CHAPTER XVI 



THE SYMPATHETIC NERVOUS SYSTEM 



BEFORE we can extend our analysis of the conduction paths 

 into the realm of the visceral activities of the body we must con- 

 sider briefly the sympathetic nervous system through which the 

 regulatory control of these activities is effected. Most of the 

 visceral activities are performed either unconsciously or with 

 very imperfect awareness. The nervous mechanisms of many 

 of them are still obscure. Nevertheless the visceral functions 

 as a whole are of enormous importance, not only in the mainte- 

 nance of the physical welfare of the body, but also as the organic 

 background of the entire conscious life (see p. 259). 



Many of the visceral functions can be performed quite apart 

 from any nervous control whatever by the intrinsic mechanisms 

 of the viscera themselves. The heart musculature, for instance, 

 beats automatically with a characteristic rhythm, and most of 

 the other visceral muscles have the power of automatic rhythmic 

 contraction. Some of the glands of the body may be excited 

 to secretion by chemical substances dissolved in the blood. For 

 instance, when food enters the small intestine from the stomach, 

 the intestinal glands are directly excited to activity by the pres- 

 ence of the food. Some of their secretions are poured out into 

 the intestine to act as digestive juices; others are absorbed di- 

 rectly by the blood (internal secretions). Among the latter is 

 secretin, a substance which is carried by the blood-stream to the 

 pancreas and there excites the secretory activity of this organ to 

 the formation of pancreatic juice, which is, in turn, poured into 

 the intestine. The very complex secretory activities involved in 

 the formation of the intestinal and pancreatic juices under the 

 stimulus offered by the presence of food in the intestine, there- 

 fore, are not directly excited by the nervous system, though they 

 may be brought under nervous control in a secondary way. 



Most of the viscera are, however, under immediate nervous 

 control of two sorts. This control is partly derived from the 



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