216 life's beginning on the earth 



consists of an immense number of nerve fibers, billions of 

 them. These form pathways which serve as outward con- 

 ductors for the nerve impulses that enter the brain, by way 

 of the sensory nerves, from such organs as the eyes and ears. 

 The chief work of the brain is to select the pathways along 

 which the nerve impulses travel through this jungle of fiber 

 network. The pathway selected determines which muscles 

 or other organs will move in following the stimulation of 

 any given sense-organ. 



Incidentally let us remember, at this point, that the nerve 

 impulse which travels along a nerve fiber is an electric 

 disturbance of a peculiar type but essentially the same as 

 the electric charge which travels along a wire. 



As experiments show, the pathway of an impulse depends 

 largely on the pathways traveled by previous impulses. A 

 subsequent impulse travels more easily over the pathway 

 than did its predecessor. Since multitudes of nerve im- 

 pulses from touch, seeing and hearing pass through the 

 human nervous system all day long, each impulse soon has 

 its established pathway. A pathway is much complicated 

 by its branching in many directions, and even more by 

 the "association tracts" which open up between several 

 pathways. 



Still another complication is the inhibition of one im- 

 pulse by another incoming stimulus. The result is that 

 many impulses do not cause any active movements at all. 

 But they do prepare new pathways for impulses traveling 

 over them later, which will elicit movements. 



In a new-born baby, numerous tracts and associations of 

 tracts form and constantly develop further. Later, inter- 

 connecting tracts open up between them. Memory and 

 mental activity thus begin to develop. A number of as- 

 sociated tracts produce definite "memory images." Mem- 

 ory images once associated with each other are called into 

 play on each subsequent occasion. Thus, the odor of a 



