STRUCTUKE OF THE BEAIN. 



815 



exactly to similar parts met with even in the brain of a reptile. Where, 

 then, are we to look for those grand differences whereby the Hammi- 

 ferous brain is peculiarly characterized ? The peculiarities of the brain 

 of a Mammal are entirely due, first, to the increased proportional deve- 

 lopment of the cerebral hemispheres ; and secondly, to the existence 

 of lateral cerebellic lobes, in connexion with both of which additional 

 structures become requisite. 



(2399.) In those Marsupial tribes that form the connecting links 

 between the Oviparous and Placental Yertebrata, the brain still ex- 

 hibits a conformation nearly allied to that of the Bird, and the great 

 commissures required in the more perfect encephalon are even yet defi- 

 cient ; but in the simplest brain of a Placental Mammifer the charac- 

 teristic differences are at once apparent. 



(2400.) In the Eabbit, for example (fig. 408), the cerebral hemi- 

 spheres (6) are found very ma- 

 terially to have increased in Fig. 411. 

 their proportionate dimensions ; 

 and although, even yet, con- 

 volutions upon the surface of 

 the cerebrum are scarcely in- 

 dicated, additional means of in- 

 tercommunication between the 

 hemispheric masses become in- 

 dispensable. The corpus cal- 

 losum, therefore, or great trans- 

 verse commissure of the hemi- 

 spheres (fig. 408, c), is now 

 superadded to those previously 

 in existence ; while other me- 

 dullary layers, called by various 

 ridiculous names, bring into 

 unison remote portions of the 

 cerebral lobes. 



(2401.) In proportion as in- Brain of the Lion. 



telligence advances, the surface 



of the cerebral hemispheres, becoming more extensive, is thrown into 

 numerous convolutions separated by deep sulci ; until at length in the 

 Carnivora, as, for instance, in the Lion (fig. 411), the cerebrum (e e) 

 attains such enormous dimensions that the other elements of the en- 

 cephalon are, as it were, hidden among its folds. 



(2402.) But, in addition to this increased complexity of the cere- 

 brum, the cerebellum likewise has assumed a proportionate importance. 

 In the Oviparous races this important element of the brain consisted 

 only of the mesial portion, so that no cerebellic commissure was requi- 

 site : but in the Mammal it exhibits in addition two large lateral lobes 



