Fig. 90731) lies deeply in the lateral
or Sylvian fissure, and can only be seen when the lips of that fissure
are widely separated, since it is overlapped and hidden by the gyri
which bound the fissure. These gyri are termed the opercula of
the insula; they are separated from each other by the three rami
of the lateral fissure, and are named the orbital, frontal, frontoparietal,
and temporal opercula. The orbital operculum lies below the
anterior horizontal ramus of the fissure, the frontal between
this and the anterior ascending ramus, the parietal between
the anterior ascending ramus and the upturned end of the posterior
ramus, and the temporal below the posterior ramus. The frontal
operculum is of small size in those cases where the anterior horizontal
and ascending rami of the lateral fissure arise from a common stem.
The insula is surrounded by a deep circular sulcus which separates
it from the frontal, parietal, and temporal lobes. When the opercula
have been removed, the insula is seen as a triangular eminence, the
apex of which is directed toward the anterior perforated substance.
It is divided into a larger anterior and a smaller posterior part
by a deep sulcus, which runs backward and upward from the apex of
the insula. The anterior part is subdivided by shallow sulci into
three or four short gyri, while the posterior part is formed
by one long gyrus, which is often bifurcated at its upper end.
The cortical gray substance of the insula is continuous with that
of the different opercula, while its deep surface corresponds with
the lentiform nucleus of the corpus striatum.


FIG. 90731– The
insula of the left side, exposed by removing the opercula.
 Limbic Lobe (Fig. 90727).—The
term limbic lobe was introduced by Broca, and under it he included
the cingulate and hippocampal gyri, which together arch around the
corpus callosum and the hippocampal fissure. These he separated on
the morphological ground that they are well-developed in animals possessing
a keen sense of smell (osmatic animals), such as the dog and fox.
They were thus regarded as a part of the rhinencephalon, but it is
now recognized that they belong to the neopallium; the cingulate gyrus
is therefore sometimes described as a part of the frontal lobe, and
the hippocampal as a part of the temporal lobe.		   The cingulate gyrus (gyrus cinguli; callosal
convolution) is an arch-shaped convolution, lying in close relation
to the superficial surface of the corpus callosum, from which it is
separated by a slit-like fissure, the callosal fissure. It
commences below the rostrum of the corpus callosum, curves around
in front of the genu, extends along the upper surface of the body,
and finally turns downward behind the splenium, where it is connected
by a narrow isthmus with the hippocampal

gyrus. It is separated from the medial part of the superior frontal
gyrus by the cingulate sulcus, and from the precuneus by the subparietal
sulcus.		   The hippocampal gyrus (gyrus hippocampi)
is bounded above by the hippocampal fissure, and below by the anterior
part of the collateral fissure. Behind, it is continuous superiorly,
through the isthmus, with the cingulate gyrus and inferiorly with
the lingual gyrus. Running in the substance of the cingulate and hippocampal
gyri, and connecting them together, is a tract of arched fibers, named
the cingulum (page 843). The anterior extremity of the hippocampal
gyrus is recurved in the form of a hook (uncus), which is separated
from the apex of the temporal lobe by a slight fissure, the incisura
temporalis. Although superficially continuous with the hippocampal
gyrus, the uncus forms morphologically a part of the rhinencephalon.		   The Hippocampal Fissure (fissura hippocampi;
dentate fissure) begins immediately behind the splenium of the
corpus callosum, and runs forward between the hippocampal and dentate
gyri to end in the uncus. It is a complete fissure (page 819), and
gives rise to the prominence of the hippocampus in the inferior cornu
of the lateral ventricle.


FIG. 90732– Scheme
of rhinencephalon.
 Rhinencephalon (Fig. 90732).—The
rhinencephalon comprises the olfactory lobe, the uncus,
the subcallosal and supracallosal gyri, the fascia
dentata hippocampi, the septum pellucidum, the fornix,
and the hippocampus.		   1. The Olfactory Lobe (lobus olfactorius)
is situated under the inferior or orbital surface of the frontal lobe.
In many vertebrates it constitutes a well-marked portion of the hemisphere
and contains an extension of the lateral ventricle; but in man and
some other mammals it is rudimentary. It consists of the olfactory
bulb and tract, the olfactory trigone, the parolfactory
area of Broca, and the anterior perforated substance.		   (a) The olfactory bulb (bulbus olfactorius)
is an oval, reddish-gray mass which rests on the cribriform plate
of the ethmoid and forms the anterior expanded extremity of the olfactory
tract. Its under surface receives the olfactory nerves, which pass
upward through the cribriform plate from the olfactory region of the
nasal cavity. Its minute structure is described on page 848.		   (b) The olfactory tract (tractus olfactorius)
is a narrow white band, triangular on coronal section, the apex being
directed upward. It lies in the olfactory sulcus on the inferior surface
of the frontal lobe, and divides posteriorly into two striæ,
a medial and a lateral. The lateral stria is directed across
the lateral part of the

