THE BRAIN 353 



triele, 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 aqneduct, or iter a tertio ad qnar- 



tum ventriculmn, is the cavity or ventricle of 



the mid-brain, 

 ii. The optic lobes, or corpora qoadrigemina, 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. 



D 



Fig. 45. — Lepns cnnicnlus. Transveise section of the brain, passing 

 through the middle of the length of the cerehellum. The shading 

 indicates diagranuuaticaUy the distribution of white and grey 

 matter, (a. m. m.) 



A. Tennis, or middle lobe of the oerebellam. B, lateral lobe of the cere- 

 bellum. C fioccnlar lobe of the cerebellum. D, transrerse fibres of the pons 

 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 dovrawards. It has a 

 very thin roof, but thick floor and sides. 



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

 sens, is a thin membrane extending from the 



A A 



