204 STUDIES IN PHYSIOLOGY 



walk or a fifteen-mile bicycle ride. 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 in 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 been 

 already given in our study of the circulatory system (p. 153). 



The best forms of exercise are those that call into play 

 the greatest number of muscles. For this reason gymnasium 

 training is better than many kinds of outdoor sports. In 

 the gymnasium, too, special forms of exercise can be taken to 

 develop any muscles found to be weak. On the other hand, 

 lawn tennis, golf, rowing and football 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. To secure the full effect of any 

 kind of exercise, 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 nervous system. Before the muscles of 

 the arm or leg contract, a " message " must come to them 

 from the brain or spinal cord ; hence nerve tissue is likewise 

 developed by exercise. 



Rest. If physical exertion is carried beyond a certain 

 point, exhaustion results, and the muscles cannot be made to 

 contract until after a period of rest. Since all muscular 

 contraction involves metabolism of tissue, periods of rest must 

 be allowed for the muscles to get rid of their wastes and to 

 build up new tissue in place of the old. The feeling of 

 weariness after long-continued exercise is probably due to 



