THE NERVOUS SYSTEM OF MAN 1 57 



three nerves in the whole body of the higher 

 animals that are not mixed, in that they transmit 

 both sensory and motor stimuli. These three are 

 the olfactory, the nerve of smell; the optic, the 

 nerve of sight (even here two kinds exist), and 

 the nerve of hearing. Injury to a nerve in the 

 arm may thus not only destroy all sensation in the 

 hand but also paralyze the muscles of the fingers. 



In discussing the subject of reproduction refer- 

 ence was made to the formation of the nervous 

 system from the superficial body cells. (Fig. 26 

 G-I.) At this state of development, the embryonic 

 nerve cells do not have processes. But before 

 birth or hatching, all of the processes essential to 

 the fundamental vital activities are formed and 

 have taken their place in all parts of the body. 

 The total number of nerve cells found in the nerv- 

 ous system of any given animal, including man, is 

 nearly all grown before birth. The sum of these 

 microscopic units in the cerebrum of man is esti- 

 mated at the incredible number of 9280 million, 

 all of which were formed before birth. There 

 are many million additional nerve cells in the 

 remainder of the human brain, which also take up 

 their position before birth. This means that the 

 nerve cell machinery for the body of man is ready 

 for use when the child is born. 



Man, like all other animals, and in just the 

 same fashion, is surrounded by physical vibrations 



