310 THE POPULAR SCIENCE MONTHLY. 



favorable effect upon all the functions which, take part in the 

 changes through which the food must pass before being con- 

 verted into tissue, namely, digestion, absorption, circulation, oxi- 

 dation, and assimilation ; but it likewise aids in preventing de- 

 rangement of these functions that is, a large number of diseases. 

 The following resume of the effects of exercise will show that its 

 value has not been overestimated : 



First. Muscular contraction exerts a pressure upon the veins 

 and lymphatics, thus pushing forward and facilitating the flow 

 of venous blood and lymph, to the heart. In this manner the ex- 

 cretion of the products of tissue waste is enhanced. These mat- 

 ters are washed out of the tissues by the blood and lymph, and 

 after their return to the heart pass through the lungs, where the 

 carbonic acid is given off, then through the general circulation, 

 the remaining substances being eliminated by the skin and kid- 

 neys. When these matters, some of which are highly poisonous, 

 collect in abnormal quantities in the system, they become more 

 or less dangerous ; even such mild symptoms as headache, drowsi- 

 ness, and general lassitude in those who lead a sedentary life may 

 probably, in many instances, be traced to their toxic effects. By 

 muscular exercise, which hastens the elimination of these sub- 

 stances, therefore, many slight ailments, which, however, are suf- 

 ficient to make labor burdensome and rob life of many of its 

 pleasures, may be avoided. 



Second. The circulation is controlled mainly by the action of 

 the heart. When the activity of this organ is increased, there- 

 fore, the general circulation will be improved. Now, the heart is 

 stimulated to action by the presence of blood in its cavities, and 

 muscular exercise, by hastening the flow of venous blood, will be 

 instrumental in sending more fluid through them in a given 

 period of time, and consequently in stimulating the organ to in- 

 creased activity. As many diseases, prominent among which are 

 those of the abdominal and pelvic organs, are the consequences of 

 congestion, and as good circulation does much for the prevention 

 of such congestion, muscular exercise, by improving the general 

 circulation both by increasing the activity of the heart and aiding 

 in the venous return, will do much to prevent a large class of 

 diseases. 



Third. The respiratory center is increased in activity when the 

 blood is more venous than usual that is, when the amount of oxy- 

 gen is diminished and the carbonic acid increased. Now, as an 

 organ consumes more oxygen and gives off more carbonic acid 

 when it is actively engaged in the performance of its functions, it 

 follows that exercise exerts a stimulating effect upon respiration 

 by making the blood more venous. When the activity of respira- 

 tion increases, a larger quantity of oxygen enters the system ; 



