NERVE IMPULSES AND REFLEX ACTION 316 



these nerve centers in any new way after a person has grown 

 up. Education of the body and the mind is thus first of all 

 the training of reflexes. Since one must employ these ser- 

 vants all the rest of his life, it is of the greatest importance 

 that they be trained aright. This is the reason why it is 

 necessary to give so much attention to education, to physical 

 training and to all discipline that develops useful habits of 

 thought or action. 



THE NERVE IMPULSE 



We have mentioned the fact that nerve impulses pass over 

 the nerve fibres, and considering the differences in the re- 

 sults they produce, one would naturally suppose that there 

 would be corresponding differences in the impulses; e. g. that 

 the impulse which produces a sensation of light must be very 

 different from one producing a sensation of sound. This is 

 not the case, however. An electric current through a wire 

 may produce different results according to the different kinds 

 of apparatus employed at the end of the wire: it may ring a 

 bell, or produce light in a bulb, or sound in a telephone; but 

 though the results are various, the electric current is essen- 

 tially the same in all cases. So in the body, the impulse 

 which travels over a nerve is always essentially the same. 



No one need be told that nerve messages travel over the 

 nerves very rapidly. One can appreciate no lapse of time 

 between willing to move the fingers and their actual motion, 

 and yet the message must meantime have passed from the 

 brain to the muscles concerned. The rate at which messages 

 pass into the central nervous system, i. e. the rate over sen- 

 sory nerves, is greater than that at which messages pass out 

 over motor nerves. The former travel inward about 140 

 feet per second, while the latter travel outward at a rate of 

 about 110 feet per second. 



But what is the impulse which travels over nerves? The 

 old idea was that the nerves were hollow and filled with a 



