226 Life and Health 



great networks, or plexuses, from which branches pass to 

 the stomach, the liver, the intestines, the kidneys, and other 

 abdominal organs. A similar network of fibers is situated 

 lower down in the pelvis, from which branches are dis- 

 tributed to the pelvic organs. 



(I 343. The General Functions of the Sympathetic System. 

 The sympathetic system exercises a superintending influ- 

 ence over the greater part of the internal organs of the 

 body, controlling to a certain extent the functions of diges- 

 tion, nutrition, circulation, and respiration. 



This influence is generally different from that conveyed 

 to the same organs by fibers running in the spinal or cranial 

 nerves, or is even opposed to it. Thus, irritation of certain 

 cranial nerve fibers, as those of the vagus, slows the heart 

 beat, but the irritation of certain sympathetic fibers which 

 go to the heart quickens its beat. 



The controlling influence of the sympathetic system does 

 not originate in the system itself, but is derived from the 

 spinal cord or from the brain. We may, for convenience, 

 think of this system, not as a separate nervous system, but 

 as an outlying part of the cerebro-spinal system, a series of 

 ganglia through which the fibers of a part of the trunk 

 of each spinal nerve pass on their way to the important 

 internal organs of the body. 



/ 344. Some Special Functions of the Sympathetic System. 

 The sympathetic nerves affect directly only those processes 

 that are beyond the control of the will. 



If itwere not for this action of the sympathetic nerves, 

 the heart would stop beating during sleep, digestion would 

 cease, and breathing would te suspended. Gentle irritation 

 of these nerves, induced by contact of food in the stomach, 

 causes that organ to begin the churning motion needed for 

 digestion. 



