282 THE HUMAN MECHANISM 



others diminish activity or abolish it altogether (p. 160). 

 The beat of the heart is quickened by one set of nerves 

 and slowed by another; the circular muscular fibers of the 

 arterioles are excited to contract by vasomotor nerves, their 

 tonic constriction is paralyzed or inhibited by vasodilators, 

 and many other examples might be drawn from the action 

 of neurones on peripheral organs of the body. 



Precisely the same thing is true in the brain and spinal 

 cord. Afferent impulses may not only reflexly excite neu-, 

 rones to activity but may also inhibit the existing or threat- 

 ened activity of other neurones, as when a sneeze is stopped 

 by biting the upper lip or by pinching the nose; or an 

 action may be inhibited by a volitional impulse from the 

 cerebrum, as when the breathing movements are voluntarily 

 stopped for a while, or when we similarly stop a wink or 

 a sneeze. These are all examples of inhibition, not of the 

 skeletal muscles concerned but of the neurones which inner- 

 vate them in other words, of the inhibition of one neurone 

 by another. 



It must be understood that inhibition is as essential a 

 part of the activity of the nervous system as is excitation. 

 Just as the driver of a team must urge on one horse while 

 he restrains another, so in all more complicated actions, 

 probably in all actions, reflex or volitional, the orderly 

 movement is as much the result of holding one neurone in 

 check as of stimulating another one to work, or to work 

 harder. Consciousness proves its presence most conclusively 

 by suppressing reflexes which would otherwise inevitably 

 occur and by bringing about new movements to meet the 

 desired end. Even in the highest processes of the most 

 highly organized of nervous systems, namely, those in which 

 human action originates, the man reveals his character and 

 influences the world around him by what he does not do 

 by what he refrains from doing, sometimes at the cost of 

 severe struggle against impulse, instinct, or passion quite 



