The =cerebral hemispheres= (Hemisphæria) form the greater part of the
fully developed brain. Viewed from above (Fig. 498) they form an ovoid
mass, of which the broader end is posterior, and the greatest transverse
diameter is a little behind the middle. The two hemispheres are
separated by a deep median cleft, the =longitudinal fissure= of the
cerebrum, which is occupied by a sickle-shaped fold of dura mater, the
falx cerebri. In front the separation is complete, and it appears to be
behind also, but here the two hemispheres are attached to each other
over a small area by the pia mater. When the hemispheres are gently
drawn apart, it is seen that the fissure is interrupted in its middle
part at a depth of a little more than an inch (ca. 3 cm.) by a white
commissural mass, the =corpus callosum=; this connects the hemispheres
for about half of their length. The =transverse fissure= separates the
hemispheres from the cerebellum, and contains the tentorium cerebelli.
The =convex= or =dorso-lateral surface=[195] conforms closely to the
cranial wall. The =medial= or =internal surface= is flat and sagittal
and bounds the longitudinal fissure; to a large extent it is in contact
with the falx cerebri, but behind the great cerebral vein the two
hemispheres are in contact and are attached to each other over a small
area as noted above. In well hardened specimens there is usually an
impression for the vein in front of the area of adhesion. The =base= or
=ventral surface= (Fig. 499) is irregular. Its anterior two-thirds is
adapted to the cerebral fossa of the cranial floor. Crossing this area
in front of the optic tract is a depression, the =transverse fossa=
(vallecula or fossa of Sylvius), which leads outward to the =lateral
fissure= (fissure of Sylvius), and lodges the middle cerebral artery. In
front of the fossa there is a considerable rounded elevation known as
the =trigonum olfactorium=. The trigonum and the inner part of the fossa
are pierced by numerous openings for the passage of small blood-vessels
and are equivalent to the locus perforatus anticus of man. Behind the
outer part of the fossa is the rounded anterior end of the =pyriform
lobe=. Traced backward the lobe curves upward and inward over the optic
tract and the thalamus to the tentorial aspect of the hemisphere; its
continuation, the hippocampus, forms part of the floor of the lateral
ventricle, and will be examined later.
The posterior part or =tentorial area= is flattened, faces inward and
backward as well as downward, and rests largely on the tentorium
cerebelli; on its anterior part there is a shallow depression adapted to
the corpora quadrigemina and the pineal body.
The =frontal pole= or anterior extremity (exclusive of the olfactory
bulb) is compressed laterally, and the =occipital pole= or posterior
extremity forms a blunt point.
[Illustration:
FIG. 507.—LEFT CEREBRAL HEMISPHERE OF HORSE, LATERAL VIEW. THE
OLFACTORY BULB IS CUT OFF.
_1_, Lateral fissure (of Sylvius); _2_, _3_, _4_, middle, posterior,
and anterior branches of _1_; _5_, presylvian fissure; _6_, _6′_,
sulcus rhinalis, anterior et posterior; _7_, suprasylvian fissure;
_8_, ectomarginal fissure; _9_, _9′_, ectosylvian fissure.
]
The hemisphere comprises: (1) the =pallium= or =mantle=, which consists
of an outer layer of gray matter, the =cortex= (Substantia corticalis),
covering a large mass of white matter (Centrum semiovale); (2) the
=rhinencephalon= or olfactory portion of the brain; (3) the =corpus
callosum= and =fornix=, the great commissural white masses; (4) the
=lateral ventricle= and certain important structures associated
therewith.
The =pallium= is thrown into numerous folds, the =gyri cerebri=, which
are separated by =sulci= or =fissures= of varying depth. The general
pattern of the gyri and sulci is similar in normal brains of the same
species, but the details are very variable and are never alike on the
two hemispheres of the same brain. In the horse the arrangement is
complicated by the existence of numerous short accessory fissures which
cut into the gyri at right angles and tend to confuse the observer. The
principal fissures and sulci of the convex surface (Figs. 497, 507) are
as follows:
1. The =lateral fissure= (Fissura lateralis Sylvii) ascends on the
lateral surface of the hemisphere as the continuation of the fossa
transversa in front of the pyriform lobe. After crossing the external
olfactory tract it divides into three branches; of these one passes
upward, one runs obliquely forward and upward, and the third is directed
upward and backward.
2. The =suprasylvian fissure= (F. suprasylvia)[196] is long and divides
a large part of the convex surface of the hemisphere into dorsal and
lateral portions. It begins on the dorso-medial border near its anterior
end and, inclining gradually downward, passes back to end on reaching
the tentorial surface. It is continuous internally with the transverse
fissure usually and in front with the presylvian fissure.
3. The =presylvian fissure= (F. præsylvia) is the continuation forward
of the preceding. It passes forward, outward, and downward almost to the
frontal pole, and then inclines backward to end at the groove which
marks the upper limit of the rhinencephalon (Sulcus rhinalis).
4. The =marginal fissure= (F. marginalis) extends along the dorso-medial
border. It begins a little in front of the middle of the border and
turns around the occipital pole to end on its tentorial aspect.
5. The =entomarginal fissure= (F. entomarginalis) lies internal to the
dorso-medial border. It does not extend quite as far forward as the
marginal fissure, from which it is separated by a narrow gyrus.
6. The =ectomarginal fissure= (F. ectomarginalis) lies about midway
between the marginal fissure and the posterior part of the suprasylvian
fissure.
7. The =sulcus rhinalis= is a very distinct furrow on the lower part of
the lateral surface which marks off the olfactory part of the brain
(rhinencephalon) from the rest of the hemisphere. It is undulating and
is highest where it is crossed by the lateral fissure (of Sylvius).[197]
On the medial surface (Fig. 508) the main fissures and sulci are as
follows:
1. The =calloso-marginal fissure= (F. calloso-marginalis) is extensive
and well defined. It is approximately parallel to the dorso-medial
border of the hemisphere, from which it is about half an inch distant.
It begins in front a short distance below and in front of the anterior
bend (genu) of the corpus callosum and forms a =C=-shaped curve, its
posterior part extending on the tentorial surface to a point behind the
depression for the corpora quadrigemina. It separates the marginal gyri
above from the gyrus fornicatus, which extends down to the corpus
callosum.
2. The =transverse fissure= (F. transversa) begins a little behind the
middle of the calloso-marginal fissure, passes obliquely upward and
forward to the dorso-medial border—into which it cuts deeply—and usually
joins the suprasylvian fissure.[198]
3. The =sublimbic fissure= (F. sublimbica) curves over the gyrus
fornicatus a short distance above the corpus callosum. Its middle part
is commonly indistinct, and it is often divided into anterior and
posterior parts.
4. The =callosal sulcus= (Sulcus corporis callosi) separates the corpus
callosum from the gyrus fornicatus.
The =hippocampus= (or cornu Ammonis) is a gyrus which curves from the
deep face of the pyriform lobe around the thalamus and forms the
posterior part of the floor of the lateral ventricle. It can be
displayed by cutting away the brain stem up to the optic tract and the
interventricular foramen. Viewed from below the hippocampal gyrus is
seen to form a semicircular curve from the apex of the pyriform lobe to
the angle of divergence of the posterior pillars of the fornix, _i. e._,
to a point under the central part of the corpus callosum. It is
separated deeply by the hippocampal fissure from the =gyrus dentatus=.
Along the concave margin of the latter is a band of white matter, termed
the =fimbria=, which is the prolongation of the greater part of the
posterior pillar of the fornix into this region. The ventricular surface
of the hippocampus (Fig. 511) is covered with a thin layer of white
matter, the =alveus=, which is also derived from the posterior pillar of
the fornix, and is therefore continuous with the fimbria. The two
hippocampi are connected at their highest parts by transverse fibers
which constitute the =hippocampal commissure=.
The interval between the hippocampus and fimbria on the one hand and the
brain stem on the other is a lateral continuation of the great
transverse fissure of the brain, and is termed the =chorioid fissure=.
It is occupied by a fold of pia mater, the =tela chorioidea= of the
third ventricle, or =velum interpositum=. This fold is triangular in
outline and its apex reaches to the interventricular foramen. Its base
is continuous at the transverse fissure with the pia which covers the
surface of the brain. Its middle part lies over the epithelial roof of
the third ventricle, as has been seen (Fig. 506). The lateral borders
will be seen on the floor of the lateral ventricles, where they form
thick rounded bands containing convolutions of blood-vessels, known as
the =chorioid plexuses= of the lateral ventricles (Fig. 511).
[Illustration: