NERVOUS SYSTEM 391 



food. It is not necessary, however, for us to eat the 

 food after it is in the mouth, or even to pick it up. 

 We may decide to le^ve it. In voluntary actions, the 

 mind controls the stimuli which are sent out. 



When we are awake countless nerve impulses keep 

 pouring into our brains. Of some of these we are con- 

 scious, and we may see, hear, or taste. The impressions 

 which they produce are more or less lasting. Some are 

 retained for only, a few minutes, but others may be 

 retained for years. In some way these impressions are 

 stored away in our brains and constitute our memory. 



There is much about the activity of the nervous 

 system which at present no one seems to know. For 

 example, we can not explain exactly how nerve cur- 

 rents pass along the nerves, or from one cell to another. 

 We often compare nerve currents with electric cur- 

 rents. They are, however, not the same. Nerve 

 currents travel only about one hundred feet per second ; 

 electric currents travel 'thousands of times more rap- 

 idly. Nerve currents wear out the nerve tissue; elec- 

 tricity seems to have no permanent effect on the wire 

 over which it passes. We likewise do not know just 

 how impressions are formed in our brains, or how they 

 can be retained there. 



Habits. When we first attempt to do a thing, we 

 do it slowly and awkwardly. Each time we repeat the 

 same thing we do it more rapidly and more accurately. 

 The first time a nerve current passes over a certain set 

 of nerves and through certain centers it does so slowly. 