anterior perforated substance and then bends abruptly medialward toward
the uncus of the hippocampal gyrus. The medial stria turns
medialward behind the parolfactory area and ends in the subcallosal
gyrus; in some cases a small intermediate stria is seen running
backward to the anterior perforated substance.		   (c) The olfactory trigone (trigonum
olfactorium) is a small triangular area in front of the anterior
perforated substance. Its apex, directed forward, occupies the posterior
part of the olfactory sulcus, and is brought into view by throwing
back the olfactory tract.		   (d) The parolfactory area of Broca (area
parolfactoria) is a small triangular field on the medial surface
of the hemisphere in front of the subcallosal gyrus, from which it
is separated by the posterior parolfactory sulcus; it is continuous
below with the olfactory trigone, and above and in front with the
cingulate gyrus; it is limited anteriorly by the anterior parolfactory
sulcus.		   (e) The anterior perforated substance
(substantia perforata anterior) is an irregularly quadrilateral
area in front of the optic tract and behind the olfactory trigone,
from which it is separated by the fissure prima; medially and
in front it is continuous with the subcallosal gyrus; laterally it
is bounded by the lateral stria of the olfactory tract and is continued
into the uncus. Its gray substance is confluent above with that of
the corpus striatum, and is perforated anteriorly by numerous small
bloodvessels.		   2. The Uncus has already been described (page
826) as the recurved, hook-like portion of the hippocampal gyrus.		   3. The Subcallosal, Supracallosal, and Dentate
Gyri form a rudimentary arch-shaped lamina of gray substance extending
over the corpus callosum and above the hippocampal gyrus from the
anterior perforated substance to the uncus.		   (a) The subcallosal gyrus (gyrus subcallosus;
peduncle of the corpus callosum) is a narrow lamina on the medial
surface of the hemisphere in front of the lamina terminalis, behind
the parolfactory area, and below the rostrum of the corpus callosum.
It is continuous around the genu of the corpus callosum with the supracallosal
gyrus.		   (b) The supracallosal gyrus (indusium
griseum; gyrus epicallosus) consists of a thin layer of gray substance
in contact with the upper surface of the corpus callosum and continuous
laterally with the gray substance of the cingulate gyrus. It contains
two longitudinally directed strands of fibers termed respectively
the medial and lateral longitudinal striæ. The
supracallosal gyrus is prolonged around the splenium of the corpus
callosum as a delicate lamina, the fasciola cinerea, which
is continuous below with the fascia dentata hippocampi.		   (c) The fascia dentata hippocampi (gyrus
dentatus) is a narrow band extending downward and forward above
the hippocampal gyrus but separated from it by the hippocampal fissure;
its free margin is notched and overlapped by the fimbria—the
fimbriodentate fissure intervening. Anteriorly it is continued
into the notch of the uncus, where it forms a sharp bend and is then
prolonged as a delicate band, the band of Giacomini, over the
uncus, on the lateral surface of which it is lost.		   The remaining parts of the rhinencephalon, viz., the
septum pellucidum, fornix, and hippocampus, will be described in connection
with the lateral ventricle.		 Interior of the Cerebral Hemispheres.—If the upper part
of either hemisphere be removed, at a level about 1.25 cm. above the
corpus callosum, the central white substance will be exposed as an
oval-shaped area, the centrum ovale minus, surrounded by a
narrow convoluted margin of gray substance, and studded with numerous
minute red dots (puncta vasculosa), produced by the escape
of blood from divided bloodvessels. If the remaining portions of the
hemispheres be slightly drawn apart a broad band of white substance,
the corpus callosum, will be observed, connecting them at the
bottom of the longitudinal fissure; the margins of the hemispheres
which overlap the corpus callosum are called the labia cerebri.
Each labrium is

part of the cingulate gyrus already described; and the slit-like interval
between it and the upper surface of the corpus callosum is termed
the callosal fissure (Fig. 90727).
If the hemispheres be sliced off to a level with the upper surface
of the corpus callosum, the white substance of that structure will
be seen connecting the two hemispheres. The large expanse of medullary
matter now exposed, surrounded by the convoluted margin of gray substance,
is called the centrum ovale majus.		   The Corpus Callosum (Fig. 90733) is the great transverse commissure which unites the cerebral
hemispheres and roofs in the lateral ventricles. A good conception
of its position and size is obtained by examining a median sagittal
section of the brain (Fig. 90720), when
it is seen to form an arched structure about 10 cm. long. Its anterior
end is about 4 cm. from the frontal pole, and its posterior end about
6 cm. from the occipital pole of the hemisphere.


FIG. 90733– Corpus
callosum from above.
   The anterior end is named the genu, and
is bent downward and backward in front of the septum pellucidum; diminishing
rapidly in thickness, it is prolonged backward under the name of the
rostrum, which is connected below with the lamina terminalis.
The anterior cerebral arteries are in contact with the under surface
of the rostrum; they then arch over the front of the genu, and are
carried backward above the body of the corpus callosum.		   The posterior end is termed the splenium
and constitutes the thickest part of the corpus callosum. It overlaps
the tela chorioidea of the third ventricle and the mid-brain, and
ends in a thick, convex, free border. A sagittal section of

the splenium shows that the posterior end of the corpus callosum is
acutely bent forward, the upper and lower parts being applied to each
other.		   The superior surface is convex from before backward,
and is about 2.5 cm. wide. Its medial part forms the bottom of the
longitudinal fissure, and is in contact posteriorly with the lower
border of the falx cerebri. Laterally it is overlapped by the cingulate
gyrus, but is separated from it by the slit-like callosal fissure.
It is traversed by numerous transverse ridges and furrows, and is
covered by a thin layer of gray matter, the supracallosal gyrus,
which exhibits on either side of the middle line the medial and lateral
longitudinal striæ, already described (page 827).		   The inferior surface is concave, and forms on
either side of the middle line the roof of the lateral ventricle.
Medially, this surface is attached in front to the septum pellucidum;
behind this it is fused with the upper surface of the body of the
fornix, while the splenium is in contact with the tela chorioidea.		   On either side, the fibers of the corpus callosum radiate
in the white substance and pass to the various parts of the cerebral
cortex; those curving forward from the genu into the frontal lobe
constitute the forceps anterior, and those curving backward
into the occipital lobe, the forceps posterior. Between these
two parts is the main body of the fibers which constitute the tapetum
and extend laterally on either side into the temporal lobe, and cover
in the central part of the lateral ventricle.


FIG. 90734– Scheme
showing relations of the ventricles to the surface of the brain.
 The Lateral Ventricles (ventriculus lateralis) (Fig. 90734).—The two lateral ventricles are irregular cavities
situated in the lower and medial parts of the cerebral hemispheres,
one on either side of the middle line. They are separated from each
other by a median vertical partition, the septum pellucidum,
but communicate with the third ventricle and indirectly with each
other through the interventricular foramen. They are lined
by a thin, diaphanous membrane, the ependyma, covered by ciliated
epithelium, and contain cerebrospinal fluid, which, even in health,
may be secreted in considerable amount. Each lateral ventricle consists
of a central part or body, and three prolongations from
it, termed cornua (Figs. 90735, 90736).		   The central part (pars centralis ventriculi
lateralis; cella) (Fig. 90737) of the
lateral ventricle extends from the interventricular foramen to the
splenium of the corpus

callosum. It is an irregularly curved cavity, triangular on transverse
section, with a roof, a floor, and a medial wall. The roof is formed
by the under surface of the corpus callosum; the floor by the following
parts, enumerated in their order of position, from before backward:
the caudate nucleus of the corpus striatum, the stria terminalis and
the terminal vein, the lateral portion of the upper surface of the
thalamus, the choroid plexus, and the lateral part of the fornix;
the medial wall is the posterior part of the septum pellucidum, which
separates it from the opposite ventricle.


FIG. 90735– Drawing
of a cast of the ventricular cavities, viewed from above. (Retzius.)


