NERVOUS SYSTEM OF VERTEBRATA. 559 



on the surface ; and from the interposition of a 

 substance of a grey colour between the laminae 

 of the white medullary matter, a section of the 

 cerebellum presents the curious appearance 

 (seen in Fig. 459), denominated, from its fancied 

 resemblance to a tree, the Arbor VitcB. 



Thus far we have followed an obvious gradation 

 in the developement and concentration of the dif- 

 ferent parts of the brain ; but on arriving at Man, 

 the continuity of the series is suddenly disturbed 

 by the great expansion of the hemispheres, 

 (Fig. 461), which, compared with those of quad- 

 rupeds, bear no sort of proportion to the rest of 

 the nervous system. Both Aristotle and Pliny 

 have asserted that the absolute, as well as the 

 comparative size of the human brain is greater 

 than in any other known animal ; exceptions, 

 however, occur in the case of the Elephant, and 

 also in that of the Whale^ whose brains are cer- 

 tainly of greater absolute bulk than that of man. 

 But all the large animals, with which we are 

 familiarly acquainted, have brains considerably 

 smaller ; as will readily appear from an exami- 

 nation of their skulls, which are narrow and com- 

 pressed at the part occupied by the brain ; the 

 greater part of the head being taken up by the 

 developement of the face and jaws. In Man, on 

 the other hand, the bones of the skull rise per- 

 pendicularly from the forehead, and are extended 

 on each side, so as to form a capacious globular 



