DISSHECTION OF THE BRAIN. 85 
Cutting through the middle commissure, the 3d 
ventricle can be seen to pass downward and forward 
toward the base of the brain, ending finally in the in- 
fundibulum. This can be seen best in a median longi- 
tudinal section of the brain. Posteriorly the 3d ven- 
tricle passes into the agueduct of Sylvzus just beneath 
the posterior commissure. A bristle can be passed 
backward easily along the aqueduct into the 4th ven- 
tricle. 
By separating the posterior portion of the cerebral 
hemisphere completely from the optic thalamus a good 
view of the latter can be obtained. and the way in 
which the optic tract comes around from the ventral 
surface of the brain to end partly in the thalamus and 
partly in the corpora quadrigemina ts nicely shown. 
3. The Mid-brain or Mesencephalon. 
a. THE CORPORA QUADRIGEMINA. They form the 
dorsal surface of the mid-brain, and surround the aque- 
duct of Sylvius. 
b. THE CRURA CEREBRI form the base of the mid- 
brain. They can be seen passing forward from the 
anterior border of the pons to the optic thalamus. 
THE ANTERIOR CRURA OF THE CEREBELLUM pass 
forward along the sides of the 4th ventricle from the 
cerebellum to the corpora quadrigemina. The middle 
and posterior crura have already been seen, but ought 
to be located again at this point to get a complete 
idea of all the connections of the cerebellum. 
