406 THE NERVES AND THE SENSE ORGANS 



the body. It can be used for getting new impressions; 

 we can learn, and we can think. Memory belongs to the 

 cerebrum alone. But the work of getting new impressions 

 is very taxing to the cerebrum, so there is a device for 

 saving work here also. If we voluntarily perform a 

 certain act again and again, our memory becomes more 

 and more definite regarding the act, and the brain needs 

 to make less and less effort to have it repeated. So we 

 finally acquire the power of reflex action in doing a thing 

 that was at first entirely voluntary. Such acts are called 

 acquired reflex acts, to distinguish them from reflex acts 

 that are natural to us. We call acquired reflex acts 

 habits. 



The power of forming habits is a great economy for the mind, for 

 the reason that the nervous effort required in the learning of a new 

 trick or trade, or in controlling our tempers, is too great to be kept 

 up always. So the mind, using its power of choice, directs the cere- 

 brum again and again to make the proper " connections " until the act 

 becomes reflex. After that time the mind depends on the acquired 

 " conduction pathways" to carry the correct impulses to the muscles. 



While nature makes a good act easier and easier to perform, it visits 

 its punishment upon us if we perform bad acts, for they become easier 

 and easier, too. Finally, a bad act, which we performed at first only 

 by making a conscious effort, and which filled us with shame and 

 disgust, becomes a masterful evil habit that we find very hard to 

 break. We need to keep up our power of forming habits, and to use 

 that power, deliberately, to form good habits, so that it will be easier 

 and easier for us to do right, and harder and harder to do wrong. 



407. Effects of Alcohol and Tobacco on the Nervous 

 System. We have already learned some of the effects of 

 alcohol and tobacco (cf. 371 and 391). Alcohol inter- 

 feres with digestion, injures the liver and kidneys, and 



