302 THE NERVOUS SYSTEM 



The spinal cord centers, or those of the medulla, send 

 efferent impulses to the glands that they may secrete the 

 digestive juices, when afferent impulses have been excited 

 by food taken into the mouth or the stomach. The mus- 

 cular movements of the alimentary canal, called peristal- 

 sis, are reflex. So are the contraction and dilation of 

 the blood vessels in response to sensations of temperature 

 conveyed by afferent nerves to the reflex centers. The 

 growth of the cells throughout the body is also presided 

 over by these centers. 



The muscular movement of reflex action is often greatly 

 out of proportion to the strength of the stimulus received. 

 A slight irritation of the mucous membrane of the trachea 

 may result in a fit of coughing so violent as to bring into 

 action most of the muscles of the trunk and limbs. The 

 mere prick of a pin may cause a man to bound from his 

 chair and execute a series of movements involving hun- 

 dreds of muscles. It is as if the nervous irritation which 

 is brought by an afferent nerve to one of the nerve centers 

 overflowed the cells and fibers of that segment and stimu- 

 lated neighboring centers also. 



440. Voluntary Movements may become Reflex. When a 

 child begins to learn to walk, each separate movement is 

 slowly and carefully directed by the intelligence of the 

 brain. In time, however, the brain is relieved by the 

 spinal cord of nearly all attention to the locomotion of 

 the body, and action which was once voluntary is better 

 done under control of the reflex centers. If we undertake 

 to descend a stairway rapidly, watching and directing each 

 step by the will, we are far more likely to make a misstep 

 and fall than if we pay no attention to the separate move- 

 ments required. So in performing upon musical instru- 

 ments, in writing with a pen or a typewriter, or in riding 



