268 PHYSIOLOGY. 



thing is very much less, showing that, in many cases, 

 fatigue is mental rather than muscular. What we do from 

 habit, and cheerfully, is easily done. Hence the desira- 

 bility of forming good habits, that we may, without un- 

 necessary effort, that is, without loss of energy, do 

 what is needed for our well-being. 



Fatigue from Standing. We are not conscious of 

 expending energy in standing until we begin to be weary ; 

 but the fact that a blow on the head causes one to fall 

 reveals the fact that the brain is constantly sending mes- 

 sages 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. 



The Usefulness of Resting. 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 knowledge is 

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

 learned. 



Nervous System compared to a Telegraph System. 



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). 



Efferent Currents. We have concerned ourselves thus 

 far chiefly with efferent nerve fibers and efferent currents. 

 These efferent currents are sent mainly to muscles, to 

 make them shorten or to relax, or to gland cells, to control 



