286 APPLIED PHYSIOLOGY 



amount of gentle impulses for the arteries and intestine. When poisons 

 or spoiled food irritate the intestine to a dangerous degree, the sensa- 

 tion goes beyond the ganglia and excites the spinal cells to action. In 

 response they send out direct orders which cause energetic and painful 

 peristalsis to remove the food, in marked contrast with the gentle action 

 caused by the ganglia alone. 



515. Influence of the brain. The brain has some power over 

 the ganglia. Excitement or fear may influence the spinal cord so that 

 it in turn modifies the impulses going through the ganglia. Sorrow 

 seems to depress the ganglia so that the processes of digestion and 

 assimilation are not so well performed, and the nutrition and growth of 

 the cells of the body are diminished. But nature has arranged that after 

 leaving the brain, mental influences shall act through two sets of nerve 

 cells before they can directly affect the nutrition of the body. Thus 

 man's body is protected against injury from his ever-changing moods. 



516. Connection of organs with each other. By means of 

 the sympathetic system, a nervous influence in one organ is spread over 

 all the rest. Because other organs seem to share in the sickness when 

 one is deranged, the nerves controlling them are called sympathetic 

 nerves. Thus, when one organ is deranged, the others act less strongly 

 and impose less work upon the disabled part. 



517. Injury to sympathetic nerves. The sympathetic 

 nerves are less influenced by outside impressions than any 

 other nerves in the body, and great violence is needed to 

 impair their action seriously. Poisons which are swallowed 

 or produced during disease may injure them so that the 

 ganglia almost cease to send out their orders. Then life is 

 endangered, and strong nerve stimulants like strychnine 

 are needed. 



Aside from poisons, almost the only grave danger which 

 may threaten the sympathetic system is a blow upon the 

 abdomen or neck. A hard blow or great pressure just 

 below the ribs may paralyze the solar plexus. The arteries 

 then enlarge and hold so much blood that too little goes 

 to the head and brain. So there is danger of sickness and 

 of death. A blow upon the side of the neck may injure 



