156 MAN, THE ANIMAL 



the conditions under which they work. All 

 nerves consist of numerous microscopic threads, 

 each of which is wholly isolated from all others. 

 One has but to think of a telephone cable with 

 several different lines, each of which may be used 

 simultaneously or one or two at a time. There is 

 this fundamental distinction, however, that nerve 

 fibers carry messages only one way. In higher 

 animals every stimulus must enter into connection 

 with the central organ before a reply can be made. 



When a message starts out over a given route 

 in a nerve, it does not switch off and continue on 

 any other of the several similar pathways leading 

 to the central nervous organ, because nerve im- 

 pulses pass across the synapse in one direction 

 only. Thus the expert can designate the several 

 possible pathways over which touch, heat, cold, or 

 pain pass until they reach the brain. This map- 

 ping out of the central nervous organ is of great 

 value in detecting the exact location and extent of 

 an injury or disease in the brain. 



When the sensory nerve brings its message into 

 the brain, associational nerve cells distribute it to 

 all parts. If a definite response is thus called forth, 

 the motor nerves carry a message to the muscles, 

 which stimulates them to act in a given manner. 

 For some distance sensory nerve fibers and motor 

 fibers may be found in the same nerve bundle of 

 the arm or leg. It is impossible to find more than 



