2Q6 ELEMENTS OF PHYSIOLOGY 



pure food, the habit of stuffing, of awaking the nerves with 

 stimulating condiments, of using alcoholic stimulants, of 

 using tobacco, are easily acquired, but are lost with 

 difficulty. Good habits are good friends; bad habits are 

 enemies. It often requires years of constant effort to root 

 out bad habits, but it is very easy to keep them out in the 

 first place. Yet if the wish and will for a better habit is 

 really strong, one need never despair, for on account of the 

 large size and great activity of the brain, and its prepon- 

 derance over the lower reflex centers, man is distinguished 

 above all other animals by his power of forming new habits. 

 If our ideals are high, we can go on forming better habits, 

 and intrusting them to the keeping of the nerve centers, and 

 thus make step after step toward our ideal. If our ideals 

 are not high, or if they are mere theories and never affect 

 our acts, we may never improve, but may even degenerate. 

 The brain and the spinal cord with the nerves that branch 

 from them form the cerebro-spinal system. 



THE SYMPATHETIC, OR GANGLIONIC, SYSTEM 



548. This System consists of numerous ganglia situated 

 apart from the brain and spinal cord, and many gray nerve 

 fibers which connect these ganglia with the spinal cord, 

 with each other, and with the cells controlled through this 

 system. (What is lacking in the gray fibers that is present 

 in white fibers ?) The sympathetic nervous system con- 

 trols the functions of unconscious life, while the brain and 

 spinal cord control the functions of conscious life. 



549. The chief functions controlled through the sympa- 

 thetic system are the peristalsis of the intestine, the growth 

 of cells, and the contraction of the heart, arteries, and veins. 

 The nerves that go to the arteries and veins are named 

 the vasomotor nerves. The ganglia of the system vary 

 in size from minute specks to the size of a grain of wheat, 

 a pea, or a bean. There is a double row of the large ones 