FIG. 90736– Drawing
of a cast of the ventricular cavities, viewed from the side. (Retzius.)
   The anterior cornu (cornu anterius; anterior
horn; precornu) (Fig. 90736) passes
forward and lateralward, with a slight inclination downward, from
the interventricular foramen into the frontal lobe, curving around
the anterior end of the caudate nucleus. Its floor is formed by the
upper surface of the reflected portion of the

corpus callosum, the rostrum. It is bounded medially by the
anterior portion of the septum pellucidum, and laterally by the head
of the caudate nucleus. Its apex reaches the posterior surface of
the genu of the corpus callosum.		   The posterior cornu (cornu posterius; postcornu)
(Figs. 90737, 90788)
passes into the occipital lobe, its direction being backward and lateralward,
and then medialward. Its roof is formed by the fibers of the corpus
callosum passing to the temporal and occipital lobes. On its medial
wall is a longitudinal eminence, the calcar avis (hippocampus
minor), which is an involution of the ventricular wall produced
by the calcarine fissure. Above this the forceps posterior of the
corpus callosum, sweeping around to enter the occipital lobe, causes
another projection, termed the bulb of the posterior cornu.
The calcar avis and bulb of the posterior cornu are extremely variable
in their degree of development; in some cases they are ill-defined,
in others prominent.


FIG. 90737– Central
part and anterior and posterior cornua of lateral ventricles exposed
from above.
   The inferior cornu (cornu inferior; descending
horn; middle horn; medicornu) (Fig. 90739),
the largest of the three, traverses the temporal lobe of the brain,
forming in its course a curve around the posterior end of the thalamus.
It passes at first backward, lateralward, and downward, and then curves
forward to within 2.5 cm. of the apex of the temporal lobe, its direction
being fairly well indicated on the surface of the brain by that of
the superior temporal sulcus. Its roof is formed chiefly by the inferior
surface of the tapetum of the corpus callosum, but the tail of the
caudate nucleus and the stria terminalis also extend forward in the
roof of the inferior cornu to its extremity; the tail of the caudate
nucleus joins the

putamen. Its floor presents the following parts: the hippocampus,
the fimbria hippocampi, the collateral eminence, and the choroid plexus.
When the choroid plexus is removed, a cleft-like opening is left along
the medial wall of the inferior cornu; this cleft constitutes the
lower part of the choroidal fissure.


FIG. 90738– Coronal
section through posterior cornua of lateral ventricle.


FIG. 90739– Posterior
and inferior cornua of left lateral ventricle exposed from the side.
   The hippocampus (hippocampus major) (Figs.
90739, 90740) is a curved eminence, about
5 cm. long, which extends throughout the entire length of the floor
of the

inferior cornu. Its lower end is enlarged, and presents two or three
rounded elevations or digitations which give it a paw-like appearance,
and hence it is named the pes hippocampi. If a transverse section
be made through the hippocampus, it will be seen that this eminence
is produced by the folding of the wall of the hemisphere to form the
hippocampal fissure. The main mass of the hippocampus consists of
gray substance, but on its ventricular surface is a thin white layer,
the alveus, which is continuous with the fimbria hippocampi.		   The collateral eminence (eminentia collateralis)
(Fig. 90740) is an elongated swelling lying
lateral to and parallel with the hippocampus. It corresponds with
the middle part of the collateral fissure, and its size depends on
the depth and direction of this fissure. It is continuous behind with
a flattened triangular area, the trigonum collaterale, situated
between the posterior and inferior cornua.		   The fimbria hippocampi is a continuation of the crus
of the fornix, and will be discussed with that body; a description
of the choroid plexus will be found on page 840.


FIG. 90740– Inferior
and posterior cornua, viewed from above.


FIG. 90741– Two
views of a model of the striatum: A, lateral aspect; B,
mesal aspect.
   The corpus striatum has received its name from
the striped appearance which a section of its anterior part presents,
in consequence of diverging white fibers being mixed with the gray
substance which forms its chief mass. A part of the corpus striatum
is imbedded in the white substance of the hemisphere, and is therefore
external to the ventricle; it is termed the extraventricular portion,
or the lentiform nucleus; the remainder, however, projects
into the ventricle, and is named the intraventricular portion,
or the caudate nucleus (Fig. 90737).		   The caudate nucleus (nucleus caudatus; caudatum)
(Figs. 90741, 90742)
is a pear-shaped, highly arched gray mass; its broad extremity, or
head, is directed forward into the anterior cornu of the lateral
ventricle, and is continuous with the anterior perforated substance
and with the anterior end of the lentiform nucleus; its narrow end,
or tail, is directed backward on the lateral side of the thalamus,
from which it is

separated by the stria terminalis and the terminal vein. It is then
continued downward into the roof of the inferior cornu, and ends in
the putamen near the apex of the temporal lobe. It is covered by the
lining of the ventricle, and crossed by some veins of considerable
size. It is separated from the lentiform nucleus, in the greater part
of its extent, by a thick lamina of white substance, called the internal
capsule, but the two portions of the corpus striatum are united
in front (Figs. 90743, 90744).


FIG. 90742– Horizontal
section of right cerebral hemisphere.
   The lentiform nucleus (nucleus lentiformis;
lenticular nucleus; lenticula) (Fig. 90741)
is lateral to the caudate nucleus and thalamus, and is seen only in
sections of the hemisphere. When divided horizontally, it exhibits,
to some extent, the appearance of a biconvex lens (Fig. 90742), while a coronal section of its central part presents a somewhat
triangular outline. It is shorter than the caudate nucleus and does
not extend as far forward. It is bounded laterally by a lamina of
white substance called the external capsule, and lateral to
this is a thin layer of gray substance termed the claustrum.
Its anterior end is continuous with the lower part of the head of
the caudate nucleus and with the anterior perforated substance.		   In a coronal section through the middle of the lentiform
nucleus, two medullary laminæ are seen dividing it into
three parts. The lateral and largest part is of a reddish color, and
is known as the putamen, while the medial and intermediate
are of

a yellowish tint, and together constitute the globus pallidus;
all three are marked by fine radiating white fibers, which are most
distinct in the putamen (Fig. 90744).		   The gray substance of the corpus striatum is traversed
by nerve fibers, some of which originate in it. The cells are multipolar,
both large and small; those of the lentiform nucleus contain yellow
pigment. The caudate and lentiform nuclei are not only directly continuous
with each other anteriorly, but are connected to each other by numerous
fibers. The corpus striatum is also connected: (1) to the cerebral
cortex, by what are termed the corticostriate fibers; (2) to
the thalamus, by fibers which pass through the internal capsule, and
by a strand named the ansa lentiformis; (3) to the cerebral
peduncle, by fibers which leave the lower aspect of the caudate and
lentiform nuclei.


