NERVOUS SYSTEM OF VERTEBRATES. 



75 



Fig. 40. — Brain of Ornitho- 

 rhynchus. (Natural size.) 

 (After Parker.) 



nerve force. The former, therefore, is the higher. The 

 organization of the brain is tested by the relative amount 

 of gray matter. Comparing again 

 to electricity, the electro-motive 

 force vanes as the gray matter. 

 Further, this relative amount, 

 other things being equal, is ex- 

 pressed by the number and depth 

 of the convolutions. Now, of all 

 animals the number and depth of 

 the convolutions of the cerebrum 

 is by far greatest in man. In 

 higher mammals, especially an- 

 thropoid apes, the convolutions 

 are well marked, but they become 

 less and less so as we go down 

 the mammalian scale, until they entirely disappear before 

 we reach the lozvest mammals (Fig. 40). Therefore all 

 below mammals — i.e., all birds, reptiles, and fishes — have 

 smooth brains. 



There is, however, a modifying cause here which 

 must not be neglected ; it is, that the brains of small 

 animals tend to smoothness irrespective of deficiency of 

 gray matter. The reason is that bulk varies as the cube, 

 while surface only as the square of the diameter, and 

 therefore a small sphere has proportionately a greater 

 surface than a large sphere. It follows from this that 

 the brain of a small animal, though smooth, may have 

 as much gray matter proportionately as the highly con- 

 voluted brain of a large animal. Thus all small mam- 

 mals have smooth brains, while large mammals have all 

 convoluted brains. The brain of an elephant, or even 

 of a whale, is wonderfully convoluted, almost as much 

 so as that of man. 



As we go back in the embryonic series the brains of 



