COORDINATION 87 



and thus added on to the stock of inherited reflexes which 

 make part of the equipment with which we begin life. 



16. The complexity of the mechanisms of the nervous 

 system. Such actions as we have been studying whether 

 the inherited reflex of winking, even when this is combined 

 with other acts like the secretion of tears, or the acquired 

 conditioned reflex secretion of saliva from the stimulation 

 of the skin by cold are comparatively simple, as compared 

 with many other actions of daily life, such, for example, 

 as the throwing of a stone. Here not only muscles which 

 produce motion at the shoulder, elbow, wrist, and finger 

 joints are called into play, but also muscles which maintain 

 the erect position and balance of the body as a whole. 

 The entire nervous mechanism involved baffles the imagi- 

 nation to conceive ; and yet any boy can perform the act. 

 He can do it, however, because his motor neurones are 

 grouped together into a perfectly well-organized army which 

 executes at once the bidding of its commander in chief 

 the will. 



We have given in the foregoing pages a mere glimpse 

 into the complexity of one part of the wonderful nervous 

 mechanism. No watch, no machine which man has ever 

 invented or constructed can for a moment compare with this 

 living machine in complexity or in perfection. Yet, like all 

 machines, this one can be abused ; it can get out of order ; it 

 can even break down. And we have already learned enough 

 to understand why this is so. Some neurones may be injured 

 by overwork or may degenerate from disuse ; indulgence in 

 stimulants or narcotics may poison the governing nerve cells ; 

 above all, constant failure to lead a normal life may deprive 

 these cells of their sole means of repair. The human body 

 is a machine designed for use, even for hard use, and it 

 thrives upon right use ; but it is a machine too delicate and 

 too complex to be abused with impunity. 