FIG. 90743– Coronal
section through anterior cornua of lateral ventricles.
   The claustrum (Figs.
90742, 90744) is a thin layer of gray
substance, situated on the lateral surface of the external capsule.
Its transverse section is triangular, with the apex directed upward.
Its medial surface, contiguous to the external capsule, is smooth,
but its lateral surface presents ridges and furrows corresponding
with the gyri and sulci of the insula, with which it is in close relationship.
The claustrum is regarded as a detached portion of the gray substance
of the insula, from which it is separated by a layer of white fibers,
the capsula extrema (band of Baillarger). Its cells
are small and spindle-shaped, and contain yellow pigment; they are
similar to those of the deepest layer of the cortex.		   The nucleus amygdalæ (amygdala) (Fig. 90741), is an ovoid gray mass, situated at the lower end of the
roof of the inferior cornu. It is merely a localized thickening of
the

gray cortex, continuous with that of the uncus; in front it is continuous
with the putamen, behind with the stria terminalis and the tail of
the caudate nucleus.		   The internal capsule (capsula interna)
(Figs. 90745, 90746)
is a flattened band of white fibers, between the lentiform nucleus
on the lateral side and the caudate nucleus and thalamus on the medial
side. In horizontal section (Figs. 90742)
it is seen to be somewhat abruptly curved, with its convexity inward;
the prominence of the curve is called the genu, and projects
between the caudate nucleus and the thalamus. The portion in front
of the genu is termed the frontal part, and separates the lentiform
from the caudate nucleus; the portion behind the genu is the occipital
part, and separates the lentiform nucleus from the thalamus.


FIG. 90744– Coronal
section of brain through anterior commissure.
   The frontal part of the internal capsule contains:
(1) fibers running from the thalamus to the frontal lobe; (2) fibers
connecting the lentiform and caudate nuclei; (3) fibers connecting
the cortex with the corpus striatum; and (4) fibers passing from the
frontal lobe through the medial fifth of the base of the cerebral
peduncle to the nuclei pontis. The fibers in the region of the genu
are named the geniculate fibers; they originate in the motor
part of the cerebral cortex, and, after passing downward through the
base of the cerebral peduncle with the cerebrospinal fibers, undergo
decussation and end in the motor nuclei of the cranial nerves of the
opposite side. The anterior two-thirds of the occipital part of the
internal capsule contains the cerebrospinal fibers, which arise
in the motor area of the cerebral cortex and, passing downward through
the middle three-fifths of the base of the cerebral peduncle, are
continued into the pyramids of the medulla oblongata. The posterior
third of the occipital part contains: (1) sensory fibers, largely
derived from the thalamus, though some may be continued upward from

the medial lemniscus; (2) the fibers of optic radiation, from the
lower visual centers to the cortex of the occipital lobe; (3) acoustic
fibers, from the lateral lemniscus to the temporal lobe; and (4) fibers
which pass from the occipital and temporal lobes to the nuclei pontis.		   The fibers of the internal capsule radiate widely as
they pass to and from the various parts of the cerebral cortex, forming
the corona radiata (Fig. 90745) and
intermingling with the fibers of the corpus callosum.		   The external capsule (capsula externa)
(Fig. 90742) is a lamina of white substance,
situated lateral to the lentiform nucleus, between it and the claustrum,
and continuous with the internal capsule below and behind the lentiform
nucleus. It probably contains fibers derived from the thalamus, the
anterior commissure, and the subthalamic region.


FIG. 90745– Dissection
showing the course of the cerebrospinal fibers. (E. B. Jamieson.)
   The substantia innominata of Meynert is a stratum
consisting partly of gray and partly of white substance, which lies
below the anterior part of the thalamus and lentiform nucleus. It
consists of three layers, superior, middle, and inferior. The superior
layer is named the ansa lentiformis, and its fibers, derived
from the medullary lamina of the lentiform nucleus, pass medially
to end in the thalamus and subthalamic region, while others are said
to end in the tegmentum and red nucleus. The middle layer consists
of nerve cells and nerve fibers; fibers enter it from the parietal
lobe through the external capsule, while others are said to connect
it with the medial longitudinal fasciculus. The inferior layer
forms the main part of the inferior stalk of the thalamus, and connects
this body with the temporal lobe and the insula.		   The stria terminalis (tænia semicircularis)
is a narrow band of white substance situated in the depression between
the caudate nucleus and the thalamus. Anteriorly, its fibers are partly
continued into the column of the fornix; some, however, pass over
the anterior commissure to the gray substance between the caudate

nucleus and septum pellucidum, while others are said to enter the
caudate nucleus. Posteriorly, it is continued into the roof of the
inferior cornu of the lateral ventricle, at the extremity of which
it enters the nucleus amygdalæ. Superficial to it is a large
vein, the terminal vein (vein of the corpus striatum),
which receives numerous tributaries from the corpus striatum and thalamus;
it runs forward to the interventricular foramen and there joins with
the vein of the choroid plexus to form the corresponding internal
cerebral vein. On the surface of the terminal vein is a narrow white
band, named the lamina affixa.		   The Fornix (Figs. 90720,
90747, 90748)
is a longitudinal, arch-shaped lamella of white substance, situated
below the corpus callosum, and continuous with it behind, but separated
from it in front by the septum pellucidum. It may be described as
consisting of two symmetrical bands, one for either hemisphere. The
two portions are not united to each other in front and behind, but
their central parts are joined together in the middle line. The anterior
parts are called the columns of the fornix; the intermediate
united portions, the body; and the posterior parts, the crura.


FIG. 90746– Diagram
of the tracts in the internal capsule. Motor tract red. The sensory
tract (blue) is not direct, but formed of neurons receiving impulses
from below in the thalamus and transmitting them to the cortex.
The optic radiation (occipitothalamic) is shown in violet.
   The body (corpus fornicis) of the fornix
is triangular, narrow in front, and broad behind. The medial part
of its upper surface is connected to the septum pellucidum in front
and to the corpus callosum behind. The lateral portion of this surface
forms part of the floor of the lateral ventricle, and is covered by
the ventricular epithelium. Its lateral edge overlaps the choroid
plexus, and is continuous with the epithelial covering of this structure.
The under surface rests upon the tela chorioidea of the third ventricle,
which separates it from the epithelial roof of that cavity, and from
the medial portions of the upper surfaces of the thalami. Below, the
lateral portions of the body of the fornix are joined by a thin triangular
lamina, named the psalterium (lyra). This lamina contains
some transverse fibers which connect the two hippocampi across the
middle line and constitute the hippocampal commissure. Between
the psalterium and the corpus callosum a horizontal cleft, the so-called
ventricle of the fornix (ventricle of Verga), is sometimes
found.		   The columns (columna fornicis; anterior pillars;
fornicolumns) of the fornix arch downward in front of the interventricular
foramen and behind the anterior commissure,

