THE BRAIN 365 



middle commissure, to the corpus albicans, in 



which it ends. 

 c. The mid-brain, or mesencephalon. 



i. The Sylvian aqueduct, or iter a tertio ad quar- 



tum ventriculum, is the cavity or ventricle of 



the mid-brain, 

 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 of great thickness. 



Fig. 64. — 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. u. m.) 



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

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

 Varolii. IV, fourth ventricle. 



d. The hind-brain. 



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

 braia, is moderately wide from side to side, 

 but shallow from above downwards. It has 

 a very thin roof, but thick floor and sides. 



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. 



