BRAIX of TITF. ZriUGLODONTIDVK. 



621 



of Insochivore origin) all possess liypertropliied parafloccuH. Tins 

 convergent hypertrophy of the same cerebellar region must arise 

 from some cause common to nil. Ferrier was one of the first to 

 emphasise the functions of the cerebellum as primnrily concerned 

 with equilibration. Ingvar (1918) hrts sui)ported this conception, 

 and he finds a certain pattern of equilibratory localisation in the 

 •orgnn. This pattern is that of a "compass," a lesion in any 

 given pai't of the cerebellum entailing a defect in co-ordinating 

 muscle-movement in such a way as to resist falling in that 

 direction ; i. e., the animal falls in the direction indicated by the 

 site of the lesion. This theory seems adequate to account for 

 the p.'iralloccular expansion exlubited by marine Mammalia, for 



pedurjc/e 



Text-figure 2. 



^'f^.'^°S-y Anterior view . 



Olfactory peduncle. 



(ophthalmic n. 

 ^axillary n. 



'Optic nerve. 



sgittol s/nus. 

 Cerebral hem/sphere. 

 Gasser/srr ganglion. 



Median tentona/^ 

 depression. 



Piece of 

 matri*: 



Middle cerebral 

 vessel. 



Parafloccuius. leash. Lobus melius cerebelli. 

 Dorsal nnd anterior views of n.itufi*! endocriiiiial cast of Zeu'cflodon sensitivus, sp. iiov. 

 M. 12123. About i nat. size. 



.the parafloccuius is situated entirely laterally in the cerebellum, 

 and these cresitures are . under the necessity of resisting con- 

 tinuously the tendency to "rolling," which the lluid medium 

 postulates. In brief, it is a mechanism evolved to preserve 

 an even, keel. 



The " parafloccuius " is bounded above by the region which in 

 the subsequent account is referred to as the lobus medius. 

 Anteriorly, the parafloccuius abuts on the cerebrum and the 

 Gasserian ga.nglion ; inferiorly, upon the eighth nerve and the 

 petrous temporal and the casts of the foramen lacervim medium 

 and the foramen lacerum posterius ("jugular leash" of the 

 ■figures). Posteriorly, it is in contact with, the exoccipital bone. 