and each descends through the gray substance in the lateral wall of
the third ventricle to the base of the brain, where it ends in the
corpus mammillare. From the cells of the corpus mammillare the thalamomammillary
fasciculus (bundle of Vicq d’Azyr) takes origin and
is prolonged into the anterior nucleus of the thalamus. The column
of the fornix and the thalamomammillary fasciculus together form a
loop resembling the figure 8, but the continuity of the loop is broken
in the corpus

mammillare. The column of the fornix is joined by the stria medullaris
of the pineal body and by the superficial fibers of the stria terminalis,
and is said to receive also fibers from the septum pellucidum. Zuckerkandl
describes an olfactory fasciculus which becomes detached from
the main portion of the column of the fornix, and passes downward
in front of the anterior commissure to the base of the brain, where
it divides into two bundles, one joining the medial stria of the olfactory
tract; the other joins the subcallosal gyrus, and through it reaches
the hippocampal gyrus.


FIG. 90747– Diagram
of the fornix. (Spitzka.)


FIG. 90748– The
fornix and corpus callosum from below. (From a specimen in the Department
of Human Anatomy of the University of Oxford.)
   The crura (crus fornicis; posterior pillars)
of the fornix are prolonged backward from the body. They are flattened
bands, and at their commencement are intimately connected with the
under surface of the corpus callosum. Diverging from one another,
each curves around the posterior end of the thalamus, and passes downward
and forward into the inferior cornu of the lateral ventricle (Fig. 90750). Here it lies along the concavity of the hippocampus, on
the surface of which some of its fibers are spread out to form the
alveus, while the remainder are continued as a narrow white
band, the fimbria hippocampi, which is prolonged into the uncus
of the hippocampal gyrus. The inner edge of the fimbria overlaps the
fascia dentata hippocampi (dentate gyrus) (page 827),
from which it is separated by the fimbriodentate fissure; from
its lateral margin, which is thin and ragged, the ventricular epithelium
is reflected over the choroid plexus as the latter projects into the
chorioidal fissure.		 Interventricular Foramen (foramen of Monro).—Between
the columns of the fornix and the anterior ends of the thalami, an
oval aperture is present on either side: this is the interventricular
foramen, and through it the lateral ventricles communicate with the
third ventricle. Behind the epithelial lining of the foramen the choroid
plexuses of the lateral ventricles are joined across the middle line.		   The Anterior Commissure (precommissure)
is a bundle of white fibers, connecting the two cerebral hemispheres
across the middle line, and placed in front of the columns of the
fornix. On sagittal section it is oval in shape, its long diameter
being vertical and measuring about 5 mm. Its fibers can be traced
lateralward and backward on either side beneath the corpus striatum
into the substance of the temporal lobe. It serves in this way to
connect the two temporal lobes, but it also contains decussating fibers
from the olfactory tracts.		   The Septum Pellucidum (septum lucidum)
(Fig. 90720) is a thin, vertically placed
partition consisting of two laminæ, separated in the greater
part of their extent by a narrow chink or interval, the cavity
of the septum pellucidum. It is attached, above, to the under
surface of the corpus callosum; below, to the anterior part of the
fornix behind, and the reflected portion of the corpus callosum in
front. It is triangular in form, broad in front and narrow behind;
its inferior angle corresponds with the upper part of the anterior
commissure. The lateral surface of each lamina is directed toward
the body and anterior cornu of the lateral ventricle, and is covered
by the ependyma of that cavity.		   The cavity of the septum pellucidum (cavum
septi pellucidi; pseudocele; fifth ventricle) is generally regarded
as part of the longitudinal cerebral fissure, which has become shut
off by the union of the hemispheres in the formation of the corpus
callosum above and the fornix below. Each half of the septum therefore
forms part of the medial wall of the hemisphere, and consists of a
medial layer of gray substance, derived from that of the cortex, and
a lateral layer of white substance continuous with that of the cerebral
hemispheres. This cavity is not developed from the cavity of the cerebral
vesicles, and never communicates with the ventricles of the brain.		   The Choroid Plexus of the Lateral Ventricle (plexus
chorioideus ventriculus lateralis; paraplexus) (Fig. 90750) is a highly vascular, fringe-like process of pia mater, which
projects into the ventricular cavity. The plexus, however, is everywhere

covered by a layer of epithelium continuous with the epithelial lining
of the ventricle. It extends from the interventricular foramen, where
it is joined with the plexus of the opposite ventricle, to the end
of the inferior cornu. The part in relation to the body of the ventricle
forms the vascular fringed margin of a triangular process of pia mater,
named the tela chorioidea of the third ventricle, and projects
from under cover of the lateral edge of the fornix. It lies upon the
upper surface of the thalamus, from which the epithelium is reflected
over the plexus on to the edge of the fornix (Fig. 90723). The portion in relation to the inferior cornu lies in the
concavity of the hippocampus and overlaps the fimbria hippocampi:
from the lateral edge of the fimbria the epithelium is reflected over
the plexus on to the roof of the cornu (Fig. 90749). It consists of minute and highly vascular villous processes,
each with an afferent and an efferent vessel. The arteries
of the plexus are: (a) the anterior choroidal, a branch of
the internal carotid, which enters the plexus at the end of the inferior
cornu; and (b) the posterior choroidal, one or two small branches
of the posterior cerebral, which pass forward under the splenium.
The veins of the choroid plexus unite to form a tortuous vein,
which courses from behind forward to the interventricular foramen
and there joins with the terminal vein to form the corresponding internal
cerebral vein.


