210 THE HUMAN MECHANISM 



ordinarily produce motion, because antagonistic muscles are 

 stimulated equally; but in another way we are often con- 

 scious of this increased muscular action. Everyone knows 

 the difference between the " bracing " effects of a cool or 

 cold day and the " relaxed," " slack-twisted " feeling on a 

 warm day ; and this is largely traceable to the sensations 

 which come from the contracting muscles in the former case 

 and to the absence of such sensations from the inactive 

 muscles in the latter. To put it in another way, cold 

 increases the tone of the skeletal muscles (see p. 161). A 

 skeletal muscle on a cold day is never completely relaxed ; 

 like the unstriped muscles of the arteries, it is in a con- 

 dition somewhere between extreme contraction and extreme 

 relaxation. 



This muscular reflex also betrays itself in shivering. 

 Ordinarily the reflex contraction consists of an even, steady 

 tone, but at times it becomes more or less incoordinated, 

 and shivering results. 



25. The regulation of the body temperature a function of the 

 nervous system. We may close this brief account of thermal 

 phenomena of the body by recalling to the attention of the 

 student what must now be obvious at a glance ; namely, that 

 a constant temperature is maintained by the coordinating 

 action of very many nervous reflexes. The action of the 

 vasomotors of the skin and of the internal organs, of the 

 nerves of the sweat glands and of the motor nerves of 

 the skeletal muscles must all be so adjusted with regard 

 to one another that exactly the right balance is preserved 

 amid all the variations of heat production and of climatic 

 conditions which affect heat loss. Success in this adjustment 

 depends upon the skill with which the coordinating nervous 

 system does its part. With the single exception of muscular 

 exertion, no condition of life makes such far-reaching or 

 such imperious demands upon the system as a whole as does 

 the maintenance of the proper internal temperature. Mental 



