CEREBELLUM. 403 



ferior surface is divided by a median groove, which, receives 

 the basilar artery. 



Its structure externally is fibrous or medullary, and 

 white; the superficial fibres on the lower surface, run 

 transversely from the crus cerebelli on the one side, to the 

 crus on the opposite side, thus making the pons the great 

 commissure of the cerebellum. Beneath these transverse 

 fibres, after turning them aside, is seen a quantity of cin- 

 eritious matter, through which pass medullary fibres corre- 

 sponding to the pyramidal tracts, intermingling with the 

 transverse fibres, and proceeding onward to the crura cerebri. 



The size of the pons is always found to be in direct pro- 

 portion to the hemispheres of the cerebellum. When the 

 cerebellum is very small or entirely wanting, the pons is 

 likewise proportionally small or wholly absent. This fact 

 is confirmed by comparative anatomy, as in birds, fishes 

 and reptiles, the cerebellum being entirely wanting or 

 only rudimental, the pons is equally wanting. 



The pons is so connected with the -medulla oblongata, 

 cerebellum, and cerebrum, as to be regarded as having a 

 common relation to the whole. 



CEREBELLUM LESSER OR INFERIOR BRAIN. (Fig. 113.) 



Dissection. Kaise the posterior lobes of the cerebrum, or 

 remove them entirely, and divide the tentorium, when the 

 cerebellum will be exposed. 



It is situated beneath the tentorium in the inferior oc- 

 cipital fossae. Its size is estimated to be seven times 

 smaller than the cerebrum, and according to Grail, larger 

 in the female than the male. The average weight is about 

 four ounces and a half. Its form is that of an "ellip- 

 soid" oval in the transverse diameter, and measuring 

 from three and a half to four inches in this direction. 

 Its antero-posterior diameter is from two to two and a 

 half inches, and its vertical, in the thickest part, about 

 two inches. It is divided into two lateral and sym- 

 metrical parts called hemispheres. This division is made 

 by the anterior and posterior median fissurSj or notches, 



