ami, developed in
the anterior cerebral vesicle, but also the corpora quadrigemina and
crura cerebri developed in the mesencephalon or middle cerebral
vesicle. The cerebrum is ovoid in shape, and presents superiorly,
anteriorly and posteriorly a deep _median longitudinal fissure_, which
subdivides it into two hemispheres. Inferiorly there is a continuity
of structure between the two hemispheres across the mesial plane, and
if the two hemispheres be drawn asunder by opening out the
longitudinal fissure, a broad white band, the _corpus callosum_, may
be seen at the bottom of the fissure passing across the mesial plane
from one hemisphere to the other. The outer surface of each hemisphere
is convex, and adapted in shape to the concavity of the inner table of
the cranial bones; its inner surface, which bounds the longitudinal
fissure, is flat and is separated from the opposite hemisphere by the
falx cerebri; its under surface, where it rests on the tentorium, is
concave, and is separated by that membrane from the cerebellum and
pons. From the front of the pons two strong white bands, the _crura
cerebri_ or _cerebral peduncles_, pass forward and upward (see fig.
2). Winding round the outer side of each crus is a flat white band,
the _optic tract_. These tracts converge in front, and join to form
the _optic commissure_, from which the two _optic nerves_ arise. The
crura cerebri, optic tracts, and optic commissure enclose a
lozenge-shaped space, which includes--(a) a grey layer, which, from
being perforated by several small arteries, is called _locus
perforatus posticus_; (b) two white mammillae, the _corpora
albicantia_; (c) a grey nodule, the _tuber cinereum_, from which (d)
the _infundibulum_ projects to join the _pituitary body_. Immediately
in front of the optic commissure is a grey layer, the _lamina cinerea_
of the third ventricle; and between the optic commissure and the inner
end of each Sylvian fissure is a grey spot perforated by small
arteries, the _locus perforatus anticus_.

If a transverse section is made at right angles to the surface of the
crura cerebri it will pass right through the mesencephalon and come
out on the dorsal side through the corpora quadrigemina (see fig. 8).
The ventral part of each crus forms the crusta, which is the
continuation forward of the anterior pyramidal fibres of the medulla
and pons, and is the great motor pat