FIG. 90749– Coronal
section of inferior horn of lateral ventricle. (Diagrammatic.)
   When the choroid plexus is pulled away, the continuity
between its epithelial covering and the epithelial lining of the ventricle
is severed, and a cleft-like space is produced. This is named the
choroidal fissure; like the plexus, it extends from the interventricular
foramen to the end of the inferior cornu. The upper part of the fissure,
i.e., the part nearest the interventricular foramen is situated
between the lateral edge of the fornix and the upper surface of the
thalamus; farther back at the beginning of the inferior cornu it is
between the commencement of the fimbria hippocampi and the posterior
end of the thalamus, while in the inferior cornu it lies between the
fimbria in the floor and the stria terminalis in the roof of the cornu.		   The tela chorioidea of the third ventricle (tela
chorioidea ventriculi tertii; velum interpositum) (Fig. 90750) is a double fold of pia mater, triangular in shape, which
lies beneath the fornix. The lateral portions of its lower surface
rest upon the thalami, while its medial portion is in contact with
the epithelial roof of the third ventricle. Its apex is situated at
the interventricular foramen; its base corresponds with the splenium
of the corpus callosum, and occupies the interval between that structure
above and the corpora quadrigemina and pineal body below. This

interval, together with the lower portions of the choroidal fissures,
is sometimes spoken of as the transverse fissure of the brain.
At its base the two layers of the velum separate from each other,
and are continuous with the pia mater investing the brain in this
region. Its lateral margins are modified to form the highly vascular
choroid plexuses of the lateral ventricles. It is supplied by the
anterior and posterior choroidal arteries already described, The veins
of the tela chorioidea are named the internal cerebral veins
(venæ Galeni); they are two in number, and run backward
between its layers, each being formed at the interventricular foramen
by the union of the terminal vein with the choroidal vein. The internal
cerebral veins unite posteriorly in a single trunk, the great cerebral
vein (vena magna Galeni), which passes backward beneath
the splenium and ends in the straight sinus.


FIG. 90750– Tela
chorioidea of the third ventricle, and the choroid plexus of the
left lateral ventricle, exposed from above.
 Structure of the Cerebral Hemispheres.—The cerebral
hemispheres are composed of gray and white substance: the former covers
their surface, and is termed the cortex; the latter occupies
the interior of the hemispheres.		   The white substance consists of medullated fibers,
varying in size, and arranged in bundles separated by neuroglia. They
may be divided, according to their course and connections, into three
distinct systems. (1) Projection fibers connect the hemisphere
with the lower parts of the brain and with the medulla spinalis. (2)
Transverse or commissural fibers unite the two hemispheres.
(3) Association fibers connect different structures in the
same hemisphere; these are, in many

instances, collateral branches of the projection fibers, but others
are the axons of independent cells.		   1. The projection fibers consist of efferent
and afferent fibers uniting the cortex with the lower parts of the
brain and with the medulla spinalis. The principal efferent strands
are: (1) the motor tract, occupying the genu and anterior two-thirds
of the occipital part of the internal capsule, and consisting of (a)
the geniculate fibers, which decussate and end in the motor nuclei
of the cranial nerves of the opposite side; and (b) the cerebrospinal
fibers, which are prolonged through the pyramid of the medulla oblongata
into the medulla spinalis: (2) the corticopontine fibers, ending
in the nuclei pontis. The chief afferent fibers are: (1) those of
the lemniscus which are not interrupted in the thalamus; (2) those
of the superior cerebellar peduncle which are not interrupted in the
red nucleus and thalamus; (3) numerous fibers arising within the thalamus,
and passing through its stalks to the different parts of the cortex
(page 810); (4) optic and acoustic fibers, the former passing to the
occipital, the latter to the temporal lobe.		   2. The transverse or commissural fibers
connect the two hemispheres. They include: (a) the transverse
fibers of the corpus callosum, (b) the anterior commissure,
(c) the posterior commissure, and (d) the lyra or hippocampal
commissure; they have already been described.


FIG. 90751– Diagram
showing principal systems of association fibers in the cerebrum.
   3. The association fibers (Fig. 90751) unite different parts of the same hemisphere, and are of
two kinds: (1) those connecting adjacent gyri, short association
fibers; (2) those passing between more distant parts, long
association fibers.		   The short association fibers lie immediately
beneath the gray substance of the cortex of the hemispheres, and connect
together adjacent gyri.		   The long association fibers include the following:
(a) the uncinate fasciculus; (b) the cingulum; (c)
the superior longitudinal fasciculus; (d) the inferior longitudinal
fasciculus; (e) the perpendicular fasciculus; (f) the
occipitofrontal fasciculus; and (g) the fornix.		   (a) The uncinate fasciculus passes across
the bottom of the lateral fissure, and unites the gyri of the frontal
lobe with the anterior end of the temporal lobe.		   (b) The cingulum is a band of white matter
contained within the cingulate gyrus. Beginning in front at the anterior
perforated substance, it passes forward and upward parallel with the
rostrum, winds around the genu, runs backward above the corpus callosum,
turns around the splenium, and ends in the hippocampal gyrus.		   (c) The superior longitudinal fasciculus
passes backward from the frontal lobe above the lentiform nucleus
and insula; some of its fibers end in the occipital lobe, and others
curve downward and forward into the temporal lobe.


FIG. 90752– Dissection
of cortex and brain-stem showing association fibers and island of
Reil after removal of its superficial gray substance.
   (d) The inferior longitudinal fasciculus
connects the temporal and occipital lobes, running along the lateral
walls of the inferior and posterior cornua of the lateral ventricle.


FIG. 90753– Deep
dissection of cortex and brain-stem.
   (e) The perpendicular fasciculus runs
vertically through the front part of the occipital lobe, and connects
the inferior parietal lobule with the fusiform gyrus.		   (f) The occipitofrontal fasciculus passes
backward from the frontal lobe, along the lateral border of the caudate
nucleus, and on the mesial aspect of the corona

