CHAP. X.] THE MESOCEPHALE. 275 



only do they exist as four. In birds,, reptiles, and fishes, they are 

 only two in number, and are called optic lobes, from their con- 

 nexion with the optic nerves. They are hollow in these classes, 

 but in mammalia they are solid. 



Between each testis and the corresponding hemisphere of the 

 cerebellum, a band of fibrous matter extends processus cerebelli 

 ad testem. Each band may be traced into the crus cerebelli of the 

 same side, of which it forms the superior layer, so that its fibres are 

 doubtless continuous with some of those which form the white 

 plates of the median and lateral lobes. The connexion of these 

 processes with the testes is more apparent than real. They seem 

 rather to pass beneath them to the optic thalami ; and, therefore, 

 it has been justly remarked, they might be more appropriately named 

 processus cerebelli ad cerebrum. The valve of Vieussens occupies the 

 interval between these processes. This layer evidently results from 

 the spreading out of some of the anterior lamella of the superior 

 vermiform process. 



From the preceding description it will appear, as before stated, 

 that the stem of fibrous matter which forms the crus cerebelli 

 derives its fibres from, or is continuous with, three planes of fibrous 

 matter: the highest, or most superficial, being the processus cerebelli 

 ad testem ; the second, or middle, the restiform body; and the infe- 

 rior the fibres of the pons. By the first, the cerebellum and cere- 

 brum are connected ; by the second the cerebellum is connected 

 with the medulla oblongata; and by the third, each hemisphere is 

 brought into union with its fellow, and with the mesocephale. 

 Foville assigns other fibres as constituents of the crura cerebelli, 

 which he describes as expansions connected with the fifth and 

 auditory nerves. 



The crura cerebelli seem to emerge from the posterior angles 

 of the mesocephale. From its anterior part there proceed up- 

 wards, with a slight divergence, two similar processes, of consider- 

 able thickness, which enter each hemisphere of the brain, and 

 upon which each of those masses rests, as a mushroom upon its 

 stalk. 



A septum of a similar kind to that described in the medulla 

 oblongata is found in the mesocephale. The fibres derived from 

 the superficial layer of the pons pass backwards from the median 

 groove to the posterior and superior part of the mesocephale. 



Of the Cerebrum. The constitution of each crus cerebri may be 

 best understood by examining a transverse section made a little 

 beyond its emergence from the mesocephale. Upon the surface 



T 2 



