THE BRAIN 353 



tricle, running backwards and downwards 

 from the body of the fornix, beneath the 

 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. 45. Lepns cuniculus. Transverse section of the brain, passing 

 through the middle of the length 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 the cere- 

 bellum. C, floccular lobe of the cerebellum. D, transverse fibres of the poiis 

 Varolii. IV., fourth ventricle. 



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. 



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

 sens, is a thin membrane extending from the 



A A 