radiata; its fibers radiate in a fan-like manner and pass into the
occipital and temporal lobes lateral to the posterior and inferior
cornua. Déjerine regards the fibers of the tapetum as being derived
from this fasciculus, and not from the corpus callosum.		   (g) The fornix connects the hippocampal
gyrus with the corpus mammillare and, by means of the thalamomammillary
fasciculus, with the thalamus (see page 839). Through the fibers of
the hippocampal commissure it probably also unites the opposite hippocampal
gyri.		   The gray substance of the hemisphere is divided
into: (1) that of the cerebral cortex, and (2) that of the caudate
nucleus, the lentiform nucleus, the claustrum, and the nucleus amygdalæ.		 Structure of the Cerebral Cortex (Fig. 90754).—The cerebral cortex differs in thickness and structure
in different parts of the hemisphere. It is thinner in the occipital
region than in the anterior and posterior central gyri, and it is
also much thinner at the bottom of the sulci than on the top of the
gyri. Again, the minute structure of the anterior central differs
from that of the posterior central gyrus, and areas possessing a specialized
type of cortex can be mapped out in the occipital lobe.		   On examining a section of the cortex with a lens, it
is seen to consist of alternating white and gray layers thus disposed
from the surface inward: (1) a thin layer of white substance; (2)
a layer of gray substance; (3) a second white layer (outer band
of Baillarger or band of Gennari); (4) a second gray layer;
(5) a third white layer (inner band of Baillarger); (6) a third
gray layer, which rests on the medullary substance of the gyrus.		   The cortex is made up of nerve cells of varying size
and shape, and of nerve fibers which are either medullated or naked
axis-cylinders, imbedded in a matrix of neuroglia.		 Nerve Cells.—According to Cajal, the nerve cells are
arranged in four layers, named from the surface inward as follows:
(1) the molecular layer, (2) the layer of small pyramidal cells, (3)
the layer of large pyramidal cells, (4) the layer of polymorphous
cells.		   The Molecular Layer.—In this layer the cells
are polygonal, triangular, or fusiform in shape. Each polygonal cell
gives off some four or five dendrites, while its axon may arise directly
from the cell or from one of its dendrites. Each triangular cell gives
off two or three dendrites, from one of which the axon arises. The
fusiform cells are placed with their long axes parallel to the surface
and are mostly bipolar, each pole being prolonged into a dendrite,
which runs horizontally for some distance and furnishes ascending
branches. Their axons, two or three in number, arise from the dendrites,
and, like them, take a horizontal course, giving off numerous ascending
collaterals. The distribution of the axons and dendrites of all three
sets of cells is limited to the molecular layer.		   The Layer of Small and the Layer of Large Pyramidal
Cells.—The cells in these two layers may be studied together,
since, with the exception of the difference in size and the more superficial
position of the smaller cells, they resemble each other. The average
length of the small cells is from 10 to 15μ; that of the large
cells from 20 to 30μ. The body of each cell is pyramidal in shape,
its base being directed to the deeper parts and its apex toward the
surface. It contains granular pigment, and stains deeply with ordinary
reagents. The nucleus is of large size, and round or oval in shape.
The base of the cell gives off the axis cylinder, and this runs into
the central white substance, giving off collaterals in its course,
and is distributed as a projection, commissural, or association fiber.
The apical and basal parts of the cell give off dendrites; the apical
dendrite is directed toward the surface, and ends in the molecular
layer by dividing into numerous branches, all of which may be seen,
when prepared by the silver or methylene-blue method, to be studded
with projecting bristle-like processes. The largest pyramidal cells
are found in the upper part of the anterior central gyrus and in the
paracentral lobule; they are often arranged in groups or nests of
from three to five, and are named the giant cells of Betz.
In the former situation they may exceed 50μ in length and 40μ
in breadth, while in the paracentral lobule they may attain a length
of 65μ.		   Layer of Polymorphous Cells.—The cells in
this layer, as their name implies, are very irregular in contour;
they may be fusiform, oval, triangular, or star-shaped. Their dendrites
are directed outward, but do not reach so far as the molecular layer;
their axons pass into the subjacent white matter.		   There are two other kinds of cells in the cerebral cortex.
They are: (a) the cells of Golgi, the axons of which
divide immediately after their origins into a large number of branches,
which are directed toward the surface of the cortex; (b) the
cells of Martinotti, which are chiefly found in the polymorphous
layer; their dendrites are short, and may have an ascending or descending
course, while their axons pass out into the molecular layer and form
an extensive horizontal arborization.		 Nerve Fibers.—These fill up a large part of the intervals
between the cells, and may be medullated or non-medullated—the
latter comprising the axons of the smallest pyramidal cells and the
cells of Golgi. In their direction the fibers may be either tangential
or radial. The tangential fibers run parallel to the surface
of the hemisphere, intersecting the radial fibers at a right angle.
They constitute several strata, of which the following are the more
important: (1) a stratum of white fibers covering the superficial
aspect of the molecular layer (plexus of Exner); (2) the band
of Bechterew, in the outer part of the layer of small pyramidal cells;
(3) the band of Gennari or external band of Baillarger, running through
the layer of large pyramidal cells; (4) the internal band of Baillarger,
between the layer of large pyramidal cells and the polymorphous layer;
(5) the deep tangential fibers, in the lower part of the polymorphous
layer. The tangential fibers consist of (a) the collaterals
of the pyramidal and polymorphous cells and of the cells of Martinotti;
(b) the branching axons of Golgi’s cells; (c) the
collaterals and terminal arborizations of the projection, commissural,
or association fibers. The radial fibers.—Some of these,
viz., the axons of the pyramidal and polymorphous cells, descend into
the central white matter, while others,

the terminations of the projection, commissural, or association fibers,
ascend to end in the cortex. The axons of the cells of Martinotti
are also ascending fibers.


FIG. 90754– Cerebral
cortex. (Poirier.) To the left, the groups of cells; to the right,
the systems of fibers. Quite to the left of the figure a sensory
nerve fiber is shown.
 Special Types of Cerebral Cortex.—It has been already
pointed out that the minute structure of the cortex differs in different
regions of the hemisphere; and A. W. Campbell 126
has endeavored to prove, as the result of an exhaustive examination
of a series of human and anthropoid brains, “that there exists
a direct correlation between physiological function and histological
structure.” The principal regions where the “typical”
structure is departed from will now be referred to.		   1. In the calcarine fissure and the gyri bounding it,
the internal band of Baillarger is absent, while the band of Gennari
is of considerable thickness, and forms a characteristic feature of
this region of the cortex. If a section be examined microscopically,
an additional layer of cells is seen to be interpolated between the
molecular layer and the layer of small pyramidal cells. This extra
layer consists of two or three strata of fusiform cells, the long
axes of which are at right angles to the surface; each cell gives
off two dendrites, external and internal, from the latter of which
the axon arises and passes into the white central substance. In the
layer of small pyramidal cells, fusiform cells, identical with the
above, are seen, as well as ovoid or star-like cells with ascending
axons (cells of Martinotti). This is the visual area
of the cortex, and it has been shown by J. S. Bolton 127
that in old-standing cases of optic atrophy the thickness of Gennari’s
band is reduced by nearly 50 per cent.


