THE NERVOUS SYSTEM AS A WHOLE 403 



brush. By the interlacing of the end brush of one 

 neuron with the dendrite of another neuron there is 

 formed a conduction pathway for the nerve impulse. We 

 can compare the interlacing of end brush and dendrite 

 with the interlacing produced when we put the fingers of 

 the one hand loosely between the fingers of the other. 



From what we have just read we can understand that 

 the nerve pathway, along which an impulse must travel, 

 does not act like a continuous "wire," but like a wire 

 with "breaks" in it, or, at least, with places where there 

 is a great deal of resistance (cf. 157). Because there are 

 "breaks" in its path, some of them harder to cross, some 

 easier, the nerve impulse finds a preferred pathway. 

 This is the easiest path: the "path of least resistance." 

 In much the same way a stream of water flowing over 

 rocky soil, although it seems to have many courses from 

 which it can choose its channel, turns aside from all but 

 the one that offers the least resistance. We can also 

 compare the nerve impulse to a traveller. While the 

 traveller is inexperienced, one route looks as good as an- 

 other; but when he has travelled many times, he finds 

 certain "connections" easy, and always goes by the 

 routes that give those connections. 



How are these ''preferred pathways" laid out for our nerve im- 

 pulses? Some of them we inherit: the same impulses came to our 

 ancestors, and found the " paths of least resistance" long, long ago. 

 But others of these pathways are developed by experience. As some 

 certain stimulus moves again and again through the same chain of 

 neurons, it finds the path easier and easier to follow, until it finally 

 falls into that path, automatically, as soon as it starts on its journey 

 to or from the central system. 



