THE EVOLUTION OF THE BRAIN 95 



only about 1.9 per cent of his total weight, while that of 

 woman is about 2.3 per cent, an excess of a fraction of i 

 per cent in her favor. 



Roughly speaking, the brain of man is about 2 per cent of 

 his total weight or twice the corresponding percentage of an 

 average animal such as the cat. The percentage in man, 

 however, does not by any means reach the 5 per cent attained 

 in such small mammals as mice. Here apparently occur the 

 highest percentages known between brain and body weights, 

 a condition dependent rather upon the requirement of a 

 minimum amount of brain substance for normal function 

 than upon excessive mentahty. 



The brain is the most comphcated organ in the vertebrate 

 body. It is a most intricate arrangement of centers and 

 connections that far exceeds in complexity the most elaborate 

 telephone system. It is at once the despair and the joy of the 

 working neurologist, for its comphcations seem hmitless, 

 while the problems hidden in its details are of the first order. 



To know the brain we cannot consider it separately from 

 the spinal cord, that strand of nervous tissue which stretches 

 from the brain backward through much of the body. In the 

 fishes the brain is only a fraction of the weight of the spinal 

 cord. In the frog the brain and cord are about equal. In all 

 higher animals the brain gains over the cord till in man the 

 cord is represented by a rod of nervous tissue somewhat 

 thicker than a lead pencil, roughly a foot and a half long, and 

 with a weight of some 26 grams or about a fiftieth that of 

 the brain. These changed relations are not due to a reduction 

 in the cord but rather to an excessive development of the 

 brain. Starting in the fishes as a relatively inconspicuous 

 organ the brain grows in proportionate size till in man it far 

 overtops all other parts of the nervous system. 



To gain some acquaintance even in a superficial way with 

 the organization of the human brain, it is best to look first 

 at the brain of some simple representative vertebrate, such 

 for example as that of a frog. The brain of this animal lies 

 in a bony skull and upon exposure it is seen to consist of an 

 elongated stem or axis which expands here and there into 

 special prominences or lobes. The spinal cord of the frog, 

 which is of relatively uniform thickness, gradually enlarges 



