COMPARATIVE ANATOMY OF THE ENCEPHALON. 163 



Callosum, is deficient. There is gradually to be noticed, however, in ascend- 

 ing the scale, a backward prolongation of the Cerebral hemispheres ; so that 

 first the Optic ganglia, and then the Cerebellum, are covered by them. The 

 latter partly shows itself, however, in all but the Q,uadrumana, when we look 

 at the brain from above downwards ; in the Rabbit, which is among the lowest 

 in this respect of the true Viviparous Mammajia, nearly the whole of the Cere- 

 bellum is uncovered. In proportion to the increase of the Cerebral hemi- 

 spheres, there is a diminution in the size of the ganglia immediately connected 

 with the organs of sense ; and this in comparison, not only with the rest of the 

 Encephalon, but even with the Spinal Cord ; so that in Man the Tubercula 

 Q,uadri gemma are absolutely smaller than they are in many animals of far 

 inferior size. The internal structure of the hemispheres becomes more com- 

 plex in the same proportion as their size and the depth of the convolutions 

 increase ; and in Man all these conditions present themselves in a far higher 

 degree than in any other animal. In fact it is only among the Ruminantia, 



[Fig. 33. 



A view of the base of the Cerebrum and Cerebellum, together with their nerves; 1, anterior extremity 

 of the fissure of the hemispheres of the brain; 2, posterior extremity of the same fissure; 3. the anterior 

 lobes of the cerebrum; 4, its middle lobe; 5, the fissure of Sylvius; 6, the posterior lobe of the cerebrum; 

 7, the point of the infundibulum ; 8, its body; 9, the corpora albicantia; 10, cineritious matter; 11, the crura 

 cerebri; 12, the pons varolii; 13, the top of the medulla oblongata; 14, posterior prolongation of the pons 

 varolii; 15, middle of the cerebellum; 16, anterior part of the cerebellum; 17, its posterior part and the 

 fissure of its hemispheres; 18, superior part of the medulla spinalis; 19, middle fissure of the medulla 

 oblongata; 20, the corpus pyramidale; 21, the corpus restiforme; 22, the corpus olivare; 23, the olfactory 

 nerve; 24, its bulb; 25, its external root; 23, its middle root; 27, its internal root; 28, the optic nerve 

 beyond the chiasm; 29, the optic nerve before the chiasm; 30, the motor oculi, or third pair of nerves ; 

 31, the fourth pair, or pathetic nerves; 32, the fifth pair, or trigemini nerves; 33, the sixth pair, or motor 

 externus; 34, the facial nerve; 35, the auditory the two making the seventh pair; 36, 37, 38, the eighth 

 pair of nerves. (The ninth pair is not here seen.) ] 



