EVOLUTION OF NERVOUS SYSTEM 93 



THE NERVOUS SYSTEM IN VERTEBRATES 



The central nervous organ of the vertebrate, using this term 

 in the most limited sense, consists of the spinal cord and the 

 brain. The spinal cord throughout the vertebrate series is a 

 relatively uniform structure, but the brain exhibits a strikingly 

 progressive development. This is seen especially in two of its 

 parts, the cerebellum and the cerebral hemispheres. Both, but 

 particularly the cerebral hemispheres, grow immensely in size 

 and complication as we proceed from the fishes to man. In 

 the lower vertebrates the cerebral hemispheres are concerned 

 almost entirely with the sense of smell and their cortex, or outer 

 covering, is spoken of as the olfactory cortex, or archipallium. 

 In the mammals, however, the hemispheres have reflected 

 upon them, in addition to olfaction, practically all the sensory 

 activities of the body as well as the mechanism for the volun- 

 tary control of the musculature. These added parts constitute 

 the neopallium which shoves the olfactory archipallium into 

 the background. Thus the neopallium comes to be the great 

 central organ of the higher animals. It receives almost the 

 whole of the sensory inflow; it stores the impressions of the 

 past; and from it emanate those impulses that excite what we 

 call our volitions. 



It is a remarkable fact that in man the cerebral cortex con- 

 sists of layers of nerve cells so regularly arranged that a rough 

 estimate of their number may be made. This is believed to be 

 approximately 9,200,000,000. This prodigious number of 

 cells is estimated to weigh a little over thirteen grams and to 

 occupy the space of less than a cubic inch. When it is re- 

 membered that every human being develops from an egg cell 

 of approximately one fifth of a millimeter in diameter and that 

 this cell begins growth by dividing into two, and these two each 

 into two thus making four, and then into eight, sixteen, thirty- 

 two, and so forth, it will be seen what a stupendous process 



