THE BRAIN. 717 
of the pineal gland opens into it above ; and in 
the hinder part of its floor is a conical depres- 
sion, the infundibulum. 
iii. The aqueductus sylvii or iter a tertio ad quartum 
ventriculum: a narrow passage leading from the 
third to the fourth ventricle: it communicates 
above with the cavities or ventricles of the optic 
lobes, which are hollow. 
iv. The fourth ventricle: the large triangular cavity 
in the medulla already exposed by removal of the 
vascular membrane covering it. 
c. The ventral surface of the brain. 
Cut through the medulla at the level of the hinder end of the 
skull : carefully remove the brain from the cranial cavity, noting 
the several nerves arising from it, and cutting through these as close 
to the skull wall as possible. Lay the brain on its dorsal surface, 
and exanune and draw the ventral surface, showing :— 
i, The optic chiasma: formed by the decussation 
of the roots of the optic nerves; the point of 
crossing being opposite the hinder ends of the 
hemispheres, and immediately in front of the 
infundibulum. 
Trace back the optic nerves behind their point of crossing to 
their origins from the optic lobes. 
ii, The tuber cinereum: a small median swelling 
immediately behind the optic chiasma, and 
caused by the depression of the floor of the third 
ventricle to form the infundibulum. 
iii, The pituitary body: a flattened sac, placed behind 
and continuous with the tuber cinereum. It 
is very liable to be left behind in the skull on 
removing the brain. 
iv. The crura cerebri: two dense white columns of 
nervous matter, lying at the base of the optic 
lobes, and partly hidden by the pituitary body: 
they serve to connect the hemispheres with the 
medulla and spinal cord. 