FIG. 90755– Coronal
section of olfactory bulb. (Schwalbe.)
   A. W. Campbell says: “Histologically, two distinct
types of cortex can be made out in the occipital lobe. The first of
these coats the walls and bounding convolutions of the calcarine fissure,
and is distinguished by the well-known line of Gennari or Vicq d’Azyr;
the second area forms an investing zone a centimetre or more broad
around the first, and is characterized by a remarkable wealth of fibers,
as well as by curious pyriform cells of large size richly stocked
with chromophilic elements—cells which seem to have escaped the
observation of Ramón y Cajal, Bolton, and others who have worked
at this region. As to the functions of these two regions there is
abundant evidence, anatomical, embryological, and pathological, to
show that the first or calcarine area is that to which visual sensations
primarily pass, and we are gradually obtaining proof to the effect
that the second investing area is constituted for the interpretation
and further elaboration of these sensations. These areas therefore
deserve the names visuo-sensory and visuo-psychic.”		   2. The anterior central gyrus is characterized by the
presence of the giant cells of Betz and by “a wealth of nerve
fibers immeasurably superior to that of any other part” (Campbell),
and in these respects differs from the posterior central gyrus. These
two gyri, together with the paracentral lobule, were long regarded
as constituting the “motor areas” of the hemisphere; but
Sherrington and Grunbaum have shown 128
that in the chimpanzee the motor area never extends on to the free
face of the posterior central gyrus, but occupies the entire length
of the anterior central gyrus, and in most cases the greater part
or the whole of its width. It extends into the depth of the central
sulcus, occupying the anterior wall, and in some places the floor,
and in some extending even into the deeper part of the posterior wall
of the sulcus.		   3. In the hippocampus the molecular layer is very thick
and contains a large number of Golgi cells. It has been divided into
three strata: (a) s. convolutum or s. granulosum,
containing many tangential fibers; (b) s. lacunosum,
presenting numerous vascular spaces; (c) s. radiatum,
exhibiting a rich plexus of fibrils. The two layers of pyramidal cells
are condensed into one, and the cells are mostly of large size. The
axons of the cells in the polymorphous layer may run in an ascending,
a descending, or a horizontal direction. Between the polymorphous
layer and the ventricular ependyma is the white substance of the alveus.		   4. In the fascia dentata hippocampi or dentate gyrus
the molecular layer contains some pyramidal cells, while the layer
of pyramidal cells is almost entirely represented by small ovoid cells.		   5. The Olfactory Bulb.—In many of the lower
animals this contains a cavity which communicates through the olfactory
tract with the lateral ventricle. In man the original cavity is filled
up by neuroglia and its wall becomes thickened, but much more so on
its ventral than on its dorsal aspect. Its dorsal part contains a
small amount of gray and white substance, but it is scanty and ill-defined.
A section through the ventral part (Fig. 90755)
shows it to consist of the following layers from without inward:


FIG. 90756– Areas
of localization on lateral surface of hemisphere. Motor area in
red. Area of general sensations in blue. Auditory area in green.
Visula area in yellow. The psychic portions are in lighter tints.
   1. A layer of olfactory nerve fibers, which are the
non-medullated axons prolonged from the olfactory cells of the nasal
cavity, and reach the bulb by passing through the cribriform plate
of the ethmoid bone. At first they cover the bulb, and then penetrate
it to end by forming synapses with the dendrites of the mitral cells,
presently to be described.		   2. Glomerular Layer.—This contains numerous
spheroidal reticulated enlargements, termed glomeruli, produced
by the branching and arborization of the processes of the olfactory
nerve fibres with the descending dendrites of the mitral cells.		   3. Molecular Layer.—This is formed of a
matrix of neuroglia, imbedded in which are the mitral cells.
These cells are pyramidal in shape, and the basal part of each gives
off a thick dendrite which descends into the glomerular layer, where
it arborizes as indicated above, and others which interlace with similar
dendrites of neighboring mitral cells. The axons pass through the
next layer into the white matter of the bulb, and after becoming bent
on themselves at a right angle, are continued into the olfactory tract.		   4. Nerve Fiber Layer.—This lies next the
central core of neuroglia, and its fibers consist of the axons or
afferent processes of the mitral cells passing to the brain; some
efferent fibers are, however, also present, and end in the molecular
layer, but nothing is known as to their exact origin.		 Weight of the Encephalon.—The average weight of the
brain, in the adult male, is about 1380 gms.; that of the female,
about 1250 gms. In the male, the maximum weight out of 278 cases was
1840 gms. and the minimum weight 964 gms. The maximum weight of the
adult female brain, out of 191 cases, was 1585 gms. and the minimum
weight 879 gms. The brain increases rapidly during the first four
years of life, and reaches its maximum weight by about the twentieth
year. As age advances, the brain decreases slowly in weight; in old
age the decrease takes place more rapidly, to the extent of about
28 gms.		   The human brain is heavier than that of any of the lower
animals, except the elephant and whale. The brain of the former weighs
from 3.5 to 5.4 kilogm., and that of a whale, in a specimen 19 metres
long, weighed rather more than 6.7 kilogm.		 Cerebral Localization.—Physiological and pathological
research have now gone far to prove that a considerable part of the
surface of the brain may be mapped out into a series of more or less
definite areas, each of which is intimately connected with some well-defined
function.		   The chief areas are indicated in Figs.
90756 and 90757.		 Motor Areas.—The motor area occupies the anterior central
and frontal gyri and the paracentral lobule. The centers for the lower
limb are located on the uppermost part of the anterior central gyrus
and its continuation on to the paracentral lobule; those for the trunk
are on the upper portion, and those for the upper limb on the middle
portion of the anterior central gyrus. The facial centers are situated
on the lower part of the anterior central gyrus, those for the tongue,
larynx, muscles of mastication, and pharynx on the frontal operculum,
while those for the head and neck occupy the posterior end of the
middle frontal gyrus.


FIG. 90757– Areas
of localization on medial surface of hemisphere. Motor area in red.
Area of general sensations in blue. Visual area in yellow. Olfactory
area in purple. The psychic portions are in lighter tints.
 Sensory Areas.—Tactile and temperature senses are located
on the posterior central gyrus, while the sense of form and solidity
is on the superior parietal lobule and precuneus. With regard to the
special senses, the area for the sense of taste is probably related
to the uncus and hippocampal gyrus. The auditory area occupies the
middle third of the superior temporal gyrus and the adjacent gyri
in the lateral fissure; the visual area, the calcarine fissure and
cuneus; the olfactory area, the rhinencephalon. As special centers
of much importance may be noted: the emissive center for speech on
the left inferior frontal and anterior central gyri (Broca); the auditory
receptive center on the transverse and superior temporal gyri, and
the visual receptive center on the lingual gyrus and cuneus.

Note 125.  Elliot Smith has named the lateral occipital sulcus the sulcus lunatus; he regards it as the representative, in the human brain, of the “Affenspalte” of the brain of the ape. [back]
Note 126.  Histological Studies on the Localization of Cerebral Function, Cambridge University Press. [back]
Note 127.  Philosophical Transactions of Royal Society, Series B, cxciii, 165. [back]
Note 128.  Transactions of the Pathological Society of London, vol. liii. [back]

 


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