THE MUSCULAR SYSTEM 257 



a greater activity on the part of those that are involuntary 

 and is indirectly a means of exercising them. 



Exercise and Health. In addition to its effects upon 

 the muscles themselves, exercise is recognized as one of 

 the most fundamental factors in the preservation of the 

 health. Practically every process of the body is stimulated 

 and the body as a whole invigorated by exercise properly 

 taken. On the other hand, a lack of exercise has an effect 

 upon the entire body somewhat similar to that observed 

 upon a single muscle. It becomes weak, lacks energy, and 

 in many instances actually loses weight when exercise is 

 omitted. This shows exercise to supply an actual need 

 and to be in harmony with the nature and plan of the 

 body. 



How Exercise benefits the Body. In accounting for the 

 healthful effects of exercise, it must be borne in mind 

 that the body is essentially a motion-producing structure. 

 Furthermore, its plan is such that the movements of its 

 different parts aid indirectly the vital processes. The 

 student will recall instances of such aid, as, for example, 

 the assistance rendered by muscular contractions in the 

 circulation of the blood and lymph, due to the valves in 

 veins and lymph vessels, and the assistance rendered by ab- 

 dominal movements in the propulsion of materials through 

 the food canal. A fact not as yet brought out, however, 

 is that exercise stimulates nutritive changes in the cells, 

 thereby imparting to them new vigor and vitality. While 

 this effect of exercise cannot be fully accounted for, two 

 conditions that undoubtedly influence it are the following? 



1. Exercise causes the blood to circulate more rapidly. 



2. Exercise increases the movement of the lymph 

 through the lymph vessels. 



The increase in the flow of the blood and the lymph 



