THE SYMPATHETIC NERVOUS SYSTEM 285 



main branches in the chest and abdomen, nerves and small 

 ganglia form intricate networks, each called a plexus. 



Just back of the stomach there is a large and important 

 plexus called the solar plexus, whose nerves supply the 

 muscles of the organs of the abdomen. A plexus within 

 the heart controls the action of the heart. 



512. Sensory sympathetic nerves. The sympathetic 

 nerves carry both sensory and motor impulses, but only 

 faint impulses of pain and touch. Thus the circulation 

 of the blood and digestion of food go on almost without 

 our knowledge, but a very strong irritation may give rise 

 to an abdominal pain, as in colic. 



Sensory impulses telling of the wants of the cells and of 

 the need of secretion or movements of the arteries or digest- 

 ive organs are continually being received by the ganglia. 

 These impulses travel slowly and require some time to 

 produce an impression. Most of them travel only to the 

 cells in the ganglia, and few get farther than to the cells 

 in the spinal cord. Only very strong impressions, whose 

 cause may injure the body, reach the brain and produce a 

 feeling of pain, hunger, thirst, or fatigue. 



513. Motor sympathetic nerves. The ganglia send 

 orders to the epithelial cells of the glands to produce 

 their secretions, and to the muscles of the intestine and 

 arteries either to contract or to dilate. They do this in 

 response to information furnished by the sensory nerves. 

 They also send out orders for the growth and nutrition 

 of the cells of the body on receipt of news of their needs. 

 Most orders from the ganglia are reflex. 



514. Mode of action of the ganglia. If cut off from connec- 

 tion with the cord, the ganglia send few impulses. The cord seems 

 to furnish them with a supply of nervous energy. They seem to take 

 a small amount of its active impulses and transform it into a large 



