300 THE NERVOUS SYSTEM 



though perhaps in not so direct a manner as do the muscular 

 coats of the arteries. While intestinal peristalsis, the secretion 

 of many glands, as, for example, the production of glycogen, bile, 

 etc., cannot be shown to be absolutely dependent on sympathetic 

 connections, yet all these processes nutritive in nature have 

 their normal activity seriously impaired by withdrawal of the 

 sympathetic influence. 



The chief vaso-motor center is in the medulla, though acces- 

 sory centers exist also in the cord; all vaso-motor fibers pass out 

 from these centers and leave the cerebro- spinal axis with the 

 cranial or spinal nerves. 



The most usual mode of action of the vaso-motor nerves is 

 reflex, as when the mucous membrane of the stomach becomes 

 hyperemic upon the introduction of food; or when the salivary 

 secretion increases during mastication, or even sometimes at the 

 sight or thought of food; or when emotions are evidenced by 

 paling or blushing. 



Raising blood-pressure by stimulating the vaso-constrictors 

 and lowering it by stimulating the vaso-dilators are simply 

 mechanical results, and require no comment. 



Sleep. Sleep is closely associated with vaso-motor action. 

 Every part of the body has a function to perform, but it must 

 have some rest from that performance or it will begin to act in- 

 efficiently and finally cease altogether. For most organs these 

 periods of rest occur at approximately uniform intervals, as in 

 case of the stomach, heart or respiratory muscles; but notably 

 in case of the involuntary muscles these periods of repose have 

 no regularity i. e., a person exercises them at no regular time 

 except by accident of occupation or otherwise. But, in any case, 

 there comes a time when repose must be had, for during activity 

 the destructive processes far exceed the constructive, and in 

 order for the balance to be preserved there must be a time when 

 the opposite is true. 



