394 DISSECTION OF THE RABBIT. 



ii. The optic lobes, or corpora quadrigemina, have 

 moderately thick walls, and form the roof of 

 the Sylvian aqueduct, which is prolonged a 

 short distance into them. 



iii. The crura cerebri, which form the floor of the 

 Sylvian aqueduct, are very thick. 



d. The hind-brain. 



i. The fourth ventricle, or cavity of the hind- 

 brain, is moderately wide from side to side, 

 but shallow from above downwards. It has 

 a very thin roof, but thick floor and sides. 



Fio. 70. — lepus cuniculus. Transverse section of the brain, passing 

 through the widest part of the cerebellum. The shading indicates 

 diagrammatically the distribution of white and grey matter. 

 (a. m. m.) 



A, vermis, or middle lobe of the cerebellum. B, lateral lobe of tlie cere- 

 bellum. O. flocoular lobe of the cerebellum. D, transverse fibres of the pons 

 Varolii. IV, fourth ventricle. 



ii. ,The velum medullse anterius, or valve of Vieus- 

 sens, is a thin membrane extending from the 

 optic lobes to the cerebellum, and forming the 

 roof of the anterior half of the fourth ven- 

 tricle. 



iii. The velum meduUae posterius is a thin mem- 

 brane, forming the roof of the posterior half 

 of the fourth ventricle. 



