le,
and of the Velum Interpositum.

a, Lyra, turned back.
b, b, Posterior pillars of the fornix, turned back.
c, c, Anterior pillars of the fornix.
d, Velum interpositum and veins of Galen.
e, Fifth ventricle.
f, f, Corpus striatum.
g, g, Taenia semicircularis.
h, h, Optic thalamus.
k, Choroid plexus.
l, Taenia hippocampi.
m, Hippocampus major in descending cornu.
n, Hippocampus minor.
o, Eminentia collateralis.]

The _optic thalamus_ is a large, somewhat ovoid body situated behind
the corpus striatum, and above the crus cerebri. Its upper surface is
partly seen in the floor of the body of the lateral ventricle, but is
for the most part covered by the fornix and velum interpositum. Its
postero-inferior surface forms the roof of the descending cornu of
the ventricle, whilst its inner surface forms the side wall of the
third ventricle. At its outer and posterior part are two slight
elevations, in close relation to the optic tract, and named
respectively corpus geniculatum internum and externum.

The posterior knob-like extremity of the thalamus is called the
_pulvinar_; this, as well as the two corpora geniculata and the
superior corpus quadrigeminum, is connected with the optic tract.

The _third ventricle_ (see fig. 6) is a cavity situated in the mesial
plane between the two optic thalami. Its roof is formed by the velum
interpositum and body of the fornix; its floor by the posterior
perforated space, corpora albicantia, tuber cinereum, infundibulum,
and optic commissure; its anterior boundary by the anterior pillars of
the fornix, anterior commissure and lamina cinerea; its posterior
boundary by the corpora quadrigemina and posterior commissure. The
cavity of this ventricle is of small size in the living head, for the
inner surfaces of the two thalami are connected together by
intermediate grey matter, named the _middle_ or _soft commissure_.
Immediately in front of the corpora quadrigemina, the white fibres of
the _posterior commissure_ pass across between the two optic thalami.
If the anterior pillars of the fornix be separated from each other,
the white fibres of the _anterior commissure_ may be seen lying in
front of them.

[Illustration: From Cunningham, _Text-book of Anatomy_.

FIG. 14.--Horizontal Section through the Right Cerebral Hemisphere at
the Level of the Widest Part of the Lentic