Chap. XII.] 



BRAIN OF MAMMALS. 



425 



so-called "arbor vitse" of the human brain, it is 

 marked externally by fairly deep transverse fissures. 

 The external surface of the cerebral hemispheres is 

 smooth, but the corporate striata are very well de- 

 veloped. 



The most important and instructive changes are 

 to be seen in the brain of 

 the Mammalia ; these 

 depend chiefly on the great 

 development of the com- 

 missures, which connect 

 the two halves of the 

 brain with one another, 

 and on the gradually in- 



/ 



creasing sizeof the cerebral 



o 



hemispheres which ends 

 in their having an extra- 

 ordinary predominance 

 over the other parts of 

 the brain ; hand in hand 

 with their increase in 

 size and extent is the 

 improvement of the in- 

 tellectual faculties. But 

 the cerebral hemispheres 

 do not merely increase 

 in bulk, their surface be- 

 comes marked by grooves, 

 and the amount of sur- 

 face thereby developed is, 

 as we have alreadv said, 



> 



greatly extended without 



any corresponding or proportionate increase in the 



size of the cranial cavity. 



The olfactory lobes lie more or less below the 

 cerebral hemispheres, and diminish in proportionate 

 size as we ascend the series ; the cerebral hemispheres 



Fig. 183. Brain of Tupain, to show 

 the large Olfactory Lobe, the 

 ungrooved Cerebral Hemi- 

 spheres, and the large Cere- 

 bellum. (After Garrod. P.Z.S., 

 1879, p. 304.) 



