THE NERVES AND SPINAL CORD 



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help jumping away. This is called a reflex act, because 

 the pain seems to be reflected back as motion. 



225. Orders for the growth of cells. The spinal cord 

 also sends most of the orders for the cells to eat and grow, 

 and for the glands to work, and for the arteries to become 

 large or small, as each part of the body has need of blood. 

 These are nearly all reflex acts. When the cells of the 

 stomach feel food touching them, they tell the spinal cord, 

 and it sends word to the glands to pour out gastric juice. 



226. The sympathetic nervous system. The spinal cord 

 sends its orders for the growth of cells, for digestion, and 

 for the flow of blood through a minor set of nerve cells 

 and fibers. There are several small collections of nerve 

 cells arranged mostly in a double row down the front of 

 the backbone. Each collection looks like a grain of wheat 

 and is called a ganglion. From the ganglia fine nerves 

 go to the different arteries and glands, and to the stomach 

 and intestine. The ganglia are also connected with the 

 spinal cord and derive most of their power from it. They 

 are really its servants, just as the cord itself is the servant 

 of the brain. The ganglia and nerves are called the sym- 

 pathetic system. 



The sympathetic system sends its orders slowly and regu- 

 larly. It has but little sensation, and is but slightly affected 

 by outside influences. So ordinary causes will not disturb 

 digestion or the flow of blood. 



227. Acquired reflex acts. In order that the brain may 

 be able to do more work, it is continually teaching the 

 spinal cord how to send the proper messages alone. When 

 a baby first learns to walk, its brain has to tell the spinal 

 cord just how far to order the feet to be moved, and when 

 and in what direction. So it is hard and slow work for 

 the baby to walk, but in a little while the spinal cord learns 



