THE NERVOUS SYSTEM 279 



12. The maintenance of balance and the regulation of mus- 

 cular tone. Walking, however, involves more than the action 

 of the neuromuscular mechanisms of the legs ; for here, as 

 weU as in complicated volitional actions, the balance of the 

 body must be preserved. For this reason we swing the arms 

 and execute ever-changing contractions of the muscles of the 

 trunk. Moreover, a proper state of tonic contraction in each 

 muscle is necessary to the proper execution not only of the 

 act of walking but of other acts as well, whether these are 

 volitional or nonvolitional. Into the mechanism of this won- 

 derfully perfect function of the body we cannot go within 

 the limits of the present book; but there is good ground 

 for thinking that, at least in the mammals, the cerebellum 

 is a very important and probably the all-important organ 

 concerned in effecting these coordinations. 



13. Actions resulting from nervous processes originating 

 within the cerebrum. A very large part of the activities of 

 the body are thus fundamentally reflex actions ; they do not 

 require the aid of consciousness for then* execution. And it 

 is fortunate for us that this is the case; one has only to 

 imagine a human being who has to give his attention, or 

 "his mind," as we often say, to every adjustment of the 

 digestive, respiratory, and vascular systems required to meet 

 the changing necessities of life ; who has to keep his thoughts 

 on every movement of walking or running; who has to be 

 constantly on guard against loss of balance even when sitting 

 still. Such a being is almost inconceivable ; he would " go 

 crazy " in a single day ; but we can in this way realize to 

 what extent the reflex mechanisms of the body perform the 

 menial offices of life, leaving the mind free for higher things. 



Speech is the result of movements in which the muscles 

 of respiration, those of the larynx, those of the tongue, 

 and those of the lips cooperate to produce articulate and 

 intelligible sound. The act of writing also consists of a series 

 of movements in which the muscles of the arm and hand 



