The =diencephalon= or =inter-brain= comprises the thalamus and a number
of other structures grouped about the third ventricle, the cavity of
this division of the brain.[193] To expose its dorsal aspect, the
greater part of the cerebral hemispheres, the corpus callosum, the
fornix, the hippocampus, and the tela chorioidea of the third ventricle
must be removed.
The =thalamus= (or optic thalamus) is the principal body in this part of
the brain. It is a large ovoid gray mass placed obliquely across the
dorsal face of each cerebral peduncle, so that the long axes of the two
thalami would meet in front about at a right angle. Medially they are
fused to a large extent, and around the area of adhesion they are
separated by a sagittal circular space, the third ventricle. The dorsal
surface is convex in both directions, and is separated from the
overlying hippocampus by the tela chorioidea (or velum interpositum).
Laterally it is separated from the nucleus caudatus by an oblique groove
in which there is a band of white matter termed the =stria terminalis=
or tænia semicircularis. Internally it is bounded by a narrow band, the
=stria medullaris=, on which is a delicate ridge of ependyma termed the
=tænia thalami=. The striæ unite posteriorly and blend with the stalk of
the pituitary body. Near this point they present a small enlargement
caused by the nucleus habenulæ. Anteriorly there is a small eminence,
the =anterior tubercle=. The posterior part of the thalamus has the form
of a rounded ridge which is continuous laterally with the optic
tract.[194] Behind the point of origin of the tract, in the angle
between the thalamus and the cerebral peduncle, is the =internal
geniculate body= (Corpus geniculatum mediale), a well defined oval
prominence.
The outer surface is separated from the lenticular nucleus by the
=internal capsule=, an important mass of white matter composed of
fibers passing to and from the cerebral cortex. These fibers go to
form a large part of the ventral portion (basis) of the cerebral
peduncle. From the entire external surface of the thalamus fibers pass
into the internal capsule and radiate to reach the cerebral cortex;
similarly fibers coming from the cortex converge in the internal
capsule to enter the thalamus. This arrangement is termed the thalamic
radiation. Ventral to the thalamus proper is the =subthalamic
tegmental region=. This is the continuation of the tegmental part of
the cerebral peduncle into the diencephalon. It contains the =red
nucleus= (Nucleus ruber) an important ganglion on the course of the
motor tracts. It receives numerous fibers from the cerebral cortex and
the corpus striatum. From it fibers proceed to the thalamus and to the
spinal cord; the fibers to the cord (Tractus rubro-spinalis of
Monakow) cross to the opposite side and extend back in the tegmentum
to the lateral columns of the cord. Lateral to the red nucleus a
conspicuous lenticular area of dark gray matter is visible on
cross-sections of the subthalamic region; this is the =subthalamic
nucleus= (Nucleus hypothalamicus s. corpus Luysi), which consists of
pigmented nerve-cells scattered through a dense network of fine
medullated fibers, and is richly supplied with capillary
blood-vessels. The two nuclei are connected by a transverse commissure
(Commissura hypothalamica), which crosses the floor of the third
ventricle above the mammillary body.
The =pineal body= or =epiphysis= is a small ovoid or fusiform red-brown
mass situated in a deep central depression between the thalami and
corpora quadrigemina. It is variable in size, but is commonly about 10
to 12 mm. long and 6 mm. wide. It is attached at the postero-superior
quadrant of the third ventricle by a short stalk, in which is a small
recess of that cavity. Its base blends in front with the junction of the
striæ medullares of the thalamus. Immediately under the posterior part
of the stalk is a short transverse band of white matter, the =posterior
commissure=.
The pineal body is inclosed in a fibrous capsule from which numerous
trabeculæ pass inward, dividing the organ into spaces occupied by
round epithelial cells of the same origin as the ependyma of the
ventricle.
The =mammillary body= (Corpus mammillare) is a white round elevation a
little larger than a pea which projects ventrally at the anterior end of
the median furrow of the interpeduncular space. While it is a single
body in external form in the horse, sections show that it is double in
structure and contains a nucleus of gray matter on either side.
Three sets of fibers are connected with the mammillary body. The
anterior pillar of the fornix curves down in the lateral wall of the
third ventricle to the body and many of the fornix fibers end in it. A
bundle (Fasciculus thalamo-mammillaris) passes upward and backward
from it into the anterior part of the thalamus, and a tract
(Fasciculus pedunculo-mammillaris) extends back in the floor of the
third ventricle to the tegmentum of the mid-brain.
The =pituitary body= or =hypophysis= was mentioned as covering part of
the interpeduncular space. It is oval in outline, flattened
dorso-ventrally, and nearly an inch (ca. 2 cm.) in width. It is attached
by a delicate tubular stalk, the =infundibulum=, to the =tuber
cinereum=, a small gray prominence situated between the optic chiasm in
front and the mammillary body behind.
The pituitary body consists of two parts which can be distinguished
readily on sections by their color. The glandular (or anterior) lobe
is brown in color and is inclosed in a fibrous capsule. It is
glandular in character and there is good ground for the view that it
is an organ of internal secretion. Besides the chief cells which stain
lightly, it contains large deeply staining chromophile cells. It
arises as an outgrowth from the primitive mouth cavity. The cerebral
(or posterior) lobe is pale and is connected with the infundibulum so
as to form a rather flask-shaped arrangement. It is almost entirely
inclosed by the glandular part. It arises as an outgrowth from the
primitive diencephalon, but loses most of its earlier nervous
character.
The optic chiasm and tracts form the anterior boundary of the
interpeduncular space. The =optic chiasm= or =commissure= (Chiasma
opticum) is formed by the convergence of the optic nerves and the
crossing of the major part of the fibers of the nerve of one side to the
tract of the opposite side. From the chiasm each =optic tract= (Tractus
opticus) curves over the cerebral peduncle outward, backward, and upward
to the posterior part of the thalamus and the internal geniculate body;
some fibers reach the anterior quadrigeminal body.
[Illustration: