_Bo_, Cavity of olfactory, which communicates through the canal _I_
with a lateral ventricle; _Ca_, anterior horn, _Cm_, body, _Ci_,
ventral horn of lateral ventricle; _M_, isthmus which connects
lateral and third ventricles _Ep_, suprapineal recess, below which
is the small infrapineal recess; _Ro_, optic recess; _Rh_,
infundibular and pituitary recess; _S_, aqueduct; _Fl_, ridge
corresponding to sulcus limitans; _Rl_, lateral recess, _Rm_,
posterior recess, of fourth ventricle; _Cc_, beginning of central
canal of spinal cord. (Dexler.)
]
The =corpus callosum= is the great transverse commissure which connects
the two cerebral hemispheres through about half of their length. On
median section (Fig. 508) it is seen to be arched from before backward,
white in color, and composed substantially of transverse fibers. The
middle part or =truncus= (Truncus corporis callosi) slopes downward and
forward and is thinner than the ends. The anterior thickened end, the
=genu=, bends downward and backward and thins out to form the =rostrum=;
the latter is continuous with the lamina terminalis. The posterior end,
the =splenium=, also thick, lies at a considerably higher level than the
genu. The =upper surface= is convex in its length, concave transversely.
Its central part forms the floor of the longitudinal fissure. It is
covered by a thin layer of gray matter (Induseum griseum), in which are
strands of longitudinal fibers (Striæ longitudinales); the latter are
arranged in median and lateral bundles (Stria medialis, striæ
laterales).[200] The =lower surface= has the reverse configuration, and
forms the roof of the lateral ventricles. Along the median line the
septum pellucidum is attached to it. The fibers of the corpus callosum
(with the exception of the longitudinal striæ) run transversely and
spread out laterally in all directions in the central white matter of
the hemispheres to the cortex, forming the =radiation of the corpus
callosum= (Radiatio corporis callosi).
The =fornix= is a bilateral structure composed of white fibers which
arch chiefly over the thalamus and the third ventricle. It is described
as consisting of a body and two pairs of pillars. The =body= (Corpus
fornicis) is formed by the fusion of the two arches of which the fornix
is composed. It is triangular and overlies the anterior parts of the
thalami and the third ventricle. The upper surface gives attachment to
the septum pellucidum and on either side forms part of the floor of the
lateral ventricle. The lateral border is related to the chorioid plexus
of the lateral ventricle and forms the inner boundary of the
interventricular foramen. The =anterior pillars= or =columns= (Columnæ
fornicis) (Fig. 512) are two slender round bundles which emerge from the
body in front of the interventricular foramen and diverge slightly as
they curve downward and backward to the mammillary body.
[Illustration:
FIG. 511.—BRAIN OF HORSE, WITH LATERAL VENTRICLES OPENED BY REMOVAL OF
UPPER PART OF CEREBRAL HEMISPHERES.
]
From the mammillary body the greater part of these fornix fibers are
continued to the thalamus by the thalamo-mammillary fasciculus (or
bundle of Vicq d’Azyr). Others pass to the cerebral peduncle. A
portion of the fibers cross to the opposite thalamus and cerebral
peduncle.
The =posterior pillars= (Crura fornicis) are much larger bands which
diverge widely from the posterior angles of the body. Each curves
outward and backward over the thalamus (from which it is separated by
the tela chorioidea) and is chiefly continued as the =fimbria= along the
concave border of the hippocampus. The pillars give off fibers to form
the alveus or white matter which covers the ventricular face of the
hippocampi, and between them are transverse fibers which constitute the
hippocampal commissure.
The =septum pellucidum= is the median partition between the two lateral
ventricles. Its convex upper border blends with the corpus callosum and
its concave lower border joins the fornix. Its anterior part is received
into the genu of the corpus callosum. Traced backward it diminishes in
height and the two edges meet at an acute angle at the splenium.
The septum consists of two layers (Laminæ septi pellucidi) which are
in direct contact with each other. They consist of nerve-fibers and
gray matter. The latter exists in considerable amount in the thicker
part of the septum adjacent to the anterior pillars of the fornix.
Many of the fibers of the septum pass up through the corpus callosum
to the gyrus fornicatus. Others are connected with the subcallosal
gyrus and the parolfactory area.
[Illustration: