ves are joined to form the
_body_; but in front the two halves separate from each other, and form
two anterior pillars, which descend in front of the third ventricle to
the base of the cerebrum, where they form the _corpora albicantia_,
and from these some white fibres called the bundle of Vicq d'Azyr
ascend to the optic thalamus (see fig. 11). Behind the body the two
halves diverge much more from each other, and form the posterior
pillars, in the triangular interval between which is a thin lamina of
commissural fibres called the _lyra_ (fig. 13, a). Each posterior
pillar curves downward and outward into the descending cornu of the
ventricle, and, under the name of _taenia hippocampi_, forms the
mesial free border of the hippocampus major (fig. 13, l). Eventually
it ends in the substance of the hippocampus and in the uncus of the
temporal lobe. If the body of the fornix be now divided by a
transverse incision, its anterior part thrown forward, and its
posterior part backward, the great transverse fissure of the cerebrum
is opened into, and the velum interpositum lying in that fissure is
exposed.

The _velum interpositum_ is an expanded fold of pia mater, which
passes into the anterior of the hemispheres through the great
transverse fissure. It is triangular in shape; its base is a line with
the posterior end of the corpus callosum, where it is continuous with
the external pia mater; its lateral margins are fringed by the choroid
plexuses, which are seen in the bodies and descending cornua of the
lateral ventricles, where they are invested by the endothelial lining
of those cavities. Its apex, where the two choroid plexuses blend with
each other, lies just behind the anterior pillars of the fornix. The
interval between the apex and these pillars is the aperture of
communication between the two lateral ventricles and the third,
already referred to as the foramen of Monro. The choroid plexuses
contain the small _choroidal arteries_; and the blood from these is
returned by small veins, which join to form the _veins of Galen._
These veins pass along the centre of the velum, and, as is shown in
fig. 1, open into the straight sinus. If the velum interpositum be now
carefully raised from before backward, the optic thalami, third
ventricle, pineal body and corpora quadrigemina are exposed.

[Illustration: FIG. 13.--A deeper dissection of the Lateral Ventric