152 HUMAN BIOLOGY 



during the night the body gets rid of much of the waste carbon 

 dioxid that is formed during the day. 



225. Exercise. It seems like a contradiction to say that the 

 only way to get more and better muscle is to destroy what we al- 

 ready have. Every one knows, however, that if the muscles of the 

 arm or the leg are not used for a time, they become weak and flabby, 

 and yet every time a muscle is made to contract, some of its substance 

 is oxidized. New muscle, formed by the process of assimilation, 

 must take its place. 



A certain amount of vigorous exercise each day is essential if 

 one's body is to be kept in the best physical condition. This 

 amount, of course, varies with the individual, and it should never be 

 carried to an excess, resulting in exhaustion. Fortunate is the boy 

 who can spend the early years of his life in the country, and who 

 has been taught to do a certain amount of manual work each day out 

 of doors. Regularity in exercise is as important as regularity hi 

 eating. One cannot exercise vigorously one day and expect its good 

 effects to last for a week. We should not call upon the muscles for 

 violent exertion immediately after rising and before breakfast, nor 

 should we exercise until at least a half hour after eating. The 

 physiological reasons for these directions have already been given 

 in our study of the circulatory system (170) . 



The best forms of exercise are those that call into play the great- 

 est number of muscles. For this reason gymnasium training is 

 better than many kinds of outdoor sports. In the gymnasium, too, 

 special forms of exercise may be taken to develop any muscles found 

 to be weak. On the other hand, lawn tennis, golf, rowing, and foot- 

 ball have the additional advantage of being played in the open air, 

 and games of this sort are usually more exhilarating than are set 

 forms of exercise with apparatus. That the full effect' of any kind 

 of exercise may be attained, it should be followed by a moderately 

 warm, then by a cold, shower, or sponge bath, and by a good rubbing 

 of the body with a coarse towel. 



Muscles are not the only tissues developed by exercise Every 

 muscular contraction is directed by some kind of stimulus from the 



