DISSECTION 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- 

 fundil)ulum. This can be seen best in a median longi- 

 tudinal section of the brain. Posteriorly the 3d ven- 

 tricle passes into the aqttedttct of Sylvms just beneath 

 the posterior commissure. A bristle can be passed 

 backward easily along the aqueduct into the 4th ven- 

 tricle. 



By scparatijig the posterior portion of the cerebral 

 hemisphere completely from the optic thalamus a good 

 view of the latter can be obtained, and the zuay i7i 

 wJiich the optic tract comes around from the ventral 

 surface of the brain to end partly in the tJialamtis and 

 partly in the corpora quadrigemina is 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. 



