282 AFFERENT AND EFFERENT IMPULSES. 



constantly sending messages to the muscles to make them act. 

 The shock of the blow has stopped the sending forth of these 

 messages, and so the body is no longer supported. None of 

 the muscles that support the body have been injured or even 

 touched. 



In studying the muscles, the fact was noted that the 

 muscles are always slightly "on the stretch." This means 

 that nerve impulses are continually passing from the central 

 nervous system which tend to keep the muscles in a state 

 of slight contraction. 



We have, in youth, such a boundless store of energy that 

 we do not sufficiently consider these matters. But if one 

 wishes to follow the intellectual life long and successfully, 

 he must learn to economize energy, and to direct his forces 

 into useful channels. And one important part of this knowl- 

 edge is learning how to rest. It is an art that very few have 

 well learned. 



The brain is like a telegraph office in both receiving and 

 sending out messages. Unlike the telegraph office, it has 

 one set of fibers to bring currents in (afferent), and another 

 to carry currents outward (efferent). 



We have concerned ourselves thus far chiefly with efferent 

 nerve fibers and efferent currents. By way of review it may 

 be stated that these efferent currents are sent mainly to 

 muscles, to make them shorten or to relax, or to gland-cells 

 to control their activity. The only other efferent currents, so 

 far as known, are those which possibly go to the cells of the 

 tissues to regulate their nutrition or their heat production. 



Having given so much attention to the outgo of nerve 

 impulses, let us ask the question, " What about the incoming 

 nerve currents ? " 



" All life long the never-ceasing changes of the external 

 world continually break as waves on the peripheral endings 



