The olfactory lobes.
The cerebral hemispheres, or prosencephalic
lobes, with the pineal gland lying in the angle
where they diverge behind.
The corpora bigemina, forming the dorsal part
of the mid-brain.
The unpaired cerebellum ; and, behind it—
The medulla oblongata, covered by a highly vas-
cular plug of pia mater, forming a choroid
plexus. On removing this, the fourth cerebral
ventricle will be partly exposed on the dorsal
aspect of the medulla. Raise the posterior
free border of the cerebellum, and note that
the fourth ventricle is continued forward be-
neath it.
180. Carefully tilt the brain over toward the right,
and study the origin of the various Cranial Nerves on
the left side.
a.
The olfactory (first cranial nerve), proceeding
from the end of the olfactory lobes.
The optic or second cranial nerve (171).
The third nerve (oculo-motor) arising from the
ventral aspect (crus cerebri) of the mid-brain.
The fourth nerve (patheticus) will be first found
on the dorsal side of the crus cerebri, close
to the cerebellum ; thence it passes forward and
downward till it reaches the base of the skull, ~
on which it runs forward parallel and exter-
nal to the third nerve. The three preceding


h.
THE CRANIAL NERVES. 71


nerves all disappear through the dura mater
closing in the skull cavity in front, and sep-
arating it from the orbital cavity.
Carefully separate the soft parts beneath from
the bony roof of the temporal fossa and orbit,
and chip away the bone. Note the large ji/th
nerve (trigeminal) arising from the anterior part
of the side of the medulla, and passing through
the dura mater; also the Gassertan ganglion
(174), which may be exposed by removing the
membrane over it.
