NERVE IMPULSES AND REFLEX ACTION 317 



churning action. In the same way, when the food commences 

 to pass on into the intestine, it too begins its complicated 

 movements. One should always remember that these occur 

 only because certain nerve centers are acting. 



Reflex Centers as Servants of Conscious Centers. It is 

 hardly possible to overestimate the value of the reflex centers; 

 it is only through them that the conscious centers can carry 

 on the multitude of duties which devolve upon them. One's 

 attention cannot possibly be given to all the little details ot 

 living, so these are turned over to the lower parts of the brain 

 and cord, which thus act as a great coterie of servants. To 

 them the whole control of many activities is given after the 

 decision has been made in the conscious centers. Take the 

 common act of walking: one simply directs the lower center 

 to put the walking muscles into proper motion, and then his 

 attention may be wholly devoted to thinking or talking, 

 while the reflex center will superintend the walking move- 

 ments until he tells it to stop. 



That the reflex centers are servants and not independent 

 units is plain when we notice that the higher centers always 

 keep the upper hand, so to speak. With adults, walking be- 

 comes a reflex. Still, by an interference, the conscious 

 centers can interrupt the reflex centers at any time, and one 

 can stand in one place as long as he wishes, starting the walk- 

 ing reflex again when he chooses. The eyes may be getting 

 dry, for example, and the tendency of the neurons in reflex 

 centers is to make the winking muscles contract; neverthe- 

 less, by giving the matter conscious thought, one can inhibit 

 the winking muscles even until he really suffers pain in the 

 eyeballs. One can also stop the breathing reflex center from 

 acting for a considerable period. Some impulses going out 

 from the brain may, therefore, be negative, or inhibitory as 

 we say, and a great many reflex centers, while acting with 

 partial independence of higher control, are yet constantly liable 

 to its dictates, Others, however, like the involuntary 



