. (P>/'^^ HEAD. 95 



duct of Sylvius ; the anterior presents the lamina cine- 

 rea or grey root of the optic nerves ; 2 — Aqueduct of 

 Sylvius posteriorly opens into the fourth ventricle ; 

 3 — Posterior cerebellar or fourth ventricle closed su- 

 periorly by the valve of Yieussens and of Renault. 



{c.) Stkucture : formed of three fasciculi of white 

 substance, a superior or corpora restiforma, an inferior 

 or corpora pyramidalia and a lateral or intermediate; 

 the grey substance is found in the floor of the fourth 

 ventricle, the crura cerebri, the lateral or intermediate 

 fasciculi of the bulb, the corpora quadrigemina, the 

 thalami optici and in the annular protuberance. 



Cerebellum. — {a.) External surface : one middle 

 lobe forming the anterior and posterior vermiform 

 processes ; two lateral lobes, beneath which the cho- 

 roid cerebellar plexus is applied. Its external surface 

 is covered with sulci ; (Z>.) Internal surface on section 

 shows the arbor vitae and corpus rhomboideum. 

 Structure : the white substance is internal and ramify- 

 ing, the grey substance forms cortical layer. 



Cerebrum. — Division : Two lateral hemispheres. 

 {a.) External surface shows: 1 — Interlobularor longitu- 

 dial fissure, whicli extends down to the corpus callosum 

 and is bound posteriorly by the fissure of Bichat, and 

 lodges the great vena Galeni and falx cerebri ; 2 — ce- 

 rebral hemispheres, with four faces covered with con- 